Universal Method for Creating Optically Active Nanostructures on

May 4, 2014 - Chemistry and Biochemistry Department, University of Northern Iowa, Cedar Falls, Iowa 50614-0423, United States. §. Condensed Matter ...
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Universal Method for Creating Optically Active Nanostructures on Layered Materials Timothy E. Kidd,*,† Aaron O’Shea,† Benjamin Beck,† Rui He,† Conor Delaney,† Paul M. Shand,† Laura H. Strauss,‡ Andrew Stollenwerk,† Noah Hurley,† Kyle Spurgeon,† and Genda Gu§ †

Physics Department, University of Northern Iowa, Cedar Falls, Iowa 50614-0150, United States Chemistry and Biochemistry Department, University of Northern Iowa, Cedar Falls, Iowa 50614-0423, United States § Condensed Matter Physics & Materials Science Department, Brookhaven National Laboratory, Upton, New York 11973-5000, United States ‡

S Supporting Information *

ABSTRACT: The ability to form patterned surface nanostructures has revolutionized the miniaturization of electronics and led to the discovery of emergent behaviors unseen in macroscopic systems. However, the creation of such nanostructures typically requires multiple processing steps, a high level of technical expertise, and highly sophisticated equipment. In this work, we have discovered a simple method to create nanostructures with control size and positioning in a single processing step using a standard scanning electron microscope. The technique can be applied to a wide range of systems and was successful in every layered material tested. Patterned nanostructures were formed on graphite, topological insulators, novel superconductors, and layered transition metal dichalcogenides. The nanostructures were formed via the incorporation of carbon nanoparticles into the samples in a novel form of intercalation. It appears that the electron beam interacts with residual organic molecules available on the sample surface, making it possible for them to intercalate between the layers in their crystal structure and break down into carbon. These carbon nanoparticles have strong broad-wavelength interactions in the visible light range, making these nanostructures easily detectable in an optical microscope and of interest for a range of nanoscale electro-optical devices.

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INTRODUCTION Advances in nanoscience research have led to a variety of methods for creating and modifying patterned surface nanostructures.1−7 Techniques based on electron beam (ebeam) radiation are perhaps both the most refined and utilized in current device development. E-beam radiation is used to modify or break chemical bonds, and can achieve true nanometer scale features.1,2,6,8,9 Such techniques include using radiation damage to mill away portions of the surface, altering local chemical bonds to create new features, and traditional ebeam lithography. However, these methods, like others based on scanning probe microscopy or chemical processing, typically require a large investment in both facilities and operator expertise. Here we demonstrate a fundamentally new method for creating patterned arrays of optically active surface nanostructures with nanometer scale resolution. The process can be performed with a standard scanning electron microscope (SEM) in a single processing step on the surface of seemingly any layered material including graphite, topological insulators, novel superconductors, and density wave materials. Hence this is a generic process that could be used to explore and alter the properties of exotic 2D materials with nanometer scale precision. E-beam exposure activated organic molecules © 2014 American Chemical Society

residing on the sample surfaces, which resulted in the incorporation of carbon nanocrystals between subsurface layers of these materials. Furthermore, almost no damage was incurred on the sample surface, even when forming nanostructures with heights exceeding 100 nm. The structures themselves exhibited broad wavelength photoluminescence and optical absorption characteristics in the visible range. Hence, this technique is a simple and versatile method for creating optically functional nanostructures and/or inducing nanometer length scale modifications to 2D materials in order to influence and explore electronic properties and phase transitions in low dimensional materials. E-beam radiation in an SEM has long been used to both measure and modify surface properties.10 In this work, however, we utilize e-beam radiation to form surface nanostructures in a unique way. During the course of a study involving SEM measurements of layered materials, highly two-dimensional materials like graphite defined by strong intralayer bonding and weak van der Waals interlayer coupling, we found that the electron beam exposure induced the growth of nanostructures Received: August 20, 2013 Revised: April 30, 2014 Published: May 4, 2014 5939

