The present invention relates to nanoparticle of transition metals,
and more particularly to convenient chemical syntheses of stable, monodisperse
elemental (such as hexagonal close-packed (hcp), face-centered Cubic (fcc), and
a cubic phase of cobalt, alloy (Co/Ni, NiFe, Co/Fe/Ni,) (where relative concentrations
of the elements can vary continuously) and intermetallic (Co3Pt, CoPt,
CoPt3, Fe3Pt, FePt, FePt3
etc. which are distinct
compounds with definite stoichiometries), and overcoated magnetic nanocrystals
(e.g., particles consisting of a concentric shell of material of different chemical
composition produced by a serial process) preferably having sizes substantially
within a range of about 1 to about 20 nm.
Magnetic properties of fine particles are different from those of
bulk samples due to a "finite size" effects.
Specifically, with the finite size effect, as the particle size is
reduced from micrometer to nanometer scale, the coercive forces increase and reach
a maximum at the size where the particles become single-domain.
Potential applications of small magnetic particles include not only
ultra-high density recording, nanoscale electronics, and permanent magnets, but
also their use as novel catalysts, in biomolecule labeling agents and as drug carriers.
An important goal related to each of these potential applications is to make monodisperse
magnetic domains with high durability and corrosion resistance.
A variety of physical and chemical synthetic routes have been attempted
to produce stable, monodisperse zero-valent magnetic nanocrystals. These include
sputtering, metal evaporation, grinding, metal salt reduction, and neutral organometallic
Conventionally, controlling the particle size of nanostructured metal
clusters has been limited only to late transition metals, such as Au, Ag, Pd and
Pt particles. The early transition metal particles prepared according to conventional
methods are either in aggregated powder form or are very air-sensitive, and tend
to agglomerate irreversibly. This is problematic because the air sensitivity generates
safety concerns when large quantities of the materials are present, and results
in degradation over time due to oxidation unless expensive air-free handling procedures
are employed during processing and the final product is hermetically sealed. The
irreversible agglomeration of the particles makes separation processes which could
narrow the size distribution impossible, and prevents the ready formation of smooth
thin films essential in magnetic recording applications. The agglomeration reduces
the chemically-active surface for catalysis, and seriously limits the soluability
esential for biological tagging, separation and drug delivery applications.
Thus, precise control of particle dimensions and making monodisperse
nanocrystals remain important goals in technological applications of nanomaterials.
Ferromagnetic uniaxial Cobalt-based nanomaterials (e.g., many of these materials
are tetragonal crystal structures which like the hcp structure is uniaxial) (e.g.,
such as CoPt inter-mettalics, and Co/Ta/Cr alloy) have been used in high density
recording media, while fcc cobalt-based nanoparticles or Ni/Fe alloy particles
are magnetically soft materials with low anisotropy which is advantageous in the
development of read heads and in magnetic shielding applications. It is noted that
the terms hexagonal close-packed (hcp) and face- centered cubic (fcc) refer to
the specific internal crystal structure of the particles and is important determining
the anisotropy of the magnetic properties. Additionally, these materials are anticipated
to display interesting, giant (e.g., very large) magnetoresistive properties when
organized in extended arrays, and thus are candidates, for example, for magnetoresistive
read head sensors.
Moreover, previously, the reproducible chemical synthesis of magnetic
transition metal nanocrystals uniform to better than about 5% in diameter has been
difficult or impossible. The inability to control nanocrystal size to better than
5% has in turn frustrated any efforts to prepare 2- and 3-dimensional ordered assemblies
of these uniform transition metal and metal alloy nanocrystals. Traditional methods
for the preparation of metal nanocrystals include physical methods such as mechanical
grinding, metal vapor condensation, laser ablation, electric spark erosion, and
chemical methods included solution phase reduction of metal salts, thermal decomposition
of metal carbonyl precursors, and electrochemical plating.
When any of these physical or chemical processes is performed directly
in the presence of a suitable stabilizing agent and a carrier fluid or the metal
particle deposited from the vapor phase into a carrier fluid containing a suitable
stabilizer, a magnetic colloid (e.g., ferrofluid) may result. All of the above-mentioned
techniques have been practiced for-many years and have been unable to refine the
level of control needed for the production of stable magnetic colloids of transition
metals and metal alloys to the levels demonstrated by the present inventors.
Several factors have limited the efficacy of the existing techniques.
First, the technical difficulty involved in the isolation/purification of the magnetic
colloids is high, and in fact only in the last decade have the tolerances for the
performance of materials and devices based on magnetic materials and devices narrowed
to make uniformity in size to better than 5% a distinct advantage. Secondly, the
tremendous growth in magnetic technology in medical and biotechnology industries
has opened many new applications.
Cain J L et al: 'Preparation of Acicular Alpha-Fe Nanoparticles in
Tubular Lechithin Colloids' IEEE Transactions of Mangetics, US, IEEE Inc. New
York, Vol. 32, No. 5, 1 September 1996 (1996-09-01), pages 4490-4492, XP000634047
ISSN: 0018-9464 discloses acicular α-Fe nanoparticles prepared by borohydride
reduction of Fe2+ in tubular lecithin reverse micellular solutions.
It is further disclosed that reduction of low concentration (0.025 or 0.05M FeCl2)
in the presence of a 1200 Oe magnetic field gave mostly spherical particles with
about 20% acicular particles; the acicular particles had lengths ranging from 100
to 600 nm and aspect ratios of at least 6:1. The method of forming nanoparticles
comprises steps : forming a metal precursor solution, introducing a surfactant,
precipitation of nanoparticles and adding a hydrocarbon solvent.
Petit C et al: 'Self-organisation of Magnetic Nanosized Cobalt Particles'
Advanced Materials, Vol. 10, No. 3, 11 February 1998 (1998-02-11), pages 259-261,
XP000732706 Weiheim, DE discloses a method of forming nanoparticles from cobalt
bis(2-ethylhexyl) sulphosuccinate by reduction with sodium tetrahydroborate. It
is reported that the particles are well dispersed and no aggregation occurs. The
coated particles are redispersed in pyridine.
