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Electron Paramagnetic Resonance Spectroscopy of Several
Unknown Samples

Satyam Ladva
University College London, Electrical Engineering Department: MSc EPR Report

Laboratory Demonstrator: Dr
...
By analysing the experimentally obtained
spectra of six radicals which were: silicon doped with phosphorous (Si-P), silicon doped with bismuth (Si-Bi), nitrogen
doped C60 (N-C60), manganese oxide (MnO), gamma irradiated methylalanine and perinaphthenyl, through use of an EPR
simulated spectra performed on MATLAB, values for the g-factor and hyperne constants were determined by trial and
error
...
286mT and the latter having a halfwidth of 0
...
Si-P had a g-factor of 1
...
211mT ,
with an electron spin density ∼ 10−5 per unit volume, whereas N-C60 had a g-factor of 2
...
567mT
...
Si-Bi
had a Gaussian linewidth of 0
...
600mT , with a g-factor of 2
...
650mT
...
MnO had a Gaussian linewidth of 0
...
110mT , with a g-factor of 1
...
600mT
...
260mT and a Lorentzian linewidth of 0
...
00231 and a hyperne constant
of 2
...
010mT and a Lorentzian linewidth of 0
...
00260 and two hyperne constants of 0
...
182mT
...
The hyperne splittings for transition elements and the N-C60 was a result of the spin quantum
number of the elements where overlapping was frequent whilst for more complicated organic elements, such as methylalanine
and perinaphthenyl, the hyperne splitting was a result of the electron-proton interactions
...
The presence of dangling bonds and oriented chemical bonds
were also found to limit the resolution of the EPR spectroscopy readings; creating the need for multiple hyperne coupling
models for the same sample
...


Chris Kay for demonstrating the practical implications of EPR in a

laboratory environment in addition to providing assistance and valuable insight into the purpose and nature of
EPR spectroscopy for industrial applications whilst performing the simulations
...


Steve Hudziak and Dr
...


3

Contents
I Introduction

4

II Methodology

4

1

How does EPR work?


...


4

3

How were the Samples analysed with EPR?
...
1

Hyperne Coupling
...
2

The g-factor
...
3

The Resonance line width
...


6

5

Si-Bi
...


8

7

MnO
...


9

9

Perinaphthenyl Radical

IV Conclusion


...
Introduction
Electron

Paramagnetic

Resonance

radiation, usually in the microwave radiation frequency
range
...


The process is very similar to nuclear mag-

netic resonance (NMR) but, rather than nuclei spins being excited, this resonance technique focuses on exciting
the spin of electrons
...
Though NMR is
used more widely than EPR, this technique oers greater
specicity thus making it useful for detecting free radical
components and free radical interactions
...

This investigation aims to implement EPR for the
study of several molecules in order to determine their gfactors and hyperne coupling constants through a simulation based analysis
...


This aim will

be analysed through knowledge obtained from existing
EPR investigations of the samples
...

This technique can be implemented on solid state materials as well as those in a solution or in gas phase
...

Through use of the Zeeman interaction the electrons
[2], which have spin
ergy levels in

B0

S =

1
2

,

which are

are allowed two discrete en-

Eαe (βe ) = ± 1 ge µB B0
...


2 What samples were used in this
Investigation and what is their
signicance in industry?
The samples used in this investigation, as depicted previously, must have unpaired electrons thus they can include free radicals or transitional metal ions
...
Methodology

•

Nitrogen doped in a C60 (N-C60) atom at room

1 How does EPR work?

•

Manganese Oxide (MnO) at room temperature

•

Gamma-irradiated methylalanine

•

Perinaphthenyl radical at room temperature

number of the nuclei involved as a source of explanantion
...
These local elds

The six samples used could all be measured by EPR but

are a result of the nuclear magnetic moments of nuclei,

they were of a dierent nature
...
Though EPR is one of

doping was between a group IV element and a group V el-

the most sensitive chemical detection systems at present,

ement, where the latter had an additional electron
...


line width eect, arising from Heisenberg's uncertainty

Since Si and P are next to each other on the periodic

principle
...


