Tuning carrier mobilities and polarity of charge transport in films of CuInSe(x)S(2-x) quantum dots

Colloidal Synthesis of P-Type Zn3As2 Nanocrystals

Zinc pnictides, particularly Zn3As2, hold significant promise for optoelectronic applications owing to their intrinsic p-type behavior and appropriate bandgaps. However, despite the outstanding properties of colloidal Zn3As2 nanocrystals, research in this area is lacking because of the absence of suitable precursors, occurrence of surface oxidation, and intricacy of the crystal structures. In this study, a novel and facile solution-based synthetic approach is presented for obtaining highly crystalline p-type Zn3As2 nanocrystals with accurate stoichiometry. By carefully controlling the feed ratio and reaction temperature, colloidal Zn3As2 nanocrystals are successfully obtained. Moreover, the mechanism underlying the conversion of As precursors in the initial phases of Zn3As2 synthesis is elucidated. Furthermore, these nanocrystals are employed as active layers in field-effect transistors that exhibit inherent p-type characteristics with native surface ligands. To enhance the charge transport properties, a dual passivation strategy is introduced via phase-transfer ligand exchange, leading to enhanced hole mobilities as high as 0.089 cm2 V-1 s-1. This study not only contributes to the advancement of nanocrystal synthesis, but also opens up new possibilities for previously underexplored p-type nanocrystal research.

Quantum Dots, Passivation Layer and Cocatalysts for Enhanced Photoelectrochemical Hydrogen Production

Solar-driven photoelectrochemical (PEC) hydrogen production is one potential pathway to establish a carbon-neutral society. Nowadays, quantum dots (QDs)-sensitized semiconductors have emerged as promising materials for PEC hydrogen production due to their tunable bandgap by size or morphology control, displaying excellent optical and electrical properties. Nevertheless, they still suffer from anodic corrosion during long-term cycling, offering poor stability. This Review discussed advancements to improve long-term stability of QDs particularly in terms of cocatalysts and passivation layers. The working principle of PEC cells was reviewed, along with all important configurations adopted over recent years. The equations to assess PEC performance were also described. A greater emphasized was placed on QDs and incorporation of cocatalysts or passivation layers that could enhance the PEC performance by influencing the charge transfer and surface recombination processes.

Dome-shaped mode lasing from liquid crystals for full-color lasers and high-sensitivity detection

In this communication, we report a new class of oscillation mode, dome-shaped mode (DSM), in liquid crystal (LC) microlasers. A record high Q-factor over 24 000 is achieved in LC soft-matter microlasers. We successfully presented a proof-of-concept demonstration of red, green, blue (RGB) LC-DSM microlaser pixels with a 74% broader achievable color gamut than the standard RGB color space. Besides, the detection limit for acetone vapor molecules is as low as 0.5 ppm, confirming the excellent potential of the proposed LC-DSM microlaser in ultra-high sensitivity detection.

High-Mobility Hole Transport in Single-Grain PbSe Quantum Dot Superlattice Transistors

Epitaxially-fused superlattices of colloidal quantum dots (QD epi-SLs) may exhibit electronic minibands and high-mobility charge transport, but electrical measurements of epi-SLs have been limited to large-area, polycrystalline samples in which superlattice grain boundaries and intragrain defects suppress/obscure miniband effects. Systematic measurements of charge transport in individual, highly-ordered epi-SL grains would facilitate the study of minibands in QD films. Here, we demonstrate the air-free fabrication of microscale field-effect transistors (μ-FETs) with channels consisting of single PbSe QD epi-SL grains (2-7 μm channel dimensions) and analyze charge transport in these single-grain devices. The eight devices studied show p-channel or ambipolar transport with a hole mobility as high as 3.5 cm² V^-1 s^-1 at 290 K and 6.5 cm² V^-1 s^-1 at 170-220 K, one order of magnitude larger than that of previous QD solids. The mobility peaks at 150-220 K, but device hysteresis at higher temperatures makes the true mobility-temperature curve uncertain and evidence for miniband transport inconclusive.

