A vapor phase deposition of self-assembled monolayers: Vinyl-terminated films of volatile silanes on silicon oxide substrates

Vinyl-terminated self-assembled monolayers (SAMs) offer significant flexibility for further chemical modification and can serve as a versatile starting point for a tailoring of surface properties. A vapor phase deposition of such films would offer advantages in cases where the preparation from solution is not an option or not desired, for example in connection with silicon microstructures such as micro-electromechanical systems. We show that SAMs of 9-decenyltrichlorosilane (CH₂CH(CH₂)₈SiCl₃), 10-undecenyltrichlorosilane (CH₂CH(CH₂)₉SiCl₃), 14-pentadecenyltrichlorosilane (CH₂CH(CH₂)₁₃SiCl₃), decyltrichlorosilane (CH₃(CH₂)₉SiCl₃), and octadecanetrichlorosilane (CH₃(CH₂)₁₇SiCl₃) can be prepared both from solution and from the vapor phase. The resulting layers were compared by static contact angle measurements, ellipsometry, X-ray photoelectron spectroscopy, and atomic force microscopy to determine their surface wettability, the film thickness, the smoothness and homogeneity of the respective films, and their chemical composition and each method gave films of comparable quality. Deposition of functionalized SAMs from the vapor phase is rare. Here we report the parameters for the preparation of well-ordered vinyl-terminated SAMs from the vapor phase.     

Surface characterization of plasma-treated and PEG-grafted PDMS for micro fluidic applications

The contact angle measurements have shown that polydimethyl siloxane (PDMS) surfaces treated by air plasma can recover up to about 40% of its hydrophobic nature in less than 20 min of air exposure. Therefore, poly(ethylene glycol) (PEG) silane was grafted after plasma treatment to permanently change the PDMS surface as hydrophilic in nature for micro fluidic application. The surface chemistry of plasma-treated and PEG-grafted PDMS substrate has been studied using X-ray photoelectron spectroscopy (XPS). The proportion of carbon atoms as C-Si and hydrocarbon decreased for both plasma-treated as well as PEG-grafted PDMS surfaces. The plasma treatment had increased the proportion of carbon atoms as CO and C(O)OX in C1s, whereas grafting of PEG silane decreased the proportion of C(O)OX and an increase in C-OX and CO functionalities. This is due to the interaction of OCH₃ on Si (in PEG silane) with C-OX and C(O)OX on plasma-treated PDMS by covalent bonding. Therefore, an increase in CO and C-OX functionalities and relative decrease in C(O)OX is expected. The plasma treatment of micro channels had increased the fluid velocity by a factor or four and similar measurements were observed in PEG grafted micro channel in PDMS chip. This indicates that the fluid velocity depends on the hydrophilic nature of substrate. The effect of nature of fluids on the fluid velocity in PDMS-based micro channel was also studied. It was observed that the fluid velocity was decreased with decreasing the pH values of the fluid.     

Preparation and characterization of silicate nanofilms doped with europium β-diketonate complexes

Films consisting of Eu³⁺ β-diketonate complexes were deposited onto glassy substrates by means of the spin- and dip-coating techniques, using different ion/ligand ratios. Absorption spectroscopy in the infrared region revealed the typical stretching bands of the SiOSi and SiOH bonds of the inorganic matrix as well as bands relative to the CO and CH symmetric vibration of β-diketone (dibenzoylmethane). The films displayed UV-visible absorption band at 350 nm, attributed to the organic ligand. Luminescence properties were studied by photoluminescence spectroscopy. Upon ligand excitation, the emission spectra exhibited the characteristic bands of the Eu³⁺ ion corresponding to the transition from the excited state ⁵D₀ to the ground state ⁷F_J. Scanning electron microscopy confirmed the formation of a film with average thickness ranging from 80 to 100 nm. The sol-gel process and the deposition techniques resulted in the effective formation of nanofilms, which opens up perspectives for their application in photonics.     

Bonding structure and optical properties of a-CNx:H films deposited in CH4-NH3 system

Hydrogenated amorphous carbon nitride (a-CN_x:H) films were deposited by plasma enhanced chemical vapor deposition (PECVD) in CH₄-NH₃ system. The chemical composition and bonding configuration were investigated by XPS and FTIR. The results indicated that both sp²CN and sp³CN bonds generally increased with the increase of the nitrogen concentration, and the N atoms bonded to C atoms through CN, CN and CN bonds. Remarkably, for FTIR spectra, two peaks (2125 and 2200 cm⁻¹) were obviously observed, corresponding to CN bond which was found to predominantly exist in the isonitrile structure. As more nitrogen atoms were incorporated, the optical band gap was found to vary from 1.8 to 2.5 eV. Finally, the conduction mechanisms were discussed at low and high temperature, respectively.     

