Gas phase enhancement of emission color quality in solid state LEDs

Abstract

Light-emitting materials are made from a porous light-emitting semiconductor having quantum dots (QDs) disposed within the pores. According to some embodiments, the QDs have diameters that are essentially equal in size to the width of the pores. The QDs are formed in the pores by exposing the porous semiconductor to gaseous QD precursor compounds, which react within the pores to yield QDs. According to certain embodiments, the pore size limits the size of the QDs produced by the gas-phase reactions. The QDs absorb light emitted by the light-emitting semiconductor material and reemit light at a longer wavelength than the absorbed light, thereby down-converting light from the semiconductor material.

Claims

1. A method for synthesizing quantum dots (QDs) in a light-emitting semiconductor material, the method comprising: flowing gaseous QD precursors through first pores in a semiconductor material to effect reaction of the QD precursors in the absence of liquid solvent and grow a plurality of first QDs in the first pores having diameters essentially equal to the first pore diameters; and flowing gaseous QD precursors through second pores in a semiconductor material to effect reaction of the QD precursors in the absence of liquid solvent and grow plurality of second QDs in the second pores having diameters essentially equal to the second pore diameters; wherein the second pores have diameters larger than the diameters of the first pores.

2. The method of claim 1, wherein the first pores and the second pores are about 1 nm to about 20 nm in diameter.

3. The method of claim 1, wherein the light-emitting semiconductor material comprises as GaN, AlGaAs, AlGaInP, or AlGaInN, or any derivatives thereof.

4. The method of claim 1, wherein the one or both of the first QDs and the second QDs comprise a semiconductor material selected from CdS, CdSe, ZnS, ZnSe InP, GaP Cd.sub.3P.sub.2 and In.sub.2Se.sub.3.

5. The method of claim 1, wherein the first QDs are green light-emitting QDs.

6. The method of claim 1, wherein the second QDs are red light-emitting QDs.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The foregoing summary, as well as the following detailed description, will be better understood when read in conjunction with the appended drawings. For the purpose of illustration only, there is shown in the drawings certain embodiments. It's understood, however, that the inventive concepts disclosed herein are not limited to the precise arrangements and instrumentalities shown in the figures.

(2) FIGS. 1A-1C illustrate pores etched into a semiconductor material.

(3) FIG. 2 shows the formation of QDs within pores of a semiconductor material from gaseous precursors provided with counter current gas streams.

(4) FIG. 3 shows the formation of QDs within pores of a semiconductor material from gaseous precursors provided with parallel gas streams.

(5) FIG. 4 illustrates an apparatus for providing counter-current flow of QD precursor gases.

(6) FIG. 5 illustrates an apparatus for providing parallel flow of QD precursor gases.

(7) FIG. 6 is a diagram comparing the relative size of an oxygen molecule to gas-phase precursor molecules of QDs.

(8) FIGS. 7A-7C illustrate the formation of QDs within pores selectively etched into a semiconductor material.

(9) FIG. 8 illustrates a conventional LED having nanoparticles embedded within a porous n-GaN layer.

DESCRIPTION

(10) It should be understood that the inventive concepts set forth herein are not limited in their application to the construction details or component arrangements set forth in the following description or illustrated in the drawings. It should also be understood that the phraseology and terminology employed herein are merely for descriptive purposes and should not be considered limiting. It should further be understood that any one of the described features may be used separately or in combination with other features. Other invented systems, methods, features, and advantages will be or become apparent to one with skill in the art upon examining the drawings and the detailed description herein. It is intended that all such additional systems, methods, features, and advantages be protected by the accompanying claims.

(11) The present disclosure generally relates to light emitting devices using a solid-state LED material into which pores have been etched. QDs are synthesized within those pores. When the LED material emits light (i.e., primary light) the QDs absorb some of that light and reemit light having a color determined by the size of the QDs (i.e., secondary light). The light emitted from the light-emitting device therefore includes a combination of the primary and secondary light. Various combinations of LED materials and QD materials and sizes can be used to obtain white light or to obtain other blends of light.

(12) According to some embodiments, the QD materials are synthesized within the pores of the LED material via gas phase reactions. As explained in more detail below, the gas phase QD precursor material diffuse into the pores of the LED material where they react to form QDs. The size of the QDs may be limited by the size of the pores in which the QDs form. In this way, the pores may be thought of as providing a template for QD formation. Since the color of light that a QD emits depends on the size of the QD, the color of emitted light can be tuned by controlling the size of the pores in which the QDs form.

