Near-Infrared Electroluminescence from HgTe Nanocrystals

Mid-infrared HgTe Colloidal Quantum Dot LEDs

Mid-infrared HgTe colloidal quantum dot electroluminescent devices are demonstrated. With emission at 4 μm, devices achieved an external quantum efficiency of ∼10^-3 and power conversion efficiency of ∼10^-4 under biases of a few volts. The power conversion efficiency benefited from lowering the transparent electrode resistance through the incorporation of a metal conductive grid. The average power emitted was about 16 μW at 2 V bias with 50% duty cycle and a 1 mm² device. The room-temperature electroluminescence efficiency at low current was limited by the photoluminescence efficiency of the quantum dots, while the diode structure provided efficient electron-hole recombination.

Electroluminescence from HgTe Nanocrystals and Its Use for Active Imaging

Mercury telluride (HgTe) nanocrystals are among the most versatile infrared (IR) materials with the absorption of lowest energy optical absorption which can be tuned from the visible to the terahertz range. Therefore, they have been extensively considered as near IR emitters and as absorbers for low-cost IR detectors. However, the electroluminescence of HgTe remains poorly investigated despite its ability to go toward longer wavelengths compared to traditional lead sulfide (PbS). Here, we demonstrate a light-emitting diode (LED) based on an indium tin oxide (ITO)/zinc oxide (ZnO)/ZnO-HgTe/PbS/gold-stacked structure, where the emitting layer consists of a ZnO/HgTe bulk heterojunction which drives the charge balance in the system. This LED has low turn-on voltage, long lifetime, and high brightness. Finally, we conduct short wavelength infrared (SWIR) active imaging, where illumination is obtained from a HgTe NC-based LED, and demonstrate moisture detection.

Nanoscale Photoinduced Charge Transfer with Individual Quantum Dots: Tunability through Synthesis, Interface Design, and Interaction with Charge Traps

Semiconducting colloidal quantum dots (QDs) provide an excellent platform for nanoscale charge-transfer studies. Because of their size-dependent optoelectronic properties, which can be tuned via chemical synthesis and of their versatility in surface ligand exchange, QDs can be coupled with various types of acceptors to create hybrids with controlled type (electron or hole), direction, and rate of charge flow, depending on the foreseen application, either solar harvesting, light emitting, or biosensing. This perspective highlights several examples of QD-based hybrids with controllable (tunable) rate of charge transfer obtained by various approaches, including by changing the QD core size and shell thickness by colloidal synthesis, by the insertion of molecular linkers or dielectric spacers between donor and acceptor components. We also show that subjecting QDs to external factors such as electric fields and alternate optical excitation energy is another approach to bias the internal charge transfer between charges photogenerated in the QD core and QD's surface charge traps. The perspective also provides the reader with various examples of how single nanoparticle spectroscopic studies can help in understanding and quantifying nanoscale charge transfer with QDs.

Synthesis of Nonaggregating HgTe Colloidal Quantum Dots and the Emergence of Air-Stable n-Doping

Nonaggregating HgTe colloidal quantum dots are synthesized without thiols as stabilizing ligands. The dots are spherical with a size tunability from 4.8 to 11.5 nm. When the results from optical and electrochemical measurement are combined, air-stable n-doping is observed in large sizes of HgTe quantum dots, which is attributed to the Hg-rich surface.

Efficient energy transfer in a new hybrid diphenylfluorene derivative-CdS quantum dot nanocomposite

