Colloidal Semiconductor Nanocrystals Research Articles

Metal-Solvent Complex Formation at the Surface of InP Colloidal Quantum Dots.

The surface chemistry of colloidal semiconductor nanocrystals (QDs) profoundly influences their physical and chemical attributes. The insulating organic shell ensuring colloidal stability impedes charge transfer, thus limiting optoelectronic applications. Exchanging these ligands with shorter inorganic ones enhances charge mobility and stability, which is pivotal for using these materials as active layers for LEDs, photodetectors, and transistors. Among those, InP QDs also serve as a model for surface chemistry investigations. This study focuses on group III metal salts as inorganic ligands for InP QDs. We explored the ligand exchange mechanism when metal halide, nitrate, and perchlorate salts of group III (Al, In Ga), common Lewis acids, are used as ligands for the conductive inks. Moreover, we compared the exchange mechanism for two starting model systems: InP QDs capped with myristate and oleylamine as X- and L-type native organic ligands, respectively. We found that all metal halide, nitrate, and perchlorate salts dissolved in polar solvents (such as n-methylformamide, dimethylformamide, dimethyl sulfoxide, H2O) with various polarity formed metal-solvent complex cations [M(Solvent)6]3+ (e.g., [Al(MFA)6]3+, [Ga(MFA)6]3+, [In(MFA)6]3+), which passivated the surface of InP QDs after the removal of the initial organic ligand. All metal halide capped InP/[M(Solvent)6]3+ QDs show excellent colloidal stability in polar solvents with high dielectric constant even after 6 months in concentrations up to 74 mg/mL. Our findings demonstrate the dominance of dissociation-complexation mechanisms in polar solvents, ensuring colloidal stability. This comprehensive understanding of InP QD surface chemistry paves the way for exploring more complex QD systems such as InAs and InSb QDs.

Observation of negative surface and interface energies of quantum dots

Surface energy is a fundamental property of materials and is particularly important in describing nanomaterials where atoms or molecules at the surface constitute a large fraction of the material. Traditionally, surface energy is considered to be a positive quantity, where atoms or molecules at the surface are less thermodynamically stable than their counterparts in the interior of the material because they have fewer bonds or interactions at the surface. Using calorimetric methods, we show that the surface energy is negative in some prototypical colloidal semiconductor nanocrystals, or quantum dots with organic ligand coatings. This implies that the surface atoms are more thermodynamically stable than those on the interior due to the strong bonds between these atoms and surfactant molecules, or ligands, that coat their surface. In addition, we extend this work to core/shell indium phosphide/zinc sulfide nanocrystals and show that the interfacial energy between these materials is highly thermodynamically favorable in spite of their large lattice mismatch. This work challenges many of the assumptions that have guided thinking about colloidal nanomaterial thermodynamics, investigates the fundamental stability of many technologically relevant colloidal nanomaterials, and paves the way for future experimental and theoretical work on nanocrystal thermodynamics.

Charge Transfer from Quantum-Confined 0D, 1D, and 2D Nanocrystals.

The properties of colloidal quantum-confined semiconductor nanocrystals (NCs), including zero-dimensional (0D) quantum dots, 1D nanorods, 2D nanoplatelets, and their heterostructures, can be tuned through their size, dimensionality, and material composition. In their photovoltaic and photocatalytic applications, a key step is to generate spatially separated and long-lived electrons and holes by interfacial charge transfer. These charge transfer properties have been extensively studied recently, which is the subject of this Review. The Review starts with a summary of the electronic structure and optical properties of 0D-2D nanocrystals, followed by the advances in wave function engineering, a novel way to control the spatial distribution of electrons and holes, through their size, dimension, and composition. It discusses the dependence of NC charge transfer on various parameters and the development of the Auger-assisted charge transfer model. Recent advances in understanding multiple exciton generation, decay, and dissociation are also discussed, with an emphasis on multiple carrier transfer. Finally, the applications of nanocrystal-based systems for photocatalysis are reviewed, focusing on the photodriven charge separation and recombination processes that dictate the function and performance of these materials. The Review ends with a summary and outlook of key remaining challenges and promising future directions in the field.

Constructing ZnTe Spherical Quantum Well for Efficient Light Emission.

