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).     

Synthesis of diamond using ultra-nanocrystalline diamonds as seeding layer and their electron field emission properties

A modified nucleation and growth process was adopted so as to improve the electron field emission (EFE) properties of diamonds films. In this process, a thin layer of ultra-nanocrystalline diamonds (UNCD), instead of bias-enhanced-nuclei, were used as nucleation layer for growing diamond films in H₂-plasma. The morphology of the grains changes profoundly due to such a modified CVD process. The geometry of the grains transform from faceted to roundish and the surface of grains changes from clear to spotty. The Raman spectroscopies and SEM micrographs imply that such a modified diamond films consist of UNCD clusters (~ 10–20 nm in size) on top of sp³-bonded diamond grains (~ 100 nm in size). Increasing the total pressure in CVD chamber deteriorated the Raman structure and hence degraded the EFE properties of the films, whereas either increasing the methane content in the H₂-based plasma or prolonged the growth time improved markedly the Raman structure and thereafter enhanced the EFE properties of diamond films. The EFE properties for the modified diamond films can be turned on at E₀ = 11.1 V/μm, achieving EFE current density as large as (J_e) = 0.7 mA/cm² at 25 V/μm applied field.     

Plasma post-treatment process for enhancing electron field emission properties of ultrananocrystalline diamond films

This paper demonstrated the plasma post-treatment (ppt) process for modifying the granular structure of ultrananocrystalline diamond (UNCD) films so as to improve their electron field emission (EFE) properties. The ppt-processed UNCD films exhibited improved EFE properties as turn-on field of E₀ = 7.0 V/μm (J_e = 0.8 mA/cm² at 17.8 V/μm). TEM investigation revealed that the prime factor, which enhanced the EFE properties of the UCND films, is the induction of nano-graphitic clusters due to the ppt-process. However, for achieving such a goal, the granular structure of the primary UNCD layer has to be relatively open. That is, the size of grains should be sufficiently small and the grain boundaries should be of considerable thickness, containing abundant hydro-carbon species. Such a simple and robust process for synthesizing conductive UNCD films is especially useful for practical applications.     

Electron field emission from filtered arc deposited diamond-like carbon films using Au and Ti layers

In this work, tetrahedral diamond-like carbon (DLC) films are deposited on Si, Ti/Si and Au/Si substrates by a new plasma deposition technique — filtered arc deposition (FAD). Their electron field emission characteristics and fluorescent displays of the films are tested using a diode structure. It is shown that the substrate can markedly influence the emission behavior of DLC films. An emission current of 0.1 μA is detected at electric field E_DLC/Si=5.6 V/μm, E_DLC/Au/Si=14.3 V/μm, and E_DLC/Ti/Si=5.2 V/μm, respectively. At 14.3 V/μm, an emission current density J_DLC/Si=15.2 μA/cm², J_DLC/Au/Si=0.4 μA/cm², and J_DLC/Ti/Si=175 μA/cm² is achieved, respectively. It is believed that a thin TiC transition layer exists in the interface between the DLC film and Ti/Si substrate.     

Fabrication and field emission properties of ultra-nanocrystalline diamond lateral emitters

Field emission characteristics of ultra-nanocrystalline diamond (UNCD) have recently caught much attraction due to its importance in technological applications. In this work, we have fabricated lateral-field emitters comprised of UNCD films, which were deposited in CH₄/Ar medium by microwave plasma-enhanced chemical vapor deposition method. The substrates, silicon-on-insulator (SOI) or SiO₂-coated silicon, were pre-treated by mixed-powders-ultrasonication process for forming diamond nuclei to facilitate the synthesis of UNCD films on these substrates. Lateral electron field emitters can thus be fabricated either on silicon-on-insulator (SOI) or silicon substrates. The lateral emitters thus obtained possess large field enhancement factor (β = 1500–1721) and exhibit good electron field emission properties, regardless of the substrate materials used. The electron field emission can be turned on at 5.25–5.50 V/μm, attaining 5500–6000 mA/mm² at 12.5 V/μm (100 V applied voltage).     

