Amplified emission from halide perovskite quantum dots by exciton–plasmon-coupled energy transfer in the neutral and trion states on gold nanoparticles

Abstract

Exciton–plasmon coupling enhances the absorption and emission rates, photoluminescence intensity, and device performance of quantum dots in solar cells, light-emitting diodes, lasers, and photocatalysis. While localized surface plasmon resonance coupling has been extensively studied in metal nanoparticle–quantum dot systems, the precise mechanism of plasmon-coupled exciton generation and recombination in halide perovskite quantum dots and nanocrystals remains subtle. We report chemically induced exciton–plasmon coupling between Au nanoparticles and methylammonium lead bromide (MAPbBr₃) perovskite single quantum dots, leading to efficient energy transfer from the plasmon to the neutral and charged (trion) exciton states of quantum dots, significantly enhancing the absorption and radiative relaxation rates. Time-resolved single-particle photoluminescence and transient absorption measurements reveal that Pb–Br⋯Au interactions and chemical interface damping accelerate radiative relaxation and bleach recovery rates. Additionally, focused ion beam-assisted scanning transmission electron microscopy imaging and finite-difference time-domain simulations highlight the roles of Au nanoparticles in exciton–plasmon coupled enhanced light absorption and amplified emission offering insights for high-performance plasmonic–perovskite hybrid devices.

---

Introduction

The rates of recombination, diffusion, and transfer of photogenerated excitons and charge carriers in halide perovskite (HP) systems play a significant role in optimizing the performance of perovskite-based optical, electronic, photodetector, and photovoltaic devices.^1–10 Localized surface plasmon resonance (LSPR) of metal nanostructures has been extensively applied for modulating such rates. Also, the performance of HP devices has been improved by combining them with the LSPR of metal nanoparticles.¹¹ HPs on plasmonic substrates generate strong light–matter coupled states,^12,13 enabling increased power conversion efficiency (from 16.95 to 19.05%) for an HP solar cell encompassing Au nano-octahedra with a broad LSPR peak.¹⁴ Also, 155% emission intensity enhancement and an increased spontaneous emission rate were accomplished for coaxial core/shell perovskite quantum dots (PQDs) embedded on plasmonic Au NP substrates.¹⁵ Another significant impact of exciton–plasmon-coupled systems is the ultralow lasing threshold (1.9 W cm⁻² at 120 K) in the Ag nanocube/CsPbBr₃ PQD/Al₂O₃/Au system by the plasmonic gap-induced localized Purcell enhancement.¹⁶ A similar plasmon-assisted PL enhancement was reported for HP nanowires on a SiO₂/Ag substrate.¹⁷ Generally, the performance of PQDs and their devices, such as photoluminescence (PL) and electroluminescence (EL) intensities, and light-trapping and power conversion efficiencies, can be improved and optimized using exciton–plasmon coupling. However, whether the coupling occurs in the charged or neutral ground or excited state of QDs remains unexplored.

Semiconductor QDs and organic dyes exhibit LSPR-induced PL intensity enhancement by energy transfer, which has been studied at the ensemble and single-particle levels.^18–22 For example, studies have shown significant (1340-fold) PL intensity enhancements for single molecules coupled with a bowtie Au nanoantenna.¹⁹ Trotsiuk et al. reported such PL intensity enhancements for chalcogenide QDs through hot-hole transfer from Au NPs.²⁰ Also, Ma et al. reported a 3-fold enhanced PL intensity for a single CdSe multi-shell QD on an Au NP film.²¹ More recently, Fu et al. reported suppressed PL blinking and enhanced PL intensity for avidin-CdSe/ZnS QDs linked to biotinylated Ag NPs.²² These enhancements are caused by energy transfer and the Purcell effect, which increase the spontaneous emission rate through strong near fields or hot spots.^23–29 The energy transfer and Purcell effect are described by the weak exciton–plasmon-coupling regime based on the perturbation to wave functions.^25–27 When the LSPR frequency is close to the frequency of the molecular energy level of an emitter, energy transfer occurs between the plasmon and exciton, accelerating the radiative recombination. While energy transfer to an emitter by exciton–plasmon coupling enhances the PL intensity, the reverse energy transfer quenches the PL. The effectiveness of the energy transfer process depends on the distance (d_E–M) between an emitter and a plasmonic nanostructure or substrate, with closer distances favoring energy transfer by scaling as 1/d_E–M⁶.^30,31 Bao et al. demonstrated the crucial role of the interparticle distance between Au NPs and CsPb(Br/Cl)₃ nanocrystals in PL intensity enhancement originating from hotspots by finite-difference time-domain (FDTD) simulation.³² Despite the strong theoretical and experimental explanation of exciton–plasmon coupling in metal-emitter systems, the LSPR-induced PL intensity enhancement mechanism in perovskite systems remains poorly explored.

The mechanism of plasmon-induced optical enhancement is discussed in terms of not only the electromagnetic enhancement (hot spots) but also the chemical interface dumping (CID).³³ Charge transfer and resonance energy transfer (dipole scattering) can occur in metal-semiconductors and metal-molecular adsorbates. For molecules adsorbed or conjugated on metals, a strong optical resonance caused by a chemical coupling, known as a chemical enhancement in surface-enhanced Raman scattering (SERS), between the plasmon and metal-molecule charge transfer states was observed.^34,35 For example, Michaels et al. reported the chemical enhancement of rhodamine 6G molecules on Ag nanocrystals. They correlated the SERS intensity with the chemisorption of dye molecules on the Ag surface.³⁶ Otto reported chemical enhancement for pyridine adsorbed on Ag.³⁷ The chemical bonds between metals and dye molecules induce the chemical enhancement of Raman scattering, called the first-layer effect. The resonant vibronic coupling of the dye molecule to the transition plasmon dipole generates an enhanced resonance. It is possible to promote the resonance energy transfer from metals to emitters and the Purcell effect by the interaction between the chemical coupling-induced plasmon resonance and the exciton. Halide ions in halide perovskites can also interact chemically with Ag and Au. Oliveira et al. reported that Au has the potential for strong interaction with halogen through the Au d orbitals and halogen lone pair electrons.³⁸ Pospisil et al. reported gold halide (Au–X) states at a single-crystal hybrid perovskite/Au interface upon electrically biasing the two.³⁹ The chemical reactivity of transition metals in contact with a halide perovskite can form reversible metal–halogen (M–X) states. Although the contribution of electromagnetic enhancement or charge transfer is well-discussed for PL intensity enhancement of halide perovskites,^40–43 the role of chemical coupling-induced PL intensity enhancement in the neutral and charged excitonic states of perovskites has not been addressed. Understanding the fundamental process of plasmon-induced chemically enhanced PL is the key to advancing the nanophotonic applications of halide perovskites and providing further insights into coupled exciton–plasmon states and the metal–perovskite interface.