Ultraviolet irradiation effect on the properties of leakage current and dielectric breakdown of low-dielectric-constant SiOC(H) films using comb capacitor structure

Low-dielectric constant SiOC(H) films were deposited on p-type Si(100) substrates by plasma-enhanced chemical-vapor deposition (PECVD) using dimethyldimethoxy silane (DMDMS, C₄H₁₂O₂Si) and oxygen gas as precursors. To improve the physicochemical properties of the SiOC(H) films, the deposited SiOC(H) films were exposed to ultraviolet (UV) irradiation in a vacuum. The bonding structure of the SiOC(H) films was investigated by Fourier transform infrared (FTIR) spectroscopy and X-ray photoelectron spectroscopy (XPS). The electrical characterization of SiOC(H) films were carried out through I-V measurements using the comb-like patterns of the TiN/Al/Ti/SiOC(H)/TiN/Al/Ti metal-insulator-metal (MIM) structure. Excessive UV treatment adversely affected the SiOC(H) film, which resulted in an increased dielectric constant. Our results provide insight into the UV irradiation of low-k SiOC(H) films.     

Determination of silicon nitride film chemical composition to study hydrogen desorption mechanisms

To go further in the comprehension of hydrogen desorption mechanisms from PECVD (Plasma Enhanced Chemical Vapour Deposited) silicon nitride, a method to determine the chemical composition of amorphous silicon nitride films using fast and non destructive characterization techniques has been developed. In particular, SiH, NH, SiSi and SiN bond concentrations are calculated from Fourier transform infra red spectroscopy, ellipsometry and mass measurement. Next, different PECVD silicon nitride films were annealed at 600 °C during 2 min. Results show that hydrogen desorption from PECVD silicon nitride depends on film mass density and main chemical reactions leading to hydrogen desorption are identified thanks to the determination of SiSi and SiN bond concentrations.     

Magnetron sputtering of Zr-Si-C thin films

The phase composition and chemical bonding of ZrC and ZrSiC films deposited by magnetron sputtering has been studied. The results show that the binary Zr-C films at higher carbon contents form nanocrystallites of ZrC in an amorphous carbon matrix. The addition of Si induces a complete amorphization of the films above a critical concentration of about 15 at.%. X-ray diffraction and transmission electron microscopy confirm that the amorphous films contain no nanocrystallites and therefore can be described as truly amorphous carbides. The amorphous films are thermally stable but start to crystallize above 500 °C. Analysis of the chemical bonding with X-ray photoelectron spectroscopy suggests that the amorphous films exhibit a mixture of different chemical bonds such as ZrC, ZrSi and SiC and that the electrical and mechanical properties are dependent on the distribution of these bonds. For higher carbon contents, strong SiC bonds are formed in the amorphous Zr-Si-C films making them harder than the corresponding binary Zr-C films.     

Silicon carbonitride nanolayers — Synthesis and chemical characterization

SiC_xN_y thin films were produced by plasma-enhanced chemical vapor deposition and characterized by ellipsometry, Fourier transform infrared and Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, as well as, by near-edge X-ray absorption fine structure measurements in total-reflection X-ray fluorescence geometry. The temperature of synthesis was varied between 100 °C and 800 °C, the precursors hexamethyldisilazane or hexamethylcyclotrisilazane were used with an addition of N₂, He, and NH₃, respectively. The composition of the products was determined to be constant in Si with about 20 at.%, whereas the sum of C and N results in 80 at.% (each varying between 20 and 60 at.%). Consequently, it can be stated, that in the produced silicon carbonitride a network of Si is built with SiCSi, SiCCSi, and SiNSi bridges. The comparison of the chemical composition and of the physical properties shows for the samples produced with He or N₂, respectively (without NH₃) that the refractive index and the absorption coefficient are increasing with an increasing content of carbon in the final formula SiC_4 − nN_n (with n = 1, 2, or 3).     

