Organic thin-film devices are of interest for a variety of forthcoming ubiquitous electronic applications. [1,2] In order to take full advantage of the potential of organic semiconductors, the improvement of crystallinity is indispensable. Unfortunately, promising organic molecules that have a large overlap of p-orbitals between the molecules cannot migrate freely on a substrate [3] because of the stronger cohesion between the molecules than the interaction between the molecule and the substrate. Therefore, enhancement of the molecule-substrate interaction, that is, the 'molecular wettability' should promote crystallization. Here, we show that the use of a substrate covered with an atomically flat pentacene (C 22 H 14 ) monomolecular layer can drastically increase the crystallinity of C 60 films and increase the field-effect mobilities of C 60 transistors to 2.0-4.9 cm 2 V -1 s -1 , which is a four-to fivefold improvement over C 60 films grown without a pentacene buffer. The observation of the initial growth stages indicates that control of the molecular wettability of the substrate by an atomically flat pentacene buffer caused the improvement of crystallinity in the C 60 films. Molecular-wetting-controlled substrates can thus offer a general solution to the fabrication of high-performance crystalline plastic and molecular-electronic applications. In order to fabricate complex electronic devices such as field-effect transistors (FETs), for ubiquitous electronics, organic thin films need to be grown on many different substrate materials to fit the requirements of particular applications.However, most organic compounds grown on such practical substrates as oxide dielectrics show a poor crystallinity. Because poor crystallinity of the active layers suppresses the device performance markedly, crystalline organic films that have good crystallinity and few grain boundaries are required for a high-performance operation. The molecular wettability of a substrate controls the distance of the lateral migration of the molecules at the film growth front and thus largely determines the morphology of the growing film. The wettability on a substrate surface is determined by the ratio of the cohesion strength among the molecules and the adhesion of the molecules to the substrate surface, as shown in Figure 1a. If the balance between the cohesion and adhesion forces could be controlled by inserting a thin buffer layer, we would expect to obtain organic films with improved crystallinity. Surface modification of a substrate by self-assembled monolayers (SAMs) has been used for the suppression of disorder in organic films near electrodes, [4] and for controlling the carrier density in organic FETs.
In a newly proposed switching device using polycrystalline HfO2 thin film with ion diffusion path, we have found that a Cu electrode could contribute to improved switching performance. Current–voltage measurements at room temperature revealed clear resistive switching, not accompanied by a forming process, in our Cu/HfO2/Pt structure. The current step difference from one state to the other one was in the order of 103–104, giving a sufficient on/off ratio. Voltage sweep polarity suggested that filamentary Cu paths were formed due to Cu ion diffusion and annihilated at the HfO2/Pt interface at reversed bias. This filament path formation and annihilation was the origin of the switching device performance.
The combinatorial method, which was exclusively employed for the drug discovery until recently, has invaded the field of inorganic materials and is becoming a key technology in materials science today. Phosphors, with their diversity of possible mechanisms of luminescence, represent a typical example of a field where the combinatorial approach promises to be especially fruitful. Here, we present the results of systematic combinatorial exploration of different binary and ternary ZnO:dopant systems, which resulted in identification of bright luminescence in ZnO:(Y,Eu), ZnO:V, ZnO:W, and ZnO:(W,Mg) systems. Careful “zooming in”, i.e., fabrication and screening of more detailed libraries near the identified promising compositions, allowed us to find optimum phosphor compositions for the above‐mentioned systems. The efficiency of the new phosphors in low‐voltage cathodoluminescence is high and promises their prospective use in advanced flat panel display and lighting applications.
We report the remarkable effects of physical and chemical treatments of substrate surface on the physical vapor deposition of rubrene thin films. Highly c-axis oriented rubrene thin films were fabricated by combinatorial molecular beam epitaxy on atomically flat α-Al2O3 (0001) substrates, the surface of which was partially modified with pentacene buffer film. Rubrene thin films grown at room temperature on a sapphire substrate without pentacene buffer layer did not exhibit any X-ray diffraction pattern, whereas films deposited on a pentacene buffer layer exhibited peaks of c-axis orientation. Atomic force microscope images of the crystalline films show the steps of 1.3 nm height which correspond to the half c-axis length of the rubrene crystal. Preliminary, p-type operation was observed in bottom-gate field effect transistors using this rubrene film deposited on a pentacene buffer.
The interfacial electronic states of a Pt/HfO2/Pt diode were investigated by using hard x-ray photoelectron spectroscopy under bias operation. The application of a forward bias to the Pt/HfO2/Pt diode increased the Pt–O bonding peak, providing evidence of Pt electrode oxidization and oxygen vacancy formation around the Pt/HfO2 interface. Under a reverse bias, hafnium was drawn to the Pt electrode, where it took part in Hf–Pt bonding. We achieved the direct observation of oxygen migration at a Pt/HfO2 interface under device operation, which is the key to controlling the electrical properties of metals on oxides.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.