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We report the room temperature thermal conductivity of polycrystalline twisted bilayer graphene (tBLG) as a function of grain size measured by employing a noncontact optical technique based on micro-Raman spectroscopy. Polycrystalline tBLG sheets of different grain sizes were synthesized on copper by hot filament chemical vapor deposition. The thermal conductivity values are 1305 ± 122 , 971 ± 73 , and 657 ± 42 W m − 1 K − 1 for polycrystalline tBLG with average grain sizes of 54, 21, and 8 nm, respectively. Based on these thermal conductivity values, we also estimated the grain boundary conductance, 14.43 ± 1.21 × 10 10 W m − 2 K − 1 , and the thermal conductivity for single crystal tBLG, 1510 ± 103 W m − 1 K − 1 . Our results show that the relative degradation of thermal conductivity due to grain boundaries is smaller in bilayer than in monolayer graphene. Molecular dynamics simulations indicate that interlayer interactions play an important role in the heat conductivity of polycrystalline bilayer graphene. The quantitative study of the grain size dependent thermal conductivity of polycrystalline bilayer graphene is valuable in technological applications as well as for fundamental scientific understanding.

We report the detection of the ethynyl (C2H) radical by (1 + 2) REMPI on the B 2 A ′ ( T , K a = 0 , 1 , 2 ) ← X 2 Σ + ( 0 , 3 1 , 0 ) transition. One parallel, B 2 A ′ ( T , K a = 1 ) ← X 2 Σ + ( 0 , 3 1 , 0 ) with E eV = 37 , 946 cm - 1 , and two perpendicular transitions, B 2 A ′ ( T , K a = 0 ) ← X 2 Σ + ( 0 , 3 1 , 0 ) with E eV = 37 , 927 cm - 1 and B 2 A ′ ( T , K a = 2 ) ← X 2 Σ + ( 0 , 3 1 , 0 ) with E eV = 38 , 002 cm - 1 , are observed. The C2H radical was generated by the photodissociation of C2H2 at 193 nm. By employing REMPI spectroscopy, we also studied the collisional induced rotational relaxation of C2H and measured rotational relaxation rate constants for this radical. The measured rotational relaxation rate constants for several values of J″ are ( 0.08 – 0.26 ) × 10 - 9 cm 3 molec - 1 s - 1 and ( 0.75 – 1.92 ) × 10 - 9 cm 3 molec - 1 s - 1 for nitrogen buffer gas and acetylene, respectively.

The emergence of antibiotic-resistant bacteria is a major threat to world-wide public health. Functionalized nanoparticles could offer novel strategies in this post-antibiotic era. In this study, we developed nanocomposites of silver nanoparticles decorated with graphene quantum dots (Ag-GQDs) using pulsed laser synthesis. The nanocomposites were PEGylated, which increases their biocompatibility and solubility in aqueous solutions. The HR-TEM micrographs of bare GQDs show that their size is in the range of 1.6–4nm, and the lattice spacing is 0.214nm, which corresponds to the (100) lattice fringes of graphene. The antibacterial activity of Ag-GQDs was evaluated and compared to that of bare GQDs and commercial silver nanoparticles (Ag-NPs) against both Gram-negative and Gram-positive bacteria, using Pseudomonas aeruginosa and Staphylococcus aureus as model bacteria, respectively. Concentration values of 25 and 50μg/mL are required for Ag-GQDs to inhibit the growth of S. aureus and P. aeruginosa bacteria, respectively. The fractional inhibitory concentration (FIC) index is below 0.5 indicating that there is a synergistic effect between Ag-NPs and GQDs. Kirby–Bauer tests showed that Ag-GQDs inhibit P. aeruginosa and S. aureus, in contrast to bare GQDs and Ag-NPs alone. Cell viability of normal mammalian cells treated with Ag-GQDs showed that cell viability is maintained at 100% for cells incubated with Ag-GQDs. The decoration of Ag-NPs with GQDs minimizes their cytotoxicity in mammalian cells and increases their biocompatibility. Ag-GQDs have potential applications in the fabrication of antibacterial coatings, self-sterile textiles, and personal care products.

