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In this work, a facile and effective method for controlled-layer and large-area MoS2 films encapsulated Au nanoparticle hybrids is developed. With accurate Ar plasma treatment time control, the large-area MoS2 layers can be obtained from monolayer to trilayer. The fabricated MoS2@Au NPs with higher surface area exhibit excellent Raman enhanced effect for aromatic organic molecules (rhodamine 6G and crystal violet) and achieve the best when the monolayer MoS2@AuNPs was obtained. The limit of detection is found to be as low as 1 × 10−10 M. The MoS2@AuNPs was characterized by SEM, EDS, AFM, Raman spectroscopy, UV-Vis, XRD and HRTEM.
© 2016 Optical Society of America
1. Introduction
Atomically thin 2D transition metal dichalcogenides (TMDCs) with a large surface area and small thickness offer great potential for a series of fundamental and technological studies due to their unique layered 2-dimensional structures and desirable optical/electrical properties [1–5]. Molybdenum disulfide (MoS2), one of the most promising 2D TMDCs, exhibits a unique 2D S-Mo-S structure and has attracted considerable attention [6,7]. Monolayer or few-layer MoS2 crystals are widely used in clean energy, spintronics, field-effect transistors and optoelectronic devices [1,3,8–10].
To further extend the application potentials of 2D MoS2, it is reasonable to integrating it with metal nanoparticles (NPs) which is a special way to control their optical and photoelectrical properties. At this point, noble metal (Au, Ag and Cu) NPs have been as the target study for these NPs show remarkable chemical and physical properties which was widely adopted in biosensor, catalysis and photonics [11–14]. So, it is expected that MoS2 films combine with metal NPs could walk out an added way for modulating their magnetic, optoelectronic, catalytic and electronic functionalities [15–18]. Even though the metal NPs decorated on the MoS2 flakes have been prepared successfully [17,18], the surface area of MoS2 to interface with chemical molecules was reduced. We further improved the surface area of MoS2 in this work and this is one of the contributions. Among the applications of MoS2 film combine with metal NPs, Surface enhanced Raman scattering (SERS) is one of the most promising research directions in the field of chemistry, biomedicine physics, and biology [18–23]. Considerable efforts have been made towards designing sensitive SERS substrates [19,24,25]. Nonetheless, it is still a challenge to design a sensitive and stable SERS substrate to detect aromatic organic molecules for the low adsorption characters of these molecules on metal NPs [24,26,27].
In this work, we fabricated a controlled-layer and large-area MoS2 films coated Au NPs hybrids (MoS2@Au NPs) instead of surface modification or decoration metal NPs on MoS2. Herein, the MoS2@Au NPs with active surface properties were developed by a facile two-step process. Firstly, this approach uses annealing method to obtain Au NPs. The Au NPs distributed more uniformly with narrower gaps compared to chemical reduction on MoS2. Secondly, the MoS2 is obtained by annealing the ammonium tetrathiomolybdate ((NH4)2MoS4) which is coated on the surface of Au NPs. This method makes the large-area MoS2 film shell on the Au NPs tightly and seamlessly. Besides, the structure of MoS2 coated Au NPs get the bigger surface area compared to 2D MoS2 films decorated with metal NPs, which makes them more suitable to interface with aromatic organic molecules. Thirdly, the number of MoS2 layers coated on the Au NPs can be well controlled with different Ar plasma irradiation time. Thus, the thickness effect of MoS2 for SERS can be investigated. The fabricated MoS2@AuNPs show excellent Raman enhanced effect for aromatic organic molecules (rhodamine 6G (R6G) and crystal violet (CV)) and achieve the best when the monolayer MoS2@AuNPs was obtained. The detection limit is discovered to be as low as 1 × 10−10 M. As far as we know, it is the first realization of controlled-layer 2D TMDCs coated metal NPs as effective SERS substrate. This paper pave the way for applying 2D TMDCs in a diverse detection and sensing fields including biomolecule detection, environmental monitoring, food security etc.
