Giant and Broadband THz and IR Emission in Drift-biased Graphene-Based Hyperbolic Nanostructures

We demonstrate that Cherenkov radiation can be manipulated in terms of operation frequency, bandwidth, and efficiency by simultaneously controlling the properties of drifting electrons and the photonic states supported by their surrounding media. We analytically show that the radiation rate strongly depends on the momentum of the excited photonic state, in terms of magnitude, frequency dispersion, and its variation versus the properties of the drifting carriers. This approach is applied to design and realize miniaturized, broadband, tunable, and efficient terahertz and far-infrared sources by manipulating and boosting the coupling between drifting electrons and engineered hyperbolic modes in graphene-based nanostructures. The broadband, dispersive, and confined nature of hyperbolic modes relax momentum matching issues, avoid using electron beams, and drastically enhance the radiation rate - allowing that over 90% of drifting electrons emit photons. Our findings open a new paradigm for the development of solid-state terahertz and infrared sources.Comment: 4 figure

Stabilizing single atom contacts by molecular bridge formation

Gold-molecule-gold junctions can be formed by carefully breaking a gold wire in a solution containing dithiolated molecules. Surprisingly, there is little understanding on the mechanical details of the bridge formation process and specifically on the role that the dithiol molecules play themselves. We propose that alkanedithiol molecules have already formed bridges between the gold electrodes before the atomic gold-gold junction is broken. This leads to stabilization of the single atomic gold junction, as observed experimentally. Our data can be understood within a simple spring model.Comment: 14 pages, 3 figures, 1 tabl

Electron transport through 8-oxoG: NEGF/DFT study

We present a first-principles study of the conductance of Guanine and 8-Oxoguanine (8-oxoG) attached to Au(111) electrodes. Cellular levels of 8-oxoG have been found in larger concentrations in cancer patients. The current through the structure was calculated using a DFT–NEGF formalism. We have compared flat and pyramidal electrode geometries and show that there is a measurable difference between the I–V characteristics of the pristine molecule and the 8-oxoG. For a flat electrode geometry, 8-oxoG produces a 2.57 (18.3) times increase in current than the corresponding counterpart at 3 V with a bond separation of 1.2 Å (2.4 Å). This can be attributed to molecular orbital energies shifting at the junction. Overall the flat geometry produces larger currents. We have also investigated the sensitivity of the current to the electrode molecule separation. For the flat geometry, the current dropped approximately 80% (97%) for 8-oxoG (pristine Guanine) with the doubling of the electrode separation

Fast electron transfer through a single molecule natively structured redox protein

The electron transfer properties of proteins are normally measured as molecularly averaged ensembles. Through these and related measurements, proteins are widely regarded as macroscopically insulating materials. Using scanning tunnelling microscopy (STM), we present new measurements of the conductance through single-molecules of the electron transfer protein cytochrome b562 in its native conformation, under pseudo-physiological conditions. This is achieved by thiol (SH) linker pairs at opposite ends of the molecule through protein engineering, resulting in defined covalent contact between a gold surface and a platinum–iridium STM tip. Two different orientations of the linkers were examined: a long-axis configuration (SH-LA) and a short-axis configuration (SH-SA). In each case, the molecular conductance could be ‘gated’ through electrochemical control of the heme redox state. Reproducible and remarkably high conductance was observed in this relatively complex electron transfer system, with single-molecule conductance values peaking around 18 nS and 12 nS for the SH-SA and SH-LA cytochrome b562 molecules near zero electrochemical overpotential. This strongly points to the important role of the heme co-factor bound to the natively structured protein. We suggest that the two-step model of protein electron transfer in the STM geometry requires a multi-electron transfer to explain such a high conductance. The model also yields a low value for the reorganisation energy, implying that solvent reorganisation is largely absent

Insulated molecular wires: inhibiting orthogonal contacts in metal complex based molecular junctions

Metal complexes are receiving increased attention as molecular wires in fundamental studies of the transport properties of metal|molecule|metal junctions. In this context we report the single-molecule conductance of a systematic series of d8 square-planar platinum(II) trans-bis(alkynyl) complexes with terminal trimethylsilylethynyl (C[triple bond, length as m-dash]CSiMe3) contacting groups, e.g. trans-Pt{C[triple bond, length as m-dash]CC6H4C[triple bond, length as m-dash]CSiMe3}2(PR3)2 (R = Ph or Et), using a combination of scanning tunneling microscopy (STM) experiments in solution and theoretical calculations using density functional theory and non-equilibrium Green's function formalism. The measured conductance values of the complexes (ca. 3–5 × 10−5G0) are commensurate with similarly structured all-organic oligo(phenylene ethynylene) and oligo(yne) compounds. Based on conductance and break-off distance data, we demonstrate that a PPh3 supporting ligand in the platinum complexes can provide an alternative contact point for the STM tip in the molecular junctions, orthogonal to the terminal C[triple bond, length as m-dash]CSiMe3 group. The attachment of hexyloxy side chains to the diethynylbenzene ligands, e.g. trans-Pt{C[triple bond, length as m-dash]CC6H2(Ohex)2C[triple bond, length as m-dash]CSiMe3}2(PPh3)2 (Ohex = OC6H13), hinders contact of the STM tip to the PPh3 groups and effectively insulates the molecule, allowing the conductance through the full length of the backbone to be reliably measured. The use of trialkylphosphine (PEt3), rather than triarylphosphine (PPh3), ancillary ligands at platinum also eliminates these orthogonal contacts. These results have significant implications for the future design of organometallic complexes for studies in molecular junctions

A Chirality-Based Quantum Leap

There is increasing interest in the study of chiral degrees of freedom occurring in matter and in electromagnetic fields. Opportunities in quantum sciences will likely exploit two main areas that are the focus of this Review: (1) recent observations of the chiral-induced spin selectivity (CISS) effect in chiral molecules and engineered nanomaterials and (2) rapidly evolving nanophotonic strategies designed to amplify chiral light-matter interactions. On the one hand, the CISS effect underpins the observation that charge transport through nanoscopic chiral structures favors a particular electronic spin orientation, resulting in large room-temperature spin polarizations. Observations of the CISS effect suggest opportunities for spin control and for the design and fabrication of room-temperature quantum devices from the bottom up, with atomic-scale precision and molecular modularity. On the other hand, chiral-optical effects that depend on both spin- and orbital-angular momentum of photons could offer key advantages in all-optical and quantum information technologies. In particular, amplification of these chiral light-matter interactions using rationally designed plasmonic and dielectric nanomaterials provide approaches to manipulate light intensity, polarization, and phase in confined nanoscale geometries. Any technology that relies on optimal charge transport, or optical control and readout, including quantum devices for logic, sensing, and storage, may benefit from chiral quantum properties. These properties can be theoretically and experimentally investigated from a quantum information perspective, which has not yet been fully developed. There are uncharted implications for the quantum sciences once chiral couplings can be engineered to control the storage, transduction, and manipulation of quantum information. This forward-looking Review provides a survey of the experimental and theoretical fundamentals of chiral-influenced quantum effects and presents a vision for their possible future roles in enabling room-temperature quantum technologies.ISSN:1936-0851ISSN:1936-086