Colloidal Semiconductor Nanocrystals Research Articles

Unraveling Changes to PbS Nanocrystal Surfaces Induced by Thiols

Ligand exchange reactions are commonly employed to modify the surfaces of colloidal semiconductor nanocrystals (NCs), yet multiple reaction pathways induced by the introduction of thiols to NCs have been proposed in the literature to occur independently of each other. Here, we report three separate ligand-based reaction mechanisms between oleate-capped PbS NCs and the exchange thiol ligand, undec-10-ene-1-thiol (UDT), and unveil how these three pathways are intertwined. Experimental evidence for these mechanisms is provided by the 1H NMR spectroscopic handles of oleate- and UDT-based species as well as Pb species quantified by inductively coupled plasma mass spectrometry. We find that Z-type Pb(oleate)2 ligands are displaced at low quantities of added UDT (0–100 equiv per NC) alongside the coupled binding of L-type thiol ligands to the NC surface. With increasing amounts of added UDT (100–500 equiv), L-type binding of thiol ligands not associated with Z-type displacement is inferred from comparison of the numbers of each bound and free ligand throughout the 1H NMR spectroscopic titration. Lastly, a substantial quantity of oleic acid is liberated by the reaction of NCs with 100–500 equiv of UDT, suggesting an X-type exchange mechanism, supported by Fourier transform infrared (FTIR) spectroscopic titration. This work provides fundamental insight into changes to the NC surface upon ligand exchange with thiols and proposes site-specific reactivity for each mechanism contextualized within the prior art.

Light-Emitting Diodes Based on Two-Dimensional Nanoplatelets

Colloidal semiconductor nanocrystals (NCs) attract significant interest in recent years due to their narrow and tunable emission wavelength in the visible range, as well as high photoluminescence quantum yield (PLQY), which are highly desired in display technologies. The high-quality NCs have been recognized as vital luminescent materials in realizing next-generation display devices. With further development, NCs with near-unity PLQY have been successfully synthesized through engineering of the core/shell heterostructure. However, as the external quantum efficiency (EQE) of the nanocrystal light-emitting diodes (LEDs) approaches the theoretical limit of about 20%, the low out-coupling factor proposes a challenge of enhancing the performance of a device when using the spherical QDs. Hence, the anisotropic NCs like nanoplatelets (NPLs) are proposed as promising solutions to improve the performance of nanocrystal LEDs. In this review, we will summarize the synthetic strategies of two-dimensional (2D) NPLs at first. Then, we will introduce fundamental concepts of LEDs, the main approaches to realize LEDs based on nanoplatelets, and the recent progress. Finally, the challenges and opportunities of LEDs based on anisotropic NCs are also presented.

Chemical Transformations of Colloidal Semiconductor Nanocrystals Advance Their Applications.

Recently, colloidal semiconductor nanocrystals (NCs) are finding more and more applications in optoelectronic devices. Their usage, however, is still very far from the great potential already demonstrated in many fields owing to their unique features. While researchers are still struggling to achieve a wider gamut of different semiconductor nanomaterials with more controllable properties, the library of already existing candidates is large enough to harness their potential. Modification of well-studied semiconductor NCs by means of their chemical transformations can greatly advance their practical exploitation. In this Perspective, the main types of chemical transformations represented by ligand and cation exchange reactions and their recent examples are summarized. While ligand exchange is used to adjust the surface of a semiconductor NC, cation exchange allows us to engineer its core composition. Both approaches greatly extend the range of properties of the resulting nanomaterials, advancing their further incorporation into optoelectronic devices.

Sample Concentration Affects Optical Gain Results in Colloidal Nanomaterials: Circumventing the Distortions by Below Band Gap Excitation

