Excitation-Dependent Anisotropic Raman Response of Atomically Thin Pentagonal PdSe₂

Abstract

The group-10 noble-metal dichalcogenides have recently emerged as a promising group of two-dimensional materials due to their unique crystal structures and fascinating physical properties. In this work, the resonance enhancement of the interlayer breathing mode (B1) and intralayer A_g¹ and A_g modes in atomically thin pentagonal PdSe₂ were studied using angle-resolved polarized Raman spectroscopy with 13 excitation wavelengths. Under the excitation energies of 2.33, 2.38, and 2.41 eV, the Raman intensities of both the low-frequency breathing mode B1 and high-frequency mode A_g¹ of all the thicknesses are the strongest when the incident polarization is parallel to the a axis of PdSe₂, serving as a fast identification of the crystal orientation of few-layer PdSe₂. We demonstrated that the intensities of B1, A_g, and A_g³ modes are the strongest with the excitation energies between 2.18 and 2.38 eV when the incident polarization is parallel to PdSe₂a axis, which arises from the resonance enhancement caused by the absorption. Our investigation reveals the underlying interplay of the anisotropic electron–phonon and electron–photon interactions in the Raman scattering process of atomically thin PdSe₂. It paves the way for future applications on PdSe₂-based optoelectronics.

Introduction

Pentagonal PdSe₂, a representative candidate in the two-dimensional (2D) group-10 noble-metal dichalcogenides, has received broad attention due to its layer-dependent indirect bandgaps from ∼0 eV (bulk) to ∼1.3 eV (monolayer), robust air stability, in-plane anisotropy, and high carrier mobility.^1,2 Specifically, the breakthroughs in PdSe₂ synthesis techniques such as the bulk crystal growth¹ and chemical vapor desposition^3,4 of ultrathin PdSe₂ have been demonstrated. The rapid progress ignites the studies on the physical properties and device applications of PdSe_2, such as thermoelectrics,⁵ photodetectors,⁶⁻⁸ and field-effect transistors (FET).⁹⁻¹¹ Furthermore, studies on the anisotropy of lattice vibrations of both strained¹² and unstrained PdSe₂¹³ have been reported. However, the in-depth understanding of the anisotropic Raman response of 2D PdSe₂ affected by different laser excitation energies remains underexplored.

The electron–photon and electron–phonon interactions in solids play an essential role in the resonant Raman response of anisotropic layered materials. The studies in the Raman response of the anisotropic layered materials such as black phosphorus¹⁴⁻¹⁶ (BP), ReS₂,¹⁷⁻¹⁹ and ReSe₂²⁰ are often complicated by the intricate thickness, stacking order, orientation, and excitation-energy-dependent photon–electron–phonon interactions, which arise from the highly anisotropic electronic band structures and phonon vibrations. Since resonant Raman scattering with a wide range of selective excitation energies could provide much more information on the interband electronic transitions and their interactions with phonons, it has served as a powerful tool in unveiling the anisotropic symmetry-dependent electron–phonon coupling of anisotropic 2D materials^21,22 and heterostructures.²³ For instance, Mao et al.²⁴ presented systematic studies on the resonance Raman spectroscopy of BP and suggested the explicit anisotropic resonant Raman response with the excitation energy between 2.60 and 2.73 eV. Other work also indicated the unambiguous anisotropic resonant Raman response with excitation energy of 2.81 eV (441.6 nm).^25,26 Nevertheless, the experimental investigations on the resonant Raman response of anisotropic PdSe₂ and theoretical understanding of the corresponding electron–photon and electron–phonon interactions are still limited. Recently, Puretzky and co-workers reported the unprecedently strong Raman intensities of low-frequency interlayer vibrations in PdSe₂ under 2.33 eV (532 nm excitation).¹³ This is quite unusual compared to the weak low-frequency interlayer Raman modes commonly observed in many other layered materials.^27,28 Therefore, a comprehensive study on the PdSe₂ low-frequency interlayer phonon response under different resonance conditions is also highly desired.

