Bulletin of the American Physical Society
APS March Meeting 2017
Volume 62, Number 4
Monday–Friday, March 13–17, 2017; New Orleans, Louisiana
Session E31: Graphene: Dopants, Adatoms, and Adsorbates |
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Sponsoring Units: DCMP DMP Chair: Adam Friedman, US Naval Research Laboratory Room: 294 |
Tuesday, March 14, 2017 8:00AM - 8:12AM |
E31.00001: Substitutional Doping of Graphene via Hyperthermal Ion Implantation (HyTII) Cory Cress, Scott Schmucker, Adam Friedman, Pratibaibha Dev, Jeremy Robinson The ability to manipulate materials with atomic precision is central to nanoscience. Hyperthermal ion implantation (HyTII) is a kinetic approach to doping with sub-nanometer control and is ideally suited for modifying 2D nanomaterials like graphene, yet few experimental studies have capitalized on this potential.$^{1}$ In this presentation, we experimentally investigate the effects of nitrogen ion implantation (N-HyTII) with ion energies ranging from 25 -- 100 eV and doses up to 10$^{15}$ N$^{+}$/cm$^{2}$. Following N-HyTII processing and transferring the graphene to a SiO$_{2}$/Si substrate, we collect Raman spatial maps over the entire sample surface, and perform XPS and STM analysis on a subset of the variable-energy samples along with HOPG as a control. The STM and XPS analysis confirm the substitutional incorporation of N into the graphene lattice at 45 eV, while the Raman D-peak to D`-peak ratios reveal distinct differences over the full energy range that are consistent with the different hyperthermal ion-substrate interactions pertaining to surface adsorption, substitutional doping, defect formation. We conclude this study by demonstrating the use of HyTII in graphene device processing, and highlight the effects of N-doping on the magnetotransport properties of graphene.$^{2}$ References: [1] C.D. Cress, et al. \textit{ACS Nano} \textbf{10}, 3714 (2016). [2] A.L. Friedman, et al. \textit{Phys. Rev. B}, \textbf{93} 161409(R) (2016). [Preview Abstract] |
Tuesday, March 14, 2017 8:12AM - 8:24AM |
E31.00002: Electronic Transport in Nitrogen-Doped Graphene Devices Using Hyperthermal Ion Implantation Adam Friedman, Cory Cress, Scott Schmucker, Jeremy Robinson, Olaf van 't Erve For most published studies, atomic species are chemically bonded to graphene out-of-plane, breaking the sp2 symmetry and producing functionalized graphene that is typically only chemically stable for weeks or less. Contrarily, hyperthermal ion implantation offers a controllable method of producing high-quality substitutionally-doped graphene with N, an n-type dopant that has potential for graphene electronics applications where high carrier concentration, uniform doping, and minimal vacancy defect concentration is desired. We examine the transport properties of monolayer graphene sheets as a function of beam energy and dose, observing a suppressed carrier-concentration-dependent transition from weak to strong localization. For nominally equivalent doses, increased N ion energy results magnetoresistance magnitude increases, which we discuss in the context of dopant concentration and defect formation. We use a model for the temperature dependence of the conductivity that takes into account both temperature activation, due to the formation of a transport gap, and Mott variable-range hopping, due to the formation of defects, to further study the electronic properties of the doped films as a function of dose and N ion energy. [Preview Abstract] |
Tuesday, March 14, 2017 8:24AM - 8:36AM |
E31.00003: Interactions between oxygen adsorbates on graphene Jason Bub, Dmitry Solenov Control over the state of the surface of graphene and its chemical functionality is crucial in many applications including mobility-reliant electronic applications, as well as chemical sensors. Adsorbates can dramatically change transport and chemical properties of graphene. We present results of analytical and numerical investigation of interactions between some of the most commonly found adsorbates - oxygen atoms. Interactions between these bivalent adsorbates substantially differ from simpler monovalent adsorbates, such as fluorine. Our investigation utilizes a synergistic approach combining analytical green's function analysis and first principle density functional calculations. [Preview Abstract] |
Tuesday, March 14, 2017 8:36AM - 8:48AM |