Ultrafast spectroscopy studies have been key to the development of optical materials, including colloidal semiconductor nanocrystals (NCs) engineered for lighting and light-harvesting technologies. Several physical processes, which are revealed by ultrafast spectroscopy in NCs, are highly dependent on the average number of excitons created per NC, including optical gain properties and multiexciton interactions. Consequently, proper considerations regarding NC populations are necessary to avoid misinterpretations. In this paper, we present an experimental and theoretical analysis of the influence of the sample optical density (OD) at the excitation energy on the results of ultrafast spectroscopy studies in NCs. We show that the pump beam depletion caused by high ODs can drastically change the results from transient absorption (TA) experiments leading to data misinterpretations, such as the overestimation of the optical gain threshold. Based on that, we propose a robust modification on the TA technique, which allows for an OD-independent characterization, free of distortions. The modification consists of pumping the sample below its band gap energy, limiting the electronic excitation to a two-photon absorption process, resulting in an effectively zero OD for the pump beam and a uniform excitation in the direction of the beam propagation. Consequently, an undistorted TA signal is produced, allowing for precise characterization of NCs, including the gain/absorption cross section, gain coefficient, and gain threshold. Furthermore, the uniform excitation allows for higher signal-to-noise ratio, independent of the sample concentration.

Water Effects on Colloidal Semiconductor Nanocrystals: Correlation of Photophysics and Photochemistry.

With high-quality CdSe/CdS core/shell nanocrystals as the main model system and under a controlled atmosphere, responses of photoexcited semiconductor nanocrystals to two active species (water and/or oxygen) in an ambient environment are studied systematically. Under photoexcitation, although high-quality semiconductor nanocrystals in either thin solid films or various solutions have a near-unity photoluminescence quantum yield, there is still a small probability (∼10-5 per photon absorbed) to be photoreduced by the water molecules efficiently accumulated in the highly hydrophilic nanocrystal-ligands interface. The resulting negatively charged nanocrystals are the starting point of most photophysical variations, and the hydroxyl radical─key photo-oxidation product of water─plays the main role for initiating various photochemical processes. Depending on the supplementation of water to the interface, accessibility to oxygen, photoirradiation power, type of matrices, type of measurement schemes, and solubility of nanocrystals in the solution, various photophysical/photochemical phenomena─either reported or not reported in the literature─are reproducibly observed. Results confirm that photophysical properties and photochemical reactions can be well-correlated, offering a unified and unique basis for fundamental studies and the design of processing techniques in industry.

Quantum Computing with Exciton Qubits in Colloidal Semiconductor Nanocrystals

The present work evaluates the feasibility of quantum computing with exciton qubits in coupled colloidal semiconductor nanocrystals (NCs). A strategy for manipulating two-qubit states of colloidal NC hetero-dimers is described. We show that a sequence of laser pulses with the same photon energy can bring excitonic states of a nanocrystal hetero-dimer into entanglement and perform arbitrary qubit rotations (quantum gates). Our simulations of a realistic two-particle assembly of CdSe/CdS core/shell NCs demonstrate that such two-qubit gate operations can be driven by optical parametric oscillators with a theoretical error of 0.1%. A strategy for upscaling two-qubit hetero-dimers to N-qubit exciton gates in semiconductor NC assemblies is discussed.

Anisotropic, Nonthermal Lattice Disordering Observed in Photoexcited PbS Quantum Dots

Given their nanoscale dimensions, colloidal semiconductor nanocrystals provide unique systems for investigating the dynamics controlling surface chemistry and fundamental issues regarding lattice reorganization upon changes in electron distribution. These systems are particularly amenable to ultrafast electron probes, offering an atomic level picture of the lattice reorganization involved following photoexcitation. Here, we study lead sulfide (PbS) quantum dots with ultrafast electron diffraction to characterize the atomic motions following high-intensity photoexcitation. Short-range nonthermal lattice distortions and increased atomic disorder were observed in PbS colloidal quantum dots ranging from 2.4 to 8.7 nm in size. These effects scaled inversely with size and were less pronounced in nanocrystals with a chloride-containing surface rather than only organic ligands, which is consistent with an effect arising at the surface. The anisotropic, nonthermal lattice disordering occurs preferentially along the (100) crystallographic directions, which could indicate an anisotropic distribution of localized charge between the differing lattice terminations of the {111} and {100} crystal facets. This is consistent with the larger anharmonicity for the lattice potential at lattice sites with reduced ligand coordination relative to the bulk, which has been shown to cause accelerated relaxation into dynamic and static surface trap sites. Through an exploration of quantum dot size and variation in surface termination, this work provides the missing structural details to advance our understanding and control of charge-carrier formation, trapping, and recombination processes in nanoscale semiconductor systems.

Resolving the Triexciton Recombination Pathway in CdSe/CdS Nanocrystals through State-Specific Correlation Measurements.