We report the synthesis of a novel compound, 9,9-bis(3'-aminopropyl)-2,7-diphenylfluorene (BAPDPF), and a new approach to graft the BAPDPF onto a CdS quantum dot (QD) surface via an acylation reaction. FT-IR and TGA characterizations indicate the formation of robust bonding between BAPDPF and QDs; the structures of the bare QDs and BAPDPF-CdS QD hybrid nanocomposites estimated by transmission electron microscopy (TEM) showed that they have the same size of about 3.5 nm. The extent of the spectral overlap between the emission of BAPDPF and absorption of QDs, and the change of fluorescence emission for the organic and inorganic components of the hybrid, demonstrate that the energy transfer process occurs from BAPDPF to the CdS QDs. The energy transfer of about 44% efficiency is corroborated by time-resolved fluorescence measurements, and then the solid-state photoluminescence quantum yield (PLQY) of the nanocomposite is measured using an integrating sphere and a conventional fluorimeter. Because of the fact that the PLQY of the nanocomposite is 9.1 times larger than that of the pristine QDs, due to the energy transfer between the donor and the acceptor and passivation effects on the surface of the acceptor, the presented BAPDPF-CdS QD hybrid nanocomposites are potentially interesting in nanoparticle-based light-emitting devices.

Colloidal semiconductor nanocrystals: the aqueous approach

This article summarizes the main achievements and challenges in the field of the aqueous synthesis of semiconductor quantum dots in colloidal solutions. Developments in the last two decades demonstrate the great potential of this approach to synthesize nanocrystalline materials with superior properties such as strong photoluminescence, long time stability and compatibility with biological media, and the variability in assembling and self-assembling into larger structures or on surfaces. Being relatively straightforward, the aqueous approach provides some advantages such as versatility, scalability, environmental friendliness and cost effectiveness, leading in summary to very attractive application perspectives.

High quality CdHgTe nanocrystals with strong near-infrared emission: relationship between composition and cytotoxic effects

High quality CdHgTe quasi core/shell nanocrystals (NCs) were prepared via the one-step method. The relationship between the composition, structure, and property was systematically investigated by the combination of X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission (ICP), and the photoluminescence (PL) measurements. The quantum yield (QY) was ~50% when the feed ratio of Cd(2+) to Hg(2+) was equal to 1. The PL property was further polished, and the QY was improved to ~80% through the variance of the prepared conditions such as the ratio of ligand to metal ion and HTe(-) to metal ion, pH value, and temperature. In addition, the cytotoxic effects of CdHgTe NCs were systematically studied. The results showed that, for Cd0.21Hg0.79Te NCs, its quasi core/shell structure was very stable and little cadmium ions were released. As a result, such NCs showed little cytotoxicity and would find applications in tissue imaging or detection.

Shell thickness dependent photoinduced hole transfer in hybrid conjugated polymer/quantum dot nanocomposites: from ensemble to single hybrid level

Photoinduced hole transfer is investigated in inorganic/organic hybrid nanocomposites of colloidal CdSe/ZnS quantum dots and a cationic conjugated polymer, poly(9,9'-bis(6-N,N,N-trimethylammoniumhexyl)fluorene-alt-phenylene, in solution and in solid thin film, and down to the single hybrid level and is assessed to be a dynamic quenching process. We demonstrate control of hole transfer rate in these quantum dot/conjugated polymer hybrids by using a series of core/shell quantum dots with varying shell thickness, for which a clear exponential dependency of the hole transfer rate vs shell thickness is observed, for both solution and thin-film situations. Furthermore, we observe an increase of hole-transfer rate from solution to film and correlate this with changes in quantum dot/polymer interfacial morphology affecting the hole transfer rate, namely, the donor-acceptor distance. Single particle spectroscopy experiments reveal fluctuating dynamics of hole transfer at the single conjugated polymer/quantum dot interface and an increased heterogeneity in the hole-transfer rate with the increase of the quantum dot's shell thickness. Although hole transfer quenches the photoluminescence intensity of quantum dots, it causes little or no effect on their blinking behavior over the time scales probed here.

Progress in the light emission of colloidal semiconductor nanocrystals

Some 25 years ago it was found that semiconductor nanocrystals emitted light. Since then tremendous progress has been made with respect to increasing the emission quantum yields, extending the spectral range that may be addressed, from the UV across to the near infrared, and improving the color purity. Here some major lines in these developments are reviewed, touching on milestones as well as on the principles of the most successful preparative approaches.

