Mianzeng Zhong

Synthesis and Transport Properties of Large-Scale Alloy Co0.16Mo0.84S2 Bilayer Nanosheets

Synthesis of large-scale highly crystalline two-dimensional alloys is significant for revealing properties. Here, we have investigated the vapor growth process of high-quality bilayer CoxMo1-xS2 (x = 0.16) hexagonal nanosheets systematically. As the initial loading of the sulfur increases, the morphology of the CoxMo1-xS2 (0 < x ≤ 1) nanosheets becomes hexagons from David stars step by step at 680 °C. We find that Co atoms mainly distribute at the edge of nanosheets. When the temperature increases from 680 to 750 °C, high-quality cubic pyrite-type crystal structure CoS2 grows on the surface of CoxMo1-xS2 nanosheet gradually and forms hexagonal film induced by the nanosheet. Electrical transport measurements reveal that the CoxMo1-xS2 nanosheets and CoS2 films exhibit n-type semiconducting transport behavior and half-metallic behavior, respectively. Theoretical calculations of their band structures agree well with the experimental results.

Flexible photodetectors based on phase dependent PbI2 single crystals

As a precursor of perovskites, lead iodide (PbI2) is a typical layered material with a direct bandgap. Perovskites are widely utilized in highly efficient photovoltaics, but the low-dimensional PbI2 nanostructures and their (opto)electronic properties are rarely reported. Herein, single-crystalline PbI2 nanosheets (phase I) and nanowires (phase II) are controllably synthesized via a facile physical vapor deposition method. Their different crystal morphology and crystallographic symmetry show obvious phase dependence. The corresponding photodetectors on both SiO2/Si and flexible polyethylene terephthalate (PET) substrates are investigated systematically. Compared with PbI2 nanowire based photodetectors, PbI2 nanosheet based photodetectors exhibit a relatively high sensitivity (with a high photoresponsivity of 147.6 A W−1 and fast response time) to the 450 nm laser. Both the PbI2 nanosheet and nanowire devices with flexible PET substrates exhibit comparable performance to their photodetectors fabricated on SiO2/Si, and also show excellent mechanical stability and durability. At the same time, the photoelectric properties vary greatly with different bending angles for such flexible PbI2 photodetectors. By modeling the band structures under different compressive strains, the theoretical simulations fit very well with experimental results. These findings provide a scientific basis for exploiting high-performance flexible photodetectors based on low-dimensional PbI2 single crystals.

A two-dimensional Fe-doped SnS 2 magnetic semiconductor

Magnetic two-dimensional materials have attracted considerable attention for their significant potential application in spintronics. In this study, we present a high-quality Fe-doped SnS2 monolayer exfoliated using a micromechanical cleavage method. Fe atoms were doped at the Sn atom sites, and the Fe contents are ∼2.1%, 1.5%, and 1.1%. The field-effect transistors based on the Fe0.021Sn0.979S2 monolayer show n-type behavior and exhibit high optoelectronic performance. Magnetic measurements show that pure SnS2 is diamagnetic, whereas Fe0.021Sn0.979S2 exhibits ferromagnetic behavior with a perpendicular anisotropy at 2 K and a Curie temperature of ~31 K. Density functional theory calculations show that long-range ferromagnetic ordering in the Fe-doped SnS2 monolayer is energetically stable, and the estimated Curie temperature agrees well with the results of our experiment. The results suggest that Fe-doped SnS2 has significant potential in future nanoelectronic, magnetic, and optoelectronic applications.2D materials can be doped with magnetic atoms in order to boost their potential applications in spintronics. Here, the authors fabricate Fe-doped SnS2 monolayers and show that Fe0.021Sn0.979S2 exhibits ferromagnetic behaviour with perpendicular anisotropy at 2 K, and a Curie temperature of 31 K.

Electrical and magnetic properties of FeS2 and CuFeS2 nanoplates

The properties of nanomaterials are always connected with their crystal morphologies of the crystals. Here, we report the synthesis of square-like high crystalline iron pyrite (FeS2) and chalcopyrite (CuFeS2) nanoplates with an average dimension of 70 nm by a developed hydrothermal process. TEM, XRD and Raman spectra characterization indicate that the nanoplates are high crystalline and pure phase. The optical band gaps of the FeS2 and CuFeS2 nanoplates are 0.97 and 0.52 eV, respectively. The electrical measurements indicate that the nanoplates have good electrical conductivity. With the dangling bonds at interface, the as-made nanoplates exhibit abnormal strong ferromagnetic behavior both at room temperature and low temperature (5 K). These nanoplates with unique morphology, high quality and good performance show huge potential in inexpensive nanoelectronics, such as solar cells and magnetic area.

