Heterojunction Gas Sensors Research Articles

Susceptible Detection of Organic Molecules Based on C3B/Graphene and C3N/Graphene van der Waals Heterojunction Gas Sensors.

Constructing van der Waals (vdW) heterostructures is a prospective approach that is essential for developing a new generation of functional two-dimensional (2D) materials and designing new conceptual nanodevices. Using density-functional theory combined with a nonequilibrium Green's function approach allows for the theoretical and systematic exploration of the electronic structure, transport properties, and sensitivity of organic small molecules adsorbed on 2D C3B/graphene (Gra) and C3N/Gra vdW heterojunctions. Calculations show the metallic properties of C3B/Gra and C3N/Gra after the formation of heterojunctions. Interestingly, the heterojunctions C3B/Gra (C3N/Gra) for the adsorption of small organic molecules (C2H2, C2H4, CH3OH, CH4, and HCHO) at the C3B (C3N) side are sensitive to the chemisorption of C2H2 and C2H4. Similarly, the Gra/C3B is chemisorbed for both C2H2 and C2H4 when adsorbed on Gra side, while it is only chemisorbed for C2H2 in Gra/C3N. Interestingly, all heterojunctions on different sides are physisorbed for CH3OH, CH4, and HCHO. Furthermore, the calculated I-V curves demonstrate that the devices based on the adsorption of C2H2 and C2H4 at each side of the heterojunction have remarkable anisotropy, in with the current being considerably greater in the zigzag direction than in the armchair direction. More specifically, with C2H2 adsorbed on the Gra side, the sensitivity along the armchair direction is up to 85.0% for Gra/C3B and close to 100% for Gra/C3N. This study reveals that C3B/Gra (C3N/Gra) heterojunctions with high selectivity, high anisotropy, and excellent sensitivity are highly prospective 2D materials for applications, which further contributes new insights into the development of future electronic nanodevices.

Micro-mechanism study of charge transfer at heterojunction interface based on first-principles theory: MoS2/SnO2 as the prototype

Gas sensors made of semiconductor heterojunctions have excellent gas-sensitive properties. However, sufficient theoretical evidence is still needed to prove that heterojunction improve gas sensitivity of a single-material. A typical case is studied here: MoS2/SnO2 heterojunction absorption of NO2 is investigated by first-principles calculations, to explore the enhancing-mechanism. Interestingly, it is found that electron transfer not only occurs between the material surface of the heterojunction and gas, but also involves the substrate atoms transferring electrons to the gas molecules through the surface atoms. This finding fills the response mechanism of heterojunction gas sensors and provides a theoretical basis for experiments.

Gas-Sensitive Performance Study of Metal (Au, Pd, Pt)/ZnO Heterojunction Gas Sensors for Dissolved Gases in Transformer Oil.

An oil-immersed transformer is a critical electrical device for power delivery. Online monitoring of transformer operation is the key to ensuring the regular operation of power systems. This paper proposes Au/ZnO, Pd/ZnO, and Pt/ZnO heterojunctions as new gas-sensitive materials and investigates their gas-sensitive performance to dissolved gases (C2H4, CO, and H2) in transformer oil. Upon theoretical density functional theory (DFT) calculations, the analysis of the total density of states (TDOS), partial density of states (PDOS), molecular orbital theory, and charge deformation density reveals that Au, Pd, and Pt form heterojunctions with ZnO, which enhance the electrical conductivity of the system. Meanwhile, intrinsic ZnO is unsuitable for gas detection and adsorption, while the Au/ZnO heterojunction suits C2H4 detection. In contrast, the Pd/ZnO heterojunction is suitable for H2 detection, and the Pt/ZnO heterojunction is suitable for C2H4 and CO detection. The electrical conductivity of the adsorption models is changed to varying degrees after gas adsorption. The different change rate electrical conductivity just serves as a theoretical foundation for determining the type and concentration of dissolved gases in transformer oil. The research results act as a theoretical foundation for constructing gas sensors with a ZnO-based material.

