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Two-dimensional (2-D) transition metal dichalcogenides, such as MoS2, WSe2, and WS2, are promising materials for application in field effect transistors, optoelectronics, and sensing devices. In this study, 2-D WSe2 samples with various numbers of layers were hybridized with functionalized gold nanoparticles (Au-NPs) to achieve surface-enhanced Raman scattering (SERS). The nanoscale Raman and photoluminescence spectra of the WSe2 layers and WSe2/Au-NP hybrids were measured using a high-resolution laser confocal microscope. The WSe2 exhibited distinct optical characteristics depending on the number of WSe2 layers. The intensities of the Raman characteristic modes of the WSe2 layers were significantly enhanced after hybridization with functionalized Au-NPs, indicating the SERS effect. The SERS effect weakened with increasing the number of WSe2 layers. The SERS effect was more pronounced for mono- and bi-layer WSe2 systems compared with the multi-layer WSe2 systems.
© 2016 Optical Society of America
1. Introduction
Two-dimensional (2-D) transition metal dichalcogenides (TMDCs) have attracted considerable attention owing to their low-dimensional semiconductor properties and applications to nanoscale devices [1,2]. A mono-layer TMDC system has a direct band gap in the visible range, while a multi-layer TMDC system has an indirect band gap. Therefore, TMDC systems exhibit interesting optical and electrical properties, depending on the number of layers. Mono- and multi-layer TMDC systems have been used in various devices, such as field effect transistors, light-emitting diodes, solar cells, valleytronics, and chemical-/bio-sensors [3–10].
Intra- and inter-layers of p-type 2-D tungsten diselenide (WSe2) semiconductors feature a covalent bond and the van der Waals interaction, respectively. A mono-layer WSe2 structure has a direct band in the visible range, at 1.65 eV, while bulk WSe2 exhibits an indirect band gap in the near infrared region, at 1.2 eV [11–13]. The photoluminescence (PL) peak in the mono-layer WSe2 structures was observed at 750 nm [12–14]. The Raman peak in the mono-layer WSe2 structures was observed at 250 cm−1, resulting from the overlap of two vibrational modes: the out-of-plane mode (A1g) and the in-plane mode (E12g) [13–15].
Recently, hybridization of TMDCs with metal nanostructures has been extensively studied for controlling the nanoscale optical properties and for fabricating highly efficient devices and sensors [16–19]. A significant enhancement of PL has been reported for chemical vapor deposition (CVD) grown MoS2/silver (Ag) nanodisc arrays [16] and WSe2/gold (Au) plasmonic hybrid structures [17]. Simultaneous enhancement of PL and Raman intensities (i.e., surface-enhanced Raman scattering (SERS)) in WSe2/metal hybrid nanostructures has not been reported yet.
In this study, we report the SERS effect in WSe2/gold-nanoparticle (Au-NP) hybrid structures. Functionalized Au-NPs were synthesized using the Brust method [20], and homogeneously attached to the surfaces of mechanically exfoliated WSe2 flakes. The nanoscale Raman and PL spectra of the fabricated WSe2/Au-NP hybrid nanostructures were measured using a high-resolution laser confocal microscopy (LCM) system. We observed an enhancement of the Raman signal in the WSe2/Au-NP hybrids owing to the SERS effect; the extent of this enhancement depended on the number of WSe2 layers.
2. Experimental
2.1 Preparation of functionalized Au-NPs and WSe2/Au-NP hybrid nanostructures
Dodecanethiol (C12H25SH)-functionalized Au-NPs were synthesized using the Brust method [20]. Chloroauric acid (HAuCl4∙3H2O) and tetraoctylammonium bromide (C32H68BrN) were dissolved in toluene and deionized water, respectively. The solution of C32H68BrN dissolved in toluene was mixed with an aqueous solution of the chloroauric acid. Then, dodecanethiol and sodium borohydride (NaBH4) were added to the mixed solution. The mixture was separated into two layers, a layer shown the black color includes the dodecanethiol-functionalized Au-NPs. The synthesized Au-NPs were washed in ethanol, and the washed Au-NPs were used in the experiments reported here. The functionalized Au-NPs were dried in vacuum at 40 °C for 24 h. All of the initial materials were purchased from Sigma-Aldrich Co.
