Startseite Ligand-modulated electron transfer rates from CsPbBr3 nanocrystals to titanium dioxide
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Ligand-modulated electron transfer rates from CsPbBr3 nanocrystals to titanium dioxide

  • Xiaochun Liu , Haifeng Zhao ORCID logo , Linfeng Wei , Xinjian Ren , Xinyang Zhang , Faming Li , Peng Zeng EMAIL logo und Mingzhen Liu ORCID logo EMAIL logo
Veröffentlicht/Copyright: 26. Februar 2021
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Nanophotonics
Aus der Zeitschrift Nanophotonics Band 10 Heft 8

Abstract

In most perovskite nanocrystal (PeNC)-based optoelectronic and photonic applications, surface ligands inevitably lead to a donor–bridge–acceptor charge transfer configuration. In this article, we demonstrate successful modulation of electron transfer (ET) rates from all-inorganic CsPbBr3 PeNCs to mesoporous titanium dioxide films, by using different surface ligands including single alkyl chain oleic acid and oleylamine, cross-linked insulating (3-aminopropyl)triethoxysilane and aromatic naphthoic acid molecules as the ligand-bridge. We systematically investigated the ET process through time-resolved photoluminescence spectroscopy. Calculations verified the ligand-bridge barrier effect of the three species upon the ET process. Transient absorption measurements excluded carrier-delocalization effect of the naphthoic acid ligands and confirmed the bridge-barrier effect. Our work provides a perspective for composable and appropriate ligands design for diverse practical purposes.

Keywords: bridge-barrier effect; CsPbBr3 nanocrystals; electron transfer; ligand-bridge

1 Introduction

Over the past decade, metal halide perovskites have risen to prominence in optoelectronic and photovoltaic fields for their outstanding properties that resulted in a rocketing progress in device performance [1], [2]. Recent research focus has also been directed toward low-dimensional metal halide perovskites with expectation for their unique photonic and electrical properties under quantum confinement and surface effects [3], [4], [5], [6], [7], [8]. Quasi-zero-dimensional all-inorganic CsPbX3 (X=Cl, Br, I) perovskite nanocrystals (PeNCs) soon distinguished themselves as a new family of photonic and optoelectronic materials for their narrow emission width with color tunability [9], high optical gain [4] and reduced fluorescence blinking [10]. Particularly, their unusual antibonding nature of electronic band structure endows CsPbX3 PeNCs higher tolerance to surface defects than conventional II-VI semiconductor nanocrystals [11], [12], [13]. The latter usually perform limited photoluminescence quantum yields due to defect-induced PL quenching. Therefore, together with their low-cost and ease of synthesis, CsPbX3 PeNCs are perceived as potential candidates for low-threshold lasers [7], [14], high dynamical displays [15], light-emitting diodes [8], [16] and photovoltaics [6], [17], [18].

Surface ligands are necessary and crucial to PeNCs in stability of the lattice structure and control of crystal growth [19], [20], [21], [22], [23], [24]. Inevitably, in most PeNC-based optoelectronic and photonic applications, it leads to a donor–bridge–acceptor charge transfer (CT) system, in which the anchoring ligands act as a linking bridge. Therefore, it is of significant importance to carefully choose appropriate ligands in order to tune the CT rates and thus achieve the desired performance of PeNC-based devices.

So far it has been one of the focuses to investigate the ligand-bridge effect on the CT process and find composable and appropriate ligands for diverse practical purposes [25], [26]. Ligands with high potential barriers for either electrons or holes may notably hinder the CT process, which is necessary to some photonic applications that need restrained photon-induced charge losses to surroundings, such as in lasers and displays [27]. Low-potential barrier ligands are in contrary. They boost the CT rates by acting as a conductive bridge between the NCs and carrier acceptors/donors, and this is crucial to charge extraction purpose [25], [26], [28]. One limiting case is that the ligand possesses a potential close or even lower than that of the NCs donating level, so-called carrier-delocalizing ligands. There is thus a possibility that the ligand performs as charge carrier acceptors rather than the bridge, leading to carrier trapping effect on the ligand which may potentially reduce the carrier extraction efficiency [29].

