Construction of permanent metal−molecule− metal (MMM) junctions, though technically challenging, is desirable for both fundamental investigations and applications of molecule-based electronics. In this study, we employed the nanotransfer printing (nTP) technique using perfluoropolyether (PFPE) stamps to print Au thin films onto self-assembled monolayers (SAMs) of alkanedithiol formed on Au thin films. We show that the resulting MMM junctions form permanent and symmetrical tunnel junctions, without the need for an additional protection layer between the top metal electrode and the molecular layer. This type of junction makes it possible for direct investigations into the electrical properties of the molecules and the metal−molecule interfaces. Dependence of transport properties on the length of the alkane molecules and the area of the printed Au electrodes has been examined systematically. From the analysis of the current−voltage (I−V) curves using the Simmons model, the height of tunneling barrier associated with the molecule (alkane) has been determined to be 3.5 +/- 0.2 eV, while the analysis yielded an upper bound of 2.4 eV for the counterpart at the interface (thiol). The former is consistent with the theoretical value of ∼3.5−5.0 eV. The measured I−V curves show scaling with respect to the printed Au electrode area with lateral dimensions ranging from 80 nm to 7 μm. These results demonstrate that PFPE-assisted nTP is a promising technique for producing potentially scalable and permanent MMM junctions. They also demonstrate that MMM structures (produced by the unique PFPE-assisted nTP) constitute a reliable test bed for exploring molecule-based electronics.
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Improved methods for imaging and assessment of vascular defects are needed for directing treatment of cardiovascular pathologies. In this paper, we employ magnetomotive optical coherence tomography (MMOCT) as a platform both to detect and to measure the elasticity of blood clots. Detection is enabled through the use of rehydrated, lyophilized platelets loaded with superparamagnetic iron oxides (SPIO-RL platelets) that are functional infusion agents that adhere to sites of vascular endothelial damage. Evidence suggests that the sensitivity for detection is improved over threefold by magnetic interactions between SPIOs inside RL platelets. Using the same MMOCT system, we show how elastometry of simulated clots, using resonant acoustic spectroscopy, is correlated with the fibrin content of the clot. Both methods are based upon magnetic actuation and phase-sensitive optical monitoring of nanoscale displacements using MMOCT, underscoring its utility as a broad-based platform to detect and measure the molecular structure and composition of blood clots.
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Atom probe tomography (APT) was used to quantify inhomogeneities in the distribution of Mn and Co in doped epitaxial Ge thin films for which X-ray diffraction (XRD) studies indicate single phase material. The segregation of dopants into Co and Mn-rich regions with characteristic sizes was evident upon visual inspection of the APT reconstruction and a frequency distribution analysis of the concentration of Co, Mn, and Ge atoms verified that the composition fluctuations exceeded those of a random alloy. Isoconcentration surfaces were generated to establish the connectedness of regions enriched in Mn that have been proposed to enhance the Curie temperature in dilute magnetic semiconductors. The analysis demonstrates important contributions that APT can make to the understanding of magnetism in these materials.
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Epitaxial growth and structural properties of Co_xMn_ySi_z thin films on Ge (111) substrates, including the Heusler alloy Co₂MnSi (111) have been studied using combinatorial molecular beam epitaxy (MBE) techniques. In situ reflection high energy electron diffraction and ex situ x-ray diffraction experiments show that high quality coherent MBE growth with fcc (111) stacking can be achieved over a relatively large composition space that includes Co₂MnSi. The highest structural and chemical ordering is observed near the composition of Co_0.63Mn_0.14Si_0.23 rather than that at the Heusler stoichiometry of Co₂MnSi. The in-plane crystallographic axis of the fcc film exhibits a 60 degree rotation with respect to that of the Ge substrate. The rotation appears to be originated at the film-substrate interface, as a result of the symmetry and stacking of the Ge (111) surface reconstruction.
