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Abstract
The luminescence properties of Ag2S quantum dots passivated with L-Cysteine (Ag2S/L-Cys QDs) are studied in the presence of Au nanorods passivated with cetyltrimethylammonium bromide molecules (Au/CTAB NRs). The effect of plasmonic Au/CTAB NRs on IR trap state luminescence (750 nm) is considered. It has been found that the direct interaction between the components of the plexcitonic nanostructure leads to a significant luminescence quenching of Ag2S/L-Cys QDs, with the luminescence lifetime being constant. This is the evidence for photoinduced charge transfer. The spatial separation of the components of plexcitonic nanostructures due to the introduction of a polymer – Poly(diallyldimethylammonium chloride) (PolyDADMAC) provides a means to change their mutual arrangement and achieve an increase in the IR trap state luminescence intensity and a decrease in the luminescence lifetime from 7.4 ns to 4.5 ns. With weak plexcitonic coupling in the nanostructures [Ag2S QD/L-Cys]/[PolyDADMAC]/[Au/CTAB NRs], the possibility of increasing the quantum yield of trap state luminescence for Ag2S QDs due to the Purcell effect has been demonstrated.
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1. Introduction
The formation of hybrid nanostructures based on semiconductor colloidal quantum dots (QDs) with unique photoluminescence properties, which are uncharacteristic of individual components, is relevant for photonic applications, such as optoelectronics, photovoltaics, bioimaging, and luminescent sensors [1–9]. The luminescence properties of the hybrid nanostructures, such as high luminescence quantum yield and photostability, as well as the radiative lifetime, are key parameters that determine the area and potential for these applications [1–9].
In recent years, techniques for controlling the luminescence properties of QDs by plasmon-exciton (plexitonic) interaction have been actively developed [9–27]. It has been found that the ”hybrid” luminescence properties of these nanostructures are unique due to their strong dependence on the features of the direct interaction between components of plasmon-exciton nanostructures, the distance between them, matching of spectral resonances, etc. [9–27]. The plasmon-exciton interaction provides a basis for changing the probabilities of radiative and nonradiative transitions in QDs due to the Purcell effect, transforming the shape of extinction and luminescence spectra of QDs as a result of the Rabi splitting, making Fano quantum interference possible [9–27].
These effects are most clearly demonstrated in the framework of single-molecule spectroscopy [12–29]. However, the use of plexcitonic effects in luminescence sensing requires an understanding of the conditions for their manifestation in the luminescence of quantum dot ensembles [9–11,30]. The size dispersion of QDs strongly affects the spectral resonance due to the large luminescence bandwidth [9–11,30]. In most situations described in the literature, QDs with exciton luminescence are considered [10–15,17–19,21–25]. Under conditions of significant overlap of QD luminescence and extinction bands of plasmonic nanoparticles (NPs), processes of electronic excitation exchange, in particular, nonradiative energy transfer from QDs to plasmonic NPs, play a significant role in the plasmon-exciton interaction [31–33]; charge phototransfer is also an important effect in these systems [34,35].
Thus, in some situations, the changes in the parameters of QD luminescence do not have an obvious interpretation. The least studied are manifestations of the plexcitonic interaction in the case of trap state luminescence of QDs. Recently, several works have appeared that address this problem [36–38]. Various samples of colloidal QDs, namely, CdS, CdTe, InP/ZnS, SiN, etc., and various plasmonic nanoparticles (nanorods, cones, as well as pyramidal and spherical nanoparticles made of silver and gold) have been considered. In most of the cited works, significant attention is given to quenching of trap state luminescence due to photoinduced electron transfer from plasmonic nanoparticles to QDs [34,37]. It should be also noted that in the case of trap state luminescence, electron-phonon interaction and influence of nonradiative recombination centers are significant. Their contribution can largely determine the behavior of the luminescence intensity of QDs near plasmonic NPs [36]. Thus, recognition of the effects responsible for the change in the spectral properties of hybrid plexcitonic nanostructures, which include QDs with trap state luminescence, is a relevant problem. There are a number of works where it is shown that the centers of radiative recombination are the centers of reverse saturable absorption (RSA) and nonlinear refraction. In this regard, control of the transition probability for defective luminescence is interesting from the point of view of the further perspective of control of nonlinear absorption and refraction [39–42].
