Nanoelectronic Properties of a Model System and of a Conjugated

Mar 26, 2010 - Here we present a combined topography, Kelvin probe force microscopy (KPFM), and the recently presented scanning conductive torsion ...
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J. Phys. Chem. C 2010, 114, 7161–7168

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Nanoelectronic Properties of a Model System and of a Conjugated Polymer: A Study by Kelvin Probe Force Microscopy and Scanning Conductive Torsion Mode Microscopy Ling Sun, Jianjun Wang, and Elmar Bonaccurso* Max Planck Institute for Polymer Research, Ackermannweg 10, 55118 Mainz, Germany ReceiVed: January 29, 2010; ReVised Manuscript ReceiVed: March 12, 2010

The sub-100 nm morphology of those conjugated polymers that already found their way into electronic industry applications is known to heavily influence the electromechanical properties and the performance of those materials. Properties we are talking about are charge injection, transport, recombination, trapping, and the phase behavior and mechanical robustness of the polymer blend. Of paramount importance is also the analysis of the stability of these properties over time. Electrical atomic force microscopy is an ideal tool to measure simultaneously electronic properties, surface morphology, and mechanical properties of thin films of conjugated polymers. Here we present a combined topography, Kelvin probe force microscopy (KPFM), and the recently presented scanning conductive torsion mode microscopy (SCTMM) study on a gold/polystyrene model system. Conductive gold nanoparticles are embedded into a nonconductive thin polystyrene film. This system is a mimic for conjugated polymer compounds where conductive domains are embedded in a nonconductive matrix, like for PEDOT:PSS or PPy:PSS. We control the nanoscale morphology of the model by varying the distribution of gold nanoparticles inside the polystyrene film. We study the influence of the morphology on the surface potential by KPFM and on the conductivity by SCTMM. By the knowledge we gain from the model we are able to predict the structure of a homemade PPy:PSS compound. 1. Introduction In the past three decades conjugated polymers have been intensively studied due to their potential application in the field of organic semiconductors. Compared to silicon based devices, these materials could allow for the fabrication of low-cost, mechanically compliant, and lightweight devices.1,2 Despite all of the past efforts, the disclosure of their nanoscale optoelectronic properties is still incomplete. This puts limits on the range of potential applications. Due to a large variability of the processing parameters of the materials, nanoscopic crystalline and amorphous regions will exist in conjugated polymers.2,3 Nanoscale phase separation could occur when two components are blended, like with acceptor and donor materials in solar cells.4,5 Such inhomogeneities, on the scale from a few to tens of nanometers, influences the macroscopic device performance.2,6 Hence nanoscale characterization instruments and methods, particularly to study the relation between morphology and electronic properties of conjugated polymers, are highly demanded. Atomic force microscopy (AFM) is a widely used material and surface characterization tool with nanoscale resolution in both lateral and vertical dimensions. By applying ad hoc voltage/ current sources and sensors to regular AFM instruments, one can measure simultaneously electric and mechanical properties, and surface morphology. For instance, Kelvin probe force microscopy (KPFM) and electrostatic force microscopy (EFM) enable to directly measure the surface potential7 for investigating charge injection and trapping in active devices based on organic or inorganic semiconductors.8-10 Conductive-AFM (c-AFM), scanning tunneling microscopy (STM), and the recently introduced scanning conductive torsion mode microscopy (SCTMM) probe the distribution of conducting/percolation channels of thin * To whom correspondence should be addressed. E-mail: bonaccur@ mpip-mainz.mpg.de.

films of organic materials.11-14 Although the nanoscale inhomogeneity of a number of conjugated polymers has been demonstrated by some of the above-mentioned studies, the correlation between the electronic properties, i.e., surface potential or conductivity, and the morphology, i.e., crystallinity or amorphousness, at the nanoscale is still not fully resolved. The structure of conjugated polymers is said to consist of conductive domains embedded into a nonconductive matrix, as described in the Prigodin-Epstein model.3,15 As an example, poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is composed of a conductive component that determines the electronic properties, and a nonconductive amorphous component that increases the processability.12,16 Analyzing conductive domains completely surrounded by the nonconductive matrix is not always possible by c-AFM or SCTMM, unless these domains form a channel (percolation path) through the sample. On the other hand, surface potentials measured by KPFM are a “weighted average” of the surface potentials generated by the area close to the apex of the sensing tip.17,18 This decreases the lateral resolution of KPFM to 10-100 nm. In this paper we show how to overcome the limitations of both techniques by a combined analysis. We demonstrate it first on a model system with controlled nanoscale morphology and known electronic properties. We will later apply this knowledge to describe the electronic properties of a homemade conjugated polymer: polypyrrole:polystyrene sulfonate (PPy:PSS). The model we used is a gold/polystyrene (Au/PS) structure, with conductive gold nanoparticles (AuNPs) embedded into a nonconductive polystyrene (PS) film. The AuNPs resemble the conductive domains and the PS the nonconductive polymer matrix. The average diameter, d, of AuNPs is ∼30 nm, which is similar in size to the conductive domains in some conjugated polymers.2,19 We controlled the nanoscale morphology by varying two parameters: (i) the distribution of the AuNPs inside the PS film, from individual particles to small and then to large

