Vapor Transport Synthesis Research Articles

Controllable preparation of red phosphorus nanoribbons for enhanced photocatalytic degradation of Methyl Orange and Tetracycline.

Nanoribbons (NRs), leveraging the flexibility of one-dimensional materials and the expansive surface area of two-dimensional materials, offer heightened exposure to edge sites and superior charge transfer rates. Consequently, they present promising prospects within the domain of photocatalysis. Crystalline red phosphorus (cRP), dcharacterized by its layered and fibrous structure, lends itself readily to the production of nanoribbons. Our study demonstrates a robust method for achieving high-yield, high-quality cRP by concurrently introducing mineralizing agent I2 and Si wafers into the Chemical Vapor Transport (CVT) synthesis process. Through ultrasound assistance, we transformed high-quality cRP into crystalline red phosphorus nanoribbons (cRP NRs) with an average thickness ranging from 7.5 to 17.5nm and an average width between 75 and 175nm. cRP NRs (I2 and Si) showcased impressive degradation capabilities towards Methyl Orange (MO) and Tetracycline (TC), achieving a remarkable 99% degradation of MO within 18min under the simulated visible-light irradiation. The reactive species capturing experiments confirmed that ·O2- was the primary active agent responsible for the photocatalytic degradation of MO.

Integration of layered group IV selenides: From SnSe–SnSe2-xSx core-shell crystals to complex (SnSe–SnSe2-xSx)-GeSe van der waals heterostructures

The layered semiconductor tin selenide (SnSe) has received extensive interest due to its promising thermoelectric and ferroelectric properties. Integrating SnSe with other layered crystals in heterostructures can enable the modification of charge- and thermal transport, electrical polarization, and other properties such as chemical stability, optoelectronics, and photonics. Here, we demonstrate the vapor transport synthesis of single-crystalline SnSe monochalcogenide flakes that are spontaneously encapsulated in a thin layered SnSe2-xSx dichalcogenide shell. In a second growth step, such SnSe–SnSe2-xSx heterostructures are integrated with the monochalcogenide GeSe, which is laterally stitched to the SnSe side facets while preserving the dichalcogenide shell across the basal facets. This architecture is confirmed by optical microscopy, electron microscopy and diffraction, energy dispersive X-ray and Raman spectroscopies, as well as cathodoluminescence spectroscopy. The results extend our capabilities for materials integration by forming complex heterostructures with both vertical van der Waals interfaces and covalent lateral interfaces between layered semiconductors.

Exfoliable Transition Metal Chalcogenide Semiconductor NbSe2I2.

As the field of exfoliated van der Waals electronics grows to include complex heterostructures, the variety of available in-plane symmetries and geometries becomes increasingly valuable. In this work, we present an efficient chemical vapor transport synthesis of NbSe2I2 with the triclinic space group P1̅. This material contains Nb-Nb dimers and an in-plane crystallographic angle γ = 61.3°. We show that NbSe2I2 can be exfoliated down to few-layer and monolayer structures and use Raman spectroscopy to test the preservation of the crystal structure of exfoliated thin films. The crystal structure was verified by single-crystal and powder X-ray diffraction methods. Density functional theory calculations show triclinic NbSe2I2 to be a semiconductor with a band gap of around 1 eV, with similar band structure features for bulk and monolayer crystals. The physical properties of NbSe2I2 have been characterized by transport, thermal, optical, and magnetic measurements, demonstrating triclinic NbSe2I2 to be a diamagnetic semiconductor that does not exhibit any phase transformation below room temperature.

Photonics in Multimaterial Lateral Heterostructures Combining Group IV Chalcogenide van der Waals Semiconductors.

Lateral heterostructures combining two multilayer group IV chalcogenide van der Waals semiconductors have attracted interest for optoelectronics, twistronics, and valleytronics, owing to their structural anisotropy, bulk-like electronic properties, enhanced optical thickness, and vertical interfaces enabling in-plane charge manipulation/separation, perpendicular to the trajectory of incident light. Group IV monochalcogenides support propagating photonic waveguide modes, but their interference gives rise to complex light emission patterns throughout the visible/near-infrared range both in uniform flakes and single-interface lateral heterostructures. Here, this work demonstrates the judicious integration of pure and alloyed monochalcogenide crystals into multimaterial heterostructures with unique photonic properties, notably the ability to select photonic modes with targeted discrete energies through geometric factors rather than band engineering. SnS-GeS1-xSex-GeSe-GeS1-xSex heterostructures with a GeS1-xSex active layer sandwiched laterally between GeSe and SnS, semiconductors with similar optical constants but smaller bandgaps, were designed and realized via sequential vapor transport synthesis. Raman spectroscopy, electron microscopy/diffraction, and energy-dispersive X-ray spectroscopy confirm a high crystal quality of the laterally stitched components with sharp interfaces. Nanometer-scale cathodoluminescence spectroscopy provides evidence for a facile transfer of electron-hole pairs across the lateral interfaces and demonstrates the selection of photon emission at discrete energies in the laterally embedded active (GeS1- xSex) part of the heterostructure.

