Processing, Characterization and Electronic Applications of Polymer Thin Films with Special Reference of Langmuir Blodgett Films Term paper as a part of Course requirements of MME 410

By, Subhashis Dash Senior Undergraduate, Materials and Metallurgical Engineering, IIT Kanpur

Under the guidance of

Dr. Ashish Garg, Professor, Materials and Metallurgical Engineering, IIT Kanpur

Abstract Conducting polymers is a relatively new field utilizing the unique electronic properties of a class of easily synthesized, primarily organic materials with the predominant property of high and controllable conductivity and subsidiary properties emanating from this conductivity and the associated, causative electronic structure. This paper tries to address the recent advances in the field of conducting polymers, especially Langmuir Blodgett films and attempts to address the current problems faced by the researchers by citing some specific examples from the literature. In fact, these examples can be extended to draw the timeline of research in the field of conducting polymers in the recent past.

Introduction Technological applications of new materials often require them to be in the form of thin films. Many thin film processing techniques have already been developed for the fabrication of electronic and optoelectronic components. Inorganic materials, such as silicon, gallium arsenide, silicon dioxide, and silicon nitride, currently dominate the microelectronics industry. But these current materials have a serious limitation. Advanced day silicon chips can store about 16 million bytes of data within an area less than 1cm2. Further attempts to improve the efficiency can cause overheating and crosstalk between electronic components, which can affect their performance. For efficient signal processing devices, this is likely to remain the case for many years to come. But a lot of new materials are showing promise in revolutionizing this field which include biological materials like proteins, pigments, and conducting polymers (CP). However, polymeric materials or in general organic compounds can outperform the above inorganic materials, particularly where cost is concerned, like in liquid crystal displays. Polymeric compounds are also making a mark in applications like chemical sensors, light-emitting diodes and infrared detectors. Unlike in conventional electronic materials derived from metals which have a delocalized electronic structure that can accommodate charge carriers such as electrons and holes, it has been proposed that electrical conduction in conducting polymers occurs via nonlinear (or topological) defects (solitons/ polarons) generated either during polymerization or as a consequence of doping. The first conjugated polymer, polythiazyl (SN)x, was discovered in 1975. It was observed that the material had metallic conductivity and showed superconductor at 0.29 K. However, the idea of using polymers for their electrical conducting properties actually emerged in 1977 with the findings of Shirakawa et al., when they reported that the iodine doped trans-polyacetylene, (CH)x, exhibits conductivity of 103 S/cm. Since then, a lot of research has been put up in this field of conducting polymers. As a result, other conducting polymers having p-electron conjugated structure (conjugated polymers), such as polyaniline (PAni), polypyrrole (PPy), polythiophene (PT), polyfuran (PFu), poly (phenylene) and polycarbazole have been synthesized and their properties are measured. The molecular structures of a few CPs are shown in Fig. 1. The conductivity of these polymers can be tuned from insulating regime to superconducting regime, by chemical modification, by the degree and nature of

doping. Besides these, polymers offer the advantages of lightweight, flexibility, corrosion resistivity, high chemical inertness, electrical insulation, and the ease of processing.

Figure1-Structures of some conducting polymers Conducting polymers possess a π-delocalized electronic structure, i.e.-one unpaired carbon atom per atom. Moreover, the π-bonding in which the carbon atomic orbitals are

