Christian and Kevin publish their work on PbSe nanocrystal thin film transistors

Congratulations to Christian and Kevin for publishing their paper “Chalcogenidometallate Clusters as Surface Ligands for PbSe Nanocrystal Field-Effect Transistors” in J. Phys. Chem. C.

ChaM PbSe

In this manuscript we report on a post-assembly ligand exchange strategy for converting films of PbSe nanocrystals into inorganic nanocomposites capped with chalcogenidometallate clusters (ChaMs), and implement them in field-effect transistor channels.  Previously, inorganic nanocrystal composites capped with ChaMs were prepared by replacing the nanocrystals’ organic surfactants with the inorganic clusters via a biphasic solution exchange, and then annealing films of this material at high temperatures to form an all-inorganic composite. This method works reliably for semiconductor nanocrystals like cadmium selenide, but lead chalcogenide nanocrystals do not survive solution exchange due to their unstable surface chemistry.  In this manuscript we develop a post-assembly exchange process for replacing organic surfactants with ChaMs without damaging the PbSe nanocrystals, and thereby expand the variety of surface ligand chemistries that couple with PbSe nanocrystals for optoelectronic device applications. Nanocrystal field effect transistors fabricated from our methods show mobilities as high as 1.3 cm2/V-s.  To our knowledge, this is the first solid exchange procedure to produce ChaM-functionalized PbSe nanocrystal FETs with high carrier mobility (>1 cm2/V-s) without the aid of device modifications such as in-filling with high capacitance oxides or surface doping with lead atoms. This mobility is comparable to the highest recorded figures for hydrazine-only treated devices and is an order of magnitude higher than the best ammonium thiocyanate-functionalized films. Our solid exchange process could serve as a template for facilitating ligand exchange on a wide variety of colloidal nanocrystals regardless of their surface stability/chemistry.

Dave, Bernard and Andrew publish “Detailed balance analysis of NQD solar cells”

Congratulations to Dave, Bernard, and Andrew for publishing their paper “Detailed balance analysis of the conversion efficiency of nanocrystal quantum dot solar cells” in the Journal of Applied Physics

Screen Shot 2014-03-09 at 10.31.18 PMThe paper details our theoretical calculations of the detailed balance conversion efficiency limits of nanocrystal solar cells with excitonic absorption profiles.

Intensive research efforts world-wide are currently directed towards the development of next-generation photovoltaic technologies that combine high conversion efficiency and low cost.  Much of this research is focused on novel absorber materials.  Among the various materials under investigation, nanocrystal quantum dots have garnered increasing attention and their potential has been underscored in steady advances in the conversion efficiency of prototype solar cells.  A unique aspect of nanocrystal absorbers is their size-tunable absorption profiles; this aspect has already been exploited in prototype devices such as single junction and tandem solar cells.

The excitonic absorption profile also has important implications on the conversion efficiency limit of nanocrystal solar cells, which have, to the best of our knowledge, not been previously considered.  Our calculations illustrate the fundamental relationship between the shape of the nanocrystal absorption profile and the conversion efficiency.  We determined conversion efficiency limits as a function of exciton peak and width.  Importantly, our results illustrate that photovoltaic conversion efficiency in nanocrystal solar cells requires careful consideration of the excitonic peak width: peak width relates directly to generated current density, but inversely to open circuit voltage.  We extend our calculations to show the conversion efficiency limits for nanocrystal solar cells with multiexciton generation and conversion efficiency limits in tandem nanocrystal solar cells.

Ben’s nanowire paper in press

Ben’s paper “Direct growth of germanium and silicon nanowires on metal films” is published in J. Mater. Chem. C. Congratulations Ben!

Screen Shot 2014-03-09 at 10.42.20 PM

The paper defines the foundational thermodynamic and kinetic factors governing the direct growth of semiconductor nanowires on bulk metals. This approach marks a paradigm shift in nanowire fabrication since it eliminates the need for nanostructured templates or nanoparticles to direct nanowire growth. The results from our combinatorial study answer several important questions about the basic mechanism of this novel approach to nanowire fabrication. Beyond the significa

nt scientific insights, our results also have important implications by providing guidance to advance the much-acclaimed potential of nanowires from lab-scale prototypes to scalable fabrication of emerging nanotechnologies. Key points of the paper include: (i) the first direct comparison of nanowire growth on a series of metal films under identical synthesis conditions, (ii) the introduction of a basic growth mechanism that explains the observed nanowire growth and correctly predicts nanowire growth reported on other metals.

