Red-emissive nanocrystals of Cs₄Mn_xCd_1−xSb₂Cl₁₂ layered perovskites

Abstract

Layered double perovskites are currently being investigated as emerging halide-based materials for optoelectronic applications. Herein, we present the synthesis of Cs₄Mn_xCd_1−xSb₂Cl₁₂ (0 ≤ x ≤ 1) nanocrystals (NCs). X-ray powder diffraction evidences the retention of the same crystal structure for all the inspected compositions; transmission electron microscopy revealed monodisperse particles with a mean size between 10.7 nm and 12.7 nm. The absorption spectra are mostly determined by transitions related to Sb³⁺, whereas Mn²⁺ induced a red emission in the 625–650 nm range. The photoluminescence emission intensity and position vary with the Mn²⁺ content and reach the maximum for the composition with x = 0.12. Finally, we demonstrate that the photoluminescence quantum yield of the latter NCs was increased from 0.3% to 3.9% through a post-synthesis treatment with ammonium thiocyanate. The present work expands the knowledge of colloidal layered double perovskite nanocrystals, stimulating future investigations of this emerging class of materials.

---

Federico Locardi

Dr Federico Locardi received his PhD in Chemical Science from the University of Genoa (Italy), in 2015. He completed his first postdoctoral studies at the University of Genoa and Nanochemistry Department of IIT (Istituto Italiano di Tecnologia), focusing on the synthesis of halide perovskite nanocrystals. In 2019, he moved to the Physics and Chemistry of Nanostructures group at Ghent University (Belgium); after one year, he returned to the University of Genoa, where he is an assistant professor at the Department of Chemistry and Industrial Chemistry. His research focuses on the development of new inorganic (nano)materials for photocatalysis and optoelectronics.

---

Introduction

In recent years, extensive research on lead halide perovkistes¹ has led to increasing interest in new metal halides with a wide variety of structures and compositions based on different transition metals.² In particular, the concerns about the toxicity and limited stability of lead-based compositions have inspired researchers to explore alternatives, with comparable or, ideally, even better optoelectronic properties.^3,4 Double perovskites (DPs) represent one of the first attempts towards lead substitution. In DPs, the original 3D structure of lead halides, i.e. a framework composed of corner-sharing [PbX₆] (X = Cl⁻, Br⁻, I⁻) octahedra, is retained by the alternation of octahedra formed by monovalent and trivalent cations.⁵ In this way, charge neutrality is maintained, as expressed through the general chemical formula A₂M⁺M³⁺X₆ (M = metal, X = Cl⁻, Br⁻, I⁻). Several combinations of M⁺ and M³⁺ cations have been tested, both in bulk and in nanocrystals (NCs), for example M⁺ = Ag⁺, Na⁺, K⁺ and M³⁺ = In³⁺, Bi³⁺, Sb³⁺.^6–12 In this kind of structure, the need to maintain the charge balance and the constraints imposed by the ionic radii make the introduction of bivalent metal cations (M²⁺) generally possible only at low concentrations.^12–14

Complete substitution of the M⁺ cations with M²⁺ cations is possible with the concomitant introduction of a vacancy (Vac), resulting in a general A₄M²⁺M³⁺₂X₁₂ stoichiometry; these materials, due to their peculiar structure, are commonly known as 〈111〉 oriented layered double perovskites (LDPs).² Indeed, the structure can be described as a cubic substructure of Cs⁺ cations¹⁵ filled alternatively by layers of corner-sharing [M²⁺X₆]–[M³⁺X₆]–[M²⁺X₆]–[VacX₆] octahedra along the 〈111〉 direction. The first LDP, Cs₄CuSb₂Cl₁₂, was synthesized as a bulk material by Solis-Ibarra's group in 2017 and had an optical bandgap of 1 eV, which is promising for photovoltaics;¹⁶ Cai et al. prepared the same composition as NCs, with optical properties in line with those of the corresponding powders.¹⁷ The partial substitution of Cu²⁺ with Mn²⁺ tunes the band gap to higher energy, and a low-intensity red emission was measured from Cs₄MnSb₂Cl₁₂.¹⁸ A more efficient emission has been observed with the partial introduction of Cd²⁺,¹⁹ leading to a photoluminescence quantum yield (PLQY) of 37.5% for Cs₄Cd_0.75Mn_0.25Sb₂Cl₁₂.²⁰ The replacement of Sb³⁺ with Bi³⁺ resulted in an increase of the PLQY²¹ up to 79.5% for the Cs₄Cd_1−xMn_xBi₂Cl₁₂ (x = 0.1) composition.²² Yang et al. successfully synthesized the same series in the form of nanocrystals; however, the resulting materials exhibited a 20-fold drop of the PLQY (4.6%) compared to bulk crystals.²³

