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Abstract
In this study, the ultrafast optical properties of type-II CdZnS/ZnSe core-shell quantum dots were investigated using the Z-scan and transient absorption technique with femtosecond pulses. With 800-nm wavelength excitation, the CdZnS/ZnSe quantum dots exhibited two-photon absorption, and the two-photon absorption cross section was obtained as about 3.37 × 106 GM. In addition, the transfer time of electrons and the recombination lifetime of a single exciton were obtained. For the photoluminescence of the CdZnS/ZnSe quantum dots at temperatures from 80 to 280 K, the peak position redshifted by 60 meV, width broadened by 3 meV, and intensity decreased by a factor of four.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Semiconductor nanocrystals possess several unique optical and electrical properties that make them suitable for an extensive range of applications in the fields of display devices, biological tagging, photovoltaics, and lasers [1]. This is primarily because these optical and electrical properties can be manipulated using the size, shape, dimension, and nanocrystal components [2, 3]. In fact, the nonlinear optical properties and the carrier dynamic properties of single component semiconductor and alloyed semiconductor were investigated several decades years ago [4, 5]. Following the emergence of nanocrystal, the nonlinear optical properties and quantum size effect of nanocrystal were investigated in the early 1990s [6, 7]. In the recent years, for further regulation of the nanocrystal properties, some single-component semiconductor nanocrystals are capped by other semiconductor nanocrystals and form core–shell or dot-in-rod composite structure semiconductor nanocrystals [8–12]. For core–shell nanocrystals, the distribution of electrons and holes can be confined to a different region of nanocrystals by selecting suitable core and shell semiconductor crystals [13–15]. For example, the electrons and holes are confined to the same region as type-I core–shell quantum dots, and their fluorescence efficiency is higher than those of core-only quantum dots. Meanwhile, electrons and holes are confined to different regions in type-II core–shell quantum dots, which make the crystals exhibit a long exciton recombination lifetime and suitable for photovoltaic applications [8]. In addition, the wavefunction overlap of electrons and holes can be controlled by varying the size of the core nanocrystal or thickness of the shell nanocrystal in core–shell nanocrystals [9, 10, 16, 17]. Correspondingly, the single exciton recombination rate, transfer rate of electrons and holes, and Auger recombination rate of multiexcitons can also be tuned [15]. Therefore, several properties of core–shell quantum dots can be tuned by varying the size of the nanocrystals or selecting a suitable semiconductor material. For core-shell quantum dots, in the last decades, many investigations were focus on the binary alloyed quantum dots. Compared to binary alloyed quantum dots, the ternary alloyed quantum dots have advantages were demonstrated in many fields in the recent years. However, to our best knowledge, the ultrafast optical properties of ternary alloyed core-shell quantum dots have not yet to be fully investigated. For deeply understanding the optical properties of ternary alloyed quantum dots, in this study, we synthesized type-II CdZnS/ZnSe core–shell quantum dots, and the nonlinear absorption properties and time-resolved dynamic processes of the carriers were investigated by the Z-scan technique and white light transient absorption technique with femtosecond pulses.
