Spintronics in hard drives 22



Spintronics in hard drives 22
Periodical of S.V.A.T. Astatine
Volume 5 | Number 3 | April 2011
Spintronics in hard drives
Genetically modified food
Arranging atoms
The “ATtentie” is the periodical of S.V.A.T. Astatine, which is issued four times a year. The ATtentie is distributed
among members of Astatine and employees connected to Advanced Technology at the University of Twente.
Volume: 5
Number: 3
Issue: 21
Copies: 415
Date of issue: April 2011
Editorial staff:
Pim Muilwijk Geert Folkertsma Jeroen van den Berg Daan in den Berken Monique Parfitt Editor in chief, Layout
Editor, Layout
Board member
S.V.A.T. Astatine
t.a.v. ATtentie
Post office box 217
7500 AE Enschede
Tel. 053-489 4450
Bank: 1475.73.769 (Rabobank)
[email protected]
Drukkerij Van den Bosch & Fikkert B.V. - http://www.druk-bosfik.nl
With thanks:
Brigitte Bruijns, Daan in den Berken, Geert Folkertsma, Jelmer Boter, Jeroen van den Berg, Marijke Stehouwer,
Melvin van Melzen, Pim Muilwijk, Thomas Janssen, Jelle van der Veen.
Copy can be delivered to the addresses mentioned above, in .doc(x) or .txt formats. Any figures or pictures can be
bundled with the text in a .zip or .rar file.
The deadline for the next ATtentie: 1 May 2011
© S.V.A.T. Astatine 2011, all rights reserved.
Authors remain responsible for the contents of their works.
The editors preserve the right to modify or reject received articles.
There it is. Finally! Some of you
might even have thought that this
edition would never see the light
of day anymore. Well, even though
it is quite majorly delayed (which
you will notice whilst reading some
of the pieces of the board and the
boardroom), we’d rather have it late
than never.
Normally we would blame this on
understaffing or some other plausible excuse, but this time we only
have ourselves to blame. Writing
this on a sunny day with clear blue
skies we will therefore blame it on a
severe winter depression.
Global Positioning Systems
Alledaagse chemie: E-nummers
Spintronics in hard drives
Genetically modified food
Arranging atoms in artificial
structures of complex oxides
The winter however has passed
and gone away for a while and so
has our depression, so we can now
focus all our energy on the last
remaining edition for this year.
Well, maybe not all of our energy;
we still have a study to complete
(at least, most of us do) and we
don’t want to pay that € 3000,- fine
now do we? Yes, the government
is going through with all the plans
(what a surprise) and additionally
the university thought it would be
a good idea to cut the so-called
“afstudeermaanden“ for people
who don’t receive a “prestatiebeurs“.
With thanks to the current government this means Master students,
who have seen their basic awards
disappear like snow before the
sun. This is exactly the reason why
I don’t like politics, in my view a
democracy is still just a dictatorship
by the majority.
That’s enough ranting for now, I’m
starting to sound like a bitter old
man, even though I’ve enjoyed all
the benefits that future students
will come to miss.
Instead, let’s just hope that the
weather stays this nice, so you
can read this edition outside.
Furthermore, I wish upon you lots
of ice cream and scarcely dressed
women (or men in case you’re
into that) in the remainder of the
academic year. Carpe diem!
Pim Muilwijk, editor in chief
CSI: The Future
Grolsch excursie
Interview: Arie van Houselt
KIVI Niria: Mindmapping
From the chairman
From the AT boardroom
Jelle van der Veen
Some talk about it had already been heard the three
months before this academic year. Three Advanced
Technology students (Kylie, Tom and me) had wild
plans for a new committee. In the time before,
occasionally some serious discussion had taken
place concerning interesting movies, music, food
and about everything else you can imagine. Some
committee had to be found to provide an output for
these creative feelings.
Coincidentally, in the summer of 2010 Advanced
Technology became an international study, bringing its
fair share of radical changes and foreign students. All
courses had to be translated; furthermore our Study
Association Astatine changed its language on posters
and the website to English. But the language barrier is
not the only thing that had to be overcome, students
from different countries bring different cultures,
which seemed another reason to start a committee
concerning culture.
That is why the Culture Committee (CultCo) of S.V.A.T.
Astatine was founded. The goal of this committee
is the introduction and integration of the cultures
amongst the students of Advanced Technology and to
provide an output for creative feelings of all Advanced
Technology students. Which student wouldn’t want to
visit some fine concert, watch some good movies, or
have a nice dinner? The CultCo will try to provide in all
of these needs. Of course the ATAC could organssuch
activities. But we have the feeling the CultCo has a
different goal, rather than just amuse the students,
which is valuable as well, we will try to develop the 3 C’s
in Advanced Technology students: Criticism, Creativity
and Contentment.
A way of reaching these goals is for instance the idea of
a combined movie night and a subsequent discussion.
We hope to develop people’s creativity by showing
them the possibilities within movies, possibly even
more using a movie directing workshop, criticism by
encouraging people to participate in discussion. Finally
the contentment will be provided by a good movie,
some laughs, some tears, but a satisfied feeling at the
end. (And for the people that don’t like the movie of
course some fine drinks.)
At the time of writing the CultCo has just organised
its first activity, the CultCo Tapas Night. On this night
everyone could bring a homemade dish, share it with
other people and try something new. This resulted
in a table filled with all kinds of food, from quiche to
wraps and from bread rolls to filled vegetables. Where
the contentment came in here is pretty clear. Everyone
had a great dinner with lots of good food. The criticism
was in the discussion about which dish was best and
why. To me personally, I liked the salmon quiche made
by Monique best. Creativity was present in the creation
of your own personal dish. So finally the first activity
was a great success, we hope that at future activities a
little more people will be present, though 12 people is
acceptable and the presence of Astatine’s first chairman
compensated for a lot.
The Astatine Cultco
Bring your own ideas
Even though the CultCo has just started, we hope that
we can be a contribution to Astatine in the coming time
and that we can help foreign students to get used to
studying in Enschede, and additionally bringing some
other cultures to everyone’s attention. Should you feel
like you have an important contribution to make to
the introduction of your (or any other) culture within
Astatine, please let us know, we will surely listen to any
suggestions. If you don’t have the slightest clue who we
are, I suggest you start examining our CultCo picture
here or on the Astatine website!
Daan in den Berken
Each year, several Astatine members organise a 5-day
study trip to a country in Europe. For the past 3 years
the destinations have been England/France, Sweden/
Denmark and Switzerland. Since a few days I am not
bound anymore to keep our destination a close kept
secret: we are going to Berlin and Prague! And thus
Germany and the Czech Republic can join the list of
When the day is finally over, at midnight the bus will
take us all back to where we started from. On Sunday
morning you will be back in Enschede.
To make all this happen five of us started working on
organising this trip in early November, brainstorming
on destinations, searching for interesting companies to
visit and setting up appointments with said companies.
And to make all of this affordable for everyone, finding
sponsors to make this happen.
I make you braver than ever before,
Golden power that all adore,
My cousins keep you fed,
But I keep your throat wet
I start as a batter,
Stuffed with sweet,
Burned to a crisp,
I am good to eat
I run but cannot walk,
sometimes sing but never talk,
Lack arms, but have hands,
lack a head but have a face.
I hope you sign up and to see you on May 18th when we
depart for our destinations!
(St)one, translate to german Ein Stein, Einstein
lived in Prague
Once you arrive in this city, which has been the set
of countless of movies, you will have the whole day
to yourself to explore one of the most beautiful and
unspoiled cities of Europe. You will be free to explore its
beautiful bridges, its castles, churches and vistas from
the surrounding hills, and to look at its Romanesque,
Gothic, Renaissance, Baroque, and Rococo architectural
A word numbering letters five,
Remove two
One survives
Beer, the Czech Republic has the highest per
capita consumption
We depart for our first excursion early in the morning
on May 18th. After dinner, you will be able to explore
Berlin and it’s nightlife to your heart’s content, though
keeping in mind that you will have to be up early the
next day. The next two days will be filled with 2 more excursions per day until we finally reach Prague on Friday.
Before our destination was revealed a few weeks ago, I
gave you a small series of riddles, each concerning our
destination in a way. Perhaps you were able to figure
out where we were going:
Berlinerbol, a pastry named after Berlin
But as the saying goes, “All work and no play makes Jack
a dull boy”. So what kind of fun do we have on these
trips? Well, after several hours of bus travel and one of
the above mentioned excursions, we will be near a city
with an active nightlife (fun fact: apparently our Prague
hostel is situated above a nightclub) scene. And while
we hope you will not be too hung-over the next day,
there will be some time for fun.
fltr: Peter Puttkammer, Loek Nijsten, Teun Bartelds, Mel
Burger en Daan in den Berken
Clock, the famous Orloj Clock is to be found in Prague
As the words study trip imply, the journey will have
educational value. While abroad, the group visits five
institutions, companies or universities. Last year this list
consisted of CERN, Ruag Space, ETH Zürich and Bayer.
With topics ranging from particle physics, shielding for
space rockets, robotics and chemistry, these excursions
illustrate why we are all studying so hard, as well as
what we could be working on in just a few short years.
From the chairman
Jelmer Boter
Last year, 2010, was one of the warmest years since the
start of weather monitoring. The mean temperature
over the first eight months was 14.7 ºC, the same as
in the record year 1998. The expectation is that 1998
will retain its record, because of a chilly December. It’s
December now and it is true: it is cold, it is snowing
and it is not the first year with a cold and snowy winter.
While bicycling to the campus I sometimes wonder if
there actually is a climate change. Is the world really
getting hotter? Yes, of course it is, but sometimes things
like to contradict each other and the world does not
seem to be the way it is. There is another contradiction
you all might have heard of, which will affect us all.
The Netherlands is a so-called “kenniseconomie”, which
means that the major part of economic growth comes
from (technical) knowledge and its implementation. This
is because of the Dutch students being entrepreneurs.
They develop themselves also outside their studies and
become skilled persons in a broad domain. In Holland
we are proud of our entrepreneurial students being
student board members, doing committees, taking
part in joint decision making and organising lots of
activities. It is for this reason Dutch employees are
wanted all over the world. They are not only skilled in
their own area, but got so many other competences.
Sounds nice, but here comes the contradiction.
The new minority cabinet of Prime Minister Rutte, with
CDA, VVD and the support of Wilders’ PVV, wants to
encourage quick studying. They invented the C+1 rule,
which means you will be fined if you take more than
one extra year for your study. For each extra year you
will have to pay €3.000 extra tuiton fee. While writing
this article the first modification has already been
made. You may take one more year for your bachelor
and one for your master, so a total of two years extra. It
is a step in the right direction, but we are not there yet.
The objective the cabinet wants to accomplish with
these plans, less unnecessary study delay, is not bad
at all. The education part of “het regeerakkoord” starts
with: the Netherlands have the ambition to belong to
the top five knowledge economies. This asks for better
quality of education and promotes better results. I agree
with them that 10 years is far too long to study. The
problem is they do not see the necessary expectations
to remain a real knowledge economy, e.g. for student
We are proud of our students, so please let us remain
proud of them and make it possible and attractive for
students to be active.
There is more. The government wants to discard the
student grant for Master students and replace it with
a social feudal system. Each MSc. has to pay at least
€6.000 after his graduation. They want each student
to invest in their own future in exchange for higher
qualilty education. But how do they want to improve
education with less money?
The plans are a disaster for Holland and have to change.
The Student Union founded www.kenniscrisis.nl and
the first actions are behind us. The van Heekplein
was crowded with students on the 16th of December
and on the 21st of January all students in the
Netherlands will unite or were united (depending on
the moment you are reading this) with each other in
a great manifestation on the Malieveld in The Hague.
Hopefully, the government receives this message and
will think one more time about their plans. Let old times
of student protests revive and show them what we can
Recently we got lost of some pains in the ass of the
cabinet and the number of AT graduates almost
doubled. The Bachelor graduation on the 3rd of
November was a memorable day for 24 former AT
students who received the crystal with Leonarde Da
Vinci’s helicopter. They spread out and are following
all different kinds of Masters right now. I want to
congratulate them all and wish them luck with the next
part of their education, the Master.
One of the things we may have to miss if there are no
active students is very near for us. We celebrated the
fifth birthday of Astatine on the 15th of December and
in just a few weeks there will be a week full of activities
and parties: the first Lustrum of S.V.A.T. Astatine.
Visit www.astatinelustrum.nl and find out about the
activities the lustrum committee has in mind for you.
Let this week be a hell of a succes and something to
remember. The first lustrum related activity was the
symposium CSI: The Future, which has a nice link with
the theme ”Small going big”. It was the most succesful
symposium in the history of Astatine.
I hope to see you all during the lustrum and again I will
end this article with a beautiful Dutch sentence:
“Op de Hoogste !”
From the AT boardroom
Marijke Stehouwer (study advisor)
As I am writing this, it is the beginning of January,
and everybody is still full of New Years’s resolutions.
