吸附重金屬原子之石墨烯奈米帶中自旋相關傳輸性質

自2004年被製備出來後，石墨烯作為第一個被發現之二維材料，一直是相關領域十分熱門之研究主題，而石墨烯作為傳輸材料其獨有的特性，如: 極長之自旋鬆弛時間、良好之電子移動率、內秉自旋偶合效應、製造方便…等，也使得石墨烯以及石墨烯自旋傳輸特性的相關研究對現今當紅之自旋電子學占有十分重要的地位。然而，由於石墨烯中其內秉之自旋偶合效應強度非常微弱，使得一些理論上預期出現之自旋現象較難在原始石墨烯中發現，其中較為知名的現象有二維自旋霍爾效應(也稱二維拓樸絕緣態)，一種邊緣有電流但內部為絕緣體之特殊電導狀態。此外，原始石墨烯中微弱的自旋偶合效應也非常難以被操縱並應用於製造自旋電子相關元件。為了處理以上這些問題，如何增強石墨烯其內在之自旋偶合效應便成為了一個極有潛力的研究方向。而當石墨烯上吸附重金屬原子(如:金、銦、鉈…等)之時，能大幅增強石墨烯內之自旋偶合效應強度。值得一提的是，穩定的量子自旋霍爾效應態已經被預測能在吸附銦原子或鉈原子之單層石墨烯上實現。但在石墨烯吸附金原子時，由於系統內之Rashba效應的影響大於內秉之自旋偶合效應，因此金原子被認為會破壞所吸附石墨烯之量子自旋霍爾效應。本論文以緊束縛近似法搭配非平衡狀態格林函數，模擬吸附金原子或鉈原子之石墨烯奈米帶內自旋相關之傳輸特性；並藉由這些結果比較吸附此兩種重金屬原子對石墨烯奈米帶造成之差異。在這兩種系統中引起的Rashba效應與內秉之自旋偶合效應對自旋傳輸特性；如:自旋鬆弛、電荷分佈、電導率…等均造成重大影響，而透過比較這些特性，我們有望能更深入了解二維石墨烯內之Rashba效應與內秉之自旋偶合效應之間的關係。本論文的模擬結果利用系統之自旋邊緣態以及量化電導率，驗證了吸附鉈原子之石墨烯內的量子自旋霍爾效應，並也指出吸附金原子之石墨烯將破壞量子自旋霍爾效應態。除此之外，模擬結果也證實吸附金原子之石墨烯具有較快速之自旋鬆弛。

關鍵字

吸附原子；重金屬原子；石墨烯奈米帶；非平衡狀態格林函數；量子自旋霍爾效應；自旋鬆弛；拓樸電子態

並列摘要

Graphene, a two-dimensional material which has become active since first created by Scotch tape method at 2004. Due to some advantageous characteristics in spintronic such as the long spin-relaxation time at room-temperature, carrier high mobility, and easy to fabricate, the study of the spin properties of graphene has turned into a significant topic at the last decade. However, the intrinsic spin-orbital coupling (SOC) in graphene is too weak to observe Quantum Spin Hall effect, a specific property which theoretical expected and also known as 2D Topological Insulator. Furthermore, the weak intrinsic SOC is also difficult to manipulate. Therefore, enhance intrinsic SOC or induce large extrinsic SOC in graphene, has become a new direction to investigate the spin transmission in graphene. Adsorbing heavy-metal adatoms such as Thallium (Tl), Indium (In), Gold (Au) randomly on graphene can increase a large SOC, and a robust quantum spin hall (QSH) state in Thallium(TI) adatom Graphene was predicted by an ab-initio article in 2011. Nevertheless, we can’t see QSH state in Gold (Au) adatoms graphene. It may due to the Rashba effect which is induced by the mirror symmetry-breaking at Z-plane is dominant than the intrinsic SOC and destroy QSH state in gold adatoms graphene. In this thesis, we use the Tight Binding model and Non-equilibrium Green’s Function (NEGF) approach to simulate the electron transport properties in graphene nanoribbons (GNR) which decorated with Au and TI adatoms. The aim of this study is to investigate the difference between Au and TI adatoms GNR. The distinguishable SOC and Rashba effect in both cases make a huge impact and change the spin-dependent transport properties such as spin relaxation, charge distribution, and electrical conductivity. By comparing those differences, it may help us to clarify the influences of SOC and Rashba effect in graphene. %Most importantly, adsorbing atom on graphene may be a promising direction for manipulate graphene Our results supports that the Rashba effect which is induced in gold adatom graphene nanoribbons is harmful for the QSH phase, and we also use the NEGF method to examine the quantized conductance and chiral edge state in Tl adatom graphene nanoribbons, the both prosperity under time-reversal symmetry is the evidence of the QSH phase in Tl adatom graphene nanoribbons. Moreover, the faster Spin-relaxation in gold adatom graphene is also demonstrated in our simulation.

並列關鍵字

Adatom ； Heavy-metal atoms ； Graphene nanoribbons ； Quantum spin Hall eff ect ； Spin-relaxation ； Topological electronic states

參考文獻

[1] Stefano Sanvito . ”Molecular spintronics” Chem. Soc. Rev., 2011,40, 33363355(2011)

Google Scholar

[2] T.C. Dinadayalane , J. Leszczynski ”Fundamental Structural, Electronic, and Chemical Properties of Carbon Nanostructures: Graphene, Fullerenes, Carbon