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We systematically study the effects of Gd adsorption and intercalation on the electronic band structure of double-layer epitaxial graphene on Si-terminated SiC(0001) by first-principles calculations. We show that Gd adsorption and intercalation exhibit strong effects on the coupling between the graphene layers and between the buffer layer and substrate. Different adsorption/intercalation geometries can result in very different electron band structures. The number of Dirac cones and the positions of the Dirac cones relative to the Fermi level can be effectively manipulated through controlling the Gd adsorption/intercalation geometries. Our calculations provide useful insights to guide the experimental design of graphene-based materials with desirable functionalities for applications.

We have determined the magnetic properties of epitaxially grown Dy islands on graphene/SiC(0001) that are passivated by a gold film (deposited in the ultra-high vacuum growth chamber) for ex-situ X-ray magnetic circular dichroism (XMCD). Our sum-rule analysis of the Dy M 4 , 5 XMCD spectra at low temperatures ( T = 15 K) as a function of magnetic field assuming Dy 3 + (spin configuration 6 H 15 / 2 ) indicate that the projection of the magnetic moment along an applied magnetic field of 5 T is 3.5(3) μ B . Temperature dependence of the magnetic moment (extracted from the M 5 XMCD spectra) shows an onset of a change in magnetic moment at about 175 K in proximity of the transition from paramagnetic to helical magnetic structure at T H = 179 K in bulk Dy. No feature at the vicinity of the ferromagnetic transition of hcp bulk Dy at T c = 88 K is observed. However, below ∼130 K, the inverse magnetic moment (extracted from the XMCD) is linear in temperature as commonly expected from a paramagnetic system suggesting different behavior of Dy nano-island than bulk Dy.

Recent experiments indicate that metal intercalation is a very effective method to manipulate the graphene-adatom interaction and control metal nanostructure formation on graphene. A key question is mass transport, i.e., how atoms deposited uniformly on graphene populate different areas depending on the local intercalation. Using first-principles calculations, we show that partially intercalated graphene, with a mixture of intercalated and pristine areas, can induce an alternating electric field because of the spatial variations in electron doping, and thus, an oscillatory electrostatic potential. This alternating field can change normal stochastic adatom diffusion to biased diffusion, leading to selective mass transport and consequent nucleation, on either the intercalated or pristine areas, depending on the charge state of the adatoms.

Intercalation of rare earth metals ($RE$ = Eu, Dy, and Gd) is achieved by depositing the $RE$ metal on graphene that is grown on silicon-carbide (SiC) and by subsequent annealing at high temperatures to promote intercalation. STM images of the films reveal that the graphene layer is defect free and that each of the intercalated metals has a distinct nucleation pattern. Intercalated Eu forms nano-clusters that are situated on the vertices of a Moir{\`e} pattern, while Dy and Gd form randomly distributed nano-clusters. X-ray magnetic circular dichroism (XMCD) measurements of intercalated films reveal the magnetic properties of these $RE$'s nano-clusters. Furthermore, field dependence and temperature dependence of the magnetic moments extracted from the XMCD show paramagnetic-like behaviors with moments that are generally smaller than those predicted by the Brillouin function. XMCD measurements of $RE$-oxides compared with those of the intercalated $RE$'s under graphene after exposure to air for months indicate that the graphene membranes protect these intercalants against oxidation., Comment: 9 pages, 7 figures

Graphene, a single atomic layer of graphite, has been the focus of recent intensive studies due to its novel electronic and structural properties. Metals grown on graphene also have been of interest because of their potential use as metal contacts in graphene devices, for spintronics applications, and for catalysis. All of these applications require good understanding and control of the metal growth morphology, which in part reflects the strength of the metal–graphene bond. Also of importance is whether the interaction between graphene and metal is sufficiently strong to modify the electronic structure of graphene. In this review, we will discuss recent experimental and computational studies related to deposition of metals on graphene supported on various substrates (SiC, SiO 2 , and hexagonal close-packed metal surfaces). Of specific interest are the metal–graphene interactions (adsorption energies and diffusion barriers of metal adatoms), and the crystal structures and thermal stability of the metal nanoclusters.