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optically corresponded to the presence of raised nanostructures. Longer e-beam exposures result in structures that appear darker in an optical microscope and taller when measured by AFM (Figure 1). Correlating AFM and optical images, we found that

at almost any level of e-beam radiation. The discovery was made during attempts to manipulate local chemical composition in layered dichalcogenides using long-term beam exposures.11,12 While the chemical composition was not significantly affected as measured by energy dispersive X-ray spectroscopy (EDX), subsequent measurements using an optical microscope revealed that exposed areas always appeared much darker than the surrounding portions of the sample. At first this appeared to be surface carbon contamination,10 which can occur during long-term exposures resulting from e-beaminduced hydrocarbon cracking. Hydrocarbons are found on the surface of any sample introduced into the SEM from the atmosphere. In fact, e-beam hydrocarbon cracking has been shown to be useful for the creation of nanoscale carbon features.13 However, the darkened areas seen in optical microscopy only appeared on layered materials. No effects of e-beam exposure could be detected optically on nonlayered systems like aluminum, steel, or silicon at the same exposures, which readily generated nanostructures on layered systems. In fact, extremely long-term exposures on silicon led to a milling away of the native oxide layer on the silicon sample rather than the formation of any surface nanostructures. This evidence shows that surface carbon deposition is not the source of these effects, and there must be some interaction based on the layered nature of the samples.

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METHODS

Figure 1. Structural features of nanostructures grown on dichalcogenide samples. All structures were created using 30 kV electrons at 2 nA beam current with different exposure times. (a) Optical image taken from the surface of a patterned TiS2 single crystal. Areas exposed to ebeam radiation appear darker. Heights of each visible line are shown. (b,c) AFM measurements of topography (b) and phase (c) measured from the 50 × 50 μm square indicated in panel a. The topography color scale spans 30 nm. (d) Scatter plot relating the heights of 10 μm lines created on various dichalcogenides using different exposure times.

Single crystal dichalcogenide samples were grown using iodine vapor transport as described previously.12 Single crystals of Bi2Se3, Bi2Te3, FeSexTe1−x, and optimally doped Bi2Sr2CaCu2O8+δ were grown by using a floating zone furnace.14 The graphite samples used in this study were highly ordered pyrolitic graphite (HOPG) crystals purchased from Ted Pella. SEM measurements and nanostructure synthesis were performed using a Tescan Vega II SEM. EDX measurements were performed using a Bruker Quantax system. Nanostructure patterning was achieved using DrawBeam software from Tescan. AFM measurements were performed in ambient conditions using an Agilent 5500 AFM in noncontact mode with Nanoworld NCHR-50 tips. AFM images were analyzed using Gwyddion. STM measurements were performed at 3 × 10−7 mbar in an Omicron STM using Pt-IR tips. Unless noted, samples were inserted into vacuum system for study after nanostructure formation without annealing or any other surface preparation. Differential tunneling spectra were acquired using lock-in detection as well as numerical differentiation of I−V spectra with both methods producing similar results. Electron scattering Monte Carlo simulations were performed using Electron Flight Simulator software. Samples were mounted using either double-sided carbon tape or silver epoxy. Clean surfaces were prepared by exfoliating surface layers using standard Scotch tape before inserting the samples into the SEM chamber for measurement and nanostructure synthesis. Measurements and nanostructure growth were performed with a chamber at pressures less than 2 × 10−5 mbar. Optical spectra were taken at room temperature in ambient conditions using a Horiba Labram HR Raman Microscope system with 532 nm laser excitation and a thermo-electric cooled charge-coupled device (CCD) detector. A 50× objective lens was used, and the laser power was kept below 1 mW. Spectral resolution was about 5 cm−1.

nanostructures as small as 5 nm were readily detectible with an optical microscope. The small heights of these features explained why they were so difficult to detect in an SEM. It also shows they must strongly absorb visible light in order be detectable optically. Furthermore, AFM measurements revealed that there were two effects resulting from the electron beam radiation. The most obvious effect is the growth of a surface structure with lateral dimensions approximately the same as the area of the beam spot. The second effect is a halo that appears around the main structure and can be seen faintly in topographic measurements and far more clearly in AFM phase imaging (Figure 1c). The AFM phase results, which can be used to identify the presence of different compositions on the sample,15,16 show that the influence of the e-beam radiation can extend out significantly from the area actually exposed. This halo effect is likely induced by electron scattering within the sample, which can spread electrons laterally for several micrometers,17 and appears to affect the chemical or electrical composition of the surface in some manner. At lower exposure levels, these changes to surface composition are not accompanied by significant changes to surface topography. The halos strongly resemble ring-like features found in ebeam-induced carbon deposition experiments.18,19 Surface diffusion of the residual organics on the sample surface and the particulars of the secondary and backscattered electron distributions were used to explain the formation of these rings.

