301 research outputs found
Formation and Evolution of Singularities in Anisotropic Geometric Continua
Evolutionary PDEs for geometric order parameters that admit propagating
singular solutions are introduced and discussed. These singular solutions arise
as a result of the competition between nonlinear and nonlocal processes in
various familiar vector spaces. Several examples are given. The motivating
example is the directed self assembly of a large number of particles for
technological purposes such as nano-science processes, in which the particle
interactions are anisotropic. This application leads to the derivation and
analysis of gradient flow equations on Lie algebras. The Riemann structure of
these gradient flow equations is also discussed.Comment: 38 pages, 4 figures. Physica D, submitte
Irreversible reorganization in a supercooled liquid originates from localised soft modes
The transition of a fluid to a rigid glass upon cooling is a common route of
transformation from liquid to solid that embodies the most poorly understood
features of both phases1,2,3. From the liquid perspective, the puzzle is to
understand stress relaxation in the disordered state. From the perspective of
solids, the challenge is to extend our description of structure and its
mechanical consequences to materials without long range order. Using computer
simulations, we show that the localized low frequency normal modes of a
configuration in a supercooled liquid are causally correlated to the
irreversible structural reorganization of the particles within that
configuration. We also demonstrate that the spatial distribution of these soft
local modes can persist in spite of significant particle reorganization. The
consequence of these two results is that it is now feasible to construct a
theory of relaxation length scales in glass-forming liquids without recourse to
dynamics and to explicitly relate molecular properties to their collective
relaxation.Comment: Published online: 20 July 2008 | doi:10.1038/nphys1025 Available from
http://www.nature.com/nphys/journal/v4/n9/abs/nphys1025.htm
Ultrafast supercontinuum spectroscopy of carrier multiplication and biexcitonic effects in excited states of PbS quantum dots
We examine the multiple exciton population dynamics in PbS quantum dots by
ultrafast spectrally-resolved supercontinuum transient absorption (SC-TA). We
simultaneously probe the first three excitonic transitions over a broad
spectral range. Transient spectra show the presence of first order bleach of
absorption for the 1S_h-1S_e transition and second order bleach along with
photoinduced absorption band for 1P_h-1P_e transition. We also report evidence
of the one-photon forbidden 1S_{h,e}-1P_{h,e} transition. We examine signatures
of carrier multiplication (multiexcitons for the single absorbed photon) from
analysis of the first and second order bleaches, in the limit of low absorbed
photon numbers (~ 10^-2), at pump energies from two to four times the
semiconductor band gap. The multiexciton generation efficiency is discussed
both in terms of a broadband global fit and the ratio between early- to
long-time transient absorption signals.. Analysis of population dynamics shows
that the bleach peak due to the biexciton population is red-shifted respect the
single exciton one, indicating a positive binding energy.Comment: 16 pages, 5 figure
Can impact excitation explain efficient carrier multiplication in carbon nanotube photodiodes?
We address recent experiments (Science 325, 1367 (2009)) reporting on highly
efficient multiplication of electron-hole pairs in carbon nanotube photodiodes
at photon energies near the carrier multiplication threshold (twice the
quasi-particle band gap). This result is surprising in light of recent
experimental and theoretical work on multiexciton generation in other confined
materials, such as semiconducting nanocrystals. We propose a detailed mechanism
based on carrier dynamics and impact excitation resulting in highly efficient
multiplication of electron-hole pairs. We discuss the important time and energy
scales of the problem and provide analysis of the role of temperature and the
length of the diode
Dynamically controlled deposition of colloidal nanoparticles suspension in evaporating drops using laser radiation
Dynamic control of the distribution of polystyrene suspended nanoparticles in evaporating droplets is investigated using a 2.9 μm high power laser. Under laser radiation a droplet is locally heated and fluid flows are induced that overcome the capillary flow, and thus a reversal of the coffee-stain effect is observed. Suspension particles are accumulated in a localised area, one order of magnitude smaller than the original droplet size. By scanning the laser beam over the droplet, particles can be deposited in an arbitrary pattern. This finding raises the possibility for direct laser writing of suspended particles through a liquid layer. Furthermore, a highly uniform coating is possible by manipulating the laser beam diameter and exposure time. The effect is expected to be universally applicable to aqueous solutions independent of solutes (either particles or molecules) and deposited substrates
Distribution of carrier multiplication rates in CdSe and InAs nanocrystals
The distribution of rates of carrier multiplication (CM) following photon
absorption is calculated for semiconductor nanocrystals (NCs). The NC
electronic structure is described using a screened pseudopotential method known
to give reliable description of NC excitons. The rates of biexciton generation
are calculated using the Fermi golden rule with all relevant Coulomb matrix
elements, taking into account proper selection rules. In CdSe and InAs NCs we
find a broad distribution biexciton generation rates depending strongly on the
exciton energy and size of the NC. The process becomes inefficient for NC
exceeding 3 nm in diameter in the photon energy range of 2-3 times the band
gap.Comment: 4 pages 3 fi
Structural diversity in binary nanoparticle superlattices
Assembly of small building blocks such as atoms, molecules and nanoparticles into macroscopic structures - that is, 'bottom up' assembly - is a theme that runs through chemistry, biology and material science. Bacteria(1), macromolecules(2) and nanoparticles(3) can self-assemble, generating ordered structures with a precision that challenges current lithographic techniques. The assembly of nanoparticles of two different materials into a binary nanoparticle superlattice (BNSL)(3-7) can provide a general and inexpensive path to a large variety of materials (metamaterials) with precisely controlled chemical composition and tight placement of the components. Maximization of the nanoparticle packing density has been proposed as the driving force for BNSL formation(3,8,9), and only a few BNSL structures have been predicted to be thermodynamically stable. Recently, colloidal crystals with micrometre-scale lattice spacings have been grown from oppositely charged polymethyl methacrylate spheres(10,11). Here we demonstrate formation of more than 15 different BNSL structures, using combinations of semiconducting, metallic and magnetic nanoparticle building blocks. At least ten of these colloidal crystalline structures have not been reported previously. We demonstrate that electrical charges on sterically stabilized nanoparticles determine BNSL stoichiometry; additional contributions from entropic, van der Waals, steric and dipolar forces stabilize the variety of BNSL structures.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62551/1/nature04414.pd
Genome-wide map of RNA degradation kinetics patterns in dendritic cells after LPS stimulation facilitates identification of primary sequence and secondary structure motifs in mRNAs
Drying-mediated patterns in colloid-polymer suspensions
Drying-mediated patterning of colloidal particles is a physical phenomenon that must be understood in inkjet printing technology to obtain crack-free uniform colloidal films. Here we experimentally study the drying-mediated patterns of a model colloid-polymer suspension and specifically observe how the deposit pattern appears after droplet evaporation by varying particle size and polymer concentration. We find that at a high polymer concentration, the ring-like pattern appears in suspensions with large colloids, contrary to suppression of ring formation in suspensions with small colloids thanks to colloidpolymer interactions. We attribute this unexpected reversal behavior to hydrodynamics and size dependence of colloid-polymer interactions. This finding would be very useful in developing control of drying-mediated self-assembly to produce crack-free uniform patterns from colloidal fluids.ope
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