U.S. patent application number 12/442943 was filed with the patent office on 2010-06-10 for quantum dots having composition gradient shell structure and manufacturing method thereof.
This patent application is currently assigned to EOUL NATIONAL UNIVERSITY INDUSTRY FOUNDATION. Invention is credited to Wan Ki Bae, Kookheon Char, Hyuck Hur, Seonghoon Lee.
Application Number | 20100140586 12/442943 |
Document ID | / |
Family ID | 39230355 |
Filed Date | 2010-06-10 |
United States Patent
Application |
20100140586 |
Kind Code |
A1 |
Char; Kookheon ; et
al. |
June 10, 2010 |
QUANTUM DOTS HAVING COMPOSITION GRADIENT SHELL STRUCTURE AND
MANUFACTURING METHOD THEREOF
Abstract
Provided are quantum dots having a gradual composition gradient
shell structure which have an improvedluminous efficiency and
optical stability, and a method of manufacturing the quantum dots
in a short amount of time at low cost. In the method, the quantum
dots can be manufactured in a short amount of time at low cost
using a reactivity difference between semiconductor precursors,
unlike in uneconomical and inefficient conventional methods where
shells areformed after forming cores and performing cleaning and
redispersion processes. Also, formation of the cores is followed by
formation of shells having a composition gradient. Thus, even if
the shells are formed to a large thickness, the lattice mismatch
between cores and shells is relieved. Furthermore, on the basis of
the funneling concept, electrons and holes generated in the shells
are transferred to the cores to emit light, thereby obtaining a
high luminous efficiency of 80% or more. The quantum dot structure
is not limited to Group II-IV semiconductor quantum dots but can be
applied to other semiconductors quantum dots, such as Group III-V
semiconductors quantum dots and Group IV-IV semiconductors quantum
dots. Also, the manufacturing method can be utilized in the
development of semiconductor quantum dots having different physical
properties, and in various other fields.
Inventors: |
Char; Kookheon; (Seoul,
KR) ; Lee; Seonghoon; (Seoul, KR) ; Bae; Wan
Ki; (Busan, KR) ; Hur; Hyuck;
(Chungcheongnam-do, KR) |
Correspondence
Address: |
SHERR & VAUGHN, PLLC
620 HERNDON PARKWAY, SUITE 320
HERNDON
VA
20170
US
|
Assignee: |
EOUL NATIONAL UNIVERSITY INDUSTRY
FOUNDATION
Seoul
KR
|
Family ID: |
39230355 |
Appl. No.: |
12/442943 |
Filed: |
September 21, 2007 |
PCT Filed: |
September 21, 2007 |
PCT NO: |
PCT/KR2007/004662 |
371 Date: |
February 1, 2010 |
Current U.S.
Class: |
257/14 ;
257/E21.068; 257/E21.461; 257/E29.071; 438/102; 438/478;
977/774 |
Current CPC
Class: |
C09K 11/02 20130101;
H01L 21/02474 20130101; B82Y 10/00 20130101; C01P 2004/84 20130101;
C01G 11/00 20130101; C01P 2006/40 20130101; C09K 11/565 20130101;
C09K 11/883 20130101; H01L 21/02557 20130101; H01L 21/02601
20130101; Y10S 977/774 20130101; C01B 19/002 20130101; C01P 2004/03
20130101; H01L 29/127 20130101; Y10S 438/962 20130101; H01L
21/02477 20130101; H01L 21/0256 20130101; H01L 21/0248 20130101;
H01L 29/22 20130101; B82Y 30/00 20130101; C01P 2004/04 20130101;
C01G 9/08 20130101; C01B 19/007 20130101; H01L 21/02562 20130101;
C01G 11/02 20130101 |
Class at
Publication: |
257/14 ; 438/478;
438/102; 257/E29.071; 257/E21.461; 257/E21.068; 977/774 |
International
Class: |
H01L 29/12 20060101
H01L029/12; H01L 21/36 20060101 H01L021/36; H01L 21/06 20060101
H01L021/06 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 25, 2006 |
KR |
10-2006-0093025 |
Claims
1-16. (canceled)
17. A quantum dot comprising a core and a shell, wherein: the core
comprises a first Group II-VI compound; the shell comprises a
second Group II-VI compound; the second Group II-VI compound has a
larger bandgap than the first Group II-VI compound; and the shell
has a gradual composition gradient.
