Tuning the Composition of Multicomponent Semiconductor Nanocrystals

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Cite This: Chem. Mater. 2018, 30, 1446−1461

Tuning the Composition of Multicomponent Semiconductor Nanocrystals: The Case of I−III−VI Materials Olesya Yarema, Maksym Yarema, and Vanessa Wood* Laboratory for Nanoelectronics, Department of Information Technology and Electrical Engineering, ETH Zurich, CH-8092 Zurich, Switzerland ABSTRACT: Among the advantages of multicomponent nanocrystals is the possibility to adjust their electronic and optical properties with composition as well as size. However, the synthesis of multicomponent nanocrystals is challenging due to the presence of several metal precursors in the reaction mixture. This review takes I−III−VI semiconductor materials as an example class of multicomponent nanocrystals to highlight the underestimated importance of composition, which can affect the electronic and optical properties of nanocrystals as much as size. We discuss synthetic strategies, which enable the composition control, and show that the ability to separately choose nanocrystal size and nanocrystal composition can be beneficial for many optoelectronic and biomedical applications.

1. INTRODUCTION 1.1. Multicomponent Semiconductors: Tuning Properties by Composition. An accurate composition control achievable in semiconductors has revolutionized the industry and brought in the modern electronics and optoelectronic devices we know today. For example, the III−V group semiconductors can form continuous solid solutions, which leads to variable ternary compositions and precise adjustment of optical band gap, carrier mobility, lattice constant, etc. Controlling the composition of III−V semiconductors has enabled the development of high-performing multijunction solar cells with mismatch-free interfaces and optimal band alignment,1 high-brightness blue InGaN/AlGaN light-emitting diodes,2 or multiquantum-well InGaN laser diodes.3 Composition tunability allows Hg1−xCdxTe (MCT) photodetectors to detect infrared photons in a broad spectral region from 1 to 30 μm.4 Ternary BixSb2−xTe3 semiconductor with a ratio between Bi and Sb close to 0.33 exhibits near optimal thermoelectric performance.5 Finally, varying the concentration of Ga in CuIn1−xGaxSe1−ySy thin film photovoltaics enables accurate engineering of a band gap across the absorbing layer and increase of solar cell efficiency.6 Composition-tunable properties of semiconductor materials is an established concept; however, for colloidal nanocrystals (NCs), this strategy of tuning the optical, electronic, and structural properties remains underappreciated. 1.2. Multicomponent Semiconductor Nanocrystals: An Open Promise. About 25 years ago, Cd-chalcogenide semiconductors (CdE, where E = S, Se, Te) were the first colloidal NCs developed with excellent size control.7 Most attention has been focused on understanding and tuning the quantum-confinement phenomena, which yield the well-known size-dependent optical and electronic properties of NCs.8 Since then, nearly all binary semiconductors have been prepared in the form of stable colloids, including group II−VI, IV−VI, III− V, I−VI, and II−V NCs.9,10 Further development of colloidal © 2018 American Chemical Society

nanomaterials can be realized via synthesis of more complex, multicomponent nanomaterials, which consist of three or four elements (i.e., ternary or quaternary semiconductors).11,12 To date, a large number of ternary and quaternary colloidal NCs have been synthesized (Figure 1).9,11,13−15 Multicomponent NCs can be grouped as semiconductor classes (I−III−VI, I−V−VI, etc.), by their crystal structure type (perovskites, kesterites, etc.), or by solid-state terminology (oxides, rare-earth fluorides, etc.). In many cases, multicomponent NCs have a preferred stoichiometric composition (e.g., NaYF4, CsPbBr3, Cu2ZnSnS4, etc.), but many are also stable with nonstoichiometric compositions (e.g., CuIn3Se5, AgIn5Te8, AgBiS2, etc.), which results in the semiconductor class having composition-dependent structure, electronic, and optical properties.16−18 It becomes apparent that, for multicomponent NCs, composition tunability opens an additional parameter to create NCs with specific properties and performance. 1.3. Case Study Systems: I−III−VI Colloidal Nanocrystals. In this review, we discuss the challenges and opportunities associated with the synthesis of multicomponent semiconductor NCs, focusing on ternary I−III−VI group semiconductors. We choose I−III−VI compounds because they are relatively well explored as bulk materials as well as colloidal NCs. Semiconductor I−III−VI materials are of interest for a wide variety of applications, for instance, in thin-film photovoltaics.6,19 Most importantly, ternary I−III−VI semiconductors can be found in both stoichiometric and nonstoichiometric compositions, and this composition tunability is the key property to their characteristics and device performance.20 Most literature on I−III−VI NCs is devoted to stoichiometric I−III−VI nanomaterials, such as CuInS2, CuInSe2, and Received: November 9, 2017 Revised: February 7, 2018 Published: February 8, 2018 1446

DOI: 10.1021/acs.chemmater.7b04710 Chem. Mater. 2018, 30, 1446−1461

Review

Chemistry of Materials

Figure 1. Examples of multicomponent semiconductors with distinct compositions (stoichiometries) prepared in the form of colloidal nanocrystals.9,11,13−15

Figure 2. (a) Structural relation between CdSe and Cu−In−Se semiconductors (VCu denotes Copper vacancy; CuIn and InCu, copper and indium antisite defects; Cui, copper interstitial defect).31 (b) Quasi-binary phase diagram of Cu2Se−In2Se3 (adapted with permission from ref 35; copyright 1987 Springer). Single-phase regions are shaded; dashed lines show ordered-vacancy compositions. by the Group II metal, while the cationic site in the I−III−VI materials is a mixture of a Group I metal (i.e., Ag or Cu) and a Group III metal (i.e., Al, Ga, In, or Tl). The ratio between constituent Group I and Group III metals can be tuned over a wide region.34−39 Taking the example of Cu−In−Se: when the Cu:In ratio is higher than 1, the material is referred to as “Cu-rich”, when the Cu:In ratio is lower than 1 it is “In-rich”, and when the Cu:In ratio is 1, the phase is “stoichiometric”. Figure 2a illustrates the structural relations between CdSe (i.e., II− VI) and Cu−In−Se (i.e., I−III−VI) materials. The presence of two nonisovalent cations in the I−III−VI structure leads to a small deformation of the structure in one direction. The stoichiometric I− III−VI2 phases can thus be described by tetragonal lattice with c/a ratio very close to 2 (typically, 1.91−2.01).40,41 The structure of nonstoichiometric I−III−VI compositions is stabilized by the formation of various atomic defects (Figure 2a).31,42 For the case of In-rich Cu−In−Se materials, Cu vacancies are formed,43 while Cu-rich Cu−In−Se compositions are realized through the formation of antisite and interstitial Cu defects.31,32 Figure 2b shows a quasi-binary phase diagram for the Cu2Se−In2Se3 system. Four different Cu−In−Se ternary materials can be realized (shaded regions in Figure 2b). Two of these are the room-temperature and high-temperature modifications of the CuInSe2 phase, which have chalcopyrite and zinc-blend structure, respectively. The other two represent In-rich Cu−In−Se phases, and their structure is closely related to chalcopyrite CuInSe2. For all Cu−In−Se compounds, the composition can be tuned over a broad range, enabling accurate adjustment of material properties by composition control. Finally, several In-rich Cu−In−Se compositions exhibit long-range ordering of

their Ag-containing analogues, and several recent reviews summarize the status of I−III−VI2 NCs.21−23 Nonstoichiometric I−III−VI NCs are less studied. A possible reason may be that, for bulk II−VI or III−V semiconductors, nonstoichiometry results in poorly performing device characteristics.24 However, for In-rich I−III−VI NCs the nonstoichiometry does not necessarily worsen their properties. In fact, In-rich I− III−VI NCs are better emitters than stoichiometric or Cu-rich/ Ag-rich I−III−VI compositions.18,25,26 Record photoluminescence (PL) quantum yields (QYs) belong exclusively to In-rich I−III−VI NCs, e.g., core/shell structures of AgIn5S8 (QY = 87%),27 CuIn5S8 (QY = 92%),28 Cu3In5Se9 (QY = 60%),29 and Ag3In5Se9 (QY = 73%) NCs;30 however, the reason for this efficient emission is still under debate.31,32 We structure this review to first explain the bulk structural, electronic, and optical properties of I−III−VI materials (Section 2) before explaining existing strategies to synthesize stoichiometric and nonstoichiometric I−III−VI NCs (Section 3) and the effect of composition on NC properties (Section 4).

