Photoelectrochemistry of Pristine Mono- and Few-layer MoS 2

Transcription

1 Photoelectrochemistry of Pristine Mono- and Few-layer MoS 2 Matěj Velický,* a Mark A. Bissett, a Colin R. Woods, b Peter S. Toth, a Thanasis Georgiou, c Ian A. Kinloch, d Kostya S. Novoselov, b and Robert A.W. Dryfe* a a School of Chemistry, University of Manchester, Oxford Rd, Manchester, M13 9PL, UK b School of Physics and Astronomy, University of Manchester, Oxford Rd, Manchester, M13 9PL, UK. c Manchester Nanomaterials Ltd, 83 Ducie Street, Manchester, M1 2JQ, UK d School of Materials, University of Manchester, Oxford Rd, Manchester, M13 9PL, UK * To whom correspondence should be addressed. Tel: +44 (0) ; Fax: +44 (0) , matej.velicky@manchester.ac.uk or robert.dryfe@manchester.ac.uk Supporting Information content: Supporting Figure S1 MoS 2 characterization Supporting Figure S2 Illumination power density calibration Supporting Figure S3 Raman spectra MoS 2 thickness calibration Supporting Figure S4 Si Raman and MoS 2 PL intensity dependence on flake thickness Supporting Figure S5 High-resolution XPS spectra of monolayer and bulk MoS 2 Supporting Figure S6 XPS imaging of monolayer and multilayer MoS 2 Supporting Figure S7 Heterogeneity of Raman and PL spectra in monolayer MoS 2 Supporting Figure S8 Dependence of electron transfer kinetics on electrode area Supporting Figure S9 Dependence of capacitance on electrode area Supporting Figure S10 Intralayer resistance effects on electron transfer kinetics Supporting Figure S11 Electron transfer kinetics of individual MoS 2 samples Supporting Figure S12 Capacitance dependence on scan rate Supporting Figure S13 Dependence of capacitance on illumination 1

2 Supporting Information Supporting Figure S1 MoS2 characterization. (A) Brightfield optical micrograph of an MoS2 flake (sample H). (B) Darkfield optical micrograph of the pristine flake surface prior to any electrochemical measurement. (C) AFM micrograph of a selected portion of the monolayer indicated by the turquoise square in (A). (D) Histogram analysis of the monolayer-substrate height separation. (E) and (F) Raman and PL maps, respectively, of a selected mono- and multilayer region of the flake indicated by the dark blue square in (A). (G) and (H) Raman and PL spectra, respectively, recorded at points indicated by the colored crosses in (E) and (F). 2

3 Discussion of Figure S1: This figure demonstrates the use of brightfield and darkfield microscopy for assessment of the flake quality, and Raman spectroscopy and PL measurement for determination of the number of MoS 2 layers. Fig. S1A shows a brightfield micrograph of the whole sample, with a darkfield micrograph magnifying a region (white rectangle) of the clean flake prior to electrochemical measurements (Fig. S1B). AFM micrograph of the monolayer part of the flake and the corresponding flake-substrate height histogram are shown in Fig. S1C and S1D, respectively. The flake is further characterized using Raman spectroscopy and PL measurement, as shown in Fig. S1E-F (maps) and in Fig. S1G-H (single-point spectra). 3

4 Supporting Figure S2 Illumination power density calibration. (A) Radiant power plotted vs. the illuminated area of the power density meter sensor. The inset shows the different apertures used for this measurement. (B) Change of irradiance with the microscope illumination control, with the corresponding photographs of the experimental setup shown in the inset. (C) Dependence of spectral irradiance on the wavelength of the incident light. 4

5 Discussion of Figure S2: The absolute value of illumination power density (irradiance) in relation to microscope illumination was established through the calibration shown in Fig. S2. Firstly, the radiant power (in mw, at 532 nm wavelength) was measured at different microscope aperture openings and plotted against the illuminated area of the power density meter sensor (Fig. S2A). This calibration curve was used to normalize the radiant power to the illuminated area and therefore to obtain the irradiance (in W cm 2 ). The background (ambient) irradiance, with the objective aperture completely closed, was measured to be mw cm 2. The sigmoidal dependence of the irradiance (at 532 nm wavelength) on the microscope illumination (in %), controlled continuously using a potentiometer, is shown in Fig. S2B. The yellow horizontal line indicates the solar constant as a useful comparison. 1 Finally, the spectral irradiance of the surface as a function of the wavelength is shown in Fig. S2C. Supporting Figure S3 Raman spectra MoS 2 thickness calibration. The graph shows a dependence of the E 2g ( ) and A 1g (+) Raman band shift (left-hand vertical axis) and the difference between the two bands (right-hand vertical axis), on the number of MoS 2 layers. 5