WSe2 flakes (purchased from HQ graphene) were mechanically exfoliated using a scotch tape, and deposited onto a SiO2/Si substrate using a polydimethylsiloxane stamp film. To determine mono-layer WSe2, we used a charge-coupled device (CCD; Olympus, U-RFL-T). The functionalized Au-NPs were homogeneously spin-coated on the surface of the layer at 500 revolutions per minute (rpm) for 5 s, and then at 3000 rpm for 70 s. Then, the resulting WSe2/Au-NP hybrid nanostructures were dried in vacuum at 40 °C for 8 h.
2.2 Measuring nanoscale optical properties
To investigate the sizes and surface structures of the fabricated samples, we used high-resolution transmission electron microscopy (HR-TEM; FEI, Tecnai 20). Optical absorption was measured using an ultraviolet and visible (UV-Vis) spectro-photometer (Agilent 8453) in the 190–1100 nm range.
The nanoscale PL and Raman spectra of the pristine WSe2 and WSe2/Au-NP hybrids were measured using a high-resolution LCM system. For the LCM-based PL and Raman mapping of the samples, the laser excitation wavelength was set to 514 nm. The excitation laser power values were 15 μW and 1 mW, and the exposure time values were 125 ms and 250 ms for the LCM-based measurements of PL and Raman spectra, respectively. Note that the measurement conditions for the pristine and hybrid samples were the same.
3. Results and discussion
The structural and optical absorption properties of the dodecanethiol-functionalized Au-NPs were investigated using HR-TEM and the UV-Vis spectro-photometer, respectively. Figure 1(a) shows an HR-TEM image of the functionalized Au-NPs, from which the diameter of the NPs was estimated to be in the 3–5 nm range. From the optical absorption spectrum of the functionalized Au-NPs, shown in Fig. 1(b), the surface plasmon peak was estimated to be at 510 nm. Figure 1(c) shows a schematic illustration of the WSe2/Au-NP hybrid nanostructure. The Au-NPs were homogeneously dispersed on the surface of WSe2 by spin-coating, mainly owing to the hydrophobic characteristics of the dodecanethiol-functional groups.
Fig. 1 (a) HR-TEM image of the dodecanethiol-functionalized Au-NPs. (b) Absorption spectrum of the functionalized Au-NPs. Inset: Schematic chemical structure of Au-NPs. (c) Schematic illustration of the WSe2/Au-NP hybrid nanostructure.
Because the 2-D material characteristics in this system were determined by the van der Waals inter-layer interaction, the PL intensity and position depended on the number of layers [12–14]. Figures 2(a) and 2(b) show the optical microscopy and CCD images of the pristine WSe2 on the SiO2/Si substrate, respectively. In Fig. 2(a), a WSe2 flake with different numbers of layers is distinguishable owing to the varying color intensity of the optical microscope image. From the LCM PL mapping (Fig. 2(b)), the region of mono-layer WSe2 exhibits brighter red light emission, compared with the relatively dark red of multi-layer WSe2, suggesting a higher PL efficiency.
Fig. 2 (a) Optical microscope image of the pristine WSe2. (b) LCM PL mapping image of the pristine WSe2. LCM PL spectra of the (c) mono-layer, (d) bi-layer, and (e) multi-layer WSe2.
