Hybrid systems based on porous silicon microcavities and quantum emitters (QEs) are of much interest in terms of both basic research and development of new hybrid photoluminescent (PL) materials to be used in photonic, optoelectronic, and sensing applications. In these systems, light-matter coupling is established, whose strength could be increased to achieve the strong coupling regime by enhancing the quality factor of the microcavity. Incorporation of plasmonic nanoparticles (PNPs) also promotes an increase in the coupling strength and establishment of the coupling regime via the formation of hierarchical plasmon-optical cavities. Here we present the results of a numerical study of hybrid systems comprising porous silicon microcavities and plasmonic arrays placed inside them. These hybrid systems enable hierarchical plasmon-optical coupling with exciton transitions in QEs embedded into a porous silicon microcavity. We used numerical simulations to estimate the critical parameters for achieving light-matter coupling, including the Purcell factor and expected field enhancement, as well as the spatial distribution of the electromagnetic field within the structure. We speculate that light-matter coupling between the PL of QEs and the hierarchical cavity mode is stronger than in a microcavity not containing PNPs.
The engineering of efficient photodetectors with a nonlinear response to light is an important task for modern optoelectronics because this allows the fabrication of fully light-controlled devices. The two-photon absorption (TPA) phenomenon allows 3D confinement of the excitation and thus, hence, the fabrication of 3D arrays of light-sensitive elements. Moreover, this type of excitation ensures the control of the response efficiency by tuning the intensity of the light. Current technologies mainly use photodiodes based on bulk semiconductors, but the use of colloidal semiconductor quantum dots (QDs) has more advantages due to ease of processing, precisely-controlled optoelectronic properties, quick response, and compatibility with flexible substrates. Moreover, QDs have very high two-photon absorption cross-sections in a wide spectral range in the infrared region. Nevertheless, to activate such photodetectors in the nonlinear optical mode, peak excitation intensities higher than GW/cm^2 are still required. To reduce the excitation intensity it is needed to further increase the TPA. To achieve it we designed plasmon–exciton hybrid material using plasmon nanoparticles (PNPs) with an absorption peak overlapped with excitation wavelength. We found that the effective TPA in QDs near PNPs increased by the order of magnitude due to the nonlinear energy transfer from plasmons in PNPs to excitons of QDs. Finally, we designed a near-infrared plasmon–exciton photodetector based on the designed plasmon-exciton hybrid material. We investigated the effect of plasmons on the photoresponse of the photodetector under near-infrared two-photon excitation of QDs. As a result, we have found that the photocurrent was up to 38 times higher in the detector filled with QD–PNP plasmon–exciton hybrid material than in the detector filled with QDs alone.
This study was supported by the Russian Science Foundation, grant no. 18-72-10143-П. V.K. has received funding from the European Union's Horizon 2020 research and innovation programme under the Marie Sklodowska-Curie, grant agreement no. 101025664 (QESPEM).
Semiconductor quantum dots (QDs) are known for their high capacity to nonlinear interaction with light via two-photon absorption (TPA). This allows them to absorb efficiently the infrared photons with energies lower than the bandgap energy. In addition, the TPA can be further enhanced due to interaction of QD excitons with plasmons of metal nanoparticles making it possible to design highly efficient optoelectronic devices with a nonlinear response to irradiation. To achieve this goal, we have fabricated the nonlinear photodetectors based on the QDs and silver nanoplates (SNPs) which combine both mentioned effects and demonstrate a highly efficient nonlinear photocurrent response at the excitation in the nearinfrared region of optical spectrum. In this study, we compared the photodetectors efficiency enhancement in hybrid devices based on the CdSe QDs and SNPs designed by the different ways. In one case, the SNPs were deposited on the top of 10 layers of QDs, and in the other, they were placed between these layers. We have demonstrated that both types of hybrid photodetectors operate in the two-photon regime. At the same time, we have found that the two-photon absorption efficiency was significantly higher in the sample where the SNPs were located between the QD-layers.
