Microcrystals Research Articles

Polymer Composites Entrapped Ce-Doped LiYF4 Microcrystals for High-Sensitivity X-ray Scintillation and Imaging.

Scintillators are widely used for radiation detection. The ability of converting ionizing radiation into visible photons is critical for application in X-ray security, computed tomography, and nuclear cameras. Conventional scintillators involve a high-temperature preparation and pose challenges for device integration and processability. Here, we report a facile hydrothermal synthesis of Ce-doped LiYF4 microcrystals (MCs) and their polymer composites for high-sensitivity X-ray detection and imaging application. These MC scintillators exhibit strong X-ray radioluminescence (RL) at ultraviolet wavelengths and show a high sensitivity to X-rays. Scintillating bulks based on these MCs display both strong RL and tunable emission across the visible spectrum. Further, these MC scintillators can be readily spun into a uniform film with a suitable MC content for X-ray imaging. Scintillating films can generate a strong X-ray-induced emission and long-term stability under X-ray illumination. Dose dependence of the RL intensity of our scintillating film indicates a high sensitivity to X-rays. Importantly, we exhibit an archetype application of scintillating films as X-ray radiography for a printed circuit board (PCB). Such an archetype can provide a decent spatial resolution as high as 0.54 mm. Our finding manifests MC composites of steady and efficient RL as a promising approach for X-ray radiography application.

Fine Fabrication and Optical Waveguide Characteristics of Hexagonal tris(8-hydroxyquinoline)aluminum(Ⅲ) (Alq3) Crystal

Herein, we reported on the precise growth and optical waveguide characteristics of hexagonal tris(8-hydroxyquinoline)aluminum(Ⅲ) (Alq3) micro-crystals (MCs). The hexagonal Alq3 MCs were prepared using surfactant-assisted assembly growth with the help of cetyltrimethylammoniumbromide (CTAB), in which the crystallization occurred as a result of molecular assembly and packing. Also, we adjusted the molar ratio of Alq3 and CTAB for the control degree of crystallization. The formation and structure of Alq3 MCs were investigated using field-emission scanning electron microscopy and X-ray diffraction pattern experiments, respectively. The solid-state laser confocal microscope-photoluminescence spectra and charge-coupled device images for the Alq3 MCs were measured to study the luminescence efficiency and colors, respectively. The optical waveguide performance of the hexagonal Alq3 MCs was measured for each side direction. According to our results, crystalline Alq3 micro-crystals are promising materials for application to the development of optical communication devices.

Enhanced red and near-infrared upconversion luminescence properties in CaSc2O4 microcrystals

CaSc2O4 co-doped with Yb/Er, Yb/Tm was successfully prepared through a high-temperature solid-state method. The enhanced red and near-infrared upconversion luminescence intensities in CaSc2O4 co-doped with Yb/Er, Yb/Tm were observed. As the increase of Ca2+ ion doping concentration, the crystal phase evolved from cubic Sc2O3 to orthorhombic CaSc2O4, then to cubic CaO. When Ca2+ ion doping concentration is about 40 mol%, CaSc2O4 co-doped with Yb/Er has the maximum red emission intensity (666 nm) which is increased by 39.5 times than that of Sc2O3 microcrystals (MCs). Similarly, the maximum near-infrared emission intensity (807 nm) in CaSc2O4 co-doped with Yb/Tm is enhanced by 9 times than that of Sc2O3 MCs when Ca2+ ion doping concentration is about 40 mol%. They are in accordance with that Er3+ ion and Tm3+ ion exhibit the longest luminescence lifetime in CaSc2O4 MCs than that in Sc2O3 and CaO MCs.

