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Synthesis of Monolayer MoSe2 with Controlled Nucleation via Reverse-Flow Chemical Vapor Deposition
摘要： Two-dimensional (2D) layered semiconductor materials, such as transition metal dichalcogenides (TMDCs), have attracted considerable interests because of their intriguing optical and electronic properties. Controlled growth of TMDC crystals with large grain size and atomically smooth surface is indeed desirable but remains challenging due to excessive nucleation. Here, we have synthesized high-quality monolayer, bilayer MoSe2 triangular crystals, and continuous thin films with controlled nucleation density via reverse-flow chemical vapor deposition (CVD). High crystallinity and good saturated absorption performance of MoSe2 have been systematically investigated and carefully demonstrated. Optimized nucleation and uniform morphology could be achieved via fine-tuning reverse-flow switching time, growth time and temperature, with corresponding growth kinetics proposed. Our work opens up a new approach for controllable synthesis of monolayer TMDC crystals with high yield and reliability, which promote surface/interface engineering of 2D semiconductors towards van der Waals heterostructure device applications.
关键词： controlled growth,transition metal dichalcogenides (TMDCs),MoSe2,reverse-flow chemical vapor deposition (CVD)
更新于2025-09-16 10:30:52
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Fine-tuning of spark-discharge aerosol CVD reactor for single-walled carbon nanotube growth: the role of ex situ nucleation
摘要： We report a development of recently designed apparatus equipped with a spark discharge generator of catalytic nanoparticles for robust aerosol CVD synthesis of single-walled carbon nanotubes. We achieve a profound control over the diameter distribution and the defectiveness of carbon nanotubes produced. By providing a justified comparison of the apparatus with the most abundant aerosol CVD reactor utilizing ferrocene as a catalyst precursor, we reveal the role of the activation procedure: while spark-discharge generator provides aerosol of nanoparticles (ex situ route), the ferrocene vapor decomposes in the nanotube growth zone providing an in situ formation of the catalyst. With other parameters being equal, we reveal the differences in the nanotube growth (diameter and length distribution, yield, defectiveness) employing a comprehensive set of methods (the analysis of differential mobility of the aerosol particles, optical spectroscopy, scanning and transmission electron microscopy, Raman spectroscopy, and atomic force microscopy). We show the ex situ activation in the spark discharge reactor to provide a lower utilization degree of the nanoparticles due to over-coagulation. However, the same method provides an independence of the key performance parameters of the nanotubes opening a room for scaling the apparatus.
关键词： spark-discharge generator,differential mobility analyzer,aerosol CVD synthesis,catalyst activation,floating catalyst,single-walled carbon nanotubes
更新于2025-09-16 10:30:52
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Fabrication of diamond diffractive optics for powerful CO2 lasers via replication of laser microstructures on silicon template
摘要： New approach to fabricate diamond diffractive optical elements (DOEs) with continuous relief for powerful CO2 lasers is proposed and tested. It involves short-pulse laser microstructuring of a silicon wafer, which further is used as a substrate for polycrystalline diamond growth in a microwave plasma-assisted CVD process. After fine mechanical polishing of the growth side of the diamond film, the silicon substrate is removed via chemical etching. Two different DOEs providing close to 100% diffraction efficiency were fabricated with this technique: cylindrical Fresnel lens with kinoform surface profile and three-beam splitter with continuous microrelief. Optimization of the laser processing conditions has made possible to reduce the final roughness of the structured diamond surface to 200-400 nm depending on the local relief depth (0-7 μm). Both DOEs tested with a CO2 laser have demonstrated high transparency and diffraction efficiency, as well as low radiation scattering of the IR radiation at the surface irregularities.
关键词： laser microstructuring,diffraction efficiency,CVD process,diamond diffractive optics,CO2 lasers,silicon template
更新于2025-09-12 10:27:22
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Ultrathin All-2D Lateral Graphene/GaS/Graphene UV Photodetectors by Direct CVD Growth
摘要： UV-sensitive lateral all-two-dimensional (2D) photodetecting devices are produced by growing the large band gap layered GaS between graphene electrode pairs directly using chemical vapor deposition methods. The use of prepatterned graphene electrode pairs on the Si wafer enables more than 200 devices to be fabricated simultaneously. We show that the surface chemistry of the substrate during GaS leads to selective growth in graphene gaps, forming the lateral heterostructures, rather than on the surface of graphene. The graphene/GaS/graphene lateral photodetecting devices are demonstrated to be sensitive to UV light only, with no measurable response to visible light. Furthermore, we demonstrate UV-band discrimination in photosensing, with measured photocurrents only produced for middle-UV and not for near-UV wavelength regions. The detection limit could reach down to 2.61 μW/cm2 with a photoresponsivity as high as 11.7 A/W and a photo gain of 53.7 under 270 nm excitation. Gate-dependent modulation of the photocurrent is also demonstrated. The photodetectors exhibit long-term stability and reproducible ON?OFF switching behavior, with a response time lower than 60 ms. These results provide insights into how ultrathin UV sensing devices can be created using only 2D materials by exploiting large band gap 2D semiconductors such as GaS.
