Recombination and photoluminescence mechanism in hydrogenated amorphous carbon

Hydrothermal carbonization synthesis of amorphous carbon nanoparticles (15-150 nm) with fine-tuning of the size, bulk order, and the consequent impact on antioxidant and photothermal properties

Hydrothermal carbonization (HTC) of carbohydrates has been reported as a sustainable and green technique to produce carbonaceous micro- and nano-materials. These materials have been developed for several applications, including catalysis, separation science, metal ion adsorption and nanomedicine. Carbon nanoparticles (CNPs) obtained through HTC are particularly interesting for the latter application since they exhibit photothermal properties when irradiated with near-infrared (NIR) light, act as an antioxidant by scavenging reactive oxygen species (ROS), and present good colloidal stability and biocompatibility. However, due to the highly disordered structure, there is still a poor understanding of the mechanism of synthesis of CNPs. Consequently, the modulation of the CNP properties by controlling the synthetic parameters is still a challenge. In this work, a novel and simplified HTC synthetic strategy to obtain non-aggregated glucose derived CNPs in the 15-150 nm size range with precise control of the diameter is presented, together with an advance in the understanding of the reaction mechanism behind the synthesis. Modifications of the synthetic parameters and a post-synthesis hydrothermal process were applied to increase the bulk order of CNPs, resulting in an increase of the photothermal and ROS scavenging activities, without affecting the morphological and colloidal properties of the nanomaterial.

Synthesis of Graphene and Graphene Films with Minimal Structural Defects

Graphene is a carbon material with extraordinary properties that has been drawing a significant amount of attention in the recent decade. High-quality graphene can be produced by different methods, such as epitaxial growth, chemical vapor deposition, and micromechanical exfoliation. The reduced graphene oxide route is, however, the only current approach that leads to the large-scale production of graphene materials at a reasonable cost. Unfortunately, graphene oxide reduction normally yields graphene materials with a high defect density. Here, we introduce a new route for the large-scale synthesis of graphene that minimizes the creation of structural defects. The method involves high-quality hydrogen functionalization of graphite followed by thermal dehydrogenation. We also demonstrated that the hydrogenated graphene synthesis route can be used for the preparation of high-quality graphene films on glass substrates. A reliable method for the preparation of these types of films is essential for the widespread implementation of graphene devices. The structural evolution from the hydrogenated form to graphene, as well as the quality of the materials and films, was carefully evaluated by Raman spectroscopy.

Carbon Layer Formation on Hexagonal Boron Nitride by Plasma Processing in Hydroquinone Aqueous Solution

Although hexagonal boron nitride (hBN) is a thermally conductive and electrically insulating filler in composite materials, surface modification remains difficult, which limits its dispersibility and functionalization. In this study, carbon layer formation on hBN particles by plasma processing in hydroquinone aqueous solution was investigated as a surface modification technique. Carbon components with features of polymeric hydrogenated amorphous carbon were found to be uniformly distributed on the hydroquinone-aided plasma-modified hBN (HQpBN) particles. Electron spin resonance measurements revealed abundant unpaired electrons in HQpBN, indicating that defects were formed on hBN by plasma processing and that the carbon layer contained dangling bonds. The defects on hBN could help in the attachment of the carbon layer, whereas the dangling bonds could act as reactive sites for further functionalization. The carbon layer on HQpBN was successfully functionalized with isocyanate groups, thus confirming the ability of this carbon layer to facilitate surface modification. These results demonstrate that the carbon layer formed on hBN can provide a designable interface in organic/inorganic composite materials.

Degradation of mixed cationic dye pollutant by metal free melem derivatives and graphitic carbon nitride

