Lattice Expansion in Seamless Bi layer Graphene Constrictions at High Bias

Our understanding of sp2 carbon nanostructures is still emerging and is important for the development of high performance all carbon devices. For example, in terms of the structural behavior of graphene or bi-layer graphene at high bias, little to nothing is known. To this end we investigated bi-layer graphene constrictions with closed edges (seamless) at high bias using in situ atomic resolution transmission electron microscopy. We directly observe a highly localized anomalously large lattice expansion inside the constriction. Both the current density and lattice expansion increase as the bi-layer graphene constriction narrows. As the constriction width decreases below 10 nm, shortly before failure, the current density rises to 4 \cdot 109 A cm-2 and the constriction exhibits a lattice expansion with a uniaxial component showing an expansion approaching 5 % and an isotropic component showing an expansion exceeding 1 %. The origin of the lattice expansion is hard to fully ascribe to thermal expansion. Impact ionization is a process in which charge carriers transfer from bonding states to antibonding states thus weakening bonds. The altered character of C-C bonds by impact ionization could explain the anomalously large lattice expansion we observe in seamless bi-layer graphene constrictions. Moreover, impact ionization might also contribute to the observed anisotropy in the lattice expansion, although strain is probably the predominant factor.Comment: to appear in NanoLetter

Silicon carbide-free graphene growth on silicon for lithium-ion battery with high volumetric energy density

Silicon is receiving discernable attention as an active material for next generation lithium-ion battery anodes because of its unparalleled gravimetric capacity. However, the large volume change of silicon over charge-discharge cycles weakens its competitiveness in the volumetric energy density and cycle life. Here we report direct graphene growth over silicon nanoparticles without silicon carbide formation. The graphene layers anchored onto the silicon surface accommodate the volume expansion of silicon via a sliding process between adjacent graphene layers. When paired with a commercial lithium cobalt oxide cathode, the silicon carbide-free graphene coating allows the full cell to reach volumetric energy densities of 972 and 700 Whl(-1) at first and 200th cycle, respectively, 1.8 and 1.5 times higher than those of current commercial lithium-ion batteries. This observation suggests that two-dimensional layered structure of graphene and its silicon carbide-free integration with silicon can serve as a prototype in advancing silicon anodes to commercially viable technology.

Graphene Coating of Silicon Nanoparticles with CO2-Enhanced Chemical Vapor Deposition

Understanding the growth of graphene over Si species is becoming ever more important as the huge potential for the combination of these two materials becomes more apparent, not only for device fabrication but also in energy applications, particularly in Li-ion batteries. Thus, the drive for the direct fabrication of graphene over Si is crucial because indirect approaches, by their very nature, require processing steps that, in general, contaminate, damage, and are costly. In this work, the direct chemical vapor deposition growth of few-layer graphene over Si nanoparticles is systematically explored through experiment and theory with the use of a reducer, H-2 or the use of a mild oxidant, CO2 combined with CH4. Unlike the case of CH4, with the use of CO2 as a mild oxidant in the reaction, the graphene layers form neatly over the surface and encapsulate the Si particles. SiC formation is also prevented. These structures show exceptionally good electrochemical performance as high capacity anodes for lithium-ion batteries. Density functional theory studies show the presence of CO2 not only prevents SiC formation but helps enhance the catalytic activity of the particles by maintaining an SiOx surface. In addition, CO2 can enhance graphitization.

On the Role of Vapor Trapping for Chemical Vapor Deposition (CVD) Grown Graphene over Copper

The role of sample chamber configuration for the chemical vapor deposition of graphene over copper was investigated in detail. A configuration in which the gas flow is unrestricted was shown to lead to graphene with an inhomogeneous number of layers (between 1 and 3). An alternative configuration in which one end of the inner tube (in which the sample is placed) is closed so as to restrict the gas flow leads a homogeneous graphene layer number. Depending on the sample placement, either homogeneous monolayer or bilayer graphene is obtained. Under our growth conditions, the data show local conditions play a role on layer homogeneity such that under quasi-static equilibrium gas conditions not only is the layer number stabilized, but the quality of the graphene improves. In short, our data suggests vapor trapping can trap Cu species leading to higher carbon concentrations, which determines layer number and improved decomposition of the carbon feedstock (CH4), which leads to higher quality graphene. © 2013 American Chemical Society.120231sciescopu

