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Kazuhiro Marumoto University of Tsukuba, Japan

Electrically controllable spin states in single-layer graphene: direct observation by ESR spectroscopy. Kazuhiro Marumoto University of Tsukuba, Japan Tsukuba Research Center for Energy Materials Science (TREMS), Japan.

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Kazuhiro Marumoto University of Tsukuba, Japan

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  1. Electrically controllable spin states in single-layer graphene: direct observation by ESR spectroscopy Kazuhiro Marumoto University of Tsukuba, Japan Tsukuba Research Center for Energy Materials Science (TREMS), Japan 15th Annual Congress on Materials Research and Technology (Materials Research 2018), Holiday Inn Paris, Paris, France, February 19, 2018

  2. Outline • Introduction 1) Graphene 2) Transport study of graphene under high charge density Electric double layer transistors (EDLTs) 3) Electron spin resonance (ESR) spectroscopy 4) Spin study of carbon materials by ESR • Direct observation of electrically controllable spin states in single-layer graphene using ESR spectroscopy 1) Device structure of graphene EDLTs 2) Electrically induced Pauli paramagnetism First direct ESR of Fermi-degenerate 2D electron system 3) Electrically induced ambipolar spin vanishment Decrease in carriers’ spin scatterings of atomic vacancies • Summary

  3. Graphene Flexible electronics Graphene • An ideal 2D electron system • Quantum anomalous Hall effect • Ballistic transport • Dirac point without a band gap • Flexibility Printable electronics Spintronics Graphene patterning printed by a ink-jet method http://www.rikenresearch.riken.jp/jpn/research/5551 E. B. Secor et al., J.Phys. Chem. Lett. 4(2013) 1347. Graphene has been extensively studied as a next-generation electronic material by various experimental and theoretical methods.

  4. Electric double layer transistor (EDLT) of graphene Cross section of an EDLT structure of graphene Using ionic liquid insulator, high charge density has been achieved in an EDLT structure. Device structure of a graphene EDLT Schematic of graphene EDLT J. Yeet al., PNAS 108 (2011) 13002.

  5. Transistor characteristics of graphene EDLT Gate voltage (VG) dependence of 2D sheet conductivity σ2D in EDLTs with monolayer, bilayer, and trilayer graphene 2D sheet conductivity σ2D shows non-linear transfer characteristics, which is due to high charge density by ionic liquid insulator. J. Yeet al., PNAS 108 (2011) 13002.

  6. Principal of electron spin resonance (ESR) and system Microwave generator Detector hν Electro- magnet H S Sample Cavity resonator hν gμB H0 = 0 H H 0 DHpp H0 High sensitive and precision analytical method for an unpaired electron Evaluation of materials and devices at the molecular level Principle of ESR ESR system Electron energy in magnetic field H Spin Hamiltonian: H= μBHgS = gμBHms (ms = ±1/2) E hν= gμBH0 0 0.03 meV (0.3 K) Microwave resonant absorption in magnetic field for charges with magnetic moment, spin S Resonant magnetic field: g factor A unique value for material

  7. Information obtained from ESR Intergrated intensity ESR FWHM linewidth factor g Original signal Integration Double integration 318 320 322 324 Magnetic Field (mT) g factor is a unique value for material, which determines resonance field. Characterization of spin species and orientation ESR linewidthreflects the environments around charges with spins. Characterization of charge dynamics Integrated intensity evaluates the absolute number of spins. Quantitative evaluation of the concentration of spins

  8. Features of g factor g factor originates from spin-orbital interaction in materials :spin-orbital coupling constant L: orbital angular momentum 　⇒ creation of internal field shift for resonant magnetic field g factor in tensor formula : free electron From different molecular structureswith different perturbation Lbetween grand state |0> and excited state |n>, g factor 1) determines spin species 2) clarifies spin orientation

  9. Spin study of carbon materials and devices by ESR Device: Single-walled carbon nanotube transistors Electrically controlled ESR Material: Single-layer graphene ESR spin susceptibility (Curie law) M. A. Augustyniak-Jabłokow et al., Chem. Phys. Lett. 557 (2013) 118. D. Matsumoto et al., Sci. Rep. 5 (2015) 11859. Study for spin states of electrically accumulated charges and defects in single-layer graphene device has not yet been performed except for our group.

