Previous Scanning Tunneling Microscopy Research Articles

Single-molecular diffusivity and long jumps of large organic molecules: CoPc on Ag(100).

Energy dissipation and the transfer rate of adsorbed molecules do not only determine the rates of chemical reactions but are also a key factor that often dictates the growth of organic thin films. Here, we present a study of the surface dynamical motion of cobalt phthalocyanine (CoPc) on Ag(100) in reciprocal space based on the helium spin-echo technique in comparison with previous scanning tunnelling microscopy studies. It is found that the activation energy for lateral diffusion changes from 150meV at 45-50K to ≈100meV at 250-350K, and that the process goes from exclusively single jumps at low temperatures to predominantly long jumps at high temperatures. We thus illustrate that while the general diffusion mechanism remains similar, upon comparing the diffusion process over widely divergent time scales, indeed different jump distributions and a decrease of the effective diffusion barrier are found. Hence a precise molecular-level understanding of dynamical processes and thin film formation requires following the dynamics over the entire temperature scale relevant to the process. Furthermore, we determine the diffusion coefficient and the atomic-scale friction of CoPc and establish that the molecular motion on Ag(100) corresponds to a low friction scenario as a consequence of the additional molecular degrees of freedom.

Nano-protrusions in intercalated graphite: understanding the structural and electronic effects through DFT.

Complex phenomena characterize the intercalation of ions inside stratified crystals. Their comprehension is crucial in view of exploiting the intercalation mechanism to change the transport properties of the crystal or obtaining a fine control of crystal delamination. In particular, the relationship between the concentration and nature of intercalated ions and surface structural modifications of the host stratified crystal is still under debate. Here, we discuss a theoretical effort to provide a rationale for some structural changes observed on the highly oriented pyrolytic graphite (HOPG) surface after electrochemical treatment in perchloric and sulphuric acid solutions. The formation of the so-called nano-protrusions on the basal plane of intercalated graphite was previously observed with scanning tunneling microscopy (STM). In this work, we employed both STM and density functional theory (DFT) simulations to elucidate the physical and chemical mechanisms driving the emergence of these nano-protrusions. The DFT results show that, in a bilayer graphene system, the presence of a single ion can generate a nano-protrusion with 2.49 Å height and 21.27 Å width. In the deformed area, the C-C bond length is stretched by about 2.5% more than the normal graphene bond. These values are of the same dimensional scale as those reported in previous STM experimental results.25 However, the simulated STM images obtained by increasing the amount of intercalated ions per area suggest that the presence of more than one ion is needed for the deformation of the uppermost graphite layer during the early stages of intercalation. In contrast, in a multilayer graphene system, no significant surface deformation is detected when ions are intercalated between the third and fourth layers. Charge analysis indicates an altered distribution of the charges as a consequence of the intercalation. The charge transfer from graphene layers to the intercalated ions results in a surface layer more prone to oxidation.

Elucidation of Cu-Zn Surface Alloying on Cu(997) by Machine-Learning Molecular Dynamics.

The Cu-Zn surface alloy has been extensively involved in the investigation of the true active site of Cu/ZnO/Al2O3, the industrial catalyst for methanol synthesis which remains under controversy. The challenge lies in capturing the interplay between the surface and reaction under operating conditions, which can be overcome given that the explicit dynamics of the system is known. To provide a better understanding of the dynamic of Cu-Zn surface at the atomic level, the structure and the formation process of the Cu-Zn surface alloy on Cu(997) were investigated by machine-learning molecular dynamics (MD). Gaussian process regression aided with on-the-fly learning was employed to build the force field used in the MD. The simulation reveals atomistic details of the alloying process, that is, the incorporation of deposited Zn adatoms to the Cu substrate. The surface alloying is found to start at upper and lower terraces near the step edge, which emphasize the role of steps and kinks in the alloying. The incorporation of Zn at the middle terrace was found at the later stage of the simulation. The rationalization of alloying behavior was performed based on statistics and barriers of various elementary events that occur during the simulation. It was observed that the alloying scheme at the upper terrace is dominated by the confinement of Zn step adatoms by other adatoms, highlighting the importance of step fluctuations in the alloying process. On the other hand, the alloying scheme at the lower terrace is dominated by direct exchange between the Zn step adatom and the Cu atom underneath. The alloying at the middle terrace is dominated by the wave deposition mechanism and deep confinement of Zn adatoms. The short propagation of alloyed Zn in the middle terrace was observed to proceed by means of indirect exchange instead of local exchange as proposed in the previous scanning tunneling microscopy (STM) observation. The comparison of migration rate and activation energies to the result of STM observation is also made. We have found that at a certain distance from the surface, the STM tip significantly affects the elementary events such as vacancy formation and direct exchange.

