Research Projects

Chirality is a ubiquitous feature in biological systems and occurs even in certain inorganic crystals. Interestingly, some inorganic nanocrystals (NCs) have been shown to possess chirality, despite their achiral bulk forms. Chiral metal halide perovskite NCs expanded the functionalities to areas such as circularly polarized light-emitting diodes, optical sensors, spintronics, and ferroelectric applications. To date, the synthesis of chiral perovskite NCs has been carried out mostly by using external chiral molecules in the perovskite framework, where the chirality is transferred from the chiral molecules to the perovskite crystal structure. In such extrinsic chirality, the interaction of the chiral ligands with the perovskite NCs induces chirality to the crystal lattice by distorting the packing of the atoms from the NCs surfaces. In such systems, the chirality is dominated by the defect states and unrelated to the states of NCs without defects. In other words, chirality was earlier achieved by altering/destroying the underlying crystal structure of the pristine perovskite NCs. Therefore, chirality induced spin selectivity (CISS) becomes somewhat difficult since the external chirality of the transporting medium blocks the spin ½ states to transport through the media. Thus, a key challenge in a spintronic device is to control the spin-polarized current through the chiral transporting medium with well-defined spin states. The intrinsic chirality of perovskite NCs can be stronger than the chirality induced by external chiral molecules since the chiroptical effect is realized from all atoms of intrinsic chiral NCs. However, reports on the intrinsically chiral halide perovskite NCs are indeed rare. These halide perovskite NCs offers an unprecedented opportunity to explore how intrinsic chirality provide spin control within these halide perovskite semiconductors. In this proposal, we intend to synthesize intrinsically chiral all-inorganic halide perovskite NCs for the fabrication of electron spin filters. This approach is very different from previous reports based on chiroptical activity in achiral NCs using chiral ligands. Furthermore, the dimension of the NCs will be tuned at different length scales to testify the effects on the chiroptical properties. After establishing the chiroptical properties of the NCs, we intend to use these intrinsically chiral NCs for the fabrication of electron spin filters using ferromagnetic electrodes as spin injectors. The spin polarization of injected current through the chiral perovskites will be preferential to one of the spin states depending on the handedness of the chirality of the NCs. Because the preferred orientation of the spin transmitted through the chiral NCs will depend on the direction of charge flow, preferred spin will be transmitted and can be used to induce magnetization in the NCs. It will be an interesting opportunity to explore how the intrinsic chirality provides the spin control within these halide perovskite NCs. The spin selectivity of the NCs will be tested at the local level and global level to examine spin transfer using magnetic conductive probe-atomic force microscopy (mcAFM), Kelvin probe force microscopy (KPFM), magneto-optic Kerr effect (MOKE) measurements, electron paramagnetic resonance (EPR) and electron spin resonance (ESR) spectroscopy. These measurements will unequivocally probe chiral NCs as electron spin filters. Finally, spintronic devices will be fabricated using a single or two ferromagnetic electrodes along with the chiral NCs. The successful demonstration of the high spin selectivity in such halide perovskite NCs will substantially expands the applications of the CISS effect.