Effect of exchange-type zero-bias anomaly on single-electron tunneling of Au nanoparticles

Abstract

Using cryogenic scanning tunnelling microscopy and scanning tunnelling spectroscopy we measured single electron tunnelling of isolated Au nanoparticles with 1.4 nm in radius. We observe that a gap ΔV ~ 2e/C (C is the capacitance of the Au particle) around zero bias in the tunnelling conductance spectrum, followed by a series of discrete single electron tunnelling peaks with voltage widths of E C ~ e/C at both negative and positive bias. Experimental data are well explained by taking into account the effect of exchange interaction of electrons on the single electron tunnelling of Au nanoparticles. A tunnelling peak near zero-bias was suppressed by the exchange-type zero-bias anomaly, which results in the gap ΔV ~ 2E C . Nanoparticles (NPs) are the subject of intense research at present, in part because they have opened up a new area of fundamental science and in part because of their longterm potential applications [1][2][3]. It is well-known that when put an individual nanoparticle (NP), isolated in an insulating barrier, between source and drain electrodes, then the NP acts as an island of electrons [4]. The tunnelling of electrons from the source through the particle to the drain can be inhibited at small bias voltages if the electrostatic energy e 2 /2C of a single excess electron on the particle is much larger than the thermal energy k B T, where C is the capacitance of the NP. Additionally, if the resistance R t of two tunnelling barriers between the NP and the two electrodes is much larger than quantum resistance R q = h/e 2 , which ensures that the wave function of an excess electron on the NP is localized there, the socalled Coulomb blockade (CB) could be observed. Since the pioneer experiment on CB of Fulton and Dolan [5], the single electron tunnelling behaviors of NP have attracted much attention [6][7][8][9][10][11][12][13]. Several groups explored unexpected phenomena of CB for ultrafine NP [14-16]. For example, it is found that the electronic wavefunctions of semiconductor quantum dots exhibit atomic-like symmetry and the semiconductor quantum dots can be treated as 'artificial atoms'. As a consequence, the separations of discrete single electron tunnelling for semiconductor quantum dots are determined by both the single electron charging energy and the discrete level spacings of the 'artificial atoms' [15]. These results indicate that the electronic properties of NP could influence itself the single electron tunnelling behavior.In this Letter, we measured single electron tunnelling of isolated Au nanoparticles with 1.4 nm in radius by using cryogenic scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS). A gap ΔV ~ 2e/C (C is the capacitance of the Au particle) around zero bias, followed by a series of discrete single electron tunnelling peaks with voltage widths of E C ~ e/C at both negative and positive bias, is observed in the tunnelling conductance dI/dV spectrum. The appearance of the gap ΔV ~ 2E C and the magnitude of conductance peaks in the differential conductance...