Supercritical Fluid Deposition Research Articles

Growth and Characterization of Carbon Nanopearls on a Si Substrate by CVD Process

Carbon nanopearls have been found to form on a Si substrate when grown at 850°C, using the chemical vapor deposition process (CVD). An acetylene carbon source and a Ni catalyst are the main ingredients for this procedure. The nickel was prepared and deposited on the substrate in two ways: (1) sonication and dispersion in a methanol solution, and (2) reduction of size via a reverse-micelle technique with a supercritical fluid deposition technique. The resulting carbon nanopearls were characterized using scanning electron microscopy (SEM), atomic force microscopy (AFM), and spectroscopic ellipsometry (SE). These tools enabled us to observe the structure, size, and absorption. Characterization of the carbon nanopearls is imperative for understanding of the structure and properties of these nanomaterials.

Preparation, Characterization and Catalysis Properties of Ag/SBA-15 Nanocomposite by Supercritical Fluid Deposition

Ag/SBA-15 nanocomposite was prepared by supercritical fluid deposition(SCFD) method,with AgNO3 as the precursor and ethanol or glycol as the cosolvent.AgNO3 was firstly deposited onto the SBA-15 support at 50℃,23-25MPa,and reacted for 3-24h.The resultant composite was then subjected to calcination and reduction treatment for obtaining Ag/SBA-15 nanocomposite.Characterizations with XRD and TEM show that Ag nanoparticles(with sizes of 3-7nm) or nanowires(5-9nm in width and tens of nanometers to several micrometers in length) are formed in the nanochannels of SBA-15 support.It is demonstrated that SCFD is a green and effective method for preparation of nanocomposite.With the aid of cosolvent,the supercritical carbon dioxide can dissolve the inorganic metallic salts.Moreover,the morphologies of the nanocomposite can be controlled to some extent by selecting the deposition conditions.A catalytic test on the Ag/SBA-15 nanocomposite shows that the sample has a moderate activity,with the complete conversion of CO at 300℃.

Supercritical Fluid Deposition of Conformal SrTiO[sub 3] Films with Composition Uniformity in Nanocontact Holes

SrTiO 3 with a high dielectric constant is considered a promising capacitor dielectric for dynamic random access memory. Until now, a SrTiO 3 deposition with compositional uniformity and perfect conformality inside high-aspect nanoscale holes has been difficult by chemical vapor deposition (CVD) and atomic layer deposition (ALD). In this study, we report a supercritical fluid deposition (SCFD) of SrTiO 3 . Compared to CVD SrTiO 3 , the SCFD SrTiO 3 showed perfect conformality and compositional uniformity inside nanosize holes. From these results, the SCFD is expected to be a solution for the stoichiometry and conformality issues of the CVD and ALD SrTiO 3 .

Covalent Attachment of Monochlorosilanes to Mesoporous Silica Membranes using Supercritical Fluid Deposition

We have prepared and characterized organically-modified mesoporous silica membranes using supercritical CO2 fluid deposition. Supercritical CO2 fluid deposition has the potential to increase both diffusion and concentration of the reactants at the reaction site, a particular advantage when the reaction site is inside a small pore, where access can be limited. The unmodified mesoporous silica membranes were prepared by surfactant-templated synthesis on α-alumina disk-shaped supports. These mesoporous silica supports had a mean pore diameter of 5 nm. The alkyl modification of the silica membranes was achieved by the covalent attachment of octadecyldimethylchlorosilane (ODMCS) using triethylamine (TEA) as a catalyst in supercritical-CO2. Successful attachment was confirmed through infrared spectroscopic identification of the removal of the band due to isolated silanols accompanied by the appearance of bands due to methyl and methylene groups. We also directly measured a ∼90% reduction in permeance of light gases through the modified membrane. The synthesis process did not degrade the support membrane, was relatively efficient, and was free of organic solvents required for traditional synthesis. The dominant transport mode in the membranes was Knudsen flow. Ideal separation factors for methane and propane over nitrogen were not increased relative to the bare support, indicating that neither the surface flow was enhanced nor the size selectivity improved for the membrane system studied. Supercritical CO2 fluid deposition was demonstrated as an effective technique for the reaction of the functionalized silanes on the surface of the silica membrane.

