Optical excitation of surface plasmons on Ag based alloys

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Optical excitation of surface plasmons on Ag based alloys

A. Tadjeddine, A.F. Benhabib, A. Zeghib, J. Le Bas

To cite this version:

A. Tadjeddine, A.F. Benhabib, A. Zeghib, J. Le Bas. Optical excitation of surface plasmons on Ag based alloys. Journal de Physique, 1987, 48 (10), pp.1715-1719. �10.1051/jphys:0198700480100171500�.

�jpa-00210613�

Optical excitation of surface plasmons ^on Ag ^based alloys

A. Tadjeddine (1), A. F. Benhabib (*,1), ^A. Zeghib (2) ^{and J. Le Bas} (2)

(1) Laboratoire d’Electrochimie Interfaciale du C.N.R.S., 1, place ^Aristide Briand, 92195 Meudon Principal Cedex, ^France

(2) L.E.C.A.P., Faculté des Sciences, ^B.P. 67, ⁷⁶¹³⁰ Mont-Saint-Aignan, ^France

(Requ ^{le 16} juillet 1986, révisé le 6 mai 1987, accepté ^{le 17} juin 1987)

Résumé.

Nous

étudié les propriétés optiques d’alliages ^dilués Ag1-xNix (x 0,15) par excitation des

plasmons ^{de surface} par réflexion totale atténuée. Les échantillons sont préparés par pulvérisation cathodique radiofréquence ^et caractérisés _par microscopie électronique ^de transmission, fluorescence X et sonde ionique.

Les courbes de dispersion ^des plasmons de surface ont été analysées

^liaison

^les niveaux liés virtuels qui peuvent exister dans le

d’impuretés d ^dans

^matrice s-p. Cette analyse

permis ^d’évaluer l’énergie

moyenne de

niveau lié virtuel à 2,2 ^{eV et}

largeur ^à 0,35 ^eV.

Abstract.

Dilute silver-nickel alloys ^have been studied by ^surface plasmons ^excited by attenuated total reflection. The samples ^{have been} prepared by ^radio frequence sputtering ^and characterized by TEM, X-ray

fluorescence and SIMS. The surface plasmon dispersion

^{have been} analysed in connection with the virtual bound levels which exist in such materials. This allows

to evaluate the energy and the width of this level (E0 ~ ^2.2 eV, 0394 ~ ^0.35 eV).

Classification

Physics ^Abstracts

07.60H - 78.65

Surface plasmon excitation (SP) has been proven

as a powerful ^{tool to} study ^the optical properties ^of

metal surfaces, thin films and interfaces [1, 2].

The SP resonance conditions ^are very sensitive ^to the interfacial properties ^{and can be used to follow}

their variations. For bare surface metal the SP

dispersion ^{curve is directly connected to the complex}

dielectric function and allows its measure as shown

previously for silver in contact with ^an electrolyte ^or

in ambiant atmosphere [3, 4]. If the metal is coated with a thin film of known thickness there is ^a

perturbation ^{in the SP resonance which can be used}

to determine the dielectric ^constant of this film. We have used this technique ^to study ^the optical proper- ties of semiconductor films of small thicknesses (5 ^to

30 nm) evaporated ^onto silver, specially ^{around the}

fundamental gap where ^the absorption may be small and difficult to obtain by traditional optical ^methods [5, 6]. ^However up to now, such experiments ^have

been performed only ^on pure Ag ^{metal in the} spectral range where its properties ^{are mainly gov-}

erned by its free electron _{gas. In} this paper, ^we discuss the first results obtained by ^a similar tech-

nique ^{on dilute} nickel-silver alloys (NixAg1 - x’

0 x 0.15). Alloying perturbs ^{both the} position

and the shape ^{of the SP} dispersion ^{curve. In the low}

energy range, the main effect is the shift of the

resonance position, which allows the determination of the nickel concentration x. For higher energy values, ^{we observe a} backbending which does not exist in pure silver and which could be related ^to the

absorption ^{due to} virtual bound states which may exist in the case of dilute d impurities ^{in a noble}

metal matrix [7, 8].

