An Introduction to NanoWires

1. An Introduction toNanoWires University of South Alabama Electrical engineering department And Their Applications Amir Dindar Shoeb Roman

2. An Introduction to Nanowires and their applications Introduction • Bottom-up assembled nanoscale electronics could hold the promise of powering future electronic devices that can outperform existing devices and open up totally new opportunities. • It will require conceptually new device building blocks, scalable circuit architectures, and fundamentally different fabrication strategies. • Central to the bottom-up approach are the nanoscale building blocks. University Of South Alabama, EE Department

3. An Introduction to Nanowires and their applications Introduction • 1D nanostructures represent the smallest dimension structure that can efficiently transport electrical carriers • 1D nanostructures can also exhibit critical device function, and thus can be exploited as both the wiring and device elements in future architectures for functional nanosystems • In this regard, two material classes: semiconductor nanowires (NWs) carbon nanotubes (NTs) have shown particular promise University Of South Alabama, EE Department

4. An Introduction to Nanowires and their applications Introduction Single-walled NTs have been used to fabricate field effect transistors, diodes, and logic circuits. Problems with Nanotubes to made devices: • Difficulties to control whether building blocks are semiconducting or metallic • Difficulties in manipulating individual NTs So, to date, device fabrication by NT largely is a random event, thus pose a significant barrier to achieving highly integrated nanocircuits. University Of South Alabama, EE Department

5. An Introduction to Nanowires and their applications Introduction Advantages of Nanowires: • NW devices can be assembled in a rational and predictable because: • Nanowires can be precisely controlled during synthesis, • chemical composition, • diameter, • length, • doping/electronic properties • Reliable methods exist for their parallel assembly. • It is possible to combine distinct NW building blocks in ways not possible in conventional electronics. • NWs thus represent the best-defined class of nanoscale building blocks, and this precise control over key variables has correspondingly enabled a wide range of devices and integration strategies to be pursued University Of South Alabama, EE Department

6. An Introduction to Nanowires and their applications Introduction • Semiconductor NWs have been assembled into a series of electronic electronics devices: • crossed NW p-n diodes, • crossed NW-FETs, • nanoscale logic gates and computation circuits, • optoelectronic devices • More general applications: • Interconnects for nano electronics • Magnetic devices • Chemical and biological sensors • Biological labels University Of South Alabama, EE Department

7. An Introduction to Nanowires and their applications Introduction • Diameter of nanowires range from a single atom to a few hundreds of nanometers. • Length varies from a few atoms to many microns • Different name of nanowires in literature: • Whiskers, fibers: 1D structures ranging from several nanometers to several hundred microns • Nanowires: Wires with large aspect ratios (e.g. >20), • Nanorods: Wires with small aspect ratios. • NanoContacts: short wires bridged between two larger electrodes. • Regarding to size (diameter) we have two different types of nanowires: • Classical nanowires • Quantum nanowires University Of South Alabama, EE Department

8. An Introduction to Nanowires and their applications Building Blocks Synthesis Different techniques can be generally grouped into four categories: • Spontaneous growth: • Evaporation condensation • Dissolution condensation • Vapor-Liquid-Solid growth (VLS) • Stress induced re-crystallization • Template-based synthesis: • Electrochemical deposition • Electrophoretic deposition • Colloid dispersion, melt, or solution filling • Conversion with chemical reaction • Electro-spinning • Lithography (top-down) University Of South Alabama, EE Department

9. An Introduction to Nanowires and their applications Building Blocks Synthesis, Spontaneous Growth General Idea: • Anisotropic growth is required • Crystal growth proceeds along one direction, where as there is no growth along other direction. • Uniformly sized nanowires (i.e. the same diameter along the longitudinal direction of a given nanowire) University Of South Alabama, EE Department

10. An Introduction to Nanowires and their applications Spontaneous Growth, Evaporation condensation • Referred to as Vapor-Solid (VS) technique. • Nanowires and nanorods grown by this method are commonly single crystals with fewer imperfections • The formation of nanowires or nanorods is due to the anisotropic growth. • The general idea is that the different facets in a crystal have different growth rates • There is no control on the direction of growth of nanowire in this method University Of South Alabama, EE Department

