Large-Area Synthesis of Monolayer and Few-Layer MoSe 2 Films on SiO 2 Substrates Xin Lu, †,¶ M. Iqbal Bakti Utama, †,¶ Junhao Lin, ‡,§ Xue Gong, † Jun Zhang, † Yanyuan Zhao, † Sokrates T. Pantelides, ‡,§ Jingxian Wang, ∥ Zhili Dong, ⊥ Zheng Liu, ⊥,# Wu Zhou, § and Qihua Xiong* ,†,# † Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371 ‡ Department of Physics and Astronomy, Vanderbilt University, Nashville, Tennessee 37235, United States § Materials Science & Technology Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States ∥ Energy Research Institute at NTU, Interdiciplinary Graduate School, Nanyang Technological University, Singapore 637141 ⊥ School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798 # NOVITAS, Nanoelectronics Centre of Excellence, School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 ABSTRACT: We present successful synthesis of large area atomically thin MoSe 2 films by selenization of MoO 3 in a vapor transport chemical vapor deposition (CVD) system. The homogeneous thin film can reach an area of 1 × 1 cm 2 consisting primarily of monolayer and bilayer MoSe 2 film. Scanning transmission electron microscopy (STEM) images reveal the highly crystalline nature of the thin film and the atomic structure of grain boundaries in monolayers. Raman and photoluminescence spectroscopy confirm the high quality of as-grown MoSe 2 in optics, and electronic transport measurements highlight the potential applications of the sample in nanoelectronics. KEYWORDS: molybdenum diselenide, monolayer, selenization, chemical vapor deposition, photoluminescence, Raman spectroscopy A tomically thin two-dimensional (2D) layered transition metal dichalcogenides (TMDs) have received consider- able interests in recent years due to their excellent electronic and optical properties. 1 TMD materials experience an indirect- to-direct band gap transition when the thickness becomes monolayer, 2,3 giving rise to an exceptionally high intensity of photoluminescence (PL) in single layers 4−6 and also enabling applications in optoelectronics such as for photodetector. 7,8 Moreover, the relatively large band gap of TMDs makes them promising for field effect transistor (FET) and low power electronics as compared to graphene. 9 Monolayer TMDs also possess strong spin−orbit coupling and the breaking of inversion symmetry, 10 exhibiting considerable promise for valleytronics and spintronics. 10−13 Due to the wide range of applications of monolayer TMDs, various methods have been reported to achieve mass fabrication. However, top-down methods such as liquid exfoliation and lithium-based chemical exfoliation 14,15 typically produce flakes with small areas and require the use of organic solutions, which may affect the properties of samples. Thinning of the few-layered TMD counterparts via laser, plasma, or thermal annealing can also result in single-layer flake, but such thinning processes may also damage the surface or lower the crystallinity of the sample to some degree. 16−19 Up until now, chemical vapor deposition (CVD) has been proven to be the most promising method for the synthesis of large-scale monolayer TMDs with high quality on various substrates, such as Cu foil, sapphire, mica, and even silicon. 20−28 Among a variety of synthesis methods, sulfuriza- tion/selenization of MoO 3 /WO 3 is the most common and widely used approach. Previous reports show that this method can produce high quality and large scale monolayer and few- layer TMDs. 23−26 However, the majority of papers reported on the synthesis of MoS 2 . Indeed, the lower chemical reactivity 29 of Se makes it more difficult to synthesize large-scale, atomically thin Se-based TMDs (MSe 2 , where M = Mo, W) than to synthesize large-scale thin MS 2 . As such, longer reaction time is usually needed for large-scale synthesis of MSe 2 ; however, the long time also inevitably leads to thicker films because the growth is not self-limiting. This is in contrast to graphene, whose synthesis on copper foil is a self-limited process due to the low solubility of carbon on copper and is thus suitable for monolayer growth. 30 It is, therefore, still challenging to synthesize large-scale transition-metal diselenides on conven- tional amorphous SiO 2 substrate that has no self-limiting properties. We believe that the key to solve the challenge to reproducibly synthesize large-area ultrathin Se-based TMD Received: January 8, 2014 Revised: March 8, 2014 Published: March 28, 2014 Letter pubs.acs.org/NanoLett © 2014 American Chemical Society 2419 dx.doi.org/10.1021/nl5000906 | Nano Lett. 2014, 14, 2419−2425