Preparation of metallic supported nanoparticles and films using supercritical fluid deposition

Supercritical fluid deposition is a promising technique to prepare supported metallic nanoparticles or metallic films. The technique was examined under three main categories which are dissolutioning of the metallic precursor in the SCF phase, adsorption or sorption of the metallic precursor on the substrate and reduction of the metallic precursor to its metal form. The fundamental aspects associated with each step were highlighted and critical perspectives on current status as well as suggestions for future research topics were provided. The challenges that need to be overcome were stated throughout the manuscript.

Effect of liquid additives in supercritical fluid deposition of copper for enhancing deposition chemistry

Four liquid chemical reagents (ethanol, 2-propanol, hydrogen peroxide, and formic acid) were evaluated as reducing agents for supercritical fluid deposition of Cu (Cu-SCFD) using bis(2,2,6,6-tetramethyl-3,5-heptanedionato)copper as a precursor onto a Ru-coated Si substrate for ultra large scale integration metallization. Deposition was carried out using a batch-type reactor and heating the substrate from 40 °C to 250 °C at a rate of 4.5 °C/min. 2-propanol and hydrogen peroxide yielded Cu grains, but not a continuous Cu film. Only formic acid yielded a high-purity continuous film. Solvent effect of two chemical reagents (ethanol and 2-propanol) on Cu-SCFD with H 2 as a reducing agent was also investigated by using in situ method, in which optical reflectivity of growing surface at wavelength of 770 nm was used. Ethanol addition enhanced the deposition reaction without degrading both the surface morphology and high purity of the deposited Cu film.

Water-in-Supercritical CO2Microemulsion for Synthesis of Carbon-Nanotube-Supported Pt Electrocatalyst for the Oxygen Reduction Reaction

Four electrocatalysts, including a commercial Pt on carbon black (Pt-CB), were compared for performance in the reduction of oxygen. Three of the catalysts were prepared on the basis of the deposition of Pt onto carbon nanotubes using (i) water-in-supercritical CO2 microemulsion (Pt-CNT SCME), (ii) direct supercritical CO2 fluid deposition (Pt-CNT SC), and (iii) water-in-hexane microemulsion (Pt-CNT ME). Cyclic voltammetric studies yielded an electrochemically active surface area for Pt-CNT SCME at 31.1 m2/g, which was the largest among all electrocatalysts tested in this work. Hydrodynamic polarization curves for oxygen reduction exhibited that the cell potential of the Pt-CNT SCME catalyst was over 350 mV more positive than the commercial Pt-CB system at 10 A/g of Pt. In chronoamperometric analyses, Pt-CNT SCME catalyst (6.6 × 103 A/g of Pt) generated 2.5 times more specific activity at 30 s than Pt-CNT ME (2.6 × 103 A/g of Pt) and 5 times more than the commercial Pt-CB (1.3 × 103 A/g of Pt). Tafel analy...

In situ Observation of Initial Nucleation and Growth Processes in Supercritical Fluid Deposition of Copper

The initial nucleation and coalescence of Cu by supercritical fluid deposition (SCFD) were monitored by measuring the surface reflectivity of visible white light. The reflectivity at 770 nm is sensitive to initial nucleation, thus, the nucleation and coalescence temperatures of Cu-SCFD can be easily monitored by this in situ technique. The nucleation temperature of Cu-SCFD was found to be independent of the precursor concentration, which suggests a strong adsorption and surface saturation of the source precursor at high concentration. A high H2 concentration up to 0.39 mol/L with Cu(tmhd)2 as a precursor can decrease the nucleation temperature from 215 to 180 °C. A high H2 concentration is also effective for realizing a smooth surface morphology of the deposited Cu film and for making the film thin at the coalescence stage probably because of the initial nucleation with a high number density. The fabrication of a 10-nm-thick continuous Cu film, which is required as a seed layer in ultralarge scale integration (ULSI), was successfully demonstrated with a high H2 concentration of 0.39 mol/L.

Acetone-assisted deposition of silver films in supercritical carbon dioxide

Growth of silver films was studied by supercritical fluid deposition (SCFD) using H 2 and acetone as reducing agents for (1,5-cyclooctadiene)(hexafluoroacetylacetonato)silver( I) in supercritical CO 2 (scCO 2). H 2 reduction did not yield continuous Ag films, whereas continuous films were deposited on Ru substrates by acetone-assisted reduction of 0.006–0.03 mol% precursor in the temperature range of 150–250 °C. Surface qualities of the Ag films were effectively improved by decreasing water content in acetone reagent, as well as reducing acetone and precursor concentrations in scCO 2. Ultimately, a 50 nm-thick film with shiny surface was obtained in optimized conditions. A possible mechanism for acetone reduction of Ag precursor on Ru surface was also proposed.

