Having unique characteristics, such as a nondestructive nature, ultrahigh sensitivity, and molecular specificity, surface-enhanced Raman scattering (SERS) has been widely explored in analytical detection, molecular imaging, Raman mapping, environmental monitoring, and material investigation since it was first demonstrated on silver electrodes in the 1970s by Martin Fleischmann, Patrick J. Hendra, and A. James McQuillan. It has since been investigated by several other researchers [1–3]. The inherent benefits of SERS in the near-infrared (NIR) region, such as reduced photo-bleaching, a low fluorescence background, and a greater penetration depth into biological macromolecules for NIR excitation, make it a powerful tool for biodetection and biosensing [4–7]. Two widely accepted mechanisms, the electromagnetic mechanism (EM) and the chemical mechanism (CM) were used to explain the SERS phenomenon [8]. The EM is believed to make the most contribution to SERS, where the electric field (E-field) is strongly enhanced in the localized area, forming “hot-spots” through plasmonic coupling of EM radiation with localized surface plasmon resonance (LSPR) of metallic nanostructures [9,10]. Another contribution to the SERS phenomenon is the charge transfer (CT) mechanism, which involves the transfer of an electron either from the Fermi level of a metal or from a semiconductor through interband transition to an unoccupied molecular orbital of probe molecule [11,12]. The SERS selection rules must be matched to produce resonance effects, including molecular resonance and molecule–semiconductor CT resonance belonging to CM, and such electromagnetic resonance as metal-like surface plasmon resonance and Mie scattering resonance belonging to EM under an excitation laser [13].

Metal nanoparticles or nanostructures are mostly used as SERS substrates and exhibit great SERS enhancement through EM. These substrates can produce strong plasmonic resonances due to coherent oscillations of electrons driven by an incident E-field [14]. These plasmonic systems are able to confine the E-field in the sub-wavelength region and provide extremely enhanced Raman signals [15,16]. Ordered nanoparticles arrays can be prepared to produce strong E-field enhancement for SERS measurements based on self-assembly methods and nanofabrication techniques [17]. However, different from nanoparticles arrays, intelligently designed metallic nanostructures exhibit outstanding properties, including large tunability of spectral responses from the ultraviolet (UV) to the NIR region and extreme enhancement of the localized E-field due to LSPR coupling, and produce distinct SERS intensity and enhancement factors in the wide spectral range [18–20]. Plasmonic nanoantenna is one of the most used metal nanostructures for SERS study and various antenna designs, such as Yagi–Uda antennas, spiral nanoantennas, and U-/T-shaped nanoaperture antennas; nanodisc, nanorod, and nanocube antennas have been developed and used for optical and sensing applications including biochemical sensing [21,22], spontaneous emission enhancement [23,24], solar energy conversion [25,26], and nanoscale light sources [27,28]. In particular, owing to its triangular shape, the bowtie nanoantenna (BNA) can provide an efficient LSPR coupling between the triangle tips to generate a strong locally enhanced E-field in the nanogap different from other geometric nanostructures [29–31]. As a result, the Raman scattering intensity of molecules located near the nanotip and in the nanogap of the BNA can be strongly enhanced by several orders of magnitude resulting in strong SERS [32–34]. Recently, a metal-insulator-metal (MIM) system, where a metallic nanostructure and a metallic thin film are separated by a dielectric spacer, has been studied extensively for plasmonic applications [35,36]. In plasmonic nanoantennas with MIM structures, the flexibility to tune E-field enhancement and spectral response is mainly attributed to the coordinated multiple coupling (i.e., the in-plane near-field coupling, the in-plane far-field coupling, and the out-of-plane coupling) [37,38]. Besides extreme enhancement of E-field by LSPR coupling in the gap region, the BNA possesses other advantages, including directionality, highly efficient electro-optical driving, and polarization control over other coupled dipole antenna designs [39,40]. Owing to these virtues, the BNA with an MIM structure is expected to exhibit great scientific interest and has led to a few exciting applications in nanophotonics, including SERS and biosensing.

