Shape-uniform, High-quality Monolayer MoS Crystals for Gate

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Shape-uniform, High-quality Monolayer MoS2 Crystals for Gate-tuneable Photoluminescence Xiumei Zhang, Haiyan Nan, Shaoqing Xiao, Xi Wan, Zhenhua Ni, Xiaofeng Gu, and Kostya (Ken) Ostrikov ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14189 • Publication Date (Web): 07 Nov 2017 Downloaded from http://pubs.acs.org on November 7, 2017

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ACS Applied Materials & Interfaces

Shape-uniform, High-quality Monolayer MoS2 Crystals for Gate-tuneable Photoluminescence

Xiumei Zhang†,⊥, Haiyan Nan†,‡,⊥, Shaoqing Xiao†,*, Xi Wan†, Zhenhua Ni‡, †⊥

†‡⊥

†

†

‡

Xiaofeng Gu†,* and Kostya (Ken) Ostrikov§,‖ †

Engineering Research Center of IoT Technology Applications (Ministry of Education), Department of Electronic

Engineering, Jiangnan University, Wuxi 214122, China ‡

Department of Physics and Key Laboratory of MEMS of the Ministry of Education, Southeast University, Nanjing,

211189, China §

Institute for Future Environments and School of Chemistry, Physics and Mechanical Engineering, Queensland

University of Technology, Brisbane QLD 4000, Australia. ‖

CSIRO-QUT Joint Sustainable Processes and Devices Laboratory, Commonwealth Scientific and Industrial

Research Organization, P.O. Box 218, Lindfield NSW 2070, Australia.

ABSTRACT Two-dimensional molybdenum disulfide (MoS2) has recently drawn major attention due to its promising applications in electronics and optoelectronics. Chemical vapor deposition (CVD) is a scalable method to produce large-area MoS2 monolayers, yet it is challenging to achieve shape-uniform, high-quality monolayer MoS2 grains as random, diverse crystallographic orientations and various shapes are produced in the same CVD process. Here we report the growth of high-quality MoS2 monolayers with uniform triangular shapes dominating (up to 89%) over other shapes on both SiO2/Si and sapphire substrates. The new confined-space CVD process 1

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prevents contamination and helps regulate the Mo:S ratio during the deposition. The as-grown triangle MoS2 monolayers exhibit grain sizes up to 150 µm and possess better crystalline properties and lighter n-type doping concentration than those grown by common CVD methods. The corresponding field effect transistor (FET) devices show high electron mobilities of 50-60 cm2V-1s-1 and positive threshold voltages of 21-35 V. This mild n-type behavior makes it possible to regulate the formation of excitons by back-gate voltage due to the interaction of excitons with free charge carriers in the MoS2 channel. As a result, gate-tunable photoluminescence (PL) effect, which is rarely achievable for MoS2 samples prepared by common CVD or mechanical exfoliation, is demonstrated. This study provide a simple versatile approach to fabricate monolayer crystals of MoS2 and other high-quality transition metal dichalcogenides (TMDs) and could lead to new optoelectronic devices based on gate-tunable PL effect. KEYWORDS: MoS2, confined-space chemical vapor deposition, photoluminescence, mobility, exciton

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INTRODUCTION

In recent years, MoS2, as a member of the transition metal dichalcogenides (TMDs) family, has drawn a great attention owing to its two-dimensional layer structure similar to graphene. Unlike conductive graphene and narrow gap black phosphorous,1 monolayer MoS2 is a semiconductor material with a direct bandgap of about 1.8 eV, making it promising for applications in electronic and optoelectronic devices, such as transitors,2-3 photodetectors,4 photovoltaics,5 light emitters,6 and others.7-10 Significant efforts including top-down and bottom-up methods have been devoted to produce MoS2 thin layers. However, top-down methods like mechanical exfoliation3 and chemical exfoliation11-12 typically produce flakes with small areas or rely on organic solutions, which may affect the material properties. Thinning of TMD few-layers films via laser,13 plasma,14-15 patterning method14 and thermal annealing16 has been reported to produce single-layer or few-layer flakes; however, these approaches often damage the surface or lower the crystallinity of the samples.

