Single-step Chemical Vapor Deposition Research Articles

Broadband tunable resonance modes from multi-composition monolayer MoS2(1−x)Se2x with SiO2 microsphere cavity

Two-dimensional (2D) monolayer transition metal dichalcogenides (TMDCs) that are compatible with Si-based substrates have already exhibited huge application potential in optoelectronics and photonics. The MoS2(1−x)Se2x ternary alloy consisting of two different chalcogens, as a class of lasing gain medium, enriches the family of 2D TMDC materials. Here, monolayer MoS2(1−x)Se2x ternary alloys with tunable composition have been synthesized via single-step chemical vapor deposition method. Raman and photoluminescence studies demonstrate that the bandgap of grown monolayer MoS2(1−x)Se2x alloys can be gradually tuned from 1.59 to 1.82 eV, indicating the continuous changes of the chemical composition x from 0.82 to 0. The oscillation characteristic is further investigated, where the MoS2(1−x)Se2x alloy provides optical gain for the SiO2 microsphere resonant cavity. The achieved resonance modes in a broadband range from 610 to 810 nm not only extend the range of potential TMDC-based lasers, but also drive the applications of alloy materials in various optoelectronics devices.

Single‐Step Chemical Vapor Deposition of Methyl Ammonium Lead Halide Perovskite for p–i‐n Solar Cells

AbstractMetal halide perovskite solar cells being one of the fastest emerging technologies for renewable energy still has to become more industry friendly in a way that will allow using it for thin film modules or tandems with conventional silicon devices. The simplest way to achieve this is to use chemical vapor deposition (CVD) technique for tandem production. In this work, we show a method for a single step production of MAPbI3 films in a simple two‐zone CVD reactor from lead diacetate and methyl‐ammonium iodide powders. Obtained films show highly ordered cubic MAPbI3 phase with a thickness of 400–500 nm, good photoluminescence response and absorption band edge similar to spin‐coated film. We used those films to produce p‐i‐n solar cells with ITO/NiO/MAPbI3/C60/Cu structure. The best cell showed negligible 0.23 % PCE right after the manufacturing but significantly improved to 5.5 % PCE after 8 hours of storage in the dark. The main limiting factor affecting the efficiency is low current density, which we attribute to non‐optimized growth conditions; however, our approach is a first step to a single step CVD deposition of MAPbI3 on any type of substrates including texturized silicon subcells for better overall efficiency

Quaternary CsPbX3 (X = Cl1−xBrx, Br1−xIx) alloy microplates synthesized by single-step chemical vapor deposition and their two-photon absorption (TPA) properties

Because of their well-defined light–matter interaction volume, high-quality single-crystalline nature, and precise bandgap tunability, all-inorganic cesium lead halide (CsPbX3 (X = Cl, Br, I)) perovskite (IHP) microplates are of fundamental and technological interest today.

Annealing Enhanced Phase Transition in VO2 Thin Films Deposited on Glass Substrates via Chemical Vapor Deposition

In this work, VO2 is deposited on borosilicate glass substrates via a single step Chemical Vapor Deposition (CVD). Though the structural characterization revealed the presence of phase pure polycrystalline VO2 in simple monoclinic crystal structure (P21/c), electrical measurements did not show a sharp transition. To understand the ambiguity, surface characterization was performed with X-ray Photoelectron Spectroscopy (XPS) on the as-deposited VO2 thin films. Vanadium was present in V3+, V4+ and V5+ states. Following these measurements, annealing was performed in ambience at 200 °C for 10 minutes. Subsequent electrical measurements showed an improved phase transition, enhanced by an order of magnitude. We attribute the enhancement in transition to annealing of oxygen vacancies present in VO2 thin films because of the deposition conditions. Decrease in oxygen vacancies was correlated with a decrease in the fraction of V3+ from XPS measurements. The sample annealed at 200°C for 10 minutes exhibited a higher resistance ratio (ΔA = 305) with an optimum hysteresis (4°C) and transition width(12°C), whereas the as-deposited sample exhibited a resistance ratio of ΔA = 32. The thermal treatment affected the characteristics of the transition. A narrow hysteresis of 4°C and a sharp transition width with a reasonable resistance ratio were obtained for the sample annealed at 200°C for 10 minutes. From this study, we understand that our synthesis of VO2 thin films via CVD is prone to be off-stoichiometric and therefore, contains defects such as oxygen vacancies. We could mitigate the effect of these vacancies and engineer the stoichiometry by performing a simple annealing which in turn improved the strength of the phase transition. Drawbacks such as a poor-quality transition, resulting from synthesis parameters can be mitigated if we understand the effect of vanadium and oxygen vacancies.

