273 research outputs found
Quantum Conductance and Electronic Properties of Lower Diamondoid Molecules and Derivatives
Diamondoids and their derivatives have found major applications as templates
and as molecular building blocks in nanotechnology. Applying ab initio method,
we calculated the quantum conductance and the essential electronic properties
of two lower diamondoids (adamantane and diamantane) and three of their
important derivatives (amantadine, memantine and rimantadine). We also studies
two artificial molecules that are built by substituting one hydrogen ion with
one sodium ion in both adamantane and diamantane molecules. Most of our results
are based on an infinite Au two-probe system constructed by ATK and VNL
software, which comprise TRANSTA-C package. By changing various system
structures and molecule orientations in linear Au and 2 by 2 Au probe systems,
we found that although the conductance of adamantane and diamantane are very
small, the derivatives of the lower diamondoids have considerable conductance
at specific orientations and also showed interesting electronic properties. The
quantum conductance of such molecules will change significantly by changing the
orientations of the molecules, which approves that residues like nitrogen and
sodium atoms have great effects on the conductance and electronic properties of
single molecule. There are obvious peaks near Fermi energy in the transmission
spectrums of artificial molecules, indicating the plateaus in I-V
characteristics of such molecules
Equilibrium phase behavior of polydisperse hard spheres
We calculate the phase behavior of hard spheres with size polydispersity,
using accurate free energy expressions for the fluid and solid phases. Cloud
and shadow curves, which determine the onset of phase coexistence, are found
exactly by the moment free energy method, but we also compute the complete
phase diagram, taking full account of fractionation effects. In contrast to
earlier, simplified treatments we find no point of equal concentration between
fluid and solid or re-entrant melting at higher densities. Rather, the fluid
cloud curve continues to the largest polydispersity that we study (14%); from
the equilibrium phase behavior a terminal polydispersity can thus only be
defined for the solid, where we find it to be around 7%. At sufficiently large
polydispersity, fractionation into several solid phases can occur, consistent
with previous approximate calculations; we find in addition that coexistence of
several solids with a fluid phase is also possible
Predicting phase equilibria in polydisperse systems
Many materials containing colloids or polymers are polydisperse: They
comprise particles with properties (such as particle diameter, charge, or
polymer chain length) that depend continuously on one or several parameters.
This review focusses on the theoretical prediction of phase equilibria in
polydisperse systems; the presence of an effectively infinite number of
distinguishable particle species makes this a highly nontrivial task. I first
describe qualitatively some of the novel features of polydisperse phase
behaviour, and outline a theoretical framework within which they can be
explored. Current techniques for predicting polydisperse phase equilibria are
then reviewed. I also discuss applications to some simple model systems
including homopolymers and random copolymers, spherical colloids and
colloid-polymer mixtures, and liquid crystals formed from rod- and plate-like
colloidal particles; the results surveyed give an idea of the rich
phenomenology of polydisperse phase behaviour. Extensions to the study of
polydispersity effects on interfacial behaviour and phase separation kinetics
are outlined briefly.Comment: 48 pages, invited topical review for Journal of Physics: Condensed
Matter; uses Institute of Physics style file iopart.cls (included
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