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Cite This: ACS Appl. Nano Mater. 2019, 2, 2593−2598
Analysis of Airborne Contamination on Transition Metal Dichalcogenides with Atomic Force Microscopy Revealing That Sulfur Is the Preferred Chalcogen Atom for Devices Made in Ambient Conditions Korbinian Pürckhauer,* Dominik Kirpal, Alfred J. Weymouth, and Franz J. Giessibl
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Institute of Experimental and Applied Physics, University of Regensburg, Regensburg 93053, Germany S Supporting Information *
ABSTRACT: The fabrication of devices incorporating transition metal dichalcogenides (TMDCs) is mostly done in ambient conditions, and thus the investigation of TMDCs’ cleanliness in air at the nanoscale is important. We imaged MoS2, WS2, MoSe2, and WSe2 using atomic force microscopy. Mechanical exfoliation of the TMDCs provided clean terraces on sulfides MoS2 and WS2. In contrast, the selenides appeared to be contaminated directly after cleavage in most cases. Long-term measurements on MoSe2 revealed that these unwanted adsorbates are mobile on the surface. In situ cleavage and imaging of WSe2 in ultrahigh vacuum shows clean surfaces, proving the airborne character of the adsorbed particles. KEYWORDS: AFM, air, ambient environment, mechanical exfoliation, TMDCs, atomic resolution, particles, ice-like water
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Kozbial et al. showed that the water contact angle (WCA) on MoS2 is directly influenced by the adsorption of airborne hydrocarbons, which are also known to adsorb on graphene and graphite to reduce the surface energy.15,16 They reported an initially hydrophilic bulk MoS2 surface with a WCA of 69°, which increases quickly in the first 24 h after cleavage to 89° and then slowly to 90.8° after 7 days. The diiodomethane contact angle (DCA), attenuated total reflectance Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy (XPS) measurements provided further evidence of hydrocarbon adsorption. A change in the DCA, which is influenced by the chemical composition of the hydrocarbons, from initially 15.2° to 27.1° after 1 h and 44.0° after 7 days, was discussed as a rearrangement in the structure of the hydrocarbons. Their ellipsometry data showed an overall thickness of the adsorbed hydrocarbon layer of 0.55 nm. In 2016, Gao et al. reported on the stability of TMDCs in ambient conditions.17 They grew MoS2 and WS2 monolayers using chemical vapor deposition and showed that they exhibited poor long-term stability in air (in the presence of O2 and H2O). Gradual oxidation, which was visible in scanning electron microscopy images, along with organic contaminants was verified by their XPS measurements. However, Longo et al. investigated the reaction of bulk MoS2, MoSe2, WS2, and WSe2 with O2 using both density functional theory and XPS measurements18 and found that in
ransition metal dichalcogenides (TMDCs) are a subgroup of 2D materials that show a range of interesting characteristics including a tunable band gap and a high carrier mobility.1 TMDCs are van der Waals stacked monolayers of MX2 materials, which consist of a transition metal atom M sandwiched between two chalcogen atoms X. These materials can be exfoliated to monolayers by mechanical exfoliation,2 which can be used, for example, to build single-layer transistors,3 heterostructures,4−6 and biosensors.7 Because the exfoliation and preparation of samples is often done in ambient conditions, the surfaces are usually exposed to air. Thus, depending on the reactivity and hydrophilicity, the surfaces are expected to be covered with various contaminations and hydration layers, which influence their material properties like the apparent monolayer thickness, carrier density, and photoluminescence.8−10 Thus, it is crucial to compare the properties of TMDCs in air. Besides the well-known hydration layers covering surfaces exposed to air,11,12 hydrocarbons can also adsorb from air onto graphene and be present in graphene-based heterostructures, as shown by Haigh et al. and Gass et al.6,13 Rooney et al. encapsulated molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2), tungsten disulfide (WS2), and tungsten diselenide (WSe2) between hexagonal boron nitrides and noticed a larger interlayer spacing for MoSe2 and WSe2 than that predicted by theory, which they attributed to a small population of chemically adsorbed species, intercalated between layers.14 They showed that cleaving WSe2 in an inert-gas atmosphere led to a significantly smaller interlayer spacing. © 2019 American Chemical Society
Received: March 20, 2019 Accepted: May 9, 2019 Published: May 9, 2019 2593
DOI: 10.1021/acsanm.9b00526 ACS Appl. Nano Mater. 2019, 2, 2593−2598
Letter
ACS Applied Nano Materials
Figure 1. Illustration of our study performed on mechanically cleaved bulk TMDCs. (a) Representative structure of a bulk TMDC material shown here exposed to air, with the cleanliness of the sample investigated with AFM using the qPlus sensors. (b) Image of the mechanical exfoliation process of MoS2, which is glued to a sample holder.
