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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]

It has been possible to use spectroscopic ellipsometry from the near infrared to near ultraviolet range to study the layered structure of polycrystalline diamond thin films produced by electron cyclotron resonance-assisted chemical vapor deposition. It is suggested that both high surface mobilities and weak-bond etching can give rise to relatively smooth polycrystalline diamond surfaces.

Research describing electron field emission measurements of nanocrystalline carbon films is presented. In particular a comparison of the properties of films grown under positive, negative and unbiased conditions is examined.

Research describing the microstructure of diamond thin films is presented. The study investigates differences between microwave chemical vapour deposition techniques and methods using electron cyclotron resonance.

Research describing the nucleation and growth of diamond thin films is presented. The study utilizes in-situ phase modulated ellipsometry for monitoring the different phases of the thin films.

Two excitation radiations were employed in measuring the Raman spectra of porous silicon-grown GaAs wirelike crystals. Redshifted and broadened transverse optic and longitudinal optic vibrational bands were observed. The vibrational band characteristics were found to be caused by a disorder-induced relaxation in Raman activity selection rules. The features are also attributed to penetration depth variations and defect density gradient.

A study was conducted on the influence of lead vacancies on the R-nFe transition in a Pb(B'1/3B'2/3)O3 compound. The results indicate that both the relaxor behavior and the phase transition are strongly dependent on the lead vacancies content. Moreover, a change in the polar clusters size resulting from lead vacancies formation weakens the dipolar interactions and inhibits the spontaneous relaxor to ferroelectric phase transition.

We report the fabrication of bifunctional nanocomposites consisting of ferroelectric Ba0.7Sr0.3TiO3 (BST) and ferromagnetic La0.67Sr0.33MnO3 (LSMO) at different concentrations via a high-temperature solid state route. The structural, dielectric, electrical, magnetodielectric (MD), magnetoelectric (ME) and magnetic properties of BST/LSMO nanocomposites were systematically investigated over a wide range of temperatures and frequencies. The X-Ray Diffraction analyses reveal the nanocrystalline nature of the heterostructures, wherein both perovskite phases co-exist. No parasitic phases were observed. The study of the dielectric properties shows that the nanocomposites exhibit relaxor ferroelectric character, with ferroelectric-paraelectric phase transition temperatures around 287-292K that do not follow the Curie-Weiss law. The electrical measurements indicate that ac conductivities of the nanocomposites follow the Jonscher's universal power law, with activation energies of 0.42-0.63 eV based on Arrhenius-type behavior at high temperatures. The nanocomposites exhibit well-defined ferromagnetic hysteresis loops at room temperature (RT). The MD and ME measurements at RT indicate that BST/LSMO exhibits a nonlinear ME effect at low frequencies, with a threshold near 0.5T. The magnetocapacitance (MCp) measurements evidence a quadratic dependence on magnetic field, further confirming the multiferroic nature of BST/LSMO. The order of MCp was found to be ~7% per Tesla. The analysis of the MCp measurements indicates that one of the BST/LSMO compositions studied can be considered as a new multiferroic compound. [ABSTRACT FROM AUTHOR]

Nanocrystalline diamond (NCD) thin films offer applications in various fields, but the existing synthetic approaches are cumbersome and destructive. A major breakthrough has been achieved by our group in the direction of a non-destructive, scalable, and economic process of NCD thin-film fabrication. Here, we report a cheap precursor for the growth of nanocrystalline diamond in the form of paraffin wax. We show that NCD thin films can be fabricated on a copper support by using simple, commonplace paraffin wax under reaction conditions of Hot Filament Chemical Vapor Deposition (HFCVD). Surprisingly, even the presence of any catalyst or seeding that has been conventionally used in the state-of-the-art is not required. The structure of the obtained films was analyzed by scanning electron microscopy and transmission electron microscopy. Raman spectroscopy and electron energy-loss spectroscopy recorded at the carbon K-edge region confirm the presence of nanocrystalline diamond. The process is a significant step towards cost-effective and non-cumbersome fabrication of nanocrystalline diamond thin films for commercial production. [ABSTRACT FROM AUTHOR]

