Ultra-mild synthesis of nanometric metal chalcogenides using organyl chalcogenide precursors

Abstract

Bis(trialkylsilyl) monochalcogenides and diorganyl dichalcogenides, (R₃Si)₂E and R₂E₂ (E = S, Se or Te and R = alkyl, aryl or allyl group), have emerged in the past decade as excellent reagents for the synthesis of metal chalcogenide nanoparticles (NPs) and clusters owing to their ability to transfer the chalcogenide anion (E²⁻) under ultra-mild conditions and versatility in reacting even with non-conventional metal reagents or being employed in a variety of synthetic methods. In comparison, the related non-silylated diorganyl monochalcogenides R₂E have received attention only recently for the solution phase synthesis of metal chalcogenide NPs. In spite of sharing many similarities, these three families of organyl chalcogenides are different in their coordination ability and decomposition behavior, and therefore in reactivities towards metal reagents. This feature article provides a concise overview on the use of these three families as synthons for the ultralow-temperature synthesis of metal chalcogenide nanomaterials, deliberating their different decomposition mechanisms and critically assessing their advantages for certain applications. More specifically, it discusses their usefulness in (i) affording molecular precursors with different kinetic and thermal stabilities, (ii) isolating reactive intermediates for comprehending the mechanism of molecule-to-nanoparticle transformation and, therefore, achieving fine control over the synthesis, (iii) stabilizing isolable metastable or difficult-to-achieve phases, and (iv) yielding complex ternary nanoparticles with controlled stoichiometry or composites with sensitive materials without modifying the characteristics of the latter. Besides providing a perspective on the low-temperature synthesis of nanomaterials, this overview is expected to assist further progress, particularly in the field of R₂E, leading to interesting materials including metastable ones for new applications.

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Shashank Mishra

Shashank Mishra is Associate Professor of Chemistry at the Université Claude Bernard Lyon 1, France. His research interests are in the intersection of molecular and materials chemistry and mainly concerned with the design of molecular precursors and understanding their transformations to metal chalcogenide, metal oxide and metal fluoride nanomaterials via solution or vapor phase routes for applications in optics, energy, catalysis and related fields. He has also made contributions in the field of halometallate-based perovskite-inspired materials for photovoltaics. About 85 research articles, 6 reviews, 2 book chapters, and over 35 invited talks in international conferences summarize his research activities in the above field. Being a recipient of a couple of young scientist awards in the past, he was recently named an Emerging Investigator 2022 by the ACS journal Crystal Growth & Design.

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1. Introduction

Metal chalcogenide nanomaterials have been attracting world-wide attention as semiconducting materials with size-dependent and, hence, tunable electronic and optical properties (e.g., band gap energy, carrier density, and dielectric response), which make them very attractive for several applications such as thermoelectricity, photovoltaics, photocatalysis, displays, biomedical imaging, etc.^1–16 The chemical and electronic properties of these functional materials are often governed by the synthetic methods employed. An ideal synthetic method must afford these materials in a reproducible manner and with a very high level of control over their size, shape, morphology and size dispersion.^1–5,15,16 Since the ‘hot injection’ method reported in 1993 by Murray et al. for the controlled synthesis of monodisperse nanoparticles (NPs) of CdE (E = S, Se, Te),¹⁷ several solution-phase methods involving either single source precursors or separate metal and chalcogenide reagents have been developed for the synthesis of colloidal metal chalcogenide NPs. These employ different chalcogenide ligands/reagents such as tertiary phosphine chalcogenides, chalcogenoureas, chalcogenolates, chalcogenocarbamates, xanthates, dichalcogenoimidodiphosphinates, or sulfur and selenium dissolved in octadecene or amines.^18–20 Although these precursors have many advantages such as easy availability and high solubility in organic solvents, a majority of them require high temperature, which favors the formation of thermodynamically stable species only. Lowering the synthesis temperature is a highly desirable step to use these materials in technologically advanced areas such as energy conversion and storage, catalysis, environment, biomedicine and electronics.

Unlike metal oxides,^21–24 the low-temperature synthesis of functional metal chalcogenide nanomaterials is yet to realize its potential, and an important area in this direction is the exploration and identification of the reagents and precursors that are reactive at low temperature.^25,26 Other reasons behind exploring alternative chalcogenide reagents include cost, ease of handling, toxicity, purity, and the ability to tune the nucleation and growth kinetics. In this regard, bis(trialkylsilyl) monochalcogenides (R₃Si)₂E and diorganyl dichalcogenides R₂E₂ (where E = S, Se or Te, and R = an alkyl, aryl or allyl group) have emerged as excellent entry points for the synthesis of metal chalcogenide nanomaterials and clusters, owing to (i) their ability to transfer the chalcogenide anion (E²⁻) under ultra-mild conditions, (ii) easy tunability of their reactivity and properties by varying the organyl group R, (iii) facile removal of the byproducts, (iv) commercial availability, and (v) high solubility in common organic solvents.^25–31 These versatile reagents not only are applicable in a variety of synthetic methods (co-precipitation, thermal or microwave decomposition, ionic liquid media, microemulsions, chemical vapor deposition, etc.), but can also react in a facile manner even with non-conventional metal reagents (metal oxides, metal chalcogenides, metallic nanoparticles, etc.) to yield metal chalcogenide NPs and nanoclusters. Since these reagents generate metal chalcogenide NPs under mild conditions, they can generate metastable and previously unknown crystalline phases for certain materials and can even be exploited for the synthesis of composites with sensitive materials without modifying their structural and morphological characteristics.

The related non-silylated diorganyl monochalcogenides R₂E, which exhibit many similarities to the above two families of organyl chalcogenides, have received much less attention for solution-phase low temperature synthesis of metal chalcogenide nanomaterials. While as a class R₂E does share many of the advantages listed above for (R₃Si)₂E and R₂E₂, it also differs in many features. As compared to (R₃Si)₂E, which often forms a thermally unstable R₃SiE-M bond due to the excellent leaving property of the silylated group, the neutral R₂E forms a stronger and stable bond with the metal centre, making them usually less efficient in transferring the chalcogen to metal centers and leading to the formation of stable metal organyl chalcogenide adducts.^32,33 This restricts their use mainly to the elaboration of thin films by the chemical vapor deposition technique, either as a reagent (in dual source CVD)^34–37 or as single source precursors (after complexation with metals),^38–41 requiring usually high temperature. On the other hand, R₂E is usually more reactive than R₂E₂, which has an additional E–E bond to be ruptured during the decomposition. These features make the reactivity of R₂E somewhat unique, which, simply by varying R and E, can be modulated between highly reactive (R₃Si)₂E and somewhat less reactive R₂E₂. This may have implications in the potential use of R₂E in many applications including in the synthesis of new metastable phases with unique properties and applications, as shown in the case of R₂E₂.⁴² Despite these unique features and high potential, the use of R₂E in the low-temperature solution-phase synthesis of metal chalcogenide nanomaterials has received much less attention. Exploiting the facile decomposition mechanism in some of the non-silylated diorganyl monochalcogenides R₂E, a recent study from our laboratory has shown (i) the synthesis of metal precursors that decompose at very low temperature to generate metal chalcogenide nanomaterials,⁴³ and (ii) the direct synthesis of metal chalcogenide NPs at room temperature.^44–47 In the latter case, the isolation and characterization of the reactive molecular intermediates has helped in understanding the molecule-to-nanoparticle transformation better towards the development of synthetic methods with greater control.

In spite of the great utility of these organyl chalcogenides in the solution phase synthesis of metal chalcogenide nanomaterials under ultra-mild conditions, no inclusive effort has been made so far to review the subject matter. While the use of bis(trialkylsilyl) monochalcogenides (R₃Si)₂E in generating metal chalcogenide molecular clusters has been reviewed,^28–31 their use in the synthesis of nanomaterials only finds a somewhat limited mention in a recent book chapter.⁴⁸ The use of R₂E₂ as synthons for semiconducting nanoparticles was reviewed in 2015,²⁶ but there have been some important developments in the field since then, particularly in the synthesis of metastable phases.^49–51 This review highlights and contextualizes the emerging work on the use of organyl chalcogenides as interesting chalcogenide reagents for the mild synthesis of metal chalcogenide nanomaterials in the solution phase. After a brief introduction, the section on non-silylated organyl monochalcogenides R₂E focuses on their divergent reactivity to afford either nanoparticles or molecular complexes with different kinetic and thermal stability and underlines the importance of identifying the reactive intermediates in comprehending what happens at the interface when molecules are transformed into nanomaterials and, therefore, achieving fine control over the synthesis. The advantages of mild and controlled preparation of these NPs are discussed in terms of ligand-free synthesis of metal chalcogenide–TiO₂ nanocomposites for photocatalytic applications. It is followed by a subsection on the closely related precursors (R₂M)₂E and (R′₂M)ER (M = Sb or Bi), which can be considered as derived from R₂E by replacing both or one R group(s) with the R′₂M moiety (M = Sb, Bi), for binary M₂E₃ or ternary (Sb_xBi_1−x)₂E₃ nanomaterials.⁵² The next section on bis(trialkylsilyl) monochalcogenides (R₃Si)₂E describes in detail the use of different methods and the influence of diverse reaction conditions on the synthesis of different types of metal chalcogenide nanomaterials including complex ternary nanocrystals with controlled stoichiometry. The final section on R₂E₂ compares its reactivity with R₂E and summarizes recent important advances in its use for the isolation of new metastable phases. In the end, it identifies and signifies the areas for future research in the Conclusions and looking ahead section. I hope that this article will not only provide a reference text for a wider community working on metal chalcogenide nanomaterials, including a general perspective on the low-temperature synthesis of nanomaterials, but also initiate a future progression, particularly on non-silylated R₂E, to enhance further the overall understanding of molecule-to-nanoparticle transformation and contribute to the synthesis of interesting metal chalcogenide materials for new high-tech applications.

