INDIUM OXIDE LOW-DIMENSIONAL STRUCTURES AND APPLICATIONS

Indium oxide is mostly known for its application as transparent electrode, when combined with tin to form indium tin oxide (ITO). However, over the last two decades an increasing interest has arisen for its properties as semiconductor material on its own (Bierwagen, & Speck, 2010), giving rise to a variety of new applications. There is a lot of available information on the use of ln203 and ITO as transparent electrode, which can be found elsewhere (see for instance Hofmann, Cloutet, & Hadziioannou, 2018), and therefore this section will be focused on the work done on low-dimensional ln203 structures with different applications such as gas sensing, field emission transistor, and as an active material in optics (nonlinear optics, harmonic generation, light confinement).

Indium oxide crystallizes in essentially three different phases, corresponding to the space groups /2[3, Ici-3 (cubic), and R-3 (rhombohedral) (Karazhanov et ah, 2007). The most stable phase at ambient pressure and temperature is the Iu-3 phase (hereinafter called bcc- ln203 phase), which has bixbyite structure and a unit cell of 80 atoms with 8 formula units per unit cell (see Figure 5.1a). It consists of two types of In atoms in octahedral and trigonal prismatic coordination, and one type of О atoms, located at Wyckoff positions 8b,

Schematic of the two most common phases

FIGURE 5.1 Schematic of the two most common phases: (a) bixbyite bcc-ln203 and (b) corundum rh-ln203.

Images were rendered by VESTA software (Momma, & Izumi, 2011).

24d, and 48e, respectively. The structure may be regarded as a 2 x 2 x 2 superstructure of fluorite, with ordered removal of О from one-fourth of the anion sites.

The rhombohedral modification R-3 (hereinafter rh-ln203) crystallizes with corundum structure (Figure 5.1b), with two formula units per unit cell and only one type of In atoms in trigonal biprism coordination and one type of О atoms, occupying 12c and 18e Wyckoff positions, respectively. This phase is stable only at high pressures, but it can be stabilized in ambient conditions by epitaxial growth on selected substrates, such as rhombohedral A1203 (0001), and carefully tuning the growth conditions (Wang et al., 2008; King et al., 2009). It is worth noting that XRD patterns from rh-ln203 resemble closely those produced by poorly oxidized bcc-ln203, where a combination of metallic In and bcc-ln203 peaks can be present, thus great care has to be taken before ascribing them to the rhombohedral modification. The /213 phase has been very rarely reported (Zachar- iasen, 1927) and experimental information is extremely limited; therefore, this phase will not be further discussed here.

Owing to the large predominance of the bcc-ln203 phase, this section will focus essentially on its properties and applications, while references to rhombohedral phase will be explicitly stated. The bcc-ln203 phase has a fundamental bandgap of 2.7-2.9 eV, (Bourlange et al., 2008; Walsh et al., 2008); however, for long time its value was believed to be close to 3.7 eV. The reason for this is the parity-forbidden nature, within the dipole approximation, of the direct transitions between the valence band maximum and the conduction band minimum, which causes a strong shift in the absorption onset of the Tauc plot that appears as a very wide “optical bandgap” of around 3.7 eV (Figure 5.2). Thus, while the electronic properties are essentially determined by the fundamental bandgap, the optical bandgap dominates the optical properties of indium oxide. This decoupling is largely the cause of the excellent

Experimental and theoretical

FIGURE 5.2 Experimental and theoretical (inset) absorption coefficient of (a) bcc-ln203 and (b) rh-ln203. The optical bandgap estimated from extrapolation of the absorption onset is indicated, as well as the fundamental bandgap. Reprinted figure with permission from King et al. (2009). Copyright (2009) bv the American Physical Society, (c) Calculated band structure of bcc-ln203. Green bands highlight the parity allowed for band-to-band transitions, with the first allowed valence bands lying 0.8 eV below the valence band maximum.

Reprinted figure with permission from Walsh et al. (2008). Copyright (2008) by the American Physical Society.

capabilities of indium oxide and its doped version ITO as transparent yet highly conductive electrode.

Degenerated doping may also cause some modifications on the optical bandgap due to the appearance of bandgap renormalization effects, which cause shrinkage of the fundamental gap, and the well-known Burstein-Moss shift (Walsh, Da Silva, & Wei, 2008). The latter is produced when the Fermi level lies above the conduction band minimum, leaving no empty states to be occupied by the excited electrons, which therefore require more energy to transit from the valence band maximum to empty states in the conduction band.