Mechanism for tunable broadband white photoluminescence of one-dimensional (C4N2H14)2Pb1-xMnxBr4 perovskite microcrystals

This work demonstrated that tunable photoluminescence of the low-dimensional hybrid halide perovskites via the doping strategy is feasible, and hence paves the way towards the investigation and application of high-performance single photoluminescence perovskite material with broadband white light emission. This work reported the first synthesis of one-dimensional (1D) (C4N2H14)2Pb1-xMnxBr4 perovskite microcrystals (MCs) with tunable broadband white-light emission and long-lived STE (self-trapped exciton) emission. The overall photoluminescence mechanism under the regulation of defect engineering of 1D (C4N2H14)2Pb1-xMnxBr4 MCs was formulated for the first time. Photoluminescence mechanism of these MCs involves competition of FE (free exciton) and STE and 4T1 states. This competitive mechanism further promotes photoluminescence tunability ranging from blue-white to white to orange-red for Mn-doped MCs. Moreover, time-resolved emission spectra show the changes of lifetime of these MCs at different emission wavelength. These perovskite MCs have a long-lived STE emission, which indicates that the more effective exciton combinations are occurring in them. For comparation, the Mn-doping strategy can regulate structural defect of these MCs, which promotes or suppresses transitions of FE→STE, FE→4T1, STE→FE and further tunes photoluminescence. This work paves the way towards the investigation and development of optoelectronic application of perovskite MCs material.

Excitation-Dependent Emission Color Tuning from an Individual Mn-Doped Perovskite Microcrystal.

Divalent manganese cation (Mn2+) doped perovskite materials are of great interest for their unique optical, magnetic, and electric properties. Herein, we report an excitation-dependent emission color tuning from an individual Mn-doped CsPbCl3 microcrystal (MC) with a wide color tuning range, reversible and continuous color change, and high photostability. We demonstrate that the Mn-doped CsPbCl3 MCs exhibit dual-color emission from both host excitons (blue) and Mn-dopants (orange) through an internal energy transfer (IET) process. By simple change of the laser excitation repetition rate or pulse intensity, the relative emission intensity between exciton (Iexciton) and Mn-dopant (IMn) can be continuously and reversibly tuned from IMn/Itotal (Itotal = IMn + Iexciton) = 0.9-0.8 to 0.1-0.2, corresponding to a color change from orange to blue. Such emission color tuning is enabled by the saturation of Mn-dopant emission at high excitation intensity and a linear dependence of exciton emission with excitation intensity. Transient spectroscopy and temperature-dependent photoluminescence (PL) measurements confirm that the exciton-to-dopant IET in Mn-doped CsPbCl3 MCs is mediated by some shallow trap states, rather than through a direct transfer pathway. Therefore, the saturation of Mn-dopant emission is caused by a bottlenecked energy transfer effect by saturating the mediating trap states at high excitation intensities. The Mn-doped MCs also exhibit a high photostability on the reversible switch of emission color between orange and blue for more than 300 cycles within a continuous operation time of 14 h. In view of the stable and color-switchable emission properties, Mn-doped perovskite MCs may find application in nanophotonic devices using a single MC.

Back-to-back Schottky junction photodetectors based on CVD grown CsPbBr3 microcrystalline striped films

In recent years, a new type of lead halide perovskite has attracted a lot of attention for next-generation photodetectors (PDs) with high responsivity, good detectivity, and fast photoresponse speed. Specifically, cesium based all-organic perovskites exhibit better photostability and therefore have achieved increasing success in PDs recently. For reducing the leak current and increasing the response speed of photoconductive PDs, back-to-back Schottky junction PD is designed and fabricated through a direct growth approach of CsPbBr3 microcrystal (MC) films on indium tin oxide (ITO) electrodes by the chemical vapor deposition (CVD) method. Due to the enhanced Schottky barrier height and threshold voltage between CsPbBr3 and ITO electrodes, the PD exhibits the on/off ratio of up to 104, peak responsivity of 3.9 AW−1, detectivity of 3.8 × 1012, and fast response speed of 0.22 ms (rise time) and 0.45 ms (decay time). In addition, the stability of PD is also enhanced by the high crystal quality of CVD grown CsPbBr3 MCs.

