Scanning Tunneling Microscope Studies of Ultrathin Graphitic

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J. Phys. Chem. C 2008, 112, 6681-6688

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Scanning Tunneling Microscope Studies of Ultrathin Graphitic (Graphene) Films on an Insulating Substrate under Ambient Conditions Elena Stolyarova,† Daniil Stolyarov,‡ Li Liu,† Kwang T. Rim,† Yuanbo Zhang,§ Melinda Han,| Mark Hybersten,⊥ Philip Kim,£ and George Flynn*,† Department of Chemistry, Department of Applied Physics, Department of Physics, Center for Electron Transport in Molecular Nanostructures, Columbia UniVersity, New York, New York 10027, Accelerator Test Facility, Department of Physics, Center for Functional Nanomaterials, BrookhaVen National Laboratory, Upton, New York 11973, and Department of Physics, UniVersity of California, Berkeley, California 94720 ReceiVed: September 25, 2007; In Final Form: February 7, 2008

In the present study, a scanning tunneling microscope (STM), modified to include active lateral position feedback control, is employed to image single and few layer graphene films placed on a nonconductive substrate under ambient conditions. The return path for tunneling electrons was provided by gold electrodes produced by either electron beam lithography or shadow evaporation techniques. STM images of graphene films with a thickness of two or more layers display topographs that are similar to those obtained from a bulk graphite crystal. For single layer graphene sheets, the ability to obtain atomically resolved images was found to be extremely sensitive to sample preparation methods. Graphene microdevices produced by electron beam lithography with edges covered by gold electrodes show hexagonal patterns similar to those obtained from ultrahigh vacuum STM images reported earlier. Ambient STM measurements of graphene microdevices made by shadow mask evaporation, whose edges were exposed to air, exhibited chaotic topographs caused by instability in the STM feedback control loop due to interactions between tip and sample. STM images recorded on these samples reveal “noisy” topographs that are likely not related to any real surface features.

Introduction Single and few-layer graphitic flakes can be formed by peeling a graphite crystal on a silicon dioxide substrate. These flakes are stable, chemically inert, and highly conductive under ambient conditions. Transport measurements show that electronic properties of microdevices based on these ultrathin graphitic films can be controlled by applying an external electric field. For this reason, graphitic films are considered to be one of the most promising materials for atomic scale devices. Observation of ballistic transport1 and the Quantum Hall Effect at low and room temperatures,2 proximity-induced superconductivity,3 quantum confinement in patterned graphene ribbons,4 etc. have caused an explosion of experimental and theoretical activity in this area. Suspended graphene films have been shown to have finite out-of plane corrugations.5 Moreover, transport properties of graphene cannot be precisely understood in terms of a model that treats these films as unperturbed twodimensional (2D) conductors. Surface corrugations, interactions with the underlying substrate, charges trapped in the support layer (e.g., silicon dioxide), and absorption of foreign molecules on the graphene surface can all change the mesoscopic properties * Corresponding author. E-mail: [email protected]. Phone: 212 854 4162. Fax: 212 854 8336. † Department of Chemistry and Center for Electron Transport in Molecular Nanostructures, Columbia University. ‡ Accelerator Test Facility, Department of Physics, Brookhaven National Laboratory. § University of California. | Department of Applied Physics and Center for Electron Transport in Molecular Nanostructures, Columbia University. ⊥ Center for Functional Nanomaterials, Brookhaven National Laboratory. £ Department of Physics and Center for Electron Transport in Molecular Nanostructures, Columbia University.