We fabricated large area bilayer graphene by hot filament chemical vapor deposition (HFCVD) on Cu foil. The HFCVD technique can represent a significant advantage over other techniques for industrial scaling at low cost. We performed systematic experiments to determine the best parameters to obtain uniform graphene coverage over an area of ~ 16 cm 2 . The experimental growth parameters are grouped into two distinct regions according to the products obtained: (A) continuous bilayer graphene with low defect density and (B) continuous bilayer graphene with high defect density. The optimum graphene films obtained are uniform bilayer with low defect density, greater than 90% transmittance in the visible region, and no gaps. The high quality of the bilayer graphene was confirmed by Raman spectroscopy mapping. The results show that the ratio of 2D to G peak intensities ( I 2D / I G ) is in the 0.9–1.6 range over 90% of the area. Moreover, we employed the static cling property of polyethylene terephthalate (PET) to preserve the integrity of the as-grown graphene films in the transfer process, showing that the graphene films become well attached to the SiO 2 /Si substrates while the PET films were completely peeled off.

We report photovoltaic effect and photo response in multiferroic BiFeO 3 (BFO) thin films. Pure phase of polycrystalline BFO thin films have been deposited on SRO/Pt/TiO 2 /SiO 2 /Si substrates by RF sputtering. The as grown BFO thin films showed good ferroelectric properties and antiferromagnetic characteristics at room temperature. The diode like characteristics at the interface of BFO/SRO thin films were observed through current–voltage measurements. The open circuit voltage ( V oc ) and short circuit current density ( J sc ) were measured as ∼0.08 V and ∼ −63 μA/cm 2 respectively with illumination of white light density ∼1 kW/m 2 . The photo response at zero bias voltage of BFO capacitor showed a rapid increase of the photocurrent which saturated at a value ∼ −63 μA/cm 2 followed by a rapid decrease with negligible delay to reach its original state during periodic on and off state of the light source. This high sensitivity makes it possible for the BFO films to be applied in a photosensitive capacitor.

Multi-walled carbon nanotubes (MWCNTs) dispersed onto a silicon substrate have been coated with diamond nanocrystals (DNC) and silicon carbide (SiC) from solid carbon and silicon sources exposed to H2 activated by hot filament chemical vapor deposition (HFCVD) at around 190 °C substrate temperature. MWCNT coating by DNC initiates during filament carburization process at 80 °C substrate temperature under conventional HFCVD conditions. The hybrid nanocarbon material was analyzed by scanning electron microscopy, transmission electron microscopy, energy dispersive X-ray spectroscopy, electron energy loss spectroscopy, selected area electron diffraction, X-ray diffraction and Raman spectroscopy. The structure of the MWCNTs is preserved during coating and the smooth DNC/SiC coating is highly conformal. The average grain size is below 10 nm. The growth mechanism of DNC and SiC onto MWCNT surface is discussed.

Chemical sensors based on tin dioxide-carbon nanotubes (SnO2-CNT) composite films were fabricated by hot filament chemical vapor deposition (HF-CVD) technique. The composite films consist of SnO2 nanoparticles highly dispersed on the CNTs surface. Their resistivity is highly sensitive to the presence of adsorbates, which become easily attached or detached at room temperature and ambient pressure depending on their gas phase concentration. We systematically studied the sensitivity of the SnO2-CNT composite films for ethanol, methanol and H2S. The results were also compared to those for SnO2 and CNTs separately. It is shown that the SnO2-CNT composite films can detect ethanol, methanol and H2S down to ppm levels below OSHA's permissible exposure limits at room temperature and ambient pressure. Moreover, they self-recover within 1 min without requiring any heating or energy source.

We report for the first time the fabrication of nanostructured ferromagnetic lanthanum strontium manganite La0.67Sr0.33MnO3 (LSMO) conformally coated onto bamboo-like carbon nanotubes (BCNTs) by pulsed laser deposition. Scanning electron microscopy revealed that one-dimensional LSMO/BCNTs with diameters ranging from 100-160 nm and lengths over 10 μm were obtained. Line-scanned energy-dispersive X-ray spectroscopy profiles, selected area electron diffraction rings, and energy-filtered transmission electron microscopy maps provided further insight into the hybrid nanostructures. LSMO/BCNT and BCNT were also investigated via in situ electrical probing in a transmission electron microscope using a piezo-driven scanning tunneling microscopy holder. Modeling of the I–V characteristics of individual hybrid nanotubes yielded the resistivity, bandgap, and electron density of LSMO and BCNT. The results show that the transport properties of LSMO/BCNT are superior to those of BCNT. This research advances the integration of oxide materials and carbon nanotubes, bringing forth new avenues for miniaturization and fabrication of one-dimensional core-multishell materials with multifunctional properties that can be used as building blocks in nanodevices.

