Interaction of NO2 with copper phthalocyanine thin films I: Characterization of the copper phthalocyanine films

Thin Solid Films, 219 (1992) 244 2511

244

Interaction of N O 2 with copper phthalocyanine thin films I: Characterization of the copper phthalocyanine films S. D o g o , J.-P. Germain, C. M a l e y s s o n *

and A. Pauly

Laboratoire d'Electronique (URA 830 CNRS), Universitk Blaise Pascal Clermont H, 63177 Aubikre Cedex (France)

(Received January 27, 1992; revised March 20, 1992; accepted April 24, 1992)

Abstract Thin films of copper phthalocyanine (CuPc) have been deposited by vacuum evaporation on glass or alumina substrates maintained at different temperatures. The morphology and the crystallinity of as-deposited films and of heat-treated and/or NO2-treated films have been studied by electron microscopy and X-ray diffraction. The crystallinity of the films increases with substrate temperature during deposition: films deposited at room temperature or below are rather amorphous, whereas films deposited at 80 °C or 200 °C exhibit a highly uniaxial order of their crystallites; this order is altered by heat treatment or NO2 doping. Scanning electron microscopy shows that the morphology of the film surface also depends on deposition temperature and that heat treatments induce the growth of whiskers from the surface. The structural changes are also observed through the temperatures of maximum conductivity of the CuPc films doped with NO2. The sensitivity of CuPc to NO2 could be applied to gas sensing.

1. Introduction Metal phthalocyanine (MPc) materials are p-type organic semiconductors which exhibit high sensitivity to electron acceptor gases such as NO2 even in the subp p m range: gas adsorption on the surface of crystals or films of MPc is followed by charge transfer reactions which induce generation of charge carriers and enhancement of conductivity. Thin films of MPc are currently being tested as active materials in resistive gas sensors [1-7]. PbPc thin films have been intensively studied and have proved to be superior to films of other phthalocyanines in terms of sensitivity, reproducibility and response-recovery times to changes in NO2 concentrations in air, especially after heat treatment at 360 °C in air [6-8]. However, in the case of MPc gas sensors the need for selectivity will be solved only by associating several MPcs on the same device, each MPc exhibiting a different response to gases, and by using appropriate data-processing systems. In this context, apart from PbPc, CuPc is one of the most interesting phthalocyanines because of its high sensitivity to oxidizing gases. However, the improvement in its present gas-sensing performances requires a better knowledge of its structure; as a matter of fact it is known that the structure and morphology of ph-

*Author to whom correspondence should be addressed.

0040-6090/92/$5.00

thalocyanine thin films strongly influence their gas-sensing characteristics [4, 9-12]. The aim of the present work is to contribute to the characterization of as-deposited, heat-treated a n d NO2doped CuPc thin films deposited by vacuum evaporation on alumina or glass substrates. IR spectroscopy, X-ray diffraction ( X R D ) and electron microscopy are used as investigative techniques in this work. In the next paper [13] (Part II) we shall study the variations with temperature in the conductivity of CuPc films doped with NO2 and the application of this material to gas sensing.

2. Experimental details CuPc (synthesized at ESPCI, Paris) is deposited by vacuum sublimation (10 6 mbar) on glass or alumina substrates, the latter (3 m m x 4 mm) being fitted with screen-printed interdigitated platinum electrodes on one side (with an interelectrode distance of 0.125 mm) for conductivity measurements and a screen-printed platinum resistor on the other side for heating and temperature measurement. The deposition rate is 0.2 n m s and film thickness is 260-300 nm. IR spectra are obtained with CuPc powder mixed with KBr and pressed into pellets. The crystalline structure of the films deposited on glass substrates is determined by X R D measurements (Siemens D500 diffractometer, Cu K s radiation). The surface morphology of films

~ 1992

Elsevier Sequoia. All rights reserved

S. Dogo et al. / Interaction o f N O 2 with CuPc: I

deposited on glass or alumina substrates is examined by scanning electron microscopy (Cambridge Instruments, Stereoscan 360 microscope). The d.c. conductivity of films on alumina substrates is measured using a Keithley 197 picoammeter with a d.c. voltage supply of 1 V. The films are placed in a flow ( 1.5 1 min- ~) of diluted oxidizing gas: N O 2 and C12 are supplied by Airgaz at a concentration of 1000 ppm respectively in synthetic air and in N2; they are afterwards diluted in N 2 to concentrations of 2-200 ppm; 02 is diluted at 0.2%-20% in N 2. The sample temperature ranges from - 7 0 °C (sample placed over liquid N2) to 300 °C.

