Electronic and magnetic properties of Ni nanoparticles embedded in various organic semiconductor matrices

Article abstract of DOI:10.1021/jp809777z

Ni nanoparticles with a size distribution from 2 to 6 nm, embedded in various organic matrices, were fabricated in ultrahigh vacuum. For this purpose metal free and Ni phthalocyanine, fullerene C₆₀, and pentacene were coevaporated with Ni. When

Full text of DOI:10.1021/jp809777z

J. Phys. Chem. B 2009, 113, 4565–4570
4565
Electronic and Magnetic Properties of Ni Nanoparticles Embedded in Various Organic
Semiconductor Matrices
,
†
‡
‡
‡
†
†
Bj o¨ rn Br a¨ uer,* Yana Vaynzof, Wei Zhao, Antoine Kahn, Wen Li, Dietrich R. T. Zahn,
§
§
†
C e´ sar de Juli a´ n Fern a´ ndez, Claudio Sangregorio, and Georgeta Salvan
Physics Department, Chemnitz UniVersity of Technology, 09107 Chemnitz, Germany, Department of Electrical
Engineering, Princeton UniVersity, Princeton, New Jersey 08544, and Department of Chemistry, UniVersity of
Florence and INSTM, 50019, Florence, Italy
ReceiVed: NoVember 5, 2008; ReVised Manuscript ReceiVed: January 24, 2009
Ni nanoparticles with a size distribution from 2 to 6 nm, embedded in various organic matrices, were fabricated
in ultrahigh vacuum. For this purpose metal free and Ni phthalocyanine, fullerene C60, and pentacene were
coevaporated with Ni. When coevaporated, Ni and H Pc react, leading to the formation of NiPc and Ni
2
nanoparticles. The molecular structure of the matrix was found to have negligible effect on the size of the
nanoparticles but to influence the magnetic anisotropy of the nanoparticles: Ni nanoparticles formed in the
buckyball matrix have a cubic symmetry, while nanoparticles formed in matrices consisting of planar molecules
exhibit a uniaxial symmetry. After exposure to atmosphere, photoelectron spectroscopy investigations
demonstrate the presence of metallic Ni nanoparticles accompanied by Ni oxide and the existence of a charge
transfer from the organic matrix to the particles in all investigated systems. The oxidized Ni nanoparticles
exhibit a larger magnetic anisotropy compared to the freshly prepared particles which show superparamagnetic
properties above 17 K. Moreover, photoelectron spectroscopy was used to probe the oxidation process of the
Ni nanoparticles in different organic matrices. It could thus be shown that a matrix consisting of spherical
molecules like C60 prevent the particles much better from oxidation compared to matrices of flat molecules.
Introduction
Nanosized particles of ferromagnetic metals such as Fe, Co,
and Ni are the object of growing interest because of a range of
interesting physical properties and potential applications such
as catalysts, high density magnetic recording media, ferrofluids,
1-5
and medical diagnostics.
Recently, an enhancement of
quantum efficiency of organic light emitting devices by doping
6
with magnetic nanoparticles was reported. In this context, the
energy level alignment and the presence of interface dipoles
and/or charge transfer at the interface between the matrix and
the particles is an important issue. With decreasing size down
to several nanometers, the particles behave like single magnetic
Figure 1. Molecules codeposited with Ni in this work.
spin-orbit and hyperfine interactions and thus the electron-spin
7
domains, leading to a superparamagnetic state for temperatures
polarization can be preserved on longer distances as compared
8
11-13
above the blocking temperature. In addition to size, the
to the inorganic semiconductor matrices.
Such effects have
11,14
15-17
chemical and structural properties of the interface between the
particles and the surrounding medium play an important role
been investigated in heterojunctions,
granular films,
As in the case of inorganic
and
more complex structures.1
8a-d
5,9
in determining their magnetic properties.
matrices, the chemical interactions between the nanoparticles
and the organic surrounding are essentially determining the
transport and magnetic properties.
A recent approach to fabricate magnetic nanoparticles with
size below 25 nm is based on the coevaporation of an organic
Most of the reported studies on magnetic nanoparticles dealt
with metal or oxide nanoparticles dispersed in other metal or
nonmagnetic oxide materials. Recently, attention was paid to
the use of carbon-based molecules as alternative material for
matrices. These molecules can exhibit a wide range of properties,
be it optical, fluorescence, catalytic, mechanical, or transport
properties, that can be combined to the magnetic properties in
1
9a
molecule, fullerene (C ), and the metal in vacuum. It was
60
suggested that the buckyball C60 molecules limit the grain
growth and reduce the magnetic coupling between the magnetic
particles. It is therefore of great interest to extend the investiga-
tions on the formation of metal particles in organic matrices
using coevaporation by considering molecules which contain
other atomic species besides the carbon atoms, for example,
nitrogen and hydrogen atoms.
