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Atomic healing of defects in transition metal dichalcogenides Junpeng Lu, Alexandra Carvalho, Xinhui Kim Chan, Hongwei Liu, bo liu, Eng Soon Tok, Kian Ping Loh, Antonio H. Castro Neto, and Chorng Haur Sow Nano Lett., Just Accepted Manuscript • Publication Date (Web): 29 Apr 2015 Downloaded from http://pubs.acs.org on April 30, 2015

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Atomic healing of defects in transition metal dichalcogenides Junpeng Lu1#, Alexandra Carvalho2#, Xinhui Kim Chan1, Hongwei Liu3, Bo Liu2, Eng Soon Tok1, Kian Ping Loh2,4, A.H. Castro Neto1,2* and Chorng Haur Sow1,2* 1

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

2

Center For Advanced 2D Materials and Graphene Research Center, National University of

Singapore, 6 Science Drive 2, Singapore 117546 3

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology

and Research), 3 Research Link, Singapore 117602 4

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore

117543 *Corresponding authors: Prof. A. H. Castro Neto

Prof. Sow Chorng Haur

Email: [email protected]

Email: [email protected]

Tel: (65)66012575

Tel: (65)65162957

Fax: (65)67776126

Fax: (65)67776126 1

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ABSTRACT As-grown transition metal dichalcogenides are usually chalcogen deficient, and therefore contain a high density of chalcogen vacancies, deep electron traps which can act as charged scattering centers, reducing the electron mobility. However, we show that chalcogen vacancies can be effectively passivated by oxygen, healing the electronic structure of the material. We proposed that this can be achieved by means of surface laser modification, and demonstrate the efficiency of this processing technique, which can enhance the conductivity of monolayer WSe2 by ~ 400 times, and its photoconductivity by ~ 150 times.

KEYWORDS laser, vacancy, oxygen, localized patterning, transition metal dichalcogenides, electronics and optoelectronics

Semiconducting transition metal dichalcogenides (TMDs) are amongst the most promising twodimensional materials. In monolayer form, Mo and W dichalcogenides become direct gap semiconductors and therefore attractive for optical and photoelectrical applications. In addition, the reduced screening and stronger electron-hole interaction in 2D TMDs, compared to conventional semiconductors, makes these materials spectrally robust and amenable to electrical devices.1 However, due to their compound nature and the higher volatility of the chalcogenides, TMDs are usually chalcogen-deficient. The abundant chalcogen vacancies have deep acceptor states that can act as efficient electron traps. These have been described theoretically2-8 and characterized experimentally6-9, and are one of the most common scattering centers limiting the electron mobility.10 WSe2 is an example of a material which theoretically has extraordinary intrinsic 2 ACS Paragon Plus Environment

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properties, but of which the implementation in a wider range of applications is constrained by its material quality, and comparatively low conductivity and photoconductivity. The problem of removing or neutralising unwanted carrier traps or strong scattering centers is one of the most important issues in semiconductor technology, and processes to remove harmful defects from active defect regions (‘gettering’) or to neutralise their electrical activity without removing them from the device have been developed. An example is the passivation of metal impurities and other deep level traps in silicon by in-diffusion of hydrogen.11 In this paper, we show that the same principles of defect engineering can be applied in TMDs, even without the need to use plasma or chemical etching. Simply using the atmosphere as a source of oxygen, it is possible to fill the chalcogen vacancies, creating isoelectronic substitutional oxygen defects that are electrically neutral. This process is demonstrated by application of a focused laser beam treatment to WSe2 with the sample in ambient condition. The experiment elegantly demonstrated the “open canvas” behavior of the 2D materials where defects can be easily altered and modified.12 We find that this simple focused laser treatment improves the conductivity of the sample by ~ 400 times. We start by analyzing the electronic structure of the Se vacancy and the type of defects that are formed in the presence of oxygen using density functional theory (DFT). The calculations were performed using the Quantum ESPRESSO package.13 Details of the DFT method are found in the supplementary information. Figure 1a-d shows the different oxygen defect models considered in this work and Figure 1e-i shows the bandstructures of different defects in WSe2 (Kohn-Sham eigenvalues) calculated using density functional theory. The selenium vacancy, created by removing a Se atom, gives rise to little deformation of the surrounding lattice (Figure 1a). 3 ACS Paragon Plus Environment