agnetic species [1] present in a material, individual mag-

Si-P is a semiconductor that is used in most quantum

netic moments, which arise from the spin of the unpaired

information systems and in a study by Rowe et
...
[3]

electron, become oriented parallel or anti-parallel to

B0
...
This result was sig-

this unpaired electron which allows net absorption of

inicant for use in electronic and photonic applications,

By applying a very strong magnetic eld

5

3 How were the Samples analysed with EPR?

since no localised surface plasmon resonances were reported in previous studies of Si-P
...


which is why it was doped at 10K
...
This overlapping was achieved through

ing current
...
In terms of current uses, as discussed by Mo-

proach
...
al
...
The reason for this replacement is because of
Si-Bi's anomalously strong hyperne coupling, which is
unusual for an element in its period as discussed by Mohammady et
...

N-C60, or nitrogen doped fullerenes, is signicant due
to its use as a catalyst for hydrogen fuel cells due to its
ability to absorb and partially reduce oxygen without the
presence of an activation barrier, as discussed by Gao et
...
[6]
...
1 Hyperne Coupling
Hyperne coupling is a result of the interaction between
an unpaired electron, due to its magnetic moment, with
nearby nuclear spins and this results in additional energy
states and a multi-lined spectra
...
This is a result of a dipole interaction or, more
commonly, a Fermi contact interaction; for isotropic interactions, when the electron is inside the nucleus, there
is a magnetic interaction between the electron and the

is a mineral formed from bacterial sources, thus allow-

nucleus in which it is inside
...
Information on such a chemical, as

polarisation is the key reason for this interaction which

discussed in Kim et
...
[7] can be used for detection of

is a result of the electron and nucleus spin interacting
...

Methylalanine, was measured due to its sensitivity to

In the case of

π

electron

In the computer simulations, this hyperne coupling
constant was dened by the letter
a unit of

A

corresponded to a

A where a change in
mT
...
In the

tional to the spectral line spacing thus an increase of

case of methylalanine, this emission is linear, stable over

the coupling constant results in the spacing between the

time and sensitive
...
e
...
al
...
The overall magnitude of the hyper-

this sample and was the most dominant measured
...


a lot simpler to identify
...
This substance is also used in the production of hydrouoric acid, an acid used to strip photoresin
during etching procedures for photolithography proce-

This constant

A = ai

ergy related to transition of electrons in EPR, where the
rst and second terms represent the Zeeman energy of
the electron and nucleus in the system whilst the third
term describes the hyperne coupling between the electron and nucleus
...
A further signicance of this radical is that it is
the third smallest polycyclic aromatic hydrocarbon bear-

atively unknown
...
al
...
For the purpose of this investigation, and most

lines tended to correspond to sharper peak corners
...


In the case of

method implemented in this investigation
...


vestigation, with physical reasons for the broadening not

ples and

S = 1/2,

as was the case for this in-

being a factor
...
2 The g-factor

origins that aect the broadening are unresolved hyperne splittings and strains but, for this investigation, such

The g-factor is an inherent property of the electron that

eects will not be discussed
...
The
g-factor can be determined theoretically through derivation of equation 2

Discussion
hν = (ge + ∆g) βB

β

(2)

ge = 2
...
Results and

is the Bohr magneton,

the g-factor that results from the interaction of the electron with the electronic structure of the material
...


LHS corresponds to Planck's energy
...
For organic radicals, it is already well established the the overall g-factor is very close to

ge

whilst for

transition metals, the overall g-factor varies considerably
...


4 Si-P
In order to create the overlap of the Si-P sample, the

4
...
The
1
...

A larger g-factor caused the simulation (in blue) to shift
towards the left, a region of low magnetic eld, whereas

3
...


the peaks of the simulated graphs overlapped the EPR

The linewidth is dened with respect to the magnetic in-

acquired graphs
...
These

to

wards the right, a region of increasing magnetic eld
...
286mT

The nal parameter, the line width,

half-width value
...


sorption value has half of the maximum absorption, in

second peak, at

the center of the resonance line
...
Both of the peaks correspond to the

investigation)
...
al
...
Since

7

5 Si-Bi

Fig
...
The two peaks are a result of hyperne splitting but, due to the nature of the Phosphorous, both
of the peaks are slightly anisotropic mainly due to noise
...


