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Thin Films for Sensing Applications
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Thin Films for Sensing Applications

A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Thin Films and Interfaces".

Deadline for manuscript submissions: closed (30 September 2021) | Viewed by 28508

Special Issue Editor

Dr. Joel Borges

E-Mail Website1 Website2
Guest Editor
1. Physics Center of Minho and Porto Universities (CF-UM-UP), Campus de Azurém, University of Minho, 4800-058 Guimarães, Portugal
2. LaPMET—Laboratory of Physics for Materials and Emergent Technologies, University of Minho, Campus de Gualtar, 4710-057 Braga, Portugal
Interests: plasmonics; thin films; sputtering; gold nanoparticles; plasmonic sensing
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Thin films are becoming widely used in a growing range of different applications. The change from bulk materials to thin films has arisen in many different fields, thanks to advancements in bottom-up fabrication methods and the use of nanomaterials. Depending on the type of application, there are several technologies that may be used to prepare a thin-film system, ranging from physical vapor deposition (PVD) techniques, to chemical vapor deposition (CVD) methods or atomic layer deposition (ALD), among others. Thin films are typically more sensitive to changes in a local environment than many bulk materials, which is widening their use and the research for different approaches and systems. Every single property of a thin film can be controlled/modified at the nano- or micro-scale by the deposition process, offering multiple advantages (e.g., high surface-to-volume ratios or excellent deformation behavior) over bulk counterparts. Thus, thin films have a wide range of uses in sensing applications as they are more prone to be used, e.g., for adsorbing molecules on their surfaces (important in gas- and biosensing), in flexible sensors to measure pressure fields or even to record a biosignal from a human body, or to accurately measure the temperature of a system.

This Special Issue is devoted to the dissemination of new and original knowledge on all aspects related to thin films, that selectively sense physical signals and chemical or biological species or processes. Articles should be focused on the synthesis, characterization, functionalization, and development of thin-film sensors that may be used to provide new and improved sensing applications. Papers may also address new thin-film preparation strategies or systems to enhance the response and detection limits of a given thin-film sensor. The scope of this Special Issue encompasses, but is not restricted to, the following list of materials and types of thin-film sensors:

  • Refractive index sensors based on nanoparticles, or nanopatterned films, manifesting localized surface plasmon resonances (LSPR);
  • Molecular sensors based on surface-enhanced Raman spectroscopy (SERS);
  • Surface plasmon resonance (spr) sensors;
  • Biomolecular sensors employing enhanced photocatalysis;
  • Gas sensors, using 2D materials (Graphene, Transition Metal Dichalcogenides, etc.) and metal oxides;
  • Strain sensors based on the piezoresistive effect;
  • Piezoelectric sensors;
  • Temperate sensors, based on thermoelectric effect and RTDs;
  • Flexible and paper-based thin-film transistors (TFTs) applied to sensing;
  • Biopotential electrodes (transducers) for biosignal acquisition (e.g., EMG, EEG, ECG, or EOG).

Dr. Joel Borges
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Materials is an international peer-reviewed open access semimonthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • Thin-film synthesis
  • Thin-film characterization
  • Thin-film properties
  • Thin-film sensors
  • Physical sensors
  • Chemical and biological sensors

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Published Papers (7 papers)

