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The Influence of Water on the Optical Properties of SingleLayer Molybdenum Disulfide Joseph O. Varghese, Peter Agbo, Alexander M. Sutherland, Victor W. Brar, George R. Rossman, Harry B. Gray, and James R. Heath* Single-layer molybdenum disulfide (SL-MoS2) is a noncentrosymmetric semiconductor with a direct bandgap of about 1.8 eV (bulk MoS2 has an indirect bandgap of 1.2 eV),[1] and has been recently explored as a material for transistors,[2,3] and spin-control optoelectronic devices.[4,5] Investigations of field-effect SL-MoS2 devices have also revealed the presence of novel carrier physics that could be attributed to trions, which are tightly bound quasiparticles comprised of two electrons and one hole, and for which no analogue exists in bulk semiconductors.[6,7] However, many properties of SL-MoS2 are still poorly understood. For example, the typical mobility of mechanically exfoliated SL-MoS2 (0.5–3 cm2 V−1 s−1) is smaller than anticipated from theory,[3,8] an observation that may arise from unanticipated scattering mechanisms, perhaps due to lattice defects or molecular adsorbates. Chemical doping can, in fact, strongly alter the physical properties of low-dimensional materials.[9,10] For example, a cesium carbonate (Cs2CO3) layer deposited between the source and drain electrodes of a SL-MoS2 field-effect transistor will promote trion formation in SL-MoS2 via n-type doping,[11] similar to what is seen through voltage gating.[6] Weakly bound adsorbates, such as physisorbed water, may have similar influences. For example, moisture has been found to cause hysteresis in single-layer MoS2 field-effect transistors.[12] Here we quantitatively explore how adsorbed water influences the electrical and photonic properties of SL-MoS2. Using single exfoliated pieces of SL-MoS2 on atomically flat substrates, we first use scanning probe microscopy to identify monolayers of adsorbed water that are stably trapped between the SL-MoS2 and the supporting substrate.[13] We then utilize electric force (EFM) and Kelvin probe force (KPFM) microscopies[14,15] to spatially correlate the electronic structure of SL-MoS2 with the presence or absence of adsorbed water, and show that water Dr. J. O. Varghese, Dr. P. Agbo, A. M. Sutherland, Prof. H. B. Gray, Prof. J. R. Heath Division of Chemistry and Chemical Engineering California Institute of Technology MC 127–72, Pasadena, CA 91125, USA E-mail: [email protected] Dr. V. W. Brar Kavli Nanoscience Institute and Division of Applied Physics and Materials Science California Institute of Technology MC 128–95, Pasadena, CA 91125, USA Prof. G. R. Rossman Division of Geological and Planetary Sciences California Institute of Technology MS 170–25, Pasadena, CA 91125, USA

DOI: 10.1002/adma.201500555

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monolayers trapped between SL-MoS2 and atomically flat mica serve as n-type dopants, while water nanodroplets sandwiched between SL-MoS2 and highly oriented pyrolytic graphite (HOPG) serve as weak p-type dopants. We then integrate those scanning probe tools with photoluminescence (PL) imaging microscopy to correlate the SL-MoS2 PL spectra with the presence or absence of adsorbed water, and demonstrate that a monolayer of sandwiched water strongly quenches the PL in a manner that is associated with and consistent with trion formation. An alternative potential PL quenching mechanism associated with intermolecular hydrogen-bond formation or proton transfer[16–18] between the SL-MoS2 and the trapped water is tested by probing for anticipated isotopic effects using templated D2O adlayers. That mechanism is found to have, at best, only a weak influence on the PL. Figure 1a is a representative AFM image of single-layer MoS2 exfoliated in air on a muscovite mica (001) surface at ambient conditions (≈42% relative humidity and 22 °C). Plateau-like features can be seen in the structure of MoS2 that are consistent with adlayers of ice being sandwiched between the MoS2 and mica surfaces. Mica is known to stabilize ice-like water adlayers on its surface even at room temperature, as shown by scanning polarization force microscopy (SPFM) and by graphenetemplating studies.[13,19,20] Lowering the humidity prior to MoS2 deposition on the mica decreased the size and frequency of trapped adlayers (Figure S1, Supporting Information). No adlayer structures were found in nontemplated regions, consistent with the finding that capillary effects between the probe tip and sample disturb water layers on bare surfaces.[20,21] AFM images of MoS2 templated water layers bore strong similarities to those reported for graphene.[13,22] They were consistent with 0, 1, 2 or more layers of uniform and atomically flat ice (Figure S2, Supporting Information), with nano-droplet-like structures sometimes appearing on top of the ice layers. Variations in water adlayer heights from 0.6 to 1.4 nm were larger than what was observed for graphene templated adlayers of either water or small organic molecules[23] (Figure S3, Supporting Information), perhaps arising from charges that can build up in the MoS2 insulating layer. The edges of the MoS2 templated water adlayers could slowly evolve in structure over periods of several hours, although the central regions of the adlayers were stable over time periods of at least many days (Figure S4, Supporting Information). We next sought to quantify how the templated water alters the electronic structure of SL-MoS2. Figures 1e and f are EFM images taken of the same area shown in the topograph in Figure 1b, with darker/lighter regions corresponding to attractive/repulsive tip–sample interactions, respectively. The lightdark contrast in the EFM images is seen between SL-MoS2

