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Abstract The idea of benefitting from the properties of III-V semiconductors and silicon on the same substrate has been occupying the minds of scientists for several years. Although the principle of III-V integration on a silicon-based platform is simple, it is often challenging to perform due to demanding requirements for sample preparation rising from a mismatch in physical properties between those semiconductor groups (e.g. different lattice constants and thermal expansion coefficients), high cost of device-grade materials formation and their post-processing. In this paper, we demonstrate the deposition of group-III metal and III-V semiconductors in microfabricated template structures on silicon as a strategy for heterogeneous device integration on Si. The metal (indium) is selectively electrodeposited in a 2-electrode galvanostatic configuration with the working electrode (WE) located in each template, resulting in well-defined In structures of high purity. The semiconductors InAs and InSb are obtained by vapour phase diffusion of the corresponding group-V element (As, Sb) into the liquified In confined in the template. We discuss in detail the morphological and structural characterization of the synthesized In, InAs and InSb crystals as well as chemical analysis through scanning electron microscopy (SEM), scanning transmission electron microscopy (TEM/STEM), and energy-dispersive X-ray spectroscopy (EDX). The proposed integration path combines the advantage of the mature top-down lithography technology to define device geometries and employs economic electrodeposition (ED) and vapour phase processes to directly integrate difficult-to-process materials on a silicon platform. Graphical abstract

Despite substantial interest on the influence of heat treatment on the activity of anodic TiO2 films, there is still a lack of systematic studies addressing the differences in photocatalytic and photoelectrochemical performance of thermally annealed anodic TiO2 layers. Here we addressed this problem on hierarchical anodic titanium oxide (ATO) layers consisting of an outer layer with large pores (rings) and inner layer with sub-pores were obtained by a three-step anodization of titanium. The ATO layers were formed at 20 °C in an ethylene glycol solution containing 0.38 wt% NH4F and 1.79 wt% H2O at the constant anodizing voltage of 70 V. The resulting amorphous ATO was annealed at 400, 500, and 600 °C in order to obtain photoactive anatase and anatase-rutile mixed phases. The characterization of ATO was performed by electron scanning microscopy (SEM/EDS), X-ray diffraction (XRD), low-temperature nitrogen sorption measurements (BET surface area), and electrochemical impedance spectroscopy (EIS). A gradual decrease in the BET surface area of ATO was observed with increasing annealing temperature due to the clogging of the sub-pores resulting from the sintering of anatase grains. EIS study and the Mott-Schottky analysis suggested that conductivity of ATO changed due to the reduced fluorine content in the anodic oxide layer, with donor densities decreasing from 1.78 ⋅ 10 18 cm − 3 to 4.08 ⋅ 10 17 cm − 3 . Raising the annealing temperature also caused a shift of the flat band potential towards more negative values, which is correlated with an increase in rutile content and consequently a decrease in band gap from 3.26 eV to 3.09 eV. The photoelectrochemical performance of the annealed TiO2 was studied under pulsed UV illumination. The highest photoconversion efficiencies were observed for the wavelength of 350 nm for the ATO sample with the dominant anatase content. The photocatalytic degradation of methyl red and toluidine blue dyes was highest on the ATO layers annealed at 500 °C. After 9 h, the total removal of each dye was about 80% and a decrease in total carbon (TC) by 55.3% was observed.

High-performance electronics would greatly benefit from a versatile III-V integration process on silicon. Unfortunately, integration using hetero epitaxy is hampered by polarity, lattice, and thermal expansion mismatch. This work proposes an alternative concept of III-V integration combining advantages of pulse electrodeposition, template-assisted selective epitaxy, and recrystallization from a melt. Efficient electrodeposition of nano-crystalline and stochiometric InSb in planar templates on Si (001) is achieved. The InSb deposits are analysed by high resolution scanning transmission electron microscopy (HR-STEM) and energy-dispersive X-ray spectroscopy (EDX) before and after melting and recrystallization. The results show that InSb can crystallise epitaxially on Si with the formation of stacking faults. Furthermore, X-ray photoelectron (XPS) and Auger electron (AE) spectroscopy analysis indicate that the InSb crystal size is limited by the impurity concentration resulting from the electrodeposition process.

