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
Although we didn’t win our title back, we came in second, and had some nice pie.
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 to Kevin for his paper on pentacene/ PbS NC bilayer FETs.
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 Egof PbS NCs decreases with pressure and the pressure coefficient dEg/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.
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.
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 !