Influence of field evaporation treatment on the field emission properties of carbon nanotubes array

Field evaporation was used in the post-fabrication treatment of a carbon nanotubes (CNTs) array and effectively modified the CNTs morphology in favor of the field emission under a moderate field. After the field evaporation treatment, the uniformity of the emission site distribution improved but the onset voltage rose. Using the Fowler-Nordheim theory, the actual onset field and the evaporation field around the CNT were calculated to be −4.6-5 and 9-12 V/nm, respectively. These values are close to those obtained from the individual CNT samples. The above results have provided an alternative to modify the configuration of an array sample and demonstrated the feasibility of tackling the problem of the disparity in the field emission capability of different CNTs in an array.     

Preparation of carbon nanosheets deposited on carbon nanotubes by microwave plasma-enhanced chemical vapor deposition method

Carbon nanosheets were synthesized by microwave plasma-enhanced chemical vapor deposition method on carbon nanotubes substrate which was treated by hydrogen plasma. The results showed that the diameters of carbon nanotubes first got thick and then “petal-like” carbon nanosheets were grown on the outer wall of carbon nanotubes. The diameters of carbon nanotubes without and with carbon nanosheets were 100-150 and 300-500 nm, respectively. Raman spectrum indicated the graphite structure of carbon nanotubes/carbon nanosheets. The hydrogen plasma treatment and reaction time greatly affected the growth and density of carbon nanosheets. Based on above results, carbon nanosheets/carbon nanotubes probably have important applications as cold cathode materials and electrode materials.     

Carbon nanosheets by microwave plasma enhanced chemical vapor deposition in CH4-Ar system

We employ a new gas mixture of CH₄-Ar to fabricate carbon nanosheets by microwave plasma enhanced chemical vapor deposition at the growth temperature of less than 500 °C. The catalyst-free nanosheets possess flower-like structures with a large amount of sharp edges, which consist of a few layers of graphene sheets according to the observation by transmission electron microscopy. These high-quality carbon nanosheets demonstrated a faster electron transfer between the electrolyte and the nanosheet surface, due to their edge defects and graphene structures.     

Synthesis of amorphous carbon nanowalls by DC-PECVD on different substrates and study of its field emission properties

Amorphous carbon thin films with quasi vertical nanowall-like morphologies have been synthesized via direct current plasma enhanced chemical vapor deposition on both copper and silicon substrates with acetylene as a carbon precursor. The deposition temperature and pressure were maintained at 750 °C and 5 mbar respectively. The morphology of the as-prepared samples has been investigated with the help of a field emission scanning electron microscope and an atomic force microscope, both revealing nanowall-like morphologies with thicknesses of the walls ∼6-15 nm. The as-prepared carbon nanowalls showed good field electron emission with a turn-on field as low as 1.39 V/μm. The effect of inter-electrode distance on the field electron emission has also been studied in detail.     

Simulation and fabrication of carbon nanotubes field emission pressure sensors

A novel field emission pressure sensor has been achieved utilizing carbon nanotubes (CNTs) as the electron source. The sensor consists of the anode sensing film fabricated by wet etching process and multi-wall carbon nanotubes (MWNTs) cathode in the micro-vacuum chamber. MWNTs on the silicon substrate were grown by thermal CVD. The prototype pressure sensor has a measured sensitivity of about 0.17-0.77 nA/Pa (101-550 KPa). The work shows the potential use of CNTs-based field-emitter in microsensors, such as accelerometers and tactile sensors.     

Carbon fractals grown from carbon nanotips by plasma-enhanced hot filament chemical vapor deposition

Carbon nanomaterials with different structures were prepared in a custom-designed plasma-enhanced hot filament chemical vapor deposition system using methane, hydrogen and nitrogen. They were investigated by scanning electron microscopy (SEM) and micro-Raman spectroscopy. The SEM images show that the smooth carbon nanotips are formed under a high bias current and the carbon fractals can grow from the tips of the carbon nanotips under a low bias current. The results of micro-Raman spectroscopy indicate that the graphitization of the carbon nanomaterials was improved by ion bombardment. Combined the ion bombardment, electric field enhancement and electron emission mechanisms, the formation model of the carbon fractals was suggested.     

