Improved Field Emission Performance Research Articles

Effect of N2 flux on the field emission properties of synthesized ZnO/ZrN core-shell nanostructures

In this paper, the highly oriented ZnO/ZrN nanoneedle emitters with excellent field emission performance were fabricated by sputtering ZrN particles with different N2 flux to modify ZnO nanoneedle arrays. The effects of different N2 flow on the microscopic morphology, structure and field emission properties of ZnO nanoneedles were investigated. The results shows that the ZnO/ZrN nanoneedles (ZnO/ZrN-1.6) emitter prepared when the flow of N2 gas is 1.6 SCCM exhibits the best field emission performance, with the lowest turn-on electric field Eto at 0.81 V/μm, and a field emission current density of 25 mA/cm2 which was 25 times of pure ZnO. In addition to the improvement of band structure, the improvement of field emission performance can also be attributed to the tunneling effect of electrons through heterostructure. This well-aligned ZnO/ZrN core-shell cathode with excellent emission characteristics grown on carbon cloth substrate has the potential applications in future flexible light source devices.

Field emission of GaN nanofilms on Si substrates enhanced by hydrogen plasma treatment

The (002) oriented GaN nanostructured films prepared by pulsed laser deposition (PLD) on n-type Si (100) substrates were treated by H- and O-plasma, finding both treatments do not make the sizable changes in crystalline structure and morphology of GaN nanofilms. However, the field emission (FE) performance of GaN nanofilms is considerably improved by H-plasma treatment (H-GaN) but deteriorated by O-plasma treatment (O-GaN). The turn-on fields, Eon, for H- and O-plasma treated GaN nanofilms are determined to be 0.52 and 1.79 V μm−1, respectively, in contrast with that of pristine GaN nanofilms, 0.95 V μm−1. The improvement of FE performance by the H-plasma treatments could be attributed to the reasons of the reduced surface potential barrier, the increased electron concentration, the increased surface conductivity, and the increased effective emission area. The first-principles calculations represents that the experimental and theoretical work function results have the same trend with the descending sequence of O-plasma treated GaN > pristine GaN > the H-plasma treated GaN.

Field emission enhancement of composite structure of ZnO quantum dots and CuO nanowires by Al2O3 transition layer optimization

In this paper, ZnO quantum dots (QDs)-CuO nanowires (NWs) heterostructure was synthesized by a mild solution method. The Al2O3 transition layer were deposited on CuO NWs by surface modification engineering using atomic layer deposition (ALD) to increase the nucleation probability of ZnO QDs. The field emission properties of the composites were investigated. The results reveal that the CuO@Al2O3/ZnO QDs ternary heterostructures have the turn-on field of 2.82 V/μm and the field emission enhancement factor of 5798, which are superior to that of ZnO QDs-CuO NWs and pure CuO NWs. The improvement of field emission performance is mainly attributed to the decrease of the electron transport barrier at the interface and the increase of the oxygen vacancies, making it easier to transport electrons to ZnO QDs. The use of amorphous Al2O3 in the transition layer also effectively alleviates the lattice mismatch between ZnO QDs and the substrate. This study provides an effective method for enhancing the field emission performance of the core-shell nanomaterial device.

Field emission performance of vertically aligned carbon nanotubes and their improved enhancement factors by the simple solution treatment

In this study, the vertically aligned carbon nanotubes (VACNTs) are fabricated on substrate via a simply spraying chemical vapor deposition method. The relationship between the field emission performance and the aspect ratio of the VACNTs is systematically studied. The optimal growth conditions of the VACNTs with outstanding field emission performance are obtained. Meanwhile, the field emission properties of the VACNTs are further improved via a simple acetone-densification treatment. After the densification, the turn-on electric field and threshold electric field could decrease to 0.83 V/μm and 2.45 V/μm, respectively, and the field enhancement factor will increase to 26,588. In addition, the field emission current stability of the VACNTs is also strengthened. The improvement of field emission performance could be resulted from the reduction of field shielding effect after tip densification. The high current density and good stability demonstrate that the densified VACNTs are the promising and highly efficient field electron emitters.