Based on our recent success in CsPbBr3 PeNCs synthesis with varied ligands, we demonstrate herein a control of the electron transfer (ET) process using single alkyl chain, cross-linked insulating and aromatic ligands. These ligand species correspond to conventional acid-base coordinated oleic acid (OA) and oleylamine (OAm), (3-aminopropyl)triethoxysilane (APTES) and naphthoic acid (NCA), respectively. Tuning the potential barrier by use of different ligands, we realized modulation of the ET rates from CsPbBr3 PeNCs to electron-accepting mesoporous titanium dioxide (TiO2) substrates spanning a range of fivefolds, as revealed by time-resolved PL spectroscopy. Calculations of the ligand molecule frontier orbitals verified the bridge-barrier effect on the changes of ET rates. Especially, the successful anchoring of NCA ligands revealed an appropriate potential barrier leading to the most significant ET rates with no obvious charge delocalization effect, which may be detrimental to quantum yields of the NCs.

2 Experimental section

2.1 Materials

All chemicals were used without further purification, which include: lead(II) bromide (PbBr2, 98%) and (3-aminopropyl)triethoxysilane (APTES, 98%) were purchased from Alfa Aesar; cesium carbonate (Cs2CO3, 99.9%), octadecene (ODE, 90%), oleylamine (OAm, 70%), oleic acid (OA, 90%) were purchased from Sigma-Aldrich; octane (C8H18, 99%) were purchased from Adamas Reagents; 1-naphthoic acid (NCA, 98%) was purchased from TCI; methyl acetate (MeAC, 98.0%) were purchased from Greagent.

2.2 Synthesis of CsPbBr3 PeNCs with various ligands

The preparation of OA/OAm-CsPbBr3 and OA/APTES-CsPbBr3 PeNCs were conducted following a well-established synthesis process as reported by Zhao and coworkers [30]. In brief, the Cs-oleate precursor solution was prepared by mixing 0.18 g Cs2CO3, 0.75 mL OA and 7.5 mL ODE into a 50 mL three-necked round-bottomed flask. With being stirred, the mixture was degassed at 120 °C under vacuum for 20 min. In order to remove moisture and O2 in the mixture, the flask was purged with Ar by switching on a double-row-pipe and subsequently degassing under vacuum for three times at 60 and 90 °C, respectively. Then, the reactant was heated to 150 °C under Ar flow until the solution turned clear which indicated a complete reaction of Cs2CO3 and OA. The Cs-oleate precursor was kept at 100 °C to avoid solidification before injection.

Taking OA/OAm-CsPbBr3 PeNCs as an example, a mixture of 0.1 g PbBr2, 0.75 mL OA, 0.75 mL OAm and 7.5 mL ODE were dried in a three-necked flask under vacuum, and then were heated to 120 °C with magnetic stirring. During the rise of temperature, the system was rinsed under Ar and vacuum in turn for three times at 60 and 90 °C, respectively. After PbBr2 was completely dissolved, the atmosphere was switched from vacuum to Ar, and the reaction temperature was elevated to 170 °C. Then 0.75 mL of the prepared Cs-oleate precursor was swiftly injected into the reaction mixture. The reaction was rapidly quenched down in an ice-water bath after mixing for 5 s. The crude solution was first washed by adding MeAC with a 3-times greater volume, and then was centrifuged at 10,000 rpm for 1 min to collect the as-synthesized OA/OAm-CsPbBr3 PeNCs. In the end, the products were dispersed in octane and centrifuged again at 10,000 rpm for 3 min. The supernatant was collected for all spectroscopic investigations reported here. OA/APTES-CsPbBr3 PeNCs were prepared using the same reaction procedure as the OA/OAm-CsPbBr3 PeNCs except for replacing 0.75 ml OAm with 0.75 mL APTES and lowering the inject temperature to 150 °C. For OA/OAm-capped NCs and OA/APTES-capped NCs, both the synthesis and washing procedure were performed under the same conditions, and the same amount of OA was added during the reaction. Thus it is rational to assume similar ligand densities in both OA/OAm- and OA/APTES-CsPbBr3 systems. For the case of NCA/OAm-CsPbBr3 PeNCs, they were obtained by a ligand treatment procedure, in which 0.25 mg NCA was added into 1 ml OA/OAm-CsPbBr3 PeNCs stock solution. The ligand treatment reaction was stirred for 5 min followed by centrifuging at 10,000 rpm for 3 min in order to remove the unbounded ligands. For NCA/OAm-CsPbBr3 NCs obtained through a well-established and mild ligand-treatment reaction, the total acid-ligand density (including OA and NCA) was also reasonably assumed to be similar to that of the pristine OA/OAm-CsPbBr3 system.

