Download Plasmonic phenomena in indium tin oxide and ITO–Au hybrid films

September 15, 2009 / Vol. 34, No. 18 / OPTICS LETTERS
2867
Plasmonic phenomena in indium tin oxide and
ITO–Au hybrid films
Stefan Franzen,1,* Crissy Rhodes,1 Marta Cerruti,1 Ralph W. Gerber,1 Mark Losego,2
Jon-Paul Maria,2 and D. E. Aspnes3
1
Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695, USA
2
Department of Materials Science and Engineering, North Carolina State University,
Raleigh, North Carolina 27695, USA
3
Department of Physics, North Carolina State University, Raleigh, North Carolina 27695, USA
*Corresponding author: [email protected]
Received May 28, 2009; revised August 8, 2009; accepted August 14, 2009;
posted August 27, 2009 (Doc. ID 112025); published September 15, 2009
The observation of surface-plasmon resonances in indium tin oxide (ITO) thin films is complemented with
the effects of hybrid ITO/Au conducting layers where charge densities can be tuned. Where carrier densities
are similar (ITO and nanoparticle Au), the plasmonic behavior is that of a monolithic ITO thin film. Where
the carrier density of one layer is much greater than that of the other (ITO and Au metal), boundary conditions lead to cancelation of the surface plasmon. In the latter case a capacitivelike plasmon resonance is
observed for sufficiently thin films. © 2009 Optical Society of America
OCIS codes: 240.0310, 240.5420, 240.6680, 240.6490, 260.5740.
For more than 100 years [1] the plasmonic periodic
table has been dominated by two elements, gold and
silver. Plasmonic absorption and scattering in Au and
Ag particles, also called localized surface plasmon
resonance (SPR), has a long history in optics as
effective-medium theory and is now being applied to
sensor technology [2,3]. Noble-metal nanoparticle
and island films have fascinated researchers, because
their associated plasma frequencies ␻p occur in the
visible and near ultraviolet, respectively, and their
associated surface-field-enhancement effects are
shape tunable [2,4]. In addition, plasmonic waveguides [5] and surface-enhancement phenomena [6,7]
have received increasing attention as improved fabrication methods enable more reproducible and even
tailored effects. The change in the surface plasmon
(SP) polariton signal in Au thin films is currently one
of the most widely used methods for detecting binding interactions in biological systems [8–11].
It would be advantageous to tune plasmon resonances in other ways, for example by varying the intrinsic properties of materials. A mechanism for doing this is suggested by the free-electron model of
Drude [12], which relates the carrier concentration n
of a material to ␻p. This provides a fundamental link
between the electrical and optical properties of a conducting material. Although n and therefore ␻p are
fixed for elemental metals such as Au and Ag, the effective values of n for Au and Ag can be modified
through nanostructural engineering. Alternatively,
one may use any of a number of conducting metal oxides, for example indium tin oxide (ITO), where n can
be engineered directly by altering crystal chemistry
and/or defect equilibrium. ITO is an n-type degenerate semiconductor where Sn4+ atoms at In3+ sites act
as donors when the oxygen stoichiometry is properly
controlled. However, research on these materials for
plasmonic applications has been sparing [13–18]. In
this communication we follow up on our earlier demonstration of plasmons in ITO [13,19] by combining
0146-9592/09/182867-3/$15.00
ITO and Au layers to create plasmons that are intrinsically different from those previously reported
[20–24]. The hybrid plasmon–polariton structures reported here originate in the ITO layer and are modulated by the properties of the Au layer [13–16,19,25].
The absence of band-to-band transitions in ITO
permits direct observation of the three regimes of interaction with electromagnetic radiation: conducting,
resonant, and insulating. In the conducting regime
␻ ⬍ ␻p / 冑 ␧⬁ , = ␻bp, where ␻bp is the frequency of the
(screened) bulk plasmon and ␧⬁ is the IR dielectric
constant. Here, electrons follow the incident electric
field, leading to complete reflection in the absence of
absorption losses. For metals such as Cr and Al, ␻bp
lies above 9 eV, in the ultraviolet, hence these metals
are good reflectors in the visible. The reflecting regime of ITO occurs in the near and mid-IR below 1 eV
共8000 cm−1兲 [14,15], which leads to its widespread
use as a heat-shield coating for windows [26]. The
second regime is resonance, where ␻ ⬃ ␻bp. As a longitudinal excitation, the BP cannot be optically excited in bulk material. However, as predicted 50
years ago by Abeles [27], a polariton resonance associated with the BP can be observed in films that are
sufficiently thin to allow the separated charges to
communicate across the film. This type of polariton
cannot be observed Ag or Au because of interfering
band-to-band transitions [28]. In the third regime,
␻ ⬃ ␻bp and the conductor behaves optically as an insulator. Free-electron conductors such as ITO are
transparent. However, the noble metals Cu, Ag, and
Au do not exhibit transparency due to strong bandto-band transitions, which cause significant deviations from the Drude free-electron model.
