Origins of limited electrical performance of polycrystalline Cu2O thin-film transistors

In this thesis, cuprous oxide Cu2O was investigated concerning its ability to function as ptype channel in thin-film transistors. The material was chosen for its promising electronic characteristics as bulk single crystal. In order to be competitive with other technological solutions for flexible thin-film electronics, the temperature during fabrication has to remain below 200 C. Following this approach, a tremendous gap between potential and actual electrical performance of Cu2O thin-film transistors is encountered. The aim of this thesis is to show the reasons for this discrepancy. Relevant stages during the fabrication process of a thin-film transistor were analyzed with respect to their impact on the cation oxidation state. These stages included thin film deposition, the study of interface formation to the dielectric layers as well as postdeposition annealing. Semiconducting and dielectric layers were deposited by reactive magnetron sputtering (Cu2O, Cu4O3, CuO, Bi2O3, Al2O3) and atomic layer deposition (Al2O3). An innovative approach for a thickness-dependent characterization of thin films was conducted by a combination of in situ X-ray photoelectron spectroscopy with in situ conductance measurement. Electrical properties of Cu2O films and thin-film transistors were analyzed in dependence of film thickness, temperature, oxygen partial pressure and time. It is shown, that the primary cause for the limited electrical performance is the polycrystalline morphology in conjunction with the material-inherent tendency to oxidation and reduction of the metal cation. On the one hand, metallic Cu(0) depletes the material from hole carriers and causes Fermi level pinning. On the other hand, a high conductivity in the grain boundary is caused by the presence of Cu(II). A model is presented to describe the conductivity at different film thicknesses as a function of grain size

Memristors using solution-based IGZO nanoparticles

Solution-based indium-gallium-zinc oldde (IGZO) nanoparticles deposited by spin coating have been investigated as a resistive switching layer in metal-insulator-metal structures for nonvolatile memory applications. Optimized devices show a bipolar resistive switching behavior, low programming voltages of +/- 1 V, on/off ratios higher than 10, high endurance, and a retention time of up to 104 s. The better performing devices were achieved with annealing temperatures of 200 degrees C and using asymmetric electrode materials of titanium and silver. The physics behind the improved switching properties of the devices is discussed in terms of the oxygen deficiency of IGZO. Temperature analysis of the conductance states revealed a nonmetallic filamentary conduction. The presented devices are potential candidates for the integration of memory functionality into low-cost System-on-Panel technology.National Funds through FCT - Portuguese Foundation for Science and Technology [UID/CTM/50025/2013, SFRH/BDP/99136/2013]; FEDER [POCI-01-0145-FEDER-007688]info:eu-repo/semantics/publishedVersio

New Protic Ionic Liquids as Potential Additives to Lubricate Si-Based MEMS/NEMS

Funding Information: The work was financed by the Portuguese Foundation for Science and Technology (FCT) through the projects UIDB/00100/2020, UIDP/00100/2020 and IMS-LA/P/0056/2020 and through the PhD grant SFRH/BD/140079/2018 from Mariana Donato. Additionally, this work was financed by national funds from FCT in the scope of the projects UIDB/50022/2020 (IDMEC/LAETA) and LA/P/0037/2020, UIDP/50025/2020 and UIDB/50025/2020 of the Associate Laboratory Institute of Nanostructures, Nanomodelling and Nanofabrication—i3N. Publisher Copyright: © 2023 by the authors.The motivation for this work was to develop new protic ionic liquids (PILs) as additives for the lubrication of micro and nanoelectromechanical systems (MEMS and NEMS). Ten PILs based on the combination of methylimidazolium ([MIMH]), 4-picolinium ([4-picH]), pyridinium ([PyrH]), 1,8-diazabicyclo[5.4.0]-undec-7-ene-8-ium ([DBUH]) and tetramethylguanidinium ([TMGH]) cations with hydrogen sulfate([HSO4]) and mesylate ([MeSO3]) anions were tested as additives in polyethylene glycol (PEG200) to lubricate steel/silicon and silicon/silicon contacts. The best additive was [4-picH][HSO4], which adsorbed strongly on the Si surface, leading to a protective film that reduced wear by up to 15 times compared to PEG200.publishersversionpublishe

Reversible Barrier Switching of ZnO/RuO₂ Schottky Diodes

The current-voltage characteristics of ZnO/RuO₂ Schottky diodes prepared by magnetron sputtering are shown to exhibit a reversible hysteresis behavior, which corresponds to a variation of the Schottky barrier height between 0.9 and 1.3 eV upon voltage cycling. The changes in the barrier height are attributed to trapping and de-trapping of electrons in oxygen vacancies

Solution Combustion Synthesis of Hafnium-Doped Indium Oxide Thin Films for Transparent Conductors

