Colloidal Synthesis and Optical Properties of All-inorganic Cs₂CuCl₄ Nanocrystals

1. Introduction

Lead halide perovskites NCs with a general formula of APbX₃ (A=Cs, methylammonium and formamidinium, X=Cl, Br, I) have attracted great attention owing to their excellent photoelectric properties including high PLQY, high exciton binding energy, tunable PL emission wavelength and solution processability. They have shown great potentials in a variety of optoelectronic applications, such as solar cells [1], light-emitting diodes [2] and lasers [3]. At present, the external quantum efficiencies (EQEs) of red and green perovskite light-emitting diodes (PeLEDs) have exceeded 20%, reaching the industrial standard, but the EQE of blue PeLEDs is still lower than red and green PeLEDs [4,5,6,7]. This is mainly due to the deep level defects originated from the large band gap of blue-emitting materials.

Blue PeLEDs have been progressing significantly during the past few years. Zeng et al [8] first fabricated blue PeLEDs by using CsPb(Cl/Br)₃ NCs as blue emitters. However, the EQE was only 0.07% due to intrinsic phase instabilities of mixed-halide perovskites. Subsequently, Bakr et al [9] used n-dodecylammoniumthiocyanate (DAT) as a surface passivator to improve the PLQY of CsPb(Cl/Br)₃ NCs close to 100%, and therefore PeLEDs with EQE up to 6.3% were achieved. Although defects can be reduced through optimizations of synthesis conditions, they can still easily form during crystallization due to their low formation energy in chloride-based perovskite NCs. To increase the defect formation energy, metal doping has been shown as an effective strategy to obtain high quality perovskite NCs [10,11,12]. For example, Ni doping can improve the short-range order of the perovskite lattice of CsPbCl_xBr_3-x NCs, and an EQE of 2.4% was obtained [10]. Mn ion is another effective dopant to increase the PLQY of perovskite NCs. Hou et al [11] carefully adjusted the concentration of Mn doping in CsPbCl_xBr_3-x NCs to increase the PLQY by over threefold while the pure emission at 470 nm was maintained. As a result, the EQE of blue LEDs reached up to 2.1%.

Despite of the rapid development of blue PeLEDs based on lead halide perovskites, their further applications are hindered by the toxicity of lead. Therefore, lots of efforts have been devoted to reduce or substitute lead with less or nontoxic metals. For example, Sb³⁺ and Bi³⁺ ions can form Cs₃M(III)₂X₉ (M=Sb or Bi) structures [13,14,15]. These perovskites generally exhibit deep blue emission and high air stability. In addition, Ag⁺ have been shown to be able to improve the PLQY of Cs₃Bi₂Br₉ NCs through reducing the surface trap density [14]. Cu is another promising alternative of lead by forming Cu based perovskites [16,17,18]. Deep blue PeLEDs based on zero-dimensional (0D) Cs₃Cu₂I₅ NCs have shown a high EQE of 1.12%, which was comparable to the best-performing deep blue PeLEDs based on lead halide perovskites [16].

However, the development of chloride based deep blue lead free PeLEDs is still far behind Br/I based green/red PeLEDs [19]. Recently, Yang et al [20] firstly reported the synthesis of Cs₂CuX₄ (X=Cl, Br, and Br/I) spherical quantum dots with blue-green emission by using an improved ligand-assisted reprecipitation technique at room temperature. By changing the halide composition and precursor ratios, the emission peak can be tuned from 385 nm to 504 nm. In order to solve the low solubility of CsBr and CsCl in polar solvents, Kar et al [21] used water as a solvent to synthesize square-shaped Cs₂CuCl₄ nanoplates with deep blue PL at 434 nm. Although the synthesis and optical properties of cesium copper(II) halide NCs have been reported, the preparation of colloidal Cs₂CuCl₄ NCs with high quality remains a challenge. Herein, we developed a synthesis of 0D Cs₂CuCl₄ NCs and examined its optical properties. The colloidal Cs₂CuCl₄ NCs have uniform size and excellent optical properties. We find that Cs₂CuCl₄ NCs have a wide band gap of 4.36 eV and show bright deep blue PL at 434 nm with a PLQY of 28.8% at room temperature. In addition, we show that the PLQY of Cs₂CuCl₄ NCs can be greatly improved to 42% by Ag⁺ passivation. The air stability of Ag⁺ treated Cs₂CuCl₄ NCs were also greatly improved. After 15-days storage in air (average temperature 25℃, humidity 50%), 75% of their initial PLQY was retained.

2. Materials and Methods

2.1. Materials

Cesium acetate (Cs(OAc), 99.9%), Copper(II) acetate (Cu(ac)₂, 99.9%), silver acetate (Ag(ac), 99.99%), benzoyl chloride (Bz-Cl, 98%) and benzoyl bromine (Bz-Br, 99%) were purchased from Aladdin. 1-octadecene (ODE, 90%), oleylamine (OLA, 70%) and oleic acid (OA, 90%) were purchased from Sigma-Aldrich. All the chemicals and solvents were used without further purification.

