Diphenylalanine (FF) peptide nanotubes (PNTs) represent a unique class of self-assembled functional biomaterials owing to a wide range of useful properties including nanostructural variability, mechanical rigidity and chemical stability. In addition, strong piezoelectric activity has recently been observed paving the way to their use as nanoscale sensors and actuators. In this work, we fabricated both horizontal and vertical FF PNTs and examined their optical second harmonic generation and local piezoresponse as a function of temperature. The measurements show a gradual decrease in polarization with increasing temperature accompanied by an irreversible phase transition into another crystalline phase at about 140–150 °C. The results are corroborated by the molecular dynamic simulations predicting an order–disorder phase transition into a centrosymmetric (possibly, orthorhombic) phase with antiparallel polarization orientation in neighbouring FF rings. Partial piezoresponse hysteresis indicates incomplete polarization switching due to the high coercive field in FF PNTs.

The mechanism of dc-electric-field-induced second-harmonic generation (EISH) was studied at the buried Si(111)-${\mathrm{SiO}}_2$ interface in transmission through a planar Si-${\mathrm{SiO}}_2$-Cr MOS structure. The second-harmonic contribution of the field-induced quadratic polarization generated in the space-charge region is determined. The role of the spatial distribution of the dc electric field inside the silicon space-charge region is demonstrated, as well as the influence of the oxide thickness. We have developed a phenomenological model of the EISH taking into account the interference between field-dependent and field-independent contributions to the nonlinear polarization (nonlinear interference) as well as the retardation of the EISH wave. We show that, due to these interference effects, the minima of the EISH curves do not coincide with the flatband voltage. © 1996 The American Physical Society.

The mechanism of dc-electric-field-induced second-harmonic (EFISH) generation at weakly nonlinear buried $\mathrm{Si}\mathrm{}\left(001\right)-{\mathrm{SiO}}_2$ interfaces is studied experimentally in planar $\mathrm{Si}\mathrm{}\left(001\right)-{\mathrm{SiO}}_2-\mathrm{Cr}$ MOS structures by optical second-harmonic generation spectroscopy with a tunable Ti:sapphire femtosecond laser. The spectral dependence of the EFISH contribution near the direct two-photon $E_1$ transition of silicon is extracted. A systematic phenomenological model of the EFISH phenomenon, including a detailed description of the space-charge region (SCR) at the semiconductor-dielectric interface in accumulation, depletion, and inversion regimes, has been developed. The influence of surface quantization effects, interface states, charge traps in the oxide layer, doping concentration, and oxide thickness on nonlocal screening of the dc-electric field and on breaking of inversion symmetry in the SCR is considered. The model describes EFISH generation in the SCR using a Green’s-function formalism which takes into account all retardation and absorption effects of the fundamental and second-harmonic (SH) waves, and multiple reflection interference in the ${\mathrm{SiO}}_2$ layer. The optical interference between field-dependent and -independent contributions to the SH field is considered as an internal homodyne amplifier of the EFISH effects. Good agreement between the phenomenological model and our EFISH spectroscopic results is demonstrated. Finally, low-frequency electromodulated EFISH is demonstrated as a useful differential spectroscopic technique for studies of the $\mathrm{Si}-{\mathrm{SiO}}_2$ interface in silicon-based metal-oxide-semiconductor structures.