Search

Exciton-phonon coupling (EXPC) plays a key role in the optoelectronic properties of semiconductor nanocrystals (NCs), but a microscopic picture of EXPC is still lacking, particularly regarding the magnitude and scaling with NC size, the dependence on phonon frequency, and the role of the NC surface. The computational complexity associated with accurately describing excitons and phonons has limited previous theoretical studies of EXPC to small NCs, noninteracting electron-hole models, and/or a small number of phonon modes. Here, we develop an atomistic approach for describing EXPC in NCs of experimentally relevant sizes. We validate our approach by calculating the reorganization energies, a measure of EXPC, for CdSe and CdSe-CdS core-shell NCs, finding good agreement with experimental measurements. We demonstrate that exciton formation distorts the NC lattice primarily along the coordinates of low-frequency acoustic modes. Modes at the NC surface play a significant role in smaller NCs while interior modes dominate for larger systems.

Lead chalcogenide colloidal nanocrystals (NCs) are promising materials for solution processable optoelectronics. However, there is little agreement on the identity and character of PbS NC emission for different degrees of quantum confinement-a critical parameter for realizing applications for these nanocrystals. In this work, we combine ensemble and single NC spectroscopies to interrogate preparations of lead sulfide NCs. We use solution photon correlation Fourier spectroscopy (S-PCFS) to measure the average single NC linewidth of near-infrared-emitting PbS quantum dots and find it to be dominated by homogeneous broadening. We further characterize PbS NCs using temperature-dependent linear and time-resolved emission spectroscopy which demonstrate that a kinetically accessed defect state dominates room temperature emission of highly confined emitting NCs. These experiments, taken together, demonstrate that the linewidth and Stokes shift of PbS NCs are the result of emission from two states: a thermally accessed defect-with an energetically pinned charge carrier-and an inhomogeneously broadened band-edge state.

Colloidal ZnO semiconductor nanocrystals have previously been shown to accumulate multiple delocalized conduction-band electrons under chemical, electrochemical, or photochemical reducing conditions, leading to emergent semimetallic characteristics such as quantum plasmon resonances and raising prospects for application in multielectron redox transformations. Here, we demonstrate a dramatic enhancement in the capacitance of colloidal ZnO nanocrystals through aliovalent Fe3+-doping. Very high areal and volumetric capacitances (33 μF cm–2, 233 F cm–3) are achieved in Zn0.99Fe0.01O nanocrystals that rival those of the best supercapacitors used in commercial energy-storage devices. The redox properties of these nanocrystals are probed by potentiometric titration and optical spectroscopy. These data indicate an equilibrium between electron localization by Fe3+ dopants and electron delocalization within the ZnO conduction band, allowing facile reversible charge storage and removal. As “soluble supercapacitors”, colloidal iron-doped ZnO nanocrystals constitute a promising class of solution-processable electronic materials with large charge-storage capacity attractive for future energy-storage applications. [ABSTRACT FROM AUTHOR]

In two-dimensional (2D) colloidal semiconductor nanoplatelets, which are atomically flat nanocrystals, the precise control of thickness and composition on the atomic scale allows for the synthesis of heterostructures with well-defined electron and hole wave function distributions. Introducing transition metal dopants with a monolayer precision enables tailored magnetic exchange interactions between dopants and band states. Here, we use the absorption based technique of magnetic circular dichroism (MCD) to directly prove the exchange coupling of magnetic dopants with the band charge carriers in hetero-nanoplatelets with CdSe core and manganese-doped CdS shell (CdSe/Mn:CdS). We show that the strength of both the electron as well as the hole exchange interactions with the dopants can be tuned by varying the nanoplatelets architecture with monolayer accuracy. As MCD is highly sensitive for excitonic resonances, excited level spectroscopy allows us to resolve and identify, in combination with wave function calculations, several excited state transitions including spin–orbit split-off excitonic contributions. Thus, our study not only demonstrates the possibility to expand the extraordinary physical properties of colloidal nanoplatelets toward magneto-optical functionality by transition metal doping but also provides an insight into the excited state electronic structure in this novel two-dimensional material. [ABSTRACT FROM AUTHOR]

Nonradiative Auger recombination limits the efficiency with which colloidal semiconductor nanocrystals can emit light when they are subjected to strong excitation, with important implications for the application of the nanocrystals in light-emitting diodes and lasers. This has motivated attempts to engineer the structure of the nanocrystals to minimize Auger rates. Here, we study Auger recombination rates in CdSe/CdS core/shell nanoplatelets, or colloidal quantum wells. Using time-resolved photoluminescence measurements, we show that the rate of biexcitonic Auger recombination has a nonmonotonic dependence on the shell thickness, initially decreasing, reaching a minimum for shells with thickness of 2-4 monolayers, and then increasing with further increases in the shell thickness. This nonmonotonic behavior has not been observed previously for biexcitonic recombination in quantum dots, most likely due to inhomogeneous broadening that is not present for the nanoplatelets. [ABSTRACT FROM AUTHOR]

Cesium lead halide (CsPbX3, X = Cl, Br, I) perovskite nanocrystals (PNCs) have recently become a promising material for optoelectronic applications due to their high emission quantum yields and facile band gap tunability via both halide composition and size. The spectroscopy of single PNCs enhances our understanding of the effect of confinement on excitations in PNCs in the absence of obfuscating ensemble averaging and can also inform synthetic efforts. However, single PNC studies have been hampered by poor PNC photostability under confocal excitation, precluding interrogation of all but the most stable PNCs, and leading to a lack of understanding of PNCs in the regime of high confinement. Here, we report the first comprehensive spectroscopic investigation of single PNC properties using solution-phase photon-correlation methods, including both highly confined and blue-emitting PNCs, previously inaccessible to single NC techniques. With minimally perturbative solution-phase photon-correlation Fourier spectroscopy (s-PCFS), we establish that the ensemble emission linewidth of PNCs of all sizes and compositions is predominantly determined by the intrinsic single PNC linewidth (homogeneous broadening). The single PNC linewidth, in turn, dramatically increases with increasing confinement, consistent with what has been found for II-VI semiconductor nanocrystals. With solution-phase photon antibunching measurements, we survey the biexciton-to-exciton quantum yield ratio (BX/X QY) in the absence of user-selection bias or photodegradation. Remarkably, the BX/X QY ratio depends both on the PNC size and halide composition, with values between ~2% for highly confined bromide PNCs and ~50% for intermediately confined iodide PNCs. Our results suggest a wide range of underlying Auger rates, likely due to transitory charge carrier separation in PNCs with relaxed confinement. [ABSTRACT FROM AUTHOR]

