Synthesis of Large-Scale Monolayer 1T'-MoTe2 and Its Stabilization via Scalable hBN Encapsulation.

Simona Pace,Stiven Forti,Camilla Coletti,Domenica Convertino,Vaidotas Mišeikis,Filippo Fabbri,Leonardo Martini,Sergio Pezzini,Dong Hoon Keum

doi:10.1021/acsnano.0c05936

Simona Pace, Stiven Forti + Show 7 more

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https://doi.org/10.1021/acsnano.0c05936

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Abstract

Out of the different structural phases of molybdenum ditelluride (MoTe2), the distorted octahedral 1T′ possesses great interest for fundamental physics and is a promising candidate for the implementation of innovative devices such as topological transistors. Indeed, 1T′-MoTe2 is a semimetal with superconductivity, which has been predicted to be a Weyl semimetal and a quantum spin Hall insulator in bulk and monolayer form, respectively. Large instability of monolayer 1T′-MoTe2 in environmental conditions, however, has made its investigation extremely challenging so far. In this work, we demonstrate homogeneous growth of large single-crystal (up to 500 μm) monolayer 1T′-MoTe2via chemical vapor deposition (CVD) and its stabilization in air with a scalable encapsulation approach. The encapsulant is obtained by electrochemically delaminating CVD hexagonal boron nitride (hBN) from copper foil, and it is applied on the freshly grown 1T′-MoTe2via a top-down dry lamination step. The structural and electrical properties of encapsulated 1T′-MoTe2 have been monitored over several months to assess the degree of degradation of the material. We find that when encapsulated with hBN, the lifetime of monolayer 1T′-MoTe2 successfully increases from a few minutes to more than a month. Furthermore, the encapsulated monolayer can be subjected to transfer, device processing, and heating and cooling cycles without degradation of its properties. The potential of this scalable heterostack is confirmed by the observation of signatures of low-temperature phase transition in monolayer 1T′-MoTe2 by both Raman spectroscopy and electrical measurements. The growth and encapsulation methods reported in this work can be employed for further fundamental studies of this enticing material as well as facilitate the technological development of monolayer 1T′-MoTe2.

Highlights

Out of the different structural phases of molybdenum ditelluride (MoTe2), the distorted octahedral 1T′ possesses great interest for fundamental physics and is a promising candidate for the implementation of innovative devices such as topological transistors
Monolayer 1T′-MoTe2 reported in this work has been grown via liquidprecursor chemical vapor deposition (LqP-CVD),[58] in which the transition metal precursor is dissolved in an aqueous solution and spun directly on the growth substrate
An aqueous solution containing ammonium heptamolybdate tetrahydrate (AHM), as Mo precursor, NaOH, as growth promoter, and commercial Opti Prep (OPTI), as density gradient medium, is prepared and spun directly on the SiO2 substrate, while tellurium powder is used as chalcogenide source

Summary

Introduction

Out of the different structural phases of molybdenum ditelluride (MoTe2), the distorted octahedral 1T′ possesses great interest for fundamental physics and is a promising candidate for the implementation of innovative devices such as topological transistors. The encapsulated monolayer can be subjected to transfer, device processing, and heating and cooling cycles without degradation of its properties The potential of this scalable heterostack is confirmed by the observation of signatures of low-temperature phase transition in monolayer 1T′-MoTe2 by both Raman spectroscopy and electrical measurements. Depending on the nature of the chalcogenide and the transition metal, semiconductor, semimetal, or topological insulator behavior can be observed,[1] making this class of materials an enticing platform for several fields of applications, such as electronics,[2,3] spintronics,[4] and optoelectronics.[5,6] Within such a class, MoTe2 has received increasing attention over the past few years, since it displays two thermodynamically stable polymorphs with peculiar electronic and structural properties. Few-layer 1T′-MoTe2 tends to oxidize in a selflimited process, which generates a homogeneous oxide film on the top of the underlying intact layers of 1T′-MoTe2.31 These different reactivities have allowed, to date, a number of fundamental and applicative studies to be performed for monolayer 2H MoTe2,14,31,37−40

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