Mater. Res. Soc. Symp. Proc. Vol Materials Research Society

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1 Mater. Res. Soc. Symp. Proc. Vol Materials Research Society Q CoPt 3 nanoparticles adsorbed on SiO 2 : a GISAXS and SEM study Jan I. Flege 1, Thomas Schmidt 1, Gabriela Alexe 1, Torben Clausen 1, Sigrid Bernstorff 2, Igor Randjelovic 3, Vesna Aleksandrovic 3, Andreas Kornowski 3, Horst Weller 3 and Jens Falta 1 1 Institute of Solid State Physics, University of Bremen, Otto-Hahn-Allee, Bremen, Germany 2 Sincrotrone Trieste, Strada Statale 14, km 163.5, Basovizza / Trieste, Italy 3 Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, Hamburg, Germany ABSTRACT Ultra-thin CoPt 3 nanoparticle films have been prepared on SiO 2 surfaces using a Langmuir-Blodgett (LB) deposition technique. The structural properties of the overlayers have been investigated by grazing-incidence small-angle x-ray scattering (GISAXS) and high-resolution scanning electron microscopy (SEM) for the first time. Self-assembly of the nanoparticles is found and with GISAXS an average particle-particle distance of (8.23 ± 0.06) nm is determined, in good agreement with the SEM results. A particle correlation length of (22.3 ± 1.2) nm was derived which is shown to be independent of the surface coverage. The latter quantity may be controlled by choice of a suitable retraction speed during the LB step. INTRODUCTION The preparation of ordered arrays of nanoparticles has attracted a tremendously increasing interest in the last years due to their wide range of potential applications e. g. in opto-electronics, ultra-dense magnetic storage devices and catalysis [1]. In this respect, the bottom-up concept of two-dimensional self-assembly of ex-situ prepared quantum dots from colloidal solutions with well-defined electronic properties is very promising for industrial use. In comparison to the alternative top-down approach which comprises a lithographic patterning process in which the required lateral resolution has to be enhanced to the near-atomic scale, using self-organization would allow the manufacturing of highly-integrated devices at reasonable costs. However, for an efficient and optimized fabrication of dense nanoparticle films a thorough structural characterization of the prepared overlayers as function of the parameters governing the preparation process is necessary. Here, we present a first grazing incidence small-angle x-ray scattering (GISAXS) study of CoPt 3 nanoparticles of about 5 nm in diameter which have been adsorbed on silicon wafers covered by native SiO 2 buffer layers using Langmuir-Blodgett techniques. Additionally, micro and mesoscale features of the film morphology have been investigated in real space with scanning electron microscopy (SEM). EXPERIMENTAL DETAILS Preceding the deposition of CoPt 3 nanoparticle films on native SiO 2, the nanoparticles were synthesized via the simultaneous reduction of Pt(acac) 2 and thermal decomposition of Co 2 (CO) 8

