Property Relations in “Giant” Semiconductor Nanocrystals

Dec 20, 2017 - In each case, we aim at providing insights on structure/property relationships and on how to optimize them toward improving device perf...
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Article Cite This: Acc. Chem. Res. 2018, 51, 609−618

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Structure/Property Relations in “Giant” Semiconductor Nanocrystals: Opportunities in Photonics and Electronics Fabiola Navarro-Pardo,†,‡ Haiguang Zhao,*,§ Zhiming M. Wang,*,† and Federico Rosei*,†,‡ †

Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, P.R. China ‡ Centre for Energy, Materials and Telecommunications, Institut National de la Recherche Scientifique, 1650 Boulevard Lionel-Boulet, Varennes, Québec J3X 1S2, Canada § College of Physics and The Cultivation Base for State Key Laboratory, Qingdao University, No. 308 Ningxia Road, Qingdao 266071, P.R. China CONSPECTUS: Semiconductor nanocrystals exhibit sizetunable absorption and emission ranging from the ultraviolet (UV) to the near-infrared (NIR) spectral range, high absorption coefficient, and high photoluminescence quantum yield. Effective surface passivation of these so-called quantum dots (QDs) may be achieved by growing a shell of another semiconductor material. The resulting core/shell QDs can be considered as a model system to study and optimize structure/ property relations. A special case consists in growing thick shells (1.5 up to few tens of nanometers) to produce “giant” QDs (g-QDs). Tailoring the chemical composition and structure of CdSe/CdS and PbS/CdS g-QDs is a promising approach to widen the spectral separation of absorption and emission spectra (i.e., the Stokes shift), improve the isolation of photogenerated carriers from surface defects and enhance charge carrier lifetime and mobility. However, most stable systems are limited by a thick CdS shell, which strongly absorbs radiation below 500 nm, covering the UV and part of the visible range. Modification of the interfacial region between the core and shell of g-QDs or tuning their doping with narrow band gap semiconductors are effective approaches to circumvent this challenge. In addition, the synthesis of g-QDs composed of environmentally friendly elements (e.g., CuInSe2/CuInS2) represents an alternative to extend their absorption into the NIR range. Additionally, the band gap and band alignment of g-QDs can be engineered by proper selection of the constituents according to their band edge positions and by tuning their stoichiometry during wet chemical synthesis. In most cases, the quasi-type II localization regime of electrons and holes is achieved. In this type of g-QDs, electrons can leak into the shell region, while the holes remain confined within the core region. This electron−hole spatial distribution is advantageous for optoelectronic devices, resulting in efficient electron−hole separation while maintaining good stability. This Account provides an overview of emerging engineering strategies that can be adopted to optimize structure/property relations in colloidal g-QDs for efficient photon management or charge separation/transfer. In particular, we focus on our recent contributions to this rapidly expanding field of research. We summarize the design and synthesis of a variety of colloidal g-QDs with the aim of tuning the optical properties, such as absorption/emission in a wide region of the solar spectrum, which allows enlargement of their Stokes shift. We also describe the band alignment within these systems, charge carrier dynamics, and charge transfer from g-QDs into semiconducting oxides. We show how these tailored g-QDs may be used as active components in luminescent solar concentrators, photoelectrochemical cells for hydrogen generation, QD-sensitized solar cells and optical nanothermometers. In each case, we aim at providing insights on structure/property relationships and on how to optimize them toward improving device performance. Finally, we describe perspectives for future work, sketching new directions and opportunities in this field of research at the intersection between chemistry, physics, materials science and engineering.

