D-band Intensity Research Articles

Increase in graphitization and electrical conductivity of glassy carbon nanowires by rapid thermal annealing

Glassy carbon (GC) exhibits numerous desirable properties such as high thermal and chemical stabilities, good hardness, and good thermal and electrical conductivities. Moreover, GC can be manufactured into micro-/nanostructures through the versatile microfabrication technology, or carbon-micro electromechanical systems, which includes polymer patterning and pyrolysis. However, despite these advantages, there are growing demands for enhancing the electrical conductivity of GC, so that it can compensate or be substituted for other carbon allotropes such as graphite, carbon nanotubes, and graphene. In this study, we demonstrated that simple rapid thermal annealing (RTA) can dramatically enhance the electrical conductivity of pyrolyzed GC nanostructures by ∼ 300%. In this research, two different architectures of 1D carbon nanostructures such as a suspended nanowire that was separated from the substrate with a fixed distance and a substrate-bound nanowire were fabricated using conventional UV-lithography and pyrolysis processes, and their conductivity enhancement behaviors via RTA were studied. After the RTA process, the carbon/oxygen content and G-/D-band intensity ratios, which are correlated to the electrical conductivity, were enhanced, depending on the pyrolysis temperature. GC structures pyrolyzed at relatively low temperatures became more electrically conductive after the RTA process owing to their relatively higher oxygen content. This is because carbon atoms interconnected to oxygen atoms tend to align more readily than those corresponding to other carbon compositions because of the graphene healing mechanism. In addition, the architecture of the carbon nanostructures (i.e., whether they were suspended or substrate-bound nanowires) influenced the RTA-induced increase in electrical conductivity; the former showed a greater increase in electrical conductivity owing to its larger portion of well-aligned carbon atoms at the surface compared to the latter carbon structure. This is because graphitization is initiated on the surface and then proceeds to the carbon core in the heat treatment. In addition, tensile stress generated only at the suspended carbon nanowires during the pyrolysis process is assumed to enhance further the electrical conductivity via RTA. For instance, the electrical conductivity of the suspended carbon nanowires formed by pyrolysis at 600 °C was enhanced to ∼59,000 S/m after the RTA process.

Catalyst nucleation and carbon nanotube growth from flame-synthesized Co-Al-O nanopowders at ten-second time scale

Flame-synthesized (CoO)x(Al2O3)1-x spinel nanopowders with primary particles of ∼20 nm were used to grow small diameter carbon nanotubes (CNTs). The nanopowders with x ≤ 0.35 grew few CNTs whereas that with x = 0.65 grew CNTs efficiently. Low crystalline and large-diameter multi-wall CNTs grew by annealing and chemical vapor deposition (CVD) at 800 °C for ∼10 min, whereas single-wall CNTs with high crystallinity (G-band to D-band intensity ratio of 20–100 by Raman spectroscopy) grew by annealing and CVD at ≥1000 °C for ∼10 s. The excess Co in the spinel reduced and segregated to form multiple Co nanoparticles on the surface of the single primary alumina nanoparticles in ∼10 s, yielding SWCNTs in ∼10 s. Such flame synthesized nanopowders, reduced and activated by H2, provide CNTs from C2H2, all in ten-second time scale, and as such are promising for practical, high-through-put production of small-diameter CNTs.

An insight into the effect of calcination conditions on catalytic cracking of toluene over 3Fe8Ni/palygorskite: Catalysts characterization and performance