Controlling the structure and composition of SnO2-based thin film with reactive sputtering to improve the sensitivity of semiconductor CO2 sensor

The sensitivity of SnO2 thin film-based CO2 gas sensors was enhanced by controlling the surface structure employing reactive sputtering during the deposition process to carefully adjust the oxygen partial pressure to modify the surface structure of the SnO2 films. This process increased the sensitivity, primarily due to larger surface area and improved gas adsorption capabilities. Furthermore, the effect of heterojunctions between p-type SnO and n-type SnO2 on the sensitivity was investigated using a model diagram. Both theoretical analysis and experimental data consistently demonstrated that the number of heterojunction interfaces contributes significantly to the sensitivity of SnO-SnO2 heterojunction gas sensors. These findings highlight the effectiveness of controlling the surface structure and composition ratio of thin films through reactive sputtering to enhance sensitivity. This study offers valuable insights for optimizing SnO2 thin-film-based gas sensors for CO2 detection.

Effect of ZnO thickness on gas sensing behavior of WS2-ZnO p-n heterojunction nanosheets towards reducing gases

Optimization of materials in heterojunction gas sensors is vital for achieving the highest gas-sensing performance. In this study, we synthesized pristine ZnO, pristine WS2 nanosheets (NSs), and ZnO with different thicknesses (10–50 nm) on WS2 NSs to study their gas-sensing properties. It was found that at 150°C and 300°C, the sensors with ZnO layers of 10 and 30 nm, respectively, exhibited the highest response to reducing gases. At low temperatures, the role of WS2 in gas sensing was dominant, whereas at high temperatures, ZnO played a dominant role. Therefore, the sensor with the thinnest ZnO layer demonstrated the best performance at low temperatures, while at higher temperatures, the sensor with a ZnO thickness of 30 nm exhibited optimal performance. Also, at 300°C, WS2 was oxidized to WO3. Furthermore, to decrease overall power consumption, we conducted sensing measurements under self-heating conditions on a WS2-ZnO (10 nm) p-n heterojunction NS sensor and found that the gas sensor, under a low applied voltage of 4.2 V, exhibited the highest response to reducing gases. The enhanced gas sensing of the optimized gas sensors was attributed to the formation of heterojunctions and the optimal thickness of the ZnO layer on WS2 at appropriate temperatures.

Investigation of low-temperature deposition of SnO2 thin film for improving sensitivity of LaOCl/SnO2 heterojunction CO2 sensor using electrochemical impedance spectroscopy

In this study, LaOCl/SnO2 heterojunction CO2 sensors were fabricated by spin-coating LaOCl onto a SnO2 thin film fabricated via RF magnetron sputtering. The sensitivity of LaOCl/SnO2 heterojunction CO2 sensors tended to improve with the deposition of SnO2 thin films without intentional heating (at approximately 35 °C). Electrochemical impedance spectroscopy measurements revealed that the formation of a LaOCl/SnO2 heterojunction interface is indispensable for high-sensitivity sensors, and LaOCl acted not only as a catalyst but also as a capacitive layer in the sensors. The capacitive element of the LaOCl/SnO2 heterojunction changed under a CO2 gas atmosphere. It is confirmed that the formation of heterojunction interfaces contributes to the improvement of sensitivity. The results of this study indicate the potential applications of heterojunction gas sensors.

Conductometric room temperature NOx sensor based on metal-organic framework-derived Fe2O3/Co3O4 nanocomposite