18. A method of manufacturing quantum dots, comprising: mixing a
compound comprising a core-forming Group II metal with a compound
comprising a shell-forming Group II metal to form a mixture;
heating the mixture comprising the core-forming Group II metal and
the shell-forming Group II metal at a temperature between
approximately 100.degree. C. and 350.degree. C.; forming a core by
mixing the mixture with a compound comprising a core-forming Group
VI element and a compound comprising a shell-forming Group VI
element; and forming a shell having a composition gradient by
maintaining the mixture at a temperature between approximately
100.degree. C. and 350.degree. C.
19. The method according to claim 18, wherein at least one of the
core-forming Group II metal and the shell-forming Group II metal
comprises at least one of zinc, cadmium, and mercury.
20. The method according to claim 18, wherein heating the mixture
occurs: in at least one of one of a N.sub.2 atmosphere and a Ar
atmosphere; at or below atmospheric pressure; at a temperature
between approximately 100.degree. C. and 350.degree. C.; and for a
duration between approximately 10 minutes and 600 minutes.
21. The method according to claim 18, wherein the compound
comprising the core-forming Group II metal is mixed with the
compound comprising the shell-forming Group II metal in a 1:1 molar
ratio to 1:50 molar ratio.
22. The method according to claim 18, wherein at least one of the
core-forming Group VI element and the shell-forming Group VI
element comprises at least one of sulfur, selenium, tellurium, and
polonium.
23. The method according to claim 18, wherein the compound
comprising the core-forming Group VI element is mixed with the
compound comprising the shell-forming Group VI element in a 1:1
molar ratio to 1:50 molar ratio.
24. A quantum dot comprising: a core; an outer shell having a
predetermined bandgap energy that is higher than that of the core;
and an inner shell that surrounds the core and that is surrounded
by the outer shell, wherein the inner shell has a bandgap energy
that gradually decreases in a direction from the outer shell
towards the core.
25. The quantum dot according to claim 24, wherein each of the
core, the outer shell, and the inner shell is formed by a Group
II-VI compound semiconductor comprising a Group II element and a
Group VI element.
26. The quantum dot according to claim 25, wherein: the Group II
element comprises at least one of cadmium, zinc, and mercury; and
the Group VI element comprises at least one of sulfur, selenium,
tellurium, and polonium.
27. The quantum dot according to claim 26, wherein the core and the
outer shell comprise at least one of: a cadmium selenide core and a
zinc sulfide outer shell; a cadmium selenide core and a zinc
selenide outer shell; a cadmium telluride core and a zinc sulfide
outer shell; a cadmium telluride core and zinc selenide outer
shell; a cadmium telluride core and a zinc telluride outer shell; a
cadmium telluride core and a cadmium sulfide outer shell; a cadmium
sulfide core and a zinc sulfide outer shell; and a zinc selenide
core and a zinc sulfide outer shell.
28. A method of manufacturing quantum dots, comprising: reacting
compounds to form cores; and reacting remaining compounds to form
shells having a gradual composition gradient, wherein said reacting
compounds and said reacting remaining compounds are sequentially
performed due to a difference in reactivity between the
compounds.
29. The method according to claim 28, wherein the gradual
composition gradient of the shells is achieved due to a reactivity
difference between the remaining compounds.
30. The method according to claim 28, wherein the cores and the
shells are formed by Group II-VI compound semiconductors comprising
a core-forming Group II metal, a core-forming Group VI element, a
shell-forming Group Il metal, and a shell-forming Group VI
element.
31. A method of manufacturing quantum dots, comprising: forming
cores; and reacting compounds to form shells, wherein the shells
are formed having a gradual composition gradient due to a
difference in reactivity between the compounds.
32. The method according to claim 31, wherein the cores and the
shells are formed by Group II-VI compound semiconductors comprising
a core-forming Group II metal, a core-forming Group VI element, a
shell-forming Group II metal, and a shell-forming Group VI element.
Description
TECHNICAL FIELD
[0001] The present invention relates to quantum dots having a
gradual composition gradient shell structure and a method of
manufacturing the same, and more particularly, quantum dots which
have improved luminous efficiency and optical stability owing to a
gradual composition gradient shell structure, and a method of
manufacturing the same.
BACKGROUND ART
[0002] Conventionally, quantum dots have been manufactured using a
dry chemical process, specifically, by inducing lattice mismatch
between a substrate prepared in a vacuum and a layer deposited
using a metal organic chemical vapor deposition (MOCVD) process. In
this case, although nanoparticles may be formed and arranged on the
substrate, expensive equipments are required. And it is also
difficult to produce large quantities of quantum dots with a
uniform size through conventional semiconductor fabrication
methods. In order to solve these problems, a wet chemical process
for synthesizing quantum dots with a uniform size using a surface
active agent (surfactant) has been developed.
