II–VI semiconductor QDs

Semiconductor QDs of the II–VI family, especially those based on CdSe, exhibit a tunable band-edge emission covering the visible spectrum (480–650 nm). They have been the most extensively investigated QDs and are recognized as a model system. The successful synthesis of high-quality cadmium chalcogenide (CdS, CdSe, and CdTe) QDs through a “hot injection” method, pioneered in 1993,^{Reference Murray, Norris and Bawendi4} catalyzed progress and applications of these QDs. In this method, a cold trioctylphosphine (TOP) solution containing an organometallic cadmium precursor and chalcogenide precursors were quickly injected into a hot tri-octylphosphine oxide (TOPO) solution at a temperature of 240°C or 300°C,^{Reference Murray, Norris and Bawendi4} resulting in a separation between nucleation and growth and producing uniform QDs. Nuclei of cadmium chalcogenide formed immediately after injection, followed by the growth of existing nuclei at lower temperatures without further nucleation. Further refinements, including the use of cadmium salts as precursors, organic acid surfactants such as long chain phosphonic acids or carboxylic acids, and weakly interacting solvents such as 1-octadecene (ODE) refined the size distribution of cadmium chalcogenide QDs, giving rise to homogeneous samples of core QDs.^{Reference Peng and Peng10–Reference Yu and Peng13} Analogous approaches have been applied for generating high-quality wider bandgap zinc chalcogenide (ZnS, ZnSe, and ZnTe) QDs with emission wavelengths covering the blue to the (UV) range.^{Reference Yu, Joo, Park, Baik, Kim, Kim and Hyeon14–Reference Xie, Zhong and Basché16} In these syntheses, a primary amine was needed to activate the zinc precursor, stabilize the surface, and improve the optical properties.^{Reference Yu, Joo, Park, Baik, Kim, Kim and Hyeon14–Reference Xie, Zhong and Basché16}

Despite advances in synthesizing uniform II–VI QDs, these core QDs do not meet the requirements in color downshifting applications, either in lighting and display, or in biology, due to their low PL QYs and poor stabilities. In addition to the desired tunable narrow bandgap emission, these core QDs also tend to exhibit a broad red-shifted emission originating from surface defects. The issues of quantum efficiency, stability, and color saturation can be addressed by overcoating core QDs with a semiconductor shell, especially with wider bandgap shell materials, resulting in core–shell QDs.^{Reference Reiss, Protière and Li5}

An initial core–shell structure concept, introduced in 1990, demonstrated CdSe QDs capped with ZnS and ZnS QDs capped with CdSe, synthesized in inverse micelle solutions.^{Reference Kortan, Hull, Opila, Bawendi, Steigerwald, Carroll and Brus17} This early structure already had the hallmarks of improved QY and resulted in clear band-edge emission and the nearly complete suppression of broad red-shifted trap emission. Later reports applied the improvements learned from the advances of the core synthesis to generate high-quality and color tunable ZnS capped CdSe core–shell QDs in organic solution, resulting in a PL QY of up to 50% and a color spectrum tunable throughout the visible wavelengths.^{Reference Hines and Guyot-Sionnest18,Reference Dabbousi, Rodriguez-Viejo, Mikulec, Heine, Mattoussi, Ober, Jensen and Bawendi19} High-quality CdSe QDs epitaxially coated with a CdS shell with high PL QYs and electronic access to the core were also synthesized using similar methods.^{Reference Peng, Schlamp, Kadavanich and Alivisatos20} Later refinements using air-stable shell precursors and introducing the concept of a layer-by-layer growth method further improved the quality of CdSe/CdS core–shell QDs.^{Reference Li, Wang, Guo, Keay, Mishima, Johnson and Peng21} The growth of a rod-like shell of CdS over an initial seed of CdSe provided high efficiency core–shell QDs with the addition of an emission polarized parallel to the long axis of the rod.^{Reference Talapin, Koeppe, Götzinger, Kornowski, Lupton, Rogach, Benson, Feldmann and Weller22–Reference Carbone, Nobile, Giorgi, Sala, Morello, Pompa, Hytch, Snoeck, Fiore, Franchini, Nadasan, Silvestre, Chiodo, Kudera, Cingolani, Krahne and Manna24}

