Thiocarboxylic acids for robust passivation and advanced applications of perovskite nanocrystals

Abstract

Thiocarboxylic acid ligands provide strong surface passivation, improving the thermal, water, and long-term stability of the PeNCs. An adamantane framework was adopted as a designable alkyl tail to improve the thin-film morphology owing to strong intermolecular interactions. Furthermore, light-emitting diodes were fabricated to demonstrate the broad applicability of the thiocarboxylic acid ligands.

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Carboxylic acids (CA), together with amines, constitute one of the two most fundamental families of ligands for lead halide perovskite nanocrystals (PeNCs) and have established an essential position in their surface chemistry.^1,2 The acids bind to lead ions—behaving, in other words, as anions similar to halides—and thereby passivate the dangling bonds on the surface.^3,4 These Pb-related dangling bonds are known to generate fatal trap states within the bandgap,⁵ underscoring the critical importance of CAs and supporting the notion that insufficient passivation can severely deteriorate the optical properties of PeNCs. To achieve passivation, various acidic ligands have been employed. Although CAs do not necessarily provide strong passivation, they have been widely utilized from the earliest reports to the present owing to their simplicity and versatility (over 500 moieties are available).⁶ Their straightforward synthesis also enables facile access to ligands with diverse alkyl tails. In contrast, alkyl phosphonic and sulfonic acids offer high binding energies owing to their low pK_a values, imparting high photoluminescence quantum yields (PLQY), long-term storage stability, and enhanced thermal robustness to PeNCs.^3,7–9 However, these families of sulfonic and phosphonic acids often require complicated synthetic routes with toxic reagents,^10,11 which limits the practical use of alkyl-tail engineering as a strategy to enhance the properties of PeNCs.^12–14 Consequently, there is an inherent trade-off between achieving strong passivation and maintaining structural flexibility in the alkyl tail, highlighting the need for a ligand-engineering strategy capable of reconciling both requirements.

In this study, we focused on thiocarboxylic acids (TCAs) and evaluated their applicability as surface ligands for PeNCs. Thiocarboxylic acids have a lower pK_a (stronger acids) than conventional carboxylic acids, enabling the formation of robust thiocarboxylate–Pb bonds. Moreover, a previous study on Pd(II) complexes has reported that elements with a large atomic radius, such as sulfur, lead to longer metal–sulfur bond lengths and can reduce the strain of the four-membered ring formed in bidentate chelation.¹⁵ From this viewpoint, the formation of a bidentate chelation-like interaction between TCAs and Pb ions may be more favorable than that of carboxylic acids. Importantly, TCAs can be readily synthesized in a single step from CAs via Lawesson's reagent, enabling facile diversification through the vast library of commercially available CAs.¹⁶ These structural and bonding advantages endow TCAs with excellent passivation for PeNCs. Their introduction not only enabled high PLQY but also imparted remarkable thermal, water, and long-term stabilities to the colloidal nanocrystals. In addition, to demonstrate the structural degree of freedom in the tail-group design, an adamantane framework was introduced in this study. The molecular volume of adamantane is larger than that of octane, the commonly used tail group of octanoic acid (Fig. S1), giving it a bulky yet short structure that can simultaneously provide high dispersibility and strong interparticle interactions.^17,18 The adamantane structure facilitates strong intermolecular interactions originating from dispersion force, increasing the ordering of the PeNCs and reducing interparticle distances.^19,20 Overall, this work introduces TCAs as a new ligand family for PeNCs and demonstrates that they can simultaneously offer strong passivation and structural tunability through diverse alkyl groups, providing a new perspective for ligand engineering.

The TCA ligands were synthesized following a previously reported procedure with some modifications, using 1-adamantaneacetic acid (Ada-CA) to obtain the corresponding TCA, 1-adamantanethioacetic acid (Ada-TCA). The synthesized TCAs were expected to be significantly stronger acids than the corresponding CAs, regardless of their alkyl structures, from their predicted pK_a values (Fig. S2).^21,22 These ligands were introduced into the PeNCs through a ligand-exchange process (Fig. S3).²³ Details of the ligand synthesis and PeNC preparation are provided in the SI. To clearly distinguish between CAs and TCAs, the resulting PeNCs were labelled Ada-CA/PeNCs and Ada-TCA/PeNCs, respectively.