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Research

20 pages, 6185 KiB  
Article
Molybdenum Oxide Thin Films Grown on Flexible ITO-Coated PET Substrates
by Alice Marciel, Manuel Graça, Alexandre Bastos, Luiz Pereira, Jakka Suresh Kumar, Joel Borges, Filipe Vaz, Marco Peres, Sergio Magalhães, Katharina Lorenz and Rui Silva
Materials 2021, 14(4), 821; https://doi.org/10.3390/ma14040821 - 9 Feb 2021
Cited by 13 | Viewed by 4076
Abstract
Molybdenum oxide thin films were deposited on stiff and flexible substrates by reactive DC magnetron sputtering. Two sets of samples were prepared. The first with different O2/Ar flow rate ratios and the second, fixing the oxygen content, with different time of [...] Read more.
Molybdenum oxide thin films were deposited on stiff and flexible substrates by reactive DC magnetron sputtering. Two sets of samples were prepared. The first with different O2/Ar flow rate ratios and the second, fixing the oxygen content, with different time of deposition. As the O2/Ar flow rate ratio varies from 0 up to 0.56, a threshold was found, ranging from crystalline to amorphous nature, and from a nontransparent appearance with metallic-like electrical conductivity to transparent and dielectric behaviour. From the second set, all transparent, the MoOx films present a compact/dense and featureless morphology with thickness from 190 up to 910 nm, depending on the time of deposition. Their structure was corroborated by XPS and Rutherford Backscattering Spectrometry (RBS) and density measurements were performed by RBS and X-ray reflectivity (XRR), revealing a value of 2.4 g/cm3. The surface roughness is in the order of a few nanometers and the maxima optical transmission, in the visible range, is around 89%. Electrochemical cyclic voltammograms showed noticeable color reversibility and reproducibility on the flexible substrates opening new framework possibilities for new electrochomic devices. Full article
(This article belongs to the Special Issue Thin Films for Sensing Applications)
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23 pages, 7326 KiB  
Article
Clean-Room Lithographical Processes for the Fabrication of Graphene Biosensors
by Patrícia D. Cabral, Telma Domingues, George Machado, Jr., Alexandre Chicharo, Fátima Cerqueira, Elisabete Fernandes, Emília Athayde, Pedro Alpuim and Jérôme Borme
Materials 2020, 13(24), 5728; https://doi.org/10.3390/ma13245728 - 15 Dec 2020
Cited by 18 | Viewed by 5039
Abstract
This work is on developing clean-room processes for the fabrication of electrolyte-gate graphene field-effect transistors at the wafer scale for biosensing applications. Our fabrication process overcomes two main issues: removing surface residues after graphene patterning and the dielectric passivation of metallic contacts. A [...] Read more.
This work is on developing clean-room processes for the fabrication of electrolyte-gate graphene field-effect transistors at the wafer scale for biosensing applications. Our fabrication process overcomes two main issues: removing surface residues after graphene patterning and the dielectric passivation of metallic contacts. A graphene residue-free transfer process is achieved by using a pre-transfer, sacrificial metallic mask that protects the entire wafer except the areas around the channel, source, and drain, onto which the graphene film is transferred and later patterned. After the dissolution of the mask, clean gate electrodes are obtained. The multilayer SiO2/SiNx dielectric passivation takes advantage of the excellent adhesion of SiO2 to graphene and the substrate materials and the superior impermeability of SiNx. It hinders native nucleation centers and breaks the propagation of defects through the layers, protecting from prolonged exposition to all common solvents found in biochemistry work, contrary to commonly used polymeric passivation. Since wet etch does not allow the required level of control over the lithographic process, a reactive ion etching process using a sacrificial metallic stopping layer is developed and used for patterning the passivation layer. The process achieves devices with high reproducibility at the wafer scale. Full article
(This article belongs to the Special Issue Thin Films for Sensing Applications)
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16 pages, 5817 KiB  
Article
Dry Electrodes for Surface Electromyography Based on Architectured Titanium Thin Films
by Marco S. Rodrigues, Patrique Fiedler, Nora Küchler, Rui P. Domingues, Cláudia Lopes, Joel Borges, Jens Haueisen and Filipe Vaz
Materials 2020, 13(9), 2135; https://doi.org/10.3390/ma13092135 - 5 May 2020
Cited by 33 | Viewed by 4882
Abstract
Electrodes of silver/silver chloride (Ag/AgCl) are dominant in clinical settings for surface electromyography (sEMG) recordings. These electrodes need a conductive electrolyte gel to ensure proper performance, which dries during long-term measurements inhibiting the immediate electrode’s reuse and is often linked to skin irritation [...] Read more.
Electrodes of silver/silver chloride (Ag/AgCl) are dominant in clinical settings for surface electromyography (sEMG) recordings. These electrodes need a conductive electrolyte gel to ensure proper performance, which dries during long-term measurements inhibiting the immediate electrode’s reuse and is often linked to skin irritation episodes. To overcome these drawbacks, a new type of dry electrodes based on architectured titanium (Ti) thin films were proposed in this work. The architectured microstructures were zigzags, obtained with different sputtering incidence angles (α), which have been shown to directly influence the films’ porosity and electrical conductivity. The electrodes were prepared using thermoplastic polyurethane (TPU) and stainless-steel (SS) substrates, and their performance was tested in male volunteers (athletes) by recording electromyography (EMG) signals, preceded by electrode-skin impedance measurements. In general, the results showed that both SS and TPU dry electrodes can be used for sEMG recordings. While SS electrodes almost match the signal quality parameters of reference electrodes of Ag/AgCl, the performance of electrodes based on TPU functionalized with a Ti thin film still requires further improvements. Noteworthy was the clear increase of the signal to noise ratios when the thin films’ microstructure evolved from normal growth towards zigzag microstructures, meaning that further tailoring of the thin film microstructure is a possible route to achieve optimized performances. Finally, the developed dry electrodes are reusable and allow for multiple EMG recordings without being replaced. Full article
