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The lithium phthalocyanine radical: A low-dimensional molecular band semiconductor
Synthetic Metals, 29 (1989) F65-F70
THE LITHIUM
F65
PHTHALOCYANINE RADICAL:
A
LOW-DIMENSIONAL MOLECULAR BAND
SEMICONDUCTOR
Ph. TUREK, M. MOUSSAVI, P. PETIT and d.-J. ANDRE Institut Charles Sadron, 67083 Strabourg C~dex (France)
ABSTRACT The lithium phthalocyanine neutral ~-radical molecular compound is depicted as a low-dimensional intrinsic band semiconductor whose spins are submitted to strong exchange interactions.lt is suggested that the charge/spin carriers of this system ought to be analyzed in the frame of itinerant magnetism.
INTRODUCTION The lithium phthalocyanine
neutral n-radical compound, PcLi, has been shown
to exhibit somewhat peculiar features such as e.g. intrinsic semiconducting behaviour [1,2] or extreme spin exchange narrowing [3]. The present contribution reports on further conductivity measurements either DC or microwave, performed on different samples.These results definitely assess the intrinsic character of the electrical conductivity in the semiconducting range.Some EPR experiments are reported which emphasize as well the low-dimensionality as the itinerant character, i.e. band related, of the n-spin system. RESULTS AND DISCUSSION Direct current and microwave conductivity The samples were grown either in acetone or in acetonitrile as previously described [4]. They consist in crystalline materials whose structure corres0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
F66 I
'
'
'
I
~'E10-2
'=10_ 3
10 -4
10 -5
.,
3
i
I
|
4
5
6
I
7 lO00i'lL(K "')
Fig. I. Variation of the electrical conductivity of single crystals of PcLi with the inverse of the temperature (note the logarithmic scale for o). The samples are different between the two reported experiments. Upper curve (filled circles): microwave conductivity (10 GHz); lower curve (open circles): DC conductivity (two-point contact method).
ponds to that reported [5,6]. The room-temperature conductivities are in the 10"3-10.2 ~'1.cm'1 range. A thermally activated behaviour[o=ooexp(-E,ct/kBT)] is observed (Fig. I) in all cases in the available temperature range (120-300 K).The activation energies are estimated in the 70-120 meV range whatever the sample considered and the measurement frequency (DC or 10 GHz). This is a strong indication of the intrinsic origin of the semiconducting properties since charge detrapping should manifest and affect the reported values in the case of doped insulator/extrinsic semiconductor. Influence of molecular
oxyqen
The importance of the effect of air exposure is underlined in Fig.2 for a crystal of PcLi successively air exposedand degased. The extremely narrow ESR line makes PcLi a potential material for low-field magnetometry applications [7]. T h i s effect is fully reversible and instantaneous without consecutive
F67
!
tO
f
!
i
!
--
•
i
ABpp=0.g8 G
lu,.,i
I
-r" "13
>-"13
ABpp=0.02 G t I
3380.5
I
I
3381.0
I
H[G]
I
3381.5
Fig. 2. Modification of the ESR spectrum of a single crystal of PcLi when I) air exposed, and 2) under vacuum.
degradation of the samples. The way for PcLi to interact with 02 has been recently f u l l y interpreted: the Oz molecules favorate a local ferromaanetic cou~lina between the neighbouring PcLi molecules [8]. The resulting Curie-Weiss contribution to the paramagnetic susceptibility XESRis observed at low temperature (Fig.3). Paramaqne~ic susceotibility Whereas to decrease monotonously as the Curie-Weiss contribution does implicate, XEsR goes through a minimum and increases at higher temperatures (Fig. 3). Such a behaviour is in agreement with the results of static susceptibility measurements [8]. Once the Oz induced Curie-Weiss contribution substracted i t has been qualitatively shown that the intrinsic x-spin susceptibility is thermally activated. The concept of temperature-induced local moments presented in the frame of the spin fluctuation theory of itinerant magnetism [g] is proposed in order to give a consistent physical background to the whole properties of LiPc whether magnetic or electrical as well as dynamical or static. I t will be discussed elsewhere [10]. Such a concept has been introduced in particular in order
F68
I I,LI
•
•
e°
I m•
4" 0
!
