Matching Items (5)

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Direct optical transitions at K- and H-point of Brillouin zone in bulk MoS2, MoSe2, WS2, and WSe2

Description

Modulated reflectance (contactless electroreflectance (CER), photoreflectance (PR), and piezoreflectance (PzR)) has been applied to study direct optical transitions in bulk MoS[subscript 2], MoSe[subscript 2], WS[subscript 2], and WSe[subscript 2]. In

Modulated reflectance (contactless electroreflectance (CER), photoreflectance (PR), and piezoreflectance (PzR)) has been applied to study direct optical transitions in bulk MoS[subscript 2], MoSe[subscript 2], WS[subscript 2], and WSe[subscript 2]. In order to interpret optical transitions observed in CER, PR, and PzR spectra, the electronic band structure for the four crystals has been calculated from the first principles within the density functional theory for various points of Brillouin zone including K and H points. It is clearly shown that the electronic band structure at H point of Brillouin zone is very symmetric and similar to the electronic band structure at K point, and therefore, direct optical transitions at H point should be expected in modulated reflectance spectra besides the direct optical transitions at the K point of Brillouin zone. This prediction is confirmed by experimental studies of the electronic band structure of MoS[subscript 2], MoSe[subscript 2], WS[subscript 2], and WSe[subscript 2] crystals by CER, PR, and PzR spectroscopy, i.e., techniques which are very sensitive to critical points of Brillouin zone. For the four crystals besides the A transition at K point, an A[subscript H] transition at H point has been observed in CER, PR, and PzR spectra a few tens of meV above the A transition. The spectral difference between A and A[subscript H] transition has been found to be in a very good agreement with theoretical predictions. The second transition at the H point of Brillouin zone (B[subscript H] transition) overlaps spectrally with the B transition at K point because of small energy differences in the valence (conduction) band positions at H and K points. Therefore, an extra resonance which could be related to the B[subscript H] transition is not resolved in modulated reflectance spectra at room temperature for the four crystals.

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Date Created
  • 2016-06-21

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Pressure coefficients for direct optical transitions in MoS2, MoSe2, WS2, and WSe2 crystals and semiconductor to metal transitions

Description

The electronic band structure of MoS[subscript 2], MoSe[subscript 2], WS[subscript 2], and WSe[subscript 2], crystals has been studied at various hydrostatic pressures experimentally by photoreflectance (PR) spectroscopy and theoretically within

The electronic band structure of MoS[subscript 2], MoSe[subscript 2], WS[subscript 2], and WSe[subscript 2], crystals has been studied at various hydrostatic pressures experimentally by photoreflectance (PR) spectroscopy and theoretically within the density functional theory (DFT). In the PR spectra direct optical transitions (A and B) have been clearly observed and pressure coefficients have been determined for these transitions to be: α[subscript A] = 2.0 ± 0.1 and α[subscript B] = 3.6 ± 0.1 meV/kbar for MoS[subscript 2], α[subscript A] = 2.3 ± 0.1 and α[subscript B] = 4.0 ± 0.1 meV/kbar for MoSe[subscript 2], α[subscript A] = 2.6 ± 0.1 and α[subscript B] = 4.1 ± 0.1 meV/kbar for WS[subscript 2], α[subscript A] = 3.4 ± 0.1 and α[subscript B] = 5.0 ± 0.5 meV/kbar for WSe[subscript 2]. It has been found that these coefficients are in an excellent agreement with theoretical predictions. In addition, a comparative study of different computational DFT approaches has been performed and analyzed. For indirect gap the pressure coefficient have been determined theoretically to be −7.9, −5.51, −6.11, and −3.79, meV/kbar for MoS[subscript 2], MoSe[subscript 2], WS[subscript 2], and WSe[subscript 2], respectively. The negative values of this coefficients imply a narrowing of the fundamental band gap with the increase in hydrostatic pressure and a semiconductor to metal transition for MoS[subscript 2], MoSe[subscript 2], WS[subscript 2], and WSe[subscript 2], crystals at around 140, 180, 190, and 240 kbar, respectively.

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Date Created
  • 2016-05-24

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Unusual lattice vibration characteristics in whiskers of the pseudo-one-dimensional titanium trisulfide TiS3

Description

Transition metal trichalcogenides form a class of layered materials with strong in-plane anisotropy. For example, titanium trisulfide (TiS[subscript 3]) whiskers are made out of weakly interacting TiS[subscript 3] layers, where

Transition metal trichalcogenides form a class of layered materials with strong in-plane anisotropy. For example, titanium trisulfide (TiS[subscript 3]) whiskers are made out of weakly interacting TiS[subscript 3] layers, where each layer is made of weakly interacting quasi-one-dimensional chains extending along the b axis. Here we establish the unusual vibrational properties of TiS[subscript 3] both experimentally and theoretically. Unlike other two-dimensional systems, the Raman active peaks of TiS[subscript 3] have only out-of-plane vibrational modes, and interestingly some of these vibrations involve unique rigid-chain vibrations and S–S molecular oscillations. High-pressure Raman studies further reveal that the A[subscript g][superscript S–S] S-S molecular mode has an unconventional negative pressure dependence, whereas other peaks stiffen as anticipated. Various vibrational modes are doubly degenerate at ambient pressure, but the degeneracy is lifted at high pressures. These results establish the unusual vibrational properties of TiS[subscript 3] with strong in-plane anisotropy, and may have relevance to understanding of vibrational properties in other anisotropic two-dimensional material systems.