(13) Generally, any solid-state LED semiconductor material can be used. Examples include, but are not restricted to, inorganic solid-state compound semiconductors, such as GaN (blue), AlGaAs (red), AlGaInP (orange-yellow-green), AlGaInN (green-blue), or any derivatives thereof. The characteristic emission colors of each material are provided in parentheses. The examples discussed in this disclosure primarily concern GaN, as it is common to seek to down-convert light from blue-emitting GaN.

(14) Pores can be etched in the solid-state LED semiconductor material using any means known in the art. Examples of controlled etching are contained in Cuong Dang et al., A wavelength engineered emitter incorporating CdSe-based colloidal quantum dots into nanoporous InGaN/GaN multiple quantum well matrix, Phys. Status Solidi, No. 7-8, 2337-339 (2011); Dang et al., A wafer-level integrated white-light-emitting diode incorporating colloidal quantum dots as a nanocomposite luminescent material, Adv. Materials, No. 24, 5915-18 (2012); and Chen et al., High reflectance membrane-based distributed Bragg reflectors for GaN photonics, App. Phys. Lett., No. 101, 221104 (2012). The reader is referred to those references for details concerning the etching of the LED semiconductor material. Generally, the LED semiconductor material is etched using an electrochemical method, for example, anodic etching in an oxalic acid electrolyte. The pore size and concentration can be controlled as a function of the applied voltage. Other methods of etching, such as acid etching and/or inductively coupled plasma-reactive ion (ICP-RI) etching may be used. It is found that the etching technique does not impair the semiconductor material's carrier transport and recombination capability. FIGS. 1A-1B, by way of example only, illustrate semiconductor materials having etched pores.

(15) In some embodiments, the etching technique produces pores having approximately the same diameter. For example, referring to FIG. 1A, the pores 100 can be etched to a target pore size. In one embodiment, the target pore size may be between approximately 2 nm and 10 nm. The pore size can be tuned to a uniform diameter that accommodates growth of both red-emitting QDs and green-emitting QDs. For example, the pore size can be tuned to a single diameter that accommodates growth of red-emitting Group III-V based QDs (e.g. InP, and including graded dots and alloys), and green-emitting CdSe QDs. In one embodiment, a semiconductor material for a blue-LED (e.g., GaN) is selectively etched as in FIG. 1A to accommodate growth of both red and green QDs at a level that effectively down-converts the LED to produce white light emissions.

(16) Alternatively, the semiconductor material can be selectively etched to include pores of various sizes, as shown in FIG. 1B, 110, 120. For example, a semiconductor material for a blue-LED (e.g., GaN) can be selectively etched as in FIG. 1B to accommodate growth of both red and green QDs at a level that effectively down-converts the LED to produce white light emissions.

(17) In other embodiments, as illustrated in FIG. 1C, the etching technique produces pores 130 having a small diameter on the top side of a semiconductor material, and pores 140 having a large diameter on the bottom side of the semiconductor material. For example, the bottom side of the semiconductor material can be etched first to a target depth (e.g., halfway) and pore size (e.g., larger diameter). Etching time can control the pore depth, while changing the bias voltage can control the pore size. After etching large pores 140 into the bottom layer, the semiconductor material can be turned over, and small pores 130 can be selectively etched in the top layer to a target depth (e.g., halfway) and pore size (e.g., smaller diameter). Again, etching time and bias voltage can be used to control pore depth and size. According to some embodiments, the small diameter pores 130 are etched to a size that can accommodate growth of green QDs and the large diameter pores 140 are etched to a size that can accommodate growth of red QDs. This pore architecture positions the red QDs below the green QDs to prevent reabsorption of the secondary light emitted by the QDs. In one embodiment, a semiconductor material for a blue-LED (e.g., GaN) is selectively etched as in FIG. 1C to accommodate growth of both red and green QDs at a level that effectively down-converts the LED light to produce white light.

