Electron and Phonon Confinement and New Surface Phonon Modes in CdSe-CdS Core-Shell Nanocrystals

Optical and vibrational properties of bare and CdS shelled CdSe nanocrystalline particles are investigated. To confirm the formation of such nanocrystals in our samples we estimate their average particle sizes and size distributions using TEM measurements. From the line profile analysis of the images the core-shell structure in the particles has been confirmed. The blue shift in optical absorption spectra, analyzed using theoretical estimates based on the effective bond order model, establishes the electron confinement in the nanoparticles. Unique characteristics of the nanocrystals (which are absent in the corresponding bulk material), such as confinement of optical phonons and the appearance of surface phonons, are then discussed. Making use of the dielectric response function model we are able to match the experimental and theoretical values of the frequencies of the surface phonons. We believe that our studies using optical probes provide further evidence on the existence of core-shell structures in CdSe-CdS type materials.Comment: 19 pages 8 figure

Low current MeV Au<SUP>2+</SUP> ion-induced amorphization in silicon: rutherford backscattering spectrometry and transmission electron microscopy study

The amorphization due to MeV Au2+ ion implantation in Si(1 1 1) has been studied using Rutherford backscattering spectrometry/channeling (RBS/C) and transmission electron microscopy (TEM) methods. 1.5 MeV Au2+ ions were implanted into Si(1 1 1) substrates at various fluences at low currents (0.02-0.04&#956;A cm-2) while the samples were kept at room temperature. The RBS/C results for as-implanted specimen shows the onset fluence for amorphization to be &#8776;5&#215; 1013 ions cm-2 which is much lower than the fluence reported earlier. Selected area diffraction (TEM) for a sample implanted at a of 1&#215;1014 ions cm-2 confirms the occurrence of the amorphization. Earlier, amorphization studies by Alford and Theodore, using 2.4 MeV gold ions in silicon (1 0 0) reported a threshold fluence of 1.8&#215;1015 ions cm-2 for amorphization when the implantation was carried out at higher currents (0.2-5 &#956; A cm-2) [J. Appl. Phys. 76 (1994) 7265]. The nuclear energy loss (Sn) for 1.5 MeV gold ions in silicon is &#8776;13% greater than the value for 2.4 MeV and cannot be the sole reason for lower threshold fluence for the amorphization. The amorphization at a relatively lower fluence for the low current implantations could be possible due to reduction in the dynamical annealing effects

Mechanistic details of the formation and growth of nanoscale voids in Ge under extreme conditions within an ion track

The formation of nanoscale voids in amorphous-germanium (a-Ge), and their size and shape evolution under ultra-fast thermal spikes within an ion track of swift heavy ion, is meticulously expatiated using experimental and theoretical approaches. Two step energetic ion irradiation processes were used to fabricate novel and distinct embedded nanovoids within bulk Ge. The 'bow-tie' shape of voids formed in a single ion track tends to attain a spherical shape as the ion tracks overlap at a fluence of about 1 x 10(12) ions cm(-2). The void assumes a prolate spheroid shape with major axis along the ion trajectory at sufficiently high ion fluences. Small angle x-ray scattering can provide complementary information about the primary stage of void formation hence this technique is applied for monitoring simultaneously their formation and growth dynamics. The results are supported by the investigation of cross-sectional transmission and scanning electron micrographs. The multi-time-scale theoretical approach corroborates the experimental findings and relates the bow-tie shape void formation to density variations as a result of melting and resolidification of Ge within the region of thermal spike generated along an ion track, plus non-isotropic stresses generated towards the end of the thermal spike.Peer reviewe

Crater formation in gold nanoislands due to mev self-ion irradiation

The modification of gold nanoislands, grown on silicon substrates under high-vacuum conditions, by MeV self-ion irradiation has been studied by using scanning electron microscopy, transmission electron microscopy, atomic force microscopy, and x-ray reflectivity. Upon irradiation with 1.5MeVAu2+, two types of craters are observed on the Au islands: Empty craters and craters with a central hillock. The contribution of plastic flow, pressure spike, and sputtering to the crater formation during the ion impacts on the gold islands is analyzed. Thermal spike confinement within the gold islands is also proposed to be one of the possible reasons for crater formation in nanoislands

Electrochemical Oxidation Assessment and Interaction of 2-aminoethanol and N, N-diethylethanamine Propagation in Acidic Medium

Electro�oxidation and inhibitor performance of copper specimens in 1 M hydrochloric acid solu� tion was investigated at room temperature by linear potentiodynamic polarization and gravimetric method in the presence of 2�aminoethanol (A) and N, N�diethylethanamine (D) as an inorganic inhibitor. The effect of the inhibitory concentration on the corrosion behavior of copper was studied over 288 hrs at 298°K. The inhibitory efficiency rise up to 96% for single induced and 98% for synergistic behavior. The adsorption mechanism characteristic was supported by SEM/EDX analysis and adsorption isotherm. From all indica� tion, the inhibitive efficiency of these compounds majorly depends on their molecular structure and concen� tration. The blocking effects of the surface interface were also explained on the basis of the inhibitor active action. 2�aminoethanol and N, N�diethylethanamine inhibits copper in 1 M HCl by strictly affecting both the anodic and cathodic sites. Portion of the surface covered calculated was also found to follow Langmuir adsorption isotherm

Free Energy Simulations of a GTPase: GTP and GDP Binding to Archaeal Initiation Factor 2

International audienceArchaeal initiation factor 2 (aIF2) is a protein involved in the initiation of protein biosynthesis. In its GTP-bound, "ON" conformation, aIF2 binds an initiator tRNA and carries it to the ribosome. In its GDP-bound, "OFF" conformation, it dissociates from tRNA. To understand the specific binding of GTP and GDP and its dependence on the ON or OFF conformational state of aIF2, molecular dynamics free energy simulations (MDFE) are a tool of choice. However, the validity of the computed free energies depends on the simulation model, including the force field and the boundary conditions, and on the extent of conformational sampling in the simulations. aIF2 and other GTPases present specific difficulties; in particular, the nucleotide ligand coordinates a divalent Mg(2+) ion, which can polarize the electronic distribution of its environment. Thus, a force field with an explicit treatment of electronic polarizability could be necessary, rather than a simpler, fixed charge force field. Here, we begin by comparing a fixed charge force field to quantum chemical calculations and experiment for Mg(2+):phosphate binding in solution, with the force field giving large errors. Next, we consider GTP and GDP bound to aIF2 and we compare two fixed charge force fields to the recent, polarizable, AMOEBA force field, extended here in a simple, approximate manner to include GTP. We focus on a quantity that approximates the free energy to change GTP into GDP. Despite the errors seen for Mg(2+):phosphate binding in solution, we observe a substantial cancellation of errors when we compare the free energy change in the protein to that in solution, or when we compare the protein ON and OFF states. Finally, we have used the fixed charge force field to perform MDFE simulations and alchemically transform GTP into GDP in the protein and in solution. With a total of about 200 ns of molecular dynamics, we obtain good convergence and a reasonable statistical uncertainty, comparable to the force field uncertainty, and somewhat lower than the predicted GTP/GDP binding free energy differences. The sign and magnitudes of the differences can thus be interpreted at a semiquantitative level, and are found to be consistent with the experimental binding preferences of ON- and OFF-aIF2