Proximity-induced spin–orbit coupling in graphene has led to the observation of intriguing phenom... more Proximity-induced spin–orbit coupling in graphene has led to the observation of intriguing phenomena like time-reversal invariant $${{\mathbb{Z}}}_{2}$$ Z 2 topological phase and spin-orbital filtering effects. An understanding of the effect of spin–orbit coupling on the band structure of graphene is essential if these exciting observations are to be transformed into real-world applications. In this research article, we report the experimental determination of the band structure of single-layer graphene (SLG) in the presence of strong proximity-induced spin–orbit coupling. We achieve this in high-mobility hexagonal boron nitride (hBN)-encapsulated SLG/WSe2 heterostructures through measurements of quantum oscillations. We observe clear spin-splitting of the graphene bands along with a substantial increase in the Fermi velocity. Using a theoretical model with realistic parameters to fit our experimental data, we uncover evidence of a band gap opening and band inversion in the SLG. Fur...
In this letter, we report the experimental determination of the band structure of single-layer gr... more In this letter, we report the experimental determination of the band structure of single-layer graphene (SLG) in the presence of strong proximity-induced spin-orbit coupling. We achieve this in high-mobility hBN-encapsulated SLG/WSe2 heterostructures through measurements of quantum oscillations. We observe clear spin-splitting of the graphene bands along with a substantial increase in the Fermi velocity. Using a theoretical model with realistic parameters to fit our experimental data, we uncover evidence of a bandgap opening and band inversion in the SLG. Further, we establish that the deviation of the low-energy band structure from pristine SLG is determined primarily by the valley-Zeeman SOC and Rashba SOC, with the Kane-Mele SOC being inconsequential. Despite robust theoretical predictions and observations of band-splitting, a quantitative measure of the spin-splitting of the valence and the conduction bands and the consequent low-energy dispersion relation in SLG was missing -- ...
We have investigated the electron transport properties of polycrystalline Ge 1Àx Ga x Te alloys. ... more We have investigated the electron transport properties of polycrystalline Ge 1Àx Ga x Te alloys. Ga-incorporation in GeTe has a nonmonotonic effect on its electrical conductivity which decreases slightly at low doping levels, but significantly at high doping levels (x ! 0.06) due to a concomitant reduction of hole-concentration and mobility. Plausible Ga-doping mechanisms determining these trends have been discussed. The Seebeck coefficient of GeTe does not enhance with Ga content as much as with Sb, Bi or In content at similar hole-concentrations. Based on the theoretical Pisarenko plots and estimated effective carrier masses in Ge 1Àx Ga x Te alloys, the observed behavior of Seebeck coefficient is rationalized on the basis of relative orbital energies and energy difference between valence band valleys in GeTe. 2. Experimental Section 2.1. Synthesis and Characterization To synthesize polycrystalline ingots of pristine GeTe and Ge 1Àx Ga x Te samples (x ¼ 0
WS 2 is a promising catalyst for the hydrogen evolution reaction. We have explored photocatalytic... more WS 2 is a promising catalyst for the hydrogen evolution reaction. We have explored photocatalytic properties of ternary sulphoselenides of tungsten (WS x Se 2−x) by the dye-sensitized hydrogen evolution. WS x Se 2−x solid solutions are found to exhibit high activity reaching 2339 μmol h −1 g −1 for WSSe, which is three times higher than that of WS 2 alone (866 μmol h −1 g −1). The turnover frequency is also high (0.7 h −1). Such synergistic effect of selenium substitution in WS 2 is noteworthy.
