Article pubs.acs.org/JPCC
PbS Quantum Dots in a Porous Matrix: Optical Characterization Aleksandr P. Litvin,† Peter S. Parfenov,† Elena V. Ushakova,† Anatoly V. Fedorov,† Mikhail V. Artemyev,‡ Anatol V. Prudnikau,‡ Valery V. Golubkov,§ and Alexander V. Baranov*,† †
National Research University of Information Technologies, Mechanics and Optics, 197101 Saint Petersburg, Russia Institute for Physico-Chemical Problems, Belarusian State University, 220030 Minsk, Belarus § I.V. Grebenschikov Institute of Silicate Chemistry, Russian Academy of Sciences, 199155 Saint-Petersburg, Russia ‡
ABSTRACT: We propose simple and practical method for creation of quantum dot (QD) systems in a porous matrix. The commercial filter paper is soaked in the colloidal solution of PbS QDs in carbon tetrachloride followed by drying. The samples prepared by the method demonstrate linear dependencies of optical density and photoluminescence intensity on the QD concentration, excellent homogeneity, and reproducibility. A red-shift of QD photoluminescence spectrum after their infiltration into the matrix and energy transfer between QDs of different sizes indicate formation of the close-packed QD system. Optical properties and stability of the close-packed PbS quantum dot systems are investigated at room temperature in a wide range of QD sizes. A strong reduction of average QD photoluminescence lifetime from 435 to 55 ns with decreasing QD diameter from 3.0 to 7.4 nm has been found. A blue-shift of the photoluminescence spectra accompanied by increasing the photoluminescence lifetime observed for small and medium QDs with the sample storage indicates decreasing the QD size due to oxidation of their surface.
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INTRODUCTION Over the past decades close-packed systems of semiconductor nanocrystals (quantum dots, QDs) have attracted much attention due to possibility to utilize them in a variety of optoelectronic devices, such as photovoltaic elements,1 LEDs and lasers,2 and photodetectors.3 Because of size-dependent optical properties, broad absorption spectra with high extinction and narrow emission, high quantum efficiency, and excellent photostability semiconductor QDs can replace traditional materials of optoelectronics like organic dyes or bulk semiconductors.4 QDs for near-infrared (NIR) photonics, such as PbS and PbSe QDs, attract a particular attention.5 These QDs possess a number of unique properties, such as high charges mobility, large Bohr radii, small and equal effective masses of electrons, and holes.6 PbS QDs demonstrate unusual luminescence properties, particularly large photoluminescence lifetimes7−11 and significant Stokes shift.8−10,12−14 An anomalous size dependence of PbS QDs luminescent properties has been recently explained by the existence of the size-dependent in-gap state and by the phonon-induced transitions between the fundamental and in-gap states.15 Theoretical study of thermal transitions occurring with both decrease and increase in energy at room or higher temperature was made by Rukhlenko et al.16 and Leonov et al.17,18 Blended PbS QDs of different sizes deposited on substrates or embedded in polymer films and porous matrix are very promising as efficient broadband absorbers and emitters for various applications in the NIR region, including solar energy conversion and communication.19−25 © 2013 American Chemical Society
A particular feature is that in the close-packed nanocrystal systems, or QD solids, with more or less pronounced nanocrystal size distribution appropriate conditions for Förster resonant energy transfer (FRET) are realized.26−31 The FRET, an entirely nonradiative process caused by dipole−dipole interaction,32 can change considerably optical and electrical properties of close-packed nanocrystal systems. Therefore, the FRET effects on energy spectra and dynamics of electron transitions in systems of close-packed QDs of different sizes are of great interest. There is a lack of knowledge on these topics especially for close-packed QDs of lead chalcogenides. Weak environmental stability restricts utilization of the lead chalcogenides QD solids. PbS QDs are known to be more stable under ambient conditions than PbSe QDs.33 QDs of this type are usually capped by organic passivating agents such as oleic acid and trioctylphosphine.5 This type of surface passivation is not efficient in case of dry QD solids and leads to degradation of optical properties of QDs in particular due to oxidation of QD.34 If oxidation is a prevailing process, it results in decreasing of QD size and blue-shifts in absorption and emission spectra. Numerous efforts have been made to improve the stability of PbS QD systems. Moreels et al.35 proposed a modified Cademartiri synthesis36 to get stable PbS QDs in a wide range of sizes, but there is no information about dry closepacked PbS QDs systems based on this method. Ihly et al.37 examined an atomic layer deposition (ALD) technique to cover Received: March 6, 2013 Revised: May 21, 2013 Published: May 22, 2013 12318
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Figure 1. Absorption (a) and photoluminescence (PL) (b) spectra from four randomly chosen different local areas of the porous matrix with embedded 4.9 nm PbS quantum dots.
