Method for optical cooling through semiconductor nanoparticle anti-stokes photoluminescene

Abstract

A process is disclosed for cooling a material that includes semiconductor nanoparticles in matrix material by anti-Stokes up-conversion. The semiconductor nanoparticle matrix is irradiated by a laser with a photonic wavelength matched to the anti-Stokes photoluminescence of the semiconductor nanoparticle bandgap. The semiconductor nanoparticles absorb the laser photon and phonons (heat) from lattice vibrations to photoluminescence photons with higher energy than the photon that were absorbed. A net cooling effect is generated from the lower energy and lower temperature in the material after anti-Stoke up-conversion.

Claims

1. A method of fabricating a quantum dot cooling system for a surface using a vacuum housing, a bandpass filter, and a lens to move photons to a fiber optical cable, comprising: mixing quantum dots in a polymer to form a quantum dot matrix; applying a reflective backing layer to the quantum dot matrix to direct any emitted photons away from the surface to be cooled; placing the quantum dot matrix in the vacuum housing; positioning a bandpass filter to receive any emitted photons and direct photons to the lens; and positioning the lens to receive any emitted photons from the bandpass filter and focus photons to the fiber optical cable.

2. The method of claim 1, further comprising: positioning a laser pump to impinge the quantum dot matrix with a laser through the bandpass filter.

3. A process of optically cooling, comprising: a) selecting a material containing quantum dots and a laser such that the quantum dot band gap, laser wavelength, absorption peak, excitation peak, and polymer transmissivity are optimized; b) fabricating a quantum dot cooling system by providing the material containing quantum dots and the laser selected in step a), adding a reflective layer to the selected material containing quantum dots, applying the selected material containing the quantum dots having the reflective layer to a surface to be cooled, and positioning a laser device to impinge on the selected material containing the quantum dots having the reflective layer the selected laser wavelength; and c) irradiating the selected material containing the quantum dots having the reflective layer with a laser beam having selected the laser wavelength to drive anti-Stokes photoluminescence in order to create a net cooling on the surface to be cooled.

4. The process of claim 3, further comprising: positioning a bandpass filter between the laser device and the selected material containing quantum dots having the reflective layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a general schematic of the photon absorption and anti-Stokes photoluminescence process for semiconductor nanoparticles.

(2) FIG. 2 is a simplified schematic representation of the optical cooling process using laser excitation and anti-Stokes photoluminescence.

(3) FIG. 3 is a schematic representation of an optical cooling device based on emitting anti-Stokes shifted photons in a vacuum and withdrawing them from the surface using an optical lens directed toward a fiber optic cable.

(4) FIG. 4 is an alternative schematic representation of an optical cooling device based on emitting anti-Stokes shifted photons in a vacuum and withdrawing them from the surface through a bandpass filter which allows the excitation laser to pass through while filtering the anti-Stokes shifted photons to an optical lens and fiber optic cable.

(5) FIG. 5 is a graph showing the photoluminescence intensity from 500 to 700 nm at excitation wavelengths between 500 and 630 nm

(6) FIG. 6 is a graph showing the transmission of a 640 nm laser through PMMA disks with varying QD loadings from 10-300 μg/mL

DETAILED DESCRIPTION OF THE DRAWINGS

(7) The absorption and emission process of a semiconductor is illustrated in FIG. 1. The semiconductor material has a handclap between the conduction 20 (upper) and valence (lower) bands 10. As the material absorbs a photon 40, an electron 50 is excited 60 to the conduction band 20 leaving behind a positively charged “hole” 70 or absence of an electron in the valence band 10. The excited electron 50 and hole 70 pair is called an exciton. The excited electron 50 can absorb heat in the form of a phonon 80 to increase its energy level 90. The hole can similarly be brought to an increased energy level 75 through phonon interactions with neighboring electrons. If the further excited electron 55 releases a phonon to return to its lower energy state and recombines with the hole, it will emit (or fluoresce) a photon that is lower in energy. This process is called Stokes shift and is not depicted in FIG. 1. Alternatively, if the further excited electron 55 recombines 100 with the higher energy hole 75, the emitted photon 110 will have a higher energy than the excitation photon 40. This is depicted in FIG. 1 and is called anti-Stokes or up-conversion of photons. Anti-Stokes up-conversion of photons is the basis for optical cooling.

(8) FIG. 2 depicts a simplified representation of the optical cooling device based on quantum dot anti-Stokes up-conversion. The substrate containing quantum dots 130 absorbs photons from the impinging laser 120 as well as phonons 150 (heat) from the surrounding media 140. The quantum dots then photoluminesce, which releases photons 110 with higher energy than the laser 120 photons, to generate a net cooling effect. The quantum dots are immobilized in an optically transparent matrix to form a stable substrate 130. The surface to be cooled 140 should be in thermal contact with the quantum dot substrate 130 in order to draw phonons 150 (heat) out of the underlying surface to be cooled 140.

