Gallium Sulfide Research Articles

Versatile few-cycle high-energy MID-IR OPCPA for nonlinear optics, spectroscopy and imaging

High-power, high-energy, ultrashort, mid-infrared (MID-IR) laser systems operating at high repetition rates are of considerable interest for many science applications, such as coherent vibrational spectroscopy, label-free imaging, time-resolved pump-probe and high-harmonic generation studies. We developed an optical parametric chirped-pulse amplifier (OPCPA) system employing a difference-frequency generation in a lithium gallium sulfide nonlinear crystal in the final amplifier stage, which provides in principle the possibility for passive carrier-envelop-phase (CEP) stability. The OPCPA efficiently down-converts a 1 μm 200 μJ Yb-YAG pump pulse into the MID-IR spectral range generating μJ-level pulses at a repetition rate of 200 kHz. Two modes of operations providing complimentary MID-IR pulse properties are presented. Depending on the envisaged application, one can switch between (a) a wavelength-tunable (4.2–11 μm) source and (b) a broadband source centered at ≈8.5 μm by controlling the group-delay dispersion of the signal pulse. The broadband, high-energy MID-IR pulses have a short pulse duration of 74±2 fs, which corresponds to only ≈3 optical cycles at the central wavelength of 8.5 μm.

Screening Switching Materials with Low Leakage Current and High Thermal Stability for Neuromorphic Computing

AbstractNeuromorphic computing implemented with spiking neural networks is an energy efficient computing paradigm to break through the Von Neumann bottleneck in the future. Ovonic threshold switching (OTS) selector is considered to be a promising spiking neuron candidate. As ≈1011 artificial neurons are needed for brain‐inspired computing, leakage current of OTS devices would waste enormous power. OTS devices with ultralow leakage current are deeply desired. Since the leakage current is closely related to the bandgap of OTS materials and only the amorphous one shows the volatile switching property, here binary gallium sulfide (GaS), characterized by 2.53 eV large band‐gap and ≈550 °C high crystallization temperature is singled out, as the most appealing OTS material. High‐density trap states, an essential prerequisite of the OTS material, are detected in the amorphous GaS. Indeed, the OTS behaviors of the GaS‐based device are directly observed. The simple OTS device could provide 21.23 MA cm−2 large ON current density, and ≈10−8 A low OFF current and ≤10 ns switching speed. Furthermore, it is also used to build GaS‐based leakage integrate and fire neurons for future neuromorphic computing. This study provides a new idea for material design before device preparation and takes a neuron as an example.

Displacement damage dose analysis of the output characteristics of In0.5Ga0.5P and Cu(In,Ga)(S,Se)2 solar cells irradiated with alpha ray simulated helium ions

To investigate the applicability of radiation-hard indium–gallium–phosphide (InGaP) and copper–indium–gallium–sulfide–selenide (CIGS) solar cells to dosimeter devices without any modification, we irradiated high-energy He+ ions, which were simulated α-ray particles, to an InGaP and a CIGS solar cell. We found that both types of solar cells have sufficient resistance to He+ ions. By using displacement damage dose (DDD) analysis, the obtained He+ ion-induced degradation trends were compared with those induced by high-energy electrons and protons, and we found that the degradation trends due to He+-ions, electrons, and protons aligned on the same curve when we plotted the data as a function of a modified DDD conversion equation, which originally was applied to space solar cells. The obtained DDD formulas enable us to predict the device lifetime or correction of an output signal for degradation when such solar cells are employed as a dosimeter.

Synthesis of Hexagonal Structured GaS Nanosheets for Robust Femtosecond Pulse Generation.

Gallium sulfide (GaS), with a hexagonal structure, has received extensive attention due to its graphene-like structure and derived optical properties. Here, high-quality GaS was obtained via chemical vapor synthesis and then prepared as a saturable absorber by the stamp-assisted localization-transfer technique onto fiber end face. The stability of the material and the laser damage threshold are maintained due to the optimized thickness and the cavity integration form. The potential of the GaS for nonlinear optics is explored by constructing a GaS-based Erbium-doped mode-locked fiber laser. Stable femtosecond (~448 fs) mode-locking operation of the single pulse train is achieved, and the robust mode-locked operation (>30 days) was recorded. Experimental results show the potential of GaS for multi-functional ultrafast high-power lasers and promote continuous research on graphene-like materials in nonlinear optics and photonics.

