Surface Relaxation Effects Research Articles

The structure of gold nanoparticles: molecular dynamics modeling and its verification by X-ray diffraction

Noble metal nanoparticles exhibit unique physical, chemical, biomedical, catalytic and optical properties. Understanding these properties and further development of production methods entail detailed knowledge of the structure at the atomic scale. Gold nanoparticles with multimodal size distribution were synthesized on porous silica and their atomic scale structure was studied by X-ray diffraction. The obtained experimental data are compared with molecular dynamics simulations. Spherical models of the Au nanoparticles, defined by ensembles of the Cartesian coordinates of constituent atoms, were generated and their geometry was optimized by applying the LAMMPS software. The comparison was performed in both reciprocal and real space. A good agreement is achieved for the models with disorder that can be related to surface relaxation effects and vacancy defects. The approach adopted here may have wider applications for further structural studies of other nanomaterials, offering direct verification of simulation results by experiment.

Potential-induced structural deformation at electrode surfaces

Abstract The atomic structure on the metal side of the electrochemical interface depends on the applied electric potential and the nature of the adsorbing species in the electrolyte solution. In this short article, we review some recent results probing surface stress and surface relaxation effects in single-crystal metal electrodes that are driven by potential changes. Both the potential and the structure in the electrolyte layers at the interface alter the metal electronic structure so that the surface in the electrochemical environment is strongly modified from the ultra-high vacuum counterpart. A methodology for linking experimental and theoretical approaches for a fundamental understanding of electrochemical reactions is proposed.

Band folding, strain, confinement, and surface relaxation effects on the electronic structure of GaAs and GaP: from bulk to nanowires

It is common to find materials that show strikingly different properties between its bulk and nanometric forms. In this paper we show how to link the electronic structures of two III-V systems, one a direct gap material, GaAs, and the other an indirect gap material, GaP, from their bulks right down to the shape of thin nanowires. The understanding of how these changes occur represents a scientific and technological challenge and is relevant for the design and prediction of novel nanostructured materials. GaAs and GaP bulk and nanowire systems are studied in the zinc-blende and wurtzite structures both free of strain and subjected to biaxial strains perpendicular to the [111]/[0001] direction (the kind of strain that the materials are subjected to when grown one on top of the other). We provide an interpretation of the band structure of nanowires, grown along the [111] (zinc-blende structure) and the [0001] (wurtzite structure) directions, in terms of the bulk band structures of the corresponding binary compounds. The procedure reveals the origin of the valence and conduction valleys relevant to determine the nature (direct or indirect) of the band gaps and the kind (direct and pseudodirect) of the valence to conduction transitions. Thus, by calculating only the bulk bands it is possible to describe the behavior of the nanowire bands even for very thin nanowires. The effects on the band structures due to biaxial strain are analogously analyzed, providing for bulk GaP the first results in literature. The role of confinement, and surface relaxation, in determining the nanowire electronic structure of thin nanowires are analyzed separately revealing that the change in the nature of the band gap is due mainly to surface relaxation effects, not confinement. We show that the change of the gap (indirect/direct) from the bulk to the 1D systems is mainly due to the competition between the energies of bulk conduction valleys which are differently influenced by confinement and strain. This effect is shown also by other low dimensional materials like the 2D materials extending only few atomic layers in one dimension. The competing valleys are already present in the bulk band structure. While the main effect of confinement is to open all gaps, it is not necessarily the main cause of the direct/indirect change in the nature of the electronic gap as instead is usually claimed in the literature. Our study can be used to understand and engineer the structure of many nanostructures systems by just better analysing the behavior of the bulk bands.

TEM diffraction contrast images simulation of dislocations.

