Sensortehnoloogiate labori seminar

Anna Wojsiat Soosaar

To spark Debate or make a Difference?

This talk uses gas sensor development as an example of what industrial research looks like in practice. The starting point is a phrase heard during an Exploring Careers Beyond Academia workshop: "to spark debate or make a difference." While presented as a contrast between academia and industry, being an industrial PhD student turns that "or" into "and".

Using examples from my work at Evikon MCI, a company developing and manufacturing sensor-based detection, measurement, and control instruments, I will discuss what happens to gas sensors once they arrive in the laboratory - what is tested, which parameters are measured, and what needs to be considered before a sensor can move further toward detector production.

Through several practical cases, I will show that industrial laboratory work is not only about answering the question "is this sensor good enough?" - scientific questions also come up along the way, and they have to be answered, too.

Koit Mauring

Temperature modulation of a gas sensor and lock-in signal detection – a way to faster sensorics

The presentation introduces a method for signal readout from a gas sensor with a microheater, which enables faster response time and reduced signal drift.

Outline of the talk is as follows:

• Response and recovery time of a gas sensor
• Standard methods to achieve faster sensors
• Temperature modulation and lock-in detection benefits
• Graphene decorated with V₂O₅ for NH₃ detection
• Comparison of DC and AC signals
• Sensor signal drift reduction mechanism

Valter Kiisk

Hall Effect Measurement of Graphene

A new generation of chemiresistive gas sensors is being developed on the basis of 2D materials (graphene being most prominent). Even though the morphology is much simpler (compared to traditional metal oxide sensors), there can be still different sensing mechanisms involved, related either to charge transfer upon adsorption or scattering of charge carriers on the adsorbed gas molecules. A combination of conductivity and Hall effect measurement can be used to separate the impacts on carrier density and mobility. The latter also indicate the quality of graphene.

The presentation will introduce the principle and instrumentation of Hall effect measurement (as recently set up in the Laboratory of Sensor Technologies) and some preliminary results on specially prepared CVD graphene samples.

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Taavi Repän

Analyzing formation of gold particles by laser annealing nanoporous gold films

Colleagues in Prague have fabricated films of gold particles by shining CW laser light (up to 200 mW) on a nanoporous gold film. Under laser irradiation, the gold structure melts and forms individual spherical particles on the glass substrate. We have studied these samples using scanning electron microscopy (SEM) and full-wave simulations of the particles in order to characterize the size and shape of the formed particles. In particular, we have picked three different samples obtained using laser powers of 100 mW, 150 mW, and 200 mW. From SEM images, we obtained an estimate of the particle size distribution in these samples. Then, using numerical simulations, we calculated absorption spectra for a variety of spherical gold particles on a glass substrate with different sizes and shapes (wetting angle). The numerical fitting of the simulation results against experimentally measured absorption spectra then indicates the particle shapes as a function of laser power. In the seminar, I will present the final analysis results and the developed numerical methodology, which took way more effort than originally anticipated.

Paniz Vafaei

Uptakes from the American Chemical Society (ACS) 150th anniversary symposium

• Colloidal nanocrystals for catalysis and energy applications
• Light–matter interactions in small volumes
• Interphases and interfaces in solid-state batteries, with emphasis on nanoscale control
• Designing and imaging energy flow in nanomaterials
• Quantum dots: looking back and future perspectives

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Jaroslav Otta

Nanocomposite and nanostructured materials as active layers of chemical gas sensors

This study investigates the development and optimization of nanocomposite gas sensors, focusing on the utilization of advanced materials and innovative deposition techniques. The study examines a range of active materials, including phthalocyanines [1], polythiophene derivatives [2], metal oxide nanostructures [3, 4, 5], black metals [6, 7], and poly(ionic liquid)s (PILs) [8], to enhance the sensitivity and selectivity of chemical gas sensors.
The research utilizes various deposition methods, emphasizing laser depositions and surface modifications [9, 10] to improve the performance of these sensors. Techniques such as Continuous-Wave Laser-Induced Forward Transfer (CW-LIFT), magnetron sputtering, and vacuum evaporation are explored for their potential to create active layers of gas sensors.
Key findings include the impact of deposition conditions on the structural and functional properties of the sensor materials. Through detailed analysis using electron microscopy, Raman spectral mapping, impedance spectroscopy, X-ray crystallography, and UV-Vis spectroscopy, the research presents an evaluation of material properties of the prepared sensors.
The study addresses the challenges of integrating nanostructured materials into gas sensors, proposing solutions for improving their performance. The outcomes contribute to the advancement of gas sensor technology, offering insights into the development of sensors with enhanced properties.

