U.S. patent application number 14/841167 was filed with the patent office on 2016-08-04 for multi-heterojunction nanoparticles, methods of manufacture thereof and articles comprising the same.
The applicant listed for this patent is The Board of Trustees of the University of Illinois, Dow Global Technologies LLC, Rohm and Haas Electronic Materials, LLC. Invention is credited to Kishori Deshpande, Jake Joo, Sooji Nam, Nuri Oh, Moonsub Shim, Peter Trefonas, III, You Zhai.
Application Number | 20160225946 14/841167 |
Document ID | / |
Family ID | 50280212 |
Filed Date | 2016-08-04 |
United States Patent
Application |
20160225946 |
Kind Code |
A1 |
Shim; Moonsub ; et
al. |
August 4, 2016 |
MULTI-HETEROJUNCTION NANOPARTICLES, METHODS OF MANUFACTURE THEREOF
AND ARTICLES COMPRISING THE SAME
Abstract
Disclosed herein is a semiconducting nanoparticle comprising a
one-dimensional semiconducting nanoparticle having a first end and
a second end; where the second end is opposed to the first end; a
first node that comprises a first semiconductor; where the first
node contacts a radial surface of the one-dimensional
semiconducting nanoparticle producing a first heterojunction at the
point of contact; and a second node that comprises a second
semiconductor; where the second node contacts the radial surface of
the one-dimensional semiconducting nanoparticle producing a second
heterojunction at the point of contact; where the first
heterojunction is compositionally different from the second
heterojunction.
Inventors: |
Shim; Moonsub; (Savoy,
IL) ; Oh; Nuri; (Champaign, IL) ; Zhai;
You; (Urbana, IL) ; Nam; Sooji; (Urbana,
IL) ; Trefonas, III; Peter; (Medway, MA) ;
Deshpande; Kishori; (Lake Jackson, TX) ; Joo;
Jake; (Somerville, MA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
The Board of Trustees of the University of Illinois
Rohm and Haas Electronic Materials, LLC
Dow Global Technologies LLC |
Urbana
Marlborough
Midland |
IL
MA
MI |
US
US
US |
|
|
Family ID: |
50280212 |
Appl. No.: |
14/841167 |
Filed: |
August 31, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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13834381 |
Mar 15, 2013 |
9123638 |
|
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14841167 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01L 29/0669 20130101;
H01L 29/068 20130101; H01L 29/22 20130101; H01L 29/0676 20130101;
H01L 51/50 20130101; H01L 31/0296 20130101; Y10S 977/774 20130101;
B82Y 10/00 20130101; Y10S 977/95 20130101; H01L 33/28 20130101;
H01L 31/035218 20130101; H01L 31/035227 20130101; B82Y 40/00
20130101; H01L 51/502 20130101; B82Y 30/00 20130101; H01L 33/0083
20130101; B82Y 20/00 20130101; H01L 33/06 20130101; H01L 29/127
20130101 |
International
Class: |
H01L 33/06 20060101
H01L033/06; H01L 33/00 20060101 H01L033/00; H01L 33/28 20060101
H01L033/28; H01L 29/06 20060101 H01L029/06 |
Claims
1. A semiconducting nanoparticle comprising: a one-dimensional
semiconducting nanoparticle having a first end and a second end;
where the second end is opposed to the first end; a first node that
comprises a first semiconductor; where the first node contacts a
radial surface of the one-dimensional semiconducting nanoparticle
producing a first heterojunction at the point of contact; and a
second node that comprises a second semiconductor; where the second
node contacts the radial surface of the one-dimensional
semiconducting nanoparticle producing a second heterojunction at
the point of contact; where the first heterojunction is
compositionally different from the second heterojunction.
2. The semiconducting nanoparticle of claim 1, where the first
nodes and the second nodes are randomly disposed upon the surface
of the one-dimensional semiconducting nanoparticle.
3. The semiconducting nanoparticle of claim 1, further comprising a
first endcap disposed on the first end of the one-dimensional
semiconducting nanoparticle; where the first endcap comprises a
first semiconductor.
4. The semiconducting nanoparticle of claim 1, further comprising
another first endcap disposed on the second end of the
one-dimensional semiconducting nanoparticle; where the another
first endcap has a similar composition to the first endcap disposed
on the first end of the one-dimensional semiconducting
nanoparticle.
5. The semiconducting nanoparticle of claim 1, where the another
first endcap has a different composition to the first endcap
disposed on the first end of the one-dimensional semiconducting
nanoparticle.
6. The semiconducting nanoparticle of claim 2, further comprising a
second endcap disposed upon and in contact with the first endcap;
where the second endcap has a different composition from the first
endcap.
7. The semiconducting nanoparticle of claim 2, where the
one-dimensional nanoparticle comprises CdS, the first node
comprises CdSe or CdTe and the second node comprises ZnSe.
8. The semiconducting nanoparticle of claim 1, where the first
nodes and the second nodes are spatially arranged in a
non-contiguous fashion.
9. A method comprising: reacting a first precursor to a
semiconductor with a second precursor to a semiconductor to form a
one-dimensional semiconducting nanoparticle; where the first
one-dimensional semiconducting nanoparticle has a first end and a
second end that is opposed to the first end; reacting a third
precursor to a semiconductor with the one dimensional nanoparticle
to form the first node that contacts the one-dimensional
nanoparticle at a radial surface to form a first heterojunction;
and reacting the one-dimensional nanoparticle having the first node
disposed thereon with a fourth precursor to a semiconductor to form
a second node that is disposed at the radial surface of the
one-dimensional semiconducting nanoparticle and forms a second
heterojunction; where the second heterojunction is compositionally
different from the first heterojunction.
10. An article comprising: a first electrode; a second electrode;
and a layer comprising a semiconducting nanoparticle disposed
between the first electrode and the second electrode; where the
semiconducting nanoparticle comprises: a one-dimensional
semiconducting nanoparticle having a first end and a second end;
where the second end is opposed to the first end; and a first node
that comprises a first semiconductor; where the first node contacts
a radial surface of the one-dimensional semiconducting nanoparticle
producing a first heterojunction at the point of contact; and a
second node that comprises a second semiconductor; where the second
node contacts the radial surface of the one-dimensional
semiconducting nanoparticle producing a second heterojunction at
the point of contact; where the first heterojunction is
compositionally different from the second heterojunction.
