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Lutetium bisphthalocyanine thin films for gas detection
Sensors and Acruutors
129
B, 8 ( 1992) 129- 135
Lutetium bisphthalocyanine
thin films for gas detection
M. Trometer, R. Even, J. Simon, A. Dubon and J.-Y. Lava1 ES. P. C.I. - C. N. R.S.,
IO rue Vauquelin, 75231 Paris Cidex
OS (France)
J. P. Germain, C. Maleysson, A. Pauly and H. Robert Laboratoire
d’Electronique
(V.A.
830 C.N.R.S.),
VniversitC Clertnont II, Blaise Pascal, 63177 AubPre
Gdex
(France)
(Received December 31, 1990; in revised form June 19, 1991; accepted July 16, 1991)
Abstract The bisphthalocyanine of lutetium, Pc,Lu, is the first intrinsic molecular semiconductor, with a high concentration of intrinsic charge carriers and a low thermal activation energy of conduction. The redox potentials of PC, Lu, which is more easily oxidized and reduced than conventional metal phthalocyanines, make it possible to obtain the intrinsic semiconducting properties. The possibility of using Pc,Lu as the sensitive element of a gas sensor is studied. This paper describes the absorption spectra of thin films of Pc,Lu and their changes during oxidation or reduction induced by gases such as HCI, Cl,, H,S and Sot. Special attention is given to the action of NO2 on the conductivity and the corresponding thermal activation energy, whose evolution is interpreted in terms of variations of carrier concentration and mobility.
Introduction Divalent metal phthalocyanine (PcM) thin films have been used in sensors for the detection of a large variety of gases [l-3], with a particular emphasis on NO2 [4-81. In all cases a more or less reversible conductivity change accompanies the detection of the gas. The conductivity in intrinsic molecular semiconductors may be easily modelled from the redox characteristics of the PcM molecular unit [9-l 11: PcM z$ PcM+ + e-
E,,“”
PcM+e-
EOred
z$ PcM-
The conductivity
is then given by
CJ= epNOexp( - AEI2kT)
(1)
where AE = e(E,,“” - Eord) N,, is the density of molecular
(2)
units and p is the mobility of charge carriers. For most divalent metal phthalocyanines AE 2 1.8 eV; the intrinsic density of charge carriers is therefore negligible at room temperature and 0925-4005/92/%5.00
these materials are insulators (cr < 10-‘3(Q cm) -‘) [lo]. The conductivity can be increased by doping, the gas detection being related to the amount of extrinsic charge carriers which are generated. Reversible or irreversible doping processes may be considered: PcM, IA z$ PcM+, I/,- + PcMo, Xo In all cases the detection of gases corresponds to a conductivity change between two different levels of doping: doping by the gas to be detected and doping by fortuitously present impurities. These latter are by nature difficult to master and this probably explains the difficulties encountered in the fabrication of reliable devices. The trivalent metallic bisphthalocyanine of lutetium, PcsLu, is an intrinsic molecular semiconductor [ 12- 141. The difference of redox potentials is AE = 0.48 eV and the corresponding thermal activation energy in thin films of Pc,Lu is 0.52 eV 112-141. The intrinsic conductivity is of the order of 10e5 (Cl cm)-’ for thin films; the concentration of intrinsic charge carriers is 4 x lOI cmW3 with an estimated mobility of 1 cm*/V s [ 12- 141. Because of the high density of intrinsic charge carriers, the conductivity is less @ 1992 - Elsevier Sequoia. All rights reserved
130
influenced by minute amounts of impurities. In particular, exposure of Pc,Lu thin films to dioxygen leads to a negligible conductivity change over short periods of time. However, conductivity changes, although less important in magnitude, are expected with the addition of donors or acceptors. In this case, changes between well-defined doping states should occur. Pc,Lu is also sensitive to acidic or alkaline conditions [ 151: 2Pc, Lu * Pc*Lu@, PczL6 5
Pc*LuH, PczLu@
This mechanism should lead to the detection of acidic toxic gases such as HCl or H,S. This paper describes the electrical and optical properties of thin films of Pc*Lu in the presence of various gases: HCl, Clz, 02, H2S, SO, and NO*. A brief description of the use of PczLu thin films as acidic gas detectors has been previously reported
Fig. ing).
1. SEM micrograph
of as-deposited
PcZn thin films (no
[161.
Characterization
of Pc,Lu thin films
Pc*Lu is synthesized in one step following a previously described procedure [ 17, 181. Thin films are made under vacuum ( 10F6 Torr) by sublimation at 450 “C. Different phthalocyanine samples have been compared by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM micrographs indicate that the microstructure of the as-deposited zinc monophthalocyanine PcZn corresponds to a fairly homogeneous single phase with an average particle size x50 nm (Fig. 1). The surface looks granular. The same observations on as-deposited lutetium Pc,Lu, show a very homodiphthalocyanine, geneous microstructure which corresponds to a single phase (Fig. 2). The particle size is again 50 nm. Electron diffraction in TEM reveals the absence of long-range order (Fig. 3). After annealing at 200 “C microcrystals are visible in TEM. Their size ranges from 10 to 25 nm, as shown by bright- and dark-field techniques (Fig. 4). The reduction and oxidation of Pc,Lu thin films lead to characteristic colour changes: Pc:,Lu@ is brown red, Pcz Lu” is blue and PC, Lu is green
Fig. 2. SEM micrograph ing).
