Deposited ultra-thin titanium nitride nanorod array as a plasmonic near-perfect light absorber

The transmittance, reflectance, and extinctance that correspond to the localized plasmonic resonance within TiN nanorods were investigated. The obliquely deposited TiN nanorod array shows polarization-independent admittance matching to air. Unlike noble metal nanorods, the near-field localized longitudinal and transverse plasmonic resonance of TiN nanorod arrays present polarization-dependent light extinction in the far field. The longitudinal plasmonic mode presents stronger extinction than transverse plasmonic mode. In order to have high efficient light absorption, an ultra-thin two-layered TiN nanorod array was fabricated with orthogonal deposition planes for upper layer and bottom layer to absorb different polarized light energy. The measured spectrum shows broadband and wide-angle light extinction.

www.nature.com/scientificreports/ fabricate TiN nanorod arrays. Sputtered atoms are aligned and the flux is controlled by setting a plate parallel to the substrate 23 . The grains that are initially deposited on the substrate form a shadow with respect to the incoming vapor flux so the flux atoms that are deposited on the grains obliquely form nanorods. The absorption of a glancing angle-deposited TiN nanorod array (NRA) has been demonstrated to vary with the deposition parameters 24 .
In this work, a two-layered TiN nanorod array is developed as a perfect light absorber. This obliquely deposited TiN nanorod array exhibits polarization-independent admittance matching with air. Based on analysis of the polarization-dependent absorption, which is related to the longitudinal plasmon resonance and transverse plasmon resonance 25,26 , two-layered TiN nanorod arrays with different directions of growth are stacked to perform broadband and wide-angle light absorption.