References
[1] J. Vlček et al., CrystEngComm, 2021, 23 (41).
[2] T. V. Shishkanova et al., Journal of Materials Science, 2022, 57 (37).
[3] A. Torrisi et al, Chemosensors, 2021, 9 (9).
[4] L. Piliai et al., Sensors Actuators B Chem, 2023, 397.
[5] J. Otta et al., Electronics, 2025, 14 (17).
[6] M. Hruška et al., Applied Surface Science, 2024, 647.
[7] J. Kejzlar et al., Material Advances, 2025, 6 (10).
[8] J. Otta et al., ACS Omega, 2025, 10 (1).
[9] R. Elashnikov et al., ACS Omega, 2019, 4 (3).
[10] R. Elashnikov et al., Soft Matter, 2018, 14 (23).

Anna Soosaar

Femtosecond Laser Induced Defect Engineering of CVD Graphene and its Potential for Gas detection

The development of chemically active graphene surfaces is essential for enhancing the performance of next generation chemiresistive gas sensors, particularly in terms of sensitivity and selectivity. Graphene’s unique structure provides a versatile platform, but controlled defect introduction and chemical modification require the development of special methods.

In this study, femtosecond laser irradiation was used to induce structural defects and facilitate the formation of functional groups on CVD graphene. The treatment was conducted in various gaseous environments: ambient air, dry nitrogen, and ethanol vapors with nitrogen as a carrier gas. Raman spectroscopy confirmed that the nature and density of defects were strongly dependent on the treatment atmosphere. The highest degree of functionalization was observed in an ethanol-containing environment. These modifications significantly improved sensitivity to nitrogen dioxide, with sensor responses up to ten times higher than those of pristine graphene. Further functionalization using aminoferrocene led to additional increases in response, demonstrating the effectiveness of femtosecond laser treatment as an intermediate functionalization step. These findings highlight the potential of laser treatment for tailoring graphene surfaces, with promising applications in gas sensor technology and other related fields.

Burak Baloglu

Investigation of Graphene-BN Heterostructures for Improving the Stability of Graphene-Based Gas Sensors

Degradation or sensor drift is a well-known phenomenon in chemiresistive gas sensors and is attributed to factors such as high operating temperatures, humidity, and other environmental stresses. Stability, as a measure of resistance to degradation, is a critical parameter for evaluating sensor performance and is essential for commercialization. Therefore, the properties of the surface material at the sensing interface play a pivotal role.

In this study, we investigated the use of thin boron nitride (BN) layers as protective coatings for graphene (Gr)-based gas sensors. BN, known for its high thermal stability and oxidation resistance, was applied either via pulsed laser deposition or through the transfer of CVD-grown single-layer hexagonal BN (hBN). Stability tests were carried out at elevated temperatures in synthetic air (40% relative humidity) containing 300 ppb ozone. Results showed that the atomically thin, wet-transferred hBN layer reduces the sensor degradation, compared to the uncoated reference.

Martin Lind

Applying machine learning on graphene-based MEMS gas sensors with pulsed heating

Machine learning in conjunction with signal processing techniques (such as the Fourier transform) applied to the readings of low-power graphene-based MEMS sensors can help to detect harmful gases (such as NH3) in the ambient air accurately and quickly. In this seminar, the presenter introduces the process and results of applying machine learning to the sensors recently prepared on the micro-hotplate substrates by the Lab of Sensor Technologies. It's a continuation of last week's seminar presented by Paniz Vafaei.

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Paniz Vafaei

Pulsed temperature operation of graphene-based MEMS sensors for low-power NH₃ detection

In the era of the Internet of Things, the development of gas sensors integrated with micro-hotplates using MEMS technology enables reduced power consumption, particularly when combined with novel materials and advanced measurement and signal processing techniques.
In this study, we investigated MEMS sensors based on chemical vapour deposited graphene, functionalised with atomically thin metal oxide (MOX) coatings applied via pulsed laser deposition.
We examined the effect of MOX growth conditions on NH₃ sensing performance in dry and humid air under pulsed temperature operation — a strategy aimed at significantly lowering power consumption. Among the tested heating sequences, a 500 ms pulse duration with a duty cycle of 1:20 and an optimised power consumption of 0.4 mW proved to be the most effective configuration. This setup was recommended for extended measurements under varying gas concentrations and humidity levels, with the end goal of developing machine learning-based signal processing models for fast and precise sensing.