Evolution of magnetoresistance with temperature in the insulating van der Waals compound Ta2Pd3Te5

Ta2Pd3Te5 has been recently identified as a van der Waals topological crystalline insulator candidate, but its magnetotransport properties are not reported. Here, we study electrical transport properties of Ta2Pd3Te5 single crystals. Ta2Pd3Te5 shows an insulating behavior with a trend of saturation below 20 K. The activity energy determined from the fits to polaron hopping model is Ea ∼ 36.1 meV, close to the value of 42 meV determined from previous scanning tunneling microscopy measurement. The linear Hall resistivity with a negative slope suggests an n-type conductivity for bulk Ta2Pd3Te5. The magnetoresistance (MR) is positive at 2 K, and the dip-like feature at low magnetic fields is attributed to the weak antilocalization effect. On the other hand, a negative low-field MR develops in the temperature range between 5 and 30 K, which is identified as a fingerprint of the weak localization effect. Such a transition from weak antilocalization to weak localization with increasing temperature reveals that the spin–orbit interaction plays an important role in Ta2Pd3Te5. Our results reveal highly enriched physical properties in Ta2Pd3Te5 single crystals, and they can be exploited in future magnetoresistance device design.

Electronic properties and degradation upon VUV irradiation of sodium chloride on Ag(111) studied by photoelectron spectroscopy

The growth as well as vacuum ultraviolet (VUV) radiation-induced degradation of sodium chloride (NaCl) on Ag(111) is investigated by ultraviolet and x-ray photoelectron spectroscopy. In line with previous scanning tunneling microscopy studies, our results confirm that NaCl grows initially as a bilayer before island growth starts. Simple spectroscopic methods for calibrating the closure of the NaCl bilayer are further presented. In addition, the energy level alignment is studied as a function of NaCl film thickness and VUV-light intensity. When measuring with ultra-low photon flux, a sharp interface dipole lowers the sample work function by 0.65 eV upon adsorption of the first bilayer, which is followed by vacuum level alignment for subsequently deposited layers. In contrast, measurements performed with standard photon fluxes, such as those provided by commercial He discharge lamps, shows ‘downward band-bending’-like characteristics in the NaCl films. Upon extended exposure time to the standard VUV intensity, photoemission measurements further reveal that strong modifications of the electronic properties of the NaCl surface occur. These are likely correlated with halogen emission, eventually resulting in the formation of Na clusters promoting low work function of parts of the sample surface. This study provides general guidelines for obtaining reliable spectroscopic measurements on alkali halide thin films on metals.

Bias-dependent diffusion of a H2O molecule on metal surfaces by the first-principles method under the grand-canonical ensemble

We investigate the process by which a water molecule diffuses on the surface of an Al(111) electrode under constant bias voltage by first-principles density functional theory. To understand the diffusion path of the water on the Al(111), we calculated the minimum energy path (MEP) determined by the nudged elastic band method in combination with constant electron chemical potential (constant-$\mu_{\rm e}$) methods. The simulation shows that the MEP of the water molecule, its adsorption site, and the activation barrier strongly depend on the applied bias voltage. This strong dependence of the water diffusion process on the bias voltage is in good agreement with the result of a previous scanning tunneling microscopy (STM) experiment. The agreement between the theoretical and experimental results implies that accurate treatment of bias voltage plays a significant role in understanding the interaction between the electric field and the surface of the material. Comparative studies of the diffusion process with the constant total number of electrons (constant-$N_\mathrm{e}$) scheme show that the absence of strong interaction between the molecular dipole and the electric field leads to a different understanding of how water diffuses on a metal surface. The proposed constant-$\mu_{\rm e}$ scheme is a realistic tool for the simulation of reactions under bias voltage not only using STM but also at the electrochemical interface.

Lateral Force Microscopy Reveals the Energy Barrier of a Molecular Switch.

