Cuong Ton-That

XPS and AFM surface studies of solvent-cast PS/PMMA blends

Films of polystyrene (PS) and poly(methyl methacrylate) (PMMA) blends of two different thicknesses have been examined by X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Blends with different compositions were spin-cast onto a mica substrate with chloroform as the mutual solvent. XPS measurements revealed surface enrichment of PMMA in all compositions. The thicker (66 nm) films exhibit a higher degree of PMMA surface enrichment than the thinner (17 nm) films. AFM imaging allows distinctions to be drawn between blends with differing compositions. The blend films with less than 50% PMMA bulk concentration generally exhibit pitted surfaces; the pit size varies with film thickness and bulk composition. When the PMMA bulk concentration is greater than 50%, the film surface changes to show island-like phase-separated structure. The surface segregation and morphology are explained in terms of solubilities of the two polymers in the solvent and dewetting of PMMA relative to PS. The phase domains on the film surface have also been resolved by frictional force microscopy (FFM) using hydrophilic tips bearing hydroxyl groups.

Self-discharge of carbon-based supercapacitors with organic electrolytes

Abstract The self-discharge behaviour of supercapacitors is an important factor when considering their suitability for some applications. In this paper, measurements of the self-discharge rates of carbon-based supercapacitors with organic electrolytes are presented and interpreted in terms of two mechanisms. The first is the diffusion of ions from regions of excess ionic concentration formed during the charging of the capacitor and the second is leakage of charge across the double-layer at the electrolyte–carbon interfaces in the capacitor. The dependence of the self-discharge rate on temperature and on the initial voltage across the capacitor is described.

Surface characterization and ageing of ultraviolet–ozone-treated polymers using atomic force microscopy and x-ray photoelectron spectroscopy†

Ultraviolet–ozone (UVO) treatment of poly(ethyleneterephthalate) (PET) films and polystyrene (PS) dishes of up to 10 min exposure has been studied. Surface polarity, oxygen chemisorption and topographical change were analysed by contact angle measurement, x-ray photoelectron spectroscopy and atomic force microscopy. Studies of the treated surface reveal the chemistry behind the increasing oxygen content. The oxidation process is shown to proceed via different mechanisms for the two polymers. Polyethyleneterephthalate appears to undergo a Norrish-type chain depolymerization reaction, whereas PS undergoes a much more random chain scission attack. Atomic force microscopy analysis shows an increase in the surface roughness with increasing exposure to UVO for both polymers, with grains of low-molecular-weight oxidized material (LMWOM) forming at the surface. This material can be removed by washing. Surfaces that remain after washing have a higher concentration of oxygen species than the native surfaces. Analysis of aged surfaces shows that for oxidized PET a relaxation process occurs, lowering the levels of surface oxygen. This appears to occur due to the diffusion of LMWOM into the PET bulk. Relaxation of the oxidized PS surface is less thermodynamically favourable owing to the apolar nature of the PS. Copyright

Typical low cost biosorbents for adsorptive removal of specific organic pollutants from water.

Specific organic pollutants (SOPs) such as phenolic compounds, PAHs, organic pesticides, and organic herbicides cause health and environmental problems due to their excessive toxic properties and poor biodegradability. Low-cost biosorbents are considered as a promising alternative for conventional adsorbents to remove SOPs from water. These materials have several advantages such as high sorption capacities, good modifiability and recoverability, insensitivity to toxic substances, simple operation in the treatment processes. However, previous reports on various types of biosorbents for removing SOPs are still moderately fragmented. Hence, this paper provides a comprehensive review on using typical low-cost biosorbents obtained from lignocellulose and chitin/chitosan for SOPs adsorption. Especially, their characteristics, biosorption mechanism together with utilization for eliminating SOPs are presented and discussed. The paper also gives a critical view regarding future applications of low-cost biosorbents in SOPs-contaminated water treatment.