of these films.6 As a result, probes of local graphene properties are of considerable interest.1 Bulk graphite consists of weakly bound, stacked graphene layers that are shifted with respect to each other as shown in Figure 1. As a result of this shift, two neighboring carbon atoms are not equivalent in the layered structure of bulk graphite. Three of the six atoms are located on top of a carbon atom in the layer below (A-type atoms), while the other three atoms are centered in the middle of the six carbon atom hexagon (B-type atoms).7 In the case of a single layer, the asymmetry between different atomic sites is removed. Most scanning tunneling microscope (STM) images of graphite reveal a so-called “three-for-six” pattern of dark (low tunneling current) and bright (high tunneling current) spots. Density functional theory based calculations and modeling by Tomanec et al.8,9 showed that for tunneling energies near the Fermi level the bright spot represents the position of B-type carbon atoms. The local electron density of states near the Fermi level on A-type atoms is believed to be decreased due to splitting of the π-orbitals of these atoms in the topmost layer as a result of interaction with the π-orbitals of carbon in the adjacent layer. For this reason, tunneling probability is lowered over the A-type atoms. An alternative theory explains the contrast in the STM images as arising from the difference in rigidity of A and B sites.10 In either case, for single layer samples the asymmetry between sites is removed, and formation of a honeycomb STM structure is expected and observed.11,12 Other intriguing features associated with STM imaging of graphite are superperiodic hexagonal structures. These gigantic superlattice corrugations (up to 3 Å) observed in STM images are not believed to be directly related to the topology of the surface, but rather they are formed as a result of a modification

10.1021/jp077697w CCC: $40.75 © 2008 American Chemical Society Published on Web 04/04/2008

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Figure 2. A side view of the experimental arrangement used to study graphene flakes. An STM tip is located on the top of a conductive monolayer, while the underlying silicon dioxide substrate is an insulator. Charge carriers must travel laterally through the 2D graphene film to reach the deposited gold electrodes and close the electrical circuit between a graphene microdevice and the STM tip.

quasi-2D graphene crystal that is grounded through fixed electrodes away from the STM tip. Figure 1. Shown is a schematic drawing of two adjacent graphene layers in the bulk graphite crystal. These two layers are shifted by a half period with respect to each other. For this reason, two nonequivalent sets of carbon atoms can be distinguished in the topmost graphene layer: A-type carbon atoms shown by purple circles are located on the top of carbon atoms in the graphene layer underneath, while B-type atoms (red circles) are located on the top of a hollow site.

of the local electron density due to interlayer interactions arising from rotation of the topmost graphene layer with respect to the underlying graphite. For this reason, these superstructures are often called Moire´ rotation patterns in the literature.15 The physical origin of this anomalous phenomenon has been debated, but numerous experimental studies suggest that observation of these superstructures in STM images can be used as a reliable signature for deformation or distortion of the ideal graphite crystal interlayer order.13,14 In contrast to the superstructures described above, an insertion of atomic scale defects into HOPG crystals causes formation of a x3 × x3 a, a ) 2.46 Å, pattern that decays away from a defect site within a few angstroms.15 Very recent studies of the morphology of graphene films on silicon dioxide substrates revealed that atomically resolved STM single layer graphene films exhibit a honeycomb structure that is consistent with the 6-fold symmetry of graphene.11 Those images are similar to STM topographs of single wall carbon nanotubes16 and single layer graphene films grown on SiC.17,18 It is interesting to note that a recent STM study of a single layer graphene sample on silicon dioxide done by another group reported a mixture of honeycomb and “three-for-six” patterns.12 A possible reason for this effect is that local perturbations can be strong enough to break the graphene hexagonal symmetry. For example, molecules trapped under the surface of the graphene flake or distortion of the flake as a result of chemical treatment during sample preparation and cleaning might cause such perturbations. STM studies of single layer graphene films grown on top of metal surfaces such as Ir(111),19 Pt (111)20 have also reported observation of these “three-for-six” patterns. The work described here reports STM studies of graphitic films of variable thickness produced from a bulk crystal of graphite on an oxidized silicon substrate under ambient conditions. Because oxidized silicon is an insulator, electron exchange between the graphene flake and the underlying substrate should be negligible, and tunneling current can pass only through a

