Enhancement of Two-Photon Fluorescence and ... - ACS Publications

Article Cite This: ACS Photonics 2018, 5, 1310−1317

Enhancement of Two-Photon Fluorescence and Low Threshold Amplification of Spontaneous Emission of Zn-processed CuInS2 Quantum Dots Pengqi Lu,† Ruifeng Li,† Ni Yao,‡ Xusheng Dai,† Zhenyu Ye,† Kai Zheng,† Weiguang Kong,† Wei Fang,*,‡ Shuang Li,§ Qinghua Xu,∥ and Huizhen Wu*,† †

Department of Physics and State Key Laboratory of Silicon Materials and ‡Department of Optical Science and Engineering, Zhejiang University, Hangzhou, Zhejiang 310027, People’s Republic of China § School of Materials Science and Engineering, South China University of Technology, Guangzhou, Guangdong 510000, People’s Republic of China ∥ Department of Chemistry, National University of Singapore, Singapore 117543 S Supporting Information *

ABSTRACT: Heavy-metal-free I−III−VI2 quantum dots (QDs) have recently emerged as favorable alternatives to toxic II−VI QDs for optoelectronic and biological applications, but low fluorescence efficiency is a key issue related to the promising nanocrystals. In this work, we demonstrate big enhancement of two-photon fluorescence efficiency and low threshold two-photon pumped amplification of spontaneous emission (ASE) of Zn-processed CuInS2 (CIS) QDs for the first time. The change of two-photon process caused by Zn2+ cation exchange in CIS QDs is systematically investigated. Two-photon ASE in Zn-processed CIS QDs films is achieved with a record of low threshold fluence of 5.7 μJ/cm2. With high two-photon fluorescence quantum yields and low threshold ASE, the Zn-processed CIS QDs could become a promising candidate for two-photon bioimaging, nonlinear optical and novel QDs laser devices. KEYWORDS: Zn-processed CuInS2 quantum dots, two-photon fluorescence, Z-scan, quantum yields, two-photon amplification of spontaneous emission

H

mechanisms of luminescence enhancement are still unclear.13−15 Apart from the distinguished linear optical performance of QDs, increasing attention has been focused on their nonlinear optical properties, where two-photon process in QDs attracts more interests for its promising applications in two-photon fluorescence imaging (TPFI), ultrafast all-optical switching devices, and two-photon induced lasers.16−18 Two-photon absorption (TPA) is a nonlinear process that a QD simultaneously absorbs a pair of photons with energy smaller than the QD bandgap in a time interval as short as 10−15 to 10−16s, indicating that high photon flux in a short pulse is essential for TPA.19 For bioimaging applications, utilizing a long wavelength ultrafast laser (800/1064 nm) which has deep penetration in tissues, the confocal TPFI system based on TPA provides unsurpassed signal-to-noise ratio with high resolution.20 To date, a plethora of studies on nonlinear optics of II− VI QDs have been conducted, which reveal that TPA properties are mainly determined by size, bandgap, and surface states of

eavy-metal-free I−III−VI2 ternary quantum dots (QDs) have attracted increasing attention for their low-toxicity and unique optical properties, which serves as a promising alternative for toxic cadmium based II−IV QDs. The wide wavelength tunability from near-infrared to visible spectra, long decay lifetimes, and large Stokes shift enable a variety of potential applications such as novel display technology, solar cells, and bioimaging.1−6 Benefited from the overcoming of balance of reactivity between cationic precursors, I−III−VI2 QDs were successfully synthesized using either two cationic stabilizers or excess thiol as both ligand and solvent.7−10 However, the defect nature of I−III−VI2 QDs leads to a relatively lower photoluminescence (PL) efficiency compared with its Cd-based II−VI counterparts. The quantum yields (QYs) of I−III−VI2 QDs are often lower than 10% while QYs of Cd-based QDs can be higher than 70%. Efforts have been made to improve their luminescence efficiency through different strategies, including core−shell structure, stoichiometry control, and alloying.7−12 Klimov et al. realized CuInS2/ ZnS core/shell structure and observed notable enhancement in PL of QDs.8 Structural tolerance of chalcopyrite allows for formation of quaternary I−III−II−VI QDs by introduction of Zn2+ into the ternary QDs.9,11 However, the underlying © 2018 American Chemical Society

