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A conspicuous feature of all-inorganic metal halide perovskite NCs is the tunability of their optical bandgap energy by modifying the chemical compositions. Cation- or anion exchange strategies have been actively applied. Among various methods, the anion exchange method allows for effective composition modifications owing to the fast ion migration rate of halides. Notably, the tunability of the optical bandgap leads to wide-range wavelength detection and light selectivity. In addition, this method allows for the fabrication of cubic CsPbCl₃ or CsPbI₃ NCs, which cannot be directly synthesized owing to their rapid transformation into orthorhombic or tetragonal phases at room temperature (Fig. 2a). This is because halide ions (Cl, Br, and I) readily diffuse and migrate through the vacancy sites present in perovskite NCs. When bromide ions in CsPbBr₃ NCs are exchanged with chloride or iodide, the photoluminescence (PL) peak shifts owing to the change in the optical bandgap energy (Fig. 2b, left). By exploiting these features, several research groups have successfully modulated the properties of perovskite NCs for their applications in photodetectors. For example, Ramasamy et al. added an LiCl or LiI solution to CsPbBr₃ NC suspensions to engineer the chemical composition of the NCs [14]. They fabricated visible-range photodetectors using anion-exchanged CsPbI₃ NCs and achieved a high on/off ratio (>10⁵) for the first time (Fig. 2b, right). Xu et al. developed a facile anion-exchange strategy, wherein halide ions were generated via the photolysis of a solvent (Fig. 2c) [15]. By adjusting the laser exposure time of the CsPbBr₃ NC films, their compositions were precisely engineered, and the CsPbBr₃- and CsPb(Cl/Br)₃ NC-based photodetectors exhibited high photoresponsivities at 540 nm and 470 nm, respectively (Fig. 2d).

To further improve the photodetector performance, Kwak et al. incorporated mixed halide perovskite NCs into graphene-based devices (Fig. 2e) [16]. Notably, while pristine graphene-based devices exhibit p-type doping behavior owing to oxidation, CsPb(Br/I)₃ NC-incorporated devices exhibit a negative shift in the Dirac point (Fig. 2f). Because the equilibrium condition at the interfacial layers of the two types of materials provides a better path for charge carriers, the photoresponsivity and response time of the device present improvements. In addition, Mandal et al. reported a method to transform perovskite NCs into nanosheets to improve the photodetector performance (Fig. 2g, h) [17]. The developed two-dimensional photoactive material (nanosheets) exhibited a lower compressive lattice strain and accelerated charge carrier mobility. In addition, by exchanging a small amount of Br⁻ with I⁻, the light absorption of the material could be enhanced. Thus, CsPbBr_1.5I_1.5 nanosheet-based photodetectors have achieved better responsivity and detectivity compared to conventional all-inorganic metal halide perovskites.

Fig. 2. 
(a) Schematic of the anion-exchange process of CsPbBr₃ NCs using LiX in a solution. (b) Normalized PL spectrum of anion-exchanged CsPbBr₃ NCs (left), and I–t curves (right) of anion-exchanged CsPbI₃ NC-based photodetectors (a, b = reprinted with permission from Ref. [14]. Copyright (2016) Royal Society of Chemistry). (c) Schematic of an anion-exchange process with the laser-assisted photolysis of a halocarbon solvent. (d) I–V curves of photodetectors based on CsPbBr₃ NCs and anion-exchanged CsPb(Cl/Br)₃ NCs (c, d = reprinted with permission from Ref. [15]. Copyright (2022) Springer Science + Business Media). (e) Schematic energy diagrams of graphene–CsPb(Br/I)₃ NCs in the separated state (left), contact state (middle), and contact under illumination (right). (f) I–t curves of graphene–mixed halide perovskite NC-based photodetectors (e, f = reprinted with permission from Ref. [16]. Copyright (2022) Springer Science + Business Media). (g) Photographs and schematic of the as-synthesized (left), mixed halide (middle), and completely anion-exchanged (right) perovskite nanosheets. (h) Scanning electron micrographs (left) and I–t curves (right) of CsPbBr_xI_y nanosheet-based photodetectors (g, h = reprinted with permission from Ref. [17]. Copyright (2021) American Chemical Society).