Substrate temperature effects on the electron field emission properties of nitrogen doped ultra-nanocrystalline diamond

For the purpose of improving the electron field emission properties of ultra-nanocrystalline diamond (UNCD) films, nitrogen species were doped into UNCD films by microwave plasma chemical vapor deposition (MPCVD) process at high substrate temperature ranging from 600° to 830 °C, using 10% N₂ in Ar/CH₄ plasma. Secondary ion mass spectrometer (SIMS) analysis indicates that the specimens contain almost the same amount of nitrogen, regardless of the substrate temperature. But the electrical conductivity increased nearly 2 orders of magnitude, from 1 to 90 cm^− 1 Ω^− 1, when the substrate temperature increased from 600° to 830 °C. The electron field emission properties of the films were also pronouncedly improved, that is, the turn-on field decreased from 20 V/μm to 10 V/μm and the electron field emission current density increased from less than 0.05 mA/cm² to 15 mA/cm². The possible mechanism is presumed to be that the nitrogen incorporated in UNCD films are residing at grain boundary regions, converting sp³-bonded carbons into sp²-bonded ones. The nitrogen ions inject electrons into the grain boundary carbons, increasing the electrical conductivity of the grain boundary regions, which improves the efficiency for electron transport from the substrate to the emission sites, the diamond grains.     

Effect of H2/Ar plasma on growth behavior of ultra-nanocrystalline diamond films: The TEM study

Incorporation of H₂ species into Ar plasma was observed to markedly alter the microstructure of diamond films. TEM examinations indicate that, while the Ar/CH₄ plasma produced the ultrananocrystalline diamond films with equi-axed grains (~ 5 nm), the addition of 20% H₂ in Ar resulted in grains with dendrite geometry and the incorporation of 80% H₂ in Ar led to micro-crystalline diamond with faceted grains (~ 800 nm). Optical emission spectroscopy suggests that small percentage of H₂-species (< 20%) in the plasma leads to partially etching of hydrocarbons adhered onto the diamond clusters, such that the C₂-species attach to diamond surface anisotropically, forming diamond flakes, which evolve into dendrite geometry. In contrast, high percentage of H₂-species in the plasma (80%) can efficiently etch away the hydrocarbons adhered onto the diamond clusters, such that the C₂-species can attach to diamond surface isotropically, resulting in large diamond grains with faceted geometry. The field needed to turn on the electron field emission for diamond films increases from E₀ = 22.1 V/μm (J_e = 0.48 mA/cm² at 50 V/μm applied field) for 0% H₂ samples to E₀ = 78.2 V/μm (J_e < 0.01 mA/cm² at 210 V/μm applied field) for 80% H₂ samples, as the grains grow, decreasing the proportion of grain boundaries.     

High dose N ion implantation effects on surface treated UNCD films

Ion implantation is commonly used to modify the surface or near-surface properties of materials. In this work, plasma treated ultrananocrystalline diamond (UNCD) films were implanted using 100 and 200 keV high dose (10¹⁶ ions/cm²) nitrogen ions and annealed. Detailed studies have been carried out to reveal the structural and chemical states of the surface treated UNCD films before implantation, as-implanted, and after annealing by using Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electron field emission (EFE) measurements. The high dose N ion implantation induced the formation of amorphous phase, which are converted into graphitic phase after annealing, and improved the field emission properties of UNCD films. The improved field emission is attributed to the surface charge transfer doping mechanism.     

The stability of the CNT/Ni field emission cathode fabricated by the composite plating method

A novel composite plating method has been developed for the fabrication of carbon nanotube/Ni (CNT/Ni) field emission cathode. The field emission properties of the initial CNT/Ni field emitter show a low turn-on electric field E_on of about 1.1 V/μm with an emission current density of 1 μA/cm², and a low threshold electric field E_th of about 1.7 V/μm with an emission current density of 1 mA/cm². After performing a stability test with a high emission current density in high vacuum, the corresponding microstructure and the degree of graphitization of the CNT/Ni field emitter were measured by using scanning electron microscopy and Raman spectroscopy. We found that the degree of graphitization slowly decreases with the duration time t_FE of the stability test, the size of small rod-like CNT/Ni composite structures in the film increases with t_FE, and obvious cracks appear in the film as t_FE is larger than 60 h. The degradation of the field emission properties may be explained by the Joule heating effect on the CNT/Ni field emitter under high emission current density.     