关键词： CVD,2D optoelectronics,GaS,photodetector,UV,graphene
更新于2025-09-12 10:27:22
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Diamond detector with laser-formed buried graphitic electrodes: micron-scale mapping of stress and charge collection efficiency
摘要： The paper reports the micron-scale investigation of an all-carbon detector based on synthetic single crystal CVD-diamond having an array of cylindrical graphitic buried-contacts, about 20 μm in diameter each, connected at the front side by superficial graphitic strips. To induce diamond-to-graphite transformation on both detector surface and bulk volume, direct-laser-writing technique was used. Laser-treatment parameters and cell shape have been chosen to minimize the overlapping of laser-induced stressed volumes. Optical microscopy with crossed polarizers highlighted the presence of an optical anisotropy of the treated material surrounding the embedded graphitized columns, and non-uniform stress in the buried zones being confirmed with a confocal Raman spectroscopy mapping. Dark current-voltage characterization highlights the presence of a field-assisted detrapping transport mainly related to highly-stresses regions surrounding buried columns, as well as superficial graphitized strips edges, where electric field strength is more intense, too. Notwithstanding the strain and electronic-active defects, the detector demonstrated a good charge collection produced by 3.0 and 4.5 MeV protons impinging the diamond, as well as those generated by MeV β-particles emitted by 90Sr source. Indeed, the mapping of charge collection efficiency with Ion Beam Induced Charge technique displayed that only a few micrometers thick radial region surrounding graphitic electrodes has a reduced efficiency, while most of the device volume preserves good detection properties with a charge collection efficiency around 90% at 60 V of biasing. Moreover, a charge collection efficiency of 96% was estimated under MeV electrons irradiation, indicating the good detection activity along the buried columns depth.
关键词： β-particles,3D detectors,graphitic pillars,IBIC,sc-CVD diamond,protons
更新于2025-09-11 14:15:04
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - N-Type Ge/SiGe Quantum Cascade Heterostructures for THz Emission
摘要： Terahertz (THz) semiconductor based quantum cascade lasers (QCLs) represent powerful and compact integrated light sources. To date they have been realized with various III-V materials where LO phonon emission becomes very effective as the temperature is increased, limiting the maximum operating temperature to 200K. To overcome this limitation, non-polar material systems are attractive because of their weaker e-phonon interaction. Theoretical studies have indicated n-type Ge/SiGe heterostructures where transport is associated to L electrons, as the most promising architecture [1]. Experimentally, sharp intersubband transitions in n-type strain compensated Ge/SiGe QWs have been observed in the 20-50 meV region [2]. Recently, non-equilibrium Green’s functions calculations proved to be efficient in optimising high temperature performance of GaAs/AlGaAs THZ QCLs [3]. In the effort of realizing a Ge/SiGe THz QCL, we present here a QCL structure designed by means of non-equilibrium Green’s functions [4], whose the bandstructure is reported in Fig.1(a). Epitaxial growth of strain-compensated Ge/SiGe heterostructure (≈1.5 μm thick) has been carried out by means of UHV-CVD yielding very good sample quality as visible from Fig.1(b). Mesa devices for transport experiments have been fabricated by dry etching and metallization. In Fig.1(c) a series of I-V curves as a function of the heatsink temperature display a thermally activated behaviour with Eact (cid:3) 10 meV. From low temperature (4 K) magnetotransport [5] in a magnetic field parallel to the current direction shows characteristics oscillations. we can deduce an energy distance between the lasing states of E23 =19.5 meV, in good agreement with the theoretical calculation. Optical experiments are ongoing and experimental results will be presented.
