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|Mar 10, 2009
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|Aug 11, 2005
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|Jun 6, 2001
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|Wei Chen
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|Nomadics, Inc.
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This application is a continuation of U.S. Ser. No. 10/166,313, filed Jun. 6, 2002 now U.S. Pat. No. 7,008,559, which claims priority under 35 U.S.C. � 119(e) of U.S. Provisional Application Ser. No. 60/296,333 filed Jun. 6, 2001 and entitled "UPCONVERSION OF Mn-DOPED SEMICONDUCTOR NANOPARTICLES", the contents of which are expressly incorporated herein by reference in their entirety.
The government may own certain rights in and to this application pursuant to: (i) a grant from the National Science Foundation Grant No. DMI-0060254, and (ii) an Air Force Office of Scientific Research Contract No. F49620-00-C-0058.
1. Field of the Invention
The present relates in general to upconversion luminescence ("UCL") materials and methods of making and using same and more particularly, but not meant to be limiting, to Mn2+ doped semiconductor nanoparticles for use as UCL materials. The present invention also relates in general to upconversion luminescence including two-photon absorption upconversion, and potential applications using UCL materials, including light emitting diodes, upconversion lasers, infrared detectors, chemical sensors, temperature sensors, pressure sensors, ultraviolet and radiation detectors and biological labels, all of which incorporate a UCL material.
2. Brief Description of the Related Art
The ever increasing demands of electronics and the interface between electronic and biological systems pushes frontier of electrical sciences and, in particular, the size and energy consumption of electronic elements. As the particle size of a material gets smaller and smaller, novel specific phenomena can be observed such as the shift of emission to shorter wavelength with decreasing size. Upconversion luminescence (UCL) is a type of fluorescence wherein the excitation wavelength entering a material capable of exhibiting UCL is longer than the emission wavelength of such a UCL material. Due to potential applications in lasers, laser cooling, optical communications, storage, displays, imaging techniques, optical sensing and biological probing, upconversion luminescence has been extensively investigated. See for example the references cited hereinafter, the contents of which are expressly incorporated herein by reference in their entirety.
UCL is extensively documented, in particular, in rare earth compounds in which the presence of more than one metastable excited f-f state results from the efficient shielding of the 4f electrons, including the upconversion luminescence of Er3+ in BaTiO3 nanoparticles. Recently, upconversion has been reported in some transition metal compounds as well—see e.g. H. U. Gudel et al., New photon upconversion processes in Yb 3+ doped CsMnCl 3 and RbMnCl 3, Chemical Physics Letters, (2000), 320, 639, hereinafter, the contents of which are expressly incorporated herein in their entirety. The upconversion luminescence of Mn2+ has been reported in Yb3+ doped CsMnCl3 and RbMnCl3 compounds at temperatures below 100 K.
Searching for an upconversion material, we need to consider both the stability and the upconversion efficiency. According to the energy gap law (J. M. F. van Dijk and M. F. H. Schuurmans, On the nonradiative and radiative decay rates and a modified exponential energy gap law for 4f-4f transitions in rare-earth ions, J. Chem. Phys. 1983, 78: 5317-5323 and L. A. Riseberg and H. W. Moos, Multiphonon orbit-lattice relaxation of excited states of rare-earth ions in crystals, Phys. Rev. 1968, 174: 429), lower highest lattice phonon energies of an upconversion host can reduce the rate, at which a given energy gap is crossed non-radiatively by multiphonon relaxation. That is, the lower the highest lattice phonon energy, the higher the upconversion efficiency. Most investigations of upconversion focus on oxides and halides. Oxides are air-stable but upconversion luminescence in oxides is not efficient because the phonon energies in oxides are high (most are higher than 500 cm−1) (P. Egger and J. Hulliger, Optical materials for short wavelength generation, Coordination Chemistry Review, 1999, 183: 101-115) and, thus, non-radiation rate is large, resulting in weak upconversion luminescence. In halides, the phonon energies is low (less than 350 cm−1) (P. Egger and J. Hulliger, Optical materials for short wavelength generation, Coordination Chemistry Review, 1999, 183: 101-115), non-radiation rate is low, and, of course, upconversion luminescence efficiency is high. However, halides are not stable in air because they are more or less sensitive to moisture. Group II-VI semiconductors like ZnS are very stable and their phonon energies are somewhat higher than that of halides but much lower than that of oxides (For ZnS:Mn2+, the phonon energy is 350 cm−1) (A. Anastassiadou et al., The luminescence spectrum of Zn1-xMnxS under hydrostatic pressure, Solid State Communications, 1988, 67:633-636). So, Group II-VI doped semiconductors are promising materials for upconversion due to their possibility of high upconversion efficiency and good stability. For example, the ZnS:Mn2+ nanoparticle samples used in the testing and experiments of the presently claimed and disclosed invention were made in approximately 2000, but as of the filing date of the present application still retain high luminescence and a stable structure.
Mn2+ doped semiconductors represent a class of phosphors that have already been utilized for many applications; however, no one has yet been able to report and/or demonstrate upconversion luminescence of Mn2+-doped semiconductors.
The presently claimed and disclosed invention(s) is predicated upon the observance of upconversion luminescence of Mn2+ in ZnS:Mn2+ semiconductor nanoparticles at room temperature. This upconversion luminescence is also shown to be principally due to two-photon excitation. These Mn2+-doped nanoparticles exhibiting UCL have unique and novel application as light emitting diodes, laser, optical communications, optical storage, infrared detection and imaging, chemical sensors, temperature and pressure sensing, radiation detection and biological probing. Additionally the presently disclosed and claimed invention provides for a novel methodology of distinguishing Mn2+ ions at the lattice sites and the near-surface sites in nanoparticles.
A common use of upconversion is to convert longer wavelength (infrared) to shorter wavelength (visible) and/or from a low energy to a higher energy state. Upconversion luminescence of Mn2+ is more efficient than that of rare earth ions because the d-d transition of Mn2+ can be modified via crystal field and in nanoparticles can become allowed or partly allowed, while the manipulation of the f-f transition of rare earth ions is more difficult and can only be slightly improved by crystal field or the host environments.
For infrared imaging, the use of nanoparticle upconversion is beneficial due to the low or absence of light scattering, because the light scattering intensity is proportional to the 6th power of the particle size (IμR6) (M. Kaszuba, The measurement of nanoparticles using photon correlation spectroscopy and avalanche photo diodes, Journal of Nanoparticle Research, (1999), 1, 405-409). Thus, compared to traditional micrometer-sized phosphors, light scattering in nanoparticles is nonexistent, which is ideal for imaging technology.
The surface-to-volume ratio of nanoparticles is very high. The attaching of chemicals or molecules to the nanoparticle surfaces changes the luminescence properties (intensity, emission energy and lifetime) of the nanoparticles greatly and rapidly. This provides for a new type of chemical sensors based on upconversion luminescence with high sensitivity.
The sizes of nanoparticles are comparable to the sizes of bio-molecules, and nanoparticles are soluble in water due to their small size and surface modification. Thus, highly luminescent nanoparticles are good labels for biological probing because they can combine with bio-molecules like antigens, anti-bodies, proteins or DNA or they can be inserted into biological systems such as a human or cell tissue. Some work has been done using undoped semiconductor nanoparticles. Until the present disclosure, however, no group has reported, discussed, or disclosed the use of doped nanoparticles and upconversion nanoparticles for biological systems. The application of undoped nanoparticles is based on photoluminescence in which the luminescence background and noise are very high due to the auto-fluorescence of the bio-molecules under ultraviolet excitation. Upconversion luminescence with an infrared excitation and a visible emission can avoid this shortcoming and thus improve the resolution and sensitivity of the upconversion luminescence assembly or molecule used, for example, as a biological sensor, probe or label.
At least one group (D. A. Zarling, M. J. Rossi, N. A. Peppers, J. Kane, G. W. Faris, M. J. Dyer, S. Y. Ng, and L. V. Schneider, Up-converting reporters for biological and other assays using laser excitation techniques, U.S. Pat. No. 5,891,656, Apr. 6, 1999) has reported using upconversion of the traditional micrometer sized phosphors for biological probing, but these molecules are not soluble in water and do not easily combine with biomolecules. The size, solubility and the strong upconversion luminescence of the doped nanoparticles of the presently claimed and disclosed invention, however, allow for the production and use of high quality biological probing materials and methodologies. Upconversion luminescence of doped nanoparticles has advantages over both the photoluminescence of undoped nanoparticles and the upconversion of traditional phosphors. In addition, nanoparticles can reduce light scattering intensity as discussed above, improving resolution greatly.
Temperature and pressure sensors can also be manufactured using the UCL materials of the presently claimed and disclosed invention. The UCL materials are especially well suited for temperature sensors due to the sensitivity of the upconversion luminescence intensity to temperature as shown in detail hereinafter. For example, there exists a linear or a near linear relationship between the upconversion intensity and temperature for high quality nanoparticles like the ZnS:Mn2+ nanoclusters encapsulated in zeolite. In addition, upconversion spectra, lifetime and temperature dependence measurements demonstrate that upconversion is a good method to reveal the luminescence characteristics of the Mn2+ ions at the near-surface sites of semiconductor nanoparticles.
The present relates in general to upconversion luminescence ("UCL") materials and methods of making and using same and more particularly, but not meant to be limiting, to Mn2+ doped semiconductor nanoparticles for use as UCL materials. The present invention also relates in general to upconversion luminescence including two-photon absorption upconversion, and potential applications using UCL materials, including light emitting diodes, upconversion lasers, infrared detectors, chemical sensors, temperature and pressure sensors, ultraviolet and radiation detectors and biological labels, all of which incorporate a UCL material.
Before explaining at least one embodiment of the invention in detail, it is to be understood that the invention is not limited in its application to the details of construction, experiments, exemplary data, and/or the arrangement of the components set forth in the following description or illustrated in the drawings. The invention is capable of other embodiments or of being practiced or carried out in various ways. Also, it is to be understood that the phraseology and terminology employed herein is for purpose of description and should not be regarded as limiting.
The term "nanoparticles" as used herein, can also refer to nanoclusters, clusters, particles, dots, quantum dots, small particles, nanostructured materials.
The presently disclosed and claimed invention relates, in general, to UCL materials and methods of making and using same. The UCL material is broadly defined as any material capable of UCL—i.e. any material capable of luminescence wherein the excitation wavelength is longer than the emission wavelength. More particularly, but not by way of limitation, the UCL material may be any host having a dopant or dopants operably associated therewith. In this embodiment of the UCL material, the dopant increases the fluorescence intensity or quantum efficiency of the host. In one specific embodiment, the host may be a nanoparticle, although one of ordinary skill in the art will appreciate that bulk materials may also be used. Nanoparticles do, however, have certain advantages over bulk materials—namely (i) high luminescence efficiency due to quantum confinement; (ii) emission and excitation energies tunable by size; (iii) low excitation power due to the increase of absorbance with decreasing size; (iv) low light scattering; (v) ease of production; (vi) thin film applications/ability; and (vii) wide industry application (e.g. infrared detection, safety control and surveillance, biological probing and labeling, and weather reporting and pollution control, etc.). Thus, without detracting or limiting the breadth of the presently claimed and disclosed UCL material, one of ordinary skill in the art (given this disclosure) would appreciate that UCL materials fabricated from doped nanoparticles would have certain advantages over a UCL material fabricated from bulk materials or conventional phosphors.
