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Fabrication and Electrical Characteristics Study of Several Organic Molecular Thin Film Devices

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Tutor: LiJianChang
School: Northeastern University
Course: Fluid Machinery and Engineering
Keywords: molecular electronics,metal/molecule/metal molecular junction,chargetransport,na
CLC: TN605
Type: Master's thesis
Year:  2012
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With the rapid development of nanoscience and nanotechnology, molecular electronics, aiming at molecular devices and molecular computers has been the most active field. The main focuses of recent research interest in molecular electronics are to fabricate molecular device, and to measure and control electron transport through a molecular device. Metal/molecule/metal molecular junction is the most basic and widely studied structure in molecular device. However, fabricating a simple and efficient micro molecular junction based on a limit substrate is a big challenge. Numerous high-performance photoelectric devices fabricated from organic polymers have been made including light-emitting diodes (LEDs), field effect transistors (FETs), photovoltaic cells, etc. The work principle of these devices is mainly based on physical processes involving the charge injection, the charge transport and the combination of electron-hole pairs. Theoretically, based on the understanding of the electrical characteristic of the interfacial between organic layers and electrodes of organic film devices, we introduce some charge transport mechanisms including:tunneling model, Fowler-Nordheim (F-N) tunneling model, Richardson-Schottky (R-S) thermal emission model, Poole-Frankel emission model, hopping conduction and space-charge limited conduction (SCLC) model.In this work, we systematically studied the charge transport of a series of self-assembled monolayer molecular crossbar junctions with different alkanethiol and conjugated molecules, respectively. The device was fabricated using a soft stamp-printing method. Current-voltage characteristics were measured at varied temperatures (300K down to90K) under different light illumination conditions. Strong temperature dependence and optoelectronic switching phenomena were observed in the as-fabricated junctions at both dark and light situations. Possible conduction and switching mechanisms of the molecular electronic transport were experimentally and theoretically investigated. The simulation results showed that the barrier height is strongly dependent on the substrate temperature, molecular length and conjugation structure. It is indicated that the electron transport can be regulated through tunning the external temperature, illumination condition, molecular length, and/or conjugation structure.(I) the asymmetry Au/SAMs interfaces may induce an asymmetry Au fermi level Ep and the barrier height is thus larger at the positive bias than that at the negative side.(II) Lower barrier height is observed under light illumination because the light absorption may generate carrier contributing photocurrent.(¢ó) The barrier falls off with the increase of temperature owning to the stronger stamp-printing SAMs/Au contact at higher temperature.(IV) The charge barrier decreases with the increase of both molecular length and conjugation degree. Our work may offer some useful information for the development of molecular electronic based memory devices.The nanoparticles (NPs) embedded organic thin films have been fabricated with a spin coating method based on the sol-gel and water bath produced NiO, Ni:Sn(1:2), and Ni: Zn(1:4) NPs. The morphology is characterized by SEM and SPM, and the electrical properties by a GaIn probe method. The results show that the thin films embedded with water bath produced NPs are smoother. All the thin films exhibit a switching behavior. The composite thin films embedded with water bath produced Ni:Zn(1:4) NPs show a better electrical property with an on/off ratio of102with an increase of approximately one order of magnitude. A further analysis of the ¢ñ-¢õ curves of ON and OFF-state using theoretical models shows that the off state is in compliance with the SCLC model and the on state is fitted well by Ohm¡¯s law. The charge transport mechanism of the switching behavior is demonstrated and a charge trapping/detrapping mechanism is proposed.
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