Our results are summarized in Fig. 1, which exhibits various plasmonic phenomena involving ITO and
ITO/Au hybrid thin films. These illustrate similarities between ITO and nanoparticle (NP) Au films and
also how Au can be used to affect the behavior of plasmonic phenomena of ITO. Our ITO films were pre© 2009 Optical Society of America
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OPTICS LETTERS / Vol. 34, No. 18 / September 15, 2009
Fig. 1. (Color online) Rp / Rs data illustrating the effect of ITO thicknesses and Au overlayers on plasmon–polariton structures in reflectance spectra of ITO and hybrid ITO/Au films. Angles of incidence increase from 42° to 53° in steps of 0.35°
as the colors advance from orange to violet. For each sample the configuration and layer thickness or thicknesses in nanometers are shown to the right of the corresponding data. The charge separations of the capacitive (CP) and surface (SP)
plasmons underlying the capacitive plasmon resonance (CPR) and surface plasmon resonance (SPR) structures of A, D and
C, F, respectively, are shown in E. For the reference 80 nm ITO film (B) neither the SP nor the CP is fully activated. However, either the associated CPR (A, D) or SPR (C, F) can be made to dominate by controlling the properties of the Au overlayer (A, C) or the ITO thickness (D, F). The high energy feature in (C,F) is a nonresonant (NR) contribution. A metallic Au
overlayer effectively reduces the ITO thickness, as seen by comparing (A) and (D). Alternatively, (C) and (F) show that the
thickness of the ITO layer is effectively increased when the ITO is covered by a NP-Au overlayer fabricated so its effective
carrier concentration matches that of the underlying film. The above data can be completely understood in terms of
effective-medium theory and the Fresnel equations for the three- (substrate/overlayer/ambient) and four-phase (substrate/
overlayer/overlayer/ambient) models.
pared by rf magnetron sputtering from an In2O3 target with 10 wt.% SnO2 onto BK-7 glass substrates
[19]. Reflectance spectra Rp and Rs, the reflectances
of light polarized parallel and perpendicular to the
plane of incidence, respectively, were obtained in the
Kretschman configuration and are given as the ratio
Rp / Rs for various configurations as shown to the
right of the data panels in Fig. 1. The Kretschmann
configuration itself, shown schematically in Fig. 1C,
uses a prism to couple the incident light into the film.
In the data panels, angles of incidence ␪ range from
42° (orange) to 53° (violet) in steps of 0.35°. The two
geometries shown for coupling of radiation into a thin
conducting film exhibit two distinct types of plasmonic phenomena. The SP is an electronic oscillation
with an in-plane charge motion at the interface and
can only occur within the skin depth of a conducting
film. When the appropriate condition is met the induced dipole gives rise to extinction and scattering,
reducing the reflected intensity and leading to a detectable signal that is the basis for SPR sensors. For
sufficiently thin films a second type of polariton occurs at the frequency ␻bp, where the charge motion is
orthogonal to the surface, as in a capacitor. Although
the actual configuration is different (the charge appears on either side of a single conducting film rather
than on two conducting films separated by an insulator), we refer to this as a capacitive plasmon (CP)
with an associated capacitive plasmon polariton. By
analogy with the SP, a CP driven by an electromagnetic field results in a capacitive plasmon resonance
(CPR). The CPR provides an analog of localized SPR,
where one of the principal axes of the ellipsoid has
collapsed essentially to zero. This collapse causes a
similar reduction in the reflected intensity.