2. PROPERTIES OF BULK I−III−VI MATERIALS 2.1. Crystal Structure of Bulk I−III−VI Materials. The structure of I−III−VI group semiconductors consists of two metals and a chalcogen. It can be viewed as a derivative from zinc-blende-type II− VI binary semiconductors, such as ZnS or CdTe.33 In both binary II− VI and I−III−VI materials, the anionic site is the same (i.e., S, Se, or Te). However, in binary II−VI materials, the cationic site is occupied 1447


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Figure 3. Schematic illustration of the vacancy ordering concept for the Cu−In−Se material, showing the (001) monolayer and unit cell of (a) stoichiometric CuInSe2, (b) Cu3In5Se9, and (c) CuIn3Se5 ordered-vacancy compounds. Cu and In atoms are orange; cat.% denotes cationic percentage.

Figure 4. Calculated band diagrams and electronic density of states for chalcopyrite CuInSe2 and ordered-vacancy CuIn5Se8 materials. Valence band maximum (VBM) is set to 0 eV. DOS denotes electronic density of states (adapted with permission from ref 31; copyright 1998 American Physical Society). Cu vacancies, often referred to in literature as ordered-vacancy compounds (OVCs).19 The family of Cu−In−Se OVCs consists of Cu3In5Se9, Cu2In4Se7, CuIn3Se5, and CuIn5Se8 ternary materials, the structures of which have 1 vacancy per every 9, 7, 5, and 4 cationic sites, respectively (dashed lines in Figure 2b).31 The concept of vacancy ordering can be illustrated for the (001) monolayer of the Cu−In−Se phase (Figure 2a). Figure 3a shows the (001) plane of stoichiometric CuInSe2. The cationic square net can be split, e.g., by groups of 9 (Figure 3b) or 5 atoms (Figure 3c). Removing one cation from each group leads to long-range vacancy ordering and tetragonal lattices for Cu3In5Se9 and CuIn3Se5 OVCs (appearing as squares on Figures 3b,c). Similarly to Cu−In−Se materials, other Cu−In−E and Ag−In−E systems (where E = S, Se, or Te) contain stoichiometric and In-rich I− III−VI phases with high-temperature modifications and broad solid solution range for many of them.34−39 The existence of OVCs is observed for many In-rich I−III−VI materials.31,44,45 The I−III−VI semiconductors exhibit interesting composition-dependent optical and electronic properties, stemming from structural peculiarities of these materials.31 2.2. Electronic and Optical Properties of Bulk I−III−VI Materials. Among the I−III−VI ternary semiconductors, Cu−In− Se materials are the most studied, due to their application in thin film photovoltaics.19 Stoichiometric CuInSe2 is a direct semiconductor with a band gap of 1.04 eV. Depending on growth conditions, the composition of the CuInSe2 phase can deviate from the 1:1:2 ratio, and either a p- or a n-type semiconductor can be obtained.31 In-rich

Cu−In−Se phases, such as CuIn3Se5 and CuIn5Se8, exhibit larger optical band gaps (direct, 1.21 and 1.15 eV, respectively).45 The broadening of the optical band gap is associated with repulsion weakening between Cu d and Se p valence band states, which leads to lowering of the valence band maximum for In-rich Cu−In−Se materials.31,46 Figure 4 shows calculated band diagrams and density of states for CuInSe2 and CuIn5Se8 compounds, and In-rich Cu−In−Se material indeed shows the larger band gap. The weaker p−d repulsion in CuIn5Se8 is apparent from the lack of the p−d repulsion gap seen in CuInSe2 at −2.5 eV (relative to the valence band maximum, Figure 4). Other In-containing I−III−VI materials are also direct semiconductors. Their optical band gaps range from 1 to 2 eV, increasing progressively from tellurides to selenides to sulfides as is expected due to the increasing ionic nature of the bonds (Table 1).41,44,45,47−49 Agcontaining I−III−VI materials exhibit larger optical band gaps than Cu-containing counterparts.41 Finally, In-rich I−III−VI compositions have larger band gaps than their stoichiometric counterparts, an effect which is most pronounced for sulfides.45 The structure of I−III−VI materials tolerates large concentrations of defects. Figure 5 schematically shows in-gap defect states, which can be found for CuInSe2.31 Copper vacancies (VCu) form very shallow acceptor states, while indium vacancies (VIn), antisite defects (InCu, CuIn), and interstitial copper atoms (Cui) form donor defects at various in-gap energy levels. Several of these atomic defects (such as VCu, InCu, and Cui) have particularly low formation energies.31,32 Theoretical calculations suggest that VCu and InCu are able to cluster, forming defect pairs (i.e., InCu−VCu), which can further order within 1448


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Chemistry of Materials Table 1. Experimentally Determined Optical Band Gaps of Selected I−III−VI Compounds I−III−VI

Eg, eV

ref

CuInS2 CuIn5S8 CuInSe2 CuIn3Se5 CuIn5Se8 CuInTe2 CuIn3Te5 CuIn5Te8 AgInS2 AgIn5S8 AgInSe2 AgIn5Se8 AgInTe2 AgIn5Te8

1.53 1.7 1.04 1.21 1.15 1.01 1.03 1.02 1.87 1.94 1.2 1.25 0.96−1.04 1.0

41 47 45 45 45 45 45 45 41 44 48 48 41 49

Figure 6. Examples of hard and soft acids and bases, used in colloidal synthesis of nanocrystals (reprinted with permission from ref 13; copyright 2017 American Chemical Society).

precursor and ligand selection. Metal salts of hard−soft nature decompose faster and/or at lower temperatures than hard− hard or soft−soft precursors. The presence of coordination agents can modify the reaction kinetics of precursors to different extents by achieving either soft−soft and hard−hard complexes (relatively slower decomposition) or hard−soft and soft−hard compounds (relatively faster decomposition). Understanding the synthesis of I−III−VI NCs benefits from the HSAB concept guidelines.30 Since the reaction flask contains a soft acid (M+ = Cu+ or Ag+) and a hard acid (M3+ = In3+, Ga3+, etc.), there is an intrinsic difference in the reactivity of two otherwise identical metal precursors (e.g., AgCl and InCl3). Moreover, according to HSAB theory, chalcogen anions are soft bases (Figure 6),13 and their softness increases proportionally with ionic radius (S2− < Se2− < Te2−). The synthesis of I−III−Se NCs is therefore more challenging than for I−III−S NCs, due to strong preferential bonding of Se and Cu (or Ag) precursors. The synthesis of colloidal I−III−VI tellurides represents an extreme case. 3.2. Synthesis of Stoichiometric I−III−VI2 Phases: Balancing the M+/M3+ Reactivity. Because of imbalanced M+/M3+ reactivity, composition and size control for colloidal I−III−VI NCs can be attained only by a few synthetic methods, such as alkanethiol-mediated synthesis, use of metal−sulfurbonded precursors, amide-promoted approach, water-based synthesis, and cation-exchange reaction. Before introducing these synthetic approaches in the next section, it is instructive to survey the syntheses for stoichiometric I−III−VI NCs, sorted by I−III−VI materialCuInS2, AgInS2, I−III−VI selenide (CuInSe2 and AgInSe2), and finally I−III−VI telluride (CuInTe2 and AgInTe2) NCs. CuInS2 Nanocrystals. Most syntheses of CuInS2 NCs are based on the strategy developed by Xie et al.52 This recipe comprises an injection of elemental S solution into the reaction mixture, containing Cu+ and In3+ salts, dodecanethiol, oleic acid, and octadecene at 180 °C (Figure 7). The presence of long-chain thiol is critical to the synthesis. The reactive Cu+ ion, a soft acid, is preferentially complexed by dodecanethiol, a soft base. In order to balance reactivity between the Cu+ and the