The electrostatic potential (ESP) maps of the Ada-CAs and Ada-TCAs calculated by density functional theory (DFT) using the Gaussian 16 program at the B3LYP/6-31+G** level are shown in Fig. 1(a) and (b) (computational details are provided in the SI). Both head groups carry a negative charge, and the TCA can act as a ligand in a similar manner to the CA. However, it was clearly demonstrated that the sulfur atom in the Ada-TCA exhibits a more spatially extended electron distribution than the oxygen atom in the Ada-CA. According to the hard–soft acid–base (HSAB) principle, this indicates that the deprotonated TCA behaves as a softer base, supporting its stronger interaction with the soft acid Pb²⁺.^24,25

Fig. 1 ESP potential maps of (a) Ada-CA and (b) Ada-TCA. (c) SEM image of PeNC solid drop-cast onto a silicon substrate and corresponding EDX mapping of (d) silicon, (e) sulfur, (f) cesium, (g) lead, and (h) bromide.

The coordination of the Ada-TCAs to PeNCs was confirmed by scanning electron microscopy–energy dispersive X-ray spectroscopy (SEM–EDX, Fig. 1(c)–(h)). PeNCs were drop-cast onto a silicon substrate, and elemental analysis was performed. In the brighter region on the left side of the SEM image, Cs, Pb, and Br—elements characteristic of PeNCs—were localized, whereas Si was not detected, confirming that this area corresponds to the PeNCs. Additionally, sulfur was localized in this region, which can be attributed to the Ada-TCA coordinated with the PeNCs. This observation was further supported by the ¹H NMR results (Fig. S4). In addition, quantitative analysis of the ¹H NMR spectra revealed that approximately 18% of the ligands on the PeNCs were substituted with Ada-TCA (Table S1). Although the ligand ratio may appear modest at first glance, it is sufficiently high compared with previous studies and was expected to have a positive impact on the PeNCs. Moreover, because Ada-CA/PeNCs prepared using an identical procedure were used as the control sample in the following evaluations, the effects of octanoic acid and didecyldimethylammonium bromide—ligands required for PeNC synthesis—can be neglected.

Subsequently, the fundamental morphology and optical properties of the PeNCs prepared using acidic ligands were evaluated. In this study, Ada-CA and Ada-TCA were introduced through post-treatment ligand exchange; therefore, they did not participate in PeNC nucleation and contributed solely to surface exchange reactions. Accordingly, they did not affect the particle morphology or crystal structure, as observed from the dynamic light scattering (DLS) and X-ray diffraction (XRD) results, respectively (Fig. 2(a) and (b)). Furthermore, no substantial differences were observed in the optical properties (Fig. 2(c)). However, a clear prolongation of the PL lifetime was observed, and the relative contribution of the slower τ₂ component—assigned to radiative decay—increased (Fig. 2(d) and Table S2). This behaviour indicates that Ada-TCA/PeNCs possess lower trap density than Ada-CA/PeNCs.²⁶ Such a trend is consistent with the slower bleach recovery observed in the transient absorption spectra (Fig. S5 and S6, and Table S3).

Fig. 2 The fundamental evaluations of Ada-CA/PeNCs and Ada-TCA/PeNCs from (a) DLS, (b) XRD, (c) absorption and PL spectra, and (d) PL lifetime.

To obtain deeper insight, the stability of the PeNCs was comprehensively assessed, through which the superiority of the TCA ligands was elucidated. Heat induces ligand desorption from the PeNC surface, leading to particle aggregation and a decrease in PLQY with the formation of surface and internal defects.²⁷ Thus, thermal stress provides a useful means of comparing the passivation capabilities of different ligands. The PeNC dispersions were heated in an oil bath at 100 °C, and the particle morphology and optical properties were subsequently evaluated. The particle size distributions obtained from DLS are shown in Fig. 3(a), and the sample images are provided in Fig. S7. For the reference sample, Ada-CA/PeNCs, severe aggregation was observed after 870 min of heating. In contrast, the Ada-TCA/PeNCs exhibited no detectable aggregation and successfully maintained their initial particle size. Moreover, the PLQY of the Ada-CA/PeNCs decreased to approximately 20%, whereas Ada-TCA/PeNCs retained an exceptionally high PLQY of over 70% even after more than 1200 min of heating (Fig. 3(b)). The high durability imparted to the PeNCs by the TCA ligands was also evident in the shelf-life evaluation, in which the samples were stored for 120 days under ambient conditions at room temperature. The transmission electron microscopy (TEM) images obtained before and after storage are shown in Fig. 3(c). Initially, both samples exhibited well-defined cubic particles with sharp edges. However, clear aggregation was observed for the Ada-CA/PeNCs after 120 days. This aggregation was further quantified by DLS (Fig. S8), indicating that the original nanoparticles had aggregated and were no longer present in their discrete form. In contrast, the Ada-TCA/PeNCs retained a well-defined particle morphology, with only minor aggregation observed. In addition, the crystallite size D was calculated based on the peak broadening in the XRD profiles.²⁸ A comparison of D enables an evaluation of the structural stability. According to the Scherrer's equation,