2. Experimental setup
2.1 Preparation of MoS2@AuNP substrates
Around 12 nm Au films were deposited (1 Å∕s rate, 10−3 Pa) using a thermal-evaporation system. The zone quartz tube was pumped down to 10−3 Pa before the annealing process. The samples were moved to the constant temperature when the quartz tube was heated to 600 °C with a flowing of Ar/ H2 around 100/20 sccm. The Au film was transformed into Au NPs after the annealing process. 1mL of glycol was added to high purity (NH4)2MoS4 powder (0.01 g) to gain a 1 wt% solution. The (NH4)2MoS4 solution was well dispersed after 20 min ultrasonic treatment. Then (NH4)2MoS4 film was coated on the surface of Au NPs using a spinner with 2000 rmp rotating speed. In order to obtain the MoS2 film, we carried out the next annealing process at 500°C with gas flowing (Ar/H2 around 80/20 sccm). After 60 min reaction, the samples cooled to ambient temperature slowly forming the structure of MoS2 @Au NPs.
2.2 Argon-plasma treatment
The MoS2@Au NPs substrates were treated by Ar plasma. Ar plasma (13.56 MHz RF source) with power of 10 W and pressure of 10 Pa is used to modify and thin the MoS2 films at room temperature.
2.3 Theoretical modeling
In order to investigate near-field electromagnetic field of MoS2@Au NPs with different number of MoS2 layers, the finite element method (COMSOL Multiphysics software) was used. The electric field was added in the x direction. A plane wave (532 nm) irradiates down to MoS2@Au NPs from the z direction. MoS2@Au NPs with the diameter of 35 nm Au NPs and 10 nm gaps shelled by MoS2 layer (monolayer ~0.65 nm, bilayer ~1.30 nm, trilayer ~1.95 nm) on SiO2 substrate is designed. The dielectric constant is = 5.9000 + i1.4000 at 532 nm for the MoS2 layers obtained from the experimental measurement data by Castellanos-Gomez et al [35].
2.4 SERS experiments
The R6G and CV molecules were deposited on the substrates by dipping the substrates into the R6G and CV aqueous solutions for three hours, rinsed with deionized water, and dried naturally. Then, the substrates were taken out for SERS measurements. The Raman signals were obtained under the same condition (objective lens of 100 × , 532 nm excitation, 4.8 mw laser power, acquisition time: 4 s).
2.5 Characterization
Scanning electron micros-copy (SEM, Zeiss Gemini Ultra-55) was used to observe the surface morphologies of the MoS2@Au NPs and the energy-dispersive X-ray spectroscopy (EDX) was used for chemical composition analysis. Raman spectrometer (Horiba HR Evolution 800) with laser excitation (2.4mW laser power, 532nm excitation, acquisition time:4 s etc) was used to the Raman analysis. Ultraviolet-uisible spectrophotometer (UV-Vis Spectrophotometer, U-4100) was used to collect UV-Vis absorption spectra of MoS2@Au NPs and Au NPs. Atomic force microscopy (AFM, Park XE100) was used to obtain the morphological changes of MoS2@Au NPs. The crystal structure of MoS2@Au NPs was characterized by X-ray diffraction (XRD, Rigaku D/MAX-RB). The high resolution Transmission Electron Microscope (HRTEM) images of MoS2@Au NPs were obtained using a transmission electron microscopy system (JEOL, JEM-2100) operated at 100 kV.
3. Results
Here, we obtained the Au NPs by annealing the Au film which is about 12 nm as shown below. The uniform and large-area MoS2 films were synthesized by a thermally decomposed (NH4)2MoS4 layer as shown in Fig. 1. With this approach, around three-layer MoS2 films covering the Au NPs homogeneously were easily obtained. The corresponding Raman spectra, SEM and AFM of trilayer MoS2@Au NPs are shown in Fig. 2.
Fig. 2 (a) AFM image of the Au film and its corresponding height profile (inset) (b) Raman spectrum of MoS2@Au NPs (red line) and MoS2 on SiO2 (black line). (c) SEM image of MoS2@Au NPs at a low magnification. (d) SEM image of MoS2@Au NPs at a higher magnification. Inset: The size histograms of MoS2@Au NPs. (e) EDX data of the same sample. (f) AFM images of the MoS2@Au NPs substrates.
