Application of Two-Dimensional Materials towards CMOS-Integrated Gas Sensors

Abstract

During the last few decades, the microelectronics industry has actively been investigating the potential for the functional integration of semiconductor-based devices beyond digital logic and memory, which includes RF and analog circuits, biochips, and sensors, on the same chip. In the case of gas sensor integration, it is necessary that future devices can be manufactured using a fabrication technology which is also compatible with the processes applied to digital logic transistors. This will likely involve adopting the mature complementary metal oxide semiconductor (CMOS) fabrication technique or a technique which is compatible with CMOS due to the inherent low costs, scalability, and potential for mass production that this technology provides. While chemiresistive semiconductor metal oxide (SMO) gas sensors have been the principal semiconductor-based gas sensor technology investigated in the past, resulting in their eventual commercialization, they need high-temperature operation to provide sufficient energies for the surface chemical reactions essential for the molecular detection of gases in the ambient. Therefore, the integration of a microheater in a MEMS structure is a requirement, which can be quite complex. This is, therefore, undesirable and room temperature, or at least near-room temperature, solutions are readily being investigated and sought after. Room-temperature SMO operation has been achieved using UV illumination, but this further complicates CMOS integration. Recent studies suggest that two-dimensional (2D) materials may offer a solution to this problem since they have a high likelihood for integration with sophisticated CMOS fabrication while also providing a high sensitivity towards a plethora of gases of interest, even at room temperature. This review discusses many types of promising 2D materials which show high potential for integration as channel materials for digital logic field effect transistors (FETs) as well as chemiresistive and FET-based sensing films, due to the presence of a sufficiently wide band gap. This excludes graphene from this review, while recent achievements in gas sensing with graphene oxide, reduced graphene oxide, transition metal dichalcogenides (TMDs), phosphorene, and MXenes are examined.

Keywords: 2D materials; CMOS integration; MXenes; VOCs; gas sensing; graphene oxide; molybdenum disulfide; nitrogen dioxide; phosphorene; transition metal dichalcogenides (TMDs).

Figure 1

Industries of relevance for the gas sensor market and their global financial shares by end-use in 2021. (source: https://www.grandviewresearch.com, accessed on 26 September 2022).

Figure 2

United States gas sensors market share by technology in 2019. (source: https://www.gminsights.com, accessed on 26 September 2022).

Figure 3

Typical schematic of a single SMO sensor with interface blocks. The sensor requires a heating element, a voltage follower, and an analog-to-digital converter (ADC). In order to analyze the obtained data, it is passed to a microcontroller, which is enabled with a read-only memory (ROM), random access memory (RAM), and input/output (I/O) interfaces.

Figure 4

Cross-section schematic of the layers composing the membrane of the hotplate.

Figure 5

Microheater geometries characterized and modeled over the last decades. The shapes depicted are: (a) Meander, (b) S-meander, (c) Curved, (d) S-curved, (e) Double spiral, (f) Square double spiral, (g) Drive wheel, (h) Elliptical, (i) Circular, (j) Plane plate, (k) Fin shape, (l) Honeycomb, (m) Loop shape, (n) Irregular.

Figure 6

Gas sensing and resulting band bending for a granular SMO film where (a) oxygen adsorbs onto the surface, creating a depletion region; (b) the adsorbed oxygen reacts with CO, reducing the amount of band bending; and (c) CO adsorbs directly on the surface without the presence of oxygen, forming an accumulation region.

Figure 7

Visual representation of semiconducting films that are being investigated for their potential in gas sensing devices. (Reprinted with permission from Nikolić et al. [20], CC-BY 4.0).

Figure 8

Adsorption spectra in the mid-infrared range of several molecules, including the pollutants from Table 1, and their intensities. (Reprinted with permission from Popa and Udrea [81], CC-BY 4.0).

Figure 9

Schematic of a light-activated gas sensor to achieve low power consumption and fast photodesorption after a sensing cycle. (Reprinted with permission from Suh et al. [105], CC-BY 3.0).

Figure 10

Schematic representation of gas sensors using (a) a chemiresistive configuration and (b) a back-gated FET configuration. (Reprinted with permission from Cao et al. [109], CC-BY 4.0).

Figure 11

Schematic of different types of gas sensors which use the field effect of a FET for transduction. (Reprinted with permission from Hong et al. [118]).