Thus, the conventional techniques have been unable to exercise the
required control in the production of stable magnetic colloids of transition metals
and metal alloys. The poor chemical stability of the conventional metal particle
has limited the reliability of systems in which they are incorporated and has prompted
wide-scale use of the metal oxide nanoparticles in many applications despite the
weaker magnetic properties inherent in the metal oxide particles.
In view of the foregoing and other problems of the conventional methods
and processes, an object of the present invention is to provide an inexpensive
chemical process for preparing stable monodisperse elemental, intermetallic, alloy
and over-coated nanocrystals.
Another object of the present invention is to provide nanocrystalline
materials with precisely controlled size and monodispersity for magnetic recording
applications such as for magnetic storage application (recording media, as well
as read and write heads).
Yet another object of the present invention is to make a ferrofluid.
In a first aspect of the present invention, the present inventors
have developed expensive and very convenient processes for the preparation of
monodisperse magnetic elemental and alloy nanoparticles such that high-quality
magnetic nanocrystals have been achieved.
More specifically, the invention provide a method of forming nanoparticles
according to claim 1.
In a preferred form the invention includes the steps of: forming a
metal salt precursor solution containing surfactant (optimally a nonionic surfactant
(e.g., tertiary organophosphine) and anionic surfactant (e.g., carboxylate) in
a non-reactive solvent, injecting an agent into the solution to reduce the metal
salt in situ producing colloidal metal particles; adding a flocculent to
cause nanoparticles to precipitate out of solution without permanent agglomeration
and separating the by-products of the synthesis which remain in solution; and adding
a hydrocarbon solvent to the precipitate to either redisperse or repeptize the
In an alternative embodiment the invention comprises the steps of:
forming a metal precursor solution of transition metal complex at a first temperature;
forming a surfactant solution which is heated to a temperature higher than the
first temperature; injecting the metal precursor solution to the warmer surfactant
solution, resulting in production of colloidal metal particles; adding a flocculent
to the mixture to cause nanoparticles to precipitate out of solution without permanent
agglomeration; and adding a hydrocarbon solvent to either redisperse or repeptize
Preferably, with the present invention, hexagonal close packed cobalt
particles are synthesized by use of long chain (e.g., C8-C22) dihydric alcohols
(e.g., diol) to reduce cobalt salts (e.g., carboxylate such as acetate) or beta-dikenonates
(e.g., acetylacetonate). Face-centered cobalt nanocrystals are obtained via thermal
decomposition of zero valent cobalt complexes (e.g., cobalt carbonyls and cobalt
organophosphine complexes), for example. Novel cubic phase cobalt nanoparticles
are prepared through a superhydride reduction of cobalt salts.
Further, with the invention, preferably all types of cobalt particles
are stabilized by a combination of long chain caboxilic acid (e.g., C8-C22) and
optimally oleic acid and trialkylphosphine. This stabilization is effective such
that the particles can be handled easily, either in solution phase or as solid
form under air. The particles are easily redispersible in a range of common aprotic
solvents (e.g., ethers, alkanes, arenes, and chlorinated hydrocarbons). Additionally,
size-selective precipitation (e.g., preferably by adding non-solvent (e.g., alcohol)
to the nonaprotic solvent alkane solution of the particles) isolates to monodisperse
nanocrystal fractions from the original distribution. Besides cobalt-based particles,
the present invention also is useful in producing Ni, Cu, Pd, Pt, and Ag nanoparticles.
The invention produces binary intermetallic compounds (Co3Pt, CoPt,
CoPt3, Fe3Pt, FePt, and FePt3) and binary alloys
(e.g., Co/Ni, Ni/Fe, and Co/Fe) and ternary alloys (e.g., Co/Fe/Ni). The invention
also produces over-coated (e.g., such as Co-Ag and Co-Pt) nanostructured particles.
Thus, with the unique and unobvious features of the present invention,
an inexpensive chemical process is provided for preparing stable monodisperse elemental,
intermetallic, alloy, and over-coated magnetic nanocrystals. Further, nanocrystalline
materials are efficiently produced with controlled size and monodispersity for
magnetic recording applications such as for magnetic recording media, read and
write heads, and a ferrofluid is inexpensively produced.
Thus, the present invention provides an improved procedure for preparing
monodisperse magnetic colloids (e.g., ferrofluids) comprised essentially of nanometer-sized
(e.g., substantially within a range of about 1 to about 20 nm) single crystals
(e.g., nanocrystals) of elemental cobalt, nickel, or iron, intermetallic (e.g.,
CoPt and FePt) or alloys (e.g., binary alloys such as Co/Ni, Co/Fe, and Ni/Fe,
and ternary alloys such as Co/Fe/Ni or the like), a colloidal stabilizer, and an
organic carrier fluid.
In the methods of preparing magnetic colloids according to the present
invention, several important innovations substantially improve the uniformity in
nanocrystal size, shape, and crystal structure, as well as improved resistance
of the nanocrystals to chemical degradation (e.g., oxidation).
For example, some of the innovations include: 1) controlling nucleation
phenomena by rapid injection (e.g., for purposes of the present application, rapid
represents the total delivery of the reagents in less than five seconds, and more
optimally between 0.5 and 2 sec) of solution containing at least one of the essential
reagents for reaction into a flask containing a hot solution of all other necessary
reagents which are being vigorously stirred under an inert gas atmosphere (e.g.,
preferably Ar, He or N2); 2) adding a tertiary alkylphosphine or arylphosphine
to mediate the metal particle growth; (3) changing the constitution of the reaction
medium to allow controlled growth at temperatures higher than the standard procedures
to improve crystalline quality of the individual nanocrystals; and 4) employing
size-selective precipitation and centrifugation after the initial stages of the
synthesis to narrow the particle size distribution to less than 10% (and optimally
less than 5%) standard deviation in diameter.
The above-mentioned innovations may be employed individually, or in
combination, to improve control of the composition and performance of the ferrofluid
in addition to providing a medium from which high quality magnetic nanocrystals
can be isolated.