Two distinct forms of carbon, ultra nanocrystalline diamond (UNCD) and carbon nanotubes (CNTs), were synthesized in a single-step process via hot filament chemical vapor deposition for the first time. The synthesized structure displays unique hierarchical flower-like clusters of vertically aligned carbon nanotubes with diameters ranging from 30 to 50 nm conformally coated with UNCD having a grain size in the range of 3–5 nm. The seeding employed a mixture of diamond and nickel nano powders dispersed in a polymer melt, which promoted the self-assembly of sp 2 and sp 3 carbon into hierarchical structures. The UNCD decorated tubes show good field emission properties with low turn-on field, large field enhancement factor, and an excellent current stability over a period of over 400 h. The ability to synthesize flower like structures of CNTs decorated with UNCD by a single-step process opens up new possibilities for the fabrication of robust nanoelectronic devices.

Graphene has been the subject of intense research in recent years due to its unique electrical, optical and mechanical properties. Furthermore, it is expected that quantum dots of graphene would make their way into devices due to their structure and composition which unify graphene and quantum dots properties. Graphene quantum dots (GQDs) are planar nano flakes with a few atomic layers thick and with a higher surface-to-volume ratio than spherical carbon dots (CDs) of the same size. We have developed a pulsed laser synthesis (PLS) method for the synthesis of GQDs that are soluble in water, measure 2-6 nm across, and are about 1-3 layers thick. They show strong intrinsic fluorescence in the visible region. The source of fluorescence can be attributed to various factors, such as: quantum confinement, zigzag edge structure, and surface defects. Confocal microscopy images of bacteria exposed to GQDs show their suitability as biomarkers and nano-probes in high contrast bioimaging.

Non-toxic magnetic-luminescent iron oxide Mn2+-doped zinc sulfide (Fe3O4/ZnS:Mn) composite nanoparticles were synthesized through a one-step chemical deposition route using surface-treated Fe3O4 nanoparticles. High resolution electron microscopy and X-ray diffraction analyses showed that the two individual phases co-exist with a high degree of crystalline quality, no parasitic phases were found. The average crystallite size was around 9 nm. At the doping level of 20–25 wt% Mn2+, the luminescence signal of the composite at 598 nm is very intense, comparable to that of ZnS:Mn. This remarkable effect has been evidenced solely for Fe3O4 Cd2+-based composites, which are toxic. Moreover, the superparamagnetic response of Fe3O4 with near-zero coercivity is successfully retained in the composite. This notable bifunctionality of Fe3O4/ZnS:Mn can enable both in vivo and in vitro biomedical applications.

ZnO nanowires with high density are grown over zinc foil by thermal evaporation process at relatively low temperature. Electron microscopy studies confirmed that the as-grown nanowires are of single crystal hexagonal wurtzite structure, growing preferentially in the c -axis direction. The Raman spectrum of the ZnO nanowires shows an optical-phonon E 2 mode at 440 cm −1 confirming good crystallinity for the grown nanowires. The field emission measurement indicated that ZnO nanowires have a turn-on field of 9.1 V/μm at current density of 0.001 μA/cm 2 .

Carbon nanotubes (CNTs) have been fabricated on free-standing microcrystalline diamond (FSD) films by hot filament chemical vapor deposition using Ni as catalyst. Micro Raman spectroscopy has been used as a primary technique to characterize the hybrid material, while electron microscopy and electron energy loss spectroscopy served as complementary characterization techniques. The fabricated material shows the presence of multiwall carbon nanotubes of diameters in the range of 10–15 nm with a wall thickness of 5–6 nm protruding from the free-standing microcrystalline diamond films of crystallite sizes ranging from 0.5 to 2.0 μm. The CNT-FSD films show good field emission properties with low turn-on field, large field enhancement factor, and an excellent current stability over a period of 10 days.

Spontaneously detaching self-standing diamond films were successfully fabricated using saturated hydrocarbon polymers as seeding source by hot filament chemical vapor deposition (HFCVD) technique. X-ray diffraction confirmed the crystal structure of diamond. The microcrystalline morphology of the diamond films was analyzed by atomic force microscopy (AFM) and scanning electron microscopy. Raman spectra from the front and back sides of the films show the characteristic diamond band at 1332 cm − 1 and no carbide layer. EELS spectra also show the signature features of diamond. The use of hydrocarbon polymers as seeding source will improve the cost-effectiveness of the fabrication of diamond wafers by simplifying the fabrication process.

