Jing Zhang

Nature Nanotechnology, Published online: 10 May 2021; doi:10.1038/s41565-021-00900-9

Semiconductor–superconductor hybrids are used for realizing complex quantum phenomena but are limited in the accessible magnetic field and temperature range. Now, hybrid devices made from InAs nanowires and epitaxially matched, single-crystal, atomically flat Pb films present superior characteristics, doubling the available parameter space.

Hafnium pentatelluride (Hf Te 5 ) is a layered two-dimensional material with various exotic properties. It is thought to be a topological insulator. Whereas bulk Hf Te 5 has a small band gap, single layers are predicted to be a quantum spin hall insulator with a large band gap. Here we measured band gap energies for samples with varying thicknesses and found a clear increase of gap energies for the thinner samples. With decreasing thickness an increase of the measured band gap energies from 40 to 304 meV is observed.

We report about the investigation of twisted MoSe 2 homo- and MoSe 2 –WSe 2 heterobilayers by means of low-frequency Raman spectroscopy (LFRS) and low-temperature micro photoluminescence ( µ PL). In room-temperature LFRS experiments on both, twisted MoSe 2 homobilayers and twisted MoSe 2 –WSe 2 heterobilayers, we observe moiré phonons, i.e. folded acoustic phonon modes due to the moiré superlattice. In the heterobilayers, we can identify moiré phonons of both materials, MoSe 2 and WSe 2 . While the twist angles for the homobilayers are relatively precisely known from the applied tear-and-stack preparation method, the twist angles of the heterobilayers have to be determined via second-harmonic-generation microscopy on monolayer regions of the samples, which has significant uncertainties. We show that by the moiré phonons of the heterobilayers, the relative twist angles can be determined on a local s...

Multiple magnetic phases emerge in single-crystal SnS₂ layered semiconductors. Two ferromagnetic (FM) transitions with Curie temperatures dependent on Mn-doping concentration can be distinguished based on magnetic measurements. The positive-to-negative crossover and anisotropy in magnetoresistance further confirm the FM semiconducting behavior. Mn-SnS₂ is expected to enlarge the scope of layered FM semiconductors towards practical applications in the field of spintronics.

Abstract

2D van der Waals magnetic semiconductors have emerged along with the possibilities of achieving an efficient gate tunability and a proximity effect with a high magnetic anisotropy compared with 3D counterparts. Little explored are multiple magnetic phases with a single crystallographic phase. Herein, the multiple magnetic phases in a Mn-doped SnS₂ single crystal with different doping concentrations using a one-step self-flux method are reported. Two ferromagnetic phases with a canted spin direction exist regardless of the Mn-doping concentration at up to 5 at%. Antiferromagnetism coexists with the ferromagnetic order and strengthens at high Mn-doping concentrations. A magnetoresistance measurement conducted on a 2 at% Mn-SnS₂ flake exhibits a positive-to-negative crossover with a value of as high as 50% and clear anisotropy, confirming the presence of ferromagnetic order in the material. By revealing multiple magnetic phases in Mn-doped SnS₂, the study broadens the scope of state-of-the-art research on layered magnetic semiconductors.

Monolayer MoSe₂-WSe₂ lateral heterostructures with atomically precise 1D boundaries are synthesized using a one-pot chemical vapor deposition process. Their functional properties are demonstrated in various electronic, optoelectronic, photovoltaic, and light-emitting devices.

Abstract

Lateral heterostructures of dissimilar monolayer transition metal dichalcogenides provide great opportunities to build 1D in-plane p–n junctions for sub-nanometer thin low-power electronic, optoelectronic, optical, and sensing devices. Electronic and optoelectronic applications of such p–n junction devices fabricated using a scalable one-pot chemical vapor deposition process yielding MoSe₂-WSe₂ lateral heterostructures are reported here. The growth of the monolayer lateral heterostructures is achieved by in situ controlling the partial pressures of the oxide precursors by a two-step heating protocol. The grown lateral heterostructures are characterized structurally and optically using optical microscopy, Raman spectroscopy/microscopy, and photoluminescence spectroscopy/microscopy. High-resolution transmission electron microscopy further confirms the high-quality 1D boundary between MoSe₂ and WSe₂ in the lateral heterostructure. p–n junction devices are fabricated from these lateral heterostructures and their applicability as rectifiers, solar cells, self-powered photovoltaic photodetectors, ambipolar transistors, and electroluminescent light emitters are demonstrated.

