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Strain Release Induced Novel Fluorescence Variation In CVD-Grown Monolayer WS Crystals 2

Shanghuai Feng, Ruilong Yang, Zhiyan Jia, Jianyong Xiang, Fusheng Wen, Congpu Mu, Anmin Nie, Zhisheng Zhao, Bo Xu, Chenggang Tao, Yongjun Tian, and Zhongyuan Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b09744 • Publication Date (Web): 13 Sep 2017 Downloaded from http://pubs.acs.org on September 17, 2017

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ACS Applied Materials & Interfaces

Strain Release Induced Novel Fluorescence Variation In CVD-Grown Monolayer WS2 Crystals Shanghuai Feng1#, Ruilong Yang1#, Zhiyan Jia1, Jianyong Xiang1,*, Fusheng Wen1,*, Congpu Mu1, Anmin Nie2, Zhisheng Zhao1, Bo Xu1, Chenggang Tao3, Yongjun Tian1, Zhongyuan Liu1,* 1

State Key Lab of Metastable Materials Science and Technology, Yanshan University, Qinhuangdao

066004, China 2

Shanghai University Materials Genome Institute and Shanghai Materials Genome Institute, Shanghai

University, Shanghai 200444, China 3

Department of Physics, Virginia Tech, Blacksburg, Virginia 24061, USA Abstract

Tensile strain is intrinsic to monolayer crystals of transition metal disulfides such as Mo(W)S2 grown on oxidized silicon substrates by chemical vapor deposition (CVD) owing to the much larger thermal expansion coefficient of Mo(W)S2 than that of silica. Here we report fascinating fluorescent variation in intensity with aging time in CVD-grown triangular monolayer WS2 crystals on SiO2 (300 nm)/Si substrates and formation of interesting concentric triangular fluorescence patterns in monolayer crystals of large size. The novel fluorescence aging behavior is recognized to be induced by the partial release of intrinsic tensile strain after CVD growth and the induced localized variations or gradients of strain in the monolayer crystals. The results demonstrate that strain has a dramatic impact on the fluorescence and photoluminescence of monolayer WS2 crystals and thus could potentially be utilized to tune electronic and optoelectronic properties of monolayer transition metal disulfides. Keywords: Transition metal disulfide, aging, fluorescence, strain, photoluminescence #

S.H.F and R.L.Y contributed equally to this work. 1

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1. Introduction Monolayer WS2 of a structural analogue to MoS2 is one more attractive semiconducting material with a direct bandgap among two-dimensional (2D) atomically thin transition metal disulfides (TMDCs). Similar to MoS2, monolayer WS2 exhibits rich unique electronic and optical properties with great promise of applications in electronics and optoelectronics1-4, such as field-effect transistors and photodetectors5-9, magnetoluminescence devices10, valleytronic devices11-14. Though MoS2 and WS2 monolayers exhibit photoluminescence (PL) as result of the direct bandgap, monolayer WS2 is found to have the much higher PL efficiency and narrower emission linewidth, and it is thus considered as better candidate in optoelectronic applications15-17. Direct growth of monolayer WS2 crystals in large-size and large-area scale is very important for promotion of the practical applications in electronics and optoelectronics. In the past few years, chemical vapor deposition (CVD) has been well developed to synthesize high-quality monolayer WS2 crystals18-22. Gao et al.21 reported CVD-grown monolayer WS2 crystals of the size up to millimeter on reusable Au foils under ambient pressure. Owing to the controllable size, shape and number of layers by manipulating the growth parameters, CVD-grown monolayer WS2 crystals are desirable for the potential applications in electronics and optoelectronics, superior to the mechanically exfoliated ones. Due to the many alterable parameters in the growth process, such as pressure, temperature, substrates, precursors, cooling rate and so on, however, CVD-grown monolayers MoS2 and WS2 involve the structural defects including grain boundaries, point defects, dislocations, vacancies and edges, which can produce significant heterogeneity in PL characteristics17,18, 20, 23-28. In addition to structural defects, strain in monolayers MoS2 and WS2 is found to strongly affect PL and fluorescence (FL) characteristics26, 29, 30. In CVD-grown monolayer MoS2 crystals on oxidized silicon substrates, since 2


