Sagnac interference in Carbon nanotube loops

In this paper we study electron interference in nanotube loops. The conductance as a function of the applied voltage is shown to oscillate due to interference between electron beams traversing the loop in two opposite directions, with slightly different velocities. The period of these oscillations with respect to the gate voltage, as well as the temperatures required for the effect to appear, are shown to be much larger than those of the related Fabry-Perot interference. This effect is analogous to the Sagnac effect in light interferometers. We calculate the effect of interactions on the period of the oscillations, and show that even though interactions destroy much of the near-degeneracy of velocities in the symmetric spin channel, the slow interference effects survive.Comment: 5 pages, 4 figure

Quantitative local probing of polarization with application on HfO 2 ‐based thin films

Owing to their switchable spontaneous polarization, ferroelectric materials have been applied in various fields, such as information technologies, actuators, and sensors. In the last decade, as the characteristic sizes of both devices and materials have decreased significantly below the nanoscale, the development of appropriate characterization tools became essential. Recently, a technique based on conductive atomic force microscopy (AFM), called AFM‐positive‐up‐negative‐down (PUND), is employed for the direct measurement of ferroelectric polarization under the AFM tip. However, the main limitation of AFM‐PUND is the low frequency (i.e., on the order of a few hertz) that is used to initiate ferroelectric hysteresis. A significantly higher frequency is required to increase the signal‐to‐noise ratio and the measurement efficiency. In this study, a novel method based on high‐frequency AFM‐PUND using continuous waveform and simultaneous signal acquisition of the switching current is presented, in which polarization–voltage hysteresis loops are obtained on a high‐polarization BiFeO3 nanocapacitor at frequencies up to 100 kHz. The proposed method is comprehensively evaluated by measuring nanoscale polarization values of the emerging ferroelectric Hf0.5Zr0.5O2 under the AFM tip

Band gap opening by two-dimensional manifestation of Peierls instability in graphene

Using first-principles calculations of graphene having high-symmetry distortion or defects, we investigate band gap opening by chiral symmetry breaking, or intervalley mixing, in graphene and show an intuitive picture of understanding the gap opening in terms of local bonding and antibonding hybridizations. We identify that the gap opening by chiral symmetry breaking in honeycomb lattices is an ideal two-dimensional (2D) extension of the Peierls metal-insulator transition in 1D linear lattices. We show that the spontaneous Kekule distortion, a 2D version of the Peierls distortion, takes place in biaxially strained graphene, leading to structural failure. We also show that the gap opening in graphene antidots and armchair nanoribbons, which has been attributed usually to quantum confinement effects, can be understood with the chiral symmetry breaking

Probing Electronic Properties of Carbon Nanotubes

Carbon nanotubes are quasi-one-dimensional objects that have many remarkable electronic properties. In Chapter I, an electrostatic force microscopy technique to probe the local density of states of single-walled carbon nanotubes (SWCNTs) under ambient conditions is described. Coupling the atomic force microscope tip motion with the quantum capacitance of nanotubes enables the van Hove singularities in the one-dimensional density of states to be resolved. We utilized this technique to identify individual semiconducting and metallic tubes, and further to estimate the chiral angle of a nanotube. Moreover, in order to realize a SWCNT interferometer, nanotube loop devices where a self-crossing geometry yields two electron paths that is a possible analog of the optical Sagnac interferometer are fabricated and explored in Chapter II. Scanning gate microscopy reveals for semiconducting devices a 0–50% transmission probability into the loop segment at the junction, which can be controlled by applying back gate voltage, hence shifting the Fermi level of the nanotube. Metallic loop devices having low contact resistance showed a large- scale conductance peak with fast oscillations superposed on it. Possible theoretical explanations including Sagnac-type interference, which takes the velocity difference between left and right movers in to account, and Fabry-Perot-type interference are compared with the experimental observations. In Chapter III, in accordance with increasing demand for developing spin-electronic devices, cobalt-filled multi-walled carbon nanotubes (Co–filled MWCNTs) are first synthesized and imaged by transmission electron microscopy, and also characterized by various spectroscopy tools like X–ray diffraction and energy dispersive X–ray spectrometry. Further, a Co–filled MWCNT device having reproducible switching in magnetoresistance was demonstrated. The last topic, in Chapter IV, covers the effects of a transverse electric field in MWCNT devices, where conductance fluctuations as a function of the transverse electric field were observed. The electric field spacing between the peaks of the fluctuations is in agreement with the theoretical predictions of band structure modulation by transverse electric fields. Future work following our experimental studies is proposed and discussed at the end of each chapter