In this paper, the effect of calcination conditions on catalytic cracking of toluene over 3Fe8Ni/PG (PG: palygorskite) was investigated. The experimental parameters including the calcination temperature, time and atmosphere and reaction temperature were evaluated. The catalysts were characterized using X-ray diffraction (XRD), H2 temperature-programmed reduction (TPR), transmission electron microcopy (TEM) and Raman spectroscopy. The gas composition was analyzed by a gas chromatograph (GC). The results showed that 3Fe8Ni/PG catalyst exhibited a higher catalytic reactivity for catalytic cracking of toluene than that of thermal cracking over quartz. In consideration of the thermal stability and catalytic reactivity of catalyst, a proper calcination temperature and time should be employed in the preparation of 3Fe8Ni/PG catalyst. The 3Fe8Ni/PG reduced in hydrogen at 700°C for 2h showed the highest catalytic reactivity as evidenced by the toluene conversion of 100% at the reaction temperature of 550°C. The catalytic reactivity of 3Fe8Ni/PG reduced in hydrogen for toluene conversion was superior to that of 3Fe8Ni/PG calcinated in air, 3Fe/PG, and 8Ni/PG, which revealed that the awaruite (NiFe) was the active component for the decomposition of toluene. Moreover, steam reforming of toluene over 3Fe8Ni/PG catalysts suggested that 80% toluene conversion can be achieved and most of toluene was converted into H2 and CO. Both amorphous carbon and graphitic carbon species were formed on 3Fe8Ni/PG catalysts after the catalytic reaction. The D/G ratio (the relative intensity of D-band to G-band ratio) was positively correlated to the catalytic reactivity of 3Fe8Ni/PG catalysts.

Chemical vapor deposition growth of large single-crystal bernal-stacked bilayer graphene from ethanol

Using ethanol as a precursor, single-crystal bilayer graphene domains with dimensions up to hundreds of micrometer, one of the largest reported so far, were synthesized on Cu foils by chemical vapor deposition (CVD). Raman spectroscopy analysis revealed that the bilayers are homogeneously AB-stacked, with very low D-band intensity. Selected area electron diffraction analysis also confirmed the bernal stacking order and the large area crystallinity. Surprisingly, decreasing ethanol pressure in CVD shows an unambiguous tread from self-limited single layer to a multi-layer graphene, and there exists a narrow window of ethanol pressure which is preferred for the formation of large domain bilayer graphene. This suggests the dual effect of ethanol in graphene growth and demonstrates that the formation of different layers of graphene can be controlled by carefully tuning single parameter: ethanol pressure.

Characterization of functionalized multiwalled carbon nanotubes and application as an effective filter for heavy metal removal from aqueous solutions

Filtration efficiency of Ni(II) from aqueous solution using pristine and modified MWCNTs filters was investigated as a function of Ni(II) ion concentration, pH, and filter mass. MWCNTs were synthesized by CVD method and modified using two complementary treatments, purification (using a mixture of hydrochloric acid and hydrogen peroxide) and functionalization (using nitric acid). The effect and mechanism of each treatment on the structural integrity of pristine MWCNTs has been studied. Morphology of the pristine and modified filters was investigated by Raman Spectrometry (RS), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), Fourier Transform Infrared (FTIR) spectrometry and Thermogravimetric analysis. It was found from Raman spectra that the ratio of the intensity of D-band to that of G-band decreased by purification process, and increased by functionalization process. The adsorption mechanism of Ni(II) onto the surface functional groups of modified MWCNTs was confirmed by FTIR spectrum. The filtration results showed that the removal efficiency of Ni(II) is strongly dependent on pH and could reach 85% at pH=8. Also, modified MWCNT filters can be reused through many cycles of regeneration with high performance. Functionalized MWCNTs filters may be a promising adsorbent candidate for heavy metal removal from wastewater.

Preparation of diamond reinforced metal powders as thermal spray feedstock using ball milling

Abstract This investigation envisages the preparation of diamond reinforced bronze and Mo powders as thermal spray feedstock via ball milling route. The metal powders with 10 wt.% micro-sized mono-crystalline diamond have been ball milled for up to 48 h. The powders have been characterised to get their crystal structure and stability of diamond using XRD and Raman spectroscopy. Substantial peak broadening of both powders has been observed, indicating a reduction in crystallite size. The bronze powder has turned flaky and shown cold welding of flakes. The Mo powder, on the other hand, exhibited brittle characteristics. Raman spectrum of the bronze-diamond powder indicates some changes in the sp 3 hybridised structure while the D-band has been protected. The Mo-diamond powder retained the sharp D-band up to 42 h of ball milling. After 48 h of milling, a substantial decrease in the intensity of D-band and a significant growth of G-band has been observed for this powder.

