Fabrication and Properties of Ag-nanoparticles Embedded Amorphous Carbon Nanowire/CNT Heterostructures
© The Author(s) 2010
Received: 22 April 2010
Accepted: 24 May 2010
Published: 9 June 2010
Carbon nanotubes were subjected to doping with an energetic Ag ion beam, and the carbon nanotubes on the top of the array were transformed into amorphous carbon nanowires with embedded Ag-nanoparticles. The field emission characteristics of these nanowires were investigated. The minimum turn-on and threshold fields were 0.68 and 1.09 V/μm, respectively, which were lower than those of the as-grown carbon nanotubes. This was probably because Ag-nanoparticles embedded in the carbon nanowires reduced the effective work function from 4.59 to 4.23 eV. Large doping amounts produced serious structural damage at the top of the nanowires and impaired the field emission characteristics.
Since Iijima’s landmark paper on carbon nanotubes (CNTs) in 1991 , there has been much research on their fabrications [2, 3], excellent physical and chemical characteristics [4, 5] and potential applications . Rinzler et al.  first reported the field emission characteristics of a single multi-walled CNT and pointed out that they have a very low threshold field (E th, which is a applied field at field emission current density of 10 mA/cm2) and high-field emission current density. The reports of Cheng and Zhou  showed that electrons are emitted from the top of CNTs.
Field emission characteristics associated with the structures (composition, tip sharpness, aspect ratio, etc.) and electrical parameters (conductivity, work function, etc.) of emitters. Element doping changes the composition and phase structures of CNTs and improves the field emission characteristics of CNT emitters. CNT arrays doped with boron [9, 10], nitrogen  and silicon  have been synthesized. Nitrogen-doped CNT arrays have a low turn-on field (1.60 V/μm, E on, which is an applied field at field emission current density of 10 μA/cm2) and high density of emission points . Liu et al.  reported that the gallium doping of CNTs induced the formation of new states near the Fermi level and reduced the work function and enhanced the field emission current density. Some reports  have shown that Cs-doped single-walled CNTs have a lower work function and higher field emission current density. Other reports  have shown that titanium–carbon compounds synthesized by deposition of titanium films onto the surface of CNTs reduced their work function and improved their field emission characteristics. Previous efforts have attempted to enhance the field emission characteristics of CNTs.
Energetic ion implantation technologies have been widely used to manufacture the semiconductor devices and in many fundamental researches of materials due to their excellent advantages on accurate controllability, superfine processing and no thermal limitation during element doping. Ion implantation of materials leads to defect creation (substitution, interstitial and vacancies creation), doping, re-crystallization and other interesting phenomena, depending on the ion beam parameters such as ion influence and the energy loss of ions in materials. Recent reports, such as the phase transformation from carbon onion to diamond, the synthesis of CNTs , modification of the electronic structure of semiconducting single-walled CNTs , synthesis of Zn:C solid solution nanowire/CNT heterostructures  and structural damages in nanomaterials [20, 21], have stimulated renewed interests in energetic ion beam fabricated nanomaterials systems and show that energetic ion beam technology is known as a valuable and innovative tool for engineering, synthesis of new nanostructural materials and modification of nanomaterials on molecular or atomic scale. The structure transformation between nanowires and nanotubes has been widely studied , and can be induced by chemical reaction , surface etching  and phase transformation .
In this paper, an energetic Ag ion beam process is used to fabricate amorphous carbon nanowires (CNWs) embedded with Ag-nanoparticles to improve the field emission characteristics of the CNWs.
Fabrication of Amorphous Carbon Nanowires
CNT arrays with high density and good orientation were synthesized on (100) silicon substrates with a thermal chemical vapor deposition method at 750°C for 30 min. During the synthesis of CNT arrays at ambient pressure, a Fe thin film with a thickness of 5 nm, which was prepared by using magnetron sputtering method, was used as a catalyst, and the mixing of hydrogen and acetylene with a H2 / C2H2 flow rate ratio of 6.9:1 was used as reactive sources. Energetic ion doping was carried out by using the metal vapor vacuum arc ion implanter . The incidence angle of the ion was about 45º, and the average energy of Ag ions was about 70 keV. The Ag-doped doses ranged from 1 × 1016 to 1 × 1017 cm−2. A TRIM simulation program was used to analyze ion radiation damage of CNTs.
Characterization of Structures and Properties
Scanning electron microscopy (SEM, JSM-4800), high-resolution transmission electron microscopy (HRTEM, TECNAI F30), X-ray photoelectron spectroscopy (XPS, PHI Quantera SXM) and field emission measurement system were employed to characterize morphology, chemical structure and the field emission characteristics of CNTs. The field emission measurements of samples with areas from 0.03 to 0.10 cm2 were carried out in a bipolar measurement equipment with a measurement distance of about 2,362 μm at room temperature and a base pressure in the chamber is about 1 × 10−7 Pa. The measurement data were automatically recorded by a computer connected to the measurement system.
