Improvement of the field emission of carbon nanotubes-metal nanocomposite

Carbon nanotubes (CNT) decorated with Ni and Ag performed by electroless plating, and the effect of Ni and Ag nanoparticles and coating distribution on field emission of CNT are studied. The chemical composition, microstructure of CNT/Ni and CNT/Ag nanocomposites are characterized by an energy dispersion X-ray spectroscope (EDS), a transmission electron microscope and a scanning electron microscope. The field emission properties of CNT/Ni and CNT/Ag cathodes are measured using a diode structure under a pressure of 10^?5 Pa. The experimental results show that fine and well-dispersed metallic nanoparticles and discontinuous coating of Ni and Ag on the CNT surface can be obtained by electroless plating. Moreover, the enhanced field emission properties of CNT decorated with Ni and Ag can be obtained by lowering the work function of emitters and reducing the contact resistance between cathode and substrate. The field enhancement factors as high as 24264 of CNT/Ni and 25565 of CNT/Ag emitters can be improved by the distributed nano-sized Ni and Ag formed on the CNT surface.     

High-Current-Density field emission from self-heating printed carbon nanotubes

High-current-density field emitters are considered as the potential and necessary components for compact high definition x-ray sources and high-power cold cathode microwave amplifiers. In this report, high-current-density field emission from self-heating printed carbon nanotubes is introduced. Large emission current causes large heat that increases the temperature of the emitters. The temperature is estimated to be more than ~1600 K. Localized surface field and high temperature both drive more electrons escaping from the emitters, and the maximum current density is larger than ~2.7 A/cm² that will satisfy the need of most vacuum electron devices including x-ray sources and microwave electron devices.     

Determination of beta emitters (90Sr, 14C and 3H) in routine measurements using plastic scintillation beads

Plastic scintillation has recently been shown to be a powerful alternative to liquid scintillation and Cherenkov techniques in radionuclide determination due to the good values obtained for the measurement parameters and the low amount of wastes generated. The present study evaluated the capability of plastic scintillation beads and polyethylene vials for routine measurements of beta emitters ( ⁹⁰Sr, ¹⁴C, ³H).Results show that high- and medium-energetic beta emitters can be quantified with relative errors less than 5% in low-activity aqueous samples, whereas low-energetic beta emitters can only be quantified in medium-activity samples.     

Electron field emitters based on multi-walled carbon nanotubes coated with conducting polymer/metal/metal-oxide composites

The present work describes the field emission properties of multi-walled nanotubes (MWNTs)-based conducting polymer/metal-oxide/metal/MWNTs composites (polyaniline (PANI)/SnO₂/Sn/MWNTs). MWNTs were synthesised by chemical vapour deposition technique. SnO₂/Sn/MWNTs were prepared by using chemical reduction followed by calcination. By in situ polymerisation method, surface of SnO₂/Sn/MWNTs were coated with PANI. PANI/SnO₂/Sn/MWNTs field emitters were fabricated over flexible graphitised carbon fabric substrate by spin coating technique. High-resolution transmission electron microscopy and scanning electron microscopy were used to characterise the field emitters. Field emission properties have been studied using an indigenously made facility. The fabricated PANI/SnO₂/Sn/MWNTs field emitters exhibited excellent field emission properties with a turn on field of 1.83 V µm^?1 and a field enhancement factor of 4800.     

Fabrication of multi-emitter array of CNT for enhancement of current density

We studied and compared field emission properties of two kinds of emitters of randomly oriented multi-wall carbon nanotubes (MWNTs), viz. continuous film emitter (CFE) and multi-emitter array (MEA). The CFE has a continuous film of MWNTs while the MEA consists of many equidistant small circular emitters. Both types of emitters were prepared by dispersing MWNTs over a titanium (Ti) film (for CFEs) or Ti circular islands (for MEAs) deposited on tantalum (Ta) followed by rooting of MWNTs into the Ti film or the Ti islands at high temperature. Emission properties of both types of emitters were analyzed with changing their emission areas. In case of the CFEs, current density decreased with an increase in emission area whereas consistent current densities were achieved from MEAs with different emission areas. In other words, the total emission current was achieved in proportion to the emission area in the case of MEAs. Additionally a high current density of 22 A/cm² was achieved at an electric field of 8 V/μm from MEAs, which was far better than that obtained from CFEs. The high current density in MEAs was attributed to edge effect, in which higher emission current is achieved from the edge of film emitter. The results indicate that the field emission characteristics can be greatly improved if a cathode contains many small equidistant circular emitters instead of a continuous film. The outstanding stability of the CFE and the MEA has been demonstrated for 2100 and 1007 h, respectively.     

