Progressive Micromodulation of Interlayer Coupling in Stacked WS2

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Functional Nanostructured Materials (including low-D carbon)

Progressive Micro-Modulation of Interlayer Coupling in Stacked WS2/WSe2 Heterobilayers Tailored by a Focused Laser Beam Ya Yu Lee, Zhenliang Hu, Xinyun Wang, and Chorng Haur Sow ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b12631 • Publication Date (Web): 08 Oct 2018 Downloaded from http://pubs.acs.org on October 14, 2018

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ACS Applied Materials & Interfaces

Progressive Micro-Modulation of Interlayer Coupling in Stacked WS2/WSe2 Heterobilayers Tailored by a Focused Laser Beam Ya Yu Lee^, Zhenliang Hu^,§, Xinyun Wang^,§, Chorng-Haur Sow^,§* ^Department §Center

of Physics, National University of Singapore, 2 Science Drive 3, Singapore 117542

For Advanced 2D Materials and Graphene Research Center, National University of Singapore, 6 Science Drive 2, Singapore 117546

*Address correspondence to Prof. C. H. Sow. Phone: (+65) 65162957. Fax: (+65) 67776126. E-mail: [email protected] ABSTRACT When a vertically stacked heterobilayer comprising of a WSe2 monolayer on WS2 monolayer is first fabricated, the heterobilayer behaves like two independent monolayers due to the presence of a large interlayer separation. However, after the stacked heterobilayer is subjected to a focused laser treatment, the interlayer separation between the two monolayers becomes progressively reduced which transforms the WS2/WSe2 heterostructure from the non-coupling to the strongly coupling regime. This strong coupling induces the charge transfer between two layers, thus lower the exciton recombination rate in the individual layer. This changes the optical properties of the heterobilayer from a fluorescence active species into one where the fluorescence is quenched. The focused laser beam scanning method is therefore able to serve S-1

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as a localized annealing tool to progressively modulate the interlayer separation and enable the micropatterning of the heterobilayer to achieve distinct regions with different degrees of fluorescence quenching. Systematic studies are carried out to gain an insight into the mechanism involved in the onset of the interlayer coupling in the material. Our method is also successfully extended to a WS2/WS2 homobilayer structure. KEYWORDS:

WS2,

WSe2,

Heterobilayer,

Interlayer

Fluorescence Emission


Coupling,

Laser

Modification,

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INTRODUCTION

Scientific interest in the field of monolayer transition metal dichalcogenide (TMDC) materials is largely motivated by its possession of a direct band gap 1, of which that of MoS2, WS2 and WSe2 have values between ~ 1 to 2 eV. This falls within the visible light to the near-infrared range of the electromagnetic spectrum, making them viable for a range of optoelectronic applications 2. Besides investigating the properties of monolayer TMDC materials, this field has also been progressing rapidly towards studying and uncovering the potential applications of 2D van der Waals heterostructures – materials constructed by stacking two different TMDC monolayers together. These heterobilayers have demonstrated huge potential in constructing a wide range of new materials with tuneable properties 3 by offering numerous parameters within the system that can be varied. This includes the interlayer separation, twist angle and stacking configuration between the monolayers in the heterostructure. Earlier efforts have managed to realise varying the interlayer separation experimentally through techniques such as: i) sandwiching a hexagonal boron nitride (hBN) dielectric layer of differing thicknesses between the TMD heterobilayer4, ii) irradiating the TMD heterobilayer with an ion beam fluence of



different intensities

5

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and iii) varying the thermal annealing duration or temperature after

constructing the TMD heterobilayer 6.

These techniques however, have their limitations: In the case of varying the number of sandwiched hBN layers in the heterostructure 6, it alters the properties of the heterostructure by changing its interlayer separation and correspondingly, the extent of interlayer coupling. This technique would thus only be able to achieve a global (non-localised) modification of its optical properties; additionally, the minimum variation in the interlayer separation would also be constrained by the intrinsic thickness of a 1L-hBN material. As for the thermal annealing technique, it works by causing the movement of trapped impurities between the monolayers, such that they agglomerate into bigger bubbles, or are removed from the heterostructure. This may eventually leave a sufficiently large pristine heterostructure interface with an enhanced interlayer interaction for investigation 7. By varying the annealing time and/or temperature, the extent to which this occurs can be modified. The main limitation of this process lies in its randomness which makes it hard to achieve localised and precise control.