Received: October 23, 2017 Published: January 30, 2018 1310

DOI: 10.1021/acsphotonics.7b01255 ACS Photonics 2018, 5, 1310−1317

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ACS Photonics

Figure 1. (a) Visible emission of CIS90 QDs under infrared (λ = 800 nm) femtosecond laser excitation. (b) Increasing power-dependent TPPL of CIS90 QDs. (c) A log−log plot of quadratic dependence of TPPL intenisty vs pumping power (the red one for increasing power and the blue one for decreasing power). (d) TPPL spectra of CIS QDs with different Zn reaction times.

QDs.21−23 In comparison with comprehensive understanding of II−VI QDs, less attention has been focused on the two-photon process of ternary copper indium sulfide (CIS) QDs because of the limitation of its low QYs to either one- or two-photon excited PL although CIS QDs hold many other advantages over II−VI QDs, such as low-toxicity to cells (or animals) and suitable emission wavelength which benefits the signal acquisition of tissues at an important 600−1300 nm bioimaging window.24 Therefore, the passivation of defects in CIS QDs and enhancement of luminescence efficiency are the critical issues to realize practical applications of CIS QDs in both optoelectronics and bioimaging. In this work, we utilize a strategy of passivation treatment on both surface and internal defects by forming a ZnS shell on the surface of a CIS QD and partial cation exchange of Zn2+ into QDs, which leads to a great improvement of QDs quality. Power-dependent two-photon pumped PL (TPPL) characterization confirms the enhanced TPA process of Zn-processed CIS QDs and Z-scan setup experiment further determines the TPA cross section. We find that after the Zn-processing CIS QDs achieve evident TPPL intensity enhancement over eight folds. The underlying mechanisms of the realized TPPL enhancement are revealed by the combination of optical characterizations and high-resolution transmission electron microscopy (HRTEM) observation. Thanks to the TPPL enhancement in Zn-processed CIS QDs, we are able to readily achieve two-photon induced amplified spontaneous emission (TPASE) with a simple CIS QDs film structure for the first time. The realized threshold fluence of TPASE (5.7 μJ/cm2) is much smaller than that of perovskite QDs (12 mJ/cm2)17 and CdSe/CdS (1.5 mJ/cm2).25 Such fascinating nature demonstrated by the Zn-processed CIS QDs promises its potential

applications in nonlinear optics, optoelectronic devices, and bioimaging technology.

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RESULTS AND DISCUSSION A two-step approach was used to acquire Zn-processed CIS QDs. CIS QDs were first prepared by adopting a typical synthesis method.26,27 Then two samples (CIS30 and CIS90 QDs) were synthesized by Zn2+ cation exchange with reaction times of 30 and 90 min, respectively, as described in Synthesis and Characterizations sections. Figure 1a shows a photo of the TPPL experimental setup and bright fluorescence from the CIS90 QDs sample pumped by an infrared femtosecond laser (λ = 800 nm). The bright upconversion fluorescence provides a visual witness of nonlinear two-photon process occurring in the Zn-processed CIS QDs. The second evidence of the twophoton process comes from the power-dependent PL characterization. Figure 1b and c show, respectively, the increasing power-dependent TPPL of CIS90 QDs and the log−log plot of quadratic dependence of TPPL intensity versus pumping power with a slope of 2.06 (increasing power) and 2.11 (decreasing power) obtained by linear fitting, which clearly confirms the existence of two-photon process. TPPL spectra of 5 μmol/mL CIS, CIS30, and CIS90 QDs solution are shown in Figure 1d. The blue shift of the luminescent peak with prolonged reaction time can be acsribed to two factors: (1) the increased spatial confinement of electrons, which results from the shrinking of the CIS core by the formation of ZnS shell;8,12 (2) the formation of quarternary CuInZnS2 core via Zn2+ partial cation exchange.12 Intensity of TPPL also advances sharply with increase of reaction time. When the reaction time lasts 90 min, the TPPL intensity enhancement of the CIS90 QDs sample reaches 8.4-fold. 1311