The Raman spectra in Fig. 2(b) show the E12g and A1g peaks of MoS2 at 387 cm−1 and 410 cm−1 respectively. The E12g and A1g peaks are considered as the characteristic peaks of MoS2. The frequency difference () between E12g and A1g can estimate the number of MoS2 layers [28]. The value obtained is around 23 cm−1 corresponding to the three-layer MoS2. The full width at half maximum (FWHM) of the two characteristic peaks (E12g and A1g) could reflect the crystallization quality of MoS2. Narrower FWHM demonstrate the higher crystallization of the materials. Here, the FWHM of the E12g and A1g are only approximately 3 cm−1 and 5.3 cm−1, which indicates the high crystallization of the MoS2 film. We also found that the Raman intensity of the MoS2 on the Au NPs (red line) is much stronger compared to the MoS2 on the SiO2 (black line). The Raman signals of MoS2 film are greatly enhanced, indicating the better enhancement effect of Au NPs which is barely found in 2D MoS2 films decorated with Au NPs [29, 30]. It also suggested that the MoS2 film was coated on the surface of Au NPs closely, because the space between MoS2 and Au NPs will unavoidably lead to obvious loss of electromagnetic enhancement activity. To observe MoS2@Au NPs intuitively, SEM was adopted as shown in Figs. 2(c) and 2(d). These nanoparticles with narrow gaps distribute relatively uniform. Note that the size distribution of the NPs is mostly at around 25–35 nm which accords with a Gaussian profile as shown in the inset of Fig. 2(d). The size of NPs could be controlled with different thickness of Au film and annealing temperature. EDX in Fig. 2(e) provides powerful and obvious evidence that MoS2 films wrap the Au NPs and the MoS2 film exhibits a rational chemical composition (atom ratios Mo: S around 1:2). The morphology of the MoS2@Au NPs was performed by the AFM as shown in Fig. 2(f). The top of the nanoparticles is pretty sharp and the MoS2 coated Au NPs get the bigger surface area compared to 2D MoS2 films, which is supposed to have great Raman enhancement for molecule probes.
After the successful synthesis of MoS2@Au NPs, we put the samples to Ar plasma treatment with the aim to control the number of layers of the MoS2 films. However, except direct thinning the MoS2 film, we note that the Ar plasma can modify the structure of the films effectively. We firstly treated the MoS2@Au NPs substrate five minutes using Ar plasma. Raman spectra of MoS2@Au NPs (black) and MoS2@Au NPs after Ar plasma treatment (ATMoS2@Au NPs, red line) are shown in Fig. 3(a). From the Raman spectra, we note that the frequency difference between the two prominent Raman modes (E12g and A1g) becomes larger for the treated samples by Ar plasma. The XRD was used to further characterize the crystalline structure of ATMoS2@Au NPs. Figure 3(b) exhibits the four strikingly peaks at 2θ = 38.1°, 44.3°, 64.5° and 77.5°assigned as the (111), (200), (220) and (311) of the gold crystals separately (a = 0.408 nm, c = 0.408 nm, Powder diffraction file no. 04-0784). Whereas any peaks originating from potential Au oxides cannot be observed. In addition, the broadening of the Bragg peaks demonstrates the formation of Au NPs. The characteristic peak of the MoS2 films at 15.6°which shifts 1.3°from 14.3°is observed clearly and assigns to the (002) reflections. The presence of (002), combined with the absence of (103) and (105) lines, indicates that the structure of MoS2 films are actually disordered [31]. According to the results of Raman signals and XRD, we found that the Ar plasma treatment process could create the structural disorders of MoS2 films. The plasma bombardment distorts chemical bonds in MoS2 layer and further break the original symmetry of MoS2 crystal. Shen et al. has observed similar phenomenon on mechanically exfoliated MoS2 nanoflakes [32].
