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. 2022 Nov 7;8(11):717.
doi: 10.3390/gels8110717.

Structural, Optical, and Sensing Properties of Nb-Doped ITO Thin Films Deposited by the Sol-Gel Method

Madalina Nicolescu  1 , Daiana Mitrea  1 , Cristian Hornoiu  1 , Silviu Preda  1 , Hermine Stroescu  1 , Mihai Anastasescu  1 , Jose Maria Calderon-Moreno  1 , Luminita Predoana  1 , Valentin Serban Teodorescu  2   3 , Valentin-Adrian Maraloiu  2 , Maria Zaharescu  1 , Mariuca Gartner  1
Affiliations

Structural, Optical, and Sensing Properties of Nb-Doped ITO Thin Films Deposited by the Sol-Gel Method

Madalina Nicolescu et al. Gels. .

Abstract

The aim of the present study was the development of Nb-doped ITO thin films for carbon monoxide (CO) sensing applications. The detection of CO is imperious because of its high toxicity, with long-term exposure having a negative impact on human health. Using a feasible sol-gel method, the doped ITO thin films were prepared at room temperature and deposited onto various substrates (Si, SiO2/glass, and glass). The structural, morphological, and optical characterization was performed by the following techniques: X-ray diffractometry (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV/Vis/NIR spectroscopic ellipsometry (SE). The analysis revealed a crystalline structure and a low surface roughness of the doped ITO-based thin films. XTEM analysis (cross-sectional transmission electron microscopy) showed that the film has crystallites of the order of 5-10 nm and relatively large pores (around 3-5 nm in diameter). A transmittance value of 80% in the visible region and an optical band-gap energy of around 3.7 eV were found for dip-coated ITO/Nb films on SiO2/glass and glass supports. The EDX measurements proved the presence of Nb in the ITO film in a molar ratio of 3.7%, close to the intended one (4%). Gas testing measurements were carried out on the ITO undoped and doped thin films deposited on glass substrate. The presence of Nb in the ITO matrix increases the electrical signal and the sensitivity to CO detection, leading to the highest response for 2000 ppm CO concentration at working temperature of 300 °C.

Keywords: CO detection; Nb-doped ITO thin films; Optical properties; Sol–gel.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Figure 1
Figure 1
XRD patterns of ITO films undoped and doped with 4% Nb deposited on (a) Si, (b) SiO2/glass, and (c) glass.
Figure 2
Figure 2
Topographic 2D AFM images scanned over an area of (1 µm × 1 µm), showing the morphology of the ITO/Nb films deposited on (a) glass, (b) SiO2/glass, and (c) Si.
Figure 3
Figure 3
RMS (solid fill) and average (gradient fill) roughness for the ITO/Nb films deposited on different substrates.
Figure 4
Figure 4
SEM micrographs at different magnification: (a) 20,000× and (b) 100,000× of the tilted ITO/Nb film deposited on Si, showing the smooth film surface and the film thickness.
Figure 5
Figure 5
(a) Low-magnification XTEM image of the ITO/Nb film on Si and (b) its SAED pattern.
Figure 6
Figure 6
HRTEM images of the ITO:Nb film on Si: (a) morphology of the layers and (b) crystallite size and coherent lattice fringes.
Figure 7
Figure 7
Optical constants (a) n, (b) k, (c) optical band gap—Eg, thickness, porosity—P, and (d) transmission—T of undoped and doped ITO thin films.
Figure 8
Figure 8
Nyquist plots for the ITO/Nb glass film at 300 °C for different CO concentrations.
Figure 9
Figure 9
Electrical response of (a) ITO/Nb glass and (b) ITO/glass film for various concentrations of CO versus the working temperature.
Figure 10
Figure 10
The flow chart of the ITO/Nb thin films preparation.
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