Kevin Whitham

Confined-but-connected quantum solids via controlled ligand displacement.

Confined-but-connected quantum dot solids (QDS) combine the advantages of tunable, quantum-confined energy levels with efficient charge transport through enhanced electronic interdot coupling. We report the fabrication of QDS by treating self-assembled films of colloidal PbSe quantum dots with polar nonsolvents. Treatment with dimethylformamide balances the rates of self-assembly and ligand displacement to yield confined-but-connected QDS structures with cubic ordering and quasi-epitaxial interdot connections through facets of neighboring dots. The QDS structure was analyzed by a combination of transmission electron microscopy and wide-angle and small-angle X-ray scattering. Excitonic absorption signatures in optical spectroscopy confirm that quantum confinement is preserved. Transport measurements show significantly enhanced conductivity in treated films.

Charge transport and localization in atomically coherent quantum dot solids

Epitaxial attachment of quantum dots into ordered superlattices enables the synthesis of quasi-two-dimensional materials that theoretically exhibit features such as Dirac cones and topological states, and have major potential for unprecedented optoelectronic devices. Initial studies found that disorder in these structures causes localization of electrons within a few lattice constants, and highlight the critical need for precise structural characterization and systematic assessment of the effects of disorder on transport. Here we fabricated superlattices with the quantum dots registered to within a single atomic bond length (limited by the polydispersity of the quantum dot building blocks), but missing a fraction (20%) of the epitaxial connections. Calculations of the electronic structure including the measured disorder account for the electron localization inferred from transport measurements. The calculations also show that improvement of the epitaxial connections will lead to completely delocalized electrons and may enable the observation of the remarkable properties predicted for these materials.

Direct growth of germanium and silicon nanowires on metal films

We describe the basic thermodynamic and kinetic aspects that govern the growth of Si and Ge nanowires directly on bulk metal films. We illustrate essential differences between the vapour–solid–solid and the conventional vapour–liquid–solid nanowire growth. Ge and Si nanowires were formed on a select set of metal films including Ag, Al, Au, Cr, Cu and Ni. Metals that form silicides or germanides (Cr, Cu, and Ni) generally yield higher quality nanowires compared to nanowires grown on metal films whose equilibrium phases are defined by alloyed phases below eutectic temperatures (Al, Ag, Au). Combinatorial experiments presented here provide new basic insights into nanowire formation in the context of metal germanide and silicide formation rates. The mechanism established from our experiments successfully predicts the nanowire growth under a broad range of conditions and also predicts the nanowire growth on other metals to provide guidance to future progress in nanowire synthesis.

Creating and optimizing interfaces for electric-field and photon-induced charge transfer.

We create and optimize a structurally well-defined electron donor-acceptor planar heterojunction interface in which electric-field and/or photon-induced charge transfer occurs. Electric-field-induced charge transfer in the dark and exciton dissociation at a pentacene/PCBM interface were probed by in situ thickness-dependent threshold voltage shift measurements in field-effect transistor devices during the formation of the interface. Electric-field-induced charge transfer at the interface in the dark is correlated with development of the pentacene accumulation layer close to PCBM, that is, including interface area, and dielectric relaxation time in PCBM. Further, we demonstrate an in situ test structure that allows probing of both exciton diffusion length and charge transport properties, crucial for optimizing optoelectronic devices. Competition between the optical absorption length and the exciton diffusion length in pentacene governs exciton dissociation at the interface. Charge transfer mechanisms in the dark and under illumination are detailed.

Propagation of Structural Disorder in Epitaxially Connected Quantum Dot Solids from Atomic to Micron Scale

Epitaxially connected superlattices of self-assembled colloidal quantum dots present a promising route toward exquisite control of electronic structure through precise hierarchical structuring across multiple length scales. Here, we uncover propagation of disorder as an essential feature in these systems, which intimately connects order at the atomic, superlattice, and grain scales. Accessing theoretically predicted exotic electronic states and highly tunable minibands will therefore require detailed understanding of the subtle interplay between local and long-range structure. To that end, we developed analytical methods to quantitatively characterize the propagating disorder in terms of a real paracrystal model and directly observe the dramatic impact of angstrom scale translational disorder on structural correlations at hundreds of nanometers. Using this framework, we discover improved order accompanies increasing sample thickness and identify the substantial effect of small fractions of missing epitaxial bonds on statistical disorder. These results have significant experimental and theoretical implications for the elusive goals of long-range carrier delocalization and true miniband formation.

Formation of Epitaxially Connected Quantum Dot Solids: Nucleation and Coherent Phase Transition

The formation of epitaxially connected quantum dot solids involves a complex interplay of interfacial assembly, surface chemistry, and irreversible-directed attachment. We describe the basic mechanism in the context of a coherent phase transition with distinct nucleation and propagation steps. The proposed mechanism explains how defects in the preassembled structure influence nucleation and how basic geometric relationships govern the transformation from hexagonal assemblies of isolated dots to interconnected solids with square symmetry. We anticipate that new mechanistic insights will guide future advances in the formation of high-fidelity quantum dot solids with enhanced grain size, interconnectivity, and control over polymorph structures.

Epitaxial Quantum Dot Superlattices: From Synthesis to Characterization to Electronic Structure

1. Department of Physics, Cornell University, Ithaca, NY 14853, USA. 2. School of Applied & Engineering Physics, Cornell University, Ithaca, NY 14853, USA. 3. Department of Materials Science & Engineering, Cornell University, Ithaca, NY 14853, USA. 4. School of Chemical & Biomolecular Engineering, Cornell University, Ithaca, NY 14853, USA. 5. Kavli Institute for Nanoscale Science, Cornell University, Ithaca, NY 14853, USA.

Quantitative, Real-Space Statistical Analysis of Imperfect Lattices

1. Dept. of Physics, Cornell University, Ithaca, NY 14853, USA. 2. School of Applied & Engineering Physics, Cornell University, Ithaca, NY 14853, USA. 3. Dept. of Materials Science & Engineering, Cornell University, Ithaca, NY 14853, USA. 4. School of Chemical & Biomolecular Engineering, Cornell University, Ithaca, NY 14853, USA. 5. Kavli Institute for Nanoscale Science, Cornell University, Ithaca, NY 14853, USA.

Long Range Order and Atomic Connectivity in Two-Dimensional Square PbSe Nanocrystal Superlattices

Benjamin H. Savitzky, Robert Hovden, Kevin Whitham, Tobias Hanrath and Lena F. Kourkoutis 1. Department of Physics, Cornell University, Ithaca, NY 14853, USA. 2. School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, USA. 3. Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, USA. 4. School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY 14853, USA. 5. Kavli Institute for Nanoscale Science, Cornell University, Ithaca, NY 14853, USA.

Three-dimensional arrangement and connectivity of lead-chalcogenide nanoparticle assemblies for next generation photovoltaics

1. Department of Physics, Cornell University, Ithaca, NY 14853, USA 2. Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853, USA 3. School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, NY 14853, USA 4. School of Applied and Engineering Physics, Cornell University, Ithaca, NY 14853, USA 5. Kavli Institute for Nanoscale Science, Cornell University, Ithaca, NY 14853, USA