[0003] A method of manufacturing quantum dots through a wet
chemical process includes preventing nanoparticles from aggregating
together using a surfactant and controlling the reactivity of
crystal surfaces by moderate choices of surfactants to synthesize
quantum dots with various shapes with an uniform size. In 1993, the
Bawendi group succeeded in synthesizing cadmium selenide (CdSe)
quantum dots with a uniform size by means of a wet chemical process
using trioctylphosphineoxide (TOPO) and trioctylphosphine (TOP) as
surfactants, dimethylcadmium (Me).sub.2Cd and selenium as
precursors of Group II and Group VI. Also, the Alivisatosgroup
developed a method of synthesizing CdSe quantum dots in a safer
manner using hexadecylamine (HDA), trioctylphosphineoxide, and
trioctylphosphine as surfactants, cadmium oxide (CdO), and TOPSe as
precursors of Group II and Group VI.
[0004] Subsequently, a substantial amount of research has been
conducted to passivate the surfaces of CdSe quantum dots with
compound semiconductorshaving a larger bandgap in order to improve
luminous characteristics and optical and environmental stability of
theCdSe quantum dots. For example, CdSe/ZnS (refer to J. Phys.
Chem. B, 1997, 101, 9465-9475), CdSe/ZnSe (refer to Nano Letters,
2002, 2, 781-874), CdSe/
[0005] WO CdS (refer to J. Am. Chem. Soc., 1997, 119, 7019-7029),
and ZnSe/ZnS (Korean Patent Registration No. 10-0376403) have been
proposed.
[0006] However, in conventional core-shell quantum dots, when a
thick shell is formed, an interface between the core and shell of
the quantum dot becomes unstable due to the lattice mismatch
between a core semiconductor material and a shell semiconductor
material, thereby causes formation of defects which lower quantum
efficiency of quantum dots. Thus, the conventional core-shell
quantum dots have thin shell structures. As a result, the shell
semiconductor material functions only to stabilize the surface
state of the core of the quantum dot and cannot funnel electrons
and holes to the core. Therefore the conventional core-shell
quantum dots have limitations in luminous efficiency, optical
stability, and environmental stability.
[0007] Therefore, research into the manufacture of a multishell
including an intermediate shell has been progressed in order to
minimize lattice mismatch between core structure and shell
structure of quantum dots. For example, CdSe/CdS/ZnS (Korean Patent
Registration No. 10-2005-0074779) and
CdSe/CdS/Zn.sub.0.5Cd.sub.0.5S/ZnS (refer to J. Am. Chem. Soc.
2005, 127, 7480-7488) were proposed. Since the core-multishell
quantum dots described above have high luminous efficiencies and
sufficient optical and environmental stability, they can be applied
to practical applications such as various emission materials, laser
materials, and biological marker materials. However, the
manufacture of conventional core-multishell quantum dots involves
complicated synthetic processes. Specifically, after each synthetic
steps (i.e., synthesis of cores or each intermediate shells of
quantum-dot), the synthetic process should be ended, all
surfactants and precursors should be cleaned away, and a precursor
required for intermediate shells should be injected into a new
surfactant at an appropriate temperature to react over a long
period. Also, during the sequential purification steps, some
quantum dots and nonreacted precursors are removed too, which is
counterproductive. Furthermore, in quantum dots having a stacked
shell structure obtained according to the synthesis process
described above, only excitons generated in quantum-dot cores are
used, while excitons generated in quantum-dot shells do not
contribute to light emission, which limits luminous efficiency of
quantum dots.
[0008] Accordingly, there is still room to develop highly
crystalline quantum dots having high luminous efficiency and
optical and environmental stability, and a straightforward
technique for synthesizing the quantum dots in large quantities at
low cost.
DISCLOSURE OF INVENTION
Technical Problem
[0009] The present invention is as to quantum dots of highly
luminous efficiency and optical stability which is due to a gradual
composition gradient shell structure, and this is a method of
manufacturing the quantum dots in a short time at low cost.
