Polyimide Langmuir-Blodgett Films Research Articles

Synthesis of silicon carbide in a matrix of graphite-like carbon prepared via carbonization of rigid-rod polyimide Langmuir-Blodgett films on silicon substrate.

Silicon carbide (SiC) is a wide-band gap semiconductor that exceeds other semiconducting materials (except diamond) in electrical, mechanical, chemical, and radiation stability. In this paper, we report a novel approach to fabrication of SiC nano films on a Si substrate, which is based on the endotaxial growth of a SiC crystalline phase in a graphite-like carbon (GLC) matrix. GLC films were formed by carbonization of rigid rod polyimide (PI) Langmuir-Blodgett (LB) films on a Si substrate at 1000 °C in vacuum. After rapid thermal annealing of GLC films at 1100 °C and 1200 °C, new types of heterostructures SiC(10 nm)/GLC(20 nm)/Si(111) and SiC(20 nm)/GLC(15 nm)/SiC(10 nm)/Si(111) were obtained. The SiC top layer was formed due to the Si-containing gas phase present above the surface of GLC film. An advantage of the proposed method of endotaxy is that the SiC crystalline phase is formed within the volume of the GLC film of a thickness predetermined by using PI LB films with different numbers of monolayers for carbonization. This approach allows growing SiC layers close to the 2D state, which is promising for optoelectronics, photovoltaics, spintronics.

Formation of Highly Conducting Optically Transparent Films with Multigraphene Structure via Carbonization of Polyimide Langmuir–Blodgett Films

Multigraphene films have been for the first time obtained on the surface of quartz glass via carbonization of polyimide Langmuir–Blodgett films. The Raman spectra of the films show bands G and D and a broad band at 2300–3200 cm–1, which are characteristic of graphite-like films. The transmission electron microscopy demonstrated that the films are constituted by “stacks” of layers spaced by 0.36 nm. The surface resistance of the 5-nm-thick films was 1.2 ± 0.2 kΩ, and the transmittance was 87% at λ = 550 nm. The transmittance was 97% in the IR part of the spectrum and 70–80% in the UV part.

Formation of crystalline heteroepitaxial SiC films on Si by carbonization of polyimide Langmuir–Blodgett films

High-quality crystalline nano-thin SiC films on Si substrates were prepared by carbonization of polyimide (PI) Langmuir–Blodgett (LB) films. The obtained films were characterized by Fourier transform-infrared (FTIR) spectroscopy, X-ray diffraction (XRD) analysis, Raman spectroscopy, transmission electon microscopy (TEM), transmission electron diffraction (TED), and scanning electron microscopy (SEM). We demonstrated that the carbonization of a PI film on a Si substrate at 1000 °C leads to the formation of a carbon film and SiC nanocrystals on the Si substrate. It was found that five planes in the 3C-SiC(111) film are aligned with four Si(111) planes. As a result of repeated annealing of PI films containing 121 layers at 1200 °C crystalline SiC films were formed on the Si substrate. It was shown that the SiC films (35 nm) grown on Si(111) at 1200 °C have a mainly cubic 3C-SiC structure with small amount of hexagonal polytypes. Only 3C-SiC films (30 nm) were formed on the Si(100) substrate at the same temperature. It was shown that the SiC films (30–35 nm) can cover the voids with size up to 10 µm in the Si substrate. The current–voltage (I–V) characteristics of the n-Si/n-SiC heterostructure were obtained by conductive atomic force microscopy.

Heteroepitaxial growth of SiC films by carbonization of polyimide Langmuir-Blodgett films on Si

High quality single crystal SiC films were prepared by carbonization of polyimide Langmuir-Blodgett films on Si substrate. The films formed after annealing of the polyimide films at 1000°C, 1100°C, 1200°C were studied by Fourier transform-infrared (FTIR) spectroscopy, X-ray diffraction (XRD), Raman spectroscopy, transmission electon microscopy (TEM), transmission electron diffraction (TED), and scanning electron microscopy (SEM). XRD study and HRTEM cross-section revealed that the crystalline SiC film begins to grow on Si (111) substrate at 1000°C. According to the HRTEM cross-section image five planes in 3C-SiC (111) film are aligned with four Si(111) planes at the SiC/Si interface. It was shown the SiC films (35 nm) grown on Si(111) at 1200°C have mainly cubic 3C-SiC structure with a little presence of hexagonal polytypes. Only 3C-SiC films (30 nm) were formed on Si (100) substrate at the same temperature. It was shown the SiC films (30-35 nm) are able to cover the voids in Si substrate with size up to 10 μm.

