Tuning the Luminescence Properties of Colloidal I ... - ACS Publications

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Tuning the Luminescence Properties of Colloidal I−III−VI Semiconductor Nanocrystals for Optoelectronics and Biotechnology Applications Haizheng Zhong,* Zelong Bai, and Bingsuo Zou Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems, School of Materials Science & Engineering, Beijing Institute of Technology, Beijing 100081, China ABSTRACT: In the past 5 years, colloidal I−III−VI nanocrystals such as CuInS2, CuInSe2, and AgInS2 have been intensively investigated for the potential to replace commonly available colloidal nanocrystals containing toxic elements in light-emitting and solar-harvesting applications. Many researchers from different disciplines are working on developing new synthetic protocols, performing spectroscopic studies to understand the luminescence mechanisms, and exploring various applications. To achieve enhanced performance, it is very desirable to obtain high-quality materials with tunable luminescence properties. In this Perspective, we highlight the current progress on tuning the luminescence properties of I−III−VI nanocrystals, especially focusing on the advances in the synthesis, spectroscopic properties, as well as the primary applications in light-emitting devices and bioimaging techniques. Finally, we outline the challenges concerning luminescent I−III−VI NCs and list a few important research tasks in this field.

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covering the NIR region, larger Stokes shifts, and a longer photoluminescence (PL) lifetime, as well as possible low toxicity and low cost.8−12All of these features might provide ways to improve performance in light-emitting devices and bioimaging techniques and certainly motivate the spectroscopic investigation of I−III−VI NCs. In this Perspective, we will focus on the important progress of tuning the luminescence properties of I−III−VI NCs, provide indepth comparative spectroscopic studies, and discuss the prospects of their applications. We begin by describing the synthetic protocols. Then, we discuss some of the important luminescence features that are observed in I−III−VI based NCs and described the strategies that applied to tune their luminescence properties. We also present primary results on their applications in optoelectronics and biotechnology. Finally, we summarize the current challenges and suggest several important research tasks for luminescent I−III−VI NCs. The synthesis of I−III−VI NCs was developed by adapting the understanding that learned from II−VI binary NCs. According to the literature, there are mainly two synthetic routes to fabricate luminescent I−III−VI NCs: hot-injection and noninjection methods. Since the first report by Murray et al. in 1993,13 hot-injection techniques have been the major route to synthesize high-quality NCs with controlled size and shapes by a temporal separating nucleation and growth process. The preparation of I−III−VI NCs has to balance the reactivity of two metallic cations, which is

olloidal semiconductor nanocrystals (NCs) have proven to possess great potential as light emitters and solar harvesters in optoelectronics and biotechnology.1−3 Although II−VI and IV−VI group cadmium- or lead-based NCs have been the focus of greatest interest, other materials containing, for instance, elements such as IV materials, binary III−V compounds, and ternary I−III−VI compounds have been less well studied.4 Among them, group I−III−VI semiconductors combining group I (Cu, Ag), group III (Al, Ga, In, Tl), and group VI (S, Se, Te) elements, are known as the analogues of II−VI binary compounds and have been identified as potentially less toxic materials. These compounds cover a wide range of optical and electronic properties but are related to each other for their chalcopyrite structure (see Figure 1).5 Considering the quantum confinement effects, NCs of I−III−VI semiconductors are predicted to exhibit tunable optical band gaps covering a wide wavelength range from the near-infrared (NIR) region, through visible, to the ultraviolet (UV) region.6 From the material chemistry viewpoint, an arising question is whether I−III−VI NCs can be low toxic alternatives to popular CdSe NCs. From a physical chemistry perspective, we also wonder whether I−III− VI NCs have some new chemical and physical properties and how we can understand these properties. Inspired by these questions, colloidal I−III−VI NCs have been intensively investigated in the past 5 years. An increasing number of publications have been reported in the literature, with particular emphasis on CuInS2-, CuInSe2-, and AgInS2-based NCs.7 Thanks to the efforts of many synthetic chemists, several available methods can prepare high-quality I−III−VI NCs. These materials exhibit tunable spectroscopic properties with particular properties such as a wide tunable spectral window © 2012 American Chemical Society

Received: September 4, 2012 Accepted: October 11, 2012 Published: October 11, 2012 3167

