Ar, O2, CHF3, and SF6 plasma treatments of screen-printed carbon nanotube films for electrode applications

This paper investigates the consequence of the material property and the plasma gas chemistry (herein referred to the plasma gas-feeding species and methods) on the electrode performance in plasma treatments of screen-printed carbon nanotube (CNT) films. Four plasma gases (Ar, O₂, SF₆, and CHF₃) and three gas-feeding methods were examined. The surface morphology, microstructure, and composition of 11 sample groups have been carefully characterized. Tests of the CNT film electrode subjected to gas discharge and field emission show that surface morphology modification is the most influential factor in respect of lowering the onset voltages. In detail, O₂/Ar (O₂ followed by Ar) and Ar + CHF₃ + SF₆ (mixed three gases) treatments are the best choices for ionization and field emission applications, respectively. The relevant results are even better than that of the samples of aligned CNT films prepared by chemical vapor deposition. The underlying mechanisms are modeled by two opposing processes (etching and coating), which phenomenally produce three competing effects, i.e., CNT protruding, bundle forming, and neo-nanostructure forming. The results and the correct behavior of our model suggest that the plasma gas chemistry is the most fundamental factor in the process of plasma treatments of CNT films.     

Cathodoluminescence and electron field emission of boron-doped a-C:N films

The optical and electrical properties of so-called carbon nitride films (a-C:N) and boron doped so-called carbon nitride films (a-C:N:B) are studied with cathodoluminescence (CL) spectroscopy and electron field emission measurement. The a-C:N films were first deposited on Si by a filtered cathodic arc plasma system, and then boron ions (∼1 × 10¹⁶ cm⁻²) were implanted into the a-C:N films to form a-C:N:B films by a medium current implanter. The structural and morphological properties of a-C:N and a-C:N:B films were then analyzed using secondary ion mass spectrometer, X-ray photoelectron spectroscopy, FT-IR spectra, Raman spectroscopy and atomic force microscopy. The a-C:N film exhibits luminescence of blue light (∼2.67 eV) and red light (∼1.91 eV), and the a-C:N:B film displays luminescence of blue light (∼2.67 eV) in CL spectra measured at 300 K. Furthermore, the incorporated boron atoms change the electron field emission property, which shows a higher turn on field for the a-C:N:B film (3.6 V/μm) than that for the a-C:N film (2.8 V/μm).     

Cathodoluminescence of fluorine doped amorphous carbon nanoparticles deposited by a filtered cathodic arc plasma system

The fluorine doped amorphous carbon nanoparticles (a-C:F NPs) films with sizes 50-100 nm were deposited on polyethylene terephthalate in an atmosphere of CF₄ by a 90°-bend magnetic filtered cathodic arc plasma system. The surface morphology of a-C:F NPs films was observed by field emission scanning electron microscope and atomic force microscope. The microstructure and chemical bonding nature of the a-C:F NPs films were investigated by Raman, X-ray diffraction and X-ray photoelectron spectroscopy. This work presents cathodoluminescence (CL) spectra of a-C:F NPs films obtained at 1.9-2.4 eV and verifies luminescence from a-C:F NPs films in the visible region. The incorporation of fluorine into the carbon network results in orange emission (∼2.03 eV) due to the transitions between fluorine-related electron levels and σ* states, and the red emission (∼1.97 eV) results from the recombination of carriers in the valence π and conduction π* states. The peak at ∼2.10 eV may result from the defects of the structures in a-C:F NPs films.     

Effects of hydrogen on carbon nanotube formation in CH4/H2 plasmas

Carbon nanotubes (CNTs) were synthesized using CH₄/H₂ plasmas and plasmas simulated using a one-dimensional fluid model. The thinnest and longest CNTs with the highest number density were obtained using CH₄/H₂ = 27/3 sccm at 10 Torr. These conditions allowed CNTs to grow for 90 min without any meaningful loss of catalyst activity. However, an excess H₂ supply to the CH₄/H₂ mixture plasma made the diameter distribution of the CNTs wider and the yield lower. Hydrogen concentration is considered to affect catalyst particle size and activity during the time interval before starting CNT growth (=incubation period). With CH₄/H₂ = 27/3 sccm for a growth time of 10 min efficient CNT growth was achieved because the amount of carbon atoms in the CNTs and that calculated from simulation showed good agreement. The effect of hydrogen etching on CNTs was analyzed by scanning electron microscopy and X-ray photoelectron spectroscopy by observing CNTs treated by H₂ plasma after CNT growth. It was confirmed that (a) multi-walled CNTs were not etched by the H₂ plasma, (b) the C 1s XPS spectra of the CNTs showed no chemical shift after the treatment, and (c) C-H bonds were produced in CNTs during their growth.     

