Bulk-like Features Research Articles

Determination of the Thickness of Interfacial Water by Time-Resolved Sum-Frequency Generation Vibrational Spectroscopy.

The physics and chemistry of a charged interface are governed by the structure of the electrical double layer (EDL). Determination of the interfacial water thickness (diw) of the charged interface is crucial to quantitatively describe the EDL structure, but it can be utilized with very scarce experimental methods. Here, we propose and verify that the vibrational relaxation time (T1) of the OH stretching mode at 3200 cm-1, obtained by time-resolved sum frequency generation vibrational spectroscopy with ssp polarizations, provides an effective tool to determine diw. By investigating the T1 values at the SiO2/NaCl solution interface, we established a time-space (T1-diw) relationship. We find that water has a T1 lifetime of ≥0.5 ps for diw ≤ 3 Å, while it displays bulk-like dynamics with T1 ≤ 0.2 ps for diw ≥ 9 Å. T1 decreases as diw increases from ∼3 Å to 9 Å. The hydration water at the DPPG lipid bilayer and LK15β protein interfaces has a thickness of ≥9 Å and shows a bulk-like feature. The time-space relationship will provide a novel tool to pattern the interfacial topography and heterogeneity in Ångstrom-depth resolution by imaging the T1 values.

Magnetic Levitation Patterns of Microfluidic-Generated Nanoparticle-Protein Complexes.

Magnetic levitation (MagLev) has recently emerged as a powerful method to develop diagnostic technologies based on the exploitation of the nanoparticle (NP)–protein corona. However, experimental procedures improving the robustness, reproducibility, and accuracy of this technology are largely unexplored. To contribute to filling this gap, here, we investigated the effect of total flow rate (TFR) and flow rate ratio (FRR) on the MagLev patterns of microfluidic-generated graphene oxide (GO)–protein complexes using bulk mixing of GO and human plasma (HP) as a reference. Levitating and precipitating fractions of GO-HP samples were characterized in terms of atomic force microscopy (AFM), bicinchoninic acid assay (BCA), and one-dimensional sodium dodecyl sulfate–polyacrylamide gel electrophoresis (1D SDS-PAGE), and nanoliquid chromatography–tandem mass spectrometry (nano-LC-MS/MS). We identified combinations of TFR and FRR (e.g., TFR = 35 μL/min and FRR (GO:HP) = 9:1 or TFR = 3.5 μL/min and FRR (GO:HP) = 19:1), leading to MagLev patterns dominated by levitating and precipitating fractions with bulk-like features. Since a typical MagLev experiment for disease detection is based on a sequence of optimization, exploration, and validation steps, this implies that the optimization (e.g., searching for optimal NP:HP ratios) and exploration (e.g., searching for MagLev signatures) steps can be performed using samples generated by bulk mixing. When these steps are completed, the validation step, which involves using human specimens that are often available in limited amounts, can be made by highly reproducible microfluidic mixing without any ex novo optimization process. The relevance of developing diagnostic technologies based on MagLev of coronated nanomaterials is also discussed.

Structure and dynamics of water confined in cylindrical nanopores with varying hydrophobicity.

We report a detailed study of the main structural and dynamical features of water confined in model Lennard–Jones nanopores with tunable hydrophobicity and finite length ( Å). The generic model of cylindrical confinement used is able to reproduce the wetting features of a large class of technologically and biologically relevant systems spanning from crystalline nanoporous materials, to mesoporous silica and ion channels. The aim of this work is to discuss the influence of parameters such as wall hydrophobicity, temperature, and pore size on the structural and dynamical features of confined water. Our simulation campaign confirmed the existence of a core domain in which water displays bulk-like structural features even in extreme ( Å) confinement, while dynamical properties were shown to depend non-trivially on the size and hydrophobicity of the pores.This article is part of the theme issue ‘Progress in mesoscale methods for fluid dynamics simulation’.