We investigate the electronic and transport properties of HgTe 2D colloidal quantum wells. We demonstrate that the material can be made p- or n-type depending on the capping ligands. In addition to the control of majority carrier type, the surface chemistry also strongly affects the photoconductivity of the material. These transport measurements are correlated with the electronic structure determined by high resolution X-ray photoemission. We attribute the change of majority carriers to the strong hybridization of an n-doped HgS layer resulting from capping the HgTe nanoplatelets by S2– ions. We further investigate the gate and temperature dependence of the photoresponse and its dynamics. We show that the photocurrent rise and fall times can be tuned from 100 μs to 1 ms using the gate bias. Finally, we use time-resolved photoemission spectroscopy as a probe of the transport relaxation to determine if the observed dynamics are limited by a fundamental process such as trapping. These pump probe surface photovoltage measurements show an even faster relaxation in the 100–500 ns range, which suggests that the current performances are rather limited by geometrical factors. [ABSTRACT FROM AUTHOR]

We report on the first Au-catalyzed growth of CdTe nanowires by metalorganic vapor phase epitaxy. The nanowires were obtained by a separate precursors flow process in which (i) di-isopropyl-telluride (iPr2Te) was first flowed through the reactor to ensure the formation of liquid Au–Te alloy droplets, and (ii) after purging with pure H2 to remove unreacted iPr2Te molecules from the vapor and the growth surface, (iii) dimethylcadmium (Me2Cd) was supplied to the vapor so that Cd atoms could enter the catalyst droplets, leading to nanowire self-assembly. CdTe nanowires were grown between 485 and 515 °C on (111)-B-GaAs substrates, the latter preliminary deposited with a 2 μm thick (111)-oriented CdTe buffer layer onto which Au nanoparticles were provided. As-grown CdTe nanowires were vertical ([111]-aligned) straight segments of constant diameter and showed an Au-rich nanodroplet at their tips, the contact angle between the droplets and the nanowires being ∼130°. The nanowire axial growth rate appeared kinetics-limited with an activation energy ∼57 kcal/mol. However, the growth rate turned independent from the nanowire diameter. Present data are interpreted by a theoretical model explaining the nanowire growth through the diffusion transport of Te adatoms under the assumption that their growth occurs during the Me2Cd-flow process step. Low-temperature cathodoluminescence spectra recorded from single nanowires showed a well-resolved band-edge emission typical of zincblend CdTe along with a dominant band peaked at 1.539 eV. [ABSTRACT FROM AUTHOR]

Assembly of anisotropic nanocrystals into ordered superstructures is an area of intense research interest due to its relevance to bring nanocrystal properties to macroscopic length scales and to impart additional collective properties owing to the superstructure. Numerous routes have been explored to assemble such nanocrystal superstructures ranging from self-directed to external field-directed methods. Most of the approaches require sensitive control of experimental parameters that are largely environmental and require extra processing steps, increasing complexity and limiting reproducibility. Here, we demonstrate a simple approach to assemble colloidal nanorods in situ, wherein dopant incorporation during the particle synthesis results in the formation of preassembled 2D sheets of close-packed ordered arrays of vertically oriented nanorods in solution. [ABSTRACT FROM AUTHOR]

Crystalline GaN nanosheets hold great challenge in growth and promising application in optoelectronic nanodevices. In this work, we reported an accessible template approach toward the rational synthesis of GaN nanosheets through the nitridation of metastable γ-Ga2O3 nanosheets synthesized from a hydrothermal reaction. The cubic γ-Ga2O3 nanosheets with smooth surface and decent crystallinity can be directly converted into hexagonal GaN nanosheets with similar morphology framework and comparable crystal quality in NH3 at 850 °C. UV-vis spectrum measurement reveals that the GaN nanosheets show a band gap of 3.30 eV with strong visible absorption in the range of 370-500 nm. The template synthetic strategy proposed in this work will open up more opportunities for the achievement of a variety of sheetlike nanostructures that can not be obtained through conventional routines and will undoubtedly further promote the fundamental research of newly emerging sheetlike nanostructures and nanotechnology. [ABSTRACT FROM AUTHOR]

In semiconductors, quantum confinement can greatly enhance the interaction between band carriers (electrons and holes) and dopant atoms. One manifestation of this enhancement is the increased stability of exciton magnetic polarons in magnetically doped nanostructures. In the limit of very strong 0D confinement that is realized in colloidal semiconductor nanocrystals, a single exciton can exert an effective exchange field B ex on the embedded magnetic dopants that exceeds several tesla. Here we use the very sensitive method of resonant photoluminescence (PL) to directly measure the presence and properties of exciton magnetic polarons in colloidal Cd1-x Mn x Se nanocrystals. Despite small Mn2+ concentrations (x = 0.4-1.6%), large polaron binding energies up to 26 meV are observed at low temperatures via the substantial Stokes shift between the pump laser and the resonant PL maximum, indicating nearly complete alignment of all Mn2+ spins by B ex. Temperature and magnetic field-dependent studies reveal that B ex ≈ 10 T in these nanocrystals, in good agreement with theoretical estimates. Further, the emission line widths provide direct insight into the statistical fluctuations of the Mn2+ spins. These resonant PL studies provide detailed insight into collective magnetic phenomena, especially in lightly doped nanocrystals where conventional techniques such as nonresonant PL or time-resolved PL provide ambiguous results. [ABSTRACT FROM AUTHOR]