Stable electron field emission from carbon nanotubes emitter transferred on graphene films

Carbon nanotubes (CNTs) arrays grown by microwave plasma enhanced chemical vapor deposition (MPCVD) method was transferred onto the substrate covered with graphene layer obtained by thermal chemical vapor deposition (CVD) technology. The graphene buffer layer provides good electrical and thermal contact to the CNTs. The field emission characteristics of this hybrid structure were investigated in this study. Compared with the CNTs arrays directly grown on the silicon substrate, the hybrid emitter shows better field emission performance, such as high emission current and long-term emission stability. The presence of this graphene layer was shown to improve the field emission behavior of CNTs. This work provides an effective way to realize stable field emission from CNTs emitter and similar hybrid structures.     

Field emission properties and synthesis of carbon nanotubes grown by rf plasma-enhanced chemical vapor deposition

Plasma-enhanced chemical vapor deposition (PECVD) method was employed to grow the Fe-catalyzed carbon nanotubes (CNTs). The grown CNTs with a uniform diameter in the range of about 10-20 nm and the typical lengths beyond 1 μm resulted in a very high aspect ratio. The Raman and TEM results showed that the grown CNTs contained a large amount of carbonaceous particles and crystal defects, such as pentagon-heptagon pair defects. XPS measurement indicated that the CNTs had CH covalent bonds. Field emission characteristics exhibited the low turn-on threshold field of 2.75 V/μm and the maximum emission current density of 7.75 mA/cm² at 6.5 V/μm. The growth mechanism of CNTs and the effects of hydrogen plasma on their structure were discussed.     

场致发射阴极碳纳米管的热化学气相沉积法低温生长

结合丝网印刷和过滤阴极真空电弧法、离子束溅射方法，在普通玻璃衬底上制备催化剂图案，采用低温热化学气相沉积法(CVD)生长碳纳米管/纤维(CNTs)薄膜.研究了不同种类催化剂对CNTs薄膜生长及其场发射的影响.结果表明，在a-C:Co,Ni-Cu和Cu三种催化剂上没有获得明显的CNTs，在外加电场小于4.4V/μm时没有观察到场发射；而在Ni-Fe及Ni-Cr两种催化剂上获得了大量的CNTs，并且表现出良好的场发射性能，开启电场为2.5V/μm，这种热CVD有简单、低温等优点，在CNTs场发射显示器的阴极制备中有潜在的应用价值.     

Effects on the field emission properties by variation in surface morphology of patterned photosensitive carbon nanotube paste using organic solvent

Photosensitive carbon nanotube (CNT) paste was prepared by 3-roll milling of multi-walled carbon nanotubes (MWNTs), UV-sensitive binder solution, and Ag as filler additives. Arrays of MWNT dots with a diode structure were fabricated by a combination of screen printing method and photolithography using these paste, and acetone utilized as the developer. The MWNT dots were well-defined and the organic binder materials in the dots were partially removed. The MWNT film without a heat treatment showed a high current density of 1.35 mA/cm² at 3.25 V/μm and low turn-on field of 2.2 V/μm at 100 μA/cm². Acetone can be used as an efficient developer to form patterns and to remove the organic residues in patterns, simultaneously.     

Enhanced field emission characteristics of boron doped diamond films grown by microwave plasma assisted chemical vapor deposition

Boron doped diamond films were synthesized on silicon substrates by microwave plasma chemical vapor deposition (MPCVD) technique. The effect of B₂O₃ concentration varied from 1000 to 5000 ppm on the field emission characteristics was examined. The surface morphology and quality of films were characterized by scanning electron microscope (SEM) and Raman spectroscopy. The surface morphology obtained by SEM showed variation from facetted microcrystal covered with nanometric grains to cauliflower of nanocrystalline diamond (NCD) particles with increasing B₂O₃ concentration. The Raman spectra confirm the formation of NCD films. The field emission properties of NCD films were observed to improve upon increasing boron concentration. The values of the onset field and threshold field are observed to be as low as 0.36 and 0.08 V/μm, respectively. The field emission current stability investigated at the preset value of ∼1 μA is observed to be good, in each case. The enhanced field emission properties are attributed to the better electrical conductivity coupled with the nanometric features of the diamond films.     