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However, there are three W dangling bonds originating in a localized unoccupied gap state (Figure 1e and 1f). This is a deep acceptor state with energy E(-/0) ~ Ec - 0.6 eV, where Ec is the conduction band bottom. Such state acts as an effective electron trap thus limiting the electron mobility in n-type material. Since it is close to the conduction band, it also decreases the mobility in p-type material, where it can still trap electrons (minority carriers) and, once negatively charged, becomes attractive for holes. In both cases, it is an exciton scattering center. Selenium vacancies can be passivated by introducing oxygen in the lattice site originally occupied by selenium (Ose) (Figure 1b). The first step towards this reaction likely is the chemisorption of oxygen on the surface. Adsorbed O can assume different configurations. The two most stable configurations are adsorption on the top of the Se atom (Oad-Se4, in Figure 1c) or in the layer, on the W plane (Oad-W1, in Figure 1d)14 (Here we adopt the same notation as in Ref.

14

). The Oad-W1 configuration is 0.35 eV higher in energy than the Oad-Se4 configuration.

These do not produce deep midgap states except for some localized levels very close to the bands as in the case of Oad-Se4 (Figure 1g and h). The capture of the impurity atom by a selenium vacancy, VSe + Oad → OSe, is very exoenergetic, with an enthalpy balance of −4.7 eV. This is about the double of the analogous reaction enthalpy in WS2 (−2.5 eV), and larger than the binding energy per atom of the oxygen molecule (2.25 eV). The resulting substitutional oxygen has no defect-related gap levels (Figure 1i), and is a benign isoelectronic impurity. A similar picture is found in MoS2.Thus, oxygen adsorption does not add deep mid-gap levels and the trapping of oxygen by a vacancy eliminates the vacancy acceptor state, leaving an inert substitutional oxygen defect.

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Oxygen substitution of chalcogenides may occur in normal ambient conditions. However, the reaction process would be slow and the substitution would be difficult to control. Therefore, to achieve controlled oxygen substitution in monolayer TMDs, the following experiments were designed and carried out. Firstly, as-grown WSe2 monolayers were prepared by chemical vapour deposition (CVD) method.[13] Subsequently, the as-grown monolayers are subjected to an efficient treatment to passivate chalcogen vacancies with oxygen using a facile focused laser beam technique (Detailed in supplementary information). Finally, the corresponding electronic and optoelectronic devices are fabricated and characterized to verify the effects of the substitutional oxygen. The monolayer WSe2 nanoflakes are prepared by a chemical vapor deposition method.15 The schematic diagram of our experimental setup for the growth of material is shown in supplementary information Figure S1. Optical image in Figure 2a shows the monolayer WSe2 flakes grown on sapphire substrate. Most of the flakes exhibit triangular shape while few of them adopt the David star shape, as revealed by the SEM images in the inserts. The as-grown WSe2 flakes are extensively characterized to verify that these flakes are indeed monolayer in thickness, making use of the fact that monolayer WSe2 has direct band gap.16 Figure 2b presents AFM image of an as-grown flake. The thickness of the flake is measured to be ~ 0.68 nm, indicating the as-grown flake is monolayer. The PL spectrum collected from a monolayer flake is shown in Figure 2c. The strong peak centered at 1.62 eV (765 nm) results from the direct excitonic transition of monolayer WSe2,17 and the uniformity of the PL intensity over the flake, shown by the PL mapping in the insert of Figure 2c indicates the uniform monolayer thickness of the flake. From the Raman spectrum in Figure 2d, we observe a dominant peak at 251 cm-1 with a small shoulder at 263 cm-1. This further confirms that the as5 ACS Paragon Plus Environment

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grown flake is a monolayer.15 The Raman mapping shown in the insert also indicates the uniform thickness across the entire flake. After synthesis of the monolayer WSe2 flakes, we demonstrate the functionality of the as-grown monolayer flakes as electronic and optoelectronic devices. The device fabrication involves the standard lithography, evaporation and lift-off processes. We create the device on the sapphire substrate directly, since sapphire is a highly insulating material without any photocurrent output. A high work function metal, Pd, is chosen as the metal electrodes, due to the low contact resistance and clear p-type conduction of WSe2/Pd contact.18 Electrical characteristics (Figure 3a) of the device are measured in dark and illuminated conditions. The experiments are carried out under global irradiation with different light intensities using a laser (λ = 532 nm, power from 0 to 2.8 mW, spot size ~ 2 mm2). Note that these light intensities are too weak to cause any laser modification of the flakes. The measured I-V plots as shown in Figure 3a indicate a clear increase in current when the laser beam is irradiated onto the device. With periodical light illumination of the device at a fixed bias of 5 V, the output current of the device as a function of time presents a rapid on and off behavior (Figure 3b). Photoresponsivity (Rres), one of the most important factors of a photodetector, can be calculated to be 25 mA/W, within the range of values reported in the literature. The external quantum efficiency, EQE =