Improvement and characteristics of conical silicon emitters employing wet-dry etching

A number of new technologies require conical and sharp tips to serve as electron emitters in the vacuum microelectronics. In this paper, we improved radius of curvature, height and cone angle of emitters in order to reach the enhancement result of field enhancement factor (β). We developed a fabrication process to improve geometry of emitter by employing isotropic dry etching in pure SF₆ and a mixture of SF₆ and O₂ followed by thermal oxidation technique. We successfully achieved excellent conical emitters with 5–10 nm radius of curvature, 4.4 μm height, and 30° cone angle. The conical silicon emitters current–voltage characteristics shows that E_to = 4.8 V/μm (turn-on electric field) with current density of 10 μA/cm², and maximum current density J = 60.4 μA/cm² at E = 8.14 V/μm. This study may provide a practical guideline for design and fabrication of a high-performance silicon emitter used in various industrial applications.     

Enhancing electron field emission of carbon nanoflakes by hydrogen post-annealing process

In this study, the carbon nanoflakes (CNFs) fabricated by sputtering were chosen as the field emission emitters because of their very sharp and thin edges which are potentially good electron field emission sites. The as-deposited CNFs were annealed in the furnace under hydrogen atmosphere. The results showed that the optimum field emission properties with smaller turn-on field and larger current density were obtained at annealing temperature of 600 °C for 10 min. The hydrogen thermal annealing has chemical etching on the surface of the CNFs and produces appropriate emission site density to increase the emission current density. The turn-on field was reduced from 6.7 to 5.8 V/μm and electric current density was increased from 22 to 187 μA/cm² under 8 V/μm after hydrogen thermal annealing.     

Spectral analysis of transition metal-doped MgO “matched emitters” for thermophotovoltaic energy conversion

A new, thermally excited Co/Ni-doped MgO ceramic emitter for TPV energy conversion is described in this work, and termed the matched emitter because its emissive power spectrum is very efficiently matched with the portion of the electromagnetic spectrum that can be converted directly into electrical energy by infrared responding GaSb photovoltaic cells. Ligand Field Theory calculations are used to estimate the crystal field splitting energies at high temperatures for Co and Ni-doped MgO matched emitters. Experimental measurements of the high temperature (1300–1400°C) emissive power spectrums for Co and Ni-doped MgO emitters are compared with predictions obtained from ligand field calculations for what is believed to be the first time. It was found that crystal field splitting energies of 10 Dq = 9070 cm^–1 represented the best fit for the Co-doped MgO high temperature emissive power spectrum, and 10 Dq = 7950 cm^–1 for the Ni-doped MgO spectrum. These values are only slightly lower than values reported for corresponding single crystal, transition metal doped laser materials that were measured at or well below room temperature.     

Novel ring structure for minimisation of screening effect in carbon nanotube based field emitters

Carbon Nanotubes (CNTs) are promising candidates for cold cathodes because of their high aspect ratio and robustness. However, the major hindrance in cold cathode based applications is the screening effect, which reduces the effective field at the tip and thereby the current density. The emission current can be improved by minimising the screening effect. The adverse effect of screening can be addressed by either controlling the growth density or by optimising the patterns of CNT cathodes. Here, novel patterns have been used to increase edge length per unit area in planar vertically aligned CNT bundles. Our motive was to increase the number of effective emitters, since the CNT at the edges are less screened by the proximal CNTs. By varying geometry and spacing of solid CNT dot patterns and by introducing the square ring structures; we could successfully enhance the effective emitters at the edges. It has been observed that an enhancement of edge length from 0.032 per micron to 0.2 per micron increases the current density from 0.71mA/cm² to 16.2 mA/cm² at a field of 4.5 V/μm. CNTs in dotted structure with high value of edge length per unit area emit very high current density as compared to other dotted structures with low value of edge length per unit area Simulation studies confirms our argument that CNTs at the corners are the least screened and have the maximum local electric field.     

