High-quality Crystals Research Articles

Growing single crystals of two-dimensional covalent organic frameworks enabled by intermediate tracing study

Resolving single-crystal structures of two-dimensional covalent organic frameworks (2D COFs) is a great challenge, hindered in part by limited strategies for growing high-quality crystals. A better understanding of the growth mechanism facilitates development of methods to grow high-quality 2D COF single crystals. Here, we take a different perspective to explore the 2D COF growth process by tracing growth intermediates. We discover two different growth mechanisms, nucleation and self-healing, in which self-assembly and pre-arrangement of monomers and oligomers are important factors for obtaining highly crystalline 2D COFs. These findings enable us to grow micron-sized 2D single crystalline COF Py-1P. The crystal structure of Py-1P is successfully characterized by three-dimensional electron diffraction (3DED), which confirms that Py-1P does, in part, adopt the widely predicted AA stacking structure. In addition, we find the majority of Py-1P crystals (>90%) have a previously unknown structure, containing 6 stacking layers within one unit cell.

Kinetic Study of the Thermal Quenching of the Ultraviolet Emission in Zn2GeO4 Microrods

Zn2GeO4 microrods obtained by thermal evaporation of a compacted powder mixture of ZnO and Ge exhibit quite intense UV luminescence at low temperatures. Herein, the luminescence properties of Zn2GeO4 microrods are studied for 2:1 and 1:1 ZnO:Ge ratio in the precursor mixture. In both cases, Zn2GeO4 microrods of high crystal quality produce a 355 nm emission under aforementioned bandgap excitation conditions at low temperatures. However, this emission vanishes at room temperature (RT) in the 1:1 samples while it is kept in the 2:1 ones. Herein this work, the thermal quenching of the UV luminescence is studied by means of steady and time‐resolved photoluminescence techniques from 4 K up to RT for both Zn2GeO4 microrods. The analysis of the results leads us to conclude that although the luminescence mechanisms are the same in both cases, a higher decay rate is observed in the 1:1 in both intensity and lifetime, which explain the observed thermal quenching at RT.

Circular photogalvanic effect of surface states in the topological insulator Bi2(Te0.23Se0.77)3 nanowires grown by chemical vapor deposition

Circular photogalvanic effect (CPGE) of single-crystalline ternary topological insulator Bi2(Te0.23Se0.77)3 nanowires, which are synthesized by the chemical vapor deposition, have been investigated. It is demonstrated that the distributions of the elements in the nanowires are fairly uniform, and they have high crystal quality. Compared with Bi2Se3 nanowires, the ternary Bi2(Te0.23Se0.77)3 nanowires have better responsivity to circularly polarized light. The incident angle dependence of the CPGE current indicates that the symmetry of the surface states of the nanowire belongs to C3v symmetry. The temperature dependence of the CPGE current is also investigated. As the temperature decreases from 300 to 77 K, the CPGE current first increases and then decreases, which is due to the variation of the mobility and photo-generated carrier density with temperature. Our work suggests that ternary Bi2(Te1−xSex)3 nanowires are good candidates for designing polarization-sensitive photoelectric devices.

Stable CsPbX3 mixed halide alloyed epitaxial films prepared by pulsed laser deposition

The pulsed laser deposition (PLD) technique has been proved to be able to grow oxide thin films with high structural quality with precisely controlled composition and thickness to achieve designed optical and electronical properties established in alloyed semiconductors and heterostructures. In this Letter, inorganic halide perovskite CsPb(IxClyBr1−x−y)3 epitaxial alloyed films on (001)-SrTiO3(STO) substrates were grown by PLD. The film crystal quality, phase stability, and the epitaxial relationship between the film and substrate were characterized with a detailed x-ray diffraction technique like high-resolution reciprocal spatial mapping and ϕ-scan. In addition, the photocarrier dynamics of the alloyed epitaxial films were investigated by photophysics spectroscopy, including steady and femtosecond transient optical absorption spectroscopy and temperature-dependent and time-resolved photoluminescence spectroscopy. The bandgap of the CsPbX3 films was tuned from 1.75 to 2.98 eV by substituting X with I/Br/Cl and their mixture of different ratios. Free exciton emissions were observed at a low temperature photoluminescence spectrum (PL, 10 K), which confirmed the high crystal and optical quality of the epitaxial perovskite alloyed films except the CsPbI3 film. The femtosecond transient absorption spectra also showed that such perovskite films are of very low concentration of exciton trap states. These results indicated that PLD is a powerful technology for growing high quality inorganic halide perovskite films with a tunable bandgap covering the full visible light range, which provided more options for CsPbX3 based panchromatic LED and other optoelectronic devices.