2. Non-silylated diorganyl monochalcogenides R₂E: distinctive reactivity towards metal reagents

As mentioned in the Introduction section, the utilization of nonsilylated diorganyl monochalcogenides R₂E has been mainly restricted to the elaboration of thin films in the chemical vapor deposition technique which requires high temperature.^34–41 Although generally less reactive in comparison to silylated organyl chalcogenides (R₃Si)₂E, the reactivity patterns and properties of non-silylated analogues R₂E can be modulated in a subtle manner by varying systematically the organyl group R. For instance, the reactions of FeCl₂ with diallyl sulfide (allyl)₂S, dibenzyl sulfide Bz₂S and di-tert-butyl sulfide ^tBu₂S in oleylamine at 220 °C yielded different morphologies of pyrrhotite Fe₇S₈ NPs, whereas di-phenyl sulfide Ph₂S under similar conditions did not react with FeCl₂.²⁷ Some of these organyl chalcogenides R₂E, particularly those containing di-tertiary-butyl groups ^tBu₂E (E = S, Se, Te), have been shown to undergo facile decomposition at low temperature, although their exact decomposition mechanism has been a subject of debate.^53,54 The possible initial steps in the decomposition include (i) C–E bond cleavage to give ^tBu˙ and ^tBuE˙ radicals and (ii) β-H elimination to give isobutene and tertiary-butyl chalcogenol (Scheme 1). While it was proposed using deuterium labelled experiments that ^tBu₂Se undergoes decomposition via initial Se–C bond cleavage,⁵³ recently it was shown using ab initio calculations that the non-radical β-H elimination pathway could almost fully account for the decomposition of ^tBu₂S, even though radical reactions do have an effect on the product distribution.⁵⁴ This difference in the decomposition mechanism of ^tBu₂Se and ^tBu₂S may be traced to a slightly different strengthening of the C–S and C–Se bonds, the former being thermodynamically more stable. This facile decomposition in ^tBu₂E (E = S, Se, Te) and related reagents can be exploited as a strategy to either prepare single source precursors with low thermal decomposition temperature or get metal chalcogenide NPs directly at room temperature, as described below.

Scheme 1 The possible decomposition pathways of ^tBu₂E (E = S, Se).

2.1. Stable molecular complexes with low thermal decomposition temperature

The high coordinating ability of organyl chalcogenides to afford molecular complexes is well-known.^32,33 However, the application of these complexes as single source precursors is relatively new and mostly confined to vapor phase synthesis which requires high temperature.^38–41 Employing well-characterized single-source precursors (SSPs) with well-defined composition not only overcomes the problem of controlling the stoichiometry of the constituent elements in the metal chalcogenides and paves the way for their low temperature and sustainable syntheses (therefore reducing the fabrication cost), but also provides numerous opportunities for the design and improvement in the properties of these materials.^55–57 As metal chalcogenide nanostructures and thin films are the core components of high-performance devices, the low temperature solution phase fabrication of these materials offers a unique opportunity for their incorporation in relevant areas like electronics, energy conversion and storage, environmental science, catalysis and biomedicine. As mentioned previously, the kinetic and thermal stability of these molecular complexes [MX_y(R₂E)_z]_n can vary depending on the nature of the metal (M), ancillary ligand (X) or substituent R and chalcogen atom E in the organyl chalcogenides. Using R₂E with alkyl groups that have a facile decomposition mechanism via β-hydrogen elimination, several molecular complexes with low thermal stability have been synthesized. For instance, Reid et al. have synthesized many metal halide complexes with the di-n-butyl chalcogenide ligand [MCl₄(ⁿBu₂E)₂] (M = Ti, Sn, Mo; E = S or Se), the thermogravimetric analyses (TGA) of which showed their low thermal decomposition (typically below 250 °C).^58–60 The C–Se bond is slightly weaker than the C–S bond and, therefore, the ⁿBu₂Se analogues are expectedly decomposed at slightly lower temperature than those with the ⁿBu₂S ligand. These precursors were employed in MOCVD to deposit metal chalcogenide thin films. On substituting the ⁿBu group with ^tBu, the decomposition can further be accelerated, thus lowering the decomposition temperature further. Fig. 1 shows the TGA curves of some homo- and heterometallic complexes of the coinage metals with the ^tBu₂S ligand, which exhibit one- or two-step decomposition at low temperature (typically between 110 and 140 °C) with residues that correspond to metal sulfides as the end products. While homometallic copper and silver precursors [Cu₂(TFA)₄(^tBu₂S)₂] and [Ag₄(TFA)₄(^tBu₂S)₄] (where TFA = trifluoroacetate) are decomposed to give Cu₉S₅ and Ag₂S, respectively,⁴³ the heterometallic [Ag₂Cu(TFA)₄(^tBu₂S)₄] decomposes in a single step at 120 °C to give a mixture of Ag₃CuS₂ and Ag₂S.⁴⁶ In contrast, organyl chalcogenides which do not have an alternative decomposition mechanism (e.g., Me₂E) afford metal complexes that are kinetically and thermally stable.^44,46,61 For example, the homometallic [Ag₅(TFA)₅(Me₂Se)₄] and heterometallic [Ag₂Cu(TFA)₄(Me₂Se)₄] complexes require much high temperature (∼300 °C) to decompose to yield Ag₂Se and a mixture of AgCuSe + Ag₂Se, respectively.^44,46

Fig. 1 Homo- and heterometallic coinage metal complexes with ^tBu₂S as a precursor with ultra-low decomposition temperature. Adapted with permission from ref. 43 and 46. Copyright (2021) Wiley-VCH; (2020) American Chemical Society.

2.2. Isolation of reactive molecular intermediates during the course of formation of metal chalcogenide nanoparticles

In the bottom-up approach of materials synthesis, identification of intermediate species operating at the intersection of molecular and nanometric levels is a key aspect in understanding the mechanism of molecule-to-nanoparticle formation and therefore achieving fine control over the synthetic conditions to prepare materials with controlled composition and properties. However, it is challenging to isolate and characterize these intermediates unambiguously because they are usually highly reactive in nature. Recently, we have found success in using R₂E of suitable reactivity to isolate and characterize reactive molecular intermediates during the course of formation of binary and ternary coinage metal selenide NPs.^44–46 Specifically, by reacting ^tBu₂Se with Cu(TFA)₂ or/and Ag(TFA) (where TFA = trifluoroacetate) in a variety of solvents (diethyl ether, tetrahydrofuran or toluene), we reported direct synthesis of binary and ternary coinage metal selenide NPs Cu_2−xSe, Ag₂Se and AgCuSe NPs at room temperature. We succeeded in isolating and characterizing by the single crystal X-ray technique the highly reactive intermediates [Cu₂(TFA)₂(^tBu₂Se)₃], [Ag(TFA)(^tBu₂Se)₂] and [Ag₂Cu(TFA)₄(^tBu₂Se)₄] during the course of these reactions, which provided unambiguous proof that Cu_2−xSe, Ag₂Se and AgCuSe NPs are formed via the above intermediates, respectively (Fig. 2). These isolated intermediates are unstable as they turn black within a few days even at low temperature and in an inert atmosphere, apparently due to their transformation into metal selenides. The thermal and kinetic instability of these intermediates was further confirmed by thermogravimetric (TGA) and density functional theory (DFT) calculations, respectively. For example, the TGA of the heterometallic intermediate [Ag₂Cu(TFA)₄(^tBu₂Se)₄] shows its decomposition below 100 °C with a residual mass that is consistent with the formation of 1 eq. AgCuSe and 0.5 eq. Ag₂Se as the end product, whereas a HOMO–LUMO gap of 0.25 eV was calculated from the DFT calculations.⁴⁶ The fact that this intermediate is transformed to a mixture of AgCuSe and Ag₂Se when left in air for a few hours underlines its high reactivity. In comparison, the ^tBu₂S analogue [Ag₂Cu(TFA)₄(^tBu₂S)₄] shows a better kinetic stability and can be kept intact at room temperature for several weeks. The higher stability of this analogue is attributed to a stronger C–S bond (as compared to the C–Se bond). Besides being a facile source of Se²⁻, the coordinated ^tBu₂Se ligand also plays the role of a reducing reagent to get the desirable +1 oxidation state of copper during the transformation of [Cu₂(TFA)₂(^tBu₂Se)₃] and [Ag₂Cu(TFA)₄(^tBu₂Se)₄] to Cu_2−xSe and CuAgSe + Cu_2−xSe NPs, respectively. In comparison to intermediates involving Cu(II) species, the transformation of [Ag(TFA)(^tBu₂Se)₂] to Ag₂Se NPs required no metal reduction step and, therefore, the reaction was much faster (5–30 min), more reproducible and afforded a better yield (>95%).⁴⁴ The binary Cu_2−xSe and Ag₂Se NPs on further reaction with ^tBu₂Se at RT gave compositionally tailored ternary metal chalcogenide CuAgSe NPs.⁴⁷ The above ultra-mild synthesis of binary and ternary NPs using the ^tBu₂Se ligand was further exploited to generate composites with TiO₂ (P25) for photocatalysis without compromising the structural and morphological characteristics of TiO₂ and without having any organics around NPs. As a result, the obtained metal–chalcogenide–TiO₂ nanocomposites acted as better photocatalysts than the commercially available benchmark TiO₂ (P25) for formic acid degradation under UV irradiation.^44,45,47 Some selected examples of metal chalcogenide nanoparticles synthesized under mild conditions using non-silylated organyl chalcogenides are summarized in Table 1.