Fig. 3. TEM electron
of as-deposited
diffraction
Pc,Lu
of as-deposited
thin films (no
Pc,Lu.
131
360
470
580
690
800
(nm)
Fig. 5. Absorption spectra of thin films of: Pc,Lu (-), Pc,Lue (- ~ - ), Pc,Lu@ (-.-.-.). The electrochemical oxidation and reduction is carried out in I M H,SO,.
(4
- 1.92 -1.54 -0.24 reduct . +z PczLLl- * Pc,Ltl- % PczLureddish purple dark blue blue 1.43 V vs. SCE - 0.45 0.03 1.23 * PC, Lu % PC, Lu + % ox, % 0x3 green red Each of these states is characterized by a colour and a conductivity level which can be calculated from eqns. ( 1) and (2). However, selectivity must also be considered and sensor effects should be distinguished from measuring effects. (b)
Gas detection by thin films of PczLu
Planar cells have been realized consisting of two gold electrodes separated by 0.8 mm. Pc,Lu is deposited under vacuum on the two electrodes (thickness: 1200 A). The thin films are exposed to pure or diluted gas at room temperature. For HCl, Cl2 and H2S, the apparatus used did not allow the response times to be measured accurately. Fig. 4. Thin films of Pc,Lu after annealing at 200 “C for 2h: (a) bright field; (b) dark field; (c) electron diffraction.
(Fig. 5) (the film thickness is about 1000 A). The colour change, which is always associated wii:h a conductivity change, makes it possible to cons ider optical sensors. Eight different oxidation states are electrochemically attainable [ 191:
(a) HCZ
Exposure of Pc2 Lu thin films to HCl gas (pure or diluted at about 10% in nitrogen) leads to a significant decrease of the conductivity. The optical absorption spectrum is red shifted, indicating the formation of PcZLu@ species (Fig. 6). The effect of HCl is reversible and evacuation of the gas brings back the original properties of the layer (Fig. 6(b)). The effect of film thickness was not determined.
360
470
580
(nm)
690
(a)
360
470
560
690
800
(nm)
Fig. 7. Evolution of the optical spectra of Pc,Lu thin films during Cl, adsorption and desorption. A resistance change accompanies the evolution of the optical spectra as listed below. Absorption; I, air, R = 3.4 MD; 2, air/Cl, (approximately IO vol.%,), R = 0.08 MR. Desorption: 3, Cl,, R = 7300 Ma, 4, N,, R = 2500 MQ.
SO, has no detectable effects.
Quantitative study of the effect of NO2 360
470
580
690
800
(nm)
(b) Fig. 6. Evolution of the optical spectra of Pc,Lu thin films during (a) HCI adsorption (curves I to 3) and (b) HCI desorption (curves 4 to 6). An electrical resistance change accompanies the evolution of the optical spectra as listed below: (a) Absorption: I, air, R =4.3 MR; 2, HCI/N,, R = 3.4MR; 3, HCI, R =22 MR. (b) Desorption: 4, HCI, R = 22 MR; 5, N,/HCI, R = 5.5 MR; 6, N,, R = 0.6 Ma.
(b)
Cl2
Contrary to the case of HCl, Cl2 leads to an irreversible oxidation of the Pc2Lu layer (Fig. 7). In a first stage the addition of Cl, causes a strong increase ( x 102) of the conductivity with an almost constant optical spectrum. In a second stage an important conductivity decrease is observed with a concomitant transformation into Pc2Lu@Clo, as evidenced by the optical absorption spectrum. (cl f72S
Exposure of Pc2Lu thin films to H2S at room temperature results in a very slow reaction: the conductivity decreases with time because of the compensation by H2S of the p-type charge carriers that appeared during storage in ambient atmosphere; indeed the ambient dioxygen is thought to act as a p-type dopant on Pc,Lu and create positive charge carriers.
A Pc2Lu thin film has been submitted to a flow of nitrogen containing various amounts of N02, starting from 2 ppm (m/m) (Fig. 8). The temperature dependence of the conductivity could be studied with thermistors integrated within the cell. At room temperature under pure nitrogen, the thermal activation energy of conduction is E = 0.36 eV (derived from Q = o. exp( - E/2kT)), indicating an extrinsic process due to uncontrolled ambient impurities, since the activation energy of Pc,Lu thin films under vacuum is 0.52 eV [13]. When the nitrogen is mixed with 2 ppm of N02, the conductivity of Pc2Lu thin films first increases as a function of time (Fig. 9). A maximum is observed after approximately one hour (D = 2.3 x 10-2(Rcm)-’ and E= 0.24 eV), followed by a slow decrease of the conductivity. The thin film placed under vacuum ( lop4 Torr) undergoes a partial restoration of its conductivity, which increases again up to 0 = 2.3 x lop2 (a cm) -’ before decreasing very slowly. NO2 first induces the formation of Pc2Lu@ and consequently the concentration of charge carriers increases. However, when the concentration of Pc2Lu@ predominates over that of Pc,Lu, the conductivity of the thin films must decrease, since
133
VECTOR GAS
DOPING
GAS
flASS
HFC
FLOW
_
CONT’ROLLER
1
-
INTERFACE nASS FLOW CONTROLLERS
Fig. IO. Action of H,S on Pc,Lu thin films treated with NO* (at room temperature). GAS EVACUATION
Above a certain level of doping the concentration of p-type carriers is given by TEIlPERATURE CONTROLLER
p = (NN,,,)
‘I2exp( - AE,/XT)
where N is the density of molecular units ( N 102’ cmp3), NNoz is the density of NOI molecules within the layer and AE2 is given by the difference of redox potentials:
COtlPUTER
I
1
Fig. 8. Schematic representation of the device used to study the conductivity of Pc,Lu thin films as a function of the ambient.
