6562
Langmuir 1997, 13, 6562-6567
Langmuir-Blodgett Multilayers Based on Copper Phthalocyanine as Gas Sensor Materials: Active Layer-Gas Interaction Model and Conductivity Modulation R. Rella,*,† A. Serra,‡ P. Siciliano,† A. Tepore,‡ L. Valli,‡ and A. Zocco‡ Istituto per lo Studio di Nuovi Materiali per l’Elettronica, IME-CNR, Universita´ di Lecce, Via per Arnesano, 73100 Lecce, Italy, and Dipartimento di Scienza dei Materiali, Universita` degli Studi di Lecce, Via per Arnesano, 73100 Lecce, Italy Received October 23, 1996. In Final Form: May 6, 1997X Langmuir-Blodgett films of copper(II) [tetrakis((3,3-dimethylbutoxy)carbonyl)]phthalocyanine and copper(II) [tetrakis(n-butoxycarbonyl)]phthalocyanine have been studied as sensitive elements of gas sensors. Moreover, alternate LB films of the two compounds and LB films obtained from a 1:1 mixture of them have also been investigated. In particular, after exposure to NO2 oxidizing gas, the electrical conductivity of the materials goes through a maximum at a temperature of about 170 °C corresponding to the maximum sensitivity to the gas. The sensitivity and stability of the response of the active layers to NO2 are discussed as well as the application of these materials as the sensitive elements for gas-sensing devices.
Introduction Toxic gas management has become an issue of vital importance in environment-conscious modern life. To this purpose there is an increasing need for low-cost, selective, sensitive, and reliable gas sensors. A simple method to detect gases is measuring the electrical conductivity change induced on a suitable active layer by the absorption of gas molecules on its surface. Traditional gas-sensing materials are inorganic oxide semiconductors, such as SnO2, TiO2, and WO2, and only recently has there been interest in the development of organic semiconductors such as metallophthalocyanines characterized by a ring network of π-electrons. Considering that the gas/material interaction is a surface effect, these materials, both organic and inorganic, are deposited in thin film form in order to increase their performance. Phthalocyanine thin films can be produced by a variety of techniques, such as vacuum sublimation, electron-beam evaporation, solvent evaporation from a fine suspension, spin coating, and Langmuir-Blodgett (LB) deposition.1,2 In particular, the LB technique is one of the most effective ways of depositing extremely thin films with precise molecular dimension and high structural order. We have recently shown that selective NO2 sensors can be fabricated by the LB deposition of specially designed phthalocyanine molecules.3,4 In this work we have analyzed LB films of copper(II) [tetrakis((3,3-dimethyl-1-butoxy)carbonyl)]phthalocyanine [labeled Cu(dmbc)Pc] and copper(II) [tetrakis(nbutoxycarbonyl)]phthalocyanine [labeled Cu(bc)Pc], two members of the phthalocyanine family, and films of the 1:1 mixture (molar ratio), and alternate films of the two compounds. The molecular formulas of these two different phthalocyanines are illustrated in Figure 1. The char* To whom correspondence should be addressed. † Istituto per lo Studio di Nuovi Materiali per l’Elettronica. ‡ Dipartimento di Scienza dei Materiali. X Abstract published in Advance ACS Abstracts, November 1, 1997. (1) Roberts, G. G.; Petty, M. C.; Baker, S.; Fowler, M. T.; Thomas, N. J. Sens. Actuators 1983, 4, 113. (2) Barger, W. R.; Snow, A. W.; Wohltjen, H.; N. L. Jarvis Thin Solid Films 1985, 133, 197. (3) Bott, B.; Jones, T. A. Sens. Actuators 1984, 5, 43. (4) Baker, S.; Petty, M. C.; Roberts, G. G.; Twigg, M. V. Thin Solid Films 1983, 99, 53.
