Prevention of Transition Metal Dichalcogenide Photodegradation by Encapsulation with h‑BN Layers Seongjoon Ahn,†,§ Gwangwoo Kim,†,§ Pramoda K. Nayak,†,§ Seong In Yoon,†,§ Hyunseob Lim,‡,§,∥ Hyun-Joon Shin,⊥ and Hyeon Suk Shin*,†,‡,§,∥ †
Department of Energy Engineering, ‡Department of Chemistry, §Low Dimensional Carbon Materials Center, and ∥Center for Multidimensional Carbon Materials, Institute of Basic Science, Ulsan National Institute of Science and Technology (UNIST), UNIST-gil 50, Ulsan 689-798, Republic of Korea ⊥ Beamline Division, Pohang Accelerator Laboratory, POSTECH, Pohang 790-784, Republic of Korea S Supporting Information *
ABSTRACT: Transition metal dichalcogenides (TMDs) have recently received increasing attention because of their potential applications in semiconducting and optoelectronic devices exhibiting large optical absorptions in the visible range. However, some studies have reported that the grain boundaries of TMDs can be easily degraded by the presence of oxygen in water and by UV irradiation, ozone, and heating under ambient conditions. We herein demonstrate the photodegradation of WSe2 and MoSe2 by laser exposure (532 nm) and the subsequent prevention of this photodegradation by encapsulation with hexagonal boron nitride (h-BN) layers. The photodegradation was monitored by variation in peak intensities in the Raman and photoluminescence spectra. The rapid photodegradation of WSe2 under air occurred at a laser power of ≥0.5 mW and was not observed to any extent at ≤0.1 mW. However, in the presence of a water droplet, the photodegradation of WSe2 was accelerated and took place even at 0.1 mW. We examined the encapsulation of WSe2 with h-BN and found that this prevented photodegradation. However, a single layer of h-BN was not sufficient to fully prevent this photodegradation, and so a triple layer of h-BN was employed. We also demonstrated that the photodegradation of MoSe2 was prevented by encapsulation with h-BN layers. On the basis of X-ray photoelectron spectroscopy and scanning photoemission microscopy data, we determined that this degradation was caused by the photoinduced oxidation of TMDs. These results can be used to develop a general strategy for improving the stability of 2D materials in practical applications. KEYWORDS: transition metal dichalcogenides, photodegradation, hexagonal boron nitride, encapsulation, oxidation in optoelectronic devices, such as light-emitting diodes,13,14 photodetectors,15,16 and lasers.17 However, recent studies have reported that TMDs can be easily degraded by oxygen,18,19 UV irradiation,20,21 ozone,22 and heating under ambient conditions.23 Furthermore, it has been reported that TMDs are unstable under air over 6 months,24 and it is believed that the observed degradation begins at the TMD grain boundaries.19,23 Indeed, this degradation is more serious in other 2D semiconducting materials, such as black phosphorus,25,26 and so attempts to protect these semiconducting layered materials through the deposition of metal oxides27 and polymers24 on
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n recent years, increasing attention has been paid to semiconducting two-dimensional (2D) layered materials such as transition metal dichalcogenides (TMDs),1,2 phosphorenes,3 and perovskites.4 Indeed, TMDs have been most actively researched because they can be easily synthesized by chemical vapor deposition5−8 and hydrothermal methods.9,10 TMDs are typically composed of a single transition metal (e.g., Mo or W) and two chalcogens (e.g., S, Se, or Te). In these structures, no dangling bonds are observed, as the layered surface is terminated by a lone pair of electrons from the chalcogen species.2 Interestingly, TMDs exhibit optical properties that are dependent on the number of layers present, with the monolayer providing strong photoluminescence due to the direct band gap, and the double layer exhibiting very weak or no photoluminescence due to the indirect band gap.11,12 TMDs can therefore be considered promising materials for application © 2016 American Chemical Society
Received: July 28, 2016 Accepted: August 26, 2016 Published: August 26, 2016 8973
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Scheme 1. Schematic Illustration Outlining (a) the Preparation of 3L h-BN/Pt Foil and (b) the Preparation of 3L h-BN/WSe2 and Exposure to the Laser in the Presence of a Water Droplet
Figure 1. Variation in the (a) Raman and (b) PL peak areas of WSe2 with laser power as a function of exposure time. Variation in the (c) Raman and (d) PL peak areas of WSe2 in the presence of a water droplet at 0.1 mW laser power as a function of exposure time. Insets show the Raman and PL spectra at different exposure times. Optical microscope images of WSe2 in the presence of a water droplet (e) before and (f) after laser exposure at 0.1 mW for 780 s. All time-dependent graphs are normalized to the initial peak intensity.
