Unusual and Instantaneous Overshoot of Response Transients in Gas Sensors: Fact or Artifact?

Today, a large fraction of gas sensor industry has been monopolised by Metal Oxide Semiconductor (MOS) based chemoresistive gas sensors because of their simplity, ease of fabrication, reproducible response and thermal stablilty. In these type of sensors, the sensor resistance changes upon exposure to the analyte gas and attains a saturation. While, the resistance drops to the baseline when the test gas is removed. This happens because of charge transfer from MOS to the surface state near the fermi level of the semiconductor and vice versa. This reversible reaction is technologically utilized in detecting such hazrdous and toxic gases.

In our recent work, (Analysis of Unusual and Instantaneous Overshoot of Response Transients in Gas Sensors”; Sreya Suresh, Kusuma Urs., Anupama T. Vasudevan, Sharath Sriram, Vinayak Kamble*, Physica Status Solidi : Rapid Research Letters) it is shown that, the sensor response transients exhibit unexpected overshoot for an n-type MOS for hydrogen gas. The work systematically explores the role of diffusion of the test gas into the sensor material, ie. Zinc Oxide microrods. Often such overshoots, though weak in nature, are encountered in many reports. However, no efforts are made to study or explain their origin in the sensor response. This is perhaps the first of its kind experimental study revealing the mechanism of overshoot in response of gas sensors.

The sensor researcher community is moving towards smaller structures for enhanced sensitivities, but here the authors have found very high sensitivity (about 1000% change in current) towards typically few hundred ppm of hydrogen gas. This has been attributed to single crystal like nature which is devoid of barriers of grain boundaries in the ZnO microrods. The observed unusual overshoot behavior has been attributed to the sub-surface diffusion of atomic hydrogen with a typical coefficient of diffusivity, 𝐷 being 10−15 𝑚2/ 𝑠 at 250 oC, and its reaction with lattice oxygen defects in subsurface of ZnO crystal leading to trapping of induced charge carriers.

Oxide Photonic Crystals

When light interacts with material structures, certain wavelengths can get diffracted with respect to the periodicity in the refractive index felt by incident light, like X-Ray diffraction from atomic crystals. This is because the wavelength of X-Rays and the periodicity of atomic arrangement in crystals are of the same length scales. So for larger wavelengths to be diffracted from materials, the length scale in which the dielectric function suffers a spatial variation should be comparable with the wavelength. The diffracted waves cannot pass through the material. This range of forbidden wavelengths are called Photonic Band Gap (PBG). These kinds of materials or structures with a PBG are known as Photonic crystals or Photonic band gap materials.

The band gap frequency range depends on the periodicity modulation of refractive index, the angle of incidence of light and the refractive index contrast. The refractive index contrast is defined as the ratio of higher refractive index to lower refractive index, ƞ high / ƞ low, which accounts for the dispersion strength of the two dielectric components of the photonic crystal. For a periodic dielectric structure, the wave vectors associated with the photonic band gap frequencies will have imaginary part and therefore the wave will suffer a strong attenuation; they will not propagate through.

The transition metal oxides are projected as potential material for photonic crystal application, hence we intend to make the inverse opal structures of oxides using polystyrene(PS) and silica microspheres arranges in a periodic fashion as a template. Thus, efforts was made to fabricate perfect periodic arrangements of PS and silica microspheres.

Themoelectric materials

The energy requirements have increased manifold with time. Conventional energy has failed to meet requirements due to the increase in energy consumption. The aforementioned energy crisis has led the research in the direction of the alternative unconventional energy sources. In this scenario, phenomenon of thermoelectricity has been the hot topic of research in these days owing to its ability to convert waste thermal energy into an electrical energy.

The efficiency of a thermoelectric device is represented by figure of merit (zT), which is defined by equation (1)

Figure of merit (zT) = S^2 σT / κ

where S is the thermopower (also known as Seebeck coefficient); σ is the electrical conductivity; κ is the total thermal conductivity and T is the absolute temperature.