Well-aligned periodic germanium nanoisland arrays with large areas and improved field emission performance induced by femtosecond laser

Si-based field emitters with high and stable current densities have gained increasing attention owing to their relevant compatibility with other silicon-based microelectronic and photonic devices. Currently, well-aligned Ge nanostructure arrays can be used in fabrication of Si-based field emitters with excellent performance. In this study, large-area well-aligned Ge nanoisland arrays are induced by femtosecond laser (120 fs, 800 nm, 1 kHz). Scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman analyses, and X-ray photoelectron spectroscopy (XPS) reveal that well-aligned Ge nanoisland arrays with good quality. Field emission (FE) measurements indicate Ge nanoisland arrays with high emission current densities and good stability. Overall, the excellent FE properties of the proposed Ge nanoisland arrays will make them potential for applications in high-performance field emitters integrated with Si nanodevices.

Field emission study from tantalum surfaces micro-structured with femtosecond pulsed laser irradiation

This paper presents our results on surface micro-structuring of tantalum samples using femtosecond (fs) pulsed laser irradiation and its effect on field emission performance. Micro-structuring of Ta targets has been carried by varying laser fluence in the range 0.35–0.55 J/cm2 while keeping target scan speed constant at 25 µm/s. Laser-treated surfaces have shown the formation of high density micro-protrusions and an oxide phase mainly consisting of crystalline Ta2O5. Our results showed substantial improvement in field emission performance of laser micro-structured surfaces in comparison to pristine Ta surface. Areal number density of micro-protrusions decreases from 8.8 × 105 to 3.5 × 105 protrusions/cm2 as incident laser fluence varied from 0.35 to 0.55 J/cm2. Ta sample micro-structured with 0.55 J/cm2 laser fluence has shown low turn on field (~ 4.0 ± 0.6 Vµm− 1) and high-field enhancement factor (4500 ± 500).

Array of vertically aligned Al-doped ZnO nanorods: Fabrication process and field emission performance

We report the development of a novel field emission (FE) device based on Al-doped ZnO nanorods (NRs), which can be used as an alternative tool for exhale biosensing applications. Highly crystalline hexagonal array of ZnO NRs doped with up to 5 wt% Al content were obtained using hydrothermal growth method. Field emission process with acceptable enhancement factor (761 < β < 951) were performed for 1 to 4 wt% Al-doped ZnO nanorod arrays. By increasing the Al content above 5 wt%, the morphology of nanorods started to transform into the nanoplates, demonstrating weak FE performance. When a high voltage was applied to 4 wt% Al-content ZnO NRs for a few times (4–6×), an abnormal microstructure formation with improved FE performance was observed. Significant field emission current with enormous enhancement factor (β = 5545) was demonstrated from the high Al-content reformed ZnO microstructure assembly. The formation of these microstructures was found to be due to the diffusion and electromigration of Au atoms through the grain boundaries into the NR assembly.

Efficient cold cathode emission in crystalline-amorphous hybrid: Study on carbon nanotube-cadmium selenide system

Cadmium Selenide (CdSe) quantum dot (QD) decorated amorphous carbon nanotubes (a-CNTs) hybrids have been synthesized by simple chemical process. The samples were characterized by field emission scanning and transmission electron microscopy, Fourier transformed infrared spectroscopy, Raman and UV–Vis spectroscopy. Lattice image obtained from transmission electron microscopic study confirms the successful attachment of CdSe QDs.It is seen that hybrid samples show an enhanced cold emission properties with good stability. The results have been explained in terms of increased roughness, more numbers of emitting sites and favorable band bending induced electron transport. ANSYS software based calculation has also supported the result.Also a first principle based study has been done which shows that due to the formation of hybrid structure there is a profound upward shift in the Fermi level, i.e. a decrease of work function, which is believed to be another key reason for the observed improved field emission performance.

