Correlating Spatially Resolved Photoconductivity and Luminescence in Colloidal Quantum Dot Films ^†

Abstract

We investigated the optoelectronic properties of lead sulphide (PbS) quantum dot films in lithographically tailored gold nanogap electrodes. In particular, we aimed at the correlated measurement of photoconductivity and luminescence as a tool to characterize the charge dynamics from exciton generation to carrier extraction and recombination. Combining these measurements in an optical microscope with laser scanning excitation enabled, as well, the spatially resolved observation of the involved effects.

1. Introduction

Colloidal quantum dots are stable single photon emitters with high quantum efficiency and discrete emission wavelengths that can be simply tuned via their size. This, in addition to scalable synthesis schemes and room temperature processability, has led to intense interest in quantum dots as building blocks for optoelectronic applications. One prominent application field is photodetection [1], where PbS quantum dot devices have realized record performance in the visible and near infrared spectral range [2]. Here, we investigated highly miniaturized PbS photodetectors involving only a few to a few hundred quantum dots, aiming at efficient light-to-current conversion with a nanoscale footprint. Upon local excitation, we looked for the correlation of photocurrent generation and luminescence.

2. Correlation of Photoconductivity and Luminescence

We used a laser scanning microscope to locally generate excitons in small ensembles of PbS quantum dots in gold electrode gaps 15 nm to 1.5 µm wide, fabricated by electron beam lithography. Exemplary simultaneously acquired optical transmission and photocurrent images are shown in Figure 1. On this basis, we detected, in addition, the quantum dot luminescence, aiming at revealing correlations between photocurrent generation and luminescence, depending on the illumination parameters light wavelength and fluence. In addition, the lithographic sample platform allowed the specific geometric design of the gap region to include plasmonic effects to modify the absorption and emission processes.