Lanthanum hexaboride (LaB6) nanostructures have attracted much attention in recent years because they exhibit high electrical conductivity and thermal conductivity, low work function and high chemical stability, and can be expected to be an ideal cold cathode electron source for power device applications. Although some groups have developed means to grow LaB6 nanostructures and investigated their emission properties, the moderate synthesis of the LaB6 nanostructure cathode with high-performance is still a challenging issue. Here we developed a simple one-step chemical vapour deposition (CVD) method to prepare the LaB6 nanowire cold cathode film. The LaB6 nanowires have a mean length of tens of micrometres and their average diameter is about 100 nm. The formation of the nanowires is attributed to the synergy of the vapour–liquid–solid (VLS) and vapour–solid (VS) mechanisms. The LaB6 nanowires are found to have a low turn-on (2.2 V μm−1) and threshold field (2.9 V μm−1) as well as nice field emission (FE) stability with a current fluctuation of only 1.7%. And their emission current can reach 5.6 mA (16.7 mA cm−2) at 4.1 V μm−1, which is large enough for the high-current requirements of cathodes used in power devices. Moreover, the LaB6 nanowires still retain excellent performance even if the operation temperature is raised up to 773 K. It is noted that the LaB6 nanowire film exhibits quite different emission behaviours during a temperature cycling between room temperature and 773 K. The adsorption and desorption of oxygen onto and from the nanowire's surface is suggested to explain the discrepancy of such emission properties based on a series of our designed experiments. Most importantly, the LaB6 nanowire cathode film can almost recover to the original excellent FE performances after detachment of the surface oxygen molecules, which suggests that they should be ideal cathode nanomaterials for power device applications.

A concurrently high beam current and high current density carbon nanotube (CNT) cold cathode electron gun is herein developed. A radial electron source has been realized, formed from CNTs synthesized directly on the side walls of a stainless steel truncated-cone electron gun. Experimental results evidenced a 35 kV/50 mA electron beam can achieve a beam transparency of nearly 100% through the use of double anodes and crossed electric and magnetic fields. A maximum beam current density of 3.5 A/cm2 was achieved. These results demonstrate the potential impact of coupling novel cold cathode gun architectures and emerging nanomaterials and their collective role in augmenting the performance of incumbent electron gun technologies, alongside allowing for the realization new types of field emission vacuum electron radiation sources.

We develop a simple one-step growth method to obtain a novel kind of tree-like carbon nano composite structure on a 4″ stainless steel substrate, which is a nano graphene-carbon nanotube tree (GCT). The structures are synthesized by using a plasma enhanced chemical vapor deposition method. The GCT consists of few-layer graphenes (FLG) epitaxially growing from a carbon nanotube (CNT) and they can grow uniformly on a 4″ substrate. Analysis of chemical bonding between FLG and CNT is given. In addition, electron micrographs are presented to show how a tree grows. Finally, we also develop techniques to characterize both experimentally and theoretically the electrical conductivity, field electron emission, heat conduction and heat radiation of a GCT. The electrical conductivity of a GCT is compatible to high quality CNT, the current performance of field electron emission is significantly high compared to the other individual single nanostructure, and very outstanding heat radiation effect is found. We believe that this large-area GCTs film can find potential applications such as super capacitor, terahertz wave manipulation, heat dissipation and field electron emission.Graphical abstract shows the scanning electron microscope image of a graphene-carbon nanotube tree (GCT) film morphology. The interface between few layer graphene and carbon nanotube is connected by carbon-carbon chemical bonds, proving that few layer graphenes epitaxially grow from a carbon nanotube. In situ field electron emission characteristics of a single GCT shows its outstanding capability to emit large current.Download high-res image (457KB)Download full-size image

Due to their optical magnetic and electric resonances associated with the high refractive index, dielectric silicon nanoparticles have been explored as novel nanocavities that are excellent candidates for enhancing various light–matter interactions at the nanoscale. Here, from both of theoretical and experimental aspects, we explored resonance coupling between excitons and magnetic/electric resonances in heterostructures composed of the silicon nanoparticle coated with a molecular J-aggregate shell. The resonance coupling was originated from coherent energy transfer between the exciton and magnetic/electric modes, which was manifested by quenching dips on the scattering spectrum due to formation of hybrid modes. The influences of various parameters, including the molecular oscillation strength, molecular absorption line width, molecular shell thickness, refractive index of the surrounding environment, and separation between the core and shell, on the resonance coupling behaviors were scrutinized. In particular, the resonance coupling can approach the strong coupling regime by choosing appropriate molecular parameters, where an anticrossing behavior with a mode splitting of 100 meV was observed on the energy diagram. Most interestingly, the hybrid modes in such dielectric heterostructure can exhibit unidirectional light scattering behaviors, which cannot be achieved by those in plexcitonic nanoparticle composed of a metal nanoparticle core and a molecular shell.Keywords: excitons; J-aggregates; magnetic resonances; resonance coupling; Silicon nanospheres;