Alloy cores have been typically used for producing green color-emitting core–shell QDs. For example, CdZnSe/CdZnS QDs with an emission peak centered at 520 nm showed a QY of 50–60%.^{Reference Steckel, Snee, Coe-Sullivan, Zimmer, Halpert, Anikeeva, Kim, Bulovic and Bawendi25} CdZnSe alloy QDs showed QYs up to 60–70% in the wavelength range of 520–535 nm after overcoating with a ZnS shell.^{Reference Protière and Reiss26} Recently, a CdZnSeS alloy core with a CdSZnS multishell system was reported with QYs close to unity and with an emission wavelength range of 502–563 nm.^{Reference Jun and Jang27} Core–shell QDs emitting in the blue and near-UV have been obtained using CdS and ZnSe as core materials overcoated with a ZnS shell.^{Reference Steckel, Zimmer, Coe-Sullivan, Stott, Bulovic and Bawendi28–Reference Chen, Lo, Hwang, Chang, Chen, Tasi and Wang30} The reported QYs of blue-emitting QDs are typically not comparable to red- and green-emitting QDs.^{Reference Steckel, Zimmer, Coe-Sullivan, Stott, Bulovic and Bawendi28–Reference Chen, Lo, Hwang, Chang, Chen, Tasi and Wang30} The full range of visible colors from emissive II–VI QDs has given rise to a series of images such as the one in Figure 2, highlighting the color tunability of core–shell QDs. Application of the full range of visible colors attainable using emissive core–shell II–VI QDs for color downshifting using blue-emitting GaN-based LED exciting QDs in a polymer matrix presaged today’s applications of QDs for display and lighting.^{Reference Lee, Sundar, Heine, Bawendi and Jensen31} Demonstration of the full visible color range of core–shell QDs in electroluminescence further showed their potential in emissive displays.^{Reference Anikeeva, Halpert, Bawendi and Bulovic32}

Figure 2.

Photograph of emissive CdS- and CdSe-based core–shell quantum dot solutions under UV light.

Although the PL of II–VI QDs can be bright and stable under a reasonable range of excitation light intensities, core–shell QDs universally showed significant “blinking” under the high fluxes used in single QD fluorescence spectroscopy, whereby the PL of single QDs turns “on” and “off” under continuous excitation. This single QD “blinking” not only limits the use of QDs as single photon sources but also potentially limits QDs from being a stable photoluminescent output source under relatively high fluxes.^{Reference Nirmal, Dabbous, Bawendi, Macklin, Trautman, Harris and Brus33–Reference Frantsuzov, Kuno, Janko and Marcus35} There have been efforts in recent years devoted to generating non-blinking or nearly non-blinking QDs. Hohng and Ha reported complete blinking suppression of CdSe/ZnS QDs in the presence of β-mercaptoethanol.^{Reference Hohng and Ha36} Later reports used a variety of other “anti-blinking agents.”^{Reference Hammer, Early, Sill, Odoi, Emrick and Barnes37,Reference Fomenko and Nesbitt38} However, these blinking suppressions rely on surface species and/or surface environments and are not robust; the initial blinking state is easily recovered by displacement of these “anti-blinking agents.”^{Reference Hohng and Ha36,Reference Fomenko and Nesbitt38}

A different approach to blinking suppression was explored by growing a thick CdS or CdS/CdZnS/ZnS shell (>5 nm in shell thickness) onto CdSe core QDs^{Reference Chen, Vela, Htoon, Casson, Werder, Bussian, Klimov and Hollingsworth39,Reference Mahler, Spinicelli, Buil, Quelin, Hermier and Dubertret40} with the idea of fully isolating the excited carriers from the QD surface and the surface environment. However, these QDs generally do not have a very good size distribution, exhibit broad PL spectra, and display moderate PL QYs. More recently, synthesis of CdSe/CdS QDs at a high reaction temperature (310°C) using octanethiol as a sulfur precursor resolved many of these issues.^{Reference Chen, Zhao, Chauhan, Cui, Wong, Harris, Wei, Han, Fukumura, Jain and Bawendi6} The high temperature and slow growth provide optimal conditions that lead to a nearly ideal crystal structure for the final core–shell QDs. During this shell growth reaction, particle size distributions remain tight throughout the entire reaction, the PL QYs increase to near unity, and emission line widths decrease to as narrow as 67.1 meV (FWHM ∼20 nm) (Figure 3a–h). Importantly, the resulting QDs for the first time simultaneously show high uniformity (size distribution of ∼4%) (Figure 3e–h), close to unity PL QYs (Figure 3c), narrow emission line widths (FWHM ∼20 nm) (Figure 3d), significantly suppressed blinking (94% “on” time fractions) (Figure 3i–j), along with a compact size (less than 10 nm) (Figure 3h).^{Reference Chen, Zhao, Chauhan, Cui, Wong, Harris, Wei, Han, Fukumura, Jain and Bawendi6} The ensemble luminescence photo darkening induced by statistical aging due to the blinking statistics is eliminated (Figure 3k). Owing to the combined properties of this new generation of core–shell QDs, significant improvements in the performance of QDs in a variety of applications should be anticipated.

