Structure and phases of the Au(001) surface: Absolute x-ray reflectivity

Properties of real metallic surfaces: Effects of density functional semilocality and van der Waals nonlocality

We have computed the surface energies, work functions, and interlayer surface relaxations of clean (111), (100), and (110) surfaces of Al, Cu, Ru, Rh, Pd, Ag, Pt, and Au. We interpret the surface energy from liquid metal measurements as the mean of the solid-state surface energies over these three lowest-index crystal faces. We compare experimental (and random phase approximation) reference values to those of a family of nonempirical semilocal density functionals, from the basic local density approximation (LDA) to our most advanced general purpose meta-generalized gradient approximation, strongly constrained and appropriately normed (SCAN). The closest agreement is achieved by the simplest density functional LDA, and by the most sophisticated one, SCAN+rVV10 (Vydrov-Van Voorhis 2010). The long-range van der Waals interaction, incorporated through rVV10, increases the surface energies by about 10%, and increases the work functions by about 3%. LDA works for metal surfaces through two known error cancellations. The Perdew-Burke-Ernzerhof generalized gradient approximation tends to underestimate both surface energies (by about 24%) and work functions (by about 4%), yielding the least-accurate results. The amount by which a functional underestimates these surface properties correlates with the extent to which it neglects van der Waals attraction at intermediate and long range. Qualitative arguments are given for the signs of the van der Waals contributions to the surface energy and work function. A standard expression for the work function in Kohn-Sham (KS) theory is shown to be valid in generalized KS theory. Interlayer relaxations from different functionals are in reasonable agreement with one another, and usually with experiment.

Anisotropy in surface-initiated melting of the triclinic molecular crystal 1,3,5-triamino-2,4,6-trinitrobenzene: A molecular dynamics study

Surface-initiated melting of 1,3,5-triamino-2,4,6-trinitrobenzene (TATB), a triclinic molecular crystal, was investigated using molecular dynamics simulations. Simulations were performed for the three principal crystallographic planes exposed to vacuum, with the normal vectors to the planes given by b × c, c × a, and a × b (where a, b, and c define the edge vectors of the unit cell), denoted as (100), (010), and (001), respectively. The best estimate of the normal melting temperature for TATB is 851 ± 5 K. The nature and extent of disordering of the crystal-vacuum interface depend on the exposed crystallographic face, with the (001) face exhibiting incomplete melting and superheating. This is attributed to the anisotropy of the inter-molecular hydrogen bonding and the propensity of the crystal to form stacking faults in directions approximately perpendicular to the (100) and (010) faces. For all three crystal orientations, formation of molecular vacancies in the lattice at the crystal-vacuum (or crystal-quasi-liquid layer) interface precedes the complete loss of order at the interface.

Facet selectivity in gold binding peptides: exploiting interfacial water structure

Peptide sequences that can discriminate between gold facets under aqueous conditions offer a promising route to control the growth and organisation of biomimetically-synthesised gold nanoparticles. Knowledge of the interplay between sequence, conformations and interfacial properties is essential for predictable manipulation of these biointerfaces, but the structural connections between a given peptide sequence and its binding affinity remain unclear, impeding practical advances in the field. These structural insights, at atomic-scale resolution, are not easily accessed with experimental approaches, but can be delivered via molecular simulation. A current unmet challenge lies in forging links between predicted adsorption free energies derived from enhanced sampling simulations with the conformational ensemble of the peptide and the water structure at the surface. To meet this challenge, here we use an in situ combination of Replica Exchange with Solute Tempering with Metadynamics simulations to predict the adsorption free energy of a gold-binding peptide sequence, AuBP1, at the aqueous Au(111), Au(100)(1 × 1) and Au(100)(5 × 1) interfaces. We find adsorption to the Au(111) surface is stronger than to Au(100), irrespective of the reconstruction status of the latter. Our predicted free energies agree with experiment, and correlate with trends in interfacial water structuring. For gold, surface hydration is predicted as a chief determining factor in peptide-surface recognition. Our findings can be used to suggest how shaped seed-nanocrystals of Au, in partnership with AuBP1, could be used to control AuNP nanoparticle morphology.

Surface relaxation and stress for 5d transition metals

Using the density functional theory, we present a systematic theoretical study of the layer relaxation and surface stress of 5d transition metals. Our calculations predict layer contractions for all surfaces, except for the (111) surface of face centered cubic Pt and Au, where slight expansions are obtained similarly to the case of the 4d series. We also find that the relaxations of the close packed surfaces decrease with increasing occupation number through the 5d series. The surface stress for the relaxed, most closely packed surfaces shows similar atomic number dependence as the surface energy. Using Cammarata's model and our calculated surface stress and surface energy values, we examine the possibility of surface reconstructions, which is in reasonable agreement with the experimental observations.

Multiple time scales in diffraction measurements of diffusive surface relaxation

We grew SrTiO3 on SrTiO3(001) by pulsed laser deposition, using x-ray scattering to monitor the growth in real time. The time-resolved small-angle scattering exhibits a well-defined length scale associated with the spacing between unit-cell high surface features. This length scale imposes a discrete spectrum of Fourier components and rate constants upon the diffusion equation solution, evident in multiple exponential relaxation of the "anti-Bragg" diffracted intensity. An Arrhenius analysis of measured rate constants confirms that they originate from a single activation energy.

Surface-driven bulk reorganization of gold nanorods

Molecular dynamic simulations are used to study the structural stability of gold nanorods upon heating. We show that the global stability of the rod is governed by the free energetics of its surface. In particular, an instability of surface facets nucleates a bulk instability that leads to both surface and bulk reorganization of the rod. The surface reorganizes to form new, more stable, {111} facets, while the underlying fcc lattice completely reorients to align with this new surface structure. Rods with predominantly {111} facets remain stable until melting.

Melting of icosahedral gold nanoclusters from molecular dynamics simulations

Molecular dynamics simulations show that gold clusters with about 600-3000 atoms crystallize into a Mackay icosahedron upon cooling from the liquid. A detailed surface analysis shows that the facets on the surface of the Mackay icosahedral gold clusters soften but do not premelt below the bulk melting temperature. This softening is found to be due to the increasing mobility of vertex and edge atoms with temperature, which leads to inter-layer and intra-layer diffusion, and a shrinkage of the average facet size, so that the average shape of the cluster is nearly spherical at melting.

Observed effects of a changing step-edge density on thin-film growth dynamics

We grew SrTiO(3) on SrTiO(3)(001) by pulsed laser deposition, while observing x-ray diffraction at the (00(1/2)) position. The drop DeltaI in the x-ray intensity following a laser pulse contains information about plume-surface interactions. Kinematic theory predicts DeltaI/I = -4sigma(1 - sigma), so that DeltaI /I depends only on the amount of deposited material sigma. In contrast, we observed experimentally that |DeltaI /I| < 4sigma(1 - sigma) and that DeltaI /I depends on the phase of x-ray growth oscillations. The combined results suggest a fast smoothing mechanism that depends on surface step-edge density.