Vlad Sukhovatkin

Colloidal quantum-dot photodetectors exploiting multiexciton generation.

Reaping Gain from Decay In photovoltaic devices, absorbed light excites electrons into a conduction band and thereby initiates electric current flow. Unfortunately, if the energy of the incident photons exceeds the threshold for this excitation (the bandgap), the excess tends to be wasted. Initially, a photon bearing several multiples of the bandgap energy may correspondingly promote several electrons, but before these can begin to travel through a circuit, most of them drop back down to the immobile state, transferring their packet of energy to a lone remaining carrier in a process termed “Auger decay.” Sukhovatkin et al. (p. 1542) show that a photoconductive device design can actually leverage the Auger decay process to improve sensitivity in ultraviolet detection. Their detector, a thin film assembled from lead sulfide quantum dots, improves its response by up to a factor of four when the incident light frequency rises to several multiples of the bandgap. Decay of multiple electron-hole pairs, a hindrance in photovoltaic design, proves a boon to sensitivity in a photoconductive detector. Multiexciton generation (MEG) has been indirectly observed in colloidal quantum dots, both in solution and the solid state, but has not yet been shown to enhance photocurrent in an optoelectronic device. Here, we report a class of solution-processed photoconductive detectors, sensitive in the ultraviolet, visible, and the infrared, in which the internal gain is dramatically enhanced for photon energies Ephoton greater than 2.7 times the quantum-confined bandgap Ebandgap. Three thin-film devices with different quantum-confined bandgaps (set by the size of their constituent lead sulfide nanoparticles) show enhancement determined by the bandgap-normalized photon energy, Ephoton/Ebandgap, which is a clear signature of MEG. The findings point to a valuable role for MEG in enhancing the photocurrent in a solid-state optoelectronic device. We compare the conditions on carrier excitation, recombination, and transport for photoconductive versus photovoltaic devices to benefit from MEG.

Highly luminescent lead sulfide nanocrystals in organic solvents and water through ligand exchange with poly(acrylic acid).

Hydrophobic lead sulfide quantum dots (PbS/OA) synthesized in the presence of oleic acid were transferred from nonpolar organic solvents to polar solvents such as alcohols and water by a simple ligand exchange with poly(acrylic acid) (PAA). Ligand exchange took place rapidly at room temperature When a colloidal solution of PbS/OA in tetrahydrofuran (THF) was treated with excess PAA, the PbS/PAA nanocrystals that formed were insoluble in hexane and toluene but could be dissolved in methanol or water, where they formed colloidal solutions that were stable for months. Ligand exchange was accompanied by a small blue shift in the band-edge absorption, consistent with a small reduction in particle size. While there was a decrease in quantum yield associated with ligand exchange and transfer to polar solvents, as is commonly found for colloidal quantum dots, the quantum yields determined were impressively high: PbS/OA in toluene (82%) and in THF (58%); PbS/PAA in THF (42%) and in water (24%). The quantum yields for the PbS/PAA solutions decreased over time as the solutions were allowed to age in the presence of air.

Optimization of Band Structure and Quantum-Size-Effect Tuning for Two-Photon Absorption Enhancement in Quantum Dots

The two-photon absorption, 2PA, cross sections of PbS quantum dots, QDs, are theoretically and experimentally investigated and are shown to be enhanced with increasing quantum confinement. This is in contrast to our previous results for CdSe and CdTe QDs where the reduced density of states dominated and resulted in a decrease in 2PA with a decrease in QD size. Qualitatively this trend can be understood by the highly symmetric distribution of conduction and valence band states in PbS that results in an accumulation of allowed 2PA transitions in certain spectral regions. We also measure the frequency nondegenerate 2PA cross sections that are up to five times larger than for the degenerate case. We use a k·p four-band envelope function formalism to model the increasing trend of the two-photon cross sections due to quantum confinement and also due to resonance enhancement in the nondegenerate case.

Role of symmetry breaking on the optical transitions in lead-salt quantum dots.

