RESUMEN
The complex behavior of the simplest atomic-scale conductors indicates that the electrode structure itself is significant in the design of future nanoscale devices. In this study, the structural asymmetry of metallic atomic contacts formed between two macroscopic Au electrodes at room temperature was investigated. Characteristic signatures of the structural asymmetries were detected by fast current-voltage (I-V) measurements with a time resolution of approximately 100 µs. Statistical analysis of more than 300,000 I-V curves obtained from more than 1000 contact-stretching processes demonstrates that the current rectification properties are correlated with the conductance of the nanocontacts. A substantial suppression of the variation in current rectification was observed for the atomic contacts with integer multiples of the conductance quantum. Statistical analysis of the time-resolved I-V curves revealed that the current rectification variations increased significantly from 500 µs onward before the breakage of the atomic contacts. Ab initio atomistic simulations of the stretching processes and corresponding I-V characteristics confirmed the magnitude of the rectification and related it to the structural asymmetries in the breakdown process of the junctions. Overall, we provide a better understanding of the interplay between geometric and electronic structures at atomically defined metal-metal interfaces by probing charge transport properties in extremely sensitive nanocontacts.
RESUMEN
The first report of a quantized conductance atomic threshold switch (QCATS) using an atomically-thin hexagonal boron nitride (hBN) layer is provided. This QCATS has applications in memory and logic devices. The QCATS device shows a stable and reproducible conductance quantization state at 1·G0 by forming single-atom point contact through a monoatomic boron defect in an hBN layer. An atomistic switching mechanism in hBN-QCATS is confirmed by in situ visualization of mono-atomic conductive filaments. Atomic defects in hBN are the key factor that affects the switching characteristic. The hBN-QCATS has excellent switching characteristics such as low operation voltage of 0.3 V, low "off" current of 1 pA, fast switching of 50 ns, and high endurance > 107 cycles. The variability of switching characteristics, which are the major problems of switching device, can be solved by reducing the area and thickness of the switching region to form single-atom point contact. The switching layer thickness is scaled down to the single-atom (≈0.33 nm) h-BN layer, and the switching area is limited to single-atom defects. By implementing excellent switching characteristics using single-layer hBN, the possibility of implementing stable and uniform atomic-switching devices for future memory and logic applications is confirmed.
RESUMEN
Heme proteins are ideal systems to investigate vibrational energy flow at the atomic level. Upon photoexcitation, a large amount of excess vibrational energy is selectively deposited on heme due to extremely fast internal conversion. This excess energy is redistributed to the surrounding protein moiety and then to water. Vibrational energy flow in myoglobin (Mb) was examined using picosecond time-resolved anti-Stokes ultraviolet resonance Raman (UVRR) spectroscopy. We used the Trp residue directly contacting the heme group as a selective probe for vibrationally excited populations. Trp residues were placed at different position close to the heme by site-directed mutagenesis. This technique allows us to monitor the excess energy on residue-to-residue basis. Anti-Stokes UVRR measurements for Mb mutants suggest that the dominant channel for energy transfer in Mb is the pathway through atomic contacts between heme and nearby amino acid residues as well as that between the protein and solvent water. It is found that energy flow through proteins is analogous to collisional exchange processes in solutions.
RESUMEN
The down-scaling of electrical components requires a proper understanding of the physical mechanisms governing charge transport. Here, we have investigated atomic-scale contacts and their transport characteristics on WS2 using conductive atomic force microscopy (c-AFM). We demonstrate that c-AFM can provide true atomic resolution, revealing atom vacancies, adatoms, and periodic modulations arising from electronic effects. Moreover, we find a lateral variation of the surface conductivity that arises from the lattice periodicity of WS2. Three distinct sites are identified, i.e., atop, bridge, and hollow. The current transport across these atomic metal-semiconductor interfaces is understood by considering thermionic emission and Fowler-Nordheim tunnelling. Current modulations arising from point defects and the contact geometry promise a novel route for the direct control of atomic point contacts in diodes and devices.