LDPs have been proposed as optoelectronic materials for LEDs^2,19 and high-speed photodetection^20,24 due to the strong anisotropy in their crystal structure. Moreover, preliminary studies have been conducted to investigate their magnetic behavior for potential applications in ferroelectrics and spintronics.^2,25

Motivated by the aforementioned studies and by the need to expand the range of the available stoichiometries of LDPs at the nanoscale, we devised a synthesis of Cs₄Cd_1−xMn_xSb₂Cl₁₂ in the form of NCs. Using a hot injection approach, we found that alloying Cd²⁺ and Mn²⁺ is possible (that is, x in the above formula can range from 0 to 1) with the formation of phase-pure cuboidal nanoparticles. The optical characterization has revealed a red emission centered at 625–650 nm for all the samples, except for x = 0, with the emission position and intensity depending on the Mn²⁺ content. The sample with x = 0.12 yielded the most intense emission of the series, with a PLQY of 0.3% that was further enhanced ten-fold to 3.9% by means of surface treatment with ammonium thiocyanate. Overall, our work expands the knowledge of the layered double perovskite at the nanoscale, highlighting the potential of these compounds as optoelectronic materials.

Results and discussion

We prepared Cs₄Mn_xCd_1−xSb₂Cl₁₂ NCs by a hot-injection method. Briefly, the synthesis consisted of degassing the precursors (metal acetates and carbonates) under vacuum in a mixture of dioctyl ether, oleylamine, and oleic acid for 20 minutes at 115 °C, followed by an injection of benzoyl chloride at 160 °C under a N₂ atmosphere (Scheme 1; see the ESI† for details). The resulting nanoparticles had a cuboidal shape for all the inspected samples, with a Mn²⁺ content tunable by systematically changing the Mn²⁺/(Cd²⁺ + Mn²⁺) precursor ratio from 0 to 1. The ICP-OES analysis revealed that, in general, an amount of Mn²⁺ lower than the one loaded actually enters the structure, especially when the Mn²⁺/(Cd²⁺ + Mn²⁺) ratio of the precursors is lower than 0.5 (Table S1 and Fig. S1†); a similar phenomenon was observed in Cs₄Mn_xCd_1−xBi₂Cl₁₂.²¹

Scheme 1 Colloidal synthesis of Cs₂Mn_xCd_1−xSb₂Cl₁₂ (0 ≤ x ≤ 1) NCs.