X-ray magnetic circular dichroism (XMCD) reveal the magnetic properties of intercalated europium metal under graphene on SiC(0001). Intercalation of Eu nano-clusters (average size 2.5 nm) between graphene and SiC substate are formed by deposition of Eu on epitaxially grown graphene that is subsequently annealed at various temperatures while keeping the integrity of the graphene layer. Using sum-rules analysis of the XMCD of Eu M$_{4,5}$ edges at $T = 15$ K, our samples show paramagnetic-like behavior with distinct anomaly at T $\approx$ 90 K which may be related to the N{\`e}el transition, T$_N$ = 91 K, of bulk metal Eu. We find no evidence of ferromagnetism due to EuO or antiferromagnetism due to Eu$_2$O$_3$ indicating that the graphene layer protects the intercalated metallic Eu against oxidation over months of exposure to atmospheric environment., Comment: 6 pages, 5 figures

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.

Graphene, a single atomic layer of graphite, has been a material of recent intensive studies due to its novel electronic and structural properties and its potential applications in the emerging area of carbon-based electronic devices. Metal on graphene growth is one of the current research interests, aiming at improving and manipulating the electronic and magnetic properties of graphene through metal atom adsorption or doping to meet various requirements in device applications. In this paper, we will give an overview of recent experimental and computational investigation of interaction, growth morphology, and thermal stability of various metals on graphene grown on 6H-SiC(0001) substrate.

Using first-principles calculations we show that face-centered cubic Dy(111) ultrathin films exhibit height selective stability. The origin of such height selection can be attributed to the interplay between the localized surface states and surface resonance states due to electron confinement effects. Such effect could be utilized to manipulate the film thickness at the atomic level to achieve desirable film properties or to control the growth of nanostructures on the thin film for various 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.

The use of graphene for microelectronics and spintronic applications requires strategies for metals to wet graphene and to grow layer-by-layer. This is especially important when metals will be used as electrical contacts or as spin filters. Extensive work in the literature so far has shown that this is very challenging, since practically all metals grow 3D, with multi-height islands forming easily. Reasons for the 3D morphology are the much weaker metal carbon bond when compared to the metal cohesive energy and the role of Coulomb repulsion of the poorly screened charges at the metal graphene interface. We employed the complementary techniques of SPA-LEED and STM to study the growth of Dy on graphene. It was found that under kinetic limitations it is possible to fully cover graphene with a bilayer Dy film, by growing well below room temperature in stepwise deposition experiments. The Dy film, however, is amorphous but ways to crystallize it within the 2D morphology are possible, since long range order improves at higher growth temperature.

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.

We present a theoretical analysis of selectivity of nucleation location for the two-dimensional island on top of a metal nanomesa. It has been observed experimentally that the nucleation can start either along the periphery of the mesa top or in the middle, depending on the mesa height. Such an intriguing nucleation behavior is shown to originate from the thickness-dependent mesa edge barrier for an adatom to jump off the mesa, which we attributed to be induced by the quantum size effect. Based on the experimentally observed nucleation locations, we estimate that the mesa edge barriers for the 5- and 6-layer Pb(1 1 1) mesas can differ by ∼19 ± 5 meV.

A theoretical model is proposed to describe the rapid coarsening observed for Pb islands on a Si(1 1 1) surface where classical kinetics breaks down. In this system, quantum size effects produce mesa-like Pb islands with chemical potentials depending strongly on their heights, in addition to the usual dependence on the step curvature. Furthermore, a dense wetting layer enables fast mass transport between islands. Incorporating these features, our theoretical model predicts evolution of the island height distribution in good agreement with experiments.