In addition to the extensive SERS study by EM on the formation of hot spots by different structural geometries, the CM is mainly attributed to the interfacial CT between probe molecules and substrate materials, which continues to be of great importance for SERS [41]. When the excitation wavelength matches with one of the CT processes, chemical enhancement of specific Raman modes can be observed. The Herzberg–Teller surface selection rule is the most comprehensive and widely accepted approach to explain the CT effect [42]. In recent years, owing to the CT mechanism, the materials for SERS studies are being gradually extended from noble metals and transition metals to semiconductor materials, such as metal oxide, two-dimensional (2D) materials, and semiconductor quantum dots (QDs), where SERS performance was largely improved by various semiconductor substrates [43–45]. QDs used in the SERS study especially show excellent properties such as high quantum yield, a tunable bandgap, and high stability [46–48]. As narrow bandgap semiconductors, silver sulfide (Ag2S) QDs have many excellent properties, such as broadband optical response, a high absorption coefficient, strong chemical stability, nontoxic properties, and excellent emission in the second NIR region (wavelength from 1000 to 1350 nm) [49]. It has also been widely considered as a potential candidate for optoelectronics, energy harvesting, and sensing applications [50,51]. The application of Ag2S QDs is mainly focused as NIR fluorescent probes for in-vivo imaging, and in the fabrication of the solar cells of QDs, thin-film transistors, optical detectors, infrared biological sensors, and optical filters [52–54]. However, uses for Ag2S QDs and its composite as a SERS substrate are limited [46,50,55–57].

Usually, EM enhancement is several orders of magnitude stronger than CM enhancement (including molecular resonance and CT resonance). However, whether EM–SERS enhancement or CM–SERS enhancement, the individual contribution for SERS is usually unsatisfactory. In this work, we studied the synergistic effect of CT resonance at the interface of BNA–QDs–PATP (p-aminothiophenol) and E-field enhancement around BNA arrays for SERS in the NIR spectral region using a semiconductor–QDs/metal-nanostructure composite substrate. Under 785 nm laser excitation, the Au BNA arrays with an MIM structure provide a strongly localized and enhanced E-field that coupled with a CT process at the interface of BNA–QDs–PATP to get strong Raman enhancement. The broadband LSPR coupling of plasmonic BNA arrays can provide E-field enhancements at excitation as well as Raman scattering wavelengths. Furthermore, the SERS polarization dependence effects of the BNA/Ag2S-QDs/PATP system are also studied. The polarization dependence study showed an increase in the SERS intensity with decreasing polarization angles (θpola) of excitation from p-polarization (θpola=90°) to s-polarization (θpola=0°). The finite-difference time-domain (FDTD) simulation was performed to calculate spectral E-field enhancement and spatial distribution of E-field at excitation wavelength around BNA arrays. The present study provides a guidance for the development of a composite metal–semiconductor system that could synergistically harness the strong E-field enhancement capability of metallic nanostructures and CT characteristics of semiconductors to enhance optoelectronic, electro-optical, solar energy, and sensing efficiency of different devices.

Sulfur powder (S, 99.95%) was purchased from Fuchen (Tianjin) Chemical Reagent Co., Ltd. Silver nitrate (AgNO3, 99.9995%) and oleic acid (OA, technical grade, 90%) were purchased from Sinopharm Chemical Reagent Co., Ltd. Tri-n-octylphosphine (TOP, technical grade, 90%), 1-octadecylamine (ODA, technical grade, 90%), and 1-octadecene (ODE, technical grade, 90%), were purchased from Aladdin Industrial Corp. Methyl alcohol (99.5%), ethyl alcohol (99.7%), butyl alcohol (99.5%), hexanes (98%), acetone (99.5%), and methylbenzene (99.5%) were purchased from Beijing Chemical Industry Group Co., Ltd. P-aminothiophenol (PATP, 98.0%) was purchased from Shanghai Yuanye Bio-Technology Co., Ltd. All chemicals were used as delivered without further purification.

