ELECTROCHEMICAL CHARACTERIZATION OF BILAYER LIPID-MEMBRANE SEMICONDUCTOR JUNCTIONS

Three different systems of glyceryl monooleate (GMO), bilayer lipid membrane (BLM) supported semiconductor particles have been prepared and characterized. A single composition of particulate semiconductor deposited only on one side of the BLM constituted system A, two different compositions of particulate semiconductors sequentially deposited on the same side of the BLM represented system B, and two different compositions of particulate semiconductors deposited on the opposite sides of the BLM made up system C. Effective refractive indices and optical thicknesses of GMO-BLM-supported In2S3 and ZnS particles (system A), determined by Brewster angle and reflection measurements, allowed the assessment of the maximum sizes and the volume fractions of semiconductor particles to be on the order of 1200 Å and 0.3, respectively. Semiconductor particles are highly porous structures; only the first layer of particulates penetrated into the BLM were considered in the proposed equivalent circuit and band models. Voltage-dependent-capacitance measurements of a CdS-coated GMO BLM led to Mott-Schottky plots from which flat-band potential of-1.04 V (SCE) and carrier density of 1.6 × 1017/cm3 were obtained. Cyclic voltammetry established current rectifying behavior for the semiconductor-particle-coated BLMs. CdS, ZnS, and In2S3 (system A) formed an n-type, while Cu2-(x+y)S (system A) behaved like a p-type, electrolyte-semiconductor (ES) junction. Semiconductor-semiconductor heterojunction (SS′) formation was established for system C. Treatment of the data in terms of Tafel equations allowed the assessment of the products of particle surface coverage on the BLM (Φs) and the exchange current density (Io), as well as the ideality factor of the diode (n). Values of ΦsIo ranged from 10-5 to 10-8 A/cm2, and those for n were between 2.3 and 8.7. Illumination increased the forward anodic peak current 20 times for CdS-Cu2-(x+y)S-coated GMO BLMs (system B). In contrast, the forward cathodic peak current under illumination was only 50% greater than the dark current for CdS-containing GMO BLMs (system A). Prolonged voltammetric cycling of BLMs - containing CdS on one side (system A) and CdS (or ZnS) on one side and Cu2-(x+y)S, PbS, or Ag on the other side (system C) - converted CdS (or ZnS, PbS) semiconductor junctions from n- to p-type behavior. Type conversion has been rationalized in terms of surface states formed by dopants, trapped carrier, or impurities, as well as by the termination of the space charge region by quantum-sized particles. © 1990 American Chemical Society.