Person: Bell, David
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Bell
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Bell, David
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Publication Dirac Fermions and Flat Bands in the Ideal Kagome Metal FeSn(Springer Science and Business Media LLC, 2019-12-09) Fang, Shiang; Han, Minyong; Graff, David; Kaxiras, Efthimios; Kang, Mingu; Ye, Linda; You, Jhih-Shih; Levitan, Abe; Facio, Jorge; Jozwiak, Chris; Bostwick, Aaron; Rotenberg, Eli; Chan, Mun; McDonald, Ross; Kaznatcheev, Konstantine; Vescovo, Elio; Bell, David; van den Brink, Jeroen; Richter, Manuel; Prasad Ghimire, Madhav; Checkelsky, Joseph; Comin, RiccardoA kagome lattice of 3d transition metal ions is a versatile platform for correlated topological phases hosting symmetry-protected electronic excitations and magnetic ground states. However, the paradigmatic states of the idealized two-dimensional kagome lattice – Dirac fermions and flat bands – have not been simultaneously observed. Here, we utilize angle-resolved photoemission spectroscopy and de Haas-van Alphen quantum oscillations to reveal coexisting surface and bulk Dirac fermions as well as flat bands in the antiferromagnetic kagome metal FeSn, that has spatially-decoupled kagome planes. Our band structure calculations and matrix element simulations demonstrate that the bulk Dirac bands arise from in-plane localized Fe-3d orbitals, and evidence that coexisting Dirac surface state realizes a rare example of fully spin-polarized two-dimensional Dirac fermions due to spin-layer locking in FeSn. The prospect to harness these prototypical excitations in kagome lattice is a frontier of great promise at the confluence of topology, magnetism, and strongly-correlated physics.Publication Synthetically Encoded Ultrashort-Channel Nanowire Transistors for Fast, Pointlike Cellular Signal Detection(American Chemical Society, 2012) Cohen-Karni, Tzahi; Casanova, Didier; Cahoon, James F.; Qing, Quan; Bell, David; Lieber, CharlesNanostructures, which have sizes comparable to biological functional units involved in cellular communication, offer the potential for enhanced sensitivity and spatial resolution compared to planar metal and semiconductor structures. Silicon nanowire (SiNW) field-effect transistors (FETs) have been used as a platform for biomolecular sensors, which maintain excellent signal-to-noise ratios while operating on lengths scales that enable efficient extra- and intracellular integration with living cells. Although the NWs are tens of nanometers in diameter, the active region of the NW FET devices typically spans micrometers, limiting both the length and time scales of detection achievable with these nanodevices. Here, we report a new synthetic method that combines gold-nanocluster-catalyzed vapor–liquid–solid (VLS) and vapor–solid–solid (VSS) NW growth modes to produce synthetically encoded NW devices with ultrasharp (<5 nm) n-type highly doped \((n^{++})\) to lightly doped (n) transitions along the NW growth direction, where \(n^{++}\) regions serve as source/drain (S/D) electrodes and the n-region functions as an active FET channel. Using this method, we synthesized short-channel \(n^{++}/n/n^{++}\) SiNW FET devices with independently controllable diameters and channel lengths. SiNW devices with channel lengths of 50, 80, and 150 nm interfaced with spontaneously beating cardiomyocytes exhibited well-defined extracellular field potential signals with signal-to-noise values of ca. 4 independent of device size. Significantly, these “pointlike” devices yield peak widths of \(\sim 500 \mu s\), which is comparable to the reported time constant for individual sodium ion channels. Multiple FET devices with device separations smaller than \(2 \mu m\) were also encoded on single SiNWs, thus enabling multiplexed recording from single cells and cell networks with device-to-device time resolution on the order of a few microseconds. These short-channel SiNW FET devices provide a new opportunity to create nanoscale biomolecular sensors that operate on the length and time scales previously inaccessible by other techniques but necessary to investigate fundamental, subcellular biological processes.Publication Coaxial multishell nanowires with high-quality electronic interfaces and tunable optical cavities for ultrathin photovoltaics(Proceedings of the National Academy of Sciences, 2012) Kempa, T. J.; Cahoon, J. F.; Kim, S.