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n-SnO2 porous thin film heterojunction based NO2 sensor with high selectivity and fast response
Journal Pre-proof Sputter deposited p-NiO/n-SnO2 porous thin film heterojunction based NO2 sensor with high selectivity and fast response Kiruba Mangalam S., Ann Susan Jose, Prajwal K., Prasanta Chowdhury, Harish C. Barshilia
PII:
S0925-4005(20)30177-5
DOI:
https://doi.org/10.1016/j.snb.2020.127830
Reference:
SNB 127830
To appear in:
Sensors and Actuators: B. Chemical
Received Date:
5 November 2019
Revised Date:
22 January 2020
Accepted Date:
4 February 2020
Please cite this article as: S. KM, Jose AS, K. P, Chowdhury P, Barshilia HC, Sputter deposited p-NiO/n-SnO2 porous thin film heterojunction based NO2 sensor with high selectivity and fast response, Sensors and Actuators: B. Chemical (2020), doi: https://doi.org/10.1016/j.snb.2020.127830
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Sputter deposited p-NiO/n-SnO2 porous thin film heterojunction based NO2 sensor with high selectivity and fast response Kiruba Mangalam S., Ann Susan Jose, Prajwal K., Prasanta Chowdhury, Harish C. Barshilia* Nanomaterials Research Laboratory, Surface Engineering Division, CSIR-National Aerospace Laboratories, Bangalore-560017, India
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Email: [email protected]; Phone: +91 80 2508 6248
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Highlights
NiO/SnO2 bilayer heterojunctions were deposited by pulsed DC magnetron sputtering.
NO2 sensing behavior of the single layer and the bilayer devices compared.
Heterojunction showed enhanced sensor performance than individual layers.
In-plane
and
across-plane
heterojunctions
showed
similar
response
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characteristics.
Abstract
In this work, we report fabrication and characterization of porous, single layer n-SnO2,
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p-NiO and bilayer thin film heterojunction devices developed using pulsed DC magnetron
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sputtering for NO2 detection. Template-free, NiO/SnO2 heterojunction devices were deposited both in top-bottom and in-plane electrode configurations. All the devices showed an optimum
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sensing temperature of 200 °C. Systematic comparison of the fabricated devices revealed that the heterojunction devices with top-bottom electrodes improved performance. Electrical
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characterization confirmed the formation of heterojunction across the interface. The response values of the heterojunction sensor ranged from 57 to 144 % for the NO2 concentration range
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of 2 to 10 ppm. The heterojunction device showed high selectivity against CO and NH3 with selectivity coefficients of 90 and 26, respectively. The heterojunction sensor exhibited fast
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response and recovery times of 37 and 98 s, respectively. The device showed excellent stability with < 2% variation in response for 10 cycles of transient response characteristics. The I-V characteristics of heterojunctions with top-bottom and in-plane electrodes were explained by an equivalent circuit model. The observed enhancement in various parameters can be ascribed to the formation of distributed p-n nano-heterojunctions across the interface. The developed
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NiO/SnO2 heterojunction sensor by the simple and reproducible sputtering technique is found to be a promising candidate for the detection of NO2.
Keywords: Heterojunction; NO2 sensor; Thin film; Bilayer; NiO; SnO2
1. Introduction
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Gas sensors find applications in numerous industries like automobile, chemical, textile, medical, combustion, food and aerospace. Lately, the gas sensors are becoming an indispensable component in the Internet of Things for applications pertaining to home safety
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(LPG, CO) and air quality monitoring (NO2, CO, SO2). Nitrogen dioxide (NO2), the second largest automobile pollutant [1] has detrimental effects to humans and the environment [2].
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The current applications demand the sensors to be versatile, work at low temperature, less
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power consuming, highly selective, robust and economical. Recent studies have shown that an enhancement in the performance of metal oxide gas sensors can be achieved when
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mixed/composite metal oxides are used [3]. When a physical contact is established between two electronically distinct materials, a junction barrier is formed along the interface owing to
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the differences in bandgap, Fermi energy and charge carrier type [4]. The barrier voltage across the junction will vary in the presence of an analyte gas which in turn modulates the current
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exponentially, thereby increasing the sensitivity [5]. Moreover, presence of a catalytically active material in the heterostructure can enhance the selectivity. Most of the published reports focus on developing highly dispersed 1D heterostructures
like nanorods [6], nanofibers [7], core-shell nanocables [8], core-shell spindles [9], porous strips [10], etc. because of their high surface area. However, in these kinds of heterostructures, the p-n junction is randomly distributed, and the junction is not well defined, which makes it 3
difficult to study the junction properties. Controlling the dispersion properties and stoichiometry in these mixed composites is critical and can be challenging. This can impact the commercial-scale sensor fabrication. Well-defined interfaces can be formed by stacking layers of p and n-type thin films. This type of 2D interface have high surface to volume ratio [11] compared to the bulk pellets and allows proper characterization of the junction for understanding the mechanism. Moreover, precise control over stoichiometry, morphology, film thickness and the degree of contact can be achieved by controlling the deposition parameters.
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This makes them highly reproducible even on a commercial scale. However, bi/multilayer films have limited gas accessibility to the bottom layer and the interface.
Very few attempts like glancing incidence deposition [5], micro-mask assisted
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deposition of ultra-thin p-type on n-type films [12], use of AAO templates [13] and PSS monolayer templates [14] have been made to enhance the gas accessibility in bilayer junction
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devices.
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The heterojunction devices are fabricated using different combinations of n-type (SnO2, TiO2, ZnO, WO3, In2O3) and p-type metal oxides (NiO, CuO, Cr2O3, V2O5). Tin oxide (SnO2), a wide band gap n-type material, is known to be the best sensing material for gas sensors, but
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lacks selectivity [15]. On the other hand, nickel oxide (NiO) is the most studied p-type gas sensing material in which selectivity can be tuned by manipulating its microstructural and
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catalytic properties [16]. SnO2/NiO combination as 1D heterostructure has been tested towards
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different gases like: CO [17], C2H5OH [18] , H2 [19], CH4 [20], H2S [21], acetone [22], TEA [23] and HCHO [24], n-butanol [25], etc. Few reports are available on SnO2-NiO towards NO2 sensing which are dispersed heterostructures like nanoneedles [26], nanowebs [27] and microspheres [28, 29]. Wang et al. [30] have reported sputtered thin film from SnO2:NiO (99:1) target on Au nanoparticle arrays. But pristine and annealed composite films were not sensitive. Kwon et al. [5] have deposited SnO2/NiO bilayer thin films. But they have mainly focussed on
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technology computer-aided design (TCAD) simulations of the heterojunction and got unusually similar responses for both oxidizing and reducing gases. Since most of the reports on heterojunctions are in 0/1D heterostructures where, one material is often decorated on the other, it is not feasible to systematically compare sensing properties of both the materials to the heterojunction. To the best of our knowledge, no systematic studies have been done by comparing the sensor performance of individual NiO and SnO2 layers with that of templatefree, porous, puled-DC magnetron sputtered NiO/ SnO2 thin film heterojunction towards NO2
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sensing. In this paper, we present NO2 sensing studies of sputtered, p-NiO and n-SnO2 thin films. The thin films were subjected to structural, morphological, optical and electrical
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characterizations. We have analysed their strengths and shortcomings in terms of sensitivity, selectivity, stability and reproducibility. Then, we have developed template-free, porous
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NiO/SnO2 thin film heterojunction devices. The performance of the heterojunction device as a
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function of NO2 concentration, temperature, interfering gases like CO and NH3 and cyclic response were studied. Electrical characterization was also performed to confirm the formation of heterojunction. The results were compared with their single layer counterparts. To rule out
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the role of differences in working conditions, we have done simultaneous sensing measurements using a specially designed 6-probe contact with gold-coated, spring-loaded pins
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connected to a multichannel data acquisition system.
We have also compared the
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heterojunction devices with in-plane and top-bottom electrode configurations.
2. Experimental details 2.1 Sensor fabrication Thin films for sensor application were deposited on borosilicate glass substrates after thorough cleaning as given in the Supplementary Information (S. I.). Thin films of SnO2 and NiO were deposited using the pulsed-DC magnetron reactive sputtering from Ni and Sn targets.