E31.00004: Metal intercalation-induced selective adatom mass transport on graphene Xiaojie Liu, Cai-Zhuang Wang, Myron Hupalo, Hai-Qing Lin, Kai-Ming Ho, Patricia A. Thiel, Michael C. Tringides Using first-principles calculaitons, we show that partially intercalated graphene, with a mixture of intercalated and pristine areas, can induce an alternating electric field because of the spatial variations in electron doping, and thus, an oscillatory electrostatic potential. This alternating field can change normal stochastic adatom diffusion to biased diffusion, leading to selective mass transport and consequent nucleation, on either the intercalated or pristine areas, depending on the charge state of the adatoms. [Preview Abstract] |
Tuesday, March 14, 2017 8:48AM - 9:00AM |
E31.00005: Nonlocal Transport in Graphene and Dilute Fluorinated Graphene Hua Wen, Jing Li, Jun Zhu Non-local transport measurements using H-bar geometries are powerful probes of edge state physics and spin Hall effect. Recently, non-local resistance (NLR) measurements in hydrogenated and Cu-decorated graphene have reported sizable signals that exceed the contributions from diffusive transport, from which a large spin Hall angle is inferred [1]. The origin of the NLR remains controversial to date [2]. In this talk, we report our NLR measurements of both pristine and dilute fluorinated graphene. In pristine graphene, we find that the microscopic charge density distribution plays an important role in the magnitude of the NLR near the charge neutrality point. A diverse range of behavior that deviates from the diffusive transport model is observed and we discuss the implications. In a small perpendicular magnetic field of B \textless 1 T, pristine graphene exhibits large NLR that may originate from thermoelectric effects. The NLR of fluorinated graphene devices, on the other hand, approximately follows the description of diffusive transport. No clear in-plane magnetic field dependence is found. The spin Hall angle, estimated from these preliminary studies, is likely to be small in dilute fluorinated graphene. [1] Balakrishnan, et. al. Nat. Phys. 9, 284 (2013); Balakrishnan, et. al. Nat. Commun. 5, 4748 (2014) [2] Kaverzin et. al. Phys. Rev. B 91, 165412 (2015) [Preview Abstract] |
Tuesday, March 14, 2017 9:00AM - 9:12AM |
E31.00006: Anomalous magnetism in hydrogenated graphene Noel Garcia, Jose Lado, Joaquin Fernandez-Rossier We revisit the problem of the local moment formation due to the functionalization of graphene by an individual chemisorbed Hydrogen atom. We first study the average spin magnetization as a function of the applied field, and we find that in the non-interacting case at $T=0$, the $m_s(H)$ curve is non-linear for small $H$ (at $T=0$) which makes it impossible to define a spin susceptibility. Second, we compute the net magnetic moment within the mean field Hubbard approximation. In contrast with all previous work that use finite simulation cells that give a magnetic moment of $S=1/2$, we use an embedding method that allows the modeling of a single impurity in infinite pristine graphene. Our results give a magnetic moment smaller than $1/2$. Our results highlight that the spin physics of a single Hydrogen is different from localized spin moments in gapped systems for which magnetic moment is quantized and from conductors, for which the $T=0$ spin susceptibility do exist. [Preview Abstract] |
Tuesday, March 14, 2017 9:12AM - 9:24AM |
E31.00007: Li adatoms on graphene: doping and intervalley scattering Ali Khademi, Ebrahim Sajadi, Pinder Dosanjh, Doug Bonn, Joshua Folk, Alexander Stöhr, Stiven Forti, Ulrich Starke Charge doping by alkali atoms (Li) deposited under cryogenic ultrahigh-vacuum conditions is arguably the simplest of all adatom effects on graphene. We present an experimental investigation of Li adatoms on epitaxial and CVD graphene, focusing on the crucial role of high temperature annealing prior to cryogenic deposition for efficient doping and a measurement of how the adatoms affect the intervalley scattering rate. While doping saturated at 2$\times$ 10$^{\mathrm{13}}$ e/cm$^{\mathrm{-2}}$ on unannealed graphene, independent of previous processing, a 700 K/900 K anneal prior to cryogenic deposition caused the saturated doping level to rise one order of magnitude to above 10$^{\mathrm{14}}$ e/cm$^{\mathrm{-2\thinspace }}$[1]. For the annealed samples, long range Coulomb interaction due to Li adatoms caused a dramatic enhancement of the intervalley scattering, a result that contradicts the naive expectation that short range scattering is necessary for intervalley scattering but is qualitatively consistent with theoretical predictions in Ref. [2]. [1] A. Khademi, et al.,~arXiv:1610.00301. [2] P. Boross, et al., Phys. Rev. B. 92, 035420 (2015). [Preview Abstract] |
Tuesday, March 14, 2017 9:24AM - 9:36AM |