2.3 Preparation of CsPbBr3 PeNC films

About 0.05 mL of CsPbBr3 PeNC solution in octane was spin-coated at 1500 rpm for 30 s onto a mesoporous TiO2 layer deposited onto a fluorine-doped tin oxide (FTO)-coated glass substrate, and a polymethyl methacrylate (PMMA) layer on a glass substrate, respectively. Films were for time-resolved PL and transient absorption (TA) measurements. For the mesoporous TiO2 layer, mesoporous TiO2 precursor was spin-coated on the substrates at 3800 rpm for 30 s, followed by annealing at 180 °C for 20 min and then annealing at 500 °C for 50 min in air. For the PMMA layer, PMMA dissolved in chlorobenzene was spin-coated on the substrates at 3000 rpm for 30 s, followed by annealing at 60 °C for 10 min in air.

2.4 Characterization

Absorption spectra were measured with an UV−vis spectrometer (type, PerkinElmer). Steady-state and time-resolved PL spectra were obtained by a time-correlated single photon counting instrument (FT300, PicoQuant) with a 454 nm ps pulse laser as the excitation source. Transmission electron microscope (TEM) measurements were conducted with G2F20 S-Twin Tecnai. X-ray diffraction (XRD) patterns were taken at a voltage of 40 kV and current of 40 mA on an X-ray diffractometer (D8 Advance, Bruker) with Cu Kα1/2 source and Vantec-1 detector in 2θ-Omega model. Fourier transform infrared spectra (FTIR) were obtained using a PerkinElmer Spectrum Two FTIR Spectrometer with a diamond attenuated total reflection kit. Scanning electron microscope (SEM) images were collected by an FEI Inspect F50 electron microscope with an electron energy of 10 kV.

TA measurement was carried out on a home-built femtosecond time-resolved pump-probe system using a regeneratively amplified laser source (Solstice, Spectra-Physics). In detail, a portion of 800 nm output beam with a repetition rate of 1 kHz was frequency-doubled in a BBO crystal for the generation of 400 nm pump beam. After sending through a computer-controlled delay stage, the pump beam was chopped by a chopper synchronized to 500 Hz. The continuum white-light probe beam covering a wavelength range from 440 to 800 nm was generated by focusing a portion of 800 nm beam into a 3-mm-thick sapphire window. The pump and probe pulses were spatially overlapped in a quartz cell of CsPbBr3 PeNC solution, or on the coated CsPbBr3 PeNC film side. Then pump-induced variations in absorbance of the samples at different delay times were recorded by a synchronized fiber spectrometer. The IRF (instrument response function) width of 150 fs was determined by 800 nm autocorrelation measurements at the sample position.

CsPbBr3 PeNC samples dispersed in octane were excited at a low pump flux of 16 nJ/pulse, in order to avoid any multiple-exciton-induced effects. The estimated exciton number per particle is provided in Supplementary Information (SI). All measurements were conducted at room temperature.

2.5 DFT calculations

All calculations were accomplished using density functional theory (DFT) with the Gaussian 09 program suit [31]. The electronic structure was obtained by Kohn−Sham density functional calculations using B3LYP (Beckethree–Lee–Yang–Parr) as the exchange-correlation functional and 6-311 + G(d,p) as the basis set [32].

3 Results and discussion

Our synthesis procedure was based on the conventional hot-injection method incorporating OA and OAm precursors. For OA/APTES-CsPbBr3 PeNCs, the APTES was introduced as a substitute for OAm. Its dangling silane groups are able to form cross-linked Si-O-Si via the mutual hydrolysis process. Based on our previous work [30], by a mild self-hydrolysis of the APTES precursor, we herein obtained highly uniform OA/APTES-CsPbBr3 PeNCs, in which particles remained in an individual cubic shape and were wrapped with thin silicone layers (see Figure S1 in SI). For NCA/OAm-CsPbBr3 PeNCs, a post-reaction of ligand-treatment procedure based on OA/OAm-CsPbBr3 PeNCs was introduced, which was able to maintain the same morphology and size of the nanoparticles.