CdS nanoparticle coated carbon nanotube through magnetron sputtering and its improved field emission performance

Carbon nanotubes (CNTs) are the most famous cold cathodes for display technology. However, graphene and other two dimensional carbon nanostructures and advanced II-VI semiconductor nanostructures are strongly emerging as strong competitors challenging the well established performance of CNTs in this field. Hence several technologies to improve field emission (FE) properties of CNTs are being developed. Here we report a facile technique to improve the cold emission performance of multiwalled CNTs (MWCNTs) by coating the same with chalcogenide semiconductor. MWCNT thin films were synthesized via chemical vapor deposition (CVD) technique and the same was coated by cadmium sulfide (CdS) nanoparticles via radio frequency magnetron sputtering technique. The pure and coated thin films were characterized by Fourier transform infrared spectroscopy and Raman spectroscopy to confirm presence of CdS on CNT surfaces. X-ray photoelectron spectroscopy revealed the compositional and valance state information of the as prepared samples. Field emission scanning electron microscopy and high resolution transmission electron microscopic studies were carried out to obtain information regarding morphology and nanocrystallinity of the CNTs and the coated CdS nanoparticles. Field assisted electron emission of pristine CNTs developed significantly with improved turn-on field and very high current density by CdS nanoparticle coating.

Enhanced Field-Emission Performance from Carbon Nanotube Emitters on Nickel Foam Cathodes

We present a three-dimensionally configured cathode with enhanced field-emission performance formed by combining carbon nanotube (CNT) emitters with a nickel foam (NiF) substrate via a conventional screen-printing technique. The CNT/NiF cathode has low turn-on electric field of 0.53 V μm−1 (with current density of 10 μA cm−2) and threshold electric field of 0.87 V μm−1 (with current density of 0.1 mA cm−2), and a very high field enhancement factor of 1.4 × 104. The porous structure of the NiF substrate can greatly improve the field-emission properties due to its large specific surface area that can accommodate more CNTs and increase the emitter density, as well as its high electrical and thermal conductivities that facilitate current transition and heat dissipation in the cathode. Most importantly, the local electric field was also enhanced by the multistage effect resulting from the rough metal surface, which furthermore leads to a high field enhancement factor. We believe that this improved field-emission performance makes such cathodes promising candidates for use in various field-emission applications.

Enhanced field emission of graphene–ZnO quantum dots hybrid structure

The cathode of graphene was prepared by the electrophoretic deposition (EPD) and ZnO quantum dots (QDs) were grown on the surface of graphene sheets by solution method to improve the field emission (FE) properties. The graphene/ZnO QDs hybrid emitters exhibited efficient field emission with lower turn-on field of 0.9V/μm, lower threshold field of 2.6V/μm, higher field enhancement factor of 3923 and more stable emission current stability than pristine graphene. The improved field emission performance was attributed to ZnO QDs, which introduce more defects, increase the number of emitting sites and decrease the work function. This investigation proposed that graphene/ZnO QDs composites are promising field cathodes in FE applications.

Towards graphane field emitters.

We report on the improved field emission performance of graphene foam (GF) following transient exposure to hydrogen plasma. The enhanced field emission mechanism associated with hydrogenation has been investigated using Fourier transform infrared spectroscopy, plasma spectrophotometry, Raman spectroscopy, and scanning electron microscopy. The observed enhanced electron emissionhas been attributed to an increase in the areal density of lattice defects and the formation of a partially hydrogenated, graphane-like material. The treated GF emitter demonstrated a much reduced macroscopic turn-on field (2.5 V μm-1), with an increased maximum current density from 0.21 mA cm-2 (pristine) to 8.27 mA cm-2 (treated). The treated GFs vertically orientated protrusions, after plasma etching, effectively increased the local electric field resulting in a 2.2-fold reduction in the turn-on electric field. The observed enhancement is further attributed to hydrogenation and the subsequent formation of a partially hydrogenated structured 2D material, which advantageously shifts the emitter work function. Alongside augmentation of the nominal crystallite size of the graphitic superstructure, surface bound species are believed to play a key role in the enhanced emission. The hydrogen plasma treatment was also noted to increase the emission spatial uniformity, with an approximate four times reduction in the per unit area variation in emission current density. Our findings suggest that plasma treatments, and particularly hydrogen and hydrogen-containing precursors, may provide an efficient, simple, and low cost means of realizing enhanced nanocarbon-based field emission devices via the engineered degradation of the nascent lattice, and adjustment of the surface work function.