6 Discussion of Figure S3: The graph in Figure S3, which was constructed using data in ref. 2, allowed us to accurately determine the number of layers in thin MoS 2 flakes (< 5 layers). The difference between the frequencies of the two Raman band shifts, A 1g and E 2g, which is more sensitive to thickness than the individual Raman band frequencies, was used. Supporting Figure S4 Si Raman and MoS 2 PL intensity dependence on flake thickness. (A) Si Raman band intensity at 520 cm 1 and (B) maximum MoS 2 PL intensity ( nm) as functions of the number of MoS 2 layers, for all the measured samples. The inset graphs zoom in on the lower intensity data (bilayer and thicker MoS 2 ). All the intensities are normalized to the MoS 2 A 1g peak intensity. 6

7 Discussion of Figure S4: As indicated in the main text, the observed intensity of the 520 cm 1 Raman peak, which originates from the underlying Si wafer, also strongly depends on the MoS 2 flake thickness (Fig. S4A). This is simply a result of the increased absorption of light by variedthickness MoS 2 crystals. Normalized Si peak intensity decreases with the increasing number of MoS 2 layers, with the most distinct difference observed between monolayer and bilayer MoS 2 (4-12 fold). The intensity continues to decrease for thicker flakes with ca. 10-fold difference between bilayer and bulk MoS 2. The relatively large variation of the Si intensity could be caused by variable thickness of the PMMA layer sandwiched between the MoS 2 flake and Si wafer. Similarly, the maximum PL intensity of MoS 2 strongly depends on the flake thickness (Fig. S4B) as shown previously. 3, 4 The decrease in PL intensity for a bilayer, in comparison to a monolayer, is ca fold, with further 5-10 fold decrease between bulk and bilayer. To summarize, both the Si Raman peak and MoS 2 PL intensity can be used to unambiguously determine an MoS 2 monolayer, while the identification of thicker layers is more approximate and less reliable. 7

8 Supporting Figure S5 High-resolution XPS spectra of monolayer and bulk MoS 2. (A) and (B) High-resolution XPS spectra of Mo 3d / S 2s peaks on monolayer and bulk MoS 2, respectively. (C) and (D) XPS spectra of S 2p peaks on monolayer and bulk MoS 2, respectively. The raw spectra (black curves) were fitted with individual major components (green, blue and red curves), and additional features (pink curves) to produce a fitted spectral envelope (empty circles), corrected for the background (brown curve). Discussion of Figure S5: The data in Fig. S5 are representative examples of typical highresolution XPS spectra on MoS 2 monolayer and bulk flakes. The position and shape of the Mo 3d and S 2p doublets confirms that the MoS 2 is present exclusively in the form of the semiconducting 2H phase. 5, 6 There are some minor differences between the spectra of monolayer and bulk MoS 2. Notably, a shoulder often appeared for the Mo 3d 3/2 peak (pink peak 8

9 at ~234 ev in Fig. S5B) of bulk MoS 2, and a further partially resolved peak between ev, appeared for both bulk and monolayer MoS 2. These features can be attributed to mild oxidation of the molybdenum atoms (Mo 4+ Mo 6+ ). 7, 8 Furthermore, an unresolved peak between ev, which has been linked to partial oxidation of MoS 2 lattice, was observed for the S 2p peak region of bulk MoS 2 (pink curve in Fig. S5D). 7 In summary, the bulk crystals appear to be slightly more oxidized and generally less pure than the monolayer crystals, but overall, the XPS analysis of the MoS 2 surfaces revealed little difference between bulk and monolayer MoS 2. 9

10 Supporting Information Supporting Figure S6 XPS imaging of monolayer and multilayer MoS2. (A) Low-resolution XPS image of an MoS2 flake following the intensity of the Mo 3d peak. (B) High-resolution XPS image of a portion of the same flake. (C) and (D) Corresponding XPS composite images following intensities of Mo 3d, O 1s and C 1s peaks (red, green, blue, respectively). All the intensities are in counts per second and all the scale bars denote 50 µm. 10

