Unbalanced Hole and Electron Diffusion in Lead Bromide Perovskites
In collaboration with the Roy and Owen groups at Columbia, we measure the lifetime and diffusion length of single crystals of lead bromide perovskites APbBr3 (A=methylammonium, formamidinium, Cs) using scanning photocurrent microscopy and time resolved microwave conductivity spectroscopy. We find that the hole diffusion length is on average an order of magnitude longer than the electron diffusion length, irrespective of the A cation. This limitation is an important factor to consider when designing and optimizing perovskite solar cells.
Plasmon Resonances in Self-Assembled Two-Dimensional Au Nanocrystal Metamolecules
We explore the evolution of plasmonic modes in two-dimensional nanocrystal oligomer “metamolecules” as the number of nanocrystals is systematically varied. Darkfield scattering spectroscopy of individual self-assembled oligomers in combination with theoretical simulations is used to map the wavelength and character of plasmonic modes as a function of metamolecule size and degree of disorder in NC packing.
Hierarchical Materials Design by Pattern Transfer Printing of Self-Assembled Binary Nanocrystal Superlattices
We demonstrate the fabrication of hierarchical materials, structured on multiple length scales, by combining self-assembly and transfer printing of binary nanocrystal superlattices.
Engineering the Surface Chemistry of Lead Chalcogenide Nanocrystal Solids to Enhance Carrier Mobility and Lifetime in Optoelectronic Devices
We introduce a stepwise, hybrid ligand-exchange method for lead
chalcogenide nanocrystal (NC) thin films using the compact inorganic ligand thiocyanate and the short organic ligand benzenediothiolate. We show that hybrid exchange enhances both carrier mobility and lifetime in NC thin films and allows the demonstration of optoelectronic devices
with enhanced power conversion and quantum efficiency.
High-Strength Magnetically Switchable Plasmonic Nanorods Assembled from a Binary Nanocrystal Mixture
We report the fabrication of multifunctional 'smart' nanoparticle systems by combining top-down fabrication and bottom-up self-assembly methods. Particularly, we template nanorods from a mixture of superparamagnetic Zn0.2
and plasmonic Au nanocrystals. Nanoscrytal-derived superparamagnetism prevents these nanorods from spontaneous magnetic-dipole-induced aggregation, while their magnetic anisotropy makes them responsive to an external field. Ligand exchange drives Au nanocrystal fusion and forms a porous network, imparting the nanorods with high mechanical strength and polarization-dependent, infrared surface plasmon resonances. The combined superparamagnetic and plasmonic functions enable switching of the infrared transmission of a hybrid nanorod suspension using an external magnetic field.
Direct Observation of Electron-Phonon Coupling and Slow Vibrational Relaxation in Organic-Inorganic Hybrid Perovskites
We harness the large exciton binding energy in 2D perovskites to directly observe electron-phonon coupling and slow vibrational relaxation using steady-state and time-resolved optical spectroscopy.
Mapping the Competition between Exciton Dissociation and Charge Transport in Organic Solar Cells
Correlated scanning confocal photoluminescence and photocurrent microscopies are used to show that contrary to general expectation, higher photoluminesence quenching is not always indicative of higher photocurrent in bulk-heterojunction organic solar cells. This work suggests that photoluminescence-quenching measurements should be used with caution in evaluating new organic materials for solar cells.
Advanced Architecture for Colloidal PbS Quantum Dot Solar Cells Exploiting a CdSe Quantum Dot Buffer Layer
We introduce a CdI2-treated CdSe quantum dot buffer layer at the junction between ZnO nanoparticles and PbS quantum dots to improve the performance of the solar cell devices. After optimizing the carrier concentration and energy band alignment of the CdSe QD buffer layer in the heterojunction, we find suppressed interface recombination and additional photogenerated carriers attributed to the buffer layer, leading to a 25% increase in solar power conversion efficiency.
Building Devices from Colloidal Quantum Dots
Kagan, Lifshitz, Sargent, and Talapin review recent progress in tailoring colloidal quantum dots and their assemblies to build electronic and optoelectronic devices.
Roadmap on Optical Metamaterials
Kagan and Murray describe using nanoparticles as building blocks for the design of optical metamaterials.
Exploiting the Colloidal Nanocrystal Library to Construct Electronic Devices
We exploit metallic, semiconducting, and insulating colloidal nanocrystals and design the materials, interfaces, and processes to construct all-nanocrystal electronic devices on flexible plastics using solution-based processes.
At the Nexus of Food Security and Safety: Opportunities for Nanoscience and Nanotechnology
Kagan describes the opportunities for nanoscience and nanotechnology to address the grand challenge at the nexus of food security and safety in an ACS Nano editorial.
Increased Carrier Mobility and Lifetime in CdSe Quantum Dot Thin Films through Surface Trap Passivation and Doping
We investigate the effect of introducing indium in CdSe QD thin films on the dark mobility and the photogenerated carrier mobility and lifetime using field-effect transistor (FET) and time-resolved microwave conductivity (TRMC) measurements. As the amount of indium increases, the FET and TRMC mobilities and the TRMC lifetime increase. The increase in mobility and lifetime is consistent with increased indium passivating midgap and band-tail trap states and doping the films, shifting the Fermi energy closer to and into the conduction band.
Charge transport in strongly coupled quantum dot solids
Kagan and Murray review the chemical and physical origins of high mobility charge transport in quantum dot (QD) solids that is being exploited in devices. They also present future prospects for QD materials and device design.
Flexible, High-Speed CdSe Nanocrystal Integrated Circuits
We report large area, flexible, photopatterned integrated circuits based on CdSe nanocrystal semiconductors. Our results show high performance devices with transistor mobilities exceeding 10 cm2
/Vs, amplifiers with ~7 kHz bandwidth, and oscillators with sub-10 µs stage delays as well as NAND and NOR logic gates.