their surfaces have been attempted. However, as the protecting materials are significantly thicker than the existing 2D materials and as they may induce chemical interactions or doping, it has been difficult to highlight the merits of TMDs. Currently, hexagonal boron nitride (h-BN) is the most promising
protecting material because it is chemically inert, thermally stable, doping-free, and transparent.28,29 Recently, h-BN has been employed as the encapsulation layer in TMD-based fieldeffect transistor (FET) devices,30,31 effectively preventing the deterioration in TMD electrical properties by preventing 8974
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Figure 2. (a) Integrated Raman mapping of WSe2 at 0.5 mW for 1 s. The white dotted area was exposed to a 4 mW laser for 30 s to promote photodegradation. The dark color in the white dotted area indicates a low Raman intensity, confirming photodegradation of WSe2. (b−f) SPEM images of part (a): (b) W 4f peak (the sum of W4+ and W6+), (c) W4+ peak, (d) W6+ peak, (e) Se 3d peak, and (f) O 1s peak.
contrast to previous studies, our measurements were carried out below 2 mW, and as such, no thermal effects were observed. It has also been reported that the thermal effect of MoS2 induced the down-shift of the A1g peak position and an increase in the full width at half-maximum (fwhm) of the A1g peak due to bond softening and amplified phonon coupling, respectively.34 However, as shown in Figure S2, no shift in the peak position nor increase in the fwhm was observed for the A1g peak of WSe2. In contrast, upon covering the WSe2 surface with h-BN, a down-shift of ∼0.6 cm−1 was observed for the A1g peak, along with a decrease in the fwhm of ∼2 cm−1 compared to pristine WSe2 (Figure S3). This result is not related to the thermal effect but to the physical protection of WSe2 from contact with air or other contaminants.36 Thus, no thermal effect was observed for either the TMDs or h-BN-covered TMDs under the experimental conditions employed herein. To observe the photodegradation of WSe2, we employed a 532 nm laser as the excitation source for both Raman and PL spectroscopy. Figure 1a and b show the variation in peak areas of the Raman (A1g peak at 252 cm−1) and PL (peak at 1.62 eV) spectra, respectively, as a function of laser exposure time.7,8 The intensities of both the Raman and PL signals were normalized to their initial intensities upon commencing laser exposure. As previously mentioned, we employed Raman and PL spectroscopy to monitor the photodegradation of TMDs below a laser power of 2 mW (i.e., at 0.1, 0.5, 1, and 2 mW). Interestingly, similar trends were observed for both spectra. At 0.1 mW, no variation in peak intensity was observed, although above 0.1 mW changes began to take place, with greater effects being observed at higher laser powers, indicating the photodegradation of WSe2. To accelerate the photodegradation process, a water droplet was placed on the new WSe2 surface, and the Raman and PL spectra were measured. Surprisingly, the photodegradation of WSe2 occurred even at 0.1 mW, indicating that water significantly accelerates this process. As expected,
contact of the TMD surface with air. However, the diverse application of h-BN layers for the protection of 2D materials has not yet been systematically studied. Thus, we herein demonstrate the degradation of WSe2 and MoSe2 as observed by changes in the intensity of Raman and photoluminescence (PL) signals under laser exposure. In addition, based on X-ray photoelectron spectroscopy (XPS) and scanning photoemission microscopy (SPEM) data, we confirm the mechanism of this degradation. We then move on to investigate the encapsulation of TMDs with h-BN grown by chemical vapor deposition (CVD), demonstrating the effects of this encapsulation on the photodegradation of TMDs (Scheme 1). This work is expected to contribute to the improvement of the stability of 2D materials in practical applications.