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All the films were deposited at room temperature. The base pressure of the sputtering chamber was kept below 6.0x10-6 mbar. Initially, films were deposited under different Ar: O2 ratios and working pressures. Optimum deposition parameters were chosen by weighing their individual NO2 sensitivity. The sputtering power used for NiO and SnO2 films were 60 and 100 W, respectively. The deposition pressure of the chamber was maintained high at 9.3x10-3 mbar to get porous oxide films. Deposited samples were then air annealed (Carbolite Gero) at 350 °C for 3 hrs with a ramp rate of 10 °C/min.
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Devices were fabricated in different configurations to study the individual layers and the heterojunctions. The stack profiles of the deposited sensors are shown in Fig. 1. Desired patterns were achieved by masking with cleaned microslides. The electrodes were deposited
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by sputtering 20/200 nm of Ti/Au in the heterojunction samples. 2.2 Sensor characterization
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Deposited samples were subjected to various characterizations and the details of the
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instrumentation used are given in S. I. The gas sensing measurements were done using an inhouse developed gas sensing characterization setup (Fig. S1). The sensing measurements were
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done at a total flow rate of 500 sccm in dynamic mode with air as the diluent gas. The precalibrated gases were allowed inside the chamber at desirable flow rates through a static gas mixture unit using computer-controlled mass flow controllers. The chamber is equipped with
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a heater and probes for the measurements. The electrical connections were secured using spring
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loaded probes either to the SMU or the multichannel DAQ depending on the type of measurements.
3. Results and discussions 3.1 Morphological, structural compositional and optical characterizations Scanning electron micrographs of pristine and annealed SnO2 and NiO samples showed porous and uniformly distributed columnar microstructure, conforming to the zone structure 6
model [31] for the given deposition conditions. For gas sensing analyses, SnO2 thin films of around 500 nm were deposited and annealed. Figs. 2(a) and (b) show the surface of asdeposited and annealed SnO2 thin films, respectively. NiO films of around 120 nm were deposited on both bare substrates and on SnO2 layer (for heterojunction devices). The FESEM images of as-deposited and annealed NiO thin films are shown in Figs. 2(c) and (d), respectively. The surface and cross-sectional micrographs of the p-n heterojunction are presented in Figs. 2(e) and (f). The cross-section of the bilayer also shows columnar
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microstructure with pinholes. The adhesion of NiO layers on SnO2 thin film was found to be good as per the ‘tape-pull’ test.
XRD characterization of SnO2 and NiO films showed crystallinity, conforming to the
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rutile tetragonal (ICDD: 01-41-1445) and cubic (ICDD: 00-47-1049) phases, respectively. The average grain sizes of the annealed SnO2 and NiO films, calculated using Scherrer formula [32],
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were 4 and 11 nm, respectively. The XRD patterns of individual layers and NiO/SnO2 bilayers
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are presented in Fig. S2. The heterojunction showed only individual phases and no mixed phases were found. The XPS analyses of the samples confirmed pure oxide films (Fig. S3). The optical bandgap values calculated from Tauc’s plots [33] for the annealed SnO2 and NiO
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thin films were 4.03 and 3.85 eV, respectively (Fig. S4). The measured values closely match with the bandgaps reported in the literature [34, 35]. The effect of annealing on the optical
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bandgap was negligible.
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3.2 Sensor characterization
Gas sensor characterization was performed for different gases such as nitrogen dioxide
(NO2), ammonia (NH3) and carbon monoxide (CO) at three different temperatures 200, 225 and 250 °C in the concentration range of 2-10 ppm. The sensor response [36] was calculated as per Eqn. 1. ΔR
…. (1)
Response = ( ) ∗ 100 Ra
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where, ΔR = (R a − R g ) or (R g − R a ), Ra and Rg are the resistances of the sensing elements in the presence of air and test gas, respectively. The response and recovery time of a sensor is the time taken by it to reach 90% of the steady state resistance in the presence of the gas and air, respectively [12]. Similarly, selectivity coefficient can be defined as the ratio between sensitivity towards the test gas to the interfering gas. When pristine, n- or p-type materials interact with air, the oxygen molecules present in the air get adsorbed on the material surface by trapping the free electrons available on the surface. The charge on the adsorbed oxygen (O− 2,
below 300 °C, the most plausible reaction is given by Eqn. 2 − O2(g) + 2e− (s) ↔ 2O(s)
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O2-, O-) depends on the working temperature of the sensor [16]. For operating temperatures
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…. (2)
Loss of surface electrons creates an electron depletion layer (EDL) in the n-type
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material and a hole accumulation layer (HAL) in the p-type material [37]. Thus, in air, the ntype material will have semiconducting core and resistive shell, enabling a serial conduction
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pathway. The p-type material, on the other hand, will have resistive core and semiconducting shell, enabling a parallel conduction pathway [38]. A decrease in depletion layer width will
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decrease the resistance of the sample in an n-type material and converse is true for a p-type material. When a reducing gas (e. g., NH3, CO, H2S) is introduced in the vicinity of the material,
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more electrons are injected on to the surface which will reduce the resistance in n-type and increase the resistance in p-type materials, respectively. The reverse is true for an oxidizing gas
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(e.g., NO2, SO2, CO2). So, in the presence of NO2, the resistance of NiO will decrease and that of SnO2 will increase. 3.2.1 Single layer thin films SnO2 and NiO films deposited in a high oxygen environment responded better towards NO2. We have observed that samples which show better response towards oxygen, exhibit
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better sensitivity towards the analyte gas as well. The influence of change in oxygen partial pressure was found to be negligible with respect to the analyte gas. The transient response characteristics of Devices-A and B with individual layers of SnO2 and NiO at 200 °C are shown in Fig. 3(a) and (b), respectively. The response of SnO2 varied from 132 to 628% for the NO2 concentration range of 2-6 ppm. On the other hand, the response of NiO for 2-10 ppm of NO2 was found to be around 8 to 33%. The NO2 response of SnO2 was much higher than NiO. The high degree of variation in response could be due to intrinsic nature, conduction pathway or a
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combination of both [38]. Hübner relation [39] states that the sensitivity of p-type material fabricated under the same conditions as an n-type material is the square root of sensitivity of the n-type material. In this case, the films are different in terms of thickness, grain size, catalytic
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activity, etc. So, obvious differences in the sensitivity are expected.
For both the devices, the ‘ON’ time was 5 min. In SnO2, the resistance variation kept
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on increasing an increase in exposure time. From Fig. 3(c), one can clearly see that, an increase
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in ‘ON’ time from 5 to 10 min increased the response of SnO2 remarkably (438 to 815%). A further increase in ‘ON ‘time resulted in even higher sensitivity. Thus, SnO2, as a single layer has limitation to show ‘steady-state’ stability. On the other hand, in NiO, when the ‘ON’ time
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was changed from 5 to 10 min, the response variation was very less (8.1 to 9.2%: Fig. 3(d)).
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The response profile was found to be reproducible. NiO exhibited excellent selectivity towards NO2 by showing no response to other
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gases like NH3 and CO in the same concentration range. SnO2 showed resistance variation to other gases and hence, comparatively less selective. The selectivity co-efficient of SnO2 was found to be 15 and 22 for NH3 and CO, respectively. Enhanced selectivity of NiO can be attributed to its oxygen affinity and selective oxidation catalytic behaviour [16]. SnO2 showed a highly varying response time and NiO showed response time of ~200 s. However, SnO2 thin films recovered faster (180-230 s) than NiO (550-600 s) upon withdrawal of the perturbing gas. 9
Zhang et al. [40] have also observed slow recovery in NiO which is attributed to its NO2 adsorption mechanism through nickel vacancy (Eqn. 3) which results in the formation of nitrates as per Eqn. 4. ′′ ′ NO2(s) + VNi ↔ NO− 2(s) + VNi
…. (3)
′′ ′ 2NO2(s) + 2VNi — O𝑜 ↔ 2NO− 3(s) + 2VNi
.… (4)
′′ ′ ′′ where VNi is doubly negative nickel vacancy, VNi is singly negative nickel vacancy, VNi — O𝑜
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is lattice oxygen bonded to nickel vacancy, the subscripts (g) and (s) represent the molecule in gaseous and surface adsorbed forms. The conventions of Krӧger-Vink notation can be found elsewhere [41]. The I-V characteristics of Devices-A and B are given in Figs. 4(a) and (b),
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respectively. Similar I-V curves for the oxide thinfilms have been reported in the literature [42]. The current variation behaviour reflects the transient response characteristics and the
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response was found to be independent of the excitation voltage. The insets of Fig. 4 show the
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resistance as a function of temperature, revealing the semiconducting nature of the annealed thin films.