The present work focuses on the formation of Ni nanoparticles
in various matrices consisting of small molecules, outlined in
Figure 1. The properties of systems based on phthalocyanine
10
such a way to obtain novel multifunctional materials. These
materials can be particularly interesting for the development of
“molecular” spintronic devices. The benefit in using organic
molecules for matrices stems from the fact that they have small
*
To whom correspondence should be addressed. Email: bb@stanford.edu.
Chemnitz University of Technology.
Princeton University.
†
‡
§
University of Florence and INSTM.
1
0.1021/jp809777z CCC: $40.75  2009 American Chemical Society
Published on Web 03/12/2009

4
566 J. Phys. Chem. B, Vol. 113, No. 14, 2009
Br a¨ uer et al.
Figure 2. (a) Typical phase-contrast bright-field TEM image recorded for a Ni/H
2
Pc film and (b) the statistic size distribution calculated for more
than 100 particles. The inset of panel a shows an electron diffraction pattern of the same sample.
2
matrices, that is, phthalocyanine (H Pc) and nickel phthalocya-
nine (NiPc), are discussed in greater detail, based on the results
of structural, Raman spectroscopy, photoelectron spectroscopy,
and magnetic investigations. The electronic and magnetic
properties of the systems based on the other organic matrices
are then comparatively discussed. Special emphasis will be put
on studying the ability of various molecular matrices to prevent
the Ni nanoparticles from air oxidation.
Experimental Section
The organic source material was thermally evaporated from
a Knudsen cell and Ni was evaporated using an electron beam
evaporator. The deposition rates were around 2 Å/min for the
organic material and 1.5 Å/min for Ni, leading to a ratio of Ni
atoms to the number of organic molecules of about 10:1. The
films were grown to a nominal thickness of 50 nm onto Si(111)
with 2 nm native oxide for the Raman spectroscopy and
magnetic susceptibility studies. Photoelectron spectroscopy
Figure 3. Raman spectra of thin films of (a) NiPc+Ni, (b) NiPc, (c)
Pc. The spectra were normalized to the intensity
of the band at 1553 cm . The band observed at 521.5 cm in the
spectra b and c stems from the optical phonon mode of the Si substrate.
2
H Pc+Ni, and (d) H
2
-
1
-1
Results and Discussion
(
PES) investigations were performed on 7 nm thick films
2
Structural and Electronic Properties. a. H Pc and NiPc
deposited onto highly doped hydrogen passivated n-Si(111). The
substrate was pretreated with a 5% HF solution for 2 min. The
high resolution transmission electron microscopy (HR-TEM)
and electron diffraction investigations of the NiPc + Ni samples
were performed using a Philips CM 20 FEG electron microscope.
Matrices. High resolution transmission electron microscopy
(TEM) combined with electron diffraction studies showed the
formation of Ni nanoparticles in all investigated matrices. The
size distribution is similar within the experimental error for all
the systems: the nanoparticle size ranges between 2 and 6 nm
and is centered around 3-4 nm. Exemplary, Figure 2 shows
the results for the systems obtained by codeposition of Ni and
2
The Raman spectra presented in this work were recorded in
a macro-configuration at room temperature. The 647.1 nm (1.916
+
eV) line of a Kr laser was used for excitation. Its energy lies
H Pc.
in the lower energy tail of the Q-band optical transition of the
used phthalocyanines. The scattered light was collected using
a Dilor XY 800 spectrometer with a multichannel charge
Raman spectroscopy was used to identify the chemical
composition of the mixed films. It should be noted that the
spectrum of the bare phthalocyanine is identical to that of the
powder source materials, confirming that the molecular structure
is preserved upon thermal deposition. The Raman spectra of
-1
coupled device detector. The spectral resolution was ∼4 cm
as determined from the full-width at half-maximum (fwhm) of
the laser line. The magnetic measurements were performed using
a Cryogenic S600 superconducting quantum interference device
the mixed films obtained by codeposition of Ni and H
and of Ni and NiPc (c) are compared in Figure 3 to the spectra
of the bare phthalocyanine films (40 nm) (H Pc (d) and NiPc
b)).