11. An Introduction to Nanowires and their applications Spontaneous Growth, Evaporation condensation (Picture from: “Nanostructures of zinc oxide,” by Zhon Lin Wang, http://www.materialstoday.com/pdfs_7_6/zhang.pdf) University Of South Alabama, EE Department

12. An Introduction to Nanowires and their applications Spontaneous Growth, Evaporation condensation Mesoporous, single-crystal ZnO nanowires. (Picture from: “Nanostructures of zinc oxide,” by Zhon Lin Wang, http://www.materialstoday.com/pdfs_7_6/zhang.pdf) University Of South Alabama, EE Department

13. An Introduction to Nanowires and their applications Spontaneous Growth, Evaporation condensation Picture : Measuring the Work Function at a Nanobelt Tip and at a Nanoparticle surface, http://www.nanoscience.gatech.edu/zlwang/paper/2003/03_NL_2.pdf University Of South Alabama, EE Department

14. An Introduction to Nanowires and their applications Spontaneous Growth, Evaporation condensation Ultra-narrow ZnO nanobelts. (Picture from: “Nanostructures of zinc oxide,” by Zhon Lin Wang, http://www.materialstoday.com/pdfs_7_6/zhang.pdf) University Of South Alabama, EE Department

15. An Introduction to Nanowires and their applications Spontaneous Growth, Dissolution condensation • Differs from Evaporation-condensation • The growth species first dissolve into a solvent or a solution, and then diffuse through the solvent or solution and deposit onto the surface resulting in the growth of nanorods or nanowires. • The nanowires in this method can have a mean length of <500 nm and a mean diameter of ~60 nm University Of South Alabama, EE Department

16. An Introduction to Nanowires and their applications Spontaneous Growth, Vapor Liquid Solid Growth (VLS) General Idea: A second phase material, commonly referred to as catalyst, is introduces to direct and confine the crystal growth on a specific orientation and within a confined area. • Catalyst forms a liquid droplet by itself • Acts as a trap for growth species • The growth species is evaporated first and then diffuses and dissolves into a liquid droplet • It precipitates at the interface between the substrate and the liquid University Of South Alabama, EE Department

17. An Introduction to Nanowires and their applications Spontaneous Growth, Vapor Liquid Solid Growth (VLS) Growth species in the catalyst droplets subsequently precipitates at the growth surface resulting in the one-directional growth University Of South Alabama, EE Department

18. An Introduction to Nanowires and their applications Spontaneous Growth, Vapor Liquid Solid Growth (VLS) Picture : “A Non-Traditional Vapor-Liquid-Solid Method for Bulk Synthesis of Semiconductor Nanowires,” Shashank Sharma, and Mahendra K. Sunkara, http://www.cvd.louisville.edu/Publications/recentpublications/proceedings_mrs_fall2001.pdf University Of South Alabama, EE Department

19. An Introduction to Nanowires and their applications Spontaneous Growth, Vapor Liquid Solid Growth (VLS) TEM and selected area diffraction image of a single crystal ZnO nanorod.(~20 nm width). Pictures : “ZnO nanowire growth and devices,” Y.W. Heoa, D.P. Nortona, et al., Materials Science and Engineering R 47 (2004) 1–47 University Of South Alabama, EE Department

20. An Introduction to Nanowires and their applications Spontaneous Growth, Vapor Liquid Solid Growth (VLS) Z-contrast scanning transmission electron microscopy image of a (Zn,Mg)O nanorod with a Ag catalyst particle at the rod tip. Pictures : “ZnO nanowire growth and devices,” Y.W. Heoa, D.P. Nortona, et al., Materials Science and Engineering R 47 (2004) 1–47 University Of South Alabama, EE Department

