Session H47: Chemical Physics of Graphene and Carbon

Adsorption Characteristics of Binary Mixtures of Two Halomethanes on Graphite Surface

Understanding the physisorption mechanism of mixtures of small molecules on graphite substrate has been a growing interest in materials science in order to investigate the changes in adsorption behavior of mixtures near interfaces vs. the individual components. Using atomic-scale molecular dynamics simulations, we have studied the structure and dynamics of multilayer adsorption of binary mixtures of two halomethanes (CF4 and CF3Cl) on graphite substrates for different bulk compositions of CF4. Simulations were performed in the temperature range 60-120K. The goal of this study is to explore how the compositions of individual components as well as temperature, affect the structure of films near the interface, the mobility of molecules, the molecular orientation and the substrate affinity. Preliminary results suggest a strong influence of the concentration of CF4 and temperature on the structure and mobility of molecules in first adsorbed layer on the graphite surface. In agreement with the recent experimental results, CF4 displaces CF3Cl from the first absorbed layer at all temperatures in our range and becomes the leading component in the mixture at high temperature for large CF4 concentrations in the mixture.   

Exfoliation and Dispersion of 2-Dimensional Materials by Elevating Temperature

It is known that graphene and other 2-dimensional materials are hard to dissolve in water without using chemicals or surfactants. Here, we present a facile method to exfoliate and disperse those materials in water by simply controlling temperature. Graphene, when sonicated in water at high temperature (60°C), was edge-functionalized due to the extremely high temperature and pressure locally induced by ultrasonic cavitation, and dissolved in water stably even for longer than 1 month. However, it was not dispersed at low temperature(30°C) because of less cavitation and reduced sonochemical reaction. Other 2-dimensional materials, such as h-BN, MoS2, and other layered metal chalcogenides, were also well dissolved in water as graphene, but even at low temperature. Their stable solution is from the electric double layer because their relatively high insulating property. Also elevated storage temperature (60°C) improved the long-term dispersion stability compared to lower storage temperature (20°C)   

New insights into the opening band gap of graphene oxides

Electronic properties of oxygen absorbed few-layer graphenes are investigated using first-principle calculations. They are very sensitive to the changes in the oxygen concentration, number of graphene layer, and stacking configuration. The feature-rich band structures exhibit the destruction or distortion of the Dirac cone, opening of band gap, anisotropic energy dispersions, O- and (C,O)-dominated energy dispersions, and extra critical points. The band decomposed charge distributions reveal the $\pi $-bonding dominated energy gap. The orbital-projected density of states (DOS) have many special structures mainly coming from a composite energy band, the parabolic and partially flat ones. The DOS and spatial charge distributions clearly indicate the critical orbital hybridizations in O-O, C-O and C-C bonds, being responsible for the diversified properties. All of the few-layer graphene oxides are semi-metals except for the semiconducting monolayer ones.   

Raman Spectra Study and the Corresponding Strain Dependence of Graphyne and Graphdiyne

Graphynes, composed of\textit{ sp-sp}$^{2}$ carbon atoms, have attracted increasing interest of research due to particular optical, electrical and mechanical properties they might have. According to recent theoretical studies prediction, the systhesis of graphyne and graphdiyne are difficult but offer more possible compared to other graphynes, and they have been tried to form and got initial achievement$^{\mathrm{1}}$. For new materials, their widespread application is impossible without a convinient, fast, non-destructive characterization tool. Raman spectroscopy has performed remarkable ability for studing the properties of \textit{sp}$^{2}$ and \textit{sp}$^{3}$ carbon materials, such as diamond, graphite, carbon fibers and nanotubes. Naturally, we may expect it is also work in \textit{sp-sp}$^{2}$ carbon materials$^{\mathrm{2}}$. In our work, the Raman features of graphyne and graphdiyne are studied systematically and their variations versus mechanical strain are also investigated by group theory and first-principles calculations. 1. Guoxing Li, et al. \textit{Chem. Commun.} 2010, 46: 3256 2. Jinying Wang*, Shuqing Zhang*, et al. \textit{Phys. Chem. Chem. Phys. }2014, 16 (23): 11303   

