Source: https://pubs.rsc.org/en/content/articlehtml/2015/cs/c5cs00285k
Timestamp: 2019-04-24 14:24:23+00:00

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Colloidal nanocrystals – produced in a growing variety of shapes, sizes and compositions – are rapidly developing into a new generation of photonic materials, spanning light emitting as well as energy harvesting applications. Precise tailoring of their optoelectronic properties enables them to satisfy disparate application-specific requirements. However, the presence of toxic heavy metals such as cadmium and lead in some of the most mature nanocrystals is a serious drawback which may ultimately preclude their use in consumer applications. Although the pursuit of non-toxic alternatives has occurred in parallel to the well-developed Cd- and Pb-based nanocrystals, synthetic challenges have, until recently, curbed progress. In this review, we highlight recent advances in the development of heavy-metal-free nanocrystals within the context of specific photonic applications. We also describe strategies to transfer some of the advantageous nanocrystal features such as shape control to non-toxic materials. Finally, we present recent developments that have the potential to make substantial impacts on the quest to attain a balance between performance and sustainability in photonics.
Joel Grim joined the NanoCrystal Photonics Lab within the Nanochemistry Department at the Istituto Italiano di Tecnologia in September 2013. His postdoctoral research centers on investigating the optoelectronic properties of zero-, one-, and two-dimensional shape-controlled colloidal nanocrystals using ultrafast and nonlinear optical techniques. In addition to these fundamental studies, he is pursuing photonic applications such as low-threshold lasers. He obtained his PhD in Physics at Wake Forest University (USA) in 2012 where he studied nonlinear recombination processes in materials used for high-energy detection. He has BSc degrees in Mathematics, and Ag. and Biological Engineering from the Pennsylvania State University (USA).
Liberato Manna received his PhD in Chemistry in 2001 from the University of Bari (Italy) and worked at the University of California Berkeley as a visiting student and then at the Lawrence Berkeley Lab as a postdoc until 2003. He was then a scientist at the National Nanotechnology Lab in Lecce (Italy) and in 2009 he moved to the Istituto Italiano di Tecnologia (Genova) as the director of the Nanochemistry Department. Since 2010 he has been also working as a part-time professor at TU Delft (The Netherlands). His research interests are the synthesis and assembly of inorganic nanostructures for applications in various fields and the study of structural and chemical transformations in nanocrystals.
Iwan Moreels obtained his PhD degree in applied physics at Ghent University (Belgium) in 2009, studying near-infrared lead salt quantum dots. He then continued with a postdoc project on quantum dot emission dynamics at Ghent University and the IBM Zurich research lab (Switzerland). In 2012 he joined the Istituto Italiano di Tecnologia (Italy), where he now leads the NanoCrystal Photonics Lab in the Nanochemistry Department. He has published about 50 papers in peer-reviewed journals, with research that spans from the synthesis of novel fluorescent nanocrystals to ultrafast optical spectroscopy and photonic applications based on colloidal quantum dots.
As the global population continues to grow, and with it energy consumption and pressure on natural resources, sustainable development of technology has become a critical issue. Photonic and optoelectronic applications are particularly relevant in this regard, with lighting and personal electronics contributing to more than 20% of electricity consumed worldwide.1 The development of economically viable solutions requires scalable, inexpensive earth-abundant materials, with minimal energy required for the development and integration into applications. Material toxicity also plays a crucial role, both with regard to minimizing possible contact in consumer applications as well as reducing hazardous waste.
Although many NC applications are targeted at reducing energy consumption (for instance, more efficient lighting) or reliance on fossil fuels via solar energy harvesting, the principles of sustainability42 start before and extend beyond the end-application. The potential benefits at the moment still do not outweigh the most significant issue facing colloidal NCs, which is the presence of toxic heavy metals such as Cd, Pb and Hg. In this review, we will discuss the current status of NC-based photonic applications, and focus on the promising benign alternatives, contrasting their performance against the (generally) superior heavy-metal containing NCs and discuss potential pathways toward future developments.
The requirement for attaining a balance between performance and sustainability using NCs is multifaceted. Their low-energy, solution-based synthesis enables scalable and facile incorporation into devices, while tunable optoelectronic properties allow fine adjustments to optimize performance to meet application-specific requirements. Established semiconductor competitors for NCs in the photonic applications discussed in this review can be broadly categorized as inorganic bulk single-crystals (e.g. Si in PVs), epitaxially grown thin films, quantum wells and dots (e.g. GaN in LEDs), and organics (e.g. poly(9,9′-dioctylfluorene) polymers for OLEDs). NCs have a clear advantage over the inorganic competitors in terms of material and production costs with the potential to rival their performance. Conversely, the current cost of scaled-up NC production is about $10 per gram while mass-produced organic small molecules can be produced at 10–100 times lower cost.43,44 However, organic molecules and inorganic single-crystal or epitaxial semiconductors are unable to match the precisely controllable emission and absorption spectra provided by NCs. From a performance and sustainability point of view, NCs thus sit somewhere between inorganic bulk or epitaxially grown semiconductors and organic molecules.