Synthesis of a composite consisting of carbon nanotubes and graphite shell-encapsulated cobalt nanoparticles using plasma-enhanced chemical vapor deposition

We report a simple and effective one-step synthesis route for synthesizing a composite consisted of carbon nanotubes (CNTs) and graphite shell-encapsulated cobalt nanoparticles using plasma-enhanced chemical vapor deposition on Si (1 0 0) substrate covered with catalyst Co particles, discharging a mixture of H₂ and CH₄ gas, and characterize the obtained composite by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high resolution transmission electron microscope, and X-ray photoelectron spectroscopy. The results show that CNTs align perpendicularly to the substrate and graphite shell-encapsulated Co nanoparticles clung to the external surfaces of aligned CNTs. The diameter of the graphite shell-encapsulated Co nanoparticles increases with increasing the H₂ content in H₂ and CH₄ carbonaceous gas. A possible growth mechanism of the CNTs and graphite shell-encapsulated cobalt nanoparticles composite has been explored.     

Growth kinetics of low temperature single-wall and few walled carbon nanotubes grown by plasma enhanced chemical vapor deposition

Single-wall, double walled or few walled nanotubes (FWNT) are grown by electron cyclotron resonance plasma enhanced chemical vapor deposition (ECR-PECVD) at temperature as low as 600 °C. Most of these structures are isolated and self-oriented perpendicular to the substrate. The growth mechanism observed for single-wall and few walled (less than seven walls) nanotubes is the “base-growth” mode. Their grow kinetics is investigated regarding two parameters namely the growth time and the synthesis temperature. It is shown that nucleation and growth rate is correlated with the number of walls into FWNT. It also provides an evidence of a critical temperature for FWNT synthesis.     

Oxygen contaminants affecting on the electronic structures of the carbon nano tubes grown by rapid thermal chemical vapor deposition

Oxygen-related electronic structures of CNTs (carbon nanotubes) grown by rapid thermal chemical vapor deposition (RT-CVD) have been investigated by using partial electron yield near edge X-ray absorption spectroscopy (PEY-NEXAFS) and X-ray photoelectron spectroscopy (XPS). On the CNT surface with increased oxygen resulting from e-beam irradiation under the O₂ gas environment, C k-edge NEXAFS spectra showed an increase of the oxygen-related resonance peaks ranging from 287 to 289 eV whereas the sp² related peak at 285.4 eV was nearly unchanged. After the complete removal process of the oxygen atom on the surface by annealing the sample at 500 °C for 30 min, C K-edge spectra showed an abrupt decrease of the oxygen-related resonance peaks in 287-289 eV and an increase of the sp² related peak at 285.4 eV, indicating that the degree of crystallinity in the CNT sample was improved.     

Field emission properties of carbon nanotubes film grown on NiCr alloy films

Carbon nanotubes (CNTs) are prepared on NiCr alloy films by low pressure thermal chemical vapor deposition at 600 °C. NiCr alloy films are deposited by magnetic co-sputtering method, and the various thickness and Ni/NiCr ratios are controlled by sputtering power. The diameter and length of CNTs, as well as the roughness of the CNTs films, mainly depend on the Ni/NiCr ratio. The field emission current density of the CNTs film increases with the increasing Ni/NiCr ratio from 65 wt% to 83 wt%, and decreases when the Ni/NiCr ratio is more than 87 wt% in the alloy film.     

InN纳米线的低压化学气相沉积及其场发射特性研究

利用低压化学气相沉积方法在以Au作催化剂的Si衬底上生长了InN纳米线. 扫描电子显微镜分析表明，这些纳米线的直径在60—100 nm的范围内, 而其长度大于1 μm.高分辨透射电子显微镜图像表明，合成的纳米线中含有六方相和立方相的InN晶体.这些InN纳米线具有良好的场发射特性和稳定的场发射电流，其开启场为10.02 V/μm(电流密度为10 μA/cm²)，在24 V/μm 的电场下，其电流密度达到5.5 mA/cm².此外，对InN纳米线的场发射机理也进行了讨论. 关键词： InN纳米线 场电子发射 非线性Fower-Nordheim曲线     

