Nanowires

This report gives a brief insight into Nanowires, how it is manufactured and its applications.

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PROJECT REPORT
ON
NANOTECHNOLOGY
NANOWIRES

Under the guidance of professor Dr. Leela Arava


Submitted by
Varun Jadhav
Gk0862

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ABSTRACT

Nanotechnology as defined by size is naturally very broad, including fields of science as diverse
as surface science, organic chemistry, molecular biology, semiconductor physics, energy storage,

microfabrication, molecular engineering, etc. The associated research and applications are
equally diverse, ranging from extensions of conventional device physics to completely new
approaches based upon molecular self-assembly, from developing new materials with
dimensions on the nanoscale to direct control of matter on the atomic scale.
Nanowires are developed from nanotechnology, which is helping the current technology to
become more effective and more compact in size. Using nanowire in space technology has
reduce the total weight of the spacecrafts in order improve the efficient of the spacecraft. An
introductory part talks about nanowires in general and how they can affect the evolution of next
generation electronics. In this report we will discuss what are nanowires, how they are synthesize
and later their applications, advantages and disadvantages.

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CONTENT
SR NO DESCRIPTION PAGE
NO
1 Introduction 3
2 Method of synthesis 4
2.1 Types of Method 4
3 Vapor Liquid Solid Synthesis 5
3.1 Requirements for catalyst particles
3.2 Growth Mechanism
3.2.1 Catalyst droplet formation
3.2.2 Nano whisker diameter
3.2.3 Whisker growth kinetics

4 Mechanical properties
4.1 Young’s Modulus of Nanowire
4.2 Yield Strength of Nanowire
8
5 Applications
5.1 Transistor
5.2 Photovoltaic Cell
5.3 Quantum dot
5.4 Flexible Flat Screen display
5.5 Light Emitting Diode
5.6 Nano Laser
11
6 Conclusion 14
7 References 15

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1.INTRODUCTION
Nanowires
In the past decade, semiconductor nanowires have been extensively investigated for the
next generation of devices including photodetectors, photocatalysis, photovoltaics,
thermoelectric, and quantum information processing, among other applications. Nanowires are
rods or pillars of semiconductor material with one dimension (the nanowire diameter) on the
order of 10 nm to a few hundred nanometers, and length that is much greater than the diameter
(on the order of microns or longer).
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 the quantum mechanical
effects are important which coined the term “Quantum Wires”

Fig. Silicon nanowire on Bulk Silicon Wafer
Typical nanowires exhibit aspect ratios of 1000 or more. Nanowires have many
interesting properties that are not seen in bulk or three-dimensional 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 many applications where nanowires may become important in electronic, opto-
electronic and nanoelectromechanical devices, as additive in advanced composites, for metallic
interconnects in nanoscale quantum devices, as field emitters and as leads in nanosensors.

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2. METHOD OF SYNTHESIS
The nanowires are typically synthesis using two basic approaches, namely the “top-
down” and “bottom-up” approach. The top-down approach basically reduces a large piece of
material block into small pieces like carving a sculpture from a block of stone, however on a
nanoscale. The top down approach can be achieved by various means like lithography, milling or
thermal oxidation. Secondly the bottom up approach synthesizes the nanowire by combining
constituent adatoms


Fig Top-down and Bottom-up Process

2.1 TYPES OF METHOD
➢ Suspension method
➢ Electrochemical Deposition method
➢ Vapor deposition method
➢ Ion track technology
➢ Solution phase synthesis
➢ Non-Catalytic growth
➢ DNA templated Metallic Nanowire synthesis
➢ VLS Growth (Vapor Liquid Solid Method)
➢ VSS (Vapor Solid Solid Method)
➢ OAG (Oxide assisted growth)

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3.VAPOR LIQUID SOLID SYNTHESIS
The vapor–liquid–solid method (VLS) is a mechanism for the growth of one-dimensional
structures, such as nanowires, from chemical vapor deposition. The growth of a crystal
through direct adsorption of a gas phase on to a solid surface is generally very slow. The
VLS mechanism circumvents this by introducing a catalytic liquid alloy phase which can
rapidly adsorb a vapor to supersaturation levels, and from which crystal growth can
subsequently occur from nucleated seeds at the liquid–solid interface. The physical
characteristics of nanowires grown in this manner depend, in a controllable way, upon the
size and physical properties of the liquid alloy.








