Article pubs.acs.org/JPCC
Electrophoresis of Nanodiamond on the Growth of Ultrananocrystalline Diamond Films on Silicon Nanowires and the Enhancement of the Electron Field Emission Properties Ting-Hsun Chang,† Kalpataru Panda,‡ B. K. Panigrahi,‡ Shiu-Cheng Lou,§ Chulung Chen,§ Huang-Chin Chan,∥ I-Nan Lin,*,∥ and Nyan-Hwa Tai*,† †
Department of Materials Science and Engineering, National Tsing-Hua University, Hsin-Chu, Taiwan 300, R.O.C. Materials Physics Division, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India § Department of Photonics Engineering, Yuan-Ze University, Chung-Li 32003, Taiwan, R.O.C. ∥ Department of Physics, Tamkang University, New-Taipei 251, Taiwan, R.O.C. ‡
ABSTRACT: Electrophoresis deposition (EPD) of nanodiamond was found to be a very efficient process in forming the nucleation sites for the growth of ultrananocrystalline diamond (UNCD) films. Transmission electron microscopic investigations showed that the EPD-derived UNCD films grown on planar Si substrates contain uniform granular structure with sharp, smooth, and conductive UNCD-to-Si interface layer, which helps in better electron field emission (EFE) properties of the UNCD films. Moreover, contrary to ultrasonication process for nucleating the diamond, the EPD process is a gentle one that induced no damage to the silicon nanowire arrays (SiNWs), facilitating the formation of nuclei for growing UNCD films on SiNWs. Such a gentle process significantly enhanced the EFE properties of UNCD/SiNWs. The EPD-derived UNCD/SiNW emitters show superior EFE performances to the planar UNCD films, that is, a turn-on field of 7.19 V/μm and a large EFE current density of 2.21 mA/ cm2 at 15.0 V/μm. Furthermore, a parallel plate plasma device fabricated using the EPD-derived UNCD/SiNW nanostructures as cathode shows a high Ar plasma current value of 3.5 mA/cm2 at a low applied field of 0.35 V/μm. These results demonstrate that these EPD-derived UNCD/SiNW nanostructures have great potential for the applications in flat panel displays due to their superior EFE properties and plasma illumination performances.
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nano-sized diamond grains.13,14 The UNCD films have more possibility to integrate with various materials. The direct deposition of UNCD films on SiNW templates and the related EFE properties has already been studied by Tseng et al.15 However, in this process, the UNCD particulates are scarcely distributed on the SiNW surface, and the related EFE properties are not satisfactory. It is due to the fact that the diamond grains are not uniformly and densely populated on the SiNW surface due to the absence of active emission sites. Pretreatment techniques such as ultrasonication were also reported, which can enhance the growth of UNCD films on SiNWs.16 However, the tips of SiNWs broke, and the length of the SiNWs reduced due to the rigid ultrasonication process, and hence the related EFE properties did not perform as well as expected. This implies that the SiNWs are not strong enough to survive the severe abrasive ultrasonication process. Hence a gentler nucleation technique that is efficient enough to form active nucleation sites for further deposition of UNCD films is required.