hcRres , is calculated to eλ

be 5.8%. In order to gain a greater insight into the photoresponse mechanism of the device, we investigate the optoelectronic property using a scanning photocurrent microscopy (SPCM) setup with the spatial resolution of ~ 1 µm. The spatial photocurrent map is obtained by scanning the focused laser beam over the entire device while recording its photocurrent at a fixed bias of 5 V (Figure 6 ACS Paragon Plus Environment

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3c). Note that during this experiment the left electrode is positively biased with respect to the right electrode. Evidently, the pronounced photocurrent signal is detected at the junction between the left electrode and the monolayer WSe2. In Figure 3d, we present the photocurrent line scan with the laser beam raster across the device along the horizontal line added in Figure 3c. A narrow photocurrent peak accompanied by a broad shoulder appears in the line scan. This indicates that most of the photocurrent originates from a small region, most likely the Schottky junction. Despite the high work function of Pd, there is still a small Schottky barrier at the Pd/WSe2 interface. The observed photocurrent of the device can be explained by the energy band diagram shown in Figure S2. In the dark, the device is characterized by small Schottky barriers at the equilibrium state.18 When the laser light is irradiated onto the device, the WSe2 flake absorbs light and generates electron-hole pairs, which are separated by the external bias. Most of the photocurrent originates from the depletion region, where it is separated by the combined action of band bending and applied bias. In the reverse-biased region, the electron-hole pairs are separated and flow towards the opposite direction. In this case, the charge carriers travel across the sample with high number of charge trappers, hindering the flow of the charges. Hence the forward-biased contact region dominates the photoresponse of the device. To realize the Ose in WSe2 monolayers, we developed an efficient method to passivate chalcogen vacancies with oxygen using a focused laser beam technique. The detailed roles of the setup of this technique will be described later in Figure 7. Briefly, as shown by the schematic in Figure 7a, we used a solid state laser with a wavelength of λ = 532 nm, output laser power of 300 mW and a focused laser spot size of 1 µm. This technique represents a controllable way to enhance the reaction of WSe2 monolayers with oxygen, producing monolayer material with higher conductivity and photoconductivity than the as-grown ones. The higher quality of the WSe2 7 ACS Paragon Plus Environment

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monolayers after Ose realization is verified by the absorption spectra (Figure S3). Evidently, the laser healed WSe2 monolayer presents four obvious absorption peaks which can be assigned to the excitonic peaks A (756 nm), B (590 nm) and A’ (500 nm) and B’ (426 nm), the splitting of the ground and excited states of A and B transitions. This is almost in accordance with previously reported absorbance of exfoliated single crystalline WSe2 monolayer.19 In stark contrast, the characteristic absorption peaks are not clearly shown in the pristine sample, indicating the lower quality of the as-grown WSe2 monolayer. This result is consistent with our theoretical calculations. We also demonstrate the improvement of the photoconductivity of the samples, by comparing the performance of photodetector devices prepared using as-grown monolayer WSe2 flakes before and after laser treatment. For comparison, we subject the device to a global modification by focused laser beam with high intensity (~ 2×107 W cm-2). After laser modification, the morphology of the monolayer device does not show any significant change. However, the changes in the conductivity and photoresponse are striking. The typical I-V curves of laser modified and pristine devices are measured and shown in Figure 4a. Remarkably, the output current increases to 70 nA at 5 V and becomes ~ 400-fold higher than the output current in the pristine device at 5V as well. Further, with periodical illumination of the whole device, the modified device also presents obvious on and off output behavior (Figure 4c and d) and the photocurrent magnitude is much higher than pristine device (Figure 4c). The corresponding photoresponsivity is calculated to be 3717 mA/W. This value is around 150 times higher than the pristine device and even higher than all other photodetectors based on WSe2.20, 21 The EQE is calculated to be as high as ~ 860% without accounting for the low light absorption of the monolayer flake. The phenomenon is also observed in our previous study on the MoS2 8 ACS Paragon Plus Environment

