Comparison between organic sensitizers containing stilbene and azo group as bridging unit for dye sensitized solar cells

Article abstract of DOI:10.1166/jnn.2014.9554

While azo dyes have been widely used in dye industry, the azo dyes have been seldom applied as sensitizers to dye sensitized solar cells. In this study, new metal-free organic sensitizers, ST and AZ, which are same structures except bridging units, were synthesized and evaluated. ST containing stilbene as bridging unit gave higher energy conversion efficiency than AZ containing azo group as bridging unit. As a result of density functional theory (DFT) calculations, ST displayed more localized frontier molecular orbitals at lowest unoccupied molecular orbital (LUMO) states than AZ.

Full text of DOI:10.1166/jnn.2014.9554

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Nanoscience and Nanotechnology
Vol. 14, 7502–7507, 2014
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Comparison Between Organic Sensitizers Containing
Stilbene and Azo Group as Bridging Unit for
Dye Sensitized Solar Cells
Se Woong Park1ꢀ2, Jin Woong Namgoong1, Il Keun Kwon2ꢀ∗, and Jae Pil Kim1ꢀ∗
1Department of Material Science and Engineering, Seoul National University Daehak-dong,
Kwanak-gu, Seoul 151-744, Republic of Korea
2Department of Maxillofacial Biomedical Engineering and Institute of Oral Biology, School of Dentistry,
Kyung Hee University, 26 Kyunghee-daero, Dongdaemun-gu, Seoul 130-701, Republic of Korea
While azo dyes have been widely used in dye industry, the azo dyes have been seldom applied as
sensitizers to dye sensitized solar cells. In this study, new metal-free organic sensitizers, ST and AZ,
which are same structures except bridging units, were synthesized and evaluated. ST containing
stilbene as bridging unit gave higher energy conversion efficiency than AZ containing azo group
as bridging unit. As a result of density functional theory (DFT) calculations, ST displayed more
localized frontier molecular orbitals at lowest unoccupied molecular orbital (LUMO) states than AZ.
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Keywords: Dye Sensitized, Solar Cell, Azo Dye, Organic Sensitizer.
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Copyright: American Scientific Publishers
1. INTRODUCTION
as electron acceptor ST consists of the stilbene structure as
bridging unit while AZ have azo group for connecting the
electron donor and acceptor. The synthesized dyes were
evaluated and studied using solar cell devices and DFT
calculations.
Dye sensitized solar cells (DSSCs)1 are attracting much
interest because of its high energy conversion efficiency
and its low cost. Solar cell sensitized by Ru-complex sen-
sitizers (N719, black dye, etc.) have achieved energy con-
version efficiency over 12%.2 In addition, co-sensitizations
using two or more dyes for panchromatic absorption
have been studied.3ꢀ4 Recently metal-free organic sensi-
tizer have been studied actively owing to its high extinction
coefficient, easy tuning HOMO–LUMO level and a variety
of colors. Several organic dye sensitized solar cells hav-
ing efficiency up to 9% was investigated.5ꢀ6 Structures of
most reported organic dyes were composed of an electron
donation group (mostly amino group derivatives), an elec-
tron withdrawing and anchoring group (mostly cyanoacetic
acid) and ꢁ-conjugation bridges (benzene ring, methine
unit, thiophene, etc.).
2. EXPERIMENTAL DETAILS
2.1. Equipment and Measurement
1H NMR spectra were recorded by Bruker Avance 500
at 500 MHz using CDCl3 and MeOD as the solvent
and TMS as the internal standard. Mass spectra were
measured with JEOL JMS-600W Agilent 6890 Series.
UV-Visible absorption spectra were measured with an
HP 8452A spectrophotometer and emission spectra were
recorded on a Quanta-Master™ UV VIS spectrofluorome-
ter. Cyclic voltammetry (CV) was performed with a three-
electrode electrochemical cell on a Potentostat/Gavanostat
Model 273A. Glassy-carbon, platinum wire, and Ag/Ag+
were employed as working, counter, and reference elec-
trodes, respectively. Tetrabutylammonium tetrafluoroborate
(TBATFB) was used as the supporting electrolyte and fer-
rocene was added as the internal reference for calibration.
Photocurrent–voltage (I–V ) measurements were per-
formed using a Keithly model 2400 source measure unit.
