Colloidal Synthesis and Optical Properties of Cs₂CuCl₄ Nanocrystals

Abstract

Lead-free copper halide perovskite nanocrystals (NCs) are emerging materials with excellent photoelectric properties. Herein, we present a colloidal synthesis route for orthorhombic Cs₂CuCl₄ NCs with a well-defined cubic shape and an average diameter of 24 ± 2.1 nm. The Cs₂CuCl₄ NCs exhibited bright, deep blue photoluminescence, which was attributed to the Cu(II) defects. In addition, passivating the Cs₂CuCl₄ NCs by Ag⁺ could effectively improve the photoluminescence quantum yield (PLQY) and environmental stability.

1. Introduction

Lead halide perovskite nanocrystals (NCs) with a general formula of APbX₃ (A = Cs, methylammonium or formamidinium, X = Cl, Br or I) have attracted a lot of attention for a variety of optoelectric applications such as solar cells [1], light-emitting diodes (LEDs) [2], lasers [3], photocatalysis [4,5] and photodetectors [6]. They have many advantages, including low-temperature solution processability, tunable direct bandgap, high optical absorption coefficients, high defect tolerance, long carrier lifetimes and diffusion length [7,8,9,10]. Their properties can be conveniently tuned by changing the A site anions or X site cations. Lead perovskites, especially perovskite LEDs (PeLEDs), are widely studied due to their advantages of high color purity, wide color gamut, high efficiency and low cost [11,12,13,14,15]. At present, the external quantum efficiencies (EQEs) of red and green PeLEDs have exceeded 20%, reaching the industrial standard. However, the EQEs of blue PeLEDs are lower than red and green PeLEDs [16,17,18,19,20,21,22], which is mainly due to the deep level defects originating from the large band-gap of blue-emitting perovskite NCs, and the highest EQE (12.3%) of blue PeLEDs was achieved by growing a bipolar shell to strongly confine CsPbBr₃ NCs and reduce the trap density [22].

Despite the rapid development of blue PeLEDs, their further applications are hindered by the inherent toxicity of the metal element lead [23]. Therefore, a lot of effort has been devoted to reducing or substituting lead with less- or nontoxic metals. For example, using Sn²⁺ or Ge²⁺ to substitute Pb²⁺ and form CsSnX₃ or CsGeX₃ NCs has been widely studied [24,25]. However, the stability of this technique is relatively low due to the Sn²⁺ or Ge²⁺ being easily oxidized into Sn⁴⁺ or Ge⁴⁺ in air and generating defects in Sn/Ge based perovskites [26,27,28]. Using isoelectronic Bi³⁺ or Sb³⁺ to replace Pb²⁺ is another approach. For example, Sb³⁺ and Bi³⁺ ions can form Cs₃M(III)₂X₉ (M = Sb or Bi) structures [29,30,31]. These perovskites generally exhibit deep blue emission and high air stability, and Ag⁺ has been shown to be able to improve the PLQY of Cs₃Bi₂Br₉ NCs through reducing the surface trap density [30]. In addition, combining a univalent ion with a trivalent to replace two Pb ions can form double perovskites Cs₂M(I)M(III)X₆ (M⁺ = Ag⁺, Na⁺; M³⁺ = In³⁺, Bi³⁺; X = Cl, Br, I), which are promising alternative lead-free materials. However, due to their inherent indirect band gap property, their PLQYs are lower than lead halide perovskites [32,33,34,35]. Compared with Cs₂M(I)M(III)X₆ perovskites, the vacancy-ordered layered double perovskites with a formula of Cs₄M(II)M(III)₂X₁₂ (M(II) = Cu²⁺, Mn²⁺, etc.) have the advantages of a highly tunable composition, direct band gap and outstanding structural stability [36,37,38,39]. They consist of a layer of [M(II)X₆]⁴⁻ octahedrons sandwiched between two layers of [M(III)X₆]³⁻ octahedrons. Cs₄CuSb₂Cl₁₂ NCs with a direct band gap of 1.79 eV has been successfully synthesized using the hot injection method [36]. The Cs₄CuSb₂Cl₁₂ NCs can be used as high-speed photodetectors with ultra-fast photo response and narrow bandwidth due to their superior charge transport characteristics and solution processability [36]. Similarly, Cs₄CuIn₂Cl₁₂ NCs were synthesized with the water-assisted hot injection method [37]. However, they generally exhibited low PLQY due to the strong self-trapping exciton effect originating from their layered structure [39].

