Myron Hupalo

Femtosecond Population Inversion and Stimulated Emission of Dense Dirac Fermions in 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.

Uniform island height selection in the low temperature growth of Pb/Si(111)-(7×7)

Self-organized, uniform-height, Pb islands with flat tops and steep edges form on Si(111)-(7 x 7) at low temperatures 120<T<250 K. Islands of heights differing by bilayer height increments are observed depending on growth conditions. The formation of these structures is highly unusual since at low temperatures thermal diffusion is suppressed. The origin of the regular structures is believed to be quantum size effects (i.e. effects related to the quantization of the electron energy levels in the islands). We have studied with two complementary techniques (i.e. high resolution spot profile analysis low energy electron diffraction and variable temperature scanning tunneling microscopy) how the preferred island heights depend on the growth parameters (i.e. temperature, coverage, kinetic pathway etc.). We have constructed a kinetic phase diagram in the coverage-temperature plane which indicates the type of islands formed under different growth conditions. The phase diagram can be used as a guide so the island height can be easily controlled.

Metal Nanostructure Formation on Graphene: Weak versus Strong Bonding

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).

Strong metal adatom-substrate interaction of Gd and Fe with graphene.

Graphene is a unique 2D system of confined electrons with an unusual electronic structure of two inverted Dirac cones touching at a single point, with high electron mobility and promising microelectronics applications. The clean system has been studied extensively, but metal adsorption studies in controlled experiments have been limited; such experiments are important to grow uniform metallic films, metal contacts, carrier doping, etc. Two non-free-electron-like metals (rare earth Gd and transition metal Fe) were grown epitaxially on graphene as a function of temperature T and coverage θ. By measuring the nucleated island density and its variation with growth conditions, information about the metal-graphene interaction (terrace diffusion, detachment energy) is extracted. The nucleated island densities at room temperature (RT) are stable and do not coarsen, at least up to 400 °C, which shows an unusually strong metal-graphene bond; most likely it is a result of C atom rebonding from the pure graphene sp(2) C-C configuration to one of lower energy.

Growth of fcc(111) Dy multi-height islands on 6H-SiC(0001) graphene

Graphene based spintronic devices require an understanding of the growth of magnetic metals. Rare earth metals have large bulk magnetic moments so they are good candidates for such applications, and it is important to identify their growth mode. Dysprosium was deposited on epitaxial graphene, prepared by thermally annealing 6H-SiC(0001). The majority of the grown islands have triangular instead of hexagonal shapes. This is observed both for single layer islands nucleating at the top of incomplete islands and for fully completed multi-height islands. We analyze the island shape distribution and stacking sequence of successively grown islands to deduce that the Dy islands have fcc(111) structure, and that the triangular shapes result from asymmetric barriers to corner crossing.

Strain effect on the adsorption, diffusion, and molecular dissociation of hydrogen on Mg (0001) surface

The adsorption, diffusion, and molecular dissociation of hydrogen on the biaxially strained Mg (0001) surface have been systematically investigated by the first principle calculations based on density functional theory. When the strain changes from the compressive to tensile state, the adsorption energy of H atom linearly increases while its diffusion barrier linearly decreases oppositely. The dissociation barrier of H2 molecule linearly reduces in the tensile strain region. Through the chemical bonding analysis including the charge density difference, the projected density of states and the Mulliken population, the mechanism of the strain effect on the adsorption of H atom and the dissociation of H2 molecule has been elucidated by an s-p charge transfer model. With the reduction of the orbital overlap between the surface Mg atoms upon the lattice expansion, the charge transfers from p to s states of Mg atoms, which enhances the hybridization of H s and Mg s orbitals. Therefore, the bonding interaction of H with Mg surface is strengthened and then the atomic diffusion and molecular dissociation barriers of hydrogen decrease accordingly. Our works will be helpful to understand and to estimate the influence of the lattice deformation on the performance of Mg-containing hydrogen storage materials.

Stabilization of the Pb/Si(1 1 1)-(7×7) uniform height islands to higher temperatures with oxygen adsorption

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<190 K) extends the temperature range of their stability. The evolution of the island height and size with temperature is monitored with SPA-LEED. The presence of oxygen suppresses Pb diffusion to higher levels and results in sharper island height distribution, when compared to the height distribution on the clean Pb/Si(1 1 1)-(7×7) system. Most likely this is because the barrier at the island edges, which controls the transfer of atoms from lower levels to the top of the islands, is increased with the adsorption of oxygen.

Wetting Layer Super-Diffusive Motion and QSE Growth in Pb/Si

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.

Surface diffusion experiments with STM: equilibrium correlations and non-equilibrium low temperature growth

Measurements of surface diffusion depend on the state of the system whether the state is equilibrium versus non-equilibrium. Equilibrium experiments carried out in 2-d overlayers measure the collective diffusion coefficient D(c) and can test theoretical predictions in two-dimensional statistical mechanics. Growth experiments typically carried out at low temperatures and/or high flux rates probe systems under non-equilibrium conditions where novel diffusion mechanisms can potentially exist. The use of STM to study both types of measurements is discussed. D(c) can be measured from the autocorrelation of time-dependent tunneling current fluctuations generated by atom motion in and out of the tunneling area. Controlled experiments as function of temperature, coverage and tip-surface separation confirm that the signal is diffusive. For growth experiments the unusually uniform height island (for Pb/Si(111) In/Si(111)) has revealed a novel and intriguing type of diffusion at low temperatures that accounts for the high degree of the self organization. By monitoring the evolution of the stable islands out of a mixture of stable and unstable islands the unusual role of the wetting layer surrounding the growing islands is revealed.

Quantum size effect dependent critical size cluster and finite size effects

Pb nucleation on top of a unique Pb island grown on Si(7×7) (in the form of a “hub”-“moat”-ring) confirms that electron confinement causes large variations in critical size cluster ic with island height. Because of smaller radial dimensions (less than 20 nm), the large variation of the nucleated island density on different layers cannot be a result of differences in terrace diffusion coefficients but ic. These results have important implications on how adsorption can be dramatically modified by quantum size effects.