Band gap measurements of monolayer h-BN and insights into carbon-related point defects

Abstract

Being a flexible wide band gap semiconductor, hexagonal boron nitride (h-BN) has great potential for technological applications like efficient deep ultraviolet (DUV) light sources, building block for two-dimensional heterostructures and room temperature single photon emitters in the UV and visible spectral range. To enable such applications, it is mandatory to reach a better understanding of the electronic and optical properties of h-BN and the impact of various structural defects. Despite the large efforts in the last years, aspects such as the electronic band gap value, the exciton binding energy and the effect of point defects remained elusive, particularly when considering a single monolayer.Here, we directly measured the density of states of a single monolayer of h-BN epitaxially grown on highly oriented pyrolytic graphite, by performing low temperature scanning tunneling microscopy (LT-STM) and spectroscopy (STS). The observed h-BN electronic band gap on defect-free regions is (6.8 ± 0.2) eV. Using optical spectroscopy to obtain the h-BN optical band gap, the exciton binding energy is determined as being of (0.7 ± 0.2) eV. In addition, the locally excited cathodoluminescence and photoluminescence show complex spectra that are typically associated to intragap states related to carbon defects. Moreover, in some regions of the monolayer h-BN we identify, using STM, point defects which have intragap electronic levels around 2.0 eV below the Fermi level.

Scalable Synthesis of Monolayer Hexagonal Boron Nitride on Graphene with Giant Bandgap Renormalization

“…[ 45,55 ] This trend illustrates that the screening depends only on the adjacent graphene layers as previously reported for other vdW heterostructures. [ 56 ] Our predictions also agree well with the E g = 6.8 ± 0.2 eV recently measured with STM for monolayer hBN, [ 33 ] and the variation of the bandgap among different stacking configurations is small, less than 0.1 eV.…”

“…[ 48–50 ] Recently, the emissions with higher peak energies (above 5.96 eV) were attributed to the carrier transition and recombination processes in monolayer hBN with a direct bandgap. [ 32,33,51 ] However, there is a large difference between the experimentally measured emission (6–6.15 eV) [ 32,33 ] and the theoretically predicted bandgap (8 eV) for a monolayer hBN. [ 34–36 ] To explain the 6.12 eV emission resonance from a monolayer hBN/HOPG heterostructure, we use first‐principles calculations based on density functional theory (DFT) and many‐body perturbation theory.…”

“…The unique optical properties of monolayer hBN result from the extraordinary strong light-matter interaction. [33][34][35][43][44][45][46] Therefore, we characterize our hBN/HOPG samples further by using temperature-variable PL spectroscopy, as schematically shown in Figure 3a. The measured, time-integrated PL spectrum at 12 K (blue curve) and the reflectance spectrum at 300 K (red curve) are presented in Figure 3b for the monolayer hBN sample of Figure 1g.…”

Polytypes of sp2-Bonded Boron Nitride

“…The photoluminescence of the BN monolayer emits at 6.085 eV at low temperature, which was recently confirmed by the direct measurement of the density of states of a single monolayer of h-BN epitaxially grown on HOPG. This was achieved by performing low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) [153]. According to group theory, the D 3h point group authorizes piezoelectric behavior, which was observed experimentally [154].…”

Room temperature deep UV photoluminescence from low dimensional hexagonal boron nitride prepared using a facile synthesis

“…Recently, experiments performed on h-BN/Graphite using scanning tunneling microscopy/spectroscopy (STM/STS) suggested an energy gap of 6.8 ± 0.2 eV. 38 The optical energy gap calculations of monolayer h-BN are determined by the exciton energy and a range of energy gap values from 5.30 -6.30eV have been calculated. 36 h-BN is also successfully synthesized using a variety of other techniques such as atomic layer deposition, 39 chemical vapour deposition 40 ,molecular beam epitaxy 30,41 and layer-by-layer sputtering process.…”