Tuning of electronic properties in IV–VI colloidal nanostructures by alloy composition and architecture

Shabaev A., Hellberg C.S., Efros A.L.

Solar energy production, one of the world's most important unsolved problems, has the potential to be a source of clean, renewable energy if scientists can find a way of generating cheap and efficient solar cells. Generation of multiple excitons from single photons is one way to increase the efficiency of solar energy collection, but the process suffers from low efficiency in bulk materials. An increase of multiexciton generation efficiency in nanocrystals was proposed by Nozik in 2002 and demonstrated by Schaller and Klimov in 2004 in PbSe nanocrystals. Since then, scientists have observed efficient multiexciton generation in nanostructures made of many semiconductors using various measurement techniques. Although the experimental evidence of efficient carrier multiplication is overwhelming, there is no complete theory of this phenomenon. Researchers cannot develop such a theory without a self-consistent description of the Coulomb interaction and a knowledge of mechanisms of electron and hole thermalization in nanostructures. The full theoretical description requires the strength of the Coulomb interaction between exciton and multiexciton states and the thermalization rates, which both vary with the dimensionality of the confining potential. As a result, the efficiency of multiexciton generation depends strongly on the material and the shape of the nanostructure. In this Account, we discuss the theoretical aspects of efficient carrier multiplication in nanostructures. The Coulomb interaction couples single excitons with multiexciton states. Phenomenological many-electron calculations of the evolution of single-photon excitations have shown that efficient multiexciton generation can exist only if the rate of the Coulomb mixing between photo-created exciton and biexciton states is significantly faster than the rate of exciton relaxation. Therefore, to increase multiexciton generation efficiency, we need to either increase the exciton-biexciton mixing rate or suppress the exciton relaxation rate. Following this simple recipe, we show that multiexciton generation efficiency should be higher in semiconductor nanorods and nanoplatelets, which have stronger exciton-biexciton coupling due to the enhancement of the Coulomb interaction through the surrounding medium.

Padilha L.A., Stewart J.T., Sandberg R.L., Bae W.K., Koh W., Pietryga J.M., Klimov V.I.

Nanomaterials with efficient carrier multiplication (CM), that is, generation of multiple electron-hole pairs by single photons, have been the object of intense scientific interest as potential enablers of high efficiency generation-III photovoltaics. In this work, we explore nanocrystal shape control as a means for enhancing CM. Specifically, we investigate the influence of aspect ratio (ρ) of PbSe nanorods (NRs) on both CM and the inverse of this process, Auger recombination. We observe that Auger lifetimes in NRs increase with increasing particle volume and for a fixed cross-sectional size follow a linear dependence on the NR length. For a given band gap energy, the CM efficiency in NRs shows a significant dependence on aspect ratio and exhibits a maximum at ρ ∼ 6-7 for which the multiexciton yields are a factor of ca. 2 higher than those in quantum dots with a similar bandgap energy. To rationalize our experimental observations, we analyze the influence of dimensionality on both CM and non-CM energy-loss mechanisms and offer possible explanations for the seemingly divergent effects the transition from zero- to one-dimensional confinement has on the closely related processes of Auger recombination and CM.

Vaxenburg R., Lifshitz E., Efros A.L.

We calculate the rate of nonradiative Auger recombination in InGaN/GaN quantum wells with rectangular and smooth confining potentials. The calculations show that the rate of Auger recombination in rectangular quantum wells is sufficiently high to explain the efficiency droop in nitride-based light emitting diodes (LEDs). This rate, however, can be reduced by softening of the confining potential and a three-fold suppression is demonstrated in the studied quantum wells. The suppression of the Auger recombination rate improves LED radiative efficiency and reduces the droop effect, as we show using the standard recombination (ABC) model.

Etgar L., Yanover D., Čapek R.K., Vaxenburg R., Xue Z., Liu B., Nazeeruddin M.K., Lifshitz E., Grätzel M.