DOI: 10.1021/acsphotonics.7b01255 ACS Photonics 2018, 5, 1310−1317

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Figure 2. (a) Open aperture Z-scan responses of Zn-processed CIS QDs. The red, green, and blue color represents the CIS, CIS30, and CIS90 QDs, respectively. (b) Peak positions of CWPL and absorption vs reaction time (left); QYs in percentage vs reaction time (right).

Figure 3. (a−c) HRTEM images for CIS, CIS30, and CIS90 QDs, respectively. Insets show the transformed FFT patterns of QDs. (d) X-ray diffraction (XRD) patterns from bottom to top correspond to CIS, CIS30, and CIS90 QDs. Miller indices of tetragonal CIS bulk (Reference code: 01−085−1575) and cubic ZnS (Reference code: 01−077−2100) are labeled.

and ζ = z/z0, in which z0 represents the Rayleigh range of the beam. I0 is the on-axis intensity at the focal plane. The parameters q(ζ = 0) (at center position of Z-scan) of 4.57, 6.75, and 7.45 are, respectively, obtained for the three samples by the normalized transmittance fitting of eq 1, while the TPA coefficient β can be further extracted by eq 2. Given the I0 = 226 GW/cm2 and Leff = 0.1 cm in our case, we obtained the TPA coefficient β of 0.20, 0.30, and 0.33 cm/GW for the three samples, respectively. Furthermore, the TPA cross section σ can be deduced as

To acquire the TPA cross section, which is a key parameter of two-photon process, open aperture Z-scan measurements were performed according to the optical setup described in ref 28 (Figure S1). Figure 2a illustrates the Z-scan responses of CIS, CIS30, and CIS90 QDs under incident laser power of 226 GW/cm2. The measured data are well fitted by the normalized nonlinear transmittance given below:28−30 T (z ) =

1 π q(ζ )

∞

∫−∞ n[1 + q(ζ)f (x)]dx

(1)

σ = βhν /N0

where 2

q(ζ ) = βI0Leff /(1 + ζ )

(3)

where hν is the photon energy and N0 is the particle density of QDs. Given the solution concertation of 5 μmol/mL, N0 = 3 × 1018 cm−3, we obtain the TPA cross sections σ of 2.45 × 103,

(2) 1312

DOI: 10.1021/acsphotonics.7b01255 ACS Photonics 2018, 5, 1310−1317

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Figure 4. (a) Schematic picture for the proposed TPPL process in CIS QDs: CB, conduction band; VB, valence band; XS, bound exciton states; SS, surface trap states; D, donor; A, acceptor. (b) Single-photon pumped PL decays for the CIS, CIS30, and CIS90 QDs. (c, e) Time-resolved PL spectra of CIS QDs and CIS90 QDs, respectively, and the scale bar represents the PL decay. (d, f) Temporal PL spectra of corresponding lifetime components of CIS QDs (black cubes, 10 ns; red circles, 60 ns; blue triangles, 250 ns) and CIS90 QDs (black cubes, 10 ns; red circles, 100 ns; blue triangles, 280 ns). The inset shows time evolution of PL peak energies.