10.1021/jp1008797  2010 American Chemical Society Published on Web 03/26/2010

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SCHEME 1: Cartoon of Sample Preparation Stages

clusters, and (ii) the thickness of the PS layer around and on top of the AuNPs. 2. Experimental Section 2.1. Sample Preparation. Scheme 1 shows a cartoon of the sample preparation. A highly doped silicon substrate (Boron doped, (100), resistivity 1-20 Ω · cm, Crys Tec GmbH, Berlin, Germany) was plasma cleaned (90% Argon and 10% Oxygen at 300 W, 200-G Plasma System, Technics Plasma GmbH, Mu¨nchen, Germany) for 10 min. It was then immersed in a methanol solution of 1% N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride (NR4+, 50% in methanol, ABCR ABCR/Gelest, Karlsruhe, Germany) for 30 min, washed with Milli-Q water, and baked at 95 °C for 1 h (Thermolyne Furnace model 47900, Thermo Fisher Scientific Inc., Waltham, USA). By this procedure a monolayer of positive NH4+ groups was bound to the substrate.20 We prepared an aqueous colloidal gold suspension via Frens’ method.21 The gold nanoparticles are stabilized by carboxylate ions. Samples with individual AuNPs were prepared by immersing silicon substrates in the suspension for 30 min (Scheme 1). The AuNPs on the silicon substrate were imaged by tapping mode AFM (Dimension D3100 cl, Veeco, Santa Barbara, CA) with 70 kHz silicon cantilevers (OMLAC240 TN, Olympus, Japan). Topography and phase images were acquired with the amplitude set-point set to ∼0.9. The size of the AuNPs was determined by the height difference between the particle apex and the flat substrate. The average diameter was d ≈ 30 nm (see Supporting Information, Figure S1). To prepare AuNPclusters we reimmersed the sample in the suspension for another 10 min. Since the carboxylate ions are sensitive to the pH of the suspension changing it from 6.7 to 4.0 causes an aggregation of AuNPs and leads to the formation of clusters. In the third step, PS (Mn ) 11200, Mw/Mn ) 1.06, Polymer Standards Service, Mainz. Germany) was spin-coated onto the substrate. By varying the concentration of the PS solution (4, 5, and 10 mg mL-1 in toluene) and the spin-coating speed, the PS layer thickness t varied from 14 to 133 nm. We measured the step height between the film and the substrate by tapping mode AFM after scratching the PS films with a sharp needle. To further vary t we etched the PS layer by Ar plasma (50% Argon, 60 W) controlling the etching time in subsequent steps and measured the thickness after each step. 2.2. KPFM and SCTMM. KPFM and SCTMM were performed in air on a Multimode TR-TUNA (Veeco Instrument, Santa Barbara, CA) with a IIIa controller. We used 70 kHz Pt-Ir coated tips (PPP-EFM, Nanosensors, Neuchatel, Switzerland). To avoid damage or contamination, we only calibrated the work function and the radius of curvature of some selected tips. All tips used during the measurements, however, were from the