ZnTe colloidal semiconductor nanocrystals (NCs) have shown promise for light-emitting diodes (LEDs) and displays, because they are free from toxic heavy metals (Cd). However, so far, their low photoluminescence (PL) efficiency (∼30%) has hindered their applications. Herein, we devised a novel structure of ZnTe NCs with the configuration of ZnSe (core)/ZnTe (spherical quantum well, SQW)/ZnSe (shell). The inner layer ZnTe was grown at the surface of ZnSe core with avoiding using highly active and high-risk Zn sources. Due to the formation of coherently strained heterostructure which reduced the lattice mismatch, and the thermodynamic growth of ZnTe, the surface or interface defects were suppressed. A high PL efficiency of >60% was obtained for the green light-emitting ZnSe/ZnTe/ZnSe SQWs after ZnS outer layer passivation, which is the highest value for colloidal ZnTe-based NCs. This work paves the way for the development of novel semiconductor NCs for luminescent and display applications.

Biotinylated Photocleavable Semiconductor Colloidal Quantum Dot Supraparticle Microlaser.

Luminescent supraparticles of colloidal semiconductor nanocrystals can act as microscopic lasers and are hugely attractive for biosensing, imaging, and drug delivery. However, biointerfacing these to increase functionality while retaining their main optical properties remains an unresolved challenge. Here, we propose and demonstrate red-emitting, silica-coated CdSxSe1-x/ZnS colloidal quantum dot supraparticles functionalized with a biotinylated photocleavable ligand. The success of each step of the synthesis is confirmed by scanning electron microscopy, energy dispersive X-ray and Fourier transform infrared spectroscopy, ζ-potential, and optical pumping measurements. The capture and release functionality of the supraparticle system is proven by binding to a neutravidin functionalized glass slide and subsequently cleaving off after UV-A irradiation. The biotinylated supraparticles still function as microlasers; e.g., a 9 μm diameter supraparticle has oscillating modes around 625 nm at a threshold of 58 mJ/cm2. This work is a first step toward using supraparticle lasers as enhanced labels for bionano applications.

Optically Anisotropic Films of Colloidal Nanocrystals/Photoluminescent Dye Doped Polymers

Semiconductor colloidal nanocrystals are attractive materials since they can be adapted to polymers to form hybrid materials and are compatible with many optical applications. Here, synthesis of CdSe/CdS nanorods (NRs) via hot injection method is carried out, followed by preparation of hybrid polymer films from polyethylene glycol monomethyl ether-block-poly(glycidyl methacrylate)-block-poly[2-(diethylamino)ethyl methacrylate] triblock copolymer (MPEG-b-PGMA-b-DEAEMA) at a liquid-air interface. The optical properties of the films are finely adjusted to form optically anisotropic (i.e. dual-color emissive) films by using dyes for the other emitter as desired. Thiazolo[5,4-d] thiazole (TTz)-based dye and 6-carboxy fluorescein were used for this purpose. Tunable emission of TTz dye from blue to green dependent on changing pH value resulted in blue-green emissive polymer films, while red emission of CdSe/CdS NRs caused red emissive films. Phase separation of these materials is achieved by the hexane-insoluble nature of MPEG-b-PGMA-b-DEAEMA and the high solubility of NRs in it. These dual emissive films are promising candidates for waveguides and optical sensors.

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.

In situ Surface Reconstruction in Pure Water by Ice‐Confined Freeze‐Thaw Strategy for High‐Performance Core–Shell Structural Perovskite Nanocrystals

AbstractConstructing core–shell structure is recognized as an effective approach for the conventional colloidal semiconductor nanocrystals (NCs) to improve their stability and optical performance in optoelectronic applications. It is, however, still a challenge to develop green manufacture for core–shell lead halide perovskite NCs (PeNCs). Herein, an ice‐confined freeze‐thaw strategy using pure water solvent is devised, which results in the surface reconstruction of CsPbBr3 PeNCs thereby formatting the CsPb2Br5 shell on the CsPbBr3 core to achieve core–shell CsPbBr3@CsPb2Br5. The in situ formation of the CsPb2Br5 shell is attributed to the transformation of the intermediate polyhedral structure with the help of amphiphilic 4‐dodecylbenzene sulfonic acid ligand in the precursor solution. The PeNCs show ultrahigh photoluminescence quantum yield of 95%, remarkable stability and humidity‐resistance. Moreover, both light‐emitting diodes and stretchable fluorescence hydrogels based on the PeNCs exhibit a boosted optical performance and excellent operation stability. The organic solvent‐free and recyclability of this strategy pave a novel way to the environmentally friendly and large‐scale manufacture of high‐quality PeNCs for optoelectronic application.