We demonstrate plasmon-induced 12-fold PL intensity enhancement for MAPbBr₃ single PQDs on Au NPs using steady-state and time-resolved single-molecule PL and ensemble transient absorption (TA) measurements. The PL intensity enhancement of single QDs occurs by Pb–Br⋯Au coupling in the neutral and photo-charged excitonic states of PQDs. While focused ion beam (FIB)-assisted STEM imaging helps demonstrate the PQD and Au samples, the near-field electric field distributions of PQD-Au NP systems obtained by the FDTD method correlate PL and TA results. We discuss the exciton–plasmon coupled PL intensity enhancement from the viewpoints of PQD–Au NP coupling in the neutral and charged exciton states of PQDs.

Results and discussion

The MAPbBr₃ PQDs without ligands (W-PQDs) were prepared by a spray deposition method. Generally, without ligands, microcrystals are formed in a bulk precursor solution,⁴⁴ whereas the fast crystallization process generates PQDs in precursor microdroplets without ligands.⁴⁵ Moreover, such PQDs are dispersed on a substrate by the regulated precursor concentration and separation of small droplets. These W-PQDs, directly prepared on glass/gold nanoparticle substrates, were free from aggregation. For comparison, colloidal MAPbBr₃ PQDs with ligands (L-PQD) were synthesized by the ligand-assisted reprecipitation (LARP) technique. The details of the L/W-PQD synthesis, purification, and characterization are described in the Experimental section. As shown in the electron microscope (TEM or STEM) (Fig. 1A and B) images, the LARP method yielded cubic L-PQDs with an average edge length of 10.2 ± 1.3 nm, as determined by analyzing the TEM images of 200 single particles. The powder XRD pattern of the L-PQD sample is shown in Fig. S1. In contrast, the spray method provided smaller W-PQDs with an average size of 6.3 ± 1.0 nm, as determined by analyzing the STEM images of 200 particles. Also, Au NPs were prepared by ion sputtering on glass coverslips or TEM grids using the Au plasma generated under 21.5 W s⁻¹ and vacuum (4 Pa). The distance between the target and the substrate was 32 mm. We estimated the average size of Au NPs at 8.1 ± 2.2 nm (Fig. 1C) using the cross-section from FIB-STEM data. Fig. 1D shows the absorption and PL spectra of a pristine L-PQD colloidal solution, and the inset represents the photograph of an L-PQD dispersion in toluene under UV light. The L-PQDs exhibit PL at 520 nm. The spectra of L-PQDs on a glass and an Au NP substrate (Fig. S2 and S3) show similar PL maxima. The excitonic absorption and emission wavelengths of these QDs are comparable to the particle size. Although W-PQDs on a glass substrate exhibited the maximum PL at 516 nm, which is equivalent to that of L-PQDs, it is shifted to 525 nm on Au NPs (Fig. 1E), suggesting that the ligand-free QDs interact more closely with Au NPs than L-PQDs through Pb–Br⋯Au interactions. Such an interaction determines the energy states and emission, which is comparable to a spectral shift reported by Biswas et al.⁴⁶ The well-dispersed W-PQDs on a glass substrate were also confirmed from 40–50 particles per 50 × 50 µm² area, as shown in the PL images obtained using an electron multiplication (EM)-charge coupled device (CCD) camera (Fig. 1F and S4).

Fig. 1 Characteristics of PQDs and Au NPs. (A–C) TEM and STEM images of (A) L-PQDs, (B) W-PQDs, and (C) Au NPs (insets) High resolution (B) TEM (100 nm × 100 nm) and (C) STEM images (50 nm × 50 nm) of (B) W-PQDs and (C) Au NPs. (D) Absorption and PL spectra of L-PQDs (inset: an image of a colloidal L-PQD solution under UV light). (E) PL spectra of W-PQDs on (i) a glass and (ii) an Au NP substrate. (F) A PL image of single W-PQDs on a glass substrate (λ_ex = 405 nm).

We investigated the PL blinking behavior of L- and W-PQDs using a single-particle PL measurement system. The PQDs were prepared on glass substrates using a drop-and-drag method or the spray method, as discussed in the SI. Representative PL intensity trajectories of the PQDs are shown in Fig. 2A, B and S5. L-PQDs on glass exhibited a low-frequency PL blinking with long ON events (bright states). In contrast, W-PQDs showed high-frequency PL blinking with short-lived ON and OFF states (dark states). Also, the QDs were free from photochemical degradation for several tens of minutes in air. To understand the PL blinking behavior of the PQDs, we carried out a statistical analysis of the ON/OFF-time probability distributions for more than 250 pristine L- and W-PQDs and analyzed the distribution using a truncated power law (eqn (1), Fig. 2C–F).

P(τ) = A₀τ^−αe^−τ/τ_c (1)

where τ_c is the truncation time, α is the power-law coefficient, and A₀ is a constant. We calculated the probability distribution using eqn (2).

P(τ) = 2N_i/[(τ_i+1 + τ_i) − (τ_i + τ_i−1)] (2)

Here, τ is time and N_i refers to the occurrence of the ith time. The mechanism of the ON/OFF events in PL blinking of PQDs has been explained using two models: the charging–discharging model (Type A) and activation–deactivation of nonradiative recombination centers (Type B).^47–49 In our experimental data, both L- and W-PQDs on glass exhibit a charging–discharging dominated mechanism because of an exponential truncation cutoff in the power-law relation (Fig. 2C–F). Although we do not entirely rule out the Type-B blinking, the nonradiative Auger recombination rate (Type A) seems higher than the trap-assisted nonradiative recombination (Type B).⁵⁰ The ON-τ_c and OFF-τ_c of the L-PQDs on glass were 1.22 and 0.81 s, respectively. In contrast, the ON-τ_c and OFF-τ_c of the W-PQDs on glass were 0.47 and 0.44 s, respectively. W-PQDs show shorter ON- and OFF-times than L-PQDs. The smaller size of W-PQDs (6.3 nm, Fig. 1B) than that of L-PQDs (10.2 nm, Fig. 1A) leads to stronger quantum confinement and weaker dielectric screening, providing enhanced Coulomb interaction in the W-PQDs, increasing the probability of exciton charging and discharging.

