55,738 research outputs found
The 5f localization/delocalization in square and hexagonal americium monolayers: A FP-LAPW electronic structure study
The electronic and geometrical properties of bulk americium and square and
hexagonal americium monolayers have been studied with the full-potential
linearized augmented plane wave (FP-LAPW) method. The effects of several common
approximations are examined: (1) non-spin polarization (NSP) vs. spin
polarization (SP); (2) scalar-relativity (no spin-orbit coupling (NSO)) vs.
full-relativity (i.e., with spin-orbit (SO) coupling included); (3)
local-density approximation (LDA) vs. generalized-gradient approximation (GGA).
Our results indicate that both spin polarization and spin orbit coupling play
important roles in determining the geometrical and electronic properties of
americium bulk and monolayers. A compression of both americium square and
hexagonal monolayers compared to the americium bulk is also observed. In
general, the LDA is found to underestimate the equilibrium lattice constant and
give a larger total energy compared to the GGA calculations. While spin orbit
coupling shows a similar effect on both square and hexagonal monolayer
calculations regardless of the model, GGA versus LDA, an unusual spin
polarization effect on both square and hexagonal monolayers is found in the LDA
results as compared with the GGA results. The 5f delocalization transition of
americium is employed to explain our observed unusual spin polarization effect.
In addition, our results at the LDA level of theory indicate a possible 5f
delocalization could happen in the americium surface within the same Am II (fcc
crystal structure) phase, unlike the usually reported americium 5f
delocalization which is associated with crystal structure change. The
similarities and dissimilarities between the properties of an Am monolayer and
a Pu monolayer are discussed in detail.Comment: 22 pages, 8 figure
Adhesive and conformational behaviour of mycolic acid monolayers
We have studied the pH-dependent interaction between mycolic acid (MA) monolayers and hydrophobic and hydrophilic surfaces using molecular (colloidal probe) force spectroscopy. In both cases, hydrophobic and hydrophilic monolayers (prepared by Langmuir-Blodgett and Langmuir-Schaefer deposition on silicon or hydrophobized silicon substrates, respectively) were studied. The force spectroscopy data, fitted with classical DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory to examine the contribution of electrostatic and van der Waals forces, revealed that electrostatic forces are the dominant contribution to the repulsive force between the approaching colloidal probe and MA monolayers. The good agreement between data and the DLVO model suggest that beyond a few nm away from the surface, hydrophobic, hydration, and specific chemical bonding are unlikely to contribute to any significant extent to the interaction energy between the probe and the surface. The pH-dependent conformation of MA molecules in the monolayer at the solid-liquid interface was studied by ellipsometry, neutron reflectometry, and with a quartz crystal microbalance. Monolayers prepared by the Langmuir-Blodgett method demonstrated a distinct pH-responsive behaviour, while monolayers prepared by the Langmuir-Schaefer method were less sensitive to pH variation. It was found that the attachment of water molecules plays a vital role in determining the conformation of the MA monolayers. (C) 2010 Elsevier B.V. All rights reserved
Wigner crystal diode
We study the transport properties of a Wigner crystal in one- and
two-dimensional asymmetric periodic potential. We show that the Aubry
transition takes place above a certain critical amplitude of potential with the
sliding and pinned phase below and above the transition. Due to the asymmetry
the Aubry pinned phase is characterized by the diode charge transport of the
Wigner crystal. We argue that the recent experimental observations of Aubry
transition with cold ions and colloidal monolayers can be extended to
asymmetric potentials making possible to observe Wigner crystal diode with
these physical systems and electrons on liquid helium.Comment: 11 pages, 19 figures, accepted to Phys Rev
Sensitive methods for estimating the anchoring strength of nematic liquid crystals on Langmuir-Blodgett monolayers of fatty acids
The anchoring of the nematic liquid crystal
N-(p-methoxybenzylidene)-p-butylaniline (MBBA) on Langmuir-Blodgett monolayers
of fatty acids (COOHCH) was studied as a function of the length
of the fatty acid alkyl chains, (). The monolayers were
deposited onto ITO-coated glass plates which were used to assemble sandwich
cells of various thickness that were filled with MBBA in the nematic phase. The
mechanism of relaxation from the flow-induced quasi-planar to the
surface-induced homeotropic alignment was studied for the four decreases
linearly with increasing the length of the alkyl chains which suggests that
the Langmuir-Blodgett film plays a role in the phenomenon. This fact was
confirmed by a sensitive estimation of the anchoring strength of MBBA on the
fatty acid monolayers after anchoring breaking which takes place at the
transition between two electric-field--induced turbulent states, denoted as
DSM1 and DSM2. It was found that the threshold electric field for the anchoring
breaking, which can be considered as a measure of the anchoring strength, also
decreases linearly as increases. Both methods thus possess a high
sensitivity in resolving small differences in anchoring strength. In cells
coated with mixed Langmuir-Blodgett monolayers of two fatty acids ( and
) a maximum of the relaxation speed was observed when the two acids were
present in equal amount. This observation homeotropic cells by changing the
ratio between the components of the surfactant film.Comment: LaTeX article, 20 pages, 15 figures, 17 EPS files. 1 figure added,
references moved. Submitted to Phys. Rev.