Suppressed Magnitude of Spectral Diffusion in Cube-Shaped CdSe/CdS Core/Shell Nanocrystals with Exceedingly Stable Photoluminescence.

Colloidal semiconductor nanocrystals are promising candidates for quantum light sources, yet their application has been impeded by photoluminescence instability due to blinking and spectral diffusion. This study introduces a new category of cube-shaped CdSe/CdS core/shell nanocrystals with exceptionally stable photoluminescence characteristics. Under continuous excitation, the emissive quantum state remained consistent without alterations of the charge state for 4000 s, and the average photon energy variation stayed within the bounds of spectral resolution throughout this extended duration. Systematic examination of single-nanocrystal photoluminescence, upon variation of the core and shell dimensions, revealed that a thicker CdS shell and increased core edge length significantly curtail spectral diffusion, considering that the nanocrystals possess well-controlled CdSe-CdS and facet-ligand interfaces. This study advances the optimization of colloidal semiconductor nanocrystals as high-performance quantum light sources.

Controlled Growth of Large SiO2 Shells onto Semiconductor Colloidal Nanocrystals: A Pathway Toward Photonic Integration.

The growth of SiO2 shells on semiconductor nanocrystals is an established procedure and it is widely employed to provide dispersibility in polar solvents, and increased stability or biocompatibility. However, to exploit this shell to integrate photonic components on semiconductor nanocrystals, the growth procedure must be finely tunable and able to reach large particle sizes (around 100 nm or above). Here, we demonstrate that these goals are achievable through a design of experiment approach. Indeed, the use of a sequential full-factorial design allows us to carefully tune the growth of SiO2 shells to large values while maintaining a reduced size dispersion. Moreover, we show that the growth of a dielectric shell alone can be beneficial in terms of emission efficiency for the nanocrystal. We also demonstrate that, according to our modeling, the subsequent growth of two shells with increasing refractive index leads to an improved emission efficiency already at a reduced SiO2 sphere radius.

Exploiting Organometallic Chemistry to Functionalize Small Cuprous Oxide Colloidal Nanocrystals.

The ligand chemistry of colloidal semiconductor nanocrystals mediates their solubility, band gap, and surface facets. Here, selective organometallic chemistry is used to prepare small, colloidal cuprous oxide nanocrystals and to control their surface chemistry by decorating them with metal complexes. The strategy is demonstrated using small (3-6 nm) cuprous oxide (Cu2O) colloidal nanocrystals (NC), soluble in organic solvents. Organometallic complexes are coordinated by reacting the surface Cu-OH bonds with organometallic reagents, M(C6F5)2, M = Zn(II) and Co(II), at room temperature. These reactions do not disrupt the Cu2O crystallinity or nanoparticle size; rather, they allow for the selective coordination of a specific metal complex at the surface. Subsequently, the surface-coordinated organometallic complex is reacted with three different carboxylic acids to deliver Cu-O-Zn(O2CR') complexes. Selective nanocrystal surface functionalization is established using spectroscopy (IR, 19F NMR), thermal gravimetric analyses (TGA), transmission electron microscopy (TEM, EELS), and X-ray photoelectron spectroscopy (XPS). Photoluminescence efficiency increases dramatically upon organometallic surface functionalization relative to that of the parent Cu2O NC, with the effect being most pronounced for Zn(II) decoration. The nanocrystal surfaces are selectively functionalized by both organic ligands and well-defined organometallic complexes; this synthetic strategy may be applicable to many other metal oxides, hydroxides, and semiconductors. In the future, it should allow NC properties to be designed for applications including catalysis, sensing, electronics, and quantum technologies.

2D Silver-Nanoplatelets Metasurface for Bright Directional Photoluminescence, Designed with the Local Kirchhoff's Law.

Semiconductor colloidal nanocrystals are excellent light emitters in terms of efficiency and spectral control. They can be integrated with a metasurface to make ultrathin photoluminescent devices with a reduced amount of active material and perform complex functionalities such as beam shaping or polarization control. To design such a metasurface, a quantitative model of the emitted power is needed. Here, we report the design, fabrication, and characterization of a ∼300 nm thick light-emitting device combining a plasmonic metasurface with an ensemble of nanoplatelets. The source has been designed with a methodology based on a local form of Kirchhoff's law. The source displays record high directionality and absorptivity.

