Laser Annealing Improves the Photoelectrochemical Activity of

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Laser Annealing Improves the Photoelectrochemical Activity of Ultrathin MoSe Photoelectrodes 2

Li Wang, Merranda Schmid, Zach Nilsson, Muhammad Tahir, Hua Chen, and Justin B. Sambur ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04785 • Publication Date (Web): 09 May 2019 Downloaded from http://pubs.acs.org on May 10, 2019

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ACS Applied Materials & Interfaces

Laser Annealing Improves the Photoelectrochemical Activity of Ultrathin MoSe2 Photoelectrodes Li Wang,1 Merranda Schmid,1 Zach N. Nilsson,1 Muhammad Tahir,2 Hua Chen,2,3 and Justin B. Sambur1,3* 1Department

of Chemistry, 2Department of Physics, 3School of Advanced Materials Discovery

(SAMD), Colorado State University, Fort Collins, Colorado, 80528, USA Correspondence to Prof. Justin B. Sambur. E-mail: [email protected]

KEYWORDS: transition metal dichalcogenides (TMDs), photoelectrochemical cell, laser treatment, vacancy healing, photocurrent mapping

ABSTRACT. Understanding light-matter interactions in transition metal dichalcogenides (TMDs) is critical for optoelectronic device applications. Several studies have shown that high intensity light irradiation can tune the optical and physical properties of pristine TMDs. The enhancement in optoelectronic properties has been attributed to a so-called laser annealing effect that heals chalcogen vacancies. However, it is unknown whether laser annealing improves functional properties such as photocatalytic activity. Here we show that high intensity supra band gap illumination improves the photoelectrochemical activity of MoSe2 nanosheets for iodide oxidation in ITO/MoSe2/I–, I3–/Pt liquid junction solar cells. Ensemble-level photoelectrochemical measurements show that, on average, illuminating MoSe2 thin films with 1 W/cm2 532 nm excitation increases the photoelectrochemical current by 142% and shifts the photocurrent 1 ACS Paragon Plus Environment

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response to more favorable (negative) potentials. Scanning photoelectrochemical microscopy measurements reveal pristine bilayer (2L)-MoSe2, trilayer (3L)-MoSe2 and multilayer-thick nanosheets are initially inactive for iodide oxidation. The light treatment activates 2L-MoSe2 and 3L-MoSe2 material and the activation process initiates at the edge sites. The photocurrent enhancement is more significant for 2L-MoSe2 than for 1L-MoSe2. Multilayer-thick MoSe2 remains inactive for iodide oxidation even after the laser treatment. Our microscopy measurements reveal that the laser-induced enhancement effect depends critically on MoSe2 layer thickness. Xray photoelectron spectroscopy (XPS) measurements further show that the laser treatment oxidizes Mo(IV) species that are initially associated with Se vacancies. Ambient oxygen fills the Se vacancies and removes trap states, thereby increasing the overall photogenerated carrier collection efficiency. To the best of our knowledge, this work represents the first report on using laser to enhance the photoelectrocatalytic properties of few-layer-thick TMDs. The simple and rapid laser annealing procedure is a promising strategy to tune the reactivity of TMD-based photoelectrochemical cells for electricity and chemical fuels production.

Introduction Two-dimensional transition metal dichalcogenides (TMDs) have unique electronic properties that are attractive for ultrathin opto-electronic device applications.1-3 While the basal planes of group-VI TMDs are considered chemically inert, recent studies have shown that TMDs evolve over time depending on their exposure to environmental factors such as moisture, oxygen, temperature and light.4-5 It is particularly important to understand how light influences properties of TMDs in device applications such as solar cells, photo-detectors, and light emitting diodes that all rely on light-matter interactions.4, 6 Raman and photoluminescence (PL) micro-spectroscopy are often used to characterize structural and photophysical properties of TMDs. These 2 ACS Paragon Plus Environment

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spectroscopy techniques typically require high intensity radiation doses (e.g., 1 mW incident power focused to a 1 µm diameter spot, yielding 0.13 MW/cm2 power density). Understanding to what extent high intensity light beams induce changes in TMD properties is important for optoelectronic applications. At the same time, there is also strong interest in using light to controllably modify TMD properties.3-4 High intensity irradiation modifies the physical properties of TMDs. Castellanos-Gomez et al. reported that focused laser treatments could thin multilayer-thick MoS2 to monolayers.7 Laserinduced-thinning is a promising route to fabricate and pattern large area 1L-TMD devices. Later, Hu et al. further expanded this laser-thinning procedure to the preparation of 2L- and 3L-MoS2 by controlling the laser power and laser exposure time.8 Cho et al. used focused laser treatments to induce the semiconductor-to-metal phase transition in MoTe2.9 The authors used this laser-induced polymorph engineering strategy to create ohmic homojunctions in MoTe2 transistors that exhibited 50 greater carrier mobility than unmodified devices. High intensity illumination has also been shown to induce lattice contraction in few-layer-thick TMDs.10 Theoretical and experimental results showed that above band gap illumination modifies the inter-layer van der Waals interactions and increases coupling between layers.10-11 High intensity laser illumination also alters the optical and electronic properties of TMDs. Lu et al. showed that focused laser illumination increased the conductivity and sensitivity of 1L-WSe2 and 1L-MoS2 photodetectors.12-13 The improved device characteristics were attributed to the atomic healing of chalcogen vacancies by ambient oxygen.13 Venkatakrishnan14 and Bera15 showed that high intensity laser irradiation increased the PL response of 1L-MoS2. The PL enhancement was also attributed to laser-induced healing of defect states. The increased PL suggested that light treatment removed fast non-radiative recombination pathways, which could 3 ACS Paragon Plus Environment


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be beneficial for solar energy conversion applications. On the other hand, prolonged illumination of 1L-MoS2 and 1L-WS2 has been shown to decrease the PL intensity.16-18 The decrease of PL intensity was attributed to light-induced reactions with H2O, N2, and O2 that are present in the environment under ambient conditions. Substantial research has shown that laser annealing influences the photophysical properties of TMDs.4,

