| Summary: | 碩士 === 國立中山大學 === 化學系研究所 === 103 === Graphene, a prospective material with novel band structure makes it pertinent to the future electronic technology. Chemical functionalization is effective to modulate the electronic properties of graphene. Forming covalent bonds can change the carbon hybridization from sp2 to sp3 state, in collaboration with its reaction reversibility, to the graphene band gap of graphene can be switched. In my research, the cycloaddition reactions of CVD-grown single-layer graphene with a series of π-conjugated 5-membered heterocycles were investigated.. Under ultrahigh vacuum conditions, temperature-programmed desorption (TPD) was utilized to characterize the bonding strengths and the desorbing species, and reflection-absorption infrared spectroscopy (RAIRS) measurements assisted to identify the adsorption geometry of the corresponding adsorbates. After cycloaddition with heterocycles in vacuum, the graphene samples were removed out for ex-situ Raman inspection. The relative intensities of D and G to 2D band are indicative of the functionalized graphene. Finally, density functional theory calculations were conducted to predict the bonding geometry and adsorption energies which help to interpret the experimental data.
According to Haddon’s report, graphene can be functionalized via Diels-Alder reactions attributed to its dual roles as diene or dienophile. In this study, maleic anhydride (MA) was selected as a dienophile to demonstrate the feasibility of the DA reaction. The TPD spectra show desorption peak at 420 K, reasonably attributable to the temperature for retro-DA reaction. The RAIR spectra acquired from a graphene sample reveal a transition from the sp2-CH stretch to sp3-CH stretch after background subtraction and baseline correction, a definitive evidence for graphene functionalization. In addition, the DFT and Raman spectra results of the MA-graphene system strongly suggest the possibility of surface modification.
We also tested the DA reactivity of graphene with several other 5-membered heterocycles, such as furan, pyrrole and substituted furan derivatives. The issues about the aromaticity, the electronic effects of substituted groups and the steric effects were thoroughly investigated. However, the TPD and RAIR results from sample to sample are not quite correlated, and the RAIR spectra indicated that the 5-membered heterocycles are unreactive towards the cycloaddition reactions. Graphene functionalization is not adequate to account for the observed D-band emergence in the Raman spectra. DFT calculations revealed that noncovalent interaction is dominant on the graphene surface.
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