Zexiang Chen

High emission current density microwave-plasma-grown carbon nanotube arrays by postdepositional radio-frequency oxygen plasma treatment

Highly stable field emission current densities of more than 6A∕cm2 along with scalable total field emission currents of ∼300μA per 70μm diameter carbon nanotube (CNT)-covered electron emitter dot are reported. Microwave-plasma chemical vapor deposition, along with a novel catalyst sandwich structure and postdepositional radio-frequency (rf) oxygen plasma treatment lead to well-structured vertically aligned CNTs with excellent and scalable emission properties. Scanning electron and transmission electron microscope investigations reveal that postdepositional treatment reduces not only the number but modifies the structure of the CNTs. Well-structured microwave-plasma-grown nanotubes become amorphous during rf oxygen plasma treatment and the measured work functions of CNTs change from 4.6eVto4.0eV before and after treatment, respectively. Our experiments outline a novel fabrication route for structured CNT arrays with improved and scalable field emission characteristics.

Large current carbon nanotube emitter growth using nickel as a buffer layer

A method is reported for the fabrication of vertically aligned carbon nanotube emitter arrays by microwave plasma chemical vapour deposition. The technique uses a catalyst structure in which Fe, Ni and Al, respectively, acted as the catalyst for carbon nanotube growth, a buffer layer and an isolated layer between the Fe and Ni layers. The nanotubes were suitable for large current field emission applications. SEM images showed that the length and the diameter of the CNTs were uniform. Accordingly the CNT emitter arrays presented excellent height uniformity. Furthermore, the adhesion of the CNTs to the substrate was stable, and the electrical conductance between the CNTs and the substrate was good enough to ensure the continual emission of plenty of electrons. A highly stable field emission current density of more than 2?A?cm?2 with a total field emission current of 20?mA was obtained. These results indicate a fabrication route for greatly improving the field emission characteristics of CNT-based field emitter arrays.

Synthesis and emission properties of carbon nanotubes grown by sandwich catalyst stacks

We report in this article a method to grow carbon nanotubes (CNTs), which are well aligned and vertically oriented. Furthermore, these CNTs have a uniform length and diameter. A sandwiched catalyst structure is utilized to form multiwalled carbon nanotubes. It is feasible to grow CNTs between two terminals directly and grow straight vertical carbon nanotube bundles. The transmission electron microscope image of the as-grown CNTs shows a typical multiwalled CNT lattice structure with a few defects. After treating the as-grown CNTs in an ammonia radio-frequency plasma, a highly stable field-emission current density of more than 6A∕cm2 at an electric field of 7.7V∕μm with a total field-emission current of >1.2mA was obtained. Our experiments indicate a fabrication route for largely improving the field-emission characteristics of CNT-based field emitters.

Enhanced conversion efficiency of dye-sensitized solar cells using a CNT-incorporated TiO2 slurry-based photoanode

A new titanium dioxide (TiO2) slurry formulation is herein reported for the fabrication of TiO2 photoanode for use in dye-sensitized solar cells (DSSCs). The prepared TiO2 photoanode featured a highly uniform mesoporous structure with well-dispersed TiO2 nanoparticles. The energy conversion efficiency of the resulting TiO2 slurry-based DSSC was ∼63% higher than that achieved by a DSSC prepared using a commercial TiO2 slurry. Subsequently, the incorporation of acid-treated multi-walled carbon nanotubes (CNTs) into the TiO2 slurry was examined. More specifically, the effect of varying the concentration of the CNTs in this slurry on the performance of the resulting DSSCs was studied. The chemical state of the CNTs-incorporated TiO2 photoanode was investigated by Fourier transform infrared spectroscopy and X-ray photoelectron spectroscopy. A high energy conversion efficiency of 6.23% was obtained at an optimum CNT concentration of ∼0.06 wt.%. The obtained efficiency corresponds to a 63% enhancement when compa...

Growth of uniform carbon-nanotube arrays with sandwich technology

— In this work, a novel approach to grow structured, highly oriented carbon nanotubes (CNTs), which are vertically aligned to the substrate and show large field emission is reported. Growth is performed on lithographically defined dots of catalysts, which can be deposited on metallic, semiconducting, and glass substrates. A sandwiched catalyst structure and microwave plasma chemical vapor deposition enables the formation of uniform CNT arrays of 1.6 × 1.6 μm2. The method is easily scalable to large areas. The CNT arrays exhibit a stable field emission of 20 mA and a macroscopic current density of 50 mA/cm2 at a rather low electric field of 5.33 V/μm. Modeling of space charge indicates that space charge reduces the magnitude of the CNT emission at high field strength: this agrees satisfactorily with the measurements.

Fabrication of sensor based on MWCNT for NO 2 and NH 3 detection

MWCNT arrays fabricated by microwave plasma chemical vapor deposition on Si substrate with interdigital electrodes (IDE) were integrated into a resistive sensor design. A scanning electron microscope was used for the characterizations of MWCNTs. The gas response of MWCNT arrays to NH3 and NO2 gas were investigated at room temperature, respectively. The results exhibited that the thickness of MWCNT arrays controlled by growth time evidently affected gas sensing properties, and as-prepared MWCNT arrays presented low gas sensitivity to NH3 and NO2 gas, while the gas response can be significantly improved by anneal, which may be interpreted by the removal of by-products such as amorphous carbon or other functional groups on the MWCNTs surface.

Stable field emission from carbon nanotube grown on graphene substrate

We reported an approach to grow carbon nanotube (CNT) arrays on graphene to obtain stable high emission current density along with scalable total ultrahigh emission current of more than 200 mA. Microwave plasma CVD was employed to grown multi-layered graphene directly on silicon wafer followed by synthesis of vertically well-aligned patterned CNT arrays, the excellent thermal conductivity and electric conductivity of graphene enabled CNTs to afford large emission current stably.

Field ion emission property of carbon nanotube arrays

Field ionization provides an efficient approach to generate ions, especially for compact neutron generator. We studied the property of field ion emission of carbon nanotube bundle arrays prepared by microwave plasma CVD, the results shows the as-prepared CNT arrays are capable of generating ions substantially at low external applied field, which provide a practical way to fabrication of compact neutron generator.

Microfocus X-ray tube based on CNT array

A macro-focus X-ray tube with CNT emitters was successfully developed. An anode current of 1.15mA was collected when the copper grid gate voltage was 2.7kV. A very small x-ray focal spot size with less than 39μm can be achieved, which could potentially produce very high special resolution X-ray image.

Improved field emission of CNTs on heated substrate for medical x-ray imaging application

For the next generation x-ray imaging system, small focal spot combined with fast switching electron emitter is desired to obtain high resolution images and minimize motion-induced blurring of images of moving organs such as the heart. Carbon nanotubes (CNTs) have the potential to be excellent emitters for novel x-ray imaging applications that overcome the limitations imposed by conventional thermal emitters. One of the challenges for achieving high emission current from CNT is the early breakdown of emitters due to the Joule heating of the CNT/substrate interface, field evaporation of CNTs and subsequent formation of ions leading to arcing. CNT emitters require emission stability and reproducibility.