Sekhar C. Ray

Carbon Nanoparticle-based Fluorescent Bioimaging Probes

Fluorescent nanoparticle-based imaging probes have advanced current labelling technology and are expected to generate new medical diagnostic tools based on their superior brightness and photostability compared with conventional molecular probes. Although significant progress has been made in fluorescent semiconductor nanocrystal-based biological labelling and imaging, the presence of heavy metals and the toxicity issues associated with heavy metals have severely limited the application potential of these nanocrystals. Here, we report a fluorescent carbon nanoparticle-based, alternative, nontoxic imaging probe that is suitable for biological staining and diagnostics. We have developed a chemical method to synthesise highly fluorescent carbon nanoparticles 1–10 nm in size; these particles exhibit size-dependent, tunable visible emission. These carbon nanoparticles have been transformed into various functionalised nanoprobes with hydrodynamic diameters of 5–15 nm and have been used as cell imaging probes.

The Effect of Thermal Reduction on the Photoluminescence and Electronic Structures of Graphene Oxides

Electronic structures of graphene oxide (GO) and hydro-thermally reduced graphene oxides (rGOs) processed at low temperatures (120–180°C) were studied using X-ray absorption near-edge structure (XANES), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS). C K-edge XANES spectra of rGOs reveal that thermal reduction restores C = C sp2 bonds and removes some of the oxygen and hydroxyl groups of GO, which initiates the evolution of carbonaceous species. The combination of C K-edge XANES and Kα XES spectra shows that the overlapping π and π* orbitals in rGOs and GO are similar to that of highly ordered pyrolytic graphite (HOPG), which has no band-gap. C Kα RIXS spectra provide evidence that thermal reduction changes the density of states (DOSs) that is generated in the π-region and/or in the gap between the π and π* levels of the GO and rGOs. Two-dimensional C Kα RIXS mapping of the heavy reduction of rGOs further confirms that the residual oxygen and/or oxygen-containing functional groups modify the π and σ features, which are dispersed by the photon excitation energy. The dispersion behavior near the K point is approximately linear and differs from the parabolic-like dispersion observed in HOPG.

Thiol‐Directed Synthesis of Highly Fluorescent Gold Clusters and Their Conversion into Stable Imaging Nanoprobes

Fluorescent gold clusters (FGCs) with tunable emission from blue to red and quantum yields in the range of 6-17% have been synthesized by simple modification of the conditions used for the synthesis of gold nanoparticles, namely by replacing the stronger reducing agent with a controlled amount of thiol. Various functional FGCs with hydrodynamic diameters of 5-12 nm have been successfully synthesized and used as cell labels. The results of our investigations strongly indicate that FGCs composed of Au(0) are more stable imaging probes than commonly reported red/NIR-emitting FGCs with a composition of Au(0)/Au(I), as this combination rapidly transforms into nonfluorescent large clusters on exposure to light. The FGC-based nanoprobes reported herein exhibit stable fluorescence upon continuous light exposure and can be used as imaging probes with low cytotoxicity.

Density of electronic states in amorphous carbons

Abstract In this paper we deal with the origin and features of the density of electronic states in the various types of a-C and a-C:H films. In particular we discuss the effects of disorder, distortions and composition (i.e. hydrogen and sp 2 carbon fractions) on the density of states and the main optoelectronic properties. We show that the wide differences among the various amorphous carbons can be understood by introducing three characteristic lengths of the structure (i.e. distance between neighbouring clusters, decay length of π states and ‘accommodation length’ of sp 2 clusters in the sp 3 network). Finally, we discuss additional features crucial in determining film properties, such as the correlation energy.

Graphene Supported Graphone/Graphane Bilayer Nanostructure Material for Spintronics

We report an investigation into the magnetic and electronic properties of partially hydrogenated vertically aligned few layers graphene (FLG) synthesized by microwave plasma enhanced chemical vapor deposition. The FLG samples are hydrogenated at different substrate temperatures to alter the degree of hydrogenation and their depth profile. The unique morphology of the structure gives rise to a unique geometry in which graphane/graphone is supported by graphene layers in the bulk, which is very different from other widely studied structures such as one-dimensional nanoribbons. Synchrotron based x-ray absorption fine structure spectroscopy measurements have been used to investigate the electronic structure and the underlying hydrogenation mechanism responsible for the magnetic properties. While ferromagnetic interactions seem to be predominant, the presence of antiferromagnetic interaction was also observed. Free spins available via the conversion of sp2 to sp3 hybridized structures, and the possibility of unpaired electrons from defects induced upon hydrogenation are thought to be likely mechanisms for the observed ferromagnetic orders.

