Jiuxiang Dai

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDCs), due to their unique physical properties, have a wide range of applications in the next generation of electronics, optoelectronics, and valleytronics. Large-scale preparation of high-quality TMDCs films is critical to realize these potential applications. Here we report a study on metal-organic chemical vapor deposition (MOCVD) growth of wafer-scale MoSe₂ films guided by the crystalline step edges of miscut sapphire wafers. We established that the nucleation density and growth rate of MoSe₂ films were positively correlated with the step-edge density and negatively with the growth temperature. At a certain temperature, the MoSe₂ domains on the substrate with high step-edge density grow faster than that with low density. As a result, wafer-scale and continuous MoSe₂ films can be formed in a short duration (30 min). The MoSe₂ films are of high crystalline quality, as confirmed by systematic Raman and photoluminescence (PL) measurements. The results provide an important methodology for the rapid growth of wafer-scale TMDCs, which may promote the application of 2D semiconductors.

Author(s): Yucheng Jiang, Xinglong Ma, Lin Wang, Jinlei Zhang, Zhichao Wang, Run Zhao, Guozhen Liu, Yang Li, Cheng Zhang, Chunlan Ma, Yaping Qi, Lin Wu, and Ju Gao

The switchable electric polarization is usually achieved in ferroelectric materials with noncentrosymmetric structures, which opens exciting opportunities for information storage and neuromorphic computing. In another polar system of p−n junction, there exists the electric polarization at the interf…

[Phys. Rev. Lett. 130, 196801] Published Tue May 09, 2023

An efficient amorphous strategy is developed to trigger the reconfiguration of interlayer Pt atoms in 2D PtTe₂ materials and tailor the electronic properties of Pt active sites via the orbital hybridization with oxygen dopants. O ur findings provide an efficient strategy for designing amorphous catalysts whose number of active sites and adsorption strength to reaction intermediates are significantly increased and optimized.

Abstract

Exposing active sites and optimizing their binding strength to reaction intermediates are two essential strategies to significantly improve the catalytic performance of 2D materials. However, pursuing an efficient way to achieve these goals simultaneously remains a considerable challenge. Here, using 2D PtTe₂ van der Waals material with a well-defined crystal structure and atomically thin thickness as a model catalyst, it is observed that a moderate calcination strategy can promote the structural transformation of 2D crystal PtTe₂ nanosheets (c-PtTe₂ NSs) into oxygen-doped 2D amorphous PtTe₂ NSs (a-PtTe₂ NSs). The experimental and theoretical investigations cooperatively reveal that oxygen dopants can break the inherent Pt-Te covalent bond in c-PtTe₂ NSs, thereby triggering the reconfiguration of interlayer Pt atoms and exposing them thoroughly. Meanwhile, the structural transformation can effectively tailor the electronic properties (e.g., the density of state near the Fermi level, d-band center, and conductivity) of Pt active sites via the hybridization of Pt 5d orbitals and O 2p orbitals. As a result, a-PtTe₂ NSs with large amounts of exposed Pt active sites and optimized binding strength to hydrogen intermediates exhibit excellent activity and stability in hydrogen evolution reaction.

Multifunctional GeAs/ReS₂ van der Waals (vdWs) heterojunction device is realized via modulating the doping level of GeAs. The directional rectification of the forward rectifying diode, Zener tunneling diode, and backward rectifying diodes is controlled by optimizing the effective band gap between GeAs and ReS₂. This work provides an effective strategy to achieve multifunctional 2D vdW heterojunction devices.

Abstract

Van der Waals heterojunction (vdWs) of 2D materials with integrated or extended superior characteristics, opening up new opportunities in functional electronic and optoelectric device applications. Exploring methods to achieve multifunctional vdWs heterojunction devices is one of the most promising prospects in this area. Herein, a diverse function of forward rectifying diode, Zener tunneling diode, and backward rectifying diodes are realized in GeAs/ReS₂ heterojunction by modulating the doping level of GeAs. The tunneling diode presents an interesting trend forward negative differential resistance (NDR) behavior which may facilitate the application of multi-value logic. More importantly, the GeAs/ReS₂ forward rectifying diode exhibits highly sensitive photodetection in the wide-spectrum range up to 1550 nm corresponding to a short-wave infrared (SWIR) region. In addition, as two strong anisotropic 2D materials of GeAs and ReS₂, the heterojunction exhibits strong polarization-sensitive photodetection behavior with a dichroic photocurrent ratio of 1.7. This work provides an effective strategy to achieve multifunctional 2D vdW heterojunction devices and develops more possibilities to broaden their functionalities and applications.