The powder X-ray diffraction (XRD) data indicated that all the synthesized compositions crystallize always in the trigonal R3̄m structure (Fig. 1a and S2†) and the samples with intermediate x were phase-pure and isostructural to the terminal compositions (x = 0 and 1, Fig. 1b and S2†). The shape of nanocrystals reflected the influence of the cuboidal Cs⁺ substructure, as commonly observed for other metal–halide perovskite nanocrystals.¹⁵ The slight shift of the diffraction peaks toward higher 2θ angles (Fig. 1b right) originates from the contraction of the unit cell due to the substitution of the Cd²⁺ cations with the smaller Mn²⁺ cations. Indeed, the cell parameters shifted from a = 7.586 Å, c = 37.165 Å to a = 7.549 Å, c = 36.994 Å for 0% Mn²⁺ and 100% Mn²⁺, respectively. The unit cell contracted by 1.44% in volume, and the contraction was found to be isotropic (a = −0.49%; c = −0.46%), suggesting a random Mn²⁺/Cd²⁺ replacement in the structure (Fig. S3†). These results agree with the data reported on the corresponding powders.¹⁹ Transmission electron microscopy (TEM) micrographs of Cs₄Mn_xCd_1−xSb₂Cl₁₂ NCs evidenced the formation of NCs with a cuboidal shape (Fig. 1c–f), and a mean size of 10.7 ± 3.5 nm and 12.4 ± 3.8 nm for 0% and 100% Mn²⁺, respectively; for the larger Mn²⁺ fractions (x = 0.9, 1) smoothing of the NC edges occurred (Fig. 1f and S4†). High-angle annular dark-field (HAADF) scanning TEM imaging confirmed the crystallinity of the NCs (Fig. 1g and S5†), most often composed of a single crystal domain; occasionally, multiple domains were observed in particles larger than 20 nm (Fig. S5†). Elemental analysis confirmed that all the expected elements are included in the NCs (Fig. S6†). Upon changing the injection temperature (i.e. 140–150–170 °C), we observed no changes in the crystal structure (Fig. S7a†), yet higher temperatures led to NC samples with a more polydisperse distribution of size and morphology (Fig. S7b–e†).

Fig. 1 (a) Crystal structure of Cs₄CdSb₂Cl₁₂; (b) XRD patterns of Cs₄Mn_xCd_1−xSb₂Cl₁₂ NCs; a shift to higher angles on increasing the Mn²⁺ amount was observed (right); (c–f) bright-field TEM micrographs of Cs₄Mn_xCd_1−xSb₂Cl₁₂ NCs. Scale bars are 100 nm; (g) HAADF-STEM image of a single nanocrystal of the Cs₄Mn_0.12Cd_0.88Sb₂Cl₁₂ sample.

All samples displayed an absorption spectrum characterized by three different features: two broad peaks between 300 nm and 400 nm (Fig. 2a and S8a†) that were independent of the Mn²⁺/(Cd²⁺ + Mn²⁺) ratio, and a well-defined peak at 267 nm (for x = 0) whose intensity decreased with a concomitant blue shift with the increasing Mn²⁺ amount (Fig. S8a†). The observed behavior is consistent with the corresponding bulk material except for the intensity variation and shift of the last spectral feature (250–300 nm), which was previously not observed by Solis-Ibarra's group.¹⁹ Overall, the absorption spectrum of Cs₄Mn_xCd_1−xSb₂Cl₁₂ seems to be mostly determined by transitions related to Sb³⁺, not unlike previous observations on Cs₃Sb₂Cl₉.²⁶ Sb³⁺ is an ns² ion characterized by the ¹S₀ ground state and four higher-energy levels denoted as ³P₀, ³P₁, ³P₂ and ¹P₁.²⁷ The absorption in the 300–400 nm range can be ascribed to the partially allowed ¹S₀ → ³P₁ transition, commonly named the A band, split into a doublet; similarly, the allowed ¹S₀ → ¹P₁ transition (C band) is responsible for the absorption found at ∼270 nm.^11,19,27,28 Instead, the d → d Mn²⁺ transition, generally observed for Mn-based materials between 400 nm and 550 nm, was not detected. All the Cs₄Mn_xCd_1−xSb₂Cl₁₂ compositions, except for x = 0, showed a red emission centered at around 625–650 nm under UV excitation (Fig. S8b†). The photoluminescence excitation (PLE) spectra exhibited an intense peak centered at ∼325 nm with a shoulder at ∼285 nm (Fig. 2b). The PLE behavior resembled that of absorption, indicating that the excitation occurs through the Sb³⁺ bands. Interestingly, even if Sb³⁺ is known for exhibiting optical emission in metal halides,^27,29 we did not detect any PL signals in Cs₄CdSb₂Cl₁₂, whereas the ⁴T₁ → ⁶A₁ transition of Mn²⁺ induced the red emission in all the other samples. We explain this behavior by an energy transfer between antimony(III) and manganese(II) ions, Sb³⁺ → Mn²⁺.³⁰ Indeed, the PLE profiles were the same for all the samples, with no considerable shift (Fig. S8c†), while significant differences were detected both in the PL intensity and positions (Fig. 2c, d and S8b†). No differences in the PL were observed when exciting the samples at a different wavelength (Fig. 2b). The intensity increased with the Mn²⁺ ratio (Fig. 2c) up to a maximum at the experimental composition of 11.5% Mn²⁺. Similar results were reported for the corresponding bulk system.¹⁹ By increasing the Mn²⁺ concentration the emission gradually shifted from 624 nm (6% Mn²⁺), a value higher than that in bulk Cs₄Mn_xCd_1−xSb₂Cl₁₂^19,20 but close to reports for the Bi-based counterpart Cs₂MnBi₂Cl₁₂,^21,23 to a roughly constant spectral position of ∼648 nm for x > 0.5 (Fig. 2d). The Mn²⁺ emission shift was already reported in Cs₄Mn(Bi_1−xIn_x)₂Cl₁₂³¹ and CsPbCl₃,^32,33 and ascribed to a contraction of the unit cell; according to the Tanabe–Sugano diagram for the d⁵ ion (e.g. Mn²⁺), an increase of the crystal field induces a red shift in the ⁴T₁ → ⁶A₁ transition. Indeed, the progressive substitution of Cd²⁺ with the smaller Mn²⁺ decreased the cell volume by shrinking the [MnCl₆] octahedra,¹⁹ thus inducing the observed red shift. The variation of the PL intensity can be instead explained considering that the maximum intensity is reached, in principle, when the MnCl₆ octahedra are surrounded by [SbCl₆] and [CdCl₆] octahedra, thus, avoiding any Mn–Mn self-quenching.²³