Synthesis,crystal structure and magnetic properties of a new cyanide-bridged mixed-valence copper(I)/copper(II) clathrate

A unique cyanide-bridge mixed-valence Cu^I/Cu^II clathrate of formula [Cu^I₂(CN)₃][{Cu^II(tren)}₂(μ-CN)](CF₃SO₃)₂ [tren = tris(2-aminoethyl)amine] containing cyanide-bridged [{Cu^II(tren)}₂(μ-CN)]^3 + binuclear cations stacked between anionic honeycomb layered copper(I) cyanide networks, was synthesized and structurally characterized by single crystal X-ray diffraction. Variable-temperature magnetic susceptibility studies showed that the cyanide bridge mediates a strong antiferromagnetic interaction between the copper(II) centers (J = − 160 cm^− 1, the spin Hamiltonian being defined as H = −J S_A⋅S_B).     

Long-term stability of a horizontally-aligned carbon nanotube field emission cathode coated with a metallic glass thin film

A horizontally-aligned carbon nanotube (HACNT) field emission cathode was coated with a metallic glass thin film (MGTF) to improve the stability of the field emission properties. HACNT field emission cathodes have previously been fabricated on glass substrates using composite plating and crack-formation techniques. A carbon nanotubes/nickel (CNTs/Ni) composite film is deposited onto a glass substrate at 80 °C by the composite plating technique alone. Cracks are then formed in the CNT/Ni composite film during 30 min heating at 300 °C, and HACNTs are exposed in the cracks. The field emission properties of the HACNT field emission cathode show a low turn-on electric field E_on of about 2.3 V/μm, a low threshold electric field E_th of about 4.7 V/μm at an emission current density of 1 mA/cm², and a stability time of 78 h. The degradation of the HACNT field emission cathode is prevented by using a MGTF-coating technique and superior long-term stability (i.e. >125 h, with 5 nm MGTF; >270 h, with 10 nm MGTF) for the MGTF/HACNT field emission cathode is achieved.     

Piezoresistivity of n-type conductive ultrananocrystalline diamond

Recent developments of a piezoresistive sensor prototype based on n-type conductive ultrananocrystalline diamond (UNCD) are presented. Samples were deposited using hot filament chemical vapor deposition (HFCVD) technique, with a gas mixture of H₂, CH₄ and NH₃, and were structured using multiple photolithographic and etching processes. Under controlled deposition parameters, UNCD thin films with n-type electrical conductivity at room temperature (5 × 10^− 3 – 5 × 10¹ S/cm) could be grown. Respective piezoresistive response of such films was analyzed and the gauge factor was evaluated in both transverse and longitudinal arrangements, also as a function of temperature from 25 °C up to 300 °C. Moreover, the gauge factor of piezoresistors with various sheet resistance values and test structure geometries was evaluated. The highest measured gauge factor was 9.54 ± 0.32 at room temperature for a longitudinally arranged piezoresistor with a sheet resistance of about 30 kΩ/square. This gauge factor is well comparable to that of p-type boron doped diamond; however, with a much better temperature independency at elevated temperatures compared to the boron-doped diamond and silicon. To our best knowledge, this is the first report on piezoresistive characteristics of n-type UNCD films.     

Structural characterization of ferromagnetic bridged-acetato and -dichlorido copper(II) complexes based on bicompartmental 4-t-butylphenol

The reaction of Cu(II) salts with 2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-t-butylphenol (L^t-Bu-OH) afforded two bridged-phenoxido/hydroxido complexes. The dinuclear bridged acetate species [Cu₂(μ-L^t-Bu-O)(μ-CH₃COO)](PF₆)₂ (1) and the 1D polymeric doubly-bridged-chlorido {[Cu₂(μ-L^t-Bu-OH)(μ-Cl)₂](ClO₄)₂·4H₂O}_n (2). The two complexes were structurally characterized. Both complexes revealed ferromagnetic interactions; moderate in complex 1 (J = + 30.8 cm^− 1) and very weak (J = + 2.25 cm⁻¹) in 2.     

