Jiuxiang Dai

Journal of Applied Physics, Volume 133, Issue 10, March 2023.
Graphene layers placed on SrTiO3 single-crystal substrates, i.e., templates for remote epitaxy of functional oxide membranes, were investigated using temperature-dependent confocal Raman spectroscopy. This approach successfully resolved distinct Raman modes of graphene that are often untraceable in conventional measurements with non-confocal optics due to the strong Raman scattering background of SrTiO3. Information on defects and strain states was obtained for a few graphene/SrTiO3 samples that were synthesized by different techniques. This confocal Raman spectroscopic approach can shed light on the investigation of not only this graphene/SrTiO3 system but also various two-dimensional layered materials whose Raman modes interfere with their substrates.

Grain refinement strongly strengthens the non-basal slip of Bi₂Te₃-based thermoelectric material, and then increase the carrier concentration, hindering the practical preparation of n-type Bi₂Te₃ by ball milling method. In this work, enhancing the ball milling energy can facilitate the recovery effect during hot pressing, which can reduce and stabilize the carrier concentration and achieve high thermoelectric and mechanical performances.

Abstract

The carrier concentration in n-type layered Bi₂Te₃-based thermoelectric (TE) material is significantly impacted by the donor-like effect, which would be further intensified by the nonbasal slip during grain refinement of crushing, milling, and deformation, inducing a big challenge to improve its TE performance and mechanical property simultaneously. In this work, high-energy refinement and hot-pressing are used to stabilize the carrier concentration due to the facilitated recovery of cation and anion vacancies. Based on this, combined with SbI₃ doping and hot deformation, the optimized carrier concentration and high texture degree are simultaneously realized. As a result, a peak figure of merit (zT) of 1.14 at 323 K for Bi₂Te_2.7Se_0.3 + 0.05 wt.% SbI₃ sample with the high bending strength of 100 Mpa is obtained. Furthermore, a 31-couple thermoelectric cooling device consisted of n-type Bi₂Te_2.7Se_0.3 + 0.05 wt.% SbI₃ and commercial p-type Bi_0.5Sb_1.5Te₃ legs is fabricated, which generates the large maximum temperature difference (ΔT _max) of 85 K at a hot-side temperature of 343 K. Thus, the discovery of recovery effect in high energy refinement and hot-pressing has significant implications for improving TE performance and mechanical strength of n-type Bi₂Te₃, thereby promoting its applications in harsh conditions.

WTe₂ is synthesized by tellurizing amorphous WO_x thin films deposited by atomic layer deposition on two substrates of sapphire and amorphous SiO₂. Despite the same oxide precursor films and the same reaction conditions, monolayer WTe₂ flakes are achieved only on the sapphire substrate while much thicker flakes are observed on SiO₂. The difference is attributed to the diffusivity of W on sapphire and SiO₂.

Abstract

Synthesis of transition metal dichalcogenides (TMDCs) has been achieved through the direct conversion of metal and metal-oxide films, demonstrating the ability to grow large area thin films with uniform thickness on a variety of substrates and direct control over the growth orientation (horizontal vs vertical) of the TMDC layers. However, the synthesized TMDC films often exhibit small grains and are more defective than their bulk counterparts. This is especially true for 2D telluride films due to the low reactivity between tellurium and transition metals such as W and Mo. In this work, the substrate interactions is examined for WTe₂ converted from amorphous WO _x thin films grown by atomic layer deposition, on c-plane sapphire and SiO₂ through tellurization at high temperatures. Similar to TMDC telluride MoTe₂, the formation of monolayer WTe₂ on sapphire is observed, but not on SiO₂. However, due to decreased diffusion of W on sapphire compared to Mo, the formation of WTe₂ flakes instead of continuous films is observed, providing insight into the role of the specific transition metal during the direct synthesis of TMDC telluride films.