10, 12-23

The ability to tune photophysical

properties suggests that laser annealing affects the underlying charge generation and recombination processes in 2D materials. Since charge generation and recombination are also underlying processes in photo(electro)catalysis, these data findings suggest that laser annealing may also be useful to tune functional properties such as photo(electro)catalytic activity. In addition, it is unclear how high intensity irradiation influences the reactivity of TMDs with different layer thicknesses. Here we report that focused laser annealing increases the photoelectrochemical activity of chemical vapor deposition (CVD)-grown MoSe2 films. We studied ITO/MoSe2/I–, I3–/Pt electrochemical cells because MoSe2 electrodes are photochemically stable in iodide electrolytes and the bulk MoSe2/iodide cell is a 9.4% efficient liquid junction solar cell.24 Continuous excitation of MoSe2 films with a 1 W/cm2, 2 millimeter-sized laser spot increases the photocurrent response for iodide oxidation by 142%. Quantitative photocurrent microscopy measurements indicate that the high intensity illumination improves the photocurrent response of 1L-MoSe2 and 2L-MoSe2, but the effect is significantly more pronounced in 2L-MoSe2. The laser treatment increases the light absorption of 1L-MoSe2 and 2L-MoSe2, consistent with literature reports,13 which can be attributed to removing disorder or defects in the material. However, the increases in light absorption cannot account entirely for the photocurrent enhancement effect. XPS measurements indicate that intense irradiation oxidizes Mo (IV) sites that are associated with Se 4 ACS Paragon Plus Environment

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vacancies. The light treatment removes Se vacancy mid-gap states and therefore improves charge separation efficiency. The approach demonstrated herein can be applied generally to a wide range of TMDs and photoelectrocatalytic reactions. Results and Discussion. We first studied the ensemble-level photoelectrochemical properties of the MoSe2 film in an iodide-based electrolyte. MoSe2 films were synthesized via CVD on sapphire substrates and mechanically transferred to indium doped tin oxide (ITO) substrates using a poly(methyl methacrylate) (PMMA) stamp. The MoSe2-coated ITO substrate served as working electrode in a three-electrode microfluidic electrochemical cell that was mounted on the stage of an inverted optical microscope as shown in Figure 1a. Figure 1b shows the pathways for photogenerated charge carriers in the photoelectrochemical cell. Photogenerated holes (h+) in MoSe2 are transported to the semiconductor/liquid interface and oxidize iodide to iodine (i.e., 2h+ + 2I–  I2). The oxidized product (I2) reacts with bulk I– according to I2 + I–  I3–. Photogenerated electrons (e–) in MoSe2 are extracted to the external circuit where they perform work and then reduce I3– to I– at the Pt counter electrode. Thus, the photoelectrochemical solar cell produces electricity from incident solar energy and causes no net chemical change in the electrolyte.25

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Figure 1. Ensemble-level (photo)electrochemical properties of laser-annealed MoSe2 films. (a) Schematic illustration of the experimental setup. A 532 nm laser excites the sample through the microscope objective and the electrochemical current from the entire cell is monitored by a potentiostat. The laser light reflected from the sample is collected in a backscatter geometry and detected by a Raman spectrometer. ctr = Pt wire counter electrode. ref = Ag/AgI reference electrode. (b) Energy level diagram and pathways for photogenerated electrons and holes in the ITO/MoSe2/I–, I3–/Pt photoelectrochemical cell. (c) Cyclic voltammetry (CV) scans of the ITO/MoSe2 electrode in 1 M NaI electrolyte using a scan rate of 50 mV/sec. Each cycle represents 5 dark CV scans followed by 15 light CV scans. A 2-mm diameter, 31 mW 532 nm laser source (1 W/cm2 power density) illuminated the sample. For clarity, we plot the 3rd dark CV scan and the 7th light CV scan from each cycle rather than all 1000 CV scans. (d) The dark and light CV scans from the 1st and 50th cycle. The vertical pink line represents the fixed potential for photocurrent mapping experiments (0.3 V). (e) Photocurrent versus illumination cycle number, where the photocurrent is calculated by subtracting the average current from the dark CV scans from the average current from the light CV scans. The error bars represent the error-propagated standard deviation.

To study the influence of irradiation time on the photocurrent onset potential and photocurrent efficiency for iodide oxidation, we measured the (photo)electrochemical response of the ITO/MoSe2/I–, I3–/Pt cell during continuous potential cycling and under high intensity light conditions. Specifically, 1 M NaI electrolyte was flowed into the cell and the electrochemical


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current was first measured under dark conditions for 5 consecutive cyclic voltammetry (CV) scans from 0.0 to 0.4 V vs Ag/AgI reference electrode. Then, the electrochemical current was measured for 15 CV scans under high intensity illumination conditions over the same potential range. A continuous wave 31.8 mW 532 nm laser excited a 2 mm diameter portion of the 1 cm2 MoSe2 film, yielding an incident power density of 1 W/cm2 or 3×104 photons s–1 nm–2. We refer to the 5 dark CV scans followed by the 15 light CV scans as one illumination cycle. The 532 nm laser illuminates the sample for 4 minutes during each cycle. Figure 1c shows the 3rd dark scan and the 7th light scan for 50 consecutive cycles, which represent the midpoint scans for the dark and light segments of each cycle. All CV scans exhibit a large, non-Faradaic double layer charging current that stems from the entire ITO electrode, which contains about 9 cm2 of bare ITO electrode surface and 1 cm2 of the MoSe2 film. While the total electrochemical response stems from both ITO and MoSe2 electrochemically active areas, the electrochemical current induced by photo-excitation (i.e., the photocurrent) stems from MoSe2 only. The ITO substrate produces no measurable photocurrent signal under all 532 nm excitation conditions and potential ranges employed in this study. To clearly illustrate the influence of light irradiation on the photocurrent onset potential and photocurrent efficiency for iodide oxidation, we plotted the dark and light CV scans from the 1st and 50th cycles in Figure 1d, corresponding to the (photo)electrochemical response of the pristine and 200 minutes laser-treated film. The 1st CV scan under dark conditions (solid black trace in Figure 1d) shows no anodic current upon scanning from 0.0 to 0.4 V. Upon illuminating the electrode for the first time (dashed black trace in Figure 1d), we observed an anodic current onset at 0.1 V whose magnitude increases with increasingly positive potentials. The anodic current can be

attributed

to

iodide

oxidation

to

iodine.

