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Band-edge excitons of few-layer nickel phosphorous trisulfide (NiPS3) are characterized via micro-thermal-modulated reflectance (μTR) measurements from 10 to 300 K. Prominent μTR features of the A exciton series and B are simultaneously detected near the band edge of NiPS3. The A exciton series contains two sharp A1 and A2 levels and one threshold-energy-related transition (direct gap, E∞), which are simultaneously detected at the lower energy side of NiPS3. In addition, one broadened B feature is present at the higher energy side of few-layer NiPS3. The A series excitons may correlate with majorly d-to-d transition in the Rydberg series with threshold energy of E∞ ≅ 1.511 eV at 10 K. The binding energy of A1 is about 36 meV, and the transition energy is A1 ≅ 1.366 eV at 300 K. The transition energy of B measured by μTR is about 1.894 eV at 10 K. The excitonic series A may directly transit from the top of valence band to the conduction band of NiPS3, while the B feature might originate from the spin-split-off valence band to the conduction band edge. The direct optical gap of NiPS3 is ~1.402 eV at 300 K, which is confirmed by μTR and transmittance experiments.

SnS and SnSe are renowned energy materials that are applied for photoelectric and thermoelectric conversions owing to their suitable band gap, close to 1 eV, and superior figure of merit (ZT), larg...

We investigated the ternary ReS2(1−x)Se2x alloys and fabricated few-layer ReS1.02Se0.98 nanoflakes to realize passive Q-switch and noise-like pulses operating at 1 μm wavelength. Stable Q-switching pulses can be achieved with a threshold pump power of 180 mW and a maximum pump power of 300 mW. The single pulse energy of 81.47 nJ and pulse width as narrow as 2.2 μs at 1035 nm center wavelength were achieved. By optimizing the parameters of the cavity, self-started noise-like pulses were realized, which are centered at 1031 nm. The average pulse width of the spike was 283 fs on the top of a broad pedestal and the fundamental repetition was 38.98 MHz. Our work demonstrates that ReS1.02Se0.98 has potential applications in optical modulators, and also opens up new application opportunities for these novel materials in nonlinear optics and ultrafast laser photonic technologies.

In this work, structure, optical, and thermoelectric properties of layered ZrS2&minus, xSex single crystals with selenium composition of x = 0, 1, and 2 were examined. Single crystals of zirconium dichalcogenides layer compounds were grown by chemical vapor transport method using I2 as the transport agent. X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) results indicated that ZrS2&minus, xSex (x = 0, 1, and 2) were crystalized in hexagonal CdI2 structure with one-layer trigonal (1T) stacking type. X-ray photoelectron and energy dispersive X-ray measurements revealed oxidation sensitive behavior of the chalcogenides series. Transmittance and optical absorption showed an indirect optical gap of about 1.78 eV, 1.32 eV, and 1.12 eV for the ZrS2&minus, xSex with x = 0, 1, and 2, respectively. From the result of thermoelectric experiment, ZrSe2 owns the highest figure-of merit (ZT) of ~0.085 among the surface-oxidized ZrS2&minus, xSex series layer crystals at 300 K. The ZT values of the ZrS2&minus, xSex (x = 0, 1, and 2) series also reveal increase with the increase of Se content owing to the increase of carrier concentration and mobility in the highly Se-incorporated zirconium dichalcogenides with surface states.

A wide gap semiconductor material has attracted attention as a heterophotocatalyst because of its light harvesting nature to be used in alternative energy production for the next generation. We, herein, grow and synthesize ZnS(1–x)Ox series compounds using the chemical vapor transport (CVT) method with I2 serving as the transport agent. Different crystals, such as undoped ZnS and oxygen-doped ZnS0.94O0.06 and ZnS0.88O0.12, revealed different bright palette emissions that were presented in photoluminescence spectra in our previous report. To study the electron–hole pair interaction of this sample series, the near-band-edge transitions of the sample series were characterized in detail by photoconductivity (PC) experiments. Additional results from surface photovoltage (SPV) spectra also detected the surface and defect-edge transitions from the higher oxygen-doped ZnS crystals. PC measurement results showed a red-shift in the bandgap with increasing incorporation of oxygen on ZnS. Consequently, the samples were subjected to photoirradiation by xenon lamp for the degradation of methylene blue (MNB) by acting as heterophotocatalysts. Undoped ZnS emerged as the best photocatalyst candidate with the fastest rate constant value of 0.0277 min–1. In cubic {111} ZnS [{111} c-ZnS], the polarized Zn+ → S– ions may play a vital role as a photocatalyst because of their strong electron–hole polarization, which leads to the mechanism for degradation of the MNB solution.

In search of high-performance field-effect transistors (FETs) made of atomic thin semiconductors, indium selenide (InSe) has held great promise because of its high intrinsic mobility and moderate electronic band gap (1.26 eV). Yet the performance of InSe FETs is decisively determined by the surface oxidation of InSe taking place spontaneously in ambient conditions, setting up a mobility ceiling and causing an uncontrollable current hysteresis. Encapsulation by hexagonal boron nitride (h-BN) has been currently used to cope with this deterioration. Here, we provide insights into the role of surface oxides played in device performance and introduce a dry-oxidation process that forms a dense capping layer on top, where InSe FETs exhibit a record-high two-probe mobility of 423 cm2/V·s at room temperature and 1006 cm2/V·s at liquid nitrogen temperature without the use of h-BN encapsulation or high-κ dielectric screening. Ultrahigh on/off current ratio of >108 and current density of 365 μA/μm can be readily achi...