Fig. 2 Isolation and structural characterization of reactive intermediates during the syntheses of Cu_2−xSe, Ag₂Se and AgCuSe NPs from the reactions of ^tBu₂Se with coinage metal trifluoroacetates. Adapted with permission from ref. 44–46. Copyright (2016) Wiley-VCH; (2018) The Royal Society of Chemistry; and (2020) American Chemical Society.

Table 1 Some selected examples of metal chalcogenide nanomaterials synthesized under mild conditions using non-silylated R₂E

Reagents/precursors	Materials	Synthesis (temp., solvent, capping ligand, etc.)	Ref.	Comments
[Cu2(TFA)4(^tBu2S)2]	ODT-Protected Cu9S5 NPs	145 °C, xylene, ODT	43	Precursor with one-step decomposition (single DTG peak at 110 °C)
Cu(TFA)2 + ^tBu2Se	Cu2−xSe NPs	110 °C, toluene	45	Reaction proceeds via a reactive intermediate [Cu2(TFA)2(^tBu2Se)3]
[Ag4(TFA)4(^tBu2S)4]	ODT-protected Ag2S NPs	145 °C, xylene, ODT	43	Precursor decomposes at as low temperature as 60 °C
Ag(TFA) + ^tBu2Se	Ag2S NPs	RT, Et2O	44	A reactive intermediate [Ag(TFA)(^tBu2Se)2] was isolated during the reaction and characterized structurally
FeCl2 + R2S (R = ^tBu, allyl, Bz, Ph)	Pyrrhotite Fe7S8 NPs	220 °C, OLA	27	While ^tBu2S, (allyl)2S, and (benzyl)2S yielded the pyrrhotite phase (Fe7S8) with different morphologies, Ph2S did not react under identical conditions
Cu2−xSe NPs + Ag(TFA) + ^tBu2Se	Uncapped CuAgSe NPs	RT, THF	47	Ultra-mild synthesis allows to combine CuAgSe with TiO2 without modifying the characteristics of TiO2
[CuAg2(TFA)4(^tBu2S)4]	Ag3CuS2 NPs	110 °C, toluene	46	Precursor with one-step decomposition (single DTG peak at 120 °C)
Ag(TFA) + Cu(TFA)2 + ^tBu2Se	AgCuSe NPs	RT, no solvent or capping ligand	46	A highly reactive intermediate [CuAg2(TFA)4(^tBu2Se)4] was isolated during the reaction and characterized

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2.3. Related (R′₂M)₂E and (R′₂M)₂ER (M = Sb, Bi) as precursors for Group 15 metal chalcogenides

The related (R′₂M)₂E and (R′₂M)ER, which can be considered as derived from R₂E by replacing both or one R group(s) with the R′₂M moiety (M = Sb, Bi), have recently been employed as single source precursors under a variety of mild conditions to get binary M₂E₃ and ternary Sb₂(S_1−xSe_x)₃ or Bi_xSb_1−xE₃ materials.⁵² Although it is known in the literature for quite some time that the related complexes of the type (RM)₂E (where M = Zn, Cd, Hg; E = S, Se, Te; R = Me, Ph) decompose readily to give ME and R₂M,⁶² it is only recently that Schulz et al. have carried out a systematic study on (R′₂M)₂E for group 15 binary and ternary metal chalcogenide nanomaterials.^63–68 For instance, they used (Et₂Sb)₂E (E = S, Se, Te) as a SSP for highly crystalline Sb₂E₃ NPs in different organic solvents in the presence of poly(1-vinylpyrrolidone)graft-1-hexadecene (PVP*) as a capping agent at temperatures below 170 °C.^63,64 While (Et₂Sb)₂Te decomposes upon heating at 140 °C in a straightforward manner to produce highly stoichiometric Sb₂Te₃ and Et₃Sb, the decomposition pathways of (Et₂Sb)₂E (E = S, Se) are more complex and produce Sb₂E₃, SbEt₃ and Et₂E. This leads to formation of slightly Sb-rich Sb₂S₃ and Sb₂Se₃ materials. Depending upon the nature of chalcogenide atoms, the morphology of the materials also differed significantly. Thus, Sb₂S₃ and Sb₂Se₃ were obtained as nanobundles and nanowires, respectively (Fig. 3a), whereas Sb₂Te₃ formed hexagonal, almost monodisperse, crystalline nanoplates of roughly 400 nm diameter and 35 nm thickness (Fig. 3b). The above difference in the decomposition pathways of (Et₂Sb)₂E is further manifested during their co-decomposition to get possibly ternary phases. While excellent compatibility in the decomposition behaviour and morphology of (Et₂Sb)₂S and (Et₂Sb)₂Se led to the formation of the ternary Sb₂(S, Se)₃ phase without any impurities on their simultaneous decomposition at 170 °C, co-decomposition of (Et₂Sb)₂S and (Et₂Sb)₂Te under similar conditions yielded biphasic, heterocomposite materials containing crystalline Sb₂S₃ nanobundles and Sb₂Te₃ nanoplates (Fig. 3c).⁶⁴ In contrast to the behavior of (Et₂Sb)₂E, the thermolysis of the analogous bismuth precursor (Et₂Bi)₂Te in 1,3-diisopropylbenzene (DIPB) led to the formation of Bi₄Te₃ at 80 °C (Fig. 3d) and mixtures of Bi₄Te₃ and elemental Bi at higher temperatures, most probably due to the comparably low Te–Bi binding energy and complex decomposition mechanism involving radicals.⁶⁵

Fig. 3 Diversity of (Et₂M)₂E (M = Sb, Bi; E = S, Se, Te) precursors to get binary M₂E₃ and ternary Sb₂(S_1−xSe_x)₃ or (Bi_xSb_1−x)₂Te₃ nanoparticles. Adapted with permission from ref. 63–65, 67 and 68. Copyright (2015, 2021) Wiley-VCH; (2015, 2017) The Royal Society of Chemistry; and (2012) American Chemical Society.

The quality of the above materials could further be improved by microwave-assisted decomposition of the above precursors in ionic liquids.^66,67 Thus, microwave-assisted decomposition of (Et₂Sb)₂Te in [C₄mim]Br (C₄mim = 1-butyl-3-methyl-imidazolium) produced phase pure Sb₂Te₃ NPs with improved thermoelectric properties.⁶⁵ The authors then studied in detail the influence of the nature of counter anions (Cl⁻, Br⁻, I⁻ and NTf₂⁻, where NTf₂⁻ = bis(trifluoromethanesulfonyl)amide) as well as the chain length of the alkyl group of the cation (1-alkyl-3-methylimidazolium or 1,3-dialkylimidazolium) in a set of ionic liquids.⁶⁷ While increasing the chain length resulted in better solubility of the precursor (Et₂Sb)₂Te, which enhanced the formation of less aggregated nanoparticles, the strongly basic anion such as Cl⁻ with higher coordinating capability resulted in the formation of thin Sb₂Te₃ nanoplates (Fig. 3e). As a consequence, the thermoelectric properties of the resulting Sb₂Te₃ nanoplates strongly differed. When the above precursor was decomposed in a mixture of ionic liquids containing varying amounts of Bi-containing ILs [C₄mim]₃[Bi₃I₁₂] and [C₄mim]I, highly stoichiometric and phase-pure ternary solid solutions of the type (Bi_xSb_1−x)₂Te₃ could be synthesized at 150 °C (Fig. 3f).⁶⁸ The synthetic details of these examples are summarized in Table 2.

Table 2 Binary M₂E₃ and ternary Sb₂(S_1−xSe_x)₃/Bi_xSb_1−xE₃ nanomaterials using (Et₂M)₂E or (R′₂M)E(R) as precursors

Precursor	Materials	Synthesis (temp., solvent, capping ligand, etc.)	Ref.	Comments
Abbreviations: C4mim = 1-butyl-3-methylimidazolium; C4C1Im = 1-butyl-3-methylimidazolium; DIPB = 1,3-diisopropylbenzene; NTf2^− = bis(trifluoromethanesulfonyl)amide; ILs: ionic liquids; PVP* = poly(1-vinylpyrrolidone)graft-1-hexadecene.
(Et2Sb)2S	Sb2S3 nanobundles	170 °C, DIPB, PVP*	63	Slightly Sb-rich Sb2S3 due to formation of the volatile by-product Et2S
(Et2Sb)2Se	Sb2Se3 nanowires	170 °C, DIPB, PVP*	63	Slightly Sb-rich Sb2Se3 due to formation of the volatile byproduct Et2Se
(Et2Sb)2S + (Et2Sb)2Se	Sb2(S1−xSex)3 nanowires	170 °C, DIPB, PVP*	63	Impurity-free ternary phase due to excellent compatibility in the decomposition patterns of (Et2Sb)2S and (Et2Sb)2Se
(Et2Sb)2Te	Sb2Te3 hexagonal plates	170 °C, DIPB, PVP*	64	Phase-pure product due to stoichiometric decomposition in Sb2Te3 and Et3Sb
(Et2Bi)2Te	Pure Bi4Te3 (80 °C), Bi4Te3 + metallic Bi (80–170 °C)	80-170 °C, DIPB, PVP*	65	Metallic Bi impurity at temp. higher than 80 °C due to a weak Te–Bi bond
(Et2Sb)2Te	Sb2Te3 NPs	170 °C (MW-decompos.), [C4mim]X (X = Cl^−, Br^−, I^−, NTf2^−)	66,67	Less agglomerated NPs by increasing the cation chain length of ILs (due to the enhanced solubility of the precursor) and thinner Sb2Te3 nanoplates by using strongly basic anions, e.g., Cl^−
(Et2Sb)2Te + [C4mim]3[Bi3I12]	(BixSb1−x)2Te3 NPs	150 °C (MW decompos.), [C4mim]I	68	Highly stoichiometric, phase-pure ternary NPs by using a Bi-containing IL
(Et2Bi)Te(Et)	Bi2Te NPs	100 °C, DIPB, OA	65	Bi-Rich product due to a weak Te–Bi bond
(Ph2Sb)Te(Et)	Sb2Te3 nanoplates	300 °C, oleylamine	69	High quality nanoplates at 300 °C per 2 h. A lower temp. and shorter time (250 °C per 1 h) lead to Sb2Te3 + Sb2Te mixed phases
(Ph2Sb)Te(Ph)	Sb2Te3 nanoplates	250 °C, oleylamine	69	Perfect hexagonal nanoplates under mild conditions (250 °C per 1 h). A higher temp. and time (300 °C per 2 h) distort the shape
(Et2Bi)Te(Et)	Bi2Te NPs	100 °C, DIPB or OA	65	Te-Poor Bi2Te particles due to formation of the volatile byproduct Et2Te