I
50
100
I
150
°
I
•
I
200
o
o o
I
250
Fig. 3. Thermal variation of the inverse ESR susceptibility of a single
T(K) crystal
of PcLi under vacuu•.
to explain the physical properties
of the FeSi narrow gap se•iconducting alloy
[9,11,12]. Linewidth anisotroDv The variation of the ESR linewidth as a function of the orientation of a single crystal under vacuu• in the applied •agnetic field Ho (Fig. 4) exhibits the peculiar shape of Iow-di•ensional syste•s of localized spins submitted to • agnetic exchangeinteractions, e.g. Heisenberg spin syste•s: there is a •ini•um linewidth at the •agic angle [13]. The geo•etry is such that the crystal is in the plane defined by the stacking axis and Ho. The corresponding experi•ent for the air exposed crystal reveals the characteristic behaviour of isotropic spin syste•. This is another way to underline the role of 02 in PcLi, i.e. 0z allows an interchain coupling leading to a break of the expected low-dimensionality owing to the crystalline structure. There is "a priori" a contradiction betweenthe "localized soin" point of view argued in order to analyze the ESR linewidth anisotropy, and the "itiner@nt soin" proposal •ade in the light of the electrical and •agnetic properties. The opposition of these two extre•es is not strong enough to induce confusion. Indeed the dynamical susceptibility x(q,¢) of the spin fluctuations is predominantly driven by the low q,¢ •odes in the Fourier space [9]. Accordingly the
F69
008 C~ v
c~
1.31
® )0
rn
0.04
OoqI
oC o
0 0
1.2 o
o
•
• o•
0.04 0
I
go
80
1.1
0
90
E)(deg.)l~0
Fig. 4. Anisotropy of the ESR linewidth at half-maximum. The angle 0 is that defined by the stacking axis of the macrocycles in the crystal and the axis of the static magnetic field Ho: a) under primary vacuum; b) at the atmosphere.
theory of the dynamics of low-dimensional "localized spin" systems emphasizes the role of the long time,low q modes in the Fourier transformed spin-spin correlation function related to the magnetization [13]. Qualitatively both analysis collapse in the present case, i.e. PcLi is a low-dimensional system of strongly exchange-correlated spins whoseorigin lies in spin fluctuations. We point out the fact that the unified theory of magnetism [9] allows to interpolate between the two above mentioned extremes. Moreover, although the theory of the dynamics of the spin fluctuations is s t i l l to be worked out, we presently propose to consider the physical properties of PcLi in this framework in order to conciliate both behaviours of the spin and charqe carriers in this band semiconductor. REFERENCES 1P. Turek, P. Petit, J.-J. Andrd, J. Simon, R. Even, B. Boudjema, G. Guillaud and M. Maitrot, J. Am. Chem. Soc., 109 (1987) 5119; Mol. Cryst. Liq. Cryst., to be published. 2 P. Petit, P. Turek, d.-J. Andrd, R. Even, J. Simon, R. Madru, M. AI Sadoun, G. Guillaud and M. Maitrot, Synth. Met., 29 (1989) F59 (these Proceedings). 3 P. Turek, J.-J. Andrd and J. Simon, Solid State Common., 6~ (1987) 741. 4 P. Turek, J.-J. Andrd, A. Giraudeau and J. Simon, Chem. Phys. Lett., 134 (1987) 471. 5 J. Fisher, A. De Cian and R. Weiss, personal communication.
F70
6 H. Sugimoto, M. Hori, H. Masuda and T. Taga, J. Chem. Soc. Chem. Commun., (1986) 1962. 7 D. Duret, M. Bdranger, H. Houssavi, P. Turek and J.-J. Andrd, Synth. Met., 27 (1988) B175 (these Proceedings). 8 P. Turek, M. Moussavl and J.-J. Andrd, submitted. 9 T. Horiya, in M. Cardona, P. Fulde and H. O. Queisser (eds.), SDin Fluctuations in Itinerant Maanetism, Springer Verlag, Berlin-Heidelberg, 1985,p. 153 & p.185; Y. Takahashi and T. Moriya, 3, Phys. Soc. JDn. 46 (1979) ]451. 10 P. Turek et a l , , t o be published. 11S. N. Evangelou and D. H. Edwards, O. Phys. C: Solid State Phys.. 16 (1983) 2121. 12 V. Jaccarino,
G.K.
Werthetm, J.
H. Wernick,
L.R.
Walker and S. Arajs,
Phys. Rev.. 160 (1967) 476. 13 P. M. Richards, Proc, of the International School of phvsic~ "Enrico Fermi", North Holland, Amsterdam, 1976, p. 539; P. Petit, TheSiS, Strasbour9, France, 1987, unpublished.