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INTRODUCTION

detectors, biomedical labels, luminescent solar concentrators (LSCs), photoelectrochemical (PEC) cells for hydrogen production, excitonic solar cells, and optical nanothermometers.12−25 Colloidal QDs are generally capped with organic ligands that serve to tune the reactivity of the precursors, prevent their aggregation, and passivate their surface-bound atoms.9 How-

Semiconductor nanocrystals, often referred to as quantum dots (QDs), exhibit structure-, size-, and composition-dependent optoelectronic properties resulting from quantum confinement.1−3 In particular, QDs present tunable absorption and emission ranging from the ultraviolet (UV) to the near-infrared (NIR) spectral range, high absorption coefficient and high photoluminescence quantum yield (PLQY).4−18 Due to these promising properties, QDs are widely used as building blocks in optoelectronic devices, such as light-emitting diodes, photo© 2017 American Chemical Society

Received: September 23, 2017 Published: December 20, 2017 609

DOI: 10.1021/acs.accounts.7b00467 Acc. Chem. Res. 2018, 51, 609−618

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Figure 1. Reaction schemes for the synthesis of differently structured g-QDs and their representative transmission electron microscopy (TEM) images.

system.4 Specifically, in g-QDs, the core can be doped with a narrow bandgap semiconductor or an alloyed layer can be developed between the core and outer shell.17,22,23 In this Account, we summarize our efforts to tailor the structure of colloidal g-QDs to fine-tune their optoelectronic properties via wet chemical synthesis. We describe electron and hole localization as well as charge dynamics, which deeply influence the optical and electrical behavior in g-QDs. We also highlight the potential of our optimized structures to significantly enhance the efficiency and stability of g-QDbased optoelectronic devices.

ever, these ligands are very sensitive to environmental chemical and physical conditions, and exposure to light can result in their detachment from the QD surface, leaving uncapped surface atoms, which induce significant degradation ranging from uncontrolled changes in PLQY to permanent “darkening” or photobleaching.9−11 An effective approach to passivate the QD surface is the epitaxial growth of another semiconductor to realize core/shell structures.2,16−30 Among these structures, core/thick-shell QDs (shell thickness, H, 1.5 nm up to tens of nanometers) stand out because of the core’s complete isolation from the environment and the significantly longer electron− hole pair (exciton) lifetime than those in core/thin-shell QDs (H < 1.5 nm).10,11,21 These QDs are often referred to as “giant” QDs (g-QDs) and such features can considerably improve the performance of QD-based optoelectronic devices.11,18 Typically, most stable g-QDs contain a thick CdS shell, which due to its large bandgap (bulk Eg ≈ 2.49 eV) absorbs strongly in the UV and partially in the visible (Vis) range (wavelength, λ < 500 nm) of the solar spectrum, imposing stringent limits to their use in potential applications.11,21 Another means of modifying the absorption of QDs is to tailor the chemical composition and stoichiometry to form an alloyed

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TUNABLE DESIGN OF GIANT QDs Colloidal core-only QDs are almost exclusively obtained using the hot-injection organometallic approach.1 A broad variety of semiconductors have been used to synthesize QDs, for example, II−VI (CdSe, CdTe, CdS)4,19,21 and IV−VI (PbSe, PbTe, PbS)6,7,16,18 binary compounds. Subsequently, the shell can be grown via cation exchange or successive ionic layer absorption and reaction (SILAR)8−10 techniques. Growing a wide band gap semiconductor such as CdS or ZnS (bulk Eg ≈ 610

DOI: 10.1021/acs.accounts.7b00467 Acc. Chem. Res. 2018, 51, 609−618

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Figure 2. Spatial probability distributions of the electron (e−) and hole (h+) for (a) CdSe/(CdS)x g-QDs of different shell thickness (R = 1.65 nm; H = 0.66−4.51 nm) and (b) comparison between CdSe/(CdS)6 (H = 1.96 nm) and CdSe/(CdSexS1−x)4/(CdS)2 (H = 2.07 nm) g-QDs. (c) Scheme showing the electronic evolution of the corresponding QDs. (d) UV−Vis absorption spectra and (e) PL intensity of different QDs. Adapted with permission from ref 22. Copyright 2017 John Wiley and Sons Inc.

