Thickness Of Single Crystals Research Articles

Can thick metal-halide perovskite single crystals have narrower optical bandgaps with near-infrared absorption?

Metal-halide perovskite (MHP) single crystals are emerging as potential competitors to their polycrystalline thin-film counterparts. These materials have shown the specific feature of extended absorbance towards the near-infrared (NIR) region, which promises further extension of their applications in the field of photovoltaics and photodetectors. This notable expansion of absorbance has been explained by the narrower effective optical bandgap of MHP single crystals promoted by their large thickness over several micrometres to millimetres. Herein, the attributes of the material's thickness and the measurement technique used to estimate these characteristics are discussed to elucidate the actual origins of the extended absorbance of MHP single crystals. Contrary to the general belief of the narrower bandgap of the MHP single crystals, we demonstrate that the extended NIR absorption in the MHP single crystals mainly originates from the combination of unique below-bandgap absorption of MHPs, the thickness of single crystals, and the technical limitation of the spectrophotometer, with the key attributes of (i) significantly large thickness of the MHP single crystals by suppressing the transmitted light and (ii) the detector's limited dynamic range. Combining the theoretical and experimental characterizations, we clarify the significant role of the large thickness together with the limited sensitivity of the detector in promoting the well-known red shift of the absorption onset of the MHP single crystals. The observations evidently show that in some special circumstances, the acquired absorption spectrum cannot reliably represent the optical bandgap of MHP materials. This highlights some misinterpretations in the estimation of the narrower optical bandgap of the MHP single crystals from conventional optical methods, while the optical bandgap is an inherent property independent of the thickness. The proposed broad applications of the MHP single crystals are dictated by their fascinating properties, and therefore, a deep insight into these features should be considered besides device applications, because much of their property-function relationships are still ambiguous and a subject of debate.

Preparation of solution-grown lozenge-shaped poly(p-phenylene terephthalamide) single crystals and their structural stabilization by heat treatment

In this study, the preparation of poly (p-phenylene terephthalamide) (PPTA) single crystals was examined using crystallization from dilute solutions in concentrated sulfuric acid. Lozenge-shaped PPTA single crystals were successfully prepared using a self-seeding method with a low degree of supercooling, and they consisted of modification I crystals. The a-axis direction of the crystal corresponded to the long diagonal direction of the rhombus, the b-axis direction with the short diagonal direction, and the PPTA molecular chain direction (the c-axis direction) with the crystal's thickness direction. In addition, the PPTA single crystals had a (110) growth plane, where the thickness of each single crystal was approximately equal to the molecular chain length of the PPTA. Upon heat treatment of the PPTA single crystals, the symmetry changed from P1a1 to the more stable P11n. In addition, the heat treatment caused a difference in the density of each symmetric crystal, resulting in crack formation along the b-axis direction, which is the hydrogen-bonding direction. However, the heat treatment did not change the thickness of the PPTA single crystals. Conversely, the isothermal crystallization of the PPTA caused progression in the crystallization only under a high degree of supercooling, thus yielding plate-like PPTA crystals that consisted of modification II crystals. In these plate-like PPTA crystals, the length corresponded to the crystal a-axis direction, and the electron diffraction pattern was broad. Furthermore, the equilibrium dissolution temperature of the PPTA single crystals was discussed.

Differential Space‐Limited Crystallization of Mixed‐Cation Lead Iodide Single‐Crystal Micro‐Plates Enhances the Performance of Perovskite Solar Cells

The synthesis of certain asymmetric perovskite single crystals (SCs)—including CH3NH3PbI3, which is used most commonly—for application in high‐performance perovskite solar cells (PeSCs), remains very challenging. Herein, a promising but general method, differential space‐limited crystallization (DSLC), is described for synthesizing high‐quality perovskite single‐crystal micro‐plates. The thickness of the perovskite SCs is controlled by the difference between the thicknesses of two sets of polytetrafluoroethylene (PTFE) spacers. Because the DSLC method does not require very thin spacers, it simplifies the procedure of crystal growth. More importantly, the hydrophobicity of the PTFE spacers weakens the attraction between the surfaces of the confined space and the precursor complexes, thereby increasing the rate of diffusion of the precursor ions. Accordingly, the critical nucleation step is not limited by the low rate of diffusion of the ions. This approach is used to prepare mixed‐cation lead iodide single‐crystalline micro‐plates for solar cell applications. The device performance of single‐crystal PeSCs improves after introducing formamidinium ions. The stability of the single‐crystal devices also improves relative to that of conventional thin‐film counterparts. It is anticipated that this DSLC method can also be used to synthesize different types of asymmetrical perovskite SCs for other optoelectronic applications.

Evolution of the stimulated resonance of a channeling electron beam

The motion of channeled particles in the co-moving coordinate system can be considered as a onedimensional oscillator (in the case of planar channeling) or as a two-dimensional atom (in the case of axial channeling). The transverse motion of channeled particles is characterized by the discrete spectrum; in this case, the probability of occupation of transverse-motion levels depends on the entrance angle of the charged particle with respect to the crystallographic axes of a single crystal. A charged channeled particle passing through a single crystal experiences the periodic action of lattice atoms. If the frequency of the external periodic action of the crystal lattice coincides with that of the transition of the channeled particle from one quantum state of transverse motion to another, then resonance excitation of the channeled particle can occur; it is similar to the excitation of atomic electrons by the periodic field of a monochromatic electromagnetic wave. In this paper, the resonance conditions are discussed and the dependence of the probability of the occupation of transverse-motion levels of channeled particles (electrons) on the single-crystal thickness is analyzed.

Ce,Mn:YAG单晶的生长及光学性能

Ce,Mn :YAG single crystal for white light emitting diode ( LED) was grown by Czochralski method. The structure, valence states of doped Mn and optical properties of the samples were characterized by X-ray diffraction(XRD), X-ray absorption fine structure(XAFS), absorption and photoluminescence spectra. The photoelectric parameters of white LED were also measured by changing the thickness of single crystal and the driving current. Ce,Mn :YAG single crystal shows a broad emission band peak around 526 nm and 566 nm under blue light of 460 nm. The XAFS result shows that the valence states of doped Mn are mostly Mn^(2+) in the Ce,Mn :YAG. Due to the effective energy transfer between Ce^(3+) and Mn^(2+) in YAG single crystal, the bright orange luminescence in the bands peaked at 566 nm are caused by the ^4T_1→^6A_1 radiative transitions of Mn^(2+).

Morphological observation and annealing effect of single crystals of poly(γ-methyl D-glutamate) in α-helical form

Abstract Single crystals of poly (γ-methyl D-glutamate) (PMDG) were obtained in a form of hexagonal platelet by isothermal crystallization from a mixed solution of N, N-dimethylformamide (DMF) and propionic acid (1:1 in volume). The electron diffraction pattern revealed that the single crystal is composed of α-helical molecular chains oriented perpendicularly to the plane surface. Comparison of the single crystal thickness with the length of the α-helical molecular chain calculated from the viscosity-average molecular weight showed that the molecular chains might fold at the lamellar surface. The fold formation was supported by the observation that fibrils were pulled out from the fracture surface when the single crystal was drawn on a substrate. Since the α-helical molecular chain cannot fold itself due to the helical conformation with intramolecular hydrogen bonds, it seems that the fold region is composed of non-α-helical segments. The surface of PMDG single crystals was considerably rougher than that ...