Free-standing State Research Articles

Strain engineering of antimonene by a first-principles study: Mechanical and electronic properties

In this work, we investigate the mechanical and electronic properties of monolayer antimonene in its most stable beta-phase using first-principles calculations. The upper region of its valence band is found to solely consist of lone pair p-orbital states, which are by nature more delocalized than the d-orbital states in transition metal dichalcogenides, implying superior transport performance of antimonene. The Young's and shear moduli of beta-antimonene are observed to be ~25% higher than those of bulk antimony, while the hexagonal lattice constant of the monolayer reduces significantly (~5%) from that in bulk, indicative of strong inter-layer coupling. The ideal tensile test of beta-antimonene under applied uniaxial strain highlights ideal strengths of 6 GPa and 8 GPa, corresponding to critical strains of 15% and 17% in the zigzag and armchair directions, respectively. During the deformation process, the structural integrity of the material is shown to be better preserved, albeit moderately, in the armchair direction. Interestingly, the application of uniaxial strain in the zigzag and armchair directions unveil direction-dependent trends in the electronic band structure. We find that the nature of the band gap remains insensitive to strain in the zigzag direction, while strain in the armchair direction activates an indirect-direct band gap transition at a critical strain of 4%, owing to a band switching mechanism. The curvature of the conduction band minimum increases during the transition, which suggests a lighter effective mass of electrons in the direct-gap configuration than in the free-standing state of equilibrium. The work function of free-standing beta-antimonene is 4.59 eV and it attains a maximum value of 5.07 eV under an applied biaxial strain of 4%.

Fundamentals, progress, and future directions of nitride-based semiconductors and their composites in two-dimensional limit: A first-principles perspective to recent synthesis

Potential applications of bulk GaN and AlN crystals have made possible single and multilayer allotropes of these III-V compounds to be a focus of interest recently. As of 2005, the theoretical studies have predicted that GaN and AlN can form two-dimensional (2D) stable, single-layer (SL) structures being wide band gap semiconductors and showing electronic and optical properties different from those of their bulk parents. Research on these 2D structures have gained importance with recent experimental studies achieving the growth of ultrathin 2D GaN and AlN on substrates. It is expected that these two materials will open an active field of research like graphene, silicene, and transition metal dichalcogenides. This topical review aims at the evaluation of previous experimental and theoretical works until 2018 in order to provide input for further research attempts in this field. To this end, starting from three-dimensional (3D) GaN and AlN crystals, we review 2D SL and multilayer (ML) structures, which were predicted to be stable in free-standing states. These are planar hexagonal (or honeycomb), tetragonal, and square-octagon structures. First, we discuss earlier results on dynamical and thermal stability of these SL structures, as well as the predicted mechanical properties. Next, their electronic and optical properties with and without the effect of strain are reviewed and compared with those of the 3D parent crystals. The formation of multilayers, hence prediction of new periodic layered structures and also tuning their physical properties with the number of layers are other critical subjects that have been actively studied and discussed here. In particular, an extensive analysis pertaining to the nature of perpendicular interlayer bonds causing planar GaN and AlN to buckle is presented. In view of the fact that SL GaN and AlN can be fabricated only on a substrate, the question of how the properties of free-standing, SL structures are affected if they are grown on a substrate is addressed. We also examine recent works treating the composite structures of GaN and AlN joined commensurately along their zigzag and armchair edges and forming heterostructures, δ-doping, single, and multiple quantum wells, as well as core/shell structures. Finally, outlooks and possible new research directions are briefly discussed.

Spontaneous curling of freestanding Janus monolayer transition-metal dichalcogenides.

In this paper, using molecular dynamics simulations we report spontaneous curling behaviors of freestanding Janus monolayer S-Mo-Se (MoSeS) structures. Density functional theory calculations are performed to obtain the phonon dispersion and phonon spectra of the Janus monolayer MoSeS for analyzing its structural stability. The results show that the Janus monolayer MoSeS is structurally stable. Due to the lattice mismatch between MoS and MoSe domains, the Janus monolayer MoSeS at the freestanding state always spontaneously rolls up in a constant temperature and pressure system. The direction of curling is preferred along the armchair orientation. Specifically, as for the Janus monolayer MoSeS whose size is larger than ∼30 nm, it can spontaneously roll up into a nanotube structure. The underlying physical mechanisms of these phenomena are well uncovered by using classical Timoshenko plate theory and the minimum energy principle.

