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1 Introduction

In
1960’s, nanodiamonds(NDs) were first unintentionally formed during nuclear blast
experiment in Russia, were carbon explosives were used to detonate the core
materials of the nuclear bomb. In nature, they had been in existence even
before the formation of the solar system as pre-solar materials which were found
in the meteorites. These nanosize diamond materials were in the dark until the
1980’s, after which intense research and production began in the field but it
did not last long due to imbalance between production and requirement of the
diamond nanoparticles, which led to a closure several ND based R&D centers.
However, slow research was being done on these so called Ultra Disperse
Diamonds(UDD) or Detonation Nano Diamonds(DND) and as their unique properties
and potential application in various fields were gradually understood, these
NDs have been gaining popularity in the recent years 17.

By
appearance (with naked eye), they look nothing like a shining crystal diamond,
but a pile of grayish powder as shown in figure 1. They can be examined under
an electron microscope to study its shape and behavior. A picture of individual
nanodiamonds clumped together, focused under a Transmission Electron
Microscope(TEM) at different magnification is as shown in Figure 2.

 

Figure 1. Nanodiamond
powder 22

   

Figure
2.1 Clusters of nanodiamond under TEM at 100000x magnification 22

Figure
2.2 Clusters of nanodiamond under TEM at 10000x magnification 10

2 Syntheses of Nanodiamonds

Diamond
is nothing but carbon atoms arranged in a specific manner, i.e., as tetrahedral
structures. This arrangement of carbon atoms can be obtained at high
temperature and pressure which is defined as given in the carbon phase diagram
below(Figure 3)8.

Figure 3. Carbon phase diagram 10

The
most common method devised to produce nanodiamonds is the detonation method,
where carbon containing explosives were detonated in an oxygen deficient
environment (to avoid CO2 formation). The Figure 3 shows a schematic
representation of a detonation method used to produce nanodiamonds using TNT
and RDX in the presence of an inert gas. The residual soot is collected and
sent to the purification process where it is chemically and mechanically
treated to obtain pure nanodiamond crystals8.

Figure 4. Detonation method to produce nanodiamonds

Since
then, several other methods to synthesize nanodiamonds have been devised such
as the High Pressure High Temperature technique(HPHT), Chemical Vapor
deposition method, Laser Ablation method, High energy ball milling technique
which uses micro diamonds, micro plasma method of producing NDs from ethanol
vapor, electron beam irradiation of carbon onions, ion bombardment with
graphite and ultrasound cavitation.

3 Properties of Nanodiamonds

The
size of nanodiamonds ranges from 1-10 nanometers which finds them a lot of
application in the quantum regime. The nanodiamonds have a hard core with sp3
hybridization and a surface with sp2 hybridization which provides a chemically
stable diamond core and a modifiable surface structure. The schematic for a 5nm
nanodiamond structure is as given in Figure 5. 22

Figure 5. Structure of a 5nm detonation nanodiamond
10

These
tiny diamonds are known to possess remarkable mechanical and optical
properties. They are extremely hard and chemically inert with less cytotoxicity
and biocompatible. They are very sensitive to magnetic fields and have higher
thermal resistance. Also, the cost of nanodiamonds are not as expensive as
their bulkier counterparts when produced on a large scale. Nanodiamonds can be
developed as composites by using them as a filler material to the matrix. Owing
to their extremely small and uniform size, enhanced Young’s modulus, almost
spherical shape, high surface area to increase the number of interaction sites
they are considered to be advantageous than other nanocomposites 1.

Figure 6. Nanodiamond composite 1

NDs
are also known to demonstrate good tribological behavior showing good
frictional resistance and endurance to wear and tear. These NDs have a unique
Raman signature which can be utilized in Raman spectroscopy to bring out their
exceptional optical properties. They can emit the reflected light for
infinitely long period of time and are hence known as fluorescent nanodiamonds.
The fluorescence property is mainly due to the point defect in the diamond
structure which is caused by a vacancy combined with a dopant which commonly is
Nitrogen or Silicon.

3.1 Point Defects in
Nanodiamonds

A
defect in a nanosized diamond provides the ND with unique properties which can
be exploited in the quantum field. A typical Nitrogen lattice vacancy is as
shown in the figure 6 below. The NV centers exist in two basic forms:
negatively charged Nitrogen Vacancy centers (NV-)and neutral
Nitrogen Vacancy centers (NV0). A lot of research is in progress to
determine the properties and as to how these vacancies are formed. These
defects may be found to be an in grown creation of the manufacturing process
such as chemical vapor deposition or ionization method or could have formed
during the heating process. Nowadays, high energy helium ions are bombarded
with nanodiamonds to cause these defects which prove to be less expensive than
the above-mentioned techniques. The features used to identify these NV centers
are their zero field magnetic resonance (for NV-) which can be
detected using optically detected magnetic resonance (ODMR) or electron
paramagnetic resonance(EPR) and optical zero phonon lines(ZPLs)(for both the
charge states) with the corresponding vibronic bands radiated from these ZPLs.
20

Figure 7. A typical NV lattice center in nanodiamond

These
defects are responsible for the photoluminescence property of the Nanodiamonds
which is also known as super radiance. The advantage of ND over other materials
exhibiting photoluminescence is their ability to resist photobleaching and
photo blinking as they can send out concentrated pulses of stable fluorescent
light over an infinite amount of time. 7

4 Applications of Nanodiamond

The
novel properties of nanodiamonds finds a myriad of applications in several
domains namely, automobiles, navigation system, biosensors, optical imaging
tools and optical computing, cosmetics, as theragnostic tool, solar cells and
quantum computing.