Consistent particle size circumvents the intrinsic size-dependent properties of PeNCs and allows significative discussion of ligand-bridge effect on the CT process. Herein, synthesis of CsPbBr3 PeNCs with different ligand composites was carefully controlled through adjusting hot injection temperature (see the synthesis part), in order to obtain similar-sized PeNCs. Particle sizes were further certified by TEM, as shown in Figure 1A. All CsPbBr3 PeNCs exhibit characteristic cubic shapes, and the average edge lengths are 11.2 ± 1.1 nm for OA/APTES-CsPbBr3 PeNCs, 11.3 ± 1.4 nm for OA/OAm-CsPbBr3 PeNCs, and 11.3 ± 1.3 nm for NCA/OAm-CsPbBr3 PeNCs (see Figure S2 in SI). As shown in Figure 1B, the absorption spectra for all CsPbBr3 PeNCs possess similar absorption onsets close to 510 nm, meanwhile, the photo-excited PL emission peak of OA/APTES-CsPbBr3 PeNCs is centered at 511 nm, close to that of OA/OAm-CsPbBr3 and NCA/OAm-CsPbBr3 PeNCs at 514 nm, implying comparable size-dependent photonic properties for all three CsPbBr3 PeNC systems. The absorption spectra of APTES, OA, OAm and NCA solution in ethanol all show no absorption band features above 400 nm (see Figure S3). We accordingly clarify that only CsPbBr3 PeNCs were selectively excited in the following spectroscopic measurements. It is also worth noting that, after the ligand treatment and washing procedure, the distinctive absorption onset close to 350 nm in NCA-CsPbBr3 PeNCs was consistent with that of the NCA solution, indicative of likely successful linkage of NCA ligands onto CsPbBr3 PeNCs.

Figure 1: (A) TEM images, (B) UV–Vis absorption and PL emission spectra, (C) FTIR spectra and (D) XRD patterns of OA/APTES-CsPbBr3 (green), OA/OAm-CsPbBr3 (blue) and NCA/OAm-CsPbBr3 (orange) PeNCs. Scale bars are 50 nm.
Figure 1:

(A) TEM images, (B) UV–Vis absorption and PL emission spectra, (C) FTIR spectra and (D) XRD patterns of OA/APTES-CsPbBr3 (green), OA/OAm-CsPbBr3 (blue) and NCA/OAm-CsPbBr3 (orange) PeNCs. Scale bars are 50 nm.

FTIR spectroscopy was further employed to confirm the existence of cross-linked APTES and NCA molecules in the CsPbBr3 PeNC complexes, respectively. As shown in Figure 1C, for OA/OAm-CsPbBr3 PeNCs, vibrations of C=O and NH3 + from OA and OAm ligand appear at 1536 and 1644 cm−1, respectively [33]. Two additional peaks at 1033 and 1109 cm−1 are observed for OA/APTES-CsPbBr3 PeNCs, corresponding to the vibration of Si–O–Si and Si–O–C for the wrapping of APTES on CsPbBr3 nanoparticles which are in line with our previous work [30]. The former clearly verified the formation of cross-linked APTES ligands through the hydrolysis process which resulted in a wrapping silicone layer outside the nanoparticles. As for NCA/OAm-CsPbBr3 PeNCs, vibrations of the conjugate structure C=C observed at 1512, 1573 and 1592 cm−1 are indicative of the presence of NCA molecules, as well as the vibrations of the C=O group observed at 1672 and 1623 cm−1 [34].

XRD patterns (Figure 1D) verified that the cubic phase remains unchanged for all species upon introduction of various ligands. Noticeably, a significant change was observed, that is, after partially replacing OA with NCA, the split of (100) diffraction peak centered at 15.11° for OA/OAm-CsPbBr3 PeNCs was suppressed, indicating that NCA conduces to more excellent crystallinity.

With the successful anchorage of the ligands, we investigated the ET process from CsPbBr3 PeNCs to the electron-accepting TiO2 with different ligand systems. In a typical electron donor–acceptor system, the PL of the electron donors is quenched, and its lifetime is accordingly shortened as the ET process provides an additional depopulation channel for the photoinduced charge carriers. Hence, the ET, or electron extraction rate in the CsPbBr3-TiO2 system here can be derived by comparing the PL lifetimes between CsPbBr3 PeNCs assembled with PMMA and TiO2 [35]:

(1) k ET = 1 τ CsPbBr 3 , TiO 2 − 1 τ CsPbBr 3 , PMMA

where τ CsPbBr 3 ,TiO 2 and τ CsPbBr 3 ,PMMA are the PL lifetimes for CsPbBr3 PeNCs casted on conductive mesoporous TiO2 with FTO-coated glass as substrates and on insulating polymethyl methacrylate (PMMA) with glass as substrates, respectively. As PMMA is insulating and thus able to inhibit carrier leakage, CsPbBr3 PeNCs coated on the PMMA layer provides a meaningful reference of the quenched PL decays. Here, TCSPC measurements were applied to monitor the PL decays of CsPbBr3 PeNCs, whereby the average lifetimes τ CsPbBr 3 , TiO 2 and τ CsPbBr 3 , PMMA were accessible through multiexponential fittings to the time-resolved PL dynamics following the intensity-weighting calculation [36]:

(2) τ a v = ∑ A i τ i 2 ∑ A i τ i

The collected PL decays, measured at the emission peak wavelengths for all CsPbBr3 PeNCs complexes attached to PMMA and TiO2 are presented in Figure 2A–C. The time-resolved PL decays were well fitted by using a tri-exponential model (solid lines in Figure 2A–C). Table 1 summarizes the fitting parameters and the average lifetimes. Both of the two faster decay components, τ 1 and τ 2 on a time scale shorter than 10 ns, are assigned to trap-mediated charge recombination; and the slow component τ 3 longer than 10 ns is attributed to the intrinsic carrier radiative recombination [37]. For CsPbBr3 PeNCs with no TiO2, the incorporation of APTES and NCA ligands both exhibited remarkable suppression of the trap-mediated charge recombination, and consequently induced an elongated PL lifetime as compared with OA/OAm-CsPbBr PeNCs (Figure 2D). It suggests a significant surface passivation effect from APTES and NCA than traditional OA and OAm, in line with previously reported results [30].

Figure 2: (A–C) PL decays of OA/APTES-CsPbBr3, OA/OAm-CsPbBr3 and NCA/OAm-CsPbBr3 PeNCs attached to PMMA (green) and TiO2 (orange) films. Insets: schematic diagram presenting the capping ligand on CsPbBr3 PeNCs, (D) comparison of decay curves of all three CsPbBr3 PeNCs-ligand systems coated on PMMA.
Figure 2:

(A–C) PL decays of OA/APTES-CsPbBr3, OA/OAm-CsPbBr3 and NCA/OAm-CsPbBr3 PeNCs attached to PMMA (green) and TiO2 (orange) films. Insets: schematic diagram presenting the capping ligand on CsPbBr3 PeNCs, (D) comparison of decay curves of all three CsPbBr3 PeNCs-ligand systems coated on PMMA.

Table 1:

Multiexponential fitting parameters for PL decays of CsPbBr3 PeNCs on TiO2 and PMMA.

Sample τ 1 (ns) A 1 τ 2 (ns) A 2 τ 3 (ns) A 3 τ av (ns)
OA/APTES-PMMA 2.60 0.55 8.60 0.43 40.16 0.02 10.90
OA/APTES-TiO2 2.80 0.60 8.54 0.39 34.60 0.01 9.14
OA/OAm-PMMA 1.15 0.62 3.88 0.36 17.53 0.02 5.09
OA/OAm-TiO2 0.92 0.51 3.22 0.45 10.06 0.04 3.81
NCA/OAm-PMMA 2.67 0.69 7.41 0.29 41.83 0.02 11.60
NCA/OAm-TiO2 0.77 0.57 3.90 0.39 14.94 0.04 5.44

When CsPbBr3 PeNCs were assembled with the TiO2 substrate, the PL decayed faster than that on the PMMA insulting layer especially for the cases of OA/OAm- and NCA/OAm-capped PeNCs, indicating the occurrence of the ET to TiO2. We applied the donor–bridge–acceptor model in which the surface ligands act as a transition bridge between the nanoparticle and TiO2. Comparing to the alkyl chain OA/OAm bridge, the quenching of the PL decay was inconspicuous with the cross-linked APTES ligand-bridge, while it became evidently enhanced with the incorporation of the aromatic NCA ligand-bridge. Based on Equation (1), we calculated the ET rates which were 6.6 × 107, 1.8 × 107 and 9.7 × 107 s−1 for OA/OAm-CsPbBr3, OA/APTES-CsPbBr3 and NCA/OAm-CsPbBr3 PeNCs, respectively. The aromatic NCA ligands resulted in a boosted ET rate while the cross-linked APTES significantly suppressed the extraction of electrons by a factor of over threefolds, in comparison to the commonly used long alkyl chain ligands.