Photo-excitation

of

MoSe2

induces 7

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photoelectrochemical iodide oxidation at potentials 500 mV more negative than the dark electrochemical process that onsets at 0.60 V vs Ag/AgI (Figure S1a), in agreement with the literature.26 In addition, a large reduction peak is observed at 0.26 V during the cathodic scan, which can be attributed to electrochemical reduction of photogenerated iodine on the MoSe2 surface (i.e., I2 + 2 e–  2I–). Continuous illumination of the MoSe2 film leads to a significant increase in the photocurrent response at positive potentials (e.g., 0.25 to 0.40 V). Figure 1e shows that the photocurrent response at 0.3 V, corresponding to a potential where no anodic dark currents are observed, increases monotonically with illumination time and then abruptly increases to a value that is 142% greater than the initial photocurrent magnitude. This photocurrent enhancement effect was confirmed on different films with both laser and LED illumination (Figure S2). The photocurrent enhancement stabilizes with prolonged illumination (e.g., 10 hours in Figure S3). The stable photocurrent after prolonged illumination suggests that the high intensity light illumination treatment effectively passivates surface charge recombination sites and does not induce MoSe2 photocorrosion. The abrupt photocurrent increase with illumination time could be due to a critical surface concentration of adsorbed I2 or I3– on the laser modified material.26 The abrupt increase was observed at different times for different samples (Figure S2c). The slight increase in dark anodic currents at 0.4 V and slight decrease in dark cathodic currents at 0.0 V with increasing illumination time are likely due to I3– species that accumulate in the electrochemical cell.26 The surface-adsorbed oxidation products can specifically adsorb onto charge recombination sites, thereby passivating them and increasing the photocurrent response. In addition to the significant photocurrent enhancement at 0.3 V, the anodic photocurrents in the positive potential region (e.g., > 0.2 V) shift to more negative potentials with illumination time (see Figure 1c and d and Figure 8 ACS Paragon Plus Environment

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S2b). The photocurrent increases with illumination time has not been observed for bulk MoSe2/iodide liquid junction solar cells.27 We observed no evidence of MoSe2 corrosion in bright field transmission during the illumination procedure (Figure S4). Next, we used scanning photoelectrochemical microscopy to probe the origin of the photocurrent enhancement effect at the single MoSe2 nanosheet level. In this approach, a focused 1.21 m-diameter 532 nm laser beam (20.3 kW/cm2) excites the sample in a point-by-point fashion and the electrochemical current from the entire cell is measured as a function of excitation position (Figure 1a). The contrast in photoelectrochemical current maps represents the overall charge collection efficiency upon exciting a 1 µm-diameter spot of the MoSe2 material; the maps do not report on the position of photoelectrochemical reactions on the MoSe2 surface. Since the liquid electrolyte provides intimate electrical contact to the entire MoSe2 film, our scanning photoelectrochemical microscopy approach enables high-throughput photoelectrochemical characterization of single MoSe2 nanosheets that vary in geometric area and layer thickness. Importantly, we characterize over 30 individual nanosheets that experience the same experimental conditions such as sample pre-treatment, humidity, light conditions, and temperature to clearly distinguish the effect of irradiation time, rather than other environmental factors, on the photoelectrochemical properties. Figure 2b shows a photocurrent map of the pristine MoSe2 sample region in Figure 2a. The sample contains mostly 10-15 µm wide 1L-MoSe2 and 2L-MoSe2 triangles. The dark edges at the nanosheet perimeter in Figure 2a can be attributed to few layer-thick material or water molecules that intercalate between the 2D material and the substrate.16,

28

The 3-10 µm wide dark black

objects represent multilayer-thick or bulk MoSe2. We estimate from optical density measurements that those objects contain over 15 layers. The bright regions in Figure 2b qualitatively correlate 9 ACS Paragon Plus Environment


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with 1L-MoSe2 regions. The dark regions in Figure 2b qualitatively correlate with the 2L-MoSe2 triangles, which suggests that photo-excited 2L-MoSe2 does not produce a measurable photocurrent response. In addition, pristine multilayer MoSe2 material does not produce a measurable photocurrent response. We observe inactive 2L-MoSe2 and multilayer MoSe2 material across the entire film and for different CVD-grown MoSe2 films (see additional photocurrent maps in Figure S5).

Figure 2. Scanning photocurrent microscopy of laser treated MoSe2. (a) 60× magnification bright field transmission image of the pristine MoSe2 sample. (b) Initial photocurrent map of the pristine sample region in (a). A 0.23 mW 532 nm laser excited a 1.13 µm2 spot, yielding a power density of 20.3 kW/cm2. The data was measured at 0.3 V vs Ag/AgI in 1 M NaI electrolyte. (c) Same photocurrent map in (b), but we overlaid the nanosheet structural contour from (a) onto (c). See Supplementary Information Note 1 for details. The yellow pixels represent MoSe2 boundaries. The red and blue pixels represent 1L-MoSe2 and 2L-MoSe2, respectively. (d-e) Photocurrent maps measured after the (d) first and (e) second laser annealing procedures. (f-h) Photocurrent maps measured (f) 1 day, (g) 7 days and (h) 30 days after the second laser annealing procedure. The MoSe2 sample was stored in 1M NaI electrolyte under dark conditions. (i) 60× magnification transmission image of the laser annealed sample after 30 days. j) Average photocurrent from 580 µm2 1L-MoSe2 and 520 µm2 2L-MoSe2 areas. The average photocurrent and the error bar represent the mean and standard error of the mean from 1D Gaussian fits to the distribution of photocurrents shown in Figure S6. 10 ACS Paragon Plus Environment

