Chemical Vapor Transport Method Research Articles

From Octahedron Crystals to 2D Silicon Nanosheets: Facet-Selective Cleavage and Biophotonic Applications.

2D silicon nanosheets (SiNSs) are promising materials for biomedicine but facile synthesis of SiNSs remains a challenge. Herein, by means of a sulfur-iodine co-assisted chemical vapor transport method, octahedron silicon (oct-Si) crystals with fully exposed {111} planes are prepared as precursors for efficient synthesis of SiNSs by facet-selective exfoliation. The 13 nm thick SiNSs have good biocompatibility and the sharp Raman scattering signal facilitates intracellular Raman imaging upon exposure to a near-infrared (NIR) laser. Furthermore, the SiNSs have excellent NIR photothermal characteristics such as a large extinction coefficient of 11.3 L g-1 cm-1 and high photothermal conversion efficiency of 21.4% at 1064 nm. In vitro experiments demonstrate superior NIR-II photothermal therapeutic effects in killing cancer cells. Comparing to conventional methods, the novel facet-selective cleavage strategy is more controllable and environmentally friendly boding well for the fabrication of non-van der Waals 2D materials. The multimodal photonic behavior also suggests large potential of the SiNSs pertaining to integrated multi-NIR biophotonic techniques using single nanomaterials.

Orbital-fluctuation freezing and magnetic-nonmagnetic phase transition in α-TiBr3

We present a detailed study on the structural phase transition in α-TiBr3, which is deeply connected with the lattice and orbital degree of freedoms. A chemical vapor transport method is adopted to synthesize the α-TiBr3 single crystal samples, and the structural phase transition at about 180 K is characterized by x-ray diffraction (XRD), magnetic susceptibility, and specific heat capacity. To further the understanding in the physical nature of this phase transition, a systematic Raman spectroscopic study is performed on α-TiBr3 crystals. With temperature decreasing, a large frequency blue shift and peak width narrowing are observed in the vibrational mode associated with Ti in-plane relative movement, which indicates the formation of Ti–Ti bonding and orbital-fluctuation freezing at low temperatures. These results are fully consistent with magnetic–nonmagnetic phase transition resolved by the measurement of magnetic susceptibility and lattice changes by XRD.

Topological Kagome Magnet Co3Sn2S2 Thin Flakes with High Electron Mobility and Large Anomalous Hall Effect.

Magnetic Weyl semimetals attract considerable interest not only for their topological quantum phenomena but also as an emerging materials class for realizing quantum anomalous Hall effect in the two-dimensional limit. A shandite compound Co3Sn2S2 with layered kagome-lattices is one such material, where vigorous efforts have been devoted to synthesize the two-dimensional crystal. Here, we report a synthesis of Co3Sn2S2 thin flakes with a thickness of 250 nm by chemical vapor transport method. We find that this facile bottom-up approach allows the formation of large-sized Co3Sn2S2 thin flakes of high-quality, where we identify the largest electron mobility (∼2600 cm2 V-1 s-1) among magnetic topological semimetals, as well as the large anomalous Hall conductivity (∼1400 Ω-1 cm-1) and anomalous Hall angle (∼32%) arising from the Berry curvature. Our study provides a viable platform for studying high-quality thin flakes of magnetic Weyl semimetal and stimulate further research on unexplored topological phenomena in the two-dimensional limit.