Embodiments of the invention will now be described with reference
to the accompanying drawings, in which:
- Figure 1 is a graph illustrating an X-ray powder analysis of the particles
with different size ranges from diol reduction using a sample prepared by the evaporation
of hexane solution of the particles on a silicon substrate (e.g., (100) Si);
- Figure 2 is a graph illustrating an X-ray powder patten of particles from a
superhydride reduction and shows a pattern similar to the X-ray diffraction of
β- phase of manganese (Mn) cubic phased manganese (e.g., as shown by the lower
two diffraction patterns, the heating of the β-Mn phase causes it to transform
to the known bulk phases; heating at below 400°C resulting in predominantly hcp
while the heating above 400°C produces predominantly fcc);
- Figure 3 is a transmission electron micrograph (TEM) image of 6 nm hcp cobalt
nanocrystals prepared from diol reduction which were prepared by the evaporation
of octane solution of the particles and dried under vacuum at room temperature;
- Figure 4 is a TEM image of monodisperse β-Mn type cobalt particles from
- Figure 5 is an image of 8 nm monodisperse fcc cobalt nanocrystals from decomposition
of dicobalt octacarbonyl and reveals a terrace-structure due to the attractive
forces between the particles (e.g., these attractive forces are a sum of the magnetic
dipolar interaction and van der Waals forces) with the sample being prepared at
room temperature from dodecane solution;
- Figure 6 is a TEM image showing the superlattice which forms due to the magnetic
and van der Waals interaction of fcc cobalt particles, indicating that the particles
tend to form hexagonal close-packed arrays using a sample deposited at 60°C from
- Figure 7 is a TEM image showing the response of the particles with a small
magnetic field applied parallel to the substrate during evaporation of dodecane
at 60°C where a stripe-like superlattice magnetic field pattern is formed;
- Figure 8 is a TEM image of polyvinylpyrrolidone ( PVP)-protected fcc cobalt
particles prepared by evaporation of butanol solution at room temperature;
- Figure 9 is a graph illustrating the size-dependent Zero Field Cooling (ZFC)-Field
Cooling (FC) magnetization versus temperature of hcp cobalt particles;
- Figure 10 is a graph illustrating size- and temperature-dependent hysteresis
loops of hcp cobalt nanocrystals at 5°K;
- Figure 11A illustrates a general chemical synthesis of monodisperse nanocrystals,
and Figure 11B illustrates a graph of the concentration of precursors versus reaction
- Figure 12 is a schematic diagram of an apparatus for performing the synthesis,
according to the present invention and schematically depicts a blow-up of the constituent
nanoparticles (with the nanoparticle schematic showing the critical structure of
a dense inorganic core and a layer of organic passivants on the surface);
- Figure 13 is a schematic diagram of another apparatus for performing the process
according to the present invention, and more specifically the size selective precipitation
of the particles (e.g., the slow dropwise) addition of the flocculent to the colloidal
dispersion causes the dispersion to begin to flocculate and precipitate, the precipitate
being separated by centrifugation);
- Figure 14 is a TEM of the particles output from the apparatus of Figure 13;
- Figure 15 illustrates a flowchart of the steps of the inventive process; and
- Figure 16 illustrates a flowchart of the steps of another method of the inventive
Referring now to the drawings and more particularly to Figures 1-16,
embodiments of the invention are illustrated.
Generally, the present invention is an inexpensive and efficient
process for preparing monodisperse magnetic elemental and alloy nanoparticles such
that high-quality magnetic nanocrystals are formed. As described below, hexagonal
close packed (hcp) cobalt particles, for example, are synthesized by long chain
dihydric alcohol (e.g., diol) reduction of cobalt acetate, thereby to obtain face-centered
cobalt (fcc) nanocrystals, for example, via thermal decomposition of dicobalt
octacarbonyl. Novel cubic cobalt nanoparticles are prepared through a superhydride
reduction of cobalt salts.
With the invention, all types of cobalt particles are stabilized by
a combination of oleic acid and trialkylphosphine, which is effective such that
the particles are handled easily either in solution phase or as solid form under
air. The particles are easily redispersed in aprotic solvent. Additionally, size-selective
precipitation (e.g., preferably by adding non-solvent alcohol to the alkane solution
of the particles) leads to monodisperse nanocrystals. While not being limited thereto,
the present invention also is useful in producing Ni, Cu, Pd, Pt, and Ag nanoparticles,
as described below. The invention produces intermetallics (e.g., CoPt, FePt), binary
alloys (e.g., Co/Ni, CoFe, and Fe/Ni) and ternary alloys (e.g., Co/Fe/Ni), and
over-coated (e.g., such as Co-Ag, Co-Pt, and FeNi-Ag) particles.
Thus, an inexpensive chemical process is provided for preparing prepare
stable monodisperse elemental, intermetallic, over-coated and alloy magnetic nanocrystals.
Further, nanocrystalline materials are efficiently produced with controlled size
and monodispersity for magnetic recording applications such as for disk and head
media, and a ferrofluid is inexpensively produced.
Turning to a first embodiment, trialkylphosphine is chosen as one
stabilizing ligand because it is a well-known ligand to stabilize zero valent
metal due to a σ-donating and π-back bonding characteristics.
In the present invention, a plurality of different phosphines can
be used such as symmetric tertiary phosphines (e.g., tributyl, trioctyl, triphenyl,
etc.) or asymmetric phosphines (e.g., dimethyl octyl phosphine). These phosphines
may be employed singly or if the situation warrants can they be used together.
However, the inventors have found that trialkylphosphines reduce the particle's
growth rate, but do not prevent the particle from growing to undispersable aggregates
(e.g., greater than 20 nm. at temperatures between 100°C and 350°C).
In general according to the invention, the surfactant comprises an
organic stabilizer which is a long chain organic compound that may be expressed
in the form R-X where:
Thus the stabilizers which result are:
- (1) R -a "tail group", which is either a straight or branched hydrocarbon or
flourocarbon chain. R-typically contains 8-22 carbon atoms.
- (2) X -a "head group", which is a moiety (X) which provides specific chemical
attacment to the nanoparticle surface. Active groups could be sulfinate (-SO2OH),
sulfonate (- SOOH), phosphinate (-POOH), phosphonate -OPO(OH)2 , carboxylate,
One specific preferred choice of organic stabilizer material is oleic
acid. Oleic acid is a well-known surfactant in stabilizing colloids and has been
used to protect iron nanoparticles. A relatively long (e.g. oleic acid has an 18
carbon chain which is ∼20 angstroms long; oleic acid is not aliphatic and it
has one double bond) chain of oleic acid presents a significant stearic barrier
to counteract the strong magnetic interaction between the particles. Similar long
chain carboxylic acids, such as erucic acid and linoleic acid, also have been used
in addition (e.g., any long chain organic acid with between 8 and 22 carbon atoms
may be employed singly or in combination) to oleic acid. Oleic acid is typically
preferable because it is easily available inexpensive natural sources (e.g., olive
oil). However, carboxylic acid alone cannot protect the growing Co particles for
The combination of the aforementioned phosphines and organic stabilizers
(e.g. triorganophosphine/acid) offers good control on particle growth and stabilization.