Chemical Vapor Transport Synthesis of Fibrous Red Phosphorus Crystal as Anodes for Lithium-Ion Batteries.

Red phosphorus (RP) is considered to be the most promising anode material for lithium-Ion batteries (LIBs) due to its high theoretical specific capacity and suitable voltage platform. However, its poor electrical conductivity (10-12 S/m) and the large volume changes that accompany the cycling process severely limit its practical application. Herein, we have prepared fibrous red phosphorus (FP) that possesses better electrical conductivity (10-4 S/m) and a special structure by chemical vapor transport (CVT) to improve electrochemical performance as an anode material for LIBs. Compounding it with graphite (C) by a simple ball milling method, the composite material (FP-C) shows a high reversible specific capacity of 1621 mAh/g, excellent high-rate performance and long cycle life with a capacity of 742.4 mAh/g after 700 cycles at a high current density of 2 A/g, and coulombic efficiencies reaching almost 100% for each cycle.

Tailoring the epitaxial growth of oriented Te nanoribbon arrays

As an elemental semiconductor, tellurium (Te) has been famous for its high hole-mobility, excellent ambient stability and topological states. Here, we realize the controllable synthesis of horizontal Te nanoribbon arrays (TRAs) with an angular interval of 60°on mica substrates by physical vapor deposition strategy. The growth of Te nanoribbons (TRs) is driven by two factors, where the intrinsic quasi-one-dimensional spiral chain structure promotes the elongation of their length; the epitaxy relationship between [110] direction of Te and [110] direction of mica facilitates the oriented growth and the expansion of their width. The bending of TRs which have not been reported is induced by grain boundary. Field-effect transistors based on TRs demonstrate high mobility and on/off ratio corresponding to 397cm2V-1s-1 and 1.5×105, respectively. These phenomena supply an opportunity to deep insight into the vapor-transport synthesis of low-dimensional Te and explore its underlying application in monolithic integration.

Room‐Temperature Self‐Powered Terahertz Photodetection in Ge‐Intercalated Topological Insulator GeBi4Te7

Topological insulators (TIs) are under rapid and intensive study in the fields of condensed matter physics, due to exotic electronic properties. Recent studies of photocurrent generation in topological surface states (TSS) properties have insufficient evidence to rule out that of bulk states, hindering the application in optoelectronics. Herein, the chemical vapor transport (CVT) synthesis of GeBi4Te7 crystal by the way of Ge intercalation is realized, enabling isolated Dirac points‐associated high‐mobility Dirac carriers and larger Seebeck coefficients. Then, the antenna‐coupled photodetector in GeBi4Te7 based on planar metal–TI–metal structure is reported, successfully realizing sensitive terahertz response without bias. According to the results, the GeBi4Te7‐based terahertz photodetector has a short response time and a high photoresponsivity (0.42 A W−1 at 0.107 THz and 0.6 A W−1 at 0.28 THz, respectively). The GeBi4Te7–graphene heterostructure can further improve the responsivity by more than two orders of magnitude, reaching 12 A W−1 at 0.107 THz. These findings bring up doable avenues for GeBi4Te7 materials’ low‐energy optoelectronic applications.

Quasi-one-dimensional phosphorene nanoribbons grown on silicon by space-confined chemical vapor transport.