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multilayers but the improvement in MoS2 film is only 4-5 times higher.22 Besides the distinct increase in the magnitude, the photoresponse process switches from a simple fast rising transient to a two-step process consisting of a fast transient rise followed by a slower transient rise (Figure 4c and d). The emerging slow process on top of the rapid photoresponse is attributed to the laserinduced thermal heating.23 The carrier thermalization trapping and the interaction of the surface states with laser irradiation could also contribute to the photocurrent.24,

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The photocurrent

distribution is also observed via SPCM. As observed from the photocurrent map (Figure 4e), the entire laser-modified WSe2 flake dominates the photocurrent generation rather than the Schottky junction as in the case before laser modification (Figure 3c). The photocurrent line scan curve in Figure 4f presents a broad peak corresponding to the width of the whole device. This indicates that more active areas of the WSe2 flake contribute to the photocurrent. Consequently, we attribute our observations to the passivation of vacancy-related recombination centers by laser modification in ambient condition. Simultaneously, the longer free carrier lifetime and elimination of the traps at the interface facilitates the carrier tunneling18 (Figure S4). This consequently enables more efficient photocurrent extraction (ie. higher EQE) and higher photoresponse. Notably, the electrical and photoelectrical properties of the TMDs could also be affected by the charge traps at TMDs/dielectric layer interface.26, 27 Upon vacuum heating and hermetic passivation of the dielectric layer, performance of the TMDs devices would be improved. However, our devices are fabricated on as-grown WSe2 flake on sapphire substrate. It has been subjected to high temperature annealing during the growth process whist laser healing would not affect sapphire much due to its transparent feature. Therefore, the improvement of the device performance is attributed to the laser healing of WSe2 monolayers. In addition, The photo-gating effect can also contribute to increase the photo-gain during illumination.28, 29 The 9 ACS Paragon Plus Environment

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laser beam used has energy above the band gap of WSe2, and therefore can excite carriers from the vacancy states, which according to our calculations have their (-/0) transition level at Ec-0.62 eV. However, the released carriers could be re-trapped at other vacancy states in the sample thus reduce the significance of this contributing factor. Additionally, the laser modified device is observed to show multispectral response to different wavelength light. As shown in Figure S5, the output current of the pristine device obviously increases under 405 nm (Figure S5a(i)) and 532 nm (Figure S5b(i)) light illumination while the response to 808 nm light is insignificant (Figure S5c(i)). After laser modification, the device shows more significant photoresponse to 405 nm and 532 nm light (Figure S5a,b(ii)). The enhancement of the photoresponse to 808 nm light is less significant (Figure S5c(ii)). This is attributed to the wide band gap of monolayer WSe2. To verify the role of oxygen, we carried out the laser annealing with the sample placed inside a vacuum chamber with a pressure of ~ 10-2 mbar (Figure 7a), where oxidation is minimal. Typical I-V curves after annealing are shown in Figure 5a. The output current is higher than the pristine device but much lower than that modified in ambient condition. The corresponding electrical and photoresponse characteristics measured under different irradiation intensities are shown in Figure 5b and c, respectively. The photoresponse of the device annealed in vaccum is only enhanced~ 2 times, well behind the improvement registered for the devices annealed in air. We consequently deduce that oxygen has to be involved in the defect processes responsible for the improvement of the electrical and photoelectrical properties of monolayer WSe2 annealed in air. Further evidence of the surface chemical modification is provided by XPS. XPS scans find Se vacancies in the as-grown sample and evidence of the incorporation of oxygen in the processed 10 ACS Paragon Plus Environment

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sample. Figure 6(a-f) displays the XPS scans for W 4f and Se 3d binding energies of the samples modified by focused laser beam at different conditions. The spectra are fitted with Gaussian/Lorentzian mixed curves. In the W 4f scans, 2 doublets may be observed. The first doublet at 33.85/31.7 eV corresponds to the 4f5/2 and 4f7/2 W4+ in W-Se environment. The second doublet consists of weaker peaks at 37.9/35.7 eV, which corresponds to 4f5/2 and 4f7/2 W6+/W5+ in the W-O environment.30, 31 The oxidization ratios (identified as

