bonds. [18,19] The formation of hydrogen bonds reduces the coulombic interactions between lead and bromide ions, and then ammonium ligands will fall off from the QD surface along with bromide ions (Scheme 1a left). [20] On the other hand, the deprotonated ammonium ligands cannot interact with the CsPbBr 3 QDs.Therefore, the use of a single ligand without amine groups and reversible processes could overcome the purification stability problem, as shown recently by Yassitepe et al, who developed an aminefree method using only oleic acid (OA) as ligands. [21] The resulting CsPbBr 3 QDs could be washed several times without size increase or emission shift (Scheme 1a middle); however, the QY of these QDs was relatively low and the long-term stability was poor. The low QY results from the presence of enormous surface bromide vacancies (V Br ), since V Br exhibited obvious negative exciton trapping effect (Scheme 1b middle). [22] The groups of both Xia and Alivisatos recently confirmed the effect of V Br in their very recent works and near 100% QY can be achieved swimmingly by compensating Br ions with any bromides during or after synthesis (Scheme 1b left). [23,24] Hence, it seems contradictory that considering the QY, surface V Br should be avoided, while on account of good stability, ammonium which interacts with Br atoms should be excluded. To reduce the V Br density and reversible protonation, Kovalenko et al. introduced several kinds of zwitterionic ligands. [15,25] CsPbBr 3 QDs maintained high QY values of ≈80% and exhibited good purification stability. However, the preparation procedures were complicated, and even required precursor synthesis. Tan et al. reported the preparation of highly luminescent and stable CsPbBr 3 QDs prepared with addition of octylphosphonic acid (OPA). [26] However, in addition to OPA, the preparation also required the introduction of OA and trioctylphosphine oxide (TOPO), complicating the mechanism because TOPO and OA can also passivate and efficiently stabilize the CsPbBr 3 surface. [27,28] Additionally, OPA is insoluble in octadecene and is unable to dissolve PbBr 2 without TOPO. [29] In this work, we propose a concept of equivalent ligand, hypothesizing that if a ligand can play a role similar to Br ions to a certain extent and form strong interaction with lead ions (surfactant), then the problems of V Br and weak interaction can be solved simultaneously. The simplest source of Br ions is HBr, andThe stability and optoelectronic device performance of perovskite quantum dots (Pe-QDs) are severely limited by present ligand strategies since these ligands exhibit a highly dynamic binding state, resulting in serious complications in QD purification and storage. Here, a "Br-equivalent" ligand strategy is developed in which the proposed strong ionic sulfonate heads, for example, benzenesulfonic acid, can firmly bind to the exposed Pb ions to form a steady binding state, and can also effectively eliminate the exciton trapping probability due to bromide vacancies. From these two aspects, the s...
Highly stable antimonene, as the cousin of phosphorene from group-VA, has opened up exciting realms in the two-dimensional (2D) materials family. However, pristine antimonene is an indirect band gap semiconductor, which greatly restricts its applications for optoelectronics devices. Identifying suitable materials, both responsive to incident photons and efficient for carrier transfer, is urgently needed for ultrathin devices. Herein, by means of first-principles computations we found that it is rather feasible to realize a new class of 2D materials with a direct bandgap and high carrier mobility, namely antimonene oxides with different content of oxygen. Moreover, these tunable direct bandgaps cover a wide range from 0 to 2.28 eV, which are crucial for solar cell and photodetector applications. Especially, the antimonene oxide (18Sb-18O) is a 2D topological insulator with a sizable global bandgap of 177 meV, which has a nontrivial Z topological invariant in the bulk and the topological states on the edge. Our findings not only introduce new vitality into 2D group-VA materials family and enrich available candidate materials in this field but also highlight the potential of these 2D semiconductors as appealing ultrathin materials for future flexible electronics and optoelectronics devices.
Coordination-related, 2D structural phase transitions are a fascinating facet of 2D materials with structural degeneracy. Phosphorene and its new phases, exhibiting unique electronic properties, have received considerable attention. The 2D group IV–IV monochalcogenides (i.e. GeS, GeSe, SnS and SnSe) like black phosphorous possess puckered layered orthorhombic structure. The 2D group IV–IV monochalcogenides with advantages of earth-abundance, less toxicity, environmental compatibility and chemical stability, can be widely used in optoelectronics, piezoelectrics, photodetectors, sensors, Li-batteries and thermoelectrics. In this review, we summarized recent research progress in theory and experiment, which studies the fundamental properties, applications and fabrication of 2D group IV–IV monochalcogenides and their new phases, and brings new perspectives and challenges for the future of this emerging field.
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