THE LITHIUM
F65
PHTHALOCYANINE RADICAL:
A
LOW-DIMENSIONAL MOLECULAR BAND
SEMICONDUCTOR
Ph. TUREK, M. MOUSSAVI, P. PETIT and d.-J. ANDRE Institut Charles Sadron, 67083 Strabourg C~dex (France)
ABSTRACT The lithium phthalocyanine neutral ~-radical molecular compound is depicted as a low-dimensional intrinsic band semiconductor whose spins are submitted to strong exchange interactions.lt is suggested that the charge/spin carriers of this system ought to be analyzed in the frame of itinerant magnetism.
INTRODUCTION The lithium phthalocyanine
neutral n-radical compound, PcLi, has been shown
to exhibit somewhat peculiar features such as e.g. intrinsic semiconducting behaviour [1,2] or extreme spin exchange narrowing [3]. The present contribution reports on further conductivity measurements either DC or microwave, performed on different samples.These results definitely assess the intrinsic character of the electrical conductivity in the semiconducting range.Some EPR experiments are reported which emphasize as well the low-dimensionality as the itinerant character, i.e. band related, of the n-spin system. RESULTS AND DISCUSSION Direct current and microwave conductivity The samples were grown either in acetone or in acetonitrile as previously described [4]. They consist in crystalline materials whose structure corres0379-6779/89/$3.50
© Elsevier Sequoia/Printed in The Netherlands
F66 I
'
'
'
I
~'E10-2
'=10_ 3
10 -4
10 -5
.,
3
i
I
|
4
5
6
I
7 lO00i'lL(K "')
Fig. I. Variation of the electrical conductivity of single crystals of PcLi with the inverse of the temperature (note the logarithmic scale for o). The samples are different between the two reported experiments. Upper curve (filled circles): microwave conductivity (10 GHz); lower curve (open circles): DC conductivity (two-point contact method).
ponds to that reported [5,6]. The room-temperature conductivities are in the 10"3-10.2 ~'1.cm'1 range. A thermally activated behaviour[o=ooexp(-E,ct/kBT)] is observed (Fig. I) in all cases in the available temperature range (120-300 K).The activation energies are estimated in the 70-120 meV range whatever the sample considered and the measurement frequency (DC or 10 GHz). This is a strong indication of the intrinsic origin of the semiconducting properties since charge detrapping should manifest and affect the reported values in the case of doped insulator/extrinsic semiconductor. Influence of molecular
oxyqen
The importance of the effect of air exposure is underlined in Fig.2 for a crystal of PcLi successively air exposedand degased. The extremely narrow ESR line makes PcLi a potential material for low-field magnetometry applications [7]. T h i s effect is fully reversible and instantaneous without consecutive
F67
!
tO
f
!
i
!
--
•
i
ABpp=0.g8 G
lu,.,i
I
-r" "13
>-"13
ABpp=0.02 G t I
3380.5
I
I
3381.0
I
H[G]
I
3381.5
Fig. 2. Modification of the ESR spectrum of a single crystal of PcLi when I) air exposed, and 2) under vacuum.
degradation of the samples. The way for PcLi to interact with 02 has been recently f u l l y interpreted: the Oz molecules favorate a local ferromaanetic cou~lina between the neighbouring PcLi molecules [8]. The resulting Curie-Weiss contribution to the paramagnetic susceptibility XESRis observed at low temperature (Fig.3). Paramaqne~ic susceotibility Whereas to decrease monotonously as the Curie-Weiss contribution does implicate, XEsR goes through a minimum and increases at higher temperatures (Fig. 3). Such a behaviour is in agreement with the results of static susceptibility measurements [8]. Once the Oz induced Curie-Weiss contribution substracted i t has been qualitatively shown that the intrinsic x-spin susceptibility is thermally activated. The concept of temperature-induced local moments presented in the frame of the spin fluctuation theory of itinerant magnetism [g] is proposed in order to give a consistent physical background to the whole properties of LiPc whether magnetic or electrical as well as dynamical or static. I t will be discussed elsewhere [10]. Such a concept has been introduced in particular in order
F68
I I,LI
•
•
e°
I m•
4" 0
!
