Thin film sensors: The role of defects and impurity sites in controlling sensor response and selectivity

Abstract

Thin film gas sensors are discussed which are produced from polycrystalline organic dyes, such as the phthalocyanines, perylenes, etc. or from metal oxides, such as SnO2, ZnO, etc. Both types of materials are typically used to detect gas phase analytes by virtue of the surface conductance changes which accompany chemisorption of the analyte. In the case of the thin film oxides this process is understood in terms of the change in surface charge density and band bending which accompanies chemisorption of electron donors or acceptors. Defects in the oxide structure, or intentionally added impurities in the near surface region, can enhance the charge density at the surface and provide specific chemisorption sites for various analytes, which are the main means of introducing selectivity to these sensors. In the case of the organic dye thin films, hopping processes, rather than movement of charge through valence and conduction bands, control the flow of charge. Significantly higher defect and impurity densities are anticipated, versus the metal oxide thin films, and movement of charge through trap sites accounts for a significant fraction of the charge transfered. Illumination of certain organic thin films can enhance the sensitivity of the sensor, by a factor of up to 104, depending upon the ratio of dark to photoconductivity in the solid. Addition of electron or hole traps in the near surface region of the thin film can introduce chemisorption sites which interact more selectively with the analyte, and further increase sensitivity. Examples of this approach are given, for the detection of NH3 and NO2, using selected photoconductive phthalocyanine thin films.

Novel peripherally functionalized seco-porphyrazines: synthesis, characterization and spectroscopic evaluation

“…Rotary evaporation and chromatography (EtOAc/hexanes 3:1) gave dinitrile 5 (2.1 g, 87%), a mixture of E and Z isomers, as a clear amber oil: R f 0Propanol (5 mL), Mg (16.3 mg, 0.7 mmol) and I 2 (two small crystals) were heated to reflux for 24 h under N 2 . The suspension was cooled to 208C and dipropylmaleonitrile 6 (140.7 mg, 0.87 mmol) followed by dinitrile 5 (103.7 mg, 0.13 mmol) were added and the mixture further heated to reflux under N 2 for 60 h. The deep blue suspension was allowed to cool, filtered (Celite), and the solids washed with CH2 Cl 2 . Rotary evaporation and chromatography (hexanes/EtOAc 7:3; EtOAc) gave the Mg-pz 7 (56.2 mg, 34%) as a blue solid: R f 0.7 (EtOAc); UV -vis (CH 2 Cl 2 ) l max 239, 345, 612 nm; MS (FAB) m/z 1320 [M þz ]; HRMS (FAB) calcd for C 74 H 107 MgN 10 O 10 : [MþH] þ , 1319.8022; found: [MþH] þ , 1319.7967.…”

Novel porphyrazines containing peripherally functionalized macrocyclic (N2O2, N2S2) units: Synthesis and characterization

“…The solid residue was dissolved in CHCl 3 (400 mL) and filtered through a pad of Celite. The filtrate was rotary evaporated and chromatographed (CHCl 3 -MeOH:NH 4 (8) Mg-porphyrazine 6 (0.20 g, 0.1 mmol) was dissolved in CF 3 CO 2 H (2.5 mL) and stirred at room temperature in the dark for 2 h. The solution was poured over ice (100 g) and then neutralized with 2 M NaOH. The mixture was kept in the refrigerator overnight to complete the precipitation.…”

“…It has already been shown that the incorporation of crown ethers into other tetrapyrrole derivatives, namely phthalocyanines and porphyrins, results in products attractive as potential sensors for metal ions and as building blocks in supramolecular assemblies [10,11]. In contrast to fused phthalocyanine-crown ether systems, metal ion coordination by the crown ether units in [(N 2 ,O 4 crown)] 4 pz has profound effects on UV-vis spectra and other properties, since the key crown nitrogen residues are in direct electronic contact with the macrocyclic central chromophore [12]. In addition, the attachment of oxacrown [13], azacrown or polyaza-polythia macrocycles to porphyrazine has received considerable attention since they allow for cation selectivity and complex stability to be enhanced through changing the numbers and types of macrocycle donors as well as the solubility of porphyrazines [14][15][16][17][18].…”

“…Porphyrins, phthalocyanines and related macrocycles, modified by the attachment of peripheral substituents, have found vast applications in diverse areas such as elaboration of LangmuirBlodgett films [1][2][3], chemical sensors [4], nonlinear optical materials [5], biomedical agents for diagnosis and therapy [6,7] as well as sensitizers in dye-sensitized solar cell applications [8,9]. In contrast, tetraazaporphyrazine or porphyrazines, the analogues of porphyrins and phthalocyanines, have been less well explored due to the lack of efficient synthetic methods.…”

Synthesis and characterization of new metallo and metal-free porphyrazines containing dioxadithia (O2S2) and tetrathia (S4) macrocyclic moieties

“…Phthalocyanines and related tetraazamacrocycles, modified by the attachment of peripheral substituents, are of potential interest for a wide variety of applications such as biomedical agents for diagnosis and theraphy [1], chemical sensors [2], non-linear optics [3], liquid crystals [4], Langmuir-Blodgett films [5] and ladder polymers [6].…”