Prevention of Si-contaminated nanocone formation during plasma enhanced CVD growth of carbon nanotubes

We investigated the growth behavior and morphology of vertically aligned carbon nanotubes (CNTs) on silicon (Si) substrates by direct current (DC) plasma enhanced chemical vapor deposition (PECVD). We found that plasma etching and precipitation of the Si substrate material significantly modified the morphology and chemistry of the synthesized CNTs, often resulting in the formation of tapered-diameter nanocones containing Si. Either low bias voltage (∼500 V) or deposition of a protective layer (tungsten or titanium film with 10-200 nm thickness) on the Si surface suppressed the unwanted Si etching during growth and enabled us to obtain cylindrical CNTs with minimal Si-related defects. We also demonstrated that a gate electrode, surrounding a CNT in a traditional field emitter structure, could be utilized as a protection layer to allow growth of a CNT with desirable high aspect ratio by preventing the nanocone formation.     

Characterization of diamond fluorinated by glow discharge plasma treatment

The surface fluorination of diamond by treatment in glow discharge plasmas of CF₄ for different times has been investigated. High quality diamond films were deposited onto silicon substrates using hot filament chemical vapor deposition (HFCVD). Subsequently, the films were exposed to a radiofrequency glow discharge plasma of CF₄ for times ranging from 5 min to 1 h. The effects of the plasma treatment on the surface morphology, diamond quality and elemental composition were investigated using atomic force microscopy (AFM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS), respectively. Differences in film roughness caused by the plasma treatment were detected by AFM and confirmed by scanning electron microscopy (SEM). Raman spectroscopic analyses showed that the original diamond was of high quality and that the bulk of each film was unchanged by the plasma treatment. Analyses using XPS revealed increased surface fluorination of the films at longer treatment times. In addition, the density of free radicals in the films was probed using electron paramagnetic resonance spectroscopy (EPRS), revealing that untreated diamond possesses an appreciable density of free radicals (6×10¹² g⁻¹) which initially falls with treatment time in the CF₄ plasma but increases for long treatment times.     

Deposition of amorphous carbon films from C60 fullerene sublimated in electron beam excited plasma

C₆₀ fullerene clusters are used as a carbon source for amorphous carbon films deposition in an electron beam excited plasma. C₆₀ clusters are sublimated by heating a ceramic crucible containing the C₆₀ powders up to 850 °C, which is located in a highly vacuumed process chamber. The sublimated fullerene powders are injected to the electron beam excited argon plasma and dissociated to be active species that are propelled toward the substrates. Consequently, the carbon species condense as a thin film onto the negatively biased substrates that are immersed in the plasma. Deposition rates of approximately 1.0 μm/h and the average surface roughness of 0.2 nm over an area of 400 μm² are achieved. Decomposition of the C₆₀ fullerene after injecting into the plasma is confirmed by optical emission spectroscopy that shows existence of small carbon species such as C₂ in the plasma. X-ray diffraction pattern reveals that the microstructure of the film is amorphous, while fullerene films deposited without the plasma show crystalline structure. Raman spectroscopic analysis shows that the films deposited in the plasma are one of the types of diamond-like carbon films. Different negative bias voltages have been applied to the substrate holder to examine the effect of the bias voltage to the properties of the films. The nano-indentation technique is used for hardness measurement of the films and results in hardness up to about 28 GPa. In addition, the films are droplet-free and show superior lubricity.     