One-step chemically vapor deposited hybrid 1T-MoS2/2H-MoS2 heterostructures towards methylene blue photodegradation

The photocatalytic degradation of methylene blue is a straightforward and cost-effective solution for water decontamination. Although many materials have been reported so far for this purpose, the proposed solutions inflicted high fabrication costs and low efficiencies. Here, we report on the synthesis of tetragonal (1T) and hexagonal (2H) mixed molybdenum disulfide (MoS2) heterostructures for an improved photocatalytic degradation efficiency by means of a single-step chemical vapor deposition (CVD) technique. We demonstrate that the 1T-MoS2/2H-MoS2 heterostructures exhibited a narrow bandgap ∼ 1.7 eV, and a very low reflectance (<5%) under visible-light, owing to their particular vertical micro-flower-like structure. We exfoliated the CVD-synthesised 1T-MoS2/2H-MoS2 films to assess their photodegradation properties towards the standard methylene blue dye. Our results showed that the photo-degradation rate-constant of the 1T-MoS2/2H-MoS2 heterostructures is much greater under UV excitation (i.e., 12.5 × 10−3 min−1) than under visible light illumination (i.e., 9.2 × 10−3 min−1). Our findings suggested that the intermixing of the conductive 1T-MoS2 with the semi-conducting 2H-MoS2 phases favors the photogeneration of electron-hole pairs. More importantly, it promotes a higher efficient charge transfer, which accelerates the methylene blue photodegradation process.

Large-area epitaxial growth of 2D ZrS2(1−x)Se2x semiconductor alloys with fully tunable compositions and bandgaps for optoelectronics

Bandgap engineering of transition metal dichalcogenides (TMDs) is significant for broadening their applications in electronics and optoelectronics devices. Herein, we report the new epitaxial growth of large-area uniform ZrS2(1−x)Se2x alloy films with fully tunable composition on sapphire substrates via a facile single-step chemical vapor deposition method. The ZrS2(1−x)Se2x alloys exhibit good single crystallinity and epitaxial quality, as well as uniform elemental distribution, and the epitaxial relationship with the substrate is determined to be ZrS2(1−x)Se2x (0001)[10-10]∥sapphire (0001)[11-20]. The bandgap of ZrS2(1−x)Se2x alloy exhibits a pronounced bowing behavior with continuously tunable bandgaps from 1.86 to 1.15 eV, depending on the Se composition. The ZrS2(1−x)Se2x-based photodetectors demonstrate a sensitive photoresponse to visible light with a fast response time of ∼100 µs, and their performances are significantly improved as the Se composition decreases. This work provides an efficient way to synthesize ZrS2(1−x)Se2x alloys with fully tunable bandgaps, providing great flexibility in designing TMD-based optoelectronic devices.

VS2 wrapped Si nanowires as core-shell heterostructure photocathode for highly efficient photoelectrochemical water reduction performance.

Interfacing an electrocatalyst with photoactive semiconductor surfaces is an emerging strategy to enhance the photocathode performance for the solar water reduction reaction. Herein, a core-shell heterostructure photocathode consisting of vanadium disulfide (VS2) as a 2D layered electrocatalyst directly deposited on silicon nanowire (Si NWs) surface is realized via single-step chemical vapor deposition towards efficient hydrogen evolution under solar irradiation. In an electrochemical study, 2D VS2/Si NWs photocathode exhibits a saturated photocurrent density (17mAcm-2) with a maximal photoconversion efficiency of 10.8% at -0.53V vs. RHE in neutral electrolyte condition (pH∼7). Under stimulated irradiation, the heterostructure photocathode produces a hydrogen gas evolution around 23μmolcm-2 h-1 (at 0V vs. RHE). Further, electrochemical impedance spectroscopy (EIS) analysis reveals that the high performance of the core-shell photocathode is associated with the generation of the high density of electron-hole pairs and the separation of photocarriers with an extended lifetime. Density functional theory calculations substantiate that core-shell photocathodes are active at very low Gibbs free energy (ΔGH*) with abundant hydrogen evolution reaction (HER) active sulphur sites. The charge density difference plot with Bader analysis of heterostructure reveals the accumulation of electrons on the sulphur sites via modulating the electronic band structure near the interface. Thus, facilitates the barrier-free charge transport owing to the synergistic effect of Si NWs@2D-VS2 core-shell hybrid photocatalyst for enhanced solar water reduction performance.

