Breakthroughs in the creation of materials with properties by design continue to emerge from our ability to precisely control size, shape and composition of materials at the nanometer level.  Assembling these materials into multi-component superlattices opens new horizons to create new materials with unprecedented properties. Controlling the interactions between nanocrystals in the superlattice critically depends on improved understanding of the local structure of the connections between the dots.

The results presented Ben’s paper  provide insights into the structure of binary nanocrystal superlattices at an unprecedented level of detail. We combine synchrotron-based X-ray scattering and molecular dynamics simulation to describe the structure of NCs in the hexamer of an AB₆ binary superlattice. We apply this knowledge to demonstrate, for the first time, how the AB₆ superlattice can be used as a ‘nanoreactor’ to probe the structural evolution of the body-centered hexamer into a mesostructured cluster. We point to the successful demonstration of ‘connecting the particles in the box’ as an exciting new avenue to create and study novel materials based on precisely defined clusters of nanocrystal.

Check out the paper for more details:  doi:10.1038/srep06731

Size-dependent properties of nanomaterials continue to intrigue scientist and engineers. In contrast to the optical and electronic properties, size-dependent mechanical properties remain less well understood. We discovered that defect-free colloidal semiconductor nanocrystals exhibit an unexpected compressibility with bimodal size-dependence. We analyzed this phenomenon using high-resolution synchrotron-based wide-angle X-ray scattering of colloidal NCs in a diamond-anvil pressure cell. To explain the observed trends, we introduce a core-shell model with distinct elasticity of the crystal near the surface and the core. Beyond new insights into the size-dependent mechanical properties of colloidal NCs reveal, for the first time, that the Debye temperature of PbS NCs exhibits a bimodal size dependence.

The work brings new insight into the structural and mechanical properties of colloidal PbS nanocrystals.

Check out the paper for more details: doi/abs/10.1021/jz501797y

Scientists at Cornell report the worlds smallest anvil pressure cell allowing them to study uniaxial compression of molecular bundles. 

Assemblies of nanocrystals present many interesting scientific challenges at the confluence of hard and soft matter physics. Our work demonstrates, for the first time, opportunities introduced by the use of nanocrystal superlattice as an experimental platform to probe molecular bundles under uniaxial compression.  We used the assembly itself as a nanoscale pressure cell to probe molecular bundles under uniaxial compression. Our manuscript reports a novel method to uniaxially compress molecules within specific confined spaces of a nanocrystal superlattice. We combined X-ray scattering experiments with density functional theory simulations demonstrate our method to probe the elastic force of single molecule as a function of chain length. We see this methodology as an exciting new opportunity to investigate structure-function relationships of molecules under uniaxial compression.  

Congratulations Kaifu!

DOI: 10.1021/nl501905a

Congratulations to Kevin for his paper on pentacene/ PbS NC bilayer FETs.

Probing surface states in PbS nanocrystal films using pentacene field effect transistors: controlling carrier concentration and charge transport in pentacene

We used a bilayer field effect transistor (FET) consisting of a thin PbS nanocrystals (NCs) film interfaced with vacuum-deposited pentacene to probe trap states in NCs. We interpret the observed threshold voltage shift in context of charge carrier trapping by PbS NCs and relate the magnitude of the threshold voltage shift to the number of trapped carriers. We explored a series of NC surface ligands to modify the interface between PbS NCs and pentacene and demonstrate the impact of interface chemistry on charge carrier density and the FET mobility in a pentacene FET.

 
Congratulations to Kaifu for winning the ‘Best Poster’ award at the 2014 CHESS Annual user meeting.

Establishing foundational structure-property relationships of nanocrystals and their assemblies

Kaifu Bian,^a Zhongwu Wang,^b Detlef Smilgies,^b and Tobias Hanrath,^a

^aSchool of Chemical and Biomolecular Engineering, ^bCornell High Energy Synchrotron Source, Cornell University, Ithaca, NY, 14853

This poster will summarize our recent work at the B1 beamline of CHESS. Assemblies of nanocrysals present many interesting scientific challenges at the confluence of hard and soft matter physics. The B1 station at CHESS presents unique experimental capabilities to probe novel materials under pressure. High-pressure experiments provide new insights into basic structure property relationships. The poster summarizes three recent projects: (1) pressure-dependent optical properties of PbS NCs, (2) size-dependent compressibility of PbS NCs, and (3) utilizing nanocrystal superlattice as a nanostructured pressure cell.