Data Mining for Terahertz Generation Crystals.

A data mining approach to discover and develop new organic nonlinear optical crystals that produce intense pulses of terahertz radiation is demonstrated. The Cambridge Structural Database is mined for non-centrosymmetric materials and these structural data are used in tandem with density functional theory calculations to predict new materials that efficiently generate terahertz radiation. This enables us to (in a relatively short time) discover, synthesize, and grow large, high-quality crystals of four promising materials and characterize them for intense terahertz generation. In a direct comparison to the current state-of-the-art organic terahertz generation crystals, these new materials excel. The discovery and characterization of these novel terahertz generators validate the approach of combining data mining with density functional theory calculations to predict properties of high-performance organic materials, potentially for a host of exciting applications.

Sensitivity and Detection Limit of Spectroscopic‐Grade Perovskite CsPbBr3 Crystal for Hard X‐Ray Detection

AbstractSpectroscopic‐grade single crystal detectors can register the energies of individual X‐ray interactions enabling photon‐counting systems with superior resolution over traditional photoconductive X‐ray detection systems. Current technical challenges have limited the preparation of perovskite semiconductors for energy‐discrimination X‐ray photon‐counting detection. Here, this work reports the deployment of a spectroscopic‐grade CsPbBr3 Schottky detector under reverse bias for continuum hard X‐ray detection in both the photocurrent and spectroscopic schemes. High surface barriers of ≈1 eV are formed by depositing solid bismuth and gold contacts. The spectroscopic response under a hard X‐ray source is assessed in resolving the characteristic X‐ray peak. The methodology in enhancing X‐ray sensitivity by controlling the X‐ray energies and flux, and voltage, is described. The X‐ray sensitivity varies between a few tens to over 8000 μC Gyair−1 cm−2. The detectable dose rate of the CsPbBr3 detectors is as low as 0.02 nGyair s−1 in the energy discrimination configuration. Finally, the unbiased CsPbBr3 device forms a spontaneous contact potential difference of about 0.7 V enabling high quality of the CsPbBr3 single crystals to operate in “passive” self‐powered X‐ray detection mode and the X‐ray sensitivity is estimated as 14 μC Gyair−1 cm−2. The great potential of spectroscopic‐grade CsPbBr3 devices for X‐ray photon‐counting systems is anticipated in this work.

Deconvoluting the energy transport mechanisms in all-inorganic CsPb2Br5/CsPbBr3 perovskite composite systems

CsPb2Br5/CsPbBr3 composite systems have received considerable attention among numerous lead halide perovskite materials due to their significantly enhanced photoluminescence intensity and stability against moisture. However, the luminescence mechanism of CsPb2Br5 based materials remains controversial, which significantly hinders the further material design and utilization for optoelectronic devices. In this work, to deconvolute their luminescent mechanisms, high-quality CsPb2Br5 crystals without any undesired by-products and impurities have been first prepared by a microwave-assisted synthesis method. The luminescence-inactive characteristics of the material are then confirmed by the steady-state absorption, photoluminescence, transient absorption spectra, and time-resolved terahertz spectroscopy. The prepared CsPb2Br5 crystals exhibit excellent crystallinity and enhanced thermal stability, particularly that they can maintain their crystalline structures in polar organic solvents. By simply manipulating the ratios of different precursor materials, it is witnessed that the green emission comes from the CsPbBr3 adhered, nucleated, and grown on the CsPb2Br5 crystals. Ultrafast transient absorption measurements in visible and terahertz spectral regions reveal that with the help of phonon scattering-assisted hopping at interfacial states, intersystem crossing dominates the electron transfer process in the composite crystals. As a result, the CsPb2Br5 and CsPbBr3 interact extensively with each other. Meanwhile, the Auger recombination rate and the defect-related non-radiative process are suppressed in the composite crystals, thereby enhancing the fluorescence of composite crystals. This work has not only deconvoluted the controversial and unclear luminescent mechanisms of CsPb2Br5 materials but also established a pathway to design and enhance the fluorescence of materials for technological applications.