This paper presents experimental data demonstrating the possibility to control the luminescence properties of Ag$_2$S QDs in the near field of plasmonic gold nanorods (Au NRs) by changing the plasmon-exciton coupling mode. For this purpose, a polymer was introduced to achieve the different mutual spatial arrangement of QDs and NRs. Note that for metal-conducting structures containing Ag$_2$S QDs, plexcitonic effects have not yet been considered in detail with regard to the luminescence properties of such complex systems.
2. Sample preparation and experimental techniques
Colloidal Ag$_2$S QDs have been synthesized within the framework of a one-step synthesis, which involves the use of L-cysteine (L-Cys) amino acid molecules during crystallization. These molecules act as both a sulfur source and a passivator of QD interfaces (further referred to as Ag$_2$S/L-Cys QDs, see Fig. 1). The procedure is as follows. Aqueous solutions of AgNO$_3$ and L-Cys are mixed in a molar ratio of 1:2. After that, the pH is adjusted to 10 using a 1 M NaOH solution. Then, the reaction mixture is kept for 40 minutes at a temperature of 95 $^\circ$C.
Fig. 1. Schematic representation of the techniques for the formation of Ag$_2$S/L-Cys QDs, Au NRs, and hybrid structures based on them.
The colloidal Au NRs have been synthesized in the presence of a cetyltrimethylammonium bromide (CTAB) surfactant. Its aqueous solution forms cylindrical micelles, creating anisotropic conditions for the growth of the NRs. The gold NRs have been obtained by a multistage procedure. This includes sequential preparation and mixing of the seed and growth solutions (Fig. 1). The seed solution is the solution of spherical Au NPs (3 nm), obtained by chemical reduction of HAuCl$_4$ (7 $\mathrm{\mu}$L, 0.36 M) with a NaBH$_4$ solution (1.0 mL, 5 mM) in the presence of CTAB (20 mL, 0.02 mM). The growth solution has been obtained by mixing HAuCl$_4$ (28 $\mathrm{\mu}$L, 0.36 M), CTAB (50 mL, 0.1 mM), AgNO$_3$ (100 $\mathrm{\mu}$L, 0.02 M), and C$_6$H$_8$O$_6$ (5 mL, 0.05 $\mathrm{\mu}$M). The aspect ratio (length:diameter) of the Au NRs has been controlled by adding AgNO$_3$ (50 $\mathrm{\mu}$L, 0.02 M) to the growth solution. An increase in the aspect ratio of Au NRs provides a long-wavelength shift of the extinction peak of the longitudinal plasmon resonance, providing its spectral overlap with the luminescence spectrum of Ag$_2$S/L-Cys QDs. The reaction by-products were removed from the obtained solution of Au NRs by several cycles of centrifuging-redispersion in distilled water.
The formation of hybrid structures was achieved by mixing colloidal solutions of Ag$_2$S/L-Cys QDs and Au NRs in the molar ratio of [$\nu$(NRs)]:[$\nu$(QD)]$\sim$10$^{-4}$ (mole fraction, m.f.). This concentration ratio provides decoration of plasmonic Au NRs with colloidal Ag$_2$S/L-Cys QDs (1:10$^4$ pcs). The surface chemistry of the mixture components implies their direct contact due to the electrostatic interaction of the active carboxyl and amino groups of the L-Cys ligand, which passivates the Ag$_2$S QDs interfaces, with the nitro groups of CTAB molecules, which coordinate the morphology of Au NRs (Fig. 1). Owing to the supplementary addition of 0.1 $\mathrm{\mu}$l of a cationic polymer (Poly(diallyldimethylammonium chloride PolyDADMAC) to the colloidal mixture, spatial separation of the mixture components is expected due to the deterioration of electrostatic interaction (Fig. 1).
The size and morphology of Ag$_2$S QDs and Au NRs were determined using a Libra 120 transmission electron microscope (TEM) (Carl Zeiss, Germany) and a JEOL 2000FX high-resolution TEM (HR-TEM) (JEOL Ltd., Japan). The absorption properties were studied using a USB2000+ spectrometer (Ocean Optics, USA) with a USB-DT radiation source (Ocean Optics, USA). The luminescence spectra and luminescence decays of Ag$_2$S QDs were studied using the USB2000+ device and a TimeHarp 260 Pico Single time-correlated single-photon counting board (PicoQuant, Germany) with a PMC-100-20 PMT module (Becker$\&$Hickl, Germany) with a time resolution of 0.2 ns. To excite luminescence, we used an LD PLTB450 laser diode (Osram, Germany) operating at a wavelength of 445 nm (200 mW). The measurements were carried out at a temperature of 300 K.