The synthesis, optical characterization, and environmental applications of β-Ga2O3 nanowires are presented. The ambient controlled synthesis of β-Ga2O3 nanowires will be demonstrated first. With the precise growth ambient control, the luminescence property of β-Ga2O3 nanowires was modulated from UV to near-red regions. The optical and luminescence properties of β-Ga2O3 nanowires were characterized by temperature-dependent thermoreflectance spectroscopy, photoluminescence spectroscopy, Raman spectroscopy, and absorption spectroscopy. The origin of gap-state and near-band-edge transitions of β-Ga2O3 nanowires were identified and discussed. The whole band structure of β-Ga2O3 nanowires was constructed and demonstrated. In the final section, the photocatalytic activity of β-Ga2O3 nanowires will be presented. The photocatalytic activity of β-Ga2O3 nanowires was greatly enhanced and attributed to the large numbers of acceptor states associated with gallium defects. The origin of defect-enhanced photocatalytic activity in β-Ga2O3 nanowires was discussed. The results demonstrated the potential applications of β-Ga2O3 nanowires in optoelectronic and environmental applications.

Supramolecular polymers can easily control the lamellar microstructures on exfoliated tungsten diselenide nanosheets., A simple and effective method for direct exfoliation of tungsten diselenide (WSe2) into few-layered nanosheets has been successfully developed by employing a low molecular weight adenine-functionalized supramolecular polymer (A-PPG). In this study, we discover A-PPG can self-assemble into a long-range, ordered lamellar microstructure on the surface of WSe2 due to the efficient non-covalent interactions between A-PPG and WSe2. Morphological and light scattering studies confirmed the dynamic self-assembly behavior of A-PPG has the capacity to efficiently manipulate the transition between contractile and extended lamellar microstructures on the surface of metallic 1T-phase and semiconducting 2H-phase WSe2 nanosheets, respectively. The extent of WSe2 exfoliation can be easily controlled by systematically adjusting the amount of A-PPG in the composites, to obtain nanocomposites with the desired functional characteristics. In addition, the resulting composites possess unique liquid–solid phase transition behavior and excellent thermoreversible properties, revealing the self-assembled lamellar structure of A-PPG functions as a critical factor to manipulate and tailor the physical properties of exfoliated WSe2. This newly developed method of producing exfoliated WSe2 provides a useful conceptual and potential framework for developing WSe2-based multifunctional nanocomposites to extend their application in solution-processed semiconductor devices.

Rhenium disulfide (ReS2) and diselenide (ReSe2), the group 7 transition metal dichalcogenides (TMDs), are known to have a layered atomic structure showing an in-plane motif of diamond-shaped-chains (DS-chains) arranged in parallel. Using a combination of transmission electron microscopy and transport measurements, we demonstrate here the direct correlation of electron transport anisotropy in single-layered ReS2 with the atomic orientation of the DS-chains, as also supported by our density functional theory calculations. We further show that the direction of conducting channels in ReS2 and ReSe2 can be controlled by electron beam irradiation at elevated temperatures and follows the strain induced to the sample. Furthermore, high chalcogen deficiency can induce a structural transformation to a nonstoichiometric phase, which is again strongly direction-dependent. This tunable in-plane transport behavior opens up great avenues for creating nanoelectronic circuits in 2D materials.

Layered-type α- and γ-In 2 Se 3 crystals were simultaneously grown by chemical vapor transport (CVT) method using ICl 3 as the transport agent. The crystalline state, surface state, and structural polytype of the as-grown In 2 Se 3 were observed by high-resolution transmission electron microscopy (HRTEM). The direct band edges of α- and γ-In 2 Se 3 were evaluated by thermoreflectance (TR) measurements. For α-In 2 Se 3 , a surface oxidation layer α-In 2 Se 3−3 x O 3 x (0 ≤ x ≤ 1) can easily form on the crystal face in environmental air by oxygen detection (∼250 nm/day). The surface formation oxide can be easily removed by laser treatment ( λ = 266 nm, P = 60 mW) and the oxygen detection can be repeated again. For γ-In 2 Se 3 , the HRTEM and Raman measurements reveal amorphous and polycrystalline state existing in the as-grown crystals. The amorphous effect renders erasable optical-memory switching of γ ↔ α inter-phase transition occurred inside the γ-In 2 Se 3 layer with laser heating treatment. The phase transition of In 2 Se 3 is fast and easily by using the laser treatment. The phase transformation of γ ↔ α was verified by Raman and TR measurements. The optical-memory switching and oxygen detection behavior for the as-grown γ- and α-phase In 2 Se 3 are demonstrated herein.

