Designing Novel Organic Ferroelectric Crystal Shashi Poddar UNL

Acknowledgements Department of Physics and Astronomy Prof. Stephen Ducharme Department of Chemistry Prof. Alexander Sinitskii Wayz Khan and Mikhail Shekhirev NCMN X-Ray Facility Specialist Dr. Shah Valloppily

Talk Organization First part: Brief Review of new Organic systems. Second part: The focus of our work.

Why Organic Molecular Ferroelectric ? Use of expensive heavy metal as a constituent. Use of toxic elements. Use of ultra high vacuum and high processing temperatures. Unsuitable for flexible electronics. Practical drawbacks of ceramic/inorganic ferroelectrics: Advantages of Organic Molecular systems: Mostly solution processed. Optimal processing temperature. Lightweight, ecofriendly and suitable for flexible electronics.

Introduction Organic systems: C–H bonds Rochelle salt is the earliest known Organic ferroelectric system. J. Valasek, Phys. Rev., 17, 475 (1921). Several other organic systems have been investigated since then. Low molecular weight systems Example: Thiourea Polymeric systems Example: PVDF ( Piezo transducers) Hydrogen bonded organic systems Example: TGS (Pyro electric sensors) References: Horiuchi et al., Nature Materials, 7, 357 (2008) Horiuchi et al., Mol Sci., 5, A0041 (2011)

Room temperature applicability. High spontaneous polarization. Low coercive fields. High switching speeds with minimal fatigue. High dielectric constants. Affinity to form crystals easily. Ease of integration in devices. Desirable Properties

Comparison MaterialCurie Temperature (Kelvin) Dielectric constant Spontaneous Polarization (µC/cm 2 ) Coercive Field (kV/cm) K RT K max. Inorganic Ferroelectrics BaTiO X RT5 PbTiO X 10 3 RT10 Organic Ferroelectrics Thiourea K0.2 TGS X K0.9 P(VDF 70 :TrFE 30 RT500 Ref: Horiuchi et al., Nature Materials, 7, 357 (2008)

NEWER ORGANIC FERROELECTRIC SYSTEMS

Donor-Acceptor H Bond System Phenazine (Acceptor A) and Chloranilic(H 2 ca)/ Bromanilic acid(H 2 ba) (Donor D) Horiuchi et al., Nature Materials, 4, 163 (2005) Polarization reversal takes place by shuttling the H atom Between the stacked ACID-BASE co–crystals. NH ON H O

Hydrogen Bonded Systems CROCONIC ACID, C 5 O 5 H 2 Horiuchi et al., Nature, 463, 789 (2010). Croconic Acid is a π conjugated molecule and is held together by H– bonds with adjacent molecules. A proton transfer between the hydrogen bonds accompanied by a π Bond switching causes the polarization reversal.

Ionic Salt Systems DIISOPROPYLAMMONIUM BROMIDE (DIPA-Br), [C 6 H 16 N] +. Br − Da-Wei Fu et al., Science, 339, 425 (2013). Inter ionic bonds between diisopropylammonium cation with bromide anion forms an infinite one- dimensional chain. The ordering of the N atom : Polarization reversal. Size of anions : Polar direction.

Comparison Materials Transition Temperature (Kelvin) Dielectric constant Spontaneous Polarization (μC/cm 2 ) Coercive Fields (KV/cm) ε RT ε max DIPA – Br Croconic Acid>4005–2014 Phenazine – H 2 ca –

Motivation of our work Anion groups attached to the diisopropylammonium cation balance charge. Anionic volume determines the size of the unit cell along the polar direction and hence, the saturation polarization values. New ferroelectric materials. The effect of replacing the anion on: Structure, transition temperature, saturation polarization values and coercive fields.

Motivation cont. Chlorine and Bromine already used. We tried substituting iodine. Other anion groups like: NO 3 -, PO 4 3-, SO Crystals were grown by slow evaporation of aqueous solution of equimolar diisopropylammonia and the acid. All of the salts gave crystals except SO 4 2-.

Thermal Analysis of FE Crystal The as-grown crystals were characterized using standard DTA/TGA technique. Ferro–Low Symmetry; Para– High Symmetry. Expect peaks near transitions in heating and cooling FE–PE: Endothermic. PE–FE: Exothermic.

Ref: Da-Wei Fu, Hong-Ling Cai, Yuanming Liu, Qiong Ye, Wen Zhang, Yi Zhang, Xue-Yuan Chen, Gianluca Giovannetti, Massimo Capone, Jiangyu Li, and Ren-Gen Xiong, Science 339, 425 (2013).

DIPA-Iodide

DIPA-Nitrate No precursor transition from non-ferroelectric to ferroelectric phase.

Thermal analysis - DTA/TGA  Heating,  Cooling References: 1. Da-Wei Fu, Wen Zhang, Hong-Ling Cai, Jia-Zhen Ge, Yi Zhang, Ren-Gen Xiong, Advanced Materials 23, 5658 (2011). 2. Da-Wei Fu, Hong-Ling Cai, Yuanming Liu, Qiong Ye, Wen Zhang, Yi Zhang, Xue-Yuan Chen, Gianluca Giovannetti, Massimo Capone, Jiangyu Li, and Ren-Gen Xiong, Science 339, 425 (2013). MaterialT C ↑ (K) T C ↓ (K) DIPA – Cl DIPA – Br DIPA – Br (Our) DIPA – I DIPA – NO

Single Crystal XRD Materialsa ( Å )b ( Å )c ( Å ) β (α=γ=90 o ) Cell Volume ( Å 3 ) DIPA – Br o 904 DIPA – I o 998 DIPA – NO o 980 DIPA – PO o 1003 Single crystal X ray diffraction performed on as grown crystals. Bigger ionic radii of the halogens and other oxide anion groups does increase the cell volume

Future Plans Full crystal structure (Space groups) DIPA-I and DIPA-NO 3 systems. Electrical characterization (P-E, Dielectric measurements). Summary Able to grow crystals – anion substitution. Thermal Analysis – promising FE systems.