Phase Transformation on Two-Dimensional MoTe₂ Films for Surface-Enhanced Raman Spectroscopy

Abstract

Two-dimensional (2D) transition metal dichalcogenides (TMDs) have recently become attractive candidate substrates for surface-enhanced Raman spectroscopy (SERS) owing to their atomically flat surfaces and adjustable electronic properties. Herein, large-scale 2D 1T′- and 2H-MoTe₂ films were prepared using a chemical vapor deposition method. We found that phase structure plays an important role in the enhancement of the SERS performances of MoTe₂ films. 1T′-MoTe₂ films showed a strong SERS effect with a detection limit of 1 × 10⁻⁹ M for the R6G molecule, which is one order of magnitude lower than that of 2H-MoTe₂ films. We demonstrated that the SERS sensitivity of MoTe₂ films is derived from the efficient photoinduced charge transfer process between MoTe₂ and adsorbed molecules. Moreover, a prohibited fish drug could be detected by using 1T′-MoTe₂ films as SERS substrates. Our study paves the way to the development and application of high-performance SERS substrates based on TMD phase engineering.

1. Introduction

The origins of surface-enhanced Raman spectroscopy (SERS) can be traced back to 1974 when Fleischmann et al. discovered the enhanced Raman signals of pyridine molecules on an electrochemically roughened silver electrode [1]. As a powerful trace analysis technique with highly sensitive, label-free, and non-destructive detection, SERS has been successfully applied in bioanalysis, catalysis, food safety, environmental monitoring, and other fields [2,3,4,5,6]. Currently, there are two main mechanisms used to enhance SERS: the electromagnetic enhancement mechanism (EM) and the chemical enhancement mechanism (CM). Conventionally, noble metals (gold or silver) with roughened surface morphologies are widely used SERS materials, which exhibit high SERS sensitivity owing to the EM caused by a strong localized surface plasmon resonance (LSPR) effect [7]. However, their high price, poor signal uniformity, and complicated “hotspots” (e.g., high-intensity electromagnetic field regions formed at nanoscale gaps) regulation limit their widespread applications [6]. Hence, the development of high-performance noble metal-free SERS substrates is urgently needed.

In the last ten years, noble metal-free SERS materials, such as graphene [8], transition metal oxides [9,10], transition metal carbides or nitrides (MXenes) [11], transition metal borides (MBenes) [12], metal–organic frameworks [13], perovskites [14], graphdiyne [15], and organic semiconductors [16], have received great attention. The SERS effect in these noble metal-free materials is mainly ascribed to the CM, which originates from the charge transfer (CT) between the materials and the adsorbed molecules [2]. In particular, two-dimensional (2D) transition metal dichalcogenide (TMD) materials with layered features are considered as promising SERS materials to replace the noble metals due to their atomically flat surfaces, superior stability, biocompatibility, adjustable electronic properties, and ease of fabrication [6]. Until now, various TMD materials such as MoS₂ [17], MoSe₂ [18], MoTe₂ [19], WS₂ [20], WSe₂ [21], TaSe₂ [22], and NbS₂ [23] have been used as SERS materials. For instance, Meng et al. developed WS₂ SERS substrates with a limit of detection (LOD) as low as 10⁻⁷ M for rhodamine 6G (R6G) molecules [24]. Ge et al. reported that atomically thin TaSe₂ films exhibited excellent SERS performance with an LOD of 1 × 10⁻¹⁰ M for R6G molecules [22]. In order to improve the SERS sensitivity of TMD materials, some strategies such as defect engineering [21], alloy engineering [25], phase engineering [20], and heterojunction engineering [26] have been developed. For example, Tang et al. reported an alloy engineering strategy to improve the SERS efficiency of a target molecule by manipulating the energy levels of the SERS substrates [25]. Guan et al. reported phase-engineered 2M-WS₂ SERS substrates exhibited an LOD for crystal violet (CV) molecules of 10⁻⁸ M by tuning the Fermi level of WS₂ [20]. Lan et al. designed WN/monolayer MoS₂ heterostructure chips with an LOD as low as 10⁻¹⁰ M for R6G molecules [27]. However, the effect of phase transformation on chemical vapor deposition (CVD)-grown large-area 2D MoTe₂-based SERS substrates is rarely reported.

