Electron Emission from Diamondoids: A Diffusion Quantum Monte Carlo Study

We present density-functional theory (DFT) and quantum Monte Carlo (QMC) calculations designed to resolve experimental and theoretical controversies over the optical properties of H-terminated C nanoparticles (diamondoids). The QMC results follow the trends of well-converged plane-wave DFT calculations for the size dependence of the optical gap, but they predict gaps that are 1–2 eV higher. They confirm that quantum confinement effects disappear in diamondoids larger than 1 nm, which have gaps below that of bulk diamond. Our QMC calculations predict a small exciton binding energy and a negative electron affinity (NEA) for diamondoids up to 1 nm, resulting from the delocalized nature of the lowest unoccupied molecular orbital. The NEA suggests a range of possible applications of diamondoids as low-voltage electron emitters.

Figure 1

Calculated size dependence of (a) DFT HOMO-LUMO gap and DMC optical gap, and (b) DFT HOMO and LUMO eigenvalues. The inset shows the convergence of the DFT HOMO and LUMO eigenvalues with basis set.

Figure 2

Isosurface plots of the square of the (a) HOMO and (b) LUMO of ${\mathrm{C}}_{29}{\mathrm{H}}_{36}$. The green isosurfaces include 50% of the charge in each orbital.