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Multimodal Kelvin Probe Force Microscopy Investigations of a Photovoltaic WSe2/MoS2 Type-II Interface Yann Almadori, Nedjma Bendiab, and Benjamin Grevin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14616 • Publication Date (Web): 08 Dec 2017 Downloaded from http://pubs.acs.org on December 10, 2017

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ACS Applied Materials & Interfaces

Multimodal Kelvin Probe Force Microscopy Investigations of a Photovoltaic WSe2/MoS2 TypeII Interface Yann Almadori†, Nedjma Bendiab‡, Benjamin Grévin†,* †

Univ. Grenoble Alpes, CNRS, CEA, INAC-SyMMES, 38000 Grenoble, France

‡

Institut Néel, CNRS, Univ. Grenoble-Alpes, 38042 Grenoble Cedex 09, France

ABSTRACT Atomically thin transition metal dichalcogenides (TMDC) have become a new platform for the development of next-generation opto-electronic and light harvesting devices. Here, we report a Kelvin probe force microscopy (KPFM) investigation carried out on a typeII photovoltaic heterojunction based on WSe2 monolayer flakes and a bilayer MoS2 film stacked in vertical configuration on a Si/SiO2 substrate. Band offset characterized by a significant interfacial dipole is pointed out at the WSe2/MoS2 vertical junction. The photocarrier generation process and photo-transport are studied by applying a differential technique allowing to map directly 2D images of the surface photo-voltage over the vHJ and in its immediate vicinity. Differential SPV imaging reveals the impact of chemical defects on the photo-carrier generation, and that negative charges diffuse in the MoS2 a few hundreds of nanometers away from the vHJ. The analysis of the SPV data confirms unambiguously that 1 ACS Paragon Plus Environment

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light absorption results in the generation of free charge carriers that do not remain Coulomb bound at the type II interface. A truly quantitative determination of the electrons-holes (e-h) quasi-Fermi levels splitting (i.e. the open-circuit voltage) is achieved by measuring the differential vacuum level shift over the WSe2 flakes and the MoS2 layer. The dependence of the energy level splitting as a function of the optical power reveals that Shockley-Read-Hall processes significantly contribute to the interlayer recombination dynamics. Last, a newly developed time-resolved mode of the KPFM is applied to map the SPV decay time constants. The time-resolved SPV images reveal the dynamics of delayed recombination processes originating from photo-carriers trapping at the SiO2/TMDC interfaces.

KEYWORDS van der Waals heterostructure, MoS2, WSe2, type-II band alignment, nc-AFM, KPFM, surface photovoltage imaging.

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INTRODUCTION In the past few years, atomically thick two dimensional (2D) materials with optoelectronic properties complementary to those of graphene have been isolated.1 While graphene is a zero-bandgap semiconductor,2 hexagonal boron nitride (hBN) displays an isolating behavior and transition metal dichalcogenides (TMDCs) are semiconductors.1 In addition, most TMDCs (MoS2, WS2, MoSe2, WSe2 …) exhibit an indirect to direct bandgap transition when thinned down to the 2D monolayer (ML),3–5 with a bandgap width (0.75 - 2 eV range) closely related to the material composition, i.e. the nature of the transition metal (Mo, W, …) and the chalcogenide (S, Se, Te, …).6,7 A material dependent direct bandgap is strongly desirable for optoelectronic developments since it increases the luminescence quantum efficiency3 and permits to achieve systems with tunable properties by adjusting the TMDCs nature. Moreover, Britnell et al. pointed out an exceptionally strong light-matter interaction regarding to the material thickness8 opening up the way to extremely thin and highly performant optoelectronic devices such as photodetectors and phototransistors,9–11 photovoltaic cells8,12–14 and light-emitting devices.15–18 The key point in photoelectric conversion based applications is to achieve systems permitting to separate efficiently electron-hole (e-h) pairs (excitons) generated by incident photons. One of the actual routes consists in bandgap engineering allowed by the formation of vertical 2D-crystal–based van der Waals (vdW) heterostructures,1,19 where the final system takes advantage of the mixing of each component properties. Two main approaches are considered in the literature for TMDCs based 2D heterostructures:

(i)

Graphene/TMDCs1,11,14,19

and

(ii)

TMDCs/TMDCs1,13,20–23

heterojunctions. In the latter case, according to their difference in bandgap and work function, several pairs of TMDCs (e.g. MoTe2/MoS2, WS2/MoS2, WSe2/MoS2 …) are theoretically predicted to form type-II heterojunctions.6,7,24 In type-II interfaces, the valence band maximum (VBM) and the conduction band minimum (CBM) are localized in separate 3 ACS Paragon Plus Environment


