Chemical Vapor Transport Growth Research Articles

Ultrahigh-strengthened intragranular κ-Al2O3 nanoparticles dispersed Mo composite prepared via SPS sintering of core-shell Mo nanocomposite powder

Oxide dispersion strengthening (ODS) is an effective method to improve the mechanical properties of Mo-based materials. However, the mechanical properties of traditional ODS-Mo composites are always limited by the coarsening and intergranular distribution of second-phase particles. In this work, an effective nano-reinforcement dispersion strategy was developed to fabricate an ODS-Mo composite with ultrafine grain and intragranular distribution of second-phase particles. Core-shell structural Mo nanocomposite powders, with internally distributed sub-10 nm Al2O3 dispersoids, were prepared by nano atomization doping (AD) followed by a chemical vapor transport growth strategy. Then, ODS-Mo composites with ultra-fine Mo grain (below 700 nm) and high-density intragranular κ-Al2O3 (below 20 nm) nanoparticles were prepared via spark plasma sintering (SPS), in which a coherent interface between κ-Al2O3 and Mo matrix was formed. The composites present remarkably improved hardness (above 500 HV), bend strength, and compressive yield strength (above 1664 MPa) at room temperature, with a suitable strain to fracture of 27.1 %. The calculation of strengthening mechanisms indicates that the enhancement was mainly attributed to the intragranular κ-Al2O3 nanoparticles. This nano-sized reinforcement distributed within the grain can more effectively pin dislocations and achieve dispersion strengthening in ODS-Mo composites. Therefore, this strategy can efficiently construct intragranular second-phase nanoparticles and open up new avenues to fabricate high-performance ODS-Mo composites.

Seeded growth of single-crystal black phosphorus nanoribbons.

Two-dimensional materials have emerged as an important research frontier for overcoming the challenges in nanoelectronics and for exploring new physics. Among them, black phosphorus, with a combination of a tunable bandgap and high mobility, is one of the most promising systems. In particular, black phosphorus nanoribbons show excellent electrostatic gate control, which can mitigate short-channel effects in nanoscale transistors. Controlled synthesis of black phosphorus nanoribbons, however, has remained an outstanding problem. Here we report large-area growth of black phosphorus nanoribbons directly on insulating substrates. We seed the chemical vapour transport growth with black phosphorus nanoparticles and obtain uniform, single-crystal nanoribbons oriented exclusively along the [100] crystal direction. With comprehensive structural calculations, we discover that self-passivation at the zigzag edges holds the key to the preferential one-dimensional growth. Field-effect transistors based on individual nanoribbons exhibit on/off ratios up to ~104, confirming the good semiconducting behaviour of the nanoribbons. These results demonstrate the potential of black phosphorus nanoribbons for nanoelectronic devices and also provide a platform for investigating the exotic physics in black phosphorus.

Controlled p-Type Doping of Pyrite FeS2.

Pyrite FeS2 has extraordinary potential as a low-cost, nontoxic, sustainable photovoltaic but has underperformed dramatically in prior solar cells. The latter devices focus on heterojunction designs, which are now understood to suffer from problems associated with FeS2 surfaces. Simpler homojunction cells thus become appealing but have not been fabricated due to the historical inability to understand and control doping in pyrite. While recent advances have put S-vacancy and Co-based n-doping of FeS2 on a firm footing, unequivocal evidence for bulk p-doping remains elusive. Here, we demonstrate the first unambiguous and controlled p-type transport in FeS2 single crystals doped with phosphorus (P) during chemical vapor transport growth. P doping is found to be possible up to at least ∼100 ppm, inducing ∼1018 holes/cm3 at 300 K, while leaving the crystal structure and quality unchanged. As the P doping is increased in crystals natively n-doped with S vacancies, the majority carrier type inverts from n to p near ∼25 and ∼55 ppm P, as detected by Seebeck and Hall effects, respectively. Detailed temperature- and P-doping-dependent transport measurements establish that the P acceptor level is 175 ± 10 meV above the valence band maximum, explain details of the carrier inversion, elucidate the relative mobility of electrons and holes, reveal mid-gap defect levels, and unambiguously establish that the inversion to p-type occurs in the bulk and is not an artifact of hopping conduction. Such controlled bulk p-doping opens the door to pyrite p-n homojunctions, unveiling new opportunities for solar cells based on this extraordinary semiconductor.