Figure 5. Schematic illustration of defect energy levels found in the CuInSe2 structure. VCu and VIn are copper and indium vacancies; CuIn and InCu are copper and indium antisite defects; Cui is a copper interstitial defect (adapted with permission from ref 31; copyright 1998 American Physical Society). the lattice.31,32 Pairing and ordering of InCu and VCu defects leads to lowering of the defect formation energy. Simultaneously, the energy level of InCu is pushed closer to the conduction band.31 Ordered defect pairs of cationic vacancy and nearby indium are regarded as electronically benign native defect groups of In-rich I−III−VI semiconductors that do not hinder their optoelectronic performance.31,32

3. SYNTHESIS OF I−III−VI NANOCRYSTALS 3.1. Hard and Soft Acids and Bases (HSAB) Concept. The HSAB theory provides a qualitative description of reactions in inorganic and coordination chemistry. It was first introduced by R. Pearson in 1963 and refined in the 1980s.50,51 According to the HSAB concept, reactants can be classified as acids (accepting electrons) and bases (donating electrons). The acids and the bases are further categorized as hard, borderline, or soft, which describe their electron accepting/donating affinities. The HSAB theory states that hard−hard and soft− soft acid−base configurations form more stable compounds than the ones of mixed hard−soft nature. The HSAB concept can be applied to the colloidal synthesis of NCs.13 Metal centers represent acids, whereas anions and coordination ligands are bases (Figure 6). In designing a colloidal synthesis, HSAB theory may be used as a guideline for 1449


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Figure 7. (a) Scheme of the alkanethiol-mediated synthesis of CuInS2 nanocrystals. (b) Composition tuning by introducing an excess of dodecanethiol. (c) TEM images, illustrating a size control for stoichiometric CuInS2 nanocrystals (adapted with permission from ref 52; copyright 2009 American Chemical Society).

In3+, at least 5-fold excess of dodecanethiol must be added (Figure 7b).52 Follow-up modifications of the alkanethiolmediated synthesis include the use of soft-acid/soft-base CuI precursor, alternative S precursor (i.e., S-oleylamine solution), variations in injection temperature, and other reaction parameters.53−55 Dodecanethiol has also been used in several heating-up synthetic approaches of CuInS2 NCs.56−59 Due to its decomposition at high temperatures (≥200 °C), dodecanethiol can serve as a sulfur precursor. Such heating-up syntheses, however, require prolonged heating times, because of slow decomposition kinetics of dodecanethiol. Another way to balance the reaction for CuInS2 NCs is to employ metal precursors that already contain Cu−S and In−S bonds.60−62 Figure 8 lists several examples, such as ethyl-

dithiocarbamates is mixed in oleylamine and heated under inert atmosphere up to 180 °C for several minutes.63,66 The dithiocarbamate salts undergo thermal decomposition, serving as a “single” source of metals and sulfur. The reaction is nearly quantitative and yields 4−6 nm in size AgInS2 NCs with narrow size distribution. Although limited in size range, the dithiocarbamate-based synthesis provides accurate composition control for Ag−In−S NCs and excellent luminescent properties of obtained materials.67−70 CuInSe2 and AgInSe2 Nanocrystals. Compared to I−III−S NCs, approaches for CuInSe2 and AgInSe 2 NCs are substantially less developed.23 Facing the M+/M3+ reactivity problem, the methods for I−III−S NCs show limited applicability: (i) slow decomposition of long-chain thiols at high temperature leads to anion-mixed quaternary I−III-(S;Se)2 NCs;71,72 (ii) metal−Selenium bonded precursors are more difficult to synthesize than metal−sulfur precursors.60,73 To prepare stoichiometric CuInSe2 and AgInSe2 NCs, alkylamine- or phosphine-based reaction mixtures are used. Generally, elemental Se is dissolved in oleylamine or trioctylphosphine and heated with (or hot-injected to) the solution of metal salts.74−76 The reaction temperature is kept relatively high, while growth times are typically long. This reaction strategy counts on reaction completeness. However, only big sizes (typically >8−10 nm) of I−III−Se2 NCs can be obtained.74−76 For small-size CuInSe2 and AgInSe2 NCs, reaction kinetics must be accelerated. This is achieved either by introducing reaction-promoting agents to the reaction mixture (e.g., diphenylphosphine,77 lithium silylamide29,30,78) or by using highly reactive selenium source (e.g., trimethylsilyl selenide26,79). As an example, the use of diphenylphosphine allows one to tune the size of CuInSe2 NCs from ∼1 to 9.2 nm, while their stoichiometries remain constant (∼0.92−0.97 Cu:In atomic ratio, Figure 9).77 The idea is borrowed from the PbSe NC synthesis, where it was shown that an addition of secondary phosphines induces the reaction mechanism via reduction of cationic precursor to metal centers and fast homolytic cleavage of TOP:Se precursor. This ultimately speeds up the reaction and increases chemical yield.80 Generally, improving the kinetics of NC reaction represents

Figure 8. Metal−sulfur bonded precursors, used for the synthesis of CuInS2 nanocrystals: (a) copper ethylxanthate; (b) copper diethyldithiocarbamate; and (c) bis(triphenylphosphine)-copper-tetrakis(μ,μ-ethanethiol)indium.60−63

xanthates, dithiocarbamates, or single-source alkanethiolate derivatives. While the preparation of these compounds is an extra synthetic step, the use of metal−sulfur bonded precursors in the NC synthesis allows one to effectively overcome the problem of reactivity difference between M+ and M3+ ions. AgInS2 Nanocrystals. Robust synthetic strategies for AgInS2 NCs are also based on alkanethiol passivation of Ag+ ions18,64,65 as well as on the use of Ag−S bonded precursors (namely, dithiocarbamate salts, Figure 8b).63,66 While dodecanethiolmediated synthesis has been borrowed from the Cu−In−S system,52,59 the use of dithiocarbamate salts was originally described for AgInS2 NC synthesis66 and later extended to Cu− In−S materials.62 The dithiocarbamate-based synthesis of AgInS2 NCs is surprisingly easy. Briefly, an equimolar mixture of Ag and In 1450


