Spatially Selective Enhancement of Photoluminescence in MoS2 by

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Surfaces, Interfaces, and Applications

Spatially Selective Enhancement of Photoluminescence in MoS by Exciton-Mediated Adsorption and Defect Passivation 2

Saujan V. Sivaram, Aubrey T Hanbicki, Matthew R. Rosenberger, Glenn G. Jernigan, Hsun-Jen Chuang, Kathleen M. McCreary, and Berend T. Jonker ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b00390 • Publication Date (Web): 11 Apr 2019 Downloaded from http://pubs.acs.org on April 11, 2019

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ACS Applied Materials & Interfaces

Spatially Selective Enhancement of Photoluminescence in MoS2 by ExcitonMediated Adsorption and Defect Passivation Saujan V. Sivaram,1a* Aubrey T. Hanbicki,1 Matthew R. Rosenberger,1a Glenn G. Jernigan,2 Hsun-Jen Chuang,1b Kathleen M. McCreary,1 Berend T. Jonker1* 1

Materials Science & Technology Division, Naval Research Laboratory, Washington, DC 20375, USA 2 Electronics Science & Technology Division, Naval Research Laboratory, Washington, DC 20375, USA a Postdoctoral associate at the Naval Research Laboratory through the National Research Council b Postdoctoral associate at the Naval Research Laboratory through the American Society for Engineering Education *Correspondence and requests for materials should be addressed to S.V.S. (email: [email protected]) or B.T.J. (email: [email protected]) Abstract Monolayers of transition metal dichalcogenides (TMDs) are promising components for flexible optoelectronic devices due to their direct band gap and atomically thin nature. The photoluminescence (PL) from these materials is often strongly suppressed by non-radiative recombination mediated by mid-gap defect states. Here, we demonstrate up to a 200-fold increase in PL intensity from monolayer MoS2 synthesized by chemical vapor deposition (CVD) by controlled exposure to laser light in ambient. This spatially resolved passivation treatment is air and vacuum stable. Regions unexposed to laser light remain dark in fluorescence despite continuous impingement of ambient gas molecules. A wavelength dependent study confirms that PL brightening is concomitant with exciton generation in the MoS2; laser light below the optical band gap fails to produce any enhancement in the PL. We highlight the photo-sensitive nature of the process by successfully brightening with a low power broadband white light source. We decouple changes in absorption from defect passivation by examining the degree of circularly polarized PL. This measurement, which is independent of exciton generation, confirms that laser brightening reduces the rate of non-radiative recombination in the MoS2. A series of gas exposure studies demonstrate a clear correlation between PL brightening and the presence of water. We propose that H2O molecules passivate sulfur vacancies in the CVD-grown MoS2, but require photo-generated excitons to overcome a large adsorption barrier. This work represents an important step in understanding the passivation of CVD-synthesized TMDs and demonstrates the interplay between adsorption and exciton generation. Keywords Transition metal dichalcogenides; photoluminescence; adsorption

two-dimensional

materials;

ACS Paragon Plus Environment

surface

passivation;

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Introduction Transition metal dichalcogenides (TMDs) are a promising class of layered semiconductors with properties that are ideal for applications in optoelectronics, flexible electronics, and chemical sensing.1–4 At the monolayer limit, many TMDs (e.g., MX2 where M = Mo/W and X = S/Se/Te) exhibit strain and compositionally tunable direct band gaps that span the visible spectrum.5–7 Facile heterostructure formation is also possible due to the lack of out of plane bonding which relaxes the constraints of epitaxial matching.8 The breaking of inversion symmetry creates unique optical selection rules for these TMD monolayers, whereby circularly polarized light can selectively populate degenerate valleys in the Brillouin zone.9–11 This effect can be leveraged as an additional state variable in valleytronic devices. Optimizing the photoluminescence (PL) from these TMDs is a key goal for future optoelectronic applications. Central to this process is understanding and passivating non-radiative defects. Sulfur vacancies are the prevalent defect observed in both monolayer MoS2 exfoliated from bulk crystal and MoS2 monolayers synthesized directly by chemical vapor deposition (CVD).12 These vacancies create mid-gap states in the band structure and serve as non-radiative carrier recombination sites.13 An ideal passivant would remove these mid-gap states without significantly altering the carrier density (i.e., isovalent). To date, a number of studies have explored passivation strategies ranging from thiols and superacids to O2 plasma irradiation with varying success.14–17 The superacid, TFSI, has shown the most promise, but the lack of vacuum stability suggests this passivation is not robust.14,17 Concurrently, researchers have shown that the electronic properties of monolayer TMDs can be modulated by ambient gas molecules.2,18–22 These effects, such as changes to PL intensity and peak position, are transient and attributed to charge transfer from physisorbed gas molecules (e.g., O2 and H2O).20 Laser irradiation studies of TMDs have demonstrated an increase in PL intensity with an accompanying peak shift, but the high laser powers used make it challenging to decouple photo and thermal effects.18,19,23,24 Indeed, similar laser powers are used to ablate TMD layers.24


