next up previous
Next: Diamond synthesis Up: Diamond and graphite Previous: Electronic structure of diamond,

Quantum confinement

Quantum confinement is change of electronic and optical properties when the material sampled is of sufficiently small size - typically 10 nanometers or less. The bandgap increases as the size of the nanostructure decreases. Specifically, the phenomenon results from electrons and holes being squeezed into a dimension that approaches a critical quantum measurement, called the exciton Bohr raduis.

The first experimental evidence of the quantum confinement effects in clusters came from crystalline CuCl clusters grown in silicate glasses [19]. Spectroscopic studies on these clusters clearly indicated an up to 0.1 eV blueshift of the absorption spectrum relative to the bulk. In the case of CdS clusters, the absorption threshold is observed to blueshift by up to 1eV or more as the cluster size is decreased [20]. When the size of the cluster is smaller, its band gap is larger, consequently the first absorption peak is shifted closer to the blue.

A recent study [21] of the X-ray absorption spectra in nanodiamond thin films with grain diameter from 3.5 to 5 $\mu$m showed that the C 1$s$ core exciton state and conduction band edge are shifted to higher energies with decrease of the grain size especially when the crystallite radius is smaller than $\sim$1.8 nm. The conduction band of nanodiamonds with radius $R>1.8$ nm, when the crystalline contains more than 4300 C atoms, remain more or less bulklike.

Recently Raty et al [22] presented ab initio calculations based on density-functional theory (DFT) in order to investigate a quantum confinement effects in hydrogenated nanodiamonds. They detected a rapid decrease of the DFT energy gap from a value of 8.9 eV in methane to 4.3eV in C$_87$H$_76$. The last value is very close to that of the bulk diamond (4.23eV), obtained using the same method. This indicate that in contrast to Si and Ge where quantum confinement effects persists up to 6-7 nm, in diamond there is no detectable quantum confinement for sizes larger than 1-1.2 nm. In addition the authors predicted a slight influence of surface structure reconstructed by hydrogen atoms on the optical properties.


next up previous
Next: Diamond synthesis Up: Diamond and graphite Previous: Electronic structure of diamond,
sorkin anastasy 2006-10-30