Coulomb Blockade and Hopping Conduction in PbSe Quantum Dots

We report for the first time on rich and tunable transport phenomena in closed-packed arrays of PbSe colloidal nanocrystals (NCs) in the form of thin films. As the interdot coupling is increased, the system evolves from an insulating regime dominated by Coulomb blockade to a semiconducting regime, where hopping conduction is the dominant transport mechanism. The observed phenomena can be interpreted using the framework established mainly in the context of transport measurements in metallic quantum dots and disordered semiconductors.

Figure 1

$I\mathrm{\text{−}}{V}_{\mathrm{bias}}$ characteristics at three representative $T$ for a PbSe-NC array vacuum annealed in situ at 373 K. The arrows show the direction of sweeping $V_{\mathrm{bias}}$ at a rate of $100\mathrm{mV}/\mathrm{s}$. The solid lines are fits to the data using Eq. (1). The upper inset shows the scaling behavior of $I$ with normalized excess voltage $v=V/V_{\mathrm{th}}-1$, where the dotted line corresponds to $\zeta =2.4$. The lower inset shows the $T$ dependence of $V_{\mathrm{th}}$ for all samples measured. Here, the solid line is a linear fit to the data.

Figure 2

$I\mathrm{\text{−}}{V}_{\mathrm{bias}}$ characteristics versus $T$ for a PbSe-NC array vacuum annealed in situ at (a) 473 and (b) 523 K. $V_{\mathrm{bias}}$ was swept at $100\mathrm{mV}/\mathrm{s}$. The solid lines in (a) are linear fits to the data. The lower insets show $G$ (in log scale) versus the inverse of $T$. Here, the solid and dashed lines are fits of Eq. (2) to the data, using $p=1/2$ and 1, respectively. The upper insets are TEM images of PbSe-NC arrays on thin amorphous carbon films, after vacuum annealing at $0.5\mathrm{K}/\min$.