Simo Kilpeläinen

Evidence of PPII-like helical conformation and glass transition in a self-assembled solid-state polypeptide-surfactant complex: poly(L-histidine)/docylbenzenesulfonic acid.

We present lamellar self-assembly of cationic poly(L-histidine) (PLH) stoichiometrically complexed with an anionic surfactant, dodecyl benzenesulfonic acid (DBSA), which allows a stabilized conformation reminiscent of polyproline type II (PPII) left-handed helices. Such a conformation has no intrapeptide hydrogen bonds, and it has previously been found to be one source of flexibility, e.g., in collagen and elastin, as well as an intermediate in silk processing. PLH(DBSA)1.0 complexes were characterized by Fourier transform infrared spectroscopy (FTIR), circular dichroism (CD), small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and differential scanning calorimetry (DSC). The PPII-like conformation in PLH(DBSA)1.0 is revealed by characteristic CD and FTIR spectra, where the latter indicates absence of intrachain peptide hydrogen bonds. In addition, a glass transition was directly verified by DSC at ca. 135 degrees C for PLH(DBSA)1.0 and indirectly by SAXS and TEM in comparison to pure PLH at 165 degrees C, thus indicating plasticization. Glass transitions have not been observed before in polypeptide-surfactant complexes. The present results show that surfactant binding can be a simple scheme to provide steric crowding to stabilize PPII conformation to tune the polypeptide properties, plasticization and flexibility.

Si nanoparticle interfaces in Si/SiO2 solar cell materials

Novel solar cell materials consisting of Si nanoparticles embedded in SiO2 layers have been studied using positron annihilation spectroscopy in Doppler broadening mode and photoluminescence. Two positron-trapping interface states are observed after high temperature annealing at 1100u2009°C. One of the states is attributed to the (SiO2/Si bulk) interface and the other to the interface between the Si nanoparticles and SiO2. A small reduction in positron trapping into these states is observed after annealing the samples in N2 atmosphere with 5% H2. Enhanced photoluminescence is also observed from the samples following this annealing step.

Vacancy engineering by He induced nanovoids in crystalline Si

Positron annihilation spectroscopy (PAS) in the Doppler broadening mode is used to study the effects of He and B implantation, and subsequent annealing on the vacancy profile of crystalline Si. The existence of two void layers, one consisting of large voids at the projected range (Rp) of He and another containing nanovoids slightly larger than divacancies at roughly halfway between Rp of He and the surface is shown for samples implanted with He. Furthermore, the same nanovoid layer is shown to be absent from samples co-implanted with B, implying that interstitials created during B implantation get trapped in the nanovoids and fill them, thus indicating that the nanovoid layer is behind the hindrance of B diffusion.

4-Channel Digital Positron Lifetime Spectrometer for Studying Biological Samples

In this work a novel detector setup for PALS-studies on biomolecular materials is presented. When pursuing optimal detecting efficiency and lifetime component resolution one must make compromises when using only one detector pair. With smaller scintillation heads the resolution is higher, but the detecting efficiency decreases and vice versa. When measuring biological materials that do not withstand long measurement periods, sacrifices are made for gaining efficiency. The price of this optimization is low resolution of the lifetime components, namely separating the always present water’s lifetime component of ~1.8 ns from the actual material’s lifetime component, typically >2 ns. A solution to this problem is measuring the annihilation spectra with two individual detector pairs simultaneously. Using analog setup, it would require duplicate ADC-hardware that are both expensive and degrade by time. With a fully digital setup, the need for hardware is smaller and the precision of the setup is constant during its service life.

Role of excessive vacancies in transgranular stress corrosion cracking of pure copper

Abstract The role of the excessive metal vacancy generation in transgranular stress corrosion cracking (TGSCC) of pure copper was studied in relation to crack initiation and growth mechanisms. Electrochemically polarized specimens were strained at room temperature in 0.3 m NaNO2 solution. Cation vacancy redistribution under applied anodic polarization took place at the p-n junction of the duplex Cu2O oxide film, resulting in cation vacancy transport in the p-type layer of the oxide to the metal substrate interface and in excessive metal vacancy accumulation in the metal substrate. The rapid initiation of TGSCC at room temperature in the cold-worked oxygen-free high-conductivity copper samples occurred during slow strain rate tests under anodic polarization in 0.3 m NaNO2 solution. The duplex Cu2O oxide film structure formed by cathodic deposition, before applying the anodic polarization, was characterized with X-ray diffraction and field emission scanning electron microscopy/electron backscatter diffraction/energy-dispersive spectroscopy. The redistribution of cation vacancies in the p-type oxide phase due to the anodic polarization and the metal vacancy accumulation in the copper substrate interface was studied by using positron annihilation spectroscopy.