Morphology-tailored synthesis of PbSe nanocrystals and thin films from bis[N,N-diisobutyl-N′-(4-nitrobenzoyl)selenoureato]lead(II)

Article abstract of DOI:10.1002/ejic.201100190

The ligand N,N-diisobutyl-N′-(4-nitrobenzoyl)selenourea (4-NO ₂-C₆H₄CONHCSeNiBu₂) (1) and its complex {bis[N,N-diisobutyl-N′-(4-nitrobenzoyl)selenoureato]lead(II)} [(4-NO₂-C₆H₄CO

Full text of DOI:10.1002/ejic.201100190

FULL PAPER
DOI: 10.1002/ejic.201100190
Morphology-Tailored Synthesis of PbSe Nanocrystals and Thin Films from
Bis[N,N-diisobutyl-NЈ-(4-nitrobenzoyl)selenoureato]lead(II)
Javeed Akhtar,[a,b] Mohammad A. Malik,[a] Stuart K. Stubbs,[c] Paul O’Brien,*[a]
Madeleine Helliwell,[a] and David J. Binks[c]
Keywords: Lead / Selenium / Nanoparticles / Thin films / Chemical vapor deposition
The ligand N,N-diisobutyl-NЈ-(4-nitrobenzoyl)selenourea (4-
NO2-C6H4CONHCSeNiBu2) (1) and its complex {bis[N,N-di-
isobutyl-NЈ-(4-nitrobenzoyl)selenoureato]lead(II)} [(4-NO2-
C6H4CONHCSeNiBu2)PbII] (2) have been synthesized, and
their structures determined by single-crystal X-ray methods.
Compound 2 was employed as a single-source precursor
(SSP) for the deposition of PbSe thin films by aerosol-assisted
chemical vapor deposition (AACVD) in the temperature
range 250–500 °C on glass substrates. The films obtained at
low temperature consist of globular-shaped crystallites,
whereas at higher temperature (ca. 500 °C) wirelike morpho-
logies are dominant. PbSe nanocrystals were also prepared
from compound 2 by solution thermolysis at 200–250 °C. Pho-
toluminescence (PL) life time measurements based on the
time-correlated single-photon-counting (TCSPC) technique
showed significantly faster PL decay in as-prepared PbSe
nanocrystals relative to previous reports. The purity and ele-
mental composition of the as-deposited PbSe thin films and
nanocrystals were determined by energy-dispersive X-ray
analysis (EDX).
cant levels of phosphorus contamination with the phospho-
rus originating from the precursor.[59] Recently, it was
shown that cadmium complexes of N,N-dialkyl-NЈ-acyl-
thiourea or the corresponding N,N-dialkyl-NЈ-acylseleno-
urea can be used as single-source precursors for the synthe-
sis of CdS or CdSe nanoparticles.[63] Herein, we report the
synthesis of a new lead complex, bis[N,N-diisobutyl-NЈ-(4-
nitrobenzoyl)selenoureato]lead(II) [(4-NO2-C6H4CONHC-
SeNiBu2)PbII] (2), its characterization by single-crystal X-
ray crystallography and its use as a single-source precursor
for the deposition of phosphorus-free PbSe thin films by
AACVD and nanoparticles by solution thermolysis. To the
best of our knowledge, this is the first report in which this
type of complex is used as a precursor for the deposition of
thin films and nanoparticles of PbSe without any phospho-
rus contamination.