We have successfully grown crystals of Al-based silver sulfides AgAlS2 and Ag(In0.2Al0.8)S2 by chemical vapor transport with ICl3 as transport agent. The Al-based silver chalcopyrites AgAlS2 and Ag(In0.2Al0.8)S2 have obvious (112) crystal faces and needle-like morphology. As-grown AgAlS2 and Ag(In0.2Al0.8)S2 are, respectively, transparent and light-yellow under vacuum. When exposed to the atmosphere, the two crystals’ surfaces are spontaneously transformed into brownish, oxygen-deficient AgAlO2−2x and Ag(In0.2Al0.8)O2−2x , because of reaction of their surfaces with moisture. This surface reaction of AgAlS2 and Ag(In0.2Al0.8)S2 may slow or stop when the samples are kept dry. The chemical reaction between AgAlS2 and water vapor occurs rapidly on exposure to the atmosphere. Addition of a little indium weakened the surface reaction. The band-edge transitions of AgAlS2 and Ag(In0.2Al0.8)S2 crystals were characterized by temperature-dependent thermoreflectance and absorption measurements. Increasing the indium content of Ag-III(Al-In)-S2 chalcopyrite strengthens the III–S bond, preventing substitution of the sulfur atom with oxygen in the reaction of AgAlS2 with moisture.

Single crystals of undoped and Au-doped MoS2were grown by the chemical vapor transport method using iodine as a transporting agent. The doping effect of the Au was characterized by temperature-dependent piezoreflectance (PzR) spectroscopy measurements in broad range of temperatures from 25 to 300 K. The temperature dependent PzR spectra revealed prominent features in the vicinity of direct-band-edge excitons for both investigated samples. The energies and broadening parameters of the A and B excitons were determined via a detailed line shape fit of the PzR spectra. We observed that Au doping strongly reduces the splitting between A and B excitons as well as it causes the redshift of observed transitions in relation to the undoped MoS2. The origin of observed excitons was discussed.

Nitrogen doped ZnO (ZnO:N) thin films have been successfully prepared by reactive ion beam sputter deposition. The ZnO:N thin films show a preferred growth orientation along the (002) direction regardless of nitrogen flow rates. Raman spectroscopy analysis shows nitrogen related local vibration modes at 275 and 576 cm − 1 in addition to the ZnO E 2 (high) mode at 436 cm − 1 , indicating a successful incorporation of nitrogen into the ZnO. Both of the peak intensities of 275 and 576 cm − 1 reach a maximum after post-growth annealing at 500 °C. ZnO:N deposited with a 0.5 sccm nitrogen flow rate exhibits p -type conductivity with a hole concentration of 2.1 × 10 17 /cm 3 and a mobility of 3 cm 2 V − 1 s − 1 after annealing at 500 °C. Conversion to p -type conductivity was not observed on ZnO:N deposited with higher nitrogen flow rates. The p -type conductivity remains stable after it was stored at ambient conditions for more than two months. The p -type ZnO:N thin film is transparent in the visible range with a transmittance larger than 83%. SIMS analysis indicates that nitrogen concentration of less than 1.2 at.% results in the successful preparation of p -type ZnO:N. ZnO:N deposited with higher nitrogen concentration results in n -type conductivity which is likely due to the formation of molecular nitrogen replacing oxygen sites that act as double donors (N 2 ) O .

ZnGa2O4 nanowires have been synthesized by a direct vapor transport process on c-sapphire substrate without any template under low temperature. Their structural and optical properties were characterized by field-emission scanning electron microscopy (FE-SEM), energy-dispersive spectroscopy (EDS) X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), and photoluminescence (PL). Structural characterization revealed that the as-synthesized samples consist of cubic spinel ZnGa2O4 nanowires with 30–90 nm in diameter and 3–5 μm long. Room temperature photoluminescence show intense ultraviolet (375 nm) and green (530 nm) emissions originate from oxygen vacancies. Specifically, a near-band-edge emission centered at 283 nm was observed. The photocatalytic activity of ZnGa2O4 nanowires was evaluated by the degradation of methyl blue (MB) under ultraviolet light illumination. The results show that ZnGa2O4 nanowires exhibit high photocatalytic activity and is attributed to the enhanced light absorption and strong redox ability of photo-generated electron–hole pairs.

β-Ga2O3 nanobelts were synthesized using a vapor transport process in a controlled ambient. Structural characterization revealed that the as-synthesized samples consisted of monoclinic β-Ga2O3 nanobelts, and the presence of gallium-associated defects was verified using cathodoluminescence (CL). The formation of gallium-associated defects was explained by the insufficiency of the supply of cations, generating gallium vacancies on the (010) facet during growth. Furthermore, field-emission measurements indicated that β-Ga2O3 nanobelts exhibited defect-related electron emission. The turn-on fields of β-Ga2O3 nanobelts increased significantly with the degree of structural defects. For a sample prepared under 15% ambient oxygen, Fowler–Nordheim (F–N) analysis revealed two distinct field-enhancement factors of 1194 and 276, respectively. A correlation between field emission and structural defects was proposed. The experimental results demonstrate the presence of gallium-associated defects, which behave as electron traps, degrading the electron field-emission properties of β-Ga2O3 nanobelts.