Cu-based perovskite NCs are another important group of materials for replacing lead perovskites due to their abundant resources and low toxicity. Due to the relatively small ionic radius of Cu, substituting Pb with Cu tends to form zero-dimensional structures [40]. Ternary Cu halide perovskites are another group of environmentally friendly materials due to their low cost, high PL intensity and adjustable emission wavelength [41,42,43,44,45,46]. Because of the two valence states and low coordination number of Cu ions, the ternary Cu halide perovskites can form different crystal structures, including Cs₃Cu₂X₅(X = Cl, Br, I), CsCu₂I₃, A₂CuX₃(A = K, Rb; X = Cl, Br, I) and Cs₂CuX₄. Cs₃Cu₂X₅(X = Cl, Br, I) NCs and CsCu₂I₃, for example, have been widely studied for their excellent optical properties. In 2019, Han et al. synthesized CsCu₂I₃ nanorods (NRs) and Cs₃Cu₂I₅ NCs using the hot injection method at different reaction temperatures [41]. The Cs₃Cu₂I₅ NCs synthesized at 70 °C exhibit bright blue emissions at 441 nm with a PLQY of 67%, while the CsCu₂I₃ NRs exhibit yellow emissions at 553 nm with a PLQY of 5%. Deep blue PeLEDs based on Cs₃Cu₂I₅ NCs have shown a high EQE of 1.12% [42]. In contrast to lead halide perovskites, replacing I with Br and Cl tends to redshift the emission wavelength of Cs₃Cu₂X₅ NCs, and Cs₃Cu₂Br₅ and Cs₃Cu₂Cl₅ NCs exhibit bright blue and green emissions at 458 nm and 521 nm, respectively [43]. However, their poor air stability significantly limits their further applications [43]. Li et al. [44] studied the effects of reaction temperature and ligand concentration on the structure of ternary Cu halide perovskites. Since the complexes formed by Cu⁺ and OA or OLA can exhibit linear, triangular or tetrahedral shapes with different barrier potentials, the crystal structure of NCs is determined by the combined effect of reaction temperature and ligand concentration. High reaction temperatures and low ligand concentration are conducive to the formation of Cs₃Cu₂I₃, while low reaction temperatures and high ligand concentration are conducive to the formation of Cs₃Cu₂I₅. In 2021, White et al. [45] reported the synthesis of Rb₂CuX₃ (X = Cl, Br) colloidal NCs with PL peaks around 400 nm using ligand assisted reprecipitation at room temperature. Ealpert et al. [46] also synthesized K₂CuX₃ NCs with near UV to deep blue emission with the ligand-assisted reprecipitation method, and the PLQY of K₂CuCl₃ also reached near 100%.

Cs₂CuCl₄ is a recently studied material with excellent blue emission properties [47,48,49,50]. The synthesis of Cs₂CuX₄ (X = Cl, Br, and Br/I) spherical quantum dots with blue-green emission was first reported by Yang et al. [48] 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. [49] used water as a solvent to synthesize square-shaped Cs₂CuCl₄ nanoplates with deep blue PL at 434 nm. Compared with the ligand-assisted reprecipitation technique, the hot injection method could better control the reaction process and achieve NCs with a more uniform crystal size and high crystal quality [8]. Using the hot injection method, Booker et al. [50] synthesized mixed phases of CsCuCl₃ and Cs₂CuCl₄ NCs with bright green emissions. At present, the synthesis of uniform and pure Cs₂CuCl₄ NCs with the hot injection method has not been reported.

