Field Emission Measurements Research Articles

Low-Temperature Vapor-Solid Growth of ZnO Nanowhiskers for Electron Field Emission

One-dimensional zinc oxide nanostructures have aroused interest from scientists and engineers for electron field emission applications because of their experimentally accessible high aspect ratio in combination with their low work function. A comprehensive study of the vapor-solid growth of zinc oxide (ZnO) nanowhiskers by utilizing zinc acetylacetonate hydrate and oxygen at low temperature (580 °C) is reported herein. The nanowhiskers morphology was investigated by varying different growth parameters, such as temperature, substrate type and position, gas flow, precursor amount, and growth time. According to the obtained parameter dependences, the process was optimized to achieve homogenous crystalline nanowhiskers with high aspect ratios and clearly defined surface facets and tips. We show electron field emission measurements from tailor-made ZnO nanowhiskers grown on n-doped silicon, titanium thin films, and free-standing silicon nitride membranes, revealing field emission turn-on fields significantly lower compared to a perfect flat ZnO thin film. Especially the latter devices—ZnO nanowhiskers on a free-standing membrane—might pave the way into a novel nanomembrane detector unit in proteomics, which can significantly extend the mass range of current time-of-flight mass spectrometers.

Evaluation of Photocatalytic Activity in Water Pollutants and Cytotoxic Response of α-Fe2O3 Nanoparticles.

α-Fe2O3 samples were manufactured by means of the polymeric precursor method. The powders were sintered and calcined at temperatures of 300–700 °C for 2 h, respectively. In the X-ray diffraction results, the formation of the rhombohedral phase without secondary phases was exhibited. The size of the particle increased after calcination at 700 °C, exhibiting a slightly more irregular morphology for the samples calcined with the addition of NH4OH in the synthesis process. From the field-emission scanning electron microscopy measurements, the particle size was determined, showing a smaller size for the samples without NH4OH in the synthesis process. The samples calcined at 600 °C had a size of 100 nm, with the sizes for lower temperatures being smaller. The size of the nanoparticle agglomerates was largest for the samples with NH4OH; however, the zeta potential was slightly lower over time for these samples. The phase study of the α-Fe2O3 nanoparticles was confirmed by means of Raman spectroscopy, without additional bands of another crystal structure. In addition, the synthesized nanoparticles exhibited good photocatalytic activity in the degradation of rhodamine B (RhB) and atrazine (ATZ) within 40 min, with a maximum degradation of 59% for ATZ and 40% for rhodamine. The best responses in the degradation were for the samples without the addition of NH4OH in the synthesis process and in proportions lower than 0.1 g. The cytotoxic effects of the nanoparticles obtained at 600 °C were evaluated in apical cells of onion roots. The results are promising for future applications because no changes were observed in the mitosis of the cells.

Fabrication of superhydrophobic surfaces based on PDMS coated hydrothermal grown ZnO on PET fabrics

Wetting behavior of Zinc Oxide (ZnO) based nanomaterials has been the subject of intense investigations and is an active research field for various engineering applications and modifying the surface wettability of ZnO is of great interest. In this study, one-dimensional (1 D) semiconducting ZnO nanorods are grown on a superhydrophobic polyethylene terephthalate (PET) fabric using a hydrothermal method. A facile polydimethylsiloxane (PDMS) coating is applied onto the ZnO grown PET fabrics to improve the hydrophobicity. A wide range of characterization techniques such as field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), UV-vis spectroscopy and contact angle measurement are used to explore the morphology and wetting behavior of the as-prepared samples. The measured water contact angle (WCA) is >150° indicating its superhydrophobicity. This study reports an efficient way to obtain highly hydrophobic semiconducting ZnO grown on PET fabric, which can be of great interest for many future applications.