11. The article of claim 10, wherein the article emits visible
light when subjected to an electric potential and current.
Description
BACKGROUND
[0001] This disclosure relates to double heterojunction
nanoparticles, methods of manufacture thereof and to articles
comprising the same.
[0002] One of the advantages of semiconductor nanocrystals is their
potential for improving the efficiencies of optoelectronic devices.
Spherical nanocrystal heterostructures, sometimes referred to as
core-shell quantum dots, have been widely used for quantum dot
light emitting diodes (LEDs). In these core-shell heterostructures
which mainly consist of type I (straddling) band offset, the
heterojunction serves merely as a passivation layer to improve
photoluminescence efficiency. Owing to their unique optical and
electronic properties, semiconductor nanocrystals have attracted
much attention in various opto-electronic applications including
photovoltaics (PVs), LEDs, solid state lighting, and displays.
These tiny crystals have one or more dimensions that are a few
nanometers in length, which allows tuning of their electronic band
gap. The change in the band gap and the electronic energy levels
allows one to control the observed optical and electrical
properties of the semiconductor.
[0003] In addition, when two or more semiconductor materials are
brought together, one can expect new and improved optical and
electronic properties depending on their relative band offsets and
band alignment. The heterojunction that is formed at the interface
of dissimilar semiconductors can help to direct electrons and holes
as well as being an active component for a variety of devices
including PVs, LEDs and transistors. By choosing different
materials for the core and the shell, the band edge positions may
be varied. However, the effective band gap and the band offsets for
some combinations of materials may be large and may hinder carrier
injection processes. It is therefore desirable to produce
semiconducting nanoparticles that have multiple heterojunctions.
Particles having multiple heterojunctions allow the band gap and
band offsets at different interfaces to be tuned by virtue of
having more than two semiconducting materials selectively
contacting one another.
[0004] Benefits of multiple heterojunctions include facilitating
carrier injection and/or blocking while providing improved
photoluminescence yields by surface passivation of the central
"core"--that is, by creating multiple heterojunctions with a
combination of type I and type II band offsets. This facilitates
the achievement of a good barrier for one type of carrier while
facilitating injection of the other carrier type in addition to the
surface passivation benefits equivalent to type I core/shells.
SUMMARY
[0005] Disclosed herein is a semiconducting nanoparticle comprising
a one-dimensional semiconducting nanoparticle having a first end
and a second end; where the second end is opposed to the first end;
a first node that comprises a first semiconductor; where the first
node contacts a radial surface of the one-dimensional
semiconducting nanoparticle producing a first heterojunction at the
point of contact; and a second node that comprises a second
semiconductor; where the second node contacts the radial surface of
the one-dimensional semiconducting nanoparticle producing a second
heterojunction at the point of contact; where the first
heterojunction is compositionally different from the second
heterojunction.
[0006] Disclosed herein too is a method comprising reacting a first
precursor to a semiconductor with a second precursor to a
semiconductor to form a one-dimensional semiconducting
nanoparticle; where the first one-dimensional semiconducting
nanoparticle has a first end and a second end that is opposed to
the first end; reacting a third precursor to a semiconductor with
the one dimensional nanoparticle to form the first node that
contacts the one-dimensional nanoparticle at a radial surface to
form a first heterojunction; and reacting the one-dimensional
nanoparticle having the first node disposed thereon with a fourth
precursor to a semiconductor to form a second node that is disposed
at the radial surface of the one-dimensional semiconducting
nanoparticle and forms a second heterojunction; where the second
heterojunction is compositionally different from the first
heterojunction.
[0007] Disclosed herein too is an article comprising a first
electrode; a second electrode; and a layer comprising a
semiconducting nanoparticle disposed between the first electrode
and the second electrode; where the semiconducting nanoparticle
comprises a one-dimensional semiconducting nanoparticle having a
first end and a second end; where the second end is opposed to the
first end; and a first node that comprises a first semiconductor;
where the first node contacts a radial surface of the
one-dimensional semiconducting nanoparticle producing a first
heterojunction at the point of contact; and a second node that
comprises a second semiconductor; where the second node contacts
the radial surface of the one-dimensional semiconducting
nanoparticle producing a second heterojunction at the point of
contact; where the first heterojunction is compositionally
different from the second heterojunction.
BRIEF DESCRIPTION OF THE FIGURES
[0008] FIG. 1(A) is a depiction of the passivated nanocrystalline
semiconductor nanoparticles disclosed herein;
[0009] FIG. 1(B) is another depiction of the passivated
nanocrystalline semiconductor nanoparticles disclosed herein;
[0010] FIG. 2 shows how the band gap can be varied (i.e., spatially
modulated) by changing the composition of the nanoparticles. In
FIG. 2, the nanoparticle comprises a cadmium sulfide (CdS)
one-dimensional nanoparticle, with the first endcap being cadmium
selenide (CdSe) and the second endcap being zinc selenide
(ZnSe);
[0011] FIG. 3 also shows how the band gap can be varied (i.e.,
spatially modulated) by changing the composition of the
nanoparticles. In the FIG. 3, the one-dimensional nanoparticle
comprises cadmium sulfide, while the first endcap comprises cadmium
telluride and the second endcap comprises zinc selenide;
[0012] FIG. 4 is a schematic depiction of an exemplary
electroluminescence (EL) device;
[0013] FIG. 5(A) is a graph that shows the EL spectra for the
core-shell (CdSe/ZnS) quantum dots;
[0014] FIG. 5(B) is a graph that shows the EL spectra for the
nanoparticles disclosed herein (CdS nanorods, passivated by a first
endcap that comprises CdSe and a second endcap that comprises
ZnSe); and
[0015] FIG. 6 is a graph that shows the integrated EL versus
voltage for the quantum dots and for the nanoparticles disclosed
herein.
