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Oriented thin films of p-nitrophenyl nitroxyl nitroxide radical
ELSEVIER
Synthetic
Metals
103 (1999)
Oriented thin films of p-nitrophenyl
2298-2299
nitroxyl nitroxide radical
J. Fraxedas*,J. Caro, J. Santiso,A, Figueras,0. Jiirgens,C. Rovira, J. Veciana Insritut de CiPncia de Materials
de Barcelona,
CSIC. Campus de Ia UAB, 08193 Bellaterra.
Spain.
Abstract
Thin films (thickness- 2 pm) of the molecular organic radical p-nitrophenyl nitroxyl nitroxide @-NPNN) have been obtained by evaporating different crystallographicphases(a, p and y) of the p-NPNN precursor in vacuum on glassand cleaved NaCl(001) substrates. The films crystallize in the a-phase (monoclinic) and show a high degree of orientation of the c*-axis perpendicular to the substrate, independently of the precursorphase.The films undergo a spontaneousphase transition, from a to p, under certain conditions. Keywords: Structural phase transitions, evaporation
and sublimation.
1. Introduction
There has been considerable interest in the development of materials based on organic free radicals exhibiting spontaneous magnetic order [ 1,2]. The first purely organic molecular material exhibiting bulk ferromagnetism was the N-O based radical pNPNN. Four polymorphic forms are known for p-NPNN: ~1, p, y and 6. The orthorhombic P-phase is the most stable one and undergoes a bulk ferromagnetic transition at 0.6 K due to the presence of 3D intermolecular ferromagnetic interactions 131, whereas the y-phase shows 1D ferromagnetic interactions but orders antiferromagnetically at 0.65 K [2j. Weak ferromagnetism has been observed at 35.5 K in the orthorhombic P-phaseof the S-N based radicalp-NC.ChF+CNSSN [4] and highly oriented thin films of this radical, ordering in the a-phase, have been recently reported [S]. The number of polymorphs associatedwith some of these molecular compounds indicates a very fine energetic balance of a few kcal.mol-* between different structures and their associatedphysical properties [6]. Therefore the preparation and characterization of oriented thin films of well-defined crystallographic phasesis essentialfor future applications. 2.
w
A
Experimental
(004)
4Ck1.3”
Thin films of p-NPNN grown on glass and on ex situ cleaved NaCl(001) substrates have been obtained by thermal evaporation of the a-, p- and y-phasesof the p-NPNN precursor in high vacuum. The precursoris located as powder in a crucible, which is heated slowly up to 160 “C in a base pressureof about 10-hmbar. The deposition rate is typically - 0.5 pm / h and the substratesare held at room temperature [7].
, .g 2
20000f
3d) 15000: 100001
2 6
4 ;;:
°iees) lo i3‘ q-
I’
3. Results
The films grown on glass substratesexhibit 20 spherulitic morphology (Fig. la). Figure lb shows a standard X-ray diffraction (XRD) pattern of a thin film.
=f 0
10
20
50
60
(b) * This work has been supported by DGICYT (Grant no. PB96-0862~C02-01) and CICYT (Grant no. MAT951038-C02-01) and by the GRQ93-8044 program of the Generalitat de Catalunya. J. C. and 0. J. also thank the Generalitat de Catalunya for the award of doctoral fellowships.
0379-6779/99/$ - seefront matter PII: SO379-6779(98)00637-7
0 1999 Elsevier
Science
S.A.
All rights
Fig. 1 (a) Optical microscopy image (x50,-crossedpolarizers)and (b) XRD pattern of a thin film of p-NPNN grown on a glass substrate.
reserved.
J. Fraxedus
et al. I Synthetic
Only (0 0 D reflections from the u-phase (monoclinic) are identified, thus showing a high degree of orientation of the c*axis perpendicular to the substrate surface. We define the degree of orientation by the FWHM of the rocking curve of the (0 0 4) reflection. This value is 1.3” (seeinset in Fig. lb) denoting a high degree of orientation considering that the substrateis amorphous. This value should be compared to the 0.6” obtained when depositing on single-crystalsubstratessuch as NaCl (001) [7]. No tracesof other crystalline phaseswere observed.
