Interactions on Ferroelectric Materials
- Molecular adsorption on barium titanate
- Polarization dependence of physisorption on ferroelectric substrates
- Evolution of Structure and Thermodynamic Stability of the BaTiO3(001)
- Polarization dependence of molecular adsorption on lithium niobate
- Multi-Component Functional Nanostructures
The surface chemistry of single crystal barium titanate (BaTiO3) has been studied using temperature programmed desorption (TPD). TPD measurements were performed with several probe molecules, including methanol and carbon dioxide. The role of oxygen vacancies in the adsorption and reaction of these molecules was examined by annealing the crystal under oxidizing or reducing conditions prior to performing TPD. It is shown that the adsorption and reaction of methanol and carbon dioxide is enhanced on BaTiO3(001) by annealing the crystal under reducing conditions.
The ability to manipulate dipole orientation in ferroelectric oxides holds promise as a method to tailor surface reactivity for specific applications. As ferroelectric domains can be patterned at the nanoscale, domain-specific surface chemistries may provide a method for fabrication of nanoscale devices. Although studies over the past 50 years have suggested that ferroelectric domain orientation may affect the energetics of adsorption, definitive evidence is still lacking. Domain-dependent sticking coefficients are observed using temperature programmed desorption (TPD) and scanning surface potential microscopy (SSPM), supported by first-principles calculations of the reaction coordinate. The first unambiguous observations of differences in the energetics of physisorption on ferroelectric domains are presented here for CH3OH and CO2 on BaTiO3 and Pb(Ti0.52Zr0.48)O3 surfaces.
TPD on BaTiO3 thin film with in situ polarization control
|CH3OH and CH2O desorption signals for a series of TPD runs where the polarization was changed in the following order: c+, unpoled, c−.|
Precursor-mediated adsorption on defective ferroelectric oxide surfaces
|The gas-phase molecule physisorbs to the oxide surface, where it diffuses until reaching and chemisorbing at an O vacancy, or eventually desorbing.|
|Schematic diagram of the potential energy as a function of the distance from the surface.|
We report a series of new surface reconstructions on BaTiO3(001) as a function of environmental conditions, determined via scanning tunneling microscopy and low energy electron diffraction. Using density functional theory calculations and thermodynamic modeling, we construct a surface phase diagram and determine the atomic structures of the thermodynamically stable phases. Excellent agreement is found between the predicted phase diagram and experiment. The results enable prediction of surface structures and properties under the entire range of accessible environmental conditions.
Water and methanol temperature programmed desorption (TPD) measurements were performed on the positive (c+) and negative (c-) surfaces of poled ferroelectric lithium niobate (LiNbO33) single crystals. The results indicate that the molecule-surface interactions are both coverage and polarization dependent. From a comparison of the TPD spectra for the positive and negative surfaces, it is shown that the desorption temperatures of water and methanol are consistently lower on the negative surface by 15 K and 20 K, respectively. The TPD spectra were simulated using the Polanyi-Wigner equation with a coverage-dependent energy term. These calculations show that the polarization dependence of the desorption temperature is due to a difference in the zero-coverage desorption energies on the two surfaces equal to a few kJ per mole. The mechanism for the polarization effect is explored with in situ pyroelectric voltage measurements, which indicate that a surface voltage of ±2 mV develops in the LiNbO3(0001) samples during TPD measurements. The magnitude of the pyroelectric-induced surface charge is heating rate dependent.
Comparison of water TPD on the positive and negative surfaces
|Water sticking coefficient at 155 K is the same for both surfaces.|
|Desorption temperature is 15 K lower on the negative surface.|
|TPD measurements performed over a range of initial coverages.|
Comparison of methanol TPD on the positive and negative surfaces
|Methanol sticking coefficient at 155 K is the same for both surfaces.|
|Desorption temperature is 20 K lower on the negative surface.|
|TPD measurements performed over a range of initial coverages.|
Pyroelectric measurements and the electrostatic polarization effect
Ferroelectric nanolithography exploits polarization dependent surface interactions to pattern nanoparticles, but the factors that control the particle size and distribution are not sufficiently well understood to produce hybrid nanostructures. Here the effects of photon energy, photon flux, and polarization vector orientation on ferroelectric domain specific photoreactions are quantified, leading to an understanding of the nanoparticle deposition mechanism. Patterned nanoparticle arrays functionalized with optically active porphyrin complexes are configured into optoelectronic devices.
Fig. a illustrates SPM polarization patterning schematically, while Fig. b shows the relationship between the surface potential and the orientation of the domains. Fig. c compares the topographic structure and the surface potential after a pattern is poled with a biased tip. Fig. d and Fig. e are examples of domain induced production of silver and gold nanoparticles on patterned PZT surfaces.
The reactions associated with the AuNPs formation are seen in the equations here. Fig. f and g show the influence of the FE domains and photon flux of the AuNPs deposition respectively. Fig. h shows the reaction process on the surface schematically. Initial reduction occurs in the absence of a barrier for electron transfer. As the nanoparticles increase in size, an interface barrier forms and causes an electron depletion region below the particle, indicated by the shaded region. The reaction will stop (the nanoparticle will stop growing) once the interfacial barrier is large enough that the depletion depth is larger than the tunneling distance.
We take advantage of FE lithography here to fabricate NP devices. Here a schematic diagram of a patterned hybrid nanostructure device and the structure of dithiol-terminated meso-to-meso ethyne-bridged tris[(porphinato)zinc(II)] supermolecule (dithiol-PZn3) is shown in Fig. i. Fig j and k are comparisons of dark current and photoconduction with 533 nm (green) and 655 nm (red) light for two devices. Enhancement of photocurrent compared to the dark current are shown in Fig. l. The photocurrent in this device arises predominantly from the interaction of the transverse plasmon induced current.