Figure 2. FM-AFM images of MoS2 (a and b) and MoSe2 (c−e) in ambient conditions. (a) Topography overview scan of MoS2. Scan parameters: f 0 = 23.5 kHz, k = 1.8 kN/m, Δf = 54 Hz, and A = 70 pm. (b) Atomic resolution of MoS2 performed in constant-height mode. Scan parameters: f 0 = 23.5 kHz, k = 1.8 kN/m, and A = 45 pm. (c) Topography overview scan of MoSe2. Scan parameters: f 0 = 46.8 kHz, k = 3.5 kN/m, Δf = 10 Hz, and A = 500 pm. (d) Atomic structure of MoSe2 still observable between the adsorbates of part c. Scan parameters: constant height, f 0 = 46.8 kHz, k = 3.5 kN/m, and A = 65 pm. (e) Time evolution of MoSe2 (500 nm × 500 nm) over a range of 45 h. The white frame indicates identical sample spots on every image.
All AFM data presented here were recorded in frequency modulation (FM) mode with a custom-designed qPlus atomic force microscope.20,21 For FM-AFM, the sensor is driven to oscillate at a constant amplitude A, and the frequency shift Δf is measured, which is itself a measure of the force gradient kts between the tip and sample.22 As a consequence, the frequency shift can be used as a feedback parameter for height control. Along with Δf, the excitation voltage that drives the cantilever’s oscillation can be recorded, which is a signature of locally varying dissipative forces. FM-AFM with stiff qPlus sensors yielded atomic resolution on graphitic samples and was used to show that bilayer graphene exposed to air has fewer contaminants than monolayer graphene.23,24 Furthermore, FM-AFM with qPlus sensors is sensitive to hydration layers on samples and, in spite of its large stiffness, allows one to image soft and weakly immobilized biological samples.20,21,24,25 The bulk crystals were glued onto a sample holder and cleaved by mechanical exfoliation in ambient conditions (relative humidity of 30−50%). AFM measurements were performed on the freshly cleaved bulk surfaces. All AFM data
air oxidation is directly dependent on the O2 dissociative adsorption barrier. Because of the high energy barrier of defectfree TMDCs, they showed that it is unlikely that TMDCs react with O2. However, they reported a higher possibility of oxidation of selenides than of sulfides. Moreover, they suggested that oxidation of TMDCs is likely to depend on the density of the grain boundaries, defects, and steps. These results were in line with the XPS measurements of Jaegermann et al. in 1985, who saw a large enhancement of oxygen species with XPS for TMDCs with large surface impurities and also reported a higher oxidation reactivity of selenides compared to sulfides, attributed to the ionicity and hybridization strength between metal and chalcogen atoms.19 In conclusion, several authors assume that airborne contaminants adsorb on TMDCs in ambient conditions, although questions about coverage, height, local distribution, and changes over time remain to be conclusively answered. In this study, we probed MoS2, MoSe2, WS2, and WSe2 in ambient conditions by atomic force microscopy (AFM), as illustrated in Figure 1. 2594
DOI: 10.1021/acsanm.9b00526 ACS Appl. Nano Mater. 2019, 2, 2593−2598
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ACS Applied Nano Materials
