GAS SENSOR

Abstract

A gas-sensing element includes a gas-sensing surface of transition metal-doped metal oxide semiconductor of a first metal (particularly tin oxide) over a body of the metal oxide semiconductor. The gas-sensing element includes an auxiliary component of: (1) internally-disposed second metal (particularly copper, gold or silver) disposed in the gas-sensing element between the body and the gas-sensing surface, or (2) a metal chalcogenide (particularly sulfide or sulphide) disposed at the gas-sensing surface or internally disposed in the gas-sensing element between the body and the gas-sensing surface that stabilizes the second metal at the gas-sensing surface.

Description

BACKGROUND

Gas sensors have been used in various applications such as process monitoring and control and safety monitoring. As the compounds can also be flammable or explosive, gas detection sensors have also been used for leak detection where such compounds are used or manufactured. Various types of sensors have been used or proposed, including but not limited to metal oxide semiconductor (MOS) sensors, non-dispersive infrared detector (NDIR) sensors, pellistor (pelletized resistor) sensors, high-temperature solid electrolytes that are permeable to oxygen ions, and electrochemical cells.

The above types of sensors have been used with varying degrees of success in the industrial or laboratory settings where they have been employed. However, many such sensors have limitations that can impact their effectiveness in demanding new and existing applications. For example, pellistor sensors are prone to false alarms due to cross-sensitivity. NDIR sensors have been used in low-volume applications, but can be difficult and expensive to manufacture to commercial tolerances. Electrochemical sensors rely on redox reactions involving tested gas components at electrodes separated by an electrolyte that produce or affect electrical current in a circuit connecting the electrodes. However, solid state electrochemical sensors can be difficult to implement for some materials. For example, solid state electrochemical sensors testing for combustible hydrocarbons may utilize solid electrolytes formed from ceramics such as perovskite, which can require high temperatures (typically in excess of 500° C.) that render them impractical for many applications. Some electrochemical sensors that operate at lower temperatures (e.g., carbon monoxide sensors, hydrogen sulfide sensors) require the presence of water at the electrode/electrolyte interface for the electrochemical redox reactions, which can render them impractical for many applications.

MOS sensors rely on interaction between gas test components such as hydrogen sulfide or hydrocarbons with adsorbed oxygen on the metal oxide semiconductor surface. In the absence of the gas test components, the metal oxide semiconductor adsorbs atmospheric oxygen at the surface, and this adsorbed oxygen captures free electrons from the metal oxide semiconductor material, resulting in a measurable level of base resistance of the semiconductor at a relatively high level. Upon exposure to gas test components such as hydrogen sulfide or hydrocarbon, the gas test component interacts with the adsorbed oxygen, causing it to release free electrons back to the semiconductor material, resulting in a measurable decrease in resistance that can be correlated with a measured level of test gas component.

In view of the demanding requirements for gas sensors, there remains a need for new alternatives for various environments and applications.

BRIEF DESCRIPTION

According to some embodiments of the disclosure, a gas-sensing element comprises a body comprising a semiconductor that is a metal oxide of a first metal. This semiconductor is also referred to herein as a “metal oxide semiconductor” or “MOS”. The gas-sensing element includes a gas-sensing surface over the body. The gas-sensing surface comprises metal oxide semiconductor of the first metal and a dopant comprising a second metal that is a transition metal and is different than the first metal. The gas-sensing element also includes an auxiliary component comprising: (1) internally-disposed second metal disposed in the gas-sensing element between the body and the gas-sensing surface, or (2) a metal chalcogenide disposed at the gas-sensing surface or internally disposed in the gas-sensing element between the body and the gas-sensing surface, that stabilizes the second metal at the gas-sensing surface.

In some embodiments, the auxiliary component comprises: (1) internally-disposed second metal disposed in the gas-sensing element between the body and the gas-sensing surface, and metal oxide semiconductor of the first metal disposed between the internally-disposed second metal and the gas-sensing surface adjacent to the gas-sensing surface.

In some embodiments where the auxiliary component comprises (1), the gas-sensing element further comprises metal oxide semiconductor of the first metal disposed between the internally-disposed second metal and the gas-sensing surface

In any one or combination of the foregoing embodiments where the gas-sensing element comprises (1), the gas-sensing element comprises a plurality of alternating deposits of the metal oxide semiconductor of the first metal and deposits of the second metal, disposed in the gas-sensing element between the body and the gas-sensing surface.

