Electrodeposition of copper on a Pt(111) electrode in sulfuric acid containing poly(ethylene glycol) and chloride ions as probed by in situ STM

This study employed real-time in situ STM imaging to examine the adsorption of PEG molecules on Pt(111) modified by a monolayer of copper adatoms and the subsequent bulk Cu deposition in 1 M H(2)SO(4) + 1 mM CuSO(4)+ 1 mM KCl + 88 μM PEG. At the end of Cu underpotential deposition (~0.35 V vs Ag/AgCl), a highly ordered Pt(111)-(√3 × √7)-Cu + HSO(4)(-) structure was observed in 1 M H(2)SO(4) + 1 mM CuSO(4). This adlattice restructured upon the introduction of poly(ethylene glycol) (PEG, molecular weight 200) and chloride anions. At the onset potential for bulk Cu deposition (~0 V), a Pt(111)-(√3 × √3)R30°-Cu + Cl(-) structure was imaged with a tunneling current of 0.5 nA and a bias voltage of 100 mV. Lowering the tunneling current to 0.2 nA yielded a (4 × 4) structure, presumably because of adsorbed PEG200 molecules. The subsequent nucleation and deposition processes of Cu in solution containing PEG and Cl(-) were examined, revealing the nucleation of 2- to 3-nm-wide CuCl clusters on an atomically smooth Pt(111) surface at overpotentials of less than 50 mV. With larger overpotential (η > 150 mV), Cu deposition seemed to bypass the production of CuCl species, leading to layered Cu deposition, starting preferentially at step defects, followed by lateral growth to cover the entire Pt electrode surface. These processes were observed with both PEG200 and 4000, although the former tended to produce more CuCl nanoclusters. Raising [H(2)SO(4)] to 1 M substantiates the suppressing effect of PEG on Cu deposition. This STM study provided atomic- or molecular-level insight into the effect of PEG additives on the deposition of Cu.     

Towards the systematic exploration of chemical space

The discovery of biologically active small molecules is shaped, in large part, by their synthetic (or biosynthetic accessibility). However, chemists' historical exploration of chemical space has been highly uneven and unsystematic. This article describes synthetic strategies that have emerged that may allow chemical space to be explored more systematically. Particular emphasis is placed on approaches that allow the scaffolds of small molecules to be varied combinatorially. In addition, some examples of bioactive small molecules that have been discovered by screening diverse small molecule libraries are highlighted. The authors comment on the likely scope of each of the strategies to deliver skeletally-diverse libraries. In addition, the authors highlight some key challenges for the future: the extension to libraries based on hundreds of distinct scaffolds; and the development of approaches that focus overtly on drug-relevant chemical space.     

Measurement of tensor polarization in elastic electron-deuteron scattering at large momentum transfer

Tensor polarization observables ( t(20), t(21), and t(22)) have been measured in elastic electron-deuteron scattering for six values of momentum transfer between 0.66 and 1.7 (GeV/c)(2). The experiment was performed at the Jefferson Laboratory in Hall C using the electron High Momentum Spectrometer, a specially designed deuteron magnetic channel and the recoil deuteron polarimeter POLDER. The new data determine to much larger Q2 the deuteron charge form factors G(C) and G(Q). They are in good agreement with relativistic calculations and disagree with perturbative QCD predictions.     

Energy levels of paired donor impurities in SixGe1−x alloys

The dependences on alloy compositionx of the bound and resonant deep energy levels of nearest-neighbor (P,P), (As, As), and (Sb, Sb) pairs in Si _x Ge_1–x alloys are predicted. Based on these predictions, it appears likely that (P, P) and (As, As) produce at least one deep level in the gap for allx, but that (Sb, Sb) does not for anyx.     

Electron–hole superlattices in GaAs/Al x Ga1− x As multiple quantum wells

A GaAs/Al _x Ga_{1? x} As semiconductor structure is proposed, which is predicted to superconduct at T _c?≈?2?K. Formation of an alternating sequence of electron- and hole-populated quantum wells (an electron–hole superlattice) in a modulation-doped GaAs/Al _x Ga_{1? x} As superlattice is considered. This superlattice may be analogous to the layered electronic structure of high-T _c superconductors. In the structures of interest, the mean spacing between nearest electron (or hole) wells is the same as the mean distance between the electrons (or holes) in any given well. This geometrical relationship mimics a prominent property of optimally doped high-T _c superconductors. Band bending by built-in electric fields from ionized donors and acceptors induces electron and heavy-hole bound states in alternate GaAs quantum wells. A proposed superlattice structure meeting this criterion for superconductivity is studied by self-consistent numerical simulation.     

Evidence for the quantum birth of our universe

We present evidence for a nonsingular origin of the Universe with intial conditions determined by quantum physics and relativistic gravity. In particular, we establish that the present temperature of the microwave background and the present density of the Universe agree well with our predictions from these intial conditions, after evolution to the present age using the Einstein-Friedmann equation. Remarkably, the quantum origin for the Universe naturally allows its evolution at exactly the critical density. We also discuss the consequences of these results to some fundamental aspects of quantum physics in the early Universe.     

Effects on the x-ray spectra of metals due to final-state interactions with effective barriers and shape resonances

Model calculations of X-ray absorption, emission, and photoemission spectral lineshapes are reported for intrinsic and extrinsic core excitations in metals with final-state interactions that can be represented by a central attractive potential surrounded by an effective barrier on its periphery. Some of the calculated spectra exhibit novel features: XPS lineshapes with multi-electron shake-off sidebands and features characteristic of resonances and anti-resonances, and X-ray absorption and emission spectra with (i) edges that are apparently peaked (on a moderate energy scale) when the X-ray edge singularity causes them to be, in fact, suppressed on a fine scale, (ii) edges that are markedly rounded, and (iii) gross spectral features that are influenced greatly by the electronic structure of the iniial state of the Fermi sea, and only weakly by the final-state.     

Role of surface antisite defects in the formation of heterojunctions

Fermi level pinning positions were measured for Si overlayers on cleaved, n-type GaAs and GaP substrates. The observed positions are 0.73 ± 0.1 eV and 0.98 ± 0.1 eV above the top of the substrate valence band. Theoretical calculations of the surface antisite acceptor levels predict the same chemical trend on going from GaAs to GaP. This agreement and other arguments suggest an important role for surface defects in the formation of semiconductor heterojunctions.     

Electronic states of (2 × 1) and (1 × 1) (111) surfaces of Ge,Si, diamond,GaAs and Ge on Si

The “dangling-bond” surface state dispersion curves, E(k), have been calculated for the (2 × 1) and (1× 1) (111) surfaces of Ge, Si, and diamond, for (1 × 1) GaAs, and for (2 × 1) Ge on Si. The calculations employ the sp³s1 empirical tight-binding model of Vogl et al. and the atomic relaxation of Feder et al. The surface state band gaps are in good agreement with optical-absorption and electron-energy-loss measurements for Ge and Si. For the assumed epitaxial geometry, Ge on Si is predicted to shift the dangling-bond states downward by ≈0.1 to 0.4 eV.