My personal resolutions are to exercise more, run
the “Batavierenrace”, eat more cookies & chocolate
and in general: to live life to the fullest! The staff of
AT intends to keep up the good work, and improve
where possible. We’ll help Astatine celebrate their
fifth birthday in February (looks like it’s going to be
a great party!), contribute to the development of
the plans concerning the University College and film
“AT, the movie” (a new promotional movie for our
Bachelor site).
2011 promises to be a year of many changes. Due to
budget cuts the structure of the UT is going to have to
be altered. You may have heard of the plans of the UT’s
executive board to narrow the number of Bachelors
down from 23 to 10, it is likely that these plans will also
affect AT. At this moment it’s ”wait and see”, we will keep
you posted.
Some of you might join the protest march against
the government’s plans for budget cuts on higher
education in The Hague on the 21st of January. At
present a lot is still unclear and nothing is definite yet.
We will make sure that students for whom the €3000,fine might become a problem are informed and hope
that you’ll come to us with your questions.
“AT, the movie” is at the time of writing not even finished
but we already have 25 prospective AT-students who
have registered through “Studielink”. This is about twice
as much as last year at this time! An increasing number
or foreign students are finding out about Advanced
Technology and both Monique (our role-model) and
me are receiving e-mails from “all over the place”. We are
looking forward to meeting all our new students and
we hope they will all fit into HT900 and the labs.
We are also looking into housing for our international
Bachelor students. At this moment, a room cannot be
guaranteed for international Bachelor students, and
this might become a problem. One of the reasons
for this problem is that the people living in student
flats are allowed to choose their new flatmate, and
given the choice, they often prefer a Dutch student
over an international one. So, in case one of your
flatmates decides to leave before the next academic
year: please consider an international flatmate. It will
make your everyday life so much more interesting and
Global Positioning Systems
Geert Folkertsma
The system
At the boy scouts, I was taught how to find your (up to
that moment unknown) position on a map: by finding
the North Pole, identifying the type of terrain and
looking for particular crossroads and the shape of
roads and creeks. I liked it and, notwithstanding my
path-finding inability, with a map I can always find my
way. Nowadays however, it is a superfluous skill, as
most cars, cell phones and tablet PCs come with builtin navigation. All these route-finding enhancements
are based on one system: GPS.
Although the “known points” from which distance
measurements are done could also be transmitters
located on the earth, they are satellites because this
allows reception over the whole world. There are about
30 active GPS satellites to ensure good coverage.
The GPS receiver is another part of the system, because
this is where all the position calculations are done. It
includes error correction and nowadays also uses other
sources than the satellite signals to determine position.
The principle
To monitor the satellites and correct mistakes (such as
a satellite being knocked off course by space debris,
or errors in the clock of the satellites), there are also
ground stations located mainly in the United States
(GPS was first used by US military and still maintained
by them).
A position measurement with GPS (Global Positioning
System) uses the finite speed of light to determine
distances to known points in space with a Time-of-Flight
measurement. The known points in space are satellites
that send a radio signal with their current time, tsend.
The receiver compares the received timestamps with
the time of reception treceive and thus knows the travel
time or ToF. The distance is then easy: d=c · (treceive - tsend).
This defines a sphere around the satellite with radius d
on which the receiver may be. Finding a second sphere
from another satellite reduces the possible positions
to the intersection of the two spheres: a circle. A third
satellite further reduces the position to two points.
For navigation systems this is sufficient: the point that
is located on the Earth’s surface (a fourth sphere) is
the actual position! GPS receivers for aerial or space
vehicles require a fourth measurement. This method is
shown graphically in figure 1 below.
The GPS satellites (see a picture in figure 2) have but
one purpose: broadcast the time to Earth, from a
known location in space. A lot of people think that GPS
satellites are in a geostationairy orbit, but this is not
necessary and not the case: it does not matter if the
position with respect to earth is not constant, as long
as the orbit (and thus position at any given time) is
precisely known. As it is, GPS satellites have an altitude
of about 20Mm, flying the same ground track every day.
first measurement from
satellite 1 places the
receiver on a sphere (a).
A second satellite’s sphere
intersects with the first’s
to form a circle of possible
locations (b). The third
satellite reduces this to
two points (c), one of
which is also located on
the earth (d). For airbound
applications, the earth
with a fourth satellite
The first part of the message (1 “subframe”) is the exact
satellite time, which is encoded as a week number
and the seconds since the start of the week. GPS time
does not correct for the earth’s rotation, so it ignores
leap seconds and the like, resulting in an offset that
occasionally increases and currently is about 20
seconds – this difference must therefore also be sent
The second part of the message (subframe 2 and 3)
is called the ephemeral and contains precise orbital
information of the satellite, so the receiver knows
exactly where the centre of the sphere should be.
The entire ephemeral must be received to determine
the position, which could take several messages if
reception is not too good.
The last part (subframe 4 and 5) is the almanac , which
provides information on the approximate location of
all satellites – which tells the receiver which satellites
should be visible, so which satellites to listen for. Because
the almanac is quite large, it takes 25 messages to send
it completely. Therefore, newly bought GPS receivers
need quite some time to get their first location fix.
Figure 2: A GPS satellite (here without the solar panels
attached) is rather small. The protrusions on the top are
the antennas to broadcast the GPS message and to receive
updates and corrections from ground stations.
The most important parts of the satellite are the
clock and the radio transmitter. Because the time has
to be very precise (an error of 1µs leads to a distance
deviation of 300m), the GPS satellite carries a couple of
atomic clocks. They have to be corrected for relativistic
effects to keep in pace with the clocks on earth!
The satellite sends a specific message containing
not only the time, but also additional data (see GPS
message below). All the satellites’ radios transmit at the
same two frequencies, around 1.22 and 1.58 GHz (new
satellites also broadcast around 1.17 GHz). They use
CDMA spread-spectrum modulation with an encoding
specific for each satellite (see Encoding).
GPS message
Twice every minute, the satellite broadcasts a message
with a very low bitrate: only 50 bits per second. This
results in a total message length of 1.5KB, less than the
textual information on this page.
The message is sent on a frequency modulated signal
around 1GHz, which is simple: modulation at the
satellite and demodulation with the same frequency
at the receiver. This is not enough though: all satellites
broadcast at the same frequency, so it would be
impossible to distinguish between them. So, before
going to the transmitter, the message is modulated –
not with a sine such as in the radio, but with a so-called
Pseudo-Random Noise sequence, a technique called
CDMA. This is a set of zeros and ones, unique for each
satellite. Because the PRN is different for each satellite,
the individual satellite messages can be received by
“tuning in” to the right PRN (see figure 3 for a schematic
1.5 GHz
carrier signal
message 1
msg 2
msg 3
Figure 3: The message is modulated with a PRN and
then with a normal carrier signal (this graph shows the
receiving end’s demodulation setup).
This PRN is also how we civilians only get an accuracy of
a couple of metres, while the US army has receivers that
go down to centimeters: there is a second, secret and
encrypted code that contains more precise information.
This code, called the P(Y) code, is only available to the
US army. “Our” code is called C/A, Coarse Acquisition.
Ground stations
Errors are bound to creep into the satellites: the clock
will eventually deviate, and because of collisions with
space debris or gravitational pull of other objects, the
satellites will not stay perfectly on course. Therefore,
there are several ground stations that monitor the exact
orbits of the GPS satellites. They send their information
to a master control station, which updates the
ephemeral in each satellite and adjusts their internal
clocks so all GPS satellites have the same time.
If a satellite is too far off course, it will be marked
unhealthy, which means that GPS receivers don’t include
that satellite’s data in their position determination.
Then the satellite uses its built-in thrusters to get back
on course, after which the ephermal is updated and the
satellite marked healthy again.
End-user devices
The principle on which the position determination is
based is fairly simple, as described at the beginning of
this article. Of course, there’s more to it than calculating
flight times and doing some geometric calculations.
Firstly, the radio message must be decoded. The
demodulation is simple, by mixing the radio signal
with the 1.5GHz carrier signal (see figure 3 again). The
decoding of the CDMA signal is a bit harder though: the
modulation signal is no sine, but a seemingly random
sequence of bits. This means that it matters at what
point in time the PRN is mixed with the CDMA signal.
This is done by trial-and-error: the PRN signal is shifted
forward 1 bit until something else than gibberish
comes out.
Then, as soon as there is a signal on one PRN code, the
almanac has to be downloaded so the receiver knows
what other satellites (=PRNs) to look for. After its first
use, the almanac is roughly known, so this information
is already known – unless you’ve travelled a long
distance with the receiver turned off.
Finally, the ephemerals of all the satellites are
downloaded and the position can be calculated.
Because the ToF can be determined at the start of each
subframe, a complete new position can be calculated
each 6 seconds.
Figure 4: From top to bottom the GPS satellite, a
(discontinued) ground station and an example of a GPS
receiver, measuring only a couple of cm in height.
Receiver challenges
Navigation paradox
As was mentioned before, the clock needs to be very
accurate to get a good position estimation. This is no
big problem for the satellites, which are equipped
with atomic clocks and are synchronised by ground
stations. The receivers on the other hand are built
into telephones and TomToms, where no such large
and expensive clock can be used. The clock error then
causes the fourth sphere (see figure 1) to not intersect
with any of the two possible locations… Therefore,
more satellites are monitored and, combining all the
spheres into the best position estimation, the clock
error is minimised.
An interesting paradox was introduced in 1964 by
Peter Reich for airplanes, but can be expanded to
general navigation. The paradox states that increased
navigational precision (smaller errors in position
determination and path following) may result in more
collisions. It can be explained as follows: if everyone
(airplane, ship) tries to follow a certain optimal path,
usually the shortest distance, then the risk of two
vehicles actually being on that optimal route and
thus crashing into eachother increases if the vehicles
get better at sticking close to that path. A similar
effect occurs when a lot of people use a (for example)
TomTom to reach a destination, all of which calculate
the same optimal route: resulting in increased traffic on
that route and possibly queues.
Another problem is that of reception: cheap GPS
receivers (or expensive ones used indoors or between
high buildings) take a long time to get a position fix – if
they succeed at all. This is because the entire ephemeral
of each satellite is required to get a location. If there is
too much noise or the connection is interrupted during
ephemeral reception, the device has to wait for the
next message. Even in the best case, it takes at least
30s before the ephemeral is received. The ephemeral
is valid up to 4 hours, so after the initial fix, a broken
connection is not disastrous: the old one can be used
and every 6s the ToF is measured.
So, when we get better at navigating, we should care
to either get worse at finding fast routes, or take this
knowledge into account when finding an optimal path.
Modern GPS receivers, especially the ones in
smartphones, use so-called assisted GPS or a-GPS.
This is a general term to indicate that the receiver uses
more sources than just the satellite signals: it could
determine its general position by finding out which
GSM site it is connected to, or download the almanac
and ephemerals through the cellular data network.
Other systems
Although this article focuses on GPS – which is used
in all our smartphones, navigation systems and other
location-aware gadgets – there are more systems.
Rusland has their own system (GLONASS) which was
introduced by the Soviet Union, fallen into disrepair
and recently refurbished. Europe (the EU and ESA)
is working on Galileo, an improved version of GPS.
Modern GPS receivers should be compatible with
this system, combining GPS and Galileo satellites to
increase performance.
Then there are some local systems in China and Japan,
others are in development (India is working on one;
China is expanding their local system to become
CSI: The Future
Brigitte Bruijns
CSI, Crime Scene Investigation: almost everybody
knows the famous television series. Those series
give a nice insight in the work of a forensic scientist.
However, what you see on television is really
the desired future perspective of every forensic
CSI: The Series
In forensic institutes, such as the Netherland Forensic
Institue (NFI), the real world is very different from the
television world. The tasks of a forensic scientist are a
bit more (or just less?) complicated. Unfortunately it is
not (yet) possible to obtain a DNA-profile on the crime
scene within a few minutes. It is also not that easy to
acquire a fingerprint from every arbitrary material
possible on a crime scene.
Also the interrogation of the suspect is for the police
(tactische reserche) and the final conviction is up to the
In countries where they have the common law system
(in the Netherlands we have the civil law system) the
jury plays an important role in the conviction of the
A forensic scientist can never tell you who the
perpetrator was. They can only give a judgement of
the evidence itself. In a forensic testimony you would
never see the statement that the DNA-profile is from
the suspect. What you can find is if there is a match
between the profile found on the crime scene and
the profile of the suspect. Next to that the rarity of the
profile is given. But the expert has to watch out for the
prosecuter’s fallacy[1].
CSI: The Future
Now that a few pitfalls of forensic research have been
discussed, the focus can be on the future again. To
show the real future of crime scene investigation a
symposium was organised, with as topic the use of labon-a-chip technology in forensics.
Figure 1: Forensic research on a crime scene.
The forensic scientists in series such as CSI (Las Vegas,
Miami and New York) have four tasks: 1. Collection
of the evidence, 2. Examination of the evidence, 3.
Interrogation of the suspect and 4. Conviction of the
perpetrator. Only one of these tasks is the real job of
a forensic scientist. The crime scene is for the police
(technische reserche); only in exceptional situations a
forensic scientist will be present on a crime scene. After
collection of the evidence the obtained “SVO’s” (stuk
van overtuiging – piece of evidence) will be sent to
a forensic laboratory. The forensic expert will
examine the evidence and will write down
12 his or her findings in a forensic testimony.