Magnetotransport and magnetic properties of the layered noncollinear antiferromagnetic Cr2Se3 single crystals

We report on the growth of high-quality stoichiometric layered Cr2Se3 single crystals with metallic and noncollinear antiferromagnetic ground state using the chemical vapor transport (CVT) method. The crystals show weak ferromagnetism in the in-plane and out-of-plane directions below the Neél temperature (TN), however, the field-cooled out-of-plane magnetization at 500 Oe and 10 K (∼0.24 μB/f.u.) is approximately 15 times larger than that of the in-plane one, indicating strong c-axis easy uniaxial magnetic anisotropy, which is further supported by the in-plane and out-of-plane isothermal anisotropic magnetic hysteresis loops and the angular dependent magnetoresistance (MR). The latter also reveals a decrease of the coercive field of the crystal upon the tilting of the weak ferromagnetic easy axis away from the direction of the magnetic field. Further, the out-of-plane isothermal MR are negative below TN and show butterfly shapes for T < 10 K and couple with the magnetic hysteresis M(H) loop. These results may help researchers better understand the interplay between the weak ferromagnetism and the magnetotransport properties of 2D itinerant noncollinear antiferromagnetic systems.

Incommensurate magnet iron monophosphide FeP: Crystal growth and characterization

We report an optimized chemical vapor transport method, which allows growing FeP single crystals up to 500 mg in mass and 80 $mm^{3}$ in volume. The high quality of the crystals obtained by this method was confirmed by means of EDX, high-resolution TEM, low-temperature single crystal XRD and neutron diffraction experiments. We investigated the transport and magnetic properties of the single crystals and calculated the electronic band structure of FeP. We show both theoretically and experimentally, that the ground state of FeP is metallic. The examination of the magnetic data reveals antiferromagnetic order below T$_{N}$ =119 K while transport remains metallic in both the paramagnetic and the antiferromagnetic phase. The analysis of the neutron diffraction data shows an incommensurate magnetic structure with the propagation vector Q=(0, 0, $\pm{\delta}$), where ${\delta}$ $\sim$ 0.2. For the full understanding of the magnetic state, further experiments are needed. The successful growth of large high-quality single crystals opens the opportunity for further investigations of itinerant magnets with incommensurate spin structures using a wide range of experimental tools.

Crystalline Red Phosphorus Nanoribbons: Large-Scale Synthesis and Electrochemical Nitrogen Fixation.

Two dimensional (2D) nanoribbons constitute an emerging nanoarchitecture for advanced microelectronics and energy conversion due to the stronger size confinement effects compared to traditional nanosheets. Triclinic crystalline red phosphorus (cRP) composed by a layered structure is a promising 2D phosphorus allotrope and the tube-like substructure is beneficial to the construction of nanoribbons. In this work, few-layer cRP nanoribbons are synthesized and the effectiveness in the electrochemical nitrogen reduction reaction (NRR) is investigated. An iodine-assisted chemical vapor transport (CVT) method is developed to synthesize circa 10 g of bulk cRP lumps with a yield of over 99 %. With the aid of probe ultrasonic treatment, high-quality cRP microcrystals are exfoliated into few-layer nanoribbons (cRP NRs) with large aspect ratios. As non-metallic materials, cRP NRs are suitable for the electrochemical nitrogen reduction reaction. The ammonia yield is 15.4 μg h-1 mgcat. -1 at -0.4 V vs. reversible hydrogen electrode in a neutral electrolyte under ambient conditions and the Faradaic efficiency is 9.4 % at -0.2 V. Not only is cRP a promising catalyst, but also the novel strategy expands the application of phosphorus-based 2D structures beyond that of traditional nanosheets.

Crystalline Red Phosphorus Nanoribbons: Large‐Scale Synthesis and Electrochemical Nitrogen Fixation

AbstractTwo dimensional (2D) nanoribbons constitute an emerging nanoarchitecture for advanced microelectronics and energy conversion due to the stronger size confinement effects compared to traditional nanosheets. Triclinic crystalline red phosphorus (cRP) composed by a layered structure is a promising 2D phosphorus allotrope and the tube‐like substructure is beneficial to the construction of nanoribbons. In this work, few‐layer cRP nanoribbons are synthesized and the effectiveness in the electrochemical nitrogen reduction reaction (NRR) is investigated. An iodine‐assisted chemical vapor transport (CVT) method is developed to synthesize circa 10 g of bulk cRP lumps with a yield of over 99 %. With the aid of probe ultrasonic treatment, high‐quality cRP microcrystals are exfoliated into few‐layer nanoribbons (cRP NRs) with large aspect ratios. As non‐metallic materials, cRP NRs are suitable for the electrochemical nitrogen reduction reaction. The ammonia yield is 15.4 μg h−1 mgcat.−1 at −0.4 V vs. reversible hydrogen electrode in a neutral electrolyte under ambient conditions and the Faradaic efficiency is 9.4 % at −0.2 V. Not only is cRP a promising catalyst, but also the novel strategy expands the application of phosphorus‐based 2D structures beyond that of traditional nanosheets.

