Jinbo Pan

Buckled Germanene Formation on Pt(111)

Germanene, a 2D honeycomb lattice analogous to graphene, is fabricated on a Pt(111) surface. It exhibits a buckled configuration with a (3 × 3) superlattice coinciding with the substrates (√19 × √19) superstructure. Covalent bonds exist throughout the germanene layer. The resulting high-quality germanene enables researchers to explore the fundamentals of germanene and its potential applications.

Monolayer PtSe2, a New Semiconducting Transition-Metal-Dichalcogenide, Epitaxially Grown by Direct Selenization of Pt

Single-layer transition-metal dichalcogenides (TMDs) receive significant attention due to their intriguing physical properties for both fundamental research and potential applications in electronics, optoelectronics, spintronics, catalysis, and so on. Here, we demonstrate the epitaxial growth of high-quality single-crystal, monolayer platinum diselenide (PtSe2), a new member of the layered TMDs family, by a single step of direct selenization of a Pt(111) substrate. A combination of atomic-resolution experimental characterizations and first-principle theoretic calculations reveals the atomic structure of the monolayer PtSe2/Pt(111). Angle-resolved photoemission spectroscopy measurements confirm for the first time the semiconducting electronic structure of monolayer PtSe2 (in contrast to its semimetallic bulk counterpart). The photocatalytic activity of monolayer PtSe2 film is evaluated by a methylene-blue photodegradation experiment, demonstrating its practical application as a promising photocatalyst. Moreover, circular polarization calculations predict that monolayer PtSe2 has also potential applications in valleytronics.

The transport properties of D-σ-A molecules: A strikingly opposite directional rectification

We design the A-R rectifier based on the D-σ-A molecules to examine the rectifying performances by the first-principles method. The calculated results show that the electronic structures for all of our systems perfectly match the A-R rectifier, as expected, but their rectifying direction is very strikingly opposite and working mechanism is completely different. This behavior can be rationalized through an asymmetrical shift of molecular levels under bias of different polarities, which is because of always-existing intrinsic asymmetrical coupling effects of molecular levels to electrodes. Detailed analysis demonstrates that the rectifying direction induced by this mechanism is always in opposition to that induced by the A-R mechanism.

Current rectification induced by asymmetrical electrode materials in a molecular device

Molecular devices are constructed based on a molecule connected into both electrodes with different metal materials, and their transport properties are investigated by the first-principles method. The result shows that such devices can generate two asymmetrical Schottky barriers at contacts; the current rectification thus is created. This rectification is also fully rationalized by the calculated transmission spectra and the spatial distribution of the lowest unoccupied molecular orbital and highest occupied molecular orbital states. Our study suggests that it might be a very important way for both electrodes using different materials to realize a molecular rectification.

Rectifying performance of D-π-A molecules based on cyanovinyl aniline derivatives

Using the first-principles method, we investigate rectifying performances of D-π-A molecules based on cyanovinyl aniline derivatives. The calculated results show that different functional groups can change the location of molecular orbitals and thus change the rectifying properties of molecules. Interestingly, we find that although the electronic structure for our studied systems is in agreement with that proposed originally by Aviram and Ratner [Chem. Phys. Lett. 29, 277 (1974)], the rectifying direction is opposite from it due to the asymmetric shift of molecular levels under biases of different polarities. Only for model (M4), it shows a forward rectifying performance under larger bias.

Examinations into the contaminant-induced transport instabilities in a molecular device

We report first-principles calculations of transport behaviors for a molecular device whose electrode surface is contaminated by various diatomic groups. It has been found that such a device demonstrates less transport variations for the contamination of the group PO or SO in the whole bias range but it shows more transport variations for contamination of the group CN, HS, or NO only under low bias, which suggests that contamination of all diatomic groups studied here always affects high-bias transport properties of a device in an extremely gentle manner.Using first-principles calculations, we study the work function of single wall silicon carbide nanotube (SiCNT). The work function is found to be highly dependent on the tube chirality and diameter. It increases with decreasing the tube diameter. The work function of zigzag SiCNT is always larger than that of armchair SiCNT. We reveal that the difference between the work function of zigzag and armchair SiCNT comes from their different intrinsic electronic structures, for which the singly degenerate energy band above the Fermi level of zigzag SiCNT is specifically responsible. Our finding offers potential usages of SiCNT in field-emission devices.

End-group effects on negative differential resistance and rectifying performance of a polyyne-based molecular wire

Based on first-principles approach, the end-group effects on negative differential resistance (NDR) and rectifying performance of polyyne-based molecular wires are investigated. The NDR behaviors are observed when the polyyne is attached to asymmetric (–NO2 and –NH2) or symmetric (double –S) end groups, and rectifying performance emerges with the presence of asymmetric groups. The analysis on microscopic nature reveals the intrinsic origin of these phenomena. Our results show the possibility of a multifunctional molecular device design simultaneously with NDR and rectifying performances by using a technology of capping certain end groups to polyyne.

Kondo Effect of Cobalt Adatoms on a Graphene Monolayer Controlled by Substrate-Induced Ripples

The Kondo effect, a widely studied phenomenon in which the scattering of conduction electrons by magnetic impurities increases as the temperature T is lowered, depends strongly on the density of states at the Fermi energy. It has been predicted by theory that magnetic impurities on free-standing monolayer graphene exhibit the Kondo effect and that control of the density of states at the Fermi level by external means can be used to switch the effect on and off. However, though transport data for Co adatoms on graphene monolayers on several substrates have been reported, there exists no evidence for a Kondo effect. Here we probe the role of the substrate on the Kondo effect of Co on graphene by combining low-temperature scanning tunneling microscopy and spectroscopy measurements with density functional theory calculations. We use a Ru(0001) substrate that is known to cause graphene to ripple, yielding a moiré superlattice. The experimental data show a sharp Kondo resonance peak near the Fermi energy from only Co adatoms at the edge of atop regions of the moiré pattern. The theoretical results show that the variation of the distance from the graphene to the Ru substrate, which controls the spin polarization and local density of states at the Fermi energy, is the key factor for the appearance of the Kondo resonance. The results suggest that rippling of graphene by suitable substrates is an additional lever for tuning and selectively switching the appearance of the Kondo effect.

Intrinsically patterned two-dimensional materials for selective adsorption of molecules and nanoclusters

Two-dimensional (2D) materials have been studied extensively as monolayers, vertical or lateral heterostructures. To achieve functionalization, monolayers are often patterned using soft lithography and selectively decorated with molecules. Here we demonstrate the growth of a family of 2D materials that are intrinsically patterned. We demonstrate that a monolayer of PtSe2 can be grown on a Pt substrate in the form of a triangular pattern of alternating 1T and 1H phases. Moreover, we show that, in a monolayer of CuSe grown on a Cu substrate, strain relaxation leads to periodic patterns of triangular nanopores with uniform size. Adsorption of different species at preferred pattern sites is also achieved, demonstrating that these materials can serve as templates for selective self-assembly of molecules or nanoclusters, as well as for the functionalization of the same substrate with two different species.

Rectifying and negative differential resistance behaviors of a functionalized Tour wire: The position effects of functional groups

Based on Tour wire, we construct four D-π-A molecular devices with different positional functional groups, in an attempt to explore the position effects of functional groups on their electronic transport properties and to show that some interesting physical phenomena can emerge by only varying the position of functional groups. The first-principles calculations demonstrate that the position of functional groups can affect the rectifying behaviors (rectification direction and ratio) significantly and determines whether or not the negative differential resistance (NDR) can be observed as well as the physical origin of the NDR phenomenon.