X-ray absorption spectroscopy (XAS) study of dip deposited a-C:H(OH) thin films

This work measures the C and O K-edge x-ray absorption near-edge structure (XANES) spectra of hydrogenated amorphous carbon (a-C:H) films deposited at various baking temperatures Tb (Tb = 300–500 °C at 50 °C). The C–H σ* peak related to the content of the sp2 graphite-like bonding in the C K-edge spectra was found to yield to the C–H π* peak related to the sp3 diamond-like bonding at high temperature (500 °C). We find that the intensities of both the sp2 and sp3 features in the C K-edge XANES spectra decrease with increase of Tb, which suggests an increase of the defect concentration with Tb. The intensities of the O K-edge XANES spectra are found to decrease with increase of Tb, which suggests thermally induced decomposition of carbonyl contaminants on the surface. The elemental analysis C/O (or O/C) ratio was obtained from XPS spectra and indicates that films are not hydrogenated amorphous carbon but rather oxyhydrogenated amorphous carbon thin films.

Atomistic nucleation sites of Pt nanoparticles on N-doped carbon nanotubes

The atomistic nucleation sites of Pt nanoparticles (Pt NPs) on N-doped carbon nanotubes (N-CNTs) were investigated using C and N K-edge and Pt L3-edge X-ray absorption near-edge structure (XANES)/extended X-ray absorption fine structure (EXAFS) spectroscopy. Transmission electron microscopy and XANES/EXAFS results revealed that the self-organized Pt NPs on N-CNTs are uniformly distributed because of the relatively high binding energies of the adsorbed Pt atoms at the imperfect sites. During the atomistic nucleation process of Pt NPs on N-CNTs, stable Pt-C and Pt-N bonds are presumably formed, and charge transfer occurs at the surface/interface of the N-CNTs. The findings in this study were consistent with density functional theory calculations performed using cluster models for the undoped, substitutional-N-doped and pyridine-like-N-doped CNTs.

The Electronic Properties of Nanomaterials Elucidated by Synchrotron Radiation–Based Spectroscopy

The use of synchrotron radiation–based spectroscopy to investigate electronic and bonding structures of nanostructured materials is reviewed with focuses on the X-ray absorption spectroscopy (XAS), valence-band photoemission spectroscopy (VB-PES), and scanning photoelectron microscopy (SPEM) measurements. This review addresses the current status of synchrotron radiation–based nanoscale characterization of carbon-based and ZnO nanomaterials. Current research works that are relevant to this rapidly evolving experimental area and implications in nanoscience and nanotechnology are emphasized.

Electronic and vibrational structures of amorphous carbon nitrides

Abstract Carbon nitrides are much more complicate alloys than amorphous carbons and additional effects related to the nitrogen incorporation should be considered. This work will review how the density-of-electronic-states (DOS), the optical properties and the electron-spin-resonance features may be understood on a common basis by observing that nitrogen inclusion often leads to lone-pair (LP) electrons formation and sp 1 hybridisation. We will give a picture of the DOS of amorphous carbon nitrides in the Vis–near UV energy range and use it to evaluate the complex dielectric constants, as determined by the transitions involving the LP electrons. We will show that understanding the role of LP is necessary in order to understand the optical properties of carbon nitrides both in the visible and in the infrared (IR) photon energy ranges. In the IR, the extra-electrons related to nitrogen (i.e. the LPs) will effect the medium-range polarisation and lead to the sharp vibrational absorption features at 1000–1900 cm −1 .

Photoluminescence investigation of carbon nitride-based films deposited by reactive sputtering

Abstract In the present work, we investigate the photoluminescence (PL) of two structurally different sets of amorphous carbon nitride thin films labelled as ‘B’—bubbly and ‘C’—compact types. PL was studied at room temperature, both before and after laser soaking, with special emphasis on the role of lone-pair electrons and hydroxyl adsorbates. The film stability under irradiation and the structural modifications induced by light-soaking are investigated. The increase and subsequent saturation of the PL intensity under irradiation is approximately described by an exponential law.