3.1 REQUIREMENTS FOR CATALYST PARTICLES
• It must form a liquid solution with the crystalline material to be grown at the nanowire
growth temperature.
• The solid solubility of the catalyzing agent is low in the solid and liquid phases of the
substrate material.
• The equilibrium vapor pressure of the catalyst over the liquid alloy must be small so that
the droplet does not vaporize, shrink in volume (and therefore radius), and decrease the
radius of the growing wire until, ultimately, growth is terminated.
• The catalyst must be inert (non-reacting) to the reaction products (during CVD nanowire
growth).
• The vapor–solid, vapor–liquid, and liquid–solid interfacial energies play a key role in the
shape of the droplets and therefore must be examined before choosing a suitable catalyst;
small contact angles between the droplet and solid are more suitable for large area
growth, while large contact angles result in the formation of smaller (decreased radius)
whiskers.
Preparation of liquid
alloy droplet upon
substrate from which
a wire is to be grown
Introduction of the substance
to be grown as a vapor, which
absorbs on the liquid surface,
and diffuses into the droplet
Supersaturation and
Nucleation at the
liquid/solid interface
leading to axial crystal
growth
VLS Mechanism in three stages

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• The solid-liquid interface must be well-defined crystallographically to produce highly
directional growth of nanowires. It is also important to point out that the solid-liquid
interface cannot, however, be completely smooth. Furthermore, if the solid liquid
interface was atomically smooth, atoms near the interface trying to attach to the solid
would have no place to attach to until a new island nucleates (atoms attach at step
ledges), leading to an extremely slow growth process. Therefore, “rough” solid surfaces,
or surfaces containing a large number of surface atomic steps (ideally 1 atom wide, for
large growth rates) are needed for deposited atoms to attach and nanowire growth to
proceed.
3.1 GROWTH MECHANISM
The growth mechanism includes the following parameters in consideration
• Catalyst droplet Formation
• Nano whisker diameter
• Whisker growth kinetics
3.1.1 Catalyst droplet Formation
The materials system used, as well as the cleanliness of the vacuum system and therefore
the amount of contamination and/or the presence of oxide layers at the droplet and wafer surface
during the experiment, both greatly influence the absolute magnitude of the forces present at the
droplet/surface interface and, in turn, determine the shape of the droplets.
The shape of the droplet, i.e. the contact angle β0 can, be modeled mathematically,
however, the actual forces present during growth are extremely difficult to measure
experimentally. Nevertheless, the shape of a catalyst particle at the surface of a crystalline
substrate is determined by a balance of the forces of surface tension and the liquid–solid
interface tension.
The radius of the droplet varies with the contact angle as:

Where: - r0 is the radius of the contact area.
β0 is defined by a modified Young’s equation.

It is dependent on the surface (σs) and liquid–solid interface (σls) tensions, as well as an
additional line tension (τ) which comes into effect when the initial radius of the droplet is small
(nanosized). As a nanowire begins to grow, its height increases by an amount dh and the radius

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of the contact area decreases by an amount dr . As the growth continues, the inclination angle at
the base of the nanowires (α, set as zero before whisker growth) increases, as does β0

he line tension therefore greatly influences the catalyst contact area. The most import result from
this conclusion is that different line tensions will result in different growth modes. If the line
tensions are too large, nanohillock growth will result and thus stop the growth.

