Near-Infrared Optical Extinction of Indium Tin Oxide Structures

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Near-infrared optical extinction of indium tin oxide structures prepared by nanosphere lithography Mi Sun Kang, Mark Losego, Edward Sachet, Jon-Paul Maria, and Stefan Franzen ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.6b00649 • Publication Date (Web): 30 Aug 2016 Downloaded from http://pubs.acs.org on September 1, 2016

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Near-infrared optical extinction of indium tin oxide structures prepared by nanosphere lithography Misun Kang1, Mark Losego2, Edward Sachet3, Jon-Paul Maria3, Stefan Franzen1* 1 2 3

Department of Chemistry, North Carolina State University, Raleigh, NC 27695

School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332

Department of Materials Science and Engineering, North Carolina State University, Raleigh, NC 27695

Corresponding Author *Stefan Franzen Department of Chemistry North Carolina State University Raleigh, NC 27695 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. KEYWORDS Localized surface plasmon resonance, Epsilon-near-zero mode, Metamaterial, Nanosphere lithography.

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Abstract

Indium tin oxide (ITO) has been the most widely studied conducting metal oxide (CMO) and serves as the best candidate for proof-of-concept experiments in the field of surface plasmon resonance and studies of electric field confinement and manipulation. ITO is chemically stable and relatively easy to sputter. In this report, arrays of ITO nanostructures were produced using nanosphere lithography (NSL) which was originally developed for plasmonic applications involving noble metals. However, the experiments presented here show that patterned ITO with similar size and shape to noble metals has an observed extinction, which corresponds to the epsilon-near-zero (ENZ) mode. The carrier density of ITO nanostructure can be controlled by the post-deposition annealing process.Thus, one can prove that the optical signals on the surface are those of the ITO nanostructure by reversible on/off switching of the capacitive plasmon resonance by annealing the surfaces successively in forming gas (N2/H2) and in air. Thus, using conducting metal oxides confident of the electric field is possible not only along the z-axis perpendicular to the thin film, but within the plane of the film as well.


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Introduction Patterned conducting metal oxide (CMO) surfaces have been used in a variety of applications to control the optical properties of surfaces.

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The inspiration for shape control on surfaces

started with nanoparticles with various shapes that have been shown to have tunable localized surface plasmon resonance (LSPR).9,10 For example, changing the aspect ratio of lower symmetry Au nanoparticles leads to tunable resonant wavelengths.11,12 A number of novel applications have been discovered by pushing the development of plasmonic materials in the near- and mid-infrared (IR).

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Indium tin oxide (ITO) is an interesting near-infrared plasmonic

material that shows both the effects of surface plasmon resonance (SPR) and epsilon-near-zero (ENZ) modes in a single spectrum.

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The effects of the LSPR in ITO include surface

enhancement6 and shape-controlled extinction3,4, but also applications as waveguides

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and

metatronic circuits 7. The optical response due to the ENZ mode in ITO was first observed in 2006

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. Since this

feature lacked a context at the time it was further characterized at that time. Subsequently using Au/ITO hybrid plasmonic materials the optical feature corresponding to the ENZ mode was shown experimentally to have z-polarization (perpendicular to the thin film) in 2009.13 This feature was called a capacitive plasmon resonance (CPR) at that time to properly distinguish the polarization of the observed extinction from the SPR phenomenon, which is polarized parallel to an interface between a conductor and an insulator. Contemporaneously, the utility of the ENZ phenomenon was described in a different field of research.17 Since that time there has been a rapidly developing interest in ENZ phenomena and a specific interest in applications of the ENZ mode using ITO.18-20 These aspects of the optical response have been studied in the development of metamaterials in part because of the utility of materials that can be described by the Drude


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free electron conduction model.5 The applicability of the Drude model to ITO has been used to distinguish SPR in conducting metal oxides from SPR in Ag and Au