Review


An alkanethiol-based approach has been also applied to I− III−VI selenide and telluride NCs. For the Cu−In−Se NCs, the Cu:In ratio can be tuned between 0.49 and 1, while the size of Cu−In−Se NCs ranges from 2.5 to 6 nm.71,125,127 For Ag−In− Se NCs, an injection of Se:OLA in the mixture of Ag, In, and dodecanethiol yields Ag−In−Se NCs with tunable composition.99,101 Bigger sizes of Ag−In−Se NCs can be achieved with longer reaction times.72 Small, 2.6 nm Cu−In−Te NCs with Cu:In ratio of 0.5 are obtained by fast addition of TOP:Te to the Cu+/In3+/dodecanethiol mixture at 250 °C and immediate cooling.131 Two reports describe alkanethiol-mediated synthesis of In-rich Ag−In−Te NCs, yielding 10−15 nm NCs and 6−8 nm thick nanorods.81,103 It should be noted, however, that the success of the alkanethiol-based approach for I−III−VI selenide and telluride NCs is limited. The ability of dodecanethiol to complex M+ ions is not enough to balance the reaction of metal ions with selenium and, especially, tellurium sources. This leads to unsteady I−III−VI NC compositions during the growth and even to the formation of binary NC byproducts (e.g., Cu2Se or Ag 2 Te). 71,72,81 On the other hand, due to its slow decomposition, alkanethiol molecules provide a considerable amount of sulfur to the NC structure.81 Nevertheless, alkanethiol-mediated reactions remain the most convenient method for Cu−In−S and Ag−In−S NCs, by which size and composition can be tuned independently.18,25,27,53,55,57,92,93,104−110,115 Synthesis from Diethyldithiocarbamate Salts. In dithiocarbamates, metals are directly bonded to the S atoms.62 Dithiocarbamate salts can thus be considered as “single-source” precursors for sulfide NCs. In the synthesis of I−III−S NCs, two dithiocarbamates are mixed in coordination solvent(s), and no additional S source is required.66 I−III−VI sulfide NCs grow via codecomposition of dithiocarbamates at high temperature. Because M+ and M3+ ions must not form new M−S bonds during the NC growth, the M+/M3+ reactivity difference is insignificant.63,66−68 Figure 11 summarizes a synthesis of Ag−In−S NCs from diethyldithiocarbamates.68 Although In salt decomposes slightly faster than Ag salt, the composition of Ag−In−S NCs is a linear function of the starting concentrations of Ag and In dithiocarbamates. The composition control can be precisely tuned between 0.1:1 and stoichiometric 1:1 Ag:In atomic ratios in Ag−In−S NCs. The size of Ag−In−S NCs remains nearly constant, and narrow size distributions are frequently obtained.68 The dithiocarbamate-based synthesis of Ag−In−S NCs has been extended to quaternary Zn−Ag−In−S and Ag−In−Ga−S NC compositions.63,66,70 The decomposition of dithiocarbamates can be carried out in air, using a sonochemical method of Ag/In salts and oleylamine mixture at room temperature.69 Using Cu and In diethyldithiocarbamates, Cu−In−S NCs have been obtained by the heating-up approach.62 The size of Cu−In−S NCs can be tuned from 3 to 28 nm by changing the growth temperature and ligand concentrations. The composition of Cu−In−S NCs stays proportional to the Cu:In dithiocarbamate ratio in the reaction mixture.62 Synthesis from precursors containing the metal−sulfur bond (e.g., dithiocarbamates) is a powerful technique for the composition control of I−III−VI sulfide NCs.62 This method, however, is difficult to apply for the I−III−VI selenide and telluride NCs, due to challenges in the preparation of metal precursors, containing M−Se and M−Te bonds.132

Figure 9. Schematic illustration of stoichiometric CuInSe2 nanocrystals in the presence of diphenylphosphine (adapted with permission from ref 77; copyright 2013 American Chemical Society).

an excellent strategy to diminish the reactivity difference between M+ and M3+ ions.26,29,30,77−79 CuInTe2 and AgInTe2 Nanocrystals. Literature data for the synthesis of I−III−Te NCs is sparse. The choice of Te sources is very limited, and reactions tend to be strongly unbalanced, leading to the formation of Ag2Te byproducts.81 Stoichiometric CuInTe2 and AgInTe2 NCs can be prepared by promoting the reaction with dioctylphosphine oxide82 or by passivating M+ ions with dodecanethiol.81,83 Indium-rich I−III−Te compositions can be achieved, e.g., by an amide-promoted approach78 or through sequential cation-exchange reaction,84 which will be discussed in the next section in more detail. 3.3. Tuning Composition in I−III−VI Nanocrystals. When assessing how developed colloidal synthesis is for a certain material, we often take our ability to control particle size and shape as a measure.7,85 For I−III−VI NCs, composition control (i.e., relative concentration of metals in the particle) is equally important. We quantify the composition of I−III−VI NCs with the atomic ratio Cu:In (or Ag:In). A general formula for the I−III−VI NCs with the ratio x can therefore be written as CuxInE0.5x+1.5 (or AgxInE0.5x+1.5), where E is S, Se, or Te. To achieve composition control for I−III−VI NCs, the M+/ M3+ reactivity balance is very important. In this section, we outline synthetic strategies to achieve composition control for I−III−VI NCs. Table 2 summarizes the syntheses of composition-tunable I−III−VI NCs. In addition, we include papers in which a single nonstoichiometric composition of I− III−VI NCs is achieved. Protocols for stoichiometric I−III−VI NC compositions, discussed above, are not included in Table 2. Alkanethiol-Mediated Synthesis. The synthesis in the presence of 1-dodecanethiol is the most common method to attain composition control for I−III−VI NCs.18,25,27,53,55,57,92,93,103−110,115,125,127,131 Long-chain thiol molecules effectively complex Cu+ (or Ag+) ions, reducing their reactivity relative to In3+ ions.52 The M+ and M3+ ions thus react with the S precursor with similar rates, providing a simple way to control the composition of I−III−VI NCs by changing a precursor ratio in the reaction mixture.18,25,57,106 Figure 10 shows an example of alkanethiol-mediated composition-controlled synthesis of I−III−VI NCs. Soft−soft CuI and hard−hard In(ac)3 HSAB salts are heated with 1dodecanethiol to 200−210 °C for 30 min. The method provides 2.5−2.6 nm NCs, whereas NC composition can be tuned between 0.48 and 1.26 Cu:In ratios.57 The size of indium-rich I−III−VI NCs can be controlled by growth time or by reaction temperature parameters. The alkanethiol-mediated synthesis works well for Ag−In−S NCs.18,27,55,92,93 The entire range of indium-rich compositions and broad range of NC sizes can be achieved for Ag−In−S as well as for Cu−In−S NCs.18,25,27,53,55,57,92,93,104−110,115 1451


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Table 2. Syntheses of Indium-Rich I−III−VI Nanocrystals, Enabling Composition Tuning (sorted by year, published)a,b precursors, ligands, solvents

method, temperature, time

Ag:In (Cu:In) ratio

size, morphology

refs

Ag−In−S [(Ph3P)2AgIn(SCOPh)4], DDT, OA Ag(dedc), In(dedc)3, OLA, OctA AgNO3, In(acac)3, S, OLA, DDT/OctT, OA, ODE Ag(dedc), In(dedc)3, DDA Ag(ac), In(ac)3, Na2S, H2O, GSH AgNO3, In(NO3)3, Na2S, H2O, PAA, MAA AgNO3, InCl3, S, OA, DDT, ODE, OLA Ag(ac), In(ac)3, MeC(S)NH2, H2O, cysteine Ag2S nanocrystals, In(NO3)3, H2O, GSH AgNO3, In(ac)3, S, OLA, DDT, ODE Ag(MAA), In(MAA)3, Na2S, H2O/glycerol AgNO3, InCl3, S, DDT, ODE, OLA AgNO3, InCl3, Na2S, H2O, PEI [(PPh3)2AgIn(SeCOPh)4], OLA, DDT AgI, InI3, (Me3Si)2Se, TOP, OLA Ag2O, In(ac)3, Se, DDT, ODE, OLA, TBP AgNO3, In(ac)3, Se, ODE, DDT, OA, OLA AgI, InI3, Se, TOP, silylamide AgNO3, In(MAA)3, NaHSe, H2O, gelatin AgNO3, InCl3, Se, OLA, ODE, DDT AgNO3, In(NO3)3, SeO2, H2O, GSH/ MPA/MAA