The materials such as intermetallic alloys, clatharates, chalcogenides, skutterudites, etc., have been the primary choice for thermoelectric applications. The oxides in the past were not considered as suitable materials for thermoelectric applications due to their low carrier mobility and strong ionic character between cation and anions. However, soon after the first report of thermoelectricity by Terasaki et al on NaCo2O4 single crystal in 1997, there has been considerable recent interest in different oxides for the exploration of thermoelectric properties due to their observed reasonable figure of merit.

Although NaCo2O4 is metallic, quite large thermopower (100 μV K−1 at 300 K) is observed due to charge and different spins of cobalt ions.

Most importantly, the oxides precede over the other class of materials due to their high thermal stability, non-toxicity and are known to be cost-effective as well as energy-efficient synthesis methods though having a low thermoelectric figure of merit. The oxides, i.e., transition metal oxides are strongly correlated systems, where both the spin as well as the orbital degrees of freedom of charge carriers contribute towards the thermopower (S). Particularly, in case of cobaltate system, the spin states (i.e., low spin (LS), high spin (HS) and intermediate spin (IS)), play a crucial role in determining the thermopower (S), which is generally dependent on the temperature. The temperature may trigger a transition in such a system from LS to HS through an intermediate spin state, but still there is ambiguity regarding the different spin states with temperature.

We are currently involved in study of

1. Oxide thermoelectrics where our candidate systems are doped Zinc oxide and rare earth cobaltates having double perovskite structure.

2. Intermetallics such as tellurites are classic thermoelectric materials with excellent thermoelectric porperties i.e. high thermoelectric power conversion efficiencies and hence are system of interest. Moreover, recent advances in the field are enehancement in the thermopower of the system by exploiting the topological insulating nature of these alloys.

3. half Heusler compositions are robust systems for high temperature thermoelectric applications due to their high electrical conductivity and increased phonon scattering rates due to alloy scattering and interface tuning.

Metal Oxide Semiconductors

Metal Oxide materials are fascinating systems showing wide variety of crystal structures and thereby variety of elecronic properties which can be exploited for wide range of applications including electronic, magnetic, optical and combinations thereof.

Particularly the wide bandgap oxide semiconductors are versatile class of materials exhibiting fascinating physics as well as chemistry. Moreover nanostructurization opens a whole new scope for study and device application of these materials.

I am keen to study the semiconductor oxide transport properties arsing due to structurals pecularities of these compunds, specifically the point defects which gives rise to semiconducting nature. Moreover, the surfaces of nanostructures and thin films are a rich source of such point defectsdue to lattice periodicity inturruption.

Wide bandgap metal oxide semiconductors have been extensively explored for their use in technological applications such as solar cells, catalysis, variastors, supercapacitors and gas sensors etc. Most of the properties exploited for the device applications of these materials are usually bestowed by the structural defects and lattice imperfections. The thermodynamic stability of the crystal demands the formation of structural defects at high temperatures. The type of defects in

metal oxide systems are predominantly point defects i. e. defect associated to one lattice point, such as cation or oxygen ion vacancy. In addition, the narrowing of the bandgap by the introduction of the defects in metal oxide semiconductors opens up the possibility of their use in the visible spectrum, which is presently limited to the ultra-violet (UV) spectrum. This introduction of defects in the materials results in improved properties like visible photocatalytic activity by enhanced utilization of visible solar spectrum and higher photoluminescence in visible region. Further, it also eliminates the use of harmful and expensive UV radiation. Defects can be introduced in metal oxide crystals by doping or by varying the oxygen stoichiometry, which leads to the modification in the electronic band structure of the material.

We synthesize different oxide nanostructures and thin films and study their morphological as well as electronic properties to establish the structure property correlations. This further enables better understanding of underlying physics of perticular technological device application of these compounds such as chemical sensors, magnetic semiconductors and themoelectrics.

