Chemical Vapor Deposition Growth of Monolayer Mose2 Nanosheets Jonathan C

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 Duan1,3 ()

Nano Res., Just Accepted Manuscript • DOI: 10.1007/s12274-014-0147-z http://www.thenanoresearch.com on January 15, 2014

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Chemical vapor deposition growth of monolayer

MoSe2 nanosheets

,§ ,§ Jonathan C. Shaw1 , Hailong Zhou1 , Yu Chen2, Nathan O. Weiss2, Yuan Liu2, Yu Huang2,3, Xiangfeng Duan1,3,*

1. Department of Chemistry and Biochemistry, University of California, Los Angeles, CA 90095, USA 2. 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.

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 domains and (f) the corresponding line scans.

Triangular monolayers were always observed on the substrate, which is consistent with previous reports [19‐24, 44, 45]. This triangular crystal shape is dictated by the thermodynamically favorable edge termination of either Mo or Se atoms and the Figure 2 (a) Low magnification TEM of MoSe2 mono/bilayer three‐fold symmetry of the half unit cell of single (scale bar 2 μm). (b) Magnified view of the mono/bilayer layer dichalcogenides, compared to the six‐fold interface (scale bar 200 nm). (c) HRTEM image of the marked symmetry of bulk MoSe2 [46, 48]. Bilayer triangles surface of (b). (d) HRTEM image of a multilayer MoSe2 begin to grow from the same nucleation site at the nanosheet with its corresponding SAED pattern (inset). center of the triangle, but in an antiparallel orientation due to an inverted AA’ stacking order [49]. This here‐so‐called monolayer/bilayer

4 bilayer sample, and drops sharply for each additional layer (Figure 3a, 3d). The dual peaks at layer numbers 3 – 5 (Figure 3a) are resulted from the Davydov splitting, consistent with previous report [40]. The lower frequency and less noticeable split peaks are located at 239.0 cm‐1, 240.1 cm‐1 and

240.8 cm‐1 for 3, 4 and 5 layer MoSe2. Other less prominent modes are observed and are summarized in the supporting information (Figure S4).

Figure 3 (a) Raman spectra of MoSe2 of various number of layers; the bulk spectrum is displayed at a ten fold scale. (b) Corresponding OM image for Raman mapping in (c, d), the scale-bar is 6 μm. (c) Raman map of A1g mode showing a thickness-dependent mode shift. (d) Raman map of overall intensity of the spectrum.

The Raman spectroscopy was carried out on a region containing 1 to 5 layers and thicker (bulk) domains (Figure 3b). The most prominent and identifiable peak is the out‐of‐plane A1g mode located at 243.7 cm‐1 for bulk MoSe2, in agreement with literature [33, 34, 40, 41]. We notice a softening of the A1g mode from 243.7 cm‐1 for bulk MoSe2 to ‐1 241.2 cm for monolayer MoSe2, consistent with Figure 4 (a) Photoluminescence (PL) spectrum of MoSe2 up to previous reports [33, 40, 41]. The peaks for 3 layers. (b) OM of MoSe2 mono/bilayer “triforce” nanosheet few‐layer MoSe2 are located at 242.1 cm‐1, 242.4 cm‐1, on SiO2/Si substrate, (c) PL intensity map of (b).

242.6 cm‐1 and 242.8 cm‐1 for 2 – 5 layer MoSe2. The monolayer MoSe2 undergoes softening of the A1g Photoluminescence (PL) spectroscopy is mode due to a decreasing inter‐planar restoring arguably one of the most direct methods for force. As expected, the inter‐planar restoring force determining the band gap of monolayer TMDs [39]. is much stronger for thicker MoSe2 samples due to It is well known that bulk MoSe2 has an indirect additional layers [50]. Thus, the MoSe2 Raman band gap of 1.1 eV [8‐10]. However, recent mode blueshifts as the thickness increases (Figure theoretical and experimental studies have shown 3a, 3c). The peak intensity is strongest for the that monolayers should exhibit a direct band gap of

