I will describe the synthesis of colloidal GaP and InP nanocrystals in organic surfactant solution.  Novel precursor conversion reactivity can be used to control the formation of solutes at a desired reaction temperature and to govern their concentration structure.  That approach provides large (10-20 nm), highly crystalline GaP under mild conditions using convetional surfactant media.  The high quality of the products is confirmed by their powder diffraction and Raman spectra which are narrower than previous reports under such conditions.  The importance of the reaction temperature, solute composition, and the conversion reactivity to the final nanocrystal product will be discussed.  The results will be used to highlight the central role of crystal growth in size and polydispersity control mechanisms. Moreover, the results highlight the essential role of precursor reactivity on the reproducibility of the synthesis and the value of carefully designing and tailoring reagents that afford control over that reactivity.

Facet homogeneity in III-V nanocrystals is a critical factor for enhancing their optoelectronic properties and chemical stability. This talk explores recent advancements in synthesizing well-defined tetrahedral and tetrapodal geometries in indium-based III-V nanocrystals, emphasizing the facet-specific surface chemistry and its implications. By integrating halide-amine co-passivation strategies and advanced nuclear magnetic resonance analyses, we unveil the correlation between facet-dependent ligand dynamics and surface reactivity. These insights enable precise control over surface passivation, critical for reducing surface heterogeneity and enhancing photophysical properties. The discussion extends to the role of (111) facets in quantum confinement and their impact on device performance, including extended photoresponse in tetrahedral quantum dots. This comprehensive understanding of facet homogeneity not only addresses long-standing challenges in III-V nanocrystal synthesis but also opens new pathways for their application in next-generation optoelectronics.

Reference

[1] Semiconductor Nanocrystals: Unveiling the Chemistry behind Different Facets Acc. Chem. Res. 2023, 56, 1756

[2] Unraveling the Facet-dependent Surface Chemistry at Molecular Scale: Photo-assisted Oxidation of InP Nanocrystals J. Am. Chem. Soc. 2024, 146, 46, 31691

[3] Surface-Originated Weak Confinement in Tetrahedral Indium Arsenide Quantum Dots, J. Am. Chem. Soc. 2024, 146, 15, 10251

Indium phosphide quantum dots (QDs) have become the workhorse for visible light emission within the past decade and today, green- and red-emitting InP QDs are extensively used for color conversion in commercial displays. On the other hand, it turned out highly challenging to achieve efficient and narrow emission in the blue range around 450-470 nm, which remains an active field of research.

With a bulk band gap of 1.35 eV, InP QDs could be also of high interest for the near-infrared range, e.g., for in vivo biological imaging. However, for achieving an emission beyond 700 nm, large particle sizes > 7 nm are required, which turned out difficult to synthesize with established methods involving indium(III) halides or carboxylates and silyl- or aminophosphine precursors. While exploring the use of indium(I) halides for double use as the indium precursor and reducing agent of aminophosphine, we found that large tetrahedral InP QDs with edge lengths of around 10 nm could be obtained.[1] After overcoating with ZnS or ZnSe/ZnS shells, these QDs exhibit narrow NIR emission at wavelengths up to 730 nm. Additional coating with an alumina shell resulted in excellent chemical stability, demonstrated by transferring the QDs to the aqueous phase via surface ligand exchange while maintaining their photoluminescence quantum yield of around 40%.[2]

Going further in the near- and short-wave infrared range opens up a large space of additional applications for III-V QDs in various fields such as night-vision, plastic sorting, agriculture, surveillance and consumer electronics. Nonetheless, narrow bandgap III-V materials have been much less explored than lead chalcogenide QDs (PbS, PbSe) due to synthetic challenges related to their more covalent character, the scarcity of appropriate group-V precursors, and their high oxidation sensitivity. We extended the indium(I) halide / aminopnictogen synthetic platform to InAs and InSb QDs, which gave access to wavelengths up to 2 µm.[3] After overgrowth with appropriate shell materials, they can also act as efficient NIR/SWIR emitters.[4] 

Quantum dots (QDs) made of III-V semiconductors have been investigated for many years as a more sustainable alternative for cadmium, lead or mercury based chalcogenides for applications involving visible, short-wave infrared or mid-wave infrared light. Led by progress in QD synthesis, these efforts have mainly focused on In-based materials, including InP, InAs and InSb. These In-based pnictides are all direct band gap semiconductors covering – when accounting for size quantization – a spectral range from the mid IR to the edge of the visible. Recently, synthetic methods for Ga-based pnictides have become available. GaP, however, is an indirect semiconductor, and GaAs and GaSb only cover a relatively narrow spectral range. Hence the question what to expect from Ga-based QDs for opto-electronic applications.

