Brief Introduction to Single Crystal Growth Techniques

As a recently discovered unconventional superconductor, the heavy fermion compound UTe<sub>2</sub> has garnered significant attention due to its potential spin-triplet superconducting pairing, high-field re-entrant superconducting phases, and unique quantum critical characteristics. However, experimental results on this system exhibit significant variations and contradictions, primarily due to differences in sample quality. Key unresolved issues include whether the system exhibits multi-component superconducting order parameters, whether time-reversal symmetry is spontaneously broken, and whether multiple field-induced superconducting phases share a common origin. These controversies have hindered an in-depth understanding of the intrinsic superconducting pairing mechanism in the UTe<sub>2</sub> system.<br>This paper reviews recent advances in single-crystal growth methods for UTe<sub>2</sub>, including chemical vapor transport (CVT), Te-flux, molten salt flux (MSF), and molten salt flux liquid transport (MSFLT). We systematically analyze how growth conditions influence superconductivity and crystal quality. Although the CVT method was initially employed in UTe<sub>2</sub> studies, the samples grown by this method exhibit poor quality and significant compositional inhomogeneity, even within individual samples. Consequently, the CVT method has been progressively supplanted by the recently developed MSF method. In contrast, the MSF and MSFLT methods yield high-quality UTe<sub>2</sub> single crystals with <i>T</i>c as high as 2.1 K and <i>RRR</i> reaching up to 1000; however, the sample sizes are smaller compared to those grown by the CVT and Te-flux methods. Notably, MSF-grown samples occasionally contain magnetic impurities such as U<sub>7</sub>Te<sub>12</sub>, necessitating careful screening during sample collection. MSFLT combines the advantages of both CVT and MSF methods, enabling the growth of high-quality UTe<sub>2</sub> single crystals while also producing larger sample sizes than MSF. Our findings highlight the importance of optimizing growth parameters such as the Te/U ratio, temperature gradients, and cooling rates. For instance, lower growth temperatures and precise control of the Te/U ratio significantly enhance Tc and sample quality. High-quality MSF and MSFLT samples have resolved several controversies, including clarifying the single-component nature of the superconducting order parameter and confirming the absence of time-reversal symmetry breaking in optimized samples.<br>In conclusion, this review underscores the pivotal role of advanced single-crystal growth techniques in advancing the study of UTe<sub>2</sub>. Future research should focus on utilizing these high-quality UTe<sub>2</sub> samples grown by MSF and MSFLT methods to accurately determine superconducting order parameters, elucidate mechanisms behind high-field re-entrant superconducting phases, and explore topological properties, such as potential Majorana fermions. These efforts will deepen our understanding of unconventional superconductivity, spin fluctuations, and quantum critical phenomena in UTe<sub>2</sub> system.

Formation of low-resistivity CdS thin films by a new chemical vapour transport (CVT) method, employing a carrier gas, is reported for the first time. Film deposition was carried out in a quartz tube kept inside a tubular furnace. Chemical vapour transport was done using a screen-printed film of CdS with CdCl2 as the flux incorporated in the film. The nature of the film depends on parameters like condensation temperature, cooling rate, substrate position etc. The extrinsic films showed an electrical conductivity of the order of 101 ( Omega cm)-1 and the films which showed intrinsic nature exhibited a conductivity of the order of 10-8 ( Omega cm)-1. A new CVT by gas system has been designed, and further studies are in progress to obtain thin film solar cell structures employing this technique.

In order to grow β-FeSi2 single crystals by the chemical vapor transport (CVT) method, the growth conditions, including density of iodine transport agent and growth duration, were examined. An iodine density of 8.9 mg cm–3 was thought to be optimum in the present experiment. It was found that the growth duration should be over 4 weeks for growing β-FeSi2 single crystals. X-ray structural analysis revealed that the growth direction of the β-FeSi2 crystals was The growth rate along the directions with lower Miller indexes was slower than that along the directions with higher Miller indexes. The measured lattice constants were a = 0.986 nm, b = 0.780 nm, and c = 0.781 nm. These values are consistent with the previously reported values. Hall measurements showed that the electron mobility was over 350 cm2 V–1 s–1 at 10 K. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Growth mechanism of ZnSe single crystal by chemical vapour transport method

We synthesized single crystals of β-MoTe2 by the chemical vapor transport (CVT) method. Resistivity and magnetoresistivity (MR) have been measured. The MR displays H2 behavior at low field and linear field dependence at high field, which is different from the quadratic behavior in the sample obtained from the NaCl-flux method. It is suggested that the observed linear MR may originate from disorder. The quality and low-temperature property of β-MoTe2 clearly depend on the growth method. Moreover, the angular-dependent MR reveals the two-fold symmetry of Fermi surface.

