The Role of GaN in the Heterostructure WS₂/GaN for SERS Applications

Abstract

In the application of WS₂ as a surface–enhanced Raman scattering (SERS) substrate, enhancing the charge transfer (CT) opportunity between WS₂ and analyte is an important issue for SERS efficiency. In this study, we deposited few-layer WS₂ (2–3 layers) on GaN and sapphire substrates with different bandgap characteristics to form heterojunctions using a chemical vapor deposition. Compared with sapphire, we found that using GaN as a substrate for WS₂ can effectively enhance the SERS signal, with an enhancement factor of 6.45 × 10⁴ and a limit of detection of 5 × 10⁻⁶ M for probe molecule Rhodamine 6G according to SERS measurement. Analysis of Raman, Raman mapping, atomic force microscopy, and SERS mechanism revealed that The SERS efficiency increased despite the lower quality of the WS₂ films on GaN compared to those on sapphire, as a result of the increased number of transition pathways present in the interface between WS₂ and GaN. These carrier transition pathways could increase the opportunity for CT, thus enhancing the SERS signal. The WS₂/GaN heterostructure proposed in this study can serve as a reference for enhancing SERS efficiency.

1. Introduction

Surface-enhanced Raman scattering (SERS) technique is a powerful tool for detecting and characterizing molecules at very low concentrations [1,2]. However, the SERS signal can still be weak for some analytes and in some experimental conditions. Improving the SERS signal would enable more sensitive detection and better characterization of molecules, which can have important applications in fields such as biomedicine, environmental monitoring, and materials science [3,4,5,6]. Additionally, a better understanding of the mechanism behind SERS enhancement is important for further optimizing the technique and developing new applications. The SERS effect is generally accepted to occur through the following two mechanisms: the electromagnetic mechanism (EM) and the chemical mechanism (CM). The use of noble metals in SERS methods is commonly adopted to achieve a significant enhancement in signal due to high electromagnetic fields around metallic structures [7,8] in the case of the EM effect. The metallic structure can provide a boost of several orders of magnitude to SERS signals. The CM effect, on the other hand, refers to the chemical interaction between the analyte molecule and the substrate [9,10]. Generally, the SERS signal of the CM effect is much lower than that of the EM effect. In recent years, several studies have demonstrated that 2D materials such as graphene, boron nitride, and molybdenum disulfide (MoS₂) can be utilized as SERS substrates for biomedical detections [11,12].

Among 2D materials used for SERS measurements, tungsten disulfide (WS₂) is a two–dimensional layered material that has recently attracted great interest in electronic and optoelectronic applications due to its high electron mobility, direct band gap, and strong absorption properties in the visible and near–infrared regions [13,14,15,16]. Recent research has also pointed out that the high sensitivity, good chemical stability, and excellent biocompatibility of WS₂ make it highly promising for SERS applications in detecting probe molecules [17,18,19]. The mechanism behind the use of WS₂ as a SERS substrate to enhance the SERS signal intensity of the analyte is attributed to a CM, where the exciton generated under light illumination undergoes a CT process between the energy levels of the analyte and WS₂ at their interface, causing a significant enhancement of Raman signals achieved through charge transitions [20].

Compared to the noble metals such as Au and Ag, which are commonly used for enhancing the SERS signal through the electromagnetic mechanism, the enhancement effect of the SERS signal using CM of WS₂ is weaker and limited [21]. Therefore, it is necessary to enhance the signal intensity by using nanostructures. However, the process of producing nanostructured WS₂ is complex, and the size is difficult to control [22]. Therefore, we propose using a simple semiconductor heterostructure of WS₂/GaN to effectively enhance the SERS signal. In this study, we utilized a metal-organic chemical vapor deposition (MOCVD) system to grow GaN on sapphire, followed by the deposition of a few-layer WS₂ film on both GaN and sapphire substrates using a furnace with chemical vapor deposition (CVD) progress. After that, we compared the SERS performance of the two substrates using rhodamine (R6G) as the analyte. Our analysis, which included Raman spectroscopy, Raman mapping, atomic force microscopy (AFM), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS), showed that the quality of the WS₂ films grown on GaN was inferior to those grown on sapphire. Despite this, we observed that using GaN as a substrate for WS₂ can significantly enhance the SERS signal, with an enhancement factor of 6.45 × 10⁴ and a limit of detection of 5 × 10⁻⁶ M for the probe molecule R6G, as confirmed by SERS measurements. Through analysis of the CT mechanism, it can be inferred that an appropriate semiconductor heterostructure such as WS₂/GaN can effectively provide more paths for carrier transition, making the effect of CT more intense and further enhancing the SERS signal intensity. A series of results could be expected to contribute to the development of using semiconductor heterostructures as SERS substrates.