Herein, we synthesized large-scale 2D MoTe₂ films with different phases (1T′ and 2H phases) using a CVD method. We found that 1T′-MoTe₂ films have higher SERS sensitivity than 2H-MoTe₂ films. The Raman enhancement factor (EF) of 1T′-MoTe₂ films’ detection of R6G molecules could be up to 1.9 × 10⁶ with a limit of detection up to 10⁻⁹ M, which could be comparable to noble metal SERS substrates. The underlying enhancement mechanisms of these MoTe₂ films were also explained. In addition, a prohibited fish drug was detectable on 1T′-MoTe₂ films, indicating the potential of 1T′-MoTe₂ films in addressing the problems of food safety. This work provides the way to design high-performance TMD-based SERS substrates.

2. Results and Discussion

2.1. Characterization of MoTe₂ Films

The structures of MoTe₂ films were characterized by using Raman spectroscopy first. In the Raman spectra of 1T′-MoTe₂ films (Figure 1a), five characteristic Raman peaks of 1T′-MoTe₂ at 107.7 (A_u), 129.7 (A_g), 162.5 (B_g), 191.4 (B_g), and 260.2 cm⁻¹ (A_g) were observed [28,29]. For 2H-MoTe₂ films, the four Raman peaks at 118.1, 171.3, 234.5, and 288.8 cm⁻¹ originated from the E_1g, A_1g, E¹_2g, and B¹_2g vibration modes of 2H-MoTe₂ [29]. XPS was used to analyze the elemental compositions and chemical states of MoTe₂ films. In the survey XPS spectra of MoTe₂ films, the signals of Mo, Te, and O elements were observed (Figure 1b). For 1T′-MoTe₂, the high-resolution Mo 3d core-level photoemission peak could be fitted with three doublet peaks, which were attributed to Mo-Te (227.0, 230.2 eV), Mo-O (Mo⁵⁺) (227.8, 231.0 eV), and Mo-O (Mo⁶⁺) (231.7, 234.9 eV), respectively (Figure 1c) [30]. As shown in Figure 1d, the Mo 3d XPS spectra of 2H-MoTe₂ could be fitted by three spin–orbit doublets, corresponding to Mo-Te (227.3, 230.5 eV), Mo-O (Mo⁵⁺) (228.1, 231.3 eV), and Mo-O (Mo⁶⁺) (231.8, 235.0 eV), respectively. Compared with 1T′-MoTe₂ films, there was a shift (~0.3 eV) to a higher binding energy for 2H-MoTe₂ films, which is consistent with previous work [29]. In Figure 1e, the Te 3d XPS spectra of 1T′-MoTe₂ exhibit three spin–orbit doublets, corresponding to Mo-Te (571.7, 582.0 eV), Te (572.9, 583.2 eV), and Te-O (575.6, 586.0 eV), respectively [31]. The two spin–orbit doublets of Mo-Te (572.0, 582.3 eV) and Te-O (575.7, 586.1 eV) were observed in 2H-MoTe₂ films (Figure 1f). According to the XPS results, the rate of oxidized Mo on the surface of 1T’- and 2H-MoTe₂ was 54.2% and 12.0%, receptively. The rate of oxidized Te on the surface of 1T’- and 2H-MoTe₂ was 56.7% and 7.7%, respectively. The higher oxidation of 1T′-MoTe₂ can be attributed to the metastable 1T′ phase [32]. The MoTe₂ films were further characterized using an AFM. The AFM characterizations showed that the thicknesses of the 1T′- and 2H-MoTe₂ films were about 4.1 and 6.5 nm, respectively (Figure 2a,b). All the results indicate that the 2D 1T′- and 2H-MoTe₂ films were synthesized successfully.

2.2. SERS Performances of MoTe₂ Films

In order to characterize the Raman enhancement effect on MoTe₂ films, R6G was chosen as the probe molecule and the excitation wavelength was 532 nm. In Figure 3a, the main Raman characteristic peaks for R6G (1 × 10⁻⁵ M) molecules adsorbed on MoTe₂ can be observed at 612 (P1, C-C-C ring in-plane bend), 773 (P2, C-H out-of-plane bend), 1185 (P3, C-C stretching vibration bend), and 1363 cm⁻¹ (P4, aromatic C-C stretching), respectively [33]. By monitoring the Raman peak at 612 cm⁻¹, the Raman intensity of R6G molecules on 1T′-MoTe₂ was shown to be ~2.4 times stronger than on 2H-MoTe₂ (Figure 3b). These results indicate that 1T′-MoTe₂ films have higher SERS sensitivity than 2H-MoTe₂ films.

