nanomaterials Review Recent Progress of Black Silicon: From Fabrications to Applications Zheng Fan 1, Danfeng Cui 1, Zengxing Zhang 1, Zhou Zhao 1, Hongmei Chen 1, Yanyun Fan 1, Penglu Li 1, Zhidong Zhang 1 , Chenyang Xue 1,* and Shubin Yan 2,3,* 1 Key Laboratory of Instrumentation Science & Dynamic Measurement, Ministry of Education, North University of China, Taiyuan 030051, China; [email protected] (Z.F.); [email protected] (D.C.); [email protected] (Z.Z.); [email protected] (Z.Z.); [email protected] (H.C.); [email protected] (Y.F.); [email protected] (P.L.); [email protected] (Z.Z.) 2 The School of Electrical Engineering, Zhejiang University of Water Resources and Electric Power, Hangzhou 310018, China 3 Zhejiang-Belarus Joint Laboratory of Intelligent Equipment and System for Water Conservancy and Hydropower Safety Monitoring, Zhejiang University of Water Resources and Electric Power, Hangzhou 310018, China * Correspondence: [email protected] (C.X.); [email protected] (S.Y.) Abstract: Since black silicon was discovered by coincidence, the special material was explored for many amazing material characteristics in optical, surface topography, and so on. Because of the material property, black silicon is applied in many spheres of a photodetector, photovoltaic cell, photo-electrocatalysis, antibacterial surfaces, and sensors. With the development of fabrication technology, black silicon has expanded in more and more applications and has become a research hotspot. Herein, this review systematically summarizes the fabricating method of black silicon, including nanosecond or femtosecond laser irradiation, metal-assisted chemical etching (MACE), reactive ion etching (RIE), wet chemical etching, electrochemical method, and plasma immersion ion implantation (PIII) methods. In addition, this review focuses on the progress in multiple black silicon applications in the past 10 years. Finally, the prospect of black silicon fabricating and various Citation: Fan, Z.; Cui, D.; Zhang, Z.; applications are outlined. Zhao, Z.; Chen, H.; Fan, Y.; Li, P.; Zhang, Z.; Xue, C.; Yan, S. Recent Keywords: black silicon; light absorption enhanced; micro-nano manufacturing; nanometer surface Progress of Black Silicon: From Fabrications to Applications. Nanomaterials 2021, 11, 41. 1. Introduction https://doi.org/10.3390/ nano11010041 Black silicon material is a silicon material with a micro-scale, nano-scale, and both micro-nano scales, including various structures such as holes, needles, and columns struc- Received: 19 November 2020 tures on the surface of wafers [1–5]. Compared with polished silicon, due to the micro-nano Accepted: 23 December 2020 structure and impurity doping of the black silicon surface, it has ultralow light reflection in Published: 26 December 2020 the visible and near-infrared bands, especially in the near-infrared band. Because of the limitation of the forbidden bandwidth of the polished silicon itself, when the wavelength Publisher’s Note: MDPI stays neu- is greater than 1100 nm [6], the light absorption ability is extremely reduced. The black tral with regard to jurisdictional claims silicon material has a light absorption rate of more than 90% in the range of 1100 nm to in published maps and institutional 2500 nm [7,8]. While extending the absorption range of materials in the longer band, it also affiliations. improves the light absorption capacity. Black silicon has high absorption of light, as even the absorption rate is close to 100%. It is dark black to the naked eye. The micro-nano structure on the surface is the crucial factor to improving the absorption of this unusual Copyright: © 2020 by the authors. Li- material in the visible light band. Figure1 explains the high absorption principle of the censee MDPI, Basel, Switzerland. This material. The incident light will be reflected multiple times between the tapered structures. article is an open access article distributed Each reflection process is accompanied by transmission at the interface. Each transmission under the terms and conditions of the increases the number and probability of absorption of incident light by black silicon so that Creative Commons Attribution (CC BY) the light seems to be trapped in the micro-awl of the material surface. The existence of the license (https://creativecommons.org/ light trapping effect is the main reason why black silicon has not only extreme low reflex in licenses/by/4.0/). a broad spectral range, but also high absorption in the visible light band. Nanomaterials 2021, 11, 41. https://dx.doi.org/10.3390/nano11010041 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, x FOR PEER REVIEW 2 of 28 Nanomaterials 2021, 11, 41 existence of the light trapping effect is the main reason why black silicon has not 2only of 26 extreme low reflex in a broad spectral range, but also high absorption in the visible light band. FigureFigure 1. 