3.1.2 Nano Whisker Diameter
The diameter of the nanowire which is grown depends upon the properties of the alloy
droplet. The growth of nano-sized wires requires nano-size droplets to be prepared on the
substrate.
In an equilibrium situation this is not possible as the minimum radius of a metal droplet is
given by

where Vl is the molar volume of the droplet, σlv the liquid-vapor surface energy, and s is
the degree of supersaturation of the vapor. This equation restricts the minimum diameter of the
droplet, and of any crystals which can be grown from it, under typically conditions to well above
the nanometer level. Several techniques to generate smaller droplets have been developed,
including the use of monodispersed nanoparticles spread in low dilution on the substrate, and the
laser ablation of a substrate-catalyst mixture so to form a plasma which allows well-separated
nanoclusters of the catalyst to form as the systems cools.
3.1.3Whisker Growth Kinetics
During VLS whisker growth, the rate at which whiskers grow is dependent on the
whisker diameter: the larger the whisker diameter, the faster the nanowire grows axially. This is
because the supersaturation of the metal-alloy catalyst (Δµ) is the main driving force for
nanowhisker growth and decreases with decreasing whisker diameter (also known as the Gibbs-
Thomson effect)

Again, Δµ is the main driving force for nanowhisker growth (the supersaturation of the
metal droplet). More specifically, Δµ0 is the difference between the chemical potential of the

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depositing species in the vapor and solid whisker phase. Δµ0 is the initial difference proceeding
whisker growth (when ‘d’ goes to Infinity), while Omega is the atomic volume of Si and alpha
the specific free energy of the wire surface. Examination of the above equation, indeed reveals
that small diameters (omega <100 nm) exhibit small driving forces for whisker growth while
large wire diameters exhibit large driving forces.
4. MECHANICAL PROPERTIES
Early studies showed that micrometer-sized whiskers have yield strengths that are over
ten times that of the corresponding bulk materials. Other measurements on BNMs revealed
remarkably different properties when compared with their coarse-grain counterparts.
Nanocrystalline Cu exhibits high yield strengths (approaching 0.4 GPa) and near-perfect elasto
plasticity without work hardening or necking1, and room-temperature superplasticity, whereas
nanocrystalline nickel shows reversible X-ray-diffraction peak broadening during plastic
deformation. In contrast, much less is known about mechanical properties of free-standing
nanoscale objects such as nanowires due to difficulties associated with standard tensile or
bending tests. Several approaches have been reported and include the use of a nano stressing
stage within scanning electron microscope (SEM), transmission electron microscope (TEM) and
atomic force microscope (AFM) measurements.
Here we report an AFM-based method that unambiguously measures the full spectrum of
mechanical properties. E is essentially diameter-independent and comparable to that of bulk
gold, yield strengths are up to 100 times larger than bulk, with ultimate strengths approaching the
theoretical value of E/10 for smaller-diameter nanowires. Gold nanowires exhibit well-defined
yield points and undergo strain hardening characterized by a universal F–d curve that is traced
out by successive yield points during repeated loading–unloading cycles. Nanowire yielding, and
plastic deformation properties are dramatically different from those of BNMs with comparable
grain size, suggesting that the constrained material volume and limited number of grains across
the nanowire diameter preclude the elastoplasticity behavior.

Fig. Mechanical deformation of a 200nm Au (gold) nanowire It can be seen how the
nanowire deformed lateral bending after repetitively applied force

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Fig. F–d curves recorded during consecutive manipulation by AFM tip-induced lateral
bending of a 200-nm Au nanowire

4.1 Young’s Modulus of Nanowire
The elastic component of the stress-strain curve described by the Young’s Modulus, has
been reported for nanowires, however the modulus depends very strongly on the microstructure.
Thus a complete description of the modulus dependence on diameter is lacking. Analytically,
continuum mechanics has been applied to estimate the dependence of modulus on diameter

where E0 is Bulk modulus
rs is shell layer thickness in which the modulus is surface dependent & varies frombulk
Es is the surface Modulus and
D is diameter