S0743-7463(96)01029-3 CCC: $14.00
Figure 1. Molecular formula of the two different phthalocyanines analyzed in the present work.
acteristics of deposition and the structural properties of the LB films obtained from these compounds have been thoroughly analyzed.5-7 In this work we have investigated the effect of successive exposure of the four active layers to NO2 gas within the part per million range as well as the sensor parameters: response time and recovery time. In particular, we have analyzed the changes in the electrical conductance as a function of time t and work temperature when the LB active layers of the four analyzed systems are exposed to different concentrations, in an environment of clean air or of NO2 gas. A model is proposed to describe the conductivity variations of the four systems upon NO2 doping. Experimental Details The Cu(dmbc)Pc and Cu(bc)Pc compounds were obtained in our laboratory, and the details of the synthetic procedures have already been described.5,8 In particular toluene and chloroform (5) Pasimeni, L.; Meneghetti, M.; Rella, R.; Valli, L.; Granito, C. Troisi, L. Thin Solid Films 1995, 265, 58. (6) Manno, D.; Rella, R.; Troisi, L.; Valli, L. Thin Solid Films 1996, 280, 249. (7) Rella, R.; Serra, A.; Siciliano, P.; Tepore, A.; Valli, L.; Zocco, A. Thin Solid Films 1996, 286, 256. (8) Valli, L.; Rella, R.; Tepore, A.; Zocco, A. Mater. Sci. Eng. C, in press.
© 1997 American Chemical Society
LB Multilayers Based on Copper Phthalocyanine were used in making up the spreading solutions. The concentrations of Cu(dmbc)Pc and Cu(bc)Pc in toluene were 4.9 × 10-4 and 4.7 × 10-4 M, respectively, whereas a 1:1 mixture of the two components was dissolved in chloroform and the resulting concentration was 2.4 × 10-4 M for each compound. LB films of the compounds were deposited by a KSV5000 System 3 LB apparatus (850 cm2). Since the electrical conductivity of phthalocyanine in thin film form is generally low, alumina substrates (10 × 5 × 1 mm3) with interdigital microelectrodes were utilized for the LB deposition in order to achieve more reliable electrical measurements. The transfer of the monolayers onto the substrates was carried out by the standard vertical dipping method at a surface pressure and speed, respectively, of 15 mN/m and 4-6 mm/min in each dipping direction for Cu(dmbc)Pc, 18 mN/m and 3 mm/min during the upstroke and 8 mm/min during the downstroke for Cu(bc)Pc, and 21 mN/m and 3-4 mm/min for both directions for the 1:1 mixture. Further details about the LB deposition of our compounds have already been reported.5,6,8 For the electrical measurements 30- and 31layer-thick LB films were reproducibly constructed. The alumina substrates were preliminary prepared by depositing by thermal evaporation a patterned microelectrode array consisting of interdigital pairs of gold fingers about 40 nm thick. Gold was selected as the electrode material, since it is well-known that it forms Ohmic contacts to the phthalocyanine. The dc resistance of the various samples was measured by a Keithley model 617 electrometer. The gas effect on electrical conductivity was measured in a dynamic pressure system implemented in our laboratory where dry air at ambient pressure was used as the carrier and reference gas. The gases employed in all experiments had purity levels of 99.99%. The samples were placed in a stainless steel chamber through which a gas could be passed. The gas concentration was varied by using a MKS INSTRUMENTS mass flow controller model 647. The working parameters, such as the operating temperature and the gas flow rates and resistance of the samples, were controlled by a personal computer with an IEEE 488 board. In this way we obtained the electrical response of each sample in real time. The temperature control was carried out with an Eurotherm apparatus while a platinum thermometer measured the temperature of the sample in the test chamber. The experimental procedure was performed by flowing dry air through the sample chamber until a steady resistance reading had been obtained. Then the active gas was admitted in its lowest concentration and the resistance was recorded during a period of time necessary to obtain a saturation current and calculate the response time of the sensor. The active gas was then turned off and the sample left to recover in air. The restore at the initial value of the resistance confirmed the reversibility of the process. At this point the highest concentration of the active gas was admitted to the chamber and the measurement procedure repeated.