Fig. 2 (a) Optical absorption at 250 nm and (b) PLE and PL spectra measured at two different excitation wavelengths (320 nm and 286 nm) of the Cs₄Mn_0.12Cd_0.88Sb₂Cl₁₂ sample chosen as a representative. (c) PL intensity (normalized at the absorption at 320 nm) behavior at increasing Mn²⁺ content; the inset shows a photograph of the most emissive sample under UV light. (d) Position of the PL peak with increasing Mn²⁺ content.

The PLQY of the most emissive colloidal sample was found to be 0.3%, a value that is much lower than that reported for the corresponding bulk material (28.5%).¹⁹ The decrease of PLQY with reduced particle size has been previously observed in Cs₄Mn_xCd_1−xBi₂Cl₁₂: when lowering the dimensions of the particles from micrometers to nanometers, the PLQY collapsed from 79.5%²² to 4.6%.²³ This unfavorable effect, already documented in several metal–halide perovskites, can be ascribed to the formation of non-radiative relaxation paths induced by surface traps. To test this hypothesis, we attempted three different post-synthetic treatments to increase the PLQY of the nanocrystals: a small fraction of the nanocrystal suspension was added to (i) a mixture of CdCl₂, OA and OlAm in hexane;^32,34 (ii) a solution of didodecyldimethylammonium chloride (DDAC) in toluene;³⁵ and (iii) a mixture of ammonium thiocyanate (NH₄SCN) in hexane^36,37 (see the ESI and Table S2†). The first two treatments slightly increased the photoluminescence, while a significant enhancement was observed when using NH₄SCN (Fig. S9†); consequently, only the sample treated with NH₄SCN was investigated. During the treatment, the sample emission kept increasing for one hour (Fig. S9†), after which it started decreasing due to the progressive decomposition of the sample, which completely evolved in other phases after three hours (Fig. S10†). The XRD and TEM analyses on the brightest sample (Fig. 3a) showed that the crystalline phase was preserved, even if a small amount of a secondary phase, identified as CsCl, was present. As determined by TEM, the morphology of the NCs was largely preserved, but cuboidal shapes appeared more rounded compared to the pristine sample (Fig. 3b and c). The absorption spectrum revealed a small decrease in the 350–400 nm range and an enhancement of the C band (λ < 300 nm) (Fig. 3d). Furthermore, the PLE spectrum narrowed without a significant shift (Fig. 3e). More importantly, an enhanced PL emission was observed, which resembled the same broadness of the pristine material with a small shift of ∼2 nm (Fig. 3f and S11†). The conservation of the PLE and PL spectral shapes and positions suggests that the post-treatment does not influence the environment around Mn²⁺ (i.e., the crystal structure); as previously demonstrated in other halide perovskites,^36,38,39 the thiocyanate group passivates the surface sites removing the shallow trap and reducing the number of non-radiative relaxation channels. Indeed, the PLQY of the material after the post-synthesis treatment reached a value of 3.9% (Fig. S12†), which is 10 times higher than that in the as-synthesized particles, and comparable to that of the Bi-based counterpart.

