Dr. Ben Richards! successfully defends his thesis. Congratulations Ben!

Congratulations, Dr. Richards! for successfully defending your PhD thesis. Pictures from the defense and abstract of the presentation are below.

post-defense champagne.

graduation ceremony


Q&A session






‘the audience’








Bulk-nucleated vapor-solid-solid Silicon and Germanium Nanowires: Synthesis, Scaling, and Applications

Benjamin T. Richards

Cornell University, 2018

Nanowires are essential building blocks for many next-generation devices. This includes applications in solar cells, field effect transistors, thermoelectric generators, chemical sensors, and electrochemical cells. One application of intense promise is using silicon and germanium nanowires as anodes materials in lithium ion batteries, as these materials form stable compounds with superior lithium capacity that increases the anode charge capacity compared to current Li-ion batteries by a factor of 4 – 10. Unlike the bulk material, nanowires have been demonstrated to withstand the ~400% volume dilation due to the intercalation of lithium. To bring the acclaimed potential of nanowires to fruition, challenges in production, scaling, and electrochemical performance must be resolved. Conventional nanowire synthesis methods produce small yields of nanowires that require many processes (i.e., slurry mixing, coating, baking) to introduce into batteries. To meet the growing energy storage demands for personal electronics and electric vehicles, an improved method is required to produce nanowires.

In this work, I will introduce a growth mechanism that is simple, robust, and adaptable to high throughput processing of nucleating and growing nanowires from bulk metal films, known as bulk nucleated vapor-solid-solid (BN-VSS). This method prepares nanowires that are epitaxially connected to the metal substrate, allowing direct integration into batteries. I identify the kinetic conditions required to grow nanowires, the processes that occur during nanowire growth, and copper as a promising growth substrate material. Then, I will present a kinetic model of germanium nanowire growth from copper films using diphenylgermane as a precursor. This model was developed by performing rapid syntheses of nanowires using an inductive heating apparatus followed by ex-situ characterization of these films. Finally, I introduce a diagnostic technique of measuring the electrical resistance of the growth substrate to monitor the solid-state transformation that proceeds to germanium nanowire growth and demonstrate how this method can be used to inform kinetic conditions in high-throughput roll-to-roll reactors.

‘The pulse of electrochemical CO2 reduction’ Kevin Kimura’s paper on the cover of ChemSusChem

Kevin’s recent paper on electrochemical CO2 reduction showed that pulsing the applied potential provides a rich parameter space of previously under appreciated ‘knobs’ to tailor the selectivity of reduced products. More information can be found in the Full Paper by Kimura et al. (https://onlinelibrary.wiley.com/doi/abs/10.1002/cssc.201801130).

Electrochemical CO2 reduction reaction (CO2RR) has garnered strong interest as a promising pathway to convert CO2 emissions into higher value chemicals including fuels and hydrocarbon feedstocks. In particular, copper has been shown to produce a range of useful hydrocarbons at room temperature, which could be powered renewably. The main challenges involved with CO2RR is the low selectivity to one product, competition with the hydrogen evolution reaction (HER) and the overall high overpotentials required. Applying the electrochemical potential in a time-programmed pulse (instead of a constant potential) has interesting implications on both fundamental and applied aspects of CO2RR. From a fundamental perspective, the timing of the square wave pulsed potential provides insights into the coupled dynamics of mass transport and surface reactions. From an applied perspective, pulsed potentials significantly mitigate electrode fouling in electrolytic cells. Intrigued by the prospects of pulsed potentials applied to CO2RR we sought out to understand the underlying mechanism responsible for pulse dependent product selectivity.  

We found that the application of a pulsed potential allows us to substantially suppress the HER while shifting selectivity to CH4 and CO. We attribute the improved CO2RR selectivity to a re-arrangement of surface hydrogen coverage during the pulsing. We also establish that the size and geometry of the electrode matters; when using a small copper electrode, HER was suppressed to less than 5% and methane or CO could be selectivity produced even at fairly low potentials. To decouple the interplay of surface reactions and mass transport to and from the electrode we performed rotating disk electrode experiments and compared the results to an analytical model.  

On a fundamental level, our findings provide new insights into the timescales of competing pathways in CO2RR. From an applied perspective, our results present an opportunity for the product selectivity to be tuned by adjusting the temporal profile of the electrochemical potential. We anticipate our findings may help others to further understand the CO2RR pathways and improve performance for other electrocatalysts.

Doug and Curtis publish their paper on the relationship of mesophase behavior and magic cluster stabilization

Congratulations to Doug and Curtis for publishing their paper in JACS.