In this work, Fe2O3/Co3O4 heterostructures were prepared via the ion-exchange between Co-MOFs template and Fe3+. At room temperature (25 °C), gas-sensing measurements revealed the Fe2O3/Co3O4 composites showed high response (Ra/Rg =2.54, 97.0 ppm), excellent selectivity and low detection limit (0.97 ppm) to NOx. The special oxygen temperature-programmed desorption (O2-TPD) and hydrogen temperature-programmed reduction (H2-TPR) curves indicated addition of Fe not only damaged the structure, but also increased the active sites of composites. Mott-Schottky (MS) plots testing and Electrochemical Impedance Spectroscopy (EIS) analysis were used to further quantitatively analyze the effect of Fe content on carrier density (Nd) and charge transfer resistance (Rct). The enhanced sensing mechanism of Fe2O3/Co3O4 composites were attributed to the as-formed heterojunctions that offered numerous diffusion channels and the additional modulation of resistance. By employing a facile MOFs hybrid-assisted method to design heterojunction composites, this work comprehensively analyzed the influence of heterogeneous structure on gas sensitivity, providing insights into the understanding of heterojunction gas sensor materials.

Molecular Engineering of Silicon Phthalocyanine to Improve the Charge Transport and Ammonia Sensing Properties of Organic Heterojunction Gas Sensors

AbstractNovel organic heterostructures fabricated with a bilayer consisting of an axially substituted silicon phthalocyanine (R2‐SiPc) derivative and lutetium bis‐phthalocyanine (LuPc2) are investigated for their ammonia sensing properties. Surface and microstructure characterization of the heterostructure films reveal either compact or highly porous surface topography in (345F)2‐SiPc and Cl2‐SiPc‐based heterostructures, while electrical characterization reveals a strong influence of the axial substituent in R2‐SiPc on NH3 sensing capabilities. Electrical characterization further demonstrates an apparent energy barrier for interfacial charge transport, which is higher in the (345F)2‐SiPc/LuPc2 heterojunction device. In‐depth charge transport studies by impedance spectroscopy further reveal a resistive interface in (345F)2‐SiPc/LuPc2 and faster bulk and interfacial charge transport in Cl2‐SiPc/LuPc2 heterojunction devices. Different interfacial charge transport capabilities and surface topographies affect NH3 sensing properties of the two heterojunction devices, in which (345F)2‐SiPc/LuPc2 reveals a fast and non‐linear response with a limit of detection (LOD) of 310 ppb, while Cl2‐SiPc/LuPc2 exhibits a slow, and linear response to NH3 with LOD of 100 ppb. Finally, different metrological parameters of the two sensors are correlated to the respective gas‐material interactions, in which adsorption and diffusion regimes are modulated by the surface topography and hydrophobicity of the sensing layer.

Interplay of electrode geometry and bias on charge transport in organic heterojunction gas sensors

Modulation of charge transport in an organic heterojunction sensor is a promising strategy to enhance its gas sensing properties. Herein, a heterostructure consisting of a bilayer assembly of perfluorinated copper phthalocyanine and lutetium bis-phthalocyanine is investigated for ammonia sensor development in two different electrode geometries and in a wide range of applied bias. The microstructure of the heterostructure retains the bulk electronic and structural properties of the individual components and shows granular distribution of phthalocyanine crystallites on the surface. The charge transport of the heterojunction devices depends on the electrode geometry and the applied bias. Notably, interfacial charge transport gets faster, while bulk charge transport remains constant with increasing bias in both devices. But, the small gap between the electrodes fastens the bulk and the interfacial charge transport by 75 and 5 times, respectively, compared to the big gap electrodes. The implication of faster charge transport in the small gap electrodes-based sensor is demonstrated on its ammonia sensing properties, exhibiting higher response and shorter response time. Moreover, the response of the sensor exponentially increases with increasing bias. The high sensitivity and stability of the sensor at different relative humidity make it a suitable NH3 sensor for real environment applications.

Performance prediction of 2D vertically stacked MoS2-WS2 heterostructures base on first-principles theory and Pearson correlation coefficient

The gas sensor made of two-dimensional (2D) material heterojunction has superior gas sensitivity, but the response mechanism has not been systematically investigated for a long time. In this work, the changes in total density of states (TDOS), partial density of states (PDOS), band gap (BG), and differential charge density (DCD) are investigated by first-principles calculations, revealing the response mechanism of NO2 adsorption by V-MoS2-WS2. The theoretical response value and recovery time of the sensor are described by the Boltzmann transport theory. The theoretical response recovery time and response value are analyzed by Pearson correlation coefficient, and the main influencing factors are BG and electron transfer. Through bader charge analysis, it is found that the electron transfer in the response process of the gas sensor is not only the transfer between the material surface and gas but also that the internal metal atoms provide electrons to be transferred to the gas molecules through the non-metal atoms. This discovery effectively explains the transient and steady-state processes of sensor response, fills the gap in the response mechanism of gas sensors prepared by 2D material heterojunction, and lays the theoretical foundation for the development of 2D material heterojunction gas sensors.

