Engineering colloidal semiconductor nanocrystals for quantum information processing.

Quantum information processing-which relies on spin defects or single-photon emission-has shown quantum advantage in proof-of-principle experiments including microscopic imaging of electromagnetic fields, strain and temperature in applications ranging from battery research to neuroscience. However, critical gaps remain on the path to wider applications, including a need for improved functionalization, deterministic placement, size homogeneity and greater programmability of multifunctional properties. Colloidal semiconductor nanocrystals can close these gaps in numerous application areas, following years of rapid advances in synthesis and functionalization. In this Review, we specifically focus on three key topics: optical interfaces to long-lived spin states, deterministic placement and delivery for sensing beyond the standard quantum limit, and extensions to multifunctional colloidal quantum circuits.

One-Pot Colloidal Synthesis Enables Highly Tunable InSb Short-Wave Infrared Quantum Dots Exhibiting Carrier Multiplication.

Colloidal quantum dots (CQDs) are emerging materials for short-wave infrared (SWIR, ≈1100-3000 nm) photodetectors, which are technologically important for a broad array of applications. Unfortunately, the most developed SWIR CQD systems are Pb and Hg chalcogenides; their toxicity and regulated compositions limit their applications. InSb CQD system is a potential environmentally friendly alternative, whose bandgap in theory, is tunable via quantum confinement across the SWIR spectrum. However, InSb CQDs are difficult to exploit, due to their complex syntheses and uncommon reactive precursors, which greatly hinder their application and study. Here, a one-pot synthesis strategy is reported using commercially available precursors to synthesize-under standard colloidal synthesis conditions-high-quality, size-tunable InSb CQDs. With this strategy, the large Bohr exciton radius of InSb can be exploited for tuning the bandgap of the CQDs over a wide range of wavelengths (≈1250-1860 nm) across the SWIR region. Furthermore, by changing the surface ligands of the CQDs from oleic acid (OA) to 1-dodecanthiol (DDT), a ≈20-fold lengthening in the excited-state lifetime, efficient carrier multiplication, and slower carrier annihilation are observed. The work opens a wide range of SWIR applications to a promising class of Pb- and Hg-free CQDs.

InAs Nanorod Colloidal Quantum Dots with Tunable Bandgaps Deep into the Short-Wave Infrared.

InAs colloidal quantum dots (CQDs) have emerged as candidate lead- and mercury-free solution-processed semiconductors for infrared technology due to their appropriate bulk bandgap, which can be tuned by quantum confinement, and promising charge-carrier transport properties. However, the lack of suitable arsenic precursors and readily accessible synthesis conditions have limited InAs CQDs to smaller sizes (<7 nm), with bandgaps largely restricted to <1400 nm in the near-infrared spectral window. Conventional InAs CQD synthesis requires highly reactive, hazardous arsenic precursors, which are commercially scarce, making the synthesis hard to control and study. Here, we present a controlled synthesis strategy (using only readily available and less reactive precursors) to overcome the practical wavelength limitation of InAs CQDs, achieving monodisperse InAs nanorod CQDs with bandgaps tunable from ∼1200 to ∼1800 nm, thus crossing deep into the short-wave infrared (SWIR) region. By controlling the reactivity through in situ precursor complexation, we isolate the reaction mechanism, producing InAs nanorod CQDs that display narrow excitonic features and efficient carrier multiplication. Our work enables InAs CQDs for a wider range of SWIR applications.

Atomic-Scale Polarization and Strain at the Surface of Lead Halide Perovskite Nanocrystals.

Inorganic halide perovskite nanocrystals (NCs) are being widely explored as next-generation optoelectronic materials. Critical to understanding the optoelectronic properties and stability behavior of perovskite NCs is the material's surface structure, where the local atomic configuration deviates from that of the bulk. Through low-dose aberration-corrected scanning transmission electron microscopy and quantitative imaging analysis techniques, we directly observed the atomic structure at the surface of the CsPbBr3 NCs. CsPbBr3 NCs are terminated by a Cs-Br plane, and the surface Cs-Cs bond length decreases significantly (∼5.6%) relative to the bulk, imposing compressive strain and inducing polarization, which we also observed in CsPbI3 NCs. Density functional theory calculations suggest such a reconstructed surface contributes to the separation of holes and electrons. These findings enhance our fundamental understanding of the atomic-scale structure, strain, and polarity at the surface of inorganic halide perovskites and provide valuable insights into designing stable and efficient optoelectronic devices.

Atomically Precise Defective Copper Nanocluster Catalysts for Highly Selective C-C Cross-Coupling Reactions.

