Jiuxiang Dai

Applied Physics Letters, Volume 122, Issue 7, February 2023.
Here, we report the growth of (Sr, Ca) Nd2Fe2O7 single crystals with the Ruddlesden–Popper structure using an optical floating-zone method. A significantly anisotropic magneto-dielectric effect (MD), ab-plane and c-axial MD coefficients reaching −12.3% and −8.4% measured at 103 Hz in a 1 T magnetic field, can be obtained in a SrNd2Fe2O7 crystal at room temperature. The corresponding anisotropic MD ratio can be reached as high as 1.46. With an increase in the Ca concentration, the MD effect decreases dramatically and is eventually completely suppressed in both directions. Analysis of magnetic properties and 57Fe Mössbauer spectra suggests that the anisotropic MD effects in SrNd2Fe2O7 can be attributed to polaronic hopping between two neighboring Fe3+ ions through oxygen vacancies in an anisotropically antiferromagnetic matrix; the disappearance of the MD effect in Ca-doped SrNd2Fe2O7 is a consequence of the suppression of the antiferromagnetism. Our work suggests that the significantly anisotropic MD effect in SrNd2Fe2O7 crystals at room temperature can be used in magneto-dielectric controlled devices.

Nature Reviews Materials, Published online: 15 February 2023; doi:10.1038/s41578-022-00530-0

Ultrafast spectroscopies enable the characterization of quantum materials and of their functional properties arising from strong correlations and electronic topology. This Review discusses three emerging techniques: attosecond transient absorption spectroscopy, solid-state high-harmonic generation spectroscopy and extreme ultraviolet-second harmonic generation spectroscopy.

Nature Communications, Published online: 15 February 2023; doi:10.1038/s41467-023-36512-1

Manipulating electrical and magnetic anisotropies will stimulate multi-terminal device applications. Here, the authors discover axis dependence of current rectifications, magnetic properties and magnon modes in van der Waals multiferroic CuCrP2S6.

A novel self-templating synthesis method is presented with molten-salt protection to synthesize high-quality few-atoms-thin Mo₂C nanosheets with a low concentration of defects. The fundamental electrochemical properties of the 2D Mo₂C nanosheets show that the electrocatalytic activity of 2D Mo₂C increases with decreasing thickness.

Abstract

2D transition metal carbides (2D TMCs and MXenes) are promising candidates for applications of energy storage and catalysis. However, producing high-quality, large 2D flakes of Mo2C MXene has been challenging. Here, a new salt-assisted templating approach is reported that enables the direct synthesis of 2D Mo₂C with low defect concentrations. KCl acts as a template to form an intermediate 2D product, facilitating Mo₂C formation without coarsening upon melting. The thickness of the flakes produced can range from monolayer (0.36 nm) to 10 layers (4.55 nm), and the electrocatalytical hydrogen evolution reaction (HER) activity of 2D Mo₂C is inversely proportional to its thickness. The monolayer Mo₂C shows remarkable HER performance with a current density of ≈6800 mA cm^{− 2} at 470 mV versus reversible hydrogen electrode and an ultrahigh turnover frequency of ≈17 500 s^{− 1}. This salt-assisted synthesis approach can also produce WC and V₈C₇ nanosheets, expanding the family of 2D carbides. The new pathway eliminates the need for layered ceramic precursors, making it a versatile approach to direct synthesis of MXene-like 2D carbides.

This study demonstrates the rational manipulation of lattice strain and crystal field energy splitting in Mg₃Sb₂ epitaxial films by the choice of substrates. Theoretical and experimental efforts unambiguously validate that valence band convergence is acquired in the Mg₃Sb₂ film with large in-plane compressive strain that is grown on InP substrate, leading to significantly improved carrier effective mass and Seebeck coefficients.