Copper phthalocyanine (CuPc) is a small molecule often used in organic light emitting diodes where it is deposited on a conducting electrode. Previous scanning tunneling microscopy (STM) studies of CuPc on Cu(111) have shown that inelastic tunneling events can cause CuPc to switch between a ground state and two symmetrically equivalent metastable states in which the molecule is rotated. We investigated CuPc on Cu(111) and Ag(111) with STM and lateral force microscopy (LFM). Even without inelastic events, the presence of the tip can induce rotations and upon closer approach, causes the rotated states to be favored. Combining STM measurements at various temperatures and LFM measurements, we show that the long-range attraction of the tip changes the potential energy landscape of this molecular switch. We can also determine the geometry of the rotated and ground states. We compare our observations of CuPc on Cu(111) to CuPc on Ag(111). On Ag(111), CuPc appears flat and does not rotate. Stronger bonding typically involves shorter bond lengths, larger shifts of energy levels, and structural stability. Although the binding of CuPc to Cu(111) is stronger than that on Ag(111), the nonplanar geometry of CuPc on Cu(111) is accompanied by two metastable states which are not present on the Ag(111) surface.

Anisotropic surface phonon dispersion of a deuterium-terminated Si(110)-(1 × 1) surface studied by high-resolution electron-energy-loss spectroscopy and first-principles calculations: Isotope effect

The surface phonon dispersion of a deuterium-terminated Si(110)-(1 × 1) surface [D:Si(110)-(1 × 1)] is investigated by using high-resolution electron-energy-loss spectroscopy (HREELS) and first-principles calculations based on the density functional theory (DFT) with the local density approximation (LDA). The characteristics of D:Si(110)-(1 × 1) are unique compared to those of H:Si(110)-(1 × 1) (Matsushita et al., 2015) in terms of the resolved vibrational modes. By the HREELS, one-dimensional surface phonons consisting of D–Si stretching vibrations are observed above the bulk-phonon band energy edge of 64.5 meV. Ten modes are observed below this value, classified as surface, surface resonant, and bulk phonons according to the calculated energy dispersion as well as the depth profile of spectral density and displacement vectors. In particular, five D–Si bending modes are observed out of the seven theoretically predicted modes. The bending modes are strongly coupled with the displacements across the D and five Si layers. The DFT-LDA surface phonon dispersion is in good agreement with the experimental results except a few frequency/dispersion mismatches, as the structure optimized by DFT-LDA mismatches with the previous scanning tunneling microscopy (STM) results (Matsushita et al., 2015). D:Si(110)-(1 × 1) elucidates the nature of covalently bonded phonons and their characteristics both experimentally and theoretically.

Self-limited growth of an oxyhydroxide phase at the Fe3O4(001) surface in liquid and ambient pressure water.

Atomic-scale investigations of metal oxide surfaces exposed to aqueous environments are vital to understand degradation phenomena (e.g., dissolution and corrosion) as well as the performance of these materials in applications. Here, we utilize a new experimental setup for the ultrahigh vacuum-compatible dosing of liquids to explore the stability of the Fe3O4(001)-(√2 × √2)R45° surface following exposure to liquid and ambient pressure water. X-ray photoelectron spectroscopy and low-energy electron diffraction data show that extensive hydroxylation causes the surface to revert to a bulklike (1 × 1) termination. However, scanning tunneling microscopy (STM) images reveal a more complex situation, with the slow growth of an oxyhydroxide phase, which ultimately saturates at approximately 40% coverage. We conclude that the new material contains OH groups from dissociated water coordinated to Fe cations extracted from subsurface layers and that the surface passivates once the surface oxygen lattice is saturated with H because no further dissociation can take place. The resemblance of the STM images to those acquired in previous electrochemical STM studies leads us to believe that a similar structure exists at the solid-electrolyte interface during immersion at pH 7.

Electrical Doping Effect of Vacancies on Monolayer MoS2

Doping of transition-metal dichalcogenides (TMDCs) is an effective way to tune the Fermi level to facilitate the band engineering required for different types of devices. For TMDCs, a controversy abounds with regard to the doping role played by vacancy-type defects. Here, we report a detailed study based on first-principles calculations proposing that the native sulfur vacancies (VS) can significantly alter the electrical doping level in MoS2 and tune the material to exhibit conventional n- or p-type semiconductor characteristics. In particular, we reveal that the lower concentration of the single VS (2.8 and 6.3%) yields p-type characteristics, whereas the higher concentration of the single VS or a cluster of VS (12.5, 18.8, and 25.0%) yields n-type characteristics. The trend is consistent with previous X-ray photoelectron spectroscopy and scanning tunneling microscopy results. Employing this method of tuning the electron doping level, we modeled a commonly used metal–semiconductor interface to demonstra...

