(12) United States Patent
`Bawendi et al.
`
`I lllll llllllll Ill lllll lllll lllll lllll lllll 111111111111111111111111111111111
`US006501091Bl
`US 6,501,091 Bl
`Dec. 31, 2002
`
`(10) Patent No.:
`(45) Date of Patent:
`
`(54) QUANTUM DOT WHITE AND COLORED
`LIGHT EMITTING DIODES
`
`(75)
`
`Inventors: Moungi G. Bawendi, Boston, MA
`(US); Jason Heine, Lincoln, MA (US);
`Klavs F. Jensen, Lincoln, MA (US);
`Jeffrey N. Miller, Los Altos Hills, CA
`(US); Ronald L. Moon, Atherton, CA
`(US)
`
`(73) Assignees: Massachusetts Institute of
`Technology, Cambridge, MA (US);
`Hewlett-Packard Company, Palo Alto,
`CA(US)
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 0 days.
`
`(21) Appl. No.: 09/167,795
`
`(22) Filed:
`
`Oct. 7, 1998
`
`FOREIGN PATENT DOCUMENTS
`
`WO
`
`96/10282
`
`4/1996
`
`OTHER PUBLICATIONS
`
`Margaret et al., Synthesis and characterization of strongly
`luminescing ZnS-capped CdSe nanocrystals, J. Phys.
`Chem., vol. 100, No. 2, 1996.*
`Kortan, et al., "Nucleation and Growth of CdSe on ZnS
`Quantum Crystallite Seeds, and Vice Versa, in Inverse
`Micelle Media," J. Am. Chem. Soc. 112:1327-1332, 1990.
`Murray, et al., "Synthesis and Characterization of Nearly
`Monodisperse CdE(E=S, Se, Te) Semiconductor Nanocrys(cid:173)
`tallites," J. Am. Chem. Soc. 115(19):8706-8715, 1993.
`Colvin, et al., "Light-emitting diodes made from cadmium
`selenide nanocrystals and a semiconducting polymer,"
`Nature 370(6488):354--357, Aug. 4 1994.
`Lawless, et al., "Bifunctional Capping of CdS Nanoparticles
`and Bridging to Ti02," J. Phys. Chem. 99:10329-10335,
`1995.
`
`(List continued on next page.)
`
`Related U.S. Application Data
`(60) Provisional application No. 60/092,120, filed on Apr. 1,
`1998.
`
`Primary Examiner-Minh Loan Tran
`(74) Attorney, Agent, or Firm-Elizabeth E. Nugent;
`Choate, Hall & Stewart
`
`(51)
`
`Int. CI.7 .......................... HOlL 29/06; HOlL 33/00
`
`(57)
`
`ABSTRACT
`
`(52) U.S. Cl. ............................. 257/14; 257/89; 257/98;
`257/100
`
`(58) Field of Search .............................. 257/79, 89, 14,
`257/98, 100
`
`(56)
`
`References Cited
`
`U.S. PATENT DOCUMENTS
`* 11/1993 Hakimi et al. ................ 372/39
`3/1994 Chapple-Sokol et al.
`..... 257 /17
`10/1994 Chapple-Sokol et al.
`... 437/106
`6/1995 Bhargava ................. 250/488.1
`4/1996 Alivisatos et al. .......... 423/299
`12/1996 Huston et al.
`........... 250/483.1
`3/1997 Taira .......................... 395/800
`5/1998 Alivisatos et al. ............ 257 /64
`
`5,260,957 A
`5,293,050 A
`5,354,707 A
`5,422,489 A
`5,505,928 A
`5,585,640 A
`5,613,140 A
`5,751,018 A
`
`An electronic device comprising a population of quantum
`dots embedded in a host matrix and a primary light source
`which causes the dots to emit secondary light of a selected
`color, and a method of making such a device. The size
`distribution of the quantum dots is chosen to allow light of
`a particular color to be emitted therefrom. The light emitted
`from the device may be of either a pure (monochromatic)
`color, or a mixed (polychromatic) color, and may consist
`solely of light emitted from the dots themselves, or of a
`mixture of light emitted from the dots and light emitted from
`the primary source. The dots desirably are composed of an
`undoped semiconductor such as CdSe, and may optionally
`be overcoated to increase photoluminescence.
