U.S. patent application number 13/777549 was filed with the patent office on 2013-09-05 for densely-packed films of lanthanide oxide nanoparticles via electrophoretic deposition.
This patent application is currently assigned to VANDERBILT UNIVERSITY. The applicant listed for this patent is VANDERBILT UNIVERSITY. Invention is credited to James Dickerson, Sameer V. Mahajan.
Application Number | 20130228462 13/777549 |
Document ID | / |
Family ID | 43854146 |
Filed Date | 2013-09-05 |
United States Patent
Application |
20130228462 |
Kind Code |
A1 |
Dickerson; James ; et
al. |
September 5, 2013 |
DENSELY-PACKED FILMS OF LANTHANIDE OXIDE NANOPARTICLES VIA
ELECTROPHORETIC DEPOSITION
Abstract
A method of forming a film of lanthanide oxide nanoparticles. In
one embodiment, the method includes the steps of: (a) providing a
first substrate with a conducting surface and a second substrate
that is positioned apart from the first substrate, (b) applying a
voltage between the first substrate and the second substrate, (c)
immersing the first and the second substrates in a solution that
comprises a plurality of lanthanide oxide nanoparticles suspended
in a non-polar solvent or apolar solvent for a first duration of
time effective to form a film of lanthanide oxide nanoparticles on
the conducting surface of the first substrate, and (d) after the
immersing step, removing the first substrate from the solution and
exposing the first substrate to air while maintaining the applied
voltage for a second duration of time to dry the film of lanthanide
oxide nanoparticles formed on the conducting surface of the first
substrate.
Inventors: |
Dickerson; James;
(Nashville, TN) ; Mahajan; Sameer V.; (Chandler,
AZ) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
VANDERBILT UNIVERSITY |
Nashville |
TN |
US |
|
|
Assignee: |
VANDERBILT UNIVERSITY
NASHVILLE
TN
|
Family ID: |
43854146 |
Appl. No.: |
13/777549 |
Filed: |
February 26, 2013 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12898159 |
Oct 5, 2010 |
8405138 |
|
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13777549 |
|
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|
12587401 |
Oct 5, 2009 |
|
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|
12898159 |
|
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Current U.S.
Class: |
204/490 |
Current CPC
Class: |
H01L 29/0665 20130101;
H01L 21/0226 20130101; B82Y 30/00 20130101; B82Y 10/00 20130101;
C25D 13/02 20130101; H01L 29/94 20130101; H01L 21/02192 20130101;
H01L 29/66181 20130101; B82Y 40/00 20130101 |
Class at
Publication: |
204/490 |
International
Class: |
C25D 13/02 20060101
C25D013/02 |
Claims
1. A method of forming a film of lanthanide oxide nanoparticles,
comprising the steps of: (a) providing a first substrate with a
conducting surface and a second substrate that is positioned apart
from the first substrate; (b) applying a voltage between the first
substrate and the second substrate; (c) immersing the first
substrate and the second substrate in a solution comprising a
plurality of lanthanide oxide nanoparticles suspended in a
non-polar solvent or apolar solvent for a first duration of time
effective to form a film of lanthanide oxide nanoparticles on the
conducting surface of the first substrate; and (d) after the
immersing step, removing the first substrate from the solution and
exposing the first substrate to air while maintaining the applied
voltage for a second duration of time to dry the film of lanthanide
oxide nanoparticles formed on the conducting surface of the first
substrate.
2. The method of claim 1, wherein the first substrate comprises one
of gold-coated glass, gold-coated silicon, stainless steel (316L),
indium-tin-oxide (ITO)-coated glass, and doped silicon.
3. The method of claim 1, wherein the applied voltage, V, is in the
range of 0 volts<V.ltoreq.1000 volts.
4. The method of claim 1, wherein each of the first duration of
time, T1 and the second duration of time voltage, T2, is in the
range of 0 minutes<T1, T2.ltoreq.30 minutes.
5. The method of claim 2, wherein the first duration of time, T1
and the second duration of time voltage, T2, can be same or
different.
6. The method of claim 1, wherein the solution has a concentration
ranging from about 1.times.10.sup.14 nanoparticles per cubic
centimeter to about 10.times.10.sup.15 nanoparticles per cubic
centimeter.
7. The method of claim 1, wherein the film of lanthanide oxide
nanoparticles formed on the conducting surface of the first
substrate has a thickness ranging from about 50 to about 500
nm.
8. The method of claim 1, wherein the film of lanthanide oxide
nanoparticles formed on the conducting surface of the first
substrate comprises randomly close-packed lanthanide oxide
nanoparticles with a packing density of about 66%.
9. The method of claim 1, wherein the lanthanide oxide
nanoparticles comprise europium oxide (Eu.sub.2O.sub.3)
nanoparticles or gadolinium oxide (Gd.sub.2O.sub.3)
nanoparticles.
10. The method of claim 1, wherein the lanthanide oxide
nanoparticles have a core diameter ranging from about 2 to about 3
nm.
11. The method of claim 1, wherein the lanthanide oxide
nanoparticles are surface-passivated with oleic acid.
12. The method of claim 1, wherein the non-polar solvent or apolar
solvent comprises at least one of hexane, octane and mixtures
thereof.
Description
CROSS-REFERENCE TO RELATED PATENT APPLICATION
[0001] This application is a divisional application of and claims
benefit of U.S. patent application Ser. No. 12/898,159, filed on
Oct. 5, 2010, which is allowed and which itself is a
continuation-in-part application of U.S. patent application Ser.
No. 12/587,401, filed on Oct. 5, 2009. The disclosures of the above
applications are incorporated herein by reference in their
entireties.
[0002] Some references, which may include patents, patent
applications and various publications, are cited in a reference
list and discussed in the description of this invention. The
citation and/or discussion of such references is provided merely to
clarify the description of the present invention and is not an
admission that any such reference is "prior art" to the invention
described herein. All references listed, cited and/or discussed in
this specification are incorporated herein by reference in their
entireties and to the same extent as if each reference was
individually incorporated by reference. In terms of notation,
hereinafter, bracketed "n" represents the nth reference cited in
the reference list. For example, [71] represents the 71st reference
cited in the reference list, namely, [71] S. V. Mahajan and J. H.
Dickerson, Nanotechnology 18, 325605 (2007).
FIELD OF THE INVENTION
[0003] The present invention relates generally to films of
nanoparticles, in particular, to films of lanthanide oxide
nanoparticles and methods of forming same.
BACKGROUND
[0004] Lanthanide oxides such as europium oxide (Eu.sub.2O.sub.3)
and gadolinium oxide (Gd.sub.2O.sub.3) are known for their light
emitting and high-.kappa. dielectric properties, respectively [76,
58, 77]. The Eu.sup.3+-doped Gd.sub.2O.sub.3, in microcrystalline
form, has been employed in video displays and tri-color fluorescent
lamps as a red phosphor [78]. Recently, nanocrystalline form of
Eu.sup.3+-doped sesquioxides has gained research interest due to
their potential use in luminescent biological tags, efficient light
emitting devices, and high-resolution displays. Gd.sub.2O.sub.3 has
received research attention because of its high-.kappa. dielectric
properties. Gd.sub.2O.sub.3 has been proposed as silicon dioxide
replacement for gate oxide in ultra-small complementary
metal-oxide-semiconductor (CMOS) devices [77]. Most applications of
luminescent and dielectric materials require their implementation
in thin-film form. The Eu.sub.2O.sub.3 and Gd.sub.2O.sub.3
nanocrystals, made via colloidal techniques, need to be assembled
into thin-film form to study their optical and dielectric
properties.
[0005] Therefore, a heretofore unaddressed need exists in the art
to address the aforementioned deficiencies and inadequacies.
SUMMARY OF THE INVENTION
[0006] In one aspect, the present invention relates to a method of
forming a film of lanthanide oxide nanoparticles. In one
embodiment, the method includes the steps of: (a) providing a first
substrate with a conducting surface and a second substrate that is
positioned apart from the first substrate, (b) applying a voltage
between the first substrate and the second substrate, (c) immersing
the first substrate and the second substrate in a solution that
comprises a plurality of lanthanide oxide nanoparticles suspended
in a non-polar solvent or apolar solvent for a first duration of
time effective to form a film of lanthanide oxide nanoparticles on
the conducting surface of the first substrate, and (d) after the
immersing step, removing the first substrate from the solution and
exposing the first substrate to air while maintaining the applied
voltage for a second duration of time to dry the film of lanthanide
oxide nanoparticles formed on the conducting surface of the first
substrate.
[0007] In one embodiment, the first substrate is gold-coated glass,
gold-coated silicon, stainless steel (316L), indium-tin-oxide
(ITO)-coated glass, or doped silicon.
[0008] In one embodiment, the applied voltage, V, is in the range
of 0 volts<V.ltoreq.1000 volts.
[0009] In one embodiment, the non-polar solvent or apolar solvent
includes at least one of hexane, octane and mixtures thereof, and
each of the first duration of time, T1 and the second duration of
time voltage, T2, is in the range of 0 minutes<T1, T2.ltoreq.30
minutes.
[0010] In one embodiment, the first duration of time, T1 and the
second duration of time voltage, T2, can be same or different.
[0011] In one embodiment, the solution has a concentration ranging
from about 1.times.10.sup.14 nanoparticles per cubic centimeter to
about 10.times.10.sup.15 nanoparticles per cubic centimeter.
