Theoretical and Experimental Study of Tip Electronic Structures in Scanning Tunneling Microscope
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1177-Z06-03
Theoretical and Experimental Study of Tip Electronic Structures in Scanning Tunneling Microscope HeeSung Choi1, Min Huang1, J.B. Ballard2, K.T. He3, S.W. Schmucker3, J.W. Lyding3, J.N. Randall2 and Kyeongjae Cho1,* 1 Department of Materials Science & Engineering and Department of Physics, The University of Texas at Dallas, 800 West Campbell Road, Richardson, TX 75080, U.S.A 2 Zyvex Labs, LLC. 1321 North Plano Road. Richardson, TX 75081 3 Department of Electrical and Computer Engineering and Beckman Institute, University of Illinois, Urbana, Illinois 61801 *[email protected] ABSTRACT The atomic and electronic structures of pyramidal model STM tips of transition metals (W, Rh, Pd, Ir and Pt) were investigated using density functional theory (DFT) method. The calculated density of states show that d electrons of the apex atoms in the M4 (M = W, Rh, Pd, Ir, Pt) model tips behave differently near the Fermi level, with the dz2 state being dominant only for W tip. The electronic structures of pyramid structures of W and Pd single-atom tips with larger sizes are studied and compared. The density of states of Pd apex atom and W apex atom show different occupation of d-bands leading to asymmetric density of states for Pd tip. The asymmetric tunneling currents measured by W and Pt-Ir STM tips are explained by the calculated electronic structures of W and Pd model tips. INTRODUCTION Scanning Tunneling Microscopes (STMs) have played an important role in exploring the surface properties with the atomic resolution due to high sensitivity of quantum mechanical tunneling on the tip-surface distance. The basic theory of STM imaging mechanism was developed using the s-wave tip model [1,2] and further refined by the localized dz2 tip state model for transition metal STM tips [3]. The s-wave model of the tip apex state has limited accuracy in explaining the observed atom-resolved images except Au STM tip [1,2]. When the actual electronic states of transition metal tips are considered in the localized dz2 tip state model, a quantitative explanation of the observed STM images of metal surfaces was possible [5]. Therefore, the specific details of the electronic states of tip apex atoms near Fermi level are expected to play an important role in the tip-surface interactions. [6,7] Single-atom tips (SATs) have well defined apex structures [8] and the corresponding electronic properties which enables various potential applications [3, 9]. SATs can be applied to scanning probe microscopes with optimum lateral resolution and the manipulation of individual atoms on the surface [10,11,12]. Despite the desirable features of SATs, preparation of SATs still remains difficult due to the thermodynamic or chemical instability issues and only a few methods have been proposed for their preparation. Among them, Tsong and colleagues have reported a new method to make sharp W (111) single-atom tips covered with Pd [13-16]. The tips generated by this method show high thermal stability up to 1000 K and an ability to regenerate by a simple anneali
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