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In this work, we present a facile focused laser beam method to engineer the interlayer coupling, and correspondingly the optical properties of a stacked van der Waals heterobilayer consisting of 1L-WSe2 mechanically stacked atop a 1L-WS2. The following combination of materials is chosen due to their high photoluminescence (PL) efficiencies

8

and potential

applications in devices such as solar cells, p-n junctions, and field effect transistors. This technique employs the use of laser beam to controllably and progressively reduce the interlayer separation, and resultantly, its optical properties. Specifically, the heterobilayer transforms from two fluorescing independent monolayers to a coupled bilayer with quenched fluorescence. This evolution is attributed to its inherent type II (staggered) band alignment, and also to the built-in electric field between the two layers

9-10.

In addition, we have also extended

this technique to WS2/WS2 vertically stacked homo-bilayer. The main advantages of this technique include achieving: (a) a progressive adjustment of the interlayer separation, (b) ondemand localized modification of the stacked region en route to micropatterning of the heterobilayer and (c) systematic insights into the onset of interlayer coupling.

MATERIALS AND METHODS



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The monolayer WS2 and WSe2 materials are first obtained via the mechanical exfoliation technique, where a blue tape is used to cleave the WS2 or WSe2 crystals till they are sufficiently thinned down. The thinned down WS2 microflakes, comprising of monolayers and multi-layered materials, are then transferred onto SiO2/Si substrates, while that of WSe2 are then transferred onto Polydimethylsiloxane (PDMS) substrates. Through a series of spectroscopy techniques, the monolayers present on the substrates are then identified. Subsequently, they are stacked together using the dry transfer technique with a micromanipulator and the aid of a heating pad. To modify the heterobilayer with the focused laser beam method, a standard setup shown in supporting information Fig. S1 is used. The set-up consists of an upright optical microscope with a side port that allows a parallel beam of laser beam to enter which is reflected towards the objective lens by a beam splitter. A DPSS laser emitting light at a wavelength of 532nm is utilized in this work. The laser scanning process is carried out by fixing the incident laser beam and moving the motorized stage holding the sample. The incident laser spot size is ~1 μm after the laser beam is focused by an objective lens (magnification of 100x). The scanning speed


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adopted in this work is 20µm/s. As shown in Fig S1, the sample can be modified under ambient or controlled conditions by placing it inside a transparent chamber. Further Characterizations. Optical microscope (OM) images of the sample are obtained using an Olympus BX51 microscope. Fluorescence microscope (FM, Olympus BX51 Microscope) images with yellow light excitation are obtained using U-MWIY2 (530-580 nm) filter cubes. The thickness of the WS2 or WSe2 microflakes are measured by Bruker ScanAsyst Atomic Force Microscope. Raman and PL spectroscopy are obtained by Renishaw inVia 2000 with a laser wavelength of 532 nm. Raman mappings are obtained from the same Reinshaw inVia 2000 system. Each Raman mapping series consists of 400 measuring points, each with an integration time of 1s. The PL mapping images and spectra are obtained on a confocal microPL set-up with an excitation wavelength of 532 nm. The Kelvin Probe Force Microscopy (KPFM) images are obtained using a contact potential difference (LCPD) method by measuring the intrinsic fermi level offset between the AFM tip and a sample surface. It is measured by the Bruker Dimension Icon SPM in PeakForce Tapping Mode through highly doped Si tips (PFQNE-AL). During the measurement, the lift height is kept around 20-45 nm.



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RESULTS AND DISCUSSION

Fig. 1(a) shows a WS2 sample that was exfoliated on the SiO2 (300 nm)/Si substrate with the monolayer outlined by the black dashed lines. Its corresponding FM image taken using yellow (530-580 nm) light excitation is shown in Fig. 1(c). The 1L-WS2 fluoresces very brightly under the FM - a common observation for monolayer TMDC materials

11.

Atomic Force Microscope

(AFM) analysis (Fig. S2) also indicates a measured thickness of ~1 nm for the exfoliated 1LWS2. Though slightly larger than its theoretical thickness of ~0.8 nm 6, the slight discrepancy is attributed to the substrate’s inherent roughness and the existence of forces between the substrate and monolayer 12.

