New electrochemically improved tetrahedral amorphous carbon films for biological applications

Carbon based materials have been frequently used to detect different biomolecules. For example high sp3 containing hydrogen free diamond-like carbon (DLC) possesses many properties that are beneficial for biosensor applications. Unfortunately, the sensitivities of the DLC electrodes are typically low. Here we demonstrate that by introducing topography on the DLC surface and by varying its layer thickness, it is possible to significantly increase the sensitivity of DLC thin film electrodes towards dopamine. The electrode structures are characterized in detail by atomic force microscopy (AFM) and conductive atomic force microscopy (C-AFM) as well as by transmission electron microscopy (TEM) combined with electron energy loss spectroscopy (EELS). With cyclic voltammetry (CV) measurements we demonstrate that the new improved DLC electrode has a very wide water window, but at the same time it also exhibits fast electron transfer rate at the electrode/solution interface. In addition, it is shown that the sensitivity towards dopamine is increased up to two orders of magnitude in comparison to the previously fabricated DLC films, which are used as benchmarks in this investigation. Finally, it is shown, based on the cyclic voltammetry measurements that dopamine exhibits highly complex behavior on top of these carbon electrodes.The authors T.L, V.P., S.S., T.P., and J.K., would like to acknowledge the National Agency for Technology and Innovation (grant number 211488) and Aalto University (grant number 902380) for the financial support

The X-ray Point Source Population Hosted by Globular Clusters in the Elliptical Galaxy NGC 4261

Utilising archival Chandra X-ray Observatory data and Hubble Space Telescope globular cluster catalogues, we probe the time-domain properties of the low mass X-ray binary population in the elliptical galaxy NGC 4261. Of the 98 unique X-ray sources identified in this study, 62 sources are within the optical field of view and, of those, 33% are aligned with an optical cluster counterpart. We find twenty X-ray sources coincident with globular clusters; two are previously discovered ultra-luminous X-ray sources (ULXs) and eighteen are low mass X-ray binaries (GCLMXBs) with $L_X < 10^{39}$ erg s$^{-1}$. ULXs are a heterogeneous class of extremely bright X-ray binaries ($L_X > 10^{39}$ erg s$^{-1}$) and ULXs located in globular clusters (GCULXs) and may be indicators of black holes. Identifying these unusually X-ray bright sources and measuring their optical properties can provide valuable constraints on the progenitors of gravitational wave sources. We compare observations of these sources to the twenty previously-studied GCULXs from five other early-type galaxies, and find that GCULXs in NGC 4261 are of similar colour and luminosity and do not significantly deviate from the rest of the sample in terms of distance from the galaxy centre or X-ray luminosity. Both the GCULX and low mass X-ray binary (GCLMXB) populations of NGC 4261 show long term variability; the former may have implications for fast radio bursts originating in globular clusters and the latter will likely introduce additional scatter into the low mass end of GCLMXB X-ray luminosity functions.Comment: Accepted to MNRA

Dislocation core structures in Si-doped GaN

Aberration-corrected scanning transmission electron microscopy was used to investigate the core structures of threading dislocations in plan-view geometry of GaN films with a range of Si-doping levels and dislocation densities ranging between (5 ± 1) × 108 and (10 ± 1) × 109 cm−2. All a-type (edge) dislocation core structures in all samples formed 5/7-atom ring core structures, whereas all (a + c)-type (mixed) dislocations formed either double 5/6-atom, dissociated 7/4/8/4/9-atom, or dissociated 7/4/8/4/8/4/9-atom core structures. This shows that Si-doping does not affect threading dislocation core structures in GaN. However, electron beam damage at 300 keV produces 4-atom ring structures for (a + c)-type cores in Si-doped GaN.This work was funded in part by the Cambridge Commonwealth trust, St. John's College, British Federation of Women Graduates and the EPSRC. M.A.M. acknowledges the support from the Royal Society through a University Research Fellowship. Additional support was provided by the EPSRC through the UK National Facility for Aberration-Corrected STEM (SuperSTEM).This is the author accepted manuscript. The final version is available from AIP via http://dx.doi.org/10.1063/1.493745

Segregation of In to dislocations in InGaN.

