For two years now, our team has looked into the subtle physical and chemical properties of quantum dots with a clear aim: find the way to exploit them for the design of nanoscale biological sensors. But at the start, we did not expect these little objects to unveil so much wealth. How rich their fundamental properties are! thanks to the only spatial confinement of their charge carriers, the so called excitons…

Exciting, that is the word. That is how we had to establish a strategy for characterizing these inorganic/organic hybrid systems, in spite of their small size (around 4 nm) and the low amount of organic matter (their ligands).

We started working with cadmium telluride quantum dots. These are known to be easily functionalized. For all that, the fine examination of their surface chemistry has proved to be very hard a job. Obviously, it is not accessible by the common optical techniques like UV-visible absorption and fluorescence spectroscopies since they are only sensitive to the electronic, that is excitonic, behaviour of the semiconductor nanocristals. This is when we came up with the idea of using two-colour nonlinear optical sum-frequency generation spectroscopy (2C-SFG). This surface-specific technique allows the probing of interfaces at different scales, from atoms to proteins, with the possibility to continuously change the  colour of the incident visible light and to combine it with an infrared excitation. Thanks to these two tunable light beams (visible and infrared), the samples experience both electronic and vibrational stimulations. It is unfortunate that 2C-SFG has largely been unexploited so far, whereas it enables us to probe all kind of vibroelectronic coupling within nanoscale hybrid interfaces, whether these are metals or semiconductors.

Thence, we probed the quantum dots with all the rainbow colours, from violet to red, as provided by our unique setup, available in the Laboratoire de Chimie Physique (Orsay, France). What a joy it was when we observed that the vibrational spectrum of the ligands was closely correlated to the optical response function of the quantum dots, so excited in the visible! From a metaphorical point of view, the molecules begin to dance in tune as soon as the nanocristals shed their light on them. In this way, it is true to say that molecules wake up at dawn.

This energy transfer from excitons to vibrations can be explained by dipolar coupling: as an oscillating dipole, each exciton locally creates an electric field in its direct surroundings, and thus influences the molecular polarizability of the ligands. This is what we evidenced from an experimental point of view and described theoretically thanks to an analytical modelling based on dielectrics and nonlinear optics. The whole study is reported in the following article: https://www.nature.com/articles/s42004-018-0079-y.

Written by Christophe Humbert and Thomas Noblet, who invite you to discover their setup within CLIO (Infrared Laser Center of Orsay), Laboratoire de Chimie Physique (France):