-K.; Day, R. W.; Bell, David; Park, Hong Gyu; Lieber, CharlesSilicon nanowires (NWs) could enable low-cost and efficient photovoltaics, though their performance has been limited by nonideal electrical characteristics and an inability to tune absorption properties. We overcome these limitations through controlled synthesis of a series of polymorphic core/multishell NWs with highly crystalline, hexagonally-faceted shells, and well-defined coaxial Graphic (p/n) and p/intrinsic/n (p/i/n) diode junctions. Designed 200–300 nm diameter p/i/n NW diodes exhibit ultralow leakage currents of approximately 1 fA, and open-circuit voltages and fill-factors up to 0.5 V and 73%, respectively, under one-sun illumination. Single-NW wavelength-dependent photocurrent measurements reveal size-tunable optical resonances, external quantum efficiencies greater than unity, and current densities double those for silicon films of comparable thickness. In addition, finite-difference-time-domain simulations for the measured NW structures agree quantitatively with the photocurrent measurements, and demonstrate that the optical resonances are due to Fabry-Perot and whispering-gallery cavity modes supported in the high-quality faceted nanostructures. Synthetically optimized NW devices achieve current densities of 17 mA/cm2 and power-conversion efficiencies of 6%. Horizontal integration of multiple NWs demonstrates linear scaling of the absolute photocurrent with number of NWs, as well as retention of the high open-circuit voltages and short-circuit current densities measured for single NW devices. Notably, assembly of 2 NW elements into vertical stacks yields short-circuit current densities of 25 mA/cm2 with a backside reflector, and simulations further show that such stacking represents an attractive approach for further enhancing performance with projected efficiencies of > 15% for 1.2 μm thick 5 NW stacks.Publication Strengthening of Ceramic-based Artificial Nacre via Synergistic Interactions of 1D Vanadium Pentoxide and 2D Graphene Oxide Building Blocks(Nature Publishing Group, 2017) Knöller, Andrea; Lampa, Christian P.; Cube, Felix von; Zeng, Tingying Helen; Bell, David; Dresselhaus, Mildred S.; Burghard, Zaklina; Bill, JoachimNature has evolved hierarchical structures of hybrid materials with excellent mechanical properties. Inspired by nacre’s architecture, a ternary nanostructured composite has been developed, wherein stacked lamellas of 1D vanadium pentoxide nanofibres, intercalated with water molecules, are complemented by 2D graphene oxide (GO) nanosheets. The components self-assemble at low temperature into hierarchically arranged, highly flexible ceramic-based papers. The papers’ mechanical properties are found to be strongly influenced by the amount of the integrated GO phase. Nanoindentation tests reveal an out-of-plane decrease in Young’s modulus with increasing GO content. Furthermore, nanotensile tests reveal that the ceramic-based papers with 0.5 wt% GO show superior in-plane mechanical performance, compared to papers with higher GO contents as well as to pristine V2O5 and GO papers. Remarkably, the performance is preserved even after stretching the composite material for 100 nanotensile test cycles. The good mechanical stability and unique combination of stiffness and flexibility enable this material to memorize its micro- and macroscopic shape after repeated mechanical deformations. These findings provide useful guidelines for the development of bioinspired, multifunctional systems whose hierarchical structure imparts tailored mechanical properties and cycling stability, which is essential for applications such as actuators or flexible electrodes for advanced energy storage.Publication Topological Graphene Imaging and Fabrication of Devices(Microscopy Society of America, 2012) Bell, David; Wang, Wei; Bhandari, Sagar; Westervelt, Robert; Kaxiras, EfthimiosPublication Direct Imaging of Atomic-Scale Ripples in Few-Layer Graphene(American Chemical Society, 