Technical Solution
[0010] In their efforts to overcome the disadvantages of
conventional quantum dots and methods of manufacturing them, the
present inventors have developed a new method of economically
manufacturing quantum dots having a gradual composition gradient
shell structure througha single-step process using the reactivity
difference between Group II metal-containing compounds and that
between Group VI element-containing compounds. Also, the present
inventors confirm that optically stable quantum dots of highly
luminous efficiency can be manufactured by this method.
[0011] One aspect of the present invention provides aquantum dot
including a Group II-VI compound core and a Group II-VI compound
shell. A Group II-VI compound forming the shell has a larger
bandgap than a Group II-VI compound forming the core and the Group
II-VI compound shell has a gradual composition gradient.
[0012] Another aspect of the present invention provides a method of
manufacturing quantum dots. The method includes the steps of: (1)
mixing a Group II metal-containing compound forming a core with a
Group II metal-containing compound forming a shell; (2) heating the
mixture at a temperature of 100 to 350.degree. C. (3) mixing the
heated mixture with a Group VI element-containing compound forming
the core and a Group VI element-containing compound forming the
shell; and (4) forming the shell to have a composition gradient by
maintaining the mixture of step (3) at a temperature of 100 to
350.degree. C.
[0013] Still another aspect of the present invention provides a
quantum dot including: a core; an outer shell having a
predetermined bandgap energy higher than that of the core; and an
inner shell surrounding the core and surrounded by the outer shell,
the inner shell having a bandgap energy that gradually decreases
from the outer shell toward the core.
[0014] Yet another aspect of the present invention provides a
method of manufacturing quantum dots including the steps of: (a)
reacting compounds to form cores; and (b) reacting remaining
compounds to form shells having a gradual composition gradient,
wherein step (a) and step (b) are sequentially performed due to a
difference in reactivity between the compounds.
[0015] Still yet another aspect of the present invention provides a
method of manufacturing quantum dots including the steps of: (a)
forming cores; and (b) reacting compounds to form shells, wherein,
in step (b), the shells are formed to have a gradual composition
gradient due to a difference in reactivity between the
compounds.
Advantageous Effects
[0016] According to the present invention, quantum dots can be
manufactured in a short amount of time at low cost using a
difference in reactivity between semiconductor precursors, unlike
in uneconomical and inefficient conventional methods where shells
are formed after forming cores through performing consecutive
cleaning and redispersion processes. Also, according to the present
invention, formation of cores is spontaneously followed by
formation of shells having a composition gradient. Thus, even if
shells are formed to a large thickness, the lattice mismatch
between core and shell does not occur, unlike in conventional
core-shell quantum dots. Furthermore, the funnel concept is applied
to the present invention, so that electrons and holes absorbed in
the shells are transferred to the cores to emit light, thereby a
high luminous efficiency of 80% or higher is obtained. Moreover,
the use of a surfactant can be minimized to facilitate a subsequent
cleaning process, and the surfaces of synthesized quantum dots can
be easily substituted, so that the quantum dots can be applied in
different environments. As described above, the present invention
provides new quantum dots having cores and shells with a gradual
composition gradient, which can be applied to not only Group II-IV
semiconductor quantum dots but also other semi-conductor quantum
dots, such as Group III-V semiconductor quantum dots and Group
IV-IV semiconductor quantum dots, and can be utilized in the
development of semi-conductor quantum dots having different
physical properties and in various other fields.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a flowchart illustrating a process of
manufacturing quantum dots according to an exemplary embodimentof
the present invention.
[0018] FIG. 2 is a diagram of a composition gradient and energy
band of a quantum dot having a gradual composition gradient shell
structure according to an exemplary embodiment of the present
invention.
[0019] FIG. 3 is a diagram illustrating a process of manufacturing
quantum dots according to an exemplary embodiment of the present
invention.
[0020] FIG. 4 is a diagram of experimental equipment used for
manufacturing quantum dots according to the present invention.
[0021] FIG. 5 is a transmission electron microscope (TEM)
photograph showing changes in quantum dots manufactured according
to the present invention as a function of reaction time.
[0022] FIG. 6 is a graph showing radial composition distribution of
a quantum dot having a core and shell manufactured according to the
exemplary embodiment of the present invention.
[0023] FIG. 7 is a graph showing the luminous efficiency of a
quantum dot manufactured according to the present invention as a
function of reaction time.
MODE FOR THE INVENTION
[0024] Hereinafter, exemplary embodiments of the present invention
will be described in detail.