Single crystals of Cu2ZnSn(SxSe1−x)4 (CZTSSe) solid solutions were grown by chemical vapor transport technique using iodine trichloride as a transport agent. As confirmed by X-ray investigations, the as-grown CZTSSe solid solutions are single phase and crystallized in kesterite structure. The lattice parameters of CZTSSe were determined and the S contents of the obtained crystals were estimated by Vegard’s law. The composition dependent band gaps of CZTSSe solid solutions were studied by electrolyte electroreflectance (EER) measurements at room temperature. From a detailed lineshape fit of the EER spectra, the band gaps of CZTSSe were determined accurately and were found to decrease almost linearly with the increase of Se content, which agreed well with the recent theoretical first-principle calculations by S. Chen, A. Walsh, J.H. Yang, X.G. Gong, L. Sun, P. X. Yang, J.H. Chu, S.H. Wei, Phys. Rev. B 83 (2011) 125201 (5pp).

Direct optical evidence of Burstein–Moss shift (BMS) in conduction band of cubic c-In2O3 nanowires is reported herein. The optical property of degenerate semiconducting oxide c-In2O3 has been investigated by thermoreflectance (TR) spectroscopy. Low-temperature TR spectra clearly show a series of band-edge excitons for c-In2O3. The threshold energy for the exciton series was determined. One transition feature of direct gap caused by BMS was detected in all TR spectra from 20 to 300 K. The Femi-level (EF) energy above conduction-band edge (EC) is determined to be EF – EC ≈ 92 meV. The energy value causes a calculated electron density of ∼2.1 × 1019 cm–3. The direct gap of c-In2O3 with BMS is 3.43 eV at 300 K. Free exciton, bound exciton complexes, donor–acceptor pair transition, and defect emissions have been evaluated by photoluminescence (PL) measurements from 10 to 300 K. Photoresistivity change (i.e., 5.71 × 10–2 → 4.85 × 10–2 Ω-cm) under the illumination of halogen lamp (5 mW·cm–2) for c-In2O3 thin-fil...

Single crystals of S- and Se-incorporated As 2 Te 3 have been grown by vertical Bridgman method. The electronic structure and optical property of As 2 (Te 1− x S x ) 3 [ATS] and As 2 (Te 1− x Se x ) 3 [ATSe] series compounds have been characterized experimentally by thermoreflectance (TR) measurements in a wide energy range of 0.7–6 eV. X-ray diffraction measurements showed that the diffraction peaks of sulfur- and selenium-incorporated As 2 (Te 1− x S x ) 3 0 ≤ x ≤ 0.3 and As 2 (Te 1− x Se x ) 3 0 ≤ x ≤ 0.6 crystals shift to higher diffraction angles with the increase of the sulfur or selenium incorporations. The analysis of X-ray measurement revealed similar crystalline phase for the As 2 Te 3 and those of the S- or Se-incorporated As 2 Te 3 . The experimental TR spectra of As 2 (Te 1− x S x ) 3 (0 ≤ x ≤ 1) and As 2 (Te 1− x Se x ) 3 (0 ≤ x ≤ 1) exhibit a lot of derivative-like spectral features in the vicinity of band edge as well as in the higher-lying bands. Transition energies and broadening parameters of the TR features at 40 and 300 K were analyzed. Compositional dependences of band gap and interband transition energies of the ATS and ATSe series were evaluated. The origins for the interband transitions in the ATS and ATSe are assigned. Based on the experimental analyses, the electronic structure of the diarsenic trichalcogenides, As 2 (Te 1− x Se x ) 3 and As 2 (Te 1− x S x ) 3 , is hence being realized.

A systematic optical characterization of a series of Mo1-xWxS2 (0 ≤ x ≤ 1) layered mixed crystals grown by chemical vapour transport method were carried out by using piezoreflectance (PzR) and Raman scattering measurements. From a detailed lineshape fit of the PzR spectra over an energy range from 1.6 to 5.0 eV, the energies of the band-edge excitonic and higher lying interband transitions were determined accurately. The transition energies and their splittings vary smoothly with the tungsten composition x indicating that the nature of the band structure is similar for the Mo1-xWxS2 series compounds. The peaks of the two dominant first-order Raman-active modes, and , and several second-order bands are observed in the range of 250-450 cm-1. The peaks corresponding to mode show a one-mode type behavior, while the peaks of mode demonstrate two-mode type behavior for the entire series of Mo1-xWxS2 crystals. These behaviors were discussed on the basis of the lattice vibrational properties of 2H-MoS2 and 2H-WS2 compounds.

β-Ga2O3 nanobelts have been synthesized by a vapor transport process under different ambient oxygen. Their structural and optical properties were characterized by field-emission scanning electron microscopy, X-ray diffraction, photoluminescence (PL), and absorbance measurements. Structural characterization revealed that the as-synthesized samples consist of monoclinic β-Ga2O3 nanobelts with comparable surface area. Samples prepared under high oxygen ambient possessed large numbers of gallium-associated defects confirmed by PL and absorbance measurements. The photocatalytic activity of β-Ga2O3 nanobelts was evaluated by the degradation of organic dyes (rhodamine B and methyl blue) under ultraviolet light illumination. The results demonstrated that β-Ga2O3 nanobelts exhibited superior photocatalytic activity and stability as compared with commercial β-Ga2O3 powder. The photocatalytic activity of β-Ga2O3 nanobelts was greatly enhanced and attributed to the large numbers of acceptor states associated with gallium defects. The origin of enhanced photocatalytic activity of β-Ga2O3 nanobelts was discussed.