Temperature-dependent photoluminescence of Cs2AgxNa1-xInCl6 microcrystals

Cs2AgxNa1-xInCl6 microcrystals (MCs) are synthesized using a solvothermal method. The regular variation of Na incorporation is characterized according to the intensity change of the X-ray diffraction and ultraviolet visible absorption peaks. Temperature-dependent photoluminescence (PL) properties of Cs2AgxNa1-xInCl6 MCs are studied by use of steady-state PL and time-resolved PL (TRPL) measurements. From steady-state PL, it is verified that the white-light emission primarily originates from self-trapped excitons (STEs) at low temperature (80–290 K). When the electron-phonon interactions become stronger, STEs are promoted to generate from free excitons (FEs). And the high temperature-dependent (293–450 K) PL reveals the thermal quenching process. Temperature-dependent TRPL demonstrates that the change of lifetimes is attributed to band-edge FEs state and STEs state. It is found that the electrically-driven PL decreases to 70% then becomes stable from 0 to 120 min when light-emitting diodes (LEDs) are fabricated to explore the thermal stability. Our results suggest these white-emission Cs2AgxNa1-xInCl6 MCs can be used as excellent materials for LEDs and provide reference to engineer such MCs toward practical optoelectronic applications.

Improved thermal stability of photoluminescence in Cs4PbBr6 microcrystals/CsPbBr3 nanocrystals

Here, Cs4PbBr6 microcrystals (MCs) are synthesized using a two-phase liquid-liquid immiscible method. In order to unravel the bright green photoluminescence (PL) mechanism for in-situ Cs4PbBr6 MCs, the thermal stability of PL spectra for the supernatant and precipitate of reactants is investigated comparatively. Exciton binding energy, exciton-phonon (EP) coefficient and PL lifetime all indicate that the PL of the precipitate has similar temperature dependence as that of the supernatant. It is found that, according to its structural and optical characteristics, the supernatant of the reactants is CsPbBr3 nanocrystals (NCs). We also find that the bright green PL from the precipitate of the reactants is due to CsPbBr3 NCs embedded into Cs4PbBr6 MCs. Experimental results further reveal the size of CsPbBr3 NCs embedded into Cs4PbBr6 MCs is larger than that of CsPbBr3 NCs in the supernatant. The surface passivation of the composites can thus help to suppress the thermal quenching of PL for CsPbBr3 NCs, which opens a new avenue for enhancing the thermal stability of PL for perovskite NCs.

Chemical Vapor Deposition Method Grown All-Inorganic Perovskite Microcrystals for Self-Powered Photodetectors.

Self-powered photodetectors (SPPDs) have attracted lots of attention due to their various advantages including no external power sources, high-sensitivity, fast response speed, and so on. This study reports the fabrication and characterization results of CsPbBr3 microcrystals (MCs) grown by chemical vapor deposition (CVD) method, and the SPPDs have been fabricated on the basis of the CsPbBr3 MCs layer with the sandwich structure of GaN/CsPbBr3 MCs/ZnO. Such designed SPPD shows the detectivity ( D*) of 1014 Jones, on/off ratio of up to 105, peak responsivity ( R) of 89.5 mA/W, and enhanced stability at the incident wavelength of 540 nm. The photodetector enables the fast photoresponse speed of 100 μs rise time and 140 μs decay time. The performances of the SPPD are comparable to the best ones ever reported for CsPbBr3 based PDs but do not need external power supplies, which mainly benefit from the low trap density, long carrier diffusion of high quality CsPbBr3MCs film, and the built-in electric fields in the sandwich structure of GaN/CsPbBr3/ZnO layers.

Facile synthesized ZnO microcrystals for random microlasers and incandescent-type light sources

Well-crystallized ZnO microcrystals (MCs) with a well-faceted, smooth surface were successfully synthesized and employed to construct ultraviolet microsized random lasers.

Phase and morphology evolution of luminescent NaLnF4 (Ln = La to Yb) micro-crystals: understanding the ionic radii and surface energy-dependent solution growth mechanism

Lanthanide (Ln)-based NaLnF4 micro-crystals (MCs) exhibit great advantages and strong feasibility for integration into micron-sized photonic devices.

Controlling upconversion luminescence patterns in space with red emission enhancement from a single fluoride microcrystal by tuning the excitation mode.