3.61 eV) shell for a CdSe (bulk Eg ≈ 1.74 eV) or a PbS (bulk Eg ≈ 0.4 eV) core allows us to engineer the QD’s band gap and band energy levels, which in turn influences the final optoelectronic properties.1−30 Due to the toxicity of the compounds involved in QD synthesis, environmental and health concerns have also arisen.31 Current efforts focus on developing alternative, greener synthetic routes,28,29 leading to heavy-metal free QDs24,27 such as CuInSe2/CuInS2 (CISe/ CIS) core/shell g-QDs. Figure 1 summarizes the synthetic protocols employed within our group to obtain several types of core/shell g-QDs whose absorption spectrum spans from the UV to the Vis range in the case of those containing pure CdS shell, enhanced absorption for the alloyed ones and up to the NIR spectral range in CISe/CIS g-QDs. For example, a thin CdS shell was grown on PbS QDs via cation exchange. Subsequently a thicker shell was formed using SILAR; control of precursor stoichiometry (Cd/S ratio from 1:1 to 1:0.8) during this sequential shell growth enabled the possibility to develop double- or single-emission g-QDs.23

from the core region into the shell, whereas the hole remained confined in the CdSe core.22 In contrast, CdSe/(CdSexS1−x)4/ (CdS)2 g-QDs behaved like single composition QDs, where both electron and hole are delocalized over the entire structure (Figure 2b). Similar calculations (Figure 3) showed that upon increasing the CIS shell thickness in a CISe/CIS g-QD, the quasi-type-II band alignment was also present.24 The quasi-type II band alignment is advantageous for optoelectronic devices, as electrons can leak into the shell while the holes are still

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CHARGE CARRIER LOCALIZATION IN GIANT QDs Depending on the relative alignment of conduction- and valence-band edges of the constituent binary compounds, core/ shell QDs may be classified as (a) type-I, where electrons and holes are confined in the same region (core), (b) type-II, where electrons and holes are spatially separated, and (c) quasi-type II, where one carrier is fully delocalized over the entire volume and the other is localized in the core or the shell.32 One of the benefits of core/shell g-QDs consists in the ease to control their band structure by tuning core size, shell thickness, and chemical composition of both core and shell.33 Calculations of the electron and hole wave functions in CdSe/ (CdS)x g-QDs (Figure 2a) showed that by increasing the shell thickness, the electron wave function exhibits increased leakage

Figure 3. (a) Band structure of a CISe/CIS g-QD (R = 1.65 nm; H = 2 nm). (b) Normalized radial distribution wave functions of CISe/CIS core/shell QDs (R = 1.65 nm; H = 0−6 nm). (c) Squared overlap integrals in CISe/CIS core/shell QDs as a function of shell layer thickness. Reproduced with permission from ref 24. Copyright 2017 John Wiley and Sons Inc. 611

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QDs, a graded interface including a PbxCd1−xS alloy might be obtained.23 This alloyed interfacial layer provides an energy gradient, leading to fast release of the hole from the CdS Wz shell to the PbS core, inhibiting direct exciton radiative recombination within the CdS shell and leading to singleemission g-QDs.

confined within the core region; this structure leads to efficient electron−hole separation while maintaining good stability.5,14,33

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INFLUENCE OF THE THICK SHELL ON THE OPTICAL PROPERTIES Quasi-type II g-QDs exhibit a lower degree of quantum confinement, which causes a strong PL red shift and an increase in PL lifetime, leading to enlarged Stokes shift.11,13,34 However, in these systems the thick shell dominates the absorption range.11 The shell absorption can be enhanced by introducing an interfacial CdxPb1−xS layer into CdSe/CdS QDs; depending on degree of doping, these g-QDs could absorb λ ≤ 850 nm at the expense of a decreased PLQY.17 Pb doping within CdSe/ CdS/CdxPb1−xS/CdS g-QDs can be optimized to improve absorption (λ ≥ 600 nm), increase PL lifetime, and attain similar high PLQY as those found in CdSe/CdS QDs.17 Another approach to broaden the absorption spectrum spanning across the UV−Vis−NIR regions consisted in synthesizing g-QDs with heavy-metal free narrow bandgap semiconductors, that is, CISe/CIS.24 Additionally, increased CIS shell thickness was shown to enhance the PL lifetime in this g-QDs. In most cases, the shell does not emit radiation, whereas in a few cases, double-emission may occur in core/shell QDs.20,23,30,35 Double-emission can be achieved in PbS/CdS g-QDs containing two distinct CdS phases, namely, cubic zinc blende (Zb) and hexagonal wurtzite (Wz).20,30 In this core/ shell/shell asymmetric structure, the Wz phase is spatially separated from the PbS core (Figure 4c), decreasing the probability of hole localization.36 Therefore, such asymmetry leads to the independent and simultaneous radiative exciton recombination in the PbS core and the CdS Wz shell (Figure 4b). Furthermore, by tailoring the stoichiometric Cd/S ratio of the SILAR shells grown onto the core/shell QDs, their doubleto single-emission can be controlled (Figure 4a). In these g-

