Free-standing Geometry Research Articles

A Quantum Monte Carlo Study of the Structural, Energetic, and Magnetic Properties of Two-Dimensional H and T Phase VSe2.

Previous works have controversially claimed near-room-temperature ferromagnetism in two-dimensional (2D) VSe2, with conflicting results throughout the literature. These discrepancies in magnetic properties between both phases (T and H) of 2D VSe2 are most likely due to the structural parameters being coupled to the magnetic properties. Specifically, both phases have a close lattice match and similar total energies, which makes it difficult to determine which phase is being observed experimentally. In this study, we used a combination of density functional theory, highly accurate diffusion Monte Carlo (DMC), and a surrogate Hessian line-search optimization technique to resolve the previously reported discrepancy in structural parameters and relative phase stability. With DMC accuracy, we determined the free-standing geometry of both phases and constructed a phase diagram. Our findings demonstrate the successes of the DMC method coupled with the surrogate Hessian structural optimization technique when applied to a 2D magnetic system.

Freestanding Nitrogen-Doped Carbons with Hierarchical Porosity for Environmental Applications: A Green Templating Route with Bio-Based Precursors.

Powdery hierarchical porous carbons serve as cost‐effective, functional materials in various fields, namely energy storage, heterogeneous catalysis, electrochemistry, and water/wastewater treatment. Such powdered activated carbons (PAC) limit new module designs and require further preparation steps, for example, adding polymeric binders, to be shaped into a standalone geometry. Polymeric binders, however, can block PACs’ catalytic and active sites and, more importantly, pose the risk of secondary pollution for environmental purposes, especially in the context of clean water supply. This study introduces a novel synthesis method for fabricating freestanding nitrogen‐doped carbons with hierarchical porosity using chitosan and sucrose as green precursors. Chitosan supplies nitrogen and acts as a backbone, giving a freestanding geometry to the final product, and sucrose is a carbon‐rich precursor. The proposed method employs ice‐ and hard‐templating for macropores and mesopores and combines carbonization and activation steps with no required activating agent. Final freestanding carbons function as adsorbents for removing persistent pollutants, as binder‐free electrodes with high specific surface area and capacitive current, and as tubular gas diffusion electrodes for oxygen reduction reactions. These freestanding carbons enable new module designs and can be scaled‐up by numbering‐up, serving as bio‐based functional materials for a wide range of applications involving porous heteroatom‐doped carbons.

2D and 3D Structuring of Freestanding Metallic Wires Enabled by Room-Temperature Welding for Soft and Stretchable Electronics.

In this work, a facile and cost-effective approach to assemble metallic wires into two-dimensional (2D) and three-dimensional (3D) freestanding geometries by room-temperature welding is demonstrated. The low melting point of gallium (29.8 °C) enables the welding at room temperature without the aid of high-energy sources required for high-melting-point metals and alloys. The welding enables assembly of solid gallium wires into 2D and 3D geometries that could create freestanding architectures with multiple junctions along any inclined direction. These 2D and 3D freestanding metallic structures are freeze-cast in soft elastomers to obtain stretchable and soft devices: a 2D stretchable resistive and capacitive sensor patterned with parallel metal lines, a 2D stretchable capacitive sensor patterned with an interdigitated metal structure with capacitive changes on stretching in both x- and y-axes, and a 3D compressive sensor by assembly of liquid metal helices, which could sense foot pressure compression. We also developed a facile method to interconnect between soft circuits and external electronics, suppressing stress during mechanical deformation.

Dense Electron-Hole Plasma Formation and Ultralong Charge Lifetime in Monolayer MoS2 via Material Tuning.