Introduction
Lead selenide is a IV–VI semiconductor with a small di-
rect band gap (0.27 eV) and large bulk exciton Bohr radius
(46 nm).[1] Strong confinement of the electron–hole pair re-
sults in a large optical nonlinearity.[2] Lead selenide par-
ticles or thin films with critical dimensions best measured
in nanometres have potential for application in optical
switches,[2–4] communication devices,[5–9] photovoltaic
cells,[10–22] biological imaging[23–28] and photodetec-
tors.[29–36] Murray et al first synthesized monodispersed
PbSe quantum dots,[37] but a number of other strategies
have now been developed.[38–56] Interestingly, all these
methods involve the use of separate metal and chalcogenide
sources.[38–58]
We have reported the use of [bis(diselenoimidodialkyl-
phosphinato)lead(II)] [Pb{N(PR2Se)2}2], [bis(dialkylseleno-
phosphinato)lead(II)] [Pb(R2PSe2)2] and [bis(diethyldiselen-
ocarbamato)lead(II)] [Pb(Se2CNEt2)2] complexes as single-
source precursors for the deposition of PbSe thin films by
low-pressure LP-CVD and aerosol-assisted (AACVD) ex-
periments.[59–62] However, the deposited films have signifi-
Results and Discussion
Ligand 1 was prepared by a simple method from the re-
action of KSeCN, 4-nitrobenzoyl chloride and diisobutyl-
amine at room temperature and its complex 2 from its lead
nitrate solution. Both the ligand and lead complex are
stable in open atmosphere and can be conveniently stored
at room temperature for months. The lead complex is solu-
ble in most organic solvents including toluene or THF. Suit-
able crystals of 1 and 2 were grown from recrystallization
from ethanol and THF/ethanol (1:1), respectively, by slow
evaporation of the solvent at room temperature. The crystal
structures of compounds 1 and 2 obtained by single-crystal
[a] School of Chemistry and the Materials Science Centre, The
University of Manchester,
Oxford Road, Manchester M13 9PL, UK
Fax: +44-161-275-4598
E-mail: paul.obrien@manchester.ac.uk
[b] Present address: Nanoscience and Materials Synthesis Lab,
Quaid-i-Azam University,
Islamabad, Pakistan
[c] School of Physics and Astronomy and the Photon Science
Institute, The University of Manchester,
Oxford Road, Manchester M13 9PL, UK
2984
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Morphology-Tailored Synthesis of PbSe Nanocrystals and Thin Films
X-ray crystallography are shown in Figure 1. The crystal Crystal Structure of N,N-Diisobutyl-NЈ-(4-nitrobenzoyl)-
refinement parameters are given in Table 1, while selected selenourea (1)
bond lengths and bond angles of 2 are given in Table 2.
N,N-Diisobutyl-NЈ-(4-nitrobenzoyl)selenourea has a mo-
noclinic space group, P2(1)/c. The bond length of C=Se
(1.824 Å) in this compound is significantly larger than that
in the sulfur analogue (1.67 Å).[64–67] This enlargement of
the C=Se bond length is due to the bigger size of the Se
atom than that of the S atom. There is intermolecular hy-
drogen bonding between the carbonyl oxygen atom and the
hydrogen atom of the selenoamide of the neighbouring mo-
lecule [symmetry operators #1: x, –y + 3/2, z + 1/20, N(2)–
H(2)···O(1)#1 2.792 Å]. This feature is unusual relative to
previous reports,[64–67] where the hydrogen bond is present
between the selenium atom of one molecule and the hydro-
gen atom of the selenoamide of the adjacent molecule,
known as resonance-assisted hydrogen bonding (RAHB) or
bond cooperativity. The structure of 1 is shown in Fig-
ure 1a.
Crystal Structures of Bis[N,N-diisobutyl-NЈ-(4-nitrobenzoyl)-
selenoureato]lead(II) (2)
The structure of the complex 2 shows that each lead
atom is surrounded by two selenium and two oxygen atoms,
from each chelating N,N-diisobutyl-NЈ-(4-nitrobenzoyl)sele-
nourea ligand as shown in Figure 1b. The lead atom has a
cis distorted square-planar geometry. Two of the Pb–Se
bonds are significantly longer than the two Pb–O bonds, i.e.
Pb(1)–Se(1)#1 2.8577, Pb(1)–Se(1) 2.8578 Å; Pb(1)–O(1)#1
2.428, Pb(1)–O(1) 2.428 Å. This shows that oxygen atoms
coordinate more strongly with Pb. The C–N bonds are all
Figure 1. (a) Crystal structure of ligand N,N-diisobutyl-NЈ-(4-nitro-
benzoyl)selenourea. (b) Crystal structure of bis[N,N-diisobutyl-NЈ-
(4-nitrobenzoyl)selenoureato]lead(II).
Table 1. Crystal refinement parameters for compounds 1 and 2.
