An analysis of exciton transport, electron tunneling, and electromigration in nanotube and nanowire systems

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Chemistry, 2016."February 2016." Vita. Cataloged from PDF version of thesis.Includes bibliographical references (pages 204-213).Herein three systems - electromigration in metal nanowires, electron tunneling in single-molecules, and carbon nanotube photovoltaics - are investigated. In the first area, electromigrative failure of metal nanowires has been shown to form single-molecule tunnel junctions, but the process has remained unpredictable, limiting the yield of devices under current methods. Electromigration in micron diameter and larger wires is well understood as the migration of vacancies in the bulk crystal, but both the quantitative predictions and qualitative features of that mechanism break down at the nanometer scale. We propose, and validate against experimental data, that as the wire diameter falls below a micron,- the increased surface-area-to-volume ratio and the low barrier to surface atom translation shift the dominant mechanism of electromigration from bulk transport to surface transport. We then apply the model to design a process controller to guide gradual electromigration. We then turn to investigating the tunnel junctions themselves. Diverse physical insights have been gained from electron tunneling measurements of single molecules, but to date all observations have been static i.e. subject to long integration times. We performed temporally resolved measurements, revealing underlying molecule dynamics. In particular we find that molecules can stochastically switch between discrete inelastic transport states, suggesting discretized molecule reconfiguration consistent with the body of literature from Scanning Tunneling Microscopy. Finally, we investigate carbon nanotube (CNT) network solar cells. The large parametric space associated with the nanometer-scale heterogeneous material, including the mixture of nanotube length, chirality, orientation, etc., has prevented proof-of-concept devices from revealing a research pathway to practical efficiencies. To address this empirical limitation, we derived a model of CNT photovoltaic steady-state operation from the light absorption and exciton transport behaviors of single and aggregate nanotubes. To do so, we treated single nanotube properties as random variables, describing the nanotube network as distributions of those properties. Applying the model to different solar cell architectures, we predict that efficiencies will be dramatically higher in high density films of verticallyaligned nanotubes. We also show that the film thickness must be at an optimum, and that as a rule of thumb the film thickness should be approximately the exciton diffusion length.by Darin O. Bellisario.Ph. D

A Quantitative and Predictive Model of Electromigration-Induced Breakdown of Metal Nanowires

An isothermal model of electromigration breakdown of metal nanowires 80–700 nm in diameter is developed and validated using experimental data obtained from isolated cylindrical Au nanowires. The model considers electromigration from an applied current producing a net flux of metal atoms, reducing the nanowire radius and conductivity precipitously and accounting for both mass and electronic carrier transport. The model successfully predicts the observed critical failure current, the correct scaling with nanowire radius to 3/2 power, and the impedance evolution prior to breakdown. Application to the case where feedback control is employed to limit the rate of nanowire thinning reproduces key features, including slowed necking, a threshold current and voltage after which lower bias is required to advance formation, and the dependence of these values on feedback parameters

An Analytical Solution for Exciton Generation, Reaction, and Diffusion in Nanotube and Nanowire-Based Solar Cells

Excitonic solar cells based on aligned or unaligned networks of nanotubes or nanowires offer advantages with respect of optical absorption, and control of excition and electrical carrier transport; however, there is a lack of predictive models of the optimal orientation and packing density of such devices to maximize efficiency. Here-in, we develop a concise analytical framework that describes the orientation and density trade-off on exciton collection computed from a deterministic model of a carbon nanotube (CNT) photovoltaic device under steady-state operation that incorporates single- and aggregate-nanotube photophysics published earlier (Energy Environ Sci, 2014, 7, 3769). We show that the maximal film efficiency is determined by a parameter grouping, α, representing the product of the network density and the effective exciton diffusion length, reflecting a cooperativity between the rate of exciton generation and the rate of exciton transport. This allows for a simple, master plot of EQE versus film thickness, parametric in α allowing for optimal design. This analysis extends to any excitonic solar cell with anisotropic transport elements, including polymer, nanowire, quantum dot, and nanocarbon photovoltaics

Importance of Kinetics in Surface Alloying: A Comparison of the Diffusion Pathways of Pd and Ag Atoms on Cu(111)

