Tailored growth of single-crystalline InP tetrapods.

Despite the technological importance of colloidal covalent III-V nanocrystals with unique optoelectronic properties, their synthetic process still has challenges originating from the complex energy landscape of the reaction. Here, we present InP tetrapod nanocrystals as a crystalline late intermediate in the synthetic pathway that warrants controlled growth. We isolate tetrapod intermediate species with well-defined surfaces of (110) and ([Formula: see text]) via the suppression of further growth. An additional precursor supply at low temperature induces [Formula: see text]-specific growth, whereas the [110]-directional growth occurs over the activation barrier of 65.7 kJ/mol at a higher temperature, thus finalizes into the (111)-faceted tetrahedron nanocrystals. We address the use of late intermediates with well-defined facets at the sub-10 nm scale for the tailored growth of covalent III-V nanocrystals and highlight the potential for the directed approach of nanocrystal synthesis.

Room-Temperature Lasing in Colloidal Nanoplatelets via Mie-Resonant Bound States in the Continuum.

Solid-state room-temperature lasing with tunability in a wide range of wavelengths is desirable for many applications. To achieve this, besides an efficient gain material with a tunable emission wavelength, a high quality-factor optical cavity is essential. Here, we combine a film of colloidal CdSe/CdZnS core-shell nanoplatelets with square arrays of nanocylinders made of titanium dioxide to achieve optically pumped lasing at visible wavelengths and room temperature. The all-dielectric arrays support bound states in the continuum (BICs), which result from lattice-mediated Mie resonances and boast infinite quality factors in theory. In particular, we demonstrate lasing from a BIC that originates from out-of-plane magnetic dipoles oscillating in phase. By adjusting the diameter of the cylinders, we tune the lasing wavelength across the gain bandwidth of the nanoplatelets. The spectral tunability of both the cavity resonance and nanoplatelet gain, together with efficient light confinement in BICs, promises low-threshold lasing with wide selectivity in wavelengths.

Electrically control amplified spontaneous emission in colloidal quantum dots.

Colloidal quantum dots (CQDs) are highly promising materials for light amplification thanks to their efficient photoluminescence, tunable emission wavelength and low-cost synthesis. Unfortunately, CQDs are suffering from band-edge state degeneracy which demands multiple excitons to achieve population inversion. As a result, non-radiative Auger recombination increases the lasing threshold and limits the gain lifetime. Here, benefiting from the negative charging, we demonstrate that the amplified spontaneous emission (ASE) threshold is controllable in a device where CQD film is exposed to an external electric field. Specifically, singly charged CQDs lower the threshold due to the preexisting electron in the conduction band, while strongly enhanced Auger recombination in doubly charged CQDs stymies the ASE. Experimental results and kinetic equation model show that ASE threshold reduces 10% even if our device only charges ~17% of the CQD population. Our results open new possibilities for controlling exciton recombination dynamics and achieving electrically pumped CQD lasers.

Ultrathin Colloidal Quantum Dot Films for Optical Amplification: The Role of Modal Confinement and Heat Dissipation.

We demonstrate optical pumping lasers based on colloidal quantum dots, with a very thin geometry consisting of a ≈20 nm thick film. Obstacles in ultrasmall laser devices come from the limitation of gain materials and the size of cavities for lasing modes, which requires a minimum thickness of the gain media (typically greater than 50-100 nm). Here we introduce dielectric waveguide structures with a high refractive index, in order to reduce the thickness of quantum dot gain media as well as their threshold energy (≈39 % compared to the original gain medium). Finite-difference time-domain simulations show that the modal confinement factor of thinner quantum dot films can be improved by the presence of an adjacent waveguide layer. We also discuss the possible role of dielectric waveguide layers for efficient heat dissipation during optical pumping. Integrating an extremely thin colloidal quantum dot gain medium into optical waveguides is a promising platform for downscaling on-chip photonic integrated devices, as well as investigating extreme interactions between light and matter such as surface plasmon-photon coupling.