Figure 8. Pictorial representation of application of
nanodiamonds 25

4.1 Nanodiamonds In
Automobile Industry

Nanodiamonds
are known to show good tribological behavior which makes them a good lubricant
with reducing friction, antiwear load bearing capabilities at high temperature
and high load. Hence, they are used as additive to engine oils where they act
as ball bearing lubricant such that the movement between the engine components
become rolling/sliding thereby reducing friction and extending the life of the
parts of the engine. The reduction in wear of a material with added percentage
of nanodiamonds is as shown in the graph below (Figure 7). 23

Figure 9. Wear vs ND percentage weight concentration
24

4.2 Nanodiamonds In Navigation

Nanodiamonds
being extremely sensitive to magnetic field, act like compass, their deflection
aligned to the Earth’s magnetic field. They emit out light, the intensity of
which depends on the magnetic field of Earth, which can be amplified and interpreted
9. This property can be used to build magnetic sensors to be used in aircraft
which can precisely locate the plane instead of relying on satellites.

4.3 Nanodiamonds in
cosmetics

The
high absorption rate of nanodiamonds facilitates increased absorption of the
cosmetic ingredient than our skin would absorb on its own and aids in deeper
penetration into the skin by helping them to work at their peak potential.

Nanodiamonds
are also found to be hydrophilic thereby forming strong bonds with water. When
these tiny water-loving diamonds are used to make moisturizing lotions, they
tend to keep the skin hydrated for a prolonged time.

4.4 Biosensing And
Bioimaging Using Nanodiamonds

The
nanodiamonds have tunable surface structures and owing to their low
cytotoxicity, these surfaces can be functionally modified specific to certain
protein, peptides or DNA to be conjugated with them to image their molecular
functions. Compared to the conventional molecular dyes used for the process,
nanodiamonds prove to be effective due to their fluorescence property. 16

 

Figure
10.1 Conventional imaging                
Figure 10.2 Nanodiamond bioimaging    

The
NV color centers in nanodiamond is used to measure the strength of the protein
molecules by measuring the orientation of electron spin during electrostatic
interaction of the nanodiamonds with the protein molecules. Also, the magnetic
sensitivity of the nanodiamonds is used to improve the signal-noise ratio and
obtain a better resolution of the image4. A company called Bikanta is working
on developing this technology using nanodiamonds. The figure shows imaging of
lymph nodes using conventional imaging technique (Figure 9.1) and using nanodiamonds
(Figure 9.2) by Bikanta Technologies.

4.5 Nanodiamond As a
Theragnostic Tool

The
ability of a single tool to be used for both diagnosis and cure is known as a
theragnostic tool and nanodiamonds can perform both. Their biosensing ability
can be used to detect the defective cells inside the living body and can be
made to deliver the drugs to the respective cells without affecting the healthy
cells. The nanodiamonds are made target specific by bonding them with receptors
that bind only with those specific cells. This method can be used to treat
cancer cells by making the nanodiamonds specific to cancer cells without
affecting the healthy cells. Therefore, acting as a theragnostic tool
nanodiamonds can just destroy an abnormality just as they discover it. Also,
they can stay inside the cells for an extended period of time, unlike the previous
specimens used, without interrupting the regular cellular mechanisms. The only
issue holding back is their inert nature which makes it difficult to be
degraded and eradicate them out of the body14.

4.6 Optical
Processing and Quantum Computing using Nanodiamonds

Optical
processing utilizes light beams in place of electrons as flow regulators, the
speed of light being much higher than that of electrons, thereby providing an
incredible processing speed. Nanodiamonds can be used for optical processing by
deflecting the light focused on these crystals in the desired manner. For
example, nanodiamond with a single NV lattice center can be used as an optical
transistor. When a beam of green laser light in focused on it, the light would
be transmitted when it is in ‘ON’ position and during the ‘OFF’ position, a
near infrared laser is shone on the crystal which changes the way the green
light is transmitted. When a bunch of these nanodiamonds are integrated
together along with a light guide for the lasers shone on them, a full integrated
circuit can be built, which would run on light instead of electricity. The
Figure 11 shows a illustrative model of a nanodiamond optical transistor 3.

Figure 11. Nanodiamond as optical transistor (Image courtesy of Carlo Bradac, Macquarie
University)

The
ability to fathom the characteristics of the NV- centers and
manipulate their pattern in the diamond structure would provide the ND a
pivotal role in quantum computing. Other desirable properties which add on to
their application in quantum computing are its photostability, source of
single-photon and ability to form a solid-state drive in room temperature by utilizing
the defect center as a quantum bit 10.

5 Conclusion

A
profound knowledge of these precious little specimen is still under
investigation and an even wider potential of their application is to be
explored as a greater control of their properties can only be obtained with an
in depth understanding of their traits. The challenges debated
above would provide the brightest prospects for the future of nanodiamonds.
They hold versatile solutions for complicated setbacks with their fundamental attributes
thereby proving to be an impactive material with an array of opportunities.

 

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