C4N3H monolayer: A two-dimensional organic Dirac material with high Fermi velocity

Searching for two-dimensional (2D) organic Dirac materials, which have more adaptable practical applications in comparing with inorganic ones, is of great significance and has been ongoing. However, only two kinds of these materials with low Fermi velocity have been discovered so far. Herein, we report the design of an organic monolayer with C$_4$N$_3$H stoichiometry which possesses fascinating structure and good stability in its free-standing state. More importantly, we demonstrate that this monolayer is a semimetal with anisotropic Dirac cones and very high Fermi velocity. This Fermi velocity is roughly one order of magnitude larger than that in 2D organic Dirac materials ever reported, and is comparable to that in graphene. The Dirac states in this monolayer arise from the extended $\pi$-electron conjugation system formed by the overlapping 2\emph{p}$_z$ orbitals of carbon and nitrogen atoms. Our finding opens a door for searching more 2D organic Dirac materials with high Fermi velocity.

Gas-like adhesion of two-dimensional materials onto solid surfaces

The adhesion of two-dimensional (2D) materials onto other surfaces is usually considered a solid-solid mechanical contact. Here, we conduct both atomistic simulations and theoretical modeling to show that there in fact exists an energy conversion between heat and mechanical work in the attachment/detachment of two-dimensional materials on/off solid surfaces, indicating two-dimensional materials adhesion is a gas-like adsorption rather than a pure solid-solid mechanical adhesion. We reveal that the underlying mechanism of this intriguing gas-like adhesion is the configurational entropy difference between the freestanding and adhered states of the two-dimensional materials. Both the theoretical modeling and atomistic simulations predict that the adhesion induced entropy difference increases with increasing adhesion energy and decreasing equilibrium binding distance. Our findings provide a fundamental understanding of the adhesion of two-dimensional materials, which is important for designing two-dimensional materials based devices and may have general implications for nanoscale efficient actuators.

Two-dimensional boron: structures, properties and applications.

Situated between metals and non-metals in the periodic table, boron is one of the most chemically versatile elements, forming at least sixteen bulk polymorphs composed of interlinked boron polyhedra. In low-dimensionality, boron chemistry remains or becomes even more intriguing since boron clusters with several to tens of atoms favor planar or cage-like structures, which are similar to their carbon counterparts in terms of conformation and electronic structure. The similarity between boron and carbon has raised a question of whether there exists stable two-dimensional (2D) boron, as a conceptual precursor, from which other boron nanostructures may be built. Here, we review the current theoretical and experimental progress in realizing boron atomic layers. Starting by describing a decade-long effort towards understanding the size-dependent structures of boron clusters, we present how theory plays a role in extrapolating boron clusters into 2D form, from a freestanding state to that on substrates, as well as in exploring practical routes for their synthesis that recently culminated in experimental realization. While 2D boron has been revealed to have unusual mechanical, electronic and chemical properties, materializing its potential in practical applications remains largely impeded by lack of routes towards transfer from substrates and controlled synthesis of quality samples.

(Invited) The Mechanisms By Which Graphene Nano-Pigments Inhibit Corrosion-Driven Organic Coating Delamination