Light: Science & Applications, Published online: 08 May 2023; doi:10.1038/s41377-023-01159-6

A collective control of oxygen vacancies is proposed to implement reliable and gradual resistive switching in ultrathin LaAlO₃/SrTiO₃ (LAO/STO) heterostructures. By controlling the concentration of the potential-constrained surface oxygen vacancies, the filament-free LAO/STO device shows the analog resistive switching property with a high accuracy of over 91% which is beneficial for neuromorphic applications.

Abstract

Filamentary resistive switching in oxides is one of the key strategies for developing next-generation non-volatile memory devices. However, despite numerous advantages, their practical applications in neuromorphic computing are still limited due to non-uniform and indeterministic switching behavior. Given the inherent stochasticity of point defect migration, the pursuit of reliable switching likely demands an innovative approach. Herein, a collective control of oxygen vacancies is introduced in LaAlO₃/SrTiO₃ (LAO/STO) heterostructures to achieve reliable and gradual resistive switching. By exploiting an electrostatic potential constraint in ultrathin LAO/STO heterostructures, the formation of conducting filaments is suppressed, but instead precisely control the concentration of oxygen vacancies. Since the conductance of the LAO/STO device is governed by the ensemble concentration of oxygen vacancies, not their individual probabilistic migrations, the resistive switching is more uniform and deterministic compared to conventional filamentary devices. It provides direct evidence for the collective control of oxygen vacancies by spectral noise analysis and modeling by Monte-Carlo simulation. As a proof of concept, the significantly-improved analog switching performance of the filament-free LAO/STO devices is demonstrated, revealing potential for neuromorphic applications. The results establish an approach to store information by point defect concentration, akin to biological ionic channels, for enhancing switching characteristics of oxide materials.

The existence of giant magnetocaloric effect (MCE) and its strain tunability in monolayer magnets such as CrX₃ (X = F, Cl, Br, I), CrAX (A = O, S, Se; X = F, Cl, Br, I), and Fe₃GeTe₂ are revealed through multiscale calculations. MCE of 2D magnets is theoretically comparable to that of bulk materials and can be remarkably tuned by strain. In particular, CrF₃ possesses the state-of-the-art MCE at low temperatures. These findings advocate the giant-MCE monolayer magnets, opening new opportunities for magnetic cooling at nanoscale.

Abstract

2D magnets can potentially revolutionize information technology, but their potential application to cooling technology and magnetocaloric effect (MCE) in a material down to the monolayer limit remain unexplored. Herein, it is revealed through multiscale calculations the existence of giant MCE and its strain tunability in monolayer magnets such as CrX₃ (X = F, Cl, Br, I), CrAX (A = O, S, Se; X = F, Cl, Br, I), and Fe₃GeTe₂. The maximum adiabatic temperature change (ΔTadmax$\Delta T_{{\rm{ad}}}^{\max }$), maximum isothermal magnetic entropy change, and specific cooling power in monolayer CrF₃ are found as high as 11 K, 35 µJ m⁻² K⁻¹, and 3.5 nW cm⁻² under a magnetic field of 5 T, respectively. A 2% biaxial and 5% a-axis uniaxial compressive strain can remarkably increase ΔTadmax$\Delta T_{{\rm{ad}}}^{\max }$ of CrCl₃ and CrOF by 230% and 37% (up to 15.3 and 6.0 K), respectively. It is found that large net magnetic moment per unit area favors improved MCE. These findings advocate the giant-MCE monolayer magnets, opening new opportunities for magnetic cooling at nanoscale.