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It is critical to understand the interaction between gaseous molecules and monolayer TMDs because these materials are typically prepared and characterized in ambient air. As oxygen is isovalent with sulfur and selenium, it is widely suspected that H2O or O2 can passivate chalcogen vacancies;13,25 however, it is unclear whether the activation barriers towards adsorption and dissociation are overcome at room temperature.26 Additionally, frequent exposure of the TMD to light and heat during device fabrication makes it challenging to assess whether the intrinsic material properties are being investigated. In this work, we demonstrate an air and vacuum stable passivation of CVD synthesized MoS2 by exposure to low power (~ 2.5 W/m2) laser light in ambient. This spatially resolved passivation treatment results in a PL intensity increase of up to 200x with no peak shift. We highlight the photo-sensitive nature of the process by successfully brightening MoS2 with a low power broadband white light source. A wavelength dependent study confirms that PL brightening is dependent upon exciton generation in the MoS2. Cryogenic studies show that the treatment results in an increase of both the bound and neutral exciton intensities. We decouple changes in absorption from defect passivation by examining the degree of circularly polarized PL, which confirms that the laser brightening reduces non-radiative recombination in the MoS2. Results MoS2 Brightening in Ambient Air We first synthesize monolayer MoS2 triangles (edge length ~75 m) on SiO2/Si via atmospheric pressure CVD. Prior to the laser brightening treatment, the as-grown MoS2 is annealed in H2 for 30 minutes at 300–500 ˚C, which is a common step in TMD passivation studies.14,20 Figure 1A shows the time-dependent PL of the MoS2 monolayer excited by a continuous wave (cw) 532 nm laser (~2.5 W/m2) and measured in ambient. We observe a rapid increase in PL intensity by two orders of magnitude within the first 60 seconds of laser illumination and minimal intensity change thereafter. The raw PL spectra acquired at 1 s and 220 s are shown in Figure 1B plotted on a linear scale. We note that the CVD-grown MoS2 monolayer appears to be under tensile strain indicated by a red shift in the peak position (Eo = 1.85 eV) compared to exfoliated MoS2 monolayers.14,20


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Normalized PL spectra plotted in Figure 1C confirm that neither the peak position nor the peak width changes appreciably over the course of the 5 minute exposure.

Figure 1. Laser brightening of 1L MoS2 in air. (A) Photoluminescence (PL) intensity as a function of laser exposure time. Semi-log plot shows orders of magnitude increase in PL over five minute exposure. Acquisition time is 1 second and spectra are recorded every 2 seconds. Inset: Cartoon illustrates spatially-controlled brightening of MoS2 triangle. (B) Plot of spectra acquired at 1 and 220 seconds demonstrates 200x increase in PL intensity. (C) Normalized spectra acquired at the indicated time and offset for clarity. Dashed line is a guide to the eye and indicates the PL peak position does not shift. (D) Optical image of MoS2 after laser brightening in air. Scale bar, 20 μm. (E) Fluorescence image shows brightened regions displaying the letters “NRL.” Scale bar, 20 μm. MoS2 monolayers interrogated before H2 annealing, show a similar, but weaker (~3x), laser brightening effect, illustrated in Figure S1. The fact that the peak position does not shift indicates that the annealing does not impart any additional degree of strain or significantly dope the TMD. For example, one expects a ~30 meV red shift (blue shift) and subsequent peak broadening (sharpening) with a shift from neutral (charged exciton, or trion) exciton to charged trion (neutral exciton).27 No such shift is observed. Time elapsed Raman spectra shown in Figure S2 corroborates this lack of charge transfer/doping. The lack of A1g peak shifting or broadening indicates that the carrier density is not appreciably changing.28 We note that this laser brightening


