Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session S40: Focus Session: Morphology and Evolution at Surfaces: Wires and Self-Assembly |
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Sponsoring Units: DMP DCMP Chair: Pantelis Kelires, University of Crete Room: LACC 408A |
Wednesday, March 23, 2005 2:30PM - 3:06PM |
S40.00001: Semiconductor Nanowires from Materials Science and Device Physics Perspectives Invited Speaker: Realization of extremely down-scaled devices gives tough challenges related to technology and materials science. One reason for the concern is that top-down fabricated nano-devices tend to have their properties dominated by process-induced damage, rendering ultra-small devices not so useful. Alternatively, bottom-up fabrication methods may allow dimensions on the scale even below 10 nm, still with superb device properties. I will in this talk describe our research on catalytically induced growth of semiconductor nanowires. Our method uses catalytic gold nanoparticles, allowing tight control of diameter as well as position of where the nanowire grows, with our work completely focused on epitaxially nucleated nanowires in which the nanowire structure can be seen as a coherent, monolithic extension of the crystalline substrate material. One of the most important achievements in this field of research is the realization of atomically abrupt heterostructures within nanowires, in which the material composition can be altered within only one or a few monolayers, thus allowing 1D heterostructure devices to be realized. This has allowed a variety of quantum devices to be realized, such as single-electron transistors, resonant tunneling devices as well as memory storage devices. A related recent field of progress has been the realization of ideally nucleated III-V nanowires on Si substrates, cases where we have also reported functioning III-V heterostructure device structures on Si. All of these device related challenges evolve from an improved understanding of the materials science involved in nucleation of nanowires, in altering of composition of the growing nanowire, in control of the growth direction etc. I will give examples of these materials science issues and will especially dwell on the opportunities to form new kinds of materials, e.g. as 3D complex nanowire structures, resembling nanotrees or nanoforests. [Preview Abstract] |
Wednesday, March 23, 2005 3:06PM - 3:18PM |
S40.00002: STM of Gold-Induced Chains on the Si(775) Surface. Laura Pedri, Laura Toppozini, Mark Gallagher Atomic scale wires on semiconductor surfaces are an attractive system to study the novel physics of one-dimensional (1D) metallic conduction. Recently, quasi-1D chains have been produced by depositing small amounts of Au onto vicinal Si(111) surfaces. The gold-induced chains run parallel to step edges, and photoemission from these surfaces reveals highly 1D metallic bands. The Si(775)-Au surface exhibits 1D chains running along $[1\bar {1}0]$ spaced 2.13~nm apart. It has been previously reported that the (775) surface is prepared by depositing 0.25~$\pm $~0.07~ML of gold onto a silicon surface tilted 8.5$^{o}$ towards $[11\bar {2}]$ [1]. Furthermore, it has been argued that the unit cell contains two Au atoms per-unit cell similar to the on axis Si(111)5x2-Au reconstruction. We have used scanning tunneling microscopy to further investigate the atomic structure and the electronic properties of the (775)-Au surface. In particular, we have used STM with Auger spectroscopy, and LEED to examine the stochiometry of the chain structure. [1] Crain et al., \textit{Phys. Rev. B} \textbf{69}, 125401 (2004). [Preview Abstract] |
Wednesday, March 23, 2005 3:18PM - 3:30PM |
S40.00003: From Multiply-Twinned Particles to Epitaxial Nanocrystals: the crucial role of interface on the structure of Ag nanoparticles Boquan Li, Jian-Min Zuo The effect of substrate on the structure of nanometer-sized metal particles is investigated for Ag on silicon surfaces. The nanometer-sized Ag particles, formed by vapor deposition of Ag on hydrogen-terminated Si (001) surfaces at room temperature, adopt multiply-twined structures, while the Ag particles of similar sizes on H-Si (111) surfaces assume the face-centered cubic (fcc) crystal structure. Upon annealing, the multiply-twined Ag nano-particles on H-Si (001) transform into fcc nanocrystals. The fcc Ag takes up the cube-on-cube epitaxy, with the orientation relationship of Ag (001)//Si (001) and Ag[110]//Si[110]. An energetic model is developed to account for the transition from multiply-twined particles to fcc epitaxial crystals, in which the interface energy change on H-Si(001) due to hydrogen desorption plays a crucial role. A pair-correlation function study will also be presented in an effort to accurately determine the inter-atomic distances and strain in the multiply-twinned Ag nanoparticles. [Preview Abstract] |