Graphitic carbon nitride (GCN), a polymeric metal free catalyst is widely used to degrade the toxic organic dye from the aqueous pollution. However, its catalytic efficiency and effective simultaneous reduction of mixed dye is still a challenge. Here, we have tuned the physiochemical properties of the GCN and melem derivatives by facilely tuning the degree of polycondensation and examined their catalytic activity towards the removal of cationic dye individually and together in solution. Catalysts were synthesized by thermal treatment of low-cost melamine and characterized by XRD, FTIR, RAMAN, FE-SEM, EDX, UV-DRS, and FL spectroscopy to confirm materials' structure, phase, morphology and optical properties. A suitable phase of the catalyst (M-450) exhibited superior removal capacity with a high-rate constant compared to others. The results demonstrate that M-450 has a maximum loading efficacy of 2.13 and 1.12 mg g^-1 for methylene blue (MB) and Rhodamine B (RhB) dyes respectively in a single dye system. Attractively, when MB and RhB co-exist in the solution, the efficacy increased by 14% (2.44 mg g^-1) and 27% (1.43 mg g^-1) for MB and RhB respectively. The adsorption kinetics, stability, effect of pH and reusability of M-450 catalyst was testified. Further, radical scavenger experiments and terephthalic acid tests were carried out to explain the reaction mechanism involved in the degradation of textile dyes. Moreover, electron paramagnetic resonance (EPR) analysis validated the availability of hydroxyl radicals in the photocatalytic reaction. Excellent stability and reusability were attained even after five successive cycles, demonstrating a suitable photocatalyst for the efficient degradation of mixed dye.

A comprehensive study on the effect of carbonization temperature on the physical and chemical properties of carbon fibers

Carbon fibers were successfully fabricated via the electrospinning technique, followed by stabilizing and carbonizing electrospun PAN fibers. A wide range of analytical techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Diffuse reflectance spectroscopy (DRS), photoluminescence spectroscopy (PL), vibrating sample magnetometer (VSM) techniques, and Hall effect were performed to study of the effect of carbonization temperature on the physical and chemical characterization of carbon fibers. The SEM images of the PAN precursor exhibit a smooth outer surface, after the stabilization and carbonization process, along with a broken fiber at higher carbonization temperature about 1400 °C. Morphological characterization based on the recorded TEM images of carbonized fibers at 1000 °C and 1400 °C, showed that the obtained morphology can be classified as fiber structures, where their diameters ranged from 196 to 331 nm. The XRD patterns of PAN-based carbon fibers confirm the structural changes from linear structure into a graphite-like structure. The DRS study indicates the possible π–π*/σ–π* and n–π* transitions. The presence of the surface functional groups and different trapped radiative recombination on the emission bands is confirmed by the PL. VSM results shows the weak ferromagnetic nature of the carbon fibers.

The Effect of Surface Treatment on Structural Properties of CVD Diamond Layers with Different Grain Sizes Studied by Raman Spectroscopy

Extensive Raman spectroscopy studies combined with scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) measurements were performed to investigate structural and chemical changes in diamond layers deposited by chemical vapour deposition (CVD) upon post-growth treatment with hydrogen. The aim of this study is to characterize the changes in micro-structural properties of diamond layers with different grain sizes and different contents of sp2 carbon phase. Hydrogenation or oxidization of diamond layer surface is often performed to modify its properties; however, it can also strongly affect the surface structure. In this study, the impact of hydrogenation on the structure of diamond layer surface and its chemical composition is investigated. Owing to their polycrystalline nature, the structural properties of CVD diamond layers can strongly differ within the same layer. Therefore, in this project, in order to compare the results before and after hydrogen treatment, the diamond layers are subjected to Raman spectroscopy studies in the vicinity of a T-shape marker fabricated on the surface of each diamond layer studied.

Structural and Mechanical Properties of a-BCN Films Prepared by an Arc-Sputtering Hybrid Process

Amorphous boron carbon nitride (a-BCN) films exhibit excellent properties such as high hardness and high wear resistance. However, the correlation between the film structure and its mechanical properties is not fully understood. In this study, a-BCN films were prepared by an arc-sputtering hybrid process under various coating conditions, and the correlations between the film’s structure and mechanical properties were clarified. Glow discharge optical emission spectroscopy, X-ray photoelectron spectroscopy, Fourier-transform infrared spectroscopy, and Raman spectroscopy were used to analyze the structural properties and chemical composition. Nanoindentation and ball-on-disc tests were performed to evaluate the hardness and to estimate the friction coefficient and wear volume, respectively. The results indicated that the mechanical properties strongly depend on the carbon content in the film; it decreases significantly when the carbon content is <90%. On the other hand, by controlling the contents of boron and nitrogen to a very small amount (up to 2.5 at.%), it is possible to synthesize a film that has nearly the same hardness and friction coefficient as those of an amorphous carbon (a-C) film and better wear resistance than the a-C film.