Empirical formula
Formula weight [gmol–1]
Temperature [K]
C16H23N3O3Se
384.33
100(2)
C32H44N6O6Se2Pb
973.84
100(2)
Wavelength [Å]
0.71073
0.71073
Crystal system, space group
a [Å]
b [Å]
c [Å]
α [º]
monoclinic, P21/c
7.5909(15)
24.000(5)
9.916(2)
90
orthorhombic, Iba2
17.853(3)
19.592(3)
10.5264(16)
90
β [º]
98.320(4)
90
1787.4(6)
4, 1.428
2.118
792
90
90
γ [º]
V [Å3]
3681.9(10)
4, 1.757
6.608
Z, calculated density [Mg/m3]
Absorption coefficient [mm–1]
F(000)
1904
Crystal size [mm]
θ range for data collection [º]
Limiting indices
0.25ϫ0.20ϫ0.04
1.70–28.29
8 Յ h Յ 10, –29 Յ k Յ 31, –12 Յ l Յ 9
11238/4162 [R(int) = 0.1010]
θ = 26.42, 99.8
0.20ϫ0.06ϫ0.04
2.08–26.42
–22 Յ h Յ 18, –24 Յ k Յ 21, –11 Յ l Յ 13
10418/3525 [R(int) = 0.0572]
θ = 25.00, 99.4
Reflections collected/unique
Completeness to θ [%]
Max. and min. transmission
Refinement method
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [IϾ2σ(I)]
R indices (all data)
Largest diff. peak and hole [eA–3]
0.9201 and 0.6195
1.000 and 0.745
full-matrix least-squares on F2
4162/0/212
full-matrix least-squares on F2
3525/448/410
0.725
1.010
R1 = 0.0474, wR2 = 0.0764
R1 = 0.1020, wR2 = 0.0876
0.503 and –0.432
R1 = 0.0375, wR2 = 0.0613
R1 = 0.0621, wR2 = 0.0671
1.043 and –0.434
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FULL PAPER
Table 2. Selected bond lengths [Å] and bond angles [°] for compound 2.
Bond length [Å]
Bond angle [°]
Pb(1)–O(1)#1
Pb(1)–O(1)
Pb(1)–Se(1)#1
Pb(1)–Se(1)
Pb(1)–O(1)#1
Pb(1)–O(1)
Pb(1)–Se(1)#1
C(8)–Se(1)–Pb(1)
2.428(6)
2.428(6)
2.857(1)
2.857(1)
2.428(6)
2.428(6)
2.857(1)
101.2(3)°
O(1)#1–Pb(1)–O(1)
O(1)#1–Pb(1)–Se(1)#1
O(1)–Pb(1)–Se(1)#1
O(1)–Pb(1)–Se(1)#1
O(1)#1–Pb(1)–Se(1)
O(1)–Pb(1)–Se(1)
150.0(4)
78.95(6)
80.42(8)
80.42(8)
80.42(8)
78.95(6)
92.22(5)
92.22(5)
Se(1)#1–Pb(1)–Se(1)
Se(1)#1–Pb(1)–Se(1)
shorter than the average C–N bond with a length of analysis confirms that all deposited films have a slight ex-
1.472(5) Å as shown in Table 2.[64] The alkyl-substituted se- cess of selenium (1–3%) relative to lead.
lenourea C–N bond [C9–N2 1.475(8) Å] is significantly
longer than the amide bond [C7–N1 1.327(4) Å] and the
acyl-substituted selenourea C–N bond [C8–N1 1.335(9) Å].
This trend is opposite to that seen for the seleno-palladium
and sulfur analogues.[65–67]
Thermogravemetric Analysis
Mass losses in multiple steps between 152–370 °C were
observed in the thermogravimetric analysis (TGA) (N2 at-
mosphere at 10 °C min–1) of compound 2 (Figure 4a). The
mass of the residue obtained from complex 2 is 38.5%,
which is close to the theoretically predicted value of
37.30%.
PbSe Thin Films
Deposition was carried out between 200 and 500 °C with
Figure 2. XRD pattern of the as-deposited PbSe thin films from
an argon flow rate of 160 sccm for 1 h at atmospheric pres-
sure. The reactor was purged with argon for 10 min at the
required deposition temperature before deposition was car-
ried out in order to avoid any in situ oxidation. The de-
posited grey/black films of PbSe weakly adhere to the sub-
strate. The films deposited were analyzed by using XRD,
which conclusively shows the formation of the normal cubic
form of PbSe (ICDD: 06–3540) at all deposition tempera-
tures (Figure 2). The films were highly textured along the
(200) plane. The films deposited show different morphol-
ogies at different growth temperatures. The PbSe thin films
obtained at 300 °C consist of globular crystallites with a
size of ca. 3 μm (Figure 3a). As the growth temperature was
increased to 400 °C, two morphologies were observed on
the same substrate, which could be a consequence of the
temperature gradient in the heating furnace. At the edges
of the substrate, the film consists of cubic particles (Fig-
ure 3b), which have wirelike structures growing out of their
exposed faces. The insert in Figure 3b shows a HR-SEM
image of such particles. In the middle of the same film, a
network of wirelike structures can be observed (Figure 3c).