Herein, we investigated the excitation-energy-dependent and polarized Raman response of both interlayer and intralayer vibrations of atomically thin (1–7 L) pentagonal PdSe₂ with 13 excitation laser lines (454–648 nm). We demonstrate that with the excitation energies of 2.41 eV (514 nm), 2.38 eV (520 nm), and 2.33 eV (532 nm), the maximum Raman intensities of B1 and A_g¹ modes appear when the incident laser polarization is along the PdSe₂a axis for all the thicknesses. Hence, the crystal orientation of PdSe₂ can be identified unambiguously. Via the analysis of resonant excitation profiles, we unveil that Raman intensities of B1, A_g and A_g³ modes are the strongest under the excitation energy of 2.33 eV (532 nm) when the incident polarization is along the a axis of PdSe₂. In addition, the A_g mode shows a second strongest Raman intensity under the excitation energy of 2.54 eV (488 nm). Furthermore, the measured absorption results show two broad absorption peaks at 1.9–2.3 and 2.6–3.0 eV, respectively, where the Raman polarizability is promoted and enhances Raman intensities. Therefore, the maximum Raman intensities of B1, A_g¹, and A_g modes appearing at different energy ranges and specific crystal orientations could be attributed to the anisotropic electron–phonon coupling and the symmetry-dependent excited states involved in the resonance Raman scattering. Our work lays the foundation for further studies and applications on PdSe₂-based optoelectronic devices.

Results and Discussion

The PdSe₂ crystal structure with a unique pentagonal configuration is illustrated in Figure 1(A). A single layer of PdSe₂ has a periodic corrugated structure; each Pd atom is connected to four Se atoms, while each Se atom is bonded with two Pd atoms and another Se atom. Pentagonal PdSe₂ exhibits layer-dependent crystal symmetries:¹³ the bulk PdSe₂ crystal (> 20 L) belongs to Pbca (#61) space group symmetry and D_2h point group symmetry; for the odd-layer PdSe₂, its crystal symmetry reduces to the point group C_2h (2/m) with the space group belonging to P2₁/c (#14). In contrast, even-layer PdSe₂ belongs to space group Pca2₁ (#29) and point group C_2v (mm2).

Figure 1.

Illustration of the PdSe₂ crystal structure and identification of PdSe₂ crystal orientations and thicknesses. (A) Crystallographic structure of a 1L 2D pentagonal PdSe₂. (B) Optical images of the exfoliated 1–4L PdSe₂ studied in this work. (C) Illustration of atomic displacements of B1, A_g¹, and A_g phonon modes. (D) Raman spectra of 1–4L PdSe₂ where the laser (532 nm excitation) polarization is along the a axis of PdSe₂.

Figure 1(B) shows a typical exfoliated PdSe₂ sample on a 285 nm SiO₂/Si substrate, with the black arrow representing the horizontal polarization of the incident laser as explained in the Methods section. Note that different numbers of layers of PdSe₂ in the same flake have the same crystallographic orientation. The exfoliated PdSe₂ flakes tend to cleave along their two principal crystal axes ([100] and [010]), which often results in the rectangular shapes, where the long and short edges are along the PdSe₂a axis ([100] direction) and the b axis ([010] direction), respectively.^3,12 Also, the two principal PdSe₂ crystal axes were further characterized and shown in Figure S1(A–D), where the maximum Raman intensities of B1 (2 – 4 L) and A_g¹ modes (1 – 4 L) appear when the laser polarization is parallel to the PdSe₂a axis (long edge) under 532 nm excitation, which agrees with previous studies.^12,13

Figure 1(C) illustrates the atomic displacements of the low-frequency breathing mode B1 and two high-frequency modes A_g¹ and A_g: B1 represents all the atoms vibrating out of the plane, A_g¹ is affected by the in-plane vibration of covalently bonded Pd and Se atoms, and A_g is affected by the in-plane vibration of two neighboring covalently bonded Se atoms. Figure S2 includes more details of the vibration patterns of A_g¹ and A_g modes.

Figure 1(D) shows the corresponding Raman spectra of different locations, and their thicknesses are determined explicitly by their low-frequency (LF) interlayer Raman fingerprints (<100 cm^–1). Following the previous works,^12,13 the LF modes’ notations are assigned, and their frequencies are summarized in Figure S3(A). Moreover, for monolayer PdSe₂, A_g¹ and A_g Raman frequencies are at ∼150 and ∼263 cm^–1, respectively; for 2L PdSe₂ and above thicknesses, their Raman frequencies are at ∼145 and ∼258 cm^–1, respectively, which agrees with previous work.¹ In addition, the Raman spectra of monolayer and three bulk PdSe₂ flakes (F1–F3) in the same crystal are shown in Figure S3(B) for comparison.