There has been significant recent interest in the development of nano-pigments based on graphene (graphene nano-pigments or GNPs) as possible additives to corrosion-resistant organic coatings. It has been claimed that lyophilized graphene particles dispersed in organic polymer coatings can effectively inhibit under-coating corrosion and corrosion-driven coating delamination. It has been further proposed that the inhibitory effect of GNPs results from either a) their blocking the through-coating diffusion of reactive species such as oxygen or b) the displacement of electrochemical processes (specifically cathodic oxygen reduction) away from the metal-coating interface and onto the intra-coating graphene surface. In this study a commercially available surface-modified GNP based on exfoliated graphite is used to systematically study the effect of GNPs on electrochemical and mass-transport processes occurring in GNP pigmented polymer films both in a free-standing state and as applied to iron and zinc surfaces. The GNP used consists of small secondary particles with a typical planar diameter of 0.3-5 microns and a thickness < 50nm. These secondary particles are in turn composed of varying number of graphene nanoplatelets. The GNP supplied had been subject to plasma processing intended to improve dispersibility in oxygenated solvents and promote compatibility with organic polymers bearing oxygen functionality. The GNP pigmented coatings consist of 30 micron thick solution-cast polyvinylbutyral (PVB) composites films. The PVB/GNP ratio is systematically varied to produce a series of composites with GNP volume fractions between zero and 0.2. The O2 permeability of these composites is determined using a photochemical method based on the atmospheric re-oxidation of leucomethylene blue in conjunction with free-standing films to follow the kinetics of through-coating oxygen diffusion. It is thus shown that O2permeability decreases sharply (by 1-2 orders of magnitude) as the GNP volume fraction is increased from zero towards 0.15. An in-situ scanning Kelvin probe technique is used to study the effect of varying GNP pigment volume fractions on the development of metal-coating contact potentials and on the kinetics of corrosion-driven coating (cathodic) delamination. Under circumstances where the GNP volume fraction is sufficiently high as to allow charge percolation through a continuous network of GNP particles contact potentials are measurable between the metal substrate and PVB/GNP composite coating on both iron and zinc. In both cases these contact potentials are consistent with the work function difference between the relevant metal and graphene/graphite. In the case of zinc this contact potential is significant (>0.5V) and is consistent with the GNP network being the cathode in any galvanic couple involving zinc and the GNPs. In the case of iron the contact potential is relatively small. When a penetrative defect is created in the PVB/GNP coating and the substrate exposed to aqueous sodium chloride electrolyte in an atmosphere of 95% relative humidity for up to 100 hours. On both iron and zinc it is shown that the rate of cathodic delamination becomes reduced by > 90% relative to the unpigmented case as at a GNP volume fraction increases. Typical SKP-derived time-dependent potential (Ecorr) versus distance (from the penetrative defect edge) profiles for unpigmented PVB and PVB/GNP composite coatings are presented. Delamination kinetics are quantified by plotting the position of the delamination front as a function of time for coatings comprising various GNP volume fractions. Similar dependence of delamination kinetics on GNP volume fraction are observed for composite coatings adherent to both iron and zinc. Analysis of these kinetics suggests that addition of GNP at > 2.5% volume fraction changes the delamination rate law from parabolic (half order) to linear (zero order) with respect to time. This is consistent with the rate limiting step changing from under-film cation mass transport to an interfacial processes such as O2 reduction becoming rate limiting. The mechanism of GNP inhibition is discussed in relation to the electrochemical and mass transfer-blocking properties of graphene. It is concluded that, although a displacement of cathodic O2 reduction away from the metal coating interface is possible (and in fact predicted) the observed inhibition effects are immediately consistent with the measured reduction in O2 permeability at the relevant GNP volume fraction.

Earthquake response of solitary slender freestanding intake towers

Earthquake response of solitary slender intake towers is investigated considering an idealized hollow intake tower with its circular footing submerged in water. The tower is studied in anchored (fixed base) and unanchored (freestanding) states placed on undeformable soil foundation. The water-structure interaction is modeled by the Eulerian-Lagrangian approach, using the pressure-based elements for the water and the displacement-based elements for the structure. The only source of nonlinearity is the contact at the base joint between the tower's footing and the ground. This contact is modeled using Coulomb friction model which allows the tower to slide and uplift. The system is three-dimensionally analyzed using finite element method under static and dynamic earthquake loads. A detailed parametric study is conducted to assess the importance of system characteristics including surrounding and inside water levels, ratio of tower height to footing radius, base joint friction coefficient, water compressibility, footing flexibility, and vertical ground motion.

Two-dimensional hexagonal layers of A N B 8–N compounds on semiconductors

Using the parabolic model of the electronic spectrum of the substrate and the low-energy approximation of the dispersion law for two-dimensional hexagonal compounds ANB8‒N, the density of states of an epitaxial layer has been investigated as a function of the band gap of the substrate, the band gap of the graphene-like compound in a free-standing state, their mutual arrangement, and the dimensionless “layer–substrate” coupling constant C. It has been shown that, when the coupling constant C exceeds critical values, the density of states of the epitaxial layer undergoes qualitative changes. Both flat and buckled epitaxial layers have been considered. Estimates of the charge redistribution due to the transformation of the density of states of the graphene-like compound have been presented.

Highly Conductive Stretchable Electrodes Prepared by In Situ Reduction of Wavy Graphene Oxide Films Coated on Elastic Tapes

A facile method is reported to prepare highly conductive stretchable wavy reduced graphene oxide (rGO) films by in situ reducing blade‐coated graphene oxide films on elastic tapes. The sheet resistance of a 1.20 μm thick rGO film coated elastic tape is measured to be as low as 21 Ω sq−1 and it does not change during 1000 cycles of stretching/releasing with a maximum strain of 100%. The wavy rGO film can also be detached from its elastic substrate into a freestanding state, keeping its excellent elasticity. This in situ reduction method does not involve film transfer and high‐temperature annealing; thus it is convenient and scalable. A solid‐state electrochemical capacitor with these rGO electrodes shows large areal specific capacitance, and excellent electrochemical stability and flexibility.