materials on both side of the junction. Thus, one can expect a spatial charge separation of photo-excited electrons and holes across the interface21–23 leading eventually to photocurrent generation via contacting electrodes.13 The ultimate aim of controlling photo-induced charge transfers in such vdW heterostructures primarily requires a fine understanding of the 2D interface physics at the atomic scale. The first fundamental question, already discussed in several works22 but still open, concerns the relative positioning of energy levels at the 2D interface since conventional approaches, like Schottky or p-n junction models, are not suitable to describe atomically thick vertical junctions. Another point which is just as crucial is that interfacial charge separation is not a priori expected to lead to free carrier generation, but instead to the formation of tightly Coulomb bound interlayer excitons due to poor screening of the electrostatic potential in 2D geometry.25 Understanding the nature of the mechanisms behind the generation of free photocarriers26 is essential for using van der Waals heterojunctions in photovoltaic energy conversion. Last, it is also of great importance to investigate the photo-carrier recombination and trapping mechanisms as well as their dynamics. Indeed, solar cells efficiency is mainly governed by recombination processes,27–30 and the photo-carrier de-trapping dynamics intrinsically limit the performances of TMDC-based phototransistors and photodetectors in terms of photo-response speed.10 Among several approaches, Kelvin probe force microscopy (KPFM) is in principle well suited to probe the band offsets of 2D heterojunctions as well as the photo-induced charge separation mechanisms. Combined with non-contact atomic force microscopy (nc-AFM), KPFM can provide a high spatial resolution for imaging surface potentials and charge distributions. This technique yields a measurement of the vacuum level variations at a sample surface,31 which are in principle directly related to the energy band bending (or offsets) at junctions between two different materials. Moreover, KPFM can be used to probe the photo4 ACS Paragon Plus Environment

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generated charge carriers by analyzing the surface potential (SP) shift under illumination, i.e. the surface photo-voltage (SPV = SPillumination - SPdark). In principle, the SPV yields a measurement of the electron-hole quasi Fermi levels splitting across any kind of interface able to dissociate the exciton into free charge carriers. This past decade, KPFM capabilities have been intensively used to investigate various kinds of organic, inorganic and hybrid photovoltaic materials and devices.32–36 By contrast, KPFM investigations of TMDC-based two dimensional type-II interfaces remain confidential,21,37–40 and very few of them deal with photo-potential measurements.21 Surface photo-voltage imaging of TMDC-based type-II heterojunctions remains almost fully unexplored, with the exception of one recent report on MoTe2/MoS2 vertical heterostructures.21 In the latter work, surface photo-voltages of a few tens of mV were observed by comparing surface potential profiles extracted from KPFM images recorded in dark and under illumination. In the present work, we report a comprehensive KPFM investigation carried out on a type II heterojunction based on WSe2 monolayers stacked in vertical configuration on a continuous MoS2 film grown on a Si/SiO2 substrate. The WSe2/MoS2 system was selected because it is a benchmark for the community working on type-II TMDC heterojunctions. To analyze the impact of defects on the photo-carrier generation, we applied a differential technique allowing to map directly 2D images of the SPV. The acquired SPV images reveal directly how defects (here in the form of oxidized WSe2 areas) can impact the photo-carrier generation process. Furthermore, a thorough analysis of the SPV magnitude and SPV contrasts as a function of the illumination intensity was carried out, allowing to discuss the interlayer recombination dynamics. Last, we applied a newly developed time-resolved mode of the KPFM to map the SPV decay dynamics.41 The time-resolved SPV images revealed the dynamics of the delayed recombination processes originating from the trapping of photo-carriers at the silicon oxide / TMDC interface. 5 ACS Paragon Plus Environment


RESULTS AND DISCUSSION In this work, we investigated vertical heterojunctions (vHJ) consisting on monolayer-thick WSe2 flakes lying on a continuous MoS2 film deposited on a SiO2/Si++ substrate (more details are given in the Methods). Figure 1.a displays an optical image of the sample, in which the area investigated by KPFM, Raman scattering and photoluminescence spectroscopies is highlighted by a dash-dotted square (the flake localization by nc-AFM/KPFM is described in the Supporting Information, Figure S1). Figure 1.b presents the schematic of the vertical WSe2/MoS2 van der Waals heterojunction and the relative position of n-type MoS2 and p-type WSe2 energy levels before band alignment. The conduction band (CB) and valence band (VB) edges have been positioned with respect to the vacuum level accordingly to Keyshar et al.42 for bilayer MoS2 and Kang et al.6 for monolayer WSe2 (the mono or bilayer character of each TMDC is discussed hereafter on the basis of Raman and photoluminescence spectroscopies). In this scheme, the Fermi level for n-type MoS2 (on SiO2) has been fixed following the results of Keyshar et al.42 This representation remains however tentative at this stage. MoS2 is indeed widely known to be n-type,43 but its doping can be modified by interface impurities and defects at the silicon oxide substrate interface.44 In the following, it will be shown that MoS2 and WSe2 behave indeed with respect to each other as n-type and p-type semiconductors. The Fermi level for WSe2 has therefore been positioned 200meV below the one of MoS2, accordingly to the average value of the interface dipole probed by KPFM in this study. For the sake of clarity, the full stack including the vHJ (WSe2/MoS2/SiO2/Si++) will be hereafter designated by the terms WSe2/MoS2 vHJ or heterojunction, while the MoS2/SiO2/Si++ stack will be referred to as MoS2 layer.


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Figure 1. (a) Optical microscopy image of the WSe2/MoS2 vertical heterojunction formed by a WSe2 flake (dark triangular shape) deposited on a continuous MoS2 film (gray background). The area investigated by nc-AFM/KPFM, Raman spectroscopy and photoluminescence spectroscopy is indicated by orange dash-dotted contours. (b) Schematic illustration of the sample geometry and of the energy levels of the TMDC materials taken separately (i.e. before band alignment). Here, it has been supposed that MoS2 and WSe2 display n-doped and p-doped characters, respectively (more details are given in the main text). Evac: vacuum energy level. EF: Fermi level. (c) Raman spectrum taken over the WSe2/MoS2 van der Waals heterojunction. Grey and red areas highlight WSe2 (A’1, 2LA) and MoS2 (E2g, A1g) characteristic vibrational modes. The inset represents a 2D map of WSe2 A’1 mode and confirms that the flake is composed of WSe2. (d) Photoluminescence spectra of the WSe2 flake (black line) and the MoS2 film (red line, magnified 5 times) with A and B components.