Figure 2: Lab-on-a-chip device for forensic investigations?
The variety of forensic traces found at a crime scene is
enormous. The term forensic science is therefore very
broad and can be divided in several classes, such as
DNA (deoxyribonucleic acid), blood, explosives and
drugs. Analyses have to be simpler, faster, more robust,
cheaper and also improved sensitivity and selectivity
is wanted. Especially devices that can be used directly
on the crime scene are wanted. Most ideal would be
a mobile forensic lab for collecting, screening and
analysis of the evidence.
CSI: The Future
Devices known as a ”lab-on-a-chip” (LOC) can speed up
the analysis, are compact and can be easily integrated
and used by people who are not technically trained.
However, micro-devices for forensic research hardly
exist. Experts in LOC technology and/or nanotechnology
do not have a lot of experience and knowledge about
forensic science. Otherwise forensic experts are in
general not very familiar with LOC devices. The two
disciplines are not yet combined in order to obtain a
LOC device for forensic research.
Severine Le Gac told us more about her PhD project
in which she developed a chip for mass spectrometry.
Also Medimate was present to give an insight in the
possibilities of on chip analysis for point-of-care devices.
CSI The Hague is the new project of the NFI and several
partners (Thales, Philips and universities). The goal of
the project is to visualise and virtualise the crime scene.
With this technique it is possible to visit the crime scene
digitally for the police, forensic investigators, but also
for legal institutions [4].
CSI: The Symposium
The first speaker of the symposium, Arian van Asten, is a
forensic scientist himself. He is head of the department
Physical and Chemical Technology of the NFI. In this
department research takes place on explosives and
explosions, weapons and all kinds of physical traces.
During his talk he told a bit more about the NFI and
he explained the work of a forensic scientist. There are
five very basic questions that are the fundamentals
of forensic evidence examination: where, what, who,
when and how.
Han Gardeniers, head of the Mesoscale Chemical
Systems group, was the second speaker of the day and
he gave the participants an insight in what is already
possible regarding lab-on-a-chip technology.
Ate Kloosterman is forensic DNA-expert at the NFI
and the expert in the field of DNA-profiles. He told
more about the MIDAS (millifluidic identification DNA
analysis system) project[2]. Part of this system, the chip
for the capillary electrophoresis, is made by Micronit.
In their presentation they went a bit more into detail
about the system, which is also shown in figure 3[3].
Figure 4: CSI: The Symposium was well attended.
It was a very successful day with a broad range of
speakers, talks and participants. Forensic research is
defnitly multidisciplinary and still new developments
are made!
[3] A.J. Hopwood, C. Hurth, J. Yang, Z. Cai, N. Moran,
J.G. Lee-Edghill, A. Nordquist, R. Lenigk, M.D. Estes, J.P.
Haley, et al. Integrated Microuidic System for Rapid
Forensic DNA Analysis: Sample Collection to DNA
Profile. Analytical Chemistry, pages 184-189.
Figure 3: The system of the MIDAS project.
KKPHk6UU, page 3
Alledaagse chemie: E-nummers
Thomas Janssen, herpublicatie uit G-mi
E-nummers zijn chemische toevoegingen aan
levensmiddelen, die veel voorkomen in kanten-klaar producten. De ‘E’ staat voor Europees
goedgekeurde toevoeging voor levensmiddelen. De
E-nummers zijn op te delen in zeven categorieën;
kleurstoffen; conserveermiddelen; anti-oxidanten en
voedingszuren; verdikkingsmiddelen, emulgatoren
en stabilisatoren; zuurteregelaars en antiklontermiddelen; smaakversterkers en overig. Overig
bestaat onder andere uit waxen, verpakkingsgassen
en zoetstoffen.
E 100-199: Kleuren
Vroeger werd er nog wel eens loodchromaat gebruikt
om melk en boter mooi geel bij te kleuren, of
loodmenie om snoepjes een mooie rode kleur te geven;
gelukkig is dat nu verboden. Er zijn 42 kleuren die een
E-nummer hebben en veel van deze kleuren hebben
een natuurlijke bron. Bijvoorbeeld E150, dat is karamel
een heeft een bruine kleur. Het is een van de oudste
kleurstoffen die voor eten gebruikt wordt en is precies
hetzelfde als de karamel in snoepjes. Het zit in heel veel
producten, van brood tot whisky en natuurlijk cola.
Figuur 1: E160b, Anatto. Dit is de vet-oplosbare component bixine, C25H30O4
Er is veel discussie over E-nummers. Sommige mensen
vinden dat ze überhaupt niet thuishoren in ons
eten en over sommige E-nummers is discussie of ze
misschien kankerverwekkend zijn of ADHD bevorderen
bij kinderen. Veel mensen zijn een beetje bang voor
E-nummers en proberen ze te vermijden.
Dit artikel gaat verder niet in op die discussie, E-nummers
zijn streng gereguleerd en de industrie springt meestal
snel in op zorgen over bepaalde E-nummers. Waar we
wel naar zullen kijken is waarom E-nummers in ons eten
zitten, wat hun functie is en waar ze vandaan komen.
De helft van de producten die we in de supermarkt
kopen is niet mogelijk zonder E-nummers, niet alleen
de kant-en-klare maaltijden en snoepjes, maar zelfs
simpele dingen als brood, beleg, wijn en tandpasta.
Veel van onze favoriete levensmiddelen zijn niet
mogelijk zonder E-nummers.
E160b heeft een oranje kleur en heet Anatto. Dit wordt
gemaakt van de zaden van de Orleaanboom. Anatto
heeft een vet-oplosbare component, bixine (zie figuur
1) en een water-oplosbare component norbixine. Het
wordt vooral gebruikt om kaas, boter en veel gebak
waar banketbakkersroom in zit bij te kleuren.
E100 is kurkuma, dit heeft een gele kleur en wordt
gemaakt van de wortel van Curcuma longa, een
plant die veel voorkomt in India. Kurkuma wordt
daar al eeuwen gebruikt en geeft kerriepoeder zijn
bekende gele kleur. Kurkuma bestaat voor 5 procent
uit curcumine, dit komt voor in een keto- en een
enolvorm (zie figuur 2 voor de eerstgenoemde). Het
is een antioxidant en heeft een ontstekingsremmende
Figuur 2: E100 of Kurkuma bestaat voor 5% uit curcumine, hier getoond in
ketovorm, C21H20O6
E140 heet chlorofyl en zit in algen, cyanobacteriën en
geeft planten hun groene kleur. Het wordt gewonnen
uit de chorella-alg en gevriesdroogd om een stabiel
poeder te verkrijgen. Er zijn zes structuren van chlorofyl;
E140 wordt gebruikt om pasta’s en Absint te kleuren
(figuur 3).
E161g geeft flamingo’s hun roze kleur. Flamingo’s
worden grijs geboren maar eten enorme hoeveelheden
brak-watergarnalen. Deze bevatten een hoge
concentratie canthaxantine (figuur 6), dat een roze
kleur heeft. Dit hoopt zich op in de veren en geeft
ze hun prachtige roze kleur. Het wordt industrieel
gesynthetiseerd uit bèta-caroteen, de kleurstof in
wortels. Het komt van nature voor in cantharellen en
wordt bijgevoerd aan kweekzalm en flamingo’s in de
dierentuin om ze een mooie roze keur te geven zoals
hun wilde soortgenoten.
Figuur 3: E140 is gewoon chorofyl, C35H30O5N4Mg
De interessantste kleurstof is misschien wel E120, dit is
een paarse kleur die Cochinille heet en dat is een soort
schildluis. Deze luis komt voor in warme landen zoals
Mexico, Chili en Peru, of iets dichter bij huis: Lanzarote.
Boeren daar groeien een speciaal soort schijfcactus die
ze infecteren met zwangere Cochinille-luizen, deze zijn
grijs en slechts 5 mm groot (zie figuur 5). Ze hebben
roodgekleurd karmijnzuur in hun bloed om vijanden
te weren. Na een tijd worden ze met de hand geoogst,
gedroogd en vermalen. Daarna wordt het karmijnzuur
geëxtraheerd (figuur 4) en gestabiliseerd met
aluminium- en calciumzouten. Op jaarbasis worden
er 20 miljard luizen geoogst. De kleur wordt in zeer
veel producten gebruikt zoals roze- en roodgekleurde
toetjes, gebak en frisdranken.
Figuur 5: De Cochinilleluis
Figuur 4: E120, cochinille, bestaat voor een groot deel uit karmijnzuur, C22H20O13
Stel je voor: de magie en kracht van de elementen, beheersbaar gemaakt tot nut van de mens. Het is
dichterbij dan je denkt. Want morgen is vandaag en dat vraagt om nieuwe toepassingen. Bijvoorbeeld
door commerciële technologie aan te wenden voor geneeskundige doelen. Onze gascentrifugetechnologie
is op verschillende manieren inzetbaar. Bijvoorbeeld om efficiënt uranium te verrijken waardoor verrijkingsfabrieken snel en veilig brandstof kunnen leveren voor de productie van kernenergie. Minder bekend is dat op
dezelfde manier ook stabiele isotopen worden geproduceerd. Hiermee kan de medische wetenschap kanker
onderzoeken en bestrijden. Alles draait om de behoeften van de moderne mens. Talenten met een passie
voor complexe technologie die ons leven veraangenaamt kijken op the futurehasarrived.nl. Nu, niet morgen.
Figuur 6: E161g is de roze kleurstof canthaxantine, C40H52O2, die flamingo’s hun roze kleur geeft.
Er zijn ook synthetische kleurstoffen die worden
gemaakt uit petroleum, deze zijn meestal stabieler
dan natuurlijke kleurstoffen. Zes van deze stoffen,
zogenaamde azo-verf, worden nu verdacht van
bijwerkingen zoals ADHD, vooral bij kinderen. Veel
producenten zijn ze daarom aan het vervangen door
natuurlijke alternatieven.
Al deze kleurstoffen worden gebruikt omdat dat veel
producten tijdens processen hun kleur verliezen.
Mensen hebben een bepaalde perceptie van
producten, en dus worden producten bijgekleurd om
aan die perceptie te voldoen. En vreemd genoeg gaan
ze daar ook beter van smaken.
E200-299: Conserveermiddelen
Conserveermiddelen zijn vooral belangrijk in
voorverpakte vleeswaren. Vroeger gebruikten we zout
voor het conserveren van vlees. Je hebt er echter veel
van nodig en dat kan de smaak beïnvloeden, maar
belangrijker is dat het een zeer gevaarlijke bacterie
niet doodt: de Clostridium botulinum. Deze bacterie
produceert het neurotoxine botuline, de veroorzaker
van botulisme. Botuline is het meeste potente gif dat
bekend is en is dodelijk binnen 24 uur. Een halve liter is
genoeg om de wereldbevolking te doden.
Figuur 7: E252 of kaliumnitraat, KNO3
E300-399: Antioxidanten
Veel van onze voedingswaren worden slecht door de
zuurstof in de lucht, het laat producten bruin worden
en zet het rottingsproces in gang. Antioxidanten
voorkomen oxidatie en vertragen de tyrosinaseenzymen die de rotting veroorzaken.
Bier en groenten en fruit in blik blijven goed door E300,
ascorbinezuur (figuur 8), ofwel vitamine C. Niet alleen
fruit en groenten gaan rotten zonder vitamine C, wijzelf
ook. Dat heet scheurbuik. In de achttiende eeuw was
een groot deel van de oorlogsslachtoffers te wijten aan
matrozen die stierven door scheurbuik.
Daarom worden er aan vlees kleine hoeveelheden
E252, kaliumnitraat (figuur 7) toegevoegd. Opgelost
wordt het nitraat om gezet in nitriet en dit is in staat om
de Clostridium botulinum te doden.
Kaliumnitraat wordt in kleine hoeveelheden gebruikt
in veel gedroogd vlees en salami, al wordt hier
tegenwoordig meestal natriumnitraat voor gebruikt.
Kaliumnitraat is vreemd genoeg ook een belangrijk
ingrediënt voor buskruit.
Figuur 8: E300 is vitamine C ofwel ascorbinezuur, C6H8O6
Na 4 tot 6 weken op zee en zonder verse groeten en fruit
kregen matrozen last van rottend tandvlees en
zweren op de huid, ze begonnen letterlijk
te rotten. In 1747 kwam scheepsarts James
Lind, na het proberen van allerlei producten erachter
dat citrusvruchten bijzonder goed werkten tegen
scheurbuik. Dat komt omdat citrusvruchten zeer veel
ascorbinezuur bevatten; ze worden ook niet bruin
wanneer ze blootgesteld worden aan de lucht.
Alleen mensen, mensapen, vleermuizen en cavia’s
moeten ascorbinezuur door hun dieet binnenkrijgen.
Andere dieren maken dit zelf uit glucose in de lever, een
mutatie bij onze voorouders heeft ervoor gezorgd dat
we dit niet meer kunnen.