in the sp2pz configuration and in which the orbitals of successive carbon atoms along the backbone overlap, leads to electron delocalization along the backbone of the polymer. This electronic delocalization provides the route for charge mobility along the backbone of the polymer chain. So chain symmetry defines the electronic structure in conducting polymers and these polymers can exhibit novel semiconducting and metallic properties. Take for example, polyacetylene (structure represented in Fig1), in which each carbon is σ-bonded to only two neighbouring carbons and one hydrogen atom with one π-electron on each carbon. If the carbon-carbon bond lengths were equal, the unpaired electron would mean a metallic state. Alternatively, if the electron-electron interactions were too strong, the chain will behave like an antiferromagnetic Mott insulator. But doping can be employed to convert these chains to metallic state which is also observed experimentally. Another example, polyacetylene, which has a dimerized structure because of Peierl’s instability with 2 carbon atoms in the repeat unit, has 2 bands- π-band and π*-bands and as each band contains 2 electrons each, the energy difference between the highest occupied state in the π-band and lowest unoccupied state in the π*-bands represents the band gap. The bond alternated structure of polyacetylene is characteristic of conjugated polymers although conjugated polymers are typically semiconductors as they have no partially filled bands. Reversible doping of conducting polymers, with associated control of the electrical conductivity over the full range from the insulator to metal, can be accomplished either by chemical doping or by electrochemical doping. As most metallic polymers are salts, their electronic conductivities result from the existence of the charge carriers through doping and from the ability of those carriers to move along the π-bonded highway. It’s even possible to dope to n-type or p-type with a higher density of carriers. Also, the attraction of electron in one repeat unit to the nuclei in the neighbouring units leads to carrier delocalization along the polymer chain and to charge carrier mobility, which is extended into three dimensions through inter-chain electron transfer. There are several methods of doping in polymers; notable among them are chemical doping by charge transfer involving redox chemistry, interfacial mechanism without counter-ions, photochemical mechanism for high performance optical materials and electrochemical mechanism which gives a process to control electrochemical potential. Polymer thin films have seen a lot of research of late as it employs novel processing techniques to prepare materials for high end applications. The ‘thin film’ refers to layers with thickness ranging from 1 nanometer to few micrometers. There are specific deposition methods for specific applications and even depends on the critical thickness of the film. It also depends on the kind of materials to be processed. Film thickness uniformity may also be essential in many instances. Conformal coverage, the ability to coat both vertical and horizontal surfaces of substrates also controls the choice of process. This becomes even more important in fabrication of integrated circuits as the semiconductor contacts and device interconnection metallization are needed to cover complex topography, with microsteps, grooves and raised stripes. Common methods of fabricating thin films are spin coatings, physical vapour deposition which include molecular beam epitaxy and sputtering, electrochemical methods, chemical vapour deposition, Langmuir-Blodgett film deposition and self assembly methods. But Langmuir Blodgett method has shown a lot of promise. In simple terms, LB technique requires ampiphiles that are insoluble in water and requires manipulation of materials on nanometer scale. They are prepared by first depositing a small quantity of an

ampiphilic compound, dissolved in a volatile solvent, on to the surface of purified water. Ampiphilic compounds are the ones with both polar and non-polar groups like long chain fatty acids. Among common transfer techniques used are the vertical-transfer “Y-Type” (vertical up and down strokes, commonly used for hydrophobic species), “X-Type” and “Z-Type” (vertical up and down strokes only), and the horizontal-transfer “lifting” technique (where the surface active species simply adheres to the horizontally placed substrate). The full exploitation of conducting polymers requires the development of processing methods for obtaining thin films out of those materials. Organization of conducting polymers in specific arrangements that would give the desired material properties is getting increasing attention, as this kind of organization is important for the development of many novel molecule based electronic devices. In this regard, Langmuir-Blodgett (LB) manipulation of conducting polymers seems promising, as it usually results in ultra thin films with known thickness and molecular orientation. This paper tries to address the recent advances in the field of conducting polymers, especially Langmuir Blodgett films and attempts to address the current problems faced by the researchers by citing some specific examples from the literature. In fact, these examples can be extended to draw the timeline of research in the field of conducting polymers in the recent past.

Processing and Characterization-Specific Examples Polypyrrole belongs to the poly (heterocycles) group and exhibits both polymeric and electroactive properties. Polypyrrole has high conductivity and environmental stability making it a promising electroactive material. The LB technique is used for the fabrication of polypyrrole ordered thin films. The characterization of these films which have been manipulated by solid state reaction to confine polypyrrole between the planes of the multilayers were done with high surface sensitivity techniques such as secondary ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy(XPS) and scanning electron microscopy (SEM). The confinement of polypyrrole in the LB structure was attained by exposing ferric stearate LB films to hydrogen chloride gas which causes precipitation of ferric chloride within the planes of the multilayer. The precipitated salts act to polymerize the pyrrole monomers, which are introduced into the film by vapor diffusion and dope the resultant polypyrrole forming conducting domains confined in between the planes of the insulating fatty acid multilayer.

