N. Ravishankar. Intriguing Structure and Transport Behaviour of Ultrathin, Single Crystalline Gold Nanowires

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1 Intriguing Structure and Transport Behaviour of Ultrathin, Single Crystalline Gold Nanowires N. Ravishankar Materials Research Centre Indian Institute of Science Bangalore India 82 nd IAS Meeting, 6 Nov 2016, IISER Bhopal

2 Acknowledgments Collaborators Aditi Halder- Au Nanowire Growth Paromita, Chandni, Arindam Ghosh- Electrical Transport Ahin Roy, Abhishek Singh - DFT Studies Aditi Halder, Subhajit Kundu Synthesis of Au nanowires Paromita Kundu, Van Tendeloo (EMAT) Microscopy Knut Muller, Andreas Rosenauer (Bremen) AC-TEM Saransh, Prita, Gururajan (IITB) Molecular Dynamics Joerg Weissmuller (Hamburg) Continuum Analysis Praveen Kumar Finite Element Modelling Aveek Bid- Sensors, Noise Facilities: Materials Research Centre, IISc Advanced Facility for Microscopy and Microanalysis, IISc Funding: Swarnajayanti Fellowship, TUE - DST NanoMission, SERB-DST

3 What we do. Porous Structures by Aggregation Controlling Nanoparticle Interactions Controlling the Composition Stability of Porous Structures Wet Chemical Methods for Nanostructure Growth Understanding Mechanisms Functional Nanostructures (Nanolett. 2014, Angew.Chem 2014, Small 2014, ACS Nano 2014, Adv.Mater 2013, Nanoscale 2013, Particle 2013, ACS Nano 2011(a,b,c); Adv. Mater 2007; Biomater 2008; J. Phys. Chem. 2006, 2009; Nanotech. 2008; J. Coll. Interf. Sci. 2008; Langmuir 2009; J. Mat. Chem. 2010,2011; J. Am. Chem. Soc 2010, 2011; Nanoscale 2010, 2011, Nanotechnology 2007; Scripta Mater 2007) (ACS Nano 2011; Nanoscale 2013, 2011, 2010; Particle 2013; J. Mater. Chem. 2011; Langmuir 2009 ) Nanoscale Hybrids Understanding heterogeneous nucleation Microwave-assisted methods (JACS 2014; Dalton Trans 2013; RSC Adv 2013; PCCP 2013; Nanoscale 2013; ACS Nano 2011; J. Mater. Chem. 2011; Chem Mater 2011; Dalton Trans 2012; Nanoscale 2011, 2012; PCCP 2011)

4 Today Ultrathin Single Crystalline Au Nanowires - Mechanism of Growth - Structure - Properties and Applications - Electrical transport - Sensing - Catalysis

5 Diameter of Au wire ~ 2 nm 5-6 atoms thick ~ 2 m long images/figure jpg 2 nm

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7 Synthesis of building blocks Wet Chemical Method for Metal Nanoparticles M n+ + Reducing Agent + Capping Agent Capped M 0 C.J. Murphy Group Size Control Shape Control Mirkin Group

8 Equilibrium Morphology Growth Morphology - Depends on anisotropy of growth rates Pyrite {100} {111} {100} FCC - Reflects point-group symmetry - Initial stages of nucleation Cuprite NaCl Heyraud and Metois Au {111}

9 Prediction of pathways for shape control of nanostructures J. Phys. Chem. C 2009

10 Fact 1: Au is a high symmetry (cubic) metal and will not grow as a wire

11 External shape related to shape of building block

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13 No unique axis for a cubic system Presence of a unique axis Will not normally like to grow as a wire Can grow as a wire Symmetry breaking!!

14 July 2007 HAuCl 4 + Oleylamine AuCl Au (particles) Wires Au nanoparticles Au Nanowires

15 Amine-Capped Nanoparticles {100} & {111} Facets Adv. Mater. 2007

16 Ultrathin Single Crystalline Nanowires Adv. Mater. 2007

17 Coalescence of droplets Regions of Different Chemical Potential Diffusion from convex to concave regions Neck Growth during Sintering of Particles Adv. Mater. 2007

18 Neck Growth by Surface Diffusion t ( x r 6 ) r 4 RT CwDσ sv Ω Average time between Collision of Brownian Particles t 3πηr ( kt 1 )( ) ρ 2 3 Adv. Mater. 2007

19 Fact 2: Au has an FCC structure

20 Fraction of surface atoms increases dramatically Introduction_to_Inorganic_Chemistry/Basic_Science_of_Nanomaterials

21 Atoms are social creatures! Like to be surrounded by their kind Atoms on the surface are unhappy less neighbours butane.chem.uiuc.edu

22 Origin of Surface Stress Tendency of surface atoms to come closer (positive surface stress) Leads to stress within the bulk of the material Stresses linked by the generalized capillary equation Weissmuller, in Electrocatalysis: Theoretical Foundations and Model Experiments, I ed

23 Lattice Parameter Changes in Small Particles ChemPhysChem 2012, 13,

24 Density Functional Theory + Electron Microscopy

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26 Density Functional Theory Results Au Nanowires Hexagonal prism morphology [111] wire axis bounded by {110} prism planes ABC stacking and periodic boundary conditions Displacements of the order of 20 pm about the mean position

27 Atomic Structure of Au Wires - Atomic defects - strain Wires on C-film: Titan 120 kv, dose e/å 2 TITAN 3 In collaboration with EMAT, Antwerp

28 Internal Wrinkling of Close-packed Planes

29 Ultrathin Au Nanowires grown on a graphene support - Minimize complications in interpretation due to substrate Aberration-corrected microscopy to study modulations

30 Experimental Image Calculated Image (STEMsim) With Knut Muller and Andreas Rosenauer

31 Molecular Dynamics Simulations Displacement localized at surface

32 Fact 3: Au is a good metal and is an excellent electrical conductor

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35 ACS Nano (2011) Metallic wire??!!

36 Fact 4: Au is very noble and unlikely to be a good catalyst

37 2010

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39 Summary Ultrathin single crystalline Au nanowires Conventional wisdom fails!! Thank You