Effects of Mono- and Bifunctional Surface Ligands of Cu-In-Se Quantum Dots on Photoelectrochemical Hydrogen Production

Semiconductor nanocrystal quantum dots (QDs) are promising materials for solar energy conversion because of their bandgap tunability, high absorption coefficient, and improved hot-carrier generation. CuInSe₂ (CISe)-based QDs have attracted attention because of their low toxicity and wide light-absorption range, spanning visible to near-infrared light. In this work, we study the effects of the surface ligands of colloidal CISe QDs on the photoelectrochemical characteristics of QD-photoanodes. Colloidal CISe QDs with mono- and bifunctional surface ligands are prepared and used in the fabrication of type-II heterojunction photoanodes by adsorbing QDs on mesoporous TiO₂. QDs with monofunctional ligands are directly attached on TiO₂ through partial ligand detachment, which is beneficial for electron transfer between QDs and TiO₂. In contrast, bifunctional ligands bridge QDs and TiO₂, increasing the amount of QD adsorption. Finally, photoanodes fabricated with oleylamine-passivated QDs show a current density of ~8.2 mA/cm², while those fabricated with mercaptopropionic-acid-passivated QDs demonstrate a current density of ~6.7 mA/cm² (at 0.6 V_RHE under one sun illumination). Our study provides important information for the preparation of QD photoelectrodes for efficient photoelectrochemical hydrogen generation.

Highly Efficient Photoelectrochemical Hydrogen Production Using Nontoxic CuIn1.5Se3 Quantum Dots with ZnS/SiO2 Double Overlayers

Quantum dots (QDs) are a promising material for photoelectrochemical (PEC) hydrogen (H₂) production because of their attractive optical properties including high optical absorption coefficient, band-gap tunability, and potential multiple exciton generation. To date, QDs containing toxic elements such as Cd or Pb have been mainly investigated for PEC H₂ production, which cannot be utilized in practice because of the environmental issue. Here, we demonstrate a highly efficient type II heterojunction photoanode of nontoxic CuIn_1.5Se₃ (CISe) QDs and a mesoporous TiO₂ film. In addition, ZnS/SiO₂ double overlayers are deposited on the photoanodes to passivate surface defect sites on the CISe QDs, leading to the enhancement of both photocurrent density and photostability. Due to a combination of a wide light absorption range of the CISe QDs and the reduced interfacial charge recombination by the overlayers, a remarkable photocurrent density of 8.5 mA cm^-2 (at 0.5 V_RHE) is obtained under 1 sun illumination, which is a record for the PEC sulfite oxidation based on nontoxic QD photoanodes.

Solution-processable integrated CMOS circuits based on colloidal CuInSe2 quantum dots

The emerging technology of colloidal quantum dot electronics provides an opportunity for combining the advantages of well-understood inorganic semiconductors with the chemical processability of molecular systems. So far, most research on quantum dot electronic devices has focused on materials based on Pb- and Cd chalcogenides. In addition to environmental concerns associated with the presence of toxic metals, these quantum dots are not well suited for applications in CMOS circuits due to difficulties in integrating complementary n- and p-channel transistors in a common quantum dot active layer. Here, we demonstrate that by using heavy-metal-free CuInSe₂ quantum dots, we can address the problem of toxicity and simultaneously achieve straightforward integration of complimentary devices to prepare functional CMOS circuits. Specifically, utilizing the same spin-coated layer of CuInSe₂ quantum dots, we realize both p- and n-channel transistors and demonstrate well-behaved integrated logic circuits with low switching voltages compatible with standard CMOS electronics.

Designing efficient toxic-element-free technologies in solution-processable CMOS electronics remains a challenge. Here, the authors demonstrate integrated logic CMOS circuits based on heavy-metal-free colloidal CuInSe₂ quantum dots with low switching voltages and with degradation-free performance on month-long time scales.

Spectroscopic and Magneto-Optical Signatures of Cu1+ and Cu2+ Defects in Copper Indium Sulfide Quantum Dots

Colloidal quantum dots (QDs) of I-III-VI ternary compounds such as copper indium sulfide (CIS) and copper indium selenide (CISe) have been under intense investigation due to both their unusual photophysical properties and considerable technological utility. These materials feature a toxic-element-free composition, a tunable bandgap that covers near-infrared and visible spectral energies, and a highly efficient photoluminescence (PL) whose spectrum is located in the reabsorption-free intragap region. These properties make them attractive for light-emission and light-harvesting applications including photovoltaics and luminescent solar concentrators. Despite a large body of literature on device-related studies of CISe(S) QDs, the understanding of their fundamental photophysical properties is surprisingly poor. Two particular subjects that are still heavily debated in the literature include the mechanism(s) for strong intragap emission and the reason(s) for a poorly defined (featureless) absorption edge, which often "tails" below the nominal bandgap. Here, we address these questions by conducting comprehensive spectroscopic studies of CIS QD samples with varied Cu-to-In ratios using resonant PL and PL excitation, femtosecond transient absorption, and magnetic circular dichroism measurements. These studies reveal a strong effect of stoichiometry on the concentration of Cu¹⁺ vs Cu²⁺ defects (occurring as Cu_In^″ and Cu_Cu^• species, respectively), and their effects on QD optical properties. In particular, we demonstrate that the increase in the relative amount of Cu²⁺ vs Cu¹⁺ centers suppresses intragap absorption associated with Cu¹⁺ states and sharpens band-edge absorption. In addition, we show that both Cu¹⁺ and Cu²⁺ centers are emissive but are characterized by distinct activation mechanisms and slightly different emission energies due to different crystal lattice environments. An important overall conclusion of this study is that the relative importance of the Cu²⁺ vs Cu¹⁺ emission/absorption channels can be controlled by tuning the Cu-to-In ratio, suggesting that the control of sample stoichiometry represents a powerful tool for achieving functionalities (e.g., strong intragap emission) that are not accessible with ideal, defect-free materials.