Defects induced magnetism in Oxide semiconductors

The existence of room temperature ferromagnetism (RTFM) along with its intrinsic semiconducting nature has envisioned wide bandgap non-transition metal oxides as strong and potential candidates for future spin coupled optoelectronic devices. The ferromagnetism observed in undoped non-transition metal oxides such as ZnO, TiO2, and HfO2 (i.e., oxides in which d orbital of metal ion are not partially filled) is termed as “d0 ferromagnetism.”The Curie temperature (TC) of these oxide materials is fairly above 300 K, which makes them better candidates in spintronics applications than III-V and II-VI compounds based dilute magnetic semiconductors (DMS) having Tc values lower than 300K (except GaN). However, the origin of this unexpected existence of RTFM in undoped oxides has been a topic of debate among various researchers worldwide from the fundamental aspect. Nevertheless, the absence of ferromagnetism in the same materials synthesised by different processing methods emphasises that this observed RTFM is strongly dependent on processing methods employed. This inspired few researchers to attribute these unforeseen ferromagnetism to possible contamination due to ineffective handling or use of stainless steel tweezers. However, the systematic dependence of RTFM in oxides on specific processing methods and observed similar degrees of magnetisation values in spite of variable probability of contamination in all the reports underlines the fact that the observed magnetism is driven by an intrinsic defect phenomenon in undoped oxides, which is predominantly contingent upon synthesis conditions.

The RTFM observed in nanostructured non-transition metal oxides, are involving high temperature or relatively anaerobic conditions during synthesis and primarily ascribe the existence of RTFM to the nano-regime dimensions and various defects present. These defects are believed to be preponderantly oxygen vacancies in n-type metal oxide semiconductors, many of which are on the surface due to high surface to volume ratio in nanostructured materials. A large number of studies on metal oxides reveal that these oxides in thin film, nanowires, and nanoparticles form exhibit RTFM even in undoped conditions due to the presence of oxygen vacancies, which act as surface trap states. However, due to lack of detailed understanding about RTFM in undoped and transition metal doped oxides, the origin remains controversial and needs additional characterizations and study for further understanding of the phenomenon.

Metal Oxide Semiconductor Gas Sensors

The MOS based sensors are used in detections of several hazardous gases including CO, NOx, SOx, NH3 and VOCs etc. Volatile Organic Compounds (VOCs) are relatively mild and very common atmospheric pollutants yet cause prolonged adverse effects on humans as well as rest of the ecosystem. VOC sensors find application in food, perfume, medicine and chemical industries. An empirical model usually used to explain the fundamental gas sensing mechanism is as follows. According to this model, a depletion layer is created at the interfaces of the sensor (oxide) as a result of adsorbed oxygen from air. Further, reducing gases are adsorbed by trapping some of the charge carriers (conduction electron). The charge transfer between a surface adsorbed gaseous species and the oxide surface atoms leads to change in carrier concentration of the oxide and widening or narrowing of the depletion layer. This phenomenon affects the current flow through the oxide matrix and further results in change in resistance of the oxide. However, other than temperature and concentration of the test gas, largely the change in resistance is determined by the interaction between the surface adsorbing species and the oxide surface. e. g. the nature of adsorbing gas molecules (oxidizing or reducing), type of functional group present, the nature of preadsorbed oxygen species I. e. molecular (O2−) or atomic oxide (O−) ions. Here it is important to note that interaction between the MO surface and the test gas involves exchange of charge carriers and hence is highly affected by polarity and electron affinities of the analyte. ![endif]–

NO2 (Oxidizing Gas) Ethanol (Reducing Gas)

The gas sensing studies of both the undoped and doped as deposited films are undertaken, which demonstrates the characteristic p-type behaviour of chromium oxide films. The film resistance decreases upon exposure to oxidising gas like NO2 and increases for the reducing gas like ethanol vapours

The p-type oxides like NiO, Cr2O3 and Co3O4 are the potential alternative materials to existing dominating n-type MO semiconductors. Fundamentally the p-type MO sensors are different from those n-type, as the surface oxygen adsorption causes formation of a surface depletion layer, which is conducting as compared to the resistive core and vice versa in n-type MOs. Most of the p-type MO semiconductors are good catalysts and exhibit selective oxidation of particular VOCs and hence, can show essential characteristic of selectivity towards certain Volatile Organic Compounds (VOCs), compared to other gases. However, the major limitation of p-type MOs is the lesser response as compared to its n-type counterparts.

Schematics of band diagrams of the Cr2O3 p-type semiconductor showing the corresponding mechanism and possible surface reaction (cartoon shown inset) of ethanol on the Cr2O3 surface involving the charge transfer leading to either the regeneration of an ethanol molecule, or the catalytic oxidation of ethanol to acetaldehyde.