Semiconductor quantum dots (QDs) have a wide absorption spectrum spreading from UV to the visible region and high photoluminescence (PL) quantum yield (QY) what determine possible use of their films for re-emitting coatings enhancing the photodetector spectral range. Unlike fluorescent organic dyes, the QDs absorption doesn’t saturate at high excitation intensities and can absorb more than one photon per particle due to the biexciton generation. However, due to the high rate of the Auger nonradiative relaxation, the QDs biexciton PL QY is much lower than the single-exciton one, what reduces the overall PL QY and the photodetector photosensitivity at the high excitation intensities. An employment of the Purcell effect in the plasmon nanocavities should increase the biexciton PL QY thus overcoming this limitation. To use this effect, we designed a thin-film plasmon–exciton material containing QDs and silver nanoplates (SNPs) in which the QDs’ PL band and the SNPs’ absorption band are overlapped. To demonstrate the advantage of the designed (QD-SNP)-film, we have compared effects of QD-film and (QD-SNP)-film on the photoresponse of the Si-based photodetector. The response of a photodetector to pulse excitation at 266 nm was negligible and increased after the deposition of the QD-film on its surface. However, at the high excitation intensities, the photosignal was reduced due to the biexcitons formation. The addition of SNPs increased the photoresponse at high excitation intensities. We attribute this improvement to a strong enhancement of QD biexciton PL in the QD-SNP material, which became predominate at high excitation intensities.
Semiconductor quantum dots (QDs) feature high values of the two-photon absorption (TPA) cross-sections, enabling their applications in biosensing and nonlinear optoelectronics. However, the efficient QD photoluminescence (PL) intensity caused by TPA requires high-intensity laser excitation which hinders these applications. Placing the QDs in the micro- or nanocavities leads to a change in their PL properties. Particularly, near plasmon nanoparticles (open nanocavities) the local field may be enhanced by the localized plasmons, which will lead to an increase of the TPA efficiency. Alternatively, placing QDs in a photonic crystal may boost an increase of their PL quantum yield due to the Purcell effect and also increase their PL intensity at the photonic mode wavelength due to the redistribution of the density of photonic states. In this study, we have fabricated thin-film hybrid materials based on QDs placed near plasmonic nanoparticles or in the photonic crystal. We have demonstrated a 4.3-fold increase of the radiative recombination rate of QDs in the photonic crystal cavity under the two-photon excitation, resulting in the increase of the PL quantum yield. In turn, the coating of the QDs films with the gold nanorods led to the 12-fold increase in TPA at the maximum of the plasmon spectrum. Our results pave the way to a strong increase of the PL efficiency of the QDs under two-photon excitation for their applications in biosensing and nonlinear optoelectronics.
Photoluminescence (PL) properties of semiconductor quantum dots (QDs) may be significantly improved by forming hybrid structures with plasmonic nanoparticles (PNPs). In general, three main effects can be observed when QDs are placed near PNPs – a local enhancement of excitation, acceleration of radiative recombination rate (Purcell effect), and acceleration of nonradiative relaxation rate due to the metal-induced energy transfer. All these effects lead to an increase in PL quantum yield (QY), excitonic (EX) and biexcitonic (BX) states and to the strong reduction of PL lifetime. In this study, we investigated the EX and BX PL parameters of single QDs in the vicinity of PNPs at different overlapping between the excitation wavelength, QDs PL, and PNPs extinction spectra. Here, we have fabricated thin films of QDs separated from the environment by the polymer spacer, and placed PNPs atop of these structures under the continuous observation of optical parameter of the same single QD. We have found that the excitation may be strongly increased in the case of a strong spectral overlap between excitation band and PNPs extinction. Nevertheless, the EX QY is strongly reduced by the energy transfer. In the case of strong spectral overlap between QDs PL and PNPs extinction, the radiative rate is increased, which leads to an increase of both EX and BX QYs and to a near-unity BX-to-EX QY ratio. Finally, we managed to combine these two effects in one material with a synergistically increased PL intensity, ultrashort PL lifetime, and levelling of EX and BX QY.
Semiconductor quantum dots (QDs) are widely used in photovoltaic and optoelectronic devices due to their unique optical properties. Photoluminescence (PL) properties of QDs can be significantly improved by their electromagnetic coupling with plasmonic nanoparticles (PNPs). The excitation of resonant localized plasmon modes leads to the enhancement of the density of photon states and increase of electromagnetic field near the surface of PNPs, what boosts the acceleration of the exciton radiative decay, known as the Purcell effect. To study the dependence of the degree of acceleration of radiative decay rate (Purcell factor) on the distance between QDs and PNPs, we fabricated thin-film hybrid structures based on CdSe(core)/ZnS/CdS/ZnS(multishell) QDs and silver or gold PNPs with a controllable distance between these components. The change in the radiative decay rate of excitons was calculated from the PL intensities and lifetimes before and after the deposition of PNPs on top of the QD thin film covered by a poly(methyl methacrylate) (PMMA) spacer. For both PNP types, the PL lifetime of underlying QDs decreased, whereas the PL intensity of the latter decreased only slightly for gold PNPs and even increased for silver PNPs. This indicates the acceleration of QDs radiative decay (Purcell effect) mediated by exciton-plasmon interaction. The Purcell factor was higher for silver PNPs than that for gold PNPs, what can be explained by the better spectral overlap between the QDs PL band and silver PNPs absorbance and the absence of interband absorption in silver at the wavelength of QDs PL. The results of this study provide better understanding of the Purcell effects in hybrid materials based on QDs and PNPs.