We report Q-switched and mode-locked erbium-doped all-fiber lasers using ternary ReS2(1−x)Se2x as saturable absorbers (SAs). The modulation depth and saturable intensity of the film SA are 1.8% and 0.046 MW/cm2. In Q-switched mechanism output, the pulse was centered at 1531.1 nm with maximum pulse energy and minimum pulse width of 28.29 nJ and 1.07 μs, respectively. In mode-locked operation, the pulse was centered at 1561.15 nm with pulse width of 888 fs, repetition rate of 2.95 MHz, and maximum pulse energy of 0.275 nJ. To the best of our knowledge, this is the first report on the mode-locked Er3+-doped fiber laser using ternary transition metal dichalcogenides. This work suggests prospective 2D-material SAs can be widely used in versatile fields due to their attractive optoelectronic and tunable energy bandgap properties.

Single crystals of Cu(AlxIn1−x)S2 (0≤x≤1) with x=0, 0.2, 0.4, 0.5, 0.6, 0.7, and 1.0 were grown by chemical vapor transport method using ICl3 as a transport agent. The results of X-ray measurement indicated that the as-grown crystals are single phase and isostructural. The formation of {1 1 2} face is preferable for the crystallization of whole series Cu(AlxIn1−x)S2 (0≤x≤1) chalcopyrite crystals. Electronic structure of Cu(AlxIn1−x)S2 (0≤x≤1) was probed experimentally by X-ray photoelectron spectroscopy (XPS) and thermoreflectance (TR) measurements. The experimental results enabled to identify the electron configuration, and determine energies of valence and conduction band edges of the chalcopyrites. The energies of near-band-edge transitions in the Cu(AlxIn1−x)S2 (0≤x≤1) were determined accurately by TR measurements. Composition dependences of transition energies near band edge were evaluated. The obtained energies were assigned to the particular interband transitions of Cu(AlxIn1−x)S2 (0≤x≤1). Based on the experimental analyses of TR and XPS, the electronic band structure near the fundamental band edge of Cu(AlxIn1−x)S2 (0≤x≤1) was thus constructed. The good correspondence of the TR and XPS results as well as the proposed band diagram verifies the crystalline quality of the chalcopyrites.

Tunability and stability in the electrical properties of 2D semiconductors pave the way for their practical applications in logic devices. A robust layered indium selenide (InSe) field-effect transistor (FET) with superior controlled stability is demonstrated by depositing an indium (In) doping layer. The optimized InSe FETs deliver an unprecedented high electron mobility up to 3700 cm2 V-1 s-1 at room temperature, which can be retained with 60% after 1 month. Further insight into the evolution of the position of the Fermi level and the microscopic device structure with different In thicknesses demonstrates an enhanced electron-doping behavior at the In/InSe interface. Furthermore, the contact resistance is also improved through the In insertion between InSe and Au electrodes, which coincides with the analysis of the low-frequency noise. The carrier fluctuation is attributed to the dominance of the phonon scattering events, which agrees with the observation of the temperature-dependent mobility. Finally, the flexible functionalities of the logic-circuit applications, for instance, inverter and not-and (NAND)/not-or (NOR) gates, are determined with these surface-doping InSe FETs, which establish a paradigm for 2D-based materials to overcome the bottleneck in the development of electronic devices.

Single crystals of β-In 2 S 3 were grown by chemical vapor transport method using ICl 3 as a transport agent. The below and above band-edge transitions in β-In 2 S 3 have been characterized using optical absorption and photoluminescence (PL) measurements in the temperature range 20–300 K. Thermoreflectance (TR) and photoconductivity (PC) measurements were carried out to verify the band-edge nature of the diindium trisulfide tetragonal crystals. Experimental analyses of the transmittance, PL, PC, and TR spectra of β-In 2 S 3 confirmed that the chalcogenide compound is a direct semiconductor with a band gap of about 1.935 eV at 300 K. β-In 2 S 3 is familiar with its defect nature. For the β-In 2 S 3 crystals, two defect emissions and two above band-edge luminescences were simultaneously detected in the PL spectra at low temperatures. The energy variations of the defect emissions showed temperature-insensitive behavior with respect to the temperature change from 20 to 300 K. Temperature dependences of transition energies of the near-band-edge (NBE) transitions below and above band gap are analyzed. The origins for the NBE transitions in the β-In 2 S 3 defect crystals are discussed.

Nitrogen-doped ZnO thin films have been prepared by reactive ion beam sputtering deposition utilizing a capillaritron ion source. X-ray diffraction (XRD) analysis of the as-deposited film exhibits a single strong ZnO (002) diffraction peak centred at 34.40°. Post-growth annealing causes increase of grain size and decrease of c-axis lattice constant. Micro-Raman spectroscopy analysis of the as-deposited film shows strong nitrogen-related local vibration mode at 275, 582, 640 and 720 cm −1 , whereas the E 2 mode of ZnO at 436 cm −1 can barely be identified. Annealing at 500–800 °C causes decrease of 275, 582, 640 and 720 cm −1 and increase of 436 cm −1 intensity, indicating out-diffusion of nitrogen and improvement of ZnO crystalline quality. Unlike un-doped ZnO, the surface roughness of nitrogen-doped ZnO deteriorates after annealing, which is also attributed to the out-diffusion of nitrogen. A nitrogen concentration of ∼10 21 /cm 3 was observed while type conversion from n-type to p-type was not achieved, which is likely due to the formation of Zn I –N O or (N 2 ) O that act as donor/double donors.