Vanadium-Doped Molybdenum Diselenide Atomic Layers with Room-Temperature Ferromagnetism.

Diluted two-dimensional magnetic semiconductors with high Curie temperature are highly sought after because of their potential applications in spintronics. Development of new techniques for preparation of high quality diluted magnetic semiconductors is critical for their applications. In this study, vanadium-doped molybdenum selenide, a new diluted magnetic semiconductor, was synthesized by a single-step chemical vapor deposition method. The merit of this method is that the molybdenum and vanadium precursors can be supplied to the growth substrate uniformly. Photoluminescence measurements reveal that the band gap of MoSe2 decreases after doping, which can be attributed to the formation of impurity energy band caused by p-type doping at the valence band maximum. Thus, the V-doped MoSe2 still maintains the semiconducting characteristics. Vibrating sample magnetometer studies clearly show the ferromagnetism of V-doped MoSe2 at room temperature. DFT calculations illustrates the joint contribution of V dopants and nearby atoms to the magnetic moments. This study provides future prospects for the multifunctional application of two-dimensional materials.

Tailoring Wettability Properties of GaN Epitaxial Layers through Surface Porosity Induced during CVD Deposition.

Porous GaN epitaxial layers were prepared using single-step chemical vapor deposition (CVD) through the direct reaction of ammonia with gallium. The degree of porosity and pore diameters in the resulting GaN were analyzed by means of SEM and AFM and were found to depend on the GaN deposition time. Furthermore, the evolution of the contact angle of a droplet of water located on the surface of these GaN epitaxial layers with the deposition time was investigated. We observe a transition from the hydrophilic regime to the hydrophobic regime for deposition times longer than 15 min. The observed dependence of GaN hydrophobicity on its degree of porosity is discussed and explained in the framework of the Cassie–Baxter model.

Single-step chemical vapour deposition of anti-pyramid MoS2/WS2 vertical heterostructures.

Van der Waals heterostructures are the fundamental building blocks of electronic and optoelectronic devices. Here we report that, through a single-step chemical vapour deposition (CVD) process, high-quality vertical bilayer MoS2/WS2 heterostructures with a grain size up to ∼60 μm can be synthesized from molten salt precursors, Na2MoO4 and Na2WO4. Instead of normal pyramid vertical heterostructures grown by CVD, this method synthesizes an anti-pyramid MoS2/WS2 structure, which is characterized by Raman, photoluminescence and second harmonic generation microscopy. Our facile CVD strategy for synthesizing anti-pyramid structures unveils a new synthesis route for the products of two-dimensional heterostructures and their devices for application.

Outcoupling efficiency enhancement of a bottom-emitting OLED with a visible parylene film

We have investigated an effective and a single-step chemical vapor deposition (CVD) method to achieve conformal visible poly-dichloro-para-xylylene (parylene C) film for light extraction enhancement in bottom-emitting organic light-emitting diodes (OLEDs) at room temperature. We report that sublimed parylene dimers pyrolyzed between 400 °C and 500 °C resulted in visible parylene films with tunable transmittance and haze, exhibiting light scattering properties due to the formation of uniformly distributed dimer crystals. We achieved a novel conformal visible parylene film with total transmittance and high haze of 79.5% and 93.6%, respectively. It is observed that the outcoupling efficiency of the OLEDs employing the visible parylene film is enhanced up to 45.8%. Additionally, the OLED with the visible parylene light extraction film shows limited angle-dependency of emission spectrum over viewing angles. The single-step room temperature fabrication process of this conformal outcoupling film paves the way to achieving commercial high-performance OLEDs.

Fused Metalloporphyrin Thin Film with Tunable Porosity via Chemical Vapor Deposition.

