Conductive Scanning Probe Characterization and Nanopatterning of

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Conductive Scanning Probe Characterization and Nanopatterning of Electronic and Energy Materials Albert L. Lipson and Mark C. Hersam* Departments of Materials Science and Engineering, Chemistry, and Medicine, Northwestern University, Evanston, Illinois 60208, United States ABSTRACT: Recent years have seen a proliferation of scanning probe microscopy (SPM) techniques that can probe and manipulate a diverse range of materials and devices. In particular, SPM methods that employ a conductive tip are well suited for probing electronic and electrochemical phenomena of direct relevance to electronic and energy technologies. Conductive SPM is also a versatile nanofabrication tool, which can create nearly arbitrary nanopatterns of oxide, metals, and organics on solid substrates. In this Feature Article, we provide an overview of recent conductive SPM work from our laboratory regarding the characterization and nanopatterning of electronic and energy materials. The discussion begins by describing the methodologies used to characterize organic photovoltaics and transparent conducting oxides. We then illustrate how different SPM techniques are applied to the more complex electrochemical environments presented by Li-ion batteries and other electrochemical systems. Lastly, the use of conductive atomic force microscopy to probe and nanopattern electronically inhomogeneous substrates, such as epitaxial graphene layers on silicon carbide, is presented. constant force.13,14 On the other hand, scanning electrochemical microscopy (SECM) uses a microelectrode integrated into the tip to spatially map changes in electrochemistry,15 while scanning ion conductance microscopy (SICM) enables noncontact imaging in electrolyte solutions.16,17 In this Feature Article, we discuss recent developments from our laboratory in conductive SPM techniques with a particular focus on applications of relevance to electronic and energy materials. We begin by delineating SPM techniques that are used to study organic photovoltaic (OPV) cell components (e.g., transparent conducting oxide electrodes) in addition to fully fabricated cells. Next, we review emerging techniques for characterizing electrochemical systems, including Li-ion batteries. Lastly, we show how conductive AFM allows spatial mapping and nanopatterning of electronically inhomogeneous samples (e.g., partially graphitized silicon carbide). Overall, we anticipate that this Feature Article will serve as an effective reference for researchers interested in employing conductive SPM methods for the nanoscale characterization and manipulation of electronic and energy materials.

1. INTRODUCTION Miniaturization to the nanometer scale is a nearly ubiquitous strategy for advancing electronic and energy technologies. For instance, the electronics industry has relentlessly reduced the size of devices in an effort to improve performance metrics and reduce cost.1 In energy storage devices, nanoscale components are also of growing importance as nanopowders are incorporated into Li-ion battery electrodes to enhance their rate performance.2 Similarly, bulk heterojunctions comprise the active layer of organic photovoltaics with features on the order of 10 nm to facilitate efficient charge separation.3−6 In addition to custom-designed nanoscale materials, there are many technologically important processes and properties that are controlled at nanometer-scale defects including corrosion,7 conductivity of transparent conducting oxides,8 and catalysis.9 In recent years, conductive scanning probe microscopy (SPM) has evolved to address the increasingly important need to understand and control these processes at the nanoscale. The origins of SPM can be traced back to the invention of the scanning tunneling microscope (STM) by Binnig et al.10,11 The STM allowed, for the first time, real-space atomic-scale spatial resolution imaging on electrically conductive substrates. The atomic force microscope (AFM) was subsequently developed to enable imaging on all surfaces regardless of electrical conductivity.12 For the past three decades, several additional capabilities and varieties of SPM have been developed to obtain more information, including the utilization of conductive SPM tips that are ideally suited for electrical and electrochemical characterization. For example, conductive AFM (cAFM) allows for a nanoscale electrical contact to be made to the sample through the conductive tip while maintaining © 2013 American Chemical Society

2. ORGANIC PHOTOVOLTAICS AND TRANSPARENT CONDUCTING OXIDES Due to their solution processability and compatibility with flexible substrates, OPVs hold promise for reducing the cost of solar electricity generation in comparison to traditional Received: December 20, 2012 Revised: February 21, 2013 Published: February 26, 2013 7953