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3.2.2. Bilayer heterojunction thin films
The sensing results of Devices-A and B show that they may not be used as such for
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practical device applications. So, p-n bilayer heterojunction sensors (Device-C) with gold electrodes were fabricated as in Fig. 1(c). The overall resistance of the heterojunction was
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found to be more than the individual layers. Sensor response of the heterojunction device as a function gas of concentration, temperature, stability and I-V characteristics are presented in Fig. 5 with the inset of Fig. 5(a) showing the sensing profile for a single gas concentration. Heterojunctions can show sensing trends of n-type [24, 43] or p-type materials [29]. The response varied from 57 to 144% at 200°C. The response was found to be linearly related to
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the gas concentration. The extrapolation of the linear calibration curve would show that the sensor is capable of measuring NO2 in sub-ppm range as well. The slope of the calibration curve in Fig. 5(b) was maximum at 200°C around 11% per ppm. Typically, sensitivity falls after a certain temperature when the equilibrium shifts towards desorption of the analyte species. The resistance of the sensor device showed a non-linear dependence on the temperature. As the device was highly resistive, measurements below 200 °
C were limited by poor signal to noise ratio and instrumental capabilities. The sensor showed
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excellent stability without requiring any de-gassing between each cycle. The overall response variation for 10 different cycles was less than 2% (Fig. 5(c)). The I-V characteristics of the sample with voltage sweep (Fig. 5(d)) showed asymmetric behaviour, indicating that a
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heterojunction is formed. The I-V graphs in the presence of different concentrations of NO2 corroborate the transient response characteristics, i. e., current decreases with an increase in
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gas concentration. It should also be noted that the sensor response is no longer independent of
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the excitation potential. Inset of the Fig. 5(d) gives I-V curves as a function of temperature in air and 10 ppm of NO2. The inset shows that with an increase in temperature, the conductivity increases, while the sensitivity decreases.
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The response of the heterojunction was found to be lying between NiO thin and SnO2 thin films. The sensitivity of the heterojunction can either be higher than both the phases [44]
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or higher than only one phase [26]. The response curves of Device-C as a function of different
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‘ON’ time:1, 2, 5, and 10 mins, were recorded and presented in Fig. S5(a). The results showed that response variation was around 3% and exhibited good stability over prolonged exposure time unlike SnO2. The heterojunction devices showed faster response and recovery towards NO2. The response time varied from a minimum of 37 s to a maximum of 68 s while the recovery time varied from a minimum of 98 s to a maximum of 114 s. A comparative plot of response in the heterojunction towards NO2, NH3 and CO is presented in Fig. S5(b). The time-
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resolved response curves towards NH3 and CO are given in Fig. S5(c) and (d). The device was highly selective to NO2 with 26 and 90 as the selectivity coefficients for NH3 and CO, respectively. Thus, the heterojunction sensors show enhanced performance in terms of selectivity, stability, response and recovery time than its single layer counterparts.
3.2.3. Parallel sensing measurements To rule out the role of measurement conditions, we have fabricated Device-D, wherein
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simultaneous sensing measurements can be performed for the individual layers and the junction. The heterojunction area in Device-D will have both the electrodes on the same plane (in-plane heterojunction). The parallel measurements also help to compare the in-plane and
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across-plane heterojunctions. The transient response curves of SnO2, NiO and the junction are given in Fig. 6 with the insets showing the sensing profiles for a single gas concentration. It
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can be seen from Figs. 6(a) and (b), that SnO2 and NiO show the same trend as in Device-A
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and B. The sensitivity of SnO2 varied from 82 to 456% and NiO varied from 14 to 23% for the concentration range of 2-10 ppm. It is noteworthy to mention that the response of SnO2 in
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Device-A is more than that observed in SnO2 region of Device-D. At the same time, the variation in the response of NiO in Devices- A and D is relatively less. This observation reiterates the lack of stability of SnO2 as mentioned earlier. The heterojunction with in-plane
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electrodes showed the trend of n-type material just like Device-C. For the junction bilayer, the
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response ranged from 28 to 124% for 2-10 ppm of NO2 (Fig. 6(c)). The in-plane junction also exhibited good stability (Fig. S6(a)). The temperature-dependent response behaviour (Fig. S6(b)) was strikingly similar to that of Device-C. The slope of the calibration curve was maximum (~12% per ppm) at 200°C. Non-linear I-V graphs for the in-plane heterojunction are presented in Fig. 6(d). The total resistance of both the heterojunction samples were higher than
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their single layer counterparts. The similarity in the sensing characteristics and difference in the I-V can be explained by an approximate equivalent model of the devices. Approximate equivalent circuit models of across-plane and in-plane heterojunctions are given in Figs. 7(a) and (b). The heterojunction device can be represented by a diode in series with the resistance of the materials. The resistances of the materials are shown as variable as they can change upon the introduction of the test gas. A leakage resistor is connected in parallel to the diode because of its non-ideal behaviour. If the p- and n-type materials do not form an
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intimate contact, then the current passes through the leakage resistor [44]. In that case, the sensitivity is merely the sum of the resistances (RSnO2, RNiO and RJ). On the other hand, if a junction is formed, the junction diode becomes active and influences the measured current. In
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forward bias, the diode conducts, and its contribution is felt in the measured current while in
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reverse bias the total current is owing to the variation in the resistances. The in-plane heterojunction can be represented as two back-to-back connected diodes as shown in Fig. 7(b).
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It should be noted that in the in-plane configuration, the resistances of the individual layers are connected in parallel. In this case, since the resistance of NiO is more than that of SnO2, the
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RNiO which in series with the RSnO2 (Fig. 7(b)) is the resistance of NiO along its thickness. At a given time, only one diode (J1 or J2) will be in forward bias and the other diode will not
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conduct. So, here, the current conduction will happen irrespective of the bias as at least one
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diode is in forward bias. This leads to a symmetric, non-linear I-V plot (Fig.6 (d)). 3.2.4. Sensing mechanism in the heterojunction device Deposition of NiO thin film on the top of porous SnO2 layer has resulted in the
formation of uniformly distributed nano-heterojunctions. The pinholes on the columnar microstructure as seen in Fig. 2(f), porosity of NiO and roughness of SnO2 enabled better gas accessibility to the bottom layer which is evident from the n-type response behaviour of the 13
heterojunction. Thus, the gas accessibility limitation in the bilayer heterojunction device is overcome by simply controlling the deposition parameters. However, deposition of NiO layer on the SnO2 has inadvertently resulted in the reduction of the effective SnO2 surface available for sensing. The gas sensing mechanism is a surface phenomenon. In single layers, when a test gas is passed, there is either depletion or accumulation of charge carriers. On the contrary, in a pn heterojunction, reactions involving depletion and accumulation of charge carriers take place
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simultaneously. The flat band structures of n- and p-type semiconductors will have different Fermi energy levels. In the presence of air, hole accumulation and electron depletion layer will form in NiO and SnO2, respectively. Fig. 8(a) depicts the effect of chemisorbed oxygen on NiO,
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SnO2 and the interface. The grains which are accessible to the gas show depletion/accumulation region. When an abrupt p-n junction is formed, the charge carriers start to flow across the
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junction and recombine until the Fermi level is equilibrated. This creates a charge depletion
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layer (CDL) on the interface (yellow region in Fig. 8(a)) and results in a potential barrier, Vb (Fig. 8(b)). In the presence of a test gas, charge density on the heterojunction can vary dynamically. When an oxidizing gas like NO2 is passed, more electrons are trapped from the
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surface, resulting in widening of the depletion and accumulation layer as in Fig. 8 (c). Transient response curve (Fig. 5 (a)) shows that the resistance variation trend in the heterojunction is
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similar to SnO2 demonstrating that there is a decrease in current density. The decrease in net
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charge density is accompanied by the increase in the depletion layer width. This would result in an increase in barrier voltage (Vb + Vb ) (Fig. 8 (d)). The exponential of the barrier voltage is in turn inversely proportional to the current [45]. Hence, the measured current of the junction decreases with an increase in NO2 concentration. Conversely, when a reducing gas is passed, both hole accumulation and electron depletion layers decrease as in Fig. 8 (e). With a net gain in the carrier density, the barrier voltage (Vb - Vb) decreases as shown in Fig. 8 (f). The extent
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of variation in the barrier voltage is determined by the sensing properties of the individual layers. The forward bias characteristics of the heterojunction device, given in Fig. 9, validates the above discussion. The barrier voltage keeps increasing upon introduction of NO2. The inset of Fig. 9 shows the variation of barrier voltage as a function of gas concentration. The response of the heterojunction was about 3 times less than that of SnO2. This is can be attributed to two factors: i) two counteracting response behaviours arising from n- and psides ii) reduced surface area of highly sensing SnO2. The gas response in a heterojunction is
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decided by the responses of the individual layers as well as the modulation of potential barrier by two complimentary responses across the interface. That is why it is not uncommon to observe n-p/ p-n inversion [46] and n-p-n [47] switching in the heterojunctions. In 0D and 1D
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heterostructures, the surface areas of n- and p-type materials are not compromised and act in synergy producing a sharp hike in sensitivity.
