Calculation of the electronic and optical properties of indium tin oxide by density functional theory

Quantum Interference at the Recombination Junction of Perovskite-Si Tandem Solar Cells Improves Efficiency

We show quantum interference effects can enhance coherent electron transmission in perovskite tandem solar cells using ultrathin indium tin oxide (ITO) layers. We develop a model for the behavior of the power conversion efficiency based on a finite difference time domain solution of the time-dependent Schrödinger equation. The modeled potential includes an imaginary part to simulate probability loss by incoherent scattering. The results agree with observations of efficiency as a function of ITO thickness, suggesting an optimized design.

Thickness-Dependent Charge Transport in Three Dimensional Ru(II)- Tris(phenanthroline)-Based Molecular Assemblies

We describe here the fabrication of large-area molecular junctions with a configuration of ITO/[Ru(Phen)3]/Al to understand temperature- and thickness-dependent charge transport phenomena. Thanks to the electrochemical technique, thin layers of electroactive ruthenium(II)-tris(phenanthroline) [Ru(Phen)3] with thicknesses of 4-16 nm are covalently grown on sputtering-deposited patterned ITO electrodes. The bias-induced molecular junctions exhibit symmetric current-voltage (j-V) curves, demonstrating highly efficient long-range charge transport and weak attenuation with increased molecular film thickness (β = 0.70 to 0.79 nm-1). Such a lower β value is attributed to the accessibility of Ru(Phen)3 molecular conduction channels to Fermi levels of both the electrodes and a strong electronic coupling at ITO-molecules interfaces. The thinner junctions (d = 3.9 nm) follow charge transport via resonant tunneling, while the thicker junctions (d = 10-16 nm) follow thermally activated (activation energy, Ea ∼ 43 meV) Poole-Frenkel charge conduction, showing a clear "molecular signature" in the nanometric junctions.

Photonic Characterisation of Indium Tin Oxide as a Function of Deposition Conditions

Indium tin oxide (ITO) has recently gained prominence as a photonic nanomaterial, for example, in modulators, tuneable metasurfaces and for epsilon-near-zero (ENZ) photonics. The optical properties of ITO are typically described by the Drude model and are strongly dependent on the deposition conditions. In the current literature, studies often make several assumptions to connect the optically measured material parameters to the electrical properties of ITO, which are not always clear, nor do they necessarily apply. Here, we present a comprehensive study of the structural, electrical, and optical properties of ITO and showed how they relate to the deposition conditions. We use guided mode resonances to determine the dispersion curves of the deposited material and relate these to structural and electrical measurements to extract all relevant material parameters. We demonstrate how the carrier density, mobility, plasma frequency, electron effective mass, and collision frequency vary as a function of deposition conditions, and that the high-frequency permittivity (ϵ∞) can vary significantly from the value of ϵ∞ = 3.9 that many papers simply assume to be a constant. The depth of analysis we demonstrate allows the findings to be easily extrapolated to the photonic characterisation of other transparent conducting oxides (TCOs), whilst providing a much-needed reference for the research area.

Tunable plasmonics on epsilon-near-zero materials: the case for a quantum carrier model

The carrier density profile in metal-oxide-semiconductor (MOS) capacitors is computed under gating using two classical models - conventional drift-diffusion (CDD) and density-gradient (DG) - and a self-consistent Schrödinger-Poisson (SP) quantum model. Once calibrated the DG model approximates well the SP model while being computationally more efficient. The carrier profiles are used in optical mode computations to determine the gated optical response of surface plasmons supported by waveguides incorporating MOS structures. Indium tin oxide (ITO) is used as the semiconductor in the MOS structures, as the real part of its optical permittivity can be driven through zero to become negative under accumulation, enabling epsilon-near-zero (ENZ) effects. Under accumulation the predictions made by the CDD and SP models differ considerably, in that the former predicts one ENZ point but the latter predicts two. Consequently, the CDD model significantly underestimates perturbations in n _{e f f} of surface plasmons (by ∼4×) and yields incorrect details in surface plasmon fields near ENZ points. The discrepancy is large enough to invalidate the CDD model in MOS structures on ENZ materials under accumulation, strongly motivating a quantum carrier model in this regime.