Phosphorene Nanosheets Exfoliated from Low-Cost and High-Quality Black Phosphorus for Hydrogen Evolution

Black phosphorus (BP) has recently attracted intense research interest due to its unique thickness-dependent and anisotropic electronic and photonic properties, and has shown promising potential in nanophotonic, nanoelectronics and energy storage applications. However, the application of BP in practical devices is hindered by its high commercial cost. To further reduce the cost and accelerate the development of BP, a highly efficient preparation strategy for low-cost BP must be found. Herein, we report such a method via a modified chemical vapor transport (CVT) method by replacing high-purity red phosphorus (RP) with a low-purity precursor counterpart. We show that this method can drastically reduce the cost of manufacturing by several orders of magnitude. Furthermore, the BP produced using low-cost RP shows nearly the same crystal quality, high purity, local chemical structure, and electronic properties, compared with those of the high-cost BP prepared by the traditional CVT method. Most importantly, exfoliated phosphorene nanosheets prepared from the low-cost BP exhibit promising hydrogen evolution reaction (HER) activity. Owing to the high quality and high conversion efficiency, the low-cost BP holds promising potential in future scientific research and industrial applications.

Synthesis and structural characterization of Ag2ZnSnS4 crystals

In today’s battle for the environment, renewable energy sources, including solar energy, plays a significant role. Therefore, it is necessary to develop new materials for solar cells. Ag2ZnSnS4 (AZTS) is a material that makes it possible to increase the efficiency of photovoltaic modules, compared to cells based on Cu2ZnSnS4. AZTS was obtained by several methods, but in these research it was synthesized by Chemical Vapor Transport method (CVT), using zinc chloride as transporting agent, for the first time. X-ray powder diffraction was used in the structural tests, which showed that through the CVT method it is possible to obtain a pure Ag2ZnSnS4 phase with a stannite crystal structure. Raman measurements were conducted to determine the crystal quality of samples and to check their phase homogeneity. Raman spectra were calculated from DFT analysis applying the LDA and GGA approximations. The measured Raman peaks were assigned to calculated ones for AZTS structure.

Phase Controlled Growth of Cd3As2 Nanowires and Their Negative Photoconductivity.

The bottom-up synthesis process often allows the growth of metastable phase nanowires instead of the thermodynamically stable phase. Herein, we synthesized Cd3As2 nanowires with a controlled three-dimensional Dirac semimetal phase using a chemical vapor transport method. Three different phases such as the body centered tetragonal (bct), and two metastable primitive tetragonal (P42/nbc and P42/nmc) phases were identified. The conversion between three phases (bct → P42/nbc → P42/nmc) was achieved by increasing the growth temperature. The growth direction is [110] for bct and P42/nbc and [100] for P42/nmc, corresponding to the same crystallographic axis. Field effect transistors and photodetector devices showed the nearly same electrical and photoelectrical properties for three phases. Differential conductance measurement confirms excellent electron mobility (2 × 104 cm2/(V s) at 10 K). Negative photoconductance was first observed, and the photoresponsivity reached 3 × 104 A/W, which is ascribed to the surface defects acting as trap sites for the photogenerated electrons.