BaTiO₃ films are grown by different thin film techniques, however, the lack of stoichiometry control makes it hard to achieve bulk-like properties. Here, hybrid molecular beam epitaxy is employed to grow BaTiO₃ films. Using this approach, a self-regulated growth window is accessed, making it ideally suited as an easy, robust, straightforward, and scalable growth method to synthesize epitaxial BaTiO₃ thin films with sufficient control over the film's stoichiometry and to realize bulk-like BaTiO₃ properties in thin film form.

Abstract

BaTiO₃ is a technologically relevant material in the perovskite oxide class with above-room-temperature ferroelectricity and a very large electro-optical coefficient, making it highly suitable for emerging electronic and photonic devices. An easy, robust, straightforward, and scalable growth method is required to synthesize epitaxial BaTiO₃ thin films with sufficient control over the film's stoichiometry to achieve reproducible thin film properties. Here the growth of BaTiO₃ thin films by hybrid molecular beam epitaxy is reported. A self-regulated growth window is identified using complementary information obtained from reflection high energy electron diffraction, the intrinsic film lattice parameter, film surface morphology, and scanning transmission electron microscopy. Subsequent optical characterization of the BaTiO₃ films by spectroscopic ellipsometry revealed refractive index and extinction coefficient values closely resembling those of stoichiometric bulk BaTiO₃ crystals for films grown inside the growth window. Even in the absence of a lattice parameter change of BaTiO₃ thin films, degradation of optical properties is observed, accompanied by the appearance of a wide optical absorption peak in the IR spectrum, attributed to optical transitions involving defect states present. Therefore, the optical properties of BaTiO₃ can be utilized as a much finer and more straightforward probe to determine the stoichiometry level present in BaTiO₃ films.

Here a thin-film assisted transfer technique that can controllably fabricate van der Waals (vdW) structures with deterministic twist angles, multi-materials, and designated positions is presented. Multilayer homo-/hetero-architectures in the geometry of series connection, spiral structure and chiral symmetry at deterministic twist angles are for the first time demonstrated. The capability of fabricating complicated vdW structures will serve as a powerful tool for twistronics.

Abstract

In 2D van der Waals (vdW) materials, rotational misalignment by a twist angle between adjacent layers can significantly affect their properties, referred as twistronics. Accurate tuning of twist angle of vdW materials in a controllable and efficient way is highly required. Here a thin-film assisted transfer (TAT) technique that can controllably fabricate vdW structures with deterministic twist angles (accuracy of 0.37°) is developed. The transparent and ultrathin film as transfer medium ensures the visible and intact manipulation of monolayer building blocks, thus constructing vdW architectures with multi-materials and designated position. Multilayer homo-/hetero-architectures in the geometry of series connection, spiral structure and chiral symmetry at deterministic twist angles are for the first time demonstrated. They can be located on various substrates for both structural analysis and device application. The capability of fabricating twisted homo-/hetero- vdW structures in a deterministic, high-throughput manner will serve as a powerful tool for twistronics.

High-temperature promoted annealing-induced intermixing across the Ag/In₂O₃:H interface is revealed by in-situ hard X-ray photoelectron spectroscopy in combination with ex-situ electron microscopy. This results in an improvement of the contact resistance rather than its often-reported deterioration.

Abstract

Hydrogen-doped In₂O₃ (In₂O₃:H) is highly conductive while maintaining extraordinary transparency, thus making it a very attractive material for applications in optoelectronic devices such as (multijunction) solar cells or light-emitting devices. However, the corresponding metal/In₂O₃:H contacts may exhibit undesirably high resistances, significantly deteriorating device performance. To gain insight into the underlying efficiency-limiting mechanism, hard X-ray photoelectron spectroscopy is employed to in-situ monitor annealing-induced changes in the chemical structure of the Ag/In₂O₃:H interface system that is further complemented by ex-situ electron microscopy analyses and contact resistance measurements. The observed evolution of the Ag- and In-related photoelectron line intensities can be explained by significant intermixing across the Ag/In₂O₃:H interface. The corresponding lineshape broadening of the Ag 3d spectra is attributed to the formation of Ag₂O and AgO, which becomes significant at temperatures above approximately 160 °C. However, after annealing to 300 °C, instead of the formation of an insulating AgO _x interfacial layer, it is found i) In to be rather homogeneously distributed in the complete Ag/In₂O₃:H stack, ii) Ag diffusing into the In₂O₃:H, and iii) an improvement of the contact resistance rather than its often-reported deterioration.