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CHARGE CARRIER DYNAMICS IN GIANT QDs Charge carrier dynamics are particularly important in heterostructures involving QDs and wide bandgap semiconductor oxides (bulk Eg ≈ 3.2−3.8 eV) such as those found in PEC cells, including solar cells.15,25,37−39 Electron (Ket) and hole (Kht) transfer rates at QD/semiconducting oxide interface can be investigated through transient fluorescence spectroscopy. Usually, a QD/ZrO2 heterostructure is considered as a benchmark system, in which charge injection does not occur, so that PL degradation can be uniquely attributed to radiative/nonradiative exciton recombination. Additionally, a QD/TiO2 heterostructure with or without the presence of a hole scavenger (electrolyte) allows to determine the charge transfer within this system. A decreased PL lifetime compared to that of QD/ZrO2 is attributed to efficient electron transfer into the TiO2 and the hole into the electrolyte. In general, fast charge mobility and efficient charge injection can minimize charge recombination processes, thus improving the photocurrent density and device performance. Interestingly, CdSe/ CdS g-QDs displayed ∼1.7-fold lower Ket and ∼4.5 times lower Kht when compared to those heterostructures conformed with CdSe QDs.21 Reduced Ket was related to the decrease of the electron density at the surface of g-QDs, whereas the decrease in Kht was attributed to the thick CdS shell, which decelerates or even blocks hole transfer. However, the electron lifetime was substantially reduced by adding an interfacial alloyed layer and tailoring the CdS shell thickness to yield CdSe/(CdSexS1−x)4/ (CdS)2 g-QDs compared to the one containing CdSe/CdS gQDs (Figure 5), consequently enhancing the Ket and injection of the electron into TiO2.22 The enhanced charge dynamics was related to favorable stepwise band alignment of the CdSe core, the interfacially alloyed layer (CdSexS1−x), and the outer CdS shell. The charge transfer dynamics can also be studied by transient differential transmission (ΔT/T) spectroscopy, which helps to improve our understanding of the structural features in gQDs.23 As seen in Figure 6a, ΔT/T spectra of double-emitting PbS/CdS g-QDs exhibited a photobleaching signal related to the excited charge carriers in the CdS shell, which raised rapidly even when pumping at low photon energies (2.38 eV). Under these conditions, below the CdS bandgap, only electrons from the core are promoted to the shell suggesting that the conduction bands of the PbS and CdS are almost aligned, thus confirming the quasi-type II localization regime. Figure 6b shows holes are promoted to the CdS shell when the pump photon energy is tuned to energies higher than 2.64 eV (λ < 470 nm). Nevertheless, also at this condition some holes relax into the PbS core, whereas some remain in the CdS shell due to a “hole-blockade” effect.36 This allows simultaneous radiative recombination at the CdS shell and at the PbS core, resulting in the double-emission. In contrast, as explained previously, in single-emitting g-QDs, the hole in the CdS shell can rapidly migrate and recombine with an electron in the PbS core.23 Additionally, as included in Figure 6c,d, ΔT/T spectra show that adding an electron scavenger (methyl viologen, MV2+) causes a faster decay of the signal in both systems, this allows to

Figure 4. (a) Absorption and PL spectra of PbS/CdS g-QDs. (b) Schematic electronic band structure of single- and double-emitting gQDs. (c) HRTEM of double-emitting g-QDs. Adapted with permission from refs 23 and 30. Copyright 2016 Elsevier and 2015 Royal Society of Chemistry. 612