HU, 125−200 °C, 2 h HU, 180−240 °C, 3−60 min HI, 100−180 °C, 3 min

0.91 0.08−1.13 0.2−1

8.5−13.9 nm, dots 3.6−4.3 nm, dots 2.6−3.9 nm, dots

86 67, 68, 70 18, 27, 55

SC, 25 °C, 5 min HI, 95 °C, 15−150 min HU, 100 °C, 10 min HI, 110 °C, >5 min HT, 110 °C, 0.5−4.5 h

0.1−9 0.2 0.1−0.35 0.17−0.99 0.2

12 nm, dots 3 nm, dots 3 nm, dots 3 nm, dots 4−6 nm, dots

69 87 88 89 90

CE, 90 °C, 0.5−3 h HI, 120−130 °C, 1−30 min RT/HU, 25/90−100 °C, 5−7 days/15−30 min HI, 180 °C, 1 h HU, 120 °C, 1 h Ag−In−Se HU, 185 °C, 17 min HI, 280 °C, 10−20 min HI, 170−230 °C, 5−300 min HI, 175 °C, 30 min HI, 260 °C, 0.25−2 min HT, 120 °C, 1 h HI, 200 °C, 20 min MW, 160 °C, 10 min

0.37−13 0.25−2 0.03−0.67 0.42 0.07−0.26

2.5−6.9 nm, dots 3−7 nm, dots 2−8 nm, dots 7 nm, dots ∼3 nm, dots

91 92, 93 94, 95 96 97

0.79−0.93 0.75 0.85−7.34 0.33−2.00 0.1−0.9 0.06−0.67 0.31−1.74 0.17

(13−20) × (30−50) nm, rods 3−6 nm, dots 9−23 nm, dots ∼5 nm, dots 2.4−6.8 nm, dots 3.1−3.4 nm, dots 2.0−2.6 nm, dots 10−15 nm, elongated

73, 98 26 72 99 30 100 101 102

81 103

Ag2O, In(ac)3, Te, TOP, DDT, ODE, OLA Ag(ac), In(ac)3, Te, TOP, DDT

Ag−In−Te HI, 170−200 °C, 0.5−360 min HU, 240 °C, 3 h

0.63−7.12 0.8−0.89

AgI, InI3, Te, TOP, silylamide

HI, 240 °C, 0.25−2 min

0.2−0.82

10.3−15 nm, dots (5.6−8.3) × (11.4−13.5) nm, rods 4.2−10.4 nm, dots

78

Cu−In−S Cu(dedc)2, In(dedc)3, OLA, OA, DDT CuI, InI3, S, ODE, OLA, DDT Cu(ac), In(ac)3, DDT, TOPO, OLA CuI, In(ac)3, DDT, (ODE)

HU, 100−250 °C, 4 min HU, 160−240 °C, 5 min HI, 240 °C, 0.5−60 min HU, 200−240 °C, 10−60 min

0.46−3 0.34−1 0.8 0.25−1.26

3−28 nm, dots 3.8 nm, dots ∼(15−20) × (30−120) nm, rods 1.8−3 nm, dots

Cu(ac)2/CuI, In(ac)3, DDT, ODE, OA Cu(NO3)2, InCl3, Na2S, H2O, GSH CuCl, InCl3, Na2S, H2O, EtOH GSH/ MAA, Na(cit) CuNO3, In(ac)3, S, OLA, OA, StA, DDT, ODE CuI, InBr3, S, TPOP, TOOP, OLA, ODE CuI, In(ac)3, DDT, (ODE) Cu(exan)2, In(exan)3, TOP, TOA Cu(ac)2, InCl3, S, OLA, OA, TOPO Cu2−xS nanocrystals, In(NO3)3, Tol, MeOH, TOP In2S3 nanoplates, CuI, Tol, DDT CuCl, InCl3, Na2S, H2O, MPA/MAA Cu(ac)2/CuI, In(ac)3, S, OLA, DDT, ODE (OA) CuCl, InCl3, Na2S, H2O, GSH, Na(cit)

HU, 210−280 °C, 7−150 min MW, 100 °C, 5 min HU, 95 °C, 40−60 min

0.12−2.9 0.07−0.72 0.08−1

2.5−4 nm, dots 2.8 nm, dots 2.1 nm, dots

62 104 53 57, 105 −109 25, 110 111 112, 113

HI, 170 °C, 10 min

0.25−1

5−8 nm, dots

54

HU, 200 °C, 0.5−30 min ST, 180 °C, 5−6 h HI, 210 °C, 30 min HI, 145 °C, 30 min CE, 25 °C, 2−5 days

0.34−0.88 0.17−1 0.11 0.69 0.68−0.71

2.9−4.1 nm, dots 2−3.6 nm, dots 3.1 nm, dots 14.5 nm, dots 2.5−4 nm, dots

114 115−117 61 118 119

CE, 25 °C, 1−3 days HU, 90−100 °C, 20−45 min HI, 150−160 °C, 3−20 min

0.72−2.89 0.18−0.95 0.1−1

1.5 × 20 nm, plates 3−4.5 nm, dots 3−4 nm, dots

120 121, 122 55, 123

MW, 95 °C, 20 min

0.08−0.31

3.2 nm, dots

124

HI, 180 °C, 10−60 min HI, 280−350 °C, quench asap

0.91 0.17−0.63

2.3−6.2 nm, dots 2.2−3.5 nm, dots

125 26

HU, 320 °C, 1.5−2.5 min HU, 80−250 °C, quench asap HI, 180−200 °C, 5−60 min HT, 120 °C, 1 h HI, 270 °C, quench asap

0.93 0.49−0.88 0.79−0.98 0.06−0.67 0.3

1.2−5.6 nm, dots 2−5 nm, dots 3−3.5 nm, dots 3.1−3.4 nm, dots 4 nm, dots

126 71 127 100 79

Cu−In−Se CuCl, InCl3, Se, TOP, DDT, ODE CuCl/CuI, InCl3/InI3, (Me3Si)2Se, TOP, OLA CuI, InCl3, Se, TOP, TOOP, ODE, HDA CuCl, InCl3, SeU, DDT, OLA, TOP, ODE CuI, In(ac)3, Se, TBP, DDT, ODE, OA CuCl2, In(MAA)3, NaHSe, H2O, gelatin CuI, InI3, (Me3Si)2Se, TOP, TOPO, HDA

1452


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Chemistry of Materials Table 2. continued precursors, ligands, solvents

Ag:In (Cu:In) ratio

method, temperature, time

size, morphology

refs

Cu−In−Se CuCl, InCl3, Se, Ph2PH, OLA CuCl, InCl3, Se, TOP, silylamide CuI, InI3, OLA-selenocarbamate, OLA CuI, In(ac)3, Te, TOP, DDT, ODE CdTe, [Cu(MeCN)4]PF6, InCl3, MeOH, Tol, TOP, ODE CuCl, InCl3, Te, TOP, silylamide

HI, 100−180 °C, 1 h HI, 240−320 °C, 0.5−10 min HU, 80−270 °C, 20 min Cu−In−Te HI, 250 °C, quench asap CE, 25/100 °C, 5/240 min