3.63 × 103, and 4.00 × 103 GM (Goeppert-Mayer units, 1 GM = 10−50 cm4·s·molecule−1·photon−1) for the CIS, CIS30, and CIS90 QDs, respectively, indicating that prolonged reaction time results in about 60% increase of CIS TPA cross section. Evidently, TPA change observed in CIS30 and CIS90 QDs is not the dominant factor to the big enhancement of TPPL shown in Figure 1d because the enhancement of fluorescence QYs of CIS90 QDs is much more prominent. As shown in Figure 1d, the absolute TPPL intensity for the Zn-processed CIS QDs is distinctly enhanced. The measured QYs for CIS QDs (11.6%), CIS30 (39.7%), and CIS90 (60.8%) are plotted in Figure 2b. The enhancements of QYs with prolonged reaction times correspond well with the previous reports,7−15 and it could also fortify the two-photon fluorescence of QDs. The QYs increase can be attributed to the passivation of surface and internal defects in CIS QDs. In I−III−VI2 QDs, high density of defects (such as surface trap states due to high surface/volume ratio and abundant intrinsic states including Cu vacancies) could be involved in synthesis.31 It is found that even if the reaction time is only 1 min (CIS01), the continuous wave PL (CWPL) peak substantially shifts from 1.78 eV of CIS to 1.90 eV of CIS01, suggesting a rapid surface

states passivation by forming a ZnS shell layer at initial stage of reaction (Figure S2). It is noted that the formation of ZnS shell could be a combined result of cation exchange and additional sulfur source (1-dodecanethiol). After the shell formation, Zn2+ partial cation exchange in CIS cores may dominate, which cures internal Cu vacancies in CIS QDs, as demonstrated by a saturation tendency of PL peak shifts from 30 to 90 min shown in Figure 2b. Our further experiment showed that increasing reaction temperature from 210 to 250 °C speeded up the reaction, leading to a faster surface passivation and partial cation exchange (Figure S3). Figure 3a−c presents the HRTEM images of CIS QDs and Zn-processed CIS QDs. Lattice structures are well distinguishable as shown in the insets. Through estimating the area of the QDs marked by the red circles in Figure 3a−c, we find that the average diameter of QDs rapidly increases from 2.20 to 2.67 nm after the first 30 min reaction, but the subsequent 60 min reaction only achieves relatively small change of QDs size from 2.67 to 2.83 nm. Figure 3d displays the XRD patterns of CIS, CIS30, and CIS90. The contents of Zn in CIS QDs can be estimated by Bragg diffraction equation, 2d sin θ = nλ, where d = (1 − x)dCIS 1313

DOI: 10.1021/acsphotonics.7b01255 ACS Photonics 2018, 5, 1310−1317

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ACS Photonics Table 1. Comparison of QDs Sizes, Bandgaps, and Lifetimes lifetime QDs

D (nm)

Eg (eV)

τ1 (ns)

A1 (%)

τ2 (ns)

A2 (%)

τ3 (ns)

A3 (%)

τa (ns)

CIS CIS30 CIS90

2.20 2.67 2.83

2.25 2.37 2.42

10.56 18.27 11.95

5.09 2.50 1.89

61.55 93.40 79.49

24.76 21.15 20.81

252.02 280.83 281.92

70.23 76.35 77.31

236.28 264.54 267.39

The remarkable enhancement of fluorescence QYs is further evidenced by time-resolved PL characterizations. Figure 4b shows PL decays of CIS, CIS30, and CIS90 QDs. The lifetimes of photoexcited carriers in the Zn-processed samples are evidently longer than the unprocessed CIS QDs. The decay curves shown in Figure 4b can be well fitted by a triexponential decay formula:

+ xdZnS. The (112) diffraction planes of CIS30 and CIS90 QDs are determined to be at 28.00° and 28.08°, respectively. Thus, the contents of Zn in CIS30 and CIS90 QDs are estimated to be 24.3% and 37.0%. The shift of XRD peaks from tetragonal CuInS2 structure toward cubic ZnS along with prolonging of reaction time indicates the formation of a core/shell structure, which is consistent with the QDs diameter changes observed by HRTEM.8,12 Now we discuss the mechanism of the increase of TPA cross section observed in Zn- processed CIS QDs. TPA cross section of QDs is closely related to the size of QDs.21,32,33 Previous theoretical and experimental studies on size-dependent TPA of II−VI QDs demonstrated that TPA increased with increscent QDs diameter in the range of 2−7 nm.21,32,33 An enlarged size of QDs gives rise to the increase of joint density of states while decrease of bandgap of QDs favors two-photon allowed transitions. It was reported that TPA cross section could have a cubic power law of QD diameter, that is, the log−log relation curve of TPA cross section versus diameter has a slope of ∼3.33 The TPA cross section versus diameter of CIS QDs is shown in Figure S4, with a slope of only 1.97. The smaller slope measured here could be related to two factors, the enlarged size and the increased bandgap of CIS QDs caused by Zn2+ cation exchange. It is known that when the diameter of QDs becomes larger, the bandgap decreases due to the reduced quantum confinement effect. Yet in this work, a blue shift of the TPPL peak with prolonged reaction time is observed, as seen in Figure 1d, indicating that, besides the enlarged QDs diameters, Zn-processed CIS QDs also have an increased bandgap, which is in agreement with the theoretical prediction.34 The absorption spectra of CIS, CIS30, and CIS90 QDs are shown in Figure S5. By extracting absorption edges from the first absorption peaks, we can estimate the bandgaps of the three QDs samples to be 2.25, 2.37, and 2.42 eV, respectively. The increased bandgap could decrease TPA cross section and result in the deviation of slope of cubic power law from ∼3 to ∼2. A simple theoretical model for the calculation of the ratio of TPA cross section for different CIS QDs is given in Supporting Information.35 The calculated TPA cross section ratio, σCIS/ σCIZS30/σCIZS90 = 1:1.47:1.59, is close to the ratio measured by Z-scan, σCIS/σCIZS30/σCIZS90 = 1:1.48:1.63. Such a result indicates that both the increased bandgap and swelled diameter of QDs could change TPA cross section of CIS QDs. A TPPL model is schematically depicted in Figure 4a to explain the TPPL enhancement. An electron in the valence band absorbs two photons with energy smaller than the QDs bandgap and transits to the conduction band. After relaxation to the conduction band edge states (such as bound exciton states, surface trap states, or intrinsic donor−acceptor pairs), the electron recombines with a hole at the valence band edge states. As a result, the TPPL intensity is determined by both the TPA cross section and the following luminescent process by radiative recombination. Thus, it can be concluded that the significant improvement of QYs in Zn-processed CIS QDs is the dominant factor of the TPPL enhancement.

⎛ t ⎞ ⎛ t ⎞ ⎛ t⎞ I(t ) = A1 exp⎜ − ⎟ + A 2 exp⎜ − ⎟ + A3 exp⎜ − ⎟ ⎝ τ2 ⎠ ⎝ τ1 ⎠ ⎝ τ3 ⎠ (4)

The average lifetime can be calculated by eq 5 given below, τa = (A1τ12 + A 2 τ22 + A3τ32)/(A1τ1 + A 2 τ2 + A3τ3)

(5)

The lifetimes together with QDs sizes and bandgaps are concluded in Table 1. The shortest component τ1 with a few nanoseconds is assigned to the bound exciton recombination because the energy of the confined excitons is higher than that of the surface trap states and the donor−acceptor (DA) pairs. The component τ2 with tens of nanoseconds is attributed to the surface trap related recombination with energy lower than the bound excitons. After 30 min reaction, the portion of τ2 (see Table 1) decreases from 24.76% to 21.15%, indicating an evident passivation of the surface trap states. Previous studies of CdSe QDs have revealed that surface passivation can effectively reduce the nonradiative recombination of the surface trap states, which significantly enhances the luminescence efficiency.36,37 The component τ3 (>200 ns) belongs to the DA pairs recombination whose energy is the lowest among the three components, which is in accordance with previous reports.8,38 After fast surface passivation from CIS to CIS30, the component τ3 exhibits a slow increase from CIS30 to CIS90 QDs. As evidenced by the HRTEM images, XRD data, and PL results, it can be concluded that the Zn introduction first passivates the surface trap states by forming a ZnS shell, then annihilates internal defects of CIS QDs via partial cation exchange, which remarkably suppresses the nonradiative recombination and improves the PL QYs, leading to the enhancement of TPPL intensity eventually. Time-resolved PL (TRPL) characterizations can further illustrate the multiple routes of radiative recombination in the Zn-processed CIS QDs. Three recombination processes can be distinguished from Figure 4d, that is, 10, 60, and 250 ns TRPL for the bound excitons, the surface trap states, and the DA pairs recombination, respectively. A pronounced red shift (30 nm) among the three components with increase of lifetime can be seen in the unprocessed CIS QDs sample. However, in the CIS90 QDs sample the red shift decreases to 12 nm as shown in Figure 4f. As the three components are related to three different recombination mechanisms, a rapid reduction of the red shift among the three components in the CIS90 QDs sample indicates that the surface trap states and the DA pairs are largely quenched by Zn2+ cation exchange. This dynamic character also agrees with the observed width reduction of one1314