The successful synthesis of one-dimensional nanostructures of a narrow band gap semiconductor, exhibiting a ferromagnetic response at room temperature, is reported. High-quality nanowires of InSb-Mn have been produced by template-assisted pulse electrodeposition. Detailed structural and spectroscopic characterizations revealed good crystallinity, a narrow size distribution of the nanostructures, and the ability to control the Mn doping level. The dominating magnetic response at a cryogenic temperature evolves with an increasing Mn concentration from paramagnetic through antiferromagnetic to ferromagnetic. A robust ferromagnetic response of InSb nanowires doped with 2.5% at. of Mn is retained up to a Curie temperature of nearly 500 K.

Presented work focuses on the tremendous and often skipped role of pulse frequency on the structural, optical and electrical properties of electrodeposited InSb films. Tailoring the pulse frequency during electrodeposition allows to obtain stoichiometric, nanocrystalline, smooth films with relatively high electrical conductivity or Sb-rich, almost insulating, ultra smooth ones. It was observed that a double coherent domain size reduction (via decrease of pulse frequency) leads to a six fold increase in resistivity of the film. Further increasing pulse on time results in increasing resistivity of the material up to ca. 540 Ω cm. Based on the FTIR and SPV measurements it was confirmed that obtained materials are characterized by small band gap and p-type conductivity. Moreover, stoichiometric, ultra smooth InSb films obtained with 10 ms pulse on time have high photovoltage amplitude and charging time constant with relatively high conductivity, which makes them a good, low-cost candidate for optoelectronic devices.

The main objective of our study was to develop an electrochemical synthesis method for obtaining Te-doped InSb thin films. The electrodeposition was performed using a pulse mode in a citrate bath containing 0.06 M InCl3, 0.045 M SbCl3 and 0–0.008 M TeO2. Firstly, the optimization of following parameters was carried out: the potential of pulse “off” (Eoff), duration of pulse “off” (toff), and the ratio of time “on” to time “off” (ton/toff). We have confirmed that appropriate selection of electrodeposition conditions allows obtaining a material with a controllable morphology and chemical composition. Based on performed studies, optimal parameters for electrodeposition of thin Te-doped InSb films were selected. In the next stage, the pulse electrodeposition of Te-doped InSb films at previously optimized conditions was conducted from a solution containing different concentrations of the TeO2 precursor. It was demonstrated that adjustment of the TeO2 content in the citrate bath allows controlling the dopant concentration in thin Te-doped InSb films. Such a controllable synthesis creates the opportunity for tuning physicochemical properties of the material. The effect of doping on the crystal structure, phase composition, surface roughness, and optical band gap of thin Te-doped InSb films was discussed in detail.

Arrays of zinc oxide (ZnO) nanowires with diameter of about 200 nm and length of >20 µm were successfully obtained by simple anodic oxidation of Zn foil in sodium bicarbonate electrolyte and thermal post-treatment. The as formed anodic film consists of overlapping nanowire bundles forming a flower-like or grass-like structure. Thermal annealing in air at temperatures higher than 150 °C results in the conversion of initially formed hydroxycarbonate precursor to crystalline wurtzite ZnO. The crystallinity of ZnO nanowires increases with increasing annealing temperature. On the other hand, optical band-gap energy of obtained semiconducting nanowires was found to be independent of the temperature applied during the thermal treatment. It is expected that the described method can be further scaled up and offers a great potential even for technological applications.

Indium antimonide is a semiconducting material with a wide range of potential applications which narrows down when a highly oxidative surface of InSb is considered. Particularly advantageous is the use of InSb in the form of nanowires. One of the most affordable methods used for their synthesis is a template-assisted electrodeposition within the pores of anodic aluminum oxide (AAO). After electrodeposition of nanowires, the AAO template is typically dissolved in a NaOH solution. The possible and undesirable dissolution of InSb nanowires can weaken electrical transport properties of nanowires. Therefore, it is essential to study corrosion behavior of InSb, which can depend on the semiconductor structure and composition. To achieve this goal, we investigated the effect of crystal size and morphology of pulse electrodeposited indium antimonide thin films on their corrosion behavior in a 0.5 M NaOH aqueous solution. Furthermore, to collect a complete set of data and to explain sometimes complicated electrochemical behavior of semiconducting films, corrosion experiments on metallic films consisting of indium or antimony were also performed. The results were compared with the electrochemical behavior of the single components - indium and antimony which were used as a reference. The open circuit potential, cyclic polarization curves, and Tafel plots were recorded in order to characterize electrochemical properties of thin films such as corrosion potential, corrosion current density, corrosion rate. The obtained results indicate that thin films composed of the single metallic component are less resistant to the corrosion than indium antimonide. Moreover, the corrosion resistance of InSb thin films depends on the crystal size of InSb which can be tuned by the duration of pulse “on”.

