Chemical Vapor Transport Method Research Articles

Room-Temperature Antisymmetric Magnetoresistance in Van Der Waals Ferromagnet Fe3GaTe2 Nanosheets.

Van der Waals (vdW) ferromagnetic materials have emerged as a promising platform for the development of 2D spintronic devices. However, studies to date are restricted to vdW ferromagnetic materials with low Curie temperature (Tc) and small magnetic anisotropy. Here, a chemical vapor transport method is developed to synthesize a high-quality room-temperature ferromagnet, Fe3GaTe2 (c-Fe3GaTe2), which boasts a high Tc = 356 K and large perpendicular magnetic anisotropy. Due to the planar symmetry breaking, an unconventional room-temperature antisymmetric magnetoresistance (MR) is first observed in c-Fe3GaTe2 devices with step features, manifesting as three distinctive states of high, intermediate, and low resistance with the sweeping magnetic field. Moreover, the modulation of the antisymmetric MR is demonstrated by controlling the height of the surface steps. This work provides new routes to achieve magnetic random storage and logic devices by utilizing the room-temperature thickness-controlled antisymmetric MR and further design room-temperature 2D spintronic devices based on the vdW ferromagnet c-Fe3GaTe2.

Structure and properties of a quasi-one-dimensional compound BaTiS3 under pressure

ABSTRACT We report the studies of structure and properties of a quasi-one-dimensional (quasi-1D) compound BaTiS 3 under pressure. The poly-crystalline samples are synthesized by solid-state reaction method and single-crystalline samples are produced by chemical vapor transport method. The XRD measurement under ambient pressure confirms the hexagonal structure, which consists of face-sharing octahedral TiS 6 chains along c-axis and displays a quasi-1D feature. When applying pressure, the synchrotron X-ray diffraction experiments show that the sample undergoes a phase transition from hexagonal phase to orthorhombic phase at around 10 GPa due to the zig–zag deformation of TiS 6 chains, while the quasi-1D structure is reserved. At ambient pressure, BaTiS 3 exhibits an insulating behavior with a band gap about 0.273 eV. When applying pressure, the sample undergoes a crossover from insulator to metal due to the enhancement of inter-chain electron hopping.

Crystal-chemical origins of the ultrahigh conductivity of metallic delafossites

Despite their highly anisotropic complex-oxidic nature, certain delafossite compounds (e.g., PdCoO2, PtCoO2) are the most conductive oxides known, for reasons that remain poorly understood. Their room-temperature conductivity can exceed that of Au, while their low-temperature electronic mean-free-paths reach an astonishing 20 μm. It is widely accepted that these materials must be ultrapure to achieve this, although the methods for their growth (which produce only small crystals) are not typically capable of such. Here, we report a different approach to PdCoO2 crystal growth, using chemical vapor transport methods to achieve order-of-magnitude gains in size, the highest structural qualities yet reported, and record residual resistivity ratios ( > 440). Nevertheless, detailed mass spectrometry measurements on these materials reveal that they are not ultrapure in a general sense, typically harboring 100s-of-parts-per-million impurity levels. Through quantitative crystal-chemical analyses, we resolve this apparent dichotomy, showing that the vast majority of impurities are forced to reside in the Co-O octahedral layers, leaving the conductive Pd sheets highly pure (∼1 ppm impurity concentrations). These purities are shown to be in quantitative agreement with measured residual resistivities. We thus conclude that a sublattice purification mechanism is essential to the ultrahigh low-temperature conductivity and mean-free-path of metallic delafossites.

Magnetic properties of Fe intercalation FexTaSe2

Intercalation of transition metal dichalcogenides with magnetic elements has been the subject of increasing research interest, aiming to explore novel magnetic materials with anisotropy and spin-orbit coupling. In this paper, two magnetic samples with varying Fe content have been prepared using different growth conditions via the chemical vapor transport method. A comprehensive investigation of the magnetic properties of the materials has been conducted using the Physical Property Measurement System (PPMS, EvercoolⅡ-9T, Quantum Design). The results reveal distinct features in the studied materials. Fe0.12TaSe2 exhibits significant ferromagnetism with a Curie transition temperature of 50 K. However, its in-plane magnetism is weak and no significant hysteresis loop is observed below the Curie temperature. On the other hand, Fe0.25TaSe2 exhibits antiferromagnetism without any hysteresis loop and has a Néel temperature up to 130 K. This finding is quite different from the intercalated iron in FexTaS2, where only an antiferromagnetic state occurs with x larger than 0.4. Our study thus provides updated insights into the magnetic properties of this new system and serves as a reference for future investigations of TaSe2 compounds with varying iron content.

