High-temperature Vapor Deposition Research Articles

Epitaxial substitution of metal iodides for low-temperature growth of two-dimensional metal chalcogenides.

The integration of various two-dimensional (2D) materials on wafers enables a more-than-Moore approach for enriching the functionalities of devices1-3. On the other hand, the additive growth of 2D materials to form heterostructures allows construction of materials with unconventional properties. Both may be achieved by materials transfer, but often suffer from mechanical damage or chemical contamination during the transfer. The direct growth of high-quality 2D materials generally requires high temperatures, hampering the additive growth or monolithic incorporation of different 2D materials. Here we report a general approach of growing crystalline 2D layers and their heterostructures at a temperature below 400 °C. Metal iodide (MI, where M = In, Cd, Cu, Co, Fe, Pb, Sn and Bi) layers are epitaxially grown on mica, MoS2 or WS2 at a low temperature, and the subsequent low-barrier-energy substitution of iodine with chalcogens enables the conversion to at least 17 different 2D crystalline metal chalcogenides. As an example, the 2D In2S3 grown on MoS2 at 280 °C exhibits high photoresponsivity comparable with that of the materials grown by conventional high-temperature vapour deposition (~700-1,000 °C). Multiple 2D materials have also been sequentially grown on the same wafer, showing a promising solution for the monolithic integration of different high-quality 2D materials.

Phase modulation using a titanium dioxide strip on lithium niobate

In this manuscript, we demonstrate the first phase modulator using titanium dioxide as a strip loaded waveguide on thin-film lithium niobate. The amorphous titanium dioxide was fabricated using RF reactive sputtering followed by a dielectric lift-off process. Two gold electrodes were deposited next to the waveguide by using the same process. This process eliminates the need for plasma etching or high-temperature vapor deposition methods, opening the door to fabrication on sensitive material systems. A propagation loss of 0.156 dB/mm and V π of 3.2 V at 1550 nm were experimentally measured and reported.

High-quality two-dimensional tellurium flakes grown by high-temperature vapor deposition

Large-area and high-quality 2D Te flakes were synthesized by high-temperature vapor deposition using In2Te3 powder as source. The thin Te flakes exhibit low electrical resistivity, high hole mobility and ultrahigh photoresponsivity in visible range.

Phase transition from Au–Te surface alloy towards tellurene-like monolayer

Two-dimensional (2D) chalcogen-based layers have proven to be the next generation of materials for potential high-tech applications, and it is very important to control their properties at the nanoscale. Herein, we discuss the structural and electronic properties of Au(111) surface after being exposed to high temperature vapor deposition of Tellurium (Te) in ultrahigh vacuum. The scenarios entailing the formation of 2D AuTe2 metal dichalcogenide or rather Au–Te alloy monolayer (ML) or even Tellurene single layer deserved to be addressed. In this purpose, low energy electron diffraction (LEED) supported by scanning tunneling microscopy (STM) shows the existence of several surface reconstructions depending on the Te film thickness in the sub-monolayer regime. We observed that the well-known spin-split Shockley state of the Au(111) surface survives the Te deposition and is even shifted to higher binding energy, suggesting a charge transfer at the interface. For a coverage of 0.33 ML of Te, new dispersive bands are observed by angle-resolved photoemission (ARPES), which arise from a strong hybridization between the electronic states of Te and Au. With a substantially low intensity and a back-folding at the boundaries of the reduced surface Brillouin zone (R-SBZ), these electronic bands represent a proof of the existence of a naturel 2D electron gas, strongly disturbed by the surface reconstruction. It is therefore possible that an Au–Te alloy is formed at the surface. By increasing the coverage to 0.5 ML, a rich, thickness-dependent transition develops from the surface alloy to Tellurene-like structure and completely excludes the growth of AuTe2 monolayer. Both the surface alloy and the Tellurene monolayer have a semiconductor character with a gap in the occupied states of about 0.65 eV.

Ionization Gas Sensor using Suspended Carbon Nanotube Beams.

An ionization sensor based on suspended carbon nanotubes (CNTs) was presented. A suspended CNT beam was fabricated by a low-temperature surface micromachining process using SU8 photoresist as the sacrificial layer. Application of a bias to the CNT beam generated very high non-linear electric fields near the tips of individual CNTs sufficient to ionize target gas molecules and initiate a breakdown current. The sensing mechanism of the CNT ionization sensor was discussed. The sensor response was tested in air, nitrogen, argon, and helium ambients. Each gas demonstrated a unique breakdown signature. Further, the sensor was tested with gaseous mixtures. The sensor exhibited good long-term stability and had comparable performance to other reported CNT-based ionization sensors in literature, which use high-temperature vapor deposition methods to grow CNTs. The sensor notably allowed for lower ionization voltages due to its reduced ionization gap size.