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This equation implies that the modulus increases as the diameter decreases. However,
various computational methods such as molecular dynamics have predicted that modulus should
decrease as diameter decreases.
Experimentally, gold nanowires have been shown to have a Young’s modulus which is
effectively diameter independent. Similarly, nano-indentation was applied to study the modulus
of silver nanowires, and again the modulus was found to be 88 GPa, very similar to the modulus
of bulk Silver (85 GPa) These works demonstrated that the analytically determined modulus
dependence seems to be suppressed in nanowire samples where the crystalline structure highly
resembles that of the bulk system.
4.2 Yield Strength of Nanowire
The plastic component of the stress strain curve (or more accurately the onset of plasticity) is
described by the yield strength. The strength of a material is increased by decreasing the number
of defects in the solid, which occurs naturally in nanomaterials where the volume of the solid is
reduced. As a nanowire is shrunk to a single line of atoms, the strength should theoretically
increase all the way to the molecular tensile strength.
Gold nanowires have been described as ‘ultrahigh strength’ due to the extreme increase
in yield strength, approaching the theoretical value of E/10. This huge increase in yield is
determined to be due to the lack of dislocations in the solid. Without dislocation motion, a
‘dislocation-starvation’ mechanism is in operation. The material can accordingly experience
huge stresses before dislocation motion is possible, and then begins to strain-harden. For these
reasons, nanowires (historically described as 'whiskers') have been used extensively in
composites for increasing the overall strength of a material. Moreover, nanowires continue to be
actively studied, with research aiming to translate enhanced mechanical properties to novel
devices in the fields of MEMS or NEMS.

Fig. The curve shows the stress vs strain properties of gold nanowire with variable
diameters, (30nm, 60nm, 70nm)

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5.APPLICATIONS OF NANOWIRES

5.1 Transistor

Fig. 10nm Nanowire Transistor Fig. I-V characteristic of 10nm Dia Nanowire
Nanowires can be used for transistors. Transistors are used widely as fundamental
building element in today's electronic circuits. As predicted by Moore's law, the dimension of
transistors is shrinking smaller and smaller into nanoscale. One of the key challenges of building
future nanoscale transistors is ensuring good gate control over the channel. Due to the high
aspect ratio, if the gate dielectric is wrapped around the nanowire channel, we can get good
control of channel electrostatic potential, thereby turning the transistor on and off efficiently.
5.2 Photovoltaic Device

The unique one-dimensional structure with remarkable optical properties, the nanowire
also opens new opportunities for realizing high efficiency photovoltaic devices. Compared with
its bulk counterparts, the nanowire solar cells are less sensitive to impurities due to bulk

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recombination, and thus silicon wafers with lower purity can be used to achieve acceptable
efficiency, leading to a reduction on material consumption.
5.3 Quantum Dot

Fig. Graphene Quantum dot
Nanowires are also being studied for use as photon ballistic waveguides as interconnects
in quantum dot/quantum effect well photon logic arrays. Photons travel inside the tube, electrons
travel on the outside shell. When two nanowires acting as photon waveguides cross each other
the juncture acts as a quantum dot.
5.4 Flexible Flat Screen Display
Conducting nanowires offer the possibility of connecting molecular-scale entities in a
molecular computer. Dispersions of conducting nanowires in different polymers are being
investigated for use as transparent electrodes for flexible flat-screen displays.
5.5 Light Emitting Diode
Nanowires can potentially be used to produce Light emitting diodes. At present scientist
have experimentally produce LED Nanowire using Gallium Nitride.
Gallium Nitride is also used as a Laser Diode, Highly sensitive atomic probe tip, small
resonators and chemical sensor.

