Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session X33: Properties of Nanostructures and Low-Dimensional Materials |
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Sponsoring Units: DCMP Chair: Arthur Baddorf, Oak Ridge National Laboratory Room: 296 |
Friday, March 17, 2017 8:00AM - 8:12AM |
X33.00001: Visualizing Quantized One Dimensional Channels in InAs Nanowire by quasi-particle interference Abhay Kumar Nayak, Jonathan Reiner, Nurit Avraham, Andrew Norris, Binghai Yan, Ion Cosma Fulga, Jung-Hyun Kang, Torsten Karzig, Hadas Shtrikman, Haim Beidenkopf Semiconducting nanowires have captured vast scientific attention ever since the first putative detection of a zero-bias conductance peak – possibly signifying a Majorana mode. It has therefore become imperative to study the underlying electronic properties of the semiconducting nanowire to further our understanding of its topological nature. A major experimental obstacle is the brittleness and reactivity of the nanowires, which oxidize once exposed to ambient conditions. By tackling this technological challenge we have been able to study spectroscopically the one-dimensional electronic states in bare InAs semiconducting nanowires using Scanning Tunneling Microscopy. We visualize the Van-Hove singularities and the corresponding quantized one dimensional channels by imaging the quasi-particle interference arising due to the scattering of the electrons from point impurities on the surface of the nanowire. We also image the standing wave pattern that emanates from the nanowire end. Its decay profile reveals strikingly different relaxation properties of the lowest quantized channel compared to higher ones, as well as an uncharted high energy regime of extended phase coherence of electrons in one dimension. [Preview Abstract] |
Friday, March 17, 2017 8:12AM - 8:24AM |
X33.00002: Extended phase coherence of hot electrons in InAs nanowires revealed by scanning tunneling microscopy Jonathan Reiner, Abhay Kumar Nayak, Nurit Avraham, Andrew Norris, Binghai Yan, Ion Cosma Fulga, Jung-Hyun Kang, Torsten Karzig, Hadas Shtrikman, Haim Beidenkopf The higher the energy of a particle is above equilibrium the faster it relaxes due to the growing phase-space of available electronic states to interact with. Upon relaxing phase coherence is lost, thus limiting high energy quantum control and manipulation. We show that the phase decoherence induced by relaxation of hot electrons in one-dimensional semiconducting nanowires evolves non-monotonically with energy such that above a certain threshold hot-electrons regain stability with increasing energy. We directly observe this phenomenon in InAs nanowires using scanning tunneling microscope by two different measurement schemes: by visualizing the phase coherence length of electronic interference patterns, and by visualizing their phase coherence time, captured by crystallographic Fabry-Perot resonators. A remarkable agreement with a theoretical model reveals that the non-monotonic behavior is driven by the unique manner in which one dimensional hot-electrons interact with the cold electrons occupying the Fermi-sea. [Preview Abstract] |
Friday, March 17, 2017 8:24AM - 8:36AM |
X33.00003: X-ray Reflectivity Study of a Highly Rough Surface: Si Nanowires Grown by Ag Nanoparticle Etching Jesse Kremenak, Christopher Arendse, Franscious Cummings, Yiyao Chen, Paul Miceli Vertically oriented Si nanowires (SiNWs) formed by Ag-assisted wet chemical etching of a Si(100) substrate was studied by X-ray reflectivity (XRR) in combination with electron microscopy. Si(100) wafers coated with Ag nanoparticles, which serve as a catalyst, were etched for different durations in a HF/H$_{2}$O$_{2}$/DI-H$_{2}$O solution. Because of the extreme roughness of these surfaces, there are challenges for using XRR methods in such systems. Therefore, significant attention is given to the analysis method of the XRR measurements. This sample-average information presents a valuable complement to electron microscopy studies, which focus on small sections of the sample. The present work shows---for the first time---the amount and distribution of Ag during the formation of SiNWs fabricated by Ag-assisted wet chemical etching, which is vital information for understanding the etching mechanisms. [Preview Abstract] |
Friday, March 17, 2017 8:36AM - 8:48AM |