Growth of highly crystalline ultrathin two-dimensional selenene

Elemental two-dimensional (2D) crystals have recently emerged as promising materials for advanced electronics and optoelectronics applications. However, it remains challenging to achieve controllable growth of high-quality, ultra-thin flakes of elemental 2D materials. Here, we demonstrate, for the first time, a seed-assisted chemical vapor transport growth of ultra-thin triangular flakes of highly crystalline trigonal selenium (t-Se) oriented in (0001) direction, with lateral size >30 µm. The polarization angle-resolved Raman spectra of bilayer selenene show in-plane isotropic properties, owing to the highly symmetric lattice resulting from its unique growth orientation. Density functional theory calculations support the experimental findings in establishing the structure and stability of the as-grown selenene. We studied the optical response of a photodetector fabricated using a bilayer selenene. Our growth strategy can be extended to other elemental 2D materials to realize their full potential in applications ranging from optoelectronics and electronics to energy conversion.

Growth of Pure and Intercalated Zrte2, Tite2 and Hfte2 Dichalcogenide Single Crystals by Isothermal Chemical Vapor Transport

We report on a modified chemical vapor transport (CVT) methodology for the growth of pure and intercalated Zr, Ti, and Hf dichalcogenide single crystals, e.g. ZrTe2, Gd0.05ZrTe2, HfTe2, and Cu0.05TiTe2. While the most common method for CVT growth is carried out in quartz tubes subjected to a temperature gradient between the charge and the growth location, the growth using this isothermal-CVT (ICVT) method takes place isothermally in sealed quartz tubes placed horizontally in box furnaces, using iodine (I2) as the transport agent. The structure and composition of crystals were determined by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), and induced coupling plasma (ICP). The crystals grown with this method can be large, and show excellent crystallinity and homogeneity. Their morphology is plate-like, and the larger dimensions can be as long as 15 mm.

Origin of the improved reactivity of MoS2 single crystal by confining lattice Fe atom in peroxymonosulfate-based Fenton-like reaction

In this study, we describe a confinement of trace amount of Fe atoms (0.66 at. ‰) in Mo lattice, via a chemical vapor transport growth of MoS2 single crystal. In the Fenton-like reaction for the degradation of atrazine, the Fe@MoS2 as catalyst to activate PMS could produce more reactive oxygen species and exhibit a rate constant (1.30 min−1) three times higher than that of pristine MoS2 (0.43 min−1). Theoretical simulation suggests that the diluted confinement of Fe atoms in Mo sites activates the inert basal plane of MoS2, creating new active sites of Mo both nearby the Fe site and afar, for the adsorption and decomposition of PMS. Our work provides a clear atomic mechanism of the improvement of the chemical reactivity of MoS2 single crystal in Fenton-like reaction via heteroatom confinement.

CVT and PVT growth and characterization of GaS crystals

Gallium sulphide (GaS) has been prepared by chemical vapour transport (CVT) and physical vapour transport (PVT). The obtained crystals show two different morphologies: large layers and little microstructures respectively, both indicated hexagonal crystal structure.Energy-dispersive X-ray spectroscopy (EDX) measurements confirm the correct stoichiometry; the hexagonal structure has been confirmed with x-ray analysis and transmission electron observations (TEM).Optical study by cathodoluminescence shows direct bandgap with energy gap at 2.5 eV for both samples.

Quantitative, experimentally-validated, model of MoS2 nanoribbon Schottky field-effect transistors from subthreshold to saturation

We investigate the channel length dependence of the electrical characteristics of chemical vapor transport (CVT)-grown MoS2 nanoribbon (NR) Schottky barrier field-effect transistors to provide insights into the transport properties of such nanostructures. The MoS2 NRs form spontaneously during the CVT growth, without the application of etching. Back gated transmission line measurement FETs were fabricated on a 45μm-long NR with channel lengths ranging between 200 nm and 3μm. Contact and sheet resistances were extracted from the electrical measurements and their back-gate bias dependence was analyzed. Numerical modeling based on a virtual probe approach combined with the Landauer formalism shows excellent agreement with the measurements. The model enables a quantitative extraction of the intrinsic FET properties, e.g., mean-free-path and electron mobility, and their dependence on carrier density and investigation of plausible trap distributions. A record electron mobility for a MoS2 NR channel of ∼81cm2/Vs was achieved.

Seeded growth of high-quality transition metal dichalcogenide single crystals via chemical vapor transport

Seeded chemical vapor transport growth gives high-quality and millimeter-sized transition metal dichalcogenide single crystals in a short period.

Chemical Vapor Transport Growth of ZrSe2 Crystals Using Cl2 as a Transport Agent

A process has been proposed for ZrSe2 crystal growth with the use of ZrOCl2 as a transport agent source. The crystals thus grown have been characterized by X-ray diffraction. It has been shown that, unlike in the case of vanadium and molybdenum diselenides, the use of zirconium chloride causes no increase in crystal size. A mechanism of the chemical vapor transport process involved is proposed.

