Optical Properties of Silicon Nanowires with TiN Coating

Abstract

Using silicon nanowire (NW) arrays for solar energy conversion has gained special interest in the past years especially for their strong light interaction. The application in photocatalysis, however, requires an additional layer of protection for the silicon surface. To that end, silicon NWs with titanium nitride (TiN) coating, which exhibit a much stronger absorption than their uncoated counterpart, have been studied. Transmission spectra of nanowires in a polydimethylsiloxane (PDMS) substrate have been measured, as well as Finite Difference Time Domain (FDTD) simulations have been performed to investigate the origin of this strong absorption. Finally, the bulk material absorption at different angles of incident light onto the sample has been modeled to estimate its influence on the spectrum at non normal incidence and its relevance regarding the observed differences between simulation and experiment.
Both experimental and simulated results show an increase in absorption at long wavelengths, and FDTD simulations suggest that this is related to the fundamental waveguiding mode HE11 in the NW, enhancing light absorption in the TiN layer. Additionally, simulated absorption spectra for different pitches show that small distances between neighboring NWs lead to broad absorption with the main features being smeared out, while arrays with longer distances show higher, sharper peaks with an overall lower base absorption. These observations lead to the conclusion that coating silicon NWs with TiN does not only increase the absorption significantly but can also be tuned similarly to the uncoated NWs, enabling further optimization for potential light harvesting applications.

Popular Abstract

In the modern world, finding new, sustainable energy sources is one of the biggest challenges, because we need to emit less greenhouse gases to limit climate change. One of the most promising ways to achieve that goal is to use sunlight because the total amount of energy we could potentially access is much larger than that of any other renewable energy source. One possible way to access this energy is by using solar cells on the roof of a house. The problem is that there are also some industrial processes that – so far – simply do not work without emitting large amounts of greenhouse gases, because they rely on chemical reactions which produce these gases as a byproduct. An example for this is the production of ammonia, which is used for fertilizers, and uses the so-called Haber-Bosch process. This process consumes a lot of energy and for the reactants for this method large amounts of CO2 are released.

With photocatalysis, one goal is now to explore how these reactions can happen without the emission of greenhouse gases. Just like in a solar cell, the energy of the sun can be used for that, too. But, instead of absorbing the light, converting this energy into usable charges, and then extracting them for electricity, the produced charges react with inserted reagents in a photo-electrochemical cell and form the desired products. All of this starts with the absorption of the sunlight in the respective catalyst, however, and so understanding how this absorption process works and how it can be optimized is crucial for the potential application of the catalyst.

In this report these absorption mechanisms have been studied for an array of silicon nanowires. A silicon nanowire is simply a silicon cylinder with dimensions in the nanometer range and putting many of those together in an ordered fashion then forms the array. Additionally, the silicon needs to be coated to protect it from corrosion. How this then changes the absorption has been studied in this report. As it turns out, the protective layer exhibits a strong absorption, which benefits from the nanowire structure of the array: Because their small dimensions are of similar size as the wavelength of the light, these nanowires interact very differently with light compared to our everyday intuition. For instance, they can absorb light that would not hit the wire from a classical perspective, leading to a much higher absorption as an unstructured silicon wafer. In future studies, it would now be important to investigate, how the enhanced absorption influences the catalysis reaction.

• author: Tritschler, Moritz
• supervisor: Jens Uhlig
• organization: Department of Chemistry
• course: KEMP30 20231
• year: 2023
• type: L3 - Miscellaneous, Projetcs etc.
• subject: Chemistry
• keywords: Chemical Physics, Silicon Nanowires, Titanium Nitride, Optical Waveguiding, Photocatalysis
• language: English
• id: 9134899
• date added to LUP: 2023-08-23 14:40:26
• date last changed: 2023-08-23 14:40:26