493 research outputs found
Shining light on hybrid perovskites for photoelectrochemical solar to fuel conversion
No full textHybrid halide perovskites (HaPs) represent a class of material with excellent optoelectronic properties providing distinct avenues for disruptive photo(-electro) catalytic technologies. However, their photocatalytic activity, selectivity and stability remains a scientific and technological hurdle. In this perspective, we discuss fundamental aspects of perovskite based photocatalytic systems, specifically for CO2 conversion and high value oxidation reactions, and highlight critical limiting factors and on-going challenges in the field. We critically assess the recent advances in designing halide perovskite hetero-interfaces and characterization methodologies which are often used to define the performance metrics. Furthermore, we outline important questions and identify emerging trends in relation to the remediation strategy towards improved photocatalytic performance and stability from halide perovskite semiconductors.S. S. acknowledges funding from the European Union’s Horizon Europe program under the Marie Skłodowska-Curie Grant Agreement No. 101067667. S. S. and V. J. acknowledge Catalisti VLAIO (Vlaanderen Agentschap Innoveren & Ondernemen) for their funding through the Moonshot SYN-CAT project (HBC.2020.2614), and the Belgian federal government through the Energy Transition Fund for T-REX project. NM would like to acknowledge the funding from the National Research Foundation (NRF), Singapore, under its Competitive Research Program (CRP) (NRF-CRP25-2020-0002). The authors would like to thank Dr Tom Aernouts and Prof. Bart Vermang for fruitful
discussions
Tunable electroluminescence for pure white emission from a perovskite-based LED
No full textRaw data for project titled "Tunable electroluminescence for pure white emission from a perovskite-based LED"</span
Multimode memtransistors as optoelectronic synapses for neuromorphic computing
No full textRecently, a neuromorphic approach to electronics has gained attention by bringing a fundamentally different approach to existing computing architectures for pattern recognition and learning applications. Emulating complex neural behavior for a synapse through conventional Si-based devices requires many elements, increasing fabrication complexity and bringing challenges to connectivity and energy consumption. Thus, there is a need to investigate alternative material systems and device architectures for emulating neural behavior. The abrupt switching physics of most two-terminal memristors (memory + resistor) limits the number of addressable states to two/binary, limiting their plasticity and storage capacity that adversely affects the trainability of artificial neural networks. The coupling of more than one control terminal is indispensable to elicit multiple programmable conductance states and non-abrupt state transitions as weighted connections to store and update weights necessary for learning algorithms. By adopting multimodal electro-optical schemes that ‘gates’ the memconductance, one can realize multi-state optoelectronic synapses with higher order weight changes that standard two-terminal devices fail to address. This thesis explores various interfacial strategies to exploit three-terminal gate-tunable memristors (aka Memtransistors) for emulating higher-order synaptic functions.Doctor of Philosoph
Tunable Electroluminescence for Pure White Emission From a Perovskite‐Based LED
No full textHalide perovskite nanocrystals are a promising candidate for lighting applications. However, the production of white light emitting diodes (LEDs) is still a major challenge due to halide ion segregation. In this work, it is demonstrated that reducing the thickness of the perovskite layer in an LED stack can modulate the recombination zone, such that a tunable emission can be obtained. This comprises of an orange electromer emission from a hole-transport layer (HTL), green electroluminescence from the perovskite active layer, and a blue monomer emission from the same HTL. Overall, a pure white emission can be achieved after successful device optimization, which is particularly challenging for LEDs in which the emission originates solely from perovskite layer. It is anticipated that this methodology could be employed on any type of green-emitting nanocrystals to fabricate white LEDs.Nanyang Technological UniversityNational Research Foundation (NRF)P.V. acknowledges a NTU, Singapore Presidential Postdoctoral Fellowship via grant 04INS00581C150. The authors acknowledge financial support from the Singapore National Research Foundation, Prime Minister’s Office, through the Competitive Research Program (CRP Award No. NRF-CRP14-2014-03)
Oxalic acid leaching and eggshell wastes adsorption for recycling of solar cells
No full textIncreasing climate change and global warming have risen concerns and drive the search
for alternative energy sources, preferably greener and renewable. Photovoltaics (PV) is
one of the promising candidates as it is clean and generates no hazardous emission. The