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The large surface area on the bare SnO2 thin film facilitated enhanced NO2 absorption
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with time leading to indiscriminate increase in sensor response (> 1000 % in 20 mins). In spite of the fact that NiO and SnO2 as single layers exhibit slow response/recovery kinetics, deposition of NiO on SnO2 has resulted in faster response/recovery due to the formation of
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nano heterojunctions [48, 49]. Kim et al. [48] suggest that in NiO/SnO2 nano-interface, the surface NiO acts as a catalyst by enabling faster chemisorption of the oxygen anions and
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transfer of the adsorbed anions to SnO2. Similarly, enhancement of adsorption and desorption
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kinetics with the addition of p-type dopants on n-type material have been reported [50]. In our case, the response/recovery time decreased from few hundreds of seconds in single layers to 37/98 s in the heterojunction. The responses of the fabricated sensors towards NH3 and CO are given in Figs. 10 (a) and (b). In the heterojunction, the response towards NH3 decreased by 5 times and that of CO decreased by 16 times when compared to SnO2. The role of NiO in enhancing the selectivity
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is apparent as the response values dropped by more than 3 times. NiO readily reacts with oxidizing gases to get Ni2+ oxidized to Ni3+ but shows less affinity towards reducing gases. Presence of NiO on the heterojunction limits the access of NH3 and CO to SnO2, both chemically and physically. The sensor response, selectivity and stability values of the in-plane heterojunctions were comparable to that of across-plane heterojunction. A variation in the response/recovery kinetics was observed when the electrode configuration was changed from top-bottom to in-plane. Comparative plots for sensor response, response time and recovery time
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as a function of different NO2 concentrations are presented in Fig. 11. The error bars shown in the plots represent the values obtained for different devices in the same configuration deposited and stored in identical conditions. Comparing the sensing behaviours of all the devices, it can
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be summarized that Device-A has the highest sensitivity and Device-B has the best selectivity but lack in other attributes. The heterojunction Device-C, in spite of showing sensitivity and
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selectivity less than Devices-A and B, has effectively overcome the limitations of single layer
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devices in terms of stability, response and recovery times. At the same time, Device-C also exhibits highly acceptable values of sensitivity and selectivity. Of the two heterojunction devices, Device-C with top-bottom electrodes was found to perform better than the Device-D
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with in-plane electrodes.
The heterojunction samples (Device-C) showed fairly good long-term stability. The
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response initially decreased for a period of 25 days and remained almost constant after that.
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The response was ~57% in day-1 and ~44% in day-38 for 2 ppm of NO2 at 200°C. The longterm stability plot is provided in Fig. S7. The sample was stored in normal atmospheric conditions (where the humidity variation was around 40-60%) before loading the sample for sensing characterization. One of the major factors for the reduction in the response with ageing can be attributed to the effect of humidity [50]. When the gas accessibility to SnO2 is reduced by depositing less porous NiO thin films, the device showed response just like NiO. We believe
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that the choice of material and the method of fabrication have a critical role to play in achieving high current modulation. At the same time, equal importance should be given to the extent of fabrication reproducibility. The deposited bilayer heterojunctions displayed enhanced sensor performance and are highly reproducible.
4. Conclusion In this work, we have deposited and systematically studied porous, single layer n-type SnO2, p-type NiO and NiO/SnO2 bi-layered thin films heterojunction towards the detection of
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NO2 in the concentration range of 2 to 10 ppm. The gas sensing studies were performed as a function of working temperatures (200, 225 and 250 °C). The formation of the heterojunction across the interface is evident from the rectifying characteristics of the junction devices. The
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NiO/SnO2 heterojunction exhibited high response for NO2 with fast response /recovery times and high selectivity against contaminants like CO and NH3. The heterojunction also showed
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good reproducibility and cyclic response stability over its single layer counterparts. Transient
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response studies for different ‘ON’ times demonstrated the steady state stability of the heterojunction devices. The improvement in the performance of the heterojunction devices in
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many aspects compared to the single layer devices can be attributed to two factors: i) resultant of two complimentary reactions and ii) the response behaviour of the interface. Simultaneous sensitivity measurements were done to study the sensing behaviour of single and bilayer thin
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films at the same working conditions. Non-linear, symmetric I-V graphs of the in-plane and
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asymmetric I-V graphs of the across-plane heterojunction were satisfactorily explained by the equivalent circuit models. The study has also revealed that the porosity of the top layer is a prerequisite for observing junction characteristics. We have established that a highly reproducible bilayer heterojunction device can be fabricated using sputtering technique by simply controlling the deposition parameters.
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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
Acknowledgements
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Ms. Kiruba M. S. thankfully acknowledges ‘Council of Scientific and Industrial Research (CSIR), India’ for the financial assistance (HRDG/ CSIR Nehru PDF/EN, Es & PS/EMR-I/02/2018). Ms. Ann Susan Jose acknowledges CEFIPRA (U-1-154) for the financial
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assistance. The authors acknowledge Dr. Venkataramana Bonu for various helps.
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List of figure captions
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Fig. 1 Stack profiles of deposited sensors at different configurations.
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Fig. 2 FESEM images of (a), (b) as-deposited and annealed SnO2 thin films, (c), (d) asdeposited and annealed NiO thin films. The insets show cross-sectional images of the annealed
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films and (e), (f) surface morphology and cross-section of annealed bilayer heterojunction.
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Fig. 3 Transient response characteristics measured at 200°C of (a) Device-A: SnO2 (b) DeviceB: NiO and sensing profiles for 5 and 10 min of ‘ON’ time in (c) Device-A: SnO2 (d) Device-
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B: NiO.
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Fig. 4 I-V characteristics of (a) Device-A: SnO2 (b) Device-B: NiO in air and different
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concentrations of NO2 at 200°C. Resistance (R) vs. Temperature (T) curves are given as insets.
Fig. 5 (a) Transient response curve of NiO/SnO2 heterojunction at 200°C. Inset shows sensing profile for a single gas concentration (b) calibration curve as a function temperature (c) cyclic response curves at 6 ppm of NO2 measured at 200°C (d) I-V characteristics of the 28
heterojunction in air and different concentrations of NO2 at 200°C. Temperature dependent I-
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V curves are given as inset.
Fig.6 Time resolved sensing curves for different concentrations of NO2 in (a) SnO2 (b) NiO
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and (c) in-plane junction regions of Device-D at 200°C. Sensing profiles for a single NO2 concentration are given as insets. (d) I-V plots of the in-plane heterojunction as a function of
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NO2 concentration at 200°C.
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Fig. 7 (a) Equivalent circuit of across-plane heterojunction (b) Equivalent circuit of in-plane
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heterojunctions.
Fig. 8 (a) NiO/SnO2 interface showing depletion and accumulation layer in the presence of air (b) band bending in the heterojunction in air (c) interface showing an increase in depletion and accumulation layer in the presence of an oxidizing gas (d) band bending in the presence of an 30
oxidizing gas (e) interface showing increase in depletion and accumulation layers in the
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presence of a reducing gas (f) band bending in the presence of a reducing gas.
Fig. 9 Forward bias characteristics of the heterojunction depicting barrier height modulation as
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a function of NO2 concentration. The inset shows barrier voltage for different NO2
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concentrations.
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Fig. 10 The response variations of SnO2, NiO, in-plane and across-plane heterojunctions
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towards (a) NH3 and (b) CO at 200°C.
