Sihan Zhao

Two-dimensional metallic NbS2: growth, optical identification and transport properties

Progress on researches of two-dimensional (2D) metals strongly relies on development of the growth technique. Studies on preparation of 2D metals have so far been limited, and this is in stark contrast to the situation of 2D semiconductors, where various layered semiconductors, including MoS2, WS2, MoSe2, WSe2, have been isolated in its monolayer form. In this work, we have developed a facile method to prepare 2D metallic transition metal dichalcogenides (TMDCs) by chemical vapor deposition (CVD) method, where direct growth of few-layered NbS2 (3R phase) on atomically flat hexagonal boron nitride (hBN) has been demonstrated. Structural characterization of the so-grown NbS2 was performed with atomic force microscopy, optical microscopy, electron microscopy and optical spectroscopy, revealing that the utilization of hBN as growth substrates is a key factor for the first successful CVD growth of 2D metallic TMDCs with large single-domain size (several μm). Electrical transport measurements have clearly shown that NbS2 atomic layers down to few-layer-thickness are metal. The current study opens up a new synthetic route for controllable growth of 2D layered metallic materials, which is of great importance in study of rich physics in 2D metals, as well as in search for novel 2D superconductors.

Manipulation of domain-wall solitons in bi- and trilayer graphene

Topological dislocations and stacking faults greatly affect the performance of functional crystalline materials1–3. Layer-stacking domain walls (DWs) in graphene alter its electronic properties and give rise to fascinating new physics such as quantum valley Hall edge states4–10. Extensive efforts have been dedicated to the engineering of dislocations to obtain materials with advanced properties. However, the manipulation of individual dislocations to precisely control the local structure and local properties of bulk material remains an outstanding challenge. Here we report the manipulation of individual layer-stacking DWs in bi- and trilayer graphene by means of a local mechanical force exerted by an atomic force microscope tip. We demonstrate experimentally the capability to move, erase and split individual DWs as well as annihilate or create closed-loop DWs. We further show that the DW motion is highly anisotropic, offering a simple approach to create solitons with designed atomic structures. Most artificially created DW structures are found to be stable at room temperature.Layer-stacking domain walls in bi- and trilayer graphene are engineered individually and moved, erased and mechanically split by means of an atomic force microscope tip.

Molecular beam epitaxy growth of monolayer niobium diselenide flakes

Monolayer niobium diselenide (NbSe2) is prepared through molecular beam epitaxy with hexagonal boron nitride (hBN) as substrates. Atomic force microscopy and the Raman spectroscopy have shown that the monolayer NbSe2 grown on the hBN possesses triangular or truncated triangular shape whose lateral size amounts up to several hundreds of nanometers. We have found that the precisely controlled supply rate and ultraflat surface of hBN plays an important role in the growth of the monolayer NbSe2.

Single-Step Functionalization and Exfoliation of Graphene with Polymers under Mild Conditions.

The simultaneous polymer functionalization and exfoliation of graphene sheets by using mild bath sonication and heat treatment at low temperature is described. In particular, free-radical polymerization of three different vinyl monomers takes place in the presence of graphite flakes. The polymerization procedure leads to the exfoliation of graphene sheets and at the same time the growing polymer chains are attached onto the graphene lattice, which gives solubility and stability to the final graphene-based hybrid material. The polymer-functionalized graphene sheets possess fewer defects as compared with previously reported polymer-functionalized graphene. The success of the covalent functionalization and exfoliation of graphene was confirmed by using a variety of complementary spectroscopic, thermal, and microscopy techniques, including Raman, IR and UV/Vis spectroscopy, thermogravimetric analysis, and transmission electron microscopy.

Rayleigh scattering studies on inter-layer interactions in structure-defined individual double-wall carbon nanotubes

Although the inter-layer coupling in layered materials has attracted considerable interest due to its importance in determining physical properties of two-dimensional systems, studies on the inter-layer coupling in one-dimensional systems have so far been limited. Double-wall carbon nanotubes (DWCNTs) are one of the most fundamental and ideal model systems to study the inter-layer coupling in one-dimensional systems. In this work, Rayleigh scattering spectroscopy and transmission electron microscopy are used to characterize the electronic transition between inner-and outer-nanotubes of the exactly same individual DWCNT. We find that the inter-layer coupling is strong, leading to downshifts in most of the optical transition energies (up to ∼0.2 eV) compared to isolated CNTs. We also find that the presence of metallic tubes lead to stronger shifts. The inter-layer screening of Coulomb interactions is one of the key factors in explaining the observed results.

Organic–Inorganic Azafullerene-Gold C59N-Au Nanohybrid: Synthesis, Characterization, and Properties

Azafullerene (C59 N) was functionalized using a Mannich-type reaction and then subsequently condensed with lipoic acid to yield dithiolane-modified C59 N. In the following step, the extended dithiolane moiety from the C59 N core was utilized to decorate the azafullerene sphere with gold nanoparticles (Au NPs). The latter were initially stabilized with dodecanothiol (DT⋅Au) and then integrated on azafullerene through a ligand exchange reaction with the dithiolane-functionalized C59 N to produce the C59 N/DT⋅Au nanohybrid. The nanohybrid was fully characterized by spectroscopy and microscopy, revealing the formation of spherical nanoparticles with a diameter in the range of 2-5 nm, as imaged by HR-TEM. In the electronic absorption spectrum of C59 N/DT⋅Au nanohybrid, the characteristic surface plasmon band (SPB) of Au NPs was observed, however, it was redshifted compared with that of DT⋅Au. The redshift of the SPB is indicative of closer interparticle proximity of Au NPs, in accordance with the formation of aggregated NPs as observed by TEM, in C59 N/DT⋅Au nanohybrid. Excited-state interactions in C59 N/DT⋅Au were probed by photoluminescence assays. It was found that the weak emission of C59 N at 819 nm was blueshifted by 14 nm in C59 N/DT⋅Au, but was stronger in intensity, thus suggesting energy transfer to C59 N, within the organic-inorganic C59 N/DT⋅Au nanohybrid. Finally, with the aid of pump-probe measurements and transient absorption spectroscopy, the formation of the singlet excited state of C59 N was identified.