Effect of Water Adsorption on the Frictional Properties of Hydrogenated Amorphous Carbon Films in Various Relative Humidities

The tribological properties of hydrogenated amorphous carbon (a-C:H) films in ambient air were investigated from the microstructural point of view. a-C:H films with various microstructures (polymer-like, diamond-like, and graphite-like structures) were prepared, and the thickness of water adsorption layers on the films was measured. The adsorption behavior of water molecules on a-C:H films could be expressed with the Brunauer-Emmett-Teller (BET) isotherm, while the thicknesses of icelike and liquidlike water layers adsorbed on the films could be determined using the BET parameters C and n_m^a. The polymer-like films exhibited the thickest icelike and liquidlike water adsorption layers, which decreased as the film structure changed to a diamond-like or a graphite-like structure. A strong relationship was observed between the thickness of water adsorption layers and the surface oxidation of the a-C:H films. The friction coefficient of the films in ambient air can be well explained by the surface oxidation and the thickness of water adsorption layers. Polymer-like films showed high friction coefficients due to the formation of a thick water layer on the films originated from the high surface oxidation of the film surface, whereas the graphite-like film exhibited a low friction coefficient due to low oxidation and a thin water adsorption layer. Furthermore, friction tests between the a-C:H films with different microstructures under ambient air were performed to determine the lowest friction pair in various relative humidities (RHs).

Unravelling the Potential of Graphene Quantum Dots in Biomedicine and Neuroscience

Quantum dots (QDs) are semiconducting nanoparticles that have been gaining ground in various applications, including the biomedical field, thanks to their unique optical properties. Recently, graphene quantum dots (GQDs) have earned attention in biomedicine and nanomedicine, thanks to their higher biocompatibility and low cytotoxicity compared to other QDs. GQDs share the optical properties of QD and have proven ability to cross the blood-brain barrier (BBB). For this reason, GQDs are now being employed to deepen our knowledge in neuroscience diagnostics and therapeutics. Their size and surface chemistry that ease the loading of chemotherapeutic drugs, makes them ideal drug delivery systems through the bloodstream, across the BBB, up to the brain. GQDs-based neuroimaging techniques and theranostic applications, such as photothermal and photodynamic therapy alone or in combination with chemotherapy, have been designed. In this review, optical properties and biocompatibility of GQDs will be described. Then, the ability of GQDs to overtake the BBB and reach the brain will be discussed. At last, applications of GQDs in bioimaging, photophysical therapies and drug delivery to the central nervous system will be considered, unraveling their potential in the neuroscientific field.

