S-Space College of Engineering/Engineering Practice School (공과대학/대학원) Dept. of Electrical and Computer Engineering (전기·정보공학부) Theses (Ph.D. / Sc.D._전기·정보공학부)
HIGHLY ELECTRICALLY CONDUCTIVE TUNGSTEN DOPED REDUCED GRAPHENE OXIDE FILM AS LARGE CURRENT DENSITY FIELD EMITTER : 높은 전기전도를 가진 텅스텐 도핑된 환원된 산화그래핀 고전류 전계방출소자
- Gun-Sik Park
- 공과대학 전기·컴퓨터공학부
- Issue Date
- 서울대학교 대학원
- rGO film ; tungsten doping ; improved physical properties ; field emission ; stability ; THz electron source
- 학위논문 (박사)-- 서울대학교 대학원 : 공과대학 전기·컴퓨터공학부, 2018. 2. Gun-Sik Park.
- Exploration of the Terahertz(THz) gap, in the field of medical, defense, communication, material processing etc, primarily suffering from the lack of high power, efficient sources, which can be supplemented vacuum electron devices (VED). But to realize practical THz range VEDs, cathode with extreme current density is in immense need.
In this thesis, we are motivated to synthesize and develop a field emitter cathode, based on graphene film that could deliver current density up to 10000A/cm2 as an electron source for THz VEDs. Previous studies were carried out on the influence of various cathode parameters such as, free standing nature, thermal and electrical conductivity, mechanical strength for better stability and long life, thickness and width for controlling the current density, to get a deeper understanding of the mechanism of field emission and the improvement & stability of emission performance. Further analysis show that the emission performance is influenced by the presence of impurities such as oxygen functional groups etc. That is if the impurity is less, emission would be higher. Also, the thermo-electrical properties and mechanical stability has an important role to play regarding the stability during operation, which is degraded by those above mentioned impurities. Considering all the before mentioned aspect, a chemical synthesis process was developed to synthesize impurity free, freestanding reduced graphene oxide (rGO) film with improved physical properties. It is known that doping of metal to rGO films improves the physical properties by several times and consequently the field emission performance. Previous reports and study have shown that if the physical properties have been improved by at least two times comparing to as prepared rGO, stable emission can be achieved even when the emission current density is being doubled or more than that. The reason behind this can be explained as, the improved properties can handle the higher thermo-electrical load generated by the larger field emission current. Prepared tungsten doped reduced graphene oxide (W-rGO) film was characterized using, Field Emission Scanning Electron Microscope (FESEM) to study the surface morphology and the thickness. X-ray Photoelectron Spectroscopy (XPS) was used to study the surface chemical & elemental information, doping & doping homogeneity, and carbon-carbon bonding status. Further, X-ray Diffraction Spectroscopy(XRD) was carried out to study the doping mechanism and layer distance information. Raman spectroscopy was utilized to study the effect of tungsten doping on carbon network morphology and improvement in carbon-carbon bond. It was found that the doped film has much more improved structure than the as-prepared film.
Experimental characterization was carried out on both, the doped and un doped films to compare the improvement in physical properties. It was found that doped film, in all three cases, such as thermal, electrical and mechanical, have much more improved properties than the non-doped films. Field emission experiment was carried out inside of an ultra-high vacuum (UHV) chamber on both of the films. The emission results are extremely positive. A very large current density of ~ 6000A/cm2 was achieved at an applied electric field of 7.4 V/µm, with a superior stability from the doped films where un-doped films, though initially delivered a current density of 4300A/cm2, eventually got degraded and was able to deliver only a current density of ~ 2300A/cm2. It is of an interesting phenomenon that the I-V characteristics have a growing nature for higher anode fields. This means that, with more efficient setup, where high voltage breakdown can be controlled, higher current density can be achieved.