J. Phys. Chem. C 2010, 114, 14427–14432
14427
GISAXS Characterization of Order in Hexagonal Monolayers of FePt Nanocrystals Andrew T. Heitsch,† Reken N. Patel,† Brian W. Goodfellow,† Detlef-M. Smilgies,‡ and Brian A. Korgel*,† Department of Chemical Engineering, Texas Materials Institute, Center for Nano- and Molecular Science and Technology, The UniVersity of Texas at Austin, Austin, Texas 78712-1062, and Cornell High Energy Synchrotron Source (CHESS), Cornell UniVersity, Ithaca, New York 14853 ReceiVed: May 26, 2010; ReVised Manuscript ReceiVed: July 15, 2010
Hexagonally close-packed monolayers of FePt nanocrystals were deposited on silicon substrates by Langmuir-Blodgett, Langmuir-Scha¨fer, or spin-coating techniques and characterized by scanning electron microscopy (SEM) and grazing incidence small-angle X-ray scattering (GISAXS). The extent of order in each nanocrystal film is quantified based on the GISAXS data and compared. The LB technique provided the most ordered superlattice films. Introduction Colloidal nanocrystals are being explored as building blocks to create new materials with unique electrical, mechanical, and optical properties.1-7 The properties of these nanocrystal-based metamaterials depend on the size-dependent properties of the individual particles as well as how they are organized, and methods for assembling nanocrystals into ordered structures covering extended surfaces are still being developed and improved.2,8-12 The structure of nanocrystal assemblies is typically characterized by transmission and scanning electron microscopy (TEM and SEM), selected area electron diffraction (SAED), and small-angle X-ray scattering (SAXS). TEM, SEM, and SAED provide a relatively localized view of the structural organization of the nanocrystals, whereas SAXS provides a more extensive (averaged) characterization of the sample. TEM, SEM, and SAXS are typically used to determine structural order in the nanocrystal assemblies but are generally not used to quantify the range of order. As better methods are developed for nanocrystal assembly, quantitative analyses of these data are needed. SAXS is widely used to characterize the mesoscopic structure of polymers, complex fluids, and colloidal crystals,13,14 including nanocrystal superlattices.15-18 Enhanced sensitivity to the sample surface and structural details about the orientation of crystallographic domains with respect to the substrate can be achieved by conducting SAXS measurements with the incident beam in a grazing incidence configurationsa technique called GISAXS. GISAXS provides long X-ray interaction path lengths that enable uniformity and structural order of very thin (∼nm’s) layers over extended substrate areas (mm2) to be measured.19,20 In comparison to SAXS measurements conducted in a transmission mode, GISAXS provides detailed information about order in the films both parallel and perpendicular to the substrate. GISAXS has been used to examine the structure of thin films and coatings,21-23 block copolymers,24,25 and nanocrystal assemblies.26-35 Lazzari has published a quantitative model to describe GISAXS data of nanostructured materials supported on substrates.36 Herein, we apply a simplified GISAXS model * Corresponding author: Tel +1-512-471-5633; Fax +1-512-471-7060; e-mail
[email protected]. † The University of Texas at Austin. ‡ Cornell University.
to the experimental data of FePt nanocrystal monolayers to quantify the range of order within the films. FePt nanocrystal films were deposited on silicon substrates by Langmuir-Blodgett (LB), Langmuir-Scha¨fer (LS) PDMS stamp transfer, or spin-coating techniques and studied by SEM and GISAXS. The order of the monolayers obtained by the three deposition methods are compared based on a quantitative analysis of the GISAXS data. LB deposition provided the most uniform and extended order of the three techniques, followed by LS deposition. LB and LS depositions were both significantly better than the spin-coating but require much more processing time. Experimental Methods Chemicals. Platinum(II) acetylacetonate (Pt(acac)2, 97%), iron pentacarbonyl (Fe(CO)5, 99.999%), oleylamine (70%), oleic acid (90%), ethanol (ACS grade), chloroform (ACS grade), acetone (ACS grade), cyclohexane (ACS grade), isopropanol (ACS grade), and hexanes (ACS grade) were purchased from Sigma-Aldrich. Phenyl ether (g98%) was purchased from Fluka. Polydimethylsiloxane (PDMS, Dow Corning, Sylgard 184) was purchased from EIS Inc. (Tridecafluoro-1,1,2,2-tetrahydrooctyl)1-trichlorosilane was purchased from Gelest. Doubly distilled deionized water (DI-H2O) was used in all preparations. All chemicals were used as received. FePt Nanocrystal Synthesis. FePt nanocrystals were synthesized using a modification of a previously published procedure.37 On a greaseless Schlenk line under inert Ar atmosphere, 0.197 g (0.50 mmol) of Pt(acac)2 was added to 10 mL of phenyl ether in a 50 mL three-neck flask and stirred at 600 rpm. The solution was degassed under vacuum (