DOI: 10.1021/acsphotonics.7b01255 ACS Photonics 2018, 5, 1310−1317

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Figure 5. (a) Power-dependent TPPL spectra of the glass/PMMA/CIS90QDs/glass sandwich structure. Inset is the schematic of experimental configuration. (b) Power-dependent intensity and fwhm of the TPPL spectra shown in (a). The orange lines are linear fitting to the measured data (green dots).

exchange, which passivates both surface trap states and internal defects in CIS QDs, we achieve over 8 folds TPPL intensity enhancement. The combination of optical and microstructural characterizations including Z-scan, time-resolved PL, and HRTEM allows us to distinguish the contributions of TPA and QYs to the observed enhancement of TPPL. It is concluded that the enhancement of QYs of Zn-processed CIS QDs plays a pivotal role. As an example of application of the Zn-processed CIS QDs, bright two-photon ASE in CIS QDs film is demonstrated with a record of threshold fluence as low as 5.7 μJ/cm2. With high two-photon fluorescence QYs and low threshold ASE, the Zn-processed CIS QDs can become promising candidate for two-photon bioimaging, nonlinear optics, and novel QDs laser devices. Further, the strategy of Zn2+ cation exchange in CIS QDs for the enhancement of twophoton process can be readily extended to other I−III−VI2 QDs.

photon pumped PL spectra as shown in Figure S2, that is, full width at half-maximum (fwhm) from 125.4 nm for CIS QDs to 102.6 nm for CIS90 QDs. The achieved enhancement of TPPL of Zn-processed CIS QDs inspires us to explore its application in two-photon related optical devices. A sandwich structure of glass/PMMA/ CIS90QDs/glass was prepared for two-photon pumped amplified spontaneous emission (ASE). The measured powerdependent TPPL spectra for the sandwich structure are plotted in Figure 5a with the experimental configuration shown in the inset. The TPPL spectra of the sandwich structure clearly demonstrate strong amplified spontaneous emission. Figure 5b shows the TPPL intensity and fwhm varying with pumping power density. As can be seen, the intensity of TPPL increases slowly in the low pump fluence, however when pump fluence exceeds a threshold point at ∼5.7 μJ/cm2, it increases sharply. A distinct narrowing of the emission spectrum can be seen as well at the threshold pump fluence. fwhm keeps unchanged when the pump fluence is over the threshold. It is noted that the twophoton pumped ASE displays low threshold though the sample has a simple sandwich structure. There have been a few reports on the two-photon pumped ASE of colloidal QDs which are mostly related to toxic CdS QDs or Pb-related materials.17,25,39 We achieved bright ASE of Zn-processed CIS QDs for the first time. The realized low threshold fluence of TPASE (5.7 μJ/ cm2) is much smaller than that of perovskite QDs (12 mJ/ cm2)17 and CdSe/CdS(1.5 mJ/cm2),25 and to our knowledge, it is a new record of low threshold for ASE of reported QDs. Besides the two-photon pumped ASE, single-photon pumped ASE measurement was also conducted. The results are presented in Figure S7(a). It can be seen that the ASE threshold fluence for single-photon pumping is approximately 82.5 nJ/cm2, which is also lower than the values reported in refs 17 and 24. The excited states of QDs is studied by the time correlated single photon counting (TCSPC) technique as shown in Figure S7(b). At pumping fluence far below the threshold (1.21 nJ/cm2), a typical PL lifetime of 14.75 ns (τ1) is observed. At pumping fluence above the threshold (360.9 nJ/ cm2), an ASE lifetime of 8.76 ns (τ1) is obtained. Such a fast decay of lifetime with increase of pumping fluence indicates a dominant contribution from the ASE effect.