Spectra c and d in Figure 3 show many differences in the
Raman shift and intensity of the bands, indicating that the
molecular structure of the mixed H Pc + Ni films and H Pc
are significantly different. On the other hand, the spectra of NiPc
b), of the NiPc + Ni films (a), and of the H Pc + Ni films (c)
2
Pc (a)
(SQUID) magnetometer. The X-ray photoelectron spectroscopy
2
(
XPS) studies were performed using the Al KR (1486.6 eV)
(
emission line with a total energy resolution of ∼0.9 eV.
Ultraviolet photoemission spectroscopy (UPS) studies were
performed using the He I (21.22 eV) and He II (40.81 eV)
photon lines of a He-discharge lamp. The photoelectrons were
counted with a double-pass cylindrical mirror analyzer. The total
energy resolution of the measurement was 0.15 eV, as deter-
mined from the width of the Fermi edge measured on a clean
poly-crystalline Au(111) substrate. The ionization energy of the
film is defined as the energy difference between the vacuum
level, determined in a standard way from the position of the
onset of photoemission spectrum, and the extrapolated leading
edge of the highest occupied molecular orbital (HOMO).
2
2
(
2
are almost identical, suggesting a similar chemical composition
of the bare NiPc and the two mixed films.
This indicates that during the deposition process a reaction
between the H Pc molecules and Ni takes place leading to the
2
formation of NiPc molecules and Ni aggregates. The reaction
may take place according to the following equation:

Ni Nanoparticles in Organic Semiconductor Matrices
J. Phys. Chem. B, Vol. 113, No. 14, 2009 4567
nNi + H Pc f NiPc + 2H + (n - 1)Ni
2
A similar reaction behavior was observed for Co atoms
reacting with monolayers of porphyrin in ultra high vacuum.1
The insertion of the Ni atom into the porphyrin is possibly
accompanied by an adsorption of H on Ni followed by the
9b
2
formation of H and its desorption. The latter hypothesis could
not be proved so far.
Slight differences are also observable between the spectra of
NiPc + Ni (a) and NiPc (b). Some bands are enhanced in
the spectra of the mixed films, cf. for example, the out-of-plane
-1
deformation vibration δ(CH) at 752 cm . A general increase
in the fwhm of the bands is observed when nanoparticles are
embedded in the system. Two prominent examples are the bands
stemming from the deformation vibration of δ(CNC) of the
-
1
isoindole fragment at 688 cm and the stretching vibration
-1
ν(NiN) at 1309 cm . Such an effect can be caused by
deformations of the NiPc molecular geometry induced by and/
or charge transfer from/to the Ni particles formed in the mixed
films. The latter band is also shifted toward higher frequencies
in the spectra of the mixed films possibly reflecting a charge
transfer from the NiPc to the Ni nanoparticles. A detailed
assignment of the Raman bands for NiPc is given in ref 20.
All the investigated samples were exposed to air for 10 days
Figure 5. Left panel: XPS spectra of the Ni 2p core levels of (a)
codeposited NiPc with Ni, (b) oxidized Ni film, (c) NiPc. Right panel:
XPS spectra of the C 1s core levels of (d) NiPc and (e) codeposited
NiPc with Ni. All films were exposed to air for 10 days.
TABLE 1: XPS Fit Parameters for the Ni 2p Core Levels
Shown in Figures 5 and 6
band
E/eV
rel. area
(
thus named “aged films”) before the PES investigations. From
UPS investigations the work function of the Ni(II) ion in NiPc
and of the oxidized Ni nanoparticles embedded in NiPc was
found to range between 3.8 and 3.9 eV range, cf. Figure 4. These
values are slightly below the value of 4.1 eV characteristic for
the oxidized Ni and well below the work function of pure Ni,
which can vary between 5.0 and 5.4 eV depending on the crystal
face.21
The Ni 2p XPS spectra of the thin films under investigation
are shown in Figure 5a-c. Selected fitting parameters are
outlined in Table 1. For comparison the spectrum of a pure 7
nm Ni film is shown in Figure 5b. The decomposition of the
Ni 2p core level emission assumes a spin-orbit splitting (2p1/2
and 2p3/2) of 17.5 eV and a statistical branching ratio of 1:2 for
p-levels. The Ni 2p3/2 spectra were fitted in order to determine
(
a) NiPc + Ni
Ni
853.4
856.4
856.8
862.0
865.0
1
2
+
Ni in NiPc
NiO
multiplet
multiplet
0.36
3.27
1.07
0.38
x
(
b) Ni
Ni
NiO
multiplet
853.4
856.8
862.0
1
0.74
0.08
x
(
c) NiPc
Ni2 in NiPc
multiplet
+
856.4
864.7
0.14
(
d) C60 + Ni
Ni
NiO
multiplet
853.5
857.1
862.2
1
1.25
0.41
the ratio of Ni to NiO
x x
where NiO stands for oxygen defective
x
NiO. The bands at 853.4 and 856.8 eV are assigned to Ni and
2+
NiO
eV and in the range from 862 to 865 eV a complex multiplet
structure occurs. The presence of both the Ni and NiO bands
x
, respectively. The band for Ni in NiPc is found at 856.4
(
e) P + Ni
Ni
NiO
multiplet
853.9
856.9
862.3
1
x
x
4.29
2.26
indicates that the nanoparticles have a metallic character and
are most probably surrounded by an oxide shell. From the total
x
area ratio of the bands assigned to Ni and NiO the relative
x
amount of NiO was estimated to almost 75% in the mixed
NiPc + Ni films. Thus most of the Ni is oxidized within a
distance from the surface equal to the information depth of XPS.