The optical properties of colloidal semiconductor nanocrystals are largely influenced by the trapping of charge carriers on the nanocrystal surface. Different concentrations of electron and hole traps and different rates of their capture to the traps provide dynamical charging of otherwise neutral nanocrystals. We study the photocharging formation and evolution dynamics in CdS colloidal quantum dots with native oleic acid surface ligands. A time-resolved technique with three laser pulses (pump, orientation, and probe) is developed to monitor the photocharging dynamics with picosecond resolution on wide time scales ranging from picoseconds to milliseconds. The detection is based on measuring the coherent spin dynamics of electrons, allowing us to distinguish the type of carrier in the QD core (electron or hole). We find that although initially negative photocharging happens because of fast hole trapping, it eventually evolves to positive photocharging due to electron trapping and hole detrapping. The positive photocharging lasts up to hundreds of microseconds at room temperature. These findings give insight into the photocharging process and provide valuable information for understanding the mechanisms responsible for the emission blinking in colloidal nanostructures. [ABSTRACT FROM AUTHOR]

The finite spectral line width of an ensemble of CdSe nanocrystals arises from size and shape inhomogeneity and the single-nanocrystal spectrum itself. This line width directly limits the performance of nanocrystal-based devices, yet most optical measurements cannot resolve the underlying contributions. We use two-dimensional electronic spectroscopy (2D ES) to measure the line width of the band-edge exciton of CdSe nanocrystals as a function of radii and surface chemistry. We find that the homogeneous width decreases for increasing nanocrystal radius and that surface chemistry plays a critical role in controlling this line width. To explore the hypothesis that unpassivated trap states serve to broaden the homogeneous line width and to explain its size-dependence, we use 3D ES to identify the spectral signatures of exciton-phonon coupling to optical and acoustic phonons. We find enhanced coupling to optical phonon modes for nanocrystals that lack electron-passivating ligands, suggesting that localized surface charges enhance exciton-phonon coupling via the Fröhlich interaction. Lastly, the data reveal that spectral diffusion contributes negligibly to the homogeneous line width on subnanosecond time scales. [ABSTRACT FROM AUTHOR]

Microlasers hold great promise for the development of photonics and optoelectronics. Among the discovered optical gain materials, colloidal quantum dots (CQDs) have been recognized as the most appealing candidate due to the facile emission tunability and solution processability. However, to date, it is still challenging to develop CQD-based microlasers with low cost yet high performance. Moreover, the poor long-term stability of CQDs remains to be the most critical issue, which may block their laser aspirations. Herein, we developed a unique but generic approach to forming a novel type of a whispering-gallery-mode (WGM) microbubble laser from the hybrid CQD/poly(methyl methacrylate) (PMMA) nanocomposites. The formation mechanism of the microbubbles was unraveled by recording the drying process of the nanocomposite droplets. Interestingly, these microbubbles naturally serve as the high-quality WGM laser resonators. By simply changing the CQDs, the lasing emission can be tuned across the whole visible spectral range. Importantly, these microbubble lasers exhibit unprecedented long-term stability (over one year), sufficient for practical applications. As a proof-of-concept, the potential of water vapor sensing was demonstrated. Our results represent a significant advance in microlasers based on the advantageous CQDs and may offer new possibilities for photonics and optoelectronics. [ABSTRACT FROM AUTHOR]

Colloidal quantum dot (CQD) materials are of interest in thin-film solar cells due to their size-tunable bandgap and low-cost solution-processing. However, CQD solar cells suffer from inefficient charge extraction over the film thicknesses required for complete absorption of solar light. Here we show a new strategy to enhance light absorption in CQD solar cells by nanostructuring the CQD film itself at the back interface. We use two-dimensional finite-difference time-domain (FDTD) simulations to study quantitatively the light absorption enhancement in nanostructured back interfaces in CQD solar cells. We implement this experimentally by demonstrating a nanoimprint-transfer-patterning (NTP) process for the fabrication of nanostructured CQD solids with highly ordered patterns. We show that this approach enables a boost in the power conversion efficiency in CQD solar cells primarily due to an increase in short-circuit current density as a result of enhanced absorption through light-trapping. [ABSTRACT FROM AUTHOR]

Typical use of colloidal quantum dots (QDs) as bright, tunable phosphors in real applications relies on engineering of their surfaces to suppress the loss of excited carriers to surface trap states or to the surrounding medium. Here, we explore the utility of QDs in an application that actually exploits their propensity toward photoionization, namely within efficient and robust photocathodes for use in next-generation electron guns. In order to establish the relevance of QD films as photocathodes, we evaluate the efficiency of electron photoemission of films of a variety of compositions in a typical electron gun configuration. By quantifying photocurrent as a function of excitation photon energy, excitation intensity and pulse duration, we establish the role of hot electrons in photoemission within the multiphoton excitation regime. We also demonstrate the effect of QD structure and film deposition methods on efficiency, which suggests numerous pathways for further enhancements. Finally, we show that QD photocathodes offer superior efficiencies relative to standard copper cathodes and are robust against degradation under ambient conditions. [ABSTRACT FROM AUTHOR]

GaAs is one of the most important semiconductors. However, colloidal GaAs nanocrystals remain largely unexplored because of the difficulties with their synthesis. Traditional synthetic routes either fail to produce pure GaAs phase or result in materials whose optical properties are very different from the behavior expected for quantum dots of direct-gap semiconductors. In this work, we demonstrate a variety of synthetic routes toward crystalline GaAs NCs. By using a combination of Raman, EXAFS, transient absorption, and EPR spectroscopies, we conclude that unusual optical properties of colloidal GaAs NCs can be related to the presence of Ga vacancies and lattice disorder. These defects do not manifest themselves in TEM images and powder X-ray diffraction patterns but are responsible for the lack of absorption features even in apparently crystalline GaAs nanoparticles. We introduce a novel molten salt based annealing approach to alleviate these structural defects and show the emergence of size-dependent excitonic transitions in colloidal GaAs quantum dots. [ABSTRACT FROM AUTHOR]

