Engineering Interfacial Charge Transfer in CsPbBr3 Perovskite Nanocrystals by Heterovalent Doping.

In this work, we demonstrated an in situ approach for doping CsPbBr3 nanocrystals (NCs) with In3+ and Cl- with a ligand-assisted precipitation method at room temperature. The In3+ and Cl- co-doped NCs are characterized by the powder x-ray diffraction patterns, ultraviolet-visible, photoluminescence (PL) spectroscopy, time-resolved PL (TRPL), ultraviolet photoelectron spectroscopy, x-ray photoelectron spectroscopy, and transmission electron microscopy. Based on PL and TRPL results, the non-radiative nature of In3+-doping induced localized impurity states is revealed. Furthermore, the impact of In3+ and Cl- doping on charge transfer (CT) from the NCs to molecular acceptors was investigated and the results indicate that the CT at the interface of NCs can be tuned and promoted by In3+ and Cl- co-doping. This enhanced CT is attributed to the enlarged energy difference between relevant states of the molecular acceptor and the NCs by In3+ and Cl- upon co-doping. This work provides insight into how to control interfacial CT in perovskite NCs, which is important for optoelectronic applications.

Perovskite nanocrystals (NCs) recently emerged as a suitable candidate for optoelectronic applications because of its simplistic synthesis approach and superior optical properties. For better device performance, the effective absorption of incident photons and the understanding of charge transfer (CT) process are the basic requirements. Herein, we investigate the interfacial charge transfer dynamics of CsPbBr3 NCs in the presence of different molecular acceptors; 7,7,8,8-Tetracyanoquinodimethane (TCNQ) and 11,11,12,12 tetracyanonaphtho-2,6-quinodimethane (TCNAQ). The vivid change in CT dynamics at the interfaces of NCs and two different molecular acceptors (TCNQ and TCNAQ) has been observed. The results demonstrate that the ground state complex formation in the presence of TCNQ acts as additional driving force to accelerate the charge transfer between the NCs and molecular acceptor. Moreover, this donor (NCs)-acceptor (TCNQ, TCNAQ) system results in the higher absorption of incident photons. Finally, the photo detector based on CsPbBr3-TCNQ system was fabricated for the first time. The device exhibited a high on–off ratio (104). Furthermore, the CsPbBr3-TCNQ photodetector shows a fast photoresponse times of 180 ms/110 ms (rise/decay time) with a specific detectivity (D*) of 5.2 × 1011 Jones. The simple synthesis and outstanding photodetection abilities of this perovskite NCs-molecular acceptor system make them potential candidates for optoelectronic applications.

Hybrid organic/inorganic perovskites have recently emerged as an important class of materials and have exhibited remarkable performance in photovoltaics. To further improve their device efficiency, an insightful understanding of the interfacial charge transfer (CT) process is required. Here, we report the first direct experimental observation of the tremendous effect that the shape of perovskite nanocrystals (NCs) has on interfacial CT in the presence of a molecular acceptor. A dramatic change in CT dynamics at the interfaces of three different NC shapes, spheres, platelets, and cubes, is recorded. Our results clearly demonstrate that the mechanism of CT is significantly affected by the NC shape. More importantly, the results demonstrate that complexation on the NC surface acts as an additional driving force not only to tune the CT dynamics but also to control the reaction mechanism at the interface. This observation opens a new venue for further developing perovskite NCs-based applications.

Facile method for the preparation of high-performance photodetectors with a GQDs/perovskite bilayer heterostructure

Interfacial charge and energy transfer critically control excited-state exciton dynamics in two-dimensional transition metal dichalcogenide (TMD) junctions and offer a promising avenue for external control via strong light-matter interactions in optical microcavities. In this work, we develop an exciton-based nonadiabatic molecular dynamics framework that combines linear-response time-dependent density functional theory with the Pauli-Fierz formulation of cavity quantum electrodynamics, and apply it to systematically investigate cavity-modified energy relaxation and charge transfer processes in H-stacked WS2/MoS2 junctions. In the absence of a microcavity, exciton energy relaxation is found to be governed by hole transfer from the K valley to the Γ valley, which constitutes the rate-limiting step due to the combined effects of energy and momentum mismatches. Upon coupling to an optical microcavity, exciton-photon hybridization generates polaritonic intermediate states with pronounced interlayer character, thereby reconstructing the excitonic relaxation pathways and accelerating both energy dissipation and charge transfer. This enhancement is most prominent in dual-mode microcavities that are simultaneously near-resonant with the intralayer bright excitons of both constituent layers, leading to the fastest relaxation dynamics. These results reveal the microscopic mechanism by which optical microcavities regulate nonequilibrium exciton dynamics in TMD junctions and establish cavity-engineered polaritonic states as an effective strategy for controlling exciton relaxation and interfacial charge transfer in low-dimensional materials.

