The hottest LB film and its application in gas sen

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LB film and its application in gas sensors

LB film is a special molecular system designed and established by artificial use of intermolecular interaction. It is a stacking technology of organic polymer monolayers. The technology used to prepare LB films is called LB film technology. Different from the traditional physical and chemical vapor phase preparation methods in vacuum, the preparation process of LB films experienced a continuous phase transition process from gas phase to liquid phase to solid phase. This phase transition process is a physical process that takes place at room temperature and at the interface

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lb film research began in the 30 years of this century, and was first proposed by ngmuir and its student odgett [1]. However, the membrane materials used at that time were mostly simple amphiphilic molecules, which greatly limited the development of membrane functions. In the early 1960s, hn[2] first used LB film technology to construct molecular ordered systems through the assembly of monolayers, and introduced amphiphilic dye molecules with photoactivity into LB films for the first time. This has a significant impact on the development of LB film research [2,3]. Known as an epoch-making contribution

by the 1980s, LB film technology has attracted widespread attention of scholars at home and abroad in the fields of physics, biology, electronics, optics, chemistry, materials science, tribology and so on, and has been applied in many aspects and made great progress [3 ~ 7]. In recent years, as a new technology in the field of high technology, LB film has attracted more and more attention from some developed countries

The characteristics of LB films can be summarized as follows: 1) the film thickness is molecular level (nano level), which has special physical and chemical properties; 2) It can be accumulated layer by layer to form multilayer molecular layers or various superlattice structures; 3) Artificial selection of different polymer materials and accumulation of different molecular layers make it have multiple functions; 4) It can be formed under normal temperature and pressure, and the required generation energy is small without breaking the ring polymer structure; 5) Almost all molecules can form LB films and can be arranged arbitrarily in order; 6) It can effectively use the organizational ability of LB film molecules to form new compounds

it is the LB film technology that can be used to design at the molecular level to prepare a structural arrangement molecular combination system with specific functions. Its insulation function is used for MIS (metal insulator semiconductor) and tunnel effect elements; Anisotropic conductor for conductive function; Semiconducting for pigment PN junction; Optical nonlinearity is used for optical modulation and hybrid switching; Piezoelectricity and thermoelectricity are used for infrared detection; Sensitivity is used for gas, biological and ion detection; Corrosion resistance for electron beam and X-ray lithography; Storage and recording functions are used for optical disks and for solving lubrication problems in magnetic recording. As a favorable means of sensor function design, it has aroused great interest of sensor and bionics researchers. The research of LB film has entered a very active stage, and has been applied in the fields of optical, electrical, magnetic, biological information conversion and gas sensitive devices

1 Preparation Technology of LB film

1.1 basic process of film formation

typical LB film-forming materials must be amphiphilic, that is, hydrophilic and hydrophobic compounds. The general LB film formation process can be divided into three basic stages: 1) the formation of monolayers on the liquid surface. Firstly, the film-forming material is dissolved in water-insoluble organic solvents such as benzene and chloroform, and then dropped on the water surface to spread out, and the material molecules are adsorbed on the air-water interface (Fig. 1a); 2) After the solvent evaporates, a movable baffle is used to reduce the area occupied by each molecule (i.e. the area of the water surface/the number of dripping molecules). Under the pressure of a certain surface, the hydrophilic groups of each molecule contact the water surface. Hydrophobic genes are in contact with one side of the air, that is, all molecules are basically aligned in pairs on the surface of the subphase and densely filled to form a monolayer (Fig. 1b); 3) By lowering the substrate at a certain speed through a mechanical device, the monolayer on the sub phase surface will be transferred to the substrate; If the substrate is lifted again, the second monolayer is transferred to the substrate (Fig. 1c)

Figure 1 Schematic diagram of LB film deposition process

a-molecules float on the water surface, and organic solvents volatilize; B- molecules are squeezed and arranged neatly on the water surface; C- the water molecules floating on the water surface cannot be transferred to such a large and strong base surface; D-multilayer transfer of molecules on the water surface

Figure 2 structural diagram of LB film preparation device

1.2 Langmuir cell device

the device used to prepare LB film is generally called Langmuir cell. As shown in Figure 2. The device is composed of a tank, a baffle, an electronic balance, a stepping motor, an electronic computer, etc. The tank is filled with pure water (specific pH value and metal ions). The dripping film-forming material spreads into a quasi graphical gaseous monolayer on the liquid subphase. The moving speed of the baffle is controlled by a stepping motor. The electronic balance measures the surface pressure of the film-forming monolayer through a precision pressure sensor. The computer controls the surface pressure of the solid substrate in the process of immersion into the subphase to keep it constant

in recent years, Langmuir slots have been greatly improved. For example, the development of multi slot tanks and movable wall tanks [8,9]. Using this kind of device, we can accumulate the monolayers of some substances on the monolayers of other substances, build a variety of composite (heterojunction) accumulation films with different molecular orientations and accumulation methods, and explore new functions. It can also greatly improve the film-forming quality, repeatability and consistency. In addition, people have done a lot of exploration and Research on the control of film drawing process parameters, which has completely realized the automation of film drawing process and significantly improved the quality of LB film

1.3 film forming method

lb film is deposited on solid substrate, and the most common method is vertical immersion method. Sometimes the horizontal immersion method is also used. According to the docking mode of organic molecular functional groups, it can be divided into X-linked method (membrane), Y-linked method (membrane) and Z-linked method (membrane). The characteristic of the vertical impregnation method is that the solid carrier is always vertical to the horizontal plane when hanging the film