Abstract

Strain engineering is demonstrated to effectively regulate the functionality of materials, such as thermoelectric, ferroelectric, and photovoltaic properties. As the straightforward approach of strain engineering, epitaxial strain is usually proposed for rationally manipulating the electronic structure and performances of thermoelectric materials, but has rarely been verified experimentally. In this study, tunable and large epitaxial strains are demonstrated, as well as the resulting valence band convergence can be achieved in the Mg₃Sb₂ epi-films with the choice of substrates. The large epitaxial strains up to 8% in Mg₃Sb₂ films represent one of the most striking results in strain engineering. The angle-resolved photoemission spectroscopy measurements and the theoretical calculations reveal the vital role of epitaxial strain in tuning the crystal field splitting and the band structure of Mg₃Sb₂. Benefiting from the appropriate manipulation of the crystal field effect via in-plane compressive strain, the valence band convergence is unambiguously discovered in the strained Mg₃Sb₂ film grown on InP(111) substrate. As a result, a state-of-the-art thermoelectric power factor of 0.94 mWm⁻¹K⁻² is achieved in the strain-engineered Mg₃Sb₂ film, well exceeding that of the strain-relaxed Mg₃Sb₂. The work paves the way for effectively manipulating epitaxial strain and band convergence for Mg₃Sb₂ and other thermoelectric films.

Journal of Applied Physics, Volume 133, Issue 6, February 2023.
Two-dimensional (2D) titanium disulfide (TiS[math]) is the lightest transition-metal dichalcogenide (TMD). It exhibits relatively better adsorption and diffusion of sodium (Na) and potassium (K) ions than other TMDs, such as MoS[math] (molybdenum disulfide) and ReS[math] (rhenium disulfide), making it a promising anode material for alkali-ion batteries. Previous studies have found that doping significantly enhances the adsorption and diffusion capabilities of 2D TMDs. For the first time, this work reports the adsorption of Na and K ions on doped TiS[math] monolayers using first-principles calculations, where the Ti atom is substituted by 3d-transition metals, including iron (Fe), cobalt (Co), nickel (Ni), and copper (Cu). Metal-atom doping induces remarkably stronger binding of alkali ions on the surface of TiS[math], with adsorption energies ranging from [math]2.07 to [math]2.48 eV for Na and [math]2.59 to [math]3.00 eV for K. The diffusion barrier energies for alkali ions decrease in the proximity of the doping site and increase as the ions travel away from the doping site for Fe-, Co-, and Ni-doped TiS[math]. The average open circuit voltage increases dramatically when Na ions are adsorbed on Fe-doped TiS[math] (by 62%) and Co-doped TiS[math] (by 61%), while K ions result in a moderate improvement of 9% and 8%, respectively. These findings suggest that metal-atom doping considerably improves the electrochemical properties of 2D TiS[math], potentially enabling its use as anode materials in Na- and K-ion batteries.

Applied Physics Letters, Volume 122, Issue 7, February 2023.

During metal–organic chemical vapor deposition of 2D semiconductor thin films, periodic alternating of low growth temperature, high ripening temperature and the intermittent supply of metal–organic precursors enable to suppress high nucleation density, and achieve large single-crystal domains.

Abstract

2D semiconductors, especially 2D transition metal dichalcogenides (TMDCs), have attracted ever-growing attention toward extending Moore's law beyond silicon. Metal–organic chemical vapor deposition (MOCVD) has been widely considered as a scalable technique to achieve wafer-scale TMDC films for applications. However, current MOCVD process usually suffers from small domain size with only hundreds of nanometers, in which dense grain boundary defects degrade the crystalline quality of the films. Here, a periodical varying-temperature ripening (PVTR) process is demonstrated to grow wafer-scale high crystalline TMDC films by MOCVD. It is found that the high-temperature ripening significantly reduces the nucleation density and therefore enables single-crystal domain size over 20 µm. In this process, no additives or etchants are involved, which facilitates low impurity concentration in the grown films. Atom-resolved electron microscopy imaging, variable temperature photoluminescence (PL) spectroscopy, and electrical transport results further confirm comparable crystalline quality to those observed in mechanically exfoliated TMDC films. The research provides a scalable route to produce high-quality 2D semiconducting films for applications in electronics and optoelectronics.