For end-on screw dislocations and inclined dislocations within thin transmission electron microscopy (TEM) foil, TEM diffraction contrast image is largely modified around piercing point due to free surface relaxation. Based on many-beam Schaeublin-Stadelmann equations, TEM diffraction contrast images simulation of inclined dislocations within thin pure Fe TEM foil are performed and the difference between isotropic and anisotropic dislocation models are studied. Image force are superposed onto bulk elastic field of dislocation in thin foil, the elastic distortions nearby the two emerging points of a straight inclined dislocation located in an elastically isotropic/anisotropic thin foil are expressed as semianalytical solution in Fourier space, and the semianalytical image stress solutions in Fourier space are implemented into CUFOUR for studying free surface relaxation effect on TEM diffraction contrast of dislocation. Simulation results suggest that the diffraction contrast at the ends of dislocation is modified drastically by image forces, and effects of anisotropy cannot be neglected. LAY DESCRIPTION: For end-on screw dislocations and inclined dislocations within thin transmission electron microscopy (TEM) foil, TEM diffraction contrast image is largely modified around piercing point due to free surface relaxation. Based on many-beam Schaeublin-Stadelmann equations, TEM diffraction contrast images simulation of inclined dislocations within thin pure Fe TEM foil are performed and the difference between isotropic and anisotropic dislocation models are studied. Image force are superposed onto bulk elastic field of dislocation in thin foil, the elastic distortions nearby the two emerging points of a straight inclined dislocation located in an elastically isotropic/anisotropic thin foil are expressed as semianalytical solution in Fourier space, and the semianalytical image stress solutions in Fourier space are implemented into CUFOUR for studying free surface relaxation effect on TEM diffraction contrast of dislocation. Simulation results suggest that the diffraction contrast at the ends of dislocation is modified drastically by image forces, and effects of anisotropy cannot be neglected.

Tunable Topological State, High Hole-Carrier Mobility, and Prominent Sunlight Absorbance in Monolayered Calcium Triarsenide.

Designing novel two-dimensional (2D) materials is highly desirable for material innovation. Here, we propose monolayered calcium triarsenide (1L CaAs3) as a new 2D semiconductor with a series of encouraging functionalities. In contrast to the ∼33 meV small band gap in bulk CaAs3, 1L CaAs3 possesses a large direct band gap of 0.92 eV with a high hole mobility of ∼104 cm2 V-1 s-1. The electronic properties of 2D CaAs3 can be manipulated significantly by the layer thickness and external strains. Remarkably, 2D CaAs3 suggests a topologically nontrivial-trivial state transition under thickness reduction and strain engineering, which is attributed to the drastic surface relaxation and pinch effect under compression. A semiconductor-semimetal transition is also revealed when the layer thickness is greater than 3L. Furthermore, 1L CaAs3 exhibits prominent visible-light absorption compared with the crystalline silicon. All these desired properties render 2D CaAs3 a promising candidate for use in electronic and photovoltaic devices.

Whole powder pattern modelling macros for TOPAS

Macros implementing the main concepts of the whole powder pattern modelling approach have been written for TOPAS. Size and strain broadening components of the diffraction line profiles can be convolved with the instrumental profile already available among the standard commands of TOPAS. Specific macros are presented with examples of applications including plastically deformed powders and atomistic simulations. A macro is presented for the modelling of surface relaxation effects in spherical nanocrystals.

Size matching effect between anion vacancies and halide ions in passive film breakdown on copper

Different types of anion vacancies were optimized in passive films on copper via first-principles calculations based on density functional theory, and numerous potentiodynamic polarizations were performed to evaluate the size matching effect between anion vacancies and halide ions in passive film breakdown on copper. The size of the anion vacancies decreased to some extent compared with the corresponding atom due to the surface relaxation effect, and electrochemical experiments showed that the pitting potential for an oxide passive film on copper followed the order F− < Cl− < Br−, while this order was Cl− < F− < Br− for a sulfide passive film, which was attributed to the integrated effect of the energy of anion vacancy expansion and the Gibbs energy of dehydration. The pitting potential orders were the same on pre-oxidized or pre-sulfided copper with that of fresh copper in the same solution. Meanwhile, iodide ions were not compared due to the formation of poorly soluble copper iodides. The ratio of pit depth to pit mouth diameter on copper decreased with increasing pit volume, resulting in shallow pits with large mouth; thus, the catalytic-occluded cell effect was not obvious for copper.

