Three-dimensional Bulk Research Articles

In-plane lattice thermal conductivity predictions of thin films within columnar grains

Polycrystalline thin films are widely used for devices and energy-related applications, such as power electronics, solar cells, and thermal management of devices. In many cases, large-scale crystallization during thin-film growth is challenging, so columnar grains are often found in metal and semiconductor thin films. These rough columnar grain boundaries may also have different phonon specularities from that for typically smoother top/bottom film surfaces. A simple analytical model to separately treat these boundaries and interfaces for phonon scattering is currently unavailable, although the in-plane thermal transport is critical to heat spreading within thin-film devices. In this paper, we extend the effective medium formulation from three-dimensional polycrystalline bulk materials to columnar-grained thin films. The model predictions agree well with those given by frequency-dependent phonon Monte Carlo simulations, considering varied phonon specularity at top/bottom film surfaces and grain-boundary phonon transmissivity. The analytical model is further used to analyze the existing data on polycrystalline ZnO thin films with columnar grains.

On the simultaneous solution of structural membranes on all level sets within a bulk domain

A mechanical model and numerical method for structural membranes implied by all isosurfaces of a level-set function in a three-dimensional bulk domain are proposed. The mechanical model covers large displacements in the context of the finite strain theory and is formulated based on the tangential differential calculus. Alongside curved two-dimensional membranes embedded in three dimensions, also the simpler case of curved ropes (cables) in two-dimensional bulk domains is covered. The implicit geometries (shapes) are implied by the level sets and the boundaries of the structures are given by the intersection of the level sets with the boundary of the bulk domain. For the numerical analysis, the bulk domain is discretized using a background mesh composed by (higher-order) elements with the dimensionality of the embedding space. The elements are by no means aligned to the level sets, i.e., the geometries of the structures, which resembles a fictitious domain method, most importantly the Trace FEM. The proposed numerical method is a hybrid of the classical FEM and fictitious domain methods which may be labelled as “Bulk Trace FEM”. Numerical studies confirm higher-order convergence rates and the potential for new material models with continuously embedded sub-structures in bulk domains.

First-principles design of ferromagnetic monolayer MnO2 at the complex interface

Rapidly increasing interest in low-dimensional materials is driven by the emerging requirement to develop nanoscale solid-state devices with novel functional properties that are not available in three-dimensional bulk phases. Among the well-known low-dimensional systems, complex transition metal oxide interface holds promise for broad applications in electronic and spintronics devices. Herein, intriguing metal-insulator and ferromagnetic-antiferromagnetic transitions are achieved in monolayer MnO2 that is sandwiched into SrTiO3-based heterointerface systems through interface engineering. By using first-principles calculations, we modeled three types of SrTiO3-based heterointerface systems with different interface terminations and performed a comparative study on the spin-dependent magnetic and electronic properties that are established in the confined MnO2 monolayer. First-principles study predicts that metal-insulator transition and magnetic transition in the monolayer MnO2 are independent on the thickness of capping layers. Moreover, 100% spin-polarized two-dimensional electron gases accompanied by robust room temperature magnetism are uncovered in the monolayer MnO2. Not only is the buried MnO2 monolayer a new interface phase of fundamental physical interest, but it is also a promising candidate material for nanoscale spintronics applications. Our study suggests interface engineering at complex oxide interfaces is an alternative approach to designing high-performance two-dimensional materials.

Cuprous Oxide-Based Cationic Hydrogel by the Integration of Enrichment and Immobilization of Radioiodine (I-, IO3-) in Aqueous Solution.

The efficient capture and immobilization of radioiodine (I-, IO3-) is of great importance in radioactive waste management. Here, a Cu2O-loaded three-dimensional bulk cationic hydrogel composite (Cu2O@CH) was successfully prepared by simple redox reactions and UV photopolymerization, which realized the rapid enrichment and efficient immobilization of I- and IO3-. The adsorption experiments showed that the maximum adsorption capacity of Cu2O@CH for I- in the solution at pH = 3 reached 416.5 mg/g, and the adsorption capacity of IO3- in the solution at pH = 6 could reach 313.4 mg/g. It exhibited extremely fast adsorption kinetics for I- and IO3-. In addition, Cu2O@CH also exhibited efficient I- and IO3- removal from simulated high-level liquid waste. The rapid capture and effective immobilization of radioiodine (I-, IO3-) were realized by the electrostatic interaction of -N+(CH3)3 groups in Cu2O@CH with I- and IO3-, as well as the chemical reactions between Cu2O and I-. The bulk cationic hydrogel composite explored the multifunctional role toward fast, high adsorption capability and easy handling, highlighting its superiority compared to the powder adsorbent, which renders it a potential adsorbent for the removal of radioactive iodine (I-, IO3-) in nuclear wastewater treatment.

