Iridescence at the Nanoscale
University of Rochester
The Institute of Optics
Iridescence is a common phenomenon that utilizes the fundamentals of wave optics. As light interacts with periodic structures, it diffracts. Depending on the wavelength of light, the angle at which the light is diffracted will change. Therefore, a surface that has periodic sub-wavelength structures or thin films will produce a rainbow or multicolor like reflection that changes with the viewing angle. Examples of artificial iridescence are depicted below with a reflective diffraction grating and thin film interference.
Common objects that has this iridescence property are shells, scales, insect wings, gemstones, and CDs. In the following, we will be examining iridescence of an abalone shell, bug wing/scale, peacock feather, and a piece of a CD. To investigate this property, a scanning electron microscope can reveal topological details at the nanoscale.
In an electron microscope, an electron beam strikes the surface of a sample to generate signals for generating an image. The electrons penetrate through the substrate and forms a tear drop shape that results from the interaction between the accelerating electrons and the atoms of the material. From this interaction volume, we have electrons that are ejected off due to elastic or inelastic collision. At the very surface, we have auger electrons, then secondary electrons, and back scattered electrons. Lastly, we also have X-ray emission from the deepest region of the interaction volume. The deeper the electrons originate from, the stronger the electron energy must be to penetrate through the material
Light microscopy is the most common microscopy technique that yields color and depth information of our sample. After preparing small pieces of our sample, we imaged them under the microsope at 10x and 20x. Due to surface uneveness, we imaged our sample at low magnification to keep a long depht of focus.
For the SEM microscope to create stunning and informative sample images at high resolution, the samples must be adequately prepared for the hostile vacuum environment and electrical potential of the target chamber. Samples need to be properly coated to ensure that the charge from the electron beam is grounded away, preventing deleterious artifacts from forming. Fortunately, our samples are dry and does not require drying methods such as critical point drying or HDMS drying.
The samples that we are investigating was coated using the gold sputter coater shown in figure (a). The sample stub was placed in the bell jar and the system started. The system was then repeatedly pumped down to approximately 100 mTorr with a rotary pump, backfilled with argon gas, and pumped down again. After six repetitions the system was pumped down to ~75 mTorr. At this level, 15 mA of current was run through the cathode for 90 seconds and a gold layer of ~100 Å was deposited. It was noted that during the sputtercoating the pressure in the chamber rose slightly due to the presence of the gold ions. Finally, our coated sample is shown in figure (b), with 2 samples of the abalone shell and a sample for the CD, peacock feather, and bug wing. The gold can be visibly seen from the reflection off the aluminum stub.
------------------------------------------------------------------ Secondary Electron Imaging
Secondary electron imaging is one of the most common SEM techniques. Using a Everhart-Thornley detector or an InLens detector, we can collect secondary electron signals at each point of the scan and record surface topology signals to form our image. The following are secondary electron images of our abalone shell, bug wing/scale, peacock feather, and CD sample.
X-ray microanalysis in the SEM is one of the few non-invasive and non-destructive methods of characterizing a material's chemical content. This technique is possible due to the interaction of the electron beam with the sample. When an electron beam spot is focused onto the surface of a sample, it creates a tear drop shape interaction volume. The deepest region in the interaction volume is origin of the characteristic X-rays. The X-ray photons are formed when a primary electron from the beam collides with an inner shell electron and forms a vacancy in the inner shell. To stabilize the structure, an outer shell electron decays to a lower energy state in the inner shell and releases an X-ray photon. In this process, elements will emit X-ray photons of energy corresponding to their atomic number and a detector will convert photon energy into electronic signals.
Once the X-ray emission emerges from the sample, an Energy Dispersive X-ray Spectroscopy (EDS) detector is used to characterize the photon energies. The EDS detector is a solid-state detector that converts X-ray photon energies into electrical charge. This electrical charge is amplified and processed through a field effect transistor. The detector is most efficient at collecting signals when it is placed at the largest solid angle from the emitted source area.
From the EDS analysis, we observe that all three samples contain large amount of carbon and oxygen, as expected with biological materials. We also observe significant signal for gold as anticipated because we coated our sample with gold. Since the abalone shell was found in sand and is composed of nacre (a form of calcium carbonate), we observe elements such as silicon and calcium from the EDS graph. Both Figure 8 and Figure 9 showed similar results; however, since the feather contains more protein than the bug wing, it emits a high nitrogen signal.------------------------------------------------------------------
Thin Film Modeling
By knowing the thickness, the material, the relative number of layers, and the structure, we can simulate the reflection of our abalone shell. From the secondary electron imaging and EDS analysis, we learn that our abalone sample has on the order of hundred of layers and is composed of carbon, oxygen, calcium, and slight bits of silicon. Using ImageJ, we can approximately measure the thickness of each layer to be between 400-420 nm. Using approximate parameters, we simulated a sample with 100 alternating layers of 400 and 420 nm thickness with refractive index of calcium carbonate (See Refractive Index), that is also alternating with a slight deviation by layer. Our model, shown in Figure 10, displays reflection by wavelength and the angle of incidence at 0,20,40, and 60 degrees. The graph shows the first order of reflection is in the near-infrared and the higher orders of reflection in the visible regime. Furthermore, the higher order reflections vary drastically with change in angle of incidence, which resembles the reflection of many bright distinct colors of an abalone shell by tilting it back and forth. *Note, this is not an accurate wavelength-reflection model of an abalone shell------------------------------------------------------------------ Colorization
All SEM images do not display color as it is a raster scan technique that collects only signal intensities. Therefore, colorization is a useful technique to convey a message when done correctly. It is important to stress that the following are false colored and only depicts my personal views.
The first image (a) is of an abalone shell tilted such that both the surface of the shell without the periodic structure can be contrasted to the cross-section of the shell. The image resembles a cliff of a canyon, so it was false colored with a light yellow to resemble sand. The second image (b) is a low magnification view of our bug wing sample with a lot of other insect scales on top because the wing was stored in a jar with many other insect samples. The contamination came as a useful surprise since I was able to image multiple sample on one. The dark green is used to depict a grassier feeling but on the more ominous side. Looking closely, you can see a moiré pattern formed by the high special frequency of the latter structure on the scales. Next, image (c) shows a warm magenta colorization of the peacock feathers, simply because feathers are warm and soft. Lastly, we have image (d), our cold blue image of the surface of a CD. For more cool SEM images or images of anything I take, please follow me on IG @thechiefray !