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RESULTS AND DISCUSSION The areas exposed to e-beam radiation were not always easily identified in an SEM, which was surprising given how easy they were to find in an optical microscope. EDX measurements also revealed no significant changes in chemical composition. This led to measurements using atomic force microscopy (AFM), which revealed that the darkened areas of the sample seen 5940

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In our work, the ratio of the halo height to the height of the main feature was consistently smaller than would be expected from the deposition experiments. Therefore, the formation of the halos may not have the same mechanism as the main nanostructure formation but instead be due to actual carbon deposition albeit at a far slower rate. Studies were performed on a number of dichalcogenides (TiS2, TiSe2, ZrSe2, and HfSe2) as well as other layered systems including graphite, novel superconductors like Bi2Sr2CaCu2O8+δ (BSCCO) and FeSexTe1−x, and topological insulators Bi2Te3 and Bi2Se3. In every case, e-beam radiation from the SEM-induced raised nanostructures that correlated with optically darker regions on the surface. Furthermore, it was found that the features of the nanostructures corresponded well to the beam parameters of the microscope. Nanostructure heights generally correlated with the number of electrons striking the sample. Higher beam voltages also led to taller structures, and also created wider halos as well. While most studies were done with 30 kV electrons, nanostructures could be formed with energies as small as 1 kV. The nanostructures’ lateral sizes extended at least a hundred nanometers beyond the diameter of the beam spot, which could be expected due to electron scattering within the sample itself. There were also distinct material dependencies for nanostructure formation. For example, in Figure 1d the heights of various nanostructures are compared for three different dichalcogenides. Nanostructure heights increased with dosage in a similar manner up to a certain point, after which the relationship became more complex. All three dichalcogenides have similar crystal structures. ZrSe2 is a semiconductor with a band gap exceeding 1 eV and both TiS2 and TiSe2 are semimetallic materials.20 The downturn seen in longer exposures on ZrSe2 could be related to the charging effects and/or e-beam induced etching as seen in other nonconductive systems.7 Density was also found to be important for nanostructure formation. In general, lower density systems exhibited shorter structures with wider halos for a given exposure setting, especially in graphite. This is consistent with Monte Carlo simulations and previous work, which show high energy electrons spread further in less dense materials and thus have less impact locally at the beam focus spot.17 Surface quality was also found to be very important. Long-term (weeks or months depending on the material) exposures to ambient conditions made it more difficult to create nanostructures. Thus, preparation of a relatively clean surface was important for nanostructure formation. In fact, it was nearly impossible to create nanostructures on samples with large amounts of surface contamination (Supporting Information Figure S1). However, once formed, the nanostructures were extremely stable and showed no signs of change over several months’ exposure to ambient conditions even for relatively reactive samples like TiS2. Studies on the relationship between nanostructure formation and beam parameters led to the discovery of beam parameters in which the nanostructure growth rate was roughly linear with dosage. With sufficient control, it was possible to create a variety of nanostructures. In Figure 2a, an optical image is shown of a grid and two potential rectangular contact pads created on the surface of a BSCCO sample. The grid was created by forming two sets of intersecting lines. Two different dosage exposure times were used to create a grid with lines running one direction 3 times as tall as their perpendicular counterparts (Supporting Figure S2). During the course of

Figure 2. Patterned surface nanostructures created with electron beam radiation. (a) Image of grid and rectangular patterns formed on BSCCO taken with optical microscope. (b) AFM topography measurement of a 60 nm tall linear structure formed on graphite. Graphite layers peeled up when the sample was exfoliated before the nanostructure was formed are clearly visible running across the nanostructure.