Experimental Section Thirteen graphene microdevices were fabricated and studied by STM under ambient conditions. These devices were prepared by mechanical exfoliation of Kish graphite (Toshiba Ceramics Co.). First, graphitic flakes were deposited on a silicon wafer with a 300 nm thick oxide surface layer. The detailed sample preparation procedure is described elsewhere.11 Usually, thousands of flakes are formed on each wafer, and only a small fraction of these flakes are few layer thick samples. Initially, an optical identification method was used to find and select flakes with the desired number of atomic layers.21 Either AFM or Raman measurements were employed to confirm the assignment done by optical identification.22 Following this step, gold electrodes were deposited on the graphene/silicon dioxide sample as shown in Figures 2 and 3. The underlying silicon dioxide is electrically insulating material, and electrodes are required to provide a return path for the STM tunnel current. It is noteworthy that in contrast to conventional STM experiments the electrons that tunnel from the STM tip to the sample must travel through the 2D graphene conductors to reach the electrode. The gold electrodes were fabricated using either shadow mask evaporation or electron beam lithography. Figure 3a represents a schematic drawing of a typical microdevice prepared using shadow mask evaporation. During fabrication, a silicon nitride membrane with etched openings is positioned over the flake. Following this, a 100 nm thick gold film was thermally deposited on top of the graphene flake. An example of an optical image of one of the microdevices used in the present study is shown in Figure 3b. The graphitic flake is in the center of the image, and several areas with different optical density, corresponding to regions of the sample with different numbers of graphene layers, can be distinguished. The area that is marked I corresponds to a single graphene layer, while the area marked II is a two layer thick flake. Raman spectroscopy performed on similar samples confirmed that the optical contrast observed here indeed corresponds to the number of identified graphene layers.22 Gold electrodes are labeled as III, and the area marked IV is a nonconductive silicon dioxide surface region. Shadow mask evaporation minimizes contamination of the graphene flake, because the mask touches the flake only at a

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Figure 4. Shown is a schematic drawing (a) and an example of an optical image (b) of a few-layer graphene device produced by electron beam lithography. The following areas can be identified in the optical image: I, single graphene layer; II, double graphene layer; III, gold electrode. The gold electrode completely covers the silicon dioxide as well as the fringes of the graphene flake. Figure 3. Shown is a schematic drawing (a) and an example of an optical image (b) of a few-layer graphene device produced by shadow mask evaporation. The following areas can be identified on the optical image: I, single graphene layer; II, double graphene layer; III, gold electrodes; IV, silicon dioxide (insulator).

few points. The microdevice was not subjected to any chemical treatment. Deposited gold electrodes form contacts with graphene only in a few areas, and most of the graphene flake edges are exposed air. For these devices it is possible to pass current between two electrodes making them ideal candidates for combined transport and STM studies. The procedure for fabrication of microdevices using electron beam lithography is described in detail in a previous report.11 A sketch of such a device, which was previously used for UHV studies,11 is shown in Figure 4a. The gold electrode covers the edges of the graphene flake completely. In the optical image (Figure 4b), the single layer flake region is marked as I, the few layer region as II, and the gold electrode as III. For samples prepared using the standard electron beam lithography fabrication method, the surface of graphene is unavoidably exposed to electron beam resist PMMA as well as other chemical contaminants. As a result, intrinsic properties of the graphene films can be changed during nanofabrication. While intensive cleaning in a hot acetone bath as well as vacuum annealing prior to STM measurements removes most of the PMMA, atomic force microscopy (AFM) measurements indicate that isolated areas on the sample are covered with organic residue. To conduct a successful STM study of samples fabricated using shadow mask evaporation, two major experimental challenges must be overcome. First, the STM tip must be positioned within a conductive region on the sample surface.