Composition-Dependent Hot Carrier Relaxation Dynamics in Cesium Lead Halide (CsPbX3 , X=Br and I) Perovskite Nanocrystals.

Cesium-based perovskite nanocrystals (NCs) have outstanding photophysical properties improving the performances of lighting devices. Fundamental studies on excitonic properties and hot-carrier dynamics in perovskite NCs further suggest that these materials show higher efficiencies compared to the bulk form of perovskites. However, the relaxation rates and pathways of hot-carriers are still being elucidated. By using ultrafast transient spectroscopy and calculating electronic band structures, we investigated the dependence of halide in Cs-based perovskite (CsPbX3 with X=Br, I, or their mixtures) NCs on the hot-carrier relaxation processes. All samples exhibit ultrafast (<0.6 ps) hot-carrier relaxation dynamics with following order: CsPbBr3 (310 fs)>CsPbBr1.5 I1.5 (380 fs)>CsPbI3 NC (580 fs). These result accounts for a reduced light emission efficiency of CsPbI3 NC compared to CsPbBr3 NC.

Thickness-Controlled Quasi-Two-Dimensional Colloidal PbSe Nanoplatelets.

We demonstrate controlled synthesis of discrete two-dimensional (2D) PbSe nanoplatelets (NPLs), with measurable photoluminescence, via oriented attachment directed by quantum dot (QD) surface chemistry. Halide passivation is critical to the growth of these (100) face-dominated NPLs, as corroborated by density functional theory studies. PbCl2 moieties attached to the (111) and (110) of small nanocrystals form interparticle bridges, aligning the QDs and leading to attachment. We find that a 2D bridging network is energetically favored over a 3D network, driving the formation of NPLs. Although PbI2 does not support bridging, its presence destabilizes the large (100) faces of NPLs, providing means for tuning NPL thickness. Spectroscopic analysis confirms the predicted role of thickness-dependent quantum confinement on the NPL band gap.

Composition-dependent trap distributions in CdSe and InP quantum dots probed using photoluminescence blinking dynamics.

Although Group II-VI quantum dots (QDs) have attracted much attention due to their wide range of applications in QD-based devices, the presence of toxic ions in II-VI QDs raises environmental concerns. To fulfill the demands of nontoxic QDs, synthetic routes for III-V QDs have been developed. However, only a few comparative analyses on optical properties of III-V QDs have been performed. In this study, the composition-related energetic trap distributions have been explored by using three different types of core/multishell QDs: CdSe-CdS (CdSe/CdS/ZnS), InP-ZnSe (InP/ZnSe/ZnS), and InP-GaP (InP/GaP/ZnS). It was shown that CdSe-CdS QDs have much larger trap densities than InP-shell QDs at higher energy states (at least 1Eg (band gap energy) above the lowest conduction band edge) based on probability density plots and Auger ionization efficiencies which are determined by analyses of photoluminescence blinking dynamics. This result suggests that the composition of encapsulated QDs is closely associated with the charge trapping processes, and also provides an insight into the development of more environmentally friendly QD-based devices.

TCNQ Interlayers for Colloidal Quantum Dot Light-Emitting Diodes.

CdSe/CdS/ZnS quantum dot light-emitting diodes (QD-LEDs) show increased brightness (from ca. 18 000 to 27 000 cd m(-2) ) with 7,7,8,8-tetracyanoquinodimethane (TCNQ) between the QD and electron-transfer layers of ZnO nanoparticles. As QD/ZnO layers are known to have interface defects, our finding leads to the importance of interface engineering for QD-LEDs. Although the photoluminescent intensity and decay lifetime of ZnO/TCNQ/QD layers are similar to those of ZnO/QD layers, cyclic voltammetry suggests improved charge transfer of TCNQ/ZnO layers compared to that of pure ZnO layers. This helps us to understand the mechanism of electrically driven QD-LED behavior, which differs from that of conventional solid-state LEDs, and enables the rational design of QD-based optoelectronic devices.