I (W 6+ W 5+ ) , I(W) is the I (W 6+ W 5+ ) + I (W 4+ )

integrated peak area) of the pristine sample, and of the samples modified in vacuum and in air

were calculated to be around 21%, 30% and 44%, respectively. The presence of oxygen in the pristine sample may be due to the occasional deposition of WO3 precursors on the substrate.15 The increasing oxidization ratio after laser annealing in air indicates that the annealing enhances greatly the introduction of oxygen into the WSe2 monolayer flakes. The stoichiometric ratio between W4+ to Se in the WSe2 sample is calculated via the integrated peak areas. It is the lowest in the pre-laser modified situation (1:1.81) and increases after laser modification in vacuum and ambient conditions (1:1.94 and 1:2.06 respectively). This is a strong indication of the elimination of the Se vacancies after laser modification. Finally, we carried out AFM imaging to probe the morphology of the monolayer, bi-layer and tri-layer WSe2 flakes after partial annealing (Figure 6(g-l)). After laser annealing, the height of these flakes increases around 0.5 nm for monolayer and 1.0 nm for bi- and tri- layers (Figure 6g, i and k). The increase in thickness is attributed to the presence of oxides. The higher thickness of the multilayers is due to their greater absorption of laser light. More obvious verification of the oxygen substitution is revealed by the AFM phase image which offers high spatial resolution and sensitive detection of compositional variations.32 As shown in Figure 6h, j and l, focused laser 11 ACS Paragon Plus Environment

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beam gives rise to obvious chemical phase modification. Thus, the combination of XPS and AFM results indicates that laser annealing enhances the introduction of oxygen in the lattice. Since the band structure of WSe2 exhibits obvious thickness-dependence,33 we therefore develop the focused laser beam technique as a method to engineer the band gap of WSe2, as we described previously in study of MoS222. During the CVD growth, if the growth conditions are optimized, most of the as-grown flakes observed are monolayers. However, in non-optimized conditions, we occasionally find the growth of multilayer domains on top of a larger and supporting monolayer, due to the AB stacking or inverted AA’ stacking.34 These multilayer domains are shown by the SEM and AFM images in Figure 7b and c. We subject the flakes with multilayer domain to a post-growth modification by focused laser beam destruction with the aim to thin down the thicker layers, as previously described for MoS222. With the flexible feature of the laser and the motorized stage, fast and large area thinning can be achieved. Figure 7d shows the optical images of the same area before (upper panel) and after (bottom panel) laser modification. The focused laser beam thins down the multilayer domains of WSe2, without leaving any obvious signs of destruction of the supporting monolayer underneath. This is attributed to the low absorption (~ 1%-8%) of monolayer WSe2 at visible light regime.15, 20 The visible optical contrast is a quick indication that the thickness of the flakes has decreased after laser modification. In general, brighter regions represent a thicker area.35 The morphology change of the flake is observed by SEM images and shown in Figure 7e. The pristine multilayer domains (left panel) show a clear triangular shape with sharp edges, while the laser treated triangles (right panel) appear more defused. The defused images may be due to the destruction of the thicker layers caused by high energy laser induced sublimation. When the energetic laser was focused onto the surface of the flake, the WSe2 absorbed the light and converted it to heat. During this 12 ACS Paragon Plus Environment

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process, both sublimation and photochemical reactions take place. As revealed by the SEM and AFM images in Figure 7f, the right part of the multilayer triangle is clearly thinned down by the focused laser beam. The reason why the supporting monolayer did not show obvious sign of destruction could be attributed to the efficient heat dissipation through the substrate.36 An additional effect of the laser thinning is the improvement of crystalline quality and passivation of selenium vacancies. After laser thinning, the region exhibits much stronger PL emission than the pristine multilayer region, as shown in Figure 7g by the orange and red curves, respectively. The PL mapping image shown in the inset further indicates that the region thinned down by laser presents stronger emission compared with the pristine region. Notably, the PL intensity is still lower than the PL emitted by as grown monolayer on the edge. The Raman spectra in Figure 7h indicates that there is no shift in the peak position before and after laser thinning despite the fact that higher intensity is shown by the modified region as observed from the spectra and Raman mapping. Remarkably, after laser modification the peak is narrower, indicating that the crystalline quality of the laser thinned region is improved. The full width at half maximum (FWHM) obtained from Gaussian-Lorentzian curve fitting is reduced from 7.81 to 6.80 cm-1. Since the width of a Raman peak can be used as an indicator of crystalline quality, with a narrower peak corresponding to a higher crystalline quality,37 this indicates a higher crystalline quality of the laser thinned region. This may be due to the higher Raman response of the remaining thinner layer after laser modification or the thermal annealing effect caused by the higher energy laser irradiation. In conclusion, we have proposed the concept of oxygen passivation of chalcogen vacancies in TMDs, a defect engineering strategy that can greatly enhance the conductivity and photoconductivity of these materials. Focused laser annealing in air was proposed as a practical 13 ACS Paragon Plus Environment