We investigate the stability of the orange band photoluminescence (PL) of bifunctional nanoparticles of Mn2+-doped ZnS at 5% (ZnS:0.05Mn). These 4-nm nanoparticles are synthesized via a one-step inorganic chemical route under ambient conditions. The phase, crystallinity, and morphology are analyzed via X-ray and electron diffractions and high-resolution electron microscopy. Based on the thermally activated carrier-transfer model, it is found that orange emission is rather stable at low temperatures and possesses thermal activation energy of ∼18 meV. The analysis of the PL decay curves suggests the coexistence of multiple lifetimes, that the shortening observed in PL lifetime is not due to the Mn2+ ions, and that the orange band decay is stable in temperature range from 10 K to 300 K. The measured M-H hysteresis loops demonstrate that ZnS:0.05Mn nanoparticles exhibit ferromagnetic ordering below 30 K, unlike its bulk counterpart. No magnetic field dependence of the Mn2+ PL intensity is observed up to 1 T. The stability of the PL signal when subject to an applied magnetic field is discussed. This study offers experimental evidence to test low-dimensional dilute magnetic semiconductor models and widens the range of applications of ZnS:Mn nanostructures beyond optoelectronics. [ABSTRACT FROM AUTHOR]

Free standing graphene-diamond hybrid films have been fabricated using saturated hydrocarbon polymers as seeding material by hot filament chemical vapor deposition technique. The films are characterized with x-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS). The XRD shows the characteristic diffraction peaks of both diamond and graphene. The Raman spectrum shows the characteristic band of diamond at 1332 cm-1 and D, G, and 2D bands of graphene at 1349, 1592, and 2687 cm-1, respectively. Both SEM and TEM depict the presence of diamond and graphene in the films. The EELS recorded in the carbon K-edge region also shows the signature peaks of diamond and graphene. The free standing hybrid films exhibit a remarkably low turn-on field of about 2.4 V/μm and a high emission current density of 0.1 mA/cm2. Furthermore, emission currents are stable over the period of 7 days. The superior field emission characteristics of the free standing graphene-diamond hybrid films are attributed to the heat sink capability of diamond and high electrical conductivity of graphene. [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]

In the ongoing quest for energy production by nonconventional methods, energy conversion by vacuum and solid-state thermionic emission devices is one of the potentially efficient pathways for converting thermal energy directly into electrical power. The realization of practical of thermionic energy conversion devices strongly depends on achieving low work function materials, which is thus far a limiting factor. In an attempt to develop a new low work function thermionic material, this work reports thermionic emission energy distributions (TEEDs) from nanocrystalline diamond (NCD) films in the temperature range from 700 to 900 °C that reveal a consistent effective work function of 3.3 eV. The NCD films also exhibit emission peaks corresponding to higher work functions as indicated by shifts in their energy position and relative intensity as a function of temperature. These shifts thus appear to be related to instabilities in the NCD’s surface chemistry. The analysis of these data yields information on the origin of the low effective work function of NCD. [ABSTRACT FROM AUTHOR]

The temporal current stability and the current fluctuations of the field emission current obtained from ultrananocrystalline diamond (UNCD) films were investigated. The films were synthesized by Ar-rich dc plasma assisted hot filament chemical vapor deposition. The field emission properties were correlated to the films’ composition (sp2 and sp3 bonded carbon volume fractions) and nanostructure. The compositional analysis was done by modeling the films’ spectroscopic ellipsometry [Ψ (E), Δ (E)] data by using a two layer structure and the Bruggeman effective medium approximation. The results indicate that the local field enhancement in UNCD films is related to their intrinsic morphology at the diamond-grain boundary-vacuum interface, and the sp3/sp2 carbon volume fraction plays a significant function on the short- and long-term current stabilities of field emitted electrons. [ABSTRACT FROM AUTHOR]

The surface topography of hydrogenated tetrahedral amorphous carbon (ta-C:H) is critical for various applications such as microelectromechanical devices, magnetic and optical storage devices, and medical implants. The surface topography of ta-C:H films deposited by distributed electron cyclotron resonance plasma from C2H2 gas precursor was investigated. The effects of pressure, together with ion flux and energy, are studied by atomic force microscopy in relation to the structural evolution of the films. The results are compared with the predictions of the Edward-Wilkinson model [Proc. R. Soc. London, Ser. A 44, 1039 (1966)] recently proposed to account for ta-C:H growth and with previous interpretations based on hypersonic shock waves. The random hillocks observed on the smooth surfaces of ta-C:H films deposited at high pressure are thought to result from the interference of high energy shock waves triggered by C4Hx+ ions that produce overlapping collision cascades and induce nonlinear effects. [ABSTRACT FROM AUTHOR]

