Enhanced Thermopower of Saturated Molecules by Noncovalent Anchor‐Induced Electron Doping of Single‐Layer Graphene Electrode

• Published in: Advanced materials (Weinheim) 2021-10, Vol.33 (41), p.e2103177-n/a
• Main Authors: Park, Sohyun, Kim, Hwa Rang, Kim, Juhee, Hong, Byung‐Hee, Yoon, Hyo Jae
• Format: Article
• Language: English
• Subjects: Alkanes; Alkylamines; Chemical bonds; Doping; EGaIn; Electrodes; Gold; Graphene; Interfacial bonding; Materials science; molecular thermoelectrics; Raman spectroscopy; Seebeck effect; self‐assembled monolayers; Thermoelectricity; Transistors
• ISSN: 0935-9648; 1521-4095
• DOI: 10.1002/adma.202103177

Enhancing thermopower is a key goal in organic and molecular thermoelectrics. Herein, it is shown that introducing noncovalent contact with a single‐layer graphene (SLG) electrode improves the thermopower of saturated molecules as compared to the traditional gold–thiolate covalent contact. Thermoelectric junction measurements with a liquid‐metal technique reveal that the value of Seebeck coefficient in large‐area junctions based on n‐alkylamine self‐assembled monolayers (SAMs) on SLG is increased up to fivefold compared to the analogous junction based on n‐alkanethiolate SAMs on gold. Experiments with Raman spectroscopy and field‐effect transistor analysis indicate that such enhancements benefit from the creation of new in‐gap states and electron doping through noncovalent interaction between the amine anchor and the SLG electrode, which leads to a reduced energy offset between the Fermi level and the transport channel. This work demonstrates that control of interfacial bonding nature in molecular junctions improves the Seebeck effect in saturated molecules. Saturated molecules are not preferred in the research of organic thermoelectrics because of intrinsically wide bandgaps and poor thermopower. Graphene electrodes are demonstrated to be able to enhance thermopower of monolayers of saturated compounds. A noncovalent amine anchor induces the creation of in‐gap states and n‐type doping of graphene, thereby leading to an increased Seebeck coefficient relative to analogous covalent gold–thiolate monolayers.