Graphene and its composites are widely investigated as supercapacitor electrodes due to their large specific surface area. However, the severe aggregation and disordered alignment of graphene sheets hamper the maximum utilization of its surface area. Here we report an optimized structure for supercapacitor electrode, i.e., the vertical graphene sheets, which have a vertical structure and open architecture for ion transport pathway. The effect of morphology and orientation of vertical graphene on the performance of supercapacitor is examined using a combination of model calculation and experimental study. Both results consistently demonstrate that the vertical graphene electrode has a much superior performance than that of lateral graphene electrode. Typically, the areal capacitances of a vertical graphene electrode reach 8.4 mF/cm2 at scan rate of 100 mV/s; this is about 38% higher than that of a lateral graphene electrode and about 6 times higher than that of graphite paper. To further improve its performance, a MnO2 nanoflake layer is coated on the surface of graphene to provide a high pseudocapacitive contribution to the overall areal capacitance which increases to 500 mF/cm2 at scan rate of 5 mV/s. The reasons for these significant improvements are studied in detail and are attributed to the fast ion diffusion and enhanced charge storage capacity. The microscopic manipulation of graphene electrode configuration could greatly improve its specific capacitance, and furthermore, boost the energy density of supercapacitor. Our results demonstrate that the vertical graphene electrode is more efficient and practical for the high performance energy storage device with high power and energy densities.Keywords: ion diffusion; morphology; orientation; supercapacitor; surface area; vertical graphene

The building formation of a one-dimensional nanostructure greatly affects its physical properties. By controlling the supersaturation of deposited molybdenum (Mo) vapor, two kinds of nanostructure building formations can be synthesized in Mo nanocones (spiral- and stacking-type) through a thermal evaporation process. The field emission performances of these two formations are vastly different, particularly with respect to their high current properties. The maximum current of a spiral-type individual Mo nanocone is five times that of the stacking-type nanocone. Electrical transport may not be the decisive factor for this difference because both types of individual Mo nanocones have similar excellent conductivities. Heat conduction during the high current emission process has been considered a primary factor, and it strongly depends on the number of internal nanostructure boundaries in the Mo nanocone. These results indicate that nanostructure building formations with fewer inner boundaries in Mo nanocones contribute to a higher current field emission performance when applied to vacuum electron devices.Keywords: electrical transport; heat conduction; high current field emission properties; molybdenum nanocone; structure building formation

A novel screw-like molybdenum nanostructure has been synthesized based on a simple thermal vapor deposition method. Each thread circle of the nanoscrews is formed by several crystalline Mo grains, which have a certain deflection with each other. The growth mechanism is described as a spiral growth mode, which depends heavily on the degree of supersaturation (σ) of deposited Mo vapors. The electrical property measurements and field emission properties on a single Mo nanoscrew show that their electrical conductivity should reach 3.44 × 104–7.74 × 104 Ω−1 cm−1 and its maximum current should reach 15.8 μA. Mo nanoscrew film is also proved to have excellent field emission properties in different voltage driver modes. The largest emission current densities can reach 106.39 mA cm−2 in the DC voltage driver mode and 0.66 A cm−2 in the pulsed mode. A low turn-on field, good site distribution and remarkable emission stability is also recorded. These experimental results show that the highly conductive molybdenum nanoscrews should have potential applications as a cold cathode material for high current vacuum electron devices.

Vertical few-layer graphene (FLG) sheets have potential applications as field emitters and super capacitors. However, it remains a challenge to controllably grow FLG with vertical and parallel orientations. We intentionally designed the substrate surface structure to manipulate the distribution of the plasma sheath and induce the direction of the built-in electric field to guide the growth direction of the FLG in two dimensions. The effects of the plasma sheath and the built-in field on the FLG growth are discussed. The electrical and field emission characteristics of single sheet FLG with parallel and random types were measured, and they demonstrate similar and good characteristics. This method provides a way to controllably grow parallel and vertical FLG arrays.