(9) FIGS. 3-4 detail a couple examples of optical cooling device architectures. However, the examples are not limiting and modifications can be made to our novel process to be applied more broadly by one skilled in the art.

(10) FIG. 3 shows an exemplary, simplified design for an optical cooling device architecture. For the optical cooling device to achieve net cooling and compete with other cooling technologies in terms of efficiency, photon management is pivotal. After the photons have been upconverted (anti-Stokes shift) and emitted from the quantum dots, any absorption of the photon by the substrate, matrix material, or device housing will only contribute to parasitic heat losses. Rejecting as many of the emitted photons as possible outside the system is key to achieving net cooling. To that end, strategies were developed to direct the emitted photons to a fiber-optic cable for rejection. Several strategies were considered including index-matched dome lens to extract photons normal to the dome, nanogap structuring, or a gradient structure to the bandgap in the quantum dot matrix. However, most of the strategies did not reject the photons outside the device house or would add significant thermal mass to the system.

(11) Therefore, an architecture was devised as depicted in FIG. 3 as a possible solution to directing photons. The system consists of a laser pump 360 impinging on a quantum dot-matrix 330, which is formed from mixing the quantum dots into a liquid polymer before the polymer sets, in thermal contact with the substrate to be cooled 370. The quantum dot-matrix is inside the device housing 380, which is under vacuum to avoid heat losses. To keep the photons emitted from the quantum dots or the laser pump from being absorbed by the housing or substrate, the quantum dot-matrix can be coated with a reflective metal 390 on all but the top surface. This ensures that the emitted photons 310 would all be rejected away from the substrate 370. The rejected photons are then focused by the use of an optical lens 300 near or at the device housing 380 boundary. The lens focuses the photons 310 to a fiber optical cable 315 where they are routed to an appropriate thermal dump, away from the cooling surface and device.

(12) A secondary architecture is depicted in FIG. 4, where a bandpass filter is used to redirect the emitted photons out and away. The bandpass filter 420 allows low-wavelength light from the laser pump 460 through but would refract wavelengths below the bandpass (anti-Stokes wavelengths) away to an optical lens 400 and fiber optic cable 410. This architecture requires a bandpass filter that filters incoming laser photons at 640 nm from the up-converted emitted photons at 620-630 nm. The difference between the two wavelengths is too small for commercial bandpass filters.

(13) FIG. 5 shows the photoluminescence spectra of CdSe quantum dots (˜3.8 nm diameter) in chloroform for varied excitation wavelengths. The vertical lines above each spectra indicate the excitation source wavelength. The dotted line 530 indicates the photoluminescence peak wavelength as it changes based on excitation. When the source is at a higher energy, or lower wavelength, than the emitted photoluminescence there is a Stokes shift and net heating occurs. When the source is at a lower energy, higher wavelength, than the emitted photoluminescence, then an anti-Stokes shift occurs leading to net cooling.

(14) To probe the propensity of quantum dots for anti-Stokes shift and cooling potential, we varied the excitation wavelength from 500 to 630 nm, based on the absorbance intensity 535, and scanned the photoluminescence spectrum from wavelengths 450 to 800 nm which corresponds to energies of 2.5 to 1.75 eV on the x-axis. The resulting photoluminescence spectra are plotted in FIG. 5 for each measured wavelength as 500 nm 1240, 520 nm 1250, 540 nm 1260, 560 nm 1270, 580 nm 1280, 590 nm 1290, 600 nm 1300, 610 nm 1310, 620 nm 1320, and 630 nm 1330. As shown, the photoluminescence peak blue-shifts to lower wavelengths (higher energies) as the excitation wavelength increases. Above 600 nm, the peak photoluminescence has a lower wavelength (higher energy) than the excitation source. Therefore, the average photon that is emitted has more energy leaving than entering the system. Excitation wavelengths of 600-620 nm display anti-Stokes shift and optical cooling can be achieved under these conditions.

(15) Although anti-Stokes shift occurs for excitation wavelengths of 600-620 nm, the total photoluminescence decreases significantly with higher excitation wavelength. This means fewer photons are contributing to cooling. At 630 nm 1330, so few photons are absorbed that the anti-Stokes photoluminescence signal is not perceivable above the signal noise. However, the cooling power per photon increases as the gap between excitation and photoluminescence peak grows. Therefore, there is a trade off in determining the optimum excitation wavelength for the overall cooling of the system. The range of anti-Stokes up-conversion shift is expected to change based on the bandgap of the quantum dots. Therefore, the bandgap of the quantum dots is tuned so the anti-Stokes region overlaps the wavelength of commercially available lasers.

(16) To incorporate the quantum dots into the optical cooling device, they need to be immobilized in a thin, matrix film. This matrix film must not inhibit or adversely affect the optical absorption or re-emission of the photons. As such, the choice of the matrix material was vital. The key requirements of the matrix material are: 1. Very high transmission in the optical region (>90% over the range of 500-700 nm). 2. Compatibility with quantum dots—needs to disperse quantum dots before setting and not require curing conditions that could negatively affect the optical properties of the quantum dots. 3. Resist photo-degradation. 4. Preferably exhibit high thermal conductivity.