Achieving highly-efficient H2S gas sensor by flower-like SnO2–SnO/porous GaN heterojunction

A flower-like SnO2–SnO/porous GaN (FSS/PGaN) heterojunction was fabricated for the first time via a facile spraying process, and the whole process also involved hydrothermal preparation of FSS and electrochemical wet etching of GaN, and SnO2–SnO composites with p–n junctions were loaded onto PGaN surface directly applied to H2S sensor. Meanwhile, the excellent transport capability of heterojunction between FSS and PGaN facilitates electron transfer, that is, a response time as short as 65 s and a release time up to 27 s can be achieved merely at 150 °C under 50 ppm H2S concentration, which has laid a reasonable theoretical and experimental foundation for the subsequent PGaN-based heterojunction gas sensor. The lowering working temperature and high sensitivity (23.5 at 200 ppm H2S) are attributed to the structure of PGaN itself and the heterojunction between SnO2–SnO and PGaN. In addition, the as-obtained sensor showed ultra-high test stability. The simple design strategy of FSS/PGaN-based H2S sensor highlights its potential in various applications.

Opposite Sensing Response of Heterojunction Gas Sensors Based on SnO2-Cr2O3 Nanocomposites to H2 against CO and Its Selectivity Mechanism.

Metal oxide semiconductor (MOS) gas sensors show poor selectivity when exposed to mixed gases. This is a challenge in gas sensors and limits their wide applications. There is no efficient way to detect a specific gas when two homogeneous gases are concurrently exposed to sensing materials. The p-n nanojunction of xSnO2-yCr2O3 nanocomposites (NCs) are prepared and used as sensing materials (x/y shows the Sn/Cr molar ratio in the SnO2-Cr2O3 composite and is marked as SnxCry for simplicity). The gas sensing properties, crystal structure, morphology, and chemical states are characterized by employing an electrochemical workstation, an X-ray diffractometer, a transmission electron microscope, and an X-ray photoelectron spectrometer, respectively. The gas sensing results indicate that SnxCry NCs with x/y greater than 0.07 demonstrate a p-type behavior to both CO and H2, whereas the SnxCry NCs with x/y < 0.07 illustrate an n-type behavior to the aforementioned reduced gases. Interestingly, the SnxCry NCs with x/y = 0.07 show an n-type behavior to H2 but a p-type to CO. The effect of the operating temperature on the opposite sensing response of the fabricated sensors has been investigated. Most importantly, the mechanism of selectivity opposite sensing response is proposed using the aforementioned characterization techniques. This paper proposes a promising strategy to overcome the drawback of low selectivity of this type of sensor.

Tailoring the selectivity of ultralow-power heterojunction gas sensors by noble metal nanoparticle functionalization

Heterojunctions are used in solar cells and optoelectronics applications owing to their excellent electrical and structural properties. Recently, these energy-efficient systems have also been employed as sensors to distinguish between individual gases within mixtures. Through a simple and versatile functionalization approach using noble metal nanoparticles, the sensing properties of heterojunctions can be controlled at the nanoscopic scale. This work reports the nanoparticle surface functionalization of TiO2/CuO/Cu2O mixed oxide heterostructures, where the gas sensing selectivity of the material is tuned to achieve versatile sensors with ultra-low power consumption. Functionalization with Ag or AgPt-nanoclusters (5–15 nm diameter), changed the selectivity from ethanol to butanol vapour, whereas Pd-nanocluster functionalization shifts the selectivity from the alcohols to hydrogen. The fabricated sensors show excellent low power consumption below 1 nW. To gain insight into the selectivity mechanism, density functional theory (DFT) calculations have been carried out to simulate the adsorption of H2, C2H5OH and n-C4H9OH at the noble metal nanoparticle decorated ternary heterostructure interface. These calculations also show a decrease in the work function by ~2.6 eV with respect to the pristine ternary heterojunctions. This work lays the foundation for the production of a highly versatile array of sensors of ultra-low power consumption with applications for the detection of individual gases in a mixture.