Quasi type-II PbSe/PbS quantum dots (QDs) are employed in a solid state high efficiency QD/TiO2 heterojunction solar cell. The QDs are deposited using layer-by-layer deposition on a half-micrometer-thick anatase TiO2 nanosheet film with (001) exposed facets. Theoretical calculations show that the carriers in PbSe/PbS quasi type-II QDs are delocalized over the entire core/shell structure, which results in better QD film conductivity compared to PbSe QDs. Moreover, PbS shell permits better stability and facile electron injection from the QDs to the TiO2 nanosheets. To complete the electrical circuit of the solar cell, a Au film is evaporated as a back contact on top of the QDs. This PbSe/PbS QD/TiO2 heterojunction solar cell produces a light to electric power conversion efficiency (η) of 4% with short circuit photocurrent (Jsc) of 17.3 mA/cm2. This report demonstrates highly efficient core/shell near infrared QDs in a QD/TiO2 heterojunction solar cell.

Lifshitz E., Vaxenburg R., Maikov G.I., Rubin-Brusilovski A., Yanover D., Tilchin J., Sashchiuk A.

Considering the significant potential of IV–VI colloidal nanostructures in various technologies, this review describes the development and characterization of unique IV–VI derivatives, consisting of core/shell heterostructures, when either the core or the shell (or both) has an alloy composition with the chemical formula PbSexS1−x/PbSeyS1−y (0≤x(y)≤1). We describe the synthetic procedures and a thorough investigation of the optical properties of the discussed materials. The experimental evidence is supported by suitable electronic band structure calculations. The special properties are expressed as fine tunability of the electronic properties, chemical and temperature stability, variation of electronphonon interactions, emission Stokes shifts and lifetimes, upon the variation of the composition and/or architecture of the heterostructures. The properties obtained are of significant importance in opto-electronic applications.

Bae W.K., Joo J., Padilha L.A., Won J., Lee D.C., Lin Q., Koh W., Luo H., Klimov V.I., Pietryga J.M.

PbSe nanocrystal quantum dots (NQDs) are a promising active material for a range of optoelectronic devices, including solar cells, high-sensitivity infrared (IR) photodetectors, and IR-emitting diodes and lasers. However, device realization has been constrained by these NQDs' chemical instability toward oxidation, which leads to uncontrollable changes in optical and electronic properties. Here, we present a simple method to enhance the stability of PbSe NQDs against oxidation and to improve their optical properties through reaction with molecular chlorine. The chlorine molecules preferentially etch out surface Se ions and react with Pb ions to form a thin (1-2 monolayers) PbCl(x) passivation layer which effectively prevents oxidation during long-term air exposure while passivating surface trap states to increase photoluminescence efficiency and decrease photocharging. Our method is simple, widely applicable to PbSe and PbS NQDs of a range of sizes, compatible with solution-based processes for fabricating NQD-based devices, and effective both in solution and in solid NQD films; thus, it is a practical protocol for facilitating advances over the full range of optoelectronic applications.

Yanover D., Čapek R.K., Rubin-Brusilovski A., Vaxenburg R., Grumbach N., Maikov G.I., Solomeshch O., Sashchiuk A., Lifshitz E.

The work focuses on the synthesis of small-sized PbSe/PbS core/shell colloidal quantum dots with the core diameter of 2–2.5 nm and the shell thickness of 0.5–1.0 nm. The PbSe/PbS core/shell CQDs are chemically stable under time-limited air exposure and have emission quantum efficiency of 60% at room temperature. The PbSe/PbS core/shell CQDs have a tunable absorption edge around 1 μm, large exciton emission Stokes shift (∼150 meV), and small exchange interaction (∼1.5 meV). Theoretical calculations associate the mentioned parameters to the small-size regime as well as to a lift of band-edge degeneracy due to slight shape anisotropy. The specific parameters are of special interest in photovoltaic applications.

Rubin-Brusilovski A., Maikov G., Kolan D., Vaxenburg R., Tilchin J., Kauffmann Y., Sashchiuk A., Lifshitz E.