Dislocations are one-dimensional topological defects that occur frequently in functional thin film materials and that are known to degrade the performance of InxGa1-xN-based optoelectronic devices. Here, we show that large local deviations in alloy composition and atomic structure are expected to occur in and around dislocation cores in InxGa(1-x)N alloy thin films. We present energy-dispersive X-ray spectroscopy data supporting this result. The methods presented here are also widely applicable for predicting composition fluctuations associated with strain fields in other inorganic functional material thin films.This work was funded in part by the Cambridge Commonwealth trust, St. John’s College and the EPSRC. SKR is funded through the Cambridge-India Partnership Fund and Indian Institute of Technology Bombay via a scholarship. MAM acknowledges support from the Royal Society through a University Research Fellowship. Additional support was provided by the EPSRC through the UK National Facility for Aberration-Corrected STEM (SuperSTEM). The Titan 80- 200kV ChemiSTEMTM was funded through HM Government (UK) and is associated with the capabilities of the University of Manchester Nuclear Manufacturing (NUMAN) capabilities. SJH acknowledges funding from the Defence Treat Reduction Agency (DTRA) USA (grant number HDTRA1-12-1-0013).This is the accepted manuscript. The final version is available at http://pubs.acs.org/doi/abs/10.1021/nl5036513

Atomic resolution imaging of dislocations in algan and the efficiency of UV LEDs

[Abstract not available

Dislocations in AlGaN: Core Structure, Atom Segregation and Optical Properties

We conducted a comprehensive investigation of dislocations in Al0.46Ga0.54N. Using aberration-corrected scanning transmission electron microscopy and energy dispersive X-ray spectroscopy, the atomic structure and atom distribution at the dislocation core have been examined. We report that the core configuration of dislocations in AlGaN is consistent with that of other materials in the III-Nitride system. However, we observed that the dissociation of mixed-type dislocations is impeded by alloying GaN with AlN, which is confirmed by our experimental observation of Ga and Al atom segregation in the tensile and compressive parts of the dislocations, respectively. Investigation of the optical properties of the dislocations shows that the atom segregation at dislocations has no significant effect on the intensity recorded by cathodoluminescence in the vicinity of the dislocations. These results are in contrast with the case of dislocations in In0.09Ga0.91N where segregation of In and Ga atoms also occurs but results in carrier localization limiting non-radiative recombination at the dislocation. This study therefore sheds light on why InGaN-based devices are generally more resilient to dislocations than their AlGaN-based counterparts

Research data supporting "Carrier Localization in the Vicinity of Dislocations in InGaN"

FIG. 1. AFM (a), SEM (b), panchromatic CL (c), and ADF-STEM (d) performed on the same micrometre-scale area. To guide the eye, a few dislocations are indicated by arrows in each picture. (e) High-resolution (HR) STEM image of the dislocation indicated by a square in (a)-(d), enabling the identification of the core structure (here dissociated 7/4/8/5-atom ring), and (f) geometric phase analysis (GPA) showing the e_xx strain component of the dislocation core region. FIG. 2. Schematic showing the electron probe in the SEM-CL scanning across a V-pit. The scale of the schematic, although indicative, is representative of the experimental conditions in which the experiment was conducted. Distance to nearest neighbor dependence of the intensity ratio (a)(c) and energy shift (b)(d) measured at the center (a)(b) and facet (c)(d) of the V-pits. FIG. 3. (a) Histogram of the number of In-N chains as a function of the number of indium atoms in the chains, located within a 10 A radius centered on the dislocation, in the case of a random distribution of indium (i.e. initial configuration of the simulation) or segregation of indium (i.e. equilibrium configuration of the simulation). Abstract representation of the data in (a), in the case of a random distribution (b) or segregation (c) of indium atoms. FIG. 4. ADF-STEM image of the clustered dislocations 26 (a) and 87 (b). The white strain-related contrast between the neighboring dislocations is indicated by an arrow. Aberration-corrected HAADF-STEM image of the core of dislocation 26 (dissociated 7/4/8/4/9-atom ring)(c) and 87 (undissociated double 5/6-atom ring)(d). An ABSF-filter (Average Background Subtraction Filter) has been applied to (c) and (d) in order to remove noise from the images. FIG. 5. 16K CL integrated intensity (a)(c) and peak emission energy (b)(d) maps of isolated (a)(b) and clustered (c)(d) dislocations. To guide the eye, the position of the bright spots, directly observable in (a) and (c), is indicated by circles in all the maps. To emphasize the relative variations in intensity and energy between isolated and clustered configurations, a common color scale is used in (a) and (c) and in (b) and (d).This work was supported by the EC [FP7/2007-2013 279361], EPSRC, ESTEEM2 and Cambridge Philosophical Society