Dual-Emission and Two Charge Transfer States in Ytterbium-Doped

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C: Energy Conversion and Storage; Energy and Charge Transport

Dual-Emission and Two Charge Transfer States in YtterbiumDoped Cesium Lead Halide Perovskite Solid Nanocrystals Lin Zhou, Taoran Liu, Jun Zheng, Kai Yu, Fan Yang, Nan Wang, Yuhua Zuo, Zhi Liu, Chunlai Xue, Chuanbo Li, Buwen Cheng, and Qiming Wang J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 07 Nov 2018 Downloaded from http://pubs.acs.org on November 7, 2018

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The Journal of Physical Chemistry

Dual-Emission and Two Charge Transfer States in Ytterbium-doped Cesium Lead Halide Perovskite Solid Nanocrystals Lin Zhou†‡, Taoran Liu†‡, Jun Zheng*†‡, Kai Yu†‡, Fan Yang†‡, Nan Wang†‡, Yuhua Zuo*†‡, Zhi Liu†‡, Chunlai Xue†‡, Chuanbo Li†‡, Buwen Cheng†‡ and Qiming Wang†‡

†State

Key Laboratory on Integrated Optoelectronics, Institute of Semiconductors, Chinese

Academy of Sciences, Beijing 100083, China. ‡

Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy

of Sciences, Beijing 100049, China. * Corresponding Author: E-mail: [email protected]; E-mail: [email protected]

KEYWORDS: Perovskite solid nanocrystals, dual-emission, rare earth, energy transfer, kinetic process, charge transfer states

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ABSTRACT: Some unusual phenomena besides near-infrared emission of Yb3+ ions have been observed in ytterbium-doped perovskite solid nanocrystals. A systematic study on doping kinetic and energy transfer processes is presented. The observed unique dual-peak PL emission of perovskite nanocrystals in the visible region can be attributed to radiative recombination in the near-surface region and the interior region of perovskite nanocrystals respectively. Insight studies based on dual-peak PL emission clarify the kinetic process of doping in perovskite nanocrystals. After dopant concentration of rare earth ions in the near-surface region is more than a certain value, dopant ions are starting to be immersed into the interior region of host nanocrystals. The unusual excitation spectra of ytterbium-doped perovskite solid nanocrystals could be explained by the presences of two charge transfer (CT) states at ~24000 cm-1 (CT1) and ~21460 cm-1 (CT2), and both of them could be observed in the near-surface region of the perovskite host. Furthermore, the lifetime of near-infrared emission of Yb3+ ions through the CT2 state is three orders faster than that through CT1 state (in millisecond) which should be fixed on the surface of perovskite nanocrystals. The results provide essential insights into the dynamic carrier behaviors and surface effects of all inorganic perovskite nanocrystals doped with rare earth ions for expanded functionality.


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1. INTRODUCTION Colloidal nanocrystals (NCs) of lead halide semiconductors with perovskite crystal structures have become a subject of extensive current interests due to their excellent optoelectronic characteristics, which are attractive for various optoelectronic applications, such as low threshold single- and multiphoton pumped gain materials for lasing, highly sensitive phototransistors, and high performance LEDs.1-11 In particular, all-inorganic CsPbX3 (X = Cl, Br, or I) NCs have exhibited higher chemical and temperature stability as well as very high photoluminescence quantum yield.5 Their optical and electronic properties can be easily controlled by adjusting their size, composition, and dimensionality.1, 4 Furthermore, the composition of perovskite NCs can be easily post-synthetically tailored through anion-exchange reactions with preservation of the size and shape of the parent NCs.12-13 In colloidal II−VI and III−V semiconductor NCs, doping with impurity ions has been extensively investigated as a way to introduce new optical, electronic, and other capabilities.14 Recently, the successful introduction of transition metal ions such as Mn2+, Bi3+ in colloidal perovskite NCs provides another synthetic tool to considerably modify the optoelectronic properties and to bestow the parent NCs with novel functionalities.15-18 Enormous efforts have been expended on understanding the doping process and designing highly efficient doped NCs.16, 19-21