When the term "nanoparticle" is used, one of ordinary skill in the art will appreciate that this term encompasses all materials with small size and quantum size confinement, generally the size is less than 100 nm. As a new type of materials, nanoparticles are luminescent with high quantum efficiency and are good candidates for upconversion luminescence. It is known that oscillator strength is a very important optical parameter that determines the absorption cross-section, recombination rate, luminescence efficiency, and the radiative lifetime in materials. The oscillator strength of the free exciton is given by (W. Chen, A. G. Joly, and Z. Z. Zhang, Upconversion luminescence of Mn 2+ in ZnS:Mn 2+ nanoparticles, Phys. Rev. B 64, 41, 202 (2001)):
where m is the electron mass, ΔE is the transition energy, μ is the transition dipole moment, and |U(0)|2 represents the probability of finding the electron and hole at the same site (the overlap factor). In nanostructured materials, the electron-hole overlap factor increases largely due to the quantum size confinement, thus yielding an increase in the oscillator strength. The oscillator strength is also related to the electron-hole exchange interaction that plays a key role in determining the exciton recombination rate. In bulk semiconductors, due to the extreme dislocation of the electron or hole, the electron-hole exchange interaction term is very small; while in molecular-size nanoparticles, due to the confinement, the exchange term should be very large. Therefore, we may expect a large enhancement of the oscillator strength from bulk to nanostructured materials.
In doped semiconductors, excitons are bound to impurity centers. The oscillator strength is given by (W. Chen, R. Sammynaiken, and Y. Huang, Luminescence enhancement of ZnS:Mn nanoclusters in zeolite, J. Appl. Phys. 88, 5188 (2000) and W. Chen, X. H. Zhang, and Y. Huang, Luminescence enhancement of EuS nanoclusters in zeolite, Appl. Phys. Lett., 2000, 24, 2328-2330):
f=f ex |∫dxF(x)|2/Ωmol,
where fex is the oscillator strength of the free exciton and Ωmol is the volume of one molecule. The oscillator strength of a bound exciton is actually given by fex multiplied by the number of molecules covered by the overlap of the electron and hole wavefunctions. Clearly, quantum size confinement will also enhance the bound exciton oscillator strength in doped nanoparticles. The luminescence efficiency is also proportional to the excition oscillator strength; therefore, it can be enhanced via quantum size confinement. Strong evidences for the above theory are from our observations on ZnS:Mn2+ (W. Chen, R. Sammynaiken, and Y. Huang, Luminescence enhancement of ZnS:Mn nanoclusters in zeolite, J. Appl. Phys. 88, 5188 (2000)) and EuS (W. Chen, X. H. Zhang, and Y. Huang, Luminescence enhancement of EuS nanoclusters in zeolite, Appl. Phys. Lett., 2000, 24, 2328-2330) nanoparticles. The luminescence intensity of the 1 nm sized ZnS:Mn2+ nanoparticles in zeolite-Y is much stronger than other nanoparticles. More interesting is that bulk EuS at room temperature is not luminescent but strong luminescence is observed when EuS nanoparticles were formed in zeolite.
The radiative decay lifetime (t) is closely related to the oscillator strength of a transition (W. Chen, R. Sammynaiken, and Y. Huang, Luminescence enhancement of ZnS:Mn nanoclusters in zeolite, J. Appl. Phys. 88, 5188 (2000)):
τ=4.5(λA 2 /nf),
where n is the refractive index and λA is the wavelength. Thus, the lifetime is shortened with decreasing size due to the increase of the oscillator strength, f. High efficiency with short decay enables nanoparticles very good candidates for luminescence sensors. In addition, small size and ultrathin films enable nanothermometry have high sensitivity and accuracy. These are expected to be the advantages of nanoparticles for upconversion. Therefore, the nanoparticle materials in this disclosure are not limiting to semiconductors but including non-semiconductors such as oxides and halides.
ZnS:Mn2+, for example, exhibits strong UCL in both bulk and nanoparticles. Indeed, the UCL is visible to the naked eye up to very low power densities. The UCL spectra in the presently claimed and disclosed invention were recorded with a laser power density of 100 mJ/cm2. The UCL is visibly detectable at power densities of at least 1 mJ/cm2. Furthermore, UCL is detectable by a photo multiplier tube (PMT) or charged coupled device (CCD) at a power density <1 mJ/cm2, such as 0.2 or 0.5 mJ/cm2. Thus, depending upon the application, the power density necessary for UCL or for use with UCL materials is unlimited. With respect to biological systems, however, one of ordinary skill in the art will realize that lasers with high power densities, such as those with power densities of >100 mJ/cm2, would create an increased risk of damage to host tissue or biomolecules. Whereas, this is not a problem by using the low power upconversion nanoparticles of the presently claimed and disclosed invention.
As mentioned previously, UCL materials that include a nanoparticle and a dopant may be particularly advantageous over UCL materials that include a bulk material and a dopant. One such doped-nanoparticle UCL material, ZnS:Mn2+, has been mentioned hereinbefore with respect to power densities and will be discussed hereinafter in detail with respect to luminescence and temperature dependence experiments. One of ordinary skill in the art would appreciate, however, that doped-nanoparticle UCL materials as contemplate for use with the presently disclosed and claimed invention are not limited to ZnS:Mn2+. Indeed, any compound of the general formula (X):(Y), wherein X is a host (e.g. a nanoparticle such as ZnS) and Y is a dopant (e.g. Mn2+), is contemplated for use as a UCL material. One of ordinary skill in the art would, however, also appreciate that a doped nanoparticle of the general formula (X):(Y) would be the most advantageous for the reasons set forth hereinabove.
Any dopant capable of increasing the fluorescence intensity or quantum efficiency of the bulk material or nanoparticle is contemplated for use in the presently claimed and disclosed invention. With respect to semiconductor nanoparticles, such as ZnS, a dopant capable of increasing fluorescence intensity or quantum efficiency due to the increase of the oscillator strength and the efficient energy transfer from the host to the dopant upon photoexcitation is contemplated for use with the presently claimed and disclosed invention. The dopant also increases the fluorescence intensity or quantum efficiency of bulk materials in the same manner.
Typically such a dopant would have a high d-d transition rate—e.g. Mn2+ has a d-d transition of 4T1→6A1. Thus, any dopant having a high d-d transition rate and that is also capable of increasing the luminescence intensity or quantum efficiency of the UCL material is contemplated for use in the presently claimed and disclosed invention. For example, but not by way of limitation, the dopants could be broadly classified as rare earth ions—e.g. Tb3+, Ce3+ or Eu3+. The dopants may also be broadly classified, however, as transition ions—e.g. Mn2+, Cu2+, Cu+, Ag+, Cr3+, Ti2+, Ni2+, or Re4+. Thus, one of ordinary skill in the art would appreciate that the dopant must be functionally defined, rather than being strictly classified according to a position in the periodic table of elements. The dopant is herein defined and it is intended to include any material or ion that, when doped into a host material such as a nanoparticle, will increase the luminescence intensity or quantum efficiency of the porous material—thereby creating an efficient UCL material.
The dopant is placed substantially within a host—i.e. a bulk material or a nanoparticle. The host may be any type of material (i.e. semiconductor, non-conducting, insulating, or a conducting material). Functionally, the host must be capable of accepting the dopant and having a wide band gap. With respect to nanoparticles, exemplary nanoparticles for use as the host include: CdTe, CdSe, ZnO, CdS, ZnS, Y2O3, MgS, CaS, SrS and BaS. The host may be any semiconductor and more specifically, but not by way of limitation, sulfide, telluride, selenide, and oxide semiconductors and their nanoparticles, such as Zn1-xMnxSy, Zn1-xMnxSey, Zn1-xMnxTey, Cd1-xMnSy, Cd1-xMnxSey, Cd1-xMnxTey, Pb1-xMnxSy, Pb1-xMnxSey, Pb1-xMnxTey, Mg1-xMnSy, Ca1-xMnxSy, Ba1-xMnxSy and Sr1-xMnxSy, etc. (wherein, 0<x≦1, and 0<y≦1). Complex compounds of the above-described semiconductors are also contemplated for use in the presently claimed and disclosed invention—e.g. (M1-zNz)1-xMnxA1-yBy(M=Zn, Cd, Pb, Ca, Ba, Sr, Mg; N=Zn, Cd, Pb, Ca, Ba, Sr, Mg; A=S, Se, Te, O; B=S, Se, Te, O; 0<x≦1, 0<y≦1, 0<z≦1). Two examples of such complex compounds are Zn0.4Cd0.4Mn0.2S and Zn0.9Mn0.1SO0.8Se0.2. Additional hosts include insulating and nonconducting materials such as BaF2, BaFBr, and BaTiO3, to name but a few exemplary compounds.
As described hereinabove, the UCL material of the present invention can be described generically by the formula (X):(Y) wherein (X) is a host and (Y) is a dopant capable of increasing the luminescence intensity or quantum efficiency of the host. The variables (X) and (Y) have been described individually heretofore, but when taken together and applied to a specific type of host—e.g. a semiconductor—preferred UCL materials would be transition and rare earth ion co-doped semiconductors. Such transition and rare earth ion co-doped semiconductors would include sulfide, telluride, selenide and oxide semiconductors and their nanoparticles, such as ZnS:Mn,Er; ZnSe:Mn,Er; MgS:Mn,ER; CaS:Mn,Er; ZnS:Mn,Yb; ZnSe:Mn,Yb; MgS:Mn,Yb; CaS:Mn,Yb etc., and their complex compounds: (M1-zNz)1-x(MnqR1-q)xA1-yBy (M=Zn, Cd, Pb, Ca, Ba, Sr, Mg; N=Zn, Cd, Pb, Ca, Ba, Sr, Mg; A=S, Se, Te, O; B=S, Se, Te, O; R=at least one rare earth ion, 0<x≦1, 0<y≦1, 0<z≦1, 0<q≦1).
Thus, a UCL material capable of upconversion luminescence at low power (e.g. <10 mJ/cm2) has been described such that one of ordinary skill in the art would be capable of making and using same. Further specific examples and resulting data are given hereinafter.
As mentioned previously, due to potential applications in lasers, laser cooling, optical communications, storage, displays, and imaging techniques, UCL has been extensively investigated. UCL is well documented in rare earth compounds in which the presence of more than one metastable excited f-f state results from the efficient shielding of the 4f electrons. Upconversion has been reported in some transition-metal compounds as well. Mn2+ doped materials represent a class of phosphors that have already been utilized for many applications; however, no one has yet disclosed nor described the use of Mn2+ (a dopant—(Y)) with a nanoparticle (host—(X)) for use as a UCL material.
