A research team led by Prof. Hilmi Volkan Demir has recently published an article in Nano Letters (vol. 19, issue 7, pp. 4297-4305, DOI: 10.1021/acs.nanolett.9b00681), in which orientation-dependent near-field interactions with colloidal quantum wells was achieved for the first time. The first author of the work, Onur Erdem, developed a self-assembly technique to deposit a single layer of colloidal quantum wells in a single desired orientation of choice: either face-up or edge-up. The resulting films are used to study the near-field nonradiative interactions between these quantum wells and another class of nanocrystals. The team showed that the strength of this interaction is greatly influenced by the orientation of the quantum wells.
Colloidal quantum wells are the latest class of size-tunable semiconductor nanocrystals. Compared to the former classes of nanocrystals, these quasi two-dimensional semiconductor structures exhibit a much stronger capability to absorb light, which is a desirable property for many optical and optoelectronic applications utilizing nanocrystals. However, due to their shape, their properties are direction-dependent, which are yet to be fully understood. To comprehend their direction-dependent properties, their orientation in solid films must be controlled, which has been challenging so far. To this end, the research team proposed and developed a self-assembly technique, which enabled them not only to control the orientation of these quantum wells, but also to deposit them in a single layer with full surface coverage over areas as large as ~20 cm2. With the help of these thin films, they studied the phenomenon of nonradiative energy transfer from another class of nanocrystals, namely spherical quantum dots, to these oriented quantum wells. Through their systematic study, they found out that the energy transfer is significantly stronger to the vertically-oriented quantum well monolayers compared to that to flat-lying ones. They also developed a theoretical model, which successfully estimates the rate of energy transfer to the quantum wells in both orientations.
The study of Prof. Demir and his research team is the first account of orientation-dependent energy transfer with colloidal quantum wells. The team states that this study reveals important conclusions about orientation-dependent properties of the colloidal quantum wells. Furthermore, their large-area self-assembly technique could also help facilitating the incorporation of these quantum wells in large-scale devices and applications.
(Artwork: Mete Duman, UNAM)