Enhanced field emission characteristics of boron doped diamond films grown by microwave plasma assisted chemical vapor deposition

Boron doped diamond films were synthesized on silicon substrates by microwave plasma chemical vapor deposition (MPCVD) technique. The effect of B₂O₃ concentration varied from 1000 to 5000 ppm on the field emission characteristics was examined. The surface morphology and quality of films were characterized by scanning electron microscope (SEM) and Raman spectroscopy. The surface morphology obtained by SEM showed variation from facetted microcrystal covered with nanometric grains to cauliflower of nanocrystalline diamond (NCD) particles with increasing B₂O₃ concentration. The Raman spectra confirm the formation of NCD films. The field emission properties of NCD films were observed to improve upon increasing boron concentration. The values of the onset field and threshold field are observed to be as low as 0.36 and 0.08 V/μm, respectively. The field emission current stability investigated at the preset value of ∼1 μA is observed to be good, in each case. The enhanced field emission properties are attributed to the better electrical conductivity coupled with the nanometric features of the diamond films.     

Enhancement of optical properties and donor-related emissions in Y-doped ZnO

The Zn_1−xY_xO nanoparticles with good optical properties have been prepared by sol–gel method. The yttrium doping effect on the structures and optical properties were investigated by XRD, SEM, XPS and low temperature photoluminescence. The UV emission intensity of yttrium doped ZnO was over 300 times stronger than that of pure ZnO, which was an exciting result in enhancing the ultraviolet near band edge emission in photoluminescence from ZnO nanoparticles. The UV emission band of doped ZnO nanoparticles exhibits a red shift from 388 to 398 nm, indicating a shallow energy level near valence band has been formed due to the yttrium doping into ZnO lattices. The defect-related band is suppressed (I_D/I_UV = 1–0.83) considerably in Zn_1−xY_xO nanoparticles, revealing the quenching of the broad yellow-orange emission. The doping effect on the optical properties is investigated by temperature dependent photoluminescence. The experimental results indicated that the donor level of yttrium is deeper than that of undoped ZnO.     

Enhanced field emission from Si doped nanocrystalline AlN thin films

Si doped and undoped nanocrystalline aluminum nitride thin films were deposited on various substrates by direct current sputtering technique. X-ray diffraction analysis confirmed the formation of phase pure hexagonal aluminum nitride with a single peak corresponding to (1 0 0) reflection of AlN with lattice constants, a = 0.3114 nm and c = 0.4986 nm. Energy dispersive analysis of X-rays confirmed the presence of Si in the doped AlN films. Atomic force microscopic studies showed that the average particle size of the film prepared at substrate temperature 200 °C was 9.5 nm, but when 5 at.% Si was incorporated the average particle size increased to ∼21 nm. Field emission study indicated that, with increasing Si doping concentration, the emission characteristics have been improved. The turn-on field (E_to) was 15.0 (±0.7) V/μm, 8.0 (±0.4) V/μm and 7.8 (±0.5) V/μm for undoped, 3 at.% and 5 at.% Si doped AlN films respectively and the maximum current density of 0.27 μA/cm² has been observed for 5 at.% Si doped nanocrystalline AlN film. It was also found that the dielectric properties were highly dependent on Si doping.     

Enhanced white light emission in Er/Tm/Yb/Li codoped Y2O3 nanocrystals

Er/Tm/Yb codoped Y₂O₃ nanocrystals and Er/Tm/Yb/Li codoped Y₂O₃ nanocrystals have been synthesized by sol-gel method, bright white light emission has been observed at 976 nm excitation. The blue, green, and red emissions, respectively, arise from the transitions ¹G₄ → ³H₆ of Tm³⁺, ²H_11/2/⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 of Er³⁺ ion. Moreover, after doping Li⁺ ions into Er/Tm/Yb codoped Y₂O₃ nanocrystals, the white light emission increase greatly. CIE coordinate of Er/Tm/Yb/Li codoped Y₂O₃ nanocrystals is X = 0.32 and Y = 0.36 at 10 W/cm² excitation, which is very close to the standard equal energy white light illuminate (X = 0.33, Y = 0.33).     