Figures 2(c)-2(e) show the LCM PL spectra of the pristine WSe2 flakes for mono-, bi-, and multi-layer systems, respectively. The LCM PL peak of the mono-layer WSe2 system is observed at 752 nm (1.65 eV), originating from the inter-band transition between the top of the valence band and the bottom of the conduction band at the Κ point with a direct energy bandgap [11,12]. Figure 2(d) shows the LCM PL spectrum of the bi-layer WSe2 system, in which two PL peaks at ~770 and ~800 nm are simultaneously observed, corresponding to the indirect inter-band transition from the top of the valence band at the Κ point, to the bottom of the conduction band at the midpoint between the Γ and Κ points [11,12]. For the multi-layer WSe2 system, Fig. 2(e) shows that the LCM PL peaks are at 770 and 830 nm, and their origin is similar to that of the bi-layer WSe2 system peaks. The measured LCM PL intensities of the WSe2 flakes changed dramatically depending on the number of layers. The maximal LCM PL intensity for the mono-layer WSe2 system was ~7000 photon counts, as shown in Fig. 2(c), while those of the bi-layer and multi-layer WSe2 systems were ~450 and ~40 photon counts, respectively. The LCM PL intensity of the mono-layer WSe2 system was the strongest, and the intensity decreased dramatically with increasing the number of layers. Our results are in accordance with previously reported results [12–14].
The LCM Raman and LCM PL spectra of the fabricated pristine WSe2 and WSe2/Au-NP hybrid nanostructures were simultaneously measured and compared for determining and comparing the optical properties. Figures 3(a) and 3(b) show the LCM Raman mapping images of the pristine WSe2 and WSe2/Au-NP hybrids, respectively. The brightness of the LCM mapping images in Figs. 3(a) and 3(b) corresponds to the Raman signal intensity. We observed a variation in the LCM Raman intensities of the fabricated hybrid samples, indicating the effect of the number of WSe2 layers and that of the hybridization of Au-NPs.
Fig. 3 LCM Raman mapping images of (a) the pristine WSe2 and (b) the WSe2/Au-NP hybrid nanostructure. LCM Raman spectra of the pristine WSe2 (black curve) and the (c) mono-layer, (d) bi-layer, and (e) multi-layer WSe2/Au-NP hybrid nanostructures (red curve). (f) Schematic illustration of energy transfer mechanism of surface plasmons.
Figures 3(c)-3(e) show the LCM Raman spectra of the pristine WSe2 and mono-, bi-, and multi-layer WSe2/Au-NP hybrid nanostructures, respectively. In Fig. 3(c), showing the LCM Raman spectra of the pristine WSe2 mono-layer and the mono-layer WSe2/Au-NP hybrid nanostructure, the Raman characteristic peak for the mono-layer structure is observed at 250 cm−1, corresponding to the overlap of the out-of-plane (A1g) and in-plane (E12g) Raman modes. The LCM Raman intensities of the pristine mono-layer WSe2 and the mono-layer WSe2/Au-NP hybrid nanostructure were ~370 and ~650 photon counts, respectively. The results indicate that the 1.76-fold enhancement of the LCM Raman intensity for the mono-layer WSe2 system occurred via the hybridization of Au-NPs, owing to the energy transfer of surface plasmons. Figure 3(d) shows the LCM Raman spectrum (black curve) for the bi-layer WSe2 system. Similar to the LCM Raman characteristics of the mono-layer WSe2 system, two overlapping LCM Raman modes (the out of plane mode and the in plane mode) were observed at ~250 cm−1, and another LCM Raman peak corresponding to the inactive B2g mode was observed at ~310 cm−1, as shown in Fig. 3(d) [13]. The LCM Raman intensities of the pristine bi-layer WSe2 system and WSe2/Au-NP hybrid layers were estimated to be 85 and 180 photon counts, respectively, indicating the 2.1-fold enhancement of the LCM Raman intensity. A similar enhancement of the LCM Raman intensity was observed for the multi-layer WSe2 structure hybridized with Au-NPs (Fig. 3(e)). However, the enhancement ratio of the Raman intensity for the multi-layer WSe2 structure was not considerably high compared with that obtained for the mono- and bi-layer structures. It is noted that the bandwidths of the Raman spectra for both WSe2 and WSe2 Au-NP hybrid samples became broader with increasing the number of WSe2 layers. The enhancement of the Raman signals for structures consisting of WSe2 layers hybridized with Au-NPs indicates the realization of the surface-enhanced Raman scattering (SERS) effect that occurs owing to the energy transfer of surface plasmons [21,22]. Figure 3(f) shows a schematic illustration of energy transfer of surface plasmons based on energy band structure.