Complete Wetting of Graphene by Biological Membrane

In a very recent study, we found that surprisingly graphene nanosheets can extract large amount of lipid molecules directly out of cell membranes thus causing serious damage in cell's integrity (Nat. Nanotechnol. 8, 594, 2013). Here through extensive molecular dynamics simulations and theoretical analyses, we show that this novel phenomenon can be categorized as a a complete wetting of graphene by membrane lipids in the medium of water. A wetting-based theory was developed to associate the free energy change during the extraction with the macroscopic spreading parameter. With a customer-designed thermodynamic cycle for detailed energetics, we demonstrated that the dispersive adhesion between graphene and lipids plays a dominant role during this extraction as manifested by the curved graphene.   

Electrical conductivity of graphite oxide nanoplatelets obtained from bamboo: Effect of the deoxidation degree.

Given the high interest in the fabrication and application of carbon-based materials, we present a new and cost-effective method for the synthesis of graphite oxide nanoplatelets (GONP) using bamboo pyroligneous acid (BPA) as source. GONP-BPA present lateral dimensions of 5-100 micro-meter and thickness less than 80 nm, as confirmed by TEM. EEL spectra show that locally the carbon is mainly in sp$^{\mathrm{2}}$ bonding configuration and confirm a short/medium range crystalline order. Elemental analysis by EDX confirms the presence of oxygen in an atomic percentage ranging from 17 to 5{\%}. For electrical characterization, single platelets were contacted by focused-ion-beam-induced deposition of Pt nanowires. The four-point probe electrical conductivity shows a direct correlation with the oxygen percentage. Three orders of magnitude conductivity rise is observed by the oxygen reduction, reaching a value of 2.3x10$^{\mathrm{3}}$ S/m at the final deoxidation degree. The results suggest that GONP-BPA could be used in the development of advanced devices and sensors.   

Simulation of Adsorption of Carbon Dioxide and Methane on Graphene Sheet.

Carbon dioxide (CO2) and Methane (CH4) constitute 90{\%} of the annual greenhouse emission. These gases are emitted from multitude of sources such as: power station, transportation fuels, industrial processes, and agricultural byproducts. Scientists around the globe are looking for materials capable of capturing, separating, and storing these gases. Graphene with its high specific surface area provides a great platform for gas adsorption and separation. Adsorption is defined as the attachment of atoms, or molecules of a gas, liquid or dissolved solid onto a surface, creating a film or monolayer of material onto the adsorbing surface. Using the Method of Grand Canonical Monte Carlo, we computed the adsorption of carbon dioxide (CO2) and methane (CH4) on a monolayer graphene sheet, at various temperatures for each gas. For each temperature, we compute the adsorption isotherm, Energy gas-surface and Energy gas-gas. We compare the uptake pressures of CO2 and CH4. Using the Ideal Adsorbed Solution Theory (IAST), we predict the selectivity of a mixture CO2/CH4.   

Structural and Compositional Study of Graphene grown on SrTiO3 by Chemical Vapour Deposition

Graphene, a monolayer of sp2 bonded carbon atom, is considered as one of the most promising candidate materials for future electronics. The most critical step in graphene research is its transfer from the growth catalyst to the dielectric substrate, many unavoidable issues in the transfer process are: contamination from etchants, photoresist residues, wrinkles, and mechanical breakage. The direct growth of graphene on the substrates without using catalyst offer new opportunities in device fabrication without any transfer process. But till now, the field of direct graphene growth on dielectrics or insulating substrates is not mature like growth on metallic catalysts using CVD. We used chemical vapour deposition to grow graphene on SrTiO3 (110) substrates. The growth was carried out in presence of methane, argon and hydrogen. Raman Spectrum clearly showed the D and G peaks which were absent in bare substrate. XPS was used to get information about the presence of necessary elements, their bonding with STO substrates. AFM imaging clearly showed graphene island formation on substrates.   