As for the WSe2 microflake exfoliated on the PDMS substrate, it is shown in Fig. S3. The PL spectrum obtained for the exfoliated 1L-WSe2 is shown in Fig. S4, which shows a single symmetric peak centred at ~745 nm. This is in good agreement with that obtained for 1L-WSe2 in other reports 13. Unlike bilayer (or even 3-layered) WSe2 samples, which have a tendency to display a broader asymmetric peak in their PL spectrum

14,


the emergence of a single distinct

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peak in Fig. S4 provides a strong evidence that it is a monolayer. The Raman spectrum collected for the same 1L-WSe2 is shown in Fig. S5 where there is a main peak and shoulder peak at ~ 251 cm-1 and ~261 cm-1 respectively. The 251 cm-1 peak can be assigned to both the E2g and A1g modes which are almost degenerate, while the latter peak, to the 2LA (M) mode 15.

After verifying their identities, the heterobilayer shown in Fig. 1(b), is constructed using the dry PDMS transfer technique. Its corresponding FM image is shown in Fig. 1(d). Clearly, the heterobilayer remains largely fluorescence active despite a mild reduction in its intensity. Note that the fluorescence for WS2 is more prominent than WSe2 in Fig. 1(d) due to the fact that the 1L-WS2 has a much higher PL intensity than the 1L-WSe2 16-17. The PL of WSe2 on the other hand, is expected to become more apparent using higher exposure times.

A clearer analysis of the PL activity of the materials can be seen by comparing the PL spectra obtained for the independent WS2 and WSe2 monolayers and also for the overlap region. This is shown in Fig. 1(g). There are two prominent peaks - centred at 616 nm and 745 nm originating from the PL of the WS2 and WSe2 monolayers respectively. The PL spectrum of the



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overlap region is largely a sum of that of its individual constituents, except for a ~50% reduction in the PL intensity and a red shift of less than 10 meV.

Figure 1 (a) OM image of (a) monolayer WS2 which is exfoliated on a SiO2(@300nm)/Si substrate, and (c) its corresponding FM image. (b) OM image of WS2/WSe2 heterostructure after dry transfer, and (d) its corresponding FM image. (e) AFM scan of heterostructure. (f) Height profile taken at the orange line shown in (e). (g) PL spectra of the individual monolayer (purple) and unmodified heterostructure region (red).

To gain an insight into the interlayer separation and topography of the sample, AFM characterization is carried out with the results shown in Fig. 1(e) and 1(f). The presence of white patches in the overlap region (Fig. 1(e)) is attributed to the presence of bubbles and wrinkles induced during the sample preparation18. The presence of wrinkles is a common


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observation due to the high flexibility of the monolayers that results in their deformation during the transfer. Similarly, bubble formation occurs frequently too due to the presence of trapped impurities, which often primarily consists of hydrocarbons 7. These regions experience a large interlayer spacing between the two monolayers, as observed from the AFM scan.

The average interlayer separation is also measured by extracting the height profile at the edge of the top monolayer to the bottom monolayer along the orange line drawn in Fig. 1(e). We ensure that the line selected for the analysis cuts across a minimal number of wrinkles and air bubbles to avoid large distortions to the measurements. The resultant height profile shown in Fig. 1(f) gives an interlayer separation of around 3.31 nm from the top of the 1L-WS2 (bottom layer) to the top of the 1L-WSe2 (top layer). Taking into account the thickness of the top 1LWSe2 (~0.7 nm

12),

the final interlayer separation can then be estimated to be ~2.6 nm. This is

significantly larger than the equilibrium interlayer separation value found in vertically stacked TMDC heterobilayer structures, which stands at ~0.6 to 0.8 nm

19-21.

As such, the lack of

effective interlayer coupling due to these wrinkles/air bubbles and the large interlayer



separation

22

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is likely to have caused the heterostructure to in general, possess the original

optical properties of its constituents.

This stacked heterobilayer is then subjected to two successive focused laser scans across the entire heterostructure, firstly at a laser power of 15 mW, and a second time at 25 mW in ambient conditions. Fig. 2(a) shows an optical image of the WS2/WSe2 heterobilayer structure before laser irradiation, while Fig. 2(b) and (c) for that obtained after the sample is subjected to the laser treatment at powers of 15 mW and 25 mW respectively. There is no visible damage induced in the sample after the laser treatment. The corresponding FM images are shown in Fig. 2(d), (e) and (f) respectively, demonstrating significant PL quenching at the overlap region after laser irradiation.