[0012] In one embodiment, the film of lanthanide oxide
nanoparticles formed on the conducting surface of the first
substrate has a thickness ranging from about 50 to about 500
nm.
[0013] In one embodiment, the film of lanthanide oxide
nanoparticles formed on the conducting surface of the first
substrate has randomly close-packed lanthanide oxide nanoparticles
with a packing density of about 66%.
[0014] In one embodiment, the lanthanide oxide nanoparticles are
europium oxide (Eu.sub.2O.sub.3) nanoparticles or gadolinium oxide
(Gd.sub.2O.sub.3) nanoparticles.
[0015] In yet another embodiment, the lanthanide oxide
nanoparticles have a core diameter ranging from about 2 to about 3
nm.
[0016] In a further embodiment, the lanthanide oxide nanoparticles
are surface-passivated with oleic acid.
[0017] In another aspect, the present invention provides an article
of manufacture having a film of the lanthanide oxide nanoparticles
made by the method set forth immediately above.
[0018] In yet another aspect, the present invention relates to a
metal-oxide-semiconductor (MOS) capacitor. In one embodiment, the
MOS capacitor has: (a) a silicon substrate having a first surface,
(b) a film of lanthanide oxide nanoparticles formed on the first
surface of the silicon substrate using the method set forth
immediately above, and (c) an aluminum film formed on the film of
lanthanide oxide nanoparticles, wherein the film of lanthanide
oxide nanoparticles comprises randomly close-packed lanthanide
oxide nanoparticles with a packing density of about 66%.
[0019] In one embodiment, the silicon substrate has p-type
silicon.
[0020] In one embodiment, the first surface of the silicon
substrate is a p-(100) surface of silicon.
[0021] In one embodiment, the MOS capacitor further has a film of
silicon oxide disposed between the silicon substrate and the film
of lanthanide oxide nanoparticles.
[0022] In one embodiment, the lanthanide oxide nanoparticles have
europium oxide (Eu.sub.2O.sub.3) nanoparticles or gadolinium oxide
(Gd.sub.2O.sub.3) nanoparticles.
[0023] In another embodiment, the film of lanthanide oxide
nanoparticles has a thickness ranging from about 50 to about 500
nm.
[0024] In yet another embodiment, the aluminum film has a thickness
of about 300 nm.
[0025] In a further embodiment, the aluminum film is formed on the
film of lanthanide oxide nanoparticles using electron-beam
evaporation.
[0026] These and other aspects of the present invention will become
apparent from the following description of the preferred embodiment
taken in conjunction with the following drawings and their
captions, although variations and modifications therein may be
affected without departing from the spirit and scope of the novel
concepts of the disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] FIG. 1 shows: (a) TEM image of Eu.sub.2O.sub.3 nanocrystals,
indicating an average diameter of about 2.4 nm; inset: electron
diffraction pattern of the nanocrystals, exhibiting features
arising from the {222} and {440} lattice planes; (b) absorption
spectrum of the Eu.sub.2O.sub.3 nanocrystals, exhibiting strong
absorption in UV region due to a transition from the ground state
to the charge-transfer state of the Eu--O bond and (inset) a weak
absorption peak at 395 nm due to the 4f.fwdarw.4f transition; and
(c) photoluminescence spectrum of the nanocrystals, exhibiting
peaks arising due to .sup.5D.sub.0.fwdarw..sup.7F.sub.J (J=0-4)
transitions.
[0028] FIG. 2 shows: (a) optical micrograph of the Eu.sub.2O.sub.3
film deposited on a gold substrate, which appears golden in colour
because of the background gold substrate and the high transparency
of the film; (b) EDS spectrum of the film deposited on a gold
substrate, which reveals the presence of europium, oxygen and
carbon that originates from the oleic-acid-functionalized
Eu.sub.2O.sub.3 nanocrystals and the gold from the substrate; and
(c) PL spectra of the Eu.sub.2O.sub.3 nanocrystal films deposited
on the anode and cathode. The spectra are identical to the spectrum
of the Eu.sub.2O.sub.3 nanocrystals. PL spectra are shifted
vertically for clarity.
[0029] FIG. 3 shows: (a) SEM image of the nanocrystal film; and (b)
AFM image of the nanocrystal film, which reveals deposition of the
nanocrystal agglomerates of about 15 nm size. RMS roughness of the
film determined from the AFM image is 1.4 nm.
[0030] FIG. 4 shows: (a) optical micrograph of the patterned
silicon substrate recorded through the nanocrystal film deposited
on ITO-coated glass substrate, which reveals high transparency of
the EPD film; and (b) transmission spectrum of a cast film of the
Eu.sub.2O.sub.3 nanocrystals, showing high transparency in the
visible region.
[0031] FIG. 5 shows the thickness of the EPD film as a function of
the applied voltage for different nanocrystal concentrations.
Average film thickness is reported from the thickness measurements
at different locations and standard deviation is employed as the
error bar. The large error bar indicates decreased film
uniformity.
[0032] FIG. 6 shows the electrophoretic mobility measurements of
the EPD suspensions with different nanocrystal concentrations.
[0033] FIGS. 7A-7D show AFM images and FIGS. 7E-7H show EDS
spectrum of the nanocrystal films deposited with the nanocrystal
suspension concentration of 4.times.10.sup.15 NC cm.sup.-3 at the
applied voltages of 250 V, 500 V, 750 V and 1000 V, respectively.
The AFM images of the films reveal the agglomerate size of about
130-160 nm and RMS roughness of about 1.6-1.8 nm. The morphology
and composition of the films were comparable.
[0034] FIG. 8 shows: (a) a schematic of the MOS capacitor
structures with NC film as the gate oxide layer according to one
embodiment of the present invention; (b) AFM image of the NC film
(1 .mu.m.sup.2 area, height: 20 nm/div) with RMS roughness of about
1.6 nm; inset: TEM image of the about 2.4 nm diameter
Gd.sub.2O.sub.3 nanocrystals (Image: 6 nm.times.6 nm); (c) EDS
spectrum of the NC film; and (d) SEM image (top view) of the MOS
capacitor structures.
[0035] FIG. 9 shows C-V characteristics of the MOS capacitors,
fabricated from NC films that were deposited on the anode and
cathode according to one embodiment of the present invention. The
thickness of the film was 116 nm.+-.10 nm, and the average area of
the capacitors was 1.96.times.10.sup.5 .mu.m.sup.2.
[0036] FIGS. 10A-10D shows C-V characteristics of the MOS
capacitors with different thicknesses of the NC films (oxide layer)
according to one embodiment of the present invention.
[0037] FIG. 11 shows a graph of capacitance versus inverse of NC
film (oxide layer) thickness for four different MOS capacitors
according to one embodiment of the present invention. The slope of
the linear regression fit was proportional to the permittivity of
the nanocrystal film and, hence, to the film's dielectric constant,
.kappa.=3.90.
DETAILED DESCRIPTION
[0038] The present invention is more particularly described in the
following examples that are intended as illustrative only since
numerous modifications and variations therein will be apparent to
those skilled in the art. Various embodiments of the invention are
now described in detail. Referring to the drawings, FIGS. 1-11,
like numbers, if any, indicate like components throughout the
views. As used in the description herein and throughout the claims
that follow, the meaning of "a", "an", and "the" includes plural
reference unless the context clearly dictates otherwise. Also, as
used in the description herein and throughout the claims that
follow, the meaning of "in" includes "in" and "on" unless the
context clearly dictates otherwise. Moreover, titles or subtitles
may be used in the specification for the convenience of a reader,
which shall have no influence on the scope of the present
invention. Additionally, some terms used in this specification are
more specifically defined below.
DEFINITIONS
[0039] The terms used in this specification generally have their
ordinary meanings in the art, within the context of the invention,
and in the specific context where each term is used. Certain terms
that are used to describe the invention are discussed below, or
elsewhere in the specification, to provide additional guidance to
the practitioner regarding the description of the invention. For
convenience, certain terms may be highlighted, for example using
italics and/or quotation marks. The use of highlighting has no
influence on the scope and meaning of a term; the scope and meaning
of a term is the same, in the same context, whether or not it is
highlighted. It will be appreciated that same thing can be said in
more than one way. Consequently, alternative language and synonyms
may be used for any one or more of the terms discussed herein, nor
is any special significance to be placed upon whether or not a term
is elaborated or discussed herein. Synonyms for certain terms are
provided. A recital of one or more synonyms does not exclude the
use of other synonyms. The use of examples anywhere in this
specification including examples of any terms discussed herein is
illustrative only, and in no way limits the scope and meaning of
the invention or of any exemplified term. Likewise, the invention
is not limited to various embodiments given in this
specification.
[0040] Unless otherwise defined, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art to which this invention pertains. In the
case of conflict, the present document, including definitions will
control.
[0041] As used herein, "around", "about" or "approximately" shall
generally mean within 20 percent, preferably within 10 percent, and
more preferably within 5 percent of a given value or range.
Numerical quantities given herein are approximate, meaning that the
term "around", "about" or "approximately" can be inferred if not
expressly stated.
[0042] As used herein, if any, the term "atomic force microscope
(AFM)" or scanning force microscope (SFM) refers to a very
high-resolution type of scanning probe microscope, with
demonstrated resolution of fractions of a nanometer, more than 1000
times better than the optical diffraction limit. The term
"microscope" in the name of "AFM" is actually a misnomer because it
implies looking, while in fact the information is gathered or the
action is taken by "feeling" the surface with a mechanical probe.
