⇦Chinese Physics Letters, 2021, Vol. 38, No. 7, Article code 074203 Surface-Enhanced Raman Scattering of Hydrogen Plasma-Treated Few-Layer MoTe$_{2}$ Xiao-Xue Jing (景小雪)1, Da-Qing Li (李大庆)2, Yong Zhang (张勇)1, Xiang-Yu Hou (侯翔宇)1, Jie Jiang (蒋杰)1, Xing-Ce Fan (范兴策)1, Meng-Chen Wang (王梦晨)1, Shao-Peng Feng (冯少朋)1, Yuan-fang Yu (于远方), Jun-Peng Lu (吕俊鹏)1, Zhen-Liang Hu (胡振良)1*, and Zhen-Hua Ni (倪振华)1* Affiliations 1Department of Physics, Southeast University, Nanjing 211189, China 2Department of Electronic Engineering, Jiangnan University, Wuxi 214122, China Received 9 April 2021; accepted 14 May 2021; published online 3 July 2021 Supported by the National Natural Science Foundation of China (Grant Nos. 91963130, 11704068, 61927808, and 61705106), the National Key R&D Program of China (Grant No. 2019YFA0308000), the Fundamental Research Funds for the Central Universities (Grant Nos. 2242021k10009, 2242021R20037, and 2242021R20035), and the China Postdoctoral Science Foundation (Grant No. 2018M632197).
*Corresponding authors. Email: phyhzl@seu.edu.cn; zhni@seu.edu.cn Citation Text: Jing X X, Li D Q, Zhang Y, Hou X Y, and Jiang J et al. 2021 Chin. Phys. Lett. 38 074203    Abstract Two-dimensional surface-enhanced Raman scattering (SERS) substrates have drawn intense attention due to their excellent spectral reproducibility, high uniformity and perfect anti-interference ability. However, the inferior detection sensitivity and low enhancement have limited the practical application of two-dimensional SERS substrates. To address this issue, we propose that the interaction between the MoTe$_{2}$ substrate and the analyte rhodamine 6G molecules could be remarkably enhanced by the introduced p-doping effect and lattice distortion of MoTe$_{2}$ via hydrogen plasma treatment. After the treatment, the SERS is greatly improved, the enhancement factor of probe molecules reaches $1.83 \times 10^{6}$ as well as the limit of detection concentration reaches $10^{-13}$ M. This method is anticipated to afford new enhancement probability for other 2D materials, even non-metal oxide semiconductor SERS substrates. DOI:10.1088/0256-307X/38/7/074203 © 2021 Chinese Physics Society Article Text As an efficient and multi-functional technology for molecular detection, surface-enhanced Raman scattering (SERS) has been broadly used in catalysis chemistry,[1,2] environmental detection,[3,4] and the biological/medical sensing field.[5] The enhancement mechanism can be attributed to two well-known theories: electromagnetic mechanism (EM) and chemical mechanism (CM).[6] Compared with the EM, which derives from the local electromagnetic field generated by the surface plasmon resonance, the CM is concerned with the charge transfer between the substrate and the dye molecules. Some traditional noble material (Au,[7–9] Ag,[10,11] Cu[12]) substrates based on EM have obtained a remarkable SERS effect, however, the extensive applications of these materials are extremely limited by their cost, chemical stability, biocompatibility and surface uniformity. In past decades, 2D material substrates based on CMs, e.g., graphene,[13,14] h-BN[14] and MoS$_{2}$,[15] have been proved to be potential candidates for SERS substrates. In addition, they are much more stable, biocompatible and uniform compared to traditional noble metals. Nevertheless, the SERS efficiency of 2D materials is inferior because 2D material SERS substrates are based on a chemical mechanism. According to previous reports, the chemical mechanism enhancement factor is in the range of $10^{1}$–$10^{2}$ times,[6] far less than the electromagnetic mechanism ($10^{6}$). As a result, one of the most critical challenges now is to explore effective methods to optimize the SERS effect of two-dimensional layered materials. Numerous methods have been employed in 2D materials to improve the SERS effect, such as doping,[16,17] phase transition[18] and heterostructure formation.[19–21] Among them, chemical doping is a common and efficient method applied to create defects in 2D materials and alter the lattice constants and electronic band structure. Appropriate and reasonable doping can enhance the charge transfer between 2D materials and foreign molecules,[22] thus improving the SERS performance of two-dimensional materials. According to previous reports, doping can be induced in 2D materials by adjusting the CVD synthesis process,[23] electron beam irradiation,[24,25] laser burning,[26] and UV ozone irradiation.