Field Emission from a Mixture of Amorphous Carbon and Carbon Nanotubes Films

A mixture of amorphous carbon and carbon nanotubes films was synthesized on stainless steel plates by a micro- wave plasma enhanced chemical vapor deposition system. The source gases were hydrogen and methane with flow rates of 100 and 16sccm,respectively,with a total pressure of 5.0kPa. The surface morphology and the structure of the films were characterized by field emission scanning electron microscopy （SEM） and Raman scattering spectroscopy. Field emission properties of as-deposited film were measured in a vacuum room below 5 ×10^ 5 Pa. The experimental results show that the initial turn-on field is 0. 9V/μm; The current density is 4.0mA/cm2 and the emission sites are dense and uniform at an electric field of 3.7V/μm. These results indicate that such a mixture of amorphous carbon and carbon nanotubes films is a promising material for field emission applications.

Effect of Radio-Frequency and Low-Frequency Bias Voltage on the Formation of Amorphous Carbon Films Deposited by Plasma Enhanced Chemical Vapor Deposition

The effect of radio-frequency （RF） or low-frequency （LF） bias voltage on the for- mation of amorphous hydrogenated carbon （a-C：H） films was studied on silicon substrates with a low methane （CH4） concentration （2-10 vol.%） in CH4＋Ar mixtures. The bias substrate was applied either by RF （13.56 MHz） or by LF （150 kHz） power supply. The highest hardness values （~18-22 GPa） with lower hydrogen content in the fihns （~20 at.%） deposited at 10 vol.% CH4, was achieved by using the RF bias, However, the films deposited using the LF bias, under similar RF plasma generation power and CH4 concentration （50 W and 10 vol.%, respectively）, displayed lower hardness （~6-12 GPa） with high hydrogen content （~40 at.%）. The structures analyzed by Fourier Transform Infrared （FTIR） and Raman scattering measurements provide an indication of trans-polyacetylene structure formation. However, its excessive formation in the films deposited by the LF bias method is consistent with its higher bonded hydrogen concentration and low level of hardness, as compared to the film prepared by the RF bias method. It was found that the effect of RF bias on the film structure and properties is stronger than the effect of the low-frequency （LF） bias under identical radio-frequency （RF） powered electrode and identical PECVD （plasma enhanced chemical vapor deposition） system configuration.

Electron emission degradation of nano-structured sp^2-bonded amorphous carbon films

The initial field electron emission degradation behaviour of original nano-structured sp^2-bonded amorphous carbon films has been observed, which can be attributed to the increase of the work function of the film in the field emission process analysed using a Fowler-Nordheim plot. The possible reason for the change of work function is suggested to be the desorption of hydrogen from the original hydrogen termination film surface due to field emission current-induced local heating. For the explanation of the emission degradation behaviour of the nano-structured sp2-bonded amorphous carbon film, a cluster model with a series of graphite （0001） basal surfaces has been presented, and the theoretical calculations have been performed to investigate work functions of graphite （0001） surfaces with different hydrogen atom and ion chemisorption sites by using first principles method based on density functional theory-local density approximation.

Optical Properties of Tetrahedral Amorphous Carbon Films and Their Potential for Lab-on-a-Chip

In this study, tetrahedral amorphous carbon (ta-C) films with thicknesses between several 100 nm and several micrometers have been deposited onto polished tungsten carbide and steel substrates by pulsed laser deposition (PLD) using an excimer laser (248 nm wavelength). We investigate the optical properties (e.g. the refractive index (n) and extinction coefficient (k) in the visible and near-infrared wavelength range) of these layers in dependence of the used laser ablation fluence on the target. It is shown that n of ~2000 nm thick ta-C films can be tuned, depending on the sp3-content, between n = 2.5 and 2.8 at a wavelength of 632 nm. Besides of this k reduces with the sp3-content and is as low as 0.03 at sp3-contents of more than 75%. We proof that this gives the opportunity to prepare coating with tailored optical properties. Furthermore, it is shown that the ta-C films have low background fluorescence in the wavelengths range of 380 - 750 nm, which make this thin films attractive for certain optical, medical and biotechnological applications. We present for the first time that one possible application is the use in Lab-on-a-Chip-systems (LOC). Within these systems, the ultrasensitive detection of fluorescence markers and dyes is a challenge. In order to increase the signal-to-noise-ratio, a setup was developed, that used the specific optical properties of ta-C films produced by PLD. We used the ta-C film as an integrated reflector that combined low background fluorescence, a low reflectivity at the excitation wavelength and the high reflectivity at the emission wavelength. We prove that this setup improves the detection of fluorescence photons.

