Optical Response of Diamond Nanocrystals as a Function of Particle Size, Shape, and Symmetry

Abstract

The optical spectra of hydrogen-passivated diamond clusters (diamondoids) precisely defined in size and shape have been measured in the gas phase, i.e., under an environment similar to boundary conditions typically assumed by theory. Characteristic optical properties evolve for these wide band-gap semiconductor nanocrystals as a function of size, shape, and symmetry in the subnanometer regime. These effects have not previously been theoretically predicted. The optical response of the tetrahedral-shaped C_{26}H_{32} diamond cluster [1(2,3)4] pentamantane is found to be remarkably similar to that of bulk diamond.

Comparison of quantum Monte Carlo with time-dependent and static density-functional theory calculations of diamondoid excitation energies and Stokes shifts

“…Then for each molecule we released all of the symmetry constraints and determined the global minimum energy at point C. We investigated the effect of symmetry relaxation in adamantane within the LDA. Optimizing the geometry without relaxation of the symmetry constraints causes the EG to be 0.67 eV larger than would otherwise be the case, leading to a significant disagreement with the experimental Stokes shift (0.1 eV from the LDA versus 0.7 eV from experiment), 10 so symmetry relaxation in the excited state gives an important contribution to the red-shift of the EGs of diamondoids.…”

“…According to Ref. 10, between adamantane and tetramantane a noticeable transition from molecular-like excitations to quasicontinuous spectra takes place. This feature is also found in our theoretical results.…”

Laser-induced fluorescence of free diamondoid molecules

“…For several higher diamondoids, absorption spectra and optical gap energies were published by Landt et al 15 and serve as a reference for this study. Since the highest attainable photon energy of the used laser system is E6.05 eV, the examined samples are restricted to diamondoids with excitation energies in this range and lower (triamantane and its higher homologues).…”

“…1 for five different photoexcitation energies selected near the absorption onset at 6.06 eV. 15 The strongest peak visible in each spectrum at high energies arises from scattered laser light. The excitation peak is followed by the fluorescence of the sample and extends about 1 eV to lower energies.…”

“…14 Moreover, valence band absorption spectra show a strong shape dependency of the optical properties. 15 The ionization potentials (IPs) of the diamondoids from adamantane to pentamantane were determined by photoion yield spectroscopy, indicating a monotonic decrease with increasing cluster size. 16 Experimental and calculated Raman spectra of a large variety of pristine diamondoids from adamantane to heptamantane show that each diamondoid has a unique vibrational structure.…”