Conducting Probe Atomic Force Microscopy: A

Research News
Conducting Probe Atomic Force
Microscopy: A Characterization Tool for
Molecular Electronics
By Tommie W. Kelley, Eric L. Granstrom, and C. Daniel Frisbie*
1. Introduction
Recently, a number of new atomic force microscopy
(AFM) techniques exploiting electrically conducting probes
have been developed to measure electrostatic forces, charge
distributions, voltage drops, capacitances, or resistances on
sub-100 nm length scales. These new AFM adaptations, e.g.,
electric force microscopy,[1] scanning capacitance microscopy,[2] and scanning potentiometry,[3] hold great promise for
electrical characterization of materials since high resolution
topographic imaging and electrical measurements are
achieved simultaneously, providing direct correlation of
electrical properties with specific topographic features.
Here we describe a variant of AFM in which conducting
probes are used to measure current±voltage (I±V) relationships and resistances (conductances) of materials. We term
this AFM technique ªconducting probe atomic force
microscopyº (CP-AFM),[4] although others have suggested
ªconducting AFMº[5] or ªscanning resistance microscopyº[6]
for similar methods. Figure 1 shows a typical CP-AFM
experiment in our laboratory in which the probed material is
a semiconducting organic crystal contacted by a microfabricated Au wire on SiO2. The conducting probe is positioned
with controlled load at desired points on the crystal and the
resistance or I±V response between the probe and the wire is
determined using external electronics. In our measurements,
the probe is held stationary during the measurement and is
used to image the sample in tapping mode before and after
the I±V data are recorded.
The aspects of CP-AFM that are most attractive for
nanoscale electrical transport measurements are 1) the
ability to image samples with high resolution before, during,
or after the measurement, 2) the ability to record I±V
relationships on samples that are highly resistive or
±
[*]
Dr. C. D. Frisbie, T. W. Kelley, E. L. Granstrom
Department of Chemical Engineering and Materials Science
University of Minnesota
421 Washington Avenue SE, Minneapolis, MN 55455 (USA)
Adv. Mater. 1999, 11, No. 3
surrounded by insulating regions, and 3) straightforward
interpretation of the tip position relative to the sample (i.e., a
measured repulsive force indicates intimate tip±sample
contact). These three characteristics make CP-AFM ideal
for studying electrical transport in microfabricated semiconductor devices, nanoparticle assemblies, and individual
molecules, for example. A deeper appreciation of these
characteristics can be obtained by comparing CP-AFM to
scanning tunneling microscopy (STM), which is a well
established technique for probing electronic properties on
nanometer length scales.
Both CP-AFM and STM share the high spatial resolution
imaging capability (STM, ~0.1 nm; CP-AFM, ~10 nm) that is
critical in linking nanoscale structure to transport properties.
An important distinction relates to the position of the tip. In
CP-AFM the tip is placed in direct contact, under controlled
load, with the material to be probed. This means that the
measured I±V relationship may be profoundly influenced by
the electronic properties of the tip±sample contact. In
contrast, an STM tip is generally not in physical contact
with the sample, so the I±V characteristic of the tunneling gap
is determined by the sample electronic structure, not tip±
sample contact properties. While reliance of STM on
tunneling allows a powerful conductance spectroscopy that
maps the electronic density of states of a material, the length
scale over which one can probe transport in resistive
materials is limited to typical tunneling distances (1±
10 nm). Also, since STM requires detection of a tunneling
current to position the tip, it is not practical to use STM to
characterize small structures surrounded by insulating
regions. In CP-AFM, force feedback decouples probe
positioning from conductivity of the sample, facilitating
transport measurements over longer distances on samples
with widely varying resistances. In addition, for some
transport experiments it is important to know the precise
location of the tip relative to the sample, and in this regard
detection of a repulsive force in CP-AFM can be easier to
interpret than a tunneling current in STM. The relative merit
of the CP-AFM and STM approaches for electrical
characterization certainly depends on specific goals and
experimental constraints, and the two techniques should be
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Fig. 1. Scheme of a typical point-contact CPAFM experiment in which a Au-coated
AFM tip is used to probe the resistance of
a thin crystal of the molecule sexithiophene
(6T). During a measurement, current is
recorded as a function of voltage applied
between the positionable conducting probe
and a fixed electrode. The inset shows the
orientation of the 6T molecules with respect
to the probe and substrate.
viewed as complementary methods for probing transport in a
variety of small-scale structures.
In our work, we have used CP-AFM to probe transport in
organic materials.[4] Heightened interest in molecule-based
electronics has underscored the need for better understanding of conduction mechanisms in organic materials,
particularly in thin film form. CP-AFM offers an ideal
characterization strategy for organic thin films because, first,
it allows direct correlation of electrical transport properties
with specific, well-defined supramolecular structures or
defects, and second, these films are often too resistive to be
probed by STM. In this article, we outline previous CP-AFM
studies and then highlight some of our recent CP-AFM
measurements on extremely thin crystals of an organic
semiconductor, as depicted in Figure 1.