Synthesis of Ternary and Quaternary Group III-Arsenide Colloidal Quantum Dots via High-Temperature Cation Exchange in Molten Salts: The Importance of Molten Salt Speciation.

Colloidal semiconductor nanocrystals are an important class of materials which have many desirable optoelectronic properties. In their bulk phases, gallium- and aluminum-containing III-V materials such as GaAs, GaP, and Al1-xGaxAs represent some of the most technologically important semiconductors. However, their colloidal synthesis by traditional methods is difficult due to the high temperatures needed to crystallize these highly covalent materials and the extreme reactivity of Ga- and Al- precursors toward organic solvents at such high temperatures. A recently developed paradigm shift in the synthesis of these materials is to use molten inorganic salts as solvents to prepare Ga- containing III-V colloidal nanocrystals by cation exchange of the corresponding indium pnictide (InPn) colloidal nanocrystals. There have been several successful applications of molten salt solvents to prepare III-phosphide colloidal nanocrystals. However, little is known about the nature of these reaction environments at the relevant reaction conditions and synthesis of III-arsenide colloidal nanocrystals remains challenging. Herein we report a detailed study on cation exchange of InPn nanocrystals using nominally Lewis basic molten salt solvents with added gallium halides. Surprisingly, these salt systems phase separate into two immiscible phases, and the nanocrystals preferentially segregate to one of the phases. Using a suite of in situ spectroscopy tools, we identify the phase the nanocrystals segregate to as Lewis neutral alkali tetrahalogallate molten salts. We apply in situ high-temperature Raman spectroscopy to identify the chemical species present in several molten salt compositions at experimentally relevant reaction conditions to elucidate a molecular basis for the reactivity observed. We then employ Lewis neutral KGaI4 molten salts to prepare high-quality In1-xGaxAs and In1-xGaxP nanocrystals and demonstrate that deviation from Lewis neutral conditions accelerate nanocrystal decomposition in the case of III-arsenide materials. Further, we expand to KAlI4-based molten salts to prepare In1-x-yGaxAlyAs nanocrystals which represent an example of solution-synthesized quaternary III-V nanocrystals. These insights provide a molecular basis for the rational development of molten salt solvents, thus allowing the preparation of a diverse array of multicomponent III-V colloidal nanocrystals.

Designer phospholipid capping ligands for soft metal halide nanocrystals

The success of colloidal semiconductor nanocrystals (NCs) in science and optoelectronics is inextricable from their surfaces. The functionalization of lead halide perovskite NCs1–5 poses a formidable challenge because of their structural lability, unlike the well-established covalent ligand capping of conventional semiconductor NCs6,7. We posited that the vast and facile molecular engineering of phospholipids as zwitterionic surfactants can deliver highly customized surface chemistries for metal halide NCs. Molecular dynamics simulations implied that ligand–NC surface affinity is primarily governed by the structure of the zwitterionic head group, particularly by the geometric fitness of the anionic and cationic moieties into the surface lattice sites, as corroborated by the nuclear magnetic resonance and Fourier-transform infrared spectroscopy data. Lattice-matched primary-ammonium phospholipids enhance the structural and colloidal integrity of hybrid organic–inorganic lead halide perovskites (FAPbBr3 and MAPbBr3 (FA, formamidinium; MA, methylammonium)) and lead-free metal halide NCs. The molecular structure of the organic ligand tail governs the long-term colloidal stability and compatibility with solvents of diverse polarity, from hydrocarbons to acetone and alcohols. These NCs exhibit photoluminescence quantum yield of more than 96% in solution and solids and minimal photoluminescence intermittency at the single particle level with an average ON fraction as high as 94%, as well as bright and high-purity (about 95%) single-photon emission.

In Situ Sintering of CdSe/CdS Nanocrystals under Electron Beam Irradiation.