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inorganic photovoltaics.18 The highest efficiency OPVs employ the bulk heterojunction (BHJ) architecture to efficiently separate photogenerated excitons preceding recombination.3−6 Following charge separation, electrons are collected by the metallic cathode, and holes are collected by the transparent conducting electrode, which is usually indium tin oxide (ITO).18 To prevent leakage currents, an electron blocking layer (EBL) and hole blocking layer (HBL) are often included at the anode and cathode, respectively, as depicted schematically in Figure 1.19

region is illuminated by a focused laser beam.22 In this manner, the local photovoltaic current can be mapped and compared to the concurrently measured topography, thereby allowing for correlated structure−property mapping at the nanometer scale. Due to the underlying nanoscale morphology of the bulk heterojunction, OPVs are a common target for pc-AFM including characterization of degraded OPVs,24 nanowire OPVs,26 and small molecule OPVs.28 2.2. Atomic Force Photovoltaic Microscopy. In addition to pc-AFM, many other SPM techniques have been employed for characterizing photovoltaics including Kelvin probe force microscopy,24,30,31 time-resolved electrostatic force microscopy,32 cAFM of the active layer,33−36 near-field scanning photocurrent microscopy,37,38 and atomic force photovoltaic microscopy (AFPM).39,40 However, among these methods, AFPM is uniquely suited for the study of lateral spatial variations in fully fabricated devices. As shown in Figure 2a, AFPM is based on the fabrication of cathode arrays on the BHJ and then addressing each device with a conductive AFM tip while illuminating the sample with a solar light simulator. In particular, a poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) BHJ cell array is characterized with AFPM in Figures 2b and 2c. Clear photocurrent differences are present between individual devices in the array, indicating micrometer-scale variations in device performance. Since the phase separation in the BHJ occurs at significantly shorter length scales, these micrometer-scale variations have been attributed to inhomogeneities in charge injection from the underlying ITO. This interpretation is consistent with independent cAFM measurements that have shown substantial variations in ITO conductivity at micrometer length scales.41 To further explore if the observed device-to-device variability is caused by the underlying ITO, the ITO can be intentionally tailored by a variety of surface treatments. For example, oxygen

Figure 1. Schematic of a typical bulk heterojunction (BHJ) organic photovoltaic (OPV) device. A transparent conducting oxide (TCO) is often employed as the anode. In addition, an electron blocking layer (EBL) and hole blocking layer (HBL) are included at the anode and cathode, respectively, to minimize leakage currents.

2.1. Photoconductive Atomic Force Microscopy. The performance of photovoltaics is quantified via a series of metrics under illumination including power conversion efficiency, short circuit current, open circuit voltage, and fill factor.20,21 Therefore, to elucidate how the microstructure and nanostructure influence these figures of merit, it is necessary to spatially map these parameters on operating devices. Toward this end, the technique of photoconductive atomic force microscopy (pc-AFM) was developed.22−29 In pc-AFM, the conductive AFM tip acts as a localized cathode, while the active

Figure 2. (a) Schematic of the apparatus for atomic force photovoltaic microscopy (AFPM). (b) Topographic map of a 7.5 μm × 7.5 μm OPV array and (c) corresponding AFPM photocurrent map under short circuit conditions. In the AFPM photocurrent map, the absolute value of the photocurrent is shown. Reprinted with permission from ref 39, copyright 2008, American Institute of Physics. 7954

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Figure 3. Pixel averaged current maps for single AFPM scans of OPV arrays with (a) HCl-treated ITO, (b) a PEDOT:PSS interlayer, and (c) cleaned but otherwise untreated ITO. Note that the absolute value of the photocurrent is shown. Reprinted with permission from ref 40, copyright 2011, American Chemical Society.