2. Previous Work
Previous CP-AFM studies have shown the ability to make
two basic kinds of electrical measurements, stationary pointcontact measurements, and two-dimensional resistance
maps. Figure 1 shows a point-contact measurement in which
resistance (or I as a function of V) is measured between the
stationary probe and a fixed contact. An important
application of point-contact CP-AFM is ªspreading resistance profilingº (SRP) of dopant concentrations in conventional semiconductors.[7] Spreading resistance is the resistance associated with current crowding near a point contact
and can be related to the dopant concentration in a material
by comparison to a calibrated standard. Measurement of the
spreading resistance as a function of point-contact location
on a semiconductor device yields the spatial distribution of
the dopant concentration. Several groups have shown in
recent publications that CP-AFM (or ªnano-SRPº) allows
determination of dopant concentration profiles with much
higher resolution than is possible with conventional point
probes commonly used in the semiconductor industry. Pointcontact CP-AFM measurements have also been used to
measure the conductances of individual semiconductor
nanoparticles,[8] Langmuir±Blodgett films,[9] adsorbed molecules on graphite,[5] and thin molecular crystals on Au.[4]
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The second type of measurement, resistance mapping,
involves simultaneous topographic imaging and resistance
measurements. Resistance maps show the resistance between the scanning probe and a second, stationary contact as
a function of probe position. Recently, Lieber and coworkers generated resistance maps of a single carbon
nanotube contacted at one end by a microfabricated Au
electrode.[10] Analysis of the maps revealed the resistance per
unit length of nanotube as well as the contact resistance
associated with the tip±tube and Au±tube junctions. A more
general result of their study was that by using hard,
conductive NbN coatings they were able to make reproducible electrical measurements while continuously scanning in
contact. Canadian, French, and Chinese workers have also
used resistance mapping to image local resistances on
semiconductor,[11] metal,[12] and metal oxide[13] surfaces,
respectively.
Key to all CP-AFM measurements is the conducting
probe. Reported conducting probes include heavily doped Si
tips and conventional Si or Si3N4 tips coated with metal (e.g.,
Ag, Au, Pt, NbN) or B-doped diamond films. Thomson and
Moreland made a detailed investigation of Ag-, Au-, and Ptcoated tips and doped Si tips, and achieved five orders of
magnitude lower contact resistance to Au surfaces with
metal-coated probes than with doped Si probes.[14] They also
noted that shear-induced abrasion of the metalized tips was
an important problem that could be mitigated by imaging
samples in tapping mode instead of contact. Compressive
and tensile forces on metal tips appear to be less damaging, as
Stalder and Durig found that the yield strength of Au point
contacts is more than one order of magnitude larger than the
macroscopic yield strength of Au.[15]
In our work, we have employed Au-coated Si probes
(700 Š Au over a 70 Š Cr adhesion layer) because Au is easily
deposited by vapor deposition and has a high work function,
facilitating low resistance contacts to many organic materials. Au tips are generally not robust to continuous scanning in
contact mode. Accordingly, we have devised an approach in
which we image our samples in tapping, or intermittent
contact mode, and make I±V measurements in stable contact
at selected points. Imaging in tapping mode preserves the Au
coating on the tip by eliminating the shear forces that tend to
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Adv. Mater. 1999, 11, No. 3
Research News
abrade the coating, and also has the advantage that we can
characterize delicate samples that could not withstand
continuous contact mode scanning.
3. Point-Contact I±V Measurements on Thin
Molecular Crystals
We have used CP-AFM to measure electrical resistances
of very thin crystals of a p-type semiconducting thiophene
oligomer, sexithiophene, or 6T.[4] The conductance of
polycrystalline films of 6T has been studied extensively
because of possible applications of this material in low cost
thin film transistors. Previously, we found that very thin
crystals of 6T may be grown on SiO2 by vacuum sublimation.
The crystals are typically several micrometers in length and
width and range from 2 to 16 nm in thickness. The 6T
molecules are arranged in layers with the long axis of the
molecule nearly perpendicular to the substrate as indicated
in Figure 1. The thickness of one molecular layer (ML) of 6T
is 2.3 nm, so the 2±16 nm thickness range corresponds to 1±
7 ML.
Figure 2A shows an AFM topograph of a 2 ML thick 6T
crystal contacted by a microfabricated Au wire on SiO2. This
image was acquired in tapping mode using a Au-coated tip.
We selected five points on the crystal, each an increasing
distance from the wire, to make point-contact I±V measurements. These points are labeled in Figure 2A and the
resulting I±V traces are shown in Figure 2B. Inspection of
Figure 2B shows that the current increases monotonically
with tip voltage over the entire 0±5 V range at each of the five
probe locations. At a given tip voltage, current decreases
with increasing distance of the probe from the wire, i.e.,
current at 5 V is less at point 2 than at point 1, as expected,
since resistance of the crystal should increase with probe±
wire separation. The nonlinearity of the I±V traces is not
surprising since the precise I±V relationship is a function of
any charge injection barriers at the tip±6T junction. These
traces were acquired in air and were extremely reproducible.