Pore sealing of SiOCH ultra low-k dielectrics with polyimide Langmuir-Blodgett film

ABSTRACTContinuous decrease of the feature size of transistors in modern integrated circuits (ICs) constrains thickness of auxiliary dielectric layers in interconnects because of their relatively high dielectric constant, which reduces the efficiency of low-k material integration. Dielectric materials used today as barrier or etch-stop layers are usually SiN (k ∼ 7.0) and SiCN (k ∼ 4.8), which k-value significantly exceeds that of recent ultra low-k materials (k < 2.2). In our work we have investigated thin films of rigid-chain polyimide (PI) with a k-value of about 3.2-3.3. This film was deposited using a Langmuir-Blodgett (LB) technique and can be as thin as several monolayers. The intermolecular interaction of densely packed precursor macromolecules within a monolayer formed at the water-air interface makes it possible to avoid penetration of precursor material inside the pores. The latter peculiarity of the deposition process results in a pore sealing effect using a 4 nm PI film.

Analysis of polar orientational order controlled by nanometer-thick polyimide Langmuir-Blodgett films

Orientational ordering processes of liquid crystalline 4- n-pentyl-4′-cyanobiphenyl (5CB) were studied by the optical second-harmonic generation measurements, in-situ, during evaporation onto silica substrates coated with n-layer polyimide Langmuir-Blodgett films ( n = 1, 3, 5, or 11). It was found that the polar orientational order, S 1 = 〈cos θ〉, of 5CB decreases as the thickness of polyimide layer increases from 0.4 nm to 2 nm, and then it saturates. It was shown that the electrostatic potential energy calculated with consideration of mirror-image charges at the interface accounts for the orientational order dependence on the polyimide layer thickness.

Observation of orientational ordering processes of 4- n-pentyl-4′-cyanobiphenyl molecules during evaporation onto polyimide Langmuir–Blodgett films by optical polarized absorption measurements

Orientational ordering processes of 4- n-pentyl-4′-cyanobiphenyl (5CB) molecules were examined, in situ, using optical polarized absorption technique whilst 5CB molecules were evaporated onto synthetic silica substrates coated with n-layer Kapton-type polyimide Langmuir–Blodgett film ( n = 3, 5, 7, and 11). It was found that the second orientational order parameter S 2 of 5CBs deposited onto the 3-layer polyimide substrate is lower than S 2 of 5CB deposited onto the 5-, 7-, and 11-layer polyimide-coated substrates when the amount of the deposited LC molecules is 10 times as much as that of a single monolayer. The results were discussed in terms of anchoring phenomena.

Structuring in mixed solvents: Study by polarized light scattering

The structure of mixed amide solvents used for the preparation of polyimide Langmuir-Blodgett films and the structural organization of water, on which nanofilms are formed, have been studied by polarized light scattering. Fluctuation supramolecular structures are found to exist in water and in its mixtures with amide solvents, and the dimensions of the structures are comparable with the light wavelength. When a mixture contains more than 20 vol % water, associates show microanisotropic properties. In snowmelt water, the dimensions of associates are larger than those in distilled water. In the DMAA-water mixture, the dimensions of supramolecular structures are virtually independent of composition.

Orientational ordering process of 4′- n-pentyl-4-cyanobiphenyl molecules deposited on polyimide Langmuir–Blodgett films

The orientational ordering processes of 4′- n-pentyl-4-cyanobiphenyl (5CB) molecules on Kapton-type polyimide Langmuir–Blodgett (LB) films were experimentally examined. During 5CB deposition, in situ observation was conducted and the orientational order of 5CB molecules was determined in terms of orientational order parameters S n ( n = 1, 2, 3) using optical second-harmonic generation (SHG) and optical polarized absorption (PA) measurements. Experimental data were discussed in connection with anchoring energy dependence on the number of polyimide layers.

