How can we observe the structure of material? This was the goal of our project. The matter consists of an arrangement of atoms which can be analyzed through the light/matter interaction. However, we have to use the most adapted radiation to the sample, which means that we have to use an electromagnetic wave whose wavelength is of the same order of magnitude as the studied object. That's the reason why X-rays are used to study materials at the atomic scale. For reasons of commodity and safety, we were interested in samples of centimeter scale, in order to study them with the light visible or the microwaves (radiations of 1 eV and 10-4 eV respectively).

 

    In a first experiment, we studied, with a white light, the surface formed by a monolayer of little transparent balls with all the same diameter, modeling a monolayer of atoms. We supposed that the luminosity transmitted by the balls is not the same if the light is just above or near a ball. To verify this hypothesis, the sample is backlit. The intensity of the transmitted white light is measured using a photoresistor which the measured resistance is inversely proportional to the luminosity. As expected, the measured light intensity is different if the sensor is just above or near a ball. We built a device allowing a millimeter displacement of the sensor. As can be seen on the graphics, we obtained different peaks, thanks to the photoresistor which scanned the surface and measured the luminosity. Each peak is associated with the center of a ball (a ball transmits the light in one direction) and then indicate the positions of the balls. It is then possible to find back the topology of the surface. If the balls have the same diameter, they should have the same focal length and then transmit the same intensity of light. The presence of a smaller ball in the surface would imply the absence of peak. Thus, our device is able to detect a defect in the monolayer. This experiment allows the description of a 2D surface. Then we wanted to study 3D structures like crystals.

 

    The objective of the second experiment was to understand how the diffraction phenomenon allows the study of a periodic structure. For the diffraction, we need a wave whose wavelength is adapted to the studied matter ie that has the same order of magnitude as the size of the studied material: we used microwaves. For this experiment, our "homemade" rotating "megacrystal" is irradiated by a monochromatic beam of micro-rays. We built lenses in paraffin in order to get parallel beam. According to the angle 2q between the transmitter and the receptor, we can observe the presence of diffraction peaks (characteristic of constructive interferences between incident and diffracted beams) on the oscilloscope connected to the receptor. The position of these peaks q is given by the Bragg law: (where l the incident micro-rays and dhkl is the distance between the atomic planes characterized by the Miller indices (h,k,l)). Thanks to this law, it is possible to determine for each angle of diffraction a specific family of planes.

    For our experiment, we built two megacrystals. The first megacrystal is a cubic lattice with arranged aluminum bars every 4 centimeters. The measurement of the intensity of the diffracted beam according to the angle of the incident micro-rays allowed us to find the four main families of planes for a cubic crystal. To go further, we simulated a defect in the material by adding a new bar at another angle. The disruption of the organization of the lattice was observed by a decrease of the intensity of the diffraction peaks due to less important constructive interferences.

    The second megacrystal is a face centered cubic lattice constituted by balls of lead, separated by glass rods. Unfortunately, the construction was difficult and the obtained lattice is not enough organized on a wide distance to allow the analyses of its structure, despite small-scale symmetry. The second megacrystal can be seen as an amorphous material from the point of view of the diffraction.

 

    These two experiments enabled us to observe the structure of the "crystallized" samples, thanks to the light/matter interactions. We modeled these phenomena to larger scale but they are used in laboratories with X-rays to probe the atomic scale.

 

Charlie Leprince, Yohann Roiron, Damien Toussaint, Lycée Pothier, A diving into the invisible

 

This is the abstract of our project, which corresponds to the second thing to send to EUCYS.

Download : A diving into the invisible

My kingdom for a rotating crystal !