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Valter Kiisk

Detecting air pollution with commercial MEMS gas sensors using modulated heating

Miniaturization of metal oxide semiconductor gas sensors over the past few decades (using MEMS fabrication) has notably reduced their power consumption. Miniaturization also enables novel approaches for sensor readout due to rapid temperature control. Hereby we investigated the performance of several commercial MEMS gas sensors utilizing an original approach of modulated heating and lock-in detection. Most sensors showed 2–3 times improvement in terms of response or recovery speed. Carbon monoxide detection also experienced improved stability and reduced moisture sensitivity.
The presentation covers the results of the experimental development project “Development of air quality sensor prototype”.

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Karl Artur Lokk (3. kursuse üliõpilane)

Using deep neural networks as fast approximations to full-wave simulations

This talk presents preliminary results from a bachelor's thesis project investigating the use of deep neural networks as fast approximations to full-wave simulations. The work involves 2D simulations of dielectric structures using the finite-difference frequency-domain (FDFD) method. A UNet architecture is trained to predict the resulting electric field distributions for specific geometries. The current state of the project is discussed, including methodology for hyperparameter optimization, network architecture choices, and an overview of preliminary results so far.

Raivo Jaaniso

Modelling the gas response of 2D materials – Part 2: Some extensions of the basic model

The seminar will provide a brief overview of some theoretical concepts for describing and interpreting changes in the electrical conductivity of two-dimensional materials due to a gas environment. In the case of two-dimensional materials, the modelling of these processes is simpler compared to bulk semiconductors, since there is no need to consider space charge or diffusion. Starting with a Langmuir basic model, we will consider which mathematical functions can be used to approximate the experimental data and within what limits the obtained parameters can be used to interpret the underlying mechanisms.

Indrek Renge

Atomic level description of graphene-based gas sensing mechanisms

In a toxic gas sensor the small molecules produce a conductivity change of graphene that is functionalized with a metal oxide (MOX) nano-phase, and optionally, exposed to UV-vis light. Bulk MOX and its surface will be treated as a collection of ions, rather than a homogeneous crystalline phase. This allows one to apply the powerful concepts of physical (in)organic chemistry, in particular, the structure-energy-property relationships (Pauling, Hammett, Palm). The qualitative rationalization of processes in participation of light quanta, electrons, protons, atoms, ions, and molecules should be in agreement with computer chemical output. Hypotheses are advanced to explain how toxic gas at concentration in parts-per-billion can ultimately generate a strong conductive response of graphene.

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Artjom Berholts

Light-enhanced sensors of oxidizing gases based on single-layer CVD graphene

(results of the PhD thesis to be defended)

Gas sensors with improved functional properties (high sensitivity, selectivity, and stability; low power consumption) are needed to detect a wide variety of gases originating from both natural and anthropogenic sources. Graphene – a material consisting of a single layer of carbon atoms - has emerged as an ideal building foundation for gas sensors since its entire surface can act as a sensing area.
This thesis focuses on investigating the influence of ultraviolet (UV) light on the functional properties of graphene-based sensors towards two oxidizing gases – oxygen (O₂) and nitrogen dioxide (NO₂). The first part of the thesis focuses on studying the light irradiation effect on pristine chemical vapor deposited (CVD) graphene. For that, the electrical properties of graphene were investigated, as well as the influence of UV light on O₂ sensing. The results demonstrated a highly beneficial effect of irradiation, which led to the activation of an initially inert sensor, making it sensitive to oxygen concentration changes and improving response and recovery speed. As a next step, both pristine and modified CVD graphene were tested as sensors for NO₂, a highly toxic polluting gas causing millions of premature deaths per year in the whole world. For graphene modification, pulsed laser deposition was used to grow thin layers of oxides (ZrO₂, TiO₂) or metal (Ag) on top of graphene. The highest sensitivity was obtained with titania-coated graphene sensors under UV light. Gas concentrations down to 10 ppb (10 parts per billion) were investigated with the estimated level of detection of 0.03 ppb. Additionally, the excellent selectivity of the graphene/TiO₂ sensor was showcased, as NO₂ results significantly overperformed those for other polluting gases (CO, SO₂, and NH₃), as well as humidity.
Overall, it was demonstrated that the irradiation with low-power UV light (365 nm) allowed a considerable increase in the sensitivity towards the studied gases and increased the response and recovery rates so that all functional parameters became within the range of real-world applications for the sensors operated at room temperature.