plasma treatments have been shown to increase the ITO work function.42 Alternatively, acid or base cleaning can be used to influence the homogeneity of the ITO electrical conductivity.41,43,44 In addition, self-assembled monolayers or other interfacial layers such as poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) provide further opportunities to influence charge injection from ITO into the BHJ.45−48 AFPM has been used to quantify the effectiveness of the aforementioned surface treatments in improving the uniformity and overall performance of OPVs.40 Specifically, Figure 3 shows AFPM current maps for P3HT:PCBM OPVs using untreated ITO, HCl acid-treated ITO, and ITO with a PEDOT:PSS interlayer. To limit the effects of variable tip−sample contact, the average current across each device is plotted in the current maps in Figure 3. Untreated ITO shows large current variations between devices, as well as many devices with substandard performance. On the other hand, the addition of a PEDOT:PSS interlayer greatly improves the performance of the OPVs and also leads to some reduction in device-to-device variability. Finally, the HCl-treated ITO shows the highest degree of spatial homogeneity, albeit with a lower average short circuit current compared to devices with a PEDOT:PSS interlayer. Overall, this study illustrates the utility of AFPM for rapid screening of OPV lateral spatial variations, which play an important role in determining the overall performance of macroscopic OPV devices. 2.3. Conductive AFM of Patterned Indium Oxide. Conductive AFM, as mentioned above, has been used to characterize spatial variations in the electrical conductivity of transparent conducting oxides (TCOs). In addition to probing inherent spatial variability, cAFM is also well suited for verifying changes in local electrical properties following nanofabrication. For example, a Ga focused ion beam (FIB) can be used to locally dope In2O3, which allows conductive lines to be nanopatterned in the otherwise highly resistive In2O3.49−51 The resulting nanowires are of potential use as interconnects in transparent electronic devices.52−56 Conductive AFM allows direct visualization and electrical characterization following Ga FIB doping as shown in Figure 4. From these images, it is evident that the FIB-patterned nanowires are electrically continuous and have greatly enhanced conductivity as compared to the unexposed In2O3 background. To gain further insight into the underlying conductivity enhancement mechanism, transmission electron microscopy is employed to reveal the microstructure following Ga FIB doping. In particular, Figure 5 shows that the In2O3 is amorphized within the FIB patterns. This result coupled with supporting electron energy loss spectroscopy data implicates oxygen vacancies as a primary

Figure 4. Conductive AFM image of a transparent conductive pattern spelling ‘‘NU TCO’’ that was created via Ga focused ion beam nanopatterning on In2O3. The surface texture was rendered using the AFM topography image, and the color map was generated from the cAFM current map. Reprinted with permission from ref 51, copyright 2009, John Wiley & Sons.

Figure 5. (a) TEM bright-field micrograph of a FIB-patterned nanowire on In2O3. The arrows in the image indicate the location of the nanowire. (b) Electron diffraction pattern from an In2O3 region without FIB exposure. (c) Electron diffraction pattern from the nanowire region that was patterned with the FIB. Adapted with permission from ref 50, copyright 2010, American Chemical Society.

contributor to the observed conductivity enhancement following Ga FIB doping.57

3. ELECTROCHEMICAL CHARACTERIZATION 3.1. Scanning Electrochemical Microscopy. Electrochemical phenomena underlie many applications including corrosion, batteries, anodization, and electroplating.58−61 7955

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UME and the pipet opening. The last step of the fabrication is to use a FIB to mill the end of the pipet and thereby expose the UME and reopen the pipet. The advantage of this technique is that the UME has small dimensions on the order of 100 nm, which enables high-resolution SECM and SICM imaging.80 Integrated SECM−SICM provides a variety of imaging modalities81,82 but was originally demonstrated using the SECM in feedback mode where a redox couple exists between the UME and the conductive regions of the sample.15,63 When this method is employed at constant tip−sample spacing, conductive substrate regions induce enhanced UME-detected redox current, whereas insulating regions reduce the UMEdetected redox current. In Figure 7, this effect is evident where