Phosphorene nanoribbons (PNRs) combine the flexibility of one-dimensional (1D) nanomaterials with the large specific surface area and the edge and electron confinement effects of two-dimensional (2D) nanomaterials. In spite of the substantial advances in bulk black phosphorus (BP) manufacturing, achieving PNRs without degradation is still a big challenge. In this work, we present a strategy for the space-confined chemical vapor transport synthesis of quasi-one-dimensional surface-passivated monocrystalline PNRs on a silicon substrate. The growth mechanism of the PNRs is proposed by combining experimental results and DFT calculations, indicating that the P4 molecules can break, restructure, and epitaxially nucleate on the surface of the Au3SnP7 catalyst, and finally prefer to grow along the zigzag (ZZ) direction to form PNRs. The low gas flow rate and an appropriate phosphorus molecule concentration allow the growth of PNRs with structural integrity, which can be regulated by the amount of red phosphorus and the confined space.

Morphology and Luminescence of Flexible Free-Standing ZnO/Zn Composite Films Grown by Vapor Transport Synthesis.

A method for fabricating flexible free-standing ZnO/Zn composite films from the vapor phase using a regular array of silicon microwhiskers as a substrate is presented. The structural and morphological peculiarities, as well as luminescent properties of the films, were studied. The films have a hybrid structure consisting of two main microlayers. The first layer is formed directly on the tops of Si whiskers and has a thickness up to 10 µm. This layer features a polycrystalline structure and well-developed surface morphology. The second layer, which makes up the front side of the films, is up to 100 µm thick and consists of large microcrystals. The films show good bending strength-in particular, resistance to repeated bending and twisting-which is provided by a zinc metallic part constituting the flexible carrier of the films. ZnO photoluminescence was observed from both surfaces of the films but with conspicuous spectral differences. In particular, a significant weakening of ZnO green luminescence (more than 10 times) at an almost constant intensity of UV near-band edge emission was found for the polycrystalline side of the films as compared to the microcrystalline side. A high degree of homogeneity of the luminescent properties of the films over their area was demonstrated. The results obtained emphasize the relevance of further studies of such ZnO structures-in particular, for application in flexible devices, sensors, photocatalysis and light generation.

Topography dependence of conductivity in electrostrictive germanium sulfide nanoribbons

Layered group IV monochalcogenides have garnered considerable attention as a new class of two-dimensional (2D) semiconducting materials owing to their unique crystal structure and novel physical properties. The present work describes the chemical vapor transport synthesis of single-crystalline GeS nanoribbons. The findings demonstrate that with incrementally applied voltage, electrostrictive deformation and highly vertical current occur more significantly. Additionally, using a 2D fast Fourier transform power spectra, we demonstrate that the horizontal distribution of topography and current is more inhomogeneous than the vertical distribution, and that their monolithic spatial correlation weakens with increasing applied voltage. Moreover, we discovered that electrostrictive deformation has a sizable effect on the monolithic vertical resistance. Furthermore, local hollow positions are more conductive than bulge positions, as demonstrated by the ‘resistor’ model and local current–voltage curve. These findings on layered GeS nanoribbons not only shed light on the topographic and electrical properties of the material but also expand the possibilities for other nanoscale electronic and electromechanical device applications.

Revealing the Quasi-Periodic Crystallographic Structure of Self-Assembled SnTiS3 Misfit Compound

Chemical vapor transport synthesis of SnTiS3 yields a self-assembled heterostructure of two distinct constituent materials, the semiconductor SnS and the semimetal TiS2. The misfit layer compound, although thermodynamically stable, is structurally complex, and precise understanding of the structure is necessary for designing nanoengineered heterojunction compound devices or for theoretical studies. In our work, we reveal the unique complexity of the quasi-periodic structure of this heterostructure by systematically investigating the misfit compound using a set of advanced electron microscopy techniques. X-ray and electron diffraction patterns along with high-resolution scanning/transmission electron microscopy images obtained from different crystallographic orientations resolve the complexity of the sublattice component layer structure and reveal the uniquely bonded alignment among interlayers and a quasi-periodic arrangement of the sublayers. Density functional theory calculations embedded with the extracted structural information provide quantitative insights into the formation of self-assembled heterojunction structures where the nonpolar van der Waals interaction is found to play a dominant role in the structural alignment over the polar interlayer interaction.

