Lloyd A. Bumm

Simulated absorption spectra of 1D, 2D, and 3D Phthalocyanine (Pc)-based organic semiconductor molecular thin film using Frenkel model Hamiltonian with long-range Coulomb couplings, for the study of exciton transport in optoelectronic devices.

Phthalocyanine (Pc)-based molecular thin films have emerged in recent years as a promising class of organic semiconductor materials for achieving a long exciton coherence length and fast exciton diffusion. However, the dependence of their exciton properties on the dimensionality and temper- ature of the Pc systems, existence of metal ions, and chemical modifications to the Pc molecule is not yet fully understood. As a first step towards a more comprehensive theoretical understand- ing of the excitonic properties of Pc thin films, we model the low-temperature exciton absorption spectra through employing the Frenkel Hamiltonian and incorporating quantum-chemistry-based site energies and exciton-exciton couplings. The predicted exciton absorption spectra of octabutoxy ph- thalocyanine (H2OBPc) was found to be strongly dependent on the dimensionality of the model as well as the distance cutoff for including the monomer-monomer exciton coupling. The best match to experimental low-temperature absorption spectra requires a 2D or 3D model and an inclusion of at least the nearest neighbor and second-nearest neighbor exciton couplings. We also caution that the widely used dipole-dipole coupling approximation could substantially overestimate the coupling of excitonic transition densities on different monomers.

The aim of this paper is to provide a simple and efficient photoassisted approach to synthesize silver nanoparticles, and to elucidate the role of the key factors (synthesis parameters, such as the concentration of TSC, irradiation time, and UV intensity) that play a major role in the photochemical synthesis of silver nanoparticles using TSC, both as a reducing and stabilizing agent. Concomitantly, we aim to provide an easy way to evaluate the particle size based on Mie theory. One of the key advantages of this method is that the synthesis can be “activated” whenever or wherever silver nanoparticles are needed, by premixing the reactants and irradiating the final solution with UV radiation. UV irradiance was determined by using Keitz’s theory. This argument has been verified by premixing the reagents and deposited them in an enclosed space (away from sunlight) at 25 °C, then checking them for three days. Nothing happened, unless the sample was directly irradiated by UV light. Further, obtained materials were monitored for 390 days and characterized using scanning electron microscopy, UV-VIS, and transmission electron microscopy.

We use dispersion-corrected density functional theory to explore the bridge-site asymmetry for methanethiol adsorbed on Au(111) with two different S–C bond orientations. We attribute the asymmetry to the intrinsic character of the Au(111) surface rather than the adsorbate. The preference for bridge-fcc versus bridge-hcp SCH₃ adsorption sites is controlled by the S–C bond orientation. The system energy difference favors the bridge-fcc site by 8.1 meV on the unrelaxed Au(111) surface. Relaxing the Au substrate increased this energy difference to 26.1 meV. This asymmetry is also reflected in the atomic displacement of the relaxed Au surface. Although in both cases, the bridge-site Au atoms shift away from the fcc 3-fold hollow site, the motion is greater for the bridge-fcc allowing a more favorable geometry for the sulfur atom to bond to the bridging atoms. We confirm that the adsorption energy is strongly dependent on the S–C bond orientation and position, which can be understood in terms of a simple coordination geometry model. This work has important implications for alkanethiol surface diffusion and the structure of their self-assembled monolayers.

We show that the adsorption site basis of the (2√3 × 3)rect. phase of n-alkanethiol self-assembled monolayers plays a key role in determining the molecular conformation of the close-packed alkyl chains. Ten proposed reconstructed Au–S interfaces are used to explore the minimized energy alkyl-chain packing of n-decanethiol molecules using molecular dynamics with the all-atom description. In this comparative study, all models have the same alkyl-chain surface density of four molecules per unit cell; thus, differences are due to the headgroup spacing within the 4-molecule basis as opposed to the average surface density. We demonstrate for the first time the 4-molecule-basis twist structure driven by the packing of alkanethiol molecules in a large simulation box (100 molecules, 25 unit cells) using molecular dynamics. Our results validate the prediction put forward by Mar and Klein that to achieve the 4-molecule-basis twist symmetry observed by the experiment, the headgroups must deviate from the high-symmetry (√3 × √3)R30° sites. The key structural parameters: tilt, twist, and end-group height, as well as their spatial order, are compared with experimental results, which we show is a highly sensitive approach that can be used to vet proposed Au–S interfacial models.

