ZnO based organic-inorganic hybrids : properties and device applications

Abstract

In the current interdisciplinary age of nanotechnology, metal oxide based semiconductors have gained substantial interest. This is mainly because of their applications in various technological areas, such as nanoelectronics, optoelectronics, sensors, etc. One of the most studied metal oxide for potential applications is zinc oxide (ZnO) due to its attractive physical and chemical properties. Also, the material characteristics such as wide bandgap, large surface area to volume ratio, intrinsic n-type conductivity, and a large exciton binding energy, have made ZnO an exciting material for various technological applications. Sensors based on ZnO are generally seen to have high chemical and thermal stability. However, they operate at high temperatures and lack selectivity [1], [2]. Hence, a stable device with high selectivity and high response at room temperature is still the concern of the research community. Interestingly, the synergetic effects between organic and inorganic components have been reported to have improved both properties and performances [3]. Therefore, our main objective is to reduce the drawbacks of ZnO based sensor, i.e. high operating temperature and low selectivity,by adding an organic moiety to it and making an organic-inorganic hybrid. In this thesis work, two ZnO based hybrid materials are synthesized, characterized and used as the active material for device fabrication. The summary of the present thesis work, in view of the main objective, is as follows:ZnO-graphene quantum dots (GQD) hybrid: o Synthesis: Synthesis of GQD, ZnO, and ZnO-GQD hybrid. o Characterization: Study of optical, elemental, electrical, crystalline and morphological properties of the grown thin films (GQD, ZnO, and ZnO-GQD hybrid). o Application: Enhanced properties for optoelectronic application.  ZnO-oligo(p-phenylenevinylene) (OPV) hybrid: o Synthesis: Synthesis of OPV, ZnO and ZnO-OPV hybrid. o Characterization: Study of structural, elemental, electrical, morphological and optical properties of the grown thin films (OPV, ZnO and ZnO-OPV hybrid). o Applications:  Fabrication of ZnO-OPV based band-selective UV photodetector.  Fabrication of ZnO-OPV based highly selectiveammonia gas sensor. Firstof all, a comparative analysis has been done on the properties of graphene quantum dots (GQDs) and ZnO-GQD organic-inorganic nanocomposite films, deposited using a simple electrochemical deposition process. The room-temperature photoluminescence (PL) spectra of the ZnO-GQD, GQDs and ZnO structures were studied. The PL peak from ZnO was observed at 380 nm. Electrodeposition process might lead to anon-uniform film deposition with various intrinsic defects which generally helps in producing a broad emission PL spectrum. ZnO film, primarily grown under oxygen-rich conditions, begets most presumptive intrinsic defects like zinc vacancies, oxygen interstitial or the extrinsic defects like trapped OH-, some functional group impurities, etc. With the excitation of 325 nm, GQD exhibits a relatively broader PL peak at 423 nm. These PL peaks are attributed from aromatic - stacking interactions. ZnO-GQD nanocomposite shows a distinct PL peak at 370 nm and looks similar to the PL peak from ZnO. However, the spectral location of PL peak (380 nm) observed in ZnO film shows a marginal blue shift towards the UV region (~370 nm) in the case of ZnO-GQD nanocomposite. Further, the band gap as obtained from PL was verified using spectroscopic ellipsometry (SE). The current-voltage (I-V) characteristics of all three samples showed a similar behaviour of photocurrent variation. Both the dark and the irradiated currents increase linearly with increasing bias voltage. The values of photosensitivity of ZnO, GQD, and ZnO-GQD nanocomposite were found to be 6.77, 51, and 99.32, respectively at room temperature. Further, in the perspective of device fabrication, the hybrids containingconjugated organic moiety have been exploited in areas such as photodetectors, transistors, sensors, etc. Blending organic molecules with the inorganic network is seen to offer extra benefits such as tuning the optical spectra, the lifetime of trap states, sensitivity and response time. Sensitivity can be optimized by controlling the structure of nano-hybrids to maximize the density of organics bound to the inorganic surface and also minimize the turbulence and discontinuities in conduction through the permeating network. This perspective inspired us to accomplish our goal of synthesizing self-assembled hybrid nanostructures having both electronically active organic (i.e. OPV) and inorganic (ZnO) components. Hence, a hybrid photodetector containing ZnO and functionalized OPV was fabricated, which was deposited electrochemically onto an ITO-coated glass electrode. X-ray diffraction (XRD) results