High-quality Single Crystals Research Articles

Hydrothermal synthesis of lanthanide ruthenate single crystals

A series of lanthanide ruthenates were synthesized as large single crystals using a high temperature hydrothermal method (600–650 °C, 200 MPa). In all cases, the lanthanide sesquioxides (Ln2O3) and RuO2 were used as starting materials in various ratios. A range of products containing ruthenium in a variety of oxidation states were isolated, including LnRuO3, Ln2Ru2O7, Ln2RuO5(OH), and Ln5Ru2O12, depending on the size of the rare earth and the relative ratio of starting materials. In most cases, the products were formed in good yield as high-quality single crystals. This approach provides a route to single crystals of the perovskite and pyrochlore structure types to enable advanced physical properties measurements. The study also introduces the Ln2RuO5(OH) structure type for Ru5+, and significantly expands the phase space for Ln5Ru2O12 to the later lanthanides, also as well-formed single crystals. Careful analyses of diffraction data suggest that the materials contain minimal site disorder or nonstoichiometry.

Growth of delafossite CuAlO2 single crystals in a reactive crucible

Delafossite oxide CuAlO2 has received great attention as a promising p-type conducting oxide. In this work, high-quality CuAlO2 single crystals with a size of several millimeters (mm) are successfully synthesized with a reactive crucible melting method. The crystals are characterized by x-ray diffraction, scanning electron microscopy with energy-dispersive spectroscopy, transport measurement, and magnetic susceptibility measurement. The CuAlO2 single crystals show semiconducting behavior with hole carriers, which is consistent with other crystals grown by the conventional slow-cooling method. This growth method we reported here eliminates the process of removing the remaining flux, allowing easy access to the high-quality single crystals. This new approach to growing high-quality delafossite oxide CuAlO2 with a few mm size is important for new technologies that demand p-type semiconductor-based device fabrication.

Pinning potential in highly performant CaKFe4As4 superconductor from DC magnetic relaxation and AC multi-frequency susceptibility studies

We have investigated the pinning potential of high-quality single crystals of superconducting material CaKFe4As4 having high critical current density and very high upper critical field using both magnetization relaxation measurements and frequency-dependent AC susceptibility. Preliminary studies of the superconducting transition and of the isothermal magnetization loops confirmed the high quality of the samples, while temperature dependence of the AC susceptibility in high magnetic fields show absolutely no dependence on the cooling conditions, hence, no magnetic history. From magnetization relaxation measurements were extracted the values of the normalized pinning potential U*, which reveals a clear crossover between elastic creep and plastic creep. The extremely high values of U*, up to 1200 K around the temperature of 20 K lead to a nearly zero value of the probability of thermally-activated flux jumps at temperatures of interest for high-field applications. The values of the creep exponents in the two creep regimes resulted from the analysis of the magnetization relaxation data are in complete agreement with theoretical models. Pinning potentials were also estimated, near the critical temperature, from AC susceptibility measurements, their values being close to those resulted (at the same temperature and DC field) from the magnetization relaxation data.

Mixed Insulating State for van der Waals CoPS3

Large-scale high-quality van der Waals CoPS3 single crystals are synthesized using a chemical vapor transport (CVT) method. The crystallographic structure and electronic properties of this layered material are systematically studied using different spectroscopic methods (XPS, NEXAFS, and resonant photoelectron spectroscopy) accompanied by density functional theory (DFT) calculations. All experimental and theoretical data allow assignment of this material to the class of mixed Mott-Hubbard/charge-transfer insulator with Udd ≅ Δ. All obtained results can enrich the information on the new class of van der Waals materials, transition metal phosphorus trichalcogenides, and help to further effectively exploit their electronic, optical, and transport properties, which are important for adopting this kind of materials into different application areas, such as spintronics and catalysis.

Improved Crystallization Quality of FAPbBr3 Single Crystals by a Seeded Solution Method.

Solution-grown hybrid perovskite, FAPbBr3, has attracted great attentions recently due to its inspiring optoelectronic properties and low-cost preparation method. However, challenges of solution growth for FAPbBr3 bulk crystals remain yet, such as uncontrollable crystalline morphologies, irregular shapes, and limited crystal sizes, which are attributed to the dense crystallization nucleus. In this work, we investigate the effects of growth conditions and seed behaviors on the crystallization quality and the yield of FAPbBr3 single crystals. First, the spontaneous nucleation is tailored by optimizing the precursor concentration and heating rate. Furthermore, the seeded crystals are introduced to solve the issues related to the morphology and the yield of single crystals. Based on the above-mentioned investigations, an optimized growth method, a seeded solution method, under a heating rate of 0.1 °C/h is proposed, and centimeter-scale FAPbBr3 single crystals with a very narrow FWHM of high-resolution X-ray diffraction rocking curves and a high yield of ∼100% of single crystals are obtained. The resulting FAPbBr3 single crystal exhibits a bulk resistivity of 3.42 × 109 Ω·cm and a superior ION/IOFF ratio over 104 under 405 nm light at a bias of 10 V. Finally, the pulse height spectra with an energy resolution of ∼21.4% are also achieved based on an AZO/FAPbBr3/Au detector, illuminated using an uncollimated 241Am@5.49 MeV α-particle source at room temperature. This modified seeded solution method shows great potential in preparing high-quality and high-yield perovskite single crystals.