Water en vet mengen niet, maar er zijn vele producten,
zoals mayonaise, waarin water en vet toch gemengd
worden en blijven. Om dit voor elkaar te krijgen zijn
er emulgatoren. Een van de bekendste is lecithine
ofwel E322 (figuur 9). Lecithine is een mengsel van
glycolipiden, triglyceriden, en fosfolipiden. Bij E322
wordt er echter een specifieke verbinding mee
bedoeld, fosfatidylcholine, een fosfolipide die heel veel
voorkomt in eidooiers. Lecithine heeft een lipofiele
kant en een polair uiteinde, waardoor het een zeer
goede emulgator is. Daarvan wordt gebruik gemaakt
in mayonaise, waaraan eidooiers worden toegevoegd
om olie en water te laten mengen. Lecithine is een
essentieel bestanddeel van iedere lichaamscel en
vormt de celwand. Lecithine wordt veel gewonnen uit
sojabonen maar ook steeds meer uit zonnebloempitten.
In een wereld waarin mensen steeds minder vet in
hun eten willen zijn de emulgatoren, stabilisatoren en
verdikkingsmiddelen heel belangrijk. Zijn kunnen bij
een lager vetgehalte of bij een minder verzadigd vet,
nog steeds het ‘rijke’ mond gevoel geven van vette
Om mengsels van vet en water stabiel te houden en
om ze de juiste dikte te geven zijn er de stabilisatoren
en verdikkingsmiddelen. Een goed voorbeeld van de
werking van een stabilisator is slasaus. De stabilisator
zorgt er voor dat de kruiden netjes verdeeld blijven en
niet naar de bodem zakken. Als je de saus schenkt wordt
hij vloeibaarder zodat hij makkelijker te gebruiken is en
als hij op de sla zit wordt hij weer viskeuzer zodat de
saus niet naar de bodem van de schaal zakt. Nog een
voorbeeld is roomijs met minder vet. Een probleem bij
ijs is dat als het in en uit de vriezer gehaald wordt, het
water ‘ontmengt’ uit het vet en de ijskristallen
steeds groter worden zodat je eindigt met
een hoop ijs en een hoop vet.
Figuur 9: E322, lecithine in de vorm van fosfatidylcholine,
C42H82NO8P is een goede emulgator.
Figuur 10: E410, Johannesbroodpitmeel, wordt gemaakt van de zaden van de Johannesbroodboom.
Een bekende stabilisator is Johannesbroodpitmeel, of
E410 (figuur 10). Dit word gemaakt van de zaden van
de Johannesbroodboom die veel in het mediterraan
gebied groeit, bijvoorbeeld op Majorca. De peulen van
de boom, ook wel Johannesbrood genoemd, bevatten
zaden waarvan het meel wordt gemaakt. Ze worden
simpelweg gescheiden van de peul, gewassen en
E 600-699: Smaakversterkers.
De bekendste, en waarschijnlijk de beruchtste stof onder
de smaakversterkers is E621, mononatriumglutamaat,
ook wel bekend als MSG of ve-tsin. MSG is in 1907
geïsoleerd door Kikune Ikeda, een scheikundige
uit Tokio. Hij vroeg zich af waarom de soep van zijn
vrouw zo goed smaakte en vond uit dat het door de
bouillon kwam die, zoals vaak in Japan, wordt gemaakt
uit zeewier. Tegenwoordig wordt het gemaakt door
fermentatie van melasse en suikerbieten.
Ve-stin wordt vaak geassocieerd met chinees eten maar
komt van nature in veel producten voor. Een hoge
concentratie is te vinden in Parmezaanse kaas, maar
ook in paddenstoelen, tomaten, broccoli en walnoten.
Figuur 11: E621 is bekend of berucht als ve-tsin. Het is
natriumglutamaat en smaakt naar “hartig”.
Bij het eten van natriumglutamaat (figuur 11) splitst
het zich in natriumionen en vrij glutamaat, de zuurrest
van het aminozuur glutamine. Het wordt ook wel
aangeduid als de vijfde smaak; ‘umami’ wat neerkomt
op ‘hartig’. Het werkt alleen in combinatie met gewoon
Natriumglutamaat wordt verdacht van het Chineesrestaurant-syndroom dat gepaard gaat met allerlei
klachten zoals rug- en nekpijn en buikkramp.
Geproduceerde natriumglutamaat, die vaak wordt
toegevoegd aan chinees eten, is identiek aan de
natuurlijk voorkomende stof. Er is tot nu toe geen
statistisch bewijs dat de klachten relateert aan
gesynthetiseerd glutamaat. Er is een mogelijkheid
dat sommige mensen allergisch zijn voor glutamaat,
synthetisch of natuurlijk.
Veel E-nummers hebben dus een natuurlijke oorsprong.
Als je alleen biologisch en zelf bereid eten zou nuttigen,
dan heb je nog steeds 90 E-nummers in je lichaam.
Bijvoorbeeld propionzuur, ofwel E280, dat zit in je
zweet, maar ook in brood tegen schimmels. In je haar
zit cysteïne, E920, wat een meelverbeteraar is en E410,
glycerol, wat glazuur op gebak zacht houdt en ook in je
lichaamsvet zit.
Overzicht E-nummers
In de tabel hieronder vind je nog een overzicht van alle
E-nummers, met de indeling in verschillende groepen
op functie en origine.
100-199: kleuren
blauw en violet
bruin en zwart
goud e.a.
200-299: conserveermiddelen
fenolen en formaten
300-399: antioxidanten en zuurteregelaars
ascorbaten (vitamine C)
tocopherol (vitamine E)
gallaten en erytorbaten
citraten en tartraten
malaten en adipaten
succinaten en fumaraten
400-499: dikmakers, stabilisatoren en emulgatoren
andere natuurlijke stoffen
natuurlijke emulgatoren
vetzuren en -verbindingen
500-599: pH-regelaars en antiklontermiddelen
minerale zuren en basen
chloriden en sulfaten
andere natuurlijke stoffen
stearaten en gluconaten
600-699: smaakverbeteraars
700-799: antibiotica
diverse synthetische antibiotica
900-999: overig
was (als in bij)
synthetische glansmiddelen
algemene wereldverbeteraars
gassen (drijfgas et cetera)
1100-1599: andere chemicaliën
Over het algemeen nieuwe chemicaliën die niet binnen
de bovengenoemde categorieën vallen.
BBC: E-numbers; An edible adventure
Geert Folkertsma
Waarom drinkt de halve wereld eigenlijk koffie?
Die vraag kwam laatst ineens in me op, toen ik nadacht
wat ik aan het doen was toen ik koffie ging zetten. Ik
haalde een grote pot water. Lekker, fris, schoon water.
Nou ja, het kwam uit een kraan in de Horst en daar valt
de kwaliteit altijd een beetje tegen, maar een doorsnee
woestijnbewoner zou er bij wijze van een moord voor
Goed, dat water dus. Ik giet het in het reservoir en
zet het apparaat aan (meestal verdelen we de taken:
iemand anders had reeds het filter geprepareerd). Dan,
na exact 7 minuten, heb je een heerlijke pot zwarte
Op de een of andere manier heeft iemand, ooit, een
procedé bedacht waarmee koffiebonen zó worden
behandeld dat ze in staat zijn die pot helder water
zwart te maken. ZWART! Een “kleur” die vrijwel niet in
de natuur voorkomt, behalve in verregaande staat van
necrose of – ook niet fijn – derdegraads brandwonden.
Koffiebonen zijn uit zichzelf mooi groen en worden
verbrand (nee, niet gébrand, dat is gewoon een
eufemisme), zodat ze zwart worden. Wat heeft ons
bezield? We eten toch ook geen verkoolde drumsticks?
Laat staan dat we er een soort Norit-bouillon van
Kleine kinderen lusten meestal geen koffie. “Omdat
ze het nog moeten leren drinken,” of “omdat kinderen
alleen van zoet houden” wordt vaak gezegd. Dat is
natuurlijk allemaal niet waar: het is vanwege een of
andere natuurlijke afweer die mensen ervoor behoedt
dit zwaar vergrafte water te drinken. Op de een of
andere manier weten veel mensen deze afweer echter
te ondermijnen, onder het mom van “koffie leren
Ik herinner me de eerste keer dat ik een kop koffie dronk
nog goed. Het was aan het begin van een opkomst
(scoutingprogramma) waarin we een dropping zouden
lopen. Omdat het al 11 uur ‘s avonds was en we nog een
hele nacht voor de boeg hadden, leek het me een goed
idee om wat cafeïne tot me te nemen, noodgedwongen
door middel van een flinke mok koffie.
Ik heb elke slok kokhalzend naar binnen moeten
Na die eerste kop is er eigenlijk geen weg meer terug.
Het kan een tijd duren, maar op een gegeven moment
dient zich weer een moment aan: je drinkt gezellig mee
met koffietijd (groepsdruk), of je voelt je nog een paar
keer genoodzaakt cafeïne tot je te nemen. Totdat het
een gewoonte wordt.
Ik ben inmiddels in het stadium waarin ik geloof dat
er ook lekkere koffie is. In mijn tweede jaar heb ik een
Nespresso-apparaat cadeau gekregen, en de espressoachtige koffie die je daarmee kunt zetten vind ik “prima
binnen te houden”. Die uitdrukking gebruiken we thuis
meestal als understatement om aan te geven dat het
eten lekker is, maar misschien schuilt er in dit geval
meer waarheid in?
Er is in ieder geval ook veel minder lekkere koffie.
Koffie, alhoewel het reeds ge- danwel verbrand is, kan
nog steeds geoxideerd worden. Dat is de reden dat
oude koffie, die al een tijd geleden gezet is, bruin, bitter
en in het algemeen smerig wordt. Wat veel mensen
vergeten is dat koffie ook oxideert in poedervorm. Dat
betekent dat, ondanks dat koffiepoeder (m.i.) erg lekker
ruikt, de zak of het blik goed gesloten moet blijven.
Anders is de koffie al bruin, bitter en in het algemeen
smerig voordat hij gezet is. Niet fijn.
De smerige koffie die de kroon spant, is die bij mijn
eigen studievereniging. Daar vergeet(?) namelijk het
gros van de mensen het deksel op het blik te doen.
Gecombineerd met de grote hoeveelheid koffie die
er in het blik zit (waardoor het lang duurt voor het
leeg is), is de kwaliteit van de koffie evenredig met de
hoeveelheid resterend poeder in het blik. Niet fijn.
Toch is koffie nuttig. Het houdt de gemiddelde
vakgroep schat ik een half tot één uur van het
werk, maar daarin wordt zonder (elektronische?)
barrière gecommuniceerd tussen onderzoekers,
docenten en studenten. Zelfs tijdens de bescheiden
onderzoeksprojectjes die ik heb gedaan, merkte ik
dat veel ingevingen tijdens die korte periode van
ontspanning komen; en discussies aan de koffietafel
leveren nieuwe inzichten.
De koffieverslaving, of het nu een fysieke drang naar
cafeïne is of een mentale zucht naar een bakkie troost,
komt dus alles ten goede. Wij, koffiedrinkers, zijn
eigenlijk helden, die onbaatzuchtig ook de nietkoffiedrinkers op sleeptouw nemen naar een
betere wereld.
Spintronics in hard drives
Pim Muilwijk
In edition 3-5 I wrote about the history, workings
and future of hard disk drives (HDDs). This article,
however, was quite qualitative and although it mentioned some of the operating principles like giant
magnetoresistance and the limit of data density due
to superparamagnetism, I never really took the time
to quantitatively explain these; nor possible alternatives. Therefore, I want to dedicate this article again
to hard disk drives and the emerging technology of
is called the transition width and is a measure of data
density, happens over the length of the Neel spikes.
This problem can be overcome by using grains, which
in theory form single magnetic domains which cannot
grow or shrink to form spikes. Therefore the transition
width will be in the order of the grain diameter, thus
increasing the data density (Figure 2). In fact, much of
the development in hard drives has been in reduction
of the grain size.
But let’s first recap how normal HDDs work. Please don’t
think that I’m a cheapskate by using some of my previous work; I’ve made it more in-depth and added helpful
pictures this time.
A dive into the hard disk drive
Figure 2: The transition width is reduced by using grains.
When you look inside an HDD you will find a spindle
with several hard circular disks called platters. These
platters are made of non-magnetic materials like
aluminium alloys or glass and are subsequently coated
with a very thin polycrystalline layer (10-20 nm) of
ferromagnetic material like CoCrPt and an outer layer
of carbon for protection. When you look closely at this
layer, it will look something like figure 1.
Now that we have the platters covered, we need
some mechanism of writing and reading bits. This is
accomplished by spinning the platters past the readand-write heads that are positioned in the order of tens
of nanometers to the surface. The speed at which these
platters rotate is measured in rotations per minute
(rpm) and generally varies from 5,400 rpm for laptop
and so-called “green” or “eco” drives (because they
consume less energy that way), up to 15,000 rpm for
hard drives used in servers that have to process a lot
of data. Generally, the faster the platters spin, the faster
data can be read and written. When reading, these
heads detect the sequential changes in the direction
of magnetisation by electromagnetic induction and
decode the data (Figure 3).
