25 research outputs found
Atomistic Hydrodynamics and the Dynamical Hydrophobic Effect in Porous Graphene
Mirroring their role in electrical and optical physics, two-dimensional
crystals are emerging as novel platforms for fluid separations and water
desalination, which are hydrodynamic processes that occur in nanoscale
environments. For numerical simulation to play a predictive and descriptive
role, one must have theoretically sound methods that span orders of magnitude
in physical scales, from the atomistic motions of particles inside the channels
to the large-scale hydrodynamic gradients that drive transport. Here, we use
constraint dynamics to derive a nonequilibrium molecular dynamics method for
simulating steady-state mass flow of a fluid moving through the nanoscopic
spaces of a porous solid. After validating our method on a model system, we use
it to study the hydrophobic effect of water moving through pores of
electrically doped single-layer graphene. The trend in permeability that we
calculate does not follow the hydrophobicity of the membrane, but is instead
governed by a crossover between two competing molecular transport mechanisms.Comment: 6 pages, 3 figure
On the Nature of Trapped-Hole States in CdS Nanocrystals and the Mechanism of their Diffusion
Recent transient absorption experiments on CdS nanorods suggest that
photoexcited holes rapidly trap to the surface of these particles and then
undergo diffusion along the rod surface. In this paper, we present a
semiperiodic DFT model for the CdS nanocrystal surface, analyze it, and comment
on the nature of both the hole-trap states and the mechanism by which the holes
diffuse. Hole states near the top of the valence band form an energetic near
continuum with the bulk, and localize to the non-bonding sp orbitals on
surface sulfur atoms. After localization, the holes form nonadiabatic small
polarons that move between the sulfur orbitals on the surface of the particle
in a series of uncorrelated, incoherent, thermally-activated hops at room
temperature. The surface-trapped holes are deeply in the weak-electronic
coupling limit and, as a result, undergo slow diffusion.Comment: 4 figure
Efficient Photon Upconversion Enabled by Strong Coupling Between Organic Molecules and Quantum Dots
Hybrid structures formed between organic molecules and inorganic quantum dots
can accomplish unique photophysical transformations by taking advantage of
their disparate properties. The electronic coupling between these materials is
typically weak, leading photoexcited charge carriers to spatially localize to a
dot or a molecule at its surface. However, we show that by converting a
chemical linker that covalently binds anthracene molecules to silicon quantum
dots from a carbon-carbon single bond to a double bond, we access a
strong-coupling regime where excited carriers spatially delocalize across both
anthracene and silicon. By pushing the system to delocalize, we design a photon
upconversion system with a higher efficiency (17.2%) and lower threshold
intensity (0.5 W/cm^2) than that of a corresponding weakly-coupled system. Our
results show that strong coupling between molecules and nanostructures achieved
through targeted linking chemistry provides a new route for tailoring
properties in materials for light-driven applications.Comment: 33 pages (20 in main text, 13 in supporting information), 12 figures
(5 in main text, 7 in supporting information
Remembering the work of Phillip L. Geissler: A coda to his scientific trajectory
Phillip L. Geissler made important contributions to the statistical mechanics
of biological polymers, heterogeneous materials, and chemical dynamics in
aqueous environments. He devised analytical and computational methods that
revealed the underlying organization of complex systems at the frontiers of
biology, chemistry, and materials science. In this retrospective, we celebrate
his work at these frontiers
Finishing the euchromatic sequence of the human genome
The sequence of the human genome encodes the genetic instructions for human physiology, as well as rich information about human evolution. In 2001, the International Human Genome Sequencing Consortium reported a draft sequence of the euchromatic portion of the human genome. Since then, the international collaboration has worked to convert this draft into a genome sequence with high accuracy and nearly complete coverage. Here, we report the result of this finishing process. The current genome sequence (Build 35) contains 2.85 billion nucleotides interrupted by only 341 gaps. It covers ∼99% of the euchromatic genome and is accurate to an error rate of ∼1 event per 100,000 bases. Many of the remaining euchromatic gaps are associated with segmental duplications and will require focused work with new methods. The near-complete sequence, the first for a vertebrate, greatly improves the precision of biological analyses of the human genome including studies of gene number, birth and death. Notably, the human enome seems to encode only 20,000-25,000 protein-coding genes. The genome sequence reported here should serve as a firm foundation for biomedical research in the decades ahead
Tetracene Aggregation on Polar and Nonpolar Surfaces: Implications for Singlet Fission
In molecular crystals that exhibit
singlet fission, quantum yields
depend strongly on intermolecular configurations that control the
relevant electronic couplings. Here, we explore how noncovalent interactions
between molecules and surfaces stabilize intermolecular structures
with strong singlet fission couplings. Using molecular dynamics simulations,
we studied the aggregation patterns of tetracene molecules on a solid
surface as a function of surface polarity. Even at low surface concentrations,
tetracene self-assembled into nanocrystallites where about 10–20%
of the clustered molecules were part of at least one herringbone structure.
The herringbone structure is the native structure of crystalline tetracene,
which exhibits a high singlet fission quantum yield. Increasing the
polarity of the surface reduced both the amount of clustering and
the relative number of herringbone configurations, but only when the
dipoles on the surface were orientationally disordered. These results
have implications for the application of singlet fission in dye-sensitized
solar cells
Carrier Transport in Heterojunction Nanocrystals Under Strain
We present a theory for carrier transport in semiconducting
nanoscale
heterostructures that emphasizes the effects of strain at the interface
between two different crystal structures. An exactly solvable model
shows that the interface region, or junction, acts as a scattering
potential that facilitates charge separation. As a case study, we
model a type-II CdS/ZnSe heterostructure. After advancing a theory
similar to that employed in model molecular conductance calculations,
we calculate the electron and hole photocurrents and conductances,
including nonlinear effects, through the junction at steady state
The Tunable Hydrophobic Effect on Electrically Doped Graphene
Using
molecular dynamics simulations, we study the hydrophobic
effect on electrically doped single layer graphene. With doping levels
measured in volts, large changes in contact angle occur for modest
voltages applied to the sheet. The effect can be understood as a renormalization
of the surface tension between graphene and water in the presence
of an electric field generated by the dopant charge, an entirely collective
effect termed electrowetting. Because the electronic density of states
scales linearly in the vicinity of the Fermi energy, the cosine of
the contact angle scales quartically with the applied voltage rather
than quadratically, as it would for a two-dimensional metal or in
multiple layer graphene. While electrowetting explains the phenomenon,
it does not account for the slight asymmetry observed in the hydrophobic
response between n- and p-doping