The ability to control the upconversion (UC) luminescence patterns in space from lanthanide-doped UC luminescence materials is very important for many applications including three-dimensional color displays, optical waveguides and optical communication. In this work, the fascinating UC luminescence patterns could be adjusted from the blue or green flower-like emission pattern to the red flame-like irradiation pattern with a red luminescence enhancement from a single β-NaYF4:Yb3+/(Tm3+ or Er3+) microcrystal (MC) by varying the excitation position. The red-to-blue (R/B) and the red-to-green (R/G) emission intensity ratios from the single MC particle that the focal point position of excitation NIR laser is on the side lace are much stronger than that in the case of the focused laser beam on the center. We think that the physical mechanism from the changes in the luminescence pattern and the emission intensity ratio is explained by the optical waveguide effect based on the total reflection effect. These results provide a new strategy for facilitating fundamental investigations of the UC micro/nano-materials, which will lead to promising applications in three-dimensional color display, optical waveguides and optical communication.

Continuous Synthesis of Highly Stable Cs4PbBr6 Perovskite Microcrystals by a Microfluidic System and Their Application in White-Light-Emitting Diodes.

In this paper, we report a simple, rapid, and stable method for the continuous synthesis of highly stable Cs4PbBr6 perovskite microcrystals (MCs) using a microfluidic system. To demonstrate the potential application of Cs4PbBr6 MCs, the sample was fabricated with K2SiF6:Mn4+ phosphor onto InGaN blue chips as white-light-emitting diodes (LEDs). Our white-LED device achieved a high National Television Standards Committee value of 119% for backlight display, which indicated that the Cs4PbBr6 MC is a promising material for future applications.

High Resolution Mapping of Two-Photon Excited Photocurrent in Perovskite Microplate Photodetector.

We fabricate photodetectors based on solution-processed single CH3NH3PbBr3 microcrystals (MCs) and map the two-photon absorption (TPA) excited photocurrent (PC) with spatial resolution of 1 μm. We find that the charge carrier transport length in the MCs depends on the applied electric field, and increasesfrom 5.7 μm for 0.02 V bias (dominated by carrier diffusion) to 23.2 μmfor 2 V bias (dominated by carrier drift). Furthermore, PC shows strong spatial variations. Combining the PC mapping results with time-resolved photoluminescence microscopy, we demonstrate that the spatial distribution of PC mainly originates from the inhomogeneous distribution of trap-states across perovskite MCs. This suggests that there is still large margin for improvement of perovskite single crystal devices by better controlling of the traps.

In vitro/vivo assessment of praziquantel nanocrystals: Formulation, characterization, and pharmacokinetics in beagle dogs

To investigate the impact of particle size on in vitro/vivo performance of praziquantel (PZQ), nanocrystals (NCs) and microcrystals (MCs) of PZQ were prepared using the methods of wet milling and jet milling, respectively. PZQ NCs and MCs were characterized with dynamic light scattering, laser particle size analyzer, transmission electron microscopy, differential scanning calorimetry, X-ray powder diffraction and fourier transform infrared spectroscopy. The average diameters of PZQ NCs and MCs were 364.4 nm and 3.7 µm, respectively. No change in crystalline form was observed. Dissolution tests were performed in two different media, where the cumulative dissolution and dissolution rate of NCs were significantly improved in comparison with those of MCs and KANGQING® in non-sink condition. Similarly, oral bioavailability of PZQ NCs in beagle dogs was 1.68 (P < 0.05) and 1.83 fold (P < 0.01) higher than that of MCs and KANGQING®. Considering the advantages of in vitro/vivo performance and facile preparation, PZQ NCs may have a great application in the treatment of schistosomiasis.

Pressure-Assisted Annealing Strategy for High-Performance Self-Powered All-Inorganic Perovskite Microcrystal Photodetectors.

Owing to their low trap-state density, high carrier mobility, and high thermal stability, CsPbBr3 perovskite microcrystals (MCs) have attracted significant attention for applications as photodetectors (PDs). However, solution synthesis processes lead to MC films with high void density, seriously limiting the performance of the PDs. Here, a pressure-assisted annealing strategy is introduced to significantly reduce the void density and decrease the surface roughness. The resulting self-powered all-inorganic CsPbBr3 perovskite MC thick-film PDs show improved performance characteristics, with responsivities and detectivities of up to 0.206 A W-1 and 7.23 × 1012 Jones, respectively. Moreover, the on/off ratios of the devices are up to 106, and the highest linear dynamic range reaches 123.5 dB. These improved results indicate that the pressure-assisted annealing method is an effective strategy to enhance the performance of solution-synthesizedperovskite MC PDs.