2021.03825.CEECIND 951774 (FOXES, H2020-EIC-FETPROACT-2019)Indium oxide (In2O3)-based transparent conducting oxides (TCOs) have been widely used and studied for a variety of applications, such as optoelectronic devices. However, some of the more promising dopants (zirconium, hafnium, and tantalum) for this oxide have not received much attention, as studies have mainly focused on tin and zinc, and even fewer have been explored by solution processes. This work focuses on developing solution-combustion-processed hafnium (Hf)-doped In2O3 thin films and evaluating different annealing parameters on TCO’s properties using a low environmental impact solvent. Optimized TCOs were achieved for 0.5 M% Hf-doped In2O3 when produced at 400 °C, showing high transparency in the visible range of the spectrum, a bulk resistivity of 5.73 × 10−2 Ω.cm, a mobility of 6.65 cm2/V.s, and a carrier concentration of 1.72 × 1019 cm−3. Then, these results were improved by using rapid thermal annealing (RTA) for 10 min at 600 °C, reaching a bulk resistivity of 3.95 × 10−3 Ω.cm, a mobility of 21 cm2/V.s, and a carrier concentration of 7.98 × 1019 cm−3, in air. The present work brings solution-based TCOs a step closer to low-cost optoelectronic applications.publishersversionpublishe

Ionically Modified Cellulose Nanocrystal Self-Assembled Films with a Mesoporous Twisted Superstructure: Polarizability and Application in Ion-Gated Transistors

FCT – Portuguese Foundation for Science and Technology through the Ph.D. scholarship SFRH/BD/125191/2016. project PapEl, reference PTDC/CTM-NAN/5172/2014. project CHIHC, reference PTDC/NAN-MAT/32558/2017. project PTDC/CTM-BIO/6178/2014, M-ERA-NET2/0007/2016 (CellColor). POR Lisboa2020 through project PTDC/CTM-REF/30529/2017. D.G. acknowledges the support from FCT – Portuguese Foundation for Science and Technology through the AdvaMTech PhD program scholarship PD/BD/52627/2014.Mesoporous structures made of cellulose nanocrystals (CNCs) and their self-assembly into films are of great interest not only due to their abundancy and sustainability but also due to their ease of chemical modification and nanoscale biomimicry capabilities. However, their implementation in (opto)electronic devices requires further understanding on how these self-assembled twisted mesoporous superstructures respond to electrical stimulus. In this regard, this work focuses on the infiltration of solid CNC droplets with three distinct alkali ions (Li+, Na+, and K+) to yield films with improved electrochemical response when compared to pristine ones, while preserving their photonic character. Electrochemical characterization shows capacitances of up to 2.5 μF cm–2 allowing for their integration as solid-state gate electrolytes in amorphous indium–gallium–zinc–oxide transistors, resulting in low operating voltages (10 cm2 V–1 s–1. Devices fabricated on Na+ and K+ infiltrated CNC films present the best characteristics, indicating pure capacitive charging of the semiconductor. The insights presented here contribute to applications in solid-state ionics in mesoporous structures or the combination of optically active electrolytes capable of providing unique functionalities in ion-gated transistors and circuitry.publishersversionpublishe

Tailoring IGZO composition for enhanced fully solution-based thin film transistors

UID/CTM/50025/2019 H2020 NMP-22-2015 project 1D-NEON Grant Agreement 685758 SFRH/BD/116047/2016 IDS-FunMat-INNO project FPA2016/EIT/EIT Raw Materials Grant Agreement 15015Solution-processed metal oxides have been investigated as an alternative to vacuum-based oxides to implement low-cost, high-performance electronic devices on flexible transparent substrates. However, their electrical properties need to be enhanced to apply at industrial scale. Amorphous indium-gallium-zinc oxide (a-IGZO) is the most-used transparent semiconductor metal oxide as an active channel layer in thin-film transistors (TFTs), due to its superior electrical properties. The present work evaluates the influence of composition, thickness and ageing on the electrical properties of solution a-IGZO TFTs, using solution combustion synthesis method, with urea as fuel. After optimizing the semiconductor properties, low-voltage TFTs were obtained by implementing a back-surface passivated 3-layer In:Ga:Zn 3:1:1 with a solution-processed high-k dielectric; AlOx. The devices show saturation mobility of 3.2 cm2 V-1 s-1, IOn/IOff of 106, SS of 73 mV dec-1 and VOn of 0.18 V, thus demonstrating promising features for low-cost circuit applications.publishe

