Researchers from the University of Amsterdam, in collaboration with their Japanese partners, have determined directly the relation between the band gap energy of single cesium lead bromide nanocrystals and their size and shape. By studying individual nanocrystals being either isolated or surrounded by neighbors, they explicitly visualized for the first time the 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 these crystals is dramatically different from that of large materials. In fact, the band gap energy, the amount of energy needed to excite electrons and make electrical conductivity possible, depends on the nanocrystal size.
The term perovskites, named after the Russian mineralogist Lev Perovski, refers to a class of materials with a specific crystal structure. Recently, perovskites have attracted much attention due to their potential for high-efficient and low-cost photovoltaics – a technique used in solar cells. In the cesium lead bromide nanocrystals, the advantages of perovskites and nanocrystals are combined, and they are therefore a promising material for various applications in optoelectronics, the construction of electronic devices that interact with light.
The state-of-the-art technique the researchers employed, is called low-loss electron energy loss spectroscopy, or EELS. Using EELS together with a transmission electron microscope with ultra high spatial resolution, allows the researchers to measure in parallel the nanocrystal’s dimensions and location with uniquely high precision. As a result, the energy absorption is directly mapped onto individual nanocrystals that are either embedded in an ensemble (i.e. that have neighbors) or that are completely isolated. In this way, an intimate relation between the nanocrystal’s size, shape and energy band gap is established.
Interaction and coupling between nearby nanocrystals
By determining the energy band gap of many individual nanocrystals as a function of their size, the researchers have found that small isolated nanocrystals appear to have a higher band gap energy than nanoocrystals of the same size surrounded by neighbors. Reversely, a large nanocrystal has lower band gap energy if isolated than when embedded in an ensemble. This result shows that two adjacent nanocrystals do not simply ‘merge’ upon interaction to pose as a larger crystal, but rather ‘average’ their band gaps. This provides direct evidence of an effective coupling between nanocrystals, where their energy band gap and therefore energy structure, is influenced by the neighbors.
These unique insights in the interacting behavior of neighboring nanocrystals pave the way towards purposeful designing of large quantum structures and quantum-dot-solids, using nanocrystals with selective properties as their building blocks.