In some embodiments, the auxiliary component comprises: (2) a metal chalcogenide disposed at the gas-sensing surface or internally disposed in the gas-sensing element between the body and the gas-sensing surface adjacent to the gas-sensing surface that stabilizes the second metal at the gas-sensing surface.

In some embodiments where the auxiliary component comprises (2), the metal chalcogenide is disposed at the gas-sensing surface.

In some embodiments where the auxiliary component comprises (2), the metal chalcogenide is internally disposed in the gas-sensing element between the body and the gas-sensing surface adjacent to the gas-sensing surface, which stabilizes the second metal at the gas-sensing surface.

In any one or combination of the foregoing embodiments where the auxiliary component comprises (2), the metal chalcogenide comprises a metal sulfide.

In any one or combination of the foregoing embodiments, the gas-sensing element comprises a first auxiliary component (1) comprising internally-disposed second metal disposed in the gas-sensing element between the body and the gas-sensing surface, and a second auxiliary component (2) comprising a metal chalcogenide disposed at the gas-sensing surface or internally disposed in the gas-sensing element between the body and the gas-sensing surface adjacent to the gas-sensing surface, that stabilizes the second metal at the gas-sensing surface.

In any one or combination of the foregoing embodiments, the second metal comprises one or more group 5 to group 11 transition metals.

In any one or combination of the foregoing embodiments, the first metal comprises any one of the commonly used metals for metal oxide semiconductors, including aluminum, bismuth, cadmium, cerium, chromium, cobalt, copper, iron, gallium, indium, molybdenum, niobium, tantalum, tin, titanium, tungsten, vanadium or zinc.

In any one or combination of the foregoing embodiments, the first metal comprises tin and the second metal comprises copper.

In some embodiments, a gas sensor comprises the gas-sensing element of any one or combination of the foregoing embodiments disposed between electrodes connected by a voltage-measuring circuit, current-measuring circuit, resistance-measuring circuit, impedance-measuring circuit, or conductance-measuring circuit.

In some embodiments, the resistance-measuring circuit of the gas sensor comprises a signal processor calibrated to determine hydrogen sulfide concentration based on measured resistance at the gas-sensing surface.

In some embodiments, a method of using the gas sensor of any one or combination of the foregoing embodiments comprises exposing the gas-sensing surface to a gas to be tested, and measuring resistance of the gas-sensing element between the electrodes to determine a presence or concentration of a gas component.

In any one or combination of the foregoing embodiments, the gas sensor tests for or is configured to test for hydrogen sulfide.

In some embodiments, a method of making a gas-sensing element comprises disposing a transition metal dopant comprising a second metal at a surface of a semiconductor that is a metal oxide of a first metal, and: (1) disposing second metal in the gas-sensing element between the surface and a body of the metal oxide semiconductor of the first metal, or (2) disposing a metal chalcogenide at the surface or in the gas-sensing element between a body comprising the metal oxide semiconductor of the first metal and the doped surface and adjacent to the doped surface.

In some embodiments where the method of making a gas-sensing element comprises (1), the method comprises depositing second metal over the body, depositing metal oxide semiconductor of the first metal over the deposited second metal, and depositing second metal over the deposited metal oxide semiconductor of the first metal.

In any one or combination of embodiments where the method of making a gas-sensing element comprises (1), the method comprises alternately depositing second metal and metal oxide semiconductor of the first metal to form a plurality of alternating deposits of second metal and metal oxide semiconductor of the first metal between the body and the doped surface.

In any one or combination of embodiments where the method of making a gas-sensing element comprises (2), the method comprises disposing the metal chalcogenide on top of the doped surface.

In any one or combination of embodiments where the method of making a gas-sensing element comprises (2), the method comprises disposing the metal chalcogenide between the body of metal oxide semiconductor of the first metal and the doped surface adjacent to the doped surface.

BRIEF DESCRIPTION OF THE DRAWINGS

Subject matter of this disclosure is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the present disclosure are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is a schematic depiction of a cross-section view of an example embodiment of a gas-sensing element;

FIG. 2 is a schematic depiction of a cross-section view of another example embodiment of a gas-sensing element;

FIG. 3 is a schematic depiction of a gas sensor;

FIGS. 4A, 4B, and 4B are plots of gas sensor outputs of tested sensing elements; and

FIGS. 5A and 5B are plots of gas sensor outputs of tested sensing elements.