A class-A solar simulator (Newport) equipped with 150 W
Azo dyes, which have been one of the most commercial
dyes for dye industry, were seldom investigated for DSSC.
In this paper, to investigate the potential utilization of azo
dyes for DSSCs, we designed, synthesized and evaluated
two dyes, ST and AZ, as shown in Figure 1, which have
dimethyl amino group as electron donor and cyano group
∗Authors to whom correspondence should be addressed.
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doi:10.1166/jnn.2014.9554

Park et al.
Comparison Between Organic Sensitizers Containing Stilbene and Azo Group as Bridging Unit for DSSCs
1
cis form. H NMR (500 MHz, CDCl3) ꢄ: 2.95 (s, 6 H),
6.36 (d, J = 12ꢅ1, 1 H), 6.57 (d, J = 8ꢅ77, 2 H), 6.63
(d, J = 12ꢅ1, 1 H), 7.11 (d, J = 8ꢅ75, 2 H), 7.41 (d, J =
8ꢅ46, 2 H), 7.51 (d, J = 8ꢅ15, 2 H).
2.2.1.3. 4-ꢂ4-ꢂdimethylaminoꢃstyrylꢃBenzaldehyde ꢂ3ꢃ.
Diisobutylaluminium hydride (1.0 M in toluene: 27 ml,
27 mmol) was added dropwise to
a solution of
Compound 2 (5.30 g, 21.34 mmol) in chloroform (250 ml)
ꢀ
at − 78 C by dry ice and acetone and the temperature
maintained for 1 h. Upon reaching room temperature, the
solution was poured into acidified water (pH = 2 by acetic
acid) and the organic layer was extracted, washed with
water and evaporated. The crude product purified by col-
umn chromatography on silica gel, eluting with chloro-
form. (Yield: 67%). (cis:trans form = 9:5).
Figure 1. Molecular structure of ST and AZ.
Xe lamp was used as a light source, where light intensity
was adjusted with an NREL-calibrated Si solar cell with
KG-1 filter for approximating 1 sun light intensity. Inci-
dent photon-to-current conversion efficiency (IPCE) was
measured as a function of wavelength from 300 nm to
800 nm using a specially designed IPCE system for the
dye-sensitized solar cell (PV measurements, Inc.). A 75 W
xenon lamp was used as a light source for generating
monochromatic beam. A calibration was performed using
a silicon photodiode, which was calibrated using the NIST-
calibrated photodiode G425 as a standard, and IPCE values
were collected under bias light at a low chopping speed of
10 Hz.
Trans form. 1H NMR (500 MHz, DMSO) ꢄ: 2.96
(s, 6 H), 6.75 (d, J = 8ꢅ8, 2 H), 7.09 (d, J = 16ꢅ4, 1 H),
7.39 (d, J = 16ꢅ4, 1 H); 7.50 (d, J = 8ꢅ73, 2 H), 7.74
(d, J = 8ꢅ21, 2 H), 7.86 (d, J = 8ꢅ23, 2 H); 9.94 (s, 1 H).
1
cis form. H NMR (500 MHz, DMSO) ꢄ: 2.89 (s, 6 H),
6.45 (d, J = 12ꢅ2, 1 H), 6.60 (d, J = 8ꢅ83, 2 H), 6.64
(d, J = 12ꢅ2, 1 H); 7.10 (d, J = 7ꢅ28, 2 H); 7.50 (d, J =
8ꢅ73, 2 H); 7.81 (d, J = 8ꢅ19, 2 H); 9.94 (s, 1 H).
2.2.1.4. ꢂEꢃ-2-Cyano-3-ꢂ4-ꢂ4-ꢂdimethylaminoꢃstyrylꢃ
phenylꢃ Acrylic Acid ꢂSTꢃ. A 150 ml acetonitrile solu-
tion of compound 3 (11.94 mmol) and 2-cyanoacetic acid
2.2. Synthesis of Dyes
(1.361 g, 16 mmol) was refluxed in the presence of piperi-
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dine (0.2 ml) for 10 h under N2 gas. After acidified water
Starting materials for the dye synthesis were purchased
from Sigma-Aldrich and were used without further purifi-
cation. N3 dye was purchase from Solaronix, Switzerand.
