plasmonic aluminum

7/25/2019 Plasmonic aluminum.

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Applied Surface Science 314 (2014) 807814

Contents lists available at ScienceDirect

Applied Surface Science

j ournal homepage: www.elsevier .com/ locate /apsusc

Tailoring ofUV/violet plasmonic properties in Ag, and Cu coated Alconcaves arrays

Magorzata Norek a,, Maksymilian Wodarskib, WojciechJ. Stepniowskia

a Department of Advanced Materials and Technologies, Faculty of Advanced Technologies and Chemistry, Military University of Technology,

Str. Kaliskiego2, 00-908 Warszawa, Polandb Institute of Optoelectronics, Military University of Technology, Str. Kaliskiego2, 00-908 Warszawa, Poland

a r t i c l e i n f o

Article history:

Received 13 May 2014

Accepted 30 June 2014

Available online 8 July 2014

Keywords:

UV plasmonics

Aluminum concaves

Anodization

Copper

Silver

Reflectivity spectra

a b s t r a c t

UV plasmonics is ofparticular interest because oflarge variety ofapplications, where the higher energy

plasmon resonances would advance scientific achievements, including surface-enhanced Raman scatter-

ing (SERS) with UV excitation, ultrasensitive label-free detection of important biomolecules absorbing

light in the UV, or the possibility for exerting control over photochemical reactions. Despite its potential,

UV plasmonics is still in its infancy, mostly due to difficulties in fabrication ofreproducible nanostruc-

tured materials operating in this high energy range. Here, we present a simple electrochemical method to

fabricate regular arrays ofaluminum concaves demonstrating plasmonic properties in UV/violet region.

The method enables the preparation of concaves with well-controlled geometrical parameters such as

interpore distance (Dc), and therefore, well controllable plasmon resonances. Moreover, the patterning

is suitable for large scale production. The UV/violet properties ofAl concaves can be further fine-tuned

by Ag and Cu metals. The refractive index sensitivity (RIS) increases after the metals deposition as com-

pared to RIS of pure Al nanohole arrays. The highest RIS of 404 nm/RIU was obtained for Cu coated Al

nanoconcaves with the Dc= 460.8 nm, which is similar or better than the RIS values previously reported

for other nanohole arrays, operating in visible/near IRrange.

2014 Published by Elsevier B.V.

1. Introduction

The interest in understanding of light interaction with ordered

arrays of nanoholes in thin metal films has been a topic of research

for number of years [14]. Surface plasmons (SPs) are essentially

the coupled oscillations of light and free electrons at the interface

between a metal, which has a negative dielectric constant, and a

positive dielectric material. They are classified as surface plasmon

polaritons (SPPs) [5,6] and localized surface plasmons (LSPs) [7].

While in metal nanoparticles of different size and shape only LSPs

is present [810], tunable and regular metal nanovoids can support

both types of resonances depending on their truncation [1114]. It

was observed that rectangular holes are considerably better than

circles or squares due to larger LSPs contribution [15].

Optimal plasmonic properties (the strongest and narrowest

resonances) are provided by metals with small imaginary part

of the dielectric constant [1618]. Most of the metals possess

Corresponding author. Tel.: +48 504628903.

E-mail addresses:[email protected], [email protected] (M. Norek).

inter- and intraband transitions that increase the imaginary part

of their dielectric constants. This phenomenon causes weaker and

broader plasmonic resonances in metals such as Ru, Pd, Pt or Ni

[1922]. Because of the transitions, gold (Au), silver (Ag), and cop-

per (Cu) do not generate plasmon resonance in UV range. On the

other hand, Au, Ag, Cu metals demonstrate excellent plasmonic

performance in the visible and near-IR region [2332] and their

plasmonic properties have been already exploited in many tech-

nological devices such as ultra-sensitive detectors, field-enhanced

fluorescence spectroscopy, or surface enhanced Raman spec-

troscopy (SERS) [3336]. There are, however, important upcoming

applications which stipulate the research on searching for mate-

rials with optimal plasmonic performance in the ultraviolet (UV)

range. UV plasmonics can be exploited in UV surface enhanced

Raman scattering spectroscopy (UV SERS), where scattering effi-

ciency is greatly improved. Biomolecules such as proteins and

nucleic acids contain residues that absorb light in UV. Plasmonic

structures operating in UV could enable the label-free detection

of those biomolecules. SPR in UV region could be used to enhance

light extraction in blue/UV light emitted devices (LEDs). Last but

not least, potential applications UV plasmonics include solar cells,

photocatalysis, or UV optical waveguides.