Layered transition metal trichalcogenides (TMTCs) are a new class of anisotropic two-dimensional ... more Layered transition metal trichalcogenides (TMTCs) are a new class of anisotropic two-dimensional materials that exhibit quasi-1D behavior. This property stems from their unique highly anisotropic crystal structure where vastly different material properties can be attained from different crystal directions. Here, we employ density functional theory predictions, atomic force microscopy, and angle-resolved Raman spectroscopy to investigate their fundamental vibrational properties which differ significantly from other 2D systems and to establish a method in identifying anisotropy direction of different types of TMTCs. We find that the intensity of certain Raman peaks of TiS3, ZrS3, and HfS3 have strong polarization dependence in such a way that intensity is at its maximum when the polarization direction is parallel to the anisotropic b-axis. This allows us to readily identify the Raman peaks that are representative of the vibrations along the b-axis direction. Interestingly, similar ang...
We report the field effect transistor characteristics of exfoliated transition metal dichalcogeni... more We report the field effect transistor characteristics of exfoliated transition metal dichalcogenide alloy tungsten sulphoselenide. WSSe is a layered material of strongly bonded S-W-Se atoms having weak interlayer van der Waals forces with a significant potential for spintronic and valleytronic applications due to its polar nature. The X-ray photoelectron spectroscopy measurements on crystals grown by the chemical vapor transport method indicate a stoichiometry of the form WSSe. We report flake thickness tunable transport mechanism with n-type behavior in thin flakes (11 nm) and ambipolarity in thicker flakes. The devices with flake thicknesses of 2.4 nm-54.8 nm exhibit a maximum electron mobility of $50 cm 2 /V s along with an I ON /I OFF ratio >10 6. The electron Schottky barrier height values of 35 meV and 52 meV extracted from low temperature I-V measurements for 3.9 nm and 25.5 nm thick flakes, respectively, indicate that an increase in hole current with thickness is likely due to lowering of the bandgap through an increase in energy of the valence band maximum.
Layered semiconductor gallium telluride (GaTe) undergoes a rapid structural transition to a degra... more Layered semiconductor gallium telluride (GaTe) undergoes a rapid structural transition to a degraded phase in ambient conditions, limiting its utility in devices such as optical switches. In this work, we demonstrate that the degradation process in GaTe flakes can be slowed down dramatically via encapsulation with graphene. Through examining Raman signatures of degradation, we show that the choice of substrate significantly impacts the degradation rate and that the process is accelerated by the transfer of GaTe to hydrophilic substrates such as SiO 2 /Si. We find that double encapsulation with both top and bottom graphene layers can extend the lifetime of the material for several weeks. The photoresponse of flakes encapsulated in this way is only reduced by 17.6 ± 0.4% after 2 weeks, whereas unencapsulated flakes display no response after this time. Our results demonstrate the potential for alternative, van der Waals material-based passivation strategies in unstable layered materials and highlight the need for careful selection of substrates for 2D electronic devices.
Optical absorption in single crystals of WSe2 has been measured at room temperature near the fund... more Optical absorption in single crystals of WSe2 has been measured at room temperature near the fundamental absorption edge using light parallel to c – axis incident normally on the basal plane. Results have been analyzed on the basis of three dimensional (3D) and two dimensional (2D) models. Absorption near the fundamental edge was found to be due to indirect and direct allowed transitions on the basis of 3D model and indirect allowed transition on the basis of 2D model. The optical energy gaps corresponding to both transitions have also been determined.
Transition metal dichalcogenides (TMDCs) MX (M=Mo,W; X= S or Se) are semiconducting compounds exh... more Transition metal dichalcogenides (TMDCs) MX (M=Mo,W; X= S or Se) are semiconducting compounds exhibiting layered 2 structure. The single crystals of MoW Se (x=0, 0.25, 1) belonging to this family have been grown by direct vapour transport x 1-x 2 (DVT) technique. The chemical composition, basic structure and morphology of the as-grown crystals have been studied by energy dispersive analysis of X-rays (EDAX), X-ray Diffraction (XRD) and optical microscopy. EDAX study indicates that the compounds are nearly stoichiometric. The optical microscopy and XRD results show that the crystals grow predominantly as 2H-hexagonal polytypes. However, MoSe and Mo W Se seem to contain a little proportion of 3R- rhombohedralpolytypes.