DETAILED DESCRIPTION
[0016] Disclosed herein are passivated nanocrystalline
semiconductor nanoparticles (hereinafter nanoparticles) that
comprise a plurality of heterojunctions and which facilitate charge
carrier injection processes that enhance light emission when used
in a device. The nanocrystalline semiconductor nanoparticles are
passivated only at some positions while being unpassivated at other
positions. These multi-heterojunction passivated nanoparticles can
serve as active elements in easy-to-fabricate, high-performance
optoelectronic devices including light emitting diodes (LEDs). The
nanoparticles comprise a one-dimensional nanoparticle that has
disposed at each end a single endcap or a plurality of endcaps that
contact the one-dimensional nanoparticle. The endcaps also contact
each other. The endcaps serve to passivate the one-dimensional
nanoparticles. The nanoparticles can be symmetrical or asymmetrical
about at least one axis. The nanoparticles can be asymmetrical in
composition, in geometric structure and electronic structure, or in
both composition and structure.
[0017] In one embodiment, the nanoparticle comprises a
one-dimensional nanoparticle that comprises an endcap at each
opposing end along its longitudinal axis. Each endcap has a
different composition, thus providing the nanoparticle with
multiple heterojunctions. In another embodiment, the nanoparticle
comprises a one-dimensional nanoparticle that comprises an endcap
at each opposing end along its longitudinal axis and further
comprises nodes disposed on a radial surface of the one-dimensional
nanoparticle or on the endcaps. The radial surface is also termed
the lateral surface of the rods. The endcaps can have similar or
different compositions from each other and/or the nodes can have
similar or different compositions from each other so long as one of
the endcaps has a different composition from either the other
endcap or from at least one of the nodes.
[0018] In one embodiment, the plurality of endcaps comprises a
first endcap and a second endcap that partially or completely
encircles the first endcap. The endcaps are three dimensional
nanoparticles, at least one of which directly contacts the
one-dimensional nanoparticle. Each endcap may or may not contact
the one-dimensional nanoparticle. The first endcap and the second
endcap can have different compositions from each other. The nodes
are also three dimensional nanoparticles that can be smaller or
larger in size than the endcaps.
[0019] The term "heterojunction" implies structures that have one
semiconductor material grown into the crystal lattice of another
semiconductor material. The term one-dimensional nanoparticle
includes objects where the mass of the nanoparticle varies with a
characteristic dimension (e.g., the length) of the nanoparticle to
the first power. This is shown in the formula (1) below:
M.alpha.L.sup.d (1)
where M is the mass of the particle, and L is the length of the
particle and d is an exponent that determines the dimensionality of
the particle. For example, when d=1, the mass of the particle is
directly proportional to the length of the particle and the
particle is termed a one-dimensional nanoparticle. When d=2, the
particles is a two-dimensional object such as a plate while d=3
defines a three-dimensional object such as a cylinder or a sphere.
The one-dimensional nanoparticles (particles where d=1) include
nanorods, nanotubes, nanowires, nanowhiskers, nanoribbons, or the
like. In one embodiment, the one-dimensional nanoparticle may be
curved or wavy (as in serpentine), i.e., have values of d that lie
between 1 and 1.5.
[0020] The one-dimensional nanoparticles have cross-sectional areas
whose characteristic thickness dimension (e.g., the diameter for a
circular cross-sectional area or a diagonal for a square or
rectangular cross-sectional area) are 1 nm to 1000 nanometers (nm),
preferably 2 nm to 50 nm, and more preferably 5 nm to 20 nm (such
as about 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, or 20
nm) in diameter. Nanorods are rigid rods that have circular
cross-sectional areas whose characteristic dimensions lie within
the aforementioned ranges. Nanowires or nanowhiskers are curvaceous
and have different serpentine or vermicular shapes. Nanoribbons
have cross-sectional areas that are bounded by four or five linear
sides. Examples of such cross-sectional areas are square,
rectangular, parallelopipeds, rhombohedrals, and the like.
Nanotubes have a substantially concentric hole that traverses the
entire length of the nanorod, thereby causing it to be tube-like.
The aspect ratios of these one-dimensional nanoparticles are
greater than or equal to 2, preferably greater than or equal to 5,
and more preferably greater than or equal to 10.
[0021] The one-dimensional nanoparticles comprise semiconductors
that include those of the group II-VI (ZnS, ZnSe, ZnTe, CdS, CdSe,
CdTe, HgS, HgSe, HgTe, and the like) and III-V (GaN, GaP, GaAs,
GaSb, InN, InP, InAs, InSb, AlAs, AlP, AlSb, and the like) and IV
(Ge, Si, Pb and the like) materials, and an alloy thereof, or a
mixture thereof.
[0022] The one-dimensional nanoparticle, the first endcap and the
second endcap each comprise semiconductors. The interface between
the nanorods and the first endcap provides a first heterojunction,
while the interface between the first endcap and the second endcap
provides a second heterojunction. In this manner the nanoparticles
may comprise a plurality of heterojunctions.
[0023] With reference now to the FIG. 1A, the nanoparticle 100
comprise a one-dimensional nanoparticle 102 that has a first end
104 and the second end 106. The first endcap 108 is disposed at the
first end 104 and the second end 106 of the one-dimensional
nanoparticle and directly contacts the one-dimensional nanoparticle
102. The interface between the first endcap 108 and the first end
104 of the one-dimensional nanoparticle forms the first
heterojunction 103. In one embodiment, the first endcap 108
contacts the ends of the one-dimensional nanoparticle 102 and does
not contact the longitudinal portion of the one-dimensional
nanoparticle 102. It is preferable that the first endcap 108 does
not surround the entire one-dimensional nanoparticle 102.
[0024] The second endcap 110 contacts the first endcap 108 and
surrounds the first endcap 108 at one or both ends of the
one-dimensional nanoparticle 102. The second endcap 110 may
partially or fully surround the first endcap 108. It is preferable
that the second endcap 110 does not surround the entire
one-dimensional nanoparticle 102.
[0025] The interface between the second endcap 110 and the first
endcap 108 forms the second heterojunction 109. The nanoparticle
100 in the FIG. 1 is therefore a double heterojunction
nanoparticle. In the event that more endcaps are disposed on the
second endcap 110, the nanoparticle 100 would have more than 2
heterojunctions. In an exemplary embodiment, the nanoparticle 100
may have 3 or more heterojunctions, preferably 4 or more
heterojunctions, or preferably 5 or more heterojunctions.