[27] However, these techniques have inevitable shortcomings. For instance, laser burning is limited by the low processing area and non-uniform surface of materials after treatment. Electron irradiation and UV ozone irradiation could effectively create defects in a large area, but it is difficult to control the doping degree precisely. The soft plasma technique, a flexible and controllable modification method for materials, has been used to create defect/sites,[28] induce phase transition,[29] and controllably thin of 2D materials.[30] Sun et al.[31] have demonstrated that plasma treatment could improve the SERS effect of MoS$_{2}$, which is attributed to p-doping effect-induced charge transfer enhancement between MoTe$_{2}$ and probe molecules. Moreover, it has been proved that plasma could modify the intrinsic characteristics of 2D substrates, such as crystal structure[32] and stoichiometry,[33] which contribute to better SERS performance. In this work, 2H-MoTe$_{2}$ flakes treated by soft hydrogen plasma present superior SERS effects with a remarkable limit of detection (LOD) as well as enhancement factor (EF). The LOD of rhodamine 6G (R6G) even reaches $10^{-13}$ M and the corresponding EF is as high as $1.83 \times 10^{6}$. Tellurium vacancies are generated by hydrogen plasma treatment and subsequently react with the ambient oxygen to produce Mo–O bonds, which result in p-type doping. The significant enhancement of MoTe$_{2}$ SERS substrates is related to the band-filling effect caused by p-type doping, which makes the top of the Mo valence band full of holes and improves the charge transfer between R6G molecules and MoTe$_{2}$ substrates. Oxygen molecules absorbed on the tellurium vacancies destroy the symmetry of the original MoTe$_{2}$ lattice and result in distortions of the lattice structure. Raman and x-ray photoelectron spectroscopy (XPS) demonstrate that lattice distortion of hydrogen plasma treated MoTe$_{2}$ and the introduction of p-doping have led to a significant SERS enhancement. The method of introducing p-doping and generating lattice distortion by plasma may be applicable to other 2D materials, even non-metal oxide semiconductor SERS substrates. Few-layer 2H-MoTe$_{2}$ samples are prepared by mechanical exfoliation and transferred on 300 nm SiO$_{2}$/Si substrates. The Raman spectra of pristine and hydrogen plasma-treated MoTe$_{2}$ (H$_{2}$-MoTe$_{2}$) films are displayed in Fig. 1(a). It shows that the pristine MoTe$_{2}$ films have three Raman vibration modes: in-plane $E_{\rm 2g}^{1}$ mode at $\sim $235 cm$^{-1}$, out-of-plane $A_{\rm g}$ mode at $\sim $174 cm$^{-1}$, and interlayer $B_{\rm 2g}$ mode at $\sim $290 cm$^{-1}$, respectively. Three Raman vibration modes indicate that the sample is in 2H phase. The Raman intensity ratio of $B_{\rm 2g}$/$E_{\rm 2g}^{1}$ of the pristine sample is 0.29, indicating that the MoTe$_{2}$ flake has three layers.[34] After hydrogen plasma process, the $E_{\rm 2g}^{1}$ and $A_{\rm g}$ peaks become broader and weaker. It can be ascribed to the defects in the materials induced by hydrogen plasma treatment.[35] On the other hand, vanishing of the $B_{\rm 2g}$ peak could be related to the weaker interlayer interaction that was induced by the distortion of MoTe$_{2}$ lattice.[36] The optical images of H$_{2}$-MoTe$_{2}$ and pristine MoTe$_{2}$ are displayed in the illustration of Fig. 1(a) and Fig. S1, respectively. The optical image shows that the hydrogen plasma treated sample is identical to the pristine one, which demonstrates that this processing technique is relatively mild. Schematic illustration of the 2D MoTe$_{2}$ materials treated by hydrogen plasma is shown in Fig. 1(b). During the treatment of the MoTe$_{2}$ surface, Te atoms overcome the binding force of Mo–Te bonds and produce the tellurium vacancies. The corresponding atomic force microscope (AFM) images are displayed in Figs. 1(c)–1(d). According to the analysis, the thickness of the pristine MoTe$_{2}$ is 2.6 nm, which can be considered as three layers as well, while the thickness increases to 3.0 nm after plasma treatment. Normally, oxygen molecules in air would not absorb on the MoTe$_{2}$ surface due to their weak physisorption. However, the stronger interaction between the tellurium vacancy and oxygen molecules promotes the quasi-chemisorption or chemisorption of oxygen molecules at the site of the vacancies.[35,37] Therefore, the increase of the layer thickness is related to the adsorption of oxygen molecules on tellurium vacancies, which is further demonstrated by XPS characterizations.
