Jian Shao

Structural transition and temperature-driven conductivity switching of single crystalline VO2(A) nanowires

Single crystalline VO2(A) nanowires were synthesized by a facile hydrothermal method. The structural transition and temperature-driven conductivity switching of the VO2(A) nanowires were investigated. Our experimental results show that VO2(A) nanowires exhibit a distinct structural transition accompanied with an order of magnitude change in resistance, and a clear temperature-dependent current switching hysteresis. In order to analyze experimental results, theoretically, the electrical conductivity behavior was found to be consistent with Motts small polaron model, the first-principles calculations also indicated that the apical V–O bond changes were mainly responsible for the band gap evolution and hence led to the conductivity switching.

Charge carrier transition in an ambipolar single-molecule junction: Its mechanical-modulation and reversibility

Precise control from the bottom-up for realizing tunable functionality is of utmost importance to facilitate the development of molecular electronic devices. Until now, however, manipulating charge carriers over single-molecule scale remains intractable. The origin of the problem is that the nature of charge carriers is often hindered by the complexity of the investigated molecular systems. Here, via ab initio simulations, we show a force-modulated and switched ambipolar single-molecule junction with Au/cyclopropane-1,2-dithiol/Au structure. The cyclopropane ring in the molecule can be opened and closed reversibly and repeatedly by the mechanical force. This structural transition from its closed state to open state enables the ambipolarity in charge carriers—from p-type to n-type. Analysis of electronic structure reveals unambiguously the force-dependent correlation between C–S bond order and the nature of charge carriers. Based on this, we design a binary interconnected junction exhibiting resistance, rectification and negative differential resistance functionalities under mechanical modulation, i.e., loading/unloading or pull/push. This interesting phenomenon provides both illuminating insight and feasible controllability into charge carriers in molecules, and a very general idea and useful approach for single-molecule junctions in practical single-molecule devices.Single-molecule switch for modular electronicsApplying force to a junction linking gold atoms leads to reversible switching of its electric properties, facilitating development of single-molecule devices. Building electronics from the molecular level up could lead to drastically smaller electronic circuits. Precisely controlling their functionality at the single-molecule scale is challenging. Yue Zheng and colleagues from China’s Sun Yat-sen University demonstrated by computer simulation that a cyclopropane-1,2-dithiol ring linking two gold atoms can be opened and closed reversibly and repeatedly using mechanical force, switching its electric properties from one that internally donates electrons to one that accepts them. Using the molecule as modular building block, they design a multifunctional junction that, with the application of mechanical force, exhibited resistance (increasing voltage leads to increased current), rectification (converts alternating current to direct) and negative differential resistance (increasing voltage leads to decreased current).

Length-dependent rectification and negative differential resistance in heterometallic n-alkanedithiol junctions

The transport properties of heterometallic n-alkanedithiol junctions have been investigated via first-principles calculations. Results show that the heterometallic n-alkanedithiol junctions exhibit significant rectification at lower voltage. A negative differential resistance was found at higher voltage, which increases with the increase of the n-alkanedithiol backbone length. In order to explain these phenomena, the molecular orbitals of n-alkanedithiol have been analyzed between certain electrodes. It is found that the rectification is induced by asymmetric orbital profiles between the heterometallic electrodes, and negative differential resistance arises when the molecular orbitals cross the band edge provided by the metal–sulfur bond.

A new insight into the electrochemical growth of Ag nanodendrites without a strong electrolyte

We have developed a simple and green electrochemistry technique to synthesize Ag nanostructures with a highly pure, crystalline and smooth surface, which takes place in a clean and slow reaction environment (just highly pure de-ionized water) without any chemical additives. Here, we report a new insight into the Ag nanodendrite (ND) synthesis via the developed electrochemistry without a strong electrolyte. It is found that Ag2CO3 nanospindles and AgNDs form in water and on a graphite negative electrode, respectively, when an Ag target is immersed in the highly pure de-ionized water under an external voltage, which is totally different from conventional electrochemistry. The reaction processes are investigated in detail both theoretically and experimentally. The results show that CO2 from air plays a crucial role in the formation of AgNDs. CO2 is dissolved into the de-ionized water to form HCO3−, weak acid anions, which results in the formation of Ag2CO3. Then, Ag+ is reduced to metallic AgNDs on the negative graphite electrode. The AgND modified graphite electrode (AgND/GE) shows a significant improvement in the electroreduction of H2O2 compared to the bare graphite electrode. These investigations suggest that introducing a weak electrolyte in the electrochemical cell offers an alternative route to design and fabricate novel nanostructures for practical applications.

Magneto-optical conductivity of Weyl semimetals with quadratic term in momentum

Weyl semimetal is a three-dimensional Dirac material whose low energy dispersion is linear in momentum. Adding a quadratic (Schrodinger) term to the Weyl node breaks the original particle-hole symmetry and also breaks the mirror symmetry between the positive and negative Landau levels in present of magnetic field. This asymmetry splits the absorption line of the longitudinal magneto-optical conductivity into a two peaks structure. It also results in an oscillation pattern in the absorption part of the Hall conductivity. The two split peaks in Reσxx (or the positive and negative oscillation in Imσxy) just correspond to the absorptions of left-handed (σ−) and right-handed (σ+) polarization light, respectively. The split in Reσxx and the displacement between the absorption of σ+ and σ− are decided by the magnitude of the quadratic term and the magnetic field.

The mechanics-modulated tunneling spectrum and low-pass effect of viscoelastic molecular monolayer

Understanding the force-induced conductance fluctuation in molecules is essential for building molecular devices with high stability. While stiffness of molecule is usually considered to be desirable for stable conductance, we demonstrate mechanical dragging in viscoelastic molecules integrates both noise resistance and mechanical controllability to molecular conductance. Via conductive atomic force microscope measurement and theoretical modeling, it’s found that viscoelastic Azurin monolayer has spectrum-like pattern of conductance corresponding to the duration and strength of applied mechanical pulse under low-frequency excitation. Conductance fluctuation is prevented under high-frequency excitation by dragging dissipation, which qualifies molecular junction with electric robustness against mechanical noise.