experimentation, a variety of linear, solid, and gradient filled shapes were achieved, some with heights exceeding 200 nm. Interestingly, nanostructure formation resulted in very little surface degradation. No damage could be seen in AFM measurements, as shown in Figure 2b. In this nanostructure, molecular step bunches can readily be seen to cross over the nanostructure formed by electron beam exposure. These nanometer-scale steps, as well as the curled feature visible on the rightmost section of the nanostructure, were formed during the exfoliation process before the sample was exposed to ebeam radiation. The preservation of these features even upon the formation of a 60 nm tall surface structure indicates that the process leaves the surface largely intact, although damage at the molecular level below the lateral resolution of the AFM is possible. Measurements taken on flat areas of various nanostructures show the typical RMS surface roughness is less than 0.2 nm extending over lateral areas exceeding 100 μm 2. Overall, the AFM measurements show that the nanostructures arise from a subsurface growth process. This is consistent with nanostructure formation only in layered materials, which have relatively open spaces that can readily incorporate additional material. Scanning tunneling microscopy (STM) was also used to analyze these structures. STM topographical images acquired from the edge of the grid in Figure 2a are shown in Figure 3a,b. Measurements taken at a tip bias of 0.2 V reveal patterned trenches rather than raised structures. The apparent depth of the trench decreased with tunneling bias up to 1.0 V, at which point the trench could not be distinguished from the surrounding BSCCO substrate (Figure 3c). Increasing the tunneling bias beyond 1.0 V had no noticeable effect on the apparent height. Potentials higher than 2.0 V resulted in increasingly unstable imaging conditions. As with the AFM measurements, no surface damage could be detected. Tunneling spectroscopy measurements were also obtained at various locations on and off the nanostructures. Representative differential tunneling spectra are shown in Figure 3d. Spectra characteristic of a metallic surface (solid black) were found far from the nanostructures on the BSCCO substrate, as expected. Most spectra on or in the vicinity of the nanostructures exhibited nonconductive energy gaps ranging from 0.5−2.5 V (solid red and blue), though metallic spectra were occasionally observed. 5941

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Figure 4. Raman and photoluminescence spectra from nanostructures. (a) Photoluminescence spectrum from a nanostructure created on graphite surface compared with a spectrum from pristine graphite. D, G, and 2D Raman bands from graphite or nanocrystalline carbon are labeled. The inset shows zoom-in spectra of the D and G bands. The spectrum from nanostructure on graphite surface (in the box) is decomposed into several Lorentzian peaks. (b) Photoluminescence spectra from nanostructures created on surfaces of different materials. The spectra are normalized to the same laser power and exposure time. The dashed vertical lines highlight the positions of the D and G Raman bands from carbon nanocrystals imbedded in different layered materials. All spectra are excited by 532 nm laser light and acquired at room temperature. All nanostructures were formed by exposing a 600 μm2 area to 30 kV electrons with 1 nA beam current for 2 h.

Figure 3. Topographical STM images acquired near the right-hand edge of the grid formed on the patterned BSCCO sample (Figure 2a) under constant current conditions (Itip = 1 nA) using a tunneling bias of (a) 0.2 Vtip and (b) 1.0 Vtip. (c) Line profiles obtained from the images depicted in a and b. (d) Representative differential tunneling current spectra. The black curve was obtained far from the nanostructures, and the blue and red curves were obtained on the patterned structures.

At first it was difficult to reconcile the 15 nm raised structures observed using AFM with the 1.5 nm deep trenches in the STM images. This discrepancy can be resolved by considering the zero or near zero density of states (DOS) of the nanostructures at lower tip biases, resulting in a decrease in tip to sample distance compared to the surrounding BSCCO. Under vacuum tunneling conditions, a dip in excess of 1 nm would cause a tip−sample impact. However, in the absence of an annealing step, a water layer can reside upon the sample even under high vacuum conditions. The existence of this layer increases the tunneling probability due to the dielectric nature of water, causing the tip-separation distance to increase to tens of nanometers. 21 Current versus height measurements (Supporting Figure S3) confirm that scans taken on metallic portions of the sample occur with a tip−sample separation greater than 20 nm, large enough for the tip to clear the raised structures even with a 1.5 nm dip. Overall, the STM results show that the nanostructures are insulating features with very smooth surfaces. Optical measurements were performed to reveal more about the properties of these nanostructures. Figure 4a shows a photoluminescence spectrum from a nanostructure formed on