As luminescence applications of colloidal semiconductor nanocrystals push toward higher excitation flux conditions, there is an increased need to both understand and potentially control emission from multiexciton states. We develop a spectrally resolved correlation method to study the triply excited state that enables direct measurements of the recombination pathway for the triexciton, rather than relying on indirect extraction of rates. We demonstrate that, for core-shell CdSe-CdS nanocrystals, triexciton emission arises exclusively from the band-edge S-like state. Time-dependent density functional theory and extended particle-in-a-sphere calculations demonstrate that reduced carrier overlap induced by the core-shell heterostructure can account for the lack of emission observed from the P-like state. These results provide a potential avenue for the control of nanocrystal luminescence, where core-shell heterostructures can be leveraged to control carrier separation and therefore maintain emission color purity over a broader range of excitation fluxes.

From CdSe Nanoplatelets to Quantum Rings by Thermochemical Edge Reconfiguration.

The variation in the shape of colloidal semiconductor nanocrystals (NCs) remains intriguing. This interest goes beyond crystallography as the shape of the NC determines its energy levels and optoelectronic properties. While thermodynamic arguments point to a few or just a single shape(s), terminated by the most stable crystal facets, a remarkable variation in NC shape has been reported for many different compounds. For instance, for the well-studied case of CdSe, close-to-spherical quantum dots, rods, two-dimensional nanoplatelets, and quantum rings have been reported. Here, we report how two-dimensional CdSe nanoplatelets reshape into quantum rings. We monitor the reshaping in real time by combining atomically resolved structural characterization with optical absorption and photoluminescence spectroscopy. We observe that CdSe units leave the vertical sides of the edges and recrystallize on the top and bottom edges of the nanoplatelets, resulting in a thickening of the rims. The formation of a central hole, rendering the shape into a ring, only occurs at a more elevated temperature.

Molecular Triplet Sensitization and Photon Upconversion Using Colloidal Semiconductor Nanocrystals

Colloidal semiconductor nanocrystals have recently emerged as a novel class of photosensitizers for molecular spin-triplet states. This sensitization strategy combines the advantages of strong and readily tunable light absorption of nanocrystals with the exceptionally long lifetime of molecular triplet states, which can find a broad range of photochemical and energy applications. A unique advantage for the application in triplet–triplet annihilation photon upconversion is that the negligible bright-to-dark exciton relaxation energy loss of nanocrystals allows for a large photon energy gain (i.e., anti-Stokes shift). This Focus Review highlights recent advances in this emerging field, mainly including a summary of the principles and mechanisms of triplet sensitization along with the factors controlling the sensitization efficiencies and an overview of the nanocrystal materials that have been applied to triplet sensitization and photon upconversion. We end the review with our perspectives on the remaining challenges in this field and the potential applications of this technology in the future.

Size-Dependent Doping Synergy and Dual-Color Emission in CsPb1-xMnxCl3 Nanocrystals

The doping of atomic impurities into colloidal semiconductor nanocrystals (NCs) is a powerful way to tune optical and electronic properties. Manganese (Mn)-doped CsPbCl3 NCs in particular show dual exciton/dopant emission in the visible spectrum, making them materials of current interest for optoelectronic applications. Here, we examine the impact of Mn doping on the photophysics of CsPbCl3 NCs from a different perspective. Using ultracentrifugation, we extract size-resolved fractions from a series of parent NC suspensions prepared at varied doping levels, and we measure the photoresponse as a function of NC size. Our results reveal synergistic radiative effects, with large enhancements in both blue (exciton) and red (dopant) quantum yield (QY) over a narrow range of Mn concentration. We ascribe the first effect to defect passivation and the latter to efficient exciton–dopant charge transfer near a critical point in the number of Mn per NC where the dopant spacing coincides with the Bohr exciton radius. Advanced computational techniques applied to Mn-doped CsPbCl3 reproduce emission colors and concentration trends, with a drop in QY at higher doping levels. Our results offer additional insight into the photophysical interplay of doping, quantum confinement, and radiative relaxation in lead-halide perovskites, while clarifying the complexity of the underlying kinetics and highlighting the critical role of the dopant number per NC at the nanoscale.

