| Summary: | 碩士 === 國立臺灣海洋大學 === 光電科學研究所 === 107 === In this thesis, we focus on the photo-assisted electron-field-emission from graphene. Usage of graphene edge as the electron source of field emission to conducts electron in vacuum by electric field, the emission properties and photoelectric characteristics was analyzed. When incident the electric field of the laser light perpendicularly on the edge of the graphene, we observed the change of the field emission current. While the analyze the change of the field emission currents we found the possible mechanism of the light-assisted electron field emission by comparing the gain of the laser-assisted electron field emission current.
The graphene electron source was used in this experiment that is chemical vapor deposited (CVD) using a high-temperature furnace tube to use copper foil as a catalyst as well as the substrate to grow graphene on it. Graphene-coated copper foil is mechanically stressed (pull-break method) to make the edge of the graphene on the copper foil protrude. We called it graphene probe, and the probe is hung on the tungsten wire as a heating source to complete the graphene electron source.
Using the graphene edge as a probe, the laser incident is on the tip of the needle, its direction is parallel to needle tip, and we observed that when the fixed tungsten heat source (H) is heated at 0.034 W and the 535 nm laser of 50 mW irradiate on the probe. The starting voltage of the electron field emission is E = 11.97 V/μm, and without laser irradiate the starting voltage is E = 14.23 V/μm. In the experiment we had known that the laser can assist electron field emission for the edge of the graphene.
We compare the change of the field emission current of the laser irradiate to the probe when laser turned on and off. We fix that tungsten heat source (H) and electric field (E) and using the 50 mW 535 nm laser irradiate on the probe. We show that the electron field emission current has a periodic change.
Under the same conditions, it's found that when lasers with different powers are used to illuminate the probe, there will be different current gains of electron field emission, and the 535 nm laser with a power of 50 mW is most stable enhancement, and can reach about 70% of the light current gain. Under the fixed electric field and the external heating source, the output of the laser power is nonlinear with the current of the electron field emission, and the field enhancement factor (β) is increased as the laser power is increased. The 50 mW, 535 nm laser field enhancement factor can reach 12675.82.
Finally, we found the field emission current variation with phase angle of the incident laser when the the direction of the electric field of the laser is perpendicular to the edge of the graphene. It will parallel to the direction of the pz orbital (free π electrons exist in the pz orbital domain), so when the electric of the laser field is superimposed on the direction of the pz orbital, it is easy to induce electrons to leave the edge of the graphene. These electrons then accelerated to the anode due to applied electric field.
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