Fig. 1 shows the emission wavelength range for a wide variety of NCs, from the bulk band gap down to the – to the best of our knowledge – shortest reported quantum confinement-induced emission wavelength. They are grouped into three categories. The first group is formed by NCs that only contain elements that are considered non-toxic (green points). The second group contains elements such as indium that are limited or geographically concentrated (orange squares), and the last group contains toxic elements which are banned by, for instance, the European Union's Restriction of Hazardous Substances Directive (RoHS), specifically cadmium, lead and mercury containing compounds (red triangles).
Fig. 1 Emission tunability of selected semiconductor NCs. In most cases, the range varies from the bulk band gap down to the shortest confinement-induced, blue shifted emission. GaN,45 ZnO,46 ZnSe,47,48 ZnTe,49,50 Zn3N2,51 Si,52,53 Ge,54,55 AgInS2,56,57 CI(Z)S,58–61 InP,62–64 InAs,65,66 InSb,67,68 CdS,69 CdSe,70,71 CdTe,69 PbS,72,73 PbSe,74 and HgTe.75 HgTe is a semi-metal, so instead of the bulk band gap, emission from large NCs is reported from ref. 75. For AIS and CI(Z)S, we report the PL position, which falls significantly below the band gap due to defect-assisted emission.
Importantly, the shape control of Zn-based NCs has extended beyond spherical QDs to 1D quantum rods82–85 and 2D nanosheets.86,87 In the former case, ZnSe, ZnS and ZnTe rods with controllable aspect ratios have been synthesized using a phosphine-free technique via a ripening process through thermodynamically driven material diffusion.82 ZnSe/ZnS dot-in-rods88 and ZnS and ZnSe/ZnS core–shell NPLs89 have been synthesized with absorption and emission in the UV-blue via cation exchange, a reaction that replaces cations within an NC lattice with those in solution. ZnSe NPLs have also been produced with both wurtzite86 and zinc-blende87 structures, with a thickness of 1.4 nm for the former, but photoluminescence has yet to be reported. Further potential of zinc-based NCs has been demonstrated with zinc nitride (Zn3N2) NCs, which belongs to the II–V semiconductor group.51 In addition to tunable emission from green to the NIR as shown in Fig. 1, they possess QEs of over 50%,51 remarkable performance considering this was the first report on the synthesis of zinc nitride NCs.
Epitaxially grown III–V semiconductors have enjoyed considerable success in photonic applications for decades, forming the basis for state-of-the-art LEDs90 and lasers,91 for example. Not surprisingly, the development of colloidal III–V semiconductor NCs has been pursued nearly as long as the II–VIs.92–94 In addition to their relatively benign elements, direct band gaps and potential to cover the full visible to the NIR spectrum makes them a very attractive class of materials. However, finding appropriate precursors for colloidal synthesis has been hampered by the high covalency of these compounds,21,95,96 requiring high temperatures for both nucleation and growth stages, resulting in large NC size-dispersions.95 For example, early synthesis protocols for InP NCs were energy intensive and yielded poor-quality particles with broad size distributions.97–99 In spite of these challenges, remarkable progress has been made with InP and InAs, initiated with a low-temperature route developed by the group of X. Peng in 2002.100 In a later work, introducing fatty amines as activation reagents had dual benefits of lowering reaction temperatures (below 200 °C) and allowing for ZnS shell growth in a one-pot synthesis to produce high quality, monodisperse InP and InP/ZnS QDs with broad size tunability.62 Importantly, more recent progress has also enabled narrow emission linewidths and high PLQE from InP/ZnS NCs,101 in addition to the wide emission tunability shown in Fig. 1.
An alternative to the direct-synthesis procedures is given by cation exchange, circumventing the challenging direct synthetic protocols, as illustrated in Fig. 2. It has recently been used to produce GaAs, InAs, GaP, and InP with relatively narrow size dispersions, while avoiding the challenges associated with the traditional so-called hot-injection synthesis.21–23 This synthetic route also has the advantage of preserving the original NC conformation, opening up the control of shape in these zinc blende NCs, including 1D rods and 2D sheets. A potential drawback is the incomplete removal of Cu that is used in an intermediate stage, and often quenches the PL.105 Recent achievements on CdTe quantum disks,106 CdSe/CdS105 and ZnSe/ZnS dot-in-rods88 suggest that it can be alleviated, however, opening an avenue for fluorescent shape-controlled III–V NCs.
Fig. 2 (a) Illustration of the cation exchange reaction used to produce monodisperse III–V NCs (including GaAs, InAs, GaP, and InP) by substitution of group 13 ions in cadmium pnictide NCs. Reproduced with permission from ref. 21. (b) Preparation of hexagonal WZ InP NPLs via a cation exchange of Cu3−xP with In3+. Adapted with permission from ref. 22.
