Characterization of Nanomaterials

Dr. Naveen Kumar Navani Assistant Professor, Department of Biotechnology Indian Institute of Technology Roorkee, Roorkee – 247 667

The future of nanotechnology rests upon approaches to making new, useful nanomaterials and testing them in complex systems. However, precise and trusting nanoparticle formation can only be promised after their thorough characterization. Here some of the basic physical and biophysical techniques are briefed which will be central to nanoparticles research.

1. X-ray Diffraction

The genesis of XRD can be traced to the suggestion of Max von Laue in 1912 that a crystal can be considered as a three-dimensional diffraction grating. It is generally used for qualitative analysis.X-ray diffraction (XRD) method is the most basic method for characterizing the crystal structures. X-rays corresponds to electromagnetic radiation in the wavelength range of 1 Å. The wavelength range is below that of ultraviolet light and above that of gamma rays.

X-rays are generally produced when electrons of several thousands of electron volts are decelerated or stopped by metals. This will produce a white radiation up to a threshold frequency corresponding to the kinetic energy of the particle. This threshold corresponds to a wavelength (in angstroms)

= 12399/V

Where,V-accelerating voltage of the electrons.

When electrons fall on matter with high energy, electrons can be ejected from various energy levels. Electron ejection from the core orbital is also accompanied by the emission of characteristic X-rays. (Fig.1)

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Figure 1- El

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Figure 3-

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2.2 Scann

Scanning of signalinteractioncompositiapplicatioimage is performinqualitativeanalysis).

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There are two bined at the image of the ohis is achievege is called thll diffracted ecular diffractee of the darke is usually us

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electron micrls at the sns reveal infion, and crysons, data are

generated thng analyses oely or semi

actions exploi

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a) B

Figure

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1. R2. T3. T4. Pr

acSEM high v

2.3Atomi

AFM pro(<10 nm)supportedbetween t

o Vio Ro

reparation fog samples areer metal or alfor eg.-carbonctive for high

ds use for vari

Routinely usedTo show spatiaTo identify pha

recise measuccomplished uoffers advant

vacuum requi

ic force micr

vides a 3D p) and surfaced on a flexiblethe probe and

ibration-free foom free of am

r SEM is a ce coated withlloy. The chon is most des

h resolution el

ied application

d to generate hal variations iases based onurement of vusing the SEMtage of wide irement of the

oscopy-

profile of the e at very she cantilever. the surface.

floor mbient magn

crucial task ina thin layer

oice of materisirable if elemlectron imagin

ns some of th

high-resolutioin chemical con qualitative cvery small fM. applicability

e order 10-5 -

surface on ahort distance The AFM tip

netic and elect

n particular fof conductin

ial for conducmental analysng application

hem are-

on images of ompositionschemical analfeatures and

and rapid dat10-6 torr.

a nanoscale, b(0.2-10 nm,

p “gently” tou

tric fields

for insulatingng material, cctive coatingsis is a prioritns.

shapes of obj

lysis and/or crobjects dow

ta acquisition

by measuring, probe-sampuches the surf

g samples. Mocommonly cas depends onty, while meta

jects

rystalline struwn to 50 nm

n but suffers f

g forces betwple separationface and reco

ost electricallarbon, gold, on the data to bal coatings ar

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from drawbac

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ly or be re

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cks of

probe obe is force

Fig 6. Block diagram of AFM

The probe is placed on the end of a cantilever (which one can think of as a spring). The

amount of force between the probe and is dependent on the spring constant (stiffness of the cantilever and the distance between the probe and the sample surface. This force can be described using Hooke’s Law: ample)

F=-k·x

F = Force

k = spring constant

x = cantilever deflection

If the spring constant of cantilever (typically ~ 0.1-1 N/m) is less than surface, the cantilever bends and deflection is monitored. Probes are typically made from Si3N4, or Si. Different cantilever lengths, materials, and shapes allow for varied spring constants and resonant frequencies. The motion of the probe across the surface is controlled using feedback loop and piezoelectronic scanners.There are 3 primary imaging modes in AFM-

(1) Contact AFM (< 0.5 nm probe-surface separation)

When cantilever bends are due to sample interaction the force on the tip is repulsive. By maintaining a constant cantilever deflection (using the feedback loops) the force between the probe and the sample remains constant and an image of the surface is obtained.

Advantages: fast scanning, good for rough samples, used in friction analysis

Disadvantages: at time forces can damage/deform soft samples (however imaging in liquids often resolves this issue)

(2) Intermittent contact (0.5-2 nm probe-surface separation)

However, in this mode the cantilever is oscillated at its resonant frequency, Figure 4. The probe lightly “taps” on the sample surface during scanning, contacting the surface at the bottom of its swing. By maintaining constant oscillation amplitude a constant tip-sample interaction is maintained and an image of the surface is obtained.

