Perspective: Graphene and other 2D materials

 （注：本文译自Kostya Novoselov等教授为我刊FOP撰写的Perspective文章，即将上线）

        过去几年石墨烯[1-3]从纯粹的实验室材料到现实生活应用，其研究取得了巨大飞跃式的进展[4,5]。这种碳同素异形体（关于碳其他形式的综述见[6]）极大地丰富了物理实验和可能应用的范围。目前，市场上出现了数百种商品，其中石墨烯在改善产品的性能和功能方面发挥着重要的（有时是主导的）作用。这里仅提供两个例子：石墨烯和其他二维材料可用于设计具有预定属性的微波屏蔽和吸收器[7]。此外，由于没有带隙和电荷载流子的非常高的迁移率，石墨烯可以用于宽带、超快光电子器件[8]和光学调制器[8,9]。而且，在这种材料的常规基础上我们仍然可以看到新的物理学正在不断被探索中。

石墨烯最重要的“性质”是它为更多二维材料的发现和研究开辟了道路[10]。现在有很多这样的材料可供使用[11]。它们共同覆盖了绝缘体、半导体、半金属和金属等很大的功能空间。最近来自各国的科学家们在二维材料中观察到了超导性[12]和铁磁性[13-15]。当达到单层极限时，通常晶体与3D对应物相比开始表现得极不寻常。例如，许多过渡金属二硫化物（TMDC）成为单层形式的直接带隙超导体，而它们在双层和较厚的晶体中具有间接带隙[16]，因此，这使得这种材料特别适用于光电应用[17]。此外，由于利用了分层结构，可以提高MoS₂的催化性能[18-20]。

众多的二维晶体结构使范德瓦尔斯异质结构的概念栩栩如生[21-24]。随着化学键饱和，二维晶体之间可以通过范德华力相互作用，从而保留了单个晶体的主要电子和结构特征。以最简单的方式，这种概念用于封装对环境敏感的原子晶体。因此，用六方氮化硼（hBN）封装的石墨烯显示出与悬浮装置[26-29]相当的电子质量[25]。在研究空气敏感材料的性质时，这种封装被证明是至关重要的，例如，用于硅光子应用的NbSe₂用于超导[13]或MoTe₂[30]。

已经通过由薄层hBN隔开的两个石墨烯单层实现了更复杂的异质结构。在这种结构中，两层中电子之间的相互作用导致库仑阻力[31]和电子-电子相关效应[32]。此外，如果hBN层足够薄-在这种异质结构中可以观察到隧道电流[33-35]。由于石墨烯的功函数可以通过外部栅极容易地控制（由于石墨烯中的低密度状态）-这种隧穿器件可以用作隧道晶体管，这已经由几个小组证实[36,37]。有趣的是，这种异质结构中的隧道电流主要取决于两个石墨烯电极之间的错误取向。如果两个石墨烯层在结晶学上对齐-可以在保持动量的情况下发生隧道效应。然而，这种影响依赖于栅极和偏置电压，并导致负微分电阻实验现象的观察[38-41]。如果两个电极在结晶学上是错误定向的-动量的守恒只能通过散射杂质[34-42]或声子[34,43,44]来实现。

通常，通过堆叠中的晶体之间的相对角度来控制这种装置的电子特性甚至电子结构的能力是范德瓦尔斯异质结构的独特特征。即使在hBN上石墨烯的最简单情况下，如果两个晶体对齐，两个晶体的原子间距离之间的紧密匹配导致莫尔结构的出现，且周期为14.5nm。在这种结构上的电子散射将导致强烈的光谱重建和在电子和孔侧的二次狄拉克点的出现[45-49]。此外，在磁场中，莫尔结构的周期性与磁通量子之间的相互作用导致明显的Brown-Zak振荡可观察到室温[50]。同样，两种不同TMDC晶体之间的相对角度对于观察间接激子至关重要[51]。此外，通过将不同的晶体堆叠在一起不单单可以改变其电子结构，还可以调控其声子结构[52]。 组装范德华异质结构时可能的主要效果是由于相邻晶体的功函数的差异导致的电荷转移。通过这种原子级薄pn结的形成能被应用到例如在开发新型光电探测器[53-55]中。

很明显，范德瓦尔斯的概念非常丰富，将导致更多令人兴奋的现象和更多新颖的应用设备。此外，随着越来越多的2D材料在理论上被预测（例如，具有直接禁带隙的材料GeAsSe和SnSbTe [56]）并且每天都在实验中发现，可能的异质结构范围也会大幅增长。此外，与其他纳米结构物体（例如γ-AlOOH纳米片作为柔性可重复使用的基底[57]）的组合可以进一步扩展这些器件的功能。尽管如此，科学家们要想将它们从实验室转移到现实生活应用还需要大量的努力工作。

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“石墨烯与其他二维材料”专题详见：

http://journal.hep.com.cn/fop/EN/collection/showCollection.do?id=191

Cover

Ultrathin GeAsSe and SnSbTe sheets own desirable electronic and optical properties. Monolayer GeAsSe and SnSbTe sheets can be easily exfoliated from the bulk crystals due to the weak interlayer binding energies. These sheets are energetically favorable and show excellent dynamical and thermal stability. Importantly, monolayer GeAsSe and SnSbTe possess moderate direct band gaps and superior hole mobility up to 20000 cm²·V⁻¹·s⁻¹, and meanwhile exhibit notable absorption in the visible region. The appropriate band edge positions ensure that layered GeAsSe and SnSbTe materials are promising photocatalysts for water splitting. For more details, please refer to the article “Monolayered semiconducting GeAsSe and SnSbTe with ultrahigh hole mobility ” by Yu Guo, Nan Gao, Yizhen Bai, Jijun Zhao, and Xiao Cheng Zeng. [Photo credits: Jijun Zhao, Dalian University of Technology]