Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session J48: Artificially Structured Materials I: Growth, Structure, and Electronic PropertiesLive
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Sponsoring Units: DCMP Chair: David Lederman, University of California, Santa Cruz |
Tuesday, March 16, 2021 3:00PM - 3:12PM Live |
J48.00001: Interfacial Ferroelectricity by van der Waals Sliding Maayan Vizner, Yuval Waschitz, Wei Cao, Iftach Nevo, Kenji Watanabe, Takashi Taniguchi, Eran Sela, Michael Urbakh, Oded Hod, Moshe Ben Shalom Despite their ionic nature, many layered diatomic crystals avoid internal electric polarization by forming a centrosymmetric lattice at their optimal anti-parallel van-der-Waals stacking. Here, we report a stable ferroelectric order emerging at the interface between two naturally-grown flakes of hexagonal-boron-nitride, which are stacked together in a metastable non-centrosymmetric parallel orientation. We observe alternating domains of inverted normal polarization, caused by a lateral shift of one lattice site between the domains. Reversible polarization switching coupled to lateral sliding is achieved by scanning a biased tip above the surface. Our calculations trace the origin of the phenomenon to a subtle interplay between charge redistribution and ionic displacement, and our minimal cohesion model predicts further venues to explore the unique "slidetronics" switching. |
Tuesday, March 16, 2021 3:12PM - 3:24PM Live |
J48.00002: Interfacial Ferroelectricity by van-der-Waals Sliding Maayan Vizner, Yuval Waschitz, Wei Cao, Iftach Nevo, Kenji Watanabe, Takashi Taniguchi, Eran Sela, Michael Urbakh, Oded Hod, Moshe Ben Shalom Despite their ionic nature, many layered diatomic crystals avoid internal electric polarization by forming a centrosymmetric lattice at their optimal anti-parallel van-der-Waals stacking. Here, we report a stable ferroelectric order emerging at the interface between two naturally-grown flakes of hexagonal-boron-nitride, which are stacked together in a metastable non-centrosymmetric parallel orientation. We observe alternating domains of inverted normal polarization, caused by a lateral shift of one lattice site between the domains. Reversible polarization switching coupled to lateral sliding is achieved by scanning a biased tip above the surface. Our calculations trace the origin of the phenomenon to a subtle interplay between charge redistribution and ionic displacement, and our minimal cohesion model predicts further venues to explore the unique "slidetronics" switching. In this talk I will discuss the origin of the effect in hexagonal-boron-nitride and discuss the switching mechanism for this new ferroelectric material. |
Tuesday, March 16, 2021 3:24PM - 3:36PM Live |
J48.00003: Electronically reconfigurable complex-oxide heterostructure free-standing membranes Muqing Yu, Kitae Eom, Jinsol Seo, Dengyu Yang, Hyungwoo Lee, Jungwoo Lee, Patrick R Irvin, Sang Ho Oh, Jeremy Levy, Chang-Beom Eom In recent years, LaAlO3/SrTiO3 interfaces have been used to create a growing family of nanoelectronic devices based on nanoscale control of LaAlO3/SrTiO3 metal-insulator transition.[1] The properties of these devices are wide-ranging, but they are restricted by nature to the underlying thick SrTiO3 substrate. Here we report the synthesis of free-standing LAO/STO membranes which can be directly integrated with other materials via van der Waals stacking. Conductive atomic force microscope lithography is shown to successfully create reversible patterns of nanoscale conducting regions that survive to milli-Kelvin temperatures. The ability to form reconfigurable conducting nanostructures on LAO/STO membranes enables new opportunities to integrate a variety of nanoelectronics with silicon-based architectures and flexible, magnetic, or superconducting materials. |
Tuesday, March 16, 2021 3:36PM - 3:48PM Live |
