A liquid crystalline homopolymer that has photoisomerizable methoxyazobenzene groups in the side chain has been synthesized and characterized. Thin films of the nematic glassy phase of this polymer have been processed in order to study the absorption spectra and the vibrational and electronic circular dichroism responses by irradiation with 488 nm circularly polarized light (CPL). Selective reflection of visible light demonstrates that the irradiation of this glassy nematic azopolymer induces a helix as a consequence of the chiral arrangement of the azobenzene units. Moreover, a wedge cell with an aligning layer for planar orientation was filled with the polymer with the aim of investigating the change in the macroscopic optical properties and optical textures of the azopolymer on irradiation with CPL. The transfer of chirality from CPL to azopolymer through chiral conformations is proposed as a model for explaining the supramolecular chirality.

No abstract

Based on micro-Raman spectroscopy (μRS) and X-ray photoelectron spectroscopy (XPS), we study the structural damage incurred in monolayer (1L) and few-layer (FL) graphene subjected to atomic-layer deposition of HfO2 and Al2O3 upon different oxygen plasma power levels. We evaluate the damage level and the influence of the HfO2 thickness on graphene. The results indicate that in the case of Al2O3/graphene, whether 1L or FL graphene is strongly damaged under our process conditions. For the case of HfO2/graphene, μRS analysis clearly shows that FL graphene is less disordered than 1L graphene. In addition, the damage levels in FL graphene decrease with the number of layers. Moreover, the FL graphene damage is inversely proportional to the thickness of HfO2 film. Particularly, the bottom layer of twisted bilayer (t-2L) has the salient features of 1L graphene. Therefore, FL graphene allows for controlling/limiting the degree of defect during the PE-ALD HfO2 of dielectrics and could be a good starting material for building field effect transistors, sensors, touch screens and solar cells. Besides, the formation of Hf-C bonds may favor growing high-quality and uniform-coverage dielectric. HfO2 could be a suitable high-K gate dielectric with a scaling capability down to sub-5-nm for graphene-based transistors.

No abstract

No abstract

No abstract

No abstract

The strain-shift coefficient used to convert Raman shifts to strain depends on multiple factors including phonon deformation potentials (PDPs). PDPs have been reported for silicon, which differ by 30%. This leads to varying strain-shift-coefficients. Using the wrong strain-shift coefficient affects the strain determined. The discrepancies in the reported PDPs were previously ascribed to surface stress relaxation and the opacity of the material to the laser radiation. This paper shows that surface orientation and scattering geometry are major factors behind the PDPs discrepancies. The work further demonstrates that different PDPs are required to accurately characterize transverse optical and longitudinal optical Raman modes. The effects of sample geometry and stress have also been studied. State of the art devices use nanowires and thin films under large values of uniaxial stress; however, previous experiments to determine PDPs and strain-shift coefficients in silicon have been limited to bulk material and stress only in the range 0–2 GPa. In this work, the strain-shift coefficient of silicon nanostructures is determined for a large range of geometries and applied stress values (0–4.5 GPa). Strain in the samples has been measured using three independent techniques: analytical calculations, finite element simulations, and by direct visual inspection of the samples elongation using scanning electron microscopy. Raman shifts have been measured using 458 nm and 364 nm laser radiations. The combination of these techniques and the large number of samples (up to 85) has allowed the accurate determination of the strain-shift coefficient for the technologically important (100) silicon surface.