5 1.55 eV [10, 33, 40, 41]. Our PL studies on Aldrich, 99.5% purity) was placed in a 5 mL monolayer MoSe2 show a prominent emission peak alumina boat at the center of the furnace with ~ 2 × at 800 +/‐ 5 nm, confirming the direct band gap of 2 cm 300nm SiO2/Si wafers (University Wafer) monolayer MoSe2 at 1.55 +/‐ 0.01 eV (blue line in Fig. placed polished side down on top of the boat. There

4a). The PL intensity for the bilayer region is were ideally 3‐4 pieces of SiO2/Si placed on the boat significantly reduced, by a factor of 35 (red line in at the center of the furnace. 0.5 g of Se powder Fig. 4a), compared to its monolayer counterpart. (Sigma Aldrich, 99.5%) was placed in a 5 mL In addition, the PL is red‐shifted to 825 +/‐ 5 nm alumina boat upstream to the edge of the furnace. (1.50 +/‐ 0.01 eV). There is no noticeable peak for the After purging for one hour the flow was reduced to trilayer PL (green line in Fig. 4a), as with increasing 65 sccm Ar and 5 sccm H2. After one hour, the layer numbers the MoSe2 transitions from a direct to furnace was heated to the growth temperature of an indirect band gap. PL mapping further confirms 750 °C at a ramp speed of 25 °C/min. When the that the monolayer region exhibits much stronger furnace reached 700 °C, the temperature of the Se at emission compared with the bilayer region (Figure the upstream edge of the tube was ~300 °C. The 4b,c) [10, 33, 40, 41]. furnace was kept at 750 °C for ten to fifteen minutes, after which it was cooled down to room Conclusion temperature. An illustration of the furnace is included in the supporting information (Figure S1). In conclusion, we have synthesized large area Characterization of single and few‐layer MoSe2 MoSe2 nanosheets using MoO3 and Se powder The MoSe2 nanoseheets grown on the SiO2/Si precursors in a mixture of Ar and H2 gases in a substrates were first characterized using optical horizontal tube furnace. H2 is found to play a critical microscopy (OM, olympus) and scanning electron role in the reaction by enhancing the reduction of microscopy (SEM, JSM‐6701F). The thickness was MoO3 with Se to form MoSe2. The triangular characterized using atomic force microscopy (AFM, single‐crystals are characterized by TEM and SAED. Veeco Dimension 5000). High‐resolution Thickness was determined using AFM and OM and transmission electron microscopy (HRTEM, FEI then analyzed using Raman and PL spectroscopy, Titan S/TEM at 300 kV) to characterize the crystal with characteristic peak shifts and a clear and structure of MoSe2 nanoseheets transferred onto a indirect–direct band gap transition. This study copper TEM grid with a carbon film. Energy represents first report of CVD growth of monolayer dispersive X‐ray spectrometry (EDAX) was used to MoSe2 directly on SiO2/Si substrates. This new determine the elemental composition. Raman synthesis of MoSe2 expands our understanding in spectroscopy and Photoluminescence (PL, Horiba, the growth of this and other 2D crystals and will 514 nm/ 1.5 mW) was carried out on MoSe2 enable researchers to further explore and engineer nanoseheets on the SiO2/Si substrates. them into novel functional materials.

Electronic Supplementary Material Methods

An illustration and description of the reaction setup, Synthesis of MoSe2 nanosheets additional OM images, EDAX measurements and The reaction took place in a 12‐inch horizontal complete Raman spectra can be found in the tube furnace (Lindberg Blue M) in a 1‐inch diameter supplementary information and is available in the quartz tube. 0.25 g of MoO3 powder (Sigma online version of this article at

6 http://dx.doi.org/10.1007/s12274‐***‐****‐* band structure calculations and photoelectron (automatically inserted by the publisher). spectroscopy. Physical Review B 1987, 35, 15. [10] Liu, L.; Kumar, S. B.; Ouyang, Y.; Guo, J.; Performance limits of monolayer transition

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