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This article reports the superparamagnetic behavior of metal-free nitrogen-doped graphene quantum dots (N-GQDs). The pulsed laser ablation (PLA) method was utilized to synthesize N-GQDs with an average diameter of 3.45 nm and a high doping level (N/C) of 1.4. Magnetic properties of as-synthesized N-GQDs were explored by performing magnetization vs magnetic field (M–H) and magnetization vs temperature (M–T) measurements. M–H plots measured in a temperature range of 2–300 K revealed the superparamagnetic behavior of N-GQDs. The value of saturation magnetization was found to be directly correlated to nitrogen concentration and a saturation magnetization up to 28.7 emu/g was obtained at room temperature (300 K). M–T measurements with zero-field-cooled (ZFC) and field-cooled (FC) conditions were employed to study anisotropy energy barriers and blocking temperature. A variation in the blocking temperature (TB) from 288 to 61 K was observed when the external magnetic field (H) was changed from 0.1 to 0.6 T. The origin of superparamagnetism was attributed to the presence of graphitic nitrogen bonding configuration and defect states. The observed superparamagnetic properties along with the optical properties of N-GQDs create an opportunity for developing materials for biomedical applications and data recording devices. [ABSTRACT FROM AUTHOR]

Thermal chemical vapor decomposition of methane on copper is the most widely employed technique for high quality and large area graphene growth. Graphene growth by this technique tends to be limited to a monolayer owing to the surface mediated self-limiting growth mechanism, and hence, it is difficult to obtain continuous bilayers and multilayers. We report the layer-controlled growth of high quality and large area polycrystalline graphene on copper foil in a hot filament chemical vapor deposition (HFCVD) reactor and demonstrate that graphene can be grown with a controlled grain size in the range of 5–16 μm. The field effect hole mobilities of the largest grain monolayer and bilayer graphene and the thickest few-layer graphene are 4310 ± 348, 2745 ± 276, and 2472 ± 185 cm2V–1s–1, respectively, which are comparable to that of high quality polycrystalline graphene and suitable for future nanoelectronics. The results of the systematic parametric study hereby presented indicate that graphene adlayers grow on top of the previously grown layer(s) and show that the HFCVD growth of graphene overcomes the difficulties to growing layer-controlled-graphene on copper. Copper vapor together with carbon radicals produced at the filaments appear to enable the adlayer graphene formation. The versatility and capability of the HFCVD for facile and inexpensive growth of graphene make it a superior technique for industrial applications. [ABSTRACT FROM AUTHOR]

The room-temperature photosensitivity of sulfur-doped micro-, submicro-, and nano-crystalline diamond films synthesized by hot-filament chemical vapor deposition was studied. The structure and composition of these diamond materials were characterized by Raman spectroscopy, scanning electron microscopy, and x-ray diffraction. The ultraviolet (UV) sensitivity and response time were studied for the three types of diamond materials using a steady-state broad UV excitation source and two pulsed UV laser radiations. It was found that they have high sensitivity in the UV region (as high as 109 s-1 mV-1 range), a linear response in a broad spectral range below 320 nm, photocurrents around ∼10-5 A, and a short response time better than 100 ns, which is independent of fluency intensity. A phenomenological model was applied to help understand the role of defects and dopant concentration on the materials' photosensitivity. [ABSTRACT FROM AUTHOR]

Nanostructured materials, such as carbon nanotubes, are excellent cold cathode emitters. Here, we report comparative field emission (FE) studies on topographically tailored few layer MoS2 films consisting of h0001i plane perpendicular (⊥) to c-axis (i.e., edge terminated vertically aligned) along with planar few layer and monolayer (1L) MoS2 films. FE measurements exhibited lower turn-on field Eto (defined as required applied electric field to emit current density of 10 µA/cm²) ~4.5 V/lm and higher current density ~1mA/cm², for edge terminated vertically aligned (ETVA) MoS2 films. However, Eto magnitude for planar few layer and 1L MoS2 films increased further to 5.7 and 11 V/µm, respectively, with one order decrease in emission current density. The observed differences in emission behavior, particularly for ETVA MoS2 is attributed to the high value of geometrical field enhancement factor (β), found to be ~1064, resulting from the large confinement of localized electric field at edge exposed nanograins. Emission behavior of planar few layers and 1L MoS2 films are explained under a two step emission mechanism. Our studies suggest that with further tailoring the microstructure of ultra thin ETVA MoS2 films would result in elegant FE properties. [ABSTRACT FROM AUTHOR]