cpl-38-7-074203-fig1.png
Fig. 1. (a) Raman spectra of the 3L pristine MoTe$_{2}$ sample and H$_{2}$-MoTe$_{2}$ sample. The inset is the optical images of trilayer MoTe$_{2}$ flake. (b) Schematic illustration of the 2D MoTe$_{2}$ materials treated by hydrogen plasma. (c) and (d) AFM images of the pristine MoTe$_{2}$ and H$_{2}$-MoTe$_{2}$ films, respectively. Height profiles of MoTe$_{2}$ flakes are indicated in the images.
To explore the change of the atomic concentrations of H$_{2}$-MoTe$_{2}$, XPS spectra of Mo, Te and O are explored, as displayed in Fig. 2. Figure 2(a) reveals that the Mo $3d$ peak from the original sample can be fitted by two peaks: Mo $3d_{3/2}$ peak at $\sim $230.9 eV and Mo $3d_{5/2}$ peak at $\sim $227.8 eV, suggesting that the Mo atoms in original samples mainly exist at the $+$4-oxidation state.[38] After hydrogen plasma treatment, new doublet peaks at $\sim $232.5 eV and $\sim $235.3 eV appear, indicating the existence of a $+$6-oxidation state of Mo.[38] As displayed in Fig. 2(b), the Te $3d$ spectra have doublet peaks of the Te $3d_{5/2}$ peak at $\sim $572.4 eV and a Te $3d_{3/2}$ peak at $\sim $582.8 eV, resulting from the Mo–Te bonds in MoTe$_{2}$. New peaks at $\sim $586.3 eV and $\sim $576 eV appear after hydrogen plasma treatment, which are related to the $+$4-oxidation state of Te.[38] More importantly, the peaks deriving from Mo $3d$ of MoTe$_{2}$ have a red shift by 0.2 eV after plasma treatment compared to the pristine one, indicating the p-type doping of hydrogen plasma.[38,39] Te $3d$ XPS spectra prove the peak position movement as well. Moreover, the Te/Mo ratio extracted from the integral peak area of XPS spectra decreases from $\sim $2.01 to $\sim $1.89 after hydrogen plasma treatment, indicating the introduction of Te vacancies. Figure 2(c) shows the O $1s$ spectra of pristine MoTe$_{2}$ as well as H$_{2}$-MoTe$_{2}$ samples. The O $1s$ spectra from original MoTe$_{2}$ has one peak at $\sim $532.2 eV, which is ascribed to the absorbed H$_{2}$O on the substrate.[40] After hydrogen plasma treatment, a new peak at $\sim $530.1 eV appears, indicating the existence of Mo–O bonds.[40] Due to the strong electronegativity of oxygen atoms, the p-doing effect in H$_{2}$-MoTe$_{2}$ is assigned to the oxygen adhesion on the Te vacancy site. The production of the oxidation state of Mo and Te can be explained by the following process: high energy charged particles bombard the MoTe$_{2}$ surface during hydrogen plasma treatment, Te atoms overcome the binding force of Mo–Te bonds and produce the Te vacancies, then the appearance of tellurium vacancies promotes the absorption of oxygen molecules at the vacancy sites. Once the plasma treated MoTe$_{2}$ is taken out of the vacuum cavity, it quickly reacts with the oxygen in the air and oxygen molecules adsorb on the MoTe$_{2}$ substrate. The way that O$_{2}$ interacts with H$_{2}$-MoTe$_{2}$ is that one oxygen atom interacts with the two adjacent Te atoms, and the other oxygen atom is attached to the vacancy.[35] In addition, the adsorption of oxygen molecules destroys the symmetry of the original MoTe$_{2}$ lattice and generate lattice distortions. XPS characterization demonstrates the existence of the Te vacancies and the distortion of the MoTe$_{2}$ lattice, in consistency with the Raman measurements.