<i>D</i> ^{6 <i>h</i>} Symmetric Radical Donor–Acceptor Nanographene Modulated Interfacial Carrier Transfer for High-Performance Perovskite Solar Cells

Hybrid van der Waals heterojunctions based on organic polymers and 2D materials have emerged as a promising solution for developing more efficient optoelectronic devices. Herein, we investigated the charge transfer (CT) dynamics at the interface of the poly[3-hexylthiophene-2,5-diyl] (P3HT) organic polymer and a MoS2 monolayer. A global picture of the charge transfer dynamics of a P3HT/MoS2/SiO2 heterojunction was elucidated from photoluminescence (PL) spectroscopy and the fluorescence lifetime decay profile. Rapid interfacial charge transfer between P3HT and MoS2 was indicated by strong PL quenching and a reduction in the average fluorescence lifetime (τav) of the P3HT/MoS2/SiO2 heterojunction. The role of specific electronic states in the interfacial CT process was investigated by applying the core hole clock approach. CT times (τCT) on femtosecond and sub-femtosecond timescales were estimated using the S1s core-hole lifetime as the internal clock. Sub-femtosecond CT was observed for electrons excited to S3pz (0.34 fs) electronic states of MoS2 and to π* (C-C) (0.45 fs) electronic states of P3HT in the P3HT/MoS2/SiO2 heterojunction. These fast bidirectional CT processes result from strong coupling between these two electronic states in the P3HT/MoS2/SiO2 heterostructure. However, the reduction of the τCT values in the heterojunction compared with those of the isolated films shows that interfacial CT from the P3HT species to MoS2 is more efficient. Interfacial CT was not observed for electrons excited to electronic states S3px,y (MoS2) and σ* (S-C) (P3HT). We conclude that the π* (C-C) electronic state of the P3HT species is the main pathway for interfacial ultrafast CT in a P3HT/MoS2/SiO2 heterojunction.

Metal halide perovskite nanocrystals (NCs), known for their strong visible-light absorption and tunable optoelectronic properties, show significant promise for photocatalytic applications. However, their efficiency is often hindered by rapid charge recombination and insufficient exciton dissociation, limiting effective catalysis. Excited-state interactions at the NC interface are critical in determining photocatalytic performance, underscoring the need for strategies that enhance charge separation and minimize recombination. To address these challenges, we developed a composite material by combining cesium lead bromide (CsPbBr3) nanocrystals with ferrocene carboxylic acid (FcA), a hole-extracting moiety. This integration enhances exciton dissociation through energy level alignment and recombination suppression, resulting in a 3-fold increase in the photocatalytic oxidation yield of benzylamine to N-benzylidenebenzylamine (35 ± 5% versus 12 ± 2% for pristine CsPbBr3). Additionally, thionyl bromide (SOBr2) surface modification strips off ligands and introduces bromide ions onto the CsPbBr3 NCs, further improving charge transfer and substrate accessibility, resulting in a 27 ± 5% yield within 3 h. While SOBr2 treatment enhances initial catalytic performance, its acidic nature may lead to reversible reactions and side products over extended reaction times. This study highlights the impact of molecular integration and surface engineering on optimizing interfacial charge dynamics, providing a pathway toward robust, high-efficiency perovskite photocatalysts for sustainable chemical transformations.

Metal halide perovskite nanocrystals (NCs) have attracted great attention in the fields of light-emitting diodes, lasers, X-ray imaging, solar cells and photoelectric detectors due to their excellent optoelectronic properties. Compared with organic-inorganic hybrid perovskite NCs, all inorganic perovskite CsPb&lt;i&gt;X&lt;/i&gt;&lt;sub&gt;3&lt;/sub&gt; (&lt;i&gt;X&lt;/i&gt; = Cl, Br, I) NCs have good photoelectric properties and high stability. To further improve the photoluminescence (PL) quantum yields (QYs) and stability of CsPb&lt;i&gt;X&lt;/i&gt;&lt;sub&gt;3&lt;/sub&gt; NCs, researchers reduced the defects as nonradiative recombination centers in NCs by the following strategies: 1) surface treatment with different ligands; 2) control of synthesis conditions with halide rich compounds; 3) doping of metal ions. Among them, metal doping is considered as a universal and effective way to adjust the optoelectronic properties of semiconductors. It is found that the type and the concentration of metal ions have great influence on the electronic band structure and PL performance of NCs after the metal ions have been doped into CsPb&lt;i&gt;X&lt;/i&gt;&lt;sub&gt;3&lt;/sub&gt; NCs. At the same time, compared with II-VI and III-V semiconductors, the unique structure of all inorganic perovskite NCs makes the doping of metal ions easier. Appropriate doping can not only enhance the intrinsic optical properties of the NCs without affecting their crystal structure, but also introduce new electronic energy levels into the NCs and new luminescent properties of doped metal ions. Based on metal ions doping strategy, the PLQYs of doped CsPb&lt;i&gt;X&lt;/i&gt;&lt;sub&gt;3&lt;/sub&gt; NCs have been enhanced to nearly 100%. In this work, we summarize recent advances in metal doping of the four typical kinds of perovskite NCs, including CsPbCl&lt;sub&gt;3&lt;/sub&gt;, CsPbBr&lt;sub&gt;3&lt;/sub&gt;, CsPbI&lt;sub&gt;3&lt;/sub&gt;, and Mn&lt;sup&gt;2+&lt;/sup&gt; doped CsPb&lt;i&gt;X&lt;/i&gt;&lt;sub&gt;3&lt;/sub&gt;, and discuss the physical mechanisms of the improved properties through doping metal ions. It should be pointed out that the doping of some metal ions such as Ni&lt;sup&gt;2+&lt;/sup&gt; and Cd&lt;sup&gt;2+&lt;/sup&gt; into the above four kinds of NC systems can effectively passivate NC defects, thus improving the PL QY and stability of NCs. In addition, we put forward some personal perspectives on the future research subjects of interest and directions of metal doping for enhanced PL of CsPb&lt;i&gt;X&lt;/i&gt;&lt;sub&gt;3&lt;/sub&gt; NCs, which needs to be further explored in order to promote extensive application of perovskite NCs to various optoelectronic devices.