Thin films of nanocrystalline diamond were grown by filament-assisted chemical-vapor deposition using methane as carbon precursor with high hydrogen dilution and hydrogen sulfide concentration ranging from 0 to 500 ppm in the gas phase. The surface topography and electronic structure of these films (n-C:S) were investigated using ultrahigh-vacuum scanning tunneling microscopy and scanning tunneling spectroscopy (STS), respectively. Topographic image analyses depict that the root-mean-square roughness of the film surface and average grain size decreases with increasing sulfur incorporation either in gas phase or solid films. High-resolution scanning tunneling microscopy images reveal the localized regions of high conductivity (white) surrounded by less conductive regions (black) pointing at the existence of inhomogeneous mixture of sp2- and sp3-bonded carbon in aggregate or clustered and dispersed state. The surface density of states was determined using scanning tunneling spectroscopy where normalized differential conductivity, i.e., (dI/dV)/(I/V) mimics local density of states (DOS). These methods were employed to understand the role of sulfur in the modification of both the surface microstructure and electronic structure near the Fermi level. The band edges were derived by taking tangents to the differential conductivity (dI/dV) within a certain potential window of ±2 eV of the Fermi level. The resulting band gap is found to be similar to that measured optically (Tauc gap). The Fermi level for undoped nanocrystalline carbon (n-C) was found just below the midgap indicating that n-C is a weakly p-type semiconductor. The STS DOS shows oscillatory behavior or peaks which we ascribe to states of the surface layer having relatively more graphitic or sp2-bonded carbon bonds. With higher sulfur addition, the Fermi level is found to move above the midgap. These results seem to agree quite well with our early work on electrical conductivity exhibiting n-type doping taking place and declining band gap at higher sulfur contents measured with spectroscopic ellipsometry technique on the same samples. [ABSTRACT FROM AUTHOR]

An open question to the community about the general consensus on the field-emission mechanism in carbon-based materials led to this study. By applying the Fowler–Nordheim (FN) model for carbon-based films, despite the fact that the microstructure and the resulting physical properties of the films can be tuned by scanning various process parameters, providing, in turn, from almost insulating (less defective) to semiconducting (highly defective) films and even a mixture of the two, the material can be categorized as electrically heterogeneous nanostructured carbon. The electrical heterogeneity arises from the different carbon hybridizations (sp2- versus sp3-bonded carbon). In an attempt to tackle these issues, we have performed a comprehensive analysis of I–V data obtained from filament-assisted chemical-vapor-deposition-grown sulfur-incorporated nanocomposite carbon thin films with different microstructures. Studies of the augmentation of the field-emission properties in this material indicated various roles of sulfur in modifying the film properties [Gupta et al., Appl. Phys. Lett. 80, 3446 (2002)]. The I–V data were fitted to various mathematical forms: I=AV2 exp(-B/V) [FN model], I=C exp(aV1/2/kT) [Schottky model], and I=Vn (n>1, for high fields) [space-charge-limited current (SCLC) model]. The goodness of fit along with the theoretical justification(s) on the electron field-emission results were taken into consideration to provide favorable indications for accepting or discarding any particular model. These findings suggest that there is an apparent crossover from SCLC to FN behavior as a function of film microstructure occurring due to the impurity incorporation as the microstructure transits smoothly from microcrystalline to nanocrystalline carbon. Other evidence in support of the aforementioned suggestion is based on the concept of percolation occurring in this nanocomposite carbon (a mix of conducting–insulating/semiconducting) material, whereby the electrons are allowed to tunnel from one conductive cluster to another separated by an insulating matrix, which is demonstrated through electrical conductivity measurements. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Sulfur incorporated nanocrystalline carbon (n-C:S) thin films were grown on molybdenum substrates by a hot-filament chemical vapor deposition technique using gas mixtures of methane, hydrogen, and a range of hydrogen sulfide (H[SUB2]S) concentrations (100-500 ppm with an interval of 100 ppm) at a fixed substrate temperature of 900 °C. They were optically characterized using Raman spectroscopy (RS) and ex situ spectroscopic phase modulated ellipsometry from near-infrared to near UV (1.5-5.0 eV) obtaining their vibrational frequencies and pseudodielectric function, respectively, as a function of [H[SUB2]S]. The ellipsometry data [〈∈[SUBr](E)〉, 〈(∈[SUBi](E)〉] were modeled using Bruggeman effective-medium theory and dispersion relations for the amorphous semiconductors: Forouhi and Bloomer (FB) parameterization model. A simplified two-layer model consisting of a top layer comprising an aggregate mixture of sp[SUP3]C+sp[SUP2]C+void and a bulk layer (L[SUB2]), defined as a dense amorphized FB-modeled material, was found to simulate the data reasonably well. Through these simulations, it was possible to estimate the dielectric function of our n-C:S material, along with the optical band gap (E[SUBg]), film thickness (d), void fraction (f[SUBυ), and roughness layer (σ) as a function of H[SUB2]S concentration. The physical interpretation of the five modeling parameters obtained in the amorphous dispersion model applied to the case of n-C:S thin films is discussed. The Raman and ellipsometry results indicate that the average size of nanocrystallites in the sulfur-incorporated carbon thin films becomes smaller with increasing H[SUB2]S concentration, consistent with atomic force microscopy measurements where the distribution of grain size yielded a gamma around 20 nm. The band gap was found to decrease systematically with increasing H[SUB2]S concentration, indicating an enhancement of π-bonded carbon (sp[SUP2]C), in agreement... [ABSTRACT FROM AUTHOR]