Electronic states realized by the interplay between Li diffusion and Co3+/Co4+ charge ordering in LixCoO2

Measurements of dc magnetization $(M)$ and electrical resistivity $(\ensuremath{\rho})$ have been carried out as a function of temperature $(T)$ for layered oxide ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2} (0.51\ensuremath{\le}x\ensuremath{\le}1.0)$ using single-crystal specimens. After slow cooling of the specimens down to 10 K, both of the $M(T)$ and $\ensuremath{\rho}(T)$ curves are found to exhibit a clear anomaly due to the occurrence of ${\mathrm{Co}}^{3+}/{\mathrm{Co}}^{4+}$ charge ordering (CO) at ${T}_{\mathrm{S}}\ensuremath{\sim}155\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ for $0.6\ensuremath{\le}x\ensuremath{\le}0.98$ (at ${T}_{\mathrm{S}}\ensuremath{\sim}180--190\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ for $0.5\ensuremath{\le}x\ensuremath{\le}0.55)$. After rapid cooling of the specimens, additional anomalies are observed related to the onset of Li diffusion at ${T}_{\mathrm{F}1}\ensuremath{\sim}370\phantom{\rule{0.28em}{0ex}}\mathrm{K}$ and/or ${T}_{\mathrm{F}2}=120--130\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. Due to phase mixing with compositions of nearly ${\mathrm{LiCoO}}_{2}$ and ${\mathrm{Li}}_{2/3}{\mathrm{CoO}}_{2}$, the specimens with $0.7\ensuremath{\lesssim}x\ensuremath{\lesssim}0.9$ show anomalies both at ${T}_{\mathrm{F}1}$ and ${T}_{\mathrm{F}2}$. For $0.6\ensuremath{\lesssim}x\ensuremath{\lesssim}0.9$, the resistivity measured after rapid cooling is found to be fairly larger than that measured after slow cooling below ${T}_{\mathrm{S}}$. The enhanced resistivity can be explained by the scenario that disordered ${\mathrm{Co}}^{3+}/{\mathrm{Co}}^{4+}$ arrangements, which have been observed and revealed to have an insulating electronic structure contrasting to the regular CO state in the previous scanning tunneling microscopy measurements [K. Iwaya et al., Phys. Rev. Lett. 111, 126104 (2013)], are realized due to the formation of an amorphouslike structure of Li ions after rapid cooling via interlayer Coulomb coupling. An electronic phase diagram for $0.5\ensuremath{\le}x\ensuremath{\le}1.0$ is proposed.

Influence of the State of the Tungsten Tip on STM Topographic Images of SnSe Surfaces

Tin selenide (SnSe) has recently attracted significant attention because of its excellent thermoelectric properties with a figure of merit (ZT) of 2.6. Previous scanning tunneling microscopy (STM) studies of SnSe surfaces showed that only Sn atoms are resolved in topographic images due to the dominant contribution of the Sn 5pz states in tunneling. However, when the state of the tungsten (W) tip changes from a typical four-lobe d state such as d xy or $${d_{{x^2} - {y^2}}}$$ to a two-lobe $${d_{{z^2}}}$$ state, the atomic features observed on the SnSe surface in STM topography can be dramatically altered. In this report, we present the results of a systematic study on the influence of the W tip’s states on the STM images of SnSe surfaces. Sn atoms are observed with much stronger corrugation amplitude and smaller apparent radius when the tip is in a $${d_{{z^2}}}$$ state. In addition, the atomic features of the Se atoms become visible because of the sharply focused shape of the W $${d_{{z^2}}}$$ state. We expect our results to provide important information for establishing a better understanding of the microscopic nature of SnSe surfaces.

