Intrinsic Exciton Photophysics of PbS Quantum Dots Revealed by Low-Temperature Single Nanocrystal Spectroscopy

Design Rules for Obtaining Narrow Luminescence from Semiconductors Made in Solution

Solution-processed semiconductors are in demand for present and next-generation optoelectronic technologies ranging from displays to quantum light sources because of their scalability and ease of integration into devices with diverse form factors. One of the central requirements for semiconductors used in these applications is a narrow photoluminescence (PL) line width. Narrow emission line widths are needed to ensure both color and single-photon purity, raising the question of what design rules are needed to obtain narrow emission from semiconductors made in solution. In this review, we first examine the requirements for colloidal emitters for a variety of applications including light-emitting diodes, photodetectors, lasers, and quantum information science. Next, we will delve into the sources of spectral broadening, including "homogeneous" broadening from dynamical broadening mechanisms in single-particle spectra, heterogeneous broadening from static structural differences in ensemble spectra, and spectral diffusion. Then, we compare the current state of the art in terms of emission line width for a variety of colloidal materials including II-VI quantum dots (QDs) and nanoplatelets, III-V QDs, alloyed QDs, metal-halide perovskites including nanocrystals and 2D structures, doped nanocrystals, and, finally, as a point of comparison, organic molecules. We end with some conclusions and connections, including an outline of promising paths forward.

Intraband Transition of HgTe Nanocrystals for Long-Wave Infrared Detection at 12 μm

The synthesis of n-doped HgTe colloidal quantum dots was optimized to produce samples with a 1S_e-1P_e intraband transition in the long-wave infrared (8-12 μm). The spin-orbit splitting of 1P_e states places the 1S_e-1P_e1/2 transition around 10 μm. The narrow line width of 130 cm^-1 at 300 K is limited by the size distribution. This narrowing leads to an absorption coefficient about 5 times stronger than is possible with the HgTe CQD interband transition at similar energies. From 300 to 80 K, the intraband transition blueshifts by 90 cm^-1, while the interband transition redshifts by 350 cm^-1. These shifts are assigned to the temperature dependence of the band structure. With ∼2 electrons/dot doping at 80 K, a photoconductive film of 80 nm thickness on a quarter wave reflector substrate showed a detectivity (D*) of ∼10⁷ Jones at 500 Hz in the 8-12 μm range.

Unusual Temperature Dependence of Bandgap in 2D Inorganic Lead-Halide Perovskite Nanoplatelets

Understanding the origin of temperature-dependent bandgap in inorganic lead-halide perovskites is essential and important for their applications in photovoltaics and optoelectronics. Herein, it is found that the temperature dependence of bandgap in CsPbBr3 perovskites is variable with material dimensionality. In contrast to the monotonous redshift ordinarily observed in bulk-like CsPbBr3 nanocrystals (NCs), the bandgap of 2D CsPbBr3 nanoplatelets (NPLs) exhibits an initial blueshift then redshift trend with decreasing temperature (290-10 K). The Bose-Einstein two-oscillator modeling manifests that the blueshift-redshift crossover of bandgap in the NPLs is attributed to the significantly larger weight of contribution from electron-optical phonon interaction to the bandgap renormalization in the NPLs than in the NCs. These new findings may gain deep insights into the origin of bandgap shift with temperature for both fundamentals and applications of perovskite semiconductor materials.

Highly versatile near-infrared emitters based on an atomically defined HgS interlayer embedded into a CdSe/CdS quantum dot

The availability of colloidal quantum dots with highly efficient, fast and 'non-blinking' near-infrared emission would benefit numerous applications, from advanced optical communication and quantum networks to biomedical diagnostics. Here, we report high-quality near-infrared emitters that are based on well known CdSe/CdS heterostructures. By incorporating an HgS interlayer at the quantum dot core/shell interface, we convert normally visible emitters into highly efficient near-infrared fluorophores. Employing thermodynamically controlled sequential deposition of metal and chalcogen ions, we achieve atomic-level precision in defining the thickness of the HgS interlayer (H). This manifests in 'quantized' jumps of the photoluminescence spectrum when H changes in discrete, atomic steps. The synthesized structures show highly efficient photoluminescence, tunable from 700 to 1,370 nm, and fast radiative rates of ~1/60 ns^-1. The emission from individual CdSe/HgS/CdS colloidal quantum dots is virtually blinking free and exhibits nearly perfect single-photon purity. In addition, when incorporated into a light-emitting-diode architecture, these quantum dots demonstrate strong electroluminescence with a sub-bandgap turn-on voltage.

