MULTIPLE BAND GAP CO-NI OXIDE COMPOSITIONS AND APPLICATIONS THEREOF

Abstract

In one aspect, metal oxide compositions having electronic structure of multiple band gaps are described. In some embodiments, a metal oxide composition comprises a (Co,Ni)O alloy having electronic structure including multiple band gaps. The (Co,Ni)O alloy can include a first band gap and a second band gap, the first band gap separating valence and conduction bands of the electronic structure.

Claims

1. A metal oxide composition comprising: a (Co,Ni)O alloy having electronic structure including multiple band gaps.

2. The metal oxide composition of claim 1, wherein the (Co,Ni)O alloy includes a first band gap and a second band gap, the first band gap separating valence and conduction bands of the electronic structure.

3. The metal oxide composition of claim 2, wherein the conduction band comprises a first band and a second band.

4. The metal oxide composition of claim 3, wherein the first band and the second band are separated by at least 0.2 eV.

5. The metal oxide composition of claim 3, wherein the first band and the second band are separated by 0.2-0.7 eV.

6. The metal oxide composition of claim 3, wherein the first band has a width of at least 0.8 eV.

7. The metal oxide composition of claim 3, wherein the first band has a width of 0.8-1.5 eV.

8. The metal oxide composition of claim 3, wherein the second band has a width of at least 0.4 eV.

9. The metal oxide composition of claim 3, wherein the second band has a width of 0.5-1.0 eV.

10. The metal oxide composition of claim 1, wherein the (Co,Ni)O alloy is of the formula Co.sub.xNi.sub.1-xO with 0.2x0.3.

11. The metal oxide composition of claim 10, wherein 0.23x0.27.

12. The metal oxide composition of claim 10, wherein x=0.25.

13. The metal oxide composition of claim 1, wherein the (Co,Ni)O alloy is of the formula Co.sub.xNi.sub.1-xO with 0.125<x<0.375.

14. The metal oxide composition of claim 1, wherein the (Co,Ni)O alloy comprises one or more dopants.

15. The metal oxide composition of claim 14, wherein the doped (Co,Ni)O alloy exhibits p-type character.

16. The metal oxide composition of claim 14, wherein the doped (Co,Ni)O alloy exhibits n-type character.

17. The metal oxide composition of claim 14, wherein the one or more dopants are selected from the group consisting of alkali metals, alkaline earth metals, transition metals and Lanthanide series metals.

18. The metal oxide composition of claim 1 having photonic emission of varying wavelengths.

19. A method of making a (Co,Ni)O composition comprising: alloying nickel (II) oxide with cobalt (II) oxide in an amount inducing multiple band gaps in the (Co,Ni)O electronic structure.

20. The method of claim 19, wherein the multiple band gaps include a first band gap and a second band gap, the first band gap separating valence and conduction bands of the electronic structure.

21. The method of claim 20, wherein the conduction band comprises a first band and a second band.

22. The method of claim 21, wherein the first band is separated from the second band by 0.2-0.7 eV.

23. The method of claim 21, wherein the first band has a width of 0.8-1.5 eV.

24. The method of claim 21, wherein the second band has a width of 0.5-1.0 eV.

25. The method of claim 19, wherein the (Co,Ni)O composition is of the formula Co.sub.xNi.sub.1-xO with 0.2x0.3.

26. The method of claim 19, wherein the (Co,Ni)O composition is of the formula Co.sub.xNi.sub.1-xO with 0.23x0.27.

27. A solar cell comprising: a photosensitive region, the photosensitive region comprising a (Co,Ni)O alloy having electronic structure including multiple band gaps.

28. A light emitting diode comprising: a light emitting region, the light emitting region comprising a (Co,Ni)O alloy having electronic structure including multiple band gaps.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] FIG. 1(a) illustrates light absorption across the band gap of a single junction semiconductor.

[0014] FIG. 1(b) illustrates light absorption across the band gaps of a multi-junction, tandem solar cell.

[0015] FIG. 1(c) illustrates light absorption across the band gaps of an intermediate band gap semiconductor.

[0016] FIG. 2(a) illustrates a unit cell for a Co.sub.0.125Ni.sub.0.875O alloy.

[0017] FIG. 2(b) illustrates a unit cell for a Co.sub.0.25Ni.sub.0.75O alloy.

[0018] FIG. 2(c) illustrates a unit cell for a C0.sub.0.375Ni.sub.0.625O alloy.