Upconversion and anti-Stokes luminescence have been reported in semiconductor quantum dots, porous silicon, CdS nanoparticles, CdSe, and InP colloidal nanoparticles (P. P. Paskov et al., Photoluminescence upconversion in InAs/GaAs self-assembled quantum dots, Appl. Phys. Lett. 77, 812 (2000); L. A. Golovan et al., Observation of two-step excitation of photoluminescence in silicon nanostructures, Pisma Zh. Eksp. Teor. Fiz. 68, 732 (1998) @JETPLett. 68, 770 (1998)#; S. A. Blanton, M. A. Hines, M. E. Schmidt, and P. Guyot-Sionnest, Two-photon spectroscopy and microscopy of II-VI semiconductor nanocrystals, J. Lumin. 70, 253 (1996); E. Poles, D. C. Selmarten, O. I. Micc, and A. J. Nozik, Anti-Stokes photoluminescence in colloidal semiconductor quantum dots, Appl. Phys. Lett. 75, 971 (1999)). These previous results indicate that semiconductor nanoparticles may work as a new type of upconversion material; however, no one has demonstrated or disclosed the UCL of doped semiconductor nanoparticles.
Recipe for Nanoparticles in Methacrylic Acid/Citric Acid Matrix
In a four-neck flask, methacrylic acid (MA) (10 ml) and 2 g citric acid (CA) were mixed with ethanol (1000 mL, 99.95%) in a solution, then the solution was stirred while purging with N2 for 2.5 hrs. Then a Na2S solution was added (8.009 g Na2S dissolved in 200 mL of 99.95% ethanol) and a mixed solution of Zn2+ and Mn2+ (30.337 g Zn(NO3)2.6(H2O) and 0.114 g Mn(NO3)2 dissolved in 200 mL 99.95% ethanol with the molar ratio of Mn2+:Zn2+ is 5:95) were added simultaneously with the same flowrate through two necks, respectively. The reaction was carried out under N2 atmosphere at 80� C. for 24 hrs. A transparent colloid of ZnS:Mn2+ was then obtained. The pH value of the final solution was 2.4. The nanoparticles were separated by centrifugation and dried in vacuum at room temperature.
Recipe for Un-Capped Nanoparticles.
The uncapped ZnS:Mn nanoparticles of 10 nm were prepared as follows: A four-neck flask was charged with 400 mL deionized water and was stirred under N2 for 2.5 hrs. An aqueous solution of 1.6 g Na2S and an aqueous solution of 5.8 g Zn(NO3)2.6(H2O) and 0.26 g Mn(NO3)2 (Mn2+:Zn2+ molar ratio 5:95) were prepared and added to the first solution simultaneously via two different necks at the same rate. After the addition, the resulting solution was stirred constantly under N2 at 80� C. for 24 hrs and a transparent colloid of ZnS:Mn was formed. The pH value of the final solution was 2.4. This relatively low pH value is required to prevent the precipitation of unwanted manganese species. The nanoparticles were separated from solution by centrifugation and dried in vacuum at room temperature.
Recipe for Nanoclusters in Zeolites
ZnS:Mn2+ nanoparticles in the cavities of an ultra-stable zeolite-Y (USY) were prepared by solid state diffusion at high temperature. Typically, 100 mg of the uncapped ZnS:Mn nanoparticles (˜10 nm in size) were mechanically mixed thoroughly with 2 g of USY powder (Si/Al ratio is 230), then the mixture was pressed into pellets. The pellets were then heated in a furnace at 900� C. in vacuum (10−5 torr) for 4 hrs.
The preparation, structure, and the luminescence properties of the Mn2+-doped ZnS (ZnS:Mn2+) nanoparticles have been described previously. (See e.g. W. Chen et al., Crystal field, Phonon coupling and emission shift of Mn 2+ in ZnS:Mn nanoparticles, J. Appl. Phys. 89, 1120 (2001). W. Chen, A. G. Joly, and Z. Z. Zhang, Upconversion luminescence of Mn 2+ in ZnS:Mn 2+ nanoparticles, Phys. Rev. B 64, 41, 202 (2001). W. Chen, R. Sammynaiken, and Y. Huang, Luminescence enhancement of ZnS:Mn nanoclusters in zeolite, J. Appl. Phys. 88, 5188 (2000), each of which is incorporated herein in their entirety by reference).
Recipe for Making CdTe and CdTe:Mn2+ Nanoparticles
The CdTe nanoparticles were prepared by a wet chemical technique which has been reported in literature (M. Kapitonov et al., Luminescence properties of tiol-stabilized CdTe nanocrystals, J. Phys. Chem. B, 1999, 103:10109-10113 and W. Chen, D. Gronquist, and J. Roark, Voltage tunable electroluminecsnce of CdTe nanoparticle light-emitting diodes, J. Nanosci. Nanotechnol., 2002, 2: 47-53). Cadmium perchlorate hydrate (Aldrich), aluminum telluride (99.5% pure, Gerac), and thioglycolic (mercaptoacetic) acid (Aldrich) were used as received. CdTe nanoparticles were prepared by the rapid mixing of precursors containing cadmium perchlorate hydrate and sodium hydrotelluride (NaHTe), cooled to 5� C., under vigorous stirring. The Cd2+ containing solution was prepared as follows: 0.73 g. of Cd(ClO4)2.H2O was dissolved in 125 mL of water. 0.3 mL of thioglycolic acid (TGA) was added to the solution and its pH was adjusted to ˜11.2 by the addition of 0.1M NaOH. The solution was then purged with nitrogen for at least 30 minutes. The solution of NaHTe was prepared in a vessel cooled with water ice to 5� C., by bubbling an excess of H2Te through 22 mL of 0.05 M NaOH for 40 minutes under nitrogen. The hydrogen telluride gas was obtained from the reaction of excessive amounts of Al2Te3 and 0.5 M H2SO4 in an inert atmosphere (nitrogen). Great care was taken to keep the NaHTe solution temperature at an average of 5� C., as well as to avoid any contact of the solutions involved with oxygen (air) at all times.
After the completion of the reaction, a yellow solution of CdTe nanocrystal nuclei was obtained. This solution was then refluxed at 100� C. to promote crystal growth. The size of the particles was controlled by the reaction time. The size of the nanoparticles created was around 4 nm as observed by high resolution transmission electron microscope (HRTEM). Most nanoparticle are spherical in shape, while some of them are nonspherical. The [1, 1, 1] lattice strings can be seen from the HRTEM images, and the spacing found is about 0.36 nm, which is in agreement with the [1, 1, 1] spacing of cubic CdTe of 0.374 nm.
The recipe for making CdTe:Mn2+ nanoparticles is the same as that for CdTe nanoparticles, while, a calculated amount of Mn(ClO4)2.H2O was dissolved in water along with a calculated amount of Cd(ClO4)2.H2O.
Recipe for Making CdS Nanoparticles in BaTiO3
Formation of CdS/BaTiO3 nanostructured materials: Ba—Ti complex alkoxide solution and CdS colloidal solution were prepared separately, and mixed together to form the composite precursor solution. To make Ba—Ti complex alkoxide solution, barium ethanoxide solution, prepared by dissolving metal barium into ethanol, and titanum isopropanide solution in 2-methoxyethanol were mixed and stirred for a few hrs. CdS colloidal solution was prepared by a selinazation process of Cd(NO3)2 and H2S flux in 2-methoxyethanol with existence of N,N-dimethlyformamide. The particle size can be controlled by the concentration of Cd(NO3)2 and N,N-dimethlyformamide. The colloid solution as prepared was added to the alkoxide solution with molar ratio of Cd/Ba=0.02, then a clear composite precursor solution was obtained. Thin films may be prepared by spin coating with the fresh precursor solution, and then dried at 100� C. in air, heat treated at 200-800� C. in N2 atmosphere.
Recipe for Making ZnS:Eu2+ and ZnS:Eu3+ Nanoparticles
The ZnS:Eu2+ nanoparticles were prepared in a water-ethanol solution as follows: In a four-neck flask, the water-ethanol solution (500 mL water and 500 mL 99.95% ethanol) was stirred while purging with N2 for 2.5 hrs, then a Na2S solution was added (8.009 g Na2S dissolved in 100 mL 99.95% ethanol and 100 mL deionized water) and a mixed solution of Zn2+ and Eu2+ (30.337 g Zn(NO3)2.6(H2O) and 0.114 g EuCl2 dissolved in a mixture of 150 mL 99.95% ethanol and 50 mL deionized water) were added at the same time with the same speed through two necks, respectively. The reaction was carried out in a N2 atmosphere at 80� C. for 24 hrs. Then a transparent colloid of ZnS:Eu2+ was then obtained. The nanoparticles were separated by centrifugation and dried in vacuum at room temperature. The pH value of the final solution is 2.4, which prevents precipitation of other Eu species outside the particles in the same manner as reported for Mn2+ doped ZnS nanoparticles.
The recipe for making ZnS:Eu3+ nanoparticles is similar to that for ZnS:Eu2+ nanoparticles. Just two different points are: it is Eu(NO)3 rather than EuCl2 that provides Eu3+; the reaction is conducted in air not in a N2 atmosphere.
Recipe for Making ZnS:Ag+ Nanoparticles
A four-neck flask is charged with a solution containing 10 mL methacrylic acid and 400 mL ethanol (99.95% Sigma). The solution is stirred under N2 for 2.5 hrs. A second solution containing 1.6 g of Na2S and 100 mL of ethanol and a third solution containing 5.8 g of Zn(NO3)2.6(H2O), 0.22 g of AgNO3, and 100 mL of ethanol (Ag+:Zn2+ molar ratio 5:95) are prepared and added to the first solution simultaneously via two different necks at the same rate. After this addition, the resulting solution is stirred constantly under N2 at 80� C. for 24 hrs. so that a transparent colloid of ZnS:Ag+ is formed. The pH value of the final solution is 3.0. This relatively low pH value is required to prevent the precipitation of unwanted Ag species from occurring.
Recipe for Making ZnS:Cu+ Nanoparticles
A four-neck flask is charged with a solution containing 10 mL methacrylic acid and 400 mL ethanol (99.95%). The solution is stirred under N2 for 2.5 hrs. A second solution containing 1.6 g of Na2S and 100 mL of ethanol and a third solution containing 5.8 g of Zn(NO3)2.6(H2O), 0.26 g of CuNO3, and 100 mL of ethanol are prepared and added to the first solution simultaneously via two different necks at the same rate. After this addition, the resulting solution is stirred constantly under N2 at 80� C. for 24 hrs. so that a transparent colloid of ZnS:Cu+ is formed. The pH value of the final solution is 2.4 This relatively low pH value is required to prevent the precipitation of unwanted Cu species from occurring.
Recipe for Making Y2O3:Tb3+, Eu3+ Nanoparticles
Solution (A) was prepared by dissolving 15.78 g of Y(NO3)3.5H2O, 0.250 g of Eu(NO3)3 and 0.350 g of Tb(NO3)3 in ethanol. The solution was stirred for 2 hrs at room temperature. Solution (B) was prepared by dissolving 0.25 mL of tween-80 poly-oxyethylene sorbitate and 0.25 mL of emulson-OG (oelsauerepolyglyscerinester) in 50 mL of aqueous ammonium hydroxode solution (pH>10) and stirred for 1hrs.at room temperature. Solution (A) was then added to solution (B) drop by drop through a burette at a controlled rate with vigorous stirring. The obtained gel was separated in a centrifuge. The aqueous solution was removed by refluxing in toluene using a water trap. The toluene was removed by evaporation, and the resulting powder was dried in the oven at 60� C. for 24 h. The final products may be isolated as white powders of different sizes after heat treatment in a tube furnace at different temperatures for 2 hrs. in ambient atmosphere.