industry has grown drastically ever since, as many switch from traditional energy source
to PV to exploit its benefits. Combined with government policies to promote PV as an
energy alternative, the growth has been further catalysed. With the increasing number of
installations and the limited lifespan of 25-30 years, it is essential that proper waste
management for End-of-Life (EoL) panels is in place to deal with the large, expected
volume of wastes. Many raw materials are present in the solar panel wastes and it is
crucial to extract and repurpose these materials. Metal extraction/dissolution has been
usually carried out with strong acids and/or bases such as nitric acid (HNO3), hydrofluoric
acid (HF) and sodium hydroxide (NaOH). The utilisation of these harsh chemicals can
have detrimental effects on the users and environment. Noxious gases like NOx can get
released in the process of extracting metal ions using HNO3, while HF and NaOH are
highly corrosive and have adverse effects to environment if not properly treated before
discharge. Therefore, it is necessary to strategize an environmentally friendlier leaching
approach. This project focus on the utilization of oxalic acid in replacement for strong
inorganic acids for leaching, and eggshell as biosorbent to remove aluminium (Al) from
the leachate solutions. Herein, we evaluated the potential of oxalic acid as leaching reagent
for solar cell waste as well as demonstrated eggshell as biosorbent to remove Al from the
collected leachate solution.Bachelor of Engineering (Materials Engineering
Direct ink writing for electroresponsive human machine interfaces
No full textTo aid with efficient and reliable communication with machines, Human-Machine Interfaces (HMIs) are crucial. Current approaches for HMIs rely on rigid, non-compliant devices. This structural non-compliance with inherently soft, curvilinear human body makes interfaces non-intuitive and limits widespread applications of HMIs. Hence, it is necessary to develop newer, compliant form factors for HMIs via development of soft sensors and responsive devices.
Existing methods for fabrication of soft devices require high temperature processing and are hence not suitable for large scale devices made with soft polymeric materials. Extrusion based additive manufacturing methods such as Direct Ink Writing (DIW) show great potential for this application. This work focuses on optimization and modification of DIW system to handle various viscosities of inks, and special functional materials, to fabricate all-printed HMI devices.
Initially, a custom DIW setup was assembled in-house. 3 axis motion stage was used for moving the dispensing head. Syringe pump and peristaltic pumps were used for low viscosity inks. Pneumatic ink dispenser was used for highly viscous shear thinning inks. The Syringe-pump system was modified to accommodate Phase Change Material (PCM) ink for tactile response devices. Formulations of various composite material systems were optimized for printing via DIW systems. First, a low viscosity conducting ink of PEDOT:PSS and MWCNT aqueous suspensions was prepared. 2 different dilutions of MWCNT, 0.5mg/ml and 1mg/ml were prepared and mixed with PEDOT:PSS in varying volume ratios. 1:3 ratio of PEDOT:PSS to 0.5 mg/ml MWCNT suspension was found out to have lowest resistivity. Subsequently, Acetylene black nanoparticles were mixed with PDMS to form viscous conducting composite ink. Percolation threshold for the network is found out to be at ~13%(w/w). To maintain printability, loading was limited to 15% and ink was prepared by adding crosslinker and thinner to the mixture. Further, Polyethylene glycol (PEG) was dispensed using a modified setup to keep it molten. Ti3C2 MXene was intercalated using LiBr and DMSO and delaminated to prepare 10mg/ml suspension which was added to PEG. The PEG-MXene composite had higher amount of nucleation sites. This helped in speeding up the phase transformation of printed patterns. For the sensing part of HMI, various tactile sensors were printed using the developed conducting inks. PEDOT:PSS-MWCNT composite was used to fabricate bending angle sensor, and strain sensor. The bending angle sensor shows fast, highly linear response with
up to 10° resolution, and a rage of 0-180°. The strain sensor successfully measures small strains with ΔR/R0=15 measured for 5% strain. Further, Acetylene black – PDMS ink was utilized for fabrication of proximity sensor. Material differentiation and water content detection using these sensors is demonstrated. Different sensor geometries were tested and upto 2% of ΔC/C0 was achieved for metallic object. PEG-MXene PCM ink was used along with silver joule heater to fabricate flexible hardness modulation based
tactile response devices. A thermally modulated transition resulting in 10 times change in hardness of the material was obtained as a response. A 3*3 array of such devices was printed to demonstrate display of letters via hardness modulation
