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Abstract
Owing to the broad spectral response and flexible choices of donors and acceptors, fluorescence resonance energy transfer (FRET) system based on quantum dots (QDs) is a potential candidate for enhancing performance of solar cells and other optoelectronic devices. Thus it is necessary to develop such FRET systems with high efficiency and understand the involved photophysical dynamics. Here, with type I CuInS2@ZnS core-shell quantum dots as the energy donor, series of CuInS2@ZnS-SQ complexes are synthesized by adjusting the acceptor (squaric acid, SQ) concentration. The FRET dynamics of the samples is systematically investigated by virtue of steady-state emission, time-resolved fluorescence decay, and transient absorption measurements. The experimental results display a positive correlation between the energy transfer efficient (η). The best energy transfer efficient achieved from experimental data is 52%. This work provides better understanding of the photophysical dynamics in similar complexes and facilitates further development of new photoelectronic devices based on relevant FRET systems.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Fluorescence resonance energy transfer (FRET) [1–5], the non-radiative photophysical process between a fluorescent donor and a suitable energy acceptor, is non-negligible as an important fluorescence attenuation pathway attributed to unavoidable charge-charge interaction between oscillating donor and acceptor dipoles instead of excited state charge transfer [6–9]. Thus it is necessary and significant to take advantage of this process and prevent such energy loss for enhancing fluorescence efficiency in associated application scenarios. Recently, for instance, efficient FRET has found prominent potential in green energy and life sciences [10–13]. Especially, FRET system based on quantum dots (QDs) has received great attention on account of its tunable spectral response, high efficiency, flexible synthesis and diversified choice of donors and acceptors [14–18]. Through suitable structure and composition control, introduction of QDs into a FRET system as the energy donor yields tunable emissive spectral and thus easy choice of energy acceptor. To make energy transfer more competitive, an applicable method is designing core/shell structure QDs [19–23]. Capped with an appropriate shell layer, charge transfer from QDs core can be suppressed due to enhanced quantum confinement. Simultaneously, reduced surface defect states and high chemical stability are available. Furthermore, adjusting the shell thickness to control suitable distance between energy donor (QDs core) and acceptor within Förster radius is beneficial to energy transfer. Among diverse QDs, with tunable fluorescence covering broad spectral range [24–27], high quantum yield [28,29], long luminescence decay [30,31], large Stokes shift [32,33] and low toxicity, CuInS2 series QDs capped by various shell layers have been proved to be an excellent fluorescent donor and triggered numerous applications for green energy and optoelectronic technologies [34–38].
Considering aforementioned background, in this work, we design a FRET system by choosing CuInS2@ZnS core-shell QDs as the energy donor and squaric acid (SQ) as the energy acceptor. The type I energy level distribution of CuInS2@ZnS sufficiently suppresses charge transfer between QD donor and SQ acceptor (Fig. 1(a, b)), thus accelerating the chance of FRET process. Details of energy level determination see Fig. S1 in Supplement 1. Besides, as shown in Fig. 1(c), the absorption spectra of CuInS2@ZnS energy donor (400∼600 nm) and SQ energy acceptor (600∼700 nm), covering a wide spectral range, synergistically facilitate efficient utilization of visible spectra. Furthermore, the emission spectral of CuInS2@ZnS well overlaps with the absorption spectra of SQ acceptor, making for more opportunity for energy transfer. As a whole, this work focus on FRET dynamics of CuInS2@ZnS-SQ complexes as the function of SQ concentration. Experimentally, through steady-state emission, time-resolved fluorescence and transient absorption measurements, the FRET efficiency is determined to be positively correlated to acceptor concentration. The maximum energy transfer efficiency (η) is about 52%. We believe this work will promote understanding of the FRET dynamics in similar complexes and meanwhile impel further exploitation of new photoelectronic devices based on such FRET systems.
Fig. 1. (a) Energy level distribution of CuInS2@ZnS core-shell QD and SQ molecule. Details of determining the energy level is shown in Fig. S1 in Supplement 1. (b) Sketch of the fluorescence decay paths in CuInS2@ZnS-SQ complexes under excitation. (c) Absorption and emission spectra of CuInS2@ZnS QDs and SQ molecule, respectively. The absorption spectrum of SQ is collected by dissolving SQ molecules in ethanol.