The configuration of CsPbBr3 PeNCs terminated with different surface ligands leads to an indirect contact interface between the nanoparticles and the electron-accepting TiO2. This was visualized by SEM measurements from which CsPbBr3 nanoparticles adsorbed onto the mesoporous scaffolding of TiO2 (See SEM images and elemental mapping images presented in Figure S4 and S5). As a result, the interparticle bridging molecules can be modeled as the potential barrier that electrons have to tunnel through in the ET process. We seek to further understand our experimentally determined k ET trends by modeling the lowest unoccupied molecular orbital (LUMO) level of the ligand-bridge as a barrier potential with respect to the conduction band (CB) of the nanoparticle [38] and the electron accepting level of TiO2 (Figure 3A) [39]. By using DFT calculations, the highest occupied molecular orbitals (HOMOs) and LUMOs of the four anchored surface ligands were compared. All LUMO levels place higher than the CB levels of both nanoparticle and TiO2, which confirms the potential barrier effect between the nanoparticle donor and TiO2 acceptor in our work. On the other hand, from the difference between LUMO and HOMO energy levels (Figure 3B), it can be inferred that the absorption peaks of APTES, OA, OAm and NCA are theoretically located at 196, 193, 197 and 291 nm, respectively. We indeed observed absorption peaks of these ligand molecules at 200, 199, 198 and 292 nm, respectively, which were in good consistency with the theoretical calculations. It suggests that the basis set and functional method chosen in our DFT calculation were reasonable.

Figure 3: (A) Schematic illustration of the ET process for CsPbBr3 PeNCs-Bridge-TiO2 system. (B) Energetic and spatial distribution of frontier orbits of ligand-bridge molecules APTES, OA, OAm and NCA. VB, valance band; CB, conduction band; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital.
Figure 3:

(A) Schematic illustration of the ET process for CsPbBr3 PeNCs-Bridge-TiO2 system. (B) Energetic and spatial distribution of frontier orbits of ligand-bridge molecules APTES, OA, OAm and NCA. VB, valance band; CB, conduction band; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital.

Both OA and OAm possess similar LUMOs. Crossed-linked APTES (dimer) exhibits the highest-lying LUMO energy level resulting in the highest potential among the four ligand species. Correspondingly, APTES leads to the slowest ET rate which was indeed experimentally observed. In contrast, NCA possesses the lowest barrier potential, consequently gave rise to the ET rate. Additionally, from the electron density distribution of HOMO and LUMO for NCA illustrated in Figure 3B, it shows a pronounced frontier orbital delocalization across the entire molecule as a result of the aromatic group; whereas the OA/OAm and APTES ligands both exhibit intensively localized frontier orbitals which are commonly insulating for charge transition. The former is more conductive and favorable to charge transition across the molecule, thus the resulted system shows a noticeably enhanced ET rate. The APTES exhibit more insulating property with high barrier potential, preventing electrons from extraction by TiO2. It should be addressed here that a dimer model of the cross-linked APTES was applied in the calculation to simulate the mild hydrolytic polymerization condition in our synthesis. In fact, more significantly high barrier potential is expected for heavily dense cross-linked APTES, which leads to further reduction in electron tunneling and subsequent transfer probability.

One concern may be raised regarding the possible delocalization of electrons caused by the NCA ligands as they exhibit low LUMO level. As discussed above, the carrier-delocalization is detrimental to the carrier extraction efficiency through the ligand-induced carrier trapping effect. We carried out ultrafast TA spectroscopy measurements to reveal the carrier delocalization effect.

Figure S6A–C presents the de-chirped TA spectra for OA/APTES-CsPbBr3, OA/OAm-CsPbBr3 and NCA/OAm-CsPbBr3 PeNC solution measured in 2 mm cuvettes, at different delay time following the 400 nm pulse excitation. Weak excitation power was delivered in order to circumvent multiexciton effect (see details in SI and Figure S8). At early stage, the spectra are dominated by a photoinduced bleach (PB) signal centered at 510 nm with two side-photoinduced absorption (PA) bands on both sides, which are consistent to the typical features of colloid CsPbBr3 PeNCs TA spectra [40], [41]. The PB peaks agree well with the absorption onsets denoted in Figure 1A, therefore it is assigned to the state-filling effects and depicts the ground-state depopulation of both electrons and holes, as for perovskite the effective masses of both carriers are similar [41]. The above-bandgap absorption feature PA1 in the region of 440–480 nm is likely from the forbidden optical transitions activated by excitons, and the fast-recovering absorption ranging from 520 to 550 nm (PA2) was induced by the local fields produced by photoexcited carriers [42]. Figure 4A presents a comparison of normalized PB kinetic traces of all three CsPbBr3 PeNC-ligand systems. Population, or the rising dynamics, of the PB feature at initial stage indicates the relaxation of excited electrons to the band-edge states in CsPbBr3 PeNCs. Delocalization ligands may provide a competing channel for excited electron relaxation which leads to faster rising of the PB signal. Whereas we indeed found a similar rising of the PB feature in all three systems, which shows no obvious carrier delocalization effect caused by NCA ligands. In addition, both the depopulation of PB signals for OA/APTES-CsPbBr3 and NCA/OAm-CsPbBr3 PeNCs were slower than OA/OAm-CsPbBr3 PeNCs, and again confirming obvious surface passivation effect from APTES and NCA than OA and OAm.