[0025] The present invention provides quantum dots having a gradual
composition gradient and high luminous efficiency and a method of
manufacturing the quantum dots in a short amount of time at low
cost. FIG. 1is a flowchart illustrating a process of manufacturing
quantum dots according to an exemplary embodiment of the present
invention.
[0026] A method of manufacturing quantum dots according to the
present invention includes mixing a Group II
metal-containingcompound forming the core of a quantum dot and a
Group II metal-containing compound forming the shell of the quantum
dot, and heating the mixture in step S1.
[0027] In the present invention, the Group II metal used in step Si
may be one selected from the group consisting of cadmium (Cd), zinc
(Zn), and mercury (Hg), and the Group II metal-containing compound
may be one selected from the group consisting of cadmium oxide,
zinc acetate, cadmium acetate, cadmium chloride, zinc chloride,
zinc oxide, mercury chloride, mercury oxide, and mercury acetate.
However, the present invention is not limited thereto and any
compound containing a Group II metal such as cadmium, zinc, or
mercury can be used.
[0028] The two compounds are mixed using an organic solvent. The
organic solvent may be one selected from the group consisting of
1-oxadecen, hexadecylamine (HDA), trioctylamine (TOA),
trioctylphosphine oxide, oleic acid, and oleyl amine, but the
present inventionis not limited thereto.
[0029] In addition to the organic solvent fatty acid may be added
when mixing the two compounds. The fatty acid may be one selected
from the group consisting of oleic acid, stearic acid, myristic
acid, lauric acid, palmitic acid, elaidicacid, eicosanoic acid,
heneicosanoic acid, tricosanoic acid, docosanoic acid,
tetracosanoic acid, hexacosanoic acid, heptacosanoic acid,
octacosanoic acid, and cis-13-docosenoic acid. However, the present
invention is not limited thereto and any fatty acid may be
used.
[0030] In the present invention, in step S1, the mixture of the
Group II metal-containing compound forming the core and the Group
II metal-containing compound forming the shell is heated in an
atmosphere of N.sub.2 or Ar, at no more than atmospheric pressure,
at a temperature of about 100 to 350.degree. C., for 10 to 600
minutes. When the mixture of the two compounds is heated under
these conditions, a metal-fatty acid complex is formed. The
metal-fatty acid complex is uniformly distributed in the solvent.
Since the mixture of the Group II metal-containing compounds may
react with oxygen, the heating process may be performed in the
nonreactive N.sub.2 or Ar atmosphere at atmospheric pressure to
suppress reactivity. Also, when the heating process is performed at
a temperature lower than 100.degree. C., a complex is not formed,
and when the heating process is performed at a temperature higher
than 350.degree. C., an organic compound may boil and evaporate or
be broken. Therefore, the heating process may be performed at a
temperature of about 100 to 350.degree. C.
[0031] After the metal-fatty acid complex is formed, the remaining
oxygen and water, and impurities with a low boiling point are
removed in a vacuum using a liquid nitrogen trap, and the
metal-unsaturated fatty acid complex is heated in an N.sub.2 or Ar
atmosphere at atmospheric pressure and a temperature of about 100
to 350.degree. C. in step S2. When the heating process is performed
at a temperature lower than 100.degree. C., forming quantum dots is
difficult, and when the heating process is performed at a
temperature higher than 350.degree. C., an organic compoundmay boil
and evaporate or be broken. Therefore, the heating process may be
performed at a temperature of about 100 to 350.degree. C.
[0032] In step S1, the Group II metal-containing compound forming
the core may be mixed with the Group II metal-containing compound
forming the shell in a 1:1 to 1:50 molar ratio. When the molar
ratio is less than 1:1, quantum dots that emit long-wavelength
light are formed. When the molar ratio is more than 1:50, quantum
dots that emit short-wavelength light are formed and a slowly
reacting material may form the core. By adjusting the molar ratio,
the wavelength range of formed quantum dots may be controlled.
[0033] Thereafter, in step S3, the heated compound obtained in step
S2 is mixed with a Group VI element-containing compound forming the
core and a Group VI element-containing compound forming the
shell.