Ternary MoSe2xTe2−2x alloy with tunable band gap for electronic and optoelectronic transistors

Two-dimensional transition metal dichalcogenide (2D TMDs) alloys, consisting of three or more elements, offer a luxury variety of chemical and physical properties through elemental ratio alteration, thus may provide ideal candidate with tunable band gap for specific electrical applications. In this work, we demonstrate a high-quality layered MoSe2xTe2−2x (x = 0 ∼ 1) alloy synthesized via one-step chemical vapor transport method for high-performance electronic and optoelectronic transistors. Our characterizations reveal the obtained ternary alloy forming high-quality single crystal layers with 2H phase. Interestingly, the electronic transistors fabricated on MoSe2xTe2−2x thin layers (6 ∼ 7 layers) display an anomalous transition from ambipolar to n-type in conductive characteristics with the increase of substitution x value. The subsequent photoelectrical measurements exhibit that high on-off ratio for every ratio (x = 0.18, 0.38, 0.67, 0.83) with optical band gap in the range of 1.6 eV and 1.1 eV (near infrared). The optimized MoSe0.37Te1.63–based transistor can achieve up to ∼107 Ion/Ioff and 105 Iph/Idark ratio, 100 mA W−1 photo-responsivity and 2.38% external quantum efficiency with high photoresponsivity. Thus, such ternary MoSe2xTe2−2x alloys may pose a great potential for 2D-based electronic and photoelectronic applications.

Tuning Electrical and Optical Properties of MoSe2 Transistors via Elemental Doping

AbstractControlled doping of transition‐metal dichalcogenide (TMDC)‐layered materials is of great importance for practical device application. Doping of TMDCs and tuning of the bandgap properties remain challenging owing to the difficulty of breaking covalent bonds. In this work, the doping of V, Fe, and Nb atoms into MoSe2 is successfully synthesized using the chemical vapor transport method. By the absorption of V or Nb dopants, the intrinsic n‐type MoSe2 is tuned to be of p type. A p‐n diode based on a homojunction is fabricated by stacking the n‐ and p‐type MoSe2 vertically. The photoresponse range of intrinsic MoSe2 is only up to 785 nm. However, the photodetector based on the doped MoSe2 flake with V, Fe, and Nb can be used up to the near‐infrared (1550 nm) region at room temperature. Furthermore, the photoresponse wavelength of a V‐doped MoSe2 could reach up to 10 µm. The results indicate that the elemental doping methods could enrich the electrical and optical application prospects of MoSe2.

Critical behavior and magnetocaloric effect of the quasi-two-dimensional room-temperature ferromagnet Cr4Te5

A high-quality ${\mathrm{Cr}}_{4}{\mathrm{Te}}_{5}$ single crystal was prepared by the chemical vapor transport method. The crystal structure, magnetic properties, and magnetocaloric effect have been investigated in detail. Its critical behavior was methodically studied by measuring isothermal magnetization curves around the ferromagnetic to paramagnetic phase transition temperature. The critical exponents $\ensuremath{\beta}$ = 0.388, $\ensuremath{\gamma}$ = 1.29, $\ensuremath{\delta}$ = 3.93, and Curie temperature $T{}_{C}$ = 318.7 K were determined with modified Arrott plots, the Kouvel-Fisher method, critical isotherm analysis, and wisdom scaling law, respectively. The obtained critical exponents were self-consistent and follow the scaling equation around $T{}_{C}$, indicating the reliability of these values. Further analysis revealed that the spin coupling inside ${\mathrm{Cr}}_{4}{\mathrm{Te}}_{5}$ exhibits three-dimensional Heisenberg-like behavior with long-range magnetic interaction and that the exchange interaction decays as $J(r)\phantom{\rule{4pt}{0ex}}\ensuremath{\approx}\phantom{\rule{4pt}{0ex}}{r}^{\ensuremath{-}4.85}$.

Quasi-Binary Transition Metal Dichalcogenide Alloys: Thermodynamic Stability Prediction, Scalable Synthesis, and Application.

