739 publications from this institution
We demonstrate that semiconductor nanorods can be used to fabricate readily processed and efficient hybrid solar cells together with polymers. By controlling nanorod length, we can change the distance on which electrons are transported directly through the thin film device. Tuning the band gap by altering the nanorod radius enabled us to optimize the overlap between the absorption spectrum of the cell and the solar emission spectrum. A photovoltaic device consisting of 7-nanometer by 60-nanometer CdSe nanorods and the conjugated polymer poly-3(hexylthiophene) was assembled from solution with an external quantum efficiency of over 54% and a monochromatic power conversion efficiency of 6.9% under 0.1 milliwatt per square centimeter illumination at 515 nanometers. Under Air Mass (A.M.) 1.5 Global solar conditions, we obtained a power conversion efficiency of 1.7%.
Abstract Review: 153 refs.
Perovskite semiconductors have emerged as a promising class of materials for optoelectronic applications. Their favorable device performances can be partly justified by the defect tolerance that originates from their electronic structure. The effect of this inherent defect tolerance, namely the absence of deep trap states, on the photoluminescence (PL) of perovskite nanocrystals (NCs) is currently not well understood. The PL emission of NCs fluctuates in time according to power law kinetics (PL intermittency, or blinking), a phenomenon that has been explored over the past two decades in a vast array of nanocrystal (NC) materials. The kinetics of the blinking process in perovskite NCs have not been widely explored. Here, PL trajectories of individual orthorhombic cesium lead bromide (CsPbBr<sub>3</sub>) perovskite NCs are measured using a range of excitation intensities. The power law kinetics of the bright NC state are observed to truncate exponentially at long durations, with a truncation time that decreases with increasing intensity before saturating at an intensity corresponding to an average formation of a single exciton. The results indicate that a diffusion-controlled electron transfer (DCET) mechanism is the most likely charge trapping process, while Auger autoionization plays a lesser role. The relevance of the multiple recombination centers (MRC) model to the results presented here cannot be ascertained, since the underlying switching mechanism is not currently available. Further experimentation and theoretical work are needed to gain a comprehensive understanding of the photophysics in these emerging materials.
Semiconducting nanocrystals smaller in size than the bulk Bohr exciton display quantum confinement, leading to a size dependent energy gap and concentration of oscillator strength into a few transitions. Given these properties these materials are being investigated for potential device applications. CdSe nanocrystals have already been implemented in tunable light emitting diodes. 1 We have performed absorption anisotropy experiments on CdSe crystals ranging in diameter from 30-50A with the goal of elucidating the symmetry of the lowest energy optical transition and the earliest events of carrier evolution. The optical anisotropy is defined as R(t)=I ‖ (t)-I ⊥ (t)/K(t), where K(t) is the total population decay (I ‖ + 2I ⊥ ). The initial value of the anisotropy, R(0), is indicative of the nature of the optical transition. For example, if the transition is spherically degenerate, as for C 60 , R(0) is expected to be 0.0. A linear dipole transition has R(0)=0.4 and for a degenerate planar transition R(0)=0.15. 2 The decay in the anisotropy may contain contributions from rotational diffusion, rotation of the transition dipole, as in an isomerization reaction, or population transfer between two excited state levels with transition dipoles of different directions. High time resolution measurements performed on nanocrystals thus give us insight into the shape of the optical transition and the earliest dynamics of the excited carriers. We’ve discovered that a competition exists between the intrinsic wurtzite crystal field and the nonspherical confinement in the crystal 3 which dictates the symmetry of the first exciton state. The crystals are thus intrinsically dichroic, a phenomenon which may be exploited in device applications.
Email Share Share with Facebook Tweet This Post on reddit Share with LinkedIn Add to CiteULike Add to Mendeley Add to BibSonomy Get Citation Copy Citation Text A. P. Alivisatos, "Nanocrystals for Optical Bio-Sensing," in Frontiers in Optics 2007/Laser Science XXIII/Organic Materials and Devices for Displays and Energy Conversion, OSA Technical Digest (CD) (Optica Publishing Group, 2007), paper SThH1. Export Citation BibTex Endnote (RIS) HTML Plain Text Citation alert Save article
Bi2S3 nanostructures with a sheaflike morphology are obtained via reaction of bismuth acetate-oleic acid complex with elemental sulfur in 1-octadecence. These structures may form by the splitting crystal growth mechanism, which is known to account for the morphology some mineral crystals assume in nature. By control of the synthetic parameters, different shapes are obtained, analogous to those which have been observed to occur by crystal splitting in minerals. These new and complex Bi2S3 nanostructures are characterized by transmission and scanning electron microscopy, and electron and X-ray diffraction.
When deposited from an evaporating solution onto a substrate, even nondescript nanoparticles can organize into intricate spatial patterns. Here we show that a simple but long-ranged anisotropy in nanoparticles' interactions can greatly enrich this scenario. In experiments with colloidal Co nanocrystals, which bear a substantial magnetic dipole, we observe assemblies quite distinct from those formed by nonmagnetic particles. Reflecting the strongly nonequilibrium nature of this process, nanocrystal aggregates also differ substantially from expected low-energy arrangements. Using coarse-grained computer simulations of dipolar nanoparticles, we have identified several dynamical mechanisms from which such unusual morphologies can arise. For particles with modest dipole moments, transient connections between growing domains frustrate phase separation into sparse and dense regions on the substrate. Characteristic length scales of the resulting cellular networks depend non-monotonically on the depth of quenches we use to mimic the effects of solvent evaporation. For particles with strong dipole moments, chain-like aggregates formed at early times serve as the agents of assembly at larger scales. Their effective interactions drive the formation of layered loop structures similar to those observed in experiments.