Enhanced Field‐Emission Behavior of Layered MoS2 Sheets

Field emission studies are reported for the first time on layered MoS₂ sheets at the base pressure of ～1 × 10^?8 mbar. The turn‐on field required to draw a field emission current density of 10 μA/cm² is found to be 3.5 V/μm for MoS₂ sheets. The turn‐on values are found to be significantly lower than the reported MoS₂ nanoflowers, graphene, and carbon nanotube‐based field emitters due to the high field enhancement factor (～1138) associated with nanometric sharp edges of MoS₂ sheet emitter surface. The emission current–time plots show good stability over a period of 3 h. Owing to the low turn‐on field and planar (sheetlike) structure, the MoS₂ could be utilized for future vacuum microelectronics/nanoelectronic and flat panel display applications.     

Enhanced and stable electron emission of carbon nanotube emitter arrays by post-growth hydrofluoric acid treatment

Carbon nanotubes (CNT) have been highlighted as possible candidates for field-emission emitters and vacuum nanoelectronic devices. In this article, we studied the effect of acid treatment of CNTs on field emission from carbon nanotube field emitter arrays (FEAs), grown using the resist-assisted patterning process (RAP). The emission current densities of as grown CNT-FEAs and those which were later immersed in hydrofluoric acid (HF) for 20 s, were 19 μA/cm² and 7.0 mA/cm², respectively, when measured at an anode field of 9.2 V/μm. Hence, the emission current densities after HF treatment are 300 times larger than those of as grown CNT-FEAs. Also, it was observed that a very stable electron emission current was obtained after stressing the CNTs with an electric field of 9.2 V/μm for 800 min in dc-mode, where the emission current non-uniformity was 0.13%. The enhancement in electron emission after HF treatment appears to be due to the effect of fluorine bonding. Also, the electron emission characteristics and structural improvement of CNT-FEAs after HF treatment are discussed.     

Effect of rapid thermal annealing treatment on the field-emission characteristics of nanocrystalline diamonds grown on various metal/silicon substrates

In this study, nanocrystalline diamond (NCD) films were deposited on various metal/silicon substrates using a microwave plasma chemical vapor deposition system. Metal layers used are chromium, titanium, aluminum and were used as the electron source for field emitters. These NCD/metal/silicon structures were subsequently annealed at 500 °C in a rapid thermal annealing (RTA) furnace. After RTA treatment, the surface of NCD films becomes flat and the grain boundaries can no longer be clearly seen. The intensity of graphitic peak is substantially decreased and the sp³ content of NCD films is increased. The chemical composition of NCD film remains unchanged after RTA treatment, but the sp³/sp² ratio in C 1s has been increased. It is found that the field-emission characteristics of diamond emitter not only can be effectively controlled by the metal used in the metal/NCD/Si structure, but also can be further enhanced by the improved microstructure of the NCD film obtained after RTA treatment.     

Detecting singly charged ions during field evaporation of tantalum at high temperatures

The field evaporation of tantalum from point emitters in a broad range of temperatures from T = 300 to 2500 K was studied using a static magnetic mass spectrometer equipped with a special field ion source. The room-temperature mass spectrum of field-evaporated particles displayed only the peaks of triply charged ions (Ta³⁺). As the temperature was increased, the charge of field-evaporated ions exhibited a decrease: at T ～ 1000 K, the peaks of doubly charged ions (Ta²⁺) prevailed. The peaks of singly charged ions (Ta⁺) were detected for the first time at temperatures in the interval 1900 K < T < 2500 K. The rate of evaporation of singly charged tantalum ions was several orders of magnitude lower than that of doubly charged ions.     

Photolithography-based carbon nanotubes patterning for field emission displays

Carbon nanotubes (CNTs) emitters were successfully patterned in small pixels (50×50 μm²) by using photolithography process on a hard metal electrode for field emission displays (FEDs) application. The CNTs particles in the patterned pixels were uniformly distributed on 2-inch diagonal substrates. The maximum diameter of CNTs particles could be controlled less than 20 μm. After patterning and heat treatment process below 300°C, most of CNTs bundles on the cathode electrode were aligned perpendicular to the substrates. The threshold electric field of emission for patterned CNTs was about 4.2 V μm⁻¹ and the field enhancement factor derived from the Fowler–Nordheim plots of the electron emissions was about 100 000 in the high voltage region. This newly developed process can be applicable to field emitter arrays for high resolution FEDs.     

Field emission from nonaligned zinc oxide nanowires

Zinc oxide (ZnO) nanowires with an average diameter of 15 nm were grown using a vapor phase transport process. Field emission was achieved from these nanowires in spite of their random orientation. The electric field for the extraction of a 10 μA/cm² current density was measured to range from 4.4 to 5.0 V/μm, and that for a 1 mA/cm² current density from 7.6 to 8.7 V/μm, depending on whether the sample was submitted to a heat treatment. The results exhibit the potential application of ZnO nanowires as field emitters in future flat panel displays.     