Growth of large ZnSe single crystal by CVT method

Growth of plate-type β-FeSi 2 single crystals by optimization of composition ratio of source materials

Fabrication of NiO bicrystals having Σ5 (310) and Σ13 (510) coincidence-site tilt boundaries was successfully carried out by a CVT (Chemical Vapor Transport) method. The CVT method was a very advantageous way to grow ultra pure crystals since it preferentially transported NiO from the source pellet to the substrate by reaction with a HCl carrying gas. Single crystal MgO was used as a substrate for epitaxial growth of NiO as the readily available MgO crystals have only a 1% lattice mismatch with NiO. Moreover, MgO is soluble in acids while NiO is not. This permitted removal of the substrate crystal after growth to provide a free-standing NiO crystal. Using two single crystals of MgO with the desired tilt orientation as a substrate, NiO bicrystals were fabricated at growth rates greater than 100 μm/hour at 1,400K using 250 torr of HCI(g) as a carrying agent. The purity of the epitaxial NiO crystals was determined by mass spectrometry and neutron activation analysis. The grain boundary in the bicrystals is exactly perpendicular to the (100) growth surface. Highly-reflective facets along the growth direction suggest high mechanical quality. High-resolution transmission electron microscopy of the Σ13 boundary revealed structure at the atomic scale that provided no evidence for segregated phases.

This report is an in-depth study of problems encountered in the growth of synthetic single crystals of zinc sulphide. The four growth procedures explored were: Chemical Vapour Transport, Vapour Transport (Sublimation), Flux Growth, and Hydrothermal Growth. The prime goal of growing centimetre-sized crystals of sphalerite was achieved by both the chemical vapour transport method, using either HCI or I 2 as the carrier, and by the vapour transport method. A few millimetre-sized single crystals of wurtzite have been obtained by the vapour transport method. The report includes sections dealing with the purification of zinc sulphide powders, the doping of zinc sulphide crystals with either Al or Fe, various analyses of powder and crystalline samples, and also some characterizations of the single crystals. The yellow-green colouration in some synthetic ZnS crystals has been found to be caused by departure of the ZnS ratio from the required 1:1 stoichiometry.

BaTiS3, a quasi-1D complex chalcogenide, has gathered considerable scientific and technological interest due to its giant optical anisotropy and electronic phase transitions. However, the synthesis of high-quality BaTiS3 crystals, particularly those featuring crystal sizes of millimeters or larger, remains a challenge. Here, we investigate the growth of BaTiS3 crystals utilizing a molten salt flux of either potassium iodide, or a mixture of barium chloride and barium iodide. The crystals obtained through this method exhibit a substantial increase in volume compared to those synthesized via the chemical vapor transport method, while preserving their intrinsic optical and electronic properties. Our flux growth method provides a promising route toward the production of high-quality, large-scale single crystals of BaTiS3, which will greatly facilitate advanced characterizations of BaTiS3 and its practical applications that require large crystal dimensions. Additionally, our approach offers an alternative synthetic route for other emerging complex chalcogenides.Graphical

Photoluminescence of Yb 3+-doped CuInS 2 single crystals prepared by In-flux and chemical vapor transport methods

Preparation of In 2O 3 single crystals by chemical vapour transport method

Optical characterisation has been performed for single crystal 2H-SnS2 synthesised by the chemical vapour transport method at low temperatures. The optical absorption spectra measured in the normal-incidence geometry at room temperature show an indirect transition with an energy gap Eg=2.17+or-0.01 eV. Photoluminescence spectra at liquid helium temperature indicate two series of phonon replica of excitons with phonon energies LO1=62 meV and LO2=70 meV, where the zero phonon lines are 2.310 eV and 2.224 eV respectively.

This study reports the synthesis of highly crystalline layered HfSe2 and CuHfSe2 using the chemical vapor transport method and the investigation of their supercapacitor behavior. Various characterizations were performed in order to confirm and ascertain the structure of the synthesized materials. The CuHfSe2 electrode exhibited better supercapacitor performance due to its higher conductivity, as confirmed by experimental analysis and density functional theory calculations. The semiconducting nature of HfSe2 with a 2H phase mainly impedes its electrochemical performance. Further electrochemical analyses were conducted to test the stability of both electrode materials. Moreover, these new nanocrystals can be regarded as promising supercapacitor electrode materials with good specific capacitance, excellent cycling stability, and high energy density due to the unique microstructure of such 2D materials.

UTe2 single crystals were grown using the chemical vapor transport method, possessing standard lattice parameters and good single crystallinity, and the effects of various growth parameters on the crystal morphology were explored.