Carbon Nanodots for Charge-Transfer Processes

In recent years, carbon nanodots (CNDs) have emerged as an environmentally friendly, biocompatible, and inexpensive class of material, whose features sparked interest for a wide range of applications. Most notable is their photoactivity, as exemplified by their strong luminescence. Consequently, CNDs are currently being investigated as active components in photocatalysis, sensing, and optoelectronics. Charge-transfer interactions are common to all these areas. It is therefore essential to be able to fine-tune both the electronic structure of CNDs and the electronic communication in CND-based functional materials. The complex, but not completely deciphered, structure of CNDs necessitates, however, a multifaceted strategy to investigate their fundamental electronic structure and to establish structure-property relationships. Such investigations require a combination of spectroscopic methods, such as ultrafast transient absorption and fluorescence up-conversion techniques, electrochemistry, and modeling of CNDs, both in the absence and presence of other photoactive materials. Only a sound understanding of the dynamics of charge transfer, charge shift, charge transport, etc., with and without light makes much-needed improvements in, for example, photocatalytic processes, in which CNDs are used as either photosensitizers or catalytic centers, possible. This Account addresses the structural, photophysical, and electrochemical properties of CNDs, in general, and the charge-transfer chemistry of CNDs, in particular. Pressure-synthesized CNDs (pCNDs), for which citric acid and urea are used as inexpensive and biobased precursor materials, lie at the center of attention. A simple microwave-assisted thermolytic reaction, performed in sealed vessels, yields pCNDs with a fairly homogeneous size distribution of ∼1-2 nm. The narrow and excitation-independent photoluminescence of pCNDs contrasts with that seen in CNDs synthesized by other techniques, making pCNDs optimal for in-depth physicochemical analyses. The atomistic and electronic structures of CNDs were also analyzed by quantum chemical modeling approaches that led to a range of possible structures, ranging from heavily functionalized, graphene-like structures to disordered amorphous particles containing small sp² domains. Both the electron-accepting and -donating performances of CNDs make the charge-transfer chemistry of CNDs rather versatile. Both covalent and noncovalent synthetic approaches have been explored, resulting in architectures of various sizes. CNDs, for example, have been combined with molecular materials ranging from electron-donating porphyrins and extended tetrathiafulvalenes to electron-accepting perylendiimides, or nanocarbon materials such as polymer-wrapped single-walled carbon nanotubes. In every case, charge-separated states formed as part of the reaction cascades initiated by photoexcitation. Charge-transfer assemblies including CNDs have also played a role in technological applications: for example, a proof-of-concept dye-sensitized solar cell was designed and tested, in which CNDs were adsorbed on the surface of mesoporous anatase TiO₂. The wide range of reported electron-donor-acceptor systems documents the versatility of CNDs as molecular building blocks, whose electronic properties are tunable for the needs of emerging technologies.

Breakdown of Kasha's Rule in a Ubiquitous, Naturally Occurring, Wide Bandgap Aluminosilicate (Feldspar)

Excitation-energy-dependent emission (EDE) is well known from photoluminescence (PL) studies of polar solvents and carbon-based nanostructures. In polar solvents, this effect known as the 'red edge effect' (REE) is understood to arise from solute-solvent interactions, whereas, in case of carbon-based nanostructures, the origin is highly debated. Understanding this effect has important bearings on the potential applications of these materials. EDE has never been reported from large crystalline materials, except very recently by our group. Here, we make detailed investigations to understand the universality and the mechanism behind the EDE in a wide band gap aluminosilicate (feldspar), which comprises more than half of the Earth's crust, and is widely used in geophotonics (e.g., optical dating). We observe EDE up to 150 nm at room temperature in our samples, which is unprecedented in rigid macroscopic structures. Based on PL investigations at 295 K and 7 K, we present a novel model that is based on photoionisation of a deep lying defect and subsequent transport/relaxation of free electrons in the sub-conduction band tail states. Our model has important implications for potential photonic applications using feldspar, measurement of band tail width in wide bandgap materials, and understanding the EDE effect in other materials.

Self-trapped exciton emission from carbon dots investigated by polarization anisotropy of photoluminescence and photoexcitation

Carbon dots have attracted tremendous attention because of their intrinsic advantages that open up opportunities to replace traditional fluorescent materials in various application fields. However, until now, the emission mechanism from carbon dots has been controversial, substantially hindering the extensive exploitation of these materials. Here, we explore systematically the essential emission behavior of carbon dots by using polarization anisotropy spectroscopy, electric-field modulation spectroscopy, and time-resolved photoluminescence measurements. We probe the momentum evolution dynamics and evaluate the decay process of the photoexcited hot carriers, which manifest characteristics that are distinct from band edge emission. We provide clear evidence that carbon dot emission originates from radiative recombination of self-trapped excitons, where the mobilization of the carriers is largely impeded due to the existence of a strong local potential field and thus the relaxation of the hot carriers is strongly suppressed. Based on the self-trapped exciton model, all the optical properties of carbon dots inferred from both steady-state and time-resolved optical spectroscopy can be interpreted consistently. Our investigation provides an alternative insight into the emission mechanisms of carbon dots, which may improve our understanding of these novel materials.