0.92−0.97 0.70−0.88 0.40−0.91

1−9.2 nm, dots 2.7−7.9 nm, dots 2.5−10 nm, dots

77, 128 29 129, 130

0.67 >1

2.6 nm, dots 2.7 nm, dots

131 84

HI, 260 °C, 0.25−3 min

0.27−0.84

4.2−6.8 nm, dots

78

a

Methods: CE, cation-exchange synthesis; HI, hot-injection colloidal synthesis; HT, hydrothermal synthesis; HU, heating-up colloidal synthesis; MW, microwave-assisted synthesis; RT, synthesis at room temperature; SC, sonochemical synthesis; ST, solvothermal synthesis. bAbbreviations for chemicals: ac, acetate; acac, acetylacetonate; cit, citrate; DDA, dodecylamide; DDT, dodecanethiol; dedc, diethyldithiocarbamate; Et, ethyl; exan, ethylxanthate; GSH, glutathione; HDA, hexadecylamine; MAA, mercaptoacetic acid/mercaptoacetate; Me, methyl; MPA, mercaptopropionic acid; OA, oleic acid; OctA, octylamine; OctT, octanethiol; ODE, octadecene; OLA, oleylamine; PAA, poly(acrylic acid); PEI, polyethylenimine; Ph, phenyl; SeU, selenourea; StA, stearic acid; TBP, tributylphosphine; TOA, trioctylamine; Tol, toluene; TOOP, trioctylphosphite; TOP, trioctylphosphine; TOPO, trioctylphosphine oxide; TPOP, triphenylphosphite.

Amide-Promoted Synthesis. The synthesis of NCs in the presence of an amide superbase is a powerful synthetic approach, which provides independent composition and size control for I−III−VI selenide and telluride NCs.29,30,78 The formation of chalcogenide NCs is promoted by an amide salt [typically lithium bis(trimethylsilyl)amide, LiN(SiMe3)2], which is coinjected with a chalcogen precursor to the mixture of metal halides and coordination ligand(s) at elevated temperatures.133 An amide superbase accelerates the nucleation process through the formation of highly reactive metal−amide intermediates.30 Consequently, amide-promoted reaction is faster and yields smaller sizes, compared to the analogous reaction conditions in the absence of amide.133 Importantly, the reactivity difference between M+ and M3+ ions can be diminished, when amide salt is introduced in an excess compared to the halide anions in the reaction mixture.30 At these conditions, the composition of I−III−VI NCs becomes linearly proportional to the metal precursor ratio. Figure 12 summarizes an amide-promoted approach for I−III− VI NCs.30,78 A broad range of indium-rich compositions can be achieved for Ag−In−Se, Ag−In−Te, Cu−In−Se, and Cu−In−

Figure 10. Composition control for Cu−In−S nanocrystals by dodecanethiol-mediated heating-up synthesis: (a) scheme of the reaction; (b) TEM images of Cu−In−S nanocrystals with different Cu:In ratio; and (c) composition control of Cu−In−S nanocrystals as a function of Cu:In precursor ratio (adapted with permission from ref 57; copyright 2016 American Chemical Society).

Figure 11. Composition control for Ag−In−S nanocrystals by the use of diethyldithiocarbamate salts: (a) composition and (b) size of Ag−In−S nanocrystals as a function of initial precursor amounts and (c) photos of colloidal solutions and (d) TEM images of Ag−In−S nanocrystals, prepared with different amounts of Ag/In precursors (adapted with permission from ref 68; copyright 2012 Royal Society of Chemistry). 1453


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Figure 12. Composition control for I−III−VI selenide and telluride nanocrystals via amide-promoted synthesis: (a) scheme of the process; (b) TEM images of various I−III−VI nanocrystals; (c, d) independent composition and size control, shown for the case of Ag−In−Se nanocrystals (reprinted and adapted with permission from refs 30 and 78; copyrights 2016 Royal Society of Chemistry and 2015 American Chemical Society).

Te NCs. The size of the NCs can be tuned for each I−III−VI stoichiometry, e.g., by the growth time or reaction temperature.29,30,78 Water-Based Synthesis. Preparation of I−III−VI NCs in the water phase is a convenient and inexpensive method.87,88,94,95,97,112,121,122 The synthesis of I−III−VI sulfide NCs is based on a precipitation reaction between water-soluble Cu (or Ag) and In salts with sodium sulfide, Na2S.87,112 The reaction is quantitative, due to very low solubility products of Cu2S and In2S3 (i.e., 10−48 and 10−75, respectively).134 Composition of I−III−VI sulfide NCs can thus be tuned, changing the ratio of metallic precursors.88,112,121 For the waterbased synthesis of Cu−In−Se and Ag−In−Se NCs, freshly made NaHSe is used instead of Na2S (Figure 13).100 The surface of growing NCs is stabilized with short-chain mercapto-acids or more environmentally friendly glytathione molecules. In some cases, water-soluble polymers are added to the reaction mixture (e.g., poly(acrylic acid) or polyethylenimine).88,97 Several microwave-assisted or hydrothermal approaches are also reported to prepare I−III−VI NCs with composition control.90,102,111,124 Water-based syntheses are advantageous because of simple handling and inexpensive upscaling (Figure 13).97,100,113 Furthermore, the I−III−VI NCs can be directly used for bioapplications (i.e., no ligand-exchange is needed to transfer NCs to the water or buffer solutions).90,111 On the other hand, I−III−VI NCs, prepared in water, exhibit relatively low luminescence efficiency due to oxidation of the NC surface.88,97,100 Synthesis via Cation-Exchange Reaction. Colloidal NCs can exchange ions with the surrounding medium.135 Owing to

Figure 13. (A, C) Large scale synthesis of ZnS-protected Cu−In−Se and Ag−In−Se core/shell nanocrystals in the water phase (TGA denotes thioglycolic acid). (B) Inexpensive equipment, like a conventional pressure cooker, can be employed (reprinted with permission from ref 100; copyright 2015 Royal Society of Chemistry).

the small size of NCs, ion-exchange processes can be fast and efficient: nearly all cations (or anions) can be replaced in a onestep process. Driven by concentration and solubility of precursors and products, ion-exchange reactions can also be designed as a reversible process.136 Colloidal I−III−VI NCs, namely, Cu−In−S, Ag−In−S, and Cu−In−Te, have been prepared by partial cation-exchange reactions.84,91,119,120 Indium-rich Cu−In−S NCs are synthe1454


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of surface atoms and hence higher energy per atom in NCs.139 The existence of phases unknown for the bulk has different origins. While in thin films it is explained by directed crystallization given by the template,140,141 in the case of colloidal NCs, either ligands or initial seed NCs can induce the crystallization in the metastable phase.62,138 The I−III−VI NCs are often reported in metastable wurtzite phases, which have hexagonal close-packing (hcp) for both the chalcogen site and statistically mixed cationic sites.30,62,78,119,137,142−144 The existence of wurtzite-type I− III−VI NCs can be attributed to the fact that many binary chalcogenides crystallize with hexagonal ordering (e.g., chalcocite Cu2S, covellite CuS, klockmannite Cu1−xSe, γIn2Se3, etc.).13,145 Figure 15 summarizes possible reaction

sized starting either from Cu2−xS (Figure 14) or from In2S3 nanoparticles.119,120 The composition of Cu−In−S NCs can be

Figure 14. Preparation of Cu−In−Se nanocrystals via partial cationexchange process. Cu2−xS nanocrystals are mixed with In(NO3)3 in a toluene−methanol mixture at room temperature for several days. A small amount of trioctylphosphine (TOP) is added to complex outgoing Cu+ ions. MeOH denotes methanol (reprinted with permission from ref 119; copyright 2014 American Chemical Society).

