Ning Lü

We perform a comprehensive first-principles study of the electronic properties of phosphorene nanoribbons, phosphorus nanotubes, multilayer phosphorene sheets, and heterobilayers of phosphorene and two-dimensional (2D) transition-metal dichalcogenide (TMDC) monolayer. The tensile strain and electric-field effects on electronic properties of low-dimensional phosphorene nanostructures are also investigated. Our calculations show that the bare zigzag phosphorene nanoribbons (z-PNRs) are metals regardless of the ribbon width, whereas the bare armchair phosphorene nanoribbons (a-PNRs) are semiconductors with indirect bandgaps and the bandgaps decrease with increasing ribbon width. We find that compressive (or tensile) strains can reduce (or enlarge) the bandgap of the bare a-PNRs while an in-plane electric field can significantly reduce the bandgap of the bare a-PNRs, leading to the semiconductor-to-metal transition beyond certain electric field. For edge-passivated PNR by hydrogen, z-PNRs become semiconductor with nearly direct bandgaps and a-PNRs are still semiconductor but with direct bandgaps. The response to tensile strain and electric field for the edge-passivated PNRs is similar to that for the edge-unpassivated (bare) a-PNRs. For single-walled phosphorus nanotubes, both armchair and zigzag nanotubes are semiconductors with direct bandgaps. With either tensile strains or transverse electric field, behavior of bandgap modulation similar to that for a-PNRs can arise. It is known that multilayer phosphorene sheets are semiconductors whose bandgaps decrease with an increase in the number of multilayers. In the presence of a vertical electric field, the bandgaps of multilayer phosphorene sheets decrease with increasing electric field and the bandgap modulation is more significant with more layers. Lastly, heterobilayers of phosphorene (p-type) with an n-type TMDC (MoS2 or WS2) monolayer are still semiconductors while their bandgaps can be reduced by applying a vertical electric field as well. We also show that the combined phosphorene/MoS2 heterolayers can be an effective solar cell material. Our estimated power conversion efficiency for the phosphorene/MoS2 heterobilayer has a theoretical maximum value of 17.5%.

Nanoscale Pt-Ni bimetallic octahedra with controlled sizes have been actively explored in recent years owning to their outstanding activity for the oxygen reduction reaction (ORR). Here we report the synthesis of uniform 9 nm Pt-Ni octahedra with the use of oleylamine and oleic acid as surfactants and W(CO)6 as a source of CO that can promote the formation of {111} facets in the presence of Ni. Through the introduction of benzyl ether as a solvent, the coverage of both surfactants on the surface of resultant Pt-Ni octahedra was significantly reduced while the octahedral shape was still attained. By further removing the surfactants through acetic acid treatment, we observed a specific activity 51-fold higher than that of the state-of-the-art Pt/C catalyst for the ORR at 0.93 V, together with a record high mass activity of 3.3 A mgPt(-1) at 0.9 V (the highest mass activity reported in the literature was 1.45 A mgPt(-1)). Our analysis suggests that this great enhancement of ORR activity could be attributed to the presence of a clean, well-preserved (111) surface for the Pt-Ni octahedra.

An effective strategy for reducing the Pt content while retaining the activity of a Pt-based catalyst is to deposit the Pt atoms as ultrathin skins of only a few atomic layers thick on nanoscale substrates made of another metal. During deposition, however, the Pt atoms often take an island growth mode because of a strong bonding between Pt atoms. Here we report a versatile route to the conformal deposition of Pt as uniform, ultrathin shells on Pd nanocubes in a solution phase. The introduction of the Pt precursor at a relatively slow rate and high temperature allowed the deposited Pt atoms to spread across the entire surface of a Pd nanocube to generate a uniform shell. The thickness of the Pt shell could be controlled from one to six atomic layers by varying the amount of Pt precursor added into the system. Compared to a commercial Pt/C catalyst, the Pd@PtnL (n = 1-6) core-shell nanocubes showed enhancements in specific activity and durability toward the oxygen reduction reaction (ORR). Density functional theory (DFT) calculations on model (100) surfaces suggest that the enhancement in specific activity can be attributed to the weakening of OH binding through ligand and strain effects, which, in turn, increases the rate of OH hydrogenation. A volcano-type relationship between the ORR specific activity and the number of Pt atomic layers was derived, in good agreement with the experimental results. Both theoretical and experimental studies indicate that the ORR specific activity was maximized for the catalysts based on Pd@Pt2-3L nanocubes. Because of the reduction in Pt content used and the enhancement in specific activity, the Pd@Pt1L nanocubes showed a Pt mass activity with almost three-fold enhancement relative to the Pt/C catalyst.

Vertical integration of two-dimensional van der Waals materials is predicted to lead to novel electronic and optical properties not found in the constituent layers. Here, we present the direct synthesis of two unique, atomically thin, multi-junction heterostructures by combining graphene with the monolayer transition-metal dichalcogenides: molybdenum disulfide (MoS2), molybdenum diselenide (MoSe2) and tungsten diselenide (WSe2). The realization of MoS2-WSe2-graphene and WSe2-MoS2-graphene heterostructures leads to resonant tunnelling in an atomically thin stack with spectrally narrow, room temperature negative differential resistance characteristics.

Tungsten diselenide (WSe2) is a two-dimensional material that is of interest for next-generation electronic and optoelectronic devices due to its direct bandgap of 1.65 eV in the monolayer form and excellent transport properties. However, technologies based on this 2D material cannot be realized without a scalable synthesis process. Here, we demonstrate the first scalable synthesis of large-area, mono and few-layer WSe2 via metal-organic chemical vapor deposition using tungsten hexacarbonyl (W(CO)6) and dimethylselenium ((CH3)2Se). In addition to being intrinsically scalable, this technique allows for the precise control of the vapor-phase chemistry, which is unobtainable using more traditional oxide vaporization routes. We show that temperature, pressure, Se:W ratio, and substrate choice have a strong impact on the ensuing atomic layer structure, with optimized conditions yielding >8 μm size domains. Raman spectroscopy, atomic force microscopy (AFM), and cross-sectional transmission electron microscopy (TEM) confirm crystalline monoto-multilayer WSe2 is achievable. Finally, TEM and vertical current/voltage transport provide evidence that a pristine van der Waals gap exists in WSe2/graphene heterostructures.