2.2. Preparation of Ag-OLA solution

Ag(ac) (25 mg) was loaded into a 50 mL three-neck flask along with ODE (10 mL) and OLA (1 mL). The mixture was degassed for 0.5 h at 30 ℃ until the Ag(ac) dissolved completely.

2.3. Synthesis of Cs₂CuCl₄ NCs

In a typical synthesis, Cs(OAc) (13.4 mg), Cu(ac)₂ (18.2 mg), OA (1 ml), OLA (0.5 ml) and ODE (5 ml) were mixed in a 50 mL flask and dried for 30 min under vacuum at 100 °C. Then the reaction flask was heated to 120 ℃ in N₂ atmosphere, when 48 ul Bz-Cl dispersed in 0.5 ml of degassed ODE were injected inside the flask. After 10 s, the solution was cooled down using an ice-water bath. The resulting mixtures of Cs₂CuCl₄ NCs were centrifuged for 5 min at 8000 rpm. The precipitate was redissolved in hexane and centrifuged at 8000 rpm for 5 min to remove undissolved species. The final supernatant was collected for further analysis.

2.4. Synthesis of the Ag passivation reagent

For absorption and PL spectra of NCs before and after Ag passivation, the as-prepared Cs₂CuCl₄ NCs (200 μL) were mixed with hexane (3 mL) in the cuvette, and then different amounts of Ag-OLA solution were injected and stirred for 2 min.

3. Results and Discussion

In this approach, Cs₂CuCl₄ NCs were synthesized using a modified hot-injection method. Figure 1a shows the crystal structure of Cs₂CuCl₄, where isolated tetrahedral CuCl₄^2- unit are separated by the surrounding Cs⁺ ions. Working with Cs/Cu/Cl molar ratio of 0.7/1/4, a pure Cs₂CuCl₄ phase was observed by X-ray diffraction (XRD) analysis. The corresponding XRD pattern (Figure 1b) confirming that the obtained sample Cs₂CuCl₄ NCs has orthorhombic crystal system. Figure 1c show a typical large scale transmission electron microscopy (TEM) image indicate a uniform size distribution of the as-prepared colloidal Cs₂CuCl₄ NCs. The Cs₂CuCl₄ NCs are cubic structure with an average size of 24 ± 2.1 nm.

The UV/Vis absorption spectrum of Cs₂CuCl₄ NCs are presented in Figure 1d. The Cs₂CuCl₄ NCs shows a strong excitonic absorption peak at around 268 nm. Besides, the band gap of the Cs₂CuCl₄ NCs was measured by using the direct band gap tauc plot in Figure 1e, which shows 4.36 eV band gap for Cs₂CuCl₄ NCs. The PL spectrum (Figure 1d) of deep blue Cs₂CuCl₄ NCs exhibit emission peak at 434 nm (PLQY=28.8%) with a full width at half-maximum (FHWM) of roughly 58 nm, which matched with Cs₂CuCl₄ nanoplates [21]. The inset shows that Cs₂CuCl₄ NCs has excellent luminescence performance that strong deep blue emission under ultraviolet irradiation. The PL excitation (PLE) spectrum (monitoring emission at 434 nm) had a peak at 270 nm. The PLE spectrum matched with the exciton peak in the absorption spectrum, showing an apparent exciton characteristic of Cs₂CuCl₄ NCs. The luminescence origin of NCs was studied by monitoring the PL spectra at different excitation wavelengths. The Figure 1f showed that PL spectra excited at different wavelength have the same characteristics, indicating that the emission originates from the same excited state relaxation.

We employed X-ray photoelectron spectroscopy (XPS) to examined the valence state of Cu in the Cs₂CuCl₄ NCs. Figure 2a displays the XPS survey spectrum in the entire binding energy region of the aggregated Cs₂CuCl₄ NCs, confirming the presence of Cs⁺, Cu²⁺ and Cl^- elements at the surface. The satellite peak in the narrow scan of Cu 2p spectrum in Figure 2c indicates the presence of Cu²⁺ ions on the surface of Cs₂CuCl₄ NCs [23].

The injection temperature is critical to the successful synthesis of Cs₂CuCl₄ NCs. Strong reducibility of oleylamine can easily reduce Cu(II) to Cu(I), while chlorine helps to stabilize the oxidation state of Cu²⁺, which was proved by XPS spectra [22]. However, when benzoyl bromine is used as halide precursor, the presence of bromine will promote the reduction of Cu²⁺ to Cu⁺, then form Cs₃Cu₂Br₅ instead of Cs₂CuBr₄ with a peak position of 454 nm [18] (Figure S1). Although high injection temperature can reduce the surface defects of NCs, the Cu(II) will be reduced to Cu(I) by oleylamine when the temperature is too high. Therefore, the highest PLQY was obtained by optimizing the experimental scheme to get an optimal injection temperature. In Figure 3a and Figure 3b, the optimal injection temperature is found to be 120℃ by comparing the UV/Vis absorption spectrum and PL spectra of Cs₂CuCl₄ NCs synthesized at different injection temperatures.