where K is the Scherrer constant (K = 0.94 was employed),²⁹λ is the X-ray wavelength, B is the full width at half maximum of the diffraction peak, and θ is the Bragg angle. The (100) peak located near 15° was used as the representative peak, and its width, B, was employed for the calculation (Fig. 3(d)). For Ada-CA/PeNCs, the crystallite size increased from 99 to 120 Å during storage, confirming that the weak passivation by CA was insufficient to suppress crystal growth. In contrast, the crystallite size of the Ada-TCA/PeNCs changed only minimally, from 101 to 100 Å, demonstrating that undesirable crystal growth was almost completely suppressed. Moreover, the strong passivation provided by the TCA ligands also demonstrated a clear advantage in terms of water stability (Fig. 3(e) and (f)). The water stability of the PeNCs was evaluated by placing the PeNC dispersion on water, allowing the PeNCs to gradually decompose at the interface. For Ada-CA/PeNCs, the PL intensity decreased to 68% of its initial value after 300 min, reflecting the inherent fragility of their ionic crystal nature. In contrast, the Ada-TCA/PeNCs exhibited markedly improved stability, retaining 93% of their initial PL intensity after 300 min. This enhanced stability is attributed to the strong adsorption of TCA ligands on the PeNC surface, which prevents ligand desorption in water and imparts hydrophobicity to the particles. These results unambiguously demonstrate the superior passivation capability of the TCA ligands.

Fig. 3 Particle size evolution measured by DLS during the heating test at 100 °C (a) and changes in PLQYs (b). TEM images (c) and XRD profiles (d) before and after the 120-day shelf-life test. Water-stability measurements of Ada-CA/PeNCs (e) and Ada-TCA/PeNCs (f). The insets show the images of the dispersions under room light and 365 nm UV illumination during testing.

The morphology and film quality of the PeNC thin films have a substantial impact on the performance of optoelectronic devices. The adamantane framework employed as the ligand tail is associated with pronounced intermolecular interactions dominated by dispersion forces. This ligand can reinforce interparticle interactions among PeNCs, promoting nanocrystal ordering and improving film quality. To investigate the interparticle interactions, the TEM images were analyzed using fast Fourier transform (FFT) to obtain information on the degree of ordering (Fig. 4(a)).³⁰ From the FFT analysis, the Ada-CA/PeNCs exhibited blurred spots, indicating weak periodicity of the PeNCs in the thin film. In contrast, the Ada-TCA/PeNCs exhibited clearer spots, including second-order harmonics. Furthermore, a one-dimensional profile converted from the FFT images revealed a shift of the peak toward higher wavenumbers, corresponding to a decrease in the center-to-center interparticle distance (Fig. 4(b)). SEM analysis was used to estimate the film coverage of the PeNCs, showing values of 80% for Ada-CA/PeNCs and 96% for Ada-TCA/PeNCs (Fig. S11), demonstrating the formation of a more uniform film. Consistently, the root-mean-square (RMS) roughness obtained from atomic force microscopy (AFM) measurements was reduced for the Ada-TCA/PeNC films (Fig. S10 and Table S4), indicating a smoother film formation. These results collectively support the presence of favourable interactions between the adamantane frameworks positioned on the PeNC surface. Although Ada-CA also contains an adamantane moiety, its weaker interaction with Pb²⁺ likely leads to ligand desorption during the film-formation process, preventing these beneficial effects.