1
2
which is equivalent to having two magnetic spin quan1
tum numbers ms = ± , each magnetic spin causes the
2
electrons to split into two
...


In its natural states, silicon is formed as

31

P,

28

Si

but

man eect causes two peaks where the peak on the right

when doped with elements such as

of gure 1 has a higher energy than the one on the left
...


which means that there are more spin up than spin down

According to Fujii, when a concentration of 0
...


P

28

Si

From section 3
...
89×10−5

per unit volume

electrons (+1 indicates all of the electrons are spin up

−1

of P was doped into the Si, at temperatures lower than

whilst

100K, several peaks appeared with the central peak, be-

the electron has a low probability of passing through the

ing located at

322
...
998

- a

indicates that they are all spin down), where

nucleus where

±1

is the highest probability
...
al
...
Fujii et
...
states that for in-

In order to create the overlap of the Si-Bi sample, the

creasing doping percentage of P, up to around 1
...
Furthermore, unlike Si-P, the lines

disappear leaving only a smooth conduction-electron fea-

present were not equally spaced indicating a hyperne

ture, represented by the horizontal lines in gure 1
...
The g-factor was then set

The hyperne splitting present in gure 1 is a result
of one of two reasons
...
65mT ,

a value

2
...
The nal parameter, the line width, was

ated from the Si-P interaction may localise at the P nu-

found to be a combination of Gaussian and Lorentzian in

cleus, leading to it interacting with the nuclear spin of

shape with a width equal to

the same element
...
6mT

0
...


line splitting into a doublet, as described in Feher's paper

Unlike the Si-P case, Si-Bi has many more lines and

[12]
...
al
...


8

6 Nitrogen doped in C60

Fig
...
The spacing is dierent as a result of hyperne coupling forces competing against the external eld whilst the line in
the center is not overlapped since it cannot be explained via hyperne coupling; it is a dangling bond
...


As a result, the magnetic spin quan-

address this issue
...


This dangling bond could

tum number can take 10 dierent values ranging from

be a result of several key processes including oxidation

9
ms = − 2 , − 7 + n , 9
2
2

n = 1, 2,
...
As

con oxide formation, or potential microcracks as a result

with the Si-P case, the higher energy lines correspond to

of the Si-Bi surface being put under extreme strain, ei-

the highest spin quantum number so the line on the far

ther thermal or mechanical in nature
...

2

right is equivalent to
left is equivalent to

whilst the line on the far

the dangling bond can be replaced with hydrogen atoms
...


al
...
05T − 0
...
The electron spin density of

1
...
In the far left of

in comparison to Si-P
...


This mixing phenomena

is present in Si-P but only at extremely low magnetic
elds of around

20mT
...
This means that

through the nucleus is larger than for Si-P
...
For Si-Bi,

In order to create the overlap of the N-C60 sample, the

if the transition frequency is a constant with respect to

hyperne coupling constant was set to

the magnetic eld, regions called cancellation resonances

smaller than Si-P
...
567mT ,

a value

are present which eect electronic and nuclear frequen-

were Gaussian in nature with a linewidth of

cies, an unexplored region for Si-Bi which makes it an

Furthermore, again like Si-P, the lines present appeared

upgrade to Si-P for qubit-pair use
...
015mT
...
00078
...
al
...
3:

There are three peaks signifying the presence of Nitrogen-14 inside the C60 whilst the equal spacing and sharp lines indicate a
spherically symmetric sample with isotropic g-factor and hyperne coupling
...


15

N

1
2 which, much like
P, would result in only two peaks forming
...
This result is also conrmed by Weidinger

0
...
11mT
...


These

peaks correspond to the spin quantum number of

55

(

53

Mn

Mn

is a radioactive isotope of Mn but it would pro-

5
2 thus the magnetic spin quantum number describes the presence of 6
duce two additional peaks) which is

peaks
...
al
...
Much like Si, oxygen in its

not only keeps its atomic electronic conguration as it

most common state,

was before doping, but it occupies the on-center posi-

of

tion of the C60, a key reason for its use as a catalyst

on gure 4, there is no peak corresponding to this oxy-

for hydrogen fuel cells
...