Whitney wins NREL summer intern poster and KAUST poster

Whitney Wenger, Cornell University KAUST-Cornell Center undergraduate researcher, is awarded 1st prize at KAUST’s 2nd Annual Research Poster Competition for Undergraduates


King Abdullah University of Science and Technology (KAUST) recently held the 2nd Annual Research Poster Competition for Undergraduates, organized by KAUST Professor Niveen M. Khashab. The winners of the undergraduate as well as graduate and postdoctoral poster competitions were honored at an award ceremony held during KAUST’s Winter Enrichment Program 2013. This year’s undergraduate winners were:

1st place: Whitney Wagner – Cornell University (USA), Chemical Engineering. Poster title: Developing a Novel Transport Layer for Quantum Dot Solar Cells
2nd place: Pierce Maguire – Trinity College (Dublin, Ireland), Physics
Poster title: Physical Characterisations of Nanoparticles Systems for Nanotoxicology Studies
3rd place: Nouf AlZahrani – KAUST (Saudi Arabia), Diagnostic Radiology
Poster title: Value of cardiac magnetic resonance imaging in the evaluation of myocardial viability

Selected from three hundred posters abstracts from around the world, including the U.S., Canada, Europe and Saudi Arabia, the top 32 undergraduate finalists were invited to present their posters at KAUST in the Kingdom of Saudi Arabia.


Whitney also won the award for best poster in recognition of her summer research at NREL.


Congratulations Whitney!!!

Whitney wins Merrill Presidential Scholar Award

whitney award

Since 1988, the Merrill Presidential Scholars Program has honored Cornell University’s most outstanding graduating seniors, while also recognizing the teachers who have played a significant role in ensuring their success. This unique program was created by the late Philip Merrill ’55 and is made possible through annual support from the Merrill Family Foundation. Cornell University is grateful to the Merrill family—Eleanor Merrill, Douglas Merrill ’89, MBA ’91, Catherine Merrill Williams ’91, and Nancy Merrill ’96—for their continued commitment to the Merrill Presidential Scholars program.

Merrill Presidential Scholars Program Convocation [MPSP]

Whitney excelled in scientific research, she demonstrated exemplary leadership in the Cornell University Sustainable Design project and, she has mastered the chemical engineering curriculum.  Based on her extensive accomplishments, Whitney stands out as one of Cornell’s most accomplished seniors.

Dr. Will Baumgardner defends his PhD!

Dr. Will Baumgardner defends his PhD!

Congratulations Will!


Abstract of Will’s thesis:

Quantum confined semiconductor nanocrystals, or quantum dots (QD), are a material class with tremendous technological potential. As research intensifies, there is a growing awareness that many of the fundamental assumptions of QD production are not well understood. A higher level of control over the processing methods that affect QD crystallinity, shape, and inter-QD coupling is necessary for the production of reliable, reproducible, and efficient devices. I present original research focused on the optimization of QD structures through the detailed investigation of chemical and physical processing procedures. The first synthesis of size tunable, monodisperse, quantum confined SnSe QDs is described. I discuss the implications of an unusual precursor injection sequence on the nucleation and growth process, and QD crystal structure. I continue my investigations of the synthesis method in a report describing shape control of PbSe QDs. I show that the shape of PbSe QDs can be tuned from pseudo-spherical to cubic by changing only the post-injection temperature ramp rate, a parameter not generally included in literature synthesis descriptions, exposing critical insights into the reproducibility and growth mechanisms of PbSe QDs. Device applications

often demand strict control over the inter-QD coupling while still maintaining quantum confinement. I present two studies on the fabrication of confined-but-connected (CBC) QD films. First, I introduce the pulsed laser annealing (PLA) processing strategy to the QD platform. After defining the process and its effect on QD films, I report the production of CBC structures by the PLA of amorphous silicon encapsulated PbSe QD films. The next report describes the fabrication of CBC PbSe films by facet specific displacement of surface ligands, resulting in epitaxial fusion of proximate QDs. Optical investigations demonstrate that the films retain quantum confinement while improving electrical conductivity by more than 3 orders of magnitude. Finally, the complex nanosized phase behavior of Au and Fe2O3 is investigated using in-situ temperature dependent scanning transmission electron microscopy (STEM). I demonstrate that heating the nanocrystals together results in a new bulk-forbidden composite phase. Using the STEM data, I present the first ever single-particle nanophase diagram with composition, temperature, and size dependent phase behavior.