Zhipeng Lian

High-Performance Planar-Type Photodetector on (100) Facet of MAPbI3 Single Crystal.

Recently, the discovery of organometallic halide perovskites provides promising routes for fabricating optoelectronic devices with low cost and high performance. Previous experimental studies of MAPbI3 optoelectronic devices, such as photodetectors and solar cells, are normally based on polycrystalline films. In this work, a high-performance planar-type photodetector fabricated on the (100) facet of a MAPbI3 single crystal is proposed. We demonstrate that MAPbI3 photodetector based on single crystal can perform much better than that on polycrystalline-film counterpart. The low trap density of MAPbI3 single crystal accounts for the higher carrier mobility and longer carrier diffusion length, resulted in a significant performance increasement of MAPbI3 photodetector. Compared with similar planar-type photodetectors based on MAPbI3 polycrystalline film, our MAPbI3 single crystal photodetector showed excellent performance with good stability and durability, broader response spectrum to near-infrared region, about 102 times higher responsivity and EQE, and approximately 103 times faster response speed. These results may pave the way for exploiting high-performance perovskites photodetectors based on single crystal.

Blue–white–orange color-tunable luminescence of Ce3+/Mn2+-codoped NaCaBO3 via energy transfer: potential single-phase white-light-emitting phosphors

Ce3+/Mn2+-codoped NaCaBO3 single-phase color-tunable phosphors were synthesized by solid-state reaction, and their photoluminescence properties were investigated in detail. Under UV excitation, two main emission bands peaking at 400 and 610 nm were realized in the NaCa0.89BO3: 0.01Ce3+, 0.10Mn2+ phosphor on the basis of the energy transfer from Ce3+ to Mn2+ with an efficiency of over 83%. The energy transfer was validated and demonstrated to be a resonant type via a dipole–dipole mechanism, and the critical distance RC values calculated by the quenching concentration method and the spectral overlap method were 22.85 and 19.41 A, respectively. On examining the Mn2+-concentration-dependent photoluminescence properties, it was observed that the emission color could be tuned from blue (0.162, 0.031) to white-light (0.335, 0.259) and eventually to orange (0.489, 0.246) through energy transfer by changing the Ce3+/Mn2+ ratio. Combining a 370 nm UV chip and a single-phase white-emitting NaCa0.96BO3: 0.01Ce3+, 0.03Mn2+ phosphor produced a white light-emitting diode with CIE chromaticity coordinates of (0.326, 0.274) and an excellent color rendering index of 90.7 at a correlated color temperature of 4046 K. These results indicate that the NaCaBO3: Ce3+, Mn2+ phosphors have potential applications as single-phase white-light-emitting phosphor-converted materials for UV-pumped light-emitting diodes.

Structure and luminescence properties of green-emitting NaBaScSi2O7:Eu2+ phosphors for near-UV-pumped light emitting diodes

Green-emitting phosphor Eu2+ doped NaBaScSi2O7 was synthesized by a solid-state reaction, and the photoluminescence properties were investigated in conjunction with a structural analysis. The crystallographic occupancy of Eu2+ in the Sc silicate NaBaScSi2O7 matrix was studied based on the Rietveld refinements results and the crystal chemistry rules. The optimum concentration of Eu2+ in the NaBaScSi2O7 phosphor was about 10 mol%, and the concentration quenching mechanism was verified to be the dipole–dipole interaction. Upon excitation at 365 nm, the composition-optimized NaBaScSi2O7:Eu2+ exhibited strong green light peaking at 501 nm with the CIE chromaticity (0.0706, 0.5540) and a high internal quantum efficiency of about 65%. The thermally stable luminescence properties were also studied and compared with those of the commercial green phosphors. A white light emitting diode (w-LED) lamp was finally fabricated by using the present green phosphor and the commercial blue and red phosphors, which exhibited a high color rendering index (Ra) of 86.5 at a correlated color temperature of 2528 K with CIE coordinates of x = 0.353, y = 0.324. These results suggest that NaBaScSi2O7:Eu2+ is a potential green phosphor candidate for near-UV-pumped w-LEDs.