Fig. 2 Blinking behavior of single PQDs. (A and B) PL intensity trajectories of (A) two L-PQDs and (B) two W-PQDs on glass substrates. The QDs were excited at 405 nm (8.5 W cm⁻²). (C–F) Log–log distributions of (C and D) ON-time and (E and F) OFF-time probabilities for (C–E) L-PQDs and (D–F) W-PQDs.

We evaluated the plasmonic effects of Au NPs on the exciton recombination processes in PQDs by placing L-PQDs or W-PQDs on Au NPs prepared on glass coverslips. The number densities of single PQDs on Au NPs were controlled by following the same sample preparation methods as those for L- and W-PQDs on glass substrates. Fewer PL spots were observed for L-PQDs on Au NPs than on glass, suggesting PL quenching for a fraction of PQDs by energy transfer to Au NPs. However, the PL intensities of many PQDs were enormously enhanced due to exciton–plasmon coupling and energy transfer from Au NPs. A representative PL intensity trajectory of a W-PQD-Au NP system is shown in Fig. 3A and B. The corresponding trajectory of an L-PQD is shown in Fig. S6. Interestingly, single W-PQDs on Au NPs showed PL intensity enhancement from the bright (ON) and dark (OFF) states (Fig. 3B), suggesting exciton–plasmon coupling of the neutral and charged excitonic states, with the transient PL intensity reaching up to 12-fold (Fig. 3A and B). We analyzed the ON-time and OFF-time probability distributions of more than 100 W-PQDs (Fig. 3C and D) and L-PQDs (Fig. S7) on Au NPs. The enhanced ON- and OFF-time distributions of W-PQDs on Au NPs were fitted using the truncated power law, indicating that the primary blinking mechanism is still Type A. Importantly, W-PQDs on Au NPs showed an ON-τ_c decrease from 0.47 to 0.2 s and an OFF-τ_c increase from 0.44 to 2.5 s compared to the W-PQDs on glass.

Fig. 3 Plasmon-induced, chemically coupled PL blinking. (A) PL intensity trajectory of W-PQDs on Au NPs and the (B) zoomed-in image showing the coupling events. (C–D) Log–log distributions of (C) enhanced ON-time and (D) enhanced OFF-time probabilities of W-PQDs on Au NPs. (E) A scheme of the plasmon-induced, chemically coupled PL blinking mechanism. Samples were excited at 404 nm.

We discuss the PL intensity enhancement from the dark and bright states of the PQD-Au NP system by correlating the LSPR coupling–decoupling in the neutral and photo-charged states, outcompeting nonradiative Auger recombination and producing bright biexciton emission. The exciton–plasmon coupling and PL blinking mechanism are shown in Fig. 3E. First, in the non-enhanced PL parts, (i) ON and (ii and iii) OFF events mainly follow the charging–discharging blinking model. For ON events, radiative recombination (k_r) and nonradiative recombination (k_nr) occur, in addition to nonradiative Auger recombination and energy transfer. In contrast, for OFF events, we considered the ground and excited states of both ionized and neutral W-PQDs. An ionized W-PQD shows OFF or dark states due to positive or negative trion generation, followed by nonradiative Auger recombination. The charged state can be assigned to autoionization or direct tunneling of the carriers to the QD surface or substrate.^48,51–54 Once an ionized PQD is neutralized, it can show high intensity (ON) states. Nevertheless, a neutral PQD can show low intensity (OFF) states due to fluorescence resonance energy transfer (FRET) to an Au NP (k_ET) promoted by the overlapping energy states of the two.⁵⁵ The rates of Auger recombination and FRET are much higher than radiative recombination rates (k_i, k_ET >> k_r).^56,57

Four types of PL blinking behaviors (ON-En_ON-ON, OFF-En_ON-OFF, OFF-En_ON-ON, and ON-En_ON-OFF) are observed (Fig. 3E) for the enhanced PL parts of single QDs. Based on Pb–Br⋯Au interactions, we hypothesize exciton–plasmon coupling through the Pb–Br⋯Au state. A large absorption cross-section in the chemically coupled resonance active mode is expected due to the cross-section of Au NPs and W-PQDs, causing PL intensity enhancement by FRET from the plasmonic to excitonic states. Nonetheless, we do not rule out PL quenching of PQDs by energy transfer to Au nanoparticles. However, such PQDs in the dark or at prolonged low-intensity levels are not included in the analyses. For En_ON events (iv, Fig. 3E), radiative recombination is promoted by the chemical coupling-assisted FRET (viii, Fig. 3E). To account for the OFF-En_ON transition, the chemical coupling is considered independent of whether a QD is charged or neutral. An ionized PQD can couple with an Au NP, spontaneously neutralizing the PQD and turning ON the PL with a high intensity. For En_ON-ON and En_ON-OFF events (Fig. 3E), there are three possibilities: (v) Auger ionization of W-PQDs, (viii) FRET from a W-PQD to an Au NP, and (ix) Pb–Br⋯Au decoupling. Auger ionization of W-PQDs and FRET from a W-PQD to an Au NP for En_ON-ON and En_ON-OFF are equivalent to the normal ON–OFF processes for a QD on a glass substrate. For En_ON-ON, we cannot rule out (v) Auger ionization and (vi) neutralization because these processes are speedier compared to the PL intensity trajectory binning time (15 to 30 ms). However, Pb–Br⋯Au decoupling can be caused by exciton dipole fluctuation and thermally-induced Pb–Br⋯Au bond breaking at a sub-nm level distance change (ix). Exciton–plasmon coupling efficiencies and durations depend on the relative orientation between a PQD dipole and the plasmon field.⁵⁸ Also, exciton diffusion causes dipole moment fluctuation and breaks prolonged high-intensity or coupled states. The coupling alone cannot explain the intensity decrease from EnON to normal ON events (Fig. 3E). The Pb–Br⋯Au state destabilization by thermal fluctuations can generate neutral or charged states without any plasmon coupling, which accounts for the switching from the high intensity (coupled) to standard ON or OFF states. Nevertheless, whether or not a Pb–Br⋯Au resonance exists in the dark states (trions) remains unexplored.