ZnO growth on sapphire by MOCVD using dimethylzinc and CO 2 as zinc and oxygen precursors was performed. A dense ZnO film with major (0 0 0 2) orientation can be prepared at 350 °C and above with high dimethylzinc flow rate. Result shows that the growth temperature suppresses the lateral growth of ZnO grains, promotes the coalescence of grains but reduces the crystal alignment. To further enhance the crystal alignment, a two-step temperature variation growth method is proposed. Using the two-step growth method, employing the initial growth at lower temperature followed by the growth at higher temperature, a densely packed ZnO film with larger grains and well-aligned (0 0 0 2) crystallographic orientation can be obtained. The effect of temperature on nucleation and growth rate, and its relation to the crystal alignment enhancement is also discussed.

For utilization of ZnO in optoelectronics, the large-area heteroepitaxy of ZnO is needed. However the lattice mismatch and anisotropy of ZnO prevent the successful heteroepitaxy of ZnO film through the conventional slow growth approach. This paper demonstrates an unconventional approach of dense ZnO film formation by high growth rate deposition using metal-organic chemical vapor deposition. The mechanism of the formation of well-oriented and dense ZnO film with large grains is illustrated in terms of dimethylzinc flow rate, oxygen flow rate, and VI/II ratio.

Structural and luminescence properties of GaSe1−x S x (0 ≤ x ≤ 1) series optical materials have been studied by X-ray diffraction, photoluminescence (PL), and piezoreflectance (PzR) measurements. Powder X-ray diffraction patterns showed the whole series layers present three different kinds of stacking formula with respect to the compositional change of sulfur from x = 0 to x = 1. The comparison of PL and PzR spectra reveals that the GaSe1−x S x layers have three different kinds of stacking phase from x = 0 to x = 1. The PL results show the whole series GaSe1−x S x layers emit the luminescences from red to blue visible region. The PL and PzR spectra of the GaSe1−x S x are analyzed. The structural variation in between the layers is discussed.

Dichroic optical and electrical behaviors of layered ReX 2 (X = S, Se) have been characterized using angular dependent polarized-absorption and resistivity measurements in the van der Waal plane. The polarized energy gaps of ReS 2 and ReSe 2 were analyzed from the polarized-absorption spectra with polarization angles from θ = 0° ( E || b -axis) to θ = 90° ( E ⊥ b -axis). The angular-dependent relationships of the polarized energy gaps of ReX 2 were analyzed. Angular dependent resistivity measurements of ReX 2 were carried out in the layer plane. The cutting edge of each sample was varied from θ = 0° (|| b ) to θ = 120° with an increment of 10° with respect to the layer crystal's b -axis. The angular dependency of the resistivities in the layered plane was analyzed. The experimental evidences of angular dependent in-plane resistivities and polarized energy gaps of ReX 2 (X = S, Se) show that for ReX 2 not only an optical dichroism but also an electrical dichroism in the van der Waal plane occurs.

As 2 (Se 1− x S x ) 3 glasses with x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0 were prepared by direct quenching from the melt solution. The structure and stochiometry of the glasses were investigated by powder X-ray diffraction, Raman spectroscopy, and electron-probe-micro-analysis (EPMA) techniques. The photodarkening effect in the As 2 (Se 1− x S x ) 3 glasses were observed under the influence of laser irradiations. The energy band gaps of whole series As 2 (Se 1− x S x ) 3 glasses were examined by thermoreflectance (TR) as well as transmission measurements. Compositional dependences of the energy gaps of the glass system were evaluated. The relationship of the direct band gaps of As 2 (Se 1− x S x ) 3 (0 ≤ x ≤ 1) glass is determined to be E g ( x ) = 1.52 + 0.28 x + 0.38 x 2 eV.

Semiconducting two-dimensional transition metal dichalcogenides are emerging as top candidates for post-silicon electronics. While most of them exhibit isotropic behaviour, lowering the lattice symmetry could induce anisotropic properties, which are both scientifically interesting and potentially useful. Here we present atomically thin rhenium disulfide (ReS2) flakes with unique distorted 1T structure, which exhibit in-plane anisotropic properties. We fabricated monolayer and few-layer ReS2 field-effect transistors, which exhibit competitive performance with large current on/off ratios (∼107) and low subthreshold swings (100 mV per decade). The observed anisotropic ratio along two principle axes reaches 3.1, which is the highest among all known two-dimensional semiconducting materials. Furthermore, we successfully demonstrated an integrated digital inverter with good performance by utilizing two ReS2 anisotropic field-effect transistors, suggesting the promising implementation of large-scale two-dimensional logic circuits. Our results underscore the unique properties of two-dimensional semiconducting materials with low crystal symmetry for future electronic applications., Many two-dimensional materials exhibit isotropic properties, but anisotropy can extend the functionality of future devices. Here, the authors fabricate field-effect transistors from single and few-layer rhenium disulfide and observe an anisotropic ratio of three to one along the two principle axes

We demonstrate a III-VI chalcogenide, polycrystalline γ-In2Se3, which simultaneously possesses the capabilities of thickness-dependent optical gaps and wide-energy-range absorption existed in the polycrystalline layers of γ-In 2 Se 3 . The amorphous effect of medium-range order (MRO) renders a structural-phase transition of γ → α occurred inside the γ-In 2 Se 3 layer with a heat treatment of about 700 °C. Photo V-I measurements of different-thickness γ-In 2 Se 3 layers propose a wide-energy-range photoelectric conversion unit ranging from visible to UV may be achieved by stacking the γ-In 2 Se 3 layers in a staircase form containing dissimilar optical gaps. The results are demonstrated here in.

