Mechanical properties of RF-sputtering MoS2 thin films

Home

Search

Collections

Journals

About

Contact us

My IOPscience

Mechanical properties of RF-sputtering MoS2 thin films

This content has been downloaded from IOPscience. Please scroll down to see the full text.

View the table of contents for this issue, or go to the journal homepage for more Download details: IP Address: 129.108.202.166 This content was downloaded on 13/06/2017 at 20:36 Please note that terms and conditions apply.

You may also be interested in: Stretching and breaking of monolayer MoS2—an atomistic simulation Tommy Lorenz, Jan-Ole Joswig and Gotthard Seifert Mechanical and structural characterization of ALD-based ZnO films K Tapily, D Gu, H Baumgart et al. Nanomechanical properties of pure and doped Ta2O5 and the effect of microwave irradiation E Atanassova, P Lytvyn, S N Dub et al. Atomistic simulations of nanoindentation on the basal plane of crystalline molybdenum disulfide (MoS2) J A Stewart and D E Spearot Nanoindentation and scratch resistance of multilayered TiO2-SiO2 coatings with different nanocolumnar structures deposited by PV-OAD J J Roa, V Rico, M Oliva-Ramírez et al. Relationships between the elastic and fracture properties of boronitrene and molybdenum disulfide and those of graphene Peter Hess Nanoindentation techniques in the measurement of mechanical properties of InP-based free-standing MEMS structures Cho Jui Tay, Chenggen Quan, Mahadevaiah Gopal et al. Plasticity resulted from phase transformation for monolayer molybdenum disulfide film during nanoindentation simulations Weidong Wang, Longlong Li, Chenguang Yang et al.

Surf. Topogr.: Metrol. Prop. 5 (2017) 025003

https://doi.org/10.1088/2051-672X/aa7421

PAPER

Mechanical properties of RF-sputtering MoS2 thin films RECEIVED

23 January 2017 RE VISED

4 May 2017 ACCEP TED FOR PUBLICATION

Manuel Ramos1,4, John Nogan2, Manuela Ortíz-Díaz1, Jose L Enriquez-Carrejo1, Claudia A Rodriguez-González1, Jose Mireles-Jr-Garcia1, Carlos Ornelas3 and Abel Hurtado-Macias3,4 1

18 May 2017 PUBLISHED

12 June 2017

2 3

4

Departamento de Física y Matemáticas, Instituto de Ingeniería y Tecnología, Universidad Autónoma de Cd. Juárez, Avenida del Charro 450 N, Cd. Juárez, Chihuahua, C.P. 32310, Mexico Center for Integrated Nanotechnologies, 1101 Eubank Bldg. SE, Albuquerque, NM 87110, United States of America Laboratorio Nacional de Nanotecnología, Centro de Investigación en Materiales Avanzados S.C., Miguel de Cervantes 120, Complejo Industrial Chihuahua, Chihuahua, C.P. 31109 Mexico Author to whom any correspondence should be addressed.

E-mail: [email protected] (MR), [email protected] (JLEC), [email protected] (CARG), [email protected] (JMG), [email protected] (AHM), [email protected] (CO) and [email protected] (JN) Keywords: thin film, electron microscopy, MoS2 sputtering, harness, elastic modulus, x-ray diffraction Supplementary material for this article is available online

Abstract We present an evaluation of the hardness and Young’s modulus properties of medium pressure sputtered molybdenum disulfide (MoS2) thin films by applying nano-indentation with continuous stiffness method combined with microstructure analysis using small angle x-ray diffraction, Raman, and an electron microscope in scanning and transmission mode. Our results indicate a vertical growth of MoS2 crystallites with stacking values of 7-laminates along the [0 0 1] direction and an average height of 105 nm, principal Raman vibrations at E12g at 378 cm−1 and A1g at 407 cm−1 and an interplanar distance of ~0.62 nm as confirmed by high-resolution transmission electron microscopy. An average hardness of H = 10.5 ± 0.1 GPa and elastic modulus of E = 136 ± 2 GPa from 0 to 90 nm of indenter penetration were found in these investigations.

