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New Materials for Colloidal Nanochemistry/Trendbericht Physikalische Chemie 2024 (2/3)

Nachrichten aus der Chemie, Mai 2024, S. 56-60, DOI, PDF. Login für Volltextzugriff.

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Zeitversetzte Laserpulse auf Moleküle verraten, wie photochemische Prozesse ablaufen – dabei sind konische Durchschneidungen entscheidend. Die kolloidale Nanochemie hat neue Materialklassen erschlossen, wie Nickelboride und frühe Übergangsmetalloxid-Katalysatoren, sowie Synthesen zu photolumineszenten III–V-Halbleitern verbessert. Mit maschinellem Lernen lassen sich am Rechner Proteine nicht mehr nur falten, sondern solche entwerfen, die nicht mit natürlichen Proteinen verwandt sind.

New Materials for Colloidal Nanochemistry

The Nobel Prize in Chemistry 2023 put research on nanoparticles back into the spotlight, with special recognition for the hot injection synthesis pioneered by Murray, Norris and Bawendi in 1993.1) This method opened up the avenues to study size-dependent effects in semiconductor nanocrystals and to a widely applicable route to nanoparticles. Research in the field has since mainly focused on the classic nanomaterials: semiconductors (e.g. CdSe, PbS), metals with plasmonic properties (Au, Ag) and magnetic particles (e.g. Fe3O4, CoPt). From the first experiments in the 1980s, physical questions drove nanoparticle research: from the need to narrow down the size distribution to observe quantum effects on the level of a nanocrystal ensemble to the refinement of nucleation theory. This is reflected by the fact that often groundbreaking physical studies were performed before full understanding of the chemistry behind particle formation was gained. The field is now well established and significant progress has been made to understand the chemistry. Hence, new material classes are being tackled again, fuelled by new research groups, and some old ones, that aim to carve out a new niche in the field for themselves.

III–V semiconductors

One of the earliest cases in which the community strayed from the classic materials are III–V semiconductors. Their synthesis has been mired by their highly covalent bond character, making the hot injection synthesis harder for such semiconductors than II-VI and IV-VI semiconductors. This is because the nucleation and growth kinetics lack the required dynamic equilibrium that allows particles to end up with a narrow size distribution and uniform crystallinity. Driven by the need for cadmium-free nanomaterials, III–V semiconductors have caught up: Samsung, the current market leader in quantum dot-based QLED displays, now bases its display technology on InP/ZnSeS core-shell quantum dots.2)

The Talapin group at the University of Chicago crucially advanced the field by annealing as-synthesised III–V semiconductor particles into crystalline and defect-poor quantum dots. They accomplished this through a phase transfer from the high-boiling organic solvents of the hot-injection synthesis into eutectic salt mixtures (e.g. AlCl3/NaCl/KCl) that allow for temperatures over 400 °C. These conditions mimic those of chemical vapour deposition or molecular beam epitaxy processes that yield such semiconductors in high quality.3,4) The resulting particles are photoluminescent, which in colloidal III–V nanocrystals has been absent or suppressed before. Additionally, the work provides conditions for stable colloidal dispersions. This is because the classical theory for the steric and electrostatic stabilization of colloids does not account for the strong interactions between ions in molten salts.5,6) So far, the absorption spectra of III–V nanocrystals have mostly lacked a clear, excitonic structure Several reasons for this have been proposed: a high defect density, an indirect band gap, or a broad distribution of size, shape or atomic structure. Recently, the Talapin group showed that Raman spectra of as-synthesised GaAs particles possess a high degree of disorder, similar to amorphous regions in GaAs wafers produced by ion bombardment. This disorder, however, does not translate into reflexes in the X-ray diffractograms that are broadened beyond a linewidth in accordance with small crystalline particles (Figure 5A,B).7) According to EPR spectroscopy, the disorder is attributed to many Ga vacancies (Fig. 5C), inferred from the low hyperfine coupling. Annealing the GaAs particles in molten salt mixtures in the presence of GaI3 mitigated the amorphous contributions to the Raman signal and recovered the peaks associated with the transverse and longitudinal optical phonons of GaAs. This caused a weak direct band gap absorption to appear in the absorption spectra of the annealed particles, which supports the hypothesis that insertion of Ga3+ ions into the lattice under high temperature conditions removes defects (see Figure 5D,E).
A) Raman spectra of as-synthesised (blue) and annealed (red) GaAs nanocrystals compared to the spectrum of the bulk phase (black). Dashed lines indicate the position of the transverse optical (TO) and longitudinal optical (LO) phonon modes. B) X-ray diffractograms of as-synthesised and annealed GaAs particles are not significantly different. C) The unit cells of cubic GaAs with and without a Ga vacancy. Absorption spectra before (D) and after (E) annealing. E contains a direct band edge transition (arrows). The inset of D) shows the band structure of GaAs. Reprinted with kind permission from The American Chemical Society.7)

More recently, the group investigated the cation diffusion and exchange in III–V semiconductor particles more broadly. They turned InAs and InP particles into ternary In1-xGaxAs and In1-xGaxP phases in a KI/Ga3I eutectic.7) The shape developed towards faceted tetrahedral particles no matter the shape and composition of the starting material. Additionally, the group observed an ion diffusion that was too fast for the ion exchange mechanism found in II–VI semiconductors and lead halide perovskites. Rather, the reaction must involve a significant mobility of both cations and anions. The group attributed this to stacking faults and twinning in the nanocrystals that survive the annealing process, unlike the Ga vacancies.

