Session DD05: V: Ferroics and Optics

The emergence of vdW synthetic ferroelectrics: fundamentals and applications

I begin with a general introduction to vdW ferroelectrics; its fundamentals and applications. Then,I report the observation of a single-element ferroelectric state in a black phosphorus-like bismuth layer, in which the ordered charge transfer and the regular atom distortion between sublattices happen simultaneously [1]. Instead of a homogenous orbital configuration that ordinarily occurs in elementary substances, we found the Bi atoms in a black phosphorous-like Bi monolayer maintain a weak and anisotropic sp orbital hybridization, giving rise to the inversion-symmetry-broken buckled structure accompanied with charge redistribution in the unit cell. As a result, the in-plane electric polarization emerges in the Bi monolayer. Using the in-plane electric field produced by scanning probe microscopy, ferroelectric switching is further visualized experimentally. This emergent single-element ferroelectricity broadens the mechanism of ferroelectrics and may enrich the applications of ferroelectronics in the future.

I will next report a van der Waals crystal, niobium oxide dichloride (NbOCl2), featuring vanishing interlayer electronic coupling and monolayer-like excitonic behaviour in the bulk form, along with a scalable second-harmonic generation intensity of up to three orders higher than that in monolayer WS2 [2]. To our knowledge, this is the first SPDC source unambiguously demonstrated in two-dimensional layered materials, and the thinnest SPDC source ever reported.

    

Electro-optic effect in thin film strontium barium niobate (SBN) grown by RF Magnetron Sputtering on SrTiO3 substrates

Ferroelectric strontium barium niobate (Sr_xBa_1-xNb₂O₆ or SBN) is a material with high electro-optic (EO) response. It is currently of interest in low voltage silicon-integrated photonics (SiPh) as electronic integrated circuits have reached their scaling and power limits. We have grown textured SBN films by RF sputtering on a (100)-oriented SrTiO₃ substrate. Grains with predominantly (310) and (001) out-of-plane orientation have been identified. We demonstrate a maximum effective EO coefficient of 230 pm/V in such mixed orientation films using a transmission geometry that is responsive only to the in-plane polarization component.  By using a TiO₂-terminated SrTiO₃ substrates, we can partly control the SBN orientation and obtain mostly (001)-oriented SBN films with out-of-plane polarization, which demonstrate a maximum effective Pockels coefficient of 88 pm/V.   

Evidence of octahedral local distortion across TN in RFeO3 (R=rare earth)

We report phonon infrared emission of  non-Jahn Teller Pbnm RFeO₃ (R=rare earth) G-type compounds as a local probe in the far infrared, highlighting anomalies across T_N (the Néel temperature) in the breathing (stretching) vibrational profile. In this frequency range, the  dielectric deconvolution reveals high temperature orbital octahedral local site distortions. High resolution X-ray diffraction patterns of RFeO₃ (R=La, Er, Lu), obtained at the ID22 ESRF beamline (Grenoble),  reveal modifications in the FeO₆ octahedra across T_N. For instance, for R=La, the room temperature delta distortion (i.e., deviation of individual distances from the average value) is 0.992x10^-4, whereas it decreases to 0.106x10^-4 at 1200 K, denoting an increment of the octahedral symmetry while keeping the Pbnm space group unchanged. This implies  intrinsic changes in bond lengths and angles involving Fe-O bonds with oxygen 2p also contributing to the magnetostructural coupling (yielding an anomalous anisotropic expansion of the unit-cell parameters) and  spin-exchange interaction pathways. Site distortion brings up a new extra perturbation in the superexchange interaction M-O-M determining antiferromagnetic transitions accounting T_N rare earth ion radius dependence [1].

[1] J.-S. Zhou and J. B. Goodenough, PRB B 77, 132104 (2008).   