Recipe for Making BaFBr:Eu3+ and BaFBr:Eu2+ Nanoparticles.
The preparation of BaFBr:Eu2+ nanoparticles is based on the following reaction in acid solutions: BaBr2+NH4F⇄BaFBr+NH4Br. In order to control the reaction rate, stabilizers (polyvinyl alcohol, Sigma) were added to the solution to control the reaction temperature. In our experience, we found the following recipe can successfully make BaFBr:Eu nanoparticles: (1) A four-neck flask was charged with 400 mL deionized water and 4 g polyvinyl alcohol and was stirred under N2 for 1 hr. The pH value was adjusted to 2 by nitrate acid. (2) 4.83 g BaBr2.2H2O and 0.137 g EuI2 were added to the solution and stirred under N2 for 1 hr at room temperature. (3) 0.70 g NaF was added to the solution and stirred under N2 for 0.5 hr at room temperature until precipitation occurred. (4) The temperature was raised to 80� C., the reaction was sustained for 2 hrs. and then, cooled rapidly to room temperature. (5) The nanoparticles were separated from solution by centrifugation, washed with dionized water, and dried in vacuum at room temperature. (6) The powder was heated at a temperature lower than 450� C. in a carbon monoxide or N2 atmosphere for 0.5 hr.
Recipe for Making BaFBr:Eu2+ nanoparticles in MCM-41 and Zeolites
For making BaFBr:Eu2+ nanoparticles in MCM-41, bulk BaFBr:Eu2+ powder was made by solid state diffusion at 800� C. for 2 hrs. Then, BaFBr:Eu2+ powder and MCM-41 powder (ratio of BaFBr:Eu2+/MCM-41 is 5:95) were mixed up and heated at 600� C. under N2 for 2 hrs. The recipe for BaFBr:Eu3+ is the same but reaction was conducted in air rather than in N2 atmosphere.
Details for Measurements
The average sizes of the ZnS:Mn2+ particles used in the following experiments, estimated from high-resolution transmission electron microscope (HRTEM) and x-ray diffraction, are approximately 1, 3, 3.5, 4.5, and 10 nm. The 1 nm-sized ZnS:Mn2+ nanoparticles are encapsulated in an ultrastable zeolite-Y (USY), while the 10 nm-sized nanoparticles are naked particles without any capping. The 3-, 3.5-, and 4.5 nm-sized particles were capped with methacrylic acid:citric acid. The size was controlled by the ratio of methacrylic acid:citric acid and by the reaction temperatures.
The photoluminescence and upconversion emission spectra and lifetime data were collected using a nanosecond optical parametric oscillator/amplifier (Spectra-Physics MOPO-730) operating at a 10-Hz repetition rate and tunable between 440 and 1800 nm. The output of the MOPO system was either frequency doubled in a potassium dihydrogen phosphate crystal for observation of the photoluminescence emission or else used directly for the upconversion measurements.
The output was directed onto the particles and emission was collected at right angles to the excitation and focused into a ⅛ meter monochromator with either a gated intensified charge coupled device detector (for emission spectra) or else a standard photo multiplier tube (for lifetime measurement). Fluorescence spectra shown in this study were collected approximately 5 μs after excitation, with a gate width of 20 μs. For lifetime measurements the output of the photo multiplier tube was directed into a digital oscilloscope with the input impedance adjusted to give a system response time of about 2 μs full width at half maximum. While fluorescence was observed on faster timescales, it has been shown previously (B. A. Smith, J. Z. Zhang, A. Joly, and J. Liu, Luminescence decay kinetics of Mn 2+-doped Zns nanoclusters grown in reverse micelles, Phys. Rev. B 62, 202+ (2000)) that much of this is emission from the ZnS itself and so was not of interest. All lifetime data reported herein were measured at or near the peak emission wavelength with a monochromator bandwidth of several nanometers. Power dependences were measured as a function of excitation energy by integrating the area of the Mn2+ emission from about 530 to 630 nm as a function of excitation power. Power levels were adjusted with a variable attenuator, measured with a power meter, and then in some cases reduced by up to five orders of magnitude using neutral density filters in order to prevent saturation effects from distorting the measurement. Due to the fact that different detector gain levels were employed and different amounts of attenuation were used after the power measurement, no comparison of the absolute intensities between different samples can be made.
Both Auger recombination and two-photon absorption can result in a quadratic dependence of the luminescence intensity on the excitation power. Auger recombination is a process in which an electron recombines with a hole and transfers the excess energy immediately to another electron or hole. The recombination of the high-energy electron or hole with another hole or electron is responsible for the upconversion. The efficiency of Auger recombination is mediated by Coulomb electron-electron interaction. In nanoparticles, the Coulomb interactions are enhanced due to size confinement. This leads to increased Auger rates in comparison with those in bulk materials. In contrast, the discrete energy-level structure in nanoparticles necessarily limits the density of final states that satisfy energy conservation so that Auger processes should be less likely unless participation of phonons or energy states outside the nanoparticle occurs coincidentally. Experimentally, it has been observed that Auger recombination occurs only at very high excitation density. (F. Wu, J. Z. Zhang, R. Kho, and R. K. Mehra, Radiative and nonradiative lifetimes of band edge states and deep trap states of CdS nanoparticles determined by time-correlated single photon counting, Chem. Phys. Lett. 330, 237 (2000)). In our measurements presented herein, the laser excitation density is lower than that normally required for Auger processes. Furthermore, it has been shown in both theoretical predictions and experimental observations that the change in upconversion luminescence intensity from Auger recombination with temperature is slower than that of the Stokes photoluminescence. In the present experiments and resulting data, the upconversion luminescence intensity is even more sensitive to temperature than that of the Stokes photoluminescence. This indicates that the upconversion luminescence is not likely to be caused by Auger processes. Therefore, it is reasonable to conclude that the upconversion luminescence of Mn2+ in ZnS:Mn2+ nanoparticles and bulk is due to a two-photon absorption process.
In order to confirm the two-photon process, we measured the emission spectra and decay lifetimes with single photon energy (383.5 nm) which is the sum energy of two photons at 767 nm. The emission spectra and lifetimes obtained at 383.5 nm excitation should be the same as that at 767 nm excitation if the upconversion luminescence is due to two-photon excitation, because the excitation energies are actually the same in this case. If the upconversion luminescence is due to Auger processes, the spectra and lifetimes excited at 383.5 nm are not expected to be the same as those at 767 nm excitation, because the two energies are not the same. The present experiments and results demonstrate that the emission spectra and lifetimes excited at 383.5 nm are almost identical as those obtained at 767 nm excitation (
As revealed by electron spin resonance (ESR), Mn2+ can occupy two sites in ZnS:Mn2+ nanoparticles: the lattice sites of Zn2+ ions and the near-surface sites. (W. Chen et al., Crystal field, Phonon coupling and emission shift of Mn 2+ in ZnS:Mn nanoparticles, J. Appl. Phys. 89, 1120 (2001). And W. Chen, R. Sammynaiken, and Y. Huang, Luminescence enhancement of ZnS:Mn nanoclusters in zeolite, J. Appl. Phys. 88, 5188 (2000)) These different sites should display different emission spectra and decay lifetimes. In the nanoparticles, the upconversion and the 383.5 nm excited photoluminescence emission maxima shift to lower energies in comparison with the photoluminescence energy when excited at 300 nm, and their decay lifetimes are shorter than the lifetimes obtained with 300 nm excitation. This could be due to site-selected excitation of Mn2+ ions or size-selected excitation of the nanoparticles. Based on the data obtained, we attribute the upconversion emission and the photoluminescence emission excited at 383.5 nm to the Mn2+ at the near-surface sites. The Mn2+ ions at the near surface sites have a faster decay lifetime and a lower emission energy than the Mn2+ ions at the bulk-like lattice sites.
One reason is that the symmetry of the near-surface sites is lower than the lattice sites. The excited levels of Mn2+ are split in the near-surface sites due to the change in the crystal field symmetry. The lowest emitting state will shift to a lower energy position, and thus the emission energy is shifted to longer wavelengths. Due to the change in the crystal field symmetry, the transition property of the emitting state is also changed, from forbidden to allowed or partly allowed. This is one reason why the upconversion and the 383.5 nm excitation decay lifetimes are shorter than the 300 nm excitation lifetimes. Another reason is that the coupling of the surface carriers to the near-surface site Mn2+ ions is stronger than to the lattice site Mn2+ ions. Thus more surface carriers are involved in the luminescence process of the near-surface site Mn2+ ions. This also may shift the emission to lower energies and shorten the decay lifetime. This is the first demonstration of distinguished emission from the two different sites of Mn2+ in ZnS:Mn2+ nanoparticles.
In bulk ZnS:Mn2+, all Mn2+ ions occupy the lattice sites and, therefore, no difference in emission energy or in lifetime is observed at different excitation wavelengths. By contrast, in the 1 nm-sized ZnS:Mn2+ particles encapsulated in zeolite-USY, almost all Mn2+ ions are at the near-surface sites as revealed by ESR, with little difference in the emission energy observed for different excitation wavelengths. In addition, the particle surface is extremely well passivated by encapsulation into the zeolite cavities at 900� C. Thus, the luminescence kinetics (decay lifetime) and temperature dependence of the 1-nm sample are different from that of other nanoparticles, as will be discussed in more detail hereinbelow.
In summary, upconversion luminescence of Mn2+ is observed in ZnS:Mn2+ bulk and nanoparticles. Based on the power dependence, the lifetime measurements, and spectra at different excitation wavelengths, the upconversion luminescence of Mn2+ in ZnS:Mn2+ is attributed to a two-photon absorption process. In nanoparticles, the redshift in the emission energy and the shortening in the lifetimes of the upconversion luminescence and the photoluminescence excited at 383.5 nm in comparison with the photoluminescence excited at 300 nm are attributed to site-selected excitation of Mn2+ ions in the nanoparticles. Mn2+ ions at the nearsurface sites are responsible for the upconversion emission and the luminescence following 383.5 nm excitation, while Mn2+ ions in the bulklike lattice sites are excited in the luminescence resulting from 300 nm excitation. Our results demonstrate that doped nanoparticles are a new type of material that display strong upconversion luminescence, and upconversion luminescence is an effective tool that can reveal the intrinsic properties of the dopants in doped nanoparticles.
Optical properties of semiconductor nanoparticles have been studied extensively in recent years. (N. S. Nalwa, Editor, Handbook of Nanostructured Materials and Nanotechnology, Academic Press, San Diego (2000), Vol. 4). Due to quantum confinement effects, large surface-to-volume ratios, and geometrical confinement of phonons, semiconductor nanoparticles behave differently from the bulk materials and therefore have novel properties. Size confinement effects lead to increased overlap between electron and hole wave functions, and decrease the density of both electronic and phonon states, resulting in different relaxation rates following electronic excitation relative to their bulk counterparts. Similarly, the increased surface-to-volume ratio necessarily allows an increase in the density of surface states, the influence of which on the energy transfer dynamics and luminescence intensity must be considered.