Relations between absorption, emission, and excited state chemical potentials from nanocrystal 2D spectra

For quantum-confined nanomaterials, size dispersion causes a static broadening of spectra that has been difficult to measure and invalidates all-optical methods for determining the maximum photovoltage that an excited state can generate. Using femtosecond two-dimensional (2D) spectroscopy to separate size dispersion broadening of absorption and emission spectra allows a test of single-molecule generalized Einstein relations between such spectra for colloidal PbS quantum dots. We show that 2D spectra and these relations determine the thermodynamic standard chemical potential difference between the lowest excited and ground electronic states, which gives the maximum photovoltage. Further, we find that the static line broadening from many slightly different quantum dot structures allows single-molecule generalized Einstein relations to determine the average single-molecule linewidth from Stokes' frequency shift between ensemble absorption and emission spectra.

Interplay of Bright Triplet and Dark Excitons Revealed by Magneto-Photoluminescence of Individual PbS/CdS Quantum Dots

A low-temperature polarization-resolved magneto-photoluminescence experiment is performed on individual PbS/CdS core/shell quantum dots (QDs). The experiment enables a direct measurement of the exciton Landé g factor and the anisotropic zero-field splitting of the lowest emissive bright exciton triplet in PbS/CdS QDs. While anisotropic splittings of individual QDs distribute randomly in 104-325 μeV range, the exciton Landé g factors increase from 0.95 to 2.70 as the emission energy of the QD increases from 1.0 to 1.2 eV. The tight-binding calculations allow to rationalize these trends as a direct consequence of reducing a cubic symmetry of QD via addition/removal of a few (<70) atoms from the surfaces of the PbS core. Furthermore, it is observed that while right (σ  ⁺ ) and left (σ   ^- ) circularly polarized photoluminescence (PL) peaks split linearly with magnetic field as expected for Zeeman effect, the energy splitting between X and Y linearly polarized PL peaks remains nearly unchanged. The theoretical study reveals rich and complex magnetic field-induced interplay of bright triplet and dark exciton states explaining this puzzling behavior. These findings fill the missing gaps in the understanding of lead salt QDs and provide foundation for development of classical and quantum light sources operating at telecommunication wavelengths.

Exciton Fine Structure in Lead Chalcogenide Quantum Dots: Valley Mixing and Crucial Role of Intervalley Electron-Hole Exchange

We study the exciton fine structure in quantum dots of multivalley lead chalcogenides. We demonstrate that intervalley electron-hole exchange interaction, ignored in previous studies, dramatically modifies the exciton fine structure and leads to appearance of the ultrabright valley-symmetric spin-triplet exciton state dominating interband optical absorption. Valley mixing leads to brightening of other symmetry-allowed spin-triplet states which dominate low-temperature photoluminescence.

Composition-, Size-, and Surface Functionalization-Dependent Optical Properties of Lead Bromide Perovskite Nanocrystals

The photoluminescence (PL), color purity, and stability of lead halide perovskite nanocrystals depend critically on surface passivation. We present a study on the temperature-dependent PL and PL decay dynamics of lead bromide perovskite nanocrystals characterized by different types of A cations, surface ligands, and nanocrystal sizes. Throughout, we observe a single emission peak from cryogenic to ambient temperature. The PL decay dynamics are dominated by surface passivation, and a postsynthesis ligand exchange with a quaternary ammonium bromide (QAB) results in more stable passivation over a larger temperature range. The PL intensity is highest from 50 to 250 K, which indicates that ligand binding competes with the thermal energy at ambient temperature. Despite the favorable PL dynamics of nanocrystals passivated with QAB ligands (monoexponential PL decay over a large temperature range, increased PL intensity and stability), surface passivation still needs to be improved to achieve maximum emission intensity in nanocrystal films.

Evidence of Skewness and Sub-Gaussian Character in Temperature-Dependent Distributions of One Million Electronic Excitation Energies in PbS Quantum Dots

Obtaining statistical distributions by sampling a large number of conformations is vital for an accurate description of temperature-dependent properties of chemical systems. However, constructing distributions with 10⁵-10⁶ samples is computationally challenging because of the prohibitively high computational cost of performing first-principles quantum mechanical calculations. In this work, we present a new technique called the effective stochastic potential configuration interaction singles (ESP-CIS) method to obtain excitation energies. The ESP-CIS method uses random matrix theory for the construction of an effective stochastic representation of the Fock operator and combines it with the CIS method. Excited-state energies of PbS quantum dots (0.75-1.75 nm) at temperatures of 10-400 K were calculated using the ESP-CIS method. Results from a total of 27 million excitation energy calculations revealed the distributions to be sub-Gaussian in nature with negative skewness, which progressively became red-shifted with increasing temperature. This study demonstrates the efficacy of the ESP-CIS method as a general-purpose method for efficient excited-state calculations.