The electronic properties of indium oxide are highly influenced by its intrinsic defect structure. The presence of oxygen vacancies (V0) are ubiquitous to the majority of oxide semiconductors and play a key role in several of their physical properties, from their electronic transport to their luminescence spectrum or their ionic diffusion. Doubly ionized oxygen vacancies in indium oxide are generally considered the origin of the high intrinsic conductivity of indium oxide, as first stated by De Wit (1973). Theoretical calculations performed by different authors (Agoston. Albe, et al., 2009; Agoston, Erhart, et al. 2009; Lany, & Zunger, 2007) have confirmed that these kind of defects have indeed the lowest formation energy, although, there is still some controversy on the energy levels they introduce in the gap. However, it is generally accepted that these are shallow levels, as measured experimentally (Bierwagen. & Speck, 2010). Vo have also been attributed as the origin of the blue-green emission of ln203, centered at approximately 2.5 eV, which is compatible with the shallow donor-like level picture. Oxygen deficiency is even more pronounce at the surface, where V0 have been reported to have exceptionally low formation energies (Lany, & Zunger, 2007), giving rise to an electron accumulation layer which can dominate the conduction of ln203 thin films. This is in contrast to its doped versions such as Sn:ln203 (ITO) or Zn:ln203 (1ZO) where an electron depletion layer is found at the surface (Bartolome, Maestre, Cremades, Amatti. & Piqueras, 2013; King et al., 2008).

A more comprehensive review of the physical properties of indium oxide by Bierwagen can be found in Bierwagen (2015).

Optics-Related Applications of Low-Dimensional In203 Structures

One of the earliest proposed applications for this material beyond its use as transparent electrode was in smart windows (Hamberg, & Granqvist, 1986) as wavelength selective filter. It is based on the large reflectivity in the infrared (1R) region presented by heavily Sn-doped indium oxide, combined with an excellent transparency in the visible range. An ITO thin-film coating can efficiently block the electromagnetic radiation in the range of 0.7-50 pm, which corresponds to the main spectral range emission of the black body at room temperature, thus hindering the radiative thermal energy transfer (Granqvist, 2007). The high 1R reflectance of heavily doped indium oxide is caused by the large carrier concentration, which is proportional to the square of the plasma frequency, co,„ within the Drude model. Thus, cop and in turn the reflectivity in the 1R region can be tuned by varying the carrier concentration of the films as shown in Figure 5.3 (Granqvist, 2007; Hamberg, & Granqvist, 1986). A different approach not involving the deposition of thin films is the use of ITO microparticle dispersions on laminated glass, which has been effectively used as heat blockers on window panels (patent ЕР 1 698 599 Bl) (Hagiwara, Nakagawa, Fukatani, Yoshioka, & Hatta, 2015).

The relatively high cop of heavily doped indium oxide is also interesting from the point of view of nonlinear optics (Liberal & Engheta, 2017). For a lossless material, any change in refractive index, n, depends on the variation of dielectric permittivity, e, as An2 - Де2/4с, and thus it is maximized when c tends to zero, a condition that is reached at the plasma frequency (Alam, Leon, & Boyd, 2016). This is usually referred as epsilon-near-zero (ENZ)

Theoretical spectral transmittance and reflectance of a 200 nm SmlmCb thin film. The nvalues indicate the electron density used for the calculations

FIGURE 5.3 Theoretical spectral transmittance and reflectance of a 200 nm SmlmCb thin film. The nc values indicate the electron density used for the calculations.

Reprinted from Hambcrg and Granqvist (1986) with permission from Л1Р Publishing.

condition. Hence, both indium oxide and ITO are expected to have strong nonlinear optical properties in the near-infrared light range (Alam et al., 2016). This phenomenon has been recently studied in ITO thin films, showing a strong (up to An ~ 0.72) and ultrafast (up to ^recovery ~360 fs) third-order nonlinear response (Alam et ah, 2016), and exploited in second (Capretti, Wang, Engheta, & Dal Negro, 2015) and third-harmonic (Capretti, Wang, Engheta. & Negro, 2015) light generation using telecommunication IR wavelengths as source light. The high transparency of ITO thin films in the visible range enables an efficient extraction of the generated light, while the ENZ condition reached at telecom wavelengths allows a complete integration with Si technology.