Except for the size, shape, and structure, the temperature also has a clear influence on the optical and electrical properties of nanocrystals. For example, the photoluminescence (PL) properties of semiconductor nanocrystals are strongly influenced by temperature. Especially, the peak position, width, and intensity of PL spectra are all strongly dependent on temperature in several semiconductor nanocrystals [18, 19]. These temperature-dependent PL properties of semiconductor nanocrystals make them suitable materials for the measurement of temperature [20]. In recent decades, there have been several reports on the PL properties of semiconductor nanocrystals under different temperatures. For example, Walker et al. reported in 2003 the PL properties of cadmium selenide quantum dots with a zinc sulfide overlayer at different temperatures. In the report, a 20-nm blueshift in emission peak and fivefold increase in intensity were reported in the PL spectra when the temperature decreased from 315 to 100 K [20]. In 2005, Valerini et al. investigated the PL temperature-dependent properties of CdSe/ZnS core–shell quantum dots. The investigation results indicated that the PL intensity decreased, the emission peak redshifted, and the PL spectra broadened with the temperature [19]. In the same year, Swarnkar et al. also reported that the PL peak shifted by 30 and 40 meV for 8 and 9-nm CdSe/CdS-ZnS core–shell quantum dots, respectively, when the temperature increased from 25 °C to 100 °C. However, the PL spectra of CsPbBr3 perovskite nanocrystals did not exhibit any change under the same temperature conditions [21]. In 2007, Morello et al. reported a redshift, broadening in the width, and decrease in intensity of the PL spectra of CdTe colloidal quantum dots with the increase in temperature, and the confinement energy of semiconductor quantum dots changed at different temperatures [18]. In 2013, Huang et al. reported that the optical band gap of a semiconductor usually decreased with temperature [22]. In 2016, Wei et al. investigated the temperature-dependent excitonic PL properties of perovskite CsPbBr3 quantum dots. The investigation results indicated that the PL spectrum exhibited a linear blueshift under excitation by two-photon absorption below 220 K, and the PL peak approached a roughly constant value independent of temperature from 220 to 380 K [23]. In 2017, Zaaboub et al. investigated the temperature-dependent PL properties of InAs/GaAs quantum dots. The investigation results indicated that the PL intensity, PL linewidth, peak energy, and emission of InAs/GaAs quantum dots were all dependent on the temperature [24].
In summary, for many nanocrystals, the PL intensity decreases, the PL width broadens, and the PL peak redshifts with the increase in temperature, respectively. For a deeper investigation of the PL properties of core–shell quantum dots, we investigated in this study the steady-state PL properties of type-II CdZnS/ZnSe core–shell quantum dots at temperatures ranging from 80 to 280 K. For the type-II CdZnS/ZnSe core–shell quantum dots, a redshift in the emission peak, broadening of the PL width, and decrease in the PL intensity with temperature increasing were observed. The investigation demonstrated that type-II CdZnS/ZnSe core–shell quantum dots are suitable materials for application to the fields of solar cells and temperature measurement.
2. Experiments
In this study, femtosecond pulses were generated from a self-mode-locked Ti:Sapphire laser (Mira 900, Coherent) with a regenerative amplifier (Legend-F, Coherent). The pulses were linearly polarized, with a pulse width of 130 fs (full-width at half-maximum (FWHM)) and wavelength of 800 nm. The frequency and spatial profile of the laser pulses were 1000 Hz and Gaussian, respectively. Details of the Z-scan experimental setup are described in our previously published paper [25]. For the investigation of the nonlinear absorption of CdZnS/ZnSe quantum dots, we conducted an open-aperture Z-scan experiment in this study. Briefly, the collimated beam was focused by a lens with a 40-cm focal length, and the beam waist radius was about 32 μm at the focal position. The sample solution was filled into a 2-mm-thick glass cell, which was fixed on a translating stage, and the molar concentration was 0.001 mol/ml. The sample was moved along the propagation direction of the laser pulses at 0.5 mm per step. With the transmittance normalized in the linear region, the open-aperture Z-scan experimental results were obtained as a function of position z.