Figure 3. FM-AFM images of WS2 (a and b) and WSe2 (c and d) in ambient conditions along with WSe2, which was cleaved and imaged in UHV (e and f). (a) Topography overview scan of WS2 revealing a clean surface. Scan parameters: f 0 = 46.8 kHz, k = 3.5 kN/m, Δf = 50 Hz, and A = 500 pm. (b) Frequency shift image with atomic resolution of WS2. Scan parameters: quasi-constant height, f 0 = 46.8 kHz, k = 3.5 kN/m, Δf = 41 Hz, and A = 170 pm. (c) Topography overview scan of WSe2 with particles on the surface. Scan parameters: f 0 = 46.8 kHz, k = 3.5 kN/m, Δf = 30 Hz, and A = 500 pm. (d) Atomic structure of WSe2 observed between contaminants. Scan parameters: quasi-constant height, f 0 = 46.8 kHz, k = 3.5 kN/m, Δf = 110 Hz, and A = 90 pm. (e) Topography overview scan of WSe2 after UHV cleavage revealing a clean surface. Scan parameters: f 0 = 27.7 kHz, k = 1.8 kN/m, Δf = 15 Hz, and A = 470 pm. (f) High-resolution scan resolving the atomic structure of WSe2 in UHV. Scan parameters: quasi-constant height, f 0 = 27.7 kHz, k = 1.8 kN/m, Δf = 15 Hz, and A = 51 pm.
specific region in every image. Initially the particles covered 80% of the surface, and after around 2 days, the coverage was reduced to 52%. At the same time, the average height increased from initially 1.5 to 2.2 nm. This indicates that the mobile structures are aggregating. This process happens relatively quickly in the first 24 h and then slows down (see the Supporting Information). The excitation channel, recorded simultaneously, revealed that there are less dissipative forces above the adsorbed particles. The dissipation did not change over time (see Figure S3). In the past, it was shown that when the tip penetrates the hydration layers on a surface, there is a measurable increase in the dissipation signal.21,29 One possibility is that the adsorbates are covered with fewer hydration layers than the MoSe2 surface. This would be the case if the MoSe2 surface was more hydrophilic and the adsorbates were more hydrophobic. This fits well to the above-mentioned reports about timeevolved wettability due to airborne contaminants.15,16 We also investigated tungsten-based TMDCs to see if they follow the trend of molybdenum-based TMDCs. Mechanical exfoliation of WS2 led to very flat surfaces, which can be seen in Figure 3a. In this study, we never observed a step edge on WS2. High-resolution images showed the hexagonal surface structure of WS2 (see Figure 3b), and the lattice constant was determined to be 0.33 nm, which fits with the literature.26 Interestingly, long-term measurements on WS2, a few weeks after mechanical exfolation, revealed a mobile, atomically resolved, steplike structure (see Figure 4). A similar steplike feature had been imaged on KBr by AFM, which had been
presented in this paper are taken with an ambient qPlus AFM equipped with sapphire tips (as described in refs 21 and 25) if not stated otherwise (further data are available in the Supporting Information). To achieve atomically resolved FM-AFM images, the scan speed was reduced by a factor of 4 to around 100 nm/s. The first measurements were performed on a freshly cleaved MoS2 crystal (purchased from 2D semiconductors). The topography overview scans revealed large clean terraces with a second MoS2 layer visible at the bottom of Figure 2a. Highresolution constant-height Δf images recorded above one terrace resolve the atomic structure of the top layer (see Figure 2b). The bright dots correspond to the top sulfur atoms forming a hexagonal crystal structure with a spacing of 0.32 nm, which fits well with the reported literature values.26−28 Long-term measurements did not show any change in cleanliness. MoSe2 (purchased from HQ graphene) was cleaved and imaged analogously to MoS2. In contrast to the clean terraces of MoS2, overview scans on MoSe2 revealed particles adsorbed on the surface (see Figure 2c). FM-AFM images between the particles showed the atomic structure of MoSe2 (see Figure 2d) with a lattice constant of 0.34 nm, which fits with the literature.26 A spot on a specific sample was investigated over 45 h, and the adsorbates were found to be mobile on the surface, as can be seen by the series of images in Figure 2e. To show the negligible effect of thermal drift, we marked an adsorbed structure with a white frame in Figure 2e and marked the 2595