RESULTS AND DISCUSSION WSe2 and MoSe2 monolayers were grown via a CVD method on c-plane sapphire substrates using WO3, MoO3, and Se powders as precursors. The resulting WSe2 and MoSe2 monolayers exhibited triangular shapes with a lateral size of 8−20 μm and were characterized by Raman and PL spectroscopy, as outlined in Figure S1. Further details regarding the growth process are described in the Methods. Raman spectroscopy has previously been used to observe the lattice vibrations of TMD (e.g., A1g and E2g modes), which are sensitive to the number of layers present.32,33 We therefore selected Raman spectroscopy with laser excitation to investigate the degradation of the TMDs through observation of peak intensities. However, prior to analysis, we examined the potential presence of a TMD thermal effect through laser irradiation, as an anharmonic lattice potential energy, and softening of TMD bonds can be caused by the heat produced from the laser.34 Indeed, some studies have demonstrated the thermal effect of TMDs under laser irradiation, where a power >2 mW was employed during Raman spectroscopy.34,35 In 8975
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Figure 3. Variation in (a) Raman peak and (b) PL peak areas for WSe2 and 1L h-BN/WSe2 as a function of exposure time at 2 mW in air. Variation in (c) Raman peak and (d) PL peak areas for MoSe2 and 1L h-BN/MoSe2 as a function of exposure time at 2 mW in air. (e) Variation in PL peak areas for WSe2, 1L h-BN/WSe2, and 3L h-BN/WSe2 in the presence of a water droplet. (f) Optical microscopy images of 3L h-BN/WSe2 in the presence of a water droplet after laser exposure at 0.1 mW for 780 s. All time-dependent graphs are normalized to the initial peak intensity.
observed a decrease in the intensity of the Se 3d binding energy peak along with an increase in the O 1s binding energy peak in the laser-exposed area, indicating detachment of Se, attachment of O, and formation of W6+ (Figure 2b−f). By XPS analysis in the SPEM images, we observed that the initial W:Se ratio of ∼1:2 (pristine WSe2) was reduced to ∼1:1 in the damaged center region of WSe2 following laser exposure (see Figure S4a and d). Similarly, the intensity of the Se 3d peak was also reduced in the damaged center region of WSe2, as shown in Figure S4b and e. Furthermore, the O 1s peak was observed at ∼532 eV, indicating the presence of the WO3 oxidation state. The results indicate that photodegradation of WSe2 is due to oxidation.
both the Raman and PL spectra showed dramatic reductions in peak intensity but no shift in peak position, as shown in Figure 1c and d. Furthermore, comparison of the optical images of WSe2 in the presence of a water droplet both before (Figure 1e) and after (Figure 1f) laser exposure confirmed that the area exposed to the laser was degraded. To verify the chemical state of the photodegraded WSe2, SPEM and XPS were then carried out (see Figures 2 and S4), as SPEM provides a particularly high spatial resolution. We conducted the image scan with a 4 mW laser for 3000 s to damage the WSe2 flakes (Figure 2a). As a result, additional signals corresponding to W6+ were clearly observed only in the regions of WSe2 damaged by laser exposure. In addition, we 8976
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Figure 4. XPS spectra of pristine WSe2, laser-exposed WSe2 in air, and laser-exposed 3L h-BN/WSe2 in air. The corresponding table shows the peak positions for the W 4f binding energies. The XPS spectra were calibrated using the gold substrate Au 4f peak.