Size-dependent nanographene oxide as a platform for efficient carboplatin release

Nanographene oxides (NGO) with well-defined sizes were produced from graphite via chemical exfoliation and separated into three different size distributions (300 nm, 200 nm, and 100 nm) using intense sonication and sucrose density gradient centrifugation. Prior to carboplatin (CP) loading, the NGO was functionalized with zero generation polyamidoamide (PAMAM) which renders improved dispersibility and stability of the nanocarrier platform in physiological media. Cell viability tests were conducted on pristine NGO samples with average widths of 200 nm and 300 nm that showed a cytotoxic effect on HeLa cancer cells and mesenchymal stem cells at low (50 μg ml-1) and high (100 μg ml-1) concentrations, while the pristine NGO sample with an average width of 100 nm revealed no significant cytotoxicity at 50 μg ml-1, and only recorded a 10% level at 100 μg ml-1. After functionalization with PAMAM, the carrier was found to be able to deliver carboplatin to the cancer cells, by enhancing the drug anticancer efficiency. Moreover, the carboplatin loaded NGO carrier shows no significant effect on the viability of mesenchymal stem cells (hMSCs) even at high concentration (100 μg ml-1). © 2013 The Royal Society of Chemistry.112151sciescopu

Chemical vapor deposition growth of large-scale hexagonal boron nitride with controllable orientation

Chemical vapor deposition (CVD) synthesis of large-domain hexagonal boron nitride (h-BN) with a uniform thickness is very challenging, mainly due to the extremely high nucleation density of this material. Herein, we report the successful growth of wafer-scale, high-quality h-BN monolayer films that have large single-crystalline domain sizes, up to similar to 72 A mu m in edge length, prepared using a folded Cu-foil enclosure. The highly confined growth space and the smooth Cu surface inside the enclosure effectively reduced the precursor feeding rate together and induced a drastic decrease in the nucleation density. The orientation of the as-grown h-BN monolayer was found to be strongly correlated to the crystallographic orientation of the Cu substrate: the Cu (111) face being the best substrate for growing aligned h-BN domains and even single-crystalline monolayers. This is consistent with our density functional theory calculations. The present study offers a practical pathway for growing high-quality h-BN films by deepening our fundamental understanding of the process of their growth by CVD. © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2015144441Nsciescopu

Few-Layer Graphene Shells and Nonmagnetic Encapsulates: A Versatile and Nontoxic Carbon Nanomaterial

In this work a simple and scalable approach to coat nonmagnetic nanoparticles with few-layer graphene is presented. In addition, the easy processing of such nanoparticles to remove their core, leaving only the 3D graphene nanoshell, is demonstrated. The samples are comprehensively characterized, as are their versatility in terms of functionalization and as a material for electrochemical storage. Indeed, these 3D graphene nanostructures are easily functionalized much as is found with carbon nanotubes and planar graphene. Electrochemical investigations indicate these nanostructures are promising for stable long-life battery applications. Finally, initial toxicological investigations suggest no acute health risk from these 3D graphene nanostructures. © 2013 American Chemical Society.123241sciescopu

Amphiphilic O-functionalized calix[4]resocinarenes with tunable structural behavior

Novel amphiphilic calix[4]resorcinarenes oxyethylated at the upper rim and alkylated at the lower rim (CR-CnH2n+1, here n is the number of carbon atoms in the alkyl substituent; n = 2,5,7,8,9,11) were synthesized, and their association behavior in water-organic solvents was explored. Surface properties and the association behavior of CRs were shown to be strongly controlled by their structure and the nature of the co-solvent. Solely CR-C5H11 demonstrates surface activity in the mixed water-DMF and water-DMSO solutions, while no surface activity occurs in the water-THF mixture. The DLS measurements revealed a very low concentration threshold of the aggregation (around 0.01 mM) for the CR series including surface inactive compounds. In water-DMF and water-DMSO solutions the CRs of low hydrophobicity were shown to associate through an open model with the formation of large aggregates of 300-400 nm, while more hydrophobic CRs can associate through a closed model and form rather small micelle-like aggregates of 10 to 20 nm. © 2014 The Royal Society of Chemistry.19101sciescopu