To improve the performance of CsPbBr₃ NC-based photodetectors, numerous research groups have engineered the chemical composition of CsPbBr₃ NCs by introducing isovalent or heterovalent metal dopants, as depicted in Fig. 3. In this regard, replacing Pb²⁺ in the metal halide perovskites with isovalent dopants, such as Sn²⁺, Ca²⁺, Sr²⁺, Cd²⁺, Zn²⁺, and Mn²⁺, can modify their optical properties or decrease the bandgap [22–24]. Typically, the introduction of trivalent metals, such as Ce³⁺, Bi³⁺, Sb³⁺, In³⁺, Eu³⁺, and Yb³⁺, leads to a longer lifetime and enhanced PL quantum yield owing to the suppression of ionic migration [25]. For example, He et al. synthesized Mn-doped CsPbBr₃ NCs by introducing HBr during their synthesis and post-treatment processes to tune the optical bandgap (Fig. 3a) [18]. The addition of HBr did not negatively affect the structural properties of the CsPbBr₃ NCs and facilitated the ionization of MnBr₂, supplying Mn²⁺ to the NCs. The Mn-doped CsPbBr₃ NCs exhibited a longer carrier lifetime, and the Mn-doped CsPbBr₃ NC-based photodetectors presented improved photoresponsivity and faster response times than photodetectors based on the as-synthesized CsPbBr₃ NCs (Fig. 3b). Wang et al. reported that Zn²⁺ doping played a role in decreasing the number of defects in CsPbBr₃ NCs (Fig. 3c, d) [19]. The dopant optimized the carrier-trapping process and weakened the “overshoot” phenomenon, that is, the appearance of sharp transient peaks in the photocurrent curves owing to rapid changes in the incident light signal.

Fig. 3. 
(a) Images of the as-synthesized (green), HBr-added (blue), Mn-doped (with HBr) (pink), and Mn-doped (with a larger amount of HBr) colloidal CsPbBr₃ NCs. (b) Performance of photodetectors based on Mn-doped CsPbBr₃ NCs (a, b = reprinted with permission from Ref. [18]. Copyright (2021) Springer Science + Business Media). (c) Schematic of the reduced “overshoot” phenomenon. (d) Time-resolved PL curves (left) and I–t curves (right) of the as-synthesized and Zn-doped CsPbBr₃ NCs (c, d = reprinted with permission from Ref. [19]. Copyright (2021) American Chemical Society). (e) Schematic of defect passivation and the Sb³⁺-doping mechanism. (f) Time-resolved PL curves of the as-synthesized and Mn-doped CsPbBr₃ NCs, and (g) photodetector response of the Mn-doped CsPbBr₃ NCs (e-g = reprinted with permission from Ref. [20]. Copyright (2022) American Chemical Society). (h) Photographs of polished 0, 0.2, 0.25, 0.4, and 1.0 at% Ag⁺/Bi³⁺ co-doped CsPbBr₃ NCs on wafers (upper), and schematic of the Fermi level, defect state density, valence, and conduction band edge position (lower). (i) logI–logV curves (left) and I–V curves (right) of 0.2 at% Ag⁺/Bi³⁺ co-doped CsPbBr₃ NCs (h, i = reprinted with permission from Ref. [21]. Copyright (2022) Elsevier).

Furthermore, the effects of trivalent dopants on the properties of the CsPbBr₃ NCs have been investigated. Subramaniam et al. reported a strategy to introduce Sb³⁺ into CsPbBr₃ NCs to fill halide vacancies (Fig. 3e) [20]. Because the Sb³⁺ species occupied the defect sites of the Br⁻ vacancies on the CsPbBr₃ NC surface, the carrier lifetime and PL quantum yield increased (Fig. 3f). Owing to the reduced number of defective surface sites, the response times of the Sb-doped CsPbBr₃ NC-based photodetectors were significantly enhanced (Fig. 3g). Gong et al. proposed an Ag⁺/Bi³⁺ co-doping strategy to prevent a decrease in the concentration of defect states and changes in the Fermi level, which were caused by the introduction of Bi³⁺ as a single dopant (Fig. 3h) [21]. Although metal dopants play a role in decreasing the number of defect sites, the optical bandgap energy can also change owing to changes in the chemical composition or structural distortions. To resolve these issues, Gong et al. introduced another metal dopant, Ag⁺, into CsPbBr₃ crystals using the vertical Bridgman technique, thereby successfully controlling the bandgap energy and improving the performance of CsPbBr₃ NC-based photodetectors (Fig. 3i).

