Topological end state and enhanced thermoelectric performance of a supramolecular device

TL;DR

This study investigates thermoelectric performance in a supramolecular device built from Su-Schrieffer-Heeger (SSH) chains with noncovalent junctions, using the non-equilibrium Green's function (NEGF) formalism to compute transport coefficients. By tuning inter-chain hopping and exploiting topological end states, the authors identify a single-end-state configuration that yields high power factors and large $ZT$, even when phonon conductance is present, with $ZT$ peaks persisting up to values on the order of a few. The work also demonstrates a pronounced switch behavior under end-state shifts, structural perturbations, and disorder, and provides guidance on robustness and optimization (e.g., weak noncovalent coupling and symmetric end-state configuration). Overall, the results highlight a viable strategy to enhance thermoelectric efficiency in nanoscale, noncovalently connected molecular devices through topological engineering.

Abstract

Supramolecular device (SMD) with topological end states and a noncovalent junction is rarely investigated but deemed promising for thermoelectric (TE) applications. We designed a new kind of SMD based on the Su-Schrieffer-Heeger (SSH) chains, and calculated TE properties of it using the non-equilibrium Green's function (NEGF) method. By scaling TE performance under different optimization conditions, we found the best scenario. Our result shows that the existing topological end states indeed give rise to a large value of power factor, rendering a dimensionless figure-of-merit ZT above 2 in a broad range of chemical potential (doping). Moreover, by imposing the system to various perturbations including end state shift, structural change and disorder, we found that the SMD system possesses a prominent switch effect, further optimizing its performance for TE applications.

Figures (6)

• Figure 1: Supramolecular device (SMD). (a) The possible real and (b) schematic SMD made of the SSH chains. The device system consists of two electrodes with one end of SSH chains adsorbed on the electrode surface and the other end crisscross connected with each other via non-covalent connections. SSH chain is zoomed in (b) to show the topological end states. (c) Schematic of SMD module for non-equilibrium Green's function (NEGF) calculations.
• Figure 2: Structure dependent electronic properties. (a) The super SSH chain (left) and SMD (right) structures. Purple ellipsoid denotes SSH chain, and the yellow band denotes the electrode connection. (b) Transmission spectral function $\mathcal{T}$. (c) Total density of states (DOS). (d) Ratio between DOS on the end of SSH chain and total DOS. In (b-d), the $ne$, $de$ and $se$ cases are represented by red dot line, blue dash line and purple solid line, respectively.
• Figure 3: Thermoelectric properties of the SMD structure in the three end states. Chemical potential $\mu$ dependence of (a) electrical conductance $\mathcal{G}$, (b) electronic thermal conductance $K_e$, (c) Seebeck coefficient $S$, and (d) figure of merit $ZT$ value. The $ne$, $de$ and $se$ cases are respectively represented by red dot line, blue dash line and purple solid line. In (a,c), only electronic thermal conductance $K_e$ is considered. $ZT$ with external thermal conductance $K_r$ = 0 and $0.1K_0$ are given on the left and right hand side of (d), respectively. $ZT$ value as a function of external thermal conductance $K_r$ and chemical potential $\mu$ are given in the (e) $ne$, (f) $de$ and (g) $se$ cases. The maximal $ZT$ value as a function of inter SSH hopping energy $t_t$ and $t_h$ are given in the (h) $ne$, (i) $de$ and (j) $se$ cases. The optimal zone [$t_h=0.01t_0$ and $t_t=0.12t_0$] for the $se$ case is marked by white dashed square on top panel of (j) and is zoomed in on bottom panel of (j). Temperature is set at 300 K.
• Figure 4: The impact of end states shift on thermoelectric properties of SSH SMD in the $se$ case. Shift of the end states from head to tail is realized via the topological phase transition of SSH chains, i.e. by tuning the $\lambda$ of the electrode as indicated by the blue dashed arrow. From top to bottom are the $\lambda$ and $\mu$ dependent electrical conductance $\mathcal{G}$, Seebeck coefficient $S$, power factor $PF$, and $ZT$ value. From left to right, (a) represent the synchronous shift of the end states of both top and bottom electrodes, (b) the shift of only one electrode, (c) the shift of only one electrode, but the end states in another electrode anchored on the tail. Temperature at 300 K. $K_r$ = $0.1K_0$. The power factor is calculated via $PF=\mathcal{G} S^2$.
• Figure 5: The impact of end states shift on thermoelectric properties of SSH SMD in the $se$ case. The shift is realized by three means: (a) vertical displacement between top and bottom leads to tune $t_h$, (b) horizontal displacement to tune $\delta_h$, and (c) vertical displacement between tail and lead to tune $t_t$. From top to bottom are tuning parameter ($t_h$, $\delta_h$, $t_t$) and $\mu$ dependent electrical conductance $\mathcal{G}$, Seebeck coefficient $S$, power factor $PF$, and $ZT$ value. Temperature is set at 300 K. The power factor is calculated via $PF=\mathcal{G} S^2$. $K_r$ = $0.1K_0$. The insets are zoomed in from the dashed square area with $t_h/t_0$ = [0, 0.02].