The AFM in general has a microscale cantilever with a tip portion
(probe) at its end that is used to scan the specimen surface. The
cantilever is typically silicon or silicon nitride with a tip
radius of curvature on the order of nanometers. When the tip is
brought into proximity of a sample surface, forces between the tip
and the sample lead to a deflection of the cantilever according to
Hooke's law. The AFM can be utilized in a variety of
applications.
[0043] As used herein, if any, the term "transmission electron
microscopy (TEM)" refers to a microscopy technique whereby a beam
of electrons is transmitted through an ultra thin specimen,
interacting with the specimen as it passes through it. An image is
formed from the electrons transmitted through the specimen,
magnified and focused by an objective lens and appears on an
imaging screen, a fluorescent screen in most TEMs, plus a monitor,
or on a layer of photographic film, or to be detected by a sensor
such as a CCD camera.
[0044] As used herein, if any, the term "scanning electron
microscope (SEM)" refers to a type of electron microscope that
images the sample surface by scanning it with a high-energy beam of
electrons in a raster scan pattern. The electrons interact with the
atoms that make up the sample producing signals that contain
information about the sample's surface topography, composition and
other properties such as electrical conductivity.
[0045] As used herein, if any, the term "energy dispersive X-ray
spectroscopy (EDS or EDX)" refers to an analytical technique used
for the elemental analysis or chemical characterization of a
sample. It is one of the variants of X-ray fluorescence
spectroscopy which analyzes X-rays emitted by the matter in
response to being hit with charged particles such as electrons or
protons, or a beam of X-rays. Its characterization capabilities are
due in large part to the fundamental principle that each element
has a unique atomic structure allowing X-rays that are
characteristic of an element's atomic structure to be identified
uniquely from one another.
[0046] As used herein, if any, the term "absorption spectroscopy"
refers to spectroscopic techniques that measure the absorption of
radiation, as a function of frequency or wavelength, due to its
interaction with a sample. The sample absorbs energy, i.e.,
photons, from the radiating field. The intensity of the absorption
varies as a function of frequency, and this variation is the
absorption spectrum. Absorption spectroscopy is employed as an
analytical chemistry tool to determine the presence of a particular
substance in a sample and, in many cases, to quantify the amount of
the substance present. Infrared and ultraviolet-visible (UV-Vis)
spectroscopy are particularly common in analytical applications.
The term "infrared spectroscopy" refers to absorption spectroscopy
in the infrared spectral region; and the term "ultraviolet-visible
(UV-Vis) spectroscopy" refers to absorption spectroscopy in the
ultraviolet-visible spectral region.
[0047] As used herein, if any, the term "photoluminescence
spectroscopy" refers to a contactless, nondestructive method of
probing the electronic structure of materials. In this method,
light is directed onto a sample, where it is absorbed and imparts
excess energy into the material in a process called
photo-excitation. One way this excess energy can be dissipated by
the sample is through the emission of light, or luminescence. In
the case of photo-excitation, this luminescence is called
photoluminescence. The intensity and spectral content of this
photoluminescence is a direct measure of various important material
properties.
[0048] As used herein, if any, the term "C-V", as in C-V profiling,
or C-V measurements, or C-V characteristics, stands for
capacitance-voltage, and refers to a technique used for
characterization of semiconductor materials and devices. The
technique uses a metal-semiconductor junction (Schottky barrier) or
a p-n junction or a MOSFET to create a depletion region, a region
which is empty of conducting electrons and holes, but may contain
ionized donors and electrically active defects or traps. The
depletion region with its ionized charges inside behaves like a
capacitor. By varying the voltage applied to the junction it is
possible to vary the depletion width. The dependence of the
depletion width upon the applied voltage provides information on
the semiconductor's internal characteristics, such as its doping
profile and electrically active defect densities. Measurements may
be done at DC, or using both DC and a small-signal AC signal (the
conductance method), or using a large-signal transient voltage.
[0049] As used herein, the term "ITO" or "ITO glass" refers to
indium tin oxide, or tin-doped indium oxide, which is a solid
solution of indium(III) oxide (In.sub.2O.sub.3) and tin(IV) oxide
(SnO.sub.2), typically 90% In.sub.2O.sub.3, 10% SnO.sub.2 by
weight. It is transparent and colorless in thin layers while in
bulk form it is yellowish to grey. In the infrared region of the
spectrum it is a metal-like mirror. Indium tin oxide is one of the
most widely used transparent conducting oxides and so has main
feature of a combination of electrical conductivity and optical
transparency. Thin films of indium tin oxide are most commonly
deposited on surfaces by electron beam evaporation, physical vapor
deposition, or a range of sputter deposition techniques.
[0050] As used herein, "nanoscopic-scale," "nanoscopic,"
"nanometer-scale," "nanoscale," "nanocomposites," "nanoparticles,"
the "nano-" prefix, and the like generally refers to elements or
articles having widths or diameters of less than about 1 .mu.m,
preferably less than about 100 nm in some cases. In all
embodiments, specified widths can be smallest width (i.e. a width
as specified where, at that location, the article can have a larger
width in a different dimension), or largest width (i.e. where, at
that location, the article's width is no wider than as specified,
but can have a length that is greater).
[0051] As used herein, "plurality" means two or more.
[0052] As used herein, the terms "comprising," "including,"
"carrying," "having," "containing," "involving," and the like are
to be understood to be open-ended, i.e., to mean including but not
limited to.
OVERVIEW OF THE INVENTION
[0053] Lanthanide oxides such as europium oxide (Eu.sub.2O.sub.3)
and gadolinium oxide (Gd.sub.2O.sub.3) are known for their light
emitting and high-.kappa. dielectric properties, respectively [76,
58, 77]. The Eu.sup.3+-doped Gd.sub.2O.sub.3, in microcrystalline
form, has been employed in video displays and tri-color fluorescent
lamps as a red phosphor [78]. Recently, nanocrystalline form of
Eu.sup.3+-doped sesquioxides has gained research interest due to
their potential use in luminescent biological tags, efficient light
emitting devices, and high-resolution displays. Gd.sub.2O.sub.3 has
received research attention because of its high-.kappa. dielectric
properties. Gd.sub.2O.sub.3 has been proposed as silicon dioxide
replacement for gate oxide in ultra-small complementary
metal-oxide-semiconductor (CMOS) devices [77]. Most applications of
luminescent and dielectric materials require their implementation
in thin-film form. The Eu.sub.2O.sub.3 and Gd.sub.2O.sub.3
nanocrystals, made via colloidal techniques, need to be assembled
into thin-film form to study their optical and dielectric
properties. Of the thin-film deposition techniques, electrophoretic
deposition (EPD) is the promising technique to deposit
nanomaterials. EPD offers a simple design set-up and provides
substantial thickness control at high deposition rates to assemble
particles site-selectively of any size and shape.
[0054] In one embodiment of the present invention, EPD technique is
employed to deposit ultra-small (<3 nm) colloidal lanthanide
oxide nanoparticles, specifically Eu.sub.2O.sub.3 and
Gd.sub.2O.sub.3 nanocrystals, to form uniform, homogeneous films.
The nanocrystals were synthesized via hot solution phase method and
purified with ethanol prior to deposition [71]. The films were
deposited onto conducting substrates such as gold-coated glass,
gold-coated silicon, stainless steel (316L),
indium-tin-oxide-coated (ITO) glass, and p-type silicon from a
suspension of the nanocrystals in hexane.
[0055] A typical EPD involves the following sequence of the steps:
application of a DC voltage to a pair of electrodes, insertion of
the pair of electrodes into the EPD suspension (area: about 18
mm.times.13 mm), deposition for 15 min, and extraction of the pair
of electrodes from the suspension, and drying in air for 5 min
while maintaining the applied voltage. The films of different
thicknesses were deposited employing different nanocrystal
suspension concentrations (about 1-10.times.10.sup.15 NC/cc) and
different applied voltages (about 250-1000 V).
[0056] Thus, in one aspect, the present invention provides a method
of forming a film of lanthanide oxide nanoparticles. In one
embodiment, the method comprises the steps of: (a) providing a
first substrate with a conducting surface and a second substrate
that is positioned apart from the first substrate, (b) applying a
voltage between the first substrate and the second substrate, (c)
immersing the first substrate and the second substrate in a
solution that comprises a plurality of lanthanide oxide
nanoparticles suspended in a non-polar solvent or apolar solvent
for a first duration of time effective to form a film of lanthanide
oxide nanoparticles on the conducting surface of the first
substrate, and (d) after the immersing step, removing the first
substrate from the solution and exposing the first substrate to air
while maintaining the applied voltage for a second duration of time
to dry the film of lanthanide oxide nanoparticles formed on the
conducting surface of the first substrate.
[0057] In one embodiment, the first substrate is gold-coated glass,
gold-coated silicon, stainless steel (316L), indium-tin-oxide
(ITO)-coated glass, or doped silicon.
[0058] In one embodiment, the applied voltage, V, is in the range
of 0 volts<V.ltoreq.1000 volts.
[0059] In one embodiment, the non-polar solvent or apolar solvent
includes at least one of hexane, octane and mixtures thereof, and
each of the first duration of time, T1 and the second duration of
time voltage, T2, is in the range of 0 minutes<T1, T2.ltoreq.30
minutes.