Effects of Cs treatment on field emission properties of capped carbon nanotubes

Within the methodology [M. Khazaei, A.A. Farajian, Y. Kawazoe, Phys. Rev. Lett. 95 (2005) 177602] based on first-principles electronic structure calculations, the effects of Cs treatment on current emissions and emission patterns of capped carbon nanotubes are considered at low deposition densities when the nanotubes are under an electric field 0.2 V/Å. The results show that the current emission from the cap with one adsorbed Cs is 3.4 times larger than the cap without any Cs. It is 9.6 times larger in the cap with two adsorbed Cs atoms. After Cs deposition the emission patterns become asymmetric (current emission from the carbon atoms located at the topmost pentagon ring close to Cs atoms is larger than the other atomic sites). There are very few localized states on Cs atoms. Hence, although the tunneling probability of electron emission from Cs atoms is significant, there is no current from Cs atoms. In addition, the effect of Cs on work function reduction of nanotubes is explained in terms of Cs deposition densities and the surface dipole moments.     

Photoluminescence properties of SiNx/Si amorphous multilayer structures grown by plasma-enhanced chemical vapor deposition

Very thin (nanometric) silicon layers were grown in between silicon nitride barriers by SiH₂Cl₂/H₂/NH₃ plasma-enhanced chemical vapor deposition (PECVD). The multilayer structures were deposited onto fused silica and silicon substrates. Deposition conditions were selected to favor Si cluster formation of different sizes in between the barriers of silicon nitride. The samples were thermally treated in an inert atmosphere for 1 h at 500 °C for dehydrogenation. Room-temperature photoluminescence (RT-PL) and optical transmission in different ranges were used to evaluate the optical properties of the structures. UV-VIS absorption spectra present two band edges. These band edges are well fitted by the Tauc model typically used for amorphous materials. RT-PL spectra are characterized by strong broad bands, which have a blue shift as a function of the deposition time of the silicon layer, even for as-grown samples. The broad luminescence could be associated with the confinement effect in the silicon clusters. After annealing of the samples, the PL bands red shift. This is probably due to the thermal decomposition of N-H bonds with further effusion of hydrogen and better nitrogen passivation of the nc-Si/SiN_x interfaces.     

Atmospheric pressure plasma treatment on graphene grown by chemical vapor deposition

We demonstrate the surface treatment of graphene using an atmospheric pressure plasma jet (APPJ) system. The graphene was synthesized by a thermal chemical vapor deposition with methane gas. A Mo foil and a SiO₂ wafer covered by Ni films were employed to synthesize monolayer and mixed-layered graphene, respectively. The home-built APPJ system was ignited using nitrogen gas to functionalize the graphene surface, and we studied the effect of different treatment times and interdistance between the plasma jet and the graphene surface. After the APPJ treatment, the hydrophobic character of graphene surface changed to hydrophilic. We found that the change is due to the formation of functionalities such as hydroxyl and carboxyl groups. Furthermore, it is worth noting that the nitrogen plasma treatment induced charge doping on graphene, and the pyridinic nitrogen component in the X-ray photoelectron spectroscopy spectrum was significantly enhanced. We conclude that the atmospheric pressure plasma treatment enables controlling the graphene properties without introducing surface defects.     

Selective growth, characterization, and field emission performance of single-walled and few-walled carbon nanotubes by plasma enhanced chemical vapor deposition

Single-walled carbon nanotubes (SWCNTs) and few-walled carbon nanotubes (FWCNTs) have been selectively synthesized by plasma enhanced chemical vapor deposition at a relative low temperature (550 °C) by tuning the thickness of iron catalyst. The parametric study and the optimization of the nanotube growth were undertaken by varying inductive power, temperature, catalyst thickness, and plasma to substrate distance. When an iron film of 3-5 nm represented the catalyst thickness for growing FWCNT arrays, SWCNTs were synthesized by decreasing the catalyst thickness to 1 nm. The nanotubes were characterized by field emission scanning electron microscopy, transmission electron microscopy, and Raman spectroscopy. Electron field emission properties of the nanotubes indicate that the SWCNTs exhibit lower turn-on field compared to the FWCNTs, implying better field emission performance.