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MoS2 has a much higher thermal expansion coefficient than silica substrate, intrinsic tensile strain is introduced owing to the significant mismatch contraction created by a fast cooling process from a high temperature to end the growth26. The intrinsic tensile strain can be partially released after fast cooling due to the combined effects of geometry and inevitable variations in interactions, leading to localized regions of less strain and the heterogeneity in PL characteristics26. Similarly, tensile strain should be intrinsic to CVD-grown WS2 crystals on oxidized silicon substrates and partially released after growth, which is lack of detailed investigation. In the present work, a series of triangular monolayer WS2 crystals were synthesized by using WO3 and S as the precursors in a CVD system (see Figure S1 in supporting information), and they were characterized with optical and fluorescence (FL) microscopies, PL and Raman spectrometers. By aging the monolayer crystals under ambient exposure, the FL variation with time and its association with the partial release of intrinsic tensile strain have been investigated in detail. 2. Results and discussion Figures 1a shows the optical image of CVD-grown monolayer WS2 crystals, and Figure 1b gives the corresponding FL image, which was immediately collected under ambient condition once the sample was taken out of the tube furnace. Corresponding to the optical images of dominantly grown monolayer WS2 crystals in triangular shape, the bright FL images are observed due to the direct bandgap. A few bilayers are occasionally observable in the optical image, and correspondingly they show no FL signal in the FL images owing to the indirect bandgap. AFM images of monolayer WS2 crystals were collected immediately after growth and one-month exposure in air, respectively (Figure 1a and Figure S2 in supporting information). From the line profiles (Insets of Figure 1a and Figure S2 in supporting information), the CVD-grown monolayer WS2 is found to have a thickness of ~1 nm, 3



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consistent with the reported ones in our previous studies8, 9. As revealed in the AFM image of Figure S2, huge amount of adsorbates appear on the edge of monolayer WS2 crystals after one-month ambient exposure. As revealed in Figure 1b, the CVD-grown triangular monolayer crystals initially exhibit strong FL in uniform intensity. With increase of aging time, however, the FL signal is noted to change continuously in intensity. As shown in Figure 2 (also Movie S1 in supporting information), the monolayer crystals exhibit obvious size-dependent FL variation with aging time. In monolayer crystals of the smaller sizes (40 µm) as represented by the marked one with red arrow, the initially collected FL image at aging time of 0 h displays uniform but much higher intensity, and the FL variation with aging becomes much more complex. At initially aging, the FL signal on the edge is firstly weakened in intensity, and with increase of aging time, the FL-weakened or dark region expands toward the center, leading to formation of interesting concentric triangular FL patterns with dark and bright FL regions and a FL-quenched interface between them. Along with expansion of dark FL region and concurrent shrinkage of bright triangular FL region toward the center, the FL signal on the edge becomes gradually stronger again, and two more triangular regions with increasing FL intensity are recognized to appear concurrently. By aging up to ~4 hours, the bright triangular FL region completely disappears in the center, and novel concentric triangular FL patterns consisting of alternating strong and weak FL regions are formed. In the aging stage from ~4 to 7 hours, the concentric triangular FL patterns are well kept, but the FL signal on the edge becomes stronger and 4