Herein, we developed the synthesis of pure phase Cs₂CuCl₄ NCs and examined its optical properties. The colloidal Cs₂CuCl₄ NCs had a uniform size and excellent optical properties. We found that Cs₂CuCl₄ NCs have a wide band gap of 4.36 eV and show a bright, deep blue PL at 434 nm with a PLQY of 28.8% at room temperature. In addition, we show here that the PLQY of Cs₂CuCl₄ NCs can be greatly improved to 42% with Ag⁺ passivation. The air stability of Ag⁺-treated Cs₂CuCl₄ NCs were also greatly improved. After 15 days’ storage in air (average temperature 25 °C, 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 (Shanghai, China). The 1-octadecene (ODE, 90%), oleylamine (OLA, 70%) and oleic acid (OA, 90%) were purchased from Aldrich (Taufkirchen, Germany). 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 °C 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 °C in N₂ atmosphere, when 48 µL 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× g rpm. The precipitate was redissolved in hexane and centrifuged at 8000× g 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 UV-Vis 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.

2.5. Characterizations

UV−Vis absorption spectra were characterized with a UV 2310-II spectrophotometer (Tianmei, Shanghai, China). The PL spectra were collected with a F-380 spectro-fluorometer (Gangdong, Tianjin, China). Absolute PLQY values were checked with an FS5 Spectrofluorometer equipped with an integrating sphere (Edinburgh Instruments, Livingston, England). Transmission electron microscopy (TEM) images were acquired with a JEM-2100 with an accelerating voltage of 200 kV (JEOL, Tokyo, Japan). X-ray diffraction (XRD) characterization was achieved with a D8 Focus X-ray diffractometer (Bruker, Billerica, MA, USA). X-ray photo-electron spectroscopy (XPS) was conducted on a QUANTERA-II SXM spectrometer (ULVAC-PHI, Maozaki, Japan). Fourier transform infrared spectroscopy (FTIR) was conducted on a TG209F1 spectrophotometer (Netzsch, Selb, Germany).

3. Results and Discussion

In this approach, Cs₂CuCl₄ NCs were synthesized using a modified hot-injection method. The optical properties of the synthesized materials were compared with those of some perovskite NCs, as shown in Table S1. Figure 1a shows the crystal structure of Cs₂CuCl₄, where isolated tetrahedral CuCl₄²⁻ units were separated by the surrounding Cs⁺ ions. Figure 1b shows the XRD data of samples synthesized with different Cs:Cu precursor ratios (0.6:1, 0.7:1 and 0.8:1). We found that a Cs: Cu ratio of 0.7:1 produced pure Cs₂CuCl₄ (PDF#71-09-01). The diffraction peaks at 20.6°, 21.6°, 22.8°, 23.4°, 24.5°, 28.3°, 28.9°, 30.7°, 33.3°, 35.1°, 37°, 42.6° and 57° corresponded to the (121), (201), (211), (130), (031), (230), (122), (311), (132), (330), (232), (332) and (541) crystal planes of orthorhombic Cs₂CuCl₄, respectively. When the ratio of the metal precursor was slightly changed to 0.6:1 and 0.8:1, additional peaks appeared in the XRD data at 43.9°, 49.4° and 54.5°, corresponding to (200), (210) and (211) of CsCl (PDF# 05-0607). This was similar to previously reported CsCl impurities in Cs₃Cu₂Cl₅ NCs [43]. Figure 1c shows a typical large-scale transmission electron microscopy (TEM) image indicating a uniform size distribution of the as-prepared colloidal Cs₂CuCl₄ NCs. The Cs₂CuCl₄ NCs had cubic structures with an average size of 24 ± 2.1 nm.

The UV-Vis absorption spectra of Cs₂CuCl₄ NCs are presented in Figure 1d. The Cs₂CuCl₄ NCs showed a strong excitonic absorption peak at around 268 nm. In addition, the band gap of the Cs₂CuCl₄ NCs was measured by using the direct band gap tauc plot in Figure 1e, which shows a 4.36 eV band gap for Cs₂CuCl₄ NCs. The PL spectrum (Figure 1d) of deep blue Cs₂CuCl₄ NCs exhibited an 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 [49]. The inset shows that Cs₂CuCl₄ NCs had excellent luminescence performances of strong deep blue emissions 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. Figure 1f shows that PL spectra excited at different wavelength had the same characteristics, indicating that the emission originated from the same excited state relaxation.

We employed X-ray photoelectron spectroscopy (XPS) to examine 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 the Cu 2p spectrum in Figure 2c indicated the presence of Cu²⁺ ions on the surface of Cs₂CuCl₄ NCs [51].

