Exciton diffusion of directed assembly of perovskite nanocrystals on patterned surfaces

Colloidal nanomaterials display a broad range of unique chemical and physical properties that make them prime candidates as nanoscale building blocks for the development of future technologies. Towards this goal, one of the... [ view full abstract ]

Colloidal nanomaterials display a broad range of unique chemical and physical properties that make them prime candidates as nanoscale building blocks for the development of future technologies. Towards this goal, one of the main challenges resides in developing methods to manipulate these materials with a level of precision comparable with their small size; it requires a transition from solution phase to solid state. During this transition the system is brought from the random and dynamic spatial distribution to an ordered, static assembly. In this work we study the effects of topography and surface chemistry on the exciton diffusion properties of perovskite nanocrystals (PNCs). The PNCs subjects of this study are cubic in shape with side length of about 10 nm. They are surrounded by organic ligands terminating with apolar groups. The PNCs solution is deposited by spin-coating on silicon substrates. If the nanocrystals are arranged in a monolayer, we can excite them with a focused laser beam. These excited nanocrystals will fluoresce and we can record the intensity profile in the far field which will have approximately a gaussian shape. If the particles are close enough, the excitons can hop from one nanocrystal to the next in this 2d network of nanocrystals. The result of this process will be an expansion of excited state ensemble. Comparing PL intensity profiles, one without exciton diffusion and one with excition diffusion we can obtain a direct measurement of the average exciton diffusion length in this system (figure 1).

The PNCs solution is also deposited on substrates that are patterned with narrow trenches and which surface is chemically functionalized to change the degree of hydrophobicity. In Figure 2, trenches between 20 and 50 nm successfully confine PNCs into ordered lines as narrow as one or two PNCs and extending for hundreds of nanometers without gaps or defects. Arranging PNCs in such ordered, 1D-like features allows careful studies of the collective mechanisms of exciton diffusion and recombination in PNCs assemblies (figure 3), which in turn determine the optoelectronic behavior of the system and offer fundamental guidance in engineering new optoelectronic devices made of PNCs.