Raman and photoluminescence spectroscopies (Figures 1.c and 1.d) and 2D mapping have been performed to ascertain the sample morphology (especially the thickness of each TMDC layer) and chemical composition. First, the monolayer character of the WSe2 flake is confirmed according to the absence of the 310 cm-1 WSe2 B2g Raman mode45–47 and to the shape and positions of the WSe2 photoluminescence peak45,47–49 and WSe2 A’1 and 2LA(M) Raman modes.48,49 Besides, Raman mapping of the WSe2 A’1 mode (inset in Fig. 1.c) confirms that WSe2 is only located over the flake. A closer look also reveals darker areas within the flake (i.e. with a less intense WSe2 A’1 mode) suggesting partial and local oxidation of the monolayer WSe2 flake.47 Last, the MoS2 A1g and E2g modes (Figure 1.c) measured respectively at 383.7 cm-1 and 406 cm-1 (∆ω = 22.3 cm-1)49–51 and the relative intensities of the A and B components of the MoS2 photoluminescence (Fig. 1.d) are characteristic of bi-layer MoS2. Thus, it turns out that the vHJ consists of a partially oxidized WSe2 monolayer stacked on a bi-layer MoS2 film. Figure 2.a displays a topographic nc-AFM image taken under ultra-high vacuum (UHV) conditions in frequency modulation mode (see the Methods) of the area of interest highlighted in Figure 1.a. In this image, the triangle shape corresponds to the WSe2 flake corner. The stacking height of the WSe2 flake on the MoS2 layer is estimated (from z-histograms, not shown) to be 1 ± 0.3 nm. This value is slightly higher than the expected thickness of a monolayer WSe2 flake (ca. 0.7 nm). It may be ascribed to the presence of intercalant species52 at the WSe2/MoS2 interface and/or to intrinsic defects such as partially oxidized areas47 as pointed out before by Raman spectroscopy. Additionally, the AFM image (Fig. 2.a) reveals the presence of structural defects within the WSe2 flake, such as holes and cracks. The surface potential image (KPFM tip compensation bias, see the Methods) acquired in dark conditions (Fig. 2.b) reveals a clear contrast between the MoS2 layer and the heterojunction. In average, the surface potential (SP) is lower (i.e. more negative) over the vHJ, the maximum surface 8 ACS Paragon Plus Environment

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potential difference between the surrounding MoS2 and the vHJ being about 200 mV (± 50 mV) as shown by the cross section profile in Figure 2.d.

Figure 2. (a) Topographic nc-AFM image of the area of interest defined in fig. 1.a. The dotted squares labelled A and B highlight two areas where nc-AFM/KPFM images have been acquired at higher magnifications (see Fig. 3 and Fig. 4). (b,c) KPFM images of the tip compensation bias Vtip acquired in dark (b) and under continuous wave illumination (c) at 515nm (Popt≈26.5 µW.mm-2). (d) KPFM compensation potential cross section profiles in dark and under illumination (profiles paths highlighted by continuous lines in b and c). Note that under illumination the compensation potential is shifted (∆Vtip) in the opposite direction to the vacuum level (∆Evac), ∆Vtip= ∆Evac/e. e: (negative) electron charge. (e) Surface photovoltage (SPV) profile calculated as the difference between the compensation bias recorded in dark and under illumination at 515nm, SPV = Vtip(515nm)- Vtip(dark). ∆W: vacuum level



shift over the WSe2/MoS2 vHJ. ∆M: vacuum level shift over the MoS2 film. (f) Schematic illustration of the sample and of the energy levels in the dark after bringing the two materials into contact. The energy level alignment (electron transfer from MoS2 to WSe2) results in the formation of an effective interface electrostatic dipole D pointing downward (upward shift of the vacuum level). Evac: vacuum energy level. EF: Fermi level. (g) Schematic illustration of the sample and of the energy levels under illumination. The photo-generation of free charge carriers (holes and electrons on the WSe2 and MoS2 sides, respectively) splits the electrons and holes Fermi levels into quasi-levels (EFe and EFh). The negative carriers diffuse within the MoS2 a few hundreds of nanometers away from the vHJ (as highlighted by the arrow in the sample illustration). Thus, it is possible to probe the full open circuit voltage (VOC) value by summing the SPV (taken in absolute value) measured over the vHJ and at its vicinity over the MoS2. For the sake of clarity, the electron quasi Fermi level has been highlighted by dotted lines only on the MoS2 film side.

