Electronic and dynamic studies of boron carbide nanowires D. N. McIlroy, Daqing Zhang, Robert M. Cohen, and J. Wharton Department of Physics and Microelectronics Research and Communications Institute, Engineering and Physics Building, University of Idaho, Moscow, Idaho 83844-0903 Yongjun Geng and M. Grant Norton School of Mechanical and Materials Engineering, Washington State University, Pullman, Washington 99164-2920 Gelsomina De Stasio ´ Institut de Physique Applique ´e, Ecole Polytechnique Fe ´de ´rale, CH-1015 Lausanne, Switzerland and Istituto di Struttura della Materia del CNR, Via Fosso del Cavaliere, I-00137 Roma, Italy B. Gilbert and L. Perfetti Institut de Physique Applique ´e, Ecole Polytechnique Fe ´de ´rale, CH-1015 Lausanne, Switzerland J. H. Streiff, B. Broocks, and Jeanne L. McHale Department of Chemistry, Renfrew Hall, University of Idaho, Moscow, Idaho 83844-2343 Received 14 December 1998 The electronic and vibrational properties of boron carbide nanowires grown by plasma-enhanced chemical vapor deposition have been examined with the techniques of near-edge x-ray absorption fine structure NEXAFS spectroscopy and Raman spectroscopy. The B 1s absorption edge is characterized by a narrow * resonance characteristic of sp / sp 2 hybridization followed by a broad * resonance characteristic of sp 3 hybridization. The C 1s NEXAFS spectrum is dominated by the * resonance indicating that C bonding in the nanowires is predominantly sp 3 in character. The NEXAFS spectra are equivalent to corresponding spectra of single-crystal (B 4 C) boron carbide, consistent with the B 4 C structure of the nanowires as determined by selected area electron diffraction. Four corresponding Raman modes of crystalline boron carbide have been observed for the boron carbide nanowires. Two previously unobserved Raman modes of boron carbide at 1365 and 1965 cm -1 have also been observed, which are specific to boron carbide nanowires. S0163-18299910231-5 I. INTRODUCTION Over the past decade significant effort has gone into de- veloping an understanding of quantum confinement and its effects on electronic transport. This has been driven by sci- entific curiosity, as well as the need to quantify the role quantum confinement plays in microprocessor performance as the architecture of these devices heads towards the na- nometer regime. Nanometer sized materials can be artifi- cially manufactured, as well grown in a self-assembled fash- ion via novel processes. One of the advantages of exploring the properties of self-assembled nanostructures is their ease of construction. Self-assembled nanostructures come in a va- riety of flavors ranging from free standing nanocrystals grown by colloidal chemistry 1,2 to carbon nanotubes, 3 to name a few, and like multilayer quantum well systems they have been found to exhibit quantum size effects. 4,5 In order to develop a full understanding of quantum size effects in self-assembled nanostructured materials a detailed understanding of their fundamental properties needs to be determined. Due to their large surface area to volume ratio, the fundamental properties of all types of nanostructured ma- terials can be anticipated to deviate from those of the bulk. In addition, the surfaces of nanostructured materials can vary significantly with size, which will inevitably affect their elec- tronic properties. An excellent example of this phenomena was illustrated with carbon nanotubes where it was shown that changes in their structure altered their electronic proper- ties from graphitic to semiconducting in character. 6 This work, as well as studies of semiconducting nanoparticles, 5 also demonstrated the feasibility of constructing nanoscale devices using self-assembled nanostructures. The work with carbon nanotubes has demonstrated the need to explore the structural properties of nanostructured materials, as well as stressed the need to examine their fundamental properties in order to develop a global understanding of their electronic transport properties relative to the bulk. In addition to quan- tum confinement, finite size effects can have a significant impact on the thermodynamic properties and vibrational den- sity of states of nanostructured materials. Consequently, it is important that finite size effects be thoroughly explored in order to gauge the different factors that influence the prop- erties of nanostructured materials. In this paper, we have examined the electronic and vibra- tional structure of single-crystal boron carbide (B 4 C) nano- wires by near-edge x-ray absorption fine structure NEX- AFS and Raman spectroscopy, respectively. Boron carbide is a refractory semiconductor material, which in addition to being chemically robust, has a very high-melting tempera- ture in excess of 2400 °C. The mechanical hardness and elec- PHYSICAL REVIEW B 15 AUGUST 1999-I VOLUME 60, NUMBER 7 PRB 60 0163-1829/99/607/48746/$15.00 4874 ©1999 The American Physical Society