Design of broadband and compact mid-infrared polarization beam splitter based on all-circular air holes with low loss

A mid-infrared (mid-IR) broadband polarization beam splitter (PBS) based on a gallium sulfide dual core highly birefringent photonic crystal fiber is proposed. The high birefringence and dual core are achieved by introducing two kinds of circular air holes with different diameters into the fiber core. The mode coupling characteristics of the even and odd modes in the proposed dual core PBS are investigated using the finite-difference time-domain method and coupled-mode theory. The numerical simulation results indicate that, when the incident light frequency is 4.5 μm and the PBS length is 277.1 μm, a wide bandwidth of 170 nm (from 4.472 to 6.642 μm) with a polarization extinction ratio (ER) >20 dB is obtained in the mid-IR waveband. The highest ER of 112.8 dB is realized. The designed PBS will find potential applications in mid-IR optical sensing, communication, and imaging systems because of its wide bandwidth and ultrashort length.

Prediction of intermediate band in Ti/V doped γ-In2S3.

Materials with an intermediate energy band (IB) introduced in the forbidden gap are viable alternatives to tandem configurations of solar cells for increasing the photon-conversion efficiency. One of the aspiring designs proposed for the intermediate band concept is hyperdoped (Ti, V):In2S3. Being very important in copper indium gallium sulfide (CIGS) solar cells, indium thiospinel (In2S3) is known for its three different temperature as well as pressure, polymorphs. The most stable β-In2S3 was experimentally shown to have an isolated intermediate band (IB) and exhibits sub-band gap absorption due to the completely filled IB after V-doping. Though experimental observation holds a positive signature, recent DFT studies did not show a metallic intermediate band for the V dopant in the 3+ charge state. In order to clarify this, we have taken incentive from experimental XRD analysis that V-doped β-In2S3 shows peaks from disordered In vacancies (either α or γ), in addition to the ordered In vacancies expected. Hence, we have carried out state-of-the-art DFT based computations on pure and Ti, V-doped In2S3 in the γ-phase which has not been studied yet. We considered the Ti and V dopants in various charge states. Our theoretical study including hybrid functional, does in fact find the IB in V-doped γ-In2S3. However, at equilibrium the IB lies in between the Fermi level (EF) and conduction band minimum (CBM).

Mid-infrared photon pair generation in AgGaS2

We demonstrate nondegenerate photon pair generation by spontaneous parametric downconversion in a silver gallium sulfide AgGaS2 crystal. By tuning the pump wavelength, we achieve phase matching over a large spectral range. This allows to generate idler photons in the mid-infrared spectral range above a 6 μm wavelength with corresponding signal photons in the visible. Also, we show photon pair generation with broad spectral bandwidth. These results are a valuable step toward the development of quantum imaging and sensing techniques in the mid-infrared.

Efficiency enhancement in photoelectrochemical water splitting: Defect passivation and boosted charge transfer kinetics of zinc oxide nanostructures via chalcopyrite/chalcogenide mix sensitization

ZnO thin films in nanorod (NR) and nanoflower (NF) morphologies were used as photoelectrode scaffolds for efficient visible-light-driven photoelectrochemical (PEC) water splitting process, where their decoration with copper indium gallium sulfide (CIGS) and indium sulfide (${\mathrm{In}}_{2}{\mathrm{S}}_{3}$) layers resulted in significant PEC performance enhancement. ZnO NF/CIGS/${\mathrm{In}}_{2}{\mathrm{S}}_{3}$ photoelectrodes exhibited a remarkably high PEC efficiency ($\ensuremath{\sim}6.0%$ applied bias photon-to-current efficiency, 83% incident photon-to-current efficiency) due to the negligible dark current, while ZnO NR/CIGS/${\mathrm{In}}_{2}{\mathrm{S}}_{3}$ generated a photocurrent density of $30.0\phantom{\rule{0.28em}{0ex}}\mathrm{mA}.\mathrm{c}{\mathrm{m}}^{--2}$ at 0.4 V (vs Ag/AgCl), being one of the highest performances reported in the literature for copper-based chalcopyrite photoelectrodes so far. The interfacial photoelectrode-electrolyte charge transport dynamics, investigated via intensity-modulated photocurrent spectroscopy, exhibited a sevenfold increase in charge transfer efficiencies with a significant drop in surface recombination kinetics for ZnO NF after CIGS/${\mathrm{In}}_{2}{\mathrm{S}}_{3}$ decoration. The obtained results show consistency with numerically modeled electric field distribution profiles and electron paramagnetic resonance results of ZnO NF, rationalizing the enhanced charge transfer rates for decorated samples and confirming the defect passivating nature of CIGS/${\mathrm{In}}_{2}{\mathrm{S}}_{3}$.