Figure 1: A close-up of the magnetic layer on a platter.
Here you can clearly see the magnetic regions that represent the bits. Such a region is typically 200 by 25 nm
and consists of about 100 grains. It is very important to
use grains rather than a continuous magnetic medium,
because in the latter formations called Neel spikes tend
to appear. These are spikes of opposite magnetisation
and form for the same reason that bar magnets tend
to align themselves in opposite direction, namely
to increase the stability of the system. This is a
problem because these spikes cancel out each
other’s magnetic field, so that the transition
22 from one magnetisation to the other, which
Figure 3: Reading data.
Writing is then accomplished by performing the
reverse operation: by running a current through the
head a magnetic field is generated which can alter the
magnetisation of the magnetic domains. The upper
part of Figure 4 shows a ring-shaped element used to
accomplish this, while the bottom part already hints to
a new technique which is used today: perpendicular
Spintronics in hard drives
Heads up
Untill now the improvements discussed had primarily
to do with the magnetic layer on the platters. However,
much of the enhancements that are used today are
focussed on the read-and-write heads. In fact, the thick
underlayer used in the perpendicular arrangement that
was mentioned earlier is often said to be part of the
Figure 4: Writing data and perpendicular recording.
Get perpendicular
Maybe some of you can still remember a Flash video
from Hitachi titled “Get Perpendicular” in which singing and dancing bits try to explain how perpendicular
recording works. According to the video this is accomplished by the bits standing upright so there is
more room to “bring their friends”, i.e. more bits and
thus creating more storage capacity. While it is true
that the bits are positioned perpendicularly rather
than longitudinally in perpendicular recording, the real
picture is somewhat more complicated and involves
superparamagnetism and magnetic coercivity - this
doesn’t make the video any less fun to watch though.
Superparamagnetism is a form of magnetism that
appears in small ferromagnetic nanoparticles like the
earlier mentioned grains in hard disk drives. The smaller
you make these particles, the larger becomes the risk
that the magnetisation of these particles will randomly
flip under the influence of temperature. This puts a limit
on the size of the particles called the superparamagnetic limit. Nowadays, the main challenge in designing
hard disk drives is to retain the magnetisation of the
particles despite these thermal fluctuations. This can
be accomplished by either increasing the particle size,
which is not beneficial for the data density, or increasing the magnetic coercivity, which is the resistance of a
ferromagnetic material to becoming magnetised.
There is a catch in increasing the magnetic coercivity though, because you will also need to increase the
field that is used to write the bits. This means that the
head must be made more efficient and this is where
perpendicular recording comes in. In a perpendicular
arrangement, a magnetically soft and relatively thick
underlayer is added underneath the hard magnetic film.
This layer guides the magnetic flux, which can therefore
be stronger. This allows for using materials with higher
coercivity, which in turn allows for decreasing the grain
size and increasing the data density. Also, since the
magnetic field needs to be stronger in order to flip the
bits, the data is less vulnerable to degradation.
Traditionally, the heads were very similar to those
used in VCR and tape recorders. They were made out
of a tiny C-shaped piece of ferrite wrapped in a wire
coil, just like what is shown in Figure 4. Ferrite is highly
magnetisable, so when a current is run through the coil,
a strong magnetic field is created in the gap of the C,
capable of writing the bits. When reading, the ferrite
core concentrates the field and a current is generated
in the coil. The size of the gap determines the minimum
size of a recorded area on the disk and since ferrite
heads are quite large, the attainable capacity was quite
small. This arrangement was improved upon by putting
a piece of metal in the gap to concentrate the field and
was appropiatly called a “Metal in Gap (MiG) head”. This
allowed for smaller features to be read and written.
Photolitography allowed even smaller features to be
created. Using this process, so-called “thin film heads”
were manufactured which were electronically similar to
ferrite heads and used the same physics but were much
smaller, thus allowing for a higher data density.
Then came a crucial step in the optimisation of the
heads; the separation of the read and write heads. This
allowed for different physical phenomena to be used
for reading and writing. Instead of electromagnetic induction, the read head now utilised magnetoresistance
(MR) to read data. As the name suggests, this effect
changes the electrical resistance of a material when an
external magnetic field is applied. This was already discovered by Lord Kelvin in 1856, when he observed that
the resistance has a maximum value when the current
is in the same direction as the applied magnetic field.
A little thing called spin
Then, in 1988 Albert Fert and Peter Grünberg observed
that the electrical resistance in thin film structures composed of alternating metallic ferromagnetic and thin
non magnetic layers (few nm) was much more affected
by an applied magnetic field than with normal MR,
hence they dubbed it giant magnetoresistance (GMR).
It was found that when no magnetic field is applied, the
magnetic moments of the successive ferromagnetic
layers, also known as spins, are antiparallel and the
overall resistance is relatively high. When a magnetic
field is applied however, the spins align and the
resistance drops significantly.
Spintronics in hard drives
This behaviour can be explained by using the Mott
model which was originally introduced to explain the
sudden increase in resistivity of ferromagnetic metals
as they are heated above the Curie temperature. In this
model Mott states that the electrical conductivity in
metals can be described in terms of two largely independent conducting channels, corresponding to the
spin up and spin down electrons. Furthermore, since
the scattering rate is proportional to the density of
states, which is not the same for spin up and spin down
electrons at the Fermi energy because of the band
structure being exchange-split, the scattering rates for
spin up and spin down and therefore the resistitivities
for electrons of different spin is different. From this line
of reasoning GMR can be explained using Figure 5.
Figure 6: A spin valve. AF, FM and NM stand for antiferromagnetic, non-magnetic and ferromagnetic respectively.
magnetic field. It is not hard to translate this to a read
head where the magnetised film on the platter alters
the magnetisation of the free ferromagnetic layer and
thus altering its resistivity. The data can now be read
by measuring the electrical resistance through the
Deeper down the quantum well
Figure 5: Different scattering rates and corresponding
Let’s assume that the scattering is strong for electrons
with spins antiparallel to the magnetisation direction
and weak for electrons with spins parallel to the magnetisation direction. Then in the parallel aligned multilayer spin up electrons will travel through the structure
almost without scattering because their spin is parallel
to the magnetisation direction, whereas the spin down
electrons will scatter strongly because their spin is antiparallel to the magnetisation direction. Since the conduction occurs in parallel for the two spin channels, the
total resistivity of the multilayer is determined mainly
by the higly conductive spin up electrons and appears
to be low. For the antiparallel aligned multilayer both
spin up and spin down electrons are scattered strongly
within one of the ferromagnetic layers, because within
one of these layers the spin is antiparallel to the magnetisation direction. The resistivity of the multilayer
in this case is therefore high. We can now exploit this
phenomenon by constructing a so-called spin valve,
which is shown in Figure 6.
In a spin valve, the magnetisation of one ferromagnetic
layer is pinned by the exchange coupling with an
adjacent antiferromagnetic layer - whereas
the magnetisation of the other ferromag24 netic layer is free to rotate with the applied
Something very interesting happens when you replace
the non magnetic spacer in a spin valve with an
insulator. In the classical world one would expect that
electrons would not be able to cross the spin valve anymore, therefore there would be no current. However,
at these length scales quantum tunnelling events can
take place, enabling the electrons to cross the insulator
and therefore a current can be generated.
This tunnel current can be manipulated by applying an
external magnetic field. As with the spin valve, the current is large when the magnetic field is parallel to the
magnetisation of the ferromagnetic layers (meaning
that the resistivity is low) and small when the magnetic
field is antiparallel to the magnetisation of the ferromagnetic layers (meaning that the resistivity is high). It
was therefore called tunnel magnetoresistance (TMR).
The reason that this effect is of interest, is because it is
much stronger than GMR (about 10 times). Do not let
the similar behaviour fool you however, because TMR is
governed by entirely different physics than GMR.
The difference lies mainly in the fact that the conductance as described in the Mott model depends on
the scattering, while the conductance in TMR can be
described using Julliere’s model, which is based on
two assumptions. Firstly, it is assumed that the spin
of the electrons is preserved during the tunnelling
process. This means that the tunnelling of spin up and
spin down electrons are two different processes, so
the conduction occurs in two different spin channels.
Therefore, electrons originating from one spin state
of the first ferromagnetic film are accepted by unfilled
states of the same spin of the second film (Figure 7).
Spintronics in hard drives
What the future holds
Even if you only have a mild interest in storage media,
you will probably have noticed that solid state drives
(SSDs) are becoming increasingly popular. Compared
to traditional HDDs they are quiet, fast and as good as
indesctructable due to the lack of moving parts such
as read- and write-heads. Instead, they use electrical
charge to store information. This has some disadvantages as well, the foremost being that the drive has a
limited amount of program-erase cycles. If only we
could combine the best of both worlds…
Figure 7: Tunneling rates are different for different spin
So, when the two ferromagnetic layers are magnetised
in parallel, the minority spins tunnel to the minority
states and the majority spins tunnel to the majority
states. Whereas if the ferromagnetic layers are magnetised antiparallel, the minority spins tunnel to the
majority states and the majority spins tunnel to the
minority states.
Secondly, it is assumed that the conductance for a particular spin orientation is proportional to the product of
the effective density of states of the two ferromagnetic
layers. The combination of these two assumptions leads
to a large current for parallel magnetised layers and a
small current for antiparallel magnetised layers. Again,
this phenomenon can be exploited by constructing a
so-called magnetic tunnel junction (MJT), as shown in
Figure 8.
Figure 8: A magnetic tunnel junction (MTJ).
Again, it should not take too much imagination to spot
a read-head in this configuration and indeed this is the
way most read-heads function in modern hard drives.
Actually, we can, by using magnetoresistive random
acces memory (MRAM) as shown in Figure 9, which
stores its information magnetically in magnetic tunnel
junctions. As you would expect, reading data is accomplished by measuring the electrical resistance of the
cell and writing is accomplished by running a current
through the corresponding “word” and “bit” line of the
target cell, much like in normal flash memory. There
are, however, still some issues with this technology.
First of all, the writing procedure requires a significant
amount of current, making it less suitable for lowpower use. Another problem arises when the device is
scaled down: this causes the magnetic field to overlap
adjacent cells, leading to false writes.
Bit Line
Write Word
Read Word Line
Figure 9: Magnetoresistive random access memory
These problems can be resolved by using a technique
called spin transfer torque (STT), which replaces the
word lines with a spin polarised current. In fact, you
can visualise STT-MRAM by connecting the bit line, MJT
and antiferromagnetic layer from Figure 9 directly to
the drain of the transistor (right N). You can now write
a bit by running a current through a thick magnetic
layer, which polarises the current, and then through the
magnetic element. The spin angular momentum
will be transferred directly and the bit will be
written. This solves all mentioned problems
and really is the hard drive of tomorrow.
Normally, the last pages of every ATtentie are dedicated to a puzzle with which our readers can enjoy
winning a gift voucher for a movie theater. This time
however, we put them in the middle.
Winner of puzzle 5-1
The puzzle in edition 5-1 was a colouring picture.
Unfortunately we only had one submission. This
subission came from Monique, making her the default
winner, congratulations Monique! Below is a picture of
her receiving her prize.
Solution to puzzle 5-2
The puzzle of edition 5-2 was a picture of a well known
AT teacher in his youth; you had to guess which teacher
it was. The correct answer was Leon Abelmann. We have
received several submissions and will ancounce the
winner in the next edition.
The new puzzle
The puzzle for this editon will be a word search puzzle.
On the next page you will find a field with 40 hidden
words, all words have something to do with the articles
in this edition. The words are hidden in horizontal, vertical or diagonal orientation and in normal or reversed
order. After you have found all the words, 3 other words
will remain. These words will form the solution. Send in
the solution before the next edition is released and you
might win that gift voucher!
Genetically modified food
Jeroen van den Berg
Genetically modified food: is it the solution to our
food shortage or will we (unintentionally) endanger
our environment and our own health? With the
increased knowledge of genetic engineering we are
able to modify the genotype of plants and animals,
for example by inserting genes of other organisms,
so that they will grow larger/faster or produce more
nutritious food. However genetically modified food
has raised some controversy among the people,
some argue that it may not help save our problems
and that it is not worth the possible health risks and
environmental damage.
Since the beginning of the 1990s there have been
several transgenic modified plant products on the
market. Modified animal products have also been
developed, but currently none of them are available on
the market. Apart from usage in food products, genetically modified organisms have been used in biological
and medical research as well as in the production of
pharmaceutical drugs. Even a genetically modified
fluorescent zebrafish has become publicly available as
a pet, under the name of GloFish.
Modifying organisms for food
Before the advances in genetic engineering people had
other means of modifying organisms for food production, the oldest of these methods is selective breeding.
Selective breeding or artificial selection is the breeding
of plants or animals with specific genetic traits. It’s opposed to natural selection, in which the environment or
“nature” acts as the selector. The term artificial selection
was first coined by Charles Darwin in his book “On the
Origins of Species” to explain how the domestication of
animals resulted in changes over time and to illustrate
the wider process of natural selection.