2. Experimental methods
2.1. Spectra measurements
For structure and composition characterization, TEM, XRD and EDX measurements are implemented using JEM-2100 (JEOL), D8 advance X-ray diffractometer (Bruker AXS) and X-ray energy dispersive spectrometer (Hitachi), respectively. For spectra tests, UV-Vis, static and time-resolved photoluminescence (PL) spectra are obtained by a SolidSpec-3700 spectrophotometer (Shimadzu) and FLS980 fluorescence spectrometer (Edinburgh), respectively. The main instruments used for transient absorption (TA) spectra are regeneratively amplified Ti:sapphire laser system (Coherent Legend, 800 nm, 85 fs, 7 mJ/pulse, and 1 kHz repetition rate) and Helios spectrometer (Ultrafast Systems LLC) for spectra generation and detection, respectively. Details of the measurements can be found in our previous work [25,39,40].
2.2. Synthesis of the FRET system
Core-shell CuInS2@ZnS QDs are prepared according to a typical synthesis process [30]. Briefly, for bare CuInS2 QDs, indium acetate (0.584 g, 1 mmol), copper iodide (0.38 g,1 mmol) and 1-dodecanethiol (DDT, 10 mL) are mixed and added into a three-neck flask. After degassed under vacuum for 5 minutes and purged with argon three times, the reaction mixture is heated up to 100 °C and kept for 10 minutes to form a clear solution. The temperature is then raised to 230 °C for nucleation and growth of CuInS2 QDs. Varying reaction time corresponds to certain QDs size. The reaction is held for 10 minutes to reach a diameter of 2.3 nm in our experiments. At the desired size, the reaction is quenched by immersing the flask in an ice-water mixture bath. The product QDs are separated by precipitating by addition of acetone, centrifuging, and decanting the supernatant. The solid QDs product is redispersible in heptane for subsequent experiments. To further synthesize CuInS2@ZnS QDs, the ZnS shell is coated through following procedure. 1 mL original solution of bare CuInS2 QDs is diluted in 4 mL 1-octadecene (ODE) and deaerated three times. 0.4 mmol zinc stearate and 0.4 mmol sulphur are mixed and dissolved in trioctylphosphine (1M solution) and 4 mL ODE is dropwise added into the reaction solution at 210°C for 20 minutes according to our preliminary experiments [41]. Solid core-shell QDs can be obtained by similar process to that of CuInS2 QDs.
To prepare the FRET system, SQ acceptors are bonded to CuInS2@ZnS donors by ultrasonic self-assembly. In short, excessive SQ molecules are added to heptane solution of CuInS2@ZnS. The solution is then ultrasonicated for 10, 20, 30 and 50 min, respectively, to form CuInS2@ZnS-SQ complexes with various concentration of SQ acceptors. Since single SQ molecule are insoluble in heptane, while those bonded to QDs through carboxy after ultrasonic are dissoluble, CuInS2@ZnS-SQ complexes can be simply obtained by filtration. Hereafter for briefness, the samples are named as S0 (CIS@ZnS), S1 (CIS@ZnS-SQ, 10 min ultrasonic), S2 (CIS@ZnS-SQ, 20 min ultrasonic), S3 (CIS@ZnS-SQ, 30 min ultrasonic), S4 (CIS@ZnS-SQ, 50 min ultrasonic).