Among bottom-up growth methods like physical vapor deposition

17

, chemical vapor deposition

(CVD),18-21 MOCVD,22 and thermolysis synthesis,23 CVD using solid state precursors of MoO3 and sulfur powder is one of the most promising methods to produce high-quality single-crystalline MoS2 monolayers on a variety of substrates.19-20,

24-27

However, MoS2 grains with random, diverse

crystallographic orientations and various shapes often coexist on the same substrate.19-20 Among them, triangle-shaped MoS2 monolayer domains are believed to be high-quality crystals which are 3



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easy to identify and use.19-20,28 It is thus important to achieve shape-uniform (e.g., triangular-shape) MoS2 monolayers to fabricate high-performance MoS2-based devices.28

To solve this problem, many efforts have been made, including using excessive amounts of sulfur29-30 and the introduction of seeding promoters.28, 31-32 In fact, it is easy to control the Mo:S ratio in the source powders but it is hard to control the real-time Mo:S ratio on the growth substrate surface since the Mo:S ratio may vary along the gas flow direction. Organic aromatic molecules such as PTAS could greatly promote the nucleation of MoS2 monolayers,8 however, chemical contamination is inevitably introduced into the growth system because the seeding promoter contains other elements in addition to sulfur and molybdenum.18, 33

In this work, we report the growth of high-quality monolayer MoS2 crystals with uniform triangle domains (up to 89%) dominating over other domains on both SiO2/Si and sapphire substrates. This is achieved by a simple and innovative confined-space CVD method, which prevents contamination and helps regulate the Mo:S ratio during the deposition process. The as-grown triangle MoS2 monolayers exhibit grain sizes up to 150 µm and possess better crystalline properties and lighter n-type doping concentration than those grown by common CVD methods. This is confirmed by systematic analytical characterization including optical microscopy, Raman, photoluminescence (PL), atomic force microscopy, and electrical transport measurements. Back-gated field-effect transistor (FET) devices based on these triangle MoS2 monolayer domains show good electron mobilities and positive threshold voltages. This mild n-type behavior makes it possible to regulate 4


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the formation of excitons by back-gate voltage due to the interaction of excitons with free charge carriers in the MoS2 channel. Consequently, gate-tunable photoluminescence (PL) effect, which is rarely achievable for MoS2 samples prepared by common CVD or mechanical exfoliation, is demonstrated.

RESULTS AND DISSCUSSION

Synthesis and Characterization of monolayer MoS2. Monolayer MoS2 grains were grown by the customized confined-space CVD process. Figure 1a shows a schematic of the growth system. In contrast to common CVD system where only one substrate was used in each experiment, we used two stacked substrates instead to confine the deposition space: one target substrate for the growth of MoS2 grains and the other serving as an assistant substrate. In common CVD systems, the whole substrate surface facing down is directly exposed to the MoO3 powders, resulting in the deposition of other contaminants like MoO2 grains on the most areas of the substrate surface although some MoS2 grains can still be observed at the edge of the substrate,34-35 as shown in Figure S1a and further supported by the corresponding optical images in Figures S1b-c and Raman spectra in Figure S1d.

Importantly, for our confined-space CVD method, the resultant samples shown in Figure S1e exhibit a very clean surface and look like a continuous film by the naked eye. Both the optical image in 5



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Figures S1f-g and the Raman spectra in Figure S1h reveal that numerous MoS2 monolayer grains with dominating triangle shape are distributed over the whole substrate surface. Interestingly, the triangle MoS2 islands merge into a partially continuous film in some places of the substrate surface as shown in Figure S1g. For confined-space CVD, the average percentage of triangle shapes on SiO2/Si substrates can reach up to 82% while that for common CVD can only reach 31%, as clearly reflected by the SEM images of three representative but independent experimental results for both CVD methods in Figures S2a-c and S2d-f as well as the histogram of the average percentage of triangle shapes on SiO2/Si substrates across 10 experiments for both CVD methods in Figure S3. Compared to the randomness of the obtained shapes for common CVD, large-area monolayer MoS2 crystals with uniform triangle shape dominating over the resultant domains and without other contaminants could be obtained by our confined-space CVD method.

The main reason for this uniform triangle shape dominating over the resultant domains is the effective control of Mo:S atom ratio on the growing surface for our confined-space CVD. Based on the principles of crystal growth, the shape of a crystal is determined by the growth rate of different crystal faces.36 The fastest growing faces either become smaller or disappear while the slowest growing faces become the largest. The most commonly observed growing faces are Mo zigzag (Mo-zz) terminations and S zigzag (S-zz) terminations for monolayer MoS2.20 In a S-rich atmosphere (Mo:S

Shape-uniform, High-quality Monolayer MoS Crystals for Gate

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