2 Q in a high-boiling coordinating solvent mixture of hexadecylamine-diphenyl ether, as described elsewhere [2, 3]. For a given set of preparational parameters, this procedure results in the formation of monodisperse (standard deviation of size distribution function between 5 10%) spherical CoPt 3 nanoparticles with adamantane carboxylic acid (ACA) and hexadecylamine as ligands of which the ACA molecules were exchanged to sorbic acid (SA) [4]. In the case presented here, particles with a size of 5.2 nm were grown as determined by x-ray powder diffraction [3]. After the redissolution of the nanocrystal precipitate in a non-polar solvent the colloids were deposited using a commercial Langmuir-Blodgett trough by retracting the substrate from the compressed monolayer [5, 6]. In the course of the present study, the retraction speed was varied in order to deduce its influence on the structural properties of the overlayer, as will be discussed below. The GISAXS experiments were performed at the SAXS beamline 5.2L [7] located at the third generation synchrotron light source ELETTRA, Trieste, Italy. Monochromatic x rays with an energy of 8 kev were irradiated on the sample at a grazing angle of α i 0.6 well above the critical angle of α c = 0.22, and the two-dimensional scattering pattern was recorded using an x-ray charge-coupled device (CCD) camera mounted at a sample-detector distance of 1.08 m. The q space calibration was achieved by the analysis of the scattering pattern of silver behenate, a commonly employed reference material [8]. Due to the much higher scattering intensity at q = 0 because of the specularly reflected beam and the diffuse reflectivity in comparison to the GISAXS signal at finite values for q, an aluminum absorber foil was used as attenuator in order to optimally match the measured intensity with the given dynamic range of the CCD detector. Real-space microscopic images of the film morphology were obtained using a commercial FEG high-resolution scanning electron microscope (LEO 1550, Gemini type) which has a resolution limit of about 1 nm. DISCUSSION In Figure 1(a), a 2D GISAXS scattering image is shown for a sample which was prepared by Langmuir-Blodgett coating employing a retraction speed of v = 1mm/min. The Al attenuator thickness was chosen in such a way that the intensities of the specularly reflected beam and the so-called Yoneda peak [9] match the intensity of the features found at q = ±0.76 nm 1 on which we will concentrate in the following. These features are a direct evidence for a lateral correlation of the CoPt 3 nanoparticles adsorbed on the SiO 2 surface. Moreover, the shape of the scattering features under consideration is rod-like with decreasing intensity in the +q z direction which is compatible with the model of a monolayer of nanoparticles. For a comprehensive description of the scattering phenomena close to the critical angle α c of x-ray reflection, a thorough theoretical treatment within the distorted-wave Born approximation is required, for a review see e. g. [10] and references therein. Recently, Rauscher et al.have extended this general approach to the case of supported islands [11]. However, for incident and exit angles α i and α e much greater than α c, theoretical simulations according to the full DWBA theory [12] and within the framework of the simple Born approximation yield that the latter may be applied. Hence, the lateral distance between the non-specular first-order scattering satellites q is directly related to the average distance d of the scattering objects, i. e. the CoPt 3 nanoparticles adsorbed on the surface, as given by d = (4π)/ q. Furthermore, details about the

3 Q q [nm 1 ] specular beam Yoneda peak 0.5 satellite satellite 100 nm q ll [nm 1 ] (a) (b) Figure 1: (a) GISAXS image of CoPt 3 nanoparticles adsorbed on SiO 2 /Si(001) by Langmuir- Blodgett coating taken at an incident angle of α i 0.6 using 8 kev photons. The vertical black stripe indicates the position of the semi-transparent Al absorber. (b) High-resolution SEM image for the same sample as investigated in (a). Nanoparticles appear bright in contrast. ordering properties may be extracted from the full width at half maximum δq of the first-order scattering peaks by defining a correlation length ξ (2π)/δq [13]. At this point it should be pointed out that for the sample systems under investigation the convolution of the experimentally obtained GISAXS pattern due to the finite lateral slit size may be neglected; nevertheless, for highly correlated overlayers of monodisperse particles deconvolution algorithms [14] have to be used. The analysis of the data presented in Figure 1(a) according to the simple Born approximation yield an average particle-particle distance of d = (8.23 ± 0.06) nm and a correlation length of ξ = (22.3 ± 1.2) nm, respectively. In Figure 1(b) a high-resolution SEM image of the same sample is displayed. Qualitatively, the particles form a defective close-packed assembly. In accordance with the determined value for the correlation length, long-range order is absent. By calculating the autocorrelation function of the SEM data an average particle-particle distance of approx. 7.8 ± 0.5 nm could be derived, in good agreement with the GISAXS result. In the close-up view shown, a fraction of the sample surface is shown in which almost the complete substrate is covered by the nanoparticle film. However, this is not representative for the whole sample surface as will be discussed in the following paragraph. In Figure 2 a series of GISAXS and large-scale SEM images for CoPt 3 -SA adsorbed on SiO 2 using different retraction speeds v is presented. The analysis of the GISAXS images yields virtually identical particle-particle distances d of (8.23 ± 0.06) nm, (8.21 ± 0.08) nm and (8.23 ± 0.15) nm, respectively. Since the corresponding correlation lengths only vary between (22.3 ± 1.2) nm and (23.7 ± 4.9) nm which lie within the error bar, the ordering properties of the nanoparticle films may be regarded as independent of the retraction speed. Hence, the particle ordering on the surface cannot be improved by choosing a slow retraction speed. These findings