The effect of the CH4 level on the morphology,microstructure, phase purity and electrochemical properties of carbon films deposited by microwave-assisted CVD from Ar-rich source gas mixtures

Carbon thin films were deposited on Si substrates by microwave-assisted chemical vapor deposition (CVD) using variable CH₄ levels in an Ar/H₂ (Ar-rich) source gas mixture. The relationship between the CH₄ concentration (0.5 to 3 vol.%) in the source gas and the resulting film morphology, microstructure, phase purity and electrochemical behavior was investigated. The H₂ level was maintained constant at 5% while the Ar level ranged from 92 to 94.5%. The films used in the electrochemical measurements were boron-doped with 2 ppm B₂H₆ while those used in the structural studies were undoped. Boron doping at this level had no detectable effect on the film morphology or microstructure. Relatively smooth ultrananocrystalline diamond (UNCD) thin films, with a nominal grain size of ca. 15 nm, were only formed at a CH₄ concentration of 1%. At the lower CH₄ concentration (0.5%), faceted microcrystalline diamond was the predominant phase formed with a grain size of ca. 0.5 µm. At the higher CH₄ concentration (2%), a diamond-like carbon film was produced with mixture of sp²-bonded carbon and UNCD. Finally, the film grown with 3% CH₄ was essentially nanocrystalline graphite. The characteristic voltammetric features of high quality diamond (low and featureless voltammetric background current, wide potential window, and weak molecular adsorption) were observed for the film grown with 1% CH_4, not the films' grown with higher CH₄ levels. The C₂ dimer level in the source gas was monitored using the Swan band optical emission intensity at 516 nm. The emission intensity and the film growth rate both increased with the CH₄ concentration in the source gas, consistent with the dimer being involved in the film growth. Importantly, C₂ appears to be involved in the growth of the different carbon microstructures including microcrystalline and ultrananocrystalline diamond, amorphous or diamond-like carbon, and nanocrystalline graphite. In summary, the morphology, microstructure, phase purity and electrochemical properties of the carbon films formed varied significantly over a narrow range of CH₄ concentrations in the Ar-rich source gas. The results have important implications for the formation of UNCD from Ar-rich source gas mixtures, and its application in electrochemistry. Characterization data by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), visible-Raman spectroscopy and electrochemical methods are presented.     

Thermal analysis of submicron nanocrystalline diamond films

The thermal properties of sub-μm nanocrystalline diamond films in the range of 0.37–1.1 μm grown by hot filament CVD, initiated by bias enhanced nucleation on a nm-thin Si-nucleation layer on various substrates, have been characterized by scanning thermal microscopy. After coalescence, the films have been outgrown with a columnar grain structure. The results indicate that even in the sub-μm range, the average thermal conductivity of these NCD films approaches 400 W m^− 1 K^− 1. By patterning the films into membranes and step-like mesas, the lateral component and the vertical component of the thermal conductivity, k_lateral and k_vertical, have been isolated showing an anisotropy between vertical conduction along the columns, with k_vertical ≈ 1000 W m^− 1 K^− 1, and a weaker lateral conduction across the columns, with k_lateral ≈ 300 W m^− 1 K^− 1.     

Composite diamond films for short-mm wave and THz traveling wave tube windows

With an increase in frequency, the diamond thickness of the microwave windows for short-mm wave and THz traveling wave tubes (TWTs) approaches 100 μm or even tens of μm. This poses problems of mechanical strength and air tightness to the polycrystalline diamond (PCD) window. To overcome these problems, we have studied a composite diamond film that consists of PCD and ultra-nanocrystalline diamond (UNCD). First, SEM was used to examine the early growing process of UNCD on PCD. The 5 μm thick UNCD grown on 40 μm PCD exhibited a hillock structure with densely packed ≤ 20 nm granules, in contrast to the PCD layer showing randomly packed, micrometer sized grains. Then, the effect of UNCD thickness on fracture strength and thermal conductivity was studied using the test samples with thin layers of UNCD having thicknesses of 1, 2.5, 5, and 10 μm on 100 μm thick PCD films, respectively. The fracture strengths of all the films are 2–3 times higher than that of the PCD films, which is 350 ± 150 MPa. As expected, the thermal conductivity of the samples measured at ~ 20 °C decreases with an increase in UNCD thickness, particularly in the range of 0 to 2.5 μm. At a thickness of 10 μm, the thermal conductivity was found to be ~ 10 W/cm 1 K. Finally, a 100 μm sandwich-like structure with a total UNCD thickness of 10 μm was fabricated and two 180 GHz TWT windows were assembled. RF tests show that for the operating frequency range of 175 to 185 GHz, the transmission loss (S₂₁) was found to be ≤ 1.22 and ≤ 1.71 dB, respectively, indicating an excellent RF performance. Mechanical strength and air tightness of the windows were also found improved and able to meet the requirement of the device. This work provides a novel approach for fabricating relatively thin diamond films for RF applications, such as TWT windows.     