The photoluminescence of individual CdSe/CdS/ZnS core/shell nanocrystals has been investigated under external forces. After mutual alignment of a correlative atomic force and confocal microscope, individual particles were colocalized and exposed to a series of force cycles by using the tip of the AFM cantilever as a nanoscale piston. Thus, force-dependent changes of photophysical properties could be tracked on a single particle level. Remarkably, individual nanocrystals either shifted to higher or to lower emission energies with no indications of multiple emission lines under applied force. The direction and magnitude of these reversible spectral shifts depend on the orientation of nanocrystal axes relative to the external anisotropic force. Maximum pressures derived from the applied forces within a simple contact-mechanical model lie in the GPa range, comparable to values typically emerging in diamond anvil cells. Average spectral shift parameters of -3.5 meV/GPa and 3.0 meV/GPa are found for red- and blue-shifting species, respectively. Our results clearly demonstrate that the emission energy of single nanocrystals can be reversibly tuned over an appreciable wavelength range without degradation of their performance which appears as a promising feature with respect to tunable single photon sources or the creation of coherently coupled particle dimers. [ABSTRACT FROM AUTHOR]

Understanding the effects of pressure-induced deformations on the optoelectronic properties of nanomaterials is important not only from the fundamental point of view but also for potential applications such as stress sensors and electromechanical devices. Here, we describe the novel insights into these piezochromic effects gained from using a linear-scaling density functional theory framework and an electronic enthalpy scheme, which allow us to accurately characterize the electronic structure of CdS nanocrystals with a zincblende-like core of experimentally relevant size. In particular, we focus on unravelling the complex interplay of size and surface (phenyl) ligands with pressure. We show that pressure-induced deformations are not simple isotropic scaling of the original structures and that the change in HOMO-LUMO gap with pressure results from two competing factors: (i) a bulk-like linear increase due to compression, which is offset by (ii) distortions and disorder and, to a lesser extent, orbital hybridization induced by ligands affecting the frontier orbitals. Moreover, we observe that the main peak in the optical absorption spectra is systematically red-shifted or blue-shifted, as pressure is increased up to 5 GPa, depending on the presence or absence of phenyl ligands. These heavily hybridize the frontier orbitals, causing a reduction in overlap and oscillator strength, so that at zero pressure, the lowest energy transition involves deeper hole orbitals than in the case of hydrogen-capped nanocrystals; the application of pressure induces greater delocalization over the whole nanocrystals bringing the frontier hole orbitals into play and resulting in an unexpected red shift for the phenyl-capped nanocrystals, in part caused by distortions. In response to a growing interest in relatively small nanocrystals that can be difficult to accurately characterize with experimental techniques, this work exemplifies the detailed understanding of structure-property relationships under pressure that can be obtained for realistic nanocrystals with state-of-the-art first-principles methods and used for the characterization and design of devices based on these and similar nanomaterials. [ABSTRACT FROM AUTHOR]

Femtosecond two-dimensional Fourier transform spectroscopy is used to determine the static bandgap inhomogeneity of a colloidal quantum dot ensemble. The excited states of quantum dots absorb light, so their absorptive two-dimensional (2D) spectra will typically have positive and negative peaks. It is shown that the absorption bandgap inhomogeneity is robustly determined by the slope of the nodal line separating positive and negative peaks in the 2D spectrum around the bandgap transition; this nodal line slope is independent of excited state parameters not known from the absorption and emission spectra. The absorption bandgap inhomogeneity is compared to a size and shape distribution determined by electron microscopy. The electron microscopy images are analyzed using new 2D histograms that correlate major and minor image projections to reveal elongated nanocrystals, a conclusion supported by grazing incidence small-angle X-ray scattering and high-resolution transmission electron microscopy. The absorption bandgap inhomogeneity quantitatively agrees with the bandgap variations calculated from the size and shape distribution, placing upper bounds on any surface contributions. [ABSTRACT FROM AUTHOR]

Although various colloidal quantum dot (QD) coating and patterning techniques have been developed to meet the demands in optoelectronic applications over the past years, each of the previously demonstrated methods has one or more limitations and trade-offs in forming multicolor, high-resolution, or large-area patterns of QDs. In this study, we present an alternative QD patterning technique using conventional photolithography combined with charge-assisted layer-by-layer (LbL) assembly to solve the trade-offs of the traditional patterning processes. From our demonstrations, we show repeatable QD patterning process that allows multicolor QD patterns in both large-area and microscale. Also, we show that the QD patterns are robust against additional photolithography processes and that the thickness of the QD patterns can be controlled at each position. To validate that this process can be applied to actual device applications as an active material, we have fabricated inverted, differently colored, active QD light-emitting device (QD-LED) on a pixelated substrate, which achieved maximum electroluminescence intensity of 23 770 cd/m², and discussed the results. From our findings, we believe that our process provides a solution to achieving both high-resolution and large-scale QD pattern applicable to not only display, but also to practical photonic device research and development. [ABSTRACT FROM AUTHOR]

Colloidal quantum dot (CQD) solar cells are solution-processed photovoltaics with broad spectral absorption tunability. Major advances in their efficiency have been made via improved CQD surface passivation and device architectures with enhanced charge carrier collection. Herein, we demonstrate a new strategy to improve further the passivation of CQDs starting from the solution phase. A cosolvent system is employed to tune the solvent polarity in order to achieve the solvation of methylammonium iodide (MAI) and the dispersion of hydrophobic PbS CQDs simultaneously in a homogeneous phase, otherwise not achieved in a single solvent. This process enables MAI to access the CQDs to confer improved passivation. This, in turn, allows for efficient charge extraction from a thicker photoactive layer device, leading to a certified solar cell power conversion efficiency of 10.6%, a new certified record in CQD photovoltaics. [ABSTRACT FROM AUTHOR]

