1
Thin films of Ag-Au nanoparticles dispersed in TiO
2
: Influence of composition and
microstructure on the LSPR and SERS responses
Joel Borges
1,a)
, Catarina G. Ferreira
1,a)
, João P.C. Fernandes
1
, Marco S. Rodrigues
1
, Manuela
Proença
1
, Mihai Apreutesei
2
, Eduardo Alves
3
, Nuno P. Barradas
4
, Cacilda Moura
1
, Filipe Vaz
1
1
Centro de Física, Universidade do Minho, Campus de Gualtar, 4710 - 057 Braga, Portugal
2
Université de Lyon, INSA-Lyon, MATEIS UMR CNRS 5510, 7 Avenue Jean Capelle, 69621
Villeurbanne Cedex, France
3
Instituto de Plasmas e Fusão Nuclear, Campus Tecnológico e Nuclear, Instituto Superior Técnico,
Universidade de Lisboa, E.N. 10 (km 139,7), 2695-066 Bobadela LRS, Portugal
4
Centro de Ciências e Tecnologias Nucleares, Instituto Superior Técnico, Universidade de Lisboa,
E.N. 10 (km 139,7), 2695-066 Bobadela LRS, Portugal
a)
The first two authors contributed equally to this work
Corresponding author: Joel Borges. E-mail: joelborges@fisica.uminho.pt
2
Abstract
Thin films containing monometallic (Ag,Au) and bimetallic (Ag-Au) noble nanoparticles were
dispersed in TiO
2
, using reactive magnetron sputtering and post-deposition thermal annealing. The
influence of metal concentration and thermal annealing in the (micro)structural evolution of the films
was studied, and its correlation with the Localized Surface Plasmon Resonance (LSPR) and Surface
Enhanced Raman Spectroscopy (SERS) behaviours was evaluated. The Ag/TiO
2
films presented
columnar to granular microstructures, developing Ag clusters at the surface for higher annealing
temperatures. In some cases, the films presented dendrite-type fractal geometry, which led to an
almost flat broadband optical response. The Au/TiO
2
system revealed denser microstructures, with
Au nanoparticles dispersed in the matrix, whose size increased with annealing temperature. This
microstructure led to the appearance of LSPR bands, although some Au segregation to the surface
hindered this effect for higher concentrations. The structural results of the Ag-Au/TiO
2
system
suggested the formation of bimetallic Ag-Au nanoparticles, which presence was supported by the
appearance of a single narrow LSPR band. In addition, the Raman spectra of Rhodamine-6G
demonstrated the viability of these systems for SERS applications, with some indication that the
Ag/TiO
2
system might be preferential, contrasting to the notorious behaviour of the bimetallic system
in terms of LSPR response.
Keywords: Thin Films; Ag-Au Nanoparticles; TiO
2
; Localized surface plasmon resonance (LSPR),
Surface enhanced Raman spectroscopy (SERS).
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1. Introduction
When materials are tailored at the nanoscale, as the case of those containing nanoparticles (NPs),
their physical and chemical properties may differ significantly from those of the corresponding bulk
materials. In particular, some noble metal NPs exhibit strong absorption bands in the visible, near-
infrared or near-ultraviolet regions of the electromagnetic spectrum, associated to the localized
surface plasmon resonance (LSPR) phenomenon [1,2]. Surface plasmons (SPs) are collective
electronic excitations that occur at a metal-dielectric interface and are accompanied by an oscillating
electromagnetic field. They can propagate along the surface of the conductor, being designated by
surface plasmon polaritons (SPPs), or be confined to a metallic nanoparticle or nanostructure, in
which case are mentioned as localized surface plasmons (LSPs) [3,4].
LSPR leads to the appearance of strong absorption bands, the enhancement of the electromagnetic
(EM) field near the nanoparticles and the appearance of scattering to the far field [1,5]. Beyond some
known decorative applications [6,7], plasmonic effects are important in a wide range of technological
domains, which include plasmon-based photodetectors [8] and modulators [9], gas sensors
[10–12],
biosensors [13–15], photothermal therapy [16], photodynamic therapy [17], LSP-enhanced solar cells
[18,19], surface enhanced Raman spectroscopy (SERS) [20–22], plasmonic nanoscopy [23], thermal
emitters [24], light-emitting diodes [25] and magnetic storage [26].