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Another related class (R₂M)ER′ (M = Sb, Bi) has also been employed as SSPs for M₂E₃ materials. Kim et al. investigated the thermal decomposition of Ph₂SbTeR (R = Et, Ph) in oleylamine.⁶⁹ In the case of Ph₂SbTeEt, high-quality Sb₂Te₃ nanoplates were obtained after heating at 300 °C for 2 h, whereas lower reaction temperatures (250 °C) and shorter reaction times (1 h) resulted in the formation of a mixture of Sb₂Te as the major component and Sb₂Te₃ powders, indicating a rather complex decomposition mechanism. On the contrary, pure Sb₂Te₃ nanoplates of hexagonal shape were obtained using Ph₂SbTePh at 250 °C for 1 h, whereas an increasing reaction time (2 h) and temperature (300 °C) yielded shape-distorted Sb₂Te₃ nanoplates. In contrast, the thermolysis of Et₂BiTeEt in DIPB or OA gave pseudo-cubic Bi₂Te particles, possibly due to the formation of the volatile byproduct Et₂Te, leading to Te-poor Bi₂Te particles.⁶⁵ Interestingly, the use of a similar precursor in MOCVD led to the formation of phase-pure Bi₂Te₃ films on Si(100) at a very low substrate temperature of 230 °C, indicating that the decomposition mechanism for this precursor is very different in the solution and gas phases.⁷⁰ EDX and XRD studies revealed the formation of highly stoichiometric and phase pure Bi₂Te₃ films at 215 and 230 °C, while higher substrate temperatures (245 °C) yielded Bi-rich materials with Bi₄Te₃ as the additional phase.

3. Bis(trialkylsilyl) monochalcogenides (R₃Si)₂E as facile chalcogenide-transfer reagents

The chalcogen-transfer ability of the silylated diorganyl monochalcogenides (R₃Si)₂E is well-known, which has allowed an easy access to metal chalcogenide nanoparticles or clusters, depending on the reaction conditions. These silylated chalcogenides (R₃Si)₂E react readily with a metal reagent MX_n (X = alkyl, halide, amide, carboxylate, oxide), where facile and homogeneous delivery of the chalcogenide (E²⁻) to the metal center is promoted by the affinity of silicon(IV) for oxygen and halides as well as the volatility of the formed byproduct R₃SiX (Scheme 2).^71–76 When the reaction is performed in the presence of an ancillary ligand L (e.g., PR₃), it often leads to the formation of high nuclear nanoclusters [M_xE_y(L)_z].^28–31 The reaction is supposed to proceed through the formation of chalcogenolate intermediates, although the high thermal instability of the M–ESiR₃ bonds makes it difficult to isolate these intermediates before they afford E²⁻. To some extent, the stability of these chalcogenolate intermediates can be increased by using a strongly bonded ancillary ligand such as N-heterocyclic carbine (NHC) or cyclic diamine, as indicated by the isolation and structural characterization of [(IPr)M(ESiMe₃)], [(tmeda)Zn(ESiMe₃)₂] (M = Cu, Ag; E = S, Se, Te; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene; tmeda = tetramethylethylenediamine), [(NHC)Ag–ESiMe₃] (E = S, Se) and [(NHC)Ag–S–Hg–S–Ag(NHC)] at low temperature.^77–79 Besides stabilizing the molecular intermediates, these ancillary ligands can also stabilize the monodisperse NPs formed at the end of the reaction, as exemplified by the synthesis of phase-pure and highly monodisperse coinage metal chalcogenide QDs (Ag₂E, Cu_2−xE; E = S, Se) from the room temperature reaction of (Me₃Si)₂E with metal halide synthons containing N-heterocyclic carbene (NHC) ligands.⁸⁰ Alternatively, reagents with mixed silylated and non-silylated alkyl groups (R₃Si)ER′ can be exploited to introduce surface chalcogenolate groups (Scheme 2). These ligand-protected clusters can be considered as borderline species between molecules and nanoparticles, which may serve as precursors for well-defined metal chalcogenide NPs.²⁹

Scheme 2 Bis(trialkylsilyl) monochalcogenides as facile chalcogenide-transfer reagents.

The high reactivity of these reagents ensures not only a high yield of the products, but also a fast growth and, therefore, well-defined shape of the NPs. While the high reactivity ensures that these silylated reagents react quantitatively with the metal reagents, the good leaving property of the trialkylsilyl group makes sure that it does not act as a surface ligand (unlike other reagents such as trioctylphosphine (TOP) that can form a complex with the metal center and, therefore, limit the yield of metal chalcogenides). These properties make the bis silylchalcogenides (R₃Si)₂E the reagents of choice for the synthesis of metal chalcogenide NPs, despite the perceived difficulty in their handling due to high volatility and air/moisture sensitivity.³¹ The following subsections describe the versatility of these reagents in the synthesis of metal chalcogenide NPs under mild conditions.

3.1. Metal–organic decomposition at low-to-moderate temperatures

3.1.1. Influence of the choice of reagents and their ratio. The main principles developed so far in the synthesis of NPs with various morphologies and shapes can be understood by taking the example of lead chalcogenides,^81–89 which as NIR-active materials show great potential in many applications such as photovoltaics, optoelectronics, sensors and bio-electronics.⁹⁰ A typical synthetic procedure involves the injection of (Me₃Si)₂E into the solution of lead reagents PbX₂ (where X = halide or oleate) at moderate temperatures of 60–140 °C, which yields PbE NPs (E = S, Se, Te) with a quantum efficiency of 20–30% and a narrow size distribution (10 ± 15%) (Fig. 4a). The mechanism of the formation of PbS from the reaction of lead(II) oleate and (Me₃Si)₂S has been studied in detail.⁹¹ According to the proposed mechanism, the reaction is proceeded by the formation of an intermediate complex [(OA)Pb(SSiMe₃)₂] with Pb–S bonds that, upon the injection of a protic solvent, forms further Pb–S containing intermediates. However, due to their highly reactive nature, these intermediate complexes could not be isolated and studied unambiguously.

Fig. 4 Synthesis of different forms of lead chalcogenides using (Me₃Si)₂E as chalcogen reagents. Reproduced with permission from ref. 85–89 and 96. Copyright (2014, 2016, 2018) American Chemical Society and (2010) The Royal Society of Chemistry.

When the quantity of (Me₃Si)₂S is less than that of the lead precursor, usually lead oleate produced in situ by the reaction of lead oxide and oleic acid (OA), the produced PbS QDs are Pb-rich with the excess of lead precursor located on the surface of the QDs and passivated by the oleic acid, leading to a stable colloidal solution.^81,82 In contrast, noncolloidal aggregated suspensions of PbS NPs are formed on using an excess of (Me₃Si)₂S, which removes the oleic acid ligands from the surface of the particles and sulfurizes the surface.⁸⁵ These S²⁻ anions on the surface of QDs bind with the Pb atoms of adjacent QDs, thus decreasing the inter-QD spacing drastically inside the spin-coated films, leading to complete disappearance of the exciton absorption peak (Fig. 4b).

On using (Me₃Si)₂Se, the selenization of the QDs was also possible (i.e. the presence of Se²⁻ instead of S²⁻ on the surface) as confirmed by the red-shift of the absorption onset in the selenized QDs with respect to the sulfurized dots. This is in accordance with the narrower bandgap of PbSe as compared to PbS. It was noted that a chalcogen-rich termination of the PbS QD surface promoted oxidation and facilitated fusion/necking of neighboring QDs. On using PbX₂ (X = Cl, Br, I) as the Pb precursors, it was possible to produce halide-terminated PbS and PbSe QDs which were air-stable (Fig. 4c).^86,87 This low-temperature synthesis also helps in scaling-up of the products, as demonstrated by Zhang et al. in the synthesis of about 47 g of PbS QDs with a clear excitonic absorption peak and good size distribution within a few minutes (Fig. 4d).⁸⁶

By exploiting the relative reaction rates of the chalcogenide precursors (Me₃Si)₂E (E = S, Se, Te), which allow for the homogeneous incorporation of the chalcogenide anions, synthesis and characterization of composition-tunable ternary lead chalcogenide alloys PbSe_xTe_1−x, PbS_xTe_1−x, and PbS_xSe_1−x were reported (Fig. 4e).^92,93 The resulting QDs were Pb-rich but the Pb/anion ratio was size- and composition-dependent and consistent with the reaction rates of the anion precursors.

Without using thiols as stabilizing ligands, Shen et al. recently synthesized HgTe colloidal quantum dots with much improved dispersion which can be stored in the cleaned solution for months without any precipitate (Fig. 5a).⁹⁴ The authors suggested that the reason behind the colloidal stability was the use of a 2-fold excess of the mercury precursor. As the reaction with highly reactive (Me₃Si)₂Te is quantitative, there is always an excess of Hg precursor which acts as a ligand that binds to the surface Te sites. The high reactivity of (Me₃Si)₂Te ensures a fast growth of the particles and results in spherical quantum dots (QDs), which is in contrast with the strongly faceted HgTe QDs synthesized using TOPTe (where slow growth ensures that slowest growing facets dominate the final structures).