Gas-pressure chemical vapor transport growth of millimeter-sized c-BAs single crystals with moderate thermal conductivity

We have grown c-BAs single crystals up to 1000 μm size by the chemical vapor transport (CVT) technique using combined As and I2 transport agents with the As:I ratio of 1:3 under gas pressures of up to 35 atm. Raman spectroscopy revealed a very sharp (∼2.4 cm−1) P1 phonon mode and an interesting splitting behavior of P1 from detailed polarization studies. Electron paramagnetic resonance (EPR) experiments revealed no evidence for EPR active growth-related defects under the experimental resolution. Finally, a moderate thermal conductivity value of ∼132 W/m-K was obtained using a transient thermal grating technique. These results suggest that although the high As gas vapor pressure environment in CVT growth can increase the transport rate of c-BAs significantly, it may not be efficient in reducing the defects and enhancing the thermal conductivity in c-BAs significantly.

Polymorphic Layered MoTe2 from Semiconductor, Topological Insulator, to Weyl Semimetal

Large size (∼2 cm) single crystals of layered MoTe2 in both 2H- and 1T′-types were synthsized using TeBr4 as the source of Br2 transport agent in chemical vapor transport growth. The crystal structures of the as-grown single crystals were fully characterized by X-ray diffraction, Raman spectroscopy, scanning transmission electron microscopy, scanning tunneling microscopy (STM), and electrical resistivity (ρ) measurements. The resistivity ρ(T), magnetic susceptibility χ(T), and heat capacity Cp(T) measurement results reveal a first order structural phase transition near ∼240 K for 1T′-MoTe2, which has been identified to be the orthorhombic Td-phase of MoTe2 as a candidate of Weyl semimetal. The STM study revealed different local defect geometries found on the surface of 2H- and Td-types of MoTe6 units in trigonal prismatic and distorted octahedral coordination, respectively.

High-pressure melt growth and transport properties of SiP, SiAs, GeP, and GeAs 2D layered semiconductors

Silicon and Germanium monopnictides SiP, SiAs, GeP and GeAs form a family of 2D layered semiconductors. We have succeeded in growing bulk single crystals of these compounds by melt-growth under high pressure (0.5-1GPa) in a cubic anvil hot press. Large (mm-size), shiny, micaceous crystals of GeP, GeAs and SiAs were obtained, and could be exfoliated into 2D flakes. Small and brittle crystals of SiP were yielded by this method. High-pressure sintered polycrystalline SiP and GeAs have also been successfully used as a precursor in the Chemical Vapor Transport growth of these crystals in the presence of I2 as a transport agent. All compounds are found to crystallize in the expected layered structure and do not undergo any structural transition at low temperature, as shown by Raman spectroscopy down to T=5K. All materials exhibit a semiconducting behavior. The electrical resistivity of GeP, GeAs and SiAs is found to depend on temperature following a 2D-Variable Range Hopping conduction mechanism. The availability of bulk crystals of these compounds opens new perspectives in the field of 2D semiconducting materials for device applications.

Introduction to the Growth of Bulk Single Crystals of Two-Dimensional Transition-Metal Dichalcogenides

Semiconducting two-dimensional transition-metal dichalcogenides (MX2) are attracting much attention as promising materials for a new generation of optical and electronic devices. MX2 compounds are complementary or competitive to graphene because of the existence of a native band gap. The growth of large and high-quality bulk single crystals is one of the critical issues for the application of MX2 compounds, whose bulk crystals are generally grown by the chemical vapor transport (CVT) method. In the present review, I introduce experimental techniques required for the CVT growth of high-quality MX2 single crystals.

The Role of Transport Agents in MoS2 Single Crystals

We report resistivity, thermoelectric power, and thermal conductivity of MoS2 single crystals prepared by the chemical vapor transport (CVT) method using I2, Br2, and TeCl4 as transport agents. The material presents low-lying donor and acceptor levels, which dominate the in-plane charge transport. Intercalates into the van der Waals gap strongly influence the interplane resistivity. Thermoelectric power displays the characteristics of strong electron–phonon interaction. A detailed theoretical model of thermal conductivity reveals the presence of a high number of defects in the MoS2 structure. We show that these defects are inherent to CVT growth method, coming mostly from the transport agent molecules inclusion as identified by total reflection X-ray fluorescence analysis (TXRF) and in-beam activation analysis (IBAA).

Shining light on CuO for exploring high-Tc multiferroics

Searching for a 3D multiferroic material with a strong magnetoelectric coupling and high critical temperature is a major challenge in modern condensed matter research. CuO is the building block of high-temperature superconductors and triggered a new interest when it was established as potential high temperature multiferroic. We have succeeded in growing high quality single crystals of CuO with two different methods, namely the floating zone under high oxygen pressure and the chemical vapor transport growth. The fact that we are able to grow crystals of the same compound by different techniques makes it possible to study the effect of slightly different chemical compositions, various kinds of defects and variable strain on the final properties of the compound. Optical spectroscopy has been deployed to study the optical response of cupric oxide. Thereby we achieved a deeper insight of the optical, electronic and structural properties by measuring the infrared reflectivity under a magnetic field and the Raman shift under hydrostatic high pressure.