First-principles calculation of interfacial stability, energy, electronic properties, ideal tensile strength and fracture toughness of SiC/BN interface

The interfacial properties and electronic structure of $$\beta$$ -SiC (111)/ $$h$$ -BN (0001), including work of adhesion (Wad), interface energy, bonding nature, ideal tensile strength and fracture toughness, were investigated using first-principles calculations. Eight interface models, with two different terminations and four stacking sites, were investigated. The $$\beta$$ -SiC (111) slab, with 10 atomic layers, and the $$h$$ -BN (0001) slab, with two atomic layers, exhibited bulk-like interior features, respectively. The Case II interface, in which the Si atom was located at the center of the B-N bond, had the largest work of adhesion (2.749 J/m2), the smallest interfacial distance (2.02 A), the minimum interface energy (0.473 J/m2) and, thus, the best stability. The valence electron density and partial density of states indicated that there was a strong chemical and electrical interaction between the two sides of the Case II interface. The electronic structure analysis suggested that the interfacial bonding was mainly attributed to the Si-N ionic interaction and the valence electronic hybridization between Si-sp and B-sp. The ideal tensile strength and fracture toughness of the Case II interface were predicted as 13.26 GPa and 0.68–1.887 MPa•m1/2, respectively.

Palmitic acid-coated magnetite nanocubes with high-quality crystallinity and bulk-like magnetic features

We have demonstrated the synthesis of high-quality monocrystalline magnetite nanocubes through the introduction of palmitic acid as surfactant in a thermal decomposition synthesis of an organic Fe precursor. Unlike the standard thermal decomposition synthesis route, we report the avoid of the reducing agent and a modification in the synthesis heating ramp. Structural and magnetic properties were investigated showing well defined cubic shaped nanoparticles with a ∼ 40 nm edge and magnetic features close to bulk magnetite. We associate the bulk-like magnetic performance and properties to the highly crystalline structure of the nanocubes.In addition, we introduce a facile way to make a ligand exchange of nanocubes initial surfactant to citric acid in order to obtain biocompatible hydrophilic nanocubes. The potential application of the obtained sample in magnetic hyperthermia therapy is shown through calorimetric heating measurements on liquid dispersions of the nanocubes. We compute the Specific Absorption Rate to quantify the heating efficiency of the nanocubes.

First-principles calculations on electronic structure and interfacial stability of Mg/NbB2 heterogeneous nucleation interface

ABSTRACT The atomic structure, electronic property, work of adhesion and interface energy of the interface between α-Mg and NbB2 were studied by the first-principles calculations to investigate the heterogeneous nucleation potential of NbB2 particles as nucleation substrate for α-Mg grains. The convergence test shows that the NbB2 (0001) slabs with 9 atomic layers exhibit bulk-like interior feature. The stacking sequences of Mg atoms onto the surface of NbB2 (0001) slabs were analyzed. It was found that the “HCP” stacking sequence was most stable due to the maximum bonding work and minimum interfacial distance. The results of differential charge density and partial density of states (PDOS) study indicate that the B-terminated interface contains strong covalent/weak metal bonds, and the Nb-terminated interface exist strong metal bond characteristics. The calculation results of Mg/NbB2 interface energy elucidating that NbB2 particles can act as effective nucleation substrate on α-Mg grains to refine α-Mg grains under certain conditions.

Toward Free-Standing Lonsdaleite and Diamond Few Layers: The Nitrogen Effect

Strong, directional, N bonding led surface stabilization is induced to obtain free-standing two-dimensional (2D) layered assemblies of Lonsdaleite and diamond-phase carbon by surface termination at three coordinated C sites. These assemblies have strong bonding and surface N lone pair induced self-protection and achieve respective bulk-like electronic, mechanical, and crystallographic properties at as low as seven atomic layer thickness. Lonsdaleite phase subnano thickness free-standing as small as three layered assembly shows bulk-like thermal stability up to 2900 K. Size confinement effect dominates up to four layered assembly, and then electronic and crystallographic properties are dominated by bulk stabilization which saturates toward respective bulk-like features from seven layered assembly. Similarly, free-standing hybrid assemblies of Lonsdaleite phase carbon with BeF (C5NBeF) and BS (C5NBS) layer are also proposed along with low symmetric Cm-C3N4 layered assembly. The combination of N and O is sho...