Properties of boron and phosphorous incorporated tetrahedral amorphous carbon films grown using filtered cathodic vacuum arc process

This paper reports the electrical, mechanical, structural and field emission properties of as grown and also boron and phosphorous incorporated tetrahedral amorphous carbon (ta-C) films, deposited using a filtered cathodic vacuum arc process. The effect of varying boron and phosphorous content (up to 2.0 at.% in to ta-C) on the conductivity (σ_D), activation energy (ΔE₁), hardness, microstructure, emission threshold (E_turn-ON) and emission current density (J) at 12.5 V/μm of ta-C: B and ta-C: P films deposited at a high negative substrate bias of −300 V are reported. It is observed that both boron and phosphorous incorporation leads to a nearly an order increase in σ_D and corresponding decrease in ΔE₁ and a slight increase in hardness as compared to as grown ta-C films. In the case of field assisted electron emission, it is observed that E_turn-ON increases and J decreases. The changes are attributed to the changes in the sp³/sp² ratio of the films due to boron and phosphorous incorporation. The effect of boron on ta-C is to give a p-type effect whereas the effect of phosphorous gives n-type doping effect.     

Infrared and visible spectroscopic studies of the ytterbium doped borate Li6Y(BO3)3

The spectroscopic study of trivalent ytterbium doped Li₆Y(BO₃)₃ is conducted in the UV-visible and infrared range. An excitation in the charge transfer band of ytterbium has been selected in order to reduce the reabsorption effect on the IR emission intensity. The maximum of the emission is located at 972 nm for an excitation at 230 nm. The energy level assignment has been successfully conducted using vibrational spectroscopy to distinguish the pure electronic transitions from the phonon-assisted ones. The splitting of the ²F_5/2 and ²F_7/2 components is equal to 523 cm⁻¹ and 676 cm⁻¹, respectively. The decay time dependence as a function of the concentration is also reported. The calculated value τ_rad is about (1.03 ± 0.01) ms for the 1% doped material. For the highest concentration, an IR excitation gives rise to the observation of a blue-green luminescence caused by two mechanisms: an erbium emission at 550 nm after upconversion and a cooperative luminescence of ytterbium ions.     

The enhanced field-emission properties of screen-printed single-wall carbon-nanotube film by electrostatic field

In this work, we improved the field-emission properties of a screen-printed single-wall carbon-nanotube (SWCNT) film by applying a strong electrostatic field during the drying process after the printing. By applying the strong field, more tips of SWCNTs could emerge from the screen-printed film and turn somewhat toward the erecting direction because of the repulsive force among the SWCNTs. The field-emission properties of the film were thus improved obviously. The improved field emitters sample has low electron emission turn-on field (E_to = 1.22 V/μm), low electron emission threshold field (E_th = 2.32 V/μm) and high brightness with good uniformity and stability. The lowest operating field of the improved sample is below 1.0 V/μm and its optimum current density exceeds 3.5 mA/cm².     

Synthesis and field emission properties of GaN nanowires

Gallium nitride (GaN) nanowires grown on nickel-coated n-type Si (1 0 0) substrates have been synthesized using chemical vapor deposition (CVD), and the field emission properties of GaN nanowires have been studied. The results show that (1) the grown GaN nanowires, which have diameters in the range of 50-100 nm and lengths of several micrometers, are uniformly distributed on Si substrates. The characteristics of the grown GaN nanowires have been investigated using X-ray diffraction (XRD) and transmission electron microscopy (TEM), and through these investigations it was found that the GaN nanowires are of a good crystalline quality (2) When the emission current density is 100 μA/cm², the necessary electric field is an open electric field of around 9.1 V/μm (at room temperature). The field enhancement factor is ∼730. The field emission properties of GaN nanowires films are related both to the surface roughness and the density of the nanowires in the film.     

Electron field emission properties of conducting polymer coated multi walled carbon nanotubes

The present work describes the field emission characteristics of conducting polymer coated multi walled carbon nanotubes (MWNTs) field emitters fabricated over flexible graphitized carbon cloth. Nanocomposites involving the combination of MWNTs and conducting polymers polyaniline (PANI) and polypyrrole (PPy) have been prepared by in-situ polymerization method and have been characterized using scanning electron microscopy and transmission electron microscopy. Using spin coating method, field emitters based on PANI/MWNTs and PPy/MWNTs over flexible graphitized carbon cloth have been prepared. The field emission characteristics have been studied using an indigenously fabricated set up in a vacuum chamber with a base pressure of 2 × 10⁻⁵ Pa and the results are discussed. Our results display that the field emission performance of the emitters depends strongly on the work function of the emitting material. Low turn on emission field of 2.12 V/μm at 10 μA/cm² and high emission current density of 1 mA/cm² at 3.04 V/μm have been observed for PANI/MWNTs field emitter.     