Exploring the Thickness-Dependence of the Properties of Layered Gallium Sulfide.

Group III layered monochalcogenide gallium sulfide, GaS, is one of the latest additions to the two-dimensional (2D) materials family, and of particular interest for visible-UV optoelectronic applications due to its wide bandgap energy in the range 2.35–3.05 eV going from bulk to monolayer. Interestingly, when going to the few-layer regime, changes in the electronic structure occur, resulting in a change in the properties of the material. Therefore, a systematic study on the thickness dependence of the different properties of GaS is needed. Here, we analyze mechanically exfoliated GaS layers transferred to glass substrates. Specifically, we report the dependence of the Raman spectra, photoluminescence, optical transmittance, resistivity, and work function on the thickness of GaS. Those findings can be used as guidance in designing devices based on GaS.

Two-Dimensional Gallium Sulfide as a Novel Saturable Absorber for Broadband Ultrafast Photonics Applications.

Two-dimensional (2D) gallium sulfide (GaS) offers a plethora of exceptional electrical and optical properties, allowing it to be used in a wide range of applications, including photodetectors, hydrogen generation, and nonlinear optical devices. In this paper, ultrathin 2D GaS nanosheets are synthesized using the liquid-phase exfoliation method, and the structure, morphology, and chemical composition of the as-prepared nanosheets are extensively investigated. After depositing 2D GaS nanosheets on side polished fibers, successful saturable absorbers (SAs) are fabricated for the first time. The realized modulation depths are 10 and 5.3% at 1 and 1.5 μm, respectively, indicating the wideband saturable absorption performance of the prepared SAs. By integrating GaS-SAs into three different wavelength-based fiber laser cavities, stable mode-locked pulses are achieved, having pulse durations of 46.22 ps (1 μm), 614 fs (1.5 μm), and 1.02 ps (2 μm), respectively. Additionally, different orders of harmonic mode-locked pulses with the highest repetition rate of 0.55 GHz (45th order) and Q-switched pulses with the shortest pulse duration of 2.2 μs are obtained in the telecommunication waveband. These findings suggest that 2D GaS has a lot of potential for broadband ultrafast photonics in nonlinear photonics devices.

Photoluminescence Stability Enhancement of Ag–In–Ga–S/GaSx Core/Shell Quantum Dots with Thicker Shells by the Addition of Gallium Diethyldithiocarbamate

A spectrally narrow band edge emission is generated in the green region from cadmium-free quantum dots comprised of silver indium gallium sulfide coated by gallium sulfide shells. The thickness of the gallium sulfide shell is increased by using gallium tris(diethyldithiocarbamate) as a supplementary precursor showing a strong adhesion with the quantum dot cores. Photoluminescence intensity and spectral shape of the core/shell quantum dots with the thicker shells are unchanged after 90 days.

Ultrafast Nonlinear Optical Response and Carrier Dynamics in Layered Gallium Sulfide (GaS) Single-Crystalline Thin Films

Gallium sulfide (GaS) is a layered metal monochalcogenide semiconductor that has recently garnered considerable attention in various fields. In this study, we investigated the nonlinear absorption characteristics of multilayer β-GaS thin films on sapphire substrate by using femtosecond open-aperture Z-scan method. The β-GaS films exhibit saturable absorption behavior at 532 nm while nonlinear absorption appears under 650 nm excitation. The nonlinear absorption coefficient of β-GaS was determined to be −1.8 × 10–8 m/W and 4.9 × 10–8 m/W at 532 and 650 nm, respectively. The carrier dynamics of β-GaS films was studied via femtosecond transient absorption (TA) measurements. The TA results demonstrated that β-GaS films have broad photo-induced absorption in the visible regime and sub-nanosecond lifetime. Our results indicate that gallium sulfide has large nonlinear optical response and long carrier lifetime, which could be applied in future photonic devices.

