Two samples of nanocluster-assembled Ge - Al thin films on quartz substrates have been fabricated by co-evaporation and inert-gas condensation. The thin films are amorphous and single phase, and the mean diameters of the nanoclusters are and , respectively. The composition of the nanoclusters in the two films is about 98.7 at.% Ge and 1.3 at.% Al, and the ratio of Ge and oxygen atoms is 1:1.8 (O). From the absorption spectra of the two samples, we estimate the optical gaps to be about 2.8 eV and 1.6 eV from Tauc plots, respectively; these are dependent on the nanocluster sizes and are much larger than those of vacuum-evaporated amorphous Ge and amorphous Ge - Al thin films. Under 3.32 eV (374 nm) excitation, photoluminescence (PL) peaks at 2.80 and 3.00 eV appear for the thin film with the mean diameter of the nanoclusters ; these can be interpreted as manifesting electron transitions from the optical gap (2.8 eV) and mobility gap (3.0 eV). In the absorption spectra of the two samples, a shoulder peak appears at 5.06 eV (245 nm), corresponding to the absorption band of one of the germanium oxygen-deficient centres (GODCs), namely GODC-1. The PL peak at 3.10 eV (400 nm), which arises from GODC-1, is also observed for the two samples.
Two samples of thin films, assembled by CuCl nanoclusters, have been prepared by the gas evaporation technique. The CuCl nanoclusters were deposited on monocrystalline silicon and quartz substrates and then coated with a layer of NaCl to prevent oxidation of the CuCl nanoclusters. From transmission electron microscope and selected area diffraction, it is clear that the two samples consist of CuCl nanocrystals and Cu aggregates and the mean diameters are about 3 nm and 6 nm. From the absorption and photoluminescence (PL) spectra of the two samples carried out at ~300 K, we found that the peaks of exciton absorption do not show up and luminescence peaks shift to lower energies, possibly due to the strong coupling between exciton and phonon. At 77 K, the PL peaks of excitons broadened by exciton–phonon coupling have been observed, in approximate agreement with the values calculated by the quantum confinement model. The broad peaks of emission from trapped states or bands have also been observed at 77 K. In addition, the interaction between exciton and phonon is stronger in the sample with mean diameter about 3 nm according to a larger broadening and redshift of the PL peak from excitons, in qualitative agreement with theoretical prediction.
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