Two organic dyes having different bridging units of stil-
bene (ST) and azo group (AZ) were synthesized as fol-
lowing procedures.7–11
Copyright: American Scientific Publishers
(pH = 2 by HCl) was added to the solution, the precipitate
was filtered. After the crude product was dried in vacuo,
the residue was purified by column chromatography over
silica gel, eluting with chloroform/methanol.
1H NMR (500 MHz, DMSO) ꢄ: 2.96 (s, 6 H), 6.74
(d, J = 8ꢅ86, 2 H), 7.07 (d, J = 16ꢅ3, 1 H), 7.39 (d, J =
16ꢅ3, 1 H), 7.49 (d, J = 8ꢅ80, 2 H), 7.72 (d, J = 8ꢅ44,
2 H), 8.02 (d, J = 8ꢅ43, 2 H), 8.24 (s, 1 H). HRMS-EI
(m/z): [M]+ 318.2873.
2.2.1. Synthesis of ST Dye
2.2.1.1. ꢂ4-cyanobenzylꢃTriphenylphosphonium Bromide
(1). To toluene (250 ml) were added triphenylphosphine
(26.754 g, 102 mmol) and 4-(bromomethyl)benzonitrile
(20 g, 102 mmol). The mixture was refluxed for 24 h under
a nitrogen atmosphere. After cooling to room temperature,
the white powder precipitate was filtered. After dried in
vacuo, the product was used next step without any purifi-
cation. (Yield: 96%).
2.2.2. Synthesis of AZ Dye
2.2.2.1.
4-aminobenzaldehyde
ꢂ4ꢃ. p-Nitrobenzal
dehyde (3.02 g, 0.02 mol) and SnCl2 · H2O (22.55 g,
0.1 mol) (20 g, 102 mmol) were added to ethanol (40 ml).
Then the mixture was stirred for 30 min under a nitrogen
atmosphere. After the solution was cooled to room tem-
perature, ice water (100 ml) was poured to the solution.
The white powder precipitate was filtered. After dried
in vacuo, the product was used next step without any
purification. The pH was made slightly basic (pH 8) by
addition of 5% aqueous sodium bicarbonate and the result-
ing basic mixture kept one hour under stirring. The aque-
ous mixture is extracted three times with ethyl acetate,
the organic phase thoroughly washed with brine and dried
over sodium sulphate. The solvent was removed by rotary
evaporation. The product was used next step without any
purification. (Yield: 89%)
2.2.1.2. 4-ꢂ4-ꢂdimethylaminoꢃstyryl)Benzonitrile ꢂ2ꢃ.
Compound 1 (13.433 g, 31 mmol), 4-(dimethylamino)
benzaldehyde (4.476 g, 30 mmol), and potassuim tert-
butoxide (3.366 g, 30 mmol) in methanol (2ꢀ00 ml) was
ꢀ
refluex at 80 C for 24 h. After cooling to 0 C, the pre-
cipitate was filtered. After washed with methanol 2 times,
the product was dried in vacuo. (Yield: 71%). (cis: trans
form =9:5).
Trans form. 1H NMR (500 MHz, CDCl3) ꢄ: 3.01
(s, 6 H), 6.72 (d, J = 8ꢅ76, 2 H), 6.89 (d, J = 16ꢅ2, 1 H),
7.16 (d, J = 16ꢅ2, 1 H, C
C
H), 7.43 (d, J = 9ꢅ78,
2 H), 7.52 (d, J = 7ꢅ98, 2 H), 7.59 (d, J = 8ꢅ32, 2 H).
J. Nanosci. Nanotechnol. 14, 7502–7507, 2014
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Comparison Between Organic Sensitizers Containing Stilbene and Azo Group as Bridging Unit for DSSCs
Park et al.
2.2.2.2. 3-ꢂ4-aminophenylꢃ-2-Cyanoacrylic Acid ꢂ5ꢃ.
A mixture of compound 4 (2.13 g, 17.7 mmol) and
2-cyanoacetic acid (1.5 g, 17.7 mmol) in dioxane (50 ml)
was refluxed for 15 h. After cooled down to 0 ꢀC,
the precipitate was filtered under reduced pressure. After
the crude product was dispersed to methylene chloride
(50 ml), the product was dissolved by decreasing the pH
of the solution using 10% HCl. After the precipitate was
filtered under reduced pressure, the product was dried in
vacuo. (Yield 50%)
Amount of the adsorbed dyes were analyzed by detach-
ing the adsorbed dye using 100 ml NaOH solution and
measuring the peak intensity using UV-vis. spectroscopy.