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estimate the charge transfer.23 The charge transfer rate obtained was ∼2 × 107 s−1 for a single-emitting g-QD and ∼3 × 107 s−1 for a double-emitting g-QD, which was consistent with PL decay measurements.23 To summarize, tailoring the thickness, chemical composition, and crystalline structure of the CdS shell not only allows determination of the absorption/emission phenomena but also defines the efficiency of the photogenerated electrons to be transferred or injected into the semiconducting oxides and hole scavengers, which is a determining factor to achieve high performance in several optoelectronic devices.

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APPLICATIONS IN OPTOELECTRONIC DEVICES Due to the feasibility of finely tuning the structure of g-QDs to optimize their optical and electrical properties, these semiconductor nanocrystals are considered particularly attractive as active components for a number of prospective optoelectronic devices.3,12−25 In the following section, we will discuss LSCs, PEC cells for hydrogen generation, QD-sensitized solar cells (QDSCs), and applications in nanoscale thermometry. Luminescent Solar Concentrators

An LSC may consist of luminescent species embedded within a transparent polymer matrix. Upon illumination, radiation is down-converted to longer wavelengths and then concentrated due to total internal reflection by the polymer and emitted through the edges of the LSC toward an integrated photovoltaic device. Emission in quasi-type II g-QDs is dominated by recombination of core excitons, whereas absorption is related to the shell. The tunable separation of the emission and absorption functions within quasi-type II g-QDs make them relevant to alleviate the readsorption losses within LSCs, which is one of the most critical challenges toward fabricating high efficiency devices.13 Consequently, CdSe/CdS/CdxPb1−xS/CdS g-QDs have shown potential to realize LSCs with low reabsorption losses thanks to the widened Stokes shift and enhanced light absorption due to the Pb doping into the CdS thick shell (Figure 7).17 The theoretical optical efficiency (ηopt) defined as the number of photons emitted from the LSC edge over the total number of photons impinging on the LSC through the top surface was enhanced by ∼20% in LSCs

Figure 5. (a) HRTEM image of CdSe/CdS g-QDs. (b) TEM image of TiO2 nanoparticles sensitized with CdSe/CdS g-QDs. PL intensity decay for (c) QDs deposited in TiO2 and ZrO2 mesoporous films and (d) g-QDs of different shell thickness (λex = 444 nm). (e) Calculated electron lifetime as a function of shell thickness. (f) Electron transfer rate variation with CdS shell thickness. Adapted with permission from ref 22. Copyright 2017 John Wiley and Sons Inc.

Figure 6. ΔT/T dynamics for double-emission g-QDs (a,b) at different pump and probe wavelengths; red and blue solid line, pump at 2.64 eV (470 nm), and green dotted line, pump at 2.38 eV (520 nm). ΔT/T dynamics without (red curves) and with MV2+ (blue curves) for (c) single-emitting gQDs and (d) double-emitting g-QDs, at CdS band probe energy (500 nm), together with fit (dotted lines). Adapted with permission from refs 30 and 23. Copyright 2015 Royal Society of Chemistry and 2016 Elsevier. 613

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Figure 7. (a) Scheme of a QD-based LSC coupled with a silicon cell. Dependence of (b) the optical efficiency and (c) LQE with varying LSC length. Adapted with permission from ref 17. Copyright 2016 John Wiley and Sons.

fabricated with CdSe/CdS/CdxPb1−xS/CdS g-QDs compared to that in CdSe/CdS g-QD-based LSCs. Additionally, the luminescence quantum efficiency (LQE), which represents the number of photons emitted by the edge of the LSC over the number of photons absorbed by the LSC, also displayed a similar trend of enhanced performance in the QD-based LSCs when using interfacially alloyed g-QDs. QD-Based PEC Cells for Hydrogen Generation