On the effect of treating poly(acrylic acid) with argon and tetrafluoromethane plasmas: Kinetics and degradation mechanism

Poly(acrylic acid) (PAAc) films were treated with either an argon or a tetrafluoromethane (CF₄) plasma and subsequently analyzed with X-ray photoelectron spectroscopy (XPS). PAAc films were decarboxylated during both types of plasma treatments. In addition, during the CF₄ plasma treatment, the PAAc films became fluorinated. The plasma phase during the argon plasma treatment of PAAc films was investigated with optical emission spectroscopy. It was shown that during this plasma treatment carbon dioxide, water, and possibly hydrogen were liberated from the PAAc surface. By covering the surface of PAAc films with different materials (lithium fluoride, UV fused silica, and glass) during the plasma treatment, it was possible to differentiate between photochemically induced and particle-induced changes of the surface. This method was used to show that decarboxylation during the argon plasma treatment was caused by vacuum UV radiation (wavelength < 150 nm) and the decarboxylation/fluorination during the CF₄ plasma treatment was induced by reactive fluorine-containing species from the plasma phase. Furthermore, during both processes, etching of the PAAc surface occurred. Based on these mechanisms, kinetic models were derived that could be used to describe the measured kinetic data adequately. © 1994 John Wiley & Sons, Inc.     

Growth and electrical characterization of high-aspect-ratio carbon nanotube arrays

The remarkable properties of carbon nanotubes (CNTs) make them attractive for microelectronic applications, especially for interconnects and nanoscale devices. In this paper, we describe a microelectronics compatible process for growing high-aspect-ratio CNT arrays with application to vertical electrical interconnects. A lift-off process was used to pattern catalyst (Al₂O₃/Fe) islands to diameters of 13 or 20 μm. After patterning, chemical vapor deposition (CVD) was involved to deposit highly aligned CNT arrays using ethylene as the carbon source, and argon and hydrogen as carrier gases. The as-grow CNTs were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The results demonstrated that the CNTs have high purity, and form densely-aligned arrays with controllable array size and height. Two-probe electrical measurements of the CNT arrays indicate a resistivity of ∼0.01 Ω cm, suggesting possible use of these CNTs as interconnect materials.     

A parametric study of carbon nanotubes production from tetrachloroethylene using a supersonic thermal plasma jet

In this study we produce carbon nanotubes (CNT) using a DC non-transferred plasma torch operated at a power of 30 kW in argon and producing a supersonic jet. Tetrachloroethylene (TCE) is used as the carbon raw material. As an initial demonstration of the supersonic plasma jet approach and in an attempt to simplify the flow/nucleation fields of metal catalyst nanoparticles, the erosion of the torch tungsten electrodes is used as a source of metal vapours nucleating in situ into catalytic nanoparticles within the plasma jet. CNT mass yield values are based on thermogravimetry analysis correlated using electron microscopy and Raman spectroscopy. A parametric study is made to evaluate the influence of the different operating parameters on the yields of carbon nanotubes. The rapid quench generated by the supersonic jet, the high vapour pressures of carbon and a control of the temperature profile within the torch nozzle enable the rapid growth of CNT. Decreasing the reactor pressure from 0.66 to 0.26 atm leads to a CNT yield increase by 22%. High vapour pressure of carbon is obtained by increasing the TCE feed rate. From 0.05 to 0.15 mol/min, the yield of CNT is improved from 38.6 to 53.7 wt%. Beyond 0.15 mol TCE/min, the yield of CNT levels to around 50 wt%. In the start-up phase, the time of operation controls the temperature profile, which in turn drastically increases the yield of CNT from 0 to 8.2 wt% between the 3rd and the 4th minute of operation and to 53.7 wt% after the 5th minute.     

Etching process of hydrogenated amorphous carbon (a-C:H) thin films in a dual ECR–r.f. nitrogen plasma

Hydrogenated amorphous carbon (a-C:H) films deposited from CH₄ in a dual electron cyclotron resonance (ECR)–r.f. plasma were treated in N₂ plasma at different r.f. substrate bias voltages after deposition. The etching process of a-C:H films in N₂ plasma was observed by in situ kinetic ellipsometry, mass spectroscopy (MS), and optical emission spectroscopy (OES). Ex situ atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were used to characterize the etched film surface. XPS analysis proves that the nitrogen treatment on the a-C:H film, induced by r.f. substrate bias, causes a direct nitrogen incorporation in the film surface up to 15–17 at.% to a depth of about 20–40 Å depending on the r.f. bias. Various bonding states between carbon and nitrogen, such as tetrahedral sp³ C–N, and trigonal sp² C–N were confirmed by the deconvolution analysis of C 1s and N 1s core level spectra. The evolution of etching rate and the surface roughness in the film measured by AFM exhibit a clear dependence on the applied r.f. bias. MS and OES show the various neutral species in the N₂ plasma such as HCN, CN, and C₂N₂, which may be considered as the chemical etching products during the N₂ plasma treatment of a-C:H film.     