Local Electronic Structure of Single-Walled Carbon Nanotubes from Electrostatic Force Microscopy

An atomic force microscope was used to locally perturb and detect the charge density in carbon nanotubes. Changing the tip voltage varied the Fermi level in the nanotube. The local charge density increased abruptly whenever the Fermi level was swept through a van Hove singularity in the density of states, thereby coupling the cantilever's mechanical oscillations to the nanotube's local electronic properties. By using our technique to measure the local band gap of an intratube quantum-well structure, created by a nonuniform uniaxial strain, we have estimated the nanotube chiral angle. Our technique does not require attached electrodes or a specialized substrate, yielding a unique high-resolution spectroscopic tool that facilitates the comparison between local electronic structure of nanomaterials and further transport, optical, or sensing experiments

NHC-Catalyzed 1,2-Selective Hydroboration of Quinolines

© 2020 American Chemical Society.Selective dearomative transformation of readily available N-heteroarenes is a powerful tool accessing useful synthetic building units. Described herein is the NHC-catalyzed 1,2-selective hydroboration of quinolines with high functional group tolerance. Dihydroquinoline products could be isolated as their amide derivatives upon in situ N-protection, thus offering high synthetic utility of the current procedure. Combined experimental and computational studies revealed that the observed regioselectivity can be rationalized by proposing a six-membered transition state that collectively incorporates NHC catalyst, hydroborane reductant, and protonated quinoline substrate11sciescopu

Parallel synaptic design of ferroelectric tunnel junctions for neuromorphic computing

We propose a novel synaptic design of more efficient neuromorphic edge-computing with substantially improved linearity and extremely low variability. Specifically, a parallel arrangement of ferroelectric tunnel junctions (FTJ) with an incremental pulsing scheme provides a great improvement in linearity for synaptic weight updating by averaging weight update rates of multiple devices. To enable such design with FTJ building blocks, we have demonstrated the lowest reported variability: σ / μ = 0.036 for cycle to cycle and σ / μ = 0.032 for device among six dies across an 8 inch wafer. With such devices, we further show improved synaptic performance and pattern recognition accuracy through experiments combined with simulations

Graphene and Thin-Film Semiconductor Heterojunction Transistors Integrated on Wafer Scale for Low-Power Electronics

Graphene heterostructures in which graphene is combined with semiconductors or other layered 2D materials are of considerable interest, as a new class of electronic devices has been realized. Here we propose a technology platform based on graphene–thin-film-semiconductor–metal (GSM) junctions, which can be applied to large-scale and power-efficient electronics compatible with a variety of substrates. We demonstrate wafer-scale integration of vertical field-effect transistors (VFETs) based on graphene–In–Ga–Zn–O (IGZO)–metal asymmetric junctions on a transparent 150 × 150 mm² glass. In this system, a triangular energy barrier between the graphene and metal is designed by selecting a metal with a proper work function. We obtain a maximum current on/off ratio (<i>I</i>_on/<i>I</i>_off) up to 10⁶ with an average of 3010 over 2000 devices under ambient conditions. For low-power logic applications, an inverter that combines complementary n-type (IGZO) and p-type (Ge) devices is demonstrated to operate at a bias of only 0.5 V

Hybrid Deep Learning Crystallographic Mapping of Polymorphic Phases in Polycrystalline Hf0.5Zr0.5O2 Thin Films

By controlling the configuration of polymorphic phases in high-k Hf0.5Zr0.5O2 thin films, new functionalities such as persistent ferroelectricity at an extremely small scale can be exploited. To bolster the technological progress and fundamental understanding of phase stabilization (or transition) and switching behavior in the research area, efficient and reliable mapping of the crystal symmetry encompassing the whole scale of thin films is an urgent requisite. Atomic-scale observation with electron microscopy can provide decisive information for discriminating structures with similar symmetries. However, it often demands multiple/multiscale analysis for cross-validation with other techniques, such as X-ray diffraction, due to the limited range of observation. Herein, an efficient and automated methodology for large-scale mapping of the crystal symmetries in polycrystalline Hf0.5Zr0.5O2 thin films is developed using scanning probe-based diffraction and a hybrid deep convolutional neural network at a 2 nm(2) resolution. The results for the doped hafnia films are fully proven to be compatible with atomic structures revealed by microscopy imaging, not requiring intensive human input for interpretation