DOI: 10.1021/acsanm.9b00526 ACS Appl. Nano Mater. 2019, 2, 2593−2598
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ACS Applied Nano Materials
Figure 4. High-resolution raw FM-AFM data of the mobile layer growing on WS2 2 weeks after cleavage. Scan parameters: quasi-constant height mode, f 0 = 26.2 kHz, k = 1.8 kN/m, Δf = 290 Hz, and A = 112 pm. (a) Frequency shift image resolving the atomic structure of WS2 measured with the tip moving from left to right (forward). On the left side, there is a structure visible with higher repulsion that follows the underlying WS2 structure. (b) Height profile along the green bar. (c) Δf distance curve performed on the point marked by a green star. (d) Backward frequency shift image (the tip is measured from right to left) of WS2. On the left side, there is a smaller bright stripe corresponding to higher repulsion, the structure of which is identical with that of the underlying WS2. (e) Frequency shift image (forward direction) taken 1.5 min after image a. It shows again, on the left side, a terrace with a higher frequency shift represented by a brighter color. (f) Schematic drawing of the qPlus sensor and WS2 surface, which we propose has an icelike hydration layer on the left side and floating H2O molecules above the whole surface.
attributed to a hydration layer on the surface.29 Atomically ordered hydration layers had been previously observed by 3D AFM measurements on, among others, mica, calcite, and DNA.30 The mobile atomically resolved, steplike structure is visible as a bright area in parts a, d, and e of Figure 4. On the basis of a line profile (see Figure 4b and the position marked by a green line in Figure 4a), the step on the left-hand side is known to be more repulsive and therefore closer to the tip, with an average frequency shift of 300 Hz compared to the lower step on the right-hand side, which shows an average frequency shift of around 265 Hz. From Δf distance curves (see Figure 4c and the acquisition spot marked by a green star in Figure 4a), we were able to link the Δf difference between 300 and 265 Hz to a height difference of 0.25 nm, which fits to the height of an ordered hydration layer.29,30 Thus, we propose that the steplike feature shown in Figure 4 can be explained by the presence of an ordered hydration layer on the WS2 surface. Additionally, a comparison of the forward (from left to right, Figure 4a) and backward (from right to left, Figure 4b) images shows hysteresis in the step of the ordered hydration layer. This directional dependence could be caused by destruction of the ice-like layer in the fast scan direction as the tip approaches an upward step and preservation of the layer when the tip is moving towards a downward step. A second possibility is asymmetry of the tip apex. Notably, the atomically resolved lattice in the bright area is identical with the lattice in the darker regions, which can be assigned to the bare WS2 surface. This interpretation is further illustrated by the overlay drawing of the atomic structure of WS2 and the H2O molecules sitting