(18) Once the LED semiconductor material is etched to provide pores, QDs are formed within those pores by reacting gas phase QD precursor compounds together within the pores. The precursors may be used to synthesize QDs including, but not restricted to, the following materials: Group II-VI nanoparticles (e.g., CdS, CdSe, ZnS, ZnSe), Group III-V nanoparticles (e.g., InP, GaP), Group II-V nanoparticles (e.g., Cd.sub.3P.sub.2), and Group III-VI nanoparticles (e.g., In.sub.2Se.sub.3). In one embodiment, suitable gas-phase precursors may include, but are not restricted to, a Group II or Group III cation source, (e.g., R.sub.2Cd/Zn; R.sub.3Ga/In (R=organic group)), and a Group V or Group VI anion source, (e.g., H.sub.2S, H.sub.2Se or H.sub.3P). In yet another embodiment, the flow rate of the gas-phase precursors may be controlled using a carrier gas. The carrier gas may include, but is not limited to, an inert gas (e.g., He, N.sub.2 or Ar), or a reducing gas (e.g., H.sub.2).

(19) The pores in the semiconductor material allow the gas phase precursors to diffuse throughout the material. The nucleation and growth of QDs from gaseous precursors may proceed in any pores. Furthermore, since QD stability increases with particle size, under suitable reaction conditions particle growth may continue until all the space is occupied. Therefore, the size of the nanoparticles can be restricted by the pore diameter. By way of example only, QDs having an approximately 5 nm diameter can form in approximately 5 nm pores. In one embodiment, QDs having uniform dimensions can grow in the pores. In another embodiment, QDs having variable diameters grow in the pores. In one embodiment, both red and green QDs grow in the pores of a semiconductor material for a blue-LED (e.g., GaN) at a level that effectively down-converts the LED to produce white light emissions. The resulting material is free of liquid solvents because the QD-producing reactions involve only gas phase precursors.

(20) QDs may be prepared by the reaction of gas phase QD precursors as described in N. L. Pickett et al., in J. Mater. Chem., 1997, 7, 1855 and in J. Mater. Chem., 1996, 6, 507. The size of the resultant QDs may be varied by careful control of the reaction conditions (e.g., temperature, time, etc.), and the addition of pyridine in the gas phase. Likewise, the methods used to synthesize QDs in polymer matrices described by Haggata et al. S. W. Haggata et al., J. Mater. Chem., 1996, 6, 1771 and J. Mater. Chem., 1997, 7, 1996 may be adapted to synthesize QDs in the pores of the LED semiconductor material. The Pickett and Haggata references cited in this paragraph are hereby incorporated by reference in their entirety.

(21) Generally, the gas phase QD precursors are exposed to the pores in parallel or counter flow and allowed to react within the pores. In one embodiment, the pores have variable sizes to accommodate both red and green QD growth. In another embodiment, the reaction conditions are controlled to produce both red and green QDs. In yet another embodiment, the QDs may be formed in the semiconductor material at a level that effectively down-converts the semiconductor material to produce white light emissions.

(22) Gas phase reaction conditions can be used to control QD growth within the semiconductor material. For example, pyridine and higher temperatures may be used to inhibit nanoparticle growth as reported by Pickett et al., Effect of pyridine upon gas-phase reactions between H.sub.2S and Me.sub.2Cd; control of nanoparticle growth, J. Mater. Chem., No. 6, 507-09 (1996). Thus, in one embodiment, the gas-phase synthesis can be carried out in the presence of a Lewis base in the gas phase. For example, the Lewis base can coordinate to the surface of the QDs and control their size. Higher concentrations of a Lewis base can be used to synthesize smaller QDs. Suitable Lewis bases may include, but are not restricted to, pyridine gas. In still another embodiment, the semiconductor may comprise a material that may act as a Lewis base. In another embodiment, the reaction may be carried out at a certain temperature. Suitable temperatures may include, but are not restricted to, approximately 25 C. to 200 C. Higher temperatures can be used to produce smaller QDs. In still another embodiment, a Lewis base concentration and temperature are adjusted during gas-phase synthesis in order to synthesize different size QDs within the semiconductor material. In one embodiment, the Lewis base concentration and temperature can be selectively adjusted to a level that results in synthesis of both red and green QDs within the pores of a semiconductor material for a blue-LED (e.g., GaN) at a level that effectively down-converts the LED to produce white light emissions.