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To quantitively compare the photoelectrochemical activity enhancement as a function of layer thickness, we overlay quantitatively the structural information contained in transmission images (Figure 2a) onto the photocurrent maps (Figure 2c). Supplementary Information Note 1 and Figure S7 in the Supporting Information describe the detailed overlay procedure. The quantitative overlay procedure enables us to determine the photocurrent response from all illuminated 1L-MoSe2 and 2L-MoSe2 areas. Figure 2j shows that pristine 1L-MoSe2 produces about 2× more photocurrent than 2L-MoSe2 even though 1L-MoSe2 absorbs about half as many photons as 2L-MoSe2 We note that the average photocurrent response for each layer thickness was determined from 1D Gaussian fits to the photocurrent distribution (Figure S6). The poor photoelectrochemical activity of pristine 2L-MoSe2 is surprising because (1) 2L-MoSe2 absorbs more than 1L-MoSe2, and (2) the barrier height for electron injection into the ITO electrode is smaller for 2L-MoSe2 than for 1L-MoSe2.29-31 Thus, we expected to generate more carriers in 2L-MoSe2 than 1L-MoSe2 under identical illumination conditions and collect more carriers from 2L-MoSe2 due to the reduced electron injection barrier height. However, the majority of photo-excited charge carriers recombine in 2LMoSe2 instead of participating in photoelectrochemical iodide oxidation. Next, we perform the focused laser treatment experiments by increasing the sample irradiation time by a factor of 10 at each location. Specifically, a 20.3 kW/cm2, 1.21 µm-diameter laser spot illuminated the sample in a point-by-point fashion for 5 s per step instead of 0.5 s per step for photocurrent mapping experiments. The photocurrent map after the first laser treatment shows an abrupt enhancement for both 1L-MoSe2 and 2L-MoSe2 (Figure 2d, j). Interestingly, we observe that 2L-MoSe2 apparently activates from edge sites because the inner-most 2L-MoSe2 areas remain inactive following the first laser treatment (Figure 2d). The photocurrent map after the second 11 ACS Paragon Plus Environment


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focused laser treatment (Figure 2e) shows that all 2L-MoSe2 is activated and produces a photocurrent response that exceeds 1L-MoSe2 (Figure 2j). Specifically, the laser treated 2L-MoSe2 photocurrent response increases by 540% compared to the pristine 2L-MoSe2 material. On the other hand, the laser treated 1L-MoSe2 photocurrent response increases by 160% (Figure 2j). The photocurrent enhancement persists for 7 days (Figures 2g) and 30 days (Figures 2h) following the second laser treatment, which suggests that the laser radiation irreversibly modifies the MoSe2 material. Although the photocurrent response decreases slightly over time, the laser annealed material is still more active than the pristine material. The 2L-MoSe2 remains more active than 1L-MoSe2 (average enhancements for 2L-MoSe2 and 1L-MoSe2 stabilize at around 280% and 110% compared to the pristine map). The decrease in photoelectrochemical activity over long dormant periods in fresh 1 M NaI electrolyte (Figure 2h versus e) could be due to the absence of adsorbed I3– on laser-modified active sites,26,

32

as we discussed above. The sample shows no obvious

signatures of photocorrosion after all photocurrent maps and focused laser treatments (Figure 2i). Significantly higher photon fluxes are needed to induce sample damage such as photo-induced thinning of multilayer MoSe2 (e.g., 100× greater power density or 1.9 MW/cm2 and 60× longer exposure times or 10 min at each pixel (Figure S8). Additional photocurrent maps are provided in Figure S9 of the Supporting Information. Since our focused laser treatment likely heats the MoSe2 material but does not ablate it, we refer to our laser treatment procedure as a laser annealing treatment. The heating effect is supported by numerical simulations by Lee and co-workers who showed that the temperature of a photo-excited MoS2/quartz sample in air increased to 253 ºC at the center of the laser spot and the temperature gradually decreased with distance from the excitation spot.21 In addition, Hu et al simulated how the sample temperature increases with focused laser irradiation power and layer thickness.8 Below we discuss how laser-induced heating


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can enhance the filling of Se vacancies by O species. To summarize the results in Figure 2, our scanning photocurrent microscopy methods reveals that laser annealing enhances the photocurrent responses of both 1L and 2L-MoSe2. The photocurrent enhancement effect is more significant for 2L-MoSe2 and it appears to initiate at edge sites. The laser annealing treatment activates trilayer (3L)-MoSe2 but it does not enhance bulk-like MoSe2 material. Figure S10 shows that focused laser annealing increases the photocurrent response of 3L-MoSe2. There is some indication from this data that the enhancement effect is smaller for 3L-MoSe2 than for 2L-MoSe2. However, large 3L-MoSe2 triangles are sporadically distributed throughout the film and it is difficult to locate and study many 3L-MoSe2 triangles as we did for 1L-MoSe2 and 2L-MoSe2. At this time, it remains unclear whether the photocurrent enhancement effect is more significant for 2L-MoSe2 than for 3L-MoSe2. In addition, we could not determine whether laser annealing activates 4L or 5L-MoSe2 objects because we are unable to locate and unambiguously characterize these nanosheets. Since we33 and others26-27, 32 have shown that bulk MoSe2 electrodes synthesized via chemical vapor transport (CVT) oxidize iodide photoelectrochemically with extremely high efficiency (the quantum yield exceeds 50%), it is reasonable to assume that CVD-grown MoSe2 contains more defects than bulk CVT-grown crystals.5,

18

The CVD synthesis likely introduces defects that promote charge carrier

recombination and limit charge carrier transport across multiple layers. Our measurements show that multiple laser annealing treatments do not enhance multilayer MoSe2 (>15 layers, see dark black objects in Figure 2a and maps in Figure 2d and e). To further demonstrate that laser annealing can spatially and temporally modify ultrathin MoSe2 material, we demonstrate nanosheet-by-nanosheet activation in Figure 3. Figure 3a-b shows a bright field transmission image and a photocurrent map of the same pristine MoSe2 region. The 13 ACS Paragon Plus Environment


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photocurrent map indicates that 2L-MoSe2 and multilayer MoSe2 are inactive for photoelectrochemical iodide oxidation. Then, we position the focused laser on a single 2L-MoSe2 nanosheet for 460 s. Figure 3c shows the photocurrent map of the same region after the localized laser annealing procedure. The photocurrent response of the laser treated 2L-MoSe2 triangle increased by about 200% while the photocurrent response of the 2L-MoSe2 triangle located 40 µm away is unchanged. Then, we laser annealed the second 2L-MoSe2 triangle and observed a significant photocurrent enhancement effect (Figure 3d). Thus, the applied electrochemical potential does not induce the photocurrent enhancement effect because the potential is applied simultaneously to all nanosheets on the ITO electrode in Figure 3. Significantly more anodic potentials would be required to influence materials properties. For example, previous studies have shown that applied potentials of +1.0 V vs Ag/AgCl and +2 V vs SCE can so-called polish TMD materials34 and induce the semiconducting to metallic phase transition35 for MoS2, respectively. Thus, the focused laser annealing procedure can be used to controllably modify the photoelectrochemical properties of ultrathin TMDs.