Colloidal semiconductor nanocrystals have attracted widespread attention due to their tremendous electrical and optical properties. Nanoparticles exhibit a strong tendency to aggregate and sinter in a short period of time during processing or use due to their large surface area-to-volume ratio, which may lead to significant changes in their required performance. Therefore, it is of great significance to conduct in-depth research on the sintering process and mechanism of nanoparticles to maintain their stability. Here, the sintering process of CdSe/CdS core/shell nanocrystals under continuous electron beam irradiation was studied using in situ transmission electron microscopy (TEM). In the early stages of sintering, CdSe/CdS nanocrystals approached each other at a distance of approximately 1-2 nm. As the exposure time to the electron beam increased, the movement of surface atoms on the nanocrystals led to contact between them. Subsequently, the atoms on the contact surfaces underwent rapid motion, resulting in the rapid formation of the neck between the particles. The neck formation between adjacent particles provides strong evidence of a sintering mechanism dominated by surface atom diffusion rather than Ostwald ripening. Further research in this area could lead to the development of improved methods to prevent sintering and enhance the stability of nanocrystals, ultimately contributing to the advancement of nanomaterial-based devices and materials with long-lasting performance.

Micrometer-Scale Carrier Transport in the Solid Film of Giant CdSe/CdS Nanocrystals Imaged by Transient Absorption Microscopy.

For the practical applications in solar cells and photodetectors, semiconductor colloidal nanocrystals (NCs) are assembled into a high-concentration film with carrier transport characteristics, the full understanding and effective control of which are critical for the achievement of high light-to-electricity conversion efficiencies. Here we have applied transient absorption microscopy to the solid film of giant CdSe/CdS NCs and discovered that at high pump fluences the carrier transport could reach a long distance of ∼2 μm within ∼30 ps after laser pulse excitation. This intriguing behavior is attributed to the metal-insulator transition and the associated bandlike transport, which are promoted by the enhanced electronic coupling among neighboring NCs with extended wave functions overlap of the excited-state charge carriers. Besides providing fundamental transport information in the regime of high laser pump fluences, the above findings shed light on the rational design of high-power light detecting schemes based on colloidal NCs.

ZnO Quantum Photoinitiators as an All-in-One Solution for Multifunctional Photopolymer Nanocomposites.

Nanocomposites are constructed from a matrix material combined with dispersed nanosized filler particles. Such a combination yields a powerful ability to tailor the desired mechanical, optical, electrical, thermodynamic, and antimicrobial material properties. Colloidal semiconductor nanocrystals (SCNCs) are exciting potential fillers, as they display size-, shape-, and composition-controlled properties and are easily embedded in diverse matrices. Here we present their role as quantum photoinitiators (QPIs) in acrylate-based polymer, where they act as a catalytic radical initiator and endow the system with mechanical, photocatalytic, and antimicrobial properties. By utilizing ZnO nanorods (NRs) as QPIs, we were able to increase the tensile strength and elongation at break of poly(ethylene glycol) diacrylate (PEGDA) hydrogels by up to 85%, unlike the use of the same ZnO NRs acting merely as fillers. Simultaneously, we endowed the PEGDA hydrogels with post-polymerization photocatalytic and antimicrobial activities and showed their ability to decompose methylene blue and significantly eradicate antibiotic-resistant bacteria and viral pathogens. Moreover, we demonstrate two fabrication showcase methods, traditional molding and digital light processing printing, that can yield hydrogels with complex architectures. These results position SCNC-based systems as promising candidates to act as all-in-one photoinitiators and fillers in nanocomposites for diverse biomedical applications, where specific and purpose-oriented characteristics are required.

(Invited) Photoluminescence up-Conversion in CsPbBr3 Nanocrystals

Photoluminescence up-conversion is a near ubiquitous process in colloidal semiconductor nanocrystals. Observed efficient up-conversion in high emission quantum yield CsPbBr3 nanocrystals suggest their possible use in optical refrigeration as well as light harvesting. We discuss efforts to understand the mechanism behind efficient up-conversion as well as studies into the identity of intermediate states responsible for the phenomenon. Here, photothermal measurements suggest real intermediate states deep in the gap that are responsible for efficient up-conversion.