Nature Nanotechnology, Published online: 03 May 2021; doi:10.1038/s41565-021-00913-4

Twisted bilayer graphene enables the realization of Josephson junctions and single electron transistors in a single, crystalline material using electric field gating only, thereby avoiding interfaces between dissimilar materials.

Nature Nanotechnology, Published online: 03 May 2021; doi:10.1038/s41565-021-00896-2

In situ electrostatic control of two-dimensional superconductivity is commonly limited due to large charge carrier densities. Now, by means of local gates, electrostatic gating can define a Josephson junction in a magic-angle twisted bilayer graphene device, a single-crystal material.

Nature Nanotechnology, Published online: 03 May 2021; doi:10.1038/s41565-021-00904-5

Atomically sharp interfaces in van der Waals heterostructures enable the realization of ultrafast non-volatile memory devices.

Nature Nanotechnology, Published online: 06 May 2021; doi:10.1038/s41565-021-00887-3

Graphene promises long-distance transfer of spin information with concomitant high charge carrier mobility. Proximity coupling of bilayer graphene with the 2D interlayer antiferromagnetic CrSBr now enables active generation of spin currents in graphene both electrically and thermally.

Hybridisation of electronic bands of two-dimensional materials, assembled into twistronic heterostructures, enables one to tune their optoelectronic properties by selecting conditions for resonant interlayer hybridisation. Resonant interlayer hybridisation qualitatively modifies the excitons in such heterostructures, transforming these optically active modes into superposition states of interlayer and intralayer excitons. For MoSe 2 /WSe 2 heterostructures, strong hybridization of both single particle and excitonic states can occur via single particle tunnelling. Here we use resonance Raman scattering to provide direct evidence for the hybridisation of excitons in twistronic MoSe 2 /WSe 2 structures, by observing scattering of specific excitons by phonons in both WSe 2 and MoSe 2 . We also demonstrate that resonance Raman scattering spectroscopy opens up a wide range of possibilities for quantifying the layer composition o...

Semiconducting transition metal dichalcogenides have significant nonlinear optical effects. In this work we have used second-harmonic generation and the four-wave mixing spectroscopy in resonance with the excitons in MoS 2 , MoSe 2 , and WS 2 monolayers to characterize the nonlinear optical properties of these materials. We show that trions and excitons are responsible for enhancing the nonlinear optical response and determine the exciton and trion energies by comparing with the photoluminescence spectra. Moreover, we extract the second- and third-order optical sheet susceptibility ( χ (2) and χ (3) ) across exciton energies and compare with values found in the literature. We also demonstrate the ability to generate different nonlinear effects in a wide spectral range in the visible region for monolayer MoS 2 , opening the possibility of using two-dimensional materials for nonlinear optoelectronic and photoni...

Van der Waals (vdW) heterostructure constructed from atomically thin layered materials provides quantum material platforms with emergent physical phenomena and novel device applications. While stacking atomically thin vdW layers in combination with automate machine vision identification and semi-automated stacking have been demonstrated, a combination of machine learning based automatic detection/identification assembly capability is necessary to further advance vdW heterostructure fabrication. Here, we developed a new automatic optical detection technique with a deep neural network (DNN) incorporated into a motorized microscope that automatically scans entire silicon wafers to detect and identify two-dimensional (2D) materials. We demonstrated the automated combination on an optical microscope (OM) with a DNN algorithm that enables identification and classification of graphene with different sizes, shapes and thicknesses. For this purpose, we trained a representative DNN for obj...

This work reviews the recent advances in 2D rare earth materials, which are the rising star in 2D applications. The crystal structure, synthesis methods, properties, and applications of 2D rare earth materials are summarized. Finally, the problems, future challenges, and new opportunities of this area are also discussed.