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Date Created
  • 2016-09-22

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Unusual dimensionality effects and surface charge density in 2D Mg(OH)2

Description

We present two-dimensional Mg(OH)[subscript 2] sheets and their vertical heterojunctions with CVD-MoS[subscript 2] for the first time as flexible 2D insulators with anomalous lattice vibration and chemical and physical properties.

We present two-dimensional Mg(OH)[subscript 2] sheets and their vertical heterojunctions with CVD-MoS[subscript 2] for the first time as flexible 2D insulators with anomalous lattice vibration and chemical and physical properties. New hydrothermal crystal growth technique enabled isolation of environmentally stable monolayer Mg(OH)[subscript 2] sheets. Raman spectroscopy and vibrational calculations reveal that the lattice vibrations of Mg(OH)[subscript 2] have fundamentally different signature peaks and dimensionality effects compared to other 2D material systems known to date. Sub-wavelength electron energy-loss spectroscopy measurements and theoretical calculations show that Mg(OH)[subscript 2] is a 6 eV direct-gap insulator in 2D, and its optical band gap displays strong band renormalization effects from monolayer to bulk, marking the first experimental confirmation of confinement effects in 2D insulators. Interestingly, 2D-Mg(OH)[subscript 2] sheets possess rather strong surface polarization (charge) effects which is in contrast to electrically neutral h-BN materials. Using 2D-Mg(OH)[subscript 2] sheets together with CVD-MoS[subscript 2] in the vertical stacking shows that a strong change transfer occurs from n-doped CVD-MoS[subscript 2] sheets to Mg(OH)[subscript 2], naturally depleting the semiconductor, pushing towards intrinsic doping limit and enhancing overall optical performance of 2D semiconductors. Results not only establish unusual confinement effects in 2D-Mg(OH)[subscript 2], but also offer novel 2D-insulating material with unique physical, vibrational, and chemical properties for potential applications in flexible optoelectronics.

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Date Created
  • 2016-02-05

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Novel Growth Routes and Fundamental Understanding of Pseudo-One-Dimensional Materials

Description

Recently, two-dimensional (2D) materials have emerged as a new class of materials with highly attractive electronic, optical, magnetic, and thermal properties. However, there exists a sub-category of 2D layers wherein

Recently, two-dimensional (2D) materials have emerged as a new class of materials with highly attractive electronic, optical, magnetic, and thermal properties. However, there exists a sub-category of 2D layers wherein constituent metal atoms are arranged in a way that they form weakly coupled chains confined in the 2D landscape. These weakly coupled chains extend along particular lattice directions and host highly attractive properties including high thermal conduction pathways, high-mobility carriers, and polarized excitons. In a sense, these materials offer a bridge between traditional one-dimensional (1D) materials (nanowires and nanotubes) and 2D layered systems. Therefore, they are often referred as pseudo-1D materials, and are anticipated to impact photonics and optoelectronics fields.

This dissertation focuses on the novel growth routes and fundamental investigation of the physical properties of pseudo-1D materials. Example systems are based on transition metal chalcogenide such as rhenium disulfide (ReS2), titanium trisulfide (TiS3), tantalum trisulfide (TaS3), and titanium-niobium trisulfide (Nb(1-x)TixS3) ternary alloys. Advanced growth, spectroscopy, and microscopy techniques with density functional theory (DFT) calculations have offered the opportunity to understand the properties of these materials both experimentally and theoretically. The first controllable growth of ReS2 flakes with well-defined domain architectures has been established by a state-of-art chemical vapor deposition (CVD) method. High-resolution electron microscopy has offered the very first investigation into the structural pseudo-1D nature of these materials at an atomic level such as the chain-like features, grain boundaries, and local defects.

Pressure-dependent Raman spectroscopy and DFT calculations have investigated the origin of the Raman vibrational modes in TiS3 and TaS3, and discovered the unusual pressure response and its effect on Raman anisotropy. Interestingly, the structural and vibrational anisotropy can be retained in the Nb(1-x)TixS3 alloy system with the presence of phase transition at a nominal Ti alloying limit. Results have offered valuable experimental and theoretical insights into the growth routes as well as the structural, optical, and vibrational properties of typical pseudo-1D layered systems. The overall findings hope to shield lights to the understanding of this entire class of materials and benefit the design of 2D electronics and optoelectronics.

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Date Created
  • 2018