(23) In an alternative embodiment, QDs having same size but different wavelength emissions can be grown within the pores of a semiconductor material. For example, nanoparticle precursors can be selected to grow both Group III-V based QDs (e.g. InP, and including graded dots and alloys) and CdSe QDs. InP QDs emitting at a particular wavelength are relatively smaller than CdSe QDs emitting at the same wavelength. Thus, in an embodiment, InP and CdSe QDs can grow to the same size but emit different wavelengths. In one embodiment, the InP and CdSe QDs grow within pores having uniform diameter, wherein the InP QDs emit red light and the CdSe QDs emit green light. In an embodiment, the concentration of precursors for red-emitting QDs and green-emitting QDs can be selectively adjusted to a level that results in synthesis of both red and green QDs within the pores of a semiconductor material for a blue-LED (e.g., GaN) at a level that effectively down-converts the LED to produce white light emissions.

(24) In one embodiment, a porous semiconductor material 200 is placed in the middle of two streams of gas flowing from opposite directions, 201 and 202, respectively, as illustrated in FIG. 2. The gas streams can include precursors to QDs 204. Referring to FIG. 2, as the gas streams flows through the semiconductor material 200, nanoparticle nucleation and growth may ensue in the material's pores 203. Nanoparticle sizes can be restricted by the size of the pores 203 they grow in. In an alternative embodiment, nanoparticle sizes may be restricted by reaction conditions, including adjustment to Lewis base concentration and/or temperature. In an embodiment, the precursor gas streams flow in an alternating pattern. In another embodiment, the precursor gas streams flow simultaneously.

(25) In another embodiment, a porous semiconductor material 300 is placed in the stream of two parallel gas sources 301, 302, as illustrated in FIG. 3A. The gas streams may be allowed to flow either sequentially or in tandem. As in the method described in FIG. 2, the gas streams can include precursors to QDs 304. As illustrated in FIG. 3, as the gas streams flow through the semiconductor material 300, nanoparticle nucleation and growth may ensue in the material's pores 303. Again, nanoparticle sizes can be restricted by the size of the pores 303 they grow in. In an alternative embodiment, nanoparticle sizes may be restricted by reaction conditions, including adjustment to Lewis base concentration and/or temperature. In an embodiment, the precursor gas streams flow in an alternating pattern. In another embodiment, the precursor gas streams flow simultaneously.

(26) FIGS. 4 and 5 illustrate embodiments of apparatuses for the gas-phase synthesis of QDs. In the apparatus 400 illustrated in FIG. 4A, a semiconductor material 401 is inserted into a quartz tube 402, which is then positioned in a tube furnace 403. QD precursor gasses are provided by lines 404 and 405 to opposite sides of the semiconductor material. The gas streams can flow simultaneously or in an alternating pattern. For example, line 404 may provide a gas phase QD precursor such as H.sub.2S, H.sub.2Se, or PH.sub.3, and line 405 may provide a QD precursor such as R.sub.2Zn, R.sub.2Cd, R.sub.3Ga or R.sub.3In. Apparatus 400 can also include lines 406 and 407 for carrier gasses. Apparatus 400 may also include a source 408 for providing a Lewis base. Precursor gas lines may include a reactor 409 for generating gaseous precursors. Any or all of the gas lines may be provided with gas-flow meters 410 and 411. Exhaust lines 412 and 413 may be provided with scrubbers 414 and 415, respectively, and with pressure controllers 416 and 417 respectively.

(27) In the apparatus 500 illustrated in FIG. 5, a semiconductor material 501 is positioned into a quartz tube 502, which is positioned in tube furnace 503. The semiconductor material is exposed to parallel streams of QD precursor gas provided by lines 504 and 505. The gas streams can flow simultaneously or in an alternating pattern. The apparatus may also include one or more lines 506 providing additional reagents, such as a Lewis base. Lines 504 and 505 are connected to sources of QD precursor gasses 507 and 508, respectively. In apparatus 500, line 506 can be connected to a source of Lewis base 509. As in the apparatus illustrated in FIG. 4, example precursor gasses for apparatus 500 include H.sub.2S, H.sub.2Se, or PH.sub.3, and R.sub.2Zn, R.sub.2Cd, R.sub.3Ga or R.sub.3In. Any of the gas lines can also be provided with a source of carrier gas 510 and additional equipment, such as gas-flow meters 511 and 512. Quartz tube 502 may contain glass wool 513 up stream of exhaust line 514. Exhaust line 514 may be equipped with monitoring, control, or processing equipment, such as one or more scrubbers 515 and pressure controller 516.