Furthermore, a series of different concentrations of R6G ranging from 1 × 10⁻⁵ to 1 × 10⁻⁹ M was adsorbed on 1T′-MoTe₂ (Figure 4a). The Raman signals of R6G can still be detectable even down to the concentration of 1 × 10⁻⁹ M, which is one order of magnitude lower compared with 2H-MoTe₂ (Figure S1). Figure 4b shows the working curves of the R6G concentration versus the Raman intensity in their logarithm scale. A well-defined linear response to the molecular concentrations in the range of 1 × 10⁻⁵ to 1 × 10⁻⁹ M with a correlation coefficient (R²) of 0.978 was obtained, which indicated 1T′-MoTe₂ films have great potential for SERS quantitative analysis. The Raman EF could be calculated by using the follow formula [34]:

$$\mathrm{EF}={\displaystyle \frac{I_{\mathrm{SERS}}}{I_{\mathrm{Raman}}}}\times {\displaystyle \frac{N_{\mathrm{Raman}}}{N_{\mathrm{SERS}}}},$$

(1)

in which I_Raman and I_SERS are the intensities of the selected Raman peak in the normal Raman spectra and SERS, respectively. N_Raman and N_SERS are the average number of molecules in the scattering area for normal Raman and SERS measurements, respectively. The corresponding EFs of the 2H- and 1T′-MoTe₂ films for R6G were calculated to be 5.2 × 10⁵ and 1.9 × 10⁶, respectively (details of the calculation process can be found in the Supplementary Materials and Figure S2). The SERS sensitivity of 1T′-MoTe₂ films could be comparable to those of reported 2D materials (Table S1) [10,20,22,24,25,33,34,35,36,37,38,39].

The relative standard deviations (RSDs) were calculated by randomly selecting 20 points on 1T′-MoTe₂ films (Figure 5a). The RSD values of characteristic Raman peaks at 612 and 1360 cm⁻¹ were 8.6% and 7.7% (Figure 5b,c), respectively, indicating the good signal uniformity. In addition, CV is a prohibited fish drug, which can be detected by using 1T′-MoTe₂ as a SERS substrate. As shown in Figure 5d, four characteristic Raman peaks of CV at 808 (R1, ring C-H bend), 915 (R2, ring skeletal vibration of radical orientation), 1178 (R3, ring C-H bend), and 1372 cm⁻¹ (R4, N–phenyl stretching) could be observed [40], indicating that MoTe₂ has a promising application in food safety.

2.3. Raman Enhancement Mechanism of MoTe₂ Films

The electromagnetic enhancement contribution is excluded in the MoTe₂-induced SERS effect, because the LSPR of MoTe₂ can hardly be excited at this laser wavelength (532 nm) [19]. Figure 6a shows the Raman spectra of R6G adsorbed on bare SiO₂, 1T′-, and 2H-MoTe₂ films without baseline correction. A large fluorescence background was observed on bare SiO₂, and the Raman signals of R6G could not be detected. However, the fluorescence background was suppressed when the R6G molecules adsorbed on MoTe₂ films. The strong quenching effects provide the evidence for the efficient CT processes between R6G and MoTe₂ [15]. In addition, no Raman signal of the R6G molecule was observed on pristine Mo and Te films (Figure S3). The Raman intensities of molecules are proportional to the square of the polarizability tensor α, which can be expressed as α = A + B + C, where the A-term can only amplify the totally symmetric Raman modes, which follows the Franck–Condon selection rules. The B-term and C-term represent the CT from the molecules to the substrates and the CT process from the substrates to the molecules, respectively [2]. The schematic energy level diagrams and CT processes in the 1T-MoTe₂-R6G system under the irradiation of a 532 nm laser are shown in Figure 6b,c. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) levels of R6G are at −3.40 and −5.70 eV, respectively. The Fermi level of 1T′-MoTe₂ is located at −4.87 eV [41]. The conduction band (CB) and valence band (VB) of 2H-MoTe₂ are at −4.48 eV and −5.68 eV, respectively [42]. In the 1T′-MoTe₂-R6G system, the excited electrons can be transferred from the Fermi level of 1T′-MoTe₂ to the HOMO of R6G, but also from the LOMO level of R6G to the Fermi level of 1T′-MoTe₂ (Figure 6b). In the 2H-MoTe₂-R6G system, the transfer of excited electrons from the HOMO level of R6G to the CB of 2H-MoTe₂, from the VB of 2H-MoTe₂ to the LOMO level of R6G, and from the CB of 2H-MoTe₂ to the VB of 2H-MoTe₂ could occur (Figure 6c). According to Herzberg–Teller coupling theory, A-term in the sum of the B-term or C-term can be described as [6]

$$R_{\mathrm{mol}-\mathrm{CT}}\left(\omega \right)={\displaystyle \frac{{\mu }_{\mathrm{mol}}{\mu }_{\mathrm{CT}}{h}_{\mathrm{mol}-\mathrm{CT}}\left.〈i\right|Q_{\mathrm{K}}\left|f\right.〉}{\left({\left({\epsilon }_1\left(\omega \right)+2{\epsilon }_0\right)}^2+{\epsilon }_2^2\right)\left(\left({\omega }_{\mathrm{CT}}^2-{\omega }^2\right)+{\gamma }_{\mathrm{CT}}^2\right)\left(\left({\omega }_{\mathrm{mol}}^2-{\omega }^2\right)+{\gamma }_{\mathrm{mol}}^2\right)}},$$