J48.00004: Monolithically integrated GaN nanoscale air channel field emission diode and transistor KESHAB SAPKOTA, Albert A Talin, Francois Leonard, Barbara A Kazanowska, Kevin S Jones, Brendan P Gunning, George T Wang GaN has many superior properties for nanoscale field emission devices, including low electron affinity (χ), high thermal and chemical stability, and high break down field. In particular, the significantly lower electron affinity of GaN compared to typical emitter materials (Si, Mo, W) can potentially enable dramatic improvement of field emission current. Here, we demonstrate monolithically fabricated GaN nanoscale vacuum electron diodes and transistors with ultra-low voltage operation (diode turn on (Von) ~ 0.24 V) in air. Some notable findings are: 1) the vacuum diodes exhibit nearly linear dependence of Von with nanogap size (d), and 2) the emission current shows complex pressure dependency even for d < electron mean free path. The field emission transistors are fabricated on single, bi-, and tri-gate configurations, where tri-gate allows independent control of vacuum barrier & GaN work function. We will present results for the design and characteristics of field emission diodes and transistor at various nanogap sizes and operating pressures. |
Tuesday, March 16, 2021 3:48PM - 4:00PM Live |
J48.00005: Electronic Structure of Mixed-Dimensional Heterojunctions from Optimally-Tuned Range-Separated Hybrid Functionals Qunfei Zhou, Zhenfei Liu, Pierre Darancet Mixed-dimensional heterojunctions (MDHJ) comprised of molecules deposited on 2D materials are actively being explored for optoelectronic applications. A predictive theory of the electronic structure of MDHJ is challenging due to their numerous competing energy scales, including local and non-local electronic correlations, interfacial charge transfer and orbital hybridization. Here we study the electronic structure of 0D-2D (metallophthalocyanines-MoS2) heterojunctions by density functional theory calculations, using optimally tuned range-separated hybrid functionals. This new method allows for optimally and non-empirically tuning the range-separation parameters for accurate description of both short-range and long-range electron interactions, as well as including the dielectric screening effect from electrostatic model. We obtain electronic structures in good agreement with known experimental results, and show the importance of phthalocyanine non-frontier orbitals in tuning the interface properties. |
Tuesday, March 16, 2021 4:00PM - 4:12PM Live |
J48.00006: Epitaxial Growth of Superlattices of Co2MnSi, Fe2MnSi, and Co2FeSi Heusler Compounds Frank Tsui, Ethan Fenwick Superlattices containing two Heusler alloys (including Co2MnSi, Fe2MnSi, and Co2FeSi) were grown on Ge substrates by molecular beam epitaxy techniques. Epitaxial growth via atomic layer-by-layer sequential deposition of various stacking sequences (e.g. the Heusler L21 and “inverse” Heusler) with superlattice periods of 1, 1.5, and 2 unit cell of the constituent crystals, and the corresponding surface structures and morphology were studied in real time using scanning RHEED technique. For example, the unit cell of the Heusler compounds along [111] consists of 12 atomic layers (AL), and for a superlattice with the same period of 12 AL, there are numerous possible atomic layer sequences for Co2MnSi and Fe2MnSi, such as Mn-Co-Si-Co-Mn-Co-Si-Co-Mn-Fe-Si-Fe, Co-Co-Mn-Si-Co-Co-Mn-Si-Fe-Mn-Fe-Si, Co-Mn-Co-Si-Co-Mn-Co-Si-Fe-Mn-Fe-Si, Mn-Co-Si-Co-Si-Fe-Mn-Fe-Si-Fe-Mn-Fe, etc. Dependences on the different atomic stacking sequences, nominal compositions, growth parameters, and crystallographic orientations were examined. In addition, means to optimize the growth and characterization of multiple superlattice stacks on the same substrate under the same conditions were explored. |
Tuesday, March 16, 2021 4:12PM - 4:24PM Live |
J48.00007: Tuning the Electronic Structure of Two-Dimensional Materials through Molecular Assembly Qunfei Zhou, Bukuru Anaclet, Trevor Steiner, Michele Kotiuga, Pierre Darancet The areal dipole of organic monolayers assembled on surfaces is known to alter significantly the surface workfunction. In this work, inspired by recent progresses in molecular assembly and in two-dimensional (2D) device synthesis, we examine how organic molecules can be used to tune the electronic structure of 2D materials through higher moments of their charge densities. |
Tuesday, March 16, 2021 4:24PM - 4:36PM Live |
J48.00008: Localized eigenstates in one-dimensional quasiperiodic and limit-periodic potentials Chongbin Zheng, Joshua Socolar We study the single-particle eigenstates of ordered, nonperiodic potentials in one dimension, focusing on the scaling of the inverse participation rate (IPR) of the ground state with the overall amplitude of the potential. For quasiperiodic potentials given by the sum of three incommensurate cosine waves, the scaling exponent depends on the incommensurate ratios of wavelengths in the potential. For a limit-periodic potential given by the sum of cosine waves with wavenumbers 2-nk0, we find a rich set of states that are technically classified as extended but have large IPR’s. We will discuss the scaling of the IPR with the strength of the potential for the ground state and for the highest energy states in a finite system with periodic boundary conditions. |