Metal borides

Like III–V semiconductors, borides are difficult to synthesize because of highly covalent and complex bonds, like 3-centre-2-electron bonds, as well as high reaction temperatures over 1000 °C. Borides are mechanically hard and thermally stable compounds, and so are they potential thermoelectric materials and catalysts, and are used as field emitters.8) The synthesis of nanoparticles strives for chemical purity, crystallinity and a narrow size distribution. Protesescu et al. have so far achieved crystallinity and phase purity by solid-state synthesis.9) In this, NaBH4 powder serves both as a boron source and a reducing agent.

At 400 °C, which is mild for solid state reactions, NaBH4 is reacted with either NiCl2 or metallic Ni powder (Figure 6). This leads to Ni3B and Ni2B nanoparticles, respectively, with a size between 10 and 80 nm. In both products, the nanoparticle cores consist of a single crystalline phase, but are covered with an amorphous oxide layer that was further characterised with X-ray photoelectron spectroscopy (XPS). The stoichiometric Ni:B ratio governs whether the reaction forms either of the two phases or an amorphous product, which points towards a complete consumption of the available boron.
A) Synthesis that leads to Ni2B and Ni3B inks from solid Ni precursors and NaBH4. B) Scanning transmission electron micrographs of a Ni3B particle, including a magnification of the crystalline core and the corresponding Fourier transformation. C) X-ray photoemission spectra of Ni2B and Ni3B particles obtained from the Ni2p, Cl2p and B1s edges. The particles contain the elements with mixed oxidation states and coordination environments. Source: Hong et al., licensed under CC-BY 4.0.10)

Although this reaction does not yet yield size distributions as narrow as the hot-injection route, the group produced dispersions and inks from the nanopowder that were colloidally stable for several days. For this, particle surfaces were treated with NOBF4 in DMSO. Subsequently, they exchanged the ligand to oleic acid in a phase transfer reaction. These data demonstrate that boride materials are accessible for processing in colloidal form, albeit not via direct colloidal synthesis. Other borides may be accessible as nanoparticles, too. Recently, the first successful synthesis of cubic boron arsenide was reported,10,11) which is an indirect semiconductor with high electron and hole mobility as well as thermal conductivity. Chemical vapour transport realised the bulk synthesis. A more straight-forward precursor thermolysis had failed around 25 years prior, due to the high covalent bond character in Boron arsenide (BAs), difficult redox chemistry, and impure reaction product.12) These issues may be circumvented by the solid-state route.

Early transition metal oxides

Except from TiO2, the colloidal nanoparticle community has ignored the early transition metal oxides. However, the group of Jonathan De Roo prepared and analysed zirconium and hafnium oxide particles. These particles catalyse the formation of esters and amides, in which the nanocrystal surface ligands simultaneously act as reactants.13) Recently, they have extended their work to atomically defined zirconia and hafnia clusters.14) These bridge both the transition from colloidal chemistry to inorganic cluster chemistry and to the study of atomically defined clusters.15,16) Early transition metal oxide nanoparticles are less affected by size-dependent quantum effects than other, common nanomaterials like semiconductors or metals. Hence, the advantage of a nanomaterial compared to the bulk oxide likely lies in increased catalytic activity due to the larger surface-to-volume ratio. Small clusters are too small to be analysed by either electron microscopy or X-ray diffraction. Instead, the group used pair-distribution function (PDF) analysis. Performing a Fourier transform on an extended X-ray diffractogram yielded peaks that correspond to inter-atomic distances between scattering centres in the structure. Through PDF analysis, the group confirmed two structure candidates, a monomeric Zr6 cluster and a Zr12 dimer which form in response to the steric demand for the fatty acid capping ligands. These ligands are important for directing the cluster formation, and it was imperative to include them in the modelling of the PDF signal to precisely derive the correct structure. This built on previous work by the Zobel group17,18), and confirms PDF analysis as a powerful tool for the analysis of small nanocrystals.19)

Work on new material classes is also important to verify the findings on nucleation theory. The classical nucleation theory by LaMer and Dinegar in 195020) does not sufficiently describe the findings on the formation of nanoparticles by the colloidal chemistry community. In particular, in the reaction

from the decomposition of molecular precursors P to monomeric units M and then to nanocrystals NC, the monomer is normally neither isolated nor detected. Instead, intermediates have been found, including defined (magic size) clusters14,19,21), pre-nucleation clusters22), and the De Roo group recently reported amorphous ZrO2 particles (AP). These nucleate directly from the highly reactive precursors and crystallise in a subsequent autocatalysed step by fusion with each other or with already existing nanocrystals.23)

