The Intriguing and Complex Case of Phosphorene and Silicene Deji Akinwande, TI/Jack Kilby Asst. Professor The University of Texas– Austin NSF US-EU 2D Workshop, 2015 US-EU Collaboration: Graphene Growth & Metrology Chemical Vapor Deposition Growth Evaporate Cu High-T H2 anneal High-T CH4 growth a-Cu (111) Cu (111) Cu SiO2 SiO2 SiO2 SiO2 Si Si Si Si 2D T STM courtesy ) L C R Of Wallace group a.u D G B Intensity ( Intensity Ref. L. Tao, J. Lee, H. Chou, M. Holt, R. S. Ruoff, and D. Akinwande, ACS Nano, 2012. 2 Device Statistics and Technology Transition Nassibe et al., ACS Nano, 2014 >10,000 devices 300mm Aixtron Tool EU Graphene Flagship 300 mm 150 mm 100 mm 3 2D Nanoelectronics: The Future is Flexible watches Smart Flexible Smart Smart Smart Systems phones Consumer Applications Consumer Defense Applications Defense Flexible, Stretchable, Wearable, Origami- inspired Foldable Smart Systems by2D Carbon Akinwande, et al nature comm. Graphene Smartphone Smartphone Graphene invited review (Dec 2014) >50K units sold 4 Why 2D Sheets for Flexible Nanoelectronics? - Atomic sheets offer maximum flexibility, transparency, electrostatics Comparison 2D Materials Other Materials Conclusions: - Graphene/Phosphorene/TMDs good for RF circuits - Phosphorene & TMDs good for digital circuits Akinwande, Petrone, Hone, nature comm. invited review (Dec. 2014) 5 Recent Results on Synthesized Monolayer MoS2 With Sanjay Banerjee TEM MoS2 ft and fmax Measurements 55 50 45 MoS2 40 35 Intrinsicm2 ft~7GHz freq=7.050GHz 30 dB(FT)=-0.3186 1mm (dB) v ~1.3x10 cm/s SiO /Si 25 sat 2 dB(FT) 21 H 20 15 10 5 L~0.3um m2 0 1E8 1E9 1E10 5E10 Freq.freq, (Hz) Hz -Extrinsic ft~3GHz -Intrinsic fmax~5GHz -Extrinsic fmax~3.7GHz 2 -theoretical v ~3.4x106cm/s uo~25cm /V-s sat 6 -theoretical TMD ~vsat~2-5x10 cm/s In review, nano letters, (2015) 6 7.050G -318.2m Phosphorene: First Flexible Devices and Circuits Zhu et al., Nano letters, 2015 Characteristics and Air-Stability Comparison with TFTs FET mobility Material (cm2/V·s) ON/OFF Film Transport ratio thickness Type (nm) µe µh 3 4 TEM 89 310 10 -10 15 ambipolar Black 3 180 104-105 5 ambipolar phosphorus 7 8 MoS2 30 - 10 -10 10 n-type 7 WSe2 24 45 10 1.5 ambipolar 6 SnSxSe2-x 12 - 10 5 n-type Pentacene - 8.85 103-104 65 p-type InGaZnO 76 - 105 23 n-type7 Phosphorene: First Flexible Devices and Circuits Zhu et al., Nano letters, 2015 Frequency Flexible Amplifiers Doublers # of Cycles 10-1 5000 500 0 A) μ -2 ( 10 d I - 10-3 -3 -2 -1 0 1 2 3 Vg (V) Radio Video https://www.youtube.com/watch?v=Sd6JGbKmvUY 8 It’s Complicated: Air-Stability Issue Kim et al., Nature Scientific Reports, 2015 Uncapped sample in air (b4 and after ~24hrs) AFM Video Uncapped sample (initial data) 9 Understanding the Complex Air-Stability Issue Kim et al., Nature With Keji Lai Scientific Reports, 2015 Air-Stability of thin capped samples studied by Near-Field Microwave Conductive AFM 104 105 ) sq / 106 ( sh Al O R 2 3 >109 5 μm P++ Si Day 1 Day 2 Day 3 Day 4 Day 5 Day 6 Day 7 50 nm 5 μm Topography 0 nm Physical Mechanisms of Air-Degradation 10 What can Silicene offer? 11 Silicene (1994-Present): The 20yr Odyssey Topological Phases -Quantum Spin Hall (QSH) -Quantum Anomalous Hall (QAH) -Spin-polarized QAH (SQAH) -Quantum spin-quantum anomalous Hall (QSQAH) -Topological single-valley semimetal (SV) 12 Silicene’s Band Structure on Ag-Free Substrate Notes: Phase dependent band-structure of Ag-free silicene similar to CNTs -Silicene on Ag loses Dirac cone due to strong Si-Ag hybridization 4x4 Silicene Scalise et al, ASS 2014 √13x √ 13 Silicene 2√3x 2√ 3 Silicene The “Swiss-knife” of 2D materials 13 Innovation: SEDNE Silicene Encapsulated Delamination Native Electrodes 2 1 transferred silicene/Ag Silicene after transfer after 7 days Silicenesilicene/Ag as growth as grown 1 Intensity (a.u.) 0 300 400 500 600 700 800 Tao et al., Nature Nanotechn., Feb 2015 Raman shift (cm-1) The 20yr Odyssey: 1st Silicene Device Results Tao et al., Nature Nano, Feb 2015 √13x√13 mixed phase silicene 11x 9 -2 2 3 -For Silicene, no~8x10 cm 4 푘푇 휋 푛표 = 2 11 -2 3 ℎ 푣퐹 -For Graphene, no~1.5x10 cm 15 15 Origin of Low Mobility: Intrinsic or Extrinsic? Predicted theoretical Desired: Uniform single- Reality: Multi-phase silicene mobility limit for ideal phase silicene, e.g √13x√13 with many phase boundaries silicene crystal √13x√13 Ki-Wook Kim’s group, PRB, 2013 ~1,200cm2/Vs at 300K ~ 30,000cm2/Vs at 50K -limited by out-of-plane acoustic (ZA) phonons Main Non-idealities - Phase boundary scattering - Remote phonon scattering from high-k interface? - Charge impurity scattering - ~low mobility is similar to early results on synthetic graphene, 10-100x lower than exfoliated 16 16 TMD Graphene Sandwich h-BN Planar 2D Diatomic Atomic Crystals Buckled Sheets: TheBuckled Twin Grand ChallengesXenes Growth Air-Stability BP Si BP Si Sustained Multi-Disciplinary Research and Collaboration is key to success! 17 Acknowledgements Thank You Collaborator: Alessandro Molle, IMM-CNR, Italy SEED Group, ARL.