Search

We study structural and electronic properties of graphene grown on silicone carbide (SiC) substrate using a scanning tunneling microscope, spot-profile-analysis low-energy electron diffraction, and angle-resolved photoemission spectroscopy. We find several new replicas of Dirac cones in the Brillouin zone. Their locations can be understood in terms of a combination of basis vectors linked to SiC 6 $\ifmmode\times\else\texttimes\fi{}$ 6 and graphene $6\sqrt{3}\ifmmode\times\else\texttimes\fi{}6\sqrt{3}$ reconstruction. Therefore, these new features originate from the moir\'e caused by the lattice mismatch between SiC and graphene. More specifically, Dirac cone replicas are caused by underlying weak modulation of the ionic potential by the substrate that is then experienced by the electrons in the graphene. We also demonstrate that this effect is equally strong in single- and trilayer graphene; therefore, the additional Dirac cones are intrinsic features rather than the result of photoelectron diffraction. These new features in the electronic structure are very important for the interpretation of recent transport measurements and can assist in tuning the properties of graphene for practical applications.

A novel type of very fast nucleation was recently found in Pb/Si(111) with 4- to 7-layer high islands becoming crystalline in an “explosive” way, when the Pb deposited amount in the wetting layer is compressed to θc ~ 1.22 ML, well above the metallic Pb(111) density. This “explosive” nucleation is very different from classical nucleation when island growth is more gradual and islands grow in size by single adatom aggregation [8]. In order to identify the key parameters that control the nucleation we used scanning tunneling microscopy (STM) and spot profile analysis low energy electron diffraction (SPA-LEED). It was found that the number and duration of steps in iterative deposition used to approach θc and the flux rate have dramatic effects on the crystallization process. Larger depositions over shorter times induce greater spatial coverage fluctuations, so local areas can reach the critical coverage θc easier. This can trigger the collective motion of the wetting layer from far away to build the Pb islands “explosively”. The SPA-LEED experiments show that even low flux experiments in iterative deposition experiments can trigger transfer of material to the superstable 7-layer islands, as seen from the stronger satellite rings close to the (00) spot.

One of the goals for growing low dimensional structures is to explore their electronic structure variation with size due to confinement. Recent experiments in Pb adsorption on Pb islands of stable and unstable height at low temperatures have shown a spectacular variation in the nucleated island density (∼60 times) as a function of height. With Monte Carlo simulations we can faithfully reproduce the observed morphologies with a single assumption that i c is different on stable and unstable islands. This shows that confinement can result in large oscillations of nucleation rates and adsorption.

The nanoscale hexagonal pattern observed in scanning tunneling microscopy (STM) for 3-layer and 4-layer Pb islands on Si(1 1 1) is studied theoretically. We found that besides thickness the atomic rearrangement at the Pb/Si interface plays an important role in determining the STM patterns. Electronic structures of the Pb film on Si(1 1 1) obtained from fully relaxed and unrelaxed Pb films are qualitatively different. Simulated STM images for Pb films with different stacking also show that the corrugation patterns are sensitive to the buried Pb–Si interfacial structure.

The T – θ phase diagram for the system Pb/Si(1 1 1) was determined in the coverage range 6/5 ML θ

The formation of numerous (∼15) phases within a narrow coverage range 6/5 θ T ∼120 K which indicates an unusual pathway of low temperature self-organization.

The comparison between measured vs calculated electronic energy levels in quantum well states (QWS) depends on the dimension of the potential well confining the electrons. The dimensions of the well can be determined with structural probes (high resolution LEED and STM) while the electronic structure can be obtained with angle resolved photoemission and STM spectroscopy. For heteroepitaxial growth the dimension of the well depends on the thickness of the wetting layer between the island and the substrate. For the Pb/Si(1 1 1)-7 × 7 system using SPA-LEED and STM, we have measured the thickness of the layer underneath the islands and the substrate to be 1 ML at all temperatures. For Pb films grown on top of the Si(1 1 1)-7 × 7 at low temperatures T

Classically, the onset of nucleation is defined in terms of a critical cluster of the condensed phase, which forms from the gradual aggregation of randomly diffusing adatoms. Experiments in Pb/Si(111) at low temperature have discovered a dramatically different type of nucleation, with perfect crystalline islands emerging "explosively" out of the compressed wetting layer after a critical coverage Θ_{c}=1.22 ML is reached. The unexpectedly high island growth rates, the directional correlations in the growth of neighboring islands and the persistence in time of where mass is added in individual islands, suggest that nucleation is a result of the highly coherent motion of the wetting layer, over mesoscopic distances.