Surface X-ray scattering and scanning–tunneling microscopy experiments, performed in situ, are used to investigate the influence of quantum size effects on the growth of Pb on the Si(111)7×7 surface. It is demonstrated that the structure of Pb islands and the associated wetting layer are consistent with the effects of quantum confinement. Moreover, these effects lead to novel behavior for the coarsening evolution of the Pb islands. For example, Ostwald ripening is not observed due to a breakdown of the classical Gibbs–Thomson effect. An unanticipated strong dependence on the deposition flux rate is found and we measure island decay times that are orders of magnitude faster than predicted. These results have important implications for understanding and controlling the growth of nanostructures.

A novel growth low temperature morphology is observed in the growth of Pb/Si(1 1 1) related to the kinetics of Quantum Size effects. This morphology is globally observed over mesoscopic distances. Bilayer height rings nucleate first at the island perimeter which spread out towards the center. With Monte Carlo simulations the morphology is reproduced with highly anisotropic diffusion (i.e., radial diffusion out of a ring of one lattice constant is slower than azimuthal diffusion by at least 10−5 at 180 K). This shows that both kinetics and energetics are responsible for the uniform height island formation and the similar temperature “window” ∼180 K independent of the initial Si interface.

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

Surface diffusion is the key process responsible for mass transport on surfaces. Experiments can be carried out to measure surface diffusion either under equilibrium or non-equilibrium conditions. Equilibrium experiments can be used to measure the coverage and temperature dependence of the diffusion coefficient D c ( θ , T ) which in turn is useful to deduce thermodynamic information about the system (adatom–adatom interactions, phase transitions, collective effects, etc.). Non-equilibrium experiments are more relevant to systems grown epitaxially because of the lower temperatures and higher deposition rates commonly used. Under these conditions metastable self-organized structures can be prepared within surprisingly short times, which suggests that some novel but still unclear kinetic pathways must exist at low temperatures. Such effects will be illustrated with the formation of uniform height Pb islands on Si ( 1 1 1 ) because of quantum size effects (QSE) and the formation of numerous “Devil's Staircase” phases in the dense Pb / Si ( 1 1 1 ) - α 3 × 3 phase. In addition, the failure of the quasi-equilibrium description of nanostructure relaxation for sufficiently small sizes will be discussed to stress the need for better knowledge of the kinetic barriers controlling the relaxation.

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

We have studied how oxygen adsorbed on top of the uniform height Pb islands, (grown on Si(1 1 1)-(7×7) at low temperatures Tl

X-ray magnetic circular dichroism (XMCD) measurements of iron nano-islands grown on graphene and covered with a Au film for passivation reveal that the oxidation through defects in the Au film spontaneously leads to the formation of magnetite nano-particles (i.e, $Fe_3$$O_4$). The Fe nano-islands (20 and 75 monolayers; MLs) are grown on epitaxial graphene formed by thermally annealing 6H-SiC(0001) and subsequently covered, in the growth chamber, with nominal 20 layers of Au. Our X-ray absorption spectroscopy and XMCD measurements at applied magnetic fields show that the thin film (20 ML) is totally converted to magnetite whereas the thicker film (75 ML) exhibits properties of magnetite but also those of pure metallic iron. Temperature dependence of the XMCD signal (of both samples) shows a clear transition at $T_{\rm V}\approx 120$ K consistent with the Verwey transition of bulk magnetite. These results have implications on the synthesis of magnetite nano-crystals and also on their regular arrangements on functional substrates such as graphene., Comment: 5 pages, 6 figures

Adsorption of Fe atom and ${\mathrm{Fe}}_{n}$ ($n=2\ensuremath{-}7$) clusters on graphene was studied by first-principles calculation and chemical bonding analysis. Various adsorption configurations, i.e., Fe clusters placed at different lateral positions and with different orientations with respect to the graphene lattice, have been optimized to locate the ground-state geometry of the Fe clusters on graphene. The calculation shows that Fe on graphene prefers a three-dimensional growth mode with a critical size of two atoms. The calculation also shows that ${\mathrm{Fe}}_{n}$ clusters on graphene exhibit ferromagnetic order and large magnetic moment. However, due to charge redistribution caused by the interaction with graphene, some ${\mathrm{Fe}}_{n}$ clusters on graphene have smaller magnetic moments compared to the corresponding free-standing clusters. Adsorption of ${\mathrm{Fe}}_{n}$ clusters also induces magnetic moments in graphene, and the induced magnetic moment on each carbon atom in graphene is correlated with the distortion of the graphene lattice.