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Figure 2. Optical image of the WS2/WSe2 heterobilayer structure (a) before and after laser irradiation with a laser power of (b) 15 mW and (c) 25 mW. Corresponding FM images are shown in (d), (e) and (f) respectively. (g) PL spectra of the WS2/WSe2 heterobilayer structure for the independent monolayers before the transfer (purple), heterostructure before modification (red), and that after modification (blue and green). (h) A zoomed-in image of the PL spectra boxed up in (g). (i) PL spectrum for the modified heterobilayer at an extended spectral range (780-910 nm).



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A more detailed analysis of the PL quenching can be seen from the PL spectra consolidated in Fig. 2 (g) and (h). They confirm the above observation - an approximately 95% drop in PL intensity after laser modification, arising from a reduction in the PL of both the WS2 and WSe2 monolayers. A small red shift of the PL spectra (of less than 10 meV) is also noted. The latter trend can be viewed more clearly in the normalized PL spectra shown in Fig. S6(a) and (b).

This PL quenching has been experimentally observed in other type II van der Waals heterostructures too

4, 21.

It is often attributed to an interlayer charge transfer process: Upon

excitation, intralayer electron-hole pairs are formed in the WS2 and WSe2 monolayers separately. Subsequently, due to the band alignment, a charge separation of the electron-hole pair would ensue. For the case of the WS2/WSe2 heterostructure, electrons will move from the WSe2 to the WS2 layer, and holes in the opposite direction, resulting in the formation of interlayer electron-hole pairs. With a much longer lifetime, it is less probable for recombination


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and therefore, re-emission of photons, thereby resulting in the PL quenching we had observed 10, 23.

This charge transfer process is pictorially illustrated in Fig. S7.

The red shift (~10 meV) observed for the above spectra has been consistently observed in 6 other heterobilayer samples fabricated in this work. It has also been reported for other TMDC van der Waals heterostructures

4, 9, 21.

This red shift is mainly attributed to two factors. i) A

change in the dielectric environment due to the presence of interlayer interaction between the monolayers, which causes a reduction in the screening effect

9, 24.

ii) Induction of stress/strain

on the monolayers; this may be created during the exfoliation and transfer process, or due to a lattice mismatch (~4%) between WS2 and WSe2 crystal structures

25-26.

In our case, it is likely

that the reduction in electrostatic screening would be the more pertinent reason. This is because this factor would have specifically led to a one-directional (red) shift of the peaks, corroborating with what is observed; the effects of stress and strain on the other hand, are likely to result in arbitrary shifts in both directions at random parts of the heterostructure.



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Aside from the PL quenching and the red shift, closer analysis of the PL spectra in Fig. 2(i) shows the formation of a new peak (~ 800 nm) after laser irradiation. This peak is commonly attributed to the less probable, but possible recombination of indirect interlayer excitons (i.e. electron-hole recombination from the conduction band minimum of one material to the valence band maximum of the other material in the heterostructure) 27.

The above analysis has been mainly conducted for the same spot on the overlap region (location 1 in Fig. S8). To verify that such a trend is consistently observed throughout the heterobilayer, we carried out a PL mapping of the heterostructure as shown in Fig. 3. Figs 3(a), (b) and (c) capture the areas in which the WS2 PL peak (~ 620 nm) is identified, with a selected wavelength range of 610-630 nm and Figs 3(d), (e) and (f) for the WSe2 PL peak (~ 745 nm), with a selected wavelength range of 740-760 nm. Comparing the various images in Fig. 3, we can observe the gradual fading of the fluorescence for both PL peaks in the overlap region after the laser treatments, confirming our single spectrum analysis. Note that during the scanning focused laser treatments, the region rastered by the laser beam covers both the overlapped and non-overlap regions (exposed monolayer regions) of the heterobilayer. It is


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evident that while the PL is quenched at the overlapped part, that of the non-overlapped 1LWS2 and 1L-WSe2 remains largely unaffected. This observation thus further supports that the PL quenching is primarily attributed to the modified interlayer interaction in the overlap region, and not due to a form of laser-induced destruction.

Figure 3: PL mapping images (selected spectral range: 610 – 630 nm) of the heterostructure: (a) before laser irradiation (b) after laser irradiation at a power of 15 mW (c) and then again at 25 mW. Corresponding PL mapping images for a different selected spectral range of 740 – 760 nm) shown in (d), (e) and (f).