[0026] In one embodiment, the heterojunction at which the one
dimensional nanoparticle contacts the first endcap has a type I or
quasi-type II band alignment. In another embodiment, the point at
which the second endcap contacts the first endcap has a type I or
quasi-type II band alignment.
[0027] The one-dimensional nanoparticle can comprise a nanorod, a
nanowire, a nanotube, a nanowhisker, or the like. In an exemplary
embodiment the nanoparticle is a nanorod. It is termed a
"one-dimensional" nanoparticle because it has a length that is
larger than its diameter and because its mass varies with its
length to the first power as shown in the Equation (1) above.
[0028] The one-dimensional nanoparticle can have a length of 10 to
100 nanometers, preferably 12 to 50 nanometers and more preferably
14 to 30 nanometers. The one-dimensional nanoparticle can have a
diameter of 2 to 10 nanometers, preferably 3 to 7 nanometers. The
one-dimensional nanoparticles have an aspect ratio that is greater
than or equal to about 3, preferably greater than or equal to about
7, and more preferably greater than or equal to about 12. The
one-dimensional nanoparticle is a nanocrystal and comprises a
binary, ternary, or quarternary semiconductor. The semiconductor
can comprise 5 or more elements if desired.
[0029] The semiconductor used in the one-dimensional nanoparticle
is a Group II-VI compound, a Group II-V compound, a Group III-VI
compound, a Group III-V compound, a Group IV-VI compound, a Group
I-III-VI compound, a Group II-IV-VI compound, or a Group II-IV-V
compound. More preferably, the one-dimensional nanoparticle may be
selected from the group consisting of Si, Ge, Pb, SiGe, ZnS, ZnSe,
ZnTe, CdS, CdSe, CdTe, HgS, HgSe, HgTe, AlN, AlP, AlAs, AlSb, GaN,
GaP, GaAs, GaSb, GaSe, InN, InP, InAs, InSb, TlN, TlP, TlAs, TlSb,
PbS, PbSe, PbTe, or the like, or a combination comprising at least
one of the foregoing semiconductors. In an exemplary embodiment,
the one-dimensional nanoparticle comprises CdS.
[0030] With reference now once again to the FIG. 1(A), the
one-dimensional nanoparticle 102 comprises a first end 104 and a
second end 106 that are opposed to each other. The first endcap 108
contacts the first end 104 and the second end 106 of the
one-dimensional nanoparticle. In one embodiment, the first endcap
108 may completely cover the first end 104 and the second end 106
of the one-dimensional nanoparticle 102. In another embodiment, the
first endcap 108 may contact the first end 104 and the second end
106 tangentially. The first endcap 108 is generally spherical or
ellipsoidal in shape and has a cross-sectional area that is
circular or elliptical. In one embodiment, the first and second
endcap can be cylindrical, but have a shorter aspect ratio than the
one dimensional nanoparticle.
[0031] The diameter of the first endcap 108 is about 0.5 to about
1.5 times, preferably about 0.7 to about 1.2 times the diameter of
the one-dimensional nanoparticle. In one embodiment, the diameter
of the first endcap is 1 to 15 nanometers, preferably 2 to 12
nanometers.
[0032] In one embodiment, as noted above, the nanoparticle can be
compositionally asymmetric while being structurally symmetrical. In
other words, the first endcap 108 and/or the second endcap 110 that
contacts the opposing ends of the one-dimensional nanoparticle 102
can have different compositions at the opposing ends, while being
dimensionally and geometrically identical. This is detailed further
in the FIG. 1(B) below.
[0033] In another embodiment (not shown here), the first endcap 108
and/or the second endcap 110 can be compositionally identical while
being of different sizes or different geometries. Such a
nanoparticle is said to be compositionally symmetrical while being
dimensionally and geometrically asymmetrical.
[0034] The second endcap 110 also contacts the one-dimensional
nanoparticle 102 in addition to contacting the first endcap 108. In
one embodiment, the second endcap contacts the first endcap without
contacting the one-dimensional nanoparticle 102. In one embodiment,
the second endcap 110 envelopes the first endcap 108 either
partially or completely. The second endcap 110 is generally
spherical or ellipsoidal in shape and has a cross-sectional area
that is circular or elliptical. While the first endcap 108 and the
second endcap 110 have cross-sectional areas that are circular or
elliptical, it is possible for these endcaps to have
cross-sectional areas that are square, rectangular, triangular or
polygonal. Cross-sectional areas that are square, rectangular,
triangular or polygonal may be synthesized by using a template,
where the one-dimensional nanoparticle is disposed in the template
during synthesis. The first endcap 108 and the second endcap 110
can contact only one end (either 104 or 106) of the one-dimensional
nanoparticle or both ends 104 and 106 of the nanoparticle. In one
embodiment, the second endcap 110 is optional, i.e., the
one-dimensional nanoparticle has only a first endcap 108 disposed
at each end of the one-dimensional nanoparticle. However, the
respective first endcaps 108 are different in composition from each
other. This is discussed in further detail in the FIG. 1(B).
[0035] In one embodiment, the first endcap 108 and the second
endcap 110 are concentrically mounted on the one-dimensional
nanoparticle 102 i.e., they are centrally located about the axis
AA' reflected by numeral 120. While the FIG. 1(A) depicts that the
one-dimensional nanoparticle 102, first endcap 108 and the second
endcap 110 are concentrically located about the axis 120, it is
possible to have the first endcap 108 and/or the second endcap 110
mounted eccentrically mounted about the one-dimensional
nanoparticle 102.
[0036] The diameter of the second endcap 110 is about 1.0 to about
3.0 times, preferably about 1.5 to about 2.7 times the diameter of
the one-dimensional nanoparticle. In one embodiment, the diameter
of the second endcap 110 is 2 to 30 nanometers, preferably 3 to 15
nanometers.