Multinary compounds, such as ternary I–III–VI2 chalcogenides are recent additions to the colloidal semiconductor NC family, but they mark a shift from simple binary NCs to more complex systems with added control provided by compositional tunability. CuIn(S, Se)2 and AgIn(S, Se)2 are prominent examples that provide access to emission from the visible to the NIR as indicated in Fig. 1, with relatively high QEs of 80% for CIS/ZnS33 and 35% for core-only AgInS2 (ref. 57). Due to the wide range of stoichiometric variations and complex crystal structures, high density sub-band gap states are formed (Fig. 3(a)). This results in characteristic broad emission linewidths and long lifetimes.107,108Ref. 109 provides a comprehensive overview of the synthesis of ternary and quaternary chalcogenides, so it will not be discussed here.
Fig. 3 (a) Left: schematic of radiative (straight arrows) and nonradiative (wavy arrows) recombination pathways for CIS NCs, with an electron in a quantized conduction band (CB), and a hole trapped at a surface (D1) or internal (D2) defect. Right: elimination of the fast, nonradiative D1 decay channel by overcoating CIS NCs with ZnS shells results in a substantial increase in quantum efficiency. Adapted with permission from ref. 110. (b) Reaction scheme depicting the synthesis of homogeneously alloyed CIZS and luminescent CIZS/ZnS core–shell alloy NCs from pristine Cu2S NCs. Reproduced with permission from ref. 58.
While III–V and I–III–VI semiconductors present realistic alternatives to the heavy-metal based NCs, the presence of critical elements is also a concern for long-term sustainable application of colloidal NCs. Indium is frequently highlighted in this regard since it is considered to be in limited supply116 making it susceptible to price instability due to the demand in a variety of other micro-electronic applications and its geographical concentration of nearly 50% of the world's resources in China.117 In our specific case, the indium supply may not be a serious issue, particularly due to the small quantities required for NC-based applications.118 Nevertheless, in the long term we are bound to search for alternatives. This makes silicon and germanium NCs especially attractive for photonic applications, providing non-toxic solutions as well as seamless integration into existing Si-based micro-electronic applications. Although bulk Si and Ge have indirect band gaps, quantum confinement induces a direct-band gap transition in NCs of these materials, enabling efficient light emission.119,120 Relatively monodisperse Si QDs have been prepared via various bottom-up synthesis procedures53,121–124 with QEs as high as 75% with surface modification,124 but emission is limited to blue and green wavelengths, with red emission (as well as generally higher QE) only obtained with top-down preparation, such as electrochemical etching of bulk Si.125 Recently, free standing two-dimensional Si nanosheets with thicknesses controllable between 1 and 13 nm have been synthesized by chemical vapour deposition.126 Emission was shown to be tunable from blue to red (387 to 775 nm), and these Si nanosheets were also used as emitting layers in a blue-emitting LED, which may be considered a significant accomplishment that will be further discussed in Section 2.3.
The 2014 Nobel Prize in Physics awarded for the invention of blue LEDs90 highlights broad impact LEDs have on providing energy efficient lighting. LED efficiency has been doubling roughly every 36 months, following what is known as Haitz's law,138 with efficiencies that are now double that of compact fluorescent lamps (CFLs).139–141 Efficiency, however, is an incomplete measure of the efficacy and sustainability of LEDs. Their cost and scalability are currently limited to a large extent by low-yield, high-energy epitaxial growth of, for example, InGaN for blue- and green-emitting LEDs.
Relatively new demands created by consumer technologies such as portable electronics and flat-panel displays have accelerated the need for facile, large area solutions. At present, LEDs are predominantly used in display applications, strongly motivating the search for efficient red, green, blue (RGB) emitters amenable to the large areas and thin dimensions required by these applications. Crucially, with already substantial but still growing demand for display technologies, special attention needs to be devoted to sustainability. Performance-wise, important metrics for LEDs are their external quantum efficiency (EQE; the number of emitted photons to the number of injected electrons), peak brightness (cd m−2), operating voltage (conversion of electrons to photons) and lifetime measured in hours, and these metrics will be used to discuss the viability of LEDs based on sustainable materials below. We only list EQE for NC-LEDs in Table 1 since the other metrics are not always reported in literature.