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Figure 3. Spatial and temporal control of the laser annealing-induced photocurrent enhancement effect. (a) 60× magnification transmission image of a pristine MoSe2 area. (b) Initial photocurrent map of the same region in (a). The 2L-MoSe2 and multilayer-MoSe2 material are inactive for photoelectrochemical iodide oxidation. (c) Photocurrent map after activating a 2L-MoSe2 triangle using a focused laser beam positioned at the object’s centroid. (d) Photocurrent map after activating another 2L-MoSe2 triangle using the same method. Both objects are illuminated with a focused 532 nm laser (20.3 kW/cm2 for 460 s).

The poor initial photoelectrochemical activity of pristine 2L-MoSe2 is not due to weak coupling between the MoSe2 layers. We investigated the interlayer coupling of these 2L-MoSe2 samples using Raman and photoluminescence (PL) micro-spectroscopy. Figure 4a shows an optical transmission image of an MoSe2 triangle that exhibits the so-called AA (0

orientation,

corresponding to 3R phase) and AB (60 orientation, 2H) stacking modes.36-37 Figure 4e shows Raman spectra measured from regions 1, 2 and 3 in Figure 4a. Spectra 1 and 2, corresponding to 1L-MoSe2 and the AA 2L-MoSe2 stacking mode, exhibit a prominent peak at 240 cm–1 that can be attributed to the out-of-plane A1g Raman mode for MoSe2.38 On the other hand, spectra 3 shows a 15 ACS Paragon Plus Environment


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prominent peak at 15 cm–1 that can be attributed to the unique interlayer shear (Eg) mode for AB 2L-MoSe2.37 Indeed, Raman mapping of the Eg peak (Figure 4c) shows uniform signal intensity across the entire AB stacking orientation. The uniform Eg peak intensity indicates that the two layers are in intimate physical contact. Furthermore, PL spectra measured from the AA and AB orientations show weak PL intensity and a distinct 20 nm red-shift compared to the prominent 1LMoSe2 PL peak at 800 nm (Figure 4f).39 The strong PL signal from 1L-MoSe2 is due to the indirectto-direct bandgap transition from multilayer to monolayer-thick MoSe2.38 The prominent PL decrease for 2L-MoSe2 indicates that there is strong interlayer coupling in these CVD-grown samples.

Figure 4. Raman and photoluminescence micro-spectroscopy of interlayer coupling in MoSe2. (a) 60× magnification optical transmission images of a MoSe2 triangle with two bilayer orientations. (b) Out-of-plane A1g peak intensity map of (a). (c) Interlayer shear mode Eg peak intensity map of (a). (d) PL peak intensity map integrated from 760 nm to 850 nm. (e) Raman and (f) PL spectra measured from spots labeled 1, 2 and 3 in (a). The spectra in (e) are offset for clarity. Spectra 1, 2, and 3, correspond to 1L-MoSe2, AA 2L-MoSe2 and AB 2L-MoSe2, respectively. The yellow dot markers in (a) represent the 690 nm diameter laser spot size that is obtained with the 100 air objective for Raman and PL measurements. 16 ACS Paragon Plus Environment

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To better understand the origin of the photocurrent enhancement effect, we studied the impact of laser annealing on the charge carrier generation and collection efficiency before and after laser annealing. Figure 5a shows a bright field transmission image of a pristine 3L-MoSe2 triangle, as confirmed by the line profile analysis in Figure 5b. Following Castellanos-Gomez et al,40 we calculate absorbance A = –log10(T), where T is the transmittance of light through the MoSe2 sample and is given by T = IMoSe2/IITO, where IMoSe2 and IITO are the light intensities transmitted through the MoSe2 and ITO substrate, respectively. Figure S11 shows the detailed image analysis procedure for the spatially resolved absorption spectroscopy measurements. The absorbance measurement does not account for the reflectance from the MoSe2 layers. Thus, we overestimate the amount of light that is absorbed by the MoSe2 layer. Figure 5c shows layer thickness-dependent absorption spectra before and after the laser annealing treatment. All 1L-, 2L-, and 3L-MoSe2 exhibit three distinct peaks centered around 790, 680 and 450 nm (indicated as A, B, and C in Figure 5c). An absorption feature at 425 nm increases with layer thickness, which can be attributed to a D exciton in 2L-MoSe2 and 3L-MoSe2.41 The A and B absorption peaks have been assigned to the excitonic transitions occurring at the K/K' points in k-space42-43. The strong C absorption peaks have been assigned to band-to-band transitions between nearly degenerate exciton states near the Γ point.44 The A and B peaks shift gradually to longer wavelengths with increasing layer thickness (Figure 5c), in agreement with the observation in MoS2.40 Interestingly, we observe significant light absorption enhancement at the A and B exciton peaks following focused laser annealing while the C and D exciton absorption remain almost constant. The light absorption increase is more significant for 1L- MoSe2 and 2L-MoSe2 than for 3L-MoSe2. Quantitative analysis of the absorption spectra changes before and after laser annealing shows that 17 ACS Paragon Plus Environment


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2L-MoSe2 absorption increases the most (Table S1). This phenomenon is supported by theoretical predictions that the absorption peaks are inversely proportional to disorder effects (Supplementary Information Note 2); less defects give rise to more light absorption. In addition, the disorderdependent absorption could be different for A and B excitons than for the C and D excitons, which could explain why the A and B exciton absorption peaks increase more than the C and D exciton regions. The significant 2L-MoSe2 absorption increase suggests that more order is introduced into those layer thicknesses than for 1L-MoSe2 and 3L-MoSe2. Lu et al. also observed light absorption increases in laser annealed 1L-WSe2 and attributed to the effect of improved film quality.13 In summary, laser annealing increases the light absorption and therefore charge carrier generation efficiency of 2L-MoSe2 more than 1L-MoSe2. The laser annealing treatment did not significantly influence the absorption properties of 3L-MoSe2.

Figure 5. Absorption and photocurrent spectroscopy of pristine and laser annealed MoSe2. (a) 60× magnification transmission image of a 3L-MoSe2 triangle. (b) Line profiles showing determination of layer thickness. (c) Absorption spectra of 1L-, 2L-, and 3L-MoSe2 before and after laser annealing treatment. The data points represent the average of 9 spectral measurements across the nanosheet (Figure S11). The absorbance measurement does not account for reflectance from the MoSe2 surface. (d) EQE spectra measured from the same 0.38 mm2 sample area before and after focused laser annealing. The monochromatic illumination power is between 7.8-30.6 W across the entire wavelength range.