21. An Introduction to Nanowires and their applications Template Base synthesis General Idea: • This is the very general method • Use in fabrication of nanorods, nanowires, and nanotubes of polymers, metals, semiconductors, and oxides. • Some porous membrane with nano-size channels (pores) are used as templates from conduct the growing of nanowires • Pore size ranging from 10 nm to 100 mm can be achieved. University Of South Alabama, EE Department

22. An Introduction to Nanowires and their applications Template Base synthesis • Electrochemical Deposition • Negative template • Positive template • This is a self-propagating process • This method can be understood as a special electrolysis resulting in the deposition of solid material on an electrode • Only applicable to electrically conductive materials: metals, alloys, semiconductors, and electrical conductive polymers. University Of South Alabama, EE Department

23. An Introduction to Nanowires and their applications Template Base synthesis, Electrochemical Deposition Negative Template • use prefabricated cylindrical nanopores in a solid material as templates • There are several ways to fill the nanopores to form nanowires, but the electrochemical method is a general and versatile method. • Electrodeposition often requires a metal film on one side of the freestanding membrane to serve as a working electrode on which electrodeposition takes place • If dissolve away the host solid material, free-standing nanowires are obtained. University Of South Alabama, EE Department

24. An Introduction to Nanowires and their applications Template Base synthesis, Electrochemical Deposition • The diameter of the nanowires is determined by the geometrical constraint of the pores • Fabrication of suitable templates is clearly a critical first step University Of South Alabama, EE Department

25. An Introduction to Nanowires and their applications Template Base synthesis, Electrochemical Deposition A porous Template Nanowire array Picture: “Fabrication of Polypyrrole Nanowire and Nanotube Arrays,” Fa-Liang Cheng*, Ming-Liang Zhang and Hong Wang, http://www.mdpi.net/sensors/papers/s5040245.pdf University Of South Alabama, EE Department

26. An Introduction to Nanowires and their applications Template Base synthesis, Electrochemical Deposition 75 nm nano wires grown in a 80nm template membrane after dissolution of the membrane. 210nm 100nm Picture: “Fabrication of Polypyrrole Nanowire and Nanotube Arrays,” Fa-Liang Cheng*, Ming-Liang Zhang and Hong Wang, http://www.mdpi.net/sensors/papers/s5040245.pdf University Of South Alabama, EE Department

27. An Introduction to Nanowires and their applications Template Base synthesis, Electrochemical Deposition Advantages • The ability to create highly conductive nanowires. Because electrodeposition relies on electron transfer, which is the fastest along the highest conductive path. • electrodeposited nanowires tend to be dense, continuous, and highly crystalline in contrast to other deposition methods. • the ability to control the aspect ratio of the metal nanowires by monitoring the total amount of passed charge. University Of South Alabama, EE Department

28. An Introduction to Nanowires and their applications Template Base synthesis, Electrochemical Deposition Three typical stages in electrodeposition process: stage I: corresponds to the electrodeposition of metal into the pores until they are filled up to the top surface of the membrane (stage I) Stage II: the pores are filled up with deposited metal, metal grow out of the pores and forms hemispherical caps on the membrane surface Stage III: When the hemispherical caps coalescence into a continuous film University Of South Alabama, EE Department

29. An Introduction to Nanowires and their applications Template Base synthesis, Electrochemical Deposition stage I Picture: Template Synthesis of Nanowires in Porous Polycarbonate Membranes: Electrochemistryand Morphology, http://www.phys.ens.fr/~bachtold/publication/wire-JPCB.pdf University Of South Alabama, EE Department

30. An Introduction to Nanowires and their applications Template Base synthesis, Electrochemical Deposition stage II Picture: Template Synthesis of Nanowires in Porous Polycarbonate Membranes: Electrochemistryand Morphology, http://www.phys.ens.fr/~bachtold/publication/wire-JPCB.pdf University Of South Alabama, EE Department

31. An Introduction to Nanowires and their applications Template Base synthesis, Electrochemical Deposition stage III Picture: Template Synthesis of Nanowires in Porous Polycarbonate Membranes: Electrochemistryand Morphology, http://www.phys.ens.fr/~bachtold/publication/wire-JPCB.pdf University Of South Alabama, EE Department