For the past decades, trivalent rare earth (RE) ion doped NCs have also been attracted much

attention. 22-26 Hence, it is expected that the all-inorganic halide perovskite NCs doped with RE ions can present the excellent optical properties of both RE ions and the perovskite NCs, expand their optical properties and applications in solar cells,27 photodetectors,28 light-emitting devices,26 modern optical telecommunication,29 and as the active material in lasers.30


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Among the trivalent ions of the lanthanide series, Yb3+ has unique properties due to its extremely simple energy-level configuration. In previous works,26-27 almost characterizations of doped NCs were focused on their dispersed colloid solution, and most applications were based their solid film.27 However, there are very few researches on the perovskite solid NCs. In this work, we successfully synthesized ytterbium-doped CsPbCl3 and CsPb(Cl/Br)3 solid NCs via the modified hot-injection method,1 which exhibited some unusual phenomena besides near-infrared emission of Yb3+ ion, such as dual-peak emission of CsPbCl3 and CsPb(Cl/Br)3 solid NCs in visible spectra, two excitation bands centered at 418 nm and 466 nm respectively for the emission of Yb3+ ions. Temperature-dependent PL measurements are used to reveal the origin of the dual-peak emission of perovskite. The kinetic process of doping in perovskite NCs has been studied in detailed via the dual-peak PL spectra. The effective near-infrared emissions indicate the existence of the exchange coupling between the host and Yb3+ ions which is sufficiently strong to make efficient energy transfer. The two different charge transfer (CT) states involved in energy transfer from NCs to Yb3+ have been used to explain the two excitation bands monitored at 990 nm. To make a further insight of CT states introduced by Yb element, ytterbium-doped CsPb(Br/Cl)3 NCs, erbium-doped CsPbCl3 NCs and erbium, ytterbium-co-doped CsPbCl3 NCs have been fabricated and characterized. Our findings enable a deeper understanding of the physics of the energy transfer process occurring in the RE-doped CsPbX3 perovskites solid NCs.

2. EXPERIMENTAL SECTION Materials: PbCl2 (99.999%, Sigma-Aldrich), PbBr2 (99.999%, Alfa-Aesar), Cs2CO3 (99.9%, Alfa-Aesar), octadecene (ODE, 90%, Alfa-Aesar), acetone (99.7%, Beijing Chemical Work), nhexane (97%, Sigma-Aldrich), YbCl3·6H2O (99.99%, Sigma-Aldrich), oleic acid (OA, 90%,


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Alfa-Aesar), oleylamine (OAm, 80~90%, Sigma-Aldrich), toluene (99.5%, Beijing Chemical Work), All the reagents were used directly without further purification. Synthesis of Cs-oleate (0.125 M): Cs2CO3 (0.814g) was loaded into 250 mL 3-neck flask along with octadecene (ODE, 40mL) and oleic acid (OA, 2.5 mL) dried under vacuum for 1h at 120 °C, and then heated to 150 °C until all Cs2CO3 reacted with OA under N2 to totally dissolve Cs2CO3 powder. Since Cs-oleate precipitates out of ODE at room-temperature, it has to be preheated to 140 °C before injection. Synthesis of CsPbCl3 perovskite NCs: ODE (50 mL), oleylamine (OAm, 6 mL), OA (6 mL) and PbCl2 (0.52 g) were loaded into 250 mL 3-neck flask and raised to 120 °C then evacuated and refilled with N2 followed by heating the solution to 120 °C for 1 hour. After complete solubilization of a PbCl2 salt, the temperature was raised to 200 °C. And at 200 °C, OAm (6 mL), OA (6 mL) were subsequently injected to solubilize the solution. Then Cs-oleate solution (5 mL, 0.125M in the ODE, prepared as described above and preheated before injection) was quickly injected and, 1 min later, the reaction mixture was cooled by the ice-water bath. After centrifugation at 8000 rpm for 10 min, the supernatant was discarded and the particles were redispersed in n-hexane. Then the NCs were precipitated with acetone and centrifuged again. The precipitate of NCs was dried under vacuum at 60 °C for 12 hours. Finally, the solid NCs were prepared. Synthesis of doped CsPbCl3:Er(y%)&Yb(x%) perovskite NCs: ODE (50 mL), oleylamine (OAm, 8 mL), OA (8 mL) , PbCl2 (0.52 g), YbCl3·6H2O (Y g) and YbCl3·6H2O (X g)(The amount of the starting materials for synthesizing perovskite NCs with different doping concentrations are shown in Table S1. The practical doping concentrations of different RE3+ ions in perovskite NCs are shown in Table S2) were loaded into 250 mL 3-neck flask and raised to