[0037] The first endcap and the second endcap are chemically
different from each other and are selected from the group
consisting of Si, Ge, Pb, SiGe, ZnO, TiO.sub.2, ZnS, ZnSe, ZnTe,
CdO, CdS, CdSe, CdTe, MgO, MgS, MgSe, MgTe, HgO, HgS, HgSe, HgTe,
AlN, AlP, AlAs, AlSb, GaN, GaP, GaAs, GaSb, InN, InP, InAs, InSb,
TlN, TlP, TlAs, TlSb, TlSb, PbS, PbSe, PbTe, or the like, or a
combination comprising at least one of the foregoing
semiconductors. In an exemplary embodiment, the first endcap is
CdTe or CdSe, while the second endcap is ZnSe.
[0038] Passivating molecules such as alkylphosphines,
alkylphosphine oxides, amines, carboxylic acids, and the like, may
be disposed on the surfaces of the nanorod heterostructures thus
allowing for the solubility and coalescence to be varied. Quantum
efficiency for photoluminescence can be varied by surface
passivating molecules and/or by inorganic endcaps.
[0039] FIG. 1(B) depicts a one-dimensional nanoparticle that has
only two first endcaps 108A and 108B disposed at opposing ends of
the one-dimensional nanoparticle. The two first endcaps 108A and
108B have compositions that are different from each other thus
providing the one-dimensional nanoparticles with two
heterojunctions. In one embodiment, the one-dimensional
nanoparticles can have nodes--a first node 122A and a second node
122B disposed on their radial surfaces. The nodes 122A and 122B
comprise semiconductors (such as those listed above for the first
endcap and the second endcap) and are randomly distributed along
the radial surface of the one-dimensional nanoparticle. During the
synthesis of the one-dimensional nanoparticles, the nodes nucleate
and grow randomly on the surface of the one-dimensional
nanoparticles. The point of contact of the node with the radial
surface of the one-dimensional nanoparticles is a
heterojunction.
[0040] It is desirable for the nodes and the endcaps to have
different compositions from each other in order to produce
multi-heterojunction nanoparticles. The nodes 122A and 122B can
have identical compositions or compositions that are different from
each other depending upon the number of heterojunctions desired.
When the nanoparticle has only two first endcaps 108 that have
identical compositions, then the nodes 122A and/or 122B have
compositions that are different from those of the endcaps 108 in
order to produce a multi-heterojunction nanoparticle. If the
respective endcaps 108A and 108B have different compositions from
each other, then the nodes can have compositions that are similar
to each other and that may be identical to one of the endcaps. In
one embodiment, the nodes can have compositions that are different
from the compositions of either of the endcaps. The heterojunctions
produced at the point of contact of the first node 122A with the
radial surface one-dimensional nanoparticle is different from the
heterojunction produced at the point of the contact of the second
node 122B with the same radial surface. By having different nodes
having different semiconductor compositions contact the radial
surface of the one-dimensional nanoparticle, the nanoparticle can
have multiple heterojunctions.
[0041] The presence of a double heterojunction in the nanoparticles
is advantageous in that it can be used to control and to vary the
emission center of the nanoparticles. This can be used to influence
emission properties and can also be used to facilitate changes in
charge mobility. The heterojunction provides a selective barrier
(depending upon the carrier type--electron or hole) while at the
same time, permitting good passivation of the core emitting center
i.e., it behaves as a good barrier for one type of carrier, while
facilitating the injection of another type of carrier in addition
to providing surface passivation benefits (equivalent to type I
core/shells).
[0042] The composition of the materials forming the heterojunction
can be used to influence the band gap and band offsets. The band
gap, also called an energy gap or bandgap, is an energy range in a
solid where no electron states can exist. In graphs of the
electronic band structure of solids, the band gap generally refers
to the energy difference (in electron volts) between the top of the
valence band and the bottom of the conduction band in insulators
and semiconductors.
[0043] This is equivalent to the energy required to free an outer
shell electron from its orbit about the nucleus to become a mobile
charge carrier, able to move freely within the solid material. The
band gap is therefore an important factor determining the
electrical conductivity of a solid. Substances with large band gaps
are generally insulators, those with smaller band gaps are
semiconductors, while conductors either have very small band gaps
or none, because the valence and conduction bands overlap. Band
offsets can also be used for charge carrier manipulation. The band
gap and band offsets can be used to determine optical properties
like the characteristic absorption/emission peak positions of
corresponding materials.
[0044] By changing the composition and size (diameter or length) of
the one-dimensional nanoparticle, the first endcap and/or the
second endcap, the energy band gap and band offsets can be varied.
Varying the energy band gap can be used to change wavelength, the
efficiency and intensity of light generation in the nanoparticles.
In one embodiment, the conduction band offset between the first
endcap and the one dimensional nanoparticle is much higher than the
conduction band offset between the first endcap and the second
endcap, and where the valence band offset between the first endcap
and the one-dimensional nanoparticle is much lower than one between
the first endcap and the second endcap. In another embodiment, the
conduction band offset between the first endcap and one-dimensional
nanoparticle is much lower than one between the first endcap and
the second endcap, and where the valence band offset between the
first endcap and one-dimensional nanoparticle is much lower than
one between the first endcap and the second endcap. In yet another
embodiment, one of two heterojunctions that are formed by the first
endcap has a smaller conduction band offset and a larger valence
band offset than others, and the other has larger conduction band
offset and smaller valence band offset.
[0045] FIGS. 2 and 3 show how the band gap can be varied (spatially
modulated) by changing the composition of the nanoparticles. In the
FIG. 2, the nanoparticle comprises a cadmium sulfide (CdS)
one-dimensional nanoparticle, with the first endcap being cadmium
selenide (CdSe) and the second endcap being zinc selenide (ZnSe).
In the FIG. 2, the interface between the cadmium sulfide
one-dimensional nanoparticle and the cadmium selenide first endcap
is the first heterojunction, while the interface between the zinc
selenide second endcap and the cadmium selenide first endcap is the
second heterojunction.
[0046] From the FIG. 2, it can be seen that the band gap between
the conduction band and the valence band of the cadmium sulfide is
greater than 2.4 electron volts, while the band gap between the
conduction band and the valence band of the cadmium selenide is
greater than 1.7 electron volts and the band gap between the
conduction band and the valence band of the zinc selenide is
greater than 2.7 electron volts.