One of the problems of working in ENZ conditions is the larger intrinsic material losses at this wavelength range. Although ITO actually presents a negative nonlinear attenuation constant at ENZ (Alam et ah, 2016), intrinsic losses can be further minimized using intrinsic ln203 instead of ITO in the visible range, where 1п20з is highly transparent. In this case, the nonlinear behavior does not rely on the high sensitivity of the refractive index in the ENZ regime, but it is originated by a strong modification of interband optical transitions upon intense carrier pumping. Contrary to the previous examples, this process produces a negative variation of the refractive index (up to An ~ -0.09) and has been demonstrated on undoped indium oxide nanorod arrays (Guo, Chang, & Schaller, 2017). Figure 5.4 shows the ln203 nanorod array obtained by vapor-liquid-solid process and the resulting modification of the array transmittance spectra at different transient times. These processes may find applications in the field of nano-photonics, plasmonics, or as saturable absorbers.

Light confinement and guiding is another application of indium oxide low-dimensional structures. As previously mentioned, the linear refractive index, n, of In203 is dependent on its plasma frequency, which can be widely varied depending on doping and V0 concentration, and therefore on its growth conditions. However, the refractive index in the visible range of undoped ln203 lies around n = 1.9-2.0 (Bartolome, Cremades, & Piqueras, 2013; Dong et al., 2009), which is high enough to support light confinement through either Fabry-Perot (FP) resonances or whispering gallery modes (WGM) in open dielectric resonator (ODR) structures. An ODR consists essentially in a structure made of a high n material with one or several dimensions close in size to the wavelength of the visible-IR light, which induces light confinement in these dimensions through multiple reflections in the inner walls of the ODR (see Figure 5.5). In the case of FP resonances, these are achieved through a large difference in n at the ODR interface, while for WGM resonances are obtained through total internal reflections.

Light confinement in the range of the visible light has been obtained for a variety of structures, from octahedral microcrystals (Dong et al., 2010) to elongated microrods with hexagonal or square cross sections (Bartolome et al., 2013; Dong et al., 2009). The photoluminescence of ln203 is employed as a source of visible light, making use of its wide emission band centered at 600 nm, which spreads from the near-infrared to the blue range (Figure 5.5c). Thus, these structures can be used as both passive (optical cavities) and active (gain medium) materials in solid-state lasers. Quality factors (a measure of the energy stored by the cavity) as high as 350 have been obtained in highly crystalline hexagonal microrods (Bartolome et al., 2013). Because light confinement on elongated microstructures is

(a) Scanning electron micrograph of ln0 nanorod array,

FIGURE 5.4 (a) Scanning electron micrograph of ln203 nanorod array, (b) and (c) Variation of its transmittance spectrum with laser fluence at two different transient times, (d) Schematic of the electronic process leading to the negative nonlinear transient process.

Visible light confinement in an ln0 rod

FIGURE 5.5 Visible light confinement in an ln203 rod. (a) Different possible resonances in a hexagonal cavity, (b) Scanning electron micrograph of an ln203 rod with hexagonal cross-section, (c) Photoluminescence (PL) spectrum of the rod containing WGM resonances and its comparison with the PL spectrum of ceramic reference, (d) Tunable WGM obtained by varying the PL excitation point along the wire.

Reproduced from Bartolomc, Crcmades ct al. (2013) with permission from the Royal Society of Chemistry.

usually produced inside their transverse cross section, the size of the optical cavity depends only on the cross section area at the excitation point and thus can be changed continuously by changing the excitation position on tapered rods, which leads to continuously tunable resonators (Figure 5.5d) (Bartolome et al., 2013) that can be used as tunable filters. Waveguiding has also been proved in elongated structures of undoped ImOj (Bartolome et al., 2013), which could facilitate the realization of all-ImCb-based optical circuits.

A different way of achieving light confinement is through surface plasmon resonances, which allows confinement at sub-wavelength scales, enabling much denser electromagnetic energy storage (lower modal volumes). 1TO nanorod arrays have been used in tunable 1R plasmonics with ultrafast switching (Guo, Schaller, Ketterson, & Chang, 2016) through localized surface plasmon resonances. In this case a combination of high free carrier density and strong directionality of the rods is used to support different localized surface plasmon resonance modes by intraband pumping of the conduction band electrons. The nonparabolic nature of ITO conduction band induces a red shift of the rods' plasmon frequency at high fluence pumping, which translates into a tunable mid infrared transmission modulation through tailoring of the sample geometry. Localized surface plasmonics on ITO nanorods find application in sensing, nanolasers, or nano-antennas.