Gold nanosponges: fascinating optical properties of a unique disorder-dominated system

Nanoporous gold is a three-dimensional bulk material that is percolated with a random network of nanometer-sized ligaments and made by selective corrosion of bimetallic alloys. It has intriguing geometric, catalytic, and optical properties that have fascinated scientists for many decades. When such a material is made into the form of small, 100-nm-sized particles, so-called nanosponges emerge that offer much flexibility in controlling their geometric, electronic, and optical properties. Importantly, these particles act as an antenna for light that can efficiently localize optical fields on a deep subwavelength scale in certain hotspots at the particle surface. This makes such nanosponges an interesting platform for plasmonic sensing, photocatalysis, and surface-enhanced Raman spectroscopy. Since the optical properties of these nanosponges can be controlled to a large degree by tuning their geometry and/or composition, they have attracted increasing attention in recent years. Here, we provide a concise overview of the current state of the art in this field, covering their fabrication, computational modeling, and specifically the linear and nonlinear optical properties of individual and hybrid nanosponges, for example, plasmon localization in randomly disordered hotspots with a size <10nm and a long lifetime with an exceptionally high Purcell factor. The resulting nonlinear optical and photoemission properties are discussed for individual and hybrid nanosponges. The results presented have strong implications for further applications of such nanosponges in photonics and photocatalysis.

Oxygen vacancy-rich nickel-iron hydroxides-derived phosphide with superaerophobic nanoarray morphology for robust overall water splitting

Multiscale engineering of efficient catalysts to optimize the adsorption energy of intermediates (atomic level) and achieve rapid mass transfer (three-dimensional (3D) bulk level) is crucial for boosting overall water splitting. In this work, we first create oxygen vacancies in nickel–iron hydroxide and then transform the hydroxide into NiFe-Vo-P having a nanoarray morphology via phosphorization. During the oxygen evolution reaction, Ni(Fe)OOH and phosphate anions can be in situ formed on the NiFe-Vo-P surface and optimize the adsorption strength of the intermediates. Consequently, NiFe-Vo-P could achieve a high current density of 1.5 A cm−2 at an overpotential of 289 mV. Moreover, the superhydrophilic/superaerophobic nanoarray morphology of NiFe phosphide effectively facilitates the mass transfer, and NiFe-Vo-P achieves current densities of 580 mA cm−2 and 1.0 A cm−2 at a cell voltage of ∼2.0 V at 25 and 70°C, respectively, over twice those of its counterpart without the superaerophobic morphology.

Gate-tunable transport in van der Waals topological insulator Bi4Br4 nanobelts

Bi4Br4 is a quasi-one-dimensional van der Waals topological insulator with novel electronic properties. Several efforts have been devoted to the understanding of its bulk form, yet it remains a challenge to explore the transport properties in low-dimensional structures due to the difficulty of device fabrication. Here we report for the first time a gate-tunable transport in exfoliated Bi4Br4 nanobelts. Notable two-frequency Shubnikov–de Haas oscillations oscillations are discovered at low temperatures, with the low- and high-frequency parts coming from the three-dimensional bulk state and the two-dimensional surface state, respectively. In addition, ambipolar field effect is realized with a longitudinal resistance peak and a sign reverse in the Hall coefficient. Our successful measurements of quantum oscillations and realization of gate-tunable transport lay a foundation for further investigation of novel topological properties and room-temperature quantum spin Hall states in Bi4Br4.

Orbital design of Berry curvature: pinch points and giant dipoles induced by crystal fields

The Berry curvature (BC)—a quantity encoding the geometric properties of the electronic wavefunctions in a solid—is at the heart of different Hall-like transport phenomena, including the anomalous Hall and the non-linear Hall and Nernst effects. In non-magnetic quantum materials with acentric crystalline arrangements, local concentrations of BC are generally linked to single-particle wavefunctions that are a quantum superposition of electron and hole excitations. BC-mediated effects are consequently observed in two-dimensional systems with pairs of massive Dirac cones and three-dimensional bulk crystals with quartets of Weyl cones. Here, we demonstrate that in materials equipped with orbital degrees of freedom local BC concentrations can arise even in the complete absence of hole excitations. In these solids, the crystals fields appearing in very low-symmetric structures trigger BCs characterized by hot-spots and singular pinch points. These characteristics naturally yield giant BC dipoles and large non-linear transport responses in time-reversal symmetric conditions.