1. TheThe reflection reflection performance performance of of (a (a)) polished polished silicon silicon and and ( (bb)) black black silicon. silicon. SinceSince the the discovery discovery of of black black silicon silicon materials materials in in the the late late 20th 20th century century [9], [9], after after more more thanthan 20 20 years years of ofdevelopment, development, black black silicon silicon has hasbeen been fabricated fabricated in a variety in a variety of ways. of Since ways. theSince black the silicon black silicon was discovered, was discovered, common common fabricating fabricating methods methods include include nanosecond nanosecond or femtosecondor femtosecond laser laser irradiation irradiation [8–11 [8],– 11metal], metal-assisted-assisted chemical chemical etching etching (MACE) (MACE) [12– [1512],– 15re-], activereactive ion ion etching etching (RIE (RIE)) [16– [1618–],18 wet], wet chemical chemical etching etching [19], [ 19and], andplasma plasma immersion immersion ion im- ion plantationimplantation (PIII) (PIII) methods. methods. Each Each fabrication fabrication method method can fabricate can fabricate black black silicon silicon with witha mi- a micro-nano surface structure. However, the surface structure of the black silicon fabricated cro-nano surface structure. However, the surface structure of the black silicon fabricated is also different using different fabrication processes. Furthermore, the material exhibits is also different using different fabrication processes. Furthermore, the material exhibits different optical characteristics and electrical characteristics. The choice of micro-nano different optical characteristics and electrical characteristics. The choice of micro-nano processing technology also determines the performance of the device. processing technology also determines the performance of the device. The development of black silicon materials has a lot of applications in different fields The development of black silicon materials has a lot of applications in different fields while the fabricating process is developing. In solar cells [20–24], the low reflectivity while the fabricating process is developing. In solar cells [20–24], the low reflectivity of of black silicon and the high absorption of visible light is of great help to promote the black silicon and the high absorption of visible light is of great help to promote the per- performance of solar cells. In catalysis [25,26], the use of black silicon as a photoelectric formance of solar cells. In catalysis [25,26], the use of black silicon as a photoelectric cata- catalytic substrate can synergize the light absorption characteristics of black silicon and lytic substrate can synergize the light absorption characteristics of black silicon and the the redox characteristics of photocatalysts to improve the performance of photocatalytic redox characteristics of photocatalysts to improve the performance of photocatalytic ma- materials. In photodetectors [27–31], due to the excellent light absorption capabilities terials.of black In siliconphotodetectors in the visible [27–31 and], due near-infrared to the excellent bands, light it absorption can fabricate capabilities the silicon-based of black siliconphotodetectors in the visible with and high near responsivity-infrared andbands, wide it responsecan fabricate ranges the in silicon the near-infrared-based photode- band. tectorsIn sensing with applications, high responsivity high-sensitivity and wide detectionresponse ran canges be achievedin the near in-infrared surface-enhanced band. In sensingRaman applications, scattering (SERS), high-sensitivity biochemical detection sensing, can gas be sensing, achieved etc. in [32 surface–35]. Figure-enhanced2 shows Ra- Nanomaterials 2021, 11, x FOR PEERman REVIEWan overview scattering of (SERS the entire), biochemical review, including sensing, gas fabrication sensing, methods etc. [32– and35]. Figure characteristics 2 shows3and ofan 28 overviewcorresponding of the applications.entire review, including fabrication methods and characteristics and cor- responding applications. Figure 2. A figure overview of this review. Figure 2. A figure overview of this review. In this paper, we review the fabrication methods and applications of this special ma- terial over the last decade. The paper compares the complexity, cost, and suitable condi- tions of different fabricating processes. In addition, this paper also summarizes the re- searches on the optical properties of black silicon samples. Finally, we propose the outlook in fabrication methods and multiple applications. 2. Fabrication