[0060] In one embodiment, the first duration of time, T1 and the
second duration of time voltage, T2, can be same or different.
[0061] In one embodiment, the solution has a concentration ranging
from about 1.times.10.sup.14 nanoparticles per cubic centimeter to
about 10.times.10.sup.15 nanoparticles per cubic centimeter.
[0062] In one embodiment, the film of lanthanide oxide
nanoparticles formed on the conducting surface of the first
substrate has a thickness ranging from about 50 to about 500
nm.
[0063] In one embodiment, the film of lanthanide oxide
nanoparticles formed on the conducting surface of the first
substrate comprises randomly close-packed lanthanide oxide
nanoparticles with a packing density of about 66%.
[0064] In one embodiment, the lanthanide oxide nanoparticles are
europium oxide (Eu.sub.2O.sub.3) nanoparticles or gadolinium oxide
(Gd.sub.2O.sub.3) nanoparticles.
[0065] In yet another embodiment, the lanthanide oxide
nanoparticles have a core diameter ranging from about 2 to about 3
nm.
[0066] In a further embodiment, the lanthanide oxide nanoparticles
are surface-passivated with oleic acid.
[0067] In another aspect, the present invention provides an article
of manufacture having a film of the lanthanide oxide nanoparticles
made by the method set forth immediately above.
[0068] In yet another aspect, the present invention provides a
metal-oxide-semiconductor (MOS) capacitor. In one embodiment, the
MOS capacitor has: (a) a silicon substrate having a first surface,
(b) a film of lanthanide oxide nanoparticles formed on the first
surface of the silicon substrate using the method set forth
immediately above, and (c) an aluminum film formed on the film of
lanthanide oxide nanoparticles, wherein the film of lanthanide
oxide nanoparticles comprises randomly close-packed lanthanide
oxide nanoparticles with a packing density of about 66%.
[0069] In one embodiment, the silicon substrate has p-type
silicon.
[0070] In one embodiment, the first surface of the silicon
substrate is a p-(100) surface of silicon.
[0071] In one embodiment, the MOS capacitor further has a film of
silicon oxide disposed between the silicon substrate and the film
of lanthanide oxide nanoparticles.
[0072] In one embodiment, the lanthanide oxide nanoparticles
include europium oxide (Eu.sub.2O.sub.3) nanoparticles or
gadolinium oxide (Gd.sub.2O.sub.3) nanoparticles.
[0073] In another embodiment, the film of lanthanide oxide
nanoparticles has a thickness ranging from about 50 to about 500
nm.
[0074] In yet another embodiment, the aluminum film has a thickness
of about 300 nm.
[0075] In a further embodiment, the aluminum film is formed on the
film of lanthanide oxide nanoparticles using electron-beam
evaporation.
[0076] Additional details are set forth below.
EXAMPLES
[0077] Aspects of the present teachings may be further understood
in light of the following examples, which should not be construed
as limiting the scope of the present teachings in any way.
Example 1
Eu.sub.2O.sub.3 Nanocrystal Films
[0078] The controlled assembly of nanomaterials into microscopic
and macroscopic structures is one of the most important and
continuously growing research directions in nanotechnology.
Efficient bottom-up assembly approaches are essential to the
development of next-generation optical, magnetic and electronic
devices that utilize the unique properties of metallic,
semiconducting or insulating nanomaterials. Currently employed
nanomaterial assembly techniques include drop-casting, spin-casting
[1], self-assembly [2-4], Langmuir-Blodgett [5, 6] and
electrophoretic deposition (EPD) [7-9]. For an assembly technique
to be commercially or industrially viable for the fabrication of
nanostructured devices, the technique must involve flexibility of
the type of material (metal/semiconductor/insulator), superior film
thickness control, high rate of assembly and site-selectivity. Of
the aforementioned deposition techniques, electrophoretic
deposition is arguably the most promising for nanomaterial
deposition, as EPD offers a simple design set-up and provides
substantial thickness control at rapid deposition rates to assemble
site-selectively particles of any size, shape and type [10]. EPD
has been employed successfully to deposit films of metallic (Au,
Pt) [11, 12], semiconducting (CdSe, ZnO) [7, 13], insulating
(TiO.sub.2, SiO.sub.2, Eu.sub.2O.sub.3) [14-18] and magnetic
(Fe.sub.3O.sub.4, Fe.sub.2O.sub.3) [9, 19] nanocrystals. Other
types of nanomaterials, such as polymer nanoparticles [20, 21] and
carbon nanotubes (CNTs) [9, 22-27], have been assembled via EPD.
Homogeneous and smooth films of nanocrystals have been reported for
the nanocrystals functionalized with surface capping ligands such
as CdSe, Fe.sub.2O.sub.3, Fe.sub.3O.sub.4 and Eu.sub.2O.sub.3 [7,
9, 16, 28]. Such nanocrystals form stable suspensions in non-polar
solvents such as hexane because of the hydrophobic surface capping
ligands, often used in colloidal nanocrystal syntheses to stabilize
the surface of the nanocrystals [29-31]. Surface-stabilized
Eu.sub.2O.sub.3 nanocrystal films have been of recent interest
because of their strong ultraviolet (UV) absorption and the
characteristic red colour emission of the nanocrystals [31].
Eu.sub.2O.sub.3 nanocrystal films have potential applications in UV
absorption coatings, photoactive coatings and fluorescent screens
[32, 33]. In these applications, the deposition of transparent
films facilitates the efficient transmission of visible light.
Since Eu.sub.2O.sub.3 nanocrystals have weak absorption in the
visible, implementation of small-diameter, light-emitting
nanocrystals should minimize light scattering losses and thereby
enhance the transmission of light throughout the visible
spectrum.
[0079] The relationship among the film thickness, the EPD
suspension parameters (particle mobility) and the EPD process
parameters (applied voltage, deposition time and nanocrystal
concentration) was originally studied by Hamaker [34]. Later, the
effect of these parameters on the deposition of various
micron-sized ceramic particles (Al.sub.2O.sub.3, TiO.sub.2,
SiO.sub.2) from their suspension in water and organic polar
solvents was investigated [14, 15, 23, 35-40]. In contrast,
research exploring the deposition of nanocrystals that are
suspended in non-polar organic solvents (hexane) is relatively
limited [7, 9, 19, 41]. Since the EPD suspension properties are
dependent on the suspension medium, the properties for nanocrystals
suspended in non-polar solvents differ from those of particles
suspended in water and organic polar solvents. Nanocrystals with
surface capping ligands form stable suspension in nonpolar solvent
(e.g. hexane) because of the steric stabilization of the
nanocrystal surface with ligands. Steric repulsive forces,
developed between the nanocrystals by the ligands, overcome the van
der Waals attraction forces to form the stable suspension [42]. The
origin of surface charges in sterically stabilized nanocrystal
suspensions also differs from that of electrostatically stabilized
particles in suspension. In electrostatically stabilized systems,
surface charges develop because of polar solvent molecules and free
ions in the solvent [43, 44]. In contrast, the charges on the
larger nanocrystals may originate from the adsorption of uncharged
ligands, ion exchange between the adsorbed ligand and the surface,
and desorption of the ionized ligands [45-47]. Thermal charging of
nanocrystals in suspension has been another debated origin of
charge [28, 48]. However, our most substantive concern regarding
surface charge arises from charge tuning on the nanocrystals
achieved through the addition of ligands and/or removal of ligands
via purification steps [2]. Since a fraction of the ligands is
detached during each step of nanocrystal purification, the
nanocrystal surface charge can become changed. The effect of the
number of nanocrystal purification steps on the quality of
electrophoretically deposited CdSe nanocrystal films has been
reported, which provides a considerable insight into the
optimization of film quality [41]. Although studies have been
conducted on the effect of EPD process parameters on the quality of
films composed of micron-sized (or larger) electrostatically
stabilized particles [49, 50], no specific report, to date, exists
exploring the effect of EPD process parameters on the quality of
sterically stabilized or electrostatically stabilized
nanocrystalline films.
[0080] In various embodiments of the present invention, certain
transparent films of Eu.sub.2O.sub.3 nanocrystals were fabricated
via electrophoretic deposition and investigated the effect of EPD
processing parameters (applied voltage, deposition time and
nanocrystal concentration) on the uniformity of the films. About
2.4 nm Eu.sub.2O.sub.3 nanocrystals that were capped with oleic
acid and were suspended in a non-polar solvent or apolar solvent
such as hexane were employed. The films were deposited onto two
types of substrates: gold-coated glass and indium-tin-oxide
(ITO)-coated glass. The deposition of the nanocrystals on the anode
and cathode was confirmed by conducting elemental analysis via
energy dispersive spectroscopy (EDS) and optical analysis using
photoluminescence spectroscopy (PL). Scanning electron microscopy
(SEM) and atomic force microscopy (AFM) confirmed the deposition of
homogeneous, topographically smooth films that were composed of
densely packed agglomerates (about 15 nm) of the Eu.sub.2O.sub.3
nanocrystals. Additionally, the effect of EPD processing parameters
on the thickness and uniformity of the transparent films were
explored. The effect that these parameters had on the thickness
homogeneity across the film provided a marked insight into the
growth mechanisms of the films.
Nanocrystal Synthesis and Purification.
