Optical quantification of interfacial charge states
Methods of generating second harmonic generation (SHG) signals from interfaces formed with, or formed over, a noncentrosymmetric material, e.g., α-quartz, are provided. The methods make use of the noncentrosymmetric material as an internal phase reference for the determination of a variety of interfacial electrostatic parameters, including interfacial potential, interfacial charge density, and the sign of the interfacial charge (i.e., net positive or net negative).
1. A method of probing an interface, the method comprising:
(a) illuminating an interface formed between a noncentrosymmetric material having a selected orientation angle with respect to a reference axis and a different material, or formed between two different materials and disposed over the noncentrosymmetric material, with light having a frequency ωunder conditions to generate a second harmonic generation (SHG) signal having frequency 2ω, and
(b) detecting the SHG signal, the SHG signal comprising a bulk second harmonic signal from the noncentrosymmetric material and an interfacial second harmonic signal from the interface.
2. The method of claim 1 , further comprising (c), comparing the detected SHG signal to a SHG signal generated from a reference interface formed between the noncentrosymmetric material and a reference material at the selected orientation angle.
3. The method of claim 1 , wherein the noncentrosymmetric material is α-quartz.
4. The method of claim 1 , wherein the noncentrosymmetric material is provided as a substrate composed entirely of the noncentrosymmetric material.
5. The method of claim 1 , wherein the noncentrosymmetric material is provided as a film on a supporting substrate.
6. The method of claim 5 , wherein the supporting substrate comprises a centrosymmetric material.
7. The method of claim 1 , wherein the different material, or one of the two different materials, comprises water.
8. The method of claim 1 , wherein the interface is the interface formed between the two different materials.
9. The method of claim 8 , wherein one of the two different materials comprises water.
10. The method of claim 8 , wherein one of the two different materials is a film formed on the noncentrosymmetric material.
11. The method of claim 10 , wherein the other of the two different materials comprises water.
12. The method of claim 1 , wherein the light is non-resonant light.
13. The method of claim 1 , wherein step (a) is carried out using an external reflection geometry.
14. The method of claim 1 , further comprising repeating steps (a) and (b) at a second, different orientation angle.
15. The method of claim 1 , wherein the different material, or one of the two different materials, comprises water, and the method further comprises carrying out steps (a) and (b) under conditions of changing ionic strength, changing pH, or both.