DETAILED DESCRIPTION

With reference now to the Figures, FIGS. 1 and 2 schematically depict an cross-section view of an example embodiment of a gas-sensing element. As shown in FIG. 1, gas-sensing element 10, 10′ includes a metal oxide semiconductor body 12 disposed on a substrate having a gas-sensing surface 14 that comprises the metal oxide semiconductor material and a transition metal dopant. Typically, the gas-sensing elements 10, 10′ are disposed on a substrate 11 as illustrated in FIGS. 1 and 2. Examples of metal oxide semiconductors include but are not limited to aluminum (III) oxide, bismuth (III) oxide, cadmium oxide, cerium (IV) oxide, chromium (III) oxide, cobalt (III) oxide, copper (II) oxide, iron (III) oxide, gallium (III) oxide, Indium (III) oxide, molybdenum (VI) oxide, niobium (V) oxide, nickel (II) oxide, tantalum (V) oxide, tin (IV) oxide, titanium (IV) oxide, tungsten (VI) oxide, vanadium (5) oxide, zinc (II) oxide and mixtures of these. Mixed metal oxides (e.g., SnO₂—CuO or other mixed oxides of the above metal oxides) can also be utilized, and the term “first metal” as used herein includes metal mixtures. Transition metal dopants are used to enhance the responsiveness of the metal oxide semiconductor to target gases being sensed for, such as hydrogen sulfide, and to allow for the target gas to be distinguished from other gases that may also produce a change in electrical resistance at the gas-sensing surface 14. In some embodiments, the dopant is a group 5 to group 11 transition metal. Examples of transition metal dopants include copper, silver, gold, iron, ruthenium, nickel, platinum, palladium, or vanadium. Although any of the above materials can exhibit a change in electrical resistance in response to exposure to various test gas components, the use of some materials for particular applications has been more widespread than other materials. For example, copper-doped tin oxide can be used for hydrogen sulfide sensing elements and platinum and palladium doping is commonly used in sensing for hydrogen or hydrocarbons. Such combinations and others are included within this disclosure. Various other materials can be included in the metal oxide semiconductor at the gas-sensing surface 14, including but not limited to noble metals (e.g., silver, gold). Dopants, metal oxide semiconductors, other materials, and combinations thereof are disclosed in Kaur, M. Aswal, D. K. and Yakhmi, J. V. “Chemiresistor Gas Sensors: Materials, Mechanisms and Fabrication” Chapter 2 in , Science and Technology of Chemiresistor Gas Sensors, Ed. Aswal, D. K. and Gupta, S. K. Nova Science Publishers, New York, 2007., and in Bochenkov, V. E. and Sergeev, G. B. “Sensitivity, Selectivity, and Stability of Gas-Sensitive Metal-Oxide Nanostructures” Chapter 2, in Metal Oxide Nanostructures and Their Applications., American Scientific Publishers, California, 2010 the disclosures of each of which is incorporated herein by reference in its entirety.

As mentioned above, the gas-sensing element includes an auxiliary component comprising: (1) internally-disposed second metal disposed in the gas-sensing element between the body and the gas-sensing surface, or (2) a metal chalcogenide disposed at the gas-sensing surface or internally disposed in the gas-sensing element between the body and the gas-sensing surface adjacent to the gas-sensing surface that stabilizes the second metal at the gas-sensing surface. An example embodiment of internally-disposed second metal 16 between the metal oxide semiconductor body 12 and the gas-sensing surface is schematically depicted in FIG. 1. In some embodiments, the element also includes metal oxide semiconductor 18 that is free of second metal (e.g., high purity metal oxide semiconductor) disposed between the internally disposed second metal 16 and the second metal-doped gas-sensing surface 14. In some embodiments, the sensing element can optionally include a plurality of deposits of second metal alternating with deposits of metal oxide semiconductor, as illustrated in FIG. 1 with additional second metal 20 and additional metal oxide semiconductor 22. Four deposits are illustrated in FIG. 1, but larger numbers (e.g., more than 10) of such alternating deposits can also be used.