[0034] The Group VI element may be one selected from the group
consisting of sulfur (S), selenium (Se), tellurium (Te), and
polonium (Po), and the Group VI element-containing compound may be
one selected from the group consisting of tri-octylphosphine
selenide, trioctylphosphine sulfide, trioctylphosphine telluride,
trioctylphosphine polonide, tributylephosphine selenide,
tributylephosphine sulfide, tributylephosphine telluride, and
tributylephosphine polonide. However, the present invention is not
limited thereto and any material containing a Group VI element such
as sulfur, selenium, tellurium, and polonium may be used. In step
S3, the Group VI element-containing compound forming the core may
be mixed with the Group VI element-containing compound forming the
shell in a 1:1 to 1:50 molar ratio. When a molar ratio is less than
1:1, quantum dots that emit long-wavelength light are formed. When
the molar ratio is more than 1:50, quantum dots that emit
short-wavelength light are formed and a slowly reacting material
may form the core. Specifically, although the compound forming the
core reacts rapidly and is formed earlier, when there is too much
compound forming the shell, part of the compound does not form the
shell and may form an additional core structure.
[0035] As described above, when the Group VI element-containing
compounds are added to the heated mixture, rapidly reacting Group
II metal and Group VI element are separated earlier in a
supersaturated state in a reactor containing themixture to generate
a nucleation reaction for forming the core. In this case, similar
nucleation reactions are facilitated at the same time, thereby
generating quantum-dot cores with a uniform size.
[0036] After adding and reacting the Group VI element-containing
compounds, the mixture obtained in step S3 is maintained at a
temperature of 100 to 350.degree. C. in step S4. When a continuous
reaction is induced by maintaining the temperature, the Group II
metal and Group VI element, which form the cores on the surface of
the quantum-dot cores, react and grow along with the Group II metal
and Group VI element, which form the shells. After 10 minutes,
reaction between rapidly reacting Group II metal and Group VI
element ends, while slowly reacting Group II metal and Group VI
element continue to react to form the shells.
[0037] The quantum dots manufactured according to the
above-described method of the present invention have Group II-VI
compoundcores and Group II-VI compound shells. The Group II-VI
compound forming the shells has a larger bandgap than the Group
II-VI compoundforming the cores, and the Group II-VI compound
shells have a gradual composition gradient.
[0038] Specifically, as shown in FIG. 2, the quantum dots
manufactured according to the present invention have shell
structures with a gradual composition gradient, so that they have
the lowest conductance-band edge and the highest covalence-band
edge in the cores. The conductance-band edge becomes gradually
higher and the covalence-band edge becomes gradually lower from an
inner portion of the shell toward an outer portion of the shell, so
that the outermost portion of the shell has the highest
conductance-band edge and the lowest covalence-band edge.
[0039] More specifically, the quantum dot manufactured according to
the present invention is divided into the core and the shell, and
the shell is divided into an inner shell and an outer shell. As
illustratedin FIG. 2, the core may be formed of CdSe, the outer
shell may be formed of ZnS, and the inner shell may be formed of
Cd.sub.1-XZn.sub.XSe.sub.1-YS.sub.Y (0<X<1, 0<Y<1, X
and Y become smaller from the outer shell toward the core). The
inner shell surrounds the core and is surrounded by the outer
shell. Each of the core and the outer shell is formed of a compound
semiconductor with a predetermined composition ratio, while the
inner shell is formed of a compound semiconductor whose composition
ratio varies with position. The core has a small bandgap energy,
and the outer shell has a largerbandgap energy than the core. Also,
the inner shell has a bandgap energy that becomes smaller from the
outer shell toward the core.
[0040] In the quantum dot described above, formation of the core is
followed by formation of the shell having a compositiongradient.
Therefore, even if the shell is formed to a large thickness, the
lattice mismatch between core and shell does not occur unlike in
conventional core-shell quantum dots. Furthermore, since the
quantum dots according to the present invention are based on the
funnel concept, electrons and holes generated in the shell are
transferred to the core to emit light, so that the quantum dots
have a high luminous efficiency of about 80% or higher.
[0041] The present invention will now be described more
explicitlyhereinafter with reference to the accompanying drawings,
in which exemplary embodiments of the invention are shown. This
invention may, however, be embodied in different forms and should
not be construed as limited to the exemplary embodiments set forth
herein.
EXEMPLARY EMBODIMENT 1
[0042] Synthesis of Quantum Dots with CdSe Cores And ZnS Shells
[0043] FIG. 3 is a diagram illustrating a process of manufacturing
quantum dots according to an exemplary embodimentof the present
invention, and FIG. 4 is a diagram of experimental equipment used
for manufacturing quantum dots according to the present invention.
Hereinafter, a process of synthesizing quantum dots with CdSe cores
and ZnS shells will be described with reference to FIGS. 3 and
4.