Figure 12

(a) Schematic of the CVD setup for the growth of ML MoS₂. (b) Shows the optical images of the grown ML at different size scales. (c,d) show the photoluminescence and Raman intensity maps, respectively, of the MoS₂ ML, grown on a SiO₂/Si substrate. (Reprinted (adapted) with permission from Shi et al. [166], CC-BY 4.0).

Figure 13

Magnetron sputter schematic of MoS₂ deposition on carbon nano-powders. (Reprinted (adapted) with permission from Rowley-Neale et al. [180]; further permissions related to this material excerpted should be directed to the ACS).

Figure 14

Schematic of the MBE setup for growing complex oxides. Metals evaporating from effusion cells are oxidized, exposing them to ozone, molecular oxygen, or atomic oxygen generated by an RF plasma source. (Reprinted (adapted) with permission from Engel-Herbert [186]).

Figure 15

Schematic of the area-selective ALD process of MoS₂ showing the impact of the MoCl₅ pulse duration on the deposition of MoS₂ on different surfaces (SiO₂ and Al). (Reprinted with permission from Ahn et al. [198]).

Figure 16

Charge density difference plots for adsorption of (a) CO, (b) NO, and (c) NO₂ on ML MoS₂. The red (green) distributions correspond to charge accumulation (depletion). (Reprinted with permission from Yue et al. [200], CC-BY 2.0).

Figure 17

Calculated band structures and corresponding PDOS of (a) pristine ML MoS₂ and (b) NO-adsorbed ML MoS₂. In (c), the corresponding PDOS is provided. The Fermi energy is set at 0 eV. (Reprinted (adapted) with permission from Wang et al. [201], CC-BY 4.0).

Figure 18

Number of published manuscripts indexed by Scopus since 2019 with “Graphene Oxide”/“rGO”, “MoS2”/“Molybdenum Disulfide”, “WS2”/“Tungsten Disulfide”, “WSe2”/“Tungsten Diselenide”, “MoSe2”/“Molybdenum Diselenide”, “phosphorene”/“Exfoliated phosphorus”, and “MXene” in the title. (Source: Scopus, 28 September 2022).

Figure 19

Detection of humidity with a piezoelectric micromachined ultrasonic transducer with a GO layer is shown. (a) The shift in frequency as a result of varying relative ambient humidity from 10% to 90% is shown. (b) The results of the relative humidity response from (a) fit nicely to an exponential equation. (Reprinted with permission from Sun et al. [226], CC-BY 4.0).

Figure 20

NO₂ detection using a paper-based rGO–chitosan composite sensor from [236]. (a) Current–voltage curves of the sensor; (b) Relative response according to NO₂ concentration; (c) Normalized specificity of the sensor towards NO₂ compared to NH₃, CO, Ethanol (EtOH), and Acetone (ACO). (Reprinted (adapted) with permission from Park et al. [236]).

Figure 21

(a) Sensing response for the differently-reduced hybrid rGO–graphene sensing films, when exposed to three different concentrations of ammonia. (b) A study on the selectivity of the rGO–graphene hybrid sensor towards ammonia, when compared to isopropanol, formaldehyde, and ethanol at 10 ppm. (Reprinted with permission from Wang et al. [245], CC-BY 4.0).

Figure 22

The relative resistance (R/R₀) in various concentrations of 400–1000 ppb NO gas using (a) pristine rGO and (b) N-doped rGO. In (c), the response to other VOCs is given for pristine and N-doped (N-rGO) rGO, including NH₃, isopropanol (IPA), ethanol (EtOH), and methanol (MeOH). (Reprinted with permission from Chang et al. [255], CC-BY 4.0).

Figure 23

CVD-fabricated monolayer MoS₂ FETs. (a) Image of the monolayer MoS₂-based FET. (b) Single MoS₂ flake with gold contacts on either side. (c) Raman spectrum of the fabricated film. (d) Typical 3D representation of a back-gated monolayer MoS₂ FET on a SiO₂ insulator. The substrate below the insulator is usually heavily doped to ensure sufficient control of the channel from the back gate. (Reprinted with permission from Ahn et al. [268], CC-BY 4.0).