Deposition of second metal or metal oxide semiconductor onto the metal oxide semiconductor body can be performed using thermal deposition techniques such as sputtering, physical vapor deposition, chemical vapor deposition, or thermal spray. Alternatively, any or all of the deposits can be grown layer by layer, for example, using solution-based epitaxy techniques such as sol-gel processing to form the individual layers. The term “layer” as used herein means any deposit of material, including islands and partial layers, as well as contiguous layers of material. Layers of internally-disposed second metal can range in thickness from 0 (meaning no contiguous layer such as where areas (e.g., islands) of deposited second metal having thicknesses as low as the mass equivalent of 0.2 Angstroms) to 20 nm. Layers of internally-disposed metal oxide semiconductor, which can be interspersed with deposits of the second metal, can range in thickness from 1 to 60 nm.

As mentioned above, the gas-sensing element includes an auxiliary component comprising: (1) internally-disposed second metal disposed in the gas-sensing element between the body and the gas-sensing surface, or (2) a metal chalcogenide disposed at the gas-sensing surface or internally disposed in the gas-sensing element between the body and the gas-sensing surface adjacent to the gas-sensing surface that stabilizes the second metal at the gas-sensing surface. An example embodiment of a metal chalcogenide disposed at the gas-sensing surface or internally disposed in the gas-sensing element between the body and the gas-sensing surface adjacent to the gas-sensing surface is schematically depicted in FIG. 2. As shown in FIG. 2, gas-sensing element 10′ includes metal oxide semiconductor body 12 and doped metal oxide semiconductor gas-sensing surface 14. In some embodiments, the metal chalcogenide can be applied internal to the gas-sensing element adjacent to the gas-sensing surface 14, as depicted by metal chalcogenide 24 in FIG. 2. In some embodiments, the metal chalcogenide can be applied over the gas-sensing surface, as depicted by metal chalcogenide 26 in FIG. 2. In some embodiments, the metal chalcogenide can be disposed (not shown) in the gas-sensing surface 14. In some embodiments, the metal chalcogenide can be disposed in a combination of more than one of the specified locations (e.g., both over and under the gas-sensing surface 14, or mixed in with and underneath the gas-sensing surface 14). In some embodiments (not shown), the auxiliary component can include both (1) internally-disposed second metal disposed in the gas-sensing element between the body and the gas-sensing surface, and (2) a metal chalcogenide disposed at the gas-sensing surface or internally disposed in the gas-sensing element between the body and the gas-sensing surface adjacent to the gas-sensing surface that stabilizes the second metal at the gas-sensing surface. For example, a gas-sensing element could have a metal chalcogenide over (24) or in the gas-sensing surface 14 and second metal disposed 16, 20 between the gas-sensing surface 14 and the metal oxide semiconductor body 12. In another example, a gas-sensing element could have a metal chalcogenide 22 internally disposed adjacent to the gas-sensing surface 14, a metal oxide semiconductor layer 18 under the metal chalcogenide 22, and second metal 16 under the metal oxide semiconductor layer 18.

Examples of chalcogens for the metal chalcogenide include sulfur, selenium and tellurium. In some embodiments, the chalcogen is a chalcogen having a higher number on the periodic table than oxygen. Metals for the metal chalcogenide include but are not limited to silver, lead, zinc, iron, cadmium or other metals that provide a stable chalcogenide at the operating temperature of the sensing element. In some embodiments, the metal chalcogenide comprises a metal sulfide. Examples of metal sulfides include but are not limited to silver sulfide, lead sulfide, zinc sulfide, iron sulfide or cadmium sulfide. In some embodiments, the metal chalcogenide comprises silver sulfide. The metal chalcogenide can be introduced by applying the metal (e.g., silver, lead, zinc, iron) below or above the gas-sensing surface 14 using sputtering or any of the techniques referenced above for application of second metal 16 or metal oxide semiconductor 18, reacting with a reactive chalcogenide such as hydrogen sulfide, and sintering. Sintering may promote spreading of the metal chalcogenide through the gas-sensing surface 14.

