Self-assembled metastable γ-Ga2O3 nanoflowers with hexagonal nanopetals for solar-blind photodetection

CO2 Stress-Driven Room Temperature Ferromagnetism of Ultrathin 2D Gallium Oxide

Spintronic devices work by manipulating the spin of electrons other than charge transfer, which is of revolutionary significance and can largely reduce energy consumption in the future. Herein, ultrathin two-dimensional (2D) non-van der Waals (non-vdW) γ-Ga2O3 with room temperature ferromagnetism is successfully obtained by using supercritical CO2 (SC CO2). The stress effect of SC CO2 under different pressures selectively modulates the orientation and strength of covalent bonds, leading to the change of atomic structure including lattice expansion, introduction of O vacancy, and transition of Ga-O coordination (GaO4 and GaO6). Magnetic measurements show that pristine γ-Ga2O3 is nonferromagnetic, whereas the SC CO2 treated γ-Ga2O3 exhibits obvious ferromagnetic behavior with an optimal magnetization of 0.025 emu g-1 and a Curie temperature of 300 K.

Tackling Disorder in γ-Ga2 O3

Ga₂ O₃ and its polymorphs are attracting increasing attention. The rich structural space of polymorphic oxide systems such as Ga₂ O₃ offers potential for electronic structure engineering, which is of particular interest for a range of applications, such as power electronics. γ-Ga₂ O₃ presents a particular challenge across synthesis, characterization, and theory due to its inherent disorder and resulting complex structure-electronic-structure relationship. Here, density functional theory is used in combination with a machine-learning approach to screen nearly one million potential structures, thereby developing a robust atomistic model of the γ-phase. Theoretical results are compared with surface and bulk sensitive soft and hard X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, spectroscopic ellipsometry, and photoluminescence excitation spectroscopy experiments representative of the occupied and unoccupied states of γ-Ga₂ O₃ . The first onset of strong absorption at room temperature is found at 5.1 eV from spectroscopic ellipsometry, which agrees well with the excitation maximum at 5.17 eV obtained by photoluminescence excitation spectroscopy, where the latter shifts to 5.33 eV at 5 K. This work presents a leap forward in the treatment of complex, disordered oxides and is a crucial step toward exploring how their electronic structure can be understood in terms of local coordination and overall structure.

Y2O3: Eu3+/PMMA hybrid film as a converter for enhanced harvesting of broadband solar-blind UV light

At present, the most common materials for solar-blind UV light detectors are wide band-gap semiconductors, which generally have high requirements and complex methods for preparation. Ordinary semiconductor materials such as silicon, TiO₂, and Cu₂O were industrialized, but they were excluded for direct harvest of solar-blind UV light due to their inability to absorb solar-blind light photons. Here, inorganic-organic hybrid film of Y₂O₃:Eu³⁺/PMMA was used as a spectral converter to realize the detection of broadband solar-blind UV light by ordinary semiconductor, converting broadband solar-blind UV luminescence to visible luminescence based on down-conversion process, after which the visible luminescence was detected by the Si photo-resister. The results show that hybrid film based on rare earth luminescence materials is particularly valuable for broadband solar-blind UV detection.

Ultrawide Band Gap Oxide Nanodots (Eg > 4.8 eV) for a High-Performance Deep Ultraviolet Photovoltaic Detector

Recently, deep ultraviolet (DUV) detectors based on gallium oxide (Ga₂O₃) have become promising in industrial and aerospace applications because of their inherently ultrawide band gaps (4.5-4.9 eV). Because most of them are difficult to be prepared, the lattice mismatch with the substrate and the expensive cost have to be taken into consideration. Because of such problems, the solution-processible nanodots (NDs) with ultrasmall size provide a solution. Here, we propose to use γ-Ga₂O₃ NDs as the DUV-sensitive layer to construct a DUV p-i-n-type detector with photovoltaic properties (p-graphene/γ-Ga₂O₃ NDs/n-SiC). The device exhibits a high photoresponsivity (5.8 mA/W) and detectivity (7.6 × 10¹⁰ jones) with a 250 nm source illumination under 0 V bias. Moreover, the DUV/UV injection ratio (R₂₅₀/R₃₆₀) reaches 10³. These results demonstrate a new way to manufacture low-cost, high-performance DUV detectors based on γ-Ga₂O₃.

