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New Techniques for Determining Electronic Properties of Nitrogen Doped Carbon
Nanospheres
Vincent Marsicano, Jonathan Keartland, William Wright, and Neil Coville
DST/NRF Centre of Excellence in Strong MaterialsSchool of Physics, WITS
School of Chemistry, WITS
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Introduction Part 1• Carbon nanomaterials are of great scientific and
technological interest at present due to their wide applicability.
• Carbon nanospheres (CNS) were produced using a horizontal CVD reactor.
• CNS of known dopant level were characterized using Electron Paramagnetic Resonance (EPR)
• Nitrogen content of unknown samples was determined using an EPR spectrometer
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Electron Magnetic Resonance• EMR (also known as ESR and EPR) experiment was
performed using the Bruker Spectrometer shown below. The experiment was performed in CW mode.
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Horizontal CVD reactor
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XPS Determination of N dopant
SDA = 0% NitrogenSDB = 0.4% NitrogenSDC = 2.5% NitrogenSDD = 5% Nitrogen
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EPR spectra of three N-doped CNS obtained. Signal strength is based on many parameters, both of the spectrometer and the samples.
Samples are carefully weighed to calculate the number of paramagnetic sites present per gram of sample.
EPR Data of Selected Samples
Magnetic Field (G)
3450 3460 3470 3480 3490 3500 3510
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EPR Spectrometer CalibrationEPR Spectrometer Calibration Curve
Nitrogen Concentration (%)
0 2 4 6
Arb
itra
ry U
nits
per
gra
m
0
500
1000
1500
2000
2500
3000
3500
4000
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A Composite EPR spectrum of N-doped CNS sample (SDD) and the EPR reference standard (DPPH) that we used to determine the g-factor. Deconvolution of the two spectra allows one to determine the g value of the original signal.
Composite EPR Spectrum of SDD and DPPH
Magnetic Field
3410 3420 3430 3440 3450 3460 3470 3480 3490
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• The g Value increases with increased nitrogen concentration.
Electron g Value vs Sample Nitrogen Concentration
Nitrogen Content (%)
-1 0 1 2 3 4 5 6
g V
alu
e
2.00260
2.00262
2.00264
2.00266
2.00268
2.00270
EPR Spectrometer Calibration
hν=gμB
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• SD1 = C2H2 at 900oC for 2 hours.
• SD2 = C2H2 at 900oC for 1.5 hours.
• SD3 = Collected from quartz tube• SD4 = Collected from quartz boat
Unknown Sample Characterisation
• SD1 = 1.710% ± 0.503%• SD2 = 1.737% ± 0.509%• SD3 = 3.362% ± 1.101%• SD4 = 3.446% ± 0.986%
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• SD1 = 2 hours.• SD2 = 1.5 hours.
• SD3 = quartz tube• SD4 = quartz boat
Unknown Sample Characterisation
Electron g Value vs Sample Nitrogen Concentration
Nitrogen Content (%)
-1 0 1 2 3 4 5 6
g V
alu
e
2.00255
2.00260
2.00265
2.00270
2.00275
2.00280
2.00285
SD1
SD2
SD3
SD4
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Introduction Part 2• Determination of Electronic Transport Properties of
CNS are criticical to their deployment in industry.• Doping contributes significantly to the transport
properties of these materials. • Resistivity of the bulk CNS samples was determined
using the Van der Pauw technique.• Sample chambers were designed and built in-house with
the use of open source communities.
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Open Source Technologies• Open Source technologies were extensively deployed in
the production of these results.
• The experiments were conducted with significant time and cost saving.
Slic3r
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Van der Pauw Greek Cross Cell
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Van der Pauw Square Cell
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Hall Effect Cell
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RepRap Ormerod Printer
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Computer Controlled 4 Channel Physical Relay Multiplexer
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VDP Results
0.0006
0.0007
0.0008
0.0009
0.001
0.0011
0.0012
50 100 150 200 250 300 350
Temperature Dependance of the Resistivity of Carbon Microspheres of Variying Nitrogen Dopant
Concentrations Determined with a Greek Cross Cell
SDDSDCSDBSDA
Resis
tivity (
Oh
m.m
)
Temperature (K)
Resistivity decreases with NitrogenDopant level.
Metal-Insulator transition likely caused by contacts.
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VDP Results
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
50 100 150 200 250 300 350
Temperature Dependance of the Resistivity of Carbon Microspheres of Variying Nitrogen Dopant
Concentrations Determined with a Square Cell
SDDSDCSDBSDA
Resis
tivity (
oh
m.m
)
Temperature (K)
Incraesing dopant level increases the semiconducting behaviour.
VRH and FIT likely models for conduction.
RH ≈ -1.1537e-07
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Conclusions• N-doped CNS have been successfully produced using a
horizontal CVD reactor.• The nitrogen is strongly paramagnetic indicating that the nitrogen
is in substitutional sites• EPR can be used as a characterisation technique for determining
dopant level. • Open source technologies allow for substantial cost savings in
producing research. • Developing an MPRI “open source” community to improve
sharing of ideas and technologies is imperative to increase research output.
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Acknowledgements
Thanks are due to the following:• DST/NRF CoE in Strong Materials, the School of
Physics for support.• School of Physics• Prof. Jonathan Keartland• Open Source Communities Mentioned