Doped nanoparticles represent a class of materials that have novel properties that may allow more flexibility when designing application devices, discussed in more detail hereinbelow. Indeed, Mn2+ doped materials already have found uses as phosphors in many applications. In particular, Mn2+ doped ZnS (ZnS:Mn2+) is of much interest because it is widely used in electroluminescence and cathodoluminescence displays. Since the report of significant emission lifetime shortening in ZnS:Mn2+ nanoparticles, (R. N. Bhargava, D. Gallagher, X. Hong and A. Nurmikko, Optical properties of Manganese-doped nanocrystals of ZnS, Phys. Rev. Lett. (1994), 72, 416) many articles on doped metal chalcogenide quantum dots have appeared, including topics such as new preparation methods, luminescence properties, and potential applications. In the field of doped nanoparticles, perhaps the most fundamentally interesting results are the luminescence enhancement and the lifetime shortening of Mn2+ emission from milliseconds in bulk to nanoseconds in ZnS:Mn2+ nanocrystals reported by R. N. Bhargava, D. Gallagher, X. Hong and A. Nurmikko, Optical properties of Manganese-doped nanocrystals of ZnS, Phys. Rev. Lett. (1994), 72, 416. However, A. A. Bol and A. Meijerink, Long-lived Mn 2+ emission in nanocrystalline ZnS:Mn 2+ , Phys. Rev. B 56, R15997 (1998), N. Murase et al., Fluorescence and EPR characteristics of Mn 2+-doped ZnS nanocrystals prepared by aqueous colloidal method, J. Phys. Chem. B 103, 754 (1999), and B. A. Smith, J. Z. Zhang, A. Joly, and J. Liu, Luminescence decay kinetics of Mn 2+-doped Zns nanoclusters grown in reverse micelles, Phys. Rev. B 62, 202+ (2000) later found that the Mn2+ emission in ZnS nanoparticles does not show a spectacular shortening of the decay time, but rather has the same decay as in bulk. The nanosecond decay time of Mn2+ emission reported by R. N. Bhargava, D. Gallagher, X. Hong and A. Nurmikko, Optical properties of Manganese-doped nanocrystals of ZnS, Phys. Rev. Left. (1994), 72, 416 was interpreted as the tail of a broad, defect-related ZnS emission. Despite the controversy surrounding the lifetime shortening and its possible mechanisms, the luminescence enhancement reported in R. N. Bhargava, D. Gallagher, X. Hong and A. Nurmikko, Optical properties of Manganese-doped nanocrystals of ZnS, Phys. Rev. Lett. (1994), 72, 416, has been observed by other groups.
Hereinabove, experiments and results were given that show that ZnS:Mn2+ nanoparticles display strong upconversion luminescence. Power dependences and lifetime measurements from multiwavelength excitation of both the photoluminescence and upconversion luminescence reveal that the mechanism for this strong upconversion is a two-photon absorption process. Upconversion luminescence is an effective method for distinguishing emissions from Mn2+ at different sites in the nanoparticles. The following experiments expand upon those outlined hereinabove and include the temperature dependence of the emission wavelength, and the emission intensity of both the photoluminescence and upconversion luminescence.
Temperature Dependence of Photoluminescence and Upconversion Luminescence
The preparation, structure, and fluorescence properties of Mn2+ doped ZnS nanoparticles have been described previously, hereinabove. The photoluminescence and upconversion emission spectra at different temperatures were measured by mounting the samples on the cold finger of a liquid helium flow-through cryostat using indium metal for thermal contact. The cold finger was equipped with a heater element and the temperature was controlled via a Lakeshore Model 330 temperature controller that monitored the temperature via a calibrated silicon diode attached to the indium metal at the sample position. Emission spectra were measured using a constant laser power density at several different temperatures between 8 and 273 K, and the integrated area of the broad Mn2+ emission was used to calculate the intensity at a specific temperature.
Temperature Dependence of the Emission Wavelength
The details of the upconversion luminescence of ZnS:Mn2+ were outlined hereinabove, but in brief, the strong upconversion luminescence observed in ZnS:Mn2+ nanoparticles is visible to the naked eye. The upconversion luminescence was recorded at a laser power density of 100 mJ/cm2, but it was detectable at a power density of 1 mJ/cm2. The upconversion emission band excited at 767 nm is red shifted from the photoluminescence emission excited at 300 nm for the nanoparticles prepared in the organic matrix. However, no shift is observed in the bulk or in the particles encapsulated in zeolite-USY. In the nanoparticles, the decay lifetimes of the upconversion emission excited at 767 nm are shorter than the 300 nm excited luminescence lifetimes, whereas in bulk the two decays are almost identical. When the photoluminescence is obtained by excitation at 383.5 nm, which is the sum energy of two photons at 767 nm, the emission spectra and the lifetimes of the two types of luminescence are almost identical.
The power dependence of the photoluminescence is linear, whereas that of the upconversion emission is quadratic. Based on these observations, two-photon excitation is responsible for the upconversion luminescence of Mn2+ in ZnS:Mn2+ nanoparticles. This conclusion is supported by the temperature dependences of the upconversion and the photoluminescence that are reported hereinafter.
In the particles prepared in MA and CA, the particle sizes are 3-5 nm, and Mn2+ ions occupy both the near-surface sites and the bulk lattice sites. A representative temperature dependence of the photoluminescence and upconversion luminescence of this type of nanoparticle is shown in
The temperature dependent luminescence spectra of bulk ZnS:Mn2+ in which all Mn2+ ions are in the lattice sites is shown in
With decreasing temperature, the emission spectra of both photo- and upconversion luminescence shift to longer wavelengths. This is due to the enhancement of the crystal field at lower temperatures that results from crystal lattice contraction. As a consequence, the emitting state, 4T1(G) of Mn2+, shifts to lower energies with decreasing temperature, shifting the emission to longer wavelengths. It is also evident from
First, the density-of-states for both the electrons and phonons decreases with size, which is likely to result in a weaker electron-phonon coupling. Similarly, the increased overlap between the electron and hole wave functions decreases the exciton-phonon coupling. In nanoparticles, the overlap between the electron and hole wave functions may decrease due to the trapping of electrons or holes to the surface states, increasing the phonon coupling strength. Thus, in the same nanoparticles, the exciton-phonon coupling in the near-surface sites is stronger than in the lattice sites. Therefore, with decreasing temperature, the enhancement of the crystal field strength at the near-surface sites is greater than at the lattice sites, and the related changes in the emission intensity and energy of the near-surface sites are more dramatic with decreasing temperature. This effect is even more pronounced in the spectra obtained following 383.5 and 767 nm excitation, because these excitation wavelengths preferentially excite the near-surface Mn2+ ions.
In contrast, even though most Mn2+ ions are at the near-surface sites in the particles formed in zeolite-USY, the temperature dependence of its emission energy is similar to that of bulk ZnS:Mn2+. These nanoparticles are encapsulated in the zeolite cavities and the particle surfaces are well passivated such that the exciton-phonon coupling contributions from surface states are less pronounced. The surface passivation of the nanoparticles encapsulated in zeolites is actually via the chemical bonding between the anions (Zn2+) at the nanoparticle surfaces and the zeolite framework oxygen ions (O2−—G. A. Ozin, NANOCHEMISTRY—SYNTHESIS IN DIMINISHING DIMENSIONS, Adv. Mater. 4, 612 (1992)). In this case, the surrounding of Mn2+ in ZnS:Mn2+/USY is similar to Mn2+ in bulk ZnS:Mn2+. This is the reason the two samples have similar luminescence temperature behaviors.
It is known from the preceding discussion that the temperature dependence of the luminescence intensity is closely related to the particle surface-interface characteristics. Better surface passivation likely results in a simplified temperature dependence. Previous work demonstrated that an inorganic coating is better than an organic coating for surface passivation. (N. S. Nalwa, Editor, Handbook of Nanostructured Materials and Nanotechnology, Academic Press, San Diego (2000), Vol. 4; G. A. Ozin, NANOCHEMISTRY—SYNTHESIS IN DIMINISHING DIMENSIONS, Adv. Mater. 4, 612 (1992)) Here, the ZnS:Mn2+ nanoparticles encapsulated in zeolite-USY are better passivated than MA/CA capped particles. This can be further evidenced from the observations that in the 3.5 nm particles, a blue emission at 425 nm is observed (data not shown) that is attributed to surface defects or deep traps. However, in the particles encapsulated in zeolite-USY, the defect-related blue emission is not observed. Also, it was reported that ultraviolet (UV) curing can enhance the luminescence intensity of MA capped ZnS:Mn2+ nanoparticles. This enhancement is attributed to better surface passivation of MA polymerization by UV curing. All these factors indicate that surface defects exist in the MA capped nanoparticles and thus surface effects must be considered for explaining the observed experimental results.
Temperature Dependence of the Emission Intensity
The temperature dependence of the luminescence intensity is more complicated than that of the emission energy.
This data further supports the conclusion that the upconversion luminescence is due to two-photon absorption. The temperature dependence of the luminescence intensity for the bulk ZnS:Mn2+ is similar to published results. (D. D. Thong and O. Goede, OPTICAL STUDY OF HIGHLY Mn-DOPED ZNS CRYSTALS, Phys. Status Solidi B 120, K145 (1983)). The gradual increase of the luminescence intensity with decreasing temperature may be explained by a weakening of phonon coupling at lower temperatures, the common theory for thermal quenching. The temperature dependence of Mn2+ photoluminescence intensity in ZnS:Mn2+ nanoparticles was reported by J. Q. Yu, H. M. Liu, Y. Y. Wang, F. E. Fernandez, and W. Y. Jia, Optical properties of ZnS:Mn 2 + nanoparticles in polymer films, J. Lumin. 76&77, 252 (1998). and M. Tanaka, and Y. Masumoto, Very weak temperature quenching in orange luminescence of ZnS:Mn2+ nanocrystals in polymer, Chem. Phys. Lett. 324, 249 (2000). J. Q. Yu, H. M. Liu, Y. Y. Wang, F. E. Fernandez, and W. Y. Jia, Optical properties of ZnS:Mn2+ nanoparticles in polymer films, J. Lumin. 76&77, 252 (1998) observed that the luminescence intensity of Mn2+ in polymer capped ZnS:Mn2+ nanoparticles increases significantly with temperature from 8 to 275 K, and then decreases with increasing temperature. However, Tanaka and Masumoto observed that the luminescence intensity of Mn2+ in ZnS:Mn2+ nanoparticles was very weakly dependent on temperature. In our observations, we find that the 300 nm excited photoluminescence of Mn2+ in the 3.5 nm sized nanoparticles is weakly dependent on temperature. In addition, in these studies we also find that the temperature behavior of ZnS:Mn2+ nanoparticles is dependent on both the particle size and the surface characteristics as evidenced from the data obtained from the 1 nm USY and 3.5 nm samples (
There are two common mechanisms for luminescence excitation in doped semiconductors. One is excitation of the impurity ions to their excited states directly. The other type is indirect excitation, i.e., excitation into the excited levels of the host, followed by energy transfer from the host to the impurity ions, thereby inducing the luminescence. This type of excitation is termed interband excitation.
We first consider the case of interband excitation following 300 nm irradiation. In this case, the luminescence intensity of Mn2+ in ZnS:Mn2+ can be expressed as:
where T is the absolute temperature, ┌ZnS(T) is the energy transfer rate from ZnS excitons to Mn2+, and σMn(T) is the luminescence efficiency of the Mn2+ 4T1→6A1 transition in the absence of energy transfer to other species or defect states. If there is energy transfer from Mn2+ to other states such as surface states in the nanoparticles, the luminescence intensity of Mn2+ in ZnS:Mn2+ should be expressed as:
where ┌ET(T) is the energy transfer rate from Mn2+ ions to other states.