[0081] Europium oxide (Eu.sub.2O.sub.3) nanocrystals were
synthesized via a two-stage solution-phase technique, as described
in detail by the inventors elsewhere [31]. In the first stage,
europium oleate was synthesized by heating a mixture of europium
(III) chloride hexahydrate (EuCl.sub.3.6H.sub.2O) and sodium oleate
(CH.sub.3(CH.sub.2).sub.7CH:CH(CH.sub.2).sub.7COONa) at 70.degree.
C. for 4 hours in a hexane-ethanol-water mixture. Next, a mixture
of europium oleate (0.5 mM) and oleic acid (0.25 mM) was heated to
350.degree. C. in tri-n-octylamine (7 ml) and maintained at that
temperature for an hour during the second stage. This synthesis
yielded Eu.sub.2O.sub.3 nanocrystals of 2.4.+-.0.3 nm core
diameter, surface-passivated with oleic acid. The nanocrystals were
isolated from the reaction mixture by a purification process that
involved sequential precipitation and centrifugation sequences. The
addition of ethanol to the reaction mixture facilitated nanocrystal
precipitation; centrifugation helped to isolate the nanocrystals.
The isolated nanocrystals were dispersed back into hexane and the
precipitation-centrifugation sequence was repeated. The
nanocrystals, purified four times (4.times.) by this procedure,
were employed for transmission electron microscopy (TEM) imaging.
The nanocrystals were purified further (10.times.) to deposit the
optimum quality film. The final nanocrystal suspensions for
employment in electrophoretic deposition were prepared in
hexane.
Electrophoretic Deposition (EPD).
[0082] The EPD technique was used to deposit Eu.sub.2O.sub.3
nanocrystals onto two types of substrate: gold-coated glass (gold
electrode) and indium-tin-oxide-coated glass (ITO electrode). Gold
electrodes were fabricated by the thermal evaporation of about 20
nm of chromium, used as an adhesion layer, onto a glass substrate
followed by about 125 nm of gold. ITO electrodes
(surface-resistant: 15-25.OMEGA.) were purchased from Delta
Technologies, Ltd. The electrodes were cut to the size of 25
mm.times.13 mm for EPD. Then, the electrodes were cleaned
sequentially using acetone, ethanol and hexane with an intermediate
drying step using a stream of nitrogen. The electrodes were mounted
in a vertical parallel-plate configuration with a gap of about 5
mm. For a single EPD, a pair of the same type of substrate was
employed as the positive and negative electrodes. A Keithley 6517A
electrometer was utilized to apply DC bias to the electrodes and to
measure current flowing through the suspension during the
deposition. A typical EPD involved the following sequence:
application of a DC voltage, insertion of an electrode pair into
the EPD suspension (deposition area: about 18 mm.times.13 mm),
deposition for 15 minutes, and extraction of the electrode pair
from the suspension and drying in air for 5 minutes while
maintaining the applied voltage.
Characterization.
[0083] The size of the Eu.sub.2O.sub.3 nanocrystals was measured
from the image of the nanocrystals, acquired with a Philips CM 20
transmission electron microscope. The absorption and
photoluminescence spectra of the nanocrystal suspensions were
recorded using a Cary 5000 spectrophotometer and a
Fluorolog-3-FL3-111 spectrophotofluorometer, respectively.
Electrophoretic mobility of the nanocrystals was measured in
suspension (hexane) from dynamic light scattering (DLS)
experiments, performed on a Malvern Nano ZS system. Optical
micrographs of the nanocrystal films were acquired with a Leitz
microscope connected to a CFM-USB-2 camera from Angstrom Sun
Technologies Inc. Surface morphologies of the films were analysed
using a Hitachi S-4200 scanning electron microscope and a Digital
Instruments Nanoscope III atomic force microscope in tapping mode.
Deposition of the nanocrystals on the electrodes was confirmed by
performing elemental analysis of the electrodes using energy
dispersive spectroscopy with a Link ISIS Series 300 microanalysis
system (Oxford Instruments). Film thickness using a Veeco Dektak
150 surface profiler was measured. The transmission and
photoluminescence spectra of the nanocrystal films were acquired
using a Cary 5000 spectrophotometer and a Fluorolog-3-FL3-111
spectrophotofluorometer, respectively.
[0084] Results and implications. FIG. 1 shows a TEM image and
electron diffraction pattern of the Eu.sub.2O.sub.3 nanocrystals,
and absorption and PL spectra of the nanocrystals that were
dispersed in hexane. The average core diameter of the
Eu.sub.2O.sub.3 nanocrystals was d.sub.NC=2.4.+-.0.3 nm, confirmed
with the TEM image (as shown in FIG. 1(a). Shown as an inset to
FIG. 1(a), an electron diffraction pattern of the nanocrystals
revealed features attributable to the {222} and {440} lattice
planes of Eu.sub.2O.sub.3. FIG. 1(b) shows the absorption spectrum
of the nanocrystals. The nanocrystals exhibited strong absorption
in the ultraviolet (UV) region but showed weak absorption in the
visible region. The strong absorption peak at 225 nm was attributed
to the transition between the ground state and the charge-transfer
state of the Eu--O bond (4f.sup.7.fwdarw.4f.sup.72p.sup.-1)
[51-53]. In addition, the weak absorption peak at 395 nm, shown in
the inset to FIG. 1(b), arose from the 4f.fwdarw.4f transition
[51]. FIG. 1(c) shows the photoluminescence spectrum of the
nanocrystals upon UV excitation (254 nm). The peaks corresponded to
the .sup.5D.sub.0.fwdarw..sup.7F.sub.J (J=0-4) radiative energy
transitions within Eu.sup.3+ ions [31]. Of these characteristic
transitions, the .sup.5D.sub.0.fwdarw..sup.7F.sub.2 is the most
sensitive transition to the location of the Eu.sup.3+ ion within
the crystal. The most intense PL peaks (612, 620 and 624 nm) were
observed [31].
[0085] Known for its production of high quality films of colloidal
nanocrystals, EPD was implemented to deposit the Eu.sub.2O.sub.3
nanocrystals from their suspension in hexane [9, 54]. Nanocrystal
films were deposited on gold and ITO electrodes. FIG. 2(a) shows a
typical optical micrograph of a Eu.sub.2O.sub.3 nanocrystal film,
deposited on a gold electrode (anode). The nanocrystal film was
cast with an applied voltage of 250 V and a nanocrystal
concentration of 2.times.10.sup.15 NC cm.sup.-3. The yellowish
colour of the film is due to its high transparency and the colour
of the underlying gold substrate. The film was continuous with no
visible defects larger than 5 .mu.m. The film that was deposited on
the cathode had a comparable appearance. The thickness of the film
was about 110 nm, which was measured using surface profilometry. To
verify deposition of Eu.sub.2O.sub.3 nanocrystals on the
electrodes, EDS was performed for elemental analysis and PL for
optical analysis. FIG. 2(b) shows the EDS spectrum of the
nanocrystal film, deposited on the anode. The presence of europium,
oxygen and carbon peaks confirmed the deposition of the
oleic-acid-functionalized Eu.sub.2O.sub.3 nanocrystals. Also, gold
was detected because of the underlying gold substrate. Similarly,
deposition of the Eu.sub.2O.sub.3 nanocrystals was confirmed on the
cathode. FIG. 2(c) shows the PL spectra of the anode and cathode
upon UV excitation (254 nm). The spectra exhibited all of the peaks
corresponding to the .sup.5D.sub.0.fwdarw.F.sub.J (J=0-4) energy
transitions of the Eu.sup.3+ ion. The spectra were identical to the
spectrum of the Eu.sub.2O.sub.3 nanocrystals (as shown in FIG.
1(c), confirming deposition of the Eu.sub.2O.sub.3 nanocrystals.
Thus, the EDS and PL spectra of cathode and anode confirmed
deposition of the Eu.sub.2O.sub.3 nanocrystal film.
[0086] Surface morphology of the nanocrystal film was probed with
SEM and AFM. FIG. 3(a) shows the SEM image of the nanocrystal film,
deposited on the anode. The nanocrystal film was topographically
smooth and uniform. The film on the cathode had comparable surface
morphology. AFM was employed to perform high-resolution surface
topological analysis of the nanocrystal film. The AFM image, shown
in FIG. 3(b), revealed that the film was composed of agglomerates
of the Eu.sub.2O.sub.3 nanocrystals, approximately 15-20 nm in
diameter. The apparent deposition of agglomerates of the
nanocrystals instead of individual nanocrystals motivated us to
identify the formation of any agglomerates in EPD suspension prior
to the deposition. New TEM samples were prepared for imaging by
drop-casting the EPD suspensions onto the grids. These new samples
confirmed the absence of any agglomerates of the Eu.sub.2O.sub.3
nanocrystals in the EPD suspension. Thus, the agglomeration of the
nanocrystals likely occurred under the influence of the electric
field during EPD. The agglomeration may have occurred at one or
more of the following stages: (a) immediate agglomeration upon
application of the voltage to the electrodes in the EPD suspension;
(b) agglomeration near the electrodes following an increase in the
nanocrystal concentration due to movement of charged nanocrystals
towards the respective electrodes and (c) reorganization of the
deposited nanocrystals at the electrode leading to agglomeration.