Results
With the deposition parameters that are mentioned in the section of methods, a uniform TiN film was deposited and its permittivity spectrum was measured; the spectrum is shown in the supplementary information. The single TiN NRA was deposited at an angle of 84 • between the vapor flux and substrate surface. As shown in Fig. 1, the average tilt angle between the rods and the normal to the substrate surface was β = 34.8 • . The deposition plane is the plane that contains the growth direction of the nanorods and the surface normal. The angle of incidence is θ and the angle between the deposition plane and incident plane is ϕ . The average width and length of TiN NRAs was estimated from the cross-sectional SEM images by using an image processing program (Image J, 1.48). The widths of rods were measured at the middle of each rod. The width and length of the rods were 26.4 nm and 121.6 nm, respectively. The porosity of the TiN NRA was approximately 25.2%. The polarization-dependent reflectance (R) and transmittance (T) spectra were measured at angles of incidence that were integer multiples of 10 • , with the plane of incidence coincident with the deposition plane, as shown in Fig. 2. The TiN nanorod array exhibits admittance matching with the air. Although the nanorods are tilted with respect to the surface normal, the p-polarized transmittance spectrum is asymmetrical about the surface normal but the s-polarized transmittance spectrum is almost symmetrical about the surface normal, as shown in Fig. 2a,b. The p-polarized reflectance is less than 10% over all wavelengths from = 400 nm to = 2000 nm and angles of incidence from |θ| = 30 • to |θ| = 60 • , as shown in Fig. 2c. At any wavelength, the reflectance has a maximum at θ = 0 • and decays to a minimum at θ = ± 60 • . At θ = ± 60 • , the reflectance is less than 4.67% over the whole range of wavelengths. The average p-polarized reflectance over the wavelengths and incident angles is 8.08%. The s-polarized reflectance is less than 17.66% at angles of incidence from θ = − 40 • to θ = 40 • , revealing good admittance matching over the wavelength range from = 400 nm to = 500 nm , as shown in Fig. 2d. The reflectance increases smoothly from = 400 nm to = 2000 nm at angles between θ = 40 • and θ = − 40 • . Unlike the p-polarized reflectance, the s-polarized reflectance has a minimum at angle of incidence www.nature.com/scientificreports/ θ = 0 • , increasing with θ , as is conventional for s-polarized reflection at an interface. The average s-polarized reflectance over the wavelengths and incident angles is 26.16%. Figure 2e,f show the extinctance E, derived from E = 1 − R − T 27,28 . The p-polarized extinctance has a high value of E = 78.1% at θ = − 60 • and = 685 nm ; in this direction, the electric field oscillates along the rods, inducing longitudinal plasmon resonance. The p-polarized extinctance of over 65% is continuously distributed in the area inside the white dash lines in Fig. 2e. When an s-polarized incident wave is incident on a tilted nanorod array, the electric field usually oscillates perpendicular to the rods at all angles of incidence, and transverse plasmon resonance is induced over a wide range of angles at a wavelength of close to 400 nm. Therefore, s-polarized extinctance is strong at 400 nm over a wide range of incident angles. An s-polarized extinctance of over 35% is continuously distributed in the area inside white dash lines in Fig. 2f. Figure 3 shows the p-polarized www.nature.com/scientificreports/ transmission images that were captured at θ = 60 • and θ = − 60 • . The image at θ = 60 • was blocked and the image at θ = − 60 • was clearly captured. Interestingly, the localized plasmonic resonances within the TiN nanorods present polarization-dependent absorption, unlike the nobel metal nanorod arrays. The longitudinal and transverse plasmon resonances of an obliquely silver nanorod array reveal not only polarization-dependent absorption but also obvious polarization-dependent reflection 29 . The polarization-dependent extinction is further investigated by measuring the reflectance and transmittance spectra with the plane of incidence perpendicular to the deposition plane ( ϕ = 90 • ), as shown in Fig. 4. The p-polarized and s-polarized reflectance spectra are similar to the spectra that were measured at ϕ = 0 • , exhibiting low reflection over a broadband and a wide range of angles. The s-polarized extinction is symmetrical about the surface normal and yields a spectrum similar to that in the case of ϕ = 0 • . Since the electric field of p-polarized light has a large component perpendicular to the rods at all angles, the p-polarized extinctance is nearly symmetrical about the surface normal and has two maxima at θ = 60 • and θ = −60 • at 630 nm, corresponding to the longitudinal plasmonic mode.
The extinctance over visible wavelengths can be increased by increasing the thickness of TiN NRA. A 281 nmthick TiN NRA that is fabricated with the same deposition parameters has higher extinction, as shown in Fig. S2 and Fig. S3. A thicker TiN NRA exhibits stronger light extinction over the visible wavelengths, with a highest extinctance of 97.6% at ( , θ) = (679 nm, − 50 • ) ) when the incident plane is oriented at ϕ = 0 • . The extinctance is reduced to be less than 30% at 2000 nm, favoring the prevention of radiation loss in some STPV systems 30 .
The reflectance, transmittance and extinctance spectra of a uniform TiN film with a thickness of 100 nm as the aforementioned TiN NRA are plotted in Fig. S4 with the permittivity function shown in Fig. S1. The sample is opaque and strong light absorption only occurs at wavelengths near 400 nm. The reflectance exceeds 60% for wavelengths over 813 nm at normal incidence. As most optical coatings, the s-polarized reflectance increases with angle of incidence and larger than the p-polarized reflectance.
Based on the aforementioned analysis, a two-layered TiN nanorod array as a broadband and wide-angle light absorber is developed here. As shown in Fig. 5a, the upper NRA was deposited by rotating the substrate by 90 • following the deposition of the bottom nanorod array, so the deposition plane of the upper NRA was perpendicular to that of the bottom NRA. The two TiN NRAs were stacked on a 39 nm-thick uniform TiN thin film that was  Figure 5a also schematically depicts the orientations of the nanorods with respect to the system coordinates. The s-polarized and p-polarized transmittance, reflectance and extinctance spectra were measured with the incident plane coincident with the x-y plane. The p-polarized extinctance was larger than 90% at angles of incidence from 0 • to − 60 • and wavelengths from = 400 nm to = 861 nm . For wavelengths of greater than 1400 nm, a high extinctance of over 78.9% obtained at angles of incidence between − 50 • and 50 • . For wavelengths from 900 to 1300 nm, the reflectance www.nature.com/scientificreports/ versus angle of incidence had a maximum at normal incidence, decaying to a minimum at angles around − 50 • -similar to conventional reflection at the Brewster angle. High extinctance from 400 to 851 nm has values of over 90.8% between angles − 60 • and 60 • . At wavelengths above 1400 nm, the extinctance has a maximum around 93.8% and decays as the angle of incidence increases. Of the two strong extinctance areas on the θplane, one is centered at ( , θ) = (498 nm, 50 • ) with an extinctance peak of 98.4% and the other is centered at ( , θ) = (506 nm, 50 • ) with an extinctance peak of 96.9%. Both areas in which the p-polarized extinctance exceeds 95% are indicated by a white dashed line in Fig. 6. The s-polarized extinctance has a strong band from 400 to 800 nm. The angular range that is associated with the strong extinctance shrinks gradually as the wavelength increases. At 400 nm, the extinctance exceeds 91.9% from − 60 • to 60 • . At 800 nm, the extinctance exceeds 86.4% from − 50 • to 50 • . The extinctance is uniformly distributed between 400 and 800 nm. At wavelengths of over 1400 nm, the extinctance rises to maximum values of 93.2% at ( , θ) = (1740 nm, − 30 • ) and 92.6% at ( , θ) = (1617 nm, 40 • ) . The s-polarized transmittance is 8-13.9% and the p-polarized transmittance is 3.2-7.3% at wavelengths above 1400 nm. A thicker uniform TiN layer underneath the nanostructured TiN films provides greater extinctance by reducing the transmittance. The extinctance versus θ and has a peak at ( , θ) = (566 nm, − 10 • ) with a maximum value of 97.0%. The s-polarized extinctance exceeds 95% in the area that is marked with a white dashed line in Fig. 6f.

