Direct observation of the band structure in bulk hexagonal boron nitride

A promising route towards nanodevice applications relies on the association of graphene and transition metal dichalcogenides with hexagonal boron nitride ($h\text{−}\mathrm{BN}$). Due to its insulating nature, $h\text{−}\mathrm{BN}$ has emerged as a natural substrate and gate dielectric for graphene-based electronic devices. However, some fundamental properties of bulk $h\text{−}\mathrm{BN}$ remain obscure. For example, the band structure and the position of the Fermi level have not been experimentally resolved. Here, we report a direct observation of parabolic dispersions of $h\text{−}\mathrm{BN}$ crystals using high-resolution angle-resolved photoemission spectroscopy (ARPES). We find that $h\text{−}\mathrm{BN}$ exfoliation on epitaxial graphene enables overcoming the technical difficulties of using ARPES with insulating materials. We show trigonal warping of the intensity maps at constant energy. The valence-band maxima are located around the K points, 2.5 eV below the Fermi level, thus confirming the residual $p$-type character of typical $h\text{−}\mathrm{BN}$.

Figure 1

Crystal structure and micro-Raman spectroscopy of multilayer $h\text{−}\mathrm{BN}$: (a) crystal structure of $h\text{−}\mathrm{BN}$, (b) and (c) micro-Raman spectra and map taken on the $h\text{−}\mathrm{BN}$/graphene layer. The Raman spectra of (b) have been measured along a terrace of the SiC substrate at the location indicated by the corresponding colored dots in (c).

Figure 2

AFM images and XPS spectra of $h\text{−}\mathrm{BN}$: (a) AFM image of $h\text{−}\mathrm{BN}$/graphene, (b) AFM height profile corresponding to the blue line in (a). (c), (d) are high-resolution XPS spectra at $h\nu =820\mathrm{eV}$ of N $1s$ and B $1s$ core levels, respectively.

Figure 3

Electronic structure of bulk $h\text{−}\mathrm{BN}$: (a) nano-ARPES spectra of $h\text{−}\mathrm{BN}$ at $h\nu =100\mathrm{eV}$ and calculated GW band structure (blue lines) of bulk $h\text{−}\mathrm{BN}$ for $k_z=0.3\pi /\mathrm{c}$. The inset of (a) shows the Brillouin zone of graphene and $h\text{−}\mathrm{BN}$ with a twist angle of 30°. (b) Second-derivative spectra of (a) to enhance the visibility of the bands. Black dashed lines indicate the Fermi level position.

Figure 4

2D structure of the bulk $h\text{−}\mathrm{BN}$ band, as measured on the sample: (a)–(c) Local $2\mathrm{D}{k}_{//}$ projection of the first Brillouin zone at binding energies of 2.4, 4, and 4.35 eV, respectively, obtained by measuring one-fourth of the BZ and mirroring two times to obtain the full BZ. (d)–(f) Theoretical calculations of (a)–(c) projections, at the same binding energies.