X33.00004: First principles study on the atomic and electronic structures of stabilized atomic layer of transition metal Young Jun Oh, Kyeongjae Cho Two-dimensional materials have gained a lot of attention due to their unique electronic properties and potential applications in electronic devices. Here, we perform first-principles calculations to investigate the stability and electronic structure of atomically thin transition metal layers. For 3d transition metal elements, silicene-like structure is the most stable among high symmetry structures that we considered. Calculated formation energy of thin transition metal layers is consistent with their tendency of melting temperature of bulk metals. For all transition metal elements, free-standing two-dimensional transition metal layers are found to be metallic. We discuss possible strategies to stabilize thin transition metal layers such as self-assembled monolayer or oxygen adsorption. [Preview Abstract] |
Friday, March 17, 2017 8:48AM - 9:00AM |
X33.00005: Two-Dimensional Quasiperiodic Nanomaterials YI- TING CHEN, DOMINIK RASTAWICKI, YAN SUN, YANG LIU, HARI C. MANOHARAN We present experiments on quasiperiodic nanostructures, assembled with scanning tunneling microscopy atom-manipulation techniques, studying the physical and electronic effects of incommensurate non-crystalline potentials on crystalline two-dimensional surfaces. Some of the resulting nanomaterials can be envisioned as extensions of quasicrystals, which have long range order but lack translational symmetry. Interest in quasiperiodic materials stems from their fascinating physical and mathematical structure and distinctive mechanical properties, which have motivated research within both hard and soft matter communities. Yet, many of the most notable features in quasiperiodic electronic structure remain unexplored. We will show new materials which extend beyond known quasicrystals by highlighting how electrons organize in two dimensions within incommensurate potentials, assembled using techniques first used for periodic non-strained and non-periodic strained molecular graphene\footnote{\textit{Nature} \textbf{483}, 306 (2012); \textit{Nature Nanotechnology} \textbf{8}, 625 (2013).}. Imaging, conductance, interference, and correlation measurements reveal unique quasi-band structures emerging from the special topology of quasiperiodic matter. [Preview Abstract] |
Friday, March 17, 2017 9:00AM - 9:12AM |
X33.00006: Imaging the modified core structure of buried Bi nanolines Jiaming Song, Bethany Hudak, Hunter Sims, Andrew Lupini, Paul Snijders Self-assembled, one-dimensional (1D) Bi nanolines on Si(100) surfaces are formed by two rows of surface Bi atoms supported by a ``Haiku'' core of reconstructed Si. These nanolines have been proposed as templates for atomic-scale wiring in nanoelectronics, or as sources of poorly soluble Bi dopants in Si films. Both applications require overgrowth of the nanolines to protect against oxidation. To understand the structure of the buried nanolines, high-resolution techniques are required. Here we employ scanning tunneling microscope (STM) and scanning transmission electron microscope (STEM) to probe Bi nanoline structures at the surface as well as at the interface with the capping layer. STM and STS data of the nanolines are consistent with the well-known Haiku core structural model. However, using high-resolution STEM we show that after depositing a Si capping layer, a modified Si core can survive depending on capping-layer growth temperature, but the Bi atoms diffuse away from their original position. The resulting 1D Si nanostructures, buried in semiconducting Si, may offer a useful nanoelectronic platform to address dopant qubits. The combination of high resolution STM and STEM provides new opportunities to guide the design of atomic-scale functional materials. [Preview Abstract] |
Friday, March 17, 2017 9:12AM - 9:24AM |
X33.00007: Understanding the behavior of buried Bi nanostructures from first principles Hunter Sims, Sokrates Pantelides, Jiaming Song, Bethany Hudak, Andrew Lupini, Paul Snijders Bismuth dopants in silicon provide several advantages over other n-type options such as phosphorus for usage as quantum bits (qubits). Self-assembled Bi nanolines on Si (100) surfaces may provide a means of introducing these dopants with greater control over placement and with less damage to the host system than is possible using ion implantation. However, these structures have thus far only been observed in vacuum, limiting their usefulness for application. We examine Bi nanolines overgrown with amorphous Si using density functional theory, comparing our findings with observations from scanning tunneling microscopy (STM) and atomic-resolution scanning transmission electron microscopy (STEM) in order to better understand the way in which the Si surface is influenced by both the Bi ad-dimers and the capping layer. We compare the thermodynamic stability of the generally accepted “haiku” defect core to the modified core that we observe and offer insight from total energy calculations into how the overgrowth process affects the nanolines. [Preview Abstract] |
Friday, March 17, 2017 9:24AM - 9:36AM |