Phenylether or n-octylether are preferably used as the solvent due to their low
cost and high boiling point although di-decyl and di-dodecylether can be employed.
The reaction can be performed at temperatures ranging from 100°C to 360°C depending
on the nanoparticles needed and boiling point of the solvent, and more preferably
at ∼240°C. If the temperature is lower than this temperature range, the particles
do not grow. If the temperature is above this range, the particles grow uncontrolled
with increased production of undesirable by-products.
A polyol process, a commonly known process in the art, involves the
reduction of metal salts by diols. The common procedure involves dissolving the
metal precursors in the neat diol and heating to initiate reduction of metal salts
and produce particles. There is no temporally discrete nucleation step and little
or no size control. Thus, the polyol process has been used to reduce metal salts
including cobalt acetate to metal particles. Ethylene glycol is the most often
used as the reducing agent. The reduction takes hours (e.g., typically hours) to
occur and stabilization of the particles is difficult except for the late transition
metal, such as Ag, Pt and Pd particles which are relatively chemically inert. In
the conventional polyol reduction of cobalt, the final product contains both hcp
and fee phases of cobalt.
Compared with ethylene glycol, long chain diols such as 1,2-octanediol,
1,2-dodecanediol and 1,2-hexadecanediol have higher boiling points (e.g., 200-300°C
as compared to the boiling point for ethylene glycol of 200°C) and, if used at
reflux, the long chain diols can easily and quickly reduce metal salts. Thus, reduction
of cobalt acetate by these diols at 200 - 240°C finishes within 20 minutes. The
most significant improvement provided by using the long chain diols is that these
molecules, when dissolved in the high boiling solvent, allow the particles to remain
dispersed during synthesis. In the conventional method, employing neat diols such
as ethylene glycol or propylene glycol fails in large part because the particles
as they are produced are insoluble in the diol and aggregate immediately. The method
and process according to the present invention avoids this by using an inert solvent
and a long chain diol. Thermal decomposition of dicobalt octacarbonyl is another
known reduction procedure used for fcc cobalt.
A variety of polymers and surfactants have been used conventionally,
to control particle growth. However, oxidation of the particles readily occurs,
and cobalt oxide particles are usually obtained. Super-hydride (LiBHEt3)
has been used to reduce metal halide in tetrahydrofuran in the presence of alkylammonium
bromide (R4NBr) at room temperature to give small particles (<about
with the present invention, reduction at approximately 100°C - 240°C
in high boiling ether (e.g., octylether or phenylether) has been used, and leads
to a well-defined X-ray powder pattern to reveal a new crystal phase of cobalt.
With the invention, stabilization of all three kinds of cobalt nanoparticles is
obtained by the combination of oleic acid and trialkylphosphine. The same principle
also applies to other metal systems, such as Ni, Cu, Pd, and Ag. Co- and Ni-based
alloy nanoparticles are particles which are relatively chemically inert. The final
product also can be easily prepared similarly, as would be known to one ordinarily
skilled in the art in light of the present specification.
Turning now to an exemplary process according to the invention, the
synthesis began with an injection of a reducing agent or an ether solution of
dicobalt octacarbonyl in the presence of long chain carboxylic acid (e.g. C8-C22)
and trialkylphosphine. The introduction of the reagents by injecting preferably
should be a single injection lasting less than 5 seconds for the delivery of the
The reduction or decomposition occurred in a short time (e.g., about
10 minutes), leading to a temporally discrete homogeneous nucleation. The growth
of the particles was finished in less than 30 minutes such that cobalt or other
metals and their alloy particles could be handled without inert atmosphere protection.
As compared to the conventional methods described above which took several hours,
the method of the present invention in growing the nanoparticles is very efficient
(e.g., on the order of much less than the conventional methods).
According to the present invention, size-selective precipitation
was performed by titrating the hexane solution of the particles with non-solvent
ethyl alcohol and providing substantially monodisperse cobalt nanocrystals which
could be easily redispersed in alkane solvent.
The crystal phase of the final product was determined by X-ray powder
diffraction, and selected are electron diffraction. There are only two stable phases
known for elemental cobalt at ambient pressures. The hcp form is stable at temperatures
below 425°C, while the fcc form is the stable structure at higher temperatures.
Figure 1 illustrates an X-ray pattern of hcp cobalt nanocrystals
produced with the above method according to the invention. The peaks that appear
at 2 &thetas; = 49, 52, 55 and 91 degrees correspond to d100 = 0.217
nm (2.17 Å), d002 = 0.202 nm (2.02 Å), d101 = 0.191 nm (1.91
Å), and d110 = 0.125 nm (1-25 Å), respectively, and are matched with
those of the hexagonal close-packed (hcp) cobalt crystal phase. The broadening
of the reflection line widths as the sample size decreases in referred to as a
finite size broadening. A detailed analysis of the line widths is commonly applied
to determination of the nanocrystal size. Figure 1 shows the X-ray diffraction
pattern of a fcc cobalt nanocrystal sample confirming the bulk fcc lattice spacings.
The X-ray pattern of the particles from superhydride reduction do not match either
hcp or fcc cobalt phase, as shown in Figure 2, but display the same symmetry as
the uncommon β phase of Mn metal. The pattern fits well to symmetry of the
β-Mn structure scaled for the difference in the size of the Mn and Cobalt
atoms. However, this structure is not stable at temperatures above 300°C. This
thermal instability can be exploited to provide a simple route to convert the nanocrystals
internal structure to either the fcc or hcp forms, thus changing the magnetic anisotropy.
