金属薄层结构相变驱动均质多层六方氮化硼化学气相沉积生长

来源：ACS美国化学会

英文原题: Chemical Vapor Deposition Growth of Uniform Multilayer Hexagonal Boron Nitride Driven by Structural Transformation of Metal Thin Film

通讯作者: Hiroki Ago 日本九州大学

作者: Yuki Uchida, Kenji Kawahara, Satoru Fukamachi and Hiroki Ago

此前的研究表明，(多层)六方氮化硼(h-BN)，可以作为阻隔材料屏蔽周围环境的影响。在由六方氮化硼和其他二维材料组合而成的异质结构中，这种屏蔽特性可以帮助人们排除环境影响而观察到那些二维材料的本征新奇物性。因此，高质量的(多层)六方氮化硼在二维材料研究中至关重要，不可或缺。然而，制备大面积高质量多层六方氮化硼仍然十分困难，且其制备生长机制也尚未明确。

在这项研究工作中，作者以沉积在蓝宝石不同晶面上的Ni-Fe薄层为催化剂，研究了该Ni-Fe薄膜催化多层六方氮化硼进行化学气相沉积(CVD)生长的过程。实验发现，在Ni-Fe/r-sapphire表面上可以生长高度均匀的多层六方氮化硼薄膜，其覆盖度远高于在沉积于蓝宝石a或者c面的Ni-Fe薄膜上生长的样品。电子背散射衍射(EBSD)和X射线衍射(XRD)表征发现，多层六方氮化硼在Ni-Fe/r-sapphire上的均匀析出过程同时伴随着Ni-Fe层一个显著的从多晶向面心立方[fcc(111)]单晶相的结构转变。截然不同的是，在c-sapphire和a-sapphire表面生长的Ni-Fe薄膜层在与borazine前驱体反应前就已经呈fcc(111)相，并在B、N析出过程中不会发现结构转变。作者认为这种特殊的Ni-Fe多晶—单晶结构转变与B、N均匀析出的过程高度相关，并在利用尖晶石作为衬底的Ni-Fe薄膜上进一步验证了这一猜想。

这项工作表明，金属催化剂薄膜的结构演化在高质量均匀六方氮化硼生长过程中扮演关键角色。它为制备合成高质量均匀多层六方氮化硼薄膜指明了一个可能的探索方向，即控制金属催化剂的晶体学性质。

图1. 在沉积于蓝宝石r表面的Ni-Fe薄膜上实现均匀多层h-BN生长。光学显微图像，截面TEM图像和拉曼光谱证实制备的六方氮化硼具有很高质量，这对二维电子学至关重要。

图2. 多层六方氮化硼的独特生长机制。Ni-Fe催化剂层在h-BN生长中的晶体结构演化过程。Ni-Fe催化剂层的晶体学转变辅助实现六方氮化硼生长由，从而导致其独特的均匀析出生长过程。

本研究的相关结果已发表于ACS Applied Electronic Materials，并被选登为期刊的内页封面文章进行报道。

英文介绍原文

Hexagonal boron nitride (h-BN), in particular multilayer h-BN, has played an important role in the research of two-dimensional materials by enabling the observation of their intrinsic and excellent physical properties via effective screening of the influences from surrounding environment.  However, it is still difficult to synthesize high-quality multilayer h-BN in large scale, and its growth mechanism is not clearly understood. 

Here, we investigated the chemical vapor deposition (CVD) growth of multilayer h-BN using thin Ni-Fe films that are deposited on sapphire substrates with different crystal planes.  The Ni-Fe film on r-plane sapphire was found to produce a uniform multilayer h-BN sheet whose surface coverage is much higher than those on c- and a-plane sapphire.  Electron back-scatter diffraction (EBSD) and X-ray diffraction (XRD) investigations revealed that the uniform segregation of multilayer h-BN on Ni-Fe/r-sapphire occurs simultaneously with a drastic structural change of the Ni-Fe thin film from polycrystalline to face-centered cubic (fcc(111)) structure.  On the other hand, the Ni-Fe films on c- and a-plane sapphire possessed the fcc(111) structure even before the reaction with borazine feedstock and did not show such structural change.  The unique crystallographic change of the Ni-Fe thin film associated with uniform h-BN segregation was further supported by the studies using spinel substrates. 

Our work indicates the essential role of dynamic evolution of the crystal structure of thin-film metal catalyst in the h-BN growth, highlighting the importance of the controlling crystallographic properties of the metal catalyst for the synthesis of high-quality and uniform multilayer h-BN films.