In this presentation, we introduce Bloch orbital expansion as a novel and unconventional computational approach to relate geometry and electronic structure in quantum dots. The method is based on the projection of the QD orbitals on bulk Bloch orbitals, and comparing the resulting QD fuzzy band structure with the bulk band structure computed at the same level of theory. Using this approach, strongly confined, delocalized QD orbitals will overlap with the bulk bands, while QD orbitals derived from bulk surface states will deviate from the bulk bands. Most notably, mid-gap surface states, which are most detrimental for the performance of opto-electronic devices, can be readily identified as falling within the bulk bands. Importantly, when using density functional theory (DFT) to compute QD orbitals for a given QD geometry, this approach provides a direct link between the QD surface termination and the appearance of such surface states.

In a first step, we apply the method to models of PbS, HgTe and HgSe QDs 3-4 nm in size. Interestingly, for all these QDs, we demonstrate that the orbitals are free from coupling to bulk surface states. This finding is rooted in the bulk band structure of these materials, and may explain why films of such QDs truly behave as printed semiconductors. Next, we use the coupling of QD orbitals to bulk surface states as an intrinsic quality-control method to screen the promise of different In- and Ga pnictides as an alternative to restricted Cd, Pb or Hg compounds. Using a fixed, 1116 atom QD model with chloride passivated (100) and (111) facets, we show that InP QDs exhibit a broad band of occupied surface states. These orbitals are related to the P-rich (-111) facets, and extend several 100 meV above the valence-band edge. Such a result can be expected for semiconductors with a p-type valence-band and an s-type conduction-band edge, and reflects charge accumulation at the (-111) facet. In line with this interpretation, we observe a gradual suppression of the coupling of QD orbitals with bulk surface states when reducing the difference in electronegativity between the anion and the cation. In particular, the frontier orbitals of GaAs and GaSb appear as delocalized states, which suggests that these compounds could be used as printed semiconductors with properties superior to In-based equivalents. We end by discussing the impact of these findings for research in III-V QDs. 

Colloidal quantum dots (QDs) are a versatile class of materials with notable potential in optoelectronics. Offering tunable optical and electronic properties and solution-processable fabrication, QDs have been widely explored in applications such as LEDs and photodetectors (PDs), where they can be used as printable inks. QDPDs are particularly interesting in the short wave infrared (SWIR) region (1 – 2µm), which is critical for many applications, such as environmental monitoring, biomedical and adverse weather imaging, and telecommunications. Current SWIR devices rely on costly fabrication techniques, but the integration of solution-processed QDs could substantially reduce the manufacturing costs of SWIR imagers, making them appealing for the consumer market.

Yet, efficient application of QDs to SWIR devices has been restrained to the use of lead sulfide (PbS), and mercury telluride (HgTe), which face significant regulatory restrictions due to their hazardous nature. In recently years, QDPDs based on III-V compounds, particularly In(As,P), have gained attention as a cost-effective, solution-based, and regulatory-compliant alternative for SWIR PDs, driving improvements on their synthetic chemistry, surface passivation and optimization of device structures. However, thus far, the performance metrics, such as quantum efficiency and dark current, of In(As,P) QDPDs have remained subpar relative to PbS-based devices, and have not significantly benefited from ligand engineering, suggesting other factors affect device performance.

Here, we investigate the relation between the properties of the In(As,P) QDs and the PD performance. QDPDs are fabricated using an established ligand exchange chemistry, involving the replacement of oleylamine and chloride by mercaptopropanediol and butylamine [1]. Current-voltage (I-V) measurements show that the resulting QDPD stacks are rectifying, and attain an external quantum efficiency (EQE) of up to 10-20% at 1050 nm. Impedance measurements are used to obtain deeper insight in the semiconductor characteristics of the In(As,P) QD film and the voltage distribution across the stack. A striking observation is the high dark current under reverse bias, which increases with adaptations to the stack that enhance the EQE. By combining temperature-dependent I-V measurements and transient absorption spectroscopy, we propose thermal generation of charge carriers within the QD film as the main source of dark current, and we discuss the prospects of adapting the QD surface termination so as to reduce dark current and enhance detectivity of PDs based on III-V QDs.