Monolayer Kagome metals AV3Sb5

Recently, layered kagome metals AV3Sb5 (A = K, Rb, and Cs) have emerged as a fertile platform for exploring frustrated geometry, correlations, and topology. Here, using first-principles and mean-field calculations, we demonstrate that AV3Sb5 can crystallize in a mono-layered form, revealing a range of properties that render the system unique. Most importantly, the two-dimensional monolayer preserves intrinsically different symmetries from the three-dimensional layered bulk, enforced by stoichiometry. Consequently, the van Hove singularities, logarithmic divergences of the electronic density of states, are enriched, leading to a variety of competing instabilities such as doublets of charge density waves and s- and d-wave superconductivity. We show that the competition between orders can be fine-tuned in the monolayer via electron-filling of the van Hove singularities. Thus, our results suggest the monolayer kagome metal AV3Sb5 as a promising platform for designer quantum phases.

Effective substrate for the growth of multilayer h-BN on sapphire—substrate off-cut, pre-growth, and post-growth conditions in metal-organic vapor phase epitaxy

The substrate is one of the key components that determines the quality of the epitaxial layers. However, the implications of growing two-dimensional layers on three-dimensional bulk substrates have not yet been fully understood, and these implications need to be studied for different combinations of materials and substrates. Here, we present a study that addresses the influence of the sapphire substrate off-cut angle on the final growth of two-dimensional layers of hexagonal boron nitride (h-BN) by metal-organic vapor phase epitaxy (MOVPE). A two-step wafer-scale process was used in one epitaxial MOVPE procedure. The main process starts with a self-limiting continuous growth of a BN buffer followed by flow-modulated epitaxy in the second step, and is used to study substrates with different off-cuts angles, pre-growth nitridation steps, and post-growth annealing. An initial nitridation step at the growth temperature allowed for the growth of an AlN sublayer. This layer is shown to smooth out the underlying sapphire and establishes an ‘effective’ sapphire/AlN substrate. This step is also responsible for enforcing a specific growth of the BN layer in a crystallographic orientation, which is shown to strongly deviate from the substrate for off-cut angles larger than 0.3°. A substrate with off-cut angle of 1° clearly yields the highest quality of h-BN layers as evidenced by the lowest amount of debris on the surface, most intense x-ray diffraction signal, minimal Raman phonon line width and thinnest amorphous BN (a-BN) at the interface with the effective substrate. Our study shows that the off-cut angles of sapphire substrates strongly influence the final epitaxial h-BN, clearly indicating the importance of optimal substrate preparation for the growth of two-dimensional BN layers. Post-growth annealing in N2 atmosphere at 800 °C improves the top surface morphology of the final stack, as well as suppresses further the presence of a-BN.

Determine ligand binding kinetics to a membrane protein using Brownian dynamics simulation.

Molecular diffusion controls the kinetics of protein-ligand binding in various biological processes. Determining the reaction rate between proteins and ligands is crucial in understanding these processes and in designing novel strategies to enhance or disrupt binding. Most previous studies focus on ligand diffusion in a three-dimensional bulk space. However, with the rapid growth of membrane protein research, a comprehensive understanding of molecular diffusion toward membrane-bound organelles or biomolecules tethered on a planar scaffold is essential. In this work, we developed a method that can predict the association rate constant of a ligand-membrane protein binding reaction according to trajectories from Brownian dynamics simulations. Our approach can be divided into two parts: 1) analytically determine the ligand association rate to a hemispherical region surrounding the membrane protein, and 2) compute the protein-ligand binding probability by Brownian dynamics simulations. We applied the method to calculate the reaction rate of biological systems with different charges. We found that the presence of a membrane does affect ligand diffusion, especially when the ligand moves close to the membrane. The critical aspects of surface diffusion, such as the forbidden zone, edge effect, and simulation criteria, were also discussed. Although the current development is based on a coarse-grained model, our method is still useful in understanding the binding kinetics of a highly complicated biological system.

Dimensionality reduction induced synergetic optimization of the thermoelectric properties in Bi2Si2X6 (X = Se, Te) monolayers.

