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Analysis and determination of crystal orientation and exposed surface facets remain a challenge in nanomaterial science. In this work we show that polarized and non-polarized Raman spectroscopy can be useful tools to determine crystal plane orientation and conveniently be applied to spatial dimensions limited only by the diffraction limit of the excitation laser. The methodology is exemplified for wurtzite structured ZnO. Three different crystal facets, (0001), (1^-100), and (11^-20) of ZnO are investigated with angle resolved polarized Raman spectroscopy. The polarization direction dependences of the main Raman peaks are characterized and related to the experimental vibrational modes in the crystal lattice and corroborated by density functional theory (DFT) calculations using two different hybrid functionals. By exploiting the symmetry of the modes and differences in Raman intensity of the optically activated phonons, a simple model is derived for determining the relation between the polar and non-polar crystal orientation. The results are generalized to allow peak intensity ratio analysis using Raman spectroscopy with a non-polarized light source, making it compatible with Raman mapping, as well as to include a critical discussion on the ability to determine the crystal plane orientation and exposed crystal facets using this model for nano dimensional ZnO and equivalent models for other nanomaterials. As the approach allows for use of non-polarized light sources, near-field excitations and local plasmons can in an extension be utilized for determination of crystal orientation and exposed planes in dimensions much smaller than the diffraction limit.

Polluted water is a severe problem in many parts of the world and is expected to cause stress in water systems in developed countries with an increased use of chemicals and rising urban densification. Advanced oxidation processes (AOPs) using photogenerated charges in semiconductors constitute an approach to reduce and oxidize pollutants, with an efficiency that, in turn, depend on the photo physics and defect chemistry of the photocatalyst. The use of visible-light-active nanostructures for AOPs is attractive, because they can offer viable opportunities for water purification by using a large part of the solar spectrum and providing intrinsically larger surface areas. Here, 3D nanostructured copper pillars are investigated together with their thin Cu₂O coating created via low-temperature oxidation in air and compared with corresponding flat surfaces. The formed copper oxide is analysed with X-ray diffraction, scanning electron microscopy (SEM) and Raman spectroscopy. Defect induced Raman scattering is analysed and corroborated by theoretical Raman spectra using linear response density functional theory (DFT) calculations for full vibrational mode analysis, revealing activation of several vibrational modes that are otherwise inactive in pristine Cu₂O. The more specific effect of different vacancies for the activation of different modes, is reviewed and analysed in more detail. The thickest surface oxide layers on the 3D structures show outgrowth of CuO nano-needles rationalized through a copper ion diffusion mechanism. All of the Cu-supported copper oxide systems exhibit effective photocatalytic performance with 3D nanopillar structures further increasing the efficiency by 34% compared to their planar counterpart.

The ability to control electronic states by utilizing quantum confinement of one of the material components in heterojunctions is a promising approach to perform energy-level matching. In this work, we report the possibility to achieve optimum energy alignment in heterojunctions made from size-controlled quantum dots (Q-dots) of ZnO in combination with three copper oxides: Cu₂O, Cu₄O₃, and CuO. Quantum confinement effects on the ZnO nanoparticles in the diameter range 2.6–7.4 nm showed that the direct optical band gap decreased from 3.99 to 3.41 eV, with a dominating shift occurring in the conduction band (CB) edge, and thus the possibility to obtain close to 0.6 eV CB edge shift by controlling the size of ZnO. The effect was utilized to align the electronic bands in the ZnO Q-dot/copper oxide heterojunctions to allow for charge transfer between the materials and to test the ability to improve the photocatalytic performance for the system, evaluated by the transformation of a dye molecule in water. The catalyst materials were investigated by X-ray diffraction, scanning electron microscopy, ultraviolet–visible (UV–vis), photoluminescence, and Raman spectroscopy. The most promising material combination was found to be the Cu₂O copper oxide in combination with an energy aligned ZnO Q-dot system with approximately 7 nm diameter, showing strong synergy effects in good agreement with the energy-level analysis, outperforming the added effect of its individual components, ZnO-Q-dots and Cu₂O, by about 140%. The results show that utilization of a heterojunction with controllable energy alignment can provide a drastically improved photocatalytic performance. Apart from increased photocatalytic activity, specific surface states of ZnO are quenched when the heterojunction is created. It is anticipated that the same approach can be utilized in several material combinations with the added benefit of a system with controllable overpotential and thus added specificity for the targeted reduction reaction.

Quantum confined semiconductors have been of interest the last decades, largely fueled bythe unique ability to tune the electronic properties, and thereby their optical response. Anotherconsequence of their low dimensions is the markedly increased surface area that canbe utilized in surface dependent phenomena such as in sensors or catalysis. In this study,zinc oxide quantum dots (Qdots) were synthesized in the size regime from 3.9 nm to 6.4 nm,with a resulting optical bandgap change from 3.61 eV to 3.43 eV. Their vibrational quantumconfinement and surface modes were assessed with Raman spectroscopy, and differentialpulse voltammetry was utilized to extract the electrochemical bandgap, the CB edge position,and the electrochemical density of states (DOS). The quantum capacitive dependence on theelectrochemical DOS is analyzed together with the potentiostatically induced Burstein-Mossshift to extract details in the conduction band (CB) properties of the Qdots. The successivenarrowing and change of density of states at the CB reveal a size dependent quantum capacitance,originating in the decrease of the electrochemically accessible states for the ZnO Qdotsupon their decrease in size.

Raman spectroscopy is an important analytical tool in materials science. Its ability to characterize transitions between rotational and vibrational states by analysis of inelastically scattered light, enables it to identify chemical bonds. As changes of the rotational and vibrational states in turn depend on secondary effects, the technique is also suitable for studying more detailed phenomena like molecular interactions, material strain, crystallinity, order and bond formations. This versatility has made it a standard tool in a large variety of science disciplines including chemistry, physics, biology, geology and medicine. There are several advantages with Raman spectroscopy including that it in most cases is non-destructive, requires no sample preparation and that almost any non-metallic type of sample can be measured. Raman scattering has however one major weakness: It is a very low probability process unless probed in resonance with an electronic excitation. To detect such a weak signal at a high spectral resolution, a very sensitive detection system is needed which subsequently leads to a high probability of picking up signal from other processes or from other origin than the measured sample. These spurious signals that sometimes occur in Raman spectra are referred to as false Raman peaks and if they are not correctly identified, they complicate the analysis of the results and increase the risk of misinterpreting the data. This work is aimed to give the fundamental principles of Raman scattering and an overview of the sources of other signals occurring in Raman spectra that include; other photon generating processes, cosmic rays, stray light, artefacts from spectrometer components, and signals from other compounds in or surrounding the sample. The origins of the false Raman peaks are explained and means and measures to identify and counteract them are given. Finally, a Raman measurement protocol that can serve as a guideline for practical confocal Raman spectroscopy measurements is presented.