However, selective breeding predates Darwin by millennia, having been practiced since the domestication
of grain by pre-Neolithic groups in Syriamore than
12,000 years ago. One of the most notable examples
of selective breeding in crops is maize, also known as
corn. Maize has been domesticated by the indigenous
people of Mesoamerica, the Aztecs and the Mayans.
They cultivated the grass teosinte, which has only a
few kernels, into the modern day maize with a lot of
external kernels, see Figure 1.
Figure 1: From left to right the evolution of the grass
teosinte to maize due to artificial selection.
In the breeding of crops, larger and better growing
plants were selected for, resulting in better crop yields.
One of the reasons that some plants grow better and
become larger is because they have more than two of
the same chromosomes inside their cells. This is called
polyploidy. Most species are diploid, meaning each
normal cell has two of each chromosome: one set from
their mother and one set from their father. Polyploidy is
fairly common among plants but very rare in animals,
most often polyploid animals are sterile. Animals can,
however, have polyploid cells in certain tissue, for
example in muscle tissue and in the liver. This is called
endopolyploidy. Wheat is an example of a plant that
has become polyploid by selective breeding and crossbreeding with other plants by humans. Polyploidisation
is also a mechanism for speciation because often polyploids are unable to reproduce with diploids.
Genetically modified food
Horizontal gene transfer
As said above, the process of genetically modifying
an organism involves incorporating genetic material
of another organism in its genotype, without it being
the offspring of the other organism. In fact, most of the
time the genetic material is from an entirely different
species. This process is known as horizontal gene transfer and occurs naturally among bacteria. It’s opposite
to vertical gene transfer, in which an organism receives
genetic material from its parent(s).
A key ingredient for this reaction is a thermostable
DNA polymerase. DNA polymerase is the enzyme
that builds the DNA strands from the single deoxynucleotide triphosphates (i.e. the basic building blocks
of DNA). Most enzymes are destroyed at temperatures
above normal body temperature due to their complex
structure; however a certain DNA polymerase called
Taq polymerase is stable up to 70° C. This enzyme
was isolated from a thermophilic bacterium named
Thermus aquaticus and is well suited for this (relatively)
high temperature reaction.
Another important ingredient is the primer. Primers are
short DNA segments which provide a binding site for
DNA polymerase. To allow amplification of a specific
DNA fragment, like the desired gene, the primer for Taq
polymerase is hybridised to the DNA right in front of the
gene (more on that later). After the DNA segment has
been duplicated, the new strand acts as template for a
new strand for the next duplication round, this method
thus allows exponential amplification of the DNA.
Figure 2: Bacteria demonstrating horizontal gene transfer.
It is thought that horizontal gene transfer contributes
significantly to the drug resistance of a bacteria
population: a resistant bacterium quickly spreads the
resistant gene to other bacteria. It also plays a part in
some reproductive processes of viruses. There are a few
documented cases of horizontal gene transfer between
bacteria and eukaryotic organisms (eukaryotes are
more complex, multicellular organisms containing a
nucleus in their cells), such as fungi. Even people have
bacterial and viral DNA in there genome, which play a
part in our immune system. Through genetic engineering techniques we are capable of artificial horizontal
gene transfer.
Polymerase chain reaction
Horizontal gene transfer is the most common type of
genetic engineering. The process is involves a few steps.
First, the desired gene has to be isolated from the other
genetic material and reproduced. The targeting and
reproduction of the gene is done with a polymerase
chain reaction. This reaction relies on thermal cycling:
the chemical concoction is repeatedly heated and
cooled to control the forming of new DNA molecules.
Figure 3: A more complex schematic representation of
DNA duplication. Notice the orange blocks that represent
the polymerase enzymes and the primer segment (in this
case it’s a RNA primer).
Recombinant DNA
Once we have our gene(s) isolated and replicated, we
need to find a method of inserting them into the host
organism. This is done via gene splicing, resulting in recombinant DNA. The basic idea of gene splicing is fairly
straightforward: we cut open the host’s DNA, insert the
desired DNA material and glue it back together. The
cutting of the DNA is again done via enzymes, so-called
restriction enzymes. Just like DNA polymerase only
binds to a primer, these restriction enzymes only bind
to specific DNA fragments. Once cut, the ends of the
DNA molecule has so-called “sticky ends”, these are the
now unpaired nucleotides to which the new DNA can
be bound by the enzyme ligase, see Figure 4.
Genetically modified food
Genetically modification of foods has also raised much
controversy among people. Some argue that genetically modified foods are the solution to the world hunger
problem; others disagree saying it won’t be enough or
that the problem lies with the distribution and politics,
and that enough food is or can be produced. Some also
argue that the real problem is the overpopulation and
not the resulting food shortage.
Figure 4: Gene splicing in a plasmid (i.e. small circular DNA
found in bacteria).
Recombinant DNA has been widely used in the pharmaceutical industry to make bacteria produce certain
medicines. An example of this is the production of
insulin for diabetes patients. In the food industry some
crops are modified to become resistant to herbicides,
pesticides or both. This way the plants can be better
protected from insects and weeds which in turn would
provide a potentially higher crop yield. Plants have
also been altered to directly increase crops rates by, for
example increasing their tolerance to cold and drought,
or to produce more nutritious foods. There are even
plants being developed that contain edible vaccines
or drugs, called pharmaceutical crops. A positive side
effect of these more resistant plants is that less fertiliser and pesticides are needed, resulting in a cleaner
Animals have mostly been modified with growth
hormones to increase their size. Some pigs have been
modified to digest plant phosphorus more efficiently
because they produce the enzyme phytase, which
helps with the digestion of the phosphorus. This would
result in lower feeding costs for pigs and less phosphorus pollution; therefore these pigs have been named
Many people are concerned with possible health issues due to for example allergic reactions or increased
toxicity. Also environmental effects may be severe.
Some people are concerned that the introduction of
new genes to the gene pool of the species might have
unforeseen effects on the environment. For example
crops of transgenic maize which are more resistant to
insect might also cause the deaths of harmless insects
like the monarch butterfly, though there are contradicting reports about this case.
Due to these concerns there are strict health and safety
regulations regarding genetically modified foods. This
is the reason why there aren’t any genetically modified
animals on the market for consumption today. Animals
can, however, be fed genetically modified plants.
Although further research is required, current studies
have not shown any traces of recombinant DNA in the
tissues of animals that have been fed genetically modified foods.
Grolsch excursie
Melvin van Melzen
When I was done fixing my bike I was already going
to be late, nothing serious but a few minutes. The first
snow had fallen and it was very cold out there. Luckily
I only had to travel half the distance the group that
had to come from the campus would have to. Against
all odds I was the first to arrive! I must have been
confused with the time because I thought someone
would already be expecting me when I got there, but
it was just me. Shortly after feeling less like an ice
cube two others arrived which were followed closely
by the main group. Apparently they missed a turn
somewhere and ended up in Boekelo, whoops.
With everyone together the tour could start. We started
with a hot cup of coffee or tea which we took to the
café. Here we watched a movie about how this brewery became such a glorious place. We split up in two
groups and went into the production area. In the first
hall we saw the top part of the massive stainless steel
tanks, used for the brewing. Also there are huge tanks
for fresh water rising all the way up to the ceiling. The
smell in here is very distinctive. Somewhat like the day
after you clean up after a party where beer was spilled
voluminously, that kind of smell: hmmm…
Now for the really spectacular part; the bottling and
canning hall! This is where robots clean, fill, label and
close the bottles, before they are put into crates or
other packages, also by robots, naturally. The finished
products are kept in the area right behind the packing
area. Everything in there is already sold and is waiting
to be picked up for transport. This massive repository is
the last place our precious green friends are kept before
being shipped to the stores, stores that make us pay
dearly for this golden liquid.
Now for the best part of our tour, back at the café we
started in. We were getting a taste of a few of the best
beers by Grolsch: Lemon 2.5 (exception in this list of
bests), Premium Pilsner, Premium Weissen, Herfstbock
and last but certainly not least Kanon. The bubble of
bliss I was in was popped by the mention of the closing
bar and the closing café shortly after that. Luckily there
was a present for all of us: a unique glass only for us,
close to the style of the new glasses used everywhere.
Nice! A quick group photo under the big (like everything in the factory) portrait of the genius who made
this beer and we are out in the cold again.
The next place was the filter area; here all solids are
removed from the beer. Everywhere are shiny pipes
which are interwoven and spaghetti-like. After a short
walk outside we got to hear a lot about how Grolsch was
being promoted since the 2nd world war. Promotion
used to be very informative, lots of text and only small
images. We took a look at how this evolved to the fancy
commercials of today. From here we went on to have a
look in the lab. The lab is always busy checking the quality of the products, important but somewhat tedious.
Next to the lab is the tasting room. In here those with
the better tastebuds are drinking all the different kinds
of beers and the same beers with minor adjustments, to
discover better recipes.
Een uniek
van fysici
Carolien Lamers
100% TMC werkondernemer
TMC Physics, het enige fysicahuis in Nederland, is gespecialiseerd in het inzetten van
werkondernemers binnen de fysica competentie.
Hiermee heeft TMC Physics een pionierspositie
verworven in haar type dienstverlening met
het inzetten van fysici op flexibele basis bij
opdrachtgevers op locatie.
Ruim twee jaar ben ik in dienst bij TMC Physics en
werk ik voor Philips Applied Technologies. Het
informele karakter en de persoonlijke omgang met
directie en account managers zorgen voor een goed
contact. Alle account managers bij TMC Physics
hebben een achtergrond in fysica. Zij begrijpen de
inhoud van mijn werk en denken met mij mee.
TMC is een jong en dynamisch bedrijf dat open staat voor vernieuwende ideeën van
Onze fysica werkondernemers zijn actief in
research, development en engineering op onder
andere de volgende gebieden: product & process
modellering, vloeistof- en gasstroming, lasers
& optica, materiaalkunde, dunne film technologie en nano technologie. Zij worden daarbij
ondersteund door een team van account managers die zelf ook een opleiding in de natuurkunde hebben genoten.
Onze klantenkring kenmerkt zich door diversiteit
en varieert van (contracted) research tot systeemen productontwikkeling in verschillende sectoren
waaronder: halfgeleiders, zonnecellen, medische
systemen, defensie, olie & gas.
TMC Physics heeft een bijzonder hoog opgeleid
en internationaal karakter. Van onze werkondernemers is 90% academisch opgeleid, waarvan het merendeel een promotie succesvol
heeft afgerond en ongeveer de helft van onze
mensen heeft een buitenlandse nationaliteit.
Een kleurrijke groep mensen verbonden door
een gezamenlijke passie: fysica.
haar werkondernemers. TMC Physics biedt bovendien een uniek netwerk van fysici
en heeft goede contacten met veel verschillende technische bedrijven.
Daarnaast combineert TMC haar professionele uitstraling met informele contacten
tussen de werkondernemers. Zo kun je na een bijeenkomst nog even met je TMC
collega’s, account manager of de directeur napraten over formele of informele zaken
onder het genot van een biertje of kopje koffie in de bar van het kantoor.
Wil je na je afstuderen aan de slag op de onderzoeks- of ontwikkelafdeling van een
high-tech bedrijf, dan heeft TMC misschien wel een passende opdracht voor jou.
Daarom kan ik je aanraden om eens bij TMC langs te komen.
TMC Physics is een businesscel van TMC Technology,
member company van TMC Group N.V..
Sinds november 2006 heeft TMC Group N.V. een notering aan Alternext Amsterdam.
TMC Physics B.V. Flight Forum 107 – Postbus 700 – 5600 AS Eindhoven – Tel. 040 239 2260 – Fax 040 239 2270 – [email protected] – www.tmc.nl
Interview: Arie van Houselt
Jeroen van den Berg & Geert Folkertsma
Na een serie van interviews met docenten
uit de natuurkundehoek (Harold Zandvliet,
Alexander Brinkman, Herman Hemmes) vond
de laatstegenoemde het tijd voor een nieuwe
interviewlijn: scheikunde. Bij wie anders te beginnen
dan bij een van de jongste AT-docenten, die een dikke
6 jaar geleden zelf nog in de collegebanken zat en
vorig jaar een Veni wist binnen te halen?
Uit wat voor gezin komt u zelf?
Ik ben de oudste van vier: een iets jonger zusje (we
hadden altijd “gezonde competitie”), een broertje
van vier jaar jonger en nog een zusje na 12 jaar. Zij is
nu bijna klaar met de middelbare school; ik kan haar
mooi bijles geven in de exacte vakken, ze woont ook
in Barneveld.
U reist elke dag hierheen met de auto. Ooit gedacht
aan verhuizen?
Wel over gedacht, maar voorlopig slaat de balans nog
door naar blijven wonen: daar zitten de jongens op
school een halve straat verderop en woont de rest van
mijn (schoon)familie.
Waar doet u de boodschappen?
Op zaterdagochtend breng ik mijn oudste naar het
zwembad voor zwemles, dan heb ik drie kwartier
de tijd om de boodschappen voor de week te doen.
De supermarkt is een afstandsminimalisatie: de AH
is het dichtste bij het zwembad. En af en toe haal ik
croissantjes bij de Aldi, maar dat moet natuurlijk niet te
vaak gebeuren.