Quantum dots (QDs) are nanometer-scale semiconductors with tunable bandgaps, making them ideal for optoelectronic devices like LEDs, photodetectors, lasers, and bioimaging tools. Their strong light absorption, tunable properties, and suitability for solution-based processing have driven interest in materials like lead halide perovskites, and cadmium, lead, and mercury-based chalcogenide QDs. However, their use is limited by toxic heavy metals, restricted by RoHS regulations. This has spurred demand for environmentally friendly alternatives, such as III-V QDs—particularly indium phosphide (InP) and indium antimonide (InSb)—which offer a wide bandgap range, high electron mobility, and strong covalent bonds. Despite their potential, the development of III-V QDs has faced challenges, including issues with precursor availability, high nucleation temperatures, and polydispersity.[1]

In our research, we have made significant advancements in the colloidal synthesis and surface chemistry of InP and InSb QDs.[2]  Using a heating-up method, we synthesized high-quality, monodisperse In-based QDs with size-tunable absorption features spanning from the visible to short-wave infrared range (445–1980 nm). To address the challenge of size tunability in InSb QDs, we developed various approaches to control their dimensions. For instance, different metal halides (InX₃, SbX₃, where X = Cl, Br, I) were employed as In and Sb precursors, with metal iodides producing the smallest InSb QDs among all tested halides. Additionally, the In-to-Sb ratio and the concentration of the reducing agent (super hydride) significantly influenced the QD size. A higher In/Sb ratio yielded larger QDs, while a higher concentration of super hydride resulted in smaller QDs, and vice versa. The resulting InSb QDs exhibited excellent colloidal and optical stability in non-polar solvents after four months. To enable their integration into highly conductive optoelectronic devices, we successfully exchanged the organic ligands of these QDs with various inorganic ligands, including metal halides, metal chalcogenides, and metal chalcogenide complexes. We elucidated the mechanisms behind the ligand exchange processes, facilitating the creation of QD inks capped with inorganic ligands. These inks were subsequently used to fabricate field-effect transistors, which exhibited enhanced conductivity. Our work marks a significant step in developing high-performance III-V-based optoelectronic devices, particularly in the infrared spectrum.

Colloidal quantum dots (QDs) offer a cost-effective and scalable solution for manufacturing thin-film semiconductors with tunable band-gaps. Among them, certain infrared-active QDs have demonstrated exceptional potential for sensing applications beyond the capabilities of conventional silicon-based sensors. Notably, QDs such as PbS, PbSe, HgTe, Ag₂Te, InAs, and InSb have been extensively studied, with some, like PbS QDs, already reaching commercialization for short-wave infrared (SWIR) applications. However, the development of these materials varies, with ongoing research focused on enhancing their performance and industrial viability. As one of the pioneers in the commercialization of QDs for SWIR sensors, we would like to provide an overview of the progress in colloidal QD sensor technology for SWIR applications. We will begin by examining the achievements of PbS QDs, addressing their limitations in industrial applications. We will then explore the current advancements in III-V QDs, with a particular focus on InAs QDs and their potential to meet industry demands and drive the next generation of SWIR sensing technologies.

InSb has a larger Bohr radius and thus offers a wider range of infrared wavelengths compared to other III-V's. Provided the complexity of achieving good shape and size control for large QDs, InSb can stay within a smaller size regime to achieve the 1400-1500 nm wavelengths of interest. 

Here the synthetic challenges associated with InSb CQDs are investigated and it is found that uncontrolled reduction of the antimony precursor hampers the controlled growth of CQDs. To overcome this, a synthetic strategy that combines nonpyrophoric precursors with zinc halide additives is developed. The experimental and computational studies show that zinc halide additives decelerate the reduction of the antimony precursor, facilitating the growth of more uniformly sized CQDs. The halide choice provides additional control over the strength of this effect. 

I will also discuss our computational efforts in understanding the surface structure of InSb dots and ligand exchanges to reduce surface oxidation.

Our further efforts are focused on understanding the nucleation process that can lead to better monodispersity. Specifically, I will discuss the kinetic models that include both anions and cations as well as real-space models that consider the geometric shape effects on nucleation and growth.

