Enhancing stability of semiconductor nanocrystal quantum dots through surface engineering

Abstract

Colloidal semiconductor nanocrystal quantum dots (NQDs) have obtained tremendous attention in past decades since their size-dependent tunable band gaps make them especially suited to various optoelectronic applications such as photovoltaics, light emitting diodes, lasings, and lightings. PbSe NQDs is one of the most intensively investigated NQDs because PbSe has a very narrow band gap (0.26 eV) ranged in infrared, large exciton Bohr radius (46 nm), and a high dielectric constant, which are essential for the optoelectronic applications using infrared regime. In particular, strong quantum confinement, efficient carrier multiplication and prolonged hot carrier lifetime due to phonon bottleneck effect of PbSe NQDs were reported, hence the interest of PbSe NQDs especially in photovoltaic devices is very high. However, further development and fabrication of reliable optoelectronic devices based on PbSe NQDs were strongly hindered because they easily oxidized under air condition. Formation of oxidized species on the surface of PbSe NQDs was reported previously. The formation of oxidized species leads to the uncontrolled changes in the optoelectronic and physicochemical properties of NQDs, and finally results in sudden failure of the NQD-based devices. Therefore, enhancing air stability of NQDs is essential for the realization of reliable NQD-based optoelectronic device. Recently, synthesis of all inorganic colloidal $CsPbX_3$ (X is Cl, Br, and I) perovskite nanocrystals (NCs) has been developed. The $CsPbX_3$ NCs showed very high photoluminescence (PL) quantum yield (QY), narrow emission linewidth, which is suitable particularly in light emitting diodes. However, $CsPbX_3$ NCs exhibit very poor stability under ambient condition. Rapid drop of PL QY and structural transformation of $CsPbX_3$ NCs have been observed. Hence, as desired in the case of PbSe NQDs, stability enhancement of $CsPbX_3$ NCs is strongly required for successful integration of the NCs in optoelectronic devices including light emitting diodes. Here, I present very simple, but highly effective surface engineering strategies to enhance the stability of PbSe NQDs and $CsPbX_3$ NCs (denoted hereafter as just NCs). The surface engineering strategies are developed based on understanding the surface structure of the NCs. Surface engineered and effectively passivated NCs exhibit extreme stability against air or any other harsh conditions. Based on the surface engineered NCs, I successfully demonstrate the fabrication of NC-based field effect transistors with long-term operation lifetime.