  10. Research contents Fabrication of single-layer graphene transistors Spin study using electrically induced ESR method Detail elucidation of spin states of charge carriers and defects in a most simple 2D electron-system single-layer graphene Direct observation of electrically controllable spin states in single-layer graphene using ESR spectroscopy 1) ESR observation of electrically induced Pauli paramagnetism First direct ESR of Fermi-degenerate 2D electron system 2) ESR observation of electrically induced ambipolar spin vanishment Decrease in carriers’ spin scatterings of atomic vacancies →high charge carrier’s mobility

  11. Raman spectra of single-layer graphene sample Graphene Quartz substrate B C A Single-layer graphene fabricated on a quartz substrate by a CVD method Raman spectra of graphite and single-layer graphene 3mm 30mm A. C. Ferrari et al, PRL97 (2006) 187401. Raman spectra of single-layer graphene of present study is consistent with that of previous study. Identification of high quality single-layer graphene Raman spectra of single-layer graphene sample fabricated by Ago group, Kyushu University, Japan

  12. 太陽電池の評価法 Single-layer graphene electric-double-layer transistors Cross section of single-layer graphene EDLT EDLT : Electric double layer transistor VG Device Sealing VD ESR direct observation of electrically controllable spin states in single-layer garaphene ESR measurements with voltage application

  13. Ion-gel insulator formed from ion liquid and copolymer Ion liquid is composed of positive and negative ions Ionic liquid: [EMIM][TFSI] 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl)imide Positive ion: EMIM Ion liquid Copolymer: PS-PMMA-PS poly(styrene-b-methyl-methacrylate-b-styrene) Negative ion: TFSI Self-assembled triblock copolymer in the presence of an ion liquid 平成27年卒業論文中間発表 J. Leeet al., J. Phys. Chem. C,113, 8972(2009). T.P. Lodge et al., Science, 321, 50 (2008).

  14. Feature of ion-gel gated transistor Field-effect transistor Ion-gel gated transistor ▪ Electric double layers are formed at the interface between charges in semiconductor and ions in ion-gel insulators ▪ Short distance d between charges dramatically increases the capacitance, causing high charge density

  15. Electrically induced ESR system VD < 0 Signal Analyzer Phase Detector Source Drain Semiconductor ＋ ＋ ＋ ＋ ＋ ＋ Microwave Bridge Insulator VG < 0 － － － － － － Gate ESR detection of accumulated charges Keithley 2612 Source meter electromagnet Device Cavity resonator JEOL RESONSNCE JES-FA200 ESR Spectrometer Simultaneous measurements of ESR and transistor characteristics

  16. Electrically controlled ESR and spin susceptibility VG dependence of ESR spectra VG dependence of spin susceptibility χ and 2D sheet conductivity σ2D Intensity and lineshape of ESR spectra depend on VG, showing a symmetry at the charge-neutral- point voltage VCNP = 0.8 V. Sci. Rep. 6 (2016) 34966.

  17. Observation of electrically induced Pauli paramagnetism in single-layer graphene using ESR spectroscopy Sci. Rep. 6 (2016) 34966.

  18. ESR spectra analysis at high |VG-VCNP| Fitting analysis of ESR spectrum ESR spectrum composes of two components: Lorentzianand Gaussian components. At high |VG-VCNP|, Lorentzian component is dominant. Sci. Rep. 6 (2016) 34966.

  19. Correlation between susceptibility and conductivity VG dependence of χL (Lorenz) Lorentzian component χL correlates with 2D sheet conductivity σ2D. Increase in χLby VGapplication ESR observation of electrically induced Pauli paramagnetism ▪ Observation of the increase in χL is due to accumulated charges ▪ Direct evidence of electrically induced carriers in Fermi- degenerate 2D electron system

  20. Electrically induced Pauli paramagnetism in single-layer graphene EDLT T dep. of spin susceptibility χ χof electrically induced ESR does not obey Curie law Pauli paramagnetism without temperature dependence VG = -1.5 V VD = 0.1 V Demonstration of the existence of Fermi-degenerate 2D electron systemin graphene from a microscopic viewpoint