Many-body interactions in photoexcited semiconductors can bring about strongly interacting electronic states, culminating in the fully ionized matter of electron-hole plasma (EHP) and electron-hole liquid (EHL). These exotic phases exhibit unique electronic properties, such as metallic conductivity and metastable high photoexcitation density, which can be the basis for future transformative applications. However, the cryogenic condition required for its formation has limited the study of dense plasma phases to a purely academic pursuit in a restricted parameter space. This paradigm can potentially change with the recent experimental observation of these phases in atomically thin MoS2 and MoTe2 at room temperature. A fundamental understanding of EHP and EHL dynamics is critical for developing novel applications on this versatile layered platform. In this work, we studied the formation and dissipation of EHP in monolayer MoS2. Unlike previous results in bulk semiconductors, our results reveal that electromechanical material changes in monolayer MoS2 during photoexcitation play a significant role in dense EHP formation. Within the free-standing geometry, photoexcitation is accompanied by an unconstrained thermal expansion, resulting in a direct-to-indirect gap electronic transition at a critical lattice spacing and fluence. This dramatic altering of the material's energetic landscape extends carrier lifetimes by 2 orders of magnitude and allows the density required for EHP formation. The result is a stable dense plasma state that is sustained with modest optical photoexcitation. Our findings pave the way for novel applications based on dense plasma states in two-dimensional semiconductors.

Contact angle measurement of free-standing square-millimeter single-layer graphene

Square millimeters of free-standing graphene do not exist per se because of thermal fluctuations in two-dimensional crystals and their tendency to collapse during the detachment from the substrate. Here we form millimeter-scale freely suspended graphene by injecting an air bubble underneath a graphene monolayer floating at the water–air interface, which allowed us to measure the contact angle on fully free-standing non-contaminated graphene. A captive bubble measurement shows that free-standing clean graphene is hydrophilic with a contact angle of 42° ± 3°. The proposed design provides a simple tool to probe and explore the wettability of two-dimensional materials in free-standing geometries and will expand our perception of two-dimensional materials technologies from microscopic to now millimeter scales.

Highly microporous-graphene aerogel monolith of unidirectional honeycomb macro-textures

The highly microporous graphene aerogel monolith of unidirectional textures is obtained from reduction and KOH activation of colloidal graphene oxide prepared with an ice-templating route. The free-standing geometry and well-aligned textures of graphene monolith are persevered even after an intensive KOH activation at 973K, although the frame structure is slightly disordered. The non-overestimated surface area of the KOH activated graphene monolith is 990m2g−1. The free-standing graphene aerogel monolith has predominant microporosity with appropriate macroporosity and a low bulk density of 8±0.5mgcm−3, being one of the lightest materials of the reported porous graphene materials.

Three-dimensional machining of carbon nanotube forests using water-assisted scanning electron microscope processing

We demonstrate that vertically aligned carbon nanotubes (CNTs) can be precisely machined in a low pressure water vapor ambient using the electron beam of an environmental scanning electron microscope. The electron beam locally damages the irradiated regions of the CNT forest and also dissociates the water vapor molecules into reactive species including hydroxyl radicals. These species then locally oxidize the damaged region of the CNTs. The technique offers material removal capabilities ranging from selected CNTs to hundreds of cubic microns. We study how the material removal rate is influenced by the acceleration voltage, beam current, dwell time, operating pressure, and CNT orientation. Milled cuts with depths between 0–100 microns are generated, corresponding to a material removal rate of up to 20.1 μm3/min. The technique produces little carbon residue and does not disturb the native morphology of the CNT network. Finally, we demonstrate direct machining of pyramidal surfaces and re-entrant cuts to create freestanding geometries.

Light-activated shape memory of glassy, azobenzene liquid crystalline polymer networks

Rapidly reconfigurable, adaptive materials are essential for the realization of “smart”, highly engineered technologies sought by aerospace, medicine, and other application areas. Shape memory observed in metal alloys and polymers (SMPs) is a primary example of shape change (adaptation). To date, nearly all shape adaptations in SMPs have been thermally triggered. A desire for isothermal, remotely cued shape adaptations of SMP has motivated examinations of other stimuli, such as light. Only a few reports document so-called light-activated SMP, in both cases exploiting photoinduced adjustments to the crosslink density of a polymer matrix with UV light of 365 nm (crosslinking) and <260 nm (decrosslinking). This work presents a distinctive approach to generating light-activated SMP by employing a glassy liquid crystal polymer network (LCN) material that is capable of rapid photo-fixing with short exposures (<5 min) of eye-safe 442 nm light. Here, linearly polarized 442 nm light is used to photo-fix temporary states in both cantilever and free-standing geometries which are then thermally or optically restored to the permanent shape. The combination of thermal and photo-fixable shape memory presented here yields substantial functionality in a single adaptive material that could reduce part count in applications. As a demonstration of the opportunities afforded by this functional material, the glassy, photoresponsive LCN is thermally fixed as a catapult and subsequently used to transduce light energy into mechanical work, demonstrated here in the “photo-fueled” launching of an object at a rate of 0.3 m s−1.