There is ^no miscibility of nickel and silver [9] ^and

their alloys ^are always metastable. It has been shown that it is possible ^to prepare NixAg1 - x thin films by radiofrequence sputtering ^on cooled, optically polished glass substrates [10, 11] ⁱⁿ argon ^atmos-

phere. For silver-rich alloys ^{which are of} interest in this work, ^the target ^{was made of a} silver disk on

which small nickel squares ^were pasted. ^{The film}

thickness (100 nm) ^was controlled in situ using ^a

calibrated quartz microbalance and the composition

was determined using X-ray fluorescence as de-

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:0198700480100171500

1716

scribed by Lebas et al. [12]. ^{The structure of the} samples depends ^{on their} composition [13] ^{and has}

been observed by Transmission Electron Microscopy (TEM). ^For high silver concentrations ( > ^{60 %} at),

the samples display ^a microcrystalline structure at room temperature. Figure ^la presents ^{a TEM dif-} fraction pattern ^for Agl-xNix (x

0.1) indicating

the existence of the alloy ^{at room} temperature. ^This

structure is destroyed ^after sample annealing ^at higher temperature (300 °C), ^{as shown in} figure ^1b

which shows the appearance of the crystallization

and phase separation by ^the presence of nickel and silver rings ^{in the TEM} diffraction pattern.

Fig. 1.

Transmission electron diffraction pattern ^of

Ag1 - xNix (x

0.1 ) ^at

temperature : a) ^before

nealing, b) ^after annealing ^{at 300 °C.}

This structure change ^under annealing ^{has also}

been observed in the bulk composition ^followed by

SIMS analysis. Measurements were carried out using

an ion probe type ^IMS3F (CAMECA) ^{with a cold}

cathode duoplasmation ^{as oxygen} primary ^source

and an electron multiplier ^{as detector.} Figure ²

Fig. ^2.

^SIMS depths profiles ^of Ag1 - xNix for various Ni and Ag isotopes : a) ^before annealing, b) ^{after anneal-} ing ^{at 300 °C.}

shows the distribution of Ni and Ag ^{as a} function of the etching depth ^{measured for x}

^0.1 (the spectra

are normalized taking Ni58 ^{as mass} reference) ^at

room temperature (a) and after annealing up ^to 300 °C (b). ^Profiles (a) ^are indicative of the good homogeneity ^{of our} samples, which sill consist in nickel-silver alloys ^{at room} temperature. ^This homogeneity ^is destroyed ^after annealing ^{as shown} by profiles (b) where the silver distribution is depth depending. ^Note the lack of silver near the surface, which may explain why ^surface plasmon ^excitation

could not be possible ^on such annealed samples.

Optical measurements have been performed by

attenuated total reflection (ATR) using ^focused light

as described previously [14, 15]. ^A monochromatic

lineary polarized beam is focused on the face of a

hemicylindrical prism ^{made of} quartz. ^{In the OTTO-} geometry used in the experiments described in this paper, the sample ^{is fixed at a} distance of the order of the wavelength ^{from the ATR} device face. The excitation of a SP appears ^{as a} minimum in the p-

polarized reflectivity Rp( ’P) ^{as a} function of the

angle of incidence at fixed wavelength (A). ^Details

on the experimental ^set up and techniques ^{can be}

found in already published papers [14, 15]. Experi-

ments have been performed ^on alloys with different Ni concentrations ranging between 0 and 15 percent.

Results and discussion.

Figure ^{3 shows} Rp (’P ) recorded for x

0.1 at diffe- rent A. The position of the minimum as a function of the _energy gives ^{the SP} dispersion ^{curve which is} plotted ⁱⁿ figure ^{4a for x}

^{0 and x}

0.1. In pure silver the dispersion ^{curve is} monotonous in the whole spectral range studied while it displays ^a backbending ^{around A}

^{540 nm for x}

^{0.1. Such}

an effect cannot be explained by ^the damping ^of

surface plasmon dispersion ^{observed on silver in the} vicinity ^{of the surface} plasmon energy by ^Arakawa

et al. [16]. ^{It cannot} either be due to surface contami- nation which is expected ^{to be} present ^{since we are} working ^{at ambiant} atmosphere ; sulfuration or

oxidation only ^{induce a} shift of the dispersion ^curve

in the visible spectral range, ^as shown in figure ^4b

where we have plotted ^{the surface} plasmon disper-

sion curves for bare silver and Ag0.9Ni0.1 ¹ samples

Fig. ^3.

p-reflected ^factor RP

function of the angle

of incidence for various A : Ag1 - xNix (x

^0.1 ).

Fig. ^4a.

^Surface plasmon dispersion

^{for bare} Ag (a) ^and Ag} - xNix (x

0.1 ) (b).

Fig. ^4b.