Figure 2- SEM micrographs of (a) fine grain structure of ferric stearate LB film; (b) small clusters of Polypyrrole contained in the stearic acid LB film.

The thin film was fabricated by spreading solution of 1mg/ml of stearic acid in chloroform into an acidic subphase of deionized water with 4x10-5 ferric chloride. After the multilayer deposition, the resultant layers were compressed with a barrier rate of 5mm/min up to a pressure of 35mN/m. the silicon slides after proper treatment were used as substrates. After proper drying, the film was exposed to hydrogen chloride vapor and then kept over pyrrole vapor for a day. The fabrication of the thin films is not easy as it will cause hydrolysis of metallic ions in the high pH region and occur under gel form. The quality of the resultant multilayer also depends on the viscoelastic properties of the substrate. However the hysteresis tests performed on the deposited films showed that the resultant hysteresis area is almost negligible thus indicating a discrete elastic behavior of the monolayer. The SEM micrograph represented a smooth film surface with a very fine grain structure. It also showed small clusters of size range of 50-300nm formed on the surface after the solid state reaction. Even larger clusters were observed and EDS analysis showed that they were Fe and Cl based. The imaging process also showed the presence of conducting layer between the insulating layers. SIMS helped in studying the surface characteristic of the first monolayer. The negative ion spectra provide very useful information on the structure and nature of the dopant species. The negative ion spectrum of the ferric stearate LB showed no nitrogen containing fragments with some signals of chloride ions. Additionally, the curves of the solid state reaction showed presence of nitrile and ammonium drrivative of ethyne. On the other hand, SIMS direct ion image corresponding to the nitrogen distribution on the surface suggests that nitrogen containing polypyrrole is distributed as small granules in the LB matrix. It also confirmed that the sporadic bigger clusters are iron and chlorine based. These findings were further confirmed by XPS which also gave complementary information on both the chemical composition and oxidation state of the polypyrrole contained in the first monolayers of the sample.

Figure 3- Negative state SIMS ion spectra in the 0-50 a.m.u. m/z range of (a) simple ferric stearate LB film; (b) Polypyrrole containing LB film.

Figure 4- SIMS direct ion image corresponding to the nitrogen distribution on the surface.

Figure 5-XPS survey spectra in the range 0-1100 eV recorded (a) on the ferric stearate LB film and (b) on the Ppy containing film.

Functionalised-acid doping has been a major breakthrough in the process of fabrication of thin polymer films although the process of fabrication is still a tedious process. This has encouraged a lot of researchers to characterize the films. The research group under Ruif is working in this field. They found that the problem of fabrication is very evident in the processing of high molecular weight polyaniline films. So efforts are on to use 16-mer to make cast films which also exhibit similar characteristic to those observed with the high molecular weight polyaniline, with the adavantage of an improved solubility in organic solvents. The 16-mer monolayer films were fabricated by mixing the monolayers of the 16-mer, processed with camphor sulphonic acid and m-cresol and Cadmium Stearate. Surface potential measurements were used for investigating macroscopic film uniformity and X-ray diffraction was used to demonstrate the formation of different phases. The LB films were also characterized by electrical conductivity measurements, Fourier transform infrared spectroscopy (FTIR) spectroscopy and ellipsometry.

Figure 6- FTIR and XRD spectra of composite LB films (21 layers) containing different weight percentages of the 16-mer polyaniline and cadmium stearate (a)25%, (b) 50% and (c) 75% of 16-mer polyaniline

