Chuancheng Jia

Covalently bonded single-molecule junctions with stable and reversible photoswitched conductivity

Stable molecular switches Many single-molecule current switches have been reported, but most show poor stability because of weak contacts to metal electrodes. Jia et al. covalently bonded a diarylethene molecule to graphene electrodes and achieved stable photoswitching at room temperature (see the Perspective by Frisbie). The incorporation of short bridging alkyl chains between the molecule and graphene decoupled their pielectron systems and allowed fast conversion of the open and closed ring states. Science, this issue p. 1443; see also p. 1394 Stable molecular conduction junctions were formed by covalently bonding single diarylethenes to graphene electrodes. Through molecular engineering, single diarylethenes were covalently sandwiched between graphene electrodes to form stable molecular conduction junctions. Our experimental and theoretical studies of these junctions consistently show and interpret reversible conductance photoswitching at room temperature and stochastic switching between different conductive states at low temperature at a single-molecule level. We demonstrate a fully reversible, two-mode, single-molecule electrical switch with unprecedented levels of accuracy (on/off ratio of ~100), stability (over a year), and reproducibility (46 devices with more than 100 cycles for photoswitching and ~105 to 106 cycles for stochastic switching).

Direct Optical Characterization of Graphene Growth and Domains on Growth Substrates

We detailed a facile detection technique to optically characterize graphene growth and domains directly on growth substrates through a simple thermal annealing process. It was found that thermal annealing transformed the naked Cu to Cu oxides while keeping graphene and graphene-covered Cu intact. This increases the interference color contrast between Cu oxides and Cu, thus making graphene easily visible under an optical microscope. By using this simple method, we studied the factors that affect graphene nucleation and growth and achieved graphene domains with the domain size as large as ~100 μm. The concept of chemically making graphene visible is universal, as demonstrated by the fact that a solution process based on selective H2O2 oxidation has been developed to achieve the similar results in a shorter time. These techniques should be valuable for studies towards elucidating the parameters that control the grains, boundaries, structures and properties of graphene.

Conductance switching and mechanisms in single-molecule junctions.

From its very start, one of the most intriguing motivations of molecular electronics is to provide unique and low-cost solutions for electronic functions based on molecules, such as diodes, transistors, switches, and memristors, since molecules are probably the smallest units still capable of offering a rich structural variety. However, the ability to control the conductance of molecules at the molecular level by an external mode is still a formidable challenge in this field. Here we report the observation of reproducible conductance switching triggered by external light on a new platform of graphene–molecule junctions, where three photochromic diarylethene derivatives with different substituents are used as key elements. Analyses of both transition voltage spectroscopy and first-principles calculations consistently reveal tunable molecule–electrode coupling, thus demonstrating the photogated inflection (Vtrans) transition when the chargetransport mechanism changes from direct to Fowler–Nordheim (F-N) tunneling. We chose diarylethene derivatives as photosensitive molecular bridges because they, as a typical family of photochromic molecules, can undergo reversible transitions between two distinct isomers with open/closed conformations when exposed to light irradiation (Figure 1a). The closed isomer is nearly planar, but the open isomer adopts a bent conformation with its thiophene rings twisted about 618 out of the plane from the cyclopentene ring. Correspondingly, these two isomers display different absorption spectra, that of the closed form extends towards longer wavelengths up to the visible region, suggesting the delocalization of p electrons over the entire structure (see Figure S1 in the Supporting Information). In the open form, however, delocalization of the p electrons is restricted to each half of the molecule and electronic communication through the unsaturated bond of the middle ring is interrupted. Another remarkable feature of the diarylethene molecules used in this study is that only negligible changes in the molecular length ( 0.2 ) happen when they switch back-and-forth between open/closed states (Figure S2 and Table S1). In conjunction with their superior thermal stability and fatigue resistance, these significant electronic and structural properties place diarylethene molecules as ideal candidates for building light-driven molecular switches as demonstrated theoretically and experimentally. However, a longstanding challenge is to conserve these promising properties in solution when the diarylethene molecules are sandwiched between solid-state molecularscale electrodes. One major reason is due to the quenching effect of the photoexcited states of the diarylethene molecules by the electrodes, which strongly stresses the importance of the molecule–electrode coupling strength to the device performance. To tailor the energy level alignments at the molecule– electrode interface, in this study we intend to modify diarylethene backbones with rationally designed side and anchoring groups (1–3 in Figure 1b). This modification has two specific considerations. The first is to substitute the hydrogenated cyclopentene in 1 by the fluorinated unit (2). In comparison with the hydrogenated cyclopentene, the fluorinated unit is electron-withdrawing and thereby decreases the electron density on the central alkene unit and increases the fatigue resistance of the photochromic properties. The second is to further introduce a methylene group (CH2) between the terminal amine group and the functional center on each side (3). The incorporation of the saturated CH2 groups can cut off p-electron delocalization, thus largely decoupling the electronic interaction between molecules and electrodes. Theoretical calculations were performed to predict the electronic structures of the molecule–electrode contacts as shown in Figure 1c (Table S2). Indeed, the energy levels of 2 are lower than those of 1 because of the electron-withdrawing effect of the fluorinated unit, which is consistent with electrochemical measurements of similar systems. For 3, the energy levels are even lower. More importantly, the calculated molecular orbital diagrams reveal a lower orbital density of states (DOS) at the C sites of the CH2 groups (Figure 1c), which implies that the CH2 groups decrease the strong electronic coupling between diarylethene molecules and electrodes. These results demonstrate the potential of molecular engineering as an efficient tool for tuning the molecule–electrode coupling strength. This tuna[*] C. Jia, J. Wang, Y. Cao, Prof. Z.-R. Liu, Prof. Z.-F. Liu, Prof. X.-F. Guo Center for NanoChemistry Beijing National Laboratory for Molecular Sciences State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking University Beijing 100871 (P. R. China) E-mail: zfliu@pku.edu.cn guoxf@pku.edu.cn