245

2033

.'2' C m Ig

0

--

4

26

6b

(a)

80

2 theta ( ' )

1574

3. Characterization 3.1. IR spectroscopy

CuPc powder is doped with 100 ppm N O 2 in synthetic air +N2 flow at room temperature and sampled

C O C

T A B L E 1. X-ray diffraction peaks of CuPc thin films (260-300 n m thick)

0

--

Td (°C)

Heat treatment

NO 2 doping

X R D peaks D(A)

20(deg)

Figure

(b)

l(b)

18784

l(c)

.=

'"L_--

-

.......

'

-

2'0

4

---I

80 2 theta

(')

Height

- 10

No

No

13.0

6.79

1600 counts

20

No

No

12.98

6.80

550 counts

80

No

No

12.94 6.54 3.28

6.82 13.54 27.16

19000 counts 1.5°/,, 2.6%

200

No

No

12.95 6.54 3.28

6.82 13.54 27.16

86000 counts 0.7% 1,4'/0

80

Yes

No

12.85 6.05 3.28 1.65

6.87 14.63 27.13 55,84

3200 counts 7,8% 56% 6,3%

l(d)

13.24 6.61 3.30

6,67 13,38 27,00

9300 counts 1.8°/,, 3.2%

4(a)

13.27 6.62 3.31 1.89

6.66 13.36 26.95 48.22

58400 counts 1.6% 2.85% 0.5°/`,

4(b)

13.37 5.66 5.01 3.31

6.60 15.64 17.68 26.90

2600 counts 14"/o 21% 22%

4(c)

m

80

200

No

No

Yes

Yes

0

4

2'0

(c)

"~'

80 2 theta ( ' )

3182

w

W o

80

Yes

Yes

T d is the temperature of the glass substrate during film deposition. Heat treatment is carried out at 240 °C for 10 h under vacuum. NO2 doping consists in exposing the film to 20 p p m NO2 diluted in N 2 + air at room temperature for 10 h. The absolute heights of the peaks are only given as a relative indication of the crystallinity since the samples do not all have rigorously the same thicknesses.

0

,,

4 (d)

2'0

66 2 theta

i 1l

80

(')

Fig. 1. X R D spectra of CuPc: (a) powder; (b) film deposited on a glass substrate at - 1 0 °C; (c) film deposited on a glass substrate at 80 °C; (d) film deposited on a glass substrate at 80 °C and heat treated at 240 °C under vacuum.

246

S. Dogo et al. / Interaction o f

at times t = 5 min, l0 min, 15 min, 30 min, 1 h, 2 h and 3 h to make pellets after mixture with KBr. On the IR spectra, in addition to the CuPc peaks, a new peak appears at 1380 cm T after 30 min of doping and increases with doping time: this peak is attributed to N O 3- [ 14].

Simultaneously, in the same NOR flOW, a CuPc thin film on an alumina substrate is doped and its conductivity cr is recorded vs. time. After 30 min of doping, cr is found to be proportional to the relative height of the NO 3 peak. It is assumed that, during NOR doping, N O 2- is formed by charge transfer with CuPc, and N O 3- is formed from NO2 and 02 (02 is contained in the layer because of its exposure to ambient air after deposition or is brought by the doping gas diluted in synthetic air and N2). N O 2f o r m a t i o n is reversible and N O 2- desorbs during the making of the pellet in air, so it cannot be seen on the IR spectra. On the contrary, NOB- is irreversibly formed in CuPc [15] and could be responsible for degradation of the response of CuPc to NO2 (see Part II, Section 4). 3.2. X-ray diffraction (1) The CuPc powder presents an X R D spectrum with few, wide peaks (Fig. l(a)) characteristic of a low crystallinity. From comparison with reference data

NO 2

with CuPc: 1

(JCPDS Card 36-1883) and with the same spectrum in ref. 16, we deduce that the powder is in the ~ phase. (2) The X R D peaks of CuPc thin films deposited from this powder onto glass substrates kept at a temperature Td during the deposition are listed in Table 1 and some X R D spectra are presented in Figs. 1 and 4. These X R D results show that films deposited at - 1 0 ' C (Fig. l(b)) or 20 °C are amorphous and that the crystallinity of fihns deposited at higher temperature (Fig. l(c) for Td = 80 °C) increases with substrate temperature Tj during the deposition phase. A high degree of order is obtained for Td = 200 ~C. For all ira examined, the X R D spectra of as-deposited films exhibit essentially one peak at D = 12.85 13.0 ~. This interplanar distance is the distance between copper atoms. As we use a powder diffractometer, we can only observe the interplanar distances between planes parallel to the substrate surface. From ref. 17 the double interplanar distance of the ~ form of CuPc is 25.92 ,~ in the "a axis" (Fig. 2). Therefore we can assume that our films are in the ~ phase, with the a axis perpendicular to the substrate plane and the CuPc stacks lying parallel to the substrate plane (Fig. 3(a)), maybe with some disorder in this plane (mosaic structure). This is in agreement with the structure proposed in refs. 10 and 16 where CuPc thin films deposited at

f-w>
(d)