Surface trap states in copper indium gallium selenide semiconductor nanocrystals (NCs), which serve as undesirable channels for nonradiative carrier recombination, remain a great challenge impeding the development of solar and optoelectronics devices based on these NCs. In order to design efficient passivation techniques to minimize these trap states, a precise knowledge about the charge carrier dynamics on the NCs surface is essential. However, selective mapping of surface traps requires capabilities beyond the reach of conventional laser spectroscopy and static electron microscopy; it can only be accessed by using a one-of-a-kind, second-generation four-dimensional scanning ultrafast electron microscope (4D S-UEM) with subpicosecond temporal and nanometer spatial resolutions. Here, we precisely map the collective surface charge carrier dynamics of copper indium gallium selenide NCs as a function of the surface trap states before and after surface passivation in real space and time using S-UEM. The time-resolved snapshots clearly demonstrate that the density of the trap states is significantly reduced after zinc sulfide (ZnS) shelling. Furthermore, the removal of trap states and elongation of carrier lifetime are confirmed by the increased photocurrent of the self-biased photodetector fabricated using the shelled NCs. [ABSTRACT FROM AUTHOR]

Doped semiconductor nanocrystals are an emerging class of materials hosting localized surface plasmon resonance (LSPR) over a wide optical range. Studies so far have focused on tuning LSPR frequency by controlling the dopant and carrier concentrations in diverse semiconductor materials. However, the influence of anisotropic nanocrystal shape and of intrinsic crystal structure on LSPR remain poorly explored. Here, we illustrate how these two factors collaborate to determine LSPR characteristics in hexagonal cesium-doped tungsten oxide nanocrystals. The effect of shape anisotropy is systematically analyzed via synthetic control of nanocrystal aspect ratio (AR), from disks to nanorods. We demonstrate the dominant influence of crystalline anisotropy, which uniquely causes strong LSPR band-splitting into two distinct peaks with comparable intensities. Modeling typically used to rationalize particle shape effects is refined by taking into account the anisotropic dielectric function due to crystalline anisotropy, thus fully accounting for the AR-dependent evolution of multiband LSPR spectra. This new insight into LSPR of semiconductor nanocrystals provides a novel strategy for an exquisite tuning of LSPR line shape. [ABSTRACT FROM AUTHOR]

Thin film solar cells, due to the low cost, high efficiency, long-term stability, and consumer applications, have been widely applied for harvesting green energy. All of these thin film solar cells generally adopt various metal thin films as the back electrode, like Mo, Au, Ni, Ag, Al, graphite, and so forth. When they contact with p-type layer, it always produces a Schottky contact with a high contact potential barrier, which greatly affects the cell performance. In this work, we report for the first time to find an appropriate p-type conductive semiconductor film, digenite Cu9S5 nanocrystalline film, as the back electrode for CdTe solar cells as the model device. Its low sheet resistance (16.6 Ω/sq) could compare to that of the commercial TCO films (6-30 O/sq), like FTO, ITO, and AZO. Different from the traditonal metal back electrode, it produces a successive gradient-doping region by the controllable Cu diffusion, which greatly reduces the contact potential barrier. Remarkably, it achieved a comparable power conversion efficiency (PCE, 11.3%) with the traditional metal back electrode (Cu/Au thin films, 11.4%) in CdTe cells and a higher PCE (13.8%) with the help of the Au assistant film. We believe it could also act as the back electrode for other thin film solar cells (a-Si, CuInS2, CIGSe, CZTS, etc.), for their performance improvement. [ABSTRACT FROM AUTHOR]

The optimization of photoluminescence spectral linewidths in semiconductor nanocrystal preparations involves minimizing both the homogeneous and inhomogeneous contributions to the ensemble spectrum. Although the inhomogeneous contribution can be controlled by eliminating interparticle inhomogeneities, far less is known about how to synthetically control the homogeneous, or single-nanocrystal, spectral linewidth. Here, we use solution photon-correlation Fourier spectroscopy (S-PCFS) to measure how the sample-averaged single-nanocrystal emission linewidth of CdSe core and core/shell nanocrystals change with systematic changes in the size of the cores and the thickness and composition of the shells. We find that the single-nanocrystal linewidth at room temperature is heavily influenced by the nature of the CdSe surface and the epitaxial shell, which have a profound impact on the internal electric fields that affect exciton-phonon coupling. Our results explain the wide variations, both experimental and theoretical, in the magnitude and size dependence in previous reports on exciton-phonon coupling in CdSe nanocrystals. Moreover, our findings offer a general pathway for achieving the narrow spectral linewidths required for many applications of nanocrystals. [ABSTRACT FROM AUTHOR]

Solution-processed quantum dots area promising material for large-scale, low-cost solar cell applications.New device architectures and improved passivation have been instrumentalin increasing the performance of quantum dot photovoltaic devices.Here we report photovoltaic devices based on inks of quantum dot onwhich we grow thin perovskite shells in solid-state films. Passivationusing the perovskite was achieved using a facile solution ligand exchangefollowed by postannealing. The resulting hybrid nanostructure createda more intrinsic CQD film, which, when incorporated into a photovoltaicdevice with graded bandstructure, achieved a record solar cell performancefor single-step-deposited CQD films, exhibiting an AM1.5 solar powerconversion efficiency of 8.95%. [ABSTRACT FROM AUTHOR]

The optoelectronictunability offered by colloidal quantum dots (CQDs) is attractivefor photovoltaic applications but demands proper band alignment atelectrodes for efficient charge extraction at minimal cost to voltage.With this goal in mind, self-assembled monolayers (SAMs) can be usedto modify interface energy levels locally. However, to be effectiveSAMs must be made robust to treatment using the various solvents andligands required for to fabricate high quality CQD solids. We reportrobust self-assembled monolayers (R-SAMs) that enable us to increasethe efficiency of CQD photovoltaics. Only by developing a processfor secure anchoring of aromatic SAMs, aided by deposition of theSAMs in a water-free deposition environment, were we able to providean interface modification that was robust against the ensuing chemicaltreatments needed in the fabrication of CQD solids. The energy alignmentat the rectifying interface was tailored by tuning the R-SAM for optimalalignment relative to the CQD quantum-confined electron energy levels.This resulted in a CQD PV record power conversion efficiency (PCE)of 10.7% with enhanced reproducibility relative to controls. [ABSTRACT FROM AUTHOR]