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To investigate the changes in the interlayer separation, AFM scans were taken and are shown in Fig. 4(a)-(c). The scans show a reduction in the number of air bubbles within the overlap region, and also a fall in the interlayer separation. The latter observation is determined by obtaining the height profiles, seen in Fig. 4(d)-(f), for the heterostructure at the various stages of the experiment. They measure the distance from the top of the bottom 1L-WS2 to the top of the 1L-WSe2 above. Referring to Fig. 4f, the height profile shows a separation value of ~1.34 nm, which translates into a final interlayer separation value of ~ 0.64 nm (by excluding the thickness of the top 1L-WSe2 of ~0.7nm

12)

after the second laser scan. This value is in good

agreement with the equilibrium interlayer distances experimentally found for other TMDC heterobilayers

19-21.

Evidently, after laser irradiation, the extent of interlayer coupling is

enhanced through a reduction in trapped impurities and average interlayer separation. The improved interlayer interaction causes a significant increase in the charge transfer processes, accounting for the observed changes in optical property.

The following preposition is further supported through an additional observation we can make from our results. Focusing our attention onto the green circle in the AFM image scan in Fig.


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4(c), we can observe that it encloses an air bubble, which is represented by a white spot. Referring to the height scale in Fig. 4(c), this represents a large interlayer separation at that point. Accordingly, that region would experience minimal coupling and no change in its optical properties (i.e. minimal PL quenching). Matching this point to the FM images previously shown in Fig. 2(f), the same region, indicated by the white circle, remains fluorescent unlike the rest of the heterostructure. This supports the notion that the fluorescence intensity is sensitive to the interlayer separation.



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Figure 4. AFM scan of the WS2/WSe2 heterobilayer structure (a) before and after laser irradiation with laser power of (b) 15 mW and (c) again with an increased power of 25 mW. The purple lines in (a), (b) and (c) indicate the positions at which the height profiles in (d), (e) and (f) are obtained to measure the interlayer separation. (g) Raman spectra of the individual monolayers, unmodified and modified heterostructures. The spectra have been normalised


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and offset with respect to one another for clarity. (h) Zoomed-in image of a section of the Raman spectra found in (g) as indicated by the black box. A new peak can be seen to emerge at approximately 310 cm-1 Raman mapping (selected spectral range of 305-315 cm-1) (i) before and after laser irradiation at (j) 15 mW and (k) subsequently at 25 mW. A possible mechanism that is likely responsible for this laser-induced reduction in interlayer separation and improved coupling is a localised laser-induced heating effect. Thermal effects caused by lasers on 2D materials have been reported and studied in numerous papers

28-29.

In

our case, it is highly probable that this heating effect causes the movement of impurities which accounts for the eventual removal of the air bubbles from the overlap region. Additionally, the thermal energy provided may also help in enabling the two monolayers to reach a thermodynamically more stable state with a smaller interlayer separation. This mechanism is very similar to what happens during the thermal annealing process in vacuum or noble gas conditions conventionally adopted in constructing vertically stacked heterostructures 6. It should be noted that our technique presents an added advantage of being able to achieve a more localized control.



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To gain further insights into the progressive increase in interlayer coupling, Raman spectroscopy with mapping is also obtained for the heterobilayer. Fig. 4(g) shows the Raman spectra for the heterostructure before (pink) and after laser irradiation (green and purple) and Fig. 4(h) shows an enlarged image of the relevant spectral range from 270-350 cm-1. By comparing the three spectra, we find that a new peak emerges at approximately 310 cm-1 after laser irradiation. The 310 cm-1 peak is known to be present in multi-layered WSe2 structures, but absent in 1L-WSe2

9, 30.

In fact, numerous reports have also observed the emergence of

this Raman peak in various coupled heterobilayer structures consisting of WSe2

14, 30-31,

which

suggests that it can be used as a spectroscopic indicator for the presence of interlayer coupling in these materials 31. This 310 cm-1 peak has also been confirmed to occur across the entire overlap region through the use of Raman mapping as indicated by the increase of the bright yellow areas in the overlap region after laser irradiation in Fig. 4(i) to (k). The prominence of the 310 cm-1 peak from Raman spectra and maps are clear indications of increased effective interlayer coupling between the two monolayers after the laser treatment.


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Besides confirming an increased interlayer interaction after laser irradiation, Raman spectroscopy also allows us to ensure that the heterostructure samples are not significantly damaged in the process

28, 32.

