Nanoscale Mapping of Layer-Dependent Surface Potential and

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Nanoscale Mapping of Layer-Dependent Surface Potential and Junction Properties of CVD Grown MoS Domains 2

Vishakha Kaushik, Deepak Varandani, and Bodh Raj Mehta J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 12 Aug 2015 Downloaded from http://pubs.acs.org on August 12, 2015

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The Journal of Physical Chemistry

Nanoscale Mapping of Layer-Dependent Surface Potential and Junction Properties of CVD Grown MoS2 Domains Vishakha Kaushik, Deepak Varandani, and B. R. Mehta* Thin Film Laboratory, Department of Physics, Indian Institute of Technology Delhi, New Delhi, 110016, India

*

Author to whom all correspondence should be addressed.

Electronic mail: [email protected]

1 ACS Paragon Plus Environment


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Abstract In the present study, nanoscale variations in the work function values and the resulting changes in junction properties of chemical vapour deposited two-dimensional (2D) MoS2 domains has been investigated as a function of number of layers using Kelvin Probe Force Microscopy (KPFM) and Conductive Atomic Force Microscopy(CAFM) techniques. Raman spectroscopy has been employed to obtain the magnitude of difference between E2g and A1g peaks which has been used as a signature of the number of layers. Surface potential of MoS2 monolayer sample exhibits a value of -427 mV (~7.2 mV for bulk) along with a large spread of about 29mV (~3mV for bulk). The present study shows that the optical and electronic properties of MoS2 1-2 layer samples exhibit a large difference from its bulk counterpart. These characteristic features remain intact even in the presence of adsorbates and defects which result in spread in surface potential values and corresponding changes in junction characteristics. These results are important for the application of Chemical Vapor Deposition (CVD) grown MoS2 monolayers for semiconductor devices.

Keywords: molybdenum disulfide, chemical vapour deposition, Kelvin probe force microscopy, Conductive atomic force microscopy, work function, monolayer, opto-electronic properties. 2 ACS Paragon Plus Environment

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1. INTRODUCTION Two-dimensional (2D) nanomaterials have recently attracted significant attention, especially due to their unusual electrical, optical, mechanical and thermal properties as compared to their bulk counterparts.1 One such material, a transition metal dichalcogenide with interesting properties in monolayer and few layer form is molybdenum disulfide (MoS2).2-3 Monolayer MoS2 with a direct bandgap of 1.9eV is a promising candidate for 2D nanoelectronic and optoelectronic devices.4 It has received increasing attention due to its potential for a range of applications like field effect transistors, electronic sensors, catalytic hydrogenation, spintronics and as building blocks for energy storage devices.5-6 Its moderate carrier mobility value, appreciable flexibility and direct band-gap allows wide applications in fabrication of transistors and electronic switches.7 The ability to form samples with a controlled number of atomic layers permits one to examine precisely the evolution of material properties with thickness. Molybdenum disulﬁde has a layered structure, in which the atoms are covalently bonded to form two-dimensional layers, stacked together through weak van der Waals interaction.8 Monolayer and few-layer MoS2 have electrical and optical properties different from the bulk counterpart due to 3D and 2D quantum confinement effects. MoS2 in bulk form is an indirect bandgap material with a bandgap of 1.2eV, whereas, monolayer MoS2 is a direct bandgap material with a bandgap of 1.9eV.9 Unlike conductive graphene and insulating hexagonal boron nitride (h-BN), atomic-layered MoS2 is a direct gap semiconductor material, offering possibilities of fabricating high-performance devices with low power consumption. Various methods have been used to prepare 2D layers of MoS2, including the top-down and bottom-up approaches of micromechanical exfoliation, solution exfoliation, hydrothermal