Fig. 4 (a) TEM images of the particles and their corresponding FFT patterns, and (b) one-dimensional profiles extracted from the FFT images. Device structure of the fabricated LEDs (c) and the corresponding measured characteristics: (d) luminance–voltage, (e) current density–voltage, and (f) EQE–voltage curves of the devices.

Finally, to demonstrate the broad applicability of the Ada-TCAs, we fabricated a prototype LED, one of the important application targets for PeNCs. The device structure is shown in Fig. 4(c), and the corresponding characteristic curves obtained from the measurements are presented in Fig. 4(d)–(f). Both the maximum luminance and maximum external quantum efficiency (EQE) were significantly enhanced by Ada-TCA, showing an approximately 2.7-fold improvement in EQE (Table S4). These results indicate that TCAs, which combine robust surface passivation with high flexibility in ligand design, can further broaden the application scope of PeNCs and contribute to advances in the ligand engineering on PeNCs.

In conclusion, TCA ligands combined with the adamantane framework provide comprehensive passivation and significantly enhance the thermal, water, and long-term stability of PeNCs. These improvements lead to superior particle orientation and a thin film morphology, demonstrating the synergistic advantages of strong surface binding and a rational tail-group design in ligands. This study further highlights how the TCA-based head group, together with flexible tail-group engineering and the ease of accessing TCA through straightforward one-step conversion from CAs, yields highly effective ligands for PeNCs.

Author contributions

Taisei Kimura – investigation, methodology, visualization, and writing – original draft; Seung Jae Jeong – investigation, data curation, and formal analysis; Yeonji Son – methodology, formal analysis, and data curation; Sieun Yoon – data curation and methodology; Takuro Iizuka – visualization; Takashi Nagata – visualization; Yuta Ito – software; Hobeom Kim – methodology, data curation, and resources; Dong Ryeol Whang – conceptualization, funding acquisition, resources, supervision, and writing – review and editing; Akito Masuhara – project administration, validation, and writing – review and editing.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: experimental methods, details of evaluations of surface ligands, optical properties of PeNCs, spectral analysis data, SEM images, AFM images, and LED performance. See DOI: https://doi.org/10.1039/d5cc07280h.

Acknowledgements

This work was supported by the Technology Innovation Program (RS-2025-02413058) through the Korea Planning & Evaluation Institute of Industrial Technology (KEIT) funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea), the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT, RS-2024-00342991) and the JSPS Program for Promoting Japan's Peak Research Universities (J-PEAKS, JPJS00420240014). T. K. acknowledges support from the Grant-in-Aid for the Japan Society for the Promotion of Science (JSPS) Fellows, JSPS KAKENHI (24KJ0450).