Figure 3.

Temporal evolution of (a) photoluminescence (PL), (b) absorption, and (c) PL quantum yields (QYs). The green square shows the original PL QY of CdSe quantum dots (QDs) and (d) FWHM of the PL peak of CdSe/CdS core–shell QDs during shell growth reaction. Transmission electron microscopy images of (e) 4.4 nm CdSe core and (f–h) CdSe/CdS core–shell QDs with a CdS shell thickness of 0.8 nm, 1.6 nm, and 2.4 nm, respectively. Scale bar is 50 nm. (i) Representative PL blinking trace of a single CdSe/CdS core–shell QD. The histogram shows the distribution of intensities observed in the trace. (j) Histogram of the blinking “on” time fraction. The average “on” time fraction is 0.94. (k) The normalized (Nor.) PL intensity trace obtained from a collection of QDs. The inset shows the Nor. PL intensity trace obtained from a collection of QDs synthesized through the conventional method. Reprinted with permission from Reference 6. © 2013 Nature Publishing Group.

IV–VI semiconductor QDs

Lead chalcogenide IV–VI semiconductor QDs are characterized by their tunable NIR emission from 600 nm to 2200 nm.^{Reference Hines and Scholes41,Reference Moreels, Justo, Geyter, Haustraete, Martins and Hens42} These QDs exhibit intrinsically strong quantum confinement and have seen deep interest in various applications such as photodetectors,^{Reference Konstantatos, Howard, Fischer, Hoogland, Clifford, Klem, Levina and Sargent43} LEDs,^{Reference Steckel, Coe-Sullivan, Bulovic and Bawendi44} and photovoltaic solar cells,^{Reference Tang and Sargent45} in which PbSe and PbS nanocrystals are widely explored. An early report of a high-quality sample of IV–VI QDs showed the controlled synthesis of PbSe QDs through high temperature (100∼200°C) co-precipitation of organometallic precursors.^{Reference Chen, Stokes, Zhou, Fang and Murray46} A later report showed the growth of colloidal PbS QDs using similar methods as the growth of II–VI core QDs.^{Reference Hines and Scholes41} Uniform 4–5 nm PbSe QDs synthesized using a hot injection process similar to that for II–VI QDs were reported and used in NIR-emitting electroluminescent devices with tunable emission from 1.32 μm to 1.56 μm.^{Reference Steckel, Coe-Sullivan, Bulovic and Bawendi44} The development of core–shell PbSe/CdSe and PbS/CdS QDs necessitated the use of cation exchange, in which Pb ions of PbSe QDs were partially exchanged with Cd ions in a solution containing Cd precursors, with properties that parallel the development of II–VI QDs,^{Reference Pietryga, Werder, Williams, Casson, Schaller, Klimov and Hollingsworth47} with demonstrations of enhanced QYs, stability, and color downshifting applications.^{Reference Geyer, Scherer, Moloto, Jaworski and Bawendi48}

II₃–V₂ semiconductor QDs

II₃–V₂ semiconductor QDs (e.g., Cd₃P₂ and Cd₃As₂) with efficient PL have only been reported very recently. Miao et al. described the synthesis of 3.5–4.5 nm Cd₃P₂ nanocrystals by injection of tris(trimethylsilyl)phosphine (P(TMS)₃) into a solution of cadmium oleate in ODE.^{Reference Miao, Hickey, Rellinghaus, Waurisch and Eychmuller49} The emission could be tuned from 600 nm to 1200 nm by adding oleylamine and/or TOP, and a maximum QY of 38% was obtained. A similar, yet simpler, protocol was described by Xie et al., who used a mixture of CdO and oleic acid in ODE, in which a mixture of P(TMS)₃ and ODE was injected at 250°C.^{Reference Xie, Zhang, Zhao, Yang and Peng50} The size was tuned from 1.6 nm to 12 nm by varying the amount of oleic acid. The obtained QY was around 30% for samples smaller than 8 nm, with the best value being 70% for 2–3 nm nanocrystals (Figure 4).

Figure 4.

Transmission electron microscopy (TEM) images of Cd₃P₂ quantum dots (QDs) with different sizes: (a) 2.5, (b) 4.0, and (c) 12 nm. High resolution (HR) TEM image of (d) Cd₃P₂ QDs (sample in (c)). Scale bar is 50 nm for TEM and 10 nm for HRTEM. (e) Photoluminescence spectra of Cd₃P₂ QDs with various sizes. (i) <1.5, (ii) <1.5, (iii) 1.5, (iv) 1.8, (v) 2.4, (vi) 3, (vii) 4, (viii) 5.5, and (ix) 7.6 nm. (f) Relation between the size of Cd₃P₂ QDs and their emission peak positions. Reprinted with permission from Reference 50. © 2010 American Chemical Society.