Effect of annealing temperature on ferroelectric electron emission of sol–gel PZT films

In this work, the influence of annealing temperature on the ferroelectric electron emission behaviors of 1.3-μm-thick sol–gel PbZr_0.52Ti_0.48O₃ (PZT) thin film emitters was investigated. The results revealed that the PZT films were crack-free in perovskite structure with columnar-like grains. Increasing annealing temperature led to the growth of the grains with improved ferroelectric and dielectric properties. The remnant polarization increased slightly from 35.3 to 39.6 μC/cm² and the coercive field decreased from the 56.4 to 54.6 kV/cm with increasing annealing temperature from 600 to 700 °C. The PZT film emitters exhibited remarkable ferroelectric electron emission behaviors at the threshold voltage above 95 V. The film annealed at 700 °C showed a relatively lower threshold voltage and higher emission current, which is related to the improved ferroelectric and dielectric properties at higher annealing temperature. The highest emission current achieved in this work was around 25 mA at the trigger voltage of 160 V.     

Open circuit voltage increase by substituted spacer and thieno[3,4-c]pyrrole-4,6-dione for polymer solar cells

We reported on two donor polymers containing thieno[3,4-c]pyrrole-4,6-dione(TPD) derivatives as electron withdrawing units for organic photovoltaics (OPVs). To control molecular weight and solubility of polymers, hexyl side chains are inserted to thiophene spacers. Due to the electron donating characteristic of hexyl side chains, highest occupied molecular orbital (HOMO) energy level of polymer is decreased as 0.18 eV, whereas the open circuit voltage is increased to 1.08 V. When bulk heterojunction devices were fabricated, the best PCE value of 0.360% (V_OC = 0.89 V, J_SC = 1.2 mA/cm², FF = 36.3%) under 100 mW/cm² irradiation.     

Superconducting properties of homoepitaxial CVD diamond

Superconductivity was achieved above 10 K in heavily boron-doped diamond thin films deposited by the microwave plasma-assisted chemical vapor deposition (CVD) method. Advantages of the CVD method are the controllability of boron concentration in a wide range, and a high boron concentration, compared to those obtained using the high-pressure high-temperature method. The superconducting transition temperatures of homoepitaxial (111) films are determined to be 11.4 K for T_C onset and 8.4 K for zero resistance from transport measurements. In contrast, the superconducting transition temperatures of (100) films T_C onset = 6.3 K and T_C zero = 3.2 K were significantly suppressed.     

Time dependent growth of ZnO nanoflowers with enhanced field emission properties

Herein, we report the facile growth of ZnO nanoflowers composed of nanorods on silicon substrate by non-catalytic thermal evaporation process. The grown nanoflowers were examined in terms of their morphological, structural, optical and field emission properties. The detailed characterizations revealed that the nanoflowers are grown in high density, possessing well-crystallinity and exhibiting wurtzite hexagonal phase. The Raman-scattering spectrum shows a sharp optical-phonon E₂ mode at 437 cm⁻¹ which confirmed the wurtzite hexagonal phase for the grown nanoflowers. The room-temperature PL spectrum depict a strong ultraviolet emission at 381 nm, revealed good optical properties for the ZnO nanoflowers. The field emission studies revealed that a turn-on field for the ZnO nanoflowers based field emission device was 4.3 V/μm and the emission current density reached to 0.075 mA/cm² at an applied electric field of 7.2 V/μm and exhibit no saturation. The field enhancement factor ‘β’ for the fabricated device was estimated from the F-N plot and found to be ~2.75×10³. Finally, systematic time-dependent experiments were performed to determine the growth process for the formation of ZnO nanoflowers composed of nanorods.