Two-Dimensional Layered Amorphous Metal Oxide Gas Sensors (LAMOS) Perspectives and Gas Sensing Properties ^†

Abstract

Two-dimensional Layered Amorphous Metal Oxide Sensors (LAMOS) represent a new class of 2D amorphous oxide (a-MOx) interfaces with unveiled properties in gas sensing applications. Herein, we report the humidity and gas sensing response of p- and n-type chemoresistive few-layered (2D) amorphous a-SnO₂, a-In₂O₃, and a-Cr₂O₃, discussing their reaction mechanisms using DFT modelling and electrical tests. LAMOS interfaces can be easily prepared by controlled oxidation in air of a large class of exfoliated 2D TMDs, MCs, and TMTH (Transition Metal Dichalcogenides, Chalcogenides, and Trihalides) like WS₂, MoS₂, SnSe₂, In₂Se₃, NiCl₂, and CrCl₃, yielding 2D amorphous a-MOx interfaces. LAMOS platforms preserving all the surface-to-volume advantages of their 2D precursors show excellent gas sensing properties representing a new class of material for gas sensing applications.

1. Introduction

The intrinsic thermodynamic instability (ΔG < 0) of 2D exfoliated TMDs/MCs/TMTHs (Transition Metal Dichalcogenides/Metal Chalcogenides/Transition Metal Trihalides), demonstrated by their spontaneous oxidation in dry/wet air laboratory conditions, represents a great opportunity to develop, via suitable thermal treatment, template-self-assembled, amorphous-metal-oxide (a-MOx) skin layers over crystalline 2D exfoliated TMDs/MCs/TMTH.

Departing from liquid-phase exfoliated TMDs/MCs/TMTHs, annealing in air at temperatures below the crystallization temperature of the native oxide, either amorphous/crystalline 2D heterostructures of a-MO/TMDs [1,2], or fully oxidized amorphous 2D a-MOx interfaces can be prepared [3] with unexploited surface properties.

Herein, we demonstrate that the oxidation/amorphization process can be extended to a large variety of exfoliated TMDs (WS₂), MCs (SnSe₂), and TMTH (CrCl₃) where sulfur, selenium, or chlorine atoms can be easily displaced by O₂ atoms under controlled oxidation conditions, producing 2D layered n-type a-WO₃, a-SnO₂, and p-type a-Cr₂O₃ 2D flakes spin-coated as thin films, with excellent sensing properties to H₂, NH₃, H₂S, and NO₂, and long-term stability properties. This research opens new perspectives for a novel generation of layered interfaces (LAMOS), exploiting new interaction mechanisms of these van der Waals amorphous semiconductor interfaces with the environment.

2. Materials and Methods

Liquid-phase exfoliated commercial SnSe2, WS2, and CrCl3 powders were annealed in air at different temperatures (180 °C–300 °C) and times (24–70 h), and spin-coated over interdigital electrodes provided with platinum electrodes and a back side heater. Platforms have been tested to sub ppm H2, NH3, H2S, NO2 gases and humidity at a 100 °C operating temperature.

3. Discussion

Figure 1a shows the SEM picture of a spin-coated thin film over interdigitated electrodes (light regions) of 2D a-SnO2 flakes of around 300 nm lateral size (Figure 1b,c), with a vertical height of approximately 20 nm, forming localized inter-sheet junctions between the flakes. Figure 1d shows the HRTEM of exfoliated flakes of 2D SnSe2 exhibiting a fully crystalline and ordered 2D texture extending up to the edge of the flake (see electronic magnification, Figure 1d). After annealing, the 2D SnSe2 is transformed in a-SnO2, whose amorphous structure is shown in Figure 1e. Grazing incidence XRD and XPS analysis of the annealed SnSe2 flakes confirms the formation of the fully amorphous a-SnO2 layer with a chemical composition matching that of SnO2.

Considering humid air as a natural background in practical gas sensing applications, we preliminary applied combined density function theory and ab initio molecular dynamics, demonstrating that a dissociative water mechanism occurs over a-MOx surfaces, leading to the formation of chemisorbed hydroxyls, as shown in Figure 2a. Experiments that aimed to investigate the humidity cross-response on NO2 and H2 sensing highlighted that increasing the relative humidity increases the degree of hydroxylation, resulting in an increase/decrease in the sensor signal response (i.e., Rg/Ra or Ra/Rg) to 1 ppm NO2 and 100 ppm H2, as shown in Figure 2b,c, respectively.

Adsorption/desorption mechanisms of water and gases over amorphous interfaces (a-MOx), investigated via theory and experiments, resulted in being congruent with those of crystalline metal oxides. Long-term stability properties of the electrical response to humidity and different gases, over a period of one year, exhibit no remarkable fluctuations in the base line resistance (BLR) or the sensor’s signal response (i.e., RRs), demonstrating that the amorphization/oxidation strategy effectively passivates the material from further degradation, while preserving an excellent gas sensing response.