Wednesday, March 23, 2005 3:30PM - 3:42PM |
S40.00004: Nucleation and aggregation in 1D with interactions Hachem Sidi Ammi, Olivier Pierre-Louis, Anna Chame, M'Hammed Touzani, Abdelilah Benyoussef, Chaouqi Misbah We present a study of the aggregation of interacting particles in one dimension. This situation for example applies to atoms trapped along linear defects at the surface of a crystal, such as crystal steps. It is therefore important for the initial stages of the formation of quantum wires. Simulations are performed with two lattice models. In the first model, the borders of atoms and islands interact in a vectorial manner. In the second model, each atom carries a dipole. These two models lead to qualitatively similar but quantitatively different behavior. In both cases, the final average island size $S_f$ does not depend on the interactions in the limits of very low and very high coverages. For intermediate coverages, $S_f$ exhibits an asymmetric behavior as a function of the interaction strength: while it saturates for attractive interactions, it decreases for repulsive interactions. A class of mean field models is designed, which allows one to retrieve the interaction dependence and the coverage dependence of the average island size with a good accuracy. Ref. H. Sidi Ammi et al, Submitted to Phys.Rev.E. [Preview Abstract] |
Wednesday, March 23, 2005 3:42PM - 4:18PM |
S40.00005: The atomic-scale processes underlying nanoscale pattern formation on solid surfaces Invited Speaker: Gary Kellogg The deposition of Pb atoms on Cu(111) produces two surface phases: a Pb-Cu surface-alloy and a Pb-overlayer. Within a specified range of Pb coverage, the two phases co-exist and spontaneously order into domain patterns with tens-of-nanometer periodicity[1]. The evolution of the domain structures with increasing Pb coverage agrees well with theoretical descriptions based on competing long- and short-range interactions. The system thus provides an ideal model system for probing the interactions underlying 2-D self-assembly. Although a self-consistent picture of the thermodynamic driving forces responsible for pattern formation now exists[2,3], a comprehensive understanding of the kinetic processes underlying self-assembly is only beginning to emerge. In this talk I will discuss recent experiments and calculations that address a key question concerning the kinetics of pattern formation: how do individual domains of each phase, which contain 10s of thousands of atoms, assemble into patterns on timescales of minutes? I will show that the self-assembly occurs by the collective motion of entire domains and present evidence that this motion is facile because (1) Pb atoms move quickly across the surface alloy due to Pb-Cu exchange and (2) Cu atoms move quickly \textit{through} the overlayer phase due to a high concentration of vacancies and Cu atoms embedded within the overlayer. The latter finding is not intuitive and may explain why the remarkable pattern formation seen for Pb/Cu(111) has not been observed for other metal-metal systems. [1] R. Plass, \textit{et al}., Nature \underline {412}, 875 (2001). [2] R. van Gastel, \textit{et al.}, Phys. Rev. Lett. \underline {91}, {\#}55503 (2003). [3] R. van Gastel, \textit{et al}, Phys. Rev. B (in press). [Preview Abstract] |
Wednesday, March 23, 2005 4:18PM - 4:30PM |