Discussion
The finite-difference time-domain (FDTD) (Lumerical FDTD Solutions 8.11.337) is adopted to simulate the near-field electric field within the two-layered structure. As shown in Fig. 7, identical TiN nanorods with geometric parameters of ( β , w, l) = (30°, 32.5 nm, 140 nm) are regularly distributed as the bottom NRA and the center-to-center distance between adjacent rods is 70 nm. The upper nanorods are connected to the bottom rods with deposition plane on x-z plane that is perpendicular to that of the bottom rods. A 39 nm-thick TiN film is arranged between the two-layered NRA and a BK7 glass substrate. Since the fan-out phenomenon observed from the SEM is obvious for the upper layer, the rod width is set to increase from 16.2 to 29.3 nm linearly. The length and tilt angle of upper nanorods are 117 nm and 30°, respectively. The permittivity of the TiN is adopted from functions of the real part and imaginary part of the permittivity shown in Fig. S1.
The electric field intensity distributions are simulated at wavelengths of 400 nm, 600 nm, 1000 nm and 1500 nm. The electric field intensity defined as |E/E i | 2 where E i and E are the amplitudes of incident electric filed and localized electric field, respectively. For p-polarization (the electric field is oscillating along the x-axis), the cross-sectional x-z and y-z planes shows that the bottom rods have stronger field enhancement than the upper www.nature.com/scientificreports/ rods, as shown in Fig. 8. The strong electric filed distributed inside each rod indicates the resonance of transverse plasmonic mode 26 . For s-polarization (the electric field is oscillating along the x-axis), the cross-sectional x-z and y-z planes shows that the upper rods have stronger field enhancement than the bottom rods, as shown in Fig. 9. The strong electric filed distributed between rods indicates the resonance of longitudinal plasmonic mode 26 . Since the permittivity of the TiN film is negative and its magnitude increases with wavelength, the optical property of the TiN film is similar to noble metals at infrared wavelengths. Therefore, the enhanced field intensities at wavelengths of 1000 nm and 1500 nm are stronger than those at 400 nm and 600 nm. The metal-like optical property of TiN NRA would enhance longitudinal plasmonic resonance and lead to increase in reflection. Therefore, the extinction at visible wavelengths is larger than that at infrared wavelengths. In summary, a broadband absorber was designed, fabricated and optically characterized. The admittance matching of an obliquely deposited TiN nanorod array reveals that the localized plasmon resonance exhibits absorption only in the far field. By varying the orientation of a TiN nanorod array with respect to the incident polarized light, the absorption spectrum can be varied to fulfill certain demand. Since TiN is a refractory material, www.nature.com/scientificreports/ the TiN nanorod absorber can be applied in an STPV system to absorb strongly visible and NIR wavelengths. A two-layered TiN nanorod array in this work exhibits broadband and wide angle light absorption. As the TiN nanorod array or TiN film underneath the nanorods becomes thicker, the absorption will increase furthermore. This work proposes an easy method for the bottom-up nanostructure fabrication on a large area. Since the optical and mechanical properties of TiN films fabricated in a sputtering system can be varied by changing the reaction gas flow rate, substrate temperature and substrate bias voltage. TiN nanorods can be tuned in design and fabrication for use as an absorber with desired optical characteristics.

Methods
The TiN film and TiN NRAs were deposited on BK7 glass substrates using a homemade DC reactive magnetron sputtering system with a 99.995% titanium target (Kurt Lesker, Jefferson Hills, PA, USA) in an argon-nitrogen environment. The base pressure in the chamber before deposition was 5 × 10 −6 Torr. The films were deposited at a pressure of 2.8 × 10 −3 Torr. The sputtering power was constant for all depositions at 200 W (DC). The deposition rate was around 1.0 Å/s and controlled with a quartz monitor. To deposit TiN, a pure Ti target was used with Ar as the sputtering gas and N 2 as the reactive gas. To obtain stoichiometric TiN, the argon flow rate and nitrogen flow rate were maintained at 15 sccm and 1.2 sccm, respectively. The deposition angle of the stage is defined as the angle between the normal to the substrate and the direction of vapor flux from the sputtering source. The uniform TiN thin film was deposited firstly at a deposition angle of 0 • . Then the bottom TiN NRA was deposited at a deposition angle of 84°. In order to control the sputtering flux rate, a stainless steel plate was placed parallel to the substrate at a distance 10 mm from it. After the deposition of bottom layer, the substrate was rotated by 90° to deposit the upper layer. The upper TiN NRA was grown at a deposition angle of 88° and the distance between the substrate and the stainless steel plate was 8 mm.
The permittivity spectrum was obtained using commercial spectroscopic ellipsometer (J. A. Woollam Co., VASE). The polarization-dependent reflectance (R) and transmittance (T) spectra were obtained using a UV-visible/NIR spectrophotometer (Hitachi High-Tech Corporation, UH4150). www.nature.com/scientificreports/ Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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