Results and Discussion
The calculated dpa of carbon atom in CNTs induced by Ag ion irradiation with different doses
1 × 1016
3 × 1016
6 × 1016
1 × 1017
dpa (displacement per atom)
During the energetic Ag ions implantation, the implanted Ag ions triggered a series of atomic collisions among the implanted Ag ions and the target C atoms. According to atomic collision theory, such a process is usually divided into two steps, i.e., atom collision and a relaxation immediately after atom collision, which lasts for a very short time period of 10−10–10−9 s. In the first step, the high energy Ag ions were dynamically launched into the carbon lattice in CNTs where they simultaneously triggered dramatic atom collisions between Ag and C atoms, resulting in the dislocation of C atoms and the formation of a highly energetic Ag and C mixture. It is commonly recognized that, during this step, a lot of defects (substitution, interstitial and vacancies) can be created to induce the formation of disordering distribution of C atoms or amorphous carbon, and the Ag-nanoparticles cannot be formed because these atoms are in violent motion. In the second step of relaxation, beginning at the termination of atom collisions, the bombarded but not moved carbon atoms would vibrate in the solid target. If the energy is released in the form of thermal energy, a global thermal spike centered at the thermal source would be formed. According to the calculation of literature , if the vibrational energy is completely transformed into thermal energy, the temperature of the globe with a diameter of 10 Å increases to more than 1,000°C. In our experiment, the Ag atoms cannot be miscible into carbon structures because of immiscibility between Ag and C, which made it possible that the Ag atoms diffused in carbon nanostructures to form Ag atom cluster instead of Ag–C compound. On the other hand, the implantation of high-energy Ag ions into CNTs induced high-density defects, especially vacancies. These vacancies enhance the diffusion of Ag atoms to form Ag-nanoparticles in carbon structures. Thus, the ANPE-CNW can be synthesized in the Ag ion-doped area of CNTs, and the tubular structures with multi-layered graphite remain in the undoped area of CNTs. Therefore, ANPE-CNW/CNT hetero nanoarrays have been synthesized by the energetic Ag ion-doping process. We assume that the field enhancement factor β of the Ag ion-doped nanoarrays is approximately the same as that of the as-grown CNT arrays, because the morphology changes in the Ag ion-doped nanoarrays are relatively small for the smaller doping amounts. According to the F–N plots in Fig. 3b, the efficient work function φeff of the ANPE-CNW/CNT nanoarrays can be calculated from the F–N equation . For the ANPE-CNW/CNT nanoarrays, the minimum φeff is about 4.23 eV and lower than that of CNTs (4.59 eV). This decrease in φeff is favorable for electron emitting from the top of the nanoarrays and enhances the field emission characteristics of the emitter. This demonstrates that the enhancement of the field emission characteristics of the nanoarrays is due to the formation of the ANPE-CNWs at the top of the nanoarrays.
High-dose implantation will create a lot of dislocation and sputtering of carbon atoms and induce the breakdown of the top microstructure in the nanoarrays due to energetic ion irradiation. According to the calculated data from Table 1, the dpa of carbon atoms is larger than 1.00 when the Ag ion-doped doses are higher than 6 × 1016 cm−2. The large dpa shows that the average displacement of every atom in the doping area of the CNTs from equilibrium lattices occurs more than once and induces serious structural damage at the top of the CNT arrays. Here, the individual structures of nanowires at the top of the nanoarrays have been destroyed and transformed into a carbon nanonet, as demonstrated by SEM analysis. The formation of the carbon nanonet structures at the top of the CNT array decreases the numbers and enlarges the sizes of emitting points. All of these changes worsen the field electron emission performance of the emitters.
Well-aligned ANPE amorphous CNW/CNT hetero nanoarrays based on CNT arrays have been fabricated by energetic ion beam processing with doses from 1 × 1016 to 3 × 1016 cm−2, in which the sizes of Ag grains are only several nanometers. Large doping amounts produce serious structural damage and cause the formation of carbon nanonet structures at the top of the nanoarrays. The formation of ANPE amorphous CNWs at the top of CNTs enhances the field emission characteristics of the CNTs. The φeff, the minimum Eon and Eth of the ANPE amorphous CNW/CNT hetero nanostructures are 4.23 eV, 0.68 and 1.09 V/μm, respectively, and these values are lower than those of the as-grown CNTs. Serious structural damage and the formation of carbon nanonet structures occurred at the top of CNT arrays, which impaired the field electron emission characteristics of the emitters.
This work was supported by National Basic Research Program of China (No: 2010CB832905) and partially by National Natural Science Foundation of China (No: 10575011) and the Key Scientific and Technological Project of Ministry of Education of China (No: 108124).
This article is distributed under the terms of the Creative Commons Attribution Noncommercial License which permits any noncommercial use, distribution, and reproduction in any medium, provided the original author(s) and source are credited.
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