Local Strain Induced Band Gap Modulation and Photoluminescence

---

Article pubs.acs.org/cm

Local Strain Induced Band Gap Modulation and Photoluminescence Enhancement of Multilayer Transition Metal Dichalcogenides Krishna P. Dhakal,† Shrawan Roy,§,‡ Houk Jang,† Xiang Chen,† Won Seok Yun,⊥ Hyunmin Kim,# JaeDong Lee,⊥ Jeongyong Kim,§,‡ and Jong-Hyun Ahn*,† †

School of Electrical and Electronic Engineering, Yonsei University, Seoul 03722, Republic of Korea Center for Integrated Nanostructure Physics, Institute for Basic Science (IBS) and ‡Department of Energy Science, Sungkyunkwan University, Suwon 16419, Republic of Korea ⊥ Department of Emerging Materials Science and #Companion Diagnostic & Medical Technology Research Group, DGIST, Daegu 42988, Republic of Korea §

S Supporting Information *

ABSTRACT: The photocarrier relaxation between direct and indirect band gaps along the high symmetry K−Γ line in the Brillion zone reveals interesting electronic properties of the transition metal dichalcogenides (TMDs) multilayer films. In this study, we reported on the local strain engineering and tuning of an electronic band structure of TMDs multilayer films along the K−Γ line by artificially creating one-dimensional wrinkle structures. Significant photoluminescence (PL) intensity enhancement in conjunction with continuously tuned optical energy gaps was recorded at the high strain regions. A direct optical band gap along K−K points and an indirect optical gap along Γ−K points measured from the PL spectra of multilayer samples monotonically decreased as the strain increased, while the indirect band gap along Λ−Γ was unaffected owing to the same level of local strain in the range of 0%−2%. The experimental results of band gap tuning were in agreement with the density functional theory calculation results. Local strain modified the band structure in which K-conduction band valley (CBV) was aligned below the Λ-CBV, and this explained the observed local PL enhancement that made the material indirect via the K−Γ transition. The study also reported experimental evidence for the funneling of photogenerated excitons toward regions of a higher strain at the top of the wrinkle geometry.

■

INTRODUCTION Two-dimensional (2D) transition metal dichalcogenides (TMDs) are inorganic semiconductor materials consisting of transition metals M (Mo, W, Sn, etc.) covalently bonded with chalcogens atoms X (S, Se, Te).1,2 Specifically, TMDs exhibit layer number dependence of their optical, electrical, and structural properties.2−10 Previous studies demonstrated several potential applications of 2D materials including field effect transistors,4 flexible electronics,11,12 optically pumped valleytronic devices5 and photodetectors.2 The monolayer and multilayer of TMDs have PL due to exciton recombination with binding energy of the order of 102 meV.13 Particularly, monolayer TMDs exhibit a quantum confinement effect at the K point of the Brillion zone that makes them ideal 2D-direct band gap materials1,13 and significantly enhance the PL from the monolayer at the direct band gap position when compared with those of multilayer counterparts.2,3 Multilayers of these TMDs involve indirect band gap materials that show significantly weaker PL.1−3 Therefore, the typical nature of the indirect band in the multilayer TMDs limits their © 2017 American Chemical Society

application in most optoelectronic devices such as lightemitting diodes, photodetectors, valleytronics devices, and lasers. Interestingly, the band structure of the TMDs can be controlled by the strain effect and thereby tunes the optical and electrical properties.9,14−24 For example, an application of the uniaxial tensile strain on monolayers MoS2, WS2, and MoSe2 induce direct to indirect band gap crossovers leading to the reduction of the direct band gap and PL intensity at the K point of the Brillion zone.9,14,16−18 However, the response of multilayer TMDs with respect to strain is not extensively studied to date.9,16−19 Previously, Conley et al. reported that a uniform tensile strain of ∼0.6% on the bilayer MoS2 resulted in the faster reduction of the indirect optical band gap when compared with that of the direct optical band gap without observable measurable changes in the PL intensity.9 AdditionReceived: February 3, 2017 Revised: May 25, 2017 Published: May 25, 2017 5124