关键词： THz,UHV-CVD,quantum cascade lasers,Ge/SiGe heterostructures,non-equilibrium Green’s functions
更新于2025-09-11 14:15:04
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Photocurrent Direction Control and Increased Photovoltaic Effects in All-2D Ultrathin Vertical Heterostructures Using Asymmetric h-BN Tunneling Barriers
摘要： Two-dimensional (2D) materials are atomically thick and without out-of-plane dangling bonds. As a result, they could break the con?nement of lattice matching, and thus can be freely mixed and matched together to construct vertical van der Waals heterostructures. Here, we demonstrated an asymmetrical vertical structure of graphene/hexagonal boron nitride (h-BN)/tungsten disul?de (WS2)/graphene using all chemical vapor deposition grown 2D materials. Three building blocks are utilized in this construction: conductive graphene as a good alternative for the metal electrode due to its tunable Fermi level and ultrathin nature, semiconducting transition-metal dichalcogenides (TMDs) as an ultrathin photoactive material, and insulating h-BNas a tunneling barrier. Such an asymmetrical vertical structure exhibits a much stronger photovoltaic e?ect than the symmetrical vertical one without h-BN. By changing the sequence of h-BN in the vertical stack, we could even control the electron ?ow direction. Also, improvement has been further made by increasing the thickness of h-BN. The photovoltaic e?ect is attributed to di?erent possibilities of excited electrons on TMDs to migrate to top and bottom graphene electrodes, which is caused by potential di?erences introduced by an insulating h-BN layer. This study shows that h-BN could be e?ectively used as a tunneling barrier in the asymmetrical vertical heterostructure to improve photovoltaic e?ect and control the electron ?ow direction, which is crucial for the design of other 2D vertical heterostructures to meet various needs of electronic and optoelectronic devices.
关键词： WS2,asymmetrical vertical heterostructure,photovoltaic,CVD,graphene,2D materials,h-BN
更新于2025-09-11 14:15:04
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Research on the mechanism of micromachining of CVD diamond by femtosecond laser
摘要： Diamond is an excellent material in terms of hardness, thermal conductivity and chemical stability. In order to broaden its application area, the interaction mechanism of CVD diamond processed by femtosecond laser is analyzed based on laser micromachining technology. Through the threshold theory, the ablation threshold of CVD diamond is calculated. At the same time, the graphitization phase transition occurred in the diamond. The internal elements before and after the ablation of diamond by femtosecond laser are characterized by X-ray energy spectrum analysis. The changes of element content and bonding form are analyzed, and the relevant graphitization mechanism is revealed.
关键词： XPS,graphitization,ablation,CVD diamond,Femtosecond laser
更新于2025-09-11 14:15:04
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Renewable and Sustainable Composites || Methodologies for Achieving 1D ZnO Nanostructures Potential for Solar Cells
摘要： One-dimensional (1D) nanostructures are generally used to describe large aspect ratio rods, wires, belts, and tubes. The 1D ZnO nanostructures have become the focus of research owing to its unique physical and technological significance in fabricating nanoscale devices. When the radial dimension of the 1D ZnO nanostructures decreases to some lengths (e.g., the light wavelength, the mean of the free path of the phonon, Bohr radius, etc.), the effect of the quantum mechanics is definitely crucial. With the large surface-to-volume ratio and the confinement of two dimensions, 1D ZnO nanostructures possess the captivating electronic, magnetic, and optical properties. Furthermore, 1D ZnO nanostructure’s large aspect ratio, an ideal candidate for the energy transport material, can conduct the quantum particles (photons, phonons, electrons) to improve the relevant technique applications. To date, many methods have been developed to synthesize 1D ZnO nanostructures. Therefore, methodologies for achieving 1D ZnO nanostructures are expressed, and the relevant potential application for solar cells are also present to highlight the attractive property of 1D ZnO nanostructures.
关键词： hydrothermal,nanostructures,one dimensional,ZnO,solar cell,chemical vapor transport and condensation (CVTC),vapor-liquid-solid (VLS),electrochemical,metal-organic chemical vapor deposition (MOCVD),chemical vapor deposition (CVD)
更新于2025-09-11 14:15:04
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[IEEE 2018 IEEE 15th International Conference on Group IV Photonics (GFP) - Cancun (2018.8.29-2018.8.31)] 2018 IEEE 15th International Conference on Group IV Photonics (GFP) - Ge<inf>0.9</inf>Sn<inf>0.1</inf> p-i-n Photodiode with Record-High Responsivity at Two-Micron-Wavelength
摘要： We demonstrate a Ge0.9Sn0.1 photodiode on Si substrate with record-high responsivity Rop of 0.17 A/W at wavelength λ of 2 μm. Fourier-transform infrared spectroscopy (FTIR) reveals that the detector has a cutoff wavelength λc at ~2.6 μm.
关键词： SWIR,CVD,photodiode,GeSn
更新于2025-09-11 14:15:04