A plasma-enhanced chemical vapor deposition method is developed to construct the 2D room-temperature magnetic MnGa₄-H crystal. Hydrogen insertion inside the MnGa₄ lattice can modulate the atomic distance and charge state, thereby ferrimagnetism can be achieved without destroying the structural configuration. 2D MnGa₄-H is high-quality and air-stable, demonstrating robust and stable room-temperature magnetism with a high Curie temperature above 620 K.

Abstract

2D room-temperature magnetic materials are of great importance in future spintronic devices while only very few are reported. Herein, a plasma-enhanced chemical vapor deposition approach is exploited to construct the 2D room-temperature magnetic MnGa₄-H single crystal with a thickness down to 2.2 nm. The employment of H₂ plasma makes hydrogen atoms can be easily inserted into the MnGa₄ lattice to modulate the atomic distance and charge state, thereby ferrimagnetism can be achieved without destroying the structural configuration. The as-obtained 2D MnGa₄-H crystal is high-quality, air-stable, and thermo-stable, demonstrating robust and stable room-temperature magnetism with a high Curie temperature above 620 K. This work enriches the 2D room-temperature magnetic family and opens up the possibility for the development of spintronic devices based on 2D magnetic alloys.

Through systematically studying the crystallographic information of nitrides/2D materials interface, it is found that graphene is the ideal buffer layer for nitrides’ remote epitaxy while WS₂ for nitrides’ van der Waals epitaxy. As a result, a suitable growth-front construction strategies and basic guidelines for high-quality 2D-materials-assisted nitrides’ epitaxy is presented. This work may open a pathway toward various semiconductors heterointegration.

Abstract

Beyond traditional heteroepitaxy, 2D-materials-assisted epitaxy opens opportunities to revolutionize future material integration methods. However, basic principles in 2D-material-assisted nitrides’ epitaxy remain unclear, which impedes understanding the essence, thus hindering its progress. Here, the crystallographic information of nitrides/2D material interface is theoretically established, which is further confirmed experimentally. It is found that the atomic interaction at the nitrides/2D material interface is related to the nature of underlying substrates. For single-crystalline substrates, the heterointerface behaves like a covalent one and the epilayer inherits the substrate's lattice. Meanwhile, for amorphous substrates, the heterointerface tends to be a van der Waals one and strongly relies on the properties of 2D materials. Therefore, modulated by graphene, the nitrides’ epilayer is polycrystalline. In contrast, single-crystalline GaN films are successfully achieved on WS₂. These results provide a suitable growth-front construction strategy for high-quality 2D-material-assisted nitrides’ epitaxy. It also opens a pathway toward various semiconductors heterointegration.

A self-powered multifunctional BiFeO₃ sensor material possesses a unique light-and temperature-controlled energy band structure and carrier behavior. The BFO sensor can not only detect both light intensity and temperature, but it can also execute three common logic gates of “AND”, “OR”, and “NOT” by converting a combination of optical and thermal inputs into electrical output.

Abstract

The growing need to process a diverse range of data has ignited effort in developing new multifunctional logic gate devices. In this article, we report a new form of all-in-one logic gate system that exploits the photoresponsivity of a self-powered multifunctional BiFeO₃ (BFO) sensor material. The BFO sensor can not only detect both light intensity and temperature, but it can also execute three common logic gates of “AND”, “OR”, and “NOT” by converting optical and thermal inputs into electrical output. The diverse functionality of the BFO logic gate sensor array utilizes the unique light-and temperature-controlled energy band structure and carrier behavior of the BFO material. To demonstrate the potential, a 3 × 3 logic gate sensor matrix is developed, which successfully detected light and temperature distributions, and accurately produced the three basic logic gate operations. This work provides a new route to construct highly integrated multifunctional electronic devices for the advancement of large sensing, communication, and computing operations.

