Luminescent Silicon Nanowires for Light-Harvesting and Environmental Sensing Applications

. Silicon nanowires (Si NWs) represent one of the most promising platforms to be integrated into modern nanodevices. The fabrication of a dense array of vertically aligned ultrathin Si NWs using a low-cost, maskless approach and compatible with Si technology will be here demonstrated. Si NWs with efficient light emission at room temperature (RT) represent a great advance industry, paving the way for a wide range of unexpected photonic applications. In this work I will show the realization of a new hybrid material based on the luminescence of Si NWs and two different dyes for light-harvesting antennas applications, without any surface functionalization and with energy transfer efficiencies higher than 90%. The luminescence of Si NWs has also been used for sensing applications by the realization of highly sensitive sensors for the detecting of low concentrations of toxic gases. These sensors allow the detection of toxic gases below the threshold limits for human health, through both optical and electrical transduction. The achievement of light emission from silicon-based materials represents a revolution in the industrial field, as it paves the way for new Si applications.


Introduction
In this text the realization of ultrathin silicon nanowires as a highly sensitive platform for photonic and sensor applications is reported.Silicon is the main material in the microelectronics thanks to its electrical properties.Furthermore, silicon is the most abundant material in the earth's surface and easy to extract, which has allowed its simple and low-cost integration into industrial processes.Consequently, the possibility of realizing Si-based sensors is a strategic result in the application field.In the literature there are already Si-based sensors that exploit the variation of the electrical properties.However as far as photoluminescence analysis is concerned, Si is a material that has not yet been studied much.Indeed obtain light emission at room temperature (RT) it is necessary to have quantum confinement.Among the quantum confined Si nanostructures, the most promising are considered porous silicon and Si nanocrystals.However, their photoluminescence is too low and inefficient to design a sensor based on photoluminescence (PL).Until a few years ago, the use of light emitting Si NWs was completely absent because with the traditional techniques it is not possible to obtain Si NWs diameters small enough to have quantum confinement and PL at RT.For this purpose, Si NWs are synthesized by "Metal Assisted Chemical Etching" (MACE), by using Au ultrathin films as catalyst.This method does not require the use of lithographic masks, is low-cost, at room temperature and compatible with the silicon technology.In this way it is possible to obtain ultrathin Si NWs (with sub-10 nm diameters) with a density of about 10 12 NW/cm 2 as shown in the scanning electron microscopy image in fig 1a.By varying the Au thickness, it is possible to control the NW average diameter, obtaining quantum confined Si NWs and therefore light emission at RT.These Si NWs exhibit a time-stable PL emission in the visible-NIR region as a function of their diameter, in agreement with the quantum confinement theory (Figure 1b) and have proved to be one of the most promising resources to be employed in modern nanodevices both in terms of fabrication costs and of PL emission.

Results and discussions
Some of the most interesting photonic and sensing applications of these luminescent Si NWs will be shown below.In particular, the realization of an artificial antenna for the light transfer and environmental sensors for the optical and electrical detection of polluting gases are both reported.

Si NWs for light harvesting antenna
The interesting characteristics of these platform have allowed for the first time in literature the realization a hybrid antenna system based on silicon nanowires and specific dyes for light-harvesting applications.[1] In particular, Si NWs were decorated with two Ru(II)-and Os(II)-based polypyridine complexes characterized an intense light emission in the NIR region.A simple and fast Si NW decoration process at RT has allowed the realization of a novel hybrid antenna for the light transfer with energy transfer efficiencies higher than 90% without any surface functionalization.The presence of the dye inside the Si NW array causes a strong NW-dye interaction and the light energy is efficiently transferred from the NW to the dye.The study of the NW and dye PL intensity ratios (Figure 2a-b) and the PL lifetime confirms this very high energy transfer efficiency with dye concentrations on the femtomolar (fM) orders.These NW-based hybrid materials offer the great advantages of compatibility with silicon technology, lowcost, stability, and high photoinduced energy transfer efficiency for various applications fields like bio-imaging, sensors, photonics, and energy.

Si NWs as environmental sensors
The extremely high Si NW surface to volume ratio has led to the design and manufacturing of Si NWs-based platforms for sensing applications.Moreover, the interesting photonic properties of these systems have contributed to the birth of novel silicon-based sensors which exploits PL as a detection mechanism.Si NWs have been tested as a platform for monitoring various toxic gases present in the environment.[2,3] The control of the concentrations of polluting and toxic gases is a particularly widespread topic for monitoring both closed and open environments.This sensing platform provided fast and reversible responses with the possibility of dual transduction, both optical and electrical.The quenching of the Si NW photoluminescence when a target gas interacts with their surface is exploited.Figure 2c shows an example of an optical Si NWs-based sensor for the nitrogen dioxide sensing.The variation of the PL as a function of the gas concentration permits a simple and extremely fast quantification of the gas target concentrations below the threshold limits for human health.Moreover, this platform has also proved to be promising in electrical detection, as shown in Figure 2d.The sensor exhibits significant responses to all gases tested by both photoluminescence and resistance changes with very short recovery times.The possibility of using the same sensor to detect small concentrations of gas through variations in both luminescence and electrical resistance makes it suitable and accessible to many users in different contexts.This is an aspect of extreme attention, especially in closed workplaces where it is important to keep air quality under control.The realization of these luminescent nanowires has paved the way for new applications of silicon-based material that were prohibited until a few years ago.The achievement of these new properties linked to the fast, easy, low cost, and compatible with silicon technology synthesis process makes our platform highly competitive with the current systems spread by the modern industry.

Fig. 1 .
Fig. 1.(a) Scanning electron microscopy of Si NWs realized via MACE; (b) PL by quantum confinement of Si NWs as a function of their size.

Fig. 2 .
Fig. 2. Variation of the emission spectra of the Si NWs-dye hybrid antenna as a function of the concentration of the Ru4 (a) and Ru3Os (b) dye; (c) quenching of the RT PL intensity of Si NWs by increasing NO2 concentration; (d) variation of the sensor electrical resistance at different NO2 concentrations.