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EXPERIMENTAL SECTION Materials. Chemicals are all commercial available: copper(I) iodide (CuI, Sigma-Aldrich, 98%), indium acetate (In(Ac)3, Alfa-Aesar, 99.99%) 1-dodecanethiol (DDT, Aladdin, 98%), oleic acid (OA, Alfa-Aesar, 90%), 1-octadecane (ODE; AlfaAesar, 90%), trioctylphosphine (TOP, Alfa-Aesar, 90%), Znstearate (Aladdin, technical grade), toluene (Sinopharm, 99.5%), ethanol (Sinopharm, 99.7%), and acetone (Sinopharm, 99.5%). Chemicals were used as received without further purification. Synthesis. A two-step approach was used to acquire Znprocessed CIS QDs. For the first step, in a typical synthesis of CIS QDs, 0.038 g CuI (0.2 mmol), 0.0584 g In(Ac)3 (0.2 mmol), 2.4 mL of DDT (10 mmol), and 0.12 mL of OA (0.4 mmol) were dissolved in 8 mL of ODE, which served as reaction solvent in a three-necked flask. The reaction mixtures were degassed under an argon flow at 100 °C for 40 min. The flask was kept at 100 °C for another 20 min until a clear yellow solution was formed. Then the reaction temperature was quickly raised to 210 °C and kept for 1 h. The color of the solution gradually changed from clear yellow to dark brown, indicating the nucleation and sequential growth of QDs. Then the reaction was stopped by fast cooling to room temperature via cold water bath. Acetone of three times of the volume of QDs solution was added to the three-necked flask and the mixture was immediately centrifuged (6000 rpm, 120 s) to separate the precipitate QDs. The precipitates were dissolved in toluene, and then acetone was added in for a second purification. After three times of purification, the CIS QDs

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CONCLUSIONS We demonstrate considerable enhancement of two-photon fluorescence efficiency and low threshold two-photon pumped amplification of spontaneous emission by a strategy of Zn processing onto CuInS2 QDs for the first time. By Zn2+ cation 1315

DOI: 10.1021/acsphotonics.7b01255 ACS Photonics 2018, 5, 1310−1317

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of 80 MHz. The laser pulses were focused to a point (of diameter ∼1.2 mm) on the QDs film coated on the glass. The direction of incident laser beam and emission detection is shown in the inset of Figure 5a. An 800 nm low-pass filter was placed in front of the detector to filter the reflected/scattered excitation light. The single photon ASE of glass/PMMA/CIS90 QDs/glass sandwich structure was measured with a PiL037X picosecond laser source of A.L.S. GmbH with the pulse width of 45 ps and repetition of 80 MHz. The laser pulses were focused to a point (of diameter ∼10 μm) on the QDs film coated on the glass. The direction of incidence and detection is the same as that utilized in two-photon ASE. A Siminics FT1010 TCSPC System and a Hamamatsu C8137 photomultiplier were employed for TCSPC measurements.