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COMMUNICATION Figure 1. Scanning probe images of single-layer MoS2 on mica. a) AFM topograph of single-layer MoS2 exfoliated on mica (42% relative humidity, room temperature). b) Zoomed AFM topograph of approximate area indicated by the blue square in (a). c) KPFM image of same area as in (b). d) Surface potential profile taken along blue line labeled 1 in (c). e) EFM image taken at −4 V. f) EFM image taken at 4 V. Lateral scale-bar: a) 1 µm, b,c,e,f) 500 nm. Z-scale: a,b) 4 nm, c) 0.15 V, e,f) 3°.

areas that do and do not sandwich adsorbed water, and is reversed when the bias is flipped, indicating that the tip is attracted to or repelled by the SL-MoS2 simply due to the presence or absence of trapped water.[14] These qualitative[15] data demonstrate strongly localized electronic effects of water on SL-MoS2. KPFM, a complementary technique to EFM, allows for more quantitative assessments[24] of differences in work function (Φ) via measurement of the contact potential between the tip and surface, and the relation ΔΦ = −e ΔV. Since the same material (SL-MoS2) is probed in the presence and absence of water, this difference indicates a variation in the Fermi level of the sample.[25,26] Previous studies have found that water can dope SL-MoS2 and other 2D materials such as graphene.[10,22,27] Our KPFM data (Figure 1c,d) shows that the Fermi level of SL-MoS2 is raised by approximately 75 meV, suggesting that the sandwiched water adlayer electron-dopes SL-MoS2. This is significant as Fermi level variations of some tens of meV can lead to distinct charge transfer effects and energy shifts of fluorescence peaks in single-layer MoS2.[6,28] We also exfoliated MoS2 on HOPG and found that SL-MoS2 templates water on that hydrophobic surface (Figure S5, Supporting Information). The water was observed as nanodroplets aligned along step edges of the HOPG, in agreement with previous studies of adsorbed water on HOPG.[21,29] As with the case for mica, the EFM images complemented each other upon reversal of the bias voltage polarity. KPFM revealed that SL-MoS2 over adsorbed water exhibited a smaller magnitude

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difference (10 meV) and a decrease in the Fermi level relative to SL-MoS2 supported by dry HOPG (Figure S5, Supporting Information). The implication is that water serves as a (weak) hole-dopant of MoS2 when HOPG is the supporting substrate. Small and sometimes mobile particles of unknown composition were often found on the edges of the SL-MoS2 sheets, and could be dragged across the surface by the AFM tip (Figure S6, Supporting Information). These were at a different surface potential—about 20 mV lower—than the rest of the MoS2 layer and the supporting substrate. These adsorbed particles were found on all the substrates we studied, and their presence may again indicate charges in the SL-MoS2 film. The negative shift observed for those adsorbates is consistent with the qualitative expectation that SL-MoS2 edge states capable of catalytic proton reduction activity should be comparatively electron-rich.[30] We should therefore expect that SL-MoS2 edges will display a tendency for attracting electron-deficient adsorbates, an effect manifested in our surface potential maps. We investigated the influence of trapped water on MoS2 photoluminescence (PL) by correlating AFM images of SL-MoS2 on mica, with fluorescence microscopy imaging of those same films. For those regions of MoS2 that sandwiched water adlayers, the luminescence is strongly quenched (Figure 2). A comparison of the AFM topograph (Figure 2a) and the PL image (Figure 2b) reveals that the quenching is directly correlated with the presence or absence of trapped water. We acquired spatially resolved PL spectra from SL-MoS2 on mica. Using an excitation wavelength of 514.3 nm, we probed

