Session D17: Focus Session: Surfaces and Interfaces in Nonoxide Nanostructures: Growth, Structure, and Characterization - Growth Dynamics

Factors that control the morphology of ice films on metal surfaces

Examination of the equilibrium nanoscale morphology of ice films provides important clues about the energetics of water-metal interactions [1]. In this talk I compare STM and DFT results for the structure of ice films on Pt(111) and Ni(111). Because the lattice constants of Ni and Pt differ by 10{\%}, this comparison allows us to probe the effect of lattice misfit on ice nucleation and growth. On both substrates, STM suggests a first molecular water layer very different from bulk ice: besides the usual hexagonal rings they also both contain a motif of pentagons and heptagons [2]. Furthermore, at 140K, thicker films on both substrates dewet the substrate to lower interfacial energy by forming 3-dimensional ice crystallites several nanometers thick [3]. However, despite these similarities, there are striking differences in the submonolayer structure and in the kinetics of the dewetting process. Using DFT calculations as a guide, I will discuss the how these differences can be related to substrate lattice constant and draw conclusions about the processes that control ice film morphology. \\[4pt] [1] A. Hodgson and S. Haq, Surf. Sci. Rep. 64, 381 (2009). \\[0pt] [2] S. Nie, P. J. Feibelman, N. C. Bartelt and K. Th\"{u}rmer, Phys. Rev. Lett. 105, 026102 (2010). \\[0pt] [3] K. Th\"{u}rmer and N. C. Bartelt, Phys. Rev. B 77, 195425 (2008); Phys. Rev. Lett. 100, 186101 (2008).   

Structure and dynamics of water nano-droplets on graphene

The wettability of graphene and the diffusion of water droplets across it is of central importance to many emerging applications in nanofluidics. Here we report an extensive set of molecular dynamics simulations for water clusters of varying sizes on graphene (20 to 2,000 water molecules), using force field parameters fitted to recent ab initio quantum Monte Carlo data [J. Ma, A. Michaelides, D. Alfe, L. Schimka, G. Kresse, and E. G. Wang, Phys. Rev. B 84, 033402 (2011)]. The contact angle for the water droplets obtained here, with our ab initio water - carbon interaction, is in very good agreement with experiments. A strong size dependence in the diffusion of the water droplets across the surface is also observed. This work is a step towards understanding surface transportation in carbon based nanofluidics.   

Large-scale simulations of glancing-angle deposition

While thin-films grown via glancing-angle deposition have interesting structural, mechanical, and optical properties, the large range of time- and length-scales makes realistic simulations difficult. Accordingly, while activated relaxation processes may be important at long time-scales, here we focus on the deposition process since we expect the effects of shadowing and deposition-induced relaxation to dominate for large deposition angles. In particular, by taking advantage of the speed of recently developed graphical-processing-units (GPUs) we have carried out ``large-scale'' GPU-enhanced MD simulations of Cu/Cu(100) growth up to 20 monolayers (ML) for deposition angles $\theta$ (corresponding to the angle with respect to the substrate normal) ranging from 50$^{\circ}$ to 85$^{\circ}$ and for both random and fixed azimuthal angles. In general, we find good agreement with experimental results for the dependence of thin-film porosity on deposition angle and film-thickness. Results for the dependence of the surface roughness, lateral correlation length and microstructure (e.g. defect density, vacancy density, and strain) on the deposition angle and film thickness will also be presented.   

Shape transitions in strained Cu islands on Ni(100): kinetics versus energetics

We examine the shape transition from compact to ramified islands observed in submonolayer Cu/Ni(100) growth. Recently, it has been argued that this transition is not due to a growth instability but can be understood in terms of energetic arguments. In order to determine the responsible mechanisms we have carried out energetics calculations as well as temperature-accelerated dynamics (TAD) and kinetic Monte Carlo (KMC) simulations. Our results indicate that the shape transition cannot be explained by equilibrium arguments, but is instead due to kinetic effects which are mediated by strain. In particular, by calculating the relevant line-tension and strain energies, we find that the equilibrium critical island-width is at least four orders of magnitude larger than the experimentally observed arm-width. In contrast, our TAD simulations indicate that unexpected concerted motions occurring at step edges are responsible. The energy barriers for these concerted motions decrease with increasing island size and appear to saturate for islands larger than 300 - 400 atoms. By including these strain-induced kinetic processes in our KMC simulations of island-growth, we have been able to explain both the temperature- and coverage-dependence of the island morphology.   