Minimizing Unintentional Strain and Doping of Single-Layer Graphene on SiO2 in Aqueous Environments by Acid Treatments.

The effects of treating SiO2/Si with either acidic or alkaline solutions on single-layer graphene were investigated using Raman microscopy. It is well-known that in air graphene on SiO2 is unintentionally strained and hole-doped to different degrees, varying widely by sample. It is also known that various amine compounds act as electron donors to graphitic materials. In this study, a SiO2/Si substrate was simply dipped in either a concentrated HCl solution or pH 9.0 NaOH solution and then rinsed, prior to transferring graphene on it. The G and 2D peaks were followed at a fixed position on a single-layer graphene flake in water and various concentrations of pH 7.4 tris(hydroxymethyl)aminomethane (Tris) buffer. The results demonstrate that these treatments reduce the sample variation, improve the stability against Tris, and even bring some graphene samples close to a freestanding state. The Raman analysis reveals that the main effect of dipping is to relieve strain. The undoping effect on some samples is explained by the HCl solution becoming trapped between the graphene and SiO2 surface.

Magnetic Moment Enhancement for Mn7 Cluster on Graphene

Mn7 cluster on graphene with different structural motifs and magnetic orders are investigated systematically by first-principles calculations. The calculations show that Mn7 on graphene prefers a two-layer motif and exhibits a ferrimagnetic coupling. The magnetic moment of the Mn7 cluster increases from 5.0 μB at its free-standing state to about 6.0 μB upon adsorption on graphene. Mn7 cluster also induces about 0.3 μB of magnetic moment in the graphene layer, leading to an overall enhancement of 1.3 μB magnetic moment for Mn7 on graphene. Detail electron transfer and bonding analysis have been carried out to investigate the origin of the magnetic enhancement.

Controllable occurrence of free-standing lipid membranes on nanograting structured supports.

Supported lipid bilayers (SLBs) have been widely used to study protein-lipid membrane interactions because their planar geometry is suitable for many surface analysis tools. However, the friction coupling between the support and the membrane can influence the properties of biomolecules in the membrane. Many studies have attempted to span SLBs over nanostructured supports to create free-standing regions in SLBs for biosensor applications. However, membranes following the support surface contour are more frequently observed than are free-standing membranes on structured supports, indicating that the parameter range suitable for formation of free-standing SLBs might be narrow and more information is necessary to understand the required conditions. The objective of this study was to estimate the system energies of free-standing and contour-following membrane states and determine which state is the most energetically favorable under various conditions. For a lipid membrane preferring to stay close to the support, an energy reward occurs when they are in close proximity; however, increasing the contact area on a structured surface can result in an energy penalty because of the bending of the lipid bilayer. Whether the energy reward or the energy penalty dominates could determine the membrane state. We used the extended Derjaguin-Landau-Verwey-Overbeek (DLVO) theory and the Helfrich bending theory to relate the energy sizes to experimentally controllable parameters. We experimentally examined whether the membrane state followed the model prediction when we used various buffer ionic strengths, various lipid types, and nanograting supports with three different geometries. Because it is difficult to observe the experimental membrane state directly at the nanoscale, we developed a method to use the fluorescence recovery shape change after photobleaching to distinguish experimental membrane states at the micrometer scale. Our experimental results closely matched the theoretical predictions, suggesting that the developed model can be used to predict suitable conditions for formation of free-standing bilayers on nanostructured solid supports.

Physical Aging of Polymer Films Quenched and Measured Free-Standing via Ellipsometry: Controlling Stress Imparted by Thermal Expansion Mismatch between Film and Support

We have developed an ellipsometry method to measure the physical aging rate of polymer films that have been thermally quenched and aged in a free-standing state where the stress imparted to the films is well-defined by the thermal-expansion mismatch between film and rigid support. For free-standing polystyrene films supported by rigid sample holders with circular openings, we demonstrate that the physical aging rate is independent of film thickness between 220 and 1800 nm when the applied stress is the same. In contrast, by comparing free-standing films supported by frames of different materials, the physical aging rate decreases by nearly a factor of 2 when the thermal-expansion mismatch, and hence stress, is reduced. We conclude that stress is key in controlling the resulting physical aging rate of free-standing films, and there is no inherent film-thickness dependence (above 220 nm) to the aging rate when stress during glass formation is held fixed.

Analysis on Stability of the Short-Pylon Cable-Stayed Bridge

This thesis took the Changshan Bridge in Dalian Changhai county as an example and adopted the general finite element software ANSYS to simulate it. Analyzed the stability of the completed bridge and the single free-standing tower state, and studied the buckling of the main girder and the thin-walled pier.