Electronic structure and polar catastrophe at the surface of LixCoO2 studied by angle-resolved photoemission spectroscopy

We report an angle-resolved photoemission spectroscopy (ARPES) study of ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ single crystals which have a hole-doped ${\mathrm{CoO}}_{2}$ triangular lattice. Similar to ${\mathrm{Na}}_{x}{\mathrm{CoO}}_{2}$, the Co $3d\phantom{\rule{4pt}{0ex}}{a}_{1g}$ band crosses the Fermi level with strongly renormalized band dispersion while the Co $3d\phantom{\rule{4pt}{0ex}}{e}_{g}^{\ensuremath{'}}$ bands are fully occupied in ${\mathrm{Li}}_{x}{\mathrm{CoO}}_{2}$ ($x=0.46$ and 0.71). At $x=0.46$, the Fermi surface area is consistent with the bulk hole concentration indicating that the ARPES result represents the bulk electronic structure. On the other hand, at $x=0.71$, the Fermi surface area is larger than the expectation which can be associated with the inhomogeneous distribution of Li reported in the previous scanning tunneling microscopy study by Iwaya et al. [Phys. Rev. Lett. 111, 126104 (2013)]. However, the Co $3d$ peak is systematically shifted towards the Fermi level with hole doping excluding phase separation between hole rich and hole poor regions in the bulk. Therefore, the deviation of the Fermi surface area at $x=0.71$ can be attributed to hole redistribution at the surface avoiding polar catastrophe. The bulk Fermi surface of Co $3d\phantom{\rule{4pt}{0ex}}{a}_{1g}$ is very robust around $x=0.5$ even in the topmost ${\mathrm{CoO}}_{2}$ layer due to the absence of the polar catastrophe.

Chemisorption of a Strained but Flexible Molecule: Cyclooctyne on Si(001).

The adsorption characteristics of a promising system for hybrid organic-inorganic interfaces, cyclooctyne on Si(001), is analyzed using density functional theory. The chemisorbed 'on-top' configuration, where a cycloadduct is formed between the ring triple bond and a surface dimer, is shown to be most stable. Less stable are 'bridge' and 'sublayer' modes featuring two molecule-surface bonds and the 'pedestal' mode with four bonds. Investigations with our recently proposed periodic energy decomposition analysis (pEDA) reveal that the four-bond configuration is destabilized by large deformation energies needed within molecule and surface as well as rather weak molecule-surface bonds. Dispersion interactions show significant influence on energy and structure of the configurations leading to an increased bending of the rather flexible molecules. Thus, features found in previous scanning tunneling microscopy experiments are conclusively explained with bent 'on-top' configurations and the 'pedestal' mode can be ruled out. A comparison to acetylene shows that the ring structure and the resulting strain of cyclooctyne are responsible for an increased reactivity of the larger adsorbate due to a pre-forming of the ring triple bond for surface bonding. In contrast, ring strain leads only to negligible electronic effects on the adsorbate-surface bonds. The computations highlight the need for in-depth theoretical analysis to understand adsorption characteristics of large, flexible molecules.

Multiple charge density wave states at the surface of TbTe3

We studied TbTe$_{3}$ using scanning tunneling microscopy (STM) in the temperature range of 298 - 355 K. As seen in previous STM measurements on RTe$_{3}$ compounds, our measurements detect a unidirectional charge density wave state (CDW) in the surface Te-layer with a wavevector consistent with that of the bulk, q$_{cdw}$ = 0.30 $\pm$ 0.01c$^{*}$. However, unlike previous STM measurements, and differing from measurements probing the bulk, we detect two perpendicular orientations for the unidirectional CDWs with no directional preference for the in-plane crystal axes (a- or c-axis) and no noticeable difference in wavevector magnitude. In addition, we find regions in which the bidirectional CDW states coexist. We propose that observation of two CDW states indicates a decoupling of the surface Te-layer from the rare-earth block layer below, and that strain variations in the Te surface layer drive the local CDW direction to the specific unidirectional or, in rare occurrences, bidirectional CDW orders observed. This indicates that similar driving mechanisms for CDW formation in the bulk, where anisotropic lattice strain energy is important, are at play at the surface. In our bias-dependent measurements, we find no contrast inversion for the CDW state between occupied and empty states. This finding differs from other quasi 2-dimensional materials containing a hidden 1-dimensional character which leads to a favorable Fermi surface nesting scenario. Our temperature-dependent measurements provide evidence for localized CDW formation above the bulk transition temperature, T$_{cdw}$.