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stronger with increase of aging time. In the aging stage from 7 to 72 hours, the FL re-enhanced region concentrically expands from the edge toward the center. By aging up to 72 hours and more, the FL becomes strong again over the monolayer crystal, but the concentric triangular FL patterns are still clearly observable. Compared to those of the smaller sizes (40 µm) exhibit not only much slower but also much more complex FL variation with aging time and formation of complex concentric triangular FL patterns. The FL aging behaviors were investigated in a series of triangular monolayer WS2 crystals, which were synthesized by changing the heating temperatures of WO3 and S precursors in the CVD-growth process (Figures S3, S4 and Movies S2-S5 in supporting information). Depending on the precise CVD-growth conditions, the triangular monolayer WS2 crystals exhibit slight differences in FL variation with aging and the induced concentric triangular FL patterns. In spite of the observable differences in detail, some common features are observed. i) The initial FL signal in monolayer crystals is uniform in intensity, and generally, it is stronger in monolayer crystals of the larger size. ii) In monolayer crystals of the smaller size, the FL signal remains uniform in intensity and becomes gradually stronger with increase of aging time, while monolayer crystals of the larger sizes exhibit much more complicated FL variation and formation of concentric triangular FL patterns. iii) The speed of FL variation with aging is size-dependent. The larger in size is the monolayer crystal, the slower is the FL variation with aging. By aging under ambient conditions, the presence of adsorbates on monolayer crystals cannot be ruled out (see the AFM image in Figure 1a). In order to examine the role of adsorbates in the observed FL variation with aging, we have carried out the following measurements. One sample of the CVD-grown triangular monolayer crystals was cut into two pieces once they were taken out of the tube 5



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furnace. One piece was immediately covered with a transparent PVC film of 0.1 mm thickness to be separated from contact with the outside environment, and the other was still left open to ambient conditions. For the covered and uncovered monolayer crystals, the FL images were collected at a series of aging times in Figures 3a and b for comparison (also see Movies S6 and S7 in supporting information). Compared to the uncovered ones, the covered monolayer crystals show the similar but slower FL variation with aging time. In the uncovered monolayers of large size, the aging-produced FL patterns are no longer observable by aging up to 40 min. In the covered monolayer crystals, the aging-induced FL variation becomes much slower, and by aging up to 30 min and above, the aging-produced FL patterns are not only obviously observed in the larger monolayers but also discernible in the smaller ones. By aging up to a long enough time, the covered monolayer crystals show no discernible FL patterns and give rise to the FL images with relatively uniform intensity, similar to the uncovered ones. Clearly, by protecting the monolayer crystals with a transparent PVC film from contact with the outside environment, the aging-induced FL variation is greatly delayed but not suppressed. This indicates that the observed FL aging behavior in CVD-grown monolayer WS2 crystals cannot be related to the adsorbates under ambient conditions, especially the formation of novel concentric FL patterns. In CVD-grown monolayer WS2 crystals, intrinsic and extrinsic defects of significant heterogeneity can be formed during growth to strongly affect the PL and FL characteristics. In monolayer WS2 crystals, suppression of PL intensity has been observed in the center region in comparison to that on the edge owing to the S-vacancies20. Enhanced PL and FL have been found on the edges and grain boundaries in WS2 monolayers owing to the presence of S vacancies in high concentration24. Recently, FL patterns of alternating dark and bright concentric triangular regions have been observed in 6


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CVD-grown WS2 monolayers, and dark triangular FL regions have been attributed to the rich S vacancies27. In our CVD-grown WS2 monolayers, concentric triangular FL patterns are induced to form by aging, and they either tend to disappear or are well retained by aging up to a long enough time, depending on the precise growth conditions. By transferring the monolayer crystals to a new oxidized silicon substrate for release of the intrinsic tensile strain, the well-kept FL patterns after aging of four days are even removed (see Figure S5 in supporting information). The chemical heterogeneity would not possibly be removed by just aging under ambient conditions once it was formed during CVD growth. Thereby, the aging-produced concentric FL patterns cannot be related to formation of the chemical heterogeneity in the monolayer WS2 crystals. As reported in previous studies26, the partial release of intrinsic tensile strain after growth leads to localized variations of strain in the CVD-grown MoS2 monolayers on oxidized silicon substrates. Similarly, intrinsic tensile strain should be built in the CVD-grown monolayer WS2 crystals on oxidized substrates. Figure 4a shows the optical image of as-grown monolayer WS2 crystals, and Figure 4b gives the corresponding FL image taken after ambient exposure of one month. Concentric FL patterns are obviously observable in monolayer crystals of large size. After one-month exposure in air, the monolayer crystals were transferred to a new oxidized silicon substrate. The optical and FL images of transferred monolayer crystals are shown in Figures 4c-f. The FL images show no observation of concentric FL patterns after transfer. The transfer-induced disappearance of FL patterns probably indicates that the formation of FL patterns after aging should not originate from the intrinsic defects of significant heterogeneity in the as-grown monolayer crystals. The PL and Raman spectra were collected from the as-grown and transferred monolayer crystals for comparison. As shown in the PL spectra of Figure 4g, a big blue shift of ~56 meV in PL peak position is produced by transferring the 7



