Xingxing Zhang

Anisotropic intraband and interband transitions in black phosphorus make it an ideal platform to study anisotropic optical and plasmonic properties in the 2D case. Here, recent development of theoretical and experimental studies on the anisotropic optical excitations and hyperbolic plasmons in black phosphorous are discussed, followed by a perspective on other anisotropic 2D materials for hyperbolic plasmons.

Abstract

In the fast growing 2D materials family, anisotropic 2D materials, with their intrinsic in‐plane anisotropy, exhibit a great potential in optoelectronics. One such typical material is black phosphorus (BP), with a layer‐dependent and highly tunable bandgap. Such intrinsic anisotropy adds a new degree of freedom to the excitation, detection, and control of light. Particularly, hyperbolic plasmons with hyperbolic q‐space dispersion are predicted to exist in BP films, where highly directional propagating polaritons with divergent densities of states are hosted. Combined with a tunable electronic structure, such natural hyperbolic surfaces may enable a series of exotic applications in nanophotonics. Herein, the anisotropic optical properties and plasmons (especially hyperbolic plasmons) of BP are discussed. In addition, other possible 2D material candidates (especially anisotropic layered semimetals) for hyperbolic plasmons are examined. This review may stimulate further research interest in anisotropic 2D materials and fully unleash their potential in flatland photonics.

Exciton polaritons (EPs) in group‐VI transition‐metal dichalcogenides (TMDs) have attracted a lot of research interest in the 2D materials and photonics communities in the past few years. Here, recent studies of EPs in TMDs, highlighting their key properties and functionalities are reviewed, and the potential directions for future research are discussed.

Abstract

Exciton polaritons (EPs) are half‐light, half‐matter quasiparticles formed due to the coupling between photons and excitons in semiconductors. Their uniqueness lies at the strong light–matter interactions and long‐distance transport, thus promising for many novel applications in photonics, information, and quantum technologies. Recently, EPs in group‐VI transition‐metal dichalcogenides (TMDs) have attracted a lot of research interest due to their room‐temperature stability, long‐distance propagation, and controllability through electric gating, valley‐selective optical pumping, and precise thickness control. In this progress report, recent studies of EPs in TMDs are reviewed, highlighting their key properties and functionalities, and then discussing the potential directions for future research.

A vertical surface‐emitting wavelength‐tunable mid‐infrared lasing is achieved from black phosphorus nanosheets embedded in distributed Bragg reflector microcavities.

Abstract

Van der Waals layered semiconductor materials own unique physical properties and have attracted intense interest in developing high‐performance electronic and photonic devices. Among them, black phosphorus (BP) is distinct for its layer number‐tuned direct band gap which spans from near‐ to mid‐infrared (MIR) waveband. In addition, the puckered honey comb crystal lattice endows the material with highly linear‐polarized emission and marked anisotropy in carrier transportation. These unique material properties render BP as an intriguing and promising building block for constructing mid‐infrared‐ranged coherent light sources. Here, a room temperature surface‐emitting MIR laser based on single crystalline BP nanosheets coupled with a distributed Bragg reflector cavity is reported. MIR stimulated emission at 3611 nm is achieved with a near‐unity linear polarization, which exhibits robust thermal stability up to 360 K. Most importantly, the lasing wavelength can be tuned from 3425 to 4068 nm by varying the cavity length via thickness control of BP layer. The demonstrated highly polarized lasing output and wavelength‐tunable capacity of the proposed device scheme in MIR spectral range opens up promising opportunities for a broad array of applications in polarization‐resolved IR imaging, range‐finding, and free space quantum communications.

In article number https://doi.org/10.1002/adma.2019070631907063, Himani Arora, Enrique Canovas, Artur Erbe, and co‐workers demonstrate broadband photodetectors based on a novel π–d conjugated Fe₃(THT)₂(NH)₃ 2D metal–organic framework (MOF), operative in the UV‐to‐NIR range. Due to the small IR bandgap of the MOF, the photodetectors are best operated at cryogenic temperatures by suppressing the thermally activated charge‐carrier population. Thus, a proof‐of‐concept MOF photodetector is reported, revealing MOFs as promising candidates for optoelectronics.

