Journal of ELECTRONIC MATERIALS, Vol. 39, No. 2, 2010
DOI: 10.1007/s11664-009-1024-8 Ó 2009 TMS
Formation of Hybrid Inorganic/Organic Nanocomposites TAMARA V. BASOVA,1 NADEZHDA M. KUROCHKINA,2 ASLAN YU. TSIVADZE,2 and ASIM K. RAY3,4 1.—Nikolaev Institute of Inorganic Chemistry, Siberian Branch of the Russian Academy of Sciences, Novosibirsk, Russia. 2.—Frumkin Institute of Physical Chemistry and Electrochemistry, Moscow, Russia. 3.—The Wolfson Centre for Materials Processing, Brunel University, Uxbridge, Middlesex UB8 3PH, UK. 4.—e-mail:
[email protected]
Hybrid inorganic/organic nanocomposites were produced by the exposure of spun films of octa-(benzo-15-crown-5)-substituted phthalocyanine of Cd(II) to an environment of hydrogen sulfide gas. The formation of cadmium sulfide (CdS) quantum dots in the metal free phthalocyanine matrix was identified in the phthalocyanine matrix from atomic force microscopy images. The mean size of CdS quantum dots was estimated to be 2–3 nm, from optical absorption and Raman spectra for the nanocomposites. Key words: Nanocomposites, phthalocyanines, cadmium sulfide
INTRODUCTION Metallated macrocyclic phthalocyanine (MPc) compounds have been extensively studied in recent years, due to their potential application in thin-film electronic devices, including light-emitting diodes,1 thin film organic transistors,2 and photovoltaic cells.3 The MPc compounds have also been widely used in the fabrication of photoreceptor devices,4 such as laser beam printers and photocopiers.5 Semiconductor quantum dots of cadmium sulfide (CdS), on the other hand, have attracted immense attention for their optoelectronic properties, which are suitable for applications in labels for biological use,6,7 light-emitting diodes,8,9 and solar cells.10–12 An interesting area for research into semiconductor CdS nanoparticles is their incorporation within polymeric or organic matrices for the development of hybrid inorganic/organic devices.13,14 For example, CdS nanoparticles embedded in a polystyrene matrix were successfully prepared by in situ thermolysis of a cadmium thiolate precursor dispersed in the polymer.15 The hybrid organic/inorganic system was formed by doping of the matrix of poly(N-vinylcarbazole) with quantum dots of CdS, and the effective carrier mobility of the resulting nanocomposite system was found to increase with the increase in nanoparticle concentration.16
(Received January 19, 2009; accepted November 24, 2009; published online December 15, 2009)
This paper describes a novel method for the solid state formation of a nano-structured composite material containing CdS quantum dots in a matrix of octa-(benzo-15-crown-5)-substituted Cd(II) phthalocyanine (CdPc) thin films (Fig. 1). The material was characterized by three independent analytical methods, and the results obtained are discussed and include details of the size and elemental composition of the CdS quantum dots. This chemical method of solid state formation of inorganic/organic nanocomposites is believed to be cost effective when compared with that of technologies such as molecular beam epitaxy. EXPERIMENTAL DETAILS Octa-(benzo-15-crown-5)-substituted phthalocyanine (H2Pc) and its complexes with Cd(II) have been synthesized as described in a previous publication.17 By the use of a photoresist spinner (Microsystem model 4000), the CdPc films were deposited onto ultrasonically cleaned substrates by the spinning of a small volume of the spreading solution at 2000 rpm. The spreading solution, having a concentration of 2 mg/ml, was prepared by dissolution of the CdPc compound in chloroform solvent. The spinning procedure was continued for 30 s, for complete evaporation of the solvent, producing a uniform phthalocyanine film. The CdPc films were then exposed to hydrogen sulfide (H2S) gas for approximately 2 h in a sealed container to produce CdS quantum dots in the films. 145
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Fig. 1. Chemical structure of octa-(benzo-15-crown-5)-substituted phthalocyanine of Cd(II).
A Shimadzu UV–VIS-a 3101PC spectrophotometer was employed to obtain ultraviolet–visible (UV–Vis) spectra for both as-deposited and H2Streated CdPc films on glass substrates. The CdPc films were deposited on silicon (100) substrates for the recording of Raman spectra with a Triplemate, SPEX spectrometer equipped with a multichannel detector, LN-1340PB, Princeton Instruments, in back-scattering geometry. The 488 nm line of a 50 mW argon laser focused to a diameter of 2 lm was used for the spectral excitation. The atomic force microscopy images of the CdPc film surfaces were obtained with a nanoscope IIIa instrument in tapping mode. Tips with typical radii of 4 nm were used; the scan frequency was approximately 0.5 s1. RESULTS AND DISCUSSION We studied the film surface morphology by taking atomic force microscopy (AFM) images at different locations on the surfaces of the CdPc films, both as-deposited and H2S treated, on Si substrates. Typical images are displayed in Fig. 2. As shown in Fig. 2b, the surface of the H2S-treated film appeared to become inhomogeneous, containing small features. The film roughness increased from 0.9 nm to 3.5 nm due to the H2S treatment, indicating the existence of heterogeneous regions of CdS clusters in an organic matrix. For further examination, the phase-contrast AFM tapping mode image was obtained (Fig. 2c). This is a widely used technique to determine inhomogeneity and discontinuity, because energy dissipation at the real contact area is sensitive to in-plane material properties and chemical composition.18 Previously observed small features shown in Fig. 2b appear as dark spots in Fig. 2c. Spots of this type on the AFM phase images are usually related to the presence of a hard phase in contrast to a soft organic regime.19 In our investigation these spots were believed to be associated with CdS clusters. The sizes of these clusters were estimated to be in the range of 10–20 nm.