To understand the kinetics of plasmon-enhanced PL, we measured the time-resolved PL and TA for a PQD on glass or Au NPs (Fig. 4). The PL decay profiles of a single W-PQD or L-PQD on glass or Au NPs are shown in Fig. 4A and S8, respectively. The PL decays were fitted using the third exponential model (eqn (3)),

γ(t) = α₀ + α₁e^(−t/τ₁) + α₂e^(−t/τ₂) + α₃e^(−t/τ₃) (3)

where α₀ is a constant, α₁, α₂, and α₃ are the amplitudes and τ₁, τ₂, and τ₃ are the different lifetime components. The goodness of fit was between 0.98 and 1.1. Generally, one component arises from radiative recombination, and the other two from nonradiative recombination processes.^59–63 The amplitude-weighted average PL lifetimes of the samples are calculated using eqn (4):

Σα_nτ_n/Σα_n (4)

Fig. 4 (A) PL decays of W-PQDs on (green) glass or (orange) Au NPs. (B) PL intensity and (C) lifetime distributions for single PQDs: (purple) L-PQDs on glass, (orange) W-PQDs on Au, and (green) W-PQDs on glass. (D) Transient absorption spectra of (D) W-PQDs and (E) W-PQDs on Au NPs. (F) TA dynamics at the GSB peak (ca. 535 nm) of W-PQDs on glass and W-PQDs on Au NPs excited at 400 nm.

The PL lifetime components of the PQD samples are summarized in Table S1. Interestingly, we observed a short PL lifetime (0.92 ns) and PL intensity enhancement for a single W-PQD on Au NPs compared to a W-PQD on a bare glass substrate (ca. 2.0 ns, Fig. 4A) or an alumina-coated glass substrate (ca. 2.1 ns). In contrast, the single L-PQD on Au NPs showed a shorter PL lifetime with PL quenching than single L-PQDs on glass (Fig. S8). The decrease in PL intensity and lifetime of L-PQDs on Au NPs suggests FRET from PQDs to Au NPs. In contrast, W-PQDs on Au NPs show an increase in PL intensity and a decrease in PL lifetime by the chemical coupling-induced FRET from the LSPR state to W-PQDs. We also examined the statistical distributions of PL lifetimes and PL intensities for more than 400 single PQDs on a glass or an Au NP substrate. Fig. 4B and C shows the single-particle PL intensity and lifetime distributions for W-PQDs on glass or Au NPs. The corresponding data for L-PQDs are compared in Fig. S9. The particle-by-particle variations of intensity and lifetime are assigned to spatial inhomogeneity and differences in the PQD–Au NP distance.⁶⁴ While the PL intensities of most single PQDs on Au NPs are higher than those on a glass substrate, the PL lifetimes are shorter for the PQDs on Au NPs. The higher PL intensities and shorter PL lifetimes of PQDs on Au suggest energy transfer from Au NPs to PQDs and an increased rate of radiative relaxation in the exciton–plasmon coupled state.

Furthermore, we investigated the interactions between PQDs and Au NPs using femtosecond pump-probe (TA) experiments for PQDs on glass and Au NPs. The time evolutions of the TA spectra of W-PQDs on glass and Au NPs are shown in Fig. 4D and E, respectively. For W-PQDs on glass, the positive peak is red-shifted from 485 nm (0.2 ps) to 520 nm (1 ns), assigned to a photoinduced excited-state absorption. The negative peak at 535 nm is attributed to ground-state bleaching (GSB). In contrast, for W-PQDs on Au NPs, the photoinduced excited-state absorption peak is red-shifted and reaches zero in 10–100 ps. The GSB peak width at 535 nm for W-PQDs on Au NPs is broader than that for W-PQDs on glass. The expansion for the lower energy side of the GSB peak is due to the plasmonic absorption band of Au NPs. Conversely, the expansion for the higher energy side of the GSB peak suggests a biexciton state. The biexciton state is blue-shifted compared to the single exciton state due to exciton–exciton interactions.⁶⁵Fig. 4F shows the TA dynamics of the GSB peak (535 nm) for W-PQDs on glass and Au NP substrates. Both curves were fitted using the biexponential model. The faster decay component of PQDs on Au NPs (τ₁: 1.3 ps, 92.2%), compared to glass (τ₁: 42 ps, 71%) can be correlated to the PL lifetime decrease. The faster GSB recovery of PQDs on Au NPs (Table S2) compared with that on glass suggests an increase in the radiative decay rate. Although we cannot rule out hot carrier transfer-induced PL intensity enhancement, the fast component (τ₁) for Au NPs is slower than the reported hot electron transfer rate.⁶⁶ The broader TA spectrum and faster kinetics for PQDs on Au NPs than those on glass indicate a biexciton recombination process.^57,65

To confirm the single- or multi-photon emission by PQDs on glass and Au NPs, we conducted single-photon coincidence measurements (Fig. 5). Fig. 5A shows a scanning confocal PL image of W-PQDs. Under high-power excitation (20 nW cm⁻²), both the histograms of PQDs on glass and Au NPs exhibit high zero-time coincidence values (Fig. 5B and C), indicating multi-photon emission. Moreover, the zero-time coincidence value is higher than the non-zero-time coincidence value for PQDs on AuNPs. Under the lowest excitation power (6.8 nW cm⁻²) in the single-photon coincidence measurements, the zero-time coincidence value was smaller than the non-zero-time coincidence for PQDs on glass. In contrast, despite the lowest excitation laser power, far below the bi-exciton threshold, the zero-time coincidence value was greater than the non-zero-time coincidence for PQDs on Au NPs (Fig. 5E). These data suggest bright biexciton emission by PQDs on Au NPs, which is consistent with the PL intensity trajectories showing high-intensity bursts.