[0019] FIG. 2(d) illustrates a unit cell for a Co.sub.0.5Ni.sub.0.5O alloy.

[0020] FIGS. 3(a) and 3(b) illustrate the multiple band gap structure of a Co.sub.0.25Ni.sub.0.75O alloy according to LDA+U and LDA+U/G.sub.0W.sub.0 calculations, respectively.

[0021] FIG. 4(a) illustrates electronic structure of a Co.sub.0.125Ni.sub.0.875O alloy according to LDA+U.

[0022] FIG. 4(b) illustrates electronic structure of a Co.sub.0.375Ni.sub.0.625O alloy according to LDA+U.

[0023] FIG. 4(c) illustrates electronic structure of a Co.sub.0.5Ni.sub.0.5O alloy according to LDA+U.

[0024] FIG. 4(d) illustrates electronic structure of a Co.sub.0.625Ni.sub.0.375O alloy according to LDA+U.

[0025] FIG. 4(e) illustrates electronic structure of a Co.sub.0.75Ni.sub.0.25O alloy according to LDA+U.

[0026] FIG. 4(f) illustrates electronic structure of a Co.sub.0.875Ni.sub.0.125O alloy according to LDA+U.

DETAILED DESCRIPTION

[0027] Embodiments described herein can be understood more readily by reference to the following detailed description, examples and drawings. Elements, apparatus, and methods described herein, however, are not limited to the specific embodiments presented in the detailed description, examples and drawings. It should be recognized that these embodiments are merely illustrative of the principles of the present invention. Numerous modifications and adaptations will be readily apparent to those of skill in the art without departing from the spirit and scope of the invention.

[0028] In one aspect, metal oxide compositions having electronic structure including multiple band gaps are described. In some embodiments, a metal oxide composition comprises a (Co,Ni)O alloy having electronic structure including multiple band gaps. The (Co,Ni)O alloy can include a first band gap and a second band gap, the first band gap separating valence and conduction bands of the electronic structure. By exhibiting two band gaps, the (Co,Ni)O alloy comprises two bands in its conduction band. As detailed further herein, the first band or intermediate band resides closer the to the valence band. The second band of the conduction band resides at higher energy. In some embodiments, the first band is separated from the second band by at least 0.2 eV. The first and second bands of the conduction band, for example, can be separated by 0.2-0.7 eV. Moreover, the first band can have a width of at least 0.8 eV, such as 0.8-1.5 eV. The second band of the conduction band can have a width of at least 0.4 eV, such as 0.5-1.0 eV.

[0029] Importantly, Co and Ni can be present in the (Co,Ni)O alloy in any amount resulting in electronic structure including multiple band gaps. In some embodiments, the (Co,Ni)O alloy is of the formula Co.sub.xNi.sub.1-xO wherein 0.2x0.3. In alternative embodiments, the value for x can be selected from Table I.

TABLE-US-00001 TABLE I Value for x in (Co.sub.xNi.sub.1x)O 0.125 < x < 0.375 0.18 x 0.35 0.22 x 0.28 0.23 x 0.27 x = 0.25
The multiple band gap electronic structure of the (Co,Ni)O alloy can permit the absorption of multiple photons in comparison to a single band gap electronic structure. Similarly, the multiple band gap electronic structure can provide the (Co,Ni)O alloy a multi-color emission profile.

[0030] Cubic NiO and CoO are stable oxides of Ni and Co, respectively. Additionally, the ionic radii of Ni.sup.2+and Co.sup.2+ deviate only 6%, thereby facilitating cubic solid solutions of Co.sub.xNi.sub.1-x at various concentrations, including the concentrations of Table I. Benchmark electronic structure calculations have identified the appropriate exchange-correlation description for treating the ground-state structure of (Co,Ni)O compositions to be the local density approximation (LDA) within LDA+U theory where UJ=4.0 eV for Co.sup.2+(optimized for Co.sup.3+in Co.sub.3O.sub.4) and UJ=3.8 eV for Ni.sup.2+(optimized for Ni.sup.2+in NiO). The optimal value for Co.sup.2+is expected to differ somewhat from Co.sup.3+, given the different oxidation states. However, previous calculations on Fe.sup.2+and Fe.sup.3+indicate this difference is only 0.5 eV for the similar transition metal Fe, which is comparable to the typical convergence threshold of 0.5 eV for calculation of UJ. Electronic structures of various (Co,Ni)O compositions were also elucidated via LDA+U/G.sub.0W.sub.0 calculations. It has been shown that LDA+U/G.sub.0 W.sub.0 theory achieved excellent agreement with experiments when it comes to the quasiparticle (QP) gap and the electronic structure of pure NiO. The spin-polarized projector augmented wave potentials used for Ni and Co ions accounted for the 1s2s2p3s3p electrons within a frozen core approximation. Various Co concentrations in the Co.sub.xNi.sub.1-xO alloys were modeled using special quasi-random structure (SQS) cells for the cases of provided in Table II.