Thermal Stability of Self-Assembled Monolayers of n-Hexanethiol on Au(111)-(1 × 1) and Au(001)-(1 × 1)
Thermal desorption in an ultrahigh vacuum of n-hexanethiol (C6T) self-assembled monolayers (SAMs) prepared from ethanolic solutions on Au(111) and Au(001) unreconstructed surfaces was investigated by X-ray photoelectron spectroscopy. The SAMs desorption was performed from room temperature (RT) to 380 K. We report that the hexanethiolate surface saturation coverage is bigger (∼0.4 ML) for the SAM on Au(001) than on Au(111) (∼0.33 ML). We identified a greater stability for C6T SAMs on Au(001). Large amounts of physisorbed species were found on preferred oriented (111) polycrystalline Au at the low coverage regime at RT, while the SAM on the Au(001) single crystal at this conditions desorbs at a steady pace. At 340 K, both SAMs remain stable at the coverage expected for the lying-down phases that maximizes the van der Waals interactions. We observe that at higher temperatures the carbon alpha-sulfur bond breaks, producing free S on both gold surfaces.Fil: Cristina, Lucila Josefina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Física del Litoral. Universidad Nacional del Litoral. Instituto de Física del Litoral; ArgentinaFil: Ruano Sandoval, Gustavo Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Física del Litoral. Universidad Nacional del Litoral. Instituto de Física del Litoral; ArgentinaFil: Salvarezza, Roberto Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Ferron, Julio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Santa Fe. Instituto de Física del Litoral. Universidad Nacional del Litoral. Instituto de Física del Litoral; Argentina. Universidad Nacional del Litoral. Facultad de Ingeniería Química. Departamento de Materiales; Argentin
Frequency control of photonic crystal membrane resonators by mono-layer deposition
We study the response of GaAs photonic crystal membrane resonators to thin
film deposition. Slow spectral shifts of the cavity mode of several nanometers
are observed at low temperatures, caused by cryo-gettering of background
molecules. Heating the membrane resets the drift and shielding will prevent
drift altogether. In order to explore the drift as a tool to detect surface
layers, or to intentionally shift the cavity resonance frequency, we studied
the effect of self-assembled monolayers of polypeptide molecules attached to
the membranes. The 2 nm thick monolayers lead to a discrete step in the
resonance frequency and partially passivate the surface.Comment: 3 pages, 4 figures, submitted to Appl. Phys. Let
The electrochemical stability of thiols on gold surfaces
In this paper we present a comparative analysis of the electrochemical stability of alkanethiols, aromatic and heterocyclic thiols on the Au(111) and Au(100) faces in relation to the theoretical energetic data. The peak potential and surface coverage are used as the key parameters to estimate the electrochemical stability while work function changes, adsorption energies and surface free energies calculated from periodic DFT, including van der Waals interactions, are used for the theoretical estimation. We find that the peak potentials do not correlate with work function changes and adsorption energies in particular for aromatic and heterocyclic thiols. In contrast, the reductive desorption potentials for the different thiols show a good correlation with the surface free energy of the SAMs estimated by density functional theory calculations. Surface coverage is a key factor that controls reductive desorption through van der Waals interactions.Fil: Salvarezza, Roberto Carlos. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas. Universidad Nacional de La Plata. Facultad de Ciencias Exactas. Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicadas; ArgentinaFil: Carro, Pilar. Universidad de La Laguna; Españ
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