Andrei Sapelkin

Influence of anneal atmosphere on ZnO-nanorod photoluminescent and morphological properties with self-powered photodetector performance

ZnO nanorods synthesised using an aqueous pH 11 solution are shown to exhibit surface-sensitive morphology post-annealing in oxygen, air, and nitrogen as shown by scanning electron microscopy and transmission electron microscopy analysis. Raman analysis confirms the nanorods were nitrogen-doped and that nitrogen incorporation takes place during the synthesis procedure in the form of N-Hx. A strong green photoluminescence is observed post-annealing for all samples, the intensity of which is dependent on the atmosphere of anneal. This luminescence is linked to zinc vacancies as recent reports have indicated that these defects are energetically favoured with the annealing conditions used herein. ZnO-nanorod/CuSCN diodes are fabricated to examine the effect of material properties on photodetector device performance. The devices exhibit a photocurrent at zero bias, creating a self-powered photodetector. A photocurrent response of 30 μA (at 6 mW cm−2 irradiance) is measured, with a rise time of ∼25 ns, and sens...

Neuron Cells Uptake of Polymeric Microcapsules and Subsequent Intracellular Release

Neuron cells uptake of biodegradable and synthetic polymeric microcapsules functionalized with aggregates of gold nanoparticles incorporated into their shells is demonstrated in situ. In addition to traditionally used optical microscopy, electron microscopy is used both for higher-resolution imaging and for confirming the uptake by focused ion beam cross-sectioning of specific cells in situ. Subsequently, physical methods of release are compared to chemical methods wherein laser-induced intracellular release of dextran molecules into the cytosol of hippocampal neuron cells is studied in comparison to biodegradation. Implications of this work for neuroscience, bio-medicine and single cell studies are discussed.

Magnetically Engineered Microcapsules as Intracellular Anchors for Remote Control Over Cellular Mobility

Living cells are anchored with magnetic microcapsules that allow in vitro manipulation via a magnetic field.

Structural and optical properties of porous nanocrystalline Ge

Nanocrystalline Ge films were prepared by isotropic chemical etching on single-crystalline Ge substrates with 100 and 111 orientations. The structural and optical properties have been investigated by transmission electron microscopy (TEM), electron diffraction (ED), Raman photoluminescence (PL), and infrared spectroscopy. The average size of nanocrystals (NCs) was estimated by fitting of the Raman spectra using a phonon-confinement model developed for spherical semiconductor NCs. Considered collectively TEM, ED, and Raman results indicate that all films contain high density of 3–4 nm diameter, diamond-structured Ge NCs with disordered surfaces. There are indications that surface of nanoparticles is mainly hydrogen terminated even for air-stabilized samples. Red PL is observed at room temperature upon excitation by 1.96 eV with peak energy of ∼1.55 eV and correlates well with recent theoretical calculations of the enlarged optical gap in Ge NCs of similar size.

On the origin of the 2.2-2.3 eV photoluminescence from chemically etched germanium

The photoluminescence (PL) at B2.2–2.3 eV from Ge-based nanocrystalline materials is described in the literature as nanocrystal size-independent. We have observed visible luminescence from two different types of stain-etched Ge samples, one prepared after Sendova-Vassileva et al. (Thin Solid Films 255 (1995) 282) in a solution of H2O2:HF at 50:1 volume ratio, and the other in a solution of HF:H3PO4:H2O2 at 34:17:1 volume ratio. Energydispersive X-rayanalysis (EDX), Raman and FTIR spectroscopy, and the near edge X-ray absorption structure (XANES), indicate that the chemically etched Ge layers of the former type of samples are composed of non-stoichometric Ge oxides, i.e. GeOx (0oxo2), and free from anyGe nanoconstructions. It is also suggested from XANES that the latter type of chemically etched Ge samples comprise 8–9 nm nanocrystals of Ge, surface-covered with mainly oxygen. Photoluminescence occurred at B2.3 eV for all samples. The PL behavior of the latter type of chemically etched Ge on annealing in different chemical environments (air or H) allowed us to conclude that the PL from these materials, as well as that from those Ge-based nanocrystalline materials reported in the literature, is from GeOxs. r 2002 Elsevier Science B.V. All rights reserved.

In Situ Synthesis of Fluorescent Carbon Dots/Polyelectrolyte Nanocomposite Microcapsules with Reduced Permeability and Ultrasound Sensitivity