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Next, we explored whether the light absorption increase in Figure 5c could account entirely for the photocurrent enhancement effect. The photocurrent i in a photoelectrochemical cell is given by i = Iabssepint,45 where Iabs is the light absorbed by the MoSe2 material, sep is the efficiency of charge separation and hole transport to the MoSe2 surface, and int is the efficiency of interfacial hole transfer to iodide. Thus, an enhancement in Iabs could account for the photocurrent enhancement effect. Figure 5d shows the external quantum efficiency (EQE) spectrum before and after the focused laser annealing treatment, where EQE()=i/qI0() and q is the electronic charge and I0() is the incident photon power at each excitation wavelength (s–1). We note that these EQE spectra were measured from a different sample area in the same photoelectrochemical cell as in Figure 1. For these EQE measurements, we probe the photocurrent response as a function of excitation wavelength using a large area (0.38 mm2), low power (7.8-30.6 W) light spot (yielding 2.1-8.1 mW/cm2 power density; significantly lower than focused laser photocurrent mapping experiments). The EQE values reported for this liquid junction MoSe2 solar cell are similar to those reported for solid-state photovoltaic systems based on 1L- and 2L-TMDs.46-47 Importantly, the EQE value at 532 nm increases following the focused laser treatment, which is consistent with ensemble-level current-voltage curves (Figure 1) and photocurrent maps (Figure 2 and 3) that are measured using a 532 nm laser. Interestingly, the EQE values for the laser annealed sample increase over the entire excitation range even though the light absorption properties are enhanced only for wavelengths longer than 500 nm. Excitation wavelength-dependent photocurrent-voltage (i-E) curves before and after laser treatment show a similar effect (Figure S12). Thus, the light absorption enhancement effect cannot account entirely for the photocurrent enhancement. This observation indicates that laser annealing modifies other MoSe2 material properties that influence either sep or int. One possible explanation for the EQE enhancement is that the laser annealing 19 ACS Paragon Plus Environment


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treatment removes mid-gap energy levels associated with Se vacancies.4, 13, 15 Removing energy levels inside the MoSe2 band gap decreases the charge carrier recombination rate and increases sep, as will be discussed in detail below. Another possible explanation is that the focused laser induces a lattice contraction between adjected MoSe2 layers.10-11 For example, Lee et al showed that the interlayer distance in mechanically transferred WS2/WSe2 bilayers decreases following focused laser treatment.20 The lattice contraction effect enhances interlayer coupling and could improve interlayer charge transport (i.e., increase sep). Unfortunately, we were unable to measure interlayer distance changes in these MoSe2 samples via atomic force microscopy (AFM) because the considerable ITO surface roughness prevents us from accurately determining the layer thickness (Figure S13). The peak shape change in the C and D exciton region for the laser annealed sample may be due to the photocurrent response from multiple layer thicknesses that are excited by the large area excitation source in these measurements (0.38 mm2 sample area). The absorbance of the C and D exciton peaks changes significantly as a function of layer thickness (Figure 5c). It remains unclear how the laser annealing-induced photocurrent enhancement scales with layer thickness beyond 2L-MoSe2 because it was not possible to measure thicker layers with our current experimental approach. Thus, the peak shape change may be due to a variable photocurrent response upon photoexcitation of C and D excitons in few-layer and multilayer thick MoSe2 nanosheets. To further investigate how laser annealing influences the chemical composition of MoSe2, we performed X-ray photoelectron spectroscopy (XPS) measurements before and after the laser annealing treatments. Figures 6a-b show XPS spectra of the Mo 3d and Se 3d binding energy regions for the pristine and laser treated MoSe2 film. Peak fitting analysis shows that the Mo 3d5/2 and Mo 3d3/2 binding energies are 228.6 and 231.7 eV, which is consistent with Mo(IV) species.48 20 ACS Paragon Plus Environment

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The Se 3d5/2 and Se 3d3/2 peaks at 54.0 and 54.9 eV (Figure 6b) are consistent with the formation of MoSe2.49 Mo 3d and Se 3d peak fitting analysis reveals a Mo:Se ratio of 1:1.69, which indicates that Se-vacancies are present in the these CVD-grown MoSe2 samples. High concentrations of chalcogen vacancies in CVD-grown MoS2, MoSe2, WSe2, and WS2 TMD films have been attributed to the high volatility of the chalcogen (Se in this case).13, 50 The lower panels in Figure 6a-b show Mo 3d and Se 3d XPS spectra from the same MoSe2 sample after laser annealing. The experimental section describes how the XPS excitation spot spatially overlapped with the laser annealed MoSe2 material. The XPS data shows three major features following laser treatment. First, a new peak appears at 235.0 eV that is consistent with oxidized Mo species (i.e., Mo5+/6+ species).48, 51 Second, the Mo 3d and Se 3d peaks shift to lower binding energy by about 0.2 and 0.1 eV, respectively, which could also be due to oxidized MoOx species in contact with MoSe2.52 Third, the Mo(IV):Se ratio increases to 1:1.84 after laser treatment even though no Se is introduced into the system during or after the laser treatment procedure. The XPS spectral changes and Mo(IV):Se ratio can be explained as follows. The pristine MoSe2 film contains Mo(IV) species associated with Se vacancies (schematically shown in Figure 6c, top panel). Mo(IV) atoms associated with Se vacancies are oxidized to Mo(V or VI)-Ox species (pink spheres in Figure 6c bottom panel), thereby increasing the Mo(IV):Se ratio. Thus, the laser treatment heals the Se vacancies by filling them with oxygen and improves the film quality. The vacancy healing effect is further confirmed by increases in PL and Raman peak intensities after laser annealing (Figure S14).13, 15, 18



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Figure 6. Chemical composition of pristine and laser annealed MoSe2 films. High resolution XPS spectra of (a) Mo 3d and (b) Se 3d binding energy regions before and after laser annealing (power density 36.5 kW/cm2, see Experimental Section for details). (c) Proposed photo-induced chemical changes to the MoSe2 sample. See the main text for a detailed discussion.