2.4. Computational Methods
To compare frontier molecular orbitals of two dyes at
LUMO states, density functional theory (DFT) calcula-
tions were carried out at the B3LYP/6-31G (3d, 3p) level.
3. RESULTS AND DISCUSSION
3.1. Optical and Electrochemical
1H NMR (500 MHz, DMSO) ꢄ: 6.60 (s, 2 H), 6.66
(d, J = 8ꢅ70, 2 H), 7.83 (d, J = 7ꢅ5, 2 H), 7.99 (s, 1 H),
13.16 (s, 1 H).
Properties of the Dyes
Figure 2 shows the absorption and emission spectra of
ST and AZ in dichloromethane and Table I shows the
values and intensities of maximum absorption and emis-
sion wavelength and HOMO-LUMO energy levels. Max-
imum absorption wavelengths (ꢉmax) are 460 nm for ST,
490 nm for AZ. AZ shows 30 nm red-shifted compared
with ST. And molar extinction coefficients (ꢊ) at ꢉmax are
18,600 M−1 cm−1 for ST and 24,720 M−1 cm−1 for AZ,
which are higher value than that of Ru based N3 or N719
dyes (∼ 13,300 M−1 cm−1ꢃ.13 The absorption band of AZ
presented red-shift compared to that of ST. Extinction
coefficient of AZ is slightly larger than that of ST. Azo
group as a chromophore seems to contribute to red-shift
2.2.2.3. 2-cyano-3-ꢂ4-ꢂꢂ4-ꢂdimethylaminoꢃphenylꢃdiazenylꢃ
phenylꢃAcrylic Acid ꢂAZꢃ. To 10% NaHCO3 aqueous
solution (50 ml) was dissolved 3-(4-Amino-phenyl)-2-
cyano-acrylic acid (1.3 g, 7 mmol). After the pH of the
solution was made pH 3 by acetic acid in ice bath, NaNO2
(0.5 g, 7 mmol) in water (2 ml) was added to the solu-
tion. After all precipitate was dissolved, the solution was
stirred at room temperature for 30 min. To the solution
was added N,N-dimethylaniline (1.045 g) in acetic acid
(10 ml). After stirring for 10 h, 150 ml of water was
added. The precipitate was filtered under reduced pressure,
washed with water 3 times and dried in vacuo. The residue
was purified by column chromatography over silica gel,
of ꢉ
and increase of extinction coefficient. Maximum
eluting with chloroform/ethyl acetate. (Yield: 91%).
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emission wavelength of both AZ and ST are 672 nm.
H NMR (500 MHz, DMSO) ꢄ: 6.08 (s, 6 H), 6.86
Copyright: American Scientific Publishers
But fluorescence intensity of ST is much higher than that
of AZ. The azo group contributes to bathochromic shift
and increasing molar extinction coefficient, but allows to
decrease the fluorescence property. Amount of adsorbed
ST and AZ were analyzed by detaching the dyes using
NaOH solution. Amount of the adsorbed ST and AZ were
2ꢅ17 × 10−7 mol/cm2 and 2ꢅ78 × 10−7 mol/cm2, respec-
tively. Amount of adsorbed AZ was larger than that of ST.
But the difference was not large enough to affect photo-
voltaic properties of DSSCs.
(d, J = 9ꢅ17 2 H), 7.83 (d, J = 9ꢅ08, 2 H), 7.86 (d, J =
8ꢅ52, 2 H), 8.01 (d, J = 9ꢅ05, 2 H), 8.03 (s, 1 H). HRMS-
EI (m/z): [M]+ 320.1274.
2.3. DSSC Cell Fabrication
Dye sensitized solar cells were fabricated as followed
procedures.12 TiO2 pastes with diameter of about 20 nm
were purchased from Solaronix, Switzerand. FTO glasses
(Pilkington, TEC-8, 8 ꢆ/sq, 2.3 mm-thick) were washed
in ethanol using an ultrasonic bath for 10 min and treated
with UV–O3 for 20 min to remove organic impurities.
After the TiO2 paste was deposited on FTO glass, the
TiO2-coated FTO was annealed at 500 ꢀC for 30 min.
Deposited TiO2 film’s thickness was 5 ꢇm, which was
measured by ꢈ-step surface profiler (KLA tencor). After
cooling to 80 ꢀC, the TiO2 electrode was immersed
overnight in ethanol solution including 0.5mM of the dye.