However, as shown

gen with the only overlap being with

55

Mn

and

17

O
...


has a spin quantum number

As opposed to Si-P, N-C60 has sharper peaks, that

5
2 and, in the case of
transition metal oxides, this isotope of oxygen is very

look very similar, thus is classied as being more spher-

useful in EPR measurements as conrmed by Che et
...


ically symmetric which conrms that the g-factor and

[18]
...
This symmetry is a key

Much like for the previous sample, the equal spacing

reason why fullerenes are very stable molecules, another

of the lines indicate isotropic behaviour of the g-factor

factor supporting its use as a catalyst
...


dierences being extremely skewed in the second graph
on gure 4, there may be some anisotropic behaviour
...
99992
...
6mT

whilst the g-factor

Much like for Si-Bi, the EPR spec-

trum was a combination of Gaussian and Lorentzian in

8 Gamma Irradiated Methylalanine
In order to create the overlap of the methyl-alanine sample, the hyperne constant was set to
g-factor was set to

2
...


2
...
4:

The six peaks correspond to 55 M n with overlaps suggesting the presence of 17 O
...


MnO samples, the EPR spectrum was a combination

likely, a proton exchange in the COOH group to make

of Gaussian and Lorentzian in shape with the Gaussian

the methyl groups behave the same [19]
...
26mT

and the Lorentzian line width

0
...


tion on the radical, coupled with gure 5, shows that the
same radical is responsible for the spectra but to achieve

Unlike the previous samples, a perfect overlap of this
sample was not possible through a single simulation but,

overlapping they must be done in 3-dimensions since the
bonds are oriented dierently in the dierent planes
...

Multiple spectroscopic lines are required since methylalanine is a combination of several dierent elements,
with a chemical formula including carbon, hydrogen and
oxygen which each contributing to the seven peaks in
dierent ways
...


al
...


Though the spacing of the peaks are very even, resembling a symmetric structure, the overlaps between the
four elements are huge with some elements not being
able to overlap with others
...

There is only one radical group measured in this sample, as discussed in section 2 of this report
...


al
...
al
...
5:

Multiple spectroscopic lines are required since methylalanine is a combination of several elements which are oriented in a 3-dimensional plane therefore 3 hyperne couplings for each plane is required
...


group has a structure such as that shown on gure 6
...
As

11

9 Perinaphthenyl Radical

such, the electronic structure of many complex molecules

Marshall [9], and the spectrum is isotropic, due to the

cannot be performed with EPR but has to be performed

equal spacing between the groups, indicating that the

through the use of other methods, such as the STM or

molecule is fairly stable
...


Fig
...
This is the only radical present why
methy-alanine is irradiated with gamma radiation
...
7:

9 Perinaphthenyl Radical
Since this radical group had 9 protons that were sepa-

This sample had 9 protons that were separated into 2
groups, one of 6 and the other 3, where the former group
represented the strongly interacting protons
...
The
lines are a result of 3 benzene rings combined to an unpaired electron
...
From gure 7, the
separation between each of the lines is one of two values i
...
they are closer or very far apart, indicating the
presence of two types of hyperne interactions with the
protons of dierent strengths
...
For the rst group of 6,
the hyperne coupling constant was

0
...
182mT
...
8:

Though the lines in gure 7 appear random, they are
grouped into 7 groups of 4
...
00260

whilst the line width was a combi-

nation of a Gaussian of width
of width

0
...
0131mT
...
Though
the lines appear random, they are grouped into 7 groups
of 4 in an arrangements presented on gure 8
...
The reason for this ratio is

that this radical is neutrally circular and contains three
benzene rings which are combined to an unpaired electron which is delocalised symmetrically over the entire
system
...
26mT

