npj: 蛋盒石墨烯—奔向電子材料的基礎性突破
摘要：他們發現這些新型石墨烯同素異形體獨特的幾何形態使得該類材料具有更高的力學強度和各異的電子性能。The major hindrances of implementing graphene in two-dimensional (2D) electronics are both mechanical (the tendency to crumble and form ripples) and electrical (the lack of a band gap). Moreover, the inevitable structural defects in graphene have a profound influence on its physical and chemical properties. Here, we propose a family of 2D egg-tray graphenes constructed by arranging pentagon and heptagon defects in the graphene lattice based on a careful analysis of the topological distribution of minima, maxima, and saddle points. First-principles calculations show that the egg-tray graphenes are dynamically stable, and their energies, which depend on the concentration of pentagons and heptagons, are the lowest among carbon allotropes. These 2D carbon allotropes exhibit a large variation in their electronic properties, ranging from semimetallic to semiconducting, including some allotropes that have Dirac cones in their band structures. Furthermore, some egg-tray graphenes are predicted to have negative Poisson’s ratios. The adsorption of Li atoms on the egg-tray graphenes is considerably stronger than the adsorption on perfect graphene, therefore they may absorb Li more effectively than graphene, which is important for improving the performance of rechargeable Li batteries.。
石墨烯是目前學術界和工業界最有前途的材料之一。儘管石墨烯表現出許多優異的性能，但有兩個關鍵問題限制了它的應用。作爲單原子層材料，石墨烯很容易摺疊或形成褶皺。另外它缺乏帶隙，很難成爲大規模集成電路的基底材料。那麼能否通過改變或調製石墨烯的幾何形態來改變它的性能，從而達到同時克服上述兩個困難的目的呢？
石墨烯之所以容易摺疊和形成皺褶是由於它只由一個原子層組成，正如一張紙容易打皺變形一樣。可是當把一個紙板壓制成有着高低起伏波紋狀結構，形如超市中用來盛裝雞蛋的紙板盒的時候，其力學強度和性能就會瞬間得到大幅度的提升。那麼關鍵的問題是能否在石墨烯的六角蜂窩狀原子晶格上構建出雞蛋盒樣的幾何形態。
由美國加州州立大學北嶺分校苗茂生教授、北京計算科學研究中心的林海清教授和吉林大學的劉靖堯教授共同組成的研究團隊，對這一有趣的重要問題展開了多年的理論計算研究。這一課題的主要承擔人是目前任教於浙江農林大學理學院的劉偉副教授。他們首先注意到，石墨烯中的五元環和七元環缺陷能夠使石墨烯片層產生不同的拓撲形變，包括極大、極小值點和鞍點。如果把這些缺陷有規律地排布，就能讓石墨烯形成類似雞蛋盒的幾何形變。通過改變缺陷的密度和排布方式，各種不同形態的蛋盒石墨烯都可以被構造出來。利用第一性原理計算方法，劉偉副教授和他的合作者們研究了一系列蛋盒石墨烯的穩定性，以及力學性質和電子結構。他們發現這些新型石墨烯同素異形體獨特的幾何形態使得該類材料具有更高的力學強度和各異的電子性能。對應於不同的幾何形態，蛋盒石墨烯可以是半金屬，也可以是具有直接或間接帶隙的半導體。還有一些蛋盒石墨烯的能帶結構中具有狄拉克錐結構。值得注意的是，除了提高力學強度外，一些蛋盒石墨烯還顯示出負的泊松比，即當蛋盒石墨烯在沿原子層平面內某一方向受壓的時候，它在該方向上以及原子層平面內該方向的垂直方向上都會發生收縮。該項研究工作提出了一種有效改善石墨烯力學和電子性能的新方法，可爲實現更多具有優良或獨特性能的新型石墨烯基材料的設計提供進一步的理論指導。
該文近期發表於npj Computational Materials 5: 71 (2019)，英文標題與摘要如下，點擊https://www.nature.com/articles/s41524-019-0211-2可以自由獲取論文PDF。
Building egg-tray shaped graphenes that have superior mechanical strength and band gap
Wei Liu, Lei Zhao, Eva Zurek, Jing Xia, Yong-hao Zheng, Hai-qing Lin, Jing-yao Liu & Mao-sheng Miao
The major hindrances of implementing graphene in two-dimensional (2D) electronics are both mechanical (the tendency to crumble and form ripples) and electrical (the lack of a band gap). Moreover, the inevitable structural defects in graphene have a profound influence on its physical and chemical properties. Here, we propose a family of 2D egg-tray graphenes constructed by arranging pentagon and heptagon defects in the graphene lattice based on a careful analysis of the topological distribution of minima, maxima, and saddle points. First-principles calculations show that the egg-tray graphenes are dynamically stable, and their energies, which depend on the concentration of pentagons and heptagons, are the lowest among carbon allotropes. These 2D carbon allotropes exhibit a large variation in their electronic properties, ranging from semimetallic to semiconducting, including some allotropes that have Dirac cones in their band structures. Furthermore, some egg-tray graphenes are predicted to have negative Poisson’s ratios. The adsorption of Li atoms on the egg-tray graphenes is considerably stronger than the adsorption on perfect graphene, therefore they may absorb Li more effectively than graphene, which is important for improving the performance of rechargeable Li batteries.