Classical Model of Surface Enhanced Infrared Absorption (SEIRA) Spectroscopy

The molecule-plasmon interaction is the key to the mechanisms of surface enhanced infrared absorption (SEIRA) and surface enhanced Raman scattering (SERS). Since plasmons are well described by Maxwell's equations, one fundamental treatment involves the classical interpretation of infrared absorption and resonance Raman spectroscopies. We can understand the molecule-plasmon interaction using electromagnetic theory if the classical field effect on a transition dipole moment or transition polarizability is properly described. In previous work, we derived the Raman excitation profile of a model molecule using a classical driven spring attached to a charged mass with a perturbative force constant due to vibrational oscillations. In this study we generalize the interactions of plasmons with molecules by considering the N₂O asymmetric stretch SEIRA signal on a Dy doped CdO (CdO:Dy) film. This semiconductor has tunable plasmon dispersion curves throughout the near-and mid-infrared that can interact directly with vibrational absorption transitions. We have demonstrated this using the Kretschmann configuration with a CaF₂ prism and a MgO substrate. The model predicts the phase behavior of SEIRA. The calculated enhancement factor relative to an Au control is 6.2, in good agreement with the value of 6.8 ± 0.5 measured under the same conditions.

The Impact of the Surface Modification on Tin-Doped Indium Oxide Nanocomposite Properties

This review provides an analysis of the theoretical methods to study the effects of surface modification on structural properties of nanostructured indium tin oxide (ITO), mainly by organic compounds. The computational data are compared with experimental data such as X-ray diffraction (XRD), atomic force microscopy (AFM) and energy-dispersive X-ray spectroscopy (EDS) data with the focus on optoelectronic and electrocatalytic properties of the surface to investigate potential relations of these properties and applications of ITO in fields such as biosensing and electronic device fabrication. Our analysis shows that the change in optoelectronic properties of the surface is mainly due to functionalizing the surface with organic molecules and that the electrocatalytic properties vary as a function of size.

Defect-Induced Tunable Permittivity of Epsilon-Near-Zero in Indium Tin Oxide Thin Films

Defect-induced tunable permittivity of Epsilon-Near-Zero (ENZ) in indium tin oxide (ITO) thin films via annealing at different temperatures with mixed gases (98% Ar, 2% O₂) was reported. Red-shift of λ_ENZ (Epsilon-Near-Zero wavelength) from 1422 nm to 1995 nm in wavelength was observed. The modulation of permittivity is dominated by the transformation of plasma oscillation frequency and carrier concentration depending on Drude model, which was produced by the formation of structural defects and the reduction of oxygen vacancy defects during annealing. The evolution of defects can be inferred by means of X-ray diffraction (XRD), atomic force microscopy (AFM), and Raman spectroscopy. The optical bandgaps (E_g) were investigated to explain the existence of defect states. And the formation of structure defects and the electric field enhancement were further verified by finite-difference time domain (FDTD) simulation.

Printed Electronics Based on Inorganic Semiconductors: From Processes and Materials to Devices

Following the ever-expanding technological demands, printed electronics has shown palpable potential to create new and commercially viable technologies that will benefit from its unique characteristics, such as, large-area and wide range of substrate compatibility, conformability and low-cost. Through the last few decades, printed/solution-processed field-effect transistors (FETs) and circuits have witnessed immense research efforts, technological growth and increased commercial interests. Although printing of functional inks comprising organic semiconductors has already been initiated in early 1990s, gradually the attention, at least partially, has been shifted to various forms of inorganic semiconductors, starting from metal chalcogenides, oxides, carbon nanotubes and very recently to graphene and other 2D semiconductors. In this review, the entire domain of printable inorganic semiconductors is considered. In fact, thanks to the continuous development of materials/functional inks and novel design/printing strategies, the inorganic printed semiconductor-based circuits today have reached an operation frequency up to several hundreds of kilohertz with only a few nanosecond time delays at the individual FET/inverter levels; in this regard, often circuits based on hybrid material systems have been found to be advantageous. At the end, a comparison of relative successes of various printable inorganic semiconductor materials, the remaining challenges and the available future opportunities are summarized.