Such damages can similarly quench the PL intensity by

increasing the density of defect trap states in the forbidden band. From Fig. 4(g), we observe that the two distinct Raman peaks, found at ~ 250 cm-1 and ~ 351 cm-1, which belong to the 1L-WSe2 and 1L-WS2 respectively, remain present in the spectrum after both laser scans. The absence of any unexplained new peaks or substantial peak shifts indicates that no significant damage has been induced in the sample after laser modification. This observation is once again further verified for the entire heterostructure through the Raman maps found in Fig. S9(a) and (c), which capture the WSe2 Raman peak at ~250 cm-1 (selected range: 240-260 cm-1), and Fig. S9(b) and (d), which capture the WS2 peak at ~351 cm-1 (selected range: 340360 cm-1). These images do not display any significant changes to the location at which the Raman peaks can be found, indicating that the 1L-WS2 and 1L-WSe2 in the sample remains largely intact.



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In this work, we have carried out the investigation on 7 stacked heterobilayers and the observations are consistently reproduced in these samples. As illustrations, the results of the detailed characterizations for the two additional samples (not discussed in detail here) are shown in Figs S10 and S11 respectively. The sample in Fig. S10 was modified with the focused laser beam in a closed chamber filled with He gas. The results therefore also demonstrate that the same effects are also achieved when modifying in He gas.

To further understand the interlayer interactions that exist between the monolayers of the heterostructure and how they enable the activation of the interlayer processes, we propose a simplified semi-classical model to describe the charge transfer. We model the heterobilayer as two infinite parallel plates with a potential difference V between them. The electric field, E between them can thus be described by E=V/d, where d corresponds to the interlayer separation. This electric field is believed to be the main driving force for the charge transfer process; In other words, the magnitude of E after laser irradiation has to reach a large enough value to overcome the corresponding electric field holding the intralayer electron-hole pair. To


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verify this, we will proceed to evaluate this electric field accordingly before and after the laser irradiation.

Firstly, the surface potential difference between the two monolayers is experimentally measured using the Kelvin Probe Force Microscopy (KPFM). The KPFM scan can be found in Fig. 5(d) which is conducted on the WS2/WSe2 heterobilayer structure shown in Fig. 5(a). This heterostructure was similarly subjected to a focused laser scan which quenched the PL intensity at the overlap section illustrated in the FM images in Fig. 5(b) and (c). The KPFM scan is only conducted on a small section of the sample which is outlined by the brown rectangular box in Fig. 5(a) to improve the accuracy of the measurements. A potential line profile is taken from the scan image shown in Fig. 5(e), obtaining a potential difference of approximately 0.56 eV between 1L-WSe2 and 1L-WS2. This is assumed to remain constant before and after the laser treatment

As for the interlayer separation values, excluding the intrinsic thickness of each layer, they have been measured to decrease from about 2.4 to 0.6 nm after laser irradiation (refer to Fig.



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S12). Combining the difference in work function value (~0.56 eV) with the interlayer separation values, the internal electric field of the vertical heterostructure before and after laser modification is calculated to be ~2×108 Vm-1 and ~9×108 Vm-1 respectively.

To determine the electrostatic field for the intralayer excitons, a simple electrostatic model of a Mott exciton is used. With the relevant parameters for the TMDs, the binding energies of the intralayer excitons is first approximated and then translated into an electrostatic field of 7 × 108 Vm-1 and 5 × 108 Vm-1 for WS2 and WSe2 respectively (Details of the calculation are presented in supporting information Section A). By comparing the values obtained, the electric field between the monolayers is only larger than that between the intralayer electron-hole pairs after the laser scan. Consequently, in agreement with our proposed model, the electron-hole pair is readily separated and hence the fluorescence is quenched as shown in Fig. 5(c).


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Figure 5. (a) OM image of a WS2/WSe2 heterobilayer before laser irradiation. Its corresponding FM images (b) before and (c) after laser treatment. (d) KPFM scan of a selected region of the unmodified heterostructure as indicated by the brown box in (a). (e) The potential line profile taken along the white line in (d). A possible mechanism that can account for the creation of this electric field is the band alignment process that would result in the creation of a contact potential. The likely presence of defects or impurities on the WS2 and WSe2 monolayers may give rise to surface charges that can further contribute to the contact potential, and correspondingly this field built up within the stacked bilayer.



