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.     

Electron transport and electron field emission of nanodiamond synthesized by explosive detonation

Electron transport and electron field emission of nanometer-size diamond powders coated on quartz and n⁺-type Si substrates have been characterized. The nanodiamond powders were synthesized by explosive detonation. The measurement of temperature-dependent conductivity shows that the conduction of the nanodiamond coating is non-Arrhenius leading to an interesting behavior at low temperatures. The material shows a good behavior of electron field emission. In the electric field range from 3 to 5 V/μm, the emission can be approximately described by the Fowler–Nordheim (F–N) equation. A stable emission current density as high as ∼95 mA/cm² was obtained under an applied field of 5 V/μm. It has been suggested that the novel electron transport and the high emission current density of the samples might originate from their non-continuous network structure of the nanodiamond particles.     

Cathodoluminescence and electron field emission of boron-doped a-C:N films

The optical and electrical properties of so-called carbon nitride films (a-C:N) and boron doped so-called carbon nitride films (a-C:N:B) are studied with cathodoluminescence (CL) spectroscopy and electron field emission measurement. The a-C:N films were first deposited on Si by a filtered cathodic arc plasma system, and then boron ions (∼1 × 10¹⁶ cm⁻²) were implanted into the a-C:N films to form a-C:N:B films by a medium current implanter. The structural and morphological properties of a-C:N and a-C:N:B films were then analyzed using secondary ion mass spectrometer, X-ray photoelectron spectroscopy, FT-IR spectra, Raman spectroscopy and atomic force microscopy. The a-C:N film exhibits luminescence of blue light (∼2.67 eV) and red light (∼1.91 eV), and the a-C:N:B film displays luminescence of blue light (∼2.67 eV) in CL spectra measured at 300 K. Furthermore, the incorporated boron atoms change the electron field emission property, which shows a higher turn on field for the a-C:N:B film (3.6 V/μm) than that for the a-C:N film (2.8 V/μm).     

Effect of surface treatment on the electron field emission property of nano-diamond films

Nano-diamond films having grain size around 20 nm were deposited by bias enhanced growth (BEG) method. Different surface treatments were carried out to increase their field emission properties and their effects are clearly noticed. Surface morphology of different surface treated nano-diamond films was examined. There was no significant change in the curve of Raman spectra of different surface treated samples. Raman spectra were typically of similar nature to nano-diamond film. Field emission results were more interesting. Biased in hydrogen plasma treated nano-diamond film has shown best electron emission behavior and low turn-on-field (E₀). The turn-on-field of bias-treated nano-diamond film was 19.5 V/μm. The decrease of turn-on-field (6 V/μm) of biased treated nano-diamond film from as-grown BEG film was attributed to the formation of thin sp² layer and more defects on the surface of film by hydrogen ion bombardment. Moreover, hydrogen-plasma treated nano-diamond film was also found to be good for electron emission but there was no improvement in electron emission as in the case of air plasma treated nano-diamond films.     

CVD growth of secondary carbon nanotubes and its effect on field electron emission

Secondary carbon nanotubes (CNTs) were grown on primary ones by simply changing the methane concentration. No additional catalyst was used throughout the whole deposition process. The CNT growth was carried out using hot filament chemical vapor deposition in a gas mixture of methane and hydrogen. The structure and surface morphology of the deposited CNTs were studied and the field emission properties of the CNTs were tested. It was found that synthesizing primary CNTs at extremely low methane concentration is the key for the secondary growth without additional catalyst. The CNT samples grown with secondary nanotubes exhibited improved field emission properties.     

Effects of hydrogen flow rate on the growth and field electron emission characteristics of diamond thin films synthesized through graphite etching

Diamond film deposition on silicon was explored using our newly developed graphite etching technology in a microwave plasma reactor. The effects of hydrogen flow rate on the growth rate, morphology and field electron emission properties of the synthesized diamond were investigated systematically. The growth rate and nucleation density of diamond films increased significantly with the decrease of hydrogen flow rate. Nanocrystalline diamond films were obtained with a low hydrogen flow rate (i.e. 3 sccm or lower). Diamond quality is improved and the growth rates are much higher in this graphite etching process, compared to conventional H₂ + 1% CH₄ gas mixture. The results suggest that diamond growth is enhanced by activated hydrocarbon radicals formed through in-situ etching of graphite by atomic hydrogen. The turn-on electric field decreased and the emission current increased with the decrease of the hydrogen flow rate. The enhanced field electron emission property of the diamond films synthesized at lower hydrogen flow rate is attributed to the decreased diamond grain size because the small diamond grains increase the electron conduction channels which facilitate the electron transport inside diamond films.     