Design and synthesis of heterostructured quantum dots with dual emission in the visible and infrared.

The unique optical properties exhibited by visible emitting core/shell quantum dots with especially thick shells are the focus of widespread study, but have yet to be realized in infrared (IR)-active nanostructures. We apply an effective-mass model to identify PbSe/CdSe core/shell quantum dots as a promising system for achieving this goal. We then synthesize colloidal PbSe/CdSe quantum dots with shell thicknesses of up to 4 nm that exhibit unusually slow hole intraband relaxation from shell to core states, as evidenced by the emergence of dual emission, i.e., IR photoluminescence from the PbSe core observed simultaneously with visible emission from the CdSe shell. In addition to the large shell thickness, the development of slowed intraband relaxation is facilitated by the existence of a sharp core-shell interface without discernible alloying. Growth of thick shells without interfacial alloying or incidental formation of homogeneous CdSe nanocrystals was accomplished using insights attained via a systematic study of the dynamics of the cation-exchange synthesis of both PbSe/CdSe and the related system PbS/CdS. Finally, we show that the efficiency of the visible photoluminescence can be greatly enhanced by inorganic passivation.

Heavily doped n-type PbSe and PbS nanocrystals using ground-state charge transfer from cobaltocene.

Colloidal nanocrystals (NCs) of lead chalcogenides are a promising class of tunable infrared materials for applications in devices such as photodetectors and solar cells. Such devices typically employ electronic materials in which charge carrier concentrations are manipulated through "doping;" however, persistent electronic doping of these NCs remains a challenge. Here, we demonstrate that heavily doped n-type PbSe and PbS NCs can be realized utilizing ground-state electron transfer from cobaltocene. This allows injecting up to eight electrons per NC into the band-edge state and maintaining the doping level for at least a month at room temperature. Doping is confirmed by inter- and intra-band optical absorption, as well as by carrier dynamics. Finally, FET measurements of doped NC films and the demonstration of a p-n diode provide additional evidence that the developed doping procedure allows for persistent incorporation of electrons into the quantum-confined NC states.

Carrier multiplication in semiconductor nanocrystals: influence of size, shape, and composition.

During carrier multiplication (CM), also known as multiexciton generation (MEG), absorption of a single photon produces multiple electron-hole pairs, or excitons. This process can appreciably increase the efficiency of photoconversion, which is especially beneficial in photocatalysis and photovoltaics. This Account reviews recent progress in understanding the CM process in semiconductor nanocrystals (NCs), motivated by the challenge researchers face to quickly identify candidate nanomaterials with enhanced CM. We present a possible solution to this problem by showing that, using measured biexciton Auger lifetimes and intraband relaxation rates as surrogates for, respectively, CM time constants and non-CM energy-loss rates, we can predict relative changes in CM yields as a function of composition. Indeed, by studying PbS, PbSe, and PbTe NCs of a variety of sizes we determine that the significant difference in CM yields for these compounds comes from the dissimilarities in their non-CM relaxation channels, i.e., the processes that compete with CM. This finding is likely general, as previous observations of a material-independent, "universal" volume-scaling of Auger lifetimes suggest that the timescale of the CM process itself is only weakly affected by NC composition. We further explore the role of nanostructure shape in the CM process. We observe that a moderate elongation (aspect ratio of 6-7) of PbSe NCs can cause up to an approximately two-fold increase in the multiexciton yield compared to spherical nanoparticles. The increased Auger lifetimes and improved charge transport properties generally associated with elongated nanostructures suggest that lead chalcogenide nanorods are a promising system for testing CM concepts in practical photovoltaics. Historically, experimental considerations have been an important factor influencing CM studies. To this end, we discuss the role of NC photocharging in CM measurements. Photocharging can distort multiexciton dynamics, leading to erroneous estimations of the CM yield. Here, we show that in addition to distorting time-resolved CM signals, photocharging also creates spectral signatures that mimic CM. This re-emphasizes the importance of a careful analysis of the potential effect of charged species in both optical and photocurrent-based measurements of this process.