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Further investigation is also conducted by taking low temperature PL measurements before and after PL quenching is achieved using the laser treatment (shown by the FM images in Fig. 6(a) and (b)). The low temperature PL spectra are shown in Fig. 6(c) and (d) which can be seen to consist of multiple peaks representing the presence of various complex excitonic forms (trions and biexcitons).


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Figure 6 (a) FM images of the heterostructure (a) before and (b) after laser irradiation. The relative positions of the monolayer WS2 and WSe2 are indicated by the white and blue dashed lines respectively. Only the relevant fragmented monolayer WS2 piece is demarcated to avoid confusion. (c) Low temperature PL spectroscopy (temperature: 77 K) of the WS2/WSe2 heterobilayer before and (d) after laser modification. Gaussian curve fitting has been carried out to both spectrums to resolve them into their constituent peaks.

Comparing the low temperature PL spectrum before and after laser irradiation in Fig. 6, we can first observe two similar changes that are previously identified from the room temperature spectra: i) a significant PL quenching for all peaks, ii) a red shift of approximately 10-20 meV. More importantly, two new observations can also be made. Firstly, the peak intensity ratio of exciton to the trion plus biexciton peaks combined increased after laser irradiation. Specifically, the ratio increased from 0.37 to 1.06 for WS2 peak cluster, and 1.80 to 3.12 for WSe2 peak cluster. (i.e. the proportion of exciton in comparison to trion and biexciton formation increased). This change in ratio is attributed to the significant reduction in concentration of exciton and thus the chances of formation for biexciton and trion are suppressed too. Secondly, the relative proportion of the intensity of the defect bound excitonic states did not increase significantly after laser irradiation; The ratio of its peak intensity to that of the sum for all other excitonic



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peaks only increased slightly from 0.047 to 0.051 after laser irradiation. This minute increase further supports the claim that laser irradiation has not resulted in any significant sample damage 33.

Besides being able to externally tune the WS2/WSe2 heterostructure from the non-coupling to the strongly coupling regime, this technique is also apt in the micro-modulation of its properties. To demonstrate its ability to achieve selective modification, a stacked heterobilayer is fabricated and only half of the overlapped region is modified by the scanning focused laser beam. Specifically, laser irradiation is carried out with a power of 15 mW at the lower left section of the heterostructure. Figs. 7(a) and (b) show the OM images before and after modification in which no visible damage is observed. The successful achievement of selective modification can be observed from the partial PL quenching at the region for which laser irradiation had been carried out on in the FM images in Fig. 7(c) and (d). Therefore, unlike the other techniques (the sandwiching of hBN dielectric layer(s) with varying thickness, the application of an ion beam fluence, or the varying of parameters in the thermal annealing process), which are only able to achieve the global tuning of its properties; the focused laser


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beam approach, with its much higher spatial resolution, is able to provide a more localised form of control.

Figure 7. OM image of another WS2/WSe2 heterobilayer structure (a) before and (b) after the laser modification. Corresponding FM images are shown in (c) and (d). Here laser scan is only carried out for the lower left part of the overlap region.

Besides being able to achieve localised modification, the focused laser beam technique also enables a more sensitive degree of control over the interlayer separation of the heterobilayer. To illustrate this attribute, we fabricate a new heterobilayer and irradiate the sample at successively increasing laser powers across the entire heterostructure. Its PL spectrum and



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FM image are measured after each stage of laser irradiation and the results are summarised in Fig. 8(i) and Figs. 8(c)-(h) respectively. The PL intensity is seen to decrease with increasing laser power with a large drop occurring after a power of 25 mW is used. The significant drop represents the transition point at which the two monolayers are close enough to create a suitably large electric field just sufficient to overcome the binding energies of the intralayer excitons. As a visual check for any laser-induced damages, Fig. 8(a) and (b) show the OM images of the heterobilayer at the initial and final stages of the modification process. These results demonstrate the potential of the focused laser beam technique to act as a more sensitive and progressive fine-tuning tool for the WS2/WSe2 heterostructure.


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Figure 8. OM image of a WS2/WSe2 heterobilayer structure (a) at the initial and (b) after the final stage of laser modification. This sample is subjected to laser irradiation at successively increasing laser powers and subsequently probed by the FM after each scan. FM image: (c) before laser modification, (d) after laser modification at a power of 16 mW, (e) followed by 25 mW, (f) 27.5 mW, (g) 32.5 mW, (h) and lastly at 40 mW. (i) Plot of the PL peak intensities (WS2 and WSe2 PL peaks) against the incident laser irradiation intensity that had been used for modification, summarising the evolution of the optical properties of the sample as the modification is conducted.