Finally, future scope of this project is discussed. Challenges for printing inks such as MXene dispersions and Liquid Metal are presented. Results of initial experiments carried out for newer printing paradigms such as 3D printing of stretchable materials and multimaterial printing is discussed. Further, newer devices such as all printed stretchable electroluminescent devices and printed active devices such as TFTs are proposed.Master of Engineerin
Metal oxide memristors for neuromorphic electronics
No full textNeuromorphic electronics aim to emulate the functionalities of the brain and enable the next generation of power efficient devices for artificial intelligence applications. Neuromorphic devices emulate various neural features including synaptic plasticity- which is the brain’s ability to re-wire itself during training and is credited for the brain’s efficiency in processing information and learning. Although silicon CMOS based approaches exist, efficient implementation of neuromorphic behavior necessitates the development of memristive devices based on novel materials and device architectures. Present memristive approaches for neuromorphic electronics utilize conventional two- terminal oxide memristors which operate based on filamentary formation and rupture. These conventional devices, although suitable for non-volatile memory applications are less suited for neuromorphic electronics since they do not natively demonstrate multiple states and critically show poor control over the temporal response. Essentially, these devices fall behind in terms of the required analogue programmability due to its stochastic nature. As memristors are devices that encode information in conductivity levels, it is important to study the properties not only of the bulk materials but also the interfaces to induce richer neuro-emulative behavior. Alternative programming modes and device structures which go beyond electrical biasing of 2 electrodes need to be explored. The overarching aim of this thesis is to explore such approaches to improve memristive device performances by tuning the number of states and temporal response. This can only be achieved through a mechanistic understanding of the charge transfer, charge transport and chemical changes in the oxide memristor device. The work presented in the thesis was able to address various challenges in the area of memristive devices for neuromorphic electronics by – (1) inducing multiple states in a memristor device by control over filament formation, (2) tuning the temporal (transient) response of the devices by utilization of electrical and optical stimulation and (3) incorporating simultaneous state and temporal control via a third electrode and optical input. Based on these approaches, devices with multi-state programmability and temporal tunability has been shown. Lastly, these properties were utilized to emulate functionalities inspired by biological systems demonstrating synaptic
Abstract
plasticity, the phenomenon of inhibition in memory formation and sensory adaptation to intensities similar to the eye.
In the first work that focuses on filamentary memristors - the controllability of intermediate switching is targeted. Resistive switching is often explained by the formation and rupturing of conductive filaments comprised of oxygen vacancies. With an ion-blocking electrode impeding the oxidative process, the suppressed anodic reaction can lead to poor controllability of the resistive switching process. By employing electrodes of high reduction potential or oxygen affinity, the memristor is shown to have improved multi-state programmability. Through spectroscopic analysis, the formation of Sn(0) species during the resistive switching process was verified ex situ. With multi-state programmability, synaptic functionalities such as short-term plasticity, long-term plasticity and spike-timing dependent plasticity are emulated. While intermediate states can be achieved, the level of temporal response in filamentary switching is insufficient for neuromorphic application.
Temporal response similar to a biological synapse was targeted using light as an additional means to modulate the conductance of the memristive systems. In the second work that focuses on photo-modulation – the inhibition of long-term weight changes is explored. Here, a photomemristor device based on the Schottky ITO/SnOx interface is fabricated. With a low-temperature low-oxidative thermal atomic layer deposition process, the SnOx thin film with high oxygen vacancies acts as the active switching layer. Through impedance measurements, the photomodulation mechanism is compared with the interfacial switching process. The switching behaviour of optical devices can be explained by the de-trapping of electrons at the oxide-oxide interface via photo-excitation. However, the same interface is also responsible for electrical based memristor switching. By applying electrical pulses before the optical pulses, we have shown that the memory retention of optical trainings can be tuned on demand via inhibition and facilitation. Lastly, with electrical pre-exposure, a self-filtering function known as latent inhibition can be demonstrated in associative learning by the synapse.
