Nannan Mao

Exploring atomic defects in molybdenum disulphide monolayers

Defects usually play an important role in tailoring various properties of two-dimensional materials. Defects in two-dimensional monolayer molybdenum disulphide may be responsible for large variation of electric and optical properties. Here we present a comprehensive joint experiment–theory investigation of point defects in monolayer molybdenum disulphide prepared by mechanical exfoliation, physical and chemical vapour deposition. Defect species are systematically identified and their concentrations determined by aberration-corrected scanning transmission electron microscopy, and also studied by ab-initio calculation. Defect density up to 3.5 × 1013 cm−2 is found and the dominant category of defects changes from sulphur vacancy in mechanical exfoliation and chemical vapour deposition samples to molybdenum antisite in physical vapour deposition samples. Influence of defects on electronic structure and charge-carrier mobility are predicted by calculation and observed by electric transport measurement. In light of these results, the growth of ultra-high-quality monolayer molybdenum disulphide appears a primary task for the community pursuing high-performance electronic devices.

Graphene: A Platform for Surface‐Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) imparts Raman spectroscopy with the capability of detecting analytes at the single-molecule level, but the costs are also manifold, such as a loss of signal reproducibility. Despite remarkable steps having been taken, presently SERS still seems too young to shoulder analytical missions in various practical situations. By the virtue of its unique molecular structure and physical/chemical properties, the rise of graphene opens up a unique platform for SERS studies. In this review, the multi-role of graphene played in SERS is overviewed, including as a Raman probe, as a substrate, as an additive, and as a building block for a flat surface for SERS. Apart from versatile improvements of SERS performance towards applications, graphene-involved SERS studies are also expected to shed light on the fundamental mechanism of the SERS effect.

Growth of Large-Area 2D MoS2(l_,)Se2, Semiconductor

Semiconducting MoS₂(₁-x) Se₂x mono-layers where x = 0-0.40 are successfully grown over large areas. A random arrangement of the S and Se atoms and a tunable bandgap photoluminescence are observed. Atomically thin, 2D semiconductor alloys with tunable bandgaps have potential applications in nano- and opto-electronics. Field-effect transistors fabricated with the monolayers exhibit high on/off ratios of >10(5).

High Responsivity and Gate Tunable Graphene-MoS2 Hybrid Phototransistor

A 2D atomic-layer-thickness phototransistor based on a graphene-MoS2 bybrid device is constructed with a photoresponse much larger than that of individual graphene or MoS2 based phototransistors. Strong and selective light absorption in the MoS2 layer creates electric charges that are transferred to graphene layers derived by a build-in electrical field, where they recirculate many times due to the high carrier mobility of graphene. Gate tunable Fermi level in graphene layer allows the responsivity of this hybrid phototransistor to be continuously tuned from 0 to about 10(4) mA/W by the gate voltage. Furthermore, large scale, flexible, and transparent 2D phototransistors with high responsivity are constructed from the CVD-grown graphene and MoS2 flakes. The high responsivity, gate-tunable sensitivity, wavelength selectivity, and compatibility with current circuit technologies of this type device give it great potential for future application in integrated nano-optoelectronic systems.

Identifying the Crystalline Orientation of Black Phosphorus Using Angle‐Resolved Polarized Raman Spectroscopy

An optical anisotropic nature of black phosphorus (BP) is revealed by angle-resolved polarized Raman spectroscopy (ARPRS), and for the first time, an all-optical method was realized to identify the crystal orientation of BP sheets, that is, the zigzag and armchair directions. We found that Raman intensities of Ag(1), B2g, and Ag(2) modes of BP not only depend on the polarization angle α, but also relate to the sample rotation angle θ. Furthermore, their intensities reach the local maximum or minimum values when the crystalline orientation is along with the polarization direction of scattered light (es). Combining with the angle-resolved conductance, it is confirmed that Ag(2) mode intensity achieves a relative larger (or smaller) local maximum under parallel polarization configuration when armchair (or zigzag) direction is parallel to es. Therefore, ARPRS can be used as a rapid, precise, and nondestructive method to identify the crystalline orientation of BP layers.