Fig. 5 Single-photon coincidence for PQDs on glass and Au NPs. (A) A confocal PL image of W-PQDs, (B–E) single-photon coincidence histograms of PQDs on (B and C) glass and (D and E) Au. The excitation laser powers were (B and D) 20 nW cm⁻² and (C and E) 6.8 nW cm⁻². The blue dotted lines indicate the average coincidence counts outside zero nanoseconds. The red lines are fitted Lorentzian functions.

Next, we examined the electric field effect of Au nanoparticles on PQDs by calculating the near-field intensity distributions around single Au NPs and PQDs on an Au NP under 400 nm excitation using the FDTD method. The details of the simulation method are given in the SI. The simulated models for relative geometries are shown in Fig. S10 (top and side views). The calculated absorption spectra and integrated electric field spectra of the simulated structures are presented in Fig. S11 and S12. We estimated the height of the Au NPs as 5 nm from the FIB-assisted cross-sectional STEM image (Fig. 6A). The active plasmon band in the absorption spectrum (Fig. 6B) is around 600 nm for Au NPs. For PQDs on Au NPs, a broad and blue-shifted peak can be seen in the absorption spectrum (Fig. 6B, orange line). Changes in the Au NP shape or size do not cause this peak. We hypothesize that the broad and blue-shifted absorption peak is the active perovskite-plasmon-based Pb–Br⋯Au resonance. The simulated model of side 4 matches the configuration in Fig. 6C and the absorption spectra in Fig. 6B and S12B. We simulated the near-field distribution on the YZ and XY cross-sectional planes for a PQD on Au NPs (Fig. 6D). The near-field distribution on the YZ and XY cross-sectional planes at 550 nm is shown in Fig. 6E and F, respectively. The FDTD simulations for Au NPs and all other models (PQD on Au) at 550 and 610 nm are shown in Fig. S13–S18. Fig. 6E and F shows that the electric field intensity is enhanced 5- to 10-fold at the PQD–Au interface. The horizontal length and interparticle distance of Au NPs are estimated at 8.0 and 7.1 nm, respectively, from STEM images. Therefore, in addition to being on one Au NP, PQDs can be in the nanogap of Au NPs, making more efficient exciton–plasmon coupling (Fig. S13). As discussed above, the exciton–plasmon coupling-induced FRET from an Au NP to a PQD depends on the dipole alignment of the plasmon field and the PQD. Thermal fluctuation-induced exciton diffusion and Pb–Br⋯Au debonding can perturb the dipole alignment. Therefore, the coupling can be deactivated spontaneously without changing the position of PQDs and Au NPs. The plasmon-induced PL intensity enhancement also depends on this dipole coupling, i.e., when the Pb–Br⋯Au interaction is active, the LSPR enhances the PQD PL efficiently. Conversely, the energy transfer becomes inactive when the Pb–Br⋯Au interaction disappears.

Fig. 6 FDTD simulation. (A) Cross-section STEM image of Au NPs on glass. (B) Experimental absorption spectra of (black) Au NPs and (orange) W-PQDs on Au NPs. (C) One of the simulation models of W-PQDs on Au NPs (side view). (D) A schematic of YZ and XY cross-sections for (E) and (F), respectively. (E and F) Near-field distribution on W-PQDs–Au NP calculated by FDTD simulation at 550 nm. (E) Cross-sectional YZ plane and (F) cross-sectional XY plane at a position of 4.2 nm above the SiO₂ surface.

Conclusion

We demonstrated up to 12-fold multi-modal PL intensity enhancement for perovskite single QDs in the neutral and charged exciton states by coupling with the LSPR of Au NPs. The coupling-enabled energy transfer and accelerated radiative relaxation suppress nonradiative Auger recombination, as observed by increased relaxation rates in time-resolved PL and TA experiments. The increased radiative relaxation is associated with increased zero-time coincidence photons, suggesting bright biexcitons in the coupled state, which is consistent with QDs showing high intensity PL, short PL lifetimes, and increased ground-state bleach-recovery rates. The QDs on Au NPs exhibit unique PL blinking behavior, characterized by enhanced ON events from the dark (OFF) and bright (ON) states with short intervals, indicating transient exciton–plasmon coupling. FDTD simulations indicated effective exciton–plasmon coupling and PL intensity enhancement for QDs on Au NPs. A combination of chemical coupling-induced energy transfer and LSPR-induced electric field enhancement resulted in the PL intensity enhancement and increased relaxation rates, which can be promising for plasmon-assisted high-efficiency perovskite photodetectors, solar cells, and LEDs.

Author contributions

V. B. conceived the project. B. M. S., T. O., and T. W. synthesized and characterized the perovskite quantum dots. B. M. S., T. O., T. W., and V. B. conducted the PL measurements. S. I. and H. Miyasaka carried out the photon-antibunching measurements. L. W. and N. T. carried out TA measurements. X. S. and H. Misawa carried out FDTD calculations. B. M. S., T. O. and V. B. wrote the manuscript. All authors contributed to the manuscript finalization.

Conflicts of interest

The authors declare no competing financial interests.

Data availability

Supplementary information (SI): experimental details and the data (Fig. S1–S18, and tables S1 and S2). See DOI: https://doi.org/10.1039/d5ta07443f.

Acknowledgements

This research was supported by the JSPS KAKENHI grant numbers JP20J00974, JP21K14580 JP18H05205, JP23H01781, JP23H05464 and JP19H02550. We acknowledge the Hokkaido University Nanotechnology PLATFORM, the Crossover Alliance to Create the Future with People, Intelligence, and Materials, and the Hokkaido University Photoexcitonix Program.