TiO2 is an extensively studied metal oxide semiconductor for various applications, including PEC devices; unfortunately, because of its large band gap (bulk Eg ≈ 3.2 eV), it only absorbs in the UV part of the solar spectrum.37,38 QDs can effectively sensitize a semiconductor like TiO2, offering new opportunities for harvesting light and improving charge separation.14,15,24,25 The PEC activity of TiO2 mesoporous films sensitized with QDs toward hydrogen evolution can be studied in a threeelectrode cell under simulated illumination, where the QD/ TiO2 photoanode acts as the working electrode in which a potential is applied with respect to a reference electrode (Figure 8). The resulting photocurrent density can be correlated with the amount of hydrogen produced at the counter-electrode.21,23,26 Although there is a reduction in the charge transfer rates as the shell thickness is increased in core/shell QDs, optimization of the structure in g-QDs may allow efficient charge transfer indicating the possibility to employ them for PEC H 2 generation. For example, CdSe/CdS g-QDs led to a 2-fold enhancement in the saturated photocurrent density compared to core-only CdSe QDs, as displayed in Figure 8.21 A further increase was achieved by effectively passivating the g-QDs with a ZnS layer, which acts as efficient corrosion resistant layer protecting the QD surface.3,8 This approach allowed an enhancement in the photocurrent density up to 10 mA·cm−2. Additionally, stability tests under long-term irradiation indicated that the photocurrent density quickly decreases yet remains at 72% of its original value after 2 h, whereas the photocurrent density in CdSe QDs decreases by 50% within 35 min.

Figure 8. (a) Scheme of the QD-based PEC hydrogen generation working principle; arrows indicate the electron and hole transfer processes; round inset showing a scheme of the device. PEC performance of TiO2 sensitized with different QDs: (b) without and (c) with ZnS passivating layers, in the dark and under one sun irradiation (AM 1.5 G, 100 mW·cm−2). (d) Stability performance of QDs applying a 0.2 V bias vs RHE. Measurements under AM 1.5 G, 100 mW·cm−2. Adapted with permission from ref 21. Copyright 2016 Elsevier.

QD-Sensitized Solar Cells

of a thick CdS shell to enhance the performance of QDSCs.22 In these devices, the power conversion efficiency (PCE) is determined by the current density and the voltage generated by the cells at the peak power divided by the power density (100 mW·cm−2) of the AM 1.5G simulated solar light. The PCE increased from 1.22% for CdSe QDs to 5.12% for CdSe/ (CdSexS1−x)4/(CdS)2 g-QDs; such improvement (Figure 9) was mainly attributed to broadening of the absorption spectrum from the UV to the Vis region (λ 400−700 nm), higher Ket, and slower rate of open circuit voltage (Voc) decay as compared to

Core/shell g-QDs have been shown to be effective for reducing recombination centers during operation in PEC cells, resulting from reduced density of surface trap states or defects and optimized electronic band alignment between core and shell.21,38 In addition, their light harvesting ability can be improved without hindering charge transport within the interfaces in PEC cells, by employing alloyed interfacial layers within the core/shell g-QDs.17,23 For instance, a CdSexS1−x alloyed layer was introduced onto the core prior to the growth 614

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Accounts of Chemical Research Nanoscale Optical Thermometry

Dual-emitting QDs are attractive for nanoscale thermal sensing applications due to their temperature dependent PL intensity ratios.20,35,40,41 The intensity ratio of their two PL peaks is found to vary linearly with the temperature, therefore allowing self-calibration.35,40 The viability to develop nanothermometers can be studied by dispersing the double-emitting g-QDs in a polymer matrix and evaluating their temperature-dependent PL.20 For example, in PbS/CdS g-QDs, the PL spectra evolution ranges over a broad temperature window, within 120−373 K (Figure 10). The PbS peak at 630 nm displayed a red shift and increase of lifetime whereas the CdS peak at 480 nm is less sensitive to temperature changes. In this g-QD, PL lifetime varies linearly with the temperature; the slope of the curve, which is related to the sensitivity of the measurement system, was equal to 14.1 ns K−1 in the range 150−350 K. The long-term stability of these g-QDs represents an additional advantage, as the PL peak position/intensity did not show any significant change after storing at 4 °C for at least one year. This type of system based in double-emitting g-QDs offers multiparametric detection due to the possibility of obtaining an independent estimate of the temperature from the ratio of two PL emissions, the lifetime of a single band, or its peak shift with respect to a reference temperature. Consequently, these features ease the detection of potential systematic errors and provide a reasonable and quantitative estimate of the accuracy of the measurement.

