SYNTHESIS OF NANOWIRES

Prepared by: Kanika MisraGuide: Dr. Bina. R. Sengupta

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Introduction• A nanowire is a structure, with the diameter of the order of a nanometer (10−9 meters). Alternatively,

nanowires can be defined as structures that have a thickness or diameter constrained to tens of nanometers or less and an unconstrained length. At these scales, quantum mechanical effects are important — which coined the term "quantum wires".

• Typical nanowires exhibit aspect ratios (length-to-width ratio) of 1000 or more. They are often referred to as one-dimensional (1-D) materials.

• Nanowires have many interesting properties that are not seen in bulk or 3-D materials. This is because electrons in nanowires are quantum confined laterally and thus occupy energy levels that are different from the traditional continuum of energy levels or bands found in bulk materials.

• There are two basic approaches of synthesizing nanowires: top-down and bottom-up approach. In a top-down approach a large piece of material is cut down to small pieces through different means such as lithography and electrophoresis. Whereas in a bottom-up approach the nanowire is synthesized by the combination of constituent ad-atoms. Most of the synthesis techniques are based on bottom-up approach.

• Nanowire structures are grown through several common laboratory techniques including suspension, deposition (electrochemical or others), and VLS growth.

1. A. T. Tilke et al. (2003). Physical Rev. B 68: 075311.

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Conceptually simple and intuitive way to synthesize nanowires. Templates contain very small cylindrical pores or voids within the host material, and the empty spaces are filled with the chosen material, which adopts pore morphology, to form nanowires.

Templates frequently used for nanowire synthesis include anodic alumina (Al2O3), nano-channel glass, ion track-etched polymers and mica films.

Porous anodic alumina templates are produced by anodizing pure Al films in various acids. Under carefully chosen anodization conditions, the resulting oxide film possesses a regular hexagonal array of parallel and nearly cylindrical channels, as shown in Fig. 1. The self-organization of the pore structure in an anodic alumina template involves two coupled processes: pore formation with uniform diameters and pore ordering.

Depending on the anodization conditions, the pore diameter can be systematically varied from < 10nm up to 200nm with a pore density in the range of 109-1011 pores/cm2.

Template assisted nanowire growth

Fig 1

Fig 2

M. S. Dresselhaus Y.M. Lin, O. Rabin, M.R. Black, G. Dresselhaus. NANOWIRES, 4 ,(January 2, 2003)

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• Employed for fabricating highly crystalline nanowires from a low-melting point material or when using porous templates with robust mechanical strength.• The nanowires are formed by pressure injecting the desired material in liquid form into the evacuated pores of the template.• The pressure P required to overcome the surface tension for the liquid material to fill

the pores with a diameter dW is determined by the Washburn equation

dW = -4γ cosθ/P

where γ is the surface tension of the liquid, and θ is the contact angle between the liquid and the template. • To reduce the required pressure and to maximize the filling factor, some surfactants are used to decrease the surface tension and the contact angle.• Nanowires produced by the pressure injection technique usually possess high crystallinity and a preferred crystal orientation along the wire axis.

Template assisted synthesis by pressure injection

XRD patterns of bismuth/anodic alumina nano-composites with average bismuth wire diameters of (a) 40 nm, (b) 52 nm, and (c) 95nm

M. S. Dresselhaus Y.M. Lin, O. Rabin, M.R. Black, G. Dresselhaus. NANOWIRES, 4 ,(January 2, 2003)

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Electrochemical deposition

(a) SEM image of a Bi2Te3 nanowire array in cross section showing a relativelyhigh pore filling factor.

(b) SEM image of a Bi2Te3 nanowire array composite along the wire axis (Sander et al., 2002).

• In the electrochemical methods, a thin conducting metal film is first coated on one side of the porous membrane to serve as the cathode for electroplating.

• In the electrochemical deposition process, the chosen template has to be chemically stable in the electrolyte during the electrolysis process. Particle track-etched mica films or polymer membranes are typical templates used in the simple dc electrolysis.