The above-described sensing element can be incorporated into a sensor 30 as schematically depicted in FIG. 3. As shown in FIG. 3, gas sensor 30 comprises the gas-sensing element 10 with metal oxide semiconductor body 12 and gas-sensing surface 14, integrated with either parallel or interdigitated (as shown, for higher gain) electrodes 32 and 34 configured to have doped metal oxide semiconductor at the gas-sensing surface 14 disposed between the interdigitated electrodes 32 and 34. The electrodes 32, 34 are depicted on top of the sensing element 10, but can also be disposed at the bottom. The electrodes are connected externally to the gas-sensing element 10 by an electrical circuit 36 that includes a signal processor 38. Signal processor 38 can be a voltmeter or ampere meter, but in many cases comprises a potentiostatic circuit, voltage divider circuit, bridge circuit, microprocessor, electronic control unit (ECU), or similar electronic device with integrated voltage and or amperage measurement functions and also can apply a voltage bias between the electrodes 32 and 34. Other sensor components including but not limited housings, mounting hardware, gas flow conduits, fluid chambers are not shown in FIG. 3, but can be incorporated into the sensor by the skilled person.

Additional disclosure is provided in the following Examples:

EXAMPLES

As demonstrated by the following non-limiting example embodiments, some embodiments can provide a technical effect that can promotes gas sensor stability and can mitigate gas sensor drift.

Example 1

This Example is directed to disposing second metal between the gas-sensing surface of a sensing element and its metal oxide semiconductor body. Sensing elements were prepared by doping a tin oxide semiconductor surface with copper deposited by physical deposition means. Sensing element A was prepared as a control with the copper and gold dopants deposited onto the surface of a tin oxide body. Sensing element 1 was prepared by depositing copper and tin oxide in alternating layers to a tin oxide body, finishing with copper. Sensing element 2 was prepared similar to sensing element 1, except that silver was deposited and sulfided after the top copper dopant application. All three sensors were sintered. The sensing elements were exposed to varying concentrations of hydrogen sulfide over time, and the sensor output was recorded in measured hydrogen sulfide content. The results are shown in FIGS. 4A (sensing element A), 4B (sensing element 1), and 4C (sensing element 2). FIGS. 4A, 4B, and 4C depict overlaying plot of delivered concentration of hydrogen sulfide and the gas sensor output result in measured hydrogen sulfide content. The plotted sensor output in FIG. 4A is typical of hydrogen sulfide sensor experiencing a phenomenon known as “sleeping”, where the sensor response to hydrogen sulfide is initially reduced, but grows stronger with each exposure. In contrast, FIG. 4B shows a longer term stabilizing effect of the layered sensor with improved stability and reduced sleep effect. In FIG. 4C, it is seen that with the silver co-catalyst at the surface produces a refined response, also with improved stability and reduced sleep effect.

Example 2

This Example is directed to disposing a metal chalcogenide at the gas-sensing surface of a surface-doped metal oxide conductor sensing element and its metal oxide semiconductor body. Sensing elements were prepared as in Example 1 by doping a tin oxide semiconductor surface with copper deposited by sputtering. Sensing element B was prepared as a control with the copper dopant deposited onto the surface of a tin oxide body. Sensing element 3 was prepared by depositing silver onto a copper top doped tin oxide body, followed by reaction with hydrogen sulfide to convert the silver to silver sulfide. The sensing elements were exposed to varying concentrations of hydrogen sulfide over time, and the sensor output was recorded in measured hydrogen sulfide content. The results are shown in FIGS. 5A (sensing element A) and 5B (sensing element 3). FIGS. 5A and 5B depict overlaying plot of delivered concentration of hydrogen sulfide and the gas sensor output result in measured hydrogen sulfide content. The plotted sensor output in FIG. 5A is typical of hydrogen sulfide sensor experiencing a phenomenon known as “sleeping”, where the sensor response to hydrogen sulfide is initially reduced, but grows stronger with each exposure. In contrast, FIG. 5B shows a longer term stabilizing effect of the layered sensor with improved stability reduced sleep effect.

While the present disclosure has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the present disclosure is not limited to such disclosed embodiments. Rather, the present disclosure can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the present disclosure. Additionally, while various embodiments of the present disclosure have been described, it is to be understood that aspects of the present disclosure may include only some of the described embodiments. Accordingly, the present disclosure is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.