Air-Stable CuInSe2 Nanocrystal Transistors and Circuits via Post-Deposition Cation Exchange

Colloidal semiconductor nanocrystals (NCs) are a promising materials class for solution-processable, next-generation electronic devices. However, most high-performance devices and circuits have been achieved using NCs containing toxic elements, which may limit their further device development. We fabricate high mobility CuInSe₂ NC field-effect transistors (FETs) using a solution-based, post-deposition, sequential cation exchange process that starts with electronically coupled, thiocyanate (SCN)-capped CdSe NC thin films. First Cu⁺ is substituted for Cd²⁺ transforming CdSe NCs to Cu-rich Cu₂Se NC films. Next, Cu₂Se NC films are dipped into a Na₂Se solution to Se-enrich the NCs, thus compensating the Cu-rich surface, promoting fusion of the Cu₂Se NCs, and providing sites for subsequent In-dopants. The liquid-coordination-complex trioctylphosphine-indium chloride (TOP-InCl₃) is used as a source of In³⁺ to partially exchange and n-dope CuInSe₂ NC films. We demonstrate Al₂O₃-encapsulated, air-stable CuInSe₂ NC FETs with linear (saturation) electron mobilities of 8.2 ± 1.8 cm²/(V s) (10.5 ± 2.4 cm²/(V s)) and with current modulation of 10⁵, comparable to that for high-performance Cd-, Pb-, and As-based NC FETs. The CuInSe₂ NC FETs are used as building blocks of integrated inverters to demonstrate their promise for low-cost, low-toxicity NC circuits.

Enhanced Photocarrier Generation with Selectable Wavelengths by M-Decorated-CuInS2 Nanocrystals (M = Au and Pt) Synthesized in a Single Surfactant Process on MoS2 Bilayers

A facile approach for the synthesis of Au- and Pt-decorated CuInS₂ nanocrystals (CIS NCs) as sensitizer materials on the top of MoS₂ bilayers is demonstrated. A single surfactant (oleylamine) is used to prepare such heterostructured noble metal decorated CIS NCs from the pristine CIS. Such a feasible way to synthesize heterostructured noble metal decorated CIS NCs from the single surfactant can stimulate the development of the functionalized heterostructured NCs in large scale for practical applications such as solar cells and photodetectors. Photodetectors based on MoS₂ bilayers with the synthesized nanocrystals display enhanced photocurrent, almost 20-40 times higher responsivity and the On/Off ratio is enlarged one order of magnitude compared with the pristine MoS₂ bilayers-based photodetectors. Remarkably, by using Pt- or Au-decorated CIS NCs, the photocurrent enhancement of MoS₂ photodetectors can be tuned between blue (405 nm) to green (532 nm). The strategy described here acts as a perspective to significantly improve the performance of MoS₂ -based photodetectors with the controllable absorption wavelengths in the visible light range, showing the feasibility of the possible color detection.