http://dx.doi.org/10.1016/j.apsusc.2014.06.192

0169-4332/ 2014 Published by Elsevier B.V.
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808 M. Norek et al./ Applied Surface Science 314 (2014) 807814

Aluminum (Al) can be the material of choice for plasmonic

application in the UV region. Al has an interband transition near

1.4eV [16], what does not affect its UV performance. The optical

spectra of Al show well-defined resonance peaks in UV or even

deep UV, which additionally are very sensitive to size and shape

of nanostructures [3739]. Though aluminum is easily oxidized,

the process is self-limiting giving a thin and stable oxide thickness

which makes it easy to work with even in atmospheres contain-

ing oxygen [4042]. Aluminum can exhibit strongly enhanced local

fields owing to its high electron density (3 conductive electron per

atom in contrast to 1 electron per atom in metals such as Au or

Ag), which was used in plasmonic nanoantennas for fluorescent

enhancement in UV [43,44]. Moreover, Al is abundant and cheaper

thanmost other metals, which canbe veryappealing for sustainable

photovoltaics.

SPs are very sensitive to the near-surface dielectric constant

(index of refraction), what has been used in a wide variety of SPR-

based sensors. In order to efficiently utilize the plasmonic field

enhancement, careful matching of plasmon resonance in a given

sensing medium to a particular wavelength of a target material is

necessary.In free-electron metallicnanoparticles SPRscan be tuned

by changing size and shape of the nanoparticles, or by prepara-

tion of hybrid or alloyed nanostructures [45]. In nanohole array the

SPR depends on the pitch size, but is also susceptible to change ofthe holes size and shape [11,46]. In order to improve the sensing

performance and broaden the application of nanoplasmonics other

approaches, including modeling of complex structures with multi-

ple SPRs [47], or development of other sensing mechanisms [48],

were proposed as well. Recently, it was demonstrated that coat-

ingof nanohole arraysin metallic thin filmwith another plasmonic

metal offer better plasmonic performance and can be used for fine

tuning of SPRs aiming to excite different fluorescent dyes [49].

Current progress in nanoplasmonics has been also focused

on searching for less-expensive fabrication methods. Typi-

cally, ion beam or electron beam lithography (EBL) is used

[4,13,15,24,25,27,34,40] to produce nanohole arrays. These meth-

ods are, however, not well suited for large-scale production (the

typical area of the periodic nanostructure is about 100m). In thiswork, we show that regular, hexagonally arranged Al nanocon-

caves, produced by a well-known electrochemical method, can be

a promising material for plasmonics operating in UV/violet range.

The preparation method is easy, reproducible, and the nanocon-

caves can be synthesized on arbitrarily large area (20 mm2 in this

paper). The material can be, thus, applied for high-throughput

based detections. Geometrical parameters, and therefore the opti-

cal properties of the Al hole arrays, can be easily controlled by

changing the operating conditions during electrochemical synthe-

sis, such as voltage, temperature or time of the first anodization.

Further tuningof plasmonic behavior canbe achieved by coating of

the Al concaves with a thin layer of other plasmonic metals, such

as Ag, and Cu.

2. Experimental

High-purity aluminum foil (99.9995% Al, Puratronic, Alfa-

Aesar) with a thickness of about 0.25mm was cut into coupons

(2cm2cm). Before the anodization process the Al foils were

degreased in acetone and ethanol and subsequently electropol-

ished in a 1:4 mixture of 60% HClO4 , and ethanol at 10C, constant

current density of 0.5 A/cm2, for 1min. Next, the samples were

rinsed with distilled water, ethanol and dried. As prepared Al

coupons were insulated at the back and the edges with acid resis-

tant tape. A Pt grid was used as a cathode,and thedistancebetween

both electrodes was kept constant (ca. 1 cm). A two-electrode elec-

trochemical cell with a Pt grid cathode was used in the anodizing

process. Two samples (Al concaves) were prepared. To obtain the

firstsample(averageinterpore distance of 246.3 nm),hard anodiza-

tion (HA) was applied according to Lee et al. [50]: the sample was

first pre-anodized in 0.3M oxalic acid solution at 40V and 0 C for

10min. Then, the voltage was gradually increased to 120 V, and the

anodization was carried out for 1 h. The anodization of the second

sample, with an average interpore distance of 456.7 nm, was per-

formed in a mixture of phosphoric acid solution (0.1M), water and

glycol (3:1, v/v), at temperature of4 C, and for 20h. As-obtained

alumina was chemically removed in a mixture of 6 wt%phosphoric

acid and 1.8 wt% chromic acid at 60C for 120min.