• In ac electrochemical deposition, although the applied voltage is sinusoidal and symmetric, the current is greater during the cathodic half-cycles, making deposition dominant over the etching, which occurs in the subsequent anodic half-cycles. • In this fashion, metals, such as Co and Fe, and semiconductors, such as CdS, have been deposited into the pores of anodic aluminum oxide templates.

• Furthermore, single-crystal Pb , Ag nanowires can be formed by pulse electrodeposition under over-potential conditions. The use of pulse currents is believed to be advantageous for the growth of crystalline wires because the metal ions in the solution can be regenerated between the electrical pulses and, therefore, uniform deposition conditions can be produced for each deposition pulse.

M. S. Dresselhaus Y.M. Lin, O. Rabin, M.R. Black, G. Dresselhaus. NANOWIRES, 4 ,(January 2, 2003)

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• One advantage of the electrochemical deposition technique is the possibility of fabricating multi-layered structures within nanowires. By varying the cathodic potentials in the electrolyte which contains two different kinds of ions, different metal layers can be controllably deposited. In this fashion, Co/Cu multi-layered nanowires have been synthesized. Figure shows TEM images of a single Co/Cu nanowire of about 40nm in diameter. The light bands represent Co-rich regions and the dark bands represent Cu-rich layers. This electrodeposition method provides a low-cost approach to prepare multi-layered 1D nanostructure.

(a) TEM image of a single Co(10 nm)/Cu(10 nm) multilayered nanowire. (b) A selected region of the sample at high magnification (Piraux et al., 1994).

• In contrast to nanowires synthesized by the pressure injection method, nanowires fab-ricated by the electrochemical process are usually polycrystalline, with no preferred crystal orientations, as observed by XRD studies. However, some exceptions exist. For example, polycrystalline CdS nanowires, fabricated by an ac electrodeposition method in anodic alumina templates possibly have a preferred wire growth orientation along the c-axis.

• Surfactants are also used with electrochemical deposition when necessary.

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Vapor deposition• Vapor deposition of nanowires includes physical vapor deposition (PVD), chemical vapor deposition

(CVD), and metallorganic chemical vapor deposition (MOCVD). • In the physical vapor deposition technique, the material to be filled is first heated to produce a vapor,

which is then introduced through the pores of the template and cooled to solidify. • Compound materials that result from two reacting gases have also be prepared by the chemical vapor

deposition (CVD) technique.

Figure shows the SEM image and XRD spectrum of InN nanowires synthesized using MOCVD.

Figure shows that source Si is condensed on the catalyst until it reaches a critical diameter as shown in Figures 4 (b) and (c) after which, the nanowire grows at the saturation point of the catalyst alloy

1. Y. Civale, L. K. Nanver, P. Hadley, E. J. G. Goudena, Aspects of Silicon Nanowire Synthesis by Aluminum-Catalyzed Vapor-Liquid-Solid Mechanism, ACS, 692-695

2. M. S. Dresselhaus Y.M. Lin, O. Rabin, M.R. Black, G. Dresselhaus. NANOWIRES, 4 ,(January 2, 2003)

Thermal Annealing

• The thermal annealing method of synthesizing nanowires is a very simple method because it does not involve complex mechanism for synthesis compared to CVD method.

• For the synthesis of gold-silica p-type silicon substrate etched with hydrofluoric acid and coated with about 50 nm of gold by sputtering gold onto the substrate surface in order to maintain uniformity.

• A thermal annealing temperature of 1100 degrees C for 1 hour is required to synthesize gold-silica nanowire in the diameter range of 30-150 nm, they also recommend a 3 % hydrogen/argon gas ambient surrounding to eliminate oxidation of nanowire.

• The temperature required for the annealing process for various nanowires can be obtained from the appropriate phase diagram.