Interface‐Engineered Bistable [2]Rotaxane‐Graphene Hybrids with Logic Capabilities

The use of high-quality graphene as a local probe in combination with photo excitation helps to establish a deep mechanistic understanding of charge generation/quenching processes under lying the graphene/environment interface. By combining a non-destructive bottom-up assembly technique with sensitive graphene-based transistors, a bistable [2]rotaxane-graphene hybrid device, which exhibits a symmetric mirror-image photoswitching effect with logic capabilities, is produced.

Stereoelectronic Effect-Induced Conductance Switching in Aromatic Chain Single-Molecule Junctions

Biphenyl, as the elementary unit of organic functional materials, has been widely used in electronic and optoelectronic devices. However, over decades little has been fundamentally understood regarding how the intramolecular conformation of biphenyl dynamically affects its transport properties at the single-molecule level. Here, we establish the stereoelectronic effect of biphenyl on its electrical conductance based on the platform of graphene-molecule single-molecule junctions, where a specifically designed hexaphenyl aromatic chain molecule is covalently sandwiched between nanogapped graphene point contacts to create stable single-molecule junctions. Both theoretical and temperature-dependent experimental results consistently demonstrate that phenyl twisting in the aromatic chain molecule produces different microstates with different degrees of conjugation, thus leading to stochastic switching between high- and low-conductance states. These investigations offer new molecular design insights into building functional single-molecule electrical devices.

Substrate-induced interfacial plasmonics for photovoltaic conversion

Surface plasmon resonance (SPR) is widely used as light trapping schemes in solar cells, because it can concentrate light fields surrounding metal nanostructures and realize light management at the nanoscale. SPR in photovoltaics generally occurs at the metal/dielectric interfaces. A well-defined interface is therefore required to elucidate interfacial SPR processes. Here, we designed a photovoltaic device (PVD) with an atomically flat TiO2 dielectric/dye/graphene/metal nanoparticle (NP) interface for quantitatively studying the SPR enhancement of the photovoltaic conversion. Theoretical and experimental results indicated that the graphene monolayer was transparent to the electromagnetic field. This transparency led to significant substrate-induced plasmonic hybridization at the heterostructure interface. Combined with interparticle plasmonic coupling, the substrate-induced plasmonics concentrated light at the interface and enhanced the photo-excitation of dyes, thus improving the photoelectric conversion. Such a mechanistic understanding of interfacial plasmonic enhancement will further promote the development of efficient plasmon-enhanced solar cells and composite photocatalysts.