Spectroscopic and optical characterization of porphyrin chromophores incorporated into ultrathin polyimide films

Polyamic acid (PAA) containing free-base porphyrin and zinc(II) porphyrin chromophores was synthesized by copolymerization of diphenylether-type tetracarboxylic dianhydride and diamines. The monolayer of the alkylamine salts of PAA (PAASs) at the air/water interface was deposited on solid substrates by the Langmuir–Blodgett (LB) technique. The PAAS LB films thus obtained were converted to polyimide (PI) LB films by chemical treatment. The fluorescence of porphyrin moieties in the PI LB film was observed, because of the weak electron-accepting properties of the diphenylether unit. Therefore, the photophysically important processes, such as photoinduced electron transfer, excitation energy transfer, and excitation energy migration could be investigated in relation to the layered nanostructures of the ultrathin PI films. The fluorescence spectrum suggested that the aggregation of porphyrin moieties in the PI LB films was effectively prevented by the use of polymeric films. The surface plasmon (SP) measurement showed that the thickness of the monolayers was 0.9–1.0 nm for PAAS films and 0.32–0.40 nm on average for PI LB films. The absorption dichroism of the Soret band of porphyrin indicated that porphyrin moieties in the PAAS and PI LB films are oriented in parallel with the substrate. These results showed that the orientation and the spatial distribution of porphyrin units can be efficiently regulated in the PI LB films in a nanometer dimension.

Polyimide 터널 장벽을 이용한 Au/polyimide/유기 단분자막/Pb 구조에서 비탄성 전자 터널링에 관한 연구

Using polyimide Langmuir-Blodgett(LB) films as a tunneling harrier, we fabricated Au/Polyimide/1-layer arachidic acid/Pb structure in order to investigate electron transport properties through a junction. It was found that 9-layer polyimide LB films function as a good tunneling harrier in a study of current-voltage(I-V) chararteristics. And several peaks originating in the vibrational modes of the constituent molecules of 1-layer arachidic acid LB films were clearly observed in d<TEX>$^2$</TEX>V/dI<TEX>$^2$</TEX>- V corves.

Patterning of photosensitive polyimide LB film and its application in the fabrication of biomolecular microphotodiode array

Ultra thin film of photosensitive polyimide having benzene and sulfonyloxyimide moieties in the main chain was prepared using a Langmuir–Blodgett (LB) technique, and then micro array pattern of the polyimide LB film on a gold substrate was obtained by deep UV lithographic technique. In order to array cytochrome c molecules along the micro-patterned gold substrate, the well-characterized monolayer of cytochrome c was immobilized with a mixed monolayer of 11-mercaptoundecanoic acid (11-MUDA) and decanethiol. The redox activity and electron transfer between cytochrome c molecular center and gold electrode interface for the self-assembled cytochrome c monolayer were investigated by cyclic voltammetry measurement. Biomolecular photodiode consisting of cytochrome c and green fluorescent protein (GFP) onto the patterned gold substrate was fabricated by self-assembly process. The integration and morphology of cytochrome c and GFP were studied from the measurements of atomic force microscopy (AFM) and fluorescence emission. Especially, current–voltage characteristics of the protein multilayers were investigated by scanning tunneling microscopy (STM) and its application in biomolecular photodiode was also examined.

Electrical breakdown of ultra-thin polyimide Langmuir–Blodgett films under a needle-plane electrode system

Electrostatic phenomena occurring at insulator–metal interfaces are interesting to the fields of electronics. In this study, we examined the surface potential of electrical insulating polyimide (PI) Langmuir–Blodgett (LB) films on metal (Al and Au) electrodes, which were charged at various voltages under a needle-plane electrode system. PI LB films were negatively charged as the biasing voltage applied to the needle-electrode increased. The surface potential saturated when the number of deposited layers was 20–30, and it was dependent on the nature of metal electrode. The interfacial electrostatic space charge at the as-deposited film/metal interface made a significant contribution to the dielectric breakdown of ultra-thin films. As a result, PI LB films deposited on Al electrode were broken at lower voltage than that on Au electrode.

Homeotropic-planar anchoring transition induced by trans-cis isomerization in ultrathin polyimide Langmuir–Blodgett films

The influence of the charge separation during the trans-cis conformational change between the surface azobenzene 6Az5PVA monolayer on polyimide Langmuir–Blodgett film on the surface charge density is investigated, using the Maxwell displacement current technique. It is theoretically shown, in the framework of the molecular model, which takes into account interactions between the surface polarization (in the quadrupole approximation) and surface electric field, that the experimentally observed changes of the surface charge density in ∼3×10−4 C/m2, caused by the charge separation in ∼2 pC, taking place during the conformational change, may lead to changing of the surface alignment of liquid crystalline molecules, such as 4-n-pentyl-4′-cyanobiphenyl (5CB), having contact with that solid substrate.

Interfacial electrostatic phenomena in polyimide Langmuir–Blodgett films in electron tunneling devices

Abstract Nanometric electrostatic space charges exist at the metal/molecular film interface due to the displacement of excessive electrons from metal to molecular films. As a result, surface potential is built across the films on metal. The distributions of the displaced electrons and the electronic density of states in films can be determined using the surface potential built. The current–voltage characteristic of tunneling device using polyimide Langmuir–Blodgett films is discussed with taking into account the space charge.