In this work, the kinetics of indium, antimony, and zinc electrodepositions were investigated by using a gold rotating disc electrode (RDE). The performed studies shed light on the influence of electrodeposition parameters (i.e., working electrode potential and hydrodynamic conditions) on the chemical composition of In-Sb and Zn-Sb films. Stoichiometric InSb and ZnSb are one of the most promising thermoelectric materials, therefore a strictly controlled chemical composition of deposits is required. During electrochemical studies, as a primary electrolyte, a citrate bath with the addition of metal ion precursors, i.e., 0.06 M InCl3 and/or 0.045 M SbCl3 and/or 0.045 M ZnCl2, was applied. Based on the linear voltammetry measurements in the RDE system, the Koutecký-Levich analyses were performed. The diffusion coefficients (D) determined at 293 K for indium, antimony, and zinc ions equal to 1.45 × 10−6, 3.76 × 10−6, and 5.94 × 10−6 cm2 s−1, respectively. The activation energy (EA) for the diffusion of metal ions was determined using D values calculated at different temperatures. EA of indium, antimony, and zinc ions equals to 25.1, 9.2, and 10.6 kJ mol−1, respectively. The number of transferred electrons (n) and rate constant for heterogeneous reactions (k) occurring in the studied electrolytes at different potentials were also determined.

Nanoporous tin oxide layers were obtained by one-step potentiostatic anodization carried out in 0.3 M H 2 C 2 O 4 followed by thermal annealing at temperatures in the range of 200–700 °C. The morphology and crystallinity of as obtained oxide layers were characterized by SEM, and XRD, respectively. It was confirmed, that after 10 min of anodic oxidation at the potential of 8 V, anodic tin oxide consists of a dense array of randomly distributed nanopores with an average diameter of about 50–60 nm. Moreover, such kind of porous structures is maintained during annealing at temperatures up to 500 °C. On the contrary, for higher annealing temperatures a transformation of nanoporous layers into particle-like structures was evident. A gradual increase of the crystallite size with increasing annealing temperature was also confirmed. Finally, photoelectrochemical properties of anodic tin oxide layers, annealed at temperatures in the range of 400–700 °C, were extensively investigated. The best photoelectrochemical performance with the maximum photocurrent densities of above 25 μA cm −2 and conversion efficiency of about 30% was observed for sample annealed at 500 °C. Higher annealing temperatures result in a gradual decrease in photocurrent densities that can be attributed to the change in the sample morphology and decrease in electrochemical reaction sites. Optical band gap energies were determined on the basis of UV–vis reflectance spectra and photocurrent measurements, and a gradual increase in the band gap energy with increasing annealing temperature was confirmed.

We present an electrochemical route to prepare nanocrystalline InSb thin films that can be transferred to an industrial scale. The morphology, composition, and crystallinity of the prepared uniform and compact thin films with a surface area of around 1 cm2 were investigated. The essential electrical characteristics such as conductivity, Seebeck coefficient, the type, concentration and mobility of charge carriers have been examined and compared with InSb nanowires obtained in the same system for electrochemical deposition (fixed pulse sequence, temperature, electrolyte composition, and system geometry). Moreover, obtained thin films show much higher band gap energy (0.53 eV) compared to the bulk material (0.17 eV) and InSb nanowires (0.195 eV).

The main objective of this paper is to demonstrate the effect of crystal size on the optical properties of InSb nanowires. Nanowires were synthesized by a template-assisted pulse electrodeposition method. The crystal size in obtained nanowires was tuned using two approaches, namely via (i) adjustment of nanowire diameter by using anodic aluminum oxide (AAO) templates with different channel diameters (i.e., 25 nm, 46 nm, and 72 nm), and (ii) annealing of nanowires at various temperatures (i.e., 623 K, 673 K, and 723 K). The morphology, crystal size, and chemical composition of nanostructures were examined using different methods of materials characterization. Finally, an effect of crystal size on the optical properties of nanowires was demonstrated. Along with increasing the annealing temperature and diameter of nanowires, an increase in crystal size and widening of band gap energy were observed. Received dependences were compared with theoretical models proposed in the literature. To explain the nature of observed widening of band gap energy, the effective energy band gap (Eg eff.), quantum effect contribution (Eg q), Burstein-Moss effect contribution (ΔEBM), and charge carrier concentration (n) were estimated for studied diameters of as-prepared and annealed nanowires.