Simultaneous probing of nanocrystal (NC)-ligand interaction-induced charge transfer/transport properties at the electron donor (lead selenide NC)/acceptor (zinc oxide) functional interface

Two-dimensional (2D) materials demonstrate exceptional sliding ferroelectricity, owing to their facilitated interface charge transfer and controllable interlayer sliding. The development of high-performance sliding ferroelectric materials necessitates substantial sliding-induced polarization alongside minimal energy barriers for fatigue resistance. However, since both the sliding-induced ferroelectric out-of-plane polarization (OOP) and energy barriers are governed by interfacial charge transfer, these two critical parameters exhibit intrinsic coupling characteristics. The absence of the underlying mechanism, compounded by the lack of sliding ferroelectricity descriptor, fundamentally impedes the rational design of high-performance sliding ferroelectrics. In this work, we find the interfacial differential charge (IDC) transfer is an intrinsic parameter to link the sliding ferroelectricity and sliding energy barrier. Tracking all of the reported sliding ferroelectric materials, the sliding-induced OOP is found to be proportional to the dipole moments of asymmetric IDC distributions, while the sliding energy barrier is proportional to the absolute difference of IDC transfer. Leveraging high-throughput screening, 45 sliding ferroelectric candidates over 2000 homobilayer junctions are identified with superior sliding ferroelectric performance than MoS2. Then, a sliding ferroelectricity descriptor is proposed, that is OOP to the ratio between sliding energy barrier and cohesion energy. We further show that moiré superlattices can suppress net IDC transfer, enabling almost zero sliding energy barrier, but OOP switching during sliding. These insights elucidate the atomic origins of sliding ferroelectricity and establish a predictive framework for designing energy-efficient, fatigue-resistant ferroelectric devices.

Carrier doping is important for semiconductor nanocrystals (NCs) as it offers a new knob to tune NCs' functionalities, in addition to size and shape control. Also, extensive studies on NC devices have revealed that under operating conditions NCs are often unintentionally doped with electrons or holes. Thus, it is essential to be able to control the doping of NCs and study the carrier dynamics of doped NCs. The extension of previously reported redox-doping methods to chemically sensitive materials, such as recently introduced perovskite NCs, has remained challenging. We introduce an "intact" carrier-doping method by performing pump-pump-probe transient absorption spectroscopy on NC-acceptor complexes. The first pump pulse is used to trigger charge transfer from the NC to the acceptor, leading to NCs doped with a band edge carrier; the following pump-probe pulses measure the dynamics of carrier-doped NCs. We performed this measurement on CsPbBr3 NCs and deduced positive and negative trion lifetimes of 220 ± 50 and 150 ± 40 ps, respectively, for 10 nm diameter NCs, both dominated by Auger recombination. It also allowed us to identify randomly photocharged excitons in CsPbBr3 NCs as positive trions.