To illustrate the versatility of our technique, we apply our method to another material, a vertically stacked WS2/WS2 homobilayer. The sample is prepared and modified in a similar fashion, and its OM image is shown in Fig. 9(a), noting that the top 1L-WS2 becomes fragmented during the transfer process. After which, this stacked homobilayer sample is



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subsequently scanned with successively increasing laser powers (a total of 8 scans with laser powers increasing from 10 to 35 mW is conducted) across the entire sample, and characterised at the initial and last stages of the experiment. Evidently, the FM images in Fig. 9(c) and (d) show a similar quenching of the PL intensity at the overlap region (highlighted by yellow box) which is consistent with the PL spectra in Fig. 9(e).

Similar to the previous preposition, the PL quenching after laser treatment is attributed to the laser-induced reduction in interlayer separation and corresponding increase in interlayer interaction. Specifically, before laser irradiation, with a large interlayer separation between the two 1L-WS2 (~5.9 nm, see AFM scan in Fig. S13), the two WS2 monolayers behaved independently. Thus, both monolayers exhibit a bright fluorescence as observed in Fig. 9(c). After which, the reduced interlayer separation of ~2.69 nm (this includes the intrinsic thickness of the top 1L-WS2) (Fig. S13) translates into an increased interlayer coupling, causing the sample to behave more similarly to a bilayer WS2 material. Due to the band structure evolution of the material - from a direct to an indirect bandgap semiconductor (notwithstanding the


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incommensurability in stacking configuration/ twist angles) - quenching results in the sample. The red shift observed in Fig. 9(e) can also be accounted for by this band structure evolution.

Interestingly, Figs. 9(c) and (d) also show an overlap (outlined by the green lines) between a multi-layered WS2 at the top and a bottom monolayer WS2. Accordingly, laser irradiation also results in an increased interaction between the top multi-layered and bottom monolayer samples in this region. This causes them to behave like a multi-layered sample with decreased PL efficiency as observed from the PL quenching at this area of the sample seen in Fig. 9(c) and (d).



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Figure 9. (a) OM image of the WS2/WS2 structure. Region bounded by the black dashed lines constitutes numerous fragmented WS2 monolayers induced during the transfer process. (b) Corresponding false colour image. (c) FM image of the WS2/WS2 homobilayer structure before and (d) after laser treatment. In c) and d), only the relevant fragmented section of the top 1LWS2 is outlined. The green-dashed lines mark an overlap region between a multi-layered WS2 at the top and the 1L-WS2 at the bottom. e) PL spectra before and after laser modification.

Conclusion

In summary, we have demonstrated the focused laser beam method to be a facile and versatile tool that can be used to engineer the interlayer coupling of a stacked van der Waals heterogeneous bilayer. This simple technique is able to progressively adjust the interlayer separation of the heterobilayer, achieve on-demand localized modification, en route to micropatterning of the stacked region and provide systematic insights into the onset of interlayer coupling in the heterobilayer. In addition, we have also shown that the technique works on a vertically stacked WS2/WS2 homobilayer material. The scanning focused laser beam technique thus represents a convenient post-stacked approach for further fine-tuning and engineering of the band structure and optical properties in van der Waals heterostructures.


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This focussed laser beam scanning method enable the micropatterning of the heterobilayer, makes it potentially applicable for microdisplay.

Associated Content Supporting Information Derivation of the electric field between electron-hole pair in intralayer excitons, schematic diagram of the focused laser beam set-up, optical microscope and atomic force microscope images of exfoliated WS2, optical microscope image of a WSe2 sample on PDMS substrate, PL and Raman spectra of WSe2, normalized PL spectra of WS2 and WSe2, schematic diagram of the charge transfer process in the WS2/WSe2 heterostructure, optical microscope image of the heterostructure, Raman mapping of the heterostructure, overview summary results for other heterostructure samples to show reproducibility of this work, AFM scan of the heterostructure, AFM scan images obtained for the WS2/WS2 homobilayer.

Acknowledgements



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The authors acknowledge the support of Singapore MOE-ARC Tier 2 grant (WBS: R-144-000357-112).


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Table of Contents


Progressive Micromodulation of Interlayer Coupling in Stacked WS2

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