`
`29 Claims, 2 Drawing Sheets
`
`(1 of 2 Drawing Sheet(s) Filed in Color)
`
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`Nanoco Technologies, Ltd
`EXHIBIT 1001
`
`

`
`US 6,501,091 Bl
`Page 2
`
`OlliER PUBLICATIONS
`
`from CdSe
`"Electro luminescence
`Dabbousi, et aI.,
`quantum-dot/polymer composites," Appl. Phys. Lett.
`66(11):1316-1318, Mar. 13 1995.
`Alivisatos, "Perspectives on the Physical Chemistry of
`Phys.
`Chern.
`Semiconductor
`Nanocrystals,"
`J.
`1996(100):13226-13239, 1996.
`Danek, et aI., "Synthesis of Luminescent Thin-Film CdSe/
`ZnSe Quantum Dot Composites Using CdSe Quantum Dots
`Passivated with an Overlayer of ZnSe," Chern. Mater.
`8(1):173-180, 1996.
`Matsumoto, et aI., "Preparation of Monodisperse CdS
`Nanocrystals by Size Selective Photocorrosion," J. Phys.
`Chern 100(32):13781-13785, 1996.
`Hines, et aI., "Synthesis and Characterization of Strongly
`Luminescing ZnS-Capped CdSe Nanocrystals," J. Phys.
`Chern. 100:468-471, Jan. 1996.
`Empedocles, et aI., "Photoluminescence Spectroscopy of
`Single CdSe Nanocrystallite Quantum Dots," Phys. Rev.
`Lett. 77(18):3873-3876, Oct. 1996.
`
`Nirmal, et aI., "Fluorescence Intermittency in single Cad(cid:173)
`mium Selenide Nanocrystals," Nature 383:802-804, Oct.
`1996.
`Diehl, "Fraunhofer LUCOLEDs to replace lamps," III-Vs
`Rev. 10(1), 1997.
`Empedocles, et aI., "Quantum-Confined Stark Effect in
`Single CdSe Nanocrystallite Quantum Dots," Science
`278:2114-2117, Dec. 1997.
`Kuno, et aI., "The band edge luminescence of surface
`modified CdSe nanocrystallites: Probing the luminescing
`state," J. Chern. Phys. 106(23):9869-9882, Jun. 1997.
`Dabbousi, et aI., "(CdSe)ZnS core-shell quantum dots:
`synthesis and characterization of a size series of highly
`luminescent nanocrystallites," J. of Phys. Chern. B
`1Dl( 46):9463-9475, Nov. 13 1997.
`Guha, et aI., "Hybrid organic-inorganic semiconductor(cid:173)
`Appl.
`Phys.
`based
`light--emitting
`diodes,"
`J.
`82(8):4126-4128, Oct. 15 1997.
`
`* cited by examiner
`
`2 of 10
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`

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`u.s. Patent
`FIG. 1
`
`Dec. 31, 2002
`
`Sheet 1 of 2
`
`US 6,501,091 Bl
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`
`Dec. 31, 2002
`
`Sheet 2 of 2
`
`US 6,501,091 Bl
`
`4 of 10
`
`

`
`1
`QUANTUM DOT WHITE AND COLORED
`LIGHT EMITTING DIODES
`
`US 6,501,091 B1
`
`This application claims benefit of U.S. Provisional
`Application No. 60/092,120, filed Apr. 1, 1998, which is 5
`incorporated herein by reference in its entirety.
`This invention was made with government support under
`Contract Number 9400034 awarded by the National Science
`Foundation. The government has certain rights in the inven(cid:173)
`tion.
`A portion of the disclosure of this patent document
`contains material which is subject to copyright protection.