Second, observed STM images must be related to mesoscopic sample features. To study mesoscopic, heterogeneous conductive samples (such as shown in Figure 3) at the atomic level while the sample is sitting on an insulating surface under ambient conditions, we have developed and tested a navigation system described below. Our apparatus, built on the base of a commercially available STM, contains an integrated lateral motion stage, vibration isolation system, direct signal access module, a personal computer based real-time data acquisition system, and point-by-point active feedback motion control software. We have modified a Veeco (Digital Instruments) Nanoscope III STM controller with SAM (Signal Access Module) option for sample imaging. The manual XY movement stage was replaced with a computer controlled motorized motion stage that includes a PCI control board (DCX-PCI 100, Thorlabs) and 2 Motorized Actuators (Z606, Thorlabs). Output signals from the Nanoscope controller were monitored by a Data Acquisition system (PCI-6229 on-board computer card and BNC-2090 signal input and output board). A program written in LabView (National Instruments) allows us to implement point-by-point active feedback motion control. The flowchart shown in Figure 5 describes the algorithm for imaging graphene microdevices used in this work. First, the STM tip is positioned over a conductive part of the sample using an optical microscope. Gold electrodes can be easily seen in such an optical image facilitating initial tip positioning. If the initial guess for the tip position is successful (i.e., the tip is placed over a conductive region), the tunneling current between tip and substrate is established. The next step requires characterization of the surface region under the tip. The choice of characterization method depends upon the materials that compose the microdevice. By measuring the dependence of the tunneling current on the bias voltage (I-V curve), metallic and

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Figure 5. A block diagram of an algorithm employed for studies of mesoscopic conductive samples on nonconductive substrates.

graphitic areas can be easily distinguished. Typically, reliable characterization is achieved within seconds. If necessary, more detailed information about the surface can be obtained by taking topographic images of the surface, which requires several minutes of processing. Because the accuracy of the initial tip alignment does not exceed a few micrometers, the exact tip position after landing in not known. However, the goal of the experiments is to find an unambiguous correspondence between recorded STM images and the mesoscopic features of the optical device image, such as shown in Figure 3a, where the exact thickness of the graphitic flakes is known. To reconstruct such a surface correspondence map, the tip must be moved with respect to the graphene sample. In this way, information about the local properties of the material under the STM tip can be collected as a function of the tip position. In the next step, the sample is moved in the XY plane with respect to the tip using the motorized motion stage to “navigate” the tip to the region of interest. As a result of the lateral motion in the XY plane, the tip can be moved into a nonconductive area of the surface. Unless a preventative algorithm is used to avoid such situations, this would result in termination of the tunneling current on which the STM feedback control relies. This in turn causes the tip to crash to the surface, which results in permanent tip damage. To avoid this problem, an edge detection method is used. If the tunneling current starts to drop, the computer control program moves the sample in the opposite direction, while at the same time the tip is moved away from the surface in the vertical direction. The edge detection parameters can be optimized for different kinds of materials. For example, near the edges of graphene flakes that were not

pinned down by gold electrodes, we found a strong increase in feedback current oscillations that provided a reliable signature of edge proximity. Once the area of interest on a device is found, atomically resolved images can be taken using commercial software (Nanoscope v. 4.30, Digital Instruments) as a final step in the experiment. Results About 1000 STM “snapshots” of the surface of graphene flakes were collected for different film thickness. For flakes with n > 1 (n is the number of atomic layers) two major types of STM images were observed. Some areas revealed “threefor-six” topological structures analogous to STM images of HOPG. A typical 5 × 5 nm image of this kind is shown in Figure 6a; this image was obtained on a film with n ) 2. Atomic-sized features were observed in 20% of these STM surface “snapshots” for the double layer flakes; for thicker flakes, atomic resolution was achieved with a probability higher than 90%. We observed no defects such as a vacancy or a pentagon-heptagon pair on these few-layer graphene flakes regardless of their thickness. No obvious signs of organic compound residue were observed on these samples. The second type of feature detected in these studies can be described as a superperiodic pattern with spatially varying periodicity. No atom-sized features were observed in these images. The probability of observing this kind of superperiodic structure increased dramatically for the thinner films. For example, these features are dominant, occurring with 80% probability in the case of a double layer film. (For the thicker flakes with n > 7, these structures were mainly observed near step edges as is typical for HOPG.23) Comparison of multiple

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Figure 6. (a) An STM image (5 nm × 5 nm) of a double layer of graphene. The microdevice was produced by the shadow evaporation technique. The observed image is indistinguishable from a typical STM image of HOPG obtained under similar conditions. Scanning conditions: I ) 2 nA, Vbias ) 52 mV. (b) An STM image (10 nm × 10 nm) of a single/double layer interface of the graphene sample that was produced by the shadow evaporation technique. Scanning conditions: I ) 1.8 nA, Vbias ) 5 mV. The left part of the image reveals two regions with the “three-for-six” pattern typical for HOPG; the right side of the image shows the tunneling current instability that was repeatedly observed on all single layer flakes.