Nanocrystals of CdS and CdTe were synthesized by aqueous chemical route. From the optical absorption spectra the particle sizes (diameter) were estimated to be around 7 and 4 nm for CdS and CdTe, respectively. The photovoltaic device was fabricated using these nanocrystalline materials on an indium tin oxide (ITO) coated glass substrate using a spin coating method. From the room temperature photoluminescence study a drastic quenching of photoluminescence the CdS–CdTe thin film was observed. Light intensity dependent current–voltage measurements of CdS–CdTe thin film shows photovoltaic effect; with increase in light intensity the current density increases, however, the open circuit voltage does not show any change. The low efficiency of the device has been explained on the basis of the defects and diffusion of Te ions into CdS.

The photodissociation of borazine, B3N3H6, following 193 nm excitation was studied by Time of Flight Mass Spectrometry (TOFMS). For one-laser experiments, the TOF mass spectrum following borazine excitation consists of m/e peaks that can be attributed exclusively to borazine ion species, i.e., no photofragments are observed. In the two-laser experiments, the TOF mass spectrum can be assigned to peaks corresponding to B3N3H6+, B3N3H5+, and a small amount of BHNH+ ions. We conclude that a significant channel of borazine photodissociation at 193 nm is the production of the B3N3H5 radical and H atom. The photodissociation process related to production of BHNH species is most likely due to a two-photon process.

Carbon nanotube (CNT)/diamond composite has been fabricated by hot filament chemical vapor deposition on a silicon substrate using iron as catalyst. The material characteristics of this monolithic structure were examined by electron microscopy, Raman spectroscopy, X-ray photoelectron spectroscopy and electron energy loss spectroscopy. The composite material shows the presence of carbon nanotubes of several microns in length together with conspicuous diamond microcrystals of sizes ranging from 0.5 to 2.0 μm. The CNTs protrude from the diamond microcrystals and become entangled around them as they grow. This monolithic CNT/diamond composite provides an intrinsic heat dissipation mechanism for CNTs during field emission and exhibits low turn on field, large field enhancement factor, and an excellent current stability over a period of 44 h.

A dual stage technique employing hot filament chemical vapor deposition (HFCVD) and radio frequency sputtering was used to synthesize SiN/BCNTs (bamboo like carbon nanotubes) on copper substrates. The films were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Electron field emission studies (EFE), charge–discharge, and cyclic voltammetry. The comprehensive characterization is consistent with a nanolayer of amorphous SiN on BCNTs. Field emission experiments confirm the excellent contact of the SiN nanolayer with the surface of the BCNTs necessary for fabrication of a coin cell. Electrochemical testing shows that SiN/BCNT electrode can deliver an initial discharge capacity of 2000 mAh g −1 which is higher than the capacity of graphite and the reversible capacity after ten cycles is 300 mAh g −1 . The cyclic voltammetry results suggest good reversibility with Li during cycling.

Sulfur-assisted hot-filament chemical vapor deposition (HFCVD) was recently employed to grow diamond nano-crystals on polyimide film [F. Piazza, G. Morell, Diamond and Related Materials, 16 (2007) 1950], unambiguously showing that the substrate temperature was below 360 °C, the polyimide glass transition temperature. This accomplishment has opened the door to employ diamond in a wide range of applications where it needs to be integrated with temperature sensitive materials. The result reported earlier relies on visible Raman spectroscopy analysis. We hereby report new additional evidences confirming the result from transmission electron microscopy (TEM), high-resolution TEM, energy dispersive X-ray analysis, electron energy loss spectroscopy and selected area electron diffraction.

The wettability and surface energy of hydrogenated amorphous carbon films (a-C:H) elaborated by distributed electron cyclotron resonance plasma were studied by contact angle measurements in relation to composition, structure and topography. Tetrahedral a-C:H (ta-C:H) showed relatively high water contact angle (CA) up to 82.3°, and low surface energy ( E s ), down to 25.3 mJ/m 2 . Hydrophobicity was found to increase with the intensity of the ion bombardment and with the tetrahedral character. A decrease of the dispersive component is responsible of the decrease of surface energy with substrate bias. Low mass-density polymer-like a-C:H (PLC) presents also a relatively high hydrophobicity with high water CA, up to 76.7°, and low surface energy values, down to 32.1 mJ/m 2 . Hydrophobicity is interpreted as resulting from a surface layer rich in sp 2 -carbon for ta-C:H and rich in C–H bonds for PLC.