Figure 4: (A) Comparison of normalized PB kinetic traces of all three CsPbBr3 PeNC-ligand solution systems measured in 2 mm cuvettes. (B–D) Normalized transient PB signal kinetics for OA/APTES-CsPbBr3, OA/OAm-CsPbBr3 and NCA/OAm-CsPbBr3 PeNC films attached to PMMA and TiO2. The gray curve in (A) represents instrument response functions of the TA instrument.
Figure 4:

(A) Comparison of normalized PB kinetic traces of all three CsPbBr3 PeNC-ligand solution systems measured in 2 mm cuvettes. (B–D) Normalized transient PB signal kinetics for OA/APTES-CsPbBr3, OA/OAm-CsPbBr3 and NCA/OAm-CsPbBr3 PeNC films attached to PMMA and TiO2.

The gray curve in (A) represents instrument response functions of the TA instrument.

Meanwhile, TA measurements were also performed on CsPbBr3 PeNC films attached to PMMA and TiO2. Figure 4B–D shows the normalized PB kinetic traces of CsPbBr3 PeNCs. There was no obvious difference in the recovery of PB signals for CsPbBr3 PeNCs assembled with and without TiO2 as the electron acceptors. These results suggest that the ET process occurs on a relatively longer time scale, beyond the detection window of our TA measurements (500 ps). Furthermore, global analysis results of the film TA dynamics supplemented to the conclusion. As indicated by the decay-associated spectra shown in Figure S7, although the noises were non-negligible, three exponential components with different lifetimes were yielded for all samples. Consistently, OA/OAm CsPbBr3 PeNCs exhibit an overall fast decay as the poor passivation effect comparing to APTES and NCA. No obvious quenching of TA decays was observed in all CsPbBr3 PeNC-TiO2 systems indicating a relative long ET process.

4 Conclusion

In this work, we have investigated the ET process from CsPbBr3 PeNCs to TiO2 with single alkyl chain OA and OAm, cross-linked insulating APTES and aromatic ligands NCA as the linking bridge, respectively. It was found that the ET rates can be effectively tuned by anchored surface ligands, based on the ligand-bridge effect between PeNCs donor and oxide acceptor. By modeling the LUMO energy level of the studied ligand-bridge with respect to the conduction band of the nanoparticle and the electron accepting level of TiO2 as their potential barrier, it allows us to further understand experimentally determined ET rate k ET trends. Meanwhile, TA measurements excluded the possibility of trapping carrier on the ligands as a consequence of the exciton-delocalizing effect. Our demonstration on bridge-barrier effect of the surface ligands provides possible options of ligands for different purposes of PeNC-based photonic and optoelectronic applications.

Corresponding authors: Peng Zeng and Mingzhen Liu, School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China; and Center for Applied Chemistry, University of Electronic Science and Technology of China, Chengdu 611731, P. R. China, Email: ,
Xiaochun Liu and Haifeng Zhao are Co-first authors.

Funding source: China Postdoctoral Science Foundation

Award Identifier / Grant number: 2020M683281

Funding source: Fundamental Research Funds for the Central Universities of China

Award Identifier / Grant number: ZYGX2019J028

Funding source: National Natural Science Foundation of China

Award Identifier / Grant number: 61604032, 61905037

Funding source: National Key R&D Program of China

Award Identifier / Grant number: 2017YFA0207400

  1. Author contributions: All the authors have accepted responsibility for the entire content of this submitted manuscript and approved submission.

  2. Research funding: This work was funded by the National Key R&D Program of China (2017YFA0207400), the National Natural Science Foundation of China (61604032 and 61905037), and the Fundamental Research Funds for the Central Universities of China (ZYGX2019J028), China Postdoctoral Science Foundation (2020M683281).

