Analysis of Airborne Contamination on Transition Metal

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Letter

Analysis of Airborne Contamination on Transition Metal Dichalcogenides with Atomic Force Microscopy Reveals that Sulfur is the Preferred Chalcogen Atom for Devices Made in Ambient Conditions Korbinian Pürckhauer, Dominik Kirpal, Alfred John Weymouth, and Franz J. Giessibl ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00526 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 11, 2019

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ACS Applied Nano Materials

Analysis of Airborne Contamination on Transition Metal Dichalcogenides with Atomic Force Microscopy Reveals that Sulfur is the Preferred Chalcogen Atom for Devices Made in Ambient Conditions Korbinian Pürckhauer,∗ Dominik Kirpal, Alfred J. Weymouth, and Franz J. Giessibl Institute of Experimental and Applied Physics, University of Regensburg, 93053 Regensburg, Germany E-mail: [email protected]

Abstract Fabrication of devices incorporating transition metal dichalcogenides (TMDCs) is mostly done in ambient conditions and thus investigation of TMDCs’ cleanliness in air at the nanoscale is important. We imaged MoS2 , WS2 , MoSe2 and WSe2 by using atomic force microscopy (AFM). 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 ultra-high vacuum shows clean surfaces, proving the airborne character of the adsorbed particles.

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Keywords AFM, ambient, mechanical exfoliation, TMDC, particles, contaminations

Transition metal dichalcogenides (TMDCs) are a subgroup of 2D materials which show a range of interesting characteristics including a tunable bandgap 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 down to monolayers by mechanical exfoliation, 2 which can be used, for example, to build single-layer transistors, 3 heterostructures 4–6 and biosensors. 7 As the exfoliation and preparation of samples is often done in ambient conditions, the surfaces are usually exposed to air. Thus, depending on reactivity and hydrophilicity, the surfaces are expected to be covered with various contaminations and hydration layers which influence their material properties like apparent monolayer thickness, carrier density and photoluminescence. 8–10 Thus it is crucial to compare the properties of TMDCs in air. Beside the well-known hydration layers covering surfaces exposed to air, 11,12 hydrocarbons can also adsorb from the air onto graphene and can be present in graphene-based heterostructures, as shown by Gass et al. and Haigh et al.. 6,13 Rooney et al. encapsulated MoS2 , MoSe2 , WS2 and WSe2 between hexagonal boron nitride (hBN) and noticed a larger interlayer spacing for MoSe2 and WSe2 than predicted by theory which they attributed to a small population of a chemically adsorbed species, intercalated between layers. 14 They could show that cleaving WSe2 in an inert gas atmosphere leads to a significantly smaller interlayer spacing. 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 an WCA of 69° which increases quickly in the first 24 hours after cleavage to 89° and then slowly to 90.8° after 7 days. Diiodomethane contact angle (DCA), attenuated total 2


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reflectance Fourier transform infrared spectroscopy (ATR-FTIR) and X-ray photoelectron spectroscopy (XPS) measurements provided further evidence of hydrocarbon adsorption. A change in DCA, which is influenced by the chemical composition of hydrocarbons, from initially 15.2° to 27.1° after one hour 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 H2 O). The gradual oxidation, which was visible in scanning electron microscopy (SEM) 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 oxygen using both density-functional theory (DFT) and XPS measurements 18 and found that in air oxidation is directly dependent on the O2 dissociative adsorption barrier. Due to the high energy barrier of defect-free TMDCs, they showed that it is unlikely that TMDCs react with oxygen. 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 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 change over time remain to be conclusively proven. In this study, we probed molybdenum disulfide (MoS2 ), molybdenum diselenide (MoSe2 ), tungsten disulfide (WS2 ) and tungsten diselenide (WSe2 ) in ambient conditions by atomic force microscopy (AFM). 3



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Figure 1: Illustration of our study performed on mechanically cleaved bulk TMDCs. (a) The representative structure of a bulk TMDC material shown here is exposed to to air and the cleanliness of the sample is visualized by AFM using the qPlus sensors. (b) Image of mechanical exfoliation process of MoS2 , which is glued to a sample holder. All AFM data presented here were recorded in frequency modulation (FM) mode with a custom-design qPlus AFM. 20,21 For FM-AFM the sensor is driven to oscillate at a constant amplitude A and the frequency shift ∆f is measured, which is itself a measure of the force gradient kts between tip and sample. 22 As a consequence, the frequency shift can be used as a feedback parameter for height control. Along with ∆f , 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 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, enables 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 to 50 %). The AFM measurements were performed on the freshly cleaved bulk surfaces. All AFM data presented in this paper are taken with an ambient qPlus AFM equipped with sapphire tips (as described in 21,25 ) if not stated otherwise (further data available in supporting information). To achieve atomically resolved FM-AFM images the scan speed was reduced by a factor of four to around 100 nm/s. 4