The influence of quantum confinement on the one- and two-photon absorption spectra (1PA and 2PA) of PbS and PbSe semiconductor quantum dots (QDs) is investigated. The results show 2PA peaks at energies where only 1PA transitions are predicted and 1PA peaks where only 2PA transitions are predicted by the often used isotropic k x p four-band envelope function formalism. The first experimentally identified two-photon absorption peak coincides with the energy of the first one photon allowed transition. This first two-photon peak cannot be explained by band anisotropy, verifying that the inversion symmetry of the wave functions is broken and relaxation of the parity selection rules has to be taken into account to explain optical transitions in lead-salt QDs. Thus, while the band anisotropy of the bulk semiconductor plays a role in the absorption spectra, especially for the more anisotropic PbSe QDs, a complete model of the absorption spectra, for both 1PA and 2PA, must also include symmetry breaking of the quantum confined wave functions. These studies clarify the controversy of the origin of spectral features in lead-salt QDs.

Carrier Relaxation Dynamics in Lead Sulfide Colloidal Quantum Dots

We report transient absorption saturation measurements on lead sulfide colloidal nanocrystals at the first and second exciton energies and fit the results to a model incorporating intraband and interband relaxation processes. We study in detail the Auger recombination from the first excited state, which takes place when more than one electron-hole pair is excited in a dot. We find an Auger coefficient of 4.5 x 10(-30) cm6/s for dots of 5.5 nm diameter, and observe saturation of the absorption bleaching when the (8-fold degenerate) first level is filled. We develop a model for the absorption dynamics using Poisson statistics and find a good fit with our experimental measurements.

A tunable colloidal quantum dot photo field-effect transistor

We fabricate and investigate field-effect transistors in which a light-absorbing photogate modulates the flow of current along the channel. The photogate consists of colloidal quantum dots that efficiently transfer photoelectrons to the channel across a charge-separating (type-II) heterointerface, producing a primary and sustained secondary flow that is terminated via electron back-recombination across the interface. We explore colloidal quantum dot sizes corresponding to bandgaps ranging from 730 to 1475 nm and also investigate various stoichiometries of aluminum-doped ZnO (AZO) channel materials. We investigate the role of trap state energies in both the colloidal quantum dot energy film and the AZO channel.

Megahertz-frequency large-area optical modulators at 1.55 µm based on solution-cast colloidal quantum dots

We report the realization of large-area, communications-wavelength electro-optic modulators made via simple solution-casting onto an arbitrary substrate. The devices employ colloidal quantum dots synthesized in, and processed from, the solution phase. Devices exhibit greater than 30% modulation depth at the 1.55 microm eye-safe wavelengths of interest in free-space optical communications. The devices retain considerable modulation depth beyond 1 MHz.

Two-photon absorption and multi-exciton generation in lead salt quantum dots

Understanding the nonlinear optical processes in semiconductor nanostructures leads to possible applications in areas including laser amplifiers, optical switches, and solar cells. Here we present a study of the frequency degenerate two-photon absorption (2PA) spectrum of a series of PbS and PbSe quantum dots (QDs). The influence of the quantum confinement is analyzed using a four-band model which considers the mixing of valence and conduction bands. In contrast to our observations of CdSe QDs, the present results point to an increase of the 2PA cross-section (normalized by the QD volume) as the quantum dot size is made smaller. This is explained by the symmetry between the valence and conduction bands which allows the density of states to remain high even for small QDs. A study of the ultrafast carrier dynamics of the PbS quantum dots is also presented. Through nondegenerate femtosecond pump-probe experiments we show evidence of multi-exciton generation with quantum yield (number of excitons generated per absorbed photon) up to 170% for excitation with hω> 3 Eg (where Eg is the bandgap energy).

Ten-fold improvement in the photoluminescence quantum efficiency of quantum-size-effect-tunable (1000-2000 nm) and solution-processed PbS nanocrystal films

PbS nanocrystals for infrared applications have previously shown to have very low photoluminescence quantum efficiency (/spl sim/ 1%) when solution-processed into the solid state. We show a ten-fold improvement through the use of polymer-nanocrystal composites.

Evidence of Symmetry Breaking in Lead Salt Quantum Dots

Experimentally observed two-photon transitions of lead-salt quantum dots, not predicted by k·p theory, are explained as symmetry forbidden one-photon transitions. Similarly features in the one-photon absorption spectra are explained as symmetry forbidden two-photon transitions.