The mechanism for the 3 x 3 distortion of Sn/ge (111)

We show that two distinct $3 \times 3$ ground states, one nonmagnetic, metallic, and distorted, the other magnetic, semimetallic (or insulating) and undistorted, compete in $\alpha$-phase adsorbates on semiconductor (111) surfaces. In Sn/Ge(111), LSDA/GGA calculations indicate, in agreement with experiment, that the distorted metallic ground state prevails. The reason for stability of this state is analysed, and is traced to a sort of bond density wave, specifically a modulation of the antibonding state filling between the adatom and a Ge-Ge bond directly underneath

Determination of the (3x3)-Sn/Ge(111) structure by photoelectron diffraction

At a coverage of about 1/3 monolayer, Sn deposited on Ge(111) below 550 forms a metastable (sqrt3 x sqrt3)R30 phase. This phase continuously and reversibly transforms into a (3x3) one, upon cooling below 200 K. The photoemission spectra of the Sn 4d electrons from the (3x3)-Sn/Ge(111) surface present two components which are attributed to inequivalent Sn atoms in T4 bonding sites. This structure has been explored by photoelectron diffraction experiments performed at the ALOISA beamline of the Elettra storage ring in Trieste (Italy). The modulation of the intensities of the two Sn components, caused by the backscattering of the underneath Ge atoms, has been measured as a function of the emission angle at fixed kinetic energies and viceversa. The bond angle between Sn and its nearest neighbour atoms in the first Ge layer (Sn-Ge1) has been measured by taking polar scans along the main symmetry directions and it was found almost equivalent for the two components. The corresponding bond lengths are also quite similar, as obtained by studying the dependence on the photoelectron kinetic energy, while keeping the photon polarization and the collection direction parallel to the Sn-Ge1 bond orientation (bond emission). A clear difference between the two bonding sites is observed when studying the energy dependence at normal emission, where the sensitivity to the Sn height above the Ge atom in the second layer is enhanced. This vertical distance is found to be 0.3 Angstroms larger for one Sn atom out of the three contained in the lattice unit cell. The (3x3)-Sn/Ge(111) is thus characterized by a structure where the Sn atom and its three nearest neighbour Ge atoms form a rather rigid unit that presents a strong vertical distortion with respect to the underneath atom of the second Ge layer.Comment: 10 pages with 9 figures, added reference

Role of defects in two-dimensional phase transitions

Imperfections and defects have a strong influence on phase transitions, especially in systems with lower dimensionality, where fluctuations can be strong enough to prevent long-range order. In quasi-ID or 2D systems that exhibit a Charge Density Wave Transition defects play a very special role. There, due the collective nature of the phenomena, a small proportion of microscopic disorder can control the global properties.Defects would produce pre-transitional effects such as charge oscillations in their vicinity, affecting CDW. On the other hand, the interaction of mobile defects with CDWwould lead to alignment of defects, or Defect Density Waves (DDW). In this dynamic model the distribution of defects would no longer be either random or static. Instead Defects would align their positions to optimize the energy of the pinned CDW.In this thesis I present a new view of the phase transitions occurring in quasi-2d Systems of ultra-thin films of a metal deposited on a semiconductor surface: α-phase ofSn/Ge(111) and α-phase of Sn/Si(111). While most of the previous studies were devoted to the elucidating the nature of the room temperature (RT) and low temperature (LT)phase, that is, electronic and lattice structure at two temperatures, above and below the transition temperature, my Variable Temperature STM observations showed how thesephases evolve into each other. From these observations it has become clear that point defects - substitutional atoms and vacancies - play a crucial role. The perturbation of the lattice and electronic structure induced as a response of the system to a defect in its vicinity has a form of density waves with the symmetry of LT phase. These waves have a short range at RT and decay exponentially with the distance from defects. When the temperature is lowered the range of these waves grows. As characteristic decay length of the perturbation reaches the average distance between defects, the density wave mediated defect-defect interaction becomes strong enough to make defects exchange their positions with their nearest neighbors. This motion brings waves originated on different defects into coherence and defects into partial ordering leading to the disorder-order phase transition in defect distribution. Defects dictate the structure of phases both above and below critical temperature. Moreover their presence smears out the critical temperature and changes the properties of the phase transition. My STM observations and modeling imply that the transition temperature of the pure system without defects should be the temperature at which the decay length of defect-induced waves becomes infinite. ForSn/Ge(lll) this temperature is ~ 70 K. This is about 140 K (!!!) lower than the temperature measured by electron diffraction and still widely cited in literature. Thesharp domain walls are the features that distinguish the low temperature phase. A newmodel is presented that allows to predict the configuration, of domain walls for a given defect distribution