Aspect ratio dependence of auger recombination and carrier multiplication in PbSe nanorods.

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.

Carrier Multiplication in Quantum Dots within the Framework of Two Competing Energy Relaxation Mechanisms.

The realization of high-yield, low-threshold carrier multiplication (CM) in semiconductor quantum dots (QDs) is a promising step toward third-generation photovoltaics (PV). Recent studies of QD solar cells have shown that CM can indeed produce greater-than-unity quantum efficiencies in photon-to-charge-carrier conversion, establishing the relevance of this process to practical PV technologies. While being appreciable, the reported CM yields are still not high enough for a significant increase in the power conversion efficiency over traditional bulk materials. At present, the design of nanomaterials with improved CM is hindered by a poor understanding of the mechanism underlying this process. Here, we present a possible solution to this problem by introducing a model that treats CM as a competition between impact-ionization-like scattering and non-CM energy losses. Importantly, it allows for evaluation of expected CM yields from fairly straightforward measurements of Auger recombination (inverse of CM) and near-band-edge carrier cooling. The validation of this model via a comparative CM study of PbTe, PbSe, and PbS QDs suggests that it indeed represents a predictive capability, which might help in the development of nanomaterials with improved CM performance.

Highly effective surface passivation of PbSe quantum dots through reaction with molecular chlorine.

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.

Thiocyanate-capped PbS nanocubes: ambipolar transport enables quantum dot based circuits on a flexible substrate.

We report the use of thiocyanate as a ligand for lead sulfide (PbS) nanocubes for high-performance, thin-film electronics. PbS nanocubes, self-assembled into thin films and capped with the thiocyanate, exhibit ambipolar characteristics in field-effect transistors. The nearly balanced, high mobilities for electrons and holes enable the fabrication of CMOS-like inverters with promising gains of ∼22 from a single semiconductor material. The mild chemical treatment and low-temperature processing conditions are compatible with plastic substrates, allowing the realization of flexible, nonsintered quantum dot circuits.

Thiocyanate-capped nanocrystal colloids: vibrational reporter of surface chemistry and solution-based route to enhanced coupling in nanocrystal solids.

Ammonium thiocyanate (NH(4)SCN) is introduced to exchange the long, insulating ligands used in colloidal nanocrystal (NC) synthesis. The short, air-stable, environmentally benign thiocyanate ligand electrostatically stabilizes a variety of semiconductor and metallic NCs in polar solvents, allowing solution-based deposition of NCs into thin-film NC solids. NH(4)SCN is also effective in replacing ligands on NCs after their assembly into the solid state. The spectroscopic properties of this ligand provide unprecedented insight into the chemical and electronic nature of the surface of the NCs. Spectra indicate that the thiocyanate binds to metal sites on the NC surface and is sensitive to atom type and NC surface charge. The short, thiocyanate ligand gives rise to significantly enhanced electronic coupling between NCs as evidenced by large bathochromic shifts in the absorption spectra of CdSe and CdTe NC thin films and by conductivities as high as (2 ± 0.7) × 10(3) Ω(-1) cm(-1) for Au NC thin films deposited from solution. NH(4)SCN treatment of PbTe NC films increases the conductivity by 10(13), allowing the first Hall measurements of nonsintered NC solids, with Hall effect mobilities of 2.8 ± 0.7 cm(2)/(V·s). Thiocyanate-capped CdSe NC thin films form photodetectors exhibiting sensitive photoconductivity of 10(-5) Ω(-1) cm(-1) under 30 mW/cm(2) of 488 nm illumination with I(photo)/I(dark) > 10(3) and form n-channel thin-film transistors with electron mobilities of 1.5 ± 0.7 cm(2)/(V·s), a current modulation of >10(6), and a subthreshold swing of 0.73 V/decade.