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Figure 2: FM-AFM images of MoS2 (a and b) and MoSe2 (c, d and e) in ambient conditions. (a) Topography overview scan of MoS2 . Scan parameters: f0 = 23.5 kHz, k = 1.8 kN/m, ∆f = 54 Hz, A = 70 pm. (b) Atomic resolution of MoS2 performed in constant-height mode. Scan parameters: f0 = 23.5 kHz, k = 1.8 kN/m, A = 45 pm. (c) Topography overview scan of MoSe2 . Scan parameters: f0 = 46.8 kHz, k = 3.5 kN/m, ∆f = 10 Hz, A = 500 pm. (d) Atomic structure of MoSe2 still observable between adsorbates of figure (c). Scan parameters: constant height, f0 = 46.8 kHz, k = 3.5 kN/m, A = 65 pm. (e) Time evolution of MoSe2 (500 nm × 500 nm) over a range of 45 h. The white frame indicates the identical sample spot on every image. The first measurements were performed on a freshly cleaved molybdenum disulfide crystal (MoS2 , purchased from 2D Semiconductors). The topography overview scans revealed large clean terraces with a second MoS2 layer visible at the bottom of Fig. 2(a). High-resolution constant-height ∆f images recorded above one terrace resolve the atomic structure of the top layer (see Fig. 2(b)). The bright dots correspond to the top sulfur atoms forming a hexagonal crystal structure with a spacing of 0.32 nm which fits well to reported literature values. 26–28 Long-term measurements did not show any change in cleanliness. Molybdenum diselenide (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 Fig. 2(c)). FM-AFM images between the particles showed the atomic structure of MoSe2 (see Fig. 2(d)) with a lattice constant of 0.34 nm which fits to literature. 26 A spot on a specific sample was investigated over 45 hours and the adsorbates were found 5



to be mobile on the surface, as can be seen by the series of images in Fig. 2(e). To show the negligible effect of thermal drift, we marked an adsorbed structure with a white frame in Fig. 2(e) and marked the specific region in every image. Initially the particles covered 80% of the surface and after around two days the coverage was reduced to 52 %. At the same time, the average height increased from initially 1.5 nm to 2.2 nm. This indicates that the mobile structures are aggregating. This process happens relatively quickly in the first 24 hours and then slows down (see 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 Fig. S3 supporting information). In the past, it was shown that when the tip penetrates 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 more hydrophobic. This fits well to the above-mentioned reports about time-evolved wettability due to airborne contaminants. 15,16 We also investigated tungsten based TMDCs to see if they follow the trend of molybdenum based TMDCs. The mechanical exfoliation of tungsten disulfide (WS2 ) led to very flat surfaces which can be seen in Fig. 3(a). In this study, we never observed a step edge on WS2 . High-resolution images showed the hexagonal surface structure of WS2 (see Fig. 3(b)) and the lattice constant was determined to be 0.33 nm which fits to literature. 26 Interestingly, long-term measurements on WS2 , a few weeks after mechanical exfolation, revealed a mobile, atomically-resolved, step-like structure (see Fig. 4). A similar step-like feature had been imaged on KBr by AFM which had been attributed to a hydration layer on the surface. 29 An atomically-ordered hydration layers have been previously observed by 3D-AFM measurements on among others mica, calcite and DNA. 30 The mobile atomically-resolved, step-like structure is visible as bright area in Fig. 4(a),Fig. 4(d) and Fig. 4(e). Based on a line profile (see Fig. 4(b), position marked by green line in Fig. 4(a)), the step on the left-hand side 6


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Figure 3: FM-AFM images of WS2 (image (a) and (b)) and WSe2 (image (c) and (d)) in ambient conditions along with WSe2 which was cleaved and imaged in UHV (image (e) and (f)). (a) Topography overview scan of WS2 reveals clean surface. Scan parameters: f0 = 46.8 kHz, k = 3.5 kN/m, ∆f = 50 Hz, A = 500 pm. (b) Frequency shift image with atomic resolution of WS2 . Scan parameters: quasi-constant height, f0 = 46.8 kHz, k = 3.5 kN/m, ∆f = 41 Hz, A = 170 pm. (c) Topography overview scan of WSe2 with particles on the surface. Scan parameters: f0 = 46.8 kHz, k = 3.5 kN/m, ∆f = 30 Hz, A = 500 pm. (d) Atomic structure of WSe2 observed between contaminants. Scan parameters: quasi-constant height, f0 = 46.8 kHz, k = 3.5 kN/m, ∆f = 110 Hz, A = 90 pm. (e) Topography overview scan of WSe2 after UHV cleavage reveals clean surface. Scan parameters: f0 = 27.7 kHz, k = 1.8 kN/m, ∆f = 15 Hz, A = 470 pm. (f) High-resolution scan resolving the atomic structure of WSe2 in UHV. Scan parameters: quasi-constant height, f0 = 27.7 kHz, k = 1.8 kN/m, ∆f = 15 Hz, A = 51 pm. 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 ∆f -distance curves (see Fig. 4(c), acquisition spot marked by green star in Fig. 4(a)) we were able to link the ∆f difference between 300 Hz 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 step-like feature shown in Fig. 4 can be explained by the presence of an ordered hydration layer on the WS2 surface. Additionally, the comparison of the forward (from left to right, Fig. 4(a)) and backward (from right to left, Fig. 4(b)) images shows a hysteresis in the step of the ordered hydration layer. This directional dependence could be caused by the destruction of the ice-like layer in the fast scan direction as the tip 7