NO* + e - + N02”
ENOzred
PC2Lu %EPc2Lu+ + e-
E,“”
AE, = E,“” - E,,,$*
A 2 PPm NO2
Yaw”“,
I 0 24 e” ,
In N2
25
/ ICC /NC /’ /’ /’ /‘\1 \ 0
(036eVl
0
,
,
‘,032.V,
,
,
2
1
ä time
uwur)
Fig. 9. Conductivity as a function of time for Pc,Lu thin films in the presence of 2 ppm NO2 at room temperature (in parentheses are the thermal activation energies of conduction E in 0 = a, exp( -E/~/CT)).
charge transport longer occur:
via electron
Pc2Lu@, Pc,Lu@ Jt,
exchange
can no
Pc2Luz+, Pc2Lu
The difference of redox potentials between PC2Lu2+ and Pc2Lu is indeed of the order of 1.26 eV.
E “‘3redis difficult to estimate since only the values in aqueous solutions are known in the literature. The oxidizing doping by NO2 can be reversed by treating the thin film with a slightly reducing agent (200 ppm H2S in N2) (Fig. 10). The introduction of H2S on a PczLu layer that has previously been slightly doped with NO2 ( 15 min doping, point Bl on Fig. 10) induces a decrease of the conductivity resulting from the partial compensation by H,S of the p-type carriers. However, if a heavily N02-doped layer (90 min doping, point B2 on Fig. 10) is exposed to H,S the conductivity of the layer is first enhanced because of the partial compensation of the p-doping (due to N02), though it reduces the number of p-type carriers, giving them a greater facility to migrate. From point B3 on Fig. 10, as the compensation becomes very large, the decrease in the number of p-type carriers is the governing process in the conductivity changes. The reproducibility of the conductivity changes in an atmosphere containing 2 ppm o,f NO2 has been studied. The shape of the c versus t curve is
134
similar for all PczLu layers, but the time needed to reach the maximum of cr and the value of crmax depend strongly on the evaporation parameters and on heat treatments before the first NO2 doping, i.e., they depend on the morphology of the layer. The role of the morphological state of Pcz.Lu on the sensor properties can be estimated, since the conductivity of Pc,Lu has been shown [ 131 to be quite insensitive to the degree of organization of the layer and has a similar value in thin films ( lop5 (0 cm)-‘) and single crystals (6 x 1O-5 (Sz cm))‘). However, the polycrystallinity of the Pc2Lu thin films (Figs. 2 and 4) has a non-negligible effect during NO2 doping, since it must influence the diffusion rate of NO* into the bulk of the film and since macroscopic conduction is governed by grain contacts and grain-surface conductivity.
Conclusions
The gas sensitivity of PczLu thin layers has been tested at room temperature for HCl, C12, NO2 and H2S. In all cases significant conductivity and optical changes have been observed. The results can be interpreted by the electrochemical properties of the molecular unit in solution. A Pc,Lu film exposed to NOz exhibits a higher concentration of oxidized molecules than conventional metallo-phthalocyanines and so a different behaviour of the conductivity as a function of exposure time can be observed. Further studies are in progress to determine the gas sensor properties of this device at higher temperatures and under different oxidation states.
References 1 R. A. Collins and K. A. Mohammed, Electrical, structural and gas sensing properties of zinc phthalocyanine thin films, Thin Solid Films, 145 (1986) 133. 2 G. G. Roberts, M. C. Pretty and I. M. Dharmadasa, Photovoltaic properties of cadmium-telluride/Langmuir-film solar cells, IEE Proc. Solid State Electron Devices, 128 (1981) 197. 3 J. D. Wright, Gas adsorption on phthalocyanines and its effects on electrical properties, Progr. Surf. Sci., 31 (1989) 1. 4 Y. Sadaoka, T. A. Jones and W. Giipel, Effect of heat pretreatment on the electrical conductance of lead phthalocyanine films for NO, gas detection. J. Mater. Sci. Lett., 8 (1989) 1095. 5 N. S. Nieuwenhuizen, A. J. Nederlof and A. Coomans, A SAW gas sensor for NO*. Chemically immobilized phthalocyanines as chemical interface, Fresenius’ 2. Anal. Chem., 330 (1988) 123.
