In the present disclosure, energy harvesters based on quantum confinement structures, such as resonant quantum wells and/or quantum dots, are described. Also disclosed are methods of harvesting energy utilizing the described energy harvester and methods of manufacturing energy harvesters. Energy harvesting is the process by which energy is taken from the environment and transformed to provide power for electronics.
What we claim is: 1. An energy -harvesting device, comprising: a first electron reservoir having a chemical potential (^<L>) and a temperature (7R<es>i); a second electron reservoir having a chemical potential (μ<κ>) and a temperature (T<Res2>), the second electron reservoir being in spaced apart relation with the first electron reservoir; a cavity having a chemical potential ( cav)<an>d a temperature (Tcav) which is greater than temperatures (r<Resl>, r<Res2>) of the first and second electron reservoirs; a first quantum confinement structure having an operative energy (E<L>), the first quantum confinement structure electrically connecting the first electron reservoir to the cavity; and a second quantum confinement structure having an operative energy (E<R>) which is different than E<L>, the second quantum confinement structure electrically connecting the second electron reservoir to the cavity. 2. The device of claim 1, wherein a bias voltage V is applied between the first electron reservoir and the second electron reservoir such that eV = μ^— μ<κ>. 3. The device of claims 1 to 2, further comprising a gate for applying a gate voltage, wherein the operative energy of the first quantum confinement structure and/or the second quantum confinement structure can be altered by providing a gate voltage at the gate. 4. The device of claims 1 to 3, further comprising: a third electron reservoir having a chemical potential (μ<3>) and a temperature (7R<eS3>), the third electron reservoir being in spaced apart relation with the second electron reservoir; a second cavity having a chemical potential (^cav<2>)<an>d a temperature (7cav<2>) which is greater than temperatures CT<Res2>, T<Res3>) of the second and third electron reservoirs; and a third quantum confinement structure having an operative energy (E<L1>), the third quantum confinement structure electrically connecting the second electron reservoir to the second cavity; and a fourth quantum confinement structure having an operative energy (E<R1>) which is different than E<L1>, the fourth quantum confinement structure electrically connecting the third electron reservoir to the second cavity. 5. The device of claim 4, wherein E<L>= E<L1>and/or E<R>= E<R1>. 6. The device of claims 4 to 5, wherein μ^<ν2>= μο 7. The device of claims 1 to 6, wherein the first quantum confinement structure and the second quantum confinement structure are quantum dots, and the operative energy of each of the first and second quantum dots is a resonant level. 8. The device of claim 7, wherein the relationship of the cavity chemical potential and the resonant levels of the first and second quantum dots is such that: μ<Άν>= (E<L>+ E<R>)/2. 9. The device of claims 7 to 8, wherein the difference (AE) between the second resonant level and the first resonant level is related to an average temperature (T = (T<Cav>+ T<Resl>+ T<Res2>)/3) of the device such that: AE = 6k<B>T, where k<B>is the Boltzmann constant. 10. The device of claims 7 to 9, wherein the resonant widths (γ) are approximately equal to k<B>T. 1 1. The device of claims 7 to 10, wherein μ<L>=— μ /2 + (E<L>+ E<R>)/2 and μ<κ>= μ /2 + (E<L>+ E<R>)/2. 12. The device of claims 7 to 11, having more than one first quantum dots connecting the first electron reservoir to the cavity and more than one second quantum dots connecting the second electron reservoir to the cavity. 13. The device of claim 12, wherein the resonant levels of the more than one first quantum dots are within the range of ±10% of £<L>and the resonant levels of each of the more than one second quantum dots is with ±10% of E<R>. 14. The device of claims 1 to 6, wherein the first quantum confinement structure and the second quantum confinement structure are quantum wells, and the operative energy of each of the first and second quantum wells is a threshold energy. 15. The device of claim 14, wherein the first and second quantum wells are intrinsically symmetric. 16. The device of claims 15, wherein the coupling strength of the first quantum well (ΓΊ) is approximately equal to the coupling strength of the second quantum well (Γ<2>). 17. The device of claims 14 to 16, wherein E<R>is substantially equal to 1.5 times a thermal energy (k<B>T), where T is a design temperature. 