Fig 1. : Schematic illustration thermoelectric effects at molecular junction

(Figure taken from https://doi.org/10.1002/adfm.201904534)

It has been reported in recent studies that almost two third of the energy generated by conventional power stations is lost as waste heat. What if we can reuse the wasted heat and convert it into usable electricity? Wouldn’t that be great? Here comes the idea of Thermoelectricity. During the past few decades as a measure against global warming, significance of recovering waste heat and converting it to electrical energy has been re-recognized as a major challenge to both science and technology while addressing the global energy crisis. Though there are various methods to recycle waste heat, much attention is being paid to ‘Thermo-Electric (TE) Energy conversion’ because of the ‘Green’ nature of conversion process (i.e., harvesting power from waste heat) and easy device maintenance due to absence of moving mechanical parts, making it technologically intriguing. Thermoelectric devices can convert heat energy directly into usable electrical energy without having any heavy moving parts like turbines, just by exploiting a temperature gradient across them.

Seebeck and Peltier Effects

Fig.2: Schematic Illustration of Seebeck effect where thermal gradient induces a voltage drop at the two junctions.

First thermoelectric effect, also known as Seebeck effect, was discovered way back in 1822 by Seebeck, where a voltage drop is induced across the junctions of two different metals kept at different temperatures. Thus, heat energy is being converted into electrical energy in this process. If  is the amount of heat generated due to  temperature difference, the Seebeck co-efficient is defined by the their ratio, i.e.,S=

Twelve years later in 1834, Peltier observed that the temperature changes at the junctions of two different metals when an electric current is passed through it. If  amount of heat is generated or absorbed due to current  flowing through a metallic branch, then these two factors are related as , where ᴨ being the Peltier co-efficient.

Fig.3.: Schematic illustration of Peltier effect where thermal gradient is being developed due to current flowing through a circuit.

Soon after this discovery in 1838 Lenz performed a simple experiment which had a great impact on technological applications of thermo-electric effect. He put a water droplet at each of the two junctions of a bismuth-antimony closed loop and passed an electric current through the system. One water droplet then freezes into ice while the other remains in the form of water. By reversing the direction of the current flowing through the loop, the ice at one junction melts into water and the water droplet at the other junction freezes into ice. This experiment indicated that the thermo-electric effect can be used for both power generation and refrigeration.

Fig.4 : Schematic diagram of a thermo-electric power generator.

Now, if a thermoelectric material is connected to two different heat baths maintaining their temperature at T_h (temperature of the hot junction) and T_c (temperature of the cold junction), then from thermodynamic argument it can be shown that the efficiency of thermo-electric power generation can be shown to be dependent on a quantity named Figure of Merit (ZT) . T is the equilibrium temperature and defined as T = (T_c + T_h ) / 2.

 What is Figure of Merit?

The parameter ZT (a dimensionless quantity) is called the Figure of Merit (FOM), which plays the crucial role in determining the quality of the thermoelectric material used. If the value of ZT is increased, the thermoelectric efficiency approaches the ideal Carnot cycle efficiency. It is defined as, ZT = GS²T/k. Here,  and k are the electrical and thermal conductances of the material respectively.  being the Seebeck co-efficient or thermopower of the thermoelectric material used. The thermal conductance includes both electronic and phononic contributions.

Evolution since inception:

In the beginning metals were obviously the first choice before the widespread usage of semi-conductors for applying these ideas in technology. But for metals the value of  was found to be much less than unity for all temperatures. So, then the attention got shifted to bulk semiconductors.

Lots of thermo-electric materials have been studied till date (including Bi₂Te_3, Mn-Si, Bi-Sb-Te-Se, etc.). But in most cases  at room temperature. For bulk samples it has not been possible to increase  much, which was also the theoretical prediction. The fact is that  can be increased for a bulk thermo-electric materials by changing the carrier concentration by doping, but that still has a limitation as G and S changes in opposite senses as carrier concentration gets changed in case of bulk materials. Therefore, to increase  low thermal conductance and at the same time enhanced and G is preferred. To obtain low thermal conductance quantum confinement is an way out. So, people started to think about using quantum wires and molecular junctions which are even smaller in sizes than regular quantum wires as prospective thermoelectric materials.

Molecules as efficient functional Thermoelectric elements:

While discussing the concept of thermo power in nano-scale junctions the main focus is given on organic molecules or self-assembled mono layers trapped between two macroscopic electrodes, but one can also consider a quantum dot (QD), nanotube, different non-trivial topological systems etc., in place of the molecular systems. Such low-dimensional nano-structured systems can have crucial significance as a room-temperature thermoelectric.

The availability, low-dimensionality and low thermal conductivity make the molecular systems a natural choice for next generation thermoelectric materials with enhanced . As molecules are automatically nano-structured and their electrical conductance ( ) and Seebeck co-efficient  can be tuned externally by applying a gate voltage or with other parameters, so they can be chosen as potential candidates for efficient and high power thermo-electric devices.

Very few experiments have been performed so far on molecular junctions for measuring thermopower. Among them in 2007, an article in Science told that the Seebeck co-efficient and the conductance can be increased simultaneously by applying a gate voltage i.e., by shifting equilibrium Fermi level  closer to the resonance peak. It strongly suggests that much higher figure of merit can be achieved in molecular systems.

Thermoelectricity and DNA

Naturally, the next challenge is to find which molecule and/or the material junction leads to the best and experimentally achievable thermoelectric properties.

DNA, the basic building block of our genetic code may exhibit large potential for application in nanotechnology. Proliferation of DNA sequencing may have a deep impact on clinical medicine, health care and criminal research. Therefore attention is also being paid for non-invasive detection of nucleotides along DNA strands, apart from the conventional Sanger sequencing method. Measuring transverse tunnel currents through a single stranded DNA as it translocates through a nanopore has been proposed as a suitable physical method for single base resolution, and it has also been shown experimentally that the four bases provide a distinguishable transverse electronic feature when measured with a Scanning Tunneling Microscope (STM) which directly detects the molecular levels of single DNA bases.

In 2011, a remarkable experiment was reported again in Science, where spin selective transmission were studied through self-assembled monolayers of double stranded DNA which provided high degree of spin polarization. Immediately after that, it was theoretically explained that in presence of spin-orbit coupling, dephasing and helical symmetry this kind of topology is quite capable of producing spontaneous spin polarization even in a two-terminal system, which makes this discovery an absolute breakthrough as it encompasses several possibilities of designing higher  functional elements using artificially synthesized DNA molecules which is the future of molecular electronics.

Let me end with a Quote by Nikola Tesla “What One Man Calls God, Another Calls the Laws of Physics”. So the journey of a physicist is through an infinite path of truth, to be one step closer to nature.