6 B. Bott and T. A. Jones, A highly sensitive NOz sensor based on electrical conductivity changes in phthalocyanine films, Sensors and Actuators, 5 ( 1984) 43. _
7 J. P. Blanc, B. Blasquez, J. P. Germain, A. Larbi, C. Maleysson and H. Robert, Behaviour of electroactive polymers in gaseous oxidizing atmospheres, Sensors and Actuators, 14 (1988) 143. 8 T. A. Temofonte and K. F. Schoch, Phthalocyanine semiconductor sensors for room temperature ppb level detection of toxic gases, J. Appl. Phys., 65 (1989) 1350. 9 L. E. Lyons, Energy gaps in organic semiconductors derived from electrochemical data, Aust. J. Chem., 33 (1980) 1717. 10 J. Simon and J. J. Andre, Molecular Semiconductors, Springer, Berlin, 1985, pp. 98-122. 11 M. Bouvet and J. Simon, Electrical properties of rare earth bisphthalocyanine and bisnaphthalocyanine complexes, Chem. Phys. Lett., 172 (1990) 299. 12 J. J. Andre, K. Holczer, P. Petit, M.-T. Riou, C. Clarisse, R. Even, M. Fourmigut and J. Simon, Electrical and magnetic properties of thin films and single crystals of bis(phthalocyaninato) lutetium, Chem. Phys. Lett., 115 (1985) 463. 13 P. Turek, P. Petit, J. J. Andre, J. Simon, R. Even, B. Boudjema, G. Guillaud and M. Maitrot, A new series of molecular semiconductors: phthalocyanine radicals, J. Am. Chem. Sot., 109 (1987) 5119. 14 C. Clarisse, M.-T. Riou and S. Robinet, Evidence for intrinsic and extrinsic semiconducting properties of Pc,Lu thin films, Synth. Met., 38 (1990) 121.
15 M. L’Her, Y. Cozien and J. Courtot-Coupez, Influence de l’acidid sur le comportement Clectrochimique et spectrophotometrique de la diphthalocyanine de lutbium en solution dans le dichloromtthane, C.R. Acad. Sci. (Paris), 300 ( 1985) 487. 16 Suwa Seikosha Co., Jpn Kokal Tokkyo Koho, Jpn. Patent No. 57 IO8 61511 CA 98 JOg54V (1983).
17 1. S. Kirin, P. N. Moskalev and Y. A. Makashev, Formation of phthalocyanines of rare earth elements, Russ. J. Inorg. Chem., IO (1965) 1065. 18 C. Clarisse and M.-T Riou, Synthesis and characterization of some lanthanide phthalocyanines, Inorg. Chim. Acta, 130 (1987) 139.
19 M. L’Her, Y. Cozien and J. Courtot-Coupez, Etude Clectrochimique de la reduction de la diphthalocyanine de lutetium en solution, C.R. Acad. Sci. (Paris), 302 (1968) 9.
Biographies
R. Even received his Doctorat d’Etat es Sciences Physiques in 1987 for his work on radical phthalocyanines as the first molecular semiconductors. He now works as a Maitre-Assistant at the Ecole Superieure de Physique et Chimie Industrielles (E.S.P.C.I.) in Paris, with research interests in photochromic molecular materials. J. Simon received his Doctorat d’Etat es Sciences Physiques in 1976 at the ULP in Strasbourg, then worked at the Centre de Recherche sur les Macromolecules in Strasbourg as a C.N.R.S. Charge de Recherche. He is presently a professor at the E.S.P.C.I. in Paris. His research activity
13.5
deals mainly with the synthesis and the study of the electrical properties of macromolecular materials, especially phthalocyanines, one of which he described as the first molecular semiconductor. A. Dubon is an Ingenieur d’Etudes of the C.N.R.S. at the Microstructures Division of the E.S.P.C.I., where he studies the microstructure of ceramics and heterophases by transmission and scanning electronic microscopy. J.-Y. Lad has a degree of Docteur es Sciences. As a Charge de Recherche at the C.N.R.S., he is presently head of the Microstructures Division at the E.S.P.C.I. His research field is the relations between properties and structures of the interfaces in ceramics and semiconductors (by analytical and high-resolution microscopy).
and optoelectronics and the applications ganic materials to gas sensors.
C. Maleysson received her Docteur-Ingenieur degree in 1984 from the University of Clermont II for her work on doping processes in electroactive polymers. As a Chargee de Recherche of the C.N.R.S. at the Laboratoire d’Electronique, she is presently in charge of the research on the potential use of organic materials as gas sensors. A. Paz& graduated from the Physics Department of the University of Clermont II and joined the Laboratoire d’Electronique in order to prepare a Doctorat d’universite on the electronic properties of phthalocyanines and their application as sensitive elements for gas sensors. Directeur de Recherche of the C.N.R.S., received his Docteur-Ingenieur degree in 1968 for research on organic semiconductors at the Laboratoire d’Electronique. For several years he worked on the doping of electroactive polymers and their application as gas sensors. His current research activities are primarily concerned with the applications of III-V semiconductors. H.