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120 °C then evacuated and refilled with N2 followed by heating the solution to 120 °C for 1 hour. After complete solubilization of a PbCl2 salt, the temperature was raised to 200 °C. And at 200 °C, OAm (8 mL), OA (8 mL) were subsequently injected to solubilize the solution. Then Csoleate solution (5 mL, 0.125M in the ODE, prepared as described above and preheated before injection) was quickly injected and, 1 min later, the reaction mixture was cooled by the ice-water bath. After centrifugation at 8000 rpm for 10 min, the supernatant was discarded and the particles were re-dispersed in n-hexane. Then the NCs were precipitated with acetone and centrifuged again. The precipitate of NCs was dried under vacuum at 60 °C for 12 hours. Finally, the doped solid NCs were prepared. Other NCs doping with the different concentrations and doped CsPb(Br/Cl)3 NCs were synthesized by similar methods. Characterization: The UV-Vis spectra of the NCs were collected using a Shimadzu UV3600PLUS UV-Vis-NIR spectrophotometer. Photoluminescence (PL) spectra, time-resolved PL decay spectra, and temperature dependent PL were measured using the Edinburgh Instruments (EI) FLS980 lifetime and steady state spectrometer. The samples were directly excited with a 450 W ozone-free xenon arc lamp. For the lifetime of Yb emission, the excitation source is the microsecond flashlamps (µF2) which used for the scale of 1 µs~10 s. The detector is NIR-PMTs whose response width is 800 ps. High-resolution transmission electron microscopy (HRTEM) images of the nanocrystals were performed using a FEI Tecnai G2 F20 S-TWIN field emission transmission electron microscope. X-ray diffraction (XRD) measurements of the perovskite nanocrystals powder were performed using a BRUKER, AXS D8 ADVANCE X-ray powder diffractometer using monochromatized Cu-Kα radiation (λ = 1.5406 Å). X-ray photoelectron spectroscopy (XPS) was performed with Axis Ultra Imaging X-ray Photoelectron Spectrometer from Kratos Analytical Ltd. Trace-Metal Analysis was carried out using


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inductively coupled plasma optical emission spectrometry (ICP-OES) on a PerkinElmer Optima 8300 ICP-optical emission spectrometer. Dried powdered samples of NPs were first acid digested (the concentrated acid of nitric acid and hydrochloric acid in 140 °C until remaining a small amount of transparent colorless liquids) and then diluted prior to measurements.

3. RESULTS AND DISCUSSION The RE-doped perovskite NCs used herein were prepared via a hot-injection method and slightly modified from that reported in the literature.1 The practical doping concentrations of different RE3+ ions in perovskite NCs have been confirmed by the inductively coupled plasma optical emission spectrometry (ICP-OES) measurements (Table S2, Supporting Information), with varied molar concentrations of Yb3+ (0.6–5.7%), and Er3+ (0.4%), which are about 25 times lower than those of starting materials (Table S1, Supporting Information). Figure 1a–d shows the typical transmission electron microscopy (TEM) images of CsPbCl3 NCs with different Yb3+ doping concentrations, which imply the large quantity and good uniformity of the as-prepared NCs (Figure S1, Supporting Information). The high-resolution transmission electron microscopy (HRTEM) images (corresponding FFT images in Figure S2, Supporting Information) for the CsPbCl3 NCs with different Yb3+ doping concentrations are presented in Figure 1e-h, corresponding to the TEM images of Figure1a-d respectively. The interplanar distances of (101) and (002) planes for un-doped NCs are respectively determined to be ~3.96 and ~2.81 Å, which are in accord with standard PDF data (PDF#73-0692). The HRTEM images highlight that the doped NCs have well-defined crystalline structures with interplanar distances of 5.51 Å for 2.1%, 5.44 Å for 4.0% and 5.36 Å for 5.7% Yb3+, respectively, corresponding to the (001) crystal faces. The slight decrease of interplanar distance with increasing concentration of Yb3+, suggests


