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Register a new userMulti-phonon diffuse scattering in solids from first-principles: Application to 2D MoS$_2$, bulk MoS$_2$, and black Phosphorous
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Time-resolved diffuse scattering experiments have gained increasing attention due to their potential to reveal non-equilibrium dynamics of crystal lattice vibrations with full momentum resolution. Although progress has been made in interpreting experimental data on the basis of one-phonon scattering, understanding the role of individual phonons can be sometimes hindered by multi-phonon excitations. In Ref. [arXiv:2103.10108] we have introduced a rigorous approach for the calculation of the all-phonon inelastic scattering intensity of solids from first-principles. In the present work, we describe our implementation in detail and show that multi-phonon interactions are captured efficiently by exploiting translational and time-reversal symmetries of the crystal. We demonstrate its predictive power by calculating the diffraction patterns of monolayer molybdenum disulfide (MoS$_2$), bulk MoS$_2$, and black phosphorus (bP), and we obtain excellent agreement with our measurements of thermal electron diffuse scattering. Remarkably, our results show that multi-phonon excitations dominate in bP across multiple Brillouin zones, while in MoS$_2$ play a less pronounced role. We expand our analysis for each system and we examine the effect of individual atomic and interatomic vibrational motion on the diffuse scattering signals. Our findings indicate that distinct features are explained by the collective displacement of MoS and specific pairs of P atoms. We further demonstrate that the special displacement method reproduces the thermally distorted configuration which generates precisely the all-phonon diffraction pattern. The present methodology opens the way for high-throughput calculations of the scattering intensity in crystals and the accurate interpretation of static and time-resolved diffuse scattering experiments.
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Transition metal dichalcogenides like MoS$_2$, MoSe$_2$, WS$_2$, and WSe$_2$ have attracted enormous interest during recent years. They are van-der-Waals crystals with highly anisotropic properties, which allows exfoliation of individual layers. Their remarkable physical properties make them promising for applications in optoelectronic, spintronic, and valleytronic devices. Phonons are fundamental to many of the underlying physical processes, like carrier and spin relaxation or exciton dynamics. However, experimental data of the complete phonon dispersion relations in these materials is missing. Here we present the phonon dispersion of bulk MoS$_2$ in the high-symmetry directions of the Brillouin zone, determined by inelastic X-ray scattering. Our results underline the two-dimensional nature of MoS$_2$. Supported by first-principles calculations, we determine the phonon displacement patterns, symmetry properties, and scattering intensities. The results will be the basis for future experimental and theoretical work regarding electron-phonon interactions, intervalley scattering, as well as phonons in related 2D materials.
To translate electrical into optical signals one uses the modulation of either the refractive index or the absorbance of a material by an electric field. Contemporary electroabsorption modulators (EAMs) employ the quantum confined Stark effect (QCSE), the field-induced red-shift and broadening of the strong excitonic absorption resonances characteristic of low-dimensional semiconductor structures. Here we show an unprecedentedly strong transverse electroabsorption (EA) signal in a monolayer of the two-dimensional semiconductor MoS2. The EA spectrum is dominated by an apparent linewidth broadening of around 15% at a modulated voltage of only Vpp = 0.5 V. Contrary to the conventional QCSE, the signal increases linearly with the applied field strength and arises from a linear variation of the distance between the strongly overlapping exciton and trion resonances. The achievable modulation depths exceeding 0.1 dBnm-1 bear the scope for extremely compact, ultrafast, energy-efficient EAMs for integrated photonics, including on-chip optical communication.
Two-dimensional (2D) MoS$_2$ has been intensively investigated for its use in the fields of microelectronics, nanoelectronics, and optoelectronics. However, intrinsic 2D MoS$_2$ is usually used as the n-type semiconductor due to the unintentional sulphur vacancies and surface gas adsorption.The synthesis and characterization of 2D MoS$_2$ semiconductor of p-type are crucial for the development of relevant p-n junction devices, as well as the practical applications of 2D MoS$_2$ in the next-generation CMOS integrated circuit. Here, we synthesize high-quality, wafer-scale, 2D p-type MoS$_2$ (Mo$_{1-x}$Nb$_x$S$_2$) with various niobium (Nb) mole fractions from 0 to 7.6% by a creative two-step method. The dielectric functions of 2D Mo1-xNbxS2 are accurately determined by spectroscopic ellipsometry. We find that the increasing fraction of Nb dopant in 2D MoS$_2$ can modulate and promote the combination of A and B exciton peaks of 2D MoS$_2$. The direct causes of this impurity-tunable combination are interpreted as the joint influence of decreasing peak A and broadening peak B. We explain the broadening peak B as the multiple transitions from the impurity-induced valance bands to the conductive band minimum at K point of Brillouin zone by comparing and analyzing the simulated electronic structure of intrinsic and 2D Nb-doped MoS$_2$. A p-type FET based on the 2D Nb-doped MoS$_2$ was fabricated for characterization, and its working performance is expected to be adjustable as a function of concentration of Nb dopant according to our theoretical research. Our study is informative for comprehending optical and electronic properties of extrinsic 2D transitional metal dichalcogenides, which is important and imperative for the development and optimization of corresponding photonics and optoelectronics devices.
Chemically and mechanically exfoliated MoS$_2$ single-layer samples have substantially different properties. While mechanically exfoliated single-layers are mono-phase (1H polytype with Mo in trigonal prismatic coordination), the chemically exfoliated samples show coexistence of three different phases, 1H, 1T (Mo in octahedral coordination) and 1T$^{}$ (a distorted $2times 1$ 1T-superstructure). By using first-principles calculations, we investigate the energetics and the dynamical stability of the three phases. We show that the 1H phase is the most stable one, while the metallic 1T phase, strongly unstable, undergoes a phase transition towards a metastable and insulating 1T$^{}$ structure composed of separated zig-zag chains. We calculate electronic structure, phonon dispersion, Raman frequencies and intensities for the 1T$^{}$ structure. We provide a microscopical description of the J$_1$, J$_2$ and J$_3$ Raman features first detected more then $20$ years ago, but unexplained up to now. Finally, we show that H adsorbates, that are naturally present at the end of the chemical exfoliation process, stabilize the 1T$^{prime}$ over the 1H one.
Molybdenum disulfide (MoS$_2$) nanosheet is a two-dimensional material with high electron mobility and with high potential for applications in catalysis and electronics. We synthesized MoS$_2$ nanosheets using a one-pot wet-chemical synthesis route with and without Re-doping. Atom probe tomography revealed that 3.8 at.% Re is homogeneously distributed within the Re-doped sheets. Other impurities are found also integrated within the material: light elements including C, N, O, and Na, locally enriched up to 0.1 at.%, as well as heavy elements such as V and W. Analysis of the non-doped sample reveals that the W and V likely originate from the Mo precursor.
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