Morphology of nanoporous aluminum and microanalysis of

chemical composition of the samples coated with various metals

was studied using field-emission scanning electron microscope FE-

SEM (FEI, Quanta) and with Carl Zeiss Leo 1530 FE-SEM equipped

with energy dispersive X-ray spectrometer (EDS). BSE images were

taken with energy selective backscattered (EsB) detector at low

(3 kV) acceleration voltage.

To obtain geometrical parameters of the fabricated Al concaves

Fast Fourier transforms (FFTs) were generated based on three SEM

images takenat thesame magnificationfor everyanodizing voltage,

andwere further used in calculationswith WSxM software [51,52].

To estimate regularity ratio, three intensity profiles were gener-

ated from each FFT image. The regularity ratio (RR) was estimatedaccording to the following formula (1) [53]:

RR =H

W1/2 n (1)

wheren is the number of pores on the analyzed image,Hthe maxi-

mal intensity valueof theFFT intensityprofile,andW1/2 isthewidth

of the intensity profile at half of its height. Interpore distance was

estimated as an inverse of the FFTs radial average. The average

interpore distance (Dc) was estimated from three FE-SEM images

for each sample. Pore diameter (Dd) of the analyzed nanostructures

was estimated from three FE-SEM images for each operating con-

ditions, using NIS-Elements software provided by Nikon Company.

Pores density (number of pores per 1m2) was evaluated based

on six FE-SEM images for a given set of experimental conditions.Detailed information on the FFTs image analysis can be found in

refs. [53,54].

The Ag, and Cu coating of as prepared Al concaves was done

by vacuum evaporation technique under high vacuum of 106 kPa

using resistively heated tungsten crucibles. The evaporation pro-

cess was performed in the same experimental conditions for all

samples: the weights of metals were kept constant, the distance

from the crucibles to the substrates was always the same. The time

of the metals deposition lasted till the crucible was empty.

Reflectivity measurement was performed using CCD spec-

trometer with fiber-optics reflection probe (Avantes) and deu-

terium/halogen light source (Ocean Optics Inc.). The probe was

set up at normal angle to measured sample. All reflectance spectra

were collected at 2001100 nm wavelength range. ElectropolishedAl coupon was used as a reference sample with 100% reflection.

3. Results and discussion

In Fig. 1 representative top view scanning electron microscopy

(SEM) images of the hemispherical Al concave arrays, synthesized

at 120 V (Fig. 1A) and at 195 V (Fig. 1B), are shown. After electro-

chemical anodization and thorough removal of the resulting oxide,

the periodic hexagonal hole arrays appear on the remaining Al foil.

The pores form a honeycombstructure, which is an exact replica of

the morphology of the AAOnanopore bottoms. Hexagonal shape of

pores is more regular in the case of the sample fabricated at 120V

as compared to the one synthesized at 195 V. The size of the pores

increases with the applied voltage. On top of the Al concaves silver


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M.Norek et al./ Applied Surface Science 314 (2014) 807814 809

Fig. 1. SEM micrographs (accelaration voltage of3 kV)of the sampleproducedat 120V (A) and 195V (B),coatedwithAg (Cand D)and Cu (E and F)metals,respectively. The

scale bar appliesto all images.

(Ag) and copper (Cu) metals were coated. Fig. 1(C) and (D) show

the top view SEM images of the sputtered Ag film, demonstrating

thatthinAg filmis composedof tiny Agnanoparticleswhich aredis-

persedon the cavities andthe edges of the apertures of theconcave

arrays. ThethinCu layerseemsto uniformlycover theentiresurface

ofthe concave arrays. Inaddition,sometextureof thelayer isvisibleon the sample anodized at 195 V (Fig. 1E and F). The compositional

contrast of the samples confirmed the above observations (Fig. 1-

S in the Supplementary data). The sample covered with Ag shows

bright contrast coming from higher atomic number Ag, which cor-

respondsto theAg nanoparticlesscatteredon loweratomicnumber

Al nanoconcaves. Onthe other hand,there is no compositional con-

trast visible in the sample covered with Cu, giving indication that

the Al materialis evenlycoated bythe Cu.The components ofthe Ag

and Cu coated Al nanobowl arrays are confirmed by energy disper-

sive X-ray spectroscopy (EDS) measurements (Fig. 2). In addition

to Al, Ag, and Cu elements, the presence of C and O were detected.