Fig. 3 (a) XRD pattern of Cs₄Mn_0.12Cd_0.88Sb₂Cl₁₂ before (black line) and after the thiocyanate treatment (red line). (b) and (c) bright-field TEM images before and after the thiocyanate treatment, respectively. (d) ABS, (e) PLE, and (f) PL of Cs₄Mn_0.12Cd_0.88Sb₂Cl₁₂ before (black line) and after thiocyanate treatment (red line). Inset in (f) shows a photograph under a UV lamp of the sample with (bright orange) and without the treatment (dark orange).

Conclusions

In summary, we successfully synthesized Cs₄Mn_xCd_1−xSb₂Cl₁₂ nanocrystals with tunable composition in the x = 0–1 range. All the samples showed a UV absorption and a red emission stemming from Sb³⁺ and Mn²⁺ ions (except for the Mn-free x = 0), respectively. The PL intensity changed with the Mn²⁺ amount, reaching a maximum for x = 0.12. The pristine sample featured a low PLQY (<1%) that was increased to 3.9% by a post-synthesis treatment with ammonium thiocyanate. The intriguing optical properties of Cs₄Mn_xCd_1−xSb₂Cl₁₂ nanocrystals should stimulate additional studies on this emerging family of layered double perovskites and their application as optoelectronic materials.

Author contributions

E.S. – conceptualization, data curation, formal analysis, investigation (synthesis, XRD, TEM, ICP-OES, and optical analysis), validation, visualization (figures) and writing (original draft); M.C. – data curation, formal analysis, investigation (synthesis, XRD, TEM, and optical analysis), and validation; D.B. – data curation, formal analysis, investigation (optical analysis), and writing (editing); M.Z. – data curation, formal analysis, and investigation (XRD, optical analysis); S.T. – formal analysis and investigation (XRD); G.D. – investigation (HAADF – STEM); M.F. – conceptualization, supervision, and writing (review and editing); Z.H. – conceptualization, supervision, and writing (review and editing); L.M. – conceptualization, supervision, and writing (review and editing); and F.L. – conceptualization, formal analysis, visualization (figures), supervision, and writing (original draft, review and editing). Contributions are assigned using CRediT taxonomy (https://casrai.org/credit/).

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the Materials Characterization Facility and Electron Microscopy Facility at the Fondazione Istituto Italiano di Tecnologia for use of XRD and TEM equipment and technical support and Filippo Drago of Nanochemistry Facility for elemental analysis using ICP-OES. Z. H. acknowledges Ghent University (GOA 01G01019) and FWO-Vlaanderen (SBO Proceed) for research funding. E. S., M. C. and F. L. thank Matilde Cirignano for the fruitful suggestions.