Langmuir, Vol. 13, No. 24, 1997 6563
Figure 2. Dynamic response curves of 31-layer-thick Cu(dmbc)Pc LB films on exposure to 500 ppm O2 (a) and to 900 ppm N2 (b) in air, respectively.
a controlled atmosphere. Similar results have been also obtained for LB films of Cu(bc)Pc. The doping effect of oxygen and absorbed nitrogen gas on the LB layer is proved by the change in the electrical resistance of the sample. In particular, we have shown that the absorbed oxygen gas behavior is based on the following charge-transfer reaction
Cu(dmbc)Pc + O2 f Cu(dmbc)Pc+ + O2with the oxidation of the phthalocyanine and the reduction of the absorbed oxygen gas. On the contrary, the absorbed nitrogen gas follows the reaction
Results
Cu(dmbc)Pc + N2 f Cu(dmbc)Pc- + N2+
Electrical Properties. It is useful to analyze the molecular properties of the phthalocyanine macrocycles and see in which way they contribute to the absorption of the gas molecules onto the active layer. The delocalized π-electron system, the presence of heteroatoms, and the metal complexes assume particular importance. The delocalized π-electron system constitutes a highly polarizable electronic cloud producing intense dispersion forces between adjacent molecules of phthalocyanine. Furthermore, the π-electrons are weakly bound to the phthalocyanine molecule so that phthalocyanines are relatively good electron donors. Consequently, the relatively low ionization energy favors the charge-transfer interactions when electron acceptor molecules were absorbed. In order to analyze the donor or acceptor behavior of the molecules of gas absorbed onto the surface of our LB films, we have exposed the active layer to mixtures of argon containing 500 ppm O2 and argon containing 900 ppm N2 and measured the electrical resistance change induced by the oxidizing and reducing gas, respectively. In Figure 2 we show the typical dynamic responses obtained at room temperature on Cu(dmbc)Pc LB films in
where we have the oxidation of the absorbed nitrogen gas and the reduction of the phthalocyanine. Taking into account the obtained experimental data, we conclude that the absorbed nitrogen gas produces on the active layer an increase in the resistance value while the absorbed oxygen gas creates a decrease in the resistance. On the basis of these experimental results, we have classified the LB films obtained in our laboratory as p-type semiconductors. NO2-Sensing Characteristics. In a previous paper7 we have shown that NO2 gas in low concentration in the test chamber produced a reversible increase in conductivity of the Cu(dmbc)Pc LB films. We have obtained the same behavior also for the other systems studied in the present work. In all cases, the interaction between NO2 and the copper phthalocyanine LB films can be explained as follows: NO2 is a well-known oxidizing gas which, on contact with the π-electron network of phthalocyanines, causes the transfer of an electron from the phthalocyanine ring to the gas. When this occurs, the phthalocyanine ring becomes positively charged and the charge carriers thus created justify the increased conductivity of the film.
6564 Langmuir, Vol. 13, No. 24, 1997
Figure 3. Variation of percentage change in conductance to 200 ppm NO2 vs work temperature of the LB films of Cu(dmbc)Pc, Cu(bc)Pc, their 1:1 mixture, and alternate multilayers of them.
Figure 4. Electrical resistance changes at the work temperature 160 °C due to NO2 absorption for a typical 31-layers thick LB film obtained from the 1:1 mixture of Cu(dmbc)Pc and Cu(bc)Pc. The arrows indicate gas injection.