1. Introduction The usage of layered 2D materials to fabricate low dimension electronic devices has brought them significant attention in the past decade; starting with the first nano-transistor made out of molybdenum disulfide (MoS2) [1], mainly due to its indirect band gap of 1.8 eV (theoretical) and 1.6 eV (experimental) [2, 3]. The crystal structure of MoS2 exhibits a hexagonal crystal (R3m) with interlayers made of S–Mo–S, sharing weak Van der Waals forces at 0.62 nm of spatial separation [4]. The anisotropic morphology leads to dangling bonds on sulfur terminated edges promoting exceptional catalytic and transport properties as described in the literature [5, 6, 17]. Additionally, one can find microanalysis studies regarding structural aspects of sheets as presented by Ponce et al, measuring the amount of accelerating radiation to destroy a MoS2 sheet, ranging above 80 kV [7]. Helveg et al, using an in situ high resolution transmission electron microscopy technique, were able to achieve the formation sulfide phase (MoS 2) during observations under hydrogen sulfide atmosphere at a 100 e Å−2 s electron dose-rate [8]. Using an exfoliation technique Casillas et al were able to attach low stacking MoS2 sheets into © 2017 IOP Publishing Ltd

an atomic force tip allocated in a transmission electron microscope (TEM) sample holder to perform in situ operando mechanical experiments; their observations describe the resilient mechanical nature of laminar sheets, obtaining a full bending at 8 GPa of applied pressure with a full recovery once the load is retracted, presenting exceptional experimental evidence of the elastic nature of MoS2 sheets [9]. Bertolazzi et al obtained a Young’s modulus of 270 GPa and a breaking strength of 16–30 GPa using an atomic force microscope (AFM) tip for MoS2 sheets suspended in micro-pores of silicon wafer [10]; similarly Castellanos-Gomez et al had found an average Young’s modulus ~E = 0.33 TPa in suspended MoS2 sheets [11]. Other theoretical approaches as presented by Jiang et al determined a Poisson’s ratio value of v = 0.29 applying first principle methods [12]. We present a report of our findings as they relate to the mechanical properties of MoS2 thin films as fabricated using a medium pressure RF-magnetron sputtering technique. Our main focus is to understand crystallite growth and structural aspects including the elastic modulus (E) and hardness (H) by way of experimental nanoindentation continuous stiffness methods (CSMs) in combination with electron microscopy and powder x-ray characterization [13].


M Ramos et al

This particular research is part of bi-national scientific and academic efforts between USA and México under umbrella non-proprietary user agreement No. 1177_07_2016 with Universidad Autónoma de Ciudad Juárez and Center for Integrated Nanotechnologies of Sandia National Labs at Albuquerque, NM, with principal investigator Dr Manuel Ramos; towards developing flexible organometallic photovoltaic templates for industrial energy applications.

2. Experimental methods

indenter tip radius of 20 ± 5 nm, depth limit of 400 nm, strain rate of 0.05 s−1, and harmonic displacement and frequency of 1 nm and 75 Hz, respectively; Poisson’s coefficient of ν = 0.22. The equipment was calibrated using a standard fused silica sample, under tests parameters of C0 = 24.06, C1 = −184.31, C2 = 6532.04, C3 = −25 482.45, and C5 = 19 015.30 as a constant area of contact, using CSM. All data was recorded by AFM Nano Vision system attached to the nanoindenter system.

3. Results and discussion

2.1. RF sputtering MoS2 thin films were deposited onto silicon oxide (SiO 2 ) substrates using a medium vacuum RFmagnetron sputtering system, argon and a 99.9% pure MoS2 target. At a 3 mTorr operating pressure and 275 W of 13.56 MHz RF power, a dwell time of 300 s produced 105 nm thin films used for this assessment. 2.2. Scanning and transmission electron microscopy Microstructure of thin films was observed by scanning and transmission electron microscopy, using a Hitachi SU5500 field emission scanning electron microscope, equipped with an energy-dispersive x-ray spectroscopy (EDS) unit, operated at 30 kV with 8 µA of current to avoid damaging the sample. To perform transmission electron microanalysis a lamella of film was prepared using a focus ion beam and coated with gallium gold to avoid fracture of MoS2 crystallites; for high-resolution transmission electron microscopy (HRTEM) observations an operational voltage of 200 kV was a field emission 2200JEOL, with STEM unit, high-angle annular dark-field (HAADF) detector, X-Twin lenses and CCD camera. All images were processed using Gatan® digital micrograph package. 2.3. Raman spectroscopy All Raman spectra were obtained on an Alpha 300RA system using a 532 nm Nd:YAG laser and a 100 × 0.9 NA objective. The laser power was varied as indicated in the corresponding figures in order to assess sample degradation. No additional sample preparation was needed. 2.4. Grazing incidence x-ray diffraction (GIXD) X-ray diffraction was collected using a Panalytical X-Pert system with source of Cu Kα radiation at 40 kV and 35 mA. The grazing incidence angle was fixed at 0.5° with 20° < θ < 80° and step size of 0.02° with a graphite flat crystal monochromator. 2.5. Nanoscale mechanical properties Nanoscale mechanical properties particularly elastic modulus (E) and hardness (H) of MoS2 thin films were evaluated using nanoindenter G200 coupled with a DCM II head instrument and Berkovich diamond