We report the new results on the direct synthesis of nanostructured silicon carbide (SiC) materials using the pulsed laser deposition technique. Scanning electron microscopy images revealed that SiC nanoholes, nanosprouts, nanowires, and nanoneedles were obtained. The crystallographic structure, chemical composition, and bond structure of the nanoscale SiC materials were investigated using X-ray diffraction, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and Raman scattering spectroscopy. The transverse optical mode and longitudinal optical mode in Raman spectra were found to become sharper as the substrate temperature was increased, while the material structure evolved from amorphous to crystalline.

The SH and CS radicals produced during sulfur-assisted hot filament chemical vapor deposition (HFCVD) were studied by cavity ringdown spectroscopy (CRDS) by detecting the A(0) ← X(0) transition near 323 and 259 nm, for SH and CS, respectively. CRDS is a non-intrusive spectroscopic diagnostic tool suitable for measuring trace amounts of gas species in hostile, reactive environments such as that of chemical vapor deposition (CVD). The SH and CS radicals are thought to play important roles in the gas-phase and heterogeneous chemistry of sulfur-assisted HFCVD synthesis of nano-structured carbon-based materials. A detailed analysis of the rotationally resolved SH and CS spectra led to an estimate of their effective rotational temperature and number density.

Continuous nanocrystalline diamond (NCD) films were grown in an argon-rich gas atmosphere with relatively high growth rates by sustaining a low power (5 W) DC plasma in a hot filament chemical vapor deposition system (HFCVD). The parameter window for the synthesis of NCD films was studied as a function of argon, methane and hydrogen concentrations, as well as substrate temperature and DC bias. The results are consistent with reports indicating that the DC plasma induces re-nucleation by ion bombardment during the initial growth step and helps to maintain the atomic H and hydrocarbon species near the growing surface. It was found that DC plasma-assisted HFCVD enables high NCD growth rates and expands the parameter window, rendering it unnecessary to heat the filament above 2800 K.

Sulfur-assisted hot-filament chemical vapor deposition (HFCVD) was employed to grow nanocrystalline diamond at low substrate temperature, ∼ 300 °C, on molybdenum (Mo), and ∼ 250 °C, on polyimide film. The polyimide films remained flexible and strong after the deposition, clearly indicating that they did not experience temperatures near or above the glass transition temperature at ∼ 360 °C. The relative intensity of the diamond peak in the Raman spectra increases when the substrate temperature is decreased from ∼ 500 to ∼ 300 °C, a result that is inverted with respect to HFCVD without sulfur. This behavior was employed to obtain microcrystalline diamond on Mo at ∼ 270 °C. Profound changes induced to the gas phase chemistry and surface reactions when a trace amount of H2S is added to the HFCVD process seem to enable these results.

We have applied Density Functional Theory (DFT) within the Generalized Gradient Approximation to study the behavior of lithium impurities in the diamond lattice. It was found that, although isolated interstitial lithium atoms in tetrahedral sites have a donor state relatively close to the conduction band at 0.35 eV, their behavior is critically affected by adjacent interstitials. Lithium atoms that occupy adjacent interstitial tetrahedral sites cluster spontaneously and induce carbon–carbon bond breaking in their neighborhood. The corresponding charge distribution and electronic density of states analyses show that charge localization takes place at the carbon atoms around the lithium clusters and deeper mid-gap states are introduced in the diamond band gap. The formation energy was found to be 1.88 eV and 3.09 eV, for clusters of two and three lithium atoms, respectively, considered in this study. The calculations also indicate that Li clustering would not take place if Li ions instead of atoms were incorporated interstitially.

The activity of n-type sulfur-doped microcrystalline CVD diamond films for electrochemical hydrogen evolution is decreased reversibly by the presence of oxygen dissolved in acidic aqueous solution. The activity is measured as a current at a constant applied potential versus a reference electrode and provides the basis for an analytical method for measuring dissolved oxygen. It is proposed that the presence of oxygen may act to decrease the number of majority carrier electrons near the surface. A possible mechanism is proposed.