Semiconductor nanocrystals (SNCs), in particular, quantum dots (QDs) and nanoplatelets (NPLs), have orders of magnitude higher two-photon absorption cross-sections (TPACS) than organic dyes, what paves the way to their advanced applications in bioimaging, sensing, and optoelectronics. Traditionally, z-scan and two-photon photoluminescence (PL) excitation spectroscopy are used to determine the TPACS values. The main disadvantage of both methods is the necessity to know the exact sample concentration. In this study, we describe an approach to the TPACS determination from the analysis of two-photon-excited (TPE) PL saturation in CdSe(core)/ZnS/CdS/ZnS(multishell) QDs and CdSe NPLs. The results obtained for NPLs using developed approach are significantly smaller than those obtained by the z-scan method and are close to the values obtained for QDs. We assume that this discrepancy occurs due to the fact, that unlike the z-scan technique, the TPE PL saturation method measures the TPACS only for single-exciton states because of the low PL quantum yields of multiexciton states. Therefore, there is no need to know the concentration, which eliminates the corresponding estimation error. Thus, the measurement of TPE PL saturation in SNCs makes it possible to determine the absolute values of the TPACS of single-exciton states, which are more informative for applications of TPE PL than the TPACS of mixed multiexciton states.
Semiconductor quantum dots (QDs) are known for their unique photophysical properties and, in particular, their ability to multiphoton emission caused by recombination of biexcitons. However, the luminescence quantum yield of biexciton states is relatively low due to the fast Auger non-radiative process. Plasmonic nanoparticles can significantly accelerate the radiative rate of QDs. In this study we demonstrate the distance-controlled enhancement of the biexciton emission of single CdSe/ZnS/CdS/ZnS QDs due to their coupling with gold nanorods. We explain this enhancement as the distancedependent trade-off between the energy transfer and the Purcell effect. Our findings constitute a reliable approach to managing the efficiency of multiphoton emission over a wide span of distances.
The photosensitive protein bacteriorhodopsin (bR) has been shown to be a promising material for optoelectronic and photovoltaic applications, but it cannot effectively absorb and utilize light energy in the near-infrared (NIR) region of the optical spectrum. Semiconductor quantum dots (QDs) have two-photon absorption cross-sections two orders of magnitude larger than those of bR and can effectively transfer the up-converted energy of two NIR photons to bR via the Förster resonance energy transfer (FRET). In this study we fabricated a hybrid material in the form of an aqueous solution of electrostatically bound complexes of QDs and purple membranes (PMs) containing bR. Efficient FRET from QDs to bR was observed in these complexes under selective two-photon excitation of QDs. Then, we fabricated a photoelectrochemical cell operating in the NIR spectral region. Measurement of the photoelectrical signals from the cell containing pure PMs, or QD–PM hybrid material has shown that the light conversion in the QD-PM hybrid material with 3:1 bR-to-QD molar ratio is more efficient than in the material with 20:1 bR-to-QD molar ratio. The results of this study may extend the use of bioinspired hybrid materials in optoelectronics, holography, and bioenergetics under the conditions of nonlinear excitation.
Semiconductor quantum dots (QDs) have high two-photon absorption cross-sections and long photoluminescence (PL) lifetimes, which make them a promising photosensitive part for fabrication of QD-based hybrid materials for two-photon bio-imaging, bio- and optoelectronics. In these areas, mode-locked femtosecond lasers are often used for two-photon excitation of QDs because of the high peak intensity of the laser pulse. However, the QD radiative lifetime usually exceeds the period between the laser pulses of such laser systems, which can affect the absorption and PL properties of QDs. In this work, we investigated the PL properties of CdSe/ZnS QDs under two-photon excitation. We have shown that using femtosecond laser excitation at a wavelength of 790 nm with a pulse repetition rate of 80 MHz and a peak intensity of more than 10 GW/cm2, the two-photon absorption in QD is saturated. However if QDs were in complexes with purple membranes (PM) containing the photosensitive protein bacteriorhodopsin (bR), saturation was not observed up to an intensity of about 27 GW/cm2. It was concluded that the difference in the saturation of two-photon absorption between QDs and QD-PM material is associated with the Förster resonance energy transfer from QD to bR and the corresponding shortening of the PL lifetime. The results obtained will allow to optimize the two-photon excitation regime of QD-PM nano-bio hybrid material which will expand the possible areas of its application in bio-imaging, bioand optoelectronics.
