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Vapor Transport Synthesis of Two-Dimensional SnS2 Nanocrystals Using a SnS2 Precursor Obtained from the Sulfurization of SnO2 Jun Cheol Park, Kyoung Ryun Lee, Hoseok Heo, Se-Hun Kwon, Jung-Dae Kwon, Myoung-Jae Lee, Woojin Jeon, Seong-Jun Jeong, and Ji-Hoon Ahn Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00447 • Publication Date (Web): 06 Jun 2016 Downloaded from http://pubs.acs.org on June 6, 2016
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Vapor Transport Synthesis of Two-Dimensional SnS2 Nanocrystals Using a SnS2 Precursor Obtained from the Sulfurization of SnO2 Jun Cheol Park, 1,† Kyoung Ryun Lee, 1,† Hoseok Heo,2 Se-Hun Kwon,3 Jung-Dae Kwon,4 Myoung-Jae Lee,5 Woojin Jeon,6 Seong-Jun Jeong,6,* and Ji-Hoon Ahn,1,* 1
Department of Electronic Material Engineering, Korea Maritime and Ocean University, 727 Taejong-
ro, Yeongdo-gu, Busan 49112, Republic of Korea 2
Division of Advanced Materials Science, Pohang University of Science and Technology, 77
Cheongam-Ro, Pohang 790-784, Republic of Korea 3
School of Materials Science and Engineering, Pusan National University, 30 Jangjeon-Dong
Geumjeong-Gu, Busan 609-735, Republic of Korea 4
Department of Advanced Functional Thin Films, Surface Technology Division, Korea Institute of
Materials Science, 797 Changwondaero, Sungsan-Gu, Changwon, Gyeongnam 51508, Republic of Korea 5
Daegu Gyeongbuk Institute of Science and Technology (DGIST), 333 Techno Jungang Daero,
Hyeonpung-Myeon, Dalseong-Gun, Daegu 42988, Republic of Korea 6
Device Lab., Device and System Research Center, Samsung Advanced Institute of Technology,
Suwon, Gyeonggi-Do 443-803, Republic of Korea
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ABSTRACT Manufacturing high-quality, two-dimensional (2D), layered materials with crystal-growth techniques is an important challenge for the advancement of 2D communication technologies. In this study, a simple method was developed for synthesizing 2D nanocrystals based on the model system of SnS2. The method involves the sulfurization of a metal oxide to a metal chalcogenide, which subsequently acts as a source of vapors for the growth of 2D crystals. The effect of the annealing conditions on the thermal sulfurization of SnO2 powder was investigated. The results showed that pure SnS2 powder could be obtained in a N2 atmosphere at 700 °C. SnS2 nanocrystals were successfully synthesized from the as-prepared SnS2 powder by the vapor transport method. The synthesized SnS2 nanocrystals had a 2D layered structure with hexagonal symmetry and exhibited typical n-type semiconducting characteristics, with an optical band gap of 2.05 eV. This novel method, which uses a preferentially prepared source for vapor transport, could provide a simple way to synthesize new types of 2D layered materials. This is because it only requires the volatilization of a source and subsequent condensation to a single crystal for the growth of 2D materials, with no complex chemical reactions occurring during vapor transport.
KEYWORDS: Chemical Vapor Transport, Two-Dimensional Materials, Tin Oxide, Tin Sulfide, Thermal Sulfurization
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INTRODUCTION In recent years, two-dimensional (2D) materials, such as graphene,1,2 hexagonal BN,3,4 and hexagonal metal dichalcogenides,5-8 have received significant attention owing to their unique physical properties that are in contrast to those of their bulk counterparts,9-12 and for potential applicability in emerging electronic and optoelectronic devices.13-15 In the past few years, the physical, electrical, and optical properties of 2D materials synthesized by exfoliation techniques have been investigated because exfoliation is one of the most accessible methods to obtain different types of high-quality layered structures. Further, mono- and few-layered-thick exfoliated 2D materials are the most suitable for studying the intrinsic properties of a material.16-19 However, in practice, there are limitations to the technique of exfoliation with respect to device integration because of the difficulty in obtaining large surface area and uniform 2D layers. Thus, because device fabrication requires single crystals with a large surface area on the order of micrometer-square scales, it is important to develop a method that can grow single crystals directly on the substrate. This is one of the most important research challenges in advancing 2D communication technologies. Many research groups have made efforts to synthesize 2D materials on a substrate, and various layered crystals, such as MoS2,7,20 MoSe2,21 WS2,22 WSe2,23 GaSe,24 and SnS2,25 have been successfully synthesized. Of all the methods used to synthesize 2D materials, the vapor transport, for which the external conditions used to realize chemical equilibrium at the position of crystallization are different from those at the position of volatilization, is one of the best methods for obtaining high-quality single crystals of metal dichalcogenides. This is because vapor transport reactions allow the simple formation of pure and highly crystalline phases. A temperature gradient is usually applied to control the volatilization and crystallization processes, and because the sublimation process occurs without any chemical reactions, it is the simplest way to control deposition by forming only one dominant species. However, most of the attempts to grow 2D materials via vapor transport have been accompanied by chemical reactions, such as the sulfurization of the metal oxide or metal precursor,25-28 that complicate the process control. Therefore, a synthetic route was developed ACS Paragon Plus Environment
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based on the preparation of a metal dichalcogenide source from a metal dioxide powder, which is normally used as the precursor for 2D growth, before performing the vapor transport method.
In this paper, we report the synthesis of 2D-structured nanocrystals of tin disulfide (SnS2), as a model system, via preparation of SnS2 nanopowder followed by vapor transport. SnS2 is one of the ntype layered semiconductors that have layered CdI2-type structure with a hexagonal unit cell. Similar to other metal chalcogenide semiconductors, SnS2 has been investigated for various applications, such as field-effect transistors (FETs),29 gas sensors,30 and optoelectronic devices.31 In the following section, we describe the preparation of SnS2 nanopowder via the thermal sulfurization of SnO2, which is then used as the vapor source. We then present the results obtained from analyzing the SnS2 nanocrystals grown by the vapor transport method.
EXPERIMENTAL SECTION Preparation of the SnS2 precursor: The SnS2 powder was prepared by a thermal sulfurization process inside a hot-wall tube furnace. Tin(IV) oxide (SnO2) nanopowder (