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Figure 1. Schematic diagram of the chalcopyrite structure and a summary of the bandgaps of I−III−VI compounds.

noninjection method to synthesize ternary I−III−VI NCs. Compounds with the general formula [(LR3)2M1 M2(ER′)4] (L = P, As; E = S, Se; M1 = Cu, Ag; M2 = In, Ga; R = aryl, alkyl; and R′ = alkyl) and M1 M2[S2CN(C2H5)2]4 have been exploited as single-source precursors to synthesize I−III−VI NCs.19−21 The presynthesis of these metal−organic precursors usually uses toxic chemicals, and the synthetic procedures are very complicated. To overcome these disadvantages, thermal decomposition of in-situformed thiolates through heating a mixture of metal sources (metal acetates, metal iodides, and/or metal oleates) with sulfur sources (dodecanethiol, DDT) was developed.9−11 In our earlier work, CuInS2 NCs with tunable emissions of 580−720 nm were obtained via thermolysis of a mixed solution of CuAc, In(Ac)3, and DDT in noncoordinating solvent 1-octadecene (ODE) at 240 °C.9 This method is scalable, reproducible, environmentally friendly, low cost, and versatile to produce high-quality luminescent CuInS2-based NCs for potential industrial applications. Many efforts have been made to modify the synthetic routes, providing superior emissive CuInS2- and AgInS2-based NCs with recorded relative PLQYs of 50−90%.11,22−26

a great challenge to control the nucleation process. If the synthetic chemistry is not well-designed, unfavorable phase separation will result in a mixture of binary products. Moreover, strong room-temperature luminescence usually is related to very small NCs in the quantum confinement regime. Therefore, the preparation of luminescent I−III−VI NCs has lagged far behind the group II−VI materials. The first successful hot-injection protocol for luminescent I−III−VI NCs was reported by Bawendi’s group in 2008.8 By using CuCl or CuI as copper precursors, InI3 or InCl3 as indium precursors, or active bis(trimethylsilyl)selenide [(TMS)2Se] as a selenium precursor in the presence of trioctylphosphine (TOP) and oleylamine (OLA) as the coordinating solvents, they produced Cu−In−Se and Ag−In−Se NCs of various stoichiometries with PL emission in the red to NIR region (from 640 to 975 nm). The synthetic route reported by Bawendi et al. requires experience with a hazardous compound [(TMS)2Se] and high temperature (∼300 °C). To overcome these problems, Cassette and his co-workers refined the synthetic chemistry to synthesize Cu−In−Se NCs with tunable emissions from 700 to ∼1100 nm by heating a mixture of CuI, InCl3, and selenourea with TOP, OLA, and dodecanethiol (DDT) as ligands at a moderate temperature.14 Scholes and Zhong et al. developed a moderate route to synthesize luminescent CuInSe2 NCs by using CuI, In(Ac)3, and selenium in TOP as precursors in the presence of dodecanethiol as the coordinating solvent at a temperature of 200 °C.15 Similar approaches were also employed to synthesize CuInS2 NCs. In 2009, Peng’s group reported the synthesis of monodisperse CuInS2 NCs with an average size ranging from 2 to 20 nm by modifying the chemical reactivity of the copper precursor using thiols.16 The resulting CuInS2/ZnS core−shell NCs exhibited size-tunable emissions from 500 to 900 nm with PL quantum yields (PLQYs) up to 30%. Very recently, Xie’s group and Pradhan’s group reported the synthesis of quaternary Cu−In−Zn−S and Cu−In−Zn−Se NCs, respectively.17,18 Impressively, PLQYs up to 70% was reported for the quaternary Cu−In−Zn−S NCs even without coating ZnS shells.17 Although hot-injection methods can produce high-quality I− III−VI NCs, the chemical process of large-scale fabrication prefers noninjection methods. Ligand-controlled thermal decomposition of single-source precursors has been an effective

To explore the potential of I−III− VI NCs as light emitters, it is very necessary to understand the luminescence mechanism, learn the factors that influence their luminescence properties, and optimize the luminescence properties. Except for the conventional thermal heating, other techniques including microwave-assisted heating,27,28 photochemical radiation,29 and ultrasonic radiation30,31 were also applied to induce the decomposition of precursors for I−III−VI NCs fabrications. Apart from the synthetic systems in the organic phase, an aqueous solvothermal decomposition synthetic protocol was recently developed to produce water-soluble CuInS2 and AgInS2 NCs with 3-mercaptopropionic acid (MPA) as the stabilizer.32,33 3168