MoSe2 belongs to a class of group II-VI chalcogenides having C7 type crystal structure, which pos... more MoSe2 belongs to a class of group II-VI chalcogenides having C7 type crystal structure, which possesses a layered structure. These MX2 (M=W or Mo, X= S, Se or Te) compounds form a structurally and chemically well defined family. The basic structure of loosely coupled X-M-X sheets makes these materials extremely interesting. This compound has attracted considerable attention due to its use in the construction of regenerative electrochemical solar cells. In present investigations, MoSe2 crystals grown by direct vapour transport method have been used. The photoconversion characteristics of n-MoSe2/I2/I–/Pt PEC solar cells have been investigated under polychromatic illumination from an incandescent lamp at various intensities, which show that the photoconversion efficiency is low. The series resistance is one of the major parameters that control the photoconversion behaviour of these solar cells. Efforts have been made to estimate its value. In addition, the effect of temperature on the...
Single crystals of tungsten diselenide have been grown by direct vapour transport technique using... more Single crystals of tungsten diselenide have been grown by direct vapour transport technique using a two zone furnace. The elemental composition and structure of grown crystals were confirmed using the energy dispersive analysis of X-rays (EDAX) and XRD. The Hall effect and thermopower measurements on these as grown crystals reveal that the crystals are p-type in nature. The relation between the thermoelectric power (TEP) and the concentration of charge carriers was also investigated along with their effective mass and effective density of states.
ABSTRACT Single crystals of Mo0.25W0.75Se2 have been grown by direct vapour transport technique. ... more ABSTRACT Single crystals of Mo0.25W0.75Se2 have been grown by direct vapour transport technique. Optical absorption spectra have been measured in the range 200-2000 nm at room temperature. Results have been analyzed on the basis of three dimensional (3D). Absorption near the fundamental edge was found to be due to indirect and direct allowed transitions on the basis of 3D model. The optical energy gaps corresponding to both transitions have also been determined. Some feeble disorder in the crystal is conceived to be present.
Band offsets between different monolayer transition metal dichalcogenides are expected to efficie... more Band offsets between different monolayer transition metal dichalcogenides are expected to efficiently separate charge carriers or rectify charge flow, offering a mechanism for designing atomically thin devices and probing exotic two-dimensional physics. However, developing such large-area heterostructures has been hampered by challenges in synthesis of monolayers and effectively coupling neighboring layers. Here, we demonstrate large-area (>tens of micrometers) heterostructures of CVD-grown WS2 and MoS2 monolayers, where the interlayer interaction is externally tuned from noncoupling to strong coupling. Following this trend, the luminescence spectrum of the heterostructures evolves from an additive line profile where each layer contributes independently to a new profile that is dictated by charge transfer and band normalization between the WS2 and MoS2 layers. These results and findings open up venues to creating new material systems with rich functionalities and novel physical e...
Monolayer Mo 1Àx W x Se 2 (x ¼ 0, 0.14, 0.75, and 1) alloys were experimentally realized from syn... more Monolayer Mo 1Àx W x Se 2 (x ¼ 0, 0.14, 0.75, and 1) alloys were experimentally realized from synthesized crystals. Mo 1Àx W x Se 2 monolayers are direct bandgap semiconductors displaying high luminescence and are stable in ambient. The bandgap values can be tuned by varying the W composition. Interestingly, the bandgap values do not scale linearly with composition. Such non-linearity is attributed to localization of conduction band minimum states around Mo d orbitals, whereas the valence band maximum states are uniformly distributed among W and Mo d orbitals. Results introduce monolayer Mo 1Àx W x Se 2 alloys with different gap values, and open a venue for broadening the materials library and applications of two-dimensional semiconductors.