The synthesis and structural and optical characterization of PbSexS1–x and PbSe/PbSexS1–x nanorods with a diameter between 2 and 4.5 nm and a length of 10 to 38 nm is reported. The energy band gap of the nanorods exhibits a pronounced variation upon the change in diameter and composition, with a minor influence on lengths beyond 10 nm. The photoluminescence spectrum of the nanorods is composed of a dominant band, accompanied by a satellite band at elevated temperatures. The dominant band shows an exceptionally small band gap temperature coefficient and negligible extension of the radiative lifetime at cryogenic temperatures compared with the photoluminescence processes in PbSe nanorods and in PbSexS1–x quantum dots with similar band gap energy. A theoretical model suggests the occurrence of independent transitions from a pair of band-edge valleys, located at the L points of Brillouin zone, related to the dominant and satellite emission processes. Each valley is four-fold degenerate and possesses a relativ...

Ip A.H., Thon S.M., Hoogland S., Voznyy O., Zhitomirsky D., Debnath R., Levina L., Rollny L.R., Carey G.H., Fischer A., Kemp K.W., Kramer I.J., Ning Z., Labelle A.J., Chou K.W., et. al.

Colloidal quantum dot (CQD) films allow large-area solution processing and bandgap tuning through the quantum size effect. However, the high ratio of surface area to volume makes CQD films prone to high trap state densities if surfaces are imperfectly passivated, promoting recombination of charge carriers that is detrimental to device performance. Recent advances have replaced the long insulating ligands that enable colloidal stability following synthesis with shorter organic linkers or halide anions, leading to improved passivation and higher packing densities. Although this substitution has been performed using solid-state ligand exchange, a solution-based approach is preferable because it enables increased control over the balance of charges on the surface of the quantum dot, which is essential for eliminating midgap trap states. Furthermore, the solution-based approach leverages recent progress in metal:chalcogen chemistry in the liquid phase. Here, we quantify the density of midgap trap states in CQD solids and show that the performance of CQD-based photovoltaics is now limited by electron-hole recombination due to these states. Next, using density functional theory and optoelectronic device modelling, we show that to improve this performance it is essential to bind a suitable ligand to each potential trap site on the surface of the quantum dot. We then develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger organic ligands. An organic crosslinking strategy is then used to form the film. Finally, we use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0%, which is a record for a CQD photovoltaic device.

Hughes B.K., Ruddy D.A., Blackburn J.L., Smith D.K., Bergren M.R., Nozik A.J., Johnson J.C., Beard M.C.

We have synthesized alkylselenide reagents to replace the native oleate ligand on PbSe quantum dots (QDs) in order to investigate the effect of surface modification on their stoichiometry, photophysics, and air stability. The alkylselenide reagent removes all of the oleate on the QD surface and results in Se addition; however, complete Se enrichment does not occur, achieving a 53% decrease in the amount of excess Pb for 2 nm diameter QDs and a 23% decrease for 10 nm QDs. Our analysis suggests that the Se ligand preferentially binds to the {111} faces, which are more prevalent in smaller QDs. We find that attachment of the alkylselenide ligand to the QD surface enhances oxidative resistance, likely resulting from a more stable bond between surface Pb atoms and the alkylselenide ligand compared to Pb-oleate. However, binding of the alkylselenide ligand produces a separate nonradiative relaxation route that partially quenches PL, suggesting the formation of a dark hole-trap.

Trinh M.T., Limpens R., de Boer W.D., Schins J.M., Siebbeles L.D., Gregorkiewicz T.