After the thickness and crystal axes were determined, the excitation-energy-dependent Raman modes of bilayer PdSe₂ were analyzed. In specific, we focused on the B1, A_g¹ (∼150 cm^–1), and A_g (∼263 cm^–1) modes due to their strong intensities among all PdSe₂ Raman modes and thicknesses.

Figure 2(A) shows the resonance excitation profiles (REPs) obtained at the a axis of bilayer PdSe₂: B1, A_g¹, and A_g Raman intensities all maximize at 2.33 eV (532 nm) excitation; A_g³ modes also show the second strongest intensities at 2.54 eV (488 nm), supported by Figure 2(D), where the mode intensities at different directions under all excitation energies were summarized. Figure 2(B) shows the representative Raman spectra of bilayer PdSe₂ excited by eight laser lines from 2.61 eV (476 nm) to 2.18 eV (568 nm). The low-frequency B1 mode shows the strongest intensity at 2.33 eV (532 nm) excitation; it then reduces notably with increasing excitation energies. Figure 2(C) shows the complete REPs obtained at the b axis of bilayer PdSe₂: the A_g mode shows its strongest Raman intensity with 2.33 eV (532 nm) excitation. However, B1 and A_g¹ Raman intensities decrease with increasing excitation energies from 2.54 eV (488 nm) and become utterly unnoticeable from 2.66 eV (466 nm) and 2.68 eV (462 nm), respectively; the A_g mode remains visible at all excitation energies despite decreasing with increasing excitation energies from 2.54 eV (488 nm). Figure 2(D) summarizes the intensities of B1, A_g¹, and A_g modes under different excitation energies of bilayer PdSe₂: between the excitation energies of 2.18 eV (568 nm) and 2.41 eV (514 nm), B1 and A_g¹ modes show significantly stronger intensities when the incident polarization is at the a axis of bilayer PdSe₂, while those of A_g are almost similar at both the a and b axes of bilayer PdSe₂ across all the excitation energies. The mode intensities of B1, A_g¹, and A_g of other few-layer samples (1 L, 3 – 7 L) and the A_g¹ and A_g of three thick flakes (30, 35, and 80 nm) are summarized in Figures S4 and S5.

Figure 2.

(A,C) Resonant excitation profiles (REPs) after normalization (by using the Raman peak intensity of a single-crystal quartz substrate at ∼465 cm^–1) of a 2L PdSe₂ sample with the laser polarization along the PdSe₂a and b axes, respectively. (B) Raman spectra of bilayer PdSe₂ are excited by eight laser lines from 476 to 568 nm, and the laser polarization is parallel to the PdSe₂a axis. (D) Summary of normalized intensities of B1, A_g¹, and A_g modes under different excitation energies after the interference effects are removed, with the laser polarization along the PdSe₂a and b axes, respectively.

The polar plots describing the angle-dependent mode intensities and mode intensity ratios are investigated to gain more insights from the anisotropic Raman intensities at different orientations. Note that similar approaches have been utilized to study the electron–phonon interactions in black phosphorus,^14−16,24 ReS₂,²⁰ and ReSe₂,²⁹ which are often intricated due to different thickness and excitation energies. The interference enhancement effects that influence the Raman intensities were computed and excluded to get the intrinsic Raman intensities (see more details in Figures S6 and S7). Figure 3(A,B) shows the angle-resolved polarized Raman results of A_g¹ and A_g phonons of bilayer PdSe₂ excited by laser photon energies from 2.18 to 2.54 eV: the A_g¹ modes always show the strongest intensities when the incident polarization is along the PdSe₂a axis, while the A_g mode intensities have negligible variations along different crystal orientations. Moreover, the mode intensities of A_g¹ and A_g of different numbers of layers (2 – 5 L) with an excitation energy of 2.33 eV (532 nm) are shown in Figure 3(C,D), suggesting the A_g¹ modes show their strongest intensities when the incident polarization is along the PdSe₂a axis for 2–5L PdSe₂, and the A_g mode intensities are insensitive to the incident polarization for 2–5L PdSe₂.

Figure 3.

(A,B) Raman intensity polar plots of A_g¹ and A_g modes of 2L PdSe₂ excited by different laser photon energies. (C,D) Raman intensity polar plots of A_g¹ and A_g modes of 2–5L PdSe₂ with 532 nm laser excitation. (E) Computed vibration patterns of PdSe₂ A_g¹ and A_g phonons.