Here we discuss how laser annealing could improve the photoelectrochemical activity of the MoSe2. First, Lu et al. reported that O atoms fill Se vacancies during high intensity irradiation of WSe2.13 This so-called oxygen healing effect can remove trap states from within the semiconductor band gap. Removing Se vacancies decreases the possibility of charge carrier recombination processes and therefore increases the overall light-to-energy conversion efficiency. The laserinduced heating effect likely enhances the filling of Se vacancies by lowering the activation barrier for O adsorption at or near the focused laser beam region, in agreement with Lee and co-workers .21 Thus, the laser annealing treatment improves photoelectrochemical activity via an indirect process: the laser-induced heating accelerates the vacancy healing effect. Second, the laserinduced oxidation chemistry could produce a favorable MoOx/MoSe2 nanoscale heterojunction whose energy level alignment could promote local charge carrier separation.52 Third, oxidized Mo


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sites can serve as local autocatalytic sites for photoelectrochemical I3– oxidation according to 2(I3–) + 2h+  3I2.32 Our photocurrent mapping data showed that photo-excited 2L-MoSe2 nanosheets are mostly inactive initially for photoelectrochemical iodide oxidation. To rationalize this observation, we consider the possibility that the total number of Se vacancies (i.e., defects) in a single MoSe2 layer is N0,Se. Then, the total number of defects for two adjacent layers is 2N0,Se. Since the total number of defects increases when two defective layers come into contact, but the interlayer surface area remains constant, then the interface between two adjacent layers has twice as many defects as a single MoSe2 layer. If the defects have energy levels inside the MoSe2 bandgap and trap photoexcited charge carriers, then the charge recombination rate will increase for 2L-MoSe2.53 Thus, we attribute the poor photoelectrochemical activity of 2L-MoSe2 to significant interlayer charge carrier recombination induced by the high density of interfacial Se vacancy defects that have energy levels inside the MoSe2 bandgap (i.e., defect states). The laser annealing treatment presumably passivates or heals the Se vacancies at the interface between the first and second MoSe2 layers and therefore diminishes that defect-assisted charge carrier recombination pathway. The healing effect likely initiates at 2L-MoSe2 edges because TMD edge sites are more reactive for the oxidation process than interior sites.5, 54 Finally, we discuss why the laser annealing treatment does not increase the photocurrent response of multi-layer MoSe2. The efficiency of the laser annealing effect is expected to depend on the penetration of both light and the mass transport of O-containing species to the defect sites in the MoSe2 layers. The charge carrier generation rate decreases approximately exponentially with layer thickness. In addition, the mass transport of O-containing species to defect sites is impeded by the multi-layer stacking configuration. Thus, it is possible that the laser annealing 23 ACS Paragon Plus Environment


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effect is not effective for buried layers in a thick multilayer MoSe2 nanosheet because both charge carrier generation and the flux of O-containing species are not sufficiently high to promote the vacancy healing effect. Conclusions In

conclusion,

high

intensity

supra-band

gap

irradiation

of

MoSe2

improves

its

photoelectrochemical performance in a liquid junction photovoltaic system. Ensemble-level photoelectrochemical measurements reveal that the photocurrent response due to iodide oxidation increases and shifts to more favorable (negative) potentials. Scanning photoelectrochemical microscopy and quantitative photocurrent image analyses reveal that laser treatments increase the light-to-energy conversion efficiency of 2L-MoSe2 more than 1L-MoSe2 (280% versus 110%). The light-induced photocurrent enhancement effect is likely due to the removal of Mo(IV) recombination centers associated with Se vacancies and O substitution at Se vacancy sites. Our study represents the first demonstration of photoelectrochemical enhancement of ultrathin MoSe2 films using a high intensity laser annealing procedure. The thickness-dependent photocurrent enhancement effect offers intriguing possibilities to tune (photo)electrocatalytic activity of TMD materials for a wide range of energy-related reactions such as H2 evolution and CO2 reduction. Experimental Section Sample preparation. CVD-grown MoSe2 films were purchased from SixCarbon Inc. (Shenzhen, China). MoO3 (99.99%, Aldrich) and Se powder (99.99%, Aldrich, ~100 mech) were used as Mo and Se precursors. The reaction took place for 10 min in a 80-mm diameter horizontal tube where MoO3 was heated to 750 oC and the upstream Se boat was maintained at approximately 260 oC. A 90%/10% argon/hydrogen mixture was used as the carrier gas. The MoSe2 films were grown on a


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1 cm2 300 nm SiO2/Si wafer that was placed 5 cm away from the MoO3 boat. The MoSe2 film composition and layer thickness was rigorously characterized by the manufacturer (http://sixcarbon.com/products-lab/mose2-en.php). The as-grown MoSe2 film was transferred to the pre-drilled ITO electrodes using PMMA stamps. Ensemble-level photoelectrochemical measurements. All ensemble-level electrochemical measurements were performed in a three-electrode electrochemical cell using a ITO/MoSe2 working electrode, Pt coil counter electrode, and Ag/AgI reference electrode. All experiments were performed in 1 M NaI (Aldrich, ACS reagent ≥99.5%) electrolyte solution in 18.2 M nanopore water. Ensemble-level CV cycling experiments were performed as follows. A single CV cycle involved 5 dark CV scans followed by 15 light scans. For the experiment shown in Figure 1 of the main text, the cell was cycled 50 times (or 1000 CV scans). A 31.8 mW 532 nm laser with a 2 mm diameter laser spot illuminated the MoSe2 film (1 W/cm2 or 3×104 photons s–1 nm–2). The scan rate was set to 50 mV/s with a sampling interval of 10 mV. The photocurrent signal was calculated by subtracting the electrochemical current under dark conditions from the electrochemical current under illumination. Focused and ensemble-level laser annealing procedure. All laser annealing experiments were carried out in the iodide/tri-iodide electrolyte. Focused laser illumination was used for single nanosheet investigations and unfocused laser illumination was used for ensemble-level studies. Specifically, a 20.3 kW/cm2, 1.21 µm-diameter laser spot (532 nm wavelength) illuminated the sample in a point-by-point fashion for 5 s per step. The focused laser illuminated the sample for 10-times longer than in photocurrent mapping experiments. For ensemble-level studies, unfocused laser illumination from the same laser source was used for annealing treatments (1 W/cm2 or 3×104 photons s–1 nm–2). The laser annealing treatment for XPS measurements was performed in air to 25 ACS Paragon Plus Environment


