The effect of Auger heating on intraband carrier relaxation in semiconductor quantum rods
Nature Physics 2, 557 (2006).

The rate at which excited charge carriers relax to their equilibrium state affects many aspects of the performance of nanoscale devices, including switching speed, carrier mobility and luminescence efficiency. A better understanding of the processes that govern carrier relaxation therefore has important technological implications. A significant increase in carrier–carrier interactions caused by strong spatial confinement of electronic excitations in semiconductor nanostructures leads to a considerable enhancement of Auger effects, which can further result in unusual, Auger-process-controlled recombination and energy relaxation regimes. Here, we report the first experimental observation of efficient Auger heating in CdSe quantum rods at high pump intensities, leading to a strong reduction of carrier cooling rates. In this regime, the carrier temperature is determined by the balance between energy outflow through phonon emission and energy inflow because of Auger heating. This equilibrium results in peculiar carrier cooling dynamics that closely correlate with recombination dynamics, an effect never seen before in bulk or nanoscale semiconductors.

Image from Nature Physics, News & Views, T. D. Krauss, Not so cool after all, 513 (2006).

Making a laser in a beaker
Advanced Materials 18, 343 (2006)

Two necessary components of a laser are an active medium that produces optical gain and an optical cavity that provides positive feedback. Normally these functions are performed by separate elements such as a laser rod containing active ions and a pair of mirrors forming an optical resonator. The synthesized composites provide both of these functions within a single structure only tens of micrometers in size, implying that one opal film can be split into multiple laser devices. Both the nanocrystal and photonic crystal properties are size tunable making such devices possible at practically any wavelength across the visible and near-infrared spectral range. The materials produced in this work exhibit efficient amplified spontaneous emission (single-pass lasing) while reducing the volume fraction of gain media by a factor of five. The composites also demonstrate amplified spontaneous emission with excitation spot sizes much smaller than is possible without incorporating photonic crystal effects, again indicating enhanced optical gain. Ultimately this work aims to produce compact, low-cost laser devices that can be targeted to any desired wavelength in the visible and near-infrared.

Multicolor Light-Emitting Diodes Based on Semiconductor Nanocrystals Encapsulated in GaN Charge Injection Layers
Nano Letters 5, 1039 (2005)

Numerous technologies including solid-state lighting, displays, and traffic signals can benefit from efficient, color-selectable light sources that are driven electrically. Semiconductor nanocrystals are attractive types of chromophores that combine size-controlled emission colors and high emission efficiencies with excellent photostability and chemical flexibility. Applications of nanocrystals in light-emitting technologies, however, have been significantly hindered by difficulties in achieving direct electrical injection of carriers. Here we report the first successful demonstration of electroluminescence from an all-inorganic, nanocrystal-based architecture in which semiconductor nanocrystals are incorporated into a p-n junction formed from GaN injection layers. The critical step in the fabrication of these nanocrystal/GaN hybrid structures is the use of a novel deposition technique, energetic neutral atom beam lithography/epitaxy, that allows for the encapsulation of nanocrystals within a GaN matrix without adversely affecting either the nanocrystal integrity or its luminescence properties. We demonstrate electroluminescence (injection efficiencies of at least 1%) in both single- and two-color regimes using structures comprising either a single monolayer or a bilayer of nanocrystals.

Light Amplification Using Inverted Core/Shell Nanocrystals: Towards Lasing in the
Single-Exciton Regime
J. Phys. Chem. B 108, 10625 (2004).

The technological potential of NCs as lasing materials is significantly diminished by highly efficient nonradiative Auger recombination of multiexcitons leading to ultrafast decay of optical gain. Here we explore a novel approach to achieve NC lasing in the Auger-recombination-free regime by using type II NC heterostructures that promote spatial separation of electrons and holes. We show that such hetero-NCs can exhibit strong repulsive exciton-exciton interactions that lead to significantly reduced excited-state absorption associated with NCs containing single electron-hole pairs. This effect leads to reduced optical-gain thresholds and can potentially allow lasing in the single-exciton regime, for which Auger recombination is inactive. We use these novel hetero-NCs to demonstrate efficient amplified spontaneous emission (ASE) that is tunable across a “difficult” range of green and blue colors.

Energy- transfer pumping of semiconductor nanocrystals using an epitaxial quantum well
Nature 429, 642 (2004).

Spectral tunability, together with large photoluminescence quantum yields and high photostability, make nanocrystals attractive for use in a variety of light-emitting technologies. An important limitation for such applications, however, is the difficulty of achieving electrical pumping. We found an approach for indirect injection of electron–hole pairs into nanocrystals by non-contact, non-radiative energy transfer from a proximal quantum well. Our theoretical and experimental results indicate that this transfer is fast enough to compete with electron–hole recombination in the quantum well, and results in greater than 50% energy transfer efficiencies in the tested structures.

High Efficiency Carrier Multiplication in PbSe Nanocrystals:
Implications for Solar Energy Conversion
Phys. Rev. Lett. 92, 186601 (2004)

We demonstrate for the first time that impact ionization (II) (the inverse of Auger recombination) occurs with very high efficiency in semiconductor nanocrystals (NCs). Interband optical excitation of PbSe NCs at low pump intensities, for which less than one exciton is initially generated per NC on average, results in the formation of two or more excitons (carrier multiplication) when pump photon energies are more than 3 times the NC band gap energy. The generation of multiexcitons from a single photon absorption event is observed to take place on an ultrafast (picosecond) time scale and occurs with up to 100% efficiency depending upon the excess energy of the absorbed photon. Efficient II in NCs can be used to considerably increase the power conversion efficiency of NC-based solar cells.