Mingguang Yao

Synthesis of ZnS nanocrystals with controllable structure and morphology and their photoluminescence property

The controllable synthesis of ZnS nanocrystals with desirable morphology and correlative structure has been carried out via the solvothermal method by simply changing the molar ratio of the reactants. The hexagonal-shaped ZnS nanosheets with a zinc-blende structure were synthesized in one step for the first time. ZnS nanorods with wurtzite structure and large ratio of length to diameter were also fabricated. We found that phase transformation is easily induced and there is a strong correlation between the morphology and structure of the ZnS nanocrystals by changing the ratio of the reactants. The photoluminescence spectra of the ZnS nanosheets and nanorods exhibit different emission bands. ZnS nanosheets show a strong emission at 534 nm while the nanorods have two emissions located at 520 and 578 nm.

Synthesis and high pressure induced amorphization of C60 nanosheets

C60 nanosheets with thicknesses in the nanometer range were synthesized by a simple method. Compared to bulk C60, the lattice of the nanosheets is expanded by about 0.4%. In situ Raman spectroscopy and energy-dispersive x-ray diffraction under high pressures have been employed to study the structure of the nanosheets. The studies indicate that the bulk modulus of the C60 nanosheets is significantly larger than that of bulk C60. The C60 cages in nanosheets can persist at pressures over 30GPa, 3GPa higher than for bulk C60. These results suggest that C60 crystals in even small size will be a potential candidate of superhard materials.

Rotational dynamics of confined C60 from near-infrared Raman studies under high pressure.

Peapods present a model system for studying the properties of dimensionally constrained crystal structures, whose dynamical properties are very important. We have recently studied the rotational dynamics of C60 molecules confined inside single walled carbon nanotube (SWNT) by analyzing the intermediate frequency mode lattice vibrations using near-infrared Raman spectroscopy. The rotation of C60 was tuned to a known state by applying high pressure, at which condition C60 first forms dimers at low pressure and then forms a single-chain, nonrotating, polymer structure at high pressure. In the latter state the molecules form chains with a 2-fold symmetry. We propose that the C60 molecules in SWNT exhibit an unusual type of ratcheted rotation due to the interaction between C60 and SWNT in the “hexagon orientation,” and the characteristic vibrations of ratcheted rotation becomes more obvious with decreasing temperature.

Pressure-induced transformation and superhard phase in fullerenes: The effect of solvent intercalation

We studied the behavior of solvated and desolvated C60 crystals under pressure by in situ Raman spectroscopy. The pressure-induced bonding change and structural transformation of C60s are similar in the two samples, both undergoing deformation and amorphization. Nevertheless, the high pressure phases of solvated C60 can indent diamond anvils while that of desolvated C60s cannot. Further experiments suggest that the solvents in the solvated C60 act as both spacers and bridges by forming covalent bonds with neighbors in 3D network at high pressure, and thus, a fraction of fullerenes may preserve the periodic arrangement in spite of their amorphization.

Reversible Polymerization in Doped Fullerides Under Pressure: The Case Of C60(Fe(C5H5)2)2

High-pressure Raman studies have been carried out on single crystalline C(60)(Fc)(2) nanosheets up to 25.4 GPa. Our results show that the charge transfer between Fc (ferrocene) and C(60) increases in the low-pressure range. Above 5 GPa, C(60) molecules start to form a chainlike polymer structure, and this polymerization is reversible upon decompression, in contrast to that of pristine C(60). The special layered structure of C(60)(Fc)(2) restricts the polymerization of C(60) molecules in some directions and explains the formation of the linear chainlike polymeric structure of the C(60) lattice under pressure. We suggest that the reversible polymerization is related to the increased charge transfer and the overridden steric repulsion of counterions.

Green synthesis of 3D SnO2/graphene aerogels and their application in lithium-ion batteries

Three-dimensional (3D) tin oxide/graphene aerogels (SnO2/GAs) were constructed by a simple, facile and environmentally friendly process. The small-sized SnO2 nanoparticles (6 nm) are encapsulated within graphene-based aerogels with interconnected 3D networks for the SnO2/GAs nanocomposite. When used as an anode material in lithium ion batteries, it delivers a high reversible capacity that is close to the theoretical capacities of SnO2 and graphene after 50 cycles. TEM observations of the samples before and after 50 cycles illustrate that the structures of the graphene network and SnO2 NPs are preserved, which explains well the good cyclic stability of the electrode. The excellent electrochemical performance of the nanocomposites can be explained by their unique 3D porous architecture and the combination of the advantages of both SnO2 and graphene in Li ion storage and transport.

Transparent, superhard amorphous carbon phase from compressing glassy carbon

Raman spectroscopy has been used to study the transformations of glassy carbon (GC) under high pressure. A GC sphere has been observed to transform into a transparent carbon phase above 33 GPa. The transformation is associated with a change in bonding character of carbon from sp2 to sp3 hybridization and an increase in hardness. The yield strength of the GC sphere reaches a value of 120 GPa at a confining pressure of 62 GPa, which is comparable to that of diamond at ambient conditions. The stress induced by the pressure medium is important for the observed transformations of GC under pressure.

Pressure-induced transformations of onion-like carbon nanospheres up to 48 GPa

Raman spectra of onion-like carbon nanospheres (OCNSs) have been studied under pressure up to 48 GPa. A transformation related to a change from sp(2) to sp(3) bonding of carbons in OCNSs was observed at pressures above 20 GPa. The Raman spectra exhibit some vibrational features similar to those of the theoretically proposed Z-carbon phase of cold-compressed graphite, while the transition pressure is obviously higher than that for graphite. In contrast to the transformations in compressed graphite, interlayer bonds are formed on the nanoscale between buckled layers in OCNSs under pressure due to the concentric configuration, and sp(2)-sp(3) conversion is incomplete even up to 48 GPa. This is confirmed by TEM observations on the decompressed samples. Moreover, the onion-like carbon structure is extremely stable and can be recovered even after a compression cycle to 48 GPa. This high stability, beyond that of other sp(2) carbon materials, is related to the unique onion-like configuration and to the interlayer bonding. The transformed material should have excellent mechanical properties so that it can sustain very high pressure.

Tailoring Building Blocks and Their Boundary Interaction for the Creation of New, Potentially Superhard, Carbon Materials

A strategy for preparing hybrid carbon structures with amorphous carbon clusters as hard building blocks by compressing a series of predesigned two-component fullerides is presented. In such constructed structures the building blocks and their boundaries can be tuned by changing the starting components, providing a way for the creation of new hard/superhard materials with desirable properties.

A New Carbon Phase Constructed by Long-Range Ordered Carbon Clusters from Compressing C70 Solvates

An ordered amorphous carbon cluster (OACC) structure with building blocks of highly deformed/collapsed C70 is synthesized by compressing C70 *m-xylene, which exhibits an exceptionally high hardness. Different from compressing C60 *m-xylene, a new structure transition is observed in C70*m-xylene at above 30 GPa, indicating the formation of a new OACC structure under pressure.