SURFACE-DERIVATIZED SEMICONDUCTORS WITH CHEMICALLY SENSITIVE LUMINESCENCE

Abstract: The presence of certain chemicals on the emitting surface of the surface-derivatized photoluminescent semicon­ ductor alters the characteristics of radiation emitted from said surface. This alteration is used to indicate the presence of those chemicals in the environment.

Description: Technical Field The present invention relates to optically coupled chemical sensing devices and to processes for detecting the presence of certain classes of chemical compounds. Background of the Art Electroluminescence occurs in semiconductor materials which are capable of emitting visible or near visible radiation when an electrical current passes through the semiconductor. Photoluminescence can also occur in these materials. If external light is used to excite the semiconductor, a characteristic wavelength of light is emitted. These characteristic wavelengths vary amongst different photoluminescent semiconductors and can be varied in a single semiconductor by doping or changing the composition of the material. Amongst the various studies on the luminescence of photostimulated or electroluminescent materials is "Luminescent Photoelectrochemical Cells", Streckert, H.H., Tong, J. and Ellis, A. B., J. Am. Chem. Soc., Vol. 104, No. 2, 1982, pp. 581-588. It is noted therein that the intensity of light emitted by electroluminescence and photoluminescence varies directly with the applied voltage. The efficiency of charge transfer and good electrical contact at the surface is also noted as important in the efficiency of the process. U.S. Patent Application Serial No. 480,471 filed on March 30, 1983 discloses semiconductor electrodes having multicolor luminescence. These semiconductors comprise solid state solutions of three elements which vary in a vertically anisotropic manner. The preferred solid state solutions are of cadmium, sulfur and selenium. U.S. Patent 4,211,586 discloses a method of forming a multicolor light-emitting array of diodes. The diodes are formed by differentially etching a graded n-type semiconductor and diffusing a p-type dopant into the surface of the n-type semiconductor to form a p-n junction diode. Summary of the Invention The present invention describes an optically coupled sensing apparatus of a photoluminescent semiconductor, a source of actinic radiation which can impinge on a radiation emitting surface of the semiconductor and a means for detecting changes in the characteristics of the radiation emitted from the radiation emitting surface. The radiation emitting surface has reacted thereon a compound capable of undergoing oxidation and/or reduction, the redox product of which reacted compound has a vertical charge distribution therein with respect to the radiation emitting surface. Detailed Description of the Invention The present invention uses the creation of electrical or ionic charges in compounds on the surface of photoluminescent semiconductors to alter or modulate the electric field of the semiconductor. The modulation of the electric field alters the photoluminescence of the semiconductor, and this alteration can be detected by sensing means. A compound can be coated or preferably reacted onto the surface of the semiconductor. The coated material or reacted compound must be capable of a further redox reaction. Furthermore, the product of the redox reaction must be a moiety or moieties with a charge thereon or a separation of charges with a vertical displacement relative to the emitting surface of the photoluminescent semiconductor. The created charge distribution must have some vertical displacement therein or there will be no variation in the electric field. For example, if the redox reaction produced a change in positive or negative ionic centers on the redox reaction product which were equidistant from the semiconductor surface, there would be no alteration in the electric field. Photoluminescent semiconductors are well known in the art. They are generally solid state solutions of at least two or three elements which, when stimulated by actinic radiation, emit radiation. Both the actinic and emitted radiation are generally visible or near visible radiation (300-900nm). When a coating over the radiation emitting surface of the semiconductor is material (such as ferrocene derivatives) which when oxidized or reduced alters the electric field in the semiconductor surface, the luminescence of the semiconductor has been found to be altered. Variations in photoluminescence are an indication of the presence of such oxidizing and reducing compounds. Particularly useful n-type semiconductors which can be used to form the detectors according to the present invention are n-GaAs, n-GaAs x P 1-x (where x is from 0 to 1), CdS, CdSe, and CdS x Se 1-x (0≦x≦1) wherein the semiconductor may have a graded composition or not. Other useful semiconductors would be ZnSe:Al, Cd x Zn 1-x S(0≦x≦1), ZnS x Se 1-x (0≦x≦1), Cd x Zn 1-x Se(0≦x≦1), and the like. The redox reactive compound can be deposited on the elements according to standard manufacturing techniques and techniques shown by Wrighton, J. Am. Chem. Soc., 1979, 101 , p. 6179. The apparatus of the invention using this phenomenon for detecting the presence of chemical compounds comprises: 1) a photoluminescent semiconductor having a reacted material bonded to a radiation emitting surface of said semiconductor, said reacted material being further capable of undergoing a redox reaction with an oxidizing or reducing compound to produce a redox product bonded to said radiation emitting surface which has an ionic charge distribution which is vertical with respect to the radiation emitting surface, said charge distribution being capable of altering the electric field in said semiconductor, 2) a source of actinic radiation which can impinge on said radiation emitting surface of the semi-conductor, and 3) a means for detecting changes in the characteristics of the radiation emitted from said radiation emitting surface. The coated semi-conductor has already been described. The actinic radiation source may be merely an opening exposing the sensor or structure to available light (room light, sunlight, etc.) or may be any internal source of radiation such as a light bulb, light emitting diode, or laser. The radiometer may be selected from amongst the many commercially available radiometers, its selection being primarily dependant upon the ultimate sensitivity desired in the final article. Fiber optics may be used to carry actinic radiation to the diode or to carry emitted radiation away from the diode. Example 1 Samples of n-GaAs were prepared for derivatization by etching in 4:1:1 H₂SO₄:H₂O₂ (30% aqueous):H₂O at 50°C for 30 seconds, rinsed in water, immersed in 7 M KOH at 50°C for 30 minutes, rinsed again, and air-dried prior to derivatization. The samples prior to preparation were noted as n-GaAs:Te with carrier concentrations of 8x10¹⁶cm⁻³ and <100> orientation. A ferrocene derivative, (1,1'-ferrocenediyl)dichlorosilane was reacted with the n-GaAs substrate in isooctane solution according to the teachings of Wrighton, J. Am. Chem. Soc., 1979, 101 , p. 6179. When the sensor was illuminated with ultrabandgap light, the bandgap photoluminescence (PL) of the GaAs substrate (865 nm in the near IR) was sensitive to gaseous oxidants. For example, gaseous I₂ or Br₂ caused the PL intensity to quench approximately 50%. This effect appeared to be irreversible in air, although strong volatile reducing agents such as hydrazine reversed the effect. The system was also regenerated by using the sample as an electrode in acetonitrile (n-Et₄N⁺BF₄⁻ supporting electrolyte) and applying a cathodic pulse to reduce the Fe to its original oxidation state. Cyclic voltammetry in this electrolyte indicated that the surface coating was present in at most several monolayers of coverage. The sensor was unaffected by gases such as air, CO, CH₄, and H₂ in both the oxidized and reduced states. Although somewhat resistant to water vapor, high concentrations of water vapor in combination with strong oxidizing agents such as Br₂ or chlorine brought about the destruction of the sensor's surface. Examples 2-4 Example 1 was repeated except that the photoluminescent semiconductors used in the substrate were CdS, GaAs 0.7 P 0.3 and CdS 0.9 Se 0.1 . The apparatus was found to detect the presence of Br₂ and NO₂.