Coordination-dependent surface strain and rational construction of robust structures

A surface relaxation model is established to study the elastic properties of nanoscale structures. This model predicts coordination-dependent strain at the surface and thickness-dependent stiffness of a material. Several atomic layers at the surface endure a significant strain gradient, which is dominated by the intrinsic properties of the material. The stiffness of low-dimensional materials is enhanced by surface relaxation effect. Surface effects on strong structures, including honeycomb structure and octet-truss structure with a high stiffness-to-weight ratio, are discussed. For these structures assembled with nanobeams, the Young’s modulus decreases with decreasing size of the struts. The coupling between Young’s modulus and relative density can be scaled down by engineering tensile strain on the struts.

N-type Rashba spin splitting in a bilayer inorganic halide perovskite with external electric field

Here, we investigated the Rashba effect of the CsPbBr3 bilayers under the external electric field (EEF) with first-principles calculations. For the PbBr2 terminated bilayer, we found that only electrons experience the Rashba splitting under EEF, while holes do not. Such an n-type Rashba effect can be ascribed to the surface relaxation effect that reverses the positions of the top valence bands. The n-type Rashba parameter can be tuned monotonically to the maximum of 0.88 eV Å at EEF of 1.35 V nm−1 at which the sequence of top valence bands recover to the bulk style. During this process the p-type spins will not survive in this 2D CsPbBr3, that indeed reveals a new way for making advanced functional spintronic devices.

First-principles calculations of different (001) surface terminations of three cubic perovskites CsCaBr3, CsGeBr3, and CsSnBr3

Three cubic bromide perovskites CsMBr3 (M = Ca, Ge, Sn) with two different surface terminations (CsBr and MBr2) were studied in this work using the first principles method. A wide range of physical properties, including electronic band structures, atom-projected density of states for each layer, surface relaxation effects, and surface energy, were evaluated for each considered surface termination. Differences between the properties of the bulk and slab models were highlighted. It was shown that surfaces with the CsBr termination have a lower energy and a more pronounced surface rumpling than those with the MBr2 termination. As a main result of this study, it was demonstrated that the CsBr-terminated surfaces appear to be energetically more stable in each of these three considered perovskites.

Lateral Interaction between Molecules Adsorbed on the Surfaces of Non-Metals

Spectral manifestations of lateral interactions between molecules adsorbed on non-metallic adsorbents, mostly oxides are considered. Static interaction, repulsive or attractive can be distinguished from the dynamic or resonance dipole–dipole interaction (RDDI) by means of isotopic dilution. Analysis of spectral data on CO adsorbed at low temperatures on ZnO, some other oxides or halogenides as well as recent quantum chemical calculations shows that static effect is enhanced by the solid due to the effect of surface relaxation induced by adsorption. Computer modelling of static and dynamic interactions enable one to explain the structure of the bands of adsorbed CO, frequency shifts, and other details in the spectra. Attractive interactions between different molecules can lead to mutual enhancement of adsorption and to the phenomena of induced Bronsted acidity and basicity, or the superacidity of some oxides exposed to gaseous acids. The theory and new experimental data on the influence of RDDI on the band shape of adsorbed SF6 and CF4 are discussed. Spectral evidence for RDDI and vibrational energy transfer between NF3 and pre-adsorbed SF6 on ZnO is reported.

Transparent and water repellent ceria film grown by atomic layer deposition

Transparent and hydrophobic ceria film was fabricated by atomic layer deposition (ALD). The ceria coatings were characterized by goniometry, atomic force microscopy, X-ray photoelectron spectroscopy and variable angles spectroscopic ellipsometry. The hydrophobicity of the ceria coatings was investigated with water contact angles achieving as high as 105°. The effect of annealing or surface relaxation on the hydrophobicity was studied. Surface chemistry analysis of the ceria surfaces was carried out to understand the surface treatment towards the wettability of the ALD coatings. The proposed ALD ceria film offers the advantages of hydrophobic coatings covering fine optical lens such as band-pass filter.

A total scattering Debye function analysis study of faulted Pt nanocrystals embedded in a porous matrix.

