Purification/annealing of graphene with 100-MeV Ag ion irradiation

Studies on interaction of graphene with radiation are important because of nanolithographic processes in graphene-based electronic devices and for space applications. Since the electronic properties of graphene are highly sensitive to the defects and number of layers in graphene sample, it is desirable to develop tools to engineer these two parameters. We report swift heavy ion (SHI) irradiation-induced annealing and purification effects in graphene films, similar to that observed in our studies on fullerenes and carbon nanotubes (CNTs). Raman studies after irradiation with 100-MeV Ag ions (fluences from 3 × 10¹⁰ to 1 × 10¹⁴ ions/cm²) show that the disorder parameter α, defined by I_D/I_G ratio, decreases at lower fluences but increases at higher fluences beyond 1 × 10¹² ions/cm². This indicates that SHI induces annealing effects at lower fluences. We also observe that the number of graphene layers is reduced at fluences higher than 1 × 10¹³ ions/cm². Using inelastic thermal spike model calculations, we estimate a radius of 2.6 nm for ion track core surrounded by a halo extending up to 11.6 nm. The transient temperature above the melting point in the track core results in damage, whereas lower temperature in the track halo is responsible for annealing. The results suggest that SHI irradiation fluence may be used as one of the tools for defect annealing and manipulation of the number of graphene layers.

PACS

60.80.x; 81.05.ue     

Synthesis of zinc oxide nanostructures on graphene/glass substrate by electrochemical deposition: effects of current density and temperature

The electrochemical growth of zinc oxide (ZnO) nanostructures on graphene on glass using zinc nitrate hexahydrate was studied. The effects of current densities and temperatures on the morphological, structural, and optical properties of the ZnO structures were studied. Vertically aligned nanorods were obtained at a low temperature of 75°C, and the diameters increased with current density. Growth temperature seems to have a strong effect in generating well-defined hexagonal-shape nanorods with a smooth top edge surface. A film-like structure was observed for high current densities above -1.0 mA/cm² and temperatures above 80°C due to the coalescence between the neighboring nanorods with large diameter. The nanorods grown at a temperature of 75°C with a low current density of -0.1 mA/cm² exhibited the highest density of 1.45 × 10⁹ cm^-2. X-ray diffraction measurements revealed that the grown ZnO crystallites were highly oriented along the c-axis. The intensity ratio of the ultraviolet (UV) region emission to the visible region emission, I_UV/I_VIS, showed a decrement with the current densities for all grown samples. The samples grown at the current density below -0.5 mA/cm² showed high I_UV/I_VIS values closer to or higher than 1.0, suggesting their fewer structural defects. For all the ZnO/graphene structures, the high transmittance up to 65% was obtained at the light wavelength of 550 nm. Structural and optical properties of the grown ZnO structures seem to be effectively controlled by the current density rather than the growth temperature. ZnO nanorod/graphene hybrid structure on glass is expected to be a promising structure for solar cell which is a conceivable candidate to address the global need for an inexpensive alternative energy source.     

The structural and electrical evolution of chemical vapor deposition grown graphene by electron beam irradiation induced disorder

The defect formation mechanism in chemical vapor deposition grown single layer graphene devices has been investigated by increasing electron beam (e-beam) irradiation doses gradually up to 750 e⁻/nm². The evolution of D peaks in Raman spectra provides an evidence of strong lattice disorder due to e-beam irradiation. Particularly, the trajectory of D and G peak intensities ratio (I_D/I_G) suggests that the transformation of graphene from crystalline to the nanocrystalline and then towards amorphous form with increasing irradiation dose. The defect parameters were calculated by phenomenological model of amorphization trajectory for graphitic materials. The mobility decreasing gradually from ∼1200 to ∼80 cm²/V s with gradual increase of irradiation dose, which implies the formation of localized states in e-beam irradiated graphene. The Dirac point is shifted towards negative gate voltage which indicates the n-doping in graphene with increasing e-beam irradiation dose.     