For the white light transient absorption experiment, each pulse generated from the regenerative amplifier was split into two equal parts using a splitter. One beam passed through a BBO crystal and the generated 400-nm-wavelength pulses were used as the pump beam. The other beam was focused into a cell filled with deuteroxide and the generated continuous white light was used as the probe beam. A half-wave plate and Glan-Taylor polarizer were inserted into the probe beam to attenuate the probe pulse energy and alter the polarization direction. The polarizer was placed in such a way that the polarization of the probe beam was perpendicular to that of the pump beam. The energy ratio of the pump beam to the probe beam was adjusted to 10:1. Two lenses with focal lengths of 40 and 20 cm were used to focus the pump and probe beam at the same region on the sample with a small angle (< 5°), respectively. The pump and probe beam radii were about 32 and 16 μm, respectively. After transmitting through the sample, the pump beam was blocked while the probe beam was received by a spectrograph. In this experiment, we used two mirrors mounted on a translating stage to introduce a delay time into the pump beam with respect to the probe beam. In the white light transient absorption experiment, the CdZnS/ZnSe quantum dot solution was also filled into a 2-mm-thick glass cell, and its concentration was as same as that used in the Z-scan experiment
For the steady-state fluorescence experiment at different temperatures, the temperature was controlled by a liquid nitrogen temperature controlling setup. The temperature could be controlled in the range from 80 to 280 K. In the investigation, the steady-state fluorescence experiment was conducted at 20° intervals from 80 to 280 K. The CdZnS/ZnSe quantum dot solution was dripped onto the surface of a metal board gilded on the sides and fixed in the temperature controlling box. The four sides of the temperature controlling box were all glass so the incident beam and emission could pass through freely and be detected. In the experiment, the exciting laser was generated by a semiconductor laser, and the output continuous laser had a wavelength of 400 nm. The emission of the sample was detected by a spectrograph.
3. Sample synthesis
Material: Stearate acid (HSt, 97%), tetramethylammonium hydroxide (TMAH, 97%), sulfur powder (S, 99.5%), cadmium acetate dihydrate (CdAc2·2H2O, 99%), 1-octadecene (ODE, 90%), selenium (Se, 99%), zinc acetate (ZnAc2, 99%), and tributylphosphine (TBP, 98%) were purchased from Aldrich. Toluene (98%), ethanol (99.7%), and hexane (95%) were purchased from Sinopharm Chemical Reagent.
CdSt2 powders were synthesized according to the previously reported method [26]. In detail, 20 mmol of HSt was neutralized with TMAH (20 mmol) in methanol (200 mL) by stirring. CdAc2·2H2O (10 mmol) dissolved in methanol (50 mL) was added dropwise to this solution under vigorous stirring. The mixture was then stirred for 20 min after finishing the dropping process to ensure complete reaction. The white precipitate indicated the formation of CdSt2. Subsequently, the precipitate was collected by filtration, washed twice with methanol, and then dried under vacuum at room temperature overnight before use. The synthesis of ZnSt2 was similar to that of CdSt2, where the CdAc2∙2H2O was replaced by ZnAc2.
CdSt2 (0.5 mmol), 5 mmol of ZnSt2, and 8 ml of ODE were placed in a 100-ml round flask. The mixture was heated to 150 °C, degassed under a pressure 100 mTorr for 60 min with Ar gas, and then rapidly heated to 300 °C to form an optically clear mixture solution. At this temperature, 0.5 M (1 ml) of S powder dissolved in 1-ODE were quickly injected into the reaction flask. After the first injection of S precursors, the temperature of the reaction flask was raised to 310 °C for the further growth of the CdxZn1–xS cores. After 5 min of reaction, 1 mmol of Se powder was dissolved in 1 ml of TBP and diluted by 2 ml of 1-ODE. The Se precursors were added dropwise at a rate of 3 mL/hr into the reactor to overcoat the existing CdxZn1–xS cores with ZnSe shells without any purification steps. After 20 min of ZnSe shell formation, it was cooled to room temperature to complete the reaction.
The resultant quantum dots were purified based on the previously reported method with some modification [27]. The original reactors were precipitated by centrifugation at 4000 rpm for 5 min, and the supernatant was carefully collected. The precipitation on the bottom was discarded because it was probably unreacted ZnSt2. Then, the supernatant was added to excess ethanol and centrifuged at 8000 rpm for 5 min. The second supernatant was discarded after centrifugation. In order to achieve pure nanocrystals, the above procedures should be repeated at least more than three times. Finally, pure quantum dots were obtained and redispersed in hexane.
4. Experimental results and discussion
The UV-vis absorption spectra and steady-state fluorescence spectra of the type-II CdZnS/ZnSe core–shell quantum dot solution at room temperature are presented in Fig. 1. It can be seen that the absorption peak is at about 450 nm. According to the band gap of the corresponding bulk semiconductor materials, most of the absorption comes from the contribution of the ZnSe shell and a fraction of the absorption comes from the contribution of the CdZnS core. The steady-state fluorescence comes from the electron–hole radiative recombination, where the electrons are in the conductor band of the CdZnS core and the holes are in the valence band of the ZnSe shell, and the fluorescence peak is at 550 nm. A schematic of the dynamic processes is illustrated in Fig. 2, followed by a detailed discussion.