To utilize carbon nanotubes (CNTs) in various commercial and scientific applications, the graphen... more To utilize carbon nanotubes (CNTs) in various commercial and scientific applications, the graphene sheets that comprise CNT surfaces are often modified to tailor properties, such as dispersion. In this article, we provide a critical review of the techniques used to explore the chemical and structural characteristics of CNTs modified by covalent surface modification strategies that involve the direct incorporation of specific elements and inorganic or organic functional groups into the graphene sidewalls. Using examples from the literature, we discuss not only the popular techniques such as TEM, XPS, IR, and Raman spectroscopy but also more specialized techniques such as chemical derivatization, Boehm titrations, EELS, NEXAFS, TPD, and TGA. The chemical or structural information provided by each technique discussed, as well as their strengths and limitations. Particular emphasis is placed on XPS and the application of chemical derivatization in conjunction with XPS to quantify functional groups on CNT surfaces in situations where spectral deconvolution of XPS lineshapes is ambiguous.
In this paper, we investigate the resistance fluctuations near the Lifshitz transition in WTe 2 d... more In this paper, we investigate the resistance fluctuations near the Lifshitz transition in WTe 2 devices. We identify the Lifshitz transition from electrical and thermal transport studies. The band structure obtained at low temperatures using quantum oscillation measurements consists of two hole pockets and two electron pockets at the Fermi energy, the hole pockets vanish above the Lifshitz transition temperature. The electrical noise shows a conspicuous peak near the Lifshitz transition temperature; we establish this noise to be arising from charge carrier scattering between the Weyl nodes and proximate areas of the bands. Our comparative study of the noise on WTe 2 devices fabricated on substrates with different scattering mechanisms elucidates the effect of interband scattering on the physics of Weyl semimetals.
Band offsets between different monolayer transition metal dichalcogenides are expected to efficie... more Band offsets between different monolayer transition metal dichalcogenides are expected to efficiently separate charge carriers or rectify charge flow, offering a mechanism for designing atomically thin devices and probing exotic two-dimensional physics. However, developing such large-area heterostructures has been hampered by challenges in synthesis of monolayers and effectively coupling neighboring layers. Here, we demonstrate large-area (>tens of micrometers) heterostructures of CVD-grown WS 2 and MoS 2 monolayers, where the interlayer interaction is externally tuned from noncoupling to strong coupling. Following this trend, the luminescence spectrum of the heterostructures evolves from an additive line profile where each layer contributes independently to a new profile that is dictated by charge transfer and band normalization between the WS 2 and MoS 2 layers. These results and findings open up venues to creating new material systems with rich functionalities and novel physical effects.
Single crystals of Mo 0.25 W 0.75 Se 2 have been grown by direct vapour transport technique. Opti... more Single crystals of Mo 0.25 W 0.75 Se 2 have been grown by direct vapour transport technique. Optical absorption spectra have been measured in the range 200 -2000nm at room temperature. Results have been analyzed on the basis of three dimensional (3D). Absorption near the fundamental edge was found to be due to indirect and direct allowed transitions on the basis of 3D model. The optical energy gaps corresponding to both transitions have also been determined. Some feeble disorder in the crystal is conceived to be present.