A preparation approach of exploring cluster ion implantation: from ultra-thin carbon film to graphene

Based on the extensive application of 2 × 1.7MV Tandetron accelerator, a low-energy cluster chamber has been built to explore for synthesizing graphene. Raman spectrum and atomic force microscopy (AFM) show that an amorphous carbon film in nanometer was deposited on the silicon by C₄ cluster implantation. And we replaced the substrate with Ni/SiO₂/Si and measured the thickness of Ni film by Rutherford backscattering spectrometry (RBS). Combined with suitable anneal conditions, these samples implanted by various small carbon clusters were made to grow graphene. Results from Raman spectrum reveal that few-layer graphene were obtained and discuss whether I_G/I_2D can contribute to explain the relationship between the number of graphene layers and cluster implantation dosage.     

Nanotwinning and structural phase transition in CdS quantum dots

Nanotwin structures are observed in high-resolution transmission electron microscopy studies of cubic phase CdS quantum dots in powder form by chemical co-precipitation method. The deposition of thin films of nanocrystalline CdS is carried out on silicon, glass, and TEM grids keeping the substrates at room temperature (RT) and 200°C by pulsed laser ablation. These films are then subjected to thermal annealing at different temperatures. Glancing angle X-ray diffraction results confirm structural phase transitions after thermal annealing of films deposited at RT and 200°C. The variation of average particle size and ratio of intensities in Raman peaks I_2LO/I_1LO with annealing temperature are studied. It is found that electron-phonon interaction is a function of temperature and particle size and is independent of the structure. Besides Raman modes LO, 2LO and 3LO of CdS at approximately 302, 603, and 903 cm⁻¹ respectively, two extra Raman modes at approximately 390 and 690 cm⁻¹ are studied for the first time. The green and orange emissions observed in photoluminescence are correlated with phase transition.     

Highly electrically and thermally conductive silicon carbide-graphene composites with yttria and scandia additives

Dense silicon carbide/graphene nanoplatelets (GNPs) and silicon carbide/graphene oxide (GO) composites with 1 vol.% equimolar Y₂O₃–Sc₂O₃ sintering additives were sintered at 2000 °C in nitrogen atmosphere by rapid hot-pressing technique. The sintered composites were further annealed in gas pressure sintering (GPS) furnace at 1800 °C for 6 h in overpressure of nitrogen (3 MPa). The effects of types and amount of graphene, orientation of graphene sheets, as well as the influence of annealing on microstructure and functional properties of prepared composites were investigated. SiC-graphene composite materials exhibit anisotropic electrical as well as thermal conductivity due to the alignment of graphene platelets as a consequence of applied high uniaxial pressure (50 MPa) during sintering. The electrical conductivity of annealed sample with 10 wt.% of GNPs oriented parallel to the measuring direction increased significantly up to 118 S·cm⁻¹. Similarly, the thermal conductivity of composites was very sensitive to the orientation of GNPs. In direction perpendicular to the GNPs the thermal conductivity decreased with increasing amount of graphene from 180 W·m⁻¹ K⁻¹ to 70 W·m⁻¹ K⁻¹, mainly due to the scattering of phonons on the graphene – SiC interface. In parallel direction to GNPs the thermal conductivity varied from 130 W·m⁻¹ K⁻¹ up to 238 W·m⁻¹ K⁻¹ for composites with 1 wt.% of GO and 5 wt.% of GNPs after annealing. In this case both the microstructure and composition of SiC matrix and the good thermal conductivity of GNPs improved the thermal conductivity of composites.     

Electrical characteristic fluctuation of 16-nm-gate high-κ/metal gate bulk FinFET devices in the presence of random interface traps

In this work, we study the impact of random interface traps (RITs) at the interface of SiO _x /Si on the electrical characteristic of 16-nm-gate high-κ/metal gate (HKMG) bulk fin-type field effect transistor (FinFET) devices. Under the same threshold voltage, the effects of RIT position and number on the degradation of electrical characteristics are clarified with respect to different levels of RIT density of state (D_it). The variability of the off-state current (I_off) and drain-induced barrier lowering (DIBL) will be severely affected by RITs with high D_it varying from 5 × 10¹² to 5 × 10¹³ eV⁻¹ cm⁻² owing to significant threshold voltage (V_th) fluctuation. The results of this study indicate that if the level of D_it is lower than 1 × 10¹² eV⁻¹ cm⁻², the normalized variability of the on-state current, I_off, V_th, DIBL, and subthreshold swing is within 5%.     

