When hit with light, semiconductors (materials that have an electrical resistance in between that of metals and insulators) generate an electric current. Semiconductors that consist of one layer or a few layers of atoms — for example, graphene, which has a single layer of carbon atoms — are of particular interest for next-generation optoelectronics because of their sensitivity to light, which can controllably alter their electrical conductivity and mechanical flexibility. But the amount of light that atomically thin semiconductors can absorb is limited, thus limiting the materials’ response to light.
To enhance the light-harvesting properties of these two-dimensional (2D) materials, tiny (10-50 atoms in diameter) semiconducting particles called quantum dots are added in the layer(s). The resulting “hybrid” nano-materials not only absorb more light, but also have interactions occurring at the interface where the two components meet. Depending on their size and composition, the light-excited quantum dots will transfer either charge or energy to the 2D material. Knowing how these two processes influence the photocurrent response of the hybrid material under different optical and electrical conditions — such as the intensity of the incoming light and applied voltage — is important to designing optoelectronic devices with properties tailored for particular applications.
An optoelectronic imaging technique was developed to study the electronic behavior of atomically thin nanomaterials exposed to light. Combined with nanoscale optical imaging, this scanning photocurrent microscopy technique provides a tool for understanding the processes affecting the generation of electrical current (photocurrent) in these materials. Such an understanding is key to improving the performance of solar cells, optical sensors, light-emitting diodes (LEDs), and other optoelectronics — electronic devices that rely on light-matter interactions to convert light into electrical signals or vice versa.
In this work, atomically thin molybdenum disulfide was combined with quantum dots. Molybdenum disulfide is one of the transition-metal dichalco-genides — semiconducting compounds with a transition-metal (in this case, molybdenum) layer sandwiched between two thin layers of a chalcogen element (in this case, sulfur). To control the interfacial interactions, two kinds of quantum dots were designed: one with a composition that favors charge transfer, and the other with a composition that favors energy transfer.
Devices were then made with the hybrid nanomaterials. To characterize the performance of these devices, scanning photocurrent microscopy studies with an optical microscope were conducted. In scanning photocurrent microscopy, a laser beam is scanned across the device while the photocurrent is measured at different points. All of these points are combined to produce an electrical current “map.” Because charge and energy transfer have distinct electrical signatures, scientists can use this technique to determine which process is behind the observed photocurrent response. The photocurrent response was highest at low light exposure for the core-only hybrid device (charge transfer) and at high light exposure for the core-shell hybrid device (energy transfer). These results suggest that charge transfer is extremely beneficial to the device functioning as a photodetector, and energy transfer is preferred for photovoltaic applications.
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