Fig. 3 (a) Raman spectrum of MoS2@Au NPs after Ar plasma treatment (red line) and MoS2@Au NPs (black line). (b) XRD patterns for the MoS2@Au NPs substrates after Ar plasma treatment.
Besides create the structural disorders of MoS2 films on Au NPs, the large-area MoS2 film thickness could be controlled by carefully controlling the Ar plasma irradiation time. Figure 4(a) shows Raman spectra of MoS2@Au NPs after Ar plasma treatment with 10 min, 15 min and 20 min. After 10 min treatment, the A1g peak red shifts, while the E1 2g peak blue shifts, and both of the two Raman peaks decrease. The frequency difference between E12g and A1g peaks changes from 23 cm−1 to 22 cm−1 (black line) and to 21 cm−1 (blue line) after 15 min which corresponds to the bilayer MoS2 film. Finally, the frequency difference becomes 19 cm−1 (nearly single-layer) after 20 min Ar plasma treatment as shown in Fig. 4(a). This possible factor may explain this thickness decrease phenomenon: The S-Mo-S bonding in MoS2 is not strong enough and the interlayer interaction between MoS2 layers is weak. Thus, the Ar plasma treatment conduce defects in the top layer MoS2 and make its structure disordered easily in 5 min and it further decreases thickness of MoS2 for the longer irradiation time. To demonstrate the homogeneity of the MoS2 film on the Au NPs after 20 min Ar plasma treatment, the 20 × 20 μm2 Raman A1g band mappings are obtained as shown in Fig. 4(b). The intensity of A1g peak located between 321 and 327. The small intensity differences and color variations of the A1g mode indicate the well uniformity of the MoS2 film. The thickness of the MoS2 shell and detailed structural information of nearby interface of MoS2@Au NPs was further exhibited by HRTEM image (Figs. 4c and 4d). Figures 4c and 4d exhibit the MoS2@Au NPs obtained from 15 min and 20 min Ar plasma treatment respectively. A HRTEM analysis of folds at the edges of MoS2 film can give the number of MoS2 layers by direct visualization. We clearly note that the MoS2 films coated on the Au NPs become bilayer after 15 min treatment shown in Fig. 4(c). Furthermore, as shown in Fig. 4(d), the HRTEM image derived from the edge of MoS2@Au NPs hybrids after 20 min treatment exhibits only one line, indicative of monolayer MoS2. The result of TEM is identical with the Raman analysis. Besides, it is apparent that Au particles are covered by thin MoS2 film seamlessly suggesting the formation of controlled-layer MoS2-wrapped Au NPs hybrids.
Fig. 4 (a) Raman spectra of the MoS2@Au NPs after Ar plasma irradiation with different time. (b) the scanning Raman A1g band mappings of the MoS2@Au NPs after 20 min Ar plasma treatment. HRTEM images of cross section of bilayer (c) and single-layer (d) MoS2 film grown on Au NPs.
We used UV−Vis spectroscopy to characterize the localized surface plasmon resonance (LSPR) properties of the MoS2@Au NPs as shown in Fig. 5(a). The positions of the wavelength of resonance absorption λR, the intensity of absorption peaks and FWHM of UV-Vis absorption spectra of each sample are also summarized in Table 1. Compared with the Au NPs, we note that the λR of the MoS2@Au NPs with three layers of MoS2 only showed slight broadening in peak width, together with a slight red-shift from 547 nm to 564 nm. The slight red-shift in the peak position likely ascribe to an increase in edge thickness with MoS2 film coating. However, the peak position of λR shift from 564 nm to 560 nm (blue shift) for bilayer-MoS2@Au NPs and to 558 nm (blue shift) for monolayer-MoS2@Au NPs. The blue-shift was due to the decreased number of MoS2 layers induced by Ar plasma treatment time. Meanwhile, the FWHM of absorption peak was narrowed from 79 nm to 78.1 nm for bilayer-MoS2@Au NPs and to 76.5 nm for monolayer-MoS2@Au NPs. This slightly change suggests that an ultrathin shell of MoS2 would not cause significant changes to the dielectric constants. Table 1 also shows the variations of absorbance of each sample, and the absorbance of MoS2@Au NPs reduces a lot compared to the Au NPs. However, it is corresponded to the high SERS activity indirectly in the absorbance spectra, which has been proved in theory by Le Ru et al [33]. We discussed it in experiments in this paper. Figure 5(b) shows the SERS performance of monolayer MoS2@Au NPs (red) and Au NPs (black). Raman scattering signal of R6G obtained from the monolayer MoS2@Au NPs is hugely enhanced with fluorescence quenching compared with the Au NPs, which is consistent with the theory.