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.

We show that strongly photoexcited graphene monolayers with 35 fs pulses quasi-instantaneously build up a broadband, inverted Dirac fermion population. Optical gain emerges and directly manifests itself via a negative conductivity at the near-infrared region for the first 200 fs, where stimulated emission completely compensates absorption loss in the graphene layer. Our experiment-theory comparison with two distinct electron and hole chemical potentials reproduce absorption saturation and gain at 40 fs, revealing, particularly, the evolution of the transient state from a hot classical gas to a dense quantum fluid with increasing the photoexcitation.

Adsorption of the alkali-, group-III, and 3$d$-transition-metal adatoms (Na, K, Al, In, V, Fe, Co, and Ni) on graphene was studied systematically by first-principles calculations. The bonding character and electron transfer between the metal adatoms and graphene were analyzed using the recently developed quasi-atomic minimal basis set orbitals (QUAMBOs) approach. The calculations showed that the interaction between alkali-metal adatoms and graphene is ionic and has minimal effects on the lattice and electronic states of the graphene layer, in agreement with previous calculations. For group-III metal adatom adsorptions, mixed covalent and ionic bonding is demonstrated. In comparison, 3$d$-transition-metal adsorption on graphene exhibits strong covalent bonding with graphene. The majority of the contributions to the covalent bonds are from strong hybridization between the ${d}_{{x}^{2}\ensuremath{-}{y}^{2}}$ and ${d}_{\mathit{yz}}$ orbitals of the 3$d$-transition-metal adatoms and ${p}_{z}$ orbitals of the carbon atoms. The strong covalent bonds cause large in-plane lattice distortions in the graphene layer. Charge redistributions upon adsorptions also induce significant electric dipole moments and affect the magnetic moments.

Graphene is an exciting material with numerous potential applications. To understand metal graphene interaction two different metals were studied. Two large Pb islands nucleate at 78K indicating fast diffusion and weak interaction(right). On the contrary, for Dysprosium a high island density is observed confirming slow diffusion and strong interaction(left).

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.

The unusual growth mode of uniform height islands discovered in Pb/Si was related to the electronic energy modulation with island height due to quantum size effects (QSEs). In addition to these energetic reasons provided by QSE, there is also the question of kinetics, i.e., how atoms move at relatively low temperatures (as low as 150 K) to build the islands in the short time of minutes. Controlled experiments with different techniques have shown the intriguing role of the dense wetting layer in transporting mass. STM experiments monitoring how unstable islands transform into stable islands have shown that the wetting layer between the islands moves selectively to the unstable islands, climbs over their sides, forms quickly rings of constant width ∼ 20 nm, and finally it completes the island top, but at a slower rate than the ring completion. This growth is independent of the starting interface, whether it is the amorphous wetting layer on the Si(111) (7 × 7) or the well-ordered Si(111)–Pb \(\alpha (\surd 3\times \surd 3)\) surface (except Pb diffusion on the latter interface is faster by a factor of ∼ 5). Real-time low-energy electron microscopy (LEEM) observations of mass transport phenomena have confirmed the fast mobility of the wetting layer in Pb/Si and in addition have revealed some unusual features that are unexpected from classical diffusion behavior. The experiment monitors the refilling of a circular vacant area generated by a laser pulse. The concentration profile does not disperse as in normal diffusion, the refilling speed \(\Delta x/\Delta t\) is constant (instead of \(\Delta x/\surd \Delta t = \mathrm{constant}\)), and the equilibration time diverges below a critical coverage, θ c, as \(1/\tau \sim (\theta _{\mathrm{c}} - \theta)^{-\kappa}\). The absolute value of the refilling speed 0.05 nm/s at 190 K is orders of magnitude higher than what is expected from Pb diffusion on Pb crystals at higher temperatures. These results are compared with predictions of three candidate models: (i) a conventional diffusion model with a step-like coverage-dependent diffusion coefficient \(D_{\mathrm{c}}(\theta)\), (ii) a model with mass transport due to adatoms on top of the wetting layer with coverage-dependent adatom vacancy formation energy, and (iii) the carpet unrolling mechanism proposed for other systems. None of these models can account for the unusual observations, which suggests that the wetting layer most likely enters a novel state of very high mobility for \(\theta > \theta _{\mathrm{c}}\), similar to a phase transition that needs to be better understood theoretically.