Superlattices represent the epitome of combinatorial materials science: their properties can transcend those of their constituent layers, such as improved mobility with modulation doped structures. Here superlattice combinations are expanded into the morphological domain by introducing direct growth of alternating amorphous and polycrystalline layers, realizing a high-quality, high mobility “heteromorphic superlattice,” originally conceived by Raphael Tsu in 1989.

Abstract

An unconventional “heteromorphic” superlattice (HSL) is realized, comprised of repeated layers of different materials with differing morphologies: semiconducting pc-In₂O₃ layers interleaved with insulating a-MoO₃ layers. Originally proposed by Tsu in 1989, yet never fully realized, the high quality of the HSL heterostructure demonstrated here validates the intuition of Tsu, whereby the flexibility of the bond angle in the amorphous phase and the passivation effect of the oxide at interfacial bonds serve to create smooth, high-mobility interfaces. The alternating amorphous layers prevent strain accumulation in the polycrystalline layers while suppressing defect propagation across the HSL. For the HSL with 7:7 nm layer thickness, the observed electron mobility of 71 cm² Vs^-1, matches that of the highest quality In₂O₃ thin films. The atomic structure and electronic properties of crystalline In₂O₃/amorphous MoO₃ interfaces are verified using ab-initio molecular dynamics simulations and hybrid functional calculations. This work generalizes the superlattice concept to an entirely new paradigm of morphological combinations.

The superconducting transition temperature is greatly enhanced to be as large as 7.5 K in bulk Mo_{1− x} Ta _x Te₂ single crystals, which is attributed to an enrichment of density of states at the Fermi level. We also find the enhanced upper critical field beyond the Pauli limit in Ta-doped Weyl semimetal T _d-MoTe₂, which is likely due to the mixed singlet–triplet superconductivity.

Abstract

2D transition metal dichalcogenides are promising platforms for next-generation electronics and spintronics. The layered Weyl semimetal (W,Mo)Te₂ series features structural phase transition, nonsaturated magnetoresistance, superconductivity, and exotic topological physics. However, the superconducting critical temperature of the bulk (W,Mo)Te₂ remains ultralow without applying a high pressure. Here, the significantly enhanced superconductivity is observed with a transition temperature as large as about 7.5 K in bulk Mo_{1− x} Ta _x Te₂ single crystals upon Ta doping (0 ≤ x ≤ 0.22), which is attributed to an enrichment of density of states at the Fermi level. In addition, an enhanced perpendicular upper critical field of 14.5 T exceeding the Pauli limit is also observed in T _d-phase Mo_{1− x} Ta _x Te₂ (x = 0.08), indicating the possible emergence of unconventional mixed singlet–triplet superconductivity owing to the inversion symmetry breaking. This work provides a new pathway for exploring the exotic superconductivity and topological physics in transition metal dichalcogenides.

Controlling the photoisomerization of donor–acceptor Stenhouse adducts (DASAs) on the surface of graphene is reported here. Carbon spacers with various lengths are introduced onto DASAs to generate tunable organic–inorganic interfaces. The microstructure and mechanical properties on the surface are demonstrated to closely affect the photoisomerization. Multistage photomodulation of 2D logic-in-memory devices are further achieved by green light.

Abstract

Logic-in-memory devices are a promising and powerful approach to realize data processing and storage driven by electrical bias. Here, an innovative strategy is reported to achieve the multistage photomodulation of 2D logic-in-memory devices, which is realized by controlling the photoisomerization of donor–acceptor Stenhouse adducts (DASAs) on the surface of graphene. Alkyl chains with various carbon spacer lengths (n = 1, 5, 11, and 17) are introduced onto DASAs to optimize the organic–inorganic interfaces: 1) Prolonging the carbon spacers weakens the intermolecular aggregation and promotes isomerization in the solid state. 2) Too long alkyl chains induce crystallization on the surface and hinder the photoisomerization. Density functional theory calculation indicates that the photoisomerization of DASAs on the graphene surface is thermodynamically promoted by increasing the carbon spacer lengths. The 2D logic-in-memory devices are fabricated by assembling DASAs onto the surface. Green light irradiation increases the drain–source current (I _ds) of the devices, while heat triggers a reversed transfer. The multistage photomodulation is achieved by well-controlling the irradiation time and intensity. The strategy based on the dynamic control of 2D electronics by light integrates molecular programmability into the next generation of nanoelectronics.