Repetitive measurements at each of the five points yielded
the same curve to within 10 %. Importantly, imaging of the
crystal after the measurements revealed no evidence of
damage to the crystal.
In Figure 2C we plot the resistance of the crystal at 5 V
(i.e., the reciprocal of the I±V slope at 5 V) as a function of
probe±wire separation. The dependence appears to be
linear, and the line on the plot is the least squares fit. Linear
dependence is somewhat surprising in light of the fact that we
are probing a crystal that can be viewed as a layered sheet of
molecules, and injected holes would be expected to spread
Adv. Mater. 1999, 11, No. 3
Fig. 2. A) Topograph of a 6T crystal connected to a microfabricated Au wire on
SiO2. This image was obtained in tapping mode with a Au-coated probe. The
locations of five point-contact I±V measurements are labeled. B) Pointcontact I±V characteristics obtained by CP-AFM at points 1±5 labeled in (A).
C) Resistance (reciprocal of I±V slope at 5 V) versus probe±wire separation
distance. The line is a least squares fit. The intercept gives a contact resistance
Rc = 82 MW.
out from the tip into the sheet en route to the wire. Detailed
modeling accounting for the distance dependence is in
progress. The importance of these data is that they can be
extrapolated to zero separation, yielding a contact resistance
Rc of 82 MW due to the tip±6T and wire±6T junctions. Large
contact resistances are typical for organic±metal junctions,
and the measurements here demonstrate that CP-AFM may
be used to measure these contact resistances directly and
perhaps in future experiments to relate them to the number
of molecular layers, specific crystal morphologies, or
molecular orientations.
4. Resistance of a Grain Boundary
Figure 3A shows another AFM topograph of a 6T crystal
in contact with a Au wire. Point-contact I±V measurements
were made at the four points labeled on the crystal. A grain
boundary (GB) is visible between points 1 and 2, and the I±V
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contacts is nearly constant for all measurements and is
<50 MW, we conclude that the 25 GW ªcontact resistanceº
associated with points 2, 3, and 4 is almost completely
attributable to the resistance of the GB. Further studies are
required to determine if this is a typical GB resistance for 6T.
The key point is that we can use CP-AFM to measure the
resistance of specific well-defined defects in molecular
materials, such as GBs, which should ultimately give us
better insight into the nature of transport in these materials.
5. Conclusion and Outlook
The combination of high spatial resolution imaging and
electrical characterization make CP-AFM a powerful
characterization tool for relating transport properties to
structure. We are particularly optimistic about the opportunities for CP-AFM in illuminating transport mechanisms in
organic materials. In the future, we can expect CP-AFM
methods to become more sophisticated. For example, it
should be possible to integrate a second, independent ªgateº
electrode on the probe tip or cantilever, which would allow
field effect measurements on the nanoscale. One can also
imagine creating conducting probes with thin tunneling
barriers on their surfaces, facilitating Coulomb blockade
experiments. Electro-optical measurements such as electroluminescence mapping also appear promising. All of these
measurements would benefit from operation in a vacuum
environment at variable temperatures. In general, CP-AFM
is a valuable complement to the other scanning probe
microscopies mentioned at the beginning of this article that
reveal the electrical properties of materials with sub-100 nm
resolution.
Fig. 3. Estimation of the resistance of a grain boundary (GB) in 6T. A) A
topograph of a 6T crystal connected to a microfabricated Au wire on SiO2. Two
GBs are indicated with arrows. Point-contact measurements were made at
points 1±4. The image was obtained in tapping mode with a au-coated probe.
B) Point contact I±V characteristics obtained by CP-AFM at points 1 and 2
labeled in (A). The inset shows an expanded view of the I±V trace at point 2.
C) Resistance (reciprocal of I±V slope at 5 V) versus probe±wire separation
distance. The linear fit through points 2, 3, and 4 has been used to estimate the
GB resistance (see text). Note the change of scale on the resistance axis.
measurements taken at points 1 and 2 are dramatically
different, as shown in Figure 3B. The current at 5 Vat point 2
is 105 times smaller than the current at point 1, indicating an
enormous resistance associated with the GB. The resistance
can be estimated by determining the nominal Rc on each side
of the GB. Figure 3C shows the resistance (at 5 V) computed
for each of the four probe locations indicated in Figure 3A.
Note the discontinuity in the resistance axis in Figure 3C,
which switches from MW to GW values. The resistance
measured at point 1 is 50 MW and we can conclude that Rc on
the near side (wire side) of the GB is therefore <50 MW. A
linear extrapolation for the measurements at points 2, 3, and
4, which are on the far side of the GB, gives Rc = 25 GW.
Assuming that the total Rc due to wire±6T and tip±6T
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±
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