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1. Singlet excitation in the intermediate magnetic equivalence regime and field-dependent study of singlet–triplet leakage

   https://doi.org/10.1039/C8CP06883F  

   Kharkov, Boris; Duan, Xueyou; Tovar, Emily S.; Canary, James W.; Jerschow, Alexej (January 2019, Physical Chemistry Chemical Physics)

   The examination and optimized preparation of nuclear spin singlet order has enabled the development of new types of applications that rely on potentially long-term polarization storage. Lifetimes several orders of magnitude longer than T 1 have been observed. The efficient creation of such states relies on special pulse sequences. The extreme cases of very large and very small magnetic equivalence received primary attention, while relatively little effort has been directed towards studying singlet relaxation in the intermediate regime. The intermediate case is of interest as it is relevant for many spin systems, and would also apply to heteronuclear systems in very low magnetic fields. Experimental evidence for singlet–triplet leakage in the intermediate regime is sparse. Here we describe a pulse sequence for efficiently creating singlets in the intermediate regime in a broad-band fashion. Singlet lifetimes are studied with a specially synthesized molecule over a wide range of magnetic fields using a home-built sample-lift apparatus. The experimental results are supplemented with spin simulations using parameters obtained from ab initio calculations. This work indicates that the chemical shift anisotropy (CSA) mechanism is relatively weak compared to singlet–triplet leakage for the proton system observed over a large magnetic field range. These experiments provide a mechanism for expanding the scope of singlet NMR to a larger class of molecules, and provide new insights into singlet lifetime limiting factors. 

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2. Atomic Structure and Electrical Activity of Grain Boundaries and Ruddlesden–Popper Faults in Cesium Lead Bromide Perovskite

   https://doi.org/10.1002/adma.201805047  

   Thind, Arashdeep Singh; Luo, Guangfu; Hachtel, Jordan A.; Morrell, Maria V.; Cho, Sung Beom; Borisevich, Albina Y.; Idrobo, Juan‐Carlos; Xing, Yangchuan; Mishra, Rohan (December 2018, Advanced Materials)

   Abstract To evaluate the role of planar defects in lead‐halide perovskites—cheap, versatile semiconducting materials—it is critical to examine their structure, including defects, at the atomic scale and develop a detailed understanding of their impact on electronic properties. In this study, postsynthesis nanocrystal fusion, aberration‐corrected scanning transmission electron microscopy, and first‐principles calculations are combined to study the nature of different planar defects formed in CsPbBr₃nanocrystals. Two types of prevalent planar defects from atomic resolution imaging are observed: previously unreported Br‐rich [001](210)∑5 grain boundaries (GBs) and Ruddlesden–Popper (RP) planar faults. The first‐principles calculations reveal that neither of these planar faults induce deep defect levels, but their Br‐deficient counterparts do. It is found that the ∑5 GB repels electrons and attracts holes, similar to an n–p–n junction, and the RP planar defects repel both electrons and holes, similar to a semiconductor–insulator–semiconductor junction. Finally, the potential applications of these findings and their implications to understand the planar defects in organic–inorganic lead‐halide perovskites that have led to solar cells with extremely high photoconversion efficiencies are discussed. 

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3. Effect of electron–electron interaction on magnitude of quantum oscillations of dissipative resistance in magnetic fields

   https://doi.org/10.1063/5.0127286  

   Abedi, Sara; Vitkalov, Sergey; Bykov, A. A.; Bakarov, A. K. (December 2022, Journal of Applied Physics)

   Magneto-intersubband resistance oscillations (MISOs) of highly mobile 2D electrons in symmetric GaAs quantum wells with two populated subbands are studied in magnetic fields [Formula: see text] tilted from the normal to the 2D electron layer at different temperatures [Formula: see text]. The in-plane component ([Formula: see text]) of the field [Formula: see text] induces magnetic entanglement between subbands, leading to beating in oscillating density of states (DOS) and to MISO suppression. Model of the MISO suppression is proposed. Within the model, a comparison of MISO amplitude in the entangled and disentangled ([Formula: see text]) 2D systems yields both difference frequency of DOS oscillations, [Formula: see text], and strength of the electron–electron interaction, described by parameter [Formula: see text], in the 2D system. These properties are analyzed using two methods, yielding consistent but not identical results for both [Formula: see text] and [Formula: see text]. The analysis reveals an additional angular dependent factor of MISO suppression. The factor is related to spin splitting of quantum levels in magnetic fields. 