Fig. 5 Versatility of (Me₃Si)₂E in the mild synthesis of different nanoforms of metal chalcogenides. Reproduced with permission from ref. 94, 95, 107 and 113. Copyright (2007, 2017) American Chemical Society; (2017) The Royal Society of Chemistry; and (2021) Wiley-VCH.

By reacting (Me₃Si)₂S with divalent lanthanide halides LnX₂ (X = Cl, Br, I) in oleylamine, Stoll et al. recently synthesized mixed-valent lanthanide sulfide Eu₃S₄ and EuSm₂S₄ NPs at 200 °C (Fig. 5b).⁹⁵ The authors reported the combination of the high reactivity of (Me₃Si)₂S and the use of low-valent metal halides as a key to stabilize the mixed-valent phases. The fact that only EuI₂ afforded phase-pure Eu₃S₄ (the use of EuCl₂ gave EuS NPs, whereas EuBr₂ yielded a contaminated sample) highlights the influence of the choice of reagent and the role of kinetics in phase stabilization.

3.1.2. Influence of solvent, reaction time and temperature. The use of olive oil as a solvent and a capping ligand represents a greener and gram scale synthetic route that not only lowers the synthetic temperature to 60 °C but also enhances the air-stability of the NPs (Fig. 4f).⁸⁸ The reaction of lead(II) oleate with (Me₃Si)₂S in a mixture of solvents 1,1,2-trichloroethane (1,1,2-TCE) and dimethylformamide (DMF) leads to the formation of PbS nanosheets with lateral dimensions of several hundred nanometers (Fig. 4g).⁸⁹ The use of 1,1,2-TCE and DMF was essential for getting nanosheets as only NPs were produced without these two solvents. By varying the reaction temperature and time, the shape and thickness of these nanosheets could be tuned. The stable colloidal suspension of the as-obtained PbS QDs is amenable to further manipulations. For example, these colloidal suspensions could easily be deposited on a glass substrate which, after drying the solvent at different temperatures, resulted in self-organization of the QDs into two- and three-dimensional (3D) superlattices in the form of flakes in parallel orientation to the substrate.⁹⁶

The reaction of (R₃Si)₂Te (R = Me, Et) with GeX₂ (X = chloride, aminoalkoxide) afforded GeTe NPs at low temperatures.⁹⁷ Specifically, the reaction of (Et₃Si)₂Te and GeCl₂·dioxane was studied in detail. The presence of protic and strongly coordinating primary or secondary amine solvents was essential for the formation of GeTe NPs with defined chemical composition, size, and shape. The nuclear magnetic resonance (NMR) studies revealed that the reaction in oleylamine (OLA) did not proceed with the elimination of Et₃SiCl. Rather, OLA reacted with (Et₃Si)₂Te at ambient temperature with the formation of first silylamine (oleylN(H)SiEt₃) and then the [Te₄]²⁻ dianion, which then reacted with GeCl₂·dioxane to afford GeTe at temperatures below 150 °C. In contrast, reactions in 1,3-diisopropylbenzene occurred with the elimination of Et₃SiCl and subsequent formation of agglomerated GeTe particles, whereas reactions in tri-n-octylamine and 1-hexadecanethiol produced elemental tellurium. In the presence of an excess of (Et₃Si)₂Te, GeTe NPs decorated with Te nanowires were obtained. Similarly, the reactions of (R₃Si)₂E (R = Me, Et; E = S, Se, Te) with metal aminoalkoxides of Sb(III) and Bi(III) at moderate temperatures afforded phase-pure M₂E₃ NPs without any additional elemental or metal-rich phases.^97,98

Ternary nanoparticles containing two different metals, i.e., AgSbS₂ NPs, were synthesized by reacting simultaneously AgNO₃ and SbCl₃ or [Sb(diethyldithiocarbamate)₃] with (Me₃Si)₂S in a mixture of OLA and OA at 180–220 °C.⁹⁹ Upon varying the reaction time between 1.5 and 5 minutes, the reaction temperature between 180 and 220 °C or the OLA/OA volume ratio in the range of 4 : 4 to 7.25 : 0.25, a wide range of various sized AgSbS₂ NPs with narrow size distributions were obtained in a very precise manner. Similarly, ternary AgInSe₂ NPs were synthesized by the hot-injection method by injecting (Me₃Si)₂Se into a solution of AgI and InI₃ in a mixture of OA and TOP at 280 °C.¹⁰⁰ The reaction of (Me₃Si)₂S with CuIn(OH)₅ in a mixture of 1-octadecene (ODE), OA and OLA yielded OA-capped CuInS₂ NPs at 200 °C, where the use of a bimetallic precursor containing a 1 : 1 ratio of the two metals ensured the desired composition in the synthesized ternary NPs.¹⁰¹

3.2. Use of ionic liquids

Ionic liquids offer an interesting synthetic route, not only as effective solvent media, but also as stabilizers and shape-directing templates. Thus, (Et₃Si)₂Se reacted with SbCl₃ in [C₄C₁Im]Cl (C₄C₁Im = 1-butyl-3-methylimidazolium) and oleic acid to give phase-pure Sb₂Se₃ at 150 °C as agglomerated nanowires with a high aspect ratio.¹⁰² Similarly, reactions of (Et₃Si)₂E with bismuth-containing ionic liquids [Cat]₃[Bi₃I₁₂] [Cat = C₄C₁Im or C₄mim] in the presence of oleylamine at 150 °C afforded phase-pure Bi₂E₃ (E = Se, Te).^68,102,103 Alternatively, these silylated organyl chalcogenides (R₃Si)₂E can easily be converted to chalcogenide-containing ionic liquids Cat⁺[ESiMe₃]⁻ (Cat⁺ = an organic cation) of comparable activity but with higher thermal stability, therefore effectively suppressing the undesired homolytic cleavage of the E–Si bond observed occasionally in the parent (R₃Si)₂E.¹⁰⁴ For instance, [C₄C₁Pyr][ESiMe₃] (C₄C₁Pyr = 1-butyl-1-methylpyrrolidinium; E = Se or Te) reacts with bismuth-containing [C₄C₁Im]₃[Bi₃I₁₂] in [C₄C₁Im]I at 150 °C to afford phase-pure Bi₂E₃ free from any elemental Se/Te impurities.¹⁰⁵

3.3. Sophisticated and intricate multicomponent architectures

One important advantage of the highly reactive nature of these silylated chalcogenides is that they generate metal chalcogenide NPs under mild conditions and therefore can be exploited to generate composites with sensitive materials. For example, these have been employed on a modified mesoporous surface such as Mobil Composition of Matter no. 41 (MCM-41) and mesocellular silica foam (MCF) to form binary CdE and ZnE or ternary Cd_xZn_1−xE (E = S, Se, Te) nanomaterials within the host framework at room temperature, and therefore without modifying its mesoporous characteristics (Fig. 5c).^106,107 Even though the surface area of the host matrix after loading with NPs diminishes, it still remains sufficiently high (e.g., 250 m² g⁻¹ in CdE loaded MCF against 715 m² g⁻¹ in MCF). More importantly, the resulting materials maintain their mesoporosity, indicating their suitability for many applications.

These synthons have also been employed to get sophisticated core@shell structures. For example, the reaction of the pre-formed Cu_2−xSe NPs with ZnEt₂ and (Me₃Si)₂S in a mixture of OLA and trioctylphosphine oxide (TOPO) afforded core@shell Cu_2−xSe@ZnS NPs at 175 °C.¹⁰⁸ Similarly, the reaction of ZnO tripods with (Me₃Si)₂S in TOP/TOPO at 250 °C yielded hollow ZnS tripods which on partial cation exchange with Cd²⁺ at 220 °C afforded ZnS@CdS core–shell tripod heterostructures.¹⁰⁹

3.4. Miscellaneous methods

As said earlier, these ligands can react with non-conventional reagents under a variety of conditions. For instance, using the microemulsion method, nanocrystalline Bi₂Te₃ with particle sizes less than 10 nm were synthesized from the reaction of BiO(ClO₄) with (Me₃Si)₂Te in a reverse micelle formed by sodium dioctyl sulfosuccinate and water in hexane in 70% yield.¹¹⁰ Similarly, these ligands can also react with pre-formed metallic or metal oxide nanoparticles. For example, (Me₃Si)₂Te reacts with pre-formed colloidal metal nanoparticles of Pd, Pt and Ni under sufficiently mild conditions (180 °C, 1 h) to form their corresponding ditellurides PdTe₂, PtTe₂, and NiTe₂.¹¹¹ Other metal nanoparticles of Co, Ag, and Rh also form crystalline metal tellurides upon reaction with (Me₃Si)₂Te, indicating that this approach to synthesizing nanoscale transition metal tellurides is general. Interestingly, the reactions of (Me₃Si)₂E with pre-formed nanocrystalline metal oxides (CdO, ZnO, NiO, Co₃O₄ and CoO) at 135–235 °C in TOP/TOPO resulted in nanocrystalline metal chalcogenides (CdS, ZnS, Ni₃S₂, Ni₅Se₅, Co₉S₈ and Co₃Se₄) with hollow, vesicle-like morphologies, as confirmed by transmission electron microscopy (TEM) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images (Fig. 5d).^108,112,113 The shape, single crystallinity, and orientation of the parent metal oxide NPs are completely preserved in the transformed particles. The authors explained the hollow morphologies on the basis of a nanoscale Kirkendall-type effect, arising from differential ion diffusion rates between ions in the reactant and product phases. All nanocrystalline chalcogenides produced were surrounded by an outer amorphous silica layer, which could help in preventing agglomeration and degradation. These organyl chalcogenides can also react with more complex precursors such as mixed-metal oxides to yield ternary metal chalcogenide NPs as demonstrated by the reaction of NiCo₂O₄ with (Me₃Si)₂S which generated NPs of heterobimetallic (Ni/Co)₉S₈ without significant phase segregation.¹¹³Table 3 summarizes some selected examples of metal chalcogenide nanomaterials synthesized under mild conditions using silylated organyl chalcogenides.