TABLE-US-00002 TABLE II Modeled Co.sub.xNi.sub.1xO alloys Co.sub.xNi.sub.1xO alloy FIG. Co.sub.0.125Ni.sub.0.875O 2(a) Co.sub.0.25Ni.sub.0.75O 2(b) Co.sub.0.375Ni.sub.0.625O 2(c) Co.sub.0.5Ni.sub.0.5O 2(d) Co.sub.0.625Ni.sub.0.375O Co.sub.0.75Ni.sub.0.25O Co.sub.0.875Ni.sub.0.175O
Unit cells for several of the modeled alloys are provided in FIGS. 2(a)-2(d). The unit cells for Co.sub.0.625Ni.sub.0.375O, C.sub.0.75Ni.sub.0.25O and C0.sub.0.875Ni.sub.0.175O are identical to FIGS. 2(a)-2(c) with Ni sites replacing Co sites and vice versa.

[0031] The cell with 0.125 Co (Ni) concentration was constructed by replacing one Ni (Co) ion with a Co (Ni) ion in the 0.25 (0.75) SQS cell. For the case with 0.375 Co (Ni) concentration, a Co (Ni) ion was replaced from the 0.5 Co (Ni) concentration SQS cell with a Ni (Co) ion. For cases with higher Ni (Co) concentration, geometry relaxation from unit cells was initiated with the lattice constant 4.180 Angstroms, corresponding to the experimental lattice constant of NiO (CoO). For the case with 0.5 Co and Ni concentrations, relaxation was started from a cell with a lattice constant of 4.21Angstroms, which lies between the lattice constants of the two transition metal oxides. Given the absence of experimental studies on the magnetism of NiOCoO alloys, it was assumed that the AFM coupling between transition-metal ions in pure NiO and CoO is preserved upon alloying to form Co.sub.xNi.sub.1-xO alloys.

[0032] The LDA+U calculations employed a 6104 -point-centered k-point mesh, 128 bands, and 700 eV kinetic energy cutoff for the plane-wave basis set. Considering the k-point sampling in a certain direction should be inversely proportional to the lattice parameter in that direction, the k-point sampling in each direction was consistent with that of pure CoO and denser than sampling implied by pure NiO, thereby providing greater accuracy. The total energy per atom was converged to within 1 meV with these parameters. The LDA+U/G.sub.0 W.sub.0 calculations used a 482 -point-centered k-point mesh, 128 bands (half of them empty) and 96 frequency points for evaluation of the response function. The QP gap was converged to within 0.1 eV with these parameters.

[0033] FIGS. 3(a) and 3(b) illustrate the multiple band gap (E.sup.1.sub.g, E.sup.2.sub.g) structure of the Co.sub.0.25Ni.sub.0.75O alloy. As illustrated in FIGS. 3(a) and 3(b), the conduction band comprises a first or intermediate band and a second band. The first band has a width of 1-1.2 eV and the second band has a width of 0.6-0.7 eV, depending on modeling employed. Moreover, the first and second bands are separated by 0.3-0.5 eV. This is in contrast to FIGS. 4(a)-4(f), where none of the (Co,Ni)O alloys exhibited a multiple band gap electronic structure. This is not to say that other Co,Ni.sub.1-xO alloy compositions unexamined in the present analysis fail to exhibit an electronic structure having multiple band gaps. When it comes to geometric structure, the lattice constant of the Co.sub.0.25Ni.sub.0.75O alloy is 4.083 Angstroms, comparable with that of NiO at 4.180 Angstroms. A closer inspection of the conduction band of this alloy shows that the intermediate band (i.e., the lower energy peak in the conduction band is a hybrid of partially empty Ni 3d (e.sub.g) and Co 3d (t.sub.2g) orbitals. Remarkably, the Ni 3d (e.sub.g) orbitals do not hybridize with the Co 3d (e.sub.g) orbitals, with which they share the same symmetry.