2. Materials and Methods

2.1. Fabrication of WS₂/GaN Heterostructure

A close–coupled showerhead MOCVD system was utilized to grow regular thin films of c–plane GaN on c–plane sapphire. A GaN nucleation layer with a thickness of around 30 nm was deposited by MOCVD at a growth temperature of 530 °C and a pressure of 600 mbar on c–plane sapphire substrates. The initial GaN layer was deposited at a growth temperature of 1030 °C and a pressure of 300 mbar. The temperature and pressure for the main GaN layer were set at 1050 °C and 150 mbar, respectively, with a V/III ratio of approximately 1200. The thickness of the GaN was approximately 2 μm. Afterward, the WS₂ film was uniformly grown on both GaN and sapphire substrates positioned at the center of the furnace through the CVD process. The Ar/H₂ carrier gas ratio was about 3:1 (150 sccm and 50 sccm). During growing few-layer WS₂, the growth gas used was a mixture of H₂S (10% to Ar) and WF₆, with a flow ratio of H₂S/WF₆ equal to 200:1.

2.2. Preparation of R6G Molecules

A solution of R6G (ACROS Organics, 99%, Geel, Belgium) in water with concentrations ranging from 10⁻² to 10⁻⁶ M was prepared and dip–coated onto the WS₂/GaN, WS₂/sapphire, and sapphire substrates. The 10⁻² M R6G solution was also placed onto a flat sapphire as a reference substrate, thereby allowing measurement of the EF. The specimens were subjected to ambient conditions and dried on a hot plate at 70 °C for 5 min.

2.3. Characterizations

A confocal Raman microscopy system (Tokyo Instruments, Nanofinder 30, Tokyo, Japan) was used to perform Raman spectra and SERS analyses. The excitation was carried out by a He–Ne laser, and the laser power used was 0.1 mW for as–grown samples and 0.3 mW for the reference substrate. The laser spot size was adjusted to 3 μm using a microscope objective with a magnification of 100× and a numerical aperture of 0.9. The acquisition time for laser measurements was configured to 20 s, while the spectrometer employed a grating with a density of 300 lines per millimeter. The resolution of Raman spectrometer was about 0.5 cm⁻¹. The spectrometer’s charge–coupled device was cooled to around −50 °C, utilizing a thermoelectric cooling chip to minimize measurement noise. Prior to measurement, the spectra were calibrated using the peak (520 cm⁻¹) of the Si on the bulk Si substrate. The surface morphology of the samples was examined using an atomic force microscope (Veeco DI–3100, Plainview, NY, USA) through an AFM scan. SEM (FEI Helios 1200+, Hillsboro, OR, USA) revealed the morphologies and nanostructures of the as-grown samples. The chemical configurations of the samples were determined using an XPS instrument (ULVAC-PHI Phi V5000, Chigasaki, Japan) equipped with an Al Kα X-ray source and applied to the samples for analysis.

3. Results and Discussion

WS₂ is a two-dimensional material in which the layers interact with each other through van der Waals forces [23]. Even though there is a bonding force between WS₂ and the underlying GaN atoms, the resulting stress has a negligible effect on the arrangement of the lattice above WS₂. Thus, we were unable to use X-ray diffraction to analyze the aforementioned information and instead were only able to further analyze the thickness of WS₂ on GaN and sapphire using Raman spectroscopy. The molecule vibration modes of the WS₂ grown on GaN and sapphire substrates under identical CVD conditions were examined through Raman spectroscopy analysis. The Raman spectra depicted in Figure 1a reveal the typical E¹_2g and A_1g vibration mode peaks of WS₂ for both WS₂/GaN and WS₂/sapphire samples [24], indicating successful deposition of the WS₂ layer on the respective substrates. A widely recognized phenomenon in the field is that the difference in wavenumber between the two vibration modes E¹_2g and A_1g of WS₂ exhibits a decreasing trend with the reduction in WS₂ thickness from bulk to few-layered structures [25,26]. Based on the observed difference in wavenumber of approximately 63 cm⁻¹ for the WS₂/GaN sample and 62 cm⁻¹ for the WS₂/sapphire sample, it can be inferred that the WS₂ thin film formed on both substrates has a thickness ranging from 2 to 3 layers. On the other hand, it has been reported by McCreary et al. that the quality of WS₂ thin film can be determined by observing the full–width at half–maximum (FWHM) of the Raman A_1g peak [27]. Therefore, the A_1g peaks of the two samples mentioned above were fitted with Gaussian curves to obtain accurate FWHM values. As shown in Figure 1b, the FWHM of the WS₂ A_1g peak grown on sapphire was 4.07 cm⁻¹, while that on GaN was approximately 4.85 cm⁻¹. This result suggests that the WS₂ grown on sapphire has better thin film quality compared to that grown on GaN. The reason for this will be discussed later.

