Raman spectroscopy has been an integral part of graphene research and can provide information about graphene structure, electronic characteristics, and electron-phonon interactions. In this study, the characteristics of the graphene Raman D-band, which vary with carrier density, are studied in detail, including the frequency, full width half-maximum, and intensity. We find the Raman D-band frequency increases for hole doping and decreases for electron doping. The Raman D-band intensity increases when the Fermi level approaches half of the excitation energy and is higher in the case of electron doping than that of hole doping. These variations can be explained by electron-phonon interaction theory and quantum interference between different Raman pathways in graphene. The intensity ratio of Raman D- and G-band, which is important for defects characterization in graphene, shows a strong dependence on carrier density.

Interface engineering is a key strategy to deal with the two-dimensional (2D)/three-dimensional (3D) hybrid heterostructure, since the properties of this atomic-layer-thick 2D material can easily be impacted by the substrate environment. In this work, the structural, electronic, and optical properties of the 2D/3D heterostructure of monolayer MoS on wurtzite GaN surface without and with nitridation interfacial layer are systematically investigated by first-principles calculation and experimental analysis. The nitridation interfacial layer can be introduced into the 2D/3D heterostructure by remote N plasma treatment to GaN sample surface prior to stacking monolayer MoS on top. The calculation results reveal that the 2D/3D integrated heterostructure is energetically favorable with a negative formation energy. Both interfaces demonstrate indirect band gap, which is a benefit for longer lifetime of the photoexcited carriers. Meanwhile, the conduction band edge and valence band edge of the MoS side increases after nitridation treatment. The modification to band alignment is then verified by X-ray photoelectron spectroscopy measurement on MoS/GaN heterostructures constructed by a modified wet-transfer technique, which indicates that the MoS/GaN heterostructure without nitridation shows a type-II alignment with a conduction band offset (CBO) of only 0.07 eV. However, by the deployment of interface nitridation, the band edges of MoS move upward for ∼0.5 eV as a result of the nitridized substrate property. The significantly increased CBO could lead to better electron accumulation capability at the GaN side. The nitridized 2D/3D heterostructure with effective interface treatment exhibits a clean band gap and substantial optical absorption ability and could be potentially used as practical photocatalyst for hydrogen generation by water splitting using solar energy.

A novel two-dimensional (2D) Ga2O3 monolayer was constructed and systematically investigated by first-principles calculations. The 2D Ga2O3 has an asymmetric configuration with a quintuple-layer atomic structure, the same as the well-studied α-In2Se3, and is expected to be experimentally synthesized. The dynamic and thermodynamic calculations show excellent stability properties of this monolayer material. The relaxed Ga2O3 monolayer has an indirect band gap of 3.16 eV, smaller than that of β-Ga2O3 bulk, and shows tunable electronic and optoelectronic properties with biaxial strain engineering. An attractive feature is that the asymmetric configuration spontaneously introduces an intrinsic dipole and thus the electrostatic potential difference between the top and bottom surfaces of the Ga2O3 monolayer, which helps to separate photon-generated electrons and holes within the quintuple-layer structure. By applying compressive strain, the Ga2O3 monolayer can be converted to a direct band gap semiconductor with a wider gap reaching 3.5 eV. Also, enhancement of hybridization between orbitals leads to an increase of electron mobility, from the initial 5000 to 7000 cm2 V–1 s–1. Excellent optical absorption ability is confirmed, which can be effectively tuned by strain engineering. With superior stability, as well as strain-tunable electronic properties, carrier mobility, and optical absorption, the studied novel Ga2O3 monolayer sheds light on low-dimensional electronic and optoelectronic device applications.