Two-Dimensional Molybdenum Tungsten Diselenide Alloys: Photoluminescence, Raman Scattering, and Electrical Transport

Two-dimensional transition-metal dichalcogenide alloys have attracted intense attention due to their tunable band gaps. In the present work, photoluminescence, Raman scattering, and electrical transport properties of monolayer and few-layer molybdenum tungsten diselenide alloys (Mo1-xWxSe2, 0 ≤ x ≤ 1) are systematically investigated. The strong photoluminescence emissions from Mo1-xWxSe2 monolayers indicate composition-tunable direct band gaps (from 1.56 to 1.65 eV), while weak and broad emissions from the bilayers indicate indirect band gaps. The first-order Raman modes are assigned by polarized Raman spectroscopy. Second-order Raman modes are assigned according to its frequencies. As composition changes in Mo1-xWxSe2 monolayers and few layers, the out-of-plane A1g mode showed one-mode behavior, while B2g(1) (only observed in few layers), in-plane E2g(1), and all observed second-order Raman modes showed two-mode behaviors. Electrical transport measurement revealed n-type semiconducting transport behavior with a high on/off ratio (>10(5)) for Mo1-xWxSe2 monolayers.

Optical Anisotropy of Black Phosphorus in the Visible Regime

The striking in-plane anisotropy remains one of the most intriguing properties for the newly rediscovered black phosphorus (BP) 2D crystals. However, because of its rather low-energy band gap, the optical anisotropy of few-layer BP has been primarily investigated in the near-infrared (NIR) regime. Moreover, the essential physics that determine the intrinsic anisotropic optical property of few-layer BP, which is of great importance for practical applications in optical and optoelectronic devices, are still in the fancy of theory. Herein, we report the direct observation of the optical anisotropy of few-layer BP in the visible regime simply by using polarized optical microscopy. On the basis of the Fresnel equation, the intrinsic anisotropic complex refractive indices (n-iκ) in the visible regime (480-650 nm) were experimentally obtained for the first time using the anisotropic optical contrast spectra. Our findings not only provide a convenient approach to measure the optical constants of 2D layered materials but also suggest a possibility to design novel BP-based photonic devices such as atom-thick light modulators, including linear polarizer, phase plate, and optical compensator in a broad spectral range extending to the visible window.

Solvatochromic Effect on the Photoluminescence of MoS2 Monolayers

The effect of surrounding solvents on the photoluminescence (PL) of MoS2 monolayers on Si/SiO2 substrates is studied. A redshift (up to -60 meV) is observed for MoS2 monolayers with nonhalogenated solvent surroundings. A blueshift (up to 60 meV) and intensity increase (2-50 times) are observed for monolayers with halogenated solvent surroundings.

CMP Aerogels: Ultrahigh‐Surface‐Area Carbon‐Based Monolithic Materials with Superb Sorption Performance

Monolithic conjugated microporous polymer (CMP) aerogels are obtained in an extremely facile way by selection of adequate reaction conditions and a freeze-drying technique. The aerogels possess an ultrahigh specific surface area and hierarchical interconnected pores, exhibiting superb gas/oil adsorption performance compared with all microporous organic polymers to date.

Growth of MoS2(1–x)Se2x (x = 0.41–1.00) Monolayer Alloys with Controlled Morphology by Physical Vapor Deposition

Transition-metal dichalcogenide (TMD) monolayer alloys are a branch of two-dimensional (2D) materials which can have large-range band gap tuning as the composition changes. Synthesis of 2D TMD monolayer alloys with controlled composition as well as controlled domain size and edge structure is of great challenge. In the present work, we report growth of MoS2(1-x)Se2x monolayer alloys (x = 0.41-1.00) with controlled morphology and large domain size using physical vapor deposition (PVD). MoS2(1-x)Se2x monolayer alloys with different edge orientations (Mo-zigzag and S/Se-zigzag edge orientations) have been obtained by controlling the deposition temperature. Large domain size of MoS2(1-x)Se2x monolayer alloys (x = 0.41-1.00) up to 20 μm have been obtained by tuning the temperature gradient in the deposition zone. Together with previously obtained MoS2(1-x)Se2x monolayer alloys (x = 0-0.40), the band gap photoluminescence (PL) is continuously tuned from 1.86 eV (i.e., 665 nm, reached at x = 0.00) to 1.55 eV (i.e., 800 nm, reached at x = 1.00). Additionally, Raman peak splitting was observed in MoS2(1-x)Se2x monolayer alloys. This work provides a way to synthesize MoS2(1-x)Se2x monolayer alloys with different edge orientations, which could be benefit to controlled growth of other 2D materials.