Dewetting Processes in Ultra-thin Epitaxial Ag Films on Si(111)

The authors have recently reported the development of a technique to grow large-area, single-crystal, atomically smooth Ag films on Si(111), and have demonstrated the utility of such films for plasmonics applications: the films support surface plasmon polaritons with extremely low damping. Although the authors have observed that films with thickness on the order of several tens of nanometers can be relatively stable against dewetting--at least on a time scale long enough for fabrication and EOT probing under ambient conditions--they have also seen that very thin Ag films (e.g., 5 nm) start dewetting under ambient conditions within about 24 hours. This raises an important question: how and why does dewetting occur? The authors have now undertaken a detailed and systematic study of the mechanism of dewetting in epitaxial Ag films on Si(111) as a function of film thickness. The current presentation will focus on this work, and will attempt to shed light on the apparent robustness of films grown using their method.   

Diffusion of a Ga adatom on the GaAs(001)-\emph{c}(4 $\times$ 4)-heterodimer surface: A first-principles study

The adsorption and diffusion behavior of a Ga adatom on the $\textrm{GaAs(001)-\emph{c}(}4\times4\textrm{)}$-heterodimer surface were studied by employing ab initio density functional theory computations in the local density approximation. Structural and bonding features of the $\textrm{\emph{c}(}4\times4\textrm{)}$-heterodimer reconstruction surface were examined. A comparison with the $\textrm{\emph{c}(}4\times4\textrm{)}$-ss reconstruction\footnote{J. L. Roehl et al, Phys. Rev. B 82, 165335 (2010).} was performed. Minimum energy sites (MES) on $\textrm{\emph{c}(}4\times4\textrm{)}$-heterodimer surface were located by mapping the potential energy surface for a Ga adatom. Barriers for diffusion of a Ga adatom between the neighboring MES were calculated by using top and exchange diffusion mechanisms. We proposed two unique diffusion pathways for a Ga adatom diffusing between the global minimums of two neighboring unit cells. Signature differences between electronic structures of top- and exchange- diffusion mechanisms were studied for relevant atoms. We observed a higher diffusion barrier for exchange mechanism compared to top hopping.\footnote{Supported by NSF DMR 0705464, CNS 0855134.}   

Voronoi Cell Patterns: theoretical model and application to submonolayer growth

We use a simple fragmentation model to describe the statistical behavior of the Voronoi cell patterns generated by a homogeneous and isotropic set of points in 1D and in 2D. In particular, we are interested in the distribution of sizes of these Voronoi cells. Our model is completely defined by two probability distributions in 1D and again in 2D, the probability to add a new point inside an existing cell and the probability that this new point is at a particular position relative to the preexisting point inside this cell. In 1D the first distribution depends on a single parameter while the second distribution is defined through a fragmentation kernel; in 2D both distributions depend on a single parameter. The fragmentation kernel and the control parameters are closely related to the physical properties of the specific system under study. We apply our model to describe the Voronoi cell patterns of island nucleation for critical island sizes $i$=0,1,2,3. Experimental results for the Voronoi cells of InAs/GaAs quantum dots are also described by our model.   

Aberration corrected Low Energy Electron Microscopy for Surface and Interface Studies

Correction of spherical and chromatic aberrations of the electron microscope objective lens constitutes one of the most significant and far-reaching breakthroughs in electron optics in the last 20 years. For instance, with the TEAM microscope it is now possible to image atoms with a spatial resolution of 50 picometers, providing a detailed real-space view of the carbon atoms in a single sheet of graphene. Similarly, the resolution on Low Energy Electron Microscopy (LEEM) has improved from a typical value of 5 nm, to less than 2 nm, at an electron energy of just a few eV. Photo Electron Emission Microscopy (PEEM) has recently achieved a resolution of 5 nm. In this talk I will discuss the successful implementation of electron-mirror based aberration correction in LEEM. Some of the details of the electron optical implementation will be discussed, in particular the unique optical properties of the electron mirror, and its mode of operation. Quantitative methods to verify proper control of the optical parameters and successful aberration correction have been developed and implemented in this new instrument. Spatial resolution has improved by more than a factor 2 as compared to the uncorrected instrument, and an ultimate spatial resolution below twice the wavelength of the electron at the sample appears to be achievable. In comparison, the highest resolution Tranmission Electron Microscopes have a spatial resolution of about 20 electron wavelengths.   