Oxygen comes from partial oxidation of the samples, which were

kept in air. The aluminum oxide with a thickness of 2.53.0 nm is

formedon the Al surface within fewhours after exposure to air[41].The process is self-limiting and its negative effect was not noticed

for the Al nanoholes arrays previously investigated [40,42]. Therole

ofAl2O3becomes important for very small nanoparticles (>10nm).

It was observed that the Al2O3 tends to red-shift the position of

SPRs in those tiny nanoparticles, but is rather undisruptive for the

extinction efficiency [39,41]. Likealuminum,copper is proneto oxi-

dation. However, some advantages of Cu surface oxidation, such as

enhancement of plasmonic response, were also noticed [55].

Fig. 2. EDS measurementsfor thesamples coated with Ag (A), andwith Cu (B). The analysis presents theresults obtained for theAl concaves synthesized at 120V.


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Fig. 3. Lower magnification SEM micrographs (accelaration voltageof 20kV) of the sample produced at 120V (A) and 195V (B) along with their respective FFT images (in

the upper, right corners).

Table 1

Poresdiameter (Dp), interpores distance(Dc), regularityratio (RR), andpores density

valuesfor thesamples anodized at 120V and 195V as determined by FFTs analysis.

Dp Dc RR Pores density

120 V 188.316.4 238.2 0.1 1.84 0.29 14.8 1.4

195 V 356.627.0 460.8 11.6 1.60 0.40 4.2 0.2

To obtain geometrical parameters of pure Al nanoconcave

arrays, relatively large SEM image areas were used in Fast Fourier

transforms (FFTs) analysis (Fig. 3). The determined values are gath-

ered in Table 1. The FFT image of the sample fabricated at 120V

demonstrates six distinct points in the corners of hexagon, con-

firming good, hexagonal arrangement of pores. In the case of the

sample anodized at 195V the points in the FFT image are moreblurred, suggesting lower degreeof pores order.The degreeof long-

range alignment of nanopores is best reflected in the regularity

ratio parameter (RR). The larger the RR, the better the alignment

[53,54]. The RR value of 1.840.29 was determined for the sam-

ple fabricated at 120 V, whereas for the sample prepared at 195 V

the RR value was 1.600.40. The worse pores arrangement in

the sample anodized at 195V is most likely linked with smaller

domain size. Within a domain high local nanoconcaves regular-

ity is observed. The domains are, however, separated by defects

and grain boundaries, where the regularities of pores drop signifi-

cantly. The bigger the domains, the higher the pores arrangement,

as indicated by the RR parameter. A domain size depends on vol-

ume expansionduring the conversion of aluminum to alumina and

the experimental conditions (i.e. temperature, time of anodiza-tion) [56]. The volume expansion is strongly influenced by a type

of used electrolyte. The larger the expansion the smaller the size

of the ordered domain [57]. During anodization process there is a

movement of negatively charged ions (OH, O2, PO43, C2O4

2),

which are attracted to the positively polarized anode. The smaller

ions combine with Al3+ cations to form a nanoporous AAO. The

larger ions from electrolyte (PO43, C2O4

2) accumulate in the

walls of the growing oxide causing gradual deterioration of pores

arrangement [58]. The ionic mobility increases with the applied

potential. It can be, therefore, expected that the larger the ions and

the higher the applied voltages the stronger the deterioration. This

explains why anodization in phosphoric acid solution (relatively

large PO43 ions) and at relatively high voltages (195V) results in

lower RR.

The resulting pores diameter (Dd) and interpores dis-

tance (Dc) determined by FFTs analysis are: 188.316.4nm

and 238.20.1 nm for the sample anodized at 120 V, and

356.627.0nm and 460.811.6 for the sample anodized at 195 V,

respectively. The increase ofDc is accompanied by the decrease

of pores density (number of pores per 1m2): it decreases from14.81.4 for sample anodized at 120V down to 4.20.2 for the

sample anodized at 195 V (Table 1).