Figure 3 shows a set of typical, reproducible UV– Vis spectra of both as-deposited and H2S-treated CdPc samples. The appearance of a Q-band at 694 nm and a Soret band below 500 nm in the spectra of as-deposited spun films is characteristic of the molecular organization of CdPc molecules in a herringbone structure.20 On treatment of the CdPc films with H2S gas, the Q-band underwent blue shift and showed the Davydov split. This characteristic feature is specific to the absorption spectra of a metal free phthalocyanine molecule21 (Fig. 3, curve b). Optical absorption was also measured for identically substituted metal free phthalocyanine (H2Pc), and the striking resemblance between two spectra (Fig. 3, curves b and c) provided further evidence of the loss of the Cd2+ central ion on H2S treatment. Curve d in Fig. 3, which was obtained from the difference between curves b and c, shows the absorption monotonically rising toward wavelengths below 412 nm. The appearance of a shoulder is very similar to the nanometer-sized CdS grown in a polystyrene matrix.22 The CdS phase is believed to be included as a fine intercalation between the H2Pc molecules in the herringbone structure. These CdS dots are, therefore, expected to be uniformly dispersed over the surface of the film. The average particle size was found to be 2–3 nm from the position of the shoulder at approximately 412 nm.23,24 This value is significantly smaller than ones obtained earlier from the AFM images. The discrepancy may be attributed to broadening of the observed features due to a blanket effect of the organic film on top of the nanoparticles and due to the finite size (4 nm) of the AFM tips used. The Raman spectra of the CdPc films on Si substrates are presented in Fig. 4. The modes in the range from 200 cm1 to 1650 cm1 correspond to intramolecular vibrations of the phthalocyanine macrocycles. The detailed assignment of the vibrations has been given in an earlier publication.17 The characteristic spectral transformations in the region
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Fig. 3. Electron absorption spectra of (a) CdPc film before H2S treatment; (b) CdPc film after H2S treatment; (c) H2Pc film; (d) subtracted spectra (b) and (c).
Fig. 4. Raman spectra of (a) CdPc film before H2S treatment; (b) CdPc film after H2S treatment; (c) H2Pc powder.
Fig. 2. AFM tapping mode images of (a) CdPc film before H2S treatment, (b) CdPc film after H2S treatment, and (c) a phase image of the CdPc film after H2S treatment. The phase image has been magnified for clarity.
from 600 cm1 to 1650 cm1 correspond to bending and stretching vibrations of phthalocyanine macrocycles. The comparison of the spectrum with that of H2Pc powder (Fig. 3c) indicates the formation of H2Pc after treatment of CdPc film by exposure to
H2S. The spectral features appearing at 301 cm1 in the CdPc film after the H2S treatment can be associated with the formulation of CdS. The frequency of 1LO (longitudinal optical) phonons is known to be 305 cm1 for bulk CdS.25 The down-frequency shift of the 1LO Raman peak is typical for formation of CdS nanoparticles.26,27 Assuming that the CdS nanoparticles are spherical semiconductor crystals of radius r surrounded by the matrix material, the most important contribution to one-phonon Raman scattering corresponds to frequencies given in the form28,29: l 2 x2n ¼ x2LO b2L n (1) r where xLO is the bulk LO phonon frequency at the Brillouin zone center, bL is a parameter describing the
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dispersion of the LO phonon in the bulk, and ln is the spherical Bessel function of the node n. The substitution of xLO = 305 cm1, bL = 5.04 9 103 ms1, and ln = 2.4 91011 seg/cm for angular momentum l = 0 and n = 1 for the bulk CdS25 into Eq. 1 gives a value of approximately 2.5 nm for the radii of the CdS nanoparticles. This value is in good agreement with ones previously determined from the UV–Vis optical absorption spectra. The particle size is smaller than the quantum size effect and is smaller than the Bohr exciton diameter of 5 nm for CdS.30 No peak associated with free sulfur ion was observed in the Raman spectra.31 It is therefore thought that, when the CdPc film was exposed to H2S, the demetallation of the central Cd ion occurred in accordance with the following possible reaction: CdPc þ H2 S ! CdS þ H2 Pc
(2)
CONCLUSIONS A well-established nanoelectronic-compatible solution processing technique was successfully employed for the solid state formation of CdS particles in a nanoscale thick-film matrix of functionally active organic phthalocyanine macromolecules. Previously demonstrated has been the formation of narrow gap IV–VI semiconductor lead sulfide quantum dots by exposure of thin spun films of a lipophilic lead phthalocyanine derivative to H2S gas.32 The fabrication method requires no involvement of colloidal chemistry for the embedding of inorganic quantum dots into an organic matrix. In addition, this solid state formation gives better control over the dispersion of quantum dots, with nearly no aggregation. This opens up the possibility of the development of an elegant, economically competitive method for the production of a range of hybrid nanocomposites containing inorganic II–VI (CdS, ZnS) and IV–VI (PbS) semiconductor nanoparticles in similarly substituted metal free phthalocyanine matrices. The size-quantized CdS dots may now be suitable for sensitization of phthalocyanine. Depending on the structural behavior of the organic matrix, the development of these nanocomposites may lead to interesting linear and nonlinear spectroscopic effects and the harnessing of electrochromic and photoelectrochromic properties for optical signal processing applications. ACKNOWLEDGEMENT This work was supported by the Royal Society (2005/R2) funding for the International Joint Project. REFERENCES 1. H.T. Lu, C.C. Tsou, and M. Yokoyama, Electrochem. Solid State Lett. 11, J31 (2008).
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