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"Scheer, Elke"
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Synthesis of graphene–transition metal oxide hybrid nanoparticles and their application in various fields
Single layer graphite, known as graphene, is an important material because of its unique two-dimensional structure, high conductivity, excellent electron mobility and high surface area. To explore the more prospective properties of graphene, graphene hybrids have been synthesised, where graphene has been integrated with other important nanoparticles (NPs). These graphene–NP hybrid structures are particularly interesting because after hybridisation they not only display the individual properties of graphene and the NPs, but also they exhibit further synergistic properties. Reduced graphene oxide (rGO), a graphene-like material, can be easily prepared by reduction of graphene oxide (GO) and therefore offers the possibility to fabricate a large variety of graphene–transition metal oxide (TMO) NP hybrids. These hybrid materials are promising alternatives to reduce the drawbacks of using only TMO NPs in various applications, such as anode materials in lithium ion batteries (LIBs), sensors, photocatalysts, removal of organic pollutants, etc. Recent studies have shown that a single graphene sheet (GS) has extraordinary electronic transport properties. One possible route to connecting those properties for application in electronics would be to prepare graphene-wrapped TMO NPs. In this critical review, we discuss the development of graphene–TMO hybrids with the detailed account of their synthesis. In addition, attention is given to the wide range of applications. This review covers the details of graphene–TMO hybrid materials and ends with a summary where an outlook on future perspectives to improve the properties of the hybrid materials in view of applications are outlined.
Journal Article
Molecular electronics
This book provides a comprehensive overview of the rapidly developing field of molecular electronics. It focuses on our present understanding of the electrical conduction in single-molecule circuits and provides a thorough introduction to the experimental techniques and theoretical concepts. It will also constitute as the first textbook-like introduction to both the experiment and theory of electronic transport through single atoms and molecules. In this sense, this publication will prove invaluable to both researchers and students interested in the field of nanoelectronics and nanoscience in general.
eBook
Modulated critical currents of spin-transfer torque-induced resistance changes in NiCu/Cu multilayered nanowires
We present a novel method combining anodic aluminum oxide template synthesis and nanolithography to selectively deposit vertically patterned magnetic nanowires on a Si substrate. With this approach we fabricated three-dimensional nanowire-based spin valve devices without the need of complex etching processes or additional spacer coating. Through this method, we successfully obtained NiCu/Cu multilayered nanowire arrays with a controlled sequence along the long axis of the nanowires. Both magnetic switching and excitation phenomena driven by spin-polarized currents were clearly demonstrated in our NiCu/Cu multilayered nanowires. Moreover, the critical currents for switching and excitation were observed to be modulated in an oscillatory manner by the magnetic field in the nanowire-based devices. We present a toy model to qualitatively explain these observations.
Journal Article
Emerging magnetic order in platinum atomic contacts and chains
The development of atomic-scale structures revealing novel transport phenomena is a major goal of nanotechnology. Examples include chains of atoms that form while stretching a transition metal contact or the predicted formation of magnetic order in these chains, the existence of which is still debated. Here we report an experimental study of the magneto-conductance (MC) and anisotropic MC with atomic-size contacts and mono-atomic chains of the nonmagnetic metal platinum. We find a pronounced and diverse MC behaviour, the amplitude and functional dependence change when stretching the contact by subatomic distances. These findings can be interpreted as a signature of local magnetic order in the chain, which may be of particular importance for the application of atomic-sized contacts in spintronic devices of the smallest possible size.
The development of magnetism in metallic atomic chains is a widely debated phenomenon, of relevance to atomic-scale spintronics. Here, Strigl
et al
. measure the magneto-conductance of platinum break junctions stretched over sub-atomic distances, evidencing the evolution of distinct magnetic order.
Journal Article
Characterization of thin-film adhesion and phonon lifetimes in Al/Si membranes by picosecond ultrasonics
We quantitatively study interfacial adhesion in a two-layer membrane system consisting of Al and Si with femtosecond time-resolved laser spectroscopy. High-frequency acoustic pulses in the sub-THz regime are utilized to characterize the membrane system. In order to explain the distinct features of the measured data, a spring model for the Al/Si interface is employed. We show that acoustic dissipation in this system needs to be included for accurate modeling of the interface adhesion over a broad frequency range. This modeling approach yields a spring constant of Al − Si = 17 kg nm − 2 s − 2 , an acoustic phonon lifetime of τ Al = 68 ps at 240 GHz in polycrystalline Al and a frequency dependence of the lifetime in Si ∝ − 1 in the frequency range from 50-800 GHz.
