Ing. Michal Kočí, Ph.D.

Graphene oxides (GOs) and hydrogen-terminated nanocrystalline diamonds (H-NCD) have attracted considerable attention due to their unique electronic structure and extraordinary physical and chemical properties in various applications, including gas sensing. Currently, there is a significant focus on air quality and the presence of pollutants (NH3, NO2, etc.), as well as volatile organic compounds (VOC) such as ethanol vapor from industry. This study examines the synthesis of GO, reduced graphene oxide (rGO), thiol-functionalized graphene oxide (SH-GO), and H-NCD thin films and their combination in heterostructures. The materials were analyzed for their ability to detect NO2, NH3, and ethanol vapor at room temperature (22 °C). Among the tested materials, the SH-GO/H-NCD heterostructure exhibited the highest sensitivity, with approximately 630% for ethanol vapor, 41% for NH3 and −19% for NO2. The SH-GO/H-NCD heterostructure also demonstrated reasonable response (272 s) and recovery (34 s) times. Cross-selectivity measurements revealed that the heterostructure’s response to ethanol vapor at 100 ppm remained dominant and was minimally affected by the presence of NH3 (100 ppm) or CO2 (100 ppm). The response variations were −1.3% for NO2 and 2.4% for NH3, respectively. These findings suggest that this heterostructure has the potential to be used as an active layer in low-temperature gas sensors. Furthermore, this research proposes a primary mechanism that explains the enhanced sensor response of the heterostructure compared with bare GOs and H-NCD layers.

Heterostructure composite sensors utilize different parameters, resulting in improved response, selectivity, and reproducibility compared to pure materials. Ensuring clean air is fundamental for upholding a favorable quality of life on Earth. Supporting that knowledge, we present here an advanced material platform for room temperature conductivity gas sensors based on H-NCD films combined with a second gas-sensitive material, such as MoS2, rGO, SH-GO, or Au NPs. This platform supports the knowledge of gas sensing technology. Most of these diamond-based heterostructures based on diamonds exhibited improved gas sensing parameters and demonstrated a significant electrical response to gases at room temperature. In particular, the combination of SH-GO / H-NCD (two p-type materials) showed up to 630 % response for ethanol vapor. The synthesis of individual layers and measurement of sensor parameters are presented, and comparison and critical evaluation of responses.

This study presents the technological progress in the deposition of diamond thin films on quartz crystal microbalance (QCM) sensors. The linear antenna microwave plasma chemical vapour deposition (CVD) technique effectively grows thin diamond films on QCM substrates (Dia-QCM) differently oriented on the substrate holder in the deposition chamber, resulting in single-sided and double-sided coated QCMs. Each of these coated QCMs offers a distinctive advantage for sensing applications. The double-sided coated QCM sensors exhibited the most effective performance in ethanol detection, demonstrating approx. a 3-fold and 12-fold higher response than single-sided diamond-coated and bare gold QCM sensors, respectively. Furthermore, the single-sided Dia-QCM aptasensors demonstrated superior performance compared to bare gold QCM sensors, with a 2-fold higher response and a lower detection limit for S-RBD protein (LODDia-QCM = 0.09 pg/mL vs. LODAu-QCM = 0.10 pg/mL). In experiments conducted in human plasma, the Dia-QCM aptasensor demonstrated the ability to detect S-RBD protein at concentrations as low as 50 pg/mL, with high percentage recoveries. These results highlight the potential of linear antenna microwave plasma CVD for the mass production of advanced diamond-coated QCM sensors with different diamond film morphologies (porous, micro- or nanocrystalline) for various applications.

Currently, great emphasis is placed on air quality and the presence of pollutants, whether on toxic substances (NH3 or CO), substances that reduce the quality of life (CO2) or chemical vapors from industries (acetone or ethanol). Attention is therefore focused on new gas-sensing materials enabling detection even at low (up to room) temperatures with sufficient response and short reaction time.Here, we investigate the suitability of hydrogen-terminated nanocrystalline diamond (H-NCD) films and their heterostructures with molybdenum disulfide (MoS2), graphene oxide (GO), reduced GO (rGO), thiolfunctionalized GO (SH-GO), or gold nanoparticles (Au NPs) for gas sensing applications. Electrical properties are measured for oxidizing gas NO2, reducing gas NH3, and chemical vapor of ethanol (C2H5OH), and at temperatures varied from room temperature to 125 °C. All tested gases were used with a concentration of up to 100 ppm. Synthetic air is used as the flushing gas. The measured parameters of the tested sensors are compared, both with each other and with commercial sensors, and subsequently evaluated. In contrast to the individual forms of employed materials with limited response to the exposed gases, the HNCD heterostructures revealed better sensing properties. In particular, the Au NPs/H-NCD heterostructures revealed a higher response at 125 °C in contrast to H-NCD, MoS2/H-NCD had quite good response even at room temperature and GO/H-NCD revealed high sensitivity to chemical vapor, which further improved for the SH-GO/HNCD.