Tuesday, March 16, 2021 4:36PM - 4:48PM Live |
J48.00009: Hidden dualities in 1D quasiperiodic models Miguel Gonçalves, Bruno Amorim, Eduardo Castro, Pedro Ribeiro We propose that Anderson localization-delocalization transitions in generic 1D quasiperiodic models are associated with local hidden duality symmetries. These symmetries are related with the Aubry-André duality through transformations that can be tackled by studying commensurate approximants of the target incommensurate system. |
Tuesday, March 16, 2021 4:48PM - 5:00PM Live |
J48.00010: Spin-structure on the surface of a sphere Delaram Nematollahi When the kinetic energy of a many-electron system is quenched, as in quantum Hall ferromagnets, electron-electron interactions can drive the system to form interesting manybody states. Such energy degeneracy occurs in quantum Hall systems under high magnetic field and low temperatures. Electrons on the surface of a sphere display similarly degenerate single particle states as a function of magnetic quantum number m for fixed orbital quantum number l. We explore the possibility of novel manybody states in this system by constructing variational wavefunctions using both a Hartree-Fock approach and by a physically motivated ansatz. We calculate a bound on the groundstate energy for the case of a contact interaction and the Coulomb interaction, comparing the energy of states with a spin texture with that of the untextured state. We investigate that the textured-spin structure can result in the lower energy of the system than the non-textured counterparts. We discuss considerations for realizing this system in semiconductor core-shell quantum dots, such as the choice of material, thickness and the role of screening. |
Tuesday, March 16, 2021 5:00PM - 5:12PM Live |
J48.00011: Physics of Strain Engineered Minigaps in III-V Digital Alloys Sheikh Ahmed, Jiyuan Zheng, Joe C Campbell, Yaohua Tan, Avik Ghosh III-V short period superlattices called "digital alloys" are being grown using state-of-the-art growth techniques. These alloys consist of binary compounds stacked alternately in a periodic manner. Some of these digital alloys have small gaps called 'minigaps" in their energy bandstructure. These minigaps along with enhanced effective mass can be used to inhibit carrier multiplication of one carrier type in avalanche photodiodes (APDs). Thus, APDs with unicarrier multiplication can be designed that have low excess noise. This enhances the detector performance. It is possible to modulate the size of these minigaps by varying the biaxial strain in these superlattice structures. The strain alters the orbital overlaps which in turn can be used to vary the minigap sizes and their position relative to the band edges. We employ an Environment-Dependent Tight Binding model that is calibrated to Density Functional Theory bandstructure and wavefunctions to compute the strain-dependent electronic bandstructure. In this talk, we discuss the physics of these minigaps and their correlation with strain. Finally, using Monte-Carlo simulation results we look at the relationship between the minigap sizes and the excess noise performance of APDs. |
Tuesday, March 16, 2021 5:12PM - 5:24PM Live |
J48.00012: InGaAs Quantum Dot Growth on InAs(111) and GaSb(111) with Molecular Beam Epitaxy Kevin Vallejo, Trent Alan Garrett, Baolai Liang, Paul J Simmonds In this study we have mapped the growth parameters for optimal homoepitaxy of InAs on InAs(111)A substrates using molecular beam epitaxy. Increasing the substrate temperature reveals a transition from 2D flat island growth to step-flow. The optimized parameters we established (substrate temperature = 500° C, growth rate = 0.12ML/s and V/III ratio = 48) produce an atomically flat surface, free of 3D imperfections. We study material quality using photoluminescence and have established a relationship between InAs(111)A surface smoothness and optical quality. This work paves the way for integrating the 6.1 Å family of semiconductors with the desirable properties of the (111) crystallographic orientation. In addition, we will present preliminary results demonstrating the self-assembly of InGaAs quantum dots on these smooth InAs(111) and GaSb(111) surfaces, indicating new paths towards ultra-low bandgap tunable light emitters for infrared optoelectronics. |