Enhanced electrical transport properties of PbTe single crystal through Ga substitution synthesized by a Pb-flux method

As a promising thermoelectric material at the medium temperature range, PbTe has attracted much attention during the past several decades. In this study, Ga-doped PbTe single crystals were synthesized by a facile Pb-flux method. The electronic structure calculations reveal that Ga substitution can introduce impurity level in the bandgap, which can effectively modulate the carrier concentration and therefore optimize the electrical conductivity. Experimentally, the carrier concentration was modulated from 1.95 to 4.46 × 1019 cm−3 at 300 K. Accordingly, the electrical conductivity reaches a maximum of 1.5 × 105 S·m−1 at 300 K for the sample with x = 0.025. As a result, maximal thermoelectric power factor 3.07 × 10−3 W·m−1·K−2 is attained at 360 K for the aforementioned sample, 62% higher than that of the undoped sample (1.89 ×10−3 W·m−1·K−2). This work provides a viable method to synthetize high-quality single crystal and it also verifies that Ga substitution can significantly improve the electrical transport properties of single crystalline PbTe thermoelectric material.

Crystal growth of “defective” half-Heusler Nb0.83CoSb

‘Defective’ half Heuslers are a new class of defect stabilized Heusler alloys. They have a cubic structure analogous to their XYZ half-Heusler parent phase but with as large as 20% vacancies at the X-site. The presence of a copious amount of vacancies has several interesting outcomes including a significantly suppressed thermal conductivity which makes them potential thermoelectric materials for high temperature applications. They also exhibit vacancy ordering. The nature of this ordering is as yet an unresolved issue. Having high-quality single-crystals of these materials can be useful for performing advanced spectroscopy experiments to throw light on this issue. In this manuscript we report the crystal growth of Nb0.83CoSb using the chemical vapor transport technique. By using iodine as a transporting agent and a temperature profile described in the manuscript, mm-sized high-quality crystals of Nb0.83CoSb are obtained. The grown crystals were characterized x-ray diffraction (powder and Laue) and electron microscopy (FESEM and HRTEM) techniques. In HRTEM, diffuse bands due to short-range vacancy ordering, and weak spots due to superstructure formation were found to coexist with the fundamental diffraction spots due to the overall hH cubic symmetry. The differential scanning calorimetry revealed a reversible phase transition near 1200 °C which is likely a vacancy order–disorder transition. The electrical resistance shows a linearly decreasing behavior upon cooling.

Ultrafast and Sensitive Self-Powered Photodetector Based on Graphene/Pentacene Single Crystal Heterostructure with Weak Light Detection Capacity.

Organic materials exhibit efficient light absorption and low-temperature, large-scale processability, and have stimulated enormous research efforts for next-generation optoelectronics. While, high-performance organic devices with fast speed and high responsivity still face intractable challenges, due to their intrinsic limitationsincluding finite carrier mobility and high exciton binding energy. Here an ultrafast and highly sensitive broadband phototransistor is demonstrated by integrating high-quality pentacene single crystal with monolayer graphene. Encouragingly, the -3dB bandwidth can reach up to 26kHz, which is a record-speed for such sensitized organic phototransistors. Enormous absorption, long exciton diffusion length of pentacene crystal, and efficient interfacial charge transfer enable a high responsivity of >105 AW-1 and specific detectivity of >1011 Jones. Moreover, self-powered weak-light detection is realized using a simple asymmetric configuration, and the obvious zero-bias photoresponses can be displayed even under 750nWcm-2 light intensity. Excellent response speed and photoresponsivity enable high-speed image sensor capability in UV-Vis ranges. The results offer a practical strategy for constructing high-performance self-powered organic hybrid photodetectors, with strong applicability in wireless, weak-light detection, and video-frame-rate imaging applications.