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The intrinsic luminescence properties of semiconductor NCs include the excitation/emission wavelength, fluorescent QYs, PL lifetime, photostability, and chemical stability in various environments. To explore the potential of I−III−VI NCs as light emitters, it is very necessary to understand the luminescence mechanism, learn the factors that influence their luminescence properties, and optimize the luminescence properties. I−III−VI NCs have been investigated by using steady and time-resolved PL spectroscopy at room temperature and low temperature. In comparison to well-known II−VI cadmiumbased NCs, I−III−VI NCs exhibit several particular luminescence properties, which are summarized in Table 1. Considering their importance, we will describe the luminescence properties of I−III−VI NCs and primarily discuss the corresponding physics involved in the emission process. Table 1. Luminescence Properties of II−VI and I−III−VI NCs properties

II−VI NCs

spectral tunable window PLQYs

UV−visible and NIR oil-soluble, >50% water-soluble, >30% 25−35 nm 50% water-soluble, 20−50% 80−120 nm 200−300 meV 100−300 ns

The basic quantum confinement effects in I−III−VI NCs provide size tunability of luminescent properties. As shown in Figure 2a, the emission peak of CuInS2 NCs gradually shifts from 640 to 850 nm (1.45−1.90 eV) with the diameter increase.9,22 The full width at half-maximum (fwhm) of PL emissions for CuInS2 NCs is about 90−120 nm, which is almost three times that of monodisperse CdSe NCs.34 Other I−III−VI NCs also have similar broad emission spectra. To reveal the origin of such broad emission, it is necessary to remove the inhomogeneous line broadening effects by narrowing the size distribution or determining the single-particle spectrum. In the size-selective precipitation experiments, no obvious narrowing was observed. A primary emission spectrum of a single CuInSe2 particle shows a fwhm of ∼40 nm (unpublished results), which is much larger than that of single CdSe particles (13−18 nm).34 Therefore, the broadened emission spectra are not due to the size inhomogeneity but mainly related to a distribution of vibrational states. Figure 2b shows a typical two-dimensional photoluminescence excitation (2D PLE) spectrum of CuInS2 NCs. An obvious gap between the excitation and emission energy of ∼290 meV due to the Stokes shifts was observed.22 In comparison to II−VI NCs, I−III−VI NCs have larger Stokes shifts, which reduce the self-absorption effects in the lightemitting and bioimaging applications. Figure 2c shows the temperature-dependent PL spectrum of CuInS2 NCs. The observed PL energy change in the temperature range from 92 to 287 K is ∼98 meV, in comparison to 10−20 meV for that of II− VI NCs.35 In Kuzuya’s work, the emission peaks of PL spectra for AgInS2 NCs were found to be excitation-power-dependent (see Figure 2d). The power dependence can be described by the power law (I ∝ Lk, K = 0.97).36 The observed large temperatureand energy-dependent energy shifts cannot be explained by the excitonic recombination mechanism that is deduced from II−VI NCs but suggested a “donor−acceptor pair” (DAP) recombination mechanism, which has been observed in the blue- and greenemitting phosphors such as ZnS/Ag,Cl and ZnS/Cu,Al,

Figure 2. (a) Size-dependent PL emission spectra of CuInS2 NCs. (b) 2D PLE spectra of CuInS2 NCs with an average diameter of 3.5 nm (the PL intensity was represented by colors). (c) PL spectra of CuInS2 cores at different temperatures of 92, 115, 138, 161, 185, 205, 249, and 287 K (from top to bottom) excited at 488 nm. The inset shows the temperature dependence of the PL emission peak energy. (d) Excitation intensity dependence of the PL spectra of AgInS2 NCs. The inset shows the relationship between the emission peak energy and the relative excitation intensity. [Adapted (b) from ref 22, (c) from ref 35 (copyright 2012, Elsevier), and (d) ref from 36 (copyright 2010, RSC Publishing)].

