Search

Tunneling electron transport through CdS nanocrystal arrays fabricated by the Langmuir-Blodgett method are studied by scanning electron spectroscopy. The effect of the matrix-annealing atmosphere on tunneling transport through the nanocrystal arrays is studied. Electron capture at traps in the case of nanocrystals annealed in vacuum is detected by tunneling current-voltage characteristics analyzed using a model relating the data of tunneling spectroscopy, photoluminescence, and quantum-mechanical calculation. Analysis shows that the nanocrystal surface is passivated by an ammonia monolayer upon annealing in an ammonia atmosphere. It is found that the substrate and surrounding non-passivated nanocrystals have an effect on the electron polarization energy.

The optical properties of CdS nanoclusters are studied for samples in which the nanoclusters are embedded in the Langmuir-Blodgett film matrix or the matrix is removed by annealing in vacuum or ammonia atmosphere. After annealing the samples in vacuum or ammonia, the bands of emission from the nanoclusters and from the surface states appear in the photoluminescence spectrum, with the peaks at 2.9 or 2.7 eV and at 1.9 or 2.1 eV, respectively. It is found that, after treating the samples with ammonia, the photoluminescence of the nanoclusters becomes more intense, whereas the photoluminescence corresponding to recombination via the levels of the surface states becomes less intense. It is established that the increase in the photoluminescence intensity for the nanoclusters and the difference between the temperature dependence of the photoluminescence peak position and the temperature dependence of the band gap of bulk CdS are due to passivation of surface states in the nanoclusters. The experimental data are interpreted in the model of recombination of nonequilibrium charge carriers in the CdS nanoclusters, taking into account the exchange of charge carriers between the nanocrystals and trapping centers at their surface and the recombination of charge carriers via the levels of the surface states.

The photoluminescence of the CdS nanoclusters formed in the matrix of a Langmuir-Blodgett film is studied in the temperature range 5–300 K. At the temperature 5 K, the photoluminescence spectrum of the nanocrystals consists of two bands, with peaks at 2.95 and 2.30 eV. The temperature dependence of the position of the high-energy photoluminescence band differs from the temperature dependence of the band gap of the CdS bulk crystal. The integrated photoluminescence intensity of this band decreases as temperature increases in the range from 5 to 75 K, increases in the range from 150 to 230 K, and decreases above 230 K. The experimental data are interpreted in the context of the model of recombination of nonequilibrium charge carriers in CdS nanoclusters with regard to the charge-carrier transport in locally coupled clusters different in size. In the model, the energy depth of the traps for electrons is estimated at 120 meV; the estimations of the activation energies of nonradiative recombination yield 5 and 100 meV.