Metal oxides, like ZrO₂ and HfO₂, and fluorides (e.g. NaYF₄ and NaGdF₄), are two classes of nanocrystals serving as hosts for optically active lanthanides ions (e.g. europium).^1-5 While doped fluorides are widely studied and their syntheses are well developed, the oxide hosts struggle with the synthetic challenge of producing colloidally stable nanocrystals with a complex architecture (as for example core/shells). In this work, we pioneer the synthesis of metal oxide core/shell nanocrystals where HfO₂ epitaxially grows onto ZrO₂, confirmed by high resolution transmission electron microscopy images and energy-dispersive Xray spectroscopy compositional maps. Furthermore, the beneficial effect of the shell on the optical properties is established by investigating the photoluminescence of ZrO₂:Eu and of ZrO₂:Eu/ZrO₂ after growing a protective zirconia shell on it. After shelling, the lifetime of the europium doped zirconia nanocrystals increases to 5.3 ± 0.1 ms, showing that we successfully shut down non-radiative pathways. The long lifetime is unprecedented for the high doping percentage (9% Eu). The method has been demonstrated to improve the local environment of europium dopants in zirconia nanocrystals, giving access to novel heterostructures with improved optical properties. This opens up possibilities for their application in different areas, from microelectronics to scintillators.

Achieving high photoluminescence quantum yield (PLQY) in magic-sized clusters (MSCs) of III-V semiconductors such as indium phosphide (InP) remains a significant challenge due to their strong oxophilicity and high sensitivity to surface defects. Here, we report the synthesis of highly luminescent InP MSCs through a novel kinetically controlled surface fluorination strategy. Utilizing the Friedel–Crafts acylation reaction, we generated hydrogen fluoride (HF) in a controlled manner, enabling effective surface passivation. This approach mitigated non-radiative recombination pathways by removing surface oxides and stabilizing surface defects, resulting in a PLQY of ~18%, the highest reported for InP MSCs. Comprehensive analyses, including PL lifetime measurement, transient absorption spectroscopy, and X-ray photoelectron spectroscopy, revealed that the enhanced luminescence arises from reduced surface trap states. Structural integrity and uniformity were confirmed through X-ray diffraction, Raman spectroscopy, and extended X-ray absorption fine structure analysis, demonstrating the preservation of the zinc-blende MSC framework. These findings not only advance the understanding of III-V MSCs but also highlight the potential of InP MSCs as environmentally benign, monodisperse, and highly efficient emissive materials for next-generation optoelectronic applications.

Advances in the synthesis of III-V quantum dots, including InP and InAs, have led to their development for current- and next-generation solid-state lighting, wide color gamut displays, and infrared optoelectronics. The most widely adopted synthesis of these III-V quantum dots involves indium carboxylates and E(SiMe₃)₃ (E = P, As) and is understood to proceed through the formation of metastable, atomically-precise intermediates that are often referred to as clusters. In this work, we investigate the synthesis and growth pathways of III-V clusters to draw conclusions about kinetic differences in their formation. Along with synthetic experimentation, we analyze the single-crystal X-ray diffraction structures of some of the reported intermediates including In₃₇P₂₀(O₂CR)₅₁, In₂₆P₁₃(O₂CR)₃₉, and In₂₆As₁₈(O₂CR)₂₄(PR^’₃)₃.[1],[2] The structural similarities between these materials and other II-VI materials of similar morphology have strong implications for understanding the landscape of accessible binary semiconductor clusters. Modifying these structures through the introduction of dopants has allowed for the formation of desirable, complex compositions including manganese, cobalt, and molybdenum. We find that the addition of L-type amine modifies the surface of these materials and leads to an amorphous single-source precursor primed for the formation of doped-InP nanomaterials. Expanding the library of cations in conjunction with exploring the basicity of the dopant-assisting L-type ligand could lead to alloys that are difficult to achieve with more conventional colloidal techniques.

InP-based quantum dots (QDs) represent the major commercial success of colloidal semiconductor nanocrystals (NCs). A combination of the robust, mostly covalent, crystal structure and the non-toxic nature of the constituent elements makes them a QD material of choice for cutting-edge display and LED technologies.[1,2] Despite successful commercial realization, InP NCs lack convenient synthesis chemistry, as illustrated by a resent quest to substitute commonly used pyrophoric and expensive tris(trimethylsilyl)phosphine precursor.[3-5] Herein, we propose solid, non-pyrophoric, and synthetically easily accessible acylphosphines as convenient phosphorus precursors for the synthesis of InP QDs. When combined with suitable anionic nucleophiles, such as arylthiolates, both triacylphosphines and indium complexes of bisacylphosphines act as efficient sources of P^3- anion, as corroborated by the results of NMR spectroscopy and powder XRD studies. This type of reactivity is utilized to synthesize uniform colloidal InP QDs with well-defined and tunable (460 – 600 nm) excitonic features in their absorption spectra. The final NCs size is controlled by the nature of acyl substituents and by the use of either indium or zinc long-chain carboxylates as ligands. Such adjustable precursor reactivity offers an improved control over the colloidal synthesis of InP, potentially opening a pathway to diverse InP-based hetero-nanostructures and InP NCs of anisotropic shapes. Furthermore, the proposed chemistry should be readily extendable to the synthesis of other metal phosphide and metal arsenide NCs.