A counter electrode was prepared on an FTO glass sub-
strate using a 0.7 mM H2PtCl6 2-propanol solution. The
dye-adsorbed electrode and the counter electrode were
assembled using 25-ꢇm thick surlyn (Dupont 1702). An
electrolyte solution was composed of 0.7 M 1-methyl-3-
propyl imidazolium iodide (PMII), 0.03 M I2, and 0.5 M
4-tert-butylpyridine (TBP) in acetonitrile (AN). TiO2 film
was about 0.210 cm2, which was measured by an image
analysis program equipped with a CCD camera (moticam
1000).
Figure 2. Absorption and emission spectra of ST(—) and AZ(–) in
dichloromethane.
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Park et al.
Comparison Between Organic Sensitizers Containing Stilbene and Azo Group as Bridging Unit for DSSCs
Table I. Optical and electrochemical properties of the dyes.
Table II. Photovoltaic performance of ST-, AZ- and N3-sensitized
solar cells.
ꢉaabs/nm
Dye (ꢊ/M−1 cm−1
ꢉaemi/nm
)
(intensity) E0b–0/V (EꢂS+/Sꢃꢃc/V (EꢂS+/S∗ꢃꢃd/V
Dye
Jsc/mA cm−2
Voc/mV
FF
ꢋa (%)
IPCEmax (%)
ST
460
(18,600)
490
672
(246,500)
672
2.16
2.08
1.42
1.40
−0ꢅ74
−0ꢅ69
ST
AZ
N3
9ꢅ2
2ꢅ9
13ꢅ0
586
474
637
0.69
0.67
0.64
3.72
0.92
5.30
52.0
10.4
61.9
AZ
(24,720)
(3,760)
Notes: aDSSC was covered with an aperture mask and measured at AM 1.5 G one
sun light intensity (100 mW/cm2).
Notes: aAbsorption and emission spectra were measured in dichloromethane; E0–0
values were determined from the threshold of absorption spectra; cOxidation poten-
tials of the dyes were measured in 0.1 M tetrabutylammonium tetrafluoroborate in
acetonitrile; dEꢂS+/S∗ꢃ was calculated by EꢂS+/Sꢃ−E0–0
b
.
Figure 4 shows monochromic incident photon to current
conversion efficiencies (IPCEs) for DSSCs based on ST,
AZ and N3. The maximum IPCE values of solar cell based
on N3, ST, AZ are 61.9%, 52.0%, 10.4%, respectively.
DSSC based on AZ having azo group have an IPCE value
below 10% under most absorption band except for TiO2
absorption range (∼ 350 nm). As ruthenium complex dyes,
such as N3, has metal-to-ligand charge transfer (MLCT)
character, they show relatively high IPCE values at broader
wavelength range compared to organic dyes having ꢁ–ꢁ∗
transition character.13
To measure electrochemical properties of two dyes,
cyclovoltammetry measurements were performed in
dichloromethane. The oxidation potential (EꢂS+/Sꢃ) is
measured by cyclic voltammetry and determined by a
mean value (E1/2) of a cathodic peak potential and an
anodic peak potential as proposed in the Ref. [14]. Zeroth–
zeroth transition energies (E0–0) were determined from the
threshold of absorption spectra. The E0–0 of ST and AZ
were 2.16 eV and 2.08 eV. The oxidation potentials of
exiton (EꢂS+/S∗ꢃ) were calculated by EꢂS+/Sꢃ − E0–0
.
EꢂS+/S∗ꢃ of ST and AZ are − 0.74 V and − 0.69 V,
respectively, both values are well-matched to conduction
band level of TiO2 (ca. − 0.5 V).15
3.3. Qualitative and Computational Study
Various factors such as extinct coefficient, maximum
absorption wavelength, and energy level influence the
efficiency of DSSCs. As dyes have higher extinction coef-
ficient, solar cells have a possibility to make higher pho-
3.2. Photovoltaic Properties of the Dyes
Figure 3 and Table II show photocurrent voltage curves
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tocurrent density. As maximum absorption wavelength of
Copyright: American Scientific Publishers
dyes is closer to 500–550 nm, which is maximum intensity
and photoelectrochemical properties of DSSCs based on
ST, AZ and N3 under AM 1.5 irradiation (100 mW cm−2).