Monolayer Mo 1Àx W x Se 2 (x ¼ 0, 0.14, 0.75, and 1) alloys were experimentally realized from syn... more Monolayer Mo 1Àx W x Se 2 (x ¼ 0, 0.14, 0.75, and 1) alloys were experimentally realized from synthesized crystals. Mo 1Àx W x Se 2 monolayers are direct bandgap semiconductors displaying high luminescence and are stable in ambient. The bandgap values can be tuned by varying the W composition. Interestingly, the bandgap values do not scale linearly with composition. Such non-linearity is attributed to localization of conduction band minimum states around Mo d orbitals, whereas the valence band maximum states are uniformly distributed among W and Mo d orbitals. Results introduce monolayer Mo 1Àx W x Se 2 alloys with different gap values, and open a venue for broadening the materials library and applications of two-dimensional semiconductors. V C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4834358]
Proximity-induced spin–orbit coupling in graphene has led to the observation of intriguing phenom... more Proximity-induced spin–orbit coupling in graphene has led to the observation of intriguing phenomena like time-reversal invariant $${{\mathbb{Z}}}_{2}$$ Z 2 topological phase and spin-orbital filtering effects. An understanding of the effect of spin–orbit coupling on the band structure of graphene is essential if these exciting observations are to be transformed into real-world applications. In this research article, we report the experimental determination of the band structure of single-layer graphene (SLG) in the presence of strong proximity-induced spin–orbit coupling. We achieve this in high-mobility hexagonal boron nitride (hBN)-encapsulated SLG/WSe2 heterostructures through measurements of quantum oscillations. We observe clear spin-splitting of the graphene bands along with a substantial increase in the Fermi velocity. Using a theoretical model with realistic parameters to fit our experimental data, we uncover evidence of a band gap opening and band inversion in the SLG. Fur...
In this letter, we report the experimental determination of the band structure of single-layer gr... more In this letter, we report the experimental determination of the band structure of single-layer graphene (SLG) in the presence of strong proximity-induced spin-orbit coupling. We achieve this in high-mobility hBN-encapsulated SLG/WSe2 heterostructures through measurements of quantum oscillations. We observe clear spin-splitting of the graphene bands along with a substantial increase in the Fermi velocity. Using a theoretical model with realistic parameters to fit our experimental data, we uncover evidence of a bandgap opening and band inversion in the SLG. Further, we establish that the deviation of the low-energy band structure from pristine SLG is determined primarily by the valley-Zeeman SOC and Rashba SOC, with the Kane-Mele SOC being inconsequential. Despite robust theoretical predictions and observations of band-splitting, a quantitative measure of the spin-splitting of the valence and the conduction bands and the consequent low-energy dispersion relation in SLG was missing -- ...
We have investigated the electron transport properties of polycrystalline Ge 1Àx Ga x Te alloys. ... more We have investigated the electron transport properties of polycrystalline Ge 1Àx Ga x Te alloys. Ga-incorporation in GeTe has a nonmonotonic effect on its electrical conductivity which decreases slightly at low doping levels, but significantly at high doping levels (x ! 0.06) due to a concomitant reduction of hole-concentration and mobility. Plausible Ga-doping mechanisms determining these trends have been discussed. The Seebeck coefficient of GeTe does not enhance with Ga content as much as with Sb, Bi or In content at similar hole-concentrations. Based on the theoretical Pisarenko plots and estimated effective carrier masses in Ge 1Àx Ga x Te alloys, the observed behavior of Seebeck coefficient is rationalized on the basis of relative orbital energies and energy difference between valence band valleys in GeTe. 2. Experimental Section 2.1. Synthesis and Characterization To synthesize polycrystalline ingots of pristine GeTe and Ge 1Àx Ga x Te samples (x ¼ 0
WS 2 is a promising catalyst for the hydrogen evolution reaction. We have explored photocatalytic... more WS 2 is a promising catalyst for the hydrogen evolution reaction. We have explored photocatalytic properties of ternary sulphoselenides of tungsten (WS x Se 2−x) by the dye-sensitized hydrogen evolution. WS x Se 2−x solid solutions are found to exhibit high activity reaching 2339 μmol h −1 g −1 for WSSe, which is three times higher than that of WS 2 alone (866 μmol h −1 g −1). The turnover frequency is also high (0.7 h −1). Such synergistic effect of selenium substitution in WS 2 is noteworthy.