Recent advances in solution-processed photodetectors based on inorganic and hybrid photo-active materials

Due to their excellent and tailorable optoelectronic performance, low cost, facile fabrication, and compatibility with flexible substrates, solution-processed inorganic and hybrid photo-active materials have attracted extensive interest for next-generation photodetector applications. This review gives a comprehensive compilation of solution-processed photodetectors. The basic structures of the device and important parameters of photodetectors will be firstly summarized. Then the development of various solution processing technologies containing solution synthesis and liquid phase film-forming processes for the preparation of semiconductor films is described. From the materials science point of view, we give a comprehensive overview about the current status of solution processed semiconductor materials including inorganic and hybrid photo-active materials for the application of photodetectors. Moreover, challenges and future trends in the field of solution-processed photodetectors are proposed.

One-Step Growth of Amorphous/Crystalline Ga2O3 Phase Junctions for High-Performance Solar-Blind Photodetection

The pursuit of high-performance photodetectors functioning in the solar-blind spectrum is motivated by both scientific and practical applications ranging from secure communication, monitoring, sensing, etc. In particular, the fabrication of heterojunctions based on the wide band gap semiconductors has emerged as an attractive strategy to promote the high-efficient photogenerated electron/hole pair separation. However, the precisely controlled growth of heterojunctions remains a huge challenge. The lattice mismatch leads to the formation of defects and/or dislocations at the interface, deteriorating the performance of devices and limiting their envisioned applications. Here, we demonstrate a simple one-step growth of amorphous/crystalline Ga₂O₃ phase junctions by using sputtering technique, yielding a large responsivity of 0.81 A/W, a superior photo-to-dark current ratio over 10⁷, and an ultrahigh response speed of ∼12 ns. Compared to the previous reported solar-blind photodetectors, the obtained detectivity ≈ 5.67 × 10¹⁴ Jones is increased by 2 orders of magnitude. Such excellent photoresponse characteristics can be understood by the interfacial built-in field-promoted electron/hole pair separation for the amorphous/crystalline Ga₂O₃ phase junctions. Our results provide a novel path toward realizing high-performance optoelectronics functioning in the solar-blind spectrum.

Hydrothermal Approach to Spinel-Type 2D Metal Oxide Nanosheets

The peculiar physical and chemical properties of 2D nanostructures have aroused global research interest in developing new members, synthetic technology, and exploring their potential applications in functional nanodevices. However, it is extremely challenging to directly obtain the 2D nanosheets for these extrinsic layered structures using conventional routines. In this work, we demonstrate the facile and general synthesis of 2D spinel-type metal oxides nanosheets through a simple hydrothermal reaction. Using this method, cubic γ-Ga₂O₃, ZnGa₂O₄ and MnGa₂O₄ nanosheets with triangular/hexagonal configuration and ultrathin thickness have been synthesized, and all these nanosheets show preferential growth along (111) plane with the minimum formation energy. Microstructural and composition analyses using HRTEM, EDS, XPS, and so on reveal that the as-synthesized 2D nanosheets are well-crystallized in cubic fcc-phase and show high purity in composition, and the formation process of MGa₂O₄ nanosheets can be regarded as the competition of M²⁺ and Ga³⁺ in tetrahedral site. Spatially resolved cathodoluminescence measurement of individual 2D nanosheet shows that the γ-Ga₂O₃, ZnGa₂O₄, and MnGa₂O₄ nanosheets exhibit distinct luminescence behavior, and ZnGa₂O₄ nanosheets show the strongest emission in visible region. It is expected that the facile synthesis of spinel-type metal oxides of γ-Ga₂O₃, ZnGa₂O₄, and MnGa₂O₄ nanosheets will further promote the exploration of a variety of semiconductor nanostructures that could not be achieved using conventional technology suitable for layered structures and will also open up some opportunities for the integration of advanced functional nanodevices such as photodetectors, phosphors on the basis of them.

