From left: Dr Indrajit Pradeepchandra Wadgaonkar, Nanyang Assistant Professor Marco Battiato, and Mr. Cai Rui. The research on perovskite excitons was led by Nanyang Assistant Professor Marco Battiato and Professor Tze-Chien Sum (not pictured). Photo credit: M. Fadly.
Transistors are the basic building blocks for modern electronics. These tiny electrical devices can switch between allowing a current to flow or blocking it, forming the 1’s and 0’s of a digital signal. However, current flow is accompanied by the generation of heat, which is one of the major factors limiting the performance of present-day computers.
But what if there is a way to reliably create digital signals without using electrical currents? Computers that do not face the heating problem of conventional electronics may be able to process information faster and more efficiently. An emerging research field, called opto-spintronics, aims to explore this possibility.
Physicists at Nanyang Technological University (NTU), Singapore, have recently shown how opto-spintronics can be implemented using materials called perovskite nanocrystals. Using a combination of experiments and computer simulations, the researchers studied particle-like entities called excitons residing in these materials, and demonstrated that they are highly suitable for encoding information, replacing electric currents in conventional electronics.
This work was led by Professor Tze Chien Sum and Nanyang Assistant Professor Marco Battiato of NTU’s School of Physical and Mathematical Sciences, and was published in the journal Nature Communications in April 2023.
The central idea in opto-spintronics is to implement digital 1’s and 0’s using “spin states”, which represent the internal angular momentum or “spin” of microscopic particles. This requires a way to switch between 1’s and 0’s in a reliable and rapid manner, which can be accomplished by shining specially-crafted light beams on the particles.
A major difficulty, however, is that spin states have a limited lifetime. After a spin state is formed, it eventually undergoes a process called “relaxation”, through interactions with the particle’s environment. This destroys the digital information stored in the spin state.
The spin states explored by the NTU researchers occur in tiny crystals made from lead halide perovskites, materials with desirable optical properties that can be used to create ultra-efficient solar cells. When light is absorbed into a lead halide perovskite crystal, it creates a hybrid particle called an exciton, consisting of an excess electron and a missing electron (or “hole”). Previous research has found that the spin states of these excitons are robust and relatively long-lived (with lifetimes of around 20 picoseconds), making them excellent candidates for opto-spintronics.
However, the precise factors determining the lifetime of these spin states – the “spin lifetime” – remained poorly understood, until now.
Fig. 1 (Left) Excitonic band structure for lead halide perovskites. Quantum beating occurs between two “dipole states”, labelled Π_x and Π_y. Right: Experimental evidence of quantum beating obtained by Sum, Battiato, and co-workers. Figure excerpted from Cai et al., Nature Communications (2023).
Professor Sum’s experimental research group studied excitons in nanocrystal colloids made from the perovskite CsPbBr3, and observed a phenomenon called quantum beating (see Fig. 1). This revealed that the excitons form spin states with properties suitable for encoding digital information. However, when they repeated the experiment at different temperatures, they discovered that the spin lifetime varies in a manner contradicting previous theoretical predictions.
To explain this unexpected finding, Nanyang Assistant Professor Battiato and his team performed simulations that provided a detailed look into the relaxation of the spin states.
The spin lifetimes of excitons is greatly affected by vibrations of the atomic lattice, which disturb the motion of the excitons by “scattering” them. At temperatures close to absolute zero, the atomic lattice hardly vibrates, so the spin lifetimes are long, giving rise to prominent quantum beating at a fixed frequency. At temperatures close to room temperature and beyond, however, lattice vibrations frequently scatter the excitons and induce spin relaxation; hence, the spin lifetime becomes inversely proportional to the temperature, as established by previous theories and experiments.
The NTU researchers discovered an intermediate temperature range, from around 100K to 300K, where both quantum beating and scattering play an important role. In this regime, scattering can play an unexpectedly beneficial role: they can prevent, rather than induce spin relaxation, through a phenomenon known as motional narrowing. This is the reason for the unexpected temperature dependence of the spin lifetime discovered by Professor Sum’s team.
Using their improved understanding, the NTU researchers have been able to develop improved nanocrystals with longer spin lifetimes. Looking ahead, they aim to use these materials to realize deterministic control over the exciton spin states, in order to use them for encoding digital information. This would represent a major step towards a practical opto-spintronic device.