Project leader Andrejs Ogurcovs
Agreement No 1.1.1.2/16/I/001
Research application No. 1.1.1.2/VIAA/4/20/590
Within the framework of this project, various 2D materials will be studied to find the best combinations between: sulfide materials - MoS2, WS2, ReS2, TaS2, VS2, TiS2, SnS2, CuS; and oxide materials - MoO3, WO3, V2O5, MnO2, etc., with the aim of developing sensor elements in the form of a field effect transistor (FET). In addition to the FET configuration, a p-n transition will be created instead of a simple S-D channel based on 2D materials, which can significantly expand the functionality of this type of element. In order to achieve a certain level of sensor selectivity, it is necessary to functionalize the working surface of the obtained elements with certain types of organic and inorganic chemicals (linkers), the level of response of such elements to the chemical reaction on their surfaces will be studied. The elements will be combined in an array, each sensitive element must respond uniquely to each substance of interest. However, instead of seeking to increase the sensitivity and selectivity of an array of individual sensor elements, which may be difficult to achieve, an option with less selective components is possible by creating a so-called 'cross-reactive' sensor array. This type of response processing of individual sensor elements will be performed using machine learning algorithms, obtaining a unique response pattern or "fingerprint". This challenging task will be solved using modern experimental methods, incl. also pulsed laser sputtering (PLD), atomic force microscopy (AFM), scanning electron microscopy (SEM). The multidisciplinary aspects of the project reflect its complex nature, which includes various chemical and physical methods of sensor fabrication, the use of a wide range of experimental methods for sensor testing, and the use of electronics and computer programming for sensor performance analysis.
The project is implemented at the Institute of Solid State Physics of the University of Latvia from 01.01.2021. until 30.06.2023. The total cost of the project is 111 504.90 EUR.
01.07.2023.
At the final stage of the project, the exploration of degenerate semiconductor materials as a field-effect transistor channel was decided. Among these materials, ITO (Indium Tin Oxide) was chosen as the semiconductor due to its widespread use in the modern semiconductor industry for obtaining transparent conductive electrodes. An 8x8 sensor matrix was used to fabricate a series of samples with ITO thicknesses ranging from 2 to 10 nm, using magnetron sputtering from an ITO target. Electrical investigations demonstrated that the most optimal thickness for the ITO layer, considering a transistor channel length of 100 micrometers, is 3.3 nanometers. The obtained samples exhibited significantly improved transport properties and greater chemical stability when compared to the previously studied ZnO, CuO, and NiO materials. Figure 1 illustrates the current-voltage characteristics of the samples based on ITO. It can be observed that at higher source-drain voltages, the device exhibits behavior similar to an electrolyte-gated Schottky field-effect transistor, which expands the potential practical applications of the obtained samples.
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Figure 1. Output (left) and transfer (right) characteristics of ITO field effect transistor.
Additionally, the development of a prototype designed for automated data acquisition and analysis from an 8x8 sensor matrix was completed. Figure 2 depicts a three-dimensional model of the device's motherboard.
Figure 2. 3D model of prototype’s motherboard.
02.05.2023.
The fabrication of metal oxide nanostructures through electrochemical deposition has been successful. However, the wide variation in the electrical parameters of the resulting thin films significantly limits their application as transistor channels. To address this issue, a switch back to magnetron sputtering method for thin film production was performed. Nevertheless, electrochemical deposition remains a fast and efficient method for obtaining nanostructured coatings over large sample areas.
Currently, research is focused on developing a multi-sensor system based on field-effect transistors with an external gate. This system comprises five IRF740N series transistors and five microscope slides measuring 26x76 mm each. A conductive layer of 250 nm thickness, composed of ITO, is deposited on the glass, followed by annealing at atmospheric pressure and a temperature of 450 °C. Subsequently, various metal oxides such as NiO, CuO, and ZnO, as well as CoO and Fe3O4 from appropriate buffer solutions, are deposited through anodic oxidation. The resulting samples are then placed in specialized holders and connected to the transistor assembly's gate terminals.
Simultaneously, the final stage of prototype development is underway to work with the aforementioned sensor matrix consisting of 64 elements.
The research outcomes were presented at “The 65th International Scientific Conference of Daugavpils University” conference through two oral presentations.
16.01.2023.
Currently, a series of experiments is being carried out, the aim of which is by using electrochemical methods create a p-n junction whose electrical parameters can be changed as a result of exposure to the analyte. The result is achieved by so-called anodic oxidation. A metallic copper layer is deposited on one of the electrodes from copper acetate (0.05 M Cu(CH3COO)2) by electrolysis with further oxidation in a concentrated KOH (0.5 M) solution. The same procedure is repeated on the other electrode, but only with zinc acetate (0.05 M Zn(O₂CCH₃)₂). The synthesis takes place in the galvanostatic mode (constant current) at a current density of 150 µA/mm2 for 15 min. Lateral growth ensures a decrease in the distance between electrodes, which allows to establish contact between different types of deposited materials. The results of the synthesis are shown below.
Figure 1. Electrochemical synthesis results for CuO (left) and for ZnO (right).
Previously obtained experimental data on the glyphosate was published in the MDPI journal "Sensors". Title of the article: Glyphosate Sensor Based on Nanostructured Water-Gated CuO Field-Effect Transistor”, DOI: 10.3390/s22228744.
14.09.2022.
