First-principles computational modeling of molecular thermoelectrics reviewed by Prof. Nikolic and collaborators

February 3, 2012 - The upcoming special issue of the Journal of Computational Electronics, which is devoted to Simulation of Thermal, Thermoelectric, and Electrothermal Phenomena in Nanostructures, features a mini-review article by Prof. Nikolic, postdoctoral researcher Dr. Saha and collaborators, Prof. K. S. Thygesen and Dr. T. Markussen, from the Center from Atomic-scale Materials Design (CAMD) at the Technical University of Denmark. The article discusses first-principles quantum transport theory and high-performance computational techniques for studying both electron and phonon transport in single-molecule nanojunctions as potentially highly efficient thermoelectric devices at low-temperatures.

Thermoelectrics transform temperature gradients into electric voltage and vice versa. Although a plethora of widespread applications has been envisioned, such as generation of electricity from waste heat to improve vehicle fuel efficiency or solid-state Peltier coolers for electronic circuits, their usage is presently limited by their small efficiency. Thus, careful tradeoffs are required to optimize the dimensionless figure-of-merit ZT=S2GT/K quantifying the maximum efficiency of a thermoelectric cycle conversion because ZT contains unfavorable combination of the thermopower S, average temperature T, electronic conductance G and the total thermal conductance K =Kel + Kph. The total thermal conductance has contributions from both electrons Kel and phonons Kph. The devices with ZT > 1 are regarded as good thermoelectrics, but ZT > 3 is required to compete with conventional mechanical power generation and refrigeration. 

The traditional efforts to increase ZT have been directed toward selective reduction of the lattice thermal conductance Kph,  using either complex  bulk materials or bulk nanostructured materials, while at the same time maintaining as optimal as possible electronic properties encoded in the so-called power factor S2G. However, decades of intense research along these lines have increased ZT of bulk materials only marginally

The nanoscale and low-dimensional devices offer additional degrees of freedom that can be tailored to achieve high ZT, as exemplified by the experiments demonstrating how rough silicon nanowires can act as efficient thermoelectrics (ZT~ 0.6 at T=300 K) although bulk silicon (ZT=0.01 at T=300 K) is not. Another example of nanoscale thermoelectrics has emerged from the recent experiments measuring thermopower of nanojunctions where thiol-end capped organic molecules are sandwiched between two gold electrodes held at different temperatures.

Besides discussing the technical details of the so-called nonequilibrium Green function formalism combined with density functional theory (NEGF-DFT) applied to both electronic and phononic transport in single-molecule nanojunctions, the article also shows examples of a new class of such devices where single organic molecule is attached to graphene nanoribbon (GNR) electrodes via highly transparent contacts. Such contacts makes possible for evanescent wavefunctions to be injected from the GNR electrodes into the molecular region, so that their overlap creates sharp peaks (i.e., resonances) in the transmission function which act favorably to optimizes the thermopower S. Moreover, evanescent mode transport is insensitive to the type of sufficiently short molecule sandwiched by the GNR electrodes, so that one can search for the molecule which reduces the phonon thermal conductance the most in order to furher increase ZT.

The maximum figure-of-merit ZT~ 3 predicted for these devices at very low temperatures T~10 K surpasses many known low-temperature bulk or nanoscale thermoelectrics, and it could be exploited for thermoelectric cooling of infrared sensors.

The analysis of these devices has been conducted via massively parallel codes MT-NEGF-DFT (for electrons) and GPAW (for phonons) on supercomputing clusters Chimera at the University of Delaware, TACC Ranger available through NSF XSEDE, and NIFLHEIM in Denmark. This research was supported by DOE EPSCoR and NSF in the US and FTP in Denmark.