Improved field emission of few-layer graphene–carbon nanotube composites by high-temperature processing

Self-assembled few-layer graphene–carbon nanotube (FLG–CNT) composites were prepared and their field emission (FE) properties before and after high-temperature (H-T) processing were investigated. It was found that their FE performance deteriorated slightly when they were processed at 1273K for 5h before the FE tests; the applied field increased ~0.05V/μm at 10mA/cm2. X-ray photoelectron spectroscopy analysis indicates that the decreased amount of SP3-hybridized defects from 39.91% to 23.87% after the H-T processing directly leads to this FE degradation. However, the largest emission current density (Jmax) of the FLG–CNT composites increased from 33.48 to 64.00mA/cm2 after the H-T processing. This improved FE performance was attributed to the enhanced adhesion between the CNT and substrates. We consider that the catalyst wrapping at the CNT foot after the H-T processing is responsible for this CNT–substrate reinforcement.

Field emission from α-Fe2O3 nanoflakes: Effect of vacuum pressure, gas adsorption and in-situ thermal treatment

The effects of vacuum pressure and gas adsorption on field emission current of α-Fe2O3 nanoflakes were studied. It was found that field emission current of α-Fe2O3 nanoflakes decreased with increasing pressure. The field emission current decreased when N2 or O2 was introduced into chamber, while no obvious change was observed for H2 gas. An in-situ thermal treatment was carried out to eradicate the effect of absorbed gas. After the in-situ thermal treatment, the field emission current density was largely enhanced from 60 to 500μA/cm2 under applied electrical field of 10MV/m and the turn on field (Eturn-on) decreased from 7.6 to 5.2MV/m. The lowered turn-on field was attributed to the decrease of surface work function induced by surface gas desorption and reduction of α-Fe2O3 nanoflakes. Moreover, the improvement of field emission performance can be retained in high vacuum condition, which indicates the in-situ thermal treatment is an efficient method to improve field emission properties of α-Fe2O3 nanoflakes.

Effective hybrid graphene/carbon nanotubes field emitters by electrophoretic deposition

Hybrid graphene and carbon nanotube (CNT) field emitters were fabricated with electrophoretic deposition (EPD). The combination of both materials was used to improve the turn-on field for pure carbon nanotubes emitters and the reliability of pure graphene emitters deposited by the same method. The CNT was envisioned to hold down the graphene flakes, like a safety belt or Velcro, at high voltages to prevent an early short circuit at relatively low voltages. These hybrid emitters were studied for their field emission performance in relation to the EPD deposition duration. It was observed that the emitters performed better when the EPD duration was increased due to the increase in the amount and density of graphene flakes. Possible reasons for the improvement of field emission performance were suggested. The roles of graphene and CNT in these hybrid emitters were also discussed.

산소 플라즈마 처리한 탄소나노튜브의 표면상태와 전계방출 특성

Multi-walled carbon nanotubes (MWCNTs) were synthesized using catalytic chemical vapor deposition (CVD) method. Oxygen plasma treatment was applied to modify surface state of the CNTs synthesized for improvement of field emission performance. Surface state of the plasma treated CNTs was studied by X-ray photoelectron spectroscopy (XPS). The surface states of the CNTs were changed as a function of plasma treatment time. The oxygen related carbon shift was moved toward higher binding energy with the plasma treatment time. This result implies that the oxygen plasma treatment changes the surface state effectively. While any shift in carbon 1s peak was not detected for the as grown CNTs, oxygen related carbon shift was detected for the plasma treated CNTs. Carbon shift implies that closed CNT tips were opened by the oxygen plasma and reacted with oxygen species. Since the field emission occurs at pentagons or dangling bonds of the CNT tips, the increase of carbon-oxygen bonds plays an important role in field emission behavior by increasing the number of electron emission sites resulting in improvement of the field emission performance.

