Self-Assembled Growth, Microstructure, and Field-Emission High-Performance of Ultrathin Diamond Nanorods Naigui Shang, †, * Pagona Papakonstantinou, † Peng Wang, ‡ Alexei Zakharov, § Umesh Palnitkar,  I-Nan Lin,  Ming Chu, ¶ and Artemis Stamboulis ¶ † Nanotechnology Research Institute, School of Electrical and Mechanical Engineering, University of Ulster, Shore Road, Newtownabbey, BT37 0QB, United Kingdom, ‡ UK SuperSTEM, Daresbury Laboratory, Cheshire, WA4 4AD, United Kingdom, § MAX-Laboratory, Lund University, Box 118, Lund S-22100, Sweden,  Department of Physics, Tamkang University, Tamsui 251, Taiwan, Republic of China, and ¶ Department of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom V arious carbon allotropes including fullerene, single/multiwall carbon nanotube (SWCNT/MWCNT), graph- ite, and diamond have received enduring attention over the last two decades because of their excellent properties and potential wide applications. Their completely differ- ent properties have been ascribed to the di- versity of C-C chemical bonds (sp 1 , sp 2 or sp 3 ). For example, sp 3 C-C bonded dia- mond is a wide band gap semiconductor exhibiting a combination of superior prop- erties such as negative electron affinity, chemical inertness, high Young’s modulus, the highest hardness and room- temperature thermal conductivity, 1 whereas sp 2 C-C bonded graphite is an ex- cellent conductor and one of the softest materials in nature. Meanwhile, the dimen- sion and size could play a critical role in de- termining the properties of such materials. For example, sp 2 -bonded SWCNT is a unique one-dimensional (1D) material, which has not only a high aspect ratio, a high thermal conductivity, the highest ten- sile intensity, and Young’s modulus, but also exhibits either metallic or semiconducting behavior with quantum electron transport. 2 The most convincing evidence of size ef- fects is that the properties of thin SWCNTs are superior to those of thick MWCNTs. Thus, high aspect-ratio, nanoscale 1D dia- mond in the form of nanotubes, nanorods, nanowires, nanofibers, nanopillars and so on has become a hot research topic in both theoretical and experimental fields, repre- sented as a counterpart of CNTs due to dif- ferent C-C bonding nature of sp 3 versus sp 2 . To date, many novel properties of dia- mond nanorods (DNRs) such as high ther- mal conductivity, a zero strain stiffness, etc. have been theoretically predicted, 3,4 fore- seeing their possible use in cross-link facili- tated heat transfer and thermal manage- ment systems. 5,6 The realization of vertically aligned conducting diamond nanorods ar- rays, which can detect picomolar concentra- tions of target DNA has triggered the devel- opment of sensors for clinical diagnostics, environmental sensing as well as other ap- plications at the interface between biology and microelectronics. 7 Like SWCNTs, DNRs may be semiconducting, semimetallic or metallic, depending on their diameter, sur- face morphology and surface functional species. Their band gaps start to be tun- able at a diameter of less than about 2.39 and 4.14 nm for the dehydrogenated and hydrogenated DNRs, respectively. 8 So far di- verse 1D diamond nanostructures have been synthesized by hydrogen plasma *Address correspondence to ngshang@hotmail.com. Received for review February 18, 2009 and accepted March 26, 2009. Published online April 3, 2009. 10.1021/nn900167p CCC: $40.75 © 2009 American Chemical Society ABSTRACT We report the growth of ultrathin diamond nanorods (DNRs) by a microwave plasma assisted chemical vapor deposition method using a mixture gas of nitrogen and methane. DNRs have a diameter as thin as 2.1 nm, which is not only smaller than reported one-dimensional diamond nanostructures (4300 nm) but also smaller than the theoretical value for energetically stable DNRs. The ultrathin DNR is encapsulated in tapered carbon nanotubes (CNTs) with an orientation relation of (111) diamond //(0002) graphite . Together with diamond nanoclusters and multilayer graphene nanowires/nano-onions, DNRs are self-assembled into isolated electron- emitting spherules and exhibit a low-threshold, high current-density (flat panel display threshold: 10 mA/cm 2 at 2.9 V/m) field emission performance, better than that of all other conventional (Mo and Si tips, etc.) and popular nanostructural (ZnO nanostructure and nanodiamond, etc.) field emitters except for oriented CNTs. The forming mechanism of DNRs is suggested based on a heterogeneous self-catalytic vaporsolid process. This novel DNRs- based integrated nanostructure has not only a theoretical significance but also has a potential for use as low-power cold cathodes. KEYWORDS: diamond nanorods · carbon nanotube · aberration-corrected TEM · HAADF · PEEM · NEXAFS · field emission ARTICLE VOL. 3 ▪ NO. 4 ▪ SHANG ET AL. www.acsnano.org 1032