Highly polarization sensitive photodetectors based on quasi-1D titanium trisulfide (TiS3)

Photodetectors with high polarization sensitivity are in great demand in advanced optical communication. Here, we demonstrate that photodetectors based on titanium trisulfide (TiS3) are extremely sensitive to polarized light (from visible to the infrared), due to its reduced in-plane structural symmetry. By density functional theory calculation, TiS3 has a direct bandgap of 1.13 eV. The highest photoresponsivity reaches 2500 A W-1. What is more, in-plane optical selection caused by strong anisotropy leads to the photoresponsivity ratio for different directions of polarization that can reach 4:1. The angle-dependent photocurrents of TiS3 clearly display strong linear dichroism. Moreover, the Raman peak at 370 cm-1 is also very sensitive to the polarization direction. The theoretical optical absorption of TiS3 is calculated by using the HSE06 hybrid functional method, in qualitative agreement with the observed experimental photoresponsivity.

Black Arsenic: A Layered Semiconductor with Extreme In‐Plane Anisotropy

2D layered materials have emerged in recent years as a new platform to host novel electronic, optical, or excitonic physics and develop unprecedented nanoelectronic and energy applications. By definition, these materials are strongly anisotropic between the basal plane and cross the plane. The structural and property anisotropies inside their basal plane, however, are much less investigated. Black phosphorus, for example, is a 2D material that has such in-plane anisotropy. Here, a rare chemical form of arsenic, called black-arsenic (b-As), is reported as a cousin of black phosphorus, as an extremely anisotropic layered semiconductor. Systematic characterization of the structural, electronic, thermal, and electrical properties of b-As single crystals is performed, with particular focus on its anisotropies along two in-plane principle axes, armchair (AC) and zigzag (ZZ). The analysis shows that b-As exhibits higher or comparable electronic, thermal, and electric transport anisotropies between the AC and ZZ directions than any other known 2D crystals. Such extreme in-plane anisotropies can potentially implement novel ideas for scientific research and device applications.

Two-dimensional plumbum-doped tin diselenide monolayer transistor with high on/off ratio

Doping can effectively regulate the electrical and optical properties of two-dimensional semiconductors. Here, we present high-quality Pb-doped SnSe2 monolayer exfoliated using a micromechanical cleavage method. X-ray photoelectron spectroscopy measurement demonstrates that Pb content of the doped sample is ∼3.6% and doping induces the downward shift of the Fermi level with respect to the pure SnSe2. Transmission electron microscopy characterization exhibits that Pb0.036Sn0.964Se2 nanosheets have a high-quality hexagonal symmetry structure and Pb element is uniformly distributed in the nanosheets. The current of the SnSe2 field effect transistors (FETs) was found to be very difficult to turn off due to the high electron density. The FETs based on the Pb0.036Sn0.964Se2 monolayer show n-type behavior with a high on/off ratio of 106 which is higher than any values of SnSe2 FETs reported at the moment. The estimated carrier concentration of Pb0.036Sn0.964Se2 is approximately six times lower than that of SnSe2. The results suggest that the method of reducing carrier concentration by doping to achieve high on/off ratio is effective, and Pb-doped SnSe2 monolayer has significant potential in future nanoelectronic and optoelectronic applications.

Large tunneling magnetoresistance in magnetic tunneling junctions based on two-dimensional CrX3 (X = Br, I) monolayers

Magnetic tunneling junctions (MTJs) have atomic thickness due to the use of two-dimensional (2D) materials. Combining density functional theory with non-equilibrium Greens function formalism, we systematically investigate the structural and magnetic properties of CrX3/h-BN/CrX3 (X = Br, I) MTJs, as well as their spin-dependent transport characteristics. Through calculation of the transmission spectrum, the large tunneling magnetoresistance (TMR) effect was observed in these MTJs. Moreover, their conductance based on two-dimensional materials was greatly improved over that of traditional MTJs. The transmission mechanism was analyzed using the symmetry of the orbit and the eigenstates of the transmitted electrons. We also discuss the problem of Schottky contact between different metal electrodes and devices. Our results suggest that MTJs based on two-dimensional ferromagnets are feasible.