Localized Surface Plasmon Resonance in Semiconductor Nanocrystals

Localized surface plasmon resonance (LSPR) in semiconductor nanocrystals (NCs) that results in resonant absorption, scattering, and near field enhancement around the NC can be tuned across a wide optical spectral range from visible to far-infrared by synthetically varying doping level, and post synthetically via chemical oxidation and reduction, photochemical control, and electrochemical control. In this review, we will discuss the fundamental electromagnetic dynamics governing light matter interaction in plasmonic semiconductor NCs and the realization of various distinctive physical properties made possible by the advancement of colloidal synthesis routes to such NCs. Here, we will illustrate how free carrier dielectric properties are induced in various semiconductor materials including metal oxides, metal chalcogenides, metal nitrides, silicon, and other materials. We will highlight the applicability and limitations of the Drude model as applied to semiconductors considering the complex band structures and crystal structures that predominate and quantum effects that emerge at nonclassical sizes. We will also emphasize the impact of dopant hybridization with bands of the host lattice as well as the interplay of shape and crystal structure in determining the LSPR characteristics of semiconductor NCs. To illustrate the discussion regarding both physical and synthetic aspects of LSPR-active NCs, we will focus on metal oxides with substantial consideration also of copper chalcogenide NCs, with select examples drawn from the literature on other doped semiconductor materials. Furthermore, we will discuss the promise that LSPR in doped semiconductor NCs holds for a wide range of applications such as infrared spectroscopy, energy-saving technologies like smart windows and waste heat management, biomedical applications including therapy and imaging, and optical applications like two photon upconversion, enhanced luminesence, and infrared metasurfaces.

Nested plasmonic resonances: extraordinary enhancement of linear and nonlinear interactions

Plasmonic resonators can provide large local electric fields when the gap between metal components is filled with an ordinary dielectric. We consider a new concept consisting of a hybrid nanoantenna obtained by introducing a resonant, plasmonic nanoparticle strategically placed inside the gap of an aptly sized metallic antenna. The system exhibits two nested, nearly overlapping plasmonic resonances whose signature is a large field enhancement at the surface and within the bulk of the plasmonic nanoparticle that leads to unusually strong, linear and nonlinear light-matter coupling.

Core-shell PbS/Sn:In2O3 and branched PbIn2S4/Sn:In2O3 nanowires in quantum dot sensitized solar cells

Core-shell PbS/Sn:In₂O₃ and branched PbIn₂S₄/Sn:In₂O₃ nanowires have been obtained via the deposition of Pb over Sn:In₂O₃ nanowires and post growth processing under H₂S between 100 °C-200 °C and 300 °C-500 °C respectively. The PbS/Sn:In₂O₃ nanowires have diameters of 50-250 nm and consist of cubic PbS and In₂O₃ while the PbIn₂S₄/Sn:In₂O₃ nanowires consist of PbIn₂S₄ branches with diameters of 10-30 nm and an orthorhombic crystal structure. We discuss the growth mechanisms and also show that the density of electrons in the n-type Sn:In₂O₃ core is strongly dependent on the thickness of the p-type PbS shell, which must be smaller than 30 nm to prevent core depletion, via the self-consistent solution of the Poisson-Schrödinger equations in the effective mass approximation. The PbS/Sn:In₂O₃ and PbIn₂S₄/Sn:In₂O₃ nanowire networks had resistances of 100-200 Ω due to the large carrier densities and exhibited defect related photoluminescence at 2.2 eV and 1.5 eV respectively. We show that PbS in contact with polysulfide electrolyte has ohmic like behavior but the PbS/Sn:In₂O₃ nanowires gave, rectifying current voltage characteristics as a counter electrode in a quantum dot sensitized solar cell using a conventional ITO/TiO₂/CdS/CdSe photo anode, an open circuit voltage of ≈0.5 V, and short circuit current density of ≈1 mA cm^-2. In contrast the branched PbIn₂S₄/Sn:In₂O₃ nanowires exhibited a higher current carrying capability of ≈7 mA cm^-2 and higher power conversion efficiency of ≈2%.

Single-Particle Plasmon Voltammetry (spPV) for Detecting Anion Adsorption

Nanoparticle and thin film surface plasmons are highly sensitive to electrochemically induced dielectric changes. We exploited this sensitivity to detect reversible electrochemical potential-driven anion adsorption by developing single-particle plasmon voltammetry (spPV) using plasmonic nanoparticles. spPV was used to detect sulfate electroadsorption to individual Au nanoparticles. By comparing both semiconducting and metallic thin film substrates with Au nanoparticle monomers and dimers, we demonstrated that using Au film substrates improved the signal in detecting sulfate electroadsorption and desorption through adsorbate modulated thin film conductance. Using single-particle surface plasmon spectroscopic techniques, we constructed spPV to sense sulfate, acetate, and perchlorate adsorption on coupled Au nanoparticles. spPV extends dynamic spectroelectrochemical sensing to the single-nanoparticle level using both individual plasmon resonance modes and total scattering intensity fluctuations.