respectively.37 In the spectroscopic study for the I−III−VI thin films, it has been learned that the emissions from DAP recombination are dominant for the In-rich samples.38 This is in accordance with the fact that only In-rich CuInS2 NCs are emissive.11 Time-resolved PL (TRPL) spectra were measured to understand the recombination progress in determination of the emission of I−III−VI NCs. The average PL lifetimes of CuInS2 core samples are 100−300 ns, which are much longer than those of II−VI NCs (CdSe NCs: 10−30 ns at room temperature). As shown in Figure 3a and b, the PL decays of CuInS2 NCs not only depend on the emission wavelength but also vary with the [Cu]/ [In] ratio for the CuInS2 NCs.9 The PL decays of CuInS2 NCs are usually not single-exponential and can be described by double- or triple-exponential analysis. The longer component of hundred of nanoseconds from the double-exponential analysis was attributed to DAP recombination. The component of tens of nanoseconds was attributed to nonradiative recombination induced by surface defects. This was confirmed by the observation that surface treatment can suppress the fast decays in I−III−VI NCs.11,12 Detailed study shows that the fraction of 3169

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Figure 3. (a) PL decays of CuInS2 NCs detected at 630, 650, 680, 710, 730, and 750 nm. (b) PL decays for CuInS2 with various [Cu]/[In] ratios as well asCuInS2/ZnS NCs. (c) Wavelength-dependent PL lifetimes of the AgInS2 NCs. [(a) from ref 9; (b) from ref 11 (copyright 2012, Wiley); and (c) from ref 39].

Size Tuning. It has been well-known that the band gaps of semiconductor NCs can be easily tuned by controlling their particle size due to the size-dependent quantum effects.6 Although the band gaps of I−III−VI compounds slightly varied with their compositions, the emission spectra of I−III−VI NCs can be systematically tuned by manipulating their sizes. In principle, size-tunability of semiconductor NCs was determined by their bulk band gap and excitonic Bohr radius. Very impressively, CuInSe2 compounds have a lower band gap of 1.04 eV and a larger excitonic Bohr radius of 10.6 nm, which gives a wide spectral range from 660 to 1100 nm in the active window for bioimaging.14 Composition Tuning. Due to the presence of three elements, composition tuning of I−III−VI NCs is more complicated than that of their II−VI counterparts. Here, we will first discuss composition variation effects on defects in I−III−VI NCs that determine their luminescence properties. I−III−VI semiconductors are known for their abundant intrinsic defects due to the easy formation of interstitial atoms or vacancies. According to Zunger’s theoretical model on defects, I−III−VI semiconductors have a large structural tolerance to the offstoichiometry, which makes copper-deficient or copper-rich samples stable.40 From the spectroscopic studies, it has been concluded that the PL emission of I−III−VI NCs mainly originates from DAP recombination. Therefore, it is deduced that the stoichiometry of I−III−VI NCs can significantly influence their luminescence properties. In an earlier work, Uehara et al. observed the PL enhancement of CuInS2 NCs by introducing lattice defects through highly off-stoichiometric effects.41 Our recent work demonstrated that Cu-rich CuInS2 NCs are nonemissive and In-rich CuInS2 NCs are emissive (see Figure 5a).11 It is also noted that the optimal CuInS2, CuInSe2, and AgInS2 NCs were synthesized with the same initial feed [Cu]/[In] and [Ag]/[In] molar ratios of 1:4.11,42,43 The PL enhancement was attributed to an increase of the population of donor−acceptor defects required for DAP recombination of excited charge carriers. The relationship between PLQYs and [Cu]/[In] ratios demonstrated that the emission intensity is proportional to the population of defects within a constant value and decreased exponentially after a specific site (see Figure 5b). This phenomenon is similar to the observation in rare-earthdoped phosphors and can be simply explained by the quenching effects that induced the interaction of nearest-neighbor DAPs.44 Composition tuning by alloying two semiconductors is an effective way to realize band gap engineering for specific

the short component in the PL decays can be obviously reduced by coating ZnS or CdS shells onto CuInS2 cores.12 Recently, Burda et al. systematically investigated the wavelength-dependent PL decays of AgInS2 cores and AgInS2/ZnS core−shell NCs.39 As shown in Figure 3c, the average size and shell coating have significant influences on the wavelength-dependent PL decays of the AgInS2 NCs. The fast decay channel in smaller AgInS2 NCs occupies a large fraction, whereas AgInS2/ZnS core−shell samples show almost no obvious spectral change. On the basis of the above results, it can be concluded that (i) the dominant luminescence mechanism is DAP recombination for I−III−VI NCs, especially for the core−shell samples and (ii) the radiative and nonradiative processes in I−III−VI NCs are related to the intrinsic and surface defects, respectively. The understanding that intrinsic and surface defects play an important role in the luminescence process provides information to tune the luminescence properties of I−III−VI NCs.