Advantages: allows high resolution of samples that are easily damaged and/or loosely held to a surface; good for biological samples

Disadvant

(3) Non-c

The probeduring scaliquid adsattractive

Advantag

Disadvantusually ne

tages: more c

contact AFM

e does not conanning. (Nosorbed on thVdW forces

ges: very low

tages: generaeed ultra-high

F

challenging to

(0.1-10 nm pr

ntact the sampte: all samplee surface). Uthe surface to

force exerted

ally lower resh vacuum (UH

Fig. 7-Plot of

o image in liqu

robe-surface

ple surface, bes unless in aUsing a feed

opography can

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but oscillates aa controlled Udback loop ton be measure

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nction of pro

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extended prob

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obe-sample s

needed

sorbed fluid laironmental ch

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lation;

3. Spectroscopy

3.1 UV-Visible spectroscopy

The word ‘spectroscopy’ is used as a collective term for all the analytical techniques based on the interaction of light and matter. Spectrophotometry is one of the branches of spectroscopy where we measure the absorption of light by molecules that are in a gas or vapour state or dissolved molecules/ions. Spectrophotometry investigates the absorption of the different substances between the wavelength limits 190 nm and 780 nm (visible spectroscopy is restricted to the wavelength range of electromagnetic radiation detectable by the human eye, that is above ~360 nm; ultraviolet spectroscopy is used for shorter wavelengths). In this wavelength range the absorption of the electromagnetic radiation is caused by the excitation (i.e. transition to a higher energy level) of the bonding and non-bonding electrons of the ions or molecules. A graph of absorbance against wavelength gives the sample’s absorption spectrum.

Spectrophotometry is used for both qualitative and quantitative investigations of samples. The wavelength at the maximum of the absorption band gives information about the structure of the molecule or ion and the extent of the absorption is proportional with the amount of the species absorbing the light.

Quantitative measurements are based on Beer’s Law which is described as follows:

A = c l

Where; A-absorbance [A = log10 (I0/I), I0 is the incident light’s intensity and I is the light intensity after it passes through the sample];

e- Molar absorbance or absorption coefficient [in dm3 mol-1 cm-1 units];

c- Concentration (molarity) of the compound in the solution [in moldm-3 units];

l - Path length of light in the sample [in cm units].

The instruments used for spectrophotometry are called photometers and Spectrophotometers. The difference between them is that we can only make measurements at a particular wavelength with a photometer, but spectrophotometers can be used for the whole wavelength range. Both types of instruments have suitable light sources, monochromator (that selects the light with the necessary wavelength) and a detector. The solution is put into a sample tube (called a “cuvette”). The light intensity

measured the readou

3.2 P

Photolumstructure othe matersample isluminesceis a direct

Photo-excelectrons light (a r(photolumthe transitis related determina

by the detecut (Fig.8)

Photolumines

minescence spof materials. Lrial in a proces through thence is calledt measure of v

citation causereturn to theiradiative pro

minescence) retion between to the relativ

ation particula

ctor is convert

Fi

scence spectr

pectroscopy iLight is direcess called phhe emission d photoluminevarious impor

es electrons wir equilibriumocess) or maelates to the dthe excited stve contributioarly in case of

ted into an el

igure 8- UV/V

roscopy

is a contactlcted onto a samhoto-excitation

of light, orescence. The rtant material

within the matm states, the exay not (a nodifference in tate and the eon of the radf semiconduc

lectric signal

Vis spectrop

less, nondestmple, where in. One way tr luminescenintensity andproperties.

terial to movexcess energy onradiative penergy levels

equilibrium stdiative procesctors.

and is displa

hotometer

tructive methit is absorbedthis excess e

nce. In the d spectral con

e into permissis released a

process). Thes between thetate (Fig.9). Tss. Photolumi

ayed as a cert

hod of probid and imparts energy can becase of pho

ntent of this p

sible excited and may inclue energy of e two electronThe quantity oinescence is

tain absorban

ing the elecexcess energ

e dissipated boto-excitationphotolumines

states. Whenude the emissi

the emitted n states involvof the emittedused for ban

nce on

tronic gy into by the n, this scence

these ion of

light ved in d light nd gap

The lightsample. Ain the samlight paswhich is reflected sources; particulartransmitsof monocexits witadjusted often meplacing thof the tra

 

t from an exA proportionmple fluoressses through

usually placincident ligincluding l

r.Filters ands light of an chromator utth a differento select wh

easured at a he sensor at

ansmitted exc

xcitation soun of the incidsce. The fluoh a second fced at 90° toght reachinglasers, photod/or monocadjustable wtilizes a diffnt angle dephich wavelen90° angle rthe line of t

citation light

Figure 9- P

urce passes dent light is

orescent lightfilter or moo the incideng the detectoodiodes, andchromators mwavelength wfraction gratipending on ngths to tranelative to ththe excitatiot. The detect

Photolumine

through a fabsorbed by

t is emitted inochromatont light beamor.Various ld lamps; xemay be uswith an adjuing, that is, cthe wavele

nsmit. As mehe excitationon light at a tor can eithe

escence

filter or mony the samplein all directir and reachm to minimilight sourcesenon arcs aed in fluor

ustable toleracollimated liength. The mentioned befn light. This 180° angle i

er be single-c

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and mercuryrimeters. A ance. The moight illuminamonochromafore, the fluo

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