bearing a water droplet was perfectly maintained after laser exposure at 0.1 mW. On the basis of these results, we could conclude that 3L h-BN was suitable for protecting TMDs from photodegradation, even in the presence of water. X-ray photoelectron spectroscopy was carried out to compare the pristine WSe2, laser-exposed WSe2, and laserexposed 3L h-BN/WSe2 (Figure 4). All WSe2 samples were transferred onto Au substrates. Pristine WSe2 exhibited W 4f7/2 and W f5/2 binding energy peaks at 32.4 and 34.55 eV, respectively, indicating the presence of the W4+ oxidation state. Indeed, similar values were previously reported for the W 4f peak of WSe2 on a metal substrate.37 In contrast, the W 4f7/2 and W 4f5/2 signals for the laser-exposed WSe2 were up-shifted slightly to 32.87 and 35.03 eV, respectively. This up-shift was also observed for the Se 3d binding energy (Figure S9). Furthermore, an additional doublet was present at 36.22 and 38.28 eV in the laser-exposed WSe2, corresponding to the W6+ oxidation state.8,37,38 However, the W6+ signal was small, due to the small area exposed to the laser compared to the larger X-ray beam size employed to record the XPS measurements. In contrast, no such effects were observed for 3L h-BN/WSe2, indicating that the oxidation of WSe2 did not take place upon exposure to the laser. We could therefore conclude that 3L hBN effectively prevents the photodegradation of TMDs.
Thus, to improve the stability and prevent the photodegradation of WSe2, CVD-grown h-BN on Pt foil was employed to cover the WSe2, which exhibits a high thermal conductivity and is highly stable in air and under heat treatment.28,29 CVD-grown h-BN was transferred to WSe2 by an electrochemical delamination method (see AFM images in Figure S5).28 Figure 3 shows the variation in Raman and PL signal intensities for WSe2 covered by a single layer of h-BN (1L h-BN/WSe2) at a laser power of 2 mW. Although the Raman peak area of pristine WSe2 decreased to ∼20% of that of the initial peak, no change was observed for 1L h-BN/WSe2 (Figure 3a). In addition, no shift in the position of the A1g peak of WSe2 was observed (Figure S6a). A similar trend was observed in the PL spectrum, as both the PL peak area and peak position remained constant for 1L h-BN/WSe2 at 2 mW (Figures 3b and S6b, respectively). These results confirmed that the presence of 1L h-BN improved the stability of WSe2 against photodegradation (see long-term stability in harsh conditions over 2 h in Figure S7). Furthermore, we also investigated the protection of MoSe2 using h-BN, to determine if h-BN is potentially suitable for protecting other 2D materials. Indeed, pristine MoSe2 also exhibited photodegradation above 0.1 mW. However, as shown in Figure 3c and d, in the presence of 1L h-BN, photodegradation was prevented even at 2 mW. Moreover, the positions of the peaks corresponding to MoSe2 in the Raman and PL spectra of 1L h-BN/MoSe2 remained constant, as shown in Figure S6c and d, respectively. As described above, the presence of water accelerated the photodegradation of WSe2 (Figure 1). Upon monitoring the photodegradation of WSe2 and 1L h-BN/WSe2 in the presence of a water droplet by time-dependent PL spectroscopy, it was clear that the use of 1L h-BN was not sufficient to protect WSe2 at 0.1 mW (Figure 3e). This contrasted with the result indicating that 1L h-BN/WSe2 was extremely stable in air, even with a laser power of 2 mW (Figure 3a−d). It therefore appeared that water was able to penetrate the h-BN layer through defects or boundaries, thus causing the oxidation of WSe2 flakes upon application of the laser.18,19 Thus, we prepared a triple h-BN layer (3L h-BN) covered WSe2 (Scheme 1). The use of 3L h-BN fully protected the WSe2 at 0.1 mW as shown in Figures 3e and S8. Furthermore, its peak intensity remained >95% even with a laser power of 2 mW. Indeed, as shown in Figure 3f, the triangular shape of the WSe2 flake