on the top sulfur atoms on the left side (see Figure 4e). In Figure 4f, there is a schematic drawing of the qPlus sensor close to the WS2 surface, which is covered with H2O molecules, and on the left side, there is an ordered hydration layer (also called an “ice-like” H2O layer) that is formed. Subsequently, freshly cleaved WSe2 was imaged in ambient conditions, shown in Figure 3c. A particle coverage of 49% over the surface was observed (see Figure S5 and a detailed discussion in the Supporting Information). In contrast to MoSe2, the adsorbed particles had a continuous height distribution, reaching a maximum of 2 nm (line profiles are included in the Supporting Information; see Figure S4). To check whether the atomic structure of WSe2 was preserved, we imaged between the contaminants. The atomically resolved image, shown in Figure 3d, shows the expected pattern with a lattice constant of 0.36 nm, which fits the reported values.26 In order to evaluate whether the particles observed on selenium-based TMDCs are intercalated contaminations between the layers and thus present also after fresh cleavage, the WSe2 sample was additionally cleaved in ultrahigh vacuum (UHV). Subsequently, AFM measurements were performed in UHV at room temperature. The scans revealed a WSe2 surface that was free of any contaminations (see Figure 3e). Highresolution FM-AFM in quasi-constant height showed the expected atomic structure of WSe 2 (see Figure 3f). Consequently, the contaminations measurable on WSe2 and MoSe2 after cleavage in ambient conditions are airborne. In conclusion, we investigated the surfaces of the TMDCs WS2, WSe2, MoS2, and MoSe2 by FM-AFM in ambient 2596
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(7) Lee, J.; Dak, P.; Lee, Y.; Park, H.; Choi, W.; Alam, M. A.; Kim, S. Two-dimensional Layered MoS2 Biosensors Enable Highly Sensitive Detection of Biomolecules. Sci. Rep. 2015, 4, 7352. (8) Peng, Z.; Yang, R.; Kim, M. A.; Li, L.; Liu, H. Influence of O2, H2O and airborne hydrocarbons on the properties of selected 2D materials. RSC Adv. 2017, 7, 27048−27057. (9) Qiu, H.; Pan, L.; Yao, Z.; Li, J.; Shi, Y.; Wang, X. Electrical characterization of back-gated bi-layer MoS2 field-effect transistors and the effect of ambient on their performances. Appl. Phys. Lett. 2012, 100, 123104. (10) Tongay, S.; Zhou, J.; Ataca, C.; Liu, J.; Kang, J. S.; Matthews, T. S.; You, L.; Li, J.; Grossman, J. C.; Wu, J. Broad-Range Modulation of Light Emission in Two-Dimensional Semiconductors by Molecular Physisorption Gating. Nano Lett. 2013, 13, 2831−2836. (11) Hu, J.; Xiao, X. D.; Ogletree, D. F.; Salmeron, M. Imaging the Condensation and Evaporation of Molecularly Thin Films of Water with Nanometer Resolution. Science 1995, 268, 267−269. (12) Santos, S.; Verdaguer, A. Imaging Water Thin Films in Ambient Conditions Using Atomic Force Microscopy. Materials 2016, 9, 182. (13) Gass, M. H.; Bangert, U.; Bleloch, A. L.; Wang, P.; Nair, R. R.; Geim, A. K. Free-standing graphene at atomic resolution. Nat. Nanotechnol. 2008, 3, 676. (14) Rooney, A. P.; Kozikov, A.; Rudenko, A. N.; Prestat, E.; Hamer, M. J.; Withers, F.; Cao, Y.; Novoselov, K. S.; Katsnelson, M. I.; Gorbachev, R.; Haigh, S. J. Observing Imperfection in Atomic Interfaces for van der Waals Heterostructures. Nano Lett. 2017, 17, 5222−5228. (15) Kozbial, A.; Gong, X.; Liu, H.; Li, L. Understanding the Intrinsic Water Wettability of Molybdenum Disulfide (MoS2). Langmuir 2015, 31, 8429−8435. (16) Li, Z.; Wang, Y.; Kozbial, A.; Shenoy, G.; Zhou, F.; McGinley, R.; Ireland, P.; Morganstein, B.; Kunkel, A.; Surwade, S. P.; Li, L.; Liu, H. Effect of airborne contaminants on the wettability of supported graphene and graphite. Nat. Mater. 2013, 12, 925. (17) Gao, J.; Li, B.; Tan, J.; Chow, P.; Lu, T.-M.; Koratkar, N. Aging of Transition Metal Dichalcogenide Monolayers. ACS Nano 2016, 10, 2628−2635. (18) Longo, R. C.; Addou, R.; KC, S.; Noh, J.