E31.00008: Transport properties and quantum criticality of magnetic vacancies and adatoms in graphene. David Ruiz-Tijerina, Luis Dias da Silva We study the effects of a low concentration of magnetic adatoms or single vacancies in the linear--response transport properties of graphene. For adatoms, we considered top-- and hollow--site adsorbates (TOP and HS). For vacancies, we studied bond--reconstructed (REC) and unreconstructed symmetric vacancies (VAC). These impurity problems map onto different power--law pseudo gap Anderson models, with distinct critical behaviors for TOP/REC and HS/VAC impurities in charge--neutral graphene. Away from charge neutrality, Kondo correlations are quintessential to all impurity types considered. We predict Kondo temperatures of up to $10\mathrm{K}$ for realistic parameters, including the cases of VAC and HS impurities, contrary to what was previously believed. Our results indicate that electronic transport is determined by usual impurity scattering with TOP and REC impurities, which do not possess the graphene $C_{3v}$ symmetry. In contrast, the presence of $C_{3v}$ and inversion symmetries for VAC and HS impurities, respectively, leads to a decoupling from electronic states at symmetry points and/or branches throughout the Brillouin zone. As a consequence, the highly--symmetric VAC and HS impurities do not contribute to the resistivity in charge neutrality. Ref: PRB 94 085425 (2016). [Preview Abstract] |
Tuesday, March 14, 2017 9:36AM - 9:48AM |
E31.00009: Electronic transport in the quantum spin Hall state due to the presence of adatoms in graphene Leandro Lima, Caio Lewenkopf Heavy adatoms, even at low concentrations, are predicted to turn a graphene sheet into a topological insulator with substantial gap. The adatoms mediate the spin-orbit coupling that is fundamental to the quantum spin Hall effect. The adatoms act as local spin-orbit scatterer inducing hopping processes between distant carbon atoms giving origin to transverse spin currents. Although there are effective models that describe spectral properties of such systems with great detail, quantitative theoretical work for the transport counterpart is still lacking. We developed a multiprobe recursive Green's function technique with spin resolution to analyze the transport properties for large geometries. We use an effective tight-binding Hamiltonian to describe the problem of adatoms randomly placed at the center of the honeycomb hexagons, which is the case for most transition metals. Our choice of current and voltage probes is favorable to experiments since it filters the contribution of only one spin orientation, leading to a quantized spin Hall conductance of $e^2/h$. We also discuss the electronic propagation in the system by imaging the local density of states and the electronic current densities. [Preview Abstract] |
Tuesday, March 14, 2017 9:48AM - 10:00AM |
E31.00010: Tunable negative magnetoresistance in hydrogenated graphene Shi-Min Cao, Chao-Yi Cai, Chuan-Wu Cao, Jian-Hao Chen The problem of unconventional magnetism in materials without d and f electrons has attracted continuous attention. In particular, a lot of efforts have been devoted to understand the origin and effects of magnetic moments induced in graphene with structure defects such as missing carbon atoms, absorption of light atoms such as hydrogen or fluorine. We have measured the magnetoresistance (MR) of graphene at low temperature with in-situ hydrogenation in ultra-high vacuum environment. Large negative MR was found in hydrogenated graphene which could be tuned by carrier density and sample temperature. Depending on the density of absorbed atomic hydrogen and carrier density, large linear negative MR was found which did not saturate up to 9 Tesla. Such negative MR could be the manifestation of local moments created by atomic hydrogen absorbed on graphene. [Preview Abstract] |
Tuesday, March 14, 2017 10:00AM - 10:12AM |
E31.00011: Modification of Berry phase in graphene with in-situ hydrogenation. Chuan-Wu Cao, Chao-Yi Cai, Shi-Min Cao, Jian-Hao Chen Surface modification has become a powerful tool to engineer spin-orbit interactions in two dimensional materials. Here we report in-situ hydrogenation of graphene devices in ultra-high vacuum environment with magnetotransport measurement. With low hydrogen concentration, the Shubnikov--de Haas oscillation is preserved in the graphene samples. By analyzing the oscillation, we find that the Berry phase of Graphene can be modified continuously by increasing hydrogenation, which may point to increasing spin-orbital coupling strength in the 2D electron system. [Preview Abstract] |
Tuesday, March 14, 2017 10:12AM - 10:24AM |