This work proposes a heteroepitaxial growth strategy of ferromagnetic CuCr₂Te₄ nanosheets on mica and Cr₂Te₃, with thickness-dependent ferromagnetism of CuCr₂Te₄ nanosheets on mica and an enhanced robust ferromagnetism with T _C up to 340 K via interfacial charge transfer in lattice-matched CuCr₂Te₄/Cr₂Te₃ heterostructure. The thickness-dependent room-temperature 2D ferromagnetic heterostructures are promising for constructing functional spintronic devices.

Abstract

Magnetic materials in 2D have attracted widespread attention for their intriguing magnetic properties. 2D magnetic heterostructures can provide unprecedented opportunities for exploring fundamental physics and novel spintronic devices. Here, the heteroepitaxial growth of ferromagnetic CuCr₂Te₄ nanosheets is reported on Cr₂Te₃ and mica by chemical vapor deposition. Magneto-optical Kerr effect measurements reveal the thickness-dependent ferromagnetism of CuCr₂Te₄ nanosheets on mica, where a decrease of Curie temperature (T _C) from 320 to 260 K and an enhancement of perpendicular magnetic anisotropy with reducing thickness are observed. Moreover, lattice-matched heteroepitaxial ultrathin CuCr₂Te₄ on Cr₂Te₃ exhibits an enhanced robust ferromagnetism with T _C up to 340 K due to the interfacial charge transfer. Stripe-type magnetic domains and single magnetic domain are discovered in this heterostructure with different thicknesses. The work provides a way to construct robust room-temperature 2D magnetic heterostructures for functional spintronic devices.

A ferroelectric-enabled van der Waals (vdWs) heterojunction through vertically stacking α-In2Se3 and black phosphorus (BP) is reported. The energy bands at the heterojunction interfaces can be aligned and flexibly engineered. Fast-response, self-driven photodetection, and three-orders of magnitude detection improvement are achieved in the switchable visible or near-infrared bands. This work provides a feasible method for the performance improvement of vdWs optoelectronic devices.

Abstract

Owing to the large built-in field for efficient charge separation, heterostructures facilitate the simultaneous realization of a low dark current and high photocurrent. The lack of an efficient approach to engineer the depletion region formed across the interfaces of heterojunctions owing to doping differences hinders the realization of high-performance van der Waals (vdW) photodetectors. This study proposes a ferroelectric-controlling van der Waals photodetector with vertically stacked two-dimensional (2D) black phosphorus (BP)/indium selenide (In₂Se₃) to realize high-sensitivity photodetection. The depletion region can be reconstructed by tuning the polarization states generated from the ferroelectric In₂Se₃ layers. Further, the energy bands at the heterojunction interfaces can be aligned and flexibly engineered using ferroelectric field control. Fast response, self-driven photodetection, and three-orders-of-magnitude detection improvements are achieved in the switchable visible or near-infrared operation bands. The results of the study are expected to aid in improving the photodetection performance of vdW optoelectronic devices.

Nature Synthesis, Published online: 13 February 2023; doi:10.1038/s44160-022-00232-z

Intercalation-based exfoliation is a reliable strategy for preparing atomically thin sheets. This Review highlights various types of intercalation-based exfoliation methods as well as the potential applications of the exfoliated nanosheets and the challenges and prospects in this emerging field.

Nature Nanotechnology, Published online: 13 February 2023; doi:10.1038/s41565-023-01326-1

High-quality crystalline two-dimensional layers of metal halides can be on mica, MoS2 or WS2 at temperatures below 400 °C.

Advances in the fabrication of periodic patterns on polymer surfaces taking advantage of anisotropic wrinkling are summarized. This review thoroughly describes the formation mechanisms and fabrication approaches for oriented wrinkles, as well as their fine-tuning in the wavelength, amplitude, and orientation control. Finally, the major applications in which oriented wrinkled interfaces are already employed or may be prospective are overviewed.

