arXiv subjects

Moungi G. Bawendi

Publications and source records attributed to Moungi G. Bawendi.

22 records · Page 2Linked to original sources

arXiv2018

Homogenization of Halide Distribution and Carrier Dynamics in Alloyed Organic-Inorganic Perovskites

Perovskite solar cells have shown remarkable efficiencies beyond 22%, through organic and inorganic cation alloying. However, the role of alkali-metal cations is not well-understood. By using synchrotron-based nano-X-ray fluorescence and complementary measurements, we show that when adding RbI and/or CsI the halide distribution becomes homogenous. This homogenization translates into long-lived charge carrier decays, spatially homogenous carrier dynamics visualized by ultrafast microscopy, as well as improved photovoltaic device performance. We find that Rb and K phase-segregate in highly concentrated aggregates. Synchrotron-based X-ray-beam-induced current and electron-beam-induced current of solar cells show that Rb clusters do not contribute to the current and are recombination active. Our findings bring light to the beneficial effects of alkali metal halides in perovskites, and point at areas of weakness in the elemental composition of these complex perovskites, paving the way to improved performance in this rapidly growing family of materials for solar cell applications.

cond-mat.mtrl-sci↗

arXiv2017

Morphology of passivating organic ligands around a nanocrystal

Semiconductor nanocrystals are a promising class of materials for a variety of novel optoelectronic devices, since many of their properties, such as the electronic gap and conductivity, can be controlled. Much of this control is achieved via the organic ligand shell, through control of the size of the nanocrystal and the distance to other objects. We here simulate ligand-coated CdSe nanocrystals using atomistic molecular dynamics, allowing for the resolution of novel structural details about the ligand shell. We show that the ligands on the surface can lie flat to form a highly anisotropic 'wet hair' layer as opposed to the 'spiky ball' appearance typically considered. We discuss how this can give rise to a dot-to-dot packing distance of one ligand length since the thickness of the ligand shell is reduced to approximately one-half of the ligand length for the system sizes considered here; these distances imply that energy and charge transfer rates between dots and nearby objects will be enhanced due to the thinner than expected ligand shell. Our model predicts a non-linear scaling of ligand shell thickness as the ligands transition from 'spiky' to 'wet hair'. We verify this scaling using TEM on a PbS nanoarray, confirming that this theory gives a qualitatively correct picture of the ligand shell thickness of colloidal quantum dots.

physics.chem-ph↗

arXiv2015

Conductance noise in nano-patterned PbS quantum dot arrays

We report unexpectedly large noise in the current from nanopatterned PbS quantum dot films. The noise is proportional to the current when the latter is varied by changing the source-drain bias, gate voltage or temperature. The spectral density of the noise is given by a power law in frequency at room temperature, but remarkably, we observe a transition to telegraph noise at lower temperatures. The probability distribution of the off-times follows a power law, reminiscent of fluorescence blinking in colloidal quantum dot systems. Our results are understood simply in terms of conductance fluctuations in a quasi-one dimensional percolation path, and more rigorously in terms of a model in which charge through the film is transmitted in discrete time intervals, with the distribution of intervals completely described by Levy statistics.

cond-mat.mes-hall↗

arXiv2005

Multi-island single-electron devices from self-assembled colloidal nanocrystal chains

We report the fabrication of multi-island single-electron devices made by lithographic contacting of self-assembled alkanethiol-coated gold nanocrystals. The advantages of this method, which bridges the dimensional gap between lithographic and NC sizes, are (1) that all tunnel junctions are defined by self-assembly rather than lithography and (2) that the ratio of gate capacitance to total capacitance is high. The rich electronic behavior of a double-island device, measured at 4.2 K, is predicted in detail by combining finite element and Monte Carlo simulations with the standard theory of Coulomb blockade with very few adjustable parameters.

cond-mat.other↗