Invisibility and cloaking based on scattering cancellation

Advances in material synthesis and in metamaterial technology offer new venues to tailor the electromagnetic properties of devices, which may go beyond conventional limits in a variety of fields and applications. Invisibility and cloaking are perhaps one of the most thought-provoking possibilities offered by these new classes of advanced materials. Here, recently proposed solutions for invisibility and cloaking using metamaterials, metasurfaces, graphene and/or plasmonic materials in different spectral ranges are reviewed and highlighted. The focus is primarily on scattering-cancellation approaches, describing material challenges, venues and opportunities for the plasmonic and the mantle cloaking techniques, applied to various frequency windows and devices. Analogies, potentials and relevant opportunities of these concepts are discussed, their potential realization and the underlying technology required to verify these phenomena are reviewed with an emphasis on the material aspects involved. Finally, these solutions are compared with other popular cloaking techniques.

Overcoming the black body limit in plasmonic and graphene near-field thermophotovoltaic systems

Near-field thermophotovoltaic (TPV) systems with carefully tailored emitter-PV properties show large promise for a new temperature range (600 – 1200K) solid state energy conversion, where conventional thermoelectric (TE) devices cannot operate due to high temperatures and far-field TPV schemes suffer from low efficiency and power density. We present a detailed theoretical study of several different implementations of thermal emitters using plasmonic materials and graphene. We find that optimal improvements over the black body limit are achieved for low bandgap semiconductors and properly matched plasmonic frequencies. For a pure plasmonic emitter, theoretically predicted generated power density of 14 W/cm2 and efficiency of 36% can be achieved at 600K (hot-side), for 0.17eV bandgap (InSb). Developing insightful approximations, we argue that large plasmonic losses can, contrary to intuition, be helpful in enhancing the overall near-field transfer. We discuss and quantify the properties of an optimal near-field photovoltaic (PV) diode. In addition, we study plasmons in graphene and show that doping can be used to tune the plasmonic dispersion relation to match the PV cell bangap. In case of graphene, theoretically predicted generated power density of 6(120) W/cm2 and efficiency of 35(40)% can be achieved at 600(1200)K, for 0.17eV bandgap. With the ability to operate in intermediate temperature range, as well as high efficiency and power density, near-field TPV systems have the potential to complement conventional TE and TPV solid state heat-to-electricity conversion devices.

Surface plasmon resonance based fiber optic sensor for the IR region using a conducting metal oxide film

Theoretical modeling of a surface plasmon resonance (SPR) based fiber optic sensor with a conducting metal oxide [indium tin oxide (ITO)] as the SPR active material is proposed. The theoretical analysis reveals that the proposed sensing probe can be utilized for sensing in the IR region, where most of the gases show their absorption regime. Comparison of sensitivity predicts that an ITO-layer-coated SPR-based fiber optic sensor is about 60% more sensitive than a gold-coated fiber optic sensor. The physical reasons behind sensitivity enhancement are provided. Apart from this, various advantageous features of the ITO over the noble metals, silver and gold, are addressed.

Suppressed blinking dynamics of single QDs on ITO

The exciton quenching dynamics of single CdSe/CdS(3ML)ZnCdS(2ML)ZnS(2ML) core/multishell QDs adsorbed on glass, In2O3, and ITO have been compared. Single QDs on In2O3 show shorter fluorescence lifetimes and higher blinking frequencies than those on glass because of interfacial electron transfer from QDs to In2O3. Compared to glass and In2O3, single QDs on ITO show suppressed blinking activity as well as reduced fluorescence lifetimes. For QDs in contact with the n-doped ITO, the equilibration of their Fermi levels leads to the formation of negatively charged QDs. In these negatively charged QDs, the off states are suppressed because of the effective removal of the valence band holes, and their fluorescence lifetimes are shortened because of exciton Auger recombination and hole transfer processes involving the additional electrons. This study shows that the blinking of single QDs can be effectively suppressed on the surface of ITO. This phenomenon may also be observable for other QDs and on different n-doped semiconductors.

Plasmonic phenomena in indium tin oxide and ITO-Au hybrid films

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.