4mT ,

is the conrmed value presented by Glarum and

Part IV
...
This was achieved by using a trial-anderror method to t the EPR curves onto the experimentally measured sample curves through manually changing
the g-factor value, the hyperne coupling constant and
through modication of the shape of the EPR simulated
graphs by determining if they were Gaussian, Lorentzian
or a combination of the two
...
These values are presented on table 1
...
99780
2
...
00078
1
...
00231
2
...
00452
−0
...
00154
−0
...
00001
0
...
211
52
...
567
8
...
280
0
...
182

0
...
300
0
...
060
0
...
010

0
0
...
110
0
...
0131

Tab
...
The g-factor of the organic elements is closest to the g-factor
of a free electron which indicates that the electrons do not interact with the electronic structure as intense as for the transition
metals
...


The g-factor of the last two organic radicals is closest

ge

which would improve spectral resolution
...


However, in all of the cases

ge

∆g
...


eect holds true when comparing the hyperne coupling

A future investigation could involve performing EPR

constant of transition elements in comparison to the or-

spectroscopy on the samples at dierent temperatures or

ganic elements since a larger linewidth corresponds to a

dierent magnetic elds in order to determine how the

larger hyperne constant with the Lorentzian linewidth

hyperne coupling and g-factor is related to temperature
...


This eect seems more intense

This would provide better accuracy into the hyperne

for the transition elements since the unpaired electrons

constants since they are deemed to change linearly with

in transition metals are more delocalised and, unlike the

temperature in many studies
...


1
2 , in order to determine how dierent spin
states eect the g-factor and the hyperne coupling
...


This was also the case when describing the

line spectra of the samples, usually a result of hyperne
splitting due to electron or proton interactions
...

EPR fails when the radical ion is present in more
than one plane or, as for Si-Bi, there are many foreign
species present that have contaminated the sample
...
This anisotropy was present somewhat in the methylalanine group and, for such a case,
the EPR signal cannot give very good accuracy
...
Weil and M
...
Electron paramagnetic
resonance (epr), Sep
...

[2] C
...
Introduction to epr spectroscopy, Dec
...

[3] David J
...
Andre Mkhoyan,
and Uwe R
...
Phosphorus-doped silicon
nanocrystals exhibiting mid-infrared localized surface
plasmon resonance
...

[4] Xiaodong Pi
...
Journal of Nanomaterials, 2012:9, 2012
...
H
...
W
...
Nazir, and T
...

Monteiro
...

Phys
...
B, 85:094404, Mar 2012
...
Spivey
...
Journal of the American
Chemical Society, 135(9):33153318, 2013
...
S
...
R
...
H
...
E
...
L
...
D
...
M
...
Villalobos
...
Astrobiology, 11:775786, Oct
...

[8] S
...
Sagstuen, M
...
Lund
...
Radiation Research, 157(2):113 
121, Feb
...

[9] Sivert H
...
Marshall
...
The Journal
of Chemical Physics, 44(8):28842890, 1966
...
Hyperne structure
of the electron spin resonance of phosphorus-doped si
nanocrystals
...
Rev
...
, 89:206805, Oct 2002
...
J
...
Ortega, A
...
Morazzani, J
...
Stievenard
...
Journal of
Luminescence, 57(16):301  313, 1993
...
Feher
...
i
...
Phys
...
,
114(5):1219  1244, Jun
...

[13] D
...
Griscom, E
...
Friebele, K
...
Long, and J
...

Fleming
...

Journal of Applied Physics, 54(7):37433762, 1983
...
H
...
W
...
S
...

Bismuth qubits in silicon: The role of epr cancellation
resonances
...
Rev
...
, 105:067602, Aug 2010
...
P
...
Haneman
...
Phys
...
B, 17:18931907, Feb 1978
...
Gembus, F
...
Janossy, H
...
P
...
Properties of n@c60 derived peapods
...

[17] A
...
Waiblinger, B
...
Almeida Murphy
...
Applied Physics A: Materials Science & Processing, 66:287
 292, 1998
...
Tench
...
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...
Kispert, Kichoon Chang, and Carolyn M
...
Electron-electron double resonance of irradiated dimethylmalonic acid, -aminoisobutyric acid, and
l-alanine single crystals: The role of methyl substituents
...




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