Porous and highly conjugated multiply fused porphyrin thin films are prepared from a fast and single-step chemical vapor deposition approach. While the solution-based coupling of porphyrins is usually undertaken at room temperature, the gas phase reaction of nickel(II) 5,15-(diphenyl)porphyrin and iron(III) chloride (FeCl3) is investigated for temperatures as high as 200 °C. Helium ion and atomic force microscopy, supported by weight and thickness measurements, shows a drastic decrease of the fused porphyrin thin film’s density accompanied by the formation of a mesoporous morphology upon increase of the reaction temperature. The increase of the film’s porosity is attributed to formation of a greater amount of HCl (originated from both the oxidative coupling and chlorination reactions) and the release of gaseous FeCl3 byproducts, i.e., Cl2, at higher deposition temperatures. In addition, high resolution mass spectrometry reveals that increase of the reaction temperature promotes a higher degree of conjugation of the fused porphyrins chains, which ensures that high electronic conductivities are maintained along with high porosity. The method reported herein could enable the engineering of fused porphyrin thin films in sensing and catalytic devices.

IR photoresponsive VO2thin films and electrically assisted transition prepared by single-step chemical vapor deposition

The infrared photodetector fabricated with VO2thin films showed a good stable photoresponse with a field-assisted thermally induced phase transition.

Facile Single-Step Fabrication of Robust Superhydrophobic Carbon Nanotube Films on Different Porous Supports

Superhydrophobic carbon nanotube (CNT) films have attracted extensive research attention because of the superior CNT properties that have a significant impact on the mass/energy transfer at the CNT/liquid interface. Until now, most reported methods for producing superhydrophobic CNT films require multiple steps, making the fabrication process very complicated and time-consuming, and may deteriorate the CNT intrinsic properties. In this study, superhydrophobic CNT films composed of short CNT strands were fabricated through a single-step chemical vapor deposition method on various types of supports, including hollow fiber and disc supports. Attributed to its firm structure, this durable superhydrophobic film could withstand water invasion and show promising stability and renewability during a 4-week wettability test. Benefiting from the preservation of CNT properties without post modifications, the film exhibited excellent chemical robustness to corrosive liquids from pH = 0–14. In addition, the application...

Electronically-Coupled Phase Boundaries in α-Fe2O3/Fe3O4 Nanocomposite Photoanodes for Enhanced Water Oxidation

Photoelectrochemical (PEC) water splitting reactions are promising for sustainable hydrogen production from renewable sources. We report here, the preparation of α-Fe2O3/Fe3O4 composite films via a single-step chemical vapor deposition of [Fe(OtBu)3]2 and their use as efficient photoanode materials in PEC setups. Film thickness and phase segregation was controlled by varying the deposition time and corroborated through cross-section Raman spectroscopy and scanning electron microscopy. The highest water oxidation activity (0.48 mA/cm2 at 1.23 V vs RHE) using intermittent AM 1.5 G (100 mW/cm2) standard illumination was found for hybrid films with a thickness of 11 μm. This phenomenon is attributed to an improved electron transport resulting from a higher magnetite content toward the substrate interface and an increased light absorption due to the hematite layer mainly located at the top surface of the film. The observed high efficiency of α-Fe2O3/Fe3O4 nanocomposite photoanodes is attributed to the close pr...

Single step synthesis of Schottky-like hybrid graphene - titania interfaces for efficient photocatalysis

The development of 2D nanomaterial coatings across metal surfaces is a challenge due to the mismatch between the metal microstructure and the nanoscale materials. The naturally occurring thin oxidative layer present across all metal surfaces, may lead to low adherence and connectivity. In this paper, graphene/titania/Titanium hybrid films were for the first time fabricated by a single step chemical vapour deposition process across Titanium foils. The presence of graphene as a dopant was found to enhance the photocatalytic performance of the final products, applied to the degradation of organic molecules and to lead to Schottky-like junction formation at the metal/oxide interface. These Schottky junctions, where vacancies are present across the titania material due to the graphene doping and where Ti3+ ions are predominantly located, yield enhanced catalytic performance. The highest degradation rate was found to be 9.66 × 10−6 min−1, achieved by the sample grown at 700 °C for 5 min, which was 62% higher than the sample just treated at that temperature without graphene growth. This work provides evidence that graphene may be grown across pure Titanium metal and opens new avenues in biomedical devices design, tribological or separation applications.