Electrochemical systems are often extremely complex with several reactions occurring that can be affected by nanometerscale electrode surface features.7,59 Consequently, conductive SPM has been employed in an effort to correlate electrochemical currents with the nanostructure of the electrode substrate. The most commonly used technique is SECM, where an ultramicroelectrode (UME) is scanned in close proximity to the surface of interest.15,62,63 For instance, SECM has been used to locally detect the location of pitting sites during the corrosion of metals and the subsequent time evolution of the corrosion process.64,65 SECM is also useful for studying catalytic activity such as the oxidation of methane using Pt catalysts.66 Furthermore, SECM has been employed for the characterization of Li-ion batteries.67,68 3.2. Combining SECM with SICM. SECM is often combined with other SPM techniques that provide independent control over tip−sample spacing. For example, shear force has been employed as the tip−sample distance feedback mechanism.69 Similarly, impedance measurements70,71 or incorporating an UME into an AFM tip72−75 can determine and control tip−sample spacing. Alternatively, scanning ion conductance microscopy (SICM) can be combined with SECM, where a SECM UME is fabricated near the tip of the SICM pipet.76−78 SICM utilizes a pipet pulled down to a nanoscale tip as the probe for SPM. The pipet is filled with electrolyte and a working electrode inserted, while the sample and a counter electrode are placed in a Petri dish filled with electrolyte. By monitoring the ion current between the pipet electrode and the counter electrode, the sample surface can be detected as it will physically occlude the opening of the pipet upon approach.16 Since the DC ion current experiences fluctuations that compromise its use as a feedback parameter for maintaining constant tip−sample spacing, the SICM pipet is typically oscillated vertically, which yields an AC component to the ion current as the tip approaches the sample that is effective as a feedback signal.79 One possible methodology for creating a combined SECM− SICM probe is shown in Figure 6. In this case, the UME is created by first depositing gold with an adhesion layer of Ti via electron beam evaporation on the side of the pipet. The pipet is then electrically insulated using atomic layer deposited aluminum oxide, which coats all surfaces and occludes the

Figure 7. (a) Scanning electron micrograph of a gold thin film on glass with 180 nm FIB-milled trenches. (b−d) SECM−SICM is performed on this sample using an electrolyte of 10 mM Ru(NH3)6Cl3, 100 mM KNO3. (b) SECM−SICM topography image. (c) SECM−SICM DC redox current. (d) SECM−SICM AC redox current. Reprinted with permission from ref 76, copyright 2010, American Chemical Society.

the SECM−SICM was used to scan patterned gold electrodes on a glass substrate. Since the pipet is oscillated vertically, tipmodulated SECM is also possible83,84 as shown in Figure 7d. 3.3. SICM Characterization of Battery Electrodes. New methods of electrical energy storage, such as Li-ion batteries, have become an important enabling technology for electrical transportation and intermittent renewable energy resources.85 However, Li-ion battery electrodes still face many challenges including limited lifetime, safety concerns, and the need for improved performance.59 To better understand the time evolution of battery electrode performance, AFM has been used extensively to study the topographical evolution of the battery electrode surface during cycling.86−92 Similarly, cAFM allows spatial mapping of the surface conductivity of the electrodes.93,94 However, neither AFM nor cAFM provides direct information about electrochemical processes, thus motivating the application of more advanced electrochemical SPM techniques such as SICM and electrochemical strain microscopy (ESM).95−97 SICM is effective at measuring ion currents through pores98 and is commonly used without scanning to probe ion currents

Figure 6. Schematic of the fabrication scheme for an integrated SECM−SICM pipet with views of the side of the pipet (top) and looking up at the pipet tip (bottom): (a) Original glass pipet after pulling to a sharp tip; (b) Pipet following deposition of a Ti adhesion layer and gold; (c) Pipet following atomic layer deposition of Al2O3 that electrically insulates the pipet; (d) Final pipet after FIB milling that exposes the gold ultramicroelectrode and reopens the pipet tip. Reprinted with permission from ref 76, copyright 2010, American Chemical Society. 7956

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microscopy (ESM). In ESM, an AFM tip detects height changes during the application of alternating current pulses. This technique allows for high-esolution imaging of electrochemically induced strain,96 in addition to gathering kinetic information.101 Furthermore, ESM can determine if the strain is caused by a reversible or irreversible reaction. In this manner, ESM is well suited for Li-ion battery systems and has been effective at probing lithium diffusion and related electrochemical phenomena.96,97,101

through biological cellular membranes in a loose patch clamp geometry.99,100 Recently, SICM has been adapted to also enable the study of Li-ion battery electrodes as shown in Figure 8.95 In