It was not long after the landmark synthesis of monodisperse QDs143 that an electrically injected NC-LED was first demonstrated in 1994.24 Since then, there have been significant improvements, including architectures employing all-inorganic as well as hybrid structures that utilize both organic and inorganic charge transport layers (CTLs).26,144–153 Notably, Kwak et al.26 developed NC-LEDs using CdSe/CdS/ZnS, CdSe@ZnS, and Cd1−xZnx@ZnS NCs to achieve peak brightness values of 23 040, 218 800, and 2250 cd m−2 for red, green, and blue emission, respectively, satisfying the requirements for both displays (100–1000 cd m−2) and general lighting (1000–10 000 cd m−2). Although the internal QE of CdSe-based NCs is close to 100% due to thermal mixing of dark and bright states, boosting the EQE has proved to be challenging. Recently, Mashford et al.146 developed a red-emitting NC-LED with an EQE of 18%, rivalling state-of-the-art OLEDs. As with other NC-LED structures,154–156 a film of ZnO NCs was used as an electron injection layer, and it was found that NC-LED performance was very sensitive to the NC emitting layer thickness. With further optimization, the performance of CdSe/CdS QD LEDs is now comparable to the state-of-the-art OLEDs, with an EQE of 20.5% and a lifetime of over 100 000 hours at 100 cd m−2.142 The device structure used in that work is shown in Fig. 4.
Fig. 4 LED device structure used to achieve record efficiencies with CdSe/CdS core–shell QDs. Adapted with permission from ref. 142.
In addition to providing competition to OLEDs in terms of power conversion efficiency at technologically relevant brightness levels, operating lifetimes that were typically in the range of 102–104 hours26,144,145,154 (compared to 103–106 hours for OLEDs157) have now been increased to over 105 hours at 100 cd m−2. While achieving sufficient operating lifetimes was one of the greatest technical challenges for the incorporation of NCs in SSL applications, the presence of Cd-based NCs in these LEDs still precludes their use in consumer applications. Nevertheless, the performance of Cd-based NC-LEDs is both a confirmation that there is a future for NC-LEDs in general and a source of knowledge that can be transferred to non-toxic systems.
InP has notably emerged as a promising alternative due to its broad spectral tunability and maturing synthesis, which is now capable of yielding monodisperse QDs with linewidths as low as 38 nm FWHM.101 In the past few years there have been numerous reports of InP-based LEDs, with rapidly improving performance.27,62,158–161 For instance, green-emitting (500–520 nm) InP cores passivated by composition gradient ZnSeS shells were incorporated into an inverted device structure (Fig. 5) to obtain an EQE of 3.46% and a peak brightness of 3900 cd m−2.158 Although high NC QEs, in the range of 60–70% have been reported for blue emission,162 and even values above 90% for red emitting InP-based NCs,118 direct charge-injection blue LEDs have not yet been reported and red LEDs exhibit inferior performance (Table 1).
Fig. 5 Architecture of InP/ZnSeS QD-based LEDs with ZnO and 4,4′,4′′-tris(N-carbazolyl)triphenylamine (TCTA) as electron and hole transport layers, respectively, used to achieve an EQE of 3.46%. Reproduced with permission from ref. 158.
In spite of the challenges associated with intra-band trap states in ternary chalcogenides, CIS-based NCs have also been successfully incorporated into direct-injection LEDs,107,166,167 albeit with modest results. The best performance to date has been achieved using yellow-emitting CIS/ZnS in a hybrid organic–inorganic architecture similar to that used for Cd-based LEDs (see Fig. 4), with EQE and peak luminance of 1.1% and 1564 cd m−2, respectively.107 Although green, yellow and red CIS-based LEDs have been reported,167 blue emission remains to be demonstrated. Another significant challenge for CIS LEDs is donor–acceptor pair-broadened emission, which is a critical issue for display applications that have high demands for colour purity. Steps toward solving this issue includes the development of CuInS2–ZnS (CIZS) alloyed cores where size-dependent shifts with concomitant short PL lifetimes (<100 ns) were observed.168 Similar CIZS cores were used with a double ZnSe/ZnS shell to obtain green, yellow and red emissions with lifetimes three orders of magnitude shorter than trap-emission lifetime,167 though emission linewidths remained broad.
Looking back at Fig. 1, there is an attractive possibility to cover the visible to the NIR spectrum using silicon NCs.119 Silicon is earth-abundant, non-toxic, and already dominates the microelectronic industry, making it a highly sustainable option. Efficient light emission from Si NCs (PLQEs in the range of 75%)124,169 is enabled by the indirect to direct band gap transition when translational symmetry is removed in the quantum confined regime. Moderate EQEs up to 1.1% have been demonstrated for red-emitting Si NC LEDs, at a noteworthy low turn-on voltage of 1.8 V.170 Broad linewidths and difficulty in achieving blue emission are among the challenges that Si NC-LEDs still face, particularly for integration into display applications.165 A promising route toward blue emission has recently been shown with two-dimensional Si nanosheets with emission tunable from 387 to 775 nm (corresponding to thicknesses ranging from 1 to 13 nm), which have been used to make a blue LED.126 Although the Si nanosheets in that work were prepared by chemical vapour deposition, it should motivate the development of colloidal analogues.