Enhancement of protein crystallization with the application of Taylor vortex and Poly(ionic liquid)s

With the increase demands for biopharmaceuticals, the development of efficient protein crystallization processes for manufacturing highly pure crystalline products has become critical for downstream biomanufacturing. This work focused on the combined effects of the uniform shear rate in a solution created by using the Taylor vortex and the macromolecular architecture of a solution developed using poly(ionic liquid)s during protein crystallization. The results were as follows: (1) the accelerated primary nucleation rate with a uniform shear force that was generated by the Taylor vortex; (2) the stability of the protein product prepared using the poly(ionic liquid)s was high in the crystallization solution; and (3) improved control of supersaturation of the polymer was achieved with the salting-out effect. The average crystal size in the control group was considerably lower than 1 μm with low quality (9.00 Å) and yield (56%) of crystals, but the average size of lysozyme crystals obtained using the Taylor vortex and poly(ionic liquid)s increased up to 5 μm with high quality (1.86 Å) of crystals and yield (86%). A higher diffraction resolution indicated a better ordered crystalline structure, demonstrating that the Taylor vortex and poly(ionic liquid)s are useful for improving the crystal quality.

Highly Transparent p‐AlGaN‐Based (326–341 nm)‐Band Ultraviolet‐A Light‐Emitting Diodes on AlN Templates: Recent Advances and Perspectives

The epitaxial growth of transparent p‐AlGaN‐based ultraviolet‐A (UVA), light‐emitting diodes (LEDs) may solve the problems of UVA light absorption through the GaN buffers and p‐GaN contact layers at (326–341 nm)‐band emission, respectively. Herein, first, an idea of conventional n‐AlGaN buffer layer (BL) and n‐AlGaN electron source layer (ESL) for the suppression of threading dislocation density (TDD) and enhancement of internal quantum efficiency (IQE) of UVA emitters, using low‐pressure metalorganic vapor‐phase epitaxy (LP‐MOVPE) is attempted. As a result, the total‐TDDs is reduced from ≈ 3 × 109 cm−2 to ≈ 1 × 109 cm−2 in the n‐AlGaN ESL of a 326 nm‐band UVA multiquantum‐wells (MQWs), and IQE is also improved from 30% to 52% at room temperature (RT). Second, an idea of Si‐doped n‐AlGaN Superlattices (SLs)‐based BL, using LP‐MOVPE is challenged. Subsequently, a record IQE of 56% at RT and high crystal quality in 341 nm‐band UVA MQWs are observed. Finally, using a well thickness ≈ 2 nm in SLs‐based UVA MQWs, the light power and external quantum efficiency (EQE), respectively, are remarkably enhanced from 3.5 mW and 0.5% to 7.5 mW and 1.4% on wafer in 341 nm‐Band UVA LED. The perspective for the improvements of UVA emitter's performances is also discussed.

Enhancement of optical property and crystal structure for GaN films on 2D MoS2 buffer layer by nitridation treatment

The growth of GaN films on layered 2D-MoS2/c-sapphire substrate by plasma-assisted molecular beam epitaxy system was investigated. Pre-nitridation process and deposition time as growth parameters were employed to exploit the heteroepitaxial growth of GaN films at the temperature of 700 °C. The characterizations of surface condition, surface morphology, chemical composition, optical properties, crystal orientation and quality of GaN epitaxial layers were conducted by reflection high energy electron diffraction (RHEED), scanning electron microscope (SEM), atomic force microscope (AFM), X-ray photoelectron Spectroscopy (XPS), Raman spectroscopy, photoluminescence spectroscopy (PL) and high-resolution X-ray diffraction (HR-XRD), respectively. The pre-nitridation process and growth duration can enhance the optical properties and crystal quality of GaN films on 2D-MoS2/c-plane sapphire. The pre-treatment of MoS2 layer by nitrogen plasma before the growth, providing a wetting layer of nitrogen for GaN growth, can eliminate the oxygen contamination and reduce the defects in the films. Band-to-band emission of Wurtzite GaN with the bandgap of 3.5 eV can be improved by pre-nitridation process, and yellow band emission can be suppressed due to the elimination of defect states in the films. Meanwhile, the crystal orientation and quality can be improved by the pre-nitridation process and longer duration of growth time. This study demonstrates a van der Waals epitaxial GaN films on 2D-MoS2 layer with higher crystal quality by pre-nitridation process, and it provides a potential solution for the future applications of GaN-based devices.