`The copyright owner has no objection to the facsimile
`reproduction by anyone of the patent document or the patent
`disclosure, as it appears in the Patent and Trademark Office 15
`patent file or records, but otherwise reserves all copyrights
`whatsoever.
`
`FIELD OF THE INVENTION
`
`The present invention relates to the use of quantum dots
`in light emitting diodes. The invention further relates to a
`light emitting diode which emits light of a tailored spectrum
`of frequencies.
`
`BACKGROUND OF THE INVENTION
`
`2
`a thin film of organic dyes, but efficiencies are low (see, for
`example, Guha, et aI., J Appl. Phys. 82(8):4126-4128, Oct.
`1997; III-Vs Review 10(1):4, 1997).
`It has also been proposed to produce LEDs of varying
`colors by the use of quantum dots. Semiconductor nanoc(cid:173)
`rystallites (quantum dots) whose radii are smaller than the
`bulk exciton Bohr radius constitute a class of materials
`intermediate between molecular and bulk forms of matter.
`Quantum confinement of both the electron and hole in all
`10 three dimensions leads to an increase in the effective band
`gap of the material with decreasing crystallite size.
`Consequently, both the optical absorption and emission of
`quantum dots shift to the blue (higher energies) as the size
`of the dots gets smaller. It has been found that a CdSe
`quantum dot, for example, can emit light in any
`monochromatic, visible color, where the particular color
`characteristic of that dot is dependent only on its size.
`Currently available light-emitting diodes and related
`devices which incorporate quantum dots use dots which
`20 have been grown epitaxially on a semiconductor layer. This
`fabrication technique is suitable for the production of infra(cid:173)
`red LEDs, but LEDs in higher-energy colors have not been
`achieved by this method. Further, the processing costs of
`epitaxial growth by currently available methods (molecular
`25 beam epitaxy and chemical vapor deposition) are quite high.
`Colloidal production of dots is a much more inexpensive
`process, but these dots have generally been found to exhibit
`low quantum efficiencies, and thus have not previously been
`considered suitable for incorporation into light-emitting
`30 diodes.
`A few proposals have been made for embedding colloi-
`dally produced quantum dots in an electrically conductive
`layer, in order to use the electroluminescence of these dots
`for an LED, but such devices require a transparent, electri(cid:173)
`cally conductive host matrix, which severely limits the
`available materials for producing LEDs by this method.
`Available host matrix materials are often themselves light(cid:173)
`emitting, which may limit the achievable colors using this
`40 method.
`
`Light emitting diodes (LEDs) are ubiquitous to modern
`display technology. More than 30 billion chips are produced
`each year and new applications, such as automobile lights
`and traffic signals, continue to grow. Conventional diodes
`are made from inorganic compound semiconductors, typi(cid:173)
`cally AlGaAs (red), AlGaInP (orange-yellow-green), and
`AlGaInN (green-blue). These diodes emit monochromatic
`light of a frequency corresponding to the band gap of the
`compound semiconductor used in the device. Thus, conven- 35
`tional LEDs cannot emit white light, or indeed, light of any
`"mixed" color, which is composed of a mixture of frequen(cid:173)
`cies. Further, producing an LED even of a particular desired
`"pure" single-frequency color can be difficult, since excel(cid:173)
`lent control of semiconductor chemistry is required.
`LEDs of mixed colors, and particularly white LEDs, have
`many potential applications. Consumers would prefer white
`light in many displays currently having red or green LEDs.
`White LEDs could be used as light sources with existing
`color filter technology to produce fall color displays. 45
`Moreover, the use of white LEDs could lead to lower cost
`and simpler fabrication than red-green-blue LED technol(cid:173)
`ogy. There is currently one technology for producing white
`LEDs, which combines a blue LED with a yellow phosphor
`to produce white light. However, color control is poor with 50
`this technology, since the colors of the LED and the phos(cid:173)
`phor cannot be varied. This technology also cannot be used
`to produce light of other mixed colors.
`It has also been proposed to manufacture white or colored
`LEDs by combining various derivatives of photolumines(cid:173)
`cent polymers such as poly(phenylene vinylene) (PPVs).