Precise determination of graphene functionalization by in situ Raman spectroscopy

The verification of a successful covalent functionalization of graphene and related carbon allotropes can easily be carried out by Raman spectroscopy. Nevertheless, the unequivocal assignment and resolution of individual lattice modes associated with the covalent binding of addends was elusive up to now. Here we present an in situ Raman study of a controlled functionalization of potassium intercalated graphite, revealing several new bands appearing in the D-region of the spectrum. The evolution of these bands with increasing degree of functionalization from low to moderate levels provides a basis for the deconvolution of the different components towards quantifying the extent of functionalization. By complementary DFT calculations we were able to identify the vibrational changes in the close proximity of the addend bearing lattice carbon atoms and to assign them to specific Raman modes. The experimental in situ observation of the developing functionalization along with the reoxidation of the intercalated graphite represents an important step towards an improved understanding of the chemistry of graphene.

Strong Photoluminescence Enhancement of Silicon Oxycarbide through Defect Engineering

The following study focuses on the photoluminescence (PL) enhancement of chemically synthesized silicon oxycarbide (SiC_xO_y) thin films and nanowires through defect engineering via post-deposition passivation treatments. SiC_xO_y materials were deposited via thermal chemical vapor deposition (TCVD), and exhibit strong white light emission at room-temperature. Post-deposition passivation treatments were carried out using oxygen, nitrogen, and forming gas (FG, 5% H₂, 95% N₂) ambients, modifying the observed white light emission. The observed white luminescence was found to be inversely related to the carbonyl (C=O) bond density present in the films. The peak-to-peak PL was enhanced ~18 and ~17 times for, respectively, the two SiC_xO_y matrices, oxygen-rich and carbon-rich SiC_xO_y, via post-deposition passivations. Through a combinational and systematic Fourier transform infrared spectroscopy (FTIR) and PL study, it was revealed that proper tailoring of the passivations reduces the carbonyl bond density by a factor of ~2.2, corresponding to a PL enhancement of ~50 times. Furthermore, the temperature-dependent and temperature-dependent time resolved PL (TDPL and TD-TRPL) behaviors of the nitrogen and forming gas passivated SiC_xO_y thin films were investigated to acquire further insight into the ramifications of the passivation on the carbonyl/dangling bond density and PL yield.

Degree of functionalisation dependence of individual Raman intensities in covalent graphene derivatives

Covalent functionalisation of graphene is a continuously progressing field of research. The optical properties of such derivatives attract particular attention. In virtually all optical responses, however, an enhancement in peak intensity with increase of sp³ carbon content, and a vanishing of the peak position shift in monolayer compared to few-layer systems, is observed. The understanding of these seemingly connected phenomena is lacking. Here we demonstrate, using Raman spectroscopy and in situ electrostatic doping techniques, that the intensity is directly modulated by an additional contribution from photoluminescent π-conjugated domains surrounded by sp³ carbon regions in graphene monolayers. The findings are further underpinned by a model which correlates the individual Raman mode intensities to the degree of functionalisation. We also show that the position shift in the spectra of solvent-based and powdered functionalised graphene derivatives originates predominantly from the presence of edge-to-edge and edge-to-basal plane interactions and is by large functionalisation independent.

Beyond Adsorption: The Effect of Sulfur Doping on Emerging Applications of Nanoporous Carbons

<p>Recently we have directed our attention to new applications of “old” materials, nanoporous carbons, as photocatalysts for oxidation of dibenzothiophenes, as water splitting catalysts, as gas sensors and as photosensitizers. Our inspiration was in graphene science. We found that both surface chemistry and porosity are crucial factors determining the specific performance. Since the effects are synergistic, it is not possible to totally separate the influence of these two factors. In terms of photoactivity and photosensitivity, surface chemistry was found as having the predominant effect on the catalytic performance. Sulfur containing groups were indicated as playing a major role in these processes. Of course physical adsorption was necessary to take place on the surface before further reactions promoted by absorption of photons occurred. Since some level of conductivity of the carbon matrix is important for an electron transfer, formation of radicals, and active oxygen species, the presence of sp² graphitic dots of 10 nm in size in the carbon matrix enhanced the photoactive performance. In the case of gas sensing where the reversibility of the signal is important, physical adsorption was a predominant factor. Here the specific polar or electrostatic interactions enhance the sensitivity and affect markedly the selectivity. A minireview of our recent work on these two emerging topics, photoactivity of carbon and their sensing application, is presented in this paper. The emphasis is on the importance of both, specific surface chemistry and developed porosity. The latter is a unique factor, which differentiates the performance of porous carbons from that of nanoforms of carbons such as graphene or carbon nanotubes.</p><p> </p><p><strong>Keywords:</strong></p><p><strong> </strong>nanoporous carbon, photoactivity, catalytic oxidations, water splitting,<br />gas sensing, surface chemistry, porosity, photosensitivity</p>