Sun et al. same batch, so that we can assume that their properties were similar. The work function of the tips (Φt ) 4.95 ( 0.05 eV) was calibrated by KPFM on freshly cleaved highly orientedpyrolytic-graphite (HOPG - SPI-3 grade, SPI Supplies, West Chester, USA). The reference work function of HOPG in air is ΦHOPG ) 4.65 eV.22 Tip radii were d, the PS film on top of the particle becomes thicker and its contribution dominates over the contribution of air. The slope of the line is then mainly related to the dielectric constant of PS. The ratio of the slopes of the linear fits (∼2.8) is indeed very close to the ratio of the dielectric constants of PS and air (∼2.5). We will demonstrate later by SCTMM measurements that in our experiments t between 25 and 40 nm represents the threshold between uncovered and covered AuNPs. In summary, in this paragraph we show the surface potential measured by KPFM is influenced by the embedding depth of the AuNPs and by the dielectric constant of the nonconductive PS matrix. Conductive domains can be detected by KPFM even when they are embedded in nonconductive matrix. For AuNPs with a d ≈ 30 nm this depth is up to ∆t ≈ 100 nm. The lateral resolution of KPFM is not as high as the vertical resolution due to the broadening (overlapping of neighboring potentials) discussed above. On a bare silicon surface the potential of a 30 nm AuNP is ∼100 nm wide and it broadens to ∼300 nm when t ) 130 nm. Thus, with KPFM only we cannot study the distribution of the percolation channels formed by the conductive domains in the nonconductive matrix or the conductivities of the channels. We need a second method, like c-AFM or SCTMM. 3.2. SCTMM Analysis of Film Thickness on Individual AuNPs. In c-AFM or SCTMM a DC bias voltage is applied to the electrode below the sample and the current flowing through the sample is measured upon the formation of a percolation path, or a channel, between the tip apex and the electrode. These techniques are thus an ideal method to study the distribution of the percolation channels formed by the conductive domains in a conjugated polymer compound. In c-AFM the conductive tip is in mechanical or ohmic contact with the surface. For this reason the technique works best for hard conducting surfaces. It is not suited to study soft polymer surfaces that might be scratched by the tip. In SCTMM the conducting tip oscillates at a distance of few nanometers from the surface.24 This noncontact technique is therefore best suited for soft surfaces. Since no ohmic contact is established between tip and surface, the measured current is due to tunneling of electrons between tip and surface (or to an electronic field emission from the tip).

The percolation threshold in conjugated polymer compounds depends on the distance between neighboring domains and on the morphology of individual domains. In our system, the formation of percolation paths is determined by the distance between the conductive tip and individual AuNPs on the silicon substrate. This distance depends on the thickness of the PS film on top of the particles. Figure 6 shows a representative series of topography and current images of individual AuNPs. We applied always a negative bias to the sample. A more negative current thus corresponds to a percolation path with higher conductivity. All AuNPs on the bare silicon surface are conducting (see correspondence between features in Figure 6a and 6b). After coating the sample with a 25 nm thick PS film less than half of the particles are still conducting. The thickness ∆t determines the barrier height for electronic tunneling. When t ) 25 nm ∆t is around 1 - 2 nm and decreases to less than 1 nm when some “larger” particles protrude out of the PS film. A comparison of topography (Figure 6c) and current image (Figure 6d) tells that only the largest AuNPs (marked by red circles) conduct a current. Upon coating, however, the current decreases from ∼6 to 0.5 pA despite we increased the bias voltage from -9 to -12 V. The thin PS film thus raises the tunneling barrier. We then plasma etched the PS film on top of the AuNPs and again imaged the sample by SCTMM at the same position. The film thickness reduced to t ) 12 nm after plasma etching and ∆h was between 17 and 25 nm. The measured current rises again to ∼5 pA (Figure 6f), which is very close to the current measured on the bare AuNPs (Figure 6b). The small difference of the current in the two experiments is probably due to the different conductivities of the tips used and to the slightly different bias voltages. We conclude that most of the particles became exposed again after plasma etching and that the nonconductive PS film, even if very thin, reduced the conductivity measured on top of the AuNPs. In this view the SCTMM imaging mode is superior to the KPFM mode in telling if there is yet a subnanometer thin dielectric film on top of a conductive sample. There are “ghosts” on the left side of some current images (e.g., Figure 6, panels b and f). They are artifacts introduced by the electronic interference of the instrument or by some mechanical vibrations. We acquired the images scanning from the right to the left. Further, the anomaly on the lower part of Figure 6f is also an artifact caused by tip wearing. We disregarded the data contained there. If t > 40 nm no tunneling current was measured by SCTMM and ∆t was always >2 nm. The resolution of the current amplifier was 60 fA,24 beyond this value we measured only noise. From our measurements we conclude that no charge

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Figure 7. Height (a) and potential (b) images of Au clusters with a 56 nm PS layer on top (after annealing); the red circled area are the same position as the red circled area on Figure 6.