[0044] In step S21, 0.0512 g (0.4 mmol) of solid cadmium oxide
(purity: 99.998%), 0.732 g (4 mmol) of zinc acetate, 4.4 ml (17.6
mmol) of oleic acid, and 13.6 ml of 1-octadecene
(CH2=CH(CH2)15CH3)were put in a 100 ml round-bottom flask 101
including a syringe 101, a thermometer 104, and a cooler 103. In
step S22, the mixture was heated at a pressure of 1 ton and a
temperature of about 320.degree. C. for 20 minutes to synthesize a
cadmium-oleic acid complex and a zinc-oleic acid complex and remove
remaining oxygen from the complexes.
[0045] Thereafter, the complexes were heated in an N.sub.2
atmosphere at a pressure of 1 atmosphere and a temperature of about
320.degree. C., and a 2 ml mixture of 0.2 ml of 2M
trioctylphosphine selenide solution obtained by dissolving selenium
in trioctylphosphine and 1.8 ml of 2M trioctylphosphine sulfide
solutionobtained by dissolving sulfur in trioctylphosphine was
rapidly injected into a reactor 101 heated to a temperature of
about 320.degree. C. In this case, rapidly reactingcadmium and
selenium are supersaturated and separated in the reactor 101. As a
result, a similar nucleation reaction is facilitated, thus
generating uniform CdSe nuclei in step S23.
[0046] Subsequently, when a continuous reaction was induced by
maintaining the reactor 101 at a temperature of about 300.degree.
C., cadmium, selenium, zinc, and sulfur reacted and grew on the
surface of the CdSe nuclei in step S24. After a predetermined
amount of time elapsed, cadmium and selenium stopped reacting, and
only zinc and sulfur reacted to form ZnS shells in step S25. As a
result, quantum dots having CdSe cores and ZnS shells with a
composition gradient were generated.
[0047] After finishing the generation of the quantum dots having
the cores and the shells with the composition gradient, the
temperature of the reactor 101 was lowered to room temperature, and
acetone was added to precipitate the quantum dots. After that, the
quantum dots were separated using a centrifugal separator, cleaned
three times using about 20 ml of acetone, and dried in a vacuum,
thereby completing manufacture of quantum dots with CdSe cores and
ZnS shells.
EXEMPLARY EMBODIMENT 2
[0048] Synthesis of Quantum Dots with CdSe Cores and ZnSe
Shells
[0049] In the present exemplary embodiment, quantum dots were
manufactured in the same manner as in the first exemplary
embodiment except that 2 ml of 2M trioctylphosphine selenide
solution was used instead of the mixtureof trioctylphosphine
selenide and trioctylphosphine sulfide used in the first exemplary
embodiment.
EXEMPLARY EMBODIMENT 3
[0050] Synthesis of Quantum Dots with CdTe Cores and ZnS Shells
[0051] In the present exemplary embodiment, quantum dots were
manufactured in the same manner as in the first exemplary
embodiment except that a 2 ml mixture of 0.2 ml of 2M
trioctylphosphine telluridesolution and 1.8 ml of 2M
trioctylphosphine sulfide solution was used instead of the mixture
of trioctylphosphine selenide and trioctylphosphine sulfide used in
the first exemplary embodiment.
EXEMPLARY EMBODIMENT 4
[0052] Synthesis of Quantum Dots with CdTe Cores and ZnSe
Shells
[0053] In the present exemplary embodiment, quantum dots were
manufactured in the same manner as in the first exemplary
embodiment except that a 2 ml mixture of 0.2 ml of 2M
trioctylphosphine telluridesolution and 1.8 ml of 2M
trioctylphosphine selenide solution was used instead of the mixture
of trioctylphosphine selenide and trioctylphosphine sulfide used in
the first exemplary embodiment.
EXEMPLARY EMBODIMENT 5
[0054] Synthesis of Quantum Dots with CdTe Cores and ZnTe
Shells
[0055] In the present exemplary embodiment, quantum dots were
manufactured in the same manner as in the first exemplary
embodiment except that 2 ml of 2M trioctylphosphine telluride
solution was used instead of the mixtureof trioctylphosphine
selenide and trioctylphosphine sulfide used in the first exemplary
embodiment.
