Complementary Resistive Switching Using Metal–Ferroelectric–Metal Tunnel Junctions

Complementary resistive switching (CRS) devices are receiving attention because they can potentially solve the current‐sneak and current‐leakage problems of memory arrays based on resistive switching (RS) elements. It is shown here that a simple anti‐serial connection of two ferroelectric tunnel junctions, based on BaTiO₃, with symmetric top metallic electrodes and a common, floating bottom nanometric film electrode, constitute a CRS memory element. It allows nonvolatile storage of binary states (“1” = “HRS+LRS” and “0” = “LRS+HRS”), where HRS (LRS) indicate the high (low) resistance state of each ferroelectric tunnel junction. Remarkably, these states have an identical and large resistance in the remanent state, characteristic of CRS. Here, protocols for writing information are reported and it is shown that non‐destructive or destructive reading schemes can be chosen by selecting the appropriate reading voltage amplitude. Moreover, this dual‐tunnel device has a significantly lower power consumption than a single ferroelectric tunnel junction to perform writing/reading functions, as is experimentally demonstrated. These findings illustrate that the recent impressive development of ferroelectric tunnel junctions can be further exploited to contribute to solving critical bottlenecks in data storage and logic functions implemented using RS elements.     

Revealing the Role of Anchoring Groups in the Electrical Conduction Through Single‐Molecule Junctions

A combined experimental and theoretical study is presented revealing the influence of metal–molecule coupling on electronic transport through single‐molecule junctions. Transport experiments through tolane molecules attached to gold electrodes via thiol, nitro, and cyano anchoring groups are performed. By fitting the experimental current–voltage characteristics to a single‐level tunneling model, we extract both the position of the molecular orbital closest to the Fermi energy and the strength of the metal–molecule coupling. The values found for these parameters are rationalized with the help of density‐functional‐theory‐based transport calculations. In particular, these calculations show that the anchoring groups determine the junction conductance by controlling not only the strength of the coupling to the metal but also the position of the relevant molecular energy levels.     

Fabrication of Reproducible,Integration‐Compatible Hybrid Molecular/Si Electronics

Reproducible molecular junctions can be integrated within standard CMOS technology. Metal–molecule–semiconductor junctions are fabricated by direct Si–C binding of hexadecane or methyl‐styrene onto oxide‐free H‐Si(111) surfaces, with the lateral size of the junctions defined by an etched SiO₂ well and with evaporated Pb as the top contact. The current density, J, is highly reproducible with a standard deviation in log(J) of 0.2 over a junction diameter change from 3 to 100 μm. Reproducibility over such a large range indicates that transport is truly across the molecules and does not result from artifacts like edge effects or defects in the molecular monolayer. Device fabrication is tested for two n‐Si doping levels. With highly doped Si, transport is dominated by tunneling and reveals sharp conductance onsets at room temperature. Using the temperature dependence of current across medium‐doped n‐Si, the molecular tunneling barrier can be separated from the Si‐Schottky one, which is a 0.47 eV, in agreement with the molecular‐modified surface dipole and quite different from the bare Si–H junction. This indicates that Pb evaporation does not cause significant chemical changes to the molecules. The ability to manufacture reliable devices constitutes important progress toward possible future hybrid Si‐based molecular electronics.     

A Translucent Nanocomposite with Liquid Inclusions of a Responsive Nanoparticle Dispersion

Active nanocomposites are created with liquid inclusions that contain plasmonic gold nanoparticles inside a polymeric matrix. The alkylthiol‐coated gold particles are designed to reversible agglomerate at certain temperatures, which changes the plasmonic coupling and thus optical properties. It is found that particles confined to the liquid inclusions inside the active composite retain this capability and cause macroscopic, temperature‐dependent color change of the solid. The transition is fully reversible for at least 100 times and tunable in temperature via particle size and ligand. This method is suitable to “package” responsive dispersion in solid composites to exploit their dynamic properties in materials.     