S40.00006: Self-organization and geometry control of Pb islands grown on anisotropic substrates Myron Hupalo, Michael Tringides The growth of uniform nanostructures requires discovering robust and reproducible ways to control their size and geometry. It is found that the growth of Pb on the anisotropic substrate In(4x1)-Si(111) leads to full control of the grown morphology from the interplay of two stabilizing mechanisms which minimize the island energy at preferred geometry. Quantum Size Effects (QSE) i.e. the dependence of the electron energy on the structure dimensions as a result of electron confinement result in uniform height 4-layer Pb islands grown at 180K. More importantly the island shape and width are controllable, with the width an integer multiple of 1.33nm (the period of the initial In(4x1)along [$\mathop 1\limits^- \mathop 1\limits^- 2$]), because the underlying reconstruction generates anisotropic potential energy relief with preferential sticking of the incoming Pb atoms along [$\mathop 1\limits^- 10$]. Growth on the different reconstructed substrate, Si(111)-In$\surd $31x$\surd $31, leads to uniform height Pb islands of hexagonal shapes thus demonstrating island shape control. [Preview Abstract] |
Wednesday, March 23, 2005 4:30PM - 4:42PM |
S40.00007: Self-assembly of isonicotinic acid molecules into supramolecular films on Ag (111) Bo Xu, Hui Li, Janice Reutt-Robey Self assembly processes of isonicotinic acid (INA) molecules into supramolecular structures on Ag (111) surface are studied with UHV-STM, XPS, and IR spectroscopy. INA molecule contains both a ring nitrogen and carboxyl tail, which lead to a tape-like molecular solid.~At room temperature, INA~molecules organize into 2D islands that exceed~100 nm on Ag (111), demonstrating 2D H-bonding interactions. A series of carboxy O-H\textbullet \textbullet \textbullet N hydrogen bonds assemble INA molecules into linear chains, while~weaker hydrogen bonds between carbonyl O and aromatic~H link the chains sideways into ordered 2D structures. Different orientational domains are observed and the domain walls (carboxyl-carboxyl coupling)~establish the molecular dipole direction. XPS spectroscopy corroborates the H-bonding interactions, while IR spectroscopy was used to assess INA molecular orientation with respect to the surface plane. [Preview Abstract] |
Wednesday, March 23, 2005 4:42PM - 4:54PM |
S40.00008: Atomically and Time Resolved Pattern Formation: S on Submonolayer Ag/Ru(0001) Bogdan Diaconescu, Katrsten Pohl Strained metallic interfaces can lead to highly ordered arrays of misfit dislocations that can be used as an elegant patterning technique for growing arrays of clusters with specific size and densities. Deposition of S on submonolayer Ag films on Ru(0001) transforms the short herringbone dislocation pattern, with a 60{\AA} x 40{\AA} unit cell, into a large scale ordered triangular array of S filled vacancy islands 50{\AA} apart as shown by STM and LEED. In this process S partially relieves the strain in Ag film as seen by the relaxation of the threading dislocation network. Atomic structure of the S islands and the coverage dependence of the atomic structure of S/Ru(0001) and S islands size is revealed by STM imaging. The low S coverage regime shows a transition from isolated, highly mobile S islands as seen through time resolved STM to an ordered islands array. As the S coverage increases beyond 0.33 ML on the Ru(0001) terrace, the ordering of the 2D S islands array is destroyed, and the compressed S phase pushes Ag onto a second layer. Supported by NSF-CAREER-DMR-0134933 and ACS-PRF-37999-G5 [Preview Abstract] |
Wednesday, March 23, 2005 4:54PM - 5:06PM |
S40.00009: MeV Ion Beam Synthesis of Nanopore Arrays in SiO2 Films Andrew Carlson, Anuranjita Tewary, Mark Brongersma, Thomas Felter, Sergei Kucheyev High energy (MeV), heavy ion irradiation can be used as a tool to deform patterned SiO$_{2}$ films in a controlled fashion. We have used this technique to deform micron-sized holes in SiO$_{2}$ films generated by photolithography and focused ion beam milling. The holes were fabricated in 2 $\mu $m thick SiO$_{2}$ films in the size range from 500 nm to 2 $\mu $m and were subsequently irradiated with 4 MeV Xe$^{ }$ions. First, a systematic study will be presented on the deformation of differently shaped holes as a function of the ion fluence. Second, we will present data on the deformation of linear and two-dimensional arrays of holes. Finally, we will test the observed deformations against currently available visco-elastic models that describe this ion irradiation-induced deformation process. This work may find application in nanoscale fashioning of SiO$_{2}$ surface features and the controlled fabrication of nanopore arrays. [Preview Abstract] |
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