This process is strongly dependent on the temperature of the active layer. Then, the first step in the study of the gas-sensing properties of our system was to determine the optimal working temperature value Tm measured for the maximum sensitivity to the gas, i.e. the maximum conductivity variation. Figure 3 shows the sensitivities to 200 ppm NO2 gas of the four systems as a function of the work temperature, where the sensitivity is defined as (Ggas - Gair)/Gair. Marked differences are apparent. For the alternate structure we have a low NO2 sensitivity of about 22% at a work temperature Tm of about 120 °C while the 1:1 mixture of Cu(dmbc)Pc and Cu(bc)Pc presents better characteristics with a sensitivity of about 2300% at a work temperature of about 170 °C. Further experimental results show also that the height of the sensitivity peak increases with the oxidizing gas concentration, but Tm is independent of it. In light of the previous results, we have not electrically characterized further on the alternate structure and our attention has been devoted to the electrical characterization of the two compounds Cu(dmbc)Pc and Cu(bc)Pc and of the promising 1:1 mixture of them. In particular, Figure 4 shows a typical response curve of the 30-layer-thick LB film of the 1:1 mixture to increasing NO2 concentrations at its operating temperature. It is clearly seen that the injection of NO2 leads
Rella et al.
Figure 5. Response time to 200 ppm NO2 gas vs the operating temperature for typical LB films of Cu(dmbc)Pc, Cu(bc)Pc, and the 1:1 mixture of them.
to a drastic drop in the resistivity that reaches its initial value after NO2 gas is shut off, and this proves that the absorption process is reversible. The curves in Figures 3 and 4 have been obtained using a constant gas flow of about 9 l h-1; if the gas flow is increased, then the response time remains practically unchanged, while the recovery time decreases. Here the response time is defined as the time required for the sample variation of conductance to reach 90% of the equilibrium value following a step increase in the test gas concentration, while the recovery time is defined as the time necessary for the sample to return to 10% above its original conductance in air following the zeroing of the test gas. In particular, Figure 5 shows the dependence of the response times of the tree LB systems on operating temperature. As one can see, the response times are a decreasing function of the work temperature. In particular, at their maximum sensitivity temperatures, the Cu(bc)Pc LB films and LB films of the 1:1 mixture exhibit a response time of about 100 s, while Cu(dmbc)Pc LB films show a response time of about 300 s. On the contrary, at the maximum sensitivity temperatures, we have obtained for the recovery time values of about 300, 500, and 1000 s for Cu(bc)Pc, Cu(dmbc)Pc, and the 1:1 mixture LB films, respectively. These high values of the recovery time are a false problem because, even though the recovery time is modifiable by introducing into the test chamber a reducing gas such as nitrogen that increases the resistance of the films, it is very hard to reduce the response time of our sensor. Finally, the curves reported in Figures 3 and 5 show that at low work temperatures LB films of Cu(dmbc)Pc and Cu(bc)Pc present relatively long values of the response time and a low sensitivity, while near the maximum sensitivity temperature the response time decreases. On the contrary, for LB films of the 1:1 mixture of the two compounds the response time remains practically constant in the investigated work temperature range. In light of the previous observations, and as reported in the literature with regard to electrical characterization of CuPc compounds,9 it is possible to note that an oxidant gas like oxygen or nitrogen dioxide produces the formation of the acceptor levels in the band gap energetically localized near the valence band; consequently the electrical con(9) Simon, J.; Andre’, J.-J. In Molecular Semiconductors Photoelectrical Properties and Solar Cells; Lehn, J. M., Rees, Ch. W., Eds.; Springer-Verlag: Berlin Heidelberg, New York, Tokyo, 1985.