Colloidal InAs quantum dots (QDs) are gaining increasing interest as optimal infrared (IR) absorbers and emitters for the next-generation optoelectronic IR commercial devices.[1] This is due to their RoHS compliance and the tunability of their optical bandgap, which can be adjusted from approximately 700 nm to over 1600 nm. To date, the most advanced synthesis strategy for InAs QDs relies on pyrophoric, toxic and costly tris-trimethylsilyl arsine (or derivatives).[2] To reduce the cost and hazardousness of InAs QD production, several less toxic and more affordable arsenic precursors have been investigated in recent years, with tris(dimethylamino)arsine (amino-As) emerging as the most promising one.[3]

The current challenge is twofold: first, to enhance control over amino-As InAs quantum dots (QDs) to fine-tune their size and size distribution, thereby producing QDs with a tunable and narrow excitonic absorption peak; second, to develop tailored InAs@shell core@shell heterostructures with customized shell materials, achieving high efficiency as infrared (IR) emitters.

In this talk, I will describe our recent efforts to address these challenges. This involved further advancing our recently published procedure for synthesizing InAs@ZnSe QDs, utilizing amino-As and ZnCl₂ as an additive.[4,5]

Metal halide perovskites (MHPs) possess a remarkable combination of properties that make them highly attractive for a wide range of optoelectronic applications. Their outstanding optical indicators such as high absorption coefficient, tunable bandgap through compositional adjustments, and strong photoluminescence are complemented by the low-cost, solution-based fabrication methods that enable scalable and economically viable production. One of the major challenges with MHPs is their inherent sensitivity to moisture, heat, and UV light, which raises concerns about their long-term stability in practical applications [1], [2]. Understanding the underlying mechanisms of degradation is therefore crucial.

In this context, a combination of in situ and ex situ electron microscopy techniques provide invaluable input. We conducted in situ experiments using both transmission electron microscopy (TEM) and scanning electron microscopy (SEM) at different scales, and compared our findings with ex situ results to corroborate the degradation pathways. In this manner, we could precisely characterize the black-to-yellow phase transition in both CsPbI₃ films and nanocrystals (NCs). Yellow phase CsPbI₃ is formed as hollow microtubes elongated along the {100} plane after NC agglomeration and cannot be transformed back into the black phase upon heating at 350 ⁰C, unlike films. Notably, our results indicate that oxygen does not play a significant role in the degradation process. Instead, one-time exposure to H₂O vapor is sufficient to initiate the detrimental phase transformation. For films, this transformation is further complicated by the formation of side phases, particularly inside pinholes. The combination of H₂O vapor and heat leads to the formation of PbO regions and decomposition of the CsPbI₃ NCs into PbI₂ (P63mc) and CsI.

Obtaining high-resolution details of these crystal phase changes is extremely challenging due to the electron beam sensitivity of MHPs, often leading to Pb/PbX₂-clusters formation or amorphization. Significant efforts have therefore been directed towards developing low-dose imaging protocols, e.g. based on “4D STEM” using event-driven direct electron detectors. Using doses of less than 500 e/Ų, we could characterize the local structure defects of MHPs. Moreover, this approach enables to visualize light elements within the perovskite lattice. As such, we observed for the first time the coexistence of CsPbI₃ and CsPbCl₃ domains, as well as the presence of mixed CsPb(I,Cl)₃ phases in perovskite/chalcohalide heterostructures.

A quantitative interpretation of TEM data is especially critical in the characterization of structural defects. For instance, stacking faults in MHPs frequently result in the formation of Ruddlesden-Popper (RP) phases. By applying statistical parameter estimation theory [3] together with molecular dynamics simulations, we have been able to quantify total column intensities and the probabilities that atomic columns belong to either the RP defect phase or the perovskite phase. This detailed analysis was vital in the study of CsPbI₃ nanocrystals (NCs), where RP-like phases were induced by dopants. The presence of these RP-like phases was correlated with phase stability measurements, offering valuable insights into the relationship between defect formation and the long-term structural stability of the material.

Finally, the influence of nanoparticle shape on the optical properties cannot be overlooked. However, 3D characterization methods such as electron tomography are electron-dose expensive. We have therefore quantified the number of atoms from a single projection and consequently modeled the shapes of perovskite NCs. Such advancements are crucial for understanding and optimizing the performance of perovskite-based devices, where both structural and morphological parameters influence their function.