Effect of Hybrid Surface Modifications on Tensile Properties of Polyacrylonitrile- and Pitch-Based Carbon Fibers

Recent interest has emerged in techniques that modify the surfaces of carbon fibers, such as carbon nanotube (CNT) grafting or polymer coating. Hybridization of these surface modifications has the potential to generate highly tunable, high-performance materials. In this study, the mechanical properties of surface-modified polyacrylonitrile (PAN)-based and pitch-based carbon fibers were investigated. Single-filament tensile tests were performed for fibers modified by CNT grafting, dipped polyimide coating, high-temperature vapor deposition polymerized polyimide coating, grafting-dipping hybridization, and grafting-vapor deposition hybridization. The Weibull statistical distributions of the tensile strengths of the surface-modified PAN- and pitch-based carbon fibers were examined. All surface modifications, especially hybrid modifications, improved the tensile strengths and Weibull moduli of the carbon fibers. The results exhibited a linear relationship between the Weibull modulus and average tensile strength on a log-log scale for all surface-modified PAN- and pitch-based carbon fibers.

Tensile Properties of Polyimide Composites Incorporating Carbon Nanotubes-Grafted and Polyimide-Coated Carbon Fibers

The tensile properties and fracture behavior of polyimide composite bundles incorporating carbon nanotubes-grafted (CNT-grafted) and polyimide-coated (PI-coated) high-tensile-strength polyacrylonitrile (PAN)-based (T1000GB), and high-modulus pitch-based (K13D) carbon fibers were investigated. The CNT were grown on the surface of the carbon fibers by chemical vapor deposition. The pyromellitic dianhydride/4,4′-oxydianiline PI nanolayer coating was deposited on the surface of the carbon fiber by high-temperature vapor deposition polymerization. The results clearly demonstrate that CNT grafting and PI coating were effective for improving the Weibull modulus of T1000GB PAN-based and K13D pitch-based carbon fiber bundle composites. In addition, the average tensile strength of the PI-coated T1000GB carbon fiber bundle composites was also higher than that of the as-received carbon fiber bundle composites, while the average tensile strength of the CNT-grafted T1000GB, K13D, and the PI-coated K13D carbon fiber bundle composites was similar to that of the as-received carbon fiber bundle composites.

The effect of high-temperature vapor deposition polymerization of polyimide coating on tensile properties of polyacrylonitrile- and pitch-based carbon fibers

Carbon fibers are widely used as a reinforcement in composite materials because of their high-specific strength and modulus. Current trends toward the development of carbon fibers have been driven in two directions; ultrahigh tensile strength fiber with a fairly high strain to failure (~2 %), and ultrahigh modulus fiber with high-thermal conductivity. Today, a number of ultrahigh strength polyacrylonitrile (PAN)-based (more than 6 GPa), and ultrahigh modulus pitch-based (more than 900 GPa) carbon fibers have been commercially available. In the present work, the tensile properties of polyimide-coated PAN-based (T1000GB, T300, and M60JB) and pitch-based (K13D and XN-05) carbon fibers have been investigated using a single-filament tensile test. The pyromellitic dianhydride/4-4′-oxydianiline polyimide coating was deposited on the carbon fiber surface using high-temperature vapor deposition polymerization (VDPH). The Weibull statistical distributions of the tensile strength were characterized. The results clearly show that the VDPH polyimide coating improves the tensile strength and the Weibull modulus of PAN- and pitch-based carbon fibers.

High-temperature vapor deposition polymerization polyimide coating for elimination of surface nano-flaws in high-strength carbon fiber

The effect of polyimide coatings on the filament tensile strength of high-strength polyacrylonitrile-based carbon fiber was studied by using dip and high-temperature vapor deposition polymerization (VDP) coating processes. Unlike a VDP on a cold substrate, high-temperature VDP has the potential to directly synthesize and isotropically deposit a polyimide, from diamine and dihydride monomers without any by-products, on a substrate heated up to 200 °C. The average filament tensile strength of the flaw-sensitive carbon fiber improved with all the polyimide coatings used. Nevertheless, for the same monomers, the high-temperature VDP coating process was advantageous for high-efficiency surface flaw healing compared to the dip-coating process, resulting in a 25% increase in the average tensile strength of the carbon fiber. These results were evident not only for the carbon fibers without artificial nano-notches but also for those with artificial notches less than 30 nm in depth. Thus, we clearly showed the potential for the VDP polyimide coating to heal surface nano-flaws of the carbon fiber. The different infiltrations of the coating into nano-notches and its effect on the filament tensile properties were characterized, as well as discussing the impact of the VDP coating with an interlayer between the coating and the fiber.