Fabrication of diamond-like carbon films using short-pulse HiPIMS

Abstract Diamond-like carbon (DLC) films were fabricated by sputtering a graphite target using a short-pulse-operated high-power impulse magnetron sputtering (HiPIMS) technology. The pulse duration was 7 μs with a maximum source voltage of − 2400 V, which efficiently facilitated the ionization of the sputtered carbon species. In order to show the effectiveness of the short pulse operation, the fabricated DLC films were investigated by Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry. XPS spectra were curve-fitted using three Gaussian functions of sp3-C, sp2-C and C–O bonding. The information from the Raman and XPS spectra showed that the sp3 fraction in the DLC films fabricated using a pulse duration of 7 μs was larger than that obtained using a pulse duration of 50 μs. It was found that the sp3/sp2 ratio was larger than 40%, which was approximately 1.5-fold compared to that obtained using a pulse duration of 50 μs. A high sp3 fraction was realized due to application of high voltage without arc generation transferred from a magnetron sputtering (MS) glow discharge. To obtain the high sp3 fraction in the fabricated DLC films, a parametric survey was performed, in which the background gas pressure, bias voltage applied to the substrate, power-source voltage and the distance between the target and the substrate were varied. There were optimum deposition parameters for lowering the D-band and G-band intensity ratio (ID/IG) in Raman spectra. The optimum source voltage was in the range of − 1200 to − 1600 V. For voltages higher than − 1600 V, energetic carbon and argon ions may deteriorate the film properties. The hardness of the fabricated DLC films at 7-μs-pulse-width and − 1200 V source-voltage were obtained as 37 GPa measured with a nano-indenter. The hardness decreased to a range from 20 to 27 GPa with increasing the pulse width in range from 40 to 60 μs.

Effects of Microwave Absorption on Long and Short Single-Walled Carbon Nanotubes at 10-6 Torr

Carbon nanotubes have been observed to emit ultraviolet, visible and infrared radiation when exposed to microwaves. We have performed experiments in which both short (0.5 μm–2 μm) and long (5 μm–30 μm) single and double-walled carbon nanotubes were exposed to 2.46 GHz microwaves at a pressure of ~ 10-6 Torr. Structural modifications of the carbon nanotubes due to microwave absorption have been studied using the Raman spectroscopy G-band and D-band intensities, which suggest that microwave irradiation at relatively low pressure results in an increase in nanotube defects, especially in the case of the long nanotubes. Furthermore, a comparison of the spectra of the radiation emitted from the nanotubes suggests that the longer nanotubes emitted radiation of much greater intensity than the shorter nanotubes. Based on the results of the experiments and results described in previous reports, the ultraviolet, visible and infrared radiation emitted as the result of microwave absorption by carbon nanotubes seems to be primarily blackbody radiation emitted due to Joule heating. However, the presence of several broad photopeaks in the spectra of the emitted radiation (which do not seem to be related to gases absorbed by the nanotubes or the presence of catalyst particles) suggest that emissions are not the result of Joule heating alone.

Microscopic And Spectroscopic Research Of The MWCNTs-Re Nanocomposites

Abstract The combination of TEM research and Raman spectroscopy to characterization of MWNTs-Re nanocomposites gives a new notion about the structure and quality of materials obtained. TEM studies indicate that the functionalization method significantly influences the morphology of obtained MWCNTs-Re nanocomposites. Due to the specific spectrum recorded for the MWCNTs they can be distinguished from other forms of carbon, furthermore comparative analysis of the results at different stages of the manufacturing process confirms the covalent modification of the MWCNTs structure. The D-band intensity compared to the G-band intensity provides valuable information about the quality of the sample, in particular indicates the existence of contamination and/or the presence of structural defects. Preliminary results suggest that the high-temperature manufacturing process of MWCNTs-Re nanocomposite improves the quality of the carbon material intended for the experiment.