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This article introduces a new functional imaging paradigm that uses optical coherence tomography (OCT) to detect rehydrated, lyophilized platelets (RL platelets) that are in the preclinical trial stage and contain superparamagnetic iron oxides (SPIOs) approved by the U.S. Food and Drug Administration. Platelets are highly functional blood cells that detect and adhere to sites of vascular endothelial damage by forming primary hemostatic plugs. By applying magnetic gradient forces, induced nanoscale displacements (magnetomotion) of the SPIO-RL platelets are detected as optical phase shifts in OCT. In this article, we characterize the iron content and magnetic properties of SPIO-RL platelets, construct a model to predict their magnetomotion in a tissue medium, and demonstrate OCT imaging in tissue phantoms and ex vivo pig arteries. Tissue phantoms containing SPIO-RL platelets exhibited >3 dB contrast/noise ratio at ≥ 1.5 x 10⁹ platelets/cm³. OCT imaging was performed on ex vivo porcine arteries after infusion of SPIO-RL platelets, and specific contrast was obtained on an artery that was surface-damaged (P < 10^-6). This may enable new technologies for in vivo monitoring of the adherence of SPIO-RL platelets to sites of bleeding and vascular damage, which is broadly applicable for assessing trauma and cardiovascular diseases.
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Magnetic anisotropy, magnetization reversal and the magnetooptic Kerr effect in Co_xMn_yGe_z have been studied over a range of compositions between 0 and 50 at.% of Ge and between 1 and 3 in the Co to Mn atomic ratio, including the Heusler alloy Co₂MnGe. A strong quadratic magnetooptic Kerr effect has been observed within a narrow region of composition centered around the Co to Mn atomic ratio of 2, which has been used to probe and quantify the magnetic anisotropy and magnetization reversal of the system. The anisotropy is sixfold with a weak uniaxial component, and it exhibits sensitive dependence on composition, especially on the atomic ratio between Co and Mn. The magnetization reversal process is consistent with the single-domain Stoner–Wohlfarth model.
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Magnetic anisotropy and magnetooptic Kerr effect for epitaxial films of Co_xMn_yGe_1−x−y grown on Ge (1 1 1) substrates have been studied systematically in the compositional vicinity of the Heusler alloy Co₂MnGe. A large quadratic magnetooptic Kerr effect has been observed within a narrow region of composition centered around the Co to Mn atomic ratio of 2. The effect has been used to probe and quantify the magnetic anisotropy of the system, which is shown to have a strong sixfold in-plane component accompanied by a weak uniaxial component at room temperature. These properties are shown to depend sensitively on atomic ratio between Co and Mn, indicating the presence of an intrinsic composition-driven phenomenon.
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Systematic investigation of structural, chemical, and magnetic properties of Co and Mn-doped Ge (001) as a function of doping concentration reveals that codoping with Co can dramatically reduce phase separation and diffusion of Mn within the Ge lattice while it magnetically complements Mn. The measured strain states indicate the critical role played by substitutional Co with its strong tendency to dimerize with interstitial Mn. Selecting appropriate codopants that form energetically stable dimers in a semiconductor host is shown to be a viable approach, thus demonstrating the feasibility for engineering stable doped magnetic semiconductors. .
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We report combinatorial molecular beam epitaxy synthesis and properties of a ternary epitaxial film of Co and Mn co-doped Ge grown on Ge (001) substrate. Structural effects were examined in situ by reflection high-energy electron diffraction and ex situ by microbeam X-ray diffraction techniques, and magnetic properties were probed by using magnetooptic Kerr effect. Ternary epitaxial phase diagrams have been studied for total doping concentrations up to 30 at.%, where regions of coherent epitaxy and rough disordered growth and those of near room temperature ferromagnetic ordering have been identified.
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We report high-resolution X-ray diffraction studies of combinatorial epitaxial Ge (001) thin-films with varying doping concentrations of Co and Mn grown on Ge (001) substrates. The crystalline structure of the epitaxial thin-film has been determined using crystal-truncation rod (CTR) measurements and fitting analysis. By analyzing the fine interference fringes in the CTR intensity profile, strain sensitivity of ~0.003% has been achieved. Using this method, the evolution of interfacial structures has been quantified as a function of doping concentration.
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The epitaxial growth of (FeCo)_xGe_1−x films on Ge and GaAs (001) substrates has been studied systematically with x in the range between 0 and 17 at. %, using combinatorial molecular beam epitaxy (MBE) techniques. Complementary doping using the two transition metal dopants into Ge (001) during MBE growth is shown to produce high quality coherent epitaxial films for transition metal concentrations as high as 11 at. %. As the doping level increases, rough growth occurs, which is accompanied by an increasing amount of stacking faults along the <111> directions. The crystal lattice that resulted from the rough growth exhibits a large out-of-plane tetragonal distortion. There are no detectable secondary phases up to a combined transition metal concentration of 17 at. %. The behaviors are shown to be invariant with respect to the choice of substrates.