Charge-Transport Mechanisms in CuInSe xS2- x Quantum-Dot Films

Colloidal quantum dots (QDs) have attracted considerable attention as promising materials for solution-processable electronic and optoelectronic devices. Copper indium selenium sulfide (CuInSe _xS_{2- x} or CISeS) QDs are particularly attractive as an environmentally benign alternative to the much more extensively studied QDs containing toxic metals such as Cd and Pb. Carrier transport properties of CISeS-QD films, however, are still poorly understood. Here, we aim to elucidate the factors that control charge conductance in CISeS QD solids and, based on this knowledge, develop practical approaches for controlling the polarity of charge transport and carrier mobilities. To this end, we incorporate CISeS QDs into field-effect transistors (FETs) and perform detailed characterization of these devices as a function of the Se/(Se+S) ratio, surface treatment, thermal annealing, and the identity of source and drain electrodes. We observe that as-synthesized CuInSe _xS_{2- x} QDs exhibit degenerate p-type transport, likely due to metal vacancies and Cu_In^'' anti-site defects (Cu¹⁺ on an In³⁺ site) that act as acceptor states. Moderate-temperature annealing of the films in the presence of indium source and drain electrodes leads to switching of the transport polarity to nondegenerate n-type, which can be attributed to the formation of In-related defects such as In_Cu^•• (an In³⁺ cation on a Cu¹⁺ site) or In_i^••• (interstitial In³⁺) acting as donors. We observe that the carrier mobilities increase dramatically (by 3 orders of magnitude) with increasing Se/(Se+S) ratio in both n- and p-type devices. To explain this observation, we propose a two-state conductance model, which invokes a high-mobility intrinsic band-edge state and a low-mobility defect-related intragap state. These states are thermally coupled, and their relative occupancies depend on both QD composition and temperature. Our observations suggest that the increase in the relative fraction of Se moves conduction- and valence band edges closer to low-mobility intragap levels. This results in increased relative occupancy of the intrinsic band-edge states and a corresponding growth of the measured mobility. Further improvement in charge-transport characteristics of the CISeS QD samples as well as their stability is obtained by infilling the QD films with amorphous Al₂O₃ using atomic layer deposition.

Compound Copper Chalcogenide Nanocrystals

This review captures the synthesis, assembly, properties, and applications of copper chalcogenide NCs, which have achieved significant research interest in the last decade due to their compositional and structural versatility. The outstanding functional properties of these materials stems from the relationship between their band structure and defect concentration, including charge carrier concentration and electronic conductivity character, which consequently affects their optoelectronic, optical, and plasmonic properties. This, combined with several metastable crystal phases and stoichiometries and the low energy of formation of defects, makes the reproducible synthesis of these materials, with tunable parameters, remarkable. Further to this, the review captures the progress of the hierarchical assembly of these NCs, which bridges the link between their discrete and collective properties. Their ubiquitous application set has cross-cut energy conversion (photovoltaics, photocatalysis, thermoelectrics), energy storage (lithium-ion batteries, hydrogen generation), emissive materials (plasmonics, LEDs, biolabelling), sensors (electrochemical, biochemical), biomedical devices (magnetic resonance imaging, X-ray computer tomography), and medical therapies (photochemothermal therapies, immunotherapy, radiotherapy, and drug delivery). The confluence of advances in the synthesis, assembly, and application of these NCs in the past decade has the potential to significantly impact society, both economically and environmentally.

Optoelectronic Properties of CuInS2 Nanocrystals and Their Origin

The capacity of fluorescent colloidal semiconductor nanocrystals for commercial application has led to the development of nanocrystals with nontoxic constituent elements as replacements for the currently available Cd- and Pb-containing systems. CuInS2 is a good candidate material because of its direct band gap in the near-infrared spectral region and large optical absorption coefficient. The ternary nature, flexible stoichiometry, and different crystal structures of CuInS2 lead to a range of optoelectronic properties, which have been challenging to elucidate. In this Perspective, the optoelectronic properties of CuInS2 nanocrystals are described and what is known of their origin is discussed. We begin with an overview of their synthesis, structure, and mechanism of formation. A complete discussion of the tunable luminescence properties and the radiative decay mechanism of this system is then presented. Finally, progress toward application of these "green" nanocrystals is summarized.

Highly Efficient Copper-Indium-Selenide Quantum Dot Solar Cells: Suppression of Carrier Recombination by Controlled ZnS Overlayers

Copper-indium-selenide (CISe) quantum dots (QDs) are a promising alternative to the toxic cadmium- and lead-chalcogenide QDs generally used in photovoltaics due to their low toxicity, narrow band gap, and high absorption coefficient. Here, we demonstrate that the photovoltaic performance of CISe QD-sensitized solar cells (QDSCs) can be greatly enhanced simply by optimizing the thickness of ZnS overlayers on the QD-sensitized TiO2 electrodes. By roughly doubling the thickness of the overlayers compared to the conventional one, conversion efficiency is enhanced by about 40%. Impedance studies reveal that the thick ZnS overlayers do not affect the energetic characteristics of the photoanode, yet enhance the kinetic characteristics, leading to more efficient photovoltaic performance. In particular, both interfacial electron recombination with the electrolyte and nonradiative recombination associated with QDs are significantly reduced. As a result, our best cell yields a conversion efficiency of 8.10% under standard solar illumination, a record high for heavy metal-free QD solar cells to date.