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5.6 Nanowire Lasers

Fig. (A, C, E) Photoluminescence microscopy images of single-nanowire lasers in three
configurations. (B, D, F) The thin arrows represent end-face reflection with low reflectivity,
while the thick arrows represent loop mirror reflection with high reflectivity
Nanowire lasers are nano-scaled lasers with potential as optical interconnects and optical
data communication on chip. Nanowire lasers are built from III–V semiconductor
heterostructures, the high refractive index allows for low optical loss in the nanowire core.
Nanowire lasers are subwavelength lasers of only a few hundred nanometers. Nanowire lasers
are Fabry–Perot resonator cavities defined by the end facets of the wire with high-reflectivity,
recent developments have demonstrated repetition rates greater than 200 GHz offering
possibilities for optical chip level communications.

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6. CONCLUSION
Nanowires provide unique tools for scaling down in semiconductor electronics and the
fundamental understanding of nanoscale physical phenomena, which are not accessible in
conventional lithography.
With the advancement in nanotechnology we can now save tremendous amount of space
in electrical applications. Measuring mechanical properties of the nanowire is still in progress
due to its nano size, the methods available at present are not capable of producing reliable
measure of nanowire strength.
Use of nanowire in commercial Aircrafts and in Spacecraft can help reduce the weight
overall. Nanowires are good candidates for many future applications. The enhanced absorption in
nanowires, the ability to build radial or axial p-n junctions, and the possibility to combine them
with organic and nanoparticle solar cells may help to build cost-effective photovoltaic devices.
The small size and the principally small power consumption of nanowires make it
feasible to build devices that can be embedded in everyday life objects such as wristwatches or
cell phones.
To make this all come true, further improvement of the nanowire material and the
nanowire device performance must be achieved.

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7.REFERENCES

[1] Electrical Properties of CuNanowires :
https://www.researchgate.net/publication/224328140_Electrical_Properties_of_Cu_Nano
wires
[2] Nanowire-based one-dimensional electronics :
https://www.sciencedirect.com/science/article/pii/S1369702106716510#bib4
[3] Wagner, R. S.; Albert P. Levitt (1975). Whisker Technology. Wiley – Interscience –
New York. ISBN 0-471-53150-2.
[4] Mayer, B., et al. "Monolithically integrated high-β nanowire lasers on silicon." Nano
letters 16.1 (2015): 152-156
[5] Wagner, R. S.; Ellis, W. C. (1964). "Vapor-liquid-solid mechanism of single crystal
growth". Appl. Phys. Lett. 4 (5): 89. Bibcode:1964ApPhL...4...89W.
doi:10.1063/1.1753975
[6] Mongillo, Massimo; Spathis, Panayotis; Katsaros, Georgios; Gentile, Pascal; De
Franceschi, Silvano (2012). "Multifunctional Devices and Logic Gates with Undoped
Silicon Nanowires". Nano Letters. 12 (6): 3074–9. arXiv:1208.1465.
Bibcode:2012NanoL..12.3074M. doi:10.1021/nl300930m. PMID 22594644
[7] Koblmüller, Gregor, et al. "GaAs–AlGaAs core–shell nanowire lasers on silicon:
invited review." Semiconductor Science and Technology 32.5 (2017): 053001
[8] Wang, Shiliang; Shan, Zhiwei; Huang, Han (2017-01-03). "The Mechanical
Properties of Nanowires". Advanced Science. 4 (4): 1600332.
doi:10.1002/advs.201600332. PMC 5396167. PMID 28435775
[9] Wu, Bin; Heidelberg, Andreas; Boland, John J. (2005-06-05). "Mechanical properties
of ultrahigh-strength gold nanowires". Nature Materials. 4 (7): 525–529.
Bibcode:2005NatMa...4..525W. doi:10.1038/nmat1403. ISSN 1476-1122. PMID
15937490.
[10] Huang, M. H.; Wu, Y; Feick, H; Tran, N.; Weber, E.; Yang, P. (2001). "Catalytic
Growth of Zinc Oxide Nanowires by Vapor Transport". Adv. Mater. 13 (2): 113–116.
doi:10.1002/1521-4095.
[11] Bhushan, Bharat. Springer Handbook of Nanotechnology. Berlin: Springer-Verlag.
p. 105. ISBN 3-540-01218-4.