Far-infrared absorption of PbSe nanorods.

Measurements of the far-infrared absorption spectra of PbSe nanocrystals and nanorods are presented. As the aspect ratio of the nanorods increases, the Fröhlich sphere resonance splits into two peaks. We analyze this splitting with a classical electrostatic model, which is based on the dielectric function of bulk PbSe but without any free-carrier contribution. Good agreement between the measured and calculated spectra indicates that resonances in the local field factors underlie the measured spectra.

Ambipolar and unipolar PbSe nanowire field-effect transistors.

Wet-chemical methods, under rigorous air-free conditions, were used to synthesize single-crystalline 10 nm diameter PbSe nanowires (NWs), and electric-field, directed assembly was employed to align NW arrays to form the semiconducting channels of field-effect transistors (FETs). Electrical measurements revealed as-aligned NWs in bottom, gold, contact FETs are predominantly p-type ambipolar, consistent with the presentation of small barriers to electron and hole injection for this low band gap semiconductor. Exposing the NW FET to UV-ozone p-doped the NWs, illustrating the sensitivity of PbSe to oxygen, but controlled oxidation allowed the fabrication of unipolar p-type FETs. Selectively exposing the contact region of as-aligned NW FETs to low to moderate concentrations of hydrazine, commonly used to n-dope nanocrystal and NW devices, switched the predominantly p- to n-type ambipolar behavior as if the entire NW channel was exposed. At these hydrazine concentrations, charge transfer doping the metal-semiconductor interface dominates the FET characteristics. Only upon exposing the NW FETs to high hydrazine concentrations did charge transfer doping of the NW channel overcome the large intrinsic, thermally generated carrier concentration of this low band gap material, modulating the NW carrier concentration and forming unipolar n-type FETs. Pulling low vacuum removed surface hydrazine returning the predominantly p-type ambipolar FET behavior. Doping and dedoping with hydrazine were repeatedly reversible. By applying surface modification to n- and p-dope PbSe NW FETs, we fabricated the first PbSe NW inverters, demonstrating the promise of these nanostructured materials in integrated circuits.

Multiscale periodic assembly of striped nanocrystal superlattice films on a liquid surface.

Self-assembly of nanocrystals (NCs) into periodically ordered structures on multiple length scales and over large areas is crucial to the manufacture of NC-based devices. Here, we report an unusual yet universal approach to rapidly assembling hierarchically organized NC films that display highly periodic, tunable microscale stripe patterns over square centimeter areas while preserving the local superlattice structure. Our approach is based on a drying-driven dynamic assembly process occurring on a liquid surface with the stripe pattern formed by a new type of contact-line instability. Periodic ordering of NCs is realized on microscopic and nanoscopic scales simultaneously without the need of any specialized equipment or the application of external fields. The striped NC superlattice films obtained can be readily transferred to arbitrary substrates for device fabrication. The periodic structure imparts interesting modulation and anisotropy to the properties of such striped NC assemblies. This assembly approach is applicable to NCs with a variety of compositions, sizes, and shapes, offering a robust, inexpensive route for large-scale periodic patterning of NCs.

Synthesis of monodisperse PbSe nanorods: a case for oriented attachment.

Monodisperse, high-quality, single-crystal PbSe nanorods were synthesized in a catalyst-free, one-pot reaction using a new phosphine selenide precursor. PbSe nanorods were assembled to provide liquid-crystalline alignment or vertical alignment under controlled evaporation conditions. The growth of nanorods was monitored by TEM and absorption spectroscopy, indicating that oriented attachment could be involved to provide anisotropic PbSe nanostructures. In-plane XRD showed an enhanced (200) peak for PbSe nanorods, indicating the preferred alignment of nanorods on the substrates and their growth along the <100> direction. Absorption and emission spectra, along with lifetime measurements, show the differences between nanoscale PbSe spheres and rods.