revealed the lamellar structure of the conjugates. As photodetectors, room temperature responsivity as high as 0.2 AW-1 at 330 nm with a bias of -20 V, themaximum external quantum efficiency of 75% at -20 V bias was observed. When the ZnO-OPV hybrid photodetector device was illuminated with a wavelength of 325 nm, excitons are generated in both OPV and ZnO. On externally applying a bias, these photo-generated excitons in ZnO or OPV dissociate at various interfaces formed between the oligomer and ZnO inside the bulk and thus form a complimentary trajectory for the electrons from oligomer to transfer onto ZnO and holes to transfer onto OPV. However, both the rise and fall time of the response were approximately 0.8 s, indicating average photoresponse characteristics. Most likely, the charge trapping at the grain boundaries of the hybrid film is detrimental to the device performance. Also, the presence of deep traps, which have longer charge release times, renders the device sluggish. Furthermore, to check the stability of the devices, the devices were exposed to air and kept in room ambience for eleven months, but no visible change in device performance was observed.Additionally, an electrochemically deposited hybrid ammonia gas sensor containing ZnO and OPV was fabricated. As a gas sensor, the response as high as 8.83 at 10 ppm of ammonia have been observed. Also, the excellent selectivity and the response time and recovery time of 9.8 s and 17.3 s, respectively, at room temperature can effectuate forthcoming high-performance hybrid gas sensor which could be cost-effective and environmentally benign. The sensing responses of the three sensors viz. OPV, ZnO and ZnO-OPV sensors were measured by exposing them to different concentrations of NH3 gas at room temperature. It is seen that, among the three sensors, Zn-OPV sensor shows the best response. For the ZnO and the OPV sensors, the responses at all the concentrations are almost similar, i.e., they do not change much. However, in the case of ZnO-OPV sensor, the change in response to the change in concentration is quite noticeable. Also, it is very clear from the results that the response of the ZnO-OPV sensor to lower concentrations of ammonia is much higher than that of bare ZnO and OPV based sensor. This implies that ZnO-OPV can be used even for sensing the lower concentrations of NH3 gas. Highselectivity is most desirable parameters for a gas sensor to be practically valuable. Therefore, the room temperature selectivity of the ZnO-OPV thinfilm towards various toxic gases such as hydrogen sulphide, carbon dioxide, and carbon monoxide was studied. The responses were 1 to 100 ppm H2S, 0.1 to 1000 ppm CO2, and 0.2 to 100 ppm CO, respectively, as compared 8.83 to 10 ppm of NH3. In hydrated state (in the presence of humidity), the ionic mobility across the peptide bonds arises due to the H+ transfer from -NHCO- to –N-COH- via the hydrogen bonding with water. The H+ transfer becomes restricted in the presence of NH3 due to the formation of [NH3...H2O] complex formation. As a consequence, H+ transfer across the peptide molecules become hindered, and resistance increases along the organic layer. Although, the exact reason for the high selectivity of ZnO-OPV towards ammonia is not clear, yet, the reason that might account for the excellent selectivity is: ammonia has a strong electron donating ability due to the presence of a lone electron pair, and thus it can readily donate the unpaired electrons. The sensing layer, consisting of the p-type OPV might have a higher binding affinity for the electron-donating ammonia. Hence, the electrons get transferred to the sensing layer, which may lead to a significant change in resistance. Also, the self-assembly of OPV and ZnO provides a continuous path for charge transfer, thus, providing a faster response. The major contributions of the work can be summarily concluded as below.  A simple and facile methodology, i.e. electrodeposition is demonstrated for synthesizing GQD and ZnO-GQD nanocomposite films. The study showed improved properties of the ZnO-GQD hybrid nanocomposites film over pure organic GQD and inorganic ZnO film.  The ZnO-OPV hybrid photodetector is demonstrated for the first time in literature. The photoresponse and external quantum efficiency of the nanohybrid photodetector were measured at different bias voltages and reached 0.2 AW-1 and 75.5%, respectively, at -20 V. These experimental findings indicate that the hybrid nanostructure of ZnO-OPV may be especially well suited for the use in high-performance nanoscale photodetectors.Sensors with sensing layers made up of OPV, ZnO and ZnO-OPV composites are successfully synthesized by electrochemical deposition. The composite based sensor exhibited the highest response of 8.83 to 10 ppm of NH3. It showed an excellent selectivity to NH3 among various test gases.