Quick and reliable colorimetric reflectometry for the thickness determination of low-dimensional GaS and GaSe exfoliated layers by optical microscopy

Interest in gallium chalcogenides, i.e., gallium sulfide (GaS) and gallium selenide (GaSe), is growing rapidly due to its layered structure compatible with the fabrication of very thin layers by mechanical exfoliation and its wide band gap desirable for the design and fabrication of visible-UV optoelectronic devices. It is well known that the properties of these materials depend on their thickness; therefore, a facile and fast method is needed to infer the thickness of layered GaS and GaSe. Here, we report and validate a colorimetric method based on optical imaging for the quick and reliable quantitative determination of the thickness of exfoliated GaS and GaSe layers although it can be extended to other layered systems. For the validation of the method, the colorimetric computational estimate of the thickness is compared to the value obtained by atomic force microscopy. Further simulation of GaS and GaSe layers on different substrates of interest for different technological applications is provided as a quick guide for the rapid and reliable thickness determination of GaS and GaSe layers on various substrates.

A Discrete Ligand-Free T3 Supertetrahedral Cluster of Gallium Sulfide.

Large discrete supertetrahedral clusters of metal chalcogenides are rare due to the difficulty of crystallizing solids in which the negative charge of the cluster is balanced by the positive charges of the countercations. Here, we describe a discrete ligand-free T3 supertetrahedral cluster, [Ga10S16(SH)4]6−, which was successfully synthesized in the presence of the superbase 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) using the neutral surfactant polyethyleneglycol (PEG)-400 as the reaction solvent. Protonated DBUH+ cations are incorporated into the crystal structure of the product, which can be formulated as [C9H17N2]6[Ga10S16(SH)4]. This compound, which represents the first example of a discrete ligand-free T3 cluster of gallium sulfide, was fully characterized by single-crystal and powder X-ray diffraction, elemental analysis, infrared spectroscopy, thermogravimetric analysis, and ultraviolet-visible diffuse reflectance. The results presented here indicate that the use of surfactants as solvents offers potential for the preparation of new compounds containing supertetrahedral clusters.

Luminescent Quaternary Ag(InxGa1-x)S2/GaSy Core/Shell Quantum Dots Prepared Using Dithiocarbamate Compounds and Photoluminescence Recovery via Post Treatment.

Cadmium-free quantum dots (QDs) consisting of silver-indium-gallium-sulfide (AIGS) quaternary semiconductors were successfully synthesized using a metal-dithiocarbamate complex with sufficiently high reactivity to produce metal sulfides. The introduction of a gallium diethyldithiocarbamate precursor decreased the reaction temperature to produce active intermediates, which were subsequently converted into AIGS QDs at 150 °C with silver and indium acetates. Because of the low reaction temperature, AIGS QDs with a tetragonal crystal phase were produced selectively, which favorably generated band-edge emission whose full width at half-maximum is smaller than 40 nm after they were coated with gallium sulfide (GaSy) shells. The compositional indium/gallium ratio was varied by changing the mixing ratio of the precursors used for the synthesis of the AIGS core, and the band-edge photoluminescence (PL) generated from the AIGS/GaSy core/shell QDs was blue-shifted with an increase in the gallium content in the core. Consequently, a pure green emission centered at 518 nm was obtained with a PL quantum yield as high as 68%.

Theoretical investigation of lattice dynamics, infrared reflectivity, polarized Raman spectra and nature of interlayer coupling in two-dimensional layered gallium sulfide

Gallium sulfide (GaS) is a highly promising two-dimensional layered semiconductor owing to its remarkable thickness dependent electronic and physical properties. In this article, we perform a comprehensive ab initio study of lattice dynamics, mode symmetry assignments, polarized Raman and infrared (IR) reflectivity spectra of GaS system. Polarized Raman spectra are obtained for different light polarization set-ups of incoming and scattered light. The frequencies of all allowed vibrational modes at the zone-centre are calculated and symmetry labels are assigned. Furthermore, the variation of frequencies & intensities of Raman/IR active modes of ultrathin GaS films (few layers) as function of film thickness is studied. In addition, we also explore the nature of weak interlayer coupling in GaS. The weak forces between the GaS layers are usually assumed to be due to interlayer van der Waals (vdW) interaction. However, this assumption has not been reasonably explained in reported experimental studies. Our study strongly suggests that weak interlayer interactions in GaS may be primarily electrostatic (Coulomb) in nature and therefore the contribution of vdW interactions to layer–layer coupling and lattice dynamics may be significantly lower than that of electrostatic interaction. The suggested nature of interlayer coupling in GaS and related III–VI semiconductors may have important implications in determination of their various physical properties.