Synthesis,Structure and Field Emission of Carbon Nanotubes

Abstract

Multiwall and single-wall nanotubes were produced by carbon arc discharge in helium, and their structures were investigated by electron microscopy. Field emission from (1) as-grown multiwall nanotubes (MWNTs), (2) purified MWNTs and (3) purified single-wall nanotubes (SWNTs) was investigated by field emission microscopy. Of the three kinds of nanotubes studied, purified MWNTs whose tips are open provided the highest current at a fixed voltage. Although purified SWNTs provided the second highest current, SWNTs are not so robust as MWNTs against ion bombardment. As an application of nanotube field emitters, we manufactured cathode ray tube (CRT) lighting-elements by replacing conventional thermionic cathodes with nanotube field emitters. Stable electron emission, adequate luminance, and long life of the emitters were substantiated.     

Nanoelectrospray emitter arrays providing interemitter electric field uniformity

Arrays of electrospray ionization (ESI) emitters have been reported previously as a means of enhancing ionization efficiency or signal intensity. A key challenge when working with multiple, closely spaced ESI emitters is overcoming the deleterious effects caused by electrical interference among neighboring emitters. Individual emitters can experience different electric fields depending on their relative position in the array, such that it becomes difficult to operate all of the emitters optimally for a given applied potential. In this work, we have developed multi-nanoESI emitters arranged with a circular pattern, which enable the constituent emitters to experience a uniform electric field. The performance of the circular emitter array was compared to a single emitter and to a previously developed linear emitter array, which verified that improved electric field uniformity was achieved with the circular arrangement. The circular arrays were also interfaced with a mass spectrometer via a matching multicapillary inlet, and the results were compared with those obtained using a single emitter. By minimizing interemitter electric field inhomogeneities, much larger arrays having closer emitter spacing should be feasible.     

Enhanced Field Emission of WS2 Nanotubes

Results on electron field emission from free standing tungsten disulfide (WS₂) nanotubes (NTs) are presented. Experiments show that the NTs protruding on top of microstructures are efficient cold emitters with turn‐on fields as low as 1 V/μm and field enhancement of few thousands. Furthermore, the emission current shows remarkable stability over more than eighteen hours of continuous operation. Such performance and long‐term stability of the WS₂ cathodes is comparable to that reported for optimized carbon nanotube (CNTs) based emitters. Besides this, it is found that the WS₂ cathodes prepared are less sensitive than CNTs in chemical reactive ambients. The high field enhancement and superior reliability achieved indicates a potential for vacuum nanoelectronics and flat panel display applications.     

A rapid and efficient method to prepare aligned SnO2 nanorod arrays for field-emission application

Aligned tin dioxide (SnO₂) nanorods have been synthesized by high-frequency inductive heating. Nanorods were grown on silicon substrates vertically in less than 3 min, using SnO₂ and graphite as the source powder. Scanning electron microscopy and transmission electron microscopy showed nanorod with diameters from 25 to 50 nm. The turn-on field needed to produce a current density of 10 μA/cm² is found to be 1.6 V/μm. This type of SnO₂ nanorods can be applied as field emitters in displays as well as vacuum electric devices.     

Mixed Lead–Tin Halide Perovskites for Efficient and Wavelength‐Tunable Near‐Infrared Light‐Emitting Diodes

Near‐infrared (NIR) light‐emitting diodes (LEDs), with emission wavelengths between 800 and 950 nm, are useful for various applications, e.g., night‐vision devices, optical communication, and medical treatments. Yet, devices using thin film materials like organic semiconductors and lead based colloidal quantum dots face certain fundamental challenges that limit the improvement of external quantum efficiency (EQE), making the search of alternative NIR emitters important for the community. In this work, efficient NIR LEDs with tunable emission from 850 to 950 nm, using lead–tin (Pb‐Sn) halide perovskite as emitters are demonstrated. The best performing device exhibits an EQE of 5.0% with a peak emission wavelength of 917 nm, a turn‐on voltage of 1.65 V, and a radiance of 2.7 W Sr^?1 m^?2 when driven at 4.5 V. The emission spectra of mixed Pb‐Sn perovskites are tuned either by changing the Pb:Sn ratio or by incorporating bromide, and notably exhibit no phase separation during device operation. The work demonstrates that mixed Pb‐Sn perovskites are promising next generation NIR emitters.