We report the switchable photovoltaic effects in graphene/BiFeO3/Pt heterostructures. Pure phase polycrystalline BiFeO3 films were deposited on Pt/TiO2/SiO2/Si substrates by pulse laser deposition. A bilayer graphene was transferred onto the BiFeO3 film which serves as transparent conducting electrodes. The heterostructures showed switchable photovoltaic effect depending on ferroelectric polarization directions indicating depolarization field induced separation of photogenerated carriers. The open circuit voltage (VOC) and short circuit current density (JSC) were measured to be ~ 110 mV, ~92 μA/cm² in positive polarity and similar values were obtained when the polarity was reversed. The JSC and VOC also showed rapid response (< 100 ms) as a function of light exposure time. [ABSTRACT FROM AUTHOR]

Tin dioxide-carbon nanotube (SnO2-CNT) composite films were synthesized on copper substrates by a one-step process using hot filament chemical vapor deposition (HFCVD) with methane gas (CH4) as the carbon source. The composite structural properties enhance the surface-to-volume ratio of SnO2 demonstrating a desirable electrochemical performance for a lithiumion battery anode. The SnO2 and CNT interactions were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transforminfrared-attenuated total reflectance (ATR-FTIR) spectroscopy. Comprehensive analysis of the structural, chemical, and electrochemical properties reveals that the material consists of self-assembled and highly dispersed SnO2 nanoparticles in CNT matrix. The process employed to develop this SnO2-CNT composite film presents a cost effective and facile way to develop anode materials for Li-ion battery technology. [ABSTRACT FROM AUTHOR]

We report the first direct synthesis of graphene on SiO2/Si by hot-filament chemical vapor deposition. Graphene deposition was conducted at low pressures (35 Torr) with a mixture of methane/hydrogen and a substrate temperature of 970 °C followed by spontaneous cooling to room temperature. A thin copper-strip was deposited in the middle of the SiO2/Si substrate as catalytic material. Raman spectroscopy mapping and atomic force microscopy measurements indicate the growth of few-layers of graphene over the entire SiO2/Si substrate, far beyond the thin copper-strip, while X-ray photoelectron spectroscopy and energy-dispersive X-ray spectroscopy showed negligible amounts of copper next to the initially deposited strip. The scale of the graphene nanocrystal was estimated by Raman spectroscopy and scanning electron microscopy. [ABSTRACT FROM AUTHOR]

Abstract: Pb0.5Sr0.5TiO3 (PST50) fine ceramic powders have been prepared by mechanosynthesis and mixed oxides methods. The dielectric, impedance, electric modulus, and ac conductivity have been carried out over a wide range of frequency and temperature i.e. 100Hz–1MHz and 100–650K, respectively. PST50 shows low dielectric constant, very low dielectric loss, and diffused ferroelectric phase transition above room temperature. The low dielectric loss behavior was continued in the paraelectric region at higher frequencies. We observed only the bulk response using impedance spectroscopy and the two distinct electrical responses by modulus spectroscopy that were attributed to the grain effects (g) and grain boundary (gb) effects in the investigated temperature region. The Master modulus spectra indicated that the role of the grain boundary capacitance becomes more prominent with the increase in temperature. The temperature-dependent Raman spectroscopy revealed a soft mode behavior below the ferroelectric phase transition and breaking in the local symmetry in the paraelectric phase. In the paraelectric region, the local symmetry breaking was associated with the polar clusters localized at the grain boundaries. The frequency-dependent conductivity data were fitted in the modified power law, and the results showed frequency dependent conduction below (450K) and evidence of two types of conduction process at elevated temperatures i.e. the low-frequency conductivity (<1kHz) is due to long-range ordering (frequency-independent) and high-frequency conduction due to the localized orientation hopping mechanism. The cross over frequency (hopping frequency) shifted towards the higher frequency side with an increase in temperature. [Copyright &y& Elsevier]