Department of Electrical and Electronics Engineering
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Item Device Modeling of Double Layered TiO2 Nanotube Array Based Resistive Vapor Sensor(IEEE, 2018) Hazra, ArnabSingle and double layered TiO 2 nanotube array were synthesized by anodic oxidation method. Anodization voltage was varied to develop double layered TiO 2 nanotube array. Developed materials were characterized structurally and morphologically by X-ray diffraction spectroscopy (XRD)and field emission scanning electron microscopy (FESEM)respectively. Sandwich structure devices with Au top electrode and Ti bottom electrode were fabricated by using both single and double layered TiO 2 nanotubes for vapor sensing application. A simplified device modeling was introduced to establish the sensing mechanism of both the TiO 2 nanotube arrays. Additional interlayer junctions in double layered TiO 2 nanotubes array, enhanced the vapor sensing performance significantly. Double layered TiO 2 nanotubes array was able to show 92.4% of response magnitude for ethanol concentration of 160 ppm at 300 K where 55.2% response was observed for mono-layered TiO 2 nanotube array.Item Influence of temperature, voltage and hydrogen on the reversible transition of electrical conductivity in sol–gel grown nanocrystalline TiO2 thin film(Springer, 2012-11) Hazra, ArnabUndoped nanocrystalline p-type TiO2 thin film was deposited by sol–gel method on a thermally oxidized p-Si (2–5 Ω cm resistivity and ⟨100⟩ orientation) substrate. The thin film was characterized by two-dimensional X-ray Diffraction (2D-XRD) and Field Emission Scanning Electron Microscopy (FESEM) to confirm the formation of stable nano crystalline anatase titania and to determine the grain size (~10 nm). Optical absorption spectroscopy was carried out to ascertain the band gap of the material. Two lateral Pd contacts were used as the metal electrodes to TiO2 thin film to study the electrical conductivity. A clear p- to n-type transition was observed at 240 °C and a bias voltage of 0.83 V and the effect was enhanced on exposure to H2 gas. The thin film showed fully n-type conductivity at 275 °C and 0.1 V. However, the reversal of the type of conductivity from n- to p-type was observed below 240 °C during lowering the temperature. The creation of oxygen vacancy and the diffusion of lattice oxygen to the surface of TiO2 thin film might be the most possible mechanism of such transitions. Presence of hydrogen enhanced the process.Item A journey towards reliability improvement of TiO2 based Resistive Random Access Memory: A review(Elsevier, 2014-03) Hazra, ArnabA Resistive Random Access Memory (RRAM), where the memory performance principally originated from ‘resistive’ change rather than ‘capacitive’ one (the case with conventional CMOS memory devices), has attracted researchers across the globe, owing to its unique features and advantages meeting the demands of future generation high-speed, ultra low power, nano dimensional memory devices. A large family of semiconducting oxides have been investigated as insulator for Resistive Random Access Memory (RRAM), amongst which TiO2 is one of the potential candidate, principally owing to some of its remarkable advantages e.g. wide band gap, high temperature stability and high dielectric constant with flexibility to offer both unipolar and bipolar switching, which are essential for RRAM device applications. In this review article, we tried to represent the long voyage of TiO2 based RRAM, towards the improvement of the reliability aspects of the device performance in a comprehensive manner. Starting with the key factors like oxygen vacancies, Ti interstitials and electroforming, which are responsible for resistive switching phenomenon, various material preparation techniques for RRAM development have been discussed with emphasis on relative merits and bottlenecks of the process. The factors like electrode material and geometry, device structuring, doping, compliance current, annealing effect etc., which play the pivotal role in determining the switching performance of the device, have been reviewed critically. Finally, the article concludes with the comparison of different TiO2 based RRAM devices followed by the prediction of possible future research trends.Item Structural and Optical Characterizations of Electrochemically Grown Connected and Free-Standing TiO2 Nanotube Array(Springer, 2014-05) Hazra, Arnab; Manjuladevi, V.; Gupta, Raj KumarA TiO2 nanotube array was grown electrochemically by using single and mixed electrolyte/s with 20 V constant potential at room temperature. Anodization was carried out for 120 min using five different electrolytes, e.g., H3PO4, NH4F, HF, NH4F with H3PO4 and HF with H3PO4. Structural characterizations of the grown titania nanotubes were conducted by using x-ray diffraction and field emission scanning electron microscopy. Optical properties of the grown nanotubes were investigated through photoluminescence (PL) spectroscopy. In the case of the 4 M H3PO4 electrolyte, no perceptible growth of nanotubes was observed. The individual electrolytes of 0.3 M NH4F and 1 M HF resulted into the formation of the wall-connected nanotubes. In contrast, the mixed electrolytes comprising the strong (NH4F, HF) and weak (H3PO4) electrolytes have been found to be efficient for the growth of wall-separated titania nanotubes. The results of the PL spectroscopy have demonstrated that the free-standing nanotubes offer low PL intensity compared to its connected counterpart owing to the lower carrier recombination rate of free-standing nanotubes.Item Room temperature alcohol sensing by oxygen vacancy controlled TiO2 nanotube array(AIP, 2014-08) Hazra, ArnabOxygen vacancy (OV) controlled TiO2 nanotubes, having diameters of 50–70 nm and lengths of 200–250 nm, were synthesized by electrochemical anodization in the mixed electrolyte comprising NH4F and ethylene glycol