Cork derived laser-induced graphene for sustainable green electronics

Funding text This work was financed by national funds from FCT—Fundação para a Ciência e a Tecnologia, I P, in the scope of the projects LA/P/0037/2020, UIDP/50025/2020 and UIDB/50025/2020 of the Associate Laboratory Institute of Nanostructures, Nanomodelling and Nanofabrication—i3N. The authors acknowledge the ERC AdG Grant 787410 from the Project DIGISMART, EC Project SYNERGY H2020-WIDESPREAD-2020-5, CSA, Proposal Number 952169, and Project FOXES, FETPROACT-EIC-05-2019—FET Proactive, Proposal Number 951774. S L Silvestre, T Pinheiro and A C Marques acknowledge PhD grant SFRH/BD/149751/2019, SFRH/BD/08606/2020 and SFRH/BD/115173/2016, respectively. The authors thank Amorim Cork Composites for providing the cork samples used in this work.The demand for smart, wearable devices has been dictating our daily life with the evolution of integrated miniaturized electronics. With technological innovations, comes the impactful human footprint left on the planet’s ecosystems. Therefore, it is necessary to explore renewable materials and sustainable methodologies for industrial processes. Here, an eco-friendly approach to producing flexible electrodes based on a single-step direct laser writing is reported. A 1.06 µm wavelength fiber laser was used for the first time to produce porous three-dimensional laser-induced graphene (LIG) on an agglomerated cork substrates. The obtained material exhibits the typical Raman spectra, along with an exceptionally low sheet resistance between 7.5 and 10 ohm sq−1. LIG on cork high electrical conductivity and the friendliness of the used production method, makes it an interesting material for future technological applications. To show its applicability, the production of planar micro-supercapacitors was demonstrated, as a proof of concept. Electrochemical performance studies demonstrate that LIG interdigitated electrodes, using PVA-H2SO4 electrolyte, achieve an area capacitance of 1.35 mF cm−2 (103.63 mF cm−3) at 5 mV s−1 and 1.43 mF cm−2 (109.62 mF cm−3) at 0.1 mA cm−2. In addition, devices tested under bending conditions exhibit a capacitance of 2.20 mF cm−2 (169.22 mF cm−3) at 0.1 mA cm−2. Here, showing that these electrodes can be implemented in energy storage devices, also successfully demonstrating LIG promising application on innovative, green, and self-sustaining platforms.publishersversionpublishe

Towards Sustainable Crossbar Artificial Synapses with Zinc-Tin Oxide

UIDB/50025/2020-2023In this article, characterization of fully patterned zinc-tin oxide (ZTO)-based memristive devices with feature sizes as small as 25 µm2 is presented. The devices are patterned via lift-off with a platinum bottom contact and a gold-titanium top contact. An on/off ratio of more than two orders of magnitude is obtained without the need for electroforming processes. Set operation is a current controlled process, whereas the reset is voltage dependent. The temperature dependency of the electrical characteristics reveals a bulk-dominated conduction mechanism for high resistance states. However, the charge transport at low resistance state is consistent with Schottky emission. Synaptic properties such as potentiation and depression cycles, with progressive increases and decreases in the conductance value under 50 successive pulses, are shown. This validates the potential use of ZTO memristive devices for a sustainable and energy-efficient brain-inspired deep neural network computation.publishersversionpublishe

Microwave Synthesis of Visible-Light-Activated g-C3N4/TiO2 Photocatalysts

Funding Information: This work was financed by national funds from FCT-Fundação para a Ciência e a Tecnologia, I.P., within the scope of projects UI/BD/151292/2021 (Ph.D. research scholarship), LA/P/0037/2020, UIDP/50025/2020 and UIDB/50025/2020 of the Associate Laboratory Institute of Nanostructures, Nanomodelling, and Nanofabrication-i3N, but also the 2021.03825.CEECIND. Acknowledgments are also given to the EC project SYNERGY H2020-WIDESPREAD-2020-5, CSA, proposal nº 952169, EMERGE-2020-INFRAIA-2020-1, proposal nº 101008701, and to the European Community’s H2020 program under grant agreement No. 787410 (ERC-2018-AdG DIGISMART). Publisher Copyright: © 2023 by the authors.The preparation of visible-light-driven photocatalysts has become highly appealing for environmental remediation through simple, fast and green chemical methods. The current study reports the synthesis and characterization of graphitic carbon nitride/titanium dioxide (g-C3N4/TiO2) heterostructures through a fast (1 h) and simple microwave-assisted approach. Different g-C3N4 amounts mixed with TiO2 (15, 30 and 45 wt. %) were investigated for the photocatalytic degradation of a recalcitrant azo dye (methyl orange (MO)) under solar simulating light. X-ray diffraction (XRD) revealed the anatase TiO2 phase for the pure material and all heterostructures produced. Scanning electron microscopy (SEM) showed that by increasing the amount of g-C3N4 in the synthesis, large TiO2 aggregates composed of irregularly shaped particles were disintegrated and resulted in smaller ones, composing a film that covered the g-C3N4 nanosheets. Scanning transmission electron microscopy (STEM) analyses confirmed the existence of an effective interface between a g-C3N4 nanosheet and a TiO2 nanocrystal. X-ray photoelectron spectroscopy (XPS) evidenced no chemical alterations to both g-C3N4 and TiO2 at the heterostructure. The visible-light absorption shift was indicated by the red shift in the absorption onset through the ultraviolet-visible (UV-VIS) absorption spectra. The 30 wt. % of g-C3N4/TiO2 heterostructure showed the best photocatalytic performance, with a MO dye degradation of 85% in 4 h, corresponding to an enhanced efficiency of almost 2 and 10 times greater than that of pure TiO2 and g-C3N4 nanosheets, respectively. Superoxide radical species were found to be the most active radical species in the MO photodegradation process. The creation of a type-II heterostructure is highly suggested due to the negligible participation of hydroxyl radical species in the photodegradation process. The superior photocatalytic activity was attributed to the synergy of g-C3N4 and TiO2 materials.publishersversionpublishe