11 Discussion of Figure S6: The XPS imaging provides a compositional confirmation of the MoS 2 flakes. Fig. S6A and S6B show the low- and high-resolution XPS images of the Mo 3d peak, respectively, clearly visualizing the MoS 2 flake. Composite XPS images in Fig. S6C and S6D, comprised of Mo 3D, O 1s, and C 1s intensity, visually enhance the chemical difference between different parts of the sample: the red/yellow/white molybdenum rich MoS 2 flakes, blue carbon rich substrate and green oxygen rich silver paint. Micrographs of the imaged areas are shown in Fig. S6E and S6F for comparison. Furthermore, the high-resolution imaging (~3 µm spatial resolution) allows different thicknesses of the flakes to be visualized as shown in Fig. S6B and S6D. 11

12 Supporting Information Supporting Figure S7 Heterogeneity of Raman and PL spectra in monolayer MoS2. (A) Raman spectra map, following position of E2g peak of MoS2. (B) Photoluminescence map, following the position of MoS2 PL peak maximum. (C) Optical micrograph of the corresponding MoS2 layers. (D) and (E) Raman and PL spectra, respectively, at three selected points, indicated by colored crosses in (A) and (B). The scale bars denote 10 µm. Discussion of Figure S7: Significant sample-to-sample variations in MoS2 electrical properties, namely conductivity and electron mobility, are often reported in the literature.9-12 We too observe sample-to-sample variations in photoelectrochemical properties of MoS2, as highlighted in the main text. In order to closer investigate these variations, we have performed a detailed analysis of the Raman and PL spectra across a monolayer MoS2 flake. We found that while the A1g Raman mode does not vary significantly, there is a substantial variation in the E2g Raman mode 12

13 and PL. The maps in Fig. S7A and S7B, which follow changes in E 2g Raman mode and maximum PL position, respectively, reveal a concordant heterogeneity across a monolayer MoS 2 flake (an optical image of the mapped flake in shown in Fig. S7C). Closer analysis of selected points on the flake surfaces (colored crosses in Fig. S7A-B), shows that it is the position and width of the Raman E 2g peak and the position of maximum photoluminescence, which undergo changes across the monolayer crystal, as shown in Fig. S7D and S7E, respectively. Such heterogeneity points suggest significant spatial variation in MoS 2 electronic structure, most likely originating in substrate-induced doping. It is possible that the adhesion of monolayer flake varies due to ripple formation and residual strain, 13 which leads to creation of regions of varied charge carrier concentration and altered band gap structure. Sensitivity of the MoS 2 Raman modes and PL to substrate identity, as well as strain, has recently been shown studied in detail

14 Supporting Figure S8 Dependence of electron transfer kinetics on electrode area. (A) ET kinetics (k 0 ) as a function of the droplet/flake interfacial area. (B) Optical micrographs of the droplets used for this measurement. The scale bars denote 20 µm, measurements were carried out at constant irradiance (0.043 W cm 2 ). Discussion of Figure S8: The electron transfer (ET) kinetics (k 0 ) is inversely proportional to the droplet/flake interfacial area, as shown in Fig. S8A. The k 0 decreases by ca. 40 % from 370 to 1720 µm 2 and it starts to plateau above 1000 µm 2. The most straightforward explanation for this dependence is absorption/dispersion of incident light by the liquid droplet and its dissolved components. As the volume of a droplet increases with the increasing droplet/flake interfacial area, a larger fraction of the incident light is absorbed. In turn, a smaller fraction of light is available to reach the MoS 2 surface and to generate free charge carriers, which results in a decrease in the measured ET kinetics. 14

15 Supporting Figure S9 Dependence of capacitance on electrode area. (A) EDLC as a function of the droplet/flake interfacial area. (B) Optical micrographs of the droplets used for the measurement. The scale bars denote 20 µm, the measurements were carried out at constant irradiance (0.043 W cm 2 ). Discussion of Figure S9: Similar to the ET kinetics, the electric double-layer capacitance (EDLC) on MoS 2 is inversely proportional to droplet/flake interfacial area (Fig. S9A). EDLC monotonously decreases by ca. 30 % from 340 to 1650 µm 2 and plateaus above 1000 µm 2. The same explanation as for the ET kinetics dependence can be applied here: the increased absorption of the incident light within the liquid lowers the concentration of photo-generated charge carriers in MoS 2 and therefore results in decreased EDLC. An alternative explanation of these effects (both for capacitance and ET) could be: 15

16 1) An ohmic drop effect due to the increased internal resistance in a larger droplet, which would also manifest itself by reduction in the measured kinetics and capacitance. 2) A surplus of photogenerated charged carriers diffusing from the photoactive MoS 2 surface surrounding a small droplet (in comparison to a larger droplet), which would increase the capacitance and ET of a small droplet via charge carrier, by an analogy with enhanced diffusion of solute to and from a small microelectrode in comparison to a larger one. 16

