Spectrumless near-field thermal radiation sensor for nanomaterial characterization – STORE

The goal of the STORE project is to develop a new characterization technique for materials and nanostructures at the intersection between atomic force microscopy (AFM) and the flourishing domain of near-field radiative heat transfer. AFM approach curves, when the tip is moved towards the contact with the sample, can be performed by monitoring simultaneously the near-field radiative flux and the hot tip-cold sample distance. It is key to note that the near-field radiative approach curves provide a signature of the materials and structures beneath the sample surface: the volume probed depends on the tip-sample distance. In addition, these approach curves can be performed as a function of the temperature of the hot tip, which is a way of shifting the spectral content that is probed. As a result, the STORE technique can probe both spectral and spatial features of the samples. A key advantage of this method is that it does not require any infrared spectrometer (FTIR) to observe the signature of materials or nanostructures buried beneath the surface. Infrared spectrometers are usually bulky and the proposed technique could therefore be much more compact.

The technique requires calibration and comparison of the experimental data with accurate numerical calculations. Indeed, the experimental signal depends on the hot tip, and convolution of the sample local density of energy above its surface with the presence of the probe is not a straightforward task. As a result, the plan is to improve the technique, systematize it and benchmark it with respect to well-established far-field infrared spectroscopy methods in order to highlight the common features observed and the differences. The consortium therefore consists in four partnering teams from two centres/institutes, to gather expertise in both experimental and numerical aspects of far- and near-field radiative heat transfer supported by specialists in properties of complex photonic structures and their nano/microfabrication.

The core idea of the project is to exploit the approach curves to trace back to morphological and material properties of the samples. Thereto, two types of data have to be obtained. First, for a constant temperature of the emitter, an approach curve allows to scan the parallel part of the wave vector up to a maximum value associated to the minimal tip-sample distance. This provides access to morphological information of the sample. Second, by measuring the flux at a given tip-sample distance as a function of the emitter temperature, one scans the frequency spectrum and thereby gains spectral information of the sample. Of course, performing approach curves for different temperatures allows getting both data in the same time.

Developing the sensor requires achieving the three following objectives: (i) analyzing the resolution of the data in the (frequency, parallel wave vector) space that the approach curves for different temperatures can provide; (ii) demonstrate that approach curves allow obtaining sub-surface sample information, especially for flat samples; (iii) show complementarity and differences between near- and far-field measurements for the characterization of samples, in particular for nanostructured surfaces where the near field data are expected to be richer. One key task to highlight the power of the new STORE technique is to design, fabricate and characterize samples of identical far-field emission properties which will significantly differ in the near field. To reach its objectives, the project is therefore divided in the four following tasks: design and modelling, nano and microfabrication, development of experimental setups and sample chararacterization. It is expected that the current project can bring the technique to a TRL level where the potential for commercialization could be assessed.