Boron-Doped Nanocrystalline Diamond-Carbon Nanospike Hybrid Electron Emission Source.

The influence of the carbon network structure of polycrystalline diamond films that were prepared from a mixture of H2, CH4, and N2 using microwave-enhanced plasma chemical vapor deposition on electron field emission has been systematically investigated. With increasing nitrogen gas flow ratio of [ N2]/[H2 + CH4 + N2], the film hardness and surface roughness of the as-grown films decreased, and the concentration ratio of amorphous sp2-bonded carbon clusters and mixed sp2−sp3 carbon structures to tetrahedrally bonded amorphous carbon phases increased. Correspondingly, the turn-on voltage for electron emission decreased. After the surface post-treatment by pure hydrogen plasma exposure, the concentration ratio was clearly found to have increased dramatically and the turn-on voltage decreased significantly for the films produced at small nitrogen flow ratio. Our results suggest that the influence of the concentration ratio on electron field emission is much more significant than that of the surface roughness of the polycrystalline diamond films studied in this paper.

A transparent titanium dioxide and carbon nanowall composite (B-CNW/TiO2) material was fabricated by growing boron-doped carbon nanowalls (B-CNWs) on quartz glass with microwave plasma-enhanced chemical vapor deposition technique, followed by sol-gel deposition using titanium isopropoxide as a TiO2 precursor. Different layer thicknesses were fabricated. Samples were investigated by spectroscopic ellipsometry and UV-VIS spectroscopy. Results shows how the B-CNW thickness affects the optical transmittance, bandgap and electrical conductivity. Full Text: PDF ReferencesWang, H., Quan, X., Yu, H. & Chen, S. Fabrication of a TiO2/carbon nanowall heterojunction and its photocatalytic ability. Carbon N. Y. 46, 1126–1132 (2008). CrossRef Fujishima, A., Zhang, X. & Tryk, D. A. TiO2 photocatalysis and related surface phenomena. Surf. Sci. Rep. 63, 515–582 (2008). CrossRef Pierpaoli, M., Giosuè, C., Ruello, M. L. & Fava, G. Appraisal of a hybrid air cleaning process. Environ. Sci. Pollut. Res. 24, 12638–12645 (2017). CrossRef Sobaszek, M. et al. Diamond Phase (sp3-C) Rich Boron-Doped Carbon Nanowalls (sp2-C): Physicochemical and Electrochemical Properties. J. Phys. Chem. C 121, 20821–20833 (2017). CrossRef Lewkowicz, A. et al. Thickness and structure change of titanium(IV) oxide thin films synthesized by the sol–gel spin coating method. Opt. Mater. (Amst). 36, 1739–1744 (2014). CrossRef Tauc, J. Optical properties and electronic structure of amorphous Ge and Si. Mater. Res. Bull. 3, 37–46 (1968). CrossRef

Si-containing crystalline carbon nitride derived from microwave plasma-enhanced chemical vapor deposition

Boron-doped nano-crystalline diamond (NCD) thin films have been successfully deposited on well-polished poly-crystalline diamond (PCD) thick films in a microwave plasma enhanced chemical vapor deposition (MPCVD) reactor for the first time. Different surface pretreatment techniques are carried out under different gas conditions (CH4, H2, Ar, and CH4/H2) to eliminate the effect of grain boundaries on the growth of a smooth NCD intrinsic layer. Well doped NCD films have been fabricated in CH4/H2/B2H6 plasma by varying the atomic ratio of B/C and the substrate temperature. Atomic force microscopy (AFM) results show that pretreatment in pure CH4 plasma at 1000°C is most effective for NCD growth, while hydrogen containing plasma is harmful to the surface smoothness of NCD thin films. Doping research indicates that the optimum parameters for the boron-doping of high-quality NCD thin films are B/C=300 ppm (10−6) and 800°C.

The physicochemical and mechanical properties of thin and freestanding heavy boron-doped diamond (BDD) nanosheets coated with a thin C:H:N:O plasma polymer were studied. First, diamond nanosheets were grown and doped with boron on a Ta substrate using the microwave plasma-enhanced chemical vapor deposition technique (MPECVD). Next, the BDD/Ta samples were covered with nylon 6.6 to improve their stability in harsh environments and flexibility during elastic deformations. Plasma polymer films with a thickness of the 500–1000 nm were obtained by magnetron sputtering of a bulk target of nylon 6.6. Hydrophilic nitrogen-rich C:H:N:O was prepared by the sputtering of nylon 6.6. C:H:N:O as a film with high surface energy improves adhesion in ambient conditions. The nylon–diamond interface was perfectly formed, and hence, the adhesion behavior could be attributed to the dissipation of viscoelastic energy originating from irreversible energy loss in soft polymer structure. Diamond surface heterogeneities have been shown to pin the contact edge, indicating that the retraction process causes instantaneous fluctuations on the surface in specified microscale regions. The observed Raman bands at 390, 275, and 220 cm−1 were weak; therefore, the obtained films exhibited a low level of nylon 6 polymerization and short-distance arrangement, indicating crystal symmetry and interchain interactions. The mechanical properties of the nylon-on-diamond were determined by a nanoindentation test in multiload mode. Increasing the maximum load during the nanoindentation test resulted in a decreased hardness of the fabricated structure. The integration of freestanding diamond nanosheets will make it possible to design flexible chemical multielectrode sensors.