Under the incident intensity of 2.7 × 102 GW/cm2, an open-aperture Z-scan experiment was conducted using 130-fs pulses at a wavelength of 800 nm. The experimental results are presented in Fig. 3. It can be seen that the CdZnS/ZnSe quantum dots exhibit reverse saturable absorption properties. For simple analysis, the photon energy E can be expressed as E = ℏω, where ℏ is the reduced Planck constant and ω is the photon circular frequency. For the 800-nm wavelength, the single photon energy is about 1.55 eV and the two-photon energy 2E is about 3.10 eV. According to the UV-vis absorption spectra, the linear absorption peak is at about 450 nm and its corresponding band gap energy Eg is 2.76 eV. The band gap energy Eg is more than E but less than 2E and satisfies the two-photon absorption condition [28]. Thus, the nonlinear absorption of CdZnS/ZnSe quantum dots is attributed to the two-photon absorption. In order to further confirm the nonlinear absorption mechanism, the experimental results were fitted to the two-photon absorption theory, which can be expressed as [29]:
where I is the incident pulse intensity, z is the sample position relative to the focus of the lens in the Z-scan experiment, α0 is the linear absorption coefficient, and β is the two-photon absorption coefficient. The linear absorption coefficient α0 can be obtained by Beer’s law. By fitting the experimental results with Eq. (1), the two-photon absorption coefficient β can be obtained, which was calculated to be about 8.2 cm/GW. According the relation of two-photon absorption coefficient and the two-photon absorption cross section, the two-photon absorption cross section was obtained about 3.37 × 106 GM [30], and the amplitude was similar with that of CdSe nanoplatelets [31]. And it was an order magnitude greater than that of CdSe/CdS core-shell quantum dots and CdSe/CdS nanodot/nanorod [30, 32].The fitting results are represented by a red solid curve in Fig. 3, and it can be seen that the experimental results agree well with the theoretical fitting results. Therefore, the nonlinear absorption mechanism of CdZnS/ZnSe quantum dots is attributed to two-photon absorption.
Fig. 3 Open-aperture Z-scan results of CdZnS/ZnSe quantum dots under an incident intensity of 2.7 × 102 GW/cm2 at 800 nm.
In order to investigate the dynamic processes of CdZnS/ZnSe quantum dots, a white light transient absorption experiment was conducted with the excitation of 130-fs pulses at a wavelength of 400 nm. Figure 4 presents the white light transient absorption experimental results of CdZnS/ZnSe quantum dots under an excitation intensity of 5 × 102 GW/cm2 at delay time of 0, 20, 100, 500, and 1000 ps. The excitation intensity was intentionally kept low to prevent the generation of multiexciton [33]. It can be seen that there are two obvious bleach signal at 470 nm and 540 nm, and there is an approximately 5% recovery at 470 nm and almost no recovery at 540 nm for the initial 20-ps delay. The single photon energy is about 3.1 eV with 400 nm wavelength which is greater than the both bang gap of CdZnS core and ZnSe shell. Therefore, combining the absorption spectra Fig. 1, the bleach signal was attributed to the electron dynamic process of ZnSe shell at 470 nm and the bleach signal was attributed to the electron dynamic process of CdZnS core at 540 nm in the Fig. 4.
Fig. 4 Transient absorption experimental results of CdZnS/ZnSe quantum dots under an excitation intensity of 5 × 102 GW/cm2 at 400 nm.