At 450 °C, the films have irregular, flakelike structures with
a micron-size diameter, randomly distributed over the sur-
face (Figure 4d). The films obtained at 500 °C comprise dis-
precursor 2 at (a) 300, (b) 350, (c) 400, (d) 450 and (e) 500 °C.
Figure 3. SEM images of PbSe thin films deposited from com-
ordered network of wires (Figure 3e, f). Quantitative EDAX pound 2 at (a) 300, (b,c) 400, (d) 450, (e,f) 500 °C.
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Morphology-Tailored Synthesis of PbSe Nanocrystals and Thin Films
PbSe Nanoparticles
The pXRD pattern of the as-prepared PbSe nanopar-
ticles at 200 °C (Figure 4b) shows a cubic (fcc) lead selenide
structure (ICCD: 06-3540). The characteristic cubic pattern
with (111), (200), (220), (311), (400) and (420) diffraction
peaks is seen. The broadening of the peaks is consistent
with the small size of the PbSe nanoparticles. At 200 °C,
nearly spherical PbSe nanoparticles form. The size of the
nanoparticles calculated by Scherrer’s equation is 11.5 nm,
which is close to the average diameter obtained from the
TEM image, 11.8 nm Ϯ 1.9 (Figure 5a–c). At 250 °C,
the predominate morphology of PbSe is cubic
(18.4 nmϮ3.1 nm), as shown in Figure 5d–f. The EDAX
profile of the as-prepared PbSe nanoparticles shows the
presence of only Pb and Se in an almost 1:1 ratio.
Figure 5. (a–c) spherical PbSe nanoparticles prepared from precur-
sor 2 at 200 °C, (d–f) cubic PbSe nanoparticles prepared at 250 °C
(all in TOP/oleicacid/octadecene).
stants associated with the fit are 10Ϯ1, 3.0Ϯ0.2 and
0.71Ϯ0.01 ns; the relative amplitudes of the constants are
8, 35 and 57%, respectively. The PL decay observed here is
significantly faster than that reported previously for PbSe
nanoparticles;[68] in that case, a largely monoexponential
decay with a 880-ns time constant was observed and attrib-
uted to electron–hole pair recombination. The multiexpon-
ential form and the much-more-rapid decay observed in the
current study suggest that processes other than electron–
hole pair recombination are significant.
Figure 4. (a) Thermogravimetric analysis (TGA) for 2 at a heating
rate of 10 °C/min under a nitrogen flow rate of 100 mL/min. (b)
Typical pXRD of PbSe nanoparticles prepared by thermolysis of 2
at 200 °C.
Figure 6. (a) UV/Vis absorption spectrum of the PbSe nanopar-
ticles prepared from precursor 2. (b) Fluorescence lifetime decay
curves of PbSe nanoparticles prepared from precursor 2.
The electronic spectrum of the PbSe nanoparticles shows
a band edge at 950 nm, 1.3 eV (Figure 6a). There is a con-
siderable blueshift in the absorption spectrum of the as-syn-
thesized PbSe relative to that of the bulk (4.6 μm, 0.27 eV).
A PL life time experiment based on the time-correlated sin-
Conclusions
The ligand N,N-diisobutyl-NЈ-(4-nitrobenzoyl)selenourea
gle-photon-counting (TCSPC) technique was carried out on and its lead complex have been characterized by single-crys-
the as-prepared PbSe nanoparticles synthesized at 200 °C. tal X-ray crystallography. The lead complex 2 deposited
The PL transient obtained is well described by a triexpon- thin films of various morphologies at temperatures between
ential decay; both the experimental data and the fitted triex- 300 and 500 °C. Nanoparticles grown by the thermolysis of
ponential function are shown in Figure 6b. The decay con- the complex in TOP (trioctylphosphane), octadecene or
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J. Akhtar, M. A. Malik, S. K. Stubbs, P. O’Brien, M. Helliwell, D. J. Binks
FULL PAPER
selenourea (1). Yield: 29%. Elemental analysis: calcd. C 50.0, H
6.0, N 10.9%; found C 48.7, H 5.3, N 10.3%. IR: 2986 υ(C–H),
1732 υ(C=O), 1079 υ(C=Se), 1223 υ(C–N) cm–1. 1H NMR (CDCl3,
400 MHz): δ = 0.89 (m, 6 H), 1.10 (m, 6 H), 2.1 (t, 2 H), 2.3 (t, 2
H), 3.4 (d, 2 H), 8.1(d, Ar, 2 H), 8.2 (s, N–H), 8.38 (d, Ar, 2 H)
ppm. MS (ES, negative scan): m/z (%) = 383 (100).
