Structural and Electrical Properties of Conducting Diamond Nanowires Kamatchi Jothiramalingam Sankaran, † Yen-Fu Lin, ‡ Wen-Bin Jian, ‡ Huang-Chin Chen, § Kalpataru Panda, ⊥ Balakrishnan Sundaravel, ⊥ Chung-Li Dong, # Nyan-Hwa Tai,* ,† and I-Nan Lin* ,§ † Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC ‡ Department of Electrophysics, National Chiao Tung University, Hsinchu 30010, Taiwan, ROC § Department of Physics, Tamkang University, Tamsui 251, Taiwan, ROC ⊥ Materials Science Group, Indira Gandhi Centre for Atomic Research, Kalpakkam 603 102, India # Scientific Research Division, National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan, ROC ABSTRACT: Conducting diamond nanowires (DNWs) films have been synthesized by N 2 -based microwave plasma enhanced chemical vapor deposition. The incorporation of nitrogen into DNWs films is examined by C 1s X-ray photoemission spectroscopy and morphology of DNWs is discerned using field-emission scanning electron microscopy and transmission electron microscopy (TEM). The electron diffraction pattern, the visible-Raman spectroscopy, and the near-edge X-ray absorption fine structure spectroscopy display the coexistence of sp 3 diamond and sp 2 graphitic phases in DNWs films. In addition, the microstructure investigation, carried out by high-resolution TEM with Fourier transformed pattern, indicates diamond grains and graphitic grain boundaries on surface of DNWs. The same result is confirmed by scanning tunneling microscopy and scanning tunneling spectroscopy (STS). Furthermore, the STS spectra of current-voltage curves discover a high tunneling current at the position near the graphitic grain boundaries. These highly conducting regimes of grain boundaries form effective electron paths and its transport mechanism is explained by the three-dimensional (3D) Mott’s variable range hopping in a wide temperature from 300 to 20 K. Interestingly, this specific feature of high conducting grain boundaries of DNWs demonstrates a high efficiency in field emission and pave a way to the next generation of high-definition flat panel displays or plasma devices. KEYWORDS: diamond nanowire films, graphitic grain boundary, high resolution transmission electron microscopy, scanning tunneling spectroscopy, hopping transport, electron field emission 1. INTRODUCTION Field-emission displays (FEDs) have received interest as the next-generation flat panel displays, substitute to the presently dominant liquid crystal, because of their potential low cost and high performance. One of the major concerns in FEDs has been the improvement of consistent, efficient cold cathode materials for electron field emitters. 1-4 Materials like semi- conductor nanowires or nanotubes, carbon nanotubes (CNTs), and diamond films have been demonstrated to show low turn- on voltages and high current densities, making them suitable for cold cathode emitter applications. Among them, diamond films have been reported to exhibit negative electron affinity, which makes diamond a more promising material for applications in FED devices. 5-8 On the other hand, one-dimensional (1D) materials showing distinct chemical and physical properties have attracted a tremendous amount of attention and have been used in many technological applications such as pH sensors, 9 ultraviolet nanolasers, 10 and nanoelectronics. 11-13 Since the discovery of CNTs in 1991, 14 a variety of 1D materials were successfully synthesized. 10,15 Among those 1D materials, diamond nanowires (DNWs) were of great interest because theoretical studies and simulations explain their structural stabilities and inspire several potential applications. 16-18 Especially, through their intrinsic dependence on size and crystallographic directions, DNWs were found to be energeti- cally favored and structurally stable at diameters ranging from 2.7 to 9.0 nm. 16,17 Recently, novel properties including high thermal conductivity and zero strain stiffness of DNWs and related carbon materials have been theoretically predicted and experimentally demonstrated, foreseeing their feasible use in cross-link facilitated heat transfer and thermal management systems. 19,20 The synthesis of DNWs is always an interesting subject since new approaches pave the way to nanoscale and atomic precision of material growth and to the search of new forms of carbon. The DNW precursor of porous diamond films was Received: October 22, 2012 Accepted: January 17, 2013 Published: January 17, 2013 Research Article www.acsami.org © 2013 American Chemical Society 1294 dx.doi.org/10.1021/am302430p | ACS Appl. Mater. Interfaces 2013, 5, 1294-1301