Residual stresses, microstructure and the surface topography of a substrate play a crucial role in the optimization of any electrodeposition process. To address this significant but often neglected problem, a detailed research focused on investigation of residual stresses in a Cu polycrystalline substrate electrochemically covered by a thin Au layer and Co coating was conducted. Additionally, the evolution of the microstructure, crystallographic texture and surface topography for the substrate and deposited layers were characterized. While the standard X-ray diffraction method showed insignificant residual stress in the Cu substrate, the multireflection X-ray diffraction methodology allowed us to determine small compressive depth-dependent residual stresses in Cu, close to the interface, when thin Co coating was deposited. Those stresses relaxed for longer deposition time, i.e. for ticker Co coating. The character of the stress distribution and microstructural investigations suggest that the stresses were generated in the Co coating due to coalescence of deposited islands.

The growing demand for clean energy catalyzes the development of new devices capable of generating electricity from renewable energy resources. One of the possible approaches focuses on the use of thermoelectric materials (TE), which may utilize waste heat, water, and solar thermal energy to generate electrical power. An improvement of the performance of such devices may be achieved through the development of composites made of an organic matrix filled with nanostructured thermoelectric materials working in a synergetic way. The first step towards such designs requires a better understanding of the fundamental interactions between available materials. In this paper, this matter is investigated and the questions regarding the change of electrical and thermal properties of nanocomposites based on low-conductive polypyrrole enriched with bismuth nanowires of well-defined geometry and morphology is answered. It is clearly demonstrated that the electrical conductivity and the Seebeck coefficient may be tuned either simultaneously or separately within particular Bi NWs content ranges, and that both parameters may be increased at the same time.

Anodic iron oxide layers were formed by anodization of the iron foil in an ethylene glycol-based electrolyte containing 0.2 M NH4F and 0.5 M H2O at 40 V for 1 h. The anodizing conditions such as electrolyte composition and applied potential were optimized. In order to examine the influence of electrolyte stirring and applied magnetic field, the anodic samples were prepared under the dynamic and static conditions in the presence or absence of magnetic field. It was shown that ordered iron oxide nanopore arrays could be obtained at lower anodizing temperatures (10 and 20 °C) at the static conditions without the magnetic field or at the dynamic conditions with the applied magnetic field. Since the as-prepared anodic layers are amorphous in nature, the samples were annealed in air at different temperatures (200–500 °C) for a fixed duration of time (1 h). The morphology and crystal phases developed after anodization and subsequent annealing were characterized using field-emission scanning electron microscopy (FE-SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. The results proved that the annealing process transforms the amorphous layer into magnetite and hematite phases. In addition, the heat treatment results in a substantial decrease in the fluorine content and increase in the oxygen content.

In this paper, we present recent results of our attempts to produce nanoporous zirconia, as well as our investigations of a hybrid material consisting of nanoporous zirconia loaded with Ag-nanoparticles, Ag-n/ZrO 2 -NT/Zr, which could be used as an active SERS substrate. The Zr-based hybrid material, as our investigations have shown, is an active and stable substrate in SERS investigations aimed at detecting various organic molecules: mercaptobenzoic acid, pyridine and two different dyes – rodhanine derivatives. The SERS spectra for the probe molecules adsorbed on silver nanoparticles on a ZrO 2 -NT/Zr platform display characteristic intensity ratios different from those measured on previously studied nanoporous substrates based on Ti and Al, which ensure a different (alternative) interaction between the investigated adsorbate and adsorbent. In order to characterize our new substrate we used high-resolution SEM and surface analytical techniques: XPS (X-ray photoelectron spectroscopy) and SERS (surface enhanced Raman spectroscopy).