Chemical vapor transport of chalcopyrite semiconductors: CuGaS2 and AgGaS2

Crystals of CuGaS2 and AgGaS2 with different isotopic compositions have been grown by chemical vapor transport (CVT) using iodine as the transport agent. Before performing the CVT growth, sulfur and copper were purified by sublimation and etching, respectively. 109Ag and the etched 71Ga isotopes were purified from oxides by vacuum annealing. Transparent yellow orange crystals of CuGaS2 and greenish yellow crystals of AgGaS2 were obtained in the shape of platelets, chunks, rods and needles in sizes of up to 8mm (CuGaS2) and 30mm (AgGaS2).These crystals were used to study their electronic, vibrational and thermodynamic properties. Higher excitonic states (n=2,3) were observed at low temperatures with wavelength-modulated reflectivity spectroscopy, thus proving an excellent surface and crystal quality. In addition, the experimentally determined non-monotonic temperature dependence of the excitonic energies can be well fitted by using two Bose–Einstein oscillators and their statistical factors, corresponding to characteristic acoustic and optical phonon frequencies. Isotopic shift of excitonic energies has also been successfully observed in these crystals.

New quaternary compound PbSbBiS4

The PbSb2S4-PbBi2S4 join of the PbS-Sb2S3-Bi2S3 system has been studied using physicochemical characterization techniques and its phase diagram has been mapped out. The join has PbSb2S4-based solid-solution series, which extends to 20 mol % PbBi2S4. At a 1: 1 ratio of the constituent sulfides, there is a congruently melting compound with the composition PbSbBiS4. PbSbBiS4 single crystals have been prepared by chemical vapor transport growth. The unit-cell parameters of PbSbBiS4 (orthorhombic structure) are a = 15.72, b = 11.36, and c = 4.41 A.

ZnO/Si arrays decorated by Au nanoparticles for surface-enhanced Raman scattering study

Large scale and highly ordered flowerlike ZnO/Si nanostructures are successfully prepared by combining two common techniques, viz. hydrothermally etch fabrication of nanoporous Si pillar array (NSPA) and self-catalytic chemical vapor transport growth of ZnO nanowires. Au nanoparticles are decorated onto the ZnO/Si nanoflowers by the hydrothermal method. The formed Au/ZnO/NSPA array is evaluated as a surface-enhanced Raman scattering SERS-active substrate, which exhibits very high sensitivity and good stability and reproducibility. The excellent SERS enhancement is mainly attributed to the strong local electromagnetic effect which is associated with the unique flowerlike nanostructures of Au/ZnO/NSPA and the formed metal-induced gap states at the Au/ZnO interfaces. The results indicated that Au/ZnO/NSPA might be employed as a promising SERS substrate for the fast detection of low-concentration biomolecules.

Electrical Properties and Magnetic Response of Cobalt Germanosilicide Nanowires

The effects of partial substitution of Ge for Si in cobalt germanosilicide (CoSi(1-x)Ge(x) and Co(2)Si(1-x)Ge(x)) nanowires (NWs) on the electrical transport, magnetic properties, and magnetoresistance (MR) have been investigated. Cobalt germanosilicide NWs were synthesized by a spontaneous chemical vapor transport growth method. The Ge concentration can be selectively controlled from 0 to 15% and 0-50% for CoSi(1-x)Ge(x) and Co(2)Si(1-x)Ge(x) NWs, respectively, by varying the reaction temperature. Electrical measurements showed that the resistivities of CoSi(1-x)Ge(x) NWs are 90, 60, 30, and 23 μΩ-cm for x = 0, 0.01, 0.05, and 0.15, respectively. Therefore, the electrical resistivity of CoSi(1-x)Ge(x) NWs was found to decrease significantly with an increasing Ge concentration, which is believed to be a result of the band gap narrowing. On the other hand, the Co(2)Si(1-x)Ge(x) (x ≤ 0.5) NWs exhibited ferromagnetism at 300 K, which is attributed to the uncoordinated Co atoms on the NW surface and spin-glass behavior at low temperature. The highest MR response of Co(2)Si(1-x)Ge(x) NWs occurred at x = 0.5, where a MR ratio of 11.7% can be obtained at 10-25 K with a magnetic field of 8 T. The enhanced physical properties of cobalt germanosilicide NWs with Ge substitution shall lead to promising application in the fabrication of nanodevices, including spintronics and serving as the gate and interconnect material.