cpl-38-7-074203-fig2.png
Fig. 2. XPS data obtained from pristine MoTe$_{2}$ flakes and H$_{2}$-MoTe$_{2}$ flakes. (a) Mo $3d$ peak, (b) Te $3d$ peak and (c) O $1s$ peak from pristine MoTe$_{2}$ flakes and H$_{2}$-MoTe$_{2}$ flakes, respectively.
cpl-38-7-074203-fig3.png
Fig. 3. (a)–(d) The O $1s$ spectra of pristine MoTe$_{2}$ and those treated with plasma for 1, 3, 5 min. (e) Raman spectra of R6G on MoTe$_{2}$ and those treated with plasma for different times. (f) The relationship between Raman intensity of the R6G molecule as well as the concentration of the Mo–O peak with plasma treatment time.
The influence of p-doping on H$_{2}$-MoTe$_{2}$ SERS substrates is explored subsequently. Figures 3(a)–3(d) display the XPS spectra of MoTe$_{2}$ samples with different plasma treatment times (1, 3, and 5 min). The Mo–O peak ($\sim $530.1 eV) appears with an exposure time of 3 min, and the peak intensity of the Mo–O increases with the exposure time. A typical dye molecule rhodamine 6G (R6G) is adopted to investigate the SERS effect of hydrogen plasma-treated MoTe$_{2}$ samples. The samples are soaked in the solution of R6G ($10^{-7}$ M) for 40 min, and then rinsed by ethanol. The van der Waals force between R6G molecule and MoTe$_{2}$ makes R6G physically adsorb on the MoTe$_{2}$ substrate. The R6G molecule structure (inset) as well as its Raman spectra under various plasma processing times is shown in Fig. 3(e). Raman intensities of R6G increase with the plasma treatment time, in other words, the SERS effect increases with the increasing concentrations of Mo–O in MoTe$_{2}$. The atomic percentages of oxygen atoms contributed from Mo–O bonds are also calculated in quantitative comparison of the SERS degree of above four samples. Figure 3(f) summarizes the intensities of characteristic Raman modes of R6G molecules ($\sim $1361 cm$^{-1}$ and $\sim $1647 cm$^{-1}$) on the H$_{2}$-MoTe$_{2}$ SERS substrates and the atomic percentage of Mo–O bonds (green curve) as a function of plasma treating time. Firstly, no Mo–O peak content variation is observed, while it increases significantly after 1 min plasma treatment (the content of Mo–O peak is calculated by the ratio of integral peak area of Mo–O peak to the average integral peak area of all $\sim $532.2 peak). Similarly, the Raman intensity of $\sim $1361 cm$^{-1}$ and $\sim $1647 cm$^{-1}$ increases quickly after 1 min treatment. As displayed in Fig. 3(f), Raman intensities as well as Mo–O bonds exhibit the same rising trend with the increasing plasma treatment time. This indicates that the p-doping effect caused by oxygen adsorption on the surface of the MoTe$_{2}$ substrate has a significant effect on the SERS performance.
Table 1. Comparison of the SERS performances among various non-metal materials.