RESUMEN
The optical control of atomic relocations in a metallic quantum point contact is of great interest because it addresses the fundamental limit of "CMOS scaling". Here, by developing a platform for combined electronics and photonics on the atomic scale, we demonstrate an optically controlled electronic switch based on the relocation of atoms. It is shown through experiments and simulations how the interplay between electrical, optical, and light-induced thermal forces can reversibly relocate a few atoms and enable atomic photodetection with a digital electronic response, a high resistance extinction ratio (70 dB), and a low OFF-state current (10 pA) at room temperature. Additionally, the device introduced here displays an optically induced pinched hysteretic current (optical memristor). The photodetector has been tested in an experiment with real optical data at 0.5 Gbit/s, from which an eye diagram visualizing millions of detection cycles could be produced. This demonstrates the durability of the realized atomic scale devices and establishes them as alternatives to traditional photodetectors.
RESUMEN
We report the development of a novel method to determine the thermopower of atomic-sized gold contacts at low temperature. For these measurements a mechanically controllable break junction (MCBJ) system is used and a laser source generates a temperature difference of a few kelvins across the junction to create a thermo-voltage. Since the temperature difference enters directly into the Seebeck coefficient S = -ΔV/ΔT, the determination of the temperature plays an important role. We present a method for the determination of the temperature difference using a combination of a finite element simulation, which reveals the temperature distribution of the sample, and the measurement of the resistance change due to laser heating of sensor leads on both sides next to the junction. Our results for the measured thermopower are in agreement with recent reports in the literature.
RESUMEN
The atom sets an ultimate scaling limit to Moore's law in the electronics industry. While electronics research already explores atomic scales devices, photonics research still deals with devices at the micrometer scale. Here we demonstrate that photonic scaling, similar to electronics, is only limited by the atom. More precisely, we introduce an electrically controlled plasmonic switch operating at the atomic scale. The switch allows for fast and reproducible switching by means of the relocation of an individual or, at most, a few atoms in a plasmonic cavity. Depending on the location of the atom either of two distinct plasmonic cavity resonance states are supported. Experimental results show reversible digital optical switching with an extinction ratio of 9.2 dB and operation at room temperature up to MHz with femtojoule (fJ) power consumption for a single switch operation. This demonstration of an integrated quantum device allowing to control photons at the atomic level opens intriguing perspectives for a fully integrated and highly scalable chip platform, a platform where optics, electronics, and memory may be controlled at the single-atom level.
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The molecular basis of life rests on the activity of biological macromolecules, mostly nucleic acids and proteins. A perhaps surprising finding that crystallized over the last handful of decades is that geometric reasoning plays a major role in our attempt to understand these activities. In this paper, we address this connection between geometry and biology, focusing on methods for measuring and characterizing the shapes of macromolecules. We briefly review existing numerical and analytical approaches that solve these problems. We cover in more details our own work in this field, focusing on the alpha shape theory as it provides a unifying mathematical framework that enable the analytical calculations of the surface area and volume of a macromolecule represented as a union of balls, the detection of pockets and cavities in the molecule, and the quantification of contacts between the atomic balls. We have shown that each of these quantities can be related to physical properties of the molecule under study and ultimately provides insight on its activity. We conclude with a brief description of new challenges for the alpha shape theory in modern structural biology.
RESUMEN
We report on the electronic transport through nanoscopic metallic contacts under the influence of external light fields. Various processes can be of relevance here, whose underlying mechanisms can be studied by comparing different kinds of atomic contacts. For this purpose two kinds of contacts, which were established by electrochemical deposition, forming a gate-controlled quantum switch (GCQS), have been studied. We demonstrate that in these kinds of contacts thermal effects resulting from local heating due to the incident light, namely thermovoltage and the temperature dependences of the electrical resistivity and the electrochemical (Helmholtz) double layer are the most prominent effects.