Cavity-Enhanced Raman Scattering from 2D Hybrid Perovskites

Two-dimensional (2D) hybrid organic-inorganic perovskites (HOIPs) are promising candidates for optoelectronic applications due to their efficient light emission properties, and strong dielectric confinement effects. Raman spectroscopy is a versatile, non-contact and often non-destructive technique, widely used to characterize crystalline materials. However, the inherently weak phonon scattering, strong background, and complex nature of the Raman signals from HOIPs present challenges in obtaining reliable signals. Further, the fragile nature of the 2D HOIP crystals results in rapid degradation upon exposure to heat, light and moisture, which presents further difficulty in enhancing Raman scattered photon signals. Herein, we report a novel approach to enhance the weak Raman scattering signals in Ruddlesden-Popper (RP) phase HOIPs by introducing an open-cavity comprising HOIP crystals on a gold substrate. We observe 15x enhancement of the Raman signals due to the Purcell effect inside the high-index HOIP crystals. Our simple approach can be extended to enhance the study of phonon scattering in other HOIP and van der Waals layered crystals.


INTRODUCTION
Two-dimensional (2D) Ruddlesden−Popper (RP) hybrid organic-inorganic perovskites (HOIPs) have attracted much attention due to their uniquely tunable structure, strong quantum confinement and extraordinary performance in optoelectronic devices 1 . The RP phase of these HOIPs have a layered crystal structure represented by the chemical formula (RNH3)2(MNH3)n-1AnX3n+1, where 'R' is an alkyl or aromatic moiety, 'M' is a methyl group (-CH3), 'A' is a metal cation (Pb), 'X' is a halide (I, Cl, Br), and 'n' is an integer that defines the number of inorganic (PbI2) layers of corner-sharing [PbI6] 4− octahedra confined by two adjacent organic layers 2 .
[PbI6] 4− layers act as quantum wells, and organic layers as dielectric spacers which play a crucial role in charge confinement, hydrophobicity, structural rigidity, and stability 3 .
Physical properties of HOIPs such as charge confinement can be tuned by the integer n, electron-phonon (e-p) coupling by organic moiety, and band structure by halide anion and external strain 3,4 . Further, the e-p coupling is responsible for scattering and provides decay channels for the excitons, which leads to the determination of absorption and emission properties of the material 5,6 . Therefore, investigation of vibrational dynamics of HOIPs is of paramount importance as they possess the information about e-p interaction, structural and compositional changes, symmetry of lattice vibration, temperature and pressure phase diagram, degree of crystallinity, dielectric screening, as well as elastic properties of the crystal 2,3 .
Raman spectroscopy plays a crucial role in the study of vibrational dynamics of materials. Unfortunately, HOIPs have inherently weak optical phonon scattering and strong background from the relaxation of electronic excited states 7 when excited above the bandgap. In addition, several overlapping and complex Raman modes smother crucial information about their vibrational spectra and properties 8 . HOIPs exposed to excessive power of laser excitation within the bandgap of the material are prone to degradation due to the thermal stress induced by the laser [8][9][10] . Degradation in HOIP crystals introduces additional Raman peaks artifacts making the characterization of the material, and the assignment of the modes very complex and challenging 3,11,12 . These material challenges/limitations compel the search and development of a consistent technique or means to characterize the vibrational spectra from such environment sensitive materials via Raman scattering.
HOIPs have crystal lattice that intrinsically exhibits low probability for Raman scattering. Enhancement in Raman scattering can be achieved by electromagnetic enhancement by placing the semiconductor over metallic nanoparticles for surfaceenhanced Raman spectroscopy, or placing them between nanogap metallic cavities for tip-enhanced Raman spectroscopy 13 . However, the aforementioned material limitations in terms of thermal and chemical stability constitute a significant drawback in enhancing the phonon scattering from the HOIP crystals.
In this paper, we fabricated an open-cavity comprising the RP perovskite flakes on a gold substrate (which acts as a reflector) and exploit the Purcell effect to increase the probability of Raman scattering by enhancing light trapping in the crystal. The extraordinary optical constants of HOIP crystals facilitate the strong light-matter coupling in thick crystals (thickness, t > 100 nm) which leads to photonic open-cavity formation in crystals themselves 1,14 . When the laser shines on the HOIP/gold, the laser undergoes multiple reflections inside the dielectric cavity created by the flat surfaces of the HOIP crystal. Due to the Purcell effect inside the crystals, the optical electric field is enhanced which results in enhanced Raman scattering by a factor of up to 15x.
Our work exhibits a simple approach to enhance Raman scattering which can be extended to enhance phonon scattering in other van der Waals crystals.