Notes and references

1. Q. A. Akkerman, G. Rainò, M. V. Kovalenko and L. Manna, Nat. Mater., 2018, 17, 1–12.
2. B. Vargas, G. Rodrĺguez-López and D. Solis-Ibarra, ACS Energy Lett., 2020, 5, 3591–3608.
3. W. Ke and M. G. Kanatzidis, Nat. Commun., 2019, 10, 1–4.
4. X. Li, X. Gao, X. Zhang, X. Shen, M. Lu, J. Wu, Z. Shi, V. L. Colvin, J. Hu, X. Bai, W. W. Yu and Y. Zhang, Adv. Sci., 2021, 8, 1–33.
5. F. Giustino and H. J. Snaith, ACS Energy Lett., 2016, 1, 1233–1240.
6. J. Luo, X. Wang, S. Li, J. Liu, Y. Guo, G. Niu, L. Yao, Y. Fu, L. Gao, Q. Dong, C. Zhao, M. Leng, F. Ma, W. Liang, L. Wang, S. Jin, J. Han, L. Zhang, J. Etheridge, J. Wang, Y. Yan, E. H. Sargent and J. Tang, Nature, 2018, 563, 541–545.
7. F. Locardi, E. Sartori, J. Buha, J. Zito, M. Prato, V. Pinchetti, M. L. Zaffalon, M. Ferretti, S. Brovelli, I. Infante, L. De Trizio and L. Manna, ACS Energy Lett., 2019, 4, 1976–1982.
8. N. R. Wolf, B. A. Connor, A. H. Slavney and H. I. Karunadasa, Angew. Chem., Int. Ed., 2021, 60, 16264–16278.
9. G. Volonakis, A. A. Haghighirad, R. L. Milot, W. H. Sio, M. R. Filip, B. Wenger, M. B. Johnston, L. M. Herz, H. J. Snaith and F. Giustino, J. Phys. Chem. Lett., 2017, 8, 772–778.
10. S. E. Creutz, E. N. Crites, M. C. De Siena and D. R. Gamelin, Nano Lett., 2018, 18, 1118–1123.
11. A. Noculak, A. Noculak, V. Morad, V. Morad, K. M. McCall, S. Yakunin, Y. Shynkarenko, S. Yakunin, Y. Shynkarenko, M. Wörle and M. V. Kovalenko, Chem. Mater., 2020, 32, 5118–5124.
12. F. Locardi, M. Cirignano, D. Baranov, Z. Dang, M. Prato, F. Drago, M. Ferretti, V. Pinchetti, M. Fanciulli, S. Brovelli, L. De Trizio and L. Manna, J. Am. Chem. Soc., 2018, 140, 12989–12995.
13. Q. Liao, J. Chen, L. Zhou, T. Wei, L. Zhang, D. Chen, F. Huang, Q. Pang and J. Z. Zhang, J. Phys. Chem. Lett., 2020, 11, 8392–8398.
14. N. K. Nila and N. Angshuman, Chem. Commun., 2018, 54, 5205–5208.
15. S. Toso, D. Baranov and L. Manna, ACS Energy Lett., 2020, 5, 3409–3414.
16. B. Vargas, E. Ramos, E. Pérez-Gutiérrez, J. C. Alonso and D. Solis-Ibarra, J. Am. Chem. Soc., 2017, 139, 9116–9119.
17. T. Cai, W. Shi, S. Hwang, K. Kobbekaduwa, Y. Nagaoka, H. Yang, K. Hills-Kimball, H. Zhu, J. Wang, Z. Wang, Y. Liu, D. Su, J. Gao and O. Chen, J. Am. Chem. Soc., 2020, 142, 11927–11936.
18. B. Vargas, R. Torres-Cadena, J. Rodríguez-Hernández, M. Gembicky, H. Xie, J. Jiménez-Mier, Y. S. Liu, E. Menéndez-Proupin, K. R. Dunbar, N. Lopez, P. Olalde-Velasco and D. Solis-Ibarra, Chem. Mater., 2018, 30, 5315–5321.
19. B. Vargas, E. Coutiño-Gonzalez, O. Ovalle-Encinia, C. Sánchez-Aké and D. Solis-Ibarra, J. Phys. Chem. Lett., 2020, 11, 10362–10367.