O

Ira,

(e)

(f)

Fig. 2. Crystalline cell and parameters of CuPc in (a) (c) the ~ phase and (d) (f) the 13 phase (from ref. 17).

S. Dogo et al. / Interaction o f NO 2 with CuPc: I

247

B

l/lit [ (a)

I substrate

I

l

!

(b)

I substrate

Fig. 3. (a) Crystalline structure of as-deposited CuPc films on glass substrates deduced from the XRD spectrum of Fig. l(c). (b) Crystalline structure of heat-treated CuPc films on glass substrates deduced from the X R D spectrum of Fig. l(d). For both figures, b is the crystallographic b axis (stacking axis of the Pc molecules).

70°C give only one X R D peak attributed to the 12.93 A spacing of the ~-CuPc (200) planes; the b axis is also supposed to lie parallel to the film plane such that only the (h00) lattice planes of the crystallites are parallel to the substrate surface. The occurrence of a single peak as we almost observe on our X R D spectra is known to result from a very high degree of uniaxial ordering of the microcrystalline domains composing the films [16]. It is known [17] that 1~ films are obtained only for Td ~>210 °C, so this is also consistent with our not observing [~-CuPc films. (3) The X R D spectrum of a film deposited at To = 80 °C is modified when the film is heat treated at 240 c~C for 10 h under vacuum (Table 1 and Fig. l(d)): the height of the D = 12.94/~ peak, i.e. the uniaxial ordering of the crystallites, is reduced and a new peak appears: D = 3.3/~. This distance corresponds to the spacing between the phthalocyanine macrocycles in the Qt or 13 phase (equal to 3.4/k from ref. 17). This implies that under heat treatment some crystallites have changed their orientation in such a way that some of the macrocycles now lie parallel to the substrate plane, which means that the stacking axis (b axis) is no longer parallel to the substrate plane but is inclined to it (Fig. 3(b)). Since the D = 12.85/k peak has not disappeared, the reorganization is not total. This transformation of the crystallite configuration has also been observed by Wright [10] in NiPc films. The 3.3 A interplanar distance is already present in the X R D spectra of as-deposited films, as well as its multiples (6.6/k and 13.2/~, the latter being masked by the high peak of the interstack distance) but the corresponding peaks are very small, so as-deposited films consist mainly of crystallites having the orientation shown in Fig. 3(a). From Figs. l(d) and 2, the film is assumed to be still in its ~ phase even after the 240 °C heat treatment under vacuum. However, a film deposited on alumina and exposed to heat treatment at 240 °C in N2 and to N O 2 doping is assumed to undergo an ~ ~ 13 trans-

I ooo

-~ '~~ I]

o 4

2b

(a)

4d

66

ao

2 theta (')

2467

c

o 4

_L I

2O

66

!

4O 2 theta (-)

(b) 2617

"

1. ;

k _

o

J

,

6~0

4 (c)

80

2 t h e t a (°)

Fig. 4. X R D spectra of CuPc after 20 ppm NO 2 doping at room temperature: (a) film deposited on a glass substrate at 80 °C (only the bottom (0% l 1%) of the full scale intensity of the 13.24 ~ peak is shown); (b) film deposited on a glass substrate at 200 °C (only the bottom (0%-4.2%) of the full scale intensity of the 13.27/~ peak is shown); (c) film deposited on a glass substrate at 80 °C and heat treated at 240 °C under vacuum.

248

S. Dogo et al. / Interaction of NO 2 with CuPc: 1

o

'

(a)

500

'

!

,

nm

'

.

'

2 IJm

2 rim

(,t)

,

, 500

nm

y,,

•

'

2

~m

'

(b)

I

L

500

'

2 j~m

nm

%

(e) (c)

I

i

2 lain

'

'

500

nm

Fig. 5. (Continued on next page.)