The TEM images, however, do not show the presence of
crystalline NiO
x x
, indicating that most of the NiO must be
present in an amorphous state.
The C 1s core level spectra of the thin films under investiga-
tion are shown in Figure 5d,e. Selected fitting parameters are
summarized in Table 2. Since the spectral resolution is limited
to ∼0.9 eV, not all components of the C 1s core level can be
fitted separately, and several atoms with similar chemical shifts
were therefore grouped in the same class. For NiPc, the C 1s
peak was resolved into three components with the following
binding energies: 284.9 eV for the aromatic carbon of the
benzene rings, 286.3 eV for the pyrrole carbon linked to
nitrogen, and 287.9 eV for the shakeup satellites of the pyrrole
Figure 4. Filled states of three Ni containing layers measured by
UPS.
22
carbon in good agreement with the literature. The codeposition

4
568 J. Phys. Chem. B, Vol. 113, No. 14, 2009
Br a¨ uer et al.
TABLE 2: XPS Fit parameters for the C 1s Core Levels
Shown in Figures 5 and 7
E/eV
rel. area
E/eV
rel. area
(a) NiPc
(b) NiPc + Ni
2
2
2
84.9
86.3
87.9
1
0.4
0.1
285.4
286.5
288.8
1
0.4
0.3
(c) C60
(d) C60 + Ni
2
2
85.2
87.7
1
0.2
285.3
288.5
1
0.2
(e) P
(f) P + Ni
2
2
2
84.5
85.4
89.6
1
0.5
0.06
285.4
286.5
289.0
1
0.2
0.2
Figure 6. XPS spectra of the Ni 2p core levels of (a) Ni in C60 matrix
and (b) Ni in pentacene matrix. All films were prepared on n-Si
substrates and exposed to air for 10 days.
of Ni and NiPc induces a shift of the C 1s signal to higher
binding energies. The two main carbon peaks are shifted by
around 0.3-0.5 eV. The satellite peak cannot be resolved since
a new superimposed peak appears at 288.8 eV. Because of its
high intensity the new peak is assumed to arise from partial
oxidation of a certain amount of NiPc molecules. This behavior
can be attributed to a partial charge transfer from the NiPc close
to the nanoparticles to the oxidized Ni nanoparticles in agree-
ment with the Raman spectroscopy results.
b. C60 and Pentacene Matrices. In Figures 6 and 7, the XPS
spectra corresponding to the Ni 2p and C 1s core level of the
thin films consisting of Ni particle in C60 and pentacene matrices
exposed to air for 10 days are shown, respectively. Selected
peak fitting parameters are summarized in Tables 1 and 2.
In all cases the presence of metallic Ni is observed,
demonstrating that Ni nanoparticles are formed in all organic
x
matrices used. The ratio of NiO to Ni peaks seems to be
strongly dependent on the molecular structure of the matrix.
Compared to the Ni nanoparticles embedded in NiPc the ratio
Figure 7. XPS spectra of the C 1s core levels of (a) C60, (b) Ni in C60
matrix, (c) P, and (d) Ni in P matrix. All films were prepared on n-Si
substrates and exposed to air for 10 days.
of NiO
and 2. For pentacene, the concentration of NiO
than for the NiPc matrix. The low amount of oxidized Ni in
60 should be due to the spherical structure of C60 that can
x
was found to be smaller for C60 matrices, cf. Table 1
x
is slightly higher
C
enclose the particles more densely than the other molecules
studied.