Experimental study of the fast scintillating behavior of ZnO:Ga crystal under MeV electron beam.

An experimental study on the scintillating behavior of ZnO:Ga crystals was conducted using a 7-MeV electron accelerator. The ZnO:Ga crystals were grown using both hydrothermal and chemical vapor transport methods. One of the ZnO:Ga crystals grown via the hydrothermal method exhibited a 1.5-ns afterglow time when excited by an MeV electron beam. The ultra-short scintillation was captured using both an ultrafast streak camera and a framed camera. The experimental results also confirm its potential for diagnosing electron beam trains with a repetition rate of hundreds of MHz.

Magnetic critical behavior of van der Waals Fe3GaTe2 with above-room-temperature ferromagnetism

Fe3GaTe2 is a promising van der Waals material for future spintronic applications because of its intrinsic above-room-temperature ferromagnetism. Herein, high quality Fe3GaTe2 single crystals were successfully grown by the chemical vapor transport method. Its magnetic critical behavior in the vicinity of paramagnetic to ferromagnetic phase transition region was systematically investigated. The critical exponents β = 0.350(1) and γ = 1.385(5) were obtained by using the Kouvel–Fisher method, and δ = 4.92(1) was obtained by a critical isotherm analysis at critical temperature TC = 358 K, which is consistent with the Widom scaling relation δ = 1 + γ/β. The critical exponents determined using different methods are self-consistent and obey the scaling equations predicted by the scaling theory. The renormalization group theory analysis based on the extracted critical exponents further reveals that the magnetic interaction in Fe3GaTe2 is of the 3D Heisenberg type with long-range magnetic coupling.

Single Crystal Growth and Nano-Structure Study in a Topological Dirac Metal, CoTe2-δ

A single crystal of a topological material, CoTe2-δ, has been grown via the chemical vapor transport method for a structural and electronic transport study. Single-crystal X-ray diffraction, powder X-ray diffraction, and high-resolution scanning electron microscope measurements confirm the high quality of the as-grown single crystals. In a high-resolution scanning electron microscopy study, a clear layered feature of the trigonal CoTe2-δ crystal was observed. Fractal features and mosaic-type nanostructures were observed on the as-grown surface and cleaved surface, respectively. The trigonal CoTe2-δ demonstrates a metallic ground state in transport measurements, with a typical carrier’s concentration in a 1021 cm−3 magnitude and a residual resistivity ratio of 1.6. Below 10 K, trigonal CoTe2-δ contains quite complicated magnetoresistance behavior as a result of the competing effect between Dirac states and possible spin fluctuations.

A flux growth technique for high quality cubic boron arsenide bulk single crystals

The growth of single crystal cubic boron arsenide (c-BAs) has attracted considerable interest due to its high room-temperature thermal conductivity and high ambipolar electrical mobility. However, currently the only growth technique reported for c-BAs crystals is the chemical vapor transport (CVT) method, which exhibits several drawbacks with regard to size scalability and crystal quality control, thereby hindering the further advancement of this semiconductor material. Herein, we report a flux growth technique using liquid arsenic (l-As) as a reaction medium at high pressures for the growth of high-quality c-BAs crystals with several millimeters size. The outstanding properties, including high uniformity, lower defect density, and lower carrier concentration of the as-grown c-BAs single crystals from flux growth, have been verified via a combination of techniques including x-ray diffraction, Raman scattering, photoluminescence spectroscopy, and electrical transport measurements, in comparison with the CVT-grown crystals.

Supercapacitor electrodes based on single crystal layered ZrX2 (X = S, Se) using chemical vapor transport method

We synthesized single crystals of layered zirconium disulfide/diselenide (ZrX2, X = S, Se) using the chemical vapor transport method. Our aim was to explore their potential as energy storage devices, specifically supercapacitors. To confirm the structures of our growth samples, we conducted various characterization tests. The semiconducting nature of ZrX2, particularly in the 1T phase, limits its electrochemical performance. However, the 1T-ZrX2 nanoflakes serve as highly stable conductive electrodes for high-performance supercapacitors. At a scan rate of 10 mV/s, ZrS2 and ZrSe2 exhibited specific capacitance values of 18.8 and 18 F/g, respectively. Notably, the metallic 1T-ZrSe2 electrode outperformed 1T-ZrS2 due to its greater conductive space observed from layer spacing, as confirmed by HRTEM analysis. The ZrSe2 devices demonstrated exceptional stability, exceeding 90%. These findings highlight the promising potential of these 2D materials as supercapacitor electrodes, offering excellent specific capacitance values, conductivity and cycling stability.