Designing and fabricating multifunctional nanocomposite microcapsules are considerable interests in both academic and industrial research aspects. This work first reports an innovative approach to in situ synthesize and assemble fluorescent carbon dots (CDs) into polyelectrolyte microcapsules, obtaining highly biocompatible nanocomposite microcapsules with excellent luminescence that facilitate imaging and identification in vitro, yet with the feasibility to load small molecules and ultrasound responsiveness to trigger their release. CDs are produced in situ in (PAH/PSS)4 microcapsule shells by carbonization of dextran molecules under relatively mild hydrothermal treatment. Compared with the collapsed and film-like (PAH/PSS)4 microcapsules, the novel composite microcapsules show a free-standing structure, smaller size, and thicker shell. CDs are proven to be fabricated and embedded in PAH/PSS multilayers, and the formed PAH/PSS/CD microcapsules are endowed with strong luminescence, as verified by the transmission electron microscopy, fluorescence spectra, and confocal laser scanning microscopy results. The in situ formation of CDs in capsule shells also empowers these capsules with ultrasound responsiveness and reduced permeability. The feasibility of encapsulation of small molecules (rhodamine B) and ultrasound-triggered release is also shown. Most importantly, due to the intrinsic biocompatible property and photostability of CDs, these fluorescent PAH/PSS/CD microcapsules show negligible cell toxicity and low photobleaching, which are impossible for capsules composited with conventional organic dyes and semiconductor quantum dots.

Structural origin of light emission in germanium quantum dots

We used a combination of optically-detected x-ray absorption spectroscopy with molecular dynamics simulations to explore the origins of light emission in small (5 nm to 9 nm) Ge nanoparticles. Two sets of nanoparticles were studied, with oxygen and hydrogen terminated surfaces. We show that optically-detected x-ray absorption spectroscopy shows sufficient sensitivity to reveal the different origins of light emission in these two sets of samples. We found that in oxygen terminated nanoparticles its the oxide-rich regions that are responsible for the light emission. In hydrogen terminated nanoparticles we established that structurally disordered Ge regions contribute to the luminescence. Using a combination of molecular dynamics simulations and optically-detected x-ray absorption spectroscopy we show that these disordered regions correspond to the disordered layer a few Å thick at the surface of the simulated nanoparticle.

Synthesis and structure of free-standing germanium quantum dots and their application in live cell imaging

Free-standing Ge quantum dots around 3 nm in size were synthesized using a bench-top colloidal method and suspended in water and ethanol. In the ethanol solution, the photoluminescence of the Ge quantum dots was observed between 650 and 800 nm. Structural and optical properties of these colloidal Ge quantum dots were investigated by utilizing X-ray diffraction, X-ray absorption spectroscopy, Raman spectroscopy, and photoluminescence spectroscopy and transmission electron microscopy. The structure of the as-prepared Ge quantum dots that were found is best described by a core–shell model with a small crystalline core and an amorphous outer shell with a surface that was terminated by hydrogen-related species. As-prepared Ge quantum dots were suspended in cell growth medium, and then loaded into cervical carcinoma (HeLa) cells. The fluorescent microscopy images were then collected using 405 nm, 488 nm, 561 nm and 647 nm wavelengths. We observed that, based on fluorescence measurements, as-prepared Ge quantum dots can remain stable for up to 4 weeks in water. Investigation of toxicity, based on a viability test, of as-prepared uncoated Ge quantum dots in HeLa cells was carried out and compared with the commercial carboxyl coated CdSe/ZnSe quantum dots. The viability tests show that Ge quantum dots are less toxic when compared to commercial carboxyl coated CdSe/ZnS quantum dots.

In situ EXAFS, X-ray diffraction and photoluminescence for high-pressure studies

A new facility for simultaneous extended X-ray absorption of fine structure (EXAFS), X-ray diffraction and photoluminescence measurements under high pressures has been developed for use on station 9.3 at the Daresbury Laboratory Synchrotron Radiation Source. This high-pressure facility can be used at any suitable beamline at a synchrotron source. Full remote operation of the rig allows simultaneous collection of optical and structural data while varying the pressure. The set-up is very flexible and can be tailored for a particular experiment, such as time- or temperature-dependent measurements. A new approach to the collection of high-pressure EXAFS data is also presented. The approach significantly shortens the experimental times and allows a dramatic increase in the quality of EXAFS data collected. It also opens up the possibility for EXAFS data collection at any pressure which can be generated using a diamond cell. The high quality of data collected is demonstrated with a GaN case study. Particular attention will be paid to the use of energy-dispersive EXAFS and quick-scanning EXAFS techniques under pressure.

The optical and structural properties of InGaN epilayers with very high indium content

Abstract We present the results of optical and structural investigations of InGaN epilayers grown by Metallorganic Vapour Phase Epitaxy (MOVPE). The peak energies of characteristic photoluminescence (PL) bands allow us to identify regions of crystal with different mean InN: (InN+GaN) fraction in the range from 0.1 to nearly 1 in selected samples. The PL peak energy and the optical absorption band edge are strongly intercorrelated, the Stokes’ shift and the Urbach tailing energy both increase with InN fraction. High-resolution energy dispersive X-ray analysis (EDX), coupled with scanning electron microscopy (SEM) and cathodoluminescence (CL) imaging, helps to establish striking microscale correlations between optical and structural properties. Finally, X-ray absorption fine structure (XAFS) at the In and Ga K-edges reveals characteristic local structure on the atomic scale for InGaN solid solutions over the available range of In:Ga composition ratios.