Burak Baloglu

Exploring the Potential of Graphene/h-BN van der Waals Heterostructures for Highly Stable Gas Sensors

The advent of graphene has revolutionized the isolation of various bulk materials into their two-dimensional (2D) forms, many of which exhibited superior gas-sensing properties compared to their bulk counterparts. Over the past decade, heterostructures (stacking) of these materials, stabilized by van der Waals (vdW) bonds, have been developed to leverage their synergistic effects and enhance the performance of gas sensors.

Graphene stands out due to its high surface area, exceptional electrical conductivity, and high carrier mobility, making it an excellent candidate for gas sensing applications. 2D hexagonal boron nitride (h-BN), on the other hand, provides excellent thermal stability and high oxidation resistance, making it an ideal complement to graphene.

We aimed to create vdW heterostructures of graphene and h-BN with the intention of detecting toxic gases (NO2, NH3, etc.) in a highly stable manner. Notably, in this work, BN was directly deposited on graphene-based gas sensors via pulsed laser deposition to serve both as a sensing layer and a protective layer for graphene. I will present the potential of such heterostructures in gas sensing and share our latest progress in this endeavor.

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Taavi Repän

Analyzing formation of gold particles by laser annealing nanoporous gold films

Colleagues in Prague have fabricated films of gold particles by shining CW laser light (up to 200 mW) on a nanoporous gold film. Under laser irradiation, the gold structure melts and forms individual spherical particles on the glass substrate. We have studied these samples using scanning electron microscopy (SEM) and full-wave simulations of the particles in order to characterize the size and shape of the formed particles. In particular, we have picked three different samples obtained using laser powers of 100 mW, 150 mW, and 200 mW. From SEM images, we obtained an estimate of the particle size distribution in these samples. Then, using numerical simulations, we calculated absorption spectra for a variety of spherical gold particles on a glass substrate with different sizes and shapes (wetting angle). The numerical fitting of the simulation results against experimentally measured absorption spectra then indicates the particle shapes as a function of laser power. In the seminar, I will present the preliminary results of this study.

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Paniz Vafaei

Gas-sensing performance of Graphene functionalized with MXene nanostructures

Among two-dimensional (2D) materials, Graphene and MXene have shown promising potentials in the field of gas sensing due to their high electrical conductivity, and large surface area. However, MXenes with tunable surface functionality, variable layer spacing, and hydrophilicity have been more advantageous. Herein, we deposited a thin layer (< 20 nm) of MXene using interfacial film formation on CVD graphene to study their gas sensing responses in exposure to different concentrations of NO₂ in an N₂ environment at room temperature, as well as heating conditions. Compared to the pristine MXene, the Graphene/Ti₃C₂T_x showed approximately twenty times higher responses to 300 ppb NO₂ at 180°C which may relate to the partial oxidation of MXenes which is different from the pristine MXene and enables selective response to NO₂.

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Martin Siebel
Technical University of Darmstadt

First principle simulation of gas adsorption on graphene / h-BN structures

Master's thesis defence presentation
Reviewers: Prof. Dr Hongbin Zhang and Assoc. Prof. Pavel Rubin

Abstract: This work made density functional theory calculations (using VASP code) for investigating the projected densities of states and adsorption energies of two different graphene / hexagonal boron nitride (h-BN) structures at the adsorption of NO₂, NH₃ and O₃ gas molecules. This was done to investigate the gas-sensing capabilities of those structures. The super-cells of the investigated structures consisted of a base layer with 72 carbon atoms. The difference between the structures was the size of the top layer, which had either 36 boron and 36 nitrogen atoms or 11 boron and 11 nitrogen atoms. In addition, the size of both structures was increased by using periodic boundary conditions. The structures have been relaxed in the first step without the gas molecules and in the second step with all of the different gas molecules. The projected densities of states, Bader charges, and adsorption energies have been calculated.