Heating this novel cubic phase can be changed to hcp cobalt phase at 300°C and
fcc cobalt at 500°C. No distinct peaks corresponding to CoO and CoP phase are detected
from X-ray analysis, and elemental analysis shows phosphorus incorporation is below
Depending on the ratio of stabilizing ligands, different particle
sizes can be prepared with smaller particles favored by a high ratio of the stabilizer/surfactant
to the metal precursor solution. For example, reduction of two equivalent (e.g.,
molar ratio) of the cobalt source in the presence of one equivalent of trialkylphosphine
and one equivalent of oleic acid (e.g., a 1:1 ratio of trialkylphosphine to oleic
acid) leads to particles up to approximately 13 nm, whereas in the presence of
more equivalent of oleic acid and trialkylphosphine (e.g., a greater than 2:1:1
ratio of the metal source to the trialkylphosphine and oleic acid), smaller particles
are obtained. There is no specific lower limit of the particle size but the smallest
cobalt containing species have too few atoms to a have a well-formed internal lattice
and more closely resemble molecular species.
The inset 100 of Figure 1 illustrates a series of X-ray patterns of
hcp cobalt particles thus prepared. Annealing the particles at 300°C in vacuum
results in the loss of stabilizing ligands yielding an insoluble mass of larger
nanocrystal whose diffraction pattern is displayed in waveform f of inset 100.
The particles diffuse and grow at this annealing condition as shown in the X-ray
diffraction patterns (e.g., see inset 100 of Figure 1), and become very air-sensitive.
If the sample is exposed to air, cobalt oxide particles are obtained instantly.
To determine the particle size and size distribution, a drop of octane
or dodecane solution containing ∼500 ppm of the product is dropped onto a carbon-coated
copper grid. Specifically, the drop of the colloid is placed onto a carbon-coated
copper TEM grid. The solution is allowed to slowly evaporate at ambient temperature
and pressure. The grid is finally dried in a vacuum chamber at room-temperature
The TEM (transmission electron micrograph) image of about 6 nm hcp
cobalt nanocrystals is shown in Figure 3. A TEM image of about 8.5 nm, β-Mn-type
cobalt particles is shown in Figure 4.
The nanocrystals in each case are very uniform in size and the particles
are separated from each other by a layer of oleic acid coordinated on the surface.
Decomposition of cobalt carbonyl gives high quality fcc cobalt nanocrystals.
As shown in Figure 5, a terrace-like multilayer is shown of 8 nm
monodisperse fcc cobalt nanocrystals on an amorphous carbon film at room temperature.
This three- dimensional ordering is only possible with monodisperse particle systems
of extremely uniform size and shape. The particles have a tendency to self-assemble
into multilayer terrace superlattice structure due to attractive magnetic and van
der Waals interactions among the particles.
If a grid is prepared at 60°C, the added thermal energy allows the
particles to diffuse to lowest energy lattice sites during evaporation and to
produce a well-defined faceted superlattice. A hexagon pattern of this superlattice
is shown in Figure 6, and is indicative of an inherent hexagonal packing of the
individual nanocrystals in the structure. If the sample for TEM study is deposited
from solution while a magnetic field is applied in the plane of the grid, the cobalt
particles tend to organize along the direction of the field, resulting in stripe-like
superlattices of cobalt particles, as shown in Figure 7.
Self-assembly of nanostructured metal particles on solid surfaces
in ordered structures constitutes a formidable preparative challenge which has
been taken up by the present invention. This challenge is driven by the prospect
of fabricating structurally uniform materials having unique electronic and/or magnetic
properties suitable for a variety of different applications including recording
and reproducing media as well as read and write sensors (e.g., disk, head, etc.
Cobalt particles prepared in accordance with the present invention
have shown a high degree of ordering. The nanocrystals tend to self-organize into
a hexagonal close packing (hcp) structure. The TEM images of the particles shows
that the distance between the centers of the cobalt cores amount to approximately
3.5 nm. The chain length of oleic acid is approximately 2.5 nm. Thus, an approximately
3.5 nm separation represents that the protective mantels on the particle surface
entangle each other. Oleic acid around the particles can be replaced by a variety
of other acids such as 1,2-hexadienoic acid and polybutadiene dicarboxylic acid.
It would be obvious to one ordinarily skilled in the art given this
disclosure and within its purview that nanoengineering spacing of magnetic quantum
dots on a solid surface should be possible simply by varying the length of the
alkyl groups of the carboxylic acid.
In pursuing other possible applications such as using the particles
in a biological system, polyvinylpyrrolidone (PVP) can be chosen as another kind
of stabilizing ligand because it contains an acetylamide group that is a basic
unit in DNA, polypeptide or other biomolecules. Results have shown that PVP can
readily replace oleic acid to form PVP-protected particles. The superlattice formed
with the oleic acid as a ligand now disappears. The particles were well-dispersed
in a typical polymer linkage pattern, as shown in Figure 8, indicating that particles
prepared in accordance with the present invention have great potential in biological
labeling and imaging. Additionally, the ready solubility of the particles is desirable
for magnetic separation of biological products.
Magnetic studies were performed using an MPMS2 Quantum Design super
conducting quantum interference device (SQUID) magnetometer. First, the sample
was dissolved either with pentane or hexane (e.g., using oleic acid as a stabilizing
ligand) or with dichloromethane (e.g., using PVP as a stabilizing ligand) and loaded
into a high-quality quartz tube.
The solvent was evaporated, and the product was dried under vacuum
at room temperature. The temperature dependence of magnetization was measured
in a 10 Oe field between 5 and 300°K according to the zero-field-cooling (ZFC)
/field-cooling (FC) processes. Since the critical sizes for cobalt is of the order
of tens of nanometers, the particles prepared here were a group of magnetic single
domains. In this ultra-fine regime, thermal fluctuations will overcome magnetocrystalline
anisotropy, making the particles magnetization fluctuate along the magnetic easy
axis as superparamagnetism occurs. This is typically shown in the temperature dependent
magnetization of the particles, as shown in the graph of Figure 9. Specifically,
Figure 9 shows magnetization versus temperature of different size cobalt particles.
As shown in Figure 9, the particles with size ranging from about 3
nm to about 10 nm are super-paramagnetic at room temperature. However, the super-paramagnetic
properties are blocked at low temperature. For particles having a size about 9
nm, the blocking temperature occurs at TB= approximately 255°K while
for about 6 nm and about 3 nm particles, their TB locate at approximately
47°K and approximately 15°K, respectively (e.g., see Figure 9), indicating the
size dependent blocking behavior.