This SP contrast implies that the WSe2 flake is in overall negatively charged. At first sight this may appear surprising since a flat band configuration is generally expected for weakly interacting van der Waals interfaces between different TMDC materials.53,54 However, several experimental reports indicate that charge transfer can occur between different TMDC materials.21,55,56 In particular, by using spatially resolved photoemission electron microscopy, Fang et al. have shown that WSe2/MoS2 vertical heterojunctions deposited on SiO2 display a built-in electric potential consistent with an electron charge transfer from MoS2 to WSe2.55 Such a band alignment is indeed consistent with p-type55 and n-type42,57 characters of WSe2 and MoS2, respectively. Since the concepts of band bending and space charge area are ambiguous in the case of atomically thick vHJs, the effects of charge transfer can be accounted by assuming the existence of an electrostatic dipole at the WSe2/MoS2 interface, as depicted in Fig 2.f. It is also clearly evident that the charge transfer is not spatially uniform over the WSe2/MoS2 interface. The SP image displays indeed bright patches with a lateral extension 10 ACS Paragon Plus Environment

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ranging from a few tens to a few hundreds of nanometers, some of them being clearly correlated with structural defects. In these parts of the vHJ the magnitude of the interface dipole becomes significantly decreased or even vanishes. The correlations between the electrostatic and topographic contrasts indicate that the “bright patches” correspond to oxidized parts of the WSe2 monolayer, which existence has already been deduced from the results of Raman spectroscopy. It has indeed been shown that the oxidation of TMDCs preferably initiates at edges or structural defects.58 The fact that the effective dipole can locally vanish implies that the WSe2 is oxidized at such a level that it behaves as an insulator, resulting in the absence of charge transfer (in the vertical direction) in these parts of the vHJ interface (this situation is depicted in Fig. S3a). Another point worth noting concerns the existence of short range spatial fluctuations of the surface potential. Such potential fluctuations have been discussed by Ghatak et al. for MoS2 monolayers on SiO2.59 They can be explained by a random and area-dependent charge trapping at the MoS2/SiO2 interface. Moreover, we assume that an imperfect screening60 of the trapped charges permits to explain the SP fluctuations observed over the WSe2/MoS2 vHJ. We now focus on the photo-response of the vHJ. Strikingly, the SP contrast between the MoS2 film and the heterojunction is dramatically reduced under continuous wave illumination at 515nm, as shown by Fig. 2.c. This SP shift is fully reversible (i.e. the surface potential returns completely to its initial state after switching off the illumination), which will be confirmed hereafter by differential SPV imaging. Cross sectional SP profiles (Fig. 2.d) show that the vacuum levels become almost aligned under illumination as sketched in figure 2.g. The SPV profile (Fig 2.e) obtained by subtracting the in dark profile from the one recorded under illumination reveals a clear positive photo-charging of the WSe2 flake (with an average SPV value of 150 ± 20 mV), which is consistent with the expected interlayer charge separation for a type-II band alignment (Fig. 2.g). The existence of a small negative SP shift 11 ACS Paragon Plus Environment


over the MoS2 layer seems however more ambiguous, since the charge separation should occur only at the WSe2/MoS2 interface. As shown hereafter, this effect can be reasonably attributed to the diffusion of negative photo-carriers within the MoS2 film. To confirm the last scenario and to investigate the impact of the defects on the carrier photo-generation, it is mandatory to map 2D images of the surface photo-voltage. Like to the calculation of cross section profiles, conventional SPV imaging proceeds by subtracting SP data recorded in dark from the ones recorded under illumination. This approach can generate artefacts due to the lateral lag between the set of images used for the SPV calculation.61 To avoid this issue, we used here a differential SPV imaging method based on the acquisition of spectroscopic curves of the SP as a function of time on a 2D grid (see also the Methods). In each pixel a single illumination pulse is applied and the SP curves are synchronously recorded. SPV images (Figure 3) can then be reconstructed by calculating the difference of the surface potential measured under illumination and in dark in each pixel (Figure 3e) of the 2D spectroscopic grid. In addition, this method can be used to investigate the photovoltage dynamics at time scales longer than the KPFM compensation bias regulation loop integration time (typically a few tens of ms). Panels a and b of the Figure 3 show respectively a topographic image and a differential SPV image acquired in the same location than the area labelled A in Figure 2.a. These data definitely confirm that a strongly positive SPV develops itself over the vHJ, and that a negative SPV exists in its neighborhood. Besides, the global SPV contrast confirms that the SP returns fully to its initial value after each illumination pulse sequence (if not one would expect some gradient in the SPV image). Moreover, it appears that the SPV is strongly negative only at the boundary between the vHJ and the MoS2, as shown by the cross section profile in Fig. 3.d. The differential SPV imaging process insures that this remarkable feature does not originate from an artefact (the latter point is obvious if one compares the SP and SPV 12 ACS Paragon Plus Environment

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cross section profiles extracted from the calculated images, see Fig. S2). The vHJ appears therefore in the SPV image as a red triangle (color code associated to positive SPV values above +150mV) surrounded by a dark blue halo (negative SPV values below -100mV). This specific contrast unveils the existence of a gradient in the negative charge carrier concentration, confirming that the electrons photo-generated at the MoS2/WSe2 interface diffuse a few hundreds of nanometers away within the MoS2 film. The existence of such mobile photo-carriers proves conclusively that interfacial charge separation leads effectively to free carrier generation, in other words all interlayer excitons do not remain permanently Coulomb bound at the MoS2/WSe2 interface (this elementary fact will be confirmed hereafter by the analysis of the open circuit voltage dependency as a function of the optical power).

