Observation of a Phase Transformation of Ga2S3 in a Quartz Effusion Cell above 1230 K by Means of Neutron Scattering

Knudsen effusion mass spectrometry: Current and future approaches

RATIONALE

Knudsen effusion mass spectrometry (KEMS) has been a powerful tool in physical chemistry since 1954. There are many excellent reviews of the basic principles of KEMS in the literature. In this review, we focus on the current status and potential growth areas for this instrumental technique.

METHODS

We discuss (1) instrumentation, (2) measurement techniques, and (3) selected novel applications of the technique. Improved heating methods and temperature measurement allow for better control of the Knudsen cell effusive source. Accurate computer models of the effusive beam and its introduction to the ionizer allow optimization of such parameters as sensitivity and removal of background signals. Computer models of the ionizer allow for optimized sensitivity and resolution. Additionally, data acquisition systems specifically tailored to a KEMS system permit improved quantity and quality of data.

RESULTS

KEMS is traditionally utilized for thermodynamic measurements of pure compounds and solutions. These measurements can now be strengthened using first principles and model-based computational thermochemistry. First principles can be used to calculate accurate Gibbs energy functions (gefs) for improving third law calculations. Calculated enthalpies of formation and dissociation energies from ab initio methods can be compared to those measured using KEMS. For model-based thermochemistry, solution parameters can be derived from measured thermochemical data on metallic and nonmetallic solutions. Beyond thermodynamic measurements, KEMS has been used for many specific applications. We select examples for discussion: measurements of phase changes, measurement/control of low-oxygen potential systems, thermochemistry of ultrahigh-temperature ceramics, geological applications, nuclear applications, applications to organic and organometallic compounds, and thermochemistry of functional room temperature materials, such as lithium ion batteries.

CONCLUSIONS

We present an overview of the current status of KEMS and discuss ideas for improving KEMS instrumentation and measurements. We discuss selected KEMS studies to illustrate future directions of KEMS.

Polymorphic Ga2S3 nanowires: phase-controlled growth and crystal structure calculations

The polymorphism of nanostructures is of paramount importance for many promising applications in high-performance nanodevices. We report the chemical vapor deposition synthesis of Ga₂S₃ nanowires (NWs) that show the consecutive phase transitions of monoclinic (M) → hexagonal (H) → wurtzite (W) → zinc blende (C) when lowering the growth temperature from 850 to 600 °C. At the highest temperature, single-crystalline NWs were grown in the thermodynamically stable M phase. Two types of H phase exhibited 1.8 nm periodic superlattice structures owing to the distinctively ordered Ga sites. They consisted of three rotational variants of the M phase along the growth direction ([001]_M = [0001]_H/W) but with different sequences in the variants. The phases shared the same crystallographic axis within the NWs, producing novel core-shell structures to illustrate the phase evolution. The relative stabilities of these phases were predicted using density functional theory calculations, and the results support the successive phase evolution. Photodetector devices based on the p-type M and H phase Ga₂S₃ NWs showed excellent UV photoresponse performance.

Single-Crystalline γ-Ga2S3 Nanotubes via Epitaxial Conversion of GaAs Nanowires

The chemical transformation of nanowire templates into nanotubes is a promising avenue toward hollow one-dimensional (1D) nanostructures. To date, high-quality single crystalline tubes of nonlayered inorganic crystals have been obtained by solid-state reactions in diffusion couples of nanowires with deposited thin film shells, but this approach presents issues in achieving single-phase tubes with a desired stoichiometry. Chemical transformations with reactants supplied from the gas- or vapor-phase can avoid these complications, allowing single-phase nanotubes to be obtained through self-termination of the reaction once the sacrificial template has been consumed. Here, we demonstrate the realization of this scenario with the transformation of zincblende GaAs nanowires into single-crystalline cubic γ-Ga₂S₃ nanotubes by reaction with sulfur vapor. The conversion proceeds via the formation of epitaxial GaAs-Ga₂S₃ core-shell structures, vacancy injection and aggregation into Kirkendall voids, elastic relaxation of the detached Ga₂S₃ shell, and finally complete incorporation of Ga in a crystalline chalcogenide tube. Absorption and luminescence spectroscopy on individual nanotubes show optoelectronic properties, notably a ∼3.1 eV bandgap and intense band-edge and near band-edge emission consistent with high-quality single crystals, along with transitions between gap-states due to the inherent cation-vacancy defect structure of Ga₂S₃. Our work establishes the transformation of nanowires via vapor-phase reactions as a viable approach for forming single-crystalline hollow 1D nanostructures with promising properties.

Nonlayered Two-Dimensional Defective Semiconductor γ-Ga2S3 toward Broadband Photodetection

Two-dimensional (2D) materials exhibit high sensitivity to structural defects due to the nature of interface-type materials, and the corresponding structural defects can effectively modulate their inherent properties in turn, giving them a wide application range in high-performance and functional devices. 2D γ-Ga₂S₃ is a defective semiconductor with outstanding optoelectronic properties. However, its controllable preparation has not been implemented yet, which hinders exploring its potential applications. In this work, we introduce nonlayered γ-Ga₂S₃ into the 2D materials family, which was successfully synthesized via the space-confined chemical vapor deposition method. Its intriguing defective structure are revealed by high-resolution transmission electron microscopy and temperature-dependent cathodoluminescence spectra, which endow the γ-Ga₂S₃-based device with a broad photoresponse from the ultraviolet to near-infrared region and excellent photoelectric conversion capability. Simultaneously, the device also exhibits excellent ultraviolet detection ability ( R_λ = 61.3 A W^-1, I_on /I_off = 851, EQE = 2.17× 10⁴ %, D* = 1.52× 10¹⁰ Jones @350 nm), and relatively fast response (15 ms). This work provides a feasible way to fabricate ultrathin nonlayered materials and explore the potential applications of a 2D defective semiconductor in high-performance broadband photodetection, which also suggests a promising future of defect creation in optimizing photoelectric properties.

Optically decomposed near-band-edge structure and excitonic transitions in Ga₂S₃

The band-edge structure and band gap are key parameters for a functional chalcogenide semiconductor to its applications in optoelectronics, nanoelectronics, and photonics devices. Here, we firstly demonstrate the complete study of experimental band-edge structure and excitonic transitions of monoclinic digallium trisulfide (Ga₂S₃) using photoluminescence (PL), thermoreflectance (TR), and optical absorption measurements at low and room temperatures. According to the experimental results of optical measurements, three band-edge transitions of E_A = 3.052 eV, E_B = 3.240 eV, and E_C = 3.328 eV are respectively determined and they are proven to construct the main band-edge structure of Ga₂S₃. Distinctly optical-anisotropic behaviors by orientation- and polarization-dependent TR measurements are, respectively, relevant to distinguish the origins of the E_A, E_B, and E_C transitions. The results indicated that the three band-edge transitions are coming from different origins. Low-temperature PL results show defect emissions, bound-exciton and free-exciton luminescences in the radiation spectra of Ga₂S₃. The below-band-edge transitions are respectively characterized. On the basis of experimental analyses, the optical property of near-band-edge structure and excitonic transitions in the monoclinic Ga₂S₃ crystal is revealed.