AlN films are synthesized by using hollow cathode discharge sputtering technique. Typical transverse optical and longitudinal optical modes in Raman spectrum of AlN sample are observed at the gas pressures of 200 Pa and 500 Pa discharge sputtering deposition, respectively. The results show evidence that the structures of the films can be controlled by variations of discharge gas pressures. High discharge electric field used for sputtering deposition of AlN film causes transverse optical mode in Raman spectrum shift toward relatively low wave numbers. After annealing at 600 °C for 5 h in air at atmospheric pressure, the profile of the Raman longitudinal optical band appears narrow, probably indicating better quality of the film.

Sulfur-assisted hot-filament chemical vapor deposition was successfully employed to synthesize diamond films at relatively low substrate temperatures, sub 500 °C, on Mo and glass substrates. This result is ascribed to sulfur-containing species that create an additional carbon transport channel to the substrate, which explicitly requires a large temperature gradient to effectively operate. The grain size and roughness were found to decrease when the Mo substrate temperature was decreased, while the quality factor only showed a slight decrease. Moreover, the diamond quality, grain size, and roughness were found to increase on glass substrates in comparison to those on Mo substrates.

Diamond film deposition at low temperature is of considerable interest in electronics. We report on the growth of high-quality microcrystalline and nanocrystalline diamond films by sulfur-assisted hot filament chemical vapor deposition at substrate temperature ∼440 °C. The effects of substrate temperature in the presence of sulfur on diamond growth and structure were investigated. The deposition rate was found to increase as the substrate temperature is reduced, accompanied by a reduction in grain size and roughness, while the quality factor decreased only slightly. When glass is used as a substrate instead of molybdenum, the diamond quality, grain size, and roughness increase. The deposition of diamond at this relatively low temperature is interpreted in terms of the profound changes induced by the presence of sulfur in the gas phase chemistry.

Boron carbonitride nanotubes (BCNNTs) were grown with high yield by DC arc discharge without catalyst particles or pre-grown template nanostructures. Two types of nanotubes (NTs) were formed: thin NTs with diameters of 10–15 nm and thick NTs with diameters of 25–50 nm. Transmission electron microscopy, electron energy loss spectroscopy, and Raman spectroscopy analyses indicate that the thin NTs are carbon NTs (CNTs) while the thick NTs are BCNNTs wrapped around CNTs. The growth kinetic appears to be faster for CNTs than for BCNNTs. Through the concerted substitution of B and N for C in the in situ grown CNTs, template growth of BCNNTs follows the CNTs growth without causing topological changes.

Device quality hydrogenated amorphous silicon (a-Si:H) thin films were grown by hot wire chemical vapor deposition (HWCVD) on glass (Corning 7059) using silane under high hydrogen dilution as a function of substrate temperature (TS) ranging 50–515 °C. As a consequence of variation of TS, the hydrogen concentration [CH] varied from 20.0% to 0.2%. They are optically examined ex situ using Raman spectroscopy (RS) and spectroscopic phase modulated ellipsometry (SPME) from near IR to near UV (1.5–5.0 eV) obtaining their vibration frequencies and pseudo-dielectric function, respectively for analyzing network disorder. The ellipsometry raw data (〈er(E)〉, 〈ei(E)〉) were modeled using Bruggeman effective medium theory (BEMT) and the dispersion relations for the amorphous semiconductors comprising a two-layer model consisting of a top surface roughness layer (dS) containing an effective medium mix of 50% a-Si:H and 50% voids and a single “bulk” layer (dB) of a-Si:H to simulate the data reasonably well. We performed these simulations by non-linear least-square regression analysis based on Marquardt–Levenberg algorithm and it was possible to estimate the true dielectric function of a-Si:H thin films and the energy band gap (Eg), besides film thickness (dSE), bulk void fraction, surface roughness layer (dS), and the confidence limits (χ2). Moreover, it is shown that the Tauc–Lorentz (TL) model fits the ellipsometry data much better than Forouhi and Bloomer (FB) and help elucidate the layered structure of a-Si:H thin films. We also compared the optical band gap (Eg) determined using ellipsometry modeling and the Tauc gap (ET) using conventional approach. We discuss the possible physical meaning and the variation of the deduced parameters in terms of role of TS (T-role) or of hydrogen (H-role) in the dispersion model abovementioned. The bandgap was found to decrease systematically with increasing TS, reconfirming the role of hydrogen as alloy in conjunction with network relaxation, which is in agreement with excitation dependent Raman spectroscopy (i.e. strongly versus weakly absorbing) results interpreted in terms of the variation of three “order parameters” relating the short- and medium-range order with respect to substrate temperature. This is since, previous Raman scattering studies in various hydrogenated amorphous silicon (a-Si:H) materials resulted in contradicting conclusions as to the role of hydrogen on the atomic-network order. Hereby, we elucidated these contradictions using the abovementioned optical probes by establishing that the surface and the bulk of HW a-Si:H films behave differently due to their different hydrogen concentrations. Micro-Raman spectroscopy (RS) and atomic force microscopy (AFM) were used to validate the simulations also. Further consequences of these findings are also discussed in terms of other optical and structural properties. These analyses led to a correlation between the films’ microstructure (or network disorder) and their electronic properties for several technological applications, in general, and for solar cells applications, in particular.