Improving the Luminescence Performance of Monolayer MoS2 by Doping Multiple Metal Elements with CVT Method.

Two-dimensional (2D) transition metal dichalcogenides (TMDCs) draw much attention as critical semiconductor materials for 2D, optoelectronic, and spin electronic devices. Although controlled doping of 2D semiconductors can also be used to tune their bandgap and type of carrier and further change their electronic, optical, and catalytic properties, this remains an ongoing challenge. Here, we successfully doped a series of metal elements (including Hf, Zr, Gd, and Dy) into the monolayer MoS2 through a single-step chemical vapor transport (CVT), and the atomic embedded structure is confirmed by scanning transmission electron microscope (STEM) with a probe corrector measurement. In addition, the host crystal is well preserved, and no random atomic aggregation is observed. More importantly, adjusting the band structure of MoS2 enhanced the fluorescence and the carrier effect. This work provides a growth method for doping non-like elements into 2D MoS2 and potentially many other 2D materials to modify their properties.

Room temperature two-dimensional ferromagnetic Ni-doped Fe5GeTe2 with tunable Tc for enhanced oxygen evolution reaction

Improving the Curie temperature and catalytic performance of two-dimensional magnetic materials through elemental doping is a feasible strategy. In this study, Ni-doped (Fe1−xNix)5GeTe2 (0 ≤ x ≤ 0.3) single crystals were grown via chemical vapor transport method and the amount of Ni doping could be precisely controlled. As the amount of Ni doping increases, the long-range ferromagnetic ordering temperature increased, and (Fe0.7Ni0.3)5GeTe2 exhibited the highest Curie temperature Tc = 492.73 K. The improvement of Tc can be attributed to the occupancy of the Fe1 site by Ni atoms. The oxygen evolution reaction (OER) activities of liquid phase exfoliated (Fe1−xNix)5GeTe2 nanoflakes were enhanced by doping Ni atoms, and the overpotential decrease to 464 mV at 10 mA/cm2 with a small Tafel slope of 41 mV/dec for the (Fe0.9Ni0.1)5GeTe2 nanoflakes. Based on the density functional theory calculations, when half of the Fe1 sites were replaced by Ni atoms, the number of enhanced OER active sites (Fe1-Te sites) reached maximum, and then, (Fe0.9Ni0.1)5GeTe2 exhibited the best catalytic performance. The experimental and theoretical calculation results indicate that replacing Fe1 site with Ni in two-dimensional magnetic Fe5GeTe2 can effectively increase the Curie temperature and enhance OER activity.

Luminescent properties of ZnSe crystals doped with group V elements and iodine

Photoluminescence (PL) spectra of ZnSe single crystals grown by the chemical vapor transport (CVT) method with iodine as a transport agent and doped with the group V elements (P, As, Sb, Bi) during the growth are investigated in the wavelength range between 420 and 750 nm at temperatures between 10 and 300 K. The effect of co-doping of ZnSe samples with iodine and the group V elements on luminescence centers and PL spectra is studied. It is established that acceptor-type associative centers of activator luminescence (ISeASe), where A represents the group V element, are formed as a result of co-doping. These centers are responsible for new wide bands of long-wavelength emission with maxima between 575 and 580 nm at room temperature. Photoluminescence mechanism that explains the behavior of the observed PL bands with temperature variation is proposed.

Review of Single Crystal Synthesis of 11 Iron-Based Superconductors.

The 11 system in the iron-based superconducting family has become one of the most extensively studied materials in the research of high-temperature superconductivity, due to their simple structure and rich physical properties. Many exotic properties, such as multiband electronic structure, electronic nematicity, topology and antiferromagnetic order, provide strong support for the theory of high-temperature superconductivity, and have been at the forefront of condensed matter physics in the past decade. One noteworthy aspect is that a high upper critical magnetic field, large critical current density and lower toxicity give the 11 system good application prospects. However, the research on 11 iron-based superconductors faces numerous obstacles, mainly stemming from the challenges associated with producing high-quality single crystals. Since the discovery of FeSe superconductivity in 2008, researchers have made significant progress in crystal growth, overcoming the hurdles that initially impeded their studies. Consequently, they have successfully established the complete phase diagrams of 11 iron-based superconductors, including FeSe1-xTex, FeSe1-xSx and FeTe1-xSx. In this paper, we aim to provide a comprehensive summary of the preparation methods employed for 11 iron-based single crystals over the past decade. Specifically, we will focus on hydrothermal, chemical vapor transport (CVT), self-flux and annealing methods. Additionally, we will discuss the quality, size, and superconductivity properties exhibited by single crystals obtained through different preparation methods. By exploring these aspects, we can gain a better understanding of the advantages and limitations associated with each technique. High-quality single crystals serve as invaluable tools for advancing both the theoretical understanding and practical utilization of high-temperature superconductivity.