CONCLUSIONS We herein observed the photodegradation and wateraccelerated photodegradation of transition metal dichalcogenides WSe2 and MoSe2 upon exposure to a laser power greater than 0.1 mW. The extent of degradation was observed by changes in the intensity of Raman and photoluminescence signals under laser exposure. X-ray photoelectron spectroscopy and scanning photoemission microscopy studies indicated that this photodegradation could be attributed to the oxidation of WSe2 upon laser exposure. We also demonstrated that the encapsulation of TMDs by hexagonal boron nitride via chemical vapor deposition effectively prevented photodegradation, with the use of 3L h-BN imparting stability even in the presence of water. We expect that this work into the encapsulation of 2D materials by h-BN could be applied to other materials, thus boosting their application in areas such as optoelectronic devices. 8977
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METHODS Growth of Single-Layer WSe2. WSe2 was grown by the chemical vapor deposition method on c-plane sapphire. Two precursors, WO3 (99.998%, Alfa Aesar) and Se (99.999%, Alfa Aesar), were used for the WSe2 growth. A 300 mg amount of Se was placed at the upstream entry of the furnace, and 120 mg of WO3 powder was placed at the center of the furnace. The sapphire substrate was located next to the crucible that contained WO3. Before the tube furnace was heated, the tube was evacuated for 30 min, and the temperature of the tube furnace increased up to 800 °C for 24 min under a steady flow of Ar gas (140 sccm) and H2 gas (20 sccm). When the furnace reached 800 °C, Se was vaporized by heating the upstream entry of the tube up to 260 °C using a heating belt. Finally, temperature of the tube furnace increased to 870 °C and was maintained for 1 h for the WSe2 growth. Afterward, the tube furnace was cooled to room temperature under the Ar flow. Growth of Single-Layer MoSe2. MoSe2 was grown by CVD on cplane sapphire. Two precursors, MoO3 (99.97%, Sigma-Aldrich) and Se (99.999%, Alfa Aesar), were used for the growth. A 150 mg amount of Se was placed at the upstream entry of the furnace, and 60 mg of MoO3 powder was placed at the center of the furnace. A crucible containing MoO3 was partially covered by SiO2/Si wafer to reduce intense evaporation of the precursor. The sapphire substrate was located next to the crucible that contained MoO3. Before the tube furnace was heated, the tube was evacuated for 30 min and filled with Ar gas to ambient pressure, and the temperature of furnace was increased up to 600 °C for 18 min under a steady flow of Ar gas (60 sccm) and H2 gas (12 sccm). When the furnace reached 600 °C, Se was vaporized by heating the upstream entry of the tube up to 270 °C using a heating belt. Finally, the temperature of the tube furnace was increased to 700 °C and maintained for 1 h for the MoSe2 growth. Afterward, the tube furnace was cooled to room temperature, while the Ar flow was maintained without H2.
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b05042. Growth methods for MoSe2 and WSe2, Raman and PL peaks for MoSe2 and WSe2, A1g peak position and fwhm of Raman time-series graph, Se 3d and O 1s peaks of pristine WSe2, laser-exposed WSe2, and laser-exposed 3L h-BN/WSe2, and XPS and SPEM of pristine WSe2 and laser-exposed WSe2 (PDF)
AUTHOR INFORMATION Corresponding Author
*E-mail (H. S. Shin):
[email protected]. Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the 2016 Research Fund (1.160075) of UNIST (Ulsan National Institute of Science & Technology), an NRF Grant (NRF- 2014R1A2A2A01007136), IBS-R019-D1, and a Grant (Code 2011-0031630) from the Center for Advanced Soft Electronics under the Global Frontier Research Program through the National Research Foundation funded by the Ministry of Science, ICT, and Future Planning, Korea. REFERENCES (1) Wang, Q. H.; Kalantar-Zadeh, K.; Kis, A.; Coleman, J. N.; Strano, M. S. Electronics and Optoelectronics of Two-dimensional Transition Metal Dichalcogenides. Nat. Nanotechnol. 2012, 7, 699−712. 8978
DOI: 10.1021/acsnano.6b05042 ACS Nano 2016, 10, 8973−8979
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DOI: 10.1021/acsnano.6b05042 ACS Nano 2016, 10, 8973−8979
