Defect engineering in metal oxides via surfaces
The present invention provides methods for controlling defects in materials, including point defects, such as interstitials and vacancies, and extended defects, including dislocations and clusters. Defect control provided by the present invention allows for fabrication and processing of materials and/or structures having a selected abundance, spatial distribution and/or concentration depth profile of one or more types of defects in a material, such as vacancies and/or interstitials in a crystalline material. Methods of the invention are useful for processing materials by controlling defects to access beneficial physical, optical, chemical and/or electronic properties.
1. A method for controlling a concentration of O interstitials or complexes thereof in a metal oxide structure, said method comprising the steps of:
providing said metal oxide structure having an exposed surface and a bulk, wherein said exposed surface is an atomically clean surface;
treating said exposed surface of said metal oxide structure with a surface modifying agent, wherein interaction between said surface modifying agent and said metal oxide structure forms molecules or atoms on the exposed surface providing defect control groups on said exposed surface;
generating a selected surface abundance of said molecules or atoms on said exposed surface; and
annealing said exposed surface populated by said selected surface abundance of said molecules or atoms at a temperature of at least 500° C. under a partial pressure of oxygen gas to provide a source of O atoms from said exposed surface to the bulk of said metal oxide structure; thereby controlling the concentration of said O interstitials or complexes thereof in said metal oxide structure.
2. The method of claim 1 , wherein said metal oxide is TiO 2 or ZnO.
3. The method of claim 2 , wherein said selected surface abundance of said molecules or atoms on the exposed surface controls a rate that said O interstitials or complexes thereof are injected into, or otherwise generated in, said TiO 2 or ZnO structure.
4. The method of claim 2 , wherein said selected surface abundance of said molecules or atoms on the exposed surface eliminates between 0.1% and 80% of dangling bonds on said exposed surface of said TiO 2 or ZnO structure.
5. The method of claim 2 , wherein said selected surface abundance of said molecules or atoms on the exposed surface controls a spatial distribution of said O interstitials or complexes thereof in said TiO 2 or ZnO structure.
6. The method of claim 2 , wherein said step of treating said exposed surface of said TiO 2 or ZnO structure with said surface modifying agent generates a selected surface abundance of chemisorbed atoms or molecules on said exposed surface of said TiO 2 or ZnO structure.
7. The method of claim 2 , wherein said step of treating said exposed surface of said TiO 2 or ZnO structure with said surface modifying agent generates sulfur atoms or sulfur-containing molecules on said exposed surface of said TiO 2 or ZnO structure.
8. The method of claim 7 , wherein said surface modifying agent is a sulfur-containing gas or a sulfur-containing liquid.
9. The method of claim 7 , wherein said surface modifying agent is elemental sulfur, CS 2 , H 2 S, SO 2 , SO 3 , SCl 2 , S 2 Cl 2 , SOCl 2 , SO 2 Cl 2 , HSO 3 Cl, a thiol, a sulfoxide, a sulfone, SF 6 or any combination of these, wherein the surface modifying agent is applied to the exposed surface as a gas.
10. The method of claim 7 , wherein said surface modifying agent is H 2 SO 4 , SOCl 2 , SO 2 Cl 2 , HSO 3 Cl, a thiol, a sulfoxide, or a sulfone, wherein the surface modifying agent is applied to the exposed surface as a liquid in a purified state or dissolved in a solvent.
11. The method of claim 7 , wherein said surface modifying agent is dimethyl sulfoxide, sulfolane or methanethiol provided as a gas or liquid.
12. The method of claim 2 , wherein said TiO 2 or ZnO structure is a single crystalline material or a polycrystalline material, having a slight excess or deficiency in oxygen atoms.
13. The method of claim 12 , wherein said TiO 2 structure has the chemical formula TiO 2-x , wherein x is less than 0.05.
14. The method of claim 2 , wherein said TiO 2 or ZnO structure is a TiO 2 or ZnO layer or a TiO 2 or ZnO particle having a cross sectional dimension selected from the range of 1 nanometer to 10 microns.
15. The method of claim 2 , wherein said step of treating said exposed surface of said TiO 2 or ZnO structure with said surface modifying agent comprises:
determining said selected surface abundance of said molecules or atoms on the exposed surface selected from the range of 5×10 12 molecules or atoms cm −2 to 2×10 15 molecules or atoms cm −2 or from the range of 0.01 monolayer to 3 monolayers, wherein said selected surface abundance of said molecules or atoms on the exposed surface is selected so as to control the concentration of said O interstitials or complexes thereof in said TiO 2 or ZnO structure so as to achieve a selected spatial distribution of said O interstitials or complexes thereof in said TiO 2 or ZnO structure; and
terminating said treating step when said selected surface abundance of said molecules or atoms on the exposed surface is reached.
16. The method of claim 2 , wherein the step of treating said exposed surface of said TiO 2 or ZnO structure with said surface modifying agent is carried out while the TiO 2 or ZnO structure is exposed to electromagnetic radiation, wherein said surface modifying agent is a photoactive surface modifying agent.
17. The method of claim 2 , wherein said TiO 2 or ZnO structure is a component of a memory resistor, a dye-sensitized solar cell, a light-emitting diode, an optical gas sensor, a photocatalytic water splitting assembly or a solid-state sensor.