Semiconductor quantum dots (QDs) are a promising “nano-antennas” capable of absorbing efficiently light energy upon one- or two-photon excitation and then transferring it to convenient energy acceptors via Förster resonance energy transfer (FRET). The photosensitive protein bacteriorhodopsin (bR) has been shown to be a promising material for optoelectronic and photovoltaic applications, but it cannot effectively absorb light in the UV, blue, and NIR regions. It was shown previously that formation of hybrid complexes of QDs and purple membranes (PMs) containing bR could significantly improve the bR capacity for utilizing light upon one- and two-photon laser excitations. Under the laser irradiation, the optical properties of bR itself remain unchanged, whereas those of QDs may be altered. Therefore, it is important to study the effects of intense laser excitation on the properties of the QD–PM hybrid material. In this study we have shown that laser irradiation can lead to an increase in the luminescence quantum yield (QY) of QDs. The fact that this irradiation does not change the QD absorption spectra means that the QD quantum yield may be optically controlled without changing the QD structure or composition. Finally, we have shown experimentally that photoinduced increase in the QY of QDs lead to the corresponding increase in the efficiency of FRET in the QD–PM hybrid material. As a result, an approach to increasing the FRET efficiency in hybrid nano-biomaterials where QDs serve as donors have been proposed.
It has been demonstrated that photo-induced changes in the optical properties of semiconductor quantum dots (QDs) can
be controlled by tuning the parameters of their laser irradiation to vary the relative contributions of photo-brightening
and photo-darkening of QDs. For this purpose, the effects of the QD size, photon energy, and intensity of irradiation of
QDs on the competing processes of photo-darkening and photo-brightening have been investigated. We have found that
photo-brightening of QDs is not accompanied by detectable growth of their photoluminescence (PL) decay time, this
process being most pronounced for QDs with an originally low PL quantum yield (QY). In this case, an increase in the
PL QY is assumed to be caused by transition of some QDs from the dark (non-emissive) state to the bright (emissive)
state. On the other hand, the photo-darkening effect, which was observed only under UV irradiation at 266 nm, was
accompanied by simultaneous drop of both the QD QY and their PL decay time. We have also found that, at a constant
dose of absorbed energy, the photo-brightening and photo-darkening processes do not depend on the excitation intensity.
Thus, the photo-induced changes in the optical properties of QDs are one-photon processes. These data may help to
optimize the QD operational conditions in practical applications requiring their intense excitation and add to
understanding the fundamental mechanisms of the irreversible photo-induced changes that occur in colloidal QDs under
illumination.
The organic ligands passivating the surface of semiconductor quantum dots (QDs) and the solvents used strongly determine the photostability of QD solutions. Highly purified QD solutions in chloroform have been shown to photodegrade upon pulsed ultraviolet (UV) irradiation, irrespectively of the type of surface ligand. However, the photostability of QDs dissolved in n-octane, a more photochemically inert solvent, strongly depends on the ligands passivating their surface. In n-octane, hexadecylamine-coated QDs are completely stable and display no photochemical response to pulsed UV laser irradiation. In solutions of octanethiol-capped QDs, the photoluminescence intensity slightly decreases under irradiation. QDs coated with trioctylphosphine oxide exhibit a more complex pattern of photobleaching, which depends on the initial value of fluorescence quantum yield of QDs. This complex pattern may be accounted for by two competing processes: (1) ligand photodesorption accompanied by photobleaching due to specific alignment of the band levels of QDs and highest occupied molecular orbital of the ligand and (2) photoinduced decrease in the population of trapping states. Furthermore, practically no thermodynamic degradation of QD solutions has been observed for the micromolar QD concentration used in the study, in contrast to lower concentrations, thus confirming the photoinduced origin of the changes caused by UV irradiation. Obtained results show that the photostability of QDs may be strongly increased by careful selection of the ligands passivating their surface and the solvents used in the experiments.
Changes of optical properties of the solutions of CdSe/ZnS quantum dots (QDs) covered with the trioctylphosphine oxide (TOPO) ligands under the pulsed ultraviolet (UV) laser irradiation are observed. The fluorescence quantum yield (QY) of QDs decreases by more than an order of magnitude when the radiation dose approaches 2 × 10-15 J per particle. This process is accompanied by a blue shift of both fluorescence and the first excitonic absorption peaks. The fluorescence quenching becomes less pronounced when the overall TOPO content in the solution is increased. When ТОРО ligands are replaced with n-hexadecylamine (HDA), QY and spectral properties are not changed at the same irradiation conditions. We assume that the above changes of the optical properties are associated with photooxidation of TOPO ligands by excited QD. Such process is less probable for the HDA ligand due to its different energy structure.
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