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treatment is comparable to the superacid treatment which exhibits a ~190x improvement in PL signal.14 Laser exposure in ambient dramatically modifies the MoS2, resulting in persistent brightening. Areas illuminated by the laser light appear bright in the post-measurement fluorescence images. The optical image in Figure 1D is taken after laser brightening and shows that laser exposure produces no visible deleterious effect. The fluorescence image in Figure 1E of the same sample shows the letters “NRL” which are not visible in the optical image. Fluorescence images are proportional to PL spatial maps but do not preserve spectral dependence. The basal plane of the large triangle unexposed to laser light appears dark (Fig. 1E). Attempts to laser brighten in vacuum (< 1 × 10–3 Torr) are unsuccessful (Figure S3) and indicate that constituents in ambient air are necessary to brighten the MoS2. Indeed, the line shape of PL intensity versus time (Fig. 1A) is similar to an adsorption isotherm curve where the saturation of PL intensity corresponds to a saturation of available adsorption sites.29 This time-dependent PL response and PL saturation is consistent with adsorption of gas molecules on the surface rather than a basic charge transfer or ionization of gas phase species. We note that the bright edges of the triangle are likely due to differences in growth chemistry or strain. Water is Responsible for Brightening Having established that MoS2 brightening is due to the confluence of ambient gas molecules and light, we first identify the component(s) in air that is necessary for MoS2 brightening. Figure 2A shows the PL intensity measured during 532 nm laser exposure (~0.6 W/m2) in different gas environments (see Methods in Supporting Information for additional details). We show that only H2O vapor has a sizeable brightening effect; H2, N2, and O2 do not have any measurable impact on the PL. This result is surprising since density functional theory suggests that only O2 adsorbs at sulfur vacancies and acts as an isovalent passivant.13 At the start of the laser exposure the PL intensity from the H2O+O2 case is greater than the other conditions. We hypothesize this difference in PL intensity stems from the rapid brightening/adsorption at the onset, which is faster than the resolution of our PL measurement (1 s). Figure 2B shows the spatial PL intensity map recorded in ambient air after the brightening attempts in Fig. 2A. The bright region in the center of the PL map


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is the result of the brightening conducted in the H2O+O2 environment. This persistent brightening is similar to the brightening conducted in ambient (Fig. 1E) and confirms that H2O is required for the increase in MoS2 PL. As we will discuss later, we postulate the H2O is directly adsorbed to the MoS2. Figure 2C shows that the peak position is not significantly modulated, which is again consistent with the brightening conducted in ambient air (Fig. 1C). The fluorescence image in Figure 2D demonstrates the sensitivity of the MoS2 to light exposure and the ability of this approach to precisely define brightened regions. The grid pattern in the MoS2 triangle is clearly observed and is the result of the PL map measured in ambient and shown in Figure 2B, C. We rule out a synergistic effect between the H2O and O2 by comparing MoS2 brightening of a nearby triangle in a H2O+N2 background. Figure S4A shows that the MoS2 successfully brightens in both environments and a larger PL intensity is measured in the H2O+O2 ambient; however, Figure S4B indicates that the degree of PL enhancement is greater for the H2O+N2 environment.


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Figure 2. Identifying constituents in air responsible for MoS2 brightening. (A) MoS2 PL intensity as a function of laser exposure time in different gas environments. Prior to measurement, the vacuum chamber is pumped to ~1 × 10-3 Torr and backfilled to ~760 Torr with the gas or gas mixture labeled in the plot. The black line corresponds to PL intensity from MoS2 in an atmosphere of 5% H2 and 95% N2. The red line refers to PL measured in a 100% O2 atmosphere. The blue line is the PL intensity obtained in an H2O and O2 environment. (B) PL intensity spatial map recorded in ambient air after brightening attempts shown in A. The bright region in the center corresponds to the brightening in the H2O + O2 environment. Scale bar, 10 μm. C) PL peak position map demonstrating the lack of peak shifting. Scale bar, 10 μm. (D) Fluorescence map recorded in ambient after PL map measurement in B and C. Bright region in the center of triangle corresponds to the same region shown in B. Surrounding grid is the result of brightening due to PL map measurement. Scale bar, 20 μm. Effect of Laser Wavelength on MoS2 Brightening We now explore the role of light exposure on MoS2 brightening in further detail. In Figure 3 we vary the incident laser wavelength and examine its effect on TMD brightening in ambient. We use fluorescence images rather than direct PL spectra to avoid the effects of incident laser power and above band gap excitation. Figure 3A shows a fluorescence image of a MoS2 triangle after H2 annealing and after being subjected to a five minute local exposure of cw laser light in ambient at 1.60 eV (775 nm) at the location indicated by the arrow. Figure 3B shows the same triangle after exposure to cw laser light at 1.65 eV (750 nm) with the same power of ~120 W/m2 measured at the sample. Figures 3A,B show that despite the relatively long duration and high laser intensity, no local PL enhancement is observed. As the laser is tuned to energies near and exceeding the Aexciton resonance, we clearly observe the brightening effect as seen in Figures 3C, D, and E. The ellipsoid laser brightened spot in Figure 3E is due to a slight out-of-focus laser beam.