The ease of processing and the possibility of light and flexible PV panels have driven the search for solution processed materials capable of competing with the state-of-the-art bulk PV materials such as silicon or CIGS. Added to the general benefits of solution processed PVs, tunable absorption spectra (extending into the NIR, beyond the Si band gap) and multiple exciton generation (MEG, the creation of more than one electron–hole pair with one incident photon) possibilities have triggered intense efforts for QD-PVs over the last decade. Stability in air during both module fabrication and implementation without the need for inert conditions provides a further advantage of QDs, especially in light of the stability issues of their organic competitors.174 Harvesting more of the solar spectrum with NCs and the prospect of MEG38 offering a route to exceed the Shockley–Queisser single-junction limit have provided good reasons for optimism. This is tempered somewhat considering that Pb-based QD-PV power conversion efficiencies (PCE) have risen from less than 1% in 2005 (ref. 175) to 9.2% in 2015 (ref. 176), still short of other emerging technologies such as dye-sensitized or perovskite-based solar cells (see Fig. 6).
Fig. 6 Comparison of record power conversion efficiencies for PVs based on CZTSSe177 and PbS QDs,176 organics, perovskites (Pb-based182 and Sn-based183,184), and more established CZTSSe and CIGS thin films and Si and GaAs bulk semiconductors (unless otherwise referenced, data were taken from the NREL Research Cell Efficiency chart185). Shockley–Queisser186 and MEG187 limits are shown using red and green dashed lines, respectively.
Interestingly, in contrast to the light-emitting applications discussed in Section 2, the performance of PV cells comprised of sustainable colloidal NCs is already comparable to the state-of-the-art in heavy-metal QD-PVs, with a PCE of 8.5% for CZTSSe NCs177 and 7.2% for CIS/ZnS QDs.37Fig. 6 compares record PCEs for QD-PVs with both solution-processed competitors and established thin film and bulk semiconductor PVs, all in single-junction mode.
Although quantum confinement and size-controlled optical properties offer tantalizing possibilities, they are not necessities for PV applications. For instance, excellent performance has been achieved by sintering all-inorganic CZTSSe NCs into thin films to create high NC packing densities for efficient charge transport,177,178 with the disadvantage of high temperatures required to add selenium lost (or replace sulphur with selenium). Strategies to increase NC packing density while avoiding energy-intensive sintering include using other NC geometries such as CuInSe2 nanowires179 (larger contact area) or organic to inorganic ligand exchange (smaller NC–NC distance).180 Another strategy to improve carrier transport is to link NCs together with composition matched inorganic ligands.181 Specifically, annealing chalcogenidometallates of Cd, Pb, and Bi with CdSe, PbSe, and Bi2Te3 NCs, respectively, at temperatures as low as 250 °C resulted in drastically enhanced electron mobilities.181 In addition to Bi2Te3, this technique can in principle be extended to other non-toxic NCs such as CZT(S, Se) and CIS. The prospect for sustainable NC-PVs is therefore multifaceted, with efforts to achieve bulk-like charge transport NCs providing an alternative means of benefitting from a solution-processed PV. With this approach, a facile preparation of bulk-like semiconductor PVs can be accomplished under relatively undemanding conditions, starting from solution-based NCs. Toward this objective, sustainable materials such as CZT(S, Se) and CIS are already well-positioned.
The traditional strengths of NCs as optically stable and highly fluorescent phosphors were recognized early on in their application as biolabels,33,34,188–190 where such properties form key requirements. The approaches discussed in the previous two sections for LEDs and solar cells relied on the efficient conversion of charge to light or vice versa, which can be circumvented for both applications by falling back on the primary use of NCs as bright and stable colour-converters.
A recent development is to use NCs in luminescent solar concentrators (LSCs) to amplify the collection efficiency of PV technologies with colour-converting phosphors. The principle is illustrated in Fig. 7, where a transparent planar waveguide slab is coated or doped with luminescent dyes or particles that absorb light and emit longer wavelength light that is channelled via total internal reflection to a solar cell.191–194 Up to a 10 fold concentration of solar light can be realized with LSCs,195 along the obvious advantage of relying on mature PV technologies for energy conversion, with an added advantage of substantially minimized area.
Fig. 7 Successful strategies for the incorporation of NCs into luminescent solar concentrators. (a) A band diagram of NC heterostructures used to shift emission outside of the absorption band. Adapted with permission from ref. 193 and (b) using dopant emission to completely avoid reabsorption. Adapted with permission from ref. 196.