LB Multilayers Based on Copper Phthalocyanine
Langmuir, Vol. 13, No. 24, 1997 6565
ductivity due to the hole concentration increases, as shown in Figure 2a. On the contrary, a reducing gas like nitrogen introduces donor levels in the band gap energetically localized near the conduction band, producing a decrease in the electrical conductivity, as shown in Figure 2b. Here we consider the band gap as the energetic separation between the occupied molecular orbitals at highest energy (valence band, HOMO) and the unoccupied molecular orbitals at lowest energy (conduction band, LUMO). In the literature is reported a value of about 2 eV for the energy gap of a phthalocyanine single crystal measured in vacuum.10 In a previous work7 we have shown that the activation energy ∆E under air for CuPc LB films was about 0.7 eV and that the conductivity in the whole temperature range studied (20-200 °C) was clearly extrinsic and thermally activated. This value of the activation energy of our LB films suggested a combined effect on the conductivity due to the formation of doping levels in the band gap of the material produced from the O2 and N2 gas absorbed onto the surface of the active layer. The admission of NO2 in small quantities in the test chamber was found to increase the conductance of the layers and lower the activation energy of the conductivity up to 0.2 eV. This value suggests the formation of doping levels in the band gap due to NO2 absorbed gas near the valence band of the material. The electrontrapping levels thus produced increase the free hole concentration in the valence band and thus the electrical conductivity of the layer. When the NO2 concentration is increased, the Fermi level moves closer to the valence band because of augmentation of the density of free charge carriers.
n gas partial pressure by the relation [NO2(ads)] )ηPNO 2 with 0 < n < 1 and η constant, eq 2 becomes
K(T) )
(j-m) γj-mPNO 2
(3)
Finally, we obtain for the hole concentration [n(1-R)+R]/2 Ch ) η(1-R)/2γR/2K(T)1/2PNO 2
(4)
where R ) j - m represents the number of NO2 molecules adsorbed onto the surface of the LB film. According to a combined statement of the first and second laws of thermodynamics, we have11
∆G ) -RT ln K ) ∆H - T∆S
(5)
where ∆G, ∆H, and ∆S are the standard Gibbs free energy, enthalpy, and entropy changes in the reaction, respectively. R is the gas constant, T is the absolute temperature, and ln K is the natural logarithm of the equilibrium constant. From this equation, the following relationship can be derived:
K(T) ) exp[∆S/R] exp[-∆H/RT]
(6)
Now, by considering that the electrical conductivity of the LB film is given by σTOT ≈ σh ) Cheµh, where µh is the electrical mobility of the holes in the phthalocyanine LB film, we have that the electrical conductivity of our LB film is given by the following equation: [n(1-R)+R]/2 σ ) η(1-R)/2γR/2K(T)1/2µhePNO 2
Model of NO2 Gas/Phthalocyanine Interaction In order to approach a gas/phthalocyanine interaction model, we suppose that NO2 gas is adsorbed on the film, the electrons are transferred from the phthalocyanine to adsorbed NO2 gas, and the holes residing in the LB film can be delocalized according to the following equilibria:
n(j-m-1) C2hη(j-m-1)PNO 2
(7)
Inserting relation 6 into eq 7 we obtain [n(1-R)+R]/2 exp[-∆H/2RT] σ ) µhAPNO 2
(8)
A ) eη(1-R)/2γR/2K(T)1/2 exp[∆S/2R]
(9)
where absorption
CuPc + jNO2(g) 798 CuPc + (j - m)NO2(abs) + charge transfer
mNO2(g) 9 7 8 CuPc + NO2 + (j - m +
-
delocalization
1)NO2(abs) + mNO2(g) 9 7 8 CuPc + NO2- + (j - m - 1)NO2(abs) + mNO2(g) + h+ (1) where j is the number of molecules of NO2 interacting with a phthalocyanine ring, m is the number of molecules not adsorbed, and h+ is the hole created in the chargetransfer reaction and available for the electrical conduction. Reaction 1 usually determines the number of positive holes by the equilibrium constant K given by the following equation
K(T) )
[NO2-]Ch[NO2(abs)]j-m-1[NO2(g)]m [NO2(g)]j
(2)
If the holes are predominantly produced only by the charge-transfer reaction, then the electroneutrality in the bulk requires [NO2-] ) Ch, where Ch is the positive hole concentration. Now, with the [NO2(g)] proportional to the partial pressure of NO2 in the mixture, [NO2(g)] ) γPNO2 with 0 < γ < 1, and assuming the concentration of adsorbed NO2 molecular units is related to the external (10) Hamann, C. Phys. Status Solidi 1967, 20, 481. Leznoff, C. C.; Lever, A. B. P. Phthalocyanine Applications; VCH: New York, 1989; Vol. I, p 133.