The ferromagnetic properties of the particles can be examined via
their hysteresis behavior. The M-H (e.g., magnetization vs. magnetic field strength)
hysteresis loop is recorded at 5°K under a field up to 1.0 T. Size-and temperature-dependent
hysteresis curves are presented in Figure 10.
For approximately 11 nm particles, the coercivity reaches 159.2 kA/m
(2000 Oe), whereas as the particle size decreases from approximately 9 nm to approximately
6 nm and further to approximately 3 nm, their coercivities are reduced from approximately
71.4 kA/m (897 Oe) to approximately 23.2 kA/m (291 Oe) and down to approximately
17.4 kA/m ((219 Oe), respectively. At room temperature, the hysteresis of particles
with sizes less than approximately 9 nm disappears, while particles having a size
of approximately 11 nm still show ferromagnetic behavior with an Hc of approximately
10.7 kA/m (135 Oe) (e.g., see Figure 10). These values correspond well with earlier
experimental results, indicating that above the superparamagnetic limit (e.g.,
around approximately 11 nm for cobalt), the coercive force of the particles drops
sharply with the decreasing particle volume.
It has been predicted that for future ultra-high-density recording
media, uniform particles with an average diameter of approximately 8-10 nm or
less and a high Hc of 2500 Oe will be required. Although elemental cobalt particles
are hardly used as such a media at room temperature due to the super-paramagnetic
limit, the inventive synthetic approach shows ferromagnetic materials that can
be applied to high density recording applications (head, disk, etc. media).
Specifically, the present inventors have been successful in extending
the inventive synthetic method to other metal systems.
For example, monodisperse fcc Ni nanocrystals, and more importantly,
monodisperse, Co-Pt intermetallic particles and alloy nanoparticles such as Co-Ni
and Ni-Fe particles (e.g., up to approximately 20 nm in size), have been made by
the reduction of relative metal acetate, or metal acetylacetonate. Ni-Fe particle
materials are used in giant magnetoresistive heads. Co-Pt alloy particles are known
as particle for "ultra-high density recording media". Making more uniform and
well-isolated Co-Pt alloy particles is an important object of the present invention,
and the inventive method contributes to making monodisperse Co-Pt nanocrystals.
Thus, as described above, the present invention achieves solution
phase, high temperature reduction of metal salts and decomposition of neutral
organometallic precursors which lead to metal nanoparticles. Stabilization of the
particles is reached by combination, for example, of oleic acid and trialkylphosphine.
Monodisperse nanocrystals can be separated by size selective precipitation. The
individual particles are well-isolated from each other by an organic layer. Thus,
intergranular exchange among these particles is greatly reduced.
Figure 11A highlights the inventive phase synthesis technique employing
an injecting of reagents to control the nucleation of nanoparticles.
Figure 11B is a standard representation of the conditions necessary
to produce monodisperse colloids to describe the growth of monodisperse micron
sized sulphur colloids. Its critical feature is the temporally discrete nucleation
event followed by slow growth on the nuclei. By designing a series of specific
chemical procedures which conform to this general reaction outline, the present
inventors have optimized the conditions for the growth of monodisperse magnetic
Figure 12 schematically show the formation of ordered arrays of nanoparticles
by the evaporation of a colloidal dispersion onto a solid substrate.
Figure 13 depicts the basic steps in size selective precipitation
as described below in the Examples.
Figure 14 displays the extreme uniformity of the fcc cobalt particles
which result from the innovations presented. The high magnification insets in the
TEM image of Figure 14 clearly show the hexagonal close-packing of the nanocrystals,
and displays one vacancy in the superlattice which confirms the three-dimensional
structure of the assembly by revealing the positions of the underlying particles.
Figure 15 illustrates a flowchart of the inventive process 150, which
represents one of the methods, and Figure 16 illustrates a second method in the
In Figure 15, the inventive method of forming nanoparticles includes
a first step 1501 of forming a metal precursor solution together with a surfactants
solution. The metal precursor solution is formed from a transition metal. As described
above and below in the Examples, the metal precursor solution may be formed from
a complex or a salt of the transition metal. In forming the metal precursor solution
and surfactants solution together, the metal precursor solution may be injected
into the surfactants solution or vice versa, as described above and below in the
Examples. The surfactant solution may be preformed at a predetermined temperature
(e.g., optimally higher than room temperature). Forming the metal precursor solution
and the surfactant solution may be performed at room temperature or at an elevated
temperature of between approximately 100°C to 300°C and most preferably about
In step 1502, the mixture (e.g., the metal precursor solution and
surfactants solution) is heated to a temperature of between approximately 100°C
to 300°C, and most preferably about 240°C.
In step 1503, a reducing agent is introduced (e.g., rapidly injected)
to the metal precursor solution and surfactants solution.
In step 1504, the mixture is cooled and a flocculent is added, as
described above and below in the Examples, to the colloidal dispersion, to cause
nanoparticles to precipitate out of solution without permanent agglomeration.
In step 1505, the precipitate (e.g., particles) is separated.
In step 1506, a solvent (e.g., an aprotic hydrocarbon solvent) is
added to the precipitate, thereby enabling the precipitate to redissolve (e.g.,
redisperse or repeptize the nanoparticles).
In step 1507, it is determined whether the size distribution is acceptable.
If so, the process ends. If the size distribution is not acceptable, steps 1504-1507
may be repeated as desired to narrow the size distribution.
Figure 16 illustrates a flowchart of another method of the inventive
process of forming nanoparticles which includes a first step 1601 of forming a
metal precursor solution from a transition metal. As described above and below
in the Examples, the metal precursor solution may be formed from a complex or a
salt of the transition metal. Such a step may be performed at room temperature
or at an elevated temperature of between approximately 100°C to 300°C, and most
preferably about 240°C.
In step 1602, a surfactant solution is formed and heated, as described
above and below in the Examples. The surfactant solution may be preformed at a
predetermined temperature (e.g., optimally higher than room temperature).