Single crystals of pyrite FeS 2 were grown by chemical vapor transport (CVT) using ICl 3 as a transport agent. Optimum conditions were given for growing large single crystals. Band-edge properties of the synthetic crystals were characterized by temperature dependent electrical and photoconductivity measurements. The Hall-effect measurement confirmed p-type semiconducting behavior of the crystal at low temperature. Temperature dependent Hall-effect measurements showed that electrical mixed conduction of electrons and holes begins to dominate at temperatures higher than 60 K. The mixed-conduction behavior is due to the ionization effect of p- and n-type imperfections in synthetic FeS 2 . The p-type state (acceptor) may come from a chalcogen-deficiency defect in synthetic FeS 2 , while the n-type level (donor) is caused by an unwanted nickel impurity involved inside the crystal. Optical properties of the synthetic p-FeS 2 are characterized using photoconductivity (PC) measurements in the temperature range between 10 and 300 K. The PC spectra at different temperatures can be decomposed into two transition features. The possible origins of the transition features A and B are assigned. Temperature dependences of the transition energies of the features are analyzed. The parameters that describe temperature variations of the transition energies in p-FeS 2 are evaluated and discussed. Based on the experimental analyses of the electrical and optical measurements, a probable band-structure scheme for describing the near-band-edge characteristic of synthetic FeS 2 is constructed.

Luminescence and structural properties of GaSe 1− x S x (0≤ x ≤1) series optical materials were characterized using photoluminescence (PL) and piezoreflectance (PzR) measurements at 300 K. From the comparison of PL and PzR spectra, the whole series GaSe 1− x S x (0≤ x ≤1) layers were confirmed to possess three different kinds of stacking phase when increasing the sulfur composition from x =0 to 1. Observing the peak positions of the PL spectra the light emissions of whole series GaSe 1− x S x are shown to cover from red to blue visible region. The PL and PzR spectra of whole series GaSe 1− x S x are analyzed. The structural variations in the layers are discussed. Based on the experimental results together with referring to previous band-structure calculations, a probable band scheme which indicating the transition assignments in the fundamental edge of GaSe 1− x S x is proposed.

Single crystals of Ge(Se1−xSx)2 with x=0x=0, 0.2, 0.4, 0.6, 0.8 and 1.0 were grown by vertical Bridgman method. The crystalline phase and stochiometry of the layered crystals were investigated by X-ray, SEM and EPMA techniques. The energy band gaps of the whole series crystals were examined by thermoreflectance (TR) and transmission measurements. Compositional dependences of the band gap and broadening parameter for Ge(Se1−xSx)2 were evaluated. The relationship for the composition-dependent direct band gaps of Ge(Se1−xSx)2 is determined to be Eg(x)=(2.43±0.03)+(0.38±0.02) x+(0.26±0.02) x2eV.

Single crystals of GaSe 1− x S x ( 0 ⩽ x ⩽ 1 ) series layered solids were grown by vertical Bridgman and chemical vapor transport methods. Crystal structure and electronic structure of the GaSe 1− x S x ( 0 ⩽ x ⩽ 1 ) series layered semiconductors were characterized using X-ray diffraction and piezoreflectance (PzR) measurements. Experimental observation of the powder X-ray diffraction patterns indicated that the whole series layers including three different kinds of stacking formula with respect to the compositional change of the sulfur incorporation. Lattice constants of the GaSe 1− x S x ( 0 ⩽ x ⩽ 1 ) are evaluated and the stacking formula for the whole series layered solids is discussed. Electronic structures of the GaSe 1− x S x ( 0 ⩽ x ⩽ 1 ) series layer compounds are characterized using PzR measurements in the energy range between 1.9 and 6 eV at 15 and 300 K, respectively. The PzR spectra of the GaSe 1− x S x ( 0 ⩽ x ⩽ 1 ) series layered semiconductors clearly indicated that there are five transition features observed at the band-edge as well as at the higher-energy interband transitions. The transition features for each composition of GaSe 1− x S x layer compounds were analyzed and identified. The compositional dependences of the transition energies of GaSe 1− x S x ( 0 ⩽ x ⩽ 1 ) series semiconductors were analyzed and discussed.

Single crystals of W- and Mo-doped ReSe2 have been grown by chemical vapor transport (CVT) process with bromine as a transporting agent. The nominal doping concentration for tungsten is 0.5% while EDX measurement indicates a 4% doping concentration for molybdenum. Single crystalline platelets up to 5 mm ×5 mm surface area and 100 μm in thickness were obtained. From the X-ray diffraction patterns, the doped crystals are found to crystallize in the triclinic-layered structure. The doping effects of the materials are characterized by temperature-dependent conductivity, optical absorption and piezoreflectance measurements. The activation energies for the impurity carriers decrease with doping. The indirect energy gaps of the doped samples show different red shifts for W- and Mo-doped samples. The direct band edge excitonic transition energies remain unchanged for W-doped samples, while for Mo-doped ReSe2 the transition energies shift toward lower energy locations. The broadening parameters of the excitonic transition features show small increase for W-doped ReSe2 due to impurity scattering, whereas for Mo-doped samples the transition features broadened significantly. The influence of the varying concentrations of the two dopants will be compared and discussed.