20 × 40 mm2 were dipped in the solution for a specified time and then were dried at ambient conditions. For all examined QDs absorbance spectra after preparation demonstrate that no changes occur with QDs during the infiltration into the porous matrix, and absorption and luminescence spectra correspond to those obtained for PbS QDs dissolved in carbon tetrachloride.15 Experimental Setup. A UV-3600 Shimadzu spectrophotometer equipped by an integrating sphere was used for measuring the absorption spectra of the samples possessing strong light scattering. The home-built setup analogous to that described by Parfenov et al.42 was used for steady-state spectral photoluminescence analysis. The laser excitation radiation at 532 nm and a power of 1.5 mW was focused on the sample. The photoluminescence was collected in 90° geometry and sent through the Acton SP-2558 monochromator (the spectral resolution was kept at 6 nm) to the Hamamatsu G5852-21 InGaAs photodiode cooled to −20 °C and with the spectral range of 0.7−2 μm. All photoluminescence spectra were corrected by the spectral sensitivity of the experimental setup, which was obtained using a blackbody spectrum.43 Before each measurement the samples were excited by the laser radiation for 60 s to avoid mistakes caused by the photobleaching effect. For transient photoluminescent analysis a home-built setup analogous to that described by us in ref 44 was used. The 640 nm radiation of a 100 ps pulsed PicoQuant LDH-P-C-640B laser operated at 4 kHz pulse repetition rate was used for the excitation. The photoluminescence passed through the relevant band-selective filter was focused on the 0.3 mm2 area of a fast Femto HCA-S-200M-IN detector based on an InGaAs pinphotodiode. Then the signal was amplified by a Stanford Research SR455A amplifier and sent to a PicoScope 3206A oscilloscope. A purpose-built computer program lets us to accumulate and average 5 million measurements, which takes about 45 min for each sample. Care was taken to avoid photobleaching during the experiments: the power density of the laser excitation did not exceed 20 μW/cm2, and photoluminescence spectra were checked after the measurements. The described setup let us carry out transient photoluminescence experiments in the spectral range of 0.8− 1.7 μm and in the time interval from 20 ns to 10 μs. Small-angle XRD measurements were performed with the homemade instrument with Cu Kα radiation with an “infinitely” high primary beam.
PbS QDs films with Al 2 O 3 . ALD-infilled QDs films demonstrate an excellent stability under photothermal treatment even in oxygen environment but only for QDs larger than 5 nm in diameter. For investigation of close-packed nanocrystal systems some sample preparation techniques were proposed: drop-casting,30 spin-casting,31 Langmuir−Blodgett technique,38 and layer-bylayer electrostatic assembly.39−41 The simplest way, dropcasting, cannot provide the desired homogeneity;31 other methods require difficult physical and chemical procedures. Because of these facts, the development of the method of formation of close-packed nanocrystal systems is an urgent task. The method should let create a uniform samples with reproducible parameters and linear dependence of optical properties on concentration of quantum dots in matrix. Here, we present the low-cost effortless method for creation of close-packed nanocrystal systems in porous matrix. The samples prepared by our method demonstrate linear dependencies of optical density and luminescence intensity on the QDs concentration and perfect homogeneity. They can be easily placed in any kind of cryostat for low-temperature experiments. A red-shift and energy transfer between QDs of different sizes indicate the formation of close-packed system in the porous matrix. Optical properties of the close-packed PbS nanocrystals systems including size dependence of the room temperature photoluminescence lifetimes are investigated. A stability of PbS QD close-packed systems prepared using the described method is considered for a variety of QD sizes.