The injection temperature is critical to the successful synthesis of Cs₂CuCl₄ NCs. Although a high injection temperature can reduce the surface defects of NCs, it can also reduce the Cu(II) to Cu(I) through oleylamine and produce impurity phases [51]. Therefore, the highest PLQY was obtained by optimizing the experimental scheme to obtain an optimal injection temperature. The UV-Vis and PL spectra of different Cs₂CuCl₄ NCs synthesized between 110 °C and 140 °C were compared in Figure 3a,b at the same concentration. The exciton absorption peak was highest for NCs synthesized between 120 °C and 130 °C, and the PL intensity was highest at 120 °C. These results indicate that the optimal injection temperature was 120 °C. We also examined the UV-Vis and PL spectra of Cs₂CuCl₄ NCs synthesized with different Cs:Cu precursor ratios (Figure 3c,d). The highest exciton absorption peak and PL intensity were obtained from the sample synthesized with a Cs:Cu ratio of 0.7:1, indicating the best crystal quality of Cs₂CuCl₄ NCs. This is consistent with the XRD result shown in Figure 1b, where other Cs:Cu ratios resulted in more CsCl impurities.

In addition, the ligands also play an important role in the synthesis of NCs. We found that the ratio between OA and OLA can affect the surface defects and stability of Cs₂CuCl₄ NCs. Figure 3e shows the UV-Vis spectrum of the sample with different OA:OLA ligand ratios (0.5 mL:0.5 mL, 1.0 mL:0.5 mL; 0.5 mL:1.0 mL and 1.0 mL:1.0 mL). Both the exciton absorption peak and PL intensity were lowest when OA = 0.5 mL and OLA = 1 mL. Increasing the amount of OA and reducing OLA could significantly increase the absorption and PL intensity. This is due to excessive OLA being able to promote the reduction of Cu(II) to Cu(I) at a high temperature, while an adequate OA ligand can reduce the defect density on the Cs₂CuCl₄ NCs surface. Therefore, in order to obtain Cs₂CuCl₄ NCs with the highest crystal quality, the amount of ligand should be controlled to OA = 1.0 mL and OLA = 0.5 mL.

The presence of OA and OLA ligands was further confirmed with Fourier transform infrared spectroscopy (FTIR) (Figure S1) [49]. The peak at 1470 cm⁻¹ represented the COO⁻ stretching vibration mode of OA. The peak at 1720 cm⁻¹ represented the asymmetrical vibration mode of OA. The peaks at 2855 cm⁻¹ and 2905 cm⁻¹ represented 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 represented the bending vibration of N-H scissors of the -NH₂ group in OLA. The above results represent the stable existence of OA and OLA ligands on the Cs₂CuCl₄ NCs surface.

We also investigated the effects of different halide precursors on the synthesis of Cs₂CuCl₄ NCs. Bz-X and TMS-X (X = Cl, Br, I) are commonly used halide precursors for lead halide perovskites. For example, Manna et al. [52] used Bz-X(X = Cl, Br, I) to synthesize APbX₃ NCs. TMS-X (X = Cl, Br, I) were also reported for the synthesis of double perovskite NCs [53]. Therefore, we focused on Bz-Cl and TMS-Cl for the synthesis of Cs₂CuCl₄ NCs. Due to the poor reactivity of TMS-Cl, the general reaction temperature was as high as 180 °C. In contrast, Bz-Cl had a higher reactivity and only needed 120 °C for the synthesis of NCs. Comparing the UV-Vis absorption and PL spectra of samples synthesized using different Cl precursors (Figure S2), Bz-Cl had a higher absorption and PL intensity than TMS-Cl. Therefore, Bz-Cl was selected as the halide precursor to form pure phase Cs₂CuCl₄ NCs with bright deep blue emissions. In addition to Cs₂CuCl₄ NCs, we also tested the synthesis of Cs₂CuBr₄ NCs. However, when using benzoyl bromine as the halide precursor, the presence of bromine will promote the reduction of Cu(II) to Cu(I), then form Cs₃Cu₂Br₅ instead of Cs₂CuBr₄. The characterization of Cs₃Cu₂Br₅ NCs are summarized in Figure S3. The Cs₃Cu₂Br₅ NCs showed a strong excitonic absorption peak at around 270 nm and a PL emission peak at 454 nm. The XPS measurement of the Cu 2p spectrum in Figure S3e indicated the presence of Cu(I) ions in Cs₃Cu₂Br₅ NCs [43].