Abstract
Finally, in the third work a system which allows for simultaneous control of states and temporal decay was demonstrated. While a strong dual mode coupling can be demonstrated in photomemristors, they lack selectivity in terms of photoresponse. With a third electrode as a gating option, doping/de-doping of the electrochromic MoO3 semiconductor with Li+ can be used to tune the optoelectronic properties and hence the photoresponse. Through spectroscopic analysis, the alteration in band structure of MoO3 can be verified. This explains how both the optical and charge transport properties can be modulated with electrochemical doping/de-doping. While electrochemical transistors have been explored as synaptic devices, the tunability of optoelectronic properties is not yet utilized. Here, with reversible electrochromic switching, multiple state programmability and temporal response tunability can be demonstrated in the electrochromic transistor device. Lastly, with the tunability in photoresponsivity, adaptation features such as scotopic and photopic vision in the eye can be emulated.
The thesis concludes with how further improvement on analog properties and the emulation of neuronal features can be explored. Lastly, with the development of functional memristors in processing and sensing, the opportunities in memristor-enabled computing and sensing platforms are discussed.Doctor of Philosoph
Self-healing materials for flexible electronic devices
No full textWith the onset of flexible and wearable electronics, devices are now put on places like the human body and various curved surfaces which doesn’t have any regular shape and sizes. Therefore, these devices need to be flexible, conformable and stretchable to fulfil all these applications. However, the continuous exposure of these devices is always subjected to higher mechanical stress, wear and tear when compared to conventional electronics. Hence the ability to recover and repair upon damage is a necessity and not a luxury for flexible electronic devices. These applications not only require good electrical conductivity but demand better mechanical properties like stretchability, flexibility and robustness where the conventional electrodes, for example, indium tin oxide (ITO) fail to perform. Hence, self-healing, flexible, transparent electrodes could transform the way of fabrication of electronic devices in future. Challenges associated with mechanical fracture of electrical conductors has hindered the realization of truly flexible, robust and high-performance wearable electronics. The remarkable achievements in transparent and flexible electrodes have raised widespread interest in research groups in flexible electronics, owing to their low-cost fabrication, easy scale-up, and unique properties. However, they still suffer from innate problems like mechanical rupture, scratching and bending torsion because of their “soft” and flexible nature related to their solid counterparts. Hence, self-healing capability would be highly desirable for these electrodes in flexible electronic devices.
In this dissertation, the demonstration of versatile transparent healable electrodes has been developed and examined to alleviate these problems. The composite electrode features a layer of interconnecting AgNWs network on a polyurethane film modified with Diels–Alder adducts (PU-DA). The PU based DA polymeric electrode can heal multiple times due to the presence of DA and retro-DA mechanism, which are based on thermo-reversibility. Surface modification using hydrophilic molecules improved adhesion of the AgNWs and resulted in mechanically robust flexible electrodes with a high figure of merit; showing low sheet resistance with good transmittance in the visible spectrum. Transparent and flexible healable heaters (TFHH) with good mechanical and thermal stability were fabricated using these electrodes for potential applications in thermochromics, electrically driven displays and defrosters. The PU-DA based healable heaters exhibited high Joule heating temperatures with a low operation voltage, rapid thermal response and enhanced robustness to withstand large repeated mechanical strain for over 500 bending cycles with small variance in resistance. After deliberate damage by a knife cut, the electrodes healed and recovered back to its original conductivity via a simple heat treatment at 120 °C. Uniquely, the healing process can also be triggered by utilizing electrical power.
The self-healing polymer with the addition of ionic liquids (ILs) may have great potential for future electronic materials. ILs would make it possible to integrate functional excipients to recognize multifunctional electrical applications; owing to the intrinsic advantages of suppressed volatility, high ionic mobility, thermal stability, and a wide range of electrochemical window. Thermally-reversible Diels-Alder (DA) mechanism for self-healing are promising but have only been demonstrated healing at high temperatures (~120 °C). However, theoretically, the DA mechanism can be triggered at temperatures as low as 50 °C, indicating that the self-healing mechanism is limited by the thermal mobility of the polymeric chains. Next, the effect of ionic liquid as a plasticizer was investigated in PU-DA in order to minimize the healing temperature and increase the mechanical properties of the polymeric composite. The incorporation of ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate (EMITFS) alleviates this challenge, rapidly accelerating healing, while concomitantly improving the dielectric constant and the mechanical properties of a polyurethane derivative based on the DA chemistry for PU. For optimized compositions, the healing temperature reduced from 120 °C to 60 °C and the maximum strain to failure significantly increased from 17.1% to 102.1%. Owing to the ionic polarizability of EMITFS, the composite exhibited highly attractive dielectric properties with the dielectric constant enhanced from 2.7 to 12.9. Finally, a highly flexible, healable and fully solution-processed electroluminescent device was demonstrated.