References

1. A. Dey, J. Ye, A. De, E. Debroye, S. K. Ha, E. Bladt, A. S. Kshirsagar, Z. Wang, J. Yin and Y. Wang, et al., State of the Art and Prospects for Halide Perovskite Nanocrystals, ACS Nano, 2021, 15, 10775–10981.
2. L. Chouhan, S. Ghimire, C. Subrahmanyam, T. Miyasaka and V. Biju, Synthesis, Optoelectronic Properties and Applications of Halide Perovskites, Chem. Soc. Rev., 2020, 49, 2869–2885.
3. S. P. Senanayak, K. Dey, R. Shivanna, W. Li, D. Ghosh, Y. Zhang, B. Roose, S. J. Zelewski, Z. Andaji-Garmaroudi and W. Wood, et al., Charge Transport in Mixed Metal Halide Perovskite Semiconductors, Nat. Mater., 2023, 22, 216–224.
4. Y. Sun, L. Ge, L. Dai, C. Cho, J. Ferrer Orri, K. Ji, S. J. Zelewski, Y. Liu, A. J. Mirabelli and Y. Zhang, et al., Bright and Stable Perovskite Light-Emitting Diodes in the Near-Infrared Range, Nature, 2023, 615, 830–835.
5. S. Liu, V. Biju, Y. Qi, W. Chen and Z. Liu, Recent Progress in the Development of High-Efficiency Inverted Perovskite Solar Cells, NPG Asia Mater., 2023, 15, 27.
6. S. L. Aneesha, F. Xia, T. Okamoto, D. Gaur, S. Seth, J. Hofkens, L. Polavarapu and V. Biju, From Halide Perovskite Nanocrystals to Supercrystals: Fundamentals and Applications, Chem. Soc. Rev., 2025, 54, 9585–9611.
7. Q. Lv, Q. Li, Z. Xu, X. Shen, K. Man, J. C. Ho and P. Guo, On-Wire Halide Perovskite-Heterostructure-Based Photodetectors for Encrypted Communication, Nano Lett., 2025, 25, 12091–12100.
8. Z. Feng, M. He, Z. Li and X. Hao, A Perspective on Spatiotemporal Engineering of Multidimensional Heterointerfaces in Perovskite Solar Cells, ACS Nano, 2025, 19, 28003–28020.
9. S. Gallagher, J. Kline, F. Jahanbakhshi, J. C. Sadighian, I. Lyons, G. Shen, B. F. Hammel, S. Yazdi, G. Dukovic and A. M. Rappe, et al., Ligand Equilibrium Influences Photoluminescence Blinking in CsPbBr₃: A Change Point Analysis of Widefield Imaging Data, ACS Nano, 2024, 18(29), 19208–19219.
10. D. Strandell, D. Dirin, D. Zenatti, P. Nagpal, A. Ghosh, G. Raino, M. V. Kovalenko and P. Kambhampati, Enhancing Multiexcitonic Emission in Metal-Halide Perovskites by Quantum Confinement, ACS Nano, 2023, 17(24), 24910–24918.
11. B. Ai, Z. Fan and Z. J. Wong, Plasmonic-Perovskite Solar Cells, Light Emitters, and Sensors, Microsyst. Nanoeng., 2022, 8, 5.
12. Z. Y. Ooi, A. Jiménez-Solano, K. Gałkowski, Y. Sun, J. Ferrer Orri, K. Frohna, H. Salway, S. Kahmann, S. Nie and G. Vega, et al., Strong Angular and Spectral Narrowing of Electroluminescence in an Integrated Tamm-Plasmon-Driven Halide Perovskite LED, Nat. Commun., 2024, 15, 5802.
13. M. Yen, C. Lee, Y. Yao, Y. Chen, S. Wu, H. Hsu, Y. Kajino, G. Lin, K. Tamada and Y. Lee, Tamm-Plasmon Exciton-Polaritons in Single-Monolayered CsPbBr₃ Quantum Dots at Room Temperature, Adv. Opt. Mater., 2023, 11, 2202326.
14. F. Juan, Y. Wu, B. Shi, M. Wang, F. Xu, J. Jia, H. Wei, T. Yang and B. Cao, Plasmonic Au Nanooctahedrons Enhance Light Harvesting and Photocarrier Extraction in Perovskite Solar Cell, ACS Appl. Energy Mater., 2021, 4, 3201–3209.
15. Z. Shi, Y. Li, S. Li, X. Li, D. Wu, T. Xu, Y. Tian, Y. Chen, Y. Zhang and B. Zhang, et al., Luminescence: Localized Surface Plasmon Enhanced All-Inorganic Perovskite Quantum Dot Light-Emitting Diodes Based on Coaxial Core/Shell Heterojunction Architecture, Adv. Funct. Mater., 2018, 28, 1707031.
16. Y. H. Hsieh, B. W. Hsu, K. N. Peng, K. W. Lee, W. C. Chu, S. W. Chang, H. W. Lin, T. J. Yen and Y. J. Lu, Perovskite Quantum Dot Lasing in a Gap-Plasmon Nanocavity with Ultralow Threshold, ACS Nano, 2020, 14, 11670–11676.
17. Q. Shang, S. Zhang, Z. Liu, J. Chen, P. Yang, C. Li, W. Li, Y. Zhang, Q. Xiong and X. Liu, et al., Surface Plasmon Enhanced Strong Exciton–Photon Coupling in Hybrid Inorganic–Organic Perovskite Nanowires, Nano Lett., 2018, 18, 3335–3343.
18. W. Knoll, M. R. Philpott, J. D. Swalen and A. Girlando, Emission of Light from Ag Metal Gratings Coated with Dye Monolayer Assemblies, J. Chem. Phys., 1981, 75, 4795–4799.
19. A. Kinkhabwala, Z. Yu, S. Fan, Y. Avlasevich, K. Müllen and W. E. Moerner, Large Single-Molecule Fluorescence Enhancements Produced by a Bowtie Nanoantenna, Nat. Photonics, 2009, 3, 654–657.
20. L. Trotsiuk, A. Muravitskaya, A. Movsesyan, A. Ramanenka, A. Prudnikau, A. Antanovich, V. Lesnyak, S. V. Gaponenko and A. O. Govorov, Nonclassical Mechanism of Metal-Enhanced Photoluminescence of Quantum Dots, Nano Lett., 2023, 23, 8524–8531.
21. X. Ma, H. Tan, T. Kipp and A. Mews, Fluorescence Enhancement, Blinking Suppression, and Gray States of Individual Semiconductor Nanocrystals Close to Gold Nanoparticles, Nano Lett., 2010, 10, 4166–4174.