Effect of mixing ratio on NO2 gas sensor response with SnO2-decorated carbon nanotube channels fabricated by one-step dielectrophoretic assembly

We fabricated nitrogen dioxide (NO2) gas sensors with p-type carbon nanotubes (CNTs) / n-type tin dioxide (SnO2) nanoparticle heterojunctions using one-step dielectrophoretic assembly and investigated the effect of CNT/SnO2 ratio on their NO2 gas detection properties. CNTs and SnO2 nanoparticles were mixed in various ratios, suspended in deionized water, and assembled by dielectrophoresis. The normalized response of fabricated CNT/SnO2 heterojunction gas sensors against 1 ppm NO2 was ∼80 in an N2 atmosphere and ∼20 in artificial air, where UV irradiation was used only for initialization. To reduce the effect of oxygen (O2), we also conducted continuous UV irradiation with various intensities during the initialization and gas detection. The CNT/SnO2 pn heterojunction gas sensor had a maximum normalized response of 19 for 1 ppm NO2 in artificial air, while that of the SnO2 sensor was 3. Furthermore, plotting the gas sensor response as a function of NO2 concentration reveals that the sensor detected an NO2 gas concentration as low as 20 ppb in artificial air.

Planar rose-like ZnO/honeycombed gallium nitride heterojunction prepared by CVD towards enhanced H2 sensing without precious metal modification

In this work, through chemical vapor deposition (CVD) method, almost indistinguishable planar rose-like ZnO (PRZ) nanostructures could be prepared on the surface of honeycombed gallium nitride (HGaN) after more than ten deposition tests, and then constructed into PRZ/HGaN heterojunction gas sensor. Subsequent gas behaviors showed that the PRZ/HGaN heterostructure constructed by CVD greatly improved the selectivity and response value (32.5 at 100 ppm) to H2, and realized the improvement of the H2 sensing performance without noble metal modification. Importantly, fast response and recovery time was 47 s and 6 s, respectively. Meanwhile, this sensor presented excellent selectivity and stability. Ultimately, the dynamic monitoring of the sensor was performed for five consecutive months, and the fluctuation ratio of the response value (sensitivity) did not exceed ±2%. It could be predicted that the as-obtained sensor was expected to promote the substantial process of industrialization of highly stable metal oxide/HGaN heterojunction sensors.

Performances of In-doped CuO-based heterojunction gas sensor

In this work, the pure and In-doped CuO nanostructure was successfully synthesized by a simple one-step hydrothermal method. X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy and X-ray photoelectric spectroscopy were employed for characterization of the structure and morphology of the as-prepared nanostructure materials. Then, the gas sensing properties of the pure and In-doped CuO nanostructure were investigated. Compared with pure CuO, the sensors based on 2 mol% In-doped CuO exhibited enhanced gas sensing and low working temperature obviously. The response to 300 ppm ethanol gas reached to 67.1 at 116 °C, which was almost 9.5 times higher than that of pure CuO. The flow and recombination of carriers at the n–p junction are the main reason for the decrease in the carrier concentration of In2O3/CuO gas sensors in reducing gas. Therefore, we believe that the change of carrier concentration and material surface caused by In doping could be responsible for the enhancement of the gas sensing properties.