The novel sulfur-containing carbon nanotubes with abundant graphene nanoflaps (S-CNTs) provide an enhanced shear interface with elastomer that dissipates the deformation energy efficiently upon stretching strain. The S-CNTs elastomer with comprehensive performance (ΔR/R ₀≈1.8 @200%, stretchability >450%, > 30 000 cycles), and excellent Joule heating efficiency (≈150 °C @12 V, > 24 h) is successfully applied in mechanical sensor and soft tongs.

Abstract

Stretchable conductors based on nanopercolation networks have garnered great attention for versatile applications. Carbon nanotubes (CNTs) are well-suited for creating high-efficiency nanopercolation networks. However, the weak interfacial shear strength (IFSS) between CNTs and elastomer hardly dissipates the deformation energy and thus deteriorates the conductive network. Herein, a novel sulfur-containing CNTs attached with abundant graphene nanoflaps using a two-step sulfidation strategy are developed. The sulfur functionality creates a strong interfacial interaction with the elastomer polymer, while the graphene nanoflaps provide an enhanced, intertwined shear interface with elastomer that is capable of efficiently dissipating the deformation energy. As a result, the optimized nanocomposite significantly improves the IFSS between nanofiller and elastomer, displaying remarkable conductive robustness (ΔR/R ₀≈1.8 under 200%), superior stretchability (> 450%), and excellent mechanical durability (≈30 000 cycles). Moreover, the nanocomposite demonstrates excellent Joule heating efficiency (≈150 °C in 12 V), stretchable heating conversion (≈200%), and long-term stability (> 24 h). To illustrate its capabilities, the nanocomposite is used to track human physiological signals and perform electric-thermal actuating as a set of soft tongs. It is believed that this innovative approach will provide value for the development of wearable/stretchable devices, as well as human-machine interaction, and bio-robotics in the future.

Dense point defects can strengthen phonon scattering, however, inevitably strengthen carrier scattering and deteriorate carrier mobility. Herein, it is demonstrated that the localized interstitial Cu can induce a synergistic effect, leading to high µ (80 cm² V⁻¹ s⁻¹) and ultralow κ_l (0.48 W m⁻¹ K⁻¹), thus approaching a high zT of ≈2.3 at 623 K in the Ge_0.93Ti_0.01Bi_0.06Te-0.01Cu.

Abstract

Dense point defects can strengthen phonon scattering to reduce the lattice thermal conductivity and induce outstanding thermoelectric performance in GeTe-based materials. However, extra point defects inevitably enlarge carrier scattering and deteriorate carrier mobility. Herein, it is found that the interstitial Cu in GeTe can result in synergistic effects, which include: 1) strengthened phonon scattering, leading to ultralow lattice thermal conductivity of 0.48 W m⁻¹ K⁻¹ at 623 K; 2) weakened carrier scattering, contributing to high carrier mobility of 80 cm² V⁻¹ s⁻¹ at 300 K; 3) optimized carrier concentration of 1.22 × 10²⁰ cm⁻³. Correspondingly, a high figure-of-merit of ≈2.3 at 623 K can be obtained in the Ge_0.93Ti_0.01Bi_0.06Te-0.01Cu, which corresponds to a maximum energy conversion efficiency of ≈10% at a temperature difference of 423 K. This study systematically investigates the doping behavior of the interstitial Cu in GeTe-based thermoelectric materials for the first time and demonstrates that the localized interstitial Cu is a new strategy to enhance the thermoelectric performance of GeTe-based thermoelectric materials.

Ceramic–metal composites prepared from technical-grade powders include different impurity elements that cause formation and growth of secondary phases. Magnetron-sputtered high-purity Al₂O₃–Nb film composites offer a path to selectively study interfacial reactions. Atom probe tomography and transmission electron microscopy shed light on the complex internal oxidation at the interface induced by oxygen transport through Nb grain boundaries.