Nevers, Douglas R., Curtis B. Williamson, Benjamin H. Savitzky, Ido Hadar, Uri Banin, Lena F. Kourkoutis, Tobias Hanrath, and Richard D. Robinson. “Mesophase Formation Stabilizes High-purity Magic-sized Clusters.” Journal of the American Chemical Society (2018). (https://pubs.acs.org/doi/abs/10.1021/jacs.7b12175)

Magic-sized clusters (MSCs), which are identical in size and resistant to conventional nanoparticle (NP) growth processes, represent an ideal synthetic product to overcome the complications resulting from the size dispersion (at least 3%) inherent to conventional NP synthesis method. However, fundamental reaction pathways toward their nucleation and stabilization still present many outstanding challenges. Here, we show that our high concentration synthesis accentuates fundamental surfactant mesophase behavior that stabilize and enable isolation of high-purity cadmium sulfide MSCs.

In this paper we address a fundamental question for MSCs: how does the precursor concentration direct the synthetic pathway between NPs and MSCs? Building upon our recently developed synthesis methods we demonstrate that a crossover in behavior is achieved by tuning concentration: ultra-high concentration synthesis promotes a well-defined reaction pathway to produce high purity MSCs (>99.9%) and forms a mesophase assembly that kinetically arrests the MSCs from NP growth. Using in-situ X-ray scattering, we find that high-purity MSC formation is accompanied by the production of a large hexagonal mesophase assembly (>100 nm grain size), containing thousands of discrete MSCs. At intermediate concentrations the mesophase assembly is less stable, resulting in NP growth at the expense of the large MSC assemblies. Collectively, these findings present an alternative result from the conventional mantra that the stability of MSCs derives from the precise arrangement of the inorganic structures (i.e., closed-shell atomic packing); we demonstrate that disordered, yet distinct, clusters can also be stabilized by self-forming fibrous mesophase assemblies. In this light we evaluate previous MSC studies and suggest that they may have been, unknowingly, observing mesophase formation or surfactant phase behavior coupled with their MSC formation. Overall, we find that the high concentration approach intensifies and showcases inherent concentration-dependent surfactant phase behavior that is not accessible in conventional (i.e., dilute) conditions.

We propose that our high concentration synthesis of MSCs provides not only a robust method to synthesize, stabilize, and study identical NP products, but also demonstrates an underappreciated stabilizing role of surfactants in NP synthesis.

Kevin’s review paper on Entropic, enthalpic and kinetic aspects of interfacial nanocrystal superlattice assembly and attachment now in press

Kevin’s review paper titled “Entropic, enthalpic and kinetic aspects of interfacial nanocrystal superlattice assembly and attachment” is now available: http://pubs.acs.org/doi/10.1021/acs.chemmater.7b04223

The directed assembly of nanoscale building blocks into complex superstructures is of widespread scientific and technological interest. Scientists and engineers have been intrigued by the prospects of tailoring self-assembly processes to create materials whose properties and function can be tuned through the interaction between constituent particles. In particular, Recent reports of epitaxially connected CQD superlattices with long-range atomic coherence have generated significant interest as a platform for novel, quasi 2D ‘designer materials’.

The coupled thermodynamic and kinetic principles governing the interfacial nanoparticle self-assembly and directed attachment present a rich, albeit complex scientific problem. In the enclosed manuscript, we describe the interesting interplay of entropic and enthalpic driving forces and the kinetic aspects of interfacial self-assembly and attachment. We present in-situ grazing incidence X-ray scattering measurements and emerging insights into the complex choreography of interfacial transport processes involved in the formation of highly ordered epitaxially connected nanocrystal solids. New understanding emerging from in-situ measurements provides process control and design principles for the selective formation of specific superlattice polymorphs. We discuss outstanding challenges that must be resolved to translate know-how from controlled assembly and attachment in the laboratory to scalable integration for emerging technological applications.

Jessica wins two awards!

Congratulations to Jessica for winning TWO awards this month: (1) the WEPAN award and (2) The Ephrahim Garcia Graduate Student Excellence in Mentoring Award.

(details below)





Cornell Engineering’s CBE Women group was recently recognized for their Women’s Outreach in Materials, Energy and Nanobiotechnology (WOMEN) event, hosted each spring at the Robert Frederick Smith School of Chemical and Biomolecular Engineering (CBE) at Cornell. The Women in Engineering ProActive Network (WEPAN) presented the program with its Women in Engineering Initiative Award June 13 at the WEPAN 2017 Change Leader Forum in Westminster, Colorado.

