Quantum dots (QDs) arecrystalline semiconductor particles that have been extensively studied as a type of nanomaterial with sizedependent electronic and optical properties [1,2]. The optoelectronic properties of QDs can be varied by tuning the band gap. The unique attributes of QDs have attracted considerable attention for photonics applications such as solid-state lighting, photovoltaics, and bioimaging [3–6]. Tuning the size, shape, composition, and structure of QDs can alter the wavelengths of optical light absorption and photoluminescence (PL) and control the location of electronic charge carriers [7–9]. The key factor for success in these applications is the surface characteristics of the QDs, which become more important with decreasing size and can affect some properties on the nanoscale. The electronic configuration of QDs can differ from that of atoms within the crystal if broken chemical bonds are present within or on the surface of the QDs. They are especially vulnerable to the creation of defect-induced trap states, which can degrade the PL and decrease the charge transfer rate [10,11]. Therefore, postsynthetic ligand exchange can be used to enhance the nanocrystal properties. Ligands of QDs can be classified as L-type (Lewis bases), Z-type (Lewis acids), or X-type (formally shared one-electron donors) [12]. The covalent bond classification of ligands suggests that Z-type ligands are the only class of ligand that completely passivates QDs, resulting in a trap-free electronic structure; only Z-type ligands affect the PL properties. By contrast, X- and L-type ligands have no effect [13].

In this work, we studied the PL enhancement effect of postsynthetic ligand exchange at various passivation temperatures on CdSe QDs. The PL was measured to observe the emission properties of CdSe QDs treated with Z-type ligands at different passivation temperatures and CdSe QDs treated without Z-type ligands. Enhanced PL was observed in the presence of Z-type ligands, and treatment without ligands did not significantly change the PL properties of CdSe QDs.

To prepare the CdSe QDs, a Cd precursor solution was prepared by mixing 0.128 g of CdO powder (1 mmol), 1.58 mL of oleic acid, and 20 mL of octadecene (ODE) in a 100 mL three-necked flask. The mixture was heated at 120 °C for 1 h under N₂ flow and then further heated at 120 °C for 1 h under vacuum conditions. Next, the temperature of the reaction mixture was increased to 250 °C and maintained at that value for 1 h to obtain a clear solution. A Se precursor solution containing 0.0789 g of Se powder (1 mmol) and 2 mL of ODE was placed in a 100 mL three-necked flask and held in vacuum. Trioctylphosphine (1 mL) was rapidly injected into the precursor solution under N₂ flow, and the mixture was heated at 120 °C for 1 h. Finally, the Se stock solution was rapidly injected into the Cd precursor solution at 203 °C, and the solutions were reacted at 195 °C for 60 min. The obtained QDs were purified using acetone and methanol and dissolved in anhydrous toluene. To prepare the CdCl₂–oleylamine stock solution, 0.18 g of CdCl₂ (1 mmol), 4.23 mL of oleylamine (9 mmol), and 10 mL of anhydrous toluene were placed in a 100 mL three-necked flask and heated at 95 °C for 1 h under N₂ flow. After the reaction was complete, the CdCl₂–oleylamine stock solution was injected into a 0.456 µmol/L CdSe QD solution in a 100 mL three-necked flask to a total volume of 3 mL in anhydrous toluene. The reaction mixture was heated at a selected passivation temperature (25, 40, 50, 60, 70, or 80 °C) for 15 min. After the reaction, the mixture was allowed to cool naturally to room temperature and then filtered through a 0.2 µm polytetrafluoroethylene syringe filter to remove excess ligands.

To confirm the passivation of Z-type CdCl₂ ligands on the CdSe QD surface, we investigated the CdSe QDs treated with Z-type ligands at different temperatures. The PL spectra of the pristine and treated CdSe QDs are shown in Fig. 1(a). The CdSe QDs treated with Z-type ligands at different passivation temperatures exhibited enhanced PL, and the QDs treated at 40 °C for 15 min showed the greatest enhancement. It was found that Lewis acidic (Z-type) ligands serving as passivation ligands in CdSe QDs are effective at passivating surface traps and can enhance the PL intensity of the QDs [13]. Figure 1(b) shows the pristine and treated CdSe QDs under a single 365 nm UV light source. Compared to the pristine CdSe QDs, the CdSe QDs treated with Z-type ligands at 40 °C for 15 min emitted bright orange light, and the other treated samples also emitted bright light of similar colors because of increased PL intensity.

Figure 1. (a) PL spectra and (b) photographs taken under UV light of pristine CdSe QDs and CdSe QDs treated with Z-type ligands at different passivation temperatures.

