Bridging energy bands to the crystalline and amorphous states of Si QDs

15 pags., 6 figs., 1 tab.The relationship between the crystallization process and opto-electronic properties of silicon quantum dots (Si QDs) synthesized by atmospheric pressure plasmas (APPs) is studied in this work. The synthesis of Si QDs is carried out by flowing silane as a gas precursor in a plasma confined to a submillimeter space. Experimental conditions are adjusted to propitiate the crystallization of the Si QDs and produce QDs with both amorphous and crystalline character. In all cases, the Si QDs present a well-defined mean particle size in the range of 1.5-5.5 nm. Si QDs present optical bandgaps between 2.3 eV and 2.5 eV, which are affected by quantum confinement. Plasma parameters evaluated using optical emission spectroscopy are then used as inputs for a collisional plasma model, whose calculations yield the surface temperature of the Si QDs within the plasma, justifying the crystallization behavior under certain experimental conditions. We measure the ultraviolet-visible optical properties and electronic properties through various techniques, build an energy level diagram for the valence electrons region as a function of the crystallinity of the QDs, and finally discuss the integration of these as active layers of all-inorganic solar cells.This work was supported by the EPSRC (EP/K022237/1, EP/M024938/1, EP/ R008841/1) and the Leverhulme International Network (IN-2012-136)

The importance of surface states in N-doped Carbon Quantum Dots

Nitrogen-doped carbon quantum dots are synthesized by a one-step atmospheric pressure microplasma process. The origin of the observed photoluminescence emission and its relationship with nitrogen doping is studied using a range of optical and chemical measurements along with verification by theoretical calculations. Nitrogen doping into the core and functionalization of surface states with nitrogen and oxygen groups gives rise to a hybrid structure which is responsible for the luminescence with quantum yields up to 33%. Carrier multiplication is observed as a step-like enhancement in the quantum yield. The analysis of visible-light emission suggests that the emission originates for the most part from surface states and not due to recombination within the quantum dot core. The role of surface functional groups is dominant over quantum confinement in determining the optical properties

Doping Independent Work Function and Stable Band Gap of Spinel Ferrites with Tunable Plasmonic and Magnetic Properties

Tuning optical or magnetic properties of nanoparticles, by addition of impurities, for specific applications is usually achieved at the cost of band gap and work function reduction. Additionally, conventional strategies to develop nanoparticles with a large band gap also encounter problems of phase separation and poor crystallinity at high alloying degree. Addressing the aforementioned trade-offs, here we report Ni–Zn nanoferrites with energy band gap (Eg) of ≈3.20 eV and a work function of ≈5.88 eV. While changes in the magnetoplasmonic properties of the Ni–Zn ferrite were successfully achieved with the incorporation of bismuth ions at different concentrations, there was no alteration of the band gap and work function in the developed Ni–Zn ferrite. This suggests that with the addition of minute impurities to ferrites, independent of their changes in the band gap and work function, one can tune their magnetic and optical properties, which is desired in a wide range of applications such as nanobiosensing, nanoparticle based catalysis, and renewable energy generation using nanotechnology

Effect of precursor pH on AuNP/MWCNT nanocomposites synthesized by plasma-induced non-equilibrium electrochemistry

In recent years, plasma-induced non-equilibrium electrochemistry (PiNE) has been increasingly used for the synthesis of nanomaterials. In this study, we investigated the effect of solution pH on the formation of AuNP/MWCNT nanocomposites synthesized by PiNE. It is found that resulting nanocomposite morphology can be manipulated by the solution pH with pH 2 giving the most uniformly distributed AuNP along the MWCNT surface during the nanocomposite formation. The detailed mechanisms of AuNP/MWCNT nanocomposites formation under different pH have been discussed. For selected AuNP/MWCNT, we further evaluated the photothermal conversion performance under a blue laser (wavelength 445 nm) and the material biocompatibility using HeLa cells. The promising photothermal capability and biocompatibility of the composite sample point to their potential future applications such as solar thermal conversion and healthcare technology

Technologies laser pour l’élaboration de matériaux carbonés pour microsystèmes analytiques environnementaux

Amorphous carbon nitride (a-CzN) material has attractor much attention in research and development. Recently, it has become a more promising electrode material than conventional carbon based electrodes in electrochemical and biosensor applications. Nitrogen containing amorphous carbon (a-C:N) thin films have been synthesized by femtosecond pulsed laser deposition (fs-PLD) coupled with plasma assistance through Direct Current (DC) bias power supply. During the deposition process, various nitrogen pressures (0 to 50 Pa) and DC bias (0 to -350 V) were used in order to explore a wide range of nitrogen content into the film. The structure and chemical composition of the films have been studied by using Multi-wavelength (MW) Roman spectroscopy, electron energy-loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTBM). The surface morphology has been studied by Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Increasing the nitrogen pressure or adding a DC bias induced an increase of the N content, up to 28 at.%. Nitrogen content increase induces a higher sp2 character of the film. However DC bias has been found to increase the film structmal disorder, which was detrimental to the electrochemical properties. Indeed the electrochemical measurern-ts, investigated by cyclic voltammetry (CV), demonstrated that the a-CzNfilms show better electron transfer kinetics, reversibility and excellent reproducibility than the pure a-C films. Electrochemical grafting from diazoniurn salts was successfully achieved on this film, with a surface coverage of covalently bonded molecules close to the dense packed monolayer of ferroceneTechnologies laser pour l’élaboration de matériaux carbonés pour microsystèmes analytiques environnementaux. Pas de résumé en français fourn

Highly pyridinic N-doped Graphene Synthesized by Ultrashort Pulsed-Laser Deposition and Thermal Annealing

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Ultrafast laser ablation of graphite in DC nitrogen plasma: time-resolved ablation plume spectroscopy and imaging

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Effect of nitrogen surrounding gas and plasma assistance on nitrogen incorporation in a-C:N films by femtosecond pulsed laser deposition

International audienceIn the context of nitrogen-rich amorphous carbon thin films ultrafast pulsed laser deposition from graphite targets in inert nitrogen or nitrogen plasma ambient, this study assesses the correlation between the ablation plume composition and dynamics and the thin films contents and structures. The use of both optical emission spectroscopy and spectrally resolved 2D imaging, coupled with intensified CCD temporal resolution, allows to precisely follow such species of the plume as CN and C-2 molecules, from their apparition to their deposition on the substrate. The results show that carbon-nitrogen bonding arises at the early time of expansion with little changes in quantity thereafter. The key role of the DC-bias is in lowering the molecular weight of the ambient gas, thus easing molecules way toward the target and interfering with the chemical reaction for CN generation. Depending on the ambient pressure, these processes will have drastically different effects on the thin films properties and contents. This work thus explains the origin of high nitrogen contents in a-C:N thin films obtained using DC-bias, and proposes an easy in situ optical observation-based way to predict and look for the best conditions to maximize those contents in future work