12
In this specific case one can show that the enthalpy change ∆H can be substituted by the activation energy ∆E necessary for hole migration. In particular, ∆H or ∆E represents the difference between the energy level of the holes in the thermally activated state and that of the holes in the equilibrium state. If the number of adsorbed molecules per unit volume is equal to Avogadro’s number NA, then the gas constant R must be replaced by the Boltzmann constant k; thus, eq 8 becomes β exp[-∆E/2kT] σ ) µhAPNO 2
(10)
where β) 1/2[n(1 - R) + R] and the activation energy ∆E is related to both the energy required for the charge transfer between two species and the energy necessary to create a mobile hole. It can be seen from eq 10 that the electrical conductivity is proportional to the power of the nitrogen dioxide pressure, and this proportionality is fixed by the stoichiometry of the reaction. So, the theoretical eq 10 is very helpful in interpreting the experimental observation made on metal phthalocyanine LB films with p-type semiconduction. In our case we have normalized (11) Goto, K. S. Solid State Electrochemistry and its applications to sensors and electronic devices; Materials Science Monographs; Elsevier: Amsterdam, Oxford, New York, Tokyo, 1988; p 45. (12) Madou, M.; Morrison, S. Chemical sensing with Solid State Devices; Academic Press: New York, 1989; Chapter 2.
6566 Langmuir, Vol. 13, No. 24, 1997
Rella et al.
Figure 6. Conductance change plotted against NO2 gas concentration for typical LB films of Cu(dmbc)Pc, Cu(bc)Pc, and the 1:1 mixture of them at the optimum sensitivity temperature Tm.
the obtained experimental data to the three different LB layers by the conductivity value of the corresponding LB film measured in dry air. In that case, eq 10, at the maximum sensitivity temperature Tm, is proportional to
σgas ∝ PβNO2 σair
(11)
which fits well our experimental results shown in Figure 6, where the log-log plots of the conductance versus NO2 concentration for the LB films of Cu(dmbc)Pc, Cu(bc)Pc, and the 1:1 mixture at their optimum operating temperatures are displayed according to eq 11. A similar relation has also been obtained by A. Pauly et al.13 As one can see, the LB films of Cu(dmbc)Pc exhibit very high sensitivity to little variations in the NO2 concentration with a slope (β value) of about 3.5 but a very high sensitivity threshold of about 100 ppm, the value obtained for the intercept at σgas/σair ) 1 of the best fit curve of the corresponding experimental data. On the contrary, we observe that the LB films of Cu(bc)Pc and of the 1:1 mixture show an intermediate situation. In particular, while LB films of Cu(bc)Pc show the lowest sensitivity to little changes of NO2 gas concentrations with a slope β ) 0.5 and a sensitivity threshold of about 10 ppm, LB films of the 1:1 mixture exhibit the better characteristics with a low sensitivity threshold of about 10 ppm and the highest sensitivity to little changes of NO2 gas concentrations (β ) 1.5). In conclusion, according to the particular technological applications, it is possible to select the active layer with the suitable characteristics. Discussion The interaction between the gas molecules and the surface of the active layer of the phthalocyanine LB film is an extremely complex phenomenon. The structure and morphology of the thin film obtained by using the Langmuir-Blodgett technique are the determinant parameters of this interaction.14 It is possible to have (13) Pauly, A.; Blanc, J. P.; Dogo, S.; Germain, J. P.; Maleysson, C.; Passard, M. Gas sensing comparative properties of LuPc2 and CuPc. In Sensors VI Technology Systems and Applications; Grattan, K.T.V. Augousti, At., Eds.; Sensors Series; Institute of Physics Publishing: Bristol and Philadelphia, 1983; p 51. (14) Wright, J. D. Prog. Surf. Sci. 1989, 31, pp 1-60.
