Effect of prior C,Si and Sn implantation on the etch rate of CVD diamond

Diamond films were implanted with C⁺, Si⁺ or Sn⁺ ions at multiple energies in order to generate a uniform layer of implantation-induced disorder. The implant energies of 60, 180, 330 and 525 keV for C⁺ ions, 200, 500 and 950 keV for Si⁺ ions and 750 and 2000 keV for Sn⁺ ions were selected to give an approximately constant vacancy concentration at depths over the range ∼ 0–0.5 μm. An analysis of the C⁺ implanted surfaces by Raman spectroscopy has shown an increase in non-diamond or sp²-bonded carbon at doses in the range 5 × 10¹³ to 5 × 10¹⁵ cm^− 2. In comparison, a completely non-diamond structure was evident after implantation with either Si⁺ ions at a dose of 5 × 10¹⁵ ions/cm² or Sn⁺ ions at ≥ 5 × 10¹⁴ cm^− 2. For a given dose, the etch rate of the diamond film was shown to increase with the mass of the implanted species in the order of C⁺, Si⁺ and Sn⁺. For a given implant species, the etch rate increased with the implant dose and the ion-induced vacancy concentration. The etch rate of the implanted diamond in various gases decreased in the order of O₂, CF₄/O₂ and CHF₃/O₂ plasmas.     

Fabrication of highly efficient TiO2/Ag/TiO2 multilayer transparent conducting electrode with N ion implantation for optoelectronic applications

Fabrication of highly conductive and transparent TiO₂/Ag/TiO₂ (referred hereafter as TAT) multilayer films with nitrogen implantation is reported. In the present work, TAT films were fabricated with a total thickness of 100 nm by sputtering on glass substrates at room temperature. The as-deposited films were implanted with 40 keV N ions for different fluences (1×10¹⁴, 5×10¹⁴, 1×10¹⁵, 5×10¹⁵ and 1×10¹⁶ ions/cm²). The objective of this study was to investigate the effect of N⁺ implantation on the optical and electrical properties of TAT multilayer films. X-ray diffraction of TAT films shows an amorphous TiO₂ film with a crystalline peak assigned to Ag (111) diffraction plane. The surface morphology studied by atomic force microscopy (AFM) and field emission scanning electron microscope (FESEM) revealed smooth and uniform top layer of the sandwich structure. The surface roughness of pristine film was 1.7 nm which increases to 2.34 nm on implantation for 1×10¹⁴ ions/cm² fluence. Beyond this fluence, the roughness decreases. The oxide/metal/oxide structure exhibits an average transmittance ~80% for pristine and ~70% for the implanted film at fluence of 1×10¹⁶ ions/cm² in the visible region. The electrical resistivity of the pristine sample was obtained as 2.04×10⁻⁴ Ω cm which is minimized to 9.62×10⁻⁵ Ω cm at highest fluence. Sheet resistance of TAT films decreased from 20.4 to 9.62 Ω/□ with an increase in fluence. Electrical and optical parameters such as carrier concentration, carrier mobility, absorption coefficient, band gap, refractive index and extinction coefficient have been calculated for the pristine and implanted films to assess the performance of films. The TAT multilayer film with fluence of 1×10¹⁶ ions/cm² showed maximum Haacke figure of merit (FOM) of 5.7×10⁻³ Ω⁻¹. X-ray photoelectron spectroscopy (XPS) analysis of N 1s and Ti 2p spectra revealed that substitutional implantation of nitrogen into the TiO₂ lattice added new electronic states just above the valence band which is responsible for the narrowing of band gap resulting in the enhancement in electrical conductivity. This study reports that fabrication of multilayer transparent conducting electrode with nitrogen implantation that exhibits superior electrical and optical properties and hence can be an alternative to indium tin oxide (ITO) for futuristic TCE applications in optoelectronic devices.     