Fig. 5 (a) the corresponding UV-Vis absorption spectra of Au NPs (red), trilayer-MoS2@Au NPs (green), bilayer-MoS2@Au NPs (black) and monolayer MoS2@Au NPs (purple). (b) SERS performance of monolayer MoS2@Au NPs (red line) and Au NPs (black line) using R6G as probe molecules.
Table 1. Results of UV-Vis analysis for samples
After successfully fabricating the MoS2@Au NPs with controllable MoS2 layers, we explored the SERS performance of these nanoparticles with different MoS2 layers. R6G molecules were deposited on the MoS2@Au NPs substrate by dipping the substrates into the R6G solutions (1 × 10−5 M) for three hours, rinsed with deionized water, and dried naturally. The R6G molecules contact with MoS2 films by the π – π bond [27, 34]. Figure 6(a) shows the Raman spectra of R6G obtained from MoS2@Au NPs substrates with monolayer, bilayer and trilayer MoS2 film coating. We note that the Raman signals of the R6G molecules slightly increase with the number of MoS2 layers decreasing. The MoS2@Au NPs with monolayer MoS2 film coating performs the best effect of Raman enhancement. Two possible factors may explain this thickness dependent Raman enhancement: (1) Chemical enhancement of MoS2 film gets better as the 2D material become thinner, which has been proved by Shen et al [32]. (2) Considering the intensity of electromagnetic field decays fast with space between Au NPs and aromatic molecules, the thinner the MoS2 film coated on the Au NPs should generate the higher SERS activity.
Fig. 6 (a) Raman spectra of R6G (10−5) on monolayer-MoS2@Au NPs, bilayer-MoS2@Au NPs and trilayer-MoS2@Au NPs respectively. The x-z views of electric field distributions on the SiO2 substrate with 35 nm MoS2 @Au NPs and 10 nm gap. (b) Au NPs covered with monolayer MoS2. (c) Au NPs covered with bilayer MoS2. (d) Au NPs covered with trilayer MoS2.
In order to investigate near-field electromagnetic field of MoS2@Au NPs with different number of MoS2 layers, the COMSOL Multiphysics software was used. The x-z views of electric field distributions on the SiO2 substrate with 35 nm MoS2@Au NPs and 10 nm gaps are shown in Fig. 6. Figure 6(b) shows the electric field distributions of Au NPs coated with monolayer MoS2. The high-density hotspot appears in the gap between the nanoparticles due to the coupling effect and the intensity of electromagnetic field can be enhanced 12.1 times. Moreover, the electromagnetic enhancement of the samples with bilayer MoS2@Au NPs is enhanced 11.6 times as shown in Fig. 6(c) which is a little smaller than that with monolayer MoS2 and higher than that with trilayer MoS2@Au NPs (11.4 times) as shown in Fig. 6(d). The extra dielectric loss of monolayer MoS2 film ( = 5.9000 + i1.4000 at 532 nm) obtained from the experimental measurement data by Castellanos-Gomez et al can be responsible for the slightly decrease of the electromagnetic field enhancement [35]. The electric field intensity of Au NPs coated with thickness of MoS2 layers (d = L × 0.65 nm) is given. Thus, the electric field enhancement produced by Au NPs would be weaker as the number of MoS2 layers increase. This result is in well accordance with the SERS performance of MoS2@Au NPs.
