Perovskite semiconductor nanocrystals (NCs) exhibit highly efficient optical gain, which is promising for laser applications. However, the intrinsic mechanism of optical gain in perovskite NCs, particularly whether more than one exciton per NCs is required, remains poorly understood. Here, we use two-dimensional electronic spectroscopy to resonantly probe the interplay between near-band-edge transitions during the buildup of optical gain in CsPbBr3 NCs. We find compelling evidence to conclude that optical gain in CsPbBr3 NCs is generated through stimulated emission from strongly interacting biexcitons. The threshold is largely determined by the competition between stimulated emission from biexcitons and excited-state absorption from single exciton to biexciton states. The findings in this work may guide future explorations of NC materials with low-threshold optical gain.

Ordered epitaxial ZrO 2 films were grown on Pt(111) and characterized by low energy electron diffraction (LEED), synchrotron radiation photoemission spectroscopy (SRPES) and X-ray photoelectron spectroscopy (XPS). The films were prepared by vapor deposition of zirconium in an O 2 atmosphere followed by annealing under ultra high vacuum. At low coverages, the films grew as discontinuous two-dimentional islands with ordered structures. The size and structure of these islands were dependent on the coverage of ZrO 2 films. At coverage <0.5 monolayer (ML), ( )

In order to optimize the formability of ferritic stainless steel (FSS), control of the crystallographic texture is essential. In the present work, texture development of cast strips with different initial solidification structure during rolling and subsequent annealing and the formability of final sheets were investigated. Fully equiaxed and columnar grained cast strips of Fe17%Cr FSS were fabricated by a pilot twin-roll strip caster. The equi-axed and columnar grained bars cut from a conventional continuous cast slab were used as comparable specimens. It was shown that in the cold rolled and annealed sheet of fully equi-axed cast strip, uniform and strong g-fiber recrystallization texture was formed and {001}͗110͘ component was almost eliminated. By contrast, the recrystallization texture in the conventional sheets with equi-axed and columnar initial solidification structures exhibited typical shift towards high-index {334}͗483͘, which led to a reduced r-value as compared to that of the equi-axed cast strip. The final annealed sheet of fully columnar cast strip, however, showed weaker g-fiber recrystallization texture and lower r-value than those of conventional sheets. It is, therefore, believed that twin-roll strip casting can improve the formability of 17 % Cr FSS only under the precondition that appropriate solidification structure is obtained.KEY WORDS: ferritic stainless steel; twin-roll strip casting; solidification structure; texture; formability. ent strips in cold rolling and annealing were studied. Equiaxed and columnar bars were cut from a continuous cast slab, and hot rolling, hot band annealing, cold rolling and final recrystallization annealing were performed for both bars to simulate the industrial processes. The texture developments of the two bars were also studied and compared with those of cast strips to understand the evolutions of microstructure, texture and properties in the cast strips. Experimental ProceduresFe17%Cr FSS with a composition of Cr, 16.9; Ni, 0.114; C, 0.03; Mn, 0.154; Si, 0.243; P, 0.016; S, 0.004 (in mass%) was used. Cast strips were produced by using a pilot twinroll strip caster, in which the molten steel was poured into a preheated tundish under Ar shield to flow through a nozzle into the rotated water-cooled rolls.15) The casting speed was set up to be 0.25 m/s and the initial roll gap was set to be 1.5 mm to produce strips with thickness of about 1.8 mm. The pool melt temperature was measured by a thermo-detector installed above the melting pool. In the present work, pool melt superheats were controlled to be about 140 K and 20 K, respectively, to produce fully columnar and fully equi-axed cast strips, Fig. 1. The average diameters of columnar and equi-axed grains were about 400 mm and 200 mm, respectively. Both strips were cold rolled by using a laboratory rolling mill with a thickness reduction of 80 %, and were annealed at 860°C for 2 min.Columnar and equi-axed bars in the size of 250 Lϫ 110 Wϫ45 T (mm) were cut from a cast slab of Cr17 FSS, Fig. 2. The average...

The effects of porous structures on the performance of a GaN UV photodetector were investigated for the first time.

Perovskite semiconductor nanocrystals of lead halides exhibit excellent electronic and optical properties that are suitable for many optoelectronic applications. In this report, we investigate the underlying mechanism of the optical response of the material by probing coherent interaction between the exciton and lattice vibration by two-dimensional electronic spectroscopy. Coherent exciton-phonon coupling has been observed with time-domain oscillations in the dynamics of signals related to either ground or excited electronic states. Furthermore, the spectra of the coherent phonon derived from the dynamics of biexciton formation have larger amplitudes for higher frequency modes, which is attributed to the phonon bottleneck effect. Moreover, the composition-dependent coherent coupling between the exciton and lattice vibration in mixed halide samples reveals a critical role played by the anharmonicity and fluctuation of lattice vibration in the excited-state dynamics of perovskite nanocrystals.

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We introduce a novel configuration for two-dimensional electronic spectroscopy (2DES) that combines the partially collinear pump-probe geometry with active phase locking. We demonstrate the method on a solution sample of CdSe/ZnS nanocrystals by employing two non-collinear optical parametric amplifiers as the pump and probe sources. The two collinear pump pulse replicas are created using a Mach-Zehnder interferometer phase stabilized by active feedback electronics. Taking the advantage of separated paths of the two pump pulses in the interferometer, we improve the signal-to-noise ratio with double modulation of the individual pump beams. In addition, a quartz wedge pair manipulates the phase difference between the two pump pulses, enabling the recovery of the rephasing and non-rephasing signals. Our setup integrates many advantages of available 2DES techniques with robust phase stabilization, ultrafast time resolution, two-color operation, long delay scan, individual polarization manipulation and the ease of implementation.

In this study, the microstructural evolution and resultant mechanical properties in a low carbon medium Mn heavy steel plate were investigated in detail. The results show that the introduction of medium manganese alloy design in the heavy steel plate has been shown to achieve the outstanding combination of strength, ductility, low-temperature impact toughness, and strain hardening capacity. It has been found that the austenite phase mainly displays at martensitic lath boundaries and shows lath shape for the heat treating at 873 K (600°C) for 1 to 10 hours or 893 K (620°C) for 2 hours, and not all the austenite phase obeys the K-S or N-W orientation relationship with respect to abutting martensitic lath. Although the microstructure in the steel after heat treating at 873 K (600°C) for 1 to 10 hours is similar to each other, the resultant mechanical properties are very different because the volume fraction and stability of retained austenite vary with the heat treatments. The best low-temperature impact toughness is achieved after heat treating at 873 K (600°C) for 2 hours due to the formation of a considerable volume fraction of retained austenite with relatively high stability, but the strain hardening capacity and ductility are disappointing because of insufficient TRIP effect. Based on enhancing TRIP effect, the two methods have been suggested. One is to increase the isothermal holding temperature to 893 K (620°C), and the other one is to prolong the isothermal holding time to 10 hours at 873 K (600°C). The two methods can significantly increase strain hardening capacity and ductility nearly without harming low-temperature impact toughness. In addition, the stability of retained austenite has been discussed by the quantitative analysis and it has been demonstrated that the stability of retained austenite is related to the chemical composition, size, and morphology. Moreover, the isothermal holding temperature has a great effect on the stability of retained austenite, while the effect of the isothermal holding time is relatively poor.