Faulted face-centred cubic platinum nanocrystals, grown within a nanoporous silica matrix, have been extensively characterized by the Debye function analysis method applied to wide-angle synchrotron X-ray total scattering data. A method for building databases of sampled interatomic distances of weakly faulted materials is proposed, maintaining statistical significance and allowing complete populations of differently sized and shaped nanocrystals to be used within the DEBUSSY approach. This study suggests that anisotropic Pt nanoclusters are formed in the presence of a shape-directing (templating) agent, and tentatively describes the effects of post-synthetic temperature treatments on fault probability, size, shape and dispersion of the nanocrystal populations. Surface relaxation effects are also observed in the smallest particles.

Structural relaxation of BaTiO3 slab with tetragonal (100) surface: Ab-initio comparison of different thickness

Symmetric BaTiO3 slabs of different thickness (6, 8 and 10-unit-cell thick) with BaO (100) surface terminations in tetragonal phase were computed by first-principles. The effects of surface termination on the structural parameters were studied. In contrast to previous literature findings, all interlayer spacings were subjected to expansions except for the outmost interlayer spacing. A comparison among three slabs with varying thickness revealed that the expansion in interlayer spacings in the 8-unit-cell thick slab is smaller than the other two slabs. The surface relaxation effects penetrated into five layers from BaO (100) surface, beyond which a trailing effect in form of small expansion of interlayer spacing was noticed and there is zero rumpling. The computed in-plane ferroelectric displacements of each layer were relatively independent of the slab thickness. In agreement of previous works, symmetric tetragonal BaTiO3 slab with BaO (100) surface terminations was predicted to possess a much smaller value of in-plane spontaneous polarization than that in the bulk case. This is due to the computed polarization value using Berry phase formalism is too small with regard to ferroelectric displacements in the TiO2 layers.

Effects of surface relaxation and reconstruction on the vibration characteristics of nanobeams

Surface effects on the free vibration characteristics of nanobeams are investigated by a modified continuum model. In this paper, the relationship between the parameters of the modified continuum model of surface effects including surface elasticity, surface density, and residual surface stresses, and the parameters of the atomistic lattice model such as surface relaxation and reconstruction in nanobeams is characterized by an atomistic lattice model. The surface effects are incorporated into nanobeams to develop a modified continuum model depicting the free vibrational behavior of nanobeams. The model is validated with the experimental data of an effective size-dependent Young’s modulus and the previous theoretical results. The results demonstrate that both surface elasticity and surface density vary exponentially with surface layer thickness. Therefore, surface elasticity and density can be affected by surface relaxation and residual surface stresses can be induced by surface reconstruction. The natural frequencies of doubly clamped nanobeams can be affected by the dimensions of the nanobeams, surface layer thickness, and residual surface stress. This work may be helpful for understanding surface effects and their influence on the vibrational behavior of nanobeams.

Powder diffraction in studies of nanocrystal surfaces: chemisorption on Pt

Atoms at the surface of nanocrystals contribute appreciably to the X-ray diffraction pattern. Phenomena like chemisorption, affecting the displacement of surface atoms with respect to their positions in the perfect crystallographic structure, cause diffraction peak shifts and intensity changes. These effects are easily measurable for small nanocrystals up to 10 nm size. This article reports diffraction effects of chemisorption of adsorbing gases H2, O2, CO and NO for a series ofin situpowder diffraction experiments on nanocrystalline Pt supported on silica. On the basis of previous diffraction observation of Pt surface reconstruction during hydrogen desorption, it was possible to quantify this effectversuscrystallite size and rationalize the observed diffraction peak shift for the other adsorbing species. This enabled the surface reconstruction to be distinguished from the surface relaxation effect, the latter depending monotonically on the adsorption energy. Even if no phase transition occurs, monitoring of a peak's position, intensity, width and gas composition (viamass spectrometry) during a carefully designed physicochemical process (including surface chemical reaction) enables insight into and understanding of the surface structure evolution (e.g.amorphization, relaxation, reconstruction or changes in the overall morphology). The proposed technique can be used as a surface science tool, allowing studies of nanocrystals under high pressure.