EXEMPLARY EMBODIMENT 6
[0056] Synthesis of Quantum Dots with CdTe Cores and CdS Shells
[0057] In the present exemplary embodiment, quantum dots were
manufactured in the same manner as in the first exemplary
embodiment except that 0.0512 g (4 mmol) of cadmium oxide, 4 ml (16
mmol) of oleic acid, 14 ml of 1-octadecene were used instead of
0.0512 g (0.4 mmol) of cadmium oxide, 0.732 g (4 mmol) of zinc
acetate, 4.4 ml (17.6 mmol) of oleic acid, 13.6 ml of 1-octadecene
used in the first exemplary embodiment. And, a 2 ml mixture of 0.2
ml of 2M trioctylphosphine telluride solution and 1.8 ml of 2M
trioctylphosphine sulfide solution was used instead of the mixture
of trioctylphosphine selenide and trioctylphosphine sulfide used in
the first exemplary embodiment.
EXEMPLARY EMBODIMENT 7
[0058] Synthesis of Quantum Dots with CdS Cores and ZnS Shells
[0059] In the present exemplary embodiment, quantum dots were
manufactured in the same manner as in the first exemplary
embodiment except that 2 ml of 2M trioctylphosphine sulfide
solution was used instead of the mixtureof trioctylphosphine
selenide and trioctylphosphine sulfide used in the first exemplary
embodiment.
EXEMPLARY EMBODIMENT 8
[0060] Synthesis of Quantum Dots with ZnSe Cores and ZnS Shells
[0061] In the present exemplary embodiment, quantum dots were
manufactured in the same manner as in the first exemplary
embodiment except that 7.32 g (4 mmol) of zinc acetate, 4 ml (16
mmol) of oleic acid, and 14 ml of 1-octadecene were used instead of
0.0512 g (0.4 mmol) of cadmium oxide, 0.732 g (4 mmol)of zinc
acetate, 4.4 ml (17.6 mmol) of oleic acid, and 13.6 ml of
1-octadecene used in the first exemplary embodiment. And, a 2 ml
mixture of 0.2ml of 2M trioctylphosphine selenide solution and 1.8
ml of 2M trioctylphosphine sulfide solution was used instead of the
mixture of trioctylphosphine selenide and trioctylphosphine sulfide
used in the first exemplary embodiment.
EXPERIMENTAL EXAMPLE
[0062] Analysis of Radial Composition Distribution and Quantum and
Luminous Efficiencies of Quantum Dots with CdSe Cores and ZnS
Shells
[0063] In the manufacture of the CdSe cores and ZnS shells with a
composition gradient according to the method described in the first
exemplary embodiment, at times of 10 seconds, 1 minute, 3 minutes,
5 minutes, and 10 minutes after injecting trioctylphosphine
selenide and trioctylphosphine sulfide into the reactor, 0.5 ml of
a reactant was extracted using a glass syringe, refined three
times, and dispersed in nucleic acid.
[0064] Each time a sample of quantum dots wasextracted on a
transmission electron microscope (TEM) grid from Jeol, a TEM
analysis was carried out using JEM-3010 from Jeol. FIG. 5 shows the
results of the TEM analysis. Also, in order to analyze the
composition of each quantum dot, quantum dots were dispersed in 1
ml of nitric acid and 19 ml of water was added to dissolve and
ionize the quantum dots. Thus, the compositions of the quantum dots
were analyzed using an ICP-Atomic Emission Spectrometer from
Shimadzu. FIG. 6 is a graph showing the distribution of Cd and Zn
relative to the TEM results and the radius of the quantum dots. In
addition, the luminous efficiencyof the quantum dots was measured
over time using an Agilent 8454 UV spectrometer from Agilent and an
Acton photomultiplier spectrometer from Acton. The light
absorptivity and fluorescence intensity of each quantum dot
solution was measured using rhodamine 6 Gpigment as shown in FIG.
7.
[0065] Referring to FIG. 5, the quantum dots grew with time and
came to have very uniform globular shapes and a uniform
distribution.
[0066] Referring to FIG. 6, as the quantum dots grew, cadmium (Cd)
mainly reacted to generate the cores. Thereafter, zinc (Zn) started
to grow to generate the shells with a gradient, and then Zn mainly
grew.
[0067] Referring to FIG. 7, it can be confirmed that when the
quantum-dot shells with a composition gradient began to form, the
luminous efficiency of the quantum dots began to gradually
increase. The luminous efficiency reached 80% in 5 minutes after
the cores and the shells with a composition gradient began to
form.
[0068] While the invention has been shown and described with
reference to certain exemplary embodiments thereof, it will be
understood by one skilled in the art that various changesin form
and details may be made therein without departing from the spirit
and scope of the invention as defined by the appended claims.
* * * * *