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

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.

Colloidal Inorganic Ligand-Capped Nanocrystals: Fundamentals, Status, and Insights into Advanced Functional Nanodevices

Colloidal nanocrystals (NCs) are intriguing building blocks for assembling various functional thin films and devices. The electronic, optoelectronic, and thermoelectric applications of solution-processed, inorganic ligand (IL)-capped colloidal NCs are especially promising as the performance of related devices can substantially outperform their organic ligand-capped counterparts. This in turn highlights the significance of preparing IL-capped NC dispersions. The replacement of initial bulky and insulating ligands capped on NCs with short and conductive inorganic ones is a critical step in solution-phase ligand exchange for preparing IL-capped NCs. Solution-phase ligand exchange is extremely appealing due to the highly concentrated NC inks with completed ligand exchange and homogeneous ligand coverage on the NC surface. In this review, the state-of-the-art of IL-capped NCs derived from solution-phase inorganic ligand exchange (SPILE) reactions are comprehensively reviewed. First, a general overview of the development and recent advancements of the synthesis of IL-capped colloidal NCs, mechanisms of SPILE, elementary reaction principles, surface chemistry, and advanced characterizations is provided. Second, a series of important factors in the SPILE process are offered, followed by an illustration of how properties of NC dispersions evolve after ILE. Third, surface modifications of perovskite NCs with use of inorganic reagents are overviewed. They are necessary because perovskite NCs cannot withstand polar solvents or undergo SPILE due to their soft ionic nature. Fourth, an overview of the research progresses in utilizing IL-capped NCs for a wide range of applications is presented, including NC synthesis, NC solid and film fabrication techniques, field effect transistors, photodetectors, photovoltaic devices, thermoelectric, and photoelectrocatalytic materials. Finally, the review concludes by outlining the remaining challenges in this field and proposing promising directions to further promote the development of IL-capped NCs in practical application in the future.

Trioctylphosphine accelerated growth of InP quantum dots at low temperature

Significant advance was realized on the economic synthesis of InP quantum dots (QDs) by using aminophosphines as phosphorus precursor. However, the low reaction activity and thermal degradation of aminophosphines bring severe difficulty for growth control of InP QDs. Here, we employed trioctylphosphine (TOP) as a surfactant to accelerate the growth of the InP QDs. The reaction mechanism study reveals that the TOP could form a reactive complex with indium halides that effectively accelerates the formation of InP monomer and reduces the demand for reaction temperature. On this basis, the effect of reaction temperature, precursors, and zinc halide additives on the growth of the TOP-InP QDs was explored. This strategy alleviates the difficulty in growth control of InP QDs and also benefits to the synthesis of luminescent InP/ZnS core-shell QDs within visible regime. A white-light emitting diode device was fabricated with the InP/ZnS QDs that demonstrates their application potential in light-emitting devices.

Impurities in Nanocrystal Thin-Film Transistors Fabricated by Cation Exchange

Cation exchange is a versatile tool used to alter the composition of nanostructures and thus to design next-generation catalysts and photonic and electronic devices. However, chemical impurities inherited from the starting materials can degrade device performance. Here, we use a sequential cation-exchange process to convert PbSe into CdSe nanocrystal thin films and study their temperature-dependent electrical properties in the platform of the thin-film transistor. We show that residual Pb impurities have detrimental effects on the device turn-on, hysteresis, and electrical stability, and as the amount increases from 2% to 7%, the activation energy for carrier transport increases from 38(3) to 62(2) meV. Selection and surface functionalization of the transistor's gate oxide layer and low-temperature atomic-layer deposition encapsulation of the thin-film channel suppress these detrimental effects. By conversion of the nanocrystal thin films layer upon layer, impurities are driven away from the gate-oxide interface and mobilities improve from 3(1) to 32(3) cm² V^-1 s^-1.

Enhanced Carrier Transport in Strongly Coupled, Epitaxially Fused CdSe Nanocrystal Solids

Strongly coupled, epitaxially fused colloidal nanocrystal (NC) solids are promising solution-processable semiconductors to realize optoelectronic devices with high carrier mobilities. Here, we demonstrate sequential, solid-state cation exchange reactions to transform epitaxially connected PbSe NC thin films into Cu₂Se nanostructured thin-film intermediates and then successfully to achieve zinc-blende, CdSe NC solids with wide epitaxial necking along {100} facets. Transient photoconductivity measurements probe carrier transport at nanometer length scales and show a photoconductance of 0.28(1) cm² V^-1 s^-1, the highest among CdSe NC solids reported. Atomic-layer deposition of a thin Al₂O₃ layer infiltrates and protects the structure from fusing into a polycrystalline thin film during annealing and further improves the photoconductance to 1.71(5) cm² V^-1 s^-1 and the diffusion length to 760 nm. We fabricate field-effect transistors to study carrier transport at micron length scales and realize high electron mobilities of 35(3) cm² V^-1 s^-1 with on-off ratios of 10⁶ after doping.