Materials with high second harmonic generation (SHG) efficiency and reduced dimensions are favorable for integrated photonics and novel nonlinear optical applications. Here, we fabricate MoS 2 nanoscrolls with different chiralities and study their SHG performances. As a 1D material, MoS 2 nanoscroll shows reduced symmetry and strong chirality dependency in the polarizationresolved SHG characterizations. This SHG performance can be well explained by the coherent superposition theory of second harmonic field of the nanoscroll walls. MoS 2 nanoscrolls with certain chiralities in our experiment can have SHG intensity up to 98 times stronger than that of monolayer MoS 2 , and the full potential can still be further exploited. The same chiralitydependent SHG can be expected for nanoscrolls or nanotubes composed of other noncentrosymmetric 2D materials, such as WS 2 , WSe 2 , and hBN. The characterization and analysis results presented here can also serve as a non-destructive technique to determine the chiralities of these nanoscrolls and nanotubes.' $%& (() (nanoscroll) = ∫ ' $%& (() :-1,3 )*++ ,1 )*++ ; (< 4566 /2>

Atomic defects with a four microsecond-long photoluminescence lifetime are created in single-layer WSe2 by focused ion beam irradiation.

The often observed p-type conduction of single carbon nanotube field-effect transistors is usually attributed to the Schottky barriers at the metal contacts induced by the work function differences or by the doping effect of the oxygen adsorption when carbon nanotubes are exposed to air, which cause the asymmetry between electron and hole injections. However, for carbon nanotube thin-film transistors, our contrast experiments between oxygen doping and electrostatic doping demonstrate that the doping-generated transport barriers do not introduce any observable suppression of electron conduction, which is further evidenced by the perfect linear behavior of transfer characteristics with the channel length scaling. On the basis of the above observation, we conclude that the environmental adsorbates work by more than simply shifting the Fermi level of the CNTs; more importantly, these adsorbates cause a poor gate modulation efficiency of electron conduction due to the relatively large trap state density near the conduction band edge of the carbon nanotubes, for which we further propose quantitatively that the adsorbed oxygen-water redox couple is responsible.

Using remote N plasma treatment to promote dielectric deposition on the dangling-bond free MoS is explored for the first time. The N plasma induced damages are systematically studied by the defect-sensitive acoustic-phonon Raman of single-layer MoS, with samples undergoing O plasma treatment as a comparison. O plasma treatment causes defects in MoS mainly by oxidizing MoS along the already defective sites (most likely the flake edges), which results in the layer oxidation of MoS. In contrast, N plasma causes defects in MoS mainly by straining and mechanically distorting the MoS layers first. Owing to the relatively strong MoS-substrate interaction and chemical inertness of MoS in N plasma, single-layer MoS shows great stability in N plasma and only stable point defects are introduced after long-duration N plasma exposure. Considering the enormous vulnerability of single-layer MoS in O plasma and the excellent stability of single-layer MoS in N plasma, the remote N plasma treatment shows great advantage as surface functionalization to promote dielectric deposition on single-layer MoS.

Surface functionalization of the dangling-bond-free MoS, WSe, and other TMDs (transition metal dichalcogenides) is of large practical importance, for example, in providing nucleation sites for the subsequent high-k dielectric integration. Of the surface functionalization methods, the reversible O or N atom adsorption on top of the chalcogen atoms is most promising. However, hazards such as severe oxidation or nitridation persist when the adsorption coverage is high. An in situ characterization technique, which can be integrated with the surface functionalization and dielectric deposition chamber, becomes valuable to enable the real-time monitoring of surface adsorption conditions. Raman spectroscopy, as a nondestructive characterization method without vacuum requirement, is a strong candidate. By utilizing first-principles calculations, Raman spectra of single-layer MoS and WSe with various O/N adsorption coverages are studied. The calculations suggest that the low-coverage O/N adsorbates will act as perturbations to the periodic lattice and activate the acoustic-phonon Raman scatterings. While high-coverage adsorptions will further activate and intensify the optical-phonon Raman scatterings of previously silent A and E modes, due to the breaking of reflection symmetry in the z direction, new phonon modes associated with the adatom oscillations are also introduced. All these pieces of evidence, together with the peak shifts of previously active A and E modes, suggest that in situ resonant Raman spectroscopy is capable of providing important information to quantify the O/N adsorption coverage and can be used as a valuable real-time characterization technique to monitor and control the surface functionalization conditions of MoS and WSe.