Tunable Molecular Electrodes for Bistable Polarization Screening

• Published in: Small (Weinheim an der Bergstrasse, Germany) Vol. 19; no. 30; pp. e2207799 - n/a
• Main Authors: Spasojevic, Irena, Santiso, José, Caicedo, José Manuel, Catalan, Gustau, Domingo, Neus
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01.07.2023
• Subjects: Adsorbates; Barium titanates; BaTiO 3 ferroelectric thin films; Bipolarity; Capacitors; Charge density; chemical electrodes; Coercivity; Electrode polarization; Electrodes; Ferroelectric materials; Ferroelectricity; Graphene; graphene ferroelectric capacitors; molecular functionalization; Nanotechnology; para‐aminobenzoic acid (pABA) molecules; Polarity; polarization screening; Screening; Surface charge; Thickness; Thin films; molecular functionalization; chemical electrodes; BaTiO 3 ferroelectric thin films; para-aminobenzoic acid (pABA) molecules; polarization screening; graphene ferroelectric capacitors
• ISSN: 1613-6810; 1613-6829; 1613-6829
• DOI: 10.1002/smll.202207799

The polar discontinuity at any ferroelectric surface creates a depolarizing field that must be screened for the polarization to be stable. In capacitors, screening is done by the electrodes, while in bare ferroelectric surfaces it is typically accomplished by atmospheric adsorbates. Although chemisorbed species can have even better screening efficiency than conventional electrodes, they are subject to unpredictable environmental fluctuations and, moreover, dominant charged species favor one polarity over the opposite. This paper proposes a new screening concept, namely surface functionalization with resonance‐hybrid molecules, which combines the predictability and bipolarity of conventional electrodes with the screening efficiency of adsorbates. Thin films of barium titanate (BaTiO3) coated with resonant para‐aminobenzoic acid (pABA) display increased coercivity for both signs of ferroelectric polarization irrespective of the molecular layer thickness, thanks to the ability of these molecules to swap between different electronic configurations and adapt their surface charge density to the screening needs of the ferroelectric underneath. Because electron delocalization is only in the vertical direction, unlike conventional metals, chemical electrodes allow writing localized domains of different polarity underneath the same electrode. In addition, hybrid capacitors composed of graphene/pABA/ferroelectric have been made with enhanced coercivity compared to pure graphene‐electode capacitors. Para aminobenzoic acid (pABA) molecules on BaTiO3 ferroelectric thin films are shown to behave as tunable “chemical electrodes” for bistable polarization screening by conforming their delocalized electronic density selectively to existent polarization orientation, therefore causing higher coercivity and enhanced stability of both ferroelectric states. Furthermore, this opens a pathway for interface engineering of 2D/ferroelectric capacitors for nanodevices.