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Figure 4: High-resolution raw FM-AFM data of mobile layer growing on WS2 two weeks after cleavage. Quasi-constant height mode, f0 = 26.2 kHz, k = 1.8 kN/m, ∆f = 290 Hz, 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 which follows the underlying WS2 structure. (b) Height profile along green bar. (c) ∆f -distance curve performed on point marked by green star. (d) Backward frequency shift image (tip measures from right to left) of WS2 . On the lift side there is a smaller bright stripe corresponding to higher repulsion which structure is identical to the underlaying WS2 . (e) Frequency shift image (forward direction) taken 1.5 minutes 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 qPlus sensor and WS2 surface which we propose has an ice-like hydration layer on the left side and floating water molecules above the whole surface. approaches an upward step and the preservation of the layer when the tip is moving towards a downward step. A second possibility is an asymmetry of the tip apex. Notably, the atomically-resolved lattice in the bright area is identical to 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 water molecules sitting on the top sulfur atoms on the left side (see Fig. 4(e)). In Fig. 4(f) there is a schematic drawing of the qPlus sensor close to the WS2 surface which is covered with water molecules and on the left side there is an ordered hydration layer (also called an "ice-like" water layer) formed. Subsequently, freshly cleaved WSe2 was imaged in ambient conditions, shown in Fig. 3(c). A particle coverage of 49% over the surface was observed (see Fig. S5 and detailed discussion 8


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in supporting information). Compared 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 Fig. S4). To check whether the atomic structure of WSe2 was preserved, we imaged between the contaminants. The atomically resolved image, shown in Fig. 3(d), shows the expected pattern with a lattice constant of 3.6 nm which fits to 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 ultra-high vacuum (UHV). Subsequently, AFM measurements were performed in UHV at room temperature (RT). The scans revealed a WSe2 surface which was free of any contaminations (see Fig. 3(e)). High-resolution FMAFM in quasi-constant height showed the expected atomic structure of WSe2 (see Fig 3(f)). 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 conditions. While the surfaces of sulfur based TMDC samples stayed clean 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 water layer on the WS2 surface after two 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 nm to 2.2 nm over time. On WSe2 the contaminants, which were up to 2nm high, covered 49 % of the surface. Our findings suggest that sulfur based TMDC materials are more appropriate for ambient-fabricated devices as compared to selenium based materials.

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Supporting Information Particle analysis on MoSe2 ; Dissipation of energy on MoSe2 ; Particle analysis on WSe2 ; Raw data of Figure 2; Raw data of Figure 3.

Acknowledgements We thank the German Research Foundation (SFB 689, SFB 1277, 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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Page 17 of 19 1 2 3 4 5 6 7 8 9 10 11 12

a

0.57 nm

b

57 Hz


200 nm

d

1 nm

0 nm

116 Hz

e

21 Hz

0.72 nm

c

400 nm

f

2.13 nm

0 nm

29 Hz

ACS Paragon Plus Environment 1 nm

104 Hz

200 nm

0 nm

400 pm

5 Hz

a

326 Hz

b


350

310

300

∆f [Hz]

∆f [Hz]

290 280

d

327 Hz

150

50 0

260

257 Hz

200

100

270

600 pm

Page 18 of 19

2.5 Å 250

300

1 2 3 4 5 6 7 8 9 10 11 12

c

320

0

e

0.5

1

x [nm]

1.5

0

2

327 Hz

2

4

6

z [nm]

8

10

f qPlus

ice-like

ACS Paragon Plus Environment 600 pm

256 Hz

600 pm

264 Hz

:S

:W

: H2O

: sulfur

: molybdenum

: selenium

PageACS 19 of Applied 19 Nano Materials 600 pm

270 pm

1 3 nm 2 ACS Paragon Plus Environment 3 100 nm 4100 nm 0 nm MoS2 in air

MoSe2 in air

Analysis of Airborne Contamination on Transition Metal

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