Chi-Yueh Kao

Magnetoresistance in an All‐Organic‐Based Spin Valve

Figure 1 . a) Device structure and b) schematic view of the energy levels of V[TCNE] x ( x ≈ 2). Spintronics is the new paradigm of electronics and utilizes the spin degree of freedom of the electron. [ 1 ] In addition to metalbased spintronic devices, which already have wide applications (e.g., read heads of hard disk drives), semiconductor spintronics provides the possibility to combine logic, communication, and storage operation using hybrid devices. [ 2 ] Recently, spintronic devices based on organic materials have attracted much attention because of the potential long spin lifetime due to low spin-orbit coupling and weak hyperfi ne interaction, as well as the ability to fabricate low-cost, light-weight, and mechanically fl exible devices. [ 3–9 ] Organic semiconductors have been used as the spacers in spin valve devices with ferromagnetic metal or transitional metal oxide contacts. One fundamental obstacle for spin injection from a ferromagnetic metal into a semiconductor is the conductivity mismatch. [ 10 ] The development of fully spinpolarized magnetic semiconductors offers a promising route to circumvent this problem. Organic-based magnets such as V[TCNE] x ( x ≈ 2, TCNE: tetracyanoethylene) provide advantages including high magnetic ordering temperature, fully spin-polarized semiconducting electronic structure, chemical tunability, and low-temperature processing. [ 11 ] Here, we show results for spin injection and detection in an all-organic-based spin valve using the organic magnetic semiconductor V[TCNE] x as both the spin injector and analyzer. We observed unusual inverted spin valve effect and propose that the negative magnetoresistance (MR) originates from the spin-dependent tunneling between highly spin-polarized bands split by Coulomb interaction. V[TCNE] x is the fi rst reported room-temperature moleculebased magnet with magnetic ordering temperature T C ≈ 400 K. [ 12 ] The magnetic order originates from the antiferromagnetic coupling between the three unpaired electrons of the V 2 +

Molecular layer deposition of an organic-based magnetic semiconducting laminate.

Organic-based magnets are intriguing materials with unique magnetic and electronic properties that can be tailored by chemical methodology. By using molecular layer deposition (MLD), we demonstrate the thin film fabrication of V[TCNE: tetracyanoethylene](x), of the first known room temperature organic-based magnet. The resulting films exhibit improvement in surface morphology, larger coercivity (80 Oe), and higher Curie temperature/thermal stability (up to 400 K). Recently, the MLD method has been widely studied to implement fine control of organic film growth for various applications. This work broadens its application to magnetic and charge transfer materials and opens new opportunities for metal-organic hybrid material development and their applications in various multilayer film device structures. Finally, we demonstrate the applicability of the multilayer V[TCNE](x) as a spin injector combining LSMO, an standard inorganic magnetic semiconductor, for spintronics applications.

Room-temperature organic-based spin polarizer

We report a magnetic tunnel junction operating at room-temperature with organic magnetic semiconductor V[TCNE]x (x ∼ 2, TCNE: tetracyanoethylene) and Fe as the spin polarizer and analyzer while 10 nm rubrene layer serves as the tunnel barrier between them. At room-temperature, the magnetoresistance (MR) presents 16.7% of its peak value at 100 K. We observed sign inversion of MR with increasing temperature, while the sign of the MR is independent of the polarity of the bias voltages. Our results suggest that V[TCNE]x is a promising material for room-temperature spintronic applications.