(17) The matrix materials we considered included silicone, siloxane, PMMA, and glass. PMMA has been successfully used as a matrix material for a quantum dot film achieving 95% quantum yield. PMMA also has a high optical transmission (>90%) in the optical range. In addition, PMMA is relatively straightforward to cure at low temperatures (<70° C.). Therefore, we down-selected PMMA as the matrix material to immobilize the quantum dots for the optical cooling film.

(18) The PMMA-quantum dot matrix was formed by adding quantum dots dispersed in toluene (1 mg/mL) to solution of 0.1% by weight 2,2′-azobis(2-methylpropionitrile) (AIBN) in 50 mL methyl methacrylate. The AIBN acts as a free-radical generator to accelerate the polymerization of the methyl methacrylate. The solution was heated to 80° C. for 10 minutes to initiate the free radical generation and then maintained at 60° C. for 20 hours for the polymer to cure. The resulting PMMA-quantum dot matrix disc was 2.5 inches (6.35 cm) in diameter and 0.33 inches (0.84 cm) thick. The discs were then mechanically polished to a 0.3 μm finish to achieve optical transparency. The resulting discs are homogenous in color and clarity with an optically clear surface free of scratches or blemishes.

(19) The high transmission of light through PMMA is important because anything less than 100% is lost as parasitic heating. This can be seen from the following equation for expected cooling power (P.sub.cool):

(20) $P_{\mathrm{cool}}=P_{\mathrm{laser}}\left(\mathrm{QY}\frac{{\lambda }_{\mathrm{abs}}}{{\lambda }_{\mathrm{PL}}}{A}_{\mathrm{QD}}-A_{\mathrm{matrix}}\right)$
where P.sub.laser is the power of the impinging laser, λ.sub.abs is the wavelength of light absorbed by the material, λ.sub.PL is the wavelength of photoluminescence, and A is the absorbance percentage for the quantum dots and the PMMA matrix material. In this case, λ.sub.abs is the wavelength of the laser and λ.sub.PL is the anti-Stokes wavelength. For high cooling power, we want the amount of light absorbed by the PMMA to be low and the amount absorbed by the quantum dots to be high.

(21) The transmission spectrum as a function of wavelength is shown in FIG. 6 for varying quantum dot loading concentrations of 0 μg/mL 1650, 10 μg/mL 1660, 100 μg/mL 1670, and 300 μg/mL 1680 in PMMA. A laser wavelength as 640 nm 1640 because is a common commercially available laser (helium-neon) in the range of expected anti-Stokes up-conversion for CdSe quantum dots which also has high transmission up to 100 μg/mL of quantum dots in PMMA.

(22) A mirror-like thin film applied to the backside of the PMMA-quantum dot disc can be used to reflect both laser photons and anti-Stokes emitted photons away from the substrate to be cooled. To demonstrate the feasibility of the mirror coating, we performed thermal evaporation of chromium and aluminum on PMMA solids. Trials to thermally evaporate chromium on PMMA resulted in a powdery coating that was not well adhered, nor reflective. The best results were obtained by rapid deposition (>5 nm/s) of aluminum. The resulting film coating is uniform, reflective, and well adhered to the PMMA-quantum dot surface.

(23) From the material properties, we calculated a materials-level coefficient of cooling performance (COP.sub.cool) based on the following equation:

(24) ${\mathrm{COP}}_{\mathrm{cool}}=\frac{P_{\mathrm{cool}}}{P_{\mathrm{laser}}}=\left(\mathrm{QY}\frac{{\lambda }_{\mathrm{abs}}}{{\lambda }_{\mathrm{PL}}}{A}_{\mathrm{QD}}-A_{\mathrm{matrix}}\right)$

(25) The material properties measured are tabulated below. All of these properties were measured at room temperature (˜293 K). Therefore the calculated COP.sub.cool=0.12 is for room temperature. It should be noted that the properties listed are expected to be a function of temperature. For example, the QY is known to increase with decreasing temperatures. In addition, there may be other thermal loss mechanisms not accounted for in the equation used. These include Stokes shift losses, photobleaching of the quantum dots, or any thermal losses from incomplete reflection of the mirror or incomplete transmission through the optical lens. Nevertheless, the calculated COP can be approximated as an upper limit.

(26) TABLE-US-00001 Properties Quantum Yield 50% $\frac{{\lambda }_{abs}}{{\lambda }_{PL}}$ 1.02 A.sub.QD 41% A.sub.matrix  9% COP.sub.cool 0.12

(27) While we have shown and described a novel process in accordance with our invention, it should be understood that the same is susceptible to further changes and modifications without departing from the scope of our invention. Therefore, we do not want to be limited to the details shown and described herein but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.