J.-P. Germain, professor at the University of Clermont II, received his Doctorat d’Etat es Sciences in 1977 for his research in the field of liquid crystals. He is presently head of the Materials Division of the Laboratoire d’Electronique. His research activity covers both the applications of the III-V semiconductors for rapid electronics
of or-
Robert,
129
B, 8 ( 1992) 129- 135
Lutetium bisphthalocyanine
thin films for gas detection
M. Trometer, R. Even, J. Simon, A. Dubon and J.-Y. Lava1 ES. P. C.I. - C. N. R.S.,
IO rue Vauquelin, 75231 Paris Cidex
OS (France)
J. P. Germain, C. Maleysson, A. Pauly and H. Robert Laboratoire
d’Electronique
(V.A.
830 C.N.R.S.),
VniversitC Clertnont II, Blaise Pascal, 63177 AubPre
Gdex
(France)
(Received December 31, 1990; in revised form June 19, 1991; accepted July 16, 1991)
Abstract The bisphthalocyanine of lutetium, Pc,Lu, is the first intrinsic molecular semiconductor, with a high concentration of intrinsic charge carriers and a low thermal activation energy of conduction. The redox potentials of PC, Lu, which is more easily oxidized and reduced than conventional metal phthalocyanines, make it possible to obtain the intrinsic semiconducting properties. The possibility of using Pc,Lu as the sensitive element of a gas sensor is studied. This paper describes the absorption spectra of thin films of Pc,Lu and their changes during oxidation or reduction induced by gases such as HCI, Cl,, H,S and Sot. Special attention is given to the action of NO2 on the conductivity and the corresponding thermal activation energy, whose evolution is interpreted in terms of variations of carrier concentration and mobility.
Introduction Divalent metal phthalocyanine (PcM) thin films have been used in sensors for the detection of a large variety of gases [l-3], with a particular emphasis on NO2 [4-81. In all cases a more or less reversible conductivity change accompanies the detection of the gas. The conductivity in intrinsic molecular semiconductors may be easily modelled from the redox characteristics of the PcM molecular unit [9-l 11: PcM z$ PcM+ + e-
E,,“”
PcM+e-
EOred
z$ PcM-
The conductivity
is then given by
CJ= epNOexp( - AEI2kT)
(1)
where AE = e(E,,“” - Eord) N,, is the density of molecular
(2)
units and p is the mobility of charge carriers. For most divalent metal phthalocyanines AE 2 1.8 eV; the intrinsic density of charge carriers is therefore negligible at room temperature and 0925-4005/92/%5.00
these materials are insulators (cr < 10-‘3(Q cm) -‘) [lo]. The conductivity can be increased by doping, the gas detection being related to the amount of extrinsic charge carriers which are generated. Reversible or irreversible doping processes may be considered: PcM, IA z$ PcM+, I/,- + PcMo, Xo In all cases the detection of gases corresponds to a conductivity change between two different levels of doping: doping by the gas to be detected and doping by fortuitously present impurities. These latter are by nature difficult to master and this probably explains the difficulties encountered in the fabrication of reliable devices. The trivalent metallic bisphthalocyanine of lutetium, PcsLu, is an intrinsic molecular semiconductor [ 12- 141. The difference of redox potentials is AE = 0.48 eV and the corresponding thermal activation energy in thin films of Pc,Lu is 0.52 eV 112-141. The intrinsic conductivity is of the order of 10e5 (Cl cm)-’ for thin films; the concentration of intrinsic charge carriers is 4 x lOI cmW3 with an estimated mobility of 1 cm*/V s [ 12- 141. Because of the high density of intrinsic charge carriers, the conductivity is less @ 1992 - Elsevier Sequoia. All rights reserved
130
influenced by minute amounts of impurities. In particular, exposure of Pc,Lu thin films to dioxygen leads to a negligible conductivity change over short periods of time. However, conductivity changes, although less important in magnitude, are expected with the addition of donors or acceptors. In this case, changes between well-defined doping states should occur. Pc,Lu is also sensitive to acidic or alkaline conditions [ 151: 2Pc, Lu * Pc*Lu@, PczL6 5
Pc*LuH, PczLu@
This mechanism should lead to the detection of acidic toxic gases such as HCl or H,S. This paper describes the electrical and optical properties of thin films of Pc*Lu in the presence of various gases: HCl, Clz, 02, H2S, SO, and NO*. A brief description of the use of PczLu thin films as acidic gas detectors has been previously reported
Fig. ing).
1. SEM micrograph
of as-deposited
PcZn thin films (no
[161.
Characterization
of Pc,Lu thin films
Pc*Lu is synthesized in one step following a previously described procedure [ 17, 181. Thin films are made under vacuum ( 10F6 Torr) by sublimation at 450 “C. Different phthalocyanine samples have been compared by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). SEM micrographs indicate that the microstructure of the as-deposited zinc monophthalocyanine PcZn corresponds to a fairly homogeneous single phase with an average particle size x50 nm (Fig. 1). The surface looks granular. The same observations on as-deposited lutetium Pc,Lu, show a very homodiphthalocyanine, geneous microstructure which corresponds to a single phase (Fig. 2). The particle size is again 50 nm. Electron diffraction in TEM reveals the absence of long-range order (Fig. 3). After annealing at 200 “C microcrystals are visible in TEM. Their size ranges from 10 to 25 nm, as shown by bright- and dark-field techniques (Fig. 4). The reduction and oxidation of Pc,Lu thin films lead to characteristic colour changes: Pc:,Lu@ is brown red, Pcz Lu” is blue and PC, Lu is green
Fig. 2. SEM micrograph ing).