the surface of graphite. A spectrum from pristine graphite is included for comparison. It is seen that the spectrum from the nanostructure shows two clear features. The first is a broad photoluminescence feature in the visible range (550−750 nm); the second is the appearance of two well-defined peaks (centered at ∼574 and ∼581 nm) that superimpose on the broad luminescence background (see the inset of Figure 4a). The broad photoluminescence features are consistent with the band gaps measured by STM. The frequencies of the two welldefined peaks (∼1350 and ∼1580 cm−1 under 532 nm laser excitation) correspond to those of the D and G Raman bands from graphite/graphene with disorders or defects.22 In the inset of Figure 4a we also see that the sharp G Raman line from pristine graphite is preserved after the fabrication of nanostructure. This sharp G peak is superimposed on the broader G Raman band from nanostructure created on the graphite surface. Both Raman peaks and photoluminescence features are nearly identical to those observed in carbon nanocrystals.23 The optical spectra were consistent in all of the layered materials. Figure 4b displays photoluminescence spectra from nanostructures on the surfaces of dichalcogenide TiS2, 5942

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al. to form nanopillars with associated rings that appear very similar to the halos seen in Figure 1.18 However, the rate of nanostructure formation in layered materials here were orders of magnitude faster than that found in the deposition work. For example, it took 2.5 min to deposit a pillar 10 nm tall with covering a lateral area much less than 0.5 μm2. Here, structures of the same height could be formed in 30 min but extending over an area of 600 μm2. If this rate were simply due to enhanced deposition rates from a larger presence of residual organics in our system, we should have easily been able to detect structures on nonlayered materials with an optical microscope. The Raman and photoluminescence spectra measured from the nanostructures in this work most closely resemble those found in amorphous carbon nanoparticles.23 However, the D and G peaks measured from the nanostructures are much sharper and more clearly defined. This indicates that the carbon bonding is better ordered and more graphitic than in the amorphous carbon nanoparticles. This information, combined with the fact that the surface of these nanostructures is flat at the molecular level both in STM and AFM, would indicate that the nanoscale carbon incorporated within the layered materials has a flattened structure rather than having 3D structure. However, the presence of photoluminescence and the insulating character of nanostructures as determined by STM rules out the possibility of the carbon forming continuous sheets within the van der Waals gaps of these materials. The resulting carbon inclusion within the layers of the sample would result in a bulging of the sample in areas exposed to e-beam radiation, as depicted in Figure 5. Given the broad

topological insulator Bi2Te3, and superconductor BSCCO. Both the D and G Raman bands (highlighted by dashed vertical lines) and broad photoluminescence features are seen in the spectra from all these samples. As might be expected, higher dosages, which induce larger structures, lead to more intense Raman and photoluminescence features (Supporting Figure S4). Therefore, the nanostructures formed by e-beam radiation are actually created by the incorporation of carbon nanocrystals into the layered materials. Furthermore, the optical spectra are consistent with nanocrystal sizes on the order of 10 nm, which can explain the ability to form nanostructures with heights measuring up to several hundred nanometers. The nanostructures produce photoluminescence at room temperature (stronger than photoluminescence from pentacene thin films used in organic light emitting diodes24,25) and thus are potential candidates for light emitting device applications. The relative intensity between Raman and photoluminescence features varies in nanostructures created on different materials. For instance, nanostructures on TiS2 and Bi2Te3 surfaces show stronger photoluminescence bands than those fabricated on graphite or BSCCO surfaces, and intensities from D and G Raman lines are stronger in the nanostructure on Bi2Te3 surface in comparison to the ones grown on other materials. This variation in the photoluminescence and Raman intensities is attributable to the difference in density and size of carbon nanocrystals intercalated in different materials, which is also consistent with the variation in energy gaps seen in STM. When taken together, the experiments clearly indicate that the nanostructure growth is not merely a surface deposition phenomenon. The most obvious evidence for this is that the phenomenon only occurs on layered materials. Measurements using optical microscopy, which could always be used to detect the presence of nanostructures in layered systems, revealed nothing on the surface of bulk metals or silicon using the same beam exposures. Furthermore, the cleanliness of the surface was very important for nanostructure formation. Surface contamination dramatically inhibits nanostructure formation, which indicates that deposition does not play a dominant role. The surface is also remarkably unchanged after nanostructure formation. Molecular step edges cleanly are undisturbed by nanostructure formation, which would be unlikely if tens or even hundreds of nanometers of carbon were simply deposited upon the surface, although the Raman measurements clearly indicate that the e-beam radiation induces the incorporation of carbon in some manner. Furthermore, the layers in the sample were more strongly bound after e-beam induced nanostructure formation (Supplemental Figure S5).26,27 For example, it was found that contact mode AFM measurements could readily induced layer-by-layer etching away of surface molecules in dichalcogenide samples. However, the surface layers of the nanostructures could not be so easily removed. In fact, when application of larger contact force was used to successfully etch away the nanostructure surface with the AFM, the result was catastrophic, uncontrolled removal of material forming pits that locally removed the entire nanostructure rather than individual layers. This also is consistent with the inclusion of material within the van der Waals gap which would strengthen interlayer bonding. Comparison of nanostructure formation rates found here with electron beam-induced carbon deposition also reveal that this process is fundamentally different. E-beam-induced carbon deposition studies were performed using nearly identical vacuum conditions and beam parameters by Rykazcewski et