“Two in one”: an unprecedented mixed anion, Ba2(C3N3O3)(CNO), with the coexistence of isolated sp and sp2 π-conjugated groups

The first mixed cyanate–cyanurate double salt Ba₂(C₃N₃O₃)(CNO) containing two types of π-conjugated groups was successfully synthesized and systematically characterized.

Oxygen vacancy modulation of two-dimensional γ-Ga2O3 nanosheets as efficient catalysts for photocatalytic hydrogen evolution

Controlling the creation of oxygen vacancies can effectively regulate the optical and electronic properties of metal oxide nanomaterials. Over the past several decades, numerous metal oxides with oxygen vacancies have been developed. However, an investigation about oxygen vacancies leading to the formation of nanosheets with different thicknesses has not been available up to now. Here, we report the oxygen vacancy modulated formation of γ-Ga₂O₃ nanosheets and demonstrate that the thickness of the nanosheets is not the decisive factor in the photocatalytic hydrogen evolution reaction of ultrathin 2D nanosheets. Detailed structural characterization indicated that γ-Ga₂O₃ prepared at 160 °C (γ-160) with a morphology of ultrathin nanosheets possesses the highest oxygen vacancy concentration and an optimal thickness of the nanosheets. The enhanced photocatalytic performance could be determined from the synergistic effects between the ultrathin 2D structure and the O-vacancies confined in the ultrathin nanosheets. This work provides an efficient strategy to regulate the formation of nanosheets at the atomic scale and enrich the study on the effect of oxygen vacancies in the photocatalytic water splitting reaction.

Phase Transition of Two-Dimensional β-Ga2O3 Nanosheets from Ultrathin γ-Ga2O3 Nanosheets and Their Photocatalytic Hydrogen Evolution Activities

Monoclinic β-Ga₂O₃ nanosheets hold great potential applications in electronic, optical, and photocatalytic fields. In this study, two-dimensional β-Ga₂O₃ nanosheets were successfully fabricated through a simple crystalline phase transition from the as-prepared ultrathin γ-Ga₂O₃ nanosheets. The photocatalytic hydrogen evolution reaction under UV light irradiation was achieved on the two kinds of photocatalysts. However, β-Ga₂O₃ with a higher crystallinity shows a lower photocatalytic activity in comparison with γ-Ga₂O₃. The average apparent quantum yield is calculated to be 0.29% for β-Ga₂O₃ nanosheets and 1.82% for γ-Ga₂O₃. More efficient separation and transfer rates of photogenerated carriers and larger specific areas were found in γ-Ga₂O₃. On the basis of the analysis of the structures of γ-Ga₂O₃ and β-Ga₂O₃, it is proposed that the disordered or defective structure contributes to the improvement of photocatalytic activity to some extent. Therefore, it is significant to develop the photocatalyst with a stable structure and a certain number of defects at the same time.

Zero-Power-Consumption Solar-Blind Photodetector Based on β-Ga2O3/NSTO Heterojunction

A solar-blind photodetector based on β-Ga₂O₃/NSTO (NSTO = Nb:SrTiO₃) heterojunctions were fabricated for the first time, and its photoelectric properties were investigated. The device presents a typical positive rectification in the dark, while under 254 nm UV light illumination, it shows a negative rectification, which might be caused by the generation of photoinduced electron-hole pairs in the β-Ga₂O₃ film layer. With zero bias, that is, zero power consumption, the photodetector shows a fast photoresponse time (decay time τ_d = 0.07 s) and the ratio I_photo/I_dark ≈ 20 under 254 nm light illumination with a light intensity of 45 μW/cm². Such behaviors are attributed to the separation of photogenerated electron-hole pairs driven by the built-in electric field in the depletion region of β-Ga₂O₃ and the NSTO interface, and the subsequent transport toward corresponding electrodes. The photocurrent increases linearly with increasing the light intensity and applied bias, while the response time decreases with the increase of the light intensity. Under -10 V bias and 45 μW/cm² of 254 nm light illumination, the photodetector exhibits a responsivity R_λ of 43.31 A/W and an external quantum efficiency of 2.1 × 10⁴ %. The photo-to-electric conversion mechanism in the β-Ga₂O₃/NSTO heterojunction photodetector is explained in detail by energy band diagrams. The results strongly suggest that a photodetector based on β-Ga₂O₃ thin-film heterojunction structure can be practically used to detect weak solar-blind signals because of its high photoconductive gain.