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CONCLUDING REMARKS AND PERSPECTIVES In this Account, we described several core/shell g-QDs systems with chemical composition- and size-tunable absorption, which typically possess a quasi-type II electron−hole spatial distribution. The decreased overlap of the electron and hole wave functions within this localization regime is related to relevant optical features such as strong PL red shift (enlarged Stokes shift) and an increase of PL lifetime. Consequently, we show how such features can considerably improve the performance of QD-based optoelectronic devices. Additionally, when tuning the properties of g-QDs, one has to consider the determining aspects that affect device performance and applicability. These are summarized as follows: In LSCs, it has been suggested that energy losses due to reabsorption are caused primarily by the Stokes shift, and variations in the PLQY play a secondary role.13 Quasi-type II gQDs offer the advantage of separating light absorption and

Figure 9. (a) Scheme of the QDSC working principle. (b) Current density vs voltage curves of QDSCs under AM 1.5 G, 100 mW·cm−2. (c) External quantum efficiency (EQE) of the corresponding devices. (d) Electron lifetime (τ) as a function of Voc decay. (e) Functional performance stability over time. Adapted with permission from ref 22. Copyright 2017 John Wiley and Sons.

CdSe/CdS g-QDs. By further tuning of the Se/S molar ratio during in situ growth of each interfacial layer, the maximum PCE value attained was 6.86%. Both CdSe/CdS g-QDs and tailored core/interfacial-alloyed-shell/shell structures exhibited excellent long-term stability, up to 240 h of intermittent operation with respect to core-only QDSCs, whose performance decayed within 96 h.

Figure 10. (a) PL spectra of selected samples at different temperatures (λex = 400 nm). (b) PL decay curves of the PbS peak at 630 nm at different temperatures. Temperature-dependent (c) intensity ratio (IPbS/ICdS) and (d) lifetime. Adapted with permission from ref 20. Copyright 2015 John Wiley and Sons Inc. 615

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QDs, to control the alloying within their interfaces, and even to deposit them into semiconductor oxides.42 The fine control of the different interfaces within g-QDs using these techniques will ensure progress in this area. Consequently, addressing the challenges of fabricating high efficiency and stable optoelectronic devices will lead to developments toward the commercialization of these emerging technologies.