Metals
103 (1999)
2298-2299
2299
reflections of the p-phase at 28 = 24.51” (2 4 0), 28= 32.66” (4 0 0) and 28 - 34” (4 2 0) are also identified. From the XRD pattern we observe that upon transformation from the c1-to the P-phase, the films are no longer preferentially oriented along a single direction. However, a certain degree of orientation is maintained because only (h k 0) reflections are observed. At ambient conditions this transformation processis rather slow, a thin film with typical dimensions of 5 mm x 5 mm transforms completely after several weeks. 4. References
[l] D. A. Dougherty, in ResearchFrontiers in Magnetochemistry (Ed.: C. J. 0’ Connor), World Scientific, Singapore, 1993; J. S. Miller, A. J. Epstein, Chem. Eng. News 73 (1995) 30. [2] M. Kinoshita, Jpn. J. Appl. Phys.33 (1994) 5718. [3] M. Tamura, Y. Nakazawa, D. Shiomi, K. Nozawa, Y. Hosokoshi, M. Ishikawa, M. Takahashi, M. Kinoshita, Chem. Phys.Lett., 186 (1991)401. [4] A. J. Banister, N. Bricklebank, I. Lavender, J. M. Rawson, C. I. Gregory, B. K. Tanner, W. Clegg, M. R. J. Elsegood, F. Palacio, Angew. Chem. Int. Ed. Eng. 35 (1996) 2533. [5] J. Caro, J. Fraxedas,J. Santiso, A. Figueras, J. M. Rawson, J. N. B. Smith, G. Antorrena, F. Palacio (unpublished results) (a)
[6] J. Dunitz, J. Bernstein, Accts. Chem. Res. (1995) 193; J. Bernstein, J. Phys.D., Appl. Phys. 26 (1993) B66. [7] J. Caro, J. Fraxedas, 0. Jiirgens, J. Santiso, C. Rovira, J. Veciana, A. Figueras, Adv. Mater. 10 (1998) 608. [8] K. Awaga, T. Inabe, U. Nagashima, Y. Maruyama, J. Chem. Sot. , Chem. Commun. (1989) 1617; Y. Nakazawa, M. Tamura, N. Shirakawa, D. Shiomi, M. Takahashi, M. Kinoshita, M. Ishikawa, Phys. Rev. B 46 (1992) 8906.
100: G 0
”
” ” ”
” ” ‘1 ” ’ ” ”
50
60
(b) Fig. 2 (a) Optical microscopy image (x50, crossedpolarizers)and (b) XRD pattern of a transformed thin film of p-NPNN grown on a glasssubstrate. At ambient conditions we observe that for most films thicker than -2 pm the 2D spherulitic morphology spontaneously transforms to a fibrilar morphology (see Fig. 2a). Figure 2b shows a standard XRD pattern of the transformed part of a film. The main peak is a doublet with components at 28 = 18.30” and 28 = 18.60”, with a FWHM of - 6”. We identify these components as the (0 4 0) and the (2 2 0) reflections of the pphase (orthorhombic) by comparing with a simulated pattern of the P-phase obtained from the atomic positions determined experimentally [8], by the observation of additional out-of-plane reflections, (2 2 0) or (0 4 0), measured at 28 = 18.3” and x = 57.4”, and by comparison with simulated patterns and available experimental data on the u-, y- and F-phases [7]. Additional
Synthetic
Metals
103 (1999)
Oriented thin films of p-nitrophenyl
2298-2299
nitroxyl nitroxide radical
J. Fraxedas*,J. Caro, J. Santiso,A, Figueras,0. Jiirgens,C. Rovira, J. Veciana Insritut de CiPncia de Materials
de Barcelona,
CSIC. Campus de Ia UAB, 08193 Bellaterra.
Spain.
Abstract
Thin films (thickness- 2 pm) of the molecular organic radical p-nitrophenyl nitroxyl nitroxide @-NPNN) have been obtained by evaporating different crystallographicphases(a, p and y) of the p-NPNN precursor in vacuum on glassand cleaved NaCl(001) substrates. The films crystallize in the a-phase (monoclinic) and show a high degree of orientation of the c*-axis perpendicular to the substrate, independently of the precursorphase.The films undergo a spontaneousphase transition, from a to p, under certain conditions. Keywords: Structural phase transitions, evaporation
and sublimation.
1. Introduction
There has been considerable interest in the development of materials based on organic free radicals exhibiting spontaneous magnetic order [ 1,2]. The first purely organic molecular material exhibiting bulk ferromagnetism was the N-O based radical pNPNN. Four polymorphic forms are known for p-NPNN: ~1, p, y and 6. The orthorhombic P-phase is the most stable one and undergoes a bulk ferromagnetic transition at 0.6 K due to the presence of 3D intermolecular ferromagnetic interactions 131, whereas the y-phase shows 1D ferromagnetic interactions but orders antiferromagnetically at 0.65 K [2j. Weak ferromagnetism has been observed at 35.5 K in the orthorhombic P-phaseof the S-N based radicalp-NC.ChF+CNSSN [4] and highly oriented thin films of this radical, ordering in the a-phase, have been recently reported [S]. The number of polymorphs associatedwith some of these molecular compounds indicates a very fine energetic balance of a few kcal.mol-* between different structures and their associatedphysical properties [6]. Therefore the preparation and characterization of oriented thin films of well-defined crystallographic phasesis essentialfor future applications. 2.
w
A
Experimental
(004)
4Ck1.3”
Thin films of p-NPNN grown on glass and on ex situ cleaved NaCl(001) substrates have been obtained by thermal evaporation of the a-, p- and y-phasesof the p-NPNN precursor in high vacuum. The precursoris located as powder in a crucible, which is heated slowly up to 160 “C in a base pressureof about 10-hmbar. The deposition rate is typically - 0.5 pm / h and the substratesare held at room temperature [7].