(This article belongs to the Special Issue Thin Films for Sensing Applications)
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15 pages, 6599 KiB  
Article
Fabrication, Characterization and Implementation of Thermo Resistive TiCu(N,O) Thin Films in a Polymer Injection Mold
by Eva Oliveira, João Paulo Silva, Jorge Laranjeira, Francisco Macedo, Senentxu Lanceros-Mendez, Filipe Vaz and Armando Ferreira
Materials 2020, 13(6), 1423; https://doi.org/10.3390/ma13061423 - 20 Mar 2020
Cited by 6 | Viewed by 2671
Abstract
This paper presents the development of metallic thermoresistive thin film, providing an innovative solution to dynamically control the temperature during the injection molding process of polymeric parts. The general idea was to tailor the signal response of the nitrogen- and oxygen-doped titanium-copper thin [...] Read more.
This paper presents the development of metallic thermoresistive thin film, providing an innovative solution to dynamically control the temperature during the injection molding process of polymeric parts. The general idea was to tailor the signal response of the nitrogen- and oxygen-doped titanium-copper thin film (TiCu(N,O))-based transducers, in order to optimize their use in temperature sensor devices. The results reveal that the nitrogen or oxygen doping level has an evident effect on the thermoresistive response of TiCu(N,O) films. The temperature coefficient of resistance values reached 2.29 × 10−2 °C−1, which was almost six times higher than the traditional platinum-based sensors. In order to demonstrate the sensing capabilities of thin films, a proof-of-concept experiment was carried out, integrating the developed TiCu(N,O) films with the best response in an injection steel mold, connected to a data acquisition system. These novel sensor inserts proved to be sensitive to the temperature evolution during the injection process, directly in contact with the polymer melt in the mold, demonstrating their possible use in real operation devices where temperature profiles are a major parameter, such as the injection molding process of polymeric parts. Full article
(This article belongs to the Special Issue Thin Films for Sensing Applications)
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11 pages, 3143 KiB  
Article
Enhancing the Sensitivity of Nanoplasmonic Thin Films for Ethanol Vapor Detection
by Marco S. Rodrigues, Joel Borges and Filipe Vaz
Materials 2020, 13(4), 870; https://doi.org/10.3390/ma13040870 - 14 Feb 2020
Cited by 7 | Viewed by 3270
Abstract
Nanoplasmonic thin films, composed of noble metal nanoparticles (gold) embedded in an oxide matrix, have been a subject of considerable interest for Localized Surface Plasmon Resonance (LSPR) sensing. Ethanol is one of the promising materials for fuel cells, and there is an urgent [...] Read more.
Nanoplasmonic thin films, composed of noble metal nanoparticles (gold) embedded in an oxide matrix, have been a subject of considerable interest for Localized Surface Plasmon Resonance (LSPR) sensing. Ethanol is one of the promising materials for fuel cells, and there is an urgent need of a new generation of safe optical sensors for its detection. In this work, we propose the development of sensitive plasmonic platforms to detect molecular analytes (ethanol) through changes of the LSPR band. The thin films were deposited by sputtering followed by a heat treatment to promote the growth of the gold nanoparticles. To enhance the sensitivity of the thin films and the signal-to-noise ratio (SNR) of the transmittance–LSPR sensing system, physical plasma etching was used, resulting in a six-fold increase of the exposed gold nanoparticle area. The transmittance signal at the LSPR peak position increased nine-fold after plasma treatment, and the quality of the signal increased six times (SNR up to 16.5). The optimized thin films seem to be promising candidates to be used for ethanol vapor detection. This conclusion is based not only on the current sensitivity response but also on its enhancement resulting from the optimization routines of thin films’ architectures, which are still under investigation. Full article
(This article belongs to the Special Issue Thin Films for Sensing Applications)
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13 pages, 2803 KiB  
Article
Under-Etched Plasmonic Disks on Indium Tin Oxide for Enhanced Refractive Index Sensing on a Combined Electrochemical and Optical Platform
by Hans Dyrnesli, Gunnar Klös and Duncan S. Sutherland
Materials 2020, 13(4), 853; https://doi.org/10.3390/ma13040853 - 13 Feb 2020
Cited by 1 | Viewed by 2995
Abstract
A simple approach to enhance the refractive index sensitivity of gold nanodisks immobilized on electrically conducting indium tin oxide (ITO) substrates has been demonstrated. A two-fold increase in sensitivity to bulk refractive index change was achieved by substrate under-etching of gold nanodisks on [...] Read more.
A simple approach to enhance the refractive index sensitivity of gold nanodisks immobilized on electrically conducting indium tin oxide (ITO) substrates has been demonstrated. A two-fold increase in sensitivity to bulk refractive index change was achieved by substrate under-etching of gold nanodisks on ITO in 50 mM sulfuric acid. The influence of an intermediate titanium adhesion layer was investigated and was found to markedly influence the etching pattern and time. Etching with an adhesion layer resulted in enhanced refractive index sensitivity on disk-on-pin like structures after long etching times, whereas etching of disks deposited directly on ITO resulted in a disk-on-pincushion like configuration and similarly enhanced sensitivity already at shorter times. The gold disks remained electrically connected to the ITO substrate throughout etching and allowed site-specific electrodeposition of poly(3-aminophenol) at the nanodisks, showing enhanced thin-film refractive index sensitivity. This work demonstrates a simple method for enhancing refractive index sensitivity of nanostructures on ITO substrates for combined electrochemical and optical platforms, and subsequently a method to modify the surface of the electrically connected nanostructures, which has potential application in biosensing. Full article
(This article belongs to the Special Issue Thin Films for Sensing Applications)
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Graphical abstract