An optical study of the band-edge property of ReX 2 (X = S, Se) has been carried out using the techniques of transmittance, photoreflectance (PR), and piezoreflectance (PzR). The polarized transmittance measurements indicate that the absorption edge of E∥b polarization presents a significant red-shift behavior with respect to that of the E? b polarization. Band-edge excitons E ex 1 and E ex 2 for ReX 2 are characterized using polarized PzR measurements. The polarization dependence of E ex 1 and E ex 2 provides conclusive evidence that the band-edge excitons are interband excitonic transitions originated from different origins. In addition higher energy excitonic series in ReX 2 layered compounds is also studied using low-temperature PR measurements at 25 K. Prominent and enlarged excitonic features positioned at higher energy side with respect to the E ex 1 and E ex 2 are observed in the PR spectra. The observed excitonic sequence has been shown to be corresponding to the Rydberg series starting with principal quantum number n = 2. From the experimental results together with the theoretical density-of-states calculations, the near band-edge property of rhenium disulfide (ReS 2 ) and diselenide (ReSe 2 ) are characterized and discussed.

Nickel-incorporated FeS2 single crystals with various Ni compositions of Fe0.99S2:Ni0.01, Fe0.98S2:Ni0.02, Fe0.96S2:Ni0.04, and Fe0.9S2:Ni0.1 were grown by chemical vapor transport (CVT) method using ICl3 as a transport agent. Physical properties of the Ni-incorporated FeS2 single crystals were characterized using X-ray diffraction, Raman spectroscopy, electrical conductivity, and photoconductivity (PC) measurements. By means of the analyses of the X-ray diffraction patterns, the whole series of Ni-doped FeS2 single crystals were determined to be single-phase and isostructural. Raman spectroscopy of the Ni-doped FeS2 crystals was carried out at room temperature. Raman resonant peaks of the Ni-doped FeS2 crystals demonstrate an energy red-shift behavior with respect to the increase of the dopant densities. Conductivity measurements show the resistivity of the Ni-doped FeS2 decreased as the doping concentration of Ni is increased. Nickel is an n-type dopant, which behaves like a donor level existed near the conduction band edge of the synthetic FeS2. On the other hand, dopant effect of nickel on the synthetic FeS2 also destroys the photoconductive sensitivity in the photoconductivity measurements.

The polarization sensitive behaviour of the band-edge transitions in ReS2 and ReSe2 layered compounds was studied using polarized-transmission and polarized-thermoreflectance (PTR) measurements with polarization angles from θ = 0° (-axis) to θ = 90° (-axis) at 300 K. The polarization dependence of the polarized energy gaps of ReS2 and ReSe2 shows a sinusoidal-like variation with respect to the angular change of the linearly polarized light. The angular dependences of the polarized energy gaps of ReS2 and ReSe2 were evaluated. The polarization sensitive behaviour of the band-edge excitons in rhenium disulfide and diselenide was characterized using angular dependent PTR measurements from θ = 0° to 90°. The polarized transition intensities of the band-edge excitons (E1ex and E2ex) of ReX2 (X = S, Se) demonstrate a sinusoidal variation with respect to the angular change of the linearly polarized light. The angular dependence of the polarized transition probabilities of E1ex and E2ex is analysed. The polarization sensitive behaviours of ReX2 (X = S, Se) layers are discussed.

We have characterized the temperature dependence of band-edge transitions of three (Zn0.38Cd0.62)1-xBexSe II–VI films with different Be concentrations x by using contactless electroreflectance (CER) and piezoreflectance (PzR) in the temperature range of 15 to 450 K. By a careful comparison of the relative intensity of PzR and CER spectra, the identification of light-hole (lh) and heavy-hole (hh) character of the excitonic transitions of the samples has been accomplished. The temperature dependence analysis yields information on the parameters that describe the temperature variations of the energy (including thermal expansion effects) and broadening parameter of the band edge transitions of ZnCdBeSe. The study shows that Be incorporation can effectively reduce the rate of temperature variation of the energy gap.

Single crystals of Mo-doped rhenium disulphide (ReS2) have been grown by the chemical vapour transport method using bromine as a transporting agent. Single-crystalline platelets of up to 5×5 mm2 surface area and 100 µm in thickness were obtained. From the x-ray diffraction patterns, the doped crystals are found to crystallize in the triclinic layered structure. The Hall coefficient measurement indicates that the samples are n-type in nature. The doping effects of the material are characterized by temperature-dependent conductivity, optical absorption and piezoreflectance measurements. The activation energies for the impurity carriers increase with doping. The indirect energy gap of the doped sample shows a slight red-shift. The direct band-edge excitonic transition energies remain unchanged, while the broadening parameter of the excitonic transition features increases due to impurity scattering.