Investigations on Diamond Nanostructuring of Different Morphologies by the Reactive-Ion Etching Process and Their Potential Applications Srinivasu Kunuku, † Kamatchi Jothiramalingam Sankaran, ‡ Cheng-Yen Tsai, § Wen-Hao Chang, § Nyan-Hwa Tai, ‡ Keh-Chyang Leou,* ,† and I-Nan Lin* ,¶ † Department of Engineering and System Science, National Tsing Hua University, Hsinchu 300, Taiwan, Republic of China ‡ Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, Republic of China § Department of Electrophysics, National Chiao Tung University, Hsinchu 300, Taiwan, Republic of China ¶ Department of Physics, Tamkang University, Tamsui 251, Taiwan, Republic of China * S Supporting Information ABSTRACT: We report the systematic studies on the fabrication of aligned, uniform, and highly dense diamond nanostructures from diamond films of various granular structures. Self-assembled Au nanodots are used as a mask in the self-biased reactive-ion etching (RIE) process, using an O 2 /CF 4 process plasma. The morphology of diamond nanostructures is a close function of the initial phase composition of diamond. Cone-shaped and tip-shaped diamond nanostructures result for microcrystalline diamond (MCD) and nanocrystalline diamond (NCD) films, whereas pillarlike and grasslike diamond nanostructures are obtained for Ar-plasma-based and N 2 -plasma-based ultrananocrystalline diamond (UNCD) films, respectively. While the nitrogen- incorporated UNCD (N-UNCD) nanograss shows the most-superior electron-field-emission properties, the NCD nanotips exhibit the best photoluminescence properties, viz, different applications need different morphology of diamond nanostructures to optimize the respective characteristics. The optimum diamond nanostructure can be achieved by proper choice of granular structure of the initial diamond film. The etching mechanism is explained by in situ observation of optical emission spectrum of RIE plasma. The preferential etching of sp 2 -bonded carbon contained in the diamond films is the prime factor, which forms the unique diamond nanostructures from each type of diamond films. However, the excited oxygen atoms (O*) are the main etching species of diamond film. KEYWORDS: diamond nanostructures, reactive ion etching, Au nanodots, ion bombardment, electron field emission, photoluminescence 1. INTRODUCTION One-dimensional (1D) nanostructures attract enormous interest in the field of optoelectronic devices and chemical and biochemical sensors, because of their high aspect ratio and low dimensional effect. 1-5 The robust applications of materials are not only a function of the bulk intrinsic properties but also a function of their surface activities. Diamond is an exclusive material for potential applications such as biological and chemical sensors, microelectromechanical devices, electron field emitters, and single photon sources, because of its wide band gap, highest hardness, outstanding thermal conductivity, tunable negative electron affinity, and virtuous chemical inertness. 6-9 Meanwhile, diamond nanostructuring is an effective method for improving these peculiar properties. Among the potential applications of diamond nanostructures, electron field emitters and single photon source are the most promising ones. The electron field emitters have great potential in the development of vacuum electronic devices such as traveling wave tubes, portable X-ray sources, and electron field emission (EFE) flat-panel displays. The diamond nanostruc- tures play a vital role for enhancing the EFE properties for practical applications. Previous studies have reported the better cathode properties of different diamond nanotips. 10-17 On the other hand, the nitrogen vacancy (NV) color center is a defect in diamond, which has been established as a proficient source for single photon emission at room temperature. 18 The single photon source is a potential core for quantum key distribution and quantum computation. The enhancement in single photon emission has been reported from diamond nanowires, which is fabricated from single-crystalline diamond. 18-20 Received: May 9, 2013 Accepted: July 12, 2013 Published: July 12, 2013 Research Article www.acsami.org © 2013 American Chemical Society 7439 dx.doi.org/10.1021/am401753h | ACS Appl. Mater. Interfaces 2013, 5, 7439-7449