More data for other thicknesses and excitation energies can be referred to Figures S8 and S9. In Figures S8(A–E), we compare Raman intensity polar plots of the low-frequency breathing modes B1, B3, and B5 with A_g symmetry and high-frequency A_g¹ phonons for different thickness samples excited by the same laser photon energy of 2.33 eV (532 nm): all the results exhibit explicit dumbbell patterns; specifically, B1 and A_g mode intensities always maximize and minimize at the PdSe₂a axis (0°) and the b axis (90°) respectively. Moreover, Figure S9(A–F) shows Raman intensity polar plots of the low-frequency B1 and high-frequency A_g¹ phonons of the same thicknesses (2 L, 4L, and 6 L) excited by the different laser photon energies from 1.91 eV (648 nm) to 2.61 eV (476 nm): mode intensities of B1 and A_g always maximize and minimize at the PdSe₂a axis (0°) and the b axis (90°), respectively. Furthermore, we inferred that only three excitation energies, 2.33 eV (532 nm), 2.38 eV (520 nm), and 2.41 eV (514 nm), would result in B1 and A_g¹ mode intensities always maximizing and minimizing at the PdSe₂a axis (0°) and the b axis (90°) through the analysis of the mode intensity ratios and shown in Figures 4(A–B) and S10(A–D). In short, the angle-dependent polarized Raman spectroscopy with the excitation energies of 2.33 eV (532 nm), 2.38 eV (520 nm), and 2.41 eV (514 nm) could serve as a fast and efficient approach for identifying the crystal orientation of few-layer PdSe₂.

Figure 4.

(A) Ratios of the normalized intrinsic Raman intensity of in 2–5L PdSe₂. (B) Ratios of the normalized intrinsic Raman intensity of in 2–5L PdSe₂.

To probe the origins of different angle-dependent behaviors of B1, A_g¹, and A_g mode intensities, we performed analysis on their computed vibrational patterns. Note that the phonon vibration direction affects the electron–phonon interactions in anisotropic 2D materials significantly.²⁴ For instance, Mao et al. differentiated the strengths of electron–phonon interactions of black phosphorus’s A_g phonons via the analysis of phonon vibrational patterns and relative mode intensity ratios.²⁴Figure 3(E) illustrates the simulation results by the density functional theory (DFT) approach: the top-view atomic displacements of A_g¹ and A_g modes in PdSe₂, respectively. The vibration of the A_g¹ mode is preferentially along the PdSe₂a axis with the Se atoms vibrating roughly 30° along the PdSe₂a axis. In contrast, the A_g mode is influenced by the Se atoms vibrating roughly 45° to both the PdSe₂a and b axes, implying that its vibration is less orientation-selective. Hence, the electronic transition dipole that is preferentially coupled to the linear laser polarization along the a axis rather than along the b axis gives rise to the stronger Raman intensity of the coupled A_g¹ phonon along the a axis; for the A_g mode, changing the laser polarization from the direction of the a to the b axis does not affect its electron–phonon coupling. Although all the atoms vibrate almost vertically out-of-plane for the B1 mode, it does not prefer a certain in-plane crystal orientation. We also noticed that the low-frequency B1 mode shares similar electron–phonon coupling behavior with the high-frequency A_g¹ mode (see the polar diagrams of the B1 mode in Figures S8 and S9). Such a similarity arises from the stronger-than-vdW interlayer coupling (due to the covalent contribution) and the unique atomic structure with two different types of atoms in each layer of PdSe₂.¹ In addition, the displacement of intralayer covalent bonds would contribute to the breathing vibrations and result in the electron–phonon coupling behavior of B1 that might resemble some high-frequency modes.¹³

To gain more insights into the anisotropic electron–phonon interactions in B1, A_g¹, and A_g phonons, we compared their Raman intensity ratios. Figure 4(A,B) shows the Raman intensity ratios and of 2–5L PdSe₂ along the a and b axes, respectively: with excitation energies from 2.18 eV (568 nm) to 2.54 eV (488 nm), both and are larger than 1. More data of the other thicknesses are shown in Figure S10(A,B): with excitation energies from 2.33 eV (532 nm) to 2.41 eV (514 nm), both and are larger than 1; Figure S10(C,D) suggests that the mode intensity ratio of for all the studied thicknesses and excitation energies is generally close to 1, which supports that A_g³ couples similarly with the incident light polarization along both the PdSe₂a and b axes, as discussed above, based on its vibration pattern.

