Lasek Kinga, Coelho Paula Mariel, Zberecki Krzysztof, Xin Yan, Kolekar Sadhu K, Li Jingfeng, Batzill Matthias
Department of Physics, University of South Florida, Tampa, Florida 33620, United States.
Faculty of Physics, Warsaw University of Technology, ul. Koszykowa 75, 00-662 Warsaw, Poland.
ACS Nano. 2020 Jul 28;14(7):8473-8484. doi: 10.1021/acsnano.0c02712. Epub 2020 Jun 29.
Material growth by van der Waals epitaxy has the potential to isolate monolayer (ML) materials and synthesize ultrathin films not easily prepared by exfoliation or other growth methods. Here, the synthesis of the early transition metal (Ti, V, and Cr) tellurides by molecular beam epitaxy (MBE) in the mono- to few-layer regime is investigated. The layered ditellurides of these materials are known for their intriguing quantum- and layer dependent- properties. Here we show by a combination of sample characterization and comparison with computational predictions that ML ditellurides with octahedral 1T structure are readily grown, but for multilayers, the transition metal dichalcogenide (TMDC) formation competes with self-intercalated compounds. CrTe, a TMDC that is known to be metastable in bulk and easily decomposes into intercalation compounds, has been synthesized successfully in the ML regime at low growth temperatures. At elevated growth temperatures or for multilayers, only the intercalation compound, equivalent to a bulk CrTe, could be obtained. ML VTe is more stable and can be synthesized at higher growth temperatures in the ML regime, but multilayers also convert to a bulk-equivalent VTe compound. TiTe is the most stable of the TMDCs studied; nevertheless, a detailed analysis of multilayers also indicates the presence of intercalated metals. Computation suggests that the intercalation-induced distortion of the TMDC-layers is much reduced in Ti-telluride compared to V-, and Cr-telluride. This makes the identification of intercalated materials by scanning tunneling microscopy more challenging for Ti-telluride. The identification of self-intercalation compounds in MBE grown multilayer chalcogenides may explain observed lattice distortions in previously reported MBE grown early transition metal chalcogenides. On the other hand, these intercalation compounds in their ultrathin limit can be considered van der Waals materials in their own right. This class of materials is only accessible by direct growth methods but may be used as "building blocks" in MBE-grown van der Waals heterostructures. Controlling their growth is an important step for understanding and studying the properties of these materials.
通过范德华外延进行材料生长有潜力分离出单层（ML）材料，并合成难以通过剥离或其他生长方法制备的超薄膜。在此，研究了通过分子束外延（MBE）在单层至少数层范围内合成早期过渡金属（Ti、V和Cr）碲化物。这些材料的层状二碲化物因其有趣的量子和层依赖性特性而闻名。在此，我们通过样品表征以及与计算预测结果的比较表明，具有八面体1T结构的ML二碲化物易于生长，但对于多层结构，过渡金属二硫属化物（TMDC）的形成与自插层化合物相互竞争。CrTe是一种已知在体相中不稳定且容易分解为插层化合物的TMDC，已在低生长温度下的ML范围内成功合成。在较高的生长温度下或对于多层结构，只能获得与体相CrTe等效的插层化合物。ML VTe更稳定，可以在ML范围内较高的生长温度下合成，但多层结构也会转变为与体相等效的VTe化合物。TiTe是所研究的TMDC中最稳定的；然而，对多层结构的详细分析也表明存在插层金属。计算表明，与V - 和Cr - 碲化物相比，Ti - 碲化物中插层引起的TMDC层畸变大大减少。这使得通过扫描隧道显微镜识别Ti - 碲化物中的插层材料更具挑战性。在MBE生长的多层硫属化物中识别自插层化合物可能解释了先前报道的MBE生长的早期过渡金属硫属化物中观察到的晶格畸变。另一方面，这些处于超薄极限的插层化合物本身可被视为范德华材料。这类材料只能通过直接生长方法获得，但可在MBE生长的范德华异质结构中用作“构建块”。控制它们的生长是理解和研究这些材料特性的重要一步。