The passivation of NCs with Ag to reduce surface defects is an effective method to improve the PLQY and stability of perovskite NCs [30,54]. For example, Li et al. [54] used the Ag-trioctylphosphine (Ag-TOP) complex to reduce surface defects and improve the PLQY and stability of CsPbBr₃ NCs significantly. However, TOP cannot coexist with Cu(II) due to its strong reducibility at room temperature [55]. Therefore, we developed a simple and effective method for the passivation of Cs₂CuCl₄ nanocrystals by combining olylamine with silver acetate to form an olylamine silver complex (Ag-OLA) as the passivation agent. We found that a small amount (μL) of Ag-OLA solution could effectively bind 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 a 3 mL Cs₂CuCl₄ NCs colloidal solution, and the absorption (Figure 4a) and PL spectra (Figure 4b) were measured 2 min later. Compared with the untreated sample, the absorbance of Ag-treated Cs₂CuCl₄ NCs increased slightly, but the peak position remained unchanged for different amounts of Ag-OLA solution. The PL intensity of Cs₂CuCl₄ NCs first increased as the amount of Ag-OLA solution increased due to the surface passivation effect. The PLQY reached the highest value of 42% with 15 μL Ag-OLA, which was nearly 70% higher than that of the untreated sample. Adding more Ag-OLA eventually led to a lower PLQY, owing to the formation of AgCl, which damaged the surface of Cs₂CuCl₄ NCs. Note that the treatment was applied at room temperature, therefore the Ag ions were unlikely to dope inside Cs₂CuCl₄ NCs. This is also supported by the unchanged PL peak position, demonstrating that no Cu-Ag alloyed NCs were formed [56].

In addition, we studied the air stability of Ag-treated Cs₂CuCl₄ NCs in an ambient environment (average temperature 25 °C, humidity 50%). As shown in Figure 4d, Ag-treated Cs₂CuCl₄ NCs maintained ~75% of their initial PLQY after a 15-day storage period, while the PLQY of untreated NCs decreased to 1% of their initial value, demonstrating the excellent air stability of the Ag-treated Cs₂CuCl₄ NCs. Importantly, as shown in Figure S4, individual OLA could not increase the PL 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, as shown in Figure 4e, where the Ag peaks can be clearly seen from the Ag-treated Cs₂CuCl₄ NCs. Figure 4f indicates that Ag existed on the surface of Cs₂CuCl₄ NCs in the form of Ag (I) in the oxidation state. The Ag-treated NCs present a regular cubic shape and size distributions at Figure 4g, with little change compared with the untreated ones.

The origin of the bright blue emission of Cs₂CuCl₄ NCs can be attributed to the Cu ion induced traps. Manna et al. [57] found that doping Cu⁺ into Cs₂ZnCl₄ NCs can achieve a bright blue emission due to the Cu(I) ions promoting the formation of trapped excitons. In addition, Xu et al. [58] proposed that the emissions from Cu-doped ZnO nanorods originated from a Cu(II) defect. Therefore, we propose the schematic model of the luminescence mechanism of Cs₂CuCl₄ NCs in Figure S5. 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. The effects of injection temperature, the Cu:Cs metal precursor ratio, the OA:OLA ligand ratio and the halide precursors were studied systematically. Through optimizing the synthesis conditions, the Cs₂CuCl₄ NCs showed a high PLQY (28.8%) with deep blue emission at 434 nm. The PLQY and stability of Cs₂CuCl₄ NCs can be further enhanced through Ag treatment using Ag-OLA complex, where Ag-treated Cs₂CuCl₄ NCs exhibit higher PLQY (42%) and much improved stability in an ambient environment. This work provides an effective strategy for the synthesis of Cu(II)-based metal halide perovskite NCs, which are promising materials for reducing the toxicity and realizing practical applications of perovskite NCs for display and lighting devices.