A molecular-level understanding of the interplay between self-assembled monolayers (SAMs) of thiolates and gold surface is of great importance to a wide range of applications in surface science and nanotechnology. Despite theoretical research progress of the past decade, an atomistic model, capable of describing key features of SAMs at reconstructed gold surfaces, is still missing. In this work, the periodic ab initio density functional theory (DFT) calculations were utilized to develop a new atomistic force field model for the alkanethiolate (AT) SAMs on the reconstructed Au(111) surface. The new force field parameters were carefully trained to reproduce the key features, including vibrational spectra and torsion energy profiles of ethylthiolate (C2S) in the bridge or staple motif model on Au(111) surface. Wherein, the force constants of the bond and angle terms were trained by matching the vibrational spectra, while the torsion parameters of the dihedral angles were trained via fitting the torsion energy profiles from DFT calculations. To validate the developed force field parameters, we performed the classical molecular dynamics (MD) simulations for both pristine and reconstructed Au-S interface models with a (2√3×3) unit cell, which includes four dodecanethiolate (C10S) on the Au(111) surface. The simulation results showed that the geometrical features of the investigated Au-S interface models and structural properties of the C10S SAMs are in good agreement with the ab initio MD studies. The newly developed atomistic force field model provides new fundamental insights of AT SAMs on the reconstructed Au(111) surface and adds advancement to existing interface research knowledge.

For radio frequency (RF) wireless integrated circuits and devices fabricated on silicon-on-insulator technologies, there has been indirect evidence of the presence of a parasitic native inversion layer, due to an effective trapped charge, “Q+,” at the interface of the buried oxide (BOX) and the underlying high resistivity p-type Si substrate in production level wafers. This inversion layer can have adverse implications for the RF performance of active devices fabricated for use in wireless technologies, notably the “RF switch.” In this study, the authors characterize the complementary-metal-oxide-semiconductor field effect RF transistor used in such a switch. The FET is built on the active epitaxial silicon layer on top of the BOX layer; its purpose is to control input and output RF signals in integrated wireless circuitry. Using scanning surface photovoltage microscopy, the authors present the first data to show quantitatively the extent of this inversion layer into the substrate. They also present device electrical performance data proving the effectiveness of the processes used to suppress the effects of Q+.

Mechanical stress is a critical parameter in the design and manufacture of devices in very large scale integrated (VLSI) circuits. Whether intentionally introduced or parasitic, mechanical stress in nanoscale silicon technologies can alter carrier mobility as by as much as 25%, which can significantly affect device performance. Currently stress metrology for in-line production is conducted only at a wafer monitor level. For design purposes, the stress state in active device regions is usually inferred from electrical data. In this paper an instrument which we have developed is described for measuring mechanical stress in nanoscale silicon devices with high spatial resolution using scanning surface photovoltage microscopy (SSPVM). Other existing techniques are generally not suitable for making such measurements on production silicon nano-device structures in situ.

In a single population of a livebearing fish, the green swordtail (Xiphophorus hellerii), a trait considered a static badge of status in males of other populations, the color of a stripe covering the lateral line, has been found to be dynamic relative to their social environment, potentially rendering this signal meaningless. Males change the color expression of their lateral stripe dynamically based on social environment. We investigated if males (n = 26) respond to visual and chemical information about other males with a color change and aggressive behavior. We found that visual information is indeed causing color change, whereas chemical information is less effective. Aggressive responses and frequency of response did not change significantly with the mode of communication. We also studied female preferences for color, but found no significant preference (n = 32). Our results lead to questions as to how interpopulation variation can affect preferences and how dynamic signaling—in this case signal transmission presumably dependent on sex and/or status—influences the behavioral interactions we might expect between X. hellerii in the field.