I
50
100
I
150
°
I
•
I
200
o
o o
I
250
Fig. 3. Thermal variation of the inverse ESR susceptibility of a single
T(K) crystal
of PcLi under vacuu•.
to explain the physical properties
of the FeSi narrow gap se•iconducting alloy
[9,11,12]. Linewidth anisotroDv The variation of the ESR linewidth as a function of the orientation of a single crystal under vacuu• in the applied •agnetic field Ho (Fig. 4) exhibits the peculiar shape of Iow-di•ensional syste•s of localized spins submitted to • agnetic exchangeinteractions, e.g. Heisenberg spin syste•s: there is a •ini•um linewidth at the •agic angle [13]. The geo•etry is such that the crystal is in the plane defined by the stacking axis and Ho. The corresponding experi•ent for the air exposed crystal reveals the characteristic behaviour of isotropic spin syste•. This is another way to underline the role of 02 in PcLi, i.e. 0z allows an interchain coupling leading to a break of the expected low-dimensionality owing to the crystalline structure. There is "a priori" a contradiction betweenthe "localized soin" point of view argued in order to analyze the ESR linewidth anisotropy, and the "itiner@nt soin" proposal •ade in the light of the electrical and •agnetic properties. The opposition of these two extre•es is not strong enough to induce confusion. Indeed the dynamical susceptibility x(q,¢) of the spin fluctuations is predominantly driven by the low q,¢ •odes in the Fourier space [9]. Accordingly the
F69
008 C~ v
c~
1.31
® )0
rn
0.04
OoqI
oC o
0 0
1.2 o
o
•
• o•
0.04 0
I
go
80
1.1
0
90
E)(deg.)l~0
Fig. 4. Anisotropy of the ESR linewidth at half-maximum. The angle 0 is that defined by the stacking axis of the macrocycles in the crystal and the axis of the static magnetic field Ho: a) under primary vacuum; b) at the atmosphere.
theory of the dynamics of low-dimensional "localized spin" systems emphasizes the role of the long time,low q modes in the Fourier transformed spin-spin correlation function related to the magnetization [13]. Qualitatively both analysis collapse in the present case, i.e. PcLi is a low-dimensional system of strongly exchange-correlated spins whoseorigin lies in spin fluctuations. We point out the fact that the unified theory of magnetism [9] allows to interpolate between the two above mentioned extremes. Moreover, although the theory of the dynamics of the spin fluctuations is s t i l l to be worked out, we presently propose to consider the physical properties of PcLi in this framework in order to conciliate both behaviours of the spin and charqe carriers in this band semiconductor. REFERENCES 1P. Turek, P. Petit, J.-J. Andrd, J. Simon, R. Even, B. Boudjema, G. Guillaud and M. Maitrot, J. Am. Chem. Soc., 109 (1987) 5119; Mol. Cryst. Liq. Cryst., to be published. 2 P. Petit, P. Turek, d.-J. Andrd, R. Even, J. Simon, R. Madru, M. AI Sadoun, G. Guillaud and M. Maitrot, Synth. Met., 29 (1989) F59 (these Proceedings). 3 P. Turek, J.-J. Andrd and J. Simon, Solid State Common., 6~ (1987) 741. 4 P. Turek, J.-J. Andrd, A. Giraudeau and J. Simon, Chem. Phys. Lett., 134 (1987) 471. 5 J. Fisher, A. De Cian and R. Weiss, personal communication.
F70
6 H. Sugimoto, M. Hori, H. Masuda and T. Taga, J. Chem. Soc. Chem. Commun., (1986) 1962. 7 D. Duret, M. Bdranger, H. Houssavi, P. Turek and J.-J. Andrd, Synth. Met., 27 (1988) B175 (these Proceedings). 8 P. Turek, M. Moussavl and J.-J. Andrd, submitted. 9 T. Horiya, in M. Cardona, P. Fulde and H. O. Queisser (eds.), SDin Fluctuations in Itinerant Maanetism, Springer Verlag, Berlin-Heidelberg, 1985,p. 153 & p.185; Y. Takahashi and T. Moriya, 3, Phys. Soc. JDn. 46 (1979) ]451. 10 P. Turek et a l , , t o be published. 11S. N. Evangelou and D. H. Edwards, O. Phys. C: Solid State Phys.. 16 (1983) 2121. 12 V. Jaccarino,
G.K.
Werthetm, J.
H. Wernick,
L.R.
Walker and S. Arajs,
Phys. Rev.. 160 (1967) 476. 13 P. M. Richards, Proc, of the International School of phvsic~ "Enrico Fermi", North Holland, Amsterdam, 1976, p. 539; P. Petit, TheSiS, Strasbour9, France, 1987, unpublished.