carriers tunneled more than 2 nm through the nonconductive PS matrix. Thus, when the AuNPs are too far away from the surface SCTMM fails. However, as shown before, in this case we can still track the particles by KPFM. In summary, by performing KPFM and SCTMM analyses on the same position of a sample we gain complementary information helping us to correlate the surface potential and the conductivity of conjugated polymers to their nanoscale morphology. By these two methods, supported by topographical images, we could determine the size of individual domains and the distance between them. However, the real structure of conjugated polymer compounds is usually not a simple 2D structure like our model. The conductive domains are inhomogeneously distributed inside the nonconductive polymer matrix. As well, the interdistance between domains varies. To model also real systems we extended our investigation to AuNPclusters embedded in a PS matrix. 3.3. KPFM and SCTMM analysis of film thickness AuNPclusters. Embedded AuNP-clusters were made by a two-step process, as described in the experimental section. We used samples where we still had some individual AuNPs and some larger clusters. After annealing the sample at 100 °C for 8 min to reduce surface roughness we determined t ≈ 55 nm by AFM. Individual AuNPs were completely covered after annealing. The measured height differences of ∆h ≈ 5 nm (Figure 7a) are due to clusters and not to individual particles. The surface potential of the clusters has a shadow-like feature (reddish area in Figure 7b) that is rather different from that of individual AuNPs. This shadow, or halo, is characterized by a smaller surface potential difference with the surrounding of only ∼15 mV. This is too

Sun et al. small for being generated by an individual AuNP embedded in a 55 nm thick PS film. In fact, we still measured a surface potential difference of 30 mV for an individual AuNP embedded in a 130 nm thick film. Hence, the halo of the surface potential results from averaging all potentials generated by AuNPs that were proximal to the tip.17,18 On the other hand, the surface potential difference between some clusters and the surrounding is up to ∼150 mV in some cases (red circled areas). This is larger than the surface potential of bare individual AuNPs, which was only ∼100 mV. This strong signal could be generated by two vertically stacked particles. In the previous paragraph we found that the current stopped when t > 40 nm. This means that we should not be able to measure currents on individual particles here as well, since t ≈ 55 nm. Let us therefore compare KPFM and SCTMM images acquired on exactly the same position, as confirmed by the corresponding height images (Figures 7a and 8a). We marked similar features/areas on both by red circles. Inside these areas we measured single spot currents of up to ∼10 pA applying a bias voltage of -10 V. Such high currents demonstrate that conducting channels were formed by two or more vertically stacked AuNPs forming percolation channels between tip and silicon surface. The number of percolation channels increased when we partially reduced t to ∼50 nm by plasma etching (Figure 8d). In this case we measured again ∼10 pA, but we applied only a bias voltage of -8.5 V. This means that the conductivity of the channels has increased. After further plasma etching we reduced t to ∼24 nm. Now all clusters and almost all individual AuNPs conduct a current. The shape of the clusters in the current image (Figure 8f) is similar to that in the height images (Figure 8, panels a, c, and e), indicating that the PS film was removed. No halos are present in the current image (Figure 8f), which is one main difference with the surface potential image (Figure 7b). The current flows only through individual channels and is not influenced by channels nearby. SCTMM thus allows imaging with higher lateral resolution than KPFM. The diameter of conducting channels and their distribution across the surface can be measured, and the channels can be counted. Such information is essential for a quantitative analysis of the charge transport routes at the nanometer scale and of their distribution in conjugated polymers.2,5,6 The surface roughness increases from Figure 8a-e, possibly due to inhomogeneous plasma etching. The elevated bump (bright yellow

Figure 8. Thickness dependence of the Au/PS system by SCTMM: (a and b) height and current images of Au clusters with t ) 56 nm PS layer, current bias ) -10.5 V; (c and d) height and current images of Au clusters after 4 min plasma etching, residual PS layer thickness t ∼50 nm, current bias ) -8.5 V; (e and f) height and current images of Au clusters after 12 min plasma etching, PS layer thickness t ∼24 nm, current bias ) -6.0 V.