On the importance of the electrostatic environment for the transport properties offreestanding multiwall carbon nanotubes

Electrical measurements of freestanding multiwall carbon nanotubes using highresistance tunnelling contacts reveal a power law behaviour, , with α as high as 5.2, followed by a transition to an offset ohmic behaviour. The freestandingelectrode geometry allows for a distinction between the predictions from Luttinger liquidand environmental quantum fluctuation (EQF) theories to be made. The high values ofexponents found are explained within the EQF formulism, where reflections resulting fromthe impedance discontinuity caused by the freestanding geometry are included.

Development and characterization of thermoelectron enhanced microplasma devices for the generation of plasmas in micro-sized capillaries

Microplasmas offer the advantage of small size, low power consumption as well as higher rate of materials processing and are therefore promising for new applications in materials processing. While many different types of microplasma devices have been presented in recent years, the effects of the smaller scale on the properties of the plasma (gas and electron temperatures), distribution of electrons, radicals and ions, and the implications of these characteristics for materials processing have not been studied in detail yet. Here we present the generation and characterization by optical emission spectroscopy (OES) as well as the numerical simulation of atmospheric pressure argon microplasmas inside capillaries with inner diameters between 50 micro m and 1 mm using two different types of thennoelectron enhanced microplasma (TEMP) devices. The first type has a freestanding geometry whereas the second type is planar, fabricated by a surface mount technique. The latter allows for future size reduction of the microplasma setup and for an easier integration with other apparatuses and devices.

First-principles calculation of valence band offset and the interface states in GaAs/GaN(001) superlattice

A first-principles scalar relativistic version of full-potential self-consistent linear muffin tin orbital (LMTO) method, together with a density functional theory in local density approximation, has been employed for calculating the electronic structure of the superlattices formed from two different types of semiconductors. In particular, we report here the results for the lattice mismatched GaAs/GaN(001) superlattices. The calculation has been performed for the two strain modes: (i) free-standing interface, where both GaAs and GaN are strained; and (ii) GaN (strained) deposited on the unrestrained GaAs. The calculated band offsets reveal strong sensitivity of the atomic positions in the interfacial region. The valence band offset increases (decreases) with decrease (increase) in GaN (GaAs) bond length. There appear a number of interface states of localized and/or resonance types which may be responsible for the sensitivity of the band offsets on the atomic positions. The calculated band offset ranging from 1.32 eV to 1.86 eV for the GaAs substrate and the free-standing geometries, respectively, are quite close to the experimental value of about 1.7–1.8 eV reported by Ding et al.

Landau theory of the size-driven phase transition in ferroelectrics

The influence of size on the dielectric behaviour of ferroelectrics is discussed using phenomenological Landau theory. Three types of free-standing geometry are used in our calculations-film, cylinder and sphere. For films, there is a size-driven phase transition (i.e. a transition from a ferroelectric state to a paraelectric state as the thickness of the film is decreased) so long as the surface ferroelectricity is weaker than that of bulk. The polarization becomes zero below a critical size at which the susceptibility has a maximum. Otherwise the susceptibility decreases as the film thickness decreases and no size-driven phase transition exists. However, for cylinders and spheres there is always a size-driven phase transition, and so the dielectric susceptibility is always enhanced at small size. The sphere geometry has the largest critical size amongst the three geometries. In order to fit the experimental measurements on fine-grained samples, we renormalize our calculations for the sphere geometry using a Gaussian distribution function to represent the variation of particle size. The renormalization rounds the peak in the susceptibility and noticeably shifts its position to smaller size if the standard deviation of the size distribution is comparable with its mean. The critical size judged from dielectric measurements could therefore be smaller than that of an isolated sphere. Our calculations are in qualitative agreement with experimental measurements on the susceptibility of barium titanate and lead titanate.