INTRODUCTION The EFE properties of nanostructured materials, such as carbon nanotubes (CNTs), Si nanowires, nanostructured diamond films, and so on have caught great attention from researchers.1−4 Among them, silicon (Si)-based nanoemitters such as silicon nanowires (SiNWs) are advantageous over other nanowire templates because of their compatibility with Si materials and have greater potential for integration with Si devices to form active electron field emitters.3,4 Over the past years, researchers have reported successful synthesis of diamond tip using aligned silicon tip arrays as templates, which were prepared by the conventional chemical vapor deposition, thermal evaporation/decomposition, and laser ablation techniques, resulting in good electron field emission (EFE).5−7 However, most of these techniques require sophisticated equipments and complicated processing steps, which are time-consuming and expensive. Several methods have been proposed to enhance the EFE properties of diamond films8−10 and diamond-coated SiNW emitters.11,12 However, conformal coating of diamond films on SiNWs is extremely difficult. Ultrananocrystalline diamond (UNCD) is a special form of diamond with nanometer size grain (2−5 nm) and have high EFE performances as compared with micrometer- or © 2012 American Chemical Society
Received: June 20, 2012 Revised: August 12, 2012 Published: August 30, 2012 19867
dx.doi.org/10.1021/jp306086b | J. Phys. Chem. C 2012, 116, 19867−19876
The Journal of Physical Chemistry C
Article
using field-emission scanning electron microscope (FESEM; Joel 6500). The bonding structure of the films was characterized at room temperature using Raman spectroscopy (Renishaw) using a 632.8 nm laser as the excitation source. The detail cross-sectional microstructure of UNCD/Si films was examined using transmission electron microscopy (TEM). In the preparation of thin foil for TEM investigation, several pieces of UNCD/Si samples were glue together, sliced, and then polished by using emery paper to a thickness ∼100 μm, followed by ion milling process (Getan) until the samples started to perforate that rendered the regions in the vicinity of the hole thin enough (about tens of nanometers) for TEM investigation. It should be noted that the UNCD/SiNW cannot be prepared in such a process, and the as-prepared UNCD/ SiNWs are too thick to be examined in TEM. The EFE properties of the UNCD/Si or UNCD/SiNW films were measured using a parallel plate setup, in which the sampleto-anode distance was adjusted using a micrometer. The current density-electric field (J−E) characteristics curves were measured using an electrometer (Keithley 237) at 10−6 Torr base pressure. The EFE characteristics were modeled using a Fowler−Nordheim model.19,20
In this context, we developed a gentler nucleation process for growing UNCD films on aligned SiNWs to synthesize UNCD nanoemitters that possess EFE properties comparable to those of the carbon nanotubes but with markedly better processing reliability. A systematic study of the effect of pretreatment techniques such as EPD and ultrasonication processes on the EFE performances of the UNCD/SiNW nano emitters is studied, and the optimized parameters are reported. How the processing parameters in EPD technique influence the EFE properties of the UNCD/SiNW structures is investigated by examining the microstructure of UNCD films and the characteristics of UNCD-to-Si interface of the films. Moreover, the benefit of UNCD coating with good EFE properties on exciting the Ar plasma in a device with parallel-plate configuration is demonstrated.
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EXPERIMENTAL SECTION The SiNW arrays were conventionally prepared by a galvanic displacement reaction in a HF/AgNO3 solution using silicon wafer as the substrate.15 The drawback of this process is that the Ag clusters easily aggregated, forming large Ag particles of various sizes, which results in nonuniformity in distribution of SiNWs. To improve the uniformity of the SiNW arrays, Au nanoparticles are used to replace the Ag nanoparticles, followed by the same galvanic displacement process. N-type Si(100) substrates were cut into 1.0 × 1.0 cm2 squares and cleaned using standard RCA process, 17 which includes rinsing sequentially the Si wafer in water-diluted hydrogen peroxide/ ammonium hydroxide and hydrogen peroxide/hydrochloric acid solution. The cleaned (100) Si samples were sputtercoated with a thin gold film (∼10 nm), followed by postannealing in an argon atmosphere at 1073 K for 15 min to form gold nanoparticles (AuNPs) on the Si substrate. The AuNPs containing Si wafers were then immersed into a HFbased aqueous solution (12 mL of 48% HF and 0.103 M of AgNO3) for 30 min, followed by cleaning with nitric acid as well as deionized water and then dried. Vertically aligned SiNWs were thus formed on the Si substrate. The alignment of SiNWs seems not correlated with the (100) orientation of the SiNWs. Nanodiamond particulates were deposited on the asgrown SiNWs prior to the deposition of UNCD films for better smoothness, uniformity, and adhesion. To facilitate the comparison, we used different pretreatment techniques such as ultrasonication and EPD of nanodiamond. In ultrasonication method, the planar Si or SiNWs were dipped in nanodiamondand titanium-powder-containing methanol solution and ultrasonicated for 45 min, followed by cleaning in acetone.16 In the EPD process, the single digit diamond particulates (Plasma Chem) aqueous solution with concentration of 0.1 g/L were used. The Si (or SiNW) substrates were biased at about +20 V with respect to the reference electrode, the Pt, for 10 to 60 s. The UNCD films were coated on the prenucleated planar-Si or SiNWs by using microwave plasma-enhanced chemical vapor deposition (MPECVD) process (IPLAS-Cyrannus) using CH4 (1%)/Ar plasma as precursors.18 The pressure, flow rate, and power were maintained at 120 Torr, 100 sccm, and 1200 W, respectively. The growth process was carried out at low temperature (