32. An Introduction to Nanowires and their applications Template Base synthesis, Electrochemical Deposition • To have freely standing nanowires we have to remove the template hosts after forming the nanowires in the templates by dissolving away the template materials in a suitable solvent. • If want to separate the nanowires from the metal films on which the nanowire are grown, a common method is to first deposit a sacrificial metal. University Of South Alabama, EE Department

33. An Introduction to Nanowires and their applications Template Base synthesis, Electrochemical Deposition • Positive Template Method • Use wire-like nanostructures, such as DNA and carbon nanotubes as templates. • Nanowires are formed on the outer surface of the templates • Diameter of the nanowires is not restricted by the template sizes and can be controlled by adjusting the amount of materials deposited on the templates • Removing the templates after deposition, wire-like and tube-like structures can be formed University Of South Alabama, EE Department

34. An Introduction to Nanowires and their applications Template Base synthesis, Electrochemical Deposition • DNA based template • DNA is an excellent choice as a template to fabricate nanowires because its diameter is ~2 nm and its length and sequence can be precisely controlled • General procedure: • Fix a DNA strand between two electrical contacts • Exposed to a solution containing some ions • Ions bind to DNA and are then form some nanoparticles decorating along the DNA chain University Of South Alabama, EE Department

35. An Introduction to Nanowires and their applications Template Base synthesis, Electrochemical Deposition • DNA based template • General procedure: • Fix a DNA strand between two electrical contacts • Exposed to a solution containing some ions • Ions bind to DNA and are then form some nanoparticles decorating along the DNA chain University Of South Alabama, EE Department

36. An Introduction to Nanowires and their applications Template Base synthesis, Electrophoretic Deposition Differs from electrochemical deposition in several aspects The deposit need not be electrically conductive Particularly for oxide nanowires: SiO2, TiO2, Bi2O3, etc. Different sizes of TiO2 nanorods grown in a membrane by sol electrophoretic deposition. Diameters: (A) 180 nm, (B) 90 nm, (C) 45 nm Picture: “A study on the growth of TiO2 nanorods using sol electrophoresis,” S. J. LIMMER, T. P. CHOU, G. Z. CAO, University of Washington, http://faculty.washington.edu/gzcao/publications/papers/31.pdf University Of South Alabama, EE Department

37. An Introduction to Nanowires and their applications Template Base synthesis, Electrophoretic Deposition • Method: • over the surface of nanoparticles develops an electrical charge via some chemical techniques. This combination is typically called Counter-Ion • Upon application of an external electric filed to a system of charged nanosize particle system, the particles are set in motion in response to the electric filed • This type of motion is referred to as electrophoresis. • The rest of this technique, in general, is the same as electrochemical deposition. University Of South Alabama, EE Department

38. An Introduction to Nanowires and their applications Template Base synthesis, Surface Step-Edge Templates • General Idea • Atomic-scale steps on a crystal surface can be used as templates to grow nanowires. • The method takes the advantage of the fact that deposition of many materials on the surface often starts preferentially at defect sites, such as surface step-edges. • The problem is that these nanowires can not be easily removed from the surface on which they are deposited University Of South Alabama, EE Department

39. An Introduction to Nanowires and their applications Properties and Application of Nanowires Nanowires are promising materials for many novel applications Not only because of their unique geometry, but also because they possess many uniquephysical properties, including : • electrical • magnetic • optical • mechanical University Of South Alabama, EE Department

40. An Introduction to Nanowires and their applications Properties and Application of Nanowires Different Nanowires We can categorize different types of nanowires regarding to the materials as follows: • Metal nanowires • Semiconductor nanowires (Silicon nanowires) • Oxide nanowires • Multi-segment nanowires • Semiconductor quantum wires University Of South Alabama, EE Department