Interaction between graphene and copper substrate: The role of lattice orientation

We present a comprehensive study of graphene grown by chemical vapor deposition on copper single crystals with exposed (1 0 0), (1 1 0) and (1 1 1) faces. Direct examination of the as-grown graphene by Raman spectroscopy using a range of visible excitation energies and microRaman mapping shows distinct strain and doping levels for individual Cu surfaces. Comparison of results from Raman mapping with X-ray diffraction techniques and atomic force microscopy shows it is neither the crystal quality nor the surface topography responsible for the specific strain and doping values, but it is the Cu lattice orientation itself. We also report an exceptionally narrow Raman 2D band width caused by the interaction between graphene and metallic substrate. The appearance of this extremely narrow 2D band with full-width-at-half maximum (FWHM) as low as 16 cm⁻¹ is correlated with flat and undoped regions on the Cu(1 0 0) and (1 1 0) surfaces. The generally compressed (∼0.3% of strain) and n-doped (Fermi level shift of ∼250 meV) graphene on Cu(1 1 1) shows the 2D band FWHM minimum of ∼20 cm⁻¹. In contrast, graphene grown on Cu foil under the same conditions reflects the heterogeneity of the polycrystalline surface and its 2D band is accordingly broader with FWHM >24 cm⁻¹.     

Ion implantation and anneal to produce low resistance metal–diamond contacts

Contacts to boron-doped, (100)-oriented diamond implanted with Si or with Si and B were formed and the effects of dose, implantation energy and anneal treatment on the specific contact resistance were examined. Ti/Au contacts on heavily implanted diamond (10¹⁶ Si ions cm⁻², E_i=30 keV or 10¹⁷ Si and B ions cm⁻², E_i=15 keV (Si) and E_i=10 keV (B)) had a specific contact resistance lower than the best contacts produced on unimplanted diamond. A specific contact resistance of (1.4±6.4)×10⁻⁷ Ω cm⁻² was achieved following a 450°C anneal. The results were consistent with a reduction in barrier height brought about by silicide formation. Light silicon implantation (10¹³ ions cm⁻²) or relatively light dual implantation (B, Si<10¹⁶ ions cm⁻²) did not reduce the specific contact resistance. Increasing the diamond conductivity by 4×10⁴ decreased the specific contact resistance by over three orders of magnitude, in agreement with the trend observed by Prins (J.F. Prins, J. Phys. D 22 (1989) 1562).     

Self-limited growth of large-area monolayer graphene films by low pressure chemical vapor deposition for graphene-based field effect transistors

During the last decade, fabrication of high-quality graphene films by chemical vapor deposition (CVD) for nanoelectronics and optoelectronic applications has attracted increasing attention. However, processing of large-area monolayer and defect-free graphene films is still challenging. In this work, we have studied the effect of processing conditions on the self-limited growth of graphene monolayers on copper foils during low pressure CVD both experimentally and theoretically based on thermokinetics and kinetics of Langmuir adsorption. The effect of copper pre-treatment, growth time, and carbon potential of the atmosphere (indicated by the methane-to-hydrogen gas ratio, r) on the quality of graphene nanosheets (number of layers, surface roughness and the lateral size) were studied. Microscopic studies show that careful pre-treatment of the copper foil by electropolishing provides a suitable condition for the self-limited growth of graphene with minimum surface roughness and defects. Raman spectroscopy and atomic force microscopy determine that the number of graphene sheets decreases with increasing the carbon potential while smother surfaces are attained. Large-area monolayer graphene films are obtained at relatively high carbon potential (r=1) and controlled growth time (10 min) at 1000 °C. Measurement of the electrical response of the prepared monolayer graphene films on SiO₂ (300 nm)/Si substrates in a field effect transistor (FET) device shows a high mobility of 2780 cm² V⁻¹ s⁻¹. Interestingly, the device exhibits p-type semiconducting behavior with the Dirac point at a gate voltage of 25 V. The finding show a great promise for graphene-based FET devices for future nanoelectronics.     