Bulk cadmium arsenide (Cd₃As₂) presents an inverted band structure with a bandgap of –0.19 eV and an exciton Bohr radius estimated to be rather large (47 nm), making this material perhaps qualitatively different from other QD materials. Harris et al. reported the synthesis of Cd₃As₂ nanocrystals emitting in a broad spectral range of 530–2000 nm by reacting cadmium myristate with tris(trimethylsilyl)arsine (As(TMS)₃) at 175°C.^{Reference Harris, Allen, Han, Walker, Lee and Bawendi51} The method chosen to inject the As precursor—an initial fast injection to form small nuclei followed by a slow and continuous addition of precursors to promote growth—played an important role in generating Cd₃As₂ QDs with emission in the NIR range. The highest QY of 85% was reached for Cd₃As₂ QDs, with an average size of 2.5 nm and an emission wavelength of ∼900 nm, with more typical values ranging from 20% to 60%.^{Reference Harris, Allen, Han, Walker, Lee and Bawendi51} Air-sensitivity was noted, leading to a strong decrease in emission intensity after exposure to ambient conditions. Good quality core–shell QDs for this class of NIR and IR-emitting QDs would enhance their properties and are lacking thus far.

III–V semiconductor QDs

Reports on the synthesis of III–V semiconductor QDs largely focus on indium phosphide QDs. They are an attractive alternative to CdSe QDs due to their size-dependent emission in the visible and NIR spectral range. In initial synthetic routes,^{Reference Micic, Curtis, Jones, Sprague and Nozik52–Reference Guzelian, Banin, Kadavanich, Peng and Alivisatos54} the method established for cadmium chalcogenide QDs^{Reference Murray, Norris and Bawendi4} was adapted to InP, but longer reaction times (three to seven days) were necessary to yield particles of good crystallinity. In 2002, Battaglia and Peng reported a new protocol, also applicable to the synthesis of InAs QDs, which is based on fatty acids as stabilizers in combination with the non-coordinating solvent ODE instead of TOPO/TOP.^{Reference Battaglia and Peng55} The use of this medium provided a fast and controllable reaction, yielding InP QDs of low size dispersion but of limited size range (2.5–3.5 nm). Similar results were obtained when organometallic In precursors were used in combination with ester type solvents.^{Reference Xu, Kumar and Nann56} InP/ZnS core–shell QDs have been prepared by thermal cycling during the shell growth^{Reference Xie, Battaglia and Peng57} and by the addition of zinc carboxylate during the core synthesis.^{Reference Xu, Ziegler and Nann58} In both cases, modifications of the core synthesis led to samples covering the visible and some of the NIR (480–740 nm) and having a maximum QY of 40–60%. One of the simplest methods for the synthesis of high-quality InPZnS QDs consists of mixing both the InP core and ZnS shell precursors (indium myristate, P(TMS)₃, zinc stearate, dodecanethiol) at room temperature in ODE and heating this mixture to 250–300°C. The obtained QDs possess an alloyed core^{Reference Huang, Demadrille, Silly, Sirotti, Reiss and Renault59} and emit in the range of 480–600 nm with a QY up to 70% (Figure 5).^{Reference Li and Reiss60} More recent works focus on the overgrowth of these QDs with additional shells (e.g., ZnSeS graded or ZnSe/ZnS double shells) in order to increase their photostability, while no improved values of PL QY have been reported.^{Reference Lim, Bae, Lee, Nam, Jung, Lee, Char and Lee61,Reference Kim, Lee, Ahn and Jeong62}

Figure 5.

Single-step one-pot synthesis of InPZnS quantum dots. (a) Evolution of the photoluminescence (PL) (λ_ex 400 nm) and absorption spectra with reaction time (vertically shifted for clarity). (b) Photograph of samples obtained using the indicated reaction times under UV light. (c) Evolution of the fluorescence quantum yield and of the PL line width (increasing reaction time from left to right). Reproduced with permission from Reference 60. © 2008 American Chemical Society.