Tuesday, March 16, 2021 5:24PM - 5:36PM Live |
J48.00013: Growth and characterization of type-II CdTe quantum dots in a ZnCdSe matrix for the use in an intermediate band solar cell Vasilios Deligiannakis, Gehan Ranepura, Milan Begliarbekov, Igor Kuskovsky, Maria C Tamargo Intermediate band solar cells (IBSC) have been proposed for being able to overcome the Schockley-Quiesser limit for single junction photovoltaics. The formation of an IB can be accomplished by either Quantum Dots (QDs) or impurities. Here we report on the growth and characterization of CdTe fractional monolayer QDs embedded in a ZnCdSe host matrix grown by a combination of migration enhanced epitaxy (MEE) and molecular beam epitaxy. The proximity of the parameters necessary for an ideal IBSC makes this material system a remarkable candidate for a successful practical device. Some unique attractive features of this material are its binary composition, simplifying growth; the absence of a deleterious interfacial layer, in spite of the lack of common ion; and the presence of strain, which allows for strain engineering of the IB. A superlattice structure of CdTe/ZnCdSe QDs (100 periods) is presented in which the size and the strain of the QDs are analyzed by X-ray diffraction and photoluminescent spectroscopy. Results suggest that thicker QDs are necessary to match the parameters for an ideal IBSC. This can be achieved by adjusting the growth parameters during MEE process. |
Tuesday, March 16, 2021 5:36PM - 5:48PM Live |
J48.00014: Decoding the structure–property relationships of quasi-two-dimensional semiconductor nanoplatelets Sergio Mazzotti, Arin R Greenwood, David J Norris, Giulia Galli Nanoplatelets (NPLs)—colloidal quantum wells—exhibit photophysical properties generally attributed to their precisely tunable thickness of a few atomic layers. However, recent experiments show that their structural and optical properties are affected by organic ligands passivating their facets. Here we report a theoretical study aimed at identifying the main factors determining the photophysical properties of NPLs. We focus on CdSe NPLs passivated by chloride ligands, and we use many body perturbation theory at the G0W0 level and solve the Bethe Salpeter equation to obtain electronic and excitonic properties. Our results are in good agreement with recent experimental reports. We then present a simple model that permits us to disentangle the effects of quantum confinement, ligand-induced strain, and dielectric contrast on the NPL electronic properties. We find that to accurately reproduce our first-principles results, it is critical to account for strain and to consider a finite (rather than infinite) potential barrier, as well as energy-dependent effective masses, when describing quantum confinement. The model presented here can be easily generalized to describe NPL heterostructures. |
Tuesday, March 16, 2021 5:48PM - 6:00PM Live |
J48.00015: Nanoporous Metal Foams as Efficient Particulate Filters James Malloy, Alberto Quintana, Kai Liu Nanostructured metal foams offer exciting potential for applications in diverse fields such as catalysts, electronics, heat exchange, structural materials, and filtration due to their extremely high surface to volume ratios. We have achieved metallic foams with tunable porosity and density (0.1%-30% of bulk density)[1] using electrochemical methods, with strong mechanical stability. Uniaxial compression tests reveal varying structural strengths depending on the relative density. Additional physical characteristics and applications have been explored. We have also investigated using such foams as efficient filtration membranes for micron and sub-micron sized particles. Over 99% of airborne micron-sized particles are found to be filtered after passing through just 1 mm of metallic foam. The foams are also found to be extremely effective for filtering out 0.2-0.4 micron sized nanoparticles. When filtering such deep submicron particles, the pressure drops across the foams are found to be comparable to an N-95 respirator for the same level of filtration. |
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