Charge Generation and Transport in CdSe Semiconductor Quantum Dot Solids
- PDF / 404,435 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 27 Downloads / 210 Views
capped QDs were prepared by repeated dissolution and precipitation of the dots from a neat solution of the new capping group, followed by washing to remove the excess cap once the exchange was complete. Quantum dot radii and inter-particle spacings were determined from published optical and transmission electron micrograph results[1,8]. All measurements were performed in vacuum in a cryostat. Polished sapphire optical flats and degenerately doped silicon substrates with a 600 nm thermally grown, gate oxide were used as substrates for the photoconductivity measurements. Dark conductivity measurements were made on silicon with a 350 nm gate oxide. Gold bar electrodes (200 x 800 x 0.1 Jim) with separations varying from I to 20 [tm were deposited on the substrates using standard photolithography techniques. DC photoconductivity of the QD solid was typically recorded in steps of 10 V with 10 s delay between each step to allow the current to settle. The light source was either the excitation beam from a SPEX fluorimeter or a low intensity beam from an Ar+ laser (typical intensity - 2 mW/cm 2). Several experiments were performed comparing the I-V curve when both the sample and electrodes were illuminated to the I-V curve when only the sample was illuminated. Because there were no qualitative differences in the results, for all subsequent experiments the entire sample pattern was illuminated. To develop a complete picture of the charge generation process, the photoconductivity results were correlated with a variety of optical :t-_ Cn C-measurements. Linear absorption and (0 photoluminescence (PL) quantum yield .E measurements were made using a SPEX 0.33 m monochromator with a Cu) (D 150 groove/mm grating, OMA multichannel analyzer and a Hg-Xe 0 C) 0 lamp. Quenching of the fluorescence in Cn an electric field was measured using an 0~ a-• (D Co Ar+ laser (typical intensity - 20 .N 2 mW/cm ) for excitation and detecting the PL using a far field epi-fluorescence 0 microscope described elsewhere[10]. Z Only the PL from the center region between the electrodes was collected 1.75 2.00 2.25 2.50 2.75 3.00 3.25 and dispersed to measure changes in the PL spectra. Energy (eV) Figure 1: Spectral dependence of the photocurrent at 10 K.The symbols indicate the photocurrent at a fixed electric field of± 2.5 x 105 V/cm. Lines are the corresponding measured linear absorption spectrum for the sample. The photocurrent was normalized to match the first absorption feature. The spectral response is independent of applied field and temperature (T
Data Loading...