emission functions between two distinct interfaces of the system, with one serving as a light-harvesting antenna and the other as a low-energy emitter. In addition, we showed that alloying with low bandgap semiconductors can enhance the absorption of g-QDs, making them optically active in the NIR region, consequently also improving the performance within the LSCs. A major challenge in LSCs is the decreased efficiency of these devices as their area increases. The smaller the LSCs’ area, the more similar the LQE is to the original PLQY of the gQDs. Enhancing photon absorption of g-QDs helps to increase the photocurrent in the optoelectronic devices. Photoanodes comprising TiO2 sensitized with QDs allow a greater volume of the light-absorbing species than other systems such as those composed of thin-film colloidal QDs and simultaneously permit optimization of the electron mobility.14 In this regard, g-QDs are advantageous compared to other systems, due to the core’s complete isolation from the environment and the significantly longer electron−hole pair lifetime than those in QDs with thinner shells. These factors are crucial to increase the efficiency in both QDSCs and PEC devices for hydrogen generation, as well as the stability of these systems. Nevertheless, there is still room to address challenges in these technologies, to improve their overall performance. For example, the PCE of QDSCs is significantly lower than that of commercial silicon solar cells (typically 20%−40%) and the photocurrent density achieved in QD-PEC devices still falls short compared to commercial target values of hydrogen generation. In addition, double-emission g-QDs can be used as nanoscale thermometers which exploit an optical effect. By simultaneously monitoring their emission peaks at different wavelengths it is possible to accurately measure the intensity ratio of the PL signals over a broad temperature range. However, in these gQDs, the dual-emission is usually achieved in Cd-based g-QDs, consequently its toxicity might limit its broad application.31,41 In general, there are several challenges to be addressed within the structure of g-QDs, such as further reduction of trap states or PL quenching due to electron leakage to the surface by coating the QDs with additional nontoxic shells, namely, SiO2 or ZnS. More systematic studies to overcome the difficulties of shell growth via aqueous synthetic routes are also needed. In addition, the design and synthesis of asymmetric g-QDs with different shape, such as dot-in-rod, or dot-in-pyramid shape may enhance charge separation and transfer. Recent progress in the synthesis and application of cadmium and lead chalcogenide based g-QDs is promising and may lead to further enhancement of the efficiency and stability of optoelectronic devices. The structure in these g-QDs is noticeably advantageous due to their efficient isolation of the core from the surrounding chemical environment by the thick shell, leading to superior photophysical/chemical stability, significantly suppressed nonradiative recombination, improved PLQY, and prolonged exciton lifetime compared to both pure QDs and core/thin shell QDs. Alternative materials and green synthetic approaches have also recently emerged toward low cost and environmentally friendly procedures and technologies. New fundamental insights will rely on the combination of multidisciplinary studies to fully tailor the functionalities of these semiconducting nanocrystals using wet chemical synthesis. Additionally, there are other potential routes to explore such as focused ion beam and molecular beam epitaxy to selfassemble and create size and shape selective noncolloidal g-

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Fabiola Navarro-Pardo: 0000-0003-4673-001X Federico Rosei: 0000-0001-8479-6955 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. Biographies Fabiola Navarro-Pardo is a postdoctoral fellow working under the auspices of the UNESCO Chair in Materials and Technologies for Energy Conversion, Saving and Storage (MATECSS) in the joint laboratory between the INRS-EMT in Canada and the IFFS at UESTC in China. She received her Ph.D. degree in Materials Science from the Autonomous University of the State of Mexico (UAEM) in 2013. Her research interests include the design of hybrid nanostructures and the optimization of their functional properties for solar technologies. Haiguang Zhao is Professor at Qingdao University since September 2017. He received his Ph.D. degree in Materials Science from INRS in 2012. His research interests focus on the synthesis of semiconductor materials (including metal oxides, quantum dots, and perovskites) for solar energy applications, such as photovoltaics, LSCs, and solar-driven hydrogen production. Zhiming M. Wang is Professor of the National 1000-Talent Program at UESTC. He received his Ph.D. degree in Condensed Matter Physics from the Institute of Semiconductors at the Chinese Academy of Sciences in Beijing, China, in 1998. His research focuses on the optoelectronic properties of low-dimensional semiconductor nanostructures and corresponding applications in photovoltaic devices. Federico Rosei is Professor and Director of the Centre for Energy, Materials and Telecommunications of INRS. He holds the UNESCO Chair MATECSS and the Senior Canada Research Chair in Nanostructured Materials. He received his Ph.D. degree in Physics from the University of Rome “La Sapienza” in 2001. His research interests involve the understanding of the fundamental processes that govern nanoscale systems and the development and exploitation of the exceptional properties of nanostructured materials for diverse applications.

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ACKNOWLEDGMENTS We acknowledge funding from the Natural Science and Engineering Research Council of Canada (NSERC, Discovery Grants). F.R. acknowledges partial salary support and funding 616

DOI: 10.1021/acs.accounts.7b00467 Acc. Chem. Res. 2018, 51, 609−618

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Accounts of Chemical Research

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from the Canada Research Chairs program. F.R. is also grateful to the government of China for a Chang Jiang scholar short term award and to Sichuan province for a 1000 Talents Plan short-term award. F.N.P. acknowledges the UNESCO Chair MATECSS for a Postdoctoral Excellence Scholarship and funding from the UESTC and the National Natural Science Foundation of China (Grant No. 5171101224).

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DOI: 10.1021/acs.accounts.7b00467 Acc. Chem. Res. 2018, 51, 609−618