2

The images taken at 30 kV from the scanning electron microscope indicate a vertical alignment growth of MoS2 crystallites, as presented in figure 1 (inset: Energy Dispersed Spectrum confirming Mo, S atoms on sample) and are in agreement with Kong et al [14] who achieved the fabrication and characterization of thin films using HRTEM and solid-gas reaction by continuous hydrogen sulfide (H2S) gas flow into molybdenum oxide powder, one typical sulfide reaction in HDS catalysis [15]. Furthermore, the crystallography of thin films were evaluated using a grazing incidence technique, finding principal diffraction at (0 0 2), (1 0 0), (2 0 1), (1 1 0) and (2 2 1) as presented in figure 2, confirming a R3mhexagonal space group [15, 20]. The inset corresponds to Raman spectra indicating modes of vibration at (measured at ~4.6 mW), two well-known mode of vibrations are clearly present at E12g at 378 cm−1 and A1g at 407 cm−1 in agreement with Kong et al [14] as well with Lince and Fleischauer, who grew MoS2 thin films using RF sputtering techniques [18]. From HRTEM measurements it was possible to confirm a vertical growth of crystallites with (1 0 0) surface termination and basal plane (0 0 1) stacking (inset: molecular model) as presented in figure 3, with stacking along [0 0 1] of 7– 10 MoS2 laminates with interplanar average spacing of 0.62 nm, in agreement with the metrics presented before when using this microscopy technique [15, 19]. The mechanical properties were set to evaluate both H and E of the MoS2 thin films using a comparison with silicon substrate (0 0 1) surface termination and applying the CSM as described extensively by Oliver and Pharr [16]. The equation to determine the S in CSM method is: −1

S=

1

F0 Z0

1 − 2 Kf cos ϕ − (Ks − mω )

(1)

where ω is the excitation frequency, Zo the displacement amplitude, ϕ the phase angle, and F0 is the excitation amplitude. All of those values can be obtained if the machine parameters load-frame stiffness K f and stiffness of springs Ks, as well the mass m, are known input values during the nanoindentation test. Using this methodology is possible to obtain with high accuracy quantities such as hardness and elastic modulus as a function of continuous surface penetration at


M Ramos et al

Figure 1. Scanning electron micrograph at 30 kV. One can observe a vertically aligned growth of MoS2 crystallites; inset: profile of one flake thickness of 14.77 nm. Energy dispersed spectrum confirming Mo, S atoms on the surface of region under observation.

Figure 2. Small angle x-ray diffraction pattern with principal diffraction at (0 0 2), (1 0 0), (2 0 1), (1 1 0) and (2 2 1). Inset: Raman spectra with two characteristic modes of vibrations at E12g at 378 cm−1 and A1g at 407 cm−1, in agreement with [14].

3


M Ramos et al

Figure 3. High-resolution transmission electron microscopy confirming a vertical growth of crystallites along [1 0 1] and stacking of seven MoS2 laminates per crystallite d-spacing of 0.62 nm in (0 0 2) basal plane, in agreement with [15].

Figure 4. Left: nanoindentation curves measured by CSM at three regions (I, II, III). The region I corresponds to elastic values of E = 136 ± 2 GPa at 0–90 nm of penetration depth at MoS2 thin film (no influence from silicon substrate). Right: the region I corresponds to hardness values of H = 10.5 ± 0.1 GPa at 0–90 nm of penetration depth at MoS2 thin film (no influence from silicon substrate). Zones II and III for both elastic modulus and hardness correspond to partial and full influence from the substrate; the reason why an increase on both values are observed.

