Phonon thermal transport

Heat is all around us. Unsurprisingly, throughout history, the ability to control heat has been pivotal to human progress. Learning to use fire for cooking and heating is a great example of this. In the modern world, understanding and manipulating thermal transport continues to be critical to technological development.

Devices have gotten smaller and we can now alter materials at the nanoscale. It follows that heat transport also needs to be understood at the atomistic scale if we want to be able to predict and control heat to improve existing technologies and invent new ones.

One area of interest in our group is phononics — this is an emerging field concerning the nature of lattice vibrations, or phonons, in conventional and artificially structured materials. The command of light and electricity we have acquired since the late 19th century has ushered in numerous new technologies. Can we learn to control heat as we have electricity and light, and use it to encode, transmit, and process information? What fascinating new technologies would this yield?

 

 

Gif: Wave packet propagating through off-set pores in perfectly crystalline Si (classical molecular dynamics)

Porous coordination compounds

Porous coordination compounds tend to be electrical insulators, and applications requiring electrical conductivity have thus been largely overlooked. In our group, we are interested in investigating the chemical, electrical, and thermal properties of porous coordination compounds. In particular, we aim to establish guidelines on how to design efficient electrically conducting porous coordination compounds that can lead to the development of smart materials (e.g., wearable devices, biomedical implants, portable electronics), and for energy harvesting applications, such as thermoelectrics — materials and devices that convert (often otherwise wasted) heat into electricity!

Porous coordination compounds, such as metal–organic frameworks (MOFs) or covalent organic frameworks (COFs), are very porous net-like frameworks of molecules held together by strong chemical bonds. Their porosity makes them promising for gas sorption and storage applications, such as carbon sequestration and hydrogen storage (e.g., to fuel a car). Most remarkably, these compounds are inherently modular and there can be innumerable combinations of molecular linkers, nodes (the vertices in the framework), and framework topologies. Linkers and nodes are somewhat like Legos pieces that come in many shapes and colors, and can be assembled in many different ways.

Figure: MIL-53 is a flexible metal–organic framework. The nodes or secondary building units (SBUs) and the linkers can be of different types and can be assembled into numerous topologies. The choice of linker, node and topology defines the properties of the framework and therefore its possible applications.

Nanoscale ‘mechanical’ functionality

Isomerizing molecules respond to stimuli by changing shape, a process known as isomerization. Light, heat, pressure, changes in pH, mechanical forces, or electric currents are all mechanisms that can cause isomerization. Isomers are idealized as molecular constituents for artificial molecular machines, such as molecular motors, or switches. They have also been incorporated as linkers or hosts in metal–organic frameworks to increase their functionality.

Whether we would like to investigate isomerizing moieties in porous frameworks or their potential as molecular machine parts, we need to be able to predict both (1) the isomer transition pathways (the trajectory of the transformation) and quantum yields (how likely they are to isomerize), and (2) how the local environment and forces exerted on these molecules change their behavior.

Isomerizing molecules do not respond to their local environment in a mechanical way, and in order to predict their ‘mechanical’ shape-shifting behavior, one must probe their chemistry under perturbed dynamics conditions. Our aim is to study and learn to predict how molecular actuators work and how to further functionalize materials through the incorporation of isomerizing moieties.

Movie: Azobenze molecules simulated with the same classical potential but starting with only slightly different initial conditions describe very different trajectories and would therefore have different quantum yields and isomerizing pathways.