Point defects in nanoparticles have long been hypothesized to play an important role in governing the particle's electronic structure and physicochemical properties. However, single point defects in material systems usually exist with other heterogeneities, obscuring the chemical role of the effects. Herein, we report the synthesis of novel atomically precise, copper hydride nanoclusters (NCs), [Cu28 H10 (C7 H7 S)18 (TPP)3 ] (Cu28 ; TPP: triphenylphosphine; C7 H7 S: o-thiocresol) with a defined defect in the gram scale via a one-pot reduction method. The Cu28 acts as a highly selective catalyst for C-C cross-couplings. The work highlights the potential of defective NCs as model systems for investigating individual defects, correlating defects with physiochemical properties, and rationally designing new nanoparticle catalysts.

Lecithin Capping Ligands Enable Ultrastable Perovskite-Phase CsPbI3 Quantum Dots for Rec. 2020 Bright-Red Light-Emitting Diodes.

Bright-red light-emitting diodes (LEDs) with a narrow emission line width that emit between 620 and 635 nm are needed to meet the latest industry color standard for wide color gamut displays, Rec. 2020. CsPbI3 perovskite quantum dots (QDs) are one of the few known materials that are ideally suited to meet these criteria. Unfortunately, CsPbI3 perovskite QDs are prone to transform into a non-red-emitting phase and are subject to further degradation mechanisms when their luminescence wavelength is tuned to match that of the Rec. 2020 standard. Here, we show that zwitterionic lecithin ligands can stabilize the perovskite phase of CsPbI3 QDs for long periods in air for at least 6 months compared to a few days for control samples. LEDs fabricated with our ultrastable lecithin-capped CsPbI3 QDs exhibit an external quantum efficiency (EQE) of 7.1% for electroluminescence centered at 634 nm─a record for all-inorganic perovskite nanocrystals in Rec. 2020 red. Our devices achieve a maximum luminance of 1391 cd/m2 at 7.5 V, and their operational half-life is 33 min (T50) at 200 cd/m2─a 10-fold enhancement compared to control samples. Density functional theory results suggest that the surface strain in CsPbI3 QDs capped with the conventional ligands, oleic acid and oleylamine, contributes to the instability of the perovskite structural phase. On the other hand, lecithin binding induces virtually no surface strain and shows a stronger binding tendency for the CsPbI3 surface. Our study highlights the tremendous potential of zwitterionic ligands in stabilizing the perovskite phase and particle size of CsPbI3 QDs for various optoelectronic applications.

The ultralow thermal conductivity and tunable thermoelectric properties of surfactant-free SnSe nanocrystals.

Most studies to date on SnSe thermal transport are focused on single crystals and polycrystalline pellets that are obtained using high-temperature processing conditions and sophisticated instruments. The effects of using sub-10 nm-size SnSe nanocrystals on the thermal transport and thermoelectric properties have not been studied to the best of our knowledge. Here, we report the synthesis of sub-10 nm colloidal surfactant-free SnSe NCs at a relatively low temperature (80 °C) and investigate their thermoelectric properties. Pristine SnSe NCs exhibit p-type transport but have a modest power factor of 12.5 µW m-1 K-2 and ultralow thermal conductivity of 0.1 W m-1 K-1 at 473 K. Interestingly, the one-step post-synthesis treatment of NC film with methylammonium iodide can switch the p-type transport of the pristine film to n-type. The power factor improved significantly to 20.3 µW m-1 K-2, and the n-type NCs show record ultralow thermal conductivity of 0.14 W m-1 K-1 at 473 K. These surfactant-free SnSe NCs were then used to fabricate flexible devices that show superior performance to rigid devices. After 20 bending cycles, the flexible device shows a 34% loss in the power factor at room temperature (295 K). Overall, this work demonstrates p- and n-type transport in SnSe NCs via the use of simple one-step post-synthesis treatment, while retaining ultralow thermal conductivity.

Postsynthesis Mn-doping in CsPbI3 nanocrystals to stabilize the black perovskite phase.