Theefficiency and stability of emission from semiconductor nanocrystalquantum dots (QDs) is negatively affected by “blinking”on the single-nanocrystal level, that is, random alternation of brightand dark periods. The time scales of these fluctuations can be aslong as many seconds, orders of magnitude longer than typical lifetimesof exciton states in QDs. In this work, we investigate photoluminescencefrom QDs delayed over microseconds to milliseconds. Our results provethe existence of long-lived charge-separated states in QDs. We studythe properties of delayed emission as a direct way to learn aboutcharge carrier separation and recovery of the exciton state. A newmicroscopic model is developed to connect delayed emission to excitonrecombination and blinking from which we conclude that bright periodsin blinking are in fact not characterized by uninterrupted opticalcycling as often assumed. [ABSTRACT FROM AUTHOR]

We demonstrate therelation between the optical blinking of colloidal semiconductor nanocrystals(NCs) and their electrical charge blinking for which we provide thefirst experimental observation of power-law statistics. To show this,we harness the performance of CdSe/ZnS NCs coupled with carbon nanotubefield-effect transistors (CNTFETs), which act as single charge-sensitiveelectrometers with submillisecond time resolution, at room temperature.A random telegraph signal (RTS) associated with the NC single-trapcharging is observed and exhibits power-law temporal statistics (τ–α, with α in the range of ∼1–3),and a Lorentzian current noise power spectrum with a well-defined1/f2corner. The spectroscopic analysisof the NC–CNTFET devices is consistent with the charging ofNC defect states with a charging energy of Ec≥ 200 meV. These results pave the way for a deeperunderstanding of the physics and technology of nanocrystal-based optoelectronicdevices. [ABSTRACT FROM AUTHOR]

A new class of chiral nanoparticlesis of great interest not onlyfor nanotechnology, but also for many other fields of scientific endeavor.Normally the chirality in semiconductor nanocrystals is induced bythe initial presence of chiral ligands/stabilizer molecules. Herewe report intrinsic chirality of ZnS coated CdSe quantum dots (QDs)and quantum rods (QRs) stabilized by achiral ligands. As-preparedensembles of these nanocrystals have been found to be a racemic mixtureof d- and l-nanocrystals which also includes a portionof nonchiral nanocrystals and so in total the solution does not showa circular dichroism (CD) signal. We have developed a new enantioselectivephase transfer technique to separate chiral nanocrystals using anappropriate chiral ligand and obtain optically active ensembles ofCdSe/ZnS QDs and QRs. After enantioselective phase transfer, the nanocrystalsisolated in organic phase, still capped with achiral ligands, nowdisplay circular dichroism (CD). We propose that the intrinsic chiralityof CdSe/ZnS nanocrystals is caused by the presence of naturally occurringchiral defects. [ABSTRACT FROM AUTHOR]

We calculate the rate of nonradiative Auger recombination in negatively charged CdSe nanocrystals (NCs). The rate is nonmonotonic, strongly oscillating with NC size, and sensitive to the NC surface. The oscillations result in nonexponential decay of carriers in NC ensembles. Using a standard single-exponential approximation of the decay dynamics, we determine the apparent size dependence of the Auger rate in an ensemble and derive CdSe surface parameters consistent with the experimental dependence on size. [ABSTRACT FROM AUTHOR]

Optical activity is a common natural phenomenon, which occurs in individual molecules, biomolecules, biological species, crystalline solids, liquid crystals, and various nanosized objects, leading to numerous important applications in almost every field of modern science and technology. Because this activity can hardly be altered, creation of artificial active media with controllable optical properties is of paramount importance. Here, for the first time to the best of our knowledge, we theoretically demonstrate that optical activity can be inherent to many semiconductor nanowires, as it is induced by chiral dislocations naturally developing during their growth. By assembling such nanowires in two- or three-dimensional periodic lattices, one can create optically active quantum supercrystals whose activity can be varied in many ways owing to the size quantization of the nanowires? energy spectra. We believe that this research is of particular importance for the future development of semiconducting nanomaterials and their applications in nanotechnology, chemistry, biology, and medicine. [ABSTRACT FROM AUTHOR]

Conventional solar cells exhibitlimited efficiencies in part dueto their inability to absorb the entire solar spectrum. Sub-band-gapphotons are typically lost but could be captured if a material thatperforms up-conversion, which shifts photon energies higher, is coupledto the device. Recently, molecular chromophores that undergo triplet–tripletannihilation (TTA) have shown promise for efficient up-conversionat low irradiance, suitable for some types of solar cells. However,the molecular systems that have shown the highest up-conversion efficiencyto date are ill suited to broadband light harvesting, reducing theirapplicability. Here we overcome this limitation by combining an organicTTA system with highly fluorescent CdSe semiconductor nanocrystals.Because of their broadband absorption and spectrally narrow, size-tunablefluorescence, the nanocrystals absorb the radiation lost by the TTAchromophores, returning this energy to the up-converter. The resultingnanocrystal-boosted system shows a doubled light-harvesting ability,which allows a green-to-blue conversion efficiency of ∼12.5%under 0.5 suns of incoherent excitation. This record efficiency atsubsolar irradiance demonstrates that boosting the TTA by light-emittingnanocrystals can potentially provide a general route for up-conversionfor different photovoltaic and photocatalytic applications. [ABSTRACT FROM AUTHOR]