Surface characterisation of ultraviolet-ozone treated PET using atomic force microscopy and X-ray photoelectron spectroscopy

The effects of ultraviolet-ozone (UVO) oxidation of polyethyleneterephthalate (PET) surfaces have been studied using atomic force microscopy and X-ray photoelectron spectroscopy. Surface oxygen increases from 26 at.% (untreated) to 37 at.% for the most oxidised surfaces produced and an increase in mean surface roughness and grain size is also observed. The larger grains appear to result from the formation of low molecular weight oxidised species by PET chain scission at the ester group and the material formed is therefore COOH/COOR rich. These species are mobile on the polymer surface and coalesce to form 200 nm grains which can be partially removed by water washing. The surface beneath is still rougher and more highly oxidised than untreated PET but the increase in stable oxygen is due to the formation of COH/COR groups.

Surface feature size of spin cast PS/PMMA blends

Thin films of polystyrene (PS)/polymethylmethacrylate (PMMA) blends have been spin cast on mica from chloroform solutions. When the concentration of PMMA in the casting solution is less than that of PS a pitted morphology is formed. The average sizes of the pits are shown to increase with both the total concentration of the casting solution and the relative concentration of PMMA. The change in pit size is explained in terms of incomplete dewetting of a PMMA solution from an underlying PS solution. For a given ratio of PMMA/PS the average pit diameters appear to increase linearly with the square of the film thickness, the gradient of which is dependent on the film composition.

Cathodoluminescence inhomogeneity in ZnO nanorods

Luminescence properties of vertically aligned, crystalline ZnO nanorods are studied by cathodoluminescence (CL) spectroscopy and microscopy. Results show that luminescence characteristics vary dramatically with location on the nanorod as well as CL excitation depth. CL inhomogeneity is observed between the nanorod tip and sidewalls, accompanied by a variation in the chemical environment of surface oxygen ions as probed by photoemission spectroscopy. Our findings demonstrate that CL can provide useful information on the local optical properties of nanostructured materials, which is simply beyond the capability of other methods.

Surface electronic properties of ZnO nanoparticles

The surface electronic structure of ZnO nanoparticles has been studied with photoemission and x-ray absorption spectroscopies. Contrary to expectation, ZnO:Zn phosphor nanoparticles were found to contain a lower oxygen vacancy density on the surface than undoped ZnO counterparts, but oxygen vacancies are in different chemical environments. Cathodoluminescence shows intense green luminescence from the ZnO:Zn surface, while the undoped nanoparticles exhibit only the near-band-edge emission. The results indicate the roles of surface oxygen vacancies and their environment in the previously unexplained green luminescence from the ZnO:Zn material.

Single photon emission from ZnO nanoparticles

Room temperature single photon emitters are very important resources for photonics and emerging quantum technologies. In this work, we study single photon emission from defect centers in 20 nm zinc oxide (ZnO) nanoparticles. The emitters exhibit bright broadband fluorescence in the red spectral range centered at 640 nm with polarized excitation and emission. The studied emitters showed continuous blinking; however, bleaching can be suppressed using a polymethyl methacrylate coating. Furthermore, hydrogen termination increased the density of single photon emitters. Our results will contribute to the identification of quantum systems in ZnO.

Luminescent properties of ZnO nanowires and as-grown ensembles

Vertically aligned ZnO nanowires were synthesized on a sapphire ([Formula: see text]) substrate by vapour deposition and their light-emitting properties were characterized using photoluminescence and cathodoluminescence spectroscopies. Besides the nanowires, there exists a mosaic ZnO film on the substrate surface. Comparison of the luminescent properties of the as-grown ensemble and the nanowires extracted from it shows that the mosaic film is the major source of the defect-related green emission while the nanowires possess highly crystalline quality with virtually no defects. Photoemission spectroscopy shows that the valence band states associated with O 2p from the as-grown sample are diminished compared with those from the extracted nanowires. These findings suggest that the green emission partly arises from oxygen vacancies located on the surface of the mosaic film.