Abstract

2D rare earth (RE) materials have received considerable attention in recent years due to the fascinating luminescence, magnetism, and electric properties originated from RE associated with sharp and various emission peaks, intrinsic 2D ferromagnetism, and incommensurate charge density wave. These materials might open up a new prospect in next‐generation lighting, magnetic devices, and phototransistors. Herein, a comprehensive review of 2D RE materials is presented, focusing on their recent progresses. First, the crystal structures of 2D RE materials are discussed. Then, typical synthesis methods such as mechanical exfoliation, molecular beam epitaxy, pulsed laser deposition, and chemical vapor deposition are introduced. Furthermore, various properties in luminescence, magnetism, and electronics are summarized. The recently reported RE‐based 2D novel photodetectors are outlined as three constructions: MoS₂/RE, graphene/RE, and perovskite/RE, which show promising applications for both narrow and broad band detection arised from the special absorption windows of different RE elements. Finally, the conclusions and outlook of this area are proposed, such as exploring novel 2D RE compounds, improving stability, and broadening applications.

Here, a 2D material-enhanced flexible and self-healable photodetector is demonstrated with a decent and stable photoresponse to a broad light spectrum under large and severe mechanical deformation and damage, based on which a large-area photodetection array is further fabricated for pattern recognition.

Abstract

Flexible photodetectors are fundamental elements to develop flexible/wearable systems, which can be widely used for in situ health and environmental monitoring, human–machine interacting, flexible displaying, etc. However, the degraded performance or even malfunction under severe mechanical deformation and/or damage remains a key challenge for current flexible photodetectors. In this article, a flexible photodetector is developed with strong self-healing capability and stable performance under large deformation. This photodetector is made of the 2D material self-healing film by mixing 2D materials homogenously with the self-healing polymer of imidazolium-based norbornene polymerized with ionic liquids and counterions. The 2D material self-healing films show enhanced light absorption, and thus, decent photoresponse as compared to the pure self-healing film. The achieved photoresponse remains stable and even increases under small tensile strain within 150%, while decreases slightly under large tensile strain up to 1000%. Moreover, the photodetector not only can be fully recovered from repeated mechanical cuttings, but also presents excellent long-term stability in ambient condition for 500 days without showing any obvious degraded performance. Furthermore, a large-area 2D material self-healing photodetection array is designed with adjustable pixel size, which successfully recognizes the patterns of “T”, “J”, and “U”.

Flexible MXene films with 3D porous structures are prepared by synchronous reduction and self-assembly of MXene sheets on the Zn foil surface. The MXene films demonstrate high electrical conductivity, large specific surface area, and excellent mechanical properties. Both of the sandwich and microsized supercapacitors based on MXene films display excellent electrochemical performance even under different bending states.

Abstract

The self-assembly of large-area MXene films is the main step to realize their applications in various energy storage devices. However, the scalable self-assembly of flexible thin MXene films with high conductivity as well as excellent mechanical and electrochemical properties is still a challenge. Herein, a synchronous reduction and self-assembly strategy to fabricate flexible MXene films is developed, where MXene films are synchronously reduced and self-assembled on the Zn foil surface. Furthermore, the self-assembly of MXene films can be scaled up by controlling the area of Zn substrates. By adjusting the patterns of Zn substrates, the interdigital MXene patterns can also be obtained via a selectively reducing/assembling process. The resultant MXene films demonstrate high electrical conductivity, large specific surface area, and excellent mechanical properties. Thus they can serve as the electrodes of flexible supercapacitor devices directly. As a proof of concept, flexible sandwich and microsized supercapacitors are designed based on the above MXene film electrodes. Both sandwich and microsized supercapacitors display stable electrochemical performance under various bending states. This study provides a route to achieve large-area MXene-based films or microsized structures for applications in the field of energy storage.

Both light and X-ray mediated persistent luminescence are enhanced in uniform nanocrystals via engineering their electron trap. Through non-equivalency substitution of zinc ions with lithium ions in Zn₂GeO₄ crystals, more electron traps are introduced into the PLNPs, leading to enhanced persistent luminescence. Such enhanced dual-mode persistent luminescence allows for novel light/X-ray orthogonally encrypted spatio-temporal dual-dimensional optical anti-counterfeiting strategies.