Fabrication and characterization and biosensor application of gold nanoparticles on the carbon nanotubes

Gold nanoparticles (Au NPs) were synthesized by co-deposition of RF-sputtering and RF-PECVD from acetylene gas and Au target on the carbon nanotubes (CNTs). The CNTs were prepared by thermal chemical vapor deposition (TCVD) and Pd nanoparticles catalyst. TEM image shows that high-density and uniform distribution of Au NPs were grown on the CNTs. XRD analysis indicates that Au NPs have fcc crystal structure and CNTs have a good graphite structure. Raman spectroscopy results suggest that our sample includes double-walled CNTs. It is resulted that intensity of D-band reduces and G-band intensity raises and radial breathing mode (RBM) is changed by immobilizing of Au NPs on the CNTs. Raman results that the free electrons on the CNTs increase by immobilizing of Au NPs. Also localized surface plasmon resonance (LSPR) peak of Au NPs has red-shift and broadens when it is immobilized on the CNTs. The change in LSPR peak rises from decreasing of local electrons density on the Au NPs surface. Raman and LSPR spectroscopy suggest that there are charge transferring and chemical bonds between Au NPs and CNTs. We applied the prepared sensor chips of Au NPs@CNTs to detect DNA primer at femtomolar concentration based on the LSPR technique.

Role of humidity in reducing sliding friction of multilayered graphene

The sliding friction of multilayered graphene was examined by conducting pin-on-disk tests in ambient air with different relative humidity (RH) levels (10–45% RH) and in dry N2 (0% RH) where a high coefficient of friction (COF) of 0.52 was measured. Tests carried out at 10% RH showed a low steady state COF of 0.17. Progressively lower friction values were observed when the RH of the atmosphere was increased, with the lowest COF of 0.11 reached at 45% RH. Micro-Raman analyses of sliding surfaces of graphene transfer layers formed on the Ti–6Al–4V counterfaces showed an increase in the intensity of D-band with an increase in the RH suggesting defect formations in the C-network as a result of sp2 to sp3 transformation and amorphization during sliding. The H and OH passivation of graphene, as indicated by X-ray photoelectron spectra (XPS) of the contact surfaces, was a possible reason for low friction of graphene in humid atmospheres. Transfer layers incorporated stringers of graphene stacks with d-spacings of 0.34–0.38nm, which were larger than that of pristine graphite suggesting that repulsion between the adsorbed molecules caused an increase in the lattice spacing.

Disorder-induced double resonant Raman process in graphene

An analytical study is presented of the double resonant Raman scattering process in graphene, responsible for the D and D$^{\prime}$ features in the Raman spectra. This work yields analytical expressions for the D and D$^{\prime}$ integrated Raman intensities that explicitly show the dependencies on laser energy, defect concentration, and electronic lifetime. Good agreement is obtained between the analytical results and experimental measurements on samples with increasing defect concentrations and at various laser excitation energies. The use of Raman spectroscopy to identify the nature of defects is discussed. Comparison between the models for the edge-induced and the disorder-induced D band intensity suggests that edges or grain boundaries can be distinguished from disorder by the different dependence of their Raman intensity on laser excitation energy. Similarly, the type of disorder can potentially be identified not only by the intensity ratio $I_{\mathrm{D}}/I_{\mathrm{D}^{\prime}}$, but also by its laser energy dependence. Also discussed is a quantitative analysis of quantum interference effects of the graphene wavefunctions, which determine the most important phonon wavevectors and scattering processes responsible for the D and D$^{\prime}$ bands.

