Zinc oxide, ZnO, is an intriguing material with applications spanning from simple to highly advanced and sophisticated technologies. Its wide direct band gap has made it useful as UV-absorbing additive in everything from sunscreens and rubber to advanced plastics.  Various nanoscale morphologies of ZnO have also emerged as promising candidates for a large set of new high-tech applications. Among these are UV-lasers, light-emitting diodes, field emitters, piezoelectric and spintronic devices, gas sensors, transparent conductors, photovoltaics, and photocatalysis.  Several of these nanoscale applications benefit from the control of the energy states where the position, nature, and relation between the states in the material affect the optical behavior. This is especially true for low-dimensional ZnO nanoparticles where the properties of the states will be a function of particle size if dimensions are made small enough.[1] Here we present experimental methods to extract the optical band edges and fluorescing trap states in ZnO quantum dots by combining electrochemistry and UV-spectroscopy.[2,3] We present the shift of band gap with particle size and how the absolute band edges shifts for up to 18 different sizes of low-dimensional  ZnO nanoparticles below the quantum confinement size regime. Time-resolved fluorescence data in the quantum confined regime and the possibility for surface stabilized excitons will be presented.[4]The development of collective vibrations and eventually phonons in the materials are also presented by combining experimental Raman spectroscopy and theoretical simulations where the vibrational quantum confinement regime as larger than the corresponding electronic quantum confinement. [5] We finally present a quantum confined Stark effect that disappears when dimensions of the particles approaches the bulk band gap regime at around 9 nm.[6] We will also briefly touch upon our more recent work on utilizing Raman spectroscopy to extract electron-phonon coupling in ZnO, and optical quantum confinement and exciton emission in 2D lead halide perovskites.

[1] Edvinsson, T. Optical Quantum Confinement and Photocatalytic Properties in Two-, One- and Zero-Dimensional Nanostructures. Royal. Soc. open sci. 2018, 5: 180387.
[2] Jacobsson, T. J.,  Edvinsson, T.  Photoelectrochemical Determination of the Absolute Band Edge Positions as a Function of Particle Size for ZnO Quantum Dots, J. Phys. Chem. C, 2012, 116, 15692.
[3] Jacobsson, T. J.; Edvinsson, T.  A Spectroelectrochemical Method for Locating Fluorescence Trap States in Nanoparticles and Quantum Dots, J. Phys. Chem. C 2013, 117, 5497.
[4] Jacobsson, T. J.; Viarbitskaya, S.; Mukhtar, E.; Edvinsson, T.,  A Size Dependent Discontinuous Decay Rate for the Exciton Emission in ZnO Quantum Dots,  Phys. Chem.
Chem. Phys.  2014, 16, 13849.
[5] Raymand, D.,  Jacobsson, T.J., Hermansson, K., and  Edvinsson, T. Investigation of Vibrational Modes and Phonon Density of States in, ZnO Quantum Dots, J. Phys. Chem. C  2012, 116, 6893.
[6] Jacobsson, T. J.; Edvinsson, T., Quantum Confined Stark Effects in ZnO Quantum  Dots Investigated with Photoelectrochemical Methods, J. Phys. Chem. C 2014, 118, 12061.