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Photon Energy (eV) Figure 2. Scanning probe and PL images, and PL spectra of single-layer MoS2 on mica. a) AFM topograph of single-layer MoS2 exfoliated on mica (31% relative humidity and room temperature). b) PL image of same area as in (a). c) Representative PL spectra of single-layer MoS2 on mica acquired over water-covered (black curve) and water-free (red curve) areas. (Spectra represent 3 accumulations). Upper inset: Close-up of quenched PL spectra acquired over water-covered area. Lower inset: Optical image of sample—red (water-free) and black (water-covered) spots indicate where depicted spectra were acquired. (Laser alignment crosshairs digitally removed for clarity.) Lateral scale-bar: a,b) 5 µm, inset) 10 µm. Z-scale: a) 5 nm.

both water-covered and water-free areas with a spatial resolution of 2 µm and power density of about 7 kW cm−2. For water-free regions, a strong fluorescence feature, centered near 1.88 eV, was recorded, with a significantly weaker feature observed at 1.96 eV (Figure 2c). These two features originate from spin–orbit splitting of the valence band.[1,31,32] The main resonance peak is well-approximated by a Lorentzian lineshape (Figure 3a), with a full-width at half maximum of around 50 meV, implying that atomically flat MoS2 on mica exhibits homogeneously broadened emission across the direct band gap with a lifetime of about 70 fs. For regions in which SL-MoS2 covers a water adlayer, the primary PL is reduced in intensity by a factor of 30–50 (Figure 2c). The primary peak is slightly redshifted to about 1.86 eV and exhibits a Gaussian (heterogeneously broadened) profile (Figure 3b), while the smaller peak remains at

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1.96 eV. This quenching of MoS2 excited states via nonemissive pathways has been ascribed to trion formation.[6,11,28] An alternative explanation, which has been attributed to fluorescence quenching of various organic,[17,18,33] inorganic,[16] and nanoparticle[34] fluorophores, is that of excited state relaxation induced by intermolecular hydrogen-bond formation or proton transfer. Such quenching mechanisms can exhibit strong isotope effects; fluorescence quenching through O–H bonds is much more efficient than through O–D bonds. Of course, if the observed quenching of SL-MoS2 fluorescence arises from trion formation, no isotope effect should be observed. To test this hypothesis, we templated physisorbed D2O on mica in order to assess whether a similar quenching mechanism was operable when SL-MoS2 templates physisorbed H2O on mica (Experimental Section).

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Figure 3. PL spectra of single-layer MoS2 on mica. a) When probing water-free areas, the primary resonance peak (centered at 1.88 eV) in the PL spectra of single-layer MoS2 is best fit by a Lorentzian. b) With water present, the PL spectra are best fit by a Gaussian. The abscissa of the peak-center is redshifted to approximately 1.86 eV. (Spectra in (a) and (b) acquired at 7 kW cm−2.) c) Color-coded quenched PL spectra obtained at 9 kW cm−2 at various spots (shown in corresponding colors) on sample. Two peak features appear at approximately 1.83 and 1.87 eV, indicated by dashed vertical lines. (Spectra represent 3 accumulations.) Inset: Optical image of spots on sample where spectra were acquired. (Laser alignment crosshairs digitally removed for clarity.) Lateral scale-bar: inset) 10 µm.

SL-MoS2 templated D2O on mica displays a distinct morphology compared to SL-MoS2 templated H2O (Figure 4a,b). The templated D2O appears as a layer of ice clusters instead of smooth adlayers. In fact, thin layers of D2O have been shown to exist as ice clusters on various surfaces,[35–38] and so while the images of Figure 4 are structurally revealing, they are also consistent with previous reports. It appears that the ice clusters form on top of an already existing D2O layer about 2–3 Å in height on the mica surface. The D2O clusters exhibit some slight variations in shape when probed over a few-hour period. A Fourier transform of the AFM topograph (Figure S7, Supporting Information) revealed a characteristic spacing of approximately 140 nm between clusters, with cluster heights of about 6–7 Å. In regions where the SL-MoS2 templates D2O, the PL is quenched, similar to the case for templated H2O adlayers (Figure 4c). PL spectra are mostly consistent with those obtained for H2O samples, with a primary excitonic peak occurring at about 1.87 eV. This peak is largely quenched by the sandwiched D2O adlayers, and slightly shifted to approximately 1.85 eV (Figure 4d). The quenched PL spectra, however, exhibit reduced lineshape variations relative to H2O templating films, when probed with the same power density of 9 kW cm−2