For the synthesis of Ag2S quantum dots [51], 16.99 g of AgNO3 was dissolved separately in 100 mL tri-n-octylphosphine (TOP) to obtain Ag-TOP. A mixture of 6.4 mL of oleic acid (OA), 5.4 g of 1-octadecylamine (ODA), 12.8 mL of 1-octadecene (ODE), and 72 mg of sulfur powder was then combined in a 100 mL round-bottom flask, degassed, flushed three times with N2 to remove water and oxygen, and heated to 70°C under N2 until the powders dissolved. Then 4 mL of Ag-TOP was quickly injected at 197°C, and the reaction was allowed to proceed for 12 min at 167°C. The growth was quenched in an ice bath, and 25 mL of butanol was added to prevent solidification of the reaction mixture as it cooled. The nanocrystals were isolated by precipitation with ethanol and redispersed in hexane. To remove excess ligands and impurities in the raw materials, the synthesized Ag2S QDs were further purified. The purification process was as follows: 30 mL of methanol was added into 5 mL of the product, and the resulting mixture was thoroughly mixed through vigorous ultrasonication followed by separation of the product through centrifugation. The centrifuged product was redispersed into 2 mL of toluene through ultrasonication followed by addition of 30 mL of methanol, further ultrasonication for 30 min, and separation of the product through centrifugation. This step was repeated three times to remove impurities and excess ligands and solvents. The methanol was then replaced by acetone and the purification process was repeated for two more cycles. After five purifications, the Ag2S QDs were dispersed in n-hexane for use.

A 100 nm thin gold film was first deposited on the SiO2 substrate (300 nm thick SiO2 on the top of the silicon wafer) using a high vacuum resistance evaporation coating system. Then, a 20 nm of silica spacer layer was deposited on the surface of the gold film using a plasma-enhanced chemical vapor deposition (PECVD) system. Next, a photoresist layer was spin-coated on the surface of silica (20 nm) and patterned using an electron beam lithography (EBL) system. A 100 nm thick gold film was next deposited on the surface of the photoresist that has been developed. Finally, a lift-off process was used to fabricate the structure.

Ag2S QDs were deposited on the surface of a Si/SiO2 wafer, Au film, and BNA arrays substrates by spin-coating at 2500 r/min for 35 s in an air environment. The Ag2S-QDs coated sample was immersed in a 10−3mol/L PATP solution (dissolved in alcohol) for 2 h followed by cleaning with alcohol and drying under a N2 flow.

The Raman spectra were collected using a LabRAM HR Evolution Raman spectrometer (Horiba Jobin Yvon). The laser beam was focused on a spot approximately 1 μm in diameter using an objective microscope with 100× magnification. The data acquisition time was 5 s for all systems. The UV-Vis spectral measurements were carried out using a Cary 5000 UV-vis-NIR spectrometer (Agilent). X-ray diffraction (XRD) spectra were collected using a Bruker D8 FOCUS X-ray diffractometer. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analysis was done by the scanning electron microscope (Phenom ProX). X-ray photoelectron spectroscopy (XPS) data were obtained using an X-ray photoelectron spectrometer (Thermo ESCALAB 250Xi). Transmission electron microscopy (TEM) images of QDs were taken using a JEOL JSM- 6700F transmission electron microscope. The relative standard deviation (SD) data of the main vibrating peaks, intensity from five randomly chosen spots in the same sample are used in the calculation of the CT degree and Raman intensity to evaluate the signal reproducibility.

For the simulations, we employed an FDTD method based on Yee’s algorithms [58] for electromagnetic simulation. In the FDTD method, Maxwell’s equations are discretized in both time and space, and the central difference approximation is used. A commercial FDTD package from Lumerical Inc.’s DEVICE Suite was used to calculate spectral response and E-field distribution of BNA arrays. A perfectly matched layer (PML) was used in the z-axis direction, while periodic boundary conditions were used in the x- and y-axis directions. The designed plasmonic system was allowed to illuminate with a polarized plane wave propagating along the z axis. Two different polarization states for the incident electromagnetic wave, the s-polarization, E-field parallel to the x axis (along the antenna axis), and the p-polarization, E-field parallel to the y axis (normal to the antenna axis), were used in the FDTD simulations to investigate the light polarization effects on the SERS enhancement. Two detector boxes were placed to measure the distribution of E-field in the x–y and x–z planes. For the calculation of SERS maps, the data of the E-field distribution were extracted from detector boxes at the wavelength of excitation and Raman vibrational modes. SERS maps of E-field intensity distribution [|Eloc(excitation)/E0|2×|Eloc(Stokes)/E0|2] were plotted by MATLAB (R2014b). An ultrafine mesh size (1 nm) was used in all the simulations. The refractive index of nsup=1.0 is chosen for air to generate an asymmetric environment. The optical constants of Au, SiO2, and Si were taken from Ref. [59].