2012) Wang, Wei; Bhandari, Sagar; Yi, Wei; Bell, David; Westervelt, Robert; Kaxiras, EfthimiosGraphene has been touted as the prototypical two-dimensional solid of extraordinary stability and strength. However, its very existence relies on out-of-plane ripples as predicted by theory and confirmed by experiments. Evidence of the intrinsic ripples has been reported in the form of broadened diffraction spots in reciprocal space, in which all spatial information is lost. Here we show direct real-space images of the ripples in a few-layer graphene (FLG) membrane resolved at the atomic scale using monochromated aberration-corrected transmission electron microscopy (TEM). The thickness of FLG amplifies the weak local effects of the ripples, resulting in spatially varying TEM contrast that is unique up to inversion symmetry. We compare the characteristic TEM contrast with simulated images based on accurate first-principles calculations of the scattering potential. Our results characterize the ripples in real space and suggest that such features are likely common in ultrathin materials, even in the nanometer-thickness range.Publication Epitaxial Catalyst-Free Growth of InN Nanorods on c-Plane Sapphire(Springer-Verlag, 2009) Shalish, Ilan; Seryogin, G.; Yi, W.; Bao, J. M.; Zimmler, M. A.; Likovich, Edward Michael; Bell, David; Capasso, Federico; Narayanamurti, VenkateshWe report observation of catalyst-free hydride vapor phase epitaxy growth of InN nanorods. Characterization of the nanorods with transmission electron microscopy, and X-ray diffraction show that the nanorods are stoichiometric 2H–InN single crystals growing in the [0001] orientation. The InN rods are uniform, showing very little variation in both diameter and length. Surprisingly, the rods show clear epitaxial relations with the c-plane sapphire substrate, despite about 29% of lattice mismatch. Comparing catalyst-free with Ni-catalyzed growth, the only difference observed is in the density of nucleation sites, suggesting that Ni does not work like the typical vapor–liquid–solid catalyst, but rather functions as a nucleation promoter by catalyzing the decomposition of ammonia. No conclusive photoluminescence was observed from single nanorods, while integrating over a large area showed weak wide emissions centered at 0.78 and at 1.9 eV.Publication Ion-sculpting of Nanopores in Amorphous Metals, Semiconductors and Insulators(American Institute of Physics, 2010) George, H. Bola; Hoogerheide, David Paul; Madi, Charbel S.; Bell, David; Golovchenko, Jene; Aziz, MichaelWe report the closure of nanopores to single-digit nanometer dimensions by ion sculpting in a range of amorphous materials including insulators (SiO\(_2\) and SiN), semiconductors (a-Si), and metallic glasses (Pd\(_{80}\)Si\(_{20}\)) — the building blocks of a single-digit nanometer electronic device. Ion irradiation of nanopores in crystalline materials (Pt and Ag) does not cause nanopore closure. Ion irradiation of c-Si pores below 100 °C and above 600 °C, straddling the amorphous-crystalline dynamic transition temperature, yields closure at the lower temperature but no mass transport at the higher temperature. Ion beam nanosculpting appears to be restricted to materials that either are or become amorphous during ion irradiation.Publication Etching of Graphene Devices with a Helium Ion Beam(American Chemical Society, 2009) Lemme, Max C.; Bell, David; Williams, James R.; Stern, Lewis A.; Baugher, Britton W. H.; Jarillo-Herrero, Pablo; Marcus, CWe report on the etching of graphene devices with a helium ion beam, including in situ electrical measurement during lithography. The etching process can be used to nanostructure and electrically isolate different regions in a graphene device, as demonstrated by etching a channel in a suspended graphene device with etched gaps down to about 10 nm. Graphene devices on silicon dioxide \((SiO_2)\) substrates etch with lower He ion doses and are found to have a residual conductivity after etching, which we attribute to contamination by hydrocarbons.Publication Nanowire-Induced Wurtzite InAs Thin Film on Zinc-Blende InAs Substrate(Wiley-Blackwell, 2009) Bao, Jiming; Bell, David; Capasso, Federico; Erdman, Natasha; Wei, Dongguang; Froeberg, Linus; Martensson, Thomas; Samuelson, Lars