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further that the small contraction happens in the doped sample due to the smaller Yb3+ (87 pm) occupied the sites of larger Pb2+ (six-coordinate effective ionic radius 119 pm).31 The X-ray diffraction (XRD) patterns (Figure 1i) show that all the NCs have the cubic phase (PDF#730692), and the patterns gradually shift toward large angle side because of the introduction of Yb3+ ions, which is the result of progressive lattice contraction as the concentration of Yb3+ ions increases, as expected for substitutional replacement of Pb2+ ions with smaller Yb3+ ions. This is consistent with the results of HRETM characterization. The XPS survey spectrum (Figure 1j) shows the presence of Cs, Pb, Cl, Er, and Yb elements, indicating the existence of RE ions in the CsPbCl3 NCs. Therefore, combining the results of HRTEM, XRD patterns, XPS and according to previous literature,26-27, 32 it can be reasonably concluded that Yb3+ may replace the sites of Pb2+ in the CsPbCl3 host.


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Figure 1. (a-d) TEM images of re-dispersible perovskite NCs doping with different percentages of Yb3+ ions: a) without doping; b) CsPbCl3: Yb (2.1%); c) CsPbCl3: Yb (4.0%); d) CsPbCl3: Yb (5.7%); Scale bar: 50 nm. (e-h) HR-TEM images of the perovskite NCs corresponding to a-d) respectively. Scale bar: 5 nm. (i) XRD patterns of perovskite NCs and the careful examination of the (110) plane. (j) XPS spectra of Yb3+-doped CsPbCl3 and Yb3+, Er3+ co-doped CsPbCl3.

Dual photoluminescence peaks in the visible region. Figure 2a shows that UV-vis absorption spectra of various Yb3+ ions doped perovskite CsPbCl3 colloid NCs. Compared with un-doped NCs, all ytterbium-doped NCs show new extra absorption peaks (ca. at 403 nm). The extra absorption peaks introduced by Yb3+ ions may imply an energy state occur in doped NCs. As


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shown in absorption spectra of various Yb3+ ions doped perovskite CsPbCl3 solid NCs (Figure 2b), after the introduction of Yb element in the CsPbCl3 host, the UV–vis absorption edge slightly shifts toward blue due to the gradually increased band gap from the original 2.934 eV to 2.973 eV (Figure S3, Supporting Information), resulting from the partial substitution of Pb by Yb element. However, it should be noted that the particle size shifts from 12.5±3.2 to 14.5±3.3 nm and average particle size is independent of the ytterbium doping concentration, which might also lead to the shift of band gap in the perovskite NCs based on the quantum confinement effect, as shown in Figure S1 (Supporting Information). To evaluate these two effects, we calculated the variation of band gap depending on the quantum confinement effect from the Equation (1).1, 33-34 The result shows that the band gap merely shifts less than 1 meV (ca. 0.73 meV) as the particle size changes from 14.5 to 12.5 nm, which is much smaller than the practical change of 40 meV, thus indicating that the quantum confinement effect is not the dominant mechanism for the shift of band gap. ∆𝐸 =

ℏ2𝜋2

1.76𝑒2

― 4𝜋𝜀0𝜀𝑅 , 2𝑚 𝑅2 𝑟

Equation (1).

Where R is the particle radius, mr is the effective mass of the exciton, and ε is the relative dielectric constant of CsPbCl3 bulk material.