Figures 2(a)and 2(b) show the PL spectra and intensity of the pristine CdSe QDs and the CdSe QDs treated with and without Z-type lig- Figure 2. (a) PL spectra of pristine CdSe QDs and CdSe QDs treated with and without Z-type ligands at 40 °C for 15 min. The inset shows photographs of each sample under UV light. (b) PL intensity of pristine CdSe QDs and CdSe QDs treated with and without Z-type ligands at 40 °C for 15 min. ands at 40 °C for 15 min. The CdSe QDs treated without ligands at 40 °C for 15 min exhibited PL quenching, which is attributed to surface defect states created by the loss of capping ligands owing to detachment. It is commonly known that decrease on capping agents make it possible to form surface states in the forbidden gap, and surface defect formation can quench the luminescence of QDs [14]. Thermal annealing can also cause Ostwald ripening and coalescence of CdSe QDs; consequently, the emission wavelength of the CdSe QDs treated without ligands at 40 °C for 15 min was slightly red-shifted [15,16]. These results are consistent with the images in the inset of Fig. 2(a), which shows photographs of the samples under UV light. The CdSe QDs treated without ligands at 40 °C for 15 min were less bright than those treated with ligands at 40 °C for 15 min.

Figure 2. (a) PL spectra of pristine CdSe QDs and CdSe QDs treated with and without Z-type ligands at 40 °C for 15 min. The inset shows photographs of each sample under UV light. (b) PL intensity of pristine CdSe QDs and CdSe QDs treated with and without Z-type ligands at 40 °C for 15 min.

Figure 3 shows the PL intensity of pristine CdSe QDs, CdSe QDs passivated at different temperatures, and CdSe QDs treated without ligands at 40 °C for 15 min. The results show that ligand passivation can be controlled by choosing an appropriate passivation temperature using CdCl₂ as Z-type ligands with oleylamine as L-type ligands. The PL intensity of the CdSe QDs treated with ligands at 40 °C for 15 min was ~2 times stronger because of surface trap state passivation. By contrast, the PL intensity of CdSe QDs treated without ligands at 40 °C for 15 min decreased by approximately 20 % compared to that of the pristine QDs. Zincblende CdSe QDs with a carboxylatepassivated surface can have a nonstoichiometric cadmium-rich composition resulting from the presence of neutral cadmium carboxylate (CdX₂) complexes on the QD surfaces. These CdX₂ complexes can be identified as Lewis acid acceptors, which bind unpassivated Se sites and stabilize the surfaces [17]. The introduction of cadmium halides or cadmium carboxylates, which can increase the surface Cd/Se ratio, can passivate the defect states and enhance the PL quantum yield [18].

Figure 3. PL intensity of CdSe QDs treated with Z-type ligands at different passivation temperatures. The star and square represent the PL intensity of the pristine CdSe QDs and CdSe QDs treated without Z-type ligands at 40 °C for 15 min, respectively.

Figure 4(a) shows the PL peak positions of the pristine CdSe QDs, CdSe QDs treated at various passivation temperatures, and CdSe QDs treated without ligands at 40 °C for 15 min. The PL peak position of the pristine CdSe QDs was 593.4 nm. The PL peaks of the CdSe QDs treated at temperatures of 25, 40, 50, 60, 70, and 80 °C appear at 591.0, 590.4, 590.7, 591.0, 591.4, and 592.0 nm, respectively. The slight differences between the PL peaks of the CdSe QDs treated at different passivation temperatures can be attributed to surface modification [19]. The PL peak of the CdSe QDs treated without ligands at 40 °C for 15 min was red-shifted slightly to 594.7 nm because the QDs became larger. Figure 4(b) shows the full width at half-maximum (FWHM) of the pristine CdSe QDs, CdSe QDs treated at different passivation temperatures, and CdSe QDs treated without ligands at 40 °C for 15 min. The FWHM of the pristine QDs was 29.78 nm. The FWHMs of the passivated QDs were 31.88 nm (25 °C), 31.36 nm (40 °C), 31.16 nm (50 °C), 31.12 nm (60 °C), 30.96 nm (70 °C), and 31.00 nm (80 °C). The slight variation in the FWHM of the CdSe QDs treated at different passivation temperatures can be attributed to differences in the surface atomic ratio [20]. The FWHM of the CdSe QDs treated without ligands at 40 °C for 15 min was almost the same as that of the pristine CdSe QDs.

Figure 4. (a) PL peak position and (b) FWHM of CdSe QDs treated with Z-type ligands at different passivation temperatures. The stars and squares represent PL peak position and FWHM of pristine CdSe QDs and CdSe QDs treated without Z-type ligands at 40 °C for 15 min, respectively.

We investigated the effects of passivation by Z-type ligands on the optical properties of CdSe QDs treated at various passivation temperature. The binding of cadmium halide complexes in the QDs strongly affected the PL properties of the QDs. The PL of CdSe QDs treated with ligands at 40 °C for 15 min increased owing to the effects of surface passivation. The small variations in spectral peak position and FWHM of the CdSe QDs treated at various passivation temperatures may have resulted from changes in the surface properties and surface atomic ratio. The surface control of colloidal semiconductor QDs can offer clues for understanding and tailoring their physical properties and chemical behavior. Our findings will contribute to increased understanding of Z-type ligand coordination and its effects on the surface stoichiometry and electrostatics of QDs.

This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (NRF- 2021R1A2C1003074).

The authors declare no conflict of interest.