The quartz crystal microbalance (QCM) represents an acoustic (mass) analytical platform suitable for the detection of gases and biomolecules in air and liquid, respectively. Several studies have shown that the sensing performance, sensitivity, and selectivity of QCMs are further enhanced by using an appropriate functional layer. Among others, diamond is one such attractive material thanks to its extraordinary properties. However, the use of standard CVD diamond growth procedures at 800 °C cannot be applied to the QCM substrate due to its low phase transition temperature (570 °C). An alternative solution here offers a low-temperature (400 °C) linear antenna microwave plasma. In this work, we demonstrate technological progress in diamond growth on QCM substrates in terms of maintaining their functionality and the ability to cover both sides in one deposition cycle due to the low pressure (<15 mbar) and placement of the QCM substrate in the so-called cold plasma region.

The nanocrystalline diamond (NCD) film reveals a unique combination of physical, chemical, and optoelectronic properties, which makes it a promising material for various sensing applications. To improve a gas sensor's response, selectivity, or reproducibility, its surface is often modified with specific terminations, functional groups or (bio)molecules, thin films, etc. In this work, the NCD surface modification was achieved by a) layer morphology variation using two different types of chemical vapor deposition (CVD) systems, b) top surface termination (H-NCD and O-NCD), and c) Au nanoparticles (Au NPs). The properties of each structure are measured, compared and subsequently evaluated. The electrical properties (resistance changes) are measured for two types of active gas (oxidizing gas NO2 and reducing gas NH3) in a temperature range from 22 °C to 125 °C. Neutral synthetic air (80 % nitrogen and 20 % oxygen) was applied for flushing and resetting the sensors. Thin film fabrication, surface analysis (scanning electron microscopy and Raman spectroscopy), and measurement of electrical properties are described. Surface morphology greatly influences gas response because a large active surface area (higher roughness or 3D-like surface) enhances interaction with gas molecules. While the termination of the NCD with hydrogen is essential for the functionality of the gas sensor, the Au NPs further enhanced the dynamic response of the sensor and magnitude.

Molybdenum disulfide (MoS2) and nanocrystalline diamond (NCD) have attracted considerable attention due to their unique electronic structure and extraordinary physical and chemical properties in many applications, including sensor devices in gas sensing applications. Combining MoS2 and H-terminated NCD (H-NCD) in a heterostructure design can improve the sensing performance due to their mutual advantages. In this study, the synthesis of MoS2 and H-NCD thin films using appropriate physical/chemical deposition methods and their analysis in terms of gas sensing properties in their individual and combined forms are demonstrated. The sensitivity and time domain characteristics of the sensors were investigated for three gases: oxidizing NO2, reducing NH3, and neutral synthetic air. It was observed that the MoS2/H-NCD heterostructure-based gas sensor exhibits improved sensitivity to oxidizing NO2 (0.157%·ppm–1) and reducing NH3 (0.188%·ppm–1) gases compared to pure active materials (pure MoS2 achieves responses of 0.018%·ppm–1 for NO2 and −0.0072%·ppm–1 for NH3, respectively, and almost no response for pure H-NCD at room temperature). Different gas interaction model pathways were developed to describe the current flow mechanism through the sensing area with/without the heterostructure. The gas interaction model independently considers the influence of each material (chemisorption for MoS2 and surface doping mechanism for H-NCD) as well as the current flow mechanism through the formed P–N heterojunction.