The understanding that intrinsic and surface defects play an important role in the luminescence process provides information to tune the luminescence properties of I−III−VI NCs. Having discussed the luminescence mechanism of I−III−VI NCs, we now discuss the strategies to tune their luminescence properties. As schematically described in Figure 4, there are mainly three strategies, size tuning, composition tuning, and surface tuning.

Figure 4. Schematic diagrams of strategies (size tuning, composition tuning, and surface tuning) that applied to synthesize the highly luminescent I−III−VI NCs. 3170

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Several experimental results have also demonstrated that ZnS− AgInS2 and ZnS−CuInS2 alloyed NCs also exhibit enhanced PL intensity with respect to pure NCs.17,30,31 Surface Tuning. As we discussed in the spectroscopic analysis, the PL of I−III−VI NCs is dominated by the recombination processes that related to the intrinsic donor−acceptor defects and surface defects. The recombination pathway to surface defects is nonradiative. Therefore, the PLQYs and material stability of semiconductor NCs can be enhanced through passivation of dangling bonds on the NC surface due to the elimination of surface trap states. A considerable increase in PL intensity of I−III−VI NCs is evident upon surface coating of ZnS or CdS layers.9−12 It is noted that the shell coating process of I− III−VI NCs often results in an obvious blue shift of their emisison spectra. This fact and the post treatment experiments imply that the surface tuning process may also invlove a cation exchange reaction or even an interdiffusion alloying process.24,47 Following the above discussion, the luminescence properties of I−III−VI NCs (emission spectra and PLQYs) can be finely tuned by applying a combination of these strategies. Recently, CuInS2- and AgInS2-based NCs with high PLQYs and tunable emissions were fabricated (PLQYs: oil-soluble, >50%; watersoluble, 20−50%; Zn−Cu−In−S: 500−800 nm; Zn−Ag−In−S: 500−700 nm; also see Figure 6a−d). Although I−III−VI NCs exhibit inherent broad emission spectra, their other luminescence properties are comparable or even better than the welldeveloped CdSe-based NCs for many applications. It is of great interest to determine their performance as low toxic alternatives in the field of optoelectronics and biotechnology. In this Perspective, we focus on the progress in light-emitting devices (see Figure 6e,f) and bioimaging techniques (see Figure 6h). Light-Emitting Devices. Semiconductor NCs can be optically or electrically excited to fabricate light-emitting devices for newgeneration solid-state lighting and display technology.49 Because of the finely tunable emissions, thin film displays with improved color saturation and white lighting with a high color rendering index have been achieved by using well-developed CdSe NCs.49 The exploration of I−III−VI NCs in light-emitting devices has attracted great attention, resulting in high color rendering whitelight-emitting diodes (WLEDs)50 and flat electroluminescence (EL) devices.11,51,52 EL ranging from green to deep red was

Figure 5. (a) Photograph of the resulting CuInS2 samples with different [Cu]/[In] molar ratios in toluene and the corresponding picture under a UV lamp. (b) A schematic diagram used to demonstrate the PL properties of CuInS2NCs. [Adapted from ref 11 (copyright 2012, Wiley)].

applications. This approach can extend the tunable spectral ranges and improve the PLQYs of resulting NCs. By adapting the nucleation- and diffusion-controlled alloying strategies, the incorporation of Zn or Ga into CuInS2 or AgInS2 NCs has been well-developed to tune the optical bandgap as well as the emission spectra, covering the whole visible spectrum.17,45−48

Figure 6. Highly luminescent and color-tunable I−III−VI NCs and the examples of their applications in optoelectronics and biotechnology such as EL devices, WLEDs, displays, and bioimaging. [Adapted (c) from ref 31 (copyright 2012, Wiley), (e) from ref 11 (copyright 2012, Wiley), (g) from ref 56 (copyright 2012, RSC), and (h) from ref 63). 3171