TEM images of the Ag2S QDs are shown in Fig. 1(a), while the inset shows a representative high-resolution TEM (HRTEM) image of a quantum dot with lattice spacing of 0.241 nm. Figure 1(b) exhibits the particle size distribution of Ag2S QDs by histogram. It shows that the majority of as-prepared Ag2S QDs were spherical or spheroidal in shape, with an average size of 3.32 nm and a narrow size dispersion of 0.86 nm. Figure 1(c) shows the XRD characterization result of the as-prepared Ag2S QDs. Almost all of the peaks in the XRD patterns matched those of monoclinic Ag2S [46]. To further investigate the purity and composition of Ag2S coated BNA arrays, XPS images were also studied (see Appendix A). Figure 1(d) shows the UV-vis-NIR absorption spectrum of Ag2S QDs, which indicates a broad absorption range until the NIR region.

Figure 2(a) shows the SEM image of the Au BNA arrays with an enlarged view in the inset. The BNA arrays consist of a periodic arrangement of a pair of equilateral triangles (known as a bowtie) as a unit cell. The side length of equilateral triangle is 420 nm, and the gap distance between a pair of equilateral triangles is 160 nm. The separation between two-unit cells (transverse period) is ∼580nm, and the separation between two lines made of BNA (vertical period) is ∼2μm. The EDS confirmed the elemental contents of the Ag2S QDs coated Au BNA arrays substrate, shown in Fig. 2(b). Insets of Fig. 2(b), from left to right, show the scanned region for EDS, and elemental mapping of S, Ag, and Au. From the EDS mapping of S [(inset (ii)] and Ag [inset (iii)] in the scanned region, we can say that the Ag2S QDs are uniformly distributed on the surface of BNA substrate.

The theoretical modeling is performed using full-wave simulation with the FDTD method (Lumerical Inc.). A 3D model is used to study the E-field enhancement calculated as |E|2/|E0|2, where E0 is the incident laser field and E is the resulting near field. An s-polarized plane wave (E-field direction parallel to the x axis) is used in the simulation. The plasmonic BNA arrays locate above the 100 nm Au thin film separated by a 20 nm SiO2 spacer layer on the SiO2 substrate (300 nm SiO2 on the top of Si wafer), as displayed in Fig. 3(a), and the shape of BNA is an equilateral triangle. The parameters of the BNA arrays, as obtained by the SEM image [Fig. 2(a)], include a side length of 420 nm, a gap distance of 160 nm, a triangle thickness of 100 nm, a transverse period of 580 nm, and a vertical period of 2 μm consistent with the experiment result.

Figure 3(b) shows the E-field enhancement spectrum of the BNA arrays with MIM structure at the center of the bowtie gap (P0 point, inset), which exhibits three distinct peaks corresponding to different LSPR modes. The primarily LSPR1 mode corresponding to the dipolar LSPR mode, located around 802 nm with the largest E-field enhancement, is close to the pump wavelength at 785 nm used in SERS measurements. The E-field of the LSPR1 mode is mainly localized in the bowtie gap, which is caused by LSPR coupling between closely spaced nanoprisms, as shown in Fig. 3(c). Moreover, a moderate level of E-field enhancement is also observed around the outer corners and edges of the triangles. The large spatial distribution of the E-field enhancement at 802 nm shows comparatively larger available surface regions for E-field-based SERS with excitation close to this wavelength [Fig. 3(c)]. The other two higher-order LSPR modes are located at 726 nm (LSPR2) and 686 nm (LSPR3) with a comparatively lower E-field enhancement. The E-fields of these two modes are localized in the gap and outer corners of nanoprisms [Figs. 3(d) and 3(e)] [60]. The E-field distribution of three LSPR modes in the x–z plane and the distribution of the E-field enhancement at 802 nm (LSPR1 mode) along the x, y, and z directions are discussed in detail (see Appendix B). The inset curve in Fig. 3(b) shows an enlarged view of the E-field enhancement spectrum in the spectral range of 780 to 930 nm that includes an excitation laser wavelength of 785 nm used in the SERS study, as well as Raman scattering modes used in SERS measurements [wavelengths range from 837 nm (or 800cm−1) to 914 nm (or 1800cm−1), the starting and ending points of SERS measurements] of PATP probe molecules. The enhancement factor (EF) of a SERS substrate is defined as EF=|Eloc(excitation)/E0|2×|Eloc(Stokes)/E0|2 (i.e., the product of the near field enhancement at the excitation and Stokes wavelengths) [61]. Therefore, a broadband LSPR absorption ranging from excitation wavelength to Raman scattering signal in a plasmonic system is important to get high E-field EF in SERS. The position of the first resonant mode with its peak (LSPR1, 802 nm) close to the excitation wavelength (785 nm) along with a broadband E-field enhancement covering the region of Raman scattering of the probe molecule makes the designed plasmonic system an ideal candidate to enhance the E-field of excitation as well as Raman scattering signals for E-field-enhancement-based SERS measurements.