Fig. 3. TEM electron
of as-deposited
diffraction
Pc,Lu
of as-deposited
thin films (no
Pc,Lu.
131
360
470
580
690
800
(nm)
Fig. 5. Absorption spectra of thin films of: Pc,Lu (-), Pc,Lue (- ~ - ), Pc,Lu@ (-.-.-.). The electrochemical oxidation and reduction is carried out in I M H,SO,.
(4
- 1.92 -1.54 -0.24 reduct . +z PczLLl- * Pc,Ltl- % PczLureddish purple dark blue blue 1.43 V vs. SCE - 0.45 0.03 1.23 * PC, Lu % PC, Lu + % ox, % 0x3 green red Each of these states is characterized by a colour and a conductivity level which can be calculated from eqns. ( 1) and (2). However, selectivity must also be considered and sensor effects should be distinguished from measuring effects. (b)
Gas detection by thin films of PczLu
Planar cells have been realized consisting of two gold electrodes separated by 0.8 mm. Pc,Lu is deposited under vacuum on the two electrodes (thickness: 1200 A). The thin films are exposed to pure or diluted gas at room temperature. For HCl, Cl2 and H2S, the apparatus used did not allow the response times to be measured accurately. Fig. 4. Thin films of Pc,Lu after annealing at 200 “C for 2h: (a) bright field; (b) dark field; (c) electron diffraction.
(Fig. 5) (the film thickness is about 1000 A). The colour change, which is always associated wii:h a conductivity change, makes it possible to cons ider optical sensors. Eight different oxidation states are electrochemically attainable [ 191:
(a) HCZ
Exposure of Pc2 Lu thin films to HCl gas (pure or diluted at about 10% in nitrogen) leads to a significant decrease of the conductivity. The optical absorption spectrum is red shifted, indicating the formation of PcZLu@ species (Fig. 6). The effect of HCl is reversible and evacuation of the gas brings back the original properties of the layer (Fig. 6(b)). The effect of film thickness was not determined.
360
470
580
(nm)
690
(a)
360
470
560
690
800
(nm)
Fig. 7. Evolution of the optical spectra of Pc,Lu thin films during Cl, adsorption and desorption. A resistance change accompanies the evolution of the optical spectra as listed below. Absorption; I, air, R = 3.4 MD; 2, air/Cl, (approximately IO vol.%,), R = 0.08 MR. Desorption: 3, Cl,, R = 7300 Ma, 4, N,, R = 2500 MQ.
SO, has no detectable effects.
Quantitative study of the effect of NO2 360
470
580
690
800
(nm)
(b) Fig. 6. Evolution of the optical spectra of Pc,Lu thin films during (a) HCI adsorption (curves I to 3) and (b) HCI desorption (curves 4 to 6). An electrical resistance change accompanies the evolution of the optical spectra as listed below: (a) Absorption: I, air, R =4.3 MR; 2, HCI/N,, R = 3.4MR; 3, HCI, R =22 MR. (b) Desorption: 4, HCI, R = 22 MR; 5, N,/HCI, R = 5.5 MR; 6, N,, R = 0.6 Ma.
(b)
Cl2
Contrary to the case of HCl, Cl2 leads to an irreversible oxidation of the Pc2Lu layer (Fig. 7). In a first stage the addition of Cl, causes a strong increase ( x 102) of the conductivity with an almost constant optical spectrum. In a second stage an important conductivity decrease is observed with a concomitant transformation into Pc2Lu@Clo, as evidenced by the optical absorption spectrum. (cl f72S
Exposure of Pc2Lu thin films to H2S at room temperature results in a very slow reaction: the conductivity decreases with time because of the compensation by H2S of the p-type charge carriers that appeared during storage in ambient atmosphere; indeed the ambient dioxygen is thought to act as a p-type dopant on Pc,Lu and create positive charge carriers.
A Pc2Lu thin film has been submitted to a flow of nitrogen containing various amounts of N02, starting from 2 ppm (m/m) (Fig. 8). The temperature dependence of the conductivity could be studied with thermistors integrated within the cell. At room temperature under pure nitrogen, the thermal activation energy of conduction is E = 0.36 eV (derived from Q = o. exp( - E/2kT)), indicating an extrinsic process due to uncontrolled ambient impurities, since the activation energy of Pc,Lu thin films under vacuum is 0.52 eV [13]. When the nitrogen is mixed with 2 ppm of N02, the conductivity of Pc2Lu thin films first increases as a function of time (Fig. 9). A maximum is observed after approximately one hour (D = 2.3 x 10-2(Rcm)-’ and E= 0.24 eV), followed by a slow decrease of the conductivity. The thin film placed under vacuum ( lop4 Torr) undergoes a partial restoration of its conductivity, which increases again up to 0 = 2.3 x lop2 (a cm) -’ before decreasing very slowly. NO2 first induces the formation of Pc2Lu@ and consequently the concentration of charge carriers increases. However, when the concentration of Pc2Lu@ predominates over that of Pc,Lu, the conductivity of the thin films must decrease, since
133
VECTOR GAS
DOPING
GAS
flASS
HFC
FLOW
_
CONT’ROLLER
1
-
INTERFACE nASS FLOW CONTROLLERS
Fig. IO. Action of H,S on Pc,Lu thin films treated with NO* (at room temperature). GAS EVACUATION
Above a certain level of doping the concentration of p-type carriers is given by TEIlPERATURE CONTROLLER
p = (NN,,,)
‘I2exp( - AE,/XT)
where N is the density of molecular units ( N 102’ cmp3), NNoz is the density of NOI molecules within the layer and AE2 is given by the difference of redox potentials:
COtlPUTER
I
1
Fig. 8. Schematic representation of the device used to study the conductivity of Pc,Lu thin films as a function of the ambient.