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synthesis, electrochemical synthesis, and thermolysis of the precursor containing Mo and S atoms.10-15 CVD, a bottom up approach has been used to synthesize large-area, continuous, crystalline triangular MoS2 domains. In this technique it is possible to control the thickness of the layers by controlling the deposition parameters.16-18 For using the MoS2 layer in device applications, it is important to study the electrical properties and its dependence on thickness or number of layers.19-20 Because of the large surface area of CVD grown layers, defects and adsorbates are an integral part of the microstructure of 2D material. In most of the top down methods, the chemicals used during synthesis can also affect the electrical properties of 2D layers. In CVD method, the use of gaseous precursors, unreacted species or products remaining on the surface can alter the electrical properties. Therefore, it is important to study the changes in electrical properties of 2D MoS2 layers due to these effects. Previously, KPFM has been used to determine the surface potential of MoS2 flakes obtained by micromechanical exfoliation and study its dependence on number of layers.21-22 In this regard it is important to note that there is no analogous study in case of CVD grown MoS2. Moreover, the influence of surface effects on the distribution of surface potential values has not been addressed in any reported study. As already mentioned, the nature of the surface defects or adsorbates can be quite different in mechanically exfoliated MoS2 layers in comparison to CVD grown MoS2 layers. The present study addresses the above important aspects by investigating the variation in surface potential as a function of number of layers in CVD grown MoS2. In order to study the effect of surface potential variation on device properties, the dependence of junction current in Pt-MoS2 junction has also been investigated for MoS2 layers having different thicknesses. How the distribution of surface potential values varies as a function of number of layers due to surface effects has also been investigated.


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In the present work, CVD grown MoS2 domains, having different thicknesses have been investigated as a function of number of layers using Raman Spectroscopy, Kelvin Probe Force Microscopy (KPFM), and Photoluminescence (PL) techniques. Further, Conductive Atomic Force Microscopy (CAFM) has been utilized to explore the nanoscale electrical characteristics of Pt-MoS2 junction. Nanoscale variations in surface potential and junction characteristics have been observed to depend upon the number of MoS2 layers and have been explained on the basis of adsorbate and surface defects. 2. EXPERIMENTAL SECTION The experimental setup for the low-pressure chemical vapour deposition (LPCVD) synthesis of MoS2 consists of a two-temperature zone furnace equipped with a quartz tube, an upstream zone for the evaporation of sulphur (S2) powder, and a downstream zone for solidification of reactive species in the outflow. Growth temperature and choice of substrate critically affect the growth of MoS2. If we consider the direct reaction of gaseous Mo and S reactants as an example, then the process can be described as follows. Firstly, a precursor Molybdenum-Trioxide (MoO3) powder and a suitable growth substrate (in our case SiO2/Si) are placed in downstream locations. After 15 min of argon purging, temperature of the downstream zone was increased from room temperature to 450˚C in 45 min. Thereafter, the temperature of upstream zone was increased to 150˚C in parallel with the downstream zone temperature being increased from 450˚C to 800˚C in 75 min. The MoO3 is then partially reduced by sulphur vapor which is placed in the upstream location to form a volatile MoO3-x species, which is subsequently transported downstream by the carrier gas and reacts with sulphur to deposit MoS2 on the substrate.



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The substrate used is an oxidized Silicon(Si) wafer with the thickness of Silicon- dioxide (SiO2) taken around 300 nm. Since a layered material is difficult to identify using an optical microscope, the choice of oxide thickness of the substrate is intentional for identifying the grown MoS2 layers optically, which exhibit a distinct color contrast with the SiO2/Si substrate. Due to the large difference in melting points of moly-oxide and sulphur, there is a difference in the vapor pressures of the two materials. Therefore, the deposition temperature was optimized to be kept around 800˚C for the reaction between gaseous precursors to take place. The reaction takes place in presence of an inert gas (Argon) to form continuous films of MoS2 on the substrate. The variation in deposition time from 10 minutes to 30 minutes yields single to few layers of MoS2 films on the substrate. It may be mentioned that the samples chosen for the scan, have predominantly one layer thickness (1, 3, 4, 5 or 7) spread over the surface of the substrate. The measurements have been performed on domains having a particular thickness identified by Raman spectroscopy. The prepared samples were systematically characterized using Optical microscopy, Micro-Raman Spectroscopy, AFM, KPFM, PL spectroscopy and CAFM. The MoS2 ﬁlms were investigated by Raman spectroscopy using a Renishaw invia confocal Raman microscope with a 532 nm laser wavelength and an 1800 lines per mm grating. The diffraction system uses a holographic diffraction grating with confocality control and the detection system utilizes a high sensitivity ultra-low noise CCD. A laser power of

Nanoscale Mapping of Layer-Dependent Surface Potential and

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