One of the most frequently used methods for characterizing thin films is UV–Vis absorption. The near-edge region can be fitted to a simple expression in which the intercept gives the band-gap and the fitting exponent identifies the electronic transition as direct or indirect (see Tauc et al., Phys. Status Solidi 15, 627 (1966); these are often called “Tauc” plots). While the technique is powerful and simple, the accuracy of the fitted band-gap result is seldom stated or known. We tackle this question by refitting a large number of Tauc plots from the literature and look for trends. Nominally pure zinc oxide (ZnO) was chosen as a material with limited intrinsic deviation from stoichiometry and which has been widely studied. Our examination of the band gap values and their distribution leads to a discussion of some experimental factors that can bias the data and lead to either smaller or larger apparent values than would be expected. Finally, an easily evaluated figure-of-merit is defined that may help guide more accurate Tauc fitting. For samples with relatively sharper Tauc plot shapes, the population yields Eg(ZnO) as 3.276 ± 0.033 eV, in good agreement with data for single crystalline material.

Evaluation of the Tauc method for optical absorption edge determination: ZnO thin films as a model system

One of the most frequently used methods for characterizing thin films is UV–Vis absorption. The near-edge region can be fitted to a simple expression where the intercept gives the band gap and the fitting exponent identifies the electronic transition as direct or indirect. [See Tauc et al., Physica Status Solidi 15, 627 (1966); naturally, these are usually called “Tauc” plots.] In earlier work, we found that direct band gaps fitted using Tauc's method can be quite accurate, to ∼1% [see Viezbicke et al., Phys. Status Solidi B 252, 1700 (2015)]. Still, slopes of these Tauc plots are less frequently quantified, even though the slopes are directly rooted in key band-structure parameters. In this study, we examine the reproducibility of Tauc plot slopes for ZnO as a model direct-gap material and compare these experimental values with the theoretically derived slope. In contrast to the band gap accuracy, the experimental slope values varied by several orders of magnitude. The histogram of slope values was significantly more compact for Tauc plots exhibiting less Urbach tail contribution. In these cases, the Tauc slopes can provide an order-of-magnitude quantification of other key band characteristics such as carrier effective mass.

Assessing Tauc plot slope quantification: ZnO thin films as a model system

Controlling the properties of nanostructured zinc oxide (ZnO) is an interesting way to broaden its multifunctionality. ZnO nanostructured films were grown on glass substrates under different conditions by a simple two-step wet chemical method. A low-cost successive ionic layer adsorption and reaction (SILAR) method was used to grow ZnO seed layers at 80 °C. Then, different hierarchical based ZnO nanostructured thin films were deposited onto the ZnO seed layers by chemical bath deposition (CBD). The influence of deposition time (tD) and pH on the surface morphology, wettability behavior, structural and optical properties of the ZnO nanostructured films were systematically investigated. The structural, morphological, optical and wetting properties were studied by X-ray diffraction (XRD), field emission scanning microscopy (FE-SEM), UV-Vis spectrophotometer, and water contact angle (WCA) measurement, respectively. The surface morphology revealed a complex and orientated hierarchical based ZnO nanostructured films with diverse shapes from hexagonal nanorods to hexagonal nanoplates and even much more complex plates/rods and flower-like shapes by changing deposition time and pH of the precursor. XRD results confirm the synthesis of nanostructured ZnO of the hexagonal structure with a preferential orientation along the (002) lattice plane. The average crystallite size, D, is altered between 41.41 to 68.43 nm dependent on the morphology of the ZnO film and pH of the precursor. At pH 6.5, the films are hydrophilic for 10 h ≤ tD ≤ 6 h and hydrophobic for 6 h < tD < 10 h. The wetting properties of the films were enhanced by increasing or decreasing pH around 6.5. Morphology and thickness of the ZnO nanostructure could efficiently control the transmittance, absorbance, optical band gap, and the extinction coefficient of the films. The optical band gap is blue shifted from 2.45 to 3.62 eV@pH 6.5 as the deposition time increased from 2 to 8 h and blue shifted from 2.72 to 3.62 eV@8 h as the pH value increased from 5.5 to 6.5. The existence of stable hydrophobic zinc oxide nanostructured films at room temperature at a large-scale and with band gaps around 3.62 eV supports their use in self-cleaning and gas sensing applications.

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Nanostructured ZnO thin films for self-cleaning applications

VO2/Si-Al gel nanocomposite thermochromic smart foils: Largely enhanced luminous transmittance and solar modulation

Optical and electrical properties of ITO thin films sputtered on flexible FEP substrate as passive thermal control system for space applications

ZnO thin film synthesis by reactive radio frequency magnetron sputtering

Optical characterization of deposited ITO thin films on glass and PET substrates

Deposition of MgF2 thin films for antireflection coating by using thermionic vacuum arc (TVA)

This study is focused on the growth of graphene doped ZnO (ZnO:Gr) nanocomposite thin films by a thermionic vacuum arc (TVA) technique. ZnO:Gr nanocomposite thin films were deposited onto glass and silicon (Si) substrates. The influence of the dopant effect on structural, optical, morphological properties of the ZnO:Gr nanocomposite thin films were investigated by using various analysis techniques such as interferometer, UV–Visible spectrophotometer, X-ray diffraction (XRD), Raman spectrometer, Fourier Transform Infrared spectroscopy (FTIR), Photoluminescence (PL) spectroscopy, Field Emission Scanning Electron Microscopy (FESEM), and Atomic Force Microscopy (AFM). Thickness values of the ZnO:Gr nanocomposite thin films for the deposited layers onto glass and Si substrates were measured as to be 50 nm and 20 nm, respectively. In XRD patterns, the reflections of the ZnO and carbon nanostructures were observed. Using UV–Vis spectrophotometer and optical interferometer, the refractive index, reflectance, transmittance, absorbance and optical band gap graphs of the ZnO:Gr nanocomposite thin films were determined. Optical band gap of the ZnO:Gr nanocomposite thin film that deposited on glass substrate was determined as to be 3.15 eV via optical method and the result is in good harmony with PL measurement. PL spectrum showed an ultraviolet (UV) emission peak at 397 nm (3.12 eV). From the Raman analysis of the ZnO:Gr nanocomposite thin films, ZnO, D and 2D peaks of the graphene were observed. FTIR spectroscopy was used to analyze the chemical composition of the samples. According to AFM and FESEM analysis, ZnO:Gr nanocomposite thin films are smooth, flat, granular, uniform and dense form. Due to the larger crystallite sizes ZnO:Gr nanocomposite thin film onto Si substrate has lower resistivity according to the ZnO:Gr nanocomposite thin film deposited onto glass substrate. As a result, ZnO:Gr nanocomposite thin films are promising material for potential applications as to be a transparent conducting oxide (TCO) material.

Saliha Elmas

Papers

ZnO thin film synthesis by reactive radio frequency magnetron sputtering

Optical characterization of deposited ITO thin films on glass and PET substrates

Deposition of MgF2 thin films for antireflection coating by using thermionic vacuum arc (TVA)

Determination of physical properties of graphene doped ZnO (ZnO:Gr) nanocomposite thin films deposited by a thermionic vacuum arc technique

Sn doped ZnO thin film deposition using thermionic vacuum arc technique