`One device which has been proposed involves a PPV coating
`over a blue GaN LED, where the light from the LED
`stimulates emission in the characteristic color of the PPV, so
`that the observed light is composed of a mixture of the
`characteristic colors of the LED and the PPY. However, the
`maximum theoretical quantum yield for PPV-based devices
`is 25%, and the color control is often poor, since organic
`materials tend to fluoresce in rather wide spectra.
`Furthermore, PPVs are rather difficult to manufacture
`reliably, since they are degraded by light, oxygen, and water.
`Related approaches use blue GaN-based LEDs coated with
`
`SUMMARY OF THE INVENTION
`
`In one aspect, this invention comprises an electronic
`device, comprising a solid-state light source, and a popula(cid:173)
`tion of quantum dots disposed in a host matrix. The quantum
`dots are characterized by a band gap smaller than the energy
`of at least a portion of the light from the light source. The
`matrix is disposed in a configuration that allows light from
`the source to pass therethrough. When the host matrix is
`irradiated by light from the source, that light causes the
`quantum dots to photo luminesce secondary light. The color
`of this light is a function of the size of the quantum dots.
`In one embodiment of this aspect, the quantum dots
`comprise CdS, CdSe, CdTe, ZnS, or ZnSe and may option-
`55 ally be overcoated with a material comprising ZnS, ZnSe,
`CdS, CdSe, CdTe, or MgSe. The quantum dots may be
`further coated with a material having an affinity for the host
`matrix. The host matrix may be a polymer such as
`polystyrene, polyimide, or epoxy, a silica glass, or a silica
`60 gel. The primary light source may be a light-emitting diode,
`a solid-state laser, or a solid-state ultraviolet source. The
`color of the device is determined by the size distribution of
`the quantum dots; this distribution may exhibit one or more
`narrow peaks. The quantum dots, for example, may be
`65 selected to have no more than a 10% rms deviation in the
`size of the dots. The light may be of a pure color, or a mixed
`color, including pure white.
`
`5 of 10
`
`

`
`3
`In a related aspect, the invention comprises a method of
`producing an electronic device as described above. In this
`method, a population of quantum dots is provided, and these
`dots are dispersed in a host matrix. A solid-state light source
`is then provided to illuminate the dots, thereby causing them 5
`to photo luminesce light of a color characteristic of their size
`distribution. The dots may be colloidally produced (i.e., by
`precipitation and/or growth from solution), and may com(cid:173)
`prise CdS, CdSe, CdTe, ZnS, or ZnSe. They may further
`comprise an overcoat comprising ZnS, ZnSe, CdS, CdSe, 10
`CdTe, or MgSe. The host matrix may be any material in
`which quantum dots may be dispersed in a configuration in
`which they may be illuminated by the primary light source.
`Some examples of host matrix materials are polymers such
`as polystyrene, polyimide, or epoxy, silica glasses, or silica 15
`gels. Any solid-state light source capable of causing the
`quantum dots to photo luminesce may be used; some
`examples are light-emitting diodes, solid-state lasers, and
`solid-state ultraviolet sources.
`It may be desirable to tailor the size distribution of the
`quantum dots in order to tailor the color of light which is
`produced by the device. In one embodiment, the dots exhibit
`no more than a 10% rms deviation in diameter. The light
`may be of a pure color (corresponding to a monodisperse
`size distribution of quantum dots), or a mixed color 25
`(corresponding to a polydisperse size distribution of quan(cid:173)
`tum dots) including white.
`In a further aspect, the invention comprises a quantum dot
`colloid, in which quantum dots are disposed in a noncon(cid:173)
`ductive host matrix. The quantum dots may be coated with
`a material having an affinity for the host matrix. When
`illuminated by a primary source of light of a higher energy
`than the band gap energy of the dots, the quantum dots
`photoluminesce in a color characteristic of their size distri-
`bution.
