Juanxia Wu

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.

Probing the Effect of Molecular Orientation on the Intensity of Chemical Enhancement Using Graphene- Enhanced Raman Spectroscopy

A rational approach to investigate the effect of molecular orientation on the intensity of chemical enhancement using graphene-enhanced Raman spectroscopy (GERS) is developed. A planar molecule, copper phthalocyanine (CuPc), is used as probe molecule. Annealing allows the CuPc molecule in a Langmuir-Blodgett film to change orientation from upstanding to lying down. The UV-visible absorption spectra prove the change of the molecular orientation, as well as the variation of the interaction between the CuPc molecule and graphene. The Raman spectra of the molecule in the two different orientations are compared and analyzed. The results show that chemical enhancement is highly sensitive to the molecular orientation. The different molecular orientations induce different magnitudes of the interaction between the molecule and graphene. The stronger the interaction, the more the Raman signal is enhanced. Furthermore, the sensitivity of GERS to molecular orientation is promising to determine the orientation of the molecule on graphene. Based on this molecular orientation sensitive Raman enhancement, quantitative calculation of the magnitude of the chemical enhancement is implemented for a series of Pc derivatives. It shows that the magnitude of the chemical enhancement can be used to evaluate the degree of interaction between the molecules and graphene. Moreover, an understanding of the chemical enhancement in GERS is promoted and the effect of molecular orientation on the intensity of chemical enhancement is explained.

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.

Hierarchical Hydrogen Bonds Directed Multi‐Functional Carbon Nanotube‐Based Supramolecular Hydrogels

Supramolecular hydrogels (SMHs) are three-dimensional networks filled with a large amount of water. The crosslinking force in the 3D network is always constructed by relatively weak and dynamic non-covalent interactions, and thus SMHs usually possess extremely high susceptibility to external environment and can show extraordinary stimuli-responsive, self-healing or other attractive properties. However, the overall crosslinking force in hydrogel networks is difficult to flexibly modulate, and this leads to limited functions of the SMHs. In this regard, hierarchical hydrogen bonds, that is, the mixture of relatively strong and relatively weak hydrogen bonds, are used herein as crosslinking force for the hydrogel preparation. The ratio of strong and weak hydrogen bonds can be finely tuned to tailor the properties of resultant gels. Thus, by delicate manipulation of the overall crosslinking force in the system, a hydrogel with multiple (thermal, pH and NIR light) responsiveness, autonomous self-healing property and interesting temperature dependent, reversible adhesion behavior is obtained. This kind of hierarchical hydrogen bond manipulation is proved to be a general method for multiple-functionality hydrogel preparation, and the resultant material shows potential for a range of applications.

Nanodiode based on a multiwall CNx/carbon nanotube intramolecular junction

We prepared multiwall carbon nanotubes (MWCNTs) from the pyrolysis of ferrocene, and CN(x) nanotubes from a mixture of ferrocene and melamine. Under well chosen synthesis conditions, massive multiwall CN(x)/carbon nanotube intramolecular junctions were successfully fabricated. The individual nanotubes were used as conductance channels to obtain their transport characteristic information. Measurement results showed that the current-voltage (I-V) curve of the CN(x)/CNT junction is highly asymmetric, behaving like a diode. Moreover, the devices are very stable in ambient environment. We attribute this nonlinear property of the CN(x)/CNT junctions to their two different atomic and electronic sections.

Birefringence-Directed Raman Selection Rules in 2D Black Phosphorus Crystals.

The incident and scattered light engaged in the Raman scattering process of low symmetry crystals always suffer from the birefringence-induced depolarization. Therefore, for anisotropic crystals, the classical Raman selection rules should be corrected by taking the birefringence effect into consideration. The appearance of the 2D anisotropic materials provides an excellent platform to explore the birefringence-directed Raman selection rules, due to its controllable thickness at the nanoscale that greatly simplifies the situation comparing with bulk materials. Herein, a theoretical and experimental investigation on the birefringence-directed Raman selection rules in the anisotropic black phosphorus (BP) crystals is presented. The abnormal angle-dependent polarized Raman scattering of the Ag modes in thin BP crystal, which deviates from the normal Raman selection rules, is successfully interpreted by the theoretical model based on birefringence. It is further confirmed by the examination of different Raman modes using different laser lines and BP samples of different thicknesses.