Scanning tunneling microscopic and field emission microscopic studies of nanostructured molybdenum film synthesized by electron cyclotron resonance plasma Vishwas S. Purohit a , A.B. Bhise a , Shirshendu Dey a , M.A. More a , C.V. Dharmadhikari a , D.S. Joag a , Renu Pasricha b , S.V. Bhoraskar a, * a Center for Advanced Studies in Materials Science and Condensed Matter Physics, Department of Physics, University of Pune, Pune-41007, India b National Chemical Laboratory, Homi Bhabha Road, Pune-411008, India article info Article history: Received 30 March 2007 Received in revised form 2 April 2008 Accepted 14 April 2008 Keywords: Mo nanoparticles ECR plasma Hollow cathode chemical sputtering Scanning tunneling microscopy Field emission microscopy abstract Cathodic sputtering is demonstrated to be effective in synthesizing thin films of molybdenum nano- particles. An electron cyclotron resonance plasma reactor has been used as the source. The particle size distribution is found to be controllable by proper choice of the cathodic bias potential. Sizes ranging between 20 and 30 nm deposited at the optimum bias potential are found to exhibit a self assembled structure as observed by scanning tunneling microscopy. Field emission microscopic studies on these films supported on W have exhibited very stable emission current over a period of 3 h. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Molybdenum is one of the important refractory metals [1], which finds application as a thermionic emitter, secondary electron emitter and field emitter. It also finds applications as a catalyst [2] and is extensively used in refining of petroleum. The catalytic ac- tivities are enhanced for nanomaterials on account of the high surface area per unit volume. Enhanced catalytic properties also may result from changes in the band gap energy of the metal as the particle size decreases [3]. Such behavior of molybdenum nano- particles has been reported by Perez-Mendoza et al. [4] during the growth of carbon nanotubes. Films of nanoparticles have attracted interest because of their outstanding properties and potential ap- plications as catalysts in promoting chemical reactions. Apart from being an anticorrosive material [5], molybdenum is also valuable in the form of sulphide and oxide. Both molybdenum disulphide (MoS 2 ) and molybdenum sulphide (MoS) are useful as lubricants, especially at high temperatures, where oil decomposition is prob- lematic. Molybdenum nanoparticles’ templates are, therefore, useful in the formation of oxides or sulphides. Growth of molybdenum nanoparticles has been reported by other methods such as electrical explosion [6] and hotwire chemical vapor de- position [7] where a molybdenum hexacarbonyl precursor is used [8,9]. Field electron emission from nanostructured materials has attracted the attention of researchers in the past few years [10]. Applications of these emitters for flat panel display devices and as electron sources in vacuum microelectronic devices comprise the principal reasons for the growing interest in the synthesis and characterization of nanostructured materials and their study as field emitters [11]. Field emission studies on molybdenum nano- wires have been carried out by Zhou et al. [12]. Here, we report the synthesis of nanostructured molybdenum by a microwave assisted electron cyclotron resonance (ECR) plasma induced chemical sputtering process using hydrogen as the ionizing gas. The variation in the crystallite/particle size is studied as a function of the bias voltage applied to the cathode during the sputtering process. As a result, the bias voltage could be optimized in order to achieve a narrow size distribution. Scanning tunneling microscope (STM) was used to investigate the finer details of the nanostructure. The STM images reveal that the nanoparticles consisted of still smaller crystallites, which then seem to give rise to self assembled linear arrays. Field emission measurements carried out on the molybdenum nanoparticles deposited on tungsten tips and flat silicon surfaces were found to produce stable current. * Corresponding author. Fax: þ91 020 25691684. E-mail address: svb@physics.unipune.ernet.in (S.V. Bhoraskar). Contents lists available at ScienceDirect Vacuum journal homepage: www.elsevier.com/locate/vacuum 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.04.077 Vacuum 83 (2009) 435–443