Crucially, the model is consistent with an initial burst of nucleation that determines the number of resulting nanocrystals, as in LaMer’s model, but replaces the nucleation from free monomer with a second-order reaction between amorphous intermediates that is strongly concentration dependent. Thus, further nucleation is halted after the initial nucleation event in favour of growth by consumption of the intermediate. Hence, the number of amorphous intermediates controls the final particle size and number through the reactivity of the molecular precursor. Again, the group used PDF analysis to separate the signal of crystalline particles with long-range order from the amorphous intermediates and reaction by-products (Figure 7).
A) Pair-distribution function (PDF) analysis of reaction aliquots of ZnO2 nanoparticles made from a mixture of ZrCl4 and ZrCl2(OiPr)2 in tri-n-octylphosphine oxide (TOPO). The signals above 5 Å are caused by long-range order from the increasingly crystalline sample. B) The signal of the amorphous phase was isolated by subtraction normalised patterns for the crystalline product and the reaction by-product ZrCl4·2 TOPO. C) Schematic of the reaction mechanism from molecular precursors via amorphous intermediates to ZrO2 nanocrystals. Source: Pokratath et al., licensed under CC-BY 4.0.24)

Phase-change materials

Amorphous phases are important for phase-change materials (PCMs).

In PCMs, optical or electric pulses trigger a reversible conversion between a crystalline and an amorphous phase. They are used in permanent memory and solid-state data storage devices as well as in switchable surfaces. All of them benefit from nanotechnology for miniaturisation and processing. The Yarema group synthesised hot-injection-based ternary metal tellurides with PCM properties.24) In the synthesis, a lithium amide, LiN(TMS)2, is added which mitigates different reactivities between the two metal precursors in an initial burst of nucleation. The group synthesised a library of MxGeyTez nanoparticles (M = Sn, Pb, In, Bi, Ag, Co) with high composition and size control. Especially SnxGe1−xTe showed an reflectivity contrast of more than 0.7 between the amorphous and crystalline phase. The amount of the metal M proved crucial for the phase-change properties of the material because it influences the rhombohedral distortion from the cubic crystal structure. This tunes the crystallisation temperature and, thus, the transition threshold.

Spinel phases

Spinel structures have seen increased interest due to their versatile use in catalysis, optoelectronics and sensing. The nanosciences focussed on magnetic ferrites like magnetite. Recently, the Della Gaspera group at RMIT University in Melbourne synthesised pure-phase copper gallium oxide CuGa2O4 which possesses a 2.5 eV band gap with strong p-type character.25) The ternary structure gives an additional dimension to tune the parameter space of the material via its copper content. This affects the amount of doping, and thereby the optical and electronic properties, such as the size of the band gap, density of states, and photocurrent. Spinel-type oxide nanocrystals are highly processable and tuneable semiconductors. Thin films show a significant photocurrent and they are attractive materials for hole-conducting layers in LEDs and photovoltaic devices. However, the shape control needs to be improved: it currently produces spheroid, branched particles with good control over the thickness of the branches, but greater variance in the overall size. The approach should nevertheless already be applicable to a range of spinel-type oxides and give access to new nanomaterials.


Further work is being undertaken by many groups, including higher chalcogenide spinel structures, I-III-VI2 semiconductors that could replace the toxic cadmium by a mono- and a trivalent cation. Similarly, efforts are currently being made to substitute lead in lead halide perovskites with less toxic alternatives. The new nanomaterials will undoubtedly lead to the improvement of nucleation theory and nano-analytical techniques.

Drei Fragen an den Autor: Klaus Boldt

Was würden Sie gerne entdecken oder herausfinden?

Ich würde gerne neue Wege finden, Lichtenergie mit Hilfe der spannenden Eigenschaften nanoskaliger Materie gezielter in gerichtete Prozesse umzuwandeln, neben Solarenergie also auch chemische Reaktionen oder Bewegung in künstlichen Muskeln.

Welcher Trend ist in den letzten zwölf Monaten aufgekommen, den Sie so nicht erwartet haben?

Mich hat schon überrascht, wie plötzlich die künstliche Intelligenz den Sprung von der Erforschung zu einer breit anwendbaren und verfügbaren Methode gemacht hat. Ich sehe dies als großartige und spannende Möglichkeit für neue fundamentale Entdeckungen auf der ganzen Breite der Wissenschaft und als sehr nützliches Werkzeug für uns Chemiker. Insbesondere begeistern mich Entwicklungen, die Problemlösungsansätze der neuronalen Netzwerke zu entschlüsseln und nachzuvollziehen.

Was sind derzeit Ihre Hauptforschungsprojekte?

Wir versuchen, komplexe Nanopartikel und Heterostrukturen mit hoher Chemo- und Regioselektivität herzustellen und ihre Struktur-Eigenschafts-Beziehungen direkt abzubilden und auszunutzen, zum Beispiel für photokatalytische Reaktionen.

Klaus Boldt hat in Hamburg Chemie studiert und dort im Jahr 2011 bei Horst Weller promoviert. Nach Postdoc-Aufenthalten in Melbourne und Dresden war er sieben Jahre an der Universität Konstanz tätig, bevor er im Jahr 2022 eine Heisenberg-Professur in Rostock antrat.

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