Fig. 3 several deposition methods of LB film

1.3.1 x-hanging method (or x-film) (Fig. 3a)

the carrier only hangs the monolayer when falling, but does not hang when rising. The multilayer monolayers thus prepared are called X-type films. The characteristic of this membrane is that the hydrophobic surface of each membrane is in contact with the hydrophilic surface of the adjacent layer. The carrier surface for preparing X-type membrane should be hydrophobic. If hydrophilic glass is used as carrier substrate, hydrophobic treatment is required in advance

1.3.2 y-hanging method (Fig. 3b)

first lower the slide (the surface is the drainage surface), hang the first layer, and then rise. Hang the second layer again, and so on. Hang the multilayer monolayers. It can also be raised from underwater (the surface of the film is the water surface), hung on the first layer, and then hung on other layers in turn (this situation is not given in Figure 2). The multilayer monolayers thus prepared are called Y-shaped films. It is characterized by the connection of Pro water surface and sparse water surface between layers. This hanging method is the most common

1.3.3 z-hanging method (Fig. 3C)

contrary to x-hanging method, the film hangs when the slide rises and does not hang when it falls. The surface of the carrier is required to be hydrophilic. The overall characteristics of the Z-shaped film are the same as those of the X-shaped film

1.4 the main factors affecting the quality of LB film

in addition to the film-forming material itself, the preparation of LB film is also related to the pH value of water subphase, the type and concentration of salts, the surface pressure during accumulation, the temperature of water phase, the up and down moving speed of solid substrate, the type of substrate and the surface state. The water resistivity is generally 15 ~ 20m, and three-stage distillation and two ion filtration are required. In order to form more stable monolayer LB films, the method of changing the water subphase is usually used. Such as changing the pH of the subphase, adding metal ions to the subphase and forming salts with film-forming materials. Soluble polymers are dissolved in the subphase, and LB films are stabilized by electrostatic attraction. In order to improve the heterogeneity of the film, the amorphous monolayer was developed on the high-temperature water surface, and then the water temperature was reduced to obtain a uniform crystalline monolayer. During the formation of LB film, the choice of developing solvent is also very important. When hexadecane and octane are added to the solution, the speed of functional molecular construction during the formation or compression of monolayers on the water surface can be adjusted, and the size and morphology of aggregates and second-order crystals in the whole membrane can be changed

as solid lining materials, there are alkali free glass, quartz glass, calcium fluoride, silicon wafer, etc. According to the properties of LB films and the requirements of high structural stiffness of junction equipment, they can be changed into hydrophilic or hydrophobic water type by chemical treatment. If the hydrophobic sheet is prepared, treat it with toluene solution of 5% Phenyltrichlorosilane

1.5 design and synthesis of LB film-forming materials

polymer LB film materials are different, and the manufacturing methods are also slightly different. If organic molecular monomers are deposited on the substrate and then polymerized, the polymerized LB films can be divided into three categories according to their formation process: copolymerized LB films, polycondensated LB films and polymerized LB films. According to their preparation methods, they can be divided into two categories: one is to make LB films of monomer molecules and then initiate polymerization with ultraviolet light; The second is LB films directly prepared from polymers. Although the ordering of monomer molecules is conducive to the polymerization reaction, because the polymerization reaction is accompanied by volume changes, this will lead to the increase of the internal application of the membrane, which will even rupture the membrane [10]. Preparing LB films directly from polymers can avoid the increase of internal stress, but there are still some difficulties in the preparation method

another method is to polymerize the monomer on the water surface (photopolymerization or chemical polymerization) and then deposit it on the substrate. Or first dissolve the charged water-soluble polymer in water, so that the amphiphilic molecules with reverse charge expand on the water surface to form a polyionic polymer method. In addition, it is necessary to prepare various organic molecules that can be matched, embedded and complexed with each other according to the requirements of performance; Synthesis of various functional molecules sensitive to physical and chemical parameters such as light, sound, electricity, heat, gas and humidity; Prepare molecules that are firmly bonded to the substrate, resistant to high temperature and long life. At present, people are exploring how to prepare films with good stability, good mechanical properties and firm adhesion to meet the requirements of different occasions

1.6 a (surface pressure area) curve determination

in the preparation of LB films, curves are usually used to characterize the state of LB films. On the other hand, it reflects the phase transition physical process of LB film deposition. The correct analysis and determination of the curve of LB film is the key to form a compact and uniform defect free monolayer. Surface pressure refers to the reduction of surface tension compared with pure water. The surface area corresponding to pressure t0 refers to the area occupied by each organic molecule on the water surface (the area of the water surface/the number of molecules dropped). Dissolve the film-forming molecules in organic solvents such as benzene and chloroform. When the droplets slowly drip on the water surface of Langmuir tank, they spread out on the gas-water interface. After the standby solvent evaporates, a monolayer is left. When initially unfolding on the water surface, the surface area of the molecules is large enough, and the molecules are isolated from each other in a quasi two-dimensional large gas state (a). With the extrusion of the baffle on the monolayer, the surface area (a) is reduced, the interaction between molecules is gradually enhanced, and the surface pressure () is increased, which is commonly known as two-dimensional solid state (b). This is Langmuir membrane (L membrane for short). This is the single homogeneous molecular membrane required. When a is reduced again, the membrane overlaps (called collapse state d) because it crosses the elastic limit of the monolayer. However, due to different film-forming materials and conditions, the overlapping films also form superlattice structure (E). This is the relationship between them. Different states have inflection points and mutations on the curve. By analyzing the surface pressure in the states of a, B, C, D and e of the curve, the best surface pressure accumulated by the monolayer can be determined, and a complete, uniform and defect free monolayer can be obtained.

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