4 Q GISAXS SEM (a) 30 µm (b) 30 µm (c) 30 µm Figure 2: GISAXS scattering patterns and large-scale SEM images of CoPt 3 nanoparticles adsorbed on SiO 2 /Si(001) by Langmuir-Blodgett techniques using different retraction speeds v. The GISAXS data were taken at an incident angle of α i 0.6 using 8 kev photons with identical exposures. (a): v = 1mm/min. (b): v = 5mm/min. (c): v = 10 mm/min. The nanoparticle islands appear bright in the SEM images. suggest that the particle order on the substrate surface is already pre-defined by the self-organization of the nanoparticles in the compressed monolayer floating at the top surface of the Langmuir-Blodgett trough. However, the satellite peak intensity I norm tremendously decreases as a function of the retraction speed, as can be seen in Table 1. Therefore, it is concluded that the change in intensity is directly related to a change in the overall nanoparticle coverage. This interpretation is justified by inspection of the corresponding series of SEM images as given in Figure 2. From these, a steep decrease in surface coverage with increasing retraction speed is observed. Interestingly, even for the slowest retraction speed employed in the sample preparation no fully coherent film could be produced. This result indicates that even higher coverages may be obtained by further reducing the retraction speed below 1 mm/min.

5 Q Sample v [mm/min] d [nm] ξ [nm] I norm [a. u.] a ± ± b ± ± c ± ± Table 1: Structural parameters derived from GISAXS data as functions of the retraction speed v for CoPt 3 nanoparticles adsorbed on SiO 2 by Langmuir-Blodgett coating. CONCLUSIONS We have shown that ordered CoPt 3 nanoparticle films with sorbic acid as ligands may be prepared on native SiO 2 substrates using Langmuir-Blodgett techniques. The structural properties of the overlayers have been characterized in reciprocal and real space with GISAXS and SEM. The GISAXS analysis revealed particle-particle distances of (8.23 ± 0.06) nm, in good agreement with the evaluation of the SEM data which gave a slightly smaller value of (7.8 ± 0.5) nm. This distance as well as the correlation length of (22.3 ± 1.2) nm are preserved even if the retraction speed during the Langmuir-Blodgett process is varied by an order of magnitude. However, it could be shown by a quantitative analysis of the GISAXS peak intensities that the nanoparticle coverage may directly be controlled by the retraction speed. Hence, the combination of GISAXS and SEM as complimentary methods allows a qualitative and quantitative structural characterization of ultra-thin nanoparticle overlayers with high resolution which is a prerequisite for the optimized fabrication of high-quality nanoparticle-based devices. REFERENCES [1] Nanoparticles: From Theory to Application, edited by G. Schmid (Wiley-VCH Verlag, Weinheim, 2004). [2] E. V. Shevchenko, D. V. Talapin, A. L. Rogach, A. Kornowski, M. Haase, and H. Weller, J. Am. Chem. Soc. 124, (2003). [3] E. V. Shevchenko, D. V. Talapin, H. Schnablegger, A. Kornowski, O. Festin, P. Svedlindh, M. Haase, and H. Weller, J. Am. Chem. Soc. 125, 9090 (2003). [4] V. Aleksandrovic et al.,inprep. [5] M. Achermann, M. A. Petruska, S. A. Crooker, and V. I. Klimov, J. Phys. Chem. B 107, (2003). [6] F. C. Meldrum, N. A. Kotov, and J. H. Fendler, Langmuir 10, 2035 (1994). [7] H. Amenitsch, M. Rappolt, M. Kriechbaum, H. Mio, P. Laggner, and S. Bernstorff, J. Synchrotron Rad. 5, 506 (1998). [8] T. Huang, H. Toraya, T. Blanton, and Y. Wu, J. Appl. Cryst. 26, 180 (1993).

6 Q [9] Y. Yoneda, Phys. Rev. 131, 2010 (1963). [10] S. Dietrich and A. Haase, Phys. Rep. 260, 1 (1995). [11] M. Rauscher, R. Paniago, H. Metzger, Z. Kovats, J. Domke, H. D. Pfannes, J. Schulze, and I. Eisele, J. Appl. Phys. 86, 6763 (1999). [12] R. Lazarri, J. Appl. Cryst. 35, 406 (2002). [13] A. Guinier, X-Ray Diffraction (Freeman, San Francisco, 1968). [14] G. Glatter and O. Kratky, Small-Angle X-ray Scattering (Academic Press, New York, 1982).