Table 3 Some selected examples of metal chalcogenide nanomaterials synthesized under mild conditions using (Me₃Si)₂E

Reagents/precursors	Materials	Synthesis (temp., solvent, capping ligand, etc.)	Ref.	Comments
Abbreviations: AOT = sodium dioctyl sulfosuccinate; C4C1Im = 1-butyl-3-methylimidazolium; DIPB = 1,3-diisopropylbenzene; dmea = 1,1-dimethylethanolamine; DMF = dimethylformamide; OA = oleic acid; ODE = 1-octadecene; ODT = octadecane thiol; OLA = oleylamine; TOP = trioctylphosphine; TOPO = trioctylphosphine oxide; MW = microwave; RT = room temperature; NHC = N-heterocyclic carbene; NPs = nanoparticles; QDs = quantum dots.
Binary metal chalcogenides
Pb(oleate)2 in excess + (Me3Si)2S	Pb-Rich PbS QDs	115 °C, OA, ODE	81 and 82	Stable colloidal solution due to the presence of oleic acid-protected lead ions on the surface of QDs
PbI2 + (Me3Si)2E in excess	E-Stabilized PbS NPs (E = S, Se)	125 °C, DMF, OA, ODE, OLA	85	The E^2− anions on the surface of QDs bind with the Pb atoms of adjacent QDs, which decrease the inter-QD spacing and lead to complete disappearance of the exciton absorption peak
PbX2 (X = Cl, Br, I) + (Me3Si)2E	Halide-stabilized and air-stable PbE (E = S, Se) QDs	100–140 °C, OLA	86 and 87	Facile scaled-up synthesis (∼47 g of high-quality PbS QDs within a few min)
Pb(oleate)2 + (Me3Si)2S	Olive oil-stabilized PbS NPs	60 °C, olive oil, OA, ODE	88	Use of olive oil as a green solvent lowers further the synthetic temp. and makes NPs more air-stable
Pb(oleate)2 + (Me3Si)2S	PbS nanosheets	90 °C, Ph2O, TOP, 1,1,2-TCE, DMF	89	Use of 1,1,2-TCE and DMF essential for getting nanosheets
GeCl2·dioxane + (Et3Si)2Te	GeTe NPs	160 °C, OLA	97	Oleylamine essential for the formation of GeTe NPs with defined chemical composition, size, and shape
M(dmea)3 (M = Sb, Bi) + (R3Si)2E (R = Me, Et; E = S, Se, Te)	M2E3 NPs (M = Sb, Bi)	120-140 °C, DIPB	98	Thermally stable but chemically reactive metal aminoalkoxides lead to phase-pure Bi2Te3 without any Bi impurity
BiO(ClO4) + (Me3Si)2Te	Bi2Te3 NPs	RT, reverse micelles in AOT, H2O and hexane	110	The microemulsion method facilitates RT synthesis of monodisperse, sub-10 nm NPs
SbCl3 + (Et3Si)2Se	Sb2Se3 nanowires	150 °C, OLA, [C4C1Im]Cl	102	Reaction easily scalable up to 10 g of NPs
(Et3Si)2E + [C4C1Im]3[Bi3I12] or [C4mim]3[Bi3I12]	Bi2E3 (E = Se, Te)	150 °C, OLA, [C4C1Im]Cl or [C4mim]I	68, 102 and 103	Oleylamine effectively suppresses the homolytic Se–Si bond cleavage to afford the phase-pure product
Zn(OAc)2(3,5-Me2Py)2 + (Me3Si)2E (E = S, Se, Te) + MCM-41	ZnE NPs (E = S, Se, Te) within MCM-41	RT, toluene, CH2Cl2	106	RT synthesis ensures no modification of the mesoporous characteristics of MCM-41
Cd(OAc)2(P^nBu3)2 + (Me3Si)2E (E = S, Se, Te) + MCM-41/MCF	CdE (E = S, Se, Te) within the mesoporous hosts	RT, CH2Cl2	107	The organic ligand, which anchors the CdE material within MCM-41/MCF, is cleaved on heating beyond 135 °C, resulting in the aggregation of CdE outside of the pores
HgCl2 in excess + (Me3Si)2Te	Non-aggregating HgTe QDs	100–120 °C, OLA	94	Use of an excess of HgCl2 ensures a well-dispersed and stable colloidal suspension
EuCl2 + (Me3Si)2S	EuS NPs	200 °C, OLA	95	—
EuI2 + (Me3Si)2S	Mixed-valent Eu3S4	200 °C, OLA	95	Initially Eu S is formed at lower temperatures, which is converted to Eu3S4 over time
SmI2 + (Me3Si)2S	Mixed-valent Sm3S4	300 °C, OLA	95	Higher reaction temperature due to the greater redox potential of samarium
EuI2 + SmI2 + (Me3Si)2S	Mixed-valent EuSm2S4	300 °C, OLA	95	Based on XPS, the authors described the materials as (Eu^2+)(Eu^3+,Sm^3+)2S4
(NHC)MBr (M = Cu, Ag) + (Me3Si)2S	NHC-protected Cu2−xS or Ag2S QDs	RT, CH2Cl2, ODE	80	Formation of NHC–Ag2S QDs via the intermediate complex [NHC–AgSAg–NHC]
(NHC)MBr (M = Cu, Ag) + (^tBuMe2Si)2Se	NHC-protected Cu2−xSe or Ag2Se QDs	RT, CH2Cl2, THF	80	NHC ligands containing long carbon chains have superior competence over commonly used organic capping ligands (OA, OLA) for obtaining highly monodisperse QDs
Metal NPs (Pd, Pt, Ni) + (Me3Si)2Te	MTe2 NPs (M = Pd, Pt, Ni)	180 °C, TOPO, OLA	111	The high reactivity of silylated (Me3Si)2Te ensures formation of the telluride-rich phase MTe2 over MTe
(Me3Si)2S + ZnO NPs	ZnS NPs with hollow morphology	235 °C, TOP/TOPO	112	The tetrapod morphology of ZnO is preserved in ZnS
(Me3Si)2S + CdO NPs	CdS NPs with hollow morphology	130 °C, TOP/TOPO	109	The pseudo-spherical shape and crystallinity of CdO are preserved in the CdS NPs
(Me3Si)2E (E = S, Se) + NiO NPs	Ni3S2 or Ni5Se5 NPs with hollow morphology	130 °C, TOP	113	Silylated organyl chalcogenides generate a protecting amorphous silica layer around NPs
Co3O4 or CoO NPs + (Me3Si)2E (E = S, Se)	Co9S8 or Co3Se4 NPs with hollow morphology	130 °C, TOP	113	Hollow morphologies due to the nanoscale Kirkendall-type effect arising from different ion diffusion rates
Cu(TFA)2 + (Me3Si)2S	ODT-capped CuS NPs	RT, ODT, Et2O	43	The high reactivity of (Me3Si)2S ensures the formation of phase-pure CuS in 90% yield
Ag(TFA) + (Me3Si)2S	ODT-capped Ag2S NPs	RT, ODT, Et2O	43	Instant formation of phase-pure Ag2S in 95% yield
Ternary metal chalcogenides
Pb(oleate)2 + (Me3Si)2E (E = S, Se, Te)	PbSexTe1−x, PbSxTe1−x, and PbSxSe1−x	140 °C, OA and ODA	92	Composition-tunable ternary alloys due to excellent compatibility among (Me3Si)2E
Pb(oleate)2 + (Me3Si)2S + (Me3Si)2Se	PbSxSe1−x (x = 0.2–0.8)	130 °C, olive oil, ODA	93	Comparable reactivities of (Me3Si)2S and (Me3Si)2Se allow homogeneous incorporation of the chalcogenide anions
AgNO3 + SbCl3 + (Me3Si)2S	AgSbS2 NPs	180–220 °C, OLA, OA	99	Various sized AgSbS2 NPs with narrow size distributions as a function of reaction conditions (time, temp. or the OLA/OA volume)
(Me3Si)2S + NiCo2O4 NPs	(Ni/Co)9S8 NPs with hollow morphologies	130 °C, TOP	113	High reactivity of silylated (Me3Si)2S ensures no significant phase segregation
CuIn(OH)5 +(Me3Si)2S	OA-Capped CuInS2 NPs	200 °C, ODE, OA, OLA	101	NPs remain well-dispersed in non-polar solvents for several months
Cu2−xSe NPs + ZnEt2 + (Me3Si)2S	Core@shell Cu2−xSe@ZnS NPs	175 °C, OLA, TOPO	108	Stable plasmonic behavior by the core–shell structure

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4. Diorganyl dichalcogenides

Since their first use in the synthesis of SnSe and SnTe nanoparticles in 2002,¹¹⁴ diorganyl dichalcogenides R₂E₂ have emerged as facile chalcogenide reagents in the synthesis of binary, ternary and multinary metal chalcogenide NPs of controlled size and shape.^26,27