Colloidal quantum dots (QDs) arepromising candidates for the nextgeneration of photovoltaic (PV) technologies. Much of the progressin QD PVs is based on using PbS QDs, partly because they are stableunder ambient conditions. There is considerable interest in extendingthis work to PbSe QDs, which have shown an enhanced photocurrent dueto multiple exciton generation (MEG). One problem complicating suchdevice-based studies is a poor stability of PbSe QDs toward exposureto ambient air. Here we develop a direct cation exchange synthesisto produce PbSe QDs with a large range of sizes and with in situ chlorideand cadmium passivation. The synthesized QDs have excellent air stability,maintaining their photoluminescence quantum yield under ambient conditionsfor more than 30 days. Using these QDs, we fabricate high-performancesolar cells without any protection and demonstrate a power conversionefficiency exceeding 6%, which is a current record for PbSe QD solarcells. [ABSTRACT FROM AUTHOR]

Colloidalquantum-dots are bright, tunable emitters that are idealfor studying near-field quantum-optical interactions. However, theircolloidal nature has hindered their facile and precise placement atdesired near-field positions, particularly on the structured substratesprevalent in plasmonics. Here, we use high-resolution electro-hydrodynamicprinting (<100 nm feature size) to deposit countable numbers ofquantum dots on both flat and structured substrates with a few nanometerprecision. We also demonstrate that the autofocusing capability ofthe printing method enables placement of quantum dots preferentiallyat plasmonic hot spots. We exploit this control and design diffraction-limitedphotonic and plasmonic sources with arbitrary wavelength, shape, andintensity. We show that simple far-field illumination can excite thesenear-field sources and generate fundamental plasmonic wave-patterns(plane and spherical waves). The ability to tailor subdiffractionsources of plasmons with quantum dots provides a complementary techniqueto traditional scattering approaches, offering new capabilities fornanophotonics. [ABSTRACT FROM AUTHOR]

The use of colloidal semiconductornanocrystals for optical amplificationand lasing has been limited by the need for high input power densities.Here we show that colloidal nanoplatelets produce amplified spontaneousemission with thresholds as low as 6 μJ/cm2and gainas high as 600 cm–1, both a significant improvementover colloidal nanocrystals; in addition, gain saturation occurs atpump fluences 2 orders of magnitude higher than the threshold. Weattribute this exceptional performance to large optical cross-sections,slow Auger recombination rates, and narrow ensemble emission linewidths. [ABSTRACT FROM AUTHOR]

Semiconductor nanocrystals are keymaterials for achieving localizedsurface plasmon resonance (LSPR) excitation in the extended spectralranges beyond visible light, which are critical wavelengths for chemicalsensing, infrared detection, and telecommunications. Unlike metalnanoparticles which are already widely exploited in plasmonics, littleis known about the near-field behavior of semiconductor nanocrystals.Near-field interactions are expected to vary greatly with nanocrystalcarrier density and mobility, in addition to properties such as nanocrystalsize, shape, and composition. Here we demonstrate near-field couplingbetween anisotropic disk-shaped nanocrystals composed of Cu2–xS, a degenerately doped semiconductor whose electronicproperties can be modulated by Cu content. Assembling colloidal nanocrystalsinto mono- and multilayer films generates dipole–dipole LSPRcoupling between neighboring nanodisks. We investigate nanodisks ofvarying crystal phases (Cu1.96S, Cu7.2S4, and CuS) and find that nanodisk orientation produces a dramaticchange in the magnitude and polarization direction of the localizedfield generated by LSPR excitation. This study demonstrates the potentialof semiconductor nanocrystals for the realization of low-cost, active,and tunable building blocks for infrared plasmonics and for the investigationof light–matter interactions at the nanoscale. [ABSTRACT FROM AUTHOR]

We report measurements of electricaltransport through single CdSe/CdScore/shell colloidal quantum dots (cQDs) connected to source and draincontacts. We observe telegraphic switching noise showing few plateausat room temperature. We model and interpret these results as chargetrapping of individual trap states, and therefore we resolve individualcharge defects in these high-quality low-strain cQDs. The small numberof observed defects quantitatively validates the passivation methodbased on thick CdS shells nearly lattice-matched to CdSe cores firstdeveloped to suppress photoluminescence blinking. Finally, we introducea figure of merit useful to efficiently distinguish telegraphic noisefrom noise with a Gaussian distribution. [ABSTRACT FROM AUTHOR]

Previous single-particle spectroscopicstudies of colloidal quantumdots have indicated a significant spread in biexciton lifetimes acrossan ensemble of nominally identical nanocrystals. It has been speculatedthat in addition to dot-to-dot variation in physical dimensions, thisspread is contributed to by variations in the structure of the quantumdot interface, which controls the shape of the confinement potential.Here, we directly evaluate the effect of the composition of the core–shellinterface on single- and multiexciton dynamics via side-by-side measurementsof individual core–shell CdSe/CdS nanocrystals with a sharpversus smooth (graded) interface. To realize the latter type of structureswe incorporate a CdSexS1–xalloy layer of controlled composition and thicknessbetween the CdSe core and the CdS shell. We observe that while havingessentially no effect on single-exciton decay, the interfacial alloylayer leads to a systematic increase in biexciton lifetimes, whichcorrelates with the increase in the biexciton emission efficiency,as inferred from two-photon correlation measurements. These observationsprovide direct experimental evidence that in addition to the sizeof the quantum dot, its interfacial properties also significantlyaffect the rate of Auger recombination, which governs biexciton decay.These findings help rationalize previous observations of a significantheterogeneity in the biexciton lifetimes across similarly sized quantumdots and should facilitate the development of “Auger-recombination-free”colloidal nanostructures for a range of applications from lasers andlight-emitting diodes to photodetectors and solar cells. [ABSTRACT FROM AUTHOR]

Solutionprocessing of photovoltaic semiconducting layers offersthe potential for drastic cost reduction through improved materialsutilization and high device throughput. One compelling solution-basedprocessing strategy utilizes semiconductor layers produced by sinteringnanocrystals into large-grain semiconductors at relatively low temperatures.Using n-ZnO/p-CdTe as a model system, we fabricate sintered CdTe nanocrystalsolar cells processed at 350 °C with power conversion efficiencies(PCE) as high as 12.3%. JSCof over 25mA cm–2are achieved, which are comparable or higherthan those achieved using traditional, close-space sublimated CdTe.We find that the VOCcan be substantiallyincreased by applying forward bias for short periods of time. Capacitancemeasurements as well as intensity- and temperature-dependent analysisindicate that the increased VOCis likelydue to relaxation of an energetic barrier at the ITO/CdTe interface. [ABSTRACT FROM AUTHOR]