Fabrication and properties of film-under-gate field emission arrays with SnO2 emitters for flat lamp

The film-under-gate field emission arrays (FEAs) have been fabricated on the glass substrates by conventional photolithography, anodic oxidation and lift-off method. SnO₂ emitters were deposited on the cathode electrodes of under-gate triode by screen printing. The image of film-under-gate field emission arrays with SnO₂ emitters was measured by the optical microscopy and field emission scanning electron microscopy (FESEM). The electric field distributions and electron trajectories of film-under-gate triode were simulated in the same anode voltage and different gate voltage by ANSYS. I-V characteristics of film-under-gate triode with SnO₂ emitters were investigated. It indicated that the SnO₂ emitters by screen printing uniformly distributed on the surface of cathode electrodes. The maximum anode current in this triode structure could come to 385 μA and the highest lightness was approximately 270 cd/m² as the gate and anode voltage was 140 V and 2000 V, respectively, at the anode-cathode spacing of 1100 μm. Moreover, the emission current fluctuation was less than 5% for 8 h. It showed that the fabricated device has a good stability of field emission performance and long lifetime, which may lead to practical applications for field emission electron source based on flat lamp for back light units (BLUs) in liquid crystal display (LCD).     

Carbon nanotube field emission cathodes fabricated with trivalent chromium conversion coated substrates

The properties of carbon nanotube (CNT) field emission cathodes fabricated by a dip coating method with trivalent chromium conversion coated substrates are studied. Two kinds of substrates with different morphologies, one with a rough crackled surface and the other with a smooth surface, were used for making the CNT cathodes, and their I-V curves and emission patterns were evaluated. The results show that, as compared to the smooth substrate surface, the rough surface with self-assembled sub-micro-cracks on the substrate can dramatically enhance the uniformity of the emission pattern and the emission efficiency. The cathode fabricated with the crackled substrate shows good field emission properties such as high brightness, good uniformity, a low turn-on field (0.86 V/μm) and a high current density of 10 mA/cm² at 2.5 V/μm.     

Thermally stable blue-emitting NaSrPO4:Eu phosphor for near UV white LEDs

A novel blue light emitting NaSr_1 − xPO₄:Eu²⁺_x (x = 0.001 to 0.02) phosphors were prepared by solid-state reaction method to investigate its optical properties and thermal stability for its application in white light-emitting diodes (w-LEDs). The excitation and emission spectra of the prepared phosphor reveal a broad emission peak centered at 460 nm which arises due to 4f-5d transitions of Eu²⁺ upon the near ultra-violet (n-UV) excitation wavelength at 380 nm. The effect of Eu²⁺ doping concentration and sintering temperature on the emission intensity of NaSrPO₄:Eu²⁺ was investigated along with its chromaticity coordinates. The temperature dependent luminescence properties of the prepared phosphor show better results than that of the commercial YAG:Ce³⁺phosphor. Besides, their XRD, FT-IR, SEM, TG, and DTA profiles have also been analyzed to explore its structural details.     