Hoe zag uw middelbareschooltijd eruit?
Ik heb gymnasium gedaan in Amersfoort, met in mijn
vakkenpakket natuurkunde, wiskunde en scheikunde.
Ik heb ook nog even economie geprobeerd, maar dat
werd geen succes: we kregen vanwege de klassieke
talen in de onderbouw geen economie, maar werden
later wel geacht daar al dan niet voor te kiezen. Ik koos
het niet, maar heb het in de zesde toch gedaan. Het
eindexamen wel gehaald, maar ik vond het niets: geef
mij maar natuur- en scheikunde.
Welke studie werd dat dan?
Wie bent u en waar komt u vandaan?
Mijn naam is Arie van Houselt, ik ben in 1980 geboren
in Rotterdam. Daar heb ik de eerste paar jaar van mijn
leven ook gewoond, daarna een tijd in het oosten van
de Veluwe; later in het midden ervan, in Barneveld – en
daar woon ik nu nog, samen met mijn vrouw Agatha,
wat “lieflijke”betekent, maar ik weet niet meer in
welke taal. En met mijn 3 zonen van 6 jaar, 4 jaar en 4
Op de middelbare school vond ik scheikunde altijd
leuker dan natuurkunde. Natuurkunde was vooral
formules invullen: ik vond het interessant dat je dingen
kunt uitrekenen, maar bij scheikunde had ik echt het
idee dat ik iets leerde van de dingen die om je heen
Ik was dus van plan scheikunde te gaan studeren, maar
ontdekte dat in Utrecht een combinatie natuurkundescheikunde mogelijk was: dat werd het.
Interview: Arie van Houselt
En dat beviel goed?
Ja, alhoewel mijn eerdere bevindingen tijdens
de studietijd juist omdraaiden: daar kwam ik
erachter dat scheikunde eigenlijk toegepaste
natuurkunde is, terwijl natuurkunde echt dingen
probeert te verklaren. Uiteindelijk ben ik in 5,5 jaar
afgestudeerd: voor natuurkunde heb ik onderzoek
naar rendementsverbetering bij zonnecellen gedaan,
bij scheikunde hield ik me bezig met colloïdale
Colloïden hebben een “jasje” van koolwaterstoffen; bij
deze halfgeleiders hing het luminiscentiegedrag af van
het type jasje. Uiteindelijk bleek bij de werktemperatuur
net een faseovergang van het omhulsel plaats te
vinden, waardoor dit werd veroorzaakt.
Had u nog tijd voor dingen naaast de studie; werk
of activisme?
Beide: ik heb jarenlang als zaterdagbezorger bij PTT post
gewerkt en was daarnaast actief lid bij Solidamentum,
een reformatorische studentenvereniging. Ik ben daar
meerdere jaren voorzitter geweest.
En hobby’s?
Ik speel orgel, heb wel eens als organist opgetreden.
Tegenwoordig is het meer hobby’en: ik heb thuis
een elektronisch orgel staan. Ik ben nu bezig om
het opgenomen geluid van echte pijporgels in het
elektronische orgel te stoppen.
Verder besteed ik nu mijn vrije tijd aan de kinderen:
die eisen ze gelukkig ook op als ik thuis kom. Tijdens
mijn promotie had ik gelukkig ook genoeg tijd voor ze;
ik had het geluk al vroeg resultaten te hebben en heb
tijdens de rit al veel dingen opgeschreven. Ik moest er
aan het eind wel iets harder aan trekken, maar dat deed
ik door vroeger op te staan.
Wringt de combinatie geloof-wetenschap wel
eens? Moet u zich bij de ene partij voor de andere
Nee, het wringt helemaal niet. Het geloof geeft
antwoorden op zingevings-vragen, zoals waarom
we op de wereld zijn. De wetenschap probeert
systematische oorzaken te zoeken en dingen te
verklaren en verbanden te leggen. Er is een duidelijke
scheiding: wetenschap beantwoordt de vraag “Hoe?”
en religie de vraag “Waarom?”.
U heeft zelf bijna nominaal gestudeerd en was toch
actief buiten de studie. Denkt u dat de op handen
zijnde langstudeer-regeling voor minder activisme
gaat zorgen?
Nee, ik denk dat activisme prima gecombineerd kan
worden met studeren. Bepaalde types activisme
tenminste: het type “bier drinken” wordt misschien
lastig… Het is een kwestie van keuzes maken. Ik denk
dat studenten die echt die ervaring op willen doen, nog
steeds wel voor bestuur of commissies kiezen.
De maatregel is dus wel goed?
Ik ben op zich niet tegen het bevorderen van snel
studeren, maar zoals het nu gaat is het wel erg abrupt:
wij kregen 5 jaar stufi, maar mochten 10 jaar over de
studie doen voor we het hoefden terug te betalen.
De manier waarop het gepresenteerd is, is ook niet
helemaal juist: langstudeerders kosten eigenlijk niet
zoveel geld, de fiscus krijgt alleen wat minder lang geld
binnen. Die bezuiniging valt dus wel mee.
En andere maatregelen, zoals het Bindend
StudieAdvies? Dat draait nu al enige tijd bij ST.
Daar zitten ook voordelen aan, maar aan de andere
kant is het ook wat dwingend, er zijn studenten die echt
even tijd nodig hebben om erin te komen; het is lastig
te voorspellen welke studenten dat zijn, en welke het
echt niet gaan halen. Je loopt het risico om veel koren
met het kaf weg te gooien.
Terug naar uw eigen studie: na het afstuderen bent
u gaan promoveren.
Ja, ik heb hier op de UT onderzoek gedaan dat in
het verlengde lag van mijn afstuderen: daar deed
ik nuldimensionale halfgeleiders, de bolletjes,
halfgeleiders: platina en gouden nanodraden op een
germaniumoppervlak. Ik heb ze bestudeerd met een
STM (scanning tunnelling microscope, red.)
Wat heeft u na uw promotie gedaan?
Ik heb twee maanden postdoc gedaan bij FOM
plasmafysica in Rijnhuizen; onderzoek naar meerlaagse
spiegels voor röntgenstraling. Daarna ben ik via mijn
promotor hier aangetreden bij de vakgroep Catalytic
Processes and Materials. Daar zit ik nu twee en een half
Vorig jaar heeft u een Veni binnengehaald, voor
onderzoek naar reacties aan metaaloppervlak
onder water.
Die beurs geeft me de kans onderzoek in een richting
te doen die ik zelf leuk vind. Het ligt een beetje in de
lijn van mijn promotieonderzoek: in dit onderzoek
gebruiken we ook een STM. Toen in vacuüm, nu in
Interview: Arie van Houselt
Een STM heeft een terugkoppeling die de stroom
constant houdt door de naald te bewegen en zo
een afbeelding van het oppervlak te maken. Deze
terugkoppeling beperkt de tijdsresolutie, waardoor
bewegende structuren vaak als ruis zichtbaar zijn. Door
zo’n gebiedje met ruis op te zoeken en de regellus uit
te schakelen, kun je door de stroomvariaties te meten
toch op hoge snelheid meten. Je kunt zo de dynamica
van processen bestuderen.
Deze truc is wel vaker toegepast, maar voor zover
ik weet nog nooit onder water, en ook nog nooit
voor chemische processen. Dat komt denk ik omdat
natuurkundigen, die deze techniek toepassen, zich niet
met reacties bezighouden; terwijl chemici het apparaat
juist meer als biologen benaderen: druk op de knop en
neem een meting. De gecombineerde opleiding die
ik heb gevolgd, maakt dat ik me voor beide aspecten
Gezien uw brede interesse: zou u AT zijn gaan
studeren, als het destijds al had bestaan?
Met de ervaring die ik nu heb, zou ik een opleiding
willen doen die meer natuurkunde bevat dan AT. Ik
denk dat ik eerder voor puur natuurkunde zou kiezen
dan voor Advanced Technology. Qua scheikunde zou ik
wel wat meer katalyse gehad willen hebben: dat geef
ik nu aan AT, maar heb ik eigenlijk zelf nooit geleerd…
Die katalyse geeft u bij AT en ST beide, bij ons in het
vak Interfaces and Catalysis. Merkt u verschil tussen
de studenten bij AT en ST?
Er zijn wel verschillen tussen de studenten, maar
het is lastig te expliceren. Ik ben zelf natuurlijk geen
doorsnee scheikundige, omdat ik ook veel met
natuurkunde doe, dus misschien sluit dat beter aan
bij AT-studenten. Er is denk ik ook een cultuurverschil
tussen de opleidingen; Scheikundige Technologie
is wat strakker georganiseerd. Ik vind het met beide
groepen studenten plezierig werken.
Ik vind het sowieso leuk om met studenten om te gaan.
Ik leer ook van ze, als ze tijdens college goede vragen
stellen of opmerkingen maken.
Als u een nieuw vak zou mogen geven bij AT, waar
zou dat dan over gaan?
een inhoudelijke motivatie vanuit de opleiding helder
te hebben.
Dat is sowieso iets wat je jezelf continu moet afvragen:
Waarom geven we AT? Waarvoor willen we opleiden?
Het maakt bijvoorbeeld veel verschil of je aan het
bedrijfsleven of onderzoek wilt leveren. Die visie mis
ik bij AT, terwijl dat veel duidelijkheid zou scheppen. Ik
denk ook dat het goed is voor het studierendement als
nieuwe studenten beter weten waar ze aan beginnen.
Je kunt beter 30 studenten krijgen die allemaal
doorgaan, dan 50 waarvan de helft afhaakt.
Helpt het idee van de nieuwe, modulaire
bacheloropleidingen daar wel of juist niet aan mee?
Er zijn denk ik voordelen van de clustering van
opleidingen, als je het op de goede manier doet. In
de opleidingen TN en ST zitten veel overeenkomsten,
maar je moet wel de diepgang voor beide stromingen
behouden; anders kun je het ook niet verkopen aan
Het vraagt ook veel van de docent: de verschijnselen
die bij natuurkunde en scheikunde worden behandeld
zijn vaak hetzelfde, maar de taal is heel anders. De
docenten moeten geleerd worden bruggen te slaan.
vakgebieden voor, in het ene geval voor een slinger
en in het andere geval bij stromingen. Je moet er voor
zorgen dat scheikundigen geen slingers hoeven door
te rekenen, en natuurkundigen geen stromingen.
U staat er dus wel positief tegenover?
In principe wel, maar het moet organisatorisch wel gaan
lukken. “In het verleden behaalde resultaten bieden
geen garanties voor de toekomst”, en in het verleden
was het ook niet altijd zo rooskleurig.
Ten slotte de laatste twee vragen: wie is uw
“favoriete wetenschapper”?
Dan heb ik twee voorbeelden, die ik bewonder vanwege
hun didactische vaardigheden. De ene kennen jullie
vast, Richard Feynman. Ik vind het erg leuk om zijn
lectures te lezen, of te gebruiken bij mijn vakken.
Ik denk over reactiviteit aan kristaloppervlakken, een
soort vervolg op Interfaces and Catalysis. Of hebben
jullie geen quantummechanica gehad? Dan zou ik de
basis quantummechanica wel willen doceren.
De andere is Walter Lewin, emeritus hoogleraar van MIT.
Zoek zijn physics lectures maar eens op, bijvoorbeeld
over klassieke mechanica. Hij is didactisch heel goed, ik
heb het idee dat hij studenten weet te bereiken en de
ideeën duidelijk over weet te brengen.
U heeft geen duidelijk overzicht van wat ATstudenten leren?
En wie gaan wij voor de volgende editie interviewen?
Nee, vanuit de staf heb ik nooit een korte en duidelijke
visie op AT gehad, over de inhoud en het doel. Ik ben
destijds gewoon in I&C gerold… Het zou goed zijn om
Hebben jullie Mireille Claessens al gehad? Het lijkt
me leuk als jullie die doen.
KIVI NIRIA: Mind mapping
Daan in den Berken
A couple of weeks ago KIVI NIRIA organised a workshop on mind mapping. Intrigued by the poster, yet
not expecting much, I attended the workshop and
was pleasantly surprised. Mind mapping is a note
making strategy that tries to utilise certain mnemonic
strategies, allegedly being so effective because it
attempts to utilise both hemispheres of your brain.
By representing information in a graphic, non-linear
manner the mind maps aid in recalling the information. It is not only used for making notes, but also
for visualising problems, in studying or organising
information or brain storming sessions.
And that is exactly what mind mapping attempts.
Instead of making a summary the old fashioned way,
it suggests you should make it in a more visual way.
Mind mapping works on the premise of lateralisation
and trying to utilise the right hemisphere in tasks
dominated by the left. The left part of the brain is said to
dominate in linear reasoning, reading and making lists,
while the right hemisphere is more specialised in spatial manipulation, creativity and processing pictures.
Though critics say this is pseudoscience, when applied
to mind mapping. The general idea is transforming a
list of words into a more graphical representation of the
content, so instead of only relying on verbal recall by
the left hemisphere, you also stimulate the right with
spatial and iconographical aspects.
To make your own mind map, start with rotating your
piece of paper (landscape, like the printer option) and
write down your subject in the middle. Then surround
it with sub-topics radially arranged around this word,
with braches connecting it to the main subject.