Layered transition metal trichalcogenides (TMTCs) are a new class of anisotropic two-dimensional ... more Layered transition metal trichalcogenides (TMTCs) are a new class of anisotropic two-dimensional materials that exhibit quasi-1D behavior. This property stems from their unique highly anisotropic crystal structure where vastly different material properties can be attained from different crystal directions. Here, we employ density functional theory predictions, atomic force microscopy, and angle-resolved Raman spectroscopy to investigate their fundamental vibrational properties which differ significantly from other 2D systems and to establish a method in identifying anisotropy direction of different types of TMTCs. We find that the intensity of certain Raman peaks of TiS3, ZrS3, and HfS3 have strong polarization dependence in such a way that intensity is at its maximum when the polarization direction is parallel to the anisotropic b-axis. This allows us to readily identify the Raman peaks that are representative of the vibrations along the b-axis direction. Interestingly, similar ang...
We report the field effect transistor characteristics of exfoliated transition metal dichalcogeni... more We report the field effect transistor characteristics of exfoliated transition metal dichalcogenide alloy tungsten sulphoselenide. WSSe is a layered material of strongly bonded S-W-Se atoms having weak interlayer van der Waals forces with a significant potential for spintronic and valleytronic applications due to its polar nature. The X-ray photoelectron spectroscopy measurements on crystals grown by the chemical vapor transport method indicate a stoichiometry of the form WSSe. We report flake thickness tunable transport mechanism with n-type behavior in thin flakes (11 nm) and ambipolarity in thicker flakes. The devices with flake thicknesses of 2.4 nm-54.8 nm exhibit a maximum electron mobility of $50 cm 2 /V s along with an I ON /I OFF ratio >10 6. The electron Schottky barrier height values of 35 meV and 52 meV extracted from low temperature I-V measurements for 3.9 nm and 25.5 nm thick flakes, respectively, indicate that an increase in hole current with thickness is likely due to lowering of the bandgap through an increase in energy of the valence band maximum.
Layered semiconductor gallium telluride (GaTe) undergoes a rapid structural transition to a degra... more Layered semiconductor gallium telluride (GaTe) undergoes a rapid structural transition to a degraded phase in ambient conditions, limiting its utility in devices such as optical switches. In this work, we demonstrate that the degradation process in GaTe flakes can be slowed down dramatically via encapsulation with graphene. Through examining Raman signatures of degradation, we show that the choice of substrate significantly impacts the degradation rate and that the process is accelerated by the transfer of GaTe to hydrophilic substrates such as SiO 2 /Si. We find that double encapsulation with both top and bottom graphene layers can extend the lifetime of the material for several weeks. The photoresponse of flakes encapsulated in this way is only reduced by 17.6 ± 0.4% after 2 weeks, whereas unencapsulated flakes display no response after this time. Our results demonstrate the potential for alternative, van der Waals material-based passivation strategies in unstable layered materials and highlight the need for careful selection of substrates for 2D electronic devices.
Optical absorption in single crystals of WSe2 has been measured at room temperature near the fund... more Optical absorption in single crystals of WSe2 has been measured at room temperature near the fundamental absorption edge using light parallel to c – axis incident normally on the basal plane. Results have been analyzed on the basis of three dimensional (3D) and two dimensional (2D) models. Absorption near the fundamental edge was found to be due to indirect and direct allowed transitions on the basis of 3D model and indirect allowed transition on the basis of 2D model. The optical energy gaps corresponding to both transitions have also been determined.
Transition metal dichalcogenides (TMDCs) MX (M=Mo,W; X= S or Se) are semiconducting compounds exh... more Transition metal dichalcogenides (TMDCs) MX (M=Mo,W; X= S or Se) are semiconducting compounds exhibiting layered 2 structure. The single crystals of MoW Se (x=0, 0.25, 1) belonging to this family have been grown by direct vapour transport x 1-x 2 (DVT) technique. The chemical composition, basic structure and morphology of the as-grown crystals have been studied by energy dispersive analysis of X-rays (EDAX), X-ray Diffraction (XRD) and optical microscopy. EDAX study indicates that the compounds are nearly stoichiometric. The optical microscopy and XRD results show that the crystals grow predominantly as 2H-hexagonal polytypes. However, MoSe and Mo W Se seem to contain a little proportion of 3R- rhombohedralpolytypes.