4. EPITAXIAL GRAPHENE ON SILICON CARBIDE 4.1. Conductive AFM Determination of Graphene Coverage. Graphene, a two-dimensional hexagonal lattice of carbon atoms, has recently received significant attention due to its exceptional electrical, mechanical, and chemical properties.102−104 In particular, epitaxial graphene on SiC (EG/SiC), which is formed via high-temperature annealing of SiC in an inert atmosphere, is of high interest for wafer-scale highfrequency electronics and related applications.105−108 Between the top EG layer and the underlying SiC substrate, an interface layer is present that possesses a (6√3 × 6√3)R30° surface reconstruction. In the event that the SiC graphitization process is incomplete, this 6√3 × 6√3 interface layer will be partially exposed at the surface. Consequently, it is important to be able to rapidly differentiate the exposed 6√3 × 6√3 interface layer from EG to optimize the graphitization procedure. While this spatial mapping can be accomplished at the atomic scale with ultrahigh vacuum (UHV) scanning tunneling microscopy (STM), this method is not practical for high-speed characterization.109 As an alternative to UHV STM, ambient cAFM and lateral force microscopy (LFM) can be used to distinguish graphene from exposed regions of the 6√3 × 6√3 interface layer. For example, Figure 10 shows AFM topography, LFM, and cAFM images of partially graphitized EG/SiC. The regions of low current in the cAFM image are attributed to the exposed 6√3 × 6√3 interface layer since the high contact resistance between 6√3 × 6√3 and the cAFM tip reduces current flow compared to the highly conductive EG domains. Contrast can also be seen

Figure 8. Schematic diagram of the SICM apparatus used to characterize battery electrodes. Reprinted with permission from ref 95, copyright 2011, John Wiley & Sons.

this setup, the battery electrode is attached to the ground of the microscope and is placed in a Petri dish filled with the desired battery electrolyte. The pipet uses a lithiated tin electrode to act as a source or a sink for Li+ ions and is filled with the same battery electrolyte. The AC current feedback signal maintains a constant current tip−sample spacing and thereby measures topography, while the DC current yields information about the local ion current. SICM thus allows spatial mapping of the electrochemical activity of battery electrodes as demonstrated in Figure 9. In this case, the substrate consists of lithographically defined tin stripes, where tin is a high capacity battery anode material, on a thin film of Cu on glass. These stripes are directly visualized in the SICM topographic image and correlate with regions of higher ion current in the corresponding SICM current image as expected. SICM has also been employed to image Si nanoparticles and the in situ evolution of tin thin films during lithiation.95 3.4. Electrochemical Strain Microscopy. Another emerging technique that probes the strain that is introduced during electrochemical processes is electrochemical strain

Figure 9. (a) SICM topographic image and (b) DC current map (with the minimum current subtracted from the current values) of tin stripes lithographically defined on a thin film of Cu on a glass substrate. (c) Extracted topography and current profiles perpendicular to the stripes averaged over 128 pixels. Reprinted with permission from ref 95, copyright 2011, John Wiley & Sons. 7957