Application of resonant ultrasound spectroscopy to determine elastic constants of plasma-sprayed coatings with high internal friction

We present a novel modification of resonant ultrasound spectroscopy (RUS) for analysis of elastic constants of plasma-sprayed coatings in the ‘as sprayed’ state, i.e. without removing the substrate. This modification is suitable for coatings with high internal friction (Q−1≳10−2), which cannot be measured by RUS in the free-standing state due to strong damping. In combination with through-transmission measurements, this modification is able to provide full anisotropic tensor of elastic constants of the coating. As an illustrative example, the proposed methodology is applied to two steel coatings prepared by water-stabilized plasma (WSP) spraying. In addition to the moduli, also the internal friction parameters of the coatings are obtained. The results of RUS are compared to moduli determined by four-point bending tests, and it is shown that the anisotropic elasticity of both examined materials exhibits an elliptic form of transversal isotropy with four independent elastic coefficients.

Magnetic property of a one-dimensional FePt atomic chain on a NiAl(110) surface

Through the full potential linearized augmented plane wave (FLAPW) method, the magnetic property of a one-dimensional (1D) FePt diatomic chain has been explored. We find a ferromagnetic (FM) ground state for both free-standing and supported systems on a NiAl(110) surface. In the free-standing state, Fe and Pt atoms have magnetic moments of 3.47 and 0.74 µB, respectively. On a NiAl(110) surface, the magnetic moment of Fe becomes 2.95 µB, and there is no spin polarization in the Pt atom. In contrast, the surface Ni has an induced magnetic moment of 0.2 µB. The direction of magnetization and the strength of magnetic anisotropy energy are found to be substantially affected by the NiAl(110) surface. For instance, the magnetization aligns perpendicular to the chain axis for the free-standing state, and the calculated magnetic anisotropy energy is 18.33 meV/cell, however the magnetization direction of a supported system aligns parallel to the chain axis with an anisotropy energy of 0.568 meV/cell. Overall, we find that the magnetic anisotropy of a 1D FePt atomic chain is substantially influenced by the presence of the NiAl(110) substrate.

Structural and electronic properties of graphene-based junctions for spin-filtering: The graphene/Al/Ni(1 1 1) intercalation-like system

The atomic and electronic structure of the graphene/Al/Ni(111) system is studied via combination of LEED and spectroscopic methods (X-ray absorption, core-level and valence-band photoelectron spectroscopy), respectively. These data demonstrating the (2×2) overstructure and the decoupling of the graphene layer from the substrate (with respect to graphene/Ni(111)) are in very good agreement with DFT calculations supporting nearly free-standing state of graphene in this system (with small n-doping). The perspectives of application of Al-based intercalation-like systems are discussed.

Importance of Quench Conditions on the Subsequent Physical Aging Rate of Glassy Polymer Films

For over a decade, research from the gas permeation community has observed faster physical aging rates with decreasing thickness of free-standing films, termed “accelerated aging”. These deviations in the aging rate from bulk behavior occur at film thicknesses of several micrometers, the largest “confinement” length scale ever reported. Here we systematically address various possible causes of this phenomenon from differences in molecular structure, quench depth below Tg, experimental technique, sample preparation, and stresses on the film. We demonstrate that the physical aging of the material is strongly dependent on conditions during the formation of the glassy state. Although supported films do not display any film thickness dependence to their aging rate at this large length scale, films quenched in a free-standing state exhibit a strong thickness dependence. We suggest differing quench conditions may impose unintended stresses trapping the glassy films into different states (potential energy minima)...

Structural and electronic properties of copper nanowire encapsulated into BeO nanotube: First-principles study

We present a systematic study on the structural and electronic properties of close-packed Cu nanowires encapsulated in a series of zigzag ( n,0) BeONTs using first-principles calculations. The initial shapes (cylindrical CuNWs and BeONTs) are preserved without any visible changes for the Cu m @( n,0) ( m=6 or 8, 8≤ n≤14) combined systems. The most stable combined systems are Cu 6@(10,0) and Cu 8@(11,0) with an optimal tube-wire distance of about 2.8 Å and a simple superposition of the band structures of their components near the Fermi level. A quantum conductance of 3G 0 is obtained for both Cu 6 and Cu 8 nanowires in either free-standing state or filled into BeONTs. The electron transport will occur only through the inner CuNW and the inert outer BeONT serves well as insulating cable sheath. So the Cu 6@(10,0) and Cu 8@(11,0) combined systems is top-priority in the ULSI circuits and MEMS devices that demand steady transport of electrons.