S. Dogo et al. / Interaction of NO 2 with CuPc: I

'

[f)

'

'

'

t

(g)

•

'

'

,

2 Ilm

500

2

a 500

nm

lam

nm

Fig. 5. Electron microscopy photographs of CuPc thin films on (a), (b), (d)--(g) alumina substrates and (c) a glass substrate: (a) film deposited at 20 °C; (b) film deposited at 215 °C; (c) film deposited at 215 °C on a glass substrate; (d) film deposited at 20 °C and transformed from ct to 13phase by NO2 doping and heat treatment; (e) film deposited at 80 °C and heat treated at 240 °C for 2 h under vacuum; (f) film deposited at 80 °C and heat treated at 240 °C for 10 h under vacuum; (g) film deposited at 80 °C and heat treated at 240 °C for 24 h under vacuum.

249

formation, observed by the evolution of the temperature of the m a x i m u m conductivity after doping (see Part II, Section 3). The apparent contradiction between these two results could arise from the influences of the nature of the substrate, of the atmosphere during heat treatment, and of the doping gas, which would facilitate the change in crystalline phase. (4) Doping CuPc thin films deposited at 80 °C or 200 °C with 20 p p m NO2 diluted in N2 + air at r o o m temperature for 10 h causes a decrease in the heights of the diffraction peaks (Table 1 and Figs. 4(a) and 4(b)), but hardly enhances the relative proportion of the D = 3.3/k peak compared with heat treatment. Thus gas doping does not cause a reorganization of the crystalline arrangement but rather induces disorder in the film structure. NO2 doping of a heat-treated film also reduces the X R D peaks (Fig. 4(c)) but new peaks (D = 5.66/~ and D = 5.01 ,~) appear, which are not yet explained, and this gives evidence of a larger range of orientations of the CuPc molecules in the film. 3.3. E l e c t r o n m i c r o s c o p y

CuPc thin films, 2 6 0 - 3 0 0 n m thick, deposited on alumina or glass substrates kept at a temperature Td during the deposition process, have been observed by scanning electron microscopy. The films have been covered with a 20 nm thick carbon layer, deposited under vacuum, to ensure sufficient conduction. The results are described below (for CuPc films deposited on alumina substrates, except where otherwise stated). (1) For Td = 2 0 ° C , a disordered phase with fine particles, as described in ref. 11, is observed (Fig. 5(a)). No difference is seen between films on alumina substrates and films on glass substrates; the larger "cell" structure (a few micrometres in size) observed for the films grown on alumina reflects the structure of the alumina substrate. This morphology is in agreement with the low crystallinity observed by X R D . (2) Td = 2 1 5 °C results in the growth of well-elongated whiskers, 150 nm in diameter and a few micrometres long (Fig. 5(b)). It should be noted that in this case the 260-300 nm film thickness measured during the deposition process by the quartz oscillator has little significance since there is a lot of empty space between these micrometre-long whiskers. In ref. 9 the same morphology is obtained for ZnPc thin films on alumina and attributed to the [3 phase. Films deposited at 215 °C on glass substrates.exhibit a completely different morphology (Fig. 5(c)): the surface of the film is relatively flat and smooth and dotted with needle-like crystals, some of them gathered in tufts. The X R D spectrum of such films has been interpreted in terms of crystallites lying with their b axis parallel to the substrate plane; this interpretation may

250

s. Dogo et al. / Interaction o f NO 2 with CuPe: 1

n o t apply to films on a l u m i n a , for which the b axis could be parallel to the whisker lengths. (3) Films deposited at Td = 20 °C a n d then transformed from ~ to [3 phase by NO2 d o p i n g a n d heat t r e a t m e n t present the same b a c k g r o u n d of fine particles as on the sample with Td = 80 °C, with the growth of whiskers, 1 5 0 n m in diameter a n d 100 n m - 1 gm long, rising from the substrate surface, very similar to those observed on the sample with Td = 2 1 5 °C, with the same diameter b u t a shorter length (Fig. 5(d)). If such whiskers are assumed to be characteristic of the 13 phase [9], then the ~ , [3 t r a n s f o r m a t i o n has really taken place on the film (see Part II, Section 3). (4) W h e n the films are deposited at T d = 20 °C or 80 °C a n d then heat treated at 240 '~C u n d e r v a c u u m for 2 h (Fig. 5(e)), 10 h (Fig. 5(f)) or 24 h (Fig. 5(g)), the fine particles e m b e d d e d in a n a m o r p h o u s structure, as seen in Fig. 5(a), are t r a n s f o r m e d by the heat t r e a t m e n t to interconnect grains which grow progressively in the form of whiskers, resulting in a structure identical to that of Fig. 5(d); the same conclusion can be d r a w n as to the phase transition. Thus it seems that heat t r e a t m e n t is sufficient to produce the ~ ,13 t r a n s f o r m a t i o n ; the a d d i t i o n of NO2 d o p i n g is not essential b u t it does not hinder this phase t r a n s f o r m a t i o n .