Raman spectra of virgin and damaged graphite edge planes

First-order Raman spectra of virgin and ion irradiated highly oriented pyrolytic graphite were performed along the basal and edge planes. Ion irradiation was performed in the low fluence regime (≈ 10¹²–10¹³ ions cm⁻²) using a 400 keV Ar⁺ beam in order to a introduce controlled amount of defects in the structure. Virgin edge Raman spectra reveal the presence of a 1351 cm⁻¹ structure (D^*) shifted 10 cm⁻¹ away from the disorder induced D-line found upon irradiating or grinding the graphite surface.The 1351 cm⁻¹ D^* structure is then considered as the feature coming from the rupture of the D_6h⁴ space group symmetry. A comparison between D^* and the classical 1360 cm⁻¹ D-line is discussed in terms of induced disorder.     

Fabrication of low-density GaN/AlN quantum dots via GaN thermal decomposition in MOCVD

With an appropriate high anneal temperature under H₂ atmosphere, GaN quantum dots (QDs) have been fabricated via GaN thermal decomposition in metal organic chemical vapor deposition (MOCVD). Based on the characterization of atomic force microscopy (AFM), the obtained GaN QDs show good size distribution and have a low density of 2.4 × 10⁸ cm^-2. X-ray photoelectron spectroscopy (XPS) analysis demonstrates that the GaN QDs were formed without Ga droplets by thermal decomposition of GaN.     

Study of Phosphorus Doped Micro/Nano Crystalline Silicon Films Deposited by Filtered Cathodic Vacuum Arc Technique

Phosphorus doped micro/nano crystalline silicon thin films have been deposited by the filtered cathodic vacuum arc technique at different substrate temperatures (T_s) ranging from room temperature (RT) to 350 ^°C. The films have been characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy, secondary ion mass spectroscopy, dark conductivity ( σ _D), activation energy ( ΔE) and optical band gap (E _g). The XRD patterns show that the RT grown film is amorphous in nature but high T_s (225 and 350 ^°C) deposited films have a crystalline structure with (111) and (220) crystal orientation. The crystallite size of the higher T_s grown silicon films evaluated was between 17 to 31 nm. Raman spectra reveal the amorphous nature of the film deposited at RT whereas higher T_s deposited films show a higher crystalline nature. The crystalline volume fraction of the silicon film deposited at higher T_s was estimated as 65.7 % and 74.4 %. The values of σ _D, ΔE and E _g of the silicon films deposited at different T_s were found to be in the range of 8.84 x 10 ^?4? 0.98 ohm ^?1 cm ^?1, 0.06 - 0.31 eV and 1.31-1.93 eV, respectively. A n-type nc-Si/p-type c-Si heterojunction diode was fabricated which showed the diode ideality factor between 1.1 to 1.5.     

Highly organised and dense vertical silicon nanowire arrays grown in porous alumina template on <100> silicon wafers

In this work, nanoimprint lithography combined with standard anodization etching is used to make perfectly organised triangular arrays of vertical cylindrical alumina nanopores onto standard <100>−oriented silicon wafers. Both the pore diameter and the period of alumina porous array are well controlled and can be tuned: the periods vary from 80 to 460 nm, and the diameters vary from 15 nm to any required diameter. These porous thin layers are then successfully used as templates for the guided epitaxial growth of organised mono-crystalline silicon nanowire arrays in a chemical vapour deposition chamber. We report the densities of silicon nanowires up to 9 × 10⁹ cm⁻² organised in highly regular arrays with excellent diameter distribution. All process steps are demonstrated on surfaces up to 2 × 2 cm². Specific emphasis was made to select techniques compatible with microelectronic fabrication standards, adaptable to large surface samples and with a reasonable cost. Achievements made in the quality of the porous alumina array, therefore on the silicon nanowire array, widen the number of potential applications for this technology, such as optical detectors or biological sensors.     