Effect of sidewall modification in the determination of friction coefficient of vertically aligned carbon nanotube films using friction force microscopy

The significance of the sidewall surface of vertically aligned carbon nanotubes (VACNTs) and the effect of humidity in the determination of VACNT film friction coefficient have been investigated. VACNT films of 2 μm thick were sidewall-modified by means of CF₄ and O₂ plasma treatments, and verified for the functionalization of the sidewalls. They were then characterized for wettability properties, as well as friction coefficient using friction force microscopy at different humidity levels. It was found that humidity had insignificant effect on the friction coefficient, and sidewall friction formed a major component of the friction force experienced by the tip. Sidewall modifications resulted in friction coefficient changes of up to 50%.     

Field emission properties from aligned carbon nanotube films with tetrahedral amorphous carbon coatings

Tetrahedral amorphous carbon (ta-C) film was coated on aligned carbon nanotube (CNT) films via filtered cathodic vacuum arc (FCVA) technique. Field electron emission properties of the CNT films and the ta-C/CNT films were measured in an ultra high vacuum system. The I–V measurements show that, with a thin ta-C film coating, the threshold electric field (E_thr) of CNTs can be significantly decreased from 5.74 V/μm to 2.94 V/μm, while thick ta-C film coating increased the E_thr of CNTs to around 8.20 V/μm. In addition, the field emission current density of CNT films reached 14.9 mA/cm² at 6 V/μm, while for CNTs film coated with thin ta-C film only 3.1 V/μm of applied electric field is required to reach equal amount of current density. It is suggested that different field emission mechanisms should be responsible for the distinction in field emission features of CNT films with different thickness of ta-C coating.     

Defect structure for the ultra-nanocrystalline diamond films synthesized in H2-containing Ar/CH4 plasma

The modification on the microstructure of diamond films due to the addition of H₂ species into the Ar/CH₄ plasma was investigated. While the Ar/CH₄ plasma produced UNCD films with equiaxed grains (about 5 nm in size), the (Ar-H₂)/CH₄ plasma produced acicular-shaped grains (about 5 × 20 nm in size). Transmission electron microscopy studies indicate that these acicular-shaped grains actually are agglomerates of diamond flakes, which contain stacking faults lying on the (111) lattice plane. Presumably, the incorporation of H₂ species in the plasma leads to partial etching of hydrocarbons adhered onto the diamond clusters, such that the C₂- (or active carbon) species contained in the plasma can attach to the diamond surface anisotropically, leading to diamond flakes. The incorporation of H₂ in Ar plasma can also suppress the formation of i-carbons, an allotropic phase of diamonds. The critical proportion of H₂ in Ar plasma for inducing the changes in the granular structure is around 0.03%. The proportion of grain boundaries was thus reduced and the electron field emission properties of the materials were thus degraded. However, the suppression of the film electrical conductivity without sacrificing the smooth surface characteristic has the applications as high-thermal-conductivity heat spreaders and substrates for surface-acoustic-wave devices.     

High-current electron emission characteristics of cathodes based on diamond films

Using different gas source, four types of diamond thin films were prepared on silicon substrate by microwave plasma chemical vapor deposition (MPCVD) technology, and characterized in detail through scanning electron microscopy (SEM), Raman spectroscopy and Fourier transform infrared (FTIR) spectroscopy. High-current pulsed emission characteristics, tested with a 2 MeV line-inducing injector, showed that all of CVD diamond films had high emission current density (> 70 A/cm²) and [100] textured B-doped microcrystalline diamond film possessed the largest emission current density of 115.1 A/cm². No obvious bright light and luminescent zones from side view CCD images indicated a possible pure field-emission mechanism of these diamond cathodes. Simultaneously, large decrease in the electron emission capability, above 15%, could be observed after several pulsed measurements, but this decrease could be completely recovered through the treatment of surface re-hydrogenation for emitted diamond cathodes, suggesting that emission performance of CVD diamond cathodes was closely relevant to hydrogen coverage ratio. The present data indicated that as-deposited CVD diamond films could be a potential candidate as cold cathode for the application in high-current electron emission field.     