Abstract

Periodically patterned surfaces can cause special surface properties and are employed as functional building blocks in many devices, yet remaining challenges in fabrication. Advancements in fabricating structured polymer surfaces for obtaining periodic patterns are accomplished by adopting “top-down” strategies based on self-assembly or physico-chemical growth of atoms, molecules, or particles or “bottom-up” strategies ranging from traditional micromolding (embossing) or micro/nanoimprinting to novel laser-induced periodic surface structure, soft lithography, or direct laser interference patterning among others. Thus, technological advances directly promote higher resolution capabilities. Contrasted with the above techniques requiring highly sophisticated tools, surface instabilities taking advantage of the intrinsic properties of polymers induce surface wrinkling in order to fabricate periodically oriented wrinkled patterns. Such abundant and elaborate patterns are obtained as a result of self-organizing processes that are rather difficult if not impossible to fabricate through conventional patterning techniques. Focusing on oriented wrinkles, this review thoroughly describes the formation mechanisms and fabrication approaches for oriented wrinkles, as well as their fine-tuning in the wavelength, amplitude, and orientation control. Finally, the major applications in which oriented wrinkled interfaces are already in use or may be prospective in the near future are overviewed.

An attractive strategy via the formation of van der Waals (vdW) contacts between atomically thin metallic and semiconducting transition-metal dichalcogenides (TMDs) is proposed to suppress strong Fermi-level pinning at the metal–semiconductor interfaces. By means of high-throughput first-principles calculations, a series of phase-engineered TMD-based vdW metal–semiconductor junctions with weak Fermi-level pining and high carrier tunneling probability have been screened.

Abstract

A main challenge for the development of two-dimensional devices based on atomically thin transition-metal dichalcogenides (TMDs) is the realization of metal–semiconductor junctions (MSJs) with low contact resistance and high charge transport capability. However, traditional metal–TMD junctions usually suffer from strong Fermi-level pinning (FLP) and chemical disorder at the interfaces, resulting in weak device performance and high energy consumption. By means of high-throughput first-principles calculations, we report an attractive solution via the formation of van der Waals (vdW) contacts between metallic and semiconducting TMDs. We apply a phase-engineering strategy to create a monolayer TMD database for achieving a wide range of work functions and band gaps, hence offering a large degree of freedom to construct TMD vdW MSJs with desired contact types. The Schottky barrier heights and contact types of 728 MSJs have been identified and they exhibit weak FLP (−0.62 to −0.90) as compared with the traditional metal–TMD junctions. We find that the interfacial interactions of the MSJs bring a delicate competition between the FLP strength and carrier tunneling efficiency, which can be utilized to screen high-performance MSJs. Based on a set of screening criteria, four potential TMD vdW MSJs (e.g., NiTe₂/ZrSe₂, NiTe₂/PdSe₂, HfTe₂/PdTe₂, TaSe₂/MoTe₂) with Ohmic contact, weak FLP, and high carrier tunneling probability have been predicted. This work not only provides a fundamental understanding of contact properties of TMD vdW MSJs but also renders their huge potential for electronics and optoelectronics.

Journal of Applied Physics, Volume 133, Issue 5, February 2023.
Secondary electron emission serves as the foundation for a broad range of vacuum electronic devices and instrumentation, from particle detectors and multipliers to high-power amplifiers. While secondary yields of at least 3–4 are required in practical applications, the emitter stability can be compromised by surface dynamics during operation. As a result, the range of practical emitter materials is limited. The development of new emitter materials with high yield and robust operation would advance the state-of-the-art and enable new device concepts and applications. In this Perspective article, I first present an analysis of the secondary emission process, with an emphasis on the influence of material properties. From this analysis, ultra-wide bandgap (UWBG) semiconductors and oxides emerge as superior emitter candidates owing to exceptional surface and transport properties that enable a very high yield of low-energy electrons with narrow energy spread. Importantly, exciting advances are being made in the development of promising UWBG semiconductors such as diamond, cubic boron nitride (c-BN), and aluminum nitride (AlN), as well as UWBG oxides with improved conductivity and crystallinity. These advances are enabled by epitaxial growth techniques that provide control over the electronic properties critical to secondary electron emission, while advanced theoretical tools provide guidance to optimize these properties. Presently, H-terminated diamond offers the greatest opportunity because of its thermally stable negative electron affinity (NEA). In fact, an electron amplifier under development exploits the high yield from this NEA surface, while more robust NEA diamond surfaces are demonstrated with potential for high yields in a range of device applications. Although c-BN and AlN are less mature, they provide opportunities to design novel heterostructures that can enhance the yield further.