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monolayer WS2 crystals to a new oxidized silicon substrate, which indicates the large tensile strain in as-grown monolayer WS2 crystals and its release after transfer. The release of intrinsic tensile strain is further confirmed in Raman spectra as shown in Figure 4h and i. Transfer-induced hardening by a blue shift of 2.74 cm-1 is determined in E2g1 (see Table SI in supporting information for the shifts of other Raman peaks). One more CVD-grown sample of monolayer crystals was cut into two pieces. One piece of monolayer crystals was transferred to a PDMS substrate after aging of a long time under ambient conditions (see the FL images collected with increase of aging time in Figure S5, supporting information), and similarly, the collected PL and Raman spectra before and after transfer reveal the release of intrinsic tensile strain by transferring (see Figure S6 and Table SI in supporting information). By transferring to the different substrates of PDMS and silica, the observed differences in the shifts of PL peak and Raman peaks could be associated to their different effects on effective strain transfer and corresponding band structure changes in WS2, as reported in the previous studies on monolayer MoS226. The intrinsic tensile strain in CVD-grown monolayer WS2 crystals comes from the much larger thermal expansion coefficient of WS2 than that of silica and the induced significant mismatch contraction between them by fast cooling from a high growth temperature. Similar to that in the CVD-grown monolayer MoS2 on silica26, partial release of intrinsic tensile strain after growth should occur in the CVD-grown monolayer WS2 crystals. To understand the role of partially released intrinsic tensile strain in the FL aging behaviors, we have carried out FL and PL examination of the CVD-grown monolayer WS2 crystals with increase of aging time. One sample was cut into two pieces immediately after it was taken out of the tube furnace. As shown in the collected FL images of one cut piece (Figure S5 in supporting information), the FL signal on the edge is firstly suppressed with aging, and simple 8


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concentric triangular FL patterns are formed with dark and bright regions. The further aging leads to expansion of dark FL region and simultaneous shrinkage of bright FL region toward the center. Figure 5a shows the optical image for one typical monolayer WS2 crystal in the other cut piece, and Figure 5b gives the corresponding FL image collected at initially aging. Four marked locations from the center to the edge were examined by PL at the aging times of 0, 1.5 and 3 h. As shown in the collected PL spectra (Figures 5c, d and e), the PL intensities from the center to the edge exhibit clear variation with aging time, similar to the FL signal (Figure S5 in supporting information), and they tend to become comparable if aging is long enough. The PL peak widths and shifts in position relative to the initial one in the center are extracted in Figures 5f and g. Figures 5h, i and j give the PL mappings of peak height, position and width immediately taken after the initial measurements of PL spectra at aging time of 0 h. Concentric triangular contrast patterns are clearly observed in the PL mappings, similar to the FL patterns (Figure 5b and Figure S5 in supporting information). From the center to the edge, as shown in Figure 5f, the PL peak width becomes narrower, and owing to its quick reduction with aging, it becomes comparable if aging is long enough. As revealed in Figure 5g, relative to the initial one in the center, the blue shift in PL peak position shows an increase from the center to the edge. At initially aging, a high blue shift of ~14 meV is observed on the edge. Toward the center, the blue shift is slightly reduced at position 3 but is hugely reduced to ~1 meV at position 2. By aging up to 1.5 h, a blue shift of ~11 meV on the edge is observed, and towards the center, the blue shift shows slight decrease at positions 3 and 2 but a sharp drop at the center. By aging up to 3 h, the blue shift on the edge decreases by just a little to ~10 meV, and towards the center, the blue shift shows a slow decrease with no sharp drop. Based on the similar aging-induced variation of PL intensity to the FL signal and the concurrent 9