After successfully fabricating the MoS2@Au NPs with monolayer MoS2 film, we investigate the feasibility of these MoS2@Au NPs as SERS substrates to detect the probe molecules such as CV and R6G. The R6G and CV molecules were deposited on the substrates by dipping the substrates into the R6G and CV aqueous solutions (1 × 10−6 M) for three hours, rinsed with deionized water, and dried naturally. For comparison, the R6G and CV molecules were also deposited onto the SiO2 substrate and MoS2 film for Raman detection under the same condition. No vibration can be observed except the strong fluorescent background for the R6G on SiO2 substrate (Fig. 7a, red curve). Lower fluorescent background can be detected in the Raman signal of R6G collected from the MoS2 film (Fig. 7a, blue curve). In comparison, the R6G spectrum obtained from MoS2@Au NPs substrate exhibit high intensity, distinct peaks and low fluorescent background (Fig. 7a, black curve). It demonstrates the significance of Au NPs in the MoS2@Au NPs SERS substrate for the Raman signal detection. We can observe the similar phenomenon in the Raman signals of CV molecules as shown in Fig. 7(b). The combination of chemical enhancement (CM) and electromagnetic enhancement (EM) contribute to the result. The EM originates from electromagnetic coupling of the localized surface plasmon resonance of Au NPs. The CM may attribute to the charge transfer between the MoS2 and probe molecules.
Fig. 7 Raman spectra of (a) R6G (10−6 M) and (b) CV (10−6 M) on SiO2, MoS2/SiO2 and MoS2@Au NPs substrates, respectively. SERS spectra of (c) R6G and (d) CV at nine different positions on the same MoS2@Au NPs substrate.
More importantly, the MoS2@Au NPs SERS substrates show excellent detection capability. For example, the characteristic peaks of analytes can be observed more clearly and the detection of lower concentration can be achieved. Compared with SERS spectra of Au NPs, the spectral features of the R6G (such as the 1317, 1368, 1513, 1656 cm−1 etc) are enhanced apparently in high frequency region of 1000–1700 cm−1. The Raman scattering signal of the MoS2@AuNPs is hugely enhanced with fluorescence quenching as shown in Fig. 5(b). Besides, the MoS2@Au NPs substrate exhibits excellent detection capacity for low concentration analytes. For example, the limit of detection for the R6G and CV is found to be as low as 1 × 10−10 M and 1 × 10−9 M respectively as shown in Figs. 8(a) and 8(b). Compared with surface modification or decoration of MoS2 SERS substrate, the detection ability of MoS2@Au NPs is stronger [29, 30].
Fig. 8 show the Raman spectra of (a) R6G and (b) CV molecules at concentration of 1 × 10−10 M and 1 × 10−9 M respectively. (c) SERS spectra of the R6G without (black) and with (red) the oxidation treatment. (d) XRD result of the MoS2@Au NPs without (black) and with (red) the oxidation treatment.
Furthermore, we measure the stability of the MoS2@Au NPs by subjecting it to aerobic exposure for 10 days. R6G with a concentration of 10−5 M was used as the probe molecules. After the expose of oxygen, the negligible change of the Raman signal suggest superior antioxidant ability of MoS2@Au NPs. We also used XRD to monitor the stability of the MoS2@Au NPs when they were exposed to a strong oxidant for 10 days as shown in Fig. 8(d). It was found that any peaks originating from potential Au oxides cannot be observed. These results indicate that the temporal stability of MoS2@Au NPs SERS substrates.
4. Summary
Herein, MoS2-wrapped Au nanoparticle hybrids (MoS2@Au NPs) were first prepared as SERS substrates. The annealing methods make large-area MoS2 film tightly and seamlessly coat the Au NPs which effectively decrease the loss of electromagnetic enhancement activity. Then, the thickness of MoS2 film can be controlled under different Ar plasma irradiation time. The fabricated MoS2@Au NPs exhibit excellent Raman enhanced effect and achieve the best when the monolayer MoS2@Au NPs was obtained. The MoS2@Au NPs was proved to be excellent SERS substrates for sensitive detection of aromatic molecules.
Funding
National Natural Science Foundation of China (NSFC) (11474187, 11274204, 11674199, 11504209); Excellent Young Scholars Research Fund of Shandong Normal University.
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