Transition metal dichalcogenide (TMDCs) alloys could have a wide range of physical and chemical properties, ranging from charge density waves to superconductivity and electrochemical activities. While many exciting behaviors of unary TMDCs have been demonstrated, the vast compositional space of TMDC alloys has remained largely unexplored due to the lack of understanding regarding their stability when accommodating different cations or chalcogens in a single-phase. Here, a theory-guided synthesis approach is reported to achieve unexplored quasi-binary TMDC alloys through computationally predicted stability maps. Equilibrium temperature-composition phase diagrams using first-principles calculations are generated to identify the stability of 25 quasi-binary TMDC alloys, including some involving non-isovalent cations and are verified experimentally through the synthesis of a subset of 12 predicted alloys using a scalable chemical vapor transport method. It is demonstrated that the synthesized alloys can be exfoliated into 2D structures, and some of them exhibit: i) outstanding thermal stability tested up to 1230 K, ii) exceptionally high electrochemical activity for the CO2 reduction reaction in a kinetically limited regime with near zero overpotential for CO formation, iii) excellent energy efficiency in a high rate Li-air battery, and iv) high break-down current density for interconnect applications. This framework can be extended to accelerate the discovery of other TMDC alloys for various applications.

N-type doping of black phosphorus single crystal by tellurium

Black phosphorus has many potential applications in optoelectronic devices because of its unique properties. Adjusting its performance by doping is an important issue of research. In this paper, we synthesized high-quality Te-doped crystals by the chemical vapor transport method. Tellurium doping with an atomic ratio of 0.1% was confirmed by X-ray photoelectron spectroscopy, X-ray diffraction, and energy dispersive X-ray analysis. The performance of field effect transistors devices shows that the hole mobility of Te-doped black phosphorous (BP) is significantly improved compared with that of undoped-BP. The highest hole mobility at room temperature is 719 cm2 V−1 s−1, and the electron mobility is 63 cm2 V−1 s−1. Te-doped BP field effect transistors show an obvious bipolar behavior.

Photothermal Antibacterial and Antibiofilm Activity of Black Phosphorus/Gold Nanocomposites against Pathogenic Bacteria

Black phosphorus (BP) as a layered two-dimensional (2D) semiconductor material with a tunable band gap has attracted growing attention for promising applications in diverse fields including biotechnology owing to its excellent physical and chemical properties. In this study, BP crystals were synthesized using a chemical vapor transport method and exfoliated into BP nanosheets in deoxygenated water or hexane. Next, monodisperse Au nanoparticles that were synthesized using a surfactant-assisted chemical reduction method were assembled on exfoliated BP nanosheets hexane to yield BP/Au nanocomposites. The photothermal antibacterial and antibiofilm activities of BP nanosheets and BP/Au nanocomposites were investigated against Enterococcus faecalis, a pathogenic biofilm-forming bacterium, by studying the photothermal effect and bacterial growth curve and using colony counting and live/dead fluorescence staining methods under near-infrared (NIR) light irradiation. Thanks to the higher photothermal conversion efficiency of BP/Au nanocomposites than that of bare BP nanosheets under NIR light irradiation, they destructed the bacterial cell membrane more efficiently than bare BP with the biofilm inhibition rate of 58%. It should be noted that this is the first study on the antibacterial and antibiofilm activity of BP/Au nanocomposites via a photothermal process under NIR light irradiation. This work shows the potential of BP/Au nanocomposites in fighting against pathogenic bacteria and paves the way for the exploration of antibacterial platforms based on the biocompatible 2D semiconductor BP.