The effect of substrate surface roughness on ZnO nanostructures growth

The ZnO nanowires have been synthesized using vapor-liquid-solid (VLS) process on Au catalyst thin film deposited on different substrates including Si(1 0 0), epi-Si(1 0 0), quartz and alumina. The influence of surface roughness of different substrates and two different environments (Ar + H₂ and N₂) on formation of ZnO nanostructures was investigated. According to AFM observations, the degree of surface roughness of the different substrates is an important factor to form Au islands for growing ZnO nanostructures (nanowires and nanobelts) with different diameters and lengths. Si substrate (without epi-taxy layer) was found that is the best substrate among Si (with epi-taxy layer), alumina and quartz, for the growth of ZnO nanowires with the uniformly small diameter. Scanning electron microscopy (SEM) reveals that different nanostructures including nanobelts, nanowires and microplates have been synthesized depending on types of substrates and gas flow. Observation by transmission electron microscopy (TEM) reveals that the nanostructures are grown by VLS mechanism. The field emission properties of ZnO nanowires grown on the Si(1 0 0) substrate, in various vacuum gaps, were characterized in a UHV chamber at room temperature. Field emission (FE) characterization shows that the turn-on field and the field enhancement factor (β) decrease and increases, respectively, when the vacuum gap (d) increase from 100 to 300 μm. The turn-on emission field and the enhancement factor of ZnO nanowires are found 10 V/μm and 1183 at the vacuum gap of 300 μm.     

Electrical and electronic properties of nitrogen doped amorphous carbon (a-CNx) thin films

Nitrogen-doped amorphous carbon thin films (a-CN_x) were prepared on silicon substrate by pulsed laser deposition process using methane (CH₄) and nitrogen (N₂) as source gas. The electrical properties of a-CN_x films changes with nitrogen concentration in the film structure. The intensity ratio of the D and G peak (I_D/I_G) increases with higher nitrogen concentration, which means that sp²-clusters were formed in these films and is responsible for the enhancement of conductivity of the a-CN_x films. We observed that the amorphous carbon (a-C) films becoming more graphitic in nature yielding higher conductivity/lower resistivity with increase of nitrogen concentration. Electron field emission result shows that the emission current density enhances with nitrogen doping that indicates the useful in electron field emission devices application.     

Synthesis, characterization, growth mechanism, photoluminescence and field emission properties of novel dandelion-like gallium nitride

Dandelion-like gallium nitride (GaN) microstructures were successfully synthesized via Ni catalyst assisted chemical vapor deposition method at 1200 °C under NH₃ atmosphere by pre-treating precursors with aqueous ammonia. The as-synthesized product was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). X-ray diffraction analysis revealed that as-synthesized dandelion-like GaN was pure and has hexagonal wurtzite structure. SEM results showed that the size of the dandelion-like GaN structure was in the range of 30-60 μm. Dandelion-like GaN microstructures exhibited reasonable field emission properties with the turn-on field of 9.65 V μm⁻¹ (0.01 mA cm⁻²) and threshold field of 11.35 V μm⁻¹ (1 mA cm⁻²) which is sufficient for applications of electron emission devices, field emission displays and vacuum micro electronic devices. Optical properties were studied at room temperature by using fluorescence spectrophotometer. Photoluminescence (PL) measurements of dandelion-like GaN showed a strong near-band-edge emission at 370.2 nm (3.35 eV) with blue band emission at 450.4 nm (2.75 eV) and 465.2 nm (2.66 eV) but with out yellow band emission. The room-temperature photoluminescence properties showed that it has also potential application in light-emitting devices. The tentative growth mechanism for the growth of dandelion-like GaN was also described.     

Fourier transform emission spectroscopy of the FΔ-XΦ system of TiF

The emission spectra of TiF have been reinvestigated in the 4200-15 000 cm⁻¹ region using the Fourier transform spectrometer associated with the National Solar Observatory at Kitt Peak. TiF was formed in a microwave discharge lamp operated with 2.5 Torr of He and a trace of TiF₄ vapor, and the spectra were recorded at a resolution of 0.02 cm⁻¹. The TiF bands observed in the 12 000-14 000 cm⁻¹ region have been assigned to a new transition, F⁴Δ-X⁴Φ. Each band consists of four sub-bands assigned as, ⁴Δ_1/2-⁴Φ_3/2, ⁴Δ_3/2-⁴Φ_5/2, ⁴Δ_5/2-⁴Φ_7/2, and ⁴Δ_7/2-⁴Φ_9/2. A rotational analysis of the 0-1, 0-0, and 1-0 bands has been obtained and spectroscopic constants have been extracted.     