Interface stability problem towards the end of THM growth of cadmium zinc telluride crystals

Growth of CdZnTe crystals by Travelling Heater Method (THM) is widely investigated by different groups. Interface shape and stability are critical parameters for the successful growth of high-quality CZT crystals. Even though interface stability could be achieved for a significant portion of the CZT ingot, the situation towards the end of the growth is not well studied and understood. In this study, interface stability and the change of crystal quality towards the end of the growth are examined. The change of the interface shape and formation of large Te inclusions are observed. Moreover, towards the end of the growth, phase separation and formation of a new zinc-based phase are detected with a needle-like structure. The decrease of crystal quality and evolution of the CZT ingot towards the end of the growth are summarized.

Large CaFeAsF single crystals of high quality grown by the oscillating temperature technique

The oscillating temperature technique was successfully employed to grow large and composition homogeneous CaFeAsF single crystals due to the reduction of nuclei number by temperature oscillation. The largest as-grown CaFeAsF single crystal was nearly 8 mm in lateral size and 0.27 mm in thickness which is far larger than previously reported in all dimensions. The full-width-at-half maximum (FWHM) of x-ray rocking curve is as small as 0.163° indicating the high quality of CaFeAsF single crystals.

Photon Echo Polarimetry of Excitons and Biexcitons in a CH3NH3PbI3 Perovskite Single Crystal

Lead halide perovskites show remarkable performance when used in photovoltaic and optoelectronic devices. However, the peculiarities of light-matter interactions in these materials in general are far from being fully explored experimentally and theoretically. Here we specifically address the energy level order of optical transitions and demonstrate photon echos in a methylammonium lead triiodide single crystal, thereby determining the optical coherence times $T_2$ for excitons and biexcitons at cryogenic temperature to be 0.79 ps and 0.67 ps, respectively. Most importantly, we have developed an experimental photon-echo polarimetry method that not only identifies the contributions from exciton and biexciton complexes, but also allows accurate determination of the biexciton binding energy of 2.4 meV, even though the period of quantum beats between excitons and biexcitons is much longer than the coherence times of the resonances. Our experimental and theoretical analysis methods contribute to the understanding of the complex mechanism of quasiparticle interactions at moderate pump density and show that even in high-quality perovskite crystals and at very low temperatures, inhomogeneous broadening of excitonic transitions due to local crystal potential fluctuations is a source of optical dephasing.

The Effect of Oxygen and Air Flow on the Characteristic of Zinc Oxide ZnO Nanorods

In this study, ZnO nanorods are synthesized by the means of chemical bath deposition (CBD). Further objectives, to study the effect of new parameters (oxygen and air flow) on ZnO nanorods on morphology and length of alignment of the nanorods. It was observed that 0.100M for both zinc nitrate hexahydrate and hexamethylenetetramine give a best result. Here, we added an oxygen gas pressure as others idea in chemical bath deposition method and a p-type silicon substrate to investigate the length of ZnO nanorods through their cross-section. It was found that the biggest grain sizes obtain from sample 14, which is 58.87nm produces a high crystal quality, among others. Sample 14 also has a long nanorods (1.434μm-1.451μm) compared to other samples. The hexagonal structure was observed by Field Emission Scanning Electron Microscope, FESEM and well aligned, compact and perpendicular ZnO nanorods grew on the surface of glass substrate. XRD data shows that it has a larger grain size with low lattice strain. The cross-sectional view of the samples demonstrates long nanorods with a range 2.763μm-2.844μm. Finally, the growth of ultra-long ZnO nanorods by using a modified chemical bath setup was successfully observed and recorded.Keywords: ZnO nanorods, CBD, Growth duration, FESEM images, pressure flow.