41. An Introduction to Nanowires and their applications Properties and Application of Nanowires The changes in properties arise from quantum confinement. • Quantum confinement describes how the electronic and optical properties change when the sampled material is in sufficiently small amounts, typically 10 nanometers or less. • Specifically, the phenomenon results from electrons and holes being squeezed into a dimension that approaches a critical quantum measurement. University Of South Alabama, EE Department

42. An Introduction to Nanowires and their applications Properties and Application of Nanowires, Magnetic Properties • Actually the magnetic properties of nanowires depend on the wire diameter and aspect ratio • It is possible to control the magnetic properties of the nanowires by controlling the fabrication parameters • Remanence ratio, which measures the remanence magnetization after switching off the external magnetic field • Coercivity, which is the coercive field required to demagnetize the magnet after full magnetization. • Giant Magnetoresistance (GMR) In a viscous solvent, magnetic field can be used to orient the growing nanowires. University Of South Alabama, EE Department

43. An Introduction to Nanowires and their applications Properties and Application of Nanowires, Optical properties • Controlling the flow of optically encoded information with nanometer-scale accuracy over distances of many microns, which may find applications in future high-density optical computing. • Silicon nanowires coated with SiC show stable photoluminescence at room temperature University Of South Alabama, EE Department

44. An Introduction to Nanowires and their applications NanoElectronic Applications of nanowires • The most important application of nanowires in nanoelectronics is using them as junctions or as multi-segment nanowires or crossed nanodevices. • Potential application of nanowires is in: • very dense logic • dense memory • optoelectronics • sensing devices University Of South Alabama, EE Department

45. An Introduction to Nanowires and their applications NanoElectronic Applications of nanowires Sensing Devices A structure for transport measurements sensor by nanowires Pictures : “ZnO nanowire growth and devices,” Y.W. Heoa, D.P. Nortona, et al., Materials Science and Engineering R 47 (2004) 1–47 University Of South Alabama, EE Department

46. An Introduction to Nanowires and their applications NanoElectronic Applications of nanowires, Quantum wire Transistor • Recent advances in formation methods allowed the fabrication of silicon quantum-wire transistors • The quantum wires have a width of 65 nm and are fully embedded in silicon dioxide. • A coulomb staircase, that is, step-like conductance versus gate voltage, was observed at temperature below 4.2 K. • Some techniques used Single electron Transistor based on a 30 nm wide Si NW, which can be operate at 77 K. The device showed clear single electron tunneling and well-defined single island and two tunnel junctions. University Of South Alabama, EE Department

47. An Introduction to Nanowires and their applications NanoElectronic Applications of nanowires, Single Electron Memory • Single electron memory cells consume extremely low power and can be realized by using the coulomb blockade effect. • Important components of such a device are a silicon nanowire as a channel, a siliconnanodot as a storage node, and a silicon nanogate as a control gate. • To realize these memory devices, narrow Si NWs need to be generated. University Of South Alabama, EE Department

48. An Introduction to Nanowires and their applications NanoElectronic Applications of nanowires, Metal Semiconductor Junction • Junctions between carbon nanotubes and silicon nanowires has been done. • To fabricate NT/SiNW junctions, SiNWs are grown from the end of the NT tips. • It has a characteristic the same as metal-semiconductor Schottkey diode. University Of South Alabama, EE Department

49. An Introduction to Nanowires and their applications NanoElectronic Applications of nanowires, Metal Semiconductor Junction (Picture from: “Controlled growth and electrical properties of heterojunctions of carbon nanotubes and silicon nanowires,” Jiangtao Hu et al. ,http://cmliris.harvard.edu/publications/1999/nature399_48.pdf) University Of South Alabama, EE Department

50. An Introduction to Nanowires and their applications NanoElectronic Applications of nanowires, Metal Nanowire SEM micrograph of single ZnO nanowire bridging two Al/Pt/Au Ohmic contact Pictures : “ZnO nanowire growth and devices,” Y.W. Heoa, D.P. Nortona, et al., Materials Science and Engineering R 47 (2004) 1–47 University Of South Alabama, EE Department