Elsevier

Carbon

Volume 50, Issue 7, June 2012, Pages 2641-2650
Carbon

Synthesis and field emission properties of vertically aligned carbon nanotube arrays on copper

https://doi.org/10.1016/j.carbon.2012.02.024Get rights and content

Abstract

We report the synthesis of periodic arrays of carbon nanotubes (CNTs) with different densities on copper substrate by employing nanosphere lithography (NSL) and plasma enhanced chemical vapor deposition. At a growth pressure of 8 torr and temperature of 520 °C, vertically aligned bamboo-like CNTs were formed with a catalyst particle on the tip. Electrical properties of CNTs with different densities were investigated for the possible applications in field emission (FE). The investigation of FE properties reveals a strong dependence on the density of CNTs. Experimental results show that NSL patterned low density CNTs exhibit better field emission properties as compared to the high density CNTs. Low-density CNTs exhibit lower turn-on and threshold electric fields, and a higher field enhancement factor. The high density of CNTs results in the deterioration of the FE properties due to the screening of the electric field by the neighboring CNTs.

Introduction

Carbon nanotubes (CNTs) exhibit intriguing mechanical, electrical, optical, thermal, and electrochemical properties and have been extensively studied since the landmark paper by Iijima in 1991 [1]. CNTs are ideal candidates for field emission applications because of their high electrical and thermal conductivities, tremendous mechanical strength, and high aspect ratio [2], [3], [4]. CNTs have shown a potential for application in field emission displays, X-ray sources, lamps, microwave amplifiers, and nanoelectronics [5], [6], [7], [8], [9]. Many of these devices require the controlled growth of vertically aligned CNTs (VACNTs) directly on the conducting substrates. The CNTs synthesized by arc discharge and laser ablation methods are highly entangled and possess a lot of impurities [10]. The plasma enhanced chemical vapor deposition (PECVD) process has garnered an appropriate attention due to its ability to synthesize vertically aligned CNT arrays at a predetermined position. It is also desirable to grow CNTs on metallic substrates like copper (Cu), which results in a lower contact resistance as compared to conventional CNTs grown on a silicon substrate. The ohmic contact between the CNTs and the metallic substrate ensures an easy electron transport, and the rigidity of these nanostructures enables the emitters to withstand high current during the process of emission. There have been efforts to produce high-performance CNT emitters by doping, plasma irradiation, thermal oxidation, laser pruning, metal coating, etc. [11], [12], [13], [14], [15].

However, it is also possible to enhance the emission properties of CNTs by varying the density of CNTs. A high-density array of CNTs results in the screening of the electric field by the neighboring CNTs and hence the reduction in the emission current, whereas the emission from a low density of CNTs is poor due to the availability of fewer number of emission sites. Selected area low-density growth of CNTs has been achieved by patterning the catalyst using nanosphere lithography (NSL), electron beam lithography, photolithography, focused ion beam lithography, chemical etching, and others [16], [17], [18], [19], [20], [21]. NSL is an inexpensive and efficient process to selectively pattern catalyst islands over which CNTs can be synthesized. A monolayer of polystyrene spheres (PS) of a specified diameter is used to create a mask over which the catalyst is deposited by e-beam evaporation, pulsed laser deposition, sputtering, etc. The PS layer is subsequently removed by wet chemical methods to obtain a honeycomb pattern of catalyst deposited in the interstices of the spheres. The size and spacing between these catalyst islands can be controlled by varying the diameter of spheres. Larger spheres result in bigger catalyst islands which are located further apart from each other as compared to the islands created by the smaller spheres. The VACNTs synthesized over these patterns exhibit better field emission behavior than the CNT thin films due to a decreased screening effect. In this study, we have employed PS of different diameters to grow CNTs of different densities on Cu substrates and used these CNTs to understand how the densities of CNTs govern the field emission properties.