The results show the theoretical possibility of detecting NO₂ using both structures via a shift of graphene's Dirac point within the density of states analyses. In the case of O₃, the detectability is given using the complete structure, while the gas dissociated on the reduced structure. In addition, the reduced structure shows promising results for detecting NH₃, which acts as a charge acceptor.

Dejan Prokop

Preparation of Black Titanium/Titanium Nitride Thin Films by Magnetron Sputtering

Faculty of Mathematics and Physics, Charles University, Prague
Institute of Physics of the Czech Academy of Sciences

Recently, black aluminium (B-Al) films proved to be very useful with their highly structured fractal-like porous surface morphology that is able to effectively trap the incident light. We also proposed some of the applications for it. Motivated by this, we tried to prepare another material – Black Titanium (B-Ti). Compared to aluminium, titanium-based materials are widely used because they possess a combination of useful mechanical, tribological, and chemical properties e.g. TiN has a high hardness, good wear, and corrosion resistance as well as thermal stability. By changing the argon to nitrogen ratio in our magnetron sputtering chamber we can modify the properties of our films. Furthermore, we are trying to implement optical emission spectroscopy to correlate the plasma parameters with resultant films. Even though we are still far from our desired goal, we were able to prepare the films and study the dependence of nitrogen content by optical diffuse reflectance/transmittance measurement and structural properties by X-ray diffraction.

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Physical and Chemical Properties of Black Gold Layers

Jan Kejzlar

University of Chemistry and Technology, Prague
Institute of Physics of the Czech Academy of Sciences

This contribution is focused on material characterization of black gold layers compared to a bulk metal. As these layers represent novel approach for gas sensing, broad material characterization is in place. The black gold layer properties are compared to the bulk metal by a high variety of methods. The impact of the nanostructured surface is discussed from the perspective of morphology, crystallography, chemical composition, reflectivity and absorbance.

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Anna Soosaar

Functionalization of CVD graphene by fs-laser treatment for gas sensing applications

(materials of BSc thesis, 10 min)

Graphene and related 2D materials can be used to construct very sensitive gas sensors. In order to achieve high selectivity for detecting different gases, it is necessary to functionalize these materials. In this work, the functionalization is carried out by femtosecond laser treatment. A new fs-laser treatment set-up at the Institute of Physics was tested, and different laser-processed chemiresistive gas sensor chips were studied. The analysis by Raman spectroscopy revealed that the laser-induced defects were created by a two-photon mechanism. The sensitivity of laser-treated sensor chips made on Si/SiO₂ substrates increased up to 10 times to 150 ppb of NO₂ gas exposure in the air.

Burak Baloglu

Graphene in blister-based LIFT

(materials of MSc thesis, 15 min)

Laser-induced forward transfer (LIFT) can provide a clean (non-contact and solvent-free) technique for the high-resolution printing of two-dimensional materials, a significant technological step for their integration into microdevices. The blister-based (BB) version of LIFT allows the transfer of ultra-thin layers without damaging the materials on the receiving substrate with intense laser light. This work investigated single-layer graphene in different roles as a donor, acceptor and release material in BB-LIFT. The transfer of graphene and ultrathin oxide (ZrO₂) layers was confirmed and characterised through optical microscopy, Raman spectroscopy and scanning electron microscopy (SEM). The crucial role of the graphene interlayer in ZrO₂ transfer was revealed. The immediate motivation for the work was the improvement of graphene-based gas sensors by transferring ultrathin oxides as gas sensor receptor layers on the CVD graphene as a transducer layer. It was found that the sensitivity of sensors to trace level NO2 gas did not practically increase after BB-LIFT of ultrathin (0.5-50 nm) ZrO₂ layers. This was interpreted as a lack of strong interactions of the transferred oxide flakes (and a small contact area) with the graphene layer of the sensor substrate. In order to increase the interaction, the sensors were annealed at a higher temperature (up to 500 ℃ in a vacuum), which significantly accelerated the gas response. The work highlights the finding that the oxide layer can be transferred only when graphene is inserted between the blister and oxide layers. This is a novel application for graphene, considerably widening the scope of materials transferable by BB-LIFT.