Optical diode effect: exploration of large effects and application to antiferromagnetic domain imaging

The optical absorption of ordinary materials is not changed by reversing the direction of light propagation. In contrast, when space-inversion and time-reversal symmetries of materials are both broken, the optical absorption for two counter-propagating light beams can be different [1,2]. This unconventional optical phenomenon, often called the optical diode (OD) effect by analogy with the semiconductor diode, has attracted interest for potential applications in novel nonreciprocal optical devices. Recent intensive studies have led to observations of the OD effect in many materials and over a wide range of wavelengths [1-5]. For the visible to near-infrared light range, however, the reported OD signals have usually been small [5] or require high magnetic fields to amplify the magnitude [4].

We are exploring materials that exhibit a large OD effect in the visible to near-infrared range. In this presentation, I will present our strategy for exploring such candidate materials. I will then present our recent achievements in the discovery of large visible-light OD signals in the antiferromagnet Bi₂CuO₄ [6]. In contrast to most previous observations, the large OD signals of Bi₂CuO₄ appear spontaneously in the absence of external fields. Furthermore, by switching multiple antiferromagnetic domains in Bi₂CuO₄ with an external field, we were able to demonstrate a three-level control of optical transparency. We will also present the results of even larger near-infrared OD signals in other antiferromagnets. Furthermore, we will show that the OD effect can be used to visualize the antiferromagnetic domains [6,7].

 

[1] G. L. J. A. Rikken et al., Nature, 390, 493 (1997).

[2] T. Arima, J. Phys.: Condens. Matter, 20, 434211 (2008).

[3] I. Kézsmárki et al., Nat. Commun. 5, 3203 (2014).

[4] S. Toyoda et al., PRL 115, 267207 (2015)

[5] T. Sato et al., PRL 124, 217402 (2020).

[6] K. Kimura et al., Nat. Commun. 13, 697 (2022).

[7] K. Kimura et al., Commun. Mater. 1, 39 (2020).   

Understanding the Stoichiometry of Two-Dimensional Metal Oxide Intercalation at Epitaxial Graphene/Silicon Carbide Interface by Octet rule and Formal Charge

Metals intercalated into epitaxial graphene on silicon carbide (EG/SiC) are a basis for the formation of metal oxides in the same confined geometry. The interaction between intercalant and substrate changes the oxidation states in the two-dimensional metal oxide, as compared to 3D bulk geometries. This talk describes how fundamental chemical principles such as the octet rule and formal charge can be used to understand the metal oxides’ oxidation number, layer-by-layer stoichiometry, preferred thickness, and (if those principles are violated) the potential bonding and doping to the graphene.

These principles are demonstrated by an example of Indium Oxide intercalation in EG/SiC. Multi-layer elemental Indium was first intercalated. The controlled oxidation allows the formation of two layers of Oxygen and one layer of Indium, while the excess Indium is expelled. The stability of the experimental product with this layering configuration was explained. Our research underscores the critical role that simple chemical concepts play in unraveling the part-covalent part-ionic intercalant-substrate interaction. It also has practical significance for theorists in constructing structural candidates. This is because geometric relaxation has fixed number of particles, which makes the knowledge of likely stoichiometry prior to relaxation very important.   

Theoretical and experimental study of the synthesis of nitrogen-doped graphene by graphene oxide and melamine.

In this study, we achieved nitrogen-doped graphene by applying a post-treatment synthesis method take as precursors graphene oxide and melamine. Graphene oxide (GO) is synthesized by the modified Hummers method, which promotes the presence of epoxy, hydroxyl, and a few more functional groups. The samples were characterized by the XPS, UV-vis, and Bohem titration and the results show the presence of expected functional groups. The nitrogen-doped graphene is obtained by heating the samples of GO under 700°C in the presence of melamine mixed in water at different levels of acidic media. Through experimental adjustments in the concentration of oxygen species within graphene oxide, achieved by modifying key parameters like the edge-area ratio of the initial materials and the synthesis reaction temperature, we were able to confirm which specific groups promote the formation of a singular nitrogen species. By DFT calculations, we investigate the mechanism to form nitrogen-doped graphene. We calculated the desorption energies of the O-based functional groups, also, we performed NEB calculations to calculate the minimum energy pathways in the N-doped process.

We thank, DGAPA-UNAM projects IA100624, and IN101523. Calculations were performed in the DGCTIC-UNAM project no. LANCAD-UNAM-DGTIC-422.