Valleytronics in a monolayer semiconductor at room temperature
(Nanowerk Spotlight) In today's electronics, semiconductors are essential to the operation of electronic circuits by carrying along the electrical charge of electrons so that bits of information – 1s and 0s – can be encoded by the presence or absence of electric charge. The use of charge, however, requires physically moving electrons from one point to another, which can consume a great deal of energy, particularly in computing applications.
Researchers are therefore searching for ways to harness other properties of electrons as data carriers – without having to physically moving the electrons – in the hope that this will lead to devices that consume much less power.
This has led to the development of spintronics, which exploits the magnetic spin properties of electrons to encode information.
In addition to manipulating the charge and spin of electrons, a third way to control electric current is by using the 'valley' degree of freedom of electrons. This novel concept is based on utilizing the wave quantum number of an electron in a crystalline material. Simply put, 'valleys' are maxima and minima of electron energies in a crystalline solid.
The basic idea of valleytronics is to pass information through two-dimensional (2D) and other very thin conducting materials using the energy valleys – or energy extrema – in their conduction and valence bands (the energy bands around which electrons orbit an atom's nucleus). Information can be transmitted by controlling an electron's association with a valley – a manipulation that can be achieved using electric fields, magnetic fields and circularly polarized light.
Valleytronics is very attractive for future electronic devices and quantum computing technology since its quantum information storage significantly surpasses existing charge or spin control technologies.
"Application of valley degree of freedom in optoelectronic devices requires the capability to access and manipulate the valley behaviors," Dr. Zilong Wu, a researcher in the Zheng Research Group at University of Texas at Austin, explains to Nanowerk. "The optical selection rule – i.e., selective coupling to photons with σ+ and σ- circular polarization at Κ and Κ' valleys, respectively – in monolayer semiconductors enables the direct addressing of a specific valley by excitation using circularly polarized lasers at cryogenic temperature."
"However" he adds, "the phonon-assisted intervalley scattering accelerates dramatically when temperature is increased, resulting in volatile valley states and significantly reduced handedness of far-field photoluminescence at room temperature."
Valley–optical cavity hybrid systems have recently shown room-temperature far-field photoluminescence of maintained handedness, which would benefit the application of valley degree of freedom. However, modulation of valley behaviors at room temperature is still limited by the requirement of precise spatial and spectral overlap between excitons and optical cavities in current approaches.
"Therefore, we have been looking for a way to solve the above challenges to enable more versatile manipulation of valley excitons," says Wu. "Our findings show that large room-temperature valley modulation can be achieved outside of strong coupling regime with more flexibility."
Specifically, the researchers demonstrated a room-temperature approach to manipulating quantum-information carriers – i.e. pairs of positive and negative charges confined at momentum valleys – which is usually volatile at room temperature, in a monolayer WSe2 semiconductor.
Schematics of modulating valley dynamics in monolayer WSe2 using chiral Purcell effects in moiré chiral metamaterials (MCMs). a) Schematic of the WSe2–MCM hybrid system. The monolayer WSe2 is represented in reciprocal space, illustrating the band structure and the optical selection rule. b) Cross-sectional schematic showing the design of the WSe2–MCM hybrid structures. c) Illustration of spin-dependent valley dynamics of monolayer WSe2. (Reprinted with permission by Wiley-VCH Verlag (click on image to enlarge)
The manipulation is enabled by the strong light-matter interactions between the quantum carriers, also known as valley excitons, and a purpose-designed plasmonic chiral metamaterial. In a previous Nanowerk Spotlight ("A new type of ultra-thin plasmonic chiral metamaterial") we have reported on the team's design work with this kind of metamaterials.
In their initial demonstration, the team was further able to actively and reversibly tune and turn ON/OFF the manipulation using cost-effective silk fibroin from Bombyx mori cocoons. These results provide a new way to control quantum information carriers in 2D materials.
Monolayer semiconductors, particularly transition metal dichalcogenides (i.e. MoS2, WSe2, WS2, etc.), feature rich valley-contrasting phenomena at Κ and Κ' points in the Brillouin zone.
"The resulting valley degree of freedom in monolayer semiconductors is promising for information storage and processing," Wu points out. "Valley excitons have also been applied for quantum computing recently. Our work offers a novel strategy to versatilely control valley excitons in monolayer semiconductors, benefiting applications in ultrathin valleytronic and optoelectronic devices."
In their paper, the team first develops a theoretical model to predict and understand the chiral Purcell effects in this plasmonic moiré chiral metamaterial, and then confirms it with experiments to show the ability to selectively control the valley dynamics (i.e. achieving active modulation of valley states on both spectral shift (∼24 nm) and ON/OFF) in an ultrathin metamaterial (∼120 nm in total thickness).
They emphasize that this model can be generalized to other metamaterial systems. It could inspire innovative approaches to address and modulate valley excitons in monolayer semiconductors for valleytronic and optoelectronic applications such as valley-polarized lasers and electrically excited valleytronic devices.
"Our findings may offer a route to solve the lack of active tunability in current valley–optical cavity hybrid systems," Wu concludes. "Active tunability of valley modulation in monolayer semiconductors through electric or optical means will enable ultrafast switching and control at room temperature."