The enhancement of carrier multiplication in semiconductor nanocrystals attracts a great deal of attention because of its potential in photovoltaic applications. Here, we present the results of investigations of a novel carrier multiplication mechanism recently proposed for closely spaced silicon nanocrystals in SiO2 on the basis of photoluminescence. Using ultrafast pump–probe spectroscopy rigorously calibrated for the number of absorbed photons, we find that adjacent nanocrystals are excited directly upon absorption of a single high-energy photon. We demonstrate efficient carrier multiplication with an onset close to the energy conservation threshold of twice the bandgap, 2Eg. Moreover, with absorption of a single high-energy photon under low pump fluence conditions, it was found that carrier–carrier interaction was significantly suppressed, but the amplitude of the signal was enhanced. We show that these results are in excellent agreement with the dependence of photoluminescence quantum yield on excitation, as reported previously for similar materials. Using a photoluminescence-based carrier multiplication mechanism recently proposed for closely spaced silicon nanocrystals in SiO2, scientists demonstrate that adjacent nanocrystals are excited directly upon absorption of a single high-energy photon. They also demonstrate efficient carrier multiplication with an onset close to the energy conservation threshold of twice the bandgap energy.

Bealing C.R., Baumgardner W.J., Choi J.J., Hanrath T., Hennig R.G.

Density functional calculations for the binding energy of oleic acid-based ligands on Pb-rich {100} and {111} facets of PbSe nanocrystals determine the surface energies as a function of ligand coverage. Oleic acid is expected to bind to the nanocrystal surface in the form of lead oleate. The Wulff construction predicts the thermodynamic equilibrium shape of the PbSe nanocrystals. The equilibrium shape is a function of the ligand surface coverage, which can be controlled by changing the concentration of oleic acid during synthesis. The different binding energy of the ligand on the {100} and {111} facets results in different equilibrium ligand coverages on the facets, and a transition in the equilibrium shape from octahedral to cubic is predicted when increasing the ligand concentration during synthesis.

Vaxenburg R., Lifshitz E.

Abel K.A., FitzGerald P.A., Wang T., Regier T.Z., Raudsepp M., Ringer S.P., Warr G.G., van Veggel F.C.

Cation-exchange reactions have greatly expanded the types of nanoparticle compositions and structures that can be prepared. For instance, cation-exchange reactions can be utilized for preparation of core/shell quantum dots with improved (photo)stability and photoluminescence quantum yield. Understanding the structure of these nanomaterials is imperative for explaining their observed properties and for their further development. Core/shell quantum dots formed by cation exchange are particularly challenging to characterize because shell growth does not lead to an increase in overall particle size that can easily be characterized by standard transmission electron microscopy (TEM). Here, we report on the direct observation of the PbSe/CdSe core/shell structure (formed by cation exchange) using high-angle annular dark field (HAADF) imaging and energy-filtered TEM (EF-TEM). These results are further confirmed by energy-dependent X-ray photoemission spectroscopy (XPS) data that show increasing Pb/Cd signal with ...

Casavola M., van Huis M.A., Bals S., Lambert K., Hens Z., Vanmaekelbergh D.

We present a study of Cd2+-for-Pb2+ exchange in PbSe nanocrystals (NCs) with cube, star, and rod shapes. Prolonged temperature-activated cation exchange results in PbSe/CdSe heterostructured nanocrystals (HNCs) that preserve their specific overall shape, whereas the PbSe core is strongly faceted with dominance of {111} facets. Hence, cation exchange proceeds while the Se anion lattice is preserved, and well-defined {111}/{111} PbSe/CdSe interfaces develop. Interestingly, by quenching the reaction at different stages of the cation exchange new structures have been isolated, such as core–shell nanorods, CdSe rods that contain one or two separated PbSe dots and fully zinc blende CdSe nanorods. The crystallographically anisotropic cation exchange has been characterized by a combined HRTEM/HAADF-STEM study of heterointerface evolution over reaction time and temperature. Strikingly, Pb and Cd are only intermixed at the PbSe/CdSe interface. We propose a plausible model for the cation exchange based on a layer-by...

Pote N., Ganguly P., Banpurkar A.

Ponomarenko Vladimir P., Popov Victor, Shuklov Ivan, Ivanov Victor V., Razumov Vladimir F.