In this study, the sample CdZnS/ZnSe quantum dots were type-II core–shell quantum dots, where the conductor band and valence band of the CdZnS core are both lower than that of the ZnSe shell. Thus, some electrons will transfer to the conductor band of the CdZnS core from the conductor band of the ZnSe shell and some holes will transfer to the valence band of the ZnSe shell from the valence band of the CdZnS core after excitation with 400-nm femtosecond pulses. However, because of the higher degeneracy of holes, the bleach of exciton bands were only caused by electrons on the conductor band [34]. The processes are illustrated in Fig. 2. For a deep investigation into the dynamic processes of the CdZnS/ZnSe quantum dots, the full white light transient absorption results are plotted for the whole 1200-ps delay time at wavelengths of 470 nm and 540 nm in Figs. 5 and 6, respectively. In Figs. 5 a clearly rapid recovery process can be seen within the initial 200-ps delay time. For estimating the recovery time, the experimental results are fitted with the following bi-exponent at 470 nm:
where ΔT(t) is the difference in transmittance in the white light transient absorption experiment at delay time t, A1 denotes the weight of one process with lifetime τ1, and A2 denotes the weight of another process with lifetime τ2. The red solid curves represent the theoretical fitting results, which agree well with the experimental results in Figs. 5. By the fitting, the two obtained lifetimes are 50 and 2300 ps at 470 nm, respectively. The 50-ps lifetime is attributed to the transfer time of electrons from the conductor band of the ZnSe shell to that of the CdZnS core, while the 2300-ps lifetime is attributed to the recombination time of electrons and holes, which is confined to the conductor band and valence band of the ZnSe shell, respectively. At 540 nm, the experimental results agree well with the following single exponent:where A denotes the weight of the dynamic process with lifetime τ.By fitting with Eq. (3), the obtained lifetime was 1500 ps, which is the recombination time of electrons on the conductor band of the CdZnS core and the holes on the valence band of the ZnSe shell. The recombination process of electrons and holes is illustrated in Fig. 2.
By the above analysis and calculation, the reason there is no recovery within the initial 20-ps delay time can be understood at 540 nm. The transfer time of electrons occur within about 50 ps. There are few electrons on the conductor band of the CdZnS core and few holes on the valence band of the ZnSe shell at an initial time within tens of ps. Thus, the difference in transmittance has no clear recovery at 540 nm at the initial delay time of 20 ps in the white light transient absorption experiment. At 470 nm, the fast process is the transfer time of electrons and the slow process is the recombination time of electrons and holes. At 540 nm, only the recombination process of electrons and holes exists, so the white light transient absorption experimental results are fitted well with the single exponent.
Except for the investigation of the dynamic processes for CdZnS/ZnSe quantum dots, the PL properties were investigated at temperatures ranging from 80 to 280 K. In the investigation, the fluorescence was generated by excitation with a continuous laser at a 400-nm wavelength. The PL spectra of CdZnS/ZnSe quantum dots are presented in Fig. 7 at temperatures ranging from 80 to 280 K. From the figure, we can see that the PL peak of CdZnS/ZnSe quantum dots redshifts and the intensity decreases with temperature. Especially, the peak of the PL shifts from 548.0 to 562.3 nm when the temperature increases from 80 to 280 K, about a 60-meV shift from 2.27 to 2.21 eV, as shown in Fig. 8. These results are similar to that of InAs/GaAs quantum dots [24].
