Electronic properties probed by scanning tunneling spectroscopy: From isolated gold nanocrystal to well-defined supracrystals

Scanning tunneling microscopy and spectroscopy at 5 K have been used to determine the electronic properties of 7-nm dodecanethiol-passivated Au nanocrystals in three different configurations: isolated nanocrystal, self-organized thin films (few nanocrystal layers), and large three-dimensional well-defined thick films (over 30 nanocrystal layers) called supracrystals. The electronic properties of both thin and thick well-ordered supracrystals are analyzed in scanning tunneling spectroscopy geometry through $dI/dV$ curves and conductance mapping at different bias voltages. The single particles exhibit a typical $dI/dV$ curve with a Coulomb gap of ∼360 meV and a Coulomb staircase. The $dI/dV$ curve of the thin supracrystals presents a Coulomb blockade feature ∼100 meV narrower in width than that of the single nanocrystal but without well-defined staircase. On the contrary, the thick supracrystals exhibit a $dI/dV$ curve showing a large Coulomb gap with a Coulomb-staircase-like structure. Generally, the conductance mapping is found to be very homogeneous for both supracrystals. Nevertheless, for some bias voltages, inhomogeneities across individual nanocrystals appear. Additionally, some of these inhomogeneities seem to be related to the supracrystal surface morphology. Finally, these slight variations in the conductance mapping across individual nanocrystals embedded in the supracrystal are discussed in terms of high degree of nanocrystal ordering, low nanocrystal size distribution, and nanocrystal crystallinity.

Figure 1

(a) TEM image of Au NCs. (b) High-resolution TEM image of Au NCs. (c) SEM image of thin (indicated by the red arrows) and thick supracrystals. Inset: SAXRD pattern of the supracrystals. (d) Tilted field-emission SEM image of thick supracrystal.

Figure 2

(a) Typical topographic STM image of large-area, thick Au supracrystals with tunneling current set point at 50 pA and bias voltage at 2.5 V. Inset: sixfold symmetry of the fast Fourier transform (FFT) pattern of the STM image. (b) Typical topographic STM image of the thin supracrystal surrounding the thick supracrystal with tunneling current set point at 200 pA and bias voltage at 0.5 V. The strips indicate that Au NCs are adsorbed on the STM tip. The numbered rectangles correspond to the regions where the average conductance curves have been deduced.

Figure 3

(a) The tunneling spectra show the Coulomb blockade and Coulomb staircase: Average $dI/dV$ curves of isolated Au NCs (curves 1 and 3) taken inside regions 1 and 3 of Fig. 2b, respectively. Curve 2 is the average $dI/dV$ curve taken inside region 2 of Fig. 2b and corresponding to the expected “$V$ shape” of HOPG. Inset: (left) schematic diagram of Coulomb blockade system where a NC is adsorbed on the STM tip and (right) the equivalent circuit of the double-barrier tunneling junction. (b) The experimental $dI/dV$ curve is fitted using the orthodox theory at different temperatures, 5 and 100 K, respectively. The parameters used in the orthodox theory are given in the inset.

Figure 4

(a) Typical 160 $\times$ 160 nm STM image of the thin supracrystal (left) on HOPG (right) at 5 K with tunneling current set point at 50 pA and bias voltage at 1 V. (b) Average $dI/dV$ of HOPG (taken inside region A of the STM image) and the thin supracrystal (taken inside region B of the STM image). (c), (d), (e) Conductance maps of the topographic STM image shown in (a) at 0, 300, and 80 mV, respectively. (f) Average $dI/dV$ taken inside region C (blue curve) and region D (red curve) of (e), respectively.

Figure 5

(a) 26 nm $\times$ 26 nm 3D STM image taken on a thin supracrystal at 5 K with tunneling current set point at 100 pA and bias voltage at 1.5 V, where the color corresponds to the conductance distribution at −40 mV bias voltage. (b) High-resolution TEM image of 7 nm Au NCs shows different nanocrystallinity labeled by 1, 2, and 3, respectively.

Figure 6

The logarithmic plot of a typical $I-V$ curve measured above a well-defined thick supracrystal at 5 K shows power-law dependence when the bias voltage is higher than 200 mV. The dashed line is guide for the eyes. Inset: corresponding linear plot of the $I-V$ curve.

Figure 7

(a) Typical 100 $\times$ 100 nm topographic STM image of the well-defined supracrystals with tunneling current set point at 130 pA and bias voltage at 2.3 V. (b) Conductance map at 800 mV bias voltage. (c) Average $dI/dV$ curves taken inside the regions indicated by the colored circles (b). (d) Average $dI/dV$ curves taken on the spots indicated by the white circles (b).