(Invited) Thermal Carbonization of Porous Silicon: The Current Status and Recent Applications
Authors: Salonen Jarno , Kaasalainen Martti , Rauhala Olli-Pekka , Lassila Lippo , Hakamies Martta , Jalkanen Tero , Hahn Robert , Schmuki P. , Mäkilä Ermei 
Published: February 2020

Porous silicon (PSi) is a versatile material for a wide range of applications. Its large surface area combined with easily adjustable pore morphology and surface chemistry offers an excellent platform for many different applications. For example, the refractive index of PSi is strictly depending on the porosity and can be controlled by the current density of electrochemical etching. This enables precise control of the refractive index and allows the fabrication of highly complex optical structures for many interesting applications.

However, for decades, the biggest stumbling block for PSi applications has been the chemical instability of PSi. As-anodized PSi is hydrogen terminated and it slowly oxidizes even at room temperature. This instability of as-anodized PSi is unacceptable in most applications. On the other hand, intentional oxidation of PSi, in order to improve the stability, usually leads to too high resistivity for electrical applications and changes the optical properties of PSi too drastically. Moreover, the oxidized PSi is not chemically stable enough for certain type of applications, especially in aqueous conditions. It may react adversely with drugs and its functionalization needs to be done through silanization chemistry which shares the silicon oxide bonding related instability, e.g., for targeted drug delivery applications.

Thermal carbonization by acetylene to stabilize PSi was introduced in 2000 [1]. It produces a non-stoichiometric silicon carbide layer on the PSi, which is very stable in harsh environments, like in NaOH, KOH, HF etc. solutions with extreme pH values. It also increases the conductivity of the PSi, which is beneficial for electrical sensor  and energy storage applications and it has been found to be nontoxic both in in vitro and in vivo applications.

Therefore, the thermally carbonized PSi (TCPSi) has been used in various different applications ranging from gas sensors and biosensors to targeted drug delivery, bioimaging and electrochemical supercapacitors since its introduction [2–9]. In the presentation, the current status and novel applications, in which thermally carbonized PSi has been used, will be discussed. In addition, some new information about the physical and chemical properties of TCPSi will be introduced.

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