Optically Active CdSe/CdS Nanoplatelets Exhibiting Both Circular Dichroism and Circularly Polarized Luminescence

AbstractLigand‐induced chirality in colloidal semiconductor nanocrystals attracts attention because of their tunable chiroptical properties. Here, the induced chirality and circularly polarized luminescence (CPL) are investigated as a function of the CdS shell growth in a range of 2D CdSe/CdS nanoplatelets (NPLs) capped with chiral ligands. Five samples of CdSe/CdS NPLs are synthesized by a one‐pot approach to vary the island‐like shell on a four‐monolayer (4 ML) CdSe NPLs core, which effectively reduces the interfacial strain energy. The successful preparation of L‐/D‐Cysteine‐capped CdSe/CdS NPLs with both tunable circular dichroism (CD) and CPL behaviors and a maximum anisotropic luminance factor (glum) of 5.29 × 10−4 is described. The induced chiroptical response shows a direct relationship with the formation of island‐like shell in the first and second stages and shows a clear signal evolution. In the third stage with a full coating shell, the CD and CPL signals are inversely proportional to the CdS shell thickness. The island‐like shell gives birth to the CPL signal, while the formation of full coating shell decreases the induced chirality. Such chiral and emissive NPLs provide an ideal platform for the rational design of semiconductor nanocrystals with chiroptical properties in areas of biomedicine, polarizers, and new generation of display devices.

Composition-Controlled Synthesis of Near-IR-Light-Emitting Ag-Ιn-Ga-Se Nanocrystals for Biological Imaging

Colloidal semiconductor nanocrystals have been intensively developed for the application to photovoltaics, light-emitting diodes, electroluminescent device, and photoluminescent markers for bio-imaging. Recently, low-toxic I-III-VI semiconductor-based multinary chalcogenide nanocrystals have attracted much attention as a promising alternative for Cd-based ones, because these nanocrystals are composed of less-toxic elements and exhibited intense photoluminescence (PL) in the visible and near-IR wavelength regions. In our previous paper, we reported that chemically synthesized Zn-Ag-In-S nanocrystals exhibited a broad PL peak (FWHM > 100 nm) originating from donor-acceptor pair radiative recombination, the wavelength of which was tunable from ca. 500 to 800 nm by changing the particle size and the chemical composition.[1] However, these multinary nanocrystals did not exhibit a band-edge emission, though narrowing PL peak has been earnestly desired for luminescent applications. In this study, we report the preparation of novel multinary Ag-In-Ga-Se nanocrystals via solution phase synthesis and control the peak wavelength of band-edge emission in the range from visible to near-IR lights. We also demonstrated the usefulness of thus-obtained Ag-In-Ga-Se nanocrystals for in vivo bio-imaging.[2] Semiconductor nanocrystals composed of Ag-In-Ga-Se (AIGSe) solid solution were prepared by the thermal decomposition of corresponding metal salts and selenourea in a hot mixture solution of oleylamine/1-dodecanethiol under an N2 atmosphere. The molar ratios of precursors were varied with the synthesis of Ag0.67InyGa(1-y)Se2 (AIGSe) in mind. The individual XRD peaks of thus-obtained AIGSe nanocrystals were located between those of tetragonal AgInSe2 and AgGaSe2 crystal structures and shifted to a higher angle with a decrease in the y value, indicating that the Ga3+ fraction in the Ag(In,Ga)Se2 solid solution increased with a decrease in the y value in preparation. TEM measurements revealed that spherical nanocrystals were formed, the average size of which were varied in the range of 3~5 nm depending on the chemical composition. The PL spectra of as-prepared AIGSe nanocrystal exhibited both a narrow band-edge emission peak and a broad defect-site emission peak. By surface-coating AIGSe nanocrystals with GaSx (AIGSe@GaS x ), a broad PL peak was remarkably diminished especially for y = 0.25-0.5, owing to the removal of the surface defect sites. With a decrease in the y value, that is, with an increase in the Ga/In ratio of nanocrystals, the peak wavelength of band-edge emission of AIGSe@GaS x nanocrystals was blue-shifted from 890 to 610 nm because of the increase in their energy gap. The PL quantum yield (QY) was enlarged by the GaS x shell coating, in which the maximum PL QY of 38% was obtained for the PL peak at 800 nm AIGSe@GaS x of y = 0.7. Thus-obtained nanocrystals were successfully used as near-IR PL probes for three-dimensional in vivo bioimaging, in which the wavelengths of excitation and detection lights could be selected in the first biological window. The imaging signals were clearly detected from AIGSe@GaS x nanocrystals injected into biological tissues by ca. 5 mm in depth. Reference Torimoto, et al., J. Phys. Chem. C, 2015, 119, 24740-24749. Torimoto, et al., ACS Appl. Nano Mater . 2020, 3, 3275-3287.