information about the average molecular orientation of the phthalocyanine macrocycles with respect to the substrate and the dipping direction by means of spectrophotometric analysis in polarized light. As reported by Ionov et al.,15 the optical transitions of the aromatic molecules are determined by the specificity of their π-electron system, and the absorption spectra carried out in the electronic transitions spectral range depend on the orientation of their transitional dipole moments and therefore on the molecular arrangement. This arrangement is sensitive to the electrostatic interactions between the π-electron systems. In a previous work we have already reported the optical spectra of the LB multilayer films of our compounds.5,6 They display a very strong QII absorption band due to aggregated species and only a weak shoulder in the region associated with the monomeric absorption band. The QII band was attributed to the development of a one-dimensional linear stacking of the phthalocyanine molecules in the form of dimers, trimers, etc. Absorptions in polarized light in the 500-700 nm spectral range were assigned to π-π* optical transitions having the dipole moment that lies in the molecular plane; the absorption spectrum with polarized light perpendicular to the dipping direction (A⊥) is more intense than the absorbance spectrum with polarized light parallel to the dipping direction (A|). This proves that the normal to the molecular plane tends to be parallel to the dipping direction. In particular, we have shown that the molecules are edge-on on the substrate, confirming the area per molecule results estimated from the Langmuir isotherms at the air-water interface. In particular we observe that the LB films of the Cu(dmbc)Pc compound evidence a less average optical order than the LB films of Cu(bc)Pc, with the dichroic ratio at normal incidence D0 ) 0.49 and D0 ) 0.21, respectively. The dichroic ratio at the ξ angle is defined by the relation Dξ ) A|/A⊥. It is possible to interpret these results by considering the chemical structure of the two single macromolecules Cu(dmbc)Pc and Cu(bc)Pc. They have different dimensions from one another because of the two different radicals connected to the Pc macrocycles, R ) COOCH2CH2CH2CH3 and R ) COOCH2CH2C(CH3)3 for Cu(bc)Pc and Cu(dmbc)Pc, respectively. Also in this connection a much larger effect of sterical hindrance is of course to be claimed in the latter case, owing to the presence of the bulky and branched tert-butyl moieties [C(CH3)3], than in the former case, where the linear groups (n-butyl radicals) are present. Consequently, it is possible to suppose that less ordered and packaged LB films are obtained with Cu(dmbc)Pc macromolecules with respect to LB films containing the Cu(bc)Pc compound. The absorption spectra of the Cu(bc)Pc LB films evidence the presence of a marked absorption band at higher values of energy (λ ) 600 nm) with respect to the ones of Cu(dmbc)Pc, where the peak is centered at about 620 nm. Such behavior was associated with the degree of association of molecules. In fact, it is well-known that the aborption band moves toward greater energies when the aggregation of molecules increases. The presence of aggregates is greater for Cu(bc)Pc LB films than in Cu(dmbc)Pc multilayers. The characteristic behavior of these two LB films of macrocyclic derivatives reverberates also on the optical properties of the 1:1 mixture and the alternate multilayers of Cu(bc)Pc and Cu(dmbc)Pc. In fact, in the case of the equimolar mixture, the ordering effect of Cu(bc)Pc predominates and the value of the dichroic ratio is D0 ) 0.29. It is noteworthy that the isotherm, deposition characteristics, and architecture of LB multilayers of this mixture are much more similar to the ones of Cu(bc)Pc than to (15) Ionof, R.; Angelova, A. J. Phys.Chem. 1995, 99, 17606 and 17593.