Figure 3. (a) Topographic nc-AFM image of the area labelled A in Fig.2a and (b) differential image of the “fast” component of the SPV (λ=515nm, Popt ≈ 26.5 µW.mm-2). This image is calculated from



the matrix of spectroscopic curves as the difference between the surface potential in dark (Vtip at t=0.4s) and the surface potential under illumination (Vtip at t=0.56s). The dotted ellipses in (a) and (b) highlight a highly oxidized part of the WSe2 flake. (c) Optical image of the area containing the WSe2 flake of interest. The black star corresponds to the measurement area « far » from the WSe2/MoS2 vHJ. (d) SPV cross section profile corresponding to the path highlighted by a black line in (b). The open circuit voltage is equal to the difference between the highest positive (SPVmax) and lowest negative (SPVmin) SPV values. (e) Spectroscopic curves of the surface potential as a function of time during the single illumination-pulse sequence recorded within the vHJ domain (red triangles) and near its periphery in a highly oxidized part of the WSe2 flake (blue squares). The arrows labelled F and S highlight the “fast” and “slow” components of the SPV (see main text). Inset: spectroscopic curve recorded on the MoS2 film several microns away (measurement area shown in c) from the first vHJ edge.

It is also worthy to note that the SPV level is not perfectly homogeneous over the vHJ. The photo-voltage image displays local minima correlated with the areas where it has been previously assumed that the WSe2 layer suffered from oxidation. This correlation is definitely established by comparing the SPV and “in-dark” SP images calculated from the 2D matrix of spectroscopic data (see Fig. S.2). An area within the vHJ domain displaying a negative SPV is highlighted by a dotted ellipse in Fig. 3.b. The SPV magnitude at that place is moreover identical to the one at the vHJ’s boundary. These elements lead us to definitely conclude that the WSe2 has been partially damaged by an oxidation process occurring preferentially at his edges and propagating in its interior due to the existence of structural defects. Due to the absence of efficient charge screening in fully oxidized WSe2 areas, the SP shift under illumination probes directly the electrostatic potential associated to the diffusion of negative charge carriers in the recessed MoS2 film. This very special situation is depicted in Fig. 2.g. (see also Fig. S.3 in the supporting information). 14 ACS Paragon Plus Environment

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Other observations are in line with a diffusion process of photo-electrons within the MoS2. The time-dependent response of the SP under illumination reveals indeed different behavior depending on the location where the spectroscopic data are acquired, as shown by Figure 3.e. Over and close (i.e. at distances not exceeding a few hundreds of nanometers) to the vHJ, the SP exhibits a quasi-instantaneous response after switching the light pulse on (i.e. a change occurring faster than the KPFM regulation loop integration time) followed by a slower evolution and a subsequent stabilization at the timescale of a few hundreds of ms. By contrast, the SP displays only a slow photo-response when the measurements are performed several tens of microns away from the heterojunction (see the inset in Fig. 3e). The SPV at equilibrium equals therefore the sum of two contributions which will be referred to hereafter for ease of reference as “fast” and “slow” SPV components. It logically follows that the “slow” SPV component is an extrinsic feature of the MoS2/SiO2/Si++ substrate, and that only the “fast” SPV originates from the carrier photo-generation at the type II interface. This assumption is strongly supported by the absence of any significant contrast in the 2D map of the slow SPV component (see Figure S.4). Equally clearly, the absence of a fast SPV component far away from the vHJ provides additional evidence that the photo-carrier diffusion length in the MoS2 film does not exceed a few hundreds of nanometers. Further studies are needed to determine conclusively whether the “extrinsic” SPV originates from band-bending induced exciton dissociation at the SiO2/Si++ interface (i.e. an extra SPV originating from the Si/SiO2 itself), or if additional photo-carriers are generated at the MoS2/SiO2 interface. The slow SPV component displays a negative polarity far away from the vHJ, and the time-constant characterizing its time evolution is on the order of a few hundreds of ms. Both facts are consistent with an electron trap filling process at the MoS2/SiO262,63 and/or at the SiO2/Si interfaces. On the other hand, the slow SPV measured over and near the vHJ displays a positive polarity at higher optical powers. A possible 15 ACS Paragon Plus Environment


explanation for that polarity inversion could be that the free carriers generated by the vHJ counteract by electrostatic screening (see Fig. S.4) the contributions of the negative carrier trapped within the MoS2/SiO2/Si++ stack. A complete electrostatic modeling (which is beyond the scope of this study) would be required to establish a more comprehensive picture of these screening effects. Further insight on the photo-response of the vHJ can be gained by analyzing the dependence of the “fast” SPV component magnitude as a function of the optical power (Popt). As shown hereafter, this procedure yields an access to the recombination processes that cannot be probed directly at the time-scale of the KPFM measurement. Figures 4.a to 4.f display a series of differential images of the “fast” SPV component acquired at various Popt values in the area highlighted by a dotted square labeled B in Figure 2.a. In addition to the contrasts already discussed and characterized by a lower SPV over the oxidized domains, it clearly appears that the SPV measured on the WSe2/MoS2 vHJ is strongly dependent of the incident optical power. However, before going any further it is crucial to remind that this is the open circuit voltage which is the relevant parameter that shall be analyzed to discuss the electron-hole recombination kinetics. From what precedes, the electron-hole quasi Fermi level splitting (in other words the VOC) is in our case equal to the sum of the surface photo-voltages in absolute value recorded over the vHJ and at its vicinity over the MoS2 (as shown by the energy levels alignment scheme in Fig 2.g). The maximum VOC value that can be extracted from an SPV image corresponds therefore to the difference between the highest positive SPV value over the WSe2 flake and the lowest negative SPV value over the MoS2. These maximum VOC values deduced from the series of SPV images scale logarithmically with the optical power as shown in Fig. 4.g. This logarithmic scaling is expected from conventional models used to describe the balance between carrier generation 16 ACS Paragon Plus Environment