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approach to implement vacancy passivation. The focused laser beam is used to promote the incorporation of oxygen and its trapping by vacancies. This process was demonstrated to improve the conductivity and photoconductivity of monolayer WSe2 respectively by 400 times and 150 times. This technique represents a controllable way to improve these properties manyfold, and can be easily adopted as a pre-processing step in 2D-TMD electronic and optoelectronic device fabrication.

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Figure 1. Oxygen defect models: (a) selenium vacancy (b) substitutional oxygen, (c) adsorbed oxygen in position Se4, and (d) adsorbed oxygen in position W1. Bandstructures of defects in WSe2 (Kohn-Sham eigenvalues) calculated using density functional theory: (e) pristine WSe2, (f) selenium vacancy (Vse)), (g,h) oxygen adsorbed in the Se4 and W1 configurations, and (i) oxygen replacing Se (Ose).

Figure 2. Morphology and structural characterization of as-synthesized products. (a) Optical images of the monolayer WSe2 nanoflakes. Inserts show the SEM images of a product with triangle shape and David star shape. (b) AFM image and height profile of a monolayer WSe2 flake. (c) PL and (d) Raman spectrum and mapping image of a monolayer WSe2 flake. All the scale bars shown are 5 µm.

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Figure 3. Photoelectrical property characterization of as-grown monolayer WSe2 flakes. (a) Typical I-V characteristics of a monolayer WSe2 flake device irradiated by different laser intensities. (b) Photoresponse characteristics of the device. (c) A photocurrent spatial map obtained as the focused laser beam is raster-scanned over the surface of the flake device. Scale bar = 5 µm. (d) Current line scan recorded as the focused laser beam is rastered across the device along the line indicated in (c).

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Figure 4. Photoelectrical property characterization of laser modified monolayer WSe2 flakes. (a) Typical dark I-V characteristics of a monolayer WSe2 flake device before and after laser modification. (b) Typical I-V characteristics of laser modified monolayer device irradiated by different laser powers. (c) Photoresponse characteristics of the device before and after laser modification. (d) Photoresponse characteristics of laser modified device at different laser power irradiation. (e) Photocurrent spatial map obtained as the focused laser beam is raster-scanned over the surface of the modified device. Scale bar = 5 µm. (f) Current line scan recorded as the focused laser beam is raster-scanned across the modified device along the line indicated in (e).

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Figure 5. Photoelectrical property characterization of WSe2 flakes modified in vacuum. (a) Typical dark I-V characteristics of a monolayer WSe2 flake device before and after modification in vacuum and ambient. (b) Typical I-V characteristics of a modified WSe2 flake device irradiated by different laser powers. (c) Photoresponse characteristics of the device.

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Figure 6. XPS scans of as-grown and modified WSe2 flakes. XPS scans for W and Se of (a) and (b) pristine sample, (c) and (d) sample modified in vacuum and (e) and (f) sample modified in ambient condition, respectively. AFM characterization of partially laser modified WSe2 flakes. For each of these flakes, the portion on the right corresponds to laser modified region. AFM height scans of (g) monolayer, (i) bi-layer and (k) tri-layer WSe2 flakes. AFM phase scans of (h) monolayer, (j) bi-layer and (l) tri-layer WSe2 flakes.

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Figure 7. Laser thinning of multilayer WSe2 flakes. (a) Schematic diagram of focused laser beam setup. (b) SEM and (c) AFM images of as-grown WSe2 flakes with multilayer domain. The comparison of WSe2 flakes before and after laser thinning shown by (d) optical and (e) SEM images. (f) SEM and AFM images of a partially thinned WSe2 flake. The right part of the multilayer domain is obviously thinned down. (g) PL and (h) Raman spectra (the peaks are fitted using Gaussian-Lorentzian curves) and mapping images of the same partially thinned WSe2 flake. Scale bars are 5 µm.