[0047] By endcapping the cadmium sulfide one-dimensional
nanoparticle with the cadmium selenide endcap, charge carriers will
be confined to cadmium selenide regions and the effective band gap
(i.e., the exciton energy level) can be reduced from 2.4 electron
volts (eV) to 1.7 eV while passivating the nanoparticle. This
energy band gap difference (i.e., the exciton energy level) can
influence the light emission characteristics of the nanoparticles
and also the light emission characteristics of any device that uses
the nanoparticles. The bandgap energy stated here is merely an
example based on the bulk properties of individual materials, and
nanoparticles can have different band gaps than the bulk material
because of quantum confinement effects.
[0048] By changing the composition of the first and the second
endcaps, the band gap between the conduction and valence bands can
be changed. For example, in the FIG. 3, it can be seen that the
band gap between the conduction and valence band can be reduced by
using a first endcap that comprises cadmium telluride. In the FIG.
3, it may be seen that the one-dimensional nanoparticle comprises
cadmium sulfide, while the first endcap comprises cadmium telluride
and the second endcap comprises zinc selenide. By endcapping the
cadmium sulfide one-dimensional nanoparticles with the cadmium
telluride first endcap, charge carriers can be potentially confined
in the cadmium telluride region under an applied bias and the
bandwidth is reduced to 1.75 eV, while the capped component is
passivated.
[0049] The reactions to produce the nanoparticles are now detailed
below. The following abbreviations are used to detail the
reactants. By "TOPO, TOP, TBP, HDA, HPA, ODPA, OA, ODE, TDPA, and
TOA" it is meant trioctylphosphine oxide, trioctylphosphine,
tri-n-butylphosphine, hexadecylamine, hexylphosphonic acid,
octadecylphosphonic acid, octylamine, octadecene,
tetradecylphosphonic acid and trioctylamine respectfully.
[0050] The nanoparticles may be manufactured by a variety of
different methods. In one embodiment, in one method of
manufacturing the nanoparticles, a first precursor to the
semiconductor (e.g., cadmium oxide) is reacted in a first solvent
(e.g., trioctylphosphine oxide) with a first surfactant (e.g.,
N-octyldecyl phosphonic acid) to form a first complex (e.g.,
Cd-ODPA-cadmium-N-octyldecyl phosphonic acid). The first surfactant
prevents the particles from contacting each other. The first
complex is formed at a temperature of 150 to 400.degree. C.,
specifically 200 to 350.degree. C. preferably in an inert
atmosphere. The inert atmosphere comprises nitrogen, argon, carbon
dioxide, or the like. In an exemplary embodiment, the inert
atmosphere comprises nitrogen or argon and substantially excludes
oxygen and water. The reaction may be conducted in a batch reactor
or in a continuous reactor. In an exemplary embodiment, the first
reaction is conducted in a batch reactor.
[0051] To a mixture comprising the first complex is added a second
precursor (e.g., sulfur (S) dissolved in TOP) to produce the
one-dimensional nanoparticles. The length and diameter of the
one-dimensional nanoparticles can be varied by controlling the
amount of the first and second precursor and that of the first
surfactant. The reaction temperature and time can also be varied to
change the dimensions of the one-dimensional nanoparticles. The
reaction temperature during the growth of the one-dimensional
nanoparticles is generally reduced during the growth of the
one-dimensional nanoparticles. In one embodiment, the reaction
temperature is reduced from 400.degree. C. to less than or equal to
350.degree. C., preferably to less than or equal to 330.degree. C.
during the growth of the one-dimensional nanoparticles. The growth
of the one-dimensional nanoparticles is terminated by reducing the
temperature to less than or equal to 300.degree. C., preferably to
less than or equal to 275.degree. C., and to preferably less than
or equal to 250.degree. C. The one-dimensional nanoparticles are
then purified and stored for the passivating process where the
first endcap and the second endcap are reacted onto them. The
purification is optional and can be conducted by precipitation
centrifugation, decanting, filtering, or the like.
[0052] The first endcap is then synthesized by adding a third
precursor (which is a precursor to the first endcap--e.g., a
selenium precursor) to a reaction mixture that comprises a solvent
and the one-dimensional nanoparticles. The formation of the first
endcap terminates the lengthwise growth of the one-dimensional
nanoparticle. The third precursor is added to a mixture of the
one-dimensional nanoparticles along with additional solvent (e.g.,
trioctylphosphine) in a reactor. The reaction temperature is
increased to greater than or equal to 100.degree. C., specifically
greater than or equal to 225.degree. C., and more specifically
greater than or equal to 250.degree. C. The reaction between the
one-dimensional nanoparticles and the third precursor produces the
first endcap on the one-dimensional nanoparticles. The
one-dimensional nanoparticles along with the first endcap can then
be separated from the remainder of the reaction mixture and
purified by the methods mentioned above. In an exemplary
embodiment, the one-dimensional nanoparticles with the first endcap
are purified by dissolving them in a solvent followed by
centrifugation.
[0053] The second endcap is then reacted onto the first endcap.
This is accomplished by growing the second endcap onto the
one-dimensional nanoparticles that are passivated by the first
endcap. The first endcap passivates the ends of the one-dimensional
nanoparticle. A fourth precursor (which is a precursor to the
second endcap--e.g., zinc acetate) is taken in a reactor along with
a solvent and a ligand or with a plurality of solvents and ligands.
The solvents may be degassed following which they are heated to a
temperature of greater than or equal to 150.degree. C. During the
heating, an intermediate (e.g., zinc oleate) may optionally be
formed. The reaction solution is then cooled to less than or equal
to 100.degree. C., preferably less than or equal to 50.degree. C.
The one-dimensional nanoparticles along with the first endcap
reacted thereto may be added to the reaction vessel along with a
fifth precursor (e.g., a selenium precursor) to form the second
endcap. The fifth precursor is slowly injected into the reaction
vessel. The temperature of the reaction vessel is increased to
greater than or equal to 200.degree. C., preferably greater than or
equal to about 250.degree. C. during the injection of the fifth
precursor. The thickness of the second endcap is determined by the
amount of the fourth and fifth precursor added to the reaction
vessel. The resulting nanoparticles (which now comprise a
one-dimensional nanoparticle endcapped by the first endcap and the
second endcap) are separated and purified as desired. The methods
of separation and purification are detailed above.
