View the related articles [J. Phys. Soc. Jpn. 76 (2007) 123601]

Clear Relation between Quantized Conductance and Structure of Au Nanowires [December 10, 2007]
Masaru Tsukada
(Graduate School of Advanced Science and Engineering, Waseda University)

   When atoms, molecules, or microclusters are in contact with biased electrodes, how does the electric current flow through these materials or how does the injected electron from one electrode move to the other? This rather fundamental question is not only of academic interest, but also of modern technological concern. In other words, in the miniaturization of integrated circuits, the possibility of devices using single molecules, atomic wires, or point contacts as individual device elements should be expected. Such structures of nanoscale materials linked with electrodes are prototypes of nanodevices, and recently various fundamental properties of these link structures have attracted considerable attention [1].

  The most important difference between these nano-link structures and the usual bulk materials or isolated molecules may be that they comprise open nonequilibrium systems. This gives rise to several novel properties in the nano-link structures; the quantization of conductance is the most remarkable among them. For nanostructures forming a very narrow constricted part, such as an atomic wire connected to two electrodes, the quantization of conductance has been observed and several researches have been performed on this phenomenon [1]. The most important consequence of the quantization of conductance is that it demonstrates that the electron behaves entirly as waves and not as a particle at the constriction part. In other words, when the electron traverses across the most constricted region, the two-dimensional electron waveform in the cross-sectional area should be an eigenmode reflecting the shape of the cross section. Eigenchannels associated with each eigenmode are introduced, and the electron transmission in the channel is the same as the one-dimensional flow of a wave or wave packet, which entails the conductance value of the channel to be

  The quantization of conductance has been observed in quantum point contacts (QPCs) and nanowires (atomic wires) of Al, Au, and other metals. The quantization of conductance is manifested as spiky peaks in the histogram of the conductance values at integer multiples of the conductance quantum unit G0. Here the histogram is constructed from the temporal change in the conductance values during a number of thinning processes for nanowires. One of nontrivial features of the nanowires is that depending on the kind of metal or the axis direction, peaks in the histogram are missing at certain integer values of the conductance (in units of G0). In some cases, the peaks in the conductance do not appear clearly. Moreover, it is not clear whether the peaks in the histogram of the conductance should be attributed to the stability of the wire structure rather than to the quantization of conductance.

Fig. 1. Histogram of conductance for [110] Au nanowires (from [2])

  To clarify these issues, a detailed direct correlation between the conductance and the atomic wire structure should be performed; however, this has been experimentally difficult so far. The study reported by Kurui, Oshima, and Takayanagi [2] has made a significant contribution to the clarification of these issues by finding a clear direct relationship between the structure and the conductance of [110] Au nanowires by making use of the simultaneous observation of TEM images and conductance in a TEM-STM apparatus. Figure 1 shows the histogram of the conductance of [110] Au nanowires reported in [2]. The shape of the hexagonal cross section of the [110] Au nanowires are defined as (n, l, m) where n, l, and m are the number of (1,1,1), (1,1,1), and (0,0,1) lattice planes in the wire, respectively. The authors have observed the histogram of the conductance for each subset of [110] Au nanowires with fixed values of n, l, or m, which are clearly defined in the TEM images. Dividing the entire histogram into the partial set of nanowires, the nanowire structures corresponding to the respective peaks in the entire histogram can be almost assigned. Such analyses based on TEM-STM observations have established the one-to-one correlation between the conductance and the nanowire structure. This enables the determination of structures causing any specific peaks in the conductance.

  Moreover, the simultaneous observation of the time development of the TEM image and the conductance values has provided interesting information on the relationship between the temporal change in the channel conduction and the wire structure. Thinning processes can be observed in the layer images obtained from TEM, and at the same time, the stepwise decrease in the conductance has been measured. It has been found that once an atomic layer becomes incomplete from one edge, even though a major part of the incomplete layer remains intact, the conductance value suddenly decreases by almost one unit. The disappearance of the conduction channel is directly correlated to the reduction in the number of Au atomic layers. The number of the Au atomic row, however, does not necessarily coincide with the number of channels. This is a very significant observation leading to a complete understanding of quantum electron transport in nanowires: the one-by-one evolution of conductance channel causes an increase in the conductance by an amount G0.

References
[1] N. Agraït, A. L. Yeyati, and J. M. van Ruitenbeek: Phys. Rep. 377 (2003) 81.
[2] Y. Kurui, Y. Oshima, and K. Takayanagi: J. Phys. Soc. Jpn. 76 (2007) 123601.

The above article should be referred as “M. Tsukada: JPSJ Online-News and Comments [Dec. 10, 2007]” when citing.

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