Microwave (mw) breakdown of atmospheric pressure microgaps is studied by a one-dimensional Particle-in-Cell Monte Carlo Collisions numerical model. The effect of both field electron emission and secondary electron emission (due to electron impact, ion impact, and primary electron reflection) from surfaces on the breakdown process is considered. For conditions where field emission is the dominant electron emission mechanism from the electrode surfaces, it is found that the breakdown voltage of mw microdischarge coincides with the breakdown voltage of direct-current (dc) microdischarge. When microdischarge properties are controlled by both field and secondary electron emission, breakdown voltage of mw microdischarge exceeds that of dc microdischarge. When microdischarge is controlled only by secondary electron emission, breakdown voltage of mw microdischarge is smaller than that of dc microdischarge. It is shown that if the interelectrode gap exceeds some critical value, mw microdischarge can be ignited only by electrons initially seeded within the gap volume. In addition, the influence of electron reflection and secondary emission due to electron impact is studied.

The bibliography includes references on field electron emission from metals and semiconductors, work functions, phase transformations, adsorption, and field ion emission. Over 100 references to technical literature for the period 1955 to 1959 are given and notations concerning English translations are provided whenever they are available. (J.R.D.)

Nanocrystalline diamond film as cathode for gas discharge sensors

The properties of capacity-type microplasma devices were significantly enhanced due to the utilisation of hybrid diamond films as cathodes. The performance of the microplasma devices was closely correlated with the electron field emission (EFE) properties of the diamond cathode materials. The nanoemitters, which were prepared by growing duplex-structured diamond films [microcrystalline diamond (MCD)/ultra-nanocrystalline diamond (UNCD)] on Si-pyramid templates via a two-step microwave plasma enhanced chemical vapour deposition (MPE-CVD) process, exhibited improved EFE properties (E0 = 5.99 V μm(-1), J(e) = 1.10 mA cm(-2) at 8.50 V μm(-1) applied field), resulting in superior microplasma device performance (with a lower threshold field of 200 V mm(-1) and a higher plasma current density of 7.80 mA cm(-2)) in comparison with UNCD film devices prepared using a single-step MPE-CVD process. The superior EFE properties of the duplex-structured MCD-UNCD films relative to those of the UNCD films can be attributed to the unique granular structure of the diamond films. High-resolution transmission electron microscopy reveals that the MCD-UNCD films consisted of abundant graphitic phases located at the periphery of large diamond aggregates and at the boundaries between the ultra-small diamond grains. The presence of the graphite phase is presumed to be the prime factor that renders these films more conductive and causes these films to exhibit higher EFE properties, thus resulting in the improved plasma illumination properties of the microplasma devices.

A new statistical method for determining the key factors of the process of field electron emission (FEE) from nanostructures based on a comparison of the microscopic measurements of the work function of electrons with the results of a macroscopic characterization of the FEE is presented. Using the example of nanocrystalline carbon films, the possibilities of this statistical method for a quantitative estimation of the influence of the local field enhancement and the work function reduction on the FEE threshold decrease are demonstrated.

Vacuum arcing (also known as breakdown), is a major limiting factor in various applications such as particle accelerators, fusion reactors etc. Although it is well-established that vacuum arcs appear after intense Field electron Emission (FE), the physical mechanism that leads from FE to the ignition of plasma is not yet understood. A common hypothesis is that intense FE leads to excessive heating of the cathode, which causes its deformation, and eventually material evaporation and plasma formation. However, this process has never been observed experimentally or fully understood theoretically. Here we present atomistic simulations of this process that give an insight to it and provide possible mechanisms that can explain the initiation of plasma. Our simulations take into account various physical processes, namely field-induced stresses, electron emission with finite-size and space-charge effects, Nottingham and Joule heating. We find that under certain conditions the cathode apex melts and the field-induced stress deforms it to become longer and sharper. This initiates a positive feedback process that leads to extremely high local temperatures, and causes evaporation of material. This mechanism might give a plausible explanation to the initiation of plasma.

To improve the process controllability and fabrication uniformity of porous silicon (PS)-based electron emission devices (EEDs), we employed an epitaxial (epi) silicon film as the functional layer, leveraging its advantages of high crystalline quality and flexibility of resistivity modulation regardless of the substrate. Precise modulation of the epi film resistivity was achieved via ion implantation. We investigated the effects of resistivity modulation on the fabrication process and device performance. This scheme enabled the formation of PS through electrochemical etching without illumination, and therefore etch self-termination. As a direct result, the etching uniformity in both the vertical and horizontal directions is enhanced. It then facilitated the optimization of the oxidation of the PS surface, which is essential for EED performance. The devices exhibited a maximum electron emission current density (Je) of 80 μA/cm2 with high stability. Driven under DC mode at a bias voltage (Vps) of 23 V, Je decreased temporarily to 28 μA/cm2 after 4 h of continuous operation. This study provides a new feasible approach for research on PS EEDs.

This paper investigated the electron emission properties of self-assembled GaAs nanowhiskers (NWs) grown by MBE which includes electron emission current, potential difference dependences in the vacuum gap between the probe tip and GaAs NWs surface. The analysis shows that the electron emission from the NWs nanostructure surface is the result of electron tunneling from the indicated above surfaces into the probe material. The question of barrier shape still remains open. Possible influence of size effects for GaAs NWs was not revealed. The estimates of the emission current density have shown that the current density is very high and equal to about 105 A/cm2

Carbon nanotubes (CNTs) were selectively grown on the proximity region of silicon tips by microwave plasma-enhanced chemical vapor deposition. The silicon substrate with silicon conical tips was sputtered with palladium to act as a nanocluster catalytic center for CNT nucleation. Curled and randomly oriented CNTs with diameters ranging from 150 to 200 nm were observed to grow selectively on surrounding areas of the silicon tips. Electron field emission tests show a low turn-on field of 3.2 V/μm. An emission current of 8.0 μA was achieved at ∼6.3 V/μm. These results show the ability to grow CNTs selectively on the silicon tip region from a catalytic metal covered surface, which may have practical applications.

Effect of plasma modulation on the nucleation and crystal evolution of nanodiamond seeds during CVD diamond growth