LB Multilayers Based on Copper Phthalocyanine
those of Cu(dmbc)Pc. As can be apparently seen, the value of the dichroic ratio in this multilayer is intermediate between the one for the most ordered film [Cu(bc)Pc] and the one of the less ordered film [Cu(dmbc)Pc]. On the contrary, in the alternate LB films, the less ordered structure of Cu(dmbc)Pc prevails (D0 ) 0.67). Therefore, each layer of Cu(bc)Pc does not succeed in bringing order on the following layer of Cu(dmbc)Pc, and the disordering effect due to steric hindrance takes priority. The electrical measurements carried out in a controlled atmosphere show that the structure and morphology of the LB films influence the sensing properties of the active layer; moreover, when the macrocycles are organized in stacks in the LB film, they may show one-dimensional energy migration and charge transport. In particular, the surface of the film will present different absorption sites for gas molecules. The distribution and concentration of these absorption sites depend strongly on the structure of the macromolecule used to realize the LB active multilayer. Transmission electron microscopy analysis, already discussed,16 has shown that both Cu(bc)Pc and Cu(dmbc)Pc phthalocyanine LB films give rise to a series of ordered domains in an amorphous matrix. Each domain is made by a series of columns placed side by side, and the structural interdomain correlation is determined by the nature of the substituent on the periphery of the macrocycle. The analysis indicates that the LB film of Cu(bc)Pc is more homogeneous than the one of Cu(dmbc)Pc. Under the same investigated area, the Cu(dmbc)Pc LB film presents a greater number of domains than the Cu(bc)Pc LB film and, consequently, a higher distribution of interdomain regions containing structural defects. Therefore, the electrical conduction mechanism appears to be influenced by the interdomain potential barriers which are produced by these defects. The absorption of oxidant gases (O2 or NO2) onto the surface of the active layer is localized preferentially in the interdomain regions. These absorbed NO2 molecules produce electron capture centers in the band gap near the valence band. This mechanism increases the concentration of holes available to conduction and at the same time reduces the height of the interdomain potential barriers. Because of structural disorder of the Cu(dmbc)Pc LB film this mechanism produces a higher sensitivity threshold to NO2; consequently, a variation in (16) Manno, D.; Valli, L.; Taurino, A.; Micocci, G. Supramol. Sci. in press.
Langmuir, Vol. 13, No. 24, 1997 6567
the conductivity is detectable as soon as the concentration of absorbed gas NO2 is sufficiently elevated. A further increase in the NO2 concentration brings a higher sensitivity. On the contrary, more ordered and homogeneous active layers like Cu(bc)Pc LB films will present more large domains and, consequently, a lower distribution of the interdomain regions. This structure assures a higher conductivity in air with respect to that of the Cu(dmbc)Pc LB film and a very low threshold sensitivity to NO2 gas. Because of the lower concentration of NO2 absorption sites on the surface we will have that, under the some variation in NO2 concentration in the mixture with air, the Cu(dmbc)Pc LB film is more sensitive than the Cu(bc)Pc LB film. The characteristic behaviour of these two LB films of macrocyclic derivatives reverberates also on the structural properties of the 1:1 mixture and the alternate multilayers of the two analyzed compounds. Consequently we conclude that the NO2 gas absorption capacity is smaller in relatively ordered structures than in structurally disordered systems. Conclusions We have presented the results of our investigations to detect NO2 gas in air with LB films of the two following compounds, Cu(dmbc)Pc and Cu(bc)Pc, and their 1:1 mixture and alternate multilayers of them. In particular, we have demonstrated that LB films of the 1:1 mixture increase the sensitivity to NO2 gas, simultaneously reducing the maximum sensitivity temperature and threshold with a very low response time. A theoretical approach to the gas/phthalocyanine interaction is taken in order to interpret the electrical behavior of our LB films in the presence of the NO2 gas in dry air. Future research will be focused on developing LB film sensors with excellent selectivity and stability. Acknowledgment. The authors are grateful to Prof. L. Troisi for the synthesis of the compounds and to F. Casino, A. R. De Bartolomeo, and L. Dimo for technical assistance. This work was partially supported by CEEFESR subprogram II, project “Materiali, Processi e Dispositivi per sensoristica, optoelettronica ed elettronica di potenza”. LA961029C