  3. Conflict of interest statement: The authors declare no conflicts of interest regarding this article.

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Supplementary Material

The online version of this article offers supplementary material (https://doi.org/10.1515/nanoph-2020-0631).

Received: 2020-11-29
Accepted: 2021-01-21
Published Online: 2021-02-26

© 2021 Xiaochun Liu et al., published by De Gruyter, Berlin/Boston

This work is licensed under the Creative Commons Attribution 4.0 International License.

Artikel in diesem Heft

  1. Frontmatter
  2. Editorial
  3. Photonics for enhanced perovskite optoelectronics
  4. Perspective
  5. Prospects of light management in perovskite/silicon tandem solar cells
  6. Reviews
  7. Ultrafast dynamics of photoexcited carriers in perovskite semiconductor nanocrystals
  8. Silicon heterojunction-based tandem solar cells: past, status, and future prospects
  9. Photon recycling in perovskite solar cells and its impact on device design
  10. Advances in Dion-Jacobson phase two-dimensional metal halide perovskite solar cells
  11. Recent advancements and perspectives on light management and high performance in perovskite light-emitting diodes
  12. Quasi-2D lead halide perovskite gain materials toward electrical pumping laser
  13. Lead-free metal-halide double perovskites: from optoelectronic properties to applications
  14. Lead-free halide perovskite photodetectors spanning from near-infrared to X-ray range: a review
  15. Research Articles
  16. Ligand-modulated electron transfer rates from CsPbBr3 nanocrystals to titanium dioxide
  17. Exploring the physics of cesium lead halide perovskite quantum dots via Bayesian inference of the photoluminescence spectra in automated experiment
  18. Comparing optical performance of a wide range of perovskite/silicon tandem architectures under real-world conditions
  19. Efficient wide-bandgap perovskite solar cells enabled by doping a bromine-rich molecule
  20. Two-dimensional perovskites with alternating cations in the interlayer space for stable light-emitting diodes
  21. Hard and soft Lewis-base behavior for efficient and stable CsPbBr3 perovskite light-emitting diodes
  22. Tailoring the electron and hole dimensionality to achieve efficient and stable metal halide perovskite scintillators
  23. Remarkable photoluminescence enhancement of CsPbBr3 perovskite quantum dots assisted by metallic thin films
https://doi.org/10.1515/nanoph-2020-0631
Schlagwörter für diesen Artikel
bridge-barrier effect; CsPbBr3 nanocrystals; electron transfer; ligand-bridge
Creative Commons
BY 4.0

Artikel in diesem Heft

  1. Frontmatter
  2. Editorial
  3. Photonics for enhanced perovskite optoelectronics
  4. Perspective
  5. Prospects of light management in perovskite/silicon tandem solar cells
  6. Reviews
  7. Ultrafast dynamics of photoexcited carriers in perovskite semiconductor nanocrystals
  8. Silicon heterojunction-based tandem solar cells: past, status, and future prospects
  9. Photon recycling in perovskite solar cells and its impact on device design
  10. Advances in Dion-Jacobson phase two-dimensional metal halide perovskite solar cells
  11. Recent advancements and perspectives on light management and high performance in perovskite light-emitting diodes
  12. Quasi-2D lead halide perovskite gain materials toward electrical pumping laser
  13. Lead-free metal-halide double perovskites: from optoelectronic properties to applications
  14. Lead-free halide perovskite photodetectors spanning from near-infrared to X-ray range: a review
  15. Research Articles
  16. Ligand-modulated electron transfer rates from CsPbBr3 nanocrystals to titanium dioxide
  17. Exploring the physics of cesium lead halide perovskite quantum dots via Bayesian inference of the photoluminescence spectra in automated experiment
  18. Comparing optical performance of a wide range of perovskite/silicon tandem architectures under real-world conditions
  19. Efficient wide-bandgap perovskite solar cells enabled by doping a bromine-rich molecule
  20. Two-dimensional perovskites with alternating cations in the interlayer space for stable light-emitting diodes
  21. Hard and soft Lewis-base behavior for efficient and stable CsPbBr3 perovskite light-emitting diodes
  22. Tailoring the electron and hole dimensionality to achieve efficient and stable metal halide perovskite scintillators
  23. Remarkable photoluminescence enhancement of CsPbBr3 perovskite quantum dots assisted by metallic thin films
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