A power law dependence I1/3of the amplitude of femtosecond diffenrential relfecltivity in highly photo-excited graphene monolayer indicates three-particle decay, allowing for generation of dense Dirac fermions beyond phase space filling within 35 fs.

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.

We present femtosecond differential reflectivity studies of few-layer graphene from weak to strong excitation regime. Temporal profiles of the differential reflectivity exhibit strong non-exponential decay. A nonlinear, cubic root dependence of their peak amplitudes on the pump fluence is clearly observed under relatively high intensity excitation, which indicates three-particle decay of transient carrier population via Auger scattering. Our results identify the critical role of such three-particle scattering in the initial electronic relaxation in photo-excited graphene with increasing phase-space filling of the correlated Dirac Fermions.

Although quantum size effects in $\mathrm{Pb}∕\mathrm{Si}(111)$ control the selection of uniform island heights, the fast formation time (after deposition of few minutes at $\mathrm{T}\ensuremath{\sim}180\phantom{\rule{0.3em}{0ex}}\mathrm{K}$) has been puzzling so far. By studying the island growth in stepwise deposition experiments, we demonstrate the important role of the wetting layer in the kinetics. Pb is transferred to unstable islands, not by independent atom hops, but by the continuous spreading of the wetting layer from the surrounding area to the island top. As Pb is moved to the top in ring-like morphology the underlying $7\ifmmode\times\else\texttimes\fi{}7$ reconstruction in the surrounding area emerges gradually.

The formation of the uniform height Pb islands on Si(111) is unusually fast and it is important to understand the driving forces responsible for it. The growth to bigger heights proceeds by bilayer rings nucleating first at the island perimeter, which spread out toward the island center. The rings have sharp growth fronts and it is challenging to explain this unusual morphology with the same set of barriers for island heights of the same stability, since nucleation is restricted to 2 layers. With Monte Carlo simulations, this nucleation morphology is reproduced and the corresponding potential-energy surface (PES) identified. The key barriers of the PES are very fast terrace diffusion (with barrier less than $50\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$), highly anisotropic diffusion (i.e., the azimuthal diffusion within a one-lattice-constant ring is 1000 times faster than radial diffusion toward the island center), and interlayer diffusion barriers for stable versus unstable islands differing by $0.11\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ (for diffusion from the island top back to the wetting layer).

A realization of the numerous phases predicted in systems with long-range repulsive interactions was recently found in Pb/Si(111). Surprisingly, these numerous phases can be grown at low temperatures {approx}40 K over macroscopic distances. This unusual observation can be explained from theoretical calculations of the collective diffusion coefficient D{sub c} in systems with long-range repulsive interactions. Instead of a gradual dependence of D{sub c} on coverage, it was found that D{sub c} has sharp maxima at low temperatures for every stable phase (i.e., for every rational value of the coverage {theta}=p/q) in agreement with the experiment.

An intriguing growth morphology of Pb islands on a Si(111) surface is observed in our STM experiments: the growth of a Pb layer on Pb islands with unstable heights starts from the periphery and moves towards the center, while the nucleation of the next layer on stable Pb islands starts away from the periphery. Using first-principles total energy calculations, we have studied the diffusion barriers of Pb adatoms on a freestanding Pb(111) film as a function of film thickness. The diffusion barriers are found to be very low (

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.