Modern Physics Letters B

Modern nanoparticle research in the field of small metallic systems has confirmed that many nanoparticles take on some Platonic and Archimedean solids related shapes. A Platonic solid looks the same from any vertex, and intuitively they appear as good candidates for atomic equilibrium shapes. A very clear example is the icosahedral (I_h) particle that only shows {111} faces that contribute to produce a more rounded structure. Indeed, many studies report the I_h as the most stable particle at the size range r≤20 Å for noble gases and for some metals. In this review, we report on the structure and shape of mono- and bimetallic nanoparticles in the wide size range from 1–300 nm. First, we present AuPd nanoparticles in the 1–2 nm size range that show dodecahedral atomic growth packing, one of the Platonic solid shapes that have not been identified before in this small size range for metallic particles. Next, with particles in the size range of 2–5 nm, we present an energetic surface reconstruction phenomenon observed also on bimetallic nanoparticle systems of AuPd and AuCu, similar to a re-solidification effect observed during cooling process in lead clusters. These binary alloy nanoparticles show the fivefold edges truncated, resulting in {100} faces on decahedral structures, an effect largely envisioned and reported theoretically, with no experimental evidence in the literature before. Next nanostructure we review is a monometallic system in the size range of ≈5 nm that we termed the decmon. We present here some detailed geometrical analysis and experimental evidence that supports our models. Finally, in the size range of 100–300 nm, we present icosahedrally derived star gold nanocrystals which resembles the great stellated dodechaedron, which is a Kepler–Poisont solid. We conclude then that the shape or morphology of some mono- and bimetallic particles evolves with size following the sequence from atoms to the Platonic solids, and with a slightly greater particle's size, they tend to adopt Archimedean related shapes. If the particle's size is still greater, they tend to adopt shapes beyond the Archimedean (Kepler–Poisont) solids, reaching at the very end the bulk structure of solids. We demonstrate both experimentally and by means of computational simulations for each case that this structural atomic growth sequence is followed in such mono- and bimetallic nanoparticles.

The Langmuir–Blodgett (LB) films of Poly (N-vinylcarbazole) (PVK) Polymers and its hybrid films with arachidic acid (AA) and C₆₀ were prepared by a vertical deposition method. Atomic Force Microscopy (AFM) and UV-Vis absorption spectroscopy were used to investigate the assembly of PVK/AA and PVK/C₆₀/AA LB films. It was found that the C₆₀ molecules could be aggregated into smaller particles on the base of PVK polymers. The photoluminescence (PL) of PVK was dramatically quenched resulting from the electron transfer from PVK to C₆₀ in their composite films.

The binding energy of positively and negatively charged excitons in GaAs quantum-well wires is calculated variationally as a function of the wire width by using a two-parameter wave function and a one-dimensional equivalent model. There is no artificial parameter added in our calculation. It is found that the binding energies are closely correlated to the sizes of the wire, and also that their magnitudes are greater than those in the two-dimensional quantum wells compared. In addition, we also calculate the average interparticle distance and the distribution of the wave function of exciton centre-of-mass as functions of the wires width. The results are discussed in detail.

The grain boundary scattering effects on carrier transport were studied analytically by considering the grains and grain boundaries that act as the series resistance in the channel of a polycrystalline silicon (poly-Si) thin-film transistor (TFT). Effective carrier mobility (μ*) and drain current (I_D) variations were analyzed using the model by changing the grain boundary width (D_GB) in the channel as a function of the gate voltage, in the linear region of the poly-Si TFT characteristic at room temperature. μ* and I_D were computed for D_GB ranging from 1 nm to 10 nm. It was found that for different values of the gate voltage, μ* and I_D increased with a decrease in grain boundary width (D_GB). A remarkable improvement was observed in device characteristics as D_GB was decreased below 2 nm. The predicted results using the present model are in a reasonably good agreement with experimental results, hence confirming the validity of the model.

The phase transitions in cholesteryl-beta-chloropropionate were studied by using thermally stimulated depolarization current (TSDC) methods. Optical transmission studies were performed over the whole temperature range. From the experimental data, useful information with reference to the conduction mechanism was obtained. The TSDC spectra displayed a peak at the solid-cholesteric phase transition temperature. Using the plots obtained when different depolarization fields were applied, the activation energies and the relaxation time were determined.

Trapping of hot electron behavior by trap centers located in the buffer layer of a wurtzite phase GaN MESFET has been simulated using an ensemble Monte Carlo simulation. The simulated results show that trap centers are responsible for current collapse in GaN MESFET at low temperatures. These electrical traps degrade the performance of the device at low temperatures. On the other hand, at high temperatures, the electrical performances are improved due to electron emission from the trap centers. The simulated device geometries and doping are matched to the nominal parameters described for the experimental structures as closely as possible, and the predicted drain current and other electrical characteristics for the simulated device including the trapping center effects show much closer agreement with the available experimental data than without trap center effects.