NO* + e - + N02”
ENOzred
PC2Lu %EPc2Lu+ + e-
E,“”
AE, = E,“” - E,,,$*
A 2 PPm NO2
Yaw”“,
I 0 24 e” ,
In N2
25
/ ICC /NC /’ /’ /’ /‘\1 \ 0
(036eVl
0
,
,
‘,032.V,
,
,
2
1
ä time
uwur)
Fig. 9. Conductivity as a function of time for Pc,Lu thin films in the presence of 2 ppm NO2 at room temperature (in parentheses are the thermal activation energies of conduction E in 0 = a, exp( -E/~/CT)).
charge transport longer occur:
via electron
Pc2Lu@, Pc,Lu@ Jt,
exchange
can no
Pc2Luz+, Pc2Lu
The difference of redox potentials between PC2Lu2+ and Pc2Lu is indeed of the order of 1.26 eV.
E “‘3redis difficult to estimate since only the values in aqueous solutions are known in the literature. The oxidizing doping by NO2 can be reversed by treating the thin film with a slightly reducing agent (200 ppm H2S in N2) (Fig. 10). The introduction of H2S on a PczLu layer that has previously been slightly doped with NO2 ( 15 min doping, point Bl on Fig. 10) induces a decrease of the conductivity resulting from the partial compensation by H,S of the p-type carriers. However, if a heavily N02-doped layer (90 min doping, point B2 on Fig. 10) is exposed to H,S the conductivity of the layer is first enhanced because of the partial compensation of the p-doping (due to N02), though it reduces the number of p-type carriers, giving them a greater facility to migrate. From point B3 on Fig. 10, as the compensation becomes very large, the decrease in the number of p-type carriers is the governing process in the conductivity changes. The reproducibility of the conductivity changes in an atmosphere containing 2 ppm o,f NO2 has been studied. The shape of the c versus t curve is
134
similar for all PczLu layers, but the time needed to reach the maximum of cr and the value of crmax depend strongly on the evaporation parameters and on heat treatments before the first NO2 doping, i.e., they depend on the morphology of the layer. The role of the morphological state of Pcz.Lu on the sensor properties can be estimated, since the conductivity of Pc,Lu has been shown [ 131 to be quite insensitive to the degree of organization of the layer and has a similar value in thin films ( lop5 (0 cm)-‘) and single crystals (6 x 1O-5 (Sz cm))‘). However, the polycrystallinity of the Pc2Lu thin films (Figs. 2 and 4) has a non-negligible effect during NO2 doping, since it must influence the diffusion rate of NO* into the bulk of the film and since macroscopic conduction is governed by grain contacts and grain-surface conductivity.
Conclusions
The gas sensitivity of PczLu thin layers has been tested at room temperature for HCl, C12, NO2 and H2S. In all cases significant conductivity and optical changes have been observed. The results can be interpreted by the electrochemical properties of the molecular unit in solution. A Pc,Lu film exposed to NOz exhibits a higher concentration of oxidized molecules than conventional metallo-phthalocyanines and so a different behaviour of the conductivity as a function of exposure time can be observed. Further studies are in progress to determine the gas sensor properties of this device at higher temperatures and under different oxidation states.
References 1 R. A. Collins and K. A. Mohammed, Electrical, structural and gas sensing properties of zinc phthalocyanine thin films, Thin Solid Films, 145 (1986) 133. 2 G. G. Roberts, M. C. Pretty and I. M. Dharmadasa, Photovoltaic properties of cadmium-telluride/Langmuir-film solar cells, IEE Proc. Solid State Electron Devices, 128 (1981) 197. 3 J. D. Wright, Gas adsorption on phthalocyanines and its effects on electrical properties, Progr. Surf. Sci., 31 (1989) 1. 4 Y. Sadaoka, T. A. Jones and W. Giipel, Effect of heat pretreatment on the electrical conductance of lead phthalocyanine films for NO, gas detection. J. Mater. Sci. Lett., 8 (1989) 1095. 5 N. S. Nieuwenhuizen, A. J. Nederlof and A. Coomans, A SAW gas sensor for NO*. Chemically immobilized phthalocyanines as chemical interface, Fresenius’ 2. Anal. Chem., 330 (1988) 123.