(Invited) Effect of Cu Doping on the Energy Structure of Dumbbell-Shaped ZnS-AgInS2 Nanocrystals

Colloidal semiconductor nanocrystals (NCs) have been intensively developed for applications in photovoltaics, light-emitting diodes, electroluminescent devices, and biological markers, due to their tunable light absorption and excellent light emission properties. Among them, group I-III-VI-based multinary semiconductor NCs, such as CuInS2, CuInSe2 and AgInS2, have received significant attention for the application to solar energy conversion systems because of their large absorption coefficient and low toxicity. Recently, we have successfully prepared ZnS-AgInS2 solid solution (ZAIS) NCs and optimized their photocatalytic activity for hydrogen production by tuning their chemical composition, size and particle morphology.1-3 The photocatalytic activity of ZAIS NCs was further enhanced by the formation of a dumbbell structure that had a type-II heterojunction at the interface between the tip and rod parts of the NCs.4 In this study, to further improve the photocatalytic activity of dumbbell-shaped ZAIS NCs, we attempted to tune the energy structure of the heterojunction by Cu doping in tip parts for effective charge separation. Rod-shaped ZAIS NCs with sizes of 4.0 × 23 nm were prepared by the previously reported method.3 The crystal growth of tip parts was carried out by heating a mixture of rod-shaped ZAIS NCs, Cu(OAc), Ag(OAc), In(OAc)3, and thiourea in an organic solvent, in which rod-shaped ZAIS NCs acted as a nucleus to form tip parts composed of (Cu y Ag(1-y)In) x Zn2(1-x)S2 solid solution. Thus-obtained dumbbell-shaped NCs were isolated from the resulting solution via size-selective precipitation. The amount of Cu doping in tip parts was controlled by changing the Cu content in metal precursors. TEM observations showed that obtained NCs had a dumbbell structure regardless of the Cu content in the metal precursor. Ellipsoidal NCs with sizes of 5.6 × 11 nm were deposited on both tips of rod-shaped ZAIS NCs. The absorption onset of the dumbbell-shaped NCs were red-shifted from 650 nm to 850 nm with an increase in the Cu fraction from y = 0 to 0.75. The photoelectron yield spectroscopy revealed that the conduction band minimum level was constant at ca. -3.2 eV, while the valence band maximum level was gradually shifted from -5.1 eV to -4.8 eV as the Cu fraction in tip parts increased from y = 0 to 0.75. Thus, it is concluded that the absorption property of dumbbell-shaped ZAIS NCs could be successfully extended from the visible to the near-IR region without changing the energy of photogenerated electrons, simply by modulating the degree of Cu doping in the tip parts. This tunability is useful for constructing efficient solar-light-driven photocatalysts for H2 evolution. T. Torimoto, T. Adachi, K. Okazaki, M. Sakuraoka, T. Shibayama, B. Ohtani, A. Kudo, S. Kuwabata, J. Am. Chem. Soc., 129 (2007) 12388-12389.T. Kameyama, T. Takahashi, T. Machida, Y. Kamiya, T. Yamamoto, S. Kuwabata, T. Torimoto, J. Phys. Chem. C, 119 (2015) 24740-24749.T. Torimoto, Y. Kamiya, T. Kameyama, H. Nishi, T. Uematsu, S. Kuwabata, T. Shibayama, ACS Appl. Mater. Inter., 8, (2016) 27151-27161.T. Kameyama, S. Koyama, T. Yamamoto, S. Kuwabata, T. Torimoto, J. Phys. Chem. C, 122 (2018) 13705-13715.

Photoexcited Carrier Transfer in CuInS2 Nanocrystal Assembly by Suppressing Resonant-Energy Transfer.

High-density assemblies or superlattice structures composed of colloidal semiconductor nanocrystals have attracted attention as key materials for next-generation photoelectric conversion devices such as quantum-dot solar cells. In these nanocrystal solids, unique transport and optical phenomena occur due to quantum coupling of localized energy states, charge-carrier hopping, and electromagnetic interactions among closely arranged nanocrystals. In particular, the photoexcited carrier dynamics in nanocrystal solids is important because it significantly affects various device parameters. In this study, we report the photoexcited carrier dynamics in a solid film of CuInS2 nanocrystals, which is one of the potential nontoxic substitutes with Cd- and Pb-free compositions. Meanwhile, these subjects have been extensively studied in nanocrystal solids formed by CdSe and PbS systems. A carrier-hopping mechanism was confirmed using temperature-dependent photoluminescence spectroscopy, which yielded a typical value of the photoexcited carrier-transfer rate of (2.2±0.6)×107 s-1 by suppressing the influence of the excitation-energy transfer.