SERS material Analyte LOD(M) EF Laser line (nm) Reference
Graphene R6G $8\times10^{-9}$ Unavailable 514 14
n-doped graphene RhB $1\times10^{-11}$ Unavailable 514 17
Partially oxidized MoS$_{2}$ R6G 10$^{-7}$ $1.4\times10^{5}$ 532 43
1T$'$-MoTe$_{2}$ R6G $4\times10^{-13}$ $1.6\times10^{8}$ 532 42
a-ZnO nanocages 4-MBA 10$^{-5}$ $6.62\times10^{5}$ 633 44
Cu$_{2}$O superstructure R6G 10$^{-9}$ $8\times10^{5}$ 647 45
W$_{18}$O$_{49}$ nanowire R6G 10$^{-7}$ $3.4\times10^{5}$ 532 46
Plasma treated MoTe$_{2}$ R6G 10$^{-13}$ $1.83\times10^{6}$ 532 This work
cpl-38-7-074203-fig4.png
Fig. 4. (a) The comparison of Raman spectra of R6G ($10^{-7}$ M) coated on H$_{2}$-MoTe$_{2}$ and MoTe$_{2}$ and SiO$_{2}$/Si substrates. (b) Raman spectra of R6G deposited on H$_{2}$-MoTe$_{2}$ flakes (3 L) with concentrations ranging from $10^{-7}$ to $10^{-13}$ M. (c) Schematic illustration of the charge transfer process in the R6G molecules with the H$_{2}$-MoTe$_{2}$ substrate. (d) Schematic illustration of the 2D MoTe$_{2}$ as platforms for Raman enhancing of analytes.
Then, the EF and LOD of H$_{2}$-MoTe$_{2}$ are explored to quantitatively determine the enhancement effect. Raman spectra of R6G molecules on the SiO$_{2}$/Si substrate, pristine and hydrogen plasma-treated MoTe$_{2}$ flakes are given in Fig. 4(a). The background noise is subtracted for better comparison. In Fig. 4(a), several Raman peaks of R6G molecules ($10^{-7}$ M) are labeled by $\sim $773, 1183, 1308, 1361, 1505, 1570 and 1647 cm$^{-1}$. The Raman intensity of R6G on pristine MoTe$_{2}$ are weak as compared to the plasma-treated one. The Raman characteristic peak of R6G cannot be detected on the SiO$_{2}$/Si substrate. To characterize the interaction between the R6G and MoTe$_{2}$ flake, the Raman signals of R6G with different concentrations are collected as shown in Fig. 4(b). The spectra remain the same except that the Raman peak intensities increase as the concentration increases. It indicates that the R6G molecules do not strongly interact with the substrates, otherwise shifts of the R6G fingerprint peaks would appear.[41] The molecules could be well preserved by the non-destructive property of 2D semiconductor substrates, which is particularly important in molecular recognition and certainly an advantage for SERS substrates. In addition, Raman signatures originating from R6G molecules can also be perceived at the concentration of $10^{-13}$ M. In comparison to the original MoTe$_{2}$ substrate with LOD of $10^{-7}$ M (Fig. S2a) as well as other semiconductor substrates (Table 1),[42–45] the ultra-low LOD of H$_{2}$-MoTe$_{2}$ makes it an ultrasensitive SERS platform. To quantitatively evaluate the SERS performance of H$_{2}$-MoTe$_{2}$ substrates, EF is calculated according to the molecules Raman strength on the plasma-treated substrate compared with the bare SiO$_{2}$/Si substrate. It is noted that an ultrahigh Raman EF ($1.83 \times 10^{6}$) is reached for 1647 cm$^{-1}$ (the calculation process is given in the Supporting Information). Further investigation demonstrates that the strong SERS effect could also be preserved by other probe molecules, such as rhodamine B (RhB). The Raman signals of RhB are distinguishable at low concentration ($10^{-11}$ M), proving the ultrahigh SERS effect of the H$_{2}$-MoTe$_{2}$ for different probes (Fig. S3). The other plasmas as Ar (O$_{2}$) are also used to treat MoTe$_{2}$ samples to explore the Ar (O$_{2}$)-MoTe$_{2}$ substrate SERS effect, as displayed in Fig. S4. SERS effect of Ar (O$_{2}$) plasma-treated MoTe$_{2}$ substrates are the same as hydrogen plasma-treated substrates. It shows that the H$_{2}$ (Ar, O$_{2}$) plasmas have the same enhancement effect on MoTe$_{2}$ SERS substrates. According to the previous reports, the SERS effect of MoTe$_{2}$ substrates can be ascribed to the photon-induced charge-transfer resonance (PICT).