A Bipedal DNA Motor that Travels Back and Forth between Two DNA Origami Tiles

In this work, the successful operation of a dynamic DNA device constructed from two DNA origami building blocks is reported. The device includes a bipedal walker that strides back and forth between the two origami tiles. Two different DNA origami tiles are first prepared separately; they are then joined together in a controlled manner by a set of DNA strands to form a stable track in high yield as confirmed by single‐molecule fluorescence (SMF). Second, a bipedal DNA motor, initially attached to one of the two origami units and operated by sequential interaction with “fuel” and “antifuel” DNA strands, moves from one origami tile to another and then back again. The operational yield, measured by SMF, was similar to that of a motor operating on a similar track embedded in a single origami tile, confirming that the transfer across the junction from one tile to the other does not result in dissociation that is any more than that of steps on a single tile. These results demonstrate that moving parts can reliably travel from one origami unit to another, and it demonstrates the feasibility of dynamic DNA molecular machines that are made of more than a single origami building block. This study is a step toward the development of motors that can stride over micrometer distances.     

Plasmonic‐Nanopore Biosensors for Superior Single‐Molecule Detection

Plasmonic and nanopore sensors have separately received much attention for achieving single‐molecule precision. A plasmonic “hotspot” confines and enhances optical excitation at the nanometer length scale sufficient to optically detect surface–analyte interactions. A nanopore biosensor actively funnels and threads analytes through a molecular‐scale aperture, wherein they are interrogated by electrical or optical means. Recently, solid‐state plasmonic and nanopore structures have been integrated within monolithic devices that address fundamental challenges in each of the individual sensing methods and offer complimentary improvements in overall single‐molecule sensitivity, detection rates, dwell time and scalability. Here, the physical phenomena and sensing principles of plasmonic and nanopore sensing are summarized to highlight the novel complementarity in dovetailing these techniques for vastly improved single‐molecule sensing. A literature review of recent plasmonic nanopore devices is then presented to delineate methods for solid‐state fabrication of a range of hybrid device formats, evaluate the progress and challenges in the detection of unlabeled and labeled analyte, and assess the impact and utility of localized plasmonic heating. Finally, future directions and applications inspired by the present state of the art are discussed.     

A Quantitative Study of the Intracellular Dynamics of Fluorescently Labelled Glyco‐Gold Nanoparticles via Fluorescence Correlation Spectroscopy

The dynamic behaviour of gold nanoparticles functionalised with glucose (Glc‐Au NPs) has been studied here by means of fluorescence correlation spectroscopy (FCS). Meaningful data on the state of aggregation and dynamics of Glc‐Au NPs fluorescently‐labelled with HiLyte Fluor647 (Glc‐Au‐Hi NPs) in the intracellular environment were obtained. Moreover, the work presented here shows that FCS can be used to visualise the presence of single NPs or NP aggregates following uptake and to estimate, locally, NP concentrations within the cell. FCS measurements become possible after applying a “prebleaching” methodology, when the immobile NP fraction has been effectively removed and thus significant FCS data has been recorded. In this study, Glc‐Au‐Hi NPs have been incubated with HepG2 cells and their diffusion time in the intracellular environment has been measured and compared with their diffusion value in water and cell media.     

Charge Transport through Ferrocene 1,1′‐Diamine Single‐Molecule Junctions

The charge transport through ferrocene 1,1′‐diamine (FDA) molecules between gold electrodes is investigated using the mechanically controllable break junction technique combined with a theoretical framework of density functional theory simulations to understand the physics of these molecular junctions. The characteristic conductances of the molecule are measured at low bias as well as current–voltage (IV) characteristics. By fitting the IV characteristics to the single‐level model, the values for the position of the molecular level, mainly responsible for the transport, and its coupling to the leads, are obtained. The influence of the binding sites, molecular conformation, and electrode distance are systematically studied from a theoretical perspective. While a strong dependence of conductance on the adsorption geometry is found, the decrease of conductance as a function of electrode distance arises mainly from a decrease of coupling strength of the molecular electronic orbitals through a reduced overlap and, to a lesser extent, from a shift of their alignment with respect to the Fermi energy.     

