Synthesis and Functionalization of Atomic Layer Boron Nitride Nanosheets for Advanced Material Applications
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Besides graphene, hexagonal boron nitride nanosheets (BNNSs) are playing an excellent role for the next generation of 2-dimentional (2D) functional nanomaterials. Due to the similar lattice parameter and identical crystalline structure to that of graphene, BNNSs are often known as white graphene. These nanosheets exhibit several unique properties that make them preferable to graphene in some ways. Unlike graphene, there was no established method for obtaining large scale of single- or few-layer BNNSs. In our current research, we installed and upgraded CO2-pulsed laser plasma deposition system and introduced new experimental parameters in order to achieve large amount of high quality, few atomic layers BNNSs at significantly low substrate temperature down to 300 oC and at short interval of deposition time (e.g. 3-5 sec). With the variation of deposition parameters such as using H2 gas as deposition environment, we controlled the thicknesses of nanosheets down to 1.5 nm, while quality and purity of the sample were still very high. Moreover, the depositions performed in H2 environment effectively prevented nanosheets from sputtering through high energy boron (B) and nitrogen (N) ions, as result, large amount of atomic defects free single-crystal and polycrystalline nanosheets were obtained. The size, shape, thickness, density, and alignment of the BNNSs were well-controlled by appropriately changing the deposition conditions. TEM images showed large area, flat and transparent BNNSs while, high resolution transmission electron microscopy (HRTEM) showed the sheets to be mostly defect-free and to have the characteristic honeycomb crystal lattice structure based on six-membered B3-N3 hexagon. From HRTEM measurements, one can clearly distinguish between the bright and slightly dull dots related to boron and nitrogen atoms arranged in a typical honeycomb network structure, similar to C-C atoms in graphene. HRTEM, electron diffraction, X-ray diffraction, Raman scattering, fast Fourier transform and Fourier transform infrared spectroscopy clearly identified hexagonal BN (h-BN). In this research, we revealed how nanostructuring of composite BNNSs can be used to provide new electronic and optical functionalities. BNNSs are used to study their applications in three different areas of nano-electronic device technology, e.g. fabrication of prototype Schottky diode, deep ultraviolet (DUV) photo detector and resistance based gas sensor. Doping with carbon elements functionalized BNNSs and current versus voltage (I-V) characteristics of Schottky diode were recorded at different temperatures (25 oC, 50 oC, 75 oC) which represented slight doping into BNNSs brought a significant change in the output current of diode. BNNSs were also treated with hydrogen plasma, which exhibit distinct and pronounced changes in its electronic properties after the plasma treatment. The band-gaps of the few layers BNNSs reduced from ∼5.6 eV (at 0 s without hydrogen treatment) to ∼4.25 eV (at 250s with hydrogen treatment), which is a signature of transition from the insulating to the semi-conductive regime. It was concluded that with the engineered of 2D materials by attaching other atoms or molecules significantly changes electronic properties. Data obtained from BNNSs-based DUV photo-detector device indicates that BNNSs are highly sensitive to deep UV light source. Response time and recover time are around 5s and 150s, respectively. Actual response time and recover time should be shorter because time delay for reaching full intensity for UV lamps after switch on lamp, or florescence after switch off the lamp affects the measurement results. Gas sensing properties of BNNSs-based gas sensor indicated that BNNSs are truly an effective material that can be used as resistance based gas sensor operates in extreme high temperature and toxic environment where properties of conventional sensors fall short. It is therefore concluded that BNNSs are highly attractive for range of applications in material science and nano-electronic device technology. Similar to diamond like carbon, BN is also fashionable in cubic structure. To synthesize cubic BN (c-BN), high temperature and high pressure condition are required. Synthesis at low temperature and low pressure was a challenge. In our study, we synthesized cubic BN at significantly low substrate temperature (450 oC) using ferrous oxide nanoparticles as catalyst. While by using nickel and cobalt nano-particles as catalyst helped in producing BN nanotubes.