There have been several reports on the theoretical simulation for the behavior of CNTs as efficient field emitters. Dionne et al. have performed a numerical investigation of the field enhancement factor of individual as well as array of CNTs [22]. It has been reported that CNT emitters have optimal total emission when the spacing between neighboring CNTs is on the order of twice their height. In an array consisting of CNTs with two different heights, there was no significant screening of the electric field when the taller CNTs were twice the height of shorter CNTs. Wang et al. have solved the Laplace equation for individual CNTs and for the hexagonal arrays of CNTs to understand the influence of intertube distance, anode–cathode distance and the structure of the tip on the field emission [23]. Considering the emission current density, the field emission can be optimal when the intertube distance of CNTs array is close to the CNTs height.

Although there have been reports of the synthesis of CNTs using Cu as catalysts [24], [25], it is still challenging to grow well graphitized VACNTs directly over Cu substrates [26], [27]. Dubosc et al. have synthesized VACNTs on 400 nm of Cu film on Si/SiO2 with Ni as catalyst and TiN as a buffer layer using PECVD [28]. Banerjee et al. synthesized coiled carbon fibers on hydrogen-fluoride etched Cu substrates by PECVD [29]. Cu has fully filled 3d-orbitals, which prevents the formation of covalent bonds with hydrocarbon molecules. Also, the small binding energy of Cu with carbon subdues the process of graphitization. Furthermore, Cu has low carbon solubility preventing the saturation of carbon atoms required to form the CNT structures [30]. To overcome these problems, a thin buffer layer of Cr or TiN should be deposited on the Cu substrate prior to the deposition of Ni catalyst layer to ensure the synthesis and good adhesion of CNTs with the substrate. In this study, we have synthesized dense arrays of VACNTs on Cu substrates with a buffer layer of Cr using PECVD process. We have also employed NSL with polystyrene spheres of different diameters to pattern Ni catalyst dots on which VACNT patterns are grown. The dense VACNT array and the patterned VACNT arrays allow us to investigate how the variation in spacing between CNTs affects their field emission behavior if the CNTs have a similar height. The growth of patterned CNTs on conducting substrates allows for the in situ fabrication of electron emitters capable of delivering stable currents under an appropriate vacuum. The enhancement of field emission by the post-growth treatment of the samples by different methods raises an incompatibility issue with the fabricated devices. Hence, it is more desirable to synthesize CNTs of specific density at a predetermined location.

Section snippets

Nanosphere lithography

The catalyst pattern was prepared by a slight modification on the reported NSL procedures as follows [16]. An oxygen-free Cu plate (1 cm × 1 cm × 1 mm) was polished with sand paper to a smooth finish, and then it was ultrasonically cleaned in acetone and ethanol baths, each for 5 min. A 15 nm buffer layer of Cr was deposited on the Cu substrate using an e-beam evaporation system operated at room temperature. Clean silicon wafers 2 cm × 2 cm (donor substrates) were treated in RCA solution for 80 min. The RCA

Nanosphere lithography

The interstices between three adjacent spheres in the PS monolayer allow a Ni deposit to form the hexagonal pattern of Ni catalyst. Assuming that the Ni catalyst takes quasi-triangular shape, the size and spacing between the Ni catalyst particles can be calculated [16]. Fig. 1a–c show schematics of the relative variation of catalyst size (da) and separation for the catalyst sites (ds) of closely packed hexagonal structures formed by spheres of diameter 0.5, 1.0, and 1.8 μm, respectively. The

Conclusions

Vertically aligned CNTs were synthesized on copper substrates with Ni catalyst nanodots patterned by nanosphere lithography. The density and location of CNTs were determined by the diameter of spheres. The as-synthesized CNTs followed a tip-growth mechanism with the Ni catalyst forming almost a core–shell like structure several nanometers long in many cases. The low-density growth of CNTs and the inherent decrease in contact resistance with the choice of metal substrate were favorable for the

Acknowledgment

W.Z. Li acknowledges the support by the National Science Foundation under grant DMR-0548061.

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