3. Results and discussion
3.1. Structure and composition characterization
To confirm the structure and chemical composition of the samples, we carry out XRD and EDX measurements. Figure 2(a) shows the XRD patterns of bare CuInS2 QDs and CuInS2@ZnS donor (S0). The three peaks of CuInS2 QDs (black line) correspond to the (112), (204)/(220) and (116)/(312) reflections of the crystal structure, consisting with previous study and implying chalcopyrite phase of the QDs [42].As for the diffraction pattern of S0 (red line), the three peaks shift to larger 2θ angle and the corresponding intensity increases due to the overlap of ZnS pattern onto that of CuInS2 QDs, also in consistent with related research [30]. To verify the chemical composition, CuInS2@ZnS QDs are deposited on a quartz glass tank. EDX spectrum in Fig. 2(b) reveals the existence of Cu, In, S and Zn. The ratio of Cu to In derived from Fig. 2(b) is 1:1.23, signifying an In-rich composition. Besides, the total of S elements comparing with stoichiometric CuInS2 and ZnS indicates an excessive S composition. C element appearing in Fig. 2(b) originates from DDT ligand for preparing the QDs. For further validating the morphology and size distribution, TEM images of bare CuInS2 QDs and S0 are shown in Fig. 2(c, d). The average sizes are 2.1 ± 0.4 nm and 5.4 ± 0.5 nm respectively. Besides, the size distribution has good homogeneity. The size of bare CuInS2 QDs determined by the location of the first exciton bleach peak (∼523 nm, Fig. 5) via the EMA theory [42] is ∼2.3 nm, well agreeing with the TEM results (2.1 ± 0.4 nm) indicated in Fig. 2(c). The characterizations manifest successful synthesis of bare CuInS2 and core-shell QDs. In addition, by defining the average size of QDs from TEM results as the edge length of tetrahedral structure, the circumradius of CuInS2 and CuInS2@ZnS is obtained and the number of ZnS layers [43] is determined to be ∼6.5, which is thick enough to effectively suppress charge transfer from QDs to SQ molecule.
Fig. 2. (a) XRD patterns of the bare CuInS2 QDs and CuInS2@ZnS donor (S0). (b) EDX spectrum and elements content of S0 by depositing it on a quartz glass tank (the overlaid and individual spatial distribution of each elements are shown as Fig. S2 in Supplement 1), (c,d) TEM images of bare CuInS2 QDs and S0.
3.2. Optical absorption
Figure 3 reveals SQ concentration-dependent absorption spectra of S0-S4. The relative ratio of SQ concentration can be determined as 0(S0): 1(S1): 3(S2): 7.2(S3): 10(S4) according to Lambert-Beer law (details see Supplement 1). For each sample, the concentration of QDs is identical except for the amount of SQ acceptor, which is confirmed by the similar traces of the absorption curves below 560 nm in Fig. 3.
Fig. 3. UV-Vis absorption spectra of CuInS2@ZnS QDs (S0) and CuInS2@ZnS-SQ (S1-S4). Samples S0-S4 contain the same concentration of QDs but increasing amount of SQ.
Fig. 4. Static emission spectra of S1-S4 containing CuInS2@ZnS-SQ (black solid line), pure CuInS2@ZnS (blue solid line), SQs (magenta solid line) and estimated emission of the contribution of QDs in corresponding CuInS2@ZnS-SQ complexes (red dash-dotted line) at 550 nm excitation. The concentration of each solution is all the same within one separate series of experiment.
3.3 Determination of FRET efficiency
Quantification of the energy transfer efficiency (η) can be realized through analyzing the static fluorescence spectra [44]. Mathematically, η satisfies the formula:
To finally quantify η, φQD and φQD-SQ are derived according to the typical method in literature [44]. ISQ and IQD-SQ of each sample are determined by series of photoluminescence (PL) spectra plotted in Fig. 4. Considering the small absorption of SQ molecules at 550 nm (excitation wavelength), direct excitation of SQ dyes in CuInS2@ZnS-SQ is firstly excluded since the pure SQ emission in SQ-ethanol solution is much weaker than that in the complex with the same SQ concentration. Consequently, the emission of SQ molecules are owed to energy transfer from excited CuInS2@ZnS rather than direct excitation by 550 nm. The rising characteristic peak at 663 nm (for SQ) and the dropping peak at ∼630 nm (for CuInS2@ZnS) along with the increase of SQ ratio indicates the strengthening of energy transfer process from donor to acceptor in the FRET system. Detailed parameters of the quantification of η are listed in Table 1. It shows a positive correlation between η and the SQ ratio. The values of η are 3%, 19%, 45% and 52% respectively.