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Figure 3. Wavelength dependent brightening of MoS2. (A, B, C, D, E) Fluorescence image of MoS2 triangle recorded after five minute exposure to laser light in ambient at the indicated excitation energy. Power measured at the sample is 120 W/m2 for all laser lines. Arrows indicate the region that was illuminated by laser light and colored arrows in (C, D, and E) highlight the region brightened by the laser at the indicated energy. Scale bars, 20 m. (F) Differential reflectivity of MoS2 measured at room temperature in ambient. The arrows correspond to the spectral energy used in A, B, C, D, and E. Dips in reflectivity are due to absorption at the “A” and “B” exciton resonances, and labeled respectively. By comparing the wavelength dependent brightening to reflectivity measurements, we demonstrate that MoS2 brightening is dependent upon photo-induced exciton generation. Figure 3F shows the room temperature reflectivity of the MoS2 monolayer illuminated by the broadband white light source. We calculate differential reflectivity, which we use a proxy for optical absorption,7,30 via the following equation: Δ𝑅 𝑅

=

𝑅𝑜 ― 𝑅 𝑅𝑜

Eq. 1

where R is the reflectivity measured on the MoS2 and Ro is the reflectivity measured on the nearby SiO2/Si substrate. The dips in reflectivity with increasing excitation energy correspond to MoS2


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optical absorption at the A and B exciton resonances located at ~1.82 and 1.98 eV, respectively. When we overlay the excitation energies used in Figure 3, we find that the laser brightening matches well with the spectral range of the A exciton. Specifically, the MoS2 does not brighten when illuminated by light far below the A exciton (Figure 3A, B). As the laser energy approaches the band edge, the TMD brightens, as shown in Figure 3F. We note that some degree of laser brightening is observed below the peak optical absorption of MoS2 (i.e., the 1.71 and 1.76 eV laser lines in Fig. 3). We attribute this effect to the broad distribution of absorption states in the MoS2 due to inhomogeneities in strain or doping. We underscore the photo-mediated nature of MoS2 brightening by exposure to a low power broadband white light source focused onto selected areas of the sample by a microscope objective. Figure S5A shows the fluorescence image of a MoS2 triangle prior to exposure to the white light source in ambient. Figure S5B clearly shows that the MoS2 triangle is brightened by this very weak source (~P < 10 nW/m2), with exposure limited to two well separated 1 m spots. We note that the bright edges of the triangle are likely due to differences in growth chemistry or strain. Persistence and Low Temperature Results To understand the nature of the H2O induced brightening, we examine the previously brightened MoS2 in vacuum and at cryogenic temperatures. Figure 4A shows a fluorescence image recorded in ambient air ~72 hours after the initial laser brightening. We find the same contrast difference between the unexposed regions and the laser brightened regions in both the “aged” sample (Figure 4A) and immediately after laser brightening (Figure 1E). This indicates that the activation barrier towards H2O adsorption and desorption is significantly greater than kT (~26 meV at 298 K). This also implies that physisorption of background gases (e.g., N2, O2, H2O, or CO2) on the surface of the MoS2 does not impart the same brightening effect without exposure to above band gap light. Figure 4B demonstrates the brightened regions are stable and remain bright in vacuum (< 1 × 105