The formation mechanisms of multi-wall carbon nanotubes over the Ni modified MCM-41 catalysts

Multi-wall carbon nanotubes (MWCNTs) were grown by thermal chemical vapor deposition (thermal CVD) of CH₄ by using Ni-MCM-41 as the catalyst. Methane pyrolysis has been performed in a quartz tube reactor over the catalyst surface to form carbon atoms via dehydrogenation process. The migration and rearrangement of the surface carbon atoms result in the formation of MWCNTs. Transmission electron microscope (TEM) and scanning electron microscope (SEM) were used to determine the morphologies and structures of CNTs, and Raman spectroscopy was exploited to analyze their purity with the relative intensity between the D-band (Disorder band) in the vicinity of 1,350 cm⁻¹ which is characteristic of the sp³ structure and G-band (Graphitic band) in vicinity of 1,580 cm⁻¹ which is characteristic of the sp² structure. In addition, the controlling factors of methane pyrolysis such as the catalyst composition; the reaction temperature, and the methane flow rate on the formation of MWCNTs were investigated to optimize the structure and yield of MWCNTs. SEM/TEM results indicate that the yield of the CNTs increases with increasing Ni concentration in the catalyst. The optimized reaction temperature to grow CNT is located between 640 and 670 °C. The uniform and narrow diameter MWCNTs form at lower flow rate of methane (∼30 sccm), and non-uniform in diameter and disorder structure of MWCNTs are observed at higher flow rate of methane. This is consistent with Raman analysis that the relative intensity of I _D/I _G increases with increasing methane flow rate. The formation mechanisms of the MWCNTs on the Ni-MCM-41 catalyst have been determined to be a Tip-Growth mode with a nanoscale catalyst particle capsulated in the tip of the CNT.     

Zinc removal from dilute solutions using a rotating cylinder electrode reactor

Mine residue recycling processes produce dilute zinc solutions suitable for metal recovery. The rotating cylinder electrode reactor behaviour sequentially followed charge transfer and diffusion control mechanisms, even with solutions contaminated with metals or organic substances. Zinc removal at low pH (0) and low concentration (2 mg dm^–3) is demonstrated. Under galvanostatic operation, the zinc deposition current efficiency in the charge transfer control region attains values up to 77.3%, whereas in the diffusion control region it decreases rapidly to values as low as 0.1%. When a potentio-static mode is used, less energy is required to deposit zinc, even at low current efficiency. The results and possible problems for continuous reactor operation under conditions of powder formation are identified and discussed using knowledge from other zinc industries such as electrowinning, plating and batteries.List of symbols A _c cylinder electrode active surface (cm²) - A _d disc electrode active surface (cm²) - c _H analytical sulfuric acid concentration (mol cm^–3) - c _Zn analytical zinc sulphate concentration (mol cm^–3) - d cylinder electrode diameter (cm) - D zinc diffusion coefficient (cm² s^–1) - F Faraday constant (96 500 C mol^–1) - I total current (A) - I _H hydrogen production current (A) - I ₁ zinc deposition limiting current (A) - j critical hydrogen current density (A cm^–2) - k zinc mass transfer coefficient (cm s^–1) - K Wark's rule constant - n number of electrons exchanged in the zinc deposition reaction - Re Reynolds number (d ²/2) - Sc Schmidt number (/D) - Sh Sherwood number (kd/D) - t time (s) - V electrolyte volume in the RCER (cm³) - solution kinematic viscosity (cm² s^–1) - zinc deposition current efficiency - rotation speed (rad s^–1)     

The situ preparation of silica nanoparticles on the surface of functionalized graphene nanoplatelets