Direct band gaps, large spin-splittings of bands, and high exciton binding energies in novel hexagonal non-centrosymmetric MSi₂Z₄ monolayers are predicted from theoretical calculations. Emerging higher conduction bands are involved in excitons with opposite selection rules and large positive g-factors. Spin-valley properties, similar to monolayer 2H transition-metal dichalcogenides, with additional valley degrees of freedom, are found in the studied compounds.

Abstract

MA₂Z₄ monolayers form a new class of hexagonal non-centrosymmetric materials hosting extraordinary spin-valley physics. While only two compounds (MoSi₂N₄ and WSi₂N₄) are recently synthesized, theory predicts interesting (opto)electronic properties of a whole new family of such two-dimensional (2D) materials. Here, the chemical trends of band gaps and spin-orbit splittings of bands in selected MSi₂Z₄ (M = Mo, W; Z = N, P, As, Sb) compounds are studied from first-principles. Effective Bethe–Salpeter-equation-based calculations reveal high exciton binding energies. Evolution of excitonic energies under external magnetic field is predicted by providing their effective g-factors and diamagnetic coefficients, which can be directly compared to experimental values. In particular, large positive g-factors are predicted for excitons involving higher conduction bands. In view of these predictions, MSi₂Z₄ monolayers yield a new platform to study excitons and are attractive for optoelectronic devices, also in the form of heterostructures. In addition, a spin-orbit induced bands inversion is observed in the heaviest studied compound, WSi₂Sb₄, a hallmark of its topological nature.

Applied Physics Letters, Volume 122, Issue 6, February 2023.
Monolayer group monochalcogenides (MX; M = Sn, Ge; X = S, Se) in the orthogonal α-phase are excellent piezoelectric materials. In this study, a configuration with bonding features similar to the α-phase is proposed (T-phase) for monolayer MX using the first-principles method. Based on the modern theory of polarization, as implemented in Vienna Ab initio Simulation Package, the T-phase is determined to be an excellent piezoelectric phase for monolayer MX. The in-plane piezoelectric coefficient d11 of T-SnS is 452.3 pm/V, which is larger than that reported for most two-dimensional binary compounds in the α-phase, including α-SnSe (∼250 pm/V). The large piezoelectric coefficients of T-MX mainly stem from its distinctive puckered configuration, which make it extraordinarily flexible along the polarization direction. The study results suggest a possibility for designing high piezoelectric coefficient materials with MX, and the potential application of T-MX in the fields of energy collection and nanoelectromechanical systems needs to be analyzed in future studies.

Applied Physics Letters, Volume 122, Issue 6, February 2023.
Cd3As2 provides an excellent platform for studying the physics of three-dimensional Dirac semimetals due to its stability as well as its compatibility with thin film growth. Crystals made using both bulk and thin film synthesis are unintentionally doped n-type, and other than introducing Zn to reduce the carrier concentration, no efforts have been reported to alter this intrinsic doping without major changes to the band structure. Here, group VI elements Te and Se are introduced during epitaxy to increase the electron concentration of the films. Starting from an unintentionally doped electron concentration of 1–2 × 1017 cm−3, concentrations of up to 3 × 1018 cm−3 are achieved. Analysis of Shubnikov–de Haas oscillations reveals good agreement in calculated effective mass and Fermi velocity of highly doped films with unintentionally doped single crystals with similar electron concentrations. The density functional theory is also performed to study the effects of group VI substitutions and confirms no strong perturbations in the electronic structure. This work ultimately demonstrates tunability in the carrier concentration using extrinsic dopants without substantial changes in the band structure, allowing for intentional design of Fermi-level position for device applications.