^Surface plasmon dispersion

^for samples figure ^{4a after}

year.

measured one year after preparation. The observed effect _upon alloying is indicative of ^a decrease of

£1 with the energy due ^to the occurrence of anomal-

ous dispersion ; ^Caroli [17] ^and Kjollestrom [18]

have described the optical properties ^{of such} alloys

in a model which takes into account two main

phenomena : ^{the resonant} scattering of the conduc- tion electrons of the matrix by ^{the d} impurities ^and

the electronic transitions from the filled states of the virtual bound levels (VBS) ^to the conduction band above the Fermi level. The first effect gives ^{rise to a} frequency-dependent relaxation time T;(w ), ^{and the}

second appears ^{as an} optical absorption band located

at the VBS mean energy Eo ^{with a width 2 A} [15].

1718

The dielectric constant can be written as [14] :

£ f, EA9 ^{and ESd} being respectively the contributions of the intraband transitions and the interband transi- tions of the conduction electrons of the Ag ^{and of}

the impurities ^{VBS. In} this paper the interband contribution of silver has not been considered, ^{as we}

are interested in frequencies below its threshold energy. So E 2 b Ag ^{is zero} while E I b Ag ^{is constant.}

Ef is given by ^a Drude relation :

where w p ^{is the} plasma frequency ^and Ta (lJ ) ^the

concentration dependent relaxation time of the

alloy :

The impurity contribution _{êSd is} given by [18] :

V being ^{a matrix} which mixes the localized d orbital of energy Eo ^{with the band states and} (IJ d is ^the

matrix element associated with the transitions :

and finally :

and

with

and

Five parameters ^{are necessary to} characterize the

optical properties ^{of the} alloy : ^the plasma frequency wpa, ^the relaxation time _Ta, the matrix element

wd, the position Eo ^{and the width 2 A of} the virtual bound level. Up ^{to now these} parameters ^{have been} obtained from E2 (to). However, the surface plasmon dispersion ^{curve is} closely dependent on £1 and its

shape ^must mainly reflect the effects of alloying ^on

El. We shall discuss this point qualitatively using ^the expression ^of El given by equation (6).

The first term is the free electron contribution which gives ^{the normal SP} dispersion ^{in the} spectral region where the SP ^wave vector increases with the

photon energy. The ^{second term} is the contribution of the virtual bound level which introduces the observed anomalous SP dispersion. ^The backbending

starts with the VBS absorption ^{and can be used to} study this effect.

In order to reach Eo ^{and A, at least} approximately,

we have compared the actual real part £1 ^obtained from the SP dispersion ^{curve with the} El car obtained

using ^a simplified model where _Esd is not taken into account :

where El Ag (Co ) is determined from SP dispersion ^at

x = 0 and _ë1 Ni (ú)) ^{from the} literature [19]. ^The difference dë]

ëcxp - ^{F,al, which accounts for} Esd, is plotted ⁱⁿ figure ^{5 for x}

^{0.1. As} expected ⁱⁿ

our analysis, el

decreases with

and displays ^two

rather well defined extrema located at about 470 and 670 nm respectively. ^{This allows}

^{to evaluate the}

Fig. ^5.

Difference A El(w) of the measured and calcu- lated real part of the dielectric ^constant of Ag1 - xNix

(x = 0.1 ).

position ^and the half width of the VBS considered as a classical oscillator. We found :

These values

different from the parameters ^of

the NiAg VBS found by Koike et al. [20, 21]

[Eo

^{1.8 eV, A}

^0.6 eV], who have studied the effects of alloying ^-on Ag based 3d-transition metal

alloys using optical absorption ^{and dc} resistivity ⁱⁿ

the air at room temperature. However, ^{no structure} analysis ^{was made} by ^{these authors wo} assumed the

samples ^{to be} homogeneous from wrong values of the impurity resistivity (see appendix, ^Ref. [21]).

Our aim in this paper ^{was to} present preliminary

results obtained on dilute silver-nickel alloys by using for the first time surface plasmon excitation in attenuated total reflection. We have shown that, up to x

0.1, ^the p-polarized ^reflected light ^{as a}

function of the angle of incidence displays ^{a well}

defined minimum which allows us to measure the SP

dispersion ^{curve. We have} analysed ^{this curve in}

terms of the real part ^{of the} alloy dielectric ^constant.

The backbending observed around 2.2 eV has been related to the absorption of the virtual bound level

on Ni impurities.

However, ^{since our results on the VBS} parameters

are different from those of Koike et al. [20], ^our analysis ^{has to be} supported by ^further experimental

results. We ^are presently performing, ^{with the same} method, ^a study ^of Ag1- xPdx alloys, ^{which are}

stable and have been studied by many groups, in order to test the possibilities of this method.

Acknowledgments.

We thank Dr. M. L. Theye and Dr. J. Lafait

(Laboratoire d’Optique ^des Solides, ^Paris VI) ^for

fruitful discussions.

References

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