The luminescence quantum yield (QY) for Ag$_2$S/L-Cys QDs was determined by the relative method using the expression
Here $QY_R$ is the quantum yield of the reference standard, $I$ and $I_R$ are the integrated luminescence intensities for the sample and reference standard, $D$ and $D_R$ are the optical densities at the excitation wavelength for the sample and reference standard (in the experiments, the corresponding value was $\sim$0.1), $n$ and $n_R$ are the refractive indices of the sample solution and the reference standard solution, respectively. Distilled water was used as a solvent for Ag$_2$S/L-Cys QDs and plexitonic structures based on them (n=1.329 at a wavelength of 750 nm at 293 K [43]). As a reference standard for measuring the luminescence quantum yield in the IR region, we used a solution of the indocyanine green (ICG) dye in DMSO with $QY_R$ = 12 % in the 800 nm region [44] ($n_R$ = 1.422 at a wavelength of 800 nm at 293 K according to the data of the work [45]). Special attention was paid to the correction for the spectral sensitivity of the spectrometer, which was determined using the emission spectrum of a standard tungsten lamp with a known color temperature. The quantum yields for A$_2$S/L-Cys QDs and their mixtures with Au NRs in the presence of PolyDADMAC were 0.2 % and 0.3 %, respectively. The relative error of this method for determining the luminescence quantum yield in our conditions was 10 %.For the study of spectral and luminescence properties of mixtures of Ag$_2$S/L-Cys QDs and Au NRs, aqueous solutions containing Ag$_2$S QDs or Au NRs with concentrations equivalent to those introduced during the formation of the mixtures were used as a reference. In the study of mixtures in the presence of the PolyDADMAC polymer, the reference samples were PolyDADMAC aqueous solutions containing Ag$_2$S QDs or Au NRs with concentrations equivalent to those introduced during the formation of the mixtures.
Analysis of the TEM images of Ag$_2$S/L-Cys QDs revealed the formation of individual nanocrystals with an average size of 2.1$\pm$0.5 nm and size dispersion of $\sim$30$\%$, which is due to the chosen approach of colloidal synthesis in an aqueous solution (Fig. 2(a)). According to the data of TEM images, the formation of Au NRs with average values of length 30$\pm$5 nm and diameter 9$\pm$2 nm was established, respectively (the ratio of the length to the diameter of the NR is 3.3). The dispersion of Au NRs in the ensemble was 25% (Fig. 2(b)). According to HR-TEM data, the approach used to obtain the mixtures of Ag$_2$S/L-Cys QDs and Au NRs ensures compatibility of the components and formation of hybrid structures. Analysis of the HR-TEM images showed the formation of particle agglomerates with interplanar distances of $\sim$0.196 nm, corresponding to the (422) crystallographic plane of the Ag$_2$S monoclinic lattice. Particles with interplanar distances of 0.237 nm were obtained as well, corresponding to the (111) crystallographic plane of the cubic face-centered lattice of Au (Fig. 2(c)). Thus, plasmonic nanoparticles are adsorption centers for QDs. In turn, QDs decorate the interfaces of Au NRs.
Fig. 2. TEM image of Ag$_2$S QDs (a); TEM image of Au NRs (b). HR-TEM image of mixtures of Ag$_2$S QDs and Au NRs (c); HR-TEM image of mixtures of Ag$_2$S QDs and Au NRs in the presence of PolyDADMAC (d).
Analysis of the HR-TEM images of hybrid structures of Ag$_2$S/L-Cys QDs and Au NRs in the presence of the PolyDADMAC polymer showed agglomeration of the particles with interplanar distances of 0.251 nm, corresponding to the (022) crystallographic plane of the Ag$_2$S monoclinic lattice, near the Au particles with interplanar distances of 0.237 nm, corresponding to the crystallographic plane (111) of the cubic face-centered lattice of gold (Fig. 2(d)). The agglomeration of nanoparticles is sparse. This is probably due to the presence of a polymer layer.