, .g 2
20000f
3d) 15000: 100001
2 6
4 ;;:
°iees) lo i3‘ q-
I’
3. Results
The films grown on glass substratesexhibit 20 spherulitic morphology (Fig. la). Figure lb shows a standard X-ray diffraction (XRD) pattern of a thin film.
=f 0
10
20
50
60
(b) * This work has been supported by DGICYT (Grant no. PB96-0862~C02-01) and CICYT (Grant no. MAT951038-C02-01) and by the GRQ93-8044 program of the Generalitat de Catalunya. J. C. and 0. J. also thank the Generalitat de Catalunya for the award of doctoral fellowships.
0379-6779/99/$ - seefront matter PII: SO379-6779(98)00637-7
0 1999 Elsevier
Science
S.A.
All rights
Fig. 1 (a) Optical microscopy image (x50,-crossedpolarizers)and (b) XRD pattern of a thin film of p-NPNN grown on a glass substrate.
reserved.
J. Fraxedus
et al. I Synthetic
Only (0 0 D reflections from the u-phase (monoclinic) are identified, thus showing a high degree of orientation of the c*axis perpendicular to the substrate surface. We define the degree of orientation by the FWHM of the rocking curve of the (0 0 4) reflection. This value is 1.3” (seeinset in Fig. lb) denoting a high degree of orientation considering that the substrateis amorphous. This value should be compared to the 0.6” obtained when depositing on single-crystalsubstratessuch as NaCl (001) [7]. No tracesof other crystalline phaseswere observed.
Metals
103 (1999)
2298-2299
2299
reflections of the p-phase at 28 = 24.51” (2 4 0), 28= 32.66” (4 0 0) and 28 - 34” (4 2 0) are also identified. From the XRD pattern we observe that upon transformation from the c1-to the P-phase, the films are no longer preferentially oriented along a single direction. However, a certain degree of orientation is maintained because only (h k 0) reflections are observed. At ambient conditions this transformation processis rather slow, a thin film with typical dimensions of 5 mm x 5 mm transforms completely after several weeks. 4. References
[l] D. A. Dougherty, in ResearchFrontiers in Magnetochemistry (Ed.: C. J. 0’ Connor), World Scientific, Singapore, 1993; J. S. Miller, A. J. Epstein, Chem. Eng. News 73 (1995) 30. [2] M. Kinoshita, Jpn. J. Appl. Phys.33 (1994) 5718. [3] M. Tamura, Y. Nakazawa, D. Shiomi, K. Nozawa, Y. Hosokoshi, M. Ishikawa, M. Takahashi, M. Kinoshita, Chem. Phys.Lett., 186 (1991)401. [4] A. J. Banister, N. Bricklebank, I. Lavender, J. M. Rawson, C. I. Gregory, B. K. Tanner, W. Clegg, M. R. J. Elsegood, F. Palacio, Angew. Chem. Int. Ed. Eng. 35 (1996) 2533. [5] J. Caro, J. Fraxedas,J. Santiso, A. Figueras, J. M. Rawson, J. N. B. Smith, G. Antorrena, F. Palacio (unpublished results) (a)
[6] J. Dunitz, J. Bernstein, Accts. Chem. Res. (1995) 193; J. Bernstein, J. Phys.D., Appl. Phys. 26 (1993) B66. [7] J. Caro, J. Fraxedas, 0. Jiirgens, J. Santiso, C. Rovira, J. Veciana, A. Figueras, Adv. Mater. 10 (1998) 608. [8] K. Awaga, T. Inabe, U. Nagashima, Y. Maruyama, J. Chem. Sot. , Chem. Commun. (1989) 1617; Y. Nakazawa, M. Tamura, N. Shirakawa, D. Shiomi, M. Takahashi, M. Kinoshita, M. Ishikawa, Phys. Rev. B 46 (1992) 8906.
100: G 0
”
” ” ”
” ” ‘1 ” ’ ” ”
50
60
(b) Fig. 2 (a) Optical microscopy image (x50, crossedpolarizers)and (b) XRD pattern of a transformed thin film of p-NPNN grown on a glasssubstrate. At ambient conditions we observe that for most films thicker than -2 pm the 2D spherulitic morphology spontaneously transforms to a fibrilar morphology (see Fig. 2a). Figure 2b shows a standard XRD pattern of the transformed part of a film. The main peak is a doublet with components at 28 = 18.30” and 28 = 18.60”, with a FWHM of - 6”. We identify these components as the (0 4 0) and the (2 2 0) reflections of the pphase (orthorhombic) by comparing with a simulated pattern of the P-phase obtained from the atomic positions determined experimentally [8], by the observation of additional out-of-plane reflections, (2 2 0) or (0 4 0), measured at 28 = 18.3” and x = 57.4”, and by comparison with simulated patterns and available experimental data on the u-, y- and F-phases [7]. Additional