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Anomalous negative differential conductance oscillation in the tunneling spectra of Ru cage-like quantum-dots


  Yehonadav Bekenstein [1,2,3]   ,  Kathy Vinokurov [2,3]  ,  Uri Banin [2,3]  ,  Oded Millo [1,3]  
 [1] Racah Institute of Physics, The Hebrew University of Jerusalem
[2] Institute of Chemistry, The Hebrew University of Jerusalem
[3] The Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem

Hybrid nanoscale inorganic cages, comprising a semiconducting Cu2S quantum-dot (QD) core encapsulated by a metallic Ru cage-like shell, and each of their individual components, are studied via scanning tunneling spectroscopy.  The bare Cu2S QDs show reproducible semiconducting-like I-V characteristics with a bandgap of ~1.5 eV similar to the reported values. The spectra acquired on the Cu2S/Ru hybrid QDs largely varied with tip position along a single dot, evolving from semiconducting-like characteristics with in-gap states to metallic-like single electron tunneling effects (SET).  Most intriguing are the tunneling spectra measured on the empty Ru cages. In addition to the expected SET features, the Coulomb blockade and staircase, the I-V characteristics exhibited, in many cases, negative differential conductance features with periodicity corresponding to the Coulomb staircase.  The conventional SET evolved, with increasing tunneling conductance, into a set of negative differential conductance peaks.  Such a unique behavior can be accounted for by considering the multiply-connected geometry of the Ru cage-like QD.