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The authors report a study of structural and chemical disorders in a ternary combinatorial epitaxial film of Co_xMn_yGe_1−x−y in the composition range that includes the Heusler alloy Co₂MnGe, using microbeam anomalous x-ray diffraction techniques. The structural and chemical ordering of the alloy has been found to be extremely stable over a large composition range, while elemental site swapping and sublattice vacancies have been identified. A model of anomalous diffraction around the Co and Ge edges is presented and shown to make possible the identification and quantification of these disorders in an epitaxial film.
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The structural and magnetic phase diagrams of Co_a(1-x)Mn_axGe_b epitaxial films near the composition of Heusler alloy Co₂MnGe have been studied using combinatorial molecular beam epitaxy techniques. Epitaxial growth on Ge (111) has been stabilized over nearly the entire composition range of the ternary system. Epitaxial constraints play an important role for the small number of observed structural phases. The variety of high-temperature magnetic properties, and the structural and chemical compatibilities with group IV elements make the ternary system promising for the science and applications of spintronics.
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Basic considerations for implementing combinatorial approach to molecular beam epitaxy (MBE) are discussed, focusing on the key issues relevant to conventional MBE synthesis using solid sources and characterization. The primary objective for implementing combinatorial approach is to make MBE do more, more able to carry out controlled and systematic work in large parameter space, without sacrificing any existing capabilities of conventional MBE. Methods for accomplishing this by integrating current instrumentation technology are described.
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We describe a high-throughput scanning X-ray fluorescence (XRF) microscopy setup using a microfocused synchrotron X-ray beam, which is optimized for in-parallel X-ray characterization of composition and crystalline structure of combinatorial samples. We present X-ray fluorescence elemental maps of a full ternary CoxMnyGe_1−x−y composition-spread thin film and discuss the quantitative analysis method used for obtaining the ternary composition.
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In metal-carbon systems with known stable compounds, carbide nanocrystals self-organize epitaxially on metal surfaces to form two-dimensional arrays during carbon deposition. The process is energetically driven by the competition between the strain and surface energies, and it appears to play an important role in the nucleation of single-walled carbon nanotubes. Interplay between energetics and kinetics controls carbon precipitation from the superlattice, such that the length scale of the carbide and superlattice appears to control the size and morphology of the precipitates. Furthermore, carbon precipitates appear to be "seedlings" of carbon nanotubes grown on top of the carbide nanocrystals. These findings reveal that the nucleation of carbon nanotubes is a nonequilibrium process and that a stable carbide superlattice can be used as an ordered template of carbon saturated "roots" for nucleating nanotube bundles with controlled diameter, spacing, and perhaps chirality.
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We report on a study of the epitaxial phase diagram of Co- and Mn-doped Ge(001) magnetic semiconductors. Complementary doping using dopants from different groups of elements can compensate for the effects of lattice strain caused by the doping species. Reducing lattice mismatch with the Ge host has been shown to be the key to stabilizing epitaxial growth and suppressing phase separation at higher doping levels. Applying this approach to other multidopant systems opens new prospects for tailoring highly doped electronic materials.
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We discuss important experimental considerations and high-throughput synchrotron-based techniques for structural characterization of binary and ternary composition-spread thin films. We apply these techniques to obtain detailed structural phase diagrams of CoMnGe ternary alloy system.
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We describe the recent discovery of promising new Ge-based magnetic semiconductors and heterostructures using combinatorial molecular-beam epitaxy for the science and applications of spintronics. We discuss key experimental considerations for implementing combinatorial synthesis and characterization, and highlight important findings in epitaxial films of (100) Ge doped by Co and Mn, specifically the ternary epitaxial phase-diagram, and the novel magnetic and electrical-transport phenomena. We illustrate the natural "marriage" between the controlled synthesis and combinatorial approach, and demonstrate the usefulness of the approach for studying complex epitaxial process.
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