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Register a new userThe optical signature of few-layer ReSe$_2$
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Optical properties of thin layers of rhenium diselenide (ReSe$_2$) with thickness ranging from mono- (1 ML) to nona-layer (9 MLs) are demonstrated. The photoluminescence (PL) and Raman scattering were measured at low ($T$=5 K) and room ($T$=300 K) temperature, respectively. The PL spectra of ReSe$_2$ layers display two well-resolved emission lines, which blueshift by about 120 meV when the layer thickness decreases from 9 MLs to a monolayer. A rich structure of the observed low-energy Raman scattering modes can be explained within a linear chain model. The two phonon modes of intralayer vibrations, observed in Raman scattering spectra at about 120 cm$^{-1}$, exhibit very sensitive and opposite evolution as a function of layer thickness. It is shown that their energy difference can serve as a convenient and reliable tool to determine the thickness of ReSe$_2$ flakes in the few-layer limit.
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Few-layer InSe draws tremendous research interests owing to the superior electronic and optical properties. It exhibits high carrier mobility up to more than 1000 cm2/Vs at room temperature. The strongly layer-tunable band gap spans a large spectral range from near-infrared to the visible. In this perspective, we systematically review the optical properties of few-layer InSe. Firstly, the intrinsic optical and electronic properties are introduced. Compared to other two-dimensional (2D) materials, the light-matter interaction of few-layer InSe is unusual. The band gap transition is inactive or extremely weak for in-plane polarized light, and the emission light is mainly polarized along the out-of-plane direction. Secondly, we will present several schemes to tune the optical properties of few-layer InSe such as external strain, surface chemical doping and van der Waals (vdW) interfacing. Thirdly, we survey the applications of few-layer InSe in photodetection and heterostructures. Overall, few-layer InSe exhibits great potential not only in fundamental research, but also in electronic and optoelectronic applications.
We have experimentally studied the optical refractive index of few-layer graphene through reflection spectroscopy at visible wavelengths. A laser scanning microscope (LSM) with a coherent supercontinuum laser source measured the reflectivity of an exfoliated graphene flake on a Si/SiO2 substrate, containing monolayer, bilayer and trilayer areas, as the wavelength of the laser was varied from 545nm to 710nm. The complex refractive index of few-layer graphene, n-ik, was extracted from the reflectivity contrast to the bare substrate and the Fresnel reflection theory. Since the SiO2 thickness enters to the modeling as a parameter, it was precisely measured at the location of the sample. It was found that a common constant optical index cannot explain the wavelength-dependent reflectivity data for single-, double- and three-layer graphene simultaneously, but rather each individual few-layer graphene possesses a unique optical index whose complex values were precisely and accurately determined from the experimental data.
We induce surface carrier densities up to $sim7cdot 10^{14}$cm$^{-2}$ in few-layer graphene devices by electric double layer gating with a polymeric electrolyte. In 3-, 4- and 5-layer graphene below 20-30K we observe a logarithmic upturn of resistance that we attribute to weak localization in the diffusive regime. By studying this effect as a function of carrier density and with ab-initio calculations we derive the dependence of transport, intervalley and phase coherence scattering lifetimes on total carrier density. We find that electron-electron scattering in the Nyquist regime is the main source of dephasing at temperatures lower than 30K in the $sim10^{13}$cm$^{-2}$ to $sim7 cdot 10^{14}$cm$^{-2}$ range of carrier densities. With the increase of gate voltage, transport elastic scattering is dominated by the competing effects due to the increase in both carrier density and charged scattering centers at the surface. We also tune our devices into a crossover regime between weak and strong localization, indicating that simultaneous tunability of both carrier and defect density at the surface of electric double layer gated materials is possible.
Since the discovery of graphene -a single layer of carbon atoms arranged in a honeycomb lattice - it was clear that this truly is a unique material system with an unprecedented combination of physical properties. Graphene is the thinnest membrane present in nature -just one atom thick- it is the strongest material, it is transparent and it is a very good conductor with room temperature charge mobilities larger than the typical mobilities found in silicon. The significance played by this new material system is even more apparent when considering that graphene is the thinnest member of a larger family: the few-layer graphene materials. Even though several physical properties are shared between graphene and its few-layers, recent theoretical and experimental advances demonstrate that each specific thickness of few-layer graphene is a material with unique physical properties.
We present a density functional theory parametrized hybrid k$cdot$p tight binding model for electronic properties of atomically thin films of transition-metal dichalcogenides, 2H-$MX_2$ ($M$=Mo, W; $X$=S, Se). We use this model to analyze intersubband transitions in $p$- and $n$-doped $2{rm H}-MX_2$ films and predict the line shapes of the intersubband excitations, determined by the subband-dependent two-dimensional electron and hole masses, as well as excitation lifetimes due to emission and absorption of optical phonons. We find that the intersubband spectra of atomically thin films of the 2H-${MX_2}$ family with thicknesses of $N=2$ to $7$ layers densely cover the infrared spectral range of wavelengths between $2$ and $30 {rm mu m}$. The detailed analysis presented in this paper shows that for thin $n$-doped films, the electronic dispersion and spin-valley degeneracy of the lowest-energy subbands oscillate between odd and even number of layers, which may also offer interesting opportunities for quantum Hall effect studies in these systems.
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