Fig. 11 Comparison plots of SnO2, NiO, in-plane and across-plane heterojunctions in terms of
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(a) response (b) response time and (c) recovery time as a function of NO2 concentration at
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200°C.
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PII:
S0925-4005(20)30177-5
DOI:
https://doi.org/10.1016/j.snb.2020.127830
Reference:
SNB 127830
To appear in:
Sensors and Actuators: B. Chemical
Received Date:
5 November 2019
Revised Date:
22 January 2020
Accepted Date:
4 February 2020
Please cite this article as: S. KM, Jose AS, K. P, Chowdhury P, Barshilia HC, Sputter deposited p-NiO/n-SnO2 porous thin film heterojunction based NO2 sensor with high selectivity and fast response, Sensors and Actuators: B. Chemical (2020), doi: https://doi.org/10.1016/j.snb.2020.127830
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Sputter deposited p-NiO/n-SnO2 porous thin film heterojunction based NO2 sensor with high selectivity and fast response Kiruba Mangalam S., Ann Susan Jose, Prajwal K., Prasanta Chowdhury, Harish C. Barshilia* Nanomaterials Research Laboratory, Surface Engineering Division, CSIR-National Aerospace Laboratories, Bangalore-560017, India
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Email: [email protected]; Phone: +91 80 2508 6248
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Highlights
NiO/SnO2 bilayer heterojunctions were deposited by pulsed DC magnetron sputtering.
NO2 sensing behavior of the single layer and the bilayer devices compared.
Heterojunction showed enhanced sensor performance than individual layers.
In-plane
and
across-plane
heterojunctions
showed
similar
response
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characteristics.
Abstract
In this work, we report fabrication and characterization of porous, single layer n-SnO2,
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p-NiO and bilayer thin film heterojunction devices developed using pulsed DC magnetron
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sputtering for NO2 detection. Template-free, NiO/SnO2 heterojunction devices were deposited both in top-bottom and in-plane electrode configurations. All the devices showed an optimum
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sensing temperature of 200 °C. Systematic comparison of the fabricated devices revealed that the heterojunction devices with top-bottom electrodes improved performance. Electrical
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characterization confirmed the formation of heterojunction across the interface. The response values of the heterojunction sensor ranged from 57 to 144 % for the NO2 concentration range
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of 2 to 10 ppm. The heterojunction device showed high selectivity against CO and NH3 with selectivity coefficients of 90 and 26, respectively. The heterojunction sensor exhibited fast
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response and recovery times of 37 and 98 s, respectively. The device showed excellent stability with < 2% variation in response for 10 cycles of transient response characteristics. The I-V characteristics of heterojunctions with top-bottom and in-plane electrodes were explained by an equivalent circuit model. The observed enhancement in various parameters can be ascribed to the formation of distributed p-n nano-heterojunctions across the interface. The developed
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NiO/SnO2 heterojunction sensor by the simple and reproducible sputtering technique is found to be a promising candidate for the detection of NO2.
Keywords: Heterojunction; NO2 sensor; Thin film; Bilayer; NiO; SnO2
1. Introduction
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Gas sensors find applications in numerous industries like automobile, chemical, textile, medical, combustion, food and aerospace. Lately, the gas sensors are becoming an indispensable component in the Internet of Things for applications pertaining to home safety
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(LPG, CO) and air quality monitoring (NO2, CO, SO2). Nitrogen dioxide (NO2), the second largest automobile pollutant [1] has detrimental effects to humans and the environment [2].
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The current applications demand the sensors to be versatile, work at low temperature, less
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power consuming, highly selective, robust and economical. Recent studies have shown that an enhancement in the performance of metal oxide gas sensors can be achieved when
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mixed/composite metal oxides are used [3]. When a physical contact is established between two electronically distinct materials, a junction barrier is formed along the interface owing to
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the differences in bandgap, Fermi energy and charge carrier type [4]. The barrier voltage across the junction will vary in the presence of an analyte gas which in turn modulates the current
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exponentially, thereby increasing the sensitivity [5]. Moreover, presence of a catalytically active material in the heterostructure can enhance the selectivity. Most of the published reports focus on developing highly dispersed 1D heterostructures
like nanorods [6], nanofibers [7], core-shell nanocables [8], core-shell spindles [9], porous strips [10], etc. because of their high surface area. However, in these kinds of heterostructures, the p-n junction is randomly distributed, and the junction is not well defined, which makes it 3
difficult to study the junction properties. Controlling the dispersion properties and stoichiometry in these mixed composites is critical and can be challenging. This can impact the commercial-scale sensor fabrication. Well-defined interfaces can be formed by stacking layers of p and n-type thin films. This type of 2D interface have high surface to volume ratio [11] compared to the bulk pellets and allows proper characterization of the junction for understanding the mechanism. Moreover, precise control over stoichiometry, morphology, film thickness and the degree of contact can be achieved by controlling the deposition parameters.
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This makes them highly reproducible even on a commercial scale. However, bi/multilayer films have limited gas accessibility to the bottom layer and the interface.
Very few attempts like glancing incidence deposition [5], micro-mask assisted
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deposition of ultra-thin p-type on n-type films [12], use of AAO templates [13] and PSS monolayer templates [14] have been made to enhance the gas accessibility in bilayer junction
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devices.
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The heterojunction devices are fabricated using different combinations of n-type (SnO2, TiO2, ZnO, WO3, In2O3) and p-type metal oxides (NiO, CuO, Cr2O3, V2O5). Tin oxide (SnO2), a wide band gap n-type material, is known to be the best sensing material for gas sensors, but
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lacks selectivity [15]. On the other hand, nickel oxide (NiO) is the most studied p-type gas sensing material in which selectivity can be tuned by manipulating its microstructural and
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catalytic properties [16]. SnO2/NiO combination as 1D heterostructure has been tested towards
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different gases like: CO [17], C2H5OH [18] , H2 [19], CH4 [20], H2S [21], acetone [22], TEA [23] and HCHO [24], n-butanol [25], etc. Few reports are available on SnO2-NiO towards NO2 sensing which are dispersed heterostructures like nanoneedles [26], nanowebs [27] and microspheres [28, 29]. Wang et al. [30] have reported sputtered thin film from SnO2:NiO (99:1) target on Au nanoparticle arrays. But pristine and annealed composite films were not sensitive. Kwon et al. [5] have deposited SnO2/NiO bilayer thin films. But they have mainly focussed on
4
technology computer-aided design (TCAD) simulations of the heterojunction and got unusually similar responses for both oxidizing and reducing gases. Since most of the reports on heterojunctions are in 0/1D heterostructures where, one material is often decorated on the other, it is not feasible to systematically compare sensing properties of both the materials to the heterojunction. To the best of our knowledge, no systematic studies have been done by comparing the sensor performance of individual NiO and SnO2 layers with that of templatefree, porous, puled-DC magnetron sputtered NiO/ SnO2 thin film heterojunction towards NO2
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sensing. In this paper, we present NO2 sensing studies of sputtered, p-NiO and n-SnO2 thin films. The thin films were subjected to structural, morphological, optical and electrical
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characterizations. We have analysed their strengths and shortcomings in terms of sensitivity, selectivity, stability and reproducibility. Then, we have developed template-free, porous
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NiO/SnO2 thin film heterojunction devices. The performance of the heterojunction device as a
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function of NO2 concentration, temperature, interfering gases like CO and NH3 and cyclic response were studied. Electrical characterization was also performed to confirm the formation of heterojunction. The results were compared with their single layer counterparts. To rule out
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the role of differences in working conditions, we have done simultaneous sensing measurements using a specially designed 6-probe contact with gold-coated, spring-loaded pins
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connected to a multichannel data acquisition system.
We have also compared the
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heterojunction devices with in-plane and top-bottom electrode configurations.
2. Experimental details 2.1 Sensor fabrication Thin films for sensor application were deposited on borosilicate glass substrates after thorough cleaning as given in the Supplementary Information (S. I.). Thin films of SnO2 and NiO were deposited using the pulsed-DC magnetron reactive sputtering from Ni and Sn targets.
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All the films were deposited at room temperature. The base pressure of the sputtering chamber was kept below 6.0x10-6 mbar. Initially, films were deposited under different Ar: O2 ratios and working pressures. Optimum deposition parameters were chosen by weighing their individual NO2 sensitivity. The sputtering power used for NiO and SnO2 films were 60 and 100 W, respectively. The deposition pressure of the chamber was maintained high at 9.3x10-3 mbar to get porous oxide films. Deposited samples were then air annealed (Carbolite Gero) at 350 °C for 3 hrs with a ramp rate of 10 °C/min.