In order to further compare the uniformity of WS₂ grown on GaN and sapphire substrates in terms of Raman results, we further utilized Raman mapping to obtain the spatial distribution of the two peaks E¹_2g and A_1g, as well as the wavenumber difference (Δω) between the two peaks. As shown in Figure 2, when WS₂ was grown on sapphire, the uniformity of both E¹_2g and A_1g peaks was significantly higher than that of GaN. In addition, the results of Δω also showed that the thickness uniformity was better when grown on sapphire. Overall, the thickness range of WS₂ grown on GaN was larger, which also verified the results of Figure 1a. Furthermore, our AFM measurement results, shown in Figure 3a,b, indicate that the root mean square (RMS) values of surface roughness for WS₂/GaN and WS₂/sapphire are 0.39 and 0.11, respectively. This suggests that the surface of WS₂/GaN is rougher than that of WS₂/sapphire, consistent with the aforementioned Raman and Raman mapping results. The two AFM images in Figure 3c,d provide further evidence that the surface of GaN is not as flat as that of sapphire, as the former has undergone MOCVD epitaxial growth while the latter has been polished. The RMS roughness of GaN is approximately 0.09, while that of sapphire is 0.06. We also conducted additional SEM observations, as shown in Figure S2a,b in the Supplementary Materials. The WS₂ grown on GaN appeared significantly rougher than the WS₂ grown on sapphire. It can be observed that the SEM results are consistent with the AFM results. This explains why the subsequent growth of WS₂ on GaN, in terms of surface uniformity and thickness uniformity, is inferior to that on sapphire. Another possible reason is that during the furnace CVD growth process, due to its higher surface activity compared to sapphire, GaN is more prone to adsorb some of the native contaminants in the furnace, such as O, C, and S atoms [28], which can affect the interface smoothness of the subsequently grown WS₂ film. Therefore, it can be concluded that the WS₂ grown on GaN exhibits inferior lattice quality and surface roughness compared to that grown on sapphire. Regarding this aspect, we provide XPS data and have a detailed discussion below.

To further understand the reason for the poor surface quality of WS₂ grown on GaN, we performed XPS experiments to analyze the content of S, C, and O atoms. The results of XPS are shown below in Figure 4a–d. Based on the signals of (a) S2p, (b) S2s, and (c) C1s, it can be concluded that when WS₂ was grown on GaN, both the amount of S and C atoms were higher than when it was grown on sapphire. This result is consistent with our AFM results mentioned above. Compared to sapphire, GaN is more prone to absorb S, C, and other atoms on its surface during the growth of WS₂, which leads to poorer surface quality. On the other hand, as shown in Figure 4d, there are relatively more O atoms in the WS₂ on the sapphire structure. It is well known that sapphire has a higher affinity for oxygen compared to GaN [29]. This is because sapphire has a more polar surface with a higher surface energy, making it easier for oxygen atoms to be absorbed or captured by the surface. In contrast, GaN has a nonpolar surface with lower surface energy, which makes it less likely for oxygen atoms to be captured or absorbed. Another possible reason for the higher concentration of O atoms could be the presence of O atoms in the sapphire substrate itself.

We conducted SERS measurements on these two types of SERS substrates using R6G of 10⁻⁴ M as the analyte, as shown in Figure 5a. The results indicate that under this range of wavenumbers, the main R6G SERS signals measured on WS₂/GaN are significantly stronger than those on WS₂/sapphire, with the peak at 611 cm⁻¹ being particularly prominent [30]. Although WS₂/sapphire has better film quality, WS₂/GaN shows a more prominent performance as a SERS substrate. Moreover, we used this WS₂/GaN heterostructure substrate to measure different concentrations of R6G solution, as shown in Figure 5b. It can be observed that a weak 611 cm⁻¹ peak signal is still present at 5 × 10⁻⁶ M, but no R6G signal can be detected at 10⁻⁶ M, indicating that the LOD of WS₂/GaN, in our case, can reach 5 × 10⁻⁶ M. Generally, the EF is an important metric for evaluating the performance of SERS because it quantifies the level of signal enhancement that occurs when analyte molecules are adsorbed onto either a semiconductor or plasmonic substrate. EF is calculated by comparing the Raman signal intensity of a given molecule on a plasmonic substrate with that of the same molecule in its bulk form. A high EF indicates that the plasmonic substrate provides a strong enhancement effect on the Raman signal of the analyte, leading to a more sensitive detection method. Therefore, the EF value is commonly used as a figure of merit to compare the performance of different SERS substrates and to optimize the SERS experimental conditions. According to the calculation formula of EF shown in the following equation [31]:

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

(1)

where I is the Raman spectral intensity and N is the number of molecules, respectively; SERS and REF denote the values obtained from WS₂/GaN and the reference sample of the sapphire substrate, respectively. As the laser spot size and exposure time were constant for all measurements, the ratio of N_REF/N_SERS was equivalent to the R6G concentration ratio for both samples. According to the above equation, it can be determined that the EF for the peak 611 cm⁻¹ of R6G measured using the WS₂/GaN SERS substrate can reach 6.45 × 10⁴. Therefore, in terms of overall SERS performance, WS₂/GaN performs better than WS₂/sapphire. In the following discussion, we will explore the advantages of using GaN in the WS₂ heterostructure for SERS measurement based on our experimental results. We also conducted measurements on the relationship between the 611 cm⁻¹ peak intensity and different R6G concentrations (ranging from 10⁻⁴ M to 5 × 10⁻⁶ M) for both samples. The results are presented in Figure S3 in the Supplementary Materials. It is apparent from the figure that the peak intensity WS₂/GaN was consistently higher than that of WS₂/sapphire in this R6G concentration range, indicating that our measurements were highly reliable.

In general, the enhancement mechanism of semiconductors as SERS substrates mainly comes from the CT between the photoexcited carriers and the analyte in the band gap of the semiconductor [32]. Lee et al. reported that WS₂ demonstrates better SERS performance as the number of atomic layers decreases [33]. From a band perspective, our Raman results indicate that WS₂ has a thickness of only about 2–3 atomic layers and a direct bandgap of about 1.6 eV [34]. The laser energy used in our experiments is approximately 1.96 eV, as shown in Figure 6, which can provide carrier transition pathways (2), (3), (5), and (6). The corresponding energy levels are the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of R6G, as well as the conduction band edge energy level (E_c) and valence band edge energy level (E_v) of WS₂. The energy differences between these pathways are all smaller than the energy of the laser, allowing WS₂ to effectively enhance the SERS signal intensity of R6G. On the other hand, when GaN is introduced as a substrate for WS₂ to form the WS₂/GaN heterostructure, the Raman results in Figure S1 show that the laser intensity can penetrate through the thin WS₂ film and measure the oscillation frequency signals E₂ and A₁ of GaN [35]. This demonstrates that the laser can also excite carriers in the interface between WS₂ and GaN with energy differences smaller than the laser energy. Compared to sapphire, GaN has a bandgap of approximately 3.4 eV, which at the interface with 1.6 eV of WS₂, can provide additional effective carrier transition pathways (7)–(9). This allows the excited electrons to have the opportunity to migrate to the interface between R6G and WS₂, increasing the probability of CT and enhancing the SERS signal intensity. For the purpose of comparison, we have listed the various pathways and their corresponding energy levels in

Table 1. Corresponding energy differences and probabilities of carrier transition pathways indicated in Figure 6.

Pathways	(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)	(10)	(11)
ΔE (eV)	2.3	1.5	0.8	2.2	0.1	1.4	1.7	0.3	1.7	3.1	3.4
Possibility	X	O	O	X	O	O	O	O	O	X	X

, along with the possibility of carrier transition.

In addition to the transition pathways between the WS₂/GaN heterostructure mentioned above that contribute to the strong SERS signal, other possibilities still need to be considered. Our AFM results show that the surface of WS₂ grown on GaN is rougher than that grown on sapphire. Therefore, the higher roughness of WS₂ on GaN also provides a larger contact area with the probe molecule R6G, which could enhance the SERS signal. Furthermore, in the above discussions, we mentioned that the native impurities O, C, and S in the furnace tube can be easily adsorbed onto the GaN surface, forming additional deep levels in the GaN bandgap. Therefore, these impurities may provide additional transition pathways for both WS₂ and GaN, thereby increasing the SERS intensity. However, our current experimental results do not confirm the contribution of these reasons to the enhancement of the SERS signal. Therefore, in the future, we will design further experiments to analyze these possible factors.

4. Conclusions

In conclusion, we propose the use of GaN as a substrate for few-layer WS₂ to form WS₂/GaN heterostructures as SERS substrates and compare their thin film quality and SERS performance with WS₂/sapphire. Through a series of experiments, including Raman, Raman mapping, AFM, and SERS mechanism investigation, our results showed that the quality of few-layer WS₂ film grown on GaN was inferior to that on sapphire. However, due to the high surface roughness associated with high surface contact and the additional carrier transition paths in the WS₂/GaN interface, WS₂/GaN heterostructures could effectively enhance SERS signals in detecting the R6G target molecule compared to WS₂/sapphire, with an EF value of 6.45 × 10⁴ and a LOD of 5 × 10⁻⁶ M. Therefore, our study suggests that designing appropriate semiconductor heterostructure SERS substrates for different target molecules can effectively increase the chance of CT and enhance SERS signals.