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bias to the cAFM tip will then drive anodic oxidation of the sample.116 This technique, called field-induced oxidation (FIO), has been employed on Si and metals to create nanopatterned regions of surface oxide.117−123 In recent work, FIO has been adapted for the nanoscale oxidation of EG/SiC as shown in Figure 11.124 The highest resolution nanopattern in Figure 11a is ∼1.2 nm tall and ∼36 nm wide, thus showing the high-resolution capabilities of this technique. The threshold voltage for nanopatterning is ∼5 V, which is similar to other substrates such as Si. Furthermore, lines and other arbitrary patterns can be created. However, upon extended voltage pulse duration, the shapes of the resulting features are considerably less isotropic than expected from previous studies on semiconductor and metal surfaces. To explore the spatial anisotropy of FIO on EG/SiC, cAFM can be used before and after nanopatterning to identify the underlying surface structure. Figure 12 shows cAFM images of a region of partially graphitized SiC before and after patterning at point A. On the basis of the magnitude of the cAFM current map, as discussed above, regions B and C can be identified as the 6√3 × 6√3 interface layer, while region A is graphene. After conducting FIO nanopatterning at point A, the measured cAFM current drops, and the topography increases in height; however, there is no apparent change to regions B and C. Consequently, it is apparent that FIO nanopatterning selectively occurs on the graphitized region of the substrate, and thus the observed nanopatterning anisotropy is reflective of the spatially anisotropic distribution of graphene on partially graphitized SiC.

Figure 10. AFM contact-mode images of partially graphitized SiC with the fast scan direction (a) from left to right and (b) from right to left. Lateral force microscopy images with the fast scan direction (c) from left to right and (d) from right to left. (e) Intermittent contact mode topographical image. (f) Conductive AFM current map at a sample bias of 0.3 V. All images are taken over the same region of the surface. Reprinted with permission from ref 109, copyright 2010, American Institute of Physics.

5. CONCLUSION In this article, we have provided an overview of the use of conductive SPM in our laboratory for the characterization and nanopatterning of electronic and energy materials. In particular, AFPM is an effective tool for rapidly screening lateral spatial variations in the performance of full fabricated OPV device arrays. Similarly, cAFM can be used to spatially map electrical conductivity at the nanometer scale, which is particularly useful for probing transparent conductive nanowires and epitaxial graphene on SiC. Furthermore, conductive SPM provides opportunities for quantifying electrochemical phenomena at the nanometer scale. Specifically, SECM and/or SICM provide insight into electrochemical systems, such as Li-ion batteries, including the time evolution of electrode topographical changes, spatial variation of electrochemical currents, and the kinetics of electrochemical reactions. Along with its extensive characterization capabilities, conductive SPM can also be used as an effective nanoscale patterning tool for technologically relevant electronic materials such as graphene. As integrated circuitry continues its relentless march toward the nanometer scale and energy conversion and storage devices increasingly include multicomponent nanoscale composite materials, conductive SPM techniques are likely to remain primary characterization methods for electronic and energy materials. Since this class of technologies requires multifunctional materials with tailored electronic, optical, chemical, thermal, and mechanical properties, hybrid SPM methods that can concurrently measure multiple signals and thereby provide a comprehensive data set in one experimental run will be especially important. Furthermore, in situ measurements that probe materials properties and performance in the native environment of an operating device (e.g., imaging in the complex electrochemical environment of a Li-ion battery) will

in the LFM image, due to frictional differences between the exposed 6√3 × 6√3 interface layer and EG. However, the LFM image suffers from cross-talk between the topographic and frictional signals that complicates the image interpretation.110 Consequently, cAFM is a more reliable and rapid method for identifying and quantifying the graphene coverage on partially graphitized SiC substrates. 4.2. Field-Induced Oxidation of Epitaxial Graphene. In addition to nanoscale characterization, SPM is an effective tool for nanopatterning. For example, atomic-scale patterning can be achieved via STM-based atomic manipulation111 or by electronstimulated desorption.112 Another method, dip-pen nanolithography, uses an AFM tip dipped in a molecular ink to subsequently deposit molecules at the nanometer-scale tip− sample contact.113 A nanoscale pipet can also be used to locally deliver inks in a technique known as nanofountain pen lithography.114 From the perspective of conductive SPM, nanopatterning methods are typically based on tip-directed electrochemistry. For instance, a pipet filled with a plating solution can be brought close enough to the substrate to form a nanoscale meniscus. Upon application of a voltage to the pipet electrode, metals are deposited with high resolution.115 Alternatively, oxidation can be achieved by using the water meniscus that forms between a cAFM tip and the sample surface as a nanoscale electrochemical cell. The application of a negative 7958

