Graphitic carbon nitride (g-C3N4), a metal-free semiconductor with a band gap of 2.7 eV, has received considerable attention owing to its fascinating photocatalytic performances under visible-light. g-C3N4 exhibits high thermal and chemical stability and non-toxicity such that it has been considered as the most promising photocatalyst for environmental improvement and energy conservation. Hence, it is of great importance to obtain high-quality g-C3N4 and gain a clear understanding of its optical properties. Herein, we report a high-yield synthesis of g-C3N4 products via heating of high vacuum-sealed melamine powder in an ampoule at temperatures between 450 and 650 °C. Using transmission electron microscopy (TEM), scanning transmission electron microscopy (STEM), electron energy loss spectroscopy (EELS), thermogravimetric analysis (TGA), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS), the chemical composition and crystallization of the as-produced g-C3N4 are demonstrated. A systematic optical study of g-C3N4 is carried out with several approaches. The optical phonon behavior of g-C3N4 is revealed by infrared and Raman spectroscopy, and the emission properties of g-C3N4 are investigated using photoluminescence (PL) spectroscopy, while the photocatalytic properties are explored by the photodegradation experiment.
Here we review the characteristics of "van der Waals epitaxy" (vdWE) as an alternative epitaxy mechanism that has been demonstrated as a viable method for circumventing the lattice matching requirements for epitaxial growth. Particular focus is given on the application of vdWE for nonplanar nanostructures. We highlight our works on the vdWE growth of nanowire arrays, tripods, and tetrapods from various semiconductors (ZnO, ZnTe, CdS, CdSe, CdSxSe1-x, CdTe, and PbS) on muscovite mica substrates, irrespective of the ensuing lattice mismatch. We then address the controllability of the synthesis and the growth mechanism of ZnO nanowires from catalyst-free vdWE in vapor transport growth. As exemplified herein with optical characterizations of ZnO and CdSe nanowires, we show that samples from vdWE may possess properties that are as excellent as those from conventional epitaxy. With our works, we aim to advocate vdWE as a prospective universal growth strategy for nonplanar epitaxial nanostructures.
Different electric field intensities were added along the growth direction of CuO nanoneedles when using a thermal oxidation process. The results show that: (1) the length of CuO nanoneedles increased with the electric field, but when the voltage was greater than a certain value, the growth stopped and (2) the diameter of CuO nanoneedles from top to root became more uniform. Therefore, it is further demonstrated the “solid state based-up diffusion growth mechanism” for CuO nanoneedles prepared by thermal oxidation. The recent study also provides a possibility for controlling the growth of metal oxide nanowires which will promote their potential applications in nanodevices.
Patterning two-dimensional materials into specific spatial arrangements and geometries is essential for both fundamental studies of materials and practical applications in electronics. However, the currently available patterning methods generally require etching steps that rely on complicated and expensive procedures. We report here a facile patterning method for atomically thin MoSe2 films using stripping with an SU-8 negative resist layer exposed to electron beam lithography. Additional steps of chemical and physical etching were not necessary in this SU-8 patterning method. The SU-8 patterning was used to define a ribbon channel from a field effect transistor of MoSe2 film, which was grown by chemical vapor deposition. The narrowing of the conduction channel area with SU-8 patterning was crucial in suppressing the leakage current within the device, thereby allowing a more accurate interpretation of the electrical characterization results from the sample. An electrical transport study, enabled by the SU-8 patterning, showed a variable range hopping behavior at high temperatures.
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