Yunan Gao

Unity quantum yield of photogenerated charges and band-like transport in quantum-dot solids

Solid films of colloidal quantum dots show promise in the manufacture of photodetectors and solar cells. These devices require high yields of photogenerated charges and high carrier mobilities, which are difficult to achieve in quantum-dot films owing to a strong electron-hole interaction and quantum confinement. Here, we show that the quantum yield of photogenerated charges in strongly coupled PbSe quantum-dot films is unity over a large temperature range. At high photoexcitation density, a transition takes place from hopping between localized states to band-like transport. These strongly coupled quantum-dot films have electrical properties that approach those of crystalline bulk semiconductors, while retaining the size tunability and cheap processing properties of colloidal quantum dots.

Photoconductivity of PbSe quantum-dot solids: dependence on ligand anchor group and length.

The assembly of quantum dots is an essential step toward many of their potential applications. To form conductive solids from colloidal quantum dots, ligand exchange is required. Here we study the influence of ligand replacement on the photoconductivity of PbSe quantum-dot solids, using the time-resolved microwave conductivity technique. Bifunctional replacing ligands with amine, thiol, or carboxylic acid anchor groups of various lengths are used to assemble quantum solids via a layer-by-layer dip-coating method. We find that when the ligand lengths are the same, the charge carrier mobility is higher in quantum-dot solids with amine ligands, while in quantum-dot solids with thiol ligands the charge carrier lifetime is longer. If the anchor group is the same, the charge carrier mobility is ligand length dependent. The results show that the diffusion length of charge carriers can reach several hundred nanometers.

Enhanced Hot-Carrier Cooling and Ultrafast Spectral Diffusion in Strongly Coupled PbSe Quantum-Dot Solids

PbSe quantum-dot solids are of great interest for low cost and efficient photodetectors and solar cells. We have prepared PbSe quantum-dot solids with high charge carrier mobilities using layer-by-layer dip-coating with 1,2-ethanediamine as substitute capping ligands. Here we present a time and energy resolved transient absorption spectroscopy study on the kinetics of photogenerated charge carriers, focusing on 0-5 ps after photoexcitation. We compare the observed carrier kinetics to those for quantum dots in dispersion and show that the intraband carrier cooling is significantly faster in quantum-dot solids. In addition we find that carriers diffuse from higher to lower energy sites in the quantum-dot solid within several picoseconds.

Fast and Efficient Photodetection in Nanoscale Quantum-Dot Junctions

We report on a photodetector in which colloidal quantum dots directly bridge nanometer-spaced electrodes. Unlike in conventional quantum-dot thin film photodetectors, charge mobility no longer plays a role in our quantum-dot junctions as charge extraction requires only two individual tunnel events. We find an efficient photoconductive gain mechanism with external quantum efficiencies of 38 electrons-per-photon in combination with response times faster than 300 ns. This compact device-architecture may open up new routes for improved photodetector performance in which efficiency and bandwidth do not go at the cost of one another.

Size-dependent electron transfer from PbSe quantum dots to SnO2 monitored by picosecond Terahertz spectroscopy.

We report the direct and unambiguous determination of electron transfer rates and efficiencies from PbSe quantum dots (QDs) to mesoporous SnO2 films. We monitor the time-dependent electron density within the oxide with picosecond time resolution using Terahertz spectroscopy, following optical excitation of the QDs using a femtosecond laser pulse. QD-oxide electron transfer occurs with efficiencies of ∼2% in our samples under 800 nm pumping with a marked dependence on QD size, ranging from ∼100 ps injection times for the smallest, ∼2 nm diameter QDs, to ∼1 ns time scale for ∼7 nm QDs. The size-dependent electron transfer rates are modeled within the framework of Marcus theory and the implications of the results for device design are discussed.

Disorder strongly enhances Auger recombination in conductive quantum-dot solids.

Auger recombination (AR) can be an important loss mechanism for optoelectronic devices, but it is typically not very efficient at low excitation densities. Here we show that in conductive quantum-dot solids, AR is the dominant charge carrier decay path even at excitation densities as low as 10−3 per quantum dot, and that AR becomes faster as the charge carrier mobility increases. Monte Carlo simulations reveal that this efficient AR results from charge carrier congregation in ‘Auger hot spots’: lower-energy sites that are present because of energy disorder. Disorder-enhanced AR is a general effect that is expected to be active in all disordered materials. The observed efficient AR is an issue of concern for devices that work at charge carrier densities in excess of ~10−3 charge carriers per quantum dot. At the same time, efficient carrier congregation could be exploited for fast optical switching or to achieve optical gain in the near infrared.

Free charges produced by carrier multiplication in strongly coupled PbSe quantum dot films.

We show that in films of strongly coupled PbSe quantum dots multiple electron-hole pairs can be efficiently produced by absorption of a single photon (carrier multiplication). Moreover, in these films carrier multiplication leads to the generation of free, highly mobile charge carriers rather than excitons. Using the time-resolved microwave conductivity technique, we observed the production of more than three electron-hole pairs upon absorption of a single highly energetic photon (5.7E(g)). Free charge carriers produced via carrier multiplication are readily available for use in optoelectronic devices even without employing any complex donor/acceptor architecture or electric fields.

Broadband and picosecond intraband absorption in lead-based colloidal quantum dots.

Using femtosecond transient absorption spectroscopy we demonstrate that lead chalcogenide nanocrystals show efficient, photoinduced absorption (PA) in a broad wavelength range starting just below the bandgap. The time-dependent decay of the PA signal correlates well with the recovery of the band gap absorption. Therefore, the same carriers are involved, which decay with the typical Auger recombination rate in these nanocrystals. Based on this, we assign this PA signal to intraband absorption, i.e., the excitation of photogenerated carriers from the bottom of the conduction band or the top of the valence band to higher energy levels in the conduction and valence band continuum. This broadband response in the commercially interesting near to mid-infrared range is very relevant for ultra-high speed all optical signal processing.

Surfaces of colloidal PbSe nanocrystals probed by thin-film positron annihilation spectroscopy

Positron annihilation lifetime spectroscopy and positron-electron momentum density (PEMD) studies on multilayers of PbSe nanocrystals (NCs), supported by transmission electron microscopy, show that positrons are strongly trapped at NC surfaces, where they provide insight into the surface composition and electronic structure of PbSe NCs. Our analysis indicates abundant annihilation of positrons with Se electrons at the NC surfaces and with O electrons of the oleic ligands bound to Pb ad-atoms at the NC surfaces, which demonstrates that positrons can be used as a sensitive probe to investigate the surface physics and chemistry of nanocrystals inside multilayers. Ab initio electronic structure calculations provide detailed insight in the valence and semi-core electron contributions to the positron-electron momentum density of PbSe. Both lifetime and PEMD are found to correlate with changes in the particle morphology characteristic of partial ligand removal.

Broadband and picosecond intraband absorption in lead based colloidal quantum dots

Using femtosecond transient absorption spectroscopy we demonstrate that lead chalcogenide nanocrystals show efficient, photoinduced absorption (PA) in a broad wavelength range starting just below the bandgap. The time-dependent decay of the PA signal correlates well with the recovery of the band gap absorption. Therefore, the same carriers are involved, which decay with the typical Auger recombination rate in these nanocrystals. Based on this, we assign this PA signal to intraband absorption, i.e., the excitation of photogenerated carriers from the bottom of the conduction band or the top of the valence band to higher energy levels in the conduction and valence band continuum. This broadband response in the commercially interesting near to mid-infrared range is very relevant for ultra-high speed all optical signal processing.