4. Conclusion T h i n films of CuPc can be obtained on glass substrates in a highly ordered, uniaxially oriented, polycrystalline form. Heat t r e a t m e n t at 2 4 0 ° C u n d e r v a c u u m causes a r e o r i e n t a t i o n o f the crystallites b u t the crystallinity remains high. NOR d o p i n g of the CuPc film damages the order of the film. S c a n n i n g electron microscopy of CuPc thin films shows that the n a t u r e of the substrate influences the film morphology. Films deposited on a l u m i n a at 20 °C exhibit a smooth g r a n u l a r surface, from which heat treatments ( a c c o m p a n i e d or n o t by NO2 doping) make whiskers grow, to increasing lengths for longer heat treatments. This m o r p h o l o g y change is related to the ~, [3 phase transition. In Part II this transition is studied t h r o u g h the temperature of the m a x i m u m conductivity of doped CuPc films.

References l Y. Sadaoka, N. Yamazoe and T. Seiyama, A gas sensor using thin films of phthalocyanine, Denki Kagaku, 46 (1978) 597 602. 2 B. Bott and T. A. Jones, A highly sensitive NO2 sensor based on electrical conductivity changes in phthalocyanine films, Sens. Actuators, 5 (1984) 43 53. 3 B. Bott and T. A. Jones, Gas-induced electrical conductivity changes in metal phthalocyanines, Sens. Aetuators, 9 (1986) 27 37. 4 A. V. Chadwick, P. B. M. Dunning and J. D. Wright, Application of organic solids to chemical sensing, Mol. Cryst. Liq. ('ryst., 134 (1986) 137 153. 5 A. Wilson and R. A. Collins, Electrical characteristics of planar phthalocyanine thin film gas sensors, Sens. Aetuators, 12 (1987) 389 403. 6 C. Hamann, A. Mrwa, M. Miiller, W. G6pel and M. Rager, Lead phthalocyanine thin films for NO2 sensors, Sens. Actuators B, 4 (1991) 73 78. 7 A. Wilson, J. D. Wright and A. V. Chadwick, A microprocessorcontrolled nitrogen dioxide sensing system, Sens. Actuators B, 4 (1991)499 504. 8 G. P. Rigby, A. Wilson, J. D. Wright and S. C. Thorpe, Fast response heat-treated lead phthalocyanine NO2 sensors. In K. T. V. Grattan (ed.), Sensors. TechnoloKv, Systems and Applications, Hilger, Bristol, 1991, pp. 121~ 126. 9 S. Pizzini, G. L. Timo, M. Beghi, N. Butta, C. M. Mari and J. Faltenmaier, Influence of the structure and morphology on the sensitivity to nitrogen oxides of phthalocyanine thin film resistivity sensors, Sens. Actuators', 17 (1989) 481-491. 10 J. D. Wright, Gas adsorption on phthalocyanines and its effects on electrical properties, Prog. Surf Sci., 31 (1989) 1-60. 11 Y. Sadaoka, T. A. Jones, G. S. Revell and W. G6pel, Influence of morphology on NOz detection in air at room temperature with phthalocyanine thin films, J. Mater. Sci., 25 (1990) 5257 5268. 12 Y. Sadaoka, M. Matsugachi, Y. Sakai, Y. Mori and W. G6pel, Effect of crystal form on the conductance in oxidative gases of metal-free and some metal phthalocyanines, Sens. Actuators B, 4 (1991) 495 498. 13 S. Dogo, J.-P. Blanc, C. Maleysson and A. Pauly, Interaction of NO~ with copper phthalocyanine thin films. 11: Application to gas sensing, Thin Solid Films, 219 (1992) 251. 14 A. D. Cross, Introduction glla Pratique de la Speetroscopie lnfi'arouge, Azoulay, Paris, 1967, p. 96. 15 Y. Sadaoka, Y. Sakai, I. Aso, N. Yamazoe and T. Seiyama, The N O 2 detecting ability of the phthalocyanine gas sensor, Denki Kagaku, 48 (1980) 486-490. 16 M. K. Debe, R. J. Poirier and K. K. Kam, Organic-thin-filminduced molecular epitaxy from the vapor phase, Thin Solid Fihns, 197 (1991) 335-347. 17 A. W. Snow and W. R. Barger, in C. C. Leznoff and A. B. P. Lever (eds.), Phthalocyanines. Properties and Applications, VCH, New York, 1989, pp. 362 367.