Photosensing based on colloidal quantum dots (CQDs) is a rapidly developing area of infrared photoelectronics. The use of colloidal quantum dots markedly simplifies the manufacture, decreases the restrictions to the pixel pitch of the photosensitive elements, and reduces the production cost, which facilitates the wide use of IR sensors in various technological systems. This paper is the first exhaustive overeview of the architectures, methods of manufacturing and basic properties of photonic sensors based on colloidal quantum dots of compounds of Group II, IV and VI elements. Characteristic features of the synthesis and roles of the ligands and CQD morphology in the design of photosensors are considered in detail. The structures of photoresistive, photodiode and phototransistor elements based on HgTe, HgSe, PbS and PbSe CQDs, which are sensitive in various spectral ranges, are described. The main parameters of the most advanced optoelectronic devices based on colloidal quantum dot structures are presented. The key trends in the development of this area are analyzed.The bibliography includes 361 references.

Pote N., Hinge S., Ganguly P., Banpurkar A.

Here, we report incorporation of spinel ferrite CoFe2O4/ZnO (ZCOF) core/shell nanocrystals (NCs) in ferroelectric liquid crystals (FLC) resulting better optical properties. The optical properties of FLC were studied using polarizing optical microscopy (POM) and photoluminescence (PL) measurements. The POM texture analysis reveals improved molecular alignment of FLC by presence of ZCOF NCs. PL emission peak show about 1.4 times improvement of area under curve ratio as a result of enhanced FLC molecular ordering. The improved emission properties of FLC were result of reduced defect state emissions by presence of ZCOF NCs. The optical memory in FLC (20 s) as well as in ZCOF-incorporated FLC (2 min) is also reported using POM texture analysis with aid of ImageJ software. Memory measurements reveal winding and unwinding of helix of FLC by external electric field takes place with discrete steps as a result of smectic subphases emergence. Study of applying staircase potential to FLC material shows critical electric field of helix unwinding is 1 kV/cm. In summary, presence of ZCOF NCs in FLC would give better contrast, better brightness, and optical memory which might be useful for future FLC-based display and electronic devices.

Zhao N., Duan Y., Yang H., Li X., Liu W., Zhao J., Han S., Shen K., Hao N., Fu J., Zhang P.

Persistent spin helix (PSH) manifests itself as an effective knob to tackle spin decay inevitably occurring in disordered two-dimensional electron gases. Here, for ordinary (110)-oriented two-subband GaInAs wells subjected to top and back gate voltages, we theoretically achieve adjusting the Dresselhaus terms of the two bands while meanwhile consistently pinning the system at symmetric configuration [i.e., locking the Rashba spin-orbit (SO) terms to zero], thus enabling simultaneous formation of two copies of PSHs of flexible control. Strikingly, we are able to stretch the pitch---spin density wave length---of PSH by far more than one period, enabling helix-stretch functional spin field-effect transistor (FET), with both on and off states protected by the PSH symmetry. Moreover, we attain a scenario in which the helicities of the two copies of PSHs are sufficiently compensated. This makes possible a new concept: ``orbit (band) filter,'' which resembles spin FET while with novel functionality of orbit filtering, opening up a new route towards spintronic and orbitronic combined applications.

Ponomarenko V.P., Popov V.S., Popov S.V.

Yang H., Wang Q., Fu J.