Drop impact upon micro- and nanostructured superhydrophobic surfaces

We experimentally investigate drop impact dynamics onto different superhydrophobic surfaces, consisting of regular polymeric micropatterns and rough carbon nanofibers, with similar static contact angles. The main control parameters are the Weber number \We and the roughness of the surface. At small \We, i.e. small impact velocity, the impact evolutions are similar for both types of substrates, exhibiting Fakir state, complete bouncing, partial rebouncing, trapping of an air bubble, jetting, and sticky vibrating water balls. At large \We, splashing impacts emerge forming several satellite droplets, which are more pronounced for the multiscale rough carbon nanofiber jungles. The results imply that the multiscale surface roughness at nanoscale plays a minor role in the impact events for small \We \apprle 120 but an important one for large \We \apprge 120. Finally, we find the effect of ambient air pressure to be negligible in the explored parameter regime \We \apprle 150Comment: 8 pages, 7 figure

The Static and Dynamic Lattice Changes Induced by Hydrogen Adsorption on NiAl(110)

Static and dynamic changes induced by adsorption of atomic hydrogen on the NiAl(110) lattice at 130 K have been examined as a function of adsorbate coverage. Adsorbed hydrogen exists in three distinct phases. At low coverages the hydrogen is itinerant because of quantum tunneling between sites and exhibits no observable vibrational modes. Between 0.4 ML and 0.6 ML, substrate mediated interactions produce an ordered superstructure with c(2x2) symmetry, and at higher coverages, hydrogen exists as a disordered lattice gas. This picture of how hydrogen interacts with NiAl(110) is developed from our data and compared to current theoretical predictions.Comment: 36 pages, including 12 figures, 2 tables and 58 reference

Aerosynthesis: Growth of Vertically-aligned Carbon Nanofibres with Air DC Plasma

Vertically-aligned carbon nanofibres (VACNFs) have been synthesized in a mixture of acetone and air using catalytic DC plasma-enhanced chemical vapour deposition. Typically, ammonia or hydrogen is used as an etchant gas in the mixture to remove carbon that otherwise passivates the catalyst surface and impedes growth. Our demonstration of the use of air as the etchant gas opens up the possibility that ion etching could be sufficient to maintain the catalytic activity state during synthesis. It also demonstrates a path toward growing VACNFs in the open atmosphere

Surfaces: a playground for physics with broken symmetry in reduced dimensionality

Abstract With our crystal ball in front of us, we attempt to articulate the opportunities and challenges for a surface physicist in the beginning of the new millennium. The challenge is quite clear: to use the unique environment of a surface or interface to do fascinating physics, while taking full advantage of the skills the community has developed over the last 30 years. The opportunities appear to be endless! In this age of Nanotechnology where the promise is to shape the world atom by atom, leading to the next industrial revolution [Nanotechnology: shaping the world atom by atom, National Science and Technology Council, Committee on Technology, 1999], surface science should be at the very forefront of both technological and scientific advances. The smaller objects become, the more important their surfaces become. In this article we focus on the role of a surface physicist in the emergence of nanoscale collective phenomena in complex materials.

Field Emission of ITO-Coated Vertically Aligned Nanowire Array

An indium tin oxide (ITO)-coated vertically aligned nanowire array is fabricated, and the field emission characteristics of the nanowire array are investigated. An array of vertically aligned nanowires is considered an ideal structure for a field emitter because of its parallel orientation to the applied electric field. In this letter, a vertically aligned nanowire array is fabricated by modified conventional UV lithography and coated with 0.1-μm-thick ITO. The turn-on electric field intensity is about 2.0 V/μm, and the field enhancement factor, β, is approximately 3,078 when the gap for field emission is 0.6 μm, as measured with a nanomanipulator in a scanning electron microscope