The content of surface states in the nanoparticles prepared in methacrylic and citric acids is much higher than in the nanoclusters encapsulated in zeolite-USY. Furthermore, it has been suggested that in weakly confined nanoparticles such as the 3.5 nm size studied, energy can transfer from the excited states [4E(4D) and 4T2(4D)] of Mn2+ to the surface states, whereas energy cannot transfer from the excited states of Mn2+ to the surface states in the strongly confined nanoclusters like those encapsulated in zeolite-USY, because the surface states in the latter are higher in energy than the Mn2+ excited states. Thus, in ZnS:Mn2+/USY and bulk ZnS:Mn2+, we will not consider the energy transfer from Mn2+ to explain the temperature dependence of the luminescence; in the 3.5 nm sized nanoparticles, the energy transfer from Mn2+ ions to surface states must be considered.
┌ZnS(T) is determined by three processes: (1) dissociation of the ZnS excitons, (2) nonradiative processes, and (3) energy transfer from the exciton to defects or surface states. In nanoparticles, due to quantum size confinement, the exciton binding energy increases and the thermal dissociation temperature usually is higher than room temperature. Therefore, in the measured range, the temperature change has little effect on exciton dissociation. The exciton nonradiative relaxation proceeds via exciton-phonon coupling, which is determined by both the quantum confinement and the surface characteristics. As described previously, the density of states and the increased overlap between electron and hole wave functions serve to weaken the electron-phonon coupling. On the other hand, the overlap between the electron and hole wave functions may decrease due to trapping of electrons or holes to the surface states, increasing the phonon coupling strength. These factors may compete with each other and determine the overall size dependence of the phonon coupling strength. Therefore, if only the exciton binding energy and the exciton-phonon coupling are considered, the temperature dependence of the 300 nm excited luminescence (which predominantly excites lattice site Mn2+ ions) is expected to be less pronounced in nanoparticles than in the bulk (Table I). Table I lists the experimentally determined ratios of highest intensity to lowest intensity within the measured temperature ranges.
300 nm
767 nm
383.5 nm
3.5 nm
For the 3.5 nm sized nanoparticles, a blue emission at 420 nm is observed. This luminescence has been assigned to the sulfur-vacancy defects, implying that a high degree of defects are present in these nanoparticles such that the foregoing process is expected to make a significant contribution to ┌ZnS(T). The luminescence quenching by this energy transfer is more efficient at lower temperatures, because thermal depopulation via phonons is decreased. This luminescence quenching may compete with the phonon quenching, which is stronger at high temperatures. Thus, the temperature dependence of the 300 nm excited luminescence in the 3.5 nm sized nanoparticles is even weaker than in ZnS:Mn2+/USY (Table I). Note that the 300 nm excited luminescence of ZnS:Mn2+/USY decreases in intensity with decreasing temperature from 145 to 10 K. Most likely, this decrease is related to the decrease in absorption intensity at 300 nm. As displayed in
The second case we consider is that due to direct excitation of the Mn2+ d-d transitions expected following excitation at either 383.5 or 767 nm. In this case, the luminescence efficiency is determined by two processes: the nonradiation relaxation of the excited states via phonon coupling and the energy transfer from Mn2+ excited states to the surface states. As previously discussed, only in the 3.5 nm sized nanoparticles should the energy transfer from Mn2+ to the surface states be considered. In bulk and ZnS:Mn2+/USY, the main temperature effect we need to consider is phonon coupling.
Thus, it is expected that the 383.5 nm luminescence and the 767 nm upconversion luminescence increase with decreasing temperature due to the decrease in phonon coupling, which is in agreement with the observations. For the 3.5 nm sized nanoparticles, however, the luminescence intensity first decreases and then increases with increasing temperature. This behavior may be understood by considering the competition between phonon quenching and surface quenching, which together may determine the overall temperature dependence of the luminescence intensity. The luminescence intensity decrease with increasing temperature can be understood based on phonon quenching, as discussed.
The luminescence increase with increasing temperature can be rationalized by considering surface state quenching, because charge carriers at the surface states or trap states can be activated or excited at higher temperatures and participate in energy transfer to the excited state of Mn2+, thereby resulting in an increase in Mn2+ luminescence. This is illustrated schematically in
In summary, the temperature dependences of the photoluminescence and upconversion luminescence of ZnS:Mn2+ nanoparticles and bulk have been described—i.e. a UCL material having the general formula (X):(Y) wherein (X) is a host such as a nanoparticle and (Y) is a dopant such as a rare earth or transition metal ion, has been described and enabled. The observed shift of the Mn2+ 4T1→6A1 emission to longer wavelengths at lower temperatures is explained by the increase in the crystal field strength, that moves the emitting state 4T1 to lower energies. The temperature dependence of the luminescence intensity shows complicated behavior, indicating the underlying interplay of energy transfer and relaxation processes in these materials. This interesting temperature behavior may be reasonably explained by considering the processes of nonradiation relaxation via phonon coupling, exciton thermal dissociation (binding energy), energy transfer, carrier trapping, and temperature dependence of the absorption spectra. The fact that the temperature dependence of the 767 nm excited upconversion luminescence is the same as the 383.5 nm excited photoluminescence for all the three samples supports the conclusion that the upconversion luminescence is due to two-photon absorption.
Upconversion Luminescence Production Assembly
Referring now to
In general, the upconversion luminescence production assembly 20 includes an electromagnetic source 26, a substrate 28, and a UCL material 30. The electromagnetic source 26 receives an input signal and in response thereto emits the excitation 24 having an excitation wavelength. The input signal can be a voltage, current or other type of signal capable of causing the electromagnetic source 26 to produce the excitation 24.
The substrate 28 is positioned within the excitation 24 emitted by the electromagnetic source 26. The UCL material 30 covers at least a portion of the substrate 28 such that the excitation 24 emitted by the electromagnetic source 26 is received by at least a portion of the UCL material 30. The UCL material 30 is formed of one or more of the doped nanoparticles discussed above. The UCL material 30 produces the emission 22 through upconversion luminescence such that the emission 22 has the emission wavelength which is shorter than the excitation wavelength of the excitation 24 received by the UCL material 30. Thus, the energy level of the emission 22 is higher than the energy level of the excitation 24.
The electromagnetic source 26 can be any device capable of producing the excitation 24, e.g., a light emitting diode, a laser, a lamp, or a heating element such as a hot wire. The excitation wavelength can vary depending on the desired emission wavelength, as well as the particular material forming the UCL material 30. For example, the excitation wavelength of the excitation 24 can form radio frequency signals, infrared light, visible light, ultraviolet light, microwaves or x-ray radiation. In one preferred embodiment, the electromagnetic source 26 is an infrared source, and the UCL material 30 converts the infrared light (excitation 24) to visible light (emission 22). In this instance, the excitation wavelength would be longer than about 800 nm and the emission wavelength would be in a range between about 360 nm to about 750 nm or shorter than 800 nm.
The substrate 28 can be any material capable of supporting the UCL material 30 such that the excitation 24 produced the electromagnetic source 26 can be received by the UCL material 30. For example, as shown in
Although the UCL material 30 has been shown in
The UCL material 30 includes one or more nanoparticles formed from the UCL material(s) discussed above. With respect to the UCL materials incorporating a dopant such as Manganese but not limited thereto, the UCL material 30 can alternatively be formed of bulk materials, such as powders as well as nanoparticles. The UCL material 30 is desirably constructed of a film positioned on the first side 32 and/or the second side 34 of the substrate 28. The thickness of the film can vary depending on the properties desired in the emission 22. That is, as the thickness of the film increases, the brightness of the emission 22 will also increase until the thickness of the film serves to limit the brightness. The thickness of the film can be 1 nanoparticle thickness or greater.
The UCL material 30 preferably has a substantially uniform thickness. However, it should be understood that the thickness of the UCL material 30 can be varied if desired. Varying the thickness of the UCL material 30 will form areas having greater and lower intensities of the emission 22.
The UCL material 30 can be applied to the substrate 28 via any suitable method or system capable of connecting or supporting the UCL material 30 on or by the substrate 28. Any system or method can be used so long as such system or method does not change the properties of the nanoparticles utilized to form the nanoparticles device.
The following methods are suitable for making nanoparticle thin films: layer-by-layer ("LBL") assembly, spin-coating or spin-casting, spray, thermal spray, drop, supercritical fluids, thermal plasma deposition, and laser ablation. The preferred methods for forming the nanoparticle thin films are spin-coating, layer-by-layer, spray and ink-jet printing. One of ordinary skill in the art would appreciate, however, that these methods are not exhaustive and any method of making a nanoparticle thin film is contemplated for use.
Layer-by-layer assembly has been initially introduced by G. Decher (G. Decher, Fuzzy nanoassemblies: toward layered polymeric multicomposites, Science, 1997, 277: 1232-1237) for oppositely charged polyelectrolytes. Being quite universal, it was later expanded to the assembly of various inorganic colloids. One of the most promising directions of this technique is that the preparation of hybrid organic-inorganic materials affords the combination of optical and electric properties of inorganic components with excellent adhesion, processability and flexibility of polymers. N. A. Kotov, I. Dekany and J. H. Fendler, Layer-by-layer self-assembly of polyelectrolyte-semiconductor nanoparticle composite films, J. Phys. Chem. 1995, 99:13065-13069 were the first to report on LBL assembly of Group II-VI semiconductor nanoparticles, where light emitting properties of quantum dots were successfully integrated into a thin hybrid film. Additionally, it was demonstrated that not only the II-VI material itself but also the order in which the layers were assembled affect photophysical and photochemical processes in the multilayer assembly. Later, it was also confirmed for the photo-induced charge transfer in LBL assemblies of CdS and graphite oxide.
The advantages of layer-by-layer thin films are:
The driving force for LBL is the electrostatic attraction of positive and negative charges located on the surface of inorganic colloids and polyelectrolytes. An important thermodynamic contribution to the film stability is also made by the van Der Waals interactions. Typically, the assembly process consists of a cyclic repetition of four steps:
The substrate 28 can be constructed of any material capable of supporting or lending support to a UCL material 30. The substrate 28 can be opaque or transparent. The substrate 28 can be constructed of a rigid material, such as quartz, or a flexible material such as natural or synthetic polymers, cloth, or combinations thereof. The substrate 28 can be formed of a solid material, a liquid material, or a gaseous material. Although the substrate 28 has been shown in
The electromagnetic source 26 is shown in
Although the electromagnetic source 26 is shown in
Furthermore, the position of the electromagnetic source 26 can be fixed with respect to the position of the substrate 28, or, the position of the electromagnetic source 26 can be varied with respect to the position of the substrate 28, or vice versa. In other words, the electromagnetic source 26 can be provided in a fixed location and the substrate 28 moveable into and/or out of the excitation 24, the substrate 28 can be provided in a fixed location and the electromagnetic source 26 moveable with respect to the substrate 28, or both the substrate 28 and the electromagnetic source 26 can be moveable with respect to each other.