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EXPERIMENTAL SECTION Sample Preparation. PbO (99.9%), oleic acid (90%), 1octadecene (90%), hexamethyldisilathiane, and carbon tetrachloride for PbS QDs synthesis were purchased from Aldrich and used without additional purification. Details of the synthesis procedure can be found elsewhere.15 PbS QDs of different sizes varied from 3 to 8.4 nm in diameter were dispersed in carbon tetrachloride with an appropriate concentration. A 388 grade Sartorius filter paper with a thickness of 0.14 ± 0.01 mm was used as a porous matrix. This filter paper is ashless and free of any residual acids. The paper porous matrix possesses sufficient transmission in 0.4−2 μm spectral region allowing precise measurements of absorption spectra of embedded PbS QDs in broad range of concentrations by subtraction of the reference spectrum and shows no traces of intrinsic luminescence bands in the spectral region of interest. For the samples preparation porous matrix stripes of 12319
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RESULTS AND DISCUSSION Matrix Properties. Absorption and photoluminescence (PL) spectra of the samples with PbS QDs of 3.0−8.4 nm in diameter, which band edge absorption and lowest energy PL transitions are in the 0.85−1.8 μm energy range, have been measured. It was found that the samples thus prepared possess very good homogeneity in the QD optical responses from areas with diameter lager than 100 μm, demonstrating excellent reproducibility of the QD absorption and PL spectra. The measured variation of the sample thickness over the strips does not exceed 10 μm. This illustrates below by example of the 4.9 nm PbS QDs with absorption and PL bands at ∼1.2 and ∼1.25 μm. In Figure 1a,b we show a very good reproducibility of the absorption and photoluminescence spectra in four randomly chosen local areas of 100 μm in diameter of the sample with 4.9 nm PbS QDs. The spectra measured for different independently prepared samples also coincide to each other. We found that concentration of embedded QDs is proportional to QD concentration in colloidal solutions. First, we kept the concentration of QDs in solution constant and changed the soaking time in solution from 5 to 60 s. We noticed that optical density of the samples did not manifest any noticeable difference, which means that saturation of the porous matrix by QDs of any sizes occurs in a very short time. Then we fixed the soaking time at 30 s and changed the QD concentration in the range of 6 × 1014−4.2 × 1015 cm−3 which resulted in linear increasing the optical density of the samples. The absorbance spectra of embedded QDs were obtained by subtraction of the pure matrix spectrum. We used the Beers law and the equation for the extinction coefficient of the 1Se1Sh transition obtained by Cademartiri et al.45 to estimate the concentration of PbS QDs in the porous matrix: c=
A 19600r 2.32l
In a specified range of QD concentrations the PL intensity possesses also a linear concentration dependency (not shown). The linearity of optical properties of the samples lets ones carrying out experiments without taking into account the reabsorption effect. While the QD absorption spectra do not change at embedding into the matrix, the PL bands of QDs in the matrix demonstrate an evident red-shift as compared with QD in diluted solutions. The red-shift, increasing from 4 to 15 nm with concentration of QDs in the matrix (Figure 3a), caused by FRET from smallest to largest QDs inside nominally monodisperse ensembles of PbS QDs is a recognizable feature, which indicates formation of system of close-packed QDs.27,29 Additional evidence of formation of QD close-packed system in the porous matrix is pronounced FRET observed in a batch of samples with embedded QDs of two different sizes: 4.9 nm (donor) and 6.4 nm (acceptor). In the experiment the concentration of QD donors was fixed while that of QD acceptors