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and recombination at p-n junctions or type-II interfaces,13,64 and the nature of the underlying recombination process can be inferred by fitting the data with the equation:

= ( )

where kB is the Boltzmann constant, T the sample temperature, q the charge of the electron and α a constant related to the nature of the recombination processes. α=1 denotes a bimolecular (Langevin-like) radiative recombination process and α=2 corresponds to monomolecular defects-mediated Shockley-Read-Hall (SRH) recombination processes. In our case α ~ 1.6 ±0.1, which is in good agreement with the results of photo-transport measurements in operating PV devices based on WSe2/MoS2 vHJs.13 This last result indicates that SRH processes constitute a significant pathway for the interlayer recombination, revealing the existence of local intra or interlayer defects even in the less defective parts of the vHJ (keeping in mind that the highest positive SPV values used for the VOC calculation correspond to the less oxidized parts of the WSe2 flake). We also emphasize that a much smaller slope (α ~ 1.2 ±0.1) would have been obtained if we had only considered the dependence of the SPV measured on the WSe2 flake taken alone (compare the SPV and VOC curves in Fig. 4.g). As already stressed by Ellison et al. in the case of organic donor-acceptor interfaces,33 this points out the limitations of the common approach which consists in treating the SPV measured above a vertical photovoltaic interface as a direct measurement of its full VOC. Last, we observe that above a certain power the VOC deviates from the logarithmic dependency and tends to saturate (see Fig. 4.g). The highest VOC value measured in this study is ca. 400mV which is smaller by 100-200mV than the one reported from transport measurements by Furchi et al. in the case of monolayer-WSe2/monolayerMoS2 heterojunctions.13 This difference is consistent with a downward shift of the conduction band minimum of bilayer MoS2 with respect to the case of the monolayer42 (reminding that 17 ACS Paragon Plus Environment


the upper limit on the VOC is imposed by the interlayer band gap, i.e. the energy difference between the MoS2 conduction band minimum and the WSe2 valence band maximum).

Figure 4. (a-f) Differential SPV images (corresponding to the area labeled B in Fig. 2a) acquired for increasing optical powers. (a) 0.4 µW/mm². (b) 0.95 µW/mm². (c) 1.69 µW/mm². (d) 4.35 µW/mm². (e) 7.69 µW/mm². (f) 26.54 µW/mm². (g) Curves of the highest positive (SPVmax, red opened triangles) and lowest negative (SPVmin, blue opened squares) SPV values (right y scale) plotted with their difference which equals the VOC (full symbols in black, left y scale) as a function of the optical power


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in semi-log scale. The dotted curves (red and black) present the results of linear fits, while the dashdotted curve (in blue) is just a guideline for the eye.

The above analysis provides an indirect access to the photo-carrier dynamics. A complementary approach consists in mapping the SPV decay time constants by performing the KPFM operations under frequency modulated illumination (FMI-KPFM).34,65 The basic principle of FMI-KPFM (Figure 5.a) consists in recording spectroscopic curves of the timeaveraged surface potential (SPav) probed by KPFM as a function of the modulation frequency (f) of the illumination source. In an ideal system characterized by an instantaneous charge build-up under illumination followed by a single photo-potential decay process in the dark state, SPav increases (in the case of a positive surface photovoltage which is illustrated in Fig. 5a) with the modulation frequency and saturates when the time between the pulses becomes shorter than the photo-potential time decay (the average potential at saturation is then equal to the one measured under continuous wave illumination). By analyzing the dependency of the average potential with respect to the modulation frequency, it becomes possible to extract time constants characterizing the photo-potential decay dynamics between the light pulses. In the following, it is important to remember that the SPV can be positive or negative (Fig. 5a displays only the case of a positive SPV, an illustration with both polarities is given in Fig. S5). One shall also keep in mind that the spectroscopic data correspond to time-averaged values of the surface potential as a function of the illumination modulation frequency, and not to a direct measurement of the surface photo-voltage (the SPV is in turn proportional to the difference between the SP recorded at the highest and lowest frequencies, as shown in Fig. S5). Recently we implemented a time-resolved imaging mode of the SPV based on the acquisition of the spectroscopic curves on a 2D matrix (we refer the reader to our previous report for a more comprehensive description of FMI-KPFM operating principles41). 19 ACS Paragon Plus Environment


Figure 5.b displays a series of SPav(f) curves extracted from the matrix of spectroscopic data acquired during the 2D dynamical imaging process. The curves acquired over the vHJ (blue squares) and over the MoS2 film (red triangles) display opposite trends (increasing towards more positive values or decreasing towards less negative values when increasing the frequency) which is consistent with the expected photo-voltage polarities. Figure 5.c presents an image of the difference between the average potentials acquired at the highest and lowest modulation frequencies. In principle, this shall yield an image of the SPV or more precisely of 90% of its full amplitude (given the 10% duty ratio of the modulated illumination, see Fig. S5). The global image contrast is indeed quite similar to the one of the former series of differential SPV images. However, the magnitude of the “FMI-KPFM SPV image” is roughly twice smaller than the one of the differential SPV image acquired under the same optical power (compare Fig. 5.c and Figure 4.f). This huge difference cannot be accounted by the sole effect of averaging the SP under modulated illumination with a 10% duty ratio. A plausible explanation for this “missing SPV” is that bi-molecular recombination processes occur at much smaller time scales that the shortest time interval (i.e. inverse of the maximum modulation frequency) allowed by our modulated illumination chain. The spectral response of the average potential probed by FMI-KPFM is thus dominated by the contribution of slower trap-delayed processes.