Journal Article
Molecular dimer junctions forming: Role of disulfide bonds and electrode‐compression‐time
Thanks to their excellent bond strength, phenyl‐based molecules with thiol anchoring groups are extensively employed to form stable single‐molecule junctions. However, two critical questions are still not answered which seriously hinder high‐yield establishing reliable molecular functional devices: (1) Whether molecular dimer junctions will be formed, and if this is the case, whether the dimerization is caused by intermolecular disulfide bonds or π–π stacking of phenyl rings; (2) Upon a mechanical‐compression force, is it possible that both anchoring groups of the molecule bond to the same electrode instead of bridging two opposite electrodes, which would drastically reduce the yield of the molecular junctions. Here, combining UV‐Vis/Raman spectroscopy of bulk molecules and conductance/flicker‐noise measurements of single molecules, we give compelling evidence that molecular dimers naturally form under ambient conditions, primarily via disulfide bonds rather than by π–π stacking. We further proposed a technique, named electrode‐compression‐hold‐on (ECHO), and reveal that the two thiol groups of phenyl‐backboned molecules will bond to the same electrode upon a compression force with a prolongated ECHO time. In contrast, the compression‐time‐dependent phenomenon is not observed for alkyl‐backboned molecules. The underlying mechanism for these unprecedented observations is elucidated, shedding light on the yield of molecular junctions.
We clarified that the molecular dimers naturally form primarily through disulfide bonds rather than π–π stacking and revealed that the yield of the thiols‐terminated molecular junction strongly depends on the mechanical electrode‐compression‐time.
Journal Article
Silicon-based MEMS/NEMS empowered by graphene: a scheme for large tunability and functionality
Integration of graphene in silicon-based micro-/nanoelectromechanical systems (MEMS/NEMS) marries the robustness of silicon-based materials with the exceptional physical properties of graphene, drastically enhancing the system’s regulation performance which now is key for many advanced applications in nanotechnology. Here, we experimentally demonstrate and theoretically analyze a powerful on-chip integration principle consisting of a hybrid graphene/silicon nitride membrane with metallic leads on top that enables an extremely large static and dynamic parameter regulation. When a static voltage is applied to the leads of the integrated structure, a spatially confined localized electrothermomechanical (ETM) effect results in ultra-wide frequency tuning, deformation (buckling transition) and regulation of the mechanical properties. Moreover, by injecting an alternating voltage to the leads, we can excite the resonator vibrating even far beyond its linear regime without a complex and space consuming actuation system. Our results prove that the scheme provides a compact integrated system possessing mechanical robustness, high controllability, and fast response. It not only expands the limit of the application range of MEMS/NEMS devices, but also enables the further miniaturization of the device.
The graphene integrated MEMS/NEMS empowered by graphene: a scheme for strong enhancements of tunability and functionality of silicon based device device consists of a hybrid graphene/silicon-nitride membrane with metallic leads that enables ultra-wide frequency tuning, spatial deflection, mechanical properties tuning and on-surface actuation.
Journal Article
Microwave-induced direct spin-flip transitions in mesoscopic Pd/Co heterojunctions
We experimentally investigate the effect of resonant microwave absorption on the magneto-conductance of tunable Co/Pd point contacts. At the interface a non-equilibrium spin accumulation is created via microwave absorption and can be probed via point contact spectroscopy. We interpret the results as a signature of direct spin-flip excitations in Zeeman-split spin-subbands within the Pd normal metal part of the junction. The inverse effect, which is associated with the emission of a microwave photon in a ferromagnet/normal metal point contact, can also be detected via its unique signature in transport spectroscopy.
Journal Article
Inelastic electron tunneling spectroscopy of difurylethene-based photochromic single-molecule junctions
Diarylethene-derived molecules alter their electronic structure upon transformation between the open and closed forms of the diarylethene core, when exposed to ultraviolet (UV) or visible light. This transformation results in a significant variation of electrical conductance and vibrational properties of corresponding molecular junctions. We report here a combined experimental and theoretical analysis of charge transport through diarylethene-derived single-molecule devices, which are created using the mechanically controlled break-junction technique. Inelastic electron tunneling (IET) spectroscopy measurements performed at 4.2 K are compared with first-principles calculations in the two distinct forms of diarylethenes connected to gold electrodes. The combined approach clearly demonstrates that the IET spectra of single-molecule junctions show specific vibrational features that can be used to identify different isomeric molecular states by transport experiments.
Journal Article