WEPAN awards “honor key individuals, programs, and organizations for accomplishments that underscore WEPAN’s mission. Honorees demonstrate extraordinary service, a significant achievement, model programs and exemplary work environments that promote a culture of inclusion and the success of women in engineering.”

The WOMEN event at Cornell brings high school girls from rural school districts in the Finger Lakes region of New York state to campus on a Saturday and provides them and their parents activities and information about some of the possibilities open if they decide to pursue college-level study in a STEM field.

The event has been run at Cornell since 2010 and was the brainchild of Jennifer Schaefer Ph.D. ’14 and Alexandra Corona M.Eng. ‘10, who were both CBE students. “I grew up in a rural community in Upstate New York,” says Schaefer, who is now an assistant professor at the University of Notre Dame. “I thought that the WOMEN event would be an opportunity for CBE to connect with the surrounding smaller school districts and provide high school girls and their parents information about career options and college preparation that may not otherwise be available to them.”

Susan Daniel, associate professor of chemical and biomolecular engineering at the Smith School, has been involved in the WOMEN event since its inception. “Many young girls are still not sure what a technical career entails and some do not have any role models to show them, especially those from rural communities,” says Daniel. “And because many parents also don’t know what these careers involve, we felt it was important to involve them.”

The Awards Committee at WEPAN recognized the WOMEN event for its effectiveness and because it can serve as a model for other institutions that would like to do something similar. Lakshmi Nathan, Ph.D. student and president of the CBE Graduate Women’s Group, along with Tyler Moeller and Jessica Akemi Cimada da Silva (also Ph.D. students) prepared the submission. Nathan accepted the award on behalf of the program.

Ben Richard’s paper on Reaction Kinetics of Germanium Nanowire Growth on Inductively Heated Copper Surfaces is published

Congratulations to Ben!

The technological potential of semiconductor nanowires has been demonstrated in a broad range of applications spanning optoelectronic, energy, and sensor technologies.  Continued progress towards meeting the large expectations generated by initial proof-of-concept demonstrations risk stalling unless outstanding challenges concerning the scalable fabrication of nanostructured materials are resolved. Inspired by the need for scalable and energy-efficient nanofabrication approaches, we investigated the growth of semiconductor nanowires on inductively heated metal surfaces.  Beyond the practical synthesis advantages, this approach also enables dynamic control over the surface temperature as an advantageous experimental platform to study nanowire growth kinetics and the fundamental reaction mechanism. In contrast to conventional nanowire growth from metal seed particles (i.e., vapor-liquid-solid), the fundamental mechanism and reaction kinetics underpinning the direct nanowire growth on bulk metal films is poorly understood.

Ben’s paper presents a systematic analysis of thermodynamic and kinetic aspects of germanium nanowire growth from copper surfaces. We disentangle the complex interplay between these steps and analyze the reaction kinetics to identify the activation energies of specific steps. These results enable deeper understanding of the basic reaction mechanism and thereby extend the control over NW growth from bulk metal films. From an applied perspective, the ability to engineer rapid, spatially and temporally programmable heating profiles without physical contact to the heater is advantageous for high-throughput processing methods like roll-to-roll manufacturing.

Ben’s paper is available here:

Chem. Mater., 2017, 29 (11), pp 4792–4800
DOI: 10.1021/acs.chemmater.7b00598

Kevin Whitham’s paper on Formation of Epitaxially Connected Quantum Dot Solids published in JPCL

The directed assembly of nanoscale building blocks into complex superstructures is of widespread scientific and technological interest. Scientists and engineers have been intrigued by the prospects of tailoring self-assembly processes to create materials whose properties and function can be tuned through the interaction between constituent particles. In particular, Recent reports of epitaxially connected CQD superlattices with long-range atomic coherence have generated significant interest as a platform for novel, quasi 2D ‘designer materials’. Experimental protocols for the formation of high quality superlattices in which constituent quantum dots are registered to within a single atomic bond length have been established; however, significant gaps persist in our fundamental understanding of several aspects of the underlying mechanism by which these structures form.

Astonishingly, the irreversible attachment of proximate particles through mutually exposed ‘sticky {100} facets’ occurs in a manner that enables the formation of structures with atomic coherence with micrometer-sized grains. In the enclosed manuscript, we present a mechanism for this transformation based on a coherent phase transition with distinct nucleation and propagation steps. Specifically, we show that the transformation is nucleated at defects (voids or grain boundaries) in the pre-assembled superlattice.

Kevin’s paper is available here:

J. Phys. Chem. Lett., 2017, 8 (12), pp 2623–2628
DOI: 10.1021/acs.jpclett.7b00846