2.1 Structural and Morphological Characterization of CCC and CCGC NCs

The main objectives of this study were to develop CCC NCs by Gd³⁺ alloying and to combine experimental and theoretical studies to better understand their structural, optical, and optoelectronic properties. The CCC NCs synthesized via the hot-injection method have a monoclinic structure with C2/c space group consisting of [CeCl₆]^3- octahedra and a Ce atom attached to six Cl atoms. The bond lengths between Ce and Cl in each octahedron (labeled Ce1 and Ce2) are shown in Figure 1a, indicating the presence of two different octahedra within the 0D structure. However, CCC NCs suffer from low stability as Ce³⁺ ions undergo quick oxidation to Ce⁴⁺ because of their relatively negative reduction potential under ambient conditions compared to other Ln³⁺ ions, such as Tb³⁺ and Pr³⁺.^[4] Therefore, the smaller Gd³⁺ ion is suitable to alloy for enhancing the stability and optical properties of CCC NCs. Monoclinic Cs₃GdCl₆ (CGC) has the same crystal structure and X-ray diffraction (XRD) pattern as CCC, as shown in Figure S1 (Supporting Information).^[3] Structural analysis of the CCGC NCs with 50% Gd³⁺ was performed via Rietveld refinement of the XRD patterns using GSAS software with refinement parameters.^[31] The refined XRD pattern is illustrated in Figure 1b. The XRD pattern of the 20% Gd³⁺-alloyed CCGC NCs was also refined, as shown in Figure S2 (Supporting Information). The values of the refinement parameters indicate the formation of single-phase CCGC NCs in both 20% and 50% Gd³⁺ without any impurities. Detailed Information related to Rietveld refinement is presented in Tables S1–S3 (Supporting Information).

These results show that 50% Gd³⁺-alloyed CCGC NCs have smaller lattice parameters than 20% Gd³⁺-alloyed CCGC NCs due to the slightly different ionic radius between Ce³⁺ and Gd³⁺ ions. As a result, the unit cell volume also decreases as the ratio of Gd³⁺ increases in Cs₃Ce_1-xGd_xCl₆. The cell volume and lattice parameters change in the stable composition of CCGC NCs at different Gd³⁺ concentrations were compared with the calculation based on Vegard's law in Figure S3 (Supporting Information). The slight deviation from Vegard's law at 0.5 can be attributed to the fact that Ce only occupied the 4d site and Gd only occupied the 4e site in the structure. Further details are discussed in the section on crystal structural study. These are distinct evidence to support that the site for Ce³⁺ can be occupied by Gd³⁺. Figure S4 in Supporting Information shows that the XRD patterns of the pure CCC NCs are identical to those of the CCGC NCs. Elemental analysis of the CCC and 50% Gd³⁺-alloyed CCGC NCs was performed via X-ray photoelectron spectroscopy (XPS), as shown in Figure 1c. The XPS spectra obtained by CCC reveal the presence of Ce (4d) and Cl (2p) elements. However, for the 50% Gd³⁺-alloyed CCGC NCs, a supplementary peak corresponding to Gd (4d) was observed in addition to the Ce and Cl peaks. This indicates the incorporation of Gd³⁺ into the CCGC NCs. In addition, a decrease in the XPS counts of Ce (5d) was observed in proportion to the ratio of Gd³⁺ in the CCC NCs, indicating that Gd³⁺ occupying occurred at the Ce site. The high-resolution XPS profiles of CCC NCs and 50% Gd³⁺-alloyed CCGC NCs for Ce (3d), Gd(4d), and Cl (2p) are added in Figure S5 (Supporting Information). The 2p_3/2 energy level containing four electrons in CCGC NCs (50% of Gd³⁺) shows a higher binding energy compared to CCC NCs. This result suggests the substitution of Ce³⁺ by Gd³⁺ because of a stronger bond between Gd³⁺ and Cl⁻ from competition between Ce³⁺ and Gd³⁺ to occupy the center site in [CeCl₆]³⁻ octahedra.

Subsequently, the morphology of the 50% Gd³⁺-alloyed CCGC NCs was analyzed using transmission electron microscopy (TEM), as shown in Figure 1d. The TEM images demonstrate that the average particle size of the 50% Gd³⁺-alloyed CCGC NCs is approximately 18 nm, with a narrow particle size distribution. Figure 1e shows a high-resolution transmission electron microscopy (HR-TEM) image of the CCGC NCs with a lattice plane distance of 0.29 nm. The calculated lattice plane distance matches the (422) plane of the CCGC crystallographic data. The selected area diffraction (SAED) patterns shown in the inset of Figure 1e, are well matched with the crystallographic data. Energy-dispersive spectroscopy (EDS) mapping, as shown in Figure 1f, further confirmed the presence of Cs, Cl, Ce, and Gd throughout the CCGC NCs.