A method for situ preparing a hybrid material consisting of silica nanoparticles (SiO₂) attached onto the surface of functionalized graphene nanoplatelets (f-GNPs) is proposed. Firstly, polyacrylic acid (PAA) was grafted to the surface of f-GNPs to increase reacting sites, and then 3-aminopropyltriethoxysilane (APTES) KH550 reacted with abovementioned product PAA-GNPs to obtain siloxane-GNPs, thus providing reaction sites for the growth of SiO₂ on the surface of GNPs. Finally, the SiO₂/graphene nanoplatelets (SiO₂/GNPs) hybrid material is obtained through introducing siloxane-GNPs into a solution of tetraethyl orthosilicate, ammonia and ethanol for hours'' reaction. The results from Fourier transform infrared spectroscopy (FTIR) showed that SiO₂ particles have situ grown on the surface of GNPs through chemical bonds as Si-O-Si. And the nanostructure of hybrid materials was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). All the images indicated that SiO₂ particles with similar sizes were grafted on the surface of graphene nanoplatelets successfully. And TEM images also showed the whole growth process of SiO₂ particles on the surface of graphene as time grows. Moreover, TGA traces suggested the SiO₂/GNPs hybrid material had stable thermal stability. And at 900°C, the residual weight fraction of polymer on siloxane-GNPs was about 94.2% and that of SiO₂ particles on hybrid materials was about 75.0%. However, the result of Raman spectroscopy showed that carbon atoms of graphene nanoplatelets became much more disordered, due to the destroyed carbon domains during the process of chemical drafting. Through orthogonal experiments, hybrid materials with various sizes of SiO₂ particles were prepared, thus achieving the particle sizes controllable. And the factors’ level of significance is as follows: the quantity of ammonia > the quantity of tetraethyl orthosilicate (TEOS) > the reaction time.     

Synthesis of multi-walled carbon nanotube in a gas-solid fluidized bed

Multi-walled carbon nano-tubes (MWCNTs) were produced by acetylene decomposition on Fe-catalyst in a fluidized bed reactor (0.056 m-IDx1.0 m-high) with a sintered metal distributor (40 μm pore size). The Fe-catalysts were tested in decomposition of the different ratios of acetylene, hydrogen and nitrogen at the temperature range of 823–973 K. The physical properties of the carbon nano-tubes were determined by HR-TEM, SEM and Raman spectroscopy. The multi-walled carbon nano-tubes produced from the fluidized bed reactor are sub-aggregates and entangled with each other. The synthesized MWCNTs have outer diameters of a few tens of nanometers at 823–973 K. It has been found that the synthesized CNT agglomerates are in good condition with less amorphous carbon with the reaction time of 30 to 60 minutes from the analyses of Raman Spectra, SEM and TEM, The ratio (I _D /I _G ) of amorphous carbon (I _D = 1,295 cm^-1) and crystalline carbon (I _G =1,590 cm^-1) range from 1.15 to 1.49.     

Quasi-free-standing monolayer and bilayer graphene growth on homoepitaxial on-axis 4H-SiC(0 0 0 1) layers

Quasi-free-standing monolayer and bilayer graphene is grown on homoepitaxial layers of 4H-SiC. The SiC epilayers themselves are grown on the Si-face of nominally on-axis semi-insulating substrates using a conventional SiC hot-wall chemical vapor deposition reactor. The epilayers were confirmed to consist entirely of the 4H polytype by low temperature photoluminescence. The doping of the SiC epilayers may be modified allowing for graphene to be grown on a conducing substrate. Graphene growth was performed via thermal decomposition of the surface of the SiC epilayers under Si background pressure in order to achieve control on thickness uniformity over large area. Monolayer and bilayer samples were prepared through the conversion of a carbon buffer layer and monolayer graphene respectively using hydrogen intercalation process. Micro-Raman and reflectance mappings confirmed predominantly quasi-free-standing monolayer and bilayer graphene on samples grown under optimized growth conditions. Measurements of the Hall properties of Van der Pauw structures fabricated on these layers show high charge carrier mobility (>2000 cm²/Vs) and low carrier density (<0.9 × 10¹³ cm⁻²) in quasi-free-standing bilayer samples relative to monolayer samples. Also, bilayers on homoepitaxial layers are found to be superior in quality compared to bilayers grown directly on SI substrates.     

