Chun Ning Lau (Jeanie) Quantum Transport! in! 2D Atomic Membranes!

Transcription

1 Chun Ning Lau (Jeanie) Quantum Transport! in! 2D Atomic Membranes!

2 2D Materials and Heterostructures! hbn MoS 2 WSe 2 Fluorographene Geim, Nature Conductors, e.g. graphene, few-layer graphene Semiconductors, e.g MoS 2, WS 2, Superconductors, Nb 2 Se 3 Insulators, e.g. hbn Charge density waves, e.g. NbSe Ferromagnets, e.g. VSe 2

3 Outline! There is still life in graphene. Beyond graphene Few Layer MoS 2 Few-layer Phosphorene

4 Dual-Gated Suspended ABC Trilayer Graphene G (µs) 200 G (µs) mobility 20,000 90,000 cm 2 / Vs T (K) mv /T (1/K) 2x10 4 Metal insulator transition, T c ~ 35K Thermal activation measurement yields Δ ~ 41 mev G(V bias ) curves at E =n=0 yield Δ 42 mev di/dv (µs) 0-40 V 40 bias (mv) (V)

5 Effect of electric and magnetic fields Differential conductance G vs source drain bias V at n=0" 40 V (mv) B (T) 30 gap educed symmetrically by E! à not layer polarized; arises from electronic interactions" gap reduced by parallel magnetic field at 30T" Y. Lee, D. Tran, K. Myhro, J. V. Jr., N. Gillgren, C. N. Lau, Y. Barlas, J. M. Poumirol, D. Smirnov, and F. Guinea, Nature Communications, 5, 5656 (2014)

6 Proposed Phase Diagram! U Quantum Valley Hall Layer Anti- Ferromagnet Canted Anti- Ferromagnet Ferromagnet B Current EU collaboration: Paco Guinea (CSIC, Spain; Machester) Frank Koppens (ICFO; Spain) Y. Lee, D. Tran, K. Myhro, J. V. Jr., N. Gillgren, C. N. Lau, Y. Barlas, J. M. Poumirol, D. Smirnov, and F. Guinea, Nature Communications, 5, 5656 (2014)

7 MoS 2 gapped, On/Off ratio >10 6 direct-to-indirect band gap transition as function of thickness valley physics But Mobility <~ cm 2 /Vs Radisavljevic et al, Nat. Nanoetchnol What is the mobility bottlenck? Wu et al, Nat. Phys and many others

8 Suspending MoS 2 F. Wang, M. Gray, P. Stepanov and C.N. Lau, Nanotechnology, in press (2015) the mobility is even lower, cm 2 /Vs gas annealing à 200 cm 2 /Vs Removing substrates does not significantly improve mobility Other mobility bottlenecks: Schottky barriers at contact impurity scattering defects

9 Ionic liquid gating of MoS 2 In collaboration with Robert Haddon at UCR Ionic liquids are molten salts with low melting point can induce high carrier density (up to cm -2 ) To date all IL gating are performed on substrate-supported devices Suspended devices enable gating from both surfaces IL" S" D" SiO 2 " Si" V ILg " IL"gate" F. Wang, M. Gray, P. Stepanov and C.N. Lau, in preparation (2015)

10 Comparing Suspended and non-suspended devices Performed IL gating of 9 suspended and 9 substrate-supported samples use DEME-TFSI all suspended devices are more conductive by at least 1-2 orders of magnitude à IL gating is more effective in freestanding devices Mechanism: 1. Higher charge density 2. Better screening F. Wang, M. Gray, P. Stepanov and C.N. Lau, in preparation (2015)

11 Transport Mechanism V Ilg =0 Schottky emission at MoS 2 -electrode interfaces 8 (a) I (µα) -16 (e) $ I exp a V Φ B & % k B T ' ) ( V ds (V) a = e e 4πε 0 ε r d I (µα) -5 slope yields ε r ~ 11 à dielectric constant of DEME-TFSI ~ 14.5 à agrees with literature values -1 1 V ds (V) Fujimoto, T.; Awaga, K. Phys Chem Chem Phys 15, 8983 (2013).

12 Charge Density Induced in Suspended MoS 2 Compare ΔV bg and ΔV IL needed to induce the same change in conductance ratio of ionic liquid gate to back gate: up to 450 à α up to 4.6x10 13 cm -2 V -1 > 2-4x previous values F. Wang, M. Gray, P. Stepanov and C.N. Lau, in preparation (2015)

13 IL-tuned Metal Insulator Transition 100 2V 1.5V V ILg =3V V ILg =3V 2V 1.5V σ s (µs) 10 1V 0V 1V 1 0V V -0.5V T (K) /T (1/K) metal insulator transition observed as V ILg is tuned At small V ILg, transport via thermal activation $ I exp a V Φ ' B & ) a = e % k B T ( e 4πε 0 ε r d obtained from I-V curves

