Investigation On The Interfacial Charge Transfer Of Graphene By Raman Spectroscopy

The two-dimentional (2D) planar structure and unique electron structure of graphene endow it many outstanding properties, especially its excellent electrical properties make it be the most potential material in future nanoelectronics. However, graphene has a large specific surface area, and every carbon atom of it is exposured outside, which makes the device performances greatly influenced by the charge transfer at graphene and its related interface. Graphene-enhanced Raman scattering (GERS) is a new Raman enhancement effect, which is strongly related to graphene's unique electronic structure. However, the exact mechanism of GERS is still not clear, which seriously restricts its future application.This dissertation aim for revealing the mechanism of GERS and the mechanism of hysteresis effect in the graphene FETs. We developed the approach of electrical-field modulated GERS, and used this method to study the relationship between the charge transfer at graphene/molecule interface and the GERS. Besides, we studied the relationship between the charge transfer at graphene/SiO2interface and the hysteresis effects in graphene FETs by using Raman spectroscopy. Based on the above findings, substrate engineering strategy for suppressing the hysteresis effect has been developed.Main research results are listed below:(1) Revealed the chemical enhancement mechanism of graphene-enhanced Raman scatteringWe revealed that the interface charge transfer at graphene/molecule interface is responsible for the mechanism of GERS through an electrical field modulated GERS approach. The Raman intensities of metal-phthalocyanine (M-Pc) molecules become weaker when the graphene Fermi level is up-shifted by applying a positive gate voltage, while they become stronger when the graphene Fermi level is down-shifted by applying a negative gate voltage. The relationship between Raman modulation and gate voltage sweep rate shows the arising of hysteresis effect. Far from having a negative effect, this phenomenon suggests the link between Raman intensity and Fermi level in graphene. Besides, from the study on the effect of energy level of molecule to GERS by comparing the modulation ability of different M-Pc molecules, which have the same molecular structure but different molecular energy level, we found that the LUMO levels of the molecules determine the Raman enhancement. Finally, we found that this modulation shows the greatest one on single-layer graphene and mainly comes from the first layer of molecules which are in direct contact with graphene. Above results prove that the Raman enhancement for GERS occurs through an charge-transfer chemical enhancement mechanism.(2) Study on the relationship between energy alignment of graphene/molecule and its charge-transfer chemical Raman enhancement.We realized the continual modulation to GERS when decrease the effect of gate hysteresis by performing the measurement under vacuum condition. The relationship between the energy alignment of graphene/molecule and the charge transfer Raman enhancement was studied by enlarging the graphene Fermi level modulation window through introducing O2(n-doping) and NH3(p-doping) atmospheres. As the graphene Fermi level is up-shift, the energy gap between graphene Fermi level and LUMO of molecule is far away from the laser energy, which causes the decrease of charge transfer ability, and the decrease of Raman enhancement. On the contrary, when the graphene Fermi level is down-shift, the energy gap between graphene Fermi level and LUMO of the molecule becomes close to the laser energy, which increases the charge transfer ability and the Raman enhancement. There is a large modulation to GERS when graphene Fermi level variation range is above the Dirac point, while that shows a smaller modulation when graphene Fermi level variation range is below the Dirac point, implying there is a large charge-transfer resonance window in GERS, and the Raman enhancement shows exponentially decrease with the charge-transfer resonance window decrease.(3) The intrinsic mechanism of hysteresis effect in graphene FETs.Based on the relationship between doping, Fermi level and Raman spectra of graphene, we used Raman spectroscopy to study the intrinsic mechanism of hysteresis effect in graphene FETs. By monitoring the doping of graphene and the gate hysteresis in graphene FETs under different atmosphere using in situ Raman spectroscopy, we confirm that the electrochemical doping graphene by O2/H2O redox couple is responsible for the hysteresis effect. In addition, Raman spectra of graphene on SiO2substrate showed stronger doping effect than that of the suspended, which indicates that SiO2substrate plays an important role on the doping of graphene. From the analysis to surface chemistry of SiO2, we propose that the doping species (H2O and O2) are bounded at the interface of graphene/SiO2substrate by hydrogen bond with the silanol groups on SiO2substrate. This work provides a clear view to the mechanism of hysteresis effect, and is useful to the design of reliable graphene FETs with suppressed hyeresis.(4) Substrate engineering for suppression of hysteresis effects and constructing graphene p-n junction.Based on the relationship between doping, Fermi level and Raman spectra of graphene, we used Raman spectroscopy to study the intrinsic mechanism of hysteresis effect in graphene FETs. By monitoring the doping of graphene and the gate hysteresis in graphene FETs under different atmosphere using in situ Raman spectroscopy, we confirm that the electrochemical doping graphene by O2/H2O redox couple is responsible for the hysteresis effect. In addition, Raman spectra of graphene on SiO2substrate showed stronger doping effect than that of the suspended, which indicates that SiO2substrate plays an important role on the doping of graphene. From the analysis to surface chemistry of SiO2, we propose that the doping species (H2O and O2) are bounded at the interface of graphene/SiO2substrate by hydrogen bond with the silanol groups on SiO2substrate. This work provides a clear view to the mechanism of hysteresis effect, and is useful to the design of reliable graphene FETs with suppressed hysteresis.