Kind Of 2D Materials Research Articles

GeAs and SiAs monolayers: Novel 2D semiconductors with suitable band structures

Two dimensional (2D) materials provide a versatile platform for nanoelectronics, optoelectronics and clean energy conversion. Based on first-principles calculations, we propose a novel kind of 2D materials – GeAs and SiAs monolayers and investigate their atomic structure, thermodynamic stability, and electronic properties. The calculations show that monolayer GeAs and SiAs sheets are energetically and dynamically stable. Their small interlayer cohesion energies (0.191eV/atom for GeAs and 0.178eV/atom for SiAs) suggest easy exfoliation from the bulk solids that exist in nature. As 2D semiconductors, GeAs and SiAs monolayers possess band gap of 2.06eV and 2.50eV from HSE06 calculations, respectively, while their band gap can be further engineered by the number of layers. The relatively small and anisotropic carrier effective masses imply fast electric transport in these 2D semiconductors. In particular, monolayer SiAs is a direct gap semiconductor and a potential photocatalyst for water splitting. These theoretical results shine light on utilization of monolayer or few-layer GeAs and SiAs materials for the next-generation 2D electronics and optoelectronics with high performance and satisfactory stability.

Two-Dimensional Semiconductor Optoelectronics Based on van der Waals Heterostructures.

Two-dimensional (2D) semiconductors such as transition metal dichalcogenides (TMDCs) and black phosphorous have drawn tremendous attention as an emerging optical material due to their unique and remarkable optical properties. In addition, the ability to create the atomically-controlled van der Waals (vdW) heterostructures enables realizing novel optoelectronic devices that are distinct from conventional bulk counterparts. In this short review, we first present the atomic and electronic structures of 2D semiconducting TMDCs and their exceptional optical properties, and further discuss the fabrication and distinctive features of vdW heterostructures assembled from different kinds of 2D materials with various physical properties. We then focus on reviewing the recent progress on the fabrication of 2D semiconductor optoelectronic devices based on vdW heterostructures including photodetectors, solar cells, and light-emitting devices. Finally, we highlight the perspectives and challenges of optoelectronics based on 2D semiconductor heterostructures.

General method for eliminating wave reflection in 2D photonic crystal waveguides by introducing extra scatterers based on interference cancellation of waves

We propose a general method for eliminating the reflection of waves in 2 dimensional photonic crystal waveguides (2D-PCWs), a kind of 2D material, by introducing extra scatterers inside the 2D-PCWs. The intrinsic reflection in 2D-PCWs is compensated by the backward-scattered waves from these scatterers, so that the overall reflection is greatly reduced and the insertion loss is improved accordingly. We first present the basic theory for the compensation method. Then, as a demonstration, we give four examples of extremely-low-reflection and high-transmission 90°bent 2D-PCWs created according to the method proposed. In the four examples, it is demonstrated by plane-wave expansion method and finite-difference time-domain method that the 90°bent 2D-PCWs can have high transmission ratio greater than 90% in a wide range of operating frequency, and the highest transmission ratio can be greater than 99.95% with a return loss higher than 43 dB, better than that in other typical 90°bent 2D-PCWs. With our method, the bent 2D-PCWs can be optimized to obtain high transmission ratio at different operating wavelengths. As a further application of this method, a waveguide-based optical bridge for light crossing is presented, showing an optimum return loss of 46.85 dB, transmission ratio of 99.95%, and isolation rates greater than 41.77 dB. The method proposed provides also a useful way for improving conventional waveguides made of cables, fibers, or metal walls in the optical, infrared, terahertz, and microwave bands.

Developing carbon-nitride nanosheets for mode-locking ytterbium fiber lasers.

Graphitic carbon nitrides (CNs) have appeared as a new type of photocatalyst for water splitting, but their optical properties (e.g., nonlinear absorption), to the best of our knowledge, have been seldom explored. Here, we report the saturable absorption effects of novel 2D carbon-nitride-type nanosheets and use them as saturable absorbers to passively mode-lock Yb-doped fiber lasers. The CN-based saturable absorber is manufactured by solution coating of 2D CN nanosheets on a gold mirror and has a modulation depth and saturation intensity of 12.5% and 7.5 MW/cm2, respectively. Two different output couplers are employed to construct ring laser cavities. With the 10% coupler, the mode-locked fiber laser produces pulses with duration of ∼310 ps, average power of 1.24 mW, and repetition rate of 7.65 MHz. The laser spectrum is centered at 1066 nm with a bandwidth of 2.4 nm. Increasing the coupling ratio to 50% improves the output power to 2.58 mW but at the same time broadens the pulse width to 420 ps. As a new kind of 2D material with strong saturable absorption, CN nanosheets will open a new way for novel photonic and optoelectronic devices.

Passively Q-switched Nd:YAlO₃nanosecond laser using MoS₂as saturable absorber.

We report on the first passively Q-switched Nd:YAlO₃ laser at ~1079.5 nm using MoS₂ as saturable absorber. The MoS₂ saturable absorber is fabricated by transferring the liquid-phase-exfoliated MoS₂ nanosheets onto a BK7 glass substrate. By inserting the glass MoS₂ saturable absorber into a plano-concave Nd:YAlO₃ laser cavity, we obtain a stable Q-switched laser operation with a maximum average output power of 0.26 W corresponding to a pulse repetition rate of 232.5 kHz, a pulse width of 227 ns and a pulse energy of about 1.11 μJ. The results experimentally confirm the promising application of the new kind of 2D material, few-layer MoS₂, in solid state lasers.

Colloidal monolayer titania quantum dots prepared by hydrothermal synthesis in supercritical water

Two-dimensional monolayer titania quantum dots (MTQDs) with ∼0.4nm thickness and ∼2nm lateral size are synthesized by supercritical water (SCW) treatment of titania nanotubes (TNTs). The morphology, chemical characteristics and the structure of MTQDs are studied. The formation mechanism of the MTQDs and the differences between SCW and low-temperature hydrothermal treatment are discussed. During the reaction, the high temperature, high pressure and high H+/OH− concentration of SCW dissolved TNTs into MTQDs, and the intercalation property of the “active” water clusters formed from the broken hydrogen bonding network facilitated the detachment of the MTQDs from the TNTs. The above two reasons lead to the capture of the dissolved tiny particles, which could hardly preserved in low-temperature hydrothermal treatment. The MTQDs may be the minimum constituent unit existing in the reality of the anatase TiO2. As a new member of the monolayer family, this new kind of 2D material may shed new light on the study of the monolayer materials.