2D Perovsktie Substrate-Assisted CsPbI3 Film Growth for High-Efficiency Solar Cells.

High-quality perovskite films are beneficial for fabricating perovskite solar cells (PSCs) with excellent photoelectric performance. The substrate on which the perovskite film grows plays a profound role in improving the crystallization quality of the perovskite film. Here, we proposed a novel method for optimizing CsPbI3 perovskite films, that is, two-dimensional (2D) perovskite substrate-assisted growth (2D-PSAG) method. The prepared PEA2PbI4 2D perovskite with proper wettability and roughness is used as a substrate to fabricate the high-quality CsPbI3 film. Moreover, it is found that PEA cations show a vertical gradient distribution within the whole CsPbI3 film because of their bottom-up self-diffusion. Also, PEA cations induce the moderate distortion of [PbI6]4- octahedron and slight lattice contraction of CsPbI3 by chemically bonding between Pb and N atoms. Surprisingly, the trace amounts of PEA cations lead to a bottom-up gradient phase transition from γ-CsPbI3 to β-CsPbI3. Therefore, the energy-level alignment becomes more matched at the interface of the perovskite layer/hole transport layer (poly3-hexylthiophene, P3HT), which denotes a large improvement of hole transport and extraction in PSCs made with the 2D-PSAG method. As a result, the CsPbI3-based PSCs with P3HT as a hole transport layer exhibit a champion efficiency of 17.13%, while the control device exhibits a PCE of only 14.16%. The PSCs made by the 2D-PSAG method retain above 70% of the initial PCE value after storage of 9 days in air (RH 10-20%), while the control device decomposes completely after 9 days. The improved stability could originate from the steric effects of PEA cations and the high crystallization quality of the mixed-phase CsPbI3 film. Therefore, 2D-PSAG is a novel and promising strategy to develop all-inorganic PSCs with high performance and stability.

High-Performance Photodetectors Based on the 2D SiAs/SnS2 Heterojunction.

Constructing 2D heterojunctions with high performance is the critical solution for the optoelectronic applications of 2D materials. This work reports on the studies on the preparation of high-quality van der Waals SiAs single crystals and high-performance photodetectors based on the 2D SiAs/SnS2 heterojunction. The crystals are grown using the chemical vapor transport (CVT) method and then the bulk crystals are exfoliated to a few layers. Raman spectroscopic characterization shows that the low wavenumber peaks from interlayer vibrations shift significantly along with SiAs’ thickness. In addition, when van der Waals heterojunctions of p-type SiAs/n-type SnS2 are fabricated, under the source-drain voltage of −1 V–1 V, they exhibit prominent rectification characteristics, and the ratio of forwarding conduction current to reverse shutdown current is close to 102, showing a muted response of 1 A/W under excitation light of 550 nm. The light responsivity and external quantum efficiency are increased by 100 times those of SiAs photodetectors. Our experimental results enrich the research on the IVA–VA group p-type layered semiconductors.

Zero-field magnetic resonance of cobalt ion pairs in ZnO nanocrystals

Cobalt-doped ZnO nanoparticles (NPs) with different Co concentrations are investigated by means of $X$- and $Q$-band electron spin resonance (ESR) near liquid-helium temperature in both parallel and perpendicular modes. The high crystal quality of the NPs allows for the hyperfine-structure resolution within the single ${\mathrm{Co}}^{2+}$ ions' ESR powder spectra. Depending on cobalt concentration, common additional weak ESR lines are detected which are here demonstrated to arise from some ${\mathrm{Co}}^{2+}$ high-spin pairs with a distance of about 4--6 \AA{}. ESR simulations show that these 3/2 spin pairs are weakly coupled by an isotropic Heisenberg Hamiltonian with either ferromagnetic or antiferromagnetic $J$ coupling constants, almost identical to those previously detected in bulk and microwire ZnO:Co. The presence of substantial (axial) single-ion anisotropy in ZnO:Co makes the different pairs' resonance positions strongly depending on the $J$ value. For resonance frequency $\ensuremath{\nu}$ in the microwave range, four cobalt pairs can satisfy the condition $|J|\ensuremath{\sim}h\ensuremath{\nu}/3$ to resonate at almost zero magnetic field. Such near-zero-field transitions notably resonate in the parallel ESR mode, which is the signature of the gapped nonlinear Zeeman effect, which is of particular interest for highly stable atomic-clock transitions.