Given the inherently small Stokes shifts in QDs, reabsorption of colour-converted light is a significant challenge for QD-LSCs, becoming a substantial loss in large-area implementations that require long optical path lengths. Fig. 7 illustrates two strategies for obviating reabsorption, ‘Stokes-shift engineering’ with core–shell NCs193,197,198 and impurity-doped NCs,196 with the emission wavelength shifted outside the absorption band in both cases. The first approach is quite general, and in principle can be extended to heavy-metal-free NCs with a voluminous large-band gap shell, confirmed by red shifted emission from InP/thick shell ZnS QDs.162 Currently, the more effective approach is to introduce transition-metal ion dopants into a NC host.197,199–201 Here, emission from intraband states formed by the dopant ions is far removed from the absorption edge of the NC. Non-toxic ZnSe/ZnS QDs doped with Mn2+ ions have been shown to be effective in LSCs, avoiding all reabsorption losses (Fig. 8).196 Although promising both from sustainability and performance perspectives, expanding to smaller-gap materials is a necessary step to harness more of the solar spectrum. InP again provides good performance, with Cu+ dopant PLQEs as high as 35–40% and tunable NIR emission obtained in InP/ZnSe QDs,202 and the results clearly show their potential for enhancing PV performance as LSCs.
Fig. 8 Possibilities for semiconductor-dopant combinations indicating potential for covering more of the solar spectrum with for instance InP doped with Cu+. Reproduced with permission from ref. 196.
Future directions should include extending the absorption deeper into the NIR, which can be accomplished by finding appropriate dopants for materials such as InAs, InSb, and Ge. Compositional tunability of multinary NCs provides another approach. In CIZS NCs for example, variations in starting ratios of Cu+ : In3+ : Zn2+ were capable of shifting the PL from 880 to 1030 nm.58 The large Stokes shift of Si NCs (absorption peak in the ultraviolet and emission possible from 300 to above 1000 nm)203 could also make them candidates for use in LSCs, though to the best of our knowledge no reports on their use in this context exists to date.
In a different sector, CFLs have been the primary competitor of the inefficient, yet aesthetically pleasing incandescent bulb invented by Thomas Edison in 1880. The presence of mercury in CFLs204 is a major concern, however, motivating the development of low-cost, heavy-metal-free white light sources. Using NCs as phosphors in combination with established InGaN blue-LEDs again relies on the intrinsic strengths of NCs while avoiding problems with inefficient charge transport.
In this regard, InP and CIS NCs in particular are beginning to offer viable solutions as colour-converters for lighting,118 where large-area implementations (50 × 50 cm) of InP/ZnS QDs have already been demonstrated.206 Their versatility for this application is further highlighted by recent demonstrations that combine intrinsic and dopant emission in a single system. For example, as illustrated in Fig. 9, a Cu-doped InP/ZnS/InP/ZnS multilayer structure yielded a WLED with a high colour-rendering index (CRI).205 Similarly, high quality white light was generated using Mn-doped CIS/ZnS NCs with bichromatic emission enabled by separating Mn2+ ions from the CIS core with a ZnS barrier.207 In both cases, the NCs down-converted blue light from commercially available InGaN LEDs that also provided the blue portion of the white-light spectrum.
Fig. 9 White light generation using dual emission (Cu dopant and InP layer) from a heavy-metal-free NC phosphor with blue light provided by the excitation LED. Reproduced with permission from ref. 205.
Lasers are arguably one of the most important technological developments of last century, finding broad, integral use in consumer electronics, industrial manufacturing, medicine and scientific research. Emerging and next-generation applications (for instance, optical computing, quantum computing, lab-on-chip devices, etc.) are creating demand for lasers that can be integrated onto a diverse array of platforms, including the micro- and nano-scales.
Solution-processed materials provide a route toward flexible, substrate-independent gain media that can be incorporated into a wide variety of cavity geometries31,212–216 using deposition techniques such as inkjet printing and spray coating. However, moving beyond laboratory demonstrations of lasing – where large, powerful pulsed pump sources are available – has proven to be a significant challenge. Laser action under continuous wave (cw) excitation or electrical injection is a crucial feasibility achievement necessary for the transfer to practical applications. The former enables the use of small, inexpensive diode laser pump sources (recently demonstrated for CdSe nanosheets28), and the latter is generally considered the ultimate goal which would permit integration into microelectronic applications.
NCs have gained significant attention for their use as gain media in large part due to their confinement-induced quantization of energy spectra that results in discrete energy levels, leading to enhanced band-edge density of states as well as low threshold lasing.31,217,218 The delta-like density of states in quantum dots further predicts a temperature independent threshold when the separation of energy levels is greater than the thermal energy, avoiding thermal depopulation.217 The primary challenge in using NCs as gain media is overcoming the exact balance between absorption and stimulated emission. For example, the doubly degenerate electron ground state of CdSe QDs results in the complete reabsorption of SE if only one carrier is excited, reaching transparency but no net gain. Therefore, a solution to break this balance and enable gain is to excite, on average, multiple electron–hole pairs (multiexcitons) per QD. However, the resulting high carrier densities in the small volume of QDs results in highly efficient Auger recombination.219 Mitigating the effects of Auger recombination has therefore been a primary objective for the realization of practical NC-based lasers.
