Quantum confinement in germanium nanocrystal thin films

Thin films of semiconductor nanocrystals with tunable optical properties are promising materials for new device applications, but progress has been slow for group IV materials. We report absorption measurements near the absorption edge of thin films of germanium nanocrystals synthesized in a plasma and impacted onto substrates. Band gaps extracted from the absorption data vary from the near-bulk value of 0.73 eV for 9.0 nm nanocrystals to over 1.0 eV for 4.7 nm nanocrystals. These values are comparable to those reported for isolated, non-interacting germanium nanocrystals, indicating minimal loss of quantum confinement upon dense film formation. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) In this Letter, the optical band gaps of dense films of germanium nanocrystals were determined from absorption spectra. By decreasing the constituent nanocrystal size from 9 nm to below 5 nm, the film band gap can be controllably tuned from 0.73 to over 1.0 eV. These values are comparable to those reported for isolated, non-interacting germanium nanocrystals, indicating that device-relevant films can be deposited without loss of quantum confinement.