E31.00012: Weak localization and weak anti-localization in graphene with in-situ hydrogenation Jian-Hao Chen, Chao-Yi Cai, Chuan-Wu Cao, Shi-Min Cao, Indra Yudhistira, Shaffique Adam, Haiwen Liu The attachment of adatoms to two dimensional materials could strongly modify the electronic properties and induce various new physics not present in these materials in the pristine form. We have measured the low field magnetoresistance (MR) of graphene at low temperature with in-situ hydrogenation in ultra-high vacuum environment. A cross-over from weak localization (WL) to weak anti-localization (WAL) is observed which closely resemble 2DEG with increasing Rashba interactions and metal thin film with increasing spin-orbit coupling. By analyzing the WL and WAL at different carrier concentration, hydrogen concentration and temperature, we have been able to identify hydrogenation as the source of increased electron dephasing and enhanced spin-orbit coupling strength in graphene. [Preview Abstract] |
Tuesday, March 14, 2017 10:24AM - 10:36AM |
E31.00013: Diverse Electronic and Magnetic Properties of Fluorine-Doped Graphene Nanoribbons. Khanh Nguyen Duy, Ming-Fa Lin The feature-rich electronic and magnetic properties of fluorine-doped graphene nanoribbons are investigated by the first principles calculations. They arise from the cooperative or competitive relations among the significant F-C bond, the finite-size quantum confinement and the edge structure. Fluorine adatoms can create the p-type metals or the narrow-gap semiconductors, depending on whether the $\pi $ bonding is seriously suppressed by the top-site chemical bonding. There exist five kinds of magnetic configurations, namely, the ferromagnetic and non-magnetic systems with the metallic and semiconducting behaviors, and the anti-ferromagnetic semiconductors. The diverse essential properties are clearly revealed in the spatial charge distribution, the spin density, and the density of states (DOS). Specifically, a lot of structures in DOS could be directly verified by the STS measurements. [Preview Abstract] |
Tuesday, March 14, 2017 10:36AM - 10:48AM |
E31.00014: 1-D Quantum Well States on Doped Graphene Nanoribbons Revealed by Transport Simulations P. Brandimarte, E. Carbonell-Sanrom\`a, R. Balog, M. Corso, S. Kawai, A. Garcia-Lekue, S. Saito, S. Yamaguchi, E. Meyer, J. I. Pascual, D. S\'anchez-Portal Quantum-well states have recently been observed in scanning tunneling microscopy experiments with chemically functionalized armchair graphene nanoribbons (AGNRs), more specifically on pristine segments confined by pairs of boron-substituted atoms into the AGNR backbone.\footnote{E. Carbonell-Sanrom\`a \emph{et al}, submitted.} Here we present a first-principles study of the electronic and transport properties of such doped AGNRs. Our results reveal that the boron pairs selectively confine the first valence band of the pristine AGNR while being almost transparent for the second one. Such band-dependent electron scattering is explained in terms of the symmetry matching between the electronic wave functions of states from the pristine AGNRs and those localized at the boron pairs. Our simulations not only reproduce the experimental measurements but also reveal the mechanism behind the observations. [Preview Abstract] |
Tuesday, March 14, 2017 10:48AM - 11:00AM |
E31.00015: Scalable Production of Biosensors Based on Aptamer-Functionalized Graphene for Detection of the HIV drug Tenofovir Ramya Vishnubhotla, Jinglei Ping, A.T. Charlie Johnson Graphene field effect transistors (GFETs) are of great interest for biosensing applications, and have shown promising results for small molecular detection due to high sensitivity and electron mobility. We describe the fabrication of a scalable array of GFETs through traditional photolithography using lab-grown graphene via chemical vapor deposition (CVD) for drug detection with an all-electronic read-out. Sensor fabrication produced~52 devices per 2 x 2 cm area, with a yield of over~90{\%}.~Our biosensors use a commercially-obtained aptamer, verified to bind to graphene via AFM, to bind to the molecules of the drug Tenofovir, a medication currently used for HIV treatment, and have proven to detect concentrations at 1 ng/mL, 10\textasciicircum 3 times lower than standard medical methods. We noted a concentration-dependent shift in the Dirac voltage for Tenofovir, and testing control drugs showed that the aptamer was only highly selective in binding to Tenofovir itself. These results are promising for potential clinical testing with urine samples, as our method is scalable and non-invasive. This work is funded by NIH through the Center for AIDS Research at the University of~Pennsylvania.~ [Preview Abstract] |
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