Parts for metallurgical applications made from refractory metal–ceramic composites offer improved thermal shock resistance due to their capability for resistive heating compared to ones made solely from ceramics such as Al₂O₃. The combination of Al₂O₃ and Nb is intriguing as both show similar thermal expansion behavior over a wide temperature range. The high affinity of Nb for O to form nonprotective oxides, however, hampers its use in oxidative environments. Formation of such phases at the ceramic–metal interface can have detrimental effects on the cohesion of the composites. For this work, nanocrystalline Nb films are deposited on sapphire substrates by magnetron sputtering to study diffusion of O and high-temperature phase formation at a refractory metal–ceramic interface during heat treatment under Ar at 1600 °C. A combined approach of atom probe tomography and transmission electron microscopy for compositional and crystallographic analyses reveals that at triple junctions of the sapphire–Nb interface with Nb grain boundaries, heterogeneous nucleation of nanoscale NbO₂ occurs, which further reacts with Al₂O₃ to form AlNbO₄, while the Nb film itself remains metallic. Fast O transport through grain boundaries leads to internal oxidation at the interface, whereas regions further away from Nb grain boundaries remain unchanged.

Quantum dots (QDs) are used as promising materials for next-generation displays due to their excellent electrical/optical properties. Strategies for patterning QDs via photolithography (conventional photolithography, lift off process, and direct photolithography) are comprehensively reviewed. This review also discusses the prospects for patterned QDs in terms of their structural and physical properties.

Abstract

Pixelating patterns of red, green, and blue quantum dots (QDs) is a critical challenge for realizing high-end displays with bright and vivid images for virtual, augmented, and mixed reality. Since QDs must be processed from a solution, their patterning process is completely different from the conventional techniques used in the organic light-emitting diode and liquid crystal display industries. Although innovative QD patterning technologies are being developed, photopatterning based on the light-induced chemical conversion of QD films is considered one of the most promising methods for forming micrometer-scale QD patterns that satisfy the precision and fidelity required for commercialization. Moreover, the practical impact will be significant as it directly exploits mature photolithography technologies and facilities that are widely available in the semiconductor industry. This article reviews recent progress in the effort to form QD patterns via photolithography. The review begins with a general description of the photolithography process. Subsequently, different types of photolithographical methods applicable to QD patterning are introduced, followed by recent achievements using these methods in forming high-resolution QD patterns. The paper also discusses prospects for future research directions.

Dense, smooth, substrate-integrated, large-area and thickness-controlled perovskite microcrystalline films are prepared. They are used as self-powered microcrystalline X-ray detectors with an impressive sensitivity of 6.1×10⁴ μC Gy_air ⁻¹ cm⁻² and a low detection limit of 1.5 nGy_air s⁻¹, leading to high-contrast X-ray imaging at an ultra-low dose rate of 67 nGy_air s⁻¹, which may further reduce the X-ray radiation risk to patients.

Abstract

Perovskite single crystals and polycrystalline films have complementary merits and deficiencies in X-ray detection and imaging. Herein, we report preparation of dense and smooth perovskite microcrystalline films with both merits of single crystals and polycrystalline films through polycrystal-induced growth and hot-pressing treatment (HPT). Utilizing polycrystalline films as seeds, multi-inch-sized microcrystalline films can be in situ grown on diverse substrates with maximum grain size reaching 100 μm, which endows the microcrystalline films with comparable carrier mobility-lifetime (μτ) product as single crystals. As a result, self-powered X-ray detectors with impressive sensitivity of 6.1×10⁴ μC Gy_air ⁻¹ cm⁻² and low detection limit of 1.5 nGy_air s⁻¹ are achieved, leading to high-contrast X-ray imaging at an ultra-low dose rate of 67 nGy_air s⁻¹. Combining with the fast response speed (186 μs), this work may contribute to the development of perovskite-based low-dose X-ray imaging.