The interfacial effect in nanosystems is crucial for diverse fields of physics. Here we explore in detail the interface associated Dresselhaus spin-orbit (SO) coupling, which comprises the usual linear ${\ensuremath{\beta}}_{\ensuremath{\nu},\mathrm{u}}$ (${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{u}}$) and cubic ${\ensuremath{\beta}}_{\ensuremath{\nu},3}$ (${\mathrm{\ensuremath{\Gamma}}}_{3}$) contributions as well as the interface-induced term ${\ensuremath{\beta}}_{\ensuremath{\nu},\mathrm{int}}$ (${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{int}}$) of intrasubband (intersubband) kinds. Focusing on ordinary ${\mathrm{Al}}_{0.48}{\mathrm{In}}_{0.52}\mathrm{As}/{\mathrm{Ga}}_{0.47}{\mathrm{In}}_{0.53}\mathrm{As}$ heterostructures with either one or two occupied subbands, we perform a self-consistent Poisson-Schr\"odinger calculation to determine all the relevant SO contributions. We observe that the interface SO term becomes intensifying for heterostructures with either a weaker bulk Dresselhaus strength of the barrier layer or a lower interface smoothness. Remarkably, it is found that the renormalized linear Dresselhaus coefficient, which accounts for the interfacial contribution, may change sign as the interface smoothness varies, opening up the feasibility of the interface-engineered topological matter of persistent skyrmion lattice [J. Y. Fu, P. H. Penteado, M. O. Hachiya, D. Loss, and J. C. Egues, Phys. Rev. Lett. 117, 226401 (2016)]. Moreover, we also determine the intersubband Dresselhaus contributions including an emergent quadratic term (${\mathrm{\ensuremath{\Gamma}}}_{2}$) depending on the interfacial effect and the parity of wave functions. As opposed to intersubband terms of the usual linear (${\mathrm{\ensuremath{\Gamma}}}_{\mathrm{u}}$) and cubic (${\mathrm{\ensuremath{\Gamma}}}_{3}$) kinds, the quadratic contribution leads to unusual avoided crossings of the band dispersion and thus may hybridize spin textures of distinct spin branches. Our results should stimulate experiments probing interface-mediated intra- and intersubband Dresselhaus SO effects and provide an extra leverage for extracting reliable bulk Dresselhaus SO parameters.

Dortaj H., Dolatyari M., Zarghami A., Alidoust F., Rostami A., Matloub S., Yadipour R.

Infrared (IR) cameras based on semiconductors grown by epitaxial methods face two main challenges, which are cost and operating at room temperature. The alternative new technologies which can tackle these two difficulties develop new and facile material and methods. Moreover, the implementation of high speed camera, which makes high resolution images with normal methods, is very expensive. In this paper, a new nanostructure based on a cost-effective solution processed technology for the implementation of the high-speed mid-infrared light camera at room temperature is proposed. To this end, the chemically synthesized PbSe–PbI2 core–shell Quantum Dots (QDs) are used. In this work, a camera including 10 × 10 pixels is fabricated and synthesized QDs spin-coated on interdigitated contact (IDC) and then the fabricated system passivated by epoxy resin. Finally, using an electronic reading circuit, all pixels are converted to an image on the monitor. To model the fabricated camera, we solved Schrodinger–Poisson equations self consistently. Then output current from each pixel is modeled based on semiconductor physics and dark and photocurrent, as well as Responsivity and Detectivity, are calculated. Then the fabricated device is examined, and dark and photocurrents are measured and compared to the theoretical results. The obtained results indicate that the obtained theoretical and measured experimental results are in good agreement together. The fabricated detector is high speed with a rise time of 100 ns. With this speed, we can get 10 million frames per second; this means we can get very high-resolution images. The speed of operation is examined experimentally using a chopper that modulates input light with 50, 100, 250, and 500 Hz. It is shown that the fabricated device operates well in these situations, and it is not limited by the speed of detector. Finally, for the demonstration of the proposed device operation, some pictures and movies taken by the camera are attached and inserted in the paper.

Subramanian A., Hussain S., Din N., Abbas G., Shuja A., Lei W., Chen J., Khan Q., Musselman K.

The vertical field-effect phototransistor (VFEPT) has received great attention because of its large current density and the low operation voltage required to achieve the desirable photodetector per...

Volk A.A., Epps R.W., Abolhasani M.