Transformations of Ionic Nanocrystals via Full and Partial Ion Exchange Reactions

ConspectusElaborate chemical synthesis methods allow the production of various types of inorganic nanocrystals (NCs) with uniform shape and size distributions. Many single-step synthesis approaches, such as the reduction of metal ions, the decomposition of metal complexes, double replacement reactions, and hydrolysis, have been adapted to promote the generation of monodisperse metal and ionic NCs. However, the question has become, how can we synthesize NCs with thermodynamically metastable phases or very complex structures? The transformation of already-synthesized NCs via elemental substitutions, such as ion exchange reactions for ionic NCs and galvanic replacement reactions for metal NCs, can overcome the difficulties facing conventional one-step syntheses. In particular, NC ion exchange reactions have been studied with numerous combinations of foreign ions and ionic NCs with various shapes. They have been investigated extensively because the reactions proceed under relatively mild conditions thanks to the large surface-to-volume ratio of the NCs relative to their bulk form. The functionality of the resulting ionic NCs, including semiconducting and plasmonic properties, can be easily tuned in a wide range, from the visible to near-infrared. Because anions generally have much larger ionic radii than cations within the frameworks of NCs, the cation exchange reactions proceed much faster than the anion exchange reactions. For ionic NCs above a critical size, the anion framework remains intact, and the original shape of the parent NCs is retained throughout the cation exchange reaction. In contrast, the anion exchange reaction often provides the new NCs with unique structures, such as hollow or anisotropically phase-segregated assemblies.This Account focuses on the full and partial ion exchange reactions involving ionic NCs, which have been thoroughly investigated by our group and others while highlighting important aspects such as the preservation of appearance and dimensions. First, we discuss how each type of ion exchange reaction progresses to understand the morphologies and crystal structures of their final products. This discussion is supported by emphasizing important examples, which help to explore the formation of NCs with thermodynamically metastable phases and complex structures, and other significant features of the ion exchange reactions leading to structure-specific functions. As a special case, we examine how the shape-dependent anionic framework (surface anion sublattice and stacking pattern) of polyhedral Cu₂O NCs determines the crystalline structure of the anion-exchanged products of hollow Cu_xS NCs. In addition, we review the characteristic anion exchange behavior of metal halide perovskite NCs observed in our laboratory and other laboratories. Finally, a general outline of the transformation of NCs via ion exchange reactions and future prospects in this field are provided.

Exploiting the Lability of Metal Halide Perovskites for Doping Semiconductor Nanocomposites

Cesium lead halides have intrinsically unstable crystal lattices and easily transform within perovskite and nonperovskite structures. In this work, we explore the conversion of the perovskite CsPbBr₃ into Cs₄PbBr₆ in the presence of PbS at 450 °C to produce doped nanocrystal-based composites with embedded Cs₄PbBr₆ nanoprecipitates. We show that PbBr₂ is extracted from CsPbBr₃ and diffuses into the PbS lattice with a consequent increase in the concentration of free charge carriers. This new doping strategy enables the adjustment of the density of charge carriers between 10¹⁹ and 10²⁰ cm^-3, and it may serve as a general strategy for doping other nanocrystal-based semiconductors.

Interplay between Chemical Transformations and Atomic Structure in Nanocrystals and Nanoclusters

ConspectusChemically induced transformations are postsynthetic processing reactions applied to first generation (as-synthesized) nanomaterials to modify property-defining factors such as atomic structure, chemical composition, surface chemistry, and/or morphology. Compared with conditions for direct synthesis of colloidal nanocrystals, postsynthetic chemical transformations can be conducted in relatively mild conditions with a more controllable process, which makes them suitable for the precise manipulation of nanomaterials and for trapping metastable phases that are typically inaccessible from the conventional synthetic routes. Each of the chemically induced transformations methods can result in substantial restructuring of the atomic structure, but their transformation pathways can be very different. And the converse is also true: the atomic structure of the parent material plays a large role in the pathway toward and the resulting chemically transformed product. Additionally, the characteristic length of the parent material greatly affects the structure, which affects the outcome of the reaction.In this Account, we show how the atomic structure and nanoscale size directs the product formation into materials that are inaccessible from analogous chemically transformations in bulk materials. Through examples from the three chemical transformation processes (cation/anion exchange, redox reactions, and ligand exchange and ligand etching), the effect of the atomic structure on chemical transformations is made apparent, and vice versa. For cation exchange, an anisotropic atomic lattice results in a unidirectional exchange boundary. And because the interface can extend through the full crystal, a substantial strain field can form, influencing the phase of the material. In the redox reaction that leads to the nanoscale Kirkendall effect, the atomic structure is the key to inverting the diffusion rates in a diffusion couple to form the hollow cores. And for ligand etching, if one of the materials in a heterostructure has a defected and\or defect-tolerant atomic structure, it can be preferentially etched and its atomic structure can undergo phase transformations while the other composition remains intact. For length scales, we show how the chemically induced transformations greatly differ between bulk, nanocrystal, and nanocluster characteristic sizes. For instance, the structural transformation on relatively large nanocrystals (2-100 nm) can be a continuous process when the activation volume is smaller than the nanocrystal, while for smaller nanoclusters (<2 nm) the transformation kinetics could be swift resulting in only discrete thermodynamic states. Comparing the two nanosystems (nanocrystals to small nanoclusters), we address how their atomic structural differences can direct the divergent transformation phenomena and the corresponding mechanisms. Understanding the nanoscale mechanisms of chemically induced transformations and how they differ from bulk processes is key to unlocking new science and for implementing this processing for functional materials.