Figure 4 shows the Raman spectra of PATP molecules adsorbed on Ag2S QDs coated BNA arrays [Fig. 4(a)] compared to those that are separately on bare BNA arrays [Fig. 4(b)] and Ag2S QDs [Fig. 4(c)] under 785 nm laser excitation. Raman spectrum of PATP powder on the glass slide is also presented as a reference [Fig. 4(d)]. As can be seen from Fig. 4, there is no Raman signal achieved when PATP molecules are adsorbed on Ag2S QDs [Fig. 4(c)]. In addition, the Raman signal of PATP molecules adsorbed on BNA arrays [Fig. 4(b)] is also very weak. However, for PATP molecules adsorbed on Ag2S QDs coated BNA arrays [Fig. 4(a)], the Raman signals are extremely enhanced. The Raman bands located at 1004cm−1 [γ(CCC)+γ(CC), 18a], 1075cm−1 [ν(CS), 7a], and 1184cm−1 [δ(CH), 9a] are assigned to the a1 mode while 1138cm−1 [δ(CH), 9b], 1299cm−1 [ν(CC)+δ(CH), 14], 1388cm−1 [δ(CH)+ν(CC), 3], 1435cm−1 [ν(CC)+δ(CH), 19], and 1574cm−1 [ν(CC), 8b] are assigned to the b2 mode of the PATP molecule. A more detailed band assignment for the SERS spectrum of PATP under different conditions is summarized in Table 1 [62].

The extremely enhanced Raman phenomenon in our study is attributed to the synergistic effects of both E-field enhancement from the BNA arrays and the CT resonance at the interface of the BNA–QDs–PATP composite system.

First, the large enhancement of the localized E-field, which is caused by the LSPR of BNA arrays, is one of the key factors that contribute to the Raman enhancement. To investigate the effect of the E-field on the SERS enhancement, we compared the SERS signal of PATP molecules adsorbed on the BNA arrays and Au thin film (30 nm). As shown in Fig. 5(a), the SERS signal of the BNA/PATP system is strongly enhanced, especially at 1078cm−1 [ν(CS), a1], compared to that of the Au-film/PATP system, where a detectable Raman signal is not observed. It is because the BNA arrays possess a strong E-field enhancement under 785 nm laser excitation, which is near the LSPR1 mode (802 nm) of BNA arrays. Moreover, the E-field of the a1 mode (1078cm−1) that is one of the most intense vibrational Raman modes of the PATP molecules, also gets strongly excited by BNA. The spatial distributions of the enhanced E-field of BNA in the x–y and x–z planes under 785 nm laser excitation are shown in Figs. 5(b) and 5(c), respectively. It can be seen from Fig. 5(b) that the E-field is mainly distributed in the central gap region with extreme enhancement, which is due to the hot spots effect generated by the LSPR coupling of a single BNA. The enhanced E-field is also distributed around the corners of triangles, which benefits from the interaction of the tip impact and pairs of BNA. The spatial distribution of the E-field in the x–z plane [Fig. 5(c)] exhibits an E-field with strong field intensity localized in an almost entire air gap formed by tip-to-tip triangles. Meanwhile, an enhanced E-field is confined below both the tips and the ridges of the BNA, which is supported by the mode coupling of the Fabry–Perot cavity formed along the z-axis in the 20 nm thin SiO2 spacer layer sandwiched between two layers of gold. The spatial distribution of the SERS EF (or SERS maps) for the bands located at 1078cm−1 [ν(CS), a1] and 1140cm−1 [δ(CH),b2] of the PATP molecule is shown in Fig. 5(d) (top view: x–y plane) and Fig. 5(e) (side view: x–z plane), respectively. The SERS E-field EFs are ∼2.28×105 and ∼1.87×105 for 1078cm−1 and 1140cm−1 Raman vibrational modes, respectively. Thus, it can be concluded that the BNA arrays can provide a strong enhanced E-field for SERS enhancement.