Compared to Ni in NiPc the shift of the C 1s peak for Ni in
C
60 matrix is much less pronounced. This fact can be explained
considering that the ionization potential of C60 (6.2 eV)23 is
24
higher than that of NiPc (5.0 eV), which prevents an effective
charge transfer from C60 to the Ni. However, the C 1s spectra
show that not all C60 molecules experience the same chemical
environment in the mixed films: a part of the C60 molecules
appears to interact stronger with the Ni nanoparticles leading
to an intense peak at 288.5 eV due to a partial charge transfer
from C60 to the oxidized Ni nanoparticles.
For pentacene the XPS spectrum was fitted using two main
peaks and a shakeup satellite peak. Shifts of approximately 1
eV were observed for the main peaks in the mixed compared
to the pure pentacene film. In addition, a new peak arises at
Figure 8. Hysteresis loops of Ni nanoparticles embedded in organic
matrices measured at 3 K.
the same value. The observed differences could therefore be
related to different percentages of metallic Ni particles in the
systems. Indeed, the ratio of Ni to NiO was found by XPS to
x
decrease in the same sequence as the magnetic moment in Figure
8 does from C60 + Ni, NiPc + Ni, to P + Ni.
2
89.0 eV in the spectrum of the mixed P + Ni film. The
Differences were also observed in the temperature dependence
of the saturation magnetization of the different samples. Figure
9 displays the temperature dependence of the saturation
magnetization (measured in a magnetic field of 5 T), after the
subtraction of the diamagnetic contribution of the substrate,
normalized to the value measured at a temperature of 3 K. Such
a field is already strong enough to saturate the samples within
the whole investigated temperature range and the temperature
dependence of the magnetization of a superparamagnetic
assembly is expected to depend on T3 (Bloch law). In Figure
appearance of the new peak around 289 eV might be due to a
partial charge transfer from some of the organic molecules to
the nanoparticles.
Magnetic Investigations. The magnetic hysteresis loops
recorded at 3 K are shown in Figure 8 for three investigated
systems after subtracting the diamagnetic contribution of the
substrate. The Ni + H Pc films behave similarly to the Ni +
2
NiPc samples. Since the particle size is similar in all samples,
we would expect to observe the magnetization to saturate at
/2

Ni Nanoparticles in Organic Semiconductor Matrices
J. Phys. Chem. B, Vol. 113, No. 14, 2009 4569
and M
r
/M
s
. As the size of the nanoparticles is in all cases smaller
25
than the critical single domain size of Ni, 50 nm, the magnetiza-
tion reversal process can be described as a coherent rotation and
thus the coercive field is proportional to the magnetic anisotropy
7
and inversely proportional to the saturation magnetization. Thus
if all the nanoparticles would have the magnetic anisotropy of the
bulk, all investigated systems should show the same H
to the experimental observation. In an ensemble of randomly
oriented noninteracting nanoparticles M /M also depends on the
nanoparticles size and on the nature of the magnetic anisotropy;
C
, contrary
r
s
2
6
Figure 9. Normalized temperature dependence of the magnetization
at an applied field of 5 T of the NiPc + Ni, C60 + Ni, and P + Ni
samples.
the values lie between 0.8 and 0.9 for cubic anisotropy while for
7
uniaxial anisotropy the value is 0.5. The kind of magnetic
anisotropy also influences the magnetic barrier and consequently
the coercive fields, being larger for uniaxial nanoparticles.
Therefore, we propose that the magnetic anisotropy in the C60
26
+
Ni is more cubic while in the NiPc + Ni and P + Ni it is more
uniaxial.
The different nature of the magnetic anisotropy can originate
from different chemical interaction between the surface of the
nanoparticles and the surrounding material, both oxide and
organic molecules. Moreover, due to the ball-like shape of C60
Ni probably grows in a cubic-like crystalline manner; the other
molecules are planar and their molecular crystals might induce
a uniaxial symmetry of the Ni nanoparticles.
The effect of oxidation on the magnetic properties will be
discussed using the example of the NiPc + Ni system by
comparing experiments performed on a fresh sample and on a
sample exposed to air for 10 days. The hysteresis loops of the
fresh and aged films measured at 3 K, cf. Figure 10, show
coercive fields of ca. 26 and 40 mT, respectively, while the
saturation magnetizations are similar. In the TEM characteriza-
tions, we do not observe the presence of core-shell nanostruc-
tures with crystalline shells, and neither crystalline phases of
Figure 10. Magnetization versus magnetic field curve measured at 3
K for a sample freshly prepared (dashed line) and after exposure to air
for 10 days (continuous line) of NiPc + Ni.
