NANO LETTERS
Atomic Polarization and Local Reactivity on Ferroelectric Surfaces: A New Route toward Complex Nanostructures
2002 Vol. 2, No. 6 589-593
S. V. Kalinin, D. A. Bonnell,* T. Alvarez, X. Lei, Z. Hu, and J. H. Ferris Department of Materials Science and Engineering, The UniVersity of PennsylVania, 3231 Walnut St, Philadelphia PennsylVania 19104
Qi Zhang and S. Dunn Building 70, Nanotechnology, AdVanced Materials, Cranfield UniVersity, Cranfield, Bedfordshire MK43 0AL, UK Received March 18, 2002; Revised Manuscript Received April 19, 2002
ABSTRACT Atomic polarization in ferroelectric compounds is manipulated to control local electronic structure and influence chemical reactivity. Ferroelectric domains are patterned with electron beams or with probe tips, and electron exchange reactions occur preferentially on positive or negative domains. Using photo reduction from aqueous solution, metal nanoparticles are produced in predefined locations on an oxide substrate. Subsequently, organic molecules are reacted selectively to the particles. The process can be repeated to develop complex structures consisting of nanosized elements of semiconductors, metals, or functional organic molecules.
The potential to assemble dissimilar molecular or nanostructural elements into structures with complex functionality has motivated considerable activity in several scientific disciplines. The library of new molecules with electrical, optical, or chemical activity grows quickly and now includes synthetic polypeptides, conjugated organic molecules, carbon and other nanotubes, metallic and semiconducting nanowires, etc.1 In recent advances in assembling nanostructures, complex device functionality has been achieved in only a few select systems.2,3 To realize the potential inherent in nanodevices, methods of assembling a wide range of dissimilar elements, connecting the resulting complex structures, and integrating them into systems must be developed. There is a need for controlling structures with diverse properties. We report here a novel approach that controls local reactivity of ferroelectric surfaces due to variations in atomic polarization. We will demonstrate first that chemical reactivity is domain specific based on control of local electronic structure. We will then show that atomic polarization can be aligned with an electron beam, as well as the local field from a probe tip. When combined with the processes associated with chemical self-assembly, these can be used to selectively position complex nanostructures. This procedure represents an important advance in the ability to * Corresponding author. 10.1021/nl025556u CCC: $22.00 Published on Web 05/03/2002
© 2002 American Chemical Society
link dissimilar nanostructures, in this case oxide substrates, metal nanoparticles, and organic molecules. Ferroelectric solids contain electric dipoles that are intrinsic to the atomic structure of the compound. For example in a cubic perovskite, the displacement of the body center cation in the unit cell produces a dipole in that structure. Dipoledipole interactions between unit cells cause polarization alignment resulting in ferroelectric domains. Polarization discontinuities in the vicinity of surfaces and interfaces result in polarization bound charge that significantly affects the properties of materials. The orientation of polarization can be altered with the application of an electric field. It is expected that interactions on ferroelectric surfaces will be influenced by the domain orientation. Since it has been demonstrated that domain orientation can be controlled to produce 10-20 nm domains, if the fundamental relationship between atomic polarization, charge compensation, and local reactivity can be understood, it could be utilized for the direct assembly of nanostructures. Atomic Polarization and Surface Reactivity. On an oriented perovskite single crystal, such as the BaTiO3 (100) surface, the atomic polarization may be directed perpendicular to the surface in the positive or negative direction or in the plane of the surface. On a randomly oriented surface the polarization vector may have intermediate orientation. In all cases the surface termination is associated with
Table 1: Deposition Conditions for Different Materials (360 W) substrate particles precursor concentration, M particle size, nm mechanical stability chemical stability reaction time, min
PZT gold HAuCl4 0.0001 30 stable in air 20
silver
barium titanate palladium
gold
AgNO3 PdCl2 0.01 0.01 100 15-20 stable with respect to rinsing by water oxidizes stable in air 30 15
silver
HAuCl4 0.00001 15-20
AgNO3 0.01 80 not stable oxidizes 3
stable in air 10
Table 2: Influence of Reaction Time on Particle Morphology (300 W) substrate
Figure 1. Piezoresponse image (a) of the BaTiO3 surface prior to the deposition in which the contrast indicates the orientation of the polarization vector, light is c+ and dark is c-. Topographic structure after silver deposition (b) and after removal of the Ag and subsequent Pd deposition (c). Note that in both cases the metal deposition pattern coincides with the positive domain structure. Schematic diagram of band bending in the paraelectric perovskite above the Curie temperature (d) and in the ferroelectric perovskite in the c- (e) and c+ (f) domain regions. For BaTiO3 and PbTiO3 Ev is the top of the band associated with oxygen 2p orbitals and Ec is the bottom of the band associated with the titanium 3d orbitals.