The magnetic proximity effect (MPE) in 2D graphene/CrBr₃ van der Waals heterostructures is probed by the Zeeman spin Hall effect via non‐local transport measurements; estimation of the Zeeman splitting energy demonstrates a significant magnetic proximity exchange field. The newly observed anomalous longitudinal resistances at the Dirac point with magnetic field may be attributed to the new phases of the ground state induced by MPE.

Abstract

2D van der Waals heterostructures serve as a promising platform to exploit various physical phenomena in a diverse range of novel spintronic device applications. Efficient spin injection is the prerequisite for these devices. The recent discovery of magnetic 2D materials leads to the possibility of fully 2D van der Waals spintronics devices by implementing spin injection through the magnetic proximity effect (MPE). Here, the investigation of MPE in 2D graphene/CrBr₃ van der Waals heterostructures is reported, which is probed by the Zeeman spin Hall effect through non‐local measurements. Quantitative estimation of the Zeeman splitting field demonstrates a significant MPE field even in a low magnetic field. Furthermore, the observed anomalous longitudinal resistance changes at the Dirac point R _XX,D with increasing magnetic field near ν = 0 may be attributed to the MPE‐induced new ground state phases. This MPE revealed in the graphene/CrBr₃ van der Waals heterostructures therefore provides a solid physics basis and key functionality for next‐generation 2D spin logic and memory devices.

A combined gate‐dependence study of the Josephson effect, carrier densities, and self‐consistent band structure calculations in thin Bi₂Te₃ topological devices reveals that the critical current in topological Josephson devices maps the band structure properties of the films when a gate voltage is applied. A coherent picture that incorporates band bending in the discussion of proximity effects for topological devices is provided.

Abstract

Thin layers of topological insulator materials are quasi‐2D systems featuring a complex interplay between quantum confinement and topological band structure. To understand the role of the spatial distribution of carriers in electrical transport, the Josephson effect, magnetotransport, and weak anti‐localization are studied in bottom‐gated thin Bi₂Te₃ topological insulator films. The experimental carrier densities are compared to a model based on the solutions of the self‐consistent Schrödinger–Poisson equations and they are in excellent agreement. The modeling allows for a quantitative interpretation of the weak antilocalization correction to the conduction and of the critical current of Josephson junctions with weak links made from such films without any ad hoc assumptions.

Nonvolatile molybdenum telluride (MoTe₂) p–n junctions with high rectification factor of 5 × 10⁵ are demonstrated by ferroelectric domains. Coupling to opposite polarized ferroelectric copolymers, electrons or holes accumulate in the corresponding region of the ambipolar MoTe₂ channel, defining a p‐n homojunction at the ferro‐electric domain wall. The p‐n junctions can be used as photodetectors and photovoltaic devices.

Abstract

Doped p–n junctions are fundamental electrical components in modern electronics and optoelectronics. Due to the development of device miniaturization, the emergence of two‐dimensional (2D) materials may initiate the next technological leap toward the post‐Moore era owing to their unique structures and physical properties. The purpose of fabricating 2D p–n junctions has fueled many carrier‐type modulation methods, such as electrostatic doping, surface modification, and element intercalation. Here, by using the nonvolatile ferroelectric field polarized in the opposite direction, efficient carrier modulation in ambipolar molybdenum telluride (MoTe₂) to form a p–n homojunction at the domain wall is demonstrated. The nonvolatile MoTe₂ p–n junction can be converted to n–p, n–n, and p–p configurations by external gate voltage pulses. Both rectifier diodes exhibited excellent rectifying characteristics with a current on/off ratio of 5 × 10⁵. As a photodetector/photovoltaic, the device presents responsivity of 5 A W⁻¹, external quantum efficiency of 40%, specific detectivity of 3 × 10¹² Jones, fast response time of 30 µs, and power conversion efficiency of 2.5% without any bias or gate voltages. The MoTe₂ p–n junction presents an obvious short‐wavelength infrared photoresponse at room temperature, complementing the current infrared photodetectors with the inadequacies of complementary metal‐oxide‐semiconductor incompatibility and cryogenic operation temperature.