Anomalous resistivity upturn in the van der Waals ferromagnet Fe5GeTe2

FenGeTe2 (n = 3, 4, and 5) has recently attracted increasing attention due to its two-dimensional van der Waals characteristic and high temperature ferromagnetism, which makes promises for spintronic devices. A Fe(1) split site is an important structural characteristic of Fe5GeTe2, which makes it very different from other FenGeTe2 (n = 3 and 4) systems. The local atomic disorder and short-range order can be induced by the split site. In this work, high-quality van der Waals ferromagnet Fe5GeTe2 single crystals were grown to study low-temperature transport properties. We found a resistivity upturn below 10 K. The temperature and magnetic field dependence of the resistivity are in good agreement with a combination of the theory of disorder-enhanced three-dimensional electron–electron and single-channel Kondo effect. The Kondo effect exists only at low magnetic fields B<3 T, while electron–electron interaction dominates the appearance for the low-temperature resistivity upturn. We believe that the enhanced three-dimensional electron–electron interaction in this system is induced by the local atomic structural disorder due to the split site of Fe(1). Our results indicate that the split site of Fe plays an important role for the exceptional transport properties.

Charge Transport in Ba1−xRbxFe2As2 Single Crystals

Recent studies in heavily hole-doped iron-based superconductor RbFe$_2$As$_2$ have suggested the emergence of novel electronic nematicity directed along the Fe-As direction, 45$^\circ$ rotated from the usual nematicity ubiquitously found in BaFe$_2$As$_2$ and related materials. This motivates us to study the physical properties of Ba$_{1-x}$Rb$_{x}$Fe$_{2}$As$_{2}$, details of which remain largely unexplored. Here we report on the normal-state charge transport in Ba$_{1-x}$Rb$_{x}$Fe$_{2}$As$_{2}$ superconductors by using high-quality single crystals in the range of Rb concentration $0.14\le x \le 1.00$. From the systematic measurements of the temperature dependence of electrical resistivity $\rho(T)$, we find a signature of a deviation from the Fermi liquid behavior around the optimal composition, which does not seem related to the antiferromagnetic quantum criticality but has a potential link to hidden nematic quantum criticality. In addition, electron correlations derived from the coefficient of $T^2$ resistivity show a marked increase with Rb content near the heavily hole-doped end, consistent with the putative Mott physics near the $3d^5$ electron configuration in iron-based superconductors.

Hybrid Bloch-Néel spiral states in Mn1.4PtSn probed by resonant soft x-ray scattering

Multiple intriguing phenomena have recently been discovered in tetragonal Heusler compounds, where $D_{2d}$ symmetry sets a unique interplay between Dzyaloshinskii-Moriya (DM) and magnetic dipolar interactions. In the prototype $D_{2d}$ compound Mn$_{1.4}$PtSn, this has allowed the stabilization of exotic spin textures such as first-reported anti-skyrmions or elliptic Bloch-type skyrmions. While less attention has so far been given to the low-field spiral state, this remains extremely interesting as a simplest phase scenario on which to investigate the complex hierarchy of magnetic interactions in this materials family. Here, via resonant small-angle soft x-ray scattering experiments on high-quality single crystals of Mn$_{1.4}$PtSn at low temperatures, we evidence how the underlying $D_{2d}$ symmetry of the DMI in this material is reflected in its magnetic texture. Our studies reveal the existence of a novel and complex metastable phase, which possibly has a mixed character of both the N\'{e}el-type cycloid and the Bloch-type helix, that forms at low temperature in zero fields upon the in-plane field training. This hybrid spin-spiral structure has a remarkable tunability, allowing to tilt its orientation beyond high-symmetry crystallographic directions and control its spiral period. These results broaden the reachness of Heusler $D_{2d}$ materials exotic magnetic phase diagram and extend its tunability, thus enhancing a relevant playground for further fundamental explorations and potential applications in energy saving technologies.

Solvated C70 single crystals for organic field effect transistors

Fullerene C70 is an n-type semiconducting material, similar to C60, used in organic field-effect transistors (OFETs). However, studies on OFETs based on C70 are limited to thin films because of the difficulty in growing high-quality single crystals. In this work, we report OFETs using m-xylene-solvated C70 single-crystal needles grown using the liquid–liquid interfacial precipitation method. The maximum electron mobility was measured to be 1.32 × 10−2 cm2/Vs although the electrical contact in the devices might be low owing to the bottom-contact. The solvated C70 single-crystal needles exhibited good transport properties, comparable to the C60 single-crystal needles.