22. Y. Fu, J. Zhang and J. R. Lakowicz, Silver-Enhanced Fluorescence Emission of Single Quantum Dot Nanocomposites, Chem. Commun., 2009, 3, 313–315.
23. Y. Chen, K. Munechika and D. S. Ginger, Dependence of Fluorescence Intensity on the Spectral Overlap between Fluorophores and Plasmon Resonant Single Silver Nanoparticles, Nano Lett., 2007, 7, 690–696.
24. E. M. Purcell, H. C. Torrey and R. V. Pound, Resonance Absorption by Nuclear Magnetic Moments in a Solid, Phys. Rev., 1946, 69, 681.
25. T. Ming, H. Chen, R. Jiang, Q. Li and J. Wang, Plasmon-Controlled Fluorescence: Beyond the Intensity Enhancement, J. Phys. Chem. Lett., 2012, 3, 191–202.
26. O. Bitton, S. N. Gupta and G. Haran, Quantum Dot Plasmonics: from Weak to Strong Coupling, Nanophotonics, 2019, 8, 559–575.
27. E. Cao, W. Lin, M. Sun, W. Liang and Y. Song, Exciton–Plasmon Coupling Interactions: from Principle to Applications, Nanophotonics, 2018, 7, 145–167.
28. X. Wang, C. Liu, C. Gao, K. Yao, S. S. M. Masouleh, R. Berté, H. Ren, L. D. S. Menezes, E. Cortés and I. C. Bicket, et al., Self-Constructed Multiple Plasmonic Hotspots on an Individual Fractal to Amplify Broadband Hot Electron Generation, ACS Nano, 2021, 15, 10553–10564.
29. V. Krivenkov, Y. P. Rakovich, P. Samokhvalov and I. Nabiev, Synergy of Excitation Enhancement and the Purcell Effect for Strong Photoluminescence Enhancement in a Thin-Film Hybrid Structure Based on Quantum Dots and Plasmon Nanoparticles, J. Phys. Chem. Lett., 2020, 11, 8018–8025.
30. S. Singh, A. Raulo, A. Singh, M. Mittal, A. Horechyy, R. Hübner, P. Formanek, R. K. Srivastava, S. Sapra and A. Fery, et al., Enhanced Photoluminescence of Gold Nanoparticle-Quantum Dot Hybrids Confined in Hairy Polymer Nanofibers, ChemNanoMat, 2021, 7, 831–841.
31. J. E. Park, J. Kim and J. M. Nam, Emerging Plasmonic Nanostructures for Controlling and Enhancing Photoluminescence, Chem. Sci., 2017, 8, 4696–4704.
32. L. Bao, W. Liu, Y. Chen, Y. Zhang and Y. Zhang, J. Mater. Chem. C, 2021, 9, 5182–5189.
33. S. A. Lee and S. Link, Chemical Interface Damping of Surface Plasmon Resonances, Acc. Chem. Res., 2021, 54, 1950–1960.
34. A. Otto, The ‘Chemical’ (Electronic) Contribution to Surface-Enhanced Raman Scattering, J. Raman Spectrosc., 2005, 36, 497–509.
35. R. D. Rodriguez, C. J. Villagómez, A. Khodadadi, S. Kupfer, A. Averkiev, L. Dedelaite, F. Tang, M. Y. Khaywah, V. Kolchuzhin and A. Ramanavicius, et al., Chemical Enhancement vs. Molecule–Substrate Geometry in Plasmon-Enhanced Spectroscopy, ACS Photonics, 2021, 8, 2243–2255.
36. A. M. Michaels, M. Nirmal and L. E. Brus, Surface Enhanced Raman Spectroscopy of Individual Rhodamine 6G Molecules on Large Ag Nanocrystals, J. Am. Chem. Soc., 1999, 121, 9932–9939.
37. A. Otto, Theory of First Layer and Single Molecule Surface Enhanced Raman Scattering (SERS), Phys. Status Solidi Appl. Res., 2001, 188, 1455–1470.
38. V. Oliveira and D. Cremer, Transition from Metal-Ligand Bonding to Halogen Bonding Involving a Metal as Halogen Acceptor: a Study of Cu, Ag, Au, Pt, and Hg Complexes, Chem. Phys. Lett., 2017, 681, 56–63.
39. J. Pospisil, A. Guerrero, O. Zmeskal, M. Weiter, J. J. Gallardo, J. Navas and G. Garci-Belmonte, Reversible Formation of Gold Halides in Single-Crystal Hybrid-Perovskite/Au Interface upon Biasing and Effect on Electronic Carrier Injection, Adv. Funct. Mater., 2019, 29, 1900881.
40. V. C. Silalahi, D. Kim, M. Kim, S. Adhikari, S. Jun, Y.-H. Cho, D. Lee, C.-L. Lee and Y. Jang, Over a Thousand-Fold Enhancement of the Spontaneous Emission Rate for Stable Core-Shell Perovskite Quantum Dots Through Coupling with Novel Plasmonic Nanogaps, Nanophotonics, 2024, 13, 369–376.
41. W. Hui, T. Ping, J. Yin, J. Li, J. Li and J. Kang, Dual-Mode Plasmonic Coupling-Enhanced Color Conversion of Inorganic CsPbBr₃ Perovskite Quantum Dot Films, ACS Appl. Mater. Interfaces, 2021, 13, 32856–32864.
42. Y. Zhang, C. K. Lim, Z. Dai, G. Yu, J. W. Haus, H. Zhang and P. N. Prasad, Photonics and Optoelectronics Using Nano-Structured Hybrid Perovskite Media and Their Optical Cavities, Phys. Rep., 2019, 795, 1–51.
43. K. Xu, Z. Zou, W. Li, L. Zhang, M. Ge, T. Wang and W. Du, Strong Linearly Polarized Light Emission by Coupling Out-of-Plane Exciton to Anisotropic Gap Plasmon Nanocavity, Nano Lett., 2024, 24, 3647–3653.
44. D. Zhang, T. Okamoto and V. Biju, Thermodynamically and Kinetically Controlled Nucleation and Growth of Halide Perovskite Single Crystals, Small, 2023, 19, 2304900.
