Fluorinated h-BN as a magnetic semiconductor

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Fluorinated h-BN as a magnetic semiconductor by Sruthi Radhakrishnan, Deya Das, Atanu Samanta, Carlos A. de los Reyes, Liangzi Deng, Lawrence B. Alemany, Thomas K. Weldeghiorghis, Valery N. Khabashesku, Vidya Kochat, Zehua Jin, Parambath M. Sudeep, Angel A. Martí, Ching-Wu Chu, Ajit Roy, Chandra Sekhar Tiwary, Abhishek K. Singh, and Pulickel M. Ajayan Science Volume 3(7):e1700842 July 14, 2017 Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

Fig. 1 Synthesis and characterization of fluorinated boron nitride. Synthesis and characterization of fluorinated boron nitride. (A) The schematic of the method used for the fluorination of h-BN is outlined. The bulk h-BN is dispersed in DMF, and Nafion is added to it. The mixture is heated in an autoclave at 200°C for 12 hours. (B) The deconvoluted XPS spectrum of B 1s peak shows two peaks, one corresponding to B-N bonding in h-BN and the other low intensity peak at higher energy corresponds to B-F bonding. (C) Deconvoluted XPS spectrum of N 1s peak also shows two peaks corresponding to N-B and N-F. (D) The IR spectra showing the broad B-N-B bending vibrations in h-BN and F-BN. In F-BN, the F-B peaks can be clearly seen interspersed with the broad B-N peak. (E) XRD showing the most intense peaks in h-BN and F-BN. (F) Raman spectra shows the E2g peak in h-BN and the shifted peak, which appears in F-BN. (G) Low-magnification bright-field TEM image of the thin F-BN sheets and the diffraction pattern of 001 orientation (inset) confirms the hexagonal structure. Sruthi Radhakrishnan et al. Sci Adv 2017;3:e1700842 Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

Fig. 2 Electronic properties and band structure of fluorinated boron nitride. Electronic properties and band structure of fluorinated boron nitride. (A) Variation of bandgap calculated from the UV-vis absorption measurements with the percentage of fluorine doping. The bandgap was calculated from the absorbance measurements using Tauc plot. (B) Variation of deconvoluted PL spectra according to the percentage of fluorine doping. Each PL spectrum was deconvoluted into three peaks, namely, A, B, and C. (C) I-V curve obtained from two-probe measurements conducted on a device fabricated on fluorinated boron nitride (F-BN) on a silicon substrate with SiO2 coating. The inset shows an optical image of the device. (D) Magnetotransport measurements as a function of temperature at zero field, low field, and at high field. The inset shows the negative magnetoresistance at two different temperatures. (E and F) Band structures and density of states with atomic contribution obtained by spin-polarized calculations for F-BN sheets having (E) 6.25% and (F) 12.5% F concentrations. (G) Change in bandgap (Eg) as a function of F concentrations calculated from DFT calculations. Inset shows variation in defect level and valence band maximum (VBM) and conduction band minimum (CBM) from doping above 5%, where defect level is seen. Sruthi Radhakrishnan et al. Sci Adv 2017;3:e1700842 Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

Fig. 3 Unconventional magnetic properties of fluorinated boron nitride. Unconventional magnetic properties of fluorinated boron nitride. (A) The room temperature hysteresis curve of F-BN with 8.1% fluorine. (B) The experimentally observed temperature-dependent susceptibility fitted with the Curie’s law. The measurement was performed at an applied dc field of 500 Oe. (C) The susceptibility variation with temperature obtained on zero field cooling (ZFC) and field cooling (FC) after subtracting the diamagnetic background. (D) The real part of the ac susceptibility against temperature for measurements at 75 and 750 Hz at an ac field of 2 Oe. The freezing temperature (Tf) is marked. Sruthi Radhakrishnan et al. Sci Adv 2017;3:e1700842 Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).

Fig. 4 DFT calculations of the magnetic moment for various configurations. DFT calculations of the magnetic moment for various configurations. The energy difference between different magnetic solutions marked by I to VII for different F concentrations and their corresponding nonmagnetic states. I, II, IV, and VI; III and V; and VII are FM, AFM, and ferrimagnetic solutions, respectively. The total magnetic moment [M(μB)] for different magnetic states is also shown in the same plot by orange diamonds, whereas axis is marked on the right side. The spin-up and spin-down densities are colored purple and yellow, respectively. The isosurface value of 0.002 e/A3 is fixed for spin-density plots. The magnetic moment is localized to the nearest six N atoms of the region, where three F atoms (red color sphere) are attached to three B atoms, and one F atom is attached in the opposite site with N atom in the middle of hexagon (as shown by dotted blue line). II (IV) and III (V) are FM and AFM solutions of the 4 × 4 × 1 (8 × 8 × 1) supercell (shown in orange dotted line) corresponding to 12.5% F concentration. VII is the FM solution of the 4 × 4 × 1 supercell corresponding to 15.625%. I and VI are not displayed because these have the same structure as II but with 9.375 and 12.5% F. Sruthi Radhakrishnan et al. Sci Adv 2017;3:e1700842 Copyright © 2017 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC).