Showing papers by "A. Paul Alivisatos published in 2012"
TL;DR: A new type of liquid cell for in situ transmission electron microscopy (TEM) based on entrapment of a liquid film between layers of graphene that facilitates atomic-level resolution imaging while sustaining the most realistic liquid conditions achievable under electron-beam radiation is introduced.
Abstract: We introduce a new type of liquid cell for in situ transmission electron microscopy (TEM) based on entrapment of a liquid film between layers of graphene. The graphene liquid cell facilitates atomic-level resolution imaging while sustaining the most realistic liquid conditions achievable under electron-beam radiation. We employ this cell to explore the mechanism of colloidal platinum nanocrystal growth. Direct atomic-resolution imaging allows us to visualize critical steps in the process, including site-selective coalescence, structural reshaping after coalescence, and surface faceting.
953 citations
TL;DR: It is demonstrated that metallic phases of WO(3-δ) nanoparticles exhibit a strong and tunable localized surface plasmon resonance, which opens up the possibility of rationally designing plasMonic tungsten oxide nanoparticles for light harvesting, bioimaging, and sensing.
Abstract: Transition-metal oxide nanocrystals are interesting candidates for localized surface plasmon resonance hosts because they exhibit fascinating properties arising from the unique character of their outer-d valence electrons. WO3−δ nanoparticles are known to have intense visible and near-IR absorption, but the origin of the optical absorption has remained unclear. Here we demonstrate that metallic phases of WO3−δ nanoparticles exhibit a strong and tunable localized surface plasmon resonance, which opens up the possibility of rationally designing plasmonic tungsten oxide nanoparticles for light harvesting, bioimaging, and sensing.
629 citations
TL;DR: A large-scale, international public effort is proposed, aimed at reconstructing the full record of neural activity across complete neural circuits, which could prove to be an invaluable step toward understanding fundamental and pathological brain processes.
527 citations
TL;DR: This work points the way to multi-Tbit/in(2) memories and provides a glimpse of the structural and electrical manifestations of ferroelectricity down to its ultimate limits.
Abstract: Ferroelectricity in finite-dimensional systems continues to arouse interest, motivated by predictions of vortex polarization states and the utility of ferroelectric nanomaterials in memory devices, actuators and other applications. Critical to these areas of research are the nanoscale polarization structure and scaling limit of ferroelectric order, which are determined here in individual nanocrystals comprising a single ferroelectric domain. Maps of ferroelectric structural distortions obtained from aberration-corrected transmission electron microscopy, combined with holographic polarization imaging, indicate the persistence of a linearly ordered and monodomain polarization state at nanometre dimensions. Room-temperature polarization switching is demonstrated down to ~5 nm dimensions. Ferroelectric coherence is facilitated in part by control of particle morphology, which along with electrostatic boundary conditions is found to determine the spatial extent of cooperative ferroelectric distortions. This work points the way to multi-Tbit/in(2) memories and provides a glimpse of the structural and electrical manifestations of ferroelectricity down to its ultimate limits.
279 citations
TL;DR: In this paper, the authors used coupled optical and electrical modeling to guide optimization of such nanostructures, which can be tailored to minimize absorption in the doped a-Si:H, improving carrier collection efficiency.
Abstract: Nanostructured light trapping has emerged as a promising route toward improved efficiency in solar cells. We use coupled optical and electrical modeling to guide optimization of such nanostructures. We study thin-film n-i-p a-Si:H devices and demonstrate that nanostructures can be tailored to minimize absorption in the doped a-Si:H, improving carrier collection efficiency. This suggests a method for device optimization in which optical design not only maximizes absorption, but also ensures resulting carriers are efficiently collected.
208 citations
TL;DR: By extending the experiment to conductively coupled ring-rod nanostructures, it is shown that a sharp Fano-like resonance feature is smeared out due to the Ti layer, suggesting the prevalence of interfacial reactions.
Abstract: Drastic chemical interface plasmon damping is induced by the ultrathin (∼2 nm) titanium (Ti) adhesion layer; alternatively, molecular adhesion is implemented for lithographic fabrication of plasmonic nanostructures without significant distortion of the plasmonic characteristics. As determined from the homogeneous linewidth of the resonance scattering spectrum of individual gold nanorods, an ultrathin Ti layer reduces the plasmon dephasing time significantly, and it reduces the plasmon scattering amplitude drastically. The increased damping rate and decreased plasmon amplitude are due to the dissipative dielectric function of Ti and the chemical interface plasmon damping where the conduction electrons are transferred across the metal–metal interface. In addition, a pronounced red shift due to the Ti adhesion layer, more than predicted using electromagnetic simulation, suggests the prevalence of interfacial reactions. By extending the experiment to conductively coupled ring-rod nanostructures, it is shown t...
190 citations
TL;DR: It was demonstrated that near monodisperse nanoparticles in the size range of 3-10 nm could be generated without using trioctylphosphine oxide, a capping ligand that shows results in phosphorus being present on the metal surface and poisoning catalyst activity in this application.
Abstract: Model cobalt catalysts for CO2 hydrogenation were prepared using colloidal chemistry. The turnover frequency at 6 bar and at 200–300 °C increased with cobalt nanoparticle size from 3 to 10 nm. It was demonstrated that near monodisperse nanoparticles in the size range of 3–10 nm could be generated without using trioctylphosphine oxide, a capping ligand that we demonstrate results in phosphorus being present on the metal surface and poisoning catalyst activity in our application.
166 citations
TL;DR: This work reports on real-time formation of two-dimensional nanoparticle arrays in the very low diffusive limit, where nanoparticles are mainly driven by capillary forces and solvent fluctuations.
Abstract: Direct imaging of nanoparticle solutions by liquid phase transmission electron microscopy has enabled unique in situ studies of nanoparticle motion and growth. In the present work, we report on real-time formation of two-dimensional nanoparticle arrays in the very low diffusive limit, where nanoparticles are mainly driven by capillary forces and solvent fluctuations. We find that superlattice formation appears to be segregated into multiple regimes. Initially, the solvent front drags the nanoparticles, condensing them into an amorphous agglomerate. Subsequently, the nanoparticle crystallization into an array is driven by local fluctuations. Following the crystallization event, superlattice growth can also occur via the addition of individual nanoparticles drawn from outlying regions by different solvent fronts. The dragging mechanism is consistent with simulations based on a coarse-grained lattice gas model at the same limit.
164 citations
TL;DR: The methodology provides an attractive shortcut for the study of various novel quantum-confined direct band gap semiconductor systems as it permits the band gap energies of a broad size range of QDs to be probed with relatively few synthetic experiments and without quantum mechanical simulations.
Abstract: This work addresses the determination of arbitrarily shaped particle size distributions (PSDs) from PbS and PbSe quantum dot (QD) optical absorbance spectra in order to arrive at a relationship between band gap energy and particle size over a large size range. Using a modified algorithm which was previously developed for ZnO, we take only bulk absorption data from the literature and match the PSDs derived from QD absorbance spectra with those from transmission electron microscopical (TEM) image analysis in order to arrive at the functional dependence of the band gap on particle size. Additional samples sized solely from their absorbance spectra with our algorithm show excellent agreement with TEM results. We investigate the influence of parameters of the TEM image analysis such as threshold value on the final result. The band gap versus size relationship developed from analysis of just two samples lies well within the bounds of a number of published data sets. We believe that our methodology provides an a...
132 citations
TL;DR: It is demonstrated that cation exchange of cadmium pnictide nanocrystals with group 13 ions yields monodisperse, crystalline III-V nanocry crystals, including GaAs, InAs, GaP, and InP.
Abstract: III–V nanocrystals displaying high crystallinity and low size dispersity are difficult to access by direct synthesis from molecular precursors. Here, we demonstrate that cation exchange of cadmium pnictide nanocrystals with group 13 ions yields monodisperse, crystalline III–V nanocrystals, including GaAs, InAs, GaP, and InP. This report highlights the versatility of cation exchange for accessing nanocrystals with covalent lattices.
132 citations
TL;DR: A new light trapping technique that exploits dielectric core-shell optical antennas to strongly enhance solar absorption and holds promise for cost reduction and efficiency improvement of solar conversion devices, including solar cells and solar-to-fuel systems.
Abstract: We demonstrate a new light trapping technique that exploits dielectric core–shell optical antennas to strongly enhance solar absorption. This approach can allow the thickness of active materials in solar cells lowered by almost 1 order of magnitude without scarifying solar absorption capability. For example, it can enable a 70 nm thick hydrogenated amorphous silicon (a-Si:H) thin film to absorb 90% of incident solar radiation above the bandgap, which would otherwise require a thickness of 400 nm in typical antireflective coated thin films. This strong enhancement arises from a controlled optical antenna effect in patterned core–shell nanostructures that consist of absorbing semiconductors and nonabsorbing dielectric materials. This core–shell optical antenna benefits from a multiplication of enhancements contributed by leaky mode resonances (LMRs) in the semiconductor part and antireflection effects in the dielectric part. We investigate the fundamental mechanism for this enhancement multiplication and de...
TL;DR: It is demonstrated that by using nanostructured electromagnetic fields, the selection rules of absorption spectroscopy could be fundamentally manipulated and forbidden transitions between discrete quantum levels in a semiconductor nanorod structure are allowed within the near-field of a noble metal nanoparticle.
Abstract: In conventional spectroscopy, transitions between electronic levels are governed by the electric dipole selection rule because electric quadrupole, magnetic dipole, and coupled electric dipole-magnetic dipole transitions are forbidden in a far field. We demonstrated that by using nanostructured electromagnetic fields, the selection rules of absorption spectroscopy could be fundamentally manipulated. We also show that forbidden transitions between discrete quantum levels in a semiconductor nanorod structure are allowed within the near-field of a noble metal nanoparticle. Atomistic simulations analyzed by an effective mass model reveal the breakdown of the dipolar selection rules where quadrupole and octupole transitions are allowed. Our demonstration could be generalized to the use of nanostructured near-fields for enhancing light-matter interactions that are typically weak or forbidden.
TL;DR: It was found that active sites around trapped holes show higher activity, stronger binding ability, and a different dissociation mechanism for the same substrate and product molecules in comparison with the active sitesaround trapped electrons.
Abstract: We used a fluorogenic reaction to study in conjunction the photocatalytic properties for both active sites (trapped photogenerated electrons and holes) on individual Sb-doped TiO2 nanorods with single-molecule fluorescence microscopy. It was found that active sites around trapped holes show higher activity, stronger binding ability, and a different dissociation mechanism for the same substrate and product molecules in comparison with the active sites around trapped electrons. These differences could be elucidated by a model involving the charged microenvironments around the active sites.
TL;DR: It is shown with molecular dynamics computer simulation that in a model of CdSe/ZnS core/shell nanocrystals the core high-pressure structure can be made metastable under ambient conditions by tuning the thickness of the shell.
Abstract: The kinetics and thermodynamics of structural transformations under pressure depend strongly on particle size due to the influence of surface free energy. By suitable design of surface structure, composition, and passivation it is possible, in principle, to prepare nanocrystals in structures inaccessible to bulk materials. However, few realizations of such extreme size-dependent behavior exist. Here we show with molecular dynamics computer simulation that in a model of CdSe/ZnS core/shell nanocrystals the core high pressure structure can be made metastable under ambient conditions by tuning the thickness of the shell. In nanocrystals with thick shells, we furthermore observe a wurtzite to NiAs transformation, which does not occur in the pure bulk materials. These phenomena are linked to a fundamental change in the atomistic transformation mechanism from heterogenous nucleation at the surface to homogenous nucleation in the crystal core. Our results suggest a new route towards expanding the range of available nanoscale materials.
TL;DR: By varying the thickness of the CdS shell, it is identified hole trapping by the ligand as the dominant charging mechanism in ChaM-capped QDs, highlighting the role of strongly stabilized trapped charges in this system.
Abstract: Fluorescence intermittency and excited-state decay measurements are carried out on single CdSe–CdS core–shell quantum dots (QD) stabilized with chalcogenidometalates (ChaMs, In2Se42–, or Sn2S64–)−. The results are used to probe the nature and distribution of charge trapping sites in the QD local environment. A comparison is made between capping by a neutral organic ligand (oleylamine) and a small inorganic ligand with high charge density (ChaMs). Overall, shorter on-state durations and longer off-state durations are observed for the ChaMs. These results indicate an increased density of charge trapping sites and increased stabilization of surface-trapped charges. By varying the thickness of the CdS shell, we identified hole trapping by the ligand as the dominant charging mechanism in ChaM-capped QDs. Faster excited-state decay rates are measured for the ChaM-capped QDs, highlighting the role of strongly stabilized trapped charges in this system. Using cyclic voltammetry measurements of the ChaMs, an energy...
TL;DR: The use of a decal transfer lithography technique to fabricate elastomeric stamps with triangular cross-sections, specifi cally triangular prisms and cones, is described in this article.
Abstract: The use of a decal transfer lithography technique to fabricate elastomeric stamps with triangular cross-sections, specifi cally triangular prisms and cones, is described. These stamps are used in demonstrations for several prototypical optical applications, including the fabrication of multiheight 3D photoresist patterns with near zero-width features using near-fi eld phase shift lithography, fabrication of periodic porous polymer structures by maskless proximity fi eld nanopatterning, embossing thin-fi lm antirefl ection coatings for improved device performance, and effi cient fabrication of substrates for surface-enhanced Raman spectroscopic sensing. The applications illustrate the utility of the triangular poly(dimethylsiloxane) decals for a wide variety of optics-centric applications, particularly those that exploit the ability of the designed geometries and materials combinations to manipulate light–matter interactions in a predictable and controllable manner.
TL;DR: This application of redox buffers for controlled doping provides a new method for the precise control of the majority carrier concentration in porous semiconductor thin films.
Abstract: Semiconductor nanocrystal solids are attractive materials for active layers in next-generation optoelectronic devices; however, their efficient implementation has been impeded by the lack of precise control over dopant concentrations. Herein we demonstrate a chemical strategy for the controlled doping of nanocrystal solids under equilibrium conditions. Exposing lead selenide nanocrystal thin films to solutions containing varying proportions of decamethylferrocene and decamethylferrocenium incrementally and reversibly increased the carrier concentration in the solid by 2 orders of magnitude from their native values. This application of redox buffers for controlled doping provides a new method for the precise control of the majority carrier concentration in porous semiconductor thin films.
Patent•
25 Jan 2012TL;DR: In this paper, a method for detecting force on the nanoscale using a tetrapod nanocrystal by exposing the tetraploid to light, which produces a luminescent response, is described.
Abstract: Systems and methods of detecting force on the nanoscale are provided. The described invention includes methods for detecting force using a tetrapod nanocrystal by exposing the tetrapod nanocrystal to light, which produces a luminescent response by the tetrapod nanocrystal. The method continues with detecting a difference in the luminescent response by the tetrapod nanocrystal relative to a base luminescent response that indicates a force between a first and second medium or stresses or strains experienced within a material. Such systems and methods find use with biological systems to measure forces in biological events or interactions.