What dose DOES a micro-crystal really absorb?

What dose DOES a micro-crystal really absorb?

Self-Powered All-Inorganic Perovskite Microcrystal Photodetectors with High Detectivity.

Organic-inorganic lead halide perovskite microcrystal (MC) films are attractive candidates for fabricating high-performance large-area self-powered photodetectors (PDs) because of their lower trap state density and higher carrier mobility than their polycrystalline counterparts and more suitability of synthesizing large lateral area films than their single-crystal counterparts. Here, we report on the fabrication of self-powered all-inorganic CsPbBr3 perovskite MC PDs with high detectivity, using a modified solution synthesis method. The MCs are up to about 10 μm in size, and the MC layer is also about 11 μm in thickness. Under 473 nm laser (100 mW) illumination, the CsPbBr3 MC PDs show responsivity values of up to 0.172 A W-1, detectivity values of up to 4.8 × 1012 Jones, on/off ratios of up to 1.3 × 105, and linear dynamic ranges of up to 113 dB. These performances are significantly better than those of PDs based on polycrystalline perovskite thin films and comparable with those of PDs based on perovskite single crystals.

Nearly temperature-independent ultraviolet light emission intensity of indirect excitons in hexagonal BN microcrystals

The radiative performance of hexagonal boron nitride (h-BN) was assessed by the spatio-time-resolved luminescence measurements on its microcrystals (MCs) annealed in an O2 gas ambient. The MCs exhibited distinct deep ultraviolet luminescence peaks higher than 5.7 eV, although h-BN is an indirect bandgap semiconductor. The result indicates a strong interaction between the indirect excitons (iXs) and LO/TO (and LA/TA) phonons at T points of the Brillouin zone. Such phonon replicas of free iXs and a luminescence band at 4.0 eV showed negligible thermal quenching, most probably assisted by the strong excitonic effect, enhanced phonon scattering, and formation of a surface BxOy layer that prevents excitons from surface recombination by the thermal excitation. Conversely, the luminescence band between 5.1 and 5.7 eV, which seems to consist of LO/TO phonon replicas of iXs localized at a certain structural singularity that are further scattered by multiple TO phonons at K points and another two emission peaks that originate from the singularity, showed the thermal quenching. In analogy with GaN and AlGaN, cation vacancy complexes most likely act as native nonradiative recombination centers (NRCs). In the present case, vacancy complexes that contain a boron vacancy (VB), such as divacancies with a nitrogen vacancy (VN), VBVN, are certain to act as NRCs. In this instance, iXs delocalized from the singularity are likely either captured by NRCs or the origin of the 4.0 eV-band; the latter is assigned to originate from a carbon on the N site or a complex between VB and an oxygen on the N site.

Direct Experimental Observation of Facet-Dependent SERS of Cu2 O Polyhedra.

Semiconductor-based surface enhanced Raman scattering (SERS) has attracted great attention due to its excellent spectral reproducibility, high uniformity, and good anti-interference ability. However, its relatively low SERS sensitivity still hinders its further developments in both performance and applications. Since the SERS is a peculiar surface effect, investigating the facet-dependent SERS activity of semiconductor nanostructures is crucial to boost their SERS signals. Although the semiconductor facet-dependent SERS effect is predicted via numerical calculations, convincing experimental evidence is scarce due to complicated and undefined surface conditions. In this work, three facet-defined ({100}, {110}, and {111} facets) Cu2 O microcrystals (MCs) with clear surface atomic configuration are utilized to investigate the facet-dependent SERS effect. The results from the Kelvin probe force microscopy measurements on single Cu2 O polyhedron, demonstrate that the facet-dependent work function plays a crucial role in the interfacial charge transfer process. Comparing with the {110} and {111} facets, the {100} facet possesses the lowest electronic work function, which enables more efficient interfacial charge transfer. The simulation results further confirm that the {100}-facets can transfer the most electrons from Cu2 O MCs to molecules due to its lowest facet work function, resulting in the largest increment of the molecular polarization.