Nature Nanotechnology, Published online: 08 May 2023; doi:10.1038/s41565-023-01360-z

Nature Electronics, Published online: 08 May 2023; doi:10.1038/s41928-023-00965-5

High-pressure induced and quantitatively regulated oxygen vacancies compensate for the limitations of lithium titanate conductivity and kinetics. In addition, the self-high-pressure quenching strategy produces abundant grain boundaries, interfaces, and vacancies, causing pseudo-capacitance effect to increase the capacity. Furthermore, the fracture structures are used as a storage of electrolyte to facilitate ion diffusion kinetics, thus improving the electrochemical performance.

Abstract

Inspired by the functional properties of ion defect induction and charge compensation in defect engineering, these methods are expected to be an effective strategy to solve the constraints of Li₄Ti₅O₁₂ (LTO) inherent conductivity and diffusion dynamics, and further improve battery rate performance. The oxygen vacancy (OV) content in LTO can be controlled quantitatively by high-pressure induction using the high-pressure and high-temperature (HPHT) method. In addition, the relationship between the electrochemical properties and OV is further explored. The theoretical calculations indicate that the OV defects cause the electrons to delocalize into the conduction band of the LTO, and thus fundamentally improve the intrinsic conductivity. In particular, the high-pressure quenching strategy of HPHT causes LTO to instantly produce crack holes with massive crystalline layers, which can be regarded as storage for the electrolyte to facilitate ion diffusion. The fabricated LTO anodes containing OVs compensate for the limitation of the poor rate performance with a capacity of 176 mAh g⁻¹ at 20 C. Pressure-induced OV defects not only open up a new perspective in the field of lithium-ion batteries (LIBs), but also provide a certain degree of freedom for the functional design characteristics of defect engineering.

Taking advantage of the optical transparency in wide bandgap semiconductors, this study introduces a unique engineering approach to develop flexible, 3D electronics which are fabricated on the front and back sides of free-standing nanomembranes. These 3D wide bandgap electronic platforms hold promise for organ-on-chip and long-term implanted applications.

Abstract

Wide bandgap (WBG) semiconductors have attracted significant research interest for the development of a broad range of flexible electronic applications, including wearable sensors, soft logical circuits, and long-term implanted neuromodulators. Conventionally, these materials are grown on standard silicon substrates, and then transferred onto soft polymers using mechanical stamping processes. This technique can retain the excellent electrical properties of wide bandgap materials after transfer and enables flexibility; however, most devices are constrained by 2D configurations that exhibit limited mechanical stretchability and morphologies compared with 3D biological systems. Herein, a stamping-free micromachining process is presented to realize, for the first time, 3D flexible and stretchable wide bandgap electronics. The approach applies photolithography on both sides of free-standing nanomembranes, which enables the formation of flexible architectures directly on standard silicon wafers to tailor the optical transparency and mechanical properties of the material. Subsequent detachment of the flexible devices from the support substrate and controlled mechanical buckling transforms the 2D precursors of wide band gap semiconductors into complex 3D mesoscale structures. The ability to fabricate wide band gap materials with 3D architectures that offer device-level stretchability combined with their multi-modal sensing capability will greatly facilitate the establishment of advanced 3D bio-electronics interfaces.

Dual-gated MoS₂ field-effect transistors are fabricated using semimetal (antimony, Sb) as top surface electrodes and hexagonal boron nitride as a top-gate (TG) dielectric. This approach achieves Fermi level pinning (FLP)-free contacts, which enable a strong modulation of pseudo-junction resistance by TG voltages, owing to the weak FLP.