was varied in the range of acceptor to donor ratio from 0.14:1 to 7:1. The significant quenching of the donor luminescence accompanied by an enhancement of the acceptor one is shown in Figure 3b for QD-donor and QD-acceptor 1:1 mixture together with individual PL spectra of donor and acceptor at the same concentrations. A red-shift of QD PL spectrum after QD infiltration into the matrix and energy transfer between QDs of different sizes indicates the formation of close-packed QD system. One more evidence comes from small-angle XRD patterns of studied porous matrix with embedded PbS QDs obtained by us. An example of the patterns for QDs of 6.4 nm mean diameter is shown in Figure 4 where interference peak at angle of 63 min proves directly that PbS QDs form close-packed ordered structure in the filter paper porous matrix.46 An average interdot distance of 8.1 nm can be easily estimated with usual diffraction relation for the first diffraction maximum d = λ/sin(θ), where λ is the X-ray wavelength of 0.154 nm and θ is the angle of the SAXS peak. It means close contact of QDs with each other if accounted for a thickness of organic molecule (oleic acid) layer of 0.8−1.0 nm present on their surface. These results confirm that the proposed low-cost method can be used for creation of closepacked nanocrystal systems with desired optical parameters. Decay of Room Temperature Photoluminescence from PbS QDs Close-Packed Systems. Samples of the porous matrix filled with PbS QDs demonstrate evident concentration dependence of average PL lifetimes. This phenomenon was considered by Lunz et al. for close-packed CdTe QDs40 and was ascribed to the FRET processes. To exclude this effect when comparing the PL lifetimes from the samples with PbS QDs of different sizes, we prepared a batch of samples with identical concentration of QDs in the matrix of 1016 cm−3. We measured the PL decay curves for the samples with PbS QDs with diameters from 3.0 to 7.4 nm with emission in the spectral range of 0.73−1.28 eV. Figure 4 shows a representative set of the PL spectra and decay curves for samples of close-packed QDs with diameters of 3.1, 4.0, 4.7, and 6.1 nm. Diameters, energies of centers of PL bands, and average PL lifetimes of these QDs are listed in the Table 1. All curves were well fitted by double-exponential decay. The average lifetimes τ were calculated as intensity weighted means τ = (I1τ12 + I2τ22)/(I1τ1 + I2τ2), where Ii and τi are the amplitude and the decay time of the ith component, respectively. It was found that the average lifetimes of the close-packed PbS QDs decreased from 435 to 55 ns with increasing QD diameter from 3.0 to 7.4 nm, respectively. The
(1)
where A is the optical density at the 1Se1Sh transition, r is the radius of QD, l = 0.014 cm is the strip thickness. The obtained linear dependence of QDs concentration in the porous matrix on QDs concentration in solution is shown in Figure 2 together with absorption spectra of pure porous matrix and matrix with embedded 4.9 nm PbS QDs.
Figure 2. Dependence of QDs concentration (CQD) in the porous matrix on QDs concentration in solution. Inset: absorption spectra of pure porous matrix (black line) and matrix with embedded 4.9 nm PbS QDs (red line). 12320
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Figure 3. (a) Increasing the red-shift of QD PL peak from 4 to 15 nm with concentration of QDs in the matrix. Mean size of QDs is 4.9 nm, and PL fwhm is about 140 nm. Dotted line is the QD PL wavelength in CCl4 solution. (b) Quenching of PL from QD donors and enhancement of luminescence from QD acceptors in 1:1 donor and acceptor mixture embedded in the porous matrix. PL spectra of 1:1 donor and acceptor mixture (solid line) and individual donor and acceptor (dotted lines) at the same concentrations.
Figure 4. Small-angle XRD pattern of samples of porous matrix with embedded PbS QDs of 6.4 nm in diameter as a function of the scattering angle θ.