[0054] In the aforementioned method, the first precursor and the
fourth precursor comprises barium, indium, zinc, cadmium,
magnesium, mercury, aluminum, gallium, thallium, or lead. The
second precursor, the third precursor and the fifth precursor
comprises selenium, tellurium, sulfur, arsenic, bismuth,
phosphorus, or tin.
[0055] In the method detailed above, the first precursor is added
to the reaction mixture in an amount of 10 to 30 weight percent,
based on the total weight of the first complex. The first
surfactant is added to the reaction mixture in an amount of 70 to
90 weight percent, based on the total weight of the first complex.
The second precursor is added to the reaction mixture in an amount
of 20 to 50 weight percent, based on the total weight of the one
dimensional nanoparticles. The molar ratio of the first precursor
to the second precursor is 4:1 to 1:1.
[0056] The third precursor is added to the reaction mixture in an
amount of 20 to 50 weight percent, based on the total weight of the
passivated nanoparticles. The fourth precursor is added to the
reaction mixture in an amount of 5 to 20 weight percent, based on
the total weight of the passivated nanoparticles. The fifth
precursor is added to the reaction mixture in an amount of 5 to 20
weight percent, based on the total weight of the passivated
nanoparticles. The molar ratio of the fourth precursor to the fifth
precursor is from 4:1 to 1:1. The heterojunctions are localized,
i.e., they are present at the ends of the one-dimensional
nanoparticle, between the first endcap and the second endcap, or at
nodes on the one-dimensional nanoparticle.
[0057] The nanoparticles having a plurality of heterojunctions may
be used in a variety of different applications. These nanoparticles
may be used in lasers, transistors, bipolar transistors, solar
cells, and the like. They can be easily processed in solution.
[0058] In one embodiment, the nanoparticles comprise two types of
heterojunctions where the type II staggered band offset allows for
the efficient injection of electrons and holes, while the type I
offset defines a recombination center for highly efficient light
emission. In addition, the anisotropic rod shape of these
nanoparticles improves nanocrystal performance. The anisotropic
shape permits alignment of the semiconductor components of the
proper charge layers within a device.
[0059] The nanoparticles can be used in EL devices. An exemplary EL
device is shown in the FIG. 4. The FIG. 4 depicts an EL device 300
comprising the nanoparticles with the double heterojunction. The
device 300 comprises a substrate 302, a first electrode 304, a hole
injection layer 306, a hole transport layer 308, a nanoparticle
layer 310 (which contains the passivated nanoparticles disclosed
herein), an electron transport layer 312 and a second electrode
314. The substrate 302 generally comprises an optically
transparent, electrically insulating glass or an optically
transparent, electrically insulating polymer. The first electrode
304 can comprise an optically transparent conductive polymer or
metal oxide. Examples of the first electrode 304 are indium tin
oxide, tin oxide, thin films of polypyrrole, polyaniline,
polythiophene, or the like. A suitable hole injection material for
use in the hole injection layer 306 is PEDOT:PSS
(poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate), which is
a polymer mixture of two ionomers.
[0060] The hole transport layer 308 comprises
poly(9,9-dioctyl-fluorene-co-N-(4-butyl-phenyl)-diphenylamine)
(TFB), poly(N,N'-bis(4-butylphenyl)-N,N'-bis(phenyl)benzidine)
(poly-TPD), poly-N-vinyl carbazole (PVK),
tetrafluoroethylene-perfuloro-3,6-dioxa-4-methyl-7-octenesulphonic
acid copolymer (PFI), or nickel oxide (NiO). The nanoparticle layer
310 comprises the nanoparticles detailed above, while the electron
transport layer 312 comprises zinc oxide or titanium oxide
nanoparticles. The second electrode 314 (which serves as the
cathode) comprises a metal film, an example of which is an aluminum
film. Other materials may be used in the first electrode 304, the
hole injection layer 306, the hole transport layer 308, the
electron transport layer 312 and the second electrode 314.
[0061] The passivated nanoparticles disclosed herein are
advantageous in that they produce a photoluminescence intensity
that compares favorably with comparative compositions that have the
same ingredients but that are not in the form of passivated
nanorods, when both compositions are illuminated with an identical
intensity of light. The passivated nanoparticles produce light in
the wavelength region of 550 to 700 nanometers, with a peak
intensity at about 630 nanometers. By changing the size of the
endcaps, the color of light emitted by the nanorods can also be
changed.
[0062] In one embodiment, the passivated nanoparticles can
self-assemble to be parallel to each other, when disposed upon a
surface. The high aspect ratio of the one-dimensional nanoparticles
permits this form of self-assembly. The self-assembly permits
increased photoluminescence efficiency and can also be used to
produce a variety of colors when illuminated by light.
[0063] The compositions and the methods disclosed herein are
exemplified by the following non-limiting examples.
EXAMPLE
Example 1
[0064] This example demonstrates the manufacturing the passivated
nanoparticles. The reactions were carried out in a standard Schlenk
line under N.sub.2 atmosphere. Technical grade trioctylphosphine
oxide (TOPO) (90%), technical grade trioctylphosphine (TOP) (90%),
technical grade octylamine (OA) (90%), technical grade octadecene
(ODE) (90%), CdO (99.5%), Zn acetate (99.99%), S powder (99.998%),
and Se powder (99.99%) were obtained from Sigma Aldrich.
N-octadecyl phosphonic acid (ODPA) was obtained from PCI Synthesis.
ACS grade chloroform, and methanol were obtained from Fischer
Scientific. Materials were used as received.