Figure 5. Schematic of nanostructure cross section. The nanostructures extend laterally beyond the focus spot of the electron beam indicated by a red cone. The van der Waals gap between the molecular layers is an space in which molecules can be incorporated. The interaction of the electron beam induces the incorporation of carbon nanoparticle derived from residual organic molecules originally present on the sample surface.

photoluminescence spectrum, significant variations in height and/or lateral extent of carbon nanoclusters would be expected. Electron scattering processes and surface diffusion of residual organics lead to interactions with residual organic material in areas outside the focused beam spot, although to lesser effect.18,19 This is consistent with the AFM measurements that showed the lateral widths of the nanostructures were typically at least a hundred nanometers larger than the beam spot diameter, which is much larger than the expected lateral resolution of the AFM tips even if worn by measurement. 5943

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CONCLUSION

The most probable mechanism is for the carbon incorporation is to begin with the intercalation of organic molecules. Hydrocarbons present on the sample surface are readily cracked by e-beam exposure. If this cracking proceeded to the ultimate reduction of pure carbon at the surface with optical properties as seen in these nanostructures, evidence of carbon surface contamination should have been seen in optical microscopy on the nonlayered materials as well. If the hydrocarbons were reduced only to shorter organic molecules, however, then these molecules would be volatile in vacuum and thus leave the surface of nonlayered materials. In layered systems, such organics could instead be incorporated into the open spaces between the layers.28−30 Once incorporated, the organics could be further reduced to pure carbon by high energy electrons, which can readily penetrate hundreds of nanometers into the sample. Then the carbon atoms located between layers could migrate, as they do in graphite systems,9 to agglomerate into nanoscale clusters. While we cannot completely rule out the presence of any carbon deposition on the surfaces of these layered materials, incorporation of carbon nanoparticles within the layers must be by far the dominant mechanism for nanostructure formation. In conclusion, we have discovered a simple and seemingly universal technique for the formation of patterned, optically active surface nanostructures on layered materials. The nanostructures can be formed with heights ranging from the angstrom scale to well over one hundred nanometers, and their lateral size and location is easily controlled in the SEM. The lateral size of the nanostructures does extend well beyond the diameter of the focused beam spot, width features having a minimum feature size exceeding 150 nm. The nanostructures have intriguing optical properties in the visible regime that could be developed into sensors, light emitting devices, or even photovoltaics. Furthermore, the technique applies to a range of materials with scientific or technological importance such as novel superconductors, topological insulators, and density wave compounds. The ability to locally modify the electronic character of these materials could lead to new insights into their properties, including the creation of localized regions to explore quantum confinement effects.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by NSF Grant No. DMR-1206530, and DOE at DE-AC02-98CH10886. Funding was also provided by the University of Northern Iowa in summer fellowships for T.K. and R.H. as well as a UNI PDA for T.K. T.K. and R.H. also acknowledge support from Iowa NASA EPSCoR under Grant No. NNX09AO66A. R.H. also acknowledges support from a UNI Provost’s Pre-Tenure Summer Fellowship Award and donors of The American Chemical Society Petroleum Research Fund Grant No. 53401-UNI10 for partial support.

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ASSOCIATED CONTENT

S Supporting Information *

This material includes supplementary figures showing the effects of surface quality on nanostructure growth, an examination of the BSCCO grid (Figure 2a) by AFM, the relationship between STM tip−sample separation and voltage, Raman and photoluminescence spectra taken from a TiS2 sample with different levels of e-beam exposure, and differences seen in AFM-induced etching between exposed and unexposed surfaces. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 5944

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