Influence of nitrogen doping on thermal stability of fluorinated amorphous carbon thin films

Nitrogen doping fluorinated amorphous carbon (a-C∶F) films were deposited using radio frequency plasma enhanced chemical vapor deposition (RF-PECVD) and annealed in Ar environment in order to investigate their thermal stability. Surface morphology and the thickness of the films before and after annealing were characterized by AFM and ellipsometer. Raman spectra and FTIR were used to analyze the chemical structure of the films. The results show that the surface of the films becomes more homogeneous either by the addition of N2 or after annealing. Deposition rate of the films increases a little at first and then decreases sharply with the increase of N2 source gas flux. It is also found that the fraction of aromatic rings structure increases and the thermal stability of the films is strengthened with the increase of N2 flux. Nitrogen doping is a feasible approach to improve the thermal stability of a-C∶F films.

Structural and mechanical properties of amorphous carbon films deposited by the dual plasma technique

Direct current metal filtered cathodic vacuum arc （FCVA） and acetylene gas （C2H2） were wielded to synthesize Ti-containing amorphous carbon films on Si （100）. The influence of substrate bias voltage and acetylene gas on the microstructure and mechanical properties of the films were investigated. The results show that the phase of TiC in the （111） preferential crystallographic orientation exists in the film,and the main existing pattern of carbon is sp2. With increasing the acetylene flow rate,the contents of Ti and TiC phase of the film gradually reduce; however,the thickness of the film increases. When the substrate bias voltage reaches -600 V,the internal stress of the film reaches 1.6 GPa. The micro-hardness and elastic modulus of the film can reach 33.9 and 237.6 GPa,respectively,and the friction coefficient of the film is 0.25.

Effects of Deposition Temperature on the Properties of Fluorinated Amorphous Carbon Films

Fluorinated amorphous carbon films (a-C:F) were prepared at different temperatures using a microwave electron cyclotron resonance chemical vapor deposition (ECR-CVD) reactor with CHF3 and C2H2 as source gases. Films were annealed at 500℃ in vacuum ambience in order to investigate the relationship of their thermal stability, optical and electrical properties with deposition temperature. Results indicate that the films deposited at high temperature have a less CFX bonding and a more cross-linking structure thus a better thermal stability. They also have a lower bandgap, higher dielectric constant and higher leakage current.

Incorporation of Nitrogen Into Amorphous Carbon Films Produced by Surface-Wave Plasma Chemical Vapor Deposition

In order to study the influence of nitrogen incorporated into amorphous carbon films, nitrogenated amorphous carbon films have been deposited by using surface wave plasma chemical vapor deposition under various ratios of N2/CH4 gas flow. Optical emission spectroscopy has been used to monitor plasma features near the deposition zone. After deposition, the samples are checked by Raman spectroscopy and x-ray photo spectroscopy (XPS). Optical emission intensities of CH and N atom in the plasma are found to be enhanced with the increase in the N2/CH4 gas flow ratio, and then reach their maximums when the N2/CH4 gas flow ratio is 5%. A contrary variation is found in Raman spectra of deposited films. The intensity ratio of the D band to the G band (Id/Ig) and the peak positions of the G and D bands all reach their minimums when the N2/CH4 gas flow ratio is 5%. These show that the structure of amorphous carbon films has been significantly modified by introduction of nitrogen.