areas within the same double layer sample shows that the period of the superstructures is not constant within a given flake. Different parts of the same sample can provide either “threefor-six” or Moire´-type images. All the observations described above were similar for both kinds of samples prepared either by means of electron beam lithography or by shadow mask evaporation. The most striking

observation of the present study is the dramatic difference in STM images for single layer samples prepared by different techniques. For devices prepared using shadow mask evaporation, the STM tip could be readily positioned in the region of a single layer flake using the algorithm described above, but no stable STM image could be recorded in this area of the sample. An example of a typical STM “image” of a graphene

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Figure 7. (a) An STM image (2 nm × 2 nm) of a single layer graphene microdevice that was made using electron beam lithography. Scanning conditions are: I ) 2 nA, Vbias ) 1 V. The observed hexagonal structure is similar to that observed in studies of the same sample under UHV conditions. (b) An STM image (10 nm × 10 nm) of the same single layer sample. I ) 1 nA, Vbias ) 1 V.

flake taken close to a single layer/double layer interface is presented in Figure 6b. On the left side of the image, two areas with atomic-sized features can be identified, although most of the image shows the featureless noise that was routinely observed for all single layer flakes. I(V) curves taken in the single layer region, although extremely unstable, exhibit conductivity. No mechanical damage or contamination of the STM tip occurred as a result of interaction of the tip with single layer films. Control AFM studies that followed the STM experiments

revealed no observable sample changes or damage as a result of STM imaging. The atomic resolution imaging was always restored as soon as the tip was moved to an area of the sample that was thicker than a single layer. This observation was repeatable for five different microdevices, suggesting that the observed phenomenon is not an experimental artifact but, rather, is characteristic of STM measurements on single layer graphene films prepared in this way. On the other hand, experiments on the sample that was produced using electron beam lithography

Graphene Films on an Insulating Substrate did not exhibit the anomalous behavior described above in the region of single layer graphene. Similar to our experiments on the same sample under UHV conditions,11 a hexagonal network can be identified in the 2 × 2 nm STM image shown in Figure 7a, and the 10 × 10 nm image (Figure 7b) reveals corrugation of the graphene film. The ambient STM images for this sample are of poorer quality than those taken in UHV at low temperature; however, this is likely due to the enhanced stability of the UHV STM device. Discussion Our results indicate that few-layer graphene flakes have a domain-like structure; the formation of Moire´ patterned domains is likely associated with the stress involved in the sample preparation procedure that separates few-layer flakes from the bulk crystal. In multilayer flakes, domains with atomic-sized features are found to have topographs that are not distinguishable from those observed for bulk graphite. This suggests that the presence of two monolayers is enough to sustain a lattice that exhibits the essential low-energy electronic structure properties associated with the 3D graphite crystal. This is born out by recent calculations of the electronic structure for one, two and three monolayers of graphene with graphitic stacking.24,25 When large area STM images were recorded for double layer samples, the roughness of the surface was found to be similar to the roughness of the underlying oxidized silicon wafer, consistent with AFM measurements. Similar results were found for single layer graphene samples studied under UHV conditions.11 At the same time, small area scans revealed atomic structure. In addition, neither few layer nor single layer flakes exhibited x3 × x3 structures that would have been indicators of strong local perturbations of the graphene lattice. Thus, it is likely that the silicon dioxide substrate only weakly perturbs the local electronic structure of graphitic flakes. We are presently pursuing a detailed analysis of graphene roughness and local electronic structure (via scanning tunneling spectroscopy) as a function of layer thickness under UHV conditions. Several hypotheses can be suggested to explain the anomalous tunneling behavior observed for single layer devices that were made by the shadow evaporation technique. First, the interaction between the tip and a graphene film can cause tip-induced mechanical instability due to Pauli repulsion, van der Waals interactions, and/or electrostatic attractive interactions. Under ambient conditions, a meniscus is likely formed (due to water vapor in the air) between tip and sample making tip-film interactions much stronger than under UHV conditions. Because the film is only weakly bound by van der Waals strength forces to the silicon oxide substrate, mechanical vibration or displacement of the film with respect to the substrate may occur. However, thermal film vibrations would likely cause instability in the double layer region of the same graphitic flake, which was not observed. For a single layer film, Pauli repulsion is likely to cause bending rather than elastic deformation of these flakes, and tunneling through multiple sites would then become possible. In the case of the devices produced by electron beam lithography, the layer of gold around the flake pins down the graphene samples firmly on the substrate and, thus, can reduce any mechanical motion of the graphene film. It is not clear at this time if mechanically stable graphene samples can be prepared with multiple single point electrode contacts that would allow successful, high-resolution STM imaging under ambient conditions. Nevertheless, of all the samples investigated in the present study, only the one with full electrode contact yielded images recognizable as those of single sheet graphene.