EXPERIMENTAL METHODS
Sample preparation: Gold template substrates were prepared by depositing a 100 nm thick film of gold on a silicon wafer, and a mixer of two epoxy components was dropped over it. Silicon substrates were placed over epoxy drops and heated to 100 C for 30 min. After cooling, template-stripped gold substrates were ready to use 15 . Electric field distribution simulation: Transfer-matrix-method (TMM) was implemented using python based on the theory presented in the reference 16 to simulate the electric field (E-field) distribution profiles of 633 and 785 nm lasers for RPn placed on quartz and gold substrates. Required optical constants of RPn have been obtained from ellipsometry measurements 14 . Figure 1a shows the schematic of the crystal structure of RP phase lead iodide based HOIP whose general chemical formula is (BA)2(MA)n-1PbnI3n+1, where BA is butylammonium and MA is methylammonium 2,3 . Therefore, RPn (n= 1, 2, 3…) can be chemically expressed as (BA)2PbI4, (BA)2(MA)Pb2I7 and (BA)2(MA)2Pb3I10 or in short, RP1, RP2 and RP3, respectively. In our previous work 1 , we reported that thick (t > 100 nm) RP HOIP crystals placed on gold show strong light-matter coupling (excitonpolariton formation) and form an open-cavity with quality factor ~250. Here, we exploit the Purcell effect in HOIP to enhance the emission of Raman scattering, using the 633 and 785 nm lasers, which are far from the excitonic absorption ( Figure S1, Supporting Information). Purcell proposed that spontaneous emission rate can be modified (enhanced) by placing the emitter inside a cavity 17 . Using the Purcell effect, enhanced radiative emission from a variety of semiconducting and atomic systems has been reported 18,19 . Here, we use the same phenomenon for enhancing the Raman scattering signals from a weak phonon scattering crystal such as 2D HOIPs.

RESULTS AND DISCUSSION
In our samples, light is trapped inside the cavity due to multireflection from the bottom gold substrate and top HOIP crystal-air interface. The required minimum volume of the cavity under diffraction limit is approximately calculated by Where λ0 is the free-space wavelength of light and n' is the refractive index of the material inside the cavity 20  rather than thin ones (t < 30 nm) ( Figure S1) as reported before [1].   Information ( Figure S2). As 785 nm laser is used for pumping which is non-resonant with the exciton-polariton modes from all three RPn used in this study ( Figure S1).  (Figures 2c,e).

Figure 2. Cavity-enhanced Raman signals from thick crystals of RPn/gold compared to RPn/quartz measured by 785 nm laser line (non-resonant to all RPn) at 80K for (a) inorganic and organic layers and (b) organic layers of HOIP crystals. Raman spectra of RP1 (c, d) and RP2 (e, f) by 633 nm laser at 80K.
To validate the universality of the open-cavity effect, Raman spectra of the same samples were recorded using 633 nm excitation laser as well (Figures 2c-f). It is worth mentioning that, the huge background in Raman spectra by 633 nm laser, unlike the flat background by 785 nm, is due to our broad cut-off wavelength in the 633 nm edge filter of the Raman system (see Figure S3, Supporting Information). At 80K, Raman signals of thick RP3 by the 633 nm laser were subjugated by polaritonic emission at ~ 645 nm (see Figure S1, Supporting Information). Compared to the 785 nm laser, Raman measurements by the 633 nm laser show more well-resolved modes at low powers (< 30 μW) due to (a) Raman scattering intensity is inversely proportional to