Hydrogenated Amorphous Carbon Films from Palmyra Sugar

A simple,highly reproducible,and environmentally friendly method is a considered approach in generating renewable energy materials.Here,hydrogenated amorphous carbon(a-C)films have been successfully prepared from palmyra liquid sugar,employing spin-coating and spraying methods.Compared with the former method,the latter shows a significance in producing a better homogeneity in particle size and film thickness.The obtained films have a thickness of approximately 1000 to 100 nm and contain an sp^(2) hexagonal structure(~70%)and sp^(3) tetrahedral configuration(~30%)of carbons.The introduction of boron(B)and nitrogen(N)as dopants has created the local structural modification of bonding,inducing a slight change of electrical conductivity,electronic energy bandgap,and optical transparency near-infrared region.The obtained α-C film features a“green”semiconducting material.

Deposition of Amorphous Carbon Films using ECR Plasma byVarying the Substrate Temperature

Amorphous hydrogenated carbon thin films have been deposited with benzene plasma in an electron cyclotron resonance (ECR) plasma enhanced chemical vapor deposition system. The characteristic of Benzene discharge plasma has been monitored by Mast spectrometry. It shows that the majority of the plasma species in the downstream ECR Plasma with benzene as gas source are acetylene, ethylene and higher mass species. In the experiments, the effects of the substrate temperature on the deposition rates have been emphatically studied. The structures of the films were analyzed by FTIR and Ramam spectrum.The results show that when the substrate temperature rises, the deposition rate drops down, the hydrogen Foment decreases, with the higher SP3 content being presented in the film.

Effect of Nickel Distributions Embedded in Amorphous Carbon Films on Transport Properties

Electrical properties of C/Ni films are studied using four mosaic targets made of pure graphite and stripes of nickel with the surface areas of 1.78,3.21,3.92 and 4.64%.The conductivity data in the temperature range of400-500 K shows the extended state conduction.The conductivity data in the temperature range of 150-300 K shows the multi-phonon hopping conduction.The Berthelot-type conduction dominates in the temperature range of 50-150 K.The conductivity of the films in the temperature range about 〈 50 K is described in terms of variable-range hopping conduction.In low temperatures,the localized density of state around Fermi level（F）for the film deposition with 3.92% nickel has a maximum value of about 56.2×10^（17）cm^（-3）eV^（-1） with the minimum average hopping distance of about 3.43 × 10^（-6） cm.

Relative Irradiance Measurement and Bonding Configurations of Amorphous Fluorinated Carbon Films Deposited by Electron Cyclotron Resonance Plasma

a-C:F films are deposited by microwave electron cyclotron resonance (ECR)plasma chemical vapor deposition (CVD) using trifluoromethane (CHF3) and benzene (C6H6) as source gases at different microwave powers. The radicals in plasma originating from source gases dissociation are analyzed by relative irradiance measurement. The bonding configurations and binding state of a-C:F films are measured with Fourier-transformed infrared spectrometer (FTIR) and x-ray photoelectron spectroscopy (XPS). The results show that a-C:F films are mainly composed of CF radical at lower powers but of CF2 radical at higher powers. The deposition of films is related to the radicals generated in plasma and the main bonding configurations are dependent on the ratio of CF to CF2 radicals in films.

Relationship Between Thermal Stability and Optical Bandgap of Fluorinated Amorphous Carbon Films

Fluorinated amorphous carbon films were deposited using microwave electron cyclotron resonance chemical vapor deposition (ECR-CVD) reactor with CF4 and C8H6 as source gas and were annealed in nitrogen ambience for the investigating of their thermal stability .The relative concentration of C=C bond and optical bandgap were obtained by Fourier Transform Infrared (FTIR) spectroscopy and Ultraviolet-Visible (UV-VIS ) spectrum, respectively. It has been demonstrated that there is a close relationship between relative concentration of C=C bond and optical bandgap, and the films deposited at a higher microwave power have a lower optical bandgap and a better thermal stability.