The characterization of mixed monolayers of the 16-mer polyaniline and cadmium stearate revealed that the pressure–area isotherms were of a condensed type, with monolayer stability decreasing with the 16-mer content. The composite monolayers were phase separated with the area per molecule decreasing linearly with the 16-mer content. Monolayers containing up to 60% of the 16-mer polyaniline could be transferred as Ytype multilayers with a near unity transfer ratio. However, upon increasing the amount of the 16-mer in the composite, a switch-over to a Z-type deposition was observed, which was also accompanied by poorer transfer ratios. Evidence that an equal amount of the 16mer was transferred during each deposition was obtained in UV-vis results which also showed that the transferred 16-mer polyaniline was in the emeraldine base, undoped form. Dedoping of the 16-mer must therefore have occurred on the neutral water surface (pH = 6.2), since the spreading solution had the 16-mer in its doped state (doped by CSA). If an acidic subphase was employed, however, the monolayer would have been doped as it has been shown to occur for the high weight polyaniline. The transfer of 16mer molecules can also be inferred by FTIR analysis. The FTIR spectra for composite, 21-layer LB films with three 16-mer compositions showed an strong absorption at 1548 cm-1 and the absence of any absorption at 1700 cm-1 indicated that the presence of cadmium stearate only and not the mixture of stearate and stearic acid in the transferred films. As the polymer content is increased, there is a relative increase in the absorption peaks of the emeraldine base form of the 16-mer (1587, 1503, 1320 and 1159 m-1) in comparison with those from cadmium stearate (1548, 1468 and 1350 cm-1). It should be noted that the absorption peaks from the 16-mer are submerged into those corresponding to the cadmium stearate in the 1600–1500 cm-1 region, when the 16-mer content is low (curve a). However, with the increasing 16-mer content in the composite, the absorption peaks corresponding to the 16-mer polyaniline could be clearly seen. Brewster angle microscopy investigation showed separate domains on the water surface for cadmium stearate and the 16-mer immediately after spreading the mixed monolayer. And the formation of such domain structures is reflected in the transferred LB films which was measured by XRD and was used for verifying the presence of the well-characterized cadmium stearate domains. The stacking order of the cadmium stearate domains is therefore being affected by the 16-mer and this effect becomes prominent as the 16-mer content is increased. For the 75% 16-mer composite LB film, in particular, no clear diffraction could be observed. It should be mentioned that the latter composite LB film was prepared from a less stable monolayer, with poor transfer ratio values in a Z-type deposition. In the UV spectroscopy which can be used for characterizing the formation of a layer-by-layer structure, the absorption of the emeraldine base 16-mer polyaniline was seen to increase linearly with the number of layers deposited, and such an absorption was also increased when the content of the 16-mer in the composite LB film was increased . Further evidence for the layer-by-layer structure comes from ellipsometry which allows the film thickness to be obtained together with the refractive index. Composite LB films were also transferred onto gold coated glass substrates for surface potential measurements. Samples with three 16-mer/cadmium stearate compositions and

of different thicknesses were employed which showed excellent film uniformity at the macroscopic level (the spatial resolution of the Kelvin probe is a few millimeters). The surface potential increases slightly with the number of layers and practically reaches a constant value after a few layers have been deposited, a feature that has been expected from different types of LB films. This is explained by the fact that the contribution from the film–substrate interface – probably from charge injection – reaches its final value within these few layers. The measured surface potential for thicker films then results from dipolar contributions from the film-forming molecules. A slight increase in potential is observed when the 16-mer content is increased, which is consistent with the small increase in the monolayer potential for the corresponding mixing ratios. It is also worth noting that the surface potential for the LB films is consistently higher, by about 70 mV, than the potential for the monolayer counterpart. However, the mixed monolayers under study had fully ionized stearic acid, whose double-layer potential is highly negative. To illustrate this point, it should be recalled that an unionized stearic acid monolayer has a surface potential of around 400 mV [15], while the fully ionized monolayer investigated here (stearic acid on water containing metal ions and at a subphase pH of about 6.0) displayed a surface potential of around 130 mV. The in-plane D.C. electrical conductivity of the as-deposited composite LB films was about 10-5 S/cm which showed that the amount of 16-mer for the lowest ratio used (25%) was already sufficient to make the LB film conductive. However, when the 16-mer ratio was increased, the conductivity did not increase significantly probably because the film organization was changed, as demonstrated by the change in orientational order in the XRD data. The conductivity was found to increase to about 10-4 S/cm when the films were submitted to an HCl vapor treatment. This value is comparable with the two or three orders of magnitude increase in conductivity for composite LB films containing high molecular weight polyaniline and also to the bulk conductivity value of the HCl treated 16-mer polyaniline.

