Congratulations to Whitney for winning NREL's summer intern poster contest for her work on metal oxide charge transport layers in quantum dot solar cells.
congratulations, the cube assembly paper"Interface-Induced Nucleation, Orientational Alignment and Symmetry Transformations in Nanocube Superlattices" by Joshua J. Choi, Kaifu Bian, William J. Baumgardner, Detlef-M. Smilgies, and Tobias Hanrath
Is published online
http://pubs.acs.org/doi/abs/10.1021/nl3026289
Dr. Joshua Choi accepted a faculty position in the Chemical Engineering Department at University of Virginia. He will be starting in 2014 after a postdoc stint at Columbia
June 20, 2012 marked the inaugural group bbq "it's almost the longest day of the year" event and Josh's farewell BBQ.
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nanocrystals under pressure. Kaifu's paper titled "Comparing the Structural Stability of PbS Nanocrystals As-sembled in fcc and bcc Superlattice Allotropes" is accepted for publication in JACS.
Using the high-pressure rock salt/ orthorhombic phase transition as a stability indicator, Kaifu discovered that PbS nanocrystals assembled into superlattices with bcc symmetry exhibit a higher structural stability than those assembled into a fcc allotrope.
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Kaifu investigated the structure of PbS NC under pressure inside a diamond anvil cell and discovered that PbS NCs assembled into superlattices with bcc symmetry exhibit a higher structural stability than those assembled into a fcc allotrope. We attribute this effect to the orientational ordering of the NC in the superlattice and the ability of the ligand matrix to absorb the loading force. The results provide new fundamental understanding of the structure-property relationship of ordered superstructures comprised of hard inorganic cores and soft matter ligands |
During Cornell's 2012 graduation ceremony Josh is hooded as a Doctor of Philosophy in recognition of his accomplishments on "NANOCRYSTAL QUANTUM DOTS AS BULDING BLOCKS FOR ARTIFICIAL SOLIDS AND THEIR APPLICATIONS IN OPTOELECTRONIC DEVICES"
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Dr. Choi successfully defended his PhD Thesis titled "NANOCRYSTAL QUANTUM DOTS AS BULDING BLOCKS FOR ARTIFICIAL SOLIDS AND THEIR APPLICATIONS IN OPTOELECTRONIC DEVICES"
Congratulations Dr. Choi !!
| Nanocrystal quantum dots exhibit size-dependent optoelectronic properties and provide intriguing scientific and technological opportunities. Most proposed technologies based on nanocrystals depend on macroscopic functional assemblies of nanocrystals in which the nanocrystals interact with each other to give rise to new collective properties - also called as artificial solids. As in the analogous atomic crystals, the optoelectronic properties of artificial solids are governed by (1) the energy levels of nanocrystals, (2) electronic coupling between nanocrystals, and (3) the symmetry of the nanocrystal superlattice. These issues add many levels of complexity to the design of artificial solids and, for the successful development of nanocrystal based technologies, it is crucial to gain deep understanding on the structure-property relationship of nanocrystals on multiple length scales. In this dissertation, we will present studies that show insights into the three governing factors of the optoelectronic properties of artificial solids mentioned above. (1) Nanocrystal energy levels: we show a direct correlation between interfacial energy level offsets between lead chalcogenide nanocrystals and ZnO layers with photovoltaic device performance. Based on obtained insights on the size dependent photovoltaic properties of lead chalcogenide nanocrystals, first demonstration of solution processed nanocrystal tandem solar cells was achieved. (2) Inter-nanocrystal electronic coupling: we probed rate of photogenerated exciton dissociation in nanocrystal assemblies as a function of inter-nanocrystal spacing. We show that excitons dissociate via tunneling induced delocalization among neighboring nanocrystals. Based on insights obtained from this work, we demonstrate drastically improved performance of solution processed nanocrystal infrared light emitting diodes. (3) Nanocrystal superlattice symmetry: interaction between ligand molecules on the surface of nanocrystals play critical roles in self-assembly process. Differences in the coverage of surface ligands bound to nanocrystals can be exploited to tune the shape of nanocrystal interaction potential during the self-assembly. Denser ligand coverage causes nanocrystals to interact as spheres and face-centered cubic structure is formed. In contrast, sparse ligand coverage amplifies the aspherical shape of the core crystallite and can cause non-close packed structures such as body-centered cubic. |
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Congratulations to Will for publishing his paper on nanocube sunthesis in 'nanoscale'
(see http://pubs.rsc.org/en/Content/ArticleLanding/2012/NR/C2NR31054F)
| The synthesis of NCs with controlled shapes is attractive since it provides structures with specific surface facets and also introduces anisotropic interactions that can be exploited during the self-assembly of superlattices. Our collaboration with Prof. Richard Hennig (MSE, Cornell) led us to predict that NC equilibrium shape is governed by the relative ligand surface coverage on {111} and {100} facets.[ACS Nano, 2012, 6 (3), pp 2118–2127 DOI: 10.1021/nn3000466] Our model suggests that it should be possible to tune the equilibrium shape of the NC from octahedral through truncated octahedral to cubes by tailoring the ligand coverage on specific NC facets. A challenge in experimentally testing this prediction is that the shape of colloidal NCs prepared by the hot-injection method is governed by the interplay of thermodynamic (e.g. nature of the ligand, precursor) and kinetic aspects (i.e.: growth rate of specific crystal facets). To gain a deeper understanding of the interplay of thermodynamic and kinetic factors, Will investigated the growth of cubic PbSe NCs under carefully controlled time-temperature profiles and demonstrated that the shape of PbSe NCs can be robustly tuned from cuboctahedra to monodisperse cubes by adjusting the post-injection ramp rate and keeping all other conditions constant. Congratulations to Will. |
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In collaboration with Wise research group, Joshua Choi and Dr. Liangfeng Sun's co-first author paper on infrared nanocrystal LED has been accepted to Nature Nanotechnology! Congratulations!
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