Effect of indium incorporation on properties of SnS thin films prepared by spray pyrolysis

Tin sulphide (SnS) thin films were deposited on glass substrate at different substrate temperature (T_s = 325 °C, 350 °C and 375 °C) by pyrolytic decomposition using stannous chloride and thiourea as precursor solutions. Also, indium-doped SnS thin films were prepared by using InCl₃ as dopant source. The dopant concentration [In/Sn] was varied from 2 at% to 6 at%. The XRD analysis revealed that the films were polycrystalline in nature having orthorhombic crystal structure with a preferred grain orientation along (1 1 1) plane. Due to In doping, the orientation of the grains in the (1 1 1) plane was found to be deteriorated. Atomic force microscopy (AFM) measurements revealed that the surface roughness of the films decreased due to indium doping. The optical properties were investigated by measuring the transmittance characteristics which were used to find the optical band gap energy, refractive index and extinction coefficient. The energy band gap value was decreased from 1.60 to 1.43 eV with increasing In concentration. The photoluminescence (PL) measurements of thin films showed strong emission band centered at 760 nm. Using Hall Effect measurements electrical resistivity, carrier concentration and Hall mobility have been determined.     

Characterization of scandia doped pressed cathode fabricated by spray drying method

Scandia doped pressed cathode was prepared by a new method of spray drying combined with two-step hydrogen reduction process. The Sc₂O₃ and barium-calcium aluminate co-doped powders have sub-micrometer size in the range of 0.1-1 μm and scandium oxide and barium-calcium aluminate are distributed evenly in the powders. The cathodes sintered by powder metallurgy at 1600 °C_b have a smooth surface and sub-micrometer grain structure with homogeneous distribution of scandium, barium, calcium and aluminum which are dispersed over and among the tungsten grains. This cathode has good emission, e.g., the current density of this cathode reaches 31.50 A/cm² at 850 °C_b. After proper activation, the cathode surface is covered by a Ba-Sc-O active substances layer with a preferable atomic ratio, leading to its good emission property. The evaporation activation energy of SDP cathode with 4.58 eV is the highest among the Ba-W, M-type and SDP cathodes, and the average evaporation velocity v_t of SDP cathode with 1.28 × 10⁻⁸ g cm⁻² s⁻¹ at 1150 °C_b is the lowest one.     

Enhanced field emission from LaB6 thin films with nanoprotrusions grown by pulsed laser deposition on Zr foil

Lanthanum hexaboride (LaB₆) films have been deposited on a zirconium foil by pulsed laser deposition method. The field emission studies of the LaB₆ deposited film have been performed in the planar diode configuration under ultra high vacuum conditions. The Fowler-Nordheim plots were found to be linear in accordance with the quantum mechanical tunneling phenomenon. A typical field emission current of 7.02 μA was drawn at an applied electric field of 2 V/μm. The field enhancement factor is calculated to be 8913 cm⁻¹, indicating that the field emission is from nanoscale protrusions present on the emitter surface. The atomic force microscope (AFM) investigation of the surface clearly shows the conical shaped nanoprotrusions of few hundred nanometers with asperities of 20-40 nm on its top. The emission current-time plot recorded at the pre-set value of emission current of 5 μA over a period of more than 3 h exhibits an initial increase and subsequent stabilization of the current. The results reveal that the LaB₆/Zr field emitter obtained by the pulsed laser deposition (PLD) is a promising cathode material for practical applications in field emission-based devices.     

Optical characterization, 1.5 μm emission and IR-to-visible energy upconversion in Er-doped fluorotellurite glasses

The optical properties of Er³⁺ ions in a novel glass based on TeO₂-PbF₂-AlF₃ oxyfluoride tellurites have been investigated using steady-state and time-resolved spectroscopies as a function of the rare-earth doping concentration. Basic optical characterizations have been performed measuring and calculating the absorption and emission spectra and the cross-sections, the Judd-Ofelt intensity parameters, the radiative probabilities and the fluorescence decays and lifetimes. Special attention has been devoted to the broad ⁴I_13/2→⁴I_15/2 emission transition at around 1.53 μm since, with a wide broadening of around 70 nm and a relative long lifetime of around 3 ms compared to others glass hosts, it shows potential applications in the design of erbium-doped fiber amplifiers. The absorption, the stimulated emission and the gain cross-sections of this transition have been obtained and compared with that obtained in different hosts. Finally, infrared-to-visible upconversion processes exciting at around 800 nm have been analyzed and different mechanisms involved in the energy conversion have been proposed.