ZnO Nanostructures Synthesized by Vapor Transport and Liquid Phase Synthesis Techniques: Growth and Properties

In this review, we briefly describe work devoted in recent years towards the effective control of morphology, structure and optical properties of ZnO nanostructures, with particular focus on cost effective and simple methods for ZnO nanowires (NWs) fabrication. For the vapor transport technique, we describe in detail mechanisms for growth precursors generation, their transport in inert and forming gas, as well as their reactions on different pretreated substrates and corresponding growth mechanisms. As for low temperature synthesis methods, three techniques are outlined: sol-gel, solvothermal and electrophoretic deposition, with emphasis on effective morphology, structure and optical properties control. In this context, we discuss recent attempts to understand the role of solvent and alkaline agents used during solvothermal synthesis of ZnO nanostructures on their morphology and photoluminescence properties. Recent success of electrophoretic deposition of ZnO nanoparticles on pre-patterned silicon substrates in the form of NWs and NW bunches is highlighted over many previous attempts to fabricate ZnO NWs with inconvenient sacrificial templates. Finally, we present a critical discussion on the current understanding of passivation mechanisms of ZnO NW surfaces by MgO shells.

Chemical vapor transport synthesis, characterization and compositional tuning of ZrSxSe2−x for optoelectronic applications

ZrS2, ZrSe2 and mixed alloy ZrSxSe2−x materials were achieved through chemical vapor transport. The incongruent melting system of Zr-S-Se formed crystalline layered flakes as a transport product that grew up to 2 cm in lateral size with cm-scale flakes consistently obtained for the entire compositional range exhibiting visible hexagonal features. Bulk flakes of the series ZrSxSe2−x (x = 0, 0.15, 0.3, 0.6, 1.05, 1.14, 1.51, 1.8 and 2) were analyzed through Raman spectroscopy revealing significant convolution of primary bonding modes and shifting of Raman features as a function of increasing sulfur composition. Additionally, activation of new modes not present in the pure compounds are observed as effects which result from disorder introduced into the crystal due to the random mixing of S-Se in the alloying process. Further structural characterization was performed via x-ray diffraction (XRD) on the layered flakes to evaluate the progression of layer spacing function of alloy composition which was found to range between 6.24 Å for ZrSe2 and 5.85 Å for ZrS2. Estimation of the compositional ratios of the alloy flakes through energy dispersive spectroscopy (EDS) large-area mapping verified the relation of the targeted source stoichiometry represented in the layered flakes. Atomic-resolution high angle annular dark field (HAADF)-scanning transmission electron microscopy (STEM) imaging was performed on the representative Zr(S0.5Se0.5)2 alloy to validate the 1T atomic structure and observe the arrangement of the chalcogenide columns stacks. Additionally, selected area diffraction pattern generated from the [0 0 0 1] zone axis revealed the in-plane lattice parameter to be approximately 3.715 Å.

Exfoliated CrPS4 with Promising Photoconductivity.

Layered semiconductors have attracted significant attention due to their diverse physical properties controlled by composition and the number of stacked layers. Herein, large crystals of the ternary layered semiconductor chromium thiophosphate (CrPS4 ) are prepared by a vapor transport synthesis. Optical properties are determined using photoconduction, absorption, photoreflectance, and photoacoustic spectroscopy exposing the semiconducting properties of the material. A simple, one-step protocol for mechanical exfoliation onto a transmission electron microscope grid is developed, and multiple layers are characterized by advanced electron microscopy methods, including atomic resolution elemental mapping confirming the structure by directly showing the positions of the columns of different elements' atoms. CrPS4 is also liquid exfoliated, and in combination with colloidal graphene, an ink-jet-printed photodetector is created. This all-printed graphene/CrPS4 /graphene heterostructure detector demonstrates a specific detectivity of 8.3 × 108 (D*). This study shows a potential application of both bulk crystal and individual flakes of CrPS4 as active components in light detection, when introduced as ink-printable moieties with a large benefit for manufacturing.

Chemical vapor transport synthesis of a selenium-based two-dimensional material

Selenium-based layered materials, and in particular transition-metal diselenides (TMDSs), have intriguing properties in the monolayer limit. Materials such as MoSe$_{2}$, WSe$_{2}$, and NbSe$_{2}$ display striking features such as spin-valley coupling at the valence-band edges and offer great potential for optoelectronics applications. Although a dozen of other TMDSs have been realized or proposed, whether two-dimensional chalcogens are possible or not is still an open challenge. In this work, we show the chemical vapor transport synthesis of a novel, atomically thin selenium-based material on oxidized silicon substrates. This new member of the two-dimensional materials family has a unique Raman spectrum similar to that of bulk selenium and has an optical gap of $\sim $1.57 eV at room temperature determined by the photoluminescence. No transition metals are found in the stoichiometry of the crystals. Analysis of high-resolution transmission electron micrographs of the monolayers reveals a distinctive set of hexagonal spots indicating a sixfold symmetry of the lattice. Atomic force microscopy measurements show the monolayer thickness to be $\sim $0.75 nm.