Another interesting phenomenon is that after the deposition of Ag2S QDs onto the surface of BNA arrays, the SERS signal of the adsorbed PATP molecules was further enhanced compared to the Raman spectra of PATP molecules adsorbed on both Ag2S QDs and BNA due to the combined effects of BNA and Ag2S QDs. Herein, the enhanced Raman signal is not only affected by the E-field of the BNA; the Ag2S QDs also play a key role in the enhancement. As shown in Fig. 4, compared to the bulk Raman signal of PATP [Fig. 4(d)] and the SERS signal of the BNA/PATP system [Fig. 4(b)], there are several bands assigned to the b2 mode of the PATP molecules, such as the1138, 1299, 1388, 1435, and 1574cm−1 bands, which are selectively enhanced in the SERS spectrum of the BNA/Ag2S-QDs/PATP system. These selectively enhanced b2 modes indicate that the CT process takes place at the interfaces of the BNA–QDs–PATP composite system, which is proven by the Herzberg-Teller contributions [42]. The CT mechanism is further explained by the complete energy level diagram under the 785 nm laser excitation in Fig. 6. The energy levels for the conduction band (CB) and the valence band (VB) of Ag2S QDs are 3.63 and 5.32 eV, respectively [63,64]. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of PATP molecules are 7.16 and 3.03 eV, respectively [65,66]. The Fermi level (Ef) of gold is 5.1 eV. Figures 6(a) and 6(b) show the energy level diagram of the Ag₂S–QDs/PATP and Au–BNA/PATP composite, respectively. The photon energy of the 785 nm laser (ca. 1.58 eV) is too low to excite the electronic transitions between the band levels at the interfaces of Ag₂S–QDs/PATP (ca. 1.69 eV) and Au–BNA/PATP (ca. 2.07 eV). Therefore, the direct CT resonance is not easily achieved at the interface of both systems because of the larger energy difference [Figs. 4(b) and 4(c)]. However, for the BNA/Ag2S-QDs/PATP system [Fig. 6(c)], the photon energy of the 785 nm laser (ca. 1.58 eV) can match the band−band transition energy of the interface (ca. 1.47 eV). The electrons in the Fermi level of Au can be photoexcited into the CB of the Ag2S QDs under 785 nm excitation. Subsequently, dynamic CT could occur by coupling with the vibrations of the molecules to easily transfer an electron to the LUMO level of a PATP molecule. This results in a similar “donor–bridge–acceptor” CT mode, which occurs more easily in the Au-Ag2S-PATP system due to the principle of energy level matching [66–68]. So the CT resonance can take place at the interfaces of the BNA–QDs–PATP system under 785 nm excitation [Fig. 4(a)]. As a result, the SERS signal of the adsorbed PATP molecules gets further enhanced.

To further evaluate the synergistic effects of the E-field and CT to the SERS enhancement, the SERS spectra of the PATP molecule adsorbed on the Ag2S QDs coated BNA arrays and Ag2S QDs coated Au thin film (30 nm) were compared. First, as shown in Fig. 7(a), the b₂ modes of these two complex systems were selectively enhanced under 785 nm laser excitation, which ascribed to the CT mechanism in both systems. The degree of charge transfer ρCT(κ) was used to investigate the CT contribution in the SERS enhancement [11,69]. The ρCT(κ) of a κ-bond can be determined using ρCT(κ)=Iκ(CT)−Iκ(SPR)Iκ(CT)+I0(SPR),(1)where the index “κ” represents individual lines in the SERS spectra. For a nonsymmetric line, Iκ(SPR) is usually quite small or zero. We defined I0(SPR) as the intensity of the 1075cm−1 band (a1 mode) and Iκ(CT) as the intensities of the bands at 1138, 1388, 1435, and 1574cm−1 (b2 mode), which are derived from nonsymmetric contributions. The degrees of CT for PATP in both BNA/Ag2S-QDs/PATP and Au-film/Ag2S-QDs/PATP systems at 1138, 1388, 1435, and 1574cm−1 with excitation at 785 nm are shown in Fig. 7(b). The CT value calculated by these peaks shows no obvious difference in these two systems, which indicates that these two systems possess nearly the same CT degrees, and the surface nanostructure of BNA will not affect the CT of the system. So the CT mechanism shows the same contribution to these two systems. However, as shown in the Fig. 7(a), the intensity of the Raman signal in the BNA/Ag2S-QDs/PATP system is much stronger than that in a Au-film/Ag2S-QDs/PATP system. Notice that the signal of PATP adsorbed on Ag2S QDs coated Au film is magnified 10 times. Such enhancement of the SERS signals in the BNA–QDs–PATP system compared to the Au film–QDs–PATP system can be attributed to the huge localized E-field of BNA arrays, as discussed above. Thus, we can conclude that the SERS enhancement of the BNA–QDs–PATP arises from the synergistic effects of the E-field and the CT mechanism.