High energy electron irradiation of interstellar carbonaceous dust analogs: Cosmic ray effects on the carriers of the 3.4 µm absorption band

The effects of cosmic rays on the carriers of the interstellar 3.4 μm absorption band have been investigated in the laboratory. This band is attributed to stretching vibrations of CH₃ and CH₂ in carbonaceous dust. It is widely observed in the diffuse interstellar medium (ISM), but disappears in dense clouds. Destruction of CH₃ and CH₂ by cosmic rays could become relevant in dense clouds, shielded from the external ultraviolet field. For the simulations, samples of hydrogenated amorphous carbon (a-C:H) have been irradiated with 5 keV electrons. The decay of the band intensity vs electron fluence reflects a-C:H dehydrogenation, which is well described by a model assuming that H₂ molecules, formed by the recombination of H atoms liberated through CH bond breaking, diffuse out of the sample. The CH bond destruction rates derived from the present experiments are in good accordance with those from previous ion irradiation experiments of HAC. The experimental simplicity of electron bombardment has allowed the use of higher energy doses than in the ion experiments. The effects of cosmic rays on the aliphatic components of cosmic dust are found to be small. The estimated cosmic ray destruction times for the 3.4 μm band carriers lie in the 10⁸ yr range and cannot account for the disappearance of this band in dense clouds, which have characteristic lifetimes of 3 × 10⁷ yr. The results invite a more detailed investigation of the mechanisms of CH bond formation and breaking in the intermediate region between diffuse and dense clouds.

Camphor-mediated synthesis of carbon nanoparticles, graphitic shell encapsulated carbon nanocubes and carbon dots for bioimaging

A green method for an efficient synthesis of water-soluble carbon nanoparticles (CNPs), graphitic shell encapsulated carbon nanocubes (CNCs), Carbon dots (CDs) using Camphor (Cinnamomum camphora) is demonstrated. Here, we describe a competent molecular fusion and fission route for step-wise synthesis of CDs. Camphor on acidification and carbonization forms CNPs, which on alkaline hydrolysis form CNCs that are encapsulated by thick graphitic layers and on further reduction by sodium borohydride yielded CDs. Though excitation wavelength dependent photoluminescence is observed in all the three carbon nanostructures, CDs possess enhanced photoluminescent properties due to more defective carbonaceous structures. The surface hydroxyl and carboxyl functional groups make them water soluble in nature. They possess excellent photostability, higher quantum yield, increased absorption, decreased cytotoxicity and hence can be utilized as a proficient bio imaging agent.

Growth and photoluminescence of oriented MoSe2nanosheets produced by hot filament CVD

Oriented MoSe₂nanosheets with varying layers and structures were synthesized on silicon substrates by hot filament chemical vapour deposition in a nitrogen environment using MoO₃and Se powders as precursors.

Nanoporous Carbons: Looking Beyond Their Perception as Adsorbents, Catalyst Supports and Supercapacitors

The discovery of carbon nanoforms, and especially graphene, has opened up new directions of science and technology. Many applications are based on the unique properties of graphene, such as its high electrical and thermal conductivity, strength, flexibility, photoactivity and transparency. Inspired by the emerging graphene science, we directed our efforts to the exploration of new applications of nanoporous (microporous) carbons. Their matrix is built of distorted graphene layers, between which pores with sizes ranging from a fraction of a nanometer to hundreds of nanometers exist. This is a very unique feature of nanoporous carbons resulting in their developed surface areas. Moreover, there are vast possibilities to modify the surface chemistry of carbons and thus their surface properties. Even though the traditional applications of porous carbons focus mainly on adsorption and separation, we decided to explore them as photocatalysts, oxygen reduction catalysts and sensors. Related to their visible-light activity, their possible application in solar energy harvesting is also indicated. This Personal Account presents our paths leading to the exploration of these directions, describing the results collected and difficulties encountered, along with the challenges remaining to be addressed.

