Magnetic Exciton Relaxation and Spin–Spin Interaction by the Time-Delayed Photoluminescence Spectra of ZnO:Mn Nanowires

ZnO:Mn nanostructures are important diluted magnetic materials, but their electronic structure and magnetic origin are still not well understood. Here we studied the time-delayed and power-dependent photoluminescence spectra of Mn­(II) doped ZnO nanowires with very low Mn concentration. From the time-delayed emission spectra, we obtained their electronic levels of single Mn ion replacement of Zn ions in ZnO nanowire. The high d-level emissions show up unusually because of the stronger p–d hybridization than that in ZnS, as well as the spin–spin coupling. After increasing Mn doping concentration, the ferromagentic cluster of the Mn–O–Mn with varied configurations can form and give individual emission peaks, which are in good agreement with the ab initio calculations. The presence of clustered Mn ions originates from their ferromagnetic coupling. The lifetimes of these d levels show strong excitation power-dependent behavior, indication of strong spin-dependent coherent emission. One-dimensional structure is critical for this coherent emission behavior. These results indicate that the d state is not within Mn ion only, but a localized exciton magnetic polaron, Mn–O–Mn coupling should be one source of ferromagnetism in ZnO:Mn lattice, the latter also can combine with free exciton for EMP and produce coherent EMP condensation and emission from a nanowire. This kind of nanowires can be expected to work for both spintronic and spin-photonic devices if we tune the transition metal ion doping concentration in it

Emulsion Synthesis of Size-Tunable CH₃NH₃PbBr₃ Quantum Dots: An Alternative Route toward Efficient Light-Emitting Diodes

We report a facile nonaqueous emulsion synthesis of colloidal halide perovskite quantum dots by controlled addition of a demulsifier into an emulsion of precursors. The size of resulting CH₃NH₃PbBr₃ quantum dots can be tuned from 2 to 8 nm by varying the amount of demulsifier. Moreover, this emulsion synthesis also allows the purification of these quantum dots by precipitation from the colloidal solution and obtains solid-state powder which can be redissolved for thin film coating and device fabrication. The photoluminescence quantum yields of the quantum dots is generally in the range of 80–92%, and can be well-preserved after purification (∼80%). Green light-emitting diodes fabricated comprising a spin-cast layer of the colloidal CH₃NH₃PbBr₃ quantum dots exhibited maximum current efficiency of 4.5 cd/A, power efficiency of 3.5 lm/W, and external quantum efficiency of 1.1%. This provides an alternative route toward high efficient solution-processed perovskite-based light-emitting diodes. In addition, the emulsion synthesis is versatile and can be extended for the fabrication of inorganic halide perovskite colloidal CsPbBr₃ nanocrystals

Controllable Transformation from Rhombohedral Cu_1.8S Nanocrystals to Hexagonal CuS Clusters: Phase- and Composition-Dependent Plasmonic Properties

Because of the rich polymorphs and lower diffusion energy barriers of copper chalcogenide systems, the phase transformation of colloidal Cu_2–<i>x</i>S (0 ≤ <i>x</i> ≤ 1) nanocrystals is critical for understanding their fundamental properties and designing convenient synthetic routes. In this work, high quality digenite Cu_1.8S nanocrystals with rhombohedral structure were synthesized at gram-scale. The as-prepared colloidal nanocrystals undergo an <i>in situ</i> phase transformation from rhombohedral Cu_1.8S nanocrystals to hexagonal CuS clusters upon keeping the resulting colloidal solution for a few days. The observed transformation was explored by a combination of structural and spectroscopic analyses, including powder X-ray diffraction, transmission electron microscopy, energy dispersive spectroscopy, and X-ray photoelectron spectroscopy characterizations. A possible mechanism is proposed and thoroughly discussed. We further determined the evolution of plasmonic absorption spectra during the transformation. The Cu_1.8S nanocrystals and CuS clusters exhibit composition-dependent local surface plasmon resonance absorption (LSPR) in the near-infrared region, which are in good agreement with calculated extinction spectra based on Mie-Drude model. Combined experimental and theoretical analyses demonstrated that both the phase induced dielectric constant change and the composition induced carrier concentration variation account for the spectroscopic evolution

Polarization-Sensitive Self-Powered Type-II GeSe/MoS₂ van der Waals Heterojunction Photodetector

Polarization-sensitive photodetectors are highly desirable for high-performance optical signal capture and stray light shielding in order to enhance the capability for detection and identification of targets in dark, haze, and other complex environments. Usually, filters and polarizers are utilized for conventional devices to achieve polarization-sensitive detection. Herein, to simplify the optical system, a two-dimensional self-powered polarization-sensitive photodetector is fabricated based on a stacked GeSe/MoS2 van der Waals (vdW) heterojunction which facilitates efficient separation and transportation of the photogenerated carriers because of type-II band alignment. Accordingly, a high-performance self-powered photodetector is achieved with merits of a very large on–off ratio photocurrent at zero bias of currently 104 and a high responsivity (Rλ) of 105 mA/W with an external quantum efficiency of 24.2%. Furthermore, a broad spectral photoresponse is extended from 380 to 1064 nm owing to the high absorption coefficient in a wide spectral region. One of the key benefits from these highly anisotropic orthorhombic structures of layered GeSe is self-powered polarization-sensitive detection with a peak/valley ratio of up to 2.95. This is realized irradiating with a 532 nm wavelength laser with which a maximum photoresponsivity of up to 590 mA/W is reached when the input polarization is parallel to the armchair direction. This work provides a facile route to fabricate self-powered polarization-sensitive photodetectors from GeSe/MoS2 vdW heterojunctions for integrated optoelectronic devices