Temperature dependence of the morphology and electronic structure of ultrathin platinum on TiO2-teminated SrTiO3 (001)

The authors investigate properties of ultrathin Pt as a function of coverage (up to 10 monolayers) on TiO2-terminated SrTiO3 (001) substrate at different temperatures (200–800 °C). In situ x-ray photoelectron spectroscopy, scanning electron microscopy, and atomic force microscopy are used to observe the electronic structure and surface morphology evolution of Pt. The authors find that although Pt will not wet SrTiO3 in the thermodynamic sense, it forms a continuous film when deposited at 200 °C due to the low surface mobility. At 800 °C, even at very low coverage, Pt forms nanoclusters showing bulk-like metallic features in the photoemission spectra. The authors compare the observed electronic structure evolution of Pt and the different growth patterns at low and high temperatures with available theoretical calculations.

Structural and optical characteristics of the hexagonal ZnO films grown on cubic MgO (001) substrates.

In this Letter, we report on the structural and optical characteristics of ZnO films with a wurtzite structure grown on MgO (001) substrates with cubic structures. The ZnO films were prepared through the molecular beam epitaxy method, and growth orientation transformation from [0001] to [10-10] direction was observed with the change of growth temperature and thickness. The x-ray diffraction pole figures and in situ RHEED patterns demonstrated that the rotational relationship among grains within the ZnO films appeared in a typical two-fold rotation of about 30° for the [0001] growth orientation and four-fold rotation of about 30° or 60° for the [10-10] growth orientation, respectively. Last, we investigated their optical properties through measuring the transmission and photoluminescence spectra of the ZnO films, which showed the bulk-like bandgap feature of the ZnO films in spite of the existing growth orientation transformation.

First-principles investigation on the electronic property and bonding configuration of NbC (111)/NbN (111) interface

The NbC (111)/NbN (111) interfaces were studied by first-principles calculation based on density functional theory to supply the theoretical basis for developing the novel NbC/NbN nano-multilayered film. Surface convergence calculations demonstrate that both NbC slab and NbN slab with more than thirteen atom-layers exhibit bulk-like interior feature. Thirty-six specific geometry models of NbC (111)/NbN(111) interface with different termination structures and stacking sequences were chosen. The calculated interfacial ideal work of adhesion and interfacial distance suggest that interface termination structure and stacking sequence both play the important role on influencing the interfacial stability, in which, C-terminated NbC is more inclined to join with the Nb-terminated NbN while the Nb-terminated NbC is more inclined to join with the N-terminated NbN. Moreover, for the C-N terminated and Nb-Nb terminated interfaces, the OT stacking sequence shows the highest stability, while for the C-Nb terminated and Nb-N terminated interfaces, the TL stacking sequence shows the highest stability. For the most stable C-Nb terminated and Nb-N terminated NbC (111)/NbN (111) interfaces, the atomic structure and the electronic transition from NbC to NbN are undertaken smoothly across interface. The chemical bond between the neighbor Nb atom and C or N atom in interface system shows both covalency and ionicity. Moreover, the transferred charge from Nb atom to N atom is larger than that from Nb atom to C atom, which may be the reason that Nb1-N terminated interface revealing the higher interfacial stability than C1-Nb terminated one.

Low-temperature growth of bismuth thin films with (111) facet on highly oriented pyrolytic graphite.

