Molecular Beam Epitaxy of Transition Metal (Ti-, V-, and Cr-) Tellurides: From Monolayer Ditellurides to Multilayer Self-Intercalation Compounds

Abstract

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 fewlayer 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 in situ 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 2 , 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 Cr 3 Te 4 , could be obtained. ML VTe 2 is more stable and can be synthesized at higher growth temperatures in the ML regime, but multilayers also convert to a bulk-equivalent V 3 Te 4 compound. TiTe 2 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.

Synthesis and Properties of Monolayer MnSe with Unusual Atomic Structure and Antiferromagnetic Ordering

“…In the case of CrTe 2 , it is difficult to grow high-quality monolayers due to Cr self-intercalation. 13 As for VSe 2 , there are several reports indicating either the presence or the lack of ferromagnetism. 16 − 19 …”

Intercalation Strategy in 2D Materials for Electronics and Optoelectronics

“…For instance, Cr 3 Te 4 with Cr intercalated within the interlayer of CrTe 2 was obtained by epitaxial growth on vdW substrates through MBE. [ 108 ] Tantalum self‐intercalated TaS y , from 25% Ta‐intercalated Ta 9 S 16 to 100% Ta‐intercalated Ta 9 Se 12 , were accessed through MBE or chemical vapor deposition (CVD). [ 40 ] Figure 7g shows the growth mechanism of Ta atoms self‐intercalated TaS y .…”

Two‐dimensional magnetic transition metal chalcogenides

“…Epitaxial Cr 2 S 3 , 58 CrSe, 56 CrTe 2 , 59 CrTe, 60 and Cr 2 Te 3 61–63 thin films with thicknesses down to several nanometers have been grown on various substrates (mica, SrTiO 3 , Si, Bi 2 Se 3 , and so on) by CVD and MBE, showing high Curie temperature T c (e.g., ~305 K for 8.7 nm CrTe 2 thin film 59 ), and promising electrical transport properties (e.g., unconventional anomalous Hall effect 60 ) for spintronic devices. The successful synthesis of 2D Cr 2 S 3 , 55,58 Cr 2 Te 3 , 57 and CrTe 2 26 with ML thickness (referring to the smallest repeating unit along the z ‐direction) have also been recently reported.…”

“…On the basis of the first‐principles calculations, it was predicted that intrinsic long‐range magnetic ordering can exist in many TMCs with M = V, Cr, Mn, Fe, Co and Ni 17,24,25 . Recently, increasing research effort has focussed on the fabrication of large‐scale 2D magnetic TMCs via methods such as chemical vapor deposition (CVD) 23 and molecular beam epitaxy (MBE) 26–28 . In particular, the successful growth of monolayer (ML) MnSe 2 , 13 VSe 2 , 27 and Cr 3 Te 4 29 with Curie temperature ( T c ) close to and even above room temperature has been demonstrated.…”