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Devices were fabricated in different configurations to study the individual layers and the heterojunctions. The stack profiles of the deposited sensors are shown in Fig. 1. Desired patterns were achieved by masking with cleaned microslides. The electrodes were deposited
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by sputtering 20/200 nm of Ti/Au in the heterojunction samples. 2.2 Sensor characterization
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Deposited samples were subjected to various characterizations and the details of the
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instrumentation used are given in S. I. The gas sensing measurements were done using an inhouse developed gas sensing characterization setup (Fig. S1). The sensing measurements were
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done at a total flow rate of 500 sccm in dynamic mode with air as the diluent gas. The precalibrated gases were allowed inside the chamber at desirable flow rates through a static gas mixture unit using computer-controlled mass flow controllers. The chamber is equipped with
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a heater and probes for the measurements. The electrical connections were secured using spring
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loaded probes either to the SMU or the multichannel DAQ depending on the type of measurements.
3. Results and discussions 3.1 Morphological, structural compositional and optical characterizations Scanning electron micrographs of pristine and annealed SnO2 and NiO samples showed porous and uniformly distributed columnar microstructure, conforming to the zone structure 6
model [31] for the given deposition conditions. For gas sensing analyses, SnO2 thin films of around 500 nm were deposited and annealed. Figs. 2(a) and (b) show the surface of asdeposited and annealed SnO2 thin films, respectively. NiO films of around 120 nm were deposited on both bare substrates and on SnO2 layer (for heterojunction devices). The FESEM images of as-deposited and annealed NiO thin films are shown in Figs. 2(c) and (d), respectively. The surface and cross-sectional micrographs of the p-n heterojunction are presented in Figs. 2(e) and (f). The cross-section of the bilayer also shows columnar
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microstructure with pinholes. The adhesion of NiO layers on SnO2 thin film was found to be good as per the ‘tape-pull’ test.
XRD characterization of SnO2 and NiO films showed crystallinity, conforming to the
-p
rutile tetragonal (ICDD: 01-41-1445) and cubic (ICDD: 00-47-1049) phases, respectively. The average grain sizes of the annealed SnO2 and NiO films, calculated using Scherrer formula [32],
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were 4 and 11 nm, respectively. The XRD patterns of individual layers and NiO/SnO2 bilayers
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are presented in Fig. S2. The heterojunction showed only individual phases and no mixed phases were found. The XPS analyses of the samples confirmed pure oxide films (Fig. S3). The optical bandgap values calculated from Tauc’s plots [33] for the annealed SnO2 and NiO
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thin films were 4.03 and 3.85 eV, respectively (Fig. S4). The measured values closely match with the bandgaps reported in the literature [34, 35]. The effect of annealing on the optical
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bandgap was negligible.
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3.2 Sensor characterization
Gas sensor characterization was performed for different gases such as nitrogen dioxide
(NO2), ammonia (NH3) and carbon monoxide (CO) at three different temperatures 200, 225 and 250 °C in the concentration range of 2-10 ppm. The sensor response [36] was calculated as per Eqn. 1. ΔR
…. (1)
Response = ( ) ∗ 100 Ra
7
where, ΔR = (R a − R g ) or (R g − R a ), Ra and Rg are the resistances of the sensing elements in the presence of air and test gas, respectively. The response and recovery time of a sensor is the time taken by it to reach 90% of the steady state resistance in the presence of the gas and air, respectively [12]. Similarly, selectivity coefficient can be defined as the ratio between sensitivity towards the test gas to the interfering gas. When pristine, n- or p-type materials interact with air, the oxygen molecules present in the air get adsorbed on the material surface by trapping the free electrons available on the surface. The charge on the adsorbed oxygen (O− 2,
below 300 °C, the most plausible reaction is given by Eqn. 2 − O2(g) + 2e− (s) ↔ 2O(s)
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O2-, O-) depends on the working temperature of the sensor [16]. For operating temperatures
-p
…. (2)
Loss of surface electrons creates an electron depletion layer (EDL) in the n-type
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material and a hole accumulation layer (HAL) in the p-type material [37]. Thus, in air, the ntype material will have semiconducting core and resistive shell, enabling a serial conduction
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pathway. The p-type material, on the other hand, will have resistive core and semiconducting shell, enabling a parallel conduction pathway [38]. A decrease in depletion layer width will
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decrease the resistance of the sample in an n-type material and converse is true for a p-type material. When a reducing gas (e. g., NH3, CO, H2S) is introduced in the vicinity of the material,
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more electrons are injected on to the surface which will reduce the resistance in n-type and increase the resistance in p-type materials, respectively. The reverse is true for an oxidizing gas
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(e.g., NO2, SO2, CO2). So, in the presence of NO2, the resistance of NiO will decrease and that of SnO2 will increase. 3.2.1 Single layer thin films SnO2 and NiO films deposited in a high oxygen environment responded better towards NO2. We have observed that samples which show better response towards oxygen, exhibit
8
better sensitivity towards the analyte gas as well. The influence of change in oxygen partial pressure was found to be negligible with respect to the analyte gas. The transient response characteristics of Devices-A and B with individual layers of SnO2 and NiO at 200 °C are shown in Fig. 3(a) and (b), respectively. The response of SnO2 varied from 132 to 628% for the NO2 concentration range of 2-6 ppm. On the other hand, the response of NiO for 2-10 ppm of NO2 was found to be around 8 to 33%. The NO2 response of SnO2 was much higher than NiO. The high degree of variation in response could be due to intrinsic nature, conduction pathway or a
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combination of both [38]. Hübner relation [39] states that the sensitivity of p-type material fabricated under the same conditions as an n-type material is the square root of sensitivity of the n-type material. In this case, the films are different in terms of thickness, grain size, catalytic
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activity, etc. So, obvious differences in the sensitivity are expected.
For both the devices, the ‘ON’ time was 5 min. In SnO2, the resistance variation kept
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on increasing an increase in exposure time. From Fig. 3(c), one can clearly see that, an increase
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in ‘ON’ time from 5 to 10 min increased the response of SnO2 remarkably (438 to 815%). A further increase in ‘ON ‘time resulted in even higher sensitivity. Thus, SnO2, as a single layer has limitation to show ‘steady-state’ stability. On the other hand, in NiO, when the ‘ON’ time
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was changed from 5 to 10 min, the response variation was very less (8.1 to 9.2%: Fig. 3(d)).
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The response profile was found to be reproducible. NiO exhibited excellent selectivity towards NO2 by showing no response to other
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gases like NH3 and CO in the same concentration range. SnO2 showed resistance variation to other gases and hence, comparatively less selective. The selectivity co-efficient of SnO2 was found to be 15 and 22 for NH3 and CO, respectively. Enhanced selectivity of NiO can be attributed to its oxygen affinity and selective oxidation catalytic behaviour [16]. SnO2 showed a highly varying response time and NiO showed response time of ~200 s. However, SnO2 thin films recovered faster (180-230 s) than NiO (550-600 s) upon withdrawal of the perturbing gas. 9
Zhang et al. [40] have also observed slow recovery in NiO which is attributed to its NO2 adsorption mechanism through nickel vacancy (Eqn. 3) which results in the formation of nitrates as per Eqn. 4. ′′ ′ NO2(s) + VNi ↔ NO− 2(s) + VNi
…. (3)
′′ ′ 2NO2(s) + 2VNi — O𝑜 ↔ 2NO− 3(s) + 2VNi
.… (4)
′′ ′ ′′ where VNi is doubly negative nickel vacancy, VNi is singly negative nickel vacancy, VNi — O𝑜
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is lattice oxygen bonded to nickel vacancy, the subscripts (g) and (s) represent the molecule in gaseous and surface adsorbed forms. The conventions of Krӧger-Vink notation can be found elsewhere [41]. The I-V characteristics of Devices-A and B are given in Figs. 4(a) and (b),
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respectively. Similar I-V curves for the oxide thinfilms have been reported in the literature [42]. The current variation behaviour reflects the transient response characteristics and the
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response was found to be independent of the excitation voltage. The insets of Fig. 4 show the
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resistance as a function of temperature, revealing the semiconducting nature of the annealed thin films.
