<< back

Interactions on Ferroelectric Materials

 

Molecular adsorption on barium titanate

Summary of TPD dataThe 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.

1) Methanol TPD measurements   2) Carbon dioxide TPD measurements
figure 2   figure3
Methanol desorbs molecularly and also decomposes to form formaldehyde, water, and carbon monoxide.  3 TPD measurements are shown here, corresponding to oxygen-annealed (O1 and O2) and vacuum-annealed (V) surfaces.  The desorption spectra of each species are plotted separately.  The intensities of the desorption signals are greatest for the vacuum-annealed surface, indicating that oxygen vacancy defects are the active sites for methanol adsorption and reaction on BaTiO3(001) at room temperature.   CO2 desorbs molecularly, producing two peaks at 360 K and 720 K.  The intensity of the high temperature peak is significantly greater on the vacuum-annealed surface (V) as compared with the oxygen-annealed surface (O2).  The high temperature desorption state corresponds to CO2 adsorbed at oxygen vacancy sites, and the low temperature state may correspond to adsorption at step edges or oxygen surface sites.

<< back

Polarization dependence of physisorption on ferroelectric substrates

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

figure 1 CH3OH and CH2O desorption signals for a series of TPD runs where the polarization was changed in the following order: c+, unpoled, c.

Influence of CO2 adsorption on surface potential of BaTiO3(001) and PZT thin films

figure 2

Precursor-mediated adsorption on defective ferroelectric oxide surfaces

figure 3a The gas-phase molecule physisorbs to the oxide surface, where it diffuses until reaching and chemisorbing at an O vacancy, or eventually desorbing.
figure 3b Schematic diagram of the potential energy as a function of the distance from the surface.

<< back

Evolution of Structure and Thermodynamic Stability of the BaTiO3(001)

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.

figure


<< back

Polarization dependence of molecular adsorption on lithium niobate

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

figure 1 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

figure 2 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

figure 3
Pyroelectric voltage measurements on the positive and negative surfaces in UHV. The sign of the pyroelectric voltage is opposite that of the surface polarization charge. The surface voltage (VS) and the sample temperature (T) are indicated by the bold blue lines and the thin black lines, respectively. The sample voltage is heating rate dependent, and VS = 2 mV per K/s.
figure 4 Schematic diagram of an electrostatic polarization effect to explain the difference in desorption temperatures on the positive and negative surfaces. Water molecules adsorb at cation sites. Pyroelectric-induced surface charge exerts an electrostatic force (FE) on the H atoms. FE is repulsive on the negative surface and attractive on the positive surface. A similar model also applies to methanol, which has more H atoms than water and exhibits a greater change in desorption temperature with polarization.


<< back

Multi-Component Functional Nanostructures

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.

figure

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.

figure

figure

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.

 

figure

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.

 

<< back