DOI: 10.1021/acs.chemmater.7b00453 Chem. Mater. 2017, 29, 5124−5133

Article

Chemistry of Materials

Figure 1. Strain localized on the top of the wrinkle studied by Raman spectroscopy of bilayer and quad-layer 2D TMDs. (a) Schematic diagram demonstrates the process of wrinkle fabrication using the flexible substrate. Buckling-induced delamination of the flexible substrate created the periodic wrinkle structures. The representative of the SEM image taken at the oblique angle showed periodic wrinkle structures. Raman spectra of (b-d) bilayer and quad-layer WSe2 and (c-e) bilayer and quad-layer WS2 films. Optical images and the correlated AFM image are shown in the left panel of their respective plots. The Raman intensity at the top of the wrinkles of each sample is increased. Spectral comparison showed that E2g1 (inplane) and A1g (out-of-plane) vibrational Raman modes are softened for the sample of wider wrinkle size. Quad-layer WSe2 has an almost overlap A1g mode, therefore indistinguishable. However, for WS2 softening of A1g vibration is clear.

ally, high strain (∼2%) created locally on 3L-4L MoS2 films via wrinkle formation led to larger reductions in direct optical band gaps when compared with that of indirect band gaps.19 As indicated by previous studies, high strain developed at the top of the wrinkle and controlled the band structure in the nanoscale.14,19,20 Furthermore, at the wrinkle region, exciton funneling occurred from the relatively unstrained region (flat) to the most strain regions (top of the wrinkle).19,20 However, there is a paucity of research examining details of the band gap modulation including PL intensity variation at both direct and indirect band gaps due to the localized strain effect on different TMDs films. The effect of strain on multilayer WSe2 presented the following distinct difference from those of MoS2: PL emission from the bilayer WSe2 film significantly enhanced and revealed the phenomena of indirect to direct band gap crossover.15 Thus, the forementioned seemingly different results between different types of 2D TMDs films implied fundamental differences in their intrinsic electronic band structure. Nearly degenerate direct and indirect band gaps in the bilayer WSe2 film (70 meV) when compared to those in the bilayer MoS2 film (300 meV) led to a contrasting strain dependent band gap crossover in the bilayer WSe2 film.9,15,25 An extant study revealed that multilayer TMDs possessed two conduction band valleys (CBV) along the Γ−K line with similar energy and relaxation of photoexcited electrons through these valleys that originated different PL bands.26 With respect to the intrinsic electronic band structure, the offset between CBV at the K point and at Λ varied as the thicknesses of multilayer TMDs between bilayer to few-layer changed, and this drove their unique optical properties.26−28 Additionally, strain on these materials directly tuned the energy state of the valleys at the conduction band and the hills at the valence bands.9,16,13,23 This indicated that different TMD multilayers are potential candidates to depict different behavior under

similar physical constraints. To date, strain dependent electronic band structures were essentially studied only with respect to the multilayer MoS2 film.9,16,19,21 Thus, although numerous investigations were conducted to explore the strain effect on the representative candidate of MoS2 films, current studies have not explored the strain effect with respect to the optical gap and emission property of other multilayer TMDs. Hence, the present study reported the change in the electronic band structure along the Γ−K line including direct and indirect optical band gaps by applying a local strain on multilayer WS2 and WSe2 films. The formation of a wrinkle structure created local strain effects on TMDs corresponding to ∼2%. Furthermore, PL spectral imaging demonstrated significantly enhanced PL intensity along the one-dimensional structure of the highly strained region with continuously tuned direct and indirect optical band gaps. Moreover, the study investigated density functional theory (DFT) calculation of a tensile strain induced band structure modification and examined the optical band gap tuning and PL enhancement mechanism. The investigation provided insights on the effect of local strain on the fundamental band structure of multilayer 2D TMDs across the high symmetry line Γ−K and further explored evidence for the exciton funneling effect of wrinkle geometry.

■

EXPERIMENTAL SECTION

Thin multilayer WSe2, WS2, MoS2, and MoSe2 flakes were exfoliated using a Scotch tape method from commercial natural crystals (supplied by a 2D semiconductor company) and deposited directly on the prestretched elastomeric substrate composed of polydimethylsiloxane (PDMS). When the prestretched substrate was relaxed, a residual compressive stress applied on the deposited thin film induced a periodic wrinkle structure of TMDs materials of various thicknesses. These were characterized by AFM imaging using sharp tips. In the study, AFM correlated Raman, PL, and absorption spectral imaging techniques were employed to examine the optical properties of the 5125