oleic acid at 200 °C gave spherical particles, whilst the
growth at 250 °C resulted in cubes. The pXRD of both the
thin films and nanoparticles showed cubic PbSe. The nano-
crystals can easily be dispersed in organic solvents such as
toluene or hexane.
Experimental Section
General: All synthetic preparations were performed under an inert
atmosphere of dry nitrogen by using standard Schlenk line tech-
niques. All reagents were purchased from Sigma–Aldrich and used
as received. Solvents were distilled and dried prior to use as neces-
sary. 1H NMR spectra were obtained by using a Bruker AC300
FT-NMR spectrometer. Mass spectra were recorded on a Kratos
concept 1S instrument. Infrared spectra were obtained on a Specac
single-reflectance ATR instrument (4000–400 cm–1). Elemental
analysis was performed by the University of Manchester microana-
lytical facility. TGA was carried out by using a Seiko SSC/S200
thermal analyzer with a heating rate of 10 °Cmin–1 under nitrogen
with flow rate of 100 mL per minute.
Thin films of lead chalcogenide material were deposited by using a
home-built AACVD reactor.[69] X-ray powder diffraction patterns
were obtained by using a Bruker D8 AXE diffractometer (with Cu-
Kα radiation). The samples were scanned between 20 and 80 º (step
size of 0.05 º, with a count rate of 9 s). Thin films were carbon
coated by using an Edwards E-306A coating system before carrying
out SEM and EDX analysis. SEM was performed with a Philips
XL 30FEG and EDX analysis were carried out with a DX4 instru-
ment. TEM samples were prepared by evaporating a dilute toluene
solution of the nanoparticles onto carbon-coated copper grids
(S160–3, Agar Scientific), and a Tchnai transmission electron
microscope (TEM) was used to obtain TEM images of the nano-
particles.
Scheme 1. Synthetic scheme for bis[N,N-diisobutyl-NЈ-(4-nitro-
benzoyl)selenoureato]lead(II).
An ethanol solution of ligand 1 (1 g, 2.6 mmol in 40 mL) with lead
nitrate (0.43 g, 1.3 mmol) in the presence of sodium acetate (0.43 g,
5.2 mmol) in water (10 mL) at room temperature gave a yellow pre-
cipitate of bis[N,N-diisobutyl-NЈ-(4-nitrobenzoyl)selenoureato]-
lead(II) (2). Yield: 54%. Elemental analysis: calcd. C 39.4, H 4.5,
N 8.6, Pb 21.3%; found C 39.7, H 4.6, N 8.74, Pb 19.9%. IR: 3064
A time-correlated single-photon-counting instrument (MiniTau,
Edinburgh Instruments) was used to determine the PL lifetimes. A
diode laser emitting pulses with a 90-ps duration at 405 nm (EPL-
405, Edinburgh Instruments) was used to excite the sample; scat-
tered excitation light was suppressed by using a long-pass filter that
cuts off wavelengths shorter than 600 nm. The PL decay transient
was measured with a commercial time-correlated single-photon-
counting instrument (MiniTau, Edinburgh Instruments). PL was
detected by using a photomultiplier tube (H4722, Hammatsu), and
the instrument response function of the system had a full-width-
half-maximum of better than 300 ps.
υ(C–H), 1645 υ(C=O), 1056 υ(C=Se), 1419 υ(C–N) cm–1 1H NMR
.
(CDCl3, 400 MHz): δ = 0.74 (d, 12 H), 0.87 (d, 12 H), 1.6 (t, 2 H),
2.1 (t, 2 H), 3.5 (d, 4 H), 3.78(d, 4 H), 8.28(m, Ar, 8 H) ppm. MS
(ES– scan): m/z (%) = 384 (100).
