Jinying Wang

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).

The rare two-dimensional materials with Dirac cones

Inspired by the great development of graphene, more and more research has been conducted to seek new two-dimensional (2D) materials with Dirac cones. Although 2D Dirac materials possess many novel properties and physics, they are rare compared with the numerous 2D materials. To provide explanation for the rarity of 2D Dirac materials as well as clues in searching for new Dirac systems, here we review the recent theoretical aspects of various 2D Dirac materials, including graphene, silicene, germanene, graphynes, several boron and carbon sheets, transition-metal oxides (VO2)n/(TiO2)m and (CrO2)n/(TiO2)m, organic and organometallic crystals, so-MoS2, and artificial lattices (electron gases and ultracold atoms). Their structural and electronic properties are summarized. We also investigate how Dirac points emerge, move, and merge in these systems. The von Neumann-Wigner theorem is used to explain the scarcity of Dirac cones in 2D systems, which leads to rigorous requirements on the symmetry, parameters, Fermi level, and band overlap of materials to achieve Dirac cones. Connections between existence of Dirac cones and the structural features are also discussed.

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

CMP Aerogels: Ultrahigh‐Surface‐Area Carbon‐Based Monolithic Materials with Superb Sorption Performance

Monolithic conjugated microporous polymer (CMP) aerogels are obtained in an extremely facile way by selection of adequate reaction conditions and a freeze-drying technique. The aerogels possess an ultrahigh specific surface area and hierarchical interconnected pores, exhibiting superb gas/oil adsorption performance compared with all microporous organic polymers to date.

Inverse relationship between carrier mobility and bandgap in graphene

A frequently stated advantage of gapless graphene is its high carrier mobility. However, when a nonzero bandgap is opened, the mobility drops dramatically. The hardness to achieve high mobility and large on∕off ratio simultaneously limits the development of graphene electronics. To explore the underlying mechanism, we investigated the intrinsic mobility of armchair graphene nanoribbons (AGNRs) under phonon scattering by combining first-principles calculations and a tight-binding analysis. A linear dependence of the effective mass on bandgap was demonstrated to be responsible for the inverse mobility-gap relationship. The deformation-potential constant was found to be determined by the strain dependence of the Fermi energy and the bandgap, resulting in two mobility branches, and is essential for the high mobility of AGNRs. In addition, we showed that the transport polarity of AGNRs can be switched by applying a uniaxial strain.

Widely tunable carrier mobility of boron nitride-embedded graphene.

The carrier transport in boron nitride-embedded graphene (BNG) is studied using density functional theory coupled with the Boltzmann transport equation. Under a phonon scattering mechanism, the intrinsic carrier mobility of BNG at room temperature is tunable from 1.7 × 10(3) to 1.1 × 10(5) cm(2) V(-1) s(-1) when the bandgap is between 0.38 and 1.39 eV. Some specific BNG materials even show ultrahigh mobility up to 6.6 × 10(6) cm(2) V(-1) s(-1) , and the transport polarity (whether it is electron or hole transport) can be tailored by the application of a uniaxial strain. The wide mobility variation of BNG is attributed to the dependence of the effective mass and the deformation potential constant on the carbon concentration and width. The results indicate that BNG can have both a large on-off ratio and high carrier mobility and is thus a promising material for electronic devices.

Dirac cones in two-dimensional systems: from hexagonal to square lattices

The influence of lattice symmetry on the existence of Dirac cones was investigated for two distinct systems: a general two-dimensional (2D) atomic crystal containing two atoms in each unit cell and a 2D electron gas (2DEG) under a periodic muffin-tin potential. A criterion was derived under a tight-binding approximation for the existence of Dirac cones in the atomic crystal. When the transfer hoppings are assumed to be single functions of the distance between atoms, it was shown that the probability of observing Dirac cones in the atomic crystal gradually decreases before being reduced to zero when the lattice changes from hexagonal to square. For a 2DEG with full square symmetry, a Dirac point exists at the Brillouin zone corners, where the energy dispersion is parabolic not linear. These results suggest that conventional Dirac fermions (such as those in graphene) are difficult to achieve in a square lattice with full symmetry (wallpaper group p4mm).

Identifying Dirac cones in carbon allotropes with square symmetry.

A theoretical study is conducted to search for Dirac cones in two-dimensional carbon allotropes with square symmetry. By enumerating the carbon atoms in a unit cell up to 12, an allotrope with octatomic rings is recognized to possess Dirac cones under a simple tight-binding approach. The obtained Dirac cones are accompanied by flat bands at the Fermi level, and the resulting massless Dirac-Weyl fermions are chiral particles with a pseudospin of S = 1, rather than the conventional S = 1∕2 of graphene. The spin-1 Dirac cones are also predicted to exist in hexagonal graphene antidot lattices.

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.

First-principles study of the transport behavior of zigzag graphene nanoribbons tailored by strain

The charge transport properties of zigzag graphene nanoribbons (ZGNRs) under uniaxial and shear strains are theoretically studied. Although all strained ZGNRs have similar metallic band structures, they show four types of transport behavior under bias voltages that depend on the type of strain and the mirror symmetry of the ZGNR. Under an applied uniaxial strain, the current of symmetric ZGNRs is consistently small, while for asymmetric ZGNRs it is large. In contrast, the current increases with increasing shear strain for symmetric ZGNRs while it decreases for asymmetric ZGNRs. The current properties merge when the shear strain exceeds a critical value, and the two systems then show similar behavior. Our results suggest that strained ZGNRs with an appropriate applied shear are ideal conducting wires.