Torr) for >18 hours and cooling to 4 K. Figure 4C compares the low temperature PL spectra

acquired from the brightened (blue) and non-brightened (red) regions of the MoS2. Upon cooling to 4 K, the single PL peak (see Figure 1B) is resolved into two peaks centered at 1.71 and 1.89 eV, which we define as the bound exciton (Xb) and free exciton (Xo) respectively.31 The “Bright”


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spectrum shows increased Xb and Xo intensity compared to the “Dark” spectrum with no significant change in peak position or peak width. The lack of peak shifting at low temperature further confirms that the laser brightening does not appreciably modify the MoS2 carrier density. We compare the contribution of the two peaks and find that the laser passivation treatment preferentially increases the bound exciton, Xb, relative to the free exciton, Xo. It is unclear if the increase of Xb relative to Xo is due to different adsorption states of the H2O on MoS2.

Figure 4. Persistence and low temperature characterization of laser brightening. (A) Fluorescence image recorded 72 hours after initial laser brightening with sample stored in N2 purged dry box between measurements. (B) Fluorescence image recorded at 4 K demonstrating laser brightening is stable at low temperature and high vacuum. (C) Low temperature PL spectra acquired with 532 nm excitation source. Blue spectrum corresponds to a laser brightened MoS2 region and the red spectrum refers to the untreated MoS2. Lower energy peak is labeled as the bound exciton, Xb, while the higher energy peak corresponds to the neutral exciton, Xo. Dotted lines are a guide for the eye and demonstrates the peak position does not change. An increase in PL intensity can be attributed to two key factors –passivation of non-radiative recombination sites and greater light absorption efficiency. To understand the relationship between these factors and their impact on PL, we use a simple rate equation to describe the conservation of excitons (NE) which includes exciton formation and non-radiative and radiative channels: 𝑑𝑁𝐸 𝑑𝑡

𝑁𝐸

= 𝛼𝑀𝑜𝑆2𝑃𝑔𝑒𝑛 ― 𝜏𝑁𝑅 ―

𝑁𝐸

(Eq. 2)

𝜏𝑅

where MoS2 is the absorption coefficient of the MoS2, Pgen is the radiant flux at the sample, NR is the non-radiative lifetime and R is the radiative lifetime. The PL is intrinsically linked to all three terms in Equation 2. While it is often assumed that adsorbed molecules only alter non-radiative


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channels, we show that both the non-radiative lifetime and absorption coefficient are changing for our MoS2 brightening. The low temperature reflectivity measurements in Figure S6 reveal that laser brightened areas absorb marginally more light at the A and B resonances compared to the untreated regions. This increase in absorption is consistent with other observations seen in the literature.21 The change in optical absorption is relatively small compared to the orders of magnitude increase in PL intensity. Therefore, H2O adsorption on MoS2 must significantly alter non-radiative recombination channels as well. In the reflectivity we observe only two absorption features, one that corresponds to the neutral exciton, Xo, at 1.89 eV and the other is the B exciton at 2.05 eV. Comparing the reflectivity in Figure S6 to the PL emission in Figure 4C, we do not observe an additional absorption feature corresponding to Xb which supports our assignment that Xb is a lower energy bound feature which originates from the A exciton. We note the significant increase in Xb PL intensity in the laser brightened spectrum relative to the modest increase in optical absorption (Figure S6). This indicates that the bound exciton feature is not an intrinsic structural defect as previously posited,32 but instead is associated with adsorbates bonded to the TMD surface. Effect of MoS2 Brightening on Valley Polarization We unambiguously decouple changes in exciton generation and exciton lifetimes by examining the valley polarization of the MoS2. This measurement involves optically exciting the sample with circularly polarized light, and analyzing the resulting PL for both positive and negative helicity. The degree of circular polarization, Pcirc, is independent of exciton generation rate and is described by the following equation:9,33

𝑃𝑐𝑖𝑟𝑐 =

𝑃𝑜 1 + 2𝜏

(Eq. 3)

𝜏𝑅𝜏𝑁𝑅 (𝜏 𝑆 𝑅 + 𝜏𝑁𝑅)

where Po is the polarization of the incoming light (> 97 %) and S is the valley relaxation lifetime. Assuming R and S are intrinsic material properties,34 and a constant Po, any perturbations to Pcirc must be a result of NR.35 Figure 5A shows low temperature circularly polarized PL spectra excited