For example, the upconversion luminescence production assembly 20 can be implemented in the form of a security system for use at clubs where it is desirable to determine whether an individual has paid the cover charge. That is, upon payment of a cover charge, the UCL material 30 can be applied to a hand or an arm of an individual (substrate 28) by any suitable process, such as by stamping. A guard or other security personnel can be provided with an electromagnetic source 26, such an infrared electromagnetic source, which such security personnel could shine on the individual's hands and/or arm to determine whether the individual has paid the cover charge. Upon receipt of the infrared light (excitation 24) the UCL material 30 would upconvert the excitation 24 to a visible light so that the security personnel could determine whether that individual's arm and/or hand had been stamped.
Shown in
Shown in
Recently, upconversion lasers have been reported in rare earth ion doped halide crystals by W. Lenth, and R. M. Macfarlane, Upconversion lasers, Optics and Photonics News, (March, 1992), pp 8-14 and P. Xie and S. C. Rand, Astigmatically compensated, high gain cooperative upconversion laser, Applied Physics Letters, 1992, 60: 3084-3086. In the present invention, it has been demonstrated that doped nanoparticles have strong upconversion luminescence. Thus, it is expected that it is possible to fabricate upconversion lasers made by these nanostructured materials.
Referring now to
The reflective surfaces M1 and M2 can be made with any suitable materials capable of reflecting energy, such as light within the laser cavity. For example, the reflective surfaces M1 and M2 can be made with silver, gold or tool steels. One of the two reflective surfaces M1 and M2 must be semi-transparent. For example, a partially silvered mirror can be used as a semi-transparent reflective surface (M1) at one end of the substrate 26 c while a totally silvered reflective surface (M2) can be used at the other end. The substrate 26 c can be made with ruby, silica or glass. Reflective surfaces of the reflective surfaces M1 and M2 at both ends of the laser cavity 40 permit energy to reflect back and forth, building up in each passage.
A UCL material 30 c formed of nanoparticles, or other size materials as discussed above can be disposed into the substrate 28 c by solid state diffusion at high temperature such as 800� C. or by ion-exchange in solution (W. Chen, R. Sammynaiken, and Y. Huang, Luminescence enhancement of ZnS:Mn nanoclusters in zeolite, J. Appl. Phys. 88, 5188 (2000). And W. Chen, X. H. Zhang, and Y. Huang, Luminescence enhancement of EuS nanoclusters in zeolite, Appl. Phys. Lett., 2000, 24, 2328-2330). Alternatively or in addition, the UCL material 30 c can be coated to an exterior surface 44 of the substrate 28 c by any suitable technique, such as layer-by-layer assembly or spray.
The laser 20 c includes the electromagnetic source 26, which in this instance can be a laser or other light source such as an infrared light-emitting diode. Upon optically pumping from the electromagnetic source 26, the emission from the UCL material 30 c will bounce back and forth between the reflective surfaces M1 and M2 until coherent light escapes from the laser cavity 40. This process is called stimulated emission. The bombarding photon and the emitted photon may then each strike other excited particles, stimulating further emission of photons, all of the same frequency and phase. It is expected that this process will produce a sudden burst of coherent radiation, that is a lasing action.
Referring to
The nanowire forming the substrate 28 d can be formed from any suitable material capable of being formed into a nanowire and receiving a dopant, such as CdS:Mn. In one preferred embodiment, the diameter of the nanowire is 5 nm with a length of 2 micrometer, for example.
An optically pumped nanowire laser has been reported by P. D. Yang et al., Room-temperature ultraviolet nanowire nanolasers, Science, 2001, 292: 1879 and P. D. Yang, Miniaturised Ultraviolet Lasers, Global Photonics Applications & Technology, 2002, pp 42-47 by self-assembling ZnO nanowires vertically on a sapphire or quartz substrate. When the nanowires were pumped optically by a fourth harmonics of neodymium-doped yttrium-aluminum-garnet (Nd:YAG) laser at 266 nm, in the absence of any fabricated mirrors, lasing at wavelengths between 370 and 395 nm was observed from the ZnO nanowires. It was found by P. D. Yang that the interface between the sapphire and ZnO and the other end of the ZnO nanowires can function as excellent laser cavity mirrors. For II-VI semiconductors, the cleaved edge of the specimen on one of the sides can be used as a mirror.
The nanowire forming the substrate 28 d includes a UCL material 30 d positioned on or within the substrate 28 d. An interface between the second substrate 46 and the substrate 28 d forms one reflecting surface M3, and the other end of the substrate 28 d forms another reflecting surface M4. A laser cavity 45 is formed between the reflecting surfaces M3 and M4.
The difference between the upconversion luminescence production assembly 20 d and the lasers reported by P. D. Yang is the following. The upconversion luminescence production assembly 20 d is based on upconversion luminescence, that is the excitation wavelength is longer than the emission wavelength, while, Yang's laser is based on photoluminescence, that is the excitation wavelength is shorter than the emission wavelength.
Upconversion Temperature Sensors
As shown in
Referring to
The receiver 48 can be any device capable of receiving the emission 22 and producing the output signal. For example, the receiver can be a CCD, one or more phototransistors, one or more photodiodes or the like.
The temperature sensor 47 can be used for determining the temperature of materials, such as biological materials. When used for biological materials, the energy power or the power density of the excitation 24 should be maintained at a low level as discussed herein to avoid damaging the biological material. The temperature sensor 47 can be used for determining the temperature of the biological materials for use in treating patient conditions, such as hyperthermia or heat treatment of tumors or cancers. For example, the UCL material 30 can be formed of bulk and/or nanosized ZnS:Mn2+.
The UCL material 30 can be bound or conjugated with biological material, such as a tumor within a human or non-human host, as discussed herein. In this instance, the electromagnetic source 26 and the receiver 48 are preferably positioned outside of the host's body. The excitation 24 will have a wavelength in the range of 700 to 1,000 nm and, more preferably, 800 nm, and which s capable of passing through at least a portion of the host's body so that the excitation 24 can be received by the UCL material 30 within the tumor. The UCL material 30 then creates the emission 22 having an emission wavelength, such as 600 nm and that is capable of passing through and exiting the host. In addition, the UCL of the nanoparticle inside the tumor or host can be excited and deflected through a tiny optical fiber.
As a temperature indicator in biological or biomedical applications, upconversion semiconductor nanoparticles have at least two advantages over organic dyes. One is semiconductor nanoparticles are more stable than organic dyes (W. C. W. Chan, S. Nie, Quantum dot bioconjugates for ultrasensitive nonisotopic detection, Science, (1998) 281, 2016 and M. J. Jr. Bruchez, M. M. Moronne, P. Gin, S. Weiss, P. A. Alivisatos, Semiconductor nanocrystals as fluorescent biological labels, Science, (1998), 281, 2013), the other is upconversion nanoparticles can provide more accurate measurement than organic fluorescent dyes because no autofluorescence is observed in upconversion measurement. As yet another example, the upconversion temperature sensor 47 can be made by coating nanoparticles to a tip or one end of an optical fiber. In this case, excitation and emission of the nanoparticles can be conducted or detected at the other end of the fiber. This kind of temperature sensor can be used for remote sensing of temperatures from a moving target like a turbine engine, a hot surface or in some hazardous conditions using a tiny fiber, this kind of temperature sensor also can be used to detect a body temperature of a human or non-human host.
Referring now to
The upconversion luminescence production assembly 20 includes the electromagnetic source 26, the substrate 28 and the UCL material 30 as discussed above. The substrate 28 is spaced a distance 56 from the substrate 28 so as to define a receiving space 58. The UCL material 30 is positioned within the excitation 24 produced by the electromagnetic source 26. An interfering object 60 is positioned within the receiving space 58 such that the interfering object 60 receives at least a portion of the excitation 24 and thereby interferes with the excitation 24. The interfering object 60 can be any article which blocks, magnifies, emits, reflects or otherwise alters the excitation 24. For example, the interfering object 60 can be a bone, a tumor or a hot spot.
At least a portion of the excitation 24 is received by the UCL material 30, which upconverts the excitation 24 to form the emission 22, as discussed above. The imaging device 52 captures one or more image of the emission 22, and provides the one or more image to the image processing system 54.
The image processing system 54 can be any device capable of executing image processing software such that the image is perceivable or otherwise usable by an individual. For example, the image processing system 54 can be a Windows-compatible or Macintosh compatible personal computer obtainable from Dell Computer Corporation of Austin Tex., Hewlett-Packard Company of Palo Alto Calif., International Business Machine of Armonk N.Y. or Apple Computer, Inc. of Cupertino Calif. The image processing software can be Photoshop Version 7.0 obtainable from Adobe Systems Incorporated of San Jose Calif.
The upconversion luminescence material of the presently claimed and disclosed invention is also useful in biological systems as a sensor, probe, or label. The use of luminescent or fluorescent material as sensors, probes, or labels is well known in the art—the use of a bioconjugated UCL nanoparticle as a biological sensor, probe or label, however, is novel and has many advantages over the prior devices. Biomodified nanoparticles from a variety of inorganic materials are known in the art and such biomodified nanoparticles can be used in biological pursuits for luminescence tagging, drug delivery, and implantable microdevices as well as for assembling hybrid protein-nanoparticle units for molecular electronics. For example, Kotou, et al. have reported the conjugation of luminescent CdTe nanoparticles with bovine serum albumin. (N. A. Kotov, I. Dekany and J. H. Fendler, Layer-by-layer self-assembly of polyelectrolyte-semiconductor nanoparticle composite films, J. Phys. Chem. 1995, 99:13065-13069 have reported the conjugation of luminescent CdTe nanoparticles with bovine serum albumin. (Shaopeng Wang, Natalia Mamedova, Nicholas A. Kotov, Wei Chen, and Joel Studer, Antigen/Antibody Immunocomplex from CdTe Nanoparticle Bioconjugates, Nanoletters, 2002, 2(8): 817-822 the contents of which are hereby expressly incorporated by reference in their entirety).
In order to make UCL sensors, probes, or labels, the upconversion luminescence material must be conjugated to a biospecific ligand and/or have the ability to bind to a biological target (i.e. bioengineered or modified to have a surface state capable of binding to a biological target, such as a protein). One of ordinary skill in the art, given the present disclosure, would appreciate that the UCL material of the presently claimed and disclosed invention could be used as a biomodified probe or label, wherein the probe or label is specifically a biomodified nanoparticle.
Bioconjugation of nanoparticles i.e. the attachment biospecific ligands to them, represents the intersection of biotechnology and nanotechnology which results in hybrid materials, processes, and devices that can utilize both the unique optical and magnetic properties of nanoparticles and highly selective binding of oligonucleotides and proteins. The combination of these features is highly desirable for current biomedical technologies. Particularly, the bioconjugation of nanoparticles with proteins can be used as (1) a method for their organization in more complex structures and (2) a pathway to new sensing and imaging technologies. Herein is disclosed the preparation of bioconjugates from an antigen and an antibody conjugated to nanoparticles of different size. The results show that the nanoparticle labeled antigen and antibody form an immunocomplex, which pairs together nanoparticles with red and green luminescence (see e.g.