First principle calculations of hydrogen sulfide adsorption and dissociation on pure Pd (111) and Au (111), and alloy Pd/Au (111) and Au/Pd (111) surfaces

The hydrogen sulfide adsorption and dissociation on pure Pd (111) and Au (111), alloy Pd / Au (111) and Au / Pd (111) surfaces have been investigated using the pseudo-potential plane wave method within the generalized-gradient approximation density functional theory (GGA+DFT). The results show that H 2 S tends to be adsorbed on top site, HS prefers to locate on bridge site, and the S and H locate on fcc site on various surfaces. Compared the adsorption of sulfur-containing species and hydrogen on pure and alloy metal surfaces, a similar trend of adsorption energies on the metal surfaces ( Pd / Au (111) > Pd (111) > Au (111) > Au / Pd (111)) is found. In addition, the dissociation process on the Pd (111) and Pd / Au (111) surfaces is predicted to be exothermic. However, on Au (111) and Au / Pd (111), the dissociation process is endothermic. The work reveals that H 2 S dissociation is more likely to happen on Pd / Au (111) surface. Finally, the adsorption energies of adsorbate on metal surfaces have strong correlation with the d-band center. The d-band center moves away from the Fermi level, and the adsorption energy decreases. According to the LDOS analysis, the inner Au atoms of Pd / Au (111) can enhance the top-layer d-band intensity, whereas the inner Pd atoms of Au / Pd (111) cause the opposite effect. The further electronic state analysis reveals the interaction between H 2 S and metal surfaces.

Effect of Embedded Graphene on Al/VOx/Cu Resistive Switching Performance

In this report, we investigated effect of embedded graphene based on VOx-RRAM. Two structures of Al/VOx/Cu, Al/graphene/VOx/Cu (G-VOx) in the same sample were fabricated to compare the effect. Then a slice of graphene was transfered to the surface of VOx/Cu. Al electrode was deposited on VOx and graphene in the same sample to form the two structures. The graphene was grown on the Cu foil by plasma enhanced chemical vapor deposition (PECVD). Surface morphology of the graphene was observed using scanning electron microscope (SEM) and Atomic Force Microscopy (AFM). Raman spectroscopy revealed the single crystals graphene to be uniform monolayers with a low D-band intensity. It shows that method can obtain high quality graphene. Finally, we found that the uniformity of G-VOx RRAM device was improved with embedded graphene compared with VOx RRAM device.

Synthesis of Carbon Nanotube Heterojunctions from the Decomposition of Ethanol

A new bimetallic catalyst of nickel-copper doped with praseodymium was used in the decomposition of ethanol. Results obtained showed the formation of a high density of carbon nanotube heterojunctions with the nanotubes having diameters ranging from 10 to 40 nm. Field emission scanning electron microscopy and transmission electron microscopy showed that the carbon nanotubes had many junctions along the surface. Raman spectra revealed that the ethanol as the carbon precursors is proficient to produce different form of carbon nanomaterials as designated by the evolution of the G-band and D-band intensities. The carbon nanotubes obtained were characterized by thermogravimetric analysis which gave a higher thermal decomposition at 520°C than carbon nanotubes without junctions previously synthesized (500°C). The percentage of weight lost is about 69%, which showed that the fairly high abundance of high purity carbon nanotube heterojunctions was synthesized.

Electrical conductivity enhancement of metallic single-walled carbon nanotube networks by CoO decoration

We report that the decoration of metallic single-walled carbon nanotube (m-SWCNT) networks with cobalt(ii) oxide (CoO) can improve the electrical conductivity of the networks. To measure the electrical conductivity, we prepared m-SWCNT networks between the source and drain electrodes of field-effect transistors (FETs). Then, the amount of CoO nanoparticles (NPs) used for decoration was controlled by treating the FETs with different volumes of a solution containing Co(NO3)2·6H2O. Atomic force microscopy imaging showed that CoO NPs were intensively deposited on the intertubular junction of the m-SWCNT networks. X-ray photoelectron spectroscopy confirmed that the oxidation state of the Co element on m-SWCNT was CoO. Raman spectra revealed that heavy decoration of CoO increased the D-band intensity of the m-SWCNT, indicating that the CoO NPs disordered the sp(2) hybridized carbon atoms of the m-SWCNT via decoration. The electrical conductivity of the m-SWCNT networks was enhanced up to 28 times after decoration, and this was attributed to the CoO NPs connecting the m-SWCNTs at junctions of the networks.

