(Invited) Optomechanical Nanoprobes in the Quest for Novel Nanoscale Electronic and Photonic Devices
Authors: Fanchini Giovanni 
Published: December 2024

In this talk we will review the efforts from our group in the development of nanoptical, nanomechanical and integrated nano-optomechanical probes that can be utilized for the purpose of developing novel nanoscale electronic and photonic devices, and related materials.[1] For example, there are very few examples of techniques based on atomic force microscopy (AFM) and scanning tumnneling microscopy (STM) capable of acquiring, with nanoscale lateral resolution, quantitative maps of the photocurrent of a thin-film solar cell, or the thermal conductivity, thermal expansivity, and nanomechanical expansivity of nanoscale heat spreaders based on 2D materials and nanostructured thin films, despite huge demand for nanoscale thermal management, for example in designing integrated circuitry for power electronics.Our methods based on scanning near-field optical microscopy (SNOM) and scanning confocal optical microscopy (SCOM) are designed to satisfy such a demand.[1] Near-field optics is a powerful technique capable of achieving nanometer-scale optical resolution beyond the diffraction limit by using evanescent waves, which is uniquely poised to be integrated into AFM and STM probes.

We will introduce ω-ω and ω-2ω near-field thermoreflectance imaging as all-optical and contactless approaches to map the thermal conductivity [2] and thermal expansion [3] coefficents at the nanoscale with precision. Testing of our techniques is performed on nanogranular films of gold and multilayer graphene (ML-G) platelets. As a case study, our recently invented ω-ω near-field scanning thermoreflectance imaging (NeSTRI) technique is here applied to multilayer graphene thin films on glass substrates. Thermal conductivity of micrometre-size multilayer graphene platelets is determined and is consistent with previous macroscopic predictions. As far as thermal expansion coefficient (TEC) is concerned, our method demonstrates that the TEC of ML-G is (-5.77±3.79) x10-6 K-1 and is assigned to in-plane vibrational bending modes.[3] A vibrational-thermal transition from graphene to graphite is observed, where the TEC becomes positive as the ML thickness increases. Basically, our nanoscale method demonstrates results in excellent agreement with its macroscopic counterparts, as well as superior capabilities to probe 2D materials and interfaces.

As far as novel photonics probes are concerned, we will present [4] a novel method capable of profiling the carrier mobility along the z axis in thin-film photovoltaics. Our setup is based on the integration of photogenerated charge extraction by linearly increasing voltage (p-CELIV) with a scanning confocal optical microscope (SCOM) toward a confocal and cross-sectional p-CELIV (cs-p-CELIV) system. As monomolecular recombination of excess carriers is the most frequent radiative pathway for electrons and holes in solar cells at low power density of illumination, while multimolecular recombination dominates at high power, enhanced multimolecular recombination occurs at the SCOM focal plane. Thus, the cs-p-CELIV signal provides enhanced information on the mobility of all of the cross-sectional layers except the focal plane. By scanning the focal plane along the z axis, the mobility profile can be derived. To demonstrate our technique, we use it to investigate the carrier mobility in hydrogenated amorphous silicon (a-Si:H) solar cells. The mobility profiles obtained by cs-p-CELIV correlate well with well-known depletion layer effects and the H content profile in a-Si:H, which is measured independently.

Collectively our methods well represent an excellent demonstration of the possibility to integrate near-field optics and scanning confocal optical microscopy towards the operando investigation of novel nanoscale electronic and photonic devices.

[1] P. Bazylewski et al (Fanchini) Appl. Sci. 7, 973 (2017)

[2] S. Ezugwu (Fanchini) Nanoscale, 9, 4097 (2017)

[3] V. Wong (Fanchini) Adv. Mater. Interfaces 2300806 (2024)

[4] N. Stocek (Fanchini) J. Appl. Phys. 135, 043104 (2024)