Different from three-dimensional bulk compounds, two-dimensional monolayer compounds exhibit much better thermoelectric performance on account of the quantum confinement and interface effect. Here, we present a systematic study on the electronic and thermal transport properties of bulk and monolayer Bi2Si2X6 (X = Se, Te) through theoretical calculations using density functional theory based on first-principles and Boltzmann transport theory. Monolayer Bi2Si2X6 are chemically, mechanically and thermodynamically stable semiconductors with suitable band gaps, and they have lower lattice thermal conductivity (κL) in the a/b direction than their bulk counterparts. The calculated κL of monolayer Bi2Si2Se6 (Bi2Si2Te6) is as low as 0.72 (0.95) W m-1 K-1 at 700 K. Moreover, monolayer Bi2Si2X6 exhibit a higher Seebeck coefficient compared with bulk Bi2Si2X6 owing to the sharper peaks in the electronic density of states (DOS). This results in a significant increase in power factor by dimensionality reduction. Combined with the synergetically suppressed thermal conductivity, the maximum ZT values for monolayer Bi2Si2Se6 and Bi2Si2Te6 are significantly enhanced up to 5.03 and 2.87 with p-type doping at 700 K, which are more than 2 times that of the corresponding bulk compounds. These results demonstrate the superb thermoelectric performance of monolayer Bi2Si2X6 for promising thermoelectric conversion applications.

Phase Engineering of Palladium Selenide Using Chalcogen Flux Control

Palladium selenides, relatively unexplored platinum group metal chalcogenides, have been attracting significant interest due to their versatility in different applications and their numerous stable phases with different crystal structures. Most vapor-based bottom-up growth approaches using furnaces have resulted in the layered PdSe2 phase due to the reactions occurring in a saturated chalcogen vapor environment, in which PdSe2 is the most stable phase. Here, we show that precise control over the Pd/Se stoichiometry can be achieved through tuning the chalcogen flux, both incoming and outgoing, resulting in control over the local Se concentration on the growth substrate. Increasing temperatures are used to increase the outgoing flux of Se, leading to a phase transition from PdSe2 to Pd17Se15. Increasing the incoming flux of Se, through high mass loading, is then used to tune the reaction back to the growth of the PdSe2 phase. Detailed material characterization is performed to prove the different crystal structures and show the ability to tailor the Pd/Se family across two stable phases, with one being layered (PdSe2) and the other being three-dimensional (3D) bulk (Pd17Se15). Finally, we demonstrate the lifecycle of phase tuning by transforming PdSe2 into Pd17Se15 and then back again, through control of temperature and Se mass loading. These results open pathways for exploring the solid-state properties of this family of Pd–Se materials in optoelectronics and catalysis, which have been limited to date for vapor-based furnace growth approaches on substrates.

Celestial holography meets twisted holography: 4d amplitudes from chiral correlators

We propose a new program for computing a certain integrand of scattering amplitudes of four-dimensional gauge theories which we call the form factor integrand, starting from 6d holomorphic theories on twistor space. We show that the form factor integrands can be expressed as sums of products of 1.) correlators of a 2d chiral algebra, related to the algebra of asymptotic symmetries uncovered recently in the celestial holography program, and 2.) OPE coefficients of a 4d non-unitary CFT. We prove that conformal blocks of the chiral algebras are in one-to-one correspondence with local operators in 4d. We use this bijection to recover the Parke-Taylor formula, the CSW formula, and certain one-loop scattering amplitudes. Along the way, we explain and derive various aspects of celestial holography, incorporating techniques from the twisted holography program such as Koszul duality. This perspective allows us to easily and efficiently recover the infinite-dimensional chiral algebras of asymptotic symmetries recently extracted from scattering amplitudes of massless gluons and gravitons in the celestial basis. We also compute some simple one-loop corrections to the chiral algebras and derive the three-dimensional bulk theories for which these 2d algebras furnish an algebra of boundary local operators.

Friction stir additive manufacturing enabling scale-up of ultrafine-grained pure copper with superior mechanical properties

Bulk manufacturing has always been a big barrier to realize industrial application of ultrafine-grained (UFG) material for a long time. In this study, a new friction stir additive manufacturing (FSAM) method was developed, and three-dimensional large-scale bulk UFG pure Cu with uniform microstructure and high mechanical property was successfully manufactured.