Exfoliation of ultrathin FePS3 layers as a promising electrocatalyst for the oxygen evolution reaction.

Few-layer ternary FePS3 nanosheets, prepared via chemical vapor transport synthesis and ball-milling exfoliation, exhibit excellent electrocatalytic performance for the oxygen evolution reaction in an alkaline medium. Combined with first principles calculations, our X-ray spectroscopy and HRTEM results clearly reveal that the introduction of in-plane defects in FePS3 layers after exfoliation and formation of a FePS3-FeOOH heterostructure during the OER process largely contribute to the catalytic activity enhancement.

Chemical vapor transport and solid-state exchange synthesis of new copper selenite bromides

A new dimorphic copper selenite bromide, Cu5(SeO3)4Br2 was obtained via chemical transport reactions. α-Cu5(SeO3)4Br2, monoclinic (1m) and β-Cu5(SeO3)4Br2, triclinic (1a) polymorphs were produced simultaneously upon reaction of amorphous, partially dehydrated copper selenite and copper bromide. 1m is similar to Cu5(SeO3)4Cl2, whereas 1a is distantly related to Ni5(SeO3)4Br2 and Co5(SeO3)4Br2. Attempts to reproduce synthesis of 1a via exchange reaction between Na2SeO3 and CuBr2 resulted in a new Na2[Cu7O2](SeO3)4Br4 (2). Current study demonstrates for the first time, that both chemical vapor and exchange reactions can be employed in preparation of new selenite halides.

Spectral-Kinetic Characteristics of ZnS Phosphors Obtained Using the Method of Vapor Transport Synthesis in a Closed System

Samples of crystalline ZnS phosphors are obtained using the method of vapor transport synthesis in a closed system. Employing X-ray phase and structural analysis, two types of the resulting sample structures are revealed. The structural composition of 70% sphalerite+30% wurtzite is obtained under the oxygen excess conditions, whereas a nearly pure wurtzite modification is produced in the deficiency of oxygen. The spectral and kinetic characteristics of the two types of samples with the photoluminescence peaks at 510 and 630 nm are studied. These are attributed to the photoluminescence mechanisms involving self-activated oxygen centers and donor-acceptor pairs. The biexponential form of the photoluminescence decay in the two types of samples is observed, related to the presence of electron capture traps near the level of interstitial zinc. The proposed method of vapor transport synthesis of ZnS in a closed system allows simple monitoring of the thermodynamic parameters of the system and provides chemical stability to the initial and final synthesis products.

Direct vapor transport synthesis of ZnGa2O4 nanowires with superior photocatalytic activity

ZnGa2O4 nanowires have been synthesized by a direct vapor transport process on c-sapphire substrate without any template under low temperature. Their structural and optical properties were characterized by field-emission scanning electron microscopy (FE-SEM), energy-dispersive spectroscopy (EDS) X-ray diffraction (XRD), high resolution transmission electron microscopy (HR-TEM), and photoluminescence (PL). Structural characterization revealed that the as-synthesized samples consist of cubic spinel ZnGa2O4 nanowires with 30–90nm in diameter and 3–5μm long. Room temperature photoluminescence show intense ultraviolet (375nm) and green (530nm) emissions originate from oxygen vacancies. Specifically, a near-band-edge emission centered at 283nm was observed. The photocatalytic activity of ZnGa2O4 nanowires was evaluated by the degradation of methyl blue (MB) under ultraviolet light illumination. The results show that ZnGa2O4 nanowires exhibit high photocatalytic activity and is attributed to the enhanced light absorption and strong redox ability of photo-generated electron–hole pairs.

Electrochromic Property of MoO3 Thin Films Deposited by Chemical Vapor Transport Synthesis

The transmittance of electrochromic MoO3 thin films by chemical vapor transport (CVT) deposition and post-annealing on indium tin oxide (ITO) glass reached 80% with low reflectivity. Optical analysis demonstrated a 3.60 eV band gap energy in MoO3 thin film. Transmittance changes of 50% between coloration and decoloration (∼30 and ∼80%) at 533 nm under the bias change frequency revealed reversible electrochromic properties and stability. A coloration efficiency of the annealed MoO3 thin film was 23.7 cm2/C. Coloration responsibility was predominant with reliable performances by bias change.