Self-lubricated Array Film of Amorphous Carbon Nanorods on an Aluminum Substrate

A self-lubricated array film of amorphous carbon nanorods was prepared by chemical catalytic pyrolysis of acetylene on the anodic aluminum oxide membrane fabricated by two-step anodization of aluminum. The tribological properties of the array film of amorphous carbon nanorods in ambient air were investigated using a ball-on-disk tester at applied loads range from 245 mN to 1960 mN at a sliding velocity of 0.2 m/s. The self-lubricated array film exhibited a small value of the friction coefficient as well as good wear resistance. The friction coefficient of array film of amorphous carbon nanorods decreased gradually with increasing the applied load. The approach proposed demonstrated a new efficient route towards enhanced the friction and wear performances of aluminum.

Metastable hybridized structure transformation in amorphous carbon films during friction-A study combining experiments and MD simulation

Amorphous carbon films have attracted substantial interest due to their exceptional mechanical and tribological properties.Previous studies revealed that the amorphous carbon films exhibited lower coefficient of friction(COF)because of the transformation in bond structure from sp^(3)-C to sp^(2)-C during friction processes.However,the mechanism for such a transformation during friction is not well understood.This study is conducted to get an insight into the metastable transformation in amorphous carbon film during friction by means of experiments and molecular dynamics(MD)simulation.Relevant wear tests showed that wear of the film changed from an abrasive wear mode to a mixture of abrasion and adhesive wear,resulting in a decrease in growth rate of the wear rate after the running-in stage.It is worth noting that the sp^(3)-C atoms were increased during the running-in stage when the films contained lower sp^(3)/sp^(2) ratios.However,the formed sp^(3)-C atoms could only be short-lived and gradually transformed to sp^(2)-C atoms with the graphitization generated on the wearing surface of the films.The radial distribution function and translational order parameter indicated that the films'high sp^(3)/sp^(2) ratio led to an increased sp^(2)-C proportion on the wear scar after friction,which caused an increased structural ordering.

Molecular dynamics simulation of the deposition process of hydrogenated diamond-like carbon (DLC) films

The deposition process of hydrogenated diamond-like carbon (DLC) film greatly affects its frictional properties. In this study, CH3 radicals are selected as source species to deposit hydrogenated DLC films for molecular dynamics simulation. The growth and structural properties of hydrogenated DLC films are investigated and elucidated in detail. By comparison and statistical analysis, the authors find that the ratio of carbon to hydrogen in the films generally shows a monotonously increasing trend with the increase of impact energy. Carbon atoms are more reactive during deposition and more liable to bond with substrate atoms than hydrogen atoms. In addition, there exists a peak value of the number of hydrogen atoms deposited in hydrogenated DLC films. The trends of the variation are opposite on the two sides of this peak point, and it becomes stable when impact energy is greater than 80 eV. The average relative density also indicates a rising trend along with the increment of impact energy, while it does not reach the saturation value until impact energy comes to 50 eV. The hydrogen content in source species is a key factor to determine the hydrogen content in hydrogenated DLC films. When the hydrogen content in source species is high, the hydrogen content in hydrogenated DLC films is accordingly high.

Synthesis and field emission properties of carbon nanotube films modified with amorphous carbon nanoparticles by a simple electrodeposition method

Amorphous carbon nanoparticles （a-CNPs） on a multi-walled carbon nanotube （MWCNT） film, deposited on a silicon substrate, were synthesized using an electrodeposition combination from a methanol suspension of polydiallyldimethylammonium chloride-modified MWCNTs. A low-voltage electropho- retic deposition of the MWCNTs and a high-voltage electrochemical deposition of the a-CNPs were carried out to yield homogenously attached a-CNPs on the surfaces of the MWCNTs, and form a composite film with good adhesion to the substrate. This scalable technology can produce a large area of a-CNP/MWCNT film. And the field emission investigations show that the a-CNP/MWCNT film has turn- on electric field of 3.17 V μm- 1 （at 10 μA cm-2） and threshold field of 4.62 V μm-1 （at 1 mA cm-2）, which are lower than those of the MWCNT film. The a-CNP/MWCNT film can be deposited simply with large areas and may be a promising cathode material applied in field emission displays.