Gas

Temperature controlled Furnace

Schematic for thermal annealing process

Roger Prasad, Dr. Zhen Guo, Synthesis and Characterization of Nanowires ,MatE 297 ( May 10, 2006) 

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• Phase diagram is usually used to determine the favorable conditions for growth of nanowires. • Melting temperature for gold is lower than silicon. In order to grow the nanowire a temperature between the two melting points is selected for the evaporation segment and a lower temperature for the synthesis segment that favors condensation into the gold-silicon alloy. • The proposed growth mechanism involves the absorption of source material from the gas phase into a liquid droplet of catalyst • Upon supersaturation of the liquid alloy, a nucleation event generates a solid precipitate of the source material. This seed serves as a preferred site for further deposition of material at the interface of the liquid droplet.

• The VLS mechanism in Figure shows that the growth happens when silicon diffuses into the alloy puddle which increasing the Au-Si interface, this enhances melting of Si into the alloy.• The composition of Au-Si in the nanowire can be selected by changing the temperature as shown in Figure. The advantage to using a mechanism utilizing catalyst is that the direction and location of growth of nanowire can be controlled .

VLS Mechanism

1. M. S. Dresselhaus Y.M. Lin, O. Rabin, M.R. Black, G. Dresselhaus. NANOWIRES, 4 ,(January 2, 2003)2. Roger Prasad, Dr. Zhen Guo, Synthesis and Characterization of Nanowires ,MatE 297 ( May 10, 2006)

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Methods for Growth of CNTsFurnace at 1200 C

Ar gas

Graphite targetLaser

Water-cooled copper collector

Nanotube growing along tip of collector

Laser Ablation Process

Arc-Discharge System

Water

Water

inin out

out

graphite, cathodegraphite anode

Water in

Hepump

Water out

Power Supply

mass flow controllerauto pressure controller

Formation of nanotubes

Note: The target may be made by pressing Si powder mixed with 0.5% iron.

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ApplicationsMaterial Growth technique Applications

Ag DNA template, redox template, pulsed ECD

Barcode tags, interconnects in electronic circuits, nanoelectrodes

Au Template, EDC Optical switching, barcode tags, colorimetric markers or sensors

Bi Stress induced, vapor phase, ECD template, pressure injection

Thermoelectric, nano-optoelectronic, cryogenic cooling applications

Ca B6 Thermal process Aerospace applications

CdS Liq. phase, recrystallization template, ac EDC

Laser LEDs & optical devices based on non-linear properties

CdSe Liq, phase, redox template, ac EDC Solar cells, optic applications

Cu Vapour deposition template, EDC Sensing of important molecules

Fe template, shadow deposition PEM fuel catalyst applications, spintronics devices, high density data storage, environmental remediation applications

GaP/GaAs VLS FET, logic gates, super current switches

GaAs Template, liq,/vapor OMVCD Rectifiers, optoelectronic application

GaN Hydroxy vapour epitaxy, template, VLS

Visible optoelectronics, lasering action

Contd…

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Material Growth Mechanism ApplicationsInP VLS P-n jnc diodes

MnO2 Template free synthesis Lithium battery applications

Mo Step decoration, EDC redox Field emission, Li-ion battery/ pseudo capacitor applications

Ni Template, EDC Barcode tags, magnetic info storage

PbSe Liquid phase Photovoltaic cells

Pd Step decoration, EDC Barcode tags, nanosensors for detecting H2

Se Liq. Phase, recrystallization template, pressure injection

Light/temp sensors, rectifiers, photocopting m/c, inorganic pigments & piezoelectric actuators

Si VLS, laser ablation VLS, oxide assisted low T VLS

Flat panel display, pH sensor, microelectronics

SnO2 Epitaxal growth(template) Li-ion batteries, dye sentitized solar cells, gas sensors

Zn Template, vapor phase template, EDC Gas sensing, FET

ZnO VLS template, ECD, CVD Accoustic transducers, sensors & actuators, lasering actions

1. M. S. Dresselhaus Y.M. Lin, O. Rabin, M.R. Black, G. Dresselhaus. NANOWIRES, 4 ,(January 2, 2003) 2. Y. Xia,P. Yang, Y. Yin, Y. Sun, Y. Wu, B. Mayer, B. Gates, F. Kim, H. Yan, 1-D Nanostuctures: synthesis, characterization &

applications, Adv. Mater., 2003,15,no. 5, (March 4)