Abstract

Persistent luminescence nanoparticles (PLNPs) are an emerging type of optical nanomaterial that possess long-lasting afterglow after the excitation has stopped. Recently, bottom-up synthesis of PLNPs has offered uniform and small nanocrystals that are desirable for various bioapplications. However, the lack of a simple method to enhance the afterglow of these PLNPs is one of the key obstacles hindering their further development and applications. Herein, a simple strategy is demonstrated that can amplify both light and X-ray charged persistent luminescence in monodispersed Zn₂GeO₄:Mn PLNPs via the non-equivalency substitution of zinc ions with lithium ions in the lattice matrix and concomitant to the electron traps tailoring. It is significant that, in addition to increasing the intensity of the afterglow, this nanoscale atomic level substitution can preserve the desirable uniform size and morphology of the PLNPs. Furthermore, since the two excitations (light and X-ray) are independent of each other, a light/X-ray orthogonally encrypted spatio-temporal dual-dimensional afterglow anti-counterfeiting is demonstrated via these nanoparticles. It is believed that this simple method offers a foundation for new opportunities to unleash the optical performance in PLNPs. This will also pave the way to the development of such PLNPs for numerous photonic and bioapplications, which are limited in existing methods.

Here, the polarization-resolved broadband MoS₂/black phosphorus/MoS₂ optoelectronic memory is prepared by exploiting oxidation induced defects in black phosphorus layer. As a result of interfacial trap-controlled charge injection, the device exhibits an ultrahigh responsivity of 1.3 × 10⁷ A W⁻¹, an ultralong retention time exceeding 6 × 10⁴ s, together with an excellent multi-bit storage capacity.

Abstract

The rapidly emerging requirement for device miniaturization and structural flexibility make 2D semiconductors and their van der Waals (vdWs) heterostructures extremely attractive for nonvolatile optoelectronic memory (NOM) applications. Although several concepts for 2D NOM have been demonstrated, multi-heterojunction devices capable of further improving storage performance have received little attention. This work reports a concept for MoS₂/black phosphorus (BP)/MoS₂ multi-heterojunction NOM with artificial trap sites through the BP oxidation, in which the trapped holes at BP/PO _x interface intrigue a persistent photoconductivity that hardly recovers within the experimental time scales (exceeding 10⁴ s). As a result of the interfacial trap-controlled charge injection, the device exhibits excellent photoresponsive memory characteristics, including a record high detectivity of ≈1.2 × 10¹⁶ Jones, a large light-to-dark switching ratio of ≈1.5 × 10⁷ _, an ultralow off-state current of ≈1.2 pA, and an outstanding multi-bit storage capacity (11 storage states, 546 nC state^–1). In addition, the middle BP layer in the multi-heterojunction enables broadband spectrum distinction (375–1064 nm), together with a high polarization ratio of 8.4. The obtained results represent the significant step toward the high-density integration of optoelectronic memories with 2D vdWs heterostructures.

Implementing compact modeling through analytical choice maps and extracting approximate their interlayer resistances in h-BN dual-gated ReS₂. Optimization and interpretation of performances in ReS₂ field-effect transistors are conducted in parallel with secondary g _m peaks, threshold voltages, subthreshold swing, mobility through DC analysis, time-dependent current, low-frequency noise, and technology computer aided design simulation analysis.

Abstract

In this study, high-performance few-layered ReS₂ field-effect transistors (FETs), fabricated with hexagonal boron nitride (h-BN) as top/bottom dual gate dielectrics, are presented. The performance of h-BN dual gated ReS₂ FET having a trade-off of performance parameters is optimized using a compact model from analytical choice maps, which consists of three regions with different electrical characteristics. The bottom h-BN dielectric has almost no defects and provides a physical distance between the traps in the SiO₂ and the carriers in the ReS₂ channel. Using a compact analyzing model and structural advantages, an excellent and optimized performance is introduced consisting of h-BN dual-gated ReS₂ with a high mobility of 46.1 cm² V⁻¹ s⁻¹, a high current on/off ratio of ≈10⁶, a subthreshold swing of 2.7 V dec⁻¹, and a low effective interface trap density (N _t,eff) of 7.85 × 10¹⁰ cm⁻² eV⁻¹ at a small operating voltage (<3 V). These phenomena are demonstrated through not only a fundamental current–voltage analysis, but also technology computer aided design simulations, time-dependent current, and low-frequency noise analysis. In addition, a simple method is introduced to extract the interlayer resistance of ReS₂ channel through Y-function method as a function of constant top gate bias.