[0034] This likely stems from the fact that the work function of pure CoO is 1 eV lower than that of pure NiO. The difference between the work functions (or equivalently the band gap center in undoped semiconductors) can lead to a reduction in the band gap upon alloying if the band gaps of the parent oxides are comparable in magnitude. However, the band gap of CoO is also 1 eV lower than that of NiO. This means that in an alloy composed of the two oxides, the Ni 3d (e.sub.g) states, which are prevalent in NiO's conduction band edge (CBE), would be aligned with the Co 3d (t.sub.2g), which make up CoO's CBE. Therefore, the CBE of the alloy consists of states that are a hybrid of these two sets of states. In contrast, since the valence band edge (VBE) of NiO lies at lower energies compared with that of CoO, hybridization at the VBE is not significant for the alloys of these two materials.

[0035] Similar to pure CoO and unlike pure NiO, the Co.sub.0.25Ni.sub.0.75O alloy does not show significant ligand-to-metal CT character. The VBE associated with Co.sub.0.25Ni.sub.0.75O has a prominent Co d character, while its CBE comprises both Ni d and Co d states (FIGS. 3(a) and 3(b)). Therefore, although no improvements to the electron-hole pair lifetime due to ligand-to-metal CT character could be expected, the possibility of metal-to-metal CT character (from Co d states at the VBE to Ni d states at the CBE of the intermediate band) could still lead to an increase in carrier lifetimes.

[0036] The ground-state LDA+U electronic structure of Co.sub.0.25Ni.sub.0.75O indicates that the width of the first or intermediate band associated with this alloy (1.2 eV) is larger than that in CoO (0.5 eV) by 0.7 eV, thereby indicating this alloy has the potential to achieve the important goal of increasing the width of the first or intermediate band. However, the actual extent of this widening should be assessed by LDA+U/G.sub.0W.sub.0 theory. LDA+U/G.sub.0W.sub.0 calculations on Co.sub.0.25Ni.sub.0.75O show that the QP corrections do not alter the double-gap structure associated with this alloy (FIG. 3(b)). The QP gap of this alloy is predicted to be 2.7 eV, which is comparable to that of pure CoO. The LDA+U/G.sub.0W.sub.0 character of the band edges and their relative prominence do not differ significantly from the ground-state electronic structure given by LDA+U (FIG. 3(a)). However, similar to the LDA+U case, the width of the intermediate band is 0.7 eV larger than that of pure CoO, while the upper band gap E.sup.2g has the same magnitude as that in pure CoO. Co.sub.0.25Ni.sub.0.75O has a much wider intermediate band than that of pure CoO and, therefore, it is more suitable for use as a potential parent material in IBSC-based in photovoltaics. By engineering proper heterojunctions and with appropriate doping, this material also has the potential for uses in LEDs and lasers.

[0037] In some embodiments, for example, (Co,Ni)0 compositions can be doped to exhibit p-type character or n-type character, while preserving the multiple band gap electronic structure. (Co,Ni)O compositions can doped with one more alkali metals, alkaline earth metals, transition metals and/or Lanthanide series metals. In some embodiments, a dopant comprises lithium, sodium or magnesium. Suitable dopants can also be selected from Groups IIB-VIA of the Periodic Table. Doping of the (Co,Ni)O composition can provide the desired heterojunction architecture for employment of the (Co,Ni)O composition in photovoltaic devices and/or light emitting diodes.

[0038] (Co,Ni)O compositions can generally be fabricated by substitutionally alloying nickel (II) oxide with Co in an amount inducing multiple band gaps in the (Co,Ni)O electronic structure. In some embodiments, (Co,Ni)O compositions can be fabricated by pulsed laser deposition (PLD), atomic layer deposition (ALD), atomic layer epitaxy (ALE) or molecular beam epitaxy (MBE). [See, e.g. Irwin et al., PNAS, Vol. 105, No. 8, pp. 2783-2787]. Colloidal synthesis techniques can also be used for creating nano-structured transition-metal oxide alloys [See, e.g. Radovanovic et al., J. Am. Chem. Soc., 2002, 124 (51), pp 15192-15193]. In some embodiments, one or more additional transition metal oxides may serve as alloying partner(s) with cobalt oxide and nickel oxide in the fabrication process as well.

[0039] Various embodiments of the invention have been described in fulfillment of the various objectives of the invention. It should be recognized that these embodiments are merely illustrative of the principles of the present invention. Numerous modifications and adaptations thereof will be readily apparent to those skilled in the art without departing from the spirit and scope of the invention.