Energy transfer versus charge separation in type-II hybrid organic-inorganic nanocomposites

Hybrid organic-inorganic nanomaterials have the potential of providing synergetic properties. Blends of semiconductor nanocrystals and conjugated polymers in particular promise novel optoelectronic properties. Effective design of tailored optoelectronic properties requires a deep understanding of the photophysics of these composite materials, which includes charge separation and Dexter and Förster energy transfer. We performed a detailed and quantitative spectroscopic investigation of a type II aligned hybrid system consisting of a blue emitting conducting polymer and CdTe nanocrystals. Although charge separation is expected from the type II alignment, we find a dominant (70% efficiency) energy transfer process. We discuss all possible de-excitation pathways for the excitons in terms of the alignment of energy levels, time scales, and physical geometry of the system. This allows us to conclude that energy transfer occurs via the Förster mechanism and provides a clear guideline for the design of novel hybrid materials.

Tuning energetic levels in nanocrystal quantum dots through surface manipulations

We demonstrate tuning of the electronic level positions with respect to the vacuum level in colloidal InAs nanocrystals using surface ligand exchange. Electrochemical as well as scanning tunneling spectroscopy measurements reveal that the tuning is largely dependent on the nanocrystal size and the surface linking group, while the polarity of the ligand molecules has a lesser effect. The implications of affecting the electronic system of nanocrystal through its capping are illustrated through prototype devices.

Infrared-emitting colloidal nanocrystals: synthesis, assembly, spectroscopy, and applications

Semiconductor nanocrystals produced by means of colloidal chemistry in a solvent medium are an attractive class of nanometer-sized building blocks from which to create complex materials with unique properties for a variety of applications. Their optical and electronic properties can be tailored easily, both by their chemical composition and particle size. While colloidal nanocrystals emitting in the infrared region have seen a burst of attention during the last decade there is clearly a paucity of review articles covering their synthesis, assembly, spectroscopic characterization, and applications. This Review comprehensively addresses these topics for II-VI, III-V, and IV-VI nanocrystals, examples being HgTe and Cd(x)Hg(1-) (x)Te, InP and InAs, and PbS, PbSe, and PbTe, respectively. Among the applications discussed here are optical amplifier media for telecommunications systems, electroluminescence devices, and noninvasive optical imaging in biology.

A Low-Temperature Synthesis for Organically Soluble HgTe Nanocrystals Exhibiting Near-Infrared Photoluminescence and Quantum Confinement

A new low-temperature, one-pot method is introduced for the preparation of organically passivated HgTe nanocrystals, without the use of highly toxic precursors. The nanocrystals show bright photoluminescence in the infrared telecommunication windows about 1300 and 1550 nm with quantum efficiencies between 55 and 60%. They have a zinc blende structure with a mean particle diameter of 3.4 nm, thus exhibiting quantum confinement effects. Particle growth is self-limited by temperature quenching, so a narrow size distribution is obtained. The measured size of the particles agrees with calculations using the pseudopotential method.

Size-Dependent Electrochemical Behavior of Thiol-Capped CdTe Nanocrystals in Aqueous Solution

Electrochemical studies of thiol-capped CdTe nanocrystals in aqueous solution have demonstrated several distinct oxidation and reduction peaks in the voltammograms, with the peak positions being dependent on the size of the nanocrystals. While the size dependence of the reduction and one of the oxidation potentials can be attributed to altering the energetic band positions owing to the quantum size effect, an extraordinary behavior was found for the oxidation peak observed at less positive potentials. In contrast to a prediction based on the quantum size effect, this peak moves to more negative potentials as the nanocrystals' size decreases. Moreover, the contribution of the charge associated with this peak compared to the total charge passed during the nanocrystal oxidation correlates well with the photoluminescence (PL) efficiency of individual fractions of the CdTe nanocrystals. These experimental observations allow a peak to be assigned to the oxidation of Te-related surface traps. The intra-band-gap energy level assigned to these Te-related trap states shifts toward the top of the valence band as the nanocrystal size increases, thus allowing the higher photostability of the larger nanocrystals to be explained. At a certain nanocrystal size, the trap level can even move out of the band gap.