Molecular Switch Controlled by Pulsed Bias Voltages

Recent experiments have shown that the current–voltage characteristics (I–V) of BPDN‐DT (bipyridyl‐dinitro oligophenylene‐ethynylene dithiol) can be switched in a very controlled manner between “on” and “off” traces by applying a pulse in a bias voltage, V_bias. Here, the polaron formation energies are calculated to check a frequently held belief, namely, that the polaron formation can explain the observed bistability. These results are not consistent with such a mechanism. Instead, a conformational reorientation is proposed. The molecule carries an intrinsic dipole moment, which couples to V_bias. Ramping V_bias exerts a force on the dipole that can reorient (“rotate”) the molecule from the ground state (“off”) into a metastable configuration (“on”) and back. By elaborated electronic structure calculations, a specific path for this rotation is identified through the molecule's conformational phase space. It is shown that this path has sufficiently high barriers to inhibit thermal instability but that the molecule can still be switched in the voltage range of the junction stability. The theoretical I–Vs qualitatively reproduce the key experimental observations. A proposal for the experimental verification of the alternative mechanism of conductance switching is presented.     

Measurement of tunnel junction resistance during formation

We have measured the characteristics of aluminum tunnel junctions during and immediately after the formation of the junction. This has permitted us to observe changes in the oxide barrier, in vacuum and in air. By observing the barrier resistance during sputtering, we were able to diagnose and correct problems due to plasma discharges which were damaging the junctions. We report preliminary results from junctions passivated with a silicon nitride cap layer.     

Time-Resolved Electrochemical Impedance Spectroscopy of Stochastic Nanoparticle Collision: Short Time Fourier Transform versus Continuous Wavelet Transform

This work demonstrates the utilization of short-time Fourier transform (STFT), and continuous wavelet transform (CWT) electrochemical impedance spectroscopy (EIS) for time-resolved analysis of stochastic collision events of platinum nanoparticles (NPs) onto gold ultramicroelectrode (UME). The enhanced electrocatalytic activity is observed in both chronoamperometry (CA) and EIS. CA provides the impact moment and rough estimation of the size of NPs. The quantitative information such as charge transfer resistance (R_ct) relevant to the exchange current density of a single Pt NP is estimated from EIS. The CWT analysis of the phase angle parameter is better for NP collision detection in terms of time resolution compared to the STFT method.     

Nanoparticle Linker‐Controlled Molecular Wire Devices Based on Double Molecular Monolayers

Highly conductive molecular wires are an important component for realizing molecular electronic devices and have to be explored in terms of interactions between molecules and electrodes in their molecular junctions. Here, new molecular wire junctions are reported to enhance charge transport through gold nanoparticle (AuNP)‐linked double self‐assembled monolayers (SAMs) of cobalt (II) bis‐terpyridine molecules (e.g., Co(II)(tpyphS)₂). Electrical characteristics of the double‐SAM devices are explored in terms of the existence of AuNP. The AuNP linker in the Co(II)(tpyphS)₂–AuNP–Co(II)(tpyphS)₂ junction acts as an electronic contact that is transparent to electrons. The weak temperature dependency of the AuNP‐linked molecular junctions strongly indicates sequential tunneling conduction through the highest occupied molecular orbitals (HOMOs) of Co(II)(tpyphS)₂ molecules. The electrochemical characteristics of the AuNP–Co(II)(tpyphS)₂ SAMs reveal fast electron transfer through molecules linked by AuNP. Density functional theory calculations reveal that the molecular HOMO levels are dominantly affected by the formation of junctions. The intermolecular charge transport, controlled by the AuNP linker, can provide a rational design for molecular connection that achieves a reliable electrical connectivity of molecular electronic components for construction of molecular electronic circuits.     

