Direct measurement of the built-in potential in a nanoscale heterostructure

We combine transmission electron microscopy with electrostatic force microscopy to determine the built-in potential across individual isolated ${\text{Cu}}_2\text{S-CdS}$ heterostructured nanorods. We observe a variation of potentials for different bicomponent nanorods, ranging from 100 to 920 mV with an average of 250 mV. Nanorods of a uniform composition with no heterojunction do not show a built-in potential, as expected. The results are particularly relevant for applications of colloidal nanocrystals in optoelectronic devices such as photovoltaics.

Figure 1

(a) A schematic of the experiment. The rods are examined in the TEM on an electron transparent silicon nitride window with a thin rear coating of conducting amorphous carbon. This substrate enables the same rods to be imaged with both the TEM for heterojunction identification and electrostatic force microscopy, which requires a conducting back plane. (b) High-resolution TEM image of ${\text{CdS-Cu}}_2\text{S}$ nanorods on a carbon film substrate. In the high magnification inset, lattice planes of both materials are visible, and the two materials induce a contrast difference over the length of the rod. Note that not all rods display a heterojunction.

Figure 2

(Color online) AFM topography image (upper right) overlaid upon a TEM image of the same sample of ${\text{CdS-Cu}}_2\text{S}$ NRs. The outlines of the rods (red online) are determined by the TEM image. Each line in the AFM image was consecutively scanned for topography and EFM measurements. The topography scan data are used to determine the boundaries of the rods and the TEM data are used to determine the presence and orientation of a heterostructure for EFM analysis.

Figure 3

(Color online) Top: TEM images and opacity profiles of rods (1) and (2) which reveal a heterojunction and rod (3) which does not. Middle: EFM data (phase shift cross sections) which are asymmetric in rods (1) and (2) but not in (3). The lines through the EFM data are the results of a finite-element electrostatic calculation modeling the tip/CdS-Cu2S rod/substrate system. Bottom: in the model, the heterojunction is represented by strips held at $-V_{bi,sim}/2$ and $V_{bi,sim}/2$, and dielectric constants are set to the bulk values. For (1), $V_{bi}=400\text{mV}$ and for (2), $V_{bi}=100\text{mV}$. The symmetric rod (3) fits best to $V_{bi}=0\text{V}$ with the same dielectric constant for each side.