Figure 5. A representative set of the decay curves and PL spectra (inset) for samples of close-packed PbS QDs with diameters of 3.1, 4.0, 4.7, and 6.1 nm.
Table 1. Parameters of PL Lifetimes Measurements diameter (nm) 1 2 3 4
3.1 4.0 4.7 6.1
± ± ± ±
0.1 0.2 0.2 0.3
PL energy (eV) 1.28 1.12 0.99 0.84
± ± ± ±
0.01 0.01 0.01 0.01
nm close-packed PbS QDs emitting at 1.12 eV, where up to 100 meV blue-shift and 20 nm increasing fwhm of the PL band are observed after 30 days of storage. At the same time we did not find any substantial shift or peak broadening for sample with the largest, 7.4 nm QDs emitting at 0.73 eV that showed only ∼1 meV shift and ∼4 nm broadening after 30 day storage. After 3 months of storage the PL band shift for small QDs reached ∼300 meV, and the first exciton peak at 1.05 eV in the QD absorption spectrum completely disappeared as shown in Figure 6b for 4.9 nm close-packed PbS QDs. It was shown37,48,49 that it can be caused by an oxidation process which leads to destruction of PbS nanocrystals and decreasing its physical size, resulting in a blue-shift of the absorption and PL spectra. Oxidation and destruction of PbS QDs may also lead to broadening the QD size distribution, since the peak shift is found to be accompanied by increasing the PL bandwidth. It is very likely that if PbS QDs are deep-embedded in the matrix, they are less prone to degradation. In this case PL from significantly destructed PbS QDs, which are close to the matrix surface, will superimpose on that from deep-embedded QDs and thereby lead to broadening the total PL spectrum. The stronger degradation of PL for small QDs as compared with large ones may be also explained by taking into account a peculiarity of electronic energy structure of PbS QDs. It was shown that electron−hole radiative recombination takes place from both the fundamental and the in-gap electronic energy states of QDs.15 Moreover, PL from the in-gap state dominates
PL lifetime (ns) 435 355 167 94
± ± ± ±
15 15 10 10
corresponding dependence is shown in Figure 7. These results agree qualitatively with size dependence of the room temperature PbS QD PL lifetime obtained by Ushakova et al.15 for QDs in liquid solution and explained in terms of sizedependent nonradiative transitions between the in-gap and the fundamental energy states of QDs.47 A 5-fold reduction of the PL lifetime for QD solids as compared with that for QD in liquid solutions15 was observed. It is not surprise since a reduction of the QD PL lifetime caused by existence of nonradiative relaxation channels from QDs to the matrix was expected for close-packed nanocrystal systems in a matrix. PbS QDs Stability. To inspect stability of the close-packed PbS QDs in the prepared samples, we measured their absorption and PL spectra immediately after preparation and some time later. During that time all samples have been stored at room temperature in an untight box without any illumination. Measurements performed after 9, 15, and 30 day storage demonstrated a significant blue-shift and increasing the width of the PL band for PbS QDs of small and medium sizes. In Figure 6a this fact is illustrated by an example of 4.0 12321
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Figure 6. (a) Blue-shift detected for 4.0 nm PbS quantum dots in the porous matrix just after preparation (1) and after 9, 15, and 30 days of storage (curves 2, 3, and 4, respectively). (b) Absorption spectra of the porous matrix filled with 4.9 nm PbS QDs measured immediately after preparation (black line) and after 3 months storing. The first exciton peak of PbS QDs in the absorption spectrum was completely washed out. Absorbance spectrum of pure matrix is shown by dash line for reference.
