Quantum size effects on the work function of metallic thin film nanostructures

In this paper, we present the direct observation of quantum size effects (QSE) on the work function in ultrathin Pb films. By using scanning tunneling microscopy and spectroscopy, we show that the very existence of quantum well states (QWS) in these ultrathin films profoundly affects the measured tunneling decay constant κ, resulting in a very rich phenomenon of “quantum oscillations” in κ as a function of thickness, L, and bias voltage, Vs. More specifically, we find that the phase of the quantum oscillations in κ vs. L depends sensitively upon the bias voltage, which often results in a total phase reversal at different biases. On the other hand, at very low sample bias (|Vs| < 0.03 V) the measurement of κ vs. L accurately reflects the quantum size effect on the work function. In particular, the minima in the quantum oscillations of κ vs. L occur at the locations where QWS cross the Fermi energy, thus directly unraveling the QSE on the work function in ultrathin films, which was predicted more than three decades ago. This further clarifies several contradictions regarding the relationship between the QWS locations and the work function.

[1]  Zhenyu Zhang,et al.  ``Electronic Growth'' of Metallic Overlayers on Semiconductor Substrates , 1998 .

[2]  M. Chou,et al.  Persistent superconductivity in ultrathin Pb films: a scanning tunneling spectroscopy study. , 2006, Physical review letters.

[3]  Chih-Kang Shih,et al.  Superconductivity at the Two-Dimensional Limit , 2009, Science.

[4]  W. Duan,et al.  Experimental observation of quantum oscillation of surface chemical reactivities , 2007, Proceedings of the National Academy of Sciences.

[5]  Yu Jia,et al.  Quantum size effects in Pb films from first principles: The role of the substrate , 2006 .

[6]  M. Chou,et al.  Quantum electronic stability of atomically uniform films. , 2001, Science.

[7]  Q. Xue,et al.  Wavevector-dependent quantum-size effect in electron decay length at Pb thin film surfaces , 2008 .

[8]  H. Zacharias,et al.  Unoccupied surface states of thec(2×2)-reconstructed3C−SiC(001)surface , 2002 .

[9]  F. K. Schulte.  A theory of thin metal films: electron density, potentials and work function , 1976 .

[10]  T. Tsong,et al.  Electronic growth of Pb islands on Si(111) at low temperature , 2002 .

[11]  M. Chou,et al.  Phase relations associated with one-dimensional shell effects in thin metal films. , 2009, Physical review letters.

[12]  M. Chou,et al.  Theory of quantum size effects in thin Pb(111) films , 2002 .

[13]  M. Chou,et al.  Atomic-layer-resolved quantum oscillations in the work function: Theory and experiment for Ag/Fe(100) , 2002 .

[14]  Zhenyu Zhang,et al.  Tuning the Quantum Stability and Superconductivity of Ultrathin Metal Alloys , 2007, Science.

[15]  James R. Thompson,et al.  Hard superconductivity of a soft metal in the quantum regime , 2006 .

[16]  C. Wang,et al.  Atomic models, domain-wall arrangement, and electronic structure of the dense Pb/Si(111)-(3)×(3) phase , 2002 .

[17]  Kuo-Jen Chao,et al.  Formation of Atomically Flat Silver Films on GaAs with a "Silver Mean" Quasi Periodicity , 1996, Science.

[18]  L. J. Chen,et al.  Correlation between quantized electronic states and oscillatory thickness relaxations of 2D Pb islands on Si(111)-(7 x 7) surfaces. , 2001, Physical review letters.

[19]  Stroscio,et al.  Electronic structure of the Si(111)2 x 1 surface by scanning-tunneling microscopy. , 1986, Physical review letters.

[20]  H. Hong,et al.  Quantum beating patterns observed in the energetics of Pb film nanostructures. , 2004, Physical review letters.

[21]  Wang,et al.  Role of the Metal/Semiconductor interface in quantum size effects: Pb /Si(111) , 2000, Physical review letters.

[22]  Q. Xue,et al.  Atomic-Layer-Resolved Local Work Functions of Pb Thin Films and Their Dependence on Quantum Well States , 2007 .