The emission color from colloidalsemiconductor nanocrystals (NCs)is usually tuned through control of particle size, while multicoloremission is obtained by mixing NCs of different sizes within an emissivelayer. Here, we demonstrate that recently introduced “dot-in-bulk”(DiB) nanocrystals can emit two-color light under both optical excitationand electrical injection. We show that the effective emission colorcan be controlled by adjusting the relative amplitudes of the coreand shell emission bands via the intensity of optical excitation orapplied bias in the cases of photoluminescence (PL) and electroluminescence(EL), respectively. To investigate the role of nonradiative carrierlosses due to trapping at intragap states, we incorporate DiB NCsinto functional light-emitting diodes and study their PL as a functionof applied bias below the EL excitation threshold. We show that voltage-dependentchanges in core and shell emissions are not due to the applied electricfield but rather arise from the transfer of charges between the anodeand the NC intragap trap sites. The changes in the occupancy of trapstates can be described in terms of the raising (lowering) of theFermi level for reverse (direct) bias. We find that the applied voltageaffects the overall PL intensity primarily via the electron-trappingchannel while bias-induced changes in hole-trapping play a less significantrole, limited to a weak effect on core emission. [ABSTRACT FROM AUTHOR]

In this report, we demonstrate for the first time photonicnanocavitiesoperating inside single biological cells. Here we develop a nanobeamphotonic crystal (PC) cavity as an advanced cellular nanoprobe, activein nature, and configurable to provide a multitude of actions forboth intracellular sensing and control. Our semiconductor nanocavityprobes emit photoluminescence (PL) from embedded quantum dots (QD)and sustain high quality resonant photonic modes inside cells. Theprobes are shown to be minimally cytotoxic to cells from viabilitystudies, and the beams can be loaded in cells and tracked for daysat a time, with cells undergoing regular division with the beams.We present in vitro label-free protein sensing with our probes todetect streptavidin as a path towards real-time biomarker and biomoleculedetection inside single cells. The results of this work will enablenew areas of research merging the strengths of photonic nanocavitieswith fundamental cell biology. [ABSTRACT FROM AUTHOR]

Sizetunability of the optical properties and inexpensive synthesismake semiconductor nanocrystals one of the most promising and versatilebuilding blocks for many modern applications such as lasers, single-electrontransistors, solar cells, and biological labels. The performance ofthese nanocrystal-based devices is however compromised by efficienttrapping of the charge carriers. This process exhibits different featuresdepending on the nanocrystal material, surface termination, size,and trap location, leading to the assumption that different mechanismsare at play in each situation. Here we revolutionize this fragmentedpicture and provide a unified interpretation of trapping dynamicsin semiconductor nanocrystals by identifying the origins of this sofar elusive detrimental process. Our findings pave the way for a generalsuppression strategy, applicable to any system, which can lead toa simultaneous efficiency enhancement in all nanocrystal-based technologies. [ABSTRACT FROM AUTHOR]

Colloidal chemistry offers an assortmentof synthetic tools fortuning the shape of semiconductor nanocrystals. While many nanocrystalarchitectures can be obtained directly via colloidal growth, othernanoparticle morphologies require alternative processing strategies.Here, we show that chemical etching of colloidal nanoparticles canfacilitate the realization of nanocrystal shapes that are topologicallyinaccessible by hot-injection techniques alone. The present methodologyis demonstrated by synthesizing a two-component CdSe/CdS nanoparticledimer, constructed in a way that both CdSe and CdS semiconductor domainsare exposed to the external environment. This structural morphologyis highly desirable for catalytic applications as it enables bothreductive and oxidative reactions to occur simultaneously on dissimilarnanoparticle surfaces. Hydrogen production tests confirmed the improvedcatalytic activity of CdSe/CdS dimers, which was enhanced 3–4times upon etching treatment. We expect that the demonstrated applicationof etching to shaping of colloidal heteronanocrystals can become acommon methodology in the synthesis of charge-separating nanocrystals,leading to advanced nanoparticles architectures for applications inareas of photocatalysis, photovoltaics, and light detection. [ABSTRACT FROM AUTHOR]

For three types of colloidal magnetic nanocrystals, wedemonstratethat postsynthetic cation exchange enables tuning of the nanocrystal’smagnetic properties and achieving characteristics not obtainable byconventional synthetic routes. While the cation exchange procedure,performed in solution phase approach, was restricted so far to chalcogenidebased semiconductor nanocrystals, here ferrite-based nanocrystalswere subjected to a Fe2+to Co2+cation exchangeprocedure. This allows tracing of the compositional modificationsby systematic and detailed magnetic characterization. In homogeneousmagnetite nanocrystals and in gold/magnetite core shell nanocrystalsthe cation exchange increases the coercivity field, the remanencemagnetization, as well as the superparamagnetic blocking temperature.For core/shell nanoheterostructures a selective doping of either theshell or predominantly of the core with Co2+is demonstrated.By applying the cation exchange to FeO/CoFe2O4core/shell nanocrystals the Neél temperature of the corematerial is increased and exchange-bias effects are enhanced so thatvertical shifts of the hysteresis loops are obtained which are superiorto those in any other system. [ABSTRACT FROM AUTHOR]

Optical pumping of semiconductor nanocrystals with femtosecondpulse sequences was performed in order to modulate multiexciton populations.We show for the first time that control of multiexciton populationsproduces high speed modulation of stimulated emission. Upon the basisof the speed of multiexcitonic processes in nanocrystals, we showmodulation rates approaching 1 THz by virtue of strong quantum confinementeffects. Employing femtosecond optical pulse sequences, we demonstrateall-optical logic using these nanocrystals in two forms: an AND gate,and an inverter, a key step toward all optical signal processing. [ABSTRACT FROM AUTHOR]