Fig. 10 (a) Illustration of single exciton gain. Top left: optical transparency but no gain when stimulated emission and absorption overlap requires the excitation of multiple electron–hole pairs in each QD to achieve gain. Top right: shifting the position of the absorption energy with respect to the emission energy of core–shell QDs lifts the requirement of generating multiple excitons, enabling single-exciton gain. Reproduced with permission from ref. 220 (b) lasing using CdSe/Cd0.5Zn0.5S core–shell QDs imbedded between distributed Bragg reflectors. Adapted with permission from ref. 31.
While requirements for efficient lasing are clear from the results obtained on shape-controlled CdSe and their heterostructures, plotting some of the record low stimulated emission thresholds (under femtosecond pulsed optical pumping) for colloidal NCs versus emission wavelength in Fig. 11 highlights the scarcity of heavy-metal-free NC gain media. The few reports of lasing using more sustainable gain materials include InP/ZnS in a liquid crystal grating cavity213 and InGaP/ZnS in a VCSEL cavity.226 Although the SE threshold shown in for InP/ZnS NCs is significantly higher than other NCs at similar wavelengths (note the log–log scale), this work predated more recent improvements in the synthesis of InP NCs. Given the substantial improvements in the past few years for InP, there are good prospects for its use as a gain medium. For instance, nanosecond pumped lasing with emission at 657 nm has been demonstrated for InGaP/ZnS core–shell QDs in a flexible VCSEL configuration,226 enabling mechanical wavelength tuning (600–660 nm in that report). Although no SE threshold was provided, the stringent gain/loss requirements of VCSEL structures make them a particularly challenging configuration for any gain media. Combined with the demonstrated nanosecond lasing, the performance of InGaP NCs in this structure demonstrates their strong potential as a green alternative to Cd-based NC gain media.
Fig. 11 Stimulated emission thresholds as a function of wavelength for InP/ZnS QDs,213 PbS QDs (1.3 μm, ref. 227 and 1.52 μm, ref. 218), CdSe/CdS dot-in-rods,217 CdZnSe/ZnS QDs,31 giant shell CdSe/CdS QDs,228 CdSe/CdZnS CQwells,225 and CdSe CQwells,28 highlighting both the lack of heavy-metal-free and the high threshold and scarce reports of NIR NC gain media.
Low-threshold, high gain ‘green’ NC lasers will ultimately depend on solving the related problems of minimizing Auger recombination, circumventing issues associated with multiple exciton generation and achieving high quantum efficiency. As discussed above, three approaches to solving these problems include enabling single-exciton lasing by engineering Type II heterostructures, smoothing confinement potentials to reduce Auger rates and most recently eliminating Auger recombination with CQwells. Auger rates in InP QDs were found to decrease as a function of CdS shell thickness (determined from increasing multiexciton lifetimes), with a simultaneous suppression of blinking.229 Furthermore, the rising QE of InP/ZnS QDs (now as high as 90% for red emission118) also places this ‘green’ material among the best NCs for this metric. Progress toward shape-controlled green materials (with a viable route offered by cation exchange) may also pave the way to low-threshold CQwell III–V NC lasers capable of, at minimum, sustaining population inversion under CW pumping.
So far, applications in LEDs, lasers and photovoltaics have been feasible with novel, heavy-metal-free, low-toxicity materials. However, the quantum nature of the colloidal NC emission also makes them attractive candidates in fields including quantum cryptography, communication and computation.41 Efficient single photon and entangled photon pair emission represent core achievements to make these new technologies a reality. Producing non-classical light from colloidal NCs hinges on a thorough understanding of their band-edge fine structure, complete suppression of blinking, and elimination of Auger recombination. In this regard, CdSe NCs are again particularly well-developed.
Fig. 12 Asymmetry-induced splitting of the 8-state band-edge fine structure of zinc blende CdSe. Reproduced with permission from ref. 237.
Fig. 13 A strategy to eliminate blinking and strongly enhance stability is realized by encapsulating core–shell NCs in gold-coated silica shells. Adapted with permission from ref. 249.
Similarly, fluorescence intermittency has been investigated in sustainable NCs, with early work conducted more than a decade ago for InP QDs.255 With respect to blinking suppression, many of the same techniques used for Cd-based NCs have been shown to be effective with non-toxic NCs. For example, overcoating InP QDs with ZnS shells was also found to strongly suppress blinking.256 In conjunction with tuning of other factors such as the particle size and the molar ratio of myristic acid/indium (MA : In), an on-time fraction up to 80% was achieved. A somewhat alternative strategy has been demonstrated with InP/CdS heterostructures.229 Here, the type II band alignment was posited for the suppression of blinking even for thin shells. However, avoiding the CdS shell necessitates the development of alternative type-II heterostructures. Investigations on PL intermittency in NIR InAs QDs with CdZnS shells have revealed blinking behaviour similar to that seen in CdSe,257 indicating that strategies to eliminate blinking are transferrable. Finally, the in principle general approach shown in Fig. 13 to eliminate blinking249 is independent of the core material, and thus provides a direction that can be applied to non-toxic NCs.