Perovskite CH3NH3PbI3(Cl) Single Crystals: Rapid Solution Growth, Unparalleled Crystalline Quality, and Low Trap Density toward 108 cm–3

Single crystal reflects the intrinsic physical properties of a material, and single crystals with high-crystalline quality are highly desired for the acquisition of high-performance devices. We found that large single crystals of perovskite CH3NH3PbI3(Cl) could be grown rapidly from chlorine-containing solutions. Within 5 days, CH3NH3PbI3(Cl) single crystal as large as 20 mm × 18 mm × 6 mm was harvested. As a most important index to evaluate the crystalline quality, the full width at half-maximum (fwhm) in the high-resolution X-ray rocking curve (HR-XRC) of as-grown CH3NH3PbI3(Cl) single crystal was measured as 20 arcsec, which is far superior to so far reported CH3NH3PbI3 single crystals (∼1338 arcsec). The unparalleled crystalline quality delivered a low trap-state density of down to 7.6 × 10(8) cm(-3), high carrier mobility of 167 ± 35 cm(2) V(-1) s(-1), and long transient photovoltaic carrier lifetime of 449 ± 76 μs. The improvement in the crystalline quality, together with the rapid growth rate and excellent carrier transport property, provides state-of-the-art single crystalline hybrid perovskite materials for high-performance optoelectronic devices.

Crystal structure refinement and luminescence properties of Ce3+ singly doped and Ce3+/Mn2+ co-doped KBaY(BO3)2 for n-UV pumped white-light-emitting diodes

A series of novel Ce3+ singly doped and Ce3+/Mn2+ co-doped color-tunable KBaY(BO3)2 phosphors were synthesized by a high-temperature solid-state reaction, and the crystal structures and luminescence properties were investigated in detail. The crystallographic cation sites occupancy of the doping Ce3+ and Mn2+ ions for KBaY(BO3)2: Ce3+, Mn2+ samples were analyzed by the Rietveld refinement method. It was demonstrated that Ce3+ and Mn2+ ions can both enter into the two types of cation sites in the KBaY(BO3)2 crystal lattice, with a slight tendency of occupancy for Ce3+ on Y3+ ion sites and Mn2+ on the Ba2+/K+ ion sites. By controlling Mn2+ ions content with a fixed Ce3+ concentration, the emission color of the phosphor varied from blue (0.160, 0.150) to pink-white (0.348, 0.272) and eventually to orange (0.490, 0.372). The critical energy transfer distance calculated by the concentration quenching theory was 16.52 A and the energy transfer from Ce3+ to Mn2+ took place via a resonance-type dipole–dipole mechanism. By combining the single-phase KBa0.97Y0.97(BO3)2: 0.01Ce3+, 0.05Mn2+ phosphor with an n-UV 370 nm InGaN chip, a phosphor-converted white LED lamp was successfully fabricated, producing a white light with a warm correlated color temperature of 4710 K and color coordinates of (0.347, 0.307).

A self-powered photodetector based on a CH3NH3PbI3 single crystal with asymmetric electrodes

Methylammonium lead iodide perovskite (MAPbI3) has made a re-entry into the literature nowadays for its extraordinary characteristics, such as high absorption of light, long carrier diffusion length, high carrier mobility, low trap-state density, low surface recombination velocity and ease of attainment. Here, we report a self-powered photodetector based on a CH3NH3PbI3 single crystal by employing asymmetric Au–Al electrodes. The key issue of this photodetector was the metal–semiconductor contacts, owing to the Schottky junction between them. By setting the channel length between the Au–Al electrodes to 30 μm for sufficient electron–hole pair separation and transportation, the device showed good performance under 1 sun illumination. The short-circuit photocurrent density and open-circuit voltage were 6.86 mA cm−2 and 0.7 V, respectively. The photocurrent was almost 2 orders of magnitude larger than that based on a perovskite polycrystalline film with a similar device structure. More importantly, the device could detect the lowest noticeable incident power density down to 1 × 10−8 W cm−2. Under this weak light intensity, the responsivity was as high as 0.24 A W−1 without any bias. The photoresponse also had a broadband ranging from 375 nm to 808 nm accompanied by a fast response speed.

