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

Ben Treml’s paper on SILAR processing of quantum dot solids is published.

Recent discoveries of the formation of epitaxially connected quasi-two-dimensional quantum dot superlattices have opened new horizons to create novel materials with properties by design. Calculations of such 2D quantum dot solids forecast a rich electronic structure with features such as Dirac cones and topological edge states. However, to date, experimental validation of the properties emerging from the delocalization electrons in these systems is still lacking. Recent studies identified the connections between the dots as a key bottleneck in the localization of electronic states. A key scientific challenge is therefore to understand and control the formation of epitaxial bonds that connect the dots in these assemblies.

Ben’s paper reports how successive ionic layer adsorption and reaction (SILAR) treatment can form new connections between dots and strengthen the bonds during the initial epitaxial attachment to enhance interdot coupling. Specifically, we show that inter-dot connectivity increases from 82 to 91% and increases the bridge width from 3.1 to 4.0 nm. Our field-effect charge transport measurements confirm a systematic increase in conductance and hole mobility with increasing SILAR growth. Collectively, our structural, optical, and electronic characterization indicates that the properties of the quantum dot solids can be tuned through control of the inorganic interdot bonds without sacrificing the long range order of the assembly.

Treml, Benjamin E., et al. “Successive Ionic Layer Absorption and Reaction for Post-Assembly Control over Inorganic Inter-Dot Bonds in Long-Range Ordered Nanocrystal Films.” ACS Applied Materials & Interfaces (2017).

http://pubs.acs.org/doi/abs/10.1021/acsami.7b01588

 

Congratulations to Kevin Kimura and Eric McShane on winning NSF graduate research fellowships

Kevin Kimura and Eric McShane (both working in the Hanrath Lab) have been awarded prestigious NSF Graduate Research Fellowships.

Kevin Kimura has joined the Hanrath Lab last year and is studying photochemical reduction of CO2 to liquid fuels. The award recognizes his his enthusiastic combination of intellectual curiosity, creativity, and passion for science and discovery. Kevin is also involved in several outreach and education activities in collaboration with the Cornell’s Center for Materials Research. 

Eric joined the Hanrath Lab as a Rawlings Cornell Presidential Scholar. Eric’s research has focused on the scalable synthesis of silicon nanostructures for applications in high-capacity next-generation Li-ion batteries. Eric will be attending graduate school in the Fall of 2016. 

The NSF graduate fellowship is awarded to just ~2,000 outstanding students out of ~17,000 applicants across all STEM fields.

Kevin Whitham’s Nature Materials paper on Charge Delocalization in Quantum Dot Solids now online

NM cover2480_x_3508_300dpiRecent discoveries of the formation of epitaxially connected quasi-two-dimensional quantum dot superlattices have opened new horizons to create novel materials with properties by design. Calculations of such 2D quantum dot solids forecast a rich electronic structure with features such as Dirac cones and topological edge states. However, to date, experimental validation of the properties emerging from the delocalization electrons in these systems is still lacking. A key scientific challenge is to understand the nature of charge localization in the best possible quantum dot superlattices that can be made today.

Kevin’s paper  integrates structural analysis (in collaboration with the Kourkoutis Group, AEP), transport measurements and electronic band-structure calculation (in collaboration with the Wise group, AEP) to examine charge delocalization in atomically coherent quantum dot solids. We fabricate superlattices with the quantum dots registered to within a single atomic bond length. Although the structure of the quantum dot solid looks nearly perfect to the human eye, to an electron trying to form a Bloch wave the tolerance for disorder is much lower. Calculations of the electronic structure that include the remaining disorder, which we directly extract from experimental data, account for the electron localization inferred from transport measurements. The calculations show that improvement of the epitaxial connections will lead to completely delocalized electrons and thereby enable observation of the remarkable properties predicted for these materials. The results presented in this paper chart a course for future research to realize the exciting predicted properties.