(28) The particular set-ups illustrated in FIGS. 4 and 5 are exemplary and schematic only. It will be readily apparent to one of skill in the art how to implement these and other geometries for providing QD precursor gasses to a semiconductor material, as described herein. The scope of the invention is not limited to any particular reactor geometry or apparatus.

(29) The methods and apparatuses described herein can grow QDs within a semiconductor material because gas phase QD precursors can diffuse into nano-size pores and react inside those pores. FIG. 6 compares the relative size of QD precursor molecules Me.sub.2Cd 601, Me.sub.2Zn 602, H.sub.2S 603, H.sub.2Se 604, PH.sub.3 605, and InMe.sub.3 606 to the size of O.sub.2 600.

(30) FIGS. 7A-7C illustrate the formation of QDs within pores selectively etched into a semiconductor material 700. Gaseous QD precursors can diffuse into pores as small as 1 nm in width or less. The QD precursors react within the pores to form QDs 702. In wider pores 703, the precursors react to form larger diameter QDs 702a. In one embodiment, these larger diameter QDs 702a can emit light that is red-shifted. In narrower pores 704, smaller diameter QDs 702b can form. In another embodiment, these smaller QDs 702b can emit green-shifted light.

(31) Referring to FIG. 7A, QDs can be grown in a semiconductor material 700 having pores with uniform diameter 710 (FIG. 1A). In another embodiment, as illustrated in FIG. 7B, QDs can be grown in a semiconductor material 700 having pores with different diameters 740, 750 (FIG. 1B). In still another embodiment, as illustrated in FIG. 7C, QDs can be grown in a semiconductor material 700 having pores with a small diameter 770 in the top half of the semiconductor material and a large diameter 780 in the bottom half of the semiconductor material (FIG. 1C).

(32) The QD precursors can diffuse into the pores and grow to a size that fills the diameter of the pores. In one embodiment, the gaseous precursors include nanoparticle precursors to produce both red 720 and green QDs 730 within the uniform-sized pores. For example, the gas may include precursors for Group III-V based QDs (e.g. InP, and including graded dots and alloys) and CdSe QDs, which will emit different wavelengths at a certain size. In an alternative embodiment, adjusting Lewis base concentration and/or temperature during synthesis can be used to selectively control QD size. In one embodiment, reaction conditions are controlled to grow red-emitting QDs 720 in the bottom half of the semiconductor material, and green-emitting QDs 730 in the top half of the semiconductor material. In yet another embodiment, green and red-emitting QDs are grown within a blue-light emitting semiconductor material having uniform pore diameter at a level that effectively down converts the semiconductor material to white light emissions.

(33) In yet another embodiment, as illustrated in FIG. 8, a conventional LED can include a selectively etched n-GaN layer with nanoparticles embedded in its pores. The LED may include a Sapphire Substrate 801, an n-GaN layer 802, a p-n junction active layer 803, a p-GaN layer 804, a p-electrode 805, and an n-electrode 806. In one embodiment, both green 807 and red-emitting QDs 808 can be embedded in the n-GaN pores. In another embodiment, green-emitting QDs 807 are embedded in the top half 809 of the n-GaN layer and red-emitting QDs 808 are embedded in the bottom half 810 of the n-GaN layer. Any of these designs can be achieved with one or more of the aforementioned methods. In still another embodiment, the QDs are embedded in the n-GaN layer with a design and at a level that results in down-converting the LED to a substantially white light emission 811.

(34) The present application presents numerous advantages over the prior art. It relies on gaseous precursors, which though larger than individual oxygen and water molecules, are of the same order of magnitude. As illustrated in FIG. 6, the gaseous precursors suggested herein are less than three times the length of an oxygen molecule (3 ) along their longest axis, which enables them to diffuse into pores less than 1 nm in diameter, i.e. below the lower limit for QD stability. As shown in FIGS. 7A-7C, if a pore is large enough such that it corresponds to a diameter within the stable QD range, nanoparticle formation may proceed. Furthermore, gaseous precursors are able to penetrate the entire semiconductor material layer. And unlike prior art techniques such as high pressure nitrogen adsorption, the embodiments herein do not assume and rely on cylindrical pores. The techniques herein can be used to formulate QDs in any pore shape. Furthermore, with the methods and apparatuses described herein, cryogenic temperatures, which may be damaging to LEDs and may be challenging and costly to maintain, are not required. Moreover, the semiconductor need not be exposed to potentially damaging high pressures. Consequently, the method does not introduce defects into the semiconductor material during gas-phase synthesis. Furthermore, since the nanoparticle size may be controlled by a number of parameters, including temperature, time, carrier gas, and the concentration of an optional Lewis base, the technique may be adapted for use with a wide range of semiconductor materials, including those used in LEDs.