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(Figure S8, Supporting Information). These differences (and similarities) between templated H2O and D2O adlayers suggest that hydroxyl coupling between SL-MoS2 and H2O (or D2O) plays a measureable but minor role in influencing the PL properties of MoS2. An alternative explanation for the water-induced PL quenching could be interfacial charge transfer with the water or D2O adlayers. Two groups have recently pointed out the potential importance of van der Waals interactions in controlling such charge transfer processes across the interface of SL-MoS2 with a second atomically thin film.[39,40] The above findings imply that the dominant PL quenching mechanism in SL-MoS2 templated water adlayers on mica is associated with trion formation. This is consistent with our observations that templated water functions as an n-type dopant of SL-MoS2. Trion formation in SL-MoS2 was first proposed by Mak et al. who showed that an applied gate voltage can split the primary excitonic peak into two resonances arising from either excitons or trions.[6] Similar evidence for two resonant peaks can be found in our quenched PL spectra. At lower power densities (7 kW cm−2) we found only one peak when probing different areas of SL-MoS2 templating water (Figure S9,

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function of anneal-induced defects and structural changes in MoS2, and does not reflect the characteristics of pristine MoS2 samples. In our system, water is trapped at the SL-MoS2-substrate interface at ambient conditions during the process of exfoliation. With no additional preparative steps, it is seen to strongly affect the PL of pristine, atomically flat SL-MoS2. Moreover, the observation of strong or quenched PL was directly 90000 d) correlated with the absence/pres6000 ence of water, for the exact same 80000 Quenched SL-MoS2 film via the integration of multiple imaging methods. Fur70000 No deuterated thermore, a small but reproducible water 60000 shift of about 1 cm−1 was observed in the out-of-plane A1g mode, 0 50000 rather than the in-plane E2g mode, 1.75 Photon Energy (eV) 2.05 of the Raman spectra of SL-MoS2 40000 on mica where water layers are present (Figure S11, Supporting 30000 Information). The A1g mode corresponds to out-of-plane vibrations 20000 of sulfur atoms with respect to the molybdenum atom.[31] Adsorbed 10000 and deposited molecules can display a much stronger electron– 0 phonon coupling to the A1g mode 1.75 1.85 1.95 2.05 than the E2g mode, in agreement with our results.[11,25,43] Our findPhoton Energy (eV) ings make qualitative sense, since Figure 4. PL spectra of single-layer MoS2–D2O-mica system. a) AFM topograph of sample. b) Zoomed adsorbed or templated molecules AFM topograph of approximate area indicated by the blue square in (a). c) PL image of same area as in will necessarily lie above or below (a). d) Representative PL spectra of single-layer MoS2 on mica acquired over D2O-covered (black curve) and D2O-free (red curve) areas. (Spectra represent 3 accumulations.) Upper inset: Close-up of quenched the MoS2 basal plane, respecPL spectra acquired over D2O-covered area. Lower inset: Optical image of sample—red (water-free) and tively, and couple with out-of-plane black (water-covered) spots indicate where spectra were acquired. (Laser alignment crosshairs digitally vibrations. removed for clarity.) Lateral scale-bar: a) 5 µm, b) 1 µm, c) 5 µm, inset) 10 µm. Z-scale: a) 10 nm, b) 4 nm. The effects of water on the electronic structure of MoS2 were spatially resolved through the use of scanning probe microscopies Supporting Information), while slightly higher power densities coupled with PL imaging of single-layer MoS2 on atomically (9 kW cm−2) revealed two peaks (Figure 3c). For those higher power densities, variations were seen in spectral lineshape, but flat substrates. In this system, the SL-MoS2 templates adsorbed two Gaussian-shaped peak features (Figure S10, Supporting water on the underlying substrate. In the absence of templated Information) were consistently recorded at approximately 1.83 water, atomically flat SL-MoS2 on mica exhibits a PL spectrum and 1.87 eV (Figure 3c). The variations are likely due to differdominated by a single homogeneously broadened feature, ences in the amount of water that is templated by the irradiated implying an excited state lifetime of around 70 fs. When water SL-MoS2. is present, templated water adlayers act as n-type dopants, shifting the Fermi level of SL-MoS2 by 75 meV. This effectively Tongay et al. reported that exposure of SL-MoS2 films to water after a 450 °C thermal annealing step enhances the quenches the SL-MoS2 PL by promoting trion formation. FluoPL,[41] although without thermal annealing, SL-MoS2 was rescence quenching via hydroxyl coupling to the underlying water adlayer is shown to be, at most, only a minor effect. The insensitive to water and other vapors. A permanent shift in the results are highly consistent with the rich optoelectronic propin-plane Raman E2g mode of annealed samples was reported, erties of MoS2 that have been previously reported, but also point suggesting the occurrence of irreversible in-plane lattice distortion. A study by Nan et al. also showed that high temperature to the need for rigorous control over the local chemical environannealing leading to increased photoluminescence correlated ment if those properties are to be fully exploited. The possibility with the appearance of cracks and defects formed in MoS2.[42] also exists, of course, to use that local chemical environment as a relatively stable means for tuning those properties. It thus appears that the acquired PL sensitivity to vapors is a