21 pages, 13866 KiB  
Article
Au-WO3 Nanocomposite Coatings for Localized Surface Plasmon Resonance Sensing
by Nuno M. Figueiredo, Filipe Vaz, Luís Cunha and Albano Cavaleiro
Materials 2020, 13(1), 246; https://doi.org/10.3390/ma13010246 - 6 Jan 2020
Cited by 17 | Viewed by 3819
Abstract
Localized surface plasmon resonance (LSPR) gas sensors are gaining increasing importance due to their unique tuneable functional properties. Au-WO3−x nanocomposite coatings, in particular, can be outstandingly sensitive to many different gases. However, a proper understanding of their optical properties and the way [...] Read more.
Localized surface plasmon resonance (LSPR) gas sensors are gaining increasing importance due to their unique tuneable functional properties. Au-WO3−x nanocomposite coatings, in particular, can be outstandingly sensitive to many different gases. However, a proper understanding of their optical properties and the way in which those properties are correlated to their structure/microstructure, is still needed. In this work, Au-WO3 nanocomposite coatings, with Au contents between 0–11 atomic percent, were grown using reactive magnetron co-sputtering technique and were characterized concerning their optical response. The precipitation of Au nanoparticles in the oxide matrix was promoted through thermal annealing treatments until 500 °C. Along with the Au nanoparticles’ morphological changes, the annealing treatments stimulated the crystallization of WO3, together with the appearance of oxygen-deficient WO3−x phases. Through theoretical simulations, we have related the LSPR effect with the different structural and morphological variations (namely, size and distribution of the nanoparticles and their local environment), which were a function of the Au content and annealing temperature. Our results suggest that local voids were present in the vicinity of the Au nanoparticles, for all temperature range, and that they should be present in a wide variety of Au-WO3 nanocomposites. A theoretical study concerning the refractive index sensitivity was carried out in order to predict the optimal coating design parameters for gas sensing experiments. Full article
(This article belongs to the Special Issue Thin Films for Sensing Applications)
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