Nano Research 1 DOINano 10.1007/s12274Res ‐014‐0147‐z Chemical vapor deposition growth of monolayer MoSe2 nanosheets Jonathan C. Shaw1, Hailong Zhou1, Yu Chen2, Nathan O. Weiss2, Yuan Liu2, Yu Huang2,3, Xiangfeng 1,3 Duan () Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0147-z http://www.thenanoresearch.com on January 15, 2014 © Tsinghua University Press 2014 Just Accepted This is a “Just Accepted” manuscript, which has been examined by the peer‐review process and has been accepted for publication. A “Just Accepted” manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides “Just Accepted” as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. 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Department of Materials Science & Engineering, University of California, Los Angeles, CA 90095, USA 3. California NanoSystems Institute, University of California, Los Angeles, CA 90095, USA §. These authors made equal contribution to this work Using chemical vapor deposition, we have synthesized monolayer MoSe2 nanosheets directly on 300 nm SiO2/Si substrates. The MoSe2 nanosheets have size dependent properties, including a transition from an indirect-to-direct band gap as the out-of plane dimensions are reduced to a single layer. 1 Nano Res DOI (automatically inserted by the publisher) Review Article/Research Article Please choose one Chemical vapor deposition growth of monolayer MoSe2 nanosheets Jonathan C. Shaw1,§ Hailong Zhou,1,§ Yu Chen,2 Nathan O. Weiss,2 Yuan Liu,2 Yu Huang,2,3 Xiangfeng Duan1,3,* 1Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095, USA 2Department of Materials Science and Engineering, University of California, Los Angeles, CA 90095, USA 3California NanoSystems Institute, University of California, Los Angeles, CA 90095, USA §These authors made equal contribution to this work Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011 ABSTRACT The synthesis of two‐dimensional (2D) layered materials with controllable thickness is of considerable interest for diverse applications. Here we report the first chemical vapor deposition growth of single‐ and few‐layer MoSe2 nanosheets. By using Se and MoO3 as the chemical vapor supply, we demonstrate that highly crystalline MoSe2 can be directly grown on the 300 nm SiO2/Si substrates to form optically distinguishable single‐ and multi‐layer nanosheets, typically in triangular shaped domains with edge lengths around 30 μm, which can merge into continuous thin films upon further growth. Micro‐Raman spectroscopy and imaging was used to probe the thickness dependent vibrational properties. Photoluminescence spectroscopy demonstrates that MoSe2 monolayers exhibit strong near band edge emission at 1.55 eV, while bilayers or multi‐layers exhibit much weaker emission, indicating of the transition to a direct band gap semiconductor as the thickness is reduced to a monolayer. KEYWORDS Chemical vapor deposition, molybdenum diselenide, two‐dimensional materials, transition metal dichalcogenide, layered materials, semiconductor Introduction have numerous applications in fields such as catalysis, energy storage, dry lubrication, Transition metal dichalcogenides (TMDs) (MX2; microelectronics and optoelectronics [1‐7]. Recently, M= W, Mo; X=S, Se, Te) encompass a large class of significant research has focused on the isolation of two‐dimensional layered materials (2DLMs), which 2D TMDs due to their unique transformation from ———————————— Address correspondence to [email protected] 2 an indirect to a direct band gap semiconductor horizontal tube furnace (Figure S1). Our studies when confined to a single layer [8‐10]. These indicate that H2 plays a critical role in the growth of atomically thin nanosheets are of particular interest MoSe2 nanosheets using solid Se and MoO3 for new types of electronic/optoelectronic devices precursors. Without H2 in the carrier gas, MoSe2 and chemical sensors [11‐18]. nanosheets were not observed on the SiO2/Si While there have been numerous reports on substrate. The introduction of a small fraction of H2 synthesis of single and few‐layer nanosheets of the (5 sccm H2 with 65 sccm Ar) is sufficient for MoSe2 sulfide family of