`In one embodiment, the dots comprise CdS, CdSe, CdTe,
`ZnS, or ZnSe, optionally overcoated with a material com(cid:173)
`prising ZnS, ZnSe, CdS, CdSe, CdTe, or MgSe. The non(cid:173)
`conductive host matrix may be a polymer such as 40
`polystyrene, polyimide, or epoxy, a silica glass, or a silica
`gel. In one embodiment, the dots are coated with a monomer
`related to a polymer component of the host matrix. The dots
`may be selected to have a size distribution exhibiting an rms
`deviation in diameter of less than 10%; this embodiment will 45
`cause the dots to photoluminesce in a pure color.
`A related aspect of the invention comprises a prepolymer
`colloid. In this aspect, the invention comprises a liquid or
`semisolid precursor material, with a population of quantum
`dots disposed therein. The colloid is capable of being
`reacted, for example by polymerization, to form a solid,
`transparent, nonconductive host matrix. The quantum dots
`may have been coated with a material having an affinity for
`the precursor material. The precursor material may be a
`monomer, which can be reacted to form a polymer. The 55
`quantum dots may comprise CdS, CdSe, CdTe, ZnS, or
`ZnSe, and may optionally be overcoated with a material
`comprising ZnS, ZnSe, CdS, CdSe, CdTe, or MgSe. The
`dots may be selected to have a size distribution having an
`rms deviation in diameter of less than 10%.
`In yet another aspect, the invention comprises a method of
`producing light of a selected color. The method comprises
`the steps of providing a population of quantum dots disposed
`in a host matrix, and irradiating the host matrix with a
`solid-state source of light having an energy high enough to
`cause the quantum dots to photo luminesce. The quantum
`dots may comprise CdS, CdSe, CdTe, ZnS, or ZnSe, and
`
`35
`
`BRIEF DESCRIPTION OF THE DRAWING
`
`The file of this patent contains at least one drawing
`executed in color. Copies of this patent with color
`drawings(s) will be provided by the Patent and Trademark
`Office request and payment of the necessary fee.
`The invention is described with reference to the several
`figures of the drawing, which are presented for the purpose
`of illustration only, and in which,
`FIG. 1 represents one embodiment of an LED according
`to the invention;
`FIG. 2 represents another embodiment of an LED accord(cid:173)
`ing to the invention; and
`FIG. 3 is a color photograph of several suspensions of
`quantum dots in hexane, illustrating the wide range of colors
`50 that can be achieved by the methods of the invention.
`
`US 6,501,091 B1
`
`4
`may further have an overcoating compnsmg ZnS, ZnSe,
`CdS, CdSe, CdTe, or MgSe. The host matrix may comprise
`polymers such as polystyrene, polyimide, or epoxy, silica
`glasses, or silica gels.
`The host matrix containing the quantum dots may be
`formed by reacting a precursor material having quantum
`dots disposed therein (for example by polymerization).
`Alternatively, two or more precursor materials may be
`provided, each having a different size distribution of quan(cid:173)
`tum dots disposed therein. These precursors may be mixed
`and reacted to form a host matrix, or alternatively, they may
`be layered to form a host matrix having different size
`distributions of quantum dots in different layers.
`As used herein, the phrase "colloidally grown" quantum
`dots refers to dots which have been produced by precipita(cid:173)
`tion and/or growth from a solution. A distinction between
`these dots and quantum dots epitaxially grown on a substrate
`is that colloidally grown dots have a substantially uniform
`surface energy, while epitaxially grown dots usually have
`20 different surface energies on the face in contact with the
`substrate and on the remainder of the dot surface.
`As used herein, the terms "pure" or "monochromatic"
`color refers to a color which is composed of light of a single
`frequency. A "mixed" or "polychromatic" color refers to a
`color which is composed of light of a mixture of different
`frequencies.
`As used herein, a "monomer" is a substance which can be
`polymerized according to techniques known in the art of
`30 materials science, and may include oligomers. A "related
`monomer" of a polymer is a component monomer of the
`polymer, or a compound capable of being incorporated into
`the backbone of the polymer chain.