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vitro and in vivo bioimaging.63 The above examples not only show the adaptability of the phase-transfer strategies for I−III− VI NCs but also demonstrate that these materials are good candidates for in vivo imaging studies. Although the NIR emissive I−III−VI NCs are only in their infant states, this attractive field will attract more interest and greatly promote the in vivo imaging studies.

obtained in the devices by sandwiching ZnCuInS/ZnSe/ZnS or CuInSe2/ZnS NCs between ITO and aluminum electrodes with organic electron and hole transport layers.11,15,52 The maxima luminance approached 2100 cd/m2 for yellow emission NCs and 1700 cd/m2 for red emission NCs.11 The color purity of EL devices is inferior to that of the cadmium-based NCs for their broad emission spectra. However, this provides a great chance to generate white light. By mixing the red emissive ZnCuInS/ZnS core−shell NCs with a blue−green emissive polymer, white lighting with a color rendering index (CRI, Ra) of ∼92 was recently obtained in Xu’s group.53 Recently, Zhao et al. invesitgated the energy-transfer process between an organic layer and CuInS2-based NCs.54 This provided a chance to optimize the material's design and thus may further improve the EL device’s performance. Phosphor-converted WLEDs (pc-WLEDs) by combing blueemitting chips with down-converting phosphors are potential candidates to replace incandescent lamps.55 By integrating welldeveloped CdSe NCs with blue-emitting chips, nanocrystalbased WLEDs (nc-WLEDs) that use semiconductor NCs as rare earth free converting materials have been demonstrated.49 The use of CuInS2-based NCs as an alternative to conventional YAG phosphors provides nc-WLEDs with a CRI of 72−75 and a luminous efficiency up to ∼60 lm/W.26,50 By using red and green emissive CuInS2-based NCs, nc-WLEDs with an excellent color rendering index of ∼95, a tunable color temperature of 4600− 5600 K, and a luminous efficiency up to 70 lm/W have been demonstrated (unpublished results). Very recently, hybrid polymer and NC composite films with good flexibility and transparency were fabricated by incorporating luminescent CuInS2-based NCs into a cellulose matrix (see Figure 6g).56 The luminescent compsoite films provide a platform for remote lighting applications. Bioconjugation and Bioimaging. Until today, cadmium-based NCs are the most popular and widely used as luminescence labels for in vitro and in vivo bioimaging studies.3 The potential toxicity of cadmium remains a major roadblock for their clinical research application. The advent of water-soluble heavy metal free I−III− VI NCs applications in biotechnology may overcome this biomedical challenge. In addition to being free of toxic elements, the extended emission in the NIR region makes I−III−VI NCs appropriate probes for long-term in vivo imaging for cancer screening and sensing. To demonstrate the applicability of obtained I−III−VI NCs for bioimaging, luminescent I−III−VI NCs synthesized in nonpolar solvents were transferred to the aqueous phase by employing the strategies that were developed for CdSe NCs. For example, Reiss et al. reported the successful application of dihydrolipoic acid (DHLA)-capped CuInS2/ZnS NCs as biomakers.10 Pons and his colleagues have demonstrated that DHLA-PEG and micelle-encapsulated coated CuInS2/ZnS NCs present a much reduced toxicity compared to that of CdTeSe/CdZnS core−shell NCs.57 Prasad’s group and Searson’s group reported the preparation of PEGylated phospholipid-encapsulated CuInS2/ZnS and Cu−In−Se/ZnS NCs for the application in in vivo imaging experiments, repectively.58,59 Chang et al. demonstrated that poly(maleic anhydride-alt-1-octadecene)-coated AgInS2/ZnS NCs were conjugated with folic acid for human liver carcinoma (HepG2) tumor cell labeling.60 Xue’s group reported preparationthe silica and amphiphilic copolymer F127 coating AgInS2 NCs for bioimaging applications.61,62 Very recently, Achilefu and Gu et al. explored the loading of oil-soluble CuInS2/ZnS NCs intofolatemodified N-succinyl-N′-octyl chitosan (FA-SOC) micelles for in

The luminescence properties of I−III−VI NCs (emission spectra and PLQYs) can be finely tuned by applying a combination of strategies.

