Thermo-optical measurements using quantitative phase microscopy.

Quantitative phase microscopy (QPM) literally images the quantitative phase shift associated with image contrast, where the phase shift can be altered by laser heating. In this study, the thermal conductivity and thermo-optic coefficient (TOC) of a transparent substrate are simultaneously determined by measuring the phase difference induced by an external heating laser using a QPM setup. The substrates are coated with a 50-nm-thick titanium nitride film to photothermally generate heat. Then, the phase difference is semi-analytically modeled based on the heat transfer and thermo-optic effect to simultaneously extract the thermal conductivity and TOC. The measured thermal conductivity and TOC agree reasonably well, indicating the potential for measuring the thermal conductivities and TOCs of other transparent substrates. The concise setup and simple modeling differentiate the advantages of our method from other techniques.

Direct Observation of Photoinduced Charge Separation at Transition-Metal Nitride-Semiconductor Interfaces.

Optically excited hot carriers from metallic nanostructures forming metal-semiconductor heterostructures are advantageous for enhancing photoelectric conversion in the sub-band gap photon energy regime. Plasmonic gold has been widely used for hot carrier excitation, but recent works have demonstrated that plasmonic transition-metal nitrides have higher efficiencies in injecting hot electrons to adjacent n-type semiconductors and are more cost-effective. To collect direct evidence of hot carrier excitation from nanostructures, imaging of hot carriers is essential. In this work, photoexcited Kelvin probe force microscopy (KPFM) is used to image hot carriers excited in transition-metal nitride nanostructures forming heterostructures with semiconductors. Among available transition-metal nitrides, we select zirconium nitride (ZrN) for this study. Additionally, both p-type and n-type titanium dioxides (TiO2) are selected to study the transport of hot holes and hot electrons. The KPFM results indicate that for ZrN and p-type TiO2 heterostructures, hot holes are injected into the p-type TiO2 across the Schottky contact. In the case of ZrN and n-type TiO2 heterostructures, hot electrons are injected into the n-type TiO2 across the ohmic contact. Because transition-metal nitrides are known to be more effective than gold at injecting hot carriers into adjacent semiconductors, unambiguously determining the mechanisms of hot carrier transportation of transition-metal nitrides using photoexcited KPFM will facilitate additional studies on hot carrier applications with transition-metal nitrides.