the spectra for small dots and negligible for the large ones for which PL from the fundamental state prevail.47 A radiative relaxation from both these states may contribute to total PL band, which will consist of two component: the blue one, which corresponds to relaxation from the fundament state, and the red one, which corresponds to relaxation from the in-gap state. It is very likely that the some surface states of QD are responsible for the in-gap state.12,15,47,50,51 A degradation of the QD surface passivation will lead to formation of additional channels of nonradiative relaxation and weakening of the red component. Thus, the center of gravity of the PL band will shift to the blue region. The fact that oxidation is only contributing but not the dominant factor leading to changes in the optical properties of lead chalcogenides solids is also reported by Quintero-Torres et al.34 In framework of this model ones can expect a decreasing the average PL lifetimes with the degradation of the QD surface passivation. To check this fact, we measured PL lifetimes for samples with QDs with diameter 4.0 and 7.4 nm after 30 day storage. We found that for sample with 7.4 nm QDs the PL lifetime of 45 ns only slightly deviates from that of 55 ns characteristic for as prepared sample and that is close to time resolution of the experimental setup. However, for sample with 4.0 nm QDs the PL lifetime increased from 325 to 465 ns that contradict to prediction of the proposed model. At the same time our finding supports the main contribution of the oxidation to changes in the optical properties of PbS QD solids. Indeed, the PL band of 4.0 nm QDs blue-shifted to ∼1020 nm after 30 day storage, as is shown in Figure 7 by the red circle, became close to that of 3.5 nm QDs as-prepared sample, which has a characteristic PL lifetime of 440 ns that agrees well with the lifetime for 4.0 nm QDs after 30 day storage. This indicates that oxidation and decreasing of QDs size take place. If the oxidation and destruction are the main reasons of the PL shift, larger shift for smaller dots can be explained taking into account the geometrical factor and surface-to-volume atom ratio. It is clear that for smallest QDs a destruction of few atomic layers will lead to more dramatic consequences in the PL band position. It is interesting to compare these results with those obtained by Ihly et al.37 The authors showed that the 2.9 nm PbS QD samples demonstrated a 146 meV blue-shift after a month from preparation without any heat or illumination in the aerobic atmosphere, while the 7.0 nm PbS QD samples had only a 54 meV blue-shift. These significantly different shifts
Figure 7. PL lifetimes for the samples of the porous matrix infilled with PbS QDs of different size (blue squares measured for 4.0 and 7.4 nm QDs PL lifetimes are shown by red circles). For 4.0 nm QDs PL band shifted by 100 meV accompanied by an increasing of the PL lifetime.
provide similar relative reduction of the QD size on 14% and 10%, respectively, analogously observed by us.
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CONCLUSIONS The low-cost effortless method for preparation of close-packed quantum dot systems is proposed. A 388 grade Sartoriusclter ashless filter paper with a thickness of 0.140 mm is used as a porous matrix. It is shown that the close-packed quantum dot systems (QD solids) can be formed in the matrix by their filling from QD colloidal solutions. The samples prepared by the method demonstrate linear dependencies of optical density and PL intensity on the QD concentration, excellent homogeneity, and reproducibility. Spectral and kinetic luminescence properties of close-packed PbS QDs in the porous matrices have been investigated. Measured average PL lifetimes decrease from 435 to 55 ns for close-packed nanocrystals systems with QDs with diameter from 3.0 to 7.4 nm, respectively. The lifetime size dependence is in qualitative agreement with that obtained for isolated PbS QDs in solution. A 5-fold reduction of the absolute lifetime values is explained by appearance of additional radiationless channels of the photoexcitation energy transfer from QDs to the matrix. Stability of PbS QDs in the closepacked systems is considered for variety of QDs sizes. A large blue-shift in PL spectra of small and medium QDs accompanied by increasing the PL lifetime indicates that oxidation of QD surface is the main factor leading to changes of QD optical properties with a storage time. 12322
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the RFBR (Grants 12-02-01263 and 12-02-00938) and the Ministry of Education and Science of the Russian Federation (Projects 11.519.11.3020, 11.519.11.3026, 14.B37.21.0741, and 14.B37.21.1954). M.V.A. and A.V.P. acknowledge the financial support from CHEMREAGENTS program.
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