The SERS EF for the PATP probe molecule is roughly calculated (see Appendix C) to evaluate the enhancement ability by the Raman intensity of the band at 1078cm−1 [ν(CS), a1] in the BNA/Ag2S-QDs/PATP system. The EF value is estimated to be about 3.5×105, which is attributed to the synergistic effects of the E-field enhancement and CT. To further estimate the enhancement ability of the synergistic effects in the BNA/Ag2S-QDs/PATP system, we compared the Raman intensities of the bands at 1078cm−1 [ν(CS), a1] and 1138cm−1 [δ(CH),b2] in the BNA/PATP, BNA/Ag2S-QDs/PATP, and Au/Ag2S-QDs/PATP systems [see Appendix C, Figs. 11(c) and 11(d)]. For the vibrational mode at 1078cm−1 [ν(CS),a1], the Raman intensity of the BNA/Ag2S-QDs/PATP system shows a 432% enhancement over the BNA/PATP system and a 3449% enhancement over the Au/Ag2S-QDs/PATP system. Moreover, for 1138cm−1 [δ(CH), b2], the Raman intensity of the BNA/Ag2S-QDs/PATP system shows a 1669% enhancement over the BNA/PATP system and a 4700% enhancement over the Au/Ag2S-QDs/PATP system. Thus, the synergistic effect of the E-field enhancement and CT results in the largest enhancement for the Ag2S-QDs/BNA-arrays system. The mapping analysis obtained by the band at 1138cm−1 indicates that the Raman enhancement in the BNA/Ag2S-QDs/PATP system under 785 nm laser excitation is much stronger near the BNA array compared to the enhancement at the non-BNA regions, which is consistent with the simulated enhancement of the E-field (see Appendix D). Meanwhile, to further study the synergistic effects of the E-field enhancement and CT, we measured the SERS spectra of the Ag2S-QDs/PATP, BNA/PATP, and BNA/Ag2S-QDs/PATP systems under 473 and 532 nm excitation wavelengths (see Appendix E). The synergistic effects of the E-field enhancement and CT resonance make the SERS intensity larger under 532 nm laser excitation compared to the 473 nm laser excitation.

Moreover, we studied the SERS polarization dependence effects of PATP adsorbed on BNA and Ag2S QDs coated BNA arrays. As discussed earlier, the SERS intensity is influenced by the EM and CT processes. Therefore, the influence of polarized light on the E-field intensity and CT process will directly lead to a change in the Raman intensity. Figure 8(a) shows the SERS spectra for PATP adsorbed on BNA arrays under different polarization angles (θpola) of light excitation (785 nm laser excitation). One can see that the SERS intensity is strongest under s-polarized (θpola=0°, E-field oscillation along the bowtie axis) light excitation. Following increased polarization angles, the SERS intensity gradually decreases and is lowest under p-polarized (θpola=90°, E-field oscillation normal to the bowtie axis) light excitation. In addition, after depositing Ag2S QDs onto the surface of BNA arrays, the SERS intensity of the PATP probe molecules gets further enhanced and the b2 mode at 1138cm−1 gets selectively enhanced due to the CT process, as shown in Fig. 8(b). Similar to the BNA/PATP system, the SERS intensity for the BNA/QDs/PATP system also gradually decreases with increasing polarization angles and has its strongest and lowest enhancements under s-polarized and p-polarized light excitation, respectively. To intuitively analyze the variation in SERS intensities for PATP under different polarization angle excitations, the peaks at 1078cm−1 [ν(CS), a1] in a BNA/PATP system, 1075cm−1 [ν(CS), a1] and 1138cm−1 [δ(CH), b2] in a BNA/QDs/PATP system were chosen as characteristic peaks for further investigation of the experimental results. As shown in Fig. 8(c), the SERS intensity at 1075cm−1 and 1138cm−1 for the BNA/QDs/PATP system has the same trend as that of 1078cm−1 for the BNA/PATP system, where Raman intensity gradually decreases with an increasing polarization angle of light excitation. Notice that, even under p-polarized (θpola=90°) light excitation, the intensity of the 1138cm−1 [δ(CH),b2] Raman mode is still selectively enhanced for the BNA/QDs/PATP system, which indicates that the CT process takes place at the interface. Figure 8(d) shows the calculated degree of CT for 1138cm−1 under different polarization angles of light excitation. It can be seen that they have nearly the same degrees of CT under different polarization angles, which means that the CT process has not been affected by light polarization. The change in SERS intensity is mainly caused by the change in the degree of the E-field enhancement in the gap region due to the change in the direction of the E-field oscillation of incident light.