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with positive helicity light at 1.96 eV (633 nm) (see Characterization section in Methods in Supporting Information for details on polarized PL measurements). The red curve signifies PL emission with a positive helicity () and the blue curve shows emission with negative () helicity. Both spectra exhibit the two emission features Xo and Xb; however, the intensity of Xo is significantly reduced in the  spectrum. Figure 5B shows the  PL intensity map of the Xo spectral region and Figure 5C plots the  PL intensity. As expected, the laser brightened regions exhibit significantly higher PL intensity with either helicity. We focus on the free exciton, Xo, since the optical selection rules are well understood for this system, and the valley polarization is not necessarily preserved for a bound multi-body excitonic features – indeed, we note that Xb exhibits no polarization.36 The degree of circular polarization is defined by the following formula: 𝐼 + (𝜆) ― 𝐼 ― (𝜆)

(Eq. 4)

𝑃𝑐𝑖𝑟𝑐(𝜆) = 𝐼 + (𝜆) + 𝐼 ― (𝜆)

where I+ and I– are the positive and negative PL emission intensity, respectively. MoS2 is well known to show a high degree of polarization,9 and we observe a polarization of ~70% in the nontreated areas. However, Figure 5D shows that the laser brightening treatment modulates this polarization. Areas that are exposed to the laser light in ambient show a ~12% reduction in valley polarization. Based on Equation 3, the reduction in valley polarization is consistent with an increased non-radiative lifetime, NR, and confirms that H2O adsorption passivates non-radiative recombination sites on the MoS2.


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Figure 5. Circularly polarized emission of MoS2 at 4 K. (A) PL spectra of MoS2 excited by circularly polarized ( 633 nm source at 4 K. Red spectrum corresponds to PL emission with positive () helicity and the blue spectrum refers to PL emission with negative () helicity. Dashed box is a guide to show the spectral range selected for maps in B, C, and D. Sharp peaks near 1.9 eV are Raman lines associated with MoS2. (B) PL map of positive helicity () emission (C) PL map of negative helicity () emission Scale bar, 5 m. (D) Map showing degree of valley polarization of MoS2 calculated using Eqn. 4. Scale bar, 5 m. Discussion Our laser brightening treatment is a novel passivation strategy that is air and vacuum stable and comparable to or better than the superacid treatment in terms of PL intensity increase.14 We have shown that this passivation is due to the exciton-mediated interaction with H2O and suspect that adsorption takes place at sulfur vacancy sites on the MoS2. The fact that this chemisorption is mediated by exciton generation allows us to spatially define passivated regions and modulate


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valley polarization. We comment on the features that make this study unique and its relation to the other findings in the literature. We observe a time-dependent PL brightening when the MoS2 is laser treated in ambient but do not observe this same effect in vacuum. Unlike other studies,18,19,23,24 our light-induced brightening uses low laser powers (< 2.5 W/m2), and we can even successfully brighten the MoS2 using a broadband white light source (Figure S5). Furthermore, the PL peak position is invariant with time, which suggests that charge transfer between the adsorbate and MoS2 is not significant (i.e., isovalent) or that the sulfur vacancy is a deep level trap that is not delocalized at room temperature.21 We therefore assert that PL brightening is not due to simply physisorption of gas molecules; despite constant exposure to ambient gases, only light exposure brightens the MoS2. We further note that the brightening occurs only for incident photon energies at or above the optical band gap, indicating that the presence of excitons is necessary to promote adsorption. This activation barrier for adsorption strongly suggests the formation covalent bonds.37 Additionally, the brightened regions are stable in vacuum and in air. We hypothesize that the H2O moiety bonds preferentially to sulfur vacancies and the exciton may catalyze the dissociation of the water molecule. Further studies are needed to determine the exact bonding configuration and distribution of adsorption states. We rule out laser heating effects due the lack of Raman peak shift (Figure S2), lack of PL peak shift (Figure 1C), and low incident laser power used.38,39 With increasing temperature, the MoS2 PL peak typically red shifts, weakens in intensity, and broadens;39 we do not observe any of these effects during the laser brightening. The fact that brightening also occurs with a broadband white light source (< 10 nW/m2) precludes thermal effects and underscores the photocatalytic nature of this brightening. The activation of MoS2 via H2 annealing appears critical for laser brightening. Similar, if not more aggressive, annealing is employed in the superacid treatment and gas sensitivity studies.14,20 Without the H2 anneal, we observe a modest, 3x increase in PL signal (Figure S1). As others have noted, the annealing likely removes species such as adsorbed hydrocarbons, and the H2 environment likely induces sulfur vacancies.20 It is well known that annealing MoS2 in a highly