9
, the temperature dependence above 75 K is typical for the
magnetic behavior of metallic Ni or NiO nanoparticles, while
x
below 50 K a significant increase of the magnetization with
decreasing temperature is observed. This increase indicates the
presence of magnetic species like paramagnetic ions or clusters
x x
of NiO with frustrated antiferromagnetic order. NiO nanopar-
the oxide were evidenced. This indicates that the NiO
in the systems is amorphous or weakly crystalline, supporting
the conclusion drawn above. Amorphous NiO , either in form
of NiO nanoparticles or NiO -Ni core-shell structures, can
x
present
ticles can exhibit a ferromagnetic-like behavior due to the
uncompensation of the antiferromagnetic order in the surface
atoms or due to the chemical disorder. The magnetic behavior
at temperatures near room temperature (RT) is mainly dominated
by the contribution from the Ni nanoparticles and the normalized
saturation magnetization is larger with the increase of the metal
to oxide ratio. In Figure 9, the normalized magnetization at RT
decreases in the order C60 + Ni > NiPc + Ni > P + Ni, which
is in good agreement with the sequence of decreasing amount
x
x
x
exhibit uncompensation in the antiferromagnetic order and can
thus make incoherent rotation processes possible,28 giving rise
C
to a strong increase of the H and of the magnetic anisotropy.
The hysteresis loops of the two samples recorded at 3 K after
cooling in a field of 6 T reveal a small displacement of the
cycles toward negative fields, cf. Figure 10. This effect can be
of Ni to NiO
Figure 8 also shows that the coercive fields (H
differ: the NiPc + Ni has the largest H (26 mT), the P + Ni has
around 20 mT, while C60 + Ni has a coercive field of only 7
mT at a temperature of 3 K. We also found that the remanence to
saturation magnetization ratio (M /M ) varies significantly from 0.7
x
ratio, according to the XPS investigations.
C
) of the films
quantified by the exchange field ∆H ) (HC+ - HC-)/2 and is
C
found to be equal to 2 mT for both samples. Such a shift is
C
H
C
characteristic for the presence of oxide-metal exchange cou-
pling at the nanoparticles surface.2
7-29
The small value of ∆H
C
observed here indicates that the coupling between the Ni
nanoparticles and the NiO present in the system is weak.
r
s
for C60 + Ni to 0.4 for P + Ni and down to 0.28 for NiPc + Ni.
The size of the Ni nanoparticles is similar in all investigated systems
x
The magnetization versus temperature behavior of freshly
and thus cannot be responsible for the observed variations in H
C
prepared codeposited NiPc + Ni sample cooled in zero magnetic
Figure 11. Temperature dependence of the magnetization for a ZFC and FC (5 mT) experiment performed on (a) a freshly prepared sample and
b) an air-exposed sample of NiPc + Ni.
(

4
570 J. Phys. Chem. B, Vol. 113, No. 14, 2009
Br a¨ uer et al.
field (ZFC) and in a magnetic field (FC) of 5 mT is shown in
Figure 11a. The observed magnetization behavior is typical for
particles with a size of just a few nanometers. The ZFC curve
exhibits a maximum at a temperature defined as the blocking
has a strong influence on their magnetic properties, leading to
higher blocking temperatures and coercivity.
This work shows that also planar molecules, similar to the
previously used fullerenes, are suitable for fabricating metal
nanoparticles by codeposition in vacuum and opens ways to
control the electronic and magnetic properties of the nanopar-
ticles by a targeted choice of the molecular structure.
temperature (T ) 17 K) above which all nanoparticles are
B
superparamagnetic. At smaller temperatures, thermal irrevers-
ibility between the ZFC and FC curves appears, indicating a
blocked behavior. The sharpness of the ZFC peak, cf. Figure
1
1a, reflects a highly monodisperse nature of the Ni nanopar-
Acknowledgment. B. Br a¨ uer thanks the Fonds der Chemi-
schen Industrie and the Marie Curie program for two Ph.D.
fellowships and the German Research Foundation (DFG) for a
postdoctoral fellowship. Dr. Steffen Schulze is acknowledged
for the HR-TEM and electron diffraction measurements. This
work was supported by the National Science Foundation (DMR-
0705920) and by the Italian FIRB project RBNE033KMA.
30,25
ticles
embedded in NiPc. In Figure 11b, the ZFC and FC
magnetization curves for the aged NiPc + Ni film are shown.