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3.2.2. Bilayer heterojunction thin films
The sensing results of Devices-A and B show that they may not be used as such for
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practical device applications. So, p-n bilayer heterojunction sensors (Device-C) with gold electrodes were fabricated as in Fig. 1(c). The overall resistance of the heterojunction was
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found to be more than the individual layers. Sensor response of the heterojunction device as a function gas of concentration, temperature, stability and I-V characteristics are presented in Fig. 5 with the inset of Fig. 5(a) showing the sensing profile for a single gas concentration. Heterojunctions can show sensing trends of n-type [24, 43] or p-type materials [29]. The response varied from 57 to 144% at 200°C. The response was found to be linearly related to
10
the gas concentration. The extrapolation of the linear calibration curve would show that the sensor is capable of measuring NO2 in sub-ppm range as well. The slope of the calibration curve in Fig. 5(b) was maximum at 200°C around 11% per ppm. Typically, sensitivity falls after a certain temperature when the equilibrium shifts towards desorption of the analyte species. The resistance of the sensor device showed a non-linear dependence on the temperature. As the device was highly resistive, measurements below 200 °
C were limited by poor signal to noise ratio and instrumental capabilities. The sensor showed
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excellent stability without requiring any de-gassing between each cycle. The overall response variation for 10 different cycles was less than 2% (Fig. 5(c)). The I-V characteristics of the sample with voltage sweep (Fig. 5(d)) showed asymmetric behaviour, indicating that a
-p
heterojunction is formed. The I-V graphs in the presence of different concentrations of NO2 corroborate the transient response characteristics, i. e., current decreases with an increase in
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gas concentration. It should also be noted that the sensor response is no longer independent of
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the excitation potential. Inset of the Fig. 5(d) gives I-V curves as a function of temperature in air and 10 ppm of NO2. The inset shows that with an increase in temperature, the conductivity increases, while the sensitivity decreases.
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The response of the heterojunction was found to be lying between NiO thin and SnO2 thin films. The sensitivity of the heterojunction can either be higher than both the phases [44]
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or higher than only one phase [26]. The response curves of Device-C as a function of different
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‘ON’ time:1, 2, 5, and 10 mins, were recorded and presented in Fig. S5(a). The results showed that response variation was around 3% and exhibited good stability over prolonged exposure time unlike SnO2. The heterojunction devices showed faster response and recovery towards NO2. The response time varied from a minimum of 37 s to a maximum of 68 s while the recovery time varied from a minimum of 98 s to a maximum of 114 s. A comparative plot of response in the heterojunction towards NO2, NH3 and CO is presented in Fig. S5(b). The time-
11
resolved response curves towards NH3 and CO are given in Fig. S5(c) and (d). The device was highly selective to NO2 with 26 and 90 as the selectivity coefficients for NH3 and CO, respectively. Thus, the heterojunction sensors show enhanced performance in terms of selectivity, stability, response and recovery time than its single layer counterparts.
3.2.3. Parallel sensing measurements To rule out the role of measurement conditions, we have fabricated Device-D, wherein
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simultaneous sensing measurements can be performed for the individual layers and the junction. The heterojunction area in Device-D will have both the electrodes on the same plane (in-plane heterojunction). The parallel measurements also help to compare the in-plane and
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across-plane heterojunctions. The transient response curves of SnO2, NiO and the junction are given in Fig. 6 with the insets showing the sensing profiles for a single gas concentration. It
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can be seen from Figs. 6(a) and (b), that SnO2 and NiO show the same trend as in Device-A
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and B. The sensitivity of SnO2 varied from 82 to 456% and NiO varied from 14 to 23% for the concentration range of 2-10 ppm. It is noteworthy to mention that the response of SnO2 in
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Device-A is more than that observed in SnO2 region of Device-D. At the same time, the variation in the response of NiO in Devices- A and D is relatively less. This observation reiterates the lack of stability of SnO2 as mentioned earlier. The heterojunction with in-plane
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electrodes showed the trend of n-type material just like Device-C. For the junction bilayer, the
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response ranged from 28 to 124% for 2-10 ppm of NO2 (Fig. 6(c)). The in-plane junction also exhibited good stability (Fig. S6(a)). The temperature-dependent response behaviour (Fig. S6(b)) was strikingly similar to that of Device-C. The slope of the calibration curve was maximum (~12% per ppm) at 200°C. Non-linear I-V graphs for the in-plane heterojunction are presented in Fig. 6(d). The total resistance of both the heterojunction samples were higher than
12
their single layer counterparts. The similarity in the sensing characteristics and difference in the I-V can be explained by an approximate equivalent model of the devices. Approximate equivalent circuit models of across-plane and in-plane heterojunctions are given in Figs. 7(a) and (b). The heterojunction device can be represented by a diode in series with the resistance of the materials. The resistances of the materials are shown as variable as they can change upon the introduction of the test gas. A leakage resistor is connected in parallel to the diode because of its non-ideal behaviour. If the p- and n-type materials do not form an
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intimate contact, then the current passes through the leakage resistor [44]. In that case, the sensitivity is merely the sum of the resistances (RSnO2, RNiO and RJ). On the other hand, if a junction is formed, the junction diode becomes active and influences the measured current. In
-p
forward bias, the diode conducts, and its contribution is felt in the measured current while in
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reverse bias the total current is owing to the variation in the resistances. The in-plane heterojunction can be represented as two back-to-back connected diodes as shown in Fig. 7(b).
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It should be noted that in the in-plane configuration, the resistances of the individual layers are connected in parallel. In this case, since the resistance of NiO is more than that of SnO2, the
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RNiO which in series with the RSnO2 (Fig. 7(b)) is the resistance of NiO along its thickness. At a given time, only one diode (J1 or J2) will be in forward bias and the other diode will not
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conduct. So, here, the current conduction will happen irrespective of the bias as at least one
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diode is in forward bias. This leads to a symmetric, non-linear I-V plot (Fig.6 (d)). 3.2.4. Sensing mechanism in the heterojunction device Deposition of NiO thin film on the top of porous SnO2 layer has resulted in the
formation of uniformly distributed nano-heterojunctions. The pinholes on the columnar microstructure as seen in Fig. 2(f), porosity of NiO and roughness of SnO2 enabled better gas accessibility to the bottom layer which is evident from the n-type response behaviour of the 13
heterojunction. Thus, the gas accessibility limitation in the bilayer heterojunction device is overcome by simply controlling the deposition parameters. However, deposition of NiO layer on the SnO2 has inadvertently resulted in the reduction of the effective SnO2 surface available for sensing. The gas sensing mechanism is a surface phenomenon. In single layers, when a test gas is passed, there is either depletion or accumulation of charge carriers. On the contrary, in a pn heterojunction, reactions involving depletion and accumulation of charge carriers take place
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simultaneously. The flat band structures of n- and p-type semiconductors will have different Fermi energy levels. In the presence of air, hole accumulation and electron depletion layer will form in NiO and SnO2, respectively. Fig. 8(a) depicts the effect of chemisorbed oxygen on NiO,
-p
SnO2 and the interface. The grains which are accessible to the gas show depletion/accumulation region. When an abrupt p-n junction is formed, the charge carriers start to flow across the
re
junction and recombine until the Fermi level is equilibrated. This creates a charge depletion
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layer (CDL) on the interface (yellow region in Fig. 8(a)) and results in a potential barrier, Vb (Fig. 8(b)). In the presence of a test gas, charge density on the heterojunction can vary dynamically. When an oxidizing gas like NO2 is passed, more electrons are trapped from the
na
surface, resulting in widening of the depletion and accumulation layer as in Fig. 8 (c). Transient response curve (Fig. 5 (a)) shows that the resistance variation trend in the heterojunction is
ur
similar to SnO2 demonstrating that there is a decrease in current density. The decrease in net
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charge density is accompanied by the increase in the depletion layer width. This would result in an increase in barrier voltage (Vb + Vb ) (Fig. 8 (d)). The exponential of the barrier voltage is in turn inversely proportional to the current [45]. Hence, the measured current of the junction decreases with an increase in NO2 concentration. Conversely, when a reducing gas is passed, both hole accumulation and electron depletion layers decrease as in Fig. 8 (e). With a net gain in the carrier density, the barrier voltage (Vb - Vb) decreases as shown in Fig. 8 (f). The extent
14
of variation in the barrier voltage is determined by the sensing properties of the individual layers. The forward bias characteristics of the heterojunction device, given in Fig. 9, validates the above discussion. The barrier voltage keeps increasing upon introduction of NO2. The inset of Fig. 9 shows the variation of barrier voltage as a function of gas concentration. The response of the heterojunction was about 3 times less than that of SnO2. This is can be attributed to two factors: i) two counteracting response behaviours arising from n- and psides ii) reduced surface area of highly sensing SnO2. The gas response in a heterojunction is
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decided by the responses of the individual layers as well as the modulation of potential barrier by two complimentary responses across the interface. That is why it is not uncommon to observe n-p/ p-n inversion [46] and n-p-n [47] switching in the heterojunctions. In 0D and 1D
-p
heterostructures, the surface areas of n- and p-type materials are not compromised and act in synergy producing a sharp hike in sensitivity.
