Session D59: Rational Design of Hybrid Organic-Inorganic Interfaces II

Can we construct a molecular multiferroic electronic device?

As the expectations for novel printable electronics grow, the design of flexible and high-density nonvolatile molecular memory devices remains a hot topic. Voltage control of "molecular spintronics" (where charge and spin both matter) is a major goal because such nonvolatile molecular structures have the very real possibility of providing a room temperature nonvolatile device on a length scale less than 10 nm (semiconductor industry goals) while delivering low power GHz nonvolatile local logic or memory operations. The successes in addressing the grand challenge of manipulating magnetically ordered states by electrical approach suggest new routes to developing novel spintronics.

While much is in its infancy, molecular spintronics has now been shown to be possible. The spin crossover (SCO) phenomenon, in 3d transition metal compounds, through the manipulation of interfacial chemistry, can be exploited to create voltage-controlled isothermal changes in the electronic structure. This has been shown for the Fe (II) spin crossover complexes interfaced with molecular ferroelectrics. This nonvolatile isothermal voltage-controlled switching, at room temperature, is evident in both spectroscopy and transport studies of thin film bilayer devices [1,2]. This comes at a lower energy cost,  faster speeds, and far less fabrication complexity than the currently commercially available nonvolatile memory based on magnetic tunnel junctions. If the molecular system(s) can be made into ink, then printable electronics are a very real possibility. Even better, the fact that molecular device fabrication is possible at room temperature from solution, means that three-dimensional memory arrays are possible - if the power dissipation is small.

But there are challenges still to be addressed. The key problem of the high impedance of the device has now been addressed through chemistry. The key take-away point is that molecular nonvolatile room temperature "memory" devices have been realized and new developments in chemistry should lead to better molecular nonvolatile electronic devices.   

Structure and Dynamics of TMDC Moiré Heterostructures: Quantum Dynamics and Machine Learning Simulations

Machine learning (ML) is revolutionizing scientific and engineering disciplines due to its ability to capture hidden patterns in large amounts of data. The recent success of ML can be attributed to increasing amount of information and data, increasing amount of simulation resources, and improving understanding of statistical inference. In this talk I will discuss two topics:

Structure Prediction of Epitaxial Organic Interfaces with Ogre, Demonstrated for Tetracyanoquinodimethane (TCNQ) on Tetrathiafulvalene (TTF)

Highly ordered epitaxial interfaces between organic semiconductors could enhance the performance of organic devices thanks to their well-controlled, uniform electronic properties and high carrier mobility. The electronic structure of epitaxial organic interfaces and their functionality are inextricably linked to their structure. We present a method for structure prediction of epitaxial organic interfaces based on lattice matching followed by surface matching, implemented in the open-source code, Ogre. The lattice matching step produces domain-matched interfaces, where commensurability is achieved with different integer multiples of the substrate and film unit cells. In the surface matching step, dispersion corrected deep neural network (DNN) interatomic potentials are used to find the interfacial distance and registry between the substrate and film. The DNN potentials are in agreement with density functional theory (DFT) for the optimal position of the film on the substrate and the ranking of putative interface structures. We investigate the epitaxial interface of TCNQ on TTF, whose electronic structure has been probed by ultraviolet photoemission spectroscopy (UPS), but whose structure had been hitherto unknown. We find that TCNQ(001) on top of TTF(100) is the most stable interface configuration, closely followed by TCNQ(010) on top of TTF(100). The density of states, calculated using DFT, is in excellent agreement with UPS, including the presence of an interface charge transfer state.   

First-principles study of transition metal-Beryllium Laves phases with distorted Be Kagome layers

Transition (T) metal-Beryllium Laves phases, characterized by hexagonal crystal symmetry, exhibit a distorted Be Kagome lattice, leading to the formation of Be trimers. To comprehend the underlying reasons behind trimer formations and explore their implications on the crystal structure, we perform density functional theory (DFT) calculations for all TBe₂ compounds with the P6₃/mmc symmetry. By utilizing projected band structure representations and density of states (DOS), we investigate the nature of local sp-orbitals at the Be sites and their hybridization with the transition metal d-orbitals. This study aims to shed light on trimer formation and the unique characteristics of these crystal structures. Through a systematic exploration of electronic band structures, Fermi surfaces, and projected DOS across the rows and columns of the transition metal in the periodic table, we aim to unveil the structure-property relationship within this class of materials. Furthermore, the 3D and 2D Fermi surfaces suggest that these materials may exhibit interesting variations in electrical and optical conductivities.   

First-Principles Studies of Two-dimensional Hybrid Inorganic-Organic Perovskites: Successes and Challenges

I will discuss our first-principles computational investigations into several interesting aspects of two-dimensional (i.e. layered) hybrid organic-inorganic perovskites, including the organic-inorganic energy level alignment, spin-orbit-coupling induced band splitting, and its coupling with lattice dynamics. In addition to the new insights and understanding we obtained from first-principles modeling in collaboration with experimental groups, I will also highlight key challenges we currently face. 

For a class of two-dimensional hybrid organic-inorganic perovskite semiconductors, the flexibility in varying the organic and inorganic components allows control over the nature, energy, and localization of carrier states in a quantum-well-like fashion. Our first-principles predictions, based on the large-scale hybrid density-functional theory with spin-orbit coupling, show that the interface between the organic and inorganic parts within a single semiconductor can be modulated systematically, enabling us to induce different energy level alignments. Additionally, we show how the spin-orbit-coupling-induced lifting of the spin degeneracy depends indirectly on constituting organic motifs. We examine experimentally synthesized structures, and the effect of the organic spacer cation is elucidated. Interestingly, we observed the momentum-independent persistent spin texture splitting rather than the conventional Rashba/Dresselhaus splitting in one case, and subtle modification to organic cation is found responsible for inducing this interesting material property. I will also discuss how lattice dynamics might induce instantaneous band-splitting and relevant timescales for such temporal changes in centrosymmetric systems. Lastly, I will touch briefly on computational/theoretical challenges for designing hybrid organic-inorganic perovskite semiconductors by discussing progress and challenges in accurate computation of the energy-level alignment and excitons of different characters.

           

    

Giant Apparent Optical Circular Dichroism in Thin Films of Bi-Based Hybrid Organic-Inorganic Metal Halide Semiconductor Through Preferred Orientation

Introducing chirality into organic/inorganic hybrid materials can impart chiroptical properties such as circular dichroism. The ability to tune chiroptical properties in self-assembled materials could have important implications for spintronic and optoelectronic applications. Here, we incorporate a chiral organic cation, (R/S)-4-methoxy-α-methylbenzylammonium, to synthesize the bismuth-based hybrid organic-inorganic metal halide semiconductor, (R/S-MeOMePMA)BiI₄. Thin films of this Bi-based compound demonstrate large chiroptical responses, with circular dichroism anisotropy (g_CD) values up to ~0.1, close to the highest value observed in another chiral metal-halide semiconductor, (R-MBA₂CuCl₄). Further investigation discloses that this large g_CD in (R/S-MeOMePMA)BiI₄ is caused by the apparent CD effect. Careful selection of deposition conditions and the concomitant thin-film orientation enable the control of g_CD, with maximum value observed when its thin film has a preferred (001) orientation parallel to the substrate. Together with other recent reports, it appears that the low symmetry plays an important role in achieving unusually large g_CD in these chiral metal-halide materials.