Bonded Carbon Atoms Research Articles

Cohesion energetics of carbon allotropes: Quantum Monte Carlo study

We have performed quantum Monte Carlo calculations to study the cohesion energetics of carbon allotropes, including sp(3)-bonded diamond, sp(2)-bonded graphene, sp-sp(2) hybridized graphynes, and sp-bonded carbyne. The computed cohesive energies of diamond and graphene are found to be in excellent agreement with the corresponding values determined experimentally for diamond and graphite, respectively, when the zero-point energies, along with the interlayer binding in the case of graphite, are included. We have also found that the cohesive energy of graphyne decreases systematically as the ratio of sp-bonded carbon atoms increases. The cohesive energy of γ-graphyne, the most energetically stable graphyne, turns out to be 6.766(6) eV/atom, which is smaller than that of graphene by 0.698(12) eV/atom. Experimental difficulty in synthesizing graphynes could be explained by their significantly smaller cohesive energies. Finally, we conclude that the cohesive energy of a newly proposed graphyne can be accurately estimated with the carbon-carbon bond energies determined from the cohesive energies of graphene and three different graphynes considered here.

Thermophysical Properties of the Novel 2D Materials Graphene and Silicene: Insights from Ab-initio Calculations

Graphene, a 2D layer of sp2 bonded carbon atoms in honeycomb arrangement, is a recently synthesized material, with exceptional mechanical, electronic, optical and thermal properties. Although theoretically studied since a long time, as it is the building block of graphite (a stacking of graphene layers), it was isolated as single or few layers only in 2004. Today graphene can be produced by several methods that depend on the quality and dimension of the material to be obtained. Applications of graphene are extensive in electronics, photonics, sensing and also in the field of energy generation and storage.Belonging to the same chemical group of carbon, also silicon could be in principle the ingredient of a 2D material similar to graphene, named silicene. It shares with graphene the peculiar optical and electronic properties and could provide a more easy compatibility with the existing silicon-based microelectronics technology.An extremely high thermal conductivity in the range 3000-5000 Wm-1K-1 has been measured for graphene. Only few results exist instead on the thermal properties of silicene.In this work we report ab-initio calculations, based on density functional theory and density functional perturbation theory, that provide the ingredients for the determination of the thermal conductivity and specific heat of graphene, silicene and 2D silicon carbide. Ab-initio calculations solve Schrödinger equation without the need of empirical parameters, only knowing the chemical nature of the system constituents. The use of consolidated approximations employed in density functional theory allows the accurate prediction of several properties of materials.

Synthesis of nitrogen-doped graphene by the thermal chemical vapor deposition method from a single liquid precursor

In this work we report the synthesis of nitrogen-doped graphene using an ambient pressure chemical vapor deposition technique on polycrystalline Ni substrates with a single liquid precursor, as the source of both carbon and nitrogen. Nitrogen atom substitution of the so-formed graphene was confirmed by X-ray photoelectron spectroscopy. The signal from sp2 bonded carbon atoms decreased in the nitrogen-doped graphene at longer deposition times. Spatially resolved Raman mapping results and transmission electron microscopy images show that the nitrogen-doped graphene formed with varying thickness from a monolayer to >10 layers. The effects of growth temperature and deposition time on the level of nitrogen doping, number of layers, and the quality of the nitrogen-doped graphene layer were investigated.

Growth Mechanism and Influences from Kinetic Factors on Carbon Materials with Cu and Silica Substrates during Atmospheric Pressure Chemical Vapor Deposition

High-quality graphene and amorphous pyrolytic carbon are deposited on Cu and silica, respectively, by atmospheric pressure chemical vapor deposition (APCVD). Growth mechanisms and influences from varied kinetic factors on the two products are analyzed and discussed. The gas-phase reaction is the decisive factor for the growth of pyrolytic carbon on silica because of poor catalytic dehydrogenation of the substrate (the electron structure of silica is the fundamental reason), while the surface reaction plays a determining role in the deposition of graphene on Cu. The formed graphene efficiently separates the substrate and aromatic hydrocarbons (AHs, gaseous products) to avoid the deposition of amorphous carbon on Cu. High H2 concentration acts as a kinetic inhibitor for both products. For the Cu substrate, H2 competes with CH4 to occupy surface active sites because of its high solubility in Cu. Moreover, excess H atoms decrease graphene quality by acting as a core for the formation of sp3 bonded carbon atoms. As for the silica substrate, the presence of H2 depresses the gas reaction, reduces the amount and size of the AHs, and leads to a lower deposition rate. In addition, the relationship between deposition rates of carbon materials and kinetic factors are studied.