There are interesting prospects for sustainable, solution processed NCs in future quantum technologies, bolstered by the knowledge gained on CdSe NCs. With maturing syntheses of green materials, the strategies already developed for CdSe NCs should accelerate their role in the future of quantum computing and communication. A key remaining question, however, is whether colloidal NCs can reach the material quality of their epitaxial analogues, which are significantly further developed as quantum emitters. For instance, electrically driven single photon emission has been demonstrated for InP grown on Si,259 and fine-structure splitting below 2 μeV for InAsP dots in InP nanowires for efficient polarized photon entanglement.260,261 To achieve such levels, clearly more work remains to be done.
Since their inception just over 30 years ago, the extraordinary progress in optimizing the synthesis, assembly and application of colloidal nanocrystals has uniquely positioned them to impact a wide array of photonic areas. Ultimate success in these applications, however, hinges as much on the sustainability of the base materials as it does on their performance. For NCs, moving beyond the well-developed Cd- and Pb-based materials therefore forms a crucial requirement.
In this review, we have highlighted some significant steps that have been taken towards sustainable NCs, including demonstrations of their use in photonic applications. The pursuit of these green materials has in many cases run in parallel to the seminal work with heavy-metal-containing NCs. For some sustainable material classes, such as III–V and IV group semiconductors, progress has lagged because of the difficulty in achieving the same material quality as the II–VIs, hindered by more demanding syntheses.21,52,95,96,262 Other heavy-metal-free options, such as ternary chalcogenides, and even Zn-based II–VIs, are by comparison in early stages of development, but are receiving considerable attention in response to the need for non-toxic NCs.
With regard to III–V NCs, notable progress in direct synthesis has been accompanied with the use of cation exchange, a method that has been generalized to other material classes. First, the synthesis of high-PL InP QDs62 with a spectral coverage that rivals CdSe, and converging performance indicates that these materials are accessible via traditional means. Cation exchange,21,23,58,83 on the other hand, is an emerging route used to obtain III–V NCs,21,22 Zn-based II–VI89 and even ternary chalcogenides.58 A prominent advantage of this approach is that it preserves the narrow size distributions as well as the shape of initial NCs, providing the possibility to obtain shape-controlled NCs that are difficult to grow via direct routes. Although the presence of toxic metals during synthesis might still form an issue, they do not remain in discernible quantities in the end-applications, and are contained in a controlled environment at the fabrication stage with appropriate waste disposal, satisfying the requirements for sustainable development of colloidal NCs.
Beyond the more traditional group IV, IV–VI, III–V, II–VI and I–III–VI NCs, there is also considerable excitement for other classes of solution processed materials. Some notable examples are represented by upconverting rare-earth doped NCs,263–267 lead halide perovskite NCs268–271 and 2D NCs from transition metal chalcogenides (TMCs).272–276 Recent work on rare-earth doped NCs has presented bright, tunable, blink-free, photostable upconverted emission from, for example NaYF4 (ref. 277), opening up possibilities for use in various photonic areas, as already demonstrated in biolabeling278 and single molecule sensors.279 Furthermore, amplified spontaneous emission (ASE) and lasing have been accomplished for red, green and blue wavelengths with upconverting core–shell NaYF4:Yb/Er@NaYF4 NCs.280 Due to the demanding conditions required to achieve population inversion for ASE and lasing, this work clearly demonstrates the substantial potential of this material class in advanced photonic applications. Considering the absence of toxic elements, these NCs should receive increasing attention over the coming years.
In another area, the extraordinary properties of graphene have led to a resurgence of interest in other layered materials. TMCs such as Mo(S, Se)2 and W(S, Se)2, for example, are not new, but remarkable optoelectronic possibilities have emerged for individual 2D layers of these materials.272–276 For example, indirect TMC bulk band gaps become direct for monolayers,284 and coupled with a very large exciton binding energy in 2D MoS2 due to reduced dielectric screening,285 their prospect as light emitters is bright. Initial synthetic schemes for TMCs have mirrored those used for graphene, utilizing both top-down exfoliation276 and bottom-up chemical vapour deposition286 yielding atomically thin sheets. Progress in colloidal synthesis has been made with the synthesis of ZrS2287 and other TMC nanosheets.288 Nevertheless, uniform nanosheets with an extended lateral size at constant thickness remains a prominent ongoing challenge.287–289 From a sustainability perspective, since TMCs are free of toxic heavy metals and thus provide an attractive route toward green photonics with 2D materials, we expect substantially increased synthesis efforts in this direction.
The research leading to these results has received funding from the European Union 7th Framework Programme under grant agreement no. 604391 GRAPHENE Flagship, and no. 614897 (ERC consolidator grant TRANS-NANO, L.M.). The present publication is further realized with the support of the Ministero degli Affari Esteri e della Cooperazione Internazionale (grant IONX-NC4SOL, I.M.).
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