First-principles density-functional investigation on the electronic properties and field emission of a hydrogenated nanodiamond

First-principles calculations using quantum-mechanical density functional theory (DFT) are carried out to investigate the geometrical structure and electronic properties for hydrogen terminated nanometer-sized diamonds. The results reveal that the size dependent feature in the electronic structures for nanodiamonds is different from that of Si clusters. The field emission properties for nanodiamonds are also explored, and it is found that under applied electric field Mulliken charges redistribute and accumulate on the emission side. Furthermore, the emission currents from the occupied orbitals for nanodiamond are calculated and it is revealed that the largest emission current comes from the third highest occupied molecular orbital.     

State of C-atoms on the modified nanodiamond surface

XPS, EEL, Auger and FTIR spectroscopies were used to testify the influence of chemical treatment upon the state of C-atoms in the core and on the surface of nanodiamond particles. The study was carried out with ND (JSC “Diamond Centre”). The different kinds of treatments were done ex-situ: with air (5 h) at 200 and 400 °C; with hydrogen (5 h) at 800, 850 and 900 °C; with fluorine (48 h) at 20 °C and 0.5 atm. Noticeable change was not found in the state of C-atoms both on the surface and up to 10 monolayers after these treatments. The concentration of F in the sample is equal to  9 at.%. The binding energy of the F 1s differs from the one in functional groups— –CF₂, –CF. Nevertheless FTIR spectra show bands that can be related to С–О, С–F bonds.     

Dual reaction capacity of hydrogenated nanodiamond

High temperature hydrogen stream treatment of nanodiamond was shown to produce nanodiamond with bifunctional surface. OH- and different CH-groups were shown to be the main functional groups on the surface. In this article we proposed two main strategies for further chemical modification of hydrogenated nanodiamond. OH-groups were shown to react with different types of acylating agents: anhydrides and chloranhydrides. Chlorination of hydrogenated nanodiamond with molecular chlorine and its subsequent treatment with alkyllithium reagents resulted in formation of alkyl-nanodiamonds. Chlorination was shown to reduce the size of nanodiamonds aggregates. We have also applied suspension-state NMR-H1 spectroscopy to study suspensions of alkyl-nanodiamonds for the first time.     

The Prato reaction on nanodiamond: Surface functionalization by formation of pyrrolidine rings

The Prato reaction is a highly valued reaction for the functionalization of fullerenes and carbon nanotubes. It uses the generation of pyrrolidine rings from azomethine ylides in the formation of a covalent grafting of functional moieties. Here we present the first application of this reaction on thermally annealed nanodiamond as a way for the grafting of a multitude of functional moieties. We used two types of azomethine ylides (aliphatic and pyridine derived) to show the broad applicability of the reaction for the surface modification of nanodiamond. The possibility to attach glycol units or alkyl chains, both with or without reactive terminal groups opens the way to tune the surface properties of such “Prato-functionalized” diamond in a very flexible way.     

Synthesis and electron field emission properties of nanodiamond films

Effects of CH₄/H₂ ratio and bias voltage of the microwave plasma-enhanced chemical vapor deposition (MPE-CVD) process on the nucleation behavior and associated characteristics of nanodiamonds were investigated. While the scanning electron microscopy (SEM) microstructure and Raman crystal structure of the films insignificantly vary with CH₄/H₂ ratio and bias voltage, electron field emission properties of the materials markedly change with these deposition parameters. The predominating factor modifying the electron field emission properties of the nanodiamond films is presumed to be the increase in the proportion of sp²-bonded grain boundaries when the grain size of the nanodiamond films decreases. Between these two major factors, the bias voltage shows more prominent effects on modifying the granular structure of the nanodiamonds than the CH₄/H₂ ratio does. The best electron field emission properties attainable are J_e=500 μA/cm² at 20 V/μm and E₀=8.5 V/μm.     

Raman characterization and UV optical absorption studies of surface plasmon resonance in multishell nanographite

Nanographite (NG) particles were produced by annealing of superpurified detonation nanodiamonds (grain size ~ 5 nm) at 1600 °C. The aim of this research was to provide Raman characterization of nanographites obtained and to investigate characteristic features of UV optical absorption in NG suspensions caused by the excitation of surface plasmon resonance (SPR) and its dependence on disorder and defectiveness of graphene shells during their transformation. The 1-st and 2-nd order Raman spectra of the NG samples excited at 514 nm were analyzed. Two different approaches applied for evaluation of the in-plane NG crystallite sizes by using the D- and G-band intensities ratio gave quite different results (~ 3.5 nm and ~ 5.5 nm) reflecting, most likely, a complicated NG structure. The changes in both intensity and position of SPR absorption peak for water suspension of NG particles may originate in structural imperfections and/or changes in aggregation of NG particles.     

Preparation of water-soluble nanographite and its application in water-based cutting fluid

Water-soluble nanographite was prepared by in situ emulsion polymerization using methacrylate as polymeric monomer. The dispersion stability and dispersion state of graphite particles were evaluated by UV-visible spectrophotometry and scanning electron microscopy, respectively. The water-soluble nanographite was then added into the water-based cutting fluid as lubricant additive. The lubrication performance of water-based cutting fluid with the nanographite additive was studied on four-ball friction tester and surface tensiometer. Results indicate that the modification method of in situ emulsion polymerization realizes the uniform and stabilized dispersion of nanographite in aqueous environment. The optimal polymerization condition is 70°C (polymerization temperature) and 5 h (polymerization time). The addition of nanographite decreases the friction coefficient and wear scar diameter by 44% and 49%. Meanwhile, the maximum non-seizure load (P_B) increases from 784 to 883 N, and the value of surface tension (32.76 × 10⁻³ N/m) is at low level. Nanographite additive improves apparently the lubrication performance of water-based cutting fluid.     

Facile fabrication of HDPE-g-MA/nanodiamond nanocomposites via one-step reactive blending

ABSTRACT: In this letter, nanocomposites based on maleic anhydride grafted high density polyethylene (HDPE-g-MA) and amine-functionalized nanodiamond (ND) were fabricated via one-step reactive melt-blending, generating a homogeneous dispersion of ND, as evidenced by transmission electron microscope observations. Thermal analysis results suggest that addition of ND does not affect significantly thermal stability of polymer matrix in nitrogen. However, it was interestingly found that incorporating pure ND decreases the thermal oxidation degradation stability temperature, but blending amino-functionalized ND via reactive processing significantly enhances it of HDPE in air condition. Most importantly, cone tests revealed that both ND additives and reactive blending greatly reduce the heat release rate of HDPE. The results suggest that ND has a potential application as flame retardant alternative for polymers. Tensile results show that adding ND considerably enhances Young's modulus, and reactive blending leads to further improvement in Young's modulus while hardly reducing the elongation at break of HDPE.     

Growth,electronic properties and applications of nanodiamond

Nanodiamond or nanocrystalline diamond is a broad term used to describe a plethora of materials. It is generally accepted that nanocrystalline diamond (NCD) consists of facets less than 100 nm in size, whereas a second term “ultrananocrystalline diamond” (UNCD) has been coined to describe material with grain sizes less than 10 nm. These differences in morphology originate in the growth process. Conventional hydrogen rich gas phases produce facetted diamond with grain size proportional to film thickness and low sp² content. If these films are thin the grains can be less than 100 nm and hence NCD. By starving the plasma of hydrogen, the reduction in etching of sp² can lead to re-nucleation. At the extreme this results in very small grain sizes of around 3–5 nm, UNCD.The electronic properties of these two materials are vastly different. NCD is basically very thin microcrystalline diamond and thus can be doped with boron. It is intrinsically transparent, with absorption increasing with doping level. UNCD is highly absorbing due to its higher sp² content, and exhibits a reduced bandgap due to disorder. By adding nitrogen to the gas phase, the density of states within the bandgap increases and ultimately metallic conductivity can be achieved. This conductivity is n-type but not doping.     

Electronic and optical properties of a nanographite ribbon in an electric field

Through the tight-binding model, the effect of electric field on electronic and optical properties of a zigzag (an armchair) H-terminated nanographite ribbon is studied. The effective electric field shifts the Fermi level (E_F), modifies the energy dispersions, alters the subband spacing, produces the new edge state, changes the band gap, and causes the semiconductor-metal transitions. First, in the metallic zigzag ribbon, the degeneracy of flat bands at E_F is lifted by the electric field. Meanwhile, an energy band gap E_g, depending on the ribbon width and the field strength, is induced. And while the field strength increases, the semiconductor-metal transition occurs in both the semiconducting armchair and the zigzag ribbons. Then, the effect of electric field on band structures is completely reflected in the features of density of states—the shift of peak position, the change of peak height and the alternation of band gap. Most importantly, the effective electric field has great influence on the low-energy absorption spectra. It can change frequency of the first peak, alter the peak height, and even produce the new peaks.     

Magnetic and high resolution TEM studies of nanographite derived from nanodiamond

The time evolution of graphitization was analyzed based on the structural and magnetic properties of nanodiamond samples annealed at 1600 °C for various time intervals. High resolution TEM and XRD show that the nanodiamond particles are converted to spherical onions for short annealing time intervals, and then they are completely transformed to polyhedral nanographite through the annealing for 120 min. The in-plane orbital susceptibility χ_orb (300 K) analyzed on the basis of Kotosonov’s equation remains fairly constant in the range −5.5 × 10⁻⁶ to −5.9 × 10⁻⁶ emu/g, suggesting that the coherent scattering region (CSR) and the Fermi energy are independent of the annealing times. It also indicates the presence of serious defects affecting the electronic structure of nanographite. As the annealing time increases from 2 to 120 min, the Pauli paramagnetic susceptibility decreases. The broad ESR signal is associated with the spins localized on the edge states of the π-electron graphite network. The ESR linewidth becomes nine times less as the heat-treatment time rises from 2 to 120 min. The relatively large linewidth is associated with the scattering of π-electrons by the edge phonon modes and magnetic interaction between the edge-localized spins. During continuous annealing the nanoparticle structure changes from more defective to less defective and the number of edge-localized spins in a particle drops from ca. 2 to ca. 1 despite the CSR size and the Fermi energy remain practically the same.     

Effect of mixed support of carbon black and nanographite on the activity of Pt catalyst for ethanol oxidation

The effect of the mixed support of carbon black and nanographite (CG) on the electrocatalytic activity and stability of the CG supported Pt (Pt/CG) catalyst for ethanol oxidation was investigated. It was found that the electrocatalytic activity and stability of the Pt/CG catalyst for ethanol oxidation depend on the weight ratio of carbon black and nanographite because the carbon black has a large surface area and nanographite has high conductivity. The Pt/CG catalyst with a weight ratio of 10:1 produces the best electrocatalytic activity and stability of the Pt/CG catalyst for ethanol oxidation.     

Technology of passivated surface channel MESFETs with modified gate structures

Surface channel MESFETs have suffered from instabilities and drift in the past. To overcome these effects a suitable device passivation seems to be one of the key aspects. In this investigation such a passivation scheme on the basis of Atomic Layer Deposition (ALD) of Al₂O₃ has been developed and combined with advanced gate structures, like a field plate and a second MOS gate (in a dual gate configuration).     

Effect of heat treatment of nanodiamonds on the scratch behavior of polyacrylic/nanodiamond nanocomposite clear coats

In the current paper, effects of heat treatment of nanodiamond particles on the scratch behavior of polyacrylic/diamond nanocomposite coatings were studied. Two types of nanodiamond, i.e. one produced by detonation synthesis (DND) and the other produced by nondetonation synthesis (NDND) were used as reinforcement phase to increase scratch resistance of polyacrylic base polymeric clear coat. Heat treatment was used as the surface modification route. Coatings containing both types of particles in two surface conditions (as-received and heat treated) were compared to each other and also to the neat polyacrylic coating. The results showed strong effect of heat treatment on scratch resistance of coatings based on the scratch width criterion. The effect of heat treatment was more pronounced on DND particles than on NDNDs. However the pendulum hardness showed a reverse trend.     

The energetic and electronic properties of atomic hydrogen on MgO(001) surface: Tight-binding and Ab initio calculations

We report the computational results of hydrogen adsorption atop Mg and O atoms on the MgO(001) surface, followed by its absorption under the target atoms, using two approaches: tight-binding and ab initio methods. We present the energetic and electronic aspects of these interactions and discuss the qualities of the non-self-consistent field tight-binding results compared with the ab initio results. There is the qualitative reproduction of ab initio results in the hydrogen adsorption phase on both types of ions. The tight-binding results are found to be more accurate in the hydrogen absorption phase than in the adsorption phase. In the adsorption calculations the introduction of the surface dipole term in the tight-binding total energy would be required to compensate for the absence of electronic relaxation in the MgO(001) surface.     

Morphological and mechanical properties of polyamide 6/nanodiamond composites prepared by melt mixing: effect of surface functionality of nanodiamond

Surface chemistry of as‐received nanodiamond (ND) was first tailored by dry thermal oxidation to obtain carboxylated ND (ND‐COOH) and by wet chemistry to obtain ethylenediamine‐functionalized ND (ND‐EDA). Then, the surface‐functionalized ND particles were dispersed in polyamide 6 (PA6) using the melt‐mixing method. Transmission optical and scanning electron microscopies indicated a fine dispersion at low nanodiamond concentrations, e.g. 0.25 wt%, particularly with ND‐EDA. Differential scanning calorimetry revealed that ND‐EDA favoured the α‐phase crystal and enhanced the degree of crystallinity of PA6. Experimental data indicated that ND‐EDA had considerably improved tensile properties at low concentration of 0.25 wt% compared to ND‐COOH, which was correlated to the fine dispersion and stronger and thicker interphase in the case of ND‐EDA. It was also found that the toughness of PA6 was improved on incorporation of ND‐EDA due to development of microcracking and crazing. © 2016 Society of Chemical Industry     

Suface grafting of polymers onto nanodiamond by ligand-exchange reaction of ferrocene moieties of polymers with polycondensed aromatic rings of the surface

To improve the dispersibility of nanodiamond (ND) in solvents and polymer matrices, the grafting of copolymers containing vinyl ferrocene (Vf) onto the surface by a ligand-exchange reaction with ferrocene moieties of the copolymer and polycondensed aromatic rings of ND surface was investigated. The copolymer containing Vf was prepared by the radical copolymerization of Vf with vinyl monomers, such as methyl methacrylate (MMA), styrene (St), and N-isopropylacrylamide (NIPAM), using 2, 2′-azobisisobutyronitrile as an initiator. It was found that by heating of ND with poly(Vf-co-MMA), poly(Vf-co-St), and poly(Vf-co-NIPAN) in the presence of AlCl₃ and Al powder as catalysts, the corresponding copolymer was successfully grafted onto the surface. On the contrary, in the absence of AlCl₃, no grafting of these copolymers was observed. The grafting of polymers onto the ND surface was confirmed by FT-IR. These polymer-grafted NDs were found to give stable dispersions in solvents for the grafted polymer. In addition, the dispersibility of poly(Vf-co-NIPAM)-grafted ND uniformly dispersed in water below 32 °C but precipitated above the temperature. Therefore, it was concluded that the dispersibility of ND in water could be controlled by the temperature of water.     

Density functional study of graphite bulk and surface properties

The structural and electronic properties of bulk graphite were compared using density functional theory calculations with the local density (LDA) and generalized gradient (GGA) approximations to determine the relative ability of each to model this material. The GGA fails to generate interplanar bonding, but the LDA does, even though the band structures obtained from both approximations were essentially identical. The atomic geometry, electronic structure, and enthalpy of the graphite (0 0 0 1) surface were then obtained using the LDA. The calculated surface energy was ∼0.075 J/m² and the calculated work function was in 4.4-5.2 eV range, both of which correspond well to published, measured and calculated values. The surface is semi-metallic, just like in the bulk, with the conduction band minimum and valence band maximum just touching with minimal overlap in the H-K region in the Brillouin Zone, so the electron affinity was identical to the work function. The (0 0 0 1) surface undergoes no noticeable relaxation and no reconstruction, as the strong covalent bonding prevents any corrugation of the basal planes.     

A DFT study of electronic structures, energies, and molecular properties of lithium bis[croconato]borate and its derivatives

Theoretical studies on electrolyte salts, lithium bis[croconato(2-)]borate (LBCB) and its derivatives, lithium [croconato(2-) salicylato(2-)]borate (LCSB), and bis[salicylato(2-)]borate (LBSB) are carried out using density functional theory (DFT) method and B3LYP theory level for the first time. Bidentate structures involving two oxygen atoms are preferred. Based on these conformations, a linear correlation was observed between the highest occupied molecular orbital (HOMO) energies and the limiting oxidation potentials measured by linear sweep voltammetry, which supports experimental results that strongly electron-withdrawing substituent anions are more resistant against oxidation than their organic counterparts. The correlations were also observed between ionic conductivity and binding energy, solubility and theoretical set of parameters of anion_, thermal stability and the hardness (η)_. Wave function analyses have been performed by natural bond orbital (NBO) method to further investigate the cation-anion interactions.     

Effects of hydrogen on electronic properties of doped diamond

We have investigated the effects of hydrogen on the electronic structure of diamond doped boron and sulfur using cluster model method within the frame of ab initio density functional theory (DFT). The results show that the presence of hydrogen results in a deep donor level with no change of conductivity type in sulfur-doped diamond samples and the formation of the multiple hydrogen-boron complexes may cause a conductivity type transition in the hydrogen-rich boron-doped diamond samples.