Based on the experimental data collected during the research, a multisensor configuration was developed, which combines 64 elements on a single substrate. A total of 255 such bases have been made (Figure 1). Thickness of base polyamide - 200 microns, length - 60 mm, width - 10 mm. The sensor electrodes are covered with an 80 nm thick gold layer, which ensures high chemical stability of the elements. The length of the channel of single element is 50 microns and the width is 2000 microns, which ensures a width/length ratio of the transistor channel equal to 40. A real picture of the sensor array can be seen in picture no. 2. A specialized device based on an STM32F103T6 (ARM Cortex-M3, 72 Mhz) microcontroller is under development, which will enable fast and efficient signal processing of the sensor array
Figure 1. Sensor array on a polyamide substrate. a) bottom view, b) top view, c) enlarged area of sensor elements
Figure 2. Real view of sensor array.
29.04.2022.
After successful experiments with Al:ZnO semiconductor material, the next stage is the use of CuO (copper oxide) as a transistor channel semiconductor. For this purpose, 6 polyimide substrates were made according to the previously mentioned scheme. On the three substrates, a 50 nm thick Cuo layer was deposited with a reactive magnetron sputtering method. On the other 3 substates, a 3 nm thick CuO precursor layer was deposited using the same reactive sputtering method. The base with the precursor was used for selective Cuo -strewing with the hydrothermal synthesis method to increase the effective surface area, which in turn will provide a greater electrical capacity of the transistor gate. An aqueous solution of glyphosate (herbicide) was chosen as the analyte in various concentrations.
Substrates with thin-film transistors with 50 nm thick magnetron sputtered CuO channel.
Substrates containing thin-film transistors with nanostructured Cuo channel obtained by hydrothermal synthesis method.
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SEM images of nanostructured CuO film surface at 1 micron and 200 nanometers scale.
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Transfer (Id-Vg) curves of thin-film CuO transistors for magnetron sputtered channel (left) and nanostructured channel (right). Nanostructures significantly improve the parameters of the transistors compared to a smooth film.
Interaction of glyphosate aqueous solution at 1 mM concentration with CuO transistor surface. The nanostructured samples demonstrate a significantly higher response level compared to a smooth Cuo surface.
Experimental results with Al:ZnO thin-film transistors were published in MDPI "Sensors" magazine. The title of the article: “Effect of DNA Aptamer Concentration on the Conductivity of a Water-Gated Al:ZnO Thin-Film Transistor-Based Biosensor” .
Research results were presented at the conference:: 38th Annual Scientific Conference at ISSP UL. Presentation title: “Water-gated Al:ZnO thin film transistor for biosensing applications” .
10.01.2022.
In the course of work on preparing MoS2-based thin-film transistors on silicon substrates, several significant limitations were identified that impede the creation of efficient devices. In this regard, further work continued with semiconductor materials based on metal oxides. Сonsidering the fact that biosensors are mainly used for liquid analytes, a new optimized design of electrolyte-gated field-effect transistor was developed. This class of semiconductor devices uses a double electric layer as an insulator between the gate electrode and the channels, which forms when the surface comes into contact with a liquid electrolyte. Depending on the composition of the electrolyte, the thickness of the electrical double layer can be in the range of 0.1 - 10 nm, and this provides an electrical capacity ranging from hundreds of pF to tens of µF per square centimetre, which, in turn, significantly reduces the control voltage levels applied to the gate. The transistor array was fabricated on a 0.25 mm thick polyamide substrate by magnetron sputtering. The channel thickness is 45 nm, the length is 175 µm, and the width is 2100 µm.
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Actual view of the EGFET array on a probe station (left image), 3D concept of singe EGFT unit (middle image), EGFET output curve (right image)
Single-stranded DNA primers from “BIONEER” OPE-01 with a length of 10mer (CCC AAG GTC C) and a molar mass of 2972.9 g / mol diluted with deionized water in five different concentrations were used as an analyte. As the aptamer concentration increases, an increase in the conductivity of the transistor channel is observed at a gate voltage of +0.7 V. Further work is aimed at optimizing the performance of the transistor.
26.07.2021.
By modifying PLD equipment and carefully adjusting sputtering parameters, a series of MoS2 samples were fabricated. Raman spectroscopy revealed good correspondence of stoichiometry between samples and PLD sputtering target. AFM surface morphology analysis of the samples indicated a homogeneous polycrystalline 11 nm thick film with a surface roughness of about 0.7 nm, crystallite size - 10-25 nm in width and 5 nm in height. Photoelectric measurements indicated relatively high sensitivity to light. Changing the lighting level from 0 to 100000 lx (solar spectrum simulator), the electrical resistance of the samples (Photo-FET configuration) reduced 10 times in a 0.3-second interval. For the films that thickness is less than 6 nm, the surface structure is non-homogenous which can be explained by the above-mentioned crystallite size limiting factor. The further work is related to improving the properties of the material, reducing the size of crystallites and providing an epitaxial growth of the film towards the surface functionalization with biomolecules (DNA primers). This goal will be achieved using GaN substrates in the presence of H2S gas.
09.04.2021
At the initial stage of this project, a literature review was conducted on the topic of two-dimensional materials based on transition metal dichalcogenides (2D TMDs) in order to determine the most optimal strategy for achieving the project objectives. Based on the above analysis, in cooperation with a partner organization (Daugavpils University), a set of metal contact masks was designed and manufactured using the laser demetallization technique. Samples were obtained by the method of pulsed laser deposition (PLD) at a substrate temperature of
400°C to 700°C. Summary of sample analysis, carried out with SEM, AFM, XRD, revealed a high quality of the material surface; however, some of the coatings turned out to be not stoichiometric as a result of the sulfur deficiency during the high-temperature deposition process. This problem will be eliminated by installing a sulfurization line in the PLD unit. To measure the electrical properties, a software and hardware complex was designed based on the Keithley 2450 SMU purchased from the project funds. The initial tests indicated high sensitivity and resolution of the setup. The results obtained so far will be presented at a conference in mid-April.