Optical properties and scanning probe microscopy study of some AgAsSSe amorphous films

Doping of AsSSe amorphous films by silver photo-dissolution leads to a decrease of the optical gap and to an increase of the refractive index in forming AgAsSSe films. The difference of the optical gap and refractive index between undoped and doped films has been found in case of Ag₁₅As₂₆S₂₉Se₃₀ film up to 0.37 eV and 0.26, respectively. Transreflectance in far infrared spectral region indicates formation of AgAsS₂ and AgAsSe₂ entities in Ag₁₅As₂₆S₂₉Se₃₀ film. Scanning probe microscopy, namely atomic force microscopy, atomic force acoustic microscopy (AFAM) and Kelvin probe force microscopy (KPFM) was used for studying AgAsSSe films. It was found that silver growth is rather three dimensional and it is reminiscent of the Stranski-Krastanov growth mode. Observed silver protuberances represent silver reservoirs responsible for a local increase of silver content. Hence, the silver growth mode enhances formation of nano/meso inhomogeneities of the surface and near surface density/stiffness, seen in AFAM, and in the surface electric potential, seen in KPFM.     

Enhanced scratch resistance of flexible carbon fiber-reinforced polymer composites by low temperature plasma-polymerized organosilicon oxynitride: The effects of nitrogen addition

An investigation is conducted on enhanced scratch resistance of flexible carbon fiber-reinforced polymer composites (FCFRPCs) by low temperature plasma-polymerized organo-silicon oxynitride (SiO_xC_yN_z) at varying N₂ concentrations. It is found that the organic-inorganic hybrid films (SiO_xC_yN_z) deposited on FCFRPCs using tetramethylsilane (TMS)O₂N₂ plasmas at low temperature dramatically improve the scratch resistance of FCFRPCs. Scratch resistance is greatly enhanced from an overwhelming presence of scratching (100%) on untreated FCFRPCs to a complete lack of scratching (0%) on TMSO₂N₂ plasma-polymerized FCFRPCs with steel wool for 250 cycles at 200 g loading. The scratch resistance of FCFRPCs is found to be highly dependent on their surface characteristics including hardness, surface morphology, surface compositions and chemical bonds of TMSO₂N₂ plasma-polymerized SiO_xC_yN_z.     

Multiferroic properties of Co and Nd co-substituted Bi5Ti3FeO15 thin films

Co and Nd co-substituted Bi₅Ti₃FeO₁₅ thin films were prepared on Pt/Ti/SiO₂/Si substrates via metal organic decomposition method. The structural and multiferroic properties of the films were investigated. It was found that Co ions enter into the lattice and occupy the Fe site. The Bi_4.15Nd_0.85Ti₃Fe_0.5Co_0.5O₁₅ films simultaneously exhibit ferroelectric and ferromagnetic properties at room temperature, and its 2P_r and 2M_r are 38 μC/cm² and 3 kA/m, respectively. Moreover, substitutions create local ferromagnetic order and antiferromagnetic order depending on whether the local bonding is FeOCo or FeOFe/CoOCo, respectively. The competing interaction of the ferromagnetic and antiferromagnetic phases results in an interesting magnetic behavior of the films.     

X-ray photoelectron spectroscopy characterization of poly(2,3-diaminophenazine) films electrosynthesised on platinum

Films of poly(2,3-diaminophenazine) (PDAPh) were obtained on platinum substrates by electropolymerization (cyclic voltammetry, CV) of the monomer in acidic solutions. Limited investigations were performed by cyclic voltammetry and quartz crystal microbalance (QCM). CV showed that phenazine units are present along the polymer chains and exhibit their peculiar electrochemistry. QCM measurements confirmed the deposition of a film on the electrode surface during cyclic voltammetry on 2,3-diaminophenazine (DAPh) solutions. Film structures were thoroughly investigated by X-ray photoelectron spectroscopy (XPS) analysis both through curve fitting of C1s and N1s signals. XPS was also performed following chemical derivatization of samples for NH₂, CO, COOH and OH groups with fluorinated markers. As a result, the presence of NH₂ groups could be excluded, COOH and CO groups were found to be minor defects, whereas OH groups, more abundant, were assigned partly to oximic groups (-CN-OH). The possible presence of other structural defects involving nitrogen (N-N and N⁺-O⁻groups) was also suggested by XPS. Finally, theoretical calculations on electron spin densities of 2,3-diaminophenazine radicals seem to support a 1,2 coupling mechanism for 2,3-diaminophenazine electrochemical polymerization.