4.1. A comparison with diorganyl monochalcogenides

These reagents enjoy mostly the same advantages described above for the R₂E reagents, i.e. easy tunability of reactivity by varying the group R, facile transfer of the chalcogenide anion under mild conditions, easily removable byproducts, good solubility in common organic solvents, etc. However, these diorganyl dichalcogenides R₂E₂ do have an additional E–E bond which is cleaved rather easily under mild conditions. As a result, they rarely act as neutral ligands towards metal centres and, unlike a rich library available of metal complexes with organyl monochalcogenides,^32,33 there are very few metal complexes reported so far with neutral R₂E₂ ligands, mostly isolated at a low temperature.^115,116 However, these complexes are not stable and quickly decompose to give monochalcogenide complexes.^115,116 While the presence of an additional E–E bond alters the decomposition pathways of R₂E₂ and makes them slightly less reactive than R₂E reagents, it also renders them more air- and temperature-stable, resulting in easy handling and more commercial availability of a wide variety of differently substituted diorganyl dichalcogenides R₂E₂ (where E = S, Se, or Te and R = alkyl, allyl, benzyl, or aryl).^25–27 Another advantage of these reagents is that they can easily be transformed in situ to monoalkyl chalcogenides R–E via E–E cleavage, which can act as capping ligands to stabilize the formed NPs. This has implications in the chemistry of RSe- and RTe-stabilized NPs as, unlike thiol ligands, which have been extensively employed as surfactant for semiconducting NPs, the selenol and tellurol ligands are usually oxidatively instable and hence are difficult to use.²⁶ With an exception of a couple of reports,^117,118 the diorganyl dichalcogenides employed so far in the synthesis of metal chalcogenides are almost entirely non-silylated. This is in contrast to diorganyl monochalcogenides where silylated derivatives form the bulk of the reported examples.

The decomposition of R₂E₂, either thermally^25–27 or photolytically,^119,120 proceeds through the radical scission of the E–E and E–C bonds. The mechanism has been studied in detail by Vela et al. who carried out a combined experimental and DFT study correlating the bond strengths in R₂E₂ with the size and shape of the obtained CdSe and CdS NPs.²⁵ They demonstrated that, by changing the R group within the series of R₂E₂ (E = S, Se), the reactivities of these chalcogenide reagents could be tuned. While the E–E bond strength was found to be relatively constant, the calculated C–E bond strengths changed significantly upon substituting the R group and determined the overall reactivity of the R₂E₂. As compared to the R₂E₂ with weaker C–E bonds which were reactive and afforded isotropic NPs, those with large C–E bond dissociation energies were less reactive and yielded anisotropic, and often morphologically metastable NPs with higher surface areas (Fig. 6). Interestingly, the strength of the C–Se bond not only influences the kinetics of R₂E₂, but also determines the nature of the crystalline phase and composition of the product. For instance, the reactions of diallyl disulfide [(allyl)₂S₂] and dibenzyl disulfide with FeCl₂ in oleylamine at 220 °C yielded the pyrite (FeS₂) and greigite (Fe₃S₄) phases, respectively, whereas the reactions of di-tert-butyl disulfide ^tBu₂S₂ and di-phenyl disulfide (Ph₂S₂) under similar conditions produced the pyrrhotite (Fe₇S₈) phase.²⁷ By exploiting this easy tunability of the reactivities by varying the R group, a number of articles have described the use of differently substituted R₂E₂ in the synthesis of binary, ternary or even multinary metal chalcogenide NPs in the last ten years. Some selected examples are summarized in Table 4.

Fig. 6 Varying C–S bond strengths and reactivities among R₂E₂ precursors (a) and its influence on the particle shape (b–d) as well as composition and crystalline phase (e–j) of metal chalcogenide nanomaterials. Adapted with permission from ref. 25 and 27. Copyright (2013, 2017) American Chemical Society.

Table 4 Some selected examples of metal chalcogenide nanomaterials synthesized under mild conditions using R₂E₂

Reagents	Materials	Synthesis (temp., solvent, capping ligand, etc.)	Ref.	Comments
Abbreviations: DDA = dodecylamine; HAD = hexadecylamine; DDT = dodecanethiol; HMDS = hexamethyldisilazane (Me3Si)2NH; OA = oleic acid; ODE = 1-octadecene; ODT = octadecane thiol; OLA = oleylamine; TOP = trioctylphosphine; TOPO = trioctylphosphine oxide; RT = room temperature; NPs = nanoparticles.
Ph3Bi + ^tBu2Se2	BiSe NPs	RT photolysis (254 nm per 4 h), DDA, P-123 copolymer	120	Ph3Bi and ^tBu2Se2 both undergo homolytic photolysis without using a reducing reagent to produce 5 nm BiSe NPs
GeI4 + Ph2Se2	GeSe nanosheets	268 °C, HDA	121	—
Metallic Sn NPs + Ph2Te2	Surfactant-free SnTe NPs	165 °C, diglyme	114	Large spherical NPs (∼60 nm) or smaller star-shaped agglomerates of needles (∼15 × 40 nm) are obtained depending on the Sn/Ph2Te2 ratio
SnCl2 + ^tBu2Se2	SnSe NPs	180 °C, DDA, DDT	122	Stoichiometric SnCl2/^tBu2Se2 ratio yielded phase-pure SnSe, whereas an excess of ^tBu2Se2 yielded SnSe2
SnI4 + GeI4 + ^tBu2Se2	Ternary SnxGe1-xSe NPs	225 °C, DDA, HMDS	123	In the absence of HMDS, only binary SnSe NPs were produced
CuCl + SnI4 + ^tBu2Se2	Wurtzite Cu2SnSe3 NPs	180 °C, DDA, DDT	42	The combination of DDT and ^tBu2Se2 is critical to obtain the wurtzite phase of Cu2SnSe3
In(acac)3 + ^tBu2S2	In2S3 nanorods	180 °C, OLA	124	Synthesis of In2S3 nanorods takes 7 h due to the less reactivity of ^tBu2S2
CuCl + In(acac)3 + ^tBu2S2	Wurtzite Cu–In–S NPs	180 °C, OLA, DDT	125	Slow formation of In2S3 leads to the isolation of a metastable wurtzite Cu–In–S phase
Cu(oleate)2 + Ph2Se2	Wurtzite-like CuInSe2 NPs	220 °C, OLA	49	The metastable phase is formed through the Cu3Se2 intermediate
InCl3 + SnCl2 + Ph2Se2	In4SnSe4 microwires	280 °C, HMDS, OLA, ODE	126	—
ZnO + ^tBu2S2	ZnS NPs	235 °C, OLA, TOP	127	ZnS with a hollow structure, induced by the nanoscale Kirkendall effect
Cd(oleate)2 + R2E2 (R = allyl, Bz, tBu, iPr, Et, Me, Ph; E = S or Se)	CdS and CdSe NPs	250 °C, OLA, OA, ODE	25	CdS and CdSe NPs with spherical, elongated or tetrapodal morphology by varying the reactivity of R2E2
CuCl2 + ^tBu2S2	Cu2−xS NPs	180-220 °C, OLA + DDT	128	Roxbyite Cu1.78S or Djurleite Cu1.96S depending on Cu concentration
Cu(acac)2 + (dodecyl)2Se2	Wurtzite Cu2−xSe NPs	155 °C, ODE	50	Metastable wurtzite Cu2−xSe NPs are irreversibly transformed to thermodynamic cubic Cu2Se at 185 °C
Cu(oleate)2 + Ph2Se2	Wurtzite Cu3Se2 NPs	220 °C, OLA	49	Metastable wurtzite Cu3Se2 NPs
Cu(acac)2 + (dodecyl)2Te2	Pseudo-cubic Cu1.5Te NPs	135 °C in a variety of solvents (ODE, OlA, OA and DOE)	51	Metastable pseudo-cubic Cu1.5Te NPs are formed at 135 °C, whereas a thermodynamic vulcanite CuTe phase is obtained at 185 °C
Cu(oleate)2 + Ph2Se2 + Zn(oleate)2 + Fe(acac)3	Wurtzite-like Cu2FeSnSe4 NPs	250 °C, OLA	129	Metastable quaternary NPs through sequential cationic replacement in the initially formed binary Cu3Se2 NPs
Cu(oleate)2 + Ph2Se2 + Zn(oleate)2 + Sn(ethylhexanoate)2	Wurtzite-like Cu2ZnSnSe4 NPs	255 °C, OLA	130	Metastable quaternary NPs through sequential cationic replacement in the initially formed binary Cu3Se2 NPs
AgNO3 + In(OAc)3 + Bz2Se2	Metastable AgInSe2 NPs	230 °C, OLA, OA, ODE	131	Presence of oleylamine essential to get the metastable phase
CuCl + Fe(acac)3 + Ph2Se2	CuFeSe2 nanosheets	255 °C, OA, OLA, ODE	132	Hierarchical nanostructures assembled from nanosheets of ∼50 nm thickness and ∼400 nm side length
FeCl2 + (allyl)2S2	Pyrite FeS2 NPs	220 °C, OLA	27	Due to a lower C–S bond energy as compared to the S–S one, (allyl)2S2 releases a persulfide (S–S)^2− ion, leading to formation of pyrite FeS2 NPs
FeCl2 + (benzyl)2S2	Greigite Fe3S4 NPs	220 °C, OLA	27	A sulphur-deficient phase is obtained through the rupture of the S–S bond
FeCl2 + R2S2 (R = ^tBu, Ph)	Pyrrhotite Fe7S8 NPs	220 °C, OLA	27	A sulphur-deficient phase is obtained through the rupture of the S–S bond
WCl4 + ^tBu2Se2	Amine-stabilized WSe2 nanosheets	225 °C, DDA	133	Addition of tetra-n-octylammonium bromide prevents agglomeration of colloidal 2H-WSe2 nanosheets
W(CO)6 + Ph2Se2	WSe2 NPs	330 °C, OA, TOPO	134	A higher OA/W(CO)6 molar ratio yields slower nucleation, larger NPs and a shift from the 2H to the 1T' crystal phase
W(CO)6 + Ph2Se2	MSe2 (M = Mo, W) nanosheets	330 or 350 °C, OA, OLA, oleyl alcohol	135	Single-layer MSe2 nanosheets obtained by using oleic acid, while multilayer nanosheets are formed with oleyl alcohol and oleylamine
MoCl5 + WCl6 + Ph2Se2	MoxW1−xSe	300 °C, OA, OLA, HMDS	136	Easily tunable compositions by varying the Mo and W ratio