References

1. L. Protesescu, S. Yakunin, M. I. Bodnarchuk, F. Krieg, R. Caputo, C. H. Hendon, R. X. Yang, A. Walsh and M. V. Kovalenko, Nano Lett., 2015, 15, 3692–3696.
2. S. Biswas, S. Akhil, N. Kumar, M. Palabathuni, R. Singh, V. V. Dutt and N. Mishra, J. Phys. Chem. Lett., 2023, 14, 1910–1917.
3. F. Zaccaria, B. Zhang, L. Goldoni, M. Imran, J. Zito, B. van Beek, S. Lauciello, L. De Trizio, L. Manna and I. Infante, ACS Nano, 2022, 16, 1444–1455.
4. I. I. G. Almeida and L. Manna, Science, 2019, 364, 833–834.
5. J. Pan, L. N. Quan, Y. Zhao, W. Peng, B. Murali, S. P. Sarmah, M. Yuan, L. Sinatra, N. M. Alyami, J. Liu, E. Yassitepe, Z. Yang, O. Voznyy, R. Comin, M. N. Hedhili, O. F. Mohammed, Z. H. Lu, D. H. Kim, E. H. Sargent and O. M. Bakr, Adv. Mater., 2016, 28, 8718–8725.
6. D. M. Kroupa, M. Voros, N. P. Brawand, B. W. McNichols, E. M. Miller, J. Gu, A. J. Nozik, A. Sellinger, G. Galli and M. C. Beard, Nat. Commun., 2017, 8, 15257.
7. H. Li, Y. Feng, M. Zhu, Y. Gao, C. Fan, Q. Cui, Q. Cai, K. Yang, H. He and X. Dai, Nat. Nanotechnol., 2024, 19, 638–645.
8. D. Sato, Y. Iso and T. Isobe, ACS Omega, 2020, 5, 1178–1187.
9. K. Xu, E. T. Vickers, L. Rao, S. A. Lindley, A. C. Allen, B. Luo, X. Li and J. Z. Zhang, Chemistry, 2019, 25, 5014–5021.
10. J. Wang, S. Xu, L. Li and J. Li, Hydrometallurgy, 2013, 137, 108–114.
11. F. P. Ballistreri, G. A. Tomaselli and R. M. Toscano, Tetrahedron Lett., 2008, 49, 3291–3293.
12. T. Kimura, R. Yamakado, N. Oshita, S. Asakura and A. Masuhara, Appl. Phys. Express, 2022, 15, 105502.
13. V. Morad, A. Stelmakh, M. Svyrydenko, L. G. Feld, S. C. Boehme, M. Aebli, J. Affolter, C. J. Kaul, N. J. Schrenker, S. Bals, Y. Sahin, D. N. Dirin, I. Cherniukh, G. Raino, A. Baumketner and M. V. Kovalenko, Nature, 2024, 626, 542–548.
14. H. Zhao, H. Chen, S. Bai, C. Kuang, X. Luo, P. Teng, C. Yin, P. Zeng, L. Hou, Y. Yang, L. Duan, F. Gao and M. Liu, ACS Energy Lett., 2021, 2395–2403,  DOI:10.1021/acsenergylett.1c00812.
15. M. Kondrashov, A. Fleckhaus, R. Gritcenko and O. F. Wendt, Struct. Rep., 2015, 71, m166–m166.
16. Y. Rao, X. Li, P. Nagorny, J. Hayashida and S. J. Danishefsky, Tetrahedron Lett., 2009, 50, 6684–6686.
17. E. V. Shevchenko, D. V. Talapin, A. L. Rogach, A. Kornowski, M. Haase and H. Weller, J. Am. Chem. Soc., 2002, 124, 11480–11485.
18. T. Chiba, S. Ishikawa, J. Sato, Y. Takahashi, H. Ebe, S. Ohisa and J. Kido, Adv. Opt. Mater., 2020, 8, 2000289.
19. M. Schoell and R. M. Carlson, Nature, 1999, 399, 15–16.
20. P. R. Schreiner, L. V. Chernish, P. A. Gunchenko, E. Y. Tikhonchuk, H. Hausmann, M. Serafin, S. Schlecht, J. E. Dahl, R. M. Carlson and A. A. Fokin, Nature, 2011, 477, 308–311.
21. Marvinsketch, version 22.22, ChemAxon, Budapest, 2022.
22. J. Manchester, G. Walkup, O. Rivin and Z. You, J. Chem. Inf. Model., 2010, 50, 565–571.
23. J. Song, J. Li, X. Li, L. Xu, Y. Dong and H. Zeng, Adv. Mater., 2015, 27, 7162–7167.
24. C. Collantes, W. Teixeira, V. González Pedro, M.-J. Bañuls and Á. Maquieira, Appl. Mater. Today, 2023, 31, 101775.
25. R. D. Hancock and A. E. Martell, Chem. Rev., 1989, 89, 1875–1914.
26. C. Bi, S. Wang, S. V. Kershaw, K. Zheng, T. Pullerits, S. Gaponenko, J. Tian and A. L. Rogach, Adv. Sci., 2019, 6, 1900462.
27. X. Yuan, X. Hou, J. Li, C. Qu, W. Zhang, J. Zhao and H. Li, Phys. Chem. Chem. Phys., 2017, 19, 8934–8940.
28. H. Ebe, R. Suzuki, S. Sumikoshi, M. Uwano, R. Moriyama, D. Yokota, M. Otaki, K. Enomoto, T. Oto, T. Chiba and J. Kido, Chem. Eng. J., 2023, 471, 144578.
29. J. I. Langford and A. Wilson, Appl. Crystallogr., 1978, 11, 102–113.
30. H. Abe, M. Uwano, R. Kobayashi, K. Narazaki, T. Akiyama, Y. Ito, D. Yokota, T. Oto, S. Nishitsuji, R. Yamakado and T. Chiba, Small, 2025, e2501159,  DOI:10.1002/smll.202501159.

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