In Situ-Generated Volatile Precursor for CVD Growth

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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

In Situ-Generated Volatile Precursor for CVD Growth of a Semimetallic 2D Dichalcogenide Zhenfei Gao,†,∥ Qingqing Ji,*,† Pin-Chun Shen,‡ Yimo Han,⊥ Wei Sun Leong,‡ Nannan Mao,‡ Lin Zhou,‡ Cong Su,†,§ Jin Niu,† Xiang Ji,‡ Mahomed Mehdi Goulamaly,‡ David A. Muller,⊥,# Yongfeng Li,*,∥ and Jing Kong*,†,‡ Downloaded via MASSACHUSETTS INST OF TECHNOLOGY on September 30, 2018 at 17:09:42 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Research Laboratory of Electronics, ‡Department of Electrical Engineering and Computer Science, and §Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States ∥ State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, P. R. China ⊥ School of Applied and Engineering Physics, Cornell University, Ithaca, New York 14850, United States # Kavli Institute at Cornell for Nanoscale Science, Ithaca, New York 14853, United States S Supporting Information *

ABSTRACT: Semimetallic-layered transition-metal dichalcogenides, such as TiS2, can serve as a platform material for exploring novel physics modulated by dimensionality, as well as for developing versatile applications in electronics and thermoelectrics. However, controlled synthesis of ultrathin TiS2 in a drychemistry way has yet to be realized because of the high oxophilicity of active Ti precursors. Here, we report the ambient pressure chemical vapor deposition (CVD) method to grow large-size, highly crystalline two-dimensional (2D) TiS2 nanosheets through in situ generating titanium chloride as the gaseous precursor. The addition of NH4Cl promoter can react with Ti powders and switch the solid-phase sulfurization reaction into a CVD process, thus enabling the controllability over the size, shape, and thickness of the TiS2 nanosheets via tuning the synthesis conditions. Interestingly, this semimetallic 2D material exhibits near-infrared surface plasmon resonance absorption and a memristorlike electrical behavior, both holding promise for further application developments. Our method hence opens a new avenue for the CVD growth of 2D metal dichalcogenides directly from metal powders and pave the way for exploring their intriguing properties and applications. KEYWORDS: chemical vapor deposition, semimetallic 2D materials, titanium disulfide, transition-metal dichalcogenides, chloride promoter

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INTRODUCTION Titanium-based dichalcogenides (TiS2, TiSe2 and TiTe2) are often referred as semimetals with vanishing indirect bandgaps.1−4 They have been established as promising excitonic materials5 that can facilitate Bose condensation,6 a macroscopic quantum phenomenon closely related to superfluidity and superconductivity. In their two-dimensional (2D) forms, the diverse electronic phases addressable by external electric fields can bring about insightful understandings on the origin and interplay of these many-body states.7 Being semimetallic, the carrier concentration and electrical conductivity in these layered materials can be facilely engineered through ion intercalation, followed with greatly improved thermoelectric figure of merits.8,9 In addition, the localized surface plasmon resonances (LSPRs) hosted by these materials are in the near-infrared range,10 which are potentially useful for biological imaging,11 photothermal therapy,12 and optical communication applications.13 These remarkable properties have made Ti-based dichalcogenides an indispensable building block in the 2D materials toolbox, as in supplement to those © XXXX American Chemical Society

metallic and semiconducting transition-metal dichalcogenides (TMDs).14−17 Although dimensionality effects have been predicted theoretically in tuning the electronic/optoelectronic properties of the Ti-based dichalcogenides,18,19 experimental explorations have been rarely reported because of the sluggish development in synthesizing the 2D materials. Chemical vapor transport (CVT), as a universal technique for bulk TMD synthesis,20 is not only time consuming, but also involves complicated processes that have indirect access to corresponding 2D crystals. Following this route, a redesigned CVT process was lately developed for synthesizing 2D TiSe2 nanosheets,21 the key of which is using a less-active transport agent, together with a much-reduced growth duration of several minutes. Nevertheless, several challenges remain to be addressed, such as the process complexity involving vacuum ampule sealing, Received: August 6, 2018 Accepted: September 18, 2018 Published: September 18, 2018 A

DOI: 10.1021/acsami.8b13428 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 1. Synthesis of TiS2 nanosheets. (a) Experimental setup for the CVD growth of TiS2 nanosheets on mica substrates. (b) Schematic illustration of the CVD growth mechanism with NH4Cl promoters. (c) Typical OM image of TiS2 nanosheets grown on mica.

strategy for synthesizing various TMD materials, under the condition that the metal powders react with hydrogen chloride vapors.

and the residue of transport agents that possibly contaminate sample surfaces. In addition, wet-chemistry methods, such as chemical exfoliation22 and solution reaction,10 were also utilized for producing TiS2 nanosheets with, however, limited lateral sizes (