45. A. P. Pushkarev, V. I. Korolev, D. I. Markina, F. E. Komissarenko, A. Naujokaitis, A. Drabavičius, V. Pakštas, M. Franckevičius, S. A. Khubezhov and D. A. Sannikov, et al., A Few-Minute Synthesis of CsPbBr₃ Nanolasers with a High Quality Factor by Spraying at Ambient Conditions, ACS Appl. Mater. Interfaces, 2019, 11, 1040–1048.
46. A. Biswas, A. Swarnkar, P. Pandey, P. Kour, S. Parmar and S. Ogale, Dynamics of Photogenerated Carriers across the Interface between CsPbBr₃ Nanocrystals and Au-Ag Nanostructured Film, and Its Control via Ultrathin MgO Interface Layer, ACS Omega, 2020, 5, 11915–11922.
47. Y.-S. Park, S. Guo, N. S. Makarov and V. I. Klimov, Room Temperature Single-Photon Emission from Individual Perovskite Quantum Dots, ACS Nano, 2015, 9, 10386–10393.
48. L. Chouhan, S. Ito, E. M. Thomas, Y. Takano, S. Ghimire, H. Miyasaka and V. Biju, Real-Time Blinking Suppression of Perovskite Quantum Dots by Halide Vacancy Filling, ACS Nano, 2021, 15, 2831–2838.
49. N. Yarita, H. Tahara, T. Ihara, T. Kawawaki, R. Sato, M. Saruyama, T. Teranishi and Y. Kanemitsu, Dynamics of Charged Excitons and Biexcitons in CsPbBr₃ Perovskite Nanocrystals Revealed by Femtosecond Transient-Absorption and Single-Dot Luminescence Spectroscopy, J. Phys. Chem. Lett., 2017, 8, 1413–1418.
50. C. Galland, Y. Ghosh, A. Steinbrück, M. Sykora, J. A. Hollingsworth, V. I. Klimov and H. Htoon, Two Types of Luminescence Blinking Revealed By Spectroelectrochemistry of Single Quantum Dots, Nature, 2011, 479, 203–207.
51. A. L. Efros, Almost Always Bright, Nat. Mater., 2008, 7, 612–613.
52. S. Yamashita, M. Hamada, S. Nakanishi, H. Saito, Y. Nosaka, S. Wakida and V. Biju, Auger Ionization Beats Photo-Oxidation of Semiconductor Quantum Dots: Extended Stability of Single-Molecule Photoluminescence, Angew. Chem., Int. Ed., 2015, 54, 3892–3896.
53. L. Chouhan, S. Ghimire and V. Biju, Blinking Beats Bleaching: The Control of Superoxide Generation by Photoionized Perovskite Nanocrystals, Angew. Chem., Int. Ed., 2019, 58, 4875–4879.
54. E. M. Thomas, S. Ghimire, R. Kohara, A. N. Anil, K. Yuyama, Y. Takano, K. G. Thomas and V. Biju, Blinking Suppression in Highly Excited CdSe/ZnS Quantum Dots by Electron Transfer Under Large Positive Gibbs (Free) Energy Change, ACS Nano, 2018, 12, 9060–9069.
55. T. Förster, Zwischenmolekulare Energiewanderung und Fluoreszenz, Ann. Phys., 1948, 437, 55–57.
56. A. Hangleiter and R. Hacker, Enhancement of Band-to-Band Auger Recombination by Electron-Hole Correlations, Phys. Rev. Lett., 1990, 65, 215–218.
57. K. Cho, T. Sato, T. Yamada, R. Sato, M. Saruyama, T. Teranishi, H. Suzuura and Y. Kanemitsu, Size Dependence of Trion and Biexciton Binding Energies in Lead Halide Perovskite Nanocrystals, ACS Nano, 2024, 18, 5723–5729.
58. Y. J. Li, Y. Hong, Q. Peng, J. Yao and Y. S. Zhao, Orientation-Dependent Exciton–Plasmon Coupling in Embedded Organic/Metal Nanowire Heterostructures, ACS Nano, 2017, 11, 10106–10112.
59. J. S. Manser, J. A. Christians and P. V. Kamat, Intriguing Optoelectronic Properties of Metal Halide Perovskites, Chem. Rev., 2016, 116, 12956–13008.
60. V. C. Nair, C. Muthu, A. L. Rogach, R. Kohara and V. Biju, Channeling Exciton Migration into Electron Transfer in Formamidinium Lead Bromide Perovskite Nanocrystal/Fullerene Composites, Angew. Chem., 2017, 129, 1234–1238.
61. B. M. Sachith, T. Okamoto, S. Ghimire, T. Umeyama, Y. Takano, H. Imahori and V. Biju, Long-Range Interfacial Charge Carrier Trapping in Halide Perovskite-C60 and Halide Perovskite-TiO2 Donor–Acceptor Films, J. Phys. Chem. Lett., 2021, 12, 8644–8651.
62. Z. Zhang, S. Ghimire, T. Okamoto, B. M. Sachith, J. Sobhanan, C. Subrahmanyam and V. BIju, Mechano-Optical Modulation of Excitons and Carrier Recombination in Self-Assembled Halide Perovskite Quantum Dots, ACS Nano, 2022, 16, 160–168.
63. S. Ghimire, L. Chouhan, Y. Takano, K. Takahashi, T. Nakamura, K. Yuyama and V. Biju, Amplified and Multicolor Emission From Films and Interfacial Layers of Lead Halide Perovskite Nanocrystals, ACS Energy Lett., 2018, 4, 133–141.
64. N. Liu, B. S. Prall and V. I. Klimov, Hybrid Gold/Silica/Nanocrystal-Quantum-Dot Superstructures: Synthesis and Analysis of Semiconductor–Metal Interactions, J. Am. Chem. Soc., 2006, 15362–15363.
65. M. N. Ashner, K. E. Shulenberger, F. Krieg, E. R. Powers, M. V. Kovalenko, M. G. Bawendi and W. A. Tisdale, Size-Dependent Biexciton Spectrum in CsPbBr₃ Perovskite Nanocrystals, ACS Energy Lett., 2019, 4, 2639–2645.
66. X. Huang, H. Li, C. Zhang, S. Tan, Z. Chen, L. Chen, Z. Lu, X. Wang and M. Xiao, Efficient Plasmon-Hot Electron Conversion in Ag–CsPbBr₃ Hybrid Nanocrystals, Nat. Commun., 2019, 10, 1163.

---

Footnote

† Equal contribution.

---