We investigated the pressure-dependent optical properties of PbS NCs. We found that the band gap E_gof PbS NCs decreases with pressure and the pressure coefficient dE_g/dP depends on the size of NCs.  Combining structural information of both atomic lattice and inter-particle separation measured by in-situ high-pressure WAXS and SAXS theoretical calculation reproduced the experimentally obtained pressure-dependent variation of band gap. A second important discovery is the disappearance of the excitonic peak as the particles undergo the high-pressure rock-salt to orthorhombic phase transition.  The pressure-induced changes in crystal structure and electronic structure are reversible. Taken together, our results provide new insights into the size- and pressure-dependent electronic structure of PbS nanocrystal quantum dots.

Our analysis of wide-angle X-ray scattering of PbS NCs under pressure also revealed that compressibility of PbS NCs, like many other properties, is size-dependent. We discovered a maximum stiffness at particle size of about 7 nm. We tentatively attribute this trend by a core-shell model. The size-dependent stiffness of nanocrystals is caused by difference in the elasticity between atoms near the center of a nanocrystal and those at the surface.

Finally, we present our recent demonstration of the nanocrystal superlattice pressure cell. We showed, for the first time, opportunities introduced by the use of nanocrystal superlattice as an experimental platform to probe moleculer bundles under uniaxial compression.  We used the assembly itself as a nanoscale pressure cell to probe molecular bundles under uniaxial compression. We report a novel method to uniaxially compress molecules within specific confined spaces of a nanocrystal superlattice. We combined X-ray scattering experiments with density functional theory simulations demonstrate our method to probe the elastic force of single molecule as a function of chain length. We see this methodology as an exciting new opportunity to investigate structure-function relationships of molecules under uniaxial compression.

Congratulations to Will! His paper on Nanoparticle metamorphosis: An in-situ high-temperature transmission electron microscopy study of the structural evolution of heterogeneous Au:Fe2O3 nanoparticles will appear in ACS Nano.

http://pubs.acs.org/doi/abs/10.1021/nn501543d

Whereas gold and rust are well-understood in their bulk form, our in-situ experiments revealed a complex and scientifically interesting phase behavior of Au and Fe₂O₃ nanoparticles. We found that binary combinations of Au and Fe₂O₃ nanoparticles exhibit intriguing and unexpected phase behavior. We discovered that Au and Fe₂O₃ fuse, in a quasi-fluid fashion, to form heterostructured particles that undergo a series of composition and temperature dependent metamorphoses. 

Kaifu’s paper on “Optical properties of PbS nanocrystal quantum dots at ambient and elevated pressure” is accepted in Phys Chem Chem Phys.

DOI: 10.1039/C4CP00395K

We combined X-ray scattering and optical spectroscopy to probe the evolution of structure and electronic properties of PbS nanocrystal quantum dots under elevated pressure. The pressure coefficient of the energy gap provides important insights into the electronic structure of the material. We discovered that the accurate description of the pressure-coefficient of the energy gap must account for the size-dependent bulk modulus of the material. A second important discovery presented in our paper is the disappearance of the excitonic peak as the particles undergo the high-pressure rock-salt to orthorhombic phase transition. The pressure-induced changes in crystal structure and electronic structure are reversible. Taken together, our results provide new insights into the size- and pressure-dependent electronic structure of PbS nanocrystal quantum dots.

Congratulations to Kaifu, Ben, and Hanqing !

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.

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.

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

The 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.

Our team is participating in the nexus-NY clean energy xcelerator .

We’re exploring commercialization opportunities for scalable manufacturing of semiconductor nanowires in emerging energy applications.