MoSe2 belongs to a class of group II-VI chalcogenides having C7 type crystal structure, which pos... more MoSe2 belongs to a class of group II-VI chalcogenides having C7 type crystal structure, which possesses a layered structure. These MX2 (M=W or Mo, X= S, Se or Te) compounds form a structurally and chemically well defined family. The basic structure of loosely coupled X-M-X sheets makes these materials extremely interesting. This compound has attracted considerable attention due to its use in the construction of regenerative electrochemical solar cells. In present investigations, MoSe2 crystals grown by direct vapour transport method have been used. The photoconversion characteristics of n-MoSe2/I2/I–/Pt PEC solar cells have been investigated under polychromatic illumination from an incandescent lamp at various intensities, which show that the photoconversion efficiency is low. The series resistance is one of the major parameters that control the photoconversion behaviour of these solar cells. Efforts have been made to estimate its value. In addition, the effect of temperature on the...
Single crystals of tungsten diselenide have been grown by direct vapour transport technique using... more Single crystals of tungsten diselenide have been grown by direct vapour transport technique using a two zone furnace. The elemental composition and structure of grown crystals were confirmed using the energy dispersive analysis of X-rays (EDAX) and XRD. The Hall effect and thermopower measurements on these as grown crystals reveal that the crystals are p-type in nature. The relation between the thermoelectric power (TEP) and the concentration of charge carriers was also investigated along with their effective mass and effective density of states.
ABSTRACT Single crystals of Mo0.25W0.75Se2 have been grown by direct vapour transport technique. ... more ABSTRACT Single crystals of Mo0.25W0.75Se2 have been grown by direct vapour transport technique. Optical absorption spectra have been measured in the range 200-2000 nm at room temperature. Results have been analyzed on the basis of three dimensional (3D). Absorption near the fundamental edge was found to be due to indirect and direct allowed transitions on the basis of 3D model. The optical energy gaps corresponding to both transitions have also been determined. Some feeble disorder in the crystal is conceived to be present.
Band offsets between different monolayer transition metal dichalcogenides are expected to efficie... more Band offsets between different monolayer transition metal dichalcogenides are expected to efficiently separate charge carriers or rectify charge flow, offering a mechanism for designing atomically thin devices and probing exotic two-dimensional physics. However, developing such large-area heterostructures has been hampered by challenges in synthesis of monolayers and effectively coupling neighboring layers. Here, we demonstrate large-area (>tens of micrometers) heterostructures of CVD-grown WS2 and MoS2 monolayers, where the interlayer interaction is externally tuned from noncoupling to strong coupling. Following this trend, the luminescence spectrum of the heterostructures evolves from an additive line profile where each layer contributes independently to a new profile that is dictated by charge transfer and band normalization between the WS2 and MoS2 layers. These results and findings open up venues to creating new material systems with rich functionalities and novel physical e...
Monolayer Mo 1Àx W x Se 2 (x ¼ 0, 0.14, 0.75, and 1) alloys were experimentally realized from syn... more Monolayer Mo 1Àx W x Se 2 (x ¼ 0, 0.14, 0.75, and 1) alloys were experimentally realized from synthesized crystals. Mo 1Àx W x Se 2 monolayers are direct bandgap semiconductors displaying high luminescence and are stable in ambient. The bandgap values can be tuned by varying the W composition. Interestingly, the bandgap values do not scale linearly with composition. Such non-linearity is attributed to localization of conduction band minimum states around Mo d orbitals, whereas the valence band maximum states are uniformly distributed among W and Mo d orbitals. Results introduce monolayer Mo 1Àx W x Se 2 alloys with different gap values, and open a venue for broadening the materials library and applications of two-dimensional semiconductors.