(35) It's understood that the above description is intended to be illustrative, and not restrictive. The material has been presented to enable any person skilled in the art to make and use the inventive concepts described herein, and is provided in the context of particular embodiments, variations of which will be readily apparent to those skilled in the art (e.g., some of the disclosed embodiments may be used in combination with each other). Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention therefore should be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms including and in which are used as the plain-English equivalents of the respective terms comprising and wherein.

EXAMPLES

Example 1: CdS

(36) CdS QDs may be formed from the gas phase reaction of helium gas streams containing Me.sub.2Cd and H.sub.2S in the presence of pyridine gas. Typical reaction conditions include a He flow rate of 600 cm.sup.3 min.sup.1 and a 30-fold excess of H.sub.2S to Me.sub.2Cd. The particle size may be controlled by varying the pyridine concentration and/or the reaction temperature. Preferably, pyridine:Me.sub.2Cd ratios in the range 1:20 to 2:1, and temperatures between room temperature and 200 C. are employed. It has been found that increasing the pyridine concentration reduces the particle size, while the particle size increases with increasing temperature.

(37) The absorption of the CdS nanoparticles may be tuned from the UV to cyan (bulk band gap 512 nm) depending on the particle size. For example, nanoparticles in the size range 2-20 nm may be expected to emit between approximately 320-500 nm, corresponding with UV to cyan light.

Example 2: CdSe

(38) Reaction conditions similar to those outlined for CdS (above) may be used to synthesize CdSe QDs [N. L. Pickett et al., J. Mater. Chem., 1997, 7, 1855], substituting H.sub.2S for H.sub.2Se. Higher pyridine concentrations may be used to control the particle size (up to 150:1 pyridine:Me.sub.2Cd).

(39) The absorption of the CdSe nanoparticles may be tuned from the blue to the deep red (bulk band gap 717 nm) depending on the particle size. Nanoparticles in the size range 2-20 nm may be expected to emit between approximately 490-700 nm, corresponding with blue to deep red light.

Example 3: ZnS

(40) Reaction conditions similar to those outlined for CdS (above) may be used to synthesize ZnS QDs [N. L. Pickett et al., J. Mater. Chem., 1997, 7, 1855], substituting Me.sub.2Cd for Me.sub.2Zn. Higher reaction temperatures (up to 300 C.) may be advantageous.

(41) The absorption of the ZnS nanoparticles may be tuned across the UV spectrum (bulk band gap 344 nm) depending on the particle size. Nanoparticles in the size range 2-20 nm may be expected to emit between approximately 235-340 nm.

Example 4: ZnSe

(42) Reaction conditions similar to those outlined for ZnS (above) may be used to synthesize ZnSe QDs [N. L. Pickett et al., J. Mater. Chem., 1997, 7, 1855], substituting H.sub.2S for H.sub.2Se. A reducing H.sub.2 carrier gas, rather than inert He, may be more effective at controlling the particle size.

(43) The absorption of the ZnS nanoparticles may be tuned from the UV to the blue (bulk band gap 459 nm) depending on the particle size. Nanoparticles in the size range 2-20 nm may be expected to emit between approximately 295-455 nm, corresponding with UV to indigo light.

Example 5: InP

(44) InP nanoparticles may be synthesized using a reaction procedure similar to those outlined for II-VI QDs (above) from Me.sub.3In and PH.sub.3 gaseous precursors.

(45) The absorption of the InP nanoparticles may be tuned from the green to the near-IR (bulk band gap 925 nm) depending on the particle size. Nanoparticles in the size range 2-20 nm may be expected to emit between approximately 520-875 nm, corresponding with green light to IR radiation.