18. The device of claim 14, wherein the coupling strength of the first quantum well (ΓΊ) is not equal to the coupling strength of the second quantum well (Γ<2>). 19. The device of claims 14 or 18, wherein Ι « 2.70Γ<2>and E<R>is substantially equal to 2 times a thermal energy (k<B>T), where T is a design temperature. 20. A method of harvesting energy from a substrate having an elevated temperature, the method comprising the steps of: providing an energy harvesting device as defined in claims 1 to 5, having: a first electron reservoir; a second electron reservoir, the second electron reservoir being in spaced apart relation with the first electron reservoir; a cavity thermally coupled to the substrate, the cavity having a chemical potential ( cav) and a temperature (Tcav) which is greater than temperatures of the first and second electron reservoirs; a first quantum confinement structure having an operative energy (E<L>), the first quantum confinement structure electrically connecting the first electron reservoir to the cavity; a second quantum confinement structure having an operative energy (E<R>) which is different than E<L>, the second quantum confinement structure electrically connecting the second electron reservoir to the cavity; and electrically connecting a load between the first and second electron reservoirs. 21. The method of claim 20, further comprising the step of applying a bias voltage (V) across the first and second electron reservoirs such that eV = μ<ί>— μ<κ>. 22. The method of claim 21, wherein V = V<stop>/2, where V<stop>is the voltage at which a heat- driven current flowing in a first direction is exactly compensated by a bias-driven current flowing in a second direction opposite to the first direction. 23. The method of claims 20 to 22, further comprising the step of applying a gate voltage using a gate. 24. A method of manufacturing an energy harvesting device as defined in claims 1 to 5, comprising the steps of: providing a first electrode layer; depositing a first quantum confinement layer on the first electrode layer, at least a portion of the first quantum confinement layer being in electrical communication with the first electrode layer and having a first operative energy (E<L>) ; depositing a central layer onto the first quantum confinement layer, the central layer being in electrical communication with at least a portion of the first quantum confinement layer; depositing a second quantum confinement layer on the central layer, at least a portion of the second quantum confinement layer being in electrical communication with the central layer and having a second operative energy (E<R>) that is different than E<L>; and depositing a second electrode layer onto the second quantum confinement layer, the second electrode layer being in electrical communication with at least a portion of the second quantum confinement layer. 25. The method of claim 24, wherein step of depositing a first quantum confinement layer on the first electrode layer comprises the sub-step of: fabricating a first quantum dot layer on the first electrode layer, the first quantum dot layer comprising a plurality of quantum dots disposed in an insulating material such that the plurality of quantum dots are not in electrical contact with each other, each quantum dot being in electrical communication with the first electrode layer and having an operative level which is substantially equal to a first resonant level (E<L>) . 26. The method of claim 25, wherein the resonant level of each quantum dot of the first quantum dot layer is ±10% of E<L>. 27. The method of claims 24 to 26, wherein step of depositing a second quantum confinement layer on the central layer comprises the sub-step of: fabricating a second quantum dot layer on the central layer, the second quantum dot layer comprising a plurality of quantum dots disposed in an insulating material such that the plurality of quantum dots are not in electrical contact with each other, each quantum dot being in electrical communication with the central layer and having an operative level which is substantially equal to a second resonant level (E<R>), and wherein E<R>is greater than an operative energy of the first quantum confinement layer. 28. The method of claim 27, wherein the resonant level of each quantum dot of the second quantum dot layer is ±10% of E<R>. 29. The method of claims 24 to 28, wherein the relationship chemical potential ( cav) of the central layer is selected such that: μ<Άν>= (E<L>+ E<R>)/2. 30. The method of claims 24 or 29, wherein the first and/or second quantum confinement layer a quantum well.