Defect-engineered two-dimensional layered gallium sulphide molecular gas sensors with ultrahigh selectivity and sensitivity

Detecting toxic gases, especially greenhouse gases, is one of the most challenging and complex tasks to maintain a sustainable environment. Based on first-principles theoritical calculations, we investigate the sensing performance of HCN, NH3 and SO2 gas molecules on GaS ML with and without Ga-/S-vacancy defects. The selectivity was investigated based on the electronic properties, charge transfer and adsorption energy of the most stable geometries of the adsorbed system. We predicted that HCN, NH3 and SO2 are physisorbed on GaS ML with and without Ga-/S-vacancy defects. The results show that Ga-/S-vacancy defective GaS ML exhibit stronger physisorption with HCN, NH3 and SO2 gas, which directly influences the sensing performance. The band structure and density of states indicate that electronic properties of Ga-/S-vacancy defective GaS ML have been altered upon gas molecules adsorption. We predicted ultrahigh selectivity with good recovery time for HCN, NH3 and SO2 gases on the defective GaS ML than pristine GaS ML. This suggests that vacancy defect is a highly active catalytic site to capture small gas molecules than pristine GaS ML. The theoretical findings suggest that Ga-/S-vacancy defective GaS ML can be used as a promising device for gas sensing applications and help guide experimentalists to develop better vacancy defective materials for efficient gas detection or sensing applications.

Mixed Solvothermal Synthesis of Tn Cluster-Based Indium and Gallium Sulfides Using Versatile Ammonia or Amine Structure-Directing Agents.

Metal chalcogenide supertetrahedral Tn clusters are of current interest for their unique compositions and structures, which rely highly on the structure-directing agents. Herein, we report four novel Tn cluster-based indium and gallium sulfides, namely, [NH(CH3)3]4In4S10H4 (1), (NH3)4Ga4S6 (2), [NH3CH2CH3]5(NH2CH2CH3)2Ga11S19 (3), and [NH3CH2CH2OH]6Ga10S18·2NH2CH2CH2OH (4). All four compounds were solvothermally synthesized in mixed amine-ethanol solutions or deep eutectic solvent (DES), where ammonia/amine molecules play significant structure-directing roles in the speciation and crystal growth. (1) Being protonated, the trimethylamine and ethanolamine molecules surround the T2-[In4S10H4]4- clusters (for 1) and [Ga10S18]n6n- open framework (for 4), respectively, compensating for the negative charge of the inorganic moieties. (2) With the lone pair of electrons, the ammonia molecules in 2 coordinate directly to corner Ga3+ ions of the {Ga4S6} cage to give a neutral T2-(NH3)4Ga4S6 cluster. (3) For compound 3, part of the ethylamine molecules act as terminating ligands for the T1 and T3 units in the [Ga11S19(NH2CH2CH3)2]n5n- layer, while the rest act as interlamellar countercations upon protonation. Theoretical studies reveal the contributions of N, C, and H to the density of states (DOS) for 2 and 3 because of their hybrid structures that combine the ammonia/amine ligands with sulfide moieties together.

Structure, optical, and electrical characterization of nanostructured gallium sulfide Ga2S3 thin films for electronic and solar cell applications

Ga2S3 thin films with different thicknesses (32, 60, 100, and 140 nm) were prepared using inert gas condensation technique using Argon gas. The structures of the used powder and films were studied using X-ray diffraction. Observation of the surface topography was carried out using transmission electron microscope, which showed that the grain size increases with film thickness for these studied samples, and diffraction patterns which illustrated that the deposited films have nanocrystalline structure. The optical transmittance, reflectance, and absorbance of these films were measured. It was found that these sample had a direct optical energy gap. This optical energy gap was calculated for these films. This energy gap increased with film thicknesses; on the other hand the Urbach tail values of these samples decreased with film thickness. The electrical resistivity for these films was determined and was found that these values decreased with film thickness.

Enhancing SiN waveguide optical nonlinearity via hybrid GaS integration

Silicon nitride (SiN) has been demonstrated as an exceptional low-loss platform for integrated photonics, however, its relatively small nonlinear refractive index limits its full potential in applications such as all-optical phase modulation, supercontinuum generation, and frequency combs. Here, we demonstrate that through hybrid integration with wide-bandgap 2D materials such as gallium sulfide (GaS), the nonlinear refractive index of the hybrid waveguide can be greatly increased (by five-fold in our experiment). We fabricate high quality factor SiN/GaS microring resonators, with low two-photon absorption at telecom wavelengths, to demonstrate enhanced all-optical modulation. We show that the hybrid platform is suitable for assessing the nonlinear properties of 2D materials, and we have used it to quantify, for the first time, the nonlinear refractive index (n 2) of GaS. For GaS, n 2 = , which is ten times larger than that of SiN.