X33.00008: Tunable and Energetically Robust PbS Nanoplatelets for Optoelectronic Applications Huashan Li, David Zhitomirsky, Jeffrey Grossman Beyond the tunable bandgaps as in CQDs, nanoplatelets (NPLs) provide unique electronic and optical properties that may overcome some of the challenges in CQD-based solar cells. Our ab-initio simulations shed light on the potential of PbS NPLs as tunable and energetically robust materials for novel optoelectronic devices. The results suggest that the broken symmetry in NPLs leads to planar wave functions and parity dependent quantum confinement effects. Compared to CQD assemblies, such pseudo-two-dimensional systems may provide stronger absorption and higher carrier mobility due to the distinct wave function distributions, large electronic couplings, and small hopping barriers. More importantly, while traps seem to be unavoidable even in slightly off-stoichiometric CQDs, both energetic and spatial traps are absent in PbS NPLs even in conditions far from charge balance, indicating an extraordinary robustness against off-stoichiometry as a result of surface homogeneity and sufficient cross-linking. Based on our findings, we propose several types of optoelectronic device architectures spanning photovoltaics and photodetectors that could take advantage of the superior properties found in NPLs. [1] Li, H. et al., Chem. Mater. 2016, 28, 1888. [Preview Abstract] |
Friday, March 17, 2017 9:36AM - 9:48AM |
X33.00009: Electronic structure of semi-metallic PtSe2 investigated with spin- and angle-resolved photoemission Jiagui Feng, O. Clark, L. Bawden, I. Marković, D. Biswas, L. Collins-McIntyre, M. S. Bahramy, Phil D. C. King The observation of extremely high and non-saturating magnetoresistance has sparked a renewed interest in compensated electron and hole pocket semimetals [1]. Here, we will present direct electronic structure measurements of 1T-structured PtSe2, a transition-metal dichalcogenide (TMD) compound. This was previously predicted to be semi-metallic with co-existing electron and hole pockets making up its the Fermi surface [2], but the details of its band structure have remained elusive to date. Unlike more intensely studied TMDs such as NbSe2\sout{ }/MoS2, its low-energy electronic structure is predicted to be dominated by chalcogen p-orbital, rather than transition-metal d-orbital, derived states. Nonetheless, we will show how spin-orbit coupling in the chalcogen shell still plays a major role in shaping its underlying electronic structure. Combining spin- with angle-resolved photoemission spectroscopy, we uncover its bulk electronic structure as well as revealing the formation of a number of topologically-protected states in this system. [1] M. N. Ali, et al. Nature 514, 205208 (2014). [2] D. Dai, et al. J. Solid State Chem. 173, 114 (2003). [Preview Abstract] |
Friday, March 17, 2017 9:48AM - 10:00AM |
X33.00010: Impurity bands and half-metal states in Mn-doped GaS layers J.T. Haraldsen, D. Parker, T. Pekarek, A.V. Balatsky In this study, we examine the magnetization, electronic band structure, and density of states for Mn doped GaS, which is a quasi-two-dimensional semiconductor. Starting with undoped GaS, we progressively added nine Mn atom into randomly determine Ga sites. We find that as the Mn doping is increased, the magnetization increases linearly with dopant, and the presence of magnetic atoms produces impurity bands in the electronic structure. Furthermore, examination of density of states shows that increase in magnetic impurity bands seems to lead to the presence of a weak, but noticeable, spin polarization at the Fermi level. This leads us to the indication of a possible half-metal state with increased Mn doping or other transition-metal atoms. [Preview Abstract] |
Friday, March 17, 2017 10:00AM - 10:12AM |
X33.00011: Optoelectronics of transition metal dichalcogenides decorated with gold nanoparticles S.B. Diefenbach, E. Parzinger, J. Kiemle, B. Miller, J. Wierzbowski, R. Csiki, A. Cattani-Scholz, M. Stutzmann, J.J. Finley, U. Wurstbauer, A.W. Holleitner We report on Raman and photoluminescence experiments on monolayers (ML) of $MoS_2$ and $WSe_2$, covered with octanethiole stabilized gold nanoparticle arrays. We observe an enhanced photoluminescence signal due to the decoration with gold nanoparticle arrays. Power-dependent Raman scattering experiments show a decrease of the normalized Raman intensity of the $A_{1g}$ and $E_{2g}^1$ phonon mode for gold nanoparticle decorated MLs. [Preview Abstract] |
Friday, March 17, 2017 10:12AM - 10:24AM |