3. Results and discussion
Let us consider the spectral properties of the components of the synthesized hybrid nanostructures and compare them with the data for the mixtures.
In the UV-Vis absorption spectrum of colloidal Ag$_2$S/L-Cys QDs, a pronounced feature is observed at 620 nm (2.00 eV), associated with the exciton absorption, which is characteristic of the quantum confinement effect in nanocrystals (Fig. 3(a), curve 1). The broadening of the distinct exciton peak in the UV-Vis absorption spectrum is caused by the size dispersion of the Ag$_2$S/L-Cys QDs in the ensemble. The spectrum corresponds to QDs with an average size of 2.1 nm.
Fig. 3. (a) UV-Vis absorption spectra of Ag$_2$S QDs (Ag$_2$S/PolyDADMAC QDs) (1), extinction spectra of Au NRs (Au/PolyDADMAC NRs) (2), sum of Ag$_2$S QD absorption and Au NR extinction spectra (3), experimental spectrum of mixtures of Ag$_2$S QDs and Au NRs (Ag$_2$S QDs and Au NRs in the presence of PolyDADMAC) (4). (b) Luminescence spectra of Ag$_2$S QDs (1), mixtures of Ag$_2$S QDs and Au NRs (2), Ag$_2$S QDs and Au NRs in the presence of PolyDADMAC (3). (c) Luminescence decays of Ag$_2$S QDs (Ag$_2$S/PolyDADMAC QDs) (1), mixtures of Ag$_2$S QDs and Au NRs (2), Ag$_2$S QDs and Au NRs in the presence of PolyDADMAC.
In the extinction spectrum of Au NRs, two peaks at 530 nm (2.34 eV) and 720 nm (1.72 eV) are observed. They correspond to the transverse and longitudinal dipole modes of localized plasmons [46] (Fig. 3(a), curve 2). Note that the presence of the PolyDADMAC polymer in the colloidal solution, which contains separately Ag$_2$S/L-Cys QDs and Au NRs, does not affect the absorption and extinction spectra.
For colloidal Ag$_2$S/L-Cys QDs, luminescence was observed with a peak at 750 nm (1.65 eV) and full width at half maximum (FWHM) 150 nm (0.31 eV) (Fig. 3(b), curve 1). One of the reasons for so large FWHM of the observed luminescence spectrum of Ag$_2$S/L-Cys QDs is large QD size dispersion [47,48]. The Stokes shift was 130 nm (0.35 eV). This is associated with the trap state luminescence of QDs. According to the data of [49], luminescence results from radiative recombination of holes and trapped electrons, localized at the levels corresponding to structural impurity defects. Thus, the geometry and size of Au NRs provide a significant overlap of the peak of the longitudinal plasmon resonance with the absorption and luminescence spectra of the Ag$_2$S QDs (Fig. 3(a),(b)).
For mixtures of Ag$_2$S/L-Cys QDs with Au NRs, the resulting extinction spectrum is not a simple sum of the spectra of individual components (Fig. 3(a), curve 3). The increase in the optical density over the entire extinction spectrum and the blue shift of the plasmon resonance peaks by 2-3 nm are not only due to the contribution from the Ag$_2$S/L-Cys QDs but also due to the interaction between the mixture components (Fig. 3(a), curves 3, 4).