Investigations of plasmonic properties of materials usually

proceed by monitoring a dipin reflectance (R), when an evanescent

light field travels through a metal thin film and excites SPs at the

metaldielectric interface. As a result, the normalreflectivity of the

metalsurfaceis greatly reduced on resonancedue to optical absorp-

tion by the metal [2]. In Fig. 4 the reflectivity spectra of the studied

samples are given. Theoverall reflectivity of the patterned sample ismuch lower thanthat ofthe continuous Al film.At the same timein

the reflectivity spectra pronounced minima are observed, the most

distinct one at a photon wavelength of254 nm and 395nm, for

the sample with Dc = 238.2 nmandDc=460.8nm, respectively. The

reflection intensity measured at the centre of the minima drops

down to 12.5% for the sample anodized at 120V and to 2.7% for

the sample anodized at 195 V as compared to100% reflectivity of

unstructured Al (Table 2).

Theminimaare signaturesof SPPsexcitations,whichare directly

connected to the symmetry of the lattice and the optical constants

of Al. At normalincidence, coupling photons of a given energy with

2-dimensional hexagonalperiodic array gives SP resonances(SPRs)

at the following wavelength (Eq. (2)) [3,22]:

=a0

43 (i

2 +j2 + ij)

mdm + d

(2)

where a0 is the pitch size (interpore distance, Dc), d is the

frequency-dependentpermittivity of the dielectric material andmis the real part of the frequencydependentpermittivity of the metal

(Alin this case). Theintegers i, j specify the orders of SP resonances.

An iterative method was applied to solve Eq. (2), to account for

frequency dependent permittivity. For mcalculation Drude modelwas applied with constants provided by ref. [59]. The analytical

wavelength calculated for (i,j) = (0,1) or(1,0)((0,1)) was: 217,3 nmfor the sample with Dc = 238.2nm, and 402.9nm for the sample

withDc =460.8nm. The theoretical (0,1)is pretty close to the mea-

sured reflection minima in the sample anodized at 195 V, whereas


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M.Norek et al./ Applied Surface Science 314 (2014) 807814 811

Fig. 4. Reflectivity spectra of Al concaves fabricated at 120V (A) and at 195V (B) pure (red dashed lines) and coated with Ag (blue dotted lines) and Cu (magenta dash dot

lines) metals. Forcomparison,the reflectivity of unstructuredAl in also provided (black lines). Theinsets show largermagnificationof thereflectivityspectra demonstrating

the minima shift after themetals deposition. (For interpretation of thecolor information in this figure legend, thereader is referred to theweb version of thearticle.)

Table 2

Position of reflectivity minima ((0,1) SPR mode),reflectance intensity at the(0,1) and refractive index sensitivity (RIS) valuesfor thesamples anodized at 120V and 195V.

Al concaves Al concaves with Ag Al concaves with Cu

120V

(0,1) [nm] 254 259 262

Reflectance at (0,1) [%] 12.5 6.2 1.7

RIS [nm/RIU] (the correlation coefficient) 189 (R2 = 0.9964) 191 (R2 = 0.9973) 199 (R2 = 0.9988)

195 V

(0,1) [nm] 395 407 425

Reflectance at (0,1) [%] 2.7 7.1 2.2

RIS [nm/RIU] (the correl ation coefficient) 296 (R2 = 0.9984) 399 (R2 = 0.9953) 404 (R2 = 0.9985)

is smaller thanthe minima related to the sample anodized at 120 V

(254nm). The (0,1) SPR mode is shifted to higher wavelength

(lower energy) most probably due to additional contributions from

localized surface plasmons. The Al concaves possess very sharp

edges (Fig. 2-S in the Supplementary data), which can sustainLSPs. The interaction between the LSPs and SPPs can generate a

strong plasmonic field, which can add to or subtract from the

SPR. Apparently, the interaction lowered the plasmonic resonance

energy in the sample characterized by better pores arrangement

as determined by FFTs analysis (RR= 1.84). For the sample fabri-

cated at 195 V, the potential LSPs effect is offset by poorer pores

arrangement (RR= 1.60) and relatively larger contribution from

grain boundaries. In the boundaries the shape of hexagons is very

irregular, or evenpentagons and heptagonsare present as an effect

of lattice defects, and therefore, the LSPs-SPPs interaction is effec-

tively suppressed.

The (0,1) reflectivity dips are slightly shifted to longer wave-

lengths after Ag and Cu metals deposition on the Al concaves (the

insets in Fig. 4). The largest shift of around 30nm is observed forCu coated Al concaves with the pitch size of 460.8nm (Table 2).