[41] In general, PICT does not emphasize the chemical interaction between the surface and the dye molecules. Instead, it mainly depends on the energy difference between the excited level of the semiconductor and the affinity level of the adsorbed molecule. For the 2D semiconductor, the photon-induced charge-transfer resonance process between the semiconductor substrate and the molecule can be described by a three-step process. First, electrons in the semiconductor valence band (VB) are excited to the excited level as well as holes being created in the VB. Then, the electrons in the excited level absorb the incident photons energy and transfer to the affinity level of the probe molecule. After a short relaxation time, the electrons return to the VB. Currently, electrons keep in an excited state. Finally, the excited electrons recombine with the holes in the VB and emits a Raman photon. Figure 4(c) gives the schematic illustration of the charge transfer process between R6G molecules and the MoTe$_{2}$ substrate. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of R6G molecules are situated at $-5.7$ and $-3.4$ eV, and the conduction band (CB) and VB levels of MoTe$_{2}$ are situated at $-3.91$ and $-4.99$ eV, respectively.[13,46] The VB of MoTe$_{2}$ is much lower than the affinity level of R6G, therefore the PICT process between the MoTe$_{2}$ substrates and R6G molecule could occur. In addition, the electron transition from HOMO levels to LUMO levels of R6G molecules $(\mu_{\rm mol}\approx 2.3\,\mathrm{eV})$ can resonate with incident photons, which also enhance the charge transfer. Hence, it can be concluded that exciton resonance ($\mu_{\rm ex}$) of H$_{2}$-MoTe$_{2}$, molecule resonance ($\mu_{\rm mol}$) of R6G as well as the photon-induced charge transfer resonance (PICT) all contribute to the Raman enhancement in the H$_{2}$-MoTe$_{2}$/R6G system. Raman and XPS results confirmed that oxygen absorption at the vacancy site after plasma treatment could induce p-doping and lattice distortion in MoTe$_{2}$. The VB of MoTe$_{2}$ has numerous holes after plasma treatment. It accelerates the recombination of the excited electrons transferred from the affinity level of R6G and holes in the VB, and thus promotes the charge transfer process and strengthen the SERS effect of MoTe$_{2}$ substrates. Furthermore, the distortion of the MoTe$_{2}$ lattice would weaken the surface potential according to previous reports.[43] More electrons would be transferred from the MoTe$_{2}$ surface to the probe molecules, and eventually improve the SERS signal. Figure 4(d) shows the schematic illustration of H$_{2}$-MoTe$_{2}$ films as SERS platforms. The SERS effect of R6G molecules on the plasma-treated MoTe$_{2}$ substrate is attributed to the charge transfer between them. In summary, we have proved that the hydrogen plasma treatment can remarkably increase the SERS performance of 2D MoTe$_{2}$ substrates. The detection limit concentration of probe molecules on H$_{2}$-MoTe$_{2}$ can reach the femtomolar level of $10^{-13}$ M, and the Raman EF can reach $1.83 \times 10^{6}$, which is far beyond previously reported SERS materials such as graphene and non-metal oxide semiconductor substrates. The SERS effect is monotonously enhanced with the increased plasma treatment time. More importantly, our results indicate that the method of generating defects by hydrogen plasma treatment could be expanded to other 2D semiconductor materials, even non-metal oxide semiconductor SERS substrates, which paves the way for the widespread application of semiconductor-based SERS substrates in various fields. 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