New devices and new physical effects with atomically thin stacks

 

Materials with layered structures can be peeled apart so that 2D crystals with thicknesses of one or a few atoms can be isolated and studied. We use techniques that take advantage of the weak out-of-plane bonds in these materials to pick up and stack them into vertical heterostructures. Hundreds of layered materials are known, but modifications of only the three most commonly studied of them— graphene, black phosphorus, and transition metal dichalcogenides such as MoS2—continuously span the range of band gaps between 0 and 2 eV.  These metals and semiconductors, combined with the large band gap insulator hexagonal boron nitride, provide the ingredients to make a wide variety of electronic and optoelectronic devices.  These flexible and nearly transparent materials also have interesting physical properties including extremely strong electron-electron interactions and a valley pseudospin degree of freedom.

 

 
2D materials, such as transition metal dichalcogenides, graphene, hexagonal boron nitride, and many others can be stacked together to form heterostructures with new physical properties.

2D materials, such as transition metal dichalcogenides, graphene, hexagonal boron nitride, and many others can be stacked together to form heterostructures with new physical properties.

Thin, transparent, and direct gap materials for optoelectronics

 

The properties of transition metal dichalcogenides and other layered materials create interesting opportunities to fabricate optoelectronic devices.  First, several transition metal dichalcogenides and black phosphorous have direct semiconducting band gap in one- to few-layer thicknesses, which leads to efficient light absorption and emission.  Second, being atomically thin makes these materials flexible, nearly transparent, and able to efficiently emit and absorb light into and from free space and other 2D or bulk materials.

We plan to investigate conventional optoelectronic devices with unconventional properites made from 2D materials, such as LEDs and photovoltaic devices, as well as new kinds of devices that rely on the two-dimensional character of these materials for their operation.  

 

Behavior of electrons at low temperatures and in low dimensions

 

Nanostructures such as quantum point contacts and quantum dots, in combination with locally applied DC-to-microwave electric and magnetic fields, have enabled exquisitely detailed probes of their host materials, from GaAs heterostructures to carbon nanotubes and many others. These nanostructures provide highly tunable control over system parameters including size, shape, lateral position, exchange and spin-orbit energies, coupling to external reservoirs, and carrier density at the level of a single charge. 

Nanoscale confinement by local electrostatic gating leverages the band gap of 2D semiconductors to create nanostructures that are difficult or impossible to fabricate in gapless graphene.  Stacking 2D materials provides a convenient method for the assembly of high-quality nanostructures based on transition metal dichalcogenides and other layered materials. 

An initial target of these experiments will be to explore the valley pseudospin degree of freedom in transition metal dichalcogenides.  The energy bands of semiconducting transition metal dichalcogenides such as MoS2 and WSe2 possess the same valley structure as graphene, with the lowest lying bands located at the vertices of the hexagonal Brillouin zone (K and Kpoints). We will explore methods to create, manipulate, and measure populations of the valley pseudospin degree of freedom (K or K) provided by this band structure. In the same way that a spintronic device operates on the spin angular momentum of electrons rather than their charge, a valleytronic device operates on the crystal momentum associated with each valley.