Ultrafast electronic heat dissipation through surface-to-bulk Coulomb coupling in quantum materials

The timescale of electronic cooling is an important parameter controlling the performance of devices based on quantum materials for optoelectronic, thermoelectric and thermal management applications. In most conventional materials, cooling proceeds via the emission of phonons, a relatively slow process that can bottleneck the carrier relaxation dynamics, thus degrading the device performance. Here we present the theory of near-field radiative heat transfer, that occurs when a two-dimensional electron system is coupled via the non-retarded Coulomb interaction to a three-dimensional bulk that can behave as a very efficient electronic heat sink. We apply our theory to study the cooling dynamics of surface states of three dimensional topological insulators, and of graphene in proximity to small-gap bulk materials. The ``Coulomb cooling'' we introduce is alternative to the conventional phonon-mediated cooling, can be very efficient and dominate the cooling dynamics under certain circumstances. We show that this cooling mechanism can lead to a sub-picosecond time scale, significantly faster than the cooling dynamics normally observed in Dirac materials.

Structural and Electronic Properties of SnO Downscaled to Monolayer.

Two-dimensional (2D) SnO is a p-type semiconductor that has received research and industrial attention for device-grade applications due to its bipolar conductivity and transparent semiconductor nature. The first-principles investigations based on the generalized gradient approximation (GGA) level of theory often failed to accurately model its structure due to interlayer Van der Waals interactions. This study is carried out to calculate structural and electronic properties of bulk and layered structures of SnO using dispersion correction scheme DFT+D3 with GGA-PBE to deal with the interactions which revealed good agreement of the results with reported data. The material in three-dimensional bulk happened to be an indirect gap semiconductor with a band gap of 0.6 eV which is increased to 2.85 eV for a two-dimensional monolayer structure. The detailed analysis of the properties demonstrated that the SnO monolayer is a promising candidate for future optoelectronics and spintronics devices, especially thin film transistors.

Doping Asymmetry and Layer-Selective Metal-Insulator Transition in Trilayer K_{3+x}C_{60}.

Thin films provide a versatile platform to tune electron correlations and explore new physics in strongly correlated materials. Epitaxially grown thin films of the alkali-doped fulleride K_{3+x}C_{60}, for example, exhibit intriguing phenomena, including Mott transitions and superconductivity, depending on dimensionality and doping. Surprisingly, in the trilayer case, a strong electron-hole doping asymmetry has been observed in the superconducting phase, which is absent in the three-dimensional bulk limit. Using density-functional theory plus dynamical mean-field theory, we show that this doping asymmetry results from a substantial charge reshuffling from the top layer to the middle layer. While the nominal filling per fullerene is close to n=3, the top layer rapidly switches to an n=2 insulating state upon hole doping, which implies a doping asymmetry of the superconducting gap. The interlayer charge transfer and layer-selective metal-insulator transition result from the interplay between crystal field splittings, strong Coulomb interactions, and an effectively negative Hund coupling. This peculiar charge reshuffling is absent in the monolayer system, which is an n=3 Mott insulator, as expected from the nominal filling.

Three-Dimensional de Sitter Holography and Bulk Correlators at Late Time.

We propose an explicitly calculable example of holography on three-dimensional de Sitter space by providing a prescription to analytic continue a higher-spin holography on three-dimensional anti-de Sitter space. Applying the de Sitter holography, we explicitly compute bulk correlation functions on three-dimensional de Sitter space at late time in a higher-spin gravity. These expressions are consistent with recent analysis based on bulk Feynman diagrams. Our explicit computations reveal how holographic computations could provide fruitful information.

Controlling and visualizing Dirac physics in topological semimetal heterostructures.

A bulk crystal of cadmium arsenide is a three-dimensional Dirac semimetal, but, in a thin film, it can behave like a three-dimensional topological insulator. This tunability provides unique opportunities to manipulate and explore a topological insulator phase. However, an obstacle to engineering such tunability is the subtlety of transport-based discriminants for topological phases. In this work, the quantum capacitance of cadmium arsenide–based heterostructures provides two direct experimental signatures of three-dimensional topological insulator physics: an insulating three-dimensional bulk and a Landau level at zero energy that does not disperse in a magnetic field. We proceed to join our ability to see these fingerprints of the topological surface states with flexibility afforded by our epitaxial heterostructures to demonstrate a route toward controlling the energy of the Dirac nodes on each surface. These results point to new avenues for engineering topological insulators based on cadmium arsenide.