In recent years, microfluidic technologies have emerged as a powerful approach for the advanced synthesis and rapid optimization of various solution-processed nanomaterials, including semiconductor quantum dots and nanoplatelets, and metal plasmonic and reticular framework nanoparticles. These fluidic systems offer access to previously unattainable measurements and synthesis conditions at unparalleled efficiencies and sampling rates. Despite these advantages, microfluidic systems have yet to be extensively adopted by the colloidal nanomaterial community. To help bridge the gap, this progress report details the basic principles of microfluidic reactor design and performance, as well as the current state of online diagnostics and autonomous robotic experimentation strategies, toward the size, shape, and composition-controlled synthesis of various colloidal nanomaterials. By discussing the application of fluidic platforms in recent high-priority colloidal nanomaterial studies and their potential for integration with rapidly emerging artificial intelligence-based decision-making strategies, this report seeks to encourage interdisciplinary collaborations between microfluidic reactor engineers and colloidal nanomaterial chemists. Full convergence of these two research efforts offers significantly expedited and enhanced nanomaterial discovery, optimization, and manufacturing.

Dibenedetto C.N., Sibillano T., Brescia R., Prato M., Triggiani L., Giannini C., Panniello A., Corricelli M., Comparelli R., Ingrosso C., Depalo N., Agostiano A., Curri M.L., Striccoli M., Fanizza E.

Fabrication of heterostructures by merging two or more materials in a single object. The domains at the nanoscale represent a viable strategy to purposely address materials’ properties for applications in several fields such as catalysis, biomedicine, and energy conversion. In this case, solution-phase seeded growth and the hot-injection method are ingeniously combined to fabricate TiO2/PbS heterostructures. The interest in such hybrid nanostructures arises from their absorption properties that make them advantageous candidates as solar cell materials for more efficient solar light harvesting and improved light conversion. Due to the strong lattice mismatch between TiO2 and PbS, the yield of the hybrid structure and the control over its properties are challenging. In this study, a systematic investigation of the heterostructure synthesis as a function of the experimental conditions (such as seeds’ surface chemistry, reaction temperature, and precursor concentration), its topology, structural properties, and optical properties are carried out. The morphological and chemical characterizations confirm the formation of small dots of PbS by decorating the oleylamine surface capped TiO2 nanocrystals under temperature control. Remarkably, structural characterization points out that the formation of heterostructures is accompanied by modification of the crystallinity of the TiO2 domain, which is mainly ascribed to lattice distortion. This result is also confirmed by photoluminescence spectroscopy, which shows intense emission in the visible range. This originated from self-trapped excitons, defects, and trap emissive states.

Boldt K., Bartlett S., Kirkwood N., Johannessen B.

Core/shell nanocrystals with a graded interface between core and shell exhibit improved opto-electronic properties compared to particles with an abrupt, sharp interface, as material gradients mitigate interfacial defects and define the shape of the confinement potential. So far few works exist that allow to quantify the width of the gradient. In this study ZnSe/CdS nanocrystals with graded shells made at different temperatures are characterized using extended X-ray absorption fine structure (EXAFS) and Raman spectroscopies. The average coordination number of the probed element with respect to the two possible counterions is fit to a simple, geometric model. It is shown that at the lower temperature limit for shell growth (260 °C) substantial interfacial alloying can be attributed mainly to cation migration. At higher temperature (290 °C) strain minimization leads to atomic ordering of the metal ions and an anomalously low degree of phase mixing.

Hyun B., Marus M., Zhong H., Li D., Liu H., Xie Y., Koh W., Xu B., Liu Y., Sun X.W.

Colloidal PbSe nanocrystals (NCs) have gained considerable attention due to their efficient carrier multiplication and emissions across near-infrared and short-wavelength infrared spectral ranges. However, the fast degradation of colloidal PbSe NCs in ambient conditions hampers their widespread applications in infrared optoelectronics. It is well-known that the inorganic thick-shell over core improves the stability of NCs. Here, we present the synthesis of PbSe/PbS core/shell NCs showing wide spectral tunability, in which the molar ratio of lead (Pb) and sulfur (S) precursors, and the concentration of sulfur and PbSe NCs in solvent have a significant effect on the efficient PbS shell growth. The infrared light-emitting diodes (IR-LEDs) fabricated with the PbSe/PbS core/shell NCs exhibit an external quantum efficiency (EQE) of 1.3 % at 1280 nm. The ligand exchange to optimize the distance between NCs and chloride treatment are important processes for achieving high performance on PbSe/PbS NC-LEDs. Our results provide evidence for the promising potential of PbSe/PbS NCs over the wide range of infrared optoelectronic applications.