A DSSC based on ST performed an energy conversion
efficiency (ꢋ) of 3.72% (Jsc = 9ꢅ2 mA cm−2, Voc = 586 mV,
FF = 0ꢅ69) which is 69.4% of efficiency if the DSSC based
on N3 (5.30%). The performance of the DSSC based on
AZ gave and ꢋ value of 0.92% (Jsc = 2ꢅ9 mA cm−2, Voc =
474 mV, FF = 0ꢅ67) which is much lower value than that
of the DSSC based on ST.
of the sun, solar cells could make higher energy conversion
efficiency. The HOMO level and LUMO level is should
be well matched to the conduction band of semiconductor
and the iodine redox potential, respectively.
The synthesized dyes, AZ and ST, mostly satis-
fied the above-mentioned conditions. Although AZ dis-
played higher molar extinction coefficients and more
bathochromic-shifted light absorption range, AZ displayed
Figure 3. I–V curves of solar cell sensitized by ST(—), AZ(–), and
N3(···) under AM 1.5 radiation (100 mW cm−2).
Figure 4. IPCE spectra of solar cell sensitized by ST(—), AZ(–), and
N3(···).
J. Nanosci. Nanotechnol. 14, 7502–7507, 2014
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Comparison Between Organic Sensitizers Containing Stilbene and Azo Group as Bridging Unit for DSSCs
Park et al.
Figure 5. Charge separations between donor and acceptor moieties of ST and AZ followed by qualitative VB theory.
significantly lower energy conversion efficiency. As shown
in IPCE spectra, conversion efficiency of AZ was under
10% at the most range of wavelength. When dyes
are excited by absorbing light, charge separation occurs
between donor and acceptor moieties. While stilbene moi-
ety of ST plays role as bridge efficiently, azo group of AZ
interrupts the donor–acceptor charge separation. According
to qualitative VB theory (Fig. 5), because the bridging unit
of ST was totally conjugated, charge separation between
donor and acceptor groups occurs directly. In case of AZ,
due to azo group, charge separation between donor and
acceptor groups are impeded.16
LUMO+1, LUMO+4 and LUMO+5, similar behaviors
were shown at both ST and AZ.
Charge transfer from excited dye to TiO2 is essential
for the operation of DSSCs. In organic dyes, delocaliza-
tion toward the anchoring group, which binds to TiO2,
at LUMO state should occur for the charge transfer to
TiO2. As shown in Figure 6, ST displayed more favor-
able isodensity plots for the charge transfer at LUMO and
LUMO+2 states, although other states were almost simi-
lar at both dyes.
Based on the qualitative VB theory and the compu-
tational study, we assumed that the azo group impeded
efficient charge transfer between the donor group and the
acceptor group on the dye for DSSC.
Figure 6 shows computational study of ST and AZ
using density functional theory (DFT) calculations per-
formed in Gaussian03 at the B3LYP/6-31G∗ level. At
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LUMO level, ST showed more localized isodensity
4. CONCLUSION
Copyright: American Scientific Publishers
plots toward the donor moiety than AZ. At LUMO + 2
Two dyes having same donor moieties (amino groups)
and acceptor moieties (acceptor groups) were synthesized
and evaluated. ST dye having the stilbene structure as
the bridging unit showed higher energy conversion effi-
ciency of 3.72% and much higher IPCE values than AZ
dye having azo groups as the bridging unit. Delocalization
between the donor and acceptor moieties occurred predom-
inantly at ST dye at the DFT calculation. We suppose that
intramolecular donor-acceptor charge transfer was inter-
rupted by azo bridging unit of AZ.
and LUMO + 3, while ST showed localized isodensity
plots, AZ showed delocalized electron distributions. At
Acknowledgment: We very thank Solar cell research
center, Korea Institute of Science and Technology (KIST)
for helping measurement of photovoltaic properties of the
synthesized dyes. This work was supported by the National
Research Foundation of Korea (NRF) grant funded by the
Ministry of Education, Science and Technology (No. NRF-
2012R1A5A2051388).
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Received: 30 September 2013. Accepted: 20 October 2013.
Delivered by Publishing Technology to: McMaster University
IP: 117.244.44.238 On: Fri, 15 Jan 2016 13:44:23
Copyright: American Scientific Publishers
J. Nanosci. Nanotechnol. 14, 7502–7507, 2014
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