The microscopic details of how metals alloy have important consequences for both their material properties and their chemical reactivity. In this study, the initial stages of alloying of Pd and Ag with Cu(111) are compared. Low-temperature scanning tunneling microscopy reveals that physical vapor deposition of Pd and Ag at or above room temperature yields remarkably different Surface alloys: Pd predominantly incorporates at the nearest ascending Cu step edge, whereas Ag appears to be able to traverse step edges rather easily and alloys into terraces both above and below its initial adsorption site. Density functional theory calculations reveal that even though Pd adatoms have a lower barrier than Ag for traversing step edges, unlike Ag they bind very strongly to ascending step edges and remain there permanently. This leads to a situation in which Pd atoms have at most a very small number of attempts to leave the terrace on which they are deposited before they are incorporated into the nearest ascending step edge. Ag adatoms, however, have many opportunities to cross step edges and can alloy at positions far from their initial starting point. This direct comparison demonstrates the importance in combining theory and experiment in order to understand complicated surface alloying mechanisms and illustrates how both the kinetics and the thermodynamics of the process must be considered to fully understand experimental observations.11Nsciescopu

Formation of High-Aspect-Ratio Helical Nanorods via Chiral Self-Assembly of Fullerodendrimers

Two novel, asymmetric methanofullerenes are presented, which self-assemble in cyclohexane upon thermal cycling to 80 °C. We show that, through the introduction of a dipeptide sequence to one terminus of the dendritic methanofullerene, it is possible to transform the assembly behavior of these molecules from poorly formed aggregates to high-aspect-ratio nanorods. These nanorods have diameters of 3.76 ± 0.52 nm and appear to be composed of interwoven helices of dendritic fullerenes. As evidenced by circular dichroism, the helicity is characterized by a preferential handedness of assembly, which is imparted by the dipeptide moiety

Experimental Observation of Real Time Molecular Dynamics Using Electromigrated Tunnel Junctions

Single molecule tunnel junctions (SMTJs) can provide important physical insights into electronic and vibrational phenomena at the molecular scale. However, observations and analysis are typically confined to sufficiently low temperatures as to suppress molecular motion and the resulting stochastic fluctuations in the tunneling current. In this work, we introduce and experimentally validate a methodology whereby a slightly higher temperature (9 K) compared to a typical SMTJ study can be used to induce sparse fluctuations in the inelastic tunneling current and provide the fingerprints of dynamics between the conformational states of the molecule. Two examples of benzene dithiol and cysteine are studied in electromigratively formed W/Au nanowire SMTJs on SiO₂ at 9 K. The second-order transform of the tunneling current reveals the expected vibrational spectra. However, we show that temporal fluctuations can be analyzed using a hidden Markov Model to reveal dynamics assigned to millisecond rearrangements of the molecule, with apparent energy barriers ranging from 35 to 66 meV, consistent with theoretical predictions. The observed transitions are consistent with a model of lateral migration of the thiol-anchored molecules in an asymmetric junction. The use of temperature in SMTJs in this way can provide new insights into molecule dynamics in confined volumes and at electrode interfaces

Emergent Properties of Nanosensor Arrays: Applications for Monitoring IgG Affinity Distributions, Weakly Affined Hypermannosylation, and Colony Selection for Biomanufacturing

It is widely recognized that an array of addressable sensors can be multiplexed for the label-free detection of a library of analytes. However, such arrays have useful properties that emerge from the ensemble, even when monofunctionalized. As examples, we show that an array of nanosensors can estimate the mean and variance of the observed dissociation constant (<i>K</i>_D), using three different examples of binding IgG with Protein A as the recognition site, including polyclonal human IgG (<i>K</i>_D μ = 19 μM, σ² = 1000 mM²), murine IgG (<i>K</i>_D μ = 4.3 nM, σ² = 3 μM²), and human IgG from CHO cells (<i>K</i>_D μ = 2.5 nM, σ² = 0.01 μM²). Second, we show that an array of nanosensors can uniquely monitor weakly affined analyte interactions <i>via</i> the increased number of observed interactions. One application involves monitoring the metabolically induced hypermannosylation of human IgG from CHO using PSA-lectin conjugated sensor arrays where temporal glycosylation patterns are measured and compared. Finally, the array of sensors can also spatially map the local production of an analyte from cellular biosynthesis. As an example, we rank productivity of IgG-producing HEK colonies cultured directly on the array of nanosensors itself