Ca N.X., Hien N.T., Tan P.M., Phan T.L., Thanh L.D., Do P.V., Bau N.Q., Lien V.T., Van H.T.

We have used wet chemical methods to fabricate colloidal CdSe/CdSm/ZnSen core/intermediate/shell (C/I/S) nanocrystals (NCs), where m = 1 and 2 and n = 1–4 are the number of monolayers. The growth was monitored by using Raman and UV–vis spectroscopy, which demonstrated the formation of correct CdSe/CdSm/ZnSen C/I/S NCs. X-ray diffraction studies proved single-phase NCs crystallized in the zincblende-type structure. Photoluminescence (PL) studies have indicated that after photoexcitation C/I/S NCs generate simultaneously type-I and type-II emissions, namely EI and EII, associated with CdSe/CdSm and CdSm/ZnSen structures, respectively. For a specific value of m = 2, we have found the redshift of both EI and EII emissions when n is increased from 1 to 4. The PL studies versus the laser-excitation power (Pex) up to about four orders of magnitude allow us to identify the origin of two emissions. A large blueshift of the EII peak is ascribed to the band bending effect resulting from the spatially separated photoexcited carriers in type-II NCs. It appears that the dependence of the EII peak on the cube root of Pex ( P e x 1 / 3 ) is linear while that of the EI peak on P e x 1 / 3 is almost constant. In this work, we also point out that the emission intensity (I) of EI and EII can be tuned by changing the values of n and Pex, where I obeys a power law I ∝ P e x k , with k = 0.7–1.0 dependent on the emission type. The nature of these phenomena is discussed in comparison with previous studies on C/S and C/I/S nanoheterostructures.

Irshad Ahamed M., Sathish Kumar K.

Abstract In this communication, we report on Cu2SnS3 quantum dots synthesized by the solvothermal process using different solvents. The optical properties of the quantum dots are analyzed by UV-Vis-NIR and photoluminescence spectroscopy. The results suggest that Cu2SnS3 material has tunable energy bandgap and appropriate wavelength for fabrication of light emitting diodes and laser diodes as sources for fiber optic communication. They exhibit wide absorption in the near infrared range. Further morphological studies with the use of atomic force microscope confirm the surface topography and the existence of quantum dots. The observed characteristics prove the efficiency of Cu2SnS3 quantum dots for O-band wavelength detection used in fiber optic communication and solar cell applications.

Zhang X., Hägglund C., Johansson M.B., Sveinbjörnsson K., Liu J., Johansson E.M.

A solar cell device architecture with top-illumination, where the light does not pass through the substrate, is advantageous for many applications. It is also specifically useful for the construction of tandem or multiple junction photovoltaic devices, with illumination through the top solar cell. Here, a top-illuminated colloidal quantum dot solar cell (TI-CQDSC) is demonstrated and compared with a conventional colloidal quantum dot solar cell (C-CQDSC) constructed on a FTO (fluorine doped tin oxide) glass substrate both theoretically and experimentally. The optical electric field distribution in the solar cells with different configuration is simulated using transfer matrix formalism and a more intense optical electric field was observed in TI-CQDSC, leading to a higher exciton generation rate within the colloidal quantum dot solid. The TI-CQDSCs are constructed on both nonconductive glass and flexible substrates, and a maximum power conversion efficiency of 6.4% and 5.6% is achieved, respectively, comparing to that of 5.9% for the C-CQDSC. The improved performance of the top illuminated solar cell is attributed to a combination of enhanced optical electric field intensity in the colloidal quantum dot solid and superior conductivity of the transparent metal film electrode.