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reducing environment leads to sulfur vacancy formation along the MoS2 edges;40 however, the activation of the basal plane has not been directly observed. Tongay et al. have used Auger spectroscopy to show a decrease in the sulfur signal after in situ annealing.20 Aggressive treatments such as high temperature H2 annealing or Ar plasma exposure can successfully modulate the Mo:S ratio, but invariably lead to secondary effects such as etching of the MoS2.41,42 At high sulfur vacancy concentrations, theory suggests the band gap will also be significantly reduced.42 Given that we do not observe any morphological changes or shifts in PL positions with the H2 annealing, we suggest sulfur vacancy formation is likely but exists at a moderate concentration ( 97% for all laser lines prior to any circularly polarized measurements, resulting in an uncertainty of ~3%.

Supporting Information


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Additional PL, Raman, fluorescence, and reflectance measurements (Figures S1-S6) . Acknowledgements The authors gratefully acknowledge funding support for this work from core programs at NRL and the NRL Nanoscience Institute and by the Air Force Office of Scientific Research under Contract AOARD 14IOA018-134141. This research was performed while S.V.S.. and M.R.R. held a National Research Council fellowship and H.-J.C. held an American Society for Engineering Education fellowship at NRL. SVS gratefully acknowledges conversations with A.R. Kulkarni on adsorption mechanisms. References (1) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-Dimensional Transition Metal Dichalcogenides. Nature Nanotechnology 2012, 7 (11), 699–712. https://doi.org/10.1038/nnano.2012.193. (2) Perkins, F. K.; Friedman, A. L.; Cobas, E.; Campbell, P. M.; Jernigan, G. G.; Jonker, B. T. Chemical Vapor Sensing with Monolayer MoS2. Nano Lett. 2013, 13 (2), 668–673. https://doi.org/10.1021/nl3043079. (3) Choi, M.; Park, Y. J.; Sharma, B. K.; Bae, S.-R.; Kim, S. Y.; Ahn, J.-H. Flexible ActiveMatrix Organic Light-Emitting Diode Display Enabled by MoS2 Thin-Film Transistor. Science Advances 2018, 4 (4), eaas8721. https://doi.org/10.1126/sciadv.aas8721. (4) Jariwala, D.; Sangwan, V. K.; Lauhon, L. J.; Marks, T. J.; Hersam, M. C. Emerging Device Applications for Semiconducting Two-Dimensional Transition Metal Dichalcogenides. ACS Nano 2014, 8 (2), 1102–1120. https://doi.org/10.1021/nn500064s. (5) Li, H.; Duan, X.; Wu, X.; Zhuang, X.; Zhou, H.; Zhang, Q.; Zhu, X.; Hu, W.; Ren, P.; Guo, P.; Ma, L.; Fan, X.; Wang, X.; Xu, J.; Pan, A.; Duan, X. Growth of Alloy MoS2xSe2(1–x) Nanosheets with Fully Tunable Chemical Compositions and Optical Properties. J. Am. Chem. Soc. 2014, 136 (10), 3756–3759. https://doi.org/10.1021/ja500069b. (6) Ruppert, C.; Aslan, O. B.; Heinz, T. F. Optical Properties and Band Gap of Single- and Few-Layer MoTe2 Crystals. Nano Lett. 2014, 14 (11), 6231–6236. https://doi.org/10.1021/nl502557g. (7) Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Atomically Thin MoS2: A New DirectGap Semiconductor. Physical Review Letters 2010, 105 (13). https://doi.org/10.1103/PhysRevLett.105.136805. (8) Novoselov, K. S.; Mishchenko, A.; Carvalho, A.; Neto, A. H. C. 2D Materials and van Der Waals Heterostructures. Science 2016, 353 (6298), aac9439. https://doi.org/10.1126/science.aac9439. (9) Mak, K. F.; He, K.; Shan, J.; Heinz, T. F. Control of Valley Polarization in Monolayer MoS2 by Optical Helicity. Nature Nanotechnology 2012, 7 (8), 494–498. https://doi.org/10.1038/nnano.2012.96.


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