Even though the nanocrystals agglomerated under the influence of an
electric field, the extent of agglomeration was limited because of
sufficient ligand coverage on the nanocrystal surface. The
deposited agglomerates packed close to each other, forming a
continuous and densely arranged film (as shown in FIG. 3(b). It was
reported a packing fraction of about 0.63 for random close packing,
also known as glassy packing, of spheres [55]. The observed packing
fraction (about 0.56) by practicing the present invention was
calculated on the basis of the AFM image and was within the lower
regime of glassy packing Nonetheless, these films were particularly
smooth. The root mean square (RMS) surface roughness, determined
from the AFM image of the film, was about 1.4 nm, which was smaller
than the diameter of one nanocrystal. The two plausible reasons for
the high smoothness of the films are: (a) a small fraction of the
nanocrystals was deposited along with the agglomerates of the
nanocrystals and (b) the agglomerates of the nanocrystals had a
degree of structural flexibility since the nanocrystal surface was
partially covered with `soft` surface capping ligands. Thus, SEM
and AFM imaging confirmed the formation of a smooth, uniform and
densely packed film of the agglomerates of the Eu.sub.2O.sub.3
nanocrystals.
[0087] To demonstrate transparency of the Eu.sub.2O.sub.3
nanocrystal film, an optical micrograph of the patterned silicon
substrate was recorded through the nanocrystal film that was
deposited on the ITO electrode (as shown in FIG. 4(a). The
patterned substrate was clearly visible, which confirmed the
formation of highly transparent film. Transmission spectroscopy was
performed on the same film to determine its transmission properties
in the visible region. Intensity oscillations of transmitted light
were seen in the visible transmission spectrum, which were due to
Bragg interference because of the thickness of the film (about 110
nm thick). To measure the transmission of the film, an about 500
.mu.m film was deposited using a drop-cast technique. FIG. 4(b)
shows the transmission spectrum of the drop-cast film, which
reveals high transparency in the visible region (>80%).
Considering the high transparency of the thick film, the
electrophoretically deposited thin film should have a comparable
transmission. High transparency of the film was achieved by
minimizing the scattering loss of visible light within the
nanocrystal film. The intensity of scattered light off a
nanoparticle within the visible region is best expressed by the
Rayleigh scattering equation, which is appropriate within the limit
x=.pi.d.sub.NC/.lamda., x<<1. For the average size (15 nm) of
agglomerates of the Eu.sub.2O.sub.3 nanocrystals, x ranges between
0.12 and 0.06 in the visible spectral region. Since the nanocrystal
films had a transparent appearance instead of hazy (e.g.
translucent), the use of Rayleigh scattering theory instead of Mie
scattering theory is valid. Rayleigh scattering intensity per
particle, I.sub.s, is written as
I S = ( 2 .pi. .lamda. ) 4 ( n 2 - 1 n 2 + 2 ) 2 ( 1 + cos 2
.theta. 2 R 2 ) ( d NC 2 ) 6 I 0 ( 1 ) ##EQU00001##
where .lamda. is the wavelength of the incident light, n is the
refractive index of the particle, .theta. is the scattering angle,
R is the distance to the particle from the point of observation,
d.sub.NC is the particle diameter and I.sub.0 is the intensity of
the incident light. Clearly, the small size of the Eu.sub.2O.sub.3
nanocrystal agglomerates within the film facilitated reduction of
the scattering loss of visible light because the scattering
intensity is proportional to the sixth power of the particle size.
Thus, the small size of the Eu.sub.2O.sub.3 nanocrystal
agglomerates was the key to achieving highly transparent films.
[0088] The ability to control the thickness of the films made
according to the various embodiments of the present invention is
extremely important while maintaining film quality. The Hamaker
equation (2) correlates the amount of particles deposited (deposit
yield, w) during EPD to the electrophoretic mobility (.mu.), the
electric field (E), the electrode area (A), the particle mass
concentration in the suspension (C) and the deposition time
(t):
w=.mu.EACt (2)
The electrophoretic mobility (.mu.) of the particles, suspended in
low-dielectric solvents, is related to the zeta potential of the
particle (.zeta.), the solvent viscosity (.eta.), relative
permittivity of the solvent (.di-elect cons..sub.r) and the
permittivity of vacuum (.di-elect cons..sub.0) of the suspension
through the Huckel equation (3):
.mu. = 2 0 r .zeta. 3 .eta. ( 3 ) ##EQU00002##
[0089] The deposition yield, as stated by the Hamaker equation, is
written as
w = 2 0 r .zeta. EACt 3 .eta. ( 4 ) ##EQU00003##
[0090] Since the solvent (hexane) and electrode set-up (deposition
area, A=18 mm.times.13 mm, 5 mm gap) for EPD were the same, the
parameters (.di-elect cons..sub.r, .eta. and A) remained constant.
The number of purification steps, employed to purify the
nanocrystals, affected the coverage of surface capping ligands on
the nanocrystals. Net charges possessed by the nanocrystals in
solution are related to the coverage of ligands on the nanocrystal
surface (steric stabilization). Hence, the Eu.sub.2O.sub.3
nanocrystals in solution were purified by the same process to
maintain a similar zeta potential of the nanocrystals for all EPDs.
Thus, the deposition yield (and film thickness) can be controlled
via the EPD process parameters such as the electric field (E), the
particle concentration (C) and the deposition time (t). For a
deposition sequence with a constant applied voltage and a fixed
initial concentration, the deposition rate decreases as the
deposition time increases [56]. A decreasing particle concentration
within the EPD suspension and an increasing voltage drop across the
growing film of insulating/semiconducting nanoparticles also
decreases the deposition rate for extended deposition times. It was
reported that the current density and deposition rate of
hydroxyapatite decreased as a function of deposition time [57]. A
decreasing current density through the particle suspension is an
indication of a decreasing deposition rate. It has been observed
that the current density through the Eu.sub.2O.sub.3 nanocrystal
suspension prepared according to various embodiments of the present
invention dropped at least 80% within ten minutes of the beginning
of the deposition run. Since the deposition rate was expected to be
low at times beyond fifteen minutes of deposition time, the
deposition time fixed at 15 minutes was maintained for all EPD
experiments and the applied voltage and the nanocrystal
concentration was varied to monitor the uniformity and thickness of
the films.
[0091] EPD of the Eu.sub.2O.sub.3 nanocrystals was performed at
different applied voltages (250, 500, 750 and 1000 V) and with
different nanocrystal concentrations (1.times.10.sup.15,
2.times.10.sup.15 and 4.times.10.sup.15 NC cm.sup.-3) to understand
their effect on the thickness and uniformity of the nanocrystal
film. Thickness measurements were conducted at five locations on
three different samples, and the average thickness was determined
with the standard deviation of the thicknesses as the error bar.
Hence, the error bar conveys the thickness uniformity of the film.
FIG. 5 shows the graph of the nanocrystal film thickness as a
function of the applied voltage for different nanocrystal
concentrations. The film thickness increased as a function of the
applied voltage and nanocrystal concentration, as expected. When
the applied voltage was increased, more nanocrystals moved toward
the electrodes under the influence of increased electric field and
deposited to form films. Similarly, when the nanocrystal
concentration was increased, more charged nanocrystals were
available for deposition, which led to the formation of thicker
films. By performing electrophoretic mobility measurements on the
EPD suspensions of different nanocrystal concentrations, it was
confirmed that more charged nanocrystals were available for
deposition as the nanocrystal concentration increased. FIG. 6 shows
that the scattering intensity of the particles increased with
nanocrystal concentration for a given electrophoretic mobility.
Subsequently, the thickness of the nanocrystal film increased with
the EPD process parameters (applied voltage and nanocrystal
concentration). During EPD, a constant applied voltage was
maintained, but the nanocrystal concentration of the EPD suspension
decreased with time as the Eu.sub.2O.sub.3 nanocrystal film grew.
The growth of the film slowed as the EPD progressed. The two
factors that slowed down the growth were: (a) the increasing
voltage drop across the growing film of the insulating
Eu.sub.2O.sub.3 nanocrystals and (b) the depletion of charged
nanocrystals from the EPD suspension. Since voltage drop across the
film increased as the film grew, the effective voltage across the
EPD suspension decreased because the applied voltage was constant.
For a given nanocrystal concentration, thicker nanocrystal films
were deposited when higher applied voltages were employed. The
application of higher voltage between the electrodes facilitated an
increased effective voltage across the EPD suspension, resulting in
thicker films. Since thicker films were deposited with higher
applied voltages for a given nanocrystal concentration, the
nanocrystal suspension was not entirely depleted of charged
nanocrystals. Thus, the increasing voltage drop across the growing
Eu.sub.2O.sub.3 nanocrystal film was primarily responsible for
restricting growth of the film.
[0092] EPD process parameters (applied voltage and nanocrystal
concentration) altered the uniformity of the Eu.sub.2O.sub.3
nanocrystal film as illustrated in FIG. 5). For a given nanocrystal
concentration, film uniformity decreased (larger error bar) as the
applied voltage increased. Also, the thickness uniformity decreased
(larger error bar) for higher nanocrystal concentrations for a
given applied voltage. Although the films were increasingly
non-uniform, we observed a particular pattern in thickness
variation. The films were thick towards the edges of the electrode
and were thin in the centre of the electrode, which suggested the
presence of strong fringe electric fields near the edges of the
electrode. Naturally the fringe field increased with applied
voltage; therefore, more nanocrystals deposited near the edges of
the electrode, increasing the non-uniformity of the film. Also, the
non-uniformity of the film increased when the nanocrystal
concentration increased. Since the same EPD setup (deposition area:
18 mm.times.13 mm electrode, 5 mm gap) was employed for all the
depositions, variation in the thickness uniformity of the film was
purely a result of changes in the EPD process parameters. Thus, the
nanocrystal concentration and the applied voltage cannot be
increased indefinitely to increase deposition rate or film
thickness because they affect the uniformity of the EPD film.