2.2 Optical and Morphological Characterization of CCGC NCs in Different Alloying Concentration

The optical and morphological properties of the CCC NCs with different alloying ratios of Gd³⁺ ions are presented in Figure 2 for analysis. The excitation and emission spectra of CCC, along with its luminescence mechanism, are shown in Figure 2a. Upon UV excitation, CCC NCs emitted intense luminescence in the UV to visible region, described as asymmetric luminescence bands at 376 and 408 nm. For the Ce³⁺ ion, the 4f¹ energy level splits into ²F_5/2 and ²F_7/2 by a spin–orbit coupling, while the 5d¹ (²D) energy level splits into T_2g and E_g as a result of crystal field effect.^{[32, 33]} The major luminescence bands at 379 nm and 408 nm originated from the transition from the lowest 5d¹ (T_2g Γ₈) to the 4f¹ (²F_5/2 and ²F_7/2) multiples.^[34] The excitation spectra of the CCC NCs were measured by monitoring the 376 nm luminescence band. Excitation bands were observed at 264 and 336 nm, which are attributed to the 4f–5d electronic transition.^[35] A well-known characteristic of Ce³⁺-based luminescent materials is their parity-allowed f–d transition exhibited by Ce³⁺ ions, typically showing a broad emission band, short PL lifetime (usually on the nanosecond scale), and a tunable emission spectrum, which are promising properties for optoelectronic devices.^[4] The [CeCl₆]³⁻ octahedra, including Cl⁻ ions as ligands, which has a dominant effect on the crystal field splitting strength, exhibits a sixfold coordination number, indicating strong splitting in the 5d of Ce³⁺ ions.^{[36, 37]}

We observed a gradual increase in the PLQY of the CCC NCs with an increase in the Gd³⁺ concentration, as shown in Figure 2b. The highest PLQY of the CCGC NCs was 96% when the Gd³⁺ concentration was 80% Figure S6, Supporting Information). This represents a substantial enhancement compared to the initial PLQY of 57% without Gd³⁺. This improvement of PLQY is attributed to the substitution of Ce³⁺ ions with Gd³⁺ ions in [CeCl₆]³⁻ octahedra, indicating the suppression of concentration quenching by providing higher distances between Ce³⁺ ions and reducing radiation reabsorption.^[38] However, after the complete substitution of Ce³⁺ with Gd³⁺ ions, the PLQY of the CGC NCs decreased to 6.2%, which is attributed to the surfactant ligands. This is because Gd³⁺ ions are not attributed to the energy transition in the UV region as shown in Figure S7 (Supporting Information) for the partial density of state (PDOS) of CGC, showing no electronic state gap. The only possible reason why CGC NCs give emissions in the UV region would be oleic acid and oleylamine on the surface of NCs. Moreover, the time-resolved photoluminescence (TRPL) decay of the CCC and CCGC NCs was measured upon excitation at 316 nm, and the emission peak at 380 nm was monitored, as shown in Figure 2c. The parameters of the decay curves are listed in Table S4 (Supporting Information). The average lifetime of the CCC NCs increased with the Gd³⁺ concentration. The fitted average lifetimes are 26.5 ns, 32.9 ns, 35.0 ns, and 42.8 ns for 0%, 20%, 40%, and 80% of Gd³⁺ ions in CCGC NCs, respectively. This further explains the improvement in the PLQY of CCGC NCs with increasing Gd³⁺. As shown in Figure 2d, the CCC NCs have significant absorbance in the UV-A region between 300 and 400 nm, with a peak at 334 nm. The full width at half maximum (FWHM) of the absorbance peak is 27 nm for the CCC NCs with good selectivity, indicating that the CCC NCs can serve as effective absorbing layers for UVPDs. However, the absorbance intensity decreased with increasing Gd³⁺ concentration in the CCGC NCs. This is because the energy transition at the UV region occurs only from 5d and 4f orbital states of Ce³⁺ ions as the Gd³⁺ does not contribute to the bandgap, as shown in Figure S11 (Supporting Information). This decrease in absorbance at 334 nm supports that Gd³⁺ ions substitute for the site of Ce³⁺ according to the alloying ratio.

Interestingly, Gd³⁺-alloyed CCGC NCs improved the optical properties and the morphology and particle size distribution, as shown in Figure 2e. Moreover, the average particle size of the CCGC NCs decreases from 29 to 18 nm with an increase of Gd³⁺ concentration from 0 to 50%. Consequently, the FWHM of the size distribution reduced from 11.1 nm to 3.2 nm (Figure S8 in Supporting Information). Note that the monodispersity with narrow FWHM in size distribution is beneficial to obtain a more uniform film without voids and pinholes with enhanced device performance.^[39] However, excessive Gd³⁺ (more than 50%) adversely affects the morphology and size distribution of the CCGC NCs, as depicted in Figure S9 (Supporting Information). Computational calculations were conducted to comprehend the underlying mechanism of the improved properties of CCGC NCs.