(2)

where ω is the frequency of the incident photons and ω_mol represents the molecular resonance frequency. Since the photon energy of a 532 nm laser (2.33 eV) is approximately equal to the band gap of R6G (ω ≈ ω_mol), the molecular transition (μ_mol) can be excited, which can further increase the Raman scattering cross-section.

AFM characterizations showed that 1T′-MoTe₂ films with different thicknesses were prepared (Figure 2a and Figure S4). The thickness-dependent Raman enhancement of 1T′-MoTe₂ films is shown in Figure S5. Compared with 16.5 nm thick 1T′-MoTe₂ films (Figure S4), the 4.1 nm thick 1T′-MoTe₂ films exhibited a larger SERS enhancement. The enhanced Raman intensities of R6G at 612 cm⁻¹ on 4.1 nm thick 1T′-MoTe₂ are ~3.7 times higher than those on 16.5 nm thick 1T′-MoTe₂ films. According to Fermi’s golden rule, an abundant density of electronic states (DOS) near the Fermi level will give rise to a high electron transition probability between the adsorbed molecules and the SERS substrates [6]. Usually, few layered TMDs have a higher DOS near the Fermi level than multilayer TMDs [20,43]. So, the thinner 1T-MoTe₂ films showed higher SERS sensitivity.

3. Experimental Section

3.1. Synthesis of MoTe₂ Films

MoTe₂ films were prepared by tellurizing the Mo films (Figure 7a) [28]. A 2 nm Mo film was deposited on Si/SiO₂ substrates by magnetron sputtering first. Then, the Mo films and high-purity Te powder (99.999%) were placed in one quartz tube. During the growth process, a mixture gas of Ar and H₂ was flowed at rates of 4 and 5 sccm in a furnace. MoTe₂ films with different structure phases were prepared by adjusting the reaction temperature and time. Typically, the growth temperature of 1T′-MoTe₂ films was kept at 700 °C for 30 min. 1T′-MoTe₂ films could be transformed into 2H-MoTe₂ films by tellurizing the 1T′-MoTe₂ films at 750 °C for 120 min. After that, the furnace was naturally cooled down to room temperature. The optical image of as-prepared MoTe₂ films is shown in Figure 7b; large-scale (10 mm × 10 mm) MoTe₂ films were prepared.

3.2. Characterization

The thicknesses of films were analyzed by using an atomic force microscope (AFM, Bruker Dimension Icon, Karlsruhe, Germany). UV–vis diffuse reflectance spectra were recorded using a UV–visible spectrophotometer (UV-2600, Shimadzu, Kyoto, Japan). X-ray photoelectron spectroscopy (XPS) spectra were recorded by a photoelectron spectrometer (ESCAlab250, Thermo Fisher Scientific, Waltham, MA, USA); the thickness explored with XPS in material is about 1 nm. Before Raman measurement, the MoTe₂ films were immersed in the solutions of Raman probes with different concentrations (1 × 10⁻⁵~1 × 10⁻⁹ M) for 1 h and were dried by N₂. All the Raman measurements were taken with a Renishaw microscopic confocal Raman spectrometer (InVia Qontor, London, UK). The excitation wavelength was 532 nm with a 600 gr mm⁻¹ grating and a 50× objective was used to focus the laser beam. The integral times were all 50 s.

4. Conclusions

In summary, the phase structure-induced Raman enhancement of CVD-grown large-scale 2D MoTe₂ films was demonstrated. A sensitive molecular sensing performance with a low LOD of 1 × 10⁻⁹ M for R6G molecules was achieved on the 2D 1T′-MoTe films, which was one order of magnitude lower than that of 2D 2H-MoTe₂ films. We demonstrated that efficient charge transfer processes at the interface between the MoTe₂ and the adsorbed molecules endow MoTe₂ with high SERS sensitivity. Furthermore, MoTe₂ also could be used to detect a prohibited fish drug. Our work’s results clearly demonstrate that the 2D MoTe₂ films would be ideal candidates for next-generation SERS substrates.