Surface and Compositional Study of Graphene grown on Lithium Niobate (LiNbO$_{3})$ substrates by Chemical Vapour Deposition

The diversity required in the designing of electronic devices motivated the community to always attempt for new functional materials and device structures. Graphene is considered as one of the most promising candidate materials for future electronics and carbon based devices. It is very exciting to combine graphene with new dielectric materials which exhibit multifunctional properties. Lithium Niobate exhibits ferro-, pyro-, and piezoelectric properties with large electro-optic, acousto-optic, and photoelastic coefficients as well as strong photorefractive and photovoltaic effects which made it one of the most extensively studied materials over the last 50 years. We used ambient pressure chemical vapour deposition to grow graphene on LiNbO$_{3}$ substrates without any catalyst. The growth was carried out in presence of methane, argon and hydrogen. AFM imaging showed very unique structures on the surface which contains triangular domains. X-ray photoelectron spectroscopy (XPS) was used to get information about the presence of necessary elements, their bonding with LiNbO$_{3}$ substrates. Detailed characterization is under process which will be presented later.   

Atomically precise nitrogen-doped graphene nanoribbons

There is a considerable interest in graphene nanoribbons (GNRs), few-nm-wide strips of graphene with high aspect ratios, because of their intriguing physical properties. For example, GNRs with zigzag edges are predicted to exhibit low-dimensional magnetism, while GNRs with armchair edges can possess large energy band gaps, making them promising materials for future electronics and photovoltaics. The ability to control structural parameters of GNRs, such as their width, edge structure and termination, with atomic precision is the key for practical realization of these intriguing nanoscale properties. Physical properties of GNRs can also be modified by their doping with heteroatoms, such nitrogen, resulting in nitrogen-doped GNRs or N-GNRs. In this talk I will discuss several types of N-GNRs with different doping levels that have been synthesized and systematically studied by spectroscopic, microscopic and transport methods. Incorporation of nitrogen atoms in graphene lattice is shown to be an effective route to affect GNRs' band gap, doping level as well as aggregation behavior.   

CVD films of narrow atomically precise graphene nanoribbons

Atomically precise graphene nanoribbons (GNRs) is a promising material for the next-generation electronics and optoelectronics. So far, solution-based and surface-assisted approaches have been the two main routes to synthesize GNRs with atomically smooth armchair edges. However, efficient processing of the resulting GNRs into uniform thin films to fabricate GNR-based functional devices remains a formidable challenge. In this presentation we will report the synthesis of narrow armchair GNRs using an alternative approach -- a radical polymerization of rationally designed molecular precursors. The technique allows fabrication of thin, transparent and conductive films of GNRs on almost any substrate. Microscopic structure and electrical properties of the fabricated GNR films will also be discussed.   

Analysis of vibrational response in graphite oxide nanoplatelets.

In this work, we present a new low-cost fabrication process to obtain graphite oxide nanoplatelets from bamboo pyroligneous acid (GO-BPA) by thermal decomposition method using a pyrolysis system for different carbonization temperatures from 673 to 973 K. The GO-BPA samples were characterized by using Raman, FTIR, XRD, SEM and TEM techniques, whose results suggest that increased carbonization temperature increases graphite conversion, boundary defects, desorption of some organic compounds and phonon response, respectively. We discuss potential applications of the GO-BPA samples involving phonon response that would benefit from a fully scaled technology, advanced electronic sensors and devices.   

Surface aging phenomena in multidimensional \textit{sp}$^{2}$ carbon allotropes

Despite the current interest in the scientific community in exploiting divergent surface properties of graphitic carbon allotropes, conclusive differentiation remains elusive even when dealing with parameters as fundamental as adhesion. Here we set out to provide conclusive experimental evidence on the time evolution of the surface properties of highly oriented pyrolytic graphite (HOPG), graphene monolayer (GML) and multiwalled carbon nanotubes (MWCNTs) as we expose these materials to airborne contaminants, by providing 1) statistically significant results based on large data-sets, i.e. thousands of force measurements, and 2) errors sufficiently self-consistent to treat the comparison between data-sets in atomic force microscopy measurements. We first consider HOPG as a model system and then employ our results to draw conclusions from the GML and MWCNT samples. We find that, in terms of surface properties and thus regarding surface functionality, aged HOPG and GML are more similar than aged HOPG and cleaved HOPG. The state of the HOPG samples is also as relevant for the comparison between HOPG and MWCNTs.