Plasmon polaritons in the near infrared on fluorine doped tin oxide films

Here we investigate plasmon polaritons in fluorine doped tin oxide (FTO) films. By fitting reflectance and transmittance measurements as a function of wavelength lambda epsilon [1.0microm, 2.5microm] we derive a Drude dispersion relation of the free electrons in the transparent conducting oxide films. Then we compute the dispersion curves for the bulk and surface modes together with a reflectance map over an extended wavelength region (lambda==>10microm). Although the surface polariton dispersion for a single FTO/air interface when neglecting damping should appear clearly in the plots in the considered region (since it is supposedly far and isolated from other resonances), a complex behaviour can arise. This is due to different characteristic parameters, such as the presence of a finite extinction coefficient, causing an enlargement and backbending of the feature, and the low film thickness, via coupling between the modes from both the glass/FTO and FTO/air interfaces. Taking into account these effects, computations reveal a general behaviour for thin and absorbing conducting films. They predict a thickness dependent transition region between the bulk polariton and the surface plasmon branches as previously reported for indium tin oxide. Finally, attenuated total reflection measurements vs the incidence angle are performed over single wavelengths lines R(theta) (lambda= 0.633,0.830,1.300,1.550microm) and over a two dimensional domain R(theta,lambda) in the near infrared region lambda epsilon [1.45microm, 1.59microm]. Both of these functions exhibit a feature which is attributed to a bulk polariton and not to a surface plasmon polariton on the basis of comparison with spectrophotometer measurements and modeling. The predicted range for the emergence of a surface plasmon polariton is found to be above lambda >or= 2.1microm, while the optimal film thickness for its observation is estimated to be around 200nm.

Thickness dependence of surface plasmon polariton dispersion in transparent conducting oxide films at 1.55 microm

We experimentally demonstrate propagation of surface plasmon polaritons in the near-IR window lambda (1.45 microm,1.59 microm) at the interface of indium-tin-oxide films with different thicknesses deposited on glass. Dispersion of such polaritons is strongly dependent on the film thickness, putting into evidence a regime in which polaritons at both films's interfaces are coupled in surface supermodes. The experimental data are shown to be in good agreement with the analytical model for thin and absorbing conducting films. Measurements on aluminum-doped zinc oxide, characterized by a redshifted plasma resonance, do not show any surface plasmon polariton excitation in the same wavelength window.

Functionalization of indium tin oxide

The preparation and functionalization of ITO surfaces has been studied using primarily X-ray photoemission spectroscopy and infrared reflection-absorption spectroscopy (IRRAS) and the reagents n-hexylamine and n-octyltrimethoxysilane (OTMS). Particular attention has been paid to characterization of the surfaces both before and after functionalization. Surfaces cleaned by ultraviolet (UV)/ozone treatment and subsequently exposed to room air have approximately 0.5-0.8 monolayers (ML) of adsorbed impurity C. Most is in the form of aliphatic species, but as much as one-half is partially oxidized and consists of C-OH, C-O-C, and/or >C=O groups. The coverage of these species can be reduced by cleaning in organic solvents prior to UV/ozone treatment. The OH coverage on the ITO surfaces studied here is relatively small (approximately 1.0 OH nm-2), based on the Si coverage after reaction with OTMS. A satellite feature in the O 1s XPS spectrum, often suggested to be a quantitative measure of adsorbed OH, receives a significant contribution from sources not directly related to hydroxylated ITO. n-Hexylamine adsorbs, at a saturation coverage of approximately 0.08 ML, via a Lewis acid-base interaction. The particular acid site has not been conclusively identified, but it is speculated that surface Sn sites may be involved. For OTMS, a saturation coverage of about 0.21 ML is found, and the C/Si atom ratios suggest that some displacement of preadsorbed organic impurities occurs during adsorption. The alkyl chain of adsorbed OTMS is disordered, with no preferred stereoisomer. However, the chain appears to lie mainly parallel to the surface with the plane defined by the terminal CH3-CH2-CH2- segment oriented essentially perpendicular to the surface.

Theoretical studies on vibrational spectra of some halides of group IVB elements

The vibrational spectra of group IVB elements halides MX4 (M=Ti(IV), Zr(IV), Hf(II); X=F, Cl, Br and I), have been investigated by ab initio RHF, MP2 and density functional theory B3LYP method with LanL2DZ basis sets. The optimized geometries, calculated vibrational frequencies and Far-IR intensities of MX4 are evaluated via comparison with experimental data. The vibrational frequencies, calculated by these methods, are compared to each other. The results indicate that B3LYP method is more reliable than RHF and MP2 methods for the frequencies calculations for these compounds. With this method, some vibrational frequencies of M2X6(2+)(M=Ti(IV), Zr(IV) and Hf(II); X=F, Cl, Br and I) are also predicted.