The graphene double-walled carbon nanotube (DWCNT) hybrid films were prepared by vacuum filtration and screen printing. Their electron field emission properties have been studied systematically. The electron emission properties of the hybrid films are much better than those of pure DWCNT films and pure graphene films. Comparing with the screen printed films, the vacuum filtered films have many advantages, such as lower turn-on field, higher emission current density, better uniformity, better long-term stability, and stronger adhesive strength with conductive substrates. The optimized hybrid films with 20% weight ratio of graphene, which were fabricated by vacuum filtration, show the best electron emission performances with a low turn-on field of 0.50 V
Field emission relies on the electron extraction from the material surface by quantum mechanical tunneling [
Due to their high aspect ratio, nanometer-sized tip radii, low work function, and high electrical conductivity, carbon nanotubes (CNTs) are considered one of the most promising materials for electron field emission [
Lots of methods for fabricating CNT field emission films are developed. CNT films prepared by the chemical vapor deposition (CVD) method [
On the other hand, graphene a two-dimensional (2D) carbon material owns the highest electron mobility of 15,000 cm2 (V·cm)−1 at room temperature [
Graphene was prepared by the reduction of graphene oxide (GO), and GO was prepared from natural graphite by modified hummer method [
The as-prepared GO (1 g) powder was dissolved in DI water (1000 mL), and then 500
The as-prepared graphene and DWCNT (Shenzhen Nanotech Port Co., Ltd., purity: CNTs > 90%, diameter < 5 nm, length of 5–15
The composite powder of graphene and DWCNTs with different weight ratios (0.2 g) mixed with ethyl cellulose and terpineol (1.4 g) was screen-printed onto silver coated substrates (1 cm
The morphology of screen-printed graphene-DWCNT hybrid cathode was examined by field emission scanning electron microscopy (FESEM) using HITACHI S-4800 at a working voltage of 5.0 KV. Field emission properties were performed in vacuum (6.5
The field emission performances of graphene-DWCNT hybrid films with different weight ratios of graphene in DWCNTs and with different preparation methods are systematically studied. The emission current density versus applied electric field of different samples with vacuum filtration and screen printing is shown in Figures
Emission current density versus applied electric field of (a) screen-printed hybrid films of different weight ratios of graphene and (b) vacuum filtered hybrid films of different weight ratios of graphene.
The field emission data of Figures
The reduced F-N equation developed by Forbes [
The Fowler-Nordheim (F-N) curves of (a) screen printed samples and (b) vacuum filtered samples.
The turn-on field values, threshold field values (Figure
(a) Turn-on fields and threshold fields of different samples and (b) field enhancement factor of different samples.
We evaluated the emission current stability (Figure
The emission current stability of VFGC-0, VFGC-20, SPGC-0, and SPGC-20.
The luminance uniformity of (a) SPGC-0, (b) VFGC-0, (c) SPGC-20, and (d) VFGC-20.
It is well known that the process of field emission includes three key steps [
In the first field emission step, electrons are transported from metal electrode to the nanocarbon hybrid films. Apparently, whether the contact interface between metal electrode and nanocarbon hybrid films is blocked or not would highly affect the sample’s emission properties. There are two important factors which affect the electrical contact between metal electrode and hybrid films. One is the Fermi energies of metal and nanocarbon hybrid films. The Fermi energies of silver, graphene, and DWCNTs are −4.3 eV, −4.4 eV, and −4.8 eV, respectively, as shown in Figure
Fermi levels of graphene, DWCNT, and silver.
The other is the adhesive strength of nanocarbon films with metal electrode. The adhesion test was performed by using a Scotch tape based on ASTM standard (D3359-02). The two kinds of DWCNT cathode surface were adhered with the same tapes and loaded with the same pressure (0.09 N). Figure
(a) Original vacuum filtered hybrid films (left) and screen-printed hybrid films (right). (b) The same pressure is loaded on adhesive tapes which are attached on hybrid films. (c) Adhesive tapes are peeled off the hybrid films.
In the second step, electrons are transported between single DWCNT in the nanocarbon films. In this process, the key factor is the conductivity of nanocarbon films. Figure
FESEM images of (a) SPGC-0, (b) VFGC-0, (c) VFGC-20, (d) VFGC-60, and (e) VFGC-100.
The conductivity of filtered samples is better than that of screen-printed samples because of the residual organic binders in screen-printed films, and the filtered films are very pure (Figures
In the last step, electrons are emitted from DWCNT tips into vacuum. There are two factors which would affect the emission properties: the number of electron emission DWCNT tips and the work function of DWCNT. Some DWCNTs are covered by graphene when the weight ratio of graphene is 20%, which is shown in Figure
We fabricated the graphene-DWCNT hybrid films by vacuum filtration and screen printing. And we systematically studied the field emission properties of graphene-DWCNT hybrid films with different weight ratios of graphene and with different preparation methods. We found that, compared with screen-printed nanocarbon hybrid films, vacuum filtered nanocarbon hybrid films show lower turn-on field and higher field enhancement factor, and the field emission performance improved significantly when adding graphene into DWCNTs as the emission material. We got the optimum FE performance with a turn-on field of 0.50 V
This work was supported by the Rising Star Program of Shanghai under Grant no. 07QA14019, the Shanghai Talent Development Fund, and the Fundamental Research Funds for the Central Universities.