Au(111)-Based Nanotemplates by Gd Alloying

A new class of nanostructured templates is obtained by submitting Au111 films to high-temperature vapor deposition of Gd in ultrahigh vacuum. In a low coverage regime, Gd atoms are embedded in the topmost Au layer, inducing a structural transformation of the herringbone reconstruction to create a network of trigons. At higher dose, the reactive deposition of Gd leads to the formation of an atomically perfect GdAu2 surface compound characterized by a long-range periodic Moire pattern. Both the trigon and Moire lattices are highly ordered nanostructures, which turned out to be robust templates to grow metal nanodots. As a test example, Co was deposited at room temperature, forming uniform dots that faithfully arrange by following the underlying trigons or Moire periodicity. For the latter, one can achieve nanodot arrays that exhibit record areal density.

The effect of a compliant polyimide nanocoating on the tensile properties of a high strength PAN-based carbon fiber

The effect of a compliant polyimide nanocoating on the tensile strength of a polyacrylonitrile-based high tensile strength (T1000GB) carbon fiber was investigated. The pyromellitic dianhydride/4-4′-oxydianiline polyimide nanocoating was deposited by high-temperature vapor deposition polymerization. The thickness of the polyimide coating was about 100 nm. The tensile strength and Weibull modulus of nanocoated and uncoated fiber bundles were evaluated using a polyimide-impregnated bundle-composite. The results clearly demonstrated that the compliant polyimide nanocoating is effective in improving the tensile strength and Weibull modulus of T1000GB carbon fiber.

Direct synthesis of straight SiO2 nanorods

The straight SiO2 nanorods with a diameter of about 200nm and smooth surface have been directly synthesized by high temperature vapor deposition method at 1300°C. The as-synthesized samples were characterized by means of scanning electron microscopy, energy dispersive x-ray, and transmission electron microscopy. The results show that as-synthesized silica nanorods have a uniform size, well-defined shape, and smooth surface. However, the morphologies and microstructures of silica nanorods are affected by synthesis conditions, such as the concentration of the SiOx and the deposition temperature. On the basis of these experimental results, a possible growth mechanism of silica nanorods in this process is proposed.

Electrical, thermal and mechanical properties of polyimide thin films prepared by high-temperature vapor deposition polymerization

Polyimide thin films were prepared on the whole surface of a substrate using a high-temperature vapor deposition polymerization method (VDP-H). In this method, pyromellitic dianhydride (PMDA) and oxydianiline (ODA), which are monomers of polyimides, are introduced into a process chamber from separate evaporation sources through inlet tubes. As the process chamber was kept at about 200 C, neither monomer could deposit on the substrate. Because the temperature of the substrate was higher than the evaporation temperature of the monomers, only polymerized films were deposited on the substrate. Infrared spectroscopy indicated that the VDP-H films were about 50% imidized. When the film thickness was less than I1 m, a uniform step coverage was observed, and above 2.5 pm a patterned surface was reduced to a planar surface. Film thickness was controlled by changing the amounts of monomers in the evaporation sources. The dielectric constant (E= 3.5), dissipation factor (tan 5 0.002), thermal characteristics (5% weight loss temperature= 551 C), and tensile strength (170 MPa) of the vDP-H films cured for 60 min at 300 C in vacuum were the same as those reported for Kapton® films. The modulus of elasticity (4.0 GPa) was larger than that of Kaptont®, and it was found that the VDP-H films showed less elongation than Kapton®.

Nucleation, growth, and structure of Al–Mn quasicrystalline thin films prepared by high-temperature vapor deposition

The nucleation and growth of quasicrystalline thin films during sequential vapor deposition of aluminum and manganese on various substrates have been studied at temperatures between 530 and 650 K. The films were analyzed by transmission electron microscopy, electron diffraction, energy dispersive x-ray analysis, replica techniques, and Auger depth profiling. The quasicrystalline phase is identified as icosahedral. It nucleates on the surfaces of the Al films. There is no indication of substantial bulk Mn diffusion. The growth process is governed by diffusion of Al to the quasicrystal surface where it reacts with the incident Mn.

Nucleation and growth of icosahedral quasicrystalline thin films during high-temperature vapour deposition

Nucleation and growth of icosahedral quasicrystalline thin films during high-temperature vapour deposition

Preparation of aluminium based icosahedral thin films by high-temperature vapour deposition

Preparation of aluminium based icosahedral thin films by high-temperature vapour deposition

Growth of Stannic Oxide Crystals from the Vapor Phase

Single crystals of stannic oxide up to 30×4×2 mm have been prepared by a high temperature vapor deposition technique. Using high purity SnO2 powder, crystallization and growth were studied under both neutral and oxidizing conditions. Neutral carrier gases, with the exception of helium, encouraged the formation of voids and inclusions. However, the use of a helium-oxygen system was found to eliminate these defects. An explanation is offered for this behavior. Three distinct variations in the crystalline habit of SnO2 have been observed, apparently dependent upon the temperature at which growth occurred.