Precision Synthesis: Designing Hot Spots over Hot Spots via Selective Gold Deposition on Silver Octahedra Edges

A major challenge in plasmonic hot spot fabrication is to efficiently increase the hot spot volumes on single metal nanoparticles to generate stronger signals in plasmon‐enhanced applications. Here, the synthesis of designer nanoparticles, where plasmonic‐active Au nanodots are selectively deposited onto the edge/tip hot spot regions of Ag nanoparticles, is demonstrated using a two‐step seed‐mediated precision synthesis approach. Such a “hot spots over hot spots” strategy leads to an efficient enhancement of the plasmonic hot spot volumes on single Ag nanoparticles. Through cathodoluminescence hyperspectral imaging of these selective edge gold‐deposited Ag octahedron (SEGSO), the increase in the areas and emission intensities of hot spots on Ag octahedra are directly visualized after Au deposition. Single‐particle surface‐enhanced Raman scattering (SERS) measurements demonstrate 10‐fold and 3‐fold larger SERS enhancement factors of the SEGSO as compared to pure Ag octahedra and non‐selective gold‐deposited Ag octahedra (NSEGSO), respectively. The experimental results corroborate well with theoretical simulations, where the local electromagnetic field enhancement of our SEGSO particles is 15‐fold and 1.3‐fold stronger than pure Ag octahedra and facet‐deposited particles, respectively. The growth mechanisms of such designer nanoparticles are also discussed together with a demonstration of the versatility of this synthetic protocol.     

Mechanically Controllable Break Junctions for Molecular Electronics

A mechanically controllable break junction (MCBJ) represents a fundamental technique for the investigation of molecular electronic junctions, especially for the study of the electronic properties of single molecules. With unique advantages, the MCBJ technique has provided substantial insight into charge transport processes in molecules. In this review, the techniques for sample fabrication, operation and the various applications of MCBJs are introduced and the history, challenges and future of MCBJs are discussed.     

Plasmonic Elastic Capsules as Colorimetric Reversible pH‐Microsensors

There is a crucial need for effective and easily dispersible colloidal microsensors able to detect local pH changes before irreversible damages caused by demineralization, corrosion, or biofilms occur. One class of such microsensors is based on molecular dyes encapsulated or dispersed either in polymer matrices or in liquid systems exhibiting different colors upon pH variations. They are efficient but often rely on sophisticated and costly syntheses, and present significant risks of leakage and photobleaching damages, which is detrimental for mainstream applications. Another approach consists of exploiting the distance‐dependent plasmonic properties of metallic nanoparticles. Still, assembling nanoparticles into dispersible colloidal pH‐sensitive sensors remains a challenge. Here, it is shown how to combine optically active plasmonic gold nanoparticles and pH‐responsive thin shells into “plasmocapsules.” Upon pH change, plasmocapsules swell or shrink. Concomitantly, the distance between the gold nanoparticles embedded in the polymeric matrix varies, resulting in an unambiguous color change. Billions of micron‐size sensors can thus be easily fabricated. They are nonintrusive, reusable, and sense local pH changes. Each plasmocapsule is an independent reversible microsensor over a large pH range. Finally, their potential use for the detection of bacterial growth is demonstrated, thus proving that plasmocapsules are a new class of sensing materials.     