Claims

1. A gas-sensing element, comprising: a body comprising a semiconductor that is a metal oxide of a first metal;a gas-sensing surface over the body, comprising metal oxide semiconductor of the first metal and a dopant comprising a second metal that is a transition metal and is different than the first metal; andan auxiliary component comprising: (1) internally-disposed second metal disposed in the gas-sensing element between the body and the gas-sensing surface, or(2) a metal chalcogenide disposed at the gas-sensing surface or internally disposed in the gas-sensing element between the body and the gas-sensing surface adjacent to the gas-sensing surface.

2. The gas-sensing element of claim 1, wherein the auxiliary component comprises: (1) internally-disposed second metal disposed in the gas-sensing element between the body and the gas-sensing surface, and metal oxide semiconductor of the first metal disposed between the internally-disposed second metal and the gas-sensing surface adjacent to the gas-sensing surface.

3. The gas-sensing element of claim 2, wherein, further comprising metal oxide semiconductor of the first metal disposed between the internally-disposed second metal and the gas-sensing surface.

4. The gas-sensing element of claim 2, comprising a plurality of alternating deposits of the metal oxide semiconductor of the first metal and deposits of the second metal, disposed in the gas-sensing element between the body and the gas-sensing surface.

5. The gas-sensing element of claim 1, wherein the auxiliary component comprises: (2) a metal chalcogenide disposed at the gas-sensing surface or internally disposed in the gas-sensing element between the body and the gas-sensing surface adjacent to the gas-sensing surface, that stabilizes the second metal at the gas-sensing surface.

6. The gas-sensing element of claim 5, wherein the metal chalcogenide is disposed at the gas-sensing surface.

7. The gas-sensing element of claim 5, wherein the metal chalcogenide is internally disposed in the gas-sensing element between the body and the gas-sensing surface adjacent to the gas-sensing surface, which stabilizes the second metal at the gas-sensing surface.

8. The gas-sensing element of claim 5, wherein the metal chalcogenide comprises a metal sulfide.

9. The gas-sensing element of claim 1, wherein the second metal comprises one or more group 5 to group 11 transition metals.

10. The gas-sensing element of claim 1, wherein the first metal comprises aluminum, bismuth, cadmium, cerium, chromium, cobalt, copper, iron, gallium, indium, molybdenum, niobium, tantalum, tin, titanium, tungsten, vanadium or zinc.

11. The gas-sensing element of claim 1, wherein the first metal comprises tin and the second metal comprises copper.

12. A gas sensor comprising the gas-sensing element of claim 1 disposed between electrodes connected by a voltage-measuring circuit, current-measuring circuit, resistance-measuring circuit, impedance-measuring circuit, or conductance-measuring circuit.

13. The gas sensor of claim 12, wherein the resistance-measuring circuit comprises a signal processor calibrated to determine hydrogen sulfide concentration based on measured resistance at the gas-sensing surface.

14. A method of using the gas sensor of claim 11, comprising exposing the gas-sensing surface to a gas to be tested, and measuring resistance of the gas-sensing element between the electrodes to determine a presence or concentration of a gas component.

15. The method of claim 14, wherein the gas component comprises hydrogen sulfide.

16. A method of making a gas-sensing element, comprising disposing a transition metal dopant comprising a second metal that is a transition metal at a surface of a semiconductor that is a metal oxide of a first metal, and: (1) disposing second metal in the gas-sensing element between the surface and a body of the metal oxide semiconductor of the first metal, or (2) disposing a metal chalcogenide on top of the doped surface or in the gas-sensing element between a body comprising the metal oxide semiconductor of the first metal and the doped surface adjacent to the doped surface.

17. The method of claim 16, wherein (1) comprises depositing second metal over the body, depositing metal oxide semiconductor of the first metal over the deposited second metal, and depositing second metal over the deposited metal oxide semiconductor of the first metal.

18. The method of claim 16, wherein (1) comprises alternately depositing second metal and metal oxide semiconductor of the first metal to form a plurality of alternating deposits of second metal and metal oxide semiconductor of the first metal between the body and the doped surface.

19. The method of claim 16, wherein (2) comprises disposing the metal chalcogenide on top of the doped surface.

20. The method of claim 16, wherein (2) comprises disposing the metal chalcogenide between the body of metal oxide semiconductor of the first metal and the doped surface adjacent to the doped surface.