Analysis of Step Voltages in Single Electron Tunneling Devices Using Organic Thin Films

During the past ten years, electron tunneling devices such as Josephson junctions and single electron tunneling devices have been fabricated using organic thin films, where the successful preparation of polyimide Langmuir-Blodgett (PI LB) films has made a significant contribution. The electrical properties of PI LB films were also examined carefully, and electrostatic phenomena, interfacial phenomena, electrical breakdown behaviors and so on have been elucidated. Based on these studies, the effects of space charge and coupling capacitance upon the operation of single electron tunneling (SET) devices using organic molecules have been investigated. The equation of step voltage is derived taking into account space charge and coupling capacitance, using an equivalent circuit model. It was concluded that knowledge of interfacial phenomena is very important for a better understanding of the operation of such devices.

Photoinduced electron transfer in nanostructures of ultrathin polyimide films containing porphyrin moieties

Photoinduced electron transfer between porphyrin moieties and pyromellitimide fragments has been investigated in multi-layered structures of ultrathin polyimide films prepared by the Langmuir–Blodgett (LB) technique. The LB films were composed of three kinds of polyimides, which contained zinc tetraphenylporphyrin (ZnTPP) unit as an electron donor (D-layer), no chromophoric groups (S-layer), and pyromellitimide fragments as an electron acceptor (A-layer). The layered structure and orientational distribution of porphyrin moieties in the LB films were evaluated by surface plasmon measurement and absorption dichroism measurement, respectively. The thickness of monolayer was estimated to be 0.9 nm for the polyamic acid films and 0.4 nm for the polyimide films. The molecular plane of porphyrin moieties was oriented in the direction parallel to the substrate plane. In the multi-layered structures of polyimide LB films, the efficiencies of photoinduced electron transfer from porphyrin moieties to pyromellitimide fragments varied sharply with the number of spacing layers, indicating that the short-range interactions such as electron transfer could be controlled by the fabric of ultrathin films. The rate of electron transfer observed by the fluorescence quenching measurements was numerically simulated for the nanostructure using the Monte Carlo method.

Ultraviolet Photoluminescence from 4H–SiC Nanocrystalline Films Deposited on Silicon Substrate

Preferred-orientated 4H–SiC nanocrystalline films on silicon substrates were successfully prepared by the pyrolysis of the polyimide Langmuir–Blodgett films containing nanometer-sized amorphous silicon nitride powders. Fourier-transform infrared spectra revealed 4H–SiC nanoparticles with interfacial silicon oxide. Ultraviolet photoluminescence with energies above the band gap of bulk 4H–SiC was observed from the films in which the mean size of 4H–SiC particles was 10 nm, while no photoluminescence was observed from the films in which the mean size of particles was 17 nm. A quantum confinement/luminescence center model is suggested to explain the origin of the ultraviolet luminescence.

Spontaneous polarization and electro-optic behaviour of a ferroelectric liquid crystal cell fabricated with polyimide Langmuir-Blodgett films

The spontaneous polarization and electro-optic response of ferroelectric liquid crystals (FLCs) were investigated in a cell fabricated with a polyimide alignment layer coated by the Langmuir-Blodgett method. The surface properties of the cured polyimide layers were monitored by contact angle measurement, and by FTIR spectroscopy and AFM for the orientation and surface roughness, respectively. The apparent spontaneous polarization of an FLC determined in a practical sandwich-cell depended on various conditions such as cell thickness, cooling rate from the smectic A to chiral smectic C phase, and deposition pressure. Electro-optic response and decay times of FLCs were also measured. Furthermore, the ions in the FLC mixture reduced the magnitude of the effective electric field, but had no effect at high frequency.

Stable bit formation in polyimide Langmuir–Blodgett film using an atomic force microscope

Nanometer scale recording bits were stably fabricated in polyimide (PI) Langmuir–Blodgett (LB) films on atomically flat Au (111) surfaces using an atomic force microscope (AFM). The bits are written by voltage application through the AFM probe only until the conductance increases. This method eliminates the influence of the variation in the transition time and therefore the degradation of the tip of the probe. 1 Mbit stable writing can be realized. The overall error rate is less than 2×10−3. The results show that an AFM based memory system with PI LB films is a hopeful candidate to realize a scanning probe microscope based memory system.