17 Supporting Figure S10 Intralayer resistance effects on electron transfer kinetics. The panels show a dependence of the normalized ET kinetics (k 0 norm) on the distance between the center of the droplet and either the contact with silver paint (A C, F) or the nearest electrically contacted bulk MoS 2 flake (D E). 17

18 Discussion of Figure S10: It has been previously shown for this type of measurement that the ET kinetics do not depend on the distance between the droplet and the electrical contact on graphene monolayer flakes. 17 This is due to the high intralayer conductivity of graphene/graphite crystals, which is sufficient to support 1 10 na currents (current densities of 1 10 ma cm 2 ), without any ohmic losses. Conductivity of MoS 2 is, however, orders of magnitude lower than that of graphene. 10, 18 Fig. S10 shows the dependence of the ET kinetics (k 0 ) on the distance between the center of a droplet and the electrical contact to the flake, which the droplet rests on. As the droplet size has earlier been shown to affect the kinetics (Fig. S8), the k 0 is normalized per droplet area (k 0 norm = k 0 (A / Ā), where A is the area of the droplet and Ā is the average area of all the droplets measured on a given flake). While the kinetics on bulk MoS 2 (>50 layers) show no obvious dependence on the distance between the droplet and the electrical contact (Fig. S10A-C), a few-layer (Fig. S10D) and monolayer (Fig. S10E) MoS 2 flakes exhibit slight decrease in k 0 norm with the increased droplet-contact distance. Furthermore, the monolayer flake in Fig. S10F, whose kinetics was orders of magnitude slower than that of Fig.S10E, showed very strong dependence on a droplet position, indicating that the MoS 2 conductivity was the main limiting factor for the slow electron transfer in this flake. Note that while for Fig. S10A C and S10F the droplet-contact distance corresponds to the distance between the center of the droplet and silver paint electrical contact, for Fig. S10D and S10E it corresponds to the distance between the center of the droplet and the nearest electrically connected bulk MoS 2 (because as shown above, the conductivity of bulk MoS 2 does not limit the ET kinetics). 18

19 Supporting Information Supporting Figure S11 Electron transfer kinetics of individual MoS2 samples. Panels (A) (H) show the ET kinetics (k0) measured for all the samples, plotted vs. the droplet/flake interfacial area. The insets show optical micrographs of the individual droplet measurements on MoS2 mono- and multilayers, all scale bars denote 50 µm. 19

20 Discussion of Figure S11: Soon after the initial measurements it was observed that the photoelectrochemical response of MoS 2 had varied greatly from sample-to-sample, most likely due to variable levels of doping in natural MoS 2 crystals. Similar variation has been observed in the literature for conductivity and electron mobility measurements in MoS We have therefore limited ourselves to comparing the ET kinetics between different MoS 2 multilayers of the same crystalline neighborhood only. This approach reduced an unwanted uncertainty due to sample-to-sample doping inhomogeneity. This also suggests that data analysis is of qualitative/semi-quantitative nature, and while general trends and conclusions can be drawn with confidence, it is difficult to extract fully quantitative information from these results. 20

21 Supporting Figure S12 Capacitance dependence on scan rate. (A), (B), (C), and (D), Dependence of the EDLC on scan rate for four individual measurements on different flakes. Discussion of Figure S12: The representative graphs in Fig. S12 show the EDLC as a function of the scan rate for four different measurements on different MoS 2 surfaces. The measured EDLC is shown to decrease with increased scan rate by % in the range of mv s 1. This slight decrease is expected from the migration-diffusion limited nature of the capacitive process. Li + and Cl ions are depleted near the surface of an MoS 2 electrode as the electric double-layer is being formed, and their replenishment through transport from bulk aqueous phase cannot keep up with their depletion at fast scan rates. This effect is not, however, very strong here due to the high concentration of LiCl and flatness of the electrode surface. 21

22 Supporting Figure S13 Dependence of capacitance on illumination. EDLC on MoS 2 expressed via cyclic voltammetry as a function of the irradiance. The wavy character of the signal is caused by an external electrical noise of ca. 2 pa in peak amplitude. Discussion of Figure S13: The EDLC is shown to have only weak dependence on illumination (Fig. 5B). Closer examination of cyclic voltammograms used for capacitance determination (Fig. S13) reveals that the EDLC is most affected by illumination at high potentials, i.e. low electron energies. As discussed in the main text, this is consistent with the generally accepted n-type doped character of natural MoS 2 crystals, which renders MoS 2 metal-like at low potentials while the semiconducting behavior (hence sensitivity to incident light) is maintained at high potentials. 22

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