Allotropic forms of carbon, in particular graphene oxide (GO) or nanocrystalline diamond (NCD), attracted the attention of many research groups due to their unique electronic structures and extraordinary physical and chemical properties, preferable for many different applications, including sensor devices. This work focuses on responses of various sensing layers (NCD with hydrogen termination (H-NCD), graphene oxide (GO), reduced graphene oxide (rGO), thiol-functionalized graphene oxide (GO-SH) and their hybrid structures to ethanol vapor with concentrations up to 100 ppm in synthetic air at room temperature. The measured parameters of the tested sensors, especially stability, reproducibility and regeneration, are compared and critically evaluated. The high sensitivity of tested sensors achieved at room temperature makes them very promising for monitoring ethanol vapor as well as other volatile substances (e.g., isopropyl-alcohol or acetone). In this view, properly designed sensing layers have great potential in many fields, such as diagnostics or environmental monitoring.

Gas sensors are an integral part of everyday life. New materials and manufacturing processes enable the production of smaller, more accurate, cheaper and more selective sensors. Compared to pure material, heterostructured or hybrid sensors allow the combination of materials’ parameters and show improved response, selectivity and reproducibility. Heterostructures consisting of nanocrystalline diamond (NCD), molybdenum disulfide (MoS2) and graphene oxide (GO) have been investigated as gas sensors that operate at room temperature and have built-in interdigital metal electrode structures to measure conductivity. The fabrication of heterostructures of MoS2/NCD, rGO/NCD or GO-SH/NCD using suitable physical/chemical deposition methods is demonstrated. Furthermore, the measured sensor parameters of the tested materials and their heterostructures, in particular response, response rate, stability and regeneration, are compared and critically evaluated. Contrary to pure sensing material, the heterostructures improved the gas sensing parameters and exhibited a remarkable electrical response to the gases at room temperature.

Gas sensing properties of a nanocrystalline diamond with a hydrogen-terminated surface (H-terminated NCD) and a molybdenum disulphide (MoS2) are investigated as conductivity sensors with built-in interdigital metal electrode structures. The H-terminated NCD was prepared by plasma-enhanced chemical vapour deposition (PECVD), and the MoS2 by a carbide-free one-zone sulfurization method. The sensor's responses were measured for oxidizing (NO2) and reducing (NH3) gases by the same equipment and setup. The parameters of the tested sensors are compared and critically evaluated. Advantageously, the MoS2/H-terminated NCD heterostructure enhances the gas sensing response at room temperature compared to the H-terminated NCD and MoS2 layers.

Gas sensors are nowadays an integral part of everyday life. New materials and fabrication processes allow the production of smaller, more accurate, cheaper, and more selective sensors. Here, gas sensing parameters of PtSe2, MoS2, and their heterostructures with nanocrystalline diamond (NCD) were measured at room temperature for oxidizing (NO2) and reducing (NH3) gases. The PtSe2 and MoS2 were prepared directly on SiO2/Si or NCD/SiO2/Si substrates by a simple selenization and a carbide-free one-zone sulfurization method. Advantageously, prepared heterostructure enhanced the gas sensing parameters and showed a notable electrical response to the examined gas types at room temperature.

Gas sensing properties of a nanocrystalline diamond with hydrogen-terminated surface (H-NCD) and a transition metal dichalcogenide (TMD) are investigated as conductivity sensors employing device design with built-in interdigital metal electrode structures. The TMDs were prepared by a carbide-free one-zone sulfurization method directly on the SiO2/Si or NCD/Si substrates. On the individual level, the sensors showed a notable electrical response to the examined gas types, i.e., oxidizing (NO2) and reducing (NH3) gases. For reference, commercially available sensor was used (conductive SnO2 or infra-red type). Advantageously, it was found that the NCD/TMD heterostructure results in enhanced signal stability and gas sensing response at room temperature compared to both bare TMD and H-NCD layers.

A nanocrystalline diamond (NCD) layer is used as an active (sensing) part of a conductivity gas sensor. The properties of the sensor with an NCD with H-termination (response and time characteristic of resistance change) are measured by the same equipment with a similar setup and compared with commercial sensors, a conductivity sensor with a metal oxide (MOX) active material (resistance change), and an infrared pyroelectric sensor (output voltage change) in this study. The deposited layer structure is characterized and analyzed by Scanning Electron Microscopy (SEM) and Raman spectroscopy. Electrical properties (resistance change for conductivity sensors and output voltage change for the IR pyroelectric sensor) are examined for two types of gases, oxidizing (NO2) and reducing (NH3). The parameters of the tested sensors are compared and critically evaluated. Subsequently, differences in the gas sensing principles of these conductivity sensors, namely H-terminated NCD and SnO2, are described.