Figure 5. Atomic force scratching at MoS2 thin film (thickness 105 nm) maximum wear deformation of 0.85 µm2 with a total groove height 125 nm; no cracks or delamination wer observed near the scratching edges, indicating a good adherence achieved by RF sputtering technique, and a scanning electron micrograph showing the indentation over the MoS2 thin film surface.

selected zones over a large nanoscale area, as presented in the supplementary material (stacks.iop.org/ STMP/5/025003/mmedia), indicating three regions (I, II, III). On the left side of figure 4 we present 4

region I corresponding to the hardness only of MoS2 crystallites with 0–90 nm of penetration depth, with no influence from substrate, with H = 10.5 ± 0.1 GPa. The second region II with 90–120 nm of penetration


M Ramos et al

depth hardness is clearly influenced by substrate, since the thin film thickness was 105 nm as measured digitally. Finally region III represents a hardness completed influenced by silicon substrate, that is to say the penetration depth reaches the substrate. The elastic modulus at each zone on the left side of figure 4 show the elastic modulus behavior as a function of the penetration depth of the MoS2, with average values of E = 136 ± 2 GPa. Indeed, the values obtained are smaller in comparison with 270 ± 100 GPa, as described by Bertolazzi et al [10], on single MoS2 layers suspended on patterning silicon substrate. We believe the discrepancy between their results and our findings occurs due to how low dimension MoS2 laminates are stronger than stacked MoS2 crystallites (as observed by scanning electron micrographs), plus the force applied was carried out over a (0 0 2)-basal plane which is not chemical bounded into silicon support. Moreover their residual value would be ±100 GPa, compared with our ±2 GPa of accuracy. In our case, from the scanning electron images we can certainly conclude that the applied force was applied over the (1 0 0)-plane (vertical alignment of layers, as observed on SEM and HRTEM images). Furthermore, we evaluate the adherence of MoS2 film using a scratching technique in AFM mode results, which indicate a maximum wear deformation 0.85 µm2 with a residual groove width of 1 µm, a total groove height 125 nm and a pile up height of 40 nm, as presented in figure 5, presenting no delamination or cracks near the groove.

Lockheed Martin Corporation, for the U.S. Department of Energy’s National Nuclear Security Administration under contract DE-AC04-94AL85000. The principal author thanks the Dirección de Planeación y Desarrollo Institucional of Universidad Autónoma de Cd. Juárez for the summer travel grant award PROFOCIE 2015. We thank Laboratorio Nacional de Nanotecnología of Centro de Investigación en Materiales Avanzados (CIMAV-Chihuahua) and ICNAM program of Kleberg Advanced Laboratory Center of University of Texas at San Antonio both for the usage of electron microscopy and characterization equipment and facilities.

Conflicts of interest All listed authors declare no conflict of interest.

Author contributions M Ramos, J Nogan and M Ortiz were in charge of RF sputtering and fabrication at CINT, J L EnríquezCarrejo performed Raman measurements, J Mireles and C Rodríguez-Gonzalez performed x-ray characterization and had insightful discussions at UACJ. Mechanical properties were measured by A HurtadoMacias and the electron microscopy in scanning transmission mode was completed by M Ramos and C Ornelas at CIMAV and UTSA. The manuscript was mainly written by M Ramos, J L Enríquez-Carrejo and A Hurtado-Macias.

4. Conclusions We present a complete study on mechanical properties by way of a nanoindentation technique using CSM on MoS 2 thin films (~105 nm) fabricated by RFmagnetron sputtering. Our results as obtained by electron microscopy indicate a vertical growth along the (1 0 0) crystallographic plane, exposing both sulfur and molybdenum surface termination with laminar stacking of 7–10 layers at 0.62 nm and high porosity between crystallites; the obtained hardness (H) and elastic modulus (E) were H = 10.5 ± 0.1 GPa and E = 136 ± 2 GPa respectively, and from scratching it was found a value of wear deformation 0.85 µm2 with no cracks or delamination. We believe this is insightful and useful information for any desired chemical etching and nanopattering when designing low dimension integrated electronics using layered molybdenum disulfide semiconducting materials.

