By Sarmiza Stanca
Engineering the nanostructured components of the infrared sensors is a prerequisite for their optimization. Several scientific studies on fractal and plasmonic structures have already pursued to find ideal infrared absorbers and to bring them at a “sub-wavelength” thin film scale in optoelectronic devices. In this regard, noble metal black represents an appreciated optical absorber material with the potential for use as a thin layer in optical sensors. Presently available optical absorbers in the wavelength region from 0.4 μm to 20 μm rely on silver black. However, silver black displays an undesired chemical reactivity to air components hindering its long term stability of performance. Our alternative is to utilize porous platinum, which together with a broad absorbance and low reflectance from ultraviolet to infrared region  exhibits the required chemical stability in air. In most cases, the insertion of the optical absorber in the construction of microelectronics requires a non-aqueous media for operation and a prospective that the material can be brought into a thin-layer. Therefore, an in-situelectro-deposition of platinum black on the microcomponents in non-aqueous media is of practical interest. Thein-situelectrochemical method achieves a great regularity and a precise localization of the metal electro-assembled in just a few seconds. Recently, we electrochemically constructed layers of platinum black (with porous aspect and not metallic shiny aspect) in isopropanol at a sub-visible-wavelength thickness (Fig. 1) on substrates such as platinum, gold, silver, stainless steel, copper, indium-tin oxide (ITO) and aluminium [1,2] and also on metallic multijunction nanolayers, which consist of silver mirror supported nickel–chrome and nickel–titanium metallic films .
Electrochemical deposition of porous platinum thin layer on different substrates:
A 20 ml electrochemical cell, equipped with two electrodes connected to a continuous current source [1-3], which assures the electrical current density of 0.1 A cm−2, was used for electrolysis. To synthesize platinum black in isopropanol media, 0.05 g PtCl4and 0.008 g Pb(CH3COO)2dissolved in 10 ml isopropanol was used as an electrolytic bath. The working electrode and the counter electrode are immersed in an electrolyte and the potential difference between the electrodes is fixed by means of an external power supply. Platinum plate serves as the anode. As cathode has been successively used: platinum plate, aluminium foil, silver wire, tin-copper alloy wire, gold layer, ITO, copper wires , and the nanolayers of NiCr 50:50 and NiCr 80:20, Ni/Ti nanolayers  supported on silver mirror-silicon substrates.
The porous platinum architectures were examined by means of scanning electron microscopy, X-Ray diffraction analysis, energy dispersive X-ray spectrometry (EDX), Fourier transform infrared spectroscopy (FTIR)[1-3]. The layers consist of platinum crystals with a polycrystalline face centred cubic structure (fcc unit cell: a=3.9231Å) with a small texture leading to an increase in the 111 reflection . Evaluating the XRD spectrum of the porous platinum the crystallite size is estimated to be approximately 10 nm. The EDX spectra clearly identified the peaks related to platinum . It was observed that the layer and sublayer thicknesses and resistivities have a substantial effect upon the porous platinum morphology and its optical properties. The augmentation of the substrate´s metallic layer electrical conductivity determines the electroformation of more compact platinum nanolayers. Moreover, the porous platinum coating of metallic nanolayers causes a considerable decrease of the reflectance in the region from 1000–8000 cm−1(Fig. 1D) .
The high absorbance and low reflectance of the porous platinum in the wavelength region from 0.4 µm to 20 μm allows for its application as a high-temperature resistant optical absorber layer for the fabrication of infrared sensors. Due to a broad resonance in the visible region, this material has the potential in the construction of sensitive solar cells. The high specific surface area of porous platinum could confer applications in catalysis.
1. Stanca S E, Hänschke F, Ihring A, Zieger G, Dellith J, Kessler E, Meyer H-G, Chemical and electrochemical synthesis of platinum black, Sci. Rep. 7(2017) 1074
2. Stanca S E, Hänschke F, Zieger G, Dellith J, Ihring A, Undisz A, Meyer H-G, Optical Assets of In situ Electroassembled Platinum Black Nanolayers, Sci. Rep. 7(2017) 14955
3. Stanca S E, Hänschke F, Zieger G, Dellith J, Dellith A, Ihring A, Belkner J, Meyer H-G, Electro-architected porous platinum on metallic multijunction nanolayers to optimize their optical properties for infrared sensor application, Nanotechnology 29 (2018) 115601 (12pp).