Interaction of Gold with a Pinwheel TiO∼1.2 Film Formed on Rh(111) Facet: STM and DFT Studies

The atomic structure of “pinwheel” TiO∼1.2 ultrathin oxide (w-TiO-UTO) layer and its reaction with gold are studied by scanning tunneling microscopy (STM) imaging and density functional theory (DFT) calculations. The UTO film was formed as an encapsulation layer on the top facet (111) of stripe-like Rh nanoparticles supported on a TiO2(110) substrate. For proposing a structural model, the previous STM, photoelectron (XPS), and ion scattering spectroscopy (LEIS) results were also taken into account. DFT calculations were carried out within the generalized gradient approximation (GGA-PBE) in the frame of the Quantum Espresso code. A Rh(111) slab of four layers with a TiO1.14 overlayer and a Rh–Ti–O stacking sequence were used. In the starting model, the ratio between hcp and fcc sites filled with Ti atoms was 1.54 (the same value for O atoms was 2.2) on the top of Rh layers. The simulation of the STM images of the relaxed structure was done following the Tersoff–Hamann approximation. The main structural cha...

Interaction of CO with PtxAg1-x/Pt(111) surface alloys: More than dilution by Ag atoms

We have investigated CO adsorption on structurally well-defined PtxAg1-x/Pt(111) surface alloys, combining temperature-programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS) as well as density functional theory (DFT) based calculations. This is part of a systematic approach including previous studies of CO adsorption on closely related Pt(111)- and Pd(111)-based surface alloys. Following changes in the adsorption properties with increasing Ag content and correlating them with structural changes allow us to assign desorption features to specific adsorption sites/ensembles identified in previous scanning tunneling microscopy (STM) measurements, and thus to identify and separate contributions from different effects such as geometric ensemble effects and electronic ligand/strain effects. DFT calculations give further insight into the nature of the metal–CO bond on these bimetallic sites. Most prominently, the growth of a new CO desorption feature at higher temperature (~550K) in the TPD spectra of Ag-rich surface alloys, which is unique for the group of Pt(111)- and Pd(111)-based surface alloys, is attributed to CO adsorption on Pt atoms surrounded by a Ag-rich neighborhood. Adsorption on these sites manifests in an IR band down-shifted to significantly lower wave number. Systematic comparison of the present results with previous findings for CO adsorption on the related Pt(111)- and Pd(111)-based surface alloys gains a detailed insight into general trends in the adsorption behavior of bimetallic surfaces.

Structural, electronic and magnetic properties of Mn3N2(0 0 1) surfaces

Spin-polarized first-principles total energy calculations have been performed to study the structural, electronic and magnetic properties of Mn3N2(001) surfaces. It is found that three surface terminations are energetically stable, in agreement with previous scanning tunneling microscopy experiments that have found three different electronic contrasts in their images. It is also found that in all three cases, the topmost layer has a MnN stoichiometry. Density of states calculations show a metallic behavior for all the stable structures with the most important contribution close to the Fermi level coming from the Mn-d orbitals. Our Tersoff–Hamann scanning tunneling microscopy simulations are in good agreement with previous experimental results.

Theoretical Study on the Upper Critical Field of a Layered Superconductor NbSe $$_2$$ 2

Based on two-band Ginzburg-Landau theory, we study the temperature and angular dependences of the upper critical field for superconducting NbSe $$_2$$ crystals. We introduce two variational parameters in our approach, and provide the explicit equations to determine the upper critical field in arbitrary direction. Our analyses strongly support previous scanning tunneling microscopy results and specific heat data, pointing to the existence of two energy gaps in this compound. Our results thus suggest that besides MgB $$_{2}$$ , NbSe $${_2}$$ is another example of two-gap superconductors.

Helium Atom Scattering from Graphene Grown on Rh(111)

Graphene growth on transition-metal surfaces leads to an ordered superlattice that appears as a series of hills and valleys in scanning tunneling microscopy. On Rh(111), the graphene forms an incommensurate epitaxial overlayer; the crystallographic axes of the graphene are closely aligned with the Rh(111) surface lattice vectors, but the Rh and graphene lattice constants are significantly different. Thus, the C atoms sit over different positions of the Rh(111) lattice and their height above the Rh(111) surface varies, but the different binding sites also affect the local electronic structure. Scanning tunneling microscopy is affected by both of these factors. He scattering is more a measure of the geometrical corrugation, the variation of the height of the graphene sheet relative to the Rh(111) surface. We compare the results of He diffraction measurements with those of previous scanning tunneling microscopy measurements to assess the relative geometric and electronic corrugations.