Highly efficient low voltage electron emission from directly spinnable carbon nanotube webs

Two methods for increasing the number of free carbon nanotube (CNT) tips in carbon nanotube webs (CNTW) and improving their field emission (FE) performance are proposed. It was observed that by laterally compressing samples by 35 percent it is possible to improve FE performance to some extent, with further compression leading to a loss of FE properties due to folding of webs and adhesion between CNT tips. Multilayered samples were also studied and it was found that samples with successive layers in which the orientation of the nanofibers in successive layers was the same, increased inter-bundle van der Waals forces, leading to more compact webs with no improvement in FE performance. However, it was found that if the fibers in successive layers were perpendicular to each other the inter-bundle attraction was minimized, maximizing the number of active CNT tips and yielding materials that exhibit the best FE performance reported for any material to date.

Simple chemical synthesis of porous carbon spheres and its improved field emission by water vapor adsorption

A very simple solvothermal approach for synthesizing porous carbon microspheres is reported here. The effect of water adsorption on the field emission properties of such samples was investigated experimentally as well as from first principle calculation. The as-synthesized microspheres were characterized by X-ray diffraction (XRD), scanning and transmission electron microscopy (TEM), atomic force microscopy and Fourier transformed infrared (FTIR) spectroscopy. XRD patterns as well as TEM lattice images confirmed that the spheres were graphitic in nature. From the microscopic analyses it was seen that the as-synthesized porous carbon spheres were actually composed of few layer thick carbon nanosheets. FTIR confirmed the adsorption of water in the sample. It has been found that samples with adsorbed water can act as a better field emitter compared to the pure ones. A first principle study showed that although no development of H2O-induced impurity states close to the Fermi level was observed due to water adsorption, there was a pronounced upward shift in the Fermi level, i.e. a decrease of work function, which is believed to be the main reason for the observed improved field emission performance.

Controlled synthesis of patterned W18O49 nanowire vertical-arrays and improved field emission performance by in situ plasma treatment

W18O49 nanowires have been intensively studied in recent years due to their good field emission (FE) performances. However, now their development has encountered difficulties, i.e., their emission uniformity is not good enough for high resolution field emission display (FED) applications. Here, we have successfully fabricated patterned, large-scale W18O49 nanowire vertical-arrays and for the first time applied an in situ plasma treatment technique to improve their field emission performance. It is found that their emission uniformity has been improved to a very high level (distribution uniformity of emission sites is higher than 96% and their brightness uniformity reaches 75.64%), while other emission properties remained unchanged compared to untreated nanowires. The bombardment of Ar plasma and the reduction of H2 plasma may be responsible for the improvement of their FE performance. It can be concluded that this developed technique is a very effective route to enhance the emission uniformity of patterned W18O49 nanowire arrays, which may pave the way for FE emission behavior improvement of other metal oxide nanowires.

Improved field emission performance of carbon nanotube by introducing copper metallic particles

To improve the field emission performance of carbon nanotubes (CNTs), a simple and low-cost method was adopted in this article. We introduced copper particles for decorating the CNTs so as to form copper particle-CNT composites. The composites were fabricated by electrophoretic deposition technique which produced copper metallic particles localized on the outer wall of CNTs and deposited them onto indium tin oxide (ITO) electrode. The results showed that the conductivity increased from 10-5 to 4 × 10-5 S while the turn-on field was reduced from 3.4 to 2.2 V/μm. Moreover, the field emission current tended to be undiminished after continuous emission for 24 h. The reasons were summarized that introducing copper metallic particles to decorate CNTs could increase the surface roughness of the CNTs which was beneficial to field emission, restrain field emission current from saturating when the applied electric field was above the critical field. In addition, it could also improve the electrical contact by increasing the contact area between CNT and ITO electrode that was beneficial to the electron transport and avoided instable electron emission caused by thermal injury of CNTs.