Ming-Chi Kan

Thermally activated electron emission from nano-tips of amorphous diamond and carbon nano-tubes

The sp2 character of graphite can carry electricity like a metal and the sp3 character of diamond can emit electrons in vacuum like an insulator. In this research, we have studied two ways of combining the electrical conductance and electron emission properties in carbon materials. In one case, the two characters are mingled atomistically to form a rather uniform mixture of amorphous diamond. Alternatively, graphite basal planes are wrapped around to form nano-tubes that exhibit a slight diamond character. Both amorphous diamond and carbon nano-tubes (CNTs) contain emission tips of nanometer sizes. When they are connected to a negative bias, they can emit electrons in vacuum toward an anode at very low turn-on field. However, when the cathode material is heated up, the responses of electron emission in vacuum are dramatically different between the two types of carbon materials. At a temperature of 300 °C, amorphous diamond can emit 13 times more electrons than it did at room temperature; but CNTs show no response to thermal agitation. The thermally sensitive emission of amorphous diamond indicated that electrons climbed up an energy ladder to reach the vacuum level for efficient emission. The energy ladder contained minute but discrete energy levels that were created by distorting the tetrahedral bonds of carbon atoms to different degrees. On the other hand, the CNTs are substantially graphitic so the energy gap between their conduction band and valence band overlap and are continuous in energy. In this case, electrons could not acquire energy to reach the vacuum energy unless the temperature could be sufficiently high. Hence, it confirms that CNTs emit electrons primarily by enhancing the applied field on nano-tips. This is in contrast to amorphous diamond that emits electrons by combining graphitic carbon atoms and diamond-like carbon atoms of various states.

Effects of substrate bias on nanocrystal-(Ti,Al)N x / amorphous-SiN y composite films

Nanocrystal-(Ti,Al)N x /amorphous-SiN y composite films were prepared in a codeposition process under different substrate bias voltages. The effects of substrate bias voltage on the deposition rate, composition, microstructure, and mechanical properties of nanocomposite films were investigated. Results indicated that the films with bias voltages caused resputtering due to the bombardment of high-energy ions on film surface. The resputtering effect had substantial influence on deposition rate, surface morphology, and composition of films. The films with (220) preferred orientation were also observed as the applied substrate bias voltages exceeded 50 V. As the substrate bias voltage increased, the nanocrystallite size increased, lattice strain raised, and the hardness decreased.

Field emission of micro aluminum cones coated by nano-tips of amorphous diamond

Abstract Nano-tips of amorphous diamond films were deposited on Al micro cones to form a metal/diamond interface structure. The amorphous diamond films were deposited by cathodic arc at a temperature less than 150 °C. The turn on applied field was 4.5 V/μm at 10 μA/cm2 for the Al/diamond structure. The turn on applied field is approximately 10 times lower than that for Al alone. The high emission current was obtained (50 mA/cm2) due to metal/diamond structure and the presence of defect band within band gap of amorphous diamond that allowed electrons to pass through with little hindrance. Moreover, the stability of electron emission was monitored at electrical field strength of 7 V/μm for 100 h. The fluctuations of long-term emission current were at ±4% during the entire duration of monitoring. The fluctuation may be due to the absorption and desorption of molecules by the dangling bonds of carbon atoms on surface of nano-tips of amorphous diamond in a vacuum chamber during electron emission. The high stability of electron emission form micro Al cones by nano-tips amorphous diamond coatings was observed.

Electron emission from nanotips of amorphous diamond

Amorphous diamond can be deposited with a high-density (4×1010 emitters/cm2) of nano-sized emitters. The turn on applied field strength was reduced by increasing aspect ratio of amorphous diamond nanotips. Moreover, the field emission was highly sensitive to the aspect ratio of tips, and relatively inert to the sp3/(sp3+sp2) ratio. The lowest turn on applied field strengths was 4.6 V/μm at the current density of 10 μA/cm2; and 11 V/μm at the current density of 10 mA/cm2. High reproducibility of field emission was also observed in this study.

Nano-tip emission of tetrahedral amorphous carbon

Abstract Various forms of tetrahedral amorphous carbon were deposited on n-type silicon (100) substrate. Tetrahedral amorphous carbon is an excellent electron emitter for field emission array (FEA) applications. The negative electron affinity (NEA) of diamond surface was believed to facilitate the electron emission for diamond-like carbon. However, in this research, we have demonstrated that the geometric enhancement factor may be even more effective to promote electron emission. Moreover, we have succeeded in depositing high-density (4×10 10 emitters/cm 2 ) of nano-sized emitters. The electrons are emitted under both the effect of enhanced field due to the sharp tips, and the optimized distortion of tetrahedral bonding of carbon atoms. The sp 3 /(sp 3 +sp 2 ) ratio of C–C bonded tetrahedral amorphous carbon films was measured by ESCA and Raman; and the geometry of emitters by AFM. The lowest turn on applied field strength of 4.6 V/μm was found at the current density of 10 μA/cm 2 . High reproducibility of field emission was also observed in this study.

Investigation of nanocrystal-(Ti,Al)N1−x/amorphous-SiNy composite films

Nanocrystal-(Ti,Al)N1−x/amorphous-SiNy composite films were prepared by the magnetron sputtering codeposition process. The effects of deposition parameters on the composition, microstructure, and mechanical properties of nanocomposite films were investigated. Results indicated that the composition of nanocomposite films could be controlled through the codeposition process. Nanocrystal (Ti,Al)N1−x embedded in amorphous SiNy matrix with (200) preferred orientation was observed. The content of amorphous SiNy has substantial influence on the nanocrystallite size, columnar structure, surface morphology, and hardness.

Amorphous Diamond Solar Cell

Amorphous diamond has the least work function for electron emission in vacuum. It is also the most thermionic material possible. Hence, amorphous diamond can be used as thermally activated energy converter, such as a solar cell. Amorphous diamond solar cells may have the potential to generate electricity with cost on the par with power plants in the near future. The implementation of amorphous diamond solar cells may usher human civilization to the availability of unlimited clean energy.