No abstract

Simple meso-scale capacitor structures have been made by incorporating thin (∼300 nm) single crystal lamellae of KTiOPO4 (KTP) between two coplanar Pt electrodes. The influence that either patterned protrusions in the electrodes or focused ion beam milled holes in the KTP have on the nucleation of reverse domains during switching was mapped using piezoresponse force microscopy imaging. The objective was to assess whether or not variations in the magnitude of field enhancement at localised “hot-spots,” caused by such patterning, could be used to both control the exact locations and bias voltages at which nucleation events occurred. It was found that both the patterning of electrodes and the milling of various hole geometries into the KTP could allow controlled sequential injection of domain wall pairs at different bias voltages; this capability could have implications for the design and operation of domain wall electronic devices, such as memristors, in the future.

Advanced organic nonlinear optical (NLO) materials have attracted increasing attention due to their multitude of applications in modern telecommunication devices. Arguably the most important advantage of organic NLO materials, relative to traditionally used inorganic NLO materials, is their short optical response time. Geminal amido esters with their donor-π-acceptor (D-π-A) architecture exhibit high levels of electron delocalization and substantial intramolecular charge transfer, which should endow these materials with short optical response times and large molecular (hyper)­polarizabilities. In order to test this hypothesis, the linear and second-order nonlinear optical properties of five geminal amido esters, (E)-ethyl 3-(X-phenylamino)-2-(Y-phenylcarbamoyl)­acrylate (1, X = 4-H, Y = 4-H; 2, X = 4-CH3, Y = 4-CH3; 3, X = 4-NO2, Y = 2,5–OCH3; 4, X = 2-Cl, Y = 2-Cl; 5, X = 4-Cl, Y = 4-Cl) were synthesized and characterized, whereby NLO structure–function relationships were established including intramolecular charge transfer characteristics, crystal field effects, and molecular first hyperpolarizabilities (β). Given the typically large errors (10–30%) associated with the determination of β coefficients, three independent methods were used: (i) density functional theory, (ii) hyper-Rayleigh scattering, and (iii) high-resolution X-ray diffraction data analysis based on multipolar modeling of electron densities at each atom. These three methods delivered consistent values of β, and based on these results, 3 should hold the most promise for NLO applications. The correlation between the molecular structure of these geminal amido esters and their linear and nonlinear optical properties thus provide molecular design guidelines for organic NLO materials; this leads to the ultimate goal of generating bespoke organic molecules to suit a given NLO device application.

The molecular design rules of organic nonlinear optical (NLO) materials are well established, especially those pertaining to the first-order molecular hyperpolarizability, β, which governs second-harmonic generation (SHG): a phenomenon that is responsible for the frequency doubling processes in many optical applications. The availability of these rational guidelines has propelled the development of new organic SHG chromophores. Conversely, the development of organometallic SHG-active complexes has not been steered so clearly. Many reports on individual series of complexes suggest a singular correlation between their structure and SHG properties. Several reviews have catalogued such results, but these have only distinguished compounds by chemical type, while their SHG properties are described one-by-one for each chemical. We herein propose a generic classification scheme that can systematically rationalize dipolar SHG properties for all organometallic complexes. This classification method stands to provide the holistic information that is needed to generate a rational set of guidelines for the systematic molecular design of dipolar SHG-active organometallic chromophores. Our scheme shows that only a simple set of molecular design rules is required to relate the chemical structure of an organometallic complex to its second-order dipolar SHG properties. This is despite the fact that these molecular design rules are rooted in a complicated panoply of ligand- and crystal-field theory, resonance structures, intramolecular charge transfer considerations, metal oxidation states, and metal coordination attributes. While the roots of these rules can be derived at the individual chemical level, their derivation via our workflow of simple decision-making processes which connect these rules within a simple classification scheme, stands to facilitate a rational molecular design approach toward the materials discovery of organometallic complexes for SHG applications.