These tests are related to the further development of immunoluminescence as a technique that affords highly sensitive and specific detection of various biological and non-biological analytes of military and civilian importance. The detection limits can be as low as 10 ppt with a linear dynamic range from 0.1 to 1000 ppb, while the utilization of antibodies enables selective detection substrates, which may differ only by a few atoms. Upconversion luminescent nanoparticles have opened new possibilities for biological labeling. It has been shown by others in the art, that nanoparticle bioconjugates selectively bind to cell components, DNA, or blood proteins and can be detected by strong luminescence which, in turn, is capable of being tuned by altering the particle size. The stability of nanoparticles makes possible not only high-contrast multiplexed imaging, but also a long-term monitoring of the environment—e.g. for biological warfare agents or natural pathogenic organisms such as cholera and E. Coli, which represent a critical problem for countries with limited water treatment capabilities. Notably, the use of UCL nanoparticle-bioconjugates does not require sample labeling/staining. Additionally, when the nanoparticle is a UCL material (i.e. doped with a rare earth or transition metal ion), the upconversion luminescence provides for an even more reliable and easily manipulated biological probing sensing, or labeling system.
Thioglycolic-acid-stabilized CdTe nanoparticles were synthesized by the modified method reported in (W. Chen, D. Gronquist, and J. Roark, Voltage tunable electroluminecsnce of CdTe nanoparticle light-emitting diodes, J. Nanosci. Nanotechnol., 2002, 2: 47-53), the contents of which are expressly incorporated herein in their entirety. Generally, two different nanoparticle dispersions were utilized—one with a luminescence peak at 555 nm and the other with a luminescence peak at 611 nm. Nanoparticle-conjugated BSA and anti-BSA IgG were prepared by the sulfo-NHS(N-hydroxysulfo-succinimide) and EDC (1-ethyl-3(3-dimethylaminopropyl)carbodiimide hydrochloride) reaction (
We tested different reaction conditions and examined the results by native and SDS-PAGE gel electrophoresis. The following is the optimal protocol developed in the course of these experiments. A reaction mixture containing 0.05 mM CdTe nanoparticle, 1.5˜2.5 mg/mL antigen or antibody, 0.05 M NHS, 0.05 M EDC in pH 7.0 PBS buffer were prepared and kept in room temperature for 2-4 hrs., then stored at 4� C. overnight, thereby allowing the unreacted EDC to hydrolyze and lose its activity. Thereafter, a small amount of precipitate is formed which is likely to consist from unconjugated nanoparticles, which are known to agglomerate and become non-emissive at low pH—e.g. pH≦7. The precipitate (if any) is removed by centrifugation. The stock, ready-to-use solution of the product was stored at 4� C. Optionally, one can dialyze it with Spectra/Por� 4 Membrane, MWCO: 12,000-14,000 (Spectrum Laboratories, Inc), in pH 7.4 PBS buffer.
The native electrophoresis results (
The mobility of the proteins in SDS-PAGE electrophoresis (
Considering SDS-PAGE data, two points are relevant to other nanoparticle bioconjugates: (1) estimates of molecular masses on the basis of gel electrophoresis results can give erroneous results for bioconjugates from highly charged nanoparticles; and (2) bioconjugation to nanoparticles may increase the stability of antibodies. A brief heat treatment for 150 seconds at 96� C. was used for the preparation of SDS-PAGE samples. The relative intensity of the SDS-PAGE bands indicates that the conjugated antibodies are more resilient to this temperature than the unlabeled ones. The unlabeled antibodies (well 3) are mostly broken apart by the heating into small fragments showing up as band at 40-50 kDa, while for nanoparticle-IgG conjugates, this band has significantly lower intensity—the antibody remains mostly intact. The latter effect was also reproduced in several control experiments. Thus, the conjunction of an antibody or other biological specimen/material results in a more stable construct capable of being used in what was previously considered adverse or unfavorable conditions.
The direct evidence of the successful conjugation of CdTe to BSA and IgG can be found from the luminescence analysis of the gel plates. The bands of nanoparticle conjugates display strong luminescence while the free proteins do not show any detectable signal in the luminescence image (
Circular dichroism (CD) spectra (
In summary, bioconjugates with complementary antigen and antibody have been prepared from thiolacid-stabilized nanoparticles. They retain substantial bioactivity and can form the corresponding immunocomplex, which pairs nanoparticles with different emission properties in one supramolecular assembly.
Nanoparticles demonstrating UCL properties have also been conjugated to biomaterials. In particular, CdTe nanoparticles exhibiting upconversion luminescence (W. Chen, D. Gronquist, and J. Roark, Voltage tunable electroluminecsnce of CdTe nanoparticle light-emitting diodes, J. Nanosci. Nanotechnol., 2002, 2: 47-53) were conjugated to dead Brucella suis and the corresponding antibody anti Brucella Suis and the conjugated bio-nanoparticle demonstrated UCL properties. According to the method described hereinabove with respect to BSA. Such a bioconjugated nanoparticle is even more useful in detection, sensor, and probe applications because of the enhanced detection and ease of detection owing to the UCL properties. The conjunction of the UCL nanoparticle with a biomaterial is detailed hereinafter.
UCL Nanoparticle Conjugated B. suis and Anti-B. suis.
For these experiments, a sample of dead Brucella suis (B. suis) [Strain 145] whole cells and the corresponding antibody Bru 38 for one of its membrane polysaccharides (Anti-GBa-O side chain) was obtained. Although the surface of the bacteria is complex, it contains numerous primary amine sites for nanoparticle binding. The B. suis whole cells were decorated with the green-emitting CdTe nanoparticles (561 nm) following the same conjugation protocol developed in the BSA experimental series described hereinabove, while Bru 38 was cross-linked to red-emitting nanoparticles (601 nm). The conjugated B. suis was purified by centrifuge and washed with 0.1 M PBS buffer three times. Native and SDS page gel electrophoresis results of the conjugated Bru 38 were similar to that of the nanoparticle-anti-BSA conjugate. The results demonstrate that the UCL nanoparticles can be bound to biomolecules.
Upconversion Luminescence of Bioconjugated CdTe Nanoparticles
In order to test the use of upconversion nanoparticles in biological labeling and biological sensors, CdTe nanoparticles were attached to the anti-B. suis as described above. Presently, it was determined that after the nanoparticles were bound to the biomaterial, upconversion luminescence was maintained. In the present experiments, bioconjugated nanoparticles (CdTe linked to anti B. suis IgG) still maintain strong upconversion luminescence and that their spectral positions were substantial the same as that of free nanoparticles (
For measuring the upconversion luminescence from the free or bioconjugated nanoparticles, the excitation wavelength from a Xenon lamp in a Spex Fluorolog fluorometer was set to 750 nm and a 600 nm optical filter was inserted to the excitation slit, and the emission was scanned from 550 nm to 700 nm. For the photoluminescence, the excitation wavelength was set to 350 nm and the emission was scanned from 550 nm to 700 nm. The shift of the upconversion emission peak from the photoluminescence peak demonstrated further that the 750 nm excited emission is due to upconversion or anti-Stokes luminescence. The shift of upconversion peak to longer wavelength is due to the involvement of surface states in the upconversion process (W. Chen, D. Gronquist, and J. Roark, Voltage tunable electroluminecsnce of CdTe nanoparticle light-emitting diodes, J. Nanosci. Nanotechnol., 2002, 2: 47-53).
One such sensing apparatus using a UCL bioconjugate material is shown in
The strength of the present UCL material for use in sensing and detecting systems is shown graphically in
Light Converting Materials
From the above disclosure, we know what is called upconversion luminescence and the advantages of upconversion nanoparticles. On the contrary, photoluminescence or fluorescence in which the excitation energy is higher (shorter) than the emission energy (wavelength), is called down-conversion luminescence. Some nanoparticles such as ZnS:Tb3+,Er3+; ZnS:Tb3+; Y203:Tb3+; Y203:Tb3+, Er3+; ZnS:Mn2+; ZnS:Mn,Er3+ have two functions, as down-conversion and upconversion luminescence. However, it should be understood that combinations of nanoparticles or bulk materials exhibiting only one or a combination of up-conversion luminescence and photoluminescence can be employed depending on the results desired. For example, one UCL material can be combined with a photoluminescing material to convert both IR light and UV light to visible light. That is, with a combination of the two types of luminescence, we can convert high energy light such as UV and low energy light such as IR to a medium energy light such as green.
Based on this idea and using the examples of the two-functional nanoparticles as mentioned (or two different nanoparticles or other sized materials each exhibiting single function), nanoparticles or other sized materials can be used to build light conveying substrates capable of being utilized as a window, light bulb, lens or for forming at least a portion of a greenhouse. It is well known that the sunshine spectrum is from UV to visible to IR, whereas most plants just take in the visible light such as green light for harvesting or growing or photosynthesis. The UV or IR light is actually not good for plant growth because the UV or IR light can damage or dry the plant tissues. If the UV and IR light are converted to visible light, this will be beneficial for growing plants like tomatoes, watermelon, grapes, etc.
As an example, referring to
The term "botanical item" when used herein means a natural or artificial herbaceous or woody plant, taken singly or in combination. The term "botanical item" also means any portion or portions of natural or artificial herbaceous or woody plants including stems, leaves, flowers, blossoms, buds, blooms, cones, propagules, other foliage or roots taken singly or in combination, or in groupings of such portions such as a bouquet or a floral grouping.
The term "propagule" when used herein means any structure capable of being propagated or acting as an agent of reproduction including seeds, shoots, stems, runners, tubers, plants, leaves, roots or spores.
The term "growing medium" when used herein means any liquid, solid or gaseous material used for plant growth or for the cultivation of botanical items 310, including organic and inorganic materials such as soil, humus, perlite, vermiculite, sand, particulate matter, water, and including the nutrients, fertilizers or hormones or combinations thereof required by the botanical items 310 for growth or survival.
The term "container" as used herein refers to any type of device used for holding a botanical item and/or growing medium. Examples of containers, used in accordance with the present invention, include but are not limited to: clay pots, wooden pots, plastic pots, foam pots, growing trays including metal trays and plastic trays, pots made from natural and/or synthetic fibers, or any combination thereof.
Shown in
The light converting material 322 can be provided on an exterior surface 326 or an interior surface 328 of the substrate 320. The substrate 320 can be made of any type of rigid, semi-rigid, crystalline, or flexible translucent or transparent material, such as silica glass, carbonate treated polymer, carbonate heated polymer, or plastic.
The light converting material 322 can be formed of one material having both downconversion and upconversion abilities, as discussed above or combinations of a UCL material and a down-converting material. For example, one UCL material as discussed above can be combined with a photoluminescing material to convert both IR light and UV light to visible light. With a combination of the two types of luminescence, high energy light such as UV and low energy light such as IR are converted to a medium energy light such as green.
Alternatively, or in addition, as shown in
Either way, the converting of the IR and UV light to visible light as discussed herein may help the botanical items 310 grow faster, higher, and/or shorten the period of time for maturation or harvesting.
Thus, it should be apparent that there has been provided in accordance with the present invention upconversion luminescence material and methods of making and using same that fully satisfies the objectives and advantages set forth above. Although the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art. Accordingly, it is intended to embrace all such alternatives, modifications, and variations that fall within the spirit and broad scope of the appended claims.
The following references, to the extent that they provide exemplary procedural or other details supplementary to those set forth herein, are specifically incorporated herein by reference in their entirety as though set forth herein in particular.
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