To utilize carbon nanotubes (CNTs) in various commercial and scientific applications, the graphen... more To utilize carbon nanotubes (CNTs) in various commercial and scientific applications, the graphene sheets that comprise CNT surfaces are often modified to tailor properties, such as dispersion. In this article, we provide a critical review of the techniques used to explore the chemical and structural characteristics of CNTs modified by covalent surface modification strategies that involve the direct incorporation of specific elements and inorganic or organic functional groups into the graphene sidewalls. Using examples from the literature, we discuss not only the popular techniques such as TEM, XPS, IR, and Raman spectroscopy but also more specialized techniques such as chemical derivatization, Boehm titrations, EELS, NEXAFS, TPD, and TGA. The chemical or structural information provided by each technique discussed, as well as their strengths and limitations. Particular emphasis is placed on XPS and the application of chemical derivatization in conjunction with XPS to quantify functional groups on CNT surfaces in situations where spectral deconvolution of XPS lineshapes is ambiguous.
In this paper, we investigate the resistance fluctuations near the Lifshitz transition in WTe 2 d... more In this paper, we investigate the resistance fluctuations near the Lifshitz transition in WTe 2 devices. We identify the Lifshitz transition from electrical and thermal transport studies. The band structure obtained at low temperatures using quantum oscillation measurements consists of two hole pockets and two electron pockets at the Fermi energy, the hole pockets vanish above the Lifshitz transition temperature. The electrical noise shows a conspicuous peak near the Lifshitz transition temperature; we establish this noise to be arising from charge carrier scattering between the Weyl nodes and proximate areas of the bands. Our comparative study of the noise on WTe 2 devices fabricated on substrates with different scattering mechanisms elucidates the effect of interband scattering on the physics of Weyl semimetals.
Band offsets between different monolayer transition metal dichalcogenides are expected to efficie... more Band offsets between different monolayer transition metal dichalcogenides are expected to efficiently separate charge carriers or rectify charge flow, offering a mechanism for designing atomically thin devices and probing exotic two-dimensional physics. However, developing such large-area heterostructures has been hampered by challenges in synthesis of monolayers and effectively coupling neighboring layers. Here, we demonstrate large-area (>tens of micrometers) heterostructures of CVD-grown WS 2 and MoS 2 monolayers, where the interlayer interaction is externally tuned from noncoupling to strong coupling. Following this trend, the luminescence spectrum of the heterostructures evolves from an additive line profile where each layer contributes independently to a new profile that is dictated by charge transfer and band normalization between the WS 2 and MoS 2 layers. These results and findings open up venues to creating new material systems with rich functionalities and novel physical effects.
Single crystals of Mo 0.25 W 0.75 Se 2 have been grown by direct vapour transport technique. Opti... more Single crystals of Mo 0.25 W 0.75 Se 2 have been grown by direct vapour transport technique. Optical absorption spectra have been measured in the range 200 -2000nm at room temperature. Results have been analyzed on the basis of three dimensional (3D). Absorption near the fundamental edge was found to be due to indirect and direct allowed transitions on the basis of 3D model. The optical energy gaps corresponding to both transitions have also been determined. Some feeble disorder in the crystal is conceived to be present.
Monolayer Mo 1Àx W x Se 2 (x ¼ 0, 0.14, 0.75, and 1) alloys were experimentally realized from syn... more Monolayer Mo 1Àx W x Se 2 (x ¼ 0, 0.14, 0.75, and 1) alloys were experimentally realized from synthesized crystals. Mo 1Àx W x Se 2 monolayers are direct bandgap semiconductors displaying high luminescence and are stable in ambient. The bandgap values can be tuned by varying the W composition. Interestingly, the bandgap values do not scale linearly with composition. Such non-linearity is attributed to localization of conduction band minimum states around Mo d orbitals, whereas the valence band maximum states are uniformly distributed among W and Mo d orbitals. Results introduce monolayer Mo 1Àx W x Se 2 alloys with different gap values, and open a venue for broadening the materials library and applications of two-dimensional semiconductors. V C 2014 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4834358]
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Papers by Deepa Narang