异质a-C:H/a-C:H:F配副的摩擦学行为研究

目的研究F元素掺杂非晶碳基薄膜与a-C:H薄膜摩擦配副的摩擦学行为机制。方法利用PECVD法在Si基底上制备a-C:H:F薄膜,与直径为6.0mm的a-C:H薄膜摩擦对偶球组成摩擦配副体系,使用往复模式的CSMTRB3摩擦机研究a-C:H:F薄膜的摩擦学特性,频率为5Hz,滑动总次数为9000,外加载荷分别为2、4、6、8、10 N。通过纳米硬度、X射线光电子能谱、傅里叶红外光谱、激光共聚焦拉曼光谱、场发射扫描电镜及CSM摩擦试验机等,分别评价a-C:H:F薄膜的结构、表面形貌、力学性能、摩擦学性能等。结果在干摩擦环境下,随着载荷的增加,a-C:H:F薄膜的摩擦因数逐渐降低,平稳后摩擦因数低至0.018。通过掺杂F元素,一方面促进了薄膜的sp^(2)-C杂化,另一方面增大了薄膜的无序度。F元素具有钝化薄膜表面和静电排斥的作用,使得a-C:H:F薄膜具有较低的摩擦因数和磨损率。结论采用PECVD方法制备的a-C:H:F薄膜具有更好的减摩降损能力。

载荷对非晶碳基薄膜腐蚀磨损性能的影响

腐蚀环境条件下的动力件经受腐蚀和磨损的双重损伤,载荷的往复作用使得金属表面的钝化层失去作用而加速损伤,非晶碳基薄膜在腐蚀和摩擦领域展现出优异的防护性能。为了研究载荷对非晶碳基薄膜腐蚀磨损性能的影响,明确各个损失分量随载荷的变化关系,采用磁控溅射在304不锈钢表面制备了a-C薄膜、a-C:Cr薄膜和a-C:Si薄膜,通过动电位极化测试3种薄膜在5.0 mol/L HNO_(3)条件下的静态腐蚀行为,同时结合3种薄膜在去离子水和5.0 mol/L HNO_(3)环境下的不同载荷作用的磨损量,分析研究了载荷对腐蚀摩擦过程中各个分量的影响。结果表明:Si掺杂使a-C薄膜的腐蚀电流密度从1.11×10^(-6 )A/cm^(2)降低至3.96×10^(-7 )A/cm^(2),Cr掺杂后腐蚀电流密度略微降低;随着载荷的增加3种薄膜的腐蚀损伤分量降低,机械磨损分量相应增加,对应的腐蚀-磨损交互作用对薄膜的损伤作用减弱。

基体表面粗糙度对非晶碳薄膜摩擦磨损性能的影响

为了探究不同基体表面粗糙度对非晶碳薄膜摩擦磨损性能的影响,改善2Cr13试样表面在601EF润滑脂润滑下的摩擦学性能,使用机加工、抛光等方式在2Cr13试样表面制备不同的表面粗糙度,然后采用磁控溅射法在不同粗糙度样件表面制备非晶碳薄膜,采用球-盘接触的摩擦磨损试验研究了在601EF润滑脂润滑条件下非晶碳薄膜及氮化硅球的摩擦磨损性能,通过光学显微镜对磨损处进行分析。结果表明:与基体表面对摩的Si_(3)N_(4)球均磨出了明显的平台,而基体表面并未出现明显的磨痕,Ra范围在0.20~0.75μm样品对偶球的磨损量要小,磨斑面积范围为0.152~0.165 mm^(2),磨损形式为黏着磨损,Ra范围在0.95~3.19μm样品对偶球的磨损量要大,磨斑面积范围为0.228~0.275 mm^(2),出现了犁沟状磨痕,磨损较为严重;在相同的工况条件下,随着基体表面粗糙度的增大,两摩擦副间的启动、平均摩擦系数以及Si_3N_4球的磨损量均呈先减小后增大再减小的趋势,当Ra为0.56μm时,非晶碳薄膜的摩擦学性能最好,摩擦系数曲线较为稳定,平均摩擦系数为0.162,其对偶球磨损量最小,为0.152 mm^(2)。基体表面粗糙度影响非晶碳薄膜的摩擦学性能,通过机加工、抛光等手段在基体表面制备合适的粗糙度可以得到具有良好减摩性能的表面。