DOI: 10.1021/acs.chemmater.7b00453 Chem. Mater. 2017, 29, 5124−5133

Article

Chemistry of Materials

Raman spectroscopy. The Raman spectra at the flat and at the top of the wrinkle were analyzed as shown in Figure 1 (b-e). As expected, the Raman spectra from the flat region contained two well-known in-plane (E12g) and out-of-plane (A1g) vibrational Raman modes. Furthermore, the peak positions, relative peak intensity, and frequency difference between each Raman peak were similar to previously reported Raman bands of WSe2 and WS2 films with respective thicknesses.15,26,28 It was also noted that Raman spectra for the monolayer and bilayer TMDs were quite decisive with respect to identifying the exact thicknesses. However, with respect to thicker samples for bilayers, trilayers, and quad-layers, the spectra closely resembled each other, and it was difficult to confirm the exact thicknesses. Typically, a comparison of the PL bands of various multilayer TMDs (bilayers, trilayers, and quad-layers) at direct and indirect band gap positions can identify the exact layer thicknesses in a more sophisticated manner.26,28 The relative positions of the PL bands for bilayer, trilayer, and quad-layer WSe2 and WS2 films were compared as shown in Table S1 (Supporting Information in section 3, Table S1) and were in agreement with the previously reported values.15,26,28 This confirmed the exact thickness of the predicted multilayer (bilayer to quad-layer) WSe2 and WS2 films. As shown in Figure 1 (b, c), Raman spectra obtained from the flat and the top of the wrinkle structures for bilayer and trilayer WS2 and WSe2 films were similar to each other relative to their respective E12g and A1g band positions with the exception of slightly enhanced Raman intensity at the wrinkle structures. Raman spectra for the trilayer samples are provided in the Supporting Information (Figures S4 and S8). At the top of the wrinkle, strain was generated, and this could modify the relative peak positions of the Raman bands between the wrinkle and flat regions. Specifically, the width of the wrinkles of the bilayer or trilayer TMDs was smaller than the size of the laser spot employed for the Raman scattering measurement. Therefore, on average, the Raman spectra obtained from the flat and wrinkle regions were observed to be similar based on their peak positions. In addition to the peak positions, an enhancement in the Raman intensity was observed from the local strain region at the top of the wrinkle structure. This was observed due to the optical interference effect between light scattered from the sample and light scattered from the PDMS substrate immediately below the wrinkle structure, which was similar to the effect observed in the bulged MoS2 films.21 The Raman spectra for quad-layer (Figure 1(d, e) and thicker TMDs films (Supporting Information, Figures S6 and S10) were further examined, and observations revealed the formation of relatively wider wrinkles (600−1000 nm) that were 2−3 times larger than the resolution limit of the confocal system at best (330 nm). Wider wrinkles were clearly visible even in the optical image as shown in Figure 1 (d, e). In these samples, a clear shift of the in-plane and out-of-plane Raman bands in addition to the enhancement in the Raman intensity measured at the top of the wrinkle structures were noted when compared to the results obtained from the flat regions. Normalized Raman spectra (Supporting Information, Figure S11) that were obtained from the wider wrinkle created on the few-layer WSe2 and WS2 films enabled in visualizing the phonon softening of the E12g and A1g Raman bands more clearly. Softening of the Raman bands at the wrinkle structures of quad-layer and fewlayer samples was similar to the previously reported Raman spectra of the wrinkle MoS2 film,19 and this was attributed to the local strain effect. It was noted that the A1g mode that was

artificially created wrinkle structures in the nanoscale. Details on the instrumentation and experimental conditions are provided in the Supporting Information in section 1.

■

RESULTS AND DISCUSSION Multilayer WSe2 and WS2 flakes were exfoliated from their natural crystals and deposited on a prestretched elastomeric substrate composed of polydimethylsiloxane (PDMS). A schematic of the local strain engineering is illustrated in Figure 1 (a). The homemade PDMS substrate was prestretched by 40%−50% (details of the sample preparation process are provided in the Supporting Information, section 1, Figure S1). Relaxations of the prestretched PDMS substrate transferred residual compressive stress on thin TMDs films deposited at the top and thereby created a periodic wrinkle structure. The wrinkles were formed due to the buckling-induced delamination of the substrate.19,20 The SEM image as shown in Figure 1(a) clearly shows the periodic wrinkle formation, creating a local strain region. The studies focused on the wrinkle structures of bilayer, trilayer, quad-layer, and few-layer WSe2 and WS2 films. The sharp AFM tip with tip radius