---

The different reactivities of diorganyl mono- and dichalcogenides can be highlighted by comparing the reactions of ^tBu₂E and ^tBu₂E₂. As compared to ^tBu₂E which often affords molecular precursors with low decomposition temperature (Fig. 1) or even NPs at room temperature (Fig. 2), the reaction of ^tBu₂E₂ requires relatively high temperature to afford NPs of binary and ternary metal chalcogenide NPs (Fig. 7). For instance, ^tBu₂Se₂ reacts with SnCl₂ or a combination of CuCl and SnI₄ to yield binary SnSe and ternary Cu₂SnSe₃ NPs, respectively, at 180 °C in the presence of dodecylamine (DDA) and dodecanethiol (DDT) (Fig. 7a and b).^42,122 While the stoichiometric ratio of Sn and Se reagents was essential for getting phase-pure SnSe NPs, the use of DDT and hexamethyldisilazane (HMDS) was critical to obtain the new wurtzite phase of Cu₂SnSe₃ NPs. Similarly, ternary Sn_xGe_1−xSe NPs were prepared by reacting ^tBu₂Se₂ with varying ratios of GeI₄ and SnI₄ in DDA and HMDS at 225 °C.¹²³ The decomposition of ^tBu₂E₂ (E = Se, Te) can be triggered photolytically also, thus decreasing the reaction temperature significantly.^119,120 For example, the photolytic decomposition at 254 nm for 4 h of the deaerated solution of Ph₃Bi and ^tBu₂Se₂ in water containing dodecylamine and a triblock copolymer P-123 yielded BiSe NPs of an average size of 5 nm (Fig. 7c).¹²⁰ As observed in the case of the R₂E series, a stronger C–S bond (in comparison to the C–Se bond) makes R₂S₂ slightly less reactive. For example, the reaction of CuCl₂ with an excess of ^tBu₂S₂ yields roxbyite Cu_2−xS NPs, where a slow but continuous growth of the nanoparticles (due to the low nucleation rate of the reagent) not only ensured accurate size control of the NPs but also allowed to follow their gradual morphology evolution from spheres and disks to tetradecahedrons and dodecahedrons (Fig. 7d).¹²⁸ The relatively slow reactivity of ^tBu₂S₂ is further manifested from its reaction with In(acac)₃ which required 7 h to afford In₂S₃ at 180 °C (Fig. 7e).¹²⁴ This slow rate was subsequently exploited to synthesize metastable wurtzite ternary CuInS NPs by performing the above reaction in the presence of CuCl and DDT (Fig. 7f).¹²⁵ Other sulphur sources such as elemental S₈ or tert-butyl thiol produced either no crystalline products or thermodynamic chalcopyrite phase.

Fig. 7 Utilization of ^tBu₂E₂ (E = S, Se) precursors to get binary and ternary metal chalcogenide nanoparticles. Adapted with permission from ref. 42, 120, 122, 124, 125, and 128. Copyright (2011) The Royal Society of Chemistry and (2009, 2009, 2010, 2011, 2012) American Chemical Society.

4.2. Isolation of metastable phases

While materials in metastable crystal structures can afford useful properties, synthesizing metastable materials necessarily requires kinetic control of the chemistry away from thermodynamic equilibrium, which is not always easy to achieve. The above results showed that variable reactivity in the R₂E₂ series, obtained simply by changing the R and E, could be utilized to control the reaction kinetics and hence predict and provide the synthetic pathways for thermodynamically or kinetically driven products. Unsurprisingly, these reagents have been employed prominently in the past few years for the synthesis and isolation of several metastable phases (Fig. 8). For instance, the reactions of Cu(acac)₂ with (dodecyl)₂E₂ (E = Se, Te) in a variety of solvents (ODE, OlAm, OA or DOE) afforded metastable wurtzite Cu_2−xSe and pseudo-cubic Cu_1.5Te NPs at 155 and 135 °C, respectively.^50,51 The right choice of the organyl group in R₂E₂ was important for isolating these metastable phases under mild synthetic conditions because at higher temperature (185 °C) these phases are irreversibly transformed into the thermodynamic cubic Cu₂Se and vulcanite CuTe phases, respectively. Similarly, using Ph₂Se₂ that possesses a higher C-Se bond strength, a “wurtzite-like” metastable phase of the ternary CuInSe₂ was obtained, whereas R₂Se₂ (R = methyl, benzyl) with weaker C–Se bonds yielded NPs with the thermodynamic chalcopyrite crystal structure.⁴⁹ These two phases are formed via distinct copper selenide intermediates, with the wurtzite-like phase forming through Cu₃Se₂ and the chalcopyrite forming through Cu_2−xSe intermediates.

Fig. 8 Isolation of metastable phases of binary and ternary metal chalcogenide NPs using R₂E₂ (R = an organyl; E = S, Se, Te) precursors. Images reproduced with permission from ref. 49–51 and 129–131. Copyright (2020) The Royal Society of Chemistry and (2018, 2019, 2020, 2021, 2021) American Chemical Society.

The fact that the reaction of Ph₂Se₂ with Cu(oleate)₂ in oleylamine yields preferentially the metastable Cu₃Se₂ intermediate which possesses a pseudo-hexagonal selenium sublattice that serves as a template for the ultimate wurtzite-like products has been exploited further to isolate many metastable phases. Thus, wurtzite-like metastable multinary Cu₂FeSnSe₄ and Cu₂ZnSnSe₄ have been isolated by in situ sequential cation exchange reactions between the intermediate Cu₃Se₂ NPs and metal precursors in solution.^128,129 Besides the reactivity of the R₂E₂ reagent, the choice of surfactants employed also influences the course of the reaction and the nature of the final products. This is demonstrated by the reaction of Bz₂Se₂ with AgNO₃ and In(OAc)₃ in the presence of either oleic acid or oleylamine, which yielded thermodynamic chalcopyrite and metastable orthorhombic phases, respectively, of the ternary AgInSe₂ NPs at 250 °C.¹³⁰ The authors explained it on the basis of the tetragonal-to-orthorhombic phase transformation of the intermediate Ag₂Se in the presence of oleylamine, which is then converted to the metastable orthorhombic phase of AgInSe₂via fast In³⁺ cation exchange due to structural similarities between the two phases. In the absence of oleylamine, the Ag₂Se intermediate is converted to the thermodynamic chalcopyrite structure of AgInSe₂via kinetically slow nontopotactic conversion processes due to structural dissimilarities between the intermediate Ag₂Se and chalcopyrite AgInSe₂.

5. Conclusions and looking ahead

This feature article provides a concise and comparative overview on the chemistry and reactivity of three related families of organyl chalcogenides as convenient reagents for the ultra-mild synthesis of metal chalcogenide nanomaterials. These synthons not only transfer the chalcogenide anion (E²⁻) to a metal center under mild conditions but also allow their reactivity and properties to be modulated. Furthermore, their commercial availability, high solubility in common organic solvents and ability to react in a facile manner even with non-conventional metal reagents make them ideal precursors for scaled-up and controlled synthesis of nanometric metal chalcogenides.

Unlike diorganyl dichalcogenides R₂E₂, the solution-phase utilization of diorganyl monochalcogenides has mostly been restricted to the silylated derivatives (R₃Si)₂E. Recent employment of nonsilylated diorganyl monochalcogenides R₂E, which are less reactive than their silylated counterparts, has allowed the isolation and characterization of reactive molecular intermediates containing coinage metals during the synthesis of metal chalcogenides. An extension of this chemistry to other metals would help develop a broader understanding of the molecule-to-nanoparticle transformation and, therefore, achieve the synthesis of even compositionally complex materials with greater control. Recent work indicates that, by using non-silylated R₂E, especially the ones that have a facile decomposition mechanism, it should also be possible to prepare a library of precursors that decompose at very low temperature to generate metal chalcogenide nanomaterials under ultra-mild conditions.^43–46 These studies should be extended to the heterometallic complexes [M_xM′_y(R₂E)_z] containing two different metals or heteroleptic complexes [M_x(R₂E)_y(R₂E′)_z] with two different organyl chalcogenide ligands. These single source precursors with enhanced properties can overcome the problem of controlling the stoichiometry of the constituent elements, which often hinders the reproducibility in the synthesis of ternary metal chalcogenides. Reagents or precursors having comparable decomposition characteristics can also be a viable option for controlled synthesis of ternary phases, and despite some recent progress in it,^63,92,93 better knowledge is required. A detailed computational and experimental study involving a systematic change of the R group to determine the variable reactivity in non-silylated R₂E reagents would further allow us to control the reaction conditions, particularly to provide new synthetic pathways for kinetically-driven products. Previous studies on the R₂E₂ series have shown that the precursors possessing strong R–E bonds and therefore being less reactive are better suited for getting metastable phases.⁴⁹ Non-silylated R₂E, which are less reactive than silylated (R₃Si)₂E, have great potential in this regard.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The financial support provided by the Université Claude Bernard Lyon 1 and the infrastructural support available at IRCELYON are thankfully acknowledged. The author also thanks his former PhD and master's students who worked in this field and whose names appear in the reference section.

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