We consider first the results for ITO alone. Figure
1B shows data obtained on an ITO film that is poised
at a thickness where neither the SP nor the CP is
fully developed. By decreasing or increasing the
thickness we realize either the CPR (Fig. 1D) or the
SPR (Fig. 1F), respectively. The charge separations
appropriate to the CP and SP, from which the CPP
and SPP derive, are shown in Fig. 1E. We see that
the CP and SP are mutually exclusive, since their associated charge motions are orthogonal. Hence the
respective polaritons must be driven by different
components of the incident field. This distinction is
further enhanced by their different dependences on
film thickness. These thickness dependences follow
from basic physics: for thick films charge communication across the film is suppressed, eliminating the
CP but at the same time establishing the necessary
condition for the SP. Thus when the SPR appears the
CPR is quenched, and vice versa. Direct experimental
evidence is seen in Figs. 1D and 1F.
We now consider the hybrid ITO/Au results. Figure
1A shows that the deposition of 50 nm Au completely
suppresses the SPR marginally seen in Fig. 1B, leaving the CPR as in Fig. 1D even though the ITO film of
Fig. 1A is substantially thicker. This follows because
the metallic Au overlayer inhibits electric field components parallel to the interface, thereby effectively
negating 50 nm of ITO and quenching any tendency
to form the SP. However, the CP is allowed because
its charge separation is perpendicular to the interface. Thus essentially angle-independent CPR is seen
in both Figs. 1A and 1D. What is more interesting is
the fact that the ITO CPR dominates the spectrum,
effectively imposing its behavior on the metallic Au
overlayer.
September 15, 2009 / Vol. 34, No. 18 / OPTICS LETTERS
For the NP-Au overlayer the opposite effect is observed. This overlayer was fabricated with alternating layers of 12 nm and 2.6 nm Au particles such that
its effective carrier density is similar to that of the
underlying ITO [29]. The result is a NP-Au overlayer
that does not affect the tangential electric field at the
interface and effectively behaves as a continuation of
the ITO film. Hence a SPR that is strongly dependent
on the angle of incidence observed in Fig. 1C, similar
to that seen in Fig. 1F. However, in this structure the
SP actually occurs in the NP-Au layer. Thus, this particular NP-Au film and the underlying ITO exhibit
the same behavior. This is a simple example of
changing the effective value of n in metallic films by
nanostructural engineering. We note that all effects
seen in Fig. 1 are well described by the Bruggeman
effective-medium theory [30] and Fresnel multilayer
models [13,19].
Returning to Fig. 1A, in the plasmon picture the
quenching of the SP in the ITO/metallic Au hybrid
structure arises because the frequency ␻p
⬃ 52,400 cm−1 in Au metal is much larger than ␻p
⬃ 18,700 cm−1 of the ITO film. Hence the Au functions as a nearly perfect reflector for the conditions
that drive the SP in ITO. The boundary condition
therefore suppresses electric field components parallel to the surface, required for the SP, while the component perpendicular to the surface is allowed, leaving only the CP. Although bulk Au cannot be tuned,
Fig. 1A shows that it can adopt the frequency of underlying resonances. Also, we have shown that the
properties of Au-based composites can be matched to
the underlying ITO by constructing a mixture of Au
and voids, as indicated in Fig. 1C. This NP-Au film
has a theoretical plasma frequency ␻p ⬃ 18,350 cm−1,
which is comparable to that of ITO. Hence, to lowest
order the NP-Au layer simply acts as a continuation
of ITO, as clearly seen in Fig. 1C.
Our demonstration of these plasma effects in ITO
and ITO/Au composites in the near-infrared expands
the repertoire of practical plasmonic materials to include hundreds of conducting metal oxides and
structured-NP layers in monolithic and composite
thin-film systems. These materials include sp-type
metal oxides (e.g., fluorine-doped tin oxide [15]),
d-type metal oxides (e.g., ruthenium oxide), and
structured layers of Au and Ag fabricated by nanoparticle depositions. The salient feature of these thin
films is that their properties can be varied systematically, hence preparation of materials such as conducting metal oxides, and nanostructures such as composite metal films, provides a wide range of
opportunities for tuning optical responses based on
properties that can be predicted.
Our observations demonstrate that these materials
provide an ideal platform to test the various limits
and effects of structural excitations used for example
in nanoparticle plasmon spectroscopy, surfaceenhanced Raman scattering, and related phenomena.
In particular, the low optical absorption and the capability of tuning various parameters in conducting
metal oxides provide a unique advantage over metals
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for the fundamental study of plasmonic phenomena
[31]. Our findings also suggest a range of new applications, since the IR plasmonic activity of metal oxides makes them suitable for SPR sensing, plasmonic
waveguides, and electromagnetic surface enhancement on silicon-based microelectronics/microphotonics platforms.
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