Effect of Gas Flow Rate on the Properties of Amorphous Carbon Thin Films Prepared by Thermal CVD Using Camphoric Carbon Precursor

Amorphous carbon (a-C) thin films have been prepared by a simple thermal CVD using camphor oil as precursor. The effects of argon gas flow rate on electrical, optical and structural properties of a-C thin films have been investigated. The a-C thin films were characterized by using current-voltage (I-V) measurement, UV-Vis-NIR spectroscopy, Raman and FTIR spectroscopy. The I-V study reveals that the electrical conductivity was increased with increasing argon gas flow rate. It was found that the optical band gap decreased from 0.88 to 0.42 eV as gas flow rate increased which indicates a microstructural disorder at different gas flow rate. Raman and FTIR studies reveal the amorphous structures which consist of a mixture of sp2 and sp3 bonded carbon atoms.

Nano‐Graphene Oxide: A Potential Multifunctional Platform for Cancer Therapy

Nano-GO is a graphene derivative with a 2D atomic layer of sp² bonded carbon atoms in hexagonal conformation together with sp³ domains with carbon atoms linked to oxygen functional groups. The supremacy of nano-GO resides essentially in its own intrinsic chemical and physical structure, which confers an extraordinary chemical versatility, high aspect ratio and unusual physical properties. The chemical versatility of nano-GO arises from the oxygen functional groups on the carbon structure that make possible its relatively easy functionalization, under mild conditions, with organic molecules or biological structures in covalent or non-covalent linkage. The synergistic effects resulting from the assembly of well-defined structures at nano-GO surface, in addition to its intrinsic optical, mechanical and electronic properties, allow the development of new multifunctional hybrid materials with a high potential in multimodal cancer therapy. Herein, a comprehensive review of the fundamental properties of nano-GO requirements for cancer therapy and the first developments of nano-GO as a platform for this purpose is presented.

Real-time sub-Ångstrom imaging of reversible and irreversible conformations in rhodium catalysts and graphene

The dynamic responses of a rhodium catalyst and a graphene sheet are investigated upon random excitation with 80 kV electrons. An extraordinary electron microscope stability and resolution allow studying temporary atom displacements from their equilibrium lattice sites into metastable sites across projected distances as short as 60 pm. In the rhodium catalyst, directed and reversible atom displacements emerge from excitations into metastable interstitial sites and surface states that can be explained by single atom trajectories. Calculated energy barriers of 0.13 eV and 1.05 eV allow capturing single atom trapping events at video rates that are stabilized by the Rh [110] surface corrugation. Molecular dynamics simulations reveal that randomly delivered electrons can also reversibly enhance the $s{p}^{3}$ and the $s{p}^{1}$ characters of the $s{p}^{2}$-bonded carbon atoms in graphene. The underlying collective atom motion can dynamically stabilize characteristic atom displacements that are unpredictable by single atom trajectories. We detect three specific displacements and use two of them to propose a path for the irreversible phase transformation of a graphene nanoribbon into carbene. Collectively stabilized atom displacements greatly exceed the thermal vibration amplitudes described by Debye-Waller factors and their measured dose rate dependence is attributed to tunable phonon contributions to the internal energy of the systems. Our experiments suggest operating electron microscopes with beam currents as small as zepto-amperes/nm${}^{2}$ in a weak-excitation approach to improve on sample integrity and allow for time-resolved studies of conformational object changes that probe for functional behavior of catalytic surfaces or molecules.

The role of carbon nanofiber defects on the electrical and mechanical properties of CNF-based resins

Heat treatment of carbon nanofibers has proven to be an effective method in removing defects from carbon nanofibers, causing a strong increase in their structural perfection and thermal stability. It affects the bonding states of carbon atoms in the nanofiber structure and causes a significant transformation in the hybridization state of the bonded carbon atoms. Nanofilled resins made of heat-treated CNF show significant increases in their electrical conductivity even at low concentrations. This confirms that enhancement in the perfection of the fiber structure with consequent change in the morphological features plays a prominent role in affecting the electrical properties. Indeed heat-treated CNFs display a stiff structure and a smooth surface which tends to lower the thickness of the unavoidable insulating epoxy layer formed around the CNF which, in turn, plays a fundamental role in the electrical transport properties along the conducting clusters. This might be very beneficial in terms of electrical conductivity but might have negligible effect on the mechanical properties.

Humidity-dependent friction mechanism in an ultrananocrystalline diamond film

Friction behaviour of an ultrananocrystalline diamond film deposited by the microwave plasma-enhanced chemical vapour deposition technique is studied in a controlled humid atmosphere. The value of friction coefficient consistently decreases while increasing the humidity level during the tribology test. This value is 0.13 under 10% relative humidity conditions, which is significantly decreased to 0.004 under 80% relative humidity conditions. Such a reduction in friction coefficient is ascribed to passivation of dangling covalent bonds of carbon atoms, which occurs due to the formation of chemical species of hydroxyl and carboxylic groups such as C–COO, CH3COH and CH2–O bonding states.

High-resolution electron spin resonance analysis of ion bombardment induced defects in advanced low-κ insulators (κ = 2.0-2.5)

Electron spin resonance analysis of defects generated by ion bombardment in different low-permittivity (low-κ) SiO2-based insulating films reveals common types of encountered defects: The EX center (g = 2.0025) and a broad line centered at g = 2.0028 tentatively associated with dangling bonds of carbon atoms backbonded to C or Si atoms. The efficiency of defect generation varies, depending on the type of bombarding ions and the technology of insulator synthesis. However, the two identified defects are observed in all studied cases, suggesting that these defects correspond to the most stable atomic configurations resulting from the network damage of the low-κ materials.

SYNTHETIC GRAPHENE GROWN BY CHEMICAL VAPOR DEPOSITION ON COPPER FOILS

The discovery of graphene, a single layer of covalently bonded carbon atoms, has attracted intense interest. Initial studies using mechanically exfoliated graphene unveiled its remarkable electronic, mechanical and thermal properties. There has been a growing need and rapid development in large-area deposition of graphene film and its applications. Chemical vapor deposition on copper has emerged as one of the most promising methods in obtaining large-scale graphene films with quality comparable to exfoliated graphene. In this paper, we review the synthesis and characterizations of graphene grown on copper foil substrates by atmospheric pressure chemical vapor deposition. We also discuss potential applications of such large-scale synthetic graphene.

Electron spin resonance analysis of sputtering-induced defects in advanced low-κ insulators (κ=2.0–2.5)

Graphical abstractDisplay Omitted HighlightsWe have carried out a low-temperature ESR study of damage induced by RF ion sputtering in dielectrics.The studied materials are three types of low-permittivity insulators grown on (100)Si.This reveals as universal damage pattern the generation of two types of point defects.These include the EX center and a broad signal ascribed to C dangling bonds in carbosilicide clusters. High-resolution electron spin resonance analysis of defects induced by RF ion sputtering has been performed on three different low-permittivity insulators, including chemical vapour deposited carbon-doped SiO2, spin-on glass with subsequent porogen removal by thermal/ultraviolet curing, and layers obtained by self-assembly. The spectra reveal, as the universal damage pattern in all three types of insulators, generation of EX-centers (g=2.0025) and a broad (ΔBpp?12-15G) signal at g=2.0028 tentatively ascribed to dangling bonds of carbon atoms in a carbosilicide cluster. The differences in defect densities observed between different discharge gasses and different materials are explained by knock off of atoms of different mass from the insulating matrix.

Free Radical Reactions in Two Dimensions: A Case Study on Photochlorination of Graphene

Graphene, a two-dimensional giant-molecule of sp(2) -bonded carbon atoms, provides a perfect platform for studying free radical reaction chemistry in two-dimensions, which holds promise to control the chemical functionality of graphene. Free-radical photochlorination of graphene is used as an example to investigate the thickness, stacking order, and single- and double-side dependent reactivity in graphene. Anomalously low reactivity is observed in the photochlorination of AB-stacked bilayer graphene in comparison with that of few-layer graphene. Double-sided chlorination of graphene shows higher reactivity and chlorine coverage than single-sided reaction. It is also experimentally and theoretically demonstrated that chlorine free radicals at low coverage prefer to form a stable charge-transfer complex with graphene, which highly enhances graphene's conductivity and simultaneously generates a pseudo-bandgap through noninvasive doping. Moreover, the initial accumulation of chlorine radicals is considered as the rate-determining step of photochlorination of graphene.

Properties of Iodine Doped Amorphous Carbon Thin Films Grown by Thermal CVD

Thin film of undoped and doped amorphous carbon has been achieved using the simple thermal CVD system in an ambient gas of Ar and Ar with I2, respectively. The electrical and optical properties of the iodine doped amorphous carbon (a-C:I) thin films were studied. The incorporation of iodine into the amorphous carbon thin film results in increase of electrical conductivity as doping temperature increase up to 400°C, which indicates that doping effect of iodine. Heterojuction is confirmed by rectifying current-voltage characteristics of a-C:I/n-Si junction. The decreasing of optical band gap from 0.54 to 0.25 eV after iodine doping was determined which contribute to induce graphitization in the films. Raman result indicates that sp2 and sp3 bonded carbon atoms were dominated in the both with and without iodine doped thin films.

Intrinsic electronic and transport properties of graphyne sheets and nanoribbons

Graphyne, a two-dimensional carbon allotrope like graphene but containing doubly and triply bonded carbon atoms, has been proven to possess amazing electronic properties as graphene. Although the electronic, optical, and mechanical properties of graphyne and graphyne nanoribbons (NRs) have been previously studied, their electron transport behaviors have not been understood. Here we report a comprehensive study of the intrinsic electronic and transport properties of four distinct polymorphs of graphyne (α, β, γ, and 6,6,12-graphynes) and their nanoribbons (GyNRs) using density functional theory coupled with the non-equilibrium Green's function (NEGF) method. Among the four graphyne sheets, 6,6,12-graphyne displays notable directional anisotropy in the transport properties. Among the GyNRs, those with armchair edges are nonmagnetic semiconductors whereas those with zigzag edges can be either antiferromagnetic or nonmagnetic semiconductors. Among the armchair GyNRs, the α-GyNRs and 6,6,12-GyNRs exhibit distinctive negative differential resistance (NDR) behavior. On the other hand, the zigzag α-GyNRs and zigzag 6,6,12-GyNRs exhibit symmetry-dependent transport properties, that is, asymmetric zigzag GyNRs behave as conductors with nearly linear current-voltage dependence, whereas symmetric GyNRs produce very weak currents due to the presence of a conductance gap around the Fermi level under finite bias voltages. Such symmetry-dependent behavior stems from different coupling between π* and π subbands. Unlike α- and 6,6,12-GyNRs, both zigzag β-GyNRs and zigzag γ-GyNRs exhibit NDR behavior regardless of the symmetry.

그래핀이 포함된 나노유체의 열전도도 특성에 대한 연구

A nanofluid is a fluid containing suspended solid particles, with sizes on the order of nanometers. Especially graphene nanoparticle that has the high thermal conductivity properties among the various nanoparticles added to the nanofluid is receiving attention. Graphene is a flat monolayer of -bonded carbon atoms tightly packed into a honeycomb lattice. And are known to have very high thermal conductivity. Therefore, we compared thermal conductivity with viscosity of graphene M-5 nanofluids and graphene M-15 nanofluids. Graphene M-5 and graphene M-15 have different average particle diameters and the other properties are the same. Two kinds of graphene nanofluids was examined by measuring thermal conductivity via transient hot-wire method. And the viscosity was measured by using a rotational digital viscometer. As a result, graphene M-5 nanofluids exhibited better thermal conductivity and viscosity than graphene M-15 nanofluids.

Relationship between Stereochemistry and Charge Density in Hydrogen Bonds with Oxygen Acceptors

An extensive survey of Cambridge Structural Database was carried out to study the directionality and stereochemistry of hydrogen bonds with an oxygen acceptor including carbonyl, alcohols, phenols, ethers, and esters groups. The results obtained through this survey are correlated with the charge density of these different chemical groups. The electron density of these different oxygen atom types shows striking dissimilarities in the electron lone pair configuration. Esters and ethers with the C–O–C oxygen atom located in an aromatic cycle display merged lone pairs lobes, which is not the case when one of the bonded carbon atoms has sp3 hybridization. The positions of the lone pairs in the deformation electron density maps derived from theoretical calculation and from experimental charge density generally agree with the notable exception of phenols and C(sp3) esters. The experimental studies show generally lone pairs lobes that are closer to each other. Differences are found within COH groups: the two electron lone pairs are slightly closer in phenol oxygen atoms compared with alcohols in theoretical electron densities. In experimental charge densities, the discrepancy is more drastic because the two lone pair lobes appear merged in phenols; this might be due to a resonance effect with the neighboring sp2 carbon atom. This difference in the configuration of the two electron lone pairs affects the directionality of hydrogen bonds. For phenols, the preferred donor hydrogen atom position is close to the COH plane, while for alcohols, it is out of plane with the direction O···Hdonor forming an angle of around 30° to the COH plane. The number of H-bonds occurring with the donor hydrogen atom pointing toward the middle of the two lone pairs is small for carbonyl, contrary to alcohols and phenols. Also H-bonds involving alcohol/phenol acceptors have a stronger tendency to occur in directions close to the electron lone pair plane than for carbonyl. As expected, the directional attraction of hydrogen bond donors toward the lone pairs is much more pronounced for short H···O distances. This study could have implications in the design of force fields, in molecular recognition, in supramolecular crystal engineering, and in drug design.

Iodine Doping of Amorphous Carbon Thin Films Deposited by Thermal CVD

A simple thermal chemical vapor deposition method is employed for the deposition of amorphous carbon thin films by natural precursor camphor oil onto the glass substrates and the iodine doping process. In this work, we have studied the effect of iodine doping on the evolution of electrical properties and the optical and structural properties of amorphous carbon thin films. The amorphous carbon thin films were characterized by using Raman spectroscopy, UV-VIS-NIR spectroscopy, current-voltage (I-V) measurement, Fourier transform infrared (FTIR) and FESEM. The I-V study reveals that the electrical conductivity was increased with the iodine doping. The iodine doped thin films induced graphitization by decreasing the optical band gap. Raman and FTIR result indicates that amorphous carbon thin films consist of a mixture of sp2 and sp3 bonded carbon atoms. The FESEM shows the amorphous nature of the thin films.

Vacuum Technology in the study of Graphene

Graphene, an allotrope of carbon is a two-dimensional sheet of covalently bonded carbon atoms that has been attracting great attention in the field of electronics. In a recent review graphene is defined as a flat monolayer of carbon atoms tightly packed into a 2-D honeycomb lattice. A survey has been made of the production processes and instrumentation for characterization of graphene. In the production of graphene, the methods mainly used are Epitaxial growth, oxide reduction, growth from metal-carbon melts, growth from sugar. In the characterization of graphene, the instruments that are mainly used to study the atomic properties, electronic properties, optical properties, spin properties are Scanning Electron Microscopy, Transmission Electron Microscopy, Raman Spectroscopy. In all these instruments high or ultra-high vacuum is required. This paper attempts to correlate vacuum technology in the production and characterization of graphene.

The Effect of Camphor Oil Amounts on the Properties of Amorphous Carbon Thin Films by Thermal Chemical Vapor Deposition

Amorphous carbon thin films have been deposited by a simple Thermal Chemical Vapor Deposition (CVD) with varying the amount of natural precursor (camphor oil) onto the glass substrates. In this work, we have investigated the effect of different amount of camphor oil on the evolution of electrical conductivity and the optical and structural properties of amorphous carbon thin films. The amorphous carbon thin films were characterized by using current-voltage (I-V) measurement, UV-VIS-NIR spectroscopy and Raman spectroscopy. The current-voltage (I-V) study reveals that the highest electrical conductivity was deposited at 3 ml camphor oil. The optical band gap is almost unchanged with the increase of camphor oil amount. Raman result indicates that amorphous carbon thin films consists a mixture of sp2 and sp3 bonded carbon atoms.