Formation of Al Nanostructures on Alq3: An in Situ Grazing Incidence Small Angle X-ray Scattering Study during Radio Frequency Sputter Deposition

Abstract

The formation of metal/organic interfaces is a complicated process involving chemical interaction, physical nucleation and diffusion, and thin film growth. It is closely related to the performance of organic electronic devices. To understand this process, we investigate the system of aluminum (Al) and tris(8-hydroxyquinolinato)-aluminum (Alq3) as a model, owing to the well-known strong chemical interaction between both and their close technological relevance to organic light emitting devices. By using grazing small angle incidence X-ray scattering (GISAXS), we follow the Al thin film development on top of Alq3 during radio frequency (rf) sputter deposition in realtime and without interrupting the growth process. Three growth stages have been clearly distinguished: Al diffusion into Alq3, Al/Alq3 complex agglomeration and self-assembled Al pillar nanostructure thin film development. Thus in situ GISAXS yields the fundamental insights into the formation of the metal/organic interface for small organic semiconductor devices, prepared via vacuum based deposition techniques. SECTION: Physical Processes in Nanomaterials and Nanostructures T he advancing of organic electronics demands a deep understanding of the interface structure and properties between metal thin films and organic layers. 1,2 In general, depending on the order of the depositionorganic-on-metal (organic/metal) and metal-on-organic (metal/organic)two types of interfaces can be formed with distinctive structures and properties. For the organic/metal interface, usually a sharp interface is found, whereas for metal/organic interfaces, a gradient metal diffusion layer within the molecular thin film can be present. Depending on the atomic or molecular interaction, different charge transfer mechanisms can be generated. 3,4 The thickness of the diffusion layer is depending on the applied metal deposition techniques. 5−8 Upon diffusing into the organic layer, the metal atoms will modify the properties of the organic film and this interaction will also tune the growth mode of the metal thin film on top.In applications, a typical organic electronic device, such as organic light emitting diodes (OLEDs), consists of a multilayer structure, incorporating different organic semiconductors and metallic materials as electrode contacts. 9 Since the first OLED d e v i c e w a s f a b r i c a t e d b y T a n g e t a l . , t r i s ( 8 -hydroxyquinolinato)aluminum (Alq3) has been one of the key materials in the research focus. 10 The interaction between metal electrodes (most commonly used are Mg, Al, Ag, and Au) and Alq3 at a molecular level has been intensively studied by different spectroscopy techniques 6−8,11−13 and theoretical studies. 14−16 All results confirm that reactive metals, such as Mg and Al, will form a compound with Alq3 via the oxygen atoms in the molecule, while noble metals, e.g., Ag and Au, are less reactive with Alq3. Recently, Fladischer et al. have shown that the combination of X-ray reflectivity (XRR) and transmission electron microscopy (TEM) can ...

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