18. The method of claim 1 , wherein said molecules or atoms on the exposed surface are selected from the group consisting of C, N, P, As, Sb, Bi, S, Se, Te, F, Cl, Br, and I and molecules containing C, N, P, As, Sb, Bi, S, Se, Te, F, Cl, Br, or I.
19. The method of claim 1 , wherein said selected surface abundance of said molecules or atoms on the exposed surface is selected from the range of 0.01 monolayer to 3 monolayers or is selected from the range of 5×10 12 molecules or atoms cm −2 to 2×10 15 molecules or atoms cm −2 .
20. The method of claim 1 , wherein the partial pressure of oxygen gas is selected from a range of 10 −6 Torr to 1500 Torr.
21. A method for controlling a concentration of defects in a material, said method comprising the steps of:
providing said material having an exposed surface and a bulk;
treating said exposed surface of said material with a surface modifying agent, wherein interaction between said surface modifying agent and said material forms molecules or atoms on the exposed surface selected from the group consisting of C, N, P, Bi, S, Se, Te, F, Cl, Br, and I and molecules containing C, N, P, Bi, S, Se, Te, F, Cl, Br, or I providing defect control groups on said exposed surface;
generating a selected surface abundance of said molecules or atoms on the exposed surface; and
annealing said exposed surface populated by said selected surface abundance of said molecules or atoms at a temperature of at least 500° C. under a partial pressure of oxygen gas to provide a source of O atoms from said exposed surface to the bulk of said material; thereby controlling the concentration of defects in said material.
22. The method of claim 21 , wherein said selected surface abundance of said molecules or atoms on the exposed surface is selected from the range of 0.01 monolayer to 3 monolayers or is selected from the range of 5×10 12 molecules or atoms cm −2 to 2×10 15 molecules or atoms cm −2 .
23. The method of claim 21 , wherein said material is a metal oxide selected from the group consisting of TiO 2 , ZrO 2 , ZnO, Co 3 O 4 , and cobalt spinels having the formula CoX 2 O 4 , wherein X is Al, Ga, or In, and wherein said metal oxide has a slight excess or deficiency of oxygen atoms.
24. The method of claim 21 , wherein said material has the formula TiO 2-x , wherein x is less than 0.05; or wherein said material is a perovskite having a slight excess or deficiency of oxygen atom; or wherein said material is a multiferroic thin film, a ferroelectric thin film, a piezoelectric thin film, or a pyroelectric thin film; or wherein said material is a superconducting thin film.
25. The method of claim 21 , wherein the partial pressure of oxygen gas is selected from a range of 10 −6 Torr to 1500 Torr.
26. A method for decreasing a concentration of vacancies in a metal oxide structure, said method comprising the steps of:
providing said metal oxide structure having an exposed surface and a bulk;
cleaning said exposed surface to produce an atomically clean surface;
providing a partial pressure of oxygen gas as a source of O atoms from said exposed surface to the bulk of the metal oxide structure, wherein at least a portion of said O atoms combine with said vacancies, thereby decreasing said concentration of vacancies in said metal oxide structure;
treating said exposed surface of said metal oxide structure with a surface modifying agent, wherein interaction between said surface modifying agent and said metal oxide structure forms molecules or atoms on the exposed surface providing defect control groups on said exposed surface;
generating a selected surface abundance of said molecules or atoms on said exposed surface; and
annealing said exposed surface populated by said selected surface abundance of said molecules or atoms at a temperature of at least 500° C.
27. The method of claim 26 , further comprising a step of equilibrating a first defect concentration within a bulk of said metal oxide structure with a second defect concentration on said atomically clean surface of said metal oxide structure.
28. The method of claim 27 , further comprising a step of monitoring the second defect concentration on said atomically clean surface of said metal oxide structure.
29. The method of claim 28 , wherein said monitoring step is performed using photoreflectance.
30. The method of claim 26 , wherein said metal oxide is TiO 2 or ZnO.
31. The method of claim 26 , wherein said metal oxide structure is a particle or thin film.
32. The method of claim 31 , wherein said particle or said thin film has a cross sectional dimension selected from the range of 1 nanometer to 10 microns.
33. The method of claim 31 , wherein said particle or said thin film has a cross sectional dimension selected from the range of 1 nanometer to 1000 nanometers.
34. The method of claim 31 , wherein said particle or said thin film has a cross sectional dimension selected from the range of 1 nanometer to 500 nanometers.
35. The method of claim 26 , wherein said metal oxide structure has a ratio of surface area to volume greater than or equal to 0.2 nm −1 .
36. The method of claim 26 , wherein said metal oxide structure has a ratio of surface area to volume selected over the range of 0.2 nm −1 to 0.002 nm −1 .
37. The method of claim 26 , wherein a space charge region at said exposed surface has a thickness selected from the range of 1 nm to 100 nm provided that the thickness of the space charge region is 80% or less of a characteristic dimension of said metal oxide structure.
38. The method of claim 26 , wherein the partial pressure of oxygen gas is selected from a range of 10 −6 Torr to 1500 Torr.