Processing of Polyoxometalates: a step toward

Processing of Polyoxometalates: a step toward molecular memories
Ph. D. Project under the supervision of Pr A. Proust and Dr F. Volatron
The objective of this project is to devise molecular-based surface materials incorporating
polyoxometalates (POMs) as electroactive molecules for applications in molecular nanosciences, in
particular nanoelectronics. Fully molecular electronics is appealing but still far from practical applications,
while a hybrid approach consisting of incorporating molecules in semi-conductor based devices provides a
mid-term alternative. Electroactive molecules like pophyrins have been proposed as good candidates for
molecular memories, in which information is stored as an electric charge depending on the discrete redox
state of the molecules.1 In the list of electroactive molecules that have been considered as promising for
charge storage in memory applications, POMs are clearly missing, though they will provide n-type
counterpart to p-type materials built on porphyrins.
Polyoxometalates are nano-scaled oxo-clusters of the early transition metals in their
highest oxidation states (MoVI, WVI, VV…). Classical structures include the
Lindqvist [M6O19]2-, the Keggin [XW12O40]n- (M = Mo, W; X = P n= 3; X= Si n=4
…) and the Wells-Dawson [X2W18O62]p- (M = Mo, W; X = P n= 6; X= S n=4 …)
types. POMs are polyanionic species that carry counter-cations, be they alkaline or
organic such as tetraalkylammonium. The electronic structure of POMs is akin to
that of oxides and account to the remarkable redox properties of POMs that display
successive (isolated molecular orbital levels and not bands) and reversible (low
structural reorganization due to the non-bonding character of the LUMO) reduction
processes. The added electrons are then delocalized on part or on the totality of the
POM scaffold explaining the robustness of the structure and ensuring endurance
upon charging/discharging cycles. Although the redox properties of POMs have
been thoroughly investigated in solution, real applications in solid-state devices are
still underdeveloped.2 POMs are indeed crystalline solids that are hard to process
and since their sublimation is not possible, they have been mainly assembled
through electrostatic interactions or simply deposited from solution on substrates.
Although very simple these procedures cannot always guarantee the homogeneity
and/or stability of the structure. Reproducibility is also a crucial issue for large
scale, commercially viable systems. Processing is thus an unavoidable prerequisite
that could have slowed down the implementation of POMs. To move forward, we
propose our expertise in the functionalization and post-functionalization of POMs,
that is in the preparation of organic-inorganic hybrids.3 This will enable us to
control the covalent grafting of POMs on substrates, their organization and
patterning and will also ensure a better control of the molecule/electrode contact.
The covalent grafting encompasses two complementary approaches: the first one is the direct grafting on the
substrate like in the spontaneous reaction of diazonium on a Si-H substrate or the grafting of thiol terminated
molecules on Au substrates; the second one proceeds in two steps, the formation of a SAM on the substrate
followed by a coupling reaction to anchor the active molecules, for example through the formation of peptide
1
Molecules for charge-based information storage, J.S. Lindsey, D. F. Bocian, Acc. Chem. Res., 2011, 44, 638.
Controlled modulation of conductance in silicon devices by molecular monolayersHe, T.; He, J. L.; Lu, M.; Chen, B.;
Pang, H.; Reus, W. F.; Nolte, W. M.; Nackashi, D. P.; Franzon, P. D.; Tour, J. M. J. Am. Chem. Soc. 2006, 128, 14537;
Design and fabrication of memory devices based on nanoscale polyoxometalate clusters, Busche, C.; Vila-Nadal, L.;
Yan, J.; Miras, H. N.; Long, D.-L.; Georgiev, V. P.; Asenov, A.; Pedersen, R. H.; Gadegaard, N.; Mirza, M. M.; Paul,
D. J.; Poblet, J. M.; Cronin, L. Nature 2014, 515, 545.
3
Functionalization and post-functionalization a step towards polyoxometalate-based materials, A. Proust, B. Matt, R.