Sensors with In203 as Active Material

One widespread application of low-dimensional semiconductor oxides is the field of sensor devices. There exist a large variety of different oxide-based sensors depending on their sensing mechanism and/or what is being sensed. Conductometric sensors are by far the most common type of oxide-based sensors and they work on the principle of sample conductance change upon exposure to chemical analytes. Other kind of sensors are field-effect transistor sensors, whose channel carrier concentration is sensitive to the presence of different chemical species, optical sensors, which are based on the change of refractive index or spectral absorption of the material, or force sensors, which measure either the change in the material stiffness or inertial mass under analyte adsorption. Several extensive reviews on the fundamentals and new advances of ln203 thin (and thick) film conductometric sensors have already been published (see for instance Korotcenkov, Brinzari, & Cho, 2016, 2018). Therefore, this subsection will be focused on the recent progress of other low-dimensional ln203-based sensors such as nanoparticles, nanowires, or rods.

Because the surface electron accumulation layer of indium oxide is produced by its large surface oxygen substoichiometry, any structure with increased surface to volume ratio, such as nanoparticles, nanowires, nanoribbons, or nanorods, would be extremely sensitive to oxidizing and/or reducing gases such as 03, NOv, NH3, or ethanol (Rombach et al., 2016). These structures have the additional advantage of highly reduced time responses as compared to thin or thick films. The reason is that adsorption processes are produced directly on the surface of the structures, so no diffusion is required for the analytes to reach the inner layers of the material. This has another implication, as surface desorption is highly enhanced in indium oxide by ultraviolet (UV) light illumination (Wang et al., 2012). Hence, UV-enhanced desorption can be used to further decrease the time response of indium oxide nanostructure- based sensors, allowing room temperature operation without heating the samples at the usual 150-300 °C, with the associated energy cost reduction.

0D (nanoparticles) (Alvarado et al., 2018; Elouali et al., 2012; Gao et al., 2016; Gu, Nie, Han, & Wang, 2015; Neri et al., 2005; Xu et al., 2015) and ID (nanowires, ribbons, rods, etc.) (Du et al.. 2007; Li et al., 2003; Liang, Kim, Yoon, Kwak, & Lee, 2015; Rout et al., 2006; Singh et al., 2010; Vomiero et al., 2007; Xing et al., 2015; Zhang et al., 2004) indium oxide conductometric and FET gas sensors have been investigated since the early 2000s. Some general insights on nanoparticle and nanowire gas sensing can be found in Huang and Choi (2007); Hung, Le, & Van Hieu, (2017); Ramgir et al. (2010); Zhang, Liu, Neri, & Pinna (2016). Gas sensing with indium oxide nanowires and nanoparticles has been pushed down to detection limits as low as few particles per billion (ppb) for highly oxidizing gases such as NOx (Zhang et al., 2004), 03 (Wang et al., 2007), or for acetone (Feng et al., 2015), with improved time response as compared to other thin or thick film counterparts. The sensing mechanisms of nanowire-based devices are, in principle, rather simple, as they usually consist of a single wire or a bunch of them connected in parallel, which change in conductivity due to the electron extraction/injection of the absorbed analytes; while on nanoparticle films analyte chemi- and/or physisorption at the grain boundaries usually dominate the transport processes, leading to more complex behaviors and interplays. However, some detailed analysis revealed, even in the early works of ln203 nanowire gas sensors, that the situation is actually more complex. Zhang et al. (2003) reported that undoped ln203 nanowire response to NH3 depends on the intrinsic carrier concentration (determined by its Vo concentration) and thus on the Fermi level of the studied wire, with «-type (increase in resistance under oxidizing conditions) sensing behavior for high carrier concentrations, and /;-type (resistance decrease) for low concentrations. The reason for this, according to Zhang et al. (2003), was the relative position of the Fermi level with respect to the surface states induced by the adsorbed NH3, which determine the role of NH3 molecules as either electron traps or donors (see Figure 5.6). Thus, careful tuning (through doping) of the Fermi level relative to the surface states

I-V curves for seemingly identical single ln0 nanowire FET devices with high

FIGURE 5.6 I-Vl[ curves for seemingly identical single ln203 nanowire FET devices with high (a) and low (b) carrier concentrations, and their different behavior upon exposure to 1% of NH3. Inset shows a schematic of the surface energy bands and the expected carrier flow.

