Effect of cooling time on the vapor liquid solid based growth of gold-catalyzed bismuth nanorods Susant Kumar Acharya 1,a,b , Alok Kumar Rai a,1 , Gil-Sung Kim a , Jung-Hwan Hyung a , Byung-Guk Ahn b , Sang-Kwon Lee a,n,2 a Department of Semiconductor Science and Technology, Basic Research Laboratory (BRL), Semiconductor Physics Research Center (SPRC), Chonbuk National University, Jeonju 561- 756, Korea b Division of Advanced Materials Engineering, Chonbuk National University, Jeonju, 561-756, Korea article info Article history: Received 9 May 2011 Received in revised form 28 November 2011 Accepted 12 December 2011 Available online 27 December 2011 abstract Deposition of single crystalline bismuth nanorods (Bi NRs) using a thermal evaporation method through vapor-liquid-solid (VLS) mechanism is reported here and the effect of sample cooling time on the growth of Bi NRs is investigated. Deposited Bi NRs have diameters varying from 100 to 400 nm and lengths extending to 3 mm in the (012) growth direction. Morphological observation indicated that the length and density of Bi NRs are strongly coupled with prolonged cooling time. A growth mechanism is suggested and discussed to describe the growth of single crystalline Bi NRs based on the morphological observations as a function of cooling temperature and time. & 2011 Elsevier B.V. All rights reserved. 1. Introduction In bulk form, bismuth (Bi) is a semimetal with unique electronic properties such as a small effective electron mass, low carrier density, long carrier mean free path, and highly anisotropic Fermi surface. It is well reported that for quantum confinement, finite-size effect, magneto resistance, and thermo- electric effect, one-dimensional (1D) structure of Bi delivers better results than other forms (2D and bulk) [1–4]. Therefore, 1D structure of Bi has received much attention in recent years from synthesis and fabrication point of view to investigate structure dependent properties. Many synthesis routes have been reported for the synthesis of 1D structure of Bi nanomaterial [5–9]. These are primarily anodic alumina membrane (AAM)- assisted methods including vacuum melting and pressure injec- tion [5]; magnetron sputtering [6]; electrochemical deposition [7]; and hydrothermal/solvothermal methods [8,9]. Boukai et al. fabricated polycrystalline Bi NRs using a micro fabrication method. [10]. Compared to the aforementioned methods, thermal evaporation is one of the simplest methods, as it provides homogenous synthesis, good quantity and high purity over a large area. To our knowledge, there is no report on the synthesis/deposi- tion of single crystalline Bi NRs using the thermal evaporation method. In this letter, we report, for the first time, the deposition of single crystalline Bi NRs by VLS-based thermal evaporation of Bi powder on gold (Au)-catalyzed Si (100) substrate. We also propose a growth mechanism based on VLS mechanism and sample cooling time, which is validated using binary Au-Bi phase diagram. 2. Experimental Details The proposed synthesis process is based on the thermal evaporation of Bi metallic powder on Au coated Si (100) substrate. High purity Bi powder (99.999%, Sigma-Aldrich, USA) was used as source material. Initially cleaned Si (100) wafer of small size (0.8  0.8 cm 2 ) was coated with 2 nm thin film of Au using e-beam evaporation, which acted as a catalyst for Bi NRs. The Bi powder was loaded in an alumina boat and placed in the center of the tubular furnace while the substrate were placed next to alumina boat in the downstream side of the furnace at distances of 7 to 12 cm from the center/boat. The furnace was heated to the desired growth temperature from 400 to 550 1C. After stabilizing the desired temperature, Ar was flown at the flow rate of 50 standard cubic centimeters per minute (sccm) and the pressure was maintained at  2.3 Torr during the deposition. Growth was continued for 2 h for all the experiments. The samples were then cooled to room temperature at different cooling times of 1, 2 and 3 h (cooling rate of  3–9 1C per minute) to study the influence of Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/physe Physica E 1386-9477/$ - see front matter & 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2011.12.009 n Corresponding author. Tel: þ82 63 270 3973; fax: þ82 63 270 3585. E-mail addresses: sk_lee@chonbuk.ac.kr, sangkwon.lee@yale.edu (S.-K. Lee). 1 These authors contributed equally to this work. 2 Currently Visiting Professor in Biomedical Engineering Department, Yale University, New Haven, CT 06511,USA. Physica E 44 (2012) 839–842