Abstract

Achieving low contact resistance (R _C) is one of the major challenges in producing 2D FETs for future CMOS technology applications. In this work, the electrical characteristics for semimetal (Sb) and normal metal (Ti) contacted MoS₂ devices are systematically analyzed as a function of top and bottom gate-voltages (V _TG and V _BG). The semimetal contacts not only significantly reduce R _C but also induce a strong dependence of R _C on V _TG, in sharp contrast to Ti contacts that only modulate R _C by varying V _BG. The anomalous behavior is attributed to the strongly modulated pseudo-junction resistance (R _jun) by V _TG, resulting from weak Fermi level pinning (FLP) of Sb contacts. In contrast, the resistances under both metallic contacts remain unchanged by V _TG as metal screens the electric field from the applied V _TG. Technology computer aided design simulations further confirm the contribution of V _TG to R _jun, which improves overall R _C of Sb-contacted MoS₂ devices. Consequently, the Sb contact has a distinctive merit in dual-gated (DG) device structure, as it greatly reduces R _C and enables effective gate control by both V _BG and V _TG. The results offer new insight into the development of DG 2D FETs with enhanced contact properties realized by using semimetals.

Double-sided processing on ultra-thin silicon provides the opportunity to double the number of devices or create dual functionalities. As a demonstration, a dual coil patterned ultra-thin silicon (DCUTS) is fabricated. DCUTS is then demonstrated as an actuating, energy harvesting, and vibrating mirror system. A multiphysics simulation model is also developed, and the reliability of the device is tested.

Abstract

Double-sided microfabrication process on an ultra-thin silicon film has rarely been attempted due to the challenges in terms of the preparation and handling of a thin film in spite of its promising fabrication potentials. Such a process allows for doubling the thin film device density or providing dual functionalities for a thin film depending on whether the front and back sides of a thin film are processed identically or distinctively. Here, a novel double-sided thin film processing strategy is introduced by realizing a dual coil patterned ultra-thin silicon film that is working as an actuating or energy harvesting system. Experimentally, a dual coil patterned thin film enabled using the introduced approach shows remarkably enhanced device performance when compared with a single coil patterned counterpart. Furthermore, a multiphysics simulation model is developed and the resultant modeling data validate the experimentally measured performance enhancement. Finally, the structural durability of the thin film upon cyclic loading is tested and its diverse vibration modes are investigated.

The ferroelectric domain of monolayer α-In₂Se₃ can be switched by applying mechanical loading or electrical biasing using the tip of piezoresponse force microscopy. The threshold forces (Fc) and voltages (Ec) for domain switching increase with the increasing writing-domain speeds, indicating a speed-dependent effect on ferroelectric domain switching.

Abstract

An electrical-biased or mechanical-loaded scanning probe written on the ferroelectric surface can generate programmable domain nanopatterns for ultra-scaled and reconfigurable nanoscale electronics. Fabricating ferroelectric domain patterns by direct-writing as quickly as possible is highly desirable for high response rate devices. Using monolayer α-In₂Se₃ ferroelectric with ≈1.2 nm thickness and intrinsic out-of-plane polarization as an example, a writing-speed dependent effect on ferroelectric domain switching is discovered. The results indicate that the threshold voltages and threshold forces for domain switching can be increased from −4.2 to −5 V and from 365 to 1216 nN, respectively, as the writing-speed increases from 2.2 to 10.6 µm s⁻¹. The writing-speed dependent threshold voltages can be attributed to the nucleations of reoriented ferroelectric domains, in which sufficient time is needed for subsequent domain growth. The writing-speed dependent threshold forces can be attributed to the flexoelectric effect. Furthermore, the electrical-mechanical coupling can be employed to decrease the threshold force, achieving as low as ≈189±41 nN, a value smaller than those of perovskite ferroelectric films. Such findings reveal a critical issue of ferroelectric domain pattern engineering that should be carefully addressed for programmable direct-writing electronics applications.

Abstract

Quasi-one-dimensional ZrS₃ nanoflakes attract intense interest attributed to their superior electrical and optical anisotropy, stemming from the low symmetry in the crystal structure. However, the conventional chemical vapor transport method for synthesizing bulk ZrS₃ is limited by morphology and size controllability. It is highly desirable to propose a facile way to precisely synthesize ZrS₃ nanoflakes. In this work, the chemical vapor deposition method is proposed as a feasible way to synthesize ZrS₃ nanoflakes. The effects of various substrates and temperatures on ZrS₃ synthesis have been investigated. For the as-grown ZrS₃, good crystallinity is confirmed with X-ray diffraction and transmission electron microscopy. The structure and interlayer coupling are investigated with Raman scattering spectroscopy. The strong in-plane anisotropy and interlayer coupling of the ZrS₃ nanoflakes are illustrated with angle-resolved Raman spectroscopy and temperature-dependent Raman characterization, respectively. This study demonstrates a feasible way for the synthesis of transition metal trisulfides, which may shed new light on the research of other two-dimensional anisotropic transition metal materials.