Synthesis of double-walled carbon nanotube films and their field emission properties

Continuous double-walled carbon nanotube (DWCNT) films were synthesized using an Fe-Mo catalyst by the arc discharge method. This new catalyst has dramatically improved the purity and selectivity of DWCNT product. High-resolution transmission electron microscopy indicates that the outer and inner diameter of DWCNT are 1.9-4.7 nm and 1.2-3.8 nm, respectively. The field emission properties of DWCNT films have been studied. The directly grown film was transferred onto quartz substrates and used as emission cathodes, and has demonstrated a quite good emission performance. Moreover, the emissions of DWCNT films have been further improved by heat treatment. The film after 400 °C oxidation shows excellent field emission property with a low turn-on (E_to = 0.6 V/μm) and threshold field (E_th = 0.9 V/μm) corresponding to the emission current density of 1 μA/cm² and 1 mA/cm², respectively.     

Dry densification of carbon nanotube bundles

A dry method for densifying vertically aligned carbon nanotube bundles is proposed and experimentally validated. The process uses the deposition of thin SiO₂ films to seal the porous CNT bundles at low pressure. When the CNT bundles are transferred back to ambient pressure they are densified by the pressure difference obtained between the inner and outer sides of the thin film. The effects of the densification have been studied for different thicknesses of SiO₂ films deposited by two different deposition techniques. The diameters of the narrowest densified sections are 26 ± 3% of their original sizes after dry densification by 50 nm thick SiO₂. The proposed dry densification method is also compared to existing liquid-based methods and its limitations are discussed.     

One-step synthesis of Ni0.5Zn0.5Fe2O4 fine-patterned films via photosensitive sol–gel route

Ni_1−xZn_xFe₂O₄ (NZFO) (x=0.0–0.7) films were prepared by a photosensitive sol–gel route utilizing nickel acetate, zinc acetate and ferric nitrate as starting materials. The saturation magnetization of the NZFO film showed a parabolic tendency with Zn substitution. For Zn substitution of 0.5, the saturation magnetization reached the maximum value of 683 emu/cm³ with a relative low coercivity of 56 Oe at room temperature. The phase constituents and surface morphology of the films were characterized by X-ray diffractometer (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM). Through a direct patterning process, a fine-patterned Ni_0.5Zn_0.5Fe₂O₄ film was obtained by a photochemical reaction between the chelated complexes and UV light.     

Enhancement of the field emission properties of low-temperature-growth multi-wall carbon nanotubes by KrF excimer laser irradiation post-treatment

Multi-wall carbon nanotube (CNT) films were fabricated by microwave plasma chemical vapor deposition at low temperatures ( 500 °C). The films when properly post-treated by laser irradiation exhibited a factor of 2–3 enhancement in the emission current, while the turn-on field (E_on) was reduced from 4.89–5.22 to 2.88–3.15 V/μm. The introduction of excessive oxygen during laser irradiation, however, degrades the performance of field emission properties drastically. Raman spectroscopy measurements revealed the intimate correlation between the parameter I_D/I_G (intensity ratio between the two representative Raman peaks seen in carbon nanotubes) and the field emission performance. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analyses showed that the irradiation-induced modification of the tube morphology and crystallinity might be responsible for the observations.     

Deposition of carbon nitride films from single-source s-triazine precursors

Trisubsituted derivatives of s-triazine (1,3,5-triazine) with N(i-Pr)₂, N(i-Bu)₂, NH(t-Bu), pyridyl, and NHNHMe ligands were used as single-source precursors to produce carbon nitride (CN_x) thin films via hot wall CVD. The precursors are either commercially available or were synthesized in straightforward, one-pot procedures, and the synthesis and characterization of tris-2,4,6-methlyhydrazino-1,3,5-triazine (TMHT) is reported for the first time. All of the precursors studied are thermally stable and volatilize below 250 °C. They thermally decompose between 500 and 1000 °C, resulting in CN_x films with x ranging from 0.95 to 0.03. The film deposition temperature and nitrogen content depend upon the structure and stability of the precursor. The film properties vary from disordered insulating structures with high nitrogen content (CN_0.95) to low nitrogen content turbostratic carbon films. The films on Si and SiO₂ substrates were characterized by Auger surface analysis, FT-IR and Raman spectroscopy, X-ray diffraction, and scanning electron microscopy.