To tailor specific multi-property materials, usually, sophisticated multi-step synthesis processes are employed and will affect the device fabrication method. With the increase in concertation of ionic liquid, the polymeric films have increased in mechanical, electrical as well as healing properties as it was observed in the case of PU-DA. Taking that concept as a motivation, a composite system with healing behaviours in polymeric systems without DA recovery mechanism was investigated. The incorporation of ionic liquid (EMITFS) in amorphous polymer like PVP, rapidly accelerates the healing, while improving the mechanical properties. The PVP with ILs composite films were compared with IL composites with another amorphous polymer, PMMA, as well as semi-crystalline polymers like PVA and PVDF-HFP to mimic the healing behaviours and mechanical properties, while incorporating EMITFS in these polymers. The effect of cation and anion present in ionic liquids was investigated, on the best polymer system (PVP), in terms of healing behaviour and mechanical properties. The different ionic liquids (EMITFB and EMITFSI) was used as compared to EMITFS since these ionic liquids have the same cation but different anions. This chapter depicts the study of the influence of ionic liquids on the different class of polymer structure and its effects on the healing properties. In the end, a novel healing mechanism with ionic liquids was demonstrated in PVP polymer without DA recovery mechanisms.Doctor of Philosoph
Novel material composites for dielectric elastomer actuators
No full textRobots have been around us for quite some time now. Robots are machines capable of performing tasks on their own with high accuracy and precision and minimal supervision to reduce human effort. Our daily lives are touched by different forms of robots around us, be it a direct interaction in form of robotic helpers at the airport or indirectly in the case of industrial arm robots assembling the electronics and machines. Traditionally, there are several component systems to robotics and one of the prominent one is actuation, which is made from hard and rigid components in these conventional robots. However, this is responsible for their intermittent motion. A new field of research, soft robotics, makes of soft materials for their fabrication and draws inspiration from natural organisms and how they produce fluidic and complex motions. There are different ways by which these soft materials can be actuated and include techniques like applying pneumatic and hydraulic pressure, applying heat, making use of a chemical process like combustion, applying varying magnetic fields and applying electric fields. Among these, the class of actuators employing soft materials deformed by applying the electric field, known as Dielectric elastomer actuators, are preferred owing to their advantages of simple fabrication, large actuation strain and control by electrical stimulus.
The architecture of these actuators resembles a variable capacitor with electrodes on either sides of the elastomer, and their performance depends on material properties like dielectric constant and mechanical stiffness. Generally, researchers have tried to modify either of these properties with addition of solid conductive or ceramic fillers, or chemical modification of the elastomer, and this has led to undesirable changes on the other material properties. Drawing inspiration from biological materials like skin, which is a soft material system filled with fluids, a novel composite is synthesized utilizing a solid polymer matrix and a high dielectric constant liquid filler, an ionic liquid. An increase in dielectric constant is observed accompanied with significant reduction in mechanical stiffness. The novel composites show significant improvement upon the actuation performance of the commonly used elastomeric system at considerably low electric fields. Owing to the judicious choice of material systems, the composites also show high stability and transparency.
With emergence of new generation technologies such as display touchscreens, touchpads, virtual reality and augmented reality, human machine interaction and haptics has become ever more important. Soft actuators and dielectric elastomer actuators (in particular), owing to their inherent softness, have convincing advantage over the conventional rigid actuators. Additionally, state-of-the-art technologies for haptics primarily seek to simulate the perception of texture change with sensory manipulation. Furthermore, dielectric elastomer actuators in their current form suffer from visible obstructiveness and non-integrable architecture. A new device architecture for dielectric elastomer actuators, capable of producing on-demand surface texture change and local topographic features, is demonstrated. Owing to its unique architecture, a highly transparent, flexible and integrable configuration of the device is showed. Out-of-plane deformations and force feedback from the devices are shown to be well above the perceptual threshold of sensors, known as mechanoreceptors, present in our fingertips.
