Supplemental Material
Comparative electron paramagnetic resonance
investigation of reduced graphene oxide and carbon
nanotubes with different chemical functionalities for
quantum dot attachment
C.V. Pham,1,2 M. Krueger,1,2,* M. Eck,1,2 S. Weber3 and E. Erdem3,*
1
Freiburg Materials Research Center (FMF), University of Freiburg, Stefan-MeierStr. 21, 79104 Freiburg, Germany.
2
Department of Microsystems Engineering (IMTEK), University of Freiburg, GeorgesKöhler-Allee 103, 79110 Freiburg, Germany.
3
Institute of Physical Chemistry I, University of Freiburg, Albertstr. 21, 79104
Freiburg, Germany.
*
Author to whom correspondence should be addressed: michael.krueger@fmf.uni-freiburg.de,
emre.erdem@physchem.uni-freiburg.de
SYNTHESIS
Thiol-functionalized reduced graphene oxide (TrGO), quantum dot Thiol-functionalized
reduced graphene oxide (QD-TrGO), carbon nanotube-oxide (CNT-O1) and thiolfunctionalized carbon nanotube (CNT-SH) synthesis
Graphene oxide (GO) was synthesized by a modified Hummers method [1]. In brief, 1 g
graphite (Merck) was oxidized at 35C for 18 hours in a mixture of 6 g KMnO4 99%, 0.5 g
NaNO3 (95%, Grussing) and 46 ml H2SO4 (98%, Sigma). Afterwards 6 ml H2O2 30% and 80
ml HCl 37% was added to reduce residual KMnO4 and MnO2 to Mn+2. The sample was
purified by extensive washing with 100 ml DI-H2O three times in succession. In the final step,
pure GO was dispersed in DI-H2O and exfoliated by 1 hour sonication to form a
homogeneous solution. Consequently the exfoliated GO was collected by vacuum evaporation
using a rotating evaporator at 70 C. GO was thiol-functionalized into TrGO by refluxing 100
mg as-received dried GO with 300 mg phosphorus pentasulfide (P4S10) (99%, Sigma) in 100
ml dimethylformamide (DMF) (HPLC reagent grade, Scharlau Chemie) at 154C for 24 h as
reported elsewhere [2]. CdSe QDs were synthesized based on a procedure described by Yuan
et al. [3] The resulting CdSe QDs have an average diameter of about 6 nm. QD-TrGO was
fabricated by a self-assembly decoration as reported in a previous publication [2]. CNT-O1
and CNT-SH were synthesized following the same procedures as described for GO and
TrGO, respectively.
Carbon nanotube (CNT-O2) and Reduced graphene oxide (rGO) synthesis
200 mg CNT (Bayer Material Science, >99%) was added in mixture of 10 ml HNO3 (Merck,
63%) and 30 ml concentrated H2SO4 (Merck, 98%) and stirred at 100C for 4 h. The dispersion
was then poured into 150 ml DI-H2O. After that CNT-O2 was collected by centrifugation at
4400 rpm for 2 h, and then purified by 3 times washing with 100 ml DI-H2O. Finally CNT-O2
was dried overnight in vacuum at room temperature. Reduced graphene oxide (rGO) was
synthesized by reducing GO by hydrazine (N2H4) in DMF: 20 mg GO was dispersed in 50 ml
DMF by sonication for at least 30 minutes. After 20 µl N2H4 (Sigma, 64%) was added and the
dispersion was stirred at 90 oC for 12 h. The resulting rGO was then purified by centrifugation
and re-dispersion three times in succession.
EXPERIMENTAL
Transmission electron microscopy (TEM, LEO 912 Omega) picture were recorded using an
accelerated voltage of 120 kV.
X-band (9.86 GHz) electron paramagnetic resonance (EPR) measurements were performed on
a Bruker EMX spectrometer. The magnetic field was detected with an NMR gaussmeter (ER
035M,
Bruker),
and
as
a
standard
magnetic-field
marker,
polycrystalline
diphenylpicrylhydrazyl (DPPH) with g = 2.0036 was used for accurate determination of the
resonance magnetic-field values and the g-factor. Following EPR experimental parameters
were used: microwave power: 1 mW, modulation amplitude: 0.5 G, time constant: 20.48 ms
and receiver gain: 2x103.
Time-depended photoluminescence (PL) quenching experiments were performed under
following conditions: The as-received trioctylphosphine oxide (TOPO)/ hexadecylamine
(HDA), TOPO/HDA, capped CdSe QDs were first post-synthetically treated with hexanoic
acid in order to reduce the amount of surface ligands by applying a procedure reported in a
previous publication [4]. Then 0.1 mg CdSe QD and 0.05 mg TrGO were added in 1 ml
chlorobenzene (CB). Afterwards the QD and TrGO in CB dispersion were stirred with a
magnetic stirring bar at 110 °C. The PL spectra of the QD-TrGO dispersion were recorded
every two minutes by utilizing a J&M FL3095 spectrometer (J&M, Germany) using an
excitation wavelength of 450 nm, band pass of 14.70 nm, integration time of 0.10 second;
emission recorded range of (550 – 800) nm, increment 1.0 nm. For QD-rGO, the experiments
were performed under the same condition as for QD-TrGO except using 0.05 mg rGO instead
of TrGO.