[0093] The microscopic morphology and the elemental composition of
the EPD films formed according to various embodiments of the
present invention were analysed as a function of film thickness,
which explained whether the nanocrystals underwent any chemical,
geometrical or topological modifications while being deposited
under the EPD electric field. It was chosen to interrogate films
produced from the highest nanocrystal concentration,
4.times.10.sup.15 NC cm.sup.-3, according to one embodiment of the
present invention, as they yielded the most substantial deviations
from uniformity in the topology, substrate coverage and roughness
in the films when assessed at the macroscopic level. It was
surmised that such film characteristics would yield the largest
changes in microscopic/nanoscale topology, morphology and
compositional changes, if any such change existed. AFM imaging and
EDS analysis were performed on the four films with different
thicknesses, which were deposited at four different applied
voltages formed according to various embodiments of the present
invention. FIGS. 7A-7D show AFM images of the EPD films deposited
at 250 V, 500 V, 750 V and 1000 V, respectively. The films were
composed of agglomerates of the Eu.sub.2O.sub.3 nanocrystals, which
were approximately 130-160 nm in diameter. These agglomerates
formed from 4.times.10.sup.15 NC cm.sup.-3 nanocrystal
concentration were much larger than the observed agglomerates
deposited from the lower 2.times.10.sup.15 NC cm.sup.-3 nanocrystal
concentration (FIG. 3(b)). The agglomerate size was consistent
across individual films and was nearly identical for different
applied voltages (FIGS. 7A-7D). The smoothness of all the films was
comparable. The RMS surface roughness, determined from an analysis
of the AFM images of the films, varied between about 1.6 and 1.8
nm. This roughness was still smaller than the diameter of one
nanocrystal. Thus, the films maintained a smooth topography as a
function of film thickness/applied voltage.
[0094] Additionally, EDS analysis of the films was performed to
juxtapose their compositions. EDS analyses were performed on small,
cleaved sections of the EPD films placed onto silicon substrates
rather than on the original ITO-coated glass substrates. This step
was utilized since the contribution of oxygen signal from the
substrate dominated the oxygen signal from the Eu.sub.2O.sub.3
nanocrystal films. Silicon substrates were chosen because their EDS
peaks do not coincide with the europium and oxygen peaks. FIGS.
7E-7H show the EDS graphs of the EPD films, deposited at 250 V, 500
V, 750 V and 1000 V, respectively. To compare the composition of
the nanocrystals, it was monitored the intensity of the oxygen peak
(K line: 0.52 keV) relative to the intensity of the europium peak
(M line: 5.84 keV). The average ratio of intensities, 2.32.+-.0.13,
was within 5% of all four of the intensity ratios, which confirmed
that the composition of the nanocrystals in the films did not
change as a function of, or because of, the applied voltage. Thus,
these analyses confirmed that the morphology, composition and
topology of the film at the microscopic level remained consistent
as the film thickness increased.
[0095] In sum, in one aspect, transparent films of about 2.4 nm
diameter Eu.sub.2O.sub.3 nanocrystals were deposited successfully
onto conducting substrates from nanocrystal suspensions, prepared
in hexane, using the EPD technique according to various embodiments
of the present invention. The films comprised agglomerates (about
15 nm) of Eu.sub.2O.sub.3 nanocrystals, which likely formed under
the influence of the electric field applied during EPD. The small
size of the agglomerates scattered a small fraction of visible
light, which reduced light scattering losses, and thus enhanced
transparency of the film (>80%) in the visible region. The films
were uniform, smooth and densely packed with a packing fraction of
about 0.56 (glassy packing regime). The films maintained very low
RMS surface roughness (about 1.4 nm). The effect of the EPD process
parameters (applied voltage and nanocrystal concentration) on
growth of these transparent films was evaluated. The thickness of
the nanocrystal films increased with applied voltage and
nanocrystal concentration; however, growth of transparent films of
the Eu.sub.2O.sub.3 nanocrystals slowed down with deposition time.
The voltage drop across the film of insulating Eu.sub.2O.sub.3
nanocrystals played a primary role and the depletion of
nanocrystals from the suspension played a secondary role in
suppressing the growth of the films. Nonuniformity of the
nanocrystal film increased with applied voltage and nanocrystal
concentration, which was attributed to the effect of a fringe
electric field. Knowledge of the effect of EPD process parameters
on the overall quality of the nanocrystal films may prove helpful
to the deposition of other nanocrystal systems. Insight gained into
the growth of high quality electrophoretically deposited films of
the colloidal nanocrystals would be beneficial in the design and
production of nanocrystal-based optical, magnetic and electronic
devices.
Example 2
Gd.sub.2O.sub.3 Nanocrystal Films
[0096] Gadolinium oxide (Gd.sub.2O.sub.3) in its crystalline and
amorphous phases has been of research interest as a replacement
gate oxide material for silicon dioxide because of its high
dielectric constant (.kappa.=14) [58]. Nanocrystals (NCs) of
Gd.sub.2O.sub.3 have been investigated for their applications as a
magnetic contrast agents [59, 60], and host materials in Eu.sup.3+-
and Tb.sup.3+-doped light emitting materials [61]. Recently,
dielectric studies of amorphous Gd.sub.2O.sub.3 films, embedded
with Gd.sub.2O.sub.3 NCs, revealed intriguing charge storage
characteristics of the NCs [62]. Similarly, metallic NCs (Au, Ru,
Ni, and Co) [63-65] and semiconducting NCs (Si and Ge) [66, 67]
have exhibited charge-storage characteristics when they were
embedded in the gate oxide layer of a metal-oxide-semiconductor
(MOS) structure for non-volatile memory (NVM) applications. NC
confined states, the states at the interface of NC-dielectric (i.e.
NC surface), and the defect sites inside NCs are responsible for
charge-storage behavior [63, 68, 69]. Colloidal NCs may possess
similar charge-storage capabilities because of the unpassivated
surface states that can arise due to the detachment of some
fraction of the nanocrystals' surface capping ligands during
cleaning procedure [70-72]. Dielectric studies of films composed
entirely of colloidal Gd.sub.2O.sub.3 NCs may provide insight into
this subject. An effective technology for the fabrication of casts
of particles, electrophoretic deposition (EPD) can produce
densely-packed films of colloidal NCs at high deposition rates [70,
73]. According to various embodiments of the present invention, EPD
was employed to produce films consisting only of colloidal
Gd.sub.2O.sub.3 NCs to be used as the gate oxide layer in MOS
architecture. Capacitance-voltage (C-V) measurements of these MOS
structures exhibited hysteresis, illustrating the charge-storage
capabilities of the films. The dielectric constant of the NC films
(.kappa.) was determined from C-V measurements of the MOS
structures with different NC film thicknesses. Since the dielectric
constant of the NC films depended on the packing density of NCs
within the films, .kappa. subsequently was used to determine the NC
packing density.
Fabrication of MOS Capacitor Structures and Characterizations.
[0097] In one embodiment of the present invention, as shown in the
schematic of FIG. 8(a), MOS capacitor structures were fabricated,
composed of films of ultra-small colloidal Gd.sub.2O.sub.3 NCs
employed as the dielectric oxide layer. The about 2.4 nm diameter
Gd.sub.2O.sub.3 nanocrystals were synthesized via a two-stage
solution-phase technique and were capped with oleic acid [71]. The
as-synthesized NCs were cleaned in ethanol using a
precipitation-centrifugation sequence, described elsewhere [71].
After purifying the Gd.sub.2O.sub.3 NCs, they were suspended in
hexane and then were deposited onto p-(100) silicon substrates [epi
layer: 20-40 .OMEGA.cm and substrate: 0.005-0.025 .OMEGA.cm] via
electrophoretic deposition. For EPD, a pair of silicon electrodes
(25 mm.times.13 mm) was mounted in a parallel-plate configuration
with a gap of about 5 mm. A DC voltage of 500 V was applied to the
electrode pair using a Keithley 6517A electrometer. The voltage was
applied for 15 min during the deposition and, subsequently, for 5
min during an air drying step of the film to improve densification
of the film [70, 74]. The NCs deposited on both the cathode and the
anode. FIG. 8(b) shows an image of the NC film (anode) captured
using a Nanoscope III atomic force microscope (AFM). The film was
continuous and topologically smooth with root mean square (RMS)
roughness of about 1.6 nm, which was smaller than the diameter of
the nanocrystals (about 2.4 nm). An image of the nanocrystals
acquired with Philips CM 20 transmission electron microscope (TEM)
is shown as an inset to FIG. 8(b). It was confirmed the deposition
of the Gd.sub.2O.sub.3 NCs by performing energy dispersive
spectroscopy (EDS) of the film using a Link ISIS series 300
microanalysis system.
