Shell Effects in Gold Nanowires at Room Temperature

1. (kFR)=1.56 (kFR)=1.2 (kFR)=1.11 Shell Effects in Gold Nanowires at Room Temperature A.I. Hulea, A.F. Otte, R.H.M. Smit, J.M. van RuitenbeekKamerlingh Onnes Laboratorium, Leiden University, The Netherlands Recently Yanson et al.,Nature 400, 144 (1999) discovered that the formation of alkali nanowires presents periodic spectrum of exceptionally stable diameters. This has been identified as a shell effect that is governing also the stability of atoms, nuclei and metallic clusters. Similar to clusters, metallic nanowires present two magical series of stable diameters given by electronic and atomic shell effects. We extended the studies of shell filling effects to gold nanowires obtained by a new mechanically controllable break junction technique (MCBJ) in ultra high vacuum (UHV) at room temperature. • Advantages: • Free choice of sample material • Optimum for study of shell effects in different metals: wide range of temperatures achievable: 70K-500K • UHV conditions maintain a clean surface The UHV-MCBJ technique: A notched wire is clamped between two bendable substrates and stretched in a controlled way by the piezoelement Simultaneously the conductance of the constriction is monitored. Stable diameters occurs for complete atomic layers –periodicity (kFR)~0.48 This shell structure occurs at large radius of the nanowires Atomic shell effect Electronic shell effect The periodicity of the stable diameters of the nanowire is related to the periodic orbits of a particle inside a cylinder Observation of shell effects in gold nanowires at T=300K We have used a histogram method that is averaging over many conductance curvesStable nanowire diameters correspond to peaks in the conductance histogram • The wire radius is obtained from the conductance as R~G1/2. A periodic shell structure can be demonstrated as a constant slope  (G1/2) vs. peak index Two sets of periodic oscillations:-electronic (kFR) =1.12-atomic: (kFR) =0.44 Crossover at G~15 G0 Periodicity (kFR)=1.18 related to electronic shell filing effect Conclusions We have obtained evidence for two sets of magic numbers in gold nanowires: electronic and atomic. The former is related to closing of electronic shells while the latter is due to the surface energy, dominating at large diameters. The electronic shell effect does not appear to be affected by non-spherical features of the gold Fermi surface. The periodicities of the peaks in the gold conductance histogram are similar to the ones obtained for alkaline nanowires (kFR)= 1.18 for electronic and (kFR)=0.44 for atomic shell effect), although often somewhat smaller. We propose that the smaller slopes can be attributed to defect scattering.