InAs is an interesting emitting material in the NIR range (700–1400 nm) for nanocrystal sizes of 1–3 nm. CdSe is an appropriate shell material due to its identical lattice parameter when compared to InAs, but ZnSe has also been investigated.^{Reference Cao and Banin63} Aharoni et al. reported the first strongly emitting system (QY >70%) in the form of InAs/CdSe/ZnSe core-shell-shell QDs in 2006.^{Reference Aharoni, Mokari, Popov and Banin64} Xie and Peng described the synthesis of InAs QDs with different shells, namely CdSe, InP, or ZnSe, to limit the sensitivity toward oxidation of the core in 2008.^{Reference Xie and Peng65} QYs up to 90% have been obtained with the InAs/CdSe core–shell system. Later, Allen et al. proposed a mixed CdZnS shell for capping small (1.4 nm) InAs QDs emitting at 800 nm for biological applications, resulting in QYs of 35–50%.^{Reference Allen, Liu, Chauhan, Lee, Ting, Fukumura, Jain and Bawendi66}

Ternary I–III–VI₂ chalcopyrite semiconductor QDs

Ternary semiconductor QDs such as the I–III–VI₂ chalcopyrites CuInSe₂ (CISe) and CuInS₂ (CIS) have emerged in the past few years as further alternative materials to cadmium-based systems without toxic elements.^{Reference Aldakov, Lefrancois and Reiss67} They are direct semiconductors and exhibit a relatively narrow bandgap (1.05 eV for CISe, 1.5 eV for CIS). Ternary CISe and CIS QDs were mainly studied because of their potential use in photovoltaics.^{Reference Nairn, Shapiro, Twamley, Pounds, von Wandruszka, Fletcher, Williams, Wang and Norton68,Reference Czekelius, Hilgendorff, Spanhel, Bedja, Lerch, Muller, Bloeck, Su and Giersig69} In contrast, their PL properties were rarely investigated before 2008. Castro and co-workers reported a synthesis method for CISe and CIS via the decomposition of the single source precursor (PPh₃)₂CuIn(SEt)₄, yielding luminescent CIS samples with a PL QY of ca. 5%.^{Reference Castro, Bailey, Raffaelle, Banger and Hepp70,Reference Castro, Bailey, Raffaelle, Banger and Hepp71} Nakamura et al. doped CIS QDs with Zn and were able to vary their PL wavelength from 570 nm to 800 nm, with QYs also in the range of 5%.^{Reference Nakamura, Kato, Uehara, Nose, Omata, Otsuka-Yao-Matsuo, Miyazaki and Maeda72} Increased fluorescence QY upon addition of Zn has also recently been observed in another example of a I–III–VI₂ semiconductor: A solid solution of ZnS and AgInS₂ exhibited emission tunable in the visible and NIR with a QY up to 24%.^{Reference Torimoto, Adachi, Okazaki, Sakuraoka, Shibayama, Ohtani, Kudo and Kuwabata73} Even higher QYs were obtained by using dodecanethiol (DDT) as both the sulfur source and surface ligand in ODE and overcoating the resulting CIS QDs with a ZnS shell.^{Reference Li, Daou, Texier, Tran, Liem and Reiss74} The obtained QDs showed tunable emission in the range of 550–815 nm with a QY up to 60%. QYs up to 67% have been reported in 2011 when using a very similar approach, but directly applying DDT instead of an ODE/DDT mixture as the solvent and using a ZnS (CdS) shell.^{Reference Li, Pandey, Werder, Khanal, Pietryga and Klimov75}

Cassette et al. reported the highest QY of CISe-based QDs.^{Reference Cassette, Pons, Bouet, Helle, Bezdetnaya, Marchal and Dubertret76} In a first step, the core QDs were prepared by reacting copper chloride with seleneourea and indium chloride in DDT. ZnS shell growth was achieved by the slow addition of a mixture containing zinc ethylxanthate and zinc oleate dispersed in ODE, TOP, and dioctylamine. The resulting core–shell QDs offer PL emission tunable from 700 nm to 1000 nm depending on the particle size, with QYs ranging from ∼50% for smaller QDs to 10% for larger QDs in organic solvents.

One drawback of all ternary chalcopyrite QDs is their comparably large emission line width, on the order of 100 nm (FWHM) in the best cases. This behavior is thought to have originated from distributions in the emitting states. Time-resolved fluorescence measurements are consistent with emissions in ternary QDs originating from transitions from quantized conduction band states to localized acceptor states or from donor-acceptor pair states involving intra-bandgap states.^{Reference Aldakov, Lefrancois and Reiss67,Reference Li, Daou, Texier, Tran, Liem and Reiss74,Reference Li, Pandey, Werder, Khanal, Pietryga and Klimov75} Hence, not only the size distribution but also particle-to-particle fluctuations in the distribution of these states likely contribute to the observed line width.