Cu films deposition by dielectric barrier discharge in supercritical CO 2, Ar and Xe environments

The high-rate deposition of Cu films was performed by generating plasmas in Cu precursor-dissolved supercritical CO 2, Ar and Xe environments. The maximum deposition rate was 500 nm/min, which is more than 10 times higher than that achieved by conventional supercritical fluid deposition. However, in the case of supercritical CO 2, more than 80 at.% contamination was observed in the films. On the other hand, by employing supercritical noble gases (Ar and Xe), the maximum Cu content increased to 83 at.%. This is attributed to the fact that in the case of supercritical noble gases, there is no plasma induced dissociation of the medium. Moreover, the maximum Cu content of the films was at an applied frequency of 10 kHz, which is considered to be the frequency resulting in the optimal balance of plasma and surface reaction rates. In addition, the film morphology underwent a transition from smooth to rough by increasing the frequency from 3 to 20 kHz.

Oriented Growth of Metal and Semiconductor Nanostructures within Aligned Mesoporous Channels

In the present work, we show how different types of inclusion chemistry can be used to generate oriented, high-aspect-ratio metal and semiconductor nanowires in insulating silica host structures prepared within anodic alumina membranes (AAMs). The structural features of the Pluronic123-templated silica filaments in the AAMs with their intriguing columnar and circular arrangement of mesopores allow for the inclusion of a variety of aligned 1D nanostructures ranging from metallic (Pt, Au, and Pd) and semiconductor (Ge) to carbon nanotubes and filaments. The synthetic techniques include wet chemical impregnation and reduction in precalcined mesopores, impregnation of surfactant-containing mesopore systems, and mass transport via supercritical fluid deposition in surfactant-containing mesopores. Important issues such as the crystallinity and continuity of the encapsulated wires as a function of material and deposition technique have been discussed.

Ultra Thin Cu film Fabrication by Supercritical Fluid Deposition for ULSI Metallization

AbstractInitial nucleation and succeeding coalescence in supercritical fluid deposition (SCFD) of Cu was studied in situ by using our devised monitoring technique, i.e. surface reflectivity measurement of visible white light. Fabrication of 10nm-thick smooth and continuous Cu film required by ULSI metallization, was succeeded. Complete filling without any seems and voids into various via patterns of 50 - 200 nm in diameter and 1 μm in depth was also achieved, which was revealed through angled polishing of patterned substrate.

Size dependent thermal properties of embedded crystalline germanium nanowires

Here we report the size-dependent melting points of crystalline germanium nanowires confined within the pores of hexagonal mesoporous silica templates. A supercritical fluid deposition technique was used to form the nanowire–template composite materials and differential thermal analysis, coupled to thermal gravimetric analysis, was used to determine the melting points of the embedded Ge nanowires with mean diameters ranging from 22 to 85 A. The melting points of the Ge nanowires within the templates were found to be higher than the melting point of bulk germanium (937 °C), typically by 60 °C, and with a broad melting range (∼80 °C). Extended X-ray absorption fine structure (EXAFS) analysis of the Ge K-edge from the nanocomposite materials revealed a linear increase in the Ge–Ge nearest neighbour distance with decreasing nanowire diameter over the size range investigated. In all cases the Ge–Ge first shell distance in the nanowires was greater than in the bulk. This observed lattice expansion can be attributed to crystallographic deformation resulting from the strain imposed on the nanowires by the template. EXAFS studies also revealed an increase in the average number of oxygen atoms at the Ge/silica interface with decreasing diameter due to the increasing surface area. Interfacial effects and interactions at the nanowires/matrix interface are believed to delay the melting point of these systems.

Recyclable and Ligandless Suzuki Coupling Catalyzed by Carbon Nanotube‐Supported Palladium Nanoparticles Synthesized in Supercritical Fluid

Carbon nanotube–supported palladium nanoparticles prepared by a supercritical fluid deposition method show high activities for catalyzing Suzuki coupling reactions, and the catalysts can be recycled and reused at least six times without losing activity.