Bimetallic Co-Pt nanorods exhibit an enhanced capacity for the production of gas from liquid-phase chemicals. Based on the systematic structural and magnetic characterization we discuss potential applications of these hybrid nanostructures for localized fuel generation in microdevices. Experimental proof of the feasibility for controlling the rate of catalytic reaction via external magnetic stimuli is shown. This unique functionality makes these hybrids promising candidates for optimizing the energy conversion rate in microfluidics fuel cells.

Correction for ‘Electrochemically deposited nanocrystalline InSb thin films and their electrical properties’ by K. E. Hnida et al., J. Mater. Chem. C, 2016, 4, 1345–1350.

Indium antimonide (InSb) is a III–V compound semiconductor that in a form of nanowires can possess improved thermoelectrical and optical properties compared to the corresponding bulk crystal. Here, we applied three electrodeposition techniques for a fast and inexpensive template-assisted fabrication of InSb nanowires from a sodium citrate-citric acid solution at room temperature. The home-made anodic aluminum oxide (AAO) templates with the pore diameter of 100 nm were used. InSb nanowires were synthesized by potentiostatic, galvanostatic and periodic pulse reverse techniques. The morphology, composition and crystallinity of as-obtained and annealed nanowires were investigated and compared with the literature data. It was found that the potentiostatic and pulse reverse methods gave crystalline nanowires. On the other hand, the constant current density deposition results in a partially amorphous nanowire material.

The hydroquinone monosulfonate-doped polypyrrole (PPy-HQS) nanowires were successfully fabricated by potentiostatic electropolymerization of pyrrole (Py) inside the pores of home-made through-hole anodic aluminum oxide (AAO) membranes. The AAO templates with a nominal pore diameter of 80 nm were prepared by a two-step anodization process. The potentiostatic electropolymerization of HQS-doped polymer nanowires was carried out in 0.1 M NaClO4, or 0.1 M LiClO4 or 0.1 M citric acid containing 0.05 M pyrrole and 0.05 M potassium hydroquinone monosulfonate. The synthesized PPy-HQS nanowire arrays were tested as potential potentiometric pH sensors. It was found that pH sensors based on PPy-HQS nanowires exhibited better electrochemical performance toward pH sensing than those based on PPy-HQS thin films. The best potentiometric response to pH changes and a very good stability in time showed the sensor based on the PPy-HQS nanowires polymerized in a 0.1 M LiClO4 solution.

We report on an effect of CoPt layer thickness on the magnetic properties of 3D CoPt-Pt multilayered nanowire arrays. The nanowires exhibit ferromagnetic behavior for the CoPt segments even only a few nanometers thick. It was found that decreasing thickness of CoPt nanosegments, by shortening the time of ‘magnetic material pulse’ during electrodeposition, leads to a stronger dependence of magnetization on temperature and the decrease of critical temperature. Also other magnetic parameters such as ground state magnetization, spin-wave parameter and Bloch exponent were determined for the CoPt-Pt nanowire arrays. The ground-state values of magnetization suggest that the mixing of Co and Pt occurs within the whole segment’s volume, however, the segments remain ferromagnetic. It is demonstrated that temperature dependencies of magnetization qualitatively follow the predictions of the spin-wave theory. Furthermore, based on the angle-dependent measurements of magnetization versus applied magnetic field, the coherent spin rotation was determined as a mechanism of magnetization reversal in the CoPt-Pt multilayered nanowire arrays.

A growing scientific interest in the fabrication of porous anodic aluminum oxide (AAO) films and their further applications for the fabrication of various devices, has given rise to many studies of porous alumina properties. Highly ordered porous alumina exhibits some unique physical and optical characteristics, especially in the visible spectrum. These properties are of technological importance for applications in the fields of micro and nanotechnology.

InSb nanowire (NW) arrays were prepared by pulsed electrodeposition combined with a porous template technique. The resulting polycrystalline material has a stoichiometric composition (In: Sb = 1:1) and a high length-to-diameter ratio. Based on a combination of Fourier transform infrared spectroscopy (FTIR) analysis and fi eld-effect measurements, the band gap, the charge carrier polarity, the carrier concentration, the mobility and the effective mass for the InSb NWs was investigated. In this preliminary work, a transition from p -type to n -type charge transport was observed when the InSb NWs were subjected to annealing.