Typically, zero-dimensional nanomaterials, such as NCs and quantum dots, require surface modification strategies, such as ligand exchanges, to improve their oxidation stability and physical properties [30]. The stability and physical properties of NCs can be determined based on their lengths and the functional (terminal) groups of their surface ligands [31]. Yan et al. reported that a conductive polymer ligand, 3,4-ethylenedioxythiophene (EDOT), improved the photocurrent response of CsPbBr₃ NC-based photodetectors by enhancing carrier transport (Fig. 4a) [26]. This was because the lone pair electrons on the S atoms of EDOT passivated the defects on the surface of the CsPbBr₃ NCs (Fig. 4b). In addition, the conjugated structure of the benzene ring of EDOT improved the stability of NCs. Gong et al. exploited 3-mercaptopropionic acid (MPA) as a ligand to improve the air stability (oxidation resistance) of CsPbX₃ NCs (Fig. 4c) [27]. They observed that the MPA ligand-passivated CsPbCl₃ NCs exhibited higher sustainability under ambient conditions, despite the lower steric hindrance of MPA compared to that of the oleate ligand (Fig. 4d). This is because the –SH groups are coupled electronically with the CsPbCl₃ NCs, preventing the degradation in charge transfer.

Fig. 4. 
(a) Schematic of the carrier transport in solid films of (left) oleic acid-passivated and (right) EDOT-passivated CsPbBr₃ NCs. (b) Time-resolved PL curves (left) and I–V curves (right) (a, b = reprinted with permission from Ref. [26]. Copyright (2021). Springer Science + Business Media). (c) Schematic of the surface passivation process, wherein the oleate ligands are replaced with MPA ligands. (d) Photocurrent measured using CsPbCl₃ NC-based photodetectors without and with passivation under ambient conditions (left), and photoresponsivity as a function of the bias voltage at different intensities of 400 nm light (right) (c, d = reprinted with permission from Ref. [27]. Copyright (2019) American Chemical Society). (e) Schematic illustrating the excitation process in a CsPbBr₃ NC-MPA-TiO₂ system following light irradiation. (f) Time-resolved PL spectra (upper) and I–t curves (lower) of CsPbBr₃ quantum dot films without and with MPA treatment (e, f = reprinted with permission from Ref. [28]. Copyright (2017) Royal Society of Chemistry). (g) Schematic of the transformation of CsPbBr₃ NCs to nanowires through a ligand-exchange strategy. (h) Time-resolved PL spectra of CsPbBr₃ (left) NCs and nanowires (middle), and I–t curves of the CsPbBr₃ NCs and NWs (g, h = reprinted with permission from Ref. [29]. Copyright (2022) American Chemical Society).

Several research groups have reported that the ligand exchange of CsPbBr₃ NCs is beneficial for the implementation of other surface modification strategies. Zhou et al. employed MPA ligands as bifunctional linkers to capture TiO₂ nanoparticles on a CsPbBr₃ NC surface (Fig. 4e) [28]. The thiol (–SH) and carboxylate (–COOH) groups of MPA interacted with the CsPbBr₃ NC and TiO₂ nanoparticles, respectively; therefore, the TiO₂ nanoparticles tethered to the NC surface. Because the excited electrons were transferred from the CsPbBr₃ NCs to the MPA ligand and then to the TiO₂ nanoparticles, the decay time of the carriers decreased (Fig. 4f). Zhao et al. reported that treatment with aminocaproic acid ligands allowed the transformation of CsPbBr₃ NCs into CsPbBr₃ nanowires (Fig. 4g) [29]. The change in the dimensions of the material from zero to one via anisotropic growth contributed to a decrease in the number of defect sites, leading to an improved on/off ratio of the photodetector (Fig. 4h).