(Invited) Synthesis and Optoelectronic Applications of Colloidal Nanorod Heterostructures

Engineering band structure and heterointerfaces has been and continues to be critical for advances in electronics, optoelectronics, photovoltaics, and displays among many high-tech applications. Achieving atomic precision through widely-accessible and cost-effective means would allow rapid advances in these critical areas. The evolution of colloidal semiconductor nanocrystals from single-composition, “spherical” particles to complex heterostructures of diverse shapes provides many opportunities for precision band structure engineering through scalable solution synthesis. With anisotropic shapes that can be exploited for assembly, charge carrier manipulation and optical anisotropy, etc., incorporating heterojunctions and other functional interfaces into colloidal nanorod heterostructures represents an especially promising direction. In this talk, general challenges to the synthesis of complex-yet-well-defined colloidal nanorod heterostructures will first be discussed. Approaches such as spatially selective solution epitaxy, catalytic growth, cation exchange and combinations thereof can be exploited to achieve unique heterostructures with useful properties. Examples of colloidal nanorod heterostructures of II-VI as well as I-III-VI semiconductors will be discussed. In addition, light-emitting diodes (LEDs) incorporating these materials are shown to not only radically improve existing function but also impart new capabilities without adding complexity in manufacturing.

(Invited) Strategies in Colloidal Catalysis for Generation of Solar Fuels

This talk will describe two separate strategies to use colloidal semiconductor nanocrystals to photocatalytically produce (i) the fuel intermediate CO from the abundant feedstock CO2, and (ii) H2 from protons within a hydrogel with pH-dependent structure. CO2 reduction is accomplished with record turnover in pure water with a combination of colloidal CuInS2 nanocrystals and a cobalt porphyrin electrocatalyst. The nanocrystals are decorated with partially protonated amine-terminated ligands, which not only provide a means of electrostatic self-assembly with the negatively charge porphyrin, but also allow pre-activation of the CO2 through formation of carbamic acid. In a separate system based on cadmium chalcogenide nanorods, the yield of proton reduction is non-monotonically dependent on the pH of the hydrogel with a large spike near the pKa of the exposed amine groups within the gel.

An Overview of Computational Studies on Colloidal Semiconductor Nanocrystals.

In the last two decades, colloidal semiconductor nanocrystals have emerged as a phenomenal research topic due to their size-dependent optoelectronic properties and to their outstanding versatility in many technological applications. In this review, we provide an historical account of the most relevant computational works that have been carried out to understand atomistically the electronic structure of these materials, including the main requirements needed for the preparation of nanocrystal models that align well with the experiments. We further discuss how the advancement of these computational tools has affected the analysis of these nanomaterials over the years. We focus our review on the three main families of colloidal semiconductor nanocrystals: group II-VI and IV-VI metal chalcogenides, group III-V metal pnictogenides and metal halides, in particular lead-based halide perovskites. We discuss the most recent research frontiers and outline the future outlooks expected in this field from a computational perspective.

Perovskite-type superlattices from lead halide perovskite nanocubes.