When the thickness of ultrathin metal films approaches the nanometer scale comparable to the coherence length of the electrons, significant effects on the structure stability and the electronic properties of the metal films emerge due to electron confinement and quantization of the allowed electronic states in the direction perpendicular to the film. Using first-principles calculations, we showed that such quantum size effects can induce oscillatory electrostatic potential and thus alternating electric field on the surface of the wedge-shaped Pb(111) films. The alternating electric field has significant influence on surface reactivity, leading to selective even- or odd-layer adsorption preference depending on the charge state of the adatoms, consistent with the odd-layer preference of higher Mg coverage on wedge-shaped Pb(111) films, as observed in experiment.

Self-organized, uniform-height, Pb islands with flat tops and steep edges form on Si(111)-(7 x 7) at low temperatures 120

We have observed the dependence of the scanning tunneling microscopy (STM) imaged atom intensity within the $(7\ifmmode\times\else\texttimes\fi{}7)$ unit cell on stepped Si(111) as a function of the tunneling voltage. Pronounced differences from the corresponding atom intensity on the flat surface are observed for the contrast of atoms on the low versus the high side of the step and for the contrast between the faulted versus unfaulted subcells of the $(7\ifmmode\times\else\texttimes\fi{}7)$ structure. These differences can be accounted for by changes in the electronic structure within the $(7\ifmmode\times\else\texttimes\fi{}7)$ subcells adjacent to the step. Calculations of the local density of states and the STM images using a tight-binding method are in excellent agreement with the experimental results.

We performed an experiment to study the thermal stability of metal on graphene. We show that Fe, Gd, Dy, and Eu on graphene exhibit island growth morphology. The 3D islands of Fe, Gd, and Dy show a small decrease in island density and an increase in the height/width aspect ratio upon thermal annealing. By contrast, coarsening of Eu islands to a close flat film is observed after annealing to 365 K. By investigating the fundamental interactions (i.e., adsorption energies and diffusion barriers) between the metal adatoms and graphene using ab initio calculations, we predict that most of the 3$d$ and group 10 transition metals, noble metals, as well as rare earth metals on graphene should exhibit a 3D growth mode as observed in experiment. Most of the metal nanostructures on graphene should also be stable against aggregation. The 3D morphology and strong thermal stability of Fe, Co, Ni, Pt, and Gd islands on graphene can serve as good candidates for surface-supported catalysis applications.

The understanding of metal nucleation on graphene is essential for promising future applications, especially of magnetic metals which can be used in spintronics or computer storage media. A common method to study the grown morphology is to measure the nucleated island density n as a function of growth parameters. Surprisingly, the growth of Fe on graphene is found to be unusual because it does not follow classical nucleation: n is unexpectedtly high, it increases continuously with the deposited amount θ and shows no temperature dependence. These unusual results indicate the presence of long range repulsive interactions. Kinetic Monte Carlo simulations and density functional theory calculations support this conclusion. In addition to answering an outstanding question in epitaxial growth, i.e., to find systems where long range interactions are present, the high density of magnetic islands, tunable with θ, is of interest for nanomagnetism applications.

The nucleation and growth of Fe on graphene is highly unusual. A constantly increasing island density indicates the presence of strong, predominantly repulsive, adatom interactions. We study these interactions by first-principles calculations to identify their origin. We find that the interactions consist of a short-range attraction and longer-range repulsion. We show that electric dipole-dipole interaction can contribute only part of the repulsive force (\ensuremath{\sim}30$%$) between Fe adatoms, and the repulsion due to the elastic interaction is small. We suggest that the dominant contribution to the repulsive energy would originate from the indirect (or RKKY) interactions mediated through the delocalized electrons on graphene.

Adsorption of rare-earth (RE) adatoms (Nd, Gd, Eu, and Yb) on graphene was studied by first-principles calculations based on the density-functional theory. The calculations show that the hollow site of graphene is the energetically favorable adsorption site for all the RE adatoms studied. The adsorption energies and diffusion barriers of Nd and Gd on graphene are found to be larger than those of Eu and Yb. Comparison with scanning tunneling microscopy experiments for Gd and Eu epitaxially grown on graphene confirms these calculated adsorption and barrier differences, since fractal-like islands are observed for Gd and flat-topped crystalline islands for Eu. The formation of flat Eu islands on graphene can be attributed to its low diffusion barrier and relatively larger ratio of adsorption energy to its bulk cohesive energy. The interactions between the Nd and Gd adatoms and graphene cause noticeable in-plane lattice distortions in the graphene layer. Adsorption of the RE adatoms on graphene also induces significant electric dipole and magnetic moments.

Scanning tunneling microscopy experiments reveal that small Pb islands with unstable heights, e.g., four layers, on a Si(111) surface decay during coarsening, whereas large islands do not decay but grow to a stable height. This bifurcation in evolution is analyzed by incorporating quantum size effects into theoretical models for island growth dynamics with appropriate geometries. The effective energy barrier for Pb atoms to reach the top of four-layer islands is estimated at about 0.26 eV.

