Nano-engineering of Atomically Thin Novel 2D Materials and Their Heterostructures
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2023
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Rahman, Sharidya
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Two dimensional layered material has attracted profound interest from material science and nanotechnology community, due to its marvelous optical, electrical and magnetic properties. In addition to Transition metal dichalcogenides (TMDs), several new ferroelectric, ferromagnetic (FM), antiferromagnetic (AFM and multiferroic layered compounds has been discovered recently, that have found potential applications from optoelectronics and photovoltaic to spintronics, computing and even medical sectors. Such advancement has been strongly influenced by novel physical phenomena and intriguing light matter coupling in these layered materials. This thesis primarily focuses on nano-engineering and nano-manufacturing of novel and emerging 2D materials using strain application and structural modulation, applied meticulously via nano wrinkles and moire heterostructures. This results in enhanced non-linear light matter interactions and magnetic properties. Then the dissertation focuses on using advanced nitrogen vacancy (NV) magnetometer to characterize 2D magnets, followed by analysis of light matter interaction in novel hybrid heterostructures involving magnetic and traditional TMDs.
Strain induced wrinkles and topological structures are mostly employed as crucial form of nano-engineering in emerging 2D layered materials, where strain could be controlled meticulously. Improved magnetic properties in strained FM material (Cr2Ge2Te6; CGT) are explored through magnetic force microscopy (MFM) mapping to precisely image the magnetic phases. large strain percentage is found to vehemently improves the Tc from cryogenic temperatures to room temperature, which is the first experimental demonstration of strain induced augmentation of Tc in layered magnetic compounds.
Effect of strain on light matter interactions are then investigated through second harmonic generation (SHG). Strain induced wrinkles, manifested in novel ferroelectric compound (CuInP2S6; CIPS) revealed extraordinary augmentation in non-linear photon emission which is dwindled in the flat layer and mitigates its potential application in non-linear optics. This effort adds CuInP2S6 among the branch of demanding non-linear optics with potential applications in optical parametric oscillators.
I have also found relative twist between two TMD monolayers can profoundly modulate the phonon dynamics by creating Moire supperlattices and periodic patterns. As an example, I studied WSe2/WS2 heterobilayers and showed that the heterostrain can lead to large displacement of phonon modes, modulated by the interlayer twist angle. Furthermore I also showed the orientation angle can also lead to significant variation in phonon position and vibration intensity, where the latter can be augmented by over 50 times due to resonant Raman process.
Second part of the thesis characterizes novel 2D magnets and heterostructures. AFM properties of a novel multiferroic compound (CuCrP2S6) has been examined using nitrogen-vacancy (NV) quantum imaging, Consequently, I showed the AFM can persist down to three layers of atomic thickness and exists in alternating layers due to uncompensated magnetism. This work can lead to interesting development of spintronic devices using multiferroic compound.
Final part of the thesis explores nano-engineering in layered materials through heterostructure formation, with an aim to improve light matter interactions. I used magnetic 2D material (CGT) to modulate the photoluminescence (PL) in conventional TMD (monolayer WS2). Heterostructures fabricated using different layers of CGT and monolayer WS2 revealed intriguing enhancement of PL spectrum. This was in part, manipulated by the strong layer dependent work function of CGT that was experimentally measured for the first time using kelvin probe force microscopy (KPFM). Dynamics of band alignment led to elusive resonance phenomena. Such hybrid heterostructures can be used potentially in solar cells improved efficiency.
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