The epitaxial growth of artificial two-dimensional metals at interfaces plays a key role in fabricating heterostructures for nanoelectronics. Here, we present the growth of bismuth nanostructures on highly oriented pyrolytic graphite (HOPG) under ultrahigh vacuum (UHV) conditions, which was investigated thoroughly by a combination of scanning tunneling microscopy (STM), ultraviolet photoemission spectroscopy (UPS), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). It was found that (111)-oriented bilayers are formed on as-cleaved high-quality HOPG at 140 K, which opens the possibility of making Bi(111) thin films on a semimetal, and this is a notable step forward from the earlier studies, which show that only Bi(110) facets could be formed at ultrathin thickness at room temperature. XPS investigation of both C 1s and Bi 4f reflects the rather weak bonding between the Bi film and the HOPG substrate and suggests a quasi layer-by-layer growth mode of Bi nanostructures on HOPG at low temperature. Moreover, the evolution of the valence band of the interface is recorded by UPS, and a transition from quantum well states to bulk-like features is observed at varying film thickness. Unlike semimetallic bulk bismuth, ultrathin Bi(111) films are expected to be topological insulators. Our study may therefore pave the way for the generation of high quality Bi nanostructures to be used in spin electronics.

Effect of post-annealing process on the optical properties of lateral composition-modulated GaInP structure grown by molecular beam epitaxy

We report the molecular beam epitaxy growth of lateral composition-modulated (LCM) GaInP structures induced by GaP/InP short-period superlattice (SPS), and the effect of post-annealing process on their optical properties. The samples are structurally analyzed using cross-sectional transmission electron microscopy which reveals the formation of SPS-induced LCM-GaInP just above the substrate and bulk-like weakly phase separated LCM-GaInP towards the sample surface. The bulk-like features are formed due to strain accumulation and relaxation during SPS growth. The grown samples are annealed under different annealing conditions and optically characterized by low-temperature photoluminescence (LT-PL) and Raman spectroscopy. LT-PL reveals a lower energy PL peak originating from LCM-GaInP and a higher energy PL peak originating from weakly phase separated LCM-GaInP. These peaks red-shift with increase in annealing duration and temperature, and the higher energy PL peak approach towards the lower energy PL peak indicating enhanced phase separation of In-rich GaInP and Ga-rich GaInP vertical layers of the constituent LCM-GaInP structure. Also there exists an optimum annealing temperature and duration at which the PL intensity of the LCM-GaInP increases. More importantly, our results suggest that post-annealing process is a viable technique to enhance the optical properties of LCM-GaInP structures.

First-principle study of interfacial properties of Ni–Ni3Si composite

In order to explore the interfacial structure of Ni/Ni3Si (001) interfaces and clarify the heterogeneous nucleation potential of α-Ni grains on Ni3Si particles in Ni–Ni3Si eutectic alloys, the work of adhesion, electronic structure, and interface energy have been studied by first-principle method based on density functional theory (DFT). Surface convergence tests show that Ni3Si (001) slabs with more than eight layers exhibit bulk-like interior feature. The values of adhesion work and interfacial distance suggest the Ni+Si-terminated interface with hollow site stacking is the most thermodynamic stable and has the largest critical stress for crack propagation among the four models, indicating the precipitates of Ni3Si can improve stability and tensile strength of alloys. This interface has covalent feature, which is mainly contributed by the hybridization of Ni-3d and Si-3p orbital electron. The calculated interface energy of Ni/Ni3Si (001) interface is smaller than that between the α-Ni and nickel melts, elucidating the excellent nucleation potency of Ni3Si particles for α-Ni grains from thermodynamic considerations.

Plasmonic characteristics of Ag and ITO/Ag ultrathin films as-grown by sputtering at room temperature and after heating

Ag and ITO/Ag ultrathin films have been prepared by magnetron sputtering on unheated glass substrates. The Ag mass thickness was varied from 6 to 30 nm by increasing the deposition time, whereas the ITO film thickness was fixed at 30 nm. The optical, structural and morphological properties of the samples were analysed as-grown at room temperature and after heating at 300 °C in flowing nitrogen. The optical characteristics of the Ag samples have been modelled according to a system which has different contributions from bulk-like features to surface-plasmon characteristics arising from spheroidal particles with reduced carrier mobilities. The as-grown Ag morphological data are consistent with particle clusters that are loosely connected at the lowest deposition times and become wetted in a smooth film only when the deposition time increases. For the uncoated Ag samples, the heating at 300 °C produced surface migrations and reorganizations that resulted in films with a higher crystallite size in preferential Ag (1 1 1) orientation, together with significant changes in the surface roughness and the optical absorption. The ITO coating deposition process induced also some morphological changes, but the optical characteristics of the ITO/Ag stacks remained practically unchanged with respect to the analogous uncoated silver samples. Besides, the ITO coating has avoided that the underlying silver could migrate and agglomerate during the thermal treatment, although it has allowed the Ag (1 1 1) recrystallization.