As a key to making effective mind maps, we were given
a few guidelines to follow.
The workshop started off with a list of 20 random objects and we were told to try to memorise them in order.
After hearing the whole list the group started writing
down the names of the objects. Ignoring the oddball
in the group, the average result was the first 3 names,
the last 3 names and the one in the middle; 7 out of 20.
And then came an explanation for a trick. You have to
create a story or some other connection between the
objects. In this list every object had a relation to its
position and so could be linked to the number. If you
thought of the number it was not that hard to recall
what object was connected to it. The second time the
group wrote down the list of objects almost everyone
had 19 or 20 correct. Even several months later, I still
remember the whole list in order. If only I could use that
for a course.
Use keywords, not full sentences
Use multiple colours, different colour per branch
Always leave open some options
To assist you in making mind maps there are several
software packages available; some freeware, some not.
There are even applications for the iPhone and other
If you are interested in mind mapping, sign up next
time a workshop is announced. It even includes a little
USB-stick with some free mind mapping software.
Arranging atoms in artificial
structures of complex oxides
Gertjan Koster & Bouwe Kuiper
Modern thin film techniques are capable of synthesising thin crystalline two-dimensional layers with a
thickness of digital precision (i.e., n=1,2,3… where n
is the number of unit cell layers of the material). One
of the research topics within the Inorganic Materials
Science group focuses on the question whether
epitaxial structures of lower dimensions can be
fabricated, such as wires (one-dimensional) or dots
An interesting class of materials for thin-film applications is the one of perovskites. These complex oxides
provide a unique toolset of materials to the structure
composition to property relationship in so-called correlated electronic systems. In essence, such materials
do not follow the simple models based on the assumption that the electrons responsible for the bonding
in the materials can be treated independently, but
experience mutual interaction. Due to this interaction
though, which strength depends on the constituent
elements, many different properties can be found in
these materials, such as colossal magnetoresistance in
LaSrMnO3 or high Tc superconductivity in YBa2Cu3O7,
making them interesting for all kinds of future applications. Structurally the perovskites consist of simple
cubic units with 4 Å ribs, see Fig. 1. The corner positions
are occupied by A-site ions. The faces contain oxygen
and the center consists of a B-site ion. The oxygen
octahedron connecting the oxygen atoms provides a
strong backbone, which is present in all perovskites,
independent of their cations and properties. This
makes that within the perovskite family, the fabrication
of heterostructures, i.e., stacks of different materials
each with a different physical property, is relatively
Thin film growth
The complex oxides thin films can be made by Pulsed
Laser deposition (PLD). A high intensity KrF excimer
(248 nm) laser is used to ablate material from a target.
The ablated material, or plasma plume, expands in a
vacuum chamber where it is deposited onto a heated
substrate. Each laser pulse results in the deposition of
a fraction of a monolayer of added material on top of
the substrate. By controlling the number of laser pulses,
the layer thickness can be precisely dialed in, down
to the scale of single unit cell layers of the material.
In this way, one essentially can build artificial crystal
structures bottom up, for example by the deposition
of superlattices and heterostructures, where either A,
B or both cations are periodically varied. Examples of
such experiments are the (Ba,Sr,Ca)TiO3 system, with
enhanced ferroelectric polarisation [1], and the LaAlO3/
SrTiO3 system, where the interfaces between the two
insulating perovskite blocks become conducting [2].
In both of these examples, the heterostructures consist
of alternating sheets of material grown on a single terminated substrate template; typically TiO2 terminated
SrTiO3, which can be obtained through well-established
chemical etching procedures [3].
Figure 1: Schematically representation of a perovskite unit
cell (a), a crystal surface showing ordered mixed surface
terminations (b) and a resulting nanowire pattern.
Lateral control
As described above, PLD tends to give flat layers or a
two-dimensional crystal structure. However, the deposition conditions and substrate surface morphology can
be chosen in such a way that it is possible to fabricate
more complicated and lower dimensional structures.
Such structures are either made using masks to deposit
only on some parts of the substrate [6] or by self-organisation [4,5]. An example of the latter is the experiment
where complicated 3D structures were created using
a solid solution of different complex oxides [5]. In this
solid solution a preferential crystal facet dominates
the ordering process resulting in one material being
embedded in the other material in for example squares,
dots, wires or ribbons. In literature, there are examples
of spontaneous self-organisation of deposited
material on a perovskite template showing
mixed A and B ions at the surface.
Arranging atoms
Here, we would like to describe a newly discovered
route to self-assembled nanowires using a controlled
substrate surface morphology. As discussed above,
a perovskite unit cell consists of two types of cations,
either one of them can be present at the surface. These
chemically distinct areas of the surface have a different
interaction with the material deposited by PLD. If on the
one hand the deposited material wets one of these surface terminations whereas on the other hand the other
termination is not wetted, the resulting thin film will
show a structure, which follows the original substrate
chemical termination morphology. This is exactly the
case for growth of SrRuO3 (a ferromagnetic metal) on
DyScO3, where the DyScO3 crystal shows ordered areas
of DyO and ScO2 termination, as depicted in figure 1b
indicated with dark (blue) and lighter (yellow) areas.
The growth results in nanowires of SrRuO3, which lie on
top of the ScO2, terminated areas, figure 1c.
By annealing the crystal at high temperature the different types of surface termination order and straighten,
an example of such an ordered crystal surface if given
in Figure 2a. Figure 2a shows a lateral force micrograph
(LFM) recorded using an Atomic Force Microscope
(AFM) of a DyScO3 substrate. This substrate was annealed at high temperatures. The colour indicates the
interaction strength between the AFM tip and the
surface. The interaction strength varies locally, caused
by the different chemical terminations on the DyScO3
surface. Vicinal terraces are about 200 nm wide and the
mixed termination ratio is about 75 percent.
After PLD growth, a nanowire pattern of SrRuO3 is observed using a Scanning Tunneling Microscope (STM),
depicted in Figure 2b. The resulting wire array shows
nanowires, which are slightly wider than the original
substrate mixed termination template and are 6-8 nm
in height. The wires are single-crystalline and have a
high aspect ratio. They are just 100 nm wide, but up to
50 micron long.
It is important to note that the SrRuO3 is electrically
conducting and commonly used as an electrode material whereas DyScO3 is a good insulator. The nanowires
are thus conducting on an insulating substrate, while
isolated over a long distance. This property was exploited by using the wire pattern as a bottom electrode
for ferroelectric PbTiO3. Figure 2c shows an AFM image
of such a PbTiO3 thin on top of a nanowire array. The
ferroelectric properties are probed using a Piezo Force
Microscope (PFM). Here we probe PbTiO3 electrical
polarisation by applying an electrical field between
the SrRuO3 bottom electrode and the PFM tip. Figure
2d shows such a PFM image. We can clearly see that a
relationship exists between the nanowire morphology
and the ferroelectric domains of the PbTiO3 film.
Monte Carlo Model
Figure 2: LFM image of a mixed terminated DyScO3 substrate (a), resulting nanowire pattern STM image (inset:
line profile) (b), AFM image of a DyScO3/SrRuO3/PbTiO3
structure (c), PFM image of structure c (d). For the colour
image, look up the digital version online.
Self assembled nanowires
The process of making the nanowire arrays requires
control over the chemical composition of DyScO3
substrate. The substrates are 5x5x0.5 mm single crystals
with one polished surface. The crystals are cut in such
a way that it shows AO and BO2 planes parallel to the
physical surface. In practice these surfaces are off by a
few tenths of a degree with respect to the crystal
lattice, resulting in unit cell high steps.
In an ideal case PLD growth results in atomically flat
two-dimensional films. For the nanowire growth we
used growth conditions, temperature, laser energy, etc
which should result in these flat films if the substrate
surface was completely single terminated. From these
thin film growth experiments we know the deposition
rate of SrRuO3. In the case of nanowire growth all the
material we deposit at this rate actually sticks to the surface and becomes part of the nanowire. In other words,
the total volume of material in the nanowire is similar to
the total volume of material we would have deposited
in a thin film. Therefore we conclude that the wires form
during growth via a diffusion process. All the material
sticks, diffuses and finally nucleates on a nanowire.
Arranging atoms
Such diffusion process can be simulated using a
kinetic Monte Carlo model or in two dimensions a
Solid-on-Solid model. A crystal lattice of 512x128 u.c.
with so-called periodic boundary conditions was used
to study the initial growth on substrates with areas of
different surface. The model is based on an Arrhenius
type equation, which relates temperature and energy
barriers for diffusion to a single diffusivity value. The
diffusivity, DS=D0 exp[-(ES+nEN)/kBT] is calculated for
each lattice site, based on an the local temperature
T, energy barrier for diffusion related to the substrate
surface, ES and a nearest neighbor interaction term, EN.
To simulate a mixed terminated substrate, the local ES
values are changed to create areas of different diffusivity. The areas with a relatively low diffusivity act as sink
sites with a high probability of nucleation, gathering
all the applied material. Figure 3 shows simulation
results for growth of 0, 1, 30 and 60 pulses of SrRuO3
on a mixed terminated substrate with an ES difference
of 0.3 eV, here 60 pulses corresponds to the amount of
material required to make two monolayers. The starting
template is depicted in Figure 3a, it has steps of 170
u.c. wide and a 50 percent mixed terminated surface
ordered along the step edges. During growth Figure
3b-d material diffuses onto areas with a low diffusivity.
When growth is continued a nanowire pattern appears,
where wires grow in height and width simultaneously,
but at different rates. Ideally one would like to combine
a strong sensitivity to different surface terminations
and a large diffusion length to create well isolated
nanowires. In other simulations, low diffusion lengths
result in undesired nucleation on all terraces. A high
growth temperature decreases the sensitivity to different chemical terminations, also resulting in undesired
nucleation. However a high temperature does increase
the diffusion length. Using the proposed mechanism,
the growth temperature can be optimised to facilitate
wire growth and prevent nucleation in between the
Conclusions & Outlook
Using the right kind of template one can grow selfassembled nanowires of single crystalline complex
oxide materials. The method that we described has the
potential to be used for various other nano-patterns
of complex oxides such as dots. Confining conducting
electrons in small structures is interesting from a fundamental point of view, since confined electrons reveal
their quantum nature. This is particularly interesting if
the electrons are of the interacting type. For applications, the method offers the possibility to fabricate prepatterned electrodes for ferroelectric switching devices
as in F-RAM (ferroelectric random access memory). The
key to the success of the method is to better control the
formation of the right kind of template surfaces, which
is one of the things that is getting a lot of attention in
the current research at IMS.
Brian Smith, Harold Zandvliet, André ten Elshof, Josée
Kleibeuker, Guus Rijnders, Jeroen Blok, IMS group,
1. Ho Nyung Lee, Hans M. Christen, Matthew F. Chisholm,
Christopher M. Rouleau & Douglas H. Lowndes, Nature
433, 395 (2005)
2. A. Ohtomo and H. Y. Hwang, Nature 427, 423 (2004)
3. G. Koster, B.L. Kropman, G. Rijnders, D.H.A. Blank and
H. Rogalla, Appl. Phys. Lett. 73 (1998) 2920-2922
4. R. Bachelet, F. Sanchez, J. Santiso, C. Munuera, C. Ocal,
J. Fontcuberta Chemistry of Materials 2009 21 (12),
5. J.L. MacManus-Driscoll Adv. Funct. Mater. 2010 20,
6. Paul te Riele, PhD thesis University of Twente.
Figure 3: Nanowire growth morphology evolution modeled by kinetic Monte Carlo, after 0, 1, 30 and 60 pulses
Morgen kunnen we sneller
chips maken. Vandaag mag
jij ons vertellen hoe.
Deep UV-licht
(193 nm)
De race om steeds meer IC-schakelingen
op de vierkante centimeter te realiseren,
is niet de enige race in de chipwereld.
Fabrikanten willen ook de chipproductie
zélf versnellen. Maar hoe voer je een
machine op, die op de nanometer
nauwkeurig moet presteren?
6 m/s
In de chip-lithografiesystemen waar ASML nu
aan werkt, wordt een
schijf fotogevoelig
silicium (de wafer)
op hoge snelheid
33 m/s2
Chips met 45-nm-details kun
je alleen maken als je - tussen
versnelling en vertraging door op de nanometer exact belicht.
1000 sensoren en 8000 actuatoren
bedwingen en daarmee
180 wafers per uur belichten.
Hoeveel software en processoren
vraagt dat? En hoe manage je de
architectuur daarvan?
33 m/s2
De wafer ligt op de
zogenoemde waferstage
(ruim 35 kilo). Die beweegt
onder het licht door. Heen en
weer, dus met een extreme
versnelling en vertraging van
33 m/s2.
Versnellen met 33 m/s2 is al een
uitdaging op zich. Welke motoren
kies je? Waar vind je versterkers met
100 kW vermogen, 120 dB SNR en
10 kHz BW? En dan begint het pas.
Want voorkom maar ’ns dat al die
warmte je systeem weer
onnauwkeurig maakt...
Voor engineers die vooruitdenken
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