Efficient thermoelectric and thermal management devices consist of complex nanostructures. Thermal properties of materials are largely determined by low-frequency phonons and their interactions, and phonon confinement effects. Therefore, it is important to understand phonon propagation in nanomaterial systems at multiple length and temporal scales for which we need a robust mathematical model.

Modeling low frequency phonons requires an accurate knowledge of the temporal behavior of a many-body system over an extended time range (up to nano or even microseconds). Modeling atomistic processes over such an extended time range is a formidable task for conventional molecular dynamics. We have developed a mathematical technique for simulation of phonon transport in nanomaterials based on the use of causal Green’s functions (CGF). This technique is referred to as the CGFMD (Causal Green's Function Molecular Dynamics) technique. Presently, we are in the process of generalizing this technique to 2D (two-dimensional) as well as 3D systems, which will be applicable to real semiconductor devices. This technique, which may be a breakthrough in the field of atomistic scale modeling of temporal processes, seems to be ideally suited for modeling of low frequency phonons and thermal processes in nanomaterials. In this technique, we expand the Hamiltonian or the potential energy up to second order terms in atomic displacements. The power of the CGFMD technique arises from the fact that the corresponding temporal equations can be solved exactly even for quadratic terms in displacements. This is in contrast to the conventional molecular dynamics based in which only the first order terms are retained. This improves the convergence of our technique by several orders of magnitude. Further, the technique is especially suitable for representing phonons because it is the second order terms that define the phononic characteristics of the system. This model can be extended to include electromagnetic effects such as plasmon-phonon coupling that may be important for electronic nanomaterials.

We propose to apply the CGF technique for multiscale modeling of thermal processes in the new 2D material systems such as silicene, germanene, molybdenum disulphide, and graphene with lattice defects and 3D systems such as nano-diamonds.  All these materials are of strong topical interest because of their potential application to the design of efficient semiconductor devices for thermal management and energy conversion.

 

key words

Causal Green’s function; Graphene and beyond; Molybdenum Disulphide; Multiscale modeling; Nanodiamonds; Nanomaterials; Phonons; Semiconductor Devices; Thermal management and energy conversion; Two-dimensional materials;

Citizenship:  Open to U.S. citizens

Level:  Open to Postdoctoral applicants