X-ray Crystallography: Single-crystal X-ray crystallography of
compounds 1 and 2 was carried out on a Bruker APEX dif-
fractometer by using graphite monochromated Mo-Kα radiation (λ
= 0.71073 Å). The structure was solved by direct methods and re-
fined by full-matrix least-squares on F2.[72] All non-H atoms were
refined anisotropically. H atoms were placed in calculated positions
by assigned isotropic thermal parameters and allowed to ride on
their parent carbon atoms. All calculations were carried out by
using the SHELXTL package.[73] CCDC-685384 (for 1) and
-733567 (for 2) contain the supplementary crystallographic data for
this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Synthesis: The preparation of ligand 1 and its lead complex 2 were
carried out by modification of the methods reported in litera-
ture[70,71] and are detailed in Scheme 1. Briefly, an acetone solution
of 4-nitrobenzoyl chloride (4.5 g, 24 mmol in 30 mL) was added
into an acetone solution of KSeCN (3.5 g, 24 mmol, in 30 mL) at
room temperature in a 250-mL two-neck flask under nitrogen. The
contents of the flask changed colour from white to greenish–yellow.
The mixture was stirred for 5 min at room temperature to ensure
completion of reaction. Addition of diisobutylamine (4.2 mL,
24 mmol in 10 mL acetone) solution into the mixture resulted in a
change of colour from dirty green to orange red. After another
5 min of stirring at room temperature, anhydrous ether (50 mL)
was added, and the flask was left undisturbed for 5 min. The N,N-
diisobutyl-NЈ-(4-nitrobenzoyl)selenourea was extracted into ether
and separated in a funnel. Upon evaporation of the ether solution,
the red solid residue obtained was re-crystallized from ethanol to
give green cubic crystals of N,N-diisobutyl-NЈ-(4-nitrobenzoyl)-
Deposition of Thin Films by AACVD: In a typical deposition experi-
ment, the precursor (0.20 g, 0.8 mmol) was dissolved in toluene
(12 mL) in a two-necked 100-mL round-bottomed flask with a gas
inlet that allows the carrier gas (argon) to pass into the solution
and to aid transport of the aerosol. This flask was connected to
the reactor tube by a piece of reinforced tubing. The argon flow
rate was controlled by a Platon flow gauge. Six glass substrates
(approx. 1ϫ3 cm) were placed inside the reactor tube and placed
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Morphology-Tailored Synthesis of PbSe Nanocrystals and Thin Films
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in a Carbolite furnace. The precursor solution in a round-bottomed
flask was kept in a water bath above the piezoelectric modulator
of a PIFCO ultrasonic humidifier (Model No. 1077). The aerosol
droplets of the precursor thus generated were transferred into the
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PbSe Nanoparticles by the Colloidal Method: TOP (12.7 g) and
oleic acid (1 mL) along with octadecene (4 mL) were placed in
three-neck flask and heated to 100 °C under vacuum for 30 min.
The flask was then flushed with nitrogen and again exposed to a
vacuum for 5 min. This procedure was repeated three times and
heating was continued until 200 °C, after which the temperature
was maintained. Bis[N,N-diisobutyl-NЈ-(4-nitrobenzoylselenoure-
ato)lead(II)] (0.70 g) was then dissolved in TOP (3 mL), and octa-
decene (1 mL) was rapidly injected. The colour of the mixture
changed instantaneously from yellow to black–brown. The hot re-
action flask was removed from the heating mantle within 60 s and
was allowed to cool at room temperature. The addition of acetone
(20 mL) into the flask gave a blackish precipitate, which was sepa-
rated by centrifugation. The obtained material appeared as brown
jelly and was suspended in toluene (5 mL) and re-precipitated by
adding an excess of acetone to wash away any impurities and excess
coating. The purified nanoparticles stayed suspended in toluene
without any coagulation for several weeks. The same reaction was
repeated at higher temperature of 250 °C.
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Acknowledgments
Javeed Akhtar thanks the Higher Education Commission (HEC)
of Pakistan for financial assistance, and we all thank the EPSRC
for funding. P. O. B. wrote this manuscript while a visiting fellow
at the IAS University of Durham. He would like to thank the Uni-
versity for the Fellowship and Collingwood College and its fellows
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Received: February 24, 2011
Published Online: May 24, 2011
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