Direct single-molecule dynamic detection of chemical reactions

The dynamic process of a nucleophilic addition reaction has been investigated in graphene-molecule single-molecule junctions. Single-molecule detection can reveal time trajectories and reaction pathways of individual intermediates/transition states in chemical reactions and biological processes, which is of fundamental importance to elucidate their intrinsic mechanisms. We present a reliable, label-free single-molecule approach that allows us to directly explore the dynamic process of basic chemical reactions at the single-event level by using stable graphene-molecule single-molecule junctions. These junctions are constructed by covalently connecting a single molecule with a 9-fluorenone center to nanogapped graphene electrodes. For the first time, real-time single-molecule electrical measurements unambiguously show reproducible large-amplitude two-level fluctuations that are highly dependent on solvent environments in a nucleophilic addition reaction of hydroxylamine to a carbonyl group. Both theoretical simulations and ensemble experiments prove that this observation originates from the reversible transition between the reactant and a new intermediate state within a time scale of a few microseconds. These investigations open up a new route that is able to be immediately applied to probe fast single-molecule physics or biophysics with high time resolution, making an important contribution to broad fields beyond reaction chemistry.

Direct observation of single-molecule hydrogen-bond dynamics with single-bond resolution

The hydrogen bond represents a fundamental interaction widely existing in nature, which plays a key role in chemical, physical and biochemical processes. However, hydrogen bond dynamics at the molecular level are extremely difficult to directly investigate. Here, in this work we address direct electrical measurements of hydrogen bond dynamics at the single-molecule and single-event level on the basis of the platform of molecular nanocircuits, where a quadrupolar hydrogen bonding system is covalently incorporated into graphene point contacts to build stable supramolecule-assembled single-molecule junctions. The dynamics of individual hydrogen bonds in different solvents at different temperatures are studied in combination with density functional theory. Both experimental and theoretical results consistently show a multimodal distribution, stemming from the stochastic rearrangement of the hydrogen bond structure mainly through intermolecular proton transfer and lactam–lactim tautomerism. This work demonstrates an approach of probing hydrogen bond dynamics with single-bond resolution, making an important contribution to broad fields beyond supramolecular chemistry.Hydrogen-bonds are widely found in many systems, such as DNAs and supramolecular assemblies, but it remains challenging to detect their dynamics at a molecular level. Here, Zhou et al. study the stochastic arrangement of hydrogen bonds using single-molecule junctions connected to graphene electrodes.

High-Efficiency Selective Electron Tunnelling in a Heterostructure Photovoltaic Diode

A heterostructure photovoltaic diode featuring an all-solid-state TiO2/graphene/dye ternary interface with high-efficiency photogenerated charge separation/transport is described here. Light absorption is accomplished by dye molecules deposited on the outside surface of graphene as photoreceptors to produce photoexcited electron-hole pairs. Unlike conventional photovoltaic conversion, in this heterostructure both photoexcited electrons and holes tunnel along the same direction into graphene, but only electrons display efficient ballistic transport toward the TiO2 transport layer, thus leading to effective photon-to-electricity conversion. On the basis of this ipsilateral selective electron tunnelling (ISET) mechanism, a model monolayer photovoltaic device (PVD) possessing a TiO2/graphene/acridine orange ternary interface showed ∼86.8% interfacial separation/collection efficiency, which guaranteed an ultrahigh absorbed photon-to-current efficiency (APCE, ∼80%). Such an ISET-based PVD may become a fundamental device architecture for photovoltaic solar cells, photoelectric detectors, and other novel optoelectronic applications with obvious advantages, such as high efficiency, easy fabrication, scalability, and universal availability of cost-effective materials.

Direct real-time detection of single proteins using silicon nanowire-based electrical circuits

We present an efficient strategy through surface functionalization to build a single silicon nanowire field-effect transistor-based biosensor that is capable of directly detecting protein adsorption/desorption at the single-event level. The step-wise signals in real-time detection of His-tag F1-ATPases demonstrate a promising electrical biosensing approach with single-molecule sensitivity, thus opening up new opportunities for studying single-molecule biophysics in broad biological systems.