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Infrared Quantum Dots: Progress, Challenges, and Opportunities Haipeng Lu, Gerard M. Carroll, Nathan R. Neale, and Matthew C. Beard*
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Chemistry & Nanoscience Center, National Renewable Energy Laboratory, Golden, Colorado 80401, United States ABSTRACT: Infrared technologies provide tremendous value to our modern-day society. The need for easy-to-fabricate, solution-processable, tunable infrared active optoelectronic materials has driven the development of infrared colloidal quantum dots, whose band gaps can readily be tuned by dimensional constraints due to the quantum confinement effect. In this Perspective, we summarize recent progress in the development of infrared quantum dots both as infrared light emitters (e.g., in light-emitting diodes, biological imaging, etc.) as well as infrared absorbers (e.g., in photovoltaics, solar fuels, photon up-conversion, etc.), focusing on how fundamental breakthroughs in synthesis, surface chemistry, and characterization techniques are facilitating the implementation of these nanostructures into exploratory device architectures as well as in emerging applications. We discuss the ongoing challenges and opportunities associated with infrared colloidal quantum dots.
T
III−V (InAs, InSb), II−VI (HgCdTe, HgSe, HgTe), I−VI (Ag2S, Ag2Se), and ternary I−III−VI (CuInS2, CuInSe