materials Review A Mini Review: Recent Advances in Surface Modification of Porous Silicon Seo Hyeon Lee 1 , Jae Seung Kang 2,3,* and Dokyoung Kim 1,4,5,6,* 1 Department of Biomedical Science, Graduate School, Kyung Hee University, Seoul 02447, Korea; [email protected] 2 Laboratory of Vitamin C and Anti-Oxidant Immunology, Department of Anatomy and Cell Biology, College of Medicine, Seoul National University, Seoul 03080, Korea 3 Institute of Allergy and Clinical Immunology, Medical Research Center, Seoul National University, Seoul 03080, Korea 4 Department of Anatomy and Neurobiology, College of Medicine, Kyung Hee University, Seoul 02447, Korea 5 Center for Converging Humanities, Kyung Hee University, Seoul 02447, Korea 6 Biomedical Science Institute, Kyung Hee University, Seoul 02447, Korea * Correspondence: [email protected] (J.S.K.), [email protected] (D.K.); Tel.: +82-02-961-0297 (D.K.) Received: 26 November 2018; Accepted: 13 December 2018; Published: 15 December 2018 Abstract: Porous silicon has been utilized within a wide spectrum of industries, as well as being used in basic research for engineering and biomedical fields. Recently, surface modification methods have been constantly coming under the spotlight, mostly in regard to maximizing its purpose of use. Within this review, we will introduce porous silicon, the experimentation preparatory methods, the properties of the surface of porous silicon, and both more conventional as well as newly developed surface modification methods that have assisted in attempting to overcome the many drawbacks we see in the existing methods. The main aim of this review is to highlight and give useful insight into improving the properties of porous silicon, and create a focused description of the surface modification methods. Keywords: surface modification; porous silicon; silicon surface; carbonization; oxidation 1. Introduction Porous silicon (abbreviated as pSi) is a silicon formulation that has introduced nanopores in its microstructure. Porous silicon was discovered in the mid-1950s, and its unique physical, chemical, optical, and biological properties allowed us to develop new disciplines [1]. Porous silicon can be generated by electrochemical etching of crystalline silicon in hydrofluoric acid (HF) containing aqueous or non-aqueous electrolytes [2]. Silicon (Si) elements in Si wafer can be 2− dissolved out to a hexafluorosilane (SiF6 ) ion in the electrochemical etching stage, with each wafer generating a different pore diameter; p-type Si wafer (micropores, <2 nm), p+/p++/n+-type Si wafer (mesopores, 2–50 nm), and n+-type Si wafer (macropores, >50 nm) [1]. So far, various types of pSi materials have been reported, including pSi chip, pSi film, and pSi micro- and nano-particles (Figure1). The porosity, pore size, pore pattern, and particle size can be controlled by the fabrication parameters; HF concentration, current density, electrolyte composition, and wafer (dopant type, dopant density, crystallographic orientation) [3]. As compared to anodization and sonication processes, recently a new concept in electroless etching of Si powder has been reported that is an easily scalable process for the generation of pSi particles [4]. The pSi materials have been widely used in various industries and basic science. Due to a significant amount of research and the discovery of quantum confinement effects, photoluminescence, Materials 2018, 11, 2557; doi:10.3390/ma11122557 www.mdpi.com/journal/materials MaterialsMaterials2018 2018, ,11 11,, 2557 x FOR PEER REVIEW 2 ofof 1414 photoluminescence, and photonic crystal properties of pSi [5–9], the focus has mostly been on andcreating photonic optoelectronic crystal properties materials of [10,11], pSi [5 –displays[12,13],9], the focus has sensors mostly [14–16], been on and creating bio-imaging optoelectronic materials materials[17,18]. Recently, [10,11], displays the pSi micro- [12,13], and sensors nano-particles [14–16], and have bio-imaging been applied materials to drug [17 ,delivery18]. Recently, systems the and pSi micro-controlled and nano-particlesrelease systems, have by using been appliedthe biodegrada to drugtion delivery property systems of pSi and [1 controlled8–22]. One release such approach, systems, by“drug using loading the biodegradation in the pore and property surface of pSifunctionalization [18–22]. One such of pSi approach, materials “drug with loading disease in targeting the pore andmoiety”, surface was functionalization one of the biggest of pSi materialsleaps in the with fiel diseased of drug targeting delivery moiety”, systems. was one In ofaddition, the biggest the leapsnanostructured in the field pSi of drug material delivery is a promising systems. In anode addition, material the nanostructuredfor high-performance