At different temperatures, the peak of the PL shift can be explained by the Varshni relation [35]:
where Eg(T) is the energy gap at temperature T, Eg0 is the energy gap at temperature 0 K, α is the temperature coefficient, and γ is close to the Debye temperature θD of the material. By theoretically fitting the experimental results, these parameters were obtained. The solid red curve in Fig. 8 represents the fitting results, and the obtained parameters are Eg0 = 2.276 eV, α = 3.5 × 10−4 eV/K, and γ = 148 K. These values are similar to those reported in the literature [18].The relative PL intensities of CdZnS/ZnSe quantum dots were measured at temperatures ranging from 80 to 280 K. Figure 10 presents the relative PL intensities using 1/kBT as abscissa (kB is the Boltzmann constant). It can be seen that the PL intensities of CdZnS/ZnSe quantum dots decreases by a factor of about four with the increase in temperature from 80 to 280 K. Several reasons can decrease the PL intensity with temperature, such as the Auger recombination, encountered thermal escape nonradiative process, Forster energy transfer between dots of different dimensions, and thermally activated process. In the experimental conditions, the Auger recombination and Forster energy transfer can be excluded. The PL of CdZnS/ZnSe quantum dots were excited under a low incident intensity with a 400-nm continuous laser. Thus, the Auger recombination process had a low probability of occurring. Based on the above white light transient absorption experiment, the transfer of electrons and holes occurred for the CdZnS/ZnSe quantum dots, not the Forster energy transfer. Therefore, the probability of Forster energy transfer can be excluded. Using the single exponent to fit the experimental results, the experimental results were fitted well, represented by the solid red curve in Fig. 9. This behavior suggests the presence of one temperature-dependent nonradiative process in the CdZnS/ZnSe quantum dots [19]. In addition, the shape of the PL intensity is similar to that reported in the literature at different temperatures [18]. In the white light transient absorption experimental results, there was no ultrafast process near the zero-delay time. This suggests that there were very few surface defect states in the type-II CdZnS/ZnSe quantum dot sample. Therefore, the decrease in PL intensity with temperature is attributed to the thermal escape process.
The FWHM of the PL spectra of the CdZnS/ZnSe quantum dots measured at temperatures ranging from 80 to 280 K is almost linearly broadened between 35.7 and 36.6 nm, as shown in Fig. 10. Using meV as the unit, the broadening is about 3 meV, which is one-order smaller than that reported in the literature [19]. For the semiconductor, there are two factors that can broaden the PL spectra: the inhomogeneous broadening and the exciton–phonon coupling. At temperature T, the broadening Γ(T) of the PL spectra of semiconductor quantum dots can be expressed as follows [36]:
where Γinh is the inhomogeneous broadening, which was determined by the size, shape, and composition of nanocrystals and is temperature-independent, σ is the exciton–acoustic phonon coupling coefficient, ΓLO is the exciton-longitudinal optical phonon coupling coefficient, and ELO is the exciton–phonon coupling energy.The FWHM is almost linearly broadened with temperature, which suggests that the broadening of PL is primarily due to the first two parts on the right of the equals sign in Eq. (4). Therefore, the PL width broadening of CdZnS/ZnSe quantum dots comes from the contribution of inhomogeneous broadening and exciton–acoustic phonon coupling. The experimental results agree well with the theoretical prediction and reported literature in zero-dimension quantum dots [18, 37]. A solid red line is plotted for reference and clarity in Fig. 10 to confirm the linear change in the broadening. In addition, the broadening value is one-order smaller than that of CdTe bulk material, which can be explained by the lattice expanding with temperature.
5. Conclusions
In this study, the nonlinear absorption properties, ultrafast dynamic properties, and PL properties of core–shell quantum dots at different temperatures were investigated with femtosecond pulses. Under 800-nm wavelength excitation, the type-II CdZnS/ZnSe core–shell quantum dots exhibited two-photon absorption, and the two-photon absorption cross section was obtained. The electron transfer time, hole transfer time, and recombination time of electrons and holes were all estimated using the white light transient absorption technique. In addition, the PL properties were investigated at temperatures ranging from 80 to 280 K. The PL peak of CdZnS/ZnSe quantum dots redshifted and the PL intensity decreased with temperature. The width of the PL of CdZnS/ZnSe broadened by 3 meV under the same temperature conditions. The investigation indicated that the type-II CdZnS/ZnSe core–shell quantum dots are suitable materials that have potential applications in the fields of solar cells and temperature measurement.
Funding
National Natural Science Fund of China (61275117, 11774141); Natural Scientific Foundation of Heilongjiang Province (QC2015068); Scientific Foundation of the Returned Overseas Scholars by Heilongjiang Province (LC2017030); Heilongjiang Postdoctoral Scientific Research Development Fund (LBH-Q15117); Heilongjiang Creative Talents Plan (UNPYSCT-2017117); Heilongjiang Basic Scientific Research Service Fund (HDRCCX-201603).
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