2D BiSeI Nanosheets for Broadband Self‐Powered Photoelectrochemical Photodetector

Broadband photodetectors are essential for the integrated design of next‐generation optical detector devices. Ternary 2D bismuth sulfide semiconductor (BiXI, X = S, Se, Te) is an ideal candidate for broadband photoelectric detection due to its narrow bandgap and excellent electrical transport performance. Here, BiSeI is grown by chemical vapor transport (CVT) method and used in photoelectrochemical (PEC) photodetection. The PEC‐type photodetector based on BiSeI nanosheets has a broadband detection ranging from 350 to 900 nm, displaying a high responsivity of 2.88 mA W−1, a high detectivity of 1.10 × 1010 Jones and a fast response time of 7/16 ms at a bias voltage of 0.5 V. Furthermore, the device also exhibits a negligible current density loss after three months, which indicates that it has good stability against air and moisture. In addition, the photodetector can work without bias potential, indicating that it is a good self‐powered device. The results demonstrate that BiSeI is a potential material in self‐powered PEC photodetector.

Pressure-induced superconductivity in Bi4(I1−x Br x )4 crystals grown by chemical vapor transport and flux methods

Achieving superconductivity in topological materials is thought as a promising route for realizing topological superconductivity, which may provide potential applications to quantum computation. Previously, rich superconducting phases have been reported in the pressurized Bi4I4 and Bi4Br4 crystals which belong to an interesting quasi-one-dimensional topological system. In this work, we have performed a high-pressure study on some Bi4(I1−x Br x )4 crystals grown by two different methods. Remarkably, crystals grown by the chemical vapor transport (CVT) method and the self-flux method show clearly different pressure effects. In the CVT-grown crystals, only one superconducting transition is observed, while three superconducting transitions can be detected in crystals grown by the flux method. Through comparisons of the pressure-dependent phase diagrams and the upper critical field behaviors in the two kinds of crystals, the higher superconducting transition (>6 K) in the flux-grown crystals is suggested to come from the residual Bi. High-pressure Raman spectroscopy measurements on both kinds of crystals have confirmed the occurrence of a similar structural transition around 10 GPa in Br-doped samples. Overall, our data could be helpful for identifying the intrinsic pressure-induced superconductivity in various Bi-based materials.

Robustness of semimetallic transport properties of TaAs against off-stoichiometric disorder

TaAs single crystals were grown by a standard chemical vapor transport method. The single-crystallinity and homogeneous distribution of elements were confirmed by transmission electron microscope and x-ray diffraction observations. Positron annihilation measurements revealed that the atomic vacancy concentration was kept below 10−5 at. %. However, inductively coupled plasma analysis showed an As-deficient (7–9 at. %) off-stoichiometry. First-principles calculations implied that the off-stoichiometry could be compensated for with excess Ta antisite defects, thereby inducing metallic states. Nevertheless, excellent semimetallic transport properties of a well-suppressed carrier density (≲1018 cm−3), ultrahigh carrier mobility (≳106 cm2/V/s), and large transverse magnetoresistance (>200000% at 9 T) with the quantum oscillation were obtained at 1.7 K. This indicated the robustness of semimetallic transport properties against the off-stoichiometric disorder and the quenching of metallic conduction associated with excess Ta atoms. The negative longitudinal magnetoresistance, which is considered evidence of a Weyl semimetal (chiral anomaly), was not observed. These data were discussed with theoretical calculations.

Ferromagnetism Induced by Magnetic Dilution in Van der Waals Material Metal Thiophosphates

AbstractMetal thio(seleno) phosphates MPX3 attract considerable attention with potential application in spintronics and other types of devices. Here, ferromagnetism in an Ising type diluted magnetic semiconductor Fe1‐xZnxPS3 is reported. Bulk single crystals of Fe1‐xZnxPS3 have been synthesized via chemical vapor transport method and characterized by X‐ray diffraction. The weak ferromagnetism associated with spin/cluster spin glass emerges at 30% Zn doping. A nearly static ferromagnetic order is observed in the highly diluted magnets Fe0.1Zn0.9PS3. X‐ray photoemission measurements indicate introduction of holes and associated Fe3+ in samples with ferromagnetic order. Density functional theory calculations confirm the Ising‐type ferromagnetic state in highly diluted compounds with hole doping. The interplay between bound magnetic polarons induces impurity‐band‐exchange and superexchange interactions can explain the evolution of magnetism with Zn doping. The findings suggest that the magnetic diluted FePS3 system is a candidate for potential applications in magnetic devices, and its quantum mechanism should be explored by using microscopic techniques in the near future.