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Figure 2. (a&b) Absorption spectra of various Yb3+ ions doped perovskite CsPbCl3 colloid NCs (a) and solid NCs (b). (c) Visible photoluminescence (PL) spectra (left, excited by 350 nm light), and near-infrared emission spectra (right, excited by 418 nm light) of various Yb3+ ions doped perovskite CsPbCl3 solid NCs. As shown in Figure 2c, visible PL (left) spectra show unusual two emission peaks in all CsPbCl3 host with various Yb3+ ions doping, which is obviously different with NCs in colloidal soltion26-27, 32. But it should not be originated by the introduction of Yb element, because both un-doped and doped solid NCs show two emission peaks. There are several possible reasons for this phenomenon, such as two kinds of the lattice phase of perovskite, two subsets of NCs with different sizes. In order to evaluate these two possible reasons, Rietveld refinement method of XRD and PL spectra of perovskite solid NCs re-dispersed in n-hexane solution have been observed as a contrast. The XRD pattern data analyzed by Rietveld refinement method (Figure S4, Supporting Information) indicates that the perovskite solid NCs is 100% CsPbCl3 of cubic phase (PDF#73-0692). Besides, if the two emission peaks originate from two subsets of NCs with different sizes and shapes, the two emission peaks should be observed in perovskite NCs redispersed in n-hexane solution. Nevertheless, the PL spectra of NCs in n-hexane solution show only single peaks appear in the visible region while the shorter-wavelength peak is disappeared, as shown in Figure S5. All two-peak emission spectra in the visible region can be fitted by two Lorentzian curves (Figure S6, Supporting Information). Then the fitting curves are summarized in Figure S7 and Figure S8 (Supporting Information) which are normalized. Since the band gap of the near-surface region is wider than interior region due to the richness of halogen at surface,35 Peak A (blue-side) may originate from the radiative recombination in the near-surface region of the NCs that is quenched in n-hexane (Figure S5), which may be due to surface passivation from


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the organic solution. But Peak A could also be ascribed the emission from defect states on the surface of solid NCs, which would be discussed later. Peak B (red-side) resulting from absorption-induced emission should occur in the interior region36-37. In the near-infrared region (Figure 2c, right), PL spectra centered at 990 nm can be observed for CsPbCl3: Yb3+ NCs, originating from the 2F5/2 → 2F7/2 transition of Yb3+ ions. The strong intrinsic emissions of Yb3+ ions excited by the absorption of perovskite host readily indicates that an efficient energy transfer occurs from the host to Yb3+ ions. To further explore the origination of Peak A and Peak B in the visible region, we have studied the temperature dependence of the PL spectra of 4.0% Yb-doped CsPbCl3 NCs. As shown in Figure S8a&9 (Supporting Information), a steady decrease in the PL intensity of dual bands in doped NCs with increasing temperature from 157 to 297 K is observed. As shown in Figure 3a, the peak position of Peak B shifts toward longer wavelength when the temperature increases from 157K to 297 K, while there is a blue shift in the case of Peak A. Peak intensities and peak positions of Peak A and Peak B as functions of temperature summarized from Fig. 3a are showed in Fig. 3b. Besides, the peak intensity ratio of A to B decreases from 1.99 to 0.94 as the temperature increases from 157 to 297 K, indicating a dominant peak transition from A to B. Since light emission (Peak B) from perovskite NCs is due to the band to band radiative recombination of electron-hole pairs in the interior region of NCs, the function of peak position and temperature should be well fit by the relation Equation (2),38 which further confirms the luminescence signal of Peak B from the interior region of NCs, as shown in Figure 3b.

𝐸𝑔𝑎𝑝(𝑇) = 𝐸𝑔𝑎𝑝,0 +𝐴 ∙

(

2 ℏ𝛺

𝑒

𝑘𝐵𝑇

)

+1 , ―1

Equation (2).