Direct formation of graphene layers on diamond by high-temperature annealing with a Cu catalyst

The formation of high-quality graphene layers on diamond was achieved based on a high-temperature annealing method using a Cu catalyst. Typical features of monolayer graphene were observed in the Raman spectra of layers formed by annealing of Cu/diamond heterostructures at 950 °C for 90 min. The coverage ratio of these graphene layers on diamond was estimated to be on the order of 85% by Raman mapping of the 2D peak. The sheet hole concentration and mobility values of the layers were estimated to be ~ 10¹³ cm^− 2 and ~ 670 cm²/Vs, respectively. These values are comparable to those previously observed for high-quality graphene layers on SiC.     

A simple PAN-based fabrication method for microstructured carbon electrodes for organic field-effect transistors

Facile and efficient fabrication of polyacrylonitrile (PAN)-based conductive graphitic carbon microstructures (GCMs) and their application to the electrodes of organic field-effect transistors (OFETs) is described. The PAN thin films spin-coated on a SiO₂-deposited Si wafer was irradiated through a pattern mask with 150 keV H⁺ ions at various fluences, and subsequently developed to form PAN microstructures. The resulting PAN microstructures were carbonized at various temperatures to create the GCMs. The analytical results revealed that the optimized fluence and carbonization temperature for well-defined GCMs was 3 × 10¹⁵ ions cm⁻² and 100 °C, respectively, and that the resulting GCMs created at the optimized condition exhibited a greatly low surface roughness of 0.36 nm, a good electrical conductivity of about 600 S cm⁻¹, and a high work function of 5.11 eV. Noticeably, the GCM electrodes-based p-type OFET showed a comparable performance to that of the gold electrode-based one, demonstrating that the practical use of GCMs as cheap electrodes to replace expensive metallic ones for organic electronic devices.     

Synthesis of large-area graphene on molybdenum foils by chemical vapor deposition

We report the synthesis of large-area graphene films on Mo foils by chemical vapor deposition. X-ray diffraction indicates that the dissolution and segregation process governs the growth of graphene on Mo foils. Among all processing parameters investigated, the cooling rate is the key one to precisely control the thickness of graphene film. By optimizing the cooling rate between 1.5 and 10 °C/s, we managed to achieve graphene films ranging from mono- to tri-layer. Their uniformity and thickness were confirmed by Raman spectroscopy and optical measurements. The carrier mobility of films reaches as high as 193 cm² V^?1 s^?1. Our experiments show that the Mo substrate has the similar simplicity and large tolerance to processing conditions as Cu.     

Synthesis and properties of an atomically thin carbon nanosheet similar to graphene and its promising use as an organic thin film transistor

We synthesize an atomically thin carbon nanosheet (CNS) analogous to graphene with properties suitable for an organic thin film transistor (OTFT). The synthesis of graphene by chemical vapor deposition has serious drawbacks such as wrinkles, grain boundaries, and defects due to catalyst removal and transfer process. Here the CNS is directly synthesized on a silicon wafer by heat-treatment of spin-coated polyacrylonitrile and shows a higher electrical conductivity (>1600 S cm⁻¹) than that of chemically converted graphene. The CNS on glass, transferred from a silicon wafer, exhibits approximately 92% optical transmittance. We have used our CNS as the electrodes of OTFTs, and recorded a mobility (0.25–0.35 cm² V⁻¹ s⁻¹) that exceeds that of gold electrodes (0.2–0.25 cm² V⁻¹ s⁻¹).     