The magnetic behavior deviates significantly from that of the
freshly prepared sample. The blocking temperature shifts to 50
K in the oxidized samples and the ZFC peak becomes broader.
B
T is proportional to the particle volume and to the effective
8
magnetic anisotropy. As the oxidation process is expected to
yield in most cases a decrease in the particle volume, the shift
of T toward higher temperatures and the broadening of the
B
ZFC peak in the aged film must be caused by a strong increase
of the magnetic anisotropy which can be produced by the
ferromagnetic-antiferromagnetic exchange. Moreover, below
References and Notes
(
(
(
1) Schultz, L.; Schitzke, K.; Wecker, J. Appl. Phys. Lett. 1990, 56, 868.
2) Lu, L.; Sui, M. L.; Lu, K. Science 2000, 287, 1463.
3) Ozaki, M. Mater. Res. Bull. 1989, XIV, 35.
(4) Gleiter, H. Nanostruct. Mater. 1992, 1, 1.
(
(
5) Bader, S. ReV. Mod. Phys. 2006, 78, 1.
6) Sun, C.-J.; Wu, Y.; Xu, Z.; Hu, B.; Bai, J.; Wang, J.-P.; Shen, J.
T
1
B
a difference in the two FC magnetization curves in Figure
1a,b is observed, which indicates a change in the magnetic
properties upon aging in air; while in the fresh sample the
magnetization decreases sharply when approaching T , in the
Appl. Phys. Lett. 2007, 90, 232110.
(7) Stoner, E. C.; Wohlfarth, E. P. Philos. Trans. R. Soc. London, Ser.
A 1948, 240, 599.
(
(
8) Bean, C. P.; Livingston, L. D. J. Appl. Phys. 1959, 30, 120S.
9) Bansmann, J.; Baker, S. H.; Binns, C.; Blackman, J. A.; Bucher,
B
aged sample the FC magnetization decrease is less pronounced.
J.-P.; Dorantes-D a´ vila, J.; Dupuis, V.; Favre, L.; Kechrakos, D.; Kleibert,
A.; Meiwes-Broer, K.-H.; Pastor, G. M.; Perez, A.; Toulemonde, O.;
Trohidou, K. N.; Tuaillon, J.; Xie, Y. Surf. Sci. Rep. 2005, 56, 189.
The latter can be indicative of a spin-glasslike behavior, as often
observed in antiferromagnetic metal oxide nanoparticles.31,32
(
10) Hueso, L. E.; Bergenti, I.; Riminucci, A.; Zhan, Y. Q.; Dediu, V.
From the above discussions we deduce that in both cases the
Ni nanoparticles are chemically and magnetically interacting
with NiO
AdV. Mater. 2008, 19, 2639.
(11) Tsukagoshi, K.; Alphenaar, B. W.; Ago, H. Nature 1999, 401, 572.
(
12) Naber, W. J. M.; Faez, S.; van der Wiel, W. G. J. Phys. D: Appl.
x
but the NiPc + Ni sample that was exposed to air
Phys. 2007, 40, R205.
13) Rocha, A. R.; Garc ´ı a-Su a´ rez, V. M.; Bailey, S. W.; Lambert, C. J.;
for 10 days has both H
C
and T larger than the fresh sample,
B
(
indicating an increase of the magnetic anisotropy. Such changes
can be ascribed to the further oxidation of the metal nanopar-
ticles. However stoichiometric changes, chemical disorder, or
structural changes in the NiO magnetically coupled to the Ni
x
nanoparticles cannot be ruled out.
Ferrer, J.; Sanvito, S. Nat. Mater. 2005, 4, 335.
(14) Xiong, Z. H.; Wu, D.; Vardeny, Z. V.; Shi, J. Nature 2004, 427, 821.
(
15) Bernien, M.; Xu, X.; Miguel, J.; Piantek, M.; Eckhold, Ph.; Luo,
J.; Kurde, J.; Kuch, W.; Baberschke, K.; Wende, H.; Srivastava, P. Phys.
ReV. B 2007, 76, 214406.
(16) Wu, Y.; Hu, B.; Li, A.-P.; Shen, J. Phys. ReV. B 2007, 75, 075413.
(
17) Kusai, H.; Miwa, S.; Mizuguchi, M.; Shinjo, T.; Suzuki, Y.;
Shiraishi, M. Chem. Phys. Lett. 2007, 448, 106.
