Nano Lett. 12, 5740–5743 (2012)

To date, research into the use of colloidal quantum dots (CQDs) as photosensitive materials has been primarily focused on their application in thin-film devices. In such systems, the extraction efficiency of photogenerated charge is dominated by the charge mobility. Now, Ferry Prins and co-workers from Delft University of Technology in The Netherlands have reported a different type of photodetector in which CQDs directly bridge nanometre-spaced electrodes. In this CQD junction, charge mobility no longer plays a role because charge extraction requires only two individual tunnelling events. Located on top of an Si/SiO2 substrate, the 10-μm-wide Ti/Au/Cr/Cr2O3 electrodes were fabricated and separated by a gap of 4 nm. A single layer of PbSe QDs measuring 4 nm in size was then deposited on top of the electrodes. The photoconductive response was seen to depend strongly on the wavelength of the incident light and closely resemble the absorption spectrum of the PbSe QDs, which demonstrates that the photoconductance is driven by optical excitation inside the QDs. For an excitation wavelength of 532 nm, the photocurrent increased linearly with laser power and saturated at an irradiance of more than 50 W cm−2. At an irradiance of 2 W cm−2, the external quantum efficiency was 10.9 and 38 electrons per photon for a bias of 750 mV and 1.5 V, respectively. The rise and fall response times of the photocurrent were 200 ns and 300 ns, respectively.