Additionally, the peak related to the samples covered by Cu (par-

ticularly to the one anodized at 120 V) becomes clearly broadened,

which is probably due to partial oxidation of Cu layer [28]. These

results indicate that the reflectivity dip is dominated by the Al

concaves and is only fine-tuned by the thin Ag, and Cu coating.

It was previously observed that in order to get a substantial con-

trast between the surrounding metal and the holes, themetal must

be opaque (optically thick), which means that the layer thick-

ness (or nanoholes depth) must be several times the skin-depth

of the metal [3,60]. The skin depth is the distance where the elec-

tric filed falls to 1/e. Typical skin-depth for noble metals in the

visible spectrum is of 20nm. When metal film is very thin, it is par-

tially transparent for the incident light, and no nanohole arrays is

necessary to obtainsignificant reflectiondrop, especially if the sur-

face is resonantly corrugated [61]. The film thickness formed by Ag

and Cu on the Al concaves can be considered as optically thin and,

therefore, plasmonic sensing performance of the combined metals

is basically determined by the aluminum nanoconcaves. Similarresults were obtained for hybrid plasmonic nanoparticles systems:

plasmonicresonances of copper- [62] and gold-shell [26] coated sil-

ver nanoparticles were only slightly shifted to longer wavelength

with respect to pure Ag NPs. Furthermore, thicker coating induced

stronger red-shift of LSPR peak.In contrast,dielectric coremetallic

shell spheroid nanoparticle demonstrated the LSPR shift to shorter

wavelength with the increase of the shell thickness [10]. Simula-

tion results showed that various plasmonic resonance modes of

nanohole arrays in metallic films (such as Ag, Al, Cu) are shifted

within 35nm after coating them with Au thin layer (10nm) [49],

which is in agreement with the results obtained in this work.

The intensity at the reflectivity minima is further reduced after

the metals deposition (Table 2). For the Cu coated Al concaves

with Dc =238.2nm the reflectivity as low as 1.7% is obtained, ascompared to unstructured, pure Al. These excellent antireflecting

properties of regular, hexagonally arranged Al nanoconcaves can

be, thus, considered for applications in solar photovoltaics. In order

to increase absorption in silicon thin films, texturing of the active

layeritself or application of various structures, including plasmonic

structures, have been widely investigated [6366].

SPRsensitivity of thecoated andpure Al nanoconcave arrays was

measured as resonance wavelength shift per refractive index unit

(RIU). After immersing the samples in liquids with different refrac-

tive index the reflectivity dips shift to longer wavelengths (Fig. 3-S

in the Supplementary data). In Fig. 5 (0,1) as a function of refrac-tive index is presented, and refractive index sensitivity (RIS) values

are determined from the slope of linear fit to the points measured

at the centre of the reflectivity minima (Table 2). It can be seen


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Fig. 5. Reflectivity dips ((0,1) ) as Figs. 4 and5 willappear in black andwhitein print andin color on theweb.Basedon this,the respective figure captionshave been updated.

Please check,and correct if necessary a functionof refractive indexfor Al concavessynthesized at120 V (A), andat 195V (B), pure (squares)andAg (circles)andCu (diamonds)

coated, along with the respective sensitivities predicted by SP theory (Eq. (2)) (black, dotted lines). The red, blue, and green lines represent linear fits to the measured data

to obtain refractive index sensitivity (RIS) values. (For interpretation of thecolor information in this figure legend, thereader is referred to theweb version of thearticle.)

that RIS increases for Ag and Cu coated Al concaves as comparedto pure Al. For the sample with Dc = 238.2nm, RIS changes from

189 nm/RIU for pure Al concaves to 199 nm/RIU for Cu coated Al

nanoconcaves arrays. Thesame trend canbe noticed forthe sample

with Dc =460.8nm: RIS399 nm/RIU and404 nm/RIU for Ag and

Cu coated Al concaves, respectively, as compared to296nm/RIU

forpure Al nanohole arrays. The increase of theplasmonic sensitiv-

itywas observed in Au coatedAg nanoparticles [26]. The plasmonic

field enhancement was also demonstrated by simulation in Au-

coated Ag, Cu and Al nanohole arrays in visible region [49]. Since

the metal coating does notchange theDc of the respectivenanohole

arrays, the increase of RIS (as well as the (0,1) shift) may resultfrom the reduction of the holes size after metal coverage, or/and

from subsequent interface effects between two metals layers. Eq.