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Figure 5. (a) Scheme of the surface potential time-response under frequency modulated illumination. The SPV decay dynamics (characterized by a time constant τd) determine the frequency-evolution of the average potential SPAV probed by KPFM. SPD and SPCW represent the in-dark surface potential and the maximum surface potential that would be measured under continuous wave illumination. Note that in this scheme the surface photo-voltage (SPV = SPcw - SPD) is positive. (b) Experimental curves of the average surface potential as a function of the illumination modulation-frequency Fmod acquired over the vHJ (open triangles) and at its vicinity over the MoS2 (open squares) during a 2D dynamical mapping. The results of the numerical fit performed to extract the SPV decay time constants (see text) are displayed by solid lines extended by dots above 50kHz (the curve fitting has been performed only on the basis of the data point below 50kHz). Inset: curve acquired over the oxidized part of the WSe2 flake. (c) Experimental image of the difference between the average surface potential values recorded



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at the extrema of the frequency range. Open symbols highlight the locations where the curves displayed in (b) have been recorded. (d) Result of the numerical fit. SPV decay time constant image recalculated from the numerical fit of the matrix of spectroscopic curves. The values below 150µs and above 1.3ms have been excluded from the color scale (their location corresponds approximately to the exclusion area defined in the error image). (e) Result of the numerical fit. Standard error image of the SPV decay time constant. The data above 50% have been arbitrarily excluded from the color scale.

In order to quantify these dynamics, we used simple exponential functions characterized by a unique time constant τd to account for the SPV decay between the light pulses. The spectroscopic curves can then be fitted by the following equation:41

() = SP + SPV . D + ( ! . . SPV (1 − e

%

(&%) '( .) )

where SPD is the “in-dark” surface potential, SPVmax the maximum photo-potential that would be measured under continuous illumination and D the illumination duty ratio. Three variable parameters are thus used to perform the fit: τd, SPD and SPVmax (SPD and SPVmax are not shown here). Actually, the spectroscopic data deviate slightly from this fitting law, which predicts that the SP shall conti5nuously increase (or decrease depending on the polarity of the SPV, see Fig. S5) when raising the frequency (and ultimately saturate at a value equal to the one measured under continuous wave illumination, as depicted in Fig. S5). This is especially apparent for the curves recorded over the MoS2, which display a slight upturn towards less negative values when increasing the frequency above 100kHz (see Figure 5b). This deviation may possibly originate from the existence of additional dynamical processes occurring at shorter time scales. However, this hypothesis cannot be confirmed, mostly due to


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the limitations of our setup in terms of maximal modulation frequency. For that reason, the fit (with a single time-constant decay) was restricted to the data points acquired below 50kHz. This procedure yields in average decay times on the order of a few hundreds of microseconds, and the associated 2D dynamical image is presented in Figure 5.d. Obviously, the fit accuracy dramatically decreases in the areas displaying too small photo-voltages, as shown by the standard error image (Fig. 5.e). A spectroscopic curve acquired over the strongly oxidized part of the WSe2 flake is displayed as an inset in Fig 5a. It features indeed a very small effective SPV (the difference between the SP taken at the extrema of the frequency range is less than 5mV). Moreover, the average potential seems to display a non-monotonic dependency as a function of the frequency which cannot be accounted by the equation used for the fit. The non-relevant data (arbitrary defined here as the ones for which the relative error is higher than 50%) have consequently been excluded from the color scale used to map the dynamical image. The dynamical image contrast is rather homogeneous on the other parts of the surface, although one can note that the average decay time is slightly higher over the vHJ especially in the periphery of the oxidized area. Taken together, these data strongly indicate that the SPV decay originate from trap-delayed recombination due to the cumulative contribution of photoelectron trapping at the MoS2/SiO2 interface, and photo-carrier trapping by interlayer defects.62,66 In that case this is the carrier de-trapping (or trap-release) time which limits the dynamics of the recombination process. Trapping processes at MoS2/dielectric interfaces have been widely investigated, and the dynamics reported in the literature are distributed over wide timescales ranging from sub-µs to s.62,67 In our case, a likely explanation for the observed SPV decays would be that photo-carrier trapped in shallow states are released within a few hundred of µs thanks to thermal activation.68 Fast, we stress that the dynamics deduced from



the FMI-KPFM measurements set an upper limit to the maximum switching rate (a few kHz) which could be achieved if the vHJ was used in a photodetector configuration.