Nanoelectronic Properties of a Model System

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Figure 9. KPFM and SCTMM images of PPy/PSS thin films: a) and b) height and potential images by KPFM; c) and d) height and current images by SCTMM, current bias ) -4.1 V, current sensitivity )10 pA/V. All images are scans of 250 × 250 nm. The current image was flattened without modifying the current data.

area on the lower side of Figures 7a and 8a,c) is probably an inhomogeneity of the PS film and not a large cluster of AuNPs, since it remains undetected in the current or surface potential signal. We analyzed also the surface potential and the current signals of individual AuNPs (Figure 7b and 8b,f), which we marked with white circles. These particles have a potential difference of ∼40 mV with the surrounding, but they are not visible in the height image (Figure 7a) and are thus completely embedded in the PS matrix. On the basis of KPFM data in Figure 7b alone we could not determine what generates the potential difference, we had to refer also to SCTMM data. Before plasma etching we measured no current in those areas (Figure 8b), while after plasma etching the current was ∼2 pA (Figure 8f). KPFM and SCTMM are thus sensitive to different regions and depths of the sample and yield complementary data. We next tested the combined use of KPFM and SCTMM on a real system, a homemade conjugated polymer compound, PPy: PSS. 3.4. KPFM and SCTMM Measurements on PPy:PSS and Prediction of Its Structure. Figure 9 shows topography, KPFM, and SCTMM images acquired on a thin film of PPy:PSS spincoated from solution. From the analysis of the height and the surface potential images we found domains (yellow areas in Figure 9a, red areas in Figure 9b) with sizes of 20-30 nm. They protruded slightly but had a lower surface potential than the surrounding surface. We assume that these features are colloidal aggregates of single PPy gel particles. These aggregates are embedded in the nonconductive continuous PSS matrix. Compared with the particles in suspension, the domain size in the dry film is much smaller. This can be reconciled when considering that the aggregates in aqueous suspension can be swollen by water while the aggregates in the film are water free. The SCTMM image shows that the percolation channels are mainly made of PPy domains (Figure 9d). The surface potential and the current images reveal also some interesting differences. The surface potential of individual PPy domains is rather uniform and of similar size, as in the topography image. The current signal of an individual PPy domain, instead, appears like a bundle of two or more channels of smaller dimensions. Based on the results of the Au/PS model system, we conclude

In this paper we presented topography, KPFM, and SCTMM studies on a model system of gold nanoparticles embedded in a polystyrene matrix. The nanoparticles were dispersed as individual particles or as clusters. The KPFM technique is very sensitive to changes of the surface potential, but its lateral resolution is of the orders of tens to hundreds of nanometers.17,18 The SCTMM technique is a valid complement to KPFM, since it can map the distribution of conducting channels with a lateral resolution of few nanometers. We applied a combination of KPFM and SCTMM on a thin film of a homemade conjugated PPy:PSS compound. We could relate surface potential and conductivity data with the nanoscale morphology of the compound. Conductive PPy agrgegates with size between 20 and 30 nm are embedded in a continuous nonconductive PSS matrix. These PPy aggregates consist of subdomains with dimensions smaller than 10 nm that also seem surrounded by thin layers of PSS. These subdomains form the actual percolation channels through the thin polymer film. The analysis we present is also consistent with information we have from the synthesis of the conjugated polymer and with the dynamics of the film formation by spin-coating. Acknowledgment. We gratefully acknowledge Hans-Ju¨rgen Butt and Gerhard Wegner for giving the starting impulse to this project and for many enlightening discussions throughout. We thank Ciba Inc. (part of BASF) for financial support, and Andreas Hafner and Andreas Mu¨hlebach for useful discussions. The authors thank Ru¨diger Berger, Stefan Weber, Maria Retschke, and Kostantinos Mpoukouvalas for instructive discussions, Maren Mu¨ller for SEM imaging, and Uwe Rietzler for all-round technical support with AFMs. Supporting Information Available: Thickness dependence study with KPFM on the Au/PS system with other PS film thickness, the size distribution of AuNPs and the phase images of the Au/PS system with two PS film thickness. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Singh, T. B.; Sariciftci, N. S. Annu. ReV. Mater. Res. 2006, 36, 199. (2) Cornil, J.; Beljonne, D.; Calbert, J. P.; Bredas, J. L. AdV. Mater. 2001, 13, 1053. (3) Prigodin, V. N.; Epstein, A. J. Synth. Met. 2001, 125, 43. (4) Yang, X.; Loos, J. Macromolecules 2007, 40, 1353. (5) Moons, E. J. Phys.: Condens. Matter 2002, 14, 12235. (6) Pingree, L. S. C.; Reid, O. G.; Ginger, D. S. AdV. Mater. 2009, 21, 19. (7) Palermo, V.; Palma, M.; Samori, P. AdV. Mater. 2006, 18, 145. (8) Slinker, J. D.; DeFranco, J. A.; Jaquith, M. J.; Silveira, W. R.; Zhong, Y. W.; Moran-Mirabal, J. M.; Craighead, H. G.; Abruna, H. D.; Marohn, J. A.; Malliaras, G. G. Nat. Mater. 2007, 6, 894. (9) Chiesa, M.; Burgi, L.; Kim, J. S.; Shikler, R.; Friend, R. H.; Sirringhaus, H. Nano Lett. 2005, 5, 559. (10) Maturova, K.; Kemerink, M.; Wienk, M. M.; Charrier, D. S. H.; Janssen, R. A. J. AdV. Funct. Mater. 2009, 19, 1379. (11) Pingree, L. S. C.; MacLeod, B. A.; Ginger, D. S. J. Phys. Chem. C 2008, 112, 7922. (12) Nardes, A. M.; Kemerink, M.; Janssen, R. A. J.; Bastiaansen, J. A. M.; Kiggen, N. M. M.; Langeveld, B. M. W.; van Breemen, A.; de Kok, M. M. AdV. Mater. 2007, 19, 1196.