The decrease of depolarization temperature and the improvement of pyroelectric properties by doping Ta in lead-free 0.94Na0.5Bi0.5TiO3-0.06BaTiO3 ceramics

Ta-doped lead-free 0.94NBT-0.06BT-xTa (x=0.0–1.0%) ceramics were synthesized by a conventional solid-state route. XRD shows that the compositions are at a morphotropic phase boundary where rhombohedral and tetragonal phases coexist. The depolarization temperature (T_d) shifted to lower temperature with the increase of Ta content. The pyroelectric coefficient (p) of doped ceramics greatly enhanced compared with undoped material and reached a maximum of 7.14×10⁻⁴ C m⁻² °C⁻¹ at room temperature (RT) and 146.1×10⁻⁴ C m⁻² °C⁻¹ at T_d at x=0.2%. The figure of merits, F_i and F_v, also showed a great improvement from 1.12×10⁻¹⁰ m v⁻¹ and 0.021 m² C⁻¹ at x=0.0 to 2.55×10⁻¹⁰ m v⁻¹ and 0.033 m² C⁻¹ at x=0.2% at RT. Furthermore, F_i and F_v show the huge improvement to 52.2×10⁻¹⁰ m v⁻¹ and 0.48×10⁻¹⁰ m v⁻¹ respectively at T_d at x=0.2%. F_C shows a value between 2.26 and 2.42 ×10⁻⁹ C cm⁻² °C⁻¹ at RT at x=0.2%. The improved pyroelectric properties make NBT-0.06BT-0.002Ta ceramics a promising infrared detector material.     

Bilayer graphene synthesis by plasma treatment of copper foils without using a carbon-containing gas

Bilayer graphene has been synthesized by using hydrogen plasma treatment of copper foils for 30 s at the temperature of 850 °C together with joule-heating treatment of the foils without using a carbon-containing gas such as methane in order to suppress the nucleation density of graphene. The effect of plasma provides active species of carbon atoms on copper substrate and a selective bilayer graphene formation of AB-stacking in a very short time. Carbon to be precipitated is delivered from the copper foil and/or the environment in the reaction chamber. The domain size of synthesized graphene, the controllability of a few layers and the electrical conductivity have been significantly improved compared with plasma chemical vapor deposition (CVD) using carbon-containing gas. The sheet resistance of bilayer graphene exhibits 951 Ω in average. The carrier mobility shows 1000 cm²/V s in maximum at room temperature. The sheet resistance of 130 ± 26 Ω has been attained after the doping by gold chloride solution.     

Development of PLZT dielectrics on base metal foils for embedded capacitors

We have deposited Pb_0.92La_0.08Zr_0.52Ti_0.48O₃ (PLZT) films on nickel and copper substrates to create film-on-foil capacitors that exhibit excellent dielectric properties and superior breakdown strength. Measurements with PLZT films on LaNiO₃-buffered Ni foils yielded the following: relative permittivity of 1300 (at 25 °C) and 1800 (at 150 °C), leakage current density of 6.6 × 10^?9 A/cm² (at 25 °C) and 1.4 × 10^?8 A/cm² (at 150 °C), and mean breakdown field strength ≈2.5 MV/cm. With PLZT deposited directly on Cu foils, we observed dielectric constant ≈1100, dielectric loss (tan δ) ≈0.06, and leakage current density of 7.3 × 10^?9 A/cm² when measured at room temperature.     

The effect of gallium implantation on the optical properties of diamond

Gallium focused-ion beam milling is a commonly used technique for sculpting diamond at the nano and micro scale. However even at fluences insufficient to cause sputtering, implanted gallium causes modification of the optical properties of the diamond substrate. We implanted 30 keV gallium ions at fluences from 1 × 10¹³ to 5 × 10¹⁴ Ga/cm² and studied the effect on the optical properties via spectroscopic ellipsometry (SE), from 0.6 to 6.5 eV, obtaining the changes in refractive index and extinction coefficient in the implanted layer as a function of operating wavelength. Here, we report the first observation of decreased refractive index for a wide spectral range in low fluence implanted diamond. In addition, we observe non-monotonic response of the refractive index with fluence, which is in disagreement with proton studies, but accords with other heavy-ion implantation reports. Such discrepancies suggest that there are different mechanisms for refractive index modification for different species and that resulting optical properties are not solely a function of damage. Further comparisons with the white light reflectance and near infra-red transmittance measurements support the SE data.     