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4. Phase segregation in inorganic mixed-halide perovskites: from phenomena to mechanisms

   https://doi.org/10.1364/PRJ.402411  

   Wang, Yutao; Quintana, Xavier; Kim, Jiyun; Guan, Xinwei; Hu, Long; Lin, Chun-Ho; Jones, Brendon Tyler; Chen, Weijian; Wen, Xiaoming; Gao, Hanwei; et al (October 2020, Photonics Research)

   Halide perovskites, such as methylammonium lead halide perovskites ( ${\mathrm{MAPbX}}_3$, $\mathrm{X}=\mathrm{I}$, Br, and Cl), are emerging as promising candidates for a wide range of optoelectronic applications, including solar cells, light-emitting diodes, and photodetectors, due to their superior optoelectronic properties. All-inorganic lead halide perovskites ${\mathrm{CsPbX}}_3$are attracting a lot of attention because replacing the organic cations with ${\mathrm{Cs}}^{+}$enhances the stability, and its halide-mixing derivatives offer broad bandgap tunability covering nearly the entire visible spectrum. However, there is evidence suggesting that the optical properties of mixed-halide perovskites are influenced by phase segregation under external stimuli, especially illumination, which may negatively impact the performance of optoelectronic devices. It is reported that the mixed-halide perovskites in forms of thin films and nanocrystals are segregated into a low-bandgap I-rich phase and a high-bandgap Br-rich phase. Herein, we present a critical review on the synthesis and basic properties of all-inorganic perovskites, phase-segregation phenomena, plausible mechanisms, and methods to mitigate phase segregation, providing insights on advancing mixed-halide perovskite optoelectronics with reliable performance. 

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5. Doping and ion substitution in colloidal metal halide perovskite nanocrystals

   https://doi.org/10.1039/C9CS00790C  

   Lu, Cheng-Hsin; Biesold-McGee, Gill V.; Liu, Yijiang; Kang, Zhitao; Lin, Zhiqun (July 2020, Chemical Society Reviews)

   null (Ed.)

   The past decade has witnessed tremendous advances in synthesis of metal halide perovskites and their use for a rich variety of optoelectronics applications. Metal halide perovskite has the general formula ABX 3 , where A is a monovalent cation (which can be either organic ( e.g. , CH 3 NH 3 + (MA), CH(NH 2 ) 2 + (FA)) or inorganic ( e.g. , Cs + )), B is a divalent metal cation (usually Pb 2+ ), and X is a halogen anion (Cl − , Br − , I − ). Particularly, the photoluminescence (PL) properties of metal halide perovskites have garnered much attention due to the recent rapid development of perovskite nanocrystals. The introduction of capping ligands enables the synthesis of colloidal perovskite nanocrystals which offer new insight into dimension-dependent physical properties compared to their bulk counterparts. It is notable that doping and ion substitution represent effective strategies for tailoring the optoelectronic properties ( e.g. , absorption band gap, PL emission, and quantum yield (QY)) and stabilities of perovskite nanocrystals. The doping and ion substitution processes can be performed during or after the synthesis of colloidal nanocrystals by incorporating new A′, B′, or X′ site ions into the A, B, or X sites of ABX 3 perovskites. Interestingly, both isovalent and heterovalent doping and ion substitution can be conducted on colloidal perovskite nanocrystals. In this review, the general background of perovskite nanocrystals synthesis is first introduced. The effects of A-site, B-site, and X-site ionic doping and substitution on the optoelectronic properties and stabilities of colloidal metal halide perovskite nanocrystals are then detailed. Finally, possible applications and future research directions of doped and ion-substituted colloidal perovskite nanocrystals are also discussed. 

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