Another attribute of the biological appendages in natural organisms is the ability to reversibly modulate its mechanical properties upon demand, which still eludes the field of soft robotics. One of the best examples of this is found in human body, where muscles modulate their stiffness in accordance to the load distribution and movement. Most of the current approaches mange to only demonstrate reversible stiffness and involves combination of a passive reversible stiffness component with the existing actuator. Hence, a novel low-temperature phase change material is identified for enabling reversible rigidity in the soft actuators owing to its low melting point, significant gap between melting and solidification, superior thermal stability, high mechanical strength and high dielectric constant. Thermal control is chosen the modulation of mechanical properties, owing to its lightweight design and scalability. The fabricated reversible rigidity composites show impressive variations in stretchability, bendability and hardness. Owing to its ability to melt and re-solidify, the composites also demonstrate a healing behaviour with the ability to regain complete structural integrity and mechanical strength.
These novel ultra-soft, high-k liquid filler-based polymer composites, reversible rigidity composites and transparent and integrable device architectures for field-driven soft actuators could pave the way for next generation applications such as shape morphing, adaptive surfaces and transparent haptic devices.Doctor of Philosoph
Halide perovskite solar cells: an investigation of ink additives on perovskite crystallization
No full textPerovskite solar cells (PSCs) are among the 3rd generation of solar cells which can compete with the conventional bulk silicon solar cells in terms of power conversion efficiency (PCE). They have been hotly researched over the past decade, with their high absorption coefficients, tunable band gaps and inexpensive facile fabrication processes as the main selling points. In just 13 years since the first PSC was successfully synthesised, the PCE has skyrocketed from 3.8% in 2009 to 25.8% (25.5% certified) in 2021, with plenty of research and keen interest ongoing to improve device performance; and also to achieve the ultimate goal of commercialising PSCs. However, the main drawback of PSCs is their poor stability in the ambient environment, which is well-documented and adversely impacts efforts to commercialise PSCs. The stability of PSCs has been the subject of numerous studies by many researchers who seek to prolong the stability of PSCs besides improving their PCE.
In 2021, Michael Gråtzel’s group added a pseudohalide additive, formamidinium formate (FAHCOO), into formamidinium lead iodide (FAPbI3), which resulted in a then-world-record PCE for single junction PSCs at 25.6% (25.2% certified), as well as improved stability. HCOO- is one of several pseudohalide additives used to improve PCE and stability of PSCs. Other examples used include BH4-, PF6-, SCN-, and CH3COO-. Another pseudohalide, the methanesulfonate (MeSf) anion, is a potential candidate to improve the PCE and stability of PSCs. There have been very limited studies of the MeSf anion, even though Cs MeSf was studied before on quasi-2D PSCs. Thus, there is a high degree of novelty in using MeSf. In this project, FA MeSf was used as the additive to FAPbI3 to investigate the effects of the MeSf anion in improving the efficiency and stability of FAPbI3. With the addition of 1 mol% FA MeSf, there is a retardation in the α-to-δ phase transformation, as well as an improved PCE of 19.63%, as compared to the control devices (17.35%). Moreover, the addition of 1 mol% FA MeSf improved the shelf life and thermal stability of FAPbI3. In particular, a higher normalised PCE of 93.8% was achieved after more than 450 hours of storage at 25°C, 35% RH in the dark as compared to the control devices (80.5%). The addition of 1 mol% FA MeSf also resulted in the highest contact angle of 47.76° and hence, the highest amount of moisture stability. Although a higher normalised PCE was also achieved after more than 250 hours of storage at 65°C, and approximately 15% RH (47.8% vs 15.9%), there is still room for improvement in the thermal stability of FAPbI3. Thus, for future outlooks, encapsulation of the PSCs could be performed to provide further protection.Bachelor of Engineering (Materials Engineering
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