with selective H2O content. The structural evolution of TiO2 nanoforms has been studied by field emission scanning electron microscopy. Variation in the formation of OVs with the variation of the structure of TiO2 nanoforms has been evaluated by photoluminescence and X-ray photoelectron spectroscopy. The sensor characteristics were correlated to the variation of the amount of induced OVs in the nanotubes. The efficient room temperature sensing achieved by the control of OVs of TiO2 nanotube array has paved the way for developing fast responding alcohol sensor with corresponding response magnitude of 60.2%, 45.3%, and 36.5% towards methanol, ethanol, and 2-propanol, respectively. This work was supported in part by CSIR, DST, AICTE, and INSA. A. Hazra, B. Bhowmik, and K. Dutta gratefully acknowledge the DST, Government of India and COE, TEQUIP-II, IIEST, Shibpur, respectively, for their fellowship for pursuing PhD program. The authors sincerely acknowledge Dr. Mallar Ray, School of Materials Science and Engineering, Indian Institute of Engineering Science and Technology (IIEST), Howrah 711 103, India for providing the facility of PL spectroscopy.Item Impedance behavior of n-type TiO2 nanotubes porous layer in reducing vapor ambient(Elsevier, 2018-06) Hazra, ArnabImpedance behavior of n-type TiO2 nanotubes array based vapor sensor is presented in this work with focus on the behavior of resistive and capacitive changes separately in presence of reducing organic vapors like methanol and acetone. The sensing layer of TiO2 nanotubes array was synthesized by anodizing Ti substrate by electrochemical route. The surface morphology, crystallinity and chemical composition of TiO2 nanotubes array were characterized with field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD) spectroscopy and X-ray photoelectron spectroscopy (XPS) respectively. Gold (Au) contact was deposited on film surface by thermal evaporation technique for developing Au/TiO2 nanotubes/Ti sensor. Cole-Cole plots of the sensor (operating at room temperature) in methanol and acetone ambient were drawn and an equivalent circuit of device was modelled to explain the sensing mechanism of the sensor.Item Selective detection of organic vapors using TiO2 nanotubes based single sensor at room temperature(Elsevier, 2019-07) Hazra, ArnabThe present study demonstrates a new measurement technique for the selective detection of multiple volatile organic compounds (VOCs) at room temperature (300 K) by using a single sensor. Electrochemically grown TiO2 nanotubes based sandwich-structured device was fabricated to realize the selectivity technique. The proposed method of selective detection of VOCs involved simultaneous measurement of both resistive and capacitive responses of the sensor upon exposure to different VOCs. By using these two types of responses, an algorithm was developed to formulate a new quantity known as ‘selectivity constant’ (S) for each VOC. This quantity exhibited a unique and concentration independent numerical value for different VOCs. The developed algorithm was tested successfully to identify different types of VOCs like methanol, ethanol, acetone and 2-propanol and their concentration.Item Synthesis of GO Loaded TiO2 Nanotubes Array by Anodic Oxidation for Efficient Detection of Organic Vapor(Springer, 2019-06) Hazra, ArnabThe present study concerns the synthesis of highly ordered graphene oxide (GO) loaded TiO2 nanotubes array by the electrochemical anodization route. Structural and morphological characterizations of pure and GO loaded TiO2 nanotubes array were carried out by x-ray diffraction spectroscopy, energy dispersive spectroscopy and field emission scanning electron microscopy study. Optical characterizations were performed with Raman spectroscopy and photoluminescence study to explore the composition of both the TiO2 nanotubes array. A conductometric solid state vapor sensing device having sandwich-type structure (Au/TiO2 nanotubes/Ti) was fabricated by using both the pure and GO-loaded TiO2 nanotube array and tested towards reducing vapor like methanol. The response was double in case of GO loaded TiO2 nanotube array (40%) as compared to pure TiO2 nanotube array (20%) based sensor at room temperature (300 K). The overall study confirmed that electrical properties of the TiO2 nanotubes array were improved due to the GO incorporation while morphological parameters were intact.Item Multiple nano-filaments based efficient resistive switching in TiO2 nanotubes array influenced by thermally induced self-doping and anatase to rutile phase transformation(IOP, 2020-12) Hazra, ArnabIn this paper, the impact of thermally induced self-doping and phase transformation in TiO2 based resistive random-access memory (ReRAM) is discussed. Instead of a thin film, a vertically aligned one-dimensional TiO2 nanotube array (TNTA) was used as a switching element. Anodic oxidation method was employed to synthesize TNTA, which was thermally treated in the air at 350 °C followed by further annealing from 350 °C to 650 °C in argon. Au/TiO2 nanotube/Ti resistive switching devices were fabricated with porous gold (Au) top electrode. The x-ray diffraction results along with Raman spectra evidently demonstrate a change in phase of crystallinity from anatase to rutile, whereas photoluminescence spectra revealed the self-doping level in terms of oxygen vacancies (OV) and Ti interstitials (Tii) as the temperature of thermal treatment gets increased. The electrical characterizations establish the bipolar and electroforming free resistive switching in all the samples. Among those, the ReRAM sample S3 thermally treated at 550 °C displayed the most effective resistive switching properties with ROFF/RON of 102 at a read voltage of −0.6 V and a SET voltage of −2.0 V. Moreover, the S3 sample showed excellent retention performance for over 106 s, where stable ROFF/RON ≈ 107 was maintained throughout the experiment