Single crystal growth and electrical transport properties of η-Mo4O11

Abstract Single crystals of quasi-two-dimensional charge-density-wave (CDW) material η-Mo4O11 were successfully grown by chemical vapor transport (CVT) method with TeCl4 and I2 as carrier gas, respectively. The obtained crystals with TeCl4 as carrier gas have a largest size of 16 × 6 × 0.8 mm3 with high quality determined by x-ray single-crystal diffraction, chemical analysis and electrical transport measurements. Temperature dependent resistivity shows that η-Mo4O11 is of highly anisotropy and undergoes successive CDW transitions at 105 K and 30 K, consistent with early reports. It is found that η-Mo4O11 with TeCl4 as carrier gas has a relative higher Residual Resistance Ratio (RRR), carrier density and mobility than that with I2 as carrier gas. This results in a much smaller magnetoresistance but pronounced Shubnikov-de Haas oscillations, providing opportunities to study the unusual oscillations of this compound beyond the quantum limit. For the Hall resistance, a drop above 10 T is reproduced corresponding to a steep increase of magnetoresistance.

Optical and Thermoelectric Properties of Surface-Oxidation Sensitive Layered Zirconium Dichalcogenides ZrS2−xSex (x = 0, 1, 2) Crystals Grown by Chemical Vapor Transport

In this work, structure, optical, and thermoelectric properties of layered ZrS2−xSex single crystals with selenium composition of x = 0, 1, and 2 were examined. Single crystals of zirconium dichalcogenides layer compounds were grown by chemical vapor transport method using I2 as the transport agent. X-ray diffraction (XRD) and high-resolution transmission electron microscope (HRTEM) results indicated that ZrS2−xSex (x = 0, 1, and 2) were crystalized in hexagonal CdI2 structure with one-layer trigonal (1T) stacking type. X-ray photoelectron and energy dispersive X-ray measurements revealed oxidation sensitive behavior of the chalcogenides series. Transmittance and optical absorption showed an indirect optical gap of about 1.78 eV, 1.32 eV, and 1.12 eV for the ZrS2−xSex with x = 0, 1, and 2, respectively. From the result of thermoelectric experiment, ZrSe2 owns the highest figure-of merit (ZT) of ~0.085 among the surface-oxidized ZrS2−xSex series layer crystals at 300 K. The ZT values of the ZrS2−xSex (x = 0, 1, and 2) series also reveal increase with the increase of Se content owing to the increase of carrier concentration and mobility in the highly Se-incorporated zirconium dichalcogenides with surface states.

Facile and fast growth of high mobility nanoribbons of ZrTe5**Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFA0300600, 2016YFA0300802, 2013CB932904, and 2016YFA0202500) and the National Natural Science Foundation of China (Grant Nos. 11574005, 11774009, and 11234001).

Recently, ZrTe5 has received a lot of attention as it exhibits various topological phases, such as weak and strong topological insulators, a Dirac semimetal, a three-dimensional quantum Hall state, and a quantum spin Hall insulator in the monolayer limit. While most of studies have been focused on the three-dimensional bulk material, it is highly desired to obtain nanostructured materials due to their advantages in device applications. We report the synthesis and characterizations of ZrTe5 nanoribbons. Via a silicon-assisted chemical vapor transport method, long nanoribbons with thickness as thin as 20 nm can be grown. The growth rate is over an order of magnitude faster than the previous method for the bulk crystals. Moreover, transport studies show that the nanoribbons are of low unintentional doping and high carrier mobility, over 30000 cm2/V⋅s, which enable reliable determination of the Berry phase of π in the ac plane from quantum oscillations. Our method holds great potential in growth of high quality ultra-thin nanostructures of ZrTe5.

Optical characterization of lead iodide grown by chemical vapor transport method

This paper presents the characteristics and optical properties of lead iodide (PbI2) grown by the chemical vapor transport method. X-ray diffraction, Raman spectra, and scanning electron microscope methods were used to characterize the crystal structure and surface morphology of PbI2 material. The temperature-dependent photoconductivity and reflection spectra are used to investigate the direct band gap energies of PbI2 in the temperature range from 20 to 300 K. The parameters that describe the temperature variation of the energies of direct band gap energies are determined. The optical response was also investigated by time-resolved photoresponse and frequency-dependent photo responsivity investigations. Possible mechanisms of the kinetic decay processes of carriers are also discussed.