Transmission electron microscope (TEM) image:
Figure S1. TEM image of CdSe quantum dot decorated thiol-functionalized reduced graphene
oxide (TrGO). The upper inserted image has been taken at lower magnification revealing the
QD attachment to the TrGO flake.
Spin counting procedure:
In order to accurately count the number of spins, there are many important issues that should
be considered before and after the EPR experiment. Crucial issues, which have to be carefully
taken into account, are: (i) Samples should be always weighed before the experiment to avoid
complications due to different sample amounts in EPR tubes. If it is not possible to have
always the same amount of sample in an EPR tube, then for normalization each spectrum
should be multiplied by a filling factor (deduced from the mass of the sample in the EPR
tube). (ii) The sample position should be always adjusted to the center of the microwave
cavity. (iii) If there is a background EPR signal (e.g., from impurities in the resonator), it has
to be subtracted from the EPR signal of the sample. (iv) One should always be careful not to
saturate the EPR signal by applying too high microwave power. The microwave phase should
be carefully adjusted during the critical coupling (tuning) of the resonator. (v) One has to
check whether there is an offset in the magnetic field and calibrate if necessary. (vi) The Q
value of the resonator has to be measured and all spectra should be referred to the same Q
value.
The number of defect centers can be quantitatively determined by the aid of EPR spectra
independent from the microwave frequency. In order to calculate the defect concentration,
one doubly integrates each EPR first-derivative signal. By comparing the integral of the
standard sample (here, MnO powder) and the measured sample one obtains the corresponding
number of spins, thus the concentration of defect centers. For an exact determination of defect
concentration, one has to normalize by taking into account the following expression including
experimental parameters of both the reference and the probe under investigation:
(1)
where NS*, RG, MF, MA, CT, P, Scans, SW, and S stand for the number of spins in reference
sample, receiver-gain, modulation frequency (in kHz), modulation amplitude (in G),
conversion time (in ms), microwave-power (in mW), field-sweep (in G), number of scans, and
spin quantum number, respectively. Note that, (*) indicates the measurement parameters for
the reference sample. Once the normalised-corrected value of NS* is obtained, via simple cross
multiplication of the NS* and the area under the EPR signals reveal the defect concentration,
ND, of the sample under investigation:
(2)
where (Area)D and ND are the area of the EPR signal of the related defect centre and the
number of spins of the sample, respectively. In this work we used MnO powder as standard
sample, which has 3.34 1019 spins/g.
In Table I, the peak-to-peak width (∆Bpp) and the spin densities were given. The spin
densities were calculated by spin-counting procedure which is described above:
Table I: Peak-to-peak linewidth (∆Bpp) and the spin densities.
Paramagnetism:
Figure S2.Temperature dependence of the EPR-susceptibility as obtained after double
integration of the obtained EPR spectra of a) CNT-O1 and b) TrGO.
Quantitatively the measured EPR spectra can be analyzed in terms of double integration of the
dispersive type continuous wave EPR which is given as EPR susceptibility. The related Curieplots were given in Figure S2. Obviously, the EPR line belong to CNT-O1 follows a
ferromagnetic behaviour with a Curie temperature of TC = 222 K. In contrast, TrGO shows a
more complicated behaviour. Below the EPR line of TrGO is temperature independent below
TC (222 K) being characteristic for a Pauli paramagnetism. Between 222- 275 K interval the
TrGO EPR line starts to tumble, which destroys the magnetic coupling of the electron spins
which is reported recently for Fullerene anions [5].
PL spectra:
Figure S3. PL spectra of CdSe QD, CdSe QD-TrGO nano-composites and a mixture of CdSe
QD and rGO dissolved in chlorobenzene respectively, with the same QD concentrations (after
2 min of incubation).
References:
[1] a) W. S. Hummers and R.E. Offeman, J. Am. Chem. Soc 80, 1339 (1958). b) D.C.
Marcano, D.V. Kosynkin, J.M. Berlin, A. Sinitskii, Z. Sun, A. Slesarev, L.B. Alemany, W.
Lu, and J.M. Tour, ACS NANO 4, 4806 (2010).
[2] C.V. Pham, M. Eck, and M. Krueger, Chemical Engineering Journal 231, 146 (2013).
[3] Yuan Y, Riehle FS, Gu H, Thomann R, Urban G, and Krüger M, J Nanosci Nanotechnol
10, 6041 (2010).
[4] Y. Zhou, F.-S. Riehle, Y. Yuan, H.-F. Schleiermacher, M. Niggemann, G. A. Urban, M.
Kruger, Appl. Phys. Lett. 96, 13304 (2010).
[5] M. B. Boeddinghaus, W. Klein, B. Wahl, P. Jakes, R.-A. Eichel, T. F. Fässler, Z. Anorg.
Allg. Chem. (in print, 2014, online available) doi: DOI: 10.1002/zaac.201300607