Synthesis of highly sensitive disordered porous SnO2 aerogel composite material by the chemical deposition method: synergistic effect of a layer of CuO thin film

In this study, a new chemical deposition method was innovatively used to prepare disordered porous CuO/SnO₂ aerogel composite material (CuO/SnO₂-ACM). The prepared material has the excellent gas sensing property.

A Facile Process for the Preparation of Three-Dimensional Hollow Zn(OH)2 Nanoflowers at Room Temperature

A facile strategy has been developed to synthesize double-shelled Zn(OH)2 nanoflowers (DNFs) at room temperature. The nanoflowers were generated via conversion of Cu2 O nanoparticles (NPs) using ZnCl2 and Na2 S2 O3 by a simple process. Outward diffusion of the Cu(2+) , produced by an oxidation process on the surface of NPs, and the inward diffusion of Zn(2+) by coordination and migration, eventually lead to a hollow cavity in the inner NPs with a double-shelled 3D hollow flower shapes. The thickness of the inner and outer shells is estimated to be about 20 nm, and the thickness of nanopetals is about 7 nm. The nanoflowers have large surface areas and excellent adsorption properties. As a proof of potential applications, the DNFs exhibited an excellent ability to remove organic molecules from aqueous solutions.

Synthesis and applications of porous non-silica metal oxide submicrospheres

A variety of metal oxide particles of spherical morphology from nano to micrometer size have been reviewed with a special emphasis on the appraisal of synthetic strategies and applications in biomedical, environmental and energy-related areas.

Enhanced ultraviolet-visible light responses of phototransistors based on single and a few ZrS₃ nanobelts

Phototransistors based on single and three ZrS3 nanobelts were fabricated on SiO2/Si wafers by photolithography and the lift-off technique, respectively, and their light-induced electric properties were investigated in detail. Both the devices demonstrate a remarkable photoresponse from ultraviolet to near infrared light. The photoswitch current ratio (PCR) of the single-nanobelt phototransistor is 13 under the illumination of 405 nm light with an optical power of 10.5 mW cm(-2) at a bias of 5 V, while the PCR of the three-nanobelt device is 210 under the illumination of 405 nm light with an optical power of 5.57 mW cm(-2) at a bias of 1 V. On comparison of the photoresponses under the same conditions, the latter is found to be superior to the former, and both the devices show a much better photoresponse than the reported flexible ZrS3-nanobelt-film photodetector.

Self-organized sheaf-like Fe3O4/C hierarchical microrods with superior lithium storage properties

Functional nanomaterials with three-dimensional hierarchical structures are of high interest for many practical applications including lithium-ion batteries (LIBs). In this work, self-organized sheaf-like Fe3O4/C microrods constructed by porous nanowires have been synthesized by a facile solvothermal method combined with a subsequent annealing treatment. The morphology of the building blocks could be easily tuned by varying the synthesis parameters. When applied as an anode material for LIBs, these sheaf-like Fe3O4/C porous microrods manifest superior electrochemical lithium storage properties in terms of high reversible capacity, stable cycling capacity retention and good rate capability.

Ordered mesoporous NiO with thin pore walls and its enhanced sensing performance for formaldehyde

A class of formaldehyde (HCHO) gas sensors with a high response were developed based on ordered mesoporous NiO, which were synthesized via the nanocasting route by directly using mesoporous silica as the hard template. A series of mesoporous NiO with different textural parameters such as specific surface area, pore size, pore wall thickness were achieved by selecting mesoporous silica with different pore sizes as templates. The gas sensing properties for formaldehyde (HCHO) of the NiO specimens were examined. The results show that this mesoporous NiO possesses a much higher response to HCHO even at low concentration levels than the bulk NiO, and a larger specific surface area and pore size as well as thinner pore walls would be beneficial for enhancing the sensing properties of NiO.