Transition from tunneling to direct contact in tungsten nanojunctions

We apply the mechanically controllable break junctions technique to investigate the transition from tunnel- ing to direct contact in tungsten. This transition is quite different from that of other metals and is determined by the local electronic properties of the tungsten surface and the relief of the electrodes at the point of their closest proximity. When flat surfaces approach each other, an avalanchelike jump to direct contact occurs at anomalously large distances z'3 -5 A. In contrast, ballistic contact between irregularly shaped electrodes is established without discontinuity in conductance curves, indicating the absence of spontaneous formation of an adhesive neck. Conductance histograms of tungsten are either featureless or show two distinct peaks related to the sequential opening of spatially separated groups of conductance channels. The role of surface states of tungsten and their contribution to the junction conductance at sub-A electrode separations are discussed. The study of transition from tunneling to direct contact and electrical transport through atomic-sized metallic con- ductors has been the object of great attention during the last decade. Different types of phenomena, related to both the quantum character of transport and the atomic discreteness of the contact, were observed in 3D nanoconstrictions produced by scanning tunneling microscopes ~STM! or mechanically controllable break junctions ~MCBJ's!. 1 In particular, con- ductance measurements of breaking nanowires demonstrated a steplike structure of conductance versus electrode separa- tion traces G(z). For single-valence s-metal conductance plateaus are close to the integer multiples of the quantum conductance unit G052e 2 /h. However, simultaneous mea- surements of both the force and the conductance for breaking gold nanowires demonstrated that jumps between plateaus in the conductance staircase are always correlated with relax- ations of the mechanical force and, therefore, with atomic rearrangements in the nanoconstriction. 2 Individual conduc- tance curves G(z) are inherently irreproducible due to the different dynamical evolution of the connective necks during the break. Therefore, analysis of experimental data includes construction of conductance histograms based on a large number of conductance traces. Peaks in the conductance his- tograms are related to the statistically more probable atomic configurations in the connective neck between the electrodes. 3 For polyvalent metals the main ~and sometimes the only! feature in the conductance histograms is a peak corresponding to one-atom point contact. 4 The conductance through such a contact is determined by a few conductance channels intimately related to atomic orbitals. 5 Transition from tunneling to single-atom contact occurs in an avalanch- elike way at an electrode separation of ;1.5 A due to the metallic adhesion forces. 6 This sudden jump in conductance precluded measurements of G(z) at sub-A distances between the electrodes for all metals studied to date. While measuring thermal expansion of MCBJ electrodes for different materials, we took notice of the unusually high stability of tungsten tunnel junctions at very close electrode separations. 7 In another study, transmission and scanning electron microscopy images showed no evidence of connec- tive neck formation between tungsten wires. 8 Moreover, measurements of adhesive forces between an atomically- defined W~111! trimer tip and a Au~111! sample revealed no spontaneous jump to contact. 9 These unusual properties of tungsten nanocontacts motivated us to carry out further ex- tensive investigations. In this paper we present our experiments with tungsten MCBJ. Our aim was to examine the behavior of tungsten nanojunctions during the transition from tunneling to direct contact and to what extend this behavior is influenced by the unique mechanical and electronic properties of tungsten. We show that very often the adhesive jumps of conductance are absent and that the transition to single atom contact is smooth. This permitted us to investigate the junction at un- precedented ultrasmall electrode separations. By employing the conductance histogram technique we determined the preferential values of conductance during the fracture of the junction to be about 1G0 and 2G0. We show that the special evolution of the conductance in tungsten junctions is mostly governed by the extreme hardness of the contact and by sur- face electronic states at the W~001! surface that occasionally terminates the junction electrodes. Also, we measured the conductance histograms of tantalum and molybdenum ~as the two nearest neighbors of tungsten in the periodic table of elements! to provide additional support for our model.