Table 1. Parameters for quantifying the energy transfer efficiency (η) of each sample
3.4 Time-resolved fluorescence
By selectively exciting at 550 nm, the time-resolved fluoresence of S0-S4 and pure SQ around 630 nm (emission center of CuInS2@ZnS) is collected in Fig. 5. The exciton decay constant (amplitude-weighted average time) of S0 is 87.9 ns. With bonded SQ molecules, fast decay components appear and the decay rates increase with the ratio of SQ to QDs, implying the quenching of CuInS2@ZnS emission by the SQ molecules involved with the energy transfer process. Note that the quenching with increased concentration of SQ indicated in Fig. 5 is much stronger than that in Fig. 4, it can be attributed to the synergistic effect of direct excitation of SQ molecule by 550 nm laser and FRET process, considering the very fast decay of pure SQ (green curve in Fig. 5). However, this doesn’t negate the validity of the results from Fig. 4 because the contribution of direct excitation of SQ has been considered and deducted from the whole fluorescence intensity.
3.5. Exciton dynamics of the complex system
To further validate the photophysics of the energy transfer process in CuInS2@ZnS-SQ complexes, transient absorption measurement (400 nm excitation) is implemented and the pseudo-color images of the transient absorption spectra for all samples are shown in Fig. S3 (Supplement 1). Accordingly, the exciton bleach signals of S0-S4 are extracted as depicted in Fig. 6(a)-(e). Figure 6(a) illustrates the first exciton bleaching (XB) signal of pure CuInS2@ZnS QDs near 523 nm dominated by state filling of conduction band electron levels. The positive signal above 630 nm corresponds to the photo-induced absorption (PA) of CuInS2@ZnS QDs. In Fig. 6(b)-(e), with bonded SQ molecules on CuInS2@ZnS QDs, new exciton bleach signals arise at ∼650 nm and the amplitudes rise along with the dropping bleach signals of CuInS2@ZnS QDs, indicating energy transfer from CuInS2@ZnS QDs to SQ. Besides, the bleach amplitude at ∼650 nm strengthening from sample S1 to S4 implies enhanced energy transfer with increased concentration of SQ molecules. After subtracting the PA signal of CuInS2@ZnS QDs, exciton bleach kinetics of QDs with or without bonded SQ molecules at ∼523 nm and the exciton bleach kinetics of SQ in the complexes at 650 nm are extracted from transient absorption spectra as Fig. 6(f). It decays much faster for CuInS2@ZnS-SQ complexes than that for pure QDs and the decay rate increases with higher ratio of SQ to QDs, which also indicates enhanced exciton quenching by energy transfer, corresponding well to the analysis of time-resolved fluorescence spectra in Fig. 5. In addition, the formation kinetics of XB in SQ accelerating along with increased ratio of SQ to QDs likewise indicate enhanced FRET from QDs to SQ.
Fig. 6. (a-e) Transient absorption spectra of pure CuInS2@ZnS QDs and S1-S4 at 400 nm excitation. (f) Extracted exciton bleach kinetics from transient absorption spectra of pure CuInS2@ZnS QDs and S1-S4 at 523 nm (XB of QDs) and 650 nm (XB of SQ), respectively.
4. Conclusions
In summary, by systematical measurements and analysis on series of steady and transient spectra, energy transfer process is confirmed to be a prominent way of exciton quenching in CuInS2@ZnS-SQ complexes. The energy transfer efficiency (η) increases with the ratio of SQ molecules to CuInS2@ZnS QDs. The highest η achieved in this work is 52%. Since the emission of CuInS2@ZnS QDs overlap highly with the absorption of SQ molecules and the absorption of CuInS2@ZnS QDs and SQ molecules covers a large part of visible spectral range, the FRET system of CuInS2@ZnS-SQ complexes is a potential candidate for applications in solar cells and other optoelectronic devices working with visible light. Our results may be instructive to understanding the photophysical mechanism of FRET process and facilitate exploiting such systems for optoelectronic applications.
Funding
National Key Research and Development Program of China (2019YFE0107200); Key Research and Development Plan of Hubei Province (2021BGE037); Teacher Research Ability Cultivation Foundation of Hubei University of Arts and Science (2020kypytd001); Hubei Key Laboratory of Low Dimensional Optoelectronic Material and Devices (HLOM222008).
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.
Supplemental document
See Supplement 1 for supporting content.
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