Long term stability of the black perovskite phase of CsPbI3 nanocrystals under ambient conditions is an important challenge for their optoelectronic applications in real life. The nanocrystalline size is found to improve the stability of the black phase at room temperature. Furthermore, doping Mn is proposed to improve the stability of the black perovskite phase of CsPbI3 nanocrystals (NCs). However, the undoped and Mn-doped CsPbI3 NCs are prepared in different batches under somewhat different synthesis conditions thus obliterating the role of Mn in the stability of the black phase of CsPbI3 NCs. Here, we elucidate the effect of Mn doping on the surface and lattice energy of CsPbI3 NCs, stabilizing the black phase. For this purpose, we employ a postsynthesis doping strategy which has an advantage that the initial host remains the same for both undoped and Mn-doped samples. Uncertainties in the size/shape, surface energy, and structure through direct synthesis of undoped and Mn-doped NCs in different batches can be neglected in our postsynthesis doping strategy, allowing us to study the effect of dopants in a more controlled manner. Our postsynthesis Mn-doping in CsPbI3 NCs shows that the black phase stability under ambient conditions improves from few days for the undoped sample to nearly a month's time for the Mn-doped sample. We found that though surface passivation with a dopant precursor improves both colloidal and phase stability of black CsPbI3 NCs, it is the contraction of the lattice upon Mn-doping that mainly stabilizes the films of black phase CsPbI3 NCs. Similarly, we found that Mn-doped CsPbBr3 NCs show improved ambient stability of photoluminescence compared to the undoped sample.

Colloidal thallium halide nanocrystals with reasonable luminescence, carrier mobility and diffusion length.

Colloidal lead halide based perovskite nanocrystals (NCs) have been recently established as an interesting class of defect-tolerant NCs with potential for superior optoelectronic applications. The electronic band structure of thallium halides (TlX, where X = Br and I) show a strong resemblance to lead halide perovskites, where both Pb2+ and Tl+ exhibit a 6s2 inert pair of electrons and strong spin-orbit coupling. Although the crystal structure of TlX is not perovskite, the similarities of its electronic structure with lead halide perovskites motivated us to prepare colloidal TlX NCs. These TlX NCs exhibit a wide bandgap (>2.5 eV or <500 nm) and the potential to exhibit a reduced density of deep defect states. Optical pump terahertz (THz) probe spectroscopy with excitation fluence in the range of 0.85-5.86 × 1013 photons per cm2 on NC films shows that the TlBr NCs possess high effective carrier mobility (∼220 to 329 cm2 V-1 s-1), long diffusion length (∼0.77 to 0.98 µm), and reasonably high photoluminescence efficiency (∼10%). This combination of properties is remarkable compared to other wide-bandgap (>2.5 eV) semiconductor NCs, which suggests a reduction in the deep-defect states in the TlX NCs. Furthermore, the ultrafast carrier dynamics and temperature-dependent reversible structural phase transition together with its influence on the optical properties of the TlX NCs are studied.

Electronic properties of (Sb;Bi)2Te3 colloidal heterostructured nanoplates down to the single particle level.

We investigate the potential use of colloidal nanoplates of Sb2Te3 by conducting transport on single particle with in mind their potential use as 3D topological insulator material. We develop a synthetic procedure for the growth of plates with large lateral extension and probe their infrared optical and transport properties. These two properties are used as probe for the determination of the bulk carrier density and agree on a value in the 2-3 × 1019 cm-3 range. Such value is compatible with the metallic side of the Mott criterion which is also confirmed by the weak thermal dependence of the conductance. By investigating the transport at the single particle level we demonstrate that the hole mobility in this system is around 40 cm2V-1s-1. For the bulk material mixing n-type Bi2Te3 with the p-type Sb2Te3 has been a successful way to control the carrier density. Here we apply this approach to the case of colloidally obtained nanoplates by growing a core-shell heterostructure of Sb2Te3/Bi2Te3 and demonstrates a reduction of the carrier density by a factor 2.5.

Origin of Unusual Excitonic Absorption and Emission from Colloidal Ag2S Nanocrystals: Ultrafast Photophysics and Solar Cell.

Colloidal Ag2S nanocrystals (NCs) typically do not exhibit sharp excitonic absorption and emission. We first elucidate the reason behind this problem by preparing Ag2S NCs from nearly monodisperse CdS NCs employing cation exchange reaction. It was found that the defect-related midgap transitions overlap with excitonic transition, blurring the absorption spectrum. On the basis of this observation, we prepared nearly defect-free Ag2S NCs using molecular precursors. These defect-free Ag2S NCs exhibit sharp excitonic absorption, emission (quantum yield 20%) in near-infrared (853 nm) region, and improved performance of Ag2S quantum-dot-sensitized solar cells (QDSSCs). Samples with lower defects exhibit photoconversion efficiencies >1% and open circuit voltage of ∼0.3 V, which are better compared with prior reports of Ag2S QDSSCs. Femtosecond transient absorption shows pump-probe two-photon absorption above 630 nm and slow-decaying excited state absorption below 600 nm. Concomitantly, open-aperture z-scan shows strong two-photon absorption at 532 nm (coefficient 55 ± 3 cm/GW).