Furthermore, the intrinsic Raman intensities of B1, A_g¹, and A_g phonons were extracted and are shown in Figure S11(A–F): when the incident polarization is along the a axis of PdSe₂, the B1 and A_g¹ modes show their strongest intensities between the excitation energies of 1.91 eV (648 nm) and 2.33 eV (532 nm); A_g modes show the strongest intensities between 2.61 eV (476 nm) and 2.68 eV (462 nm) excitation. When the incident polarization is along the b axis of PdSe₂, the maximum B1 and A_g¹ mode intensities generally appear under the excitation energies of 2.33 eV (532 nm) and 2.68 eV (462 nm); A_g modes show their strongest intensities with excitation energies between ∼2.6 eV (476 nm) and 2.68 eV (462 nm), similar to the case when the incident polarization is along the a axis of PdSe₂. On the other hand, the absorption measurements of few-layer PdSe₂ (as shown in Figure S12) indicate two broad absorption peaks centered at ∼2.3 eV (full width at half maximum (fwhm) of ∼0.3 eV) and ∼2.9 eV (fwhm of ∼0.3 eV), which match with the strong resonance conditions of B1, A_g¹, and A_g phonons, respectively. Meanwhile, a recent work³⁰ demonstrated the orientation-dependent absorption of few-layer PdSe₂, where the lower energy absorption peak exhibits a blue shift when the incident polarization switches from the PdSe₂a axis to the b axis. Such a transition matches the trend mentioned above. Therefore, the anisotropic absorption dominates the discrepancies of resonance conditions of B1, A_g¹, and A_g modes along with different incident polarizations.

Moreover, the relative strengths of the electron–phonon interaction among the B1, A_g¹, and A_g modes were compared by calculating the intensity ratios of and as a function of the excitation energy. As shown in Figures 5(A,C), for 2–5L PdSe₂, the intensity ratios of and are both larger than 1 under the excitation energies of 1.91 eV (648 nm) and 2.18 eV (568 nm) when the laser polarization is along the PdSe₂a axis, indicating the stronger electron–phonon coupling of B1 and A_g¹ along the a axis than that of A_g along the a axis. However, with larger excitation energies, the ratios start decreasing to less than 1, suggesting that the electron–phonon coupling of A_g³ along the PdSe₂a axis surpasses those of B1 and A_g modes; on the other hand, as shown in Figure 5(B,D), for 2–5L PdSe₂, the ratios of and are always less than 1 when the laser polarization is along the PdSe₂b axis, suggesting that the electron–phonon coupling of A_g³ along the b axis is always larger than those of B1 and A_g for all the excitation energies. Such a trend also agrees roughly with the previous discussion of resonant energy windows for B1, A_g¹, and A_g modes: 1.91 eV (648 nm)–2.33 eV (532 nm) for B1 and A_g¹ phonons and 2.61 eV (476 nm)–2.68 eV (462 nm) excitation for the A_g mode. More data for other thicknesses are shown in Figure S13(A–D), which show consistent results with those for 2 – 5 L. Nonetheless, no distinct trend was observed for the ratios of : as shown in Figure S14(A–D), the excitation-energy-dependent ratios of fluctuate around ∼1 for most thicknesses with incident polarization along both the PdSe₂a and b axes. To summarize, the distinct similar trend of and suggests that the electron–phonon interactions of the low-frequency B1 mode resemble that of the high-frequency A_g¹ mode, which is further supported by their mode intensity ratios .

Figure 5.

(A,B) Ratios of the normalized intrinsic Raman intensity of with the laser polarization parallel to the PdSe₂a axis (A) and the b axis (B). (C,D) Ratios of the normalized intrinsic Raman intensity of with the laser polarization along the PdSe₂a axis (C) and the b axis (D).