Stacking-mode confined growth of 2H-MoTe2/MoS2 bilayer heterostructures for UV–vis–IR photodetectors

The atomic thin, vertically-stacked 2H-MoTe2/MoS2 heterostructures are successfully synthesized using the single step chemical vapor deposition (CVD) method and a magnet-assisted secondary precursor delivery tool. The second material (MoTe2) was grown in a well-controlled, unique and epitaxial 2H-stacking mode atop the first material (MoS2), starting from the edges. This led to the construction of a vertical p-n junction with a broadband photoresponse from the ultraviolet (UV, 200 nm) to the near-infrared (IR, 1100 nm) regions. The high crystallinity of MoTe2/MoS2 heterostructures with a modulation of sulfur and tellurium distribution is corroborated by multiple characterization methods, including Raman spectroscopy, photoluminescence (PL) spectroscopy and high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM). Furthermore, the photoelectrical measurements exhibit a tremendous photoresponsivity with an external quantum efficiency (EQE) as high as 4.71 A/W and 532% at 1100 nm, while as 4.67 A/W and 1935% at 300 nm, one to two orders of magnitude higher than other exfoliated MoTe2 heterostructure devices have been reported so far. This synthetic method is a controllable stacking mode confined synthesis approach for 2D heterostructures, and paves the way for the fabrication of high-performance functional telluride-based broadband photodetectors.

Single step route to highly transparent, conductive and hazy aluminium doped zinc oxide films.

Light scattering yet transparent electrodes are important for photovoltaics as they increase device efficiency by prolonging light path lengths. Here, we present a novel single step route to highly textured Al doped ZnO thin films on glass substrates that show a minimum resistivity of ∼3 × 10−3 Ω cm and high visible light transmittance of 83% while still maintaining high haze factor of 63%. Roughness was imparted into the ZnO films during the synthetic process using acetylacetone and deionized water as additives. The highly hazy yet visible and near infrared transparent nature of the conductive ZnO:Al films allow it to be potentially used as an electrode material in amorphous and microcrystalline silicon solar cells.

Phase Segregation Behavior of Two-Dimensional Transition Metal Dichalcogenide Binary Alloys Induced by Dissimilar Substitution

Transition metal dichalcogenide (TMD) alloys form a broad class of two-dimensional (2D) layered materials with tunable bandgaps leading to interesting optoelectronic applications. In the bottom-up approach of building these atomically thin materials, atomic doping plays a crucial role. Here we demonstrate a single step CVD (chemical vapor deposition) growth procedure for obtaining binary alloys and heterostructures by tuning atomic composition. We show that a minute doping of tin during the growth phase of the Mo1–xWxS2 alloy system leads to formation of lateral and vertical heterostructure growth. High angle annular dark field scanning transmission electron microscopy (HAADF-STEM) imaging and density functional theory (DFT) calculations also support the modified stacking and growth mechanism due to the nonisomorphous Sn substitution. Our experiments demonstrate the possibility of growing heterostructures of TMD alloys whose spectral responses can be desirably tuned for various optoelectronic applications.

Graphene-ceramic hybrid nanofibers for ultrasensitive electrochemical determination of ascorbic acid

The authors describe the fabrication of a 3-dimensional interconnected edge-exposed graphene nanostructure. It was fabricated by single-step chemical vapor deposition of highly foliated graphene on a network of γ-alumina nanofibers having a single fiber diameter of around 7 nm. It is shown that a highly sensitive electrochemical sensor can be prepared for simultaneous detection of ascorbic acid, uric acid and dopamine from such graphene-coated nanofibers possessing different but controllable density of foliates. The study concentrates on the determination of ascorbic acid in a concentration range that extends from as low as 5 nM to 1 mM. The electrochemical properties of the materials were studied by cyclic voltammetry, chronoamperometry and differential pulse voltammetry using hexacyanoferrate as a redox probe. Response is linear in the 1 μM to 60 μM ascorbate concentration range, the sensitivity is 1.06 μA⋅μM−1⋅cm−2, and the detection limit is 117 nM at foliate density of 50 foliates μm−1. The sensor was successfully applied to the determination of ascorbic acid in spiked urine samples. These results demonstrate a method for controllable growth of graphene possessing tailored morphology. In our perception, the application of such graphene-decorated nanostructures paves the way to the design of ultrasensitive electrochemical sensors of high stability.