CONCLUSION

We reported a comprehensive analysis of the opto-electronic properties

of a vertical type II van der Waals heterojunction by using complementary modes of the Kelvin Probe Force. The band alignment at the monolayer-WSe2/bilayer-MoS2 interface was first determined by analyzing the surface potential images, which contrast confirmed (with the support of Raman spectroscopy outputs) the presence of oxidized area within the WSe2 flake. A differential technique was used to map directly 2D images of the surface photo-voltage over the vHJ and at its periphery. The full set of SPV data confirmed that light absorption results in the generation of free charge carriers that do not remain Coulomb bound at the type II interface. The SPV images revealed directly how defects can impact the photo-carrier generation process, as well as the diffusion of negative carriers at the vicinity of the heterojunction. SPV imaging was moreover used to perform a quantitative measurement of the electron-hole quasi Fermi levels splitting under illumination, i.e. the open circuit voltage. The nature of the recombination processes was discussed by analyzing the dependency of the VOC as a function of the optical power, and by mapping the SPV decay times with KPFM under frequency modulation illumination. In particular, the time-resolved SPV images revealed directly the existence of trap-delayed recombination processes occurring at the time scale of a few hundreds of µs. We stress that these phenomena are precisely the ones that limit the photo-response speed of TMDC-based photodetectors and phototransistors, and that it is crucial to investigate the sources of carrier losses by recombination for the development of efficient photovoltaic devices. Thus, many parameters which are highly relevant for TMDCbased photovoltaic and optoelectronic applications can be obtained with this multimodal KPFM approach. 24 ACS Paragon Plus Environment

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METHODS

Sample preparation. Both continuous MoS2 film and monolayer WSe2 flakes have been grown separately by chemical vapor deposition process on two different degenerately p-doped silicon substrates coated with 270 nm thermal silicon oxide (SiO2/Si++). The vertical WSe2/MoS2 van der Waals heterojunctions (vHJ) was then fabricated by mechanically transferring the WSe2 flakes on top of the continuous MoS2 film. The sample fabricated and provided by the 2DLayer company,69 has been fixed on a stainless steel sample holder with an ultra-high vacuum compatible electrically conductive silver epoxy paste from EPO-TEK (E4110). The MoS2 film was also contacted to the sample holder with the same silver epoxy paste in order to define the MoS2 layer as the potential reference. Before nc-AFM/KPFM measurements, the sample has been annealed overnight under UHV conditions at low temperature (T= 150 °C) in order to minimize the amount of adsorbed and intercalated species without damaging the silver epoxy paste.

Raman and photoluminescence spectroscopy. Raman measurements have been performed by using a confocal Witec alpha 500 spectrometer at 532 nm. In this setup the elastically scattered light from the sample is filtered out by a Notch filter, and the inelastically scattered light is collected and sent to a spectrometer working with a resolution less than 1 cm-1. A typical Raman spectrum is acquired in 1-10 s. To avoid laser heating, laser power is kept below 1 mW/µm2. The PL measurements have been performed on the same setup. A typical PL spectrum is acquired in 1-10 s. To avoid laser heating, the laser power is kept below 50µW/µm2.



nc-AFM/KPFM measurements. nc-AFM/KPFM experiments on TMDCs vHJ have been achieved under UHV conditions within a beam deflection Omicron VT-AFM setup. The topographic imaging was performed in frequency modulation mode with frequency shifts (negative set-points for nc-AFM) and vibration amplitude of a few Hz and a few tens of nanometers, respectively. KPFM measurements have been simultaneously acquired in frequency modulation mode (FM-KPFM) in single-pass mode. All data have been obtained with PtIr-coated silicon cantilever (EFM, Nanosensors, resonance frequency in the 45-115 kHz range) annealed under UHV in the preparation chamber of the VT-AFM to remove contaminant. The modulation bias of 0.2 mV and the KPFM compensation potential VDC were applied to the cantilever (tip bias Vtip = VDC) and the MoS2 substrate was grounded. As VDC is applied to the tip, the contact potential difference (CPD) is equal to –VDC. In this work, the tip compensation bias (Vtip = -VCPD) was used to represent the potentiometric data, and was called for simplicity KPFM potential or surface potential.

KPFM surface photovoltage imaging and dynamical measurements. An external laser module has been used for sample illumination through an optical viewport of the UHV AFM chamber. The sample was illuminated at 515 nm in front side geometry by using a PhoxXplus module from Omicron Laserage GmBH (rise and fall times 1.32s. In FMI-KPFM, one records spectroscopic curves on a 2D grid of the average surface potential as a function of the frequency-modulation of the illumination source. Illumination pulse groups are defined at pre-selected frequencies with the frequency list mode of the AWG. The performances of the illumination chain are currently limited by the edge transitions times of the pulses generated by the AWG, which limit in practice the modulation frequency to a few MHz for a 10% illumination duty ratio. More details about FMI-KPFM operations can be found in our former report.41

ACKNOWLEDGMENT Financial support by the Agence Nationale de la Recherche (France) with the 2DJ project (ANR-15-CE24-0017) is gratefully acknowledged.

Supporting Information. Flake localization by nc-AFM under UHV. KPFM and SPV images reconstructed from the matrix of spectroscopic curves. Schematic illustration of the sample and of the KPFM tip compensation bias profiles and SPV profiles. Spectroscopic curves of the surface potential, and calculated images of the fast SPV, the total SPV and the slow SPV component. Schemes and simulations of the surface potential time-response under frequency modulated illumination in the case of positive and negative surface photo-voltages.

Corresponding Author *Address correspondence to [email protected] References



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