Investigation Of The Interface Between Organic/Magnetic Metal And The Field Emission Properties Of Organic Materials

Organic electronics is an interdisciplinary branch of science. With the characteristics of low cost, low density, large numbers of types and structures, good mechanical properties, good adjustability, simple preparation procedure, organic materials have provided new concepts of electronic devices with special or multi functions. Organic spintronics utilize the long spin relaxation time of organic materials in order to develop spin electronic devices with high spin injection efficiency. However, in the organic-magnetic metal interfaces, there are always existence a dipolar layer which could produce negative effect to the spin injection efficiency of spin devices. Thus, the energy level alignment and properties of the organic-magnetic metal interface are very important not only for fundamental physics but also for application. Organic materials also have great potential application in the field of field emission, especially for organic semi-conductor materials with low work function and easy synthesis with various morphologies. Thus, the interface properties of organic/magnetic metal and the field emission properties of organic materials were investigated as follows:(1) In the bilayer system of Bepp2-FeCo, we investigated the element content variation of Bepp2/FeCo and peel-off Bepp2/FeCo interfaces using XPS with the Ar etching. The chemical states of the interface between Bepp2and FeCo are also discussed. It is observed that the chemical reaction happened for Co and Fe with Bepp2in different degrees. Moreover, Fe is easier to react with Bepp2than Co. Through analyzing the UPS results, in the interface of Bepp2-FeCo, a large downward energy shift△=-3.0eV is observed. It was attributed to the formation of an interface dipole layer. Hole injection barrier ΦpB is1.6eV, electronic injection barrier ΦnB is1.2eV, indicating that the electron injection is the dominant mechanism in the transport experimental. Moveover, only when the FeCo thickness is less than3nm, an uniaxial anisotropy can be induced on the organic layer with the investigation of magnetic optical Kerr effect (MOKE).(2) A smooth surface of Alq3layer with RMS roughness around0.9nm was fabricated through dedicated control of the experimental parameters. In the bilayer structure of Alq3-FeCo, we investigated the element Co, Fe, O and C content variation of Alq3/FeCo and peel-off Alq3/FeCo using XPS with the Ar etching time, the chemical states in the interface of Alq3-FeCo are also discussed. It is observed that the chemical reaction happened for Co and Fe with Alq3in different degrees. Moreover, Fe is easier to react with Alq3than Co. Through analyzing the UPS results, in the interface of Alq3-FeCo, a downward shift△=-0.9eV is observed, this demonstrated the formation of an interface dipole layer. Hole injection barrier ΦpB is2.16eV, electronic injection barrier ΦnB is0.76eV. This indicates that electronic injection is the dominant mechanism in the interface of Alq3-FeCo upon with a bias. Moveover, only when the FeCo thickness is less than5nm, uniaxial anisotropy can be induced on the organic layer with the investigation of MOKE.(3) Amorphous tris-(8-hydroxyquinoline)aluminum (Alq3) layers, which firstly form Alq3islands and then grow into submicron thorns, with different nominal thicknesses are investigated. The field emission characteristics of Alq3submicron thorns with the nominal thicknesses of20,50and100nm include low turn-on fields of3.2,6.8and9.0V/μm at10μA/cm2, and low threshold fields of5.1,10.0and13.0V/μm at1mA/cm2. The enhanced field emission properties are governed by the morphology of Alq3submicron thorns, which can be controlled by the evaporation rate and the layer thickness. A significant hysteresis in the cycle-testing of the current density with a rising and falling electric field process, which is an undesirable property for practical applications, is observed in the case of the considered samples. This hysteresis can be eliminated via an increase in the nominal thicknesses and tests, and this is important for practical applications. The microstructure and adsorption/desorption effect are responsible for this hysteresis phenomenon. Amorphous Alq3submicron thorns with good field emission properties are promising candidates for application as field emitters.(4) The field emission (FE) properties of vertically aligned carbon nanotube (CNT) arrays having a surface decorated with Ta layer were investigated. The CNTs with6nm thickness of Ta decoration showed improved FE properties with a low turn-on field of0.64V/μm at10μA/cm2, a threshold field of1.06V/μm at1mA/cm2and a maximum current density of7.61mA/cm2at1.6V/μm. After Ta decoration, the increased emission centres and/or defect sites on the surface of CNTs improved the field enhancement factor. The work function of CNTs with Ta decoration measured with ultraviolet photoelectron spectroscopy decreased from4.74to4.15eV with increasing Ta thickness of0nm to6nm. The decreased work function and increased field enhancement factor were responsible for the improved FE properties of the vertically aligned CNTs. Moreover, a significant hysteresis in the cycle-testing of the current density with rising and falling electric field process was observed and attributed to the adsorption/desorption effect, as confirmed by the photoelectron spectrum.