Colloidal semiconductor quantum dots (QDs) are used in a wide variety of applications, some already technologically mature such as displays and fluorescent labels. One of the areas in which QDs are particularly promising is solar energy harvesting. The efficiency of solar cell devices is bound by the Shockley–Queisser limit, which in part accounts for the loss of energy in excess of the bandgap in the form of heat before carrier extraction. Therefore, one possibility of achieving high performance solar cells is extracting high energy carriers before their excess energy is dissipated. Experimental observation of the relaxation dynamics of high energy excitons and hot excitons in different QD systems is important to provide fundamental understanding of such processes.

Over the years femtosecond transient absorption spectroscopy (TA) has been widely applied to gain insight on QDs. However, TA is not able to provide high time and excitation energy resolution simultaneously, which can be achieved using two-dimensional electronic spectroscopy (2DES). 2DES measures third-order non-linear pathways like TA, but makes use of a second pump pulse, so that each field matter interaction takes place with a different pulse. In the time interval between the two pumps these pathways show oscillatory behaviour matching the original excitation frequency, so that Fourier transform spectroscopy can be used to recover different excitation frequencies within the spectrum of the ultrashort pulse. As a result, a correlation map between signal excitation and signal emission frequencies is obtained for each TA waiting time.

The advantage of retaining high time and excitation frequency resolution can be crucial for the study of carrier thermalization processes, where different excess energies can lead to different thermalization timescales. Further, homogeneous and inhomogeneous broadening contributions can be both read from the overall 2DES lineshape, which also allows the study of spectral diffusion. Finally, couplings between different excitons appear as off-diagonal peaks, a feature that can even be used to reveal dark excitons coupled to bright ones. Because of these advantages, 2DES was already applied to study many QD systems.

Going beyond QDs, nanostructured materials such as nanorods are attracting significant attention. The geometrical differences between QDs and nanorods imply different quantum confinement and consequently different electronic properties, and nanorods are ideal building blocks for hybrid nanostructures. Applications of such systems were demonstrated over the past decade, and 2DES has potential to provide experimental evidence to the understanding of the hybrid electronic structures.

Here we report a 2DES study on CdSe nanorods that are 25.1 nm long and have a diameter of 8.4 nm in water, laying the foundation for future studies of hybrid CdSe nanorods with different materials. 2DES reveals the presence of seven different excitons between 530 and 730 nm, with the lowest energetic bright one being at 665 nm. The 2DES maps at early times include positive signal (ground state bleach and stimulated emission) at and below the diagonal, while the region above the diagonal is dominated by a negative signal (excited state absorption). The evolution of the 2DES signal tracks the exciton relaxation process revealing that the high energy excitons go through the excitons at intermediate energies on a sub-100 fs timescale before, eventually relaxing to 665 nm.