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Supporting Information. Detailed experimental methods, schematic diagram of the growth technique, corresponding band diagrams of the photoelectrical devices, micro-absorbance spectra and photoresponse characterizations to different wavelength light are supplied as Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * A. H. Castro Neto Email: [email protected] * Sow Chorng Haur Email: [email protected]. Author Contributions J.L. C.H.S. and A.H.C.N. conceived the project. J.L. and K.X.C. grew the materials and carried out the experiments. A.C. and A.H.C.N. carried out the theoretical simulation. H.L. helped to analyse the data. B.L. helped on the AFM measurement. E.S.T. and K.P.L. provided scientific suggestions. J.L. and A.C. wrote the manuscript under the supervision of C.H.S. and A.H.C.N. All the authors have contributed to the manuscript and agree with the content. #These authors contributed equally. Notes The authors declare no competing financial interests. ACKNOWLEDGMENT The authors acknowledge the National Research Foundation, Prime Minister Office, Singapore, under its Medium Sized Centre Programme.

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REFERENCES 1.

Nat Nano 2012, 7, 683-683.

2.

Caulfield, J. C.; Fisher, A. J. J. Phys.: Condens. Matter 1997, 9, 3671.

3.

Fuhr, J. D.; Saúl, A.; Sofo, J. O. Phys. Rev. Lett. 2004, 92, 026802.

4.

Yong, K. S.; Otalvaro, D. M.; Duchemin, I.; Saeys, M.; Joachim, C. Phys. Rev. B 2008,

77, 205429. 5.

Zhou, W.; Zou, X.; Najmaei, S.; Liu, Z.; Shi, Y.; Kong, J.; Lou, J.; Ajayan, P. M.;

Yakobson, B. I.; Idrobo, J.-C. Nano Lett. 2013, 13, 2615-2622. 6.

Komsa, H.-P.; Kotakoski, J.; Kurasch, S.; Lehtinen, O.; Kaiser, U.; Krasheninnikov, A.

V. Phys. Rev. Lett. 2012, 109, 035503. 7.

Tiwari, R. K.; Yang, J.; Saeys, M.; Joachim, C. Surf. Sci. 2008, 602, 2628-2633.

8.

Avsar, A.; Tan, J. Y.; Taychatanapat, T.; Balakrishnan, J.; Koon, G. K. W.; Yeo, Y.;

Lahiri, J.; Carvalho, A.; Rodin, A. S.; O’Farrell, E. C. T.; Eda, G.; Castro Neto, A. H.; Özyilmaz, B. Nat. Commun. 2014, 5, 4875. 9.

Komsa, H.-P.; Kurasch, S.; Lehtinen, O.; Kaiser, U.; Krasheninnikov, A. V. Phys. Rev. B

2013, 88, 035301. 10.

Zhu, W.; Low, T.; Lee, Y.-H.; Wang, H.; Farmer, D. B.; Kong, J.; Xia, F.; Avouris, P.

Nat. Commun. 2014, 5, 3087

22 ACS Paragon Plus Environment

Page 23 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

11.

Seibt, M.; Kveder, V., Gettering Processes and the Role of Extended Defects. In

Advanced Silicon Materials for Photovoltaic Applications, John Wiley & Sons, Ltd: 2012; pp 127-188. 12.

Zou, X.; Yakobson, B. I. Acc. Chem. Res. 2015, 48, 73-80.

13.

Paolo, G.; Stefano, B.; Nicola, B.; Matteo, C.; Roberto, C.; Carlo, C.; Davide, C.; Guido,

L. C.; Matteo, C.; Ismaila, D.; Andrea Dal, C.; Stefano de, G.; Stefano, F.; Guido, F.; Ralph, G.; Uwe, G.; Christos, G.; Anton, K.; Michele, L.; Layla, M.-S.; Nicola, M.; Francesco, M.; Riccardo, M.; Stefano, P.; Alfredo, P.; Lorenzo, P.; Carlo, S.; Sandro, S.; Gabriele, S.; Ari, P. S.; Alexander, S.; Paolo, U.; Renata, M. W. J. Phys.: Condens. Matter 2009, 21, 395502. 14.

Ataca, C.; Ciraci, S. J. Phys. Chem. C 2011, 115, 13303-13311.

15.

Huang, J.-K.; Pu, J.; Hsu, C.-L.; Chiu, M.-H.; Juang, Z.-Y.; Chang, Y.-H.; Chang, W.-H.;

Iwasa, Y.; Takenobu, T.; Li, L.-J. ACS Nano 2013, 8, 923-930. 16.