dichalcogenides (MoS2 and WS2) crystals to nucleate and grow into extended 2D [19‐24], the synthesis and characterization of the structures. A recent report on the synthesis of WS2 selenide family (MoSe2 and WSe2) remains largely and WSe2 demonstrates a similar role that H2 has in unexplored [26]. Several bottom‐up syntheses of the formation of highly crystalline 2D nanosheets thin films and clusters of MoSe2 have been reported [44, 45]. Here H2 function as an additional reducing [27‐32], yet most studies of single layers are based agent along with Se to reduce MoO3 and produce on top‐down, mechanically‐exfoliated monolayers MoSe2 nanosheets on the substrate, in a mechanism using Scotch tape [33, 34]. Combining these similar to that of H2 with S to synthesize MoS2 nanosheets into heterostructures of TMDs have fullerenes [47]. been explored theoretically [35], and can lead to The as grown MoSe2 nanosheets and domains intriguing structures designed with specifically were first observed using optical microscopy (OM) engineered properties [36, 37]. For MoSe2 to be (Figure 1a,c,d), with a thickness dependent contrast implemented in electronic devices and that can distinguish between single‐ and hetero‐integrated nanostructures, controllable multi‐layers on 300 nm SiO2/Si (Figure 1c, d). MoSe2 synthesis of large‐area, single‐ and few‐layered nanosheets were found to have nucleated randomly crystals is a necessity. on the SiO2/Si substrate with domains up to several To this end, chemical vapor deposition (CVD) tens of micrometers. Under proper conditions, the represents an attractive approach for the growth of domains can merge together to form continuous MoSe2 nanosheets on a supporting substrate by thin films up to several hundred microns across exploiting the anisotropic bonding characteristics of (Figure S2). Single layers are the most frequently layered materials. CVD is used to synthesize a observed (Figure 1c), but bilayer, trilayer and number of other 2D materials including graphene, thicker crystals are seen as well. A scanning Bi2Te3, MoS2 and WS2 [19‐25 38, 39, 45]. Here we electron micrograph (SEM) presents a cluster of report the first CVD synthesis of MoSe2 monolayers monolayer MoSe2 nanosheets (Figure 1b). Atomic by using solid Se and MoO3 powders in a reducing force microscopy (AFM) was used to characterize atmosphere under ambient pressures, with the the thickness of the MoSe2 nanosheets. The height of product depositing directly on untreated 300 nm the MoSe2 monolayer on the SiO2/Si substrate was SiO2/Si substrates. We found highly crystalline typically measured between 0.70 – 1.0 nm, MoSe2 triangular domains formed on the substrate comparable to previous reports of exfoliated MoSe2 and had edge lengths up to ~30 μm. [33, 40]. The second layer height in bilayers is consistently found to be between 0.60 ‐ 0.70 nm, Results and Discussion which is in agreement with the step height difference of 0.64 nm between monolayer and The growth is carried out in a home‐built CVD bilayer MoSe2 [43]. system within a 1‐inch diameter quartz tube in 3 “triforce” structures (Figure 1d) are predominantly observed, with subsequent layers alternating triangle direction; however, some parallel bilayer triangles are also seen, which may indicate a less favorable AB stacking order. The crystal structure of mono‐, bi‐, and multi‐layers was analyzed using high‐resolution transmission electron microscopy (HRTEM) (Figure 2a‐d). Figures 2a‐c show the interface between the mono/bilayer at increasing magnification. The selected area electron diffraction (SAED) confirms the hexagonal MoSe2 structure, which is consistent with the high‐resolution image (Figure 2d). The lattice spacing of the {1010 } and {1120 } planes are ~0.28 and ~0.16 nm, in agreement with the reported values for MoSe2 [43]. The energy dispersive X‐ray spectrometry (EDAX) independently verifies the Mo:Se atomic ratio of 1:2 (Figure S3). Figure 1 (a) OM and (b) SEM of a cluster of MoSe2 triangular domains. (c) OM image of a triangular monolayer and (d) 3 monolayer/bilayer “triforce” crystals with anti-parallel triangular layers. (e) AFM image of monolayer and bilayer