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Metal halide perovskites have generated significant attention in recent years because of their extraordinary physical properties and photovoltaic performance. Among these, inorganic perovskite quantum dots (QDs) stand out for their prominent merits, such as quantum confinement effects, high photoluminescence quantum yield, and defect-tolerant structures. Additionally, ligand engineering and an all-inorganic composition lead to a robust platform for ambient-stable QD devices. This review presents the state-of-the-art research progress on inorganic perovskite QDs, emphasizing their electronic applications. In detail, the physical properties of inorganic perovskite QDs will be introduced first, followed by a discussion of synthesis methods and growth control. Afterwards, the emerging applications of inorganic perovskite QDs in electronics, including transistors and memories, will be presented. Finally, this review will provide an outlook on potential strategies for advancing inorganic perovskite QD technologies.

Metal halide perovskites were discovered to exhibit photoconductivity in 1957 [1]. Still, it was not until the last decade that they started to attract enormous attention in the materials science community because of their extraordinary power conversion capability in photovoltaic devices [2,3,4,5]. After intensive developments, they have emerged as up-and-coming photovoltaic materials with the highest energy conversion efficiency among thin film materials and promoted the development of high-performance optoelectronic devices [6,7,8,9]. In addition, metal halide perovskites have also been exploited as active materials in other high-performance applications such as light-emitting diodes (LEDs), field-effect transistors (FET), and photoelectrochemical catalysis [10,11,12,13,14].

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Quantum dots (QDs) with tens of nanometers or smaller dimensions, which feature quantum confinement effects, have been regarded as optical materials distinct from their bulk counterparts and garnered rising research interests [15,16,17]. Typically, they exhibit eminent physical properties, including size-tunable emission wavelength [17], high photoluminescence quantum yield (PLQY) [18], delta-function-like density of states [19], large optical oscillator strength [20], and low-threshold operation [21]. By virtue of the advantageous characteristics, they are envisioned to find applications in LEDs [22, 23], solar cells [24, 25], lasers [19, 26], medical imaging [27, 28], single-photon source [29, 30] and quantum computing [31, 32]. With their color-tunable emission wavelength and high color purity privilege, major display manufacturers such as Sony, Samsung, and LG have used QDs in commercial products and produced QLED displays [33].

Semiconductor QDs such as CdSe and InP have been widely investigated in the past three decades [34, 35]. Optical absorption and photoluminescence (PL) spectroscopic studies on QDs revealed unique properties such as narrow peak widths of excitons along with exotic properties of biexcitons and higher-order excitons [36]. Although ligands can guarantee phase stabilization of QDs, the excess capping ligands on the QD surface, a typical side effect of colloidal synthesis, may hinder charge transport [37, 38]. As a result, the current of QD solar cells is restricted, and the device performance is often inferior to thin-film alternatives. Therefore, the intricate ligand engineering on QD surfaces is crucial to maintaining both phase stability and carrier transport.

Compared to many traditional semiconductor QDs, inorganic perovskite QDs preserve high-performance features in the presence of high-concentration defects [13, 48]. Furthermore, unlike their bulk and thin film counterparts, the quantum confinement effect leads to strong PL emissions with a high PLQY [49, 50]. They offer other photophysical properties, such as tunable PL emission across the entire spectral range, narrow full width at half maximum (FWHM), large multiple-photon absorption cross section, and low threshold of population inversion. The promising properties enable inorganic perovskite QDs to be used in wide-range applications, including LED, solar cells, photodetectors, nonlinear emission sources, and electro-optic modulators [51,52,53,54]. Additionally, they were also exploited in fast X-ray scintillators for ionizing radiation detection [34]. Very recently, the usage of inorganic perovskite QDs has been extended to the area of electronics. Although plenty of work has been done in this emerging area of QD applications, to the best of our knowledge, the progress on inorganic perovskite QD electronics has not been reported in any review.

Despite the benefits listed above, the main challenge confronting inorganic perovskite QDs is long-term structural stability [55]. There are three significant aspects regarding the instability of inorganic perovskite QDs [56]: (i) inorganic perovskite QDs can be degraded by polar solvents or ionic compounds, threatening the long-term structural integrity of QDs [57]; (ii) the ligand-binding is highly ionic, which causes fast ligand desorption and weakens the colloidal state and structural integrity [58]; (iii) light or electric field-induced halide migration causes bond breaking in inorganic perovskite QDs and deteriorates optoelectronic performance [59]. Hence, the maintenance of their structural integrity remains a critical issue.

In this review, we present the state-of-the-art research progress on inorganic perovskite QDs focusing on their electronic applications. The structural and physical characteristics of inorganic perovskite QDs will be thoroughly discussed. Then, the general synthetic methodologies, as well as the size and shape management of inorganic perovskite QDs, will be highlighted. Finally, inorganic perovskite QDs as an active layer in the application of transistors and memory devices will be discussed, which will be followed by a perspective on the future developments of perovskite QD electronics.