Mak, K. F.; Lee, C.; Hone, J.; Shan, J.; Heinz, T. F. Phys. Rev. Lett. 2010, 105, 136805.

17.

Tonndorf, P.; Schmidt, R.; Böttger, P.; Zhang, X.; Börner, J.; Liebig, A.; Albrecht, M.;

Kloc, C.; Gordan, O.; Zahn, D. R. T.; Michaelis de Vasconcellos, S.; Bratschitsch, R. Opt. Express 2013, 21, 4908-4916. 18.

Fang, H.; Chuang, S.; Chang, T. C.; Takei, K.; Takahashi, T.; Javey, A. Nano Lett. 2012,

12, 3788-3792. 19.

Zhao, W.; Ghorannevis, Z.; Chu, L.; Toh, M.; Kloc, C.; Tan, P.-H.; Eda, G. ACS Nano

2012, 7, 791-797. 23 ACS Paragon Plus Environment

Nano Letters

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

20.

Page 24 of 26

Ross, J. S.; Klement, P.; Jones, A. M.; Ghimire, N. J.; Yan, J.; Mandrus, D. G.;

Taniguchi, T.; Watanabe, K.; Kitamura, K.; Yao, W.; Cobden, D. H.; Xu, X. Nat Nano 2014, 9, 268-272. 21.

Baugher, B. W. H.; Churchill, H. O. H.; Yang, Y.; Jarillo-Herrero, P. Nat Nano 2014, 9,

262-267. 22.

Lu, J.; Lu, J. H.; Liu, H.; Liu, B.; Chan, K. X.; Lin, J.; Chen, W.; Loh, K. P.; Sow, C. H.

ACS Nano 2014, 8, 6334-6343. 23.

Rajesh, T.; Binni, V.; Subodh, G. M.; Eng Soon, T.; Chorng Haur, S. Nanotechnology

2011, 22, 115202. 24.

Sarkar, S.; Basak, D. Appl. Phys. Lett. 2013, 103, 041112

25.

Chang, J.-S.; Hwang, Y. K.; Ariga, K. J. Nanosci. Nanotechnol. 2010, 10, 1-2.

26.

Late, D. J.; Liu, B.; Matte, H. S. S. R.; Dravid, V. P.; Rao, C. N. R. ACS Nano 2012, 6,

5635-5641. 27.

Kim, T.-Y.; Cho, K.; Park, W.; Park, J.; Song, Y.; Hong, S.; Hong, W.-K.; Lee, T. ACS

Nano 2014, 8, 2774-2781. 28.

Hou, J. Y.; Fonash, S. J. Appl. Phys. Lett. 1992, 61, 186-188.

29.

Miller, B.; Parzinger, E.; Vernickel, A.; Holleitner, A. W.; Wurstbauer, U. Appl. Phys.

Lett. 2015, 106, 122103.

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Page 25 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Nano Letters

30.

Su, L.; Zhang, L.; Fang, J.; Xu, M.; Lu, Z. Sol. Energy Mater. Sol. Cells 1999, 58, 133-

140. 31.

Morar, J.; Himpsel, F.; Hughes, G.; Jordan, J.; McFeely, F.; Hollinger, G. Journal of

Vacuum Science & Technology A 1985, 3, 1477-1480. 32.

Garcia, R.; Magerle, R.; Perez, R. Nat Mater 2007, 6, 405-411.

33.

Sahin, H.; Tongay, S.; Horzum, S.; Fan, W.; Zhou, J.; Li, J.; Wu, J.; Peeters, F. M. Phys.

Rev. B 2013, 87, 165409. 34.

Shaw, J.; Zhou, H.; Chen, Y.; Weiss, N.; Liu, Y.; Huang, Y.; Duan, X. Nano Res. 2014,

7, 1-7. 35.

Li, H.; Wu, J. M. T.; Huang, X.; Lu, G.; Yang, J.; Lu, X.; Zhang, Q. H.; Zhang, H. Acs

Nano 2013, 7, 10344-10353. 36.

Castellanos-Gomez, A.; Barkelid, M.; Goossens, A. M.; Calado, V. E.; van der Zant, H.

S. J.; Steele, G. A. Nano Lett. 2012, 12, 3187-3192. 37.

Laskar, M. R.; Ma, L.; Kannappan, S.; Park, P. S.; Krishnamoorthy, S.; Nath, D. N.; Lu,

W.; Wu, Y.; Rajan, S. Appl. Phys. Lett. 2013, 102, 252108.

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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