The LCM PL characteristics of two different batches of the pristine WSe2 and WSe2/Au-NP hybrids were investigated, and the results are shown in Figs. 4 and 5. Figures 4(a) and 4(b) show the LCM PL mapping images of the pristine WSe2 flake and WSe2/Au-NP hybrids, respectively. Figure 4(c) shows the variation in the LCM PL intensities of the corresponding samples. We clearly observed an increase in the LCM PL intensity from 6500 to 13500 photon counts for the WSe2/Au-NP hybrid nanostructures that occurred owing to the energy transfer of surface plasmons. The results qualitatively agree with those of the SERS effect in Fig. 3. However, in the case of a different batch sample, the brightness of the LCM PL mapping image of the WSe2/Au-NP hybrid nanostructure was weaker than that of the pristine WSe2, as shown in Figs. 5(a) and 5(b). A slight decrease in the LCM PL intensity was detected for the WSe2/Au-NP hybrids (Fig. 5(c)). This reduction in the LCM PL intensity in Fig. 5 might arise owing to the inhomogeneous and relatively thicker aggregation of the functionalized Au-NPs on the surface of the different batch WSe2 sample. This fluctuation, i.e., an increase or reduction in the LCM PL intensities for different batches of hybrids, was observed for four different batch samples. However, the SERS effect was observed for all WSe2/Au-NP hybrids.
Fig. 4 LCM PL mapping images of (a) the pristine WSe2 and (b) the WSe2/Au-NP hybrid nanostructure. (c) LCM PL spectra of the pristine WSe2 (black curve) and the WSe2/Au-NP hybrid nanostructure (red curve).
Fig. 5 LCM PL mapping images of (a) the pristine WSe2 and (b) the WSe2/Au-NP hybrid nanostructure. (c) LCM PL spectra of the pristine WSe2 (black curve) and the WSe2/Au-NP hybrid (red curve). All data in Fig. 5 were obtained for a different batch of samples.
4. Conclusion
For obtaining hybrid nanostructures of WSe2/Au-NP, dodecanethiol-functionalized Au-NPs were synthesized using the Brust method and spin-coated on mechanically exfoliated WSe2 flakes. Using a high-resolution LCM system, nanoscale Raman and PL characteristics were investigated for pristine WSe2 and WSe2/Au-NP hybrids. Enhancement of the LCM Raman intensities (i.e., the SERS effect) was observed for mono-, bi-, and multi-layer WSe2 structures, after hybridization with Au-NPs. This enhancement occurred owing to the energy transfer of surface plasmons. Clear 1.76-fold and 2.1-fold enhancements of Raman signals were observed for the mono-layer and bi-layer WSe2 structures hybridized with Au-NPs, respectively. For the multi-layer WSe2 structure, the SERS effect was relatively weak owing to the reduced energy transfer rate of surface plasmons. An increase in the LCM PL intensity for WSe2/Au-NP hybrids was sample-dependent. This study indicates that nanoscale optical properties, such as PL and Raman signals of 2-D WSe2, can be tuned by hybridization with metal nanostructures. In addition, this work shows the feasibility of fabricating high-efficiency surface plasmon-based sensors using 2-D WSe2.
Funding
National Research Foundation of Korea (NRF) funded by the Korea government (MSIP) (No. 2015R1A2A2A01003805; Center for Advanced Meta-Materials (CAMM) funded by the Ministry of Science; ICT and Future Planning as Global Frontier Project (CAMM-2014M3A6B3063710).
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