-Y.; Smyth, C. M.; Barrera, D.; Zhang, C.; Hsu, J. W. P.; Wallace, R. M.; Cho, K. Intrinsic air stability mechanisms of two-dimensional transition metal dichalcogenide surfaces: basal versus edge oxidation. 2D Mater. 2017, 4, 025050. (19) Jaegermann, W.; Schmeisser, D. Reactivity of layer type transition metal chalcogenides towards oxidation. Surf. Sci. 1986, 165, 143−160. (20) Giessibl, F. J. The qPlus sensor, a powerful core for the atomic force microscope. Rev. Sci. Instrum. 2019, 90, 011101. (21) Pürckhauer, K.; Weymouth, A. J.; Pfeffer, K.; Kullmann, L.; Mulvihill, E.; Krahn, M. P.; Müller, D. J.; Giessibl, F. J. Imaging in Biologically-Relevant Environments with AFM Using Stiff qPlus Sensors. Sci. Rep. 2018, 8, 9330. (22) Albrecht, T.; Grütter, P.; Horne, D.; Rugar, D. Frequency modulation detection using high-Q cantilevers for enhanced force microscope sensitivity. J. Appl. Phys. 1991, 69, 668−673. (23) Wastl, D. S.; Speck, F.; Wutscher, E.; Ostler, M.; Seyller, T.; Giessibl, F. J. Observation of 4 nm Pitch Stripe Domains Formed by Exposing Graphene to Ambient Air. ACS Nano 2013, 7, 10032− 10037. (24) Wastl, D. S.; Weymouth, A. J.; Giessibl, F. J. Atomically resolved graphitic surfaces in air by atomic force microscopy. ACS Nano 2014, 8, 5233−5239. (25) Wastl, D. S.; Judmann, M.; Weymouth, A. J.; Giessibl, F. J. Atomic resolution of calcium and oxygen sublattices of calcite in ambient conditions by atomic force microscopy using qPlus sensors with sapphire tips. ACS Nano 2015, 9, 3858−3865. (26) Ding, Y.; Wang, Y.; Ni, J.; Shi, L.; Shi, S.; Tang, W. First principles study of structural, vibrational and electronic properties of graphene-like MX2 (M = Mo, Nb, W, Ta; X = S, Se, Te) monolayers. Phys. B 2011, 406, 2254−2260.
conditions. While the surfaces of the sulfur-based TMDC samples stayed clean for up to weeks, the surfaces of the selenium-based materials were contaminated directly after exfoliation in ambient conditions. Long-term measurements showed that sulfur-based TMDCs stayed clean in air. Interestingly, we observed an ordered H2O layer on the WS2 surface after 2 weeks, which could be manipulated with the AFM tip. Cleavage in UHV led to clean surfaces, which shows that the contaminants, visible after cleavage in ambient conditions, are airborne. Long-term measurements on MoSe2 showed that the adsorbates are mobile on the surface and aggregate to larger patches, reducing the initial coverage of 80% to 52%. The average height of the adsorbed patches increased from 1.5 to 2.2 nm over time. On WSe2, the contaminants, which were up to 2 nm high, covered 49% of the surface. Our findings suggest that sulfur-based TMDC materials are more appropriate for ambient-fabricated devices compared to selenium-based materials.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b00526. Particle analysis on MoSe2, dissipation of energy on MoSe2, particle analysis on WSe2, and raw data of Figures 2 and 3 (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Korbinian Pürckhauer: 0000-0002-5687-7464 Alfred J. Weymouth: 0000-0001-8793-9368 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank the German Research Foundation (Grants SFB 689, SFB 1277, and GRK 1570) for their support as well as Alexey Chernikov and Tobias Korn for supplying samples, valuable discussions, and helpful editorial suggestions. Additionally, we thank Alexander Liebig for proofreading and fruitful discussions.
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REFERENCES
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