In article number https://doi.org/10.1002/adma.2019064991906499, Yuyan Wang, Junying Zhang, and co‐workers achieve novel and facile electron doping of 2D WSe₂ by cetyltrimethyl ammonium bromide to form a high‐quality lateral p–n homojunction with superior optoelectronic properties. The high switching light ratio, superior photoresponsivity, and specific detectivity of the device demonstrate its promising application for high‐sensitivity photodetectors and low‐power photoelectronic devices.

The magnetic proximity effect (MPE) in 2D graphene/CrBr₃ van der Waals heterostructures is probed by the Zeeman spin Hall effect via non‐local transport measurements; estimation of the Zeeman splitting energy demonstrates a significant magnetic proximity exchange field. The newly observed anomalous longitudinal resistances at the Dirac point with magnetic field may be attributed to the new phases of the ground state induced by MPE.

Abstract

2D van der Waals heterostructures serve as a promising platform to exploit various physical phenomena in a diverse range of novel spintronic device applications. Efficient spin injection is the prerequisite for these devices. The recent discovery of magnetic 2D materials leads to the possibility of fully 2D van der Waals spintronics devices by implementing spin injection through the magnetic proximity effect (MPE). Here, the investigation of MPE in 2D graphene/CrBr₃ van der Waals heterostructures is reported, which is probed by the Zeeman spin Hall effect through non‐local measurements. Quantitative estimation of the Zeeman splitting field demonstrates a significant MPE field even in a low magnetic field. Furthermore, the observed anomalous longitudinal resistance changes at the Dirac point R _XX,D with increasing magnetic field near ν = 0 may be attributed to the MPE‐induced new ground state phases. This MPE revealed in the graphene/CrBr₃ van der Waals heterostructures therefore provides a solid physics basis and key functionality for next‐generation 2D spin logic and memory devices.

Nature Chemistry, Published online: 24 February 2020; doi:10.1038/s41557-020-0418-3

The deposition of noble metals onto two-dimensional transition metal dichalcogenides is crucial for practical applications, including in catalysis and sensing, yet this process has remained difficult to control. Now, gold and silver have been shown to grow on colloidal transition metal dichalcogenide nanosheets into either atomically thin layers or nanoparticles whose sizes and morphologies depend on the relative strengths of the interfacial noble metal–chalcogen bonds.

Nonvolatile molybdenum telluride (MoTe₂) p–n junctions with high rectification factor of 5 × 10⁵ are demonstrated by ferroelectric domains. Coupling to opposite polarized ferroelectric copolymers, electrons or holes accumulate in the corresponding region of the ambipolar MoTe₂ channel, defining a p‐n homojunction at the ferro‐electric domain wall. The p‐n junctions can be used as photodetectors and photovoltaic devices.

Abstract

Doped p–n junctions are fundamental electrical components in modern electronics and optoelectronics. Due to the development of device miniaturization, the emergence of two‐dimensional (2D) materials may initiate the next technological leap toward the post‐Moore era owing to their unique structures and physical properties. The purpose of fabricating 2D p–n junctions has fueled many carrier‐type modulation methods, such as electrostatic doping, surface modification, and element intercalation. Here, by using the nonvolatile ferroelectric field polarized in the opposite direction, efficient carrier modulation in ambipolar molybdenum telluride (MoTe₂) to form a p–n homojunction at the domain wall is demonstrated. The nonvolatile MoTe₂ p–n junction can be converted to n–p, n–n, and p–p configurations by external gate voltage pulses. Both rectifier diodes exhibited excellent rectifying characteristics with a current on/off ratio of 5 × 10⁵. As a photodetector/photovoltaic, the device presents responsivity of 5 A W⁻¹, external quantum efficiency of 40%, specific detectivity of 3 × 10¹² Jones, fast response time of 30 µs, and power conversion efficiency of 2.5% without any bias or gate voltages. The MoTe₂ p–n junction presents an obvious short‐wavelength infrared photoresponse at room temperature, complementing the current infrared photodetectors with the inadequacies of complementary metal‐oxide‐semiconductor incompatibility and cryogenic operation temperature.