We examine the relationship between covalent structure and conformational propensity among a series of β-amino acid tetramers. These experiments focus on the hairpin folding motif. Among conventional peptides, the minimum increment of β-sheet secondary structure is a "β-hairpin," in which two strands are connected via a short loop. The present studies are aimed at optimizing hairpin stability among β-peptides. Previous work from our laboratory has identified optimal substitution patterns for residues that form strands in an antiparallel β-peptide sheet (Krautha ¨user et al. J. Am. Chem. Soc. 1997, 119, 11719), and we have shown that a dinipecotic acid segment can promote sheet-type interactions between attached strand residues (Chung et al.

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The change in the transfer characteristics of amorphous In–Ga–Zn–O thin film transistors (TFTs) was investigated under light illumination at various wavelengths. The variations in the interfacial trap density (Dit) were also studied using metal-insulator-semiconductor capacitors. The transfer characteristics of the TFTs were dependent on the wavelength of illuminated light. The increase in subthreshold swing observed under light illumination of wavelengths below 550 nm (∼2.3 eV) was confirmed to be related to the increase in Dit near the conduction band edge. This Dit increase is caused by doubly ionized oxygen vacancies (VO2+) that are temporarily generated under light illumination.

Local doping of Al:ZnO into a ZnO matrix was performed vertically at various positions in a thin film using atomic layer deposition, and its influence was investigated by analyzing thin film transistor (TFT) characteristics. The position specific dopant distribution in the films was confirmed by high resolution transmission electron microscopy. It was found that doping specific locations in the active channel layer of a TFT had a different impact on its electrical characteristics. When near the semiconductor/gate dielectric interface, doping had a significant impact on the mobility of the TFT devices, which showed a gradual recovery as the doped region was moved away from the interface. The original characteristics of the device were almost completely restored once the doped region was moved more than 15 nm away from the interface, and when moved further away the output characteristics portrayed a shift in threshold voltage while preserving all other electrical characteristics. Various doping concentrations were implemented in regions both near and far away from the interface to gain a better understanding of the phenomena. The experimental results given here imply that the geographical position of doping is as important as selecting a dopant material in the device optimization of TFTs.

What are the ground states of an interacting, low-density electron system? In the absence of disorder, it has long been expected that as the electron density is lowered, the exchange energy gained by aligning the electron spins should exceed the enhancement in the kinetic (Fermi) energy, leading to a (Bloch) ferromagnetic transition. At even lower densities, another transition to a (Wigner) solid, an ordered array of electrons, should occur. Experimental access to these regimes, however, has been limited because of the absence of a material platform that supports an electron system with very high quality (low disorder) and low density simultaneously. Here we explore the ground states of interacting electrons in an exceptionally clean, two-dimensional electron system confined to a modulation-doped AlAs quantum well. The large electron effective mass in this system allows us to reach very large values of the interaction parameterrs, defined as the ratio of the Coulomb to Fermi energies. As we lower the electron density via gate bias, we find a sequence of phases, qualitatively consistent with the above scenario: a paramagnetic phase at large densities, a spontaneous transition to a ferromagnetic state whenrssurpasses 35, and then a phase with strongly nonlinear current-voltage characteristics, suggestive of a pinned Wigner solid, whenrsexceeds≃38. However, our sample makes a transition to an insulating state atrs≃27, preceding the onset of the spontaneous ferromagnetism, implying that besides interaction, the role of disorder must also be taken into account in understanding the different phases of a realistic dilute electron system.

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