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The large surface area on the bare SnO2 thin film facilitated enhanced NO2 absorption
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with time leading to indiscriminate increase in sensor response (> 1000 % in 20 mins). In spite of the fact that NiO and SnO2 as single layers exhibit slow response/recovery kinetics, deposition of NiO on SnO2 has resulted in faster response/recovery due to the formation of
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nano heterojunctions [48, 49]. Kim et al. [48] suggest that in NiO/SnO2 nano-interface, the surface NiO acts as a catalyst by enabling faster chemisorption of the oxygen anions and
ur
transfer of the adsorbed anions to SnO2. Similarly, enhancement of adsorption and desorption
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kinetics with the addition of p-type dopants on n-type material have been reported [50]. In our case, the response/recovery time decreased from few hundreds of seconds in single layers to 37/98 s in the heterojunction. The responses of the fabricated sensors towards NH3 and CO are given in Figs. 10 (a) and (b). In the heterojunction, the response towards NH3 decreased by 5 times and that of CO decreased by 16 times when compared to SnO2. The role of NiO in enhancing the selectivity
15
is apparent as the response values dropped by more than 3 times. NiO readily reacts with oxidizing gases to get Ni2+ oxidized to Ni3+ but shows less affinity towards reducing gases. Presence of NiO on the heterojunction limits the access of NH3 and CO to SnO2, both chemically and physically. The sensor response, selectivity and stability values of the in-plane heterojunctions were comparable to that of across-plane heterojunction. A variation in the response/recovery kinetics was observed when the electrode configuration was changed from top-bottom to in-plane. Comparative plots for sensor response, response time and recovery time
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as a function of different NO2 concentrations are presented in Fig. 11. The error bars shown in the plots represent the values obtained for different devices in the same configuration deposited and stored in identical conditions. Comparing the sensing behaviours of all the devices, it can
-p
be summarized that Device-A has the highest sensitivity and Device-B has the best selectivity but lack in other attributes. The heterojunction Device-C, in spite of showing sensitivity and
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selectivity less than Devices-A and B, has effectively overcome the limitations of single layer
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devices in terms of stability, response and recovery times. At the same time, Device-C also exhibits highly acceptable values of sensitivity and selectivity. Of the two heterojunction devices, Device-C with top-bottom electrodes was found to perform better than the Device-D
na
with in-plane electrodes.
The heterojunction samples (Device-C) showed fairly good long-term stability. The
ur
response initially decreased for a period of 25 days and remained almost constant after that.
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The response was ~57% in day-1 and ~44% in day-38 for 2 ppm of NO2 at 200°C. The longterm stability plot is provided in Fig. S7. The sample was stored in normal atmospheric conditions (where the humidity variation was around 40-60%) before loading the sample for sensing characterization. One of the major factors for the reduction in the response with ageing can be attributed to the effect of humidity [50]. When the gas accessibility to SnO2 is reduced by depositing less porous NiO thin films, the device showed response just like NiO. We believe
16
that the choice of material and the method of fabrication have a critical role to play in achieving high current modulation. At the same time, equal importance should be given to the extent of fabrication reproducibility. The deposited bilayer heterojunctions displayed enhanced sensor performance and are highly reproducible.
4. Conclusion In this work, we have deposited and systematically studied porous, single layer n-type SnO2, p-type NiO and NiO/SnO2 bi-layered thin films heterojunction towards the detection of
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NO2 in the concentration range of 2 to 10 ppm. The gas sensing studies were performed as a function of working temperatures (200, 225 and 250 °C). The formation of the heterojunction across the interface is evident from the rectifying characteristics of the junction devices. The
-p
NiO/SnO2 heterojunction exhibited high response for NO2 with fast response /recovery times and high selectivity against contaminants like CO and NH3. The heterojunction also showed
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good reproducibility and cyclic response stability over its single layer counterparts. Transient
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response studies for different ‘ON’ times demonstrated the steady state stability of the heterojunction devices. The improvement in the performance of the heterojunction devices in
na
many aspects compared to the single layer devices can be attributed to two factors: i) resultant of two complimentary reactions and ii) the response behaviour of the interface. Simultaneous sensitivity measurements were done to study the sensing behaviour of single and bilayer thin
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films at the same working conditions. Non-linear, symmetric I-V graphs of the in-plane and
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asymmetric I-V graphs of the across-plane heterojunction were satisfactorily explained by the equivalent circuit models. The study has also revealed that the porosity of the top layer is a prerequisite for observing junction characteristics. We have established that a highly reproducible bilayer heterojunction device can be fabricated using sputtering technique by simply controlling the deposition parameters.
17
Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper
Acknowledgements
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Ms. Kiruba M. S. thankfully acknowledges ‘Council of Scientific and Industrial Research (CSIR), India’ for the financial assistance (HRDG/ CSIR Nehru PDF/EN, Es & PS/EMR-I/02/2018). Ms. Ann Susan Jose acknowledges CEFIPRA (U-1-154) for the financial
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assistance. The authors acknowledge Dr. Venkataramana Bonu for various helps.
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List of figure captions
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Fig. 1 Stack profiles of deposited sensors at different configurations.
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Fig. 2 FESEM images of (a), (b) as-deposited and annealed SnO2 thin films, (c), (d) asdeposited and annealed NiO thin films. The insets show cross-sectional images of the annealed
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films and (e), (f) surface morphology and cross-section of annealed bilayer heterojunction.
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Fig. 3 Transient response characteristics measured at 200°C of (a) Device-A: SnO2 (b) DeviceB: NiO and sensing profiles for 5 and 10 min of ‘ON’ time in (c) Device-A: SnO2 (d) Device-
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B: NiO.
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Fig. 4 I-V characteristics of (a) Device-A: SnO2 (b) Device-B: NiO in air and different
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concentrations of NO2 at 200°C. Resistance (R) vs. Temperature (T) curves are given as insets.
Fig. 5 (a) Transient response curve of NiO/SnO2 heterojunction at 200°C. Inset shows sensing profile for a single gas concentration (b) calibration curve as a function temperature (c) cyclic response curves at 6 ppm of NO2 measured at 200°C (d) I-V characteristics of the 28
heterojunction in air and different concentrations of NO2 at 200°C. Temperature dependent I-
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V curves are given as inset.
Fig.6 Time resolved sensing curves for different concentrations of NO2 in (a) SnO2 (b) NiO
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and (c) in-plane junction regions of Device-D at 200°C. Sensing profiles for a single NO2 concentration are given as insets. (d) I-V plots of the in-plane heterojunction as a function of
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NO2 concentration at 200°C.
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Fig. 7 (a) Equivalent circuit of across-plane heterojunction (b) Equivalent circuit of in-plane
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heterojunctions.
Fig. 8 (a) NiO/SnO2 interface showing depletion and accumulation layer in the presence of air (b) band bending in the heterojunction in air (c) interface showing an increase in depletion and accumulation layer in the presence of an oxidizing gas (d) band bending in the presence of an 30
oxidizing gas (e) interface showing increase in depletion and accumulation layers in the
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presence of a reducing gas (f) band bending in the presence of a reducing gas.
Fig. 9 Forward bias characteristics of the heterojunction depicting barrier height modulation as
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a function of NO2 concentration. The inset shows barrier voltage for different NO2
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concentrations.
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Fig. 10 The response variations of SnO2, NiO, in-plane and across-plane heterojunctions
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towards (a) NH3 and (b) CO at 200°C.
Fig. 11 Comparison plots of SnO2, NiO, in-plane and across-plane heterojunctions in terms of
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(a) response (b) response time and (c) recovery time as a function of NO2 concentration at
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200°C.
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