Lead halide perovskite nanocrystals (NCs) have been widely studied for application in optoelectronic devices due to their excellent optical properties and low-cost synthesis. However, the toxicity of lead and the poor stability of the NCs hindered their practical applications. Sn2+-based perovskite with low toxicity was first developed; however, the Sn2+-based perovskite NCs are unstable in air and oxidize easily. Recently, air-stable lead-free perovskite NCs have been developed and received increasing attention. Unfortunately, the optical and optoelectronic properties of these lead-free halide perovskite NCs are generally far worse than those of lead-perovskite NCs. Understanding the charge-carrier dynamics of semiconductors is crucial to improve their optical properties. In this Account, we mainly review our recent research progress on the study of charge-carrier dynamics in air-stable lead-free perovskite NCs. The exciton trapping followed by nonradiative recombination was the major carrier relaxation pathway and resulted in a low photoluminescence quantum efficiency (PLQE). A feasible route for passivating surface traps and tuning the self-trapped excitons from "dark" (nonradiative) to "bright" (radiative) was proposed. Through this strategy, the PLQE could be increased over 100-fold. In addition, we have compared several photophysical properties of lead-free perovskite NCs with that of lead perovskite NCs, such as charge-carrier relaxation, exciton-phonon coupling, and hot-carrier cooling. In 2017, we reported the synthesis, optical properties, and charge-carrier dynamics of Cs3Bi2X9 (X: Cl, Br, I) NCs. The Cs3Bi2Br9 NCs exhibited clear exciton trapping processes with time scales in the range of 2-20 ps. The fast trapping processes could be passivated via the use of surfactants (such as oleic acid), and the PLQE increased over 20-fold (from 0.2% to 4.5%). The low PLQE may be due to the reduced dimensionality of Cs3Bi2Br9 (2D) compared with the 3D cubic perovskite structure of CsPbBr3. We next reported double perovskite Cs2AgSb1-yBiyX6 (X: Br, Cl; 0 ≤ y ≤ 1) NCs, which exhibited a similar 3D cubic perovskite structure to that of the lead-perovskite NCs. The charge-carrier dynamics indicated that the sub-band-gap exciton trapping processes were dominated by ultrafast (∼1-2 ps) intrinsic self-trapping and trapping at surface defects (∼50-100 ps). While trapping at surface defects can be passivated using surfactants, the self-trapping processes is due to the giant carrier-phonon coupling effect. By designing direct band gap double perovskite NCs to tune the sub-band-gap trapping processes, bright dual-color emission was achieved. Furthermore, the violet PLQE could be improved to 36.6%, which is comparable to that in lead halide perovskite NCs. We hope this Account will deepen the understanding of the charge-carrier dynamics in lead-free perovskite NCs and guide the design of high-performance lead-free perovskites.

Lead halide perovskite quantum dots, also known as perovskite nanocrystals, are considered one of the most promising photovoltaic materials for solar cells due to their outstanding optoelectronic properties and simple preparation techniques. The key factors restricting the photoelectric conversion efficiency of solar cell systems are the separation and transmission performances of charge carriers. Here, femtosecond time-resolved ultrafast spectroscopy was used to measure the interfacial charge transfer dynamics of different sizes of CsPbBr3 assembled with TiO2. The effect of perovskite size on the charge transfer is discussed. According to our experimental data analysis, the time constants of the interfacial electron transfer and charge recombination of the assembled systems of CsPbBr3 and titanium dioxide become larger when the size of the CsPbBr3 nanocrystals increases. We discuss the physical mechanism by which the size of perovskites affects the rate of charge transfer in detail. We expect that our experimental results provide experimental support for the application of novel quantum dots for solar cell materials.

Surface-enhanced Raman spectroscopy (SERS) has traditionally relied on noble metals, yet their prohibitive cost, easy oxidation, and poor environmental stability severely restrict scalable practical applications. Herein, the monodisperse MoO2 nanoparticles were synthesized via a facile hydrothermal method, and their SERS performance was systematically evaluated in comparison with MoS2 and MoO3. The as-prepared MoO2 exhibits superior SERS sensitivity toward both rhodamine 6G (R6G) and O-safranin (SO). For the SO molecule, MoO2 delivers an ultralow detection limit down to 10-10 M and a remarkable enhancement factor as high as 3.52 × 107. Quantitative analysis reveals that the degree of charge transfer (ρCT) exceeds 0.5 for SO adsorbed on MoO2, indicating that interfacial charge transfer (CT) dominates the SERS enhancement, as further confirmed by transient absorption spectroscopy and density functional theory (DFT) calculations. DFT calculations further reveal a larger CT quantity of 0.78 e and richer bidirectional CT pathways in the MoO2/SO system, leading to stronger chemical enhancement than that of MoO2/R6G system. Benefiting from robust intrinsic metallic and electronic properties, the MoO2 substrate maintains stable SERS performance under harsh environments including acid-alkali corrosion, thermal annealing, and physiological saline immersion, without obvious degradation in sensitivity and detection capability. This work clarifies the critical role of interfacial band alignment and CT efficiency in MoO2 SERS systems and establishes MoO2 as a high-performance substrate that combines ultrahigh sensitivity, exceptional stability, and exceptional environmental endurance, which offers a solid foundation for its practical sensing applications in complex environments.