Magnetic Plasmon Networks Programmed by Molecular Self‐Assembly

Nanoscale manipulation of magnetic fields has been a long‐term pursuit in plasmonics and metamaterials, as it can enable a range of appealing optical properties, such as high‐sensitivity circular dichroism, directional scattering, and low‐refractive‐index materials. Inspired by the natural magnetism of aromatic molecules, the cyclic ring cluster of plasmonic nanoparticles (NPs) has been suggested as a promising architecture with induced unnatural magnetism, especially at visible frequencies. However, it remains challenging to assemble plasmonic NPs into complex networks exhibiting strong visible magnetism. Here, a DNA‐origami‐based strategy is introduced to realize molecular self‐assembly of NPs forming complex magnetic architectures, exhibiting emergent properties including anti‐ferromagnetism, purely magnetic‐based Fano resonances, and magnetic surface plasmon polaritons. The basic building block, a gold NP (AuNP) ring consisting of six AuNP seeds, is arranged on a DNA origami frame with nanometer precision. The subsequent hierarchical assembly of the AuNP rings leads to the formation of higher‐order networks of clusters and polymeric chains. Strong emergent plasmonic properties are induced by in situ growth of silver upon the AuNP seeds. This work may facilitate the development of a tunable and scalable DNA‐based strategy for the assembly of optical magnetic circuitry, as well as plasmonic metamaterials with high fidelity.     

机械可控纳米劈裂法实现单分子电导值的测量

为了获得电子在单分子结中传输的特性,研制了具有高稳定性的机械可控纳米劈裂装置芯片.利用此芯片及相应机械装置,获得了能在皮米级精度上操纵间距的纳米电极对;并实现了稳定的单个金属离子所桥接的分子结(电极-分子-金属离子-分子-电极).在保持配合物不变的前提下,置换不同金属离子,测量了其所桥接的单分子结的电导值,发现Ca2+络合物的电导值>>Zn2+络合物的电导值>Ni2+络合物的电导值.结合密度泛函理论的计算,验证了所得实验装置的可靠性及实验结果的正确性.     

Programmable Organic‐Free Negative Differential Resistance Memristor Based on Plasmonic Tunnel Junction

A novel negative differential resistance (NDR) phenomenon is reported herein based on planar plasmonic tunnel junction, resulting from plasmon‐assisted long‐range electron tunneling (P‐tunneling) and electronic caching effect of Au@SiO₂ nanoparticles. The tunnel junction is made of shell‐insulated Au@SiO₂ nanoparticle nanomembrane, in which SiO₂ shells act as a tunable tunneling barrier, while the Au core not only support the plasmonic effect to enable P‐tunneling, but also act as electronic caches to render NDR responses. The NDR peak voltage and current can be programmably controlled by varying the thickness of SiO₂ shell and the size of Au core to tune barrier level for electron transport. In addition, light induced plasmonic effect can be further managed to regulate the NDR behavior by fine‐tuning P‐tunneling. The phenomenon is exploited for robust use as memristors. The work provides a new mechanism for the generation of NDR effect and may open a way for the development of robust and new conceptual nanoelectronic devices.     

Ultraviolet photodetector based on single GaN nanorod p–n junctions

The fabrication and characterization of ultraviolet photodetectors based on single GaN nanorod p–n junctions are reported. Single nanorod p–n junctions within individual GaN nanorods were grown by using the plasma-assisted molecular beam epitaxy. In order to form the p–n junction at the middle of the nanorod, the dopant was changed from Si (n-type) to Mg (p-type) during the growth procedure. These single GaN nanorod p–n junction diodes have exhibited a rectifying behavior and a photocurrent effect under ultraviolet illumination.     

Single-molecule conductance of π-conjugated rotaxane: new method for measuring stipulated electric conductance of π-conjugated molecular wire using STM break junction

An electronic conductance with small fluctuations, which is stipulated in single-molecule junctions, is necessary for the precise control of single-molecule devices. However, the suppression of conductance fluctuations in conventional molecular junctions is intrinsically difficult because the fluctuations are related to the contact fluctuations and molecular motion. In the present study involving experimental and theoretical investigations, it is found that covering a single π-conjugated wire with an α-cyclodextrin molecule is a promising technique for suppressing conductance fluctuations. The conductance histogram of the covered molecular junction measured with the scanning tunneling microscope break-junction technique shows that the conductance peak for the covered junction is sharper than that of the uncovered junction. The covering technique thus has two prominent effects: the suppression of intramolecular motion, and the elimination of intermolecular interactions. Theoretical calculations of electronic conductance clearly support these experimental observations.