were dissolved in toluene for further characterizations and reaction. For the second step, the Zn-processed CIS QDs was realized by Zn2+ cation exchange. Zn precursor was prepared by dissolving 0.632 g Zn-stearate (1 mmol) in 4 mL of ODE and 1 mL of TOP. All the as-prepared CIS QDs were mixed with 8 mL of ODE, 2.4 mL of DDT as the sulfur source, and 0.12 mL of OA in a three-necked flask. The mixture solution was heated up to 100 °C to remove both toluene and air under an argon flow. After 30 min, the heater was set to 230 °C (210 °C/250 °C) to start the reaction for incorporation of Zn2+. The CIS30 QDs were achieved with a reaction time of 30 min and other Zn-processed CIS QDs with different reaction times are similarly named. Then the reaction was stopped by fast cooling to room temperature via cold water bath. The samples were immediately washed for three times to remove the redundant solvents via the same method described above. The precipitates were dissolved in toluene for characterizations. The preparation of glass/PMMA/CIS90 QDs/glass sandwich structure is facile. First, The 5 μmol/mL CIS90 quantum dots solution was drop-casted on a piece of heated glass (1.5 cm × 1.5 cm, 80 °C) to remove toluene. After the formation of the QDs film, 10 mg/mL PMMA acetone solution was spincoated on the film (3000 rpm, 30 s). Then another piece of glass was covered on the PMMA film. Characterizations. The PL and lifetime measurements were carried out by an Edinburg Instrument FLS920 system. The QDs solution was placed in the quartz cuvette and the excitation light penetrates from the side of cuvette. The emission from the QDs was detected in the direction perpendicular to the propagation of incident light. A 400 nm Xe lamp was employed for steady-state PL and a 405 nm ns laser was employed for lifetime measurements. The quantum yields were measured by the attached integrating sphere on the FLS920 system (The attached integrating sphere is an optional upgrade of FLS920 system provided by the Edinburgh Instruments, see “http://www.edinst.com/products/fls920upgrades/”). The QDs solution was loaded in the 1 cm × 1 cm quartz cuvette, and the quartz cuvette was put in the integrating sphere for QYs measurement. The absorption spectra were measured by a Shimadzu UV2500 system. The QDs solution was placed in the 1 cm × 1 cm quartz cuvette. The HRTEM characterization was carried out in a FEI Tecnai F20 system equipped with a field emission gun working at an accelerating voltage of 200 kV. The lattice resolution is 0.14 nm and the point resolution is 0.23 nm. Samples were prepared by dropping diluted QDs solution onto carbon-coated 200 mesh Aluminum grids, with subsequent solvent evaporation. For the two-photon induced PL (TPPL) measurements and open Z-scan measurements, a MAI TAI VF-T1S femtosecond laser source of 800 nm with a pulse width of 130 fs and repetition rate of 1 kHz was employed. The incident laser was focused onto the sample by a lens and the QDs were placed in the 1 cm × 1 cm quartz cuvette. For efficient luminescence collection in TPPL measurement, we put a 50× magnification objective perpendicular to the beam path, as shown in Figure 1a. The detailed setup of the open Z-scan measurements is shown in Figure S1. The two-photon ASE of glass/PMMA/CIS90 QDs/glass sandwich structure was measured with a Mai Tai HP laser source of 900 nm with the pulse width of 100 fs and repetition

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsphotonics.7b01255. Z-scan setup, single-photon pumped CWPL spectra of CIS QDs, single-photon pumped CWPL peak shift with reaction temperature of CIS QDs, A log−log plot of dependence of TPA cross section versus diameter of CIS QDs, absorption spectra of CIS QDs, calculation of bandgap-dependent TPA coefficient, calculation of ratio of TPA cross section, temperature dependence of singlephoton pumped CWPL and lifetimes, single-photon amplified spontaneous emission, and directionality of single-photon amplified spontaneous emission (PDF).

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Pengqi Lu: 0000-0003-2278-083X Qinghua Xu: 0000-0002-4153-0767 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate Prof. Jianrong Qiu of Department of Optical Science and Engineering, Zhejiang University, for his help in the Z-scan measurement and useful discussions. This research is supported by National Science Foundation of China (Nos. 61290305 and 11374259).

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REFERENCES

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