Beside temperature, the molar ratio of Cs and Cu precursors as well as the ratio of OA and OLA are also important parameters for the synthesis of high-quality Cs₂CuCl₄ NCs. With Cs/Cu molar ratio of 0.7:1, we observed the formation of pure phase of Cs₂CuCl₄ NCs with narrow size distribution and high PLQY (Figure 3c-d). However, a molar ratio of 0.6:1 or 0.8:1 lead to reduced PLQY of Cs₂CuCl₄ NCs due to the presence of CsCl impurity phase, which was confirmed by the XRD pattern in Figure 1b. In addition, the ratio between OA and OLA can also affect the surface defect and stability of Cs₂CuCl₄ NCs. We found the optimal amount of OA and OLA was 1.0 ml and 0.5 ml, respectively (see Experimental section), which resulted in the lowest surface defects density of Cs₂CuCl₄ NCs (Figure 3f). The presence of OA and OLA ligands were further confirmed by Fourier infrared spectroscopy (FITR) (Figure S2) [21]. The peak at 1470 cm^-1 represents the COO^- stretching vibration mode of OA. The peak at 1720 cm^-1 represents the asymmetric vibration mode of OA. The peak at 2855 cm^-1, 2905 cm^-1 represents the vibration due to the stretching of the C-H bonds of -CH₃ and -CH₂ in the aliphatic hydrocarbon chain. The peak at 1624 cm^-1 represents the bending vibration of N-H scissors of -NH₂ group in OLA. The above results represent the stable existence of OA and OLA ligands on Cs₂CuCl₄ NCs surface.

Passivation of NCs with Ag to reduce surface defects is an effective method to improve PLQY. For example, Li et al [23] treated CsPbBr₃ NCs using Ag-trioctylphosphine complex to reduce surface defects and improve the PLQY of CsPbBr₃ NCs significantly. Here, the Ag-OLA solution was added for surface passivation treatment of Cs₂CuCl₄ NCs. We found a small amount of Ag-OLA solution can effectively bound chloride on the Cs₂CuCl₄ NCs surface, reducing the surface traps caused by the loss of unstable ammonium chloride. Different amounts of Ag-OLA solution were added into 3 mL Cs₂CuCl₄ NCs colloidal solution, and the absorption (Figure 4a) and PL spectra (Figure 4b) were measured 2 min later. Compared with untreated sample, the absorbance of Ag treated Cs₂CuCl₄ NCs increased slightly, but the peak position remained unchanged for different amounts of Ag-OLA. The PLQY of Ag-treated sample reached maximum value of 42% with adding 15 ul Ag-OLA, which is nearly 70% higher than that of untreated sample. In addition, we studied the air stability of Ag-treated Cs₂CuCl₄ NCs in ambient environment (average temperature 25℃, humidity 50%). As shown in Figure 4d, the PLQY of Ag-treated Cs₂CuCl₄ NCs maintained ~75% of their initial PLQY after 15 days storage period, while the PLQY of untreated NCs decreased to 1% of their initial value, demonstrating the excellent stability of the Ag-treated Cs₂CuCl₄ NCs. Importantly, as shown in Figure S3, individual OLA cannot increase the fluorescence intensity of Cs₂CuCl₄ NCs, proving that Ag plays a key role in enhancing the PLQY of Cs₂CuCl₄ NCs. The existence of Ag on the surface of Cs₂CuCl₄ NCs was verified by XPS shown in Figure 4e, where the Ag peaks can be clearly seen from the Ag-treated Cs₂CuCl₄ NCs. Figure 4f indicating that Ag exists on the surface of Cs₂CuCl₄ NCs in the form of Ag (I) in the oxidation state. The Ag-treated NCs present regular cubic shape and size distributions at Figure 4g, with little change compared with the untreated ones.

The origin of bright blue emission of Cs₂CuCl₄ NCs can be attributed to the Cu ion induced traps. Manna et al [24] found doping Cu⁺ into Cs₂ZnCl₄ NCs can achieve bright blue emission due to that the Cu(I) ions can promote the formation of trapped excitons. In addition, Xu et al [25] proposed that the emission from Cu-doped ZnO nanorods originates from Cu(II) defect. Therefore, we propose the schematic model of the luminescence mechanism of Cs₂CuCl₄ NCs in Figure S4. Under UV light excitation, electrons transition from the ground state to the photoexcited state, and then undergo a non-radiative compound transition to the Cu²⁺ defect state. The electrons at the defect level transition back to the ground state as a radiative recombination, exhibiting a broad spectrum of deep blue luminescence.

4. Conclusions

We successfully synthesized pure phase Cs₂CuCl₄ NCs with well-defined shapes via a modified hot-injection synthesis strategy. With optimized injection temperature, precursor and ligand ratios, the Cs₂CuCl₄ NCs showed high PLQY (28.8%) in the deep blue spectrum at 434 nm. The PLQY and stability of Cs₂CuCl₄ NCs can be further enhanced through Ag treatment, where Ag-treated Cs₂CuCl₄ NCs exhibit higher PLQY (42%) and better air stability in ambient environment. This work provides a useful strategy for the synthesis of Cu(II) based metal halide perovskite NCs, which are promising materials to reduce the toxicity and realize practical application of perovskite NCs for display and lighting devices.