Yingxiang Cai

Chiral recognition of zinc phthalocyanine on Cu(100) surface

The windmill-like chiral nature of individual ZnPc molecules adsorbed on Cu(100) surface at room temperature has been revealed by scanning tunneling microscopy (STM) and the origin of such chirality is attributed to asymmetrical charge transfer between the molecules and the copper surface. Such chiral enantiomers do recognize each other in molecular level and spontaneously form second-level chiral supramolecular structures with the same chirality during thermally driven movements. The interactions between the ZnPc molecules during such chiral recognition process have been discussed based on the analysis of the sub-molecule-resolution STM images.

Switching Molecular Orientation of Individual Fullerene at Room Temperature

Reversible molecular switches with molecular orientation as the information carrier have been achieved on individual fullerene molecules adsorbed on Si (111) surface at room temperature. Scanning tunneling microscopy imaging directly demonstrates that the orientation of individual fullerene with an adsorption geometry of 5-6 bond is rotated by integral times as 30 degree after a pulse bias is applied between the STM tip and the molecule. Dependences of the molecular rotation probability on the voltage and the process of applied bias reveal that the rotation of a fullerene molecule takes place in two successive steps: the bonding between the fullerene and the Si surface is firstly weakened via electronic excitation and then low energy electron bombardment causes the molecule to rotate by certain degree.

Positioning and Switching Phthalocyanine Molecules on a Cu(100) Surface at Room Temperature

Reversible molecular switches with molecular orientation as the information carrier have been achieved on individual phthalocyanine (H2Pc) molecules adsorbed on a Cu(100) surface at room temperature. Scanning tunneling microscopy (STM) imaging directly demonstrates that H2Pc molecules can be controlled to move along the [011] or [011̅] surface direction of the Cu(100) surface, and the orientation of H2Pc molecules can also be switched between two angles of ±28° with respect to the [011] surface direction by a lateral manipulation. Owing to the highly efficient control over the adsorption site and orientation of H2Pc adsorbed on the Cu(100) surface by lateral manipulation, a pyramidal array formed by 10 H2Pc molecules has been constructed on the Cu surface as a prototype of binary memory, and every molecule within such a molecular array can be individually and reversibly controlled by a STM tip.

Adsorption geometry of individual fullerene on Si surface at room-temperature

The adsorption properties of individual fullerene molecules at room-temperature have been investigated by scanning tunneling microscopy. Statistical analysis of the images demonstrates that fullerene molecules prefer to adsorb on five specific sites: corner hole, the middle of the faulted and unfaulted parts, and the top of the rest atoms within the faulted and unfaulted parts. High-resolution images of individual fullerene molecules on each of the adsorption sites show distinct intramolecular structures. By combining these internal fine structures with theoretical simulations, the molecular orientations at each of the adsorption sites can be unambiguously determined.

Encapsulation of cathode in lithium-sulfur batteries with a novel two-dimensional carbon allotrope: DHP-graphene

Sulfur cathodes in lithium-sulfur (Li-S) batteries still suffer from their low electronic conductivity, undesired dissolution of lithium polysulfide (Li2Sn, 3 ≤ n ≤ 8) species into the electrolyte, and large degree volume change during the cycle. To overcome these problems, an effective encapsulation for the sulfur cathode is necessary. By means of particle swarm optimization (PSO) and density functional theory (DFT), we have predicted a stable metallic two-dimensional sp2-hybridized carbon allotrope (DHP-graphene). This carbon sheet can prevent S atoms from cathode entering electrolyte. However, Li-ions can shuttle freely due to the increasing difference in Li-ions concentration between electrolyte and cathode along with the potential difference between cathode and anode during charge-discharge cycles. In addition, versatile electronic band structures and linear dispersion are found in DHP-graphene nanoribbons but only metallic band structure occurs for DHP-graphene nanotubes.

