Elsevier

Journal of Crystal Growth

Volume 363, 15 January 2013, Pages 122-127
Journal of Crystal Growth

Growth and characterization of large, high quality MoSe2 single crystals

https://doi.org/10.1016/j.jcrysgro.2012.10.026Get rights and content

Abstract

MoSe2 single crystals were grown by chemical vapor transport using TeCl4 as transport agents in the temperature gradient 1020–980  °C. They were characterized by scanning electron microscopy (SEM), optical microscopy, image analysis coupled with SEM, microanalysis by SEM-EDX, X-ray fluorescence, inductively coupled plasma (ICP), X-ray photoelectron spectroscopy (XPS) and electrical conductivity. The characterizations showed that single crystals are perfectly homogeneous, stoichiometric and have very few defects and clean surfaces with areas in the range of 35–100  mm2. Single crystals grown by TeCl4 showed a high electrical conductivity. Their properties were highly dependent on the quality of the polycrystalline powders used for the growth.

Highlights

► Growth of very large MoSe2 single-crystals with high electrical conductivities. ► The properties of single crystals depend on the quality of the starting material. ► TeCl4 identified as the best transport agent.

Introduction

Molybdenum diselenide (MoSe2) is a layered semiconductor compound belonging to the transition metal dichalcogenide family MX2 (M=Mo, W, X=S, Se), which crystallizes in the 2H-MoS2 structures. Because of its great potentialities for solar energy conversion, MoSe2 has been the subject of many investigations [1], which have shown that surface defects play a major role in the corrosion and photocorrosion of the material [2], [3]. Consequently, some strategies have been devoted to surface treatment of this material in order to suppress the undesired surface limitations or surface defects as much as possible [2], [3]. Unfortunately, the improvements observed with such processes vanish under prolonged illumination. Thus the best way to increase the photoconversion efficiency is the growth of single crystals with surfaces free of defects, as much as possible, and that is the aim of this work. Even though the difficulty to obtain large surfaces of MX2 single crystals by chemical vapor transport method has led some researchers to engage in electrocrystallization of binary [4], [5], [6], mixed/alloyed and ternary [7], [8], semiconducting compounds thin films on conductive support, studies on MX2 single crystals remain valuable references for improving the performance of this type of conductive substrate/ thin film/electrolyte junction.

Another major interest of transition metal dichalcogenides crystals resides in their layered structure. They have attracted recently a considerable attention as starting materials for the isolation of two-dimensional objects consisting of free standing crystals made of a single or a few atomic layers, in strong analogy with the graphene layer(s) obtained from graphite [9], [10], [11]. As low dimensional compounds, they exhibit unique electrical, mechanical and optical properties [10], [12], [13]. These 2D crystals are obtained by exfoliation or mechanical cleavage of the 3D parent compound [9], [10], [11], [14]. Improving the crystalline quality of this latter will result in 2D systems of larger lateral size, reaching the macroscopic scale, and there is therefore a direct interest in the synthesis of single crystals of large areas and high quality.

This work describes the growth and characterization of perfectly crystallized single crystals that may have fewer defects. Indeed, there is good evidence that the quality of the polycrystalline powders used as the starting material for the preparation of single crystals has a significant impact on the quality and properties of these single crystals [15]. The reproducible syntheses of various powders with homogeneous, well crystallized structure and well-defined microstructure have been the subject of a previous study [16]. Two types of polycrystalline powders were synthesized using two different experimental protocols (protocol I and protocol II). Single crystals obtained by chemical vapor transport from protocol I polycrystalline powders have been extensively characterized in our previous works [17], [18], [19], [20]. Studies showed that TeCl4 and SeCl4 are the best transport agents. Photocurrent gain was 390 A m−2 for single crystal grown by SeCl4 and 440 Am−2 for single crystals grown by TeCl4. Our goal is to increase this value by improving the single crystals quality. So, single crystals were grown from protocol II polycrystalline powders using TeCl4 as transport agents. The single crystals have been characterized by scanning electron microscopy (SEM), optical microscopy, image analysis coupled with SEM, microanalysis by SEM-EDX, X-ray fluorescence, inductively coupled plasma (ICP), X-ray photoelectron spectroscopy (XPS) and electrical conductivity. The results are compared with those of previously made single crystals from protocol I. It appears from the comparative study that the present single crystals are much better than those previously studied. The impact of polycrystalline powders on single crystals properties is discussed.

Section snippets

Synthesis of polycrystalline powders

Polycrystalline powders have been synthesized from elements in powder (Mo: Aldrich Chemical Company, 3N) or granular forms (Se: Aldrich Chemical Company, 4N), using silica glass tubes sealed under secondary vacuum of 10−4 Pa. The tubes were carefully degreased with Ajax®, etched (HF, 10%) and dried before introducing the reactants. After sealing, the ampoule (sealed tube) was set in an isothermal zone furnace (CARBOLYTE type 201). During synthesis (protocol II), the temperature was increased

Characterization and comparative study of single crystals grown by TeCl4

The growth of single crystals from both protocol I and protocol II polycrystalline powders has been successful in the chosen temperature gradient (1020–980 °C) in a furnace with three temperature zones. The transport over 120 h with a transport agent concentration of 3 g L−1 leads to single crystals growth in the cold or growth zone (TG=980 °C). There was sometimes crystal formation in the intermediate zone or even in the source zone (TS=1020 °C) (Fig. 1A). All single crystals are well developed,

Conclusions

  • (1)

    Over this study, the growth conditions allowed us getting fairly massive single crystals (35–100 mm2) which may be used as electrodes in photoelectrochemical cells.

  • (2)

    SEM, EDX microanalysis, optical microscopy and photoelectron spectroscopy (XPS) showed that cleaved crystals are very clean and homogeneous. Analyses by ICP showed that they are almost stoichiometric with a ratio Se/Mo=2.02. Electrical conductivity measurements have confirmed the semiconducting character of single crystals. Moreover,

Acknowledgments

This work was supported by the Commission Universitaire pour le Développement (CUD)-Communauté Francaise de Belgique. Their financial support is gratefully acknowledged. Moussa Bougouma would like to thank the CUD-CIUF for receiving Fellowships to stay at the Université Libre de Bruxelles within the framework of the International cooperation project between the Communauté francaise de Belgique and the University of Ouagadougou.

References (27)

  • L. Peraldo Bicceli et al.

    Surface defects on n-MoSe2 electrodes used in photoelectrochemical solar cells

    Surface Technology

    (1983)
  • H.S. White et al.

    Improvement in performance of n-WSe2 by electrochemical polymerization of o-phenylenediamine at surface imperfections

    Journal of the Electrochemical Society

    (1982)
  • S.N. Gawale et al.

    Electrosynthesis and characterisation of WSe2 thin films

    Journal of Archives of Applied Science Research

    (2010)
  • Cited by (52)

    View all citing articles on Scopus
    View full text