LEEM and STM studies of Ag on Ge (110)

The growth of Ag deposited on Ge(110) was studied with low energy electron microscopy (LEEM) and scanning tunneling microscopy (STM). The LEEM studies showed the formation of long one dimensional islands as Ag was deposited above 430\r{ }C. Island nucleation proceeded from defects in the Ge substrate. During deposition, the length of the islands increased while the width remained constant. The size and distribution of the islands was dependent on the substrate temperatures during deposition. At 480\r{ }C, islands were 100 nm wide and 1-20 $\mu $m long at 9 ML of coverage. At 530\r{ }C, islands were 200nm wide and 1-3 $\mu $m long at 9 ML of coverage. STM images showed that the islands were composed of Ag and that the surface regions between the islands exhibited a reconstruction which is characteristic of pure Ge.   

LEEM Observations of Ag/Ge(111) Structural Phases and Phase Transformations

We use low energy electron microscopy (LEEM) to study the growth of, and transformations between structural phases of Ag deposited on Ge(111) above and below the Ag desorption temperature. Ag deposited on Ge(111) forms three main surface phases above 100 \r{ }C: (4x4), ($\surd $3x$\surd $3)R30\r{ }, and (3x1). For deposition between 540-575 \r{ }C, a (3x1) phase grows. Upon the completion of the growth of the (3x1) phase, a ($\surd $3x$\surd $3)R30\r{ } phase grows. For sufficiently high Ag deposition rates, we observed the same growth sequence above the Ag desorption temperature of 575 \r{ }C , up to 640 \r{ }C. Desorption above 575 \r{ }C proceeds with the reverse sequence: the ($\surd $3x$\surd $3)R30\r{ } phase desorbs followed by desorption of the 3x1 phase. Above 640 \r{ }C, we observed the growth of the (3x1) phase but not the ($\surd $3x$\surd $3)R30\r{ }. For 4x4 and ($\surd $3x$\surd $3)R30\r{ } surfaces prepared by deposition between 200-500 \r{ }C we observe the transformation of these phases to a 1x1 disordered phase at the desorption temperature (575 \r{ }C), with desorption proceeding from the edges of disordered 1x1 domains.   

Crystalline Structure of the Pb/Si(111)7x7 Stable Wetting Layer

The wetting layer formation in the Pb/Si(111)7x7 system has attracted extensive interest because of anomalously fast kinetics, which enables the formation of quantum size effect (QSE) nanoislands [Jeffrey et al. PRL \textbf{96}, 106105 (2006)]. However, previous studies of the wetting layer by x-ray diffraction and scanning-probes have led to \textit{inconsistent} structural models; thus, the structure of this wetting layer has been unsolved. Furthermore, a recent investigation has revealed that the wetting layer is out-of-equilibrium over a surprisingly broad temperature range [Gramlich et al., PRB \textbf{84}, 075433 (2011)]. Using \textit{in situ} x-ray scattering methods, we have solved the \textit{stable}, low temperature annealed structure of the wetting layer. It exhibits a strained atomic layer where Pb atoms are in transition, from Si-7x7 sites towards 8x8-sites, with some Pb-atoms vertically closer to the Si-7x7. Interestingly, the Si adatoms shift to the edges of the unit cell. Funding is acknowledged from NSF DMR-0706278 (PFM, MWG, STH, YC, and the Ministry of Knowledge Economy of Korea 2009-F014-01 (CK). The experiments were performed on the 6IDC beam line, supported by the US-DOE (through Ames Lab, W-7405-Eng-82), at the Advanced Photon Source (US-DOE, W-31-109-Eng-38) located at Argonne National Laboratory.