Excitonic luminescence of iodine-intercalated HfS2

Photoluminescence from bulk HfS2 grown by the chemical vapor transport method is reported. A series of emission lines is apparent at low temperature in the energy range of 1.4–1.5 eV. Two groups of the observed excitonic transitions followed by their replicas involving acoustic and optical phonons are distinguished using classical intensity correlation analysis. The emission is attributed to the recombination of excitons bound to iodine (I2) molecules intercalated between layers of HfS2. The I2 molecules are introduced to the crystal during the growth as halogen transport agents in the growth process. Their presence in the crystal is confirmed by secondary ion mass spectroscopy.

Synthesis of violet phosphorus with large lateral sizes to facilitate nano-device fabrications.

Violet phosphorus has been proven to be the most stable phosphorus allotrope and has attracted much attention recently. The growth of violet phosphorus with large lateral sizes is crucial to obtain good quality violet phosphorene for nanodevice fabrication. Herein, a large number of violet phosphorus plates have been produced from molten lead using an optimized method to achieve red bronze luster. The crystal structure of the as-produced violet phosphorus was determined by single-crystal X-ray diffraction to be monoclinic with the space group P2/n (13) (CSD-2160375), identical to the one from the chemical vapor transport method (CSD-1935087). The as-produced violet phosphorus plates were found to have lateral sizes of 1.30 ± 0.41 mm2. The violet phosphorus plates were easily exfoliated and directly transferred to silicon substrates to facilitate building of a back-gate field-effect transistor. A hole mobility of 2.308 cm2 V-1 s-1 was obtained from a violet phosphorus nanosheet with a thickness of 52 nm under ambient conditions. The absolute responsivity of 130 mA W-1 with a fast response time of 27 ms was also obtained under the irradiation of a 530 nm laser.

In-plane optical anisotropy of quasi-one-dimensional layered semiconductor Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystal

Transition-metal phosphorous chalcogenide <i>M</i>PS (<i>M</i> = transition metal), an emerging type of two-dimensional (2D) van der Waals material with the unique optical and opto-electronic properties, has received much attention. The quasi-one-dimensional chain structure of Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> will possess the strong anisotropic optical and photoelectric properties. Therefore, the single crystal and low-dimensional materials of Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> have potential applications in new polarization controllers, polarization-sensitive photoelectronic detectors, etc. However, there is still a lack of research on the anisotropic optical properties of the high-quality Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystals. Herein, the millimeter-sized Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystals are successfully prepared by the chemical vapor transport method. The chemical composition, the crystal structure and the anisotropic optical properties of the Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystals are carefully analyzed. The energy dispersive X-ray spectroscopy results show that the element distribution is uniform and the element ratio is close to the stoichiometric ratio. The X-ray diffraction and the transmission electron microscopy results show a good crystallinity. The absorption spectra shows that the optical band gap of the Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystal is 1.8 eV. Interestingly, the Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystal can be mechanically exfoliated to obtain few-layer material. The thickness-dependent Raman spectra show that the Raman vibration peaks of bulk and few-layer Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> each have only a weak shift, indicating a weak interlayer interaction in the Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystal. In order to make an in-depth study of the optical properties of Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystals, the polarized-dependent Raman spectra and the femtosecond transient absorption (TA) spectra by using pump pulses and probe pulses with a wavelength of 400 nm and a wavelength range of 500–700 nm are recorded. Importantly, the polarized-dependent Raman scattering spectra with the angle-dependent measurements reveal that the intensity of Raman peak at 202 cm<sup>–1</sup> and at 489 cm<sup>–1</sup> show a 2-fold symmetry and a 4-fold symmetry in the parallel and vertical polarization configurations, respectively. Moreover, the results of ultrafast carrier dynamics with the in-plane rotation angles of Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystals in the parallel polarization configurations, clearly indicate that both the hot carrier number and the relaxation rate after photoexcitation have the in-plane anisotropic properties. These results are useful in understanding the in-plane anisotropic optical properties of Nb<sub>4</sub>P<sub>2</sub>S<sub>21</sub> single crystal, which can further promote their applications in the low-dimensional angle-dependent optoelectronics.