Effects of surface compliance and relaxation on the frictional properties of lamellar materials

We describe the results of atomic-level stick-slip friction measurements performed on chemically-modified graphite, using atomic force microscopy (AFM). Through detailed molecular dynamics simulations, coarse-grained simulations, and theoretical arguments, we report on complex indentation profiles during AFM scans involving local reversible exfoliation of the top layer of graphene from the underlying graphite sample and its effect on the measured friction force during retraction of the scanning tip. In particular, we report nearly constant lateral stick-slip magnitudes at decreasing loads, which cannot be explained within the standard framework based on continuum mechanics models for the contact area. We explain this anomalous behavior by introducing the effect of local compliance of the topmost graphene layer, which varies when interaction with the AFM tip is enhanced. Such behavior is not observed for non-lamellar materials. We extend our discussion toward the more general understanding of the effects of the top layer relaxation on the friction force under pushing and pulling loads. Our results may provide a more comprehensive understanding of the effectively negative coefficient of friction recently observed on chemically-modified graphite.

Very large dielectric response from ferroelectric nanocapacitor films due to collective surface and strain relaxation effects

Dependence of the dielectric response of ferroelectrics on defect types, particularly those with long range strain fields in confined geometries have been often mentioned, especially in interpreting experimental results in films. However, detailed discussions on the mechanisms with which defects alter properties, particularly in the presence of interfaces imposing certain boundary conditions, are seldom made. Here, we studied the thickness dependence of transition temperatures and dielectric response of Metal/BaTiO3/Metal ferroelectric nanocapacitor structures grown on SrTiO3 using a phenomenological approach accounting for the equations of electrostatics and semiconductors. Relaxation of the misfit strain via misfit dislocations amplify the surface effects in films below a critical thickness and favor electrical domains leading to very large dielectric responses in this regime. Thin film structures with relaxed misfit strain in this work are fully depleted in the presence of moderate densities of impurities ( ∼ 1025 m −3). This is due to the reduction of polarization amplitude parallel to the film normal and its mplications for near-micron thick films are discussed. Consequently, the misfit dislocation sites have nearly no free carrier localization, making the role of these sites on leakage currents highly questionable. Dielectric response of intrinsic thicker films ( &amp;gt; 40 nm) is mostly under the influence of strain relaxation only with minimal interface impact in the limit of ideal electrodes. Our results point out that control of the dislocation density can lead to non-conventional functionalities in ferroelectric thin film capacitors via electromechanical coupling of properties to structure and domain stabilization.

Evolution of a short-T2 liquid-like 1H signal during the hydration of White Portland Cement

The kinetics of cement hydration has been studied by 1H-NMR, through the analysis of the Free Induction Decay (FID) combined with quasi-continuous T1 relaxation time distributions analysis, operating a sort of 2D T1–T2 correlation analysis on a somewhat more physical than mathematical basis. The analysis allowed us to distinguish two solid components, SS (Solid-Short) with T1 of a few milliseconds and SL (Solid-Long) with T1 of the order of a second, ascribable to 1H of anisotropically bound groups in the C–S–H gel, and to 1H nuclei of the Portlandite crystals and possibly to crystal water in Ettringite crystals, respectively. A liquid-like component LS (Liquid-Short) with very short T2 (∼80μs) and T1 a few milliseconds, both quite stable during hydration, was isolated and interpreted as water confined in the inter-layer spaces of the gel. Amounts of LS and SS increase together steadily during hydration, and these, divided by LL (Liquid-Long, the signal from the water in the pores), are proportional to 1/T1 for LL, confirming a surface relaxation effect for LL.

Simulation and Modeling of Nanoparticle Surface Strain

The effects of surface relaxation in the powder diffraction pattern from metal nanoparticles are discussed. Molecular dynamics simulations are carried out to simulate the structure of a series of free-standing Al and Cu nanoparticles of different sizes and stabilization temperatures. The diffraction patterns found from considering the average atomic positions are then modeled, assuming different forms for the effects of the surface strain field. The modeling finds that the strain field in the simulated Al particles does not result in an appreciable effect on the peak broadening. However, that of the Cu particles results in anisotropic peak broadening, which is not able to be properly accounted for by the existing isotropic surface strain models.