To further investigate the dependence of the local E-field enhancement on the polarization angles of the light excitation, we performed numerical simulations by the FDTD method to calculate the local E-field distribution of BNA arrays. Figures 8(e) and 8(f) show the spatial E-field distribution (|E|2/|E0|2) in the x–y and x–z planes under different polarization angles at 785 nm light excitation. One can see that in Fig. 8(e), under s-polarized light excitation (θpola=0°), it has the strongest E-field intensity localized in the gap region of BNA, which is caused by LSPR coupling between the closely spaced antenna corresponding to the dipolar LSPR mode. This large enhancement in the E-field leads to the strongest intensity of the SERS signal. When the angle of the polarized light was increased (θpola: from 30° to 60°), the E-field intensity showed a fast decrease in the gap region, but a slight increase at the outer corners. So the intensity of SERS spectra also decreased slightly. Under p-polarized light excitation (θpola=90°), nearly no E-field localized in the central gap. However, it showed the strongest E-field intensity at the outer corner caused by the interaction of two BNA unit cells along the y-axis in the arrays. But the enhanced intensity is still far less than that under s-polarization, which leads to the lowest intensity of SERS spectra. Figure 8(f) shows the spatial E-field distribution in the x–z plane with light polarization angles. It obviously shows that the E-field intensity decreased in the gap region of the BNA.

In conclusion, the synergistic effect of interfacial CT and E-field enhancement is demonstrated in the BNA/Ag2S-QDs/PATP system for the SERS study in the NIR region based on PATP as probe molecules. The strongly enhanced SERS signals with selective enhancement in the b2 Raman modes are observed in the BNA/Ag2S-QDs/PATP system under 785 nm laser excitation, which is attributed to the synergistic effect of CT resonance at the interface of the BNA–QDs–PATP and the localized E-field enhancement around BNA arrays. The E-field enhancement in the broad spectral region ranging from the excitation line at 785 nm up to the scattered Raman signal wavelength provides enhancements in the E-fields of excitation as well as scattered Raman signals. FDTD simulation is also performed to simulate the E-field enhancement spectrum and spatial distribution of the enhanced E-field around the BNA arrays. The Ag2S-QDs/PATP, Au-film/PATP, and BNA/PATP substrate systems having either a benefit from interfacial CT or from an E-field enhancement mechanism lead to very weak or no SERS enhancement. However, the synergistic effect of CT and E-field enhancement results in a large enhancement in the Raman intensity and further improves the efficiency of SERS-based detection in a BNA/Ag2S-QDs/PATP system. Furthermore, the SERS polarization dependence effects of the BNA/Ag2S-QDs/PATP system were also investigated. The SERS spectra indicated that the polarization dependence of the substrate increased with decreasing polarization angles (θpola) of excitation from p-polarized (θpola=90°) excitation to s-polarized (θpola=0°) excitation. The synergistic effect of CT and E-field enhancement studied in the BNA/Ag2S-QDs/PATP system provides a guidance for the development of semiconductor QDs-based plasmonic SERS substrates, and may be extended to other material-nanostructure systems for optoelectronic, electro-optical, sensing, and energy applications.