Multiporous carbon allotropes transformed from symmetry-matched carbon nanotubes

Carbon nanotubes (CNTs) with homogeneous diameters have been proven to transform into new carbon allotropes under pressure but no studies on the compression of inhomogeneous CNTs have been reported. In this study, we propose to build new carbon allotropes from the bottom-up by applying pressure on symmetry-matched inhomogeneous CNTs. We find that the (3,0) CNT with point group C3v and the (6,0) CNT with point group C6v form an all sp3 hybridized hexagonal 3060-Carbon crystal, but the (4,0) CNT with point group D4h and the (8,0) CNT with point group D8h polymerize into a sp2+sp3 hybridized tetragonal 4080-Carbon structure. Their thermodynamic, mechanical and dynamic stabilities show that they are potential carbon allotropes to be experimentally synthesized. The multiporous structures, excellently mechanical properties and special electronic structures (semiconductive 3060-Carbon and semimetallic 4080-Carbon) imply their many potential applications, such as gases purification, hydrogen storage and lightweig...

Role of Thermal Process on Self-Assembled Structures of 4'-([2,2':6',2''-Terpyridin]-4'-Yl)-[1,1'-Biphenyl]-4-Carboxylic Acid on Au(III)

The role of dynamic processes on self-assembled structures of 4′-([2,2′:6′, 2″-terpyridin]-4′-yl)-[1,1′-biphenyl]-4-carboxylic acid (l) molecules on Au(III) has been studied by scanning tunneling microscopy. The as-deposited monolayer is closed-packed and periodic in a short-range due to dipole forces. A thermal annealing process at 110 degrees drives such disordered monolayer into ordered chain-like structures, determined by the combination of the dipole forces and hydrogen bonding. Further annealing at 130 degrees turns the whole monolayer into a bowknot-like structure in which hydrogen bonding plays the dominant role in the formation of assembled structures. Such dependence of an assembled structure on the process demonstrates that an assembled structure can be regulated and controlled not only by the molecular structure but also by the thermal process to form the assembled structure.

Atomic mechanism of zinc-blende to NiAs high-pressure phase transition in BeTe

The atomic mechanism of the zinc-blende to NiAs structural transition under high pressure was investigated by crystallographic analysis and first-principles pseudopotential methods within the generalized gradient approximation. The relative sliding of less-close-packed cation layers {001}C along the directions could be identified, accompanied by changes of lattice parameters and the fractional coordinates of anions, briefly referred to as the less-close-packed to most-close-packed cation layer transition mechanism. BeTe was taken as an example to study the changes of lattice parameters, enthalpy difference and the fractional coordinates of anions with the relative sliding of cations at equilibrium transition pressure. Furthermore, the change of the charge density along the transition path was investigated to characterize the formation of two new Be–Te bonds.

Chirality control of nonplanar lead phthalocyanine (PbPc) and its potential application in high-density storage: a theoretical investigation.

On single-crystal surfaces, achiral molecules may become chiral owing to confinement in two dimensions (2D). Metal phthalocyanines (MPcs) on Cu(001) and Ag(100) surfaces have exhibited a chiral electronic state. However, the chirality is not always desirable since crystal defects (grain boundaries) inevitably occur between two different chiral domains during the self-assembly of single layers. In this theoretical study, we propose to utilize metal(001) substrates with different electron configurations to mediate the azimuthal orientations of nonplanar PbPc. The results show that PbPc is chiral on Cu(001) with a partially filled s orbital (3d(10)4s(1)) but achiral on Pd(001) with a completely filled d orbital (4d(10)). The mechanism that PbPc prefers achiral azimuthal orientation rather than chiral orientation on Pd(001) is clarified. In addition, we predict that PbPc can form a (3 × 4) surface reconstruction. While it is used for data storage, the capacity is almost three orders of magnitude higher than the present storage materials.

Room-temperature tracking of chiral recognition process at the single-molecule level

The molecular-level identification of a chiral recognition process of phthalocyanine (Pc) was studied on a Cu(100) surface by scanning tunneling microscopy (STM). STM revealed that a chiral Pc molecule forms a series of metastable dimer configurations with other Pc molecules. Eventually, the Pc molecule recognizes another Pc molecule with the same chirality to form a stable dimer configuration. Homochiral dimers were found on the Cu surface, demonstrating the chiral specificity of Pc dimerization. The mechanism for this chiral recognition process is identified, disclosing the critical role of the particular adsorption geometry of the chiral dimers on the Cu surface.