tuned, for example, by the concentration of Cu (or In) salts, while the morphology of Cu−In−S NCs is solely defined by template binary NCs.120 Ag−In−S NCs are obtained, mixing spherical Ag2S NCs and In nitrate in boiling water.91 For the synthesis of Cu−In−Te NCs, a two step cation-exchange reaction is required. First, CdTe NCs are converted to Cu2Te NCs, which are subsequently mixed with In3+ solution to yield indium-rich Cu−In−Te NC compositions.84 Heterogeneous formation of I−III−VI NCs via cationexchange reaction has both advantages and disadvantages. On the plus side, starting from binary Cu and Cd chalcogenide NCs, where advanced synthesis enable size and shape control, cation-exchange synthesis facilitated making nonstoichiometric I−III−VI NCs of larger sizes and different shapes.84,91,119,120 Furthermore, since the cation-exchange process starts with one of the metals (either M+ or M3+) in the NC structure, the different reactivity of M+ and M3+ ions with chalcogen precursor is not a relevant question for cation-exchange reaction. On other hand, cation-exchange reactions require presynthesized and purified binary NCs. The cation-exchange process may also require long reaction time (i.e., up to several days), leading to the deterioration of NC size distributions.91,119,120 3.4. Controlling Crystal Structure of I−III−VI Nanocrystals. As discussed above in Section 2.1, bulk I−III−VI materials can exist in several crystal structures, depending on their composition as well as crystallization temperature (i.e., low and high temperature modifications).34−39 In most cases, the crystal structure of I−III−VI NCs is the same as that for the bulk I−III−VI composition. This can be illustrated, for example, by the Ag−In−S system, for which stoichiometric AgInS2 NCs crystallize in orthorhombic lattice, while In-rich Ag−In−S NCs have cubic spinel-type structure.18,68,86 However, nanoscale dimensions of crystals can add more complexity to the crystallization process, and a crystal structure of NCs is not solely composition-defined. One distinct feature is the existence of NCs in metastable phases, such as hightemperature modifications or crystal structures that are not known for bulk materials.137,138 The presence of hightemperature phases for NCs can be explained by suppressed temperatures of first-order transitions, such as melting or solid−solid polymorphism, stemming from a large percentage

Figure 15. Schematic representation of possible reaction pathways for I−III−VI NCs via binary seed particles or through the direct reaction. A nonexhaustive list of binary I−VI and III−VI compounds, which are stable at room temperature, is provided.

pathways for I−III−VI NCs, which can either include the formation of binary seeds or follow the direct coprecipitation of two metal precursors with chalcogen-containing reagent. If I− III−VI NCs are formed via binary intermediates, it is likely to retain the predefined structure for the final I−III−VI NC products, including metastable wurtzite modifications. A cationexchange reaction represents an ultimate case,138 where one starts with binary NCs. For example, In-rich Cu−In−S NCs can be prepared either in wurtzite or cubic phases, when started from hexagonal Cu2−xS or cubic In2S3, respectively.119,120 On the other hand, if the I−III−VI NCs form directly from molecular precursors (Figure 15), the thermodynamically stable phases are expected for NC products.68,77,88

4. COMPOSITION EFFECTS ON OPTICAL PROPERTIES OF I−III−VI NANOCRYSTALS As for binary semiconductor NCs, the optical band gap of I− III−VI NCs is size-dependent. Bohr radii for I−III−VI semiconductors of several nanometers56,127 results in a band gap broadening for small-size I−III−VI NCs, due to quantum confinement effects.8 However, the stoichiometry of I−III−VI NCs adds extra possibilities for tuning absorption and emission photon energies. Figure 16 shows size-dependent optical band gaps for defined stoichiometries of I−III−VI colloidal NCs.29,30,65,77,78,146 1455


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on PL efficiency. Figure 17 shows composition-dependent PL QY for Ag−In−S, Ag−In−Se, and Cu−In−S NCs.18,25,30 In all cases, PL efficiency peaks at certain optimal I−III−VI stoichiometries. Interestingly, best-emitting I−III−VI NCs have compositions that closely match those of ordered-vacancy compounds (e.g., I−III5−VI8, I−III3−VI5, or I3−III5−VI9).31,45 Improved PL QY can therefore be attributed to the structure ordering of I−III−VI NCs, exhibiting OVC stoichiometries. At the same time, other PL properties of OVC I−III−VI NCs (i.e., PL widths, PL lifetimes, and Stokes shifts) remain comparable to other In-rich I−III−VI NCs.147 Figure 17 suggests which compositions of I−III−VI NCs should be avoided, if luminescent materials are targeted. Highly In-rich I−III−VI NCs (e.g., Ag:In or Cu:In < 0.2) as well as stoichiometric and Cu/Ag-rich I−III−VI NCs (e.g., Ag:In or Cu:In ≥ 1.0) exhibit moderate PL efficiencies.25,30 While the former can be explained by high concentration of Cu/Ag vacancies and mechanical instability of the structure, the latter can be attributed to the presence of electronically harmful ingap defect states, such as antisite and interstitial Cu/Ag atoms.

Figure 16. Size dependences of optical band gaps for selected compositions of I−III−VI nanocrystals.29,30,65,77,78,137

5. CONCLUSIONS AND OUTLOOK In summary, ternary I−III−VI nanocrystals are an important class of semiconductors, whose properties can be adjusted not only by changing nanocrystal size but also by tuning the I−III− VI composition (i.e., the ratio between the Group I and Group III metals in its structure). In order to exploit the full potential of I−III−VI nanocrystals, composition optimization must be considered on par with the size and morphology control. Synthesis of ternary I−III−VI nanocrystals represents a challenging task due to the presence of two metal salts in the reaction mixture. Composition control for I−III−VI nanocrystals can be obtained when the reactivities of the metal precursors are balanced. Composition-controlled synthesis of I−III−VI nanocrystals is built on one of the following synthetic strategies: (i) use of long-chain alkanethiols; (ii) employment of metal− chalcogen bonded precursor; (iii) addition of amide superbase; (iv) synthesis in the water phase; and (v) heterogeneous synthesis via partial cation-exchange reaction. Optical properties of I−III−VI nanocrystals are defined by their size and composition. The effect of composition can be predicted from properties of bulk I−III−VI materials, and for each I−III−VI stoichiometry, the band gap follows the wellestablished R−2 dependence with the nanocrystal size. The efficiency of the optical emission shows interesting trends as a

Generally, composition effects on the optical band gap of I− III−VI NCs correspond to band gaps for bulk I−III−IV semiconductors (Table 1).41,44,45,47−49 In-rich I−III−VI NCs exhibit wider band gap than stoichiometric compositions (e.g., AgIn 5 S 8 vs AgInS 2 or Cu 3 In 5 Se 9 vs CuInSe 2 , Figure 16).29,65,77,146 The band gap of I−III−VI NCs broadens in a row of homologous I−III−VI tellurides−selenides−sulfides.30,78,146 Finally, optical band gaps are systematically larger for Ag-containing I−III−VI NCs than for their Cu-containing counterparts (e.g., Ag3In5Se9 vs Cu3In5Se9 or Ag2In4Se7 vs Cu2In4Se7, Figure 16).29,30,78 Stoichiometry of I−III−VI NCs has a notable effect on band gap energy, and this must be taken into account to design materials with demanded optical characteristics. Composition of I−III−VI NCs influences the luminescent properties of ternary colloids. In accordance to band gap dependences, the PL peak energy of I−III−VI NCs blue-shifts as the Cu:In (or Ag:In) ratio decreases, a manifestation of the band gap broadening for In-rich I−III−VI materials.18,22,30,57 Similarly to stoichiometric compositions, In-rich I−III−VI NCs exhibit relatively broad PL width (typically, 200−300 meV), large apparent Stokes shifts, and long-lived charge carriers (on the order of hundreds of nanoseconds).25,30,92,100,101 Composition of I−III−VI NCs has, however, a very pronounced effect