Catalyst-free growth of ZnO nanowires: structural, optical, vibrational and field emission properties

High density of ZnO nanowires is grown over zinc foil by a catalyst-free thermal evaporation process. Electron microscopy studies confirmed that the as-grown nanowires are hexagonal wurtzite structure. The nanowires diameter was ~ 50 nm with typical length of the order of μm. Photoluminescence (PL) spectrum of the wires consist of a strong blue emission peak at 2.54 eV accompanied by other peaks of relatively lower intensities. The Raman spectra of the ZnO nanowires exhibit compressive stress due to the local heating effects during the synthesis. The field emission measurement indicated that ZnO nanowires have a turn-on field of 13.5 V/μm at current density of 0.001 μA/cm2.

Electron emission characteristics of wet spun carbon nanotube fibers

Wet spun carbon nanotube fibers were characterized using both field emission and electron energy distribution measurements. Fowler-Nordheim analysis of the field emission results showed that the carbon fibers demonstrated a large effective emission area, 2 x 10-12 m2, which resulted in a reduced brightness of 1.84 x 1010 A/m2/sr/V. By considering the emission and number of carbon nanotube emitters it can be shown that the brightness is consistent with previous reports for single nanotube emitters. Additionally, using the effective emission area determined from the Fowler-Nordheim analysis an emittance value around 0.70 μm was found. These characteristics are useful metrics in determining the applicability of using wet spun carbon nanotube fibers for field emission devices.

Fabrication and characterization of flexible transparent electrodes manufactured from silver nanowire

In this work, ITO/silver nanowire (AgNW)/ITO (IAWI) and IZO/AgNW/IZO (ZAWZ) hybrid electrodes were prepared on polyethylene terephthalate (PET) substrates at low temperature. The physical properties of the flexible electrodes were studied using X-ray diffraction, spectrophotometer, field-emission scanning electron microscope, Hall, and four-point probe resistivity measurements. The Haacke index shows that an electrode has the best optoelectronic properties at an AgNW concentration of 2.0 mg/mL. At this concentration, the sheet resistances of the IAWI and ZAWZ electrodes are 18.6 [Formula: see text]/sq and 16.0 [Formula: see text]/sq, respectively. The bending test results show that the electrical properties of both ITO and IZO films became poor only after 100 bending tests. In contrast, the sheet resistances of the IAWI and ZAWZ electrodes show no obvious change after 1000 bending tests, confirming their excellent flexibility. A hyperspectral technique is developed to characterize the damages within the electrodes caused by the bending test.

Field emission properties of carbon nanowalls prepared by RF magnetron sputtering

Carbon nanowalls were prepared on silicon substrates by radio frequency magnetron sputtering under an argon–hydrogen mixture, at different hydrogen fluxes and varying the substrate temperature and deposition times. Scanning and transmission electron microscopy showed that carbon films deposited at 510 °C are nanostructured polycrystalline carbon nanowalls with crystals of about 3 nm inside the flakes. The hydrogen flow induces the growth of nanowalls oriented perpendicularly to the substrate, and the density (amount) of these nanowalls decrease as the hydrogen flow increases. Field emission measurements showed that carbon nanowalls grown in hydrogen have a turn-on voltage of 2.0 V/µm, lower than those grown in pure argon with 4.5 V/µm.

Synthesis Procedure of Highly Densely Packed Carbon Nanotube Forests on TiN.

The goal of this research was to obtain high-density single-walled carbon nanotube forests (SWNTs) on conductive substrates for different applications, including field emission. For this, dip-coating was chosen as the catalyst deposition method, to subsequently grow SWNTs by Alcohol Catalytic Chemical Vapor Deposition (AC-CVD). Si (100) was chosen as the substrate, which was then coated with a TiN thin film. By sputtering with Ar, it was possible to generate alternating TiN and Si lanes, with a different wettability and, therefore, a different affinity for the catalysts. As a result, the Mo-Co catalyst was mainly deposited on TiN and not on sputtered-Si, which allowed the selective growth of SWNT forests on the TiN conductive surfaces. These as-synthesized SWNTs were used for field emission measurements in a high vacuum chamber.