Conclusion

We presented comprehensive investigations on the resonant Raman response of atomically thin PdSe₂ (1 – 7 L) with 13 excitation wavelengths covering the visible range. Under the excitation energies of 2.33, 2.38, and 2.41 eV, the intrinsic interlayer B1 and intralayer A_g¹ mode intensities are the strongest when the incident polarization is along PdSe₂a axis, serving as a convenient method for fast and unambiguous identification of the crystal orientation of few-layer PdSe₂. In contrast, the intralayer A_g mode intensities are insensitive to the incident polarization. Our DFT simulation results reveal that the vibration of A_g³ mode is dominated by the Se atoms vibrating roughly 45° to both the PdSe₂a and b axes, suggesting a weaker mode anisotropy and leading to the similar Raman intensities measured at different orientations. Also, among all the excitation wavelengths, the intrinsic B1 and A_g mode intensities generally maximize between 520 nm (2.38 eV) and 648 nm (1.91 eV) when the incident polarization is parallel to the a axis of PdSe₂; the intrinsic A_g³ mode intensities generally maximize under 466 nm (2.66 eV) and 454 nm (2.73 eV). Meanwhile, the absorption measurements reveal two broad peaks at 1.9 – 2.3 and 2.6 – 3.0 eV, respectively. Hence, we attributed that the maximum B1, A_g, and A_g³ Raman intensities appear at different excitation wavelength ranges and specific crystal orientations to the anisotropic electron–phonon coupling and the symmetry-dependent excited states involved in the resonance Raman scattering. Moreover, we illustrated that the trend and strength of electron–phonon interactions in the low-frequency B1 mode are similar to the high-frequency mode A_g for all the thicknesses and photon energies studied here. This comprehensive investigation of the anisotropic Raman response of PdSe₂ would shed light on future Raman studies of 2D materials and the development of PdSe₂-based optoelectronics.

Methods

PdSe₂ thin flakes were first mechanically exfoliated onto a 300 nm SiO₂/Si wafer using Nitto SPV 224R blue tape from a bulk PdSe₂ single crystal that was synthesized using a self-flux method.¹ The thicknesses of the bulk flakes were characterized by a Veeco Dimension 3000 atomic force microscope (AFM), while those thinner ones (1 – 7 L) were characterized by their characteristic ultralow frequency Raman modes. All Raman measurements were performed using a Horiba T64000 triple-grating micro-Raman system with a backscattering configuration using 13 different excitation energies from 1.91 eV (648 nm) to 2.73 eV (454 nm) (Ar/Kr, Coherent Innova 70C). A 100× Olympus (NA = 0.95) objective lens and an 1800 g/mm groove grating were selected to collect and disperse Raman signals.

The angle-resolved Raman measurements were performed to find and distinguish the two main axes (i.e., a and b axes) of 10 PdSe₂ flakes (1–7 L, ∼30, ∼35, and 80 nm). All Raman spectra were collected under the parallel configuration, where the scattered light polarization is parallel to that of the incident light by using an analyzer. PdSe₂ samples were rotated every 15° to change the relative angle between their crystal orientations and the laser polarization. The laser power was controlled at 0.5 mW (±1%) for each excitation energy. All the collected Raman spectra were calibrated using the Raman peak intensity of a single-crystal quartz substrate at ∼465 cm^–1 to eliminate the discrepancy of the instrumental response under different excitation energies.

Plane-wave DFT calculations were carried out using the Vienna Ab initio Simulation Package^31,32 (VASP) with projector augmented wave (PAW) pseudopotentials^31,33,34 for electron–ion interactions, and the generalized gradient approximation (GGA) functional of Perdew, Burke, and Ernzerhof³⁴ (PBE) for exchange-correlation interactions. Based on the bulk PdSe₂ structure (Materials-project database³⁵), the original strain-free 1–3L PdSe₂ was modeled by creating a periodic slab with a vacuum separation of more than 22 Å to avoid the interactions with periodic images in the out-of-plane direction (Z direction). A cutoff energy of 350 eV and 6 × 6 × 1 k-point samplings were used for the optimization of the unit cell (both atomic positions and in-plane lattice constants) until the maximum force allowed on each atom was less than 0.001 eV/Å. The total volume of the structures was fixed during geometry optimization to avoid the structural collapse of the 2D slabs with vacuum separations.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsphyschemau.2c00007.

• Detailed descriptions of the characterizations (angle-polarized Raman spectroscopy, absorption spectroscopy, and AFM), analytical results (normalized mode Raman intensities and mode intensity ratios), and detailed DFT simulation results of Raman mode assignments, visualization of Raman mode vibration patterns, dielectric functions, refractive index, and calculations of interference enhancement (PDF)

The authors declare no competing financial interest.