Figure 17. Composition dependences of photoluminescence efficiency for various I−III−VI nanocrystals (adapted with permission from refs 18, 25, and 30; copyrights 2012 Royal Society of Chemistry, 2015 American Chemical Society, and 2012 Wiley-VCH Verlag). 1456


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tellurium) semiconductor nanocrystallites. J. Am. Chem. Soc. 1993, 115, 8706−8715. (8) Brus, L. E. Electron−electron and electron-hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state. J. Chem. Phys. 1984, 80, 4403−4409. (9) Reiss, P.; Carrière, M.; Lincheneau, C.; Vaure, L.; Tamang, S. Synthesis of Semiconductor Nanocrystals, Focusing on Nontoxic and Earth-Abundant Materials. Chem. Rev. 2016, 116, 10731−10819. (10) Kershaw, S. V.; Susha, A. S.; Rogach, A. L. Narrow bandgap colloidal metal chalcogenide quantum dots: synthetic methods, heterostructures, assemblies, electronic and infrared optical properties. Chem. Soc. Rev. 2013, 42, 3033−3087. (11) Protesescu, L.; Yakunin, S.; Bodnarchuk, M. I.; Krieg, F.; Caputo, R.; Hendon, C. H.; Yang, R. X.; Walsh, A.; Kovalenko, M. V. Nanocrystals of Cesium Lead Halide Perovskites (CsPbX3, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut. Nano Lett. 2015, 15, 3692−3696. (12) Regulacio, M. D.; Han, M.-Y. Composition-Tunable Alloyed Semiconductor Nanocrystals. Acc. Chem. Res. 2010, 43, 621−630. (13) Coughlan, C.; Ibáñez, M.; Dobrozhan, O.; Singh, A.; Cabot, A.; Ryan, K. M. Compound Copper Chalcogenide Nanocrystals. Chem. Rev. 2017, 117, 5865−6109. (14) Garnweitner, G.; Niederberger, M. Nonaqueous and SurfactantFree Synthesis Routes to Metal Oxide Nanoparticles. J. Am. Ceram. Soc. 2006, 89, 1801−1808. (15) Rui, X.; Zhao, X.; Lu, Z.; Tan, H.; Sim, D.; Hng, H. H.; Yazami, R.; Lim, T. M.; Yan, Q. Olivine-Type Nanosheets for Lithium Ion Battery Cathodes. ACS Nano 2013, 7, 5637−5646. (16) Nishi, H.; Kuwabata, S.; Torimoto, T. Composition-Dependent Photoelectrochemical Properties of Nonstoichiometric Cu2ZnSnS4 Nanoparticles. J. Phys. Chem. C 2013, 117, 21055−21063. (17) Zeng, H.; Duan, G.; Li, Y.; Yang, S.; Xu, X.; Cai, W. Blue Luminescence of ZnO Nanoparticles Based on Non-Equilibrium Processes: Defect Origins and Emission Controls. Adv. Funct. Mater. 2010, 20, 561−572. (18) Hong, S. P.; Park, H. K.; Oh, J. H.; Yang, H.; Do, Y. R. Comparisons of the structural and optical properties of o-AgInS2, tAgInS2, and c-AgIn5S8 nanocrystals and their solid-solution nanocrystals with ZnS. J. Mater. Chem. 2012, 22, 18939−18949. (19) Stanbery, B. J. Copper Indium Selenides and Related Materials for Photovoltaic Devices. Crit. Rev. Solid State Mater. Sci. 2002, 27, 73−117. (20) Jackson, P.; Wuerz, R.; Hariskos, D.; Lotter, E.; Witte, W.; Powalla, M. Effects of heavy alkali elements in Cu(In,Ga)Se2 solar cells with efficiencies up to 22.6%. Phys. Status Solidi RRL 2016, 10, 583− 586. (21) Kolny-Olesiak, J.; Weller, H. Synthesis and Application of Colloidal CuInS2 Semiconductor Nanocrystals. ACS Appl. Mater. Interfaces 2013, 5, 12221−12237. (22) Torimoto, T.; Kameyama, T.; Kuwabata, S. Photofunctional Materials Fabricated with Chalcopyrite-Type Semiconductor Nanoparticles Composed of AgInS2 and Its Solid Solutions. J. Phys. Chem. Lett. 2014, 5, 336−347. (23) Witt, E.; Kolny-Olesiak, J. Recent Developments in Colloidal Synthesis of CuInSe2 Nanoparticles. Chem. - Eur. J. 2013, 19, 9746− 9753. (24) Rudolph, P. Non-stoichiometry related defects at the melt growth of semiconductor compound crystals − a review. Cryst. Res. Technol. 2003, 38, 542−554. (25) Chen, B.; Zhong, H.; Zhang, W.; Tan, Z. a.; Li, Y.; Yu, C.; Zhai, T.; Bando, Y.; Yang, S.; Zou, B. Highly Emissive and Color-Tunable CuInS2-Based Colloidal Semiconductor Nanocrystals: Off-Stoichiometry Effects and Improved Electroluminescence Performance. Adv. Funct. Mater. 2012, 22, 2081−2088. (26) Allen, P. M.; Bawendi, M. G. Ternary I−III−VI Quantum Dots Luminescent in the Red to Near-Infrared. J. Am. Chem. Soc. 2008, 130, 9240−9241. (27) Ko, M.; Yoon, H. C.; Yoo, H.; Oh, J. H.; Yang, H.; Do, Y. R. Highly Efficient Green Zn-Ag-In-S/Zn-In-S/ZnS QDs by a Strong

function of composition, with the best emitters having stoichiometries of known ordered-vacancy compounds. In terms of outlook, the composition control becomes even more important for quaternary I−III−VI-based nanocrystals, such as anion-mixed compounds (e.g., CuInSxSe2−x) or containing three cations in the structure via mixing two I− III−VI materials (e.g., CuIn1−xGaxSe2) or via Zn-alloying (e.g., Zn−Ag−In−Se, Zn−Cu−In−S, etc.). In addition, crystal structure of I−III−VI nanocrystals can further modify their optical characteristics (i.e., change the bandgap, emission wavelength, PL efficiency, etc.). The difference in structure and optical performance between stoichiometric and In-rich I− III−VI nanocrystals is still to be understood. In particular, the charge carrier recombination mechanism remains one of intriguing questions for nonstoichiometric I−III−VI nanocrystals,148 which is also under debate for stoichiometric I−III−VI nanocrystals149 as well as for bulk I−III−VI materials.150 Tuning the composition of multicomponent nanocrystals is of prime importance for various nanocrystal-based devices, e.g., defining the doping-type of nanocrystal films,13 increasing the resistivity contrast for phase-change memory alloys,151 or improving the performance of photovoltaic,152 light-emitting,153 and thermoelectric devices.154

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AUTHOR INFORMATION

Corresponding Author

*V. Wood. E-mail: [email protected]. ORCID

Maksym Yarema: 0000-0002-2006-2466 Vanessa Wood: 0000-0001-6435-0227 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors acknowledge a financial support from the Swiss National Science foundation via Quantum Sciences and Technology (QSIT) NCCR (51NF40-160591) and an Ambizione Fellowship (PZ00P2-161249).

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