14 Conclusion Mobility not limited by substrate in current generation of devices Bottleneck: Schottky barrier at MoS2-electrode interface à critical: contact engineering Ionic liquid gating of suspended devices à ion accumulation on both surfaces à higher charge density, enhanced screening see Cui et al, arxiv (2014) Further optimization à Ultra-high density regime for new phases p-doping à spin/valley transport F. Wang, M. Gray, P. Stepanov and C.N. Lau, in preparation (2015)

15 Outline! Few Layer Graphene Few Layer MoS 2 Fabrication and annealing of suspended MoS 2 Ionic liquid gating Few-layer Phosphorene Fabrication of air-stable, high mobility devices Observation of quantum oscillation

16 Curse of 2D Materials Graphene Mobility ~ cm 2 /Vs Gapless MoS 2,WS 2, MoSe 2, WSe 2, etc Mobility ~ 100 cm 2 /Vs Gapped Black Phosphorus most stable form of phosphorus layered structure bulk mobility up to 60,000 cm 2 /Vs peroidictable.com

17 Black Phosphorus Tran et al, PRB 2014 Asahina & Morita, J. Phys. C, 1986 only other layered element Puckered atoms within layers Anisotropic Thickness dependent band gap, ev Direct band gap for all thickness

18 Few-Layer Black Phosphorus Transistors Li et al, Nature Nanotechnol 2014 ambipolar transport gapped, on/off ration ~10 5 Anisotropic Transport Mobility ~ cm 2 /Vs for thickness ~2 20 nm Best of both worlds! Liu et al, ACS Nano 2014 Xia et al, Nature Comm. 2014

19 Challenges Kroenig et al, APL 2014 Island et al, 2D Materials 2014 Instability in air react with water and O 2 to form phosphoric acid reaction accelerated by light Favor et al, arxiv 2014

20 Encapsulation for stable, high mobility Devices hexagonal boron nitride (hbn) from wikipedia atomically flat no dangling bonds à little trapped charges high mobility graphene/hbn devices demonstrated Encapsulate few-layer phosphorene with hbn? Columbia group, Nature Nanotechnol. 2012

21 Device Fabrication phosphorene hbn PDMS top gate SiO 2 hbn electrode Dry transfer to form hbn/few-layer phosphorene/hbn heterostructure sandwiches etch to expose edges of phosphorene 1D metallic contact to 2D layers Si/SiO 2 Wang et al, Science 2013

22 Device Stability Encapsulated in hbn (our data) Device left in air for 2 weeks Slight shift in charge neutrality point Only slight decrease in conductance & mobility Wood et al, Nano Letters 2014 N. Gillgren, D. Wickramaratne, Y. Shi, T. Espiritu, J.Yang, J. Hu, J. Wei, X. Liu, Z. Mao, K. Watanabe, T. Taniguchi, Marc Bockrath, Yafis Barlas, R. K. Lake, C.N. Lau, 2D Materials, 2, (2014)

23 Device mobility R xx (Ω) R xx (Ω) Ambipolar transport On/off ratio ~ 10 5 linear I-V à ohmic contact Metal-insulator transition highly hole-doped: metallic, µ up to 4000 towards band edge: insulating, µ ê with T

24 Quantum Oscillations a ΔR xx (Ω) R xx with smooth background subtracted oscillations periodic in 1/B oscillations periodic in V g ~n doubling frequency in for B>8T à Zeeman splitting c d N. Gillgren, D. Wickramaratne, Y. Shi, T. Espiritu, J.Yang, J. Hu, J. Wei, X. Liu, Z. Mao, K. Watanabe, T. Taniguchi, Marc Bockrath, Yafis Barlas, R. K. Lake, C.N. Lau, 2D Materials, (2015)

25 Temperature Dependence Quantum Oscillations Oscillations amplitude dependence on T b effective mass of charge carriers ~0.25 to 0.31 m e as Fermi energy increases towards band edge agree with DFT calculations within 50% N. Gillgren, D. Wickramaratne, Y. Shi, T. Espiritu, J.Yang, J. Hu, J. Wei, X. Liu, Z. Mao, K. Watanabe, T. Taniguchi, Marc Bockrath, Yafis Barlas, R. K. Lake, C.N. Lau, 2D Materials, (2014)

26 Conclusion Few layer phosphorene has both high mobility and band gap Stable via hbn encapsulation Outlook Physics strain-dependent band gap large anisotropy (up to factor of 60, electrical and thermal transport, thermopower) electric field effect quantum Hall effect Electronics and optoelectronics hbn encapsulation of reactive 2D materials see Cao et al, arxiv: b Number month 12

27 Acknowledgments! Graduate Students Undergraduate Students Tim Espiritu Kevin Thilahar Mason Gray Ziqi Pi Yongjin Lee " Jhao-wun Huang " Fenglin Wang" Kevin Myhro " Yanmeng Shi" Nathaniel Gillgren" UCOP Petr Stepanov " Son Tran "

28 Collaborators! UCR Physics Marc Bockrath UCR Chem. & CEE Robert Haddon UCR Physics Yafis Barlas UCR EE Roger Lake Florida Mag Lab Dmitry Smirnov Jean-Marie Poumirol Tulane Zhiqiang Mao Tulane Jiang Wei CSIC Paco Guinea