It is well known that inorganic perovskites have three main phases: cubic, tetragonal, and orthorhombic [62,63,64,65]. The preferentially stable phase at room temperature is dramatically related to the size of inorganic perovskites. For example, the tetragonal phase is most stable for bulk CsPbI3 at room temperature but will convert to a non-perovskite orthorhombic phase due to low formation energy [66,67,68]. However, when reducing CsPbI3 to the QD scale, the surface energy and the total Gibbs free energy for the non-perovskite orthorhombic phase become more considerable, thereby inhibiting the detrimental phase transition (Fig. 1b). Hence, the cubic and tetragonal phases of CsPbI3 QDs are more thermodynamically favorable than the solar-inactive non-perovskite orthorhombic phase [67]. Furthermore, the phase transformation can be triggered via various external conditions. CsPbI3 QDs with highly metastable cubic phase readily convert to the stable non-perovskite orthorhombic phase via adding extra polar additives capable of removing the surface ligands, leading to the loss of strong-emitting properties. CsPbBr3 and CsPbCl3 QDs undergo phase transformation at elevated temperatures.

One of the most essential characteristics of QDs is quantum confinement, which leads to the quantization of energy due to their nanoscale sizes. This phenomenon occurs when the wavefunctions of electrons and holes are shrunk down to dimensions smaller than the excitonic Bohr radius [70]. The confinement energy can be estimated as \(\Delta E=\frac\hslash ^2\pi ^22m^*r^2\), where \(\hslash\) is Planck constant, m* is the exciton reduced mass which relates to the Bohr radius through the effective mass approximation, and r is the particle radius. The excitonic Bohr radii of inorganic halide perovskites are quite small, in the range of a few nanometers [16]. By controlling the size of inorganic perovskite QDs, their bandgap can be tunable, further influencing their PL emission wavelengths. According to the study by Butkus et al., when the particle size decreases from 8.5 to 4.1 nm, the bandgap of CsPbBr3 QDs increases from 2.37 to 2.5 eV (Fig. 1d), accompanied by obvious blue shifts in the PL spectra [69]. However, once the size of inorganic perovskite QDs surpasses the Bohr radius, the quantum confinement effect is no longer remarkable [71].

Inorganic perovskite QDs exhibit strong and narrow PL light emissions [74, 75]. Compared with traditional semiconductor QDs such as CdSe, CdS, or PbS, inorganic perovskite QDs can achieve more than 99% PLQY without any surface passivation by the wide-bandgap epitaxial shells [76,77,78]. This remarkable trait is a symptom of the high defect tolerance in inorganic perovskite QDs, which derives from their electronic structure and the bandgap between two antibonding orbitals [71].

Synthesis methods of inorganic perovskite QDs are mainly based on solution reaction and can be divided into direct synthesis and post-synthesis [93, 94]. Direct synthesis comprises ligand-assisted reprecipitation (LARP), emulsion synthesis, hot injection, ultrasonication, microwave, solvothermal, and chemical vapor deposition (CVD). These approaches produce QDs of various forms, including nanospheres and nanocubes. Post-synthesis is an alternative approach and uses QDs that have already been manufactured as templates. Ion exchange and phase transformation are prominent examples of post-synthesis techniques.

In general, direct synthesis methods can be divided into three categories, solution based, solid based, and gas based. The solution-based synthesis includes LARP, emulsion synthesis, hot injection, ultrasonication, microwave, and solvothermal. In the typical solvent-free solid-based method, mortar and pestle were used to ground precursors in a mechanochemical process. The gas-based method mainly relies on chemical vapor deposition (CVD).

The original concept of LARP was proposed by Schmide et al. [39]. They demonstrated the preparation of MAPbX3 perovskite QDs via a facile colloidal strategy using ligands with a medium-sized chain to stabilize the colloidal phase. Inspired by this, Zhang et al. further modified the process to prepare MAPbX3 QDs and named it LARP [95]. The mechanism of LARP is shown in Fig. 2b. In a typical process, the precursor, which is a mixture of MABr, PbBr2, OA, and n-octylamine (OM) dissolved in dimethylformamide (DMF), is poured into a vessel containing toluene while stirring, and supersaturate precipitation is induced at room temperature to form a yellow-green QDs dispersion, thereby overcoming the temperature limitations of hot-injection methods. Li et al. also developed a similar method leveraging ligand assistance to generate inorganic perovskite QDs under the room temperature (Fig. 2c) [96]. The obtained inorganic perovskite QDs have a monoclinic phase structure, unlike the conventional product of hot injection. The emulsion synthesis method includes the emulsion formation and demulsion processes for producing inorganic perovskite QDs, as illustrated in Fig. 2d [97]. The surfactants, polar, and nonpolar solvents are mixed to start the emulsion, followed by adding the demulsifier into the immiscible solution to crystallize QDs. Taking Yang et al.'s study as an example, they first dissolved CsBr in deionized water (diH2O) and PbBr2 in DMF. Then, they made the "oil phase" by mixing oleic acid with n-octylamine in 10 mL hexane. After that, a dropwise mixture of the CsBr-diH2O and PbBr2-DMF solution was added into the oil phase. This caused the oil phase to progressively transform from clear to a faint white color, which resulted in the formation of an emulsion. Finally, acetone was used to initiate a demulsion process, and the QDs were collected after centrifuging the mixture [98]. Although LARP and emulsion techniques have similar mechanism, their supersaturated environments are distinct. Specifically, the solvent mixing in LARP would promote a change in solubility and result in the nucleation of QDs. In contrast, in the emulsion method, the crystallization of QDs would be triggered by microreactors that result from solvent mixing.

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