MoS2 hybrid heterostructure thin film decorated with CdTe quantum dots for room temperature NO2 gas sensor

Molybdenum disulfide (MoS2) is a very promising candidate for room temperature (RT) gas sensing applications. However, the limitation of synthesis techniques, incomplete recovery, and selectivity at RT are prime drawbacks. In this report, MoS2 nanoworms (NWs) thin film and CdTe quantum dots (QDs) decorated MoS2 NWs hybrid heterostructure thin film (CdTe QDs/MoS2 NWs) have been fabricated by scalable sputtering technique on the p-Si substrate and tested for RT NO2 gas sensing applications. The proposed CdTe QDs/MoS2 NWs hybrid heterostructure thin film sensor manifests an excellent sensor response (∼40 %), fast response time = 16 s, complete recovery (recovery time = 114 s) and highly selective towards 10 ppm NO2 at room temperature in comparison to pristine MoS2 NWs thin film sensor (response ∼26 %, response/recovery time ∼23 s/incomplete recovery). This superior gas sensing performance may be attributed to the combined effect of factors such as hybrid heteronanostructure with unique morphology, catalytic activity, synergistic effects, and p-n heterojunctions. The approach employed here may lead to the development of RT operable MoS2-based heterojunction gas sensors.

Determination of the Optimal Sensing Temperature in Pt/Ta₂O₅/MoO₃ Schottky Contacted Nanobelt Straddling Heterojunction.

Nanostructured Schottky barrier gas sensors have emerged as novel semiconductor devices with large surface areas and unique electronic characteristics. Although it is widely known that operating these gas sensors requires heating to an optimal temperature for the highest sensitivity, the fundamental mechanism that governs the temperature-dependent sensitivity has yet been well understood. In this work, we present new evidence to support that thermionic field emission (TFE) is the dominant transport mechanism for Schottky contacted nanostructured heterojunction gas sensors at their optimal sensing temperature. Through the fabrication and characterization of Pt/MoO3 Schottky contacts, and Pt/Ta2O5/MoO3 heterojunctions, we found a previously unreported connection between TFE transport and optimal gas sensing temperature. This connection enables the description of Schottky barrier gas sensing performance using transport theory, which is a major step towards systematic engineering of gas sensors with nanostructured high-k oxide layers.

Enhancement of H2S sensing performance of p-CuO nanofibers by loading p-reduced graphene oxide nanosheets

Being compared to p-n heterojunctions, less attention has been paid to p-p heterojunctions for gas sensing studies. Motivated by this fact, herein, gas sensing characteristics of p-reduced graphene oxide (RGO) loaded p-CuO nanofibers (NFs) will be presented. In order to find the optimal value of RGO loading, different amounts of RGO (0.05–1.5 wt%) were added to the electrospun CuO NFs. The results of sensing tests showed that the CuO-0.5 wt% RGO has the best sensitivity to H2S at 300 °C. Also, low response to the interfering gases demonstrated the good selectivity of the optimal sensor towards H2S. Underlying sensing mechanism is discussed in details. High response of CuO-0.5 wt% RGO sensor towards H2S was related to NF morphology, presence of RGO, high intrinsic sensitivity of CuO towards H2S gas, presence of a lot of CuO nanograins along with the existence of the plenty of p-p heterojunctions, which will be changed by transformation of CuO to CuS. The results of present study can be used for design and fabrication of novel RGO containing p-p heterojunction gas sensors.

Enhanced hydrogen sensitivity of AlGaN/GaN heterojunction gas sensors by GaN‐cap layer

We fabricated Pt-functionalised hydrogen gas sensors on AlGaN/GaN heterojunction platform and investigated the influence of GaN-cap layer on the sensing characteristics. Pt-Schottky diodes with GaN-cap layer exhibited a larger change of Schottky barrier height than ones with no GaN-cap layer when hydrogen gas was detected. Technology computer-aided design simulation indicated that the increase of electron concentration at heterojunction can be magnified by a larger change of barrier height. The AlGaN/GaN FET-type sensors with Pt catalyst on the gate area demonstrated significant enhancement of hydrogen gas sensitivity from 16 to 35% at 200°C when GaN cap layer was employed.