Mis-cut direction of substrate effect on the photoresponse characteristic of β-Ga2O3 film

β-Ga2O3 epitaxial film was grown on 6° mis-cut angle (toward <112‾0> and <101‾0> directions) c-plane sapphire substrate by metalorganic chemical vapor deposition (MOCVD). The influence of mis-cut sapphire substrate on β-Ga2O3 film and the photoresponse properties of β-Ga2O3 photodetectors were studied. Compared with non-miscut substrates, both mis-cut sapphire substrates can obtain high crystal quality β-Ga2O3 films by reducing the in-plane rotational symmetry and promoting the step flow growth mode. Compared with the mis-cut toward <112‾0> direction, the substrate of mis-cut toward <101‾0> direction showed the highest crystal quality and largest growth rate due to smaller edge step terrace width and large step density. Three β-Ga2O3 photodetectors were fabricated on films grown on the non-miscut and the two mis-cut substrates. The photoresponse characteristics of β-Ga2O3 photodetectors based on sapphire substrate of mis-cut toward <101‾0> direction can reach 65 μA (photocurrent), 3.5 × 103 (Ip-Id/Id) and 3.21 A/W (responsivity, under 254 nm light). These results demonstrate an important guidance to develop the industrialization of β-Ga2O3 materials and devices.

High-quality optically defect-free 1D ZnS nanostructures by a modified thermal evaporation method

ZnS has great potential as a valuable material for nanoscale devices because of its rich morphologies and unique structure. Although much effort has been made, the growth of high-quality ZnS crystal is still a challenge. In this paper, high-quality optically defect-free 1D ZnS nanostructures, including nanorods, nanowires, and nanobelts, were successfully synthesized on a large scale by a modified thermal evaporation method. XRD patterns and HRTEM images indicated that the ZnS nanostructures are single phases with hexagonal structures. Under optical excitation, all the ZnS nanostructures show intense UV emissions and almost no defect emissions at room temperature. Sharp UV lasing-like peaks with the FWHM as narrow as 2–3 nm are achieved for the ZnS nanobelts and nanowires. The optical transition from free exciton A, free exciton B, and their associated LO phonon replicas were determined from the evolution of the NBE emissions. These high-quality nanostructures are envisaged to be highly promising for high-efficiency light-emitting devices and lasers in the UV region.

Strain Balanced Self‐Supporting Single‐Crystalline LiNbO3 Thin Films for Flexible Electronics

AbstractFunctional single‐crystalline films with mechanical flexibility have attracted intensive interest due to excellent material quality and the wide applications in flexible electronics. However, the free‐standing single‐crystalline films with the thickness in sub‐micrometer range usually deform due to insufficient mechanical strength or internal stress. This study introduces a strain balanced model (SBM) of a sandwich structure and an ion slicing‐based strain compensation bonding method for fabricating ultrathin but self‐supporting single‐crystalline thin films. Based on the SBM and the strain compensation bonding method, a centimeter‐scale strain balanced LiNbO3 (LN) thin film (SB‐LNTF) consisting of two pieces of 550 nm single‐crystalline LN film and an intermediate layer of benzocyclobutene is successfully fabricated. In additional to flat, bendable, transparent, and lightweight, the fabricated ultrathin (&lt;10 µm) SB‐LNTF also exhibits excellent self‐supporting property. Standard piezoresponse force microscopy amplitude butterfly curve and a 180° phase switching associated with ferroelectric behavior of LN film are observed, which confirm its high crystal quality of the ion sliced LiNbO3 thin film. A flexible acoustic resonator demonstrated on SB‐LNTF shows strong resonances. In principle, the strain compensation bonding method is also applicable to epitaxial lift‐off films.