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blue shift in PL peak position, the FL aging behaviors in CVD-grown monolayer WS2 crystals are supposed to result from partial release of the large intrinsic tensile strain after growth. The large tensile strain in our CVD-grown monolayer WS2 crystals is introduced as a result of the much higher thermal expansion coefficient of WS2 than that of oxidized silicon substrate. In the growth process, the thermal expansion of monolayer WS2 is much greater than that of the oxidized silicon substrate. By fast cooling from a high growth temperature to stop the CVD growth, however, great mismatch contraction exists between oxidized silicon and CVD-grown monolayer WS2. Owing to the interactions with the oxidized silicon substrate, the significantly expanded monolayer WS2 crystal can only be partially relaxed, thus leading to just partial release of the tensile strain. As implied by the observed concurrent variations of blue shift in PL peak position and FL intensity with aging from the edge to the center, the partial release of intrinsic tensile strain in CVD-grown monolayer WS2 crystals should start on the edge and gradually go towards the center with aging. In WS2 monolayer of the smaller size, the partial release of tensile strain can be finished more quickly from the edge to the center, while it needs a longer aging time in WS2 monolayer of the larger size. During the gradual partial release of tensile strain from the edge to the center, localized strain variations or gradients can be built in monolayer WS2 crystals of the large size, and as a result, they leads to the observed concentric patterns in the FL images and PL mappings. 3. Conclusions In conclusion, we have observed novel FL aging behaviors in CVD-grown monolayer WS2 crystals on oxidized silicon substrate. In the WS2 monolayer, large intrinsic tensile strain is created during CVD growth owing to the much larger thermal expansion coefficient of WS2 than that of silica, and the partial release of tensile strain leads to a blue shift in PL peak position. After CVD growth, the partial 10


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release of intrinsic tensile strain starts from the edge toward the center by aging. In monolayer WS2 crystal of the large size, the partial release of tensile strain needs a long aging time to go from the edge to the center. Along with the gradual partial release of tensile strain from the edge to the center, localized strain variations or gradients can be created and lead to the observed interesting concentric FL patterns in the FL images. As indicated in our study, FL and PL characterizations are very useful nondestructive tools for investigation on strain in monolayer WS2 crystals. Our work also highlights one potential avenue for tailoring electronic and optoelectronic properties of monolayer transition metal disulfides through local strain engineering. 4. Experimental methods CVD growth and transfer of monolayer WS2 crystals. Monolayer WS2 crystals were grown on oxidized silicon substrates (SiO2 (300 nm)/Si) in a quartz tube furnace with low- and high-temperature zones. WO3 (2.5 g, 99.9% purity, Alfa Asar) and S (1g, 99.5% purity, Alfa Asar) powders were hold in Al2O3 crucibles as the precursors. The Al2O3 crucible holding the S powder was covered by a piece of Al film (0.5 mm in thickness) with several punched holes. The arrangement of substrates and precursors in the quartz tube is schematically shown in Figure S1. A flow of high-purity Ar gas at 35 sccm was always used in the growth process to deliver vaporized precursors onto the substrates for reaction. During growth, the WO3/S precursors were heated at the temperatures of 1050/165°C, 980/165°C, 965/165°C, 965/190°C, 965/180°C, respectively, for synthesizing a series of monolayer WS2 crystals. The growth process was ended after 50 min growth by fast cooling from the high heating temperatures. The as-grown monolayer WS2 crystals were spin-coated with a poly(methyl methacrylate) (PMMA) scaffold. The PMMA-coated monolayer WS2 crystals were separated from the SiO2 (300 nm)/Si 11



