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The effects of various treatments on dark conductivity and photoconductivity of the n-type CdSe nanocrystalline solids are investigated. The high mobility has allowed the demonstration of the operation of a field effect transistor based on an optimized film preparation.

Conduction in thin films of PbSe nanocrystals doped by electrochemical gating was studied. Charging the film, with electrons or holes, increases the conductance by order of magnitude and the temperature and electric field dependence of conductance in the film measured between 4.3 and 135 K, were both well described by a variable range hopping model.

Solid films of CdSe colloidal quantum dots are conductive when reduced by electrochemistry. The dark conductivity and the photoconductivity of these n-type CdSe nanocrystalline solids have been investigated as a function of film treatment. When the films are treated with NaOH, a carrier mobility of ∼10-2 cm2/V s and a charging/discharging time of ∼10 ms are obtained. Under an excitation intensity of ∼0.75 mW/cm2, a significant photoconductivity of ∼10-3 S cm-1 has been observed. The high mobility allowed demonstrating the operation of a field effect transistor. [ABSTRACT FROM AUTHOR]

The intraband relaxation between the 1Pe and 1Se state of CdSe colloidal quantum dots is studied by pump-probe time-resolved spectroscopy. Infrared pump-probe measurements with ∼6-ps pulses show identical relaxation whether the electron has been placed in the 1Se state by above band-gap photoexcitation or by electrochemical charging. This indicates that the intraband relaxation of the electrons is not affected by the photogenerated holes which have been trapped. However, the surface ligands are found to strongly affect the rate of relaxation in colloid solutions. Faster relaxation (<8 ps) is obtained with phosphonic acid and oleic acid ligands. Alkylamines lead to longer relaxation times of ∼10 ps and the slowest relaxation is observed for dodecanethiol ligands with relaxation times ∼30 ps. It is concluded that, in the absence of holes or when the holes are trapped, the intraband relaxation is dominated by the surface and faster relaxation correlates with larger interfacial polarity. Energy transfer to the ligand vibrations may be sufficiently effective to account for the intraband relaxation rate. [ABSTRACT FROM AUTHOR]

Photoluminescence of CdSe colloidal quantum dots were observed in the neutral and one-electron and two electron reduced states. The photoluminescence of doubly charged nanocrystals showed emission from both P and S excitons with similar intensity.

PbSe nanocrystal colloids exhibit a well-defined excitonic structure with the lowest energy exciton tuning from 0.5 to 1 eV, as a function of size. Intraband pump-probe measurements of the 1S(sub e,h)-1P(sub e,h) transition reveal a short lifetime and the absence of the phonon bottleneck.

A study was conducted to measure the lifetime for the CN(super -) stretching mode at a polycrystalline Cu and Cu(111) electrochemical interface with difference frequency generation. The result states the measurement of vibrational lifetimes as ~ 17 ps, and these lifetimes are in agreement with estimates from the image-dipole relaxation mechanism.

Both electrons and holes can be injected from an electrode into the quantum confined states of a semiconductor nanocrystal film. Hole injection is less robust than electron injection but the ability should allow the development of novel applications.

Solid films of n-type Mn2+/CdSe colloidal quantum dots show a positive magnetoresistance. At 0.3 K the film resistance is increased by up to 150% at 10 T. The variable range hopping model quantitatively accounts for the magnetoresistance at low field. [ABSTRACT FROM AUTHOR]