Microcrystalline diamond and nanocrystalline carbon thin films prepared by microwave plasma assisted chemical vapor deposition were submitted to gamma radiation in order to study the radiation-induced structural transformation and the corresponding changes in the electron field emission properties. Electron field emission measurements and microstructural characterizations, which included Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and x-ray photoelectron spectroscopy, were performed before and after gamma radiation doses of 1, 5, and 20 Mrads. Microcrystalline diamond showed a dramatic improvement in the emission properties only after a cumulative dose of 26 Mrad, while nanocrystalline carbon showed a relatively small but systematic decrease in turn-on field from 9 to 4 V/µm with increasing gamma radiation dose. The enhancement in emission characteristics associated with the radiation-induced microstructural transformation shows the critical role of defects with their associated electronic defect states, and of sp²-bonded carbon channels in the electron field emission mechanism of nanocomposite carbon materials. The results also indicate that nanocrystalline carbon tends to reach a state of damage saturation when submitted to Mrad doses of gamma radiation, suggesting the possibility of fabricating radiation buffer materials that would undergo internal sp[sup 3] - sp² interconversions while absorbing ionizing radiation without changing their average microstructure, which can be employed for preventing radiation from reaching sensitive materials. [ABSTRACT FROM AUTHOR]

Results are reported on the electron field emission properties of sulfur (S)-incorporated nanocrystalline carbon (n-C:S) thin films grown on molybdenum (Mo) substrates by hot-filament chemical vapor deposition (HFCVD) technique. In addition to the conventionally used methane (CH[sub 4]) as carbon precursor with high hydrogen (H[sub 2]) dilution, hydrogen sulfide-hydrogen (H[sub 2]S)/H[sub 2] premix gas was used for sulfur incorporation. The field emission properties for the S-incorporated films were investigated systematically as a function of substrate temperature (T[sub s]) and sulfur concentration. Lowest turn-on field achieved was observed at around 4.0 V/µm for the n-C:S sample grown at T[sub s] of 900 deg;C with 500 ppm of H[sub 2]S. These results are compared with those films grown without sulfur (n-C) at a particular T[sub s]. The turn-on field was found to be almost half for the S-assisted film thus demonstrating the effect of sulfur addition to the chemical vapor deposition process. An inverse relation between turn-on field (E[sub C]), growth temperature and sulfur concentration was found. The s incorporation also causes significant microstructural changes, as characterized with non-destructive complementary ex situ techniques: scanning electron microscopy (SEM), atomic force microscopy (AFM), and Raman spectroscopy (RS). S-assisted films show relatively smoother and finer-grained surfaces than those grown without it. These findings are discussed in terms of the dual role of sulfur in enhancing the field emission properties by controlling the sp² C cluster size and introducing substantial structural defects through its incorporation. The in-plane correlation length (L[sub a]) of sp² C cluster was determined from the intensity ratio of the D- and G-bands [I(D)/I(G)] in the visible RS as a function of deposition temperature and sulfur concentration using a phenomenological model. The turn-on field was found to decrease with increasing... [ABSTRACT FROM AUTHOR]