In summary, we have primarily presented here the important advances in synthetic chemistry and spectroscopic studies of luminescent I−III−VI NCs. Current achievements have demonstrated that I−III−VI NCs could be low toxic luminescent materials with an adequate supply. As an emerging new generation of luminescent NCs, many of the materials properties and possible uses have yet to be investigated. Even the use of I− III−VI NCs in light-emitting devices and bioimaging is at the beginning. In the next few years, we expect that the research interest of I−III−VI NCs will increased dramatically. In the following, we list a few challenges to be addressed in the future work. We believed that these unresolved questions and incoming issues could be very interesting topics in the field of physical chemistry. (1) Although special emphasis has been paid to understand the luminescence mechanism for I−III−VI NCs, further research still needs to refine the physical model to better describe the luminescence process due to the exciton recombination and carrier relaxation in a complicated confined electronic system. For example, low-temperature and single-particle spectroscopic study may further reveal the DAP recombination mechanism. (2) Although current synthetic methods have been successful in producing highly luminescent and color-tunable colloidal I− III−VI CuInS2-, CuInSe2-, and AgInS2-based NCs, several important issues remain to be resolved. To better control the composition and optimize the luminescence efficiency, we need to have a deeper understanding of the chemical reaction mechanism involved in the synthetic process. (3) Although the initial experimental results have demonstrated that I−III−VI NCs have low toxicity compared to cadmium- and lead-based NCs, there is still tremendous need to further evaluate the toxicity of I−III−VI NCs for biotechnology applications. (4) To realize the application of I−III−VI NCs in optoelectronics and biotechnology, many physical and chemical parameters need to be evaluated. For example, Critchley’s group and Xie’s group recently determined the molar extinction coefficient of CuInS2 NCs and demonstrated their size dependence. This is very important for the determination of the concentration of I−III−VI NC solutions.64,65 In addition, the stability of I−III−VI NCs under various conditions (chemical, photo, and thermal) should be further evaluated. (5) With the desirable luminescence features, there also exist attractive opportunities to explore the use of I−III−VI NCs in other applications such as photocatalysis,45 biosensors,66 photodetectors,67 and luminescent solar concentrators. 3172

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(6) With the excellent ability to tolerate intrinsic defects states, I−III−VI NCs are an ideal matrix to be doped and alloyed. This provides chances to make novel diluted magnetic semiconductor NCs. As a very excellent sample, Mn-doped I−III−VI NCs have been recently demonstrated.68,69

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Haizheng Zhong is currently an Associated Professor in the School of Materials Science & Engineering at the Beijing Institute of Technology. He received his B.E. degree from Jilin University in 2003 and Ph.D. degree from the Institute of Chemistry, Chinese Academy of Sciences (ICCAS) in 2008, under the supervision of Prof. Yongfang Li. From 2008 to 2010, he worked as a postdoctoral fellow with Prof. Gregory Scholes at the University of Toronto. He joined the Beijing Institute of Technology in 2010. His present research concerns the study of colloidal semiconductor nanocrystals for light-emitting and solarharvesting applications. http://mse.bit.edu.cn/staff.php?id=6040010 Zelong Bai is a Ph.D. candidate in the School of Materials Science & Engineering at the Beijing Institute of Technology. His major research interests are surface modification of colloidal semiconductor nanocrystals for optoelectronics and biotechnology applications. Bingsuo Zou is currently a Cheung Kong Scholars Professor at the Beijing Institute of Technology. He is the director of the Beijing Key Laboratory of Nanophotonics and Ultrafine Optoelectronic Systems and codirector of the Micro/Nano Research Center at the Beijing Institute of Technology. His interests focus on nanophotonics materials, physics, and applications.

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ACKNOWLEDGMENTS This work is supported under the National Basic Research Program of China (No. 2011CB933600, 2013CB328806), NSFC Grants (No. 51003005), and the 111 research base (BIT111-201101). H.Z. gratefully acknowledges Prof. Gregory Scholes for his helpful comments and Prof. Todd Krauss for the single-particle spectroscopic measurements. Mr. Bingkun Chen is also acknowledged for his contribution to the materials synthesis and light-emitting applications.

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REFERENCES

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