Graphene & Nanowires: Applications Kevin Babb & Petar Petrov Physics 141A Presentation March 5, 2013

What is a Nanowire? “One-dimensional” structure o Diameter: nanometers (10 -9 m) o Length: microns (10 -6 m) Exhibits crystal structure o Unlike quantum “dots” (0-dimensional) Many different materials o Metals, semiconductors, oxides Kevin Babb & Petar Petrov – Physics 141A – Spring 20132

Features of Nanowires Smallest dimension which can transport charge carriers (e -, h + ) o Can act as both nanoscale devices and wiring o Unique density of states Controlled synthesis o Diameter, length, composition o Electronic structure (band gap, doping) Size o Quantum confinement Present in some, absent in others Unique magnetic & electronic properties o Millions more transistors per microprocessor o Probe microscopic systems (e.g. cells) Kevin Babb & Petar Petrov – Physics 141A – Spring 20133

Graphene Reminder Graphene is a 2-d from of pure carbon Band gap depends on structure o Large area monolayers o Bilayers o Nanoribbons

Solar Cells Currently: silicon wafers, thin films Application of graphene: o Transparent conducting electrodes Robust, conductive, abundant Cheaper than ITO Application of nanowires: o Enhanced light trapping o Efficient charge transport (1D) Kevin Babb & Petar Petrov – Physics 141A – Spring 20135

Graphene-Nanowire Solar Cells A new design: o Layer of graphene (transparent cathode) o Conductive polymer (maintains integrity) o ZnO nanowire layer (electron transport) o PbS quantum dots (hole transport) o Au layer (anode) Efficiency approaches ITO-based solar cells o 4.2% conversion efficiency (5.1% for ITO) o Cheaper to produce Kevin Babb & Petar Petrov – Physics 141A – Spring 20136

Field Effect Transistors Challenges to scaling o Lower transconductance o Manufacturing difficulties o Quantum effects o Gate capacitance

Graphene FETs Graphene FETs Challenges o Low on-off ratios o High graphene- electrode contact resistance o Tradeoff between mobility and bandgap Advantages – High room temperature mobility – Thinner than traditional MOSFETs

Nanowire FETs Advantages o Many different nanowires with different properties o High mobility o “Bottom up” synthesis Challenges o Integrating NW into circuit o Control of growth and dopants

Light-Emitting Diodes LEDs versus conventional lighting: o Efficient: less heat, lower power consumption o Long lifetime o Cheap o No mercury How nanowires help: o Various geometries of p-n junctions available Coaxial wires Thin film/wire combinations Crossed-wire junction arrays o Unique carrier transport properties Natural waveguiding cavities o Improve extraction efficiency of light High surface area improves conductivity Kevin Babb & Petar Petrov – Physics 141A – Spring

Artificial Photosynthesis Simulate natural photosynthetic process o Convert CO 2 and H 2 O into fuels, O 2 H 2 O oxidation CO 2 reduction How nanowires help: photoelectrodes o High surface area for reaction sites o High charge mobility due to small diameter o Can be grown in large quantities Kevin Babb & Petar Petrov – Physics 141A – Spring

Touch Screen Devices Graphene is strong, transparent, highly conductive, and cheaper than traditional ITO

This is scalable!

Ultracapacitors Graphene advantages: o High surface area to weight ratio (2600 m 2 /g) o High conductivity o Measured specific capacitance 135 F/g Uses: o Electric vehicles o Backup powering o High power capability o Cell phones

References Physical Foundations of Solid State Devices, E. F. Schubert Y. J. Hwang, et al., Nano Lett., 2012, 12, 1678–1682 A. Hochbaum, Chem. Rev., 2010, 110, 527–546 H. Park, et al., Nano Lett., 2013, 13, E. Lai, et al., Nano Res., 2008, 1, D. Siburly, et al., J. Phys. Chem, 2005, 109, F. Schwarz, Nature Nanotechnology, 2010, 5, 487–496 S. Bae, et al., Nature Nanotechnology, 2010, 5, 574–578 M. Stoller, et al., Nano Lett., 2008, 8, 3498–3502 Y. Zhang, et al., Nature, 2009, 459, Kevin Babb & Petar Petrov – Physics 141A – Spring