6 B. Bott and T. A. Jones, A highly sensitive NOz sensor based on electrical conductivity changes in phthalocyanine films, Sensors and Actuators, 5 ( 1984) 43. _
7 J. P. Blanc, B. Blasquez, J. P. Germain, A. Larbi, C. Maleysson and H. Robert, Behaviour of electroactive polymers in gaseous oxidizing atmospheres, Sensors and Actuators, 14 (1988) 143. 8 T. A. Temofonte and K. F. Schoch, Phthalocyanine semiconductor sensors for room temperature ppb level detection of toxic gases, J. Appl. Phys., 65 (1989) 1350. 9 L. E. Lyons, Energy gaps in organic semiconductors derived from electrochemical data, Aust. J. Chem., 33 (1980) 1717. 10 J. Simon and J. J. Andre, Molecular Semiconductors, Springer, Berlin, 1985, pp. 98-122. 11 M. Bouvet and J. Simon, Electrical properties of rare earth bisphthalocyanine and bisnaphthalocyanine complexes, Chem. Phys. Lett., 172 (1990) 299. 12 J. J. Andre, K. Holczer, P. Petit, M.-T. Riou, C. Clarisse, R. Even, M. Fourmigut and J. Simon, Electrical and magnetic properties of thin films and single crystals of bis(phthalocyaninato) lutetium, Chem. Phys. Lett., 115 (1985) 463. 13 P. Turek, P. Petit, J. J. Andre, J. Simon, R. Even, B. Boudjema, G. Guillaud and M. Maitrot, A new series of molecular semiconductors: phthalocyanine radicals, J. Am. Chem. Sot., 109 (1987) 5119. 14 C. Clarisse, M.-T. Riou and S. Robinet, Evidence for intrinsic and extrinsic semiconducting properties of Pc,Lu thin films, Synth. Met., 38 (1990) 121.
15 M. L’Her, Y. Cozien and J. Courtot-Coupez, Influence de l’acidid sur le comportement Clectrochimique et spectrophotometrique de la diphthalocyanine de lutbium en solution dans le dichloromtthane, C.R. Acad. Sci. (Paris), 300 ( 1985) 487. 16 Suwa Seikosha Co., Jpn Kokal Tokkyo Koho, Jpn. Patent No. 57 IO8 61511 CA 98 JOg54V (1983).
17 1. S. Kirin, P. N. Moskalev and Y. A. Makashev, Formation of phthalocyanines of rare earth elements, Russ. J. Inorg. Chem., IO (1965) 1065. 18 C. Clarisse and M.-T Riou, Synthesis and characterization of some lanthanide phthalocyanines, Inorg. Chim. Acta, 130 (1987) 139.
19 M. L’Her, Y. Cozien and J. Courtot-Coupez, Etude Clectrochimique de la reduction de la diphthalocyanine de lutetium en solution, C.R. Acad. Sci. (Paris), 302 (1968) 9.
Biographies
R. Even received his Doctorat d’Etat es Sciences Physiques in 1987 for his work on radical phthalocyanines as the first molecular semiconductors. He now works as a Maitre-Assistant at the Ecole Superieure de Physique et Chimie Industrielles (E.S.P.C.I.) in Paris, with research interests in photochromic molecular materials. J. Simon received his Doctorat d’Etat es Sciences Physiques in 1976 at the ULP in Strasbourg, then worked at the Centre de Recherche sur les Macromolecules in Strasbourg as a C.N.R.S. Charge de Recherche. He is presently a professor at the E.S.P.C.I. in Paris. His research activity
13.5
deals mainly with the synthesis and the study of the electrical properties of macromolecular materials, especially phthalocyanines, one of which he described as the first molecular semiconductor. A. Dubon is an Ingenieur d’Etudes of the C.N.R.S. at the Microstructures Division of the E.S.P.C.I., where he studies the microstructure of ceramics and heterophases by transmission and scanning electronic microscopy. J.-Y. Lad has a degree of Docteur es Sciences. As a Charge de Recherche at the C.N.R.S., he is presently head of the Microstructures Division at the E.S.P.C.I. His research field is the relations between properties and structures of the interfaces in ceramics and semiconductors (by analytical and high-resolution microscopy).
and optoelectronics and the applications ganic materials to gas sensors.
C. Maleysson received her Docteur-Ingenieur degree in 1984 from the University of Clermont II for her work on doping processes in electroactive polymers. As a Chargee de Recherche of the C.N.R.S. at the Laboratoire d’Electronique, she is presently in charge of the research on the potential use of organic materials as gas sensors. A. Paz& graduated from the Physics Department of the University of Clermont II and joined the Laboratoire d’Electronique in order to prepare a Doctorat d’universite on the electronic properties of phthalocyanines and their application as sensitive elements for gas sensors. Directeur de Recherche of the C.N.R.S., received his Docteur-Ingenieur degree in 1968 for research on organic semiconductors at the Laboratoire d’Electronique. For several years he worked on the doping of electroactive polymers and their application as gas sensors. His current research activities are primarily concerned with the applications of III-V semiconductors. H.
J.-P. Germain, professor at the University of Clermont II, received his Doctorat d’Etat es Sciences in 1977 for his research in the field of liquid crystals. He is presently head of the Materials Division of the Laboratoire d’Electronique. His research activity covers both the applications of the III-V semiconductors for rapid electronics
of or-
Robert,