The formation of mixtures of Ag$_2$S/L-Cys QDs and Au NRs leads to a three fold decrease in the luminescence intensity for the Ag$_2$S QDs (Fig. 3(b), curve 2). However, the average luminescence lifetime for Ag$_2$S QDs does not change (Fig. 3(c), curves 1, 2). The presented experimental data do not confirm nonradiative resonance energy transfer between the components of the hybrid mixtures. Decreasing luminescence intensity Ag$_2$S/L-Cys QDs in the presence of Au NRs, together with constant luminescence lifetime, is the evidence for photoinduced electron transfer (PET), similar to the results of [34,35]. The positions of the quantum confinement levels of Ag$_2$S/L-Cys QDs and the Fermi level of gold imply that PET is possible in this hybrid system (Fig. 3(d)). For bulk silver sulfide, photoionization energy exceeds 5.24 eV [50]. Due to the quantum confinement effect, this value is increased. For Ag$_2$S/L-Cys QDs with an average size of 2.1 nm, the corresponding energy has been estimated to be 5.7 eV [49]. The Fermi level of Au is at 5.1 eV [51]. Therefore, the position of this Fermi level falls within the effective bandgap of the Ag$_2$S/L-Cys QDs (Fig. 3(d)). In this case, efficient PET from the Ag$_2$S/L-Cys QDs to Au NRs is possible, which leads to quenching of radiative recombination in the Ag$_2$S/L-Cys QDs. PET can also be facilitated by the complex chemical interaction of the active carboxyl groups of the L-Cys ligand, which passivates the interfaces of the Ag$_2$S QDs, with the nitro groups of the CTAB molecules, which coordinate the morphology of the Au NRs. This suggests direct contact between the Ag$_2$S QDs and Au NRs (Fig. 1). The incomplete suppression of the luminescence intensity of the Ag$_2$S/L-Cys QDs and the preservation of the luminescence lifetime, in this case, is due to the presence of QDs in the mixture, which do not participate in the charge transfer to Au NRs.
The introduction of the PolyDADMAC cationic polymer into the mixture of the Ag$_2$S/L-Cys QDs and Au NRs stabilizes the complex chemical interaction between the mixture components. It also provides spatial separation of the components, which is confirmed by the TEM data (Fig. 2(c)). In this case, an increase in the intensity of vis-IR luminescence (the peak at 750 nm) of Ag$_2$S/L-Cys is observed evenly over the entire luminescence band of the QDs, with the intensity being increased by a factor of 1.5 (Fig. 3(b), curve 3). At the same time, acceleration of the luminescence decay was observed (Fig. 3(c), curve 3). The average luminescence lifetime was determined from the experimental luminescence decays using the equation
where a$_i$ is the amplitude and $\tau _i$ is the time constant of the $i-th$ component of the luminescence decay. The values of a$_i$ and $\tau _i$ were obtained by approximating the luminescence decays by the sum of three exponentials The data indicate a decrease in the luminescence lifetime from 7.2 ns to 4.5 ns. Such behavior of the luminescent properties is the evidence for the Purcell effect [52]. Apparently, the presence of the PolyDADMAC polymer violates the conditions for charge transfer between the components of the mixture. In this case, Au NRs play the role of nanoresonators, the vibration modes of which are close to the recombination luminescence frequency of Ag$_2$S QDs. An insignificant increase in the luminescence intensity, in this case, is explained by the detuning of the luminescence spectral band of Ag$_2$S/L-Cys QDs and the resonance modes of Au NRs, taking into account their size dispersion in the ensemble [53,54]. In addition, the high concentration ratio of Ag$_2$S/L-Cys QDs to Au NRs, which is equal to 10$^4$:1, suggests the presence of QDs free from interactions with plasmon NPs, the luminescence of which will also contribute to the resulting luminescent properties of plexitonic nanostructures.The data on the change in the average lifetime and quantum yield of the trap state luminescence of Ag$_2$S/L-Cys QDs during the formation of the mixture with Au NRs provide a means to estimate the Purcell factor which characterizes the decay rate enhancement:
here $\gamma _{sp}^0$ and $\gamma _{sp}$ are the spontaneous transition rates for Ag$_2$S/L-Cys QDs in the absence and presence of NRs, $QY_0$ and $QY$ are the luminescence quantum yields for Ag$_2$S/L-Cys QDs in the absence and presence of NRs, and $\tau _0$ and $\tau$ are the average luminescence lifetimes for Ag$_2$S/L-Cys QDs in the absence and presence of NRs. Taking into account the quantum yield of trap state luminescence of the Ag$_2$S/L-Cys QDs ($QY_0$ = 0.2$\%$ and $QY$=0.3$\%$), the estimated value turns out to be $F_P$=2.8.The obtained value of $F_P$ leads us to the conclusion that the spontaneous transition rate is increased by a factor of 3 for the studied hybrid systems. This fact argues for the formation of a metal-semiconductor structure, where the weak plasmon-exciton coupling is achieved by virtue of spatial separation using the PolyDADMAC polymer.