Manipulating Electronic Structure from the Bottom-Up: Colloidal Nanocrystal-Based Semiconductors

Semiconductors assembled from colloidal nanocrystals (NCs) are often described in the same terms as their single-crystalline counterparts with references to conduction and valence band edges, doping densities, and electronic defects; however, how and why semiconductor properties manifest in these bottom-up fabricated thin films can be fundamentally different. In this Perspective, we describe the factors that determine the electronic structure in colloidal NC-based semiconductors, and comment on approaches for measuring or calculating this electronic structure. Finally, we discuss future directions for these semiconductors and highlight their potential to bridge the divide between localized quantum effects and long-range transport in thin films.

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.

Metastable CdTe@HgTe Core@Shell Nanostructures Obtained by Partial Cation Exchange Evolve into Sintered CdTe Films Upon Annealing

Partial Hg²⁺ → Cd²⁺ cation exchange (CE) reactions were exploited to transform colloidal CdTe nanocrystals (NCs, 4-6 nm in size) into CdTe@HgTe core@shell nanostructures. This was achieved by working under a slow CE rate, which limited the exchange to the surface of the CdTe NCs. In such nanostructures, when annealed at mild temperatures (as low as 200 °C), the HgTe shell sublimated or melted and the NCs sintered together, with the concomitant desorption of their surface ligands. At the end of this process, the annealed samples consisted of ligand-free CdTe sintered films containing an amount of Hg²⁺ that was much lower than that of the starting CdTe@HgTe NCs. For example, the CdTe@HgTe NCs that initially contained 10% of Hg²⁺, after being annealed at 200 °C were transformed to CdTe sintered films containing only traces of Hg²⁺ (less than 1%). This procedure was then used to fabricate a proof-of-concept CdTe-based photodetector exhibiting a photoresponse of up to 0.5 A/W and a detectivity of ca. 9 × 10⁴ Jones under blue light illumination. Our strategy suggests that CE protocols might be exploited to lower the overall costs of production of CdTe thin films employed in photovoltaic technology, which are currently fabricated at high temperatures (above 350 °C), using post-process ligand-stripping steps.

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.

Tailoring Cu+ for Ga3+ Cation Exchange in Cu2-xS and CuInS2 Nanocrystals by Controlling the Ga Precursor Chemistry

Nanoscale cation exchange (CE) has resulted in colloidal nanomaterials that are unattainable by direct synthesis methods. Aliovalent CE is complex and synthetically challenging because the exchange of an unequal number of host and guest cations is required to maintain charge balance. An approach to control aliovalent CE reactions is the use of a single reactant to both supply the guest cation and extract the host cation. Here, we study the application of GaCl₃-L complexes [L = trioctylphosphine (TOP), triphenylphosphite (TPP), diphenylphosphine (DPP)] as reactants in the exchange of Cu⁺ for Ga³⁺ in Cu_2-xS nanocrystals. We find that noncomplexed GaCl₃ etches the nanocrystals by S^2- extraction, whereas GaCl₃-TOP is unreactive. Successful exchange of Cu⁺ for Ga³⁺ is only possible when GaCl₃ is complexed with either TPP or DPP. This is attributed to the pivotal role of the Cu_2-xS-GaCl₃-L activated complex that forms at the surface of the nanocrystal at the onset of the CE reaction, which must be such that simultaneous Ga³⁺ insertion and Cu⁺ extraction can occur. This requisite is only met if GaCl₃ is bound to a phosphine ligand, with a moderate bond strength, to allow facile dissociation of the complex at the nanocrystal surface. The general validity of this mechanism is demonstrated by using GaCl₃-DPP to convert CuInS₂ into (Cu,Ga,In)S₂ nanocrystals, which increases the photoluminescence quantum yield 10-fold, while blue-shifting the photoluminescence into the NIR biological window. This highlights the general applicability of the mechanistic insights provided by our work.