  21. Elliot mechanism in electrically induced Puli paramagnetism in single-layer graphene EDLT T dep. of ESR linewidth ΔH1/2 The linewidthof electrically induced ESR signal increases with increasing T Elliott mechanism In the system with free mobile charge carriers, spin-lattice relaxation makes ESR linewidth broader when T increases. VG = -1.5 V VD = 0.1 V Increase in ESR linewidth with increasing T reflects the effect of Elliott mechanismin Fermi-degenerate 2D electron system

  22. Direct observation of electrically induced ambipolar spin vanishment in single-layer graphene using ESR spectroscopy Sci. Rep. 6 (2016) 34966.

  23. ESR spectra analysis at VCNP Fitting analysis of ESR spectrum ESR intensity shows a maximum at the charge neutral point voltage VG = 0.8 V. At VG = VCNP, Gaussian component is dominant. Sci. Rep. 6 (2016) 34966.

  24. Reverse correlation between susceptibility and conductivity VG dependence of χG (Gauss) Gaussian component χG reversely correlates with σ2D. Decrease in χGby VGapplication is due to non-magnetization of non-bonding orbital (NBO) ▪ Direct evidence for electrically induced ambipolar spin vanishment ▪ Decrease in spin scattering due to non-magnetization of NBOs

  25. Mechanism of electrically induced ambipolar spin vanishment DFT calculation of localized spin density at atomic vacancy of graphene Electrically induced ambipolar spin vanishment of atomic vacancies Y. Ma et al, New J. Phys.6 (2004) 68. ▪ Antiferromagnetic interactions between spins of atomic vacancies and accumulated electrons ▪ Discharge of atomic vacancies' spins by accumulated holes Sci. Rep. 6 (2016) 34966; ibid. 5 (2015) 11859.

  26. Correlation between electrically induced ambipolar spin vanishment and charge transport Charge neutral point (Dirac point) NBO: Non-bonding orbital Maximum of localized spin density Atomic vacancy’s spin is discharged by accumulated holes, forming non-magnetic state Atomic vacancy’s level is filled by accumulated electrons, forming non-magnetic state Non-magnetic states of NBOs decrease carriers’ spin scatterings of atomic vacancies → Increase in conductivity

  27. Summary Electrically induced ESR study of EDLTs with high quality single-layer graphene Direct ESR observation of electrically induced charge carriers due to Fermi-degenerate 2D electron systemin single-layer graphene EDLTs → Electrically induced Pauli paramagnetism Direct ESR observation of electrically induced ambipolar spin vanishmentof atomic vacancies’ spins in graphene → Similar behavior has bee observed for carbon nanotubes, which suggests that this spin vanishment is a universal phenomenon for carbon materials.

  28. Acknowledgements Collaborators: N. Fujita, D. Matsumoto, Y. Sakurai (U. Tsukuba) Dr. K. Kawahara, Prof. H. Ago (Kyushu U.) Prof. T. Takenobu (Nagoya U.) Prof. K. Yanagi (Tokyo Metropolitan U.) Funds: JST, PRESTO; JSPS; Univ. of Tsukuba; TIMS, U. Tsukuba; JST, ALCA SEI Group CSR Foundation; The Murata Science Foundation

  29. CNT薄膜のESR信号とスピン磁化率 スピン磁化率の温度依存性 ESR信号の温度依存性 キュリー則を観測 スピン間に磁気相互作用なし 原子空孔スピン濃度： 1.1× 1018 cm⁻³ スピン間距離：9.6 nm 全測定温度で単一成分のローレンツ型線形を観測 薄膜測定による表皮効果（ダイソン型線形）の排除

  30. Ion-gel Insular in EDLTs Network copolymer Ion liquid Ion gel = ＋ 攪拌、熱アニール Positive ion: [EMIM] [PS-PMMA-PS] Negative ion: [TFSI] ・絶縁物質としての機能 ・効率的な電荷注入の促進 電気二重層を作りdを非常に小さくする。 電極板間の距離dが大きく、電気容量が小さい。 ・ESR測定に支障を与えない 電気容量が大

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