Surface x-ray scattering and scanning-tunneling microscopy experiments reveal novel coarsening behavior of Pb nanocrystals grown on $\mathrm{Si}(111)\mathrm{\text{\ensuremath{-}}}(7\ifmmode\times\else\texttimes\fi{}7)$. It is found that quantum size effects lead to the breakdown of the classical Gibbs-Thomson analysis. This is manifested by the lack of scaling of the island densities. In addition, island decay times $\ensuremath{\tau}$ are orders of magnitude faster than expected from the classical analysis and have an unusual dependence on the growth flux $F$ (i.e., $\ensuremath{\tau}\ensuremath{\sim}1/F$). As a result, a highly monodispersed 7-layer island height distribution is found after coarsening if the islands are grown at high rather than low flux rates. These results have important implications, especially at low temperatures, for the controlled growth and self-organization of nanostructures.

The growth of uniform nanostructures requires discovering robust and reproducible ways to control the grown size and geometry. It is found that the growth of Pb on the anisotropic substrate Si(111)-In(4x1) leads to the control of both the island height and island width by exploiting the combined effects of two different stabilization mechanisms. Quantum size effects control the height and result in uniform four-layer islands while the strain anisotropy, due to the underlying reconstruction, controls the width selection.

The corrugation observed on top of Pb islands grown on $\mathrm{Si}(111)\text{\ensuremath{-}}\mathrm{Pb}\text{\ensuremath{-}}\ensuremath{\alpha}\text{\ensuremath{-}}\ensuremath{\surd}3\ifmmode\times\else\texttimes\fi{}\ensuremath{\surd}3$ (as a result of the Moire pattern formation at the $\mathrm{Si}∕\mathrm{Pb}$ interface) offers a different way to obtain information about the interface. Results from several probes (high resolution LEED, STM, STS, model image calculations) are compared. The origin of the corrugation is predominantly geometric (because of layer relaxation in the island) and to a lesser degree electronic (related to the confined electrons within the island). This is deduced from the hexagonal ringlike features in LEED and the quantitative comparison of the corrugation contrast in calculated vs measured STM and STS images.

The ``devil's staircase'' (DS) is one of the outstanding phase diagrams in physics because it shows a high degree of self-organization driven by a repulsive long-range adatom interaction. An infinite number of phases are predicted to exist which are built hierarchically with simple combinatorial rules. It was found with diffraction and scanning tunneling microscopy that a DS exists in Pb/Si (111) in the range $1.2l\ensuremath{\theta}l1.33$ monolayers, surprisingly at $\ensuremath{\sim}120\mathrm{K},$ and with phases which extend spatially over macroscopic distances $(\ensuremath{\sim}0.5\mathrm{mm}).$ The extraordinary amount of atom rearrangement necessary for these phases to form indicates an unexpected degree of self-organization at low temperatures.

With scanning tunneling microscopy we have found that ordered phases in $\mathrm{P}\mathrm{b}/\mathrm{S}\mathrm{i}(111)$ are one of the best examples of the ``devil's staircase'' phase diagram. Phases within a narrow coverage range ($1.2l\ensuremath{\theta}l1.3\text{ }\mathrm{\text{monolayers}}$) are constructed with the rules similar to the ones found in theoretical models.

Despite extensive work in the literature, the structure of the dense $\mathrm{Si}(111)\ensuremath{-}\mathrm{Pb}(\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3})$ phase (ideal coverage $\ensuremath{\theta}=4/3\mathrm{ML})$ is still unknown because a variety of phases can form, within few percent coverage variation. These phases differ in the way equivalent $\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ domains self-organize in different patterns with different domain-wall arrangements. With scanning tunneling microscopy and first-principles calculations we have identified the atomic structure within domains, domain-wall arrangement, and the binding site (H3 or T4) occupied by Pb atoms within each domain.

We have observed the formation of highly regular networks of Pb clusters, which extend up to 100 nm in linear size, during the growth of Pb on the $\mathrm{Si}(111)\ensuremath{-}\mathrm{Pb}\ensuremath{\alpha}\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ phase at low temperatures $115\mathrm{K}lTl250\mathrm{K}.$ The cluster networks are a metastable phase before the first preferred height of two-step Pb islands form as a result of quantum size effects. The cluster networks provide an easy kinetic pathway for the buildup of the two-step islands. The driving force for the formation of the cluster networks is most likely the long range elastic interaction between the Pb atoms mediated through the strained substrate.

Uniform-height islands, with preferred heights differing by bilayer height increments, can be grown on Pb/Si(111) at low temperatures most likely as a result of quantum size effects. With scanning-tunneling-microscope spectroscopy we have determined how the electronic structure of individual islands is related to their stability. Differences between preferred vs nonpreferred island heights are seen at the position of the Fermi level with respect to the highest occupied band and lowest unoccupied band. This difference is supported from oscillations of the measured apparent barrier $\ensuremath{\Delta}(\mathrm{ln}I)/\ensuremath{\Delta}z$ with island height.