Liquid phase collagen modified graphene that induces apoptosis

The differential interference contrast (DIC) and fluorescence confocal micrographs show collagen microfibrils attacking graphite from all sides to form a stable dispersion of collagen modified graphene, but only collagen picks up a stain.

Photophysical properties and singlet oxygen generation efficiencies of water-soluble fullerene nanoparticles

As various fullerene derivatives have been developed, it is necessary to explore their photophysical properties for potential use in photoelectronics and medicine. Here, we address the photophysical properties of newly synthesized water-soluble fullerene-based nanoparticles and polyhydroxylated fullerene as a representative water-soluble fullerene derivative. They show broad emission band arising from a wide-range of excitation energies. It is attributed to the optical transitions from disorder-induced states, which decay in the nanosecond time range. We determine the kinetic properties of the singlet oxygen ((1)O2) luminescence generated by the fullerene nanoparticles and polyhydroxylated fullerene to consider the potential as photodynamic agents. Triplet state decay of the nanoparticles was longer than (1)O2 lifetime in water. Singlet oxygen quantum yield of a series of the fullerene nanoparticles is comparably higher ranging from 0.15 to 0.2 than that of polyhydroxylated fullerene, which is about 0.06.

Origin of superlubricity in a-C:H:Si films: a relation to film bonding structure and environmental molecular characteristic

Superlubricity of Si-containing hydrogenated amorphous carbon (a-C:H:Si) films has been systematically investigated in relation to the film bonding structure and the environmental atmosphere. Structural diversity induced by hydrogen incorporation (i.e., 17.3-36.7 at. % H), namely sp(2)-bonded a-C, diamond-like or polymer-like, and tribointeractions activated by the participation of environmental gaseous molecules mainly determine the frictional behaviors of a-C:H:Si films. A suitable control of hydrogen content in the film (i.e., the inherent hydrogen coverage) is obligate to obtain durable superlubricity in a distinct gaseous atmosphere such as dry N2, reactive H2 or humid air. Rapid buildup of running-in-induced antifriction tribolayers at the contact interface, which is more feasible in self-mated sliding, is crucial for achieving a superlubric state. Superior tribological performances have been observed for the polymer-like a-C:H:Si (31.9 at. % H) film, as this hydrogen-rich sample can exhibit superlow friction in various atmospheres including dry inert N2 (μ ∼ 0.001), Ar (μ ∼ 0.012), reactive H2 (μ ∼ 0.003) and humid air (μ ∼ 0.004), and can maintain ultralow friction in corrosive O2 (μ ∼ 0.084). Hydrogen is highlighted for its decisive role in obtaining superlow friction. The occurrence of superlubricity in a-C:H:Si films is generally attributed to a synergistic effect of phase transformation, surface passivation and shear localization, for instance, the near-frictionless state occurred in dry N2. The contribution of each mechanism to the friction reduction depends on the specific intrafilm and interfilm interactions along with the atmospheric effects. These antifriction a-C:H:Si films are promising for industrial applications as lubricants.

Colloidal graphite/graphene nanostructures using collagen showing enhanced thermal conductivity

In the present study, the exfoliation of natural graphite (GR) directly to colloidal GR/graphene (G) nanostructures using collagen (CL) was studied as a safe and scalable process, akin to numerous natural processes and hence can be termed “biomimetic”. Although the exfoliation and functionalization takes place in just 1 day, it takes about 7 days for the nano GR/G flakes to stabilize. The predominantly aromatic residues of the triple helical CL forms its own special micro and nanoarchitecture in acetic acid dispersions. This, with the help of hydrophobic and electrostatic forces, interacts with GR and breaks it down to nanostructures, forming a stable colloidal dispersion. Surface enhanced Raman spectroscopy, X-ray diffraction, photoluminescence, fluorescence, and X-ray photoelectron spectroscopy of the colloid show the interaction between GR and CL on day 1 and 7. Differential interference contrast images in the liquid state clearly reveal how the GR flakes are entrapped in the CL fibrils, with a corresponding fluorescence image showing the intercalation of CL within GR. Atomic force microscopy of graphene-collagen coated on glass substrates shows an average flake size of 350 nm, and the hexagonal diffraction pattern and thickness contours of the G flakes from transmission electron microscopy confirm ≤ five layers of G. Thermal conductivity of the colloid shows an approximate 17% enhancement for a volume fraction of less than approximately 0.00005 of G. Thus, through the use of CL, this new material and process may improve the use of G in terms of biocompatibility for numerous medical applications that currently employ G, such as internally controlled drug-delivery assisted thermal ablation of carcinoma cells.