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avoid surface contamination by the concentrated iodide electrolyte. To do so, a 7.3 mW, 532 nm laser was focused through a 10× NA0.3 air objective to achieve a 5 m-diameter spot and then the laser spot was scanned across a 1 mm2 sample area (power density 36.5 kW/cm2, similar to the annealing treatment for photocurrent maps). Scanning photoelectrochemical microscopy measurements. A microfluidic electrochemical cell was fabricated according to the procedure described in our previous work.55 The cell was assembled by sandwiching adhesive tape between the ITO/MoSe2 working electrode and a glass coverslip. The cell was filled with 1 M NaI electrolyte solution and mounted on the motorized XY stage of an inverted Olympus IX73 optical microscope. In a typical photocurrent mapping experiment, we maintained a constant applied potential (+0.3 V vs Ag/AgI) and incident laser power while the electrochemical current signal was continuously monitored at 50 Hz for 0.2 s at each 1 m2 pixel. A continuous wave 532 nm laser (Ondax) was aligned through the back aperture of a 60× NA1.2 water-immersion objective (UPLANSAPO60x/W), yielding a laser spot diameter of 1.2 m (determined from the full width at half-maximum of the beam profile). The incident laser power density for photocurrent mapping experiments was 20.3 kW/cm2. The excitation laser was chopped at 8 Hz (TTI C-995 optical chopper). The modulated laser light induced a modulating photocurrent signal due to the iodide oxidation reaction. The analog current signal from the potentiostat (Ivium Compactstat) and the optical chopper frequency was fed into the input and reference channels, respectively, of a Stanford Research Systems SR830 lock-in amplifier. The lock-in detected photocurrent signal was used to produce photocurrent maps. The lock-in detected photocurrent signal is proportional to the steady-state photocurrent response, as described in our previous publication.30 At the same time and location as the photocurrent signal is acquired, the reflected laser light from the TMD/ITO electrode was collected in a backscatter geometry, passed 26 ACS Paragon Plus Environment

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through a Horiba iHR 550 spectrometer, and imaged on a Synapse back-illuminated charge coupled device (CCD) detector. The reflected laser light was used to correlate optical images and photoelectrochemical current maps, as described in Supplementary Note 1. The typical step size and dwell time in photocurrent mapping experiments was 1.0 m and 0.5 s, respectively. Spatially resolved absorption spectroscopy. Following the hyperspectral imaging method developed by Andres et al.,40 monochromatic light from a Horiba OBB Tunable PowerArc Illuminator was used to measure layer thickness-dependent absorbance spectra. The light transmitted through the TMD sample (I) and the ITO substrate (I0) were collected through a 60× microscope objective and imaged on a Photometrics Prime 95B back-illuminated CMOS camera (Figure S11). The average values from approximately 10 transmission images per excitation wavelength were used to calculate I and I0. The absorbance of the sample as a function of wavelength A() was calculated according to A() = log10T() = log10[I()/I0()], where T() is the transmittance as a function of wavelength. Photoluminescence (PL) and Raman spectroscopy. The same 532 nm excitation source and illumination path that was used for photocurrent mapping and laser reflection measurements was also used for PL and Raman mapping experiments. The laser was directed through a 100× NA0.95 air objective (Olympus PlanFL N100X) and illuminated a 0.69 m-diameter spot at the focal plane. PL and Raman signals were collected and filtered by an Ondax 532 nm THz Raman system, passed through a Horiba iHR 550 spectrometer, and then detected by the Synapse CCD detector. PL spectra were acquired for 10 min at 7.3 mW from 650 to 900 nm (power density was 1.9 MW/cm2). Raman spectra were acquired at the same power density for 60 s over a spectral range of 10 to 500 cm–1. The step size in PL and Raman mapping experiments was 1 m. All PL and Raman spectra



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were measured in air and at room temperature before or after the photoelectrochemical measurements. Photocurrent spectroscopy measurements. A Horiba OBB Tunable PowerArc Illuminator excited a 670 µm-diameter circular region of the MoSe2 film. The photocurrent was measured as a function of excitation wavelength at a fixed applied potential (+0.3 V vs Ag/AgI) using lock-in detection at 8 Hz. The excitation wavelength was scanned from 400 to 830 nm with a step size of 2 nm. The lamp spectrum was measured using a Si diode (ThorLabs, APD120A2 - Si Avalanche Photodetector, UV Enhanced) that was calibrated with a Thorlabs P100 power meter. X-ray photoelectron spectroscopy (XPS). XPS analyses were performed on a Physical Electronics PE5800 ESCA/AES (Chanhassen, MN, USA) system with a monochromatic Al Kα x-ray source (1486.6 eV), hemispherical analyzer, and multichannel detector. Survey spectra were collected from 10 to 1100 eV for 10 min for all samples with a pass energy of 187.85 eV, step size of 1.6 eV and 20 ms per step (Figure S15). High-resolution scans were performed with a pass energy of 23.5 eV and a step size of 0.10 eV/step. XPS spectral data were processed using Multipak software, version 9.3.03. All spectra were shifted using indium 3d5/2 as a reference at 444 eV. Shirley-type background subtraction was performed, and the high-resolution spectral data were fitted with Gaussian-Lorentzian peaks. Detailed peak fitting procedures are discussed in Supporting Information Note 3. A black marker was used to identify the laser illumination area on the backside of the ITO slide to ensure that the XPS spectra were measured from the same region as the laser treatment (Figure S16). Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 28 ACS Paragon Plus Environment

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Ensemble CV scans; additional photocurrent maps; image analysis procedures; absorption spectra analyses; theoretical model to explain absorbance spectra; excitation dependent i-E curves and XPS survey scans. Corresponding Author Justin B. Sambur. E-mail: [email protected] Author Contributions L. W. fabricated devices and performed experiments. L. W., Z. N. N and J. B. S discussed the experimental procedures. L. W., M. S and J. B. S discussed and analyzed the data. L. W. and J. B. S wrote the manuscript. M. T and H. C provided the theoretical model to explain the absorption increase. All authors read and approved the manuscript. Acknowledgements The authors thank Dr. Patrick McCurdy from Colorado State University’s Central Instrument Facility for XPS analysis. This material is based upon work supported by the Air Force Office of Scientific Research (AFOSR) under award number FA9550-17-1-0255. References

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