The Purcell factor for a hybrid structure with plexcitonic coupling is also known to be governed by the Q-factor of the resonance for the plasmonic nanoparticle, as well as by the mode volume [55]:
The Q-factor of the nanocavity was determined from the literature data on the FWHM of the extinction spectrum for monodisperse Au NRs [56,57] since the FWHM of the peak corresponding to the longitudinal dipole mode of the localized plasmon resonance in the real extinction spectrum of Au NRs is $\sim$120 nm (Fig. 3), which is mainly due to inhomogeneous broadening. For calculations, the FWHM of the spectral profile was taken to be 30 nm at the frequency of the extinction peak ($\omega _r=2.61\cdot 10^{15}$ s$^{-1}$), corresponding to monodisperse Au NRs with the average length and diameter of 30 and 9 nm, respectively (length-to-diameter ratio 3.3). In this case, $Q=\omega _r/ \omega _0 \approx 23$.
By substituting the value of $F_p$ obtained from (4) and the determined Q-factor into (5), we obtain the mode volume of $V_m=9 \cdot 10^6$ nm$^3$. For the average values of the length and diameter of Au NRs (30 and 9 nm), the geometric volume of an Au NR is $V_g=0.2 \cdot 10^4$ nm$^3$. In contrast to the geometric volume $V_g$, which takes into account only the physical size of the cavity, the mode volume $V_m$ is determined by the distribution of the electromagnetic field strength and has the physical meaning of the volume occupied by the field if it were uniformly distributed with the maximum energy density. Plasmonic nanocavities are characterized by a small mode volume $V_m$, comparable to the geometric volume $V_g$ of the nanocavity. The mode volume $V_m$ obtained from (5) is more times greater than the value of the geometric volume $V_g$ of the NRs. However, the calculated $V_m$ is not accurate and is rather a qualitative estimate of the maximum achievable value of the Purcell factor in this situation. The inverse proportionality of the Purcell factor $F_p$ to the mode volume $V_m$ (see (5)) makes it possible to achieve the $F_p$ values of the order of several thousand when the emitter is placed in the nanocavity. However, in practice, the Purcell factor differs significantly from the theoretical values due to the mismatch between the spectral and spatial resonance for the emitter and the maximum of the resonance mode [58]. Equation (5) does not take into account the geometry of the hybrid structure, i.e. the distance between the plasmonic nanoparticle and the QD. Moreover, to solve the problems of this kind, it is necessary to understand the effect of a nanocavity on the properties of nonradiative transitions in QDs. These questions require separate consideration and will be addressed in our future studies.
Thus, the value of the experimentally determined Purcell factor allows us to conclude that the spontaneous transition rate in the studied hybrid structures is increased by a factor of 2.8. This fact argues for the formation of a metal-semiconductor structure where the weak plasmon-exciton coupling is achieved.
4. Conclusions
To sum up, this study presents the experimental data demonstrating the manifestations of plexcitonic coupling in the luminescence properties of colloidal Ag$_2$S/L-Cys QDs in the mixtures with Au NRs. Significant quenching of the luminescence intensity has been observed for the Ag$_2$S/L-Cys QDs upon their direct interaction with the Au NRs. Based on the absence of noticeable changes in the luminescence lifetime for Ag$_2$S/L-Cys QDs, charge transfer between the mixture components has been proposed. It has been shown that the additional introduction of the PolyDADMAC cationic polymer into the hybrid mixture leads to spatial separation of the mixture components and blocks the charge transfer between them. At the same time, enhancement of the luminescence intensity of Ag$_2$S/L-Cys QDs, accompanied by a decrease in the luminescence lifetime has been found, which is the evidence for the Purcell effect. Based on the data on the luminescence lifetime and quantum yield for Ag$_2$S/L-Cys QDs, the Purcell factor was estimated to be 2.8.
Funding
Russian Science Foundation (19-12-00266); Ministry of Science and Higher Education of the Russian Federation (N 075-15-2021-1351).
Acknowledgments
This study was supported by the Ministry of Science and Higher Education of Russia under Agreement N 075-15-2021-1351 in part of structural analysis of colloidal Ag2S QDs and Au NRs. Results of TEM investigations with TEM Libra 120 were obtained on the equipment of the Center of Collective Usage of Scientific Equipment of Voronezh State University.
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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