Check out the full paper here: http://www.nature.com/nmat/journal/vaop/ncurrent/full/nmat4576.html

doi:10.1038/nmat4576

Ben Treml’s paper on µ-Rainbow on the cover of Nano Letters

nalefd_v016i002.indd
The immense technological prospects of creating nanotechnologies that utilize precisely programmed variation in quantum dot (QD) size-dependent properties have intrigued scientist and engineers for years. Despite remarkable advances in the synthesis of colloidal QD with tunable size, shape and composition, the development of enabling technologies in which size-tuned building blocks are patterned with well-defined spatial resolution have lagged behind. In particular, colloidal semiconductor QDs have been proposed for the development of low-cost optoelectronic devices including light-emitting diodes (LEDs). Much of the promise of these materials derives from their size-dependent, and hence tunable, properties.

Our manuscript reports how laser stripe annealing of CdSe QD thin films can yield structures with precisely programmed variations in QD size. We demonstrate that the size-tuned PL emission can be tuned throughout the visible range with a micrometer spatial resolution in a single processing step. Through precise control of the annealing temperature and step, we discovered that QD sintering is a kinetically limited process with a constant activation energy over two orders of magnitude.

To underscore the broader technological implications of our work, we show how periodic modulation of the QD PL properties via laser annealing can change the CIE coordinates of the QD film emission; this enables spatially programmable emission from QD thin films formed from a single feedstock. We illustrate CdSe QD as a proof of concept, we believe that both fundamental kinetic studies as well as periodic property manipulation for applications can be extended to a wide range of nanomaterials with size dependent properties.

Check out the full paper here :/doi/10.1021/acs.nanolett.5b03918

 

 

Kaifu successfully defends his PhD Thesis. Congratulations Dr. Bian.

Untitledimage-4
Dr. Bian defended his thesis titled:
RELATIONSHIPS BETWEEN PROCESSING, STRUCTURE AND PROPERTIESOF NANOCRYSTAL QUANTUM DOTS AND SUPERLATTICES
The abstract to his 422 pg thesis is at the bottom of this post.
Congratulations Kaifu.

image-6

 

 

 

 

 

 

 

image-10

 

 

 

Nanocrystal quantum dots are novel materials of great scientific and technologicalinterests. The attractive features of quantum dots include size-tunable optoelectronic properties, high optical absorption cross section and easiness of synthesis and deposition. These unique features qualify quantum dots as ideal materials for applications such as photovoltaic devices, sensors, light emitting diodes and bioimaging. In most circumstances, individual quantum dots cannot be utilized until they form macroscopic assemblies. Under proper conditions quantum dots self-assemble into periodical superlattices. The properties and performance of quantum dotassemblies depend on not only the intrinsic properties of isolated dots but also theirspatial arrangement or ordering. Consequently the relationships between processing,structure and properties of quantum dots and superlattices are of great importance inguiding the design and fabrication of novel nanomaterials with advantageous featuresusing quantum as building blocks. In this dissertation, I present my graduate researchstudying such trilateral relationships in lead chalcogenide quantum dot systems. 
The first half of this dissertation discusses the relationship between processing andstructure of self-assembled superlattices. An overview of how the ligand-ligand interaction as the major driving force along with factors including particle size, shape, ligand morphology, solvents and interfaces determine superlattice morphology isprovided and followed by specific examples. (1) By tuning surface ligand morphology of PbS quantum dots and growth conditions, the effective particles shape was alteredand therefore different symmetries (fccbcc and bct) of superlattice were achieved. (2)The translational and orientational orderings in an fcc superlattice of cuboctahedronPbS quantum dots was decoded by small and wide angle x-ray scatterings. The dots showed two distinct orientations as a result of the interplay between particle shape andligand attractions. (3) Study of the nucleation, orientational alignment and symmetry transformations of PbS nanocubes at solvent-air and solvent-substrate interfaces is presented to demonstrate the role of interfaces as templates in guiding superlattice formation.
Presented in the second half of this dissertation are my research works using high pressure, which efficiently tunes both superlattice and atomic structures without altering chemistry, to probe the relationships between structure and properties of quantum dots systems. (1) Difference in the pressure-induced atomic phase transition pressure indicated that dots in bcc superlattice are more mechanically stable than thosein fcc due to translational and orientational orderings. (2) Elastic stiffness of PbS quantum dots were found to show size-dependence which is explained by a core-shell model. (3) Size-dependent variation of band gap of PbS quantum dot under elevated pressure was observed and correlated to changes of atomic structure. (4) Quantum dots were innovatively used as a nano-scaled tool to uniaxially compress organic moleculechains and measure the force-length relationship of single molecules.