Epitaxial hexagonal boron nitride on Ir(111): A work function template

Hexagonal boron nitride (h-BN) is a prominent member in the growing family of two-dimensional materials with potential applications ranging from being an atomically smooth support for other two-dimensional materials to templating growth of molecular layers. We have studied the structure of monolayer h-BN grown by chemical vapor deposition on Ir(111) by low-temperature scanning tunneling microscopy (STM) and spectroscopy (STS) experiments and state-of-the-art density functional theory (DFT) calculations. The lattice mismatch between the h-BN and Ir(111) surface results in the formation of a moiré superstructure with a periodicity of $\sim$29 Å and a corrugation of $\sim$0.4 Å. By measuring the field emission resonances above the h-BN layer, we find a modulation of the work function within the moiré unit cell of $\sim$0.5 eV. DFT simulations for a 13-on-12 h-BN/Ir(111) unit cell confirm our experimental findings and allow us to relate the change in the work function to the subtle changes in the interaction between boron and nitrogen atoms and the underlying substrate atoms within the moiré unit cell. Hexagonal boron nitride on Ir(111) combines weak topographic corrugation with a strong work function modulation over the moiré unit cell. This makes h-BN/Ir(111) a potential substrate for electronically modulated thin film and heterosandwich structures.

Figure 1

Moiré superstructure of hexagonal boron nitride on Ir(111). (a) STM overview image of the sample showing a single h-BN domain extending over four monatomic steps. The black and white color scale is adjusted to repeatedly cover the height of two terraces. The inset is a fast Fourier transform of the image, showing six sharp spots indicative of a single rotational moiré domain. (b) Zoom-in on the moiré pattern (left), highlighting its hexagonal periodicity with an overlayed grid (right). (c) Atomically resolved STM image; the moiré and atomic unit cells are indicated in black and white, respectively. Pore and wire regions are marked by a blue “P” and a red “W”, respectively. (d) DFT simulation of h-BN on Ir(111). The moiré unit cell as well as regions where B and N atoms occupy high-symmetry positions with respect to the Ir lattice are indicated. Feedback parameters: (a) 1.66 V, 0.31 nA; (b) $-$1.71 V, 0.40 nA; and (c) 0.20 V, 3.00 nA.

Figure 2

Experimental electronic structure of h-BN on Ir(111). (a) $dI/dV$ spectra taken on the pore and on the wire of the moiré. The inset is an STM image indicating the positions for the point spectra as well as for the line spectra in (b). (b) Color-coded $dI/dV$ spectra taken along a line connecting two pores. (c) and (d) Bias-dependent STM contrast of the h-BN layer. Line profiles in (c) correspond to green lines in (d). Feedback parameters: Inset (a) 2.10 V, 0.25 nA; (d) 0.25 nA for all images, sample bias as indicated.

Figure 3

Theoretical electronic structure of the h-BN monolayer on Ir(111). (a) The projected density of states of the $p_z$ orbitals of boron and nitrogen as obtained from the DFT calculation. (b) Simulated STM images at different bias voltages.

Figure 4

Field emission resonances and $I\left(z\right)$ spectroscopy on h-BN/Ir(111). (a) STM junction under high bias in the FER regime. (b) FER and (c) $I\left(z\right)$ point spectroscopy measured on the clean Ir(111) (black) and on different parts of the h-BN moiré. FER spectra vertically offset for clarity. (d) STM topography indicating the location of the point spectra in (b) and (c). Feedback parameters: (d) 0.20 V, 1.00 nA.

Figure 5

Mapping the work function changes over the moiré unit cell. (a) STM topography image showing the location of the FER line spectra plotted in (b). (b) Color-coded $dI/dV$ FER spectra taken along the line indicated in (a). (c) Theoretical work function changes over the moiré unit cell computed from DFT. Feedback parameters: (a) 0.20 V, 1.00 nA.