Light penetration depth dependence of photocarrier life time and the Hall effect in phosphorous-doped and boron-doped homoepitaxial CVD diamond films

Photocurrent in phosphorous-doped CVD diamond film of the bandgap of 5.5 eV with the density of 2 × 10¹⁸ cm^− 3 decreases with increasing photon energy in the energy range higher than 5.8 eV at room temperature (RT). The photocarrier life time is 0.3 ms at the excitation energy of 5.8 eV and decreases with increasing excitation energy. These show that the photocarriers, ascertained to be electrons by the Hall effect of the photocurrent, are trapped near the surface. The life time of photo-excited holes in Boron-doped CVD diamond film with the density of 9 × 10¹⁷ cm^− 3 is 35 ms at RT and decreases with decreasing Boron density, which is explained from the relation between the Fermi energy and the density.     

Rapid oxidative activation of carbon nanotube yarn and sheet by a radio frequency,atmospheric pressure,helium and oxygen plasma

Carbon nanotube yarn and sheet were activated using radio frequency, atmospheric pressure, helium and oxygen plasmas. The nanotubes were exposed to the plasma afterglow, which contained 8.0 × 10¹⁶ cm⁻³ ground state O atoms, 8.0 × 10¹⁶ cm⁻³ metastable O₂ (¹Δ_g), and 1.0 × 10¹⁶ cm⁻³ ozone. X-ray photoelectron spectroscopy and infrared spectroscopy revealed that 30 s of plasma treatment converted 25.2% of the carbon atoms on the CNT surface to oxidized species, producing 17.0% alcohols, 5.9% carbonyls, and 2.3% carboxylic acids. The electrical resistivity increased linearly with the extent of oxidation of the CNT from 4 to 9 × 10⁻⁶ Ω m. On the other hand, the tensile strength of the yarn was decreased by only 27% following plasma oxidation.     

High density carbon nanotube growth using a plasma pretreated catalyst

We have developed a direct current plasma pretreatment of the Fe catalyst to increase the area density of vertically-aligned carbon nanotube forests. The carbon wall density of the double- and multi-walled nanotubes reaches 4.8 × 10¹² cm⁻², with a 40% volume occupancy and a mass density of ∼0.4 g cm⁻³. The plasma pretreatment works by reducing the sintering of the catalyst nanoparticles during growth. This treatment increases the forest density by 8 times compared to the standard growth conditions.     

Influence of irradiation by 24 GeV protons on the properties of 4H–SiC radiation detectors

We had investigated the effects of the irradiation by 24 GeV protons with the doses from 10¹³ cm^− 2 up to 10¹⁶ cm^− 2 on the properties of radiation detectors fabricated as Schottky diodes on 4H–SiC. Numbers and activities of radionuclides and isotopes produced after the irradiation were analysed. Activities of ⁷Be and ²²Na were found to be proportional to the irradiation dose and ranged from 1.3 up to 890 Bq and from 1.9 up to 950 Bq, respectively. The total amount of formed stable isotopes was from 1.2 × 10¹¹ cm^− 2 up to 5.9 × 10¹³ cm^− 2. 390 days after the irradiation the number of radiated electrons with different energies ranged from 1.0 up to 600 per second, respectively. The contact properties were investigated by means of the current–voltage analysis. At lower irradiation doses a slight decrease of the effective potential barrier height from about 0.74 eV down to < 0.7 eV took place. The reverse current of the diodes grew by up to one order of magnitude. At the doses above 3 × 10¹⁵ cm^− 2 opposite changes were observed. Irradiation by up to 1 × 10¹⁶ protons/cm^− 2, resulted in the increase of the potential barrier height up to ∼ 0.85 eV, followed by the drop of the reverse current by up to two orders of magnitude. The observed effects were explained by the appearance of the disordered material structure because of the high-energy particle bombardment.     

