Fringe structures and tunable bandgap width of 2D boron nitride nanosheets.

Fringe structures and tunable bandgap width of 2D boron nitride nanosheets Peter Feng*1, Muhammad Sajjad1, Eric Yiming Li1, Hongxin Zhang2, Jin Chu3, Ali Aldalbahi4 and Gerardo Morell1 Full Research Paper Address: 1Institute of Functional Nanomaterials and Department of Physics, College of Natural Sciences, University of Puerto Rico, San Juan, PR/USA 00936-8377, 2Globalfoundrie, 400 Stone Break Road extension, Malta, NY 12020, USA, 3Chongqing Institute of Green and Intelligent Technology, CAS, Chongqing 400714, China and 4King Abdullah Institute for Nanotechnology and Department of Chemistry, KSU, Riyadh 11451, Saudi Arabia Email: Peter Feng* - * Corresponding author Open Access Beilstein J. Nanotechnol. 2014, 5, 1186–1192. doi:10.3762/bjnano.5.130 Received: 11 March 2014 Accepted: 04 July 2014 Published: 31 July 2014 This article is part of the Thematic Series "Physics, chemistry and biology of functional nanostructures II". Guest Editor: A. S. Sidorenko © 2014 Feng et al; licensee Beilstein-Institut. License and terms: see end of document. Keywords: boron nitride sheets; fringe patterns; functionalization; tunable bandgap width Abstract We report studies of the surface fringe structures and tunable bandgap width of atomic-thin boron nitride nanosheets (BNNSs). BNNSs are synthesized by using digitally controlled pulse deposition techniques. The nanoscale morphologies of BNNSs are characterized by using scanning electron microscope (SEM), and transmission electron microscopy (TEM). In general, the BNNSs appear microscopically flat in the case of low temperature synthesis, whereas at high temperature conditions, it yields various curved structures. Experimental data reveal the evolutions of fringe structures. Functionalization of the BNNSs is completed with hydrogen plasma beam source in order to efficiently control bandgap width. The characterizations are based on Raman scattering spectroscopy, X-ray diffraction (XRD), and FTIR transmittance spectra. Red shifts of spectral lines are clearly visible after the functionalization, indicating the bandgap width of the BNNSs has been changed. However, simple treatments with hydrogen gas do not affect the bandgap width of the BNNSs. Introduction The recent successful investigation of graphene has stimulated interest in atomically thin boron nitride sheets [1,2]. Similar to the method used to produce graphene, BNNSs can be exfoliated from bulk BN crystals by simple mechanical cleavage techniques [3-5]. The problem is that the obtained hBN nanosheets are usually limited by too small size. Therefore, recently most work on synthesis of large BNNSs is based on either chemical-solution-derived method or a chemical vapor deposition (CVD) process. Many excellent results have been reported [6-9]. Systematic and comprehensive reviews of two- 1186 Beilstein J. Nanotechnol. 2014, 5, 1186–1192. dimensional (2D) boron nitride nanostructures: nanosheets, nanoribbons, nanomeshes, and hybrids with graphene have been presented by Lin [10]. Theoretically, surface treatment can effectively control the band gap of nano BN and plays a crucial role of engineering their electrical and electronic properties. For example for BN nanotubes (BNNT), 50% tube surface coverage with chemisorbed hydrogen atoms would cause the BN band gap (which was computed to be 4.29 eV in pristine BNNT) decreased to 2.01 eV [11]. For BNNSs case the adsorption behavior of a single H atom either on the top site of a B or on the top site of an N atom, or two H atoms adsorbed on adjacent B and N sites are also investigated [12]. Using first-principles computations [13] and hybrid density functional theory calculations with van der Waals correction [14], Chen and Zhang show that polar boron nitride (BN) nanoribbons can be favorably aligned via substantial hydrogen bonding at the interfaces, which induces significant interface polarizations and sharply reduces the band gap of insulating BNNSs. Based on these research, we have experimentally conducted several experiments on using digitally controlled pulse deposition technique to quick synthesis of BNNSs [15] as well as their applications for gas sensors [16] and electronic devices [17-19]. In the present paper, the focus of studies is on variation of the fringe structures and the hydrogen (H) atoms induced band gap width. Chemically shifted components were observed following H treatment, and clear evidence of tunable bandgap width was found. Briefly, the laser beam, focused with a 30 cm focal length of ZnSe lens, was incident at 45 degree relative to a rotated (speed of circa 200 rpm) pyrolytic hexagonal BN target (2.00" diameter × 0.125" thick, 99.99% purity, B/N ratio ≈1.05, density ≈1.94 g/ccm) under high vacuum (2.66 × 10−3 Pa) chamber. The purpose of the use of the long-focal-length lens is to effectively control the laser-produced plasma beams. The diameter of the focus spot of laser beam on the target was about 2 mm and could be varied by shifting focal lens. The power density of the laser on the target was 2 × 108 W/cm2 per pulse. Molybdenum (Mo) and silicon (Si) wafers (1 × 1 cm2) as substrates were used and placed 4 cm away from the target. Substrate temperature was controlled by using a thermocouple and heater. Prior to laser irradiation, substrates were rinsed in acetone and methanol in sequence. The duration for each deposition was few minutes. The as-grown samples were then characterized by using SEM, Raman scattering, X-ray diffraction, and FTIR transmittance, respectively. For studies of the nanoscale morphology of BNNSs, the samples were simply scratched off and then transferred to the grids for TEM measurement. Results and Discussion Fringe structures of boron nitride nanosheets Experimental Figure 1 shows TEM images of BNNSs with different magnifications. The sample is prepared at low temperature, around 350 °C. Each as-grown sample normally consists of a large amount of BNNSs that are partially overlapped one another. Average size of each continuous BNNS piece is around a few micrometer squares. The thickness of the BNNS varies from 1 nm to 10 nm. Each BNNS appears highly flat and transparent properties. The well-shaped edge of each BNNS piece is clearly visible as shown in Figure 1a. A pulsed CO2 laser deposition technique (CO2-PLD: wavelength: 10.6 µm, pulse width: 1–5 µs, repetition rate: 5 Hz, and pulse energy: 5 J) was used. Detailed description of PLD experimental setup can be found in our previous papers [18,19]. Figure 1b shows TEM image with a large magnification, indicating there are many tiny fringes at the edge of the BNNSs. All the fringes have almost the same orientations. Continuing to Figure 1: TEM images of BNNSs with different magnifications. 1187 Beilstein J. Nanotechnol. 2014, 5, 1186–1192. magnify the TEM image, the highly ordered fringe pattern becomes obvious (Figure 1c), where each fringe is related to a single atomic layer, and thickness of the each atomic layer i (...truncated)