In step 1603, the metal precursor solution is introduced (e.g., rapidly
injected) to the hot surfactant solution. It is noted that instead of introducing
(e.g., rapidly injecting) the metal precursor solution to the surfactant solution,
the surfactant solution could be introduced (e.g., rapidly injected) to the metal
In step 1604, the mixture (e.g., colloidal dispersion) is cooled,
and a flocculent is added thereto, as described above and below in the Examples,
for separating the precipitate (particles).
In step 1605, the precipitate is separated. Specifically, the particles
(nanoparticles) are precipitated out of solution without permanent agglomeration.
Finally, in step 1606, a solvent (e.g., preferably an aprotic hydrocarbon
solvent) is added to the precipitate to redissolve the same (redisperse or repeptize
In step 1607, it is determined whether the size distribution is acceptable.
If so, the process ends. If the size distribution is not acceptable, steps 1604-1607
may be repeated as desired to narrow the size distribution.
Thus, the method of the present invention offers a unique and unobvious
approach to producing monodisperse transition metal nanostructures, as is illustrated
by the following examples.
Monodisperse 6 nm hcp cobalt nanoparticles were synthesized as follows.
First, cobalt acetate tetrahydrate / oleic acid / PR3 / phenylether
in a ratio of 1 mmol/ 2 mmol / 2 mmol / 10 mL were mixed under a nitrogen atmosphere
in a sealed vessel (e.g., as shown in Figure 11A), and were heated to 240°C over
a period of ∼30 minutes. It is noted that a complete dissolution of the metal
precursor is indicated by the formation of a clear, dark blue solution.
An 80°C phenyl ether solution of 1,2-dodecanediol (2.5 equivalent
of cobalt was injected rapidly (1-2 seconds) though a septum into metal precursor
solution being vigorously stirred (e.g., with either a magnetic stir-bar or a mechanical
device) to initiate the reduction of the metal salts. The color of the solution
changed from dark blue to black over a period of 5 minutes as the blue cobalt salt
species was consumed and the black cobalt metal particles were formed. The black
solution was stirred vigorously at 240°C for a total of approximately 15 minutes,
to complete the growth of the particles and then the reaction mixture was cooled
to room temperature. After the reaction mixture was cooled below 60°C, methanol
was added in a dropwise manner (e.g., as shown in Figure 12) until an air-stable
magnetic black precipitate began to separate from the solution. Then, the precipitate
was separated by centrifugation (e.g., as shown in Figure 12) or filtration after
which the supernatant was discarded, and the black waxy magnetic precipitate was
redispersed in hexane in the presence of approximately 100 to 500 microliters of
oleic acid. Size-selective precipitation was performed by titrating the hexane
solution with a short chain alcohol (e.g., methanol, ethanol, propanol, and/or
isopropanol, but preferably ethanol).
Magnetic transition metal intermetallics (e.g., CoPt, Co3Pt,
etc.) and alloy particles such as, for example, Co/Ni, Ni/Fe or the like, can
be synthesized in an analogous procedure in which a mixture of metal salts are
used to prepare the metal precursor solution. The phase of the intermetallic particles
and the composition of the alloy can be easily adjusted by changing the molar ratio
of starting metal salts. Nickel acetate tetrahydrate, iron acetate, and platinum
acetylacetonate were used as Ni, Fe, and Pt sources, respectively.
Monodisperse 8 nm fcc cobalt nanocrystals were prepared as follows.
First, under a nitrogen atmosphere, oleic acid / PR3 / phenylether in the ratio
of 1 mmol / 1 mmol / 20 mL, respectively, were mixed and heated to 200°C over a
period of ~30 min. A phenyl ether solution dicobalt octacarbonyl (1 mmol) was injected
to the hot mixture as it was undergoing vigorous stirring (e.g., with either a
magnetic stir-bar or a mechanical device). A black solution was formed instantly,
indicating the decomposition of the carbonyl and the formation of cobalt particles.
The solution was stirred vigorously at 200°C for a total of 15 minutes,
and then cooled to room temperature. Dropwise addition of methanol (e.g., as shown
in Figure 12) produced an air-stable magnetic black precipitate. The air-stable
magnetic black precipitate produced was separated by centrifugation or filtration,
and the supernatant was discarded. The black magnetic waxy product was redispersed
in hexane in the presence of approximately 100 to 500 microliters of oleic acid.
Size-selective precipitation was performed by titrating the hexane solution with
In contrast to the procedure of Example 1, the procedure of Example
2 provides access to particles with different crystal phases. Additionally, the
method of Example 2 provides a better route to different crystal structures and
better initial particle size distribution than Example 1, which allows the isolation
of monodisperse particles in fewer recursive steps of size selective precipitation.
The improved initial size distribution is attained because the carbonyl
decomposition route more closely approaches the idealized growth curve for monodisperse
colloids depicted in Figure llB. The kinetics of the carbonyl decomposition are
much faster than the polyol reduction producing a more temporally discrete nucleation
event, and better separation of the nucleation and growth stages of the reaction.
The extreme uniformity of the material produce using the carbonyl decomposition
procedure is evident in the TEM image of Figure 14.
In Figure 14, an ensemble of 8-nm. diameter fcc cobalt particles is
seen organized into a regular three-dimensional array (e.g., also referred to
as a colloidal crystal or nanocrystal superlattice). A statistical analysis of
the particles in the images places a measurement limited standard deviation in
particle size to be less than 5% in diameter. The striking regularity of the assembly
is a clear indication of the uniformity of the constituent particles.
Monodisperse 10 nm cubic phase cobalt nanoparticles were produced
as follows. First, CoCl2(anhydrous) / oleic acid / tributylphosphine
/ n-octylether in a ratio of 1 mmol / 1 mmol / 3 mmol / 20 mL, respectively, were
mixed under a nitrogen atmosphere, and heated to 200°C. LiBHEt3 (superhydride)
(2 mmol) was injected into the hot blue solution under vigorous stirring. A black/brown
solution was formed instantly, indicating the reduction of CoCl2 and
the formation of cobalt particles. The solution was stirred at 200°C for approximately
15 minutes and cooled to below 60°C. The addition of ethanol produced an air-stable
magnetic black precipitate, as shown, for example, in Figure 12. The precipitate
was separated by centrifugation or filtration, and the supernatant was subsequently
discarded. The magnetic waxy product was redispersed in hexane in the presence
of approximately 100-500 micro-liters of oleic acid. Size-selective precipitation
was performed by titrating the hexane solution with ethanol.