Ab Initio Study of Confinement and Surface Effects in AlN Nanowires

Electronic structures and imaginary parts of dielectric functions of AlN nanowires are calculated using density functional theory. It is found that the surface atoms form quite localized states near the top of the valence band and the bottom of the conduction band. The dielectric function spectrum of a AlN nanowire is shown to be correlated with the surface-to-volume ratio. As the number of surface atoms to the bulk atoms decreases, the optical absorption of a AlN nanowire gradually acquires bulklike features. Furthermore, the effect of piezoelectric field on the AlN nanowire optical properties are tested. It is found that in addition to the shifts toward higher energies, as it is usual in bulk AlN, one can also obtain shifts toward lower energies depending on the frequencies. As the diameter of the nanowire increases, the reversal becomes less pronounced suggesting that the optical transition of a realistic AlN nanowire most probably manifests under stress a shift toward higher energies.

Hydration of ions under confinement

Molecular dynamics simulations are used to examine the changes in water density and hydration characteristics of NaCl solutions confined in slit-shaped graphitic pores. Using a structural signature, we define the hydration limit as the salt concentration at which a sharp drop in the hydration number is observed. At small pores (H = 8.0–10 Å), confined water does not possess bulk-like features and remains in a layered arrangement between two surfaces. Despite this high degree of confinement, ions are able to form a quasi-2D hydration shell between two surfaces. Our results indicate the strong propensity of ions to form the first hydration shell, even under extremely confined aqueous environments. The hydration of ions is seen to weakly perturb the oxygen density distributions between two surfaces. The hydration number of Na+ reduces to about 4.15 at a pore width of H = 0.8 nm, when compared with the bulk hydration number of 6.25. At larger pore widths, above H = 16 Å, where bulk-like water densities are observed in the central regions of the pore, the hydration number is above 6.

Influence of Hydrophilic Surface Specificity on the Structural Properties of Confined Water

The influence of chemical specificity of hydrophilic surfaces on the structure of confined water in the subnanometer regime is investigated using grand canonical Monte Carlo simulations. The structural variations for water confined between hydroxylated silica surfaces are contrasted with water confined between mica surfaces. Although both surfaces are hydrophilic, our study shows that hydration of potassium ions on the mica surface has a strong influence on the water structure and solvation force response of confined water. In contrast to the disrupted hydrogen bond network observed for water confined between mica surfaces, water between silica surfaces retains its hydrogen bond network displaying bulklike structural features down to surface separations as small as 0.45 nm. Hydrogen bonding of an invariant contact water layer with the surface silanol groups aids in maintaining a constant number of hydrogen bonds per water molecule for the silica surfaces. As a consequence, water depletion and rearrangement upon decreasing confinement is a strong function of the hydrophilic surface specificity, particularly at smaller separations. An oscillatory solvation force response is only observed for water confined between silica surfaces, and bulklike features are observed for both surfaces above a surface separation of about 1.2 nm. We evaluate and contrast the water density, dipole moment distributions, pair correlation functions, and solvation forces as a function of the surface separation.