Figure 7-Cyclic voltammograms of (a) LB film of polyaniline (25 layers) deposited on platinum plate and (b) an electrochemically deposited polyaniline film recorded in 0.5 M H2SO4(scan rate=50 mV/sec).

Figure 8- Plot of total redox charge per unit area, Qtotal vs. CTP in multilayer polyaniline LB films. But the electrochemical characteristics of polyaniline multilayer LB films have not been given much attention. The electrochemical behavior of multilayer polyaniline LB films have been studied by the research group on conducting polymers in IIT Bombay using cyclic voltammetry coupled with a quartz crystal microbalance. The results have been used to draw some conclusions about the packing arrangement of polymer chains in the LB film structure. The electrochemical characteristics of polyaniline LB films were studied by cyclic voltammetric measurements. The total redox charges calculated from the I-V response are found to increase linearly with increasing number of layers. Measurements indicated that multilayer polyaniline LB films exhibited slow redox kinetics (broad redox peaks) and also resulted in poor electrochromism, in comparison with electrochemically prepared thin films. Although like the electrochemically prepared film, the whole LB film is found to be electroactive, the closely packed polymer chains in the LB film structure impose restrictions on conformational changes and the movement of ions during the potential scan. Quartz crystal microbalance measurements indicate that the mass associated with anion inclusion during the anodic scan increases linearly with increasing numbers of layers, supporting the observation of increase in redox charge with increasing number of layers. These results on polyaniline LB films have direct implications on the use of these films in molecular devices. These results show that multilayer LB films are electroactive but the kinetics of counter ion transport in these films is slower than that observed in electrochemically deposited films. The multilayer LB films also show poor electrochromic switching behavior which needs a lot of research before it’s used for high end applications.

Applications Much research has been carried out on the use of CPs as active materials in molecular electronics devices. Undoped and doped conjugated polymers, such as polyacetylene, PPy, PT and its derivatives exhibit interesting semiconducting behavior that can be used

in device applications. In 1978, researchers tried to make a p–n junction device by pressure contact of a p-type polyacetylene film (doped with Na) to an n-type polyacetylene film (doped with AsF5). However, stable p–n junctions could not be developed successfully. This is because the preparation of p–n junction by diffusion of two dopants eventually led to compensation of the donor by the acceptor and thereby, led to a homogeneously insulating material. The alternative attempt to fabricate CP based rectifying junctions is to use a heterojunction between a semiconductor and a CP or Schottky junction between CP and metal. P-type Si, n-type Si, and CPs have been used as semiconductor in order to achieve stable junctions. The photovoltaic yield of CPs never exceeded about 1% making it unfit for use in photovoltaic energy conversion. In contrast to inorganic semiconductors, which exhibit low selectivity, CPs offer high selectivity, room-temperature operation and low-cost. Attempts to fabricate Schottky junctions were more successful. The undoped toluene-4-sulfonic acid doped and polyacrylic acid doped PAni behaves as a p-type semiconductor. The acid doping of PAni can cause a higher rectifying behaviour and photovoltaic conversion efficiency. On the other hand metal/polymer junction using vacuum evaporated PAni films was reported which said that the structure of PAni remains unchanged after vacuum evaporation due to polymerization and conjugation of chains broken during evaporation. This was confirmed by infrared and optical spectroscopy. The large degrees of defects possible in vacuum deposited metal/polymer junction were attributed to the fact that the defects and dangling bond readily react with various gases. Electronic properties of PPy/metal and PPy/n-Si junctions have also been investigated in detail by several researchers. The electrical and the optical data related to energy band diagrams of PPy/n-Si junction showed higher values of ideality factor in comparison to that of inorganic semiconductors were found to be due to the discontinuous structure at the interface between polymer and semiconductor. Various physical characteristics of the polymer, work function, Fermi level and carrier concentration have been estimated and Al/PPy formed by interfacial polymerization method was found to have better performance characteristics than those by other methods. On the other hand, the I–V measurements on p-type indium phosphide/ PPy Schottky diode show a deviation from thermoionic emission theory as the temperature is reduced, as evident by the increase of quality factor and the curvature in Richardson plot. Such deviations have been explained by the barrier inhomogeneity model, in which the barrier becomes voltage dependent due to the interaction of a small low-barrier region with a higher surrounding potential. In addition, leaky Schottky barriers at the interfaces between PPy and reactive metals and the absence of Schottky barrier at the junction of PPy/Al contacts have also been reported. Diodes made of PT including its derivatives and copolymers were also studied in details. Poly (3methylthiophene) was prepared using an electrochemical technique to fabricate Si based devices. The results obtained from current–voltage measurements, chronopotentiometry, SEM and Fourier-transform infrared spectroscopy suggested that the rectifying behavior was induced by covalent bond formation between P3MT and the silicon. On the contrary, I–V plots were essentially independent of voltage polarity. The observation of two stable, voltage-controlled conductance states was related to a thin depletion region formed at the device interface. It was postulated that the field-induced changes in space-charge (SC) density in the depletion layer because a change from electrode to bulk limited transport in the device. Recently, the effect of regioregularity and electron withdrawing/donating