Abstract

Multiferroics are an intriguing family of materials due to the simultaneous presence of two ferroic orderings, namely, ferroelectricity and ferromagnetism. They are scientifically and technologically important and have numerous potential applications, such as four-state logic memories and multiferroic tunneling junctions. However, the growth of epitaxial single-phase multiferroic thin films typically requires single crystalline oxide substrates, which hinders their future integration with Si-based devices. In this study, we report a generalized synthesis method that uses the polydimethylsiloxane (PDMS)-assisted wet-etching method with an Sr₃Al₂O₆ (SAO) sacrificial layer to transfer freestanding single-phase multiferroic Bi₂NiMnO₆ (BNMO) films from conventional SrTiO₃ (STO) substrates onto a Si wafer. The structures and properties of the films have been characterized before and after the transfer. These transferred films possess good multiferroic properties on Si wafers, indicating full compatibility with modern Si technology. This method can be generally applicable to other Bi-based multiferroic materials as well. Lastly, the original STO substrates after the transfer process have been recycled for preparing new batches of freestanding BNMO films, indicating a low-cost and sustainable method for manufacturing large-volume freestanding complex oxide thin films.

The introduction of 2D silicene into bendable membranes allows investigating its optothermal and piezoresistive properties when external strain is applied on a micro and macroscale, thus representing an advance toward a new generation of flexible and fully silicon-based devices.

Abstract

Due to their superior mechanical properties, 2D materials have gained interest as active layers in flexible devices co-integrating electronic, photonic, and straintronic functions altogether. To this end, 2D bendable membranes compatible with the technological process standards and endowed with large-scale uniformity are highly desired. Here, it is reported on the realization of bendable membranes based on silicene layers (the 2D form of silicon) by means of a process in which the layers are fully detached from the native substrate and transferred onto arbitrary flexible substrates. The application of macroscopic mechanical deformations induces a strain-responsive behavior in the Raman spectrum of silicene. It is also shown that the membranes under elastic tension relaxation are prone to form microscale wrinkles displaying a local generation of strain in the silicene layer consistent with that observed under macroscopic mechanical deformation. Optothermal Raman spectroscopy measurements reveal a curvature-dependent heat dispersion in silicene wrinkles. Finally, as compelling evidence of the technological potential of the silicene membranes, it is demonstrated that they can be readily introduced into a lithographic process flow resulting in the definition of flexible device-ready architectures, a piezoresistor, and thus paving the way to a viable advance in a fully silicon-compatible technology framework.

A non-centrosymmetric layer stacking phase of 2D tin sulfide (SnS) is successfully achieved on van der Waals substrates and the shift current in SnS is demonstrated. Following the PFM observation that reveals the existence of ferroelectric domains, an atomic model of the ferroelectric domain boundary is proposed.

Abstract

The shift-current photovoltaics of group-IV monochalcogenides has been predicted to be comparable to those of state-of-the-art Si-based solar cells. However, its exploration has been prevented from the centrosymmetric layer stacking in the thermodynamically stable bulk crystal. Herein, the non-centrosymmetric layer stacking of tin sulfide (SnS) is stabilized in the bottom regions of SnS crystals grown on a van der Waals substrate by physical vapor deposition and the shift current of SnS, by combining the polarization angle dependence and circular photogalvanic effect, is demonstrated. Furthermore, 180° ferroelectric domains in SnS are verified through both piezoresponse force microscopy and shift-current mapping techniques. Based on these results, an atomic model of the ferroelectric domain boundary is proposed. The direct observation of shift current and ferroelectric domains reported herein paves a new path for future studies on shift-current photovoltaics.