Villanneau, G. Guillemot, P. Gouzerh, G. Izzet, Chem. Soc. Rev. Themed issue on Polyoxometalates, 2012, 41, 7605
2
bonds. We have already investigated both approaches with POM hybrids.4 Densely packed monolayers of
POMs on glassy C, graphene and Si(100) have thus been obtained and electron transfer kinetics from the
electrode to the layer investigated.5 The conductivity inside the POM layer on Si has also been assessed by
Scanning Electrochemical Microscopy (SECM) experiments.
This project will deal with several complementary aspects: deeper understanding of the properties of POM
monolayers, control of the POM dispersion at the sub-mono layer level and characterization of the electrical
properties of the surface materials integrated in very simple devices.
On POM monolayers:
- flat and homogeneous layer of POMs on Si(100) have been obtained. However Si substrates are
sensitive and subsequent formation of native SiO2 is unavoidable. A two-step procedure for the
immobilization of the POMs will thus be considered to protect first the Si wafer by a SAM prior to
the POM anchoring. Alternatively, we could also start with a Si wafer already covered by a native
SiO2 layer to be functionalized by silanes and then the POMs. This will allow us to compare the
performance of the resulting molecular junctions with variable dielectric (SiO2 + organic tether)
thickness and to find the good balance between fast electron transfer (write/read operations) and
good retention times (refreshment);
- extend to another oxide materials used in microelectronics such as Al2O3: direct grafting will
involve POM hybrids with carboxylic or catechol endings, which will be easy to prepare, with
variable tether lengths and types (conjugated or not).
The direct grafting is not suitable to space out the POMs and decrease their surface density. However, in the
case of porphyrin layers it is known that the lower the density of the molecules on the surface the higher the
electron transfer rate. It is also worth investigating the effect of a decrease of the density on the lateral (inside
the layer) conductivity. To control the dispersion at the sub-monolayer level different strategies will be
followed:
-
4
grafting to a preformed SAM that itself will display reactive
groups statistically dispersed. This will be performed on gold,
since more characterization techniques are available, before
considering grafting on Si. Different coupling reactions will be
evaluated and improved. Effect on the vertical as well as the
lateral conductivity will be assessed by electrochemical
techniques and C-AFM.
Bifunctional Polyoxometalates for planar gold surfaces nanostructuration, D. Mercier, S. Boujday, C. Annabi, R.
Villanneau, C.-M. Pradier, A. Proust, J. Phys. Chem. C, 2012,116, 13217
5
Electrografting of Diazonium post-functionalized Polyoxometalates: Synthesis, Immobilization and Electron Transfer
Characterization from Glassy Carbon, C.Rinfray, G. Izzet, J. Pinson, S. Gam Derouich, C. Combellas, F. Kanoufi, A.
Proust, Chem. Eur. J., 2013,19, 13838
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grafting (direct or indirect) on regularly spaced Au (or other
susbtrate) nanoplots of various size from about 1µm to 10 nm
and variable separation. In that case POMs will be densely
packed on each plot but POMs to POMs interactions between
two plots will be lowered or even cancelled.
Parallel to immobilization of POMs and surface chemistry, electrical
measurements will be carried out using various techniques. Electron transfer
kinetics will be assessed through cyclo-voltammetry (conductive substrates) and
SECM (isolating substrates) in solution when applicable. At the solid state I=f(V)
curves will be obtained either by conducting atomic force microscopy C-AFM, or
using a liquid top contact like a Hg drop or a GaIn eutectic drop, or after
evaporation of a metallic electrode like Al on the top of the layer to complete the
molecular junction. This will be completed by capacitance measurements. All
together, these studies will give some insights about the layer quality
(homogeneity), charge transport mechanisms and characteristics times. Elaboration
of a two-contact capacitor-like structure will be the first step toward memory
devices.
Collaboration: Dr D. Vuillaume, Institute of Electronics Microelectronics and Nanotechnology IEMN in
Lille especially for specified electrical measurements and development of more advanced devices
Contact: Pr Anna PROUST, Dr Florence VOLATRON
Institut Parisien de Chimie Moléculaire, UMR CNRS 8232, Université Pierre et Marie Curie Paris
06, 4 Place Jussieu, Case 42, 75252 Paris Cedex 05
[email protected], Tel +33 1 44 27 30 34
[email protected], Tel +33 1 44 27 55 53