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substrate by 5% HF etching at room temperature. The floated PMMA-coated monolayer WS2 crystals were carefully transferred into deionized water for cleansing, and finally they were transferred to a new substrate of oxidized silicon or PDMS and baked at 100°C for 2 hours to enhance adhesion to the substrate. Characterization. Optical and FL images were collected by using a Leica microscopy (DM4000M) equipped with a fluorescence accessory, an excitation light source of Xenon lamp and a filter system of I3. PL and Raman measurements were carried out in a Horiba JY micro Raman Microscope system (HR Evolution) equipped with Si-based CCD detector (1800 g/mm) in a backscattering geometry, and the 532 nm laser light with a diameter of ~1 µm was used. Acknowledgements: We appreciate the support from the National Natural Science Foundation of China (Grant No. 51732010, 51271214, 51102206, 51421091, and 51571172, 11474311). Supporting information Schematic diagram of the CVD system and temperature profile in the growth process (Figure S1). AFM image and line profile for the monolayer WS2 crystal immediately collected after the growth (Figure S2). FL aging behaviors in a series of CVD-grown monolayer WS2 crystals (Figures S3, S4 and S5, Movies S1-S7). PL and Raman spectra of monolayer WS2 crystals before and after transfer (Figure S6). The determined Raman peak positions of monolayer WS2 crystals before and after transfer and transfer-induced shifts (Table SI). A bar graph to show that with the increase of aging time, the smaller triangular monolayer WS2 crystals with a side length of 40 µm show formation of FL patterns (Figure S7).

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Caption of Figures 1-5 Figure 1. Optical (a) and corresponding FL (b) images for the monolayer WS2 crystals on an oxidized silicon substrate grown at the heating temperatures of 980°C (WO3 source) and 165°C (S source). Scale bars: 200 µm. Triangular monolayer WS2 crystals have been dominantly grown. A few bilayers are occasionally observable, and no FL signals are observed (marked with arrows). Inset in (a) is the AFM image immediately taken after the sample was taken out of the tube furnace. The line profile was taken from the marked dash line. Scale bar: 2 µm. Figure 2. Optical image (a) and corresponding FL images collected at a series of aging times (b-l) for the monolayer WS2 crystals grown at the heating temperatures of 980°C (WO3 source) and 165°C (S source). Scale bars: 50 µm. Figure 3. Optical and corresponding FL images collected at a series of aging times for the monolayer WS2 crystals grown at the heating temperatures of 965°C (WO3 source) and 165°C (S source). (a)The monolayer WS2 crystals covered with a transparent PVC film of 0.1 mm thickness. (b) The uncovered monolayer WS2 crystals. Scale bar: 50 µm. Figure 4. Optical (a) and corresponding FM (b) images for as-grown monolayer WS2 crystals on an oxidized silicon substrate. Optical (c, e) and corresponding FM (d, f) images for transferred monolayer WS2 crystals to a new oxidized silicon substrate. PL (g) and Raman (h, i) spectra before and after transfer. The monolayer WS crystals were grown at the heating temperatures of 980°C (WO source) 2

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and 165°C (S source), and they were aged for one month under ambient conditions before transfer. Scale bars: 50 µm. Figure 5. PL characterization of a monolayer WS2 crystal at the aging times of 0, 1.5 and 3 h. The time at which initial PL measurements were performed is defined as the aging time of 0 h. Optical image (a) 18


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and corresponding FL (b) images. PL spectra (c, d, e) collected at the aging times of 0, 1.5 and 3 h from the marked locations with “x” in a. The extracted peak width and shift in peak position relative to that at the center (location 1) at the aging time of 0 h (f, g). PL mappings of maximum height, peak position and width (h, i, j).

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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TOC The fluorescence of monolayer WS2 shows a strong ageing effect.

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Strain Release Induced Novel Fluorescence ... - ACS Publications

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