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

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 ${\mathrm{C}}_{26}{\mathrm{H}}_{32}$ diamond cluster [1(2,3)4] pentamantane is found to be remarkably similar to that of bulk diamond.

Figure 1

Investigated diamondoids categorized by their size and divided into different geometrical families (3D, 2D, 1D). The two middle columns contain clusters of same size. Following the arrows distinguished geometries evolve representing 3D, 2D, 1D nanostructures. Only the carbon framework is shown; the hydrogen termination is omitted for clarity.

Figure 2

Optical absorption as a function of cluster size and shape. The spectra are arranged corresponding to the structures shown in Fig. 1. The optical gaps are indicated by small arrows.

Figure 3

(a) Integrated oscillator strength for the measured diamond clusters. The threshold defining the optical gap is marked by a dotted line. (b) Optical gaps as function of size. The experimental gaps are compared to optical gaps derived by QMC calculations [21]. The dashed line marks the energy gap of bulk diamond [30].

Figure 4

Comparison of the optical absorption of the 3D basic shape ([1(2,3)4]pentamantane) with the absorption of high purity type IIa diamond (dotted line) [30]. The spectra are plotted over different axes which are shifted by 0.73 eV. The spectral inset shows the enlarged onset of the bulk diamond spectra.