In article number 2009554, Man Luo, Peng Wang, Zhiming Huang, and co‐workers demonstrate a room‐temperature 2D Bi₂O₂Se photodetector with ultrafast (476 ns) and ultralow noise (0.2 pW Hz^−1/2) for broadband detection. In the infrared regions, the nonequilibrium carriers result from photo‐induced electron‐hole pairs in Bi₂O₂Se. While in the THz region, the nonequilibrium electrons are injected from the metal electrodes to Bi₂O₂Se by the electromagnetic induced well under the THz wave.

Nature Nanotechnology, Published online: 19 April 2021; doi:10.1038/s41565-021-00885-5

Antiferromagnets are interesting materials for fast spintronics applications, but control of the antiferromagnetic order has been limited to bulk materials so far. Now, uniaxial strain is shown to align the Néel vector in MnPSe3 down to the monolayer limit.

Controllable hole doping of WSe₂ by direct laser irradiation is studied systematically. Scanning transmission electron microscopy characterizations confirm the doping mechanism to be the oxidization-induced charge transfer. Different PN junction profiles can be achieved in samples with different thicknesses and under different amounts of laser irradiation. A laser-patterned NOR gate circuit is demonstrated, showing potential for large-scale circuit fabrications.

Abstract

Carrier doping is the basis of the modern semiconductor industry. Great efforts are put into the control of carrier doping for 2D semiconductors, especially the layered transition metal dichalcogenides. Here, the direct laser patterning of WSe₂ devices via light-induced hole doping is systematically studied. By changing the laser power, scan speed, and the number of irradiation times, different levels of hole doping can be achieved in the pristine electron-transport-dominated WSe₂, without obvious sample thinning. Scanning transmission electron microscopy characterization reveals that the oxidation of the laser-radiated WSe₂ is the origin of the carrier doping. Photocurrent mapping shows that after the same amount of laser irradiation, with increasing thickness, the laser patterned PN junction changes from the pure lateral to the vertical-lateral hybrid structure, accompanied by the decrease in the open circuit voltage. The vertical-lateral hybrid PN junction can be tuned to a pure lateral one by further irradiation, showing possibilities to construct complex junction profiles. Moreover, a NOR gate circuit is demonstrated by direct patterning of p-doped channels using laser irradiation without introducing passive layers and metal electrodes with different work functions. This method simplifies device fabrication procedures and shows a promising future in large scale logic circuit applications.

A new method of inkjet printing-assisted melt processing is proposed for patterned growth of liquid crystalline thin films for high-performance organic field-effect transistor (OFET) arrays and integrated circuits. The OFET arrays exhibit uniform electrical properties with high average mobility of 6.31 cm² V⁻¹ s⁻¹. Further, inverter circuits with high gain and large static noise margins of 81.3% are achieved.

Abstract

Liquid crystalline (LC) organic semiconductors having long-range-ordered LC phases hold great application potential in organic field-effect transistors (OFETs). However, to meet real device application requirements, it is a prerequisite to precisely pattern the LC film at desired positions. Here, a facile method that combines the technique of inkjet printing and melt processing to fabricate patterned LC film for achieving high-performance organic integrated circuits is demonstrated. Inkjet printing controls the deposition locations of the LC materials, while the melt processing implements phase transition of the LC materials to form high-quality LC films with large grain sizes. This approach enables to achieve patterned growth of high-quality 2,7-dioctyl[1]-benzothieno[3,2-b][1]benzothiophene (C₈-BTBT) LC films. The patterned C₈-BTBT LC film-based 7 × 7 OFET array has 100% die yield and shows high average mobility of 6.31 cm² V⁻¹ s⁻¹, along with maximum mobility up to 9.33 cm² V⁻¹ s⁻¹. As a result, the inverters based on the patterned LC films reach a high gain up to 23.75 as well as an ultrahigh noise margin over 81.3%. Given the good generality of the patterning process and the high quality of the resulting films, the proposed method paves the way for high-performance organic integrated devices.