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Where A is the temperature-independent constant (-0.403 eV for doped CsPbCl3 NCs), kB is the Boltzmann constant, Egap,0 is the energy value at 0 K determined by extrapolation (3.397 eV at 0 K for doped NCs), and ℏΩ represents an average phonon energy (0.093 eV for doped NCs), as shown in Figure 3b. And if the emission of Peak A is originated from another subset of NCs with smaller sizes or different lattice phase, it’s expected that the temperature dependence of the peak position should also follow the behavior described by Equation (2). However, a slight blue shift of the peak position is observed in Peak A, which eliminates the possibility of Peak A originated from another subset of NCs with smaller sizes or different lattice phase. And if Peak A is ascribed to the radiative recombination of localized levels formed by surface states, the position of Peak A should be temperature insensitive. Therefore, Peak A could be ascribed to the radiative recombination of energy levels from the near-surface region of NCs not the defect states. It’s consistent with the previous discussion. Comparing with the band gap in the interior region of NCs, the band gap in near-surface region has a positive temperature coefficient, which may be ascribed to surface-stress-driven lattice contraction39-41 increased with increasing temperature. The detailed originations of the blue shift need further investigation. In brief, Peak A (blue-side) may originate from the radiative recombination in the near-surface region, while the red-side PL peak (Peak B) is attributed to emission it interior region.


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Figure 3. (a) Peak intensities (dark color) and peak positions (red color) of Peak A (up) and Peak B (down) as functions of temperature summarized from Figure S9 for 4.0% Yb-doped CsPbCl3 solid NCs. (b) Peak PL intensity ratio of Peak A to B as a function of temperature (dark color) and PL peak position (eV) of Peak B as a function of the temperature.

Kinetic process of doping in perovskite nanocrystals. As discussed above and previous reports26-27, the increased band gap with the increasing dopant concentration are ascribed to the substitution of Pb2+ ions by smaller Yb3+ ions in host lattices, which would lead to the blue shift of PL spectra. As shown in Figure 4a&b, Peak A show distinct blue-shift after doping with a low dopant concentration, while Peak B shifts to red slightly. However, with further increasing dopant concentration, there are no blue-shift in Peak A but obvious blue shifts in Peak B. Based on the previous discussion, Peak A is considered to be originated from the radiative recombination in the near-surface region, while the Peak B is attributed to emission in the interior region. Furthermore, the effect of particle sizes that would lead the PL peak shift to blue less than 1 meV as we discussed above (by Equation (1)), while the blue-shift of the PL peak is over 30 meV in this case. Hence, the blue-shift of the PL peak could be mainly attributed to the


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introduction of ytterbium element. It may indicate there are two stages in the doping kinetics: Stage I and Stage II. In Stage I, when the Yb element with lower dopant concentration (less than 2.1%) is introduced, Yb3+ ions may be mainly implanted in the near-surface region of the host. Therefore the position of Peak A shifts to blue while Peak B nearly remains unchanged or just slightly shifts to red. In Stage II, when more Yb3+ ions are introduced, Yb3+ ions are starting to be immersed into the interior region of host NCs while Yb3+ ions in the near-surface region are saturated. Hence the position of Peak A exhibits no blue-shift while Peak B begins to shift to blue. Figure 4b shows the peak intensity ratio of Peak A to B and the peak position of PL spectra at the different molar ratio of Yb3+/Pb2+. The peak intensity ratio of A to B decreases from 2.26 to 0.47 with an obvious turning point as the molar ratio of Yb3+/Pb2+ increases from 0 to 5.7 at. %, indicating a dominant peak transition from A to B, which may also demonstrate there are two stages in doping kinetics. Time-resolved spectroscopy provides further details on the mechanism of optical improvement. The dynamics of un-doped and RE-doped perovskite NCs are shown in Figure S13 (Supporting Information) and fitting the data with multiexponential decay curves yields several time constants, τ1, τ2, and τave, as summarized in Table S3. As shown in Figure 4c, the lifetime of Peak B is shorter than that of Peak A except un-doped and low concentration (0.6% Yb) doped samples, which may imply two stages in doping process with the increase of dopant concentration. The lifetimes of Yb-doped perovskite NCs decrease obviously compared to the un-doped NCs. Moreover, there is a turning point near the molar ratio of Yb/Pb less than 2.1% in the curves of lifetime-dopant concentration in Figure 4c, which may demonstrate two stages in doping kinetics again. In brief, when the scarce Yb3+ ions (

Dual-Emission and Two Charge Transfer States in Ytterbium-Doped

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