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. The Drude model also

predicts the ENZ mode in ITO thin films, which permits the identification of the CPR as the ENZ mode. However, this identification raises questions regarding the significance of the extinction in nanostructured ITO when compared to the noble metals. In this work we report the extinction in ITO obtained by the nanosphere lithography (NSL) approach and show that it has the property of the ENZ mode when the feature thickness is less than the skin depth of ITO. NSL is the simplest method that can be used to obtain patterned surfaces. The technique has been used for noble metals for nearly two decades.9,11,22-29 Therefore, the use NSL in creating patterned surfaces on ITO provides insight into similarities and differences of ITO with noble metals. NSL originated from “natural lithography” introduced by Deckman in 1982 to investigate the relationship between surface roughness and optical elements.30 Deckman et al. described a monolayer made by polymer spheres using electrostatic and rheological interactions for deposition or reactive ion etching.30 In 1995, Van Duyne et al. extended the concept of natural lithography to NSL.9 By controlling nanosphere (NS) concentration and spin coating rotational velocity, this group demonstrated the ability to create different patterns using both single layer and double layer NS masks. Using this method, in 2001, the Van Duyne group demonstrated the fabrication of defect-free single layers and bilayers of 10 to 100 µm2 area for use in patterning Ag surfaces.11 The goal of NSL using noble metals has been the control of LSPR on surfaces. NSL has the correct length scale since LSPR occurs for features smaller than the wavelength of the exciting light. LSPR has become useful as a method for detecting molecules on the surface of films and promised the potential for surface enhanced spectroscopies31,32, bio- or chemical-sensors33-38, or


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optical devices.11 While most of this work has been carried out using noble metals ITO has recently found application as well. 39,40 The success of NSL as a technology for creation of LSPR in Au patterned surfaces provides inspiration to pursue a similar method for conducting metal oxides (CMOs). Semiconductor films of CMOs have transparency, chemical stability, and plasma frequencies in the mid-infrared (mid-IR) spectral region.14,16 Since the carrier density of CMOs is lower than that for noble metals, ( 140 nm. There is a shift of circa 200 cm-1 in the high wavenumber feature, but this is significantly less than the shift of more than 3,000 cm-1 observed in the SPR over a similar angle range. These two optical features have also been observed in a hybrid plasmonic material when there is a mismatch between the plasma resonance frequencies such that one material is a perfect reflector (ideal conductor).13 Specicially, when a thin gold film is deposited on ITO that is too thick to observe the ENZ mode, the presence of the second layer “turns on” the ENZ mode beucase of the surface selection rules. The surface selection rule at the interface with a perfect conductor forbids excitation of any transition parallel to the surface. The perpendicular field component will be observed, however, since the image field adds constructively. Since the early work on ITO provided an experimental proof that the high wave number extinction was polarized perpendicular to the surface it was given the name capactive plasmon resonance (CPR). This distinguishes extinction due to a perpendicular oscillation from that due to a parallel oscillation. However, the CPR nomenclature, which dates from 2009, is obsolete since this optical feature has been characterized extensively in other systems and now even in ITO as the ENZ. We therefore assign the feature in Figure 2 as the ENZ mode of the patterned ITO array. To further understand the relationship with previous work on ITO where the ENZ and LSPR on thin films were observed in the Kretschmann configuration13 we have used the FDTD approach to calculate the expected reflectance signal for films of 20 nm and 150 nm thickness. The results of this calculation are shown in Figure 5A and 5B. The sharp dip that is not highly angle dependent observed for the 20 nm thick films modeled in Figure 5A corresponds to the ENZ signal observed previously. Note that this signal is quite similar to the ITO triangles,


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although it is more intense. The intensity matches experimental observations since thin films had a much stronger extinction than the nanostructured ITO.14,16

The FDTD approach has a

numerical error of the order of 10-3 optical density units. Small numerical error in the calculation using LumericalTM accounts for the negative baseline in one of the simulated spectra in Figure 5. The ENZ mode is is distinct from the LSPR in that it is polarized perpendicularly to the plane of the film. The ENZ mode is only observed in very thin films, which is consistent with the observations made in the ITO structures with a thickness (20 nm), which is clearly much less than the skin depth of ITO (120 nm). The fact that the reflection has a minimum at ~6,200 cm-1 rather than the value of 8,900 cm-1 observed in Ref. 13 is attributable to the fact that the films prepared in this study have a charge carrier density of 9.1 x 1020 cm-3, which is significantly lower than films studied previously.14,16 The value obtained in this work is comparable to both absorption maximum of 10% Sn-doped ITO in Ref. 12 and the normalized extinction of the ITO(∆) sample shown in Figure 2. In order to show that the FDTD calculation is a robust method for this type of modeling we have calculated the optical response expected for 150 nm thick ITO film in Figure 5B. Indeed, we observe the SPR as expected based on previous work. Having made the assignment of the absorption in Figure 2 peaked at 6,300 cm-1 as the ENZ mode, it is important to note that this extinction is in the same range as the LSPR of ITO NPs made by Teranishi et al with different mole percentages of Sn from 0% to 30%.42 Based on the foregoing, we believe that the similarty of the these two spectral features is a coincidence. The ITO structures obtained using NSL are of comparable size and shape to previously studied structures obtained using Ag and Au. Indeed, the normalized extinction is comparable to the Ag and Au structures as well. However, the peak positions and widths of LSPR extinctions depend on the size of NPs for metals, such as Al, Ag, Au, and Cu with the same shapes and