`
`DETAILED DESCRIPTION OF PREFERRED
`EMBODIMENTS
`LEDs of almost any color visible to the human eye can be
`produced by the techniques of the current invention, using
`only a single undoped semiconductor material for the quan(cid:173)
`tum dots. Embodiments of the invention are illustrated in
`FIGS. 1 and 2. In general terms, the invention comprises
`providing a primary light source 10, for example an LED, a
`60 solid-state laser, or a microfabricated UV source. The pri(cid:173)
`mary light source 10 is desirably chosen so that its energy
`spectrum includes light of higher energies than the desired
`LED color. The primary light source is disposed so as to
`irradiate a host matrix 12 containing a population of quan-
`65 tum dots 14. The host matrix 12 may be any material at least
`partially transparent to visible light in which quantum dots
`can be disposed; suitable host matrices are discussed further
`
`6 of 10
`
`

`
`US 6,501,091 B1
`
`5
`below. The host matrix 12 desirably contains a dispersion of
`isolated quantum dots 14, where the dots have been size
`selected so as to produce light of a given color. Other
`configurations of quantum dots disposed in a host matrix,
`such as, for example, a two-dimensional layer on a substrate 5
`with a polymer overcoating, are also contemplated within
`the scope of the invention. Techniques for producing dots
`fluorescing brightly in a very narrow spectral distribution of
`a selected color are discussed further below and in copend(cid:173)
`ing U.S. patent application Ser. No. 08/969,302, "Highly 10
`Luminescent Color Selective Materials," Bawendi et aI, filed
`Nov. 13, 1997, the teachings of which are incorporated
`herein by reference; such techniques allow particularly fine
`color control of the final LED. However, other techniques
`for producing quantum dots and disposing them in a host 15
`matrix are also encompassed within the scope of the inven(cid:173)
`tion.
`The primary light source 10 and the size distribution of
`the quantum dots 12 are chosen in such a way that the
`radiation emitted from the device is of the desired color. The 20
`invention may be constructed with a large number of quan(cid:173)
`tum dots, whereby substantially all light from the primary
`source is absorbed and the finally emitted radiation is
`produced only by photoluminescence of the quantum dots,
`or with a smaller number of quantum dots, whereby the light 25
`emerging from the device consists of a mixture of unab(cid:173)
`sorbed primary light and of secondary light produced by
`photoluminescence of the quantum dots. A very wide range
`of both pure and mixed colors can be produced by a device
`constructed according to the principles of the invention. For 30
`example, cadmium selenide quantum dots can be produced
`which will emit in any color visible to the human eye, so that
`in combination with a source of higher frequency than the
`highest frequency of the desired color, these dots can be
`tailored to produce visible light of any spectral distribution. 35
`FIG. 3 shows several suspensions of CdSe quantum dots
`made according to the method of U.S. application Ser. No.
`08/969,302, and illustrates the very wide range of colors
`which can be achieved using the photoluminescence of these
`materials. The photoluminescent peaks in these solutions are 40
`(from left to right) (a) 470 nm, (b) 480 nm, (c) 520 nm, (d)
`560 nm, (e) 594 nm, and (t) 620 nm. The solutions are being
`irradiated by an ultraviolet lamp emitting 356 nm ultraviolet
`light.
`It is usually desirable that the each dot be isolated within
`the host matrix, particularly when the device is intended to
`emit light of a mixed color. When quantum dots of different
`sizes are in close contact, the larger dot, which has a lower
`characteristic emission frequency, will tend to absorb a large
`fraction of the emissions of the smaller dot, and the overall
`energy efficiency of the diode will be reduced, while the
`color will shift towards the red.