excitation wavelength and (b) sensitivity of charge-coupled device detector decreases
in the near-infrared region, which applies for the 785 nm laser wavelength 22 . Further, we did Raman measurements of RP3/quartz by 633 nm laser source and found that although it leads to near-resonant Raman scattering, the cavity effect is stronger ( Figure S5). Higher optical E-fields within the crystal vis-a-vis the air-crystal and crystal-substrate interface increases the probability for interaction between the laser beam and the phonons of HOIP crystal, which ultimately leads to enhanced Raman intensity 20,23 . We have also verified that surface plasmons do not play any significant role in this Raman intensity enhancement via a separate control experiment (see supporting information S6).  21,24 . It is noteworthy that as organic spacers BA and MA have similar chemical nature. Therefore, we can assume that the Raman features of (BA)2PbI4, are similar to that of MAPbI3.
A normal mode of vibration is whether Raman-or IR-active, it depends upon its symmetry 25 . Group theory analysis shows that the symmetries of the normal vibrational modes of MAPbI3 can be expressed as: Γoptic = 17Au + 21B1u + 16B2u + 21B3u + 19Ag + 14B1g + 19B2g + 14B3g. In case of inorganic PbI3 network, Au modes are expected to be silent, u modes are IR-active and g modes are Raman-active 26 . Fundamental Raman modes of BA cation fall in the R2 range (1200 -1800 cm -1 ) which comprises of the angular distortion of CH2, CH3, and NH2 groups 27,28 . Observed Raman modes in our experiments in R2 range at ≈1251 cm -1 correspond to CH2 twisting, ≈1300 cm -1 to CH2 wagging, ≈1458 cm -1 to a mixture of CH2 scissoring vibration and CH3 antisymmetric deformation, ≈1556 cm -1 to asymmetric NH3 deformation, and ≈1584 cm -1 to NH3 + degenerate deformation while the mode at ≈1323 cm -1 marked with a blue asterisk is not reported in the literature ( Figure S7) 21,24,27,28 . A plausible explanation for the 1323 cm -1 mode could be that a strong interaction between BA and MA molecules of RP2 on gold with incoming photons has led to the splitting of mode at ≈ 1320 cm -1 into 1300 and 1323 cm -1 29 . This further emphasizes that enhanced light-matter interaction leads to an increase in the intensity of the nominally weak Raman modes of HOIP crystals in addition to observation of hitherto unknown and unseen modes.

CONCLUSIONS
In summary, we show that the Raman scattering in 2D HOIP crystals can be enhanced without compromising the crystal quality or need for complex integration.

S5: Raman spectrum of RP3/quartz using 633 nm laser
Raman spectrum of RP3/quartz was measured using 633 nm laser source at 80K. As RP3 has excitonic absorption at ~ 620 nm, so, 633 nm laser is near-resonant to RP3 HOIP crystal's electronic excitation. Even in near-resonant Raman scattering condition, the Raman modes are not well resolved as compared to RP3/gold (Figures   2a-b). This shows that although 633 nm laser leads to near-resonant Raman scattering, the background emission overshadows the Raman scattering signal. Figure S5: Raman spectrum of RP3/quartz obtained using 633 nm excitation laser source at 80K. The thickness of RP3 flake is ~ 120 nm.

S6: Verification of the role of surface plasmons in Raman scattering
To find the role of surface plasmons in enhanced Raman signals, we did a control experiment of surface-enhanced Raman spectroscopy (SERS) of RP2 crystal. For that, we used ~13 nm sized gold (Au) nanoparticles (NPs) on silicon substrate, whose plasma oscillation is at 620 nm. RP2 HOIP crystals were exfoliated on gold NPs substrate and as, HOIP crystal are prone to heat, these samples were not heated.
Since, the thickness of HOIP flakes is > 100 nm, these crystals are just lying over the NPs and SERS hot spot is away from the HOIP crystal. Figures S6 (d)-(g) clearly show that there is no effect of SERS on RP2/gold-NPs.