We present ab initio quantum-mechanical molecular-dynamics simulations of the liquid-metal--amorphous-semiconductor transition in Ge. Our simulations are based on (a) finite-temperature density-functional theory of the one-electron states, (b) exact energy minimization and hence calculation of the exact Hellmann-Feynman forces after each molecular-dynamics step using preconditioned conjugate-gradient techniques, (c) accurate nonlocal pseudopotentials, and (d) Nos\'e dynamics for generating a canonical ensemble. This method gives perfect control of the adiabaticity of the electron-ion ensemble and allows us to perform simulations over more than 30 ps. The computer-generated ensemble describes the structural, dynamic, and electronic properties of liquid and amorphous Ge in very good agreement with experiment. The simulation allows us to study in detail the changes in the structure-property relationship through the metal-semiconductor transition. We report a detailed analysis of the local structural properties and their changes induced by an annealing process. The geometrical, bonding, and spectral properties of defects in the disordered tetrahedral network are investigated and compared with experiment.

Ab initio molecular-dynamics simulation of the liquid-metal-amorphous-semiconductor transition in germanium.

/pdf/self-assembled-monolayers-of-thiolates-on-metals-as-a-form-1neb0hj3k4.pdf

Self-assembled monolayers of thiolates on metals as a form of nanotechnology.

https://www.tau.ac.il/%7Eklafter1/258.pdf

The random walk's guide to anomalous diffusion: a fractional dynamics approach

http://experimentationlab.berkeley.edu/sites/default/files/AFMImages/butt_AFM.pdf

Force measurements with the atomic force microscope: Technique, interpretation and applications

PATTERNING matter on the nanometre scale is an important objective of current materials chemistry and physics. It is driven by both the need to further miniaturize electronic components and the fact that at the nanometre scale, materials properties are strongly size-dependent and thus can be tuned sensitively1. In nanoscale crystals, quantum size effects and the large number of surface atoms influence the, chemical, electronic, magnetic and optical behaviour2—4. 'Top-down' (for example, lithographic) methods for nanoscale manipulation reach only to the upper end of the nanometre regime5; but whereas 'bottom-up' wet chemical techniques allow for the preparation of mono-disperse, defect-free crystallites just 1–10 nm in size6–10, ways to control the structure of nanocrystal assemblies are scarce. Here we describe a strategy for the synthesis of'nanocrystal molecules', in which discrete numbers of gold nanocrystals are organized into spatially defined structures based on Watson-Crick base-pairing interactions. We attach single-stranded DNA oligonucleotides of defined length and sequence to individual nanocrystals, and these assemble into dimers and trimers on addition of a complementary single-stranded DNA template. We anticipate that this approach should allow the construction of more complex two-and three-dimensional assemblies.

Organization of 'nanocrystal molecules' using DNA

The creation of perfect nanometer-scale crystallites (nanocrystals), identically replicated in unlimited quantities, in a state that can be manipulated and understood as pure macromolecular substances, is an ultimate challenge of modern materials research with outstanding fundamental and potential technological consequences.[’l We report on the prediction, isolation, and characterization of a series of gold nanocrystals, passivated by self-assembled monolayers (SAMs) of straight-chain alkylthiolate molecules (RS, R

Nanocrystal gold molecules

Molecular dynamics simulations and atomic force microscopy are used to investigate the atomistic mechanisms of adhesion, contact formation, nanoindentation, separation, and fracture that occur when a nickel tip interacts with a gold surface. The theoretically predicted and experimentally measured hysteresis in the force versus tip-to-sample distance relationship, found upon approach and subsequent separation of the tip from the sample, is related to inelastic deformation of the sample surface characterized by adhesion of gold atoms to the nickel tip and formation of a connective neck of atoms. At small tipsample distances, mechanical instability causes the tip and surface to jump-to-contact, which in turn leads to adhesion-induced wetting of the nickel tip by gold atoms. Subsequent indentation of the substrate results in the onset of plastic deformation of the gold surface. The atomic-scale mechanisms underlying the formation and elongation of a connective neck, which forms upon separation, consist of structural transformations involving elastic and yielding stages.

https://science.sciencemag.org/content/sci/248/4954/454.full.pdf

Atomistic Mechanisms and Dynamics of Adhesion, Nanoindentation, and Fracture

Material structures of reduced dimensions exhibit electrical and mechanical properties different from those in the bulk. Measurements of room-temperature electronic transport in pulled metallic nanowires are presented, demonstrating that the conductance characteristics depend on the length, lateral dimensions, state and degree of disorder, and elongation mechanism of the wire. Conductance during the elongation of short wires (length l ∼ 50 angstroms) exhibits periodic quantization steps with characteristic dips, correlating with the order-disorder states of layers of atoms in the wire predicted by molecular dynamics simulations. The resistance R of wires as long as l ∼ 400 angstroms exhibits localization characteristics with In R ( l ) ∼ l 2 .

https://science.sciencemag.org/content/sci/267/5205/1793.full.pdf

Properties of Metallic Nanowires: From Conductance Quantization to Localization

Using grand-canonical molecular dynamics simulations and free energy calculations of spherical molecular confined films, we investigate the internal energy and entropic origins of density layering and solvation force oscillations. We show that these properties, as well as diffusion and rheological characteristics of such films, depend on interfacial commensurability, with commensurate films exhibiting abrupt solidification for relatively thick, 5 or 6 layer, films. {copyright} {ital 1997} {ital The American Physical Society}

Layering Transitions and Dynamics of Confined Liquid Films

Investigations of the thermal evolution of structural and dynamic properties of gold nanocrystalline clusters of variable size $({\mathrm{Au}}_{75},$ ${\mathrm{Au}}_{146},$ and ${\mathrm{Au}}_{459})$ were performed using extensive molecular-dynamics simulations employing embedded-atom interactions. These studies reveal that the thermal evolution of these clusters is punctuated by diffusionless solid-to-solid structural transformations from the low-temperature optimal structures (truncated-decahedra for ${\mathrm{Au}}_{75}$ and ${\mathrm{Au}}_{146},$ and a face-centered-cubic structure with a truncated-octahedral morphology for ${\mathrm{Au}}_{459})$ to icosahedral structures. These structural transformations are precursors to the melting transitions which occur at temperatures below the bulk melting temperature of crystalline gold, and they are intrinsic to the thermodynamics of the clusters. The melting scenario revealed by the simulations for these gold clusters differs from that involving thickening of a quasiliquid wetting surface layer, and in addition it does not involve at any stage of the thermal process dynamic coexistence where the cluster fluctuates between being entirely solid or liquid. For the larger cluster, ${\mathrm{Au}}_{459},$ a thermodynamic (icosahedral) solid-liquid coexistence state is found in the vicinity of the melting transition. The occurrence of polymorphic solid structures, that is a cluster containing simultaneously decahedral and icosahedral parts, is discussed in light of early observations of such structures via high-resolution electron microscopy.

/pdf/melting-of-gold-clusters-1zi91b7yi3.pdf

W. D. Luedtke

Papers

Nanocrystal gold molecules

Atomistic Mechanisms and Dynamics of Adhesion, Nanoindentation, and Fracture

Properties of Metallic Nanowires: From Conductance Quantization to Localization

Layering Transitions and Dynamics of Confined Liquid Films

Melting of gold clusters