`For the particular embodiment of a white LED, such an
`LED may be produced by combining a combination of sizes
`of photoluminescent quantum dots with a standard blue
`LED. Referring to FIG. 1, a blue LED 10, for example of the
`AlGaInN type, is used to provide primary light. This light
`passes through a quantum dot layer or layers, where these
`layers comprise quantum dots adapted to luminesce in a
`lower-energy range than the blue LED, embedded typically
`in a polymeric matrix. In the embodiment shown in FIG. 1,
`the primary light first passes through a layer 16 of quantum
`dots 18 of a material and size adapted to emit green
`secondary light. The primary light which has not been
`absorbed by the first layer and the secondary light then pass
`through a second layer 20 of quantum dots 22 of a material
`and size adapted to emit red secondary light. Once the light
`
`6
`has passed through this second layer, it will be composed of
`a mix of unabsorbed blue primary light, green secondary
`light, and red secondary light, and hence will appear white
`to the observer. The relative amplitudes of the red, green,
`and blue components of the light can be controlled by
`varying the thickness and quantum dot concentrations of the
`red and green layers to produce an LED of a desired color.
`In another preferred embodiment, the red-emitting quan(cid:173)
`tum dots 22 and greenemitting quantum dots 18 can be
`mixed within a single layer 12, as shown in FIG. 2. The color
`can be controlled by varying the relative concentrations of
`the different sizes of quantum dots and the thickness of the
`layer.
`In yet another embodiment, the primary light source may
`be a solid state violet or ultraviolet source, such as a solid
`state laser or a microfabricated UV source. In this
`embodiment, the quantum dot layer(s) may comprise quan(cid:173)
`tum dots emitting in a spectral range ranging from red to
`violet. By controlling the size distribution of the quantum
`dots, the spectral distribution of the resulting light may be
`controlled.
`When it is desired to produce an LED of a particular color,
`rather than a white LED, this also may be accomplished by
`the practice of the invention. Although the invention is
`expected to be particularly useful for the manufacture of
`LEDs producing polychromatic light (mixed colors), which
`are difficult to produce by traditional methods, LEDs pro(cid:173)
`ducing monochromatic light (pure colors) may also be
`produced by the practice of the invention. This may be
`desirable for purposes of ease of manufacturing, since
`substantially the same set of equipment is required to
`produce LEDs of almost any visible color, whether pure or
`mixed.
`The perception of color by the human eye is well
`understood, and formulae for mixing pure colors to produce
`any desired mixed color can be found in a number of
`handbooks. The color of light produced by a particular size
`and composition of quantum dot may also be readily cal(cid:173)
`culated or measured by methods which will be apparent to
`those skilled in the art. As an example of these measurement
`techniques, the band gaps for quantum dots of CdSe of sizes
`ranging from 12A. to 115A. are given in Murray, et aI., JAm.
`Chern. Soc. 115:8706 (1993), incorporated herein by refer-
`45 ence. These techniques allow ready calculation of an appro(cid:173)
`priate size distribution of dots and choice of primary light
`source to produce an LED of any desired color.
`When a white diode is desired, an appropriate mix of
`quantum dot sizes may be used. A white light which appears
`50 "clean" to the observer may be achieved, for example, by
`tailoring the spectral distribution to match a black body
`distribution.
`When a colored LED such as the blue AlGaInN LED
`described above is used as the primary light source, the color
`55 of that LED may or may not be included in the final
`spectrum produced by the device according to the invention,
`depending on the concentration of the quantum dots. If a
`sufficiently large number of quantum dots is provided, the
`dots will absorb substantially all of the primary light, and
`60 only secondary light in the characteristic colors of the dots
`will be observed. If a smaller number of quantum dots is
`provided, a significant quantity of primary light may be
`mixed with the secondary light emitted by the dots.
`The host matrix will typically be a polymer, a silica glass,
`65 or a silica gel, but any material which is at least somewhat
`transparent to the light emitted by the quantum dots and in
`which quantum dots can be dispersed may serve as the host
`
`7 of 10
`
`

`
`US 6,501,091 B1
`
`7
`matrix. An advantage of the present invention compared to
`light-emitting diodes based on electro luminescence of quan(cid:173)
`tum dots, rather than photoluminescence, is that the host
`matrix need not be electrically conductive. Electrolumines(cid:173)
`cent quantum dot LEDs require a transparent, electrically 5
`conductive material to serve as the host matrix. Such mate(cid:173)
`rials are rare, compared to the very large number of trans(cid:173)
`parent insulator materials available for use with the present
`invention. Suitable host matrix materials for the devices
`described herein include many inexpensive and commonly 10
`available materials, such as polystyrene, epoxy, polyimides,
`and silica glass.