J. Phys. Chem. C, Vol. 112, No. 17, 2008 6687 It is also worth mentioning that all graphene samples in this study are prepared under ambient conditions. Thus, it is likely that molecules, absorbed on the surface of the silicon dioxide wafer at the moment of flake deposition, are trapped between the flake and the silicon dioxide surface. Wafer cleaning performed less than 1 h prior to flake deposition removes the vast majority of organic contaminants. On the other hand, for samples produced using shadow mask evaporation, the sample edges are exposed to the environment. Under these conditions layers of water and organic molecules (possibly from the left over resist material) can accumulate on the surface of the wafer and migrate to the regions under the graphene flake. Although there is no direct experimental evidence for such trapped species in the case of graphene flakes on a silicon dioxide substrate, diffusion of atoms and molecules under graphene films grown on metals is a well-documented phenomenon.19 Energetic considerations suggest that atoms or molecules trapped between a graphene layer and a silicon dioxide surface will be more stable than those physisorbed on the bare substrate. Finally, the likelihood that trapped charges exist in the silicon dioxide is high, which may affect the strength of the graphene-silicon dioxide binding or even contribute directly to the observed anomalous tunneling behavior. It is worth noting that few layer graphene flakes can be reliably and reproducibly imaged under ambient conditions, which suggests that impurities trapped on the surface from exposure to air do not significantly affect STM surface probes of this material. Nevertheless, the possible affects of impurities adsorbed on single sheet graphene surfaces are as yet unexplored and may contribute in a significant way to differences between STM images of this species taken under ambient and UHV conditions. Conclusions STM studies of few layer graphitic films conducted under ambient conditions demonstrate that there is no significant difference between STM images of bulk graphite and graphene films that are more than one atomic layer thick. The possibility of obtaining STM images for single layer flakes strongly depends upon sample preparation procedures and design of the sample. Our observations suggest strong interactions between the STM tip and these thinnest graphene samples. Successfully recorded STM images of single layer graphene reveal a symmetric honeycomb structure indicating that the inequity between adjacent carbon atoms characteristic of bulk graphite is removed for single layer films. Acknowledgment. The authors thank Etienne De Poortere and Kirill Bolotin for invaluable help in the microfabrication of the samples used in this study. This work was supported by the National Science Foundation through Grant CHE-03-52582 (to G.W.F.) and the NSEC Program (CHE-06-41523), by the New York State Office of Science, Technology, and Academic Research (NYSTAR), and by the U.S. Department of Energy (DE-AC02-98CH10886 to M.S.H. and DE-FG02-88ER13937 to G.W.F.). References and Notes (1) Geim, A. K.; Novoselov, K. S. Nat Mater 2007, 6, 183. (2) Novoselov, K. S.; Jiang, Z.; Zhang, Y.; Morozov, S. V.; Stormer, H. L.; Zeitler, U.; Maan, J. C.; Boebinger, G. S.; Kim, P.; Geim, A. K. Science 2007, 315, 1379. (3) Heersche, H. B.; Jarillo-Herrero, P.; Oostinga, J. B.; Vandersypen, L. M. K.; Morpurgo, A. F. Nature 2007, 446, 56.

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