Acknowledgments This work was performed, in part, at the Center for Integrated Nanotechnologies, an Office of Science User Facility operated for the U.S. Department of Energy (DOE) Office of Science. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of 5

References [1] Radisavljevic B, Radenovic A, Brivio J, Giacometti V and Kis A 2011 Single-layer MoS2 transistors Nat. Nanotechnol. 6 147–50 [2] Mak K F, Lee C, Hone J, Shan J and Heinz T F 2010 Atomically thin MoS2: a new direct-gap semiconductor Phys. Rev. Lett. 105 136805 [3] Kam K K and Parklnclon B A 1982 Detailed photocurrent spectroscopy of the semiconducting group VI transition metal dichalcogenides J. Phys. Chem. 86 463–7 [4] Jellinek F, Brauer G and Müller H 1960 Molybdenum and niobium sulphides Nature 185 376–7 [5] Gonzalez G A, Alvarado M, Ramos M A, Berhault G and Chianelli R R 2016 Transition states energies for catalytic hydrodesulfurization reaction in Co9S8/MoS2 theoretical interface using computer-assisted simulations Comput. Mater. Sci. 121 240–7 [6] Kang K, Xie S, Huang L, Han Y, Huang P Y, Mak K F, Kim C-J, Muller D and Park J 2016 High-mobility three-atom-thick semiconducting films with wafer-scale homogeneity Nature 520 656–60 [7] Garcia A, Raya A M, Mariscal M M, Esparza R, Herrera M, Molina S I, Scavello G, Galindo P L, Jose-Yacaman M and Ponce A 2014 Analysis of electron beam damage of exfoliated MoS2 sheets and quantitative HAADF-STEM imaging Ultramicroscopy 146 33–8 [8] Hansen L P, Johnson E, Brorson M and Helveg S 2014 Growth mechanism for single- and multi-layer MoS2 nanocrystals J. Phys. Chem. C 118 22768–73 [9] Casillas G, Santiago U, Barrón H, Alducin D, Ponce A and José-Yacamán M 2015 Elasticity of MoS2 sheets by mechanical deformation observed by in situ electron microscopy J. Phys. Chem. C 119 710–15


M Ramos et al

[10] Bertolazzi S, Brivio J and Kis A 2011 Stretching and breaking of ultrathin MoS2 ACS Nano 5 9703–9 [11] Castellanos-Gomez A, Poot M, Steele G A, van der Zant H S J, Agraït N and Rubio-Bollinger G 2012 Elastic properties of freely suspended MoS2 nanosheets Adv. Mater. 24 772–5 [12] Jiang J-W, Qi Z, Park H S and Rabczuk T 2013 Elastic bending modulus of single-layer molybdenum disulfide (MoS2): finite thickness effect Nanotechnology 24 435705 [13] Xiaodong L and Bhushan B 2002 A review of nanoindentation continuous stiffness measurement technique and its applications Mater. Charact. 48 11–36 [14] Kong D, Wang H, Cha J J, Pasta M, Koski K J, Yao J and Cui Y 2013 Synthesis of MoS2 and MoSe2 films with vertically aligned layers Nano Lett. 13 1341–7 [15] Ramos M A, Berhault G, Ferrer D A, Torres B and Chianelli R R 2012 Combined HRTEM and molecular modeling of the

6

MoS2–Co9S8 interface to understand the promotion effect in bulk HDS catalytical structures Catal. Sci. Technol. 2 164–78 [16] Pharr G M, Oliver W C and Brotzen F R 1992 On the generality of the relationship among contact stiffness, contact area, and elastic modulus during indentation J. Mater. Res. 7 613–7 [17] Perea-López N 2014 CVD-grown monolayered MoS2 as an effective photosensor operating at low-voltage 2D Mater. 1 011004 [18] Lince J R and Fleischauer P D 1987 Crystallinity of rf-sputtered MoS2 films J. Mater. Res. 2 827–38 [19] Kaplan-Ashiri I, Cohen S R, Gartsman K, Rosentsveig R, Seifert G and Tenne R 2004 Mechanical behavior of individual WS2 nanotubes J. Mater. Res. 19 454–9 [20] Peng Q and De S 2013 Outstanding mechanical properties of monolayer MoS2 and its application in elastic energy storage Phys. Chem. Chem. Phys. 15 19427–37