Synthesis and luminescence mechanism of multicolor-emitting g-C3N4 nanopowders by low temperature thermal condensation of melamine

Graphite like C3N4 (g-C3N4) was synthesized facilely via the low temperature thermal condensation of melamine between 300-650°C. The results showed that the products maintained as melamine when the temperature is below 300°C. With the increase of temperature, the products were transformed into carbon nitride and amorphous g-C3N4 successively. The morphology of products was changed from spherical nanoparticles of melamine into layer carbon nitride and g-C3N4 with the increase of temperature. The photoluminescence spectra showed that the carbon nitride products have continuous tunable photoluminescence properties in the visible region with increasing temperature. With the help of steady state, transient state time-resolved photoluminescence spectra and Raman microstructural characterization, a novel tunable photoluminescence mechanism was founded systematically, which is mainly related to the two dimensional π-conjugated polymeric network and the lone pair of the carbon nitride.

Mechanism of quantum dot luminescence excitation within implanted SiO2:Si:C films

Results of the investigation of photoluminescence (PL) mechanisms for silicon dioxide films implanted with ions of silicon (100 keV; 7 × 10(16) cm(-2)) and carbon (50 keV; 7 × 10(15)-1.5 × 10(17) cm(-2)) are presented. The spectral, kinetic and thermal activation properties of the quantum dots (Si, C and SiC) formed by a subsequent annealing were studied by means of time-resolved luminescence spectroscopy under selective synchrotron radiation excitation. Independent quantum dot PL excitation channels involving energy transfer from the SiO(2) matrix point defects and excitons were discovered. A resonant mechanism of the energy transfer from the matrix point defects (E' and ODC) is shown to provide the fastest PL decay of nanosecond order. The critical distances (6-9 nm) of energy transport between the bulk defects and nanoclusters were determined in terms of the Inokuti-Hirayama model. An exchange interaction mechanism is realized between the surface defects (E(s)'-centres) and the luminescent nanoparticles. The peculiarities of an anomalous PL temperature dependence are explained in terms of a nonradiative energy transfer from the matrix excitons. It is established that resonant transfer to the luminescence centre triplet state is realized in the case of self-trapped excitons. In contrast, the PL excitation via free excitons includes the stages of energy transfer to the singlet state, thermally activated singlet-triplet conversion and radiative recombination.

Band Model for Electron Emission from Diamond and Diamond-Like Carbon

The band diagram is constructed for diamond on metal and diamond-like carbon on metal emitters, from data for electron affinities, Schottky barrier heights or band offsets. For diamond, there is a large offset for the conduction band at the back-contact, causing this to be the dominant barrier for emission. Nitrogen and perhaps grain boundaries reduce this barrier by forming a depletion layer of ionised donors, which narrows the tunnelling distance. In DLC, there is little conduction band offset at the back-contact, and the dominant barrier is at the surface. Nitrogen now lowers the emission barrier by raising the bulk Fermi level and lowering the work function. Unresolved problems are also pointed out.

Deposition and Properties of Diamond-Like Carbons

We review for diamond-like carbon the various deposition methods, the deposition mechanisms of subplantation and ion-assisted addition, the characterisation methods such as Raman and electron energy loss spectroscopy, its mechanical properties and some applications as a coating material.

Theory of Electron Field Emission From Diamond And Diamond-Ldxe Carbon

It is shown that the facile electron field emission from diamond and diamond-like carbon occurs because surface groups such as C-H can produce large changes in electron affinity, so that electric fields from the anode can be focused towards unhydrogenated surface areas of high affinity, the fields ending on negative charges in an underlying depletion layer. The resulting downwards band bending creates very large fields which cause Fowler-Nordheim emission, while not exceeding the material's breakdown field, which is the highest for any solid.