Field emission properties of single crystalline W5O14 and W18O49 nanowires

Single crystalline tungsten oxides in a form of W5O14 and W18O49 nanowires were synthesized by iodine transport method. The morphology, work functions and field emission properties of these nanowires were investigated. Work functions of the W5O14 (4.20–4.34 eV) and W18O49 (4.55–4.57 eV) nanowires (NWs) have been measured by Kelvin probe force microscopy (KPFM) in ultra-high vacuum. Field emission (FE) measurements of individual nanowires were performed in ultra-high vacuum at microscopic and macroscopic distances between the emitter and electron collector. The obtained FE curves at microscopic distances were analyzed in the framework of the Fowler–Nordheim (F–N) theory. Field enhancement factors of W5O14 at the emitter-collector distance of 2, 4 and 5 μm were calculated to be 110 ± 10, 180 ± 25 and 210 ± 30, respectively, and 125 ± 15 for W18O49 at 2 μm. At macroscopic distances, the F–N theory revealed unrealistic high field enhancement factors: for W5O14 at 1 mm it was 17,000 ± 500, and for W18O49, the field enhancement factors were 5050 ± 30 and 6450 ± 30 at 600 μm and 800 μm emitter-collector distance, respectively. Therefore, more realistic model was discussed. The lower work function and typically smaller diameter of the W5O14 nanowires in comparison with the W18O49 wires, range the W5O14 nanowires to the promising sources of electrons in field emission devices.

A Comparative Study of Field Emission From Pristine, Ion‐Treated and Tungsten Nanoparticle‐Decorated p‐Type Silicon Tips

The field electron emission characteristics of individual tips of a silicon field emitter array are analyzed. The array of conical‐shaped tips is fabricated on a p‐type silicon wafer by using reactive ion etching and sharpening oxidation. The tips are decorated with single tungsten nanoparticles at their apexes. Furthermore, the focused ion beam is also used to increase surface conductivity of some of the tips. Comparative measurements of field emission are performed by using the scanning anode probe field emission microscopy technique. All types of tips demonstrated emission activation consisting of a sudden current increase at a certain value of the applied voltage. Compared to the pristine tips, a noticeable reduction of the saturation effect in the current–voltage characteristics and a smaller light sensitivity for the decorated tips is found. For ion‐treated tips, saturation effects and light sensitivity are completely suppressed. Scanning electron microscopy observations reveal the formation of single nanoscale protrusions extending from the metal particles and from the apexes of bare ion‐treated tips after exposure under strong electric fields during the field emission measurements. The influence of protrusions growth on characteristics of silicon field emitter arrays is discussed.

Field Emission Measurements of Solid Fuel Stoves in Yunnan, China Demonstrate Dominant Causes of Uncertainty in Household Emission Inventories.

Emission factors of carbon monoxide (CO), particulate matter (PM2.5), organic carbon (OC), and elemental carbon (EC), as well as combustion efficiency and particle optical properties were measured during 37 uncontrolled cooking tests of residential stoves in Yunnan Province, China. Fuel mixtures included coal, woody biomass, and agricultural waste. Compared to previously published emission measurements of similar stoves, these measurements have higher CO and PM2.5 emission factors. Real-time data show two distinct burn phases: a devolatilization phase after fuel addition with high PM2.5 emissions and a solid-fuel combustion phase with low PM2.5 emissions. The average emission factors depend on the relative contributions of these phases, which are affected by the services provided by the stoves. Differences in stove and fuel characteristics that are not represented in emission inventories affect the variability of emission factors much more than do the type of solid fuel or stove. In developing inventories with highly variable sources such as residential solid-fuel combustion, we suggest that (1) all fuels should be accounted for, not just the primary fuel; (2) the household service provided should be emphasized rather than specific combinations of solid fuels and devices; and (3) the devolatilization phase should be explicitly measured and represented.

MEMS ion source for ion mobility spectrometry

This paper describes a miniature, silicon-glass, electron-impact ion source, in which a field emission carbon nanotube cathode is separated from the external atmosphere by a semitransparent Si3N4 membrane. High vacuum necessary for the proper operation of the ion source is generated on a chip by an integrated micropump. The experiment revealed that it is possible to obtain and maintain high vacuum inside the structure; however, some problems associated with outgassing occurred during field emission measurements. Despite this fact, both the transmission of electrons through the thin membrane and the possibility of gas ionization have been confirmed.