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(13) Kemerink, M.; Alvarado, S. F.; Muller, P.; Koenraad, P. M.; Salemink, H. W. M.; Wolter, J. H.; Janssen, R. A. J. Phys. ReV. B 2004, 70, 13. (14) Berger, R.; Butt, H.-J.; Retschke, M.; Weber, S. Macromol. Rapid Commun. 2009, 30, 1167. (15) Zuo, F.; Angelopoulos, M.; Macdiarmid, A. G.; Epstein, A. J. Phys. ReV. B 1987, 36, 3475. (16) Ionescu-Zanetti, C.; Mechler, A.; Carter, S. A.; Lal, R. AdV. Mater. 2004, 16, 385. (17) Jacobs, H. O.; Leuchtmann, P.; Homan, O. J.; Stemmer, A. J. Appl. Phys. 1998, 84, 1168. (18) Charrier, D. S. H.; Kemerink, M.; Smalbrugge, B. E.; de Vries, T.; Janssen, R. A. J. ACS Nano 2008, 2, 622. (19) O’Neil, K. D.; Shaw, B.; Semenikhin, O. A. J. Phys. Chem. B 2007, 111, 9253. (20) del Campo, A.; Boos, D.; Spiess, H. W.; Jonas, U. Angew. Chem., Int. Ed. 2005, 44, 4707. (21) Frens, G. Nat.-Phys. Sci. 1973, 241, 20. (22) Sommerhalter, C.; Matthes, T. W.; Glatzel, T.; Jager-Waldau, A.; Lux-Steiner, M. C. Appl. Phys. Lett. 1999, 75, 286. (23) Jacobs, H. O.; Knapp, H. F.; Muller, S.; Stemmer, A. Ultramicroscopy 1997, 69, 39.

Sun et al. (24) Harris, P.; Huang, L.; Su, C. “Electical testing of soft delicate samples using Torsional Resonance Mode and TUNA,” Veeco application notes AN107, 2007. (25) Weber, S.; Haberkorn, N.; Theato, P.; Berger, R. Nano Lett. 2010, in press. (26) van der Pauw, L. J. Philips Res. Rep. 1958, 13, 1. (27) Wang, J.; Sun, L.; Mpoukouvalas, K.; Fassbender, B.; Bonaccurso, E.; Brunklaus, G.; Muehlebach, A.; Rime, F.; Butt, H.-J.; Wegner, G. Macromol. Chem. Phys. 2009, 210, 1504. (28) Wang, J. J.; Sun, L.; Mpoukouvalas, K.; Lienkamp, K.; Lieberwirth, I.; Fassbender, B.; Bonaccurso, E.; Brunklaus, G.; Muehlebach, A.; Beierlein, T.; Tilch, R.; Butt, H. J.; Wegner, G. AdV. Mater. 2009, 21, 1137. (29) Barbalace, K. Periodic Table of Elements - Gold - Au. In Accessed on-line:5/22/2009; EnvironmentalChemistry.com. 1995-2009. (30) Lide, D. R. CRC Handbook of Chemistry and Physics; CRC Press: Boca Raton, FL, 2006. (31) Atkins, P. W. Physical Chemistry, 2nd ed.; Oxford University Press: Oxford, 1984. (32) Rao, N. N. Basic electromagnetics with applications; Prentice-Hall: Englewood Cliffs, NJ, 1972.

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