Sulfur-incorporated nanocrystalline carbon (n-C:S) thin films were grown on molybdenum substrates by hot-filament chemical vapor deposition technique using gas mixtures of methane, hydrogen and a range of hydrogen sulfide (H 2 S) concentrations (100–500 ppm with an interval of 100 ppm) at a fixed substrate temperature of 900 °C. They were optically characterized using ex situ spectroscopic phase modulated ellipsometry from near IR to near UV (1.5–5.0 eV) obtaining their pseudo-dielectric function as a function of [H 2 S]. The ellipsometry data (〈e r ( E )〉, 〈e i ( 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 3 C+sp 2 C+void and a bulk layer ( L 2 ), 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 bandgap ( E g ), film thickness ( d ) and an overlayer thickness ( L 1 ), which is equivalent to surface roughness layer (σ SE ) as a function of H 2 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 optical bandgap was found to decrease systematically with increasing H 2 S concentration, indicating an enhancement of π-bonded carbon (sp 2 C), in agreement with RS results. These results are compared to those obtained for films grown without sulfur (n-C), in order to demonstrate the influence of sulfur addition on film microstructure. These analyses led to a correlation between the film microstructure and its electronic properties.

We have investigated high sp2 content intrinsic and sulfur doped nanocrystalline diamond films to study field emission properties by electron emission microscopy operated in different modes. Electron emission microscopy enables real time imaging of the electron emission from a surface with a lateral resolution of ∼15 nm. The nanocrystalline intrinsic diamond films exhibit electron emission at room temperature from localized emission sites with weak temperature dependence, and a density of ∼103–104/cm2. In contrast, sulfur doped diamond films show similar emission characteristics at room temperature, but at elevated temperatures the emission significantly increases from the localized regions and a thermionic component is identified in the I/V dependence. We discuss the role of S-donor states to explain the enhanced emission of the S-doped nanocrystalline diamond.

Results are reported on the electron field emission properties of microcrystalline and nanocrystalline carbon thin films grown on molybdenum substrates by S-assisted hot-filament chemical vapor deposition technique using methane (CH 4 ), hydrogen sulfide (H 2 S), and hydrogen (H 2 ) gas mixtures. Electron field emission measurements revealed that the S-assisted thin films have substantially lower turn-on fields and steep rising currents, for both microcrystalline and nanocrystalline carbon, as compared to those grown without sulfur. In order to study the property–structure correlations, we characterized the films’ microstructure with SEM, AFM and Raman spectroscopy (RS) techniques. It was found that sulfur addition causes significant microstructural changes in both micro- and nanocrystalline thin films. S-assisted films show smoother and finer-grained surfaces than those grown without it, and a higher content of non-diamond carbon (sp 2 -bonded C). Besides, although most of the S is expected to be electrically inactive, under the high doping conditions hereby employed, there may be a significant amount in donor states. The results are discussed in terms of the dual role of S by enhancing the creation of defects and the availability of conduction electrons.