Atomically defined assemblies of dye molecules (such as H and J aggregates) have been of interest for more than 80 years because of the emergence of collective phenomena in their optical spectra1-3, their coherent long-range energy transport, their conceptual similarity to natural light-harvesting complexes4,5, and their potential use as light sources and in photovoltaics. Another way of creating versatile and controlled aggregates that exhibit collective phenomena involves the organization of colloidal semiconductor nanocrystals into long-range-ordered superlattices6. Caesium lead halide perovskite nanocrystals7-9 are promising building blocks for such superlattices, owing to the high oscillator strength of bright triplet excitons10, slow dephasing (coherence times of up to 80 picoseconds) and minimal inhomogeneous broadening of emission lines11,12. So far, only single-component superlattices with simple cubic packing have been devised from these nanocrystals13. Here we present perovskite-type (ABO3) binary and ternary nanocrystal superlattices, created via the shape-directed co-assembly of steric-stabilized, highly luminescent cubic CsPbBr3 nanocrystals (which occupy the B and/or O lattice sites), spherical Fe3O4 or NaGdF4 nanocrystals (A sites) and truncated-cuboid PbS nanocrystals (B sites). These ABO3 superlattices, as well as the binary NaCl and AlB2 superlattice structures that we demonstrate, exhibit a high degree of orientational ordering of the CsPbBr3 nanocubes. They also exhibit superfluorescence-a collective emission that results in a burst of photons with ultrafast radiative decay (22 picoseconds) that could be tailored for use in ultrabright (quantum) light sources. Our work paves the way for further exploration of complex, ordered and functionally useful perovskite mesostructures.

Photoinduced Rotation of Colloidal Semiconductor Nanocrystals in an Electric Field.

We demonstrate that solution-phase semiconductor nanocrystals (NCs) undergo photoinduced rotation in an external electric field. Present measurements backed by theoretical calculations show that the rotation of colloidal NCs is driven by the excited-state dipole moment, which is counterbalanced by the solvent viscosity drag. Corresponding angular velocities range from 0.5°/ns for cubic CsPbBr3 NCs to 3°/ns for nanoparticles with a large photoinduced charge separation (CdSe/CdS core-shell and dot-in-a-rod NCs). Because of photoinduced rotation, solution-phase semiconductor NCs exhibited an order-of-magnitude increase in the spectral changes caused by the quantum confined Stark effect (QCSE), compared to solid NC assemblies. The enhanced QCSE of colloidal NCs reflected their global alignment in solution, which could be retained in a solid environment by slow crystallization. Overall, we expect that the demonstrated phenomenon of the colloidal nanocrystal rotation in an electric field will open up new avenues for developing electro-optical and voltage-sensitive applications.

Rapid In Situ Ligand-Exchange Process Used to Prepare 3D PbSe Nanocrystal Superlattice Infrared Photodetectors.

Colloidal semiconductor nanocrystals are important building blocks for low-cost, solution-processed electronic devices with tunable functionalities. Considerable progress is made in improving charge transport through nanocrystal films by exchanging long insulating ligands with shorter passivating ligands. To take full advantage of this strategy, it is equally important to fabricate close-packed structures that reduce the average interparticle spacing. Yet it remains a challenge to retain long-range, close-packed order after ligand exchange. Here, a novel one-step in situ ligand-exchange method is demonstrated that enables rapid (5min) ligand exchange of nanocrystal films, which are more than 50 layers thick. Using this simple and efficient method, it is shown that the face-centered cubic ordering of 500 nmthick PbSe nanocrystal films is retained after ligand exchange from oleic acid to benzoic acid. Moreover, it is demonstrated that PbSe nanocrystal photodetectors with a well-ordered structure have superior optoelectronic properties compared to disordered films; ordered films have a 16× higher responsivity of ≈0.25 A W-1 at 1 Vand a 2× faster response time. As far as it is known, this is the first report to realize a rapid one-step ligand exchange through a thick superlattice film with retention of long-range order.

Ultrafast electron–hole relaxation dynamics in CdS nanocrystals

Surface traps significantly influence the charge carrier dynamics within semiconductor nanocrystals, introducing non-radiative exciton recombination channels which are detrimental for their applications. Understanding the nature of these trap states and modulating them synthetically bears immense potential in designing defect-free colloidal semiconductor nanocrystals for efficient optoelectronic devices. Thus, systems devoid of surface traps can be used to study the relaxation pathways of excitons generated within these nanocrystals. In this work, we study the ultrafast charge carrier relaxation dynamics upon near-edge resonance excitation and above-resonance excitation in CdS nanocrystals using ultrafast transient absorption spectroscopy, in order to understand intraband cooling and mid-gap trap states. The time-resolved studies reveal that the above bandgap excitation results in a three-step process, including instantaneous growth followed by a fast sub-picosecond decay and a long-lived (>1 ns) excited state or a trap state recombination. The large percentage of long-lived excitons in CdS nanocrystals elucidates the defect-free nature of the system arising from the absence of surface states.