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Exfoliation of MoS2 and Preparation of Mica, HOPG, and D2O Samples: Following similar studies, molybdenum disulfide was mechanically exfoliated on mica and HOPG under various conditions of relative humidity and an ambient room temperature of about 22 °C.[13,29] Reduced humidity values of about 4 to 5% were achieved by exfoliating MoS2 in a Sigma Aldrich Atmos glovebag that was continually purged with high-purity nitrogen. Temperature and humidity were monitored using a Fluke 971 temperature/humidity meter. The Scotch tape used for exfoliation was prepared inside the glovebag, as well as the substrates for different humidity conditions. Both mica discs (1 cm diameter muscovite, Ted Pella) and HOPG (Grade ZYB, 2SPI) were cleaved and allowed to equilibrate for 15 min before exfoliation. For preparation of D2O samples, a similar procedure was followed, except that a container of deuterium oxide (100 mL, 99.9% purity, Cambridge Isotope Laboratories Inc.) was also placed inside the glovebag. The glovebag was first purged with high-purity nitrogen, and the temperature and humidity were monitored. At reduced values of less than 4% relative humidity, the D2O container was opened and poured into a 100 mL dish and allowed to equilibrate for 30 min. In this D2O atmosphere the Scotch tape used for exfoliation was prepared. MoS2 was then exfoliated onto the mica discs, after cleaving and exposing them to D2O vapor for approximately 30 s. Identification of Single-Layer MoS2: Suitable single-layer MoS2 samples were identified by optical microscopy in transmission mode for MoS2 exfoliated on mica. A color contrast was observed in reflection mode optical microscopy for MoS2 exfoliated on HOPG, with single layers appearing a faint pink color. Thicknesses were confirmed by microRaman spectroscopy using a Renishaw M1000 microRaman spectrometer with a 514.3 nm wavelength laser, spatial resolution of 2 µm, and a 2400 lines per mm grating. AFM, EFM, and KPFM: AFM, EFM, and KPFM characterizations were performed at ambient conditions using a Digital Instruments Nanoscope IIIa in Tapping Mode. Bruker TESP tips were used with a typical resonance frequency of 320 kHz, tip radius of 8 nm, and force constant of 42 N m−1. For EFM, the probe tip was oscillated near its resonance frequency and raised to a preset height (10 nm used) above the surface after an initial pass mapping the topography, and the resulting changes in the resonant frequency due to electrostatic forces were recorded while applying a DC bias voltage. For KPFM, the tip was similarly raised to a preset height of 10 nm after an initial pass to scan the surface topography, but the mechanical oscillation of the tip was switched off. An AC voltage was then applied to the tip, and the cantilever oscillation amplitude at the AC-frequency was measured. When a DC voltage difference exists between the tip and sample, the cantilever experiences an oscillating electric force at the AC-frequency, and this amplitude of oscillation was used to map the surface potential. Calibration was performed on grade ZYB freshly cleaved HOPG surfaces obtained from 2SPI. Photoluminescence Microscopy and Spectroscopy: Fluorescence images were obtained using an Olympus IX81 microscope with a dichroic mirror with cut-on wavelength at 660 nm, an excitation bandpass filter centered at a wavelength of 620 nm with 60 nm bandwidth, and an emission bandpass filter centered at a wavelength of 700 nm with 75 nm bandwidth. A 100 W mercury lamp was used as an excitation source. Fluorescence spectra were obtained on a Renishaw M1000 microRaman spectrometer with a 514.3 nm wavelength laser, spatial resolution of 2 µm and a 2400 lines per mm grating.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgments The authors thank Dr. Jamil Tahir-Kheli for helpful discussions. The authors acknowledge grants from the Department of Energy (Grant

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No. DE-FG03–01ER46175: J.R.H.) and the National Science Foundation (Grant No. EAR-1322082: G.R.R.). Collection of PL spectra was supported in part by NSF CCI Solar Fuels grant (Grant No. CHE-1305124: H.B.G). Received: February 3, 2015 Revised: February 26, 2015 Published online: March 18, 2015

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Adv. Mater. 2015, 27, 2734–2740

The influence of water on the optical properties of single-layer molybdenum disulfide.

Adsorbed molecules can significantly affect the properties of atomically thin materials. Physisorbed water plays a significant role in altering the op...
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