The effect of oxygen plasma treatment on the field emission properties of lanthanum hexaboride tip emitter

Polycrystal lanthanum hexaboride (LaB6) tip field emitter has been prepared using the electrochemical etching technique. The effects of oxygen plasma treatment on its morphology, structure and field emission properties are mainly discussed. The results reveal that the surface morphology of emitters did not change significantly after oxygen plasma treatment. The oxygen plasma treatment does not introduce any new phase into LaB6 emitter, but eliminates surface contaminants, which lead to lower work function. The cathode treated by 2 min shows the best emission performance during the field emission measurement. It obtains 8.2 μA at 2000 V, especially exhibiting strong ability of anti oxygen ion bombardment.

Enhanced electron field emission from nanocrystalline orthorhombic boron nitride films

Nanocrystalline orthorhombic boron nitride (oBN) thin films with an island-in-honeycomb morphology were prepared on graphite substrate by radio frequency (r.f.) magnetron sputtering. The Field emission (FE) measurement results indicated that the FE properties are significantly enhanced in oBN films compared to high quality cBN films, the turn-on electric field of oBN films is decreased from 17.0 V/μm to 6 V/μm, and the highest emission current density is increased from 2.8 × 10−5 to 3 × 10−4 A/cm2. The enhanced FE properties of the oBN films can be attributed to significant reduction in effective potential barrier caused by both protruded island-in-honeycomb morphology and honeycomb-like interconnected internal structure.

Sharp GaN nanowires used as field emitter on active cantilevers for scanning probe lithography

Field emission scanning probe lithography (FE-SPL), which offers sub-10 nm resolution under ambient conditions, strongly relies on the quality and shape of the applied scanning probe tip. The technological development of AFM systems is also strongly connected to the performance of the cantilever. Thereby, the probes evolved from a simple passive deflection element to a complex MEMS through integration of functional groups, such as piezoresistive detection sensors and bimaterial based actuators. Here, the authors show actual trends and developments of miniaturization efforts for fabrication of active cantilevers with mounted sharp GaN and diamond nanowires. Both materials exhibit a higher mechanical stability than the typically applied silicon, which is supposed to increase tip lifetime and reproducibility of the lithography process. Furthermore, diamond offers a high chemical stability, whereas electrical and optical properties of GaN are tunable. They present preliminary results on the applicability of GaN nanowires and diamond tips, mounted on active scanning probes, as field emitter for FE-SPL. Therefore, field emission measurements, stability of field emission current, and exemplary high resolution features generated with the tested tips are shown.

Fabrication and Field-Emission Properties of Vertically-Aligned Tapered [110]Si Nanowire Arrays Prepared by Nanosphere Lithography and Electroless Ag-Catalyzed Etching

In this study, the controllable fabrication of a variety of vertically aligned, single-crystalline [110]-oriented Si nanowire arrays with sharp tips on (110)Si substrates is achieved using a combined self-assembled nanosphere lithography and multiple electroless Ag-catalyzed Si etching processes. All of the experiments were performed at room temperature. The morphological evolution and formation mechanism of long tapered [110]Si nanowire arrays during the multiple tip-sharpening cycle processes have been investigated by scanning electron microscopy, transmission electron microscopy and water contact angle measurements. Field emission measurements demonstrate that the field-emission behaviors of all nanowire samples produced in this study agree well with the Fowler–Nordheim theory, and the produced long tapered [110]Si nanowire array possesses superior electron emission characteristics, with a very low turn-on field of 1.4[Formula: see text]V/[Formula: see text]m and a high field enhancement factor of 3816. The simple and room temperature fabrication of the well-ordered long tapered [110]Si nanowire array and its excellent electron field emission performance suggest that it can serve as a good candidate for applications in high-performance Si-based vacuum electronic nanodevices.