Understanding the Sensing Mechanism of ZnO Nanoparticles through Identifying Intrinsic Defects

Progress in crystal growth made ZnO applicable as the next generation of inorganic light-emitting diodes and lasers, which has attracted much attention. Our previous work is a proof that Zn vacancies are occupied by three protons to form VZnH3 complexes in high-quality ZnO single crystals. In this work, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) studies on high-quality ZnO single crystals further confirm that three proton clusters or cations with three positive charges are the intrinsic defects and/or impurities related to doping in the host-lattice of high-quality ZnO single crystals. Unintentionally doped hydrogen (H) clusters might be relating to ZnO’s sensing capabilities. The sensor response of ZnO semiconductors is promoted by a decreasing size; however, the nature of the size effects has not been elucidated satisfactorily. In this work, broccoli-like ZnO nanoarrays, which can detect H2S gas in the parts per billion (ppb) region, has been synthesized under a hydrothermal method. High-resolution transmission electron microscopy (HRTEM) experimental results demonstrate that there are multiple polar faces exposed on the surfaces of ZnO nano crystals. The high catalytic active of oxygen species adsorbed on the multiple polar surfaces should respond to the high sensing performance and the high selectivity of ZnO toward reducing H2S gas. However, the enormous variations in electric current cannot be produced by releasing electrical charges from O– adsorbates into semiconductors. EPR results show that Zn+ interstitials have been produced after the reaction between H2S and O– adsorbates on ZnO. In contrast to the unintentionally doped hydrogen (H) clusters, our experimental results show that the Zn+ interstitials acting as shallow donors is what really offer the extra free-electron carriers.

A Lection in Humbleness: Crystallization of Chiral and Zwitterionic APIs Baclofen and Phenibut

Crystallization and multicomponent crystal formation of active pharmaceutical ingredients Baclofen and Phenibut with dicarboxylic acid co-formers are discussed. The crystallization process of several crystalline entities is elucidated via single crystal—as well as powder X-ray—diffraction, followed by thermal analysis and phase stability studies over time. Both APIs form increasingly complex crystalline phases with co-formers malic and tartaric acid, where phase purity of a desired compound is not necessarily a given. Therefore, the influence of different solution and milling environments during crystallization on the outcome is studied. Emphasis is laid on how molecular influences such as the chirality, propensity to form hydrates as well as low solubility of Baclofen and Phenibut impede attempts to gather high-quality single crystals. The results highlight that targeted crystallization of these compounds with dicarboxylic acids can be difficult and unreliable.

UI3 – 5f-electron magnetic van der Waals material

We grew high-quality single crystals of the 5f electron van der Waals compound UI3 and investigated them by measurements of specific heat and magnetization as function of temperature and magnetic field. UI3 behaves as an antiferromagnet with a first-order magnetic phase transition at the Neél temperature TN ​= ​2.65 ​K. It is characterized by a sharp symmetric specific-heat peak which is gradually shifted to lower temperatures by increasing magnetic field applied along either the a- or b-axis. The behavior in magnetic field reveals orthorhombic magnetocrystalline anisotropy with the hard magnetization direction along the c-axis. The 2-K a- and b-axis magnetization curves exhibit a metamagnetic transition at the critical field μ0Hc ​≈ ​3 ​T and ≈ 1.8 ​T, respectively. In higher fields, when the long-range antiferromagnetism is suppressed by a metamagnetic transition, signs of short-range magnetic ordering of antiferromagnetic correlations in the paramagnetic state show up both in specific heat and magnetization. The anomalous S-shape field dependence of a-axis magnetization well above Hc can be understood as the crossover from the correlated paramagnetic regime to the high-field polarized paramagnet. The magnetic phase diagrams for the magnetic field applied along the a- and b-axis, respectively, designed using the mentioned experimental results are presented. We have also found that the UI3 crystals are easily cleavable which predisposes this material for direct investigation of 5f electron magnetism in the 2D limit on exfoliated atomically thin samples.

Self-Modulation-Guided Growth of 2D Tellurides with Ultralow Thermal Conductivity.

Ultralow thermal conductivity materials have triggered much interest due to diverse applications in thermal insulation, thermal barrier coating, and especially thermoelectrics.Two dimensional (2D)indium tellurides with ultralow thermal conductivity provide a versatile platform for tailoring the heat transfer, exploring new candidates for thermoelectrics, and achieving miniature, lightweight, and highly integrated devices. Unfortunately, their nanostructure and structure-related heat transfer properties at a 2D scale are much less studied due to difficulties in material fabrication. The ionic character between interlayers and strong covalent bonds in 3D directions impede the anisotropic growth of indium telluride flakes; meanwhile, the low environmental stability and chemical reactivity of tellurium also limit the fabrication of high-quality tellurides, thus hindering the exploration of thermal transport properties. Here, a self-modulation-guided growth strategy to synthesize high-quality 2D In4 Te3 single crystals with ultralow thermal conductivity (0.47Wm-1 K-1 ) is developed. This strategy can also be extended to synthesize a series of highly crystallized metal tellurides, providing excellent candidates for further application in thermoelectrics.