Summary
(18) (a) Bogani, L.; Wernsdorfer, W. Nat. Mater. 2008, 7, 179. (b)
Br a¨ uer, B.; Weigend, F.; Fittipaldi, M.; Gatteschi, D.; Guerri, A.; Ciattini,
S.; Reijerse, E. J.; Salvan, G.; R u¨ ffer, T. Inorg. Chem. 2008, 47, 6633. (c)
Br a¨ uer, B.; Weigend, F.; Totti, F.; Zahn, D. R. T.; R u¨ ffer, T.; Salvan, G. J.
Phys. Chem. B 2008, 112, 5585. (d) Br a¨ uer, B.; Zahn, D. R. T.; R u¨ ffer, T.;
Salvan, G. Chem. Phys. Lett. 2006, 432, 226.
In summary the codeposition of Ni and various molecules is
shown to yield the formation of Ni nanoparticles with a controlled
size distribution between 2 and 6 nm. Since all investigated
molecules have similar size, the molecular structure of the matrix
hardly effects the size distribution of the Ni nanoparticles but has
a significant influence on their oxidation behavior.
(19) (a) Zheng, L. A.; Lairson, B. M.; Barrera, E. V. Appl. Phys. Lett.
2
000, 70, 3110. (b) Gottfried, J. M.; Flechtner, K.; Kretschmann, A.;
Lukasczyk, T.; Steinr u¨ ck, H.-P. J. Am. Chem. Soc. 2006, 128, 5644.
20) Liu, Z.; Zhang, X.; Zhang, Y.; Jiang, J. Spectrochim. Acta, Part A
(
2
Among the investigated molecules, only H Pc was found to
2
007, 67, 1232.
react with Ni when codeposited, leading to the formation of
NiPc and Ni nanoparticles as shown by Raman spectroscopy
and PES. All other molecules experience a charge transfer from
or to the Ni nanoparticles, favored by the presence of the Ni
oxide shell after exposure to atmosphere.
The formation of Ni oxide upon exposure to atmosphere
occurs in all organic matrices but its amount depends strongly
on the molecule structure and their packing in the organic matrix,
as demonstrated by PES studies. C60 was found to prevent the
nanoparticles from oxygen most effectively due to its most
densely packed structure.
In all systems the Ni nanoparticles were shown to behave
superparamagnetically. The molecular nature of the matrix was
found to influence the magnetic anisotropy of the nanoparticles;
while Ni nanoparticles grown in the buckyball matrix have a
cubic symmetry, the nanoparticles formed in matrices consisting
of planar molecules exhibit a uniaxial symmetry. The oxidation
(21) Holzl, J.; Schulze, F. K. Solid Surface Physics. In Tracts in Modern
Physics; Springer: New York, 1979; Vol. 85, p 92.
(22) Petraki, F.; Papaefthimiou, V.; Kennou, S. Org. Electron. 2007, 8, 522.
(23) Cherkashinin, G.; Krischok, S.; Himmerlich, M.; Ambacher, O.;
Schaefer, J. A. J. Phys.: Condens. Matter 2006, 18, 43, 9841.
24) Petraki, F.; Papaefthimiou, V.; Kennou, S. Org. Electron. 2007, 8,
, 522.
(
5
(
25) de Juli a´ n Fern a´ ndez, C. Phys. ReV. B 2005, 72, 054438.
(26) Walker, M. P.; Mayo, P. I.; O’Grady, K.; Charles, S. W.; Chantrell,
R. W. J. Phys.: Condens. Matter 1993, 5, 2779.
(27) Meiklejohn, W. H.; Bean, C. P. Phys. ReV. 1956, 102, 1413.
(28) Nogu e´ s, J.; Sort, J.; Langlais, V.; Skumryev, V.; Suri n˜ ach, S.;
Mu n˜ oz, J. S.; Bar o´ , M. D. Phys. Rep. 2005, 422, 65.
(29) Spasova, M.; Wiedwald, U.; Farle, M.; Radetic, T.; Dahmen, U.;
Hilgendorff, M.; Giersig, M. J. Magn. Magn. Mater. 2004, 272-276, 1508.
(
30) Sappey, R.; Vincent, E.; Hadacek, N.; Chaput, F.; Boilot, J. P.;
Zins, D. Phys. ReV. B 1997, 56, 14551.
(31) Binder, K.; Young, A. P. ReV. Mod. Phys. 1986, 58, 801.
(
32) Makhlouf, S. A.; Parker, F. T.; Spada, F. E.; Berkowitz, A. E.
J. Appl. Phys. 1997, 81, 5561.
JP809777Z