Coupling at nanoscale: resolving interaction between individual inorganic perovskite nanocrystals
Researchers from FOM and the University of Amsterdam, in collaboration with their Japanese partners, have determined directly the relation between the bandgap energy of single cesium lead bromide nanocrystals (CsPbBr3) and their size and shape. By studying individual nanocrystals being either isolated or surrounded by 'neighbors', they explicitly visualized for the first time band structure modification introduced by effective coupling between semiconductor nanocrystals upon close contact.
Nanocrystals and perovskites
Nanocrystals are extremely small, about a thousand times smaller than the width of a human hair. Due to their small size, the energy structure of the crystals is dramatically different from that of bulk material. In fact, the bandgap energy depends on the nanocrystal's size.
The term 'perovskites' refers to the class of materials with a crystal structure in the form ABX3 and are named after the Russian mineralogist Lev Perovski. Recently, perovskites attract much attention due to their potential for high-efficient and low-cost photovoltaics. In CsPbBr3 nanocrystals, the advantages of perovskites and nanocrystals are combined, and they are therefore a promising material for various optoelectronic applications.
The experimental setup
The state-of-the-art technique the researchers employed, is called low-loss electron energy loss spectroscopy (EELS) and arises from low energy excitations, i.e. valence electrons. It is therefore an analogy to absorption spectroscopy. Using EELS together with a scanning electron transmission (STEM) microscope with ultrahigh special resolution, allows the researchers to measure the nanocrystals' dimensions and location with uniquely high precision, in parallel. In that way, the energy absorption is directly mapped onto individual nanocrystals that are either embedded in an ensemble (they have neighbors) or are completely isolated. In that way, an intimate relation between the nanocrystals size, shape and energy bandgap is established.
Interaction and coupling between proximal nanocrystals
By determining the energy bandgap of many individual nanocrystals as a function of their size, the researchers have found that small isolated nanocrystals appear to have a higher bandgap energy as compared to a nanocrystal of the same size surrounded by neighbors. And reversely, a large nanocrystal has lower bandgap energy if isolated than when embedded in an ensemble. Their result shows that two adjacent nanocrystals do not simply 'merge' upon interaction and pose as a larger crystal, but rather 'average' their bandgaps. This provides direct evidence of an effective coupling between nanocrystals where their energy bandgap and therefore energy structure, is influenced by the neighbors. These unique insights in the interacting behavior of neighboring nanocrystals paves the way towards purposeful designing of large quantum structures and quantum-dot-solids, consisting of nanocrystals with selective properties serving as building blocks.
Contact information
Chris de Weerd
Leyre Gomez
Reference
Direct Observation of Bandstructure Modifications in Nanocrystals of CsPbBr3 Perovskite. Nano Lett. 2016, DOI: 10.1021/acs.nanolett.6b03552