Nanocrystalline carbon thin films were grown by hot-filament chemical vapor deposition (HFCVD) using a relatively high concentration of methane in hydrogen. The films were deposited on molybdenum substrates at 900°C, and under various substrate-biasing conditions. The optical properties were examined ex situ using spectroscopic phase-modulated ellipsometry (SPME) from the near IR to the near UV region (1.5–5.0 eV). The ellipsometry data [ψ(λi), Δ(λi)] were modeled using Bruggeman effective-medium approximation (EMA) and the dispersion relation for the amorphous semiconductor (Forouhi and Bloomer Model; Phys. Rev. B 34, 7018, 1986). We performed these simulations by least-square regression analysis (LRA) and obtained the true dielectric function of our nanocrystalline carbon material and the energy band-gap (Eg), along with the film thickness, bulk void fraction and roughness layer. We discuss the possible physical meaning of the five parameters in the amorphous dispersion model applied to the case of nanocrystalline carbon. Micro-Raman spectroscopy and profilometry were used to guide and validate the simulations.

We employed in situ phase-modulated ellipsometry in the monitoring of surface damage to monocrystalline silicon (Si) under plasma conditions typical for the chemical vapor deposition of diamond. Single-wavelength kinetic and spectroscopic ellipsometry measurements were done and complemented with Raman spectroscopy, in order to characterize the surface conditions. It was found that heating the Si substrate to 700°C in the presence of molecular hydrogen produces etching of the native oxide layer. When the hot bare silicon surface is submitted to atomic hydrogen, it becomes rough in minutes. Modeling of the spectroscopic ellipsometry provided a quantitative physical picture of the surface damage, in terms of the roughness layer thickness and void fraction. The results indicate that by the time a thin film starts to grow on these silicon surfaces, like in the chemical vapor deposition of diamond, the roughness produced by the atomic hydrogen has already determined to a large extent the rough nature of the film to be grown.

Photoluminescence (PL) and Raman spectra of thermochemically reduced and untreated cubic calcia-stabilized zirconia (CaSZ) have been investigated. The effect of the reduction process results in a decrease in Raman activity in the acoustic mode region and a shift in the maximum of the excitation (EX) spectra. These variations can be related to a broad absorption band centered at ∼365 nm. The results point out the oxygen vacancies, their aggregates and associated complex defects as the main defect involved in the reduction process.

Raman spectroscopy is an excellent technique for characterization of the crystalline quality of diamond films and the amount of graphite and amorphous carbon impurities in them. Most publications on diamond include Raman spectra to show the quality of the films through a measurement of the bandwidth (FWHM) of the feature at ca. 1332.5 cm−1 characteristic of diamond. These measurements are often done directly on the spectra without correcting them for the instrumental contribution to the bandwidth. This can lead to meaningless comparisons of diamond bandwidth measurements across research groups. In order to show to what extent interpretations based on such raw measurements can be misled, we have characterized five CVD diamond films and a natural type IIa diamond using four different sets of instrumental parameters. We present a fitting procedure to obtain the Raman bandwidth corresponding to the material under study independent of the experimental parameters. This method permits absolute comparisons among Raman bandwidths obtained under different experimental conditions, providing they are corrected for the instrumental contribution.

The network disorder of amorphous Si 1 − x C x :H films, containing carbon concentrations below 25 at.%, has been studied by means of Raman spectroscopy. Two different radiations were employed to excite the Raman scattering of these materials, one that is strongly (458 nm) absorbed and another that is weakly (581 nm) absorbed. The variation in probed depth attained with these excitation radiations together with the significant differences observed in the Raman spectra excited with them indicate the existence of silicon network disorder inhomogeneities at the short-range (bond angle) and intermediate-range (dihedral angle) levels along the axis perpendicular to the films which are considerably larger than those existing in device-quality a-Si:H. Such inhomogeneities are observed to develop abruptly at the smallest carbon concentration. The findings explain the various previous conflicting reports in the literature regarding the behavior of the Si-network disorder in these alloys. When excited with strongly absorbed radiations (e.g., 514 and 488 nm), the Raman spectra do not show a definite trend as a function of carbon concentration, although it can still be concluded that there is less order than in a-Si:H. On the other hand, the Raman spectra excited with weakly absorbed radiation show that the order actually improves in the bulk of Si-rich a-SiC:H films. The above findings indicate that silicon is forming compact clusters in the interior of the films on this compositional regime. Besides, our results show that hydrogen dilution of the gas mixture during film growth helps improve the short-range order in the bulk of the films with the smallest carbon concentrations, but has no definite effect over the intermediate-range order or over the short-range order at the near surface. It is also found that the spectral intensity enhancement observed in these materials with increasing carbon content is due to a scattering volume effect.