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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.