Synthesis of highly dense and vertically aligned array of SWCNTs using a catalyst barrier layer: High performance field emitters for devices

The authors in this research investigated the role of double catalyst layer Fe/Al in the growth of SWCNTs to obtain high current density, low turn on field and high field enhancement factor to act as efficient field emitters. We synthesise SWCNTs by PECVD technique using Fe/Al as a dual catalyst layer on Si substrate. It is seen that during the growth of SWCNTs, Al act as a barrier layer which reduces the diffusion of Fe catalyst in Si substrate resulting in high density vertically aligned growth of SWCNTs. Field emission measurements show high current density 30.5 mA/cm2 at very low turn-on field 1.2 Vμm−1 with a high field enhancement factor of 1.6 × 104. Stability and repeatability parameters of as-grown highly vertically aligned SWCNTs are also studied as a function of current density. Stability results show fairly stable field emitters for 7 h with good repeatability.

Formation of WS2 nanosheets and its field emission studies

The present study describes the results on the formation of WS2 nanosheets using the nanosecond laser ablation in a liquid environment. The laser ablation in liquid is a well-known method and is widely used for the generation of nanostructure. The nanosecond laser ablation is used to irradiate WS2 in a isopropyl alcohol to generate WS2 nanosheets. The laser ablation time was 120 min. The field emission measurement of irradiated WS2 nanostructures was performed. The Bulk and WS2 were used to characterize the Transmission Electron Microscopy (TEM) and Raman spectroscopy to reveal their surface and structural morphology. It is found that the laser ablation time is an important parameter to transform the bulk WS2 into WS2 nanosheets. The generation of WS2 nanosheets is revealed by the Raman spectra. Moreover, the TEM images indicate that the size of WS2 microsheet has been reduced to 400 nm after the laser ablation treatment. It is also found that the field emission current density of WS2 is increased after the laser ablation. The improvement in the field emission characteristics of WS2 is due to the formation of WS2 as well as the separation of layered WS2 structure into few layers of WS2. The formation of WS2 nanosheets can be used for future optoelectronic devices.

Generation of quasi continuous-wave electron beams in an L-band normal conducting pulsed RF injector for laboratory astrophysics experiments

We report on an approach to produce quasi continuous-wave (cw) electron beams with an average beam current of milliamperes and a mean beam energy of a few MeV in a pulsed RF injector. Potential applications are in the planned laboratory astrophysics programs at DESY. The beam generation is based on field emission from a specially designed metallic field emitter. A quasi cw beam profile is formed over subsequent RF cycles at the resonance frequency of the gun cavity. This is realized by debunching in a cut disk structure accelerating cavity (booster) downstream of the gun. The peak and average beam currents can be tuned in beam dynamics simulations by adjusting operation conditions of the booster cavity. Optimization of the transverse beam size at specific positions (e.g., entrance of the plasma experiment) is performed by applying magnetic focusing fields provided by solenoids along the beam line. In this paper, the design of a microtip field emitter is introduced and characterized in electromagnetic field simulations in the gun cavity. A series of particle tracking simulations are conducted for multi-parametric optimization of the parameters of the produced quasi cw electron beams. The obtained results will be presented and discussed. In addition, measurements of the parasitic field emission (PFE) current (dark current) in the PITZ gun will be exemplarily shown to distinguish its order of magnitude from the produced beam current by the designed field emitter.

Investigations on the transformation of vertically aligned CNTs to intramolecular junctions by atmospheric pressure PECVD

Atmospheric pressure glow discharge plasma assisted chemical vapor deposition process is used for the growth of carbon nanotubes (CNTs) on Inconel without using any external catalyst. Four different sets of samples are prepared by varying the growth time. The grown nanostructures are then characterized by field emission scanning electron microscopy (FESEM), high-resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. Field emission measurements are also done on the grown structures. Our investigations revealed that vertically aligned CNTs transformed into intramolecular junctions of CNTs with an increase in deposition time from 4 to 10 min. This time dependence for the transformation is investigated and correlated with other characterization results. It is seen that field emission behavior is the best for the vertically aligned CNTs but it also improves when the junctions of CNTs are present in the matrix after increasing growth time to 15 min. The investigations reveal that branching brings favorable defects in the grown CNTs that can give rise to impressive field emission current density. A schematic model has been prepared based on the experimental findings to find out the reason behind the growth of these junctions and how did they help in improving field emission.