(2) neglects those effects, although there are strong experimentalpremises that they affect the positions of SPRs [1115].

In Fig. 5 theoretical sensitivities for uncoated Al concaves,

obtained via solution of Eq. (2) by changing d values for dif-

ferent solvents, are also provided (dotted black lines). For both

samples the theoretical RIS is always higher than the experimental

one. It equals 230nm/RIU for the sample with Dc =238.2nm and

427 nm/RIU for the sample with Dc =460.8nm. The disagreement

can be a consequence of domain-like morphology of Al concaves

and lessened poresarrangement outside the domains, giving rise to

strong scatteringeffects (energy losses producedby scattering from

roughness and irregularities on metal surface and at the concave

edges). Those effects are stronger when relative contribution from

grain boundaries with respect to that from domains is larger. More-

over, the bulk SPR sensitivity of nanohole arrays depends on holesnumber contributing to the resonance. For nanohole arrays with a

low hole number the bandwidth is wider compared to nanohole

arrays with a large hole number [67,68]. Wider bandwidth results

in lower bulk SPR sensitivity. In this work the effect of holes num-

ber can also be noticed: the sample anodized at 120V with larger

pores density (14.8m2) demonstrates narrower bandwidths ofreflectivity minima as compared to the Al concaves synthesized

at 195V (pores density of 4.2m2). In other words, a number ofholes contributed to the SPR were lower in the case of the second

sample, whichmight have resulted inlowerthanexpectedSPR sen-

sitivity (bigger difference between theoretical and experimental

RIS). Besides, previously it was demonstrated that the RIS depends

strongly on whether it is measured at the transmission dip (thus

reflectionpeak) or the transmission peak (reflectiondip) [13]. From

finite-difference time-domain (FDTD) calculations, the optical pat-tern at the transmission peak showed extensive surface plasmons

field near the aperture edges (so-called short-range SPR), which

resulted in a small sensitivity, much smaller than the theoretical

one. Since in this work the sensitivity was determined from the

reflection dips (because they were better discernible), the wave-

length sensitivity was mainly constituted by the short-range SPR.

As an effect, lower than theoretically expected RISwas obtained for

both samples. The experimental RIS becomes closer to the theoret-

ical one after the deposition of Ag andCu metals. Particularlyfor Cu

coated Al concaves withDc = 460.8 nmit increasesup to404 nm/RIU

as compared to 296 nm/RIU determined for pure Al holes arrays.

This indicates that the metals coating has a positive effect on plas-

monic field enhancementand canbe used fortailoring of plasmonic

performance of Al concave arrays in UV region.The RIS values evaluated in this work are comparable or even

better than previously determined for various nanohole arrays

operating in visible/near IR range. The RIS of 252n m/RIU was

measured for Ag nanohole arrays prepared by colloidal lithogra-

phy [32], 167 nm/RIU and 286 nm/RIU for periodic Au nanohole

arrays fabricated by EBL [27] and soft interference lithography

[69], respectively, and 212 nm/RIU for Ag nanoparticles prepared

by nano-sphere lithography [8]. Au nanohole arrays with the

lattice parameter (periodicities) of 590 nm demonstrated RIS of

400nm/RIU [25], whereas RIS409 nm/RIU was obtained for 500-

nm-period Au nanhole arrays [23].

It is anticipated that upon optimization of electrochemical pro-

cess performed at high voltages the sensitivity performance of

the Al nanoconcave arrays, particularly those fabricated at 195 Vwith the pitch size of500nm, will be better. As an example, Al

nanohole square arrays with similar periodicity fabricated by EBL

and nanoimprint lithography (NIL) techniques provided more reg-

ular nanohole arrays, which, in turn, resulted in higher RIS values:

487 and 516 nm/RIU, respectively [40,42]. Perfect Al concaves

arrangement is also possible by application of imprinting or pretex-

turing techniques prior to anodization [50]. On the other hand, the

electrochemical synthesis of Al nanoconcaves with smaller pitches

gives better pores arrangements and SPRs at lower wavelengths

(higher energies). Owing to the proportionality of the resonance

wavelength to the pitch size (Eq. (2)), plasmonic sensitivity is

consequently lower in the Al concaves with the smaller Dc, but

the material is very promising with respect to many important

applications in UV range, such as improvement of blue/UV light


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