Polypyrrole nanowires (PPyNWs) were fabricated and examined as a structural component of amperometric biosensor matrix. An enzyme, tyrosinase (TYR), was immobilized onto PPyNWs using glutaraldehyde (GA). Matrix composite morphology was investigated using scanning electron microscopy. Electrochemical behavior of the prepared PPyNWs/GA/TYR biosensor towards catechol was studied and the assessment of its analytical characteristics was carried out taking into account linear range, sensitivity, repeatability, reproducibility and operational stability.

The effect of annealing temperature on crystal structure of anodic titanium dioxide (ATO) layers prepared via anodization in the ethylene glycol and glycerol based electrolytes was studied. Then samples were annealed in air at the temperatures ranging from 400 to 1000 °C. The XRD measurements proved that a gradual phase change from anatase to rutile occurs with increasing annealing temperature. The anatase-to-rutile transformation occurs between 500 and 600 °C. The changes in the average crystallite sizes of anatase and rutile occurring during heat treatment of ATO layers were correlated with the mechanism of rutile phase nucleation. It was found also that the transition to the rutile phase in the samples formed in the ethylene glycol based electrolyte is considerably retarded and takes place at higher annealing temperatures due to the higher content of the embedded fluoride ions. The photoelectrochemical performance of ATO layers were studied under pulsed UV illumination. Photocurrent vs incident light wavelength and applied potential plots were recorded. The highest photocurrents were observed for the samples annealed at 400 °C, regardless of the electrolyte. It was demonstrated that the decrease in photocurrent values is related with the decreasing amount of the anatase phase in ATO samples. The enhanced photocurrent response was observed for ATO layers decorated with Ag nanoparticles. The highest photoconversion efficiencies, determined by incident photon-to-current efficiency (IPCE) calculations, were observed for the wavelength of 350 nm.

A simple electrochemical one-step method of synthesis of polypyrrole- silver (PPy-Ag) nanowire arrays is presented. The method is based on simultaneous cathodic polymerization and silver electrodeposition in the nanopores of homemade porous anodic aluminum oxide (AAO) templates. The synthesized 80 nm in diameter PPy nanowires have embedded Ag nanoparticles. The nanowires are characterized by scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, ultraviolet-visible-near-infrared absorption spectroscopy, and electro- chemical techniques. The morphology, chemical structure, and optical, electrochemical, and catalytic properties of the synthesized PPy-Ag nanowires are examined and compared with those of PPy nanowires. The effect of annealing the PPy-Ag nanowires above the glass transition temperature of polypyrrole on properties of the nanowires is studied. A surface segregation/aggregation of silver nanoparticles and degradation of polymer chain integrity occur during annealing. The as-prepared PPy-Ag and annealed PPy-Ag nanowire arrays possess enhanced catalytic activity toward electroreduction of hydrogen peroxide and can be used as novel and inexpensive sensors for its determination. The highest sensing activity is exhibited by the freshly prepared and annealed PPy-Ag nanowire electrode because of accumulation of catalytic centers on the surface of the nanowires (segregation of silver). However, the sensitivity of amperometric determination of $H_{2}O_{2}$ for the annealed PPy-Ag nanowire array decreases dramatically after storage. On the other hand, the as-prepared PPy-Ag nanowire arrays provide a stable in time sensing activity toward reduction of hydrogen peroxide. This can be attributed to a distribution of active Ag nanoparticles deep within the polymer matrix.

In this paper, we demonstrate a distributed Bragg reflector (DBR) based on nanoporous anodic aluminum oxide (AAO) formed by pulse anodization. The AAO structure with alternating mild anodized (MA) and hard anodized (HA) layers having different porosities and thereby different refractive indices was fabricated in 0.3 M H 2 SO 4 using potential pulses of 25 and 35 V. The effective refractive index of the HA layers can be tailored by changing the porosity of the HA layers. The porosity of the HA layers can be significantly increased by selective chemical etching of HA segments in 0.52 M H 3 PO 4 . Before etching, the porous AAO structure was supported by a polymer nanorod frame. On the selected surface area pores were infiltrated with polymers (polystyrene and PMMA). The designed AAO structure consists of alternating high and low refractive index layers and behaves as a distributed Bragg mirror reflecting light in two different ranges of wavelength. This behavior is extremely important in optical communication lines where two separate spectral bands of high reflectivity in the infrared region are desired.