Preparation of flat porous carbon films from paper-thin wood shavings and control of their mechanical,electrical and magnetic properties

A method for preparing flat porous carbonized films (FPCFs) by carbonizing paper-thin wood shavings was developed, and their structures and properties were investigated. The FPCFs were binder-free and had horizontal honeycomb structures consisting of macropores of 2–50 μm arising from tracheids and pittings. The FPCFs with approximate dimensions of 120 mm × 104 mm × 113 μm showed some flexibility. They were effectively reinforced and became twistable by introducing styrene–butadiene rubber (SBR) into the macropores via impregnation. The SBR-modified FPCFs carbonized at 1023 K retained electrical conductivity on the order of 10⁻³ S/m with the inclusion of SBR concentrations up to 15 mg/cm². The electrical conductivity of FPCFs carbonized at 773 K changed from nonconductive to 10⁻² S/m by impregnating the macropores with electrically conductive carbon paste or to 10⁵ S/m with Cu chemical plating. The FPCFs became responsive to a magnetic field by impregnating the macropores with a magnetic fluid or by creating ferrites through chemical reactions within the macropores.     

Cooperative adsorption of bisphenol-A and chromium(III) ions from water on activated carbons prepared from olive-mill waste

The aim of this study was to investigate the simultaneous adsorption of bisphenol A (BPA) and chromium ions from aqueous solution on activated carbons (both commercial and prepared from olive-mill waste), analyzing both kinetic and equilibrium adsorption data. The effects of solution pH and ionic strength on the adsorption processes were also studied, as well as the chemical interactions between the carbon surface and the pollutants. The activated carbon prepared from olive-mill waste showed: a large surface area (up to 1641 m² g⁻¹), a highly heterogeneous micropore distribution, and a basic chemical nature. The pore volume diffusion model was used to predict the adsorption kinetics of both pollutants. The effective diffusion coefficients ranged between 1.15 × 10⁻⁶ and 9.18 × 10⁻⁷ cm² s⁻¹ for the BPA–Cr(III) system and between 1.65 × 10⁻⁶ and 2.8 × 10⁻⁶ cm² s⁻¹ for the Cr(III)–BPA system. The presence of both Cr(III) and BPA in the binary systems increases the adsorption effective diffusion coefficients and therefore the overall adsorption rate of each pollutant. The increased adsorption of each adsorbate when both pollutants are present is due to the in situ formation of complex compounds between Cr(III), acting as central metallic cation, and BPA, acting as ligand, during adsorption of both adsorbates on activated carbon.     

Grand canonical Monte Carlo simulations of nitrogen adsorption on graphene materials with varying layer number

Fabrication of monolayer graphene is a challenge and many processes yield few-layer or multi-layer graphene materials instead. The layer number is an important property of those materials and a quality control variable in graphene manufacture. We demonstrated that N₂ adsorption on graphene materials was used to distinguish its layer number. We performed grand canonical Monte Carlo simulation of N₂ adsorption on graphene materials with 1–10 layers to indicate the possibility of distinction of layer number by evaluating the dependence of N₂ adsorption characteristics on the layer number of graphene materials as well as the adsorption mechanism. The threshold relative pressures of monolayer adsorption of N₂ on monolayer and two-layer graphene were 1 × 10⁻³ and 2 × 10⁻⁴, respectively, while those of the others were 1 × 10⁻⁴. In contrast, the threshold pressures of second layer adsorption of N₂ were similar to each other. The difference of threshold pressures is attributed to stabilized energies induced by interactions with graphene materials. Therefore, the layer number of graphene materials could be evaluated from the threshold pressures of adsorption, providing a guide to aid fabrication of graphene materials.