In an early report, some of us have demonstrated that graphene-sheet films prepared by electrophoretic deposition (EPD) method have great potential as high-performance field emission cathode. We report that the field emission performance from such graphene-sheet films may be enhanced. We have investigated the correlation between topographic structures and local field emission characteristics of graphene-sheet films, prepared with changing EPD deposition time. Detailed experiments show that samples prepared with longer deposition time have better field emission performance. Both scanning electron microscopy and high resolution transmission electron microscopy images show that the topographic structure of the surface layer of the samples deposited with longer time is formed with higher density of graphene sheets with shorter length and fewer graphene layers, in comparison with those with shorter deposition times. Such topographic structure is found experimentally to give large field enhancement. Computer simulation further confirms that a thinner graphene sheet will give more significant geometrical field enhancement at the corner of graphene sheet. Theoretical analysis shows that in an EPD process, longer-length graphene sheets will be deposited before shorter ones, explaining why with longer deposition time, the topographic structure of surface layer consists of shorter-length graphene sheets.

An oxygen-assisted field emission treatment is introduced for improving field emission uniformity of carbon nanotube (CNT) pixel arrays. Oxygen gas is added during the field emission process, and the uniformity of both emission area and brightness of a CNT pixel array are dramatically improved by 83% and 90%, respectively, without reducing emission stability. The underlying physical mechanism for the improvements is attributed to the fact that the oxygen oxidizes the highly emitting CNTs, resulting in their burning out. As a result, the emitting CNTs having a too high current are removed and more and more emitting CNTs with weak current can be stimulated at a higher field, leading finally to a balance of emission from each pixel in the array.

Carbon nanotube (CNT) has excellent field emission characteristics and could play as a good cold cathode in the application of vacuum electronic devices. However, the practical application faces a big obstacle regarding current fluctuation and deterioration of the CNT cathode. In this research, the formation of amorphous carbon (ac) layer between the CNT film and the substrate, and the effect of the existence of this layer on field emission stability of the CNT film are studied. The formation of the ac layer could be controlled by adjustment of growth temperature and hydrocarbon flow rate. The field emission character and current stability of the CNT film without ac layer are better than those of the CNT film with ac layer. The results attribute to the ac layer a thermal disequilibrium state under high current level. Moreover, adhesion capacity of the CNT film without ac layer is also better than that with the ac layer. It is concluded that the ac layer between the CNT film and substrate is a key factor in the stability of field emission characteristics and should be eliminated before applications.Research Highlights► CNT film without ac layer has better field emission character and current stability than the CNT film with ac layer. ► The formation of ac layer is controlled by temperature and hydrocarbon flow rate. ► The thermal disequilibrium state of the sp2-bond-rich ac layer leads to current instability.

By adjusting the type of catalysts, the controlled growth of micropatterned WO2 and WO3 nanowire arrays has been first accomplished at low temperature (450−600 °C). The as-prepared WO2 and WO3 nanowires are proven to be single crystalline structures with a single phase by Raman and transmission electron microscopy (TEM) techniques. Their formation mechanisms are attributed to the vapor−liquid−solid (VLS) mechanism. It is found that both micropatterned WO2 and WO3 nanowire arrays have very excellent field emission (FE) properties with quite low turn-on field (1.36 V/μm) and threshold field (2.38 V/μm), which is very similar to carbon nanotubes (CNTs) with best FE behaviors. In addition, the physical properties of an individual WO2 and individual WO3 nanowire are compared to probe the determinant factor for their different FE behaviors and understand their FE mechanism. Their very excellent FE performance suggests that this novel growth method is a useful technique for low-temperature preparation of micropatterned nanoemitter cathode arrays.

H2 sensors able to operate at room temperature are very important for safe detection of H2 leakage. We report the large electrical response to H2 of Pt-coated WO3 (Pt−WO3) nanowire films without the need of using an external heater. More important, hydrogen sensing processes have been investigated under various conditions, including in air, vacuum filled with pure gas, and a mixture of H2 and other gases. This is carried out with both electrical and optical methods. The evidence shows that hydrogen will inject into the nanowire and create oxygen vacancy, in addition to reducing adsorbed oxygen at the surface, as is often recognized. It is experimentally demonstrated that the increase of electrical conductivity resulting from the reaction with hydrogen is hampered by coadsorption of O2, while N2 has no such effect. A model has been developed to combine these new findings to give a clearer understanding of the mechanism responsible for H2 sensing behavior.

Three-dimensional (3D) nanowire network is a potential building block for nanodevices. Films of 3D tungsten nanowire networks were found early, and the present study is to develop a technical procedure for achieving very high percentage of 3D tungsten nanowire networks in a film. We demonstrate that the content of 3D tungsten nanowire networks in a film, prepared by thermal vapor deposition, may be adjusted by controlling the temperature of substrate, and also that films of very high percentage of 3D tungsten nanowire networks may be prepared. It is found that all films exhibit stable field electron emission, but that the performance varies depending on content of 3D nanowire networks.