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thickness as the structures obtained in this study.11,57 In the present study we find that the wave number of the ITO ENZ mode observed in these extremely thin films does not have a significant dependence on angle. The difference can be understood based on the physical insight we have previously proposed to explain the difference between thin and thick films of ITO. The extinction observed in Figure 3 is an ENZ mode because the thickness of the film is much less than the skin depth. Previous work has already shown that the ENZ mode differs from the LSPR in the same thing film. Since both can be observed simultaneously in thing films it is evidenct that the ENZ mode is not strongly dependent on angle. The lack of an angle dependence is related to the electric field confinement in ENZ structures. Moreover, the fact that the ENZ has a similar position in a thin film or nanostructured surface is consistent with its perpendicular polarization. The position of the ENZ mode is dependent mainly on the out of plane thickness and charge carrier density, which is determined by the Sn doping and the annealing conditions for ITO thin films. However, an increase in surface coverage will give rise to an increase in the intensity of the extinction, but not have a significant effect on the wave number of the maximum extinction.

Conclusion We conclude that the extinction at circa 6,200 cm-1 on a patterned surface is the ENZ mode for ITO. The plasmonic nature of the extinction on a patterned ITO surface was demonstrated by the variation of the carrier concentration through ex-situ annealing in air and in N2/H2 forming gas, respectively. Hence, the ENZ mode is present in a thin film, but also on a patterened surface if the thickness is less than the skin depth. Although the observation was made on ITO it should be generally valid. Since a second layer providing optical elements made of Au or other material


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can have a fixed relationship with respect to a patterned ENZ surface this work suggests the possibility of unit cell tuning of ENZ mode optical effects.

Supporting Information SEM and AFM data showing the preparation and typical height profile of the surfaces are presented. This material is available free of charge via the internet at http://pubs.acs.org. Acknowledgments This work was supported by NSF Grant CHE 1112017.


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Figure 1. Triangular nanopatterns of ITO (a) SEM image of 14 x 12 µm, (b) AFM image. show that the observed extinction in these patterned structures corresponds to the epsilon-near-zero (ENZ) mode for ITO.


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Figure 2. IR extinction spectra recorded at incident angles ranging from 42.5º to 52º (from orange to violet, respectively) in 0.7º steps for patterned ITO films A) Spectra of a film after annealing in a 5 % H2/95 % N2 (forming) gas, B) same film as A) after annealing in 20% O2 gas, C) a film annealed in 20% xO2 gas, D) same film as C) after annealing in forming gas.


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Figure 3. FDTD calculated normalized extinction spectra of equilateral triangles of ITO 800 nm on a side [ITO(∆)] and with a thickness of 20 nm. The result is based on a normalized calculation of the reflectance of BK7 glass and patterned ITO(∆). The normalized extinction was calculated as –log(ITO(∆)/BK7). The incident light source is set below the ITO patterns with incident angles of 39° and 41° to be consistent with the Kretschmann configuration geometry.


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ACS Photonics

Figure 4. Schematic illustration of the FDTD numerical simulation based on a triangular shaped model of the ITO patterns. (a) The red-dotted square shows the simulation area. (b) The side view of (a). The red arrow represents p-polarized EM wave and the blue arrow illustrates the direction of the incident EM wave. The incident EM wave is placed below the patterned ITO to be consistent with the experimental Kretschmann configuration.


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Figure 5. The electric field intensity distributions (|E|2) are shown in the xy plane at the wavenumber of 2000 cm-1 (2.0 nm), 6250 cm-1 (1.6 nm), and 7143 cm-1 (1.4 nm). (a)-(c) are calculated with the incident angle of 39°. (d)-(f) are the results with the incident angle of 41°.


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ACS Photonics

Indium tin oxide surface patterned triangles can be reversibly converted into optically active structures by annealing in forming gas or into optically inactive structures by annealing in oxygen gas. 338x190mm (96 x 96 DPI)


Near-Infrared Optical Extinction of Indium Tin Oxide Structures

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