Chemical functionalization of III-V compound semiconductor van der Pauw (VDP) sensors is being studied by our research group for defense, biological and environmental applications. The sensing mechanism rests upon adsorbate-induced changes in charge and surface potential, and follows a Langmuir gas adsorption kinetics model. Sheet resistance changes have been monitored as a function of analyte type, concentration, dilution in air and inert gases, and for long-term operation in our initial work using AlGaN/GaN, InAs, and InP materials [1,2]. This sensor platform and capabilities in our molecular beam epitaxy growth facility to modify semiconductor attributes enable us to expediently evaluate attachment chemistry and sensor effectiveness. This lecture reports on porphyrination of VDP surfaces as a biomimetic approach to selective detection of exhaled breath by-products linked to physiological status including respiratory and cardiac ailments. Surface chemistry and sensing modality have been studied for various protoporphyrin compounds (e.g. ferric-porphyrin chloride - hemin) functionalized to the semiconductor surfaces. These functional groups can be engineered, combined, and incorporated to fit desired specifications, and thus can be tailored for selective response to a broad range of analytes. Included in the analytes of interest in our ongoing work are nitric oxide, carbon monoxide, and their variants. The porphyrins are characterized by a central metal cation operative for selective sensing, amine linking groups which may be altered chemically to limit porphyrin aggregation, and carboxylic acid ligands which may be used as surface tethers. X-ray photoelectron spectroscopy and spectroscopic ellipsometry (SE) have been used to assess and optimize surface chemistry. Modifications in the electronic and chemical state of the semiconductor surfaces provide insight into chemical bonding. Interestingly, surface analysis has revealed upward band-bending of the valence band maximum indicating chemisorption of the functional groups, and evidence of carboxylate bonding preferentially to the anion of the semiconductor. SE analysis has been performed both in-situ and ex-situ to explore variations in porphyrin molarity and benzoic acid used as a ‘spacer’ to optimize surface coverage (following the chemistry protocol developed by Wu and Cahen & co-workers [3] on GaAs). The porphyrin and benzoic acid concentration (relative to dimethlyformamide solvent), along with the chemistry duration, were determined to directly correlate to surface coverage. Benzoic acid added to the porphyrin solution appears to prevent the formation of aggregates in favor of normal-to-surface orientation. We have also begun conducting temperature-dependent sensor measurements to learn more about the effects associated with analyte-surface interactions (i.e. thermodynamics and kinetics of adsorption-desorption processes) and initiating experiments on humidity effects on analyte detection. These results will be discussed and serve as a basis for discussion of exhaled breath sensors and future applications.

[1] J. Uhlrich, M. Garcia, S.D. Wolter, A. Brown, and T. Keuch, J. Cryst. Growth, 300, 204 (2007).

[2] M.A. Garcia, M. Losurdo, S.D. Wolter, T.H. Kim, W. Lampert, J. Bonaventura, G.Bruno, M. Giangregorio, and A. Brown, J. Vac. Sci. Technol. B 25(4), 1504 (2007).

[3] D.G. Wu, D. Cahen, P. Graf, R. Naaman, A. Nitzan, and D. Shvarts, Chem. Eur. J. 7, 1743 (2001).

• Legal notice:
  

  Copyright (c) Pielaszek Research, all rights reserved.
  The above materials, including auxiliary resources, are subject to Publisher's copyright and the Author(s) intellectual rights. Without limiting Author(s) rights under respective Copyright Transfer Agreement, no part of the above documents may be reproduced without the express written permission of Pielaszek Research, the Publisher. Express permission from the Author(s) is required to use the above materials for academic purposes, such as lectures or scientific presentations.
  In every case, proper references including Author(s) name(s) and URL of this webpage: https://science24.com/paper/12653 must be provided.

1. Metastable Epitaxial Alloys