Raman spectroscopic characterization of single walled carbon nanotubes: influence of the sample aggregation state

Raman spectroscopy has been employed in analytical sciences for purity determination of carbon nanotube samples based on the consideration of G-/D-band intensity ratios. This work demonstrates the role of aggregation in these feature bands, which, in the case of single walled carbon nanotubes (SWNTs), has proved to be crucial for G-/D-band intensity ratio measurements. We have found variation in the relative intensities of G- and D-bands across a sample of SWNTs without any other treatment, discarding the possible influence of the laser beam or sample focusing. In the case of multiwalled carbon nanotubes (MWNTs), this effect is less notorious. Thus, to achieve a good representativeness of Raman measurements, it is important to consider the sample preparation procedure in order to avoid aggregation, which has an effect over the signals, making difficult the subsequent interpretation of results.

Structure and transport properties of the interface between CVD-grown graphene domains.

During the chemical vapor deposition (CVD) growth of graphene, graphene domains grown on a Cu surface merge together and form a uniform graphene sheet. For high-performance electronics and other applications, it is important to understand the interfacial structure of the merged domains, as well as their influence on the physical properties of graphene. We synthesized large hexagonal graphene domains with controlled orientations on a heteroepitaxial Cu film and studied the structure and properties of the interfaces between the domains mainly merged with the same angle. Although the merged domains have various interfaces with/without wrinkles and/or increased defect-related Raman D-band intensity, the intra-domain transport showed higher carrier mobility reaching 20,000 cm(2) V(-1) s(-1) on SiO2 at 280 K (the mean value was 7200 cm(2) V(-1) s(-1)) than that measured for inter-domain areas, 6400 cm(2) V(-1) s(-1) (mean value 2000 cm(2) V(-1) s(-1)). The temperature dependence of the mobility suggests that impurity scattering dominates at the interface even for the merged domains with the same orientation. This study highlights the importance of domain interfaces, especially on the carrier transport properties, in CVD-grown graphene.

The Dependence of Graphene Raman D-band on Carrier Density

Raman spectroscopy has been an integral part of graphene research and can provide information about graphene structure, electronic characteristics, and electron-phonon interactions. In this study, the characteristics of the graphene Raman D-band, which vary with carrier density, are studied in detail, including the frequency, full width half-maximum, and intensity. We find the Raman D-band frequency increases for hole doping and decreases for electron doping. The Raman D-band intensity increases when the Fermi level approaches half of the excitation energy and is higher in the case of electron doping than that of hole doping. These variations can be explained by electron-phonon interaction theory and quantum interference between different Raman pathways in graphene. The intensity ratio of Raman D- and G-band, which is important for defects characterization in graphene, shows a strong dependence on carrier density.

Effect of the Structure and Morphology of Natural, Synthetic and Post-processed Graphites on Their Dispersibility and Electronic Properties

Twenty-two different graphite grades of the families natural, synthetic, intercalated, expanded, amorphous, and turbostratic graphite, as well as platelets and activated carbon, were analyzed on their dispersibility in N-methylpyrrolidone, cyclohexylpyrrolidone and water-sodium cholate solution. They were characterized for their properties (grain size, density, purity, surface area, pH in water) and morphology. Thermogravimetric analysis, Raman spectroscopy studies, and electrical conductivity measurements were also used. XRPD followed by a Rietveld refinement provided information on the amounts of the rhombohedral (3R) and hexagonal (2H) phases for the crystalline part of the material and the interlayer distances. The properties of graphites favoring better dispersibility are: small grain size and bulk density, neutral pH in water. The electrical conductivity in inversely proportional to the Raman D-band intensity and is highest for the graphites with shortest interlayer spacing. The D-, G- and 2D-bands in the Raman spectra of graphites are in an exponential relation.