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).
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.
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. 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
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.