substituents on the junction properties of Schottky diode based on PT has been reported. It was found that better performance of the device could either be achieved with regioregular polymers having a short alkyl substituent or by the presence of electron withdrawing group.

Figure 9- Schematic representation of polymer based FET. CP based transistors is another application and researches have shown that it is possible to control the current flowing between source and drain through the gate as genrally the field-effect controls the current through a gate electrode thereby opening the possibility of a transistor action without requiring the existence of p–n junctions. Tsumura et al. fabricated an FET based on electrochemically grown PT thin film and showed that it is possible to modulate the drain current by more than two orders of magnitude by varying the gate voltage. Further, Garnier et al. reported fully plastic FET fabricated by using printing technique. The performances of CP based FETs are quite encouraging and recent use of these devices in logic circuits or active matrix emissive displays has geared up the research in this direction immensely. In an FET, the interface of the gate insulator and organic semiconductor layer plays an important role in charge transport. It was found that if SiO2 is replaced by cyanoethylpulluan as the gate insulator, the mobility is increase by three orders of magnitude. Conjugated polymer based FETs are much cheaper than Si based device. However, the slow response and limited lifetime of the device restrict them to replace current Si technology. Nevertheless, the possibility of making flexible and flat panel with conjugated polymers open a new area of large-area low-cost plastic electronics.

Figure 10-Current–voltage and capacitance–voltage characteristics of metal/CP junctions Another big application of conducting polymer is in the field of batteries, mostly for secondary batteries. They have been used as the anode as well as the cathode material. Polymers are light weight, presumed low cost, presumed processability into odd shapesthin films and button shaped, less corrosive nature, and compatibility with organic and solid electrolytes. They are considered primarily for low-discharge-rate applications, such as in electronic devices, rather than high rate applications like transportation systems as their very poor under high discharge rates and low specific capacities. Li/CP batteries are already commercially available and are currently used as a back up source for static RAM in intelligent telephones, intelligent FM receivers, timers for VCRs, and fax machines and as back up power sources for solar calculators and watches. Another exciting discovery which has potential application is the use of the CPs such as poly(phenylene vinylene) as emmisive layer in light emitting diodes. Methods of processing and synthesis of the polymer in its undoped state with free from defects which quenched luminescence. But this still needs a lot of research as they are still not able to match the inorganic and Si-based semiconductors in terms of their properties. The polymer LEDS require low voltage with ease in processing through inexpensive methods, and even exhibit ac-operation. Their emission colours are possibly tunable by changing the CP backbone, or by combining several multiple CPs. But it’s very difficult to adjust the hole and electron current for balance. Although it can be solved by selecting a metal electrode, especially for electron injection, it leads to much reduced efficiency of emission.

Conclusion 3 scientists, Alan Heeger (USA), Alan MacDiarmid (USA) and Hideki Shirakawa (Japan) got nobel for their contributions to the development of conducting polymers in 2000. This award has boosted research in this field and last few years have seen a lot of new discoveries in this field. Conductive polymers have high promise and when properly

researched, can replace inorganic semiconductor devices in many applications. Conductive polymers have a lot of advantages and if their demerits are properly tailored, next few years will see a lot of products based on conductive polymers. This paper is a try to on bringing to light the cutting edge research that is going in this field by reporting the literature of last 4-5 years.

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