Organic photovoltaic cells with nano-fabric heterojunction structure

“nanofabrics” are investigated. Nano-fabric heterojunctions of poly(3-hexylthiophene) and electron-transporting nanofibers significantly improve short-circuit current density in organic photovoltaic cells. The nanofibers and nanofabric are synthesized from organic electrontransporting material bis(octyl)-perylenediimide (PDI-C8). The PDI-C8 based nano-fabric’s electron mobility is measured to be 0.08 cm 2 /V s. The nanofabric improves charge collection by expanding the interfacial acceptor-donor area while simultaneously providing dedicated electron transport pathways to the LiF/Al electrodes. An increase in fill factor is observed for photovoltaic cells incorporating the nanofabric heterojunctions and is attributed to efficient removal of space charge. V C 2012 American Institute of Physics. [doi:10.1063/1.3679097]

New advances in organic spintronics

The basic components of spintronic devices are spin polarized ferromagnets and spin transporting non-magnetic spacers. Exploiting carbon-based materials for these components promises to extend functionality of information storage and processing as well as to improve device integration and fabrication. Here we present the magnetoresistance of organic semiconductor rubrene (C42H28) used as a spacer in La2/3Sr1/3MnO3 (LSMO)/organic semiconductor (OSC)/Fe heterojunctions. Efficient spin polarized tunneling through the thin layer of rubrene spacer (5 nm) was observed. As the thickness of rubrene layers is increased, device current is strongly limited by carrier injection resulting in strong temperature dependent device resistance. The carrier injection is described with thermionic field emission at the metal/OSC interface. As a next step toward organic spintronics we used an organic based magnet vanadium-tetracyanoethylene (V(TCNE)x, x~2) in tandem with LSMO in a spin- valve with 5 nm rubrene spacer. V(TCNE)x is the earliest developed room temperature molecule-based magnet (Tc ~ 400 K). Due to strong on-site Coulomb interaction and weak intermolecule overlapping their magnetic state can be described with a model of half-semiconductor in which valence and conduction bands are spin polarized. The magnetoresistance data for bulk V(TCNE)x is in agreement with the model of spin polarized valence and conduction bands. We demonstrated that an organic-based magnetic semiconductor V(TCNE)x functions very well as an electron spin polarizer in the standard spintronic device geometry.

Thin films of organic-based magnetic materials of vanadium and cobalt tetracyanoethylene by molecular layer deposition

Thin films of Co[TCNE]x and CoxVy[TCNE]z were grown by molecular layer deposition (MLD). Metallic cobalt (Co0) was observed in the Co[TCNE]x film which contributes to the magnetism showing a coercivity of 450 Oe at 5 K with magnetic ordering temperature above 300 K. A thin film of CoxVy[TCNE]z showed magnetic ordering temperature up to 300 K with a coercivity of 50 Oe at 5 K and 30 Oe at 300 K. The film growth was monitored using a quartz crystal microbalance (QCM), and chemical compositions were characterized by X-ray photoelectron microscopy (XPS). Compared to materials previously synthesized from solution and chemical vapor deposition (CVD) methods, thin films made in this work are smooth and pinhole-free that can be beneficial for the development of thin film devices. The chemical composition of MLD-made films and the presence of Co islands are also shown here.

Thin‐Film Deposition of an Organic Magnet Based on Vanadium Methyl Tricyanoethylenecarboxylate

The preparation and characterization of a new thin-film organic-based magnet V[MeTCEC]x (V = vanadium; MeTCEC = methyl tricaynoethylenecarboxylate) via low-temperature chemical vapor deposition (50 °C) is reported. These thin films exhibit room-temperature magnetic ordering and semiconducting behavior, demonstrating the ability of tuning their magnetic, and potentially spintronic, functionality via chemical modification of the organic ligand.

Investigation of thin films of organic-based magnets grown by physical vapor deposition

Thin films of organic-based magnet, V[TCNE]x (TCNE: tetracyanoethylene), were deposited by physical vapor deposition (PVD) based reactive evaporation. The growth conditions were studied in detail. A saturated composition of V[TCNE]∼1.9 was determined by optimizing the growth condition. Two sets of films with different V to TCNE ratios were characterized. Both films were magnetic ordered up to 400 K and held coercive field of 60 Oe at room temperature. With the presence of excess vanadium within the film, the increase of defects created by PVD results in significant change in electronic property.