This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), en...

Compound Copper Chalcogenide Nanocrystals

We report the synthesis of a size series of copper indium selenide quantum dots (QDs) of various stoichiometries exhibiting photoluminescence (PL) from the red to near-infrared (NIR). The synthetic method is modular, and we have extended it to the synthesis of luminescent silver indium diselenide QDs. Previous reports on QDs luminescent in the NIR region have been primarily restricted to binary semiconductor systems, such as InAs, PbS, and CdTe. This work seeks to expand the availability of luminescent QD materials to ternary I−III−VI semiconductor systems.

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Ternary I-III-VI quantum dots luminescent in the red to near-infrared.

CuGaTe_2 with a chalcopyrite structure demonstrates promising thermoelectric properties. The maximum figure of merit ZT is 1.4 at 950 K. CuGaTe_2 and related chalcopyrites are a new class of high-efficiency bulk thermoelectric material for high-temperature applications.

Chalcopyrite CuGaTe(2): a high-efficiency bulk thermoelectric material.

Temperature dependent (4.2–300 K) photoluminescence (PL) of bulk (0001)-oriented ZnO in the range of free- and bound-exciton emission is presented. Emission from several bound excitons and the free A exciton were observed from the low temperature (20 K) PL spectrum. The temperature dependence of the free-exciton peak position was fit using the Manoogian-Woolley equation and the coefficients obtained show reasonable agreement both with first-principle theoretical calculations and empirical values of the coefficients for other II–VI semiconductors. The strongest bound-exciton line with a width (full width at half maximum) of about 1 meV exhibited a thermal activation energy of approximately 14 meV, consistent with the exciton-defect binding energy. It was not observed at temperatures above 150 K. Additional analysis of this particular bound-exciton peak suggests it dissociates into a free exciton and a neutral-donor-like defect-pair complex with increasing temperature.

Temperature dependent exciton photoluminescence of bulk ZnO

Because of its unmatched resource potential, solar energy utilization currently is one of the hottest research areas. Much effort has been devoted to developing advanced materials for converting solar energy into electricity, solar fuels, active chemicals, or heat. Among them, TiO2 nanomaterials have attracted much attention due to their unique properties such as low cost, nontoxicity, good stability and excellent optical and electrical properties. Great progress has been made, but research opportunities are still present for creating new nanostructured TiO2 materials. Core–shell structured nanomaterials are of great interest as they provide a platform to integrate multiple components into a functional system, showing improved or new physical and chemical properties, which are unavailable from the isolated components. Consequently, significant effort is underway to design, fabricate and evaluate core–shell structured TiO2 nanomaterials for solar energy utilization to overcome the remaining challenges, for example, insufficient light absorption and low quantum efficiency. This review strives to provide a comprehensive overview of major advances in the synthesis of core–shell structured TiO2 nanomaterials for solar energy utilization. This review starts from the general protocols to construct core–shell structured TiO2 nanomaterials, and then discusses their applications in photocatalysis, water splitting, photocatalytic CO2 reduction, solar cells and photothermal conversion. Finally, we conclude with an outlook section to offer some insights on the future directions and prospects of core–shell structured TiO2 nanomaterials and solar energy conversion.

Core–shell structured titanium dioxide nanomaterials for solar energy utilization

Within its rich phase diagram titanium dioxide is a truly multifunctional material with a property palette that has been shown to span from dielectric to transparent-conducting characteristics, in addition to the well-known catalytic properties. At the same time down-scaling of microelectronic devices has led to an explosive growth in research on atomic layer deposition (ALD) of a wide variety of frontier thin-film materials, among which TiO2 is one of the most popular ones. In this topical review we summarize the advances in research of ALD of titanium dioxide starting from the chemistries of the over 50 different deposition routes developed for TiO2 and the resultant structural characteristics of the films. We then continue with the doped ALD-TiO2 thin films from the perspective of dielectric, transparent-conductor and photocatalytic applications. Moreover, in order to cover the latest trends in the research field, both the variously constructed TiO2 nanostructures enabled by ALD and the Ti-based hybrid inorganic-organic films grown by the emerging ALD/MLD (combined atomic/molecular layer deposition) technique are discussed. CONTENTS

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Titanium dioxide thin films by atomic layer deposition: a review

The optical vibrational modes of the ordered vacancy compounds CuIn3Se5 and CuGa3Se5 have been obtained by Raman spectra at various temperatures. The totally symmetric A1 mode appears around 155 and 167 cm−1 in CuIn3Se5 and CuGa3Se5, respectively. All the lines observed, except the B1 modes, present a reduction in their frequencies by about 10% as compared to the corresponding values in the CuInSe2 and CuGaSe2 chalcopyrites. This is due to the presence in these compounds of an array of vacancies occupying particular sites in the cation sublattice. These tend to relax the stretching forces thus reducing the vibrational frequencies.

