Contactless Scanning Near-Field Optical Dilatometry Imaging at the Nanoscale

To date, there are very few experimental techniques, if any, that are suitable for the purpose of acquiring quantitative maps of the thermal expansivity of 2D materials and nanostructured thin films with nanoscale lateral resolution in spite of huge demand for nanoscale thermal management, for example in designing integrated circuitry for power electronics. Besides, contactless analytical tools for determining the thermal expansion coefficient (TEC) are highly desirable because probes in contact with the sample significantly perturb any thermal measurements. Here, omega-2 omega near-field thermoreflectance imaging is presented as a novel, all-optical, and contactless technique to map the TEC at the nanoscale with precision. Testing of this technique is performed on nanogranular films of gold and multilayer graphene (ML-G) platelets. With omega-2 omega near-field thermoreflectance, it is demonstrated that the TEC of Au is higher at the metal-insulator interface, with an average of (17.12 +/- 2.30) x10-6 K-1 in agreement with macroscopic techniques. For ML-G, the average TEC is (-5.77 +/- 3.79) x10-6 K-1 and is assigned to in-plane vibrational bending modes. A vibrational-thermal transition from graphene to graphite is observed, where the TEC becomes positive as the ML thickness increases. The nanoscale method here reported demonstrates results in excellent agreement with its macroscopic counterparts, as well as superior capabilities to probe 2D materials and interfaces. A novel scanning near-field optical dilatometer is presented, which is contactless and capable of generating maps of both positive and negative thermal expansion coefficients of 2D materials and nanostructured thin-films at the nanoscale. This dilatometer is based on super-resolution scanning near-field optical microscopy (SNOM) and, therefore, has resolution well beyond the diffraction limit of light, even if it is all-optical in nature. image