Claims: An apparatus for detecting the presence of oxidizing or reducing chemical compounds, said apparatus comprising: 1. a photoluminescent semiconductor having bonded to a radiation-emitting surface thereof a material capable of undergoing a redox reaction with such an oxidizing or reducing chemical compound to form a redox product bonded to said surface, said redox product having an ionic charge distribution which alters the electric field in said semiconductor, 2. means for directing actinic radiation onto said radiation-emitting surface of said semiconductor, and 3. means for detecting a change in a characteristic of the radiation emitted from said radiation-emitting surface caused by an alteration in said electric field due to said redox product. The apparatus of claim 1 wherein said semiconductor comprises a solid state solution of at least two elements selected from the group of two or three elements consisting of a) cadmium, selenium and sulfur, b) zinc, selenium and sulfur, c) cadmium, zinc and selenium, d) cadmium, zinc and sulfur, e) cadmium and selenium, f) cadmium and sulfur, g) zinc and selenium doped with aluminum, h) gallium and arsenic, i) gallium, arsenic and phosphorus, and j) gallium and phosphorus. The apparatus of any one of claims 1 and 2 wherein said means for detecting changes in the characteristics of the radiation is a radiometer. Apparatus as claimed in any preceding claim wherein said material capable of undergoing a redox reaction is a ferrocene compound. Apparatus as claimed in any preceding claim wherein said radiation-emitting surface is exposed to ambient light as said active radiation. A process for detecting the presence of oxidizing or reducing chemical compounds, comprising the steps of: exposing to such a compound a photoluminescent semiconductor having bonded to a radiation-emitting surface thereof a material capable of undergoing a redox reaction with such a compound, thereby forming a redox product bonded to said surface which redox product has an ionic charge distribution which alters the electric field in said semiconductor; directing active radiation onto said radiation-emitting surface, and detecting a change in a characteristic of the radiation emitted from said radiation-emitting surface caused by an alteration in said electric field due to said redox product. A process as claimed in claim 6 wherein said active radiation is ambient light. A process as claimed in claim 6 or claim 7 when performed with apparatus as claimed in any of claims 2, 3 and 4.

Inventors: ELLIS, ARTHUR B., VAN RYSWYK, HAL
Applicants: MINNESOTA MINING AND MANUFACTURING COMPANY
Application year: 1986
Main classification: G01N21
IPC codes: G01N21