[0098] Shown in FIG. 8(c), the EDS spectrum of the film exhibits
the gadolinium, oxygen, carbon, and silicon peaks, which are
present due to the oleic acid-capped Gd.sub.2O.sub.3 NCs on the
silicon substrate. To complete the fabrication of the MOS
capacitors, aluminum contacts (500 .mu.m diameter and 300 nm thick)
were deposited on the Gd.sub.2O.sub.3 NC film via e-beam
evaporation of aluminum using a shadow mask. Aluminum was chosen as
the gate material because of its suitable work function and cost
effectiveness. FIG. 8(d) shows the image, which is a top view, of
the MOS capacitors recorded using a Hitachi S-4200 scanning
electron microscope (SEM). MOS capacitors were fabricated with
different oxide layer (NC film) thicknesses (116.+-.10 nm,
179.+-.10 nm, 276.+-.10 nm, and 397.+-.15 nm), which were verified
using a Veeco Dektak 150 surface profiler. To vary thickness of the
NC film, EPD suspensions of different NC concentrations
(1.0.times.10.sup.15 NC/cm.sup.3, 1.5.times.10.sup.15 NC/cm.sup.3,
2.0.times.10.sup.15 NC/cm.sup.3, and 2.5.times.10.sup.15
NC/cm.sup.3) were employed.
C-V Measurements, and Calculations of the Dielectric Constant
(.kappa.) and the Packing Density.
[0099] High-frequency C-V measurements of the capacitors were
performed at a frequency of 1 MHz and at a sweep rate of 50 mV/s,
using a Keithley 590 CV analyzer on a Signatone probe station. The
gate voltage was swept from -10 V (accumulation) to +5 V
(inversion) and back to -10 V (accumulation). The capacitors were
biased at -10 V for 15 min prior to the forward sweep and were
biased at +5 V for 1 min prior to the reverse sweep. FIG. 9 shows
the C-V characteristics of capacitors fabricated from 116 nm thick
NC films, deposited on the anode and cathode. C-V characteristics
of both capacitors are similar to that of a typical MOS capacitor
with distinct accumulation, depletion, and inversion regions. The
MOS capacitors exhibited a clockwise hysteresis in their C-V
characteristics as they were biased through the
accumulation-inversion-accumulation regions. The observed
hysteresis indicated the presence of charge carriers within the NC
film. The charge carriers could be immobile charges, arising from
the unpassivated surface sites (Gd.sup.3+ and O.sup.2-) of the NCs,
or mobile charges (electrons or holes), injected into the NC film.
The presence of positive charges shifts the flat-band voltage
(V.sub.FB) in negative direction, while the presence of negative
charges shifts it in positive direction. Assuming no charges in the
NC film, the ideal flat-band voltage (V.sub.FB.sup.Ideal) of -0.88
V was based on the work functions of Al and Si and the doping
concentration in the epi layer. A larger negative shift in V.sub.FB
(.DELTA.V.sub.FB is about -3.92 V, anode) during reverse sweep
(inversion-accumulation) than the positive shift in V.sub.FB
(.DELTA.V.sub.FB is about 0.05 V, anode) during forward sweep
(accumulation-inversion) suggested the presence of more positive
charges in the NC film. Electrons were injected into the NC film
from the gate electrode in the accumulation region, while electrons
were subsequently extracted (equivalent to injection of holes) from
the NC film into the gate electrode in the inversion region. To
compare the charge-storage in the NC films deposited on the anode
and cathode, it was measured the width of the hysteresis window
(.DELTA.V) for the two NC films [3.97 V (anode) and 4.19 V
(cathode)]. These values were within the statistical uncertainty
(.+-.0.13 V) when multiple MOS capacitors were characterized.
Similar to other metal [64], semiconductor [66, 67], and insulator
[62] NC-embedded MOS capacitor structures, charge-storage was
observed in the Gd.sub.2O.sub.3 NC films formed according to
various embodiments of the present invention.
[0100] FIGS. 10A-10D shows C-V characteristics of the MOS
capacitors for NC films of different thicknesses. The oxide
capacitance, C.sub.OX, in the accumulation region decreased with
increased NC film thickness, as was expected. For a given gate
dielectric material (typically oxide), the oxide capacitance (in
accumulation) has a linear relationship with the inverse of the
gate oxide thickness, as stated in equation 1 set forth below:
C OX = A .times. OX t OX ( 1 ) ##EQU00004##
[0101] In this expression, C.sub.OX is the oxide capacitance (F), A
is the gate area (cm.sup.2), t.sub.OX is the oxide thickness (cm),
and .di-elect cons..sub.OX is the permittivity of the oxide
material (F/cm). From the thickness and capacitance of the
Gd.sub.2O.sub.3 NC film, the permittivity of the NC film can be
determined. FIG. 11 shows a graph of the NC film capacitance as a
function of the inverse of the NC film thickness for MOS capacitors
with different NC film thicknesses. The data exhibited good
agreement with the linear trend. The dielectric permittivity of the
NC film was extracted from slope of the linear fit, given the area
of the gate. The dielectric constant, .kappa., of the
Gd.sub.2O.sub.3 NC film was calculated using the relation,
.di-elect cons.=.di-elect cons..sub.OX/.di-elect cons..sub.0 and
was found to be 3.90.+-.0.06. Since the NC film comprised
Gd.sub.2O.sub.3 (.kappa.=14.0), oleic acid (.kappa.=2.5), and air
(.kappa.=1.0), the effective dielectric constant of the NC film
depended on the volumetric fractions of each component in the film.
Good agreement between the data and the liner fit suggested that
all of the NC films, cast from solutions with different NC
concentrations, possessed comparable NC packing fractions. It was
calculated the volumetric packing fractions of the NC film using a
three-component Bruggeman model for the dielectric constant
(Equations 2 and 3):
f air .kappa. air - .kappa. NC film .kappa. air + 2 .kappa. NC film
+ f oleic .kappa. oleic - .kappa. NC film .kappa. oleic + 2 .kappa.
NC film + f Gd 2 O 3 .kappa. Gd 2 O 3 - .kappa. NC film .kappa. Gd
2 O 3 + 2 .kappa. NC film = 0 ( 2 ) f air + f oleic + f Gd 2 O 3 =
1 ( 3 ) ##EQU00005##
[0102] In the expression, volume fractions of air, NC film, oleic
acid, and Gd.sub.2O.sub.3 are given as f.sub.air, f.sub.NC film,
f.sub.oleic, and f.sub.Gd.sub.2.sub.O.sub.3, respectively.
Dielectric constants of air, NC film, and Gd.sub.2O.sub.3 are given
as .kappa..sub.sir, .kappa..sub.NC film, .kappa..sub.oleic, and
.kappa..sub.Gd.sub.2.sub.O.sub.3, respectively. Based on the
coverage of oleic acid on the surface of a spherical NC core, a
relationship was formed between the volumetric fractions of the
oleic acid surfactant and the Gd.sub.2O.sub.3 NC core as stated in
equation 4, where R.sub.1 is radius of the Gd.sub.2O.sub.3 NC core
(R.sub.1=1.2 nm.+-.0.1), and R.sub.2 is the radius of NC core plus
thickness of the oleic acid layer (0.3.+-.0.1 nm):
f oleic = f Gd 2 O 3 [ ( R 2 / R 1 ) 3 - 1 ] ( 4 ) ##EQU00006##
[0103] Volumetric fractions,
f.sub.Gd.sub.2.sub.O.sub.3=0.34.+-.0.02, f.sub.air=0.34.+-.0.08,
and f.sub.oleic.ltoreq.0.32.+-.0.10, were calculated from equations
2-4. The summed packing fraction for the nanocrystals
(Gd.sub.2O.sub.3 NC core plus oleic acid) is 0.66.+-.0.08 and
resides within the glassy packing regime for closely packed spheres
[75]. Thus, EPD can produce densely packed, glassy films of
ultra-small nanocrystals that exhibit potential charge storage
capabilities, surmised from their packing density and their
effective dielectric constant.
Dielectric Properties of the Colloidal Gd.sub.2O.sub.3 NC
Films.
[0104] Accordingly, in another aspect, MOS capacitor structures
with colloidal Gd.sub.2O.sub.3 NC film as oxide layer were
fabricated according to various embodiments of the present
invention. Uniformly deposited films of the purified
Gd.sub.2O.sub.3 NCs were produced via electrophoretic deposition.
C-V measurements of the MOS capacitors exhibited clockwise
hysteresis that suggested charge-storage within the NC films. The
NC films, deposited on the anode and cathode, had similar
charge-storage properties. NC films with different thicknesses
showed charge-storage behavior. Dielectric constant (.kappa.=3.90)
of the NC films was calculated from the C-V measurements of the MOS
capacitors. Packing density of the NCs within the film
(0.66.+-.0.08) was calculated from the dielectric constant of the
NC film and was found to be within glassy-packing regime, as
expected for the films deposited via EPD.
[0105] The foregoing description of the exemplary embodiments of
the invention has been presented only for the purposes of
illustration and description and is not intended to be exhaustive
or to limit the invention to the precise forms disclosed. Many
modifications and variations are possible in light of the above
teaching.
[0106] The embodiments were chosen and described in order to
explain the principles of the invention and their practical
application so as to enable others skilled in the art to utilize
the invention and various embodiments and with various
modifications as are suited to the particular use contemplated.
Alternative embodiments will become apparent to those skilled in
the art to which the present invention pertains without departing
from its spirit and scope. Accordingly, the scope of the present
invention is defined by the appended claims rather than the
foregoing description and the exemplary embodiments described
therein.
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