The product of Example 3 yields yet another crystal phase of cobalt
particles, thereby allowing more materials choices for various applications. This
material displays a crystal symmetry of the β-phase of manganese. This novel
phase of cobalt can be transformed subsequently into either the hcp or fcc crystal
structures by controlled annealing. Heating the (β-manganese) nanoparticles
at temperatures lower than 400°C converts the material to predominantly hcp particles
(e.g., hcp is the preferred bulk phase below 425°C). If the annealing is performed
at temperatures above 400°C, the nanoparticles produced by the process of Example
3 convert to fcc particles (e.g., the stable bulk phase at temperatures above 425°C).
The diffraction patterns of figure 2B document the structural transitions.
The kinetics of the alkylborohdryide reduction and rate are similar
to the carbonyl decomposition (method of Example 2), but are much faster than
the polyol reduction (e.g., method of Example 1), thereby producing a temporally
discrete nucleation event, and good separation of the nucleation and growth stages
of the reaction. The extreme uniformity of the material produced using the superhydride
reduction procedure is evident in the TEM image of Figure 4.
In Figure 4, an ensemble of 8-nm. diameter cobalt particles is seen
organized in to a regular three-dimensional array (e.g., also referred to as a
colloidal crystal or nanocrystal superlattice). The statistical analysis of the
particle in the images places a measurement- limited standard deviation in particle
size to be less than 5% in diameter. Once again, the striking regularity of the
assembly is a clear indication of the uniformity of the constituent particles.
For Ag-coated cobalt particles, the process was as follows. First,
cobalt particles synthesized as above (e.g., as in any of methods of Examples
1, 2, or 3) were redispersed in phenyl ether under N2. AgNO3
(one equivalent of Ag>NO3) were added to the dispersion. The mixture
was stirred mechanically or magnetically at room temperature for about 10 minutes
to ensure complete dissolution of the reagents. Then, the temperature was slowly
raised to 100°C in a period of 20 minutes with continuous stirring and held at
100°C for 10 minutes.
After being cooled down to room temperature, the mixture was treated
with methanol, which produced an air-stable black-brown precipitate, and the mixture
underwent centrifugation. The supernatant was discarded after centrifugation. The
product was redispersed in hexane in the presence of about 100 - 500 microliters
of oleic acid. Size-selective precipitation was performed by titrating the hexane
solution with ethanol.
A similar procedure can be used to prepare Pd- and Pt-coated Co particles
and other ferromagnetic metal-based (e.g., such as Ni-based), over-coated nanocrystals.
Merely by substituting the AgNO3 for Pd(acetate), Pt(acetate) and nickel
acetate, respectively. The resulting particles each have a surface shell of the
less chemically-active metal which improves the corrosion resistance of the particles
and allows new chemical groups to be bound to the surface.
This ability to change the surface chemistry of the particles facilitates
the attachment of new biologically active groups through a sulphide linkage. The
use of Au and Ag nanoparticles with sulphide-linked biomolecules is well established
and, with the procedure outlined above, all of the existing technology in this
area can be exploited to derivate the surface of the magnetic core-shell structure
for biological tagging and separation applications.
Thus, as is clear from the above description and the Examples, the
present invention provides a method for the chemical synthesis of magnetic transition
metal nanocrystals (colloids) and their assembly into two- and three-dimensional
ordered lattices and a method for the use of the dispersed colloids and ordered
More specifically and as described above, the present invention provides
a method for the chemical synthesis of magnetic transition metal colloids (e.g.,
such as Co, Fe, Ni and alloys thereof such as CoxFe(1-x),
CoxNi(1-x) and FexNi (1-x), wherein
x is within a range of 0 to 1 mole fraction, and CoxFeyNz
where x+y+z=1 (mole fraction) and with a diameter in the range of approximately
1-20 nm in size with a standard deviation in size of 5% in diameter or less. As
described above, the invention employs these unusually uniform dispersed magnetic
nanocrystals with significant benefit in a range of applications where currently
less uniform colloidal or granular magnetic material is employed.
It will be seen that variations of the invention are possible. For
example, there are many useful applications of both dispersed magnetic nanocrystals
and organized thin films of the nanocrystals, and thus the uses described above
should not be construed as limiting the invention in any way.
Specifically, the uniform magnetic colloids can be utilized in the
dispersed state as magnetic ink or in engineering applications including magnetorheological
fluids (e.g., ferrofluids), as electromagnetic tags for interrogation of composite
materials, as remote heat sources when under the influence of electromagnetic radiation
near the ferromagnetic resonance frequency of the individual particles, and as
the active light modulator in a magnetopheretic display.
Several important applications in the life sciences also are envisaged
for the dispersed magnetic nanocrystals as contrast-enhancing agents in magnetic
resonance imaging (MRI), remote heat sources for hyper-thermal destruction of tissue
under the influence of an external electromagnetic field, an externally-triggered
drug delivery vehicle, selective magnetic labels in high gradient magnetic separation
of purification/isolation biomolecules and cellular products, and for use in medical
diagnosis through the selective isolation of biological products which are considered
indicative of the presence of disease or bodily dysfunction.
Organized assemblies of the magnetic nanocrystals have significant
potential as high density magnetic recording media (e.g., tapes, flexible disks,
rigid disks, magnetic smart cards and the like). Close-packed assemblies of these
magnetic nanocrystals are envisioned as the active magnetoresistive medium in two
components of nonvolatile magnetic storage technologies, as the magnetoresistive
medium in magnetic sensors (e.g., read heads) exploiting the modulation spin-dependent
hopping/tunneling between neighboring nanocrystals in the presence of a modulated
external magnetic field or in proximity to a magnetic encoded pattern.
Assemblies of uniform magnetic nanocrystals also are envisioned as
the active elements in the channel of transistors for non-volatile magnetic random
access memory in which the spin-dependent (magnetoresistive) and hysteretic properties
of the nanocrystals are simultaneously exploited. For example, a strong magnetic
field (e.g., produced by write pulses or the like) is generated by on-chip current
flow which is sufficient to orient the magnetic polarization of the nanocrystals
in the channel.