Raman spectra of the ordered vacancy compounds CuIn3Se5 and CuGa3Se5

From the analysis of the Urbach tail in ${\mathrm{CuInSe}}_{2}$ and ${\mathrm{CuInTe}}_{2},$ it is found that the energy $h{\ensuremath{\nu}}_{p}$ involved in the electron--exciton-phonon interaction is not that of the ordered compound longitudinal or transverse optical modes. It is established that this energy depends on the structural and cations disorders. An expression of the form ${E}_{U}(T,P,N)=K\ensuremath{\Theta}/{\ensuremath{\sigma}}_{0}{(1+P)/2+N[\mathrm{exp}(\ensuremath{\Theta}/T)\ensuremath{-}1{]}^{\ensuremath{-}1}},$ where P and N are adjustable structural and order-disorder parameters and $\ensuremath{\Theta}$ the Einstein characteristic temperature, accurately explains the temperature dependence of the Urbach energy. From extrapolation of the linear variation of N and P with $h{\ensuremath{\nu}}_{p},$ the phonon energy for a completely ordered and disordered systems can be estimated. We discuss the physical meaning of the parameter N as due to the contribution of localized modes produced by substitutional disorder of low energy of formation. We find an intriguing relation between the temperature of the chalcopyrite-sphalerite transition and the phonons contributing to the formation of Urbach tails in the strong-disorder limit.

Effect of structural disorder on the Urbach energy in Cu ternaries

The optical properties of the ordered defect compound CuIn5Se8, which crystallizes in a hexagonal structure, have been studied by the absorption technique. The analysis of the data shows that the band gap energy EG varies from 1.23 to 1.13 eV between 10 and 300 K. It is found that the variation of EG with temperature is due to the contribution of both acoustic and optical phonons with a characteristic phonon energy of about 14 meV. The optical absorption coefficient just below the absorption edge varies exponentially with photon energy indicating the presence of Urbach’s tail. The phonon energy hνp associated with Urbach’s tail, which is found to be 53 meV, is higher than the highest optical phonon mode reported for this compound, which is about 29 meV. The origin of the additional energy is attributed to the contribution of localized modes produced by structural disorder of low energy formation. An empirical relation, also used earlier in the case of 1:1:2 and other ordered defect compounds of the 1:3:5 ...

Temperature dependence of the optical energy gap and Urbach’s energy of CuIn5Se8

The experimentally observed energy band gap difference (ΔE1) between the I–III3–VI5 and I–III–VI2 and the energy band gap difference (ΔE2) between the I–III5–VI8 and I–III–VI2 phases of Cu–In–Se, Cu–Ga–Se, Cu–In–Te, and Cu–Ga–Te systems is explained in terms of the relative shift of the conduction band minimum (CBM) and the valence band maximum (VBM) caused due to the presence of the ordered VCu and [In(Ga)Cu+2+2 VCu−1] defect pair and to the effect of the p–d hybridization. The nearly linear variation of ΔE1 and ΔE2 with p–d hybridization of the corresponding I–III–VI2 phase suggests that in selenides the lowering of the VBM predominates over that of the CBM. In the case of the Cu–In–Te system, they are very near the same magnitude, whereas in Cu–Ga–Te the lowering of the CBM predominates over that of the VBM.

Giovanni Marín

Papers

Titanium dioxide thin films by atomic layer deposition: a review

Raman spectra of the ordered vacancy compounds CuIn3Se5 and CuGa3Se5

Effect of structural disorder on the Urbach energy in Cu ternaries

Temperature dependence of the optical energy gap and Urbach’s energy of CuIn5Se8

On the band gap anomaly in I–III–VI2, I–III3–VI5, and I–III5–VI8 families of Cu ternaries