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Figure 11. AFM topography images of oxide nanopatterns on epitaxial graphene on SiC created by conductive AFM field-induced oxidation. (a) Image of a grid of oxide dots created at a constant sample voltage of 6 V, while the tip dwell time was varied from 0.1 to 4.8 s. (b) Image of a grid of oxide dots created by varying the voltage from 1 to 9 V with a constant dwell time of 4 s. (c) Image of patterned oxide lines at a write speed of 1 μm s−1 and voltages ranging from 7 to 10 V. (d) Image of a pattern that spells “Graphene/SiC” created at a sample bias of 9 V and a write speed of 1 μm s−1. These patterns were created at ∼35% relative humidity.124 Reprinted with permission from ref 124, copyright 2011, John Wiley & Sons.

Figure 12. (a) AFM topography of the partially graphitized SiC substrate and (b) corresponding conductive AFM current map. (c) AFM topography and (d) corresponding conductive AFM current map after field-induced oxidation nanopatterning was performed at point A at ∼50% relative humidity. Reprinted with permission from ref 124, copyright 2011, John Wiley & Sons.

likely prove to be the most relevant for technology optimization. Finally, further improvements in the temporal resolution of SPM imaging and spectroscopy will provide deeper physical insight into time-dependent phenomena and improve experimental throughput. While this article provided several emerging conductive SPM methods that touch on many of these frontier issues, no single technique is ideal for all problems, and thus further integration of existing conductive SPM methods and/or development of completely new

conductive SPM strategies can be expected for the foreseeable future.

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AUTHOR INFORMATION

Corresponding Author

*Address: 2220 Campus Drive, Cook Hall 2036, Evanston, IL 60208. Phone: 847-491-2696. E-mail: m-hersam@ northwestern.edu. 7959

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Notes

microscopy; SICM, scanning ion conductance microscopy; OPV, organic photovoltaic; BHJ, bulk heterojunction; TCO, transparent conducting oxide; ITO, indium tin oxide; EBL, electron blocking layer; HBL, hole blocking layer; pc-AFM, photoconductive atomic force microscopy; AFPM, atomic force photovoltaic microscopy; P3HT:PCBM, poly(3-hexylthiophene):[6,6]-phenyl-C61-butyric acid methyl ester; PEDOT:PSS, poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate); FIB, focused ion beam; UME, ultramicroelectrode; ESM, electrochemical strain microscopy; AC, alternating current; DC, direct current; EG/SiC, epitaxial graphene on silicon carbide; LFM, lateral force microscopy; FIO, field-induced oxidation

The authors declare no competing financial interest. Biographies

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REFERENCES

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Mark C. Hersam is the Bette and Neison Harris Chair in Teaching Excellence and Professor of Materials Science and Engineering, Chemistry, and Medicine at Northwestern University. He earned a B.S. in Electrical Engineering from the University of Illinois at Urbana−Champaign (UIUC) in 1996, M.Phil. in Physics from the University of Cambridge in 1997, and a Ph.D. in Electrical Engineering from UIUC in 2000. His research interests include nanofabrication, scanning probe microscopy, semiconductor surfaces, and carbon nanomaterials. Dr. Hersam cofounded NanoIntegris, which is a commercial supplier of high performance carbon nanotubes and graphene. Dr. Hersam is a Fellow of MRS, AVS, APS, and SPIE, and serves as an Associate Editor of ACS Nano.

Albert L. Lipson is a Ph.D. candidate in Prof. Mark Hersam’s research group at Northwestern University. He received a B.S. in Materials Science and Engineering from the University of California, Los Angeles (UCLA) in 2003. His research focuses on scanning ion conductance microscopy of Li-ion battery electrodes, characterization of new materials for Li-ion batteries, and nanofabrication.

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ACKNOWLEDGMENTS This work was supported by the Center for Electrical Energy Storage, an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences (Award Number DE-AC02-06CH11357).

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ABBREVIATIONS SPM, scanning probe microscopy; STM, scanning tunneling microscopy; AFM, atomic force microscopy; cAFM, conductive atomic force microscopy; SECM, scanning electrochemical 7960

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