2.3 Crystal Structural Study and Optical Stability of CCGC NCs

The XRD patterns of the CCGC NCs shown in Figure 3c confirm the superior phase stability of the 50% Gd³⁺-alloyed CCGC NCs compared with that of the pristine CCC NCs. The CCC NCs exhibited poor stability within 30 d of storage under ambient conditions, corresponding to the additional CsCl XRD peaks observed. In contrast, the 50% Gd³⁺-alloyed CCC NCs exhibited superior stability over 150 d under ambient conditions. The optical stability of the CCGC NCs was analyzed using PLQY measurements, as shown in Figure 3d. The initial PLQY of the CCC NCs decreased by 50% after 84 d. For the 50% Gd³⁺-alloyed CCC NCs, 84% of the initial PLQY was maintained after 110 d. These enhanced phase and optical stability of the CCC NCs were achieved by alloying with Gd³⁺, which is crucial for optoelectronic device fabrication.

2.4 Electrical Characterization of CCGC NCs for Photodetector

As mentioned previously, CCC NCs exhibit high absorbance in the UV-A region, making them good candidates as absorbers in PDs. The absorbance and PLQY of the absorbing layer in PDs are crucial factors determining device performance due to their significant contribution to photocurrent generation. We performed a calculation to estimate the relative external quantum efficiency (EQE) of the PD as a function of the Gd³⁺ concentration in the CCC NCs, and the two factors were assumed as the main parameters. The best performance of the PDs was predicted to be at approximately 20% of the Gd³⁺ concentration, as indicated in Figure 4a with a red line according to the tradeoff relation between the relative PLQY and absorbance. In addition, we verified whether the best performance is achieved with the 20% Gd³⁺ by comparing our calculation with experimental results obtained from fabricating a series of devices in Figure 5.

Furthermore, we calculated the electronic structures using CCC and CCGC to understand the electronic consequences of Gd³⁺ incorporation. As shown in Figure 4b, both CCC and CCGC exhibit a direct optical bandgap of 3.6 eV at the gamma point without a significant change in their band structure near the band edges. Indeed, despite the incorporation of Gd³⁺ into the CCC lattice, the conduction band minimum (CBM) of CCC and CCGC was contributed to the Ce-5d orbital while the valence band maximum (VBM) consisted of the Ce-4f orbital (Figure 4c). This can be attributed to the fact that the Gd³⁺ ions do not introduce electronic states in the bandgap of CCC, but form electronic states far away from the band edge of CCC (see Figure S11 in Supporting Information). A concentration of 25% was selected for the DFT calculations, based on the results of the EQE estimation. It was confirmed that the spin- and parity-allowed 4f–5d transition in Ce³⁺ demonstrated a nanosecond-scale PL lifetime, which has significant advantages for optoelectronic devices.

To verify the calculated relative EQE values, charge transport layers (CTLs) were selected to design the full photodiode-type device based on the energy-level diagram of the CCGC NCs. Therefore, the VBM of the 20% Gd³⁺-alloyed CCGC NCs was determined using ultraviolet photoelectron spectroscopy (UPS) measurements in Figure S15 (Supporting Information). Subsequently, the optical bandgap (3.53 eV) was estimated based on the UV–Vis absorbance spectrum using the modified Kubelka–Munk equation in Figure S16 (Supporting Information).^[45] Accordingly, the energy level diagram of the PD is shown in Figure 4e, including the optimum CCGC (20% Gd³⁺) NCs with the calculated VBM (6.12 eV) and CBM (2.59 eV). The CTLs were selected by considering the energy level of the CCGC NCs to reduce the energy barriers so that charges could be extracted efficiently.^[46] The device structure consisted of fluorine-doped tin oxide (FTO), compact titanium dioxide (c-TiO₂), mesoporous titanium dioxide (m-TiO₂), absorbing layer (CCGC NCs), molybdenum trioxide (MoO₃), and silver (Ag), as shown in Figure 4f. FTO was used as the cathode, c-TiO₂ and m-TiO₂ as electron-transport layers (ETLs), MoO₃ as the hole transport layer (HTL), and Ag as the anode.

Finally, we monitored R values to investigate the operational stability of the fabricated PDs during storage of the devices in an N₂-filled glovebox. As depicted in Figure 5f, compared to CCC PD, the PD based on CCGC (20% Gd³⁺) exhibited much more improved stability by maintaining 80% of its initial R-value for up to 370 h. CCGC NCs possess enhanced electrical durability due to phase stability and reduced internal energy. For comparison with prior research, we compiled the parameters of lead-free metal halide-based UVPDs in the region (200–400 nm), as summarized in Table S5 (Supporting Information).