Opto-Electronic Properties and Stability of Artificial Zinc Oxide Molecules

ABSTRACTThe Hartree-Fock (HF), restricted open shell HF (ROHF), and multiconfiguration self-consistent field (CI/CASSCF/MCSCF) approximations are used to study computationally the electronic properties of zinc oxide artificial molecules whose structure and composition have been derived from those of the symmetry elements of the wurtzite bulk lattice of zinc oxide. Such molecules may provide realistic models for small ZnO quantum dots (QDs) synthesized in “vacuum” or quantum confinement (such as that of well-defined nanopore arrays of silica and alumina membranes) using variety of methods in particular, supercritical fluid deposition. The computational direct optical transition energy (OTE) of the confined molecule appears to be several times smaller than that of the corresponding vacuum cluster. The charge and spin density distributions of these molecules (CDDs and SDDs, respectively) differ significantly, revealing dramatic effects of quantum confinement on electronic properties of Zn-O clusters. The obtained results suggest that manipulations with the electronic properties of the confined clusters by sophisticated design of their quantum confinement may provide means for synthesis of Zn-O – based electronic materials that combine a wide, tunable band gap with large, tunable exciton binding energy.

Novel Reducing Chemistry for Supercritical Fluid Deposition of Copper

AbstractWe study new deposition chemistry in Cu-SCFD (supercritical fluid deposition), especially effect of acetone is investigated as reducing agent and additives into H2 reducing Cu-SCFD. Acetone reduction yields Cu film deposition onto Ru coated Si substrate, because Ru has catalytic effect to generate reducing agent from acetone. When acetone is added into H2 reducing chemistry, the solubility of precursor is enhanced approximately 50 %, which is so-called “entrainer effect”. Ethanol can enhance the deposition of Cu in H2 reduction chemistry, which is a solvent effect. Two effects of entrainer effect by acetone and solvent effect by ethanol works independently, without interfering each other.

Large Pore Methylene-Bridged Periodic Mesoporous Organosilicas: Synthesis, Bifunctionalization and Their Use as Nanotemplates

Large pore, methylene-bridged, periodic mesoporous organosilicas (pmo-me's) have been synthesized using the nonionic surfactant p123 (eo(20)po(70)eo(20)) as a structure-directing agent under acidic conditions. the morphology of the pmo-me materials was observed to be strongly dependent on the acidity of the solution used in each preparation. modification of the pmo-me surface with 3-mercaptopropyltrimethoxysilane (mpts), via a supercritical fluid grafting reaction, resulted in the formation of a bifunctionalized organosilica (sh-pmo-me). these sh-pmo-me materials, coated with au clusters, were subsequently utilized as templates for the supercritical fluid deposition of germanium nanocrystals. the highly hydrophobic surfaces of the sh-pmo-me templates, and the effective penetrating power of the supercritical fluid, resulted in rapid diffusion of the germanium precursor into the mesopores to produce highly crystalline nanoparticles and nanorods. these nanocomposite materials were found to be significantly more crystalline than those formed from functionalized au-coated mesoporous silica templates (sh-sba-15), highlighting the benefits of using pmos as hosts for the encapsulation of semiconductor nanomaterials.

Low-Temperature Deposition of Polycrystalline Titanium Oxide Thin Film on Si Substrate Using Supercritical Carbon Dioxide Fluid

A novel technique for the deposition of oxide thin films using supercritical carbon dioxide (CO2) fluid was proposed for the large-scale circuit integration on a silicon (Si) substrate at a low temperature. Thin films of titanium oxide (TiO2) as a model material were fabricated from organometallic Ti(Oi-Pr)2(dpm)2 source on (100)Si substrates in supercritical CO2 fluid. The film deposition was accomplished using a flow-type supercritical fluid deposition apparatus designed on the basis of metal-organic chemical vapor deposition (MOCVD) systems. Flat films were fabricated on (100)Si substrates at a faster source delivery (fluid flow rate: 2.0–10.0 cm3/min), while granular deposits owing to homogeneous nucleation in the bulk supercritical CO2 fluid appeared on the substrates at a slower source delivery (fluid flow rate: 1.0 cm3/min). The amount of TiO2 deposited decreased gradually with increasing the temperature of CO2 fluid, which would be related to the change in the density of CO2 fluid. The crystalline TiO2 film was deposited on the (100)Si substrate at a substrate temperature of 80–120°C and fluid temperature/pressure of 40°C/8.0 MPa, at which the decomposition of organometallic Ti(Oi-Pr)2(dpm)2 source was accomplished. These results indicated that the supercritical CO2 deposition technique is suitable for the large-scale circuit integration on Si substrates.