Figure 24

(a) Three-dimensional representation of a ML MoS₂ resistor, while being exposed to a triethylamine (TEA) molecule. The sensing film is deposited on a SiO₂-on-Si wafer and is electrically contacted with Au pads. (b) Relative change in the conductivity of the ML film from (a), and CNT for comparison, after exposure to several molecules, including triethylamine (TEA), tetrahydrofuran (THF), acetone, methanol, nitrotoluene (NT), 1,5-dichloropentane (DCP), and 1,4-dichlorobenzene (DCB). (Reprinted with permission from (a) Perkins et al. [288] and (b) Wang et al. [245], CC-BY 4.0).

Figure 25

(a) Gas sensing device presented by Burman et al. [291]; (b) the response of samples with varying Au-doping concentrations in 400 ppm NH₃ at 90 °C; (c) the stability of the sensors for two months under different NH₃ concentrations; and (d) the selectivity of undoped, Au-doped, and Pt-doped MoS₂ towards several VOCs. (Reprinted (adapted) with permission from Burman et al. [291]).

Figure 26

The relaxed atomic structures of different polymorphs of phosphorene, including (a) Black, (b) Green, and (c) Blue. (d) The magnification of atomic structure of Black and Blue Phosphorene, indicating that Green Phosphorene is derived from the mixture of the black and the blue phases. (Reprinted (adapted) with permission from Kaewmaraya et al. [314]).

Figure 27

Change in the resistance of several 2D materials (phosphorene, MoS₂, and rGO) when exposed to various ambient environments, showing an increased selectivity of phosphorene and MoS₂ towards NO₂. (Reprinted with permission from Cho et al. [322]).

Figure 28

(a) Schematic of the CO gas sensing mechanism in the suspended and supported black phosphorene configurations. (b) The measured sensitivity in terms of $\mathsf{\mu }$DR/R_N2 = |R_gas− R_N2|/R_N2 at each gas concentration and their Langmuir isotherm fitting curves. (Reprinted with permission from Lee et al. [323]).

Figure 29

(a) Atomic configurations of Au-, Ag-, and Pt-doped phosphorene (with two P atoms substituted for one metal atom) with an NO gas molecule adsorbed. Corresponding PDOS are shown in (b). Balls in purple and red represent N and O atoms, respectively. The dashed line in the PDOS illustrates the Fermi level. (Reprinted with permission from Yang et al. [325]).

Figure 30

The optimized structures of vacancy defected black phosphorene MLs in their (a) armchair and (b) zigzag directions. The above row represents the top views and below row shows the side views of defected structures. The white and orange balls demonstrate H and P atoms, respectively. The current-voltage (IV) response for (c) pristine and (d) vacancy-defected phosphorene in the zigzag direction when exposed to various ambient gases. (Reprinted (adapted) with permission from Meshginqalam et al. [327]).

Figure 31

Working temperature versus gas sensing response for (a) glass/Mo₂CT_x, (b) cSi/Mo₂CT_x, and (c) pSi/Mo₂CT_x sensors for different CO₂ concentrations. (Reprinted with permission from Thomas et al. [353]).

Figure 32

Gas sensing properties of a V₂CT_x sensor at room temperature. (a) The compiled resistance variation and (b) gas response toward 100 ppm hydrogen, ethanol, acetone, methane, ammonia, and H₂S. Real-time sensing response of V₂CT_x gas sensors at varying concentrations of (c) hydrogen, (d) acetone, (e) methane, and (f) H₂S. (Reprinted with permission from Lee et al. [352]).

Figure 33

Measured gas response of Ti₃C₂T_x sensors compared with sensors based on other 2D materials, such as black phosphorene (BP), MoS₂, and rGO. (a) Real-time gas response behavior of these sensors to 100 ppm of target gases. (b) Maximum resistance change after exposure to 10 ppm acetone, ethanol, ammonia, propanal, NO₂, SO₂, and 10,000 ppm CO₂. Inset to the right displays a magnified scale to be able to see Ti₃C₂T_x and rGO. (Reprinted (adapted) with permission from Kim et al. [362]; further permissions related to this material excerpted should be directed to the ACS).

Figure 34

(a) Gas sensing FET based on a vdW heterojunction consisting of graphene and MoS₂. (b) Sensor response to a 1 ppm exposure to NO₂ when operating in reverse-bias conditions. (Reprinted with permission from Tabata et al. [374]).

Figure 35

Schematic structures and band diagrams of a WO_x-decorated MoS₂ heterojunction (a) before and (b) after UV exposure and (c) NO₂ exposure. (Reprinted with permission from Zheng et al. [375]).