Adsorption Isotherms of Water on Mica: Redistribution and Film Growth

Adsorption isotherms of water on muscovite mica are obtained using grand canonical Monte Carlo simulations over a wide range of relative vapor pressures, p/p(0) at 298 K. Three distinct stages are observed in the adsorption isotherm. A sharp rise in the water coverage occurs for 0 < p/p(0) < 0.1. This is followed by a relatively slow increase in the coverage for 0.1 < or = p/p(0) < or = 0.7. Above p/p(0) = 0.7, a second increase in the coverage occurs due to the adsorption of water with bulklike features. The derived film thickness and isotherm shape for the simple point charge (SPC) water model is in excellent agreement with recent experiments of Balmer et al. [ Langmuir 2008 , 24 , 1566 ]. A novel observation is the significant redistribution of water between adsorbed layers as the water film develops. This redistribution is most pronounced for 0.1 < or = p/p(0) < or = 0.7, where water is depleted from the inner layers and film growth is initiated on the outer layer. During this stage, potassium hydration is found to play a dominant role in the rearrangement of water near the mica surface. The analysis of structural features reveals a strongly bound first layer of water molecules occupying the ditrigonal cavities between the potassium ions. In-plane structure of oxygen in the second layer, which forms part of the first hydration shell of potassium, reveals a liquidlike structure with the oxygen-oxygen pair correlation function displaying features similar to bulk water. Isosteric heats of adsorption were found to be in good agreement with the differential microcalorimetric data of Rakhmatkariev ( Clays Clay Miner. 2006 , 54 , 402 ), over the entire range of pressures investigated. Both SPC and extended simple point charge (SPC/E) water models were found to yield qualitatively similar adsorption and structural characteristics, with the SPC/E model predicting lower coverages than the SPC model for p/p(0) > 0.7.

Surface plasmon resonance and magnetism of thiol-capped gold nanoparticles

Surface plasmon resonance measurements and magnetic characterization studies have beencarried out for two types of thiol-capped gold nanoparticles (NPs) with similar diametersbetween 2.0 and 2.5 nm and different organic molecules linked to the sulfur atom:dodecanethiol and tiopronin. In addition, Au NPs capped with tetraoctyl ammoniumbromide have also been included in the investigation since such capping molecules weaklyinteract with the gold surface atoms and, therefore, this system can be used as amodel for naked gold NPs; such particles presented a bimodal size distributionwith diameters around 1.5 and 5 nm. The plasmon resonance is non-existent fortiopronin-capped NPs, whereas a trace of such a feature is observed for NPs covered withdodecanethiol molecules and a bulk-like feature is measured for NPs capped withtetralkyl ammonium salts. These differences would indicate that the modification ofthe surface electronic structure of the Au NPs depends on the geometry andself-assembling capabilities of the capping molecules and on the electric charge transferredbetween Au and S atoms. Regarding the magnetization, dodecanethiol-cappedNPs have a ferromagnetic-like behaviour, while the NPs capped with tioproninexhibit a paramagnetic behaviour and tetralkyl ammonium-protected NPs arediamagnetic across the studied temperature range; straight chains with a well-definedsymmetry axis can induce orbital momentum on surface electrons close to the bindingatoms. The orbital momentum not only contributes to the magnetization butalso to the local anisotropy, giving rise to permanent magnetism. Due to thedomain structure of the adsorbed molecules, orbital momentum is not induced fortiopronin-capped NPs and the charge transfer only induces a paramagnetic spincomponent.

The electronic configuration and the conductance of silicon nanograins: An application of the scattering approach

The purpose of this study is the assessment of the properties of the electronic structure and of the transport characteristics of silicon nanostructures of a size comparable with the experimental ones. Accordingly, crystalline columnar grains, sandwiched between two adsorbing aluminum contacts, with a size of several hundreds atoms and linear dimensions up to a few nanometers, have been considered. The calculation method elaborates on the scattering approach, as reported in the recent chemical literature, using the extended Hückel theory for the evaluation of either the electronic charge or the transmission function. The calculations show that the binding energy has bulk-like features, i.e. its size dependence has a nearly flat asymptote with a value close to the cohesive energy of the solid. However the effect of the loosely coordinated boundary atoms is perceptible even at the sizes few hundreds atoms and produces an oscillatory behaviour of the binding energy. The comparison between this energy and the conductance indicates that both quantities increase at the same sizes and therefore the orbitals of the loosely bounded atoms are the important conductive channels.