polarization charge, σ ) P‚n, where P is polarization vector and n is unit normal to the surface. The charge may be compensated by mobile carriers in the solid and/or adsorption on the surface.4 We have shown that molecular adsorption is domain specific by measuring the temperature dependence of surface potential of individual domains after exposure to a combination of molecules.5 We have further demonstrated this by comparing the temperature dependence of molecular desorption from crystals with oriented polarization.6 These results imply that intrinsic screening can be utilized to control local chemical reactivity. Intrinsic screening by mobile charges in a semiconducting ferroelectric involves band bending in the near surface region with the formation of depletion and accumulation layers. Figure 1 illustrates how the atomic polarization direction influences local band bending. This altered electronic structure in the vicinity of the surface dictates which type of carriers are available for surface reactions, consequently chemical and photochemical reactivity is domain specific. This has been demonstrated with Ag deposition on BaTiO3.7,8 We have accomplished domain specific deposition of Ag, Rh, Pd, and Au via photo reduction of aqueous solutions on BaTiO3 and PZT single crystals, polycrystals, and thin films, Tables 1 and 2. Radiation energies are in the range of 3.3 to 4.5 eV, for BaTiO3 and PZT, respectively.9,10 Photochemical 590
PZT
Barium Titanate
Metal
Gold
Palladium
Gold
Palladium
particle size, nm concentration, ppm reaction time, min
30-70 30 5
25-200 70 5
300-800 30 30
75-200 70 30
reactivity is strongly dependent on the semiconducting properties of BaTiO3. Weakly n-doped or intrinsic polycrystalline substrates develop noticeable deposition in ∼1-10 min for 0.01 M solution. Heavily donor doped BaTiO3 is extremely active under the same conditions and can even evolve hydrogen. The solution concentration and reaction time can be varied to control the morphology of the deposited metal. Localized photoreduction produces metal nanoparticles in the range of 3-10 nm that can be sparsely spaced at (Figure 2 a) or closely packed in patterns defined by the underlying domain geometry (Figure 2b). Under more rapid reaction conditions nm sized wires can be produced (Figure 2 c) and somewhat larger particles adopt surface energy dictated geometries (Figure 2d). The mechanism for domain selective photoreduction is closely related to the intrinsic screening on ferroelectric surfaces. In the absence of vacancy or step-edge defects, transition metal oxide surfaces have a low density of surface state in the gap between the conduction band, formed predominantly from the d states of the transition metal, and valence band, formed predominantly from oxygen p states. In regions with negative polarization (c- domains) the effective surface charge becomes more negative and, therefore, upward band bending occurs. In the regions with positive polarization (c+ domains) surface charge is positive, with associated downward band bending. Irradiation with super band gap light results in the formation of an electronhole pair. In ambient conditions, the space charge field results in separation of the electron-hole pair and charge accumulation on the surface, i.e., the photovoltage effect. However, on a surface immersed in a cationic solution the electrons can reduce the metal cations preventing charge accumulation at the surface. Reduction is expected on positive domains, while oxidation is expected at negative domains, in perfect agreement with experimental results. This mechanism is confirmed by a comparison of the sign of the local piezoelectric response11 and the location of the deposited metal nanoparticles, Figure 1. The particles form preferentially at c+ domains. Nano Lett., Vol. 2, No. 6, 2002
Figure 2. Surface topography (a,b) of Ag nanoparticles from photo reduction of aqueous AgNO3. Characteristic particle size as determined from height is ∼5 nm. On the larger length scales, particles are assembled in the lamellar arrays corresponding to the underlying positive ferroelectric domains (b). SEM image of crystalline Ag nanowires occurring at longer reaction times (c). SEM images of larger Au particles from photo reduction of HAuCl4 which appear to be shape determined surface energies (d).
It is important to note that this mechanism of directed assembly differs fundamentally from those that utilize local electrostatic attraction to assemble nanostructures onto templates of patterned charge.12 In these cases the positions of the charges are not pinned therefore the pattern is susceptible to diffusion. On a ferroelectric substrate, local surface charge is due to atomic polarization and therefore is stable. More importantly, since the reaction mechanism involves controlling the surface electronic structure, the reaction product is not limited by the amount of local charge. Patterning Ferroelectric Domains. To exploit atomic polarization to the assembly of complex structures, the local orientation of the domains must be controlled. Local poling of ferroelectric domains has been explored in the context of developing high-density memory storage mechanisms. Ahn et al. have shown that oriented domains as small as 10 nm can be poled using the conducting tip of an AFM.13 The efficacy of this approach for constraining local chemical reactivity is shown in Figure 3, in which atomic polarization orientation is patterned with the local field of an AFM tip and photo reduction of Ag occurs only on c+ domains in the pattern. We have fabricated much more intricate structures with minimum polarization based feature sizes limited by the grain size, ∼100 nm, and minimum metal particle size of 3 nm. While nanofabrication with probe tips is useful in fundamental studies, it is not amenable to large-scale applications. Figure 4 shows that it is also possible to locally reorient polarization with an electron beam. Primary electrons from the beam injected into the surface interact with the solid causing secondary electrons (core electrons, valence elecNano Lett., Vol. 2, No. 6, 2002
Figure 3. Surface topography (a) and piezoresponse image (b) of PZT thin film. The inset shows that the PFM contrast is not random but is due to the small (∼50-100 nm) ferroelectric domains associated with grains. PFM image (c) of lines patterned with alternating +10 and -10 Vdc. Surface topography (d) after deposition of Ag nanoparticles. Note one-to-one correspondence between tip-induced polarization distribution and metal deposition pattern. The features consist of closely packed metal nanoparticles of 3-10 nm. Piezoresponse image of checkerboard domain structure fabricated using in-house lithographic system (e) and SEM image of corresponding silver photodeposition pattern (f).
Figure 4. Piezoresponse image of PZT surface exposed to an e-beam (a). 600 s exposure at 20 kV results in formation of positive polarization orientation (light contrast in the center), which is the consequence of negative surface charge. Short time (