pSi material lithium-ion is a promising batteries anode[23,24]. material With proper for high-performance surface modification, lithium-ion pSi mate batteriesrials have [23 shown,24]. With the suppression proper surface of pulverization, modification, pSilow materials volume expansion, have shown and the a suppression long-term cycling of pulverization, stability in low the volumelithiation expansion, and delithiation and a long-term stages as cyclingnext-generation stability inlithium-ion the lithiation batteries. and delithiation stages as next-generation lithium-ion batteries. Figure 1. Schematic illustration for the preparation of porous silicon by electrochemical etching and ultrasonication.Figure 1. Schematic A porous illustration silicon for layer the ispreparation generated of on porous the surface silicon of by the electrochemical silicon wafer using etching electro and chemicalultrasonication. etching, A and porous the layersilicon can layer be separatedis generated from on the wafersurface using of the lift-off silicon etch. wafer Porous using siliconelectro micro-chemical and etching, nano-particles and the can layer be prepared can be separated using ultrasonication from the wafer fracturing. using Thelift-off surface etch. of Porous the resulting silicon porousmicro- siliconand nano-particles is covered mainly can withbe prepared silicon-hydrogen using ultrasonication (Si–H) and partly fracturing. with silicon-oxygen The surface (Si–OH, of the Si–O–Si).resulting porous silicon is covered mainly with silicon-hydrogen (Si–H) and partly with silicon- oxygen (Si–OH, Si–O–Si). As we described above, the surface modification of pSi materials is imperative to improve the propertiesAs we of described pSi and itsabove, usage. the Essentially,surface modification freshly etched of pSi pSi materials has silicon is imperative hydrides to (Si–H) improve on thethe surfaceproperties and of residual pSi and oxides its usage. or fluorides Essentially, are removed freshly etched by the HFpSi electrolytehas silicon (Figurehydrides1). (Si–H) The reactive on the siliconsurface hydrides and residual on the oxides large or surface fluorides of pSi are is removed susceptible by tothe slow HF electrolyte oxidation in(Figure humid 1). air The [25 reactive,26]. In thesilicon field hydrides of optoelectronics on the large or surfac batterye of application, pSi is susceptible the oxidized to slow pSi oxidation could degrade in humid performance air [25,26]. ofIn materials.the field of However, optoelectronics the oxidized or battery pSi is applicatio necessaryn, in the the oxidized development pSi could of sensors, degrade photoluminescent performance of bio-imagingmaterials. However, materials, the and oxidized drug-delivery pSi is necessary systems. in Accordingly,the development surface of sensors, modification photoluminescent is the most importantbio-imaging component materials, in and terms drug-delivery of the use of pSisystems. materials. Accordingly, surface modification is the most importantIn this component review, we in have terms summarized of the use of the pSi conventional materials. surface modification methods, newly reportedIn this surface review, modification we have methods,summarized and ourthe perspectives.conventional surface modification methods, newly reported surface modification methods, and our perspectives. 2. Conventional Surface Modification Methods 2. ConventionalWe categorized Surface the well-known Modification and Methods typical surface modification methods into three categories: (i) hydrosilylationWe categorized & carbonization,the well-known (ii) an oxidation,d typical andsurface (iii) modification hydrolytic condensation. methods into three categories: (i) hydrosilylation & carbonization, (ii) oxidation, and (iii) hydrolytic condensation. 2.1. Hydrosilylation & Carbonization 2.1. HydrosilylationOver the last several& Carbonization decades, various attempts have been made at stabilizing the surface of porousOver silicon the last in order several to improvedecades, itsvarious suitability attempts for varioushave been applications made at stabilizing [27–31]. Amongthe surface them, of theporous silicon-carbon silicon in order (Si–C) to bondimprove formation its suitability yielded for avarious very stableapplications surface [27–31]. of pSi Among due to them, the low the electronegativitysilicon-carbon (Si–C) of carbon, bond which formation possesses yielded greater a kineticvery stable stability surface in comparison of pSi todue silicon-oxygen to the low (Si–O)electronegativity [32]. The most of carbon, ubiquitous which reaction possesses to make greater the Si–Ckinetic bond stability from in hydrogen-terminated comparison to silicon-oxygen pSi (Si–H) is(Si–O) hydrosilylation. [32]. The most