T. Marek

Single crystal Si layers on glass formed by ion cutting

The process of ion cutting was used to integrate single crystalline Si layers on glass for potential active matrix flat panel display and other applications. It was found that p-Si wafers implanted at 100–150 ° C with H with a dose in the order of a few times 10 16 cm −2 could be readily bonded to glass substrates when both of the surfaces were properly treated and activated. The as-implanted Si wafer surface was converted from p type to n type. Upon bonding at room temperature, annealing (300 ° C ) and exfoliation (450 ° C ), the transferred Si layer on glass and the as-exfoliated surface of the implanted Si wafer remained n type. A highly defective region was observed near the top of the Si layer on glass, however the crystalline quality was nearly defect free in the deeper region of the layer. Annealing at sequentially higher temperatures led to the recovery of p type conductivity at ∼600–650 ° C . The type conversion and the subsequent annealing behavior observed on the samples were rationalized in terms of ion enhanced oxygen diffusion and the presence of H-related shallow donors in the Si.

Evaluation of surface and microstructure of differently plasticized chitosan films

Surface and structural investigations of natural biopolymer (chitosan) films containing various conventionally applied hydrophilic plasticizers (glycerol and poly(ethylene glycol) 400) were performed and the results were compared, with the aim of acquiring new information concerning the formation of these plasticized films. The surface tests revealed that the water uptake, the water-binding properties (moisture content) and the polarity were higher for the film containing glycerol as plasticizer. Positronium lifetime measurements and NMR studies performed to evaluate the effects of the plasticizer on the polymer structure demonstrated relevant differences in the effects of the plasticizers. The influence of glycerol on the structure of the film formed was more intensive than that of PEG 400. It can be concluded that the surface properties of the films, which are very important for their storage and application, cannot be established exactly by means of structural tests. Both surface and structural tests must be performed before the formulation of this type of plasticized mucoadhesive films.

Tracking the physical aging of poly(ethylene oxide): A technical note

Summary and ConclusionsPhysical aging of 2 types of PEO could be tracked by the combination of PALS, DSC, and SEM methods. After storing the samples at 40°C±2°C and 75%±5% RH, a decrease in the o-Ps lifetime values and an increase in the melting enthalpies as a function of storage time indicated a reorientation of the polymer chains. The limitations of monitoring enthalpy relaxation confirm the importance of methods that track volume relaxation, such as PALS. Structural changes could be observed even after a short storage time (4 weeks), which highlights the effect of further investigations of the influence of physical aging on the properties of PEO-containing dosage forms.

Positronium as a tool to monitor changes of chemical structure

Abstract The applicability of positron annihilation spectroscopy for chemical structural problems is demonstrated by two examples: For the swelling dynamics of an amphiphilic polymer network, very quick structural changes were indicated by positronium lifetime parameters at very low swelling ratios and a hydration mechanism was proposed accordingly. For the series [Fe x Zn 1− x (propyltetrazole) 6 ](BF 4 ) 2 ( x =1, 0.6, 0.4, 0.1, 0), changes in the dynamic structure were revealed by positronium lifetime spectroscopy. For compounds of x >0, temperature-induced spin-crossover was detected.

Prediction of the stability of polymeric matrix tablets containing famotidine from the positron annihilation lifetime distributions of their physical mixtures

The aim of the present work was to elucidate the impact of the structural changes of polymeric excipients during the course of storage on the drug release stability of tablets containing different polymers. Matrix tablets were formulated with famotidine as a model drug, using polyvinylpyrrolidone and carbopol matrix. Dissolution tests were carried out before and after storing the tablets under stress conditions for different time intervals. Parameters characterizing the release kinetics of matrix tablets, just as difference and similarity factors, were calculated to compare the release profiles as a function of storage time. Positron annihilation lifetime measurements were carried out to track the structural changes of the physical mixtures containing polymers during the course of storage. The changes in the positron lifetime distribution curves of the famotidine-polymer mixtures were in good correlation with the significant changes of release parameters of tablets. Thus the method would be a valuable tool for the screening of possible destabilizing interactions in the preformulation phase.

The Effect of Plasticizer on the Free Volume in Metolose Systems

The quality of the pharmaceutical film-forming material (Metolose) is crucial during the tablet-coating process. The aim of this study was to establish a connection between the concentration of the plasticizer (poly-ethylene-glycol, PEG) in the cast film and the free volume. We studied the o-Ps lifetime in a series of Metolose-PEG systems as a function of PEG concentration in the widest available range. The samples were thin films cast from a dispersion of Metolose in aqueous solution of PEG. The found concentration dependence of the free volume in the Metolose-PEG systems is compared with other physical-chemical properties of the samples.

Positron annihilation study of polyphenylene dendrimers

Abstract The free volume of a series of rigid polyphenylene dendrimers was studied by positron lifetime spectroscopy and the experimental data were compared with results from molecular dynamics calculations. Contrary to dendrimers containing flexible repeat units rigid polyphenylene dendrimers proved to contain large stable “inner” voids of a distinct size between their dendritic branches. The size of the “inner” cavities increases with the size of the dendrimer. Due to these voids, polyphenylene dendrimers are potentially attractive with respect to the selective incorporation of guest molecules.

Model for the incorporation of excess arsenic into interstitial positions during the low-temperature growth of GaAs(001) layers

Using the GAUSSIAN 94 program package on a Ga16As17H36 cluster, the interaction of an As2 molecule approaching an As-terminated nonreconstructed GaAs(001) surface along the symmetry axis C2 of the cluster is studied. To describe the interaction between As2 molecule and cluster, the total energy of the system was calculated for various cluster As2 distances. The results show that the system (cluster plus molecule) has a stable conformation where one As atom resides at a tetrahedral interstitial site below the surface, i.e., inside the crystal. This result indicates the possibility of As incorporation by the particular process presented here.

Defect structure of electrodeposited chromium layers

Abstract Positron annihilation spectroscopy was applied to study the effects of pre-treatment and composition of substrates on the quality and defect structure of electrodeposited thick chromium coatings. The results show that both parameters are important, and a scenario is proposed why the mechanically polished substrate gives more defective film than the electropolished one.

Effect of spin-crossover on the parameters of the lifetime spectra of positrons and positronium in crystalline materials

Abstract Positron lifetime parameters were measured for the spin-crossover complexes [Fe(1-alkyl-1 H -tetrazole) 6 ](BF 4 ) 2 and [Fe(1,10-phenathroline) 2 (NCS) 2 ]. If there are structural free volumes large enough (as for [Fe(1-ethyl-1 H -tetrazole) 6 ](BF 4 ) 2 and Fe(1-propyl-1 H -tetrazole) 6 ](BF 4 ) 2 ), there is a significant intensity of positronium (Ps) and the lifetime of o -Ps is a detector of spin-crossover. The intensity of the shortest lifetime component also signals the spin-crossover.