Preparation of the One-Dimensional Nanoparticles-CdS Nanorods
[0065] First, 2.0 grams (g) (5.2 millimoles (mmol)) of TOPO, 0.67 g
(2.0 mmol) of ODPA and 0.13 g (2.0 mmol) of CdO were prepared in a
50 ml three-neck round-bottom flask. The mixture was degassed at
150.degree. C. for 30 minutes (min) under vacuum, and then heated
to 350.degree. C. under stirring. As Cd-ODPA complex was formed at
350.degree. C., the brown solution in the flask became optically
transparent and colorless after about 1 hour. Then, the solution
was degassed at 150.degree. C. for 10 minutes to remove by-products
of complexation including O.sub.2 and H.sub.2O. After degassing,
the solution was heated to 350.degree. C. under a N.sub.2
atmosphere. Sulfur (S) precursor containing 16 milligrams (mg) (0.5
mmol) of S dissolved in 1.5 milliliters (ml) of TOP was swiftly
injected into the flask with a syringe. Consequently, the reaction
mixture was quenched to 330.degree. C. where the CdS growth was
carried out. After 15 minutes, the CdS nanorods growth was
terminated by cooling to 250.degree. C. where the CdSe growth on
CdS nanorods was carried out. An aliquot of the CdS nanorods was
taken, and cleaned by precipitation with methanol and butanol for
analysis. The CdS/CdSe heterostructures were formed by adding Se
precursor to the same reaction flask, maintained under N.sub.2
atmosphere as described below.
Passivation of the Nanorods by the First Endcap--Cds/Cdse Nanorod
Heterostructures
[0066] Following the formation of CdS nanorods, Se precursors
containing 20 mg (0.25 mmol) of Se dissolved in 1.0 ml of TOP was
slowly injected at 250.degree. C. at a rate of 4 milliliters per
hour (ml/h) via syringe pump (total injection time .about.15
minutes). Then, the reaction mixture was aged for an additional 5
minutes at 250.degree. C. before the reaction flask was rapidly
cooled by air jet. An aliquot of CdS/CdSe nanorod heterostructures
was taken and cleaned by precipitation with methanol and butanol
for analysis. The final solution was dissolved in chloroform and
centrifuged at 2000 revolutions per minute (rpm). The precipitate
was redissolved in chloroform and stored as a solution. The CdS
band-edge absorption peak corresponds 0.75 when the solution is
diluted by a factor of 10.
Formation of the Second Endcap--CdS/CdSe/ZnSe Double Heterojunction
Nanorods
[0067] CdS/CdSe/ZnSe double heterojunction nanorods were
synthesized by growing ZnSe onto CdS/CdSe nanorod heterostructures.
For Zn precursor, 6 ml of ODE, 2 ml of OA and 0.18 g (1.0 mmol) of
Zn acetate were degassed at 100.degree. C. for 30 minutes. The
mixture was heated to 250.degree. C. under N.sub.2 atmosphere and
consequently Zn oleate was formed after 1 hour. 2 ml of previously
prepared CdS/CdSe solution was injected into Zn oleate solution
after cooling to 50.degree. C. Chloroform in the mixture was
allowed to evaporate for 30 min under vacuum. ZnSe growth was
initiated by a slow injection of Se precursor containing 20 mg
(0.25 mmol) of Se dissolved in 1.0 ml of TOP at 250.degree. C.
Thickness of ZnSe on CdS/CdSe nanorod heterostructures was
controlled by the amount of Se injected. The ZnSe growth was
terminated by removing heating mantle after injecting desired
amount of Se precursor. Cleaning procedures were same as described
for the CdS nanorods.
Alternative Method for Forming the Second Endcap--CdS/CdSe/ZnSe
Double Heterojunction Nanorods
[0068] Coordinating solvents such as TOA can alternatively be used
for growing ZnSe. 5 ml of TOA, 1.2 ml of OA and 0.18 g (1.0 mmol)
of Zn acetate were degassed at 100.degree. C. for 30 minutes. The
mixture was heated to 250.degree. C. under N.sub.2 atmosphere and
consequently Zn oleate was formed after 1 hour. 2 ml of previously
prepared CdS/CdSe solution was injected into Zn oleate solution
after cooling to 50.degree. C. Chloroform in the mixture was
allowed to evaporate for 30 min under vacuum. ZnSe growth was
initiated by a slow injection of Se precursor containing 20 mg
(0.25 mmol) of Se dissolved in 1.0 ml of TOP at 250.degree. C.
Thickness of ZnSe on CdS/CdSe nanorod heterostructures was
controlled by the amount of Se injected. The ZnSe growth was
terminated by removing heating mantle after injecting desired
amount of Se precursor. Cleaning procedures were same as described
for the CdS nanorods.
Example 2
[0069] This example was conducted to demonstrate the use of the
nanoparticles in an electroluminescent device. The device shown in
the FIG. 4 was used. The device 300 comprises a glass substrate
302, a first electrode 304 that comprises indium tin oxide, a hole
injection layer 306 comprising PEDOT:PSS, a hole transport layer
308 comprising TFB, a nanoparticle layer 310 whose contents are
detailed below, an electron transport layer 312 comprising zinc
oxide nanoparticles, and the second electrode 314 comprising
aluminum.
[0070] The nanoparticle layer 310 contains either the nanoparticles
disclosed herein (CdS nanorods, passivated by a first endcap that
comprises CdSe and a second endcap that comprises ZnSe) or a
comparative material that comprises core-shell quantum dots, where
the core is CdSe and the shell is ZnSe.
[0071] The EL performance of the EL device with the respective
materials is shown in the FIG. 5. The FIG. 5(A) show the EL spectra
for the core-shell (CdSe/ZnS) quantum dots, while the FIG. 5(B)
shows the EL spectra for the nanoparticles disclosed herein (CdS
nanorods, passivated by a first endcap that comprises CdSe and a
second endcap that comprises ZnSe).
[0072] From the FIGS. 5(A) and 5(B), it may be seen that the EL
spectra for the passivated nanorods is shifted to higher
wavelengths. The core shell quantum dots have a peak intensity at
600 nanometers, while the passivated nanorods have a peak emission
at 630 nanometers. Devices with the passivated nanorods have a
considerably lower turn-on voltage for the EL intensity of ca. 2.5V
compared to ca. 4V for the devices with the core shell quantum
dots.
[0073] The FIG. 6 is a graph that shows the integrated EL versus
the applied voltage for the core-shell CdSe/ZnS quantum dots and
the nanoparticles (CdS nanorods, passivated by a first endcap that
comprises CdSe and a second endcap that comprises ZnSe). From the
FIG. 6, it may be seen that the integrated EL is greater for the
CdS nanorods, passivated by a first endcap that comprises CdSe and
a second endcap that comprises ZnSe than for the quantum dots in
low voltage regions.
* * * * *