`A further advantage of the present invention is the manu(cid:173)
`facturing flexibility afforded by the use of multiple popula(cid:173)
`tions of quantum dots to achieve both pure and mixed colors
`of light. "Stock" solutions of different sizes of dots sus- 15
`pended in a monomer or other precursor material can be
`maintained, and mixed in varying amounts to produce
`almost any desired color. For example, three suspensions of
`CdSe quantum dots in a liquid monomer such as styrene
`could be produced: a first suspension of dots of approxi- 20
`mately 5.5 nm diameter (which will luminesce in the red), a
`second suspension of dots of approximately 4.0 nm diameter
`(which will luminesce in the green), and a third suspension
`of dots of approximately 2.3 nm diameter (which will
`luminesce in the blue). These suspensions function as a kind 25
`of "light paint"; by mixing varying amounts of these three
`suspensions, and polymerizing the resulting mixture, LEDs
`of a very wide range of colors can be produced using the
`same manufacturing techniques, varying only the starting
`materials.
`It will usually be found to be necessary to coat colloidally
`produced dots with a coating which enables them to be
`dispersed in the host matrix without flocculation. In the case
`of dispersal in a polymeric matrix, an oligomer related to the
`polymer, with a Lewis base at the end of the oligomer which 35
`is bound to the dots, has been found to allow good mixing
`of dots into a monomer solution for polymerization. Par(cid:173)
`ticular cases of this type of coating may be found in the
`Examples. In the case of dispersal into a silica glass or gel,
`any overcoating which will bind at one end to the dot, and 40
`whose other end has an affinity for the matrix, may be used.
`A number of methods of producing quantum dots are
`known in the art. Any method of producing quantum dots
`which will fluoresce in the desired colors may be used in the
`practice of the invention, but it has been found that the 45
`particular methods described in U.S. application Ser. No.
`08/969,302 can be used to produce devices with excellent
`brightness and color control. That application discloses a
`method of overcoating dots composed of CdS, CdSe, or
`CdTe with ZnS, ZnSe, or mixtures thereof. Before 50
`overcoating, the quantum dots are prepared by a method
`yielding a substantially monodisperse size distribution,
`which is described in Murray, et aI., J Am. Chern. Soc.
`115:8706 (1993). An overcoat of a controlled thickness can
`then be applied by controlling the duration and temperature 55
`of growth of the coating layer. The monodispersity of the
`core dots ensures that the dots will radiate substantially in a
`pure color, while the overcoat provides a much improved
`quantum efficiency, allowing the dots to fluoresce more
`brightly than do uncoated dots.
`The above method can be used to prepare several separate
`populations of quantum dots, where each population exhib-
`its photoluminescence in a different pure color. By mixing
`the populations so prepared, a device which fluoresces in
`any desired mixed color, including white, may be produced. 65
`The overcoating on the dots allows the device to produce a
`brighter light than would be possible using uncoated dots.
`
`8
`EXAMPLE 1
`Quantum Dots in Polystyrene
`A green LED has been constructed according to the
`principles of the invention described above. The quantum
`dots used to construct this diode were composed of a CdSe
`core and a ZnS shell. The absorption and luminescence
`properties of the quantum dots were primarily determined by
`the size of the CdSe core. The ZnS shell acted to confine
`electrons and holes in the core and to electronically and
`chemically passivate the quantum dot surface. Both the core
`and shell were synthesized using wet chemistry techniques
`involving formation of CdSe or ZnS from precursors added
`to a hot organic liquid.
`CdSe Core Synthesis
`16 mL of trioctylphosphine (TOP), 4 mL of 1 M trio(cid:173)
`ctylphosphine selenide (TOPSe) in TOP, and 0.2 mL dim(cid:173)
`ethylcadmium were mix