The main effects of oxygen vacancy defects on the electronic and optical properties of V 2 O 5 nanowires were studied through in-situ Raman, photoluminescence, absorption, and photoemission spectroscopy. Both thermal reduction and electrochemical reduction via lithium insertion leads to the creation of oxygen vacancy defects in the crystal that gives rise to new electronic mid-gap defect states at energy 0.75 eV below the conduction band edge. The defect formation results in delocalization and injection of excess electrons into the conduction band, as opposed to localized electron injection as previously suggested. Contrary to what is seen in most oxides, the presence of vacancy defects leads to band filling and an increase in the optical band gap of V 2 O 5 from 1.95 eV to 2.45 eV, which is attributed to the Burstein-Moss effect. Other observed changes in the optical properties are correlated to the changes in the electronic structure of the oxide as result of defect formation. Further, in-situ Raman measurements during the electrochemical reduction at room temperature show that the oxygen atom that is most readily reduced is the three-fold coordinated oxygen (O3). Reduction of V 2 O 5 through the formation of oxygen vacancy, V O , defects have been studied both by theoretical and experimental studies. 10,11,12,13,14,15 In this work, we address some of the key unanswered questions regarding the correlation between optical properties and electronic structure of The rise of absorbance in the NIR spectral range in reduced oxide is a result of optical excitation of V O defect related mid-gap states. In addition, using in-situ Raman measurements during reduction process, we show that the oxygen atom that most readily participates during electrochemical reduction at room-temperature and gets reduced is the three-fold coordinated oxygen (O3).Vacancies in vanadium oxide have been probed using a variety of spectroscopic techniques, such as scanning tunneling microscopy, 9,26,27 electron energy loss spectroscopy, 28,29 electron paramagnetic resonance studies, 16,12,23 X-ray absorption measurements, 30 and X-ray photoemission spectroscopy (XPS). 15,17 However, most of these techniques either require ultrahigh vacuum (UHV) or controlled environment for operation that makes it difficult to adapt it to 6 in-situ catalytic studies. Here, we use near-infrared photoluminescence (NIR-PL) and Raman spectroscopy that allows for study of vacancy defects and their interaction with redox species under in-operando electrochemical conditions at room temperature, as shown in our recent study. 31 The spectral range of various types of electronic transitions in V 2 O 5 is shown in Fig.1A.Representative PL spectra of stoichiometric and non-stoichiometric V 2 O 5 are shown in Fig.1B.The optical gap of nominally undoped V 2 O 5 is in the range of 1.9-2.5 eV and hence emission spectrum lies in the ultraviolet-to-visible part of the spectral range. In most TMOs, deviation from stoichiometry is a result of the presence of high density ...
Metal-insulator transitions in strongly correlated oxides induced by electrochemical charging have been attributed to formation of vacancy defects. However, the role of native defects in affecting these transitions is not clear. Here, we report a new type of phase transition in p-type, nonstoichiometric nickel oxide involving a semiconductor-to-insulator-to-metal transition along with the complete reversal of conductivity from p- to n-type at room temperature induced by electrochemical charging in a Li-containing electrolyte. Direct observation of vacancy-ion interactions using in situ near-infrared photoluminescence spectroscopy show that the transition is a result of passivation of native nickel (cationic) vacancy defects and subsequent formation of oxygen (anionic) vacancy defects driven by Li insertion into the lattice. Changes in the oxidation states of nickel due to defect interactions probed by X-ray photoemission spectroscopy support the above conclusions. In contrast, n-type, nonstoichiometric tungsten oxide shows only insulator-to-metal transition, which is a result of oxygen vacancy formation. The defect-property correlations shown here in these model systems can be extended to other oxides.
Effects of electrochemical charging of quantum dots (QDs) have been reported previously, wherein optical and electrical properties could be modulated through cation adsorption and electron injection into the quantum-confined 1S states. In this work, we report two different modes of electrochemical double-layer charging in CdSe QDs and their effects on the electronic and optical properties. We show that the charging mechanism at the interface involves cation intercalation for smaller ions, such as Li, Na, or K, and cation adsorption for larger bulky ions, such as tetrabutylammonium ions, where steric hindrance precludes intercalation. As a result, while cation adsorption leads to an increase in the absorbance in the mid-infrared spectral range, cation intercalation into the CdSe core results in an absorbance increase from the visible to infrared spectral range, an enhancement in radiative lifetime of e, an increase of 158% in the intensity of band-edge photoluminescence, and strong emission in the near-infrared spectral range as a result of the formation of Se vacancies. The nature of charging mechanisms is discussed using the results of combined photoluminescence, radiative lifetime, and X-ray photoemission studies. The cation-coupled electronic and optical modulation reported here in CdSe QDs have important implications for electrochromic smart windows, photovoltaics, and other devices.
A new versatile technique of synthesizing large area, vertically oriented two-dimensional (2D) layers of molybdenum nitride (MoN) is reported that involves synthesis of 2D nanosheets of MoO 3 through a hot-filament chemical vapor deposition process and subsequent phase transformation to δ-MoN. This simple two-step approach of phase transforming 2D oxide layers potentially enables easy synthesis of a wide variety of MXenes of nitrides, sulfides, and carbides of tunable composition. When used as electrodes in Li-ion batteries, the 2D layers of MoN show an unusual Li + storage mechanism. Using combined differential capacitance, X-ray diffraction, and X-ray photoemission spectroscopy, it is shown that unlike their bulk form, which behaves as conversion electrodes, 2D layers of MoN behave as insertion electrodes that do not involve chemical transformation. As a result, the electrode shows stable capacity of 320 mAh/g for more than 200 cycles without any structural and electrochemical degradation.
A hot wire chemical vapor deposition technique is described for synthesis of 1D nanostructures of a controlled morphology, stoichiometry, and composition. The synthesis involves the evaporation and condensation of metal oxide vapor through the reaction of oxygen with the hot filaments of respective transition metals. The stoichiometry and morphology of MoO 3 and WO 3 were modulated by varying the filament temperature and partial pressure of oxygen in the growth chamber. Based on the results under different conditions, a morphological phase diagram, and a growth model based on the extent of gas phase supersaturation were developed to understand the growth mechanism. Further, ternary transition metal oxide, NiMoO 4, was synthesized as a proof-of-concept for tuning the composition of deposition through simultaneous evaporation of two metal oxides.
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