The two most well studied plasmonic metals are gold (Au) and silver (Ag), which exhibit LSPR bands
in the visible range of the electromagnetic spectrum [27,28]. While Au has the advantage of being
chemically inert and biocompatible, Ag exhibits the sharpest and strongest LSPR absorption among
all metals [3,28]. On the other hand, bimetallic Ag-Au nanoparticles are attracting increasing attention
due to their improved electronic, optical and catalytic properties [29,30]. According to the preparation
method, bimetallic nanoparticles with core/shell or alloyed (where the two constituent metals are
mixed at the atomic level) structures may be obtained [31]. In the latter case, the continuous tunability
of the LSPR absorption band position between the two single metals may be achievable, by varying
the Au/Ag atomic ratio [32]. Since both materials crystalize in face centered cubic (fcc) structures
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with very similar lattice constants (407.00 pm for Au and 408.55 pm for Ag), it is possible, in
principle, to produce bimetallic Ag-Au nanoparticles of any composition and, consequently, adjust
the position of its LSPR band towards a targeted application [27,30].
Due to the unique properties of plasmonic NPs, the preparation of nanocomposite thin films where
they are randomly dispersed in a dielectric matrix is becoming an increasingly important subject in
the fields of materials science, nanoscience and nanotechnology. In such films, the optical response,
and particularly the LSPR band properties are strongly dependent on the nature, size, shape and
interparticle spacing of the NPs, as well as on the dielectric properties of the surrounding medium
[1,5,7]. To produce these nanocomposite films, Physical Vapor Deposition (PVD) techniques have
been frequently used. Magnetron sputtering is an example, and its versatility allows to tailor the
chemical and (micro)structural properties of these nanocomposite films towards the above mentioned
applications [4]. Nevertheless, when using this method to prepare plasmonic thin films, a second step
may be required, especially in the case of low preparation temperatures [5,33,34]. For this purpose, a
post-deposition in-air or in-vacuum thermal annealing treatment is often used.
Beyond the work developed by F. Vaz et al. [4,35,36], few studies addressed the synthesis of Ag/TiO
2
[37,38] and Au/TiO
2
[39,40] thin films by magnetron sputtering followed by a heat treatment, as an
efficient and versatile way to control the morphology, particle growth and optical properties of these
nanocomposites. Therefore, the application of this method to prepare bimetallic Ag-Au nanoparticles
on a TiO
2
matrix, seems to be a very promising way to achieve different Ag/Au atomic ratios in a
dielectric matrix such as TiO
2
, which, in principle, will allow to tune the LSPR peak position between
those of the corresponding monometallic counterparts.
In the present work, nanocomposite thin films containing monometallic (Ag/TiO
2
, Au/TiO
2
) and
bimetallic (Ag-Au/TiO
2
) noble nanoparticles were prepared, with different metal concentrations, in a
TiO
2
matrix. A post-deposition annealing treatment was performed in air atmosphere at different
temperatures to promote the growth of the metallic nanoparticles with different sizes and
distributions, aiming the tuning of their LSPR behaviour. The applicability of some representative
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samples of each set to SERS was also accessed in this work, through the enhancement of the Raman
signal of a Rhodamine 6G (R6G) solution.
2. Material and methods
Nanocomposite thin films containing single (Ag/TiO
2
and Au/TiO
2
) and bimetallic (Ag-Au/TiO
2
)
noble metal nanoparticles, containing different metal concentrations, were prepared by reactive DC
magnetron sputtering, using the sputtering system described in [41,42]. The thin films were deposited
on (100) oriented Si wafers (for chemical, structural and morphological characterization), glass
lamellae - ISO 8037 and fused silica (SiO
2
, to analyze the optical response and the SERS activity)
substrates. To obtain the different noble metal concentrations (Ag and/or Au), a rectangular Ti target
(200×100×6 mm
3
, 99.8% purity), containing various amounts of gold or/and silver pellets (each one
with an area of about 16 mm
2
and 1 mm thick) placed on its preferential erosion zone, were used. The
total area of pellets is listed in Table 1.
The target was sputtered in a gas atmosphere composed of Ar (partial pressure of 4.0×10
-1
Pa) and
O
2
(partial pressure of 5.6×10
-2
Pa, chosen according to the hysteresis experiment [3]), maintaining a
constant work pressure of 4.5×10
-1
Pa during 2 h of deposition. The base pressure of the system was
around 4×10
-4
Pa. The power supply was set to operate in a current regulating mode, using a constant
current density of 100 A·m
-2
applied to the target. A grounded rotating substrate holder was used,
positioned 70 mm away from the target, with a constant rotation speed of 9 rpm. No external heating
was used during the process. The target potential was measured in each deposition and the values,
presented in Table 1, remained roughly constant with the increase of the total area of pellets, showing
that the incorporation of silver/gold pellets on the target had no significant influence in the plasma.
