Scanning Tunneling Spectroscopy of Free-Standing CdS Nanocrystals Fabricated by the Langmuir–Blodgett Method

Abstract

The energy spectrum and tunneling transport through ensembles of closely packed CdS nanocrystals (NCs) obtained by the Langmuir–Blodgett (LB) method have been investigated using the scanning tunneling spectroscopy (STS) technique at room temperature. NC ensembles were obtained by annealing the solid LB matrix in a vacuum or in an ammonia atmosphere. The STS data indicate the formation of point defects attributed to an excess of Cd atoms in NC capturing tunneling electrons and the effect of the ammonia atmosphere on the defect formation process. A high density of defects arising as a result of sintering NCs that lost a protective organic shell during the migration on the substrate surface is likely to cause a band gap narrowing in NCs located near the ensemble edge. The analysis of a dependence of the zero-conduction gap value on the NC size gives evidence of a strong effect of the Coulomb interaction between the carriers located in NCs and their polarization charges in the shells on the NC energy spectrum. A dielectric constant of the shell (εout = 10–16.5) and the energy barrier height for electrons (V0 = 0.6–0.8 eV) were determined. The shell comprises ammonia and organic molecules, the ammonia molecules making a major contribution to the shell dielectric constant and the organic molecules determining the barrier height.