Figure reprinted from Li ct al. (2003) with permission from AIP Publishing.

introduced by the desired analytes should allow improvements on the selectivity and sensitivity of these devices (Singh et al., 2010). Similarly, an anomalous change from /Муре to //-type sensing has been reported for ln203 nanoparticles for increasing NOx concentrations, which was explained by the competition between adsorbed NOx_ and СГ species for the same adsorption sites (Xu et al., 2018). Each NOx~ molecule displaces two O” ions, which desorb releasing two electrons, with a net gain of one electron, leading to an increase in conductance as in a //-type material. At high NOx concentrations, the majority of adsorbed СГ species are displaced and competition between both is no longer determining the material response, which thus behave as an //-type sensor. The presence of humidity and/or different operating temperature can also modify the sensing response of ln203 to oxidizing or reducing agents (Kor- otcenkov et al., 2004).

Improved performances can be obtained also by functionalizing the structures with nanoparticle decorations of other inorganic compounds such as CuO (Liang et al.. 2015), Bi2Oj (Park, Kim, Sun, & Lee, 2015), Sn02 (Xu et al., 2015), W02 (Zachariasen, 1927; Feng et al., 2015), ТЮ2 (Wu, Chou, & Wu, 2018), La203 (Zhan, Lu, Song, Jiang, & Xu, 2007), Au (Xing et al., 2015). Ag (Zhu, Chang, et al., 2016), or Pt (Neri et al., 2007), or with graphene/reduced graphene oxide (rGO) (Gu et al., 2015). The enhancement obtained with metallic nanoparticles is usually associated to their catalytic actions, as they accelerate the chemical reactions taking place on the surface of the matrix material (i.e., ln203). However, in the case of metal oxide nanoparticles, the improvement is strongly related to the band bending at the heterojunction and its modification upon analyte exposure.

In203 nanoribbon arrays have also been used as biosensors for the detection of cardiac attack biomarkers (Liu et al., 2016). Liu et al. (2016) fabricated and patented (US2019120788A1) an ln203 nanoribbon array in an FET configuration by a lithography-free shadow masking method (see Figure 5.7a). ln203 nanoribbons demonstrated high sensitivity and negligible degradation to pH changes under wet conditions. These ribbons were subsequently functionalized by antibodies specific of three different biomolecules produced during acute myocardial infarctions which fixed the targeted antigens and, at the same time, induced an amplified pH change of the ribbons environment (Figure 5.7). These sensors showed excellent stability, reproducibility, and reusability.

A different kind of sensors consists in those based on mechanical or optical resonances. In203 optical resonators have already been briefly reviewed in the previous section, and the potential application of ODRs in sensing has been discussed elsewhere (Yang, & Guo,

ln0 nanoribbon biosensors,

FIGURE 5.7 ln203 nanoribbon biosensors, (a) Optical and scanning electron micrographs of the ribbons fabricated by shadow mask technique, (b) Scheme of antibody biofunctionalization and working principle, (c) and (d) Biosensor time response to 1, 10, and 300 pg mLH of cardiac troponin I antigens in phosphate-buffered saline, and average sensing results of three devices for the three concentrations in (c) and one concentration of troponin I in diluted human whole blood, marked as a red dot.

Figure adapted with permission from Liu ct al. (2016). Copyright (2016) American Chemical Society. (US2019120788A1).

2006; Fan et al., 2011). However, to the best of the authors’ knowledge, ln203 optical resonators have not been employed for gas sensing yet, despite their promising results.

In203 Mechanical Resonators

Mechanical resonators, on the other hand, make use of their change in resonance frequency and/or quality factor upon molecule adsorption as transducing mechanism (Eom, Park, Yoon, & Kwon, 2011). These sensors can present extremely high sensitivities, allowing detection limits down to the single molecule. Most of the mechanical resonators consist of a single or double clamped beam of a selected material, typically Si or diamond, with cross sections in the range of the nanometers to few micrometers. Other configurations based on planar or more complex structures can also be found. The resonance frequency of the resonator depends fundamentally on the beam stiffness and its mass, and therefore the variation in resonance frequency due to single-molecule adsorption/desorption could be detected if the resonance peak is narrow enough, that is, if the quality factor is high enough. Indium oxide mechanical resonators have been obtained from single crystal microrods (Figure 5.8), presenting quality factors as high as 4 x HP (Bartolome, Cremades. & Piqueras, 2015; Bartolome, Hidalgo, Maestre, Cremades, & Piqueras, 2014), comparable to other high-quality resonators, with the added advantage of high transparency in the visible and electrical conductivity. These resonators presented theoretical force detection limits of 10~16 N/Hz1/2, opening the gate to ultrasensitive ln203 mass sensors.

 
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