Electrochemical behavior of multifunctional graphene–polyimide nanocomposite film in two different electrolyte solutions

The effect of solvent on specific capacitance, bulk resistance, and charge/discharge capacity of graphene/polyimide composite films is studied by electrochemical methods. Composite films are synthesized by in situ condensation polymerization of poly (amic acid) in the presence of 50 wt % partly exfoliated graphene sheets followed by thermal curing at 250°C. Raman spectrum of the exfoliated graphene sheets show an increase in the ratio of I_D to I_G peak intensities from 0.167 to 0.222, suggesting increased defects in graphene basal planes. Electrochemical measurements carried out by using 0.4M potassium hexafluorophosphate (KPF₆) dissolved in propylene carbonate and N‐methylpyrrolidone at 25°C show that the composite system exhibits both pseudocapacitance and supercapacitance behaviors, with an average capacitance of 40 and 36.5 F g^?1, respectively. Bulk resistance of the composite obtained by using KPF₆–propylene carbonate electrolyte solution is 300% lower than that obtained in KPF₆–N‐methylpyrrolidone solution, with a fairly stable specific capacity of 85 μAhr g^?1, with 80% retention observed after 30 charge–discharge cycles. Fourier transform infrared spectroscopy measurements show shifts in the cyclic imide carbonyl peak from 1778 to 1774 cm^?1, which suggests that some form of interaction exists between the graphene and polyimide. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42673.     

Investigation of hydrogen plasma treatment for reducing defects in silicon quantum dot superlattice structure with amorphous silicon carbide matrix

We investigate the effects of hydrogen plasma treatment (HPT) on the properties of silicon quantum dot superlattice films. Hydrogen introduced in the films efficiently passivates silicon and carbon dangling bonds at a treatment temperature of approximately 400°C. The total dangling bond density decreases from 1.1 × 10¹⁹ cm^-3 to 3.7 × 10¹⁷ cm^-3, which is comparable to the defect density of typical hydrogenated amorphous silicon carbide films. A damaged layer is found to form on the surface by HPT; this layer can be easily removed by reactive ion etching.     

Effect of drag-reducing additives on the rate of mass transfer in a parallel-plate electrochemical flow reactor

The effect of polyox and CMC drag-reducing polymers on the rate of mass transfer in a parallel-plate flow cell was studied by measuring the limiting current for the cathodic reduction of potassium ferricyanide in alkaline medium. Reynolds number and polymer concentration were varied over the range 3500–21 000 and 10–200 ppm respectively. Under these conditions it was found that polyox and CMC reduce the rate of mass transfer by a maximum of 42% and 35% respectively.Nomenclature a a constant - C concentration of ferricyanide ion (g mol cm^–3) - D diffusivity of ferricyanide ion (cm²s^–1) - d _e equivalent diameter of the cell (4 x cross-sectional area/wetted perimeter) - F Faraday's constant (96 487 C mol^–1) - I limiting current density (A cm^–2) - K mass transfer coefficient (cm s^–1) - L electrode height (cm) - (Re) Reynolds number (d _e /u) - (Sc) Schmidt number (u/D) - (Sh) Sherwood number (Kd _e/D) - u solution viscosity (poise) - flow rate of the solution (cm s^–1) - Z number of electrons involved in the reaction - solution density (g cm^–3)     

Axial dispersion in flow porous electrodes

The coefficient of axial dispersionD _L in a porous electrode, composed of rolled 80-mesh platinum screen, was determined using the process of the flow electrolysis of 2.0×10^–3 M K₃Fe(CN)₆ in 1 MKCl in water. The results were analysed in the light of an earlier model for flow electrodes.List of symbols a Electrode cross-sectional area (cm²) - b Empirical constant - c ₀ Initial concentration of substrate (mol ml^–1) - D _L Axial dispersion coefficient (cm² s^–1) - D ^* Effective dispersion coefficient (cm² s^–1) - F Faraday constant (C mol^–1) - I ₁ Limiting current (A) - L Electrode height (cm) - R Limiting degree of conversion of substance - v Volume flow rate (ml s^–1) - Empirical constant - Electrode porosity