Hexagonal Crown-Capped Zinc Oxide Micro Rods: Hydrothermal Growth and Formation Mechanism

Hexagonal crown-capped ZnO micro rods were successfully prepared by a facile low-temperature hydrothermal method. The as-prepared ZnO micro rods are 4.4-5.2 μm in length and 2.4-3.6 μm in diameter, possessing a single-crystal hexagonal structure. The morphology evolution and structure changes were tracked during hydrothermal growth by field-emission scanning electron microscopy and X-ray diffraction, respectively. A three-stage growth mechanism of the hexagonal crown-capped ZnO micro rods was proposed and further verified by a growth solution renewal experiment. The room-temperature photoluminescence (PL) spectrum of the hexagonal crowns exhibits a strong UV emission at about 382 nm. The temperature dependent PL results indicate that the UV emission originates from the radiative free-exciton recombination.

Highly efficient field emission from large-scale and uniform monolayer graphene sheet supported on patterned ZnO nanorod arrays

A graphene/patterned ZnO (G/PZO) nanorod array composite was successfully fabricated by using the colloidal nanosphere monolayer templating strategy. Compared with graphene/compact ZnO (G/CZO) nanorod arrays, the G/PZO nanorod array composite exhibited a highly efficient and stable field emission with a low turn-on field. The field emission had a strong dependence on the density and the inter-distance of the emitters. By changing the diameter of polystyrene (PS) colloidal nanospheres, optimal emitter inter-distance was obtained. Graphene supported on ZnO nanorod arrays, patterned by an 886 nm PS colloidal nanosphere monolayer, displayed the best field emission with a turn-on electric field of only 6.4 V μm−1 and the field enhancement factor β was 1158. The K point in the Fowler–Nordheim plot demonstrated the electron tunneling mechanism took place in this composite material. The cyclic experiment showed that the field emission of the G/PZO nanorod array composite was of high stability.

Cs6RE2(PO4)4 (RE = Y and Gd): two new members of the alkali rare-earth double phosphates

Two new alkali rare-earth double phosphates, Cs6Y2(PO4)4 (CYP) and Cs6Gd2(PO4)4 (CGP), have been successfully synthesized using the conventional solid-state reaction method. The two compounds crystallize in the same space group, P21 (no. 4), and feature a three-dimensional network composed of CsOn (n = 8, 10 and 11) distorted polyhedra and isolated YO6 and PO4 groups. IR spectroscopy, UV-vis-NIR diffuse reflectance spectroscopy, and electronic band structures of the two compounds are reported. In addition, the Ce3+-doped CYP and CGP phosphors were synthesized and their photoluminescence properties were investigated. The results indicate that the CYP:Ce3+ and CGP:Ce3+ phosphors can be efficiently excited by ultraviolet (UV) light to realize a blue luminescence corresponding to the electric-dipole allowed 4f–5d transition of the Ce3+ ions.

Eu2+-Doped NaBa4(AlB4O9)2X3 (X = Cl, Br) phosphors with intense two-center blue emission and high color purity for n-UV pumped white light-emitting diodes

Highly efficient blue-emitting NaBa4(AlB4O9)2X3 (X = Cl, Br):Eu2+ phosphors (NBAC:Eu2+ and NBAB:Eu2+) were prepared by solid-state reaction. Two different crystallographic cation sites of Eu2+ ion occupancy were confirmed by Rietveld structural refinement, photoluminescence spectroscopy and measurement of fluorescence decay lifetimes. The NBAC:Eu2+ and NBAB:Eu2+ phosphors exhibited broad excitation ranging from 250 to 420 nm, both of which overlapped well with the emission of n-UV LED chips. Under 365 nm excitation, intense asymmetric blue emission bands were obtained for NBAC:Eu2+ and NBAB:Eu2+, peaking around 429 and 437 nm, respectively. The optimal Eu2+ doping concentration in both NBAC:xEu2+ and NBAB:xEu2+ was determined to be the same (10 mol%). The concentration quenching mechanism turned out to be electric dipole–dipole interactions. The NBAB:0.1Eu2+ and NBAC:0.1Eu2+ phosphors presented superior blue-emitting properties to the commercially available BAM:Eu2+ blue phosphor. The optimized NBAB:0.1Eu2+ and NBAC:0.1Eu2+ exhibited high external quantum efficiencies of 98.7% and 74.7% of the commercially available compound BaMgAl10O17:Eu2+ (BAM:Eu2+), respectively, while the NBAB:0.1Eu2+ phosphor demonstrated a very narrow band (FWHM ≤ 33 nm) and an ultra-high color purity of 99.58%. These results suggest that NBAB:Eu2+ and NBAC:Eu2+ can serve as promising blue-emitting candidates for n-UV pumped white LEDs.