20. C. Peng, Q. Wei, L. Chen, R. Zeng, Q. Zhang, Q. Hu and B. Zou, J. Mater. Chem. C, 2021, 9, 15522.
21. N. P. Holzapfel, J. D. Majher, T. A. Strom, C. E. Moore and P. M. Woodward, Chem. Mater., 2020, 32, 3510–3516.
22. B. Vargas, D. T. Reyes-Castillo, E. Coutino-Gonzalez, C. Sánchez-Aké, C. Ramos, C. Falcony and D. Solis-Ibarra, Chem. Mater., 2020, 32, 9307–9315.
23. H. Yang, W. Shi, T. Cai, K. Hills-Kimball, Z. Liu, L. Dube and O. Chen, Nanoscale, 2020, 12, 23191–23199.
24. T. Cai, W. Shi, D. J. Gosztol, K. Kobbekaduwa, H. Yang, N. Jin, Y. Nagaoka, L. Dube, J. Schneider, S. Hwang, J. Gao, X. Ma and O. Chen, Matter, 2021, 4, 2936–2295.
25. T. T. Tran, T. T. Tran, C. A. Pocs, Y. Zhang, Y. Zhang, M. J. Winiarski, M. J. Winiarski, J. Sun, M. Lee, T. M. McQueen and T. M. McQueen, Phys. Rev. B, 2020, 101, 235107.
26. B. Pradhan, G. S. Kumar, S. Sain, A. Dalui, U. K. Ghorai, S. K. Pradhan and S. Acharya, Chem. Mater., 2018, 30, 2135–2142.
27. Y. Jing, Y. Liu, M. Li and Z. Xia, Adv. Opt. Mater., 2021, 9, 1–15.
28. F. Locardi, M. Samoli, A. Martinelli, O. Erdem, D. V. Magalhaes, S. Bals and Z. Hens, ACS Nano, 2021, 15, 17729–17737.
29. H. Arfin, A. S. Kshirsagar, J. Kaur, B. Mondal, Z. Xia, S. Chakraborty and A. Nag, Chem. Mater., 2020, 32, 10255–10267.
30. X. Liu, X. Xu, B. Li, L. Yang, Q. Li, H. Jiang and D. Xu, Small, 2020, 16, 1–7.
31. S. He, S. Fang, T. Han, T. Lang, M. Cai, H. You, L. Peng, S. Cao, B. Liu, Q. Qiang, J. Chen and B. Lei, J. Phys. Chem. C, 2021, 125(31), 16938–16945.
32. S. Ji, X. Yuan, S. Cao, W. Ji, H. Zhang, Y. Wang, H. Li, J. Zhao and B. Zou, J. Phys. Chem. Lett., 2020, 11, 2142–2149.
33. H. Liu, Z. Wu, J. Shao, D. Yao, H. Gao, Y. Liu, W. Yu, H. Zhang and B. Yang, ACS Nano, 2017, 11, 2239–2247.
34. N. Mondal, A. De and A. Samanta, ACS Energy Lett., 2019, 4, 32–39.
35. M. Imran, J. Ramade, F. Di Stasio, M. De Franco, J. Buha, S. Van Aert, L. Goldoni, S. Lauciello, M. Prato, I. Infante, S. Bals and L. Manna, Chem. Mater., 2020, 32, 10641–10652.
36. M. Lu, J. Guo, P. Lu, L. Zhang, Y. Zhang, Q. Dai, Y. Hu, V. L. Colvin and W. W. Yu, J. Phys. Chem. C, 2019, 123, 22787–22792.
37. V. G. V. Dutt, S. Akhil and N. Mishra, ChemNanoMat, 2020, 6, 1730–1742.
38. B. A. Koscher, J. K. Swabeck, N. D. Bronstein and A. P. Alivisatos, J. Am. Chem. Soc., 2017, 139, 6566–6569.
39. J. T. Lin, Y. K. Hu, C. H. Hou, C. C. Liao, W. T. Chuang, C. W. Chiu, M. K. Tsai, J. J. Shyue and P. T. Chou, Small, 2020, 16, 2–9.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/d1nr06200j

---