X33.00012: Photocurrents and Electroluminescence at gate-defined edges in monolayer $p$--$n$ junctions Erik Lenferink, Nathaniel Stern, Kenji Watanabe, Takashi Taniguchi Charge carriers in single layer transition metal dichalcogenides (TMDs) possess coupled spin and valley degrees of freedom arising from strong spin-orbit coupling and the absence of inversion symmetry. The valley-contrasting Berry curvature and magnetic moment of these carriers enables valley currents to be created with circularly polarized light and measured electrically. However, due to a high binding energy, excitons only efficiently dissociate into free carriers in the depletion region of metal contacts, strongly impacting coupled spin, valley, and charge dynamics. To overcome this difficulty, we utilize electrostatic gating to define artificial edges with large electric field changes far from metal-semiconductor boundaries\footnote{B. W. H. Baugher, H. O. H. Churchill, Y. Yang, and P. Jarillo-Herrero. \emph{Nature Nanotech.}, \textbf{39}, 262–267 (2014).}. We explore the spatially-resolved photocurrent and electroluminescence properties of dual-gated WSe$_2$ $p$--$n$ junctions in a geometry enabling detection of transverse and longitudinal currents and discuss applications of this geometry to measuring valley-polarized opto-electronic processes of electrons and holes. [Preview Abstract] |
Friday, March 17, 2017 10:24AM - 10:36AM |
X33.00013: Giant bulk photovoltaic effect and spontaneous polarization of single-layer monochalcogenides Tonatiuh Rangel, Benjamin M. Fregoso, Bernardo S. Mendoza, Takahiro Morimoto, Joel E. Moore, Jeffrey B. Neaton We implement and use a first-principles density functional theory approach to calculate the shift current response of monolayer group-IV monochalcogenides. We find a larger effective three- dimensional effective polarization (\textasciitilde 1.9 C/m2) and shift current (\textasciitilde 200 $\mu $A/V2) than have been previously observed in common ferroelectrics. By using a one-dimensional Rice-Mele tight-binding model we investigate the shift-current tensor along the polarization axis, its relation with polarization, and the conditions under which shift-current reaches a maximum [1]. Importantly, our calculations predict that shift current can be largest in the UV visible range, indicating the potential of these materials for optoelectronic applications.[1] arXiv:1610.06589 [Preview Abstract] |
Friday, March 17, 2017 10:36AM - 10:48AM |
X33.00014: Ising Nematic in High Landau Levels Sayan Basak, Erica Carlson At low temperatures in ultraclean AlAs-AlGaAs heterojunctions, high fractional Landau levels break rotational symmetry. Lilly et al. [1] find that the transport properties are highly anisotropic at high Landau levels and low temperatures. Fradkin et al. [2] characterize the temperature dependence of the resistivity anisotropy as being in the Kosterlitz-Thouless universality class in the presence of a weak orientational symmetry breaking term. The weak symmetry breaking term rounds the transition and also allows for the development of a net resistivity anisotropy. Whereas a reasonably good match between the experiment and theory was found near and above the transition temperature (Fig. 2 of Ref. [2]), the simulation deviates significantly from the experimental data at low temperatures. We find that an Ising nematic in the presence of a weak symmetry breaking term captures the full temperature dependence.\\ \ [1] M. P. Lilly, K. B. Cooper, J. P. Eisenstein, L. N. Pfeiffer, and K. W. West, Phys. Rev. Lett. 82, 394 (1999).\\ \ [2] E. Fradkin, S. A. Kivelson, E. Manousakis, and K. Nho, Phys. Rev. Lett. 84, 1982 (2000). [Preview Abstract] |
Friday, March 17, 2017 10:48AM - 11:00AM |
X33.00015: Improved high-bias stability of single-atom contacts formed by junction closing Akira Sakai, Shin-saku Wakasugi, Shu Kurokawa Single-atom contacts (SACs) of Au exhibit a conductance very close to $G_0$, the quantum unit of conductance, and would serve as an important element in atomic-scale devices. The high-bias stability of Au SACs has already been studied by some authors who found that the average break voltage of Au SACs is around 1.2 V [1,2]. These previous experiments, however, employed the break-junction method that tends to generate a large tensile force within a produced SAC and makes it liable to break at lower voltages than SACs under weaker forces. In order to reduce the internal junction force, we fabricated Au SACs not by breaking but by closing junctions. Our experiment at 4 K showed that the average break voltage of Au SACs formed by the junction closing goes up to 1.7 V. This non-negligible increase in the break voltage confirms that the reduction of the junction force should be a key for improving the high-bias stability of SACs. [1] R. H. M. Smit \textit{et al.}, Nanotechnology \textbf{15}, S472 (2004). [2] D. Miura \textit{et al.}, e-J. Surf. Sci. Nanotech. \textbf{7}, 891 (2009). [Preview Abstract] |
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