Martensitic transformation in V3Si single crystal: 51V NMR evidence for coexistence of cubic and tetragonal phases

The Martensitic transformation (MT) in A15 binary-alloy superconductor V3Si, though studied extensively, has not yet been conclusively linked with a transition to superconductivity. Previous NMR studies have mainly been on powder samples and with little emphasis on temperature dependence during the transformation. Here we study a high-quality single crystal, where quadrupolar splitting of NMR spectra for 51V allowed us to distinguish between spectra from transverse chains of V as a function of temperature. Our data revealed that (1) the MT is not abrupt, but rather there is a microscopic coexistence of pre-transformed cubic phase and transformed tetragonal phase over a few K below and above Tm, while (2) no pre-transformed phase can be found at Tc, and (3) the Martensitic lengthening of one axis occurs predominantly in a plane perpendicular to the crystal growth axis, as twinned domains.

Realization and optimization of enhanced and spectral selective photoluminescence in size and phase controlled nanocrystalline Ga2O3 films made by pulsed laser deposition

• Nanocolumnar β-Ga 2 O 3 thin films were made by pulsed-laser deposition. • Performed detailed structural, interfacial and optical characterization of Ga 2 O 3 films. • Controlled processing conditions allow to tune the surface/interface microstructure. • Spectral selective photoluminescence demonstrated from nanotextured β-Ga 2 O 3 films. Realization and optimization of the optical properties for tunable and/or enhanced emission characteristics is critical to further advance the field of optoelectronics, photonics, and electronics for extreme environment applications. While recent advancements made to realize high-quality single crystals of Ga 2 O 3 , an interesting material for electronics and optoelectronics, strategies to obtain enhanced functionality and desired optical properties in nanocrystalline Ga 2 O 3 without needing to expensive epitaxial systems or quite cumbersome equipment or rare-earth dopants remains a challenging problem for further development. In this context we have demonstrated high yield spectral selective photoemission, from pulsed laser deposited (PLD) un-doped Ga 2 O 3 thin films with extensive control over nanocrystalline growth. Structural, interfacial and morphological investigation reveals the formation of perfect nanocrystalline seed layer promoting further growth of compact nano-columns with nano-textured atop. Optical spectroscopy demonstrates the desired control over band-edge absorption and realization of ideal photo-transition in perfectly oriented nano-columnar PLD β-Ga 2 O 3 films. Ellipsometry shows the growth parameters dependent evolution of refractive index over broad spectral range. The findings as presented and described here provide evidence for controlled growth and realization of nanocrystalline β-Ga 2 O 3 for functional device applications in optoelectronics in addition to provide a platform to further explore their futuristic technological applications.

Challenging Bendable Organic Single Crystal and Transistor Arrays with High Mobility and Durability toward Flexible Electronics.

Bendable organic single crystals are promising candidates for flexible electronics owing to their superior charge-transport properties. However, large-area high-quality organic single crystals are rarely available on the polymer substrates generally used in flexible electronics. Here, a surface-assisted assembly strategy based on a polymer modification, poly(amic acid) (PAA), is developed to grow large-area organic singe crystals on polymer substrates using a simple drop-casting method. The unique surface properties of PAA that enable molecular solution superwetting and promote molecular ordered assembly produce an extraordinary self-driven "meniscus-guided coating" behavior, enabling the fabrication of millimeter-sized, highly aligned organic single crystals for a variety of organic semiconductors. Organic field-effect transistors based on a mode molecule of 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene demonstrate the highest (average) mobility of 18.6 cm2 V-1 s-1 (15.9 cm2 V-1 s-1 ), attractively low operating voltage of -3V, and high flexible durability. The results shed light on the large-area fabrication of organic single crystals on polymer dielectrics toward high-performance and integrated plastic electronics.

Preparation of large Cu3Sn single crystal by Czochralski method

Cu3Sn was recently predicted to host topological Dirac fermions, but related research is still in its infancy. The growth of large and high-quality Cu3Sn single crystals is, therefore, highly desired to investigate the possible topological properties. In this work, we report the single crystal growth of Cu3Sn by Czochralski (CZ) method. Crystal structure, chemical composition, and transport properties of Cu3Sn single crystals were analyzed to verify the crystal quality. Notably, compared to the mm-sized crystals from a molten Sn flux, the cm-sized crystals obtained by the CZ method are free from contamination from flux materials, paving the way for the follow-up works.