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.     

The effect of hydrogen plasma chemical annealing on electron field emission of amorphous carbon films

Layer-by-layer deposition method, in which nanometer-thick film deposition and hydrogen plasma annealing processes were alternatively repeated, was applied to fabricate hydrogenated amorphous carbon films in our present work. It was found that the hydrogen plasma treatment changed the sp²/sp³ ratio due to chemical etching. Consequently, a stable vacuum electron emission with a low threshold field was achieved compared with that from conventionally deposited a-C films. The threshold electric field is as low as 2 V/μm. The influence of the hydrogen plasma chemical annealing on the field emission behavior was systematically investigated. The improvement of field emission characteristics can be attributed to the large field enhancement effect due to the inhomogeneous distribution of nanometer scale sp² clusters.     

The effect of argon on the electron field emission properties of α-C:N thin films

Amorphous carbon nitride (α-C:N) thin films were synthesized on silicon as electron emitters by the electron cyclotron resonance chemical vapor deposition (ECR-CVD) system in which a negative dc bias was applied to the graphite substrate holder and a mixture of C₂H₂ and N₂ was used as precursors. The addition of Ar combined with the application of a negative dc bias can increase nitrogen content (N/C) measured by X-ray photoelectron spectroscopy (XPS), eliminate the dangling bonds in the film determined by Fourier transform infrared (FTIR) spectroscopy, decrease the film thickness measured by field emission scanning electron microscope (FE-SEM), increase the film roughness measured by atomic force microscope (AFM) and raise the graphitic content examined by Raman spectroscopy. The result shows that the onset emission field of α-C:N with Ar addition to the precursors can be as low as 4.5 V μm⁻¹ compared with 9.5 V μm⁻¹ of the film without the addition of Ar.     

CVD growth and field electron emission of aligned carbon nanotubes on oxidized Inconel plates without addition of catalyst

Direct growth of carbon nanotubes (CNTs) on Inconel 600 sheets was investigated using plasma enhanced hot filament chemical vapor deposition in a gas mixture of methane and hydrogen. The Inconel 600 sheets were oxidized at different temperatures (800 °C, 900 °C, 1000 °C, and 1100 °C) before CNT deposition. The structure and surface morphology of the pre-treated substrate sheets and the deposited CNTs were studied by scanning electron microscopy (SEM) and X-ray diffraction. The field electron emission (FEE) properties of the CNTs were also tested. The SEM results show that well aligned CNTs have been grown on the pre-treated Inconel sheets without addition of any catalysts and the higher treatment temperature resulted in CNTs with better uniformity, indicating that the oxidation pre-treatment of the substrate is effective to enhance the CNT growth. FEE testing shows that CNTs with better height uniformity exhibit better FEE characteristics.     

Growth and electron field emission properties of ultrananocrystalline diamond on silicon nanostructures

The electron field emission (EFE) properties of Si nanostructures (SiNS), such as Si nanorods (SiNR) and Si nanowire (SiNW) bundles were investigated. Additionally, ultrananocrystalline diamond (UNCD) growth on SiNS was carried out to improve the EFE properties of SiNS via forming a combined UNCD/SiNS structure. The EFE properties of SiNS were improved after the deposition of UNCD at specific growth conditions. The EFE performance of SiNR (turn-on field, E₀ = 5.3 V/μm and current density, J_e = 0.53 mA/cm² at an applied field of 15 V/μm) was better than SiNW bundles (turn-on field, E₀ = 10.9 V/μm and current density, J_e < 0.01 mA/cm² at an applied field of 15 V/μm). The improved EFE properties with turn-on field, E₀ = 4.7 V/μm, current density, J_e = 1.1 mA/cm² at an applied field of 15 V/μm was achieved for UNCD coated (UNCD grown for 60 min at 1200 W) SiNR. The EFE property of SiNW bundles was improved to a turn-on field, E₀ = 8.0 V/μm, and current density, J_e = 0.12 mA/cm² at an applied field of 15 V/μm (UNCD grown for 30 min at 1200 W).     

Effect of structural variation on the field emission from carbon films with nano-sized constituents

We report on the structural variation and the concomitant change in the emission characteristics of carbon films with two nano-sized constituents, nanotubes and nanoparticles, induced by the addition of ammonia to the precursor of hot-filament chemical vapor deposition. The increase of the ammonia concentration resulted in a large morphological change due to the change in the relative abundance of two constituents and moreover, a deterioration of carbon layers making up the nanoparticles and nanotubes. The systematic variation of the electron emission accompanying the structural change indicated that the graphitic-sheet quality was one of the important factors that determined the emission from the nano-sized constituents.     

CVD growth and field emission characteristics of nano-structured films composed of vertically standing and mutually intersecting nano-carbon sheets

Nano-structured films composed of rather flat nano-carbon sheets which roughly stood vertically on Si wafers and which intersected each other at large angles were grown by means of a high-power microwave-plasma chemical-vapour-deposition (MWPCVD) method. The structure of the fabricated films was investigated using scanning electron microscopes and Raman spectroscopy. Field emission (FE) currents obtained from these films reached 50 mA/cm² at a macroscopic electric field of 3.6 V/μm. The observed FE characteristics were analyzed using a modified Fowler–Nordheim (F–N) equation where the field enhancement factor and effective emission area were treated as field-dependent parameters. It was found from the analysis that the vertically standing nano-carbon flat sheets had larger field enhancement factors and less FE areas, compared with those obtained for wrinkled nano-carbon sheets previously reported by the authors. It is suggested that the observed variations in the FE current without saturation behavior as a function of the macroscopic electric field can be explained in terms of an effective surface geometry phenomenon of the concerned films.     

Growth and high current field emission of carbon nanofiber films with electroplated Ni catalyst

We report high current-density field emission from carbon nanofiber (CNF) films synthesized using electroplated Ni catalysts on gold-buffer layers via hot-filament chemical vapor deposition. High-density thick CNFs which had a solid structure without hollow cores and many protrusions on the outside of CNF body were formed. The protrusions consisted of buckled small graphitic sheets, and some protrusions had very small tip radius to which we attribute good field emission from CNF films. The maximum emission current of 3.67 mA was measured from the area of 4.9 × 10^{− 3} cm², corresponding to the current density of 750 mA/cm², at the electric field of 12.5 V/μm. There was a distinctive hysteresis in emission–current curves measured while ramping up and down the bias-voltage. The deviations between up- and down-sweep emission currents, and the slope change in Fowler–Nordheim curves were most prominent in medium-voltage and -current regime. Moreover, the emission–current hysteresis showed dependence on the pressure during measurement and the voltage-sweep speed. We propose that adsorbate-enhanced field emission and adsorbate desorption during field-emission measurement were responsible for the observed emission behavior.     

Ultra-high resolution electron microscopy investigation of growth defects in CVD diamond films: twin interactions and fivefold twin centres

An elucidation of the core structure of fivefold twin centres arising from the interaction between less than five first-order twin boundaries in plasma-assisted chemical vapour deposition (CVD) of diamond films is reported. These particular twinning centres have been identified by ultra-high resolution electron microscopy at 0.12 nm resolution, with the help of image calculations. Plausible three-dimensional atomic-scale models are proposed for two specific structural variants, which have been found closely connected to high-order twin boundaries via original heptagonal or octagonal structural units. To our knowledge, this is the first time that such types of fivefold twin centres and associated structural units, which are quite representative of the growth defects observed in CVD diamond, have been reported.     

Isothermal titration calorimetry, transmission electron microscopy, and field emission scanning electron microscopy of a main-chain viologen polymer containing bromide as counterions

The interaction of a main-chain viologen polymer containing bromide as counterions with water and aqueous potassium bromide over a broad range of concentrations was studied with isothermal titration calorimetry. The dilution process of this polymer was endothermic as opposed to flexible poly(sodium acrylate) and poly(sodium styrenesulfonate). This result may be related to the different mechanism of hydration of pyridinium and bromide groups in the main chain. It also exhibited aggregation phenomenon in both water and aqueous potassium bromide solutions as detected by transmission electron microscopy like other flexible and rigid-rod polyelectrolytes. As the polymer concentrations in aqueous solutions increase, the aggregated polymer exhibited more defined ordered structures than random structures observed at low polymer concentrations. Field emission scanning electron microscopy also revealed the effect of variation of concentration of aqueous potassium bromide on the morphology of the polymer matrix. At increasing concentrations of aqueous potassium bromide, the polymer structures became more ordered than those in low concentrations.     

Syntheses and characterizations of two-dimensional polymers based on tetraimidazole tetraphenylethylene ligand with aggregation-induced emission property

Two new coordination polymers [Co(TIPE)(H₂O)₂]·Hbtc·CH₃CN·3H₂O (1) and [Cd(TIPE)_0.5(m-bdc)(H₂O)]·CH₃OH·H₂O (2) (TIPE = tetra(3-imidazoylphenyl)ethylene, H₃btc = 1,3,5-benzenetricarboxylate, m-H₂bdc = 1,3-benzenedicarboxylate) have been synthesized under hydrothermal conditions and characterized by single-crystal X-ray diffraction, elemental analysis, IR, TGA, and PXRD. Furthermore, the luminescent properties of complexes 1, 2 and TIPE ligand have also been investigated in the solid at room temperature.     

Chemical interaction of D9 alloy clad with B4C in liquid sodium: Studies employing XPS,XRD, and SEM

A cell representing typical control rod subassembly of the prototype fast breeder reactor is designed to probe the liquid sodium-mediated chemical interaction of boron carbide (B₄C) control rod with the D9 alloy clad. The cell was equilibrated at 973 K for 5000 hours with liquid sodium in the annular gap. XRD shows the formation of Cr₃C₂ and Fe₂B along with oxides of Ni, Cr, and Fe. XPS studies reveal the diffusion of boron and carbon up to a depth of about 160 and 120 μm, respectively. A boron-rich region is observed up to a depth of about 40 μm which consists of B³⁺ arising from oxides and a nearly constant elemental boron region, extending up to a depth of 160 μm. The highly reacted zone extends up to a depth of about 40 μm consisting of oxides of most of the elements of D9 clad exhibiting a higher valence states. As the cell is helium leak tested, oxygen out gassed from B₄C at 973 K played a major role in chemical oxidation of the constituents of the D9 alloy which can be minimized by using high-density B₄C.     

Investigation of alkylamine self-assembled films on iron electrodes by SEM, FT-IR, EIS and molecular simulations

Alkylamine were used to form self-assembled films for the inhibition of the corrosion of iron in 0.5 M H₂SO₄ solution. The films were characterized by electrochemical impedance spectroscopy (EIS) and surface analysis techniques including SEM and FT-IR. EIS results indicate that the inhibition ability of these alkylamine self-assembled films depends on the immersion time and the alky chain length of the adsorbate. Both longer immersion time of the electrode in the solution and longer alky chain will result in stronger inhibition ability of the films. However, when the immersion time was increased over some critical point, the inhibition ability was almost invariable. Also when the chain length reached a certain degree such as 14 carbon atoms, the inhibition ability decreased on the contrary. The ability of the corrosion inhibition of the tetradecylamine and dodecylthiol mixed films improved remarkably comparing with the tetradecylamine or dodecylthiol single films. In addition, molecular simulation was used to discuss the adsorption mechanism and good agreement with electrochemical results was obtained.     

Electrochemical synthesis of lepidocrocite thin films on gold substrate—EQCM, IRRAS, SEM and XRD study

Lepidocrocite thin films have been electrochemically synthesised on polycrystalline Au substrates following two ways, direct synthesis and synthesis via green rust (GR). The direct synthesis consists in oxidising Fe(II) species in a 0.4 M NaCl/0.02 M Met-Imidazole/0.01 M FeCl₂ solution at pH 7.5. The synthesis via GR consists in converting a green rust thin film into lepidocrocite thin film by galvanostatic oxidation. The thin films have been characterised by means of electrochemical quartz crystal microbalance, scanning electron microscopy, X-ray diffraction and infrared reflection-absorption spectroscopy.