藉由熱輔助佔據密度泛函理論對鋰吸附鋸齒型石墨烯奈米帶儲氫性質的研究

Abstract

本論文以熱輔助佔據密度泛函理論(thermally-assisted-occupation density functional theory,TAO-DFT)，在廣義密度梯度近似(generalized gradient approximation,GGA)加以色散校正(dispersion correction)下研究鋰吸附鋸齒型石墨烯奈米帶(Li-adsorbed zigzag graphene nanoribbons)之電子性質與儲氫性質。

本文發現在電子性質方面，不論是單純的鋸齒型石墨烯奈米帶或是鋰吸附鋸齒型石墨烯奈米帶，在基本能隙(Fundamental gap)、對稱馮諾伊曼熵(symmetrized von Neumann entropy)以及軌道佔據數目(orbital occupation number)的計算結果都顯示此系統具有多重參考性(multi-reference)，而鋰原子的加入會使此特性更為顯著，這點與之前研究結果類似，也正面說明本文之所以採用熱輔助佔據密度泛函理論而不使用柯恩-沈(Kohn-Sham)理論的原因。

在儲氫性質部分，本文發現在固定寬度為兩個苯環的鋰吸附鋸齒型石墨烯奈米帶的長度小於等於三個苯環時，氫氣可以分子的形式在物理吸附其上，且其吸附能量位於或接近理想範圍，此外本所研究的系統在吸附氫氣的重量百分率上亦有優勢。

This thesis employs TAO-DFT (thermally-assisted-occupation density functional theory) under the GGA (generalized gradient approximation ) with dispersion correction to study the hydrogen storage and electronic properties of Li-adsorbed zigzag graphene nanoribbons.

Regarding electronic properties, this study finds that both pristine zigzag graphene nanoribbons and Li-adsorbed zigzag graphene nanoribbons exhibit multi-reference characteristics, as evidenced by calculations of the fundamental gap,orbital occupation number, and symmetrized von Neumann entropy. The addition of lithium atoms further enhances this multi-reference nature. These findings are consistent with some previous studies and substantiate the rationale for using TAO-DFT instead of Kohn-Sham density functional theory.

When it comes to hydrogen storage , this study shows that when the width of Li-adsorbed zigzag graphene nanoribbons is fixed at two benzene rings and the length is less or than or equal to three benzene rings, hydrogen molecules are able to be physically adsorbed on the nanoribbons, with energy of adsorption falling within or near the ideal range. Furthermore, the studied system demonstrates an advantage in terms of hydrogen adsorption weight percentage.