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Two-Dimensional Materials Make “Valleytronics” Possible


The latest cutting-edge electronics research increasingly depends on 2-D materials


The newest buzzword in semiconductor research is “Valleytronics”. Like the term “Spintronics” before it, Valleytronics takes the behavior of electrons to make a portmanteau that plays off the word “Electronics”.


While spintronics is based on the intrinsic quantum spin of electrons, valleytronics is even a bit more obscure and requires a bit of definition. 


In its simplest explanation, valleytronics represents an abandonment of exploiting the electrical charge of electrons as a means for storing information and instead uses the wave quantum number of an electron in a crystalline material to encode data. Not any clearer? Hold on, it gets even more complicated.


The “valley” in valleytronics is derived from the shape of the graph you get when you plot the energy of electrons relative to their momentum. This creates a curve that features two valleys. Electrons move through the lattice of a 2-D semiconductor as a wave populating these two valleys, with each valley being characterized by a distinct momentum and quantum valley number.


Manipulating these two valleys so that one is deeper than the other yields a way for the electrons to populate one of the two valleys. The positions into which electrons fall can be used to represent the zeroes and ones in digital computing.


Okay, it’s complicated. But in the end, the aim of valleytronics, like spintronics, is to find a faster way of encoding information than moving electrical charges around at high switching rates, which is the basis of electronics.


Valleytronics is only a few years old, but until quite recently the mainstay material for achieving the effect has been diamonds. Last year, we saw the introduction of a new material called rhenium disulfide that was in fact a 3-D material but behaved like a 2-D material.


One of the applications cited for the material, besides photovoltaics, was valleytronics. Since then, the amount of research employing 2-D materials has been gradually increasing. 


Tungsten Diselenide Plays Role in Valleytronics




This year we have again seen researchers at the U.S Department of Energy (DOE) Berkeley Lab, where the rhenium disulfide research was conducted, use the 2-D material known as tungsten diselenide in combination with a phenomenon known as the “optical Stark effect” to selectively control photo-excited electrons/hole pairs—excitons—in different energy valleys. This work is seen as a new pathway to achieving valleytronics.


“This is the first demonstration of the important role the optical Stark effect can play in valleytronics,” said Feng Wang, a condensed matter physicist with Berkeley Lab’s Materials Sciences Division, in a press release. “Our technique, which is based on the use of circularly polarized femtosecond light pulses to selectively control the valley degree of freedom, opens up the possibility of ultrafast manipulation of valley excitons for quantum information applications.”


Tungsten Disulfide Throws Its Hat in the Ring


Late last year, researchers at MIT used tungsten disulfide —which belongs to a class of 2-D crystals known as transition metal dichalcogenides—as another 2-D material for valleytronics. 



What the MIT researchers were attempting to address was how you can get the electrons to populate one valley more than the other to represent the ones and zeroes for digital logic.


Electrons naturally want to settle into the lowest energy value and that could be in either of the two valleys. So, you need to find a way to generate a difference in the energies of the two electron valleys to get that digital logic. 


The idea has been that you would need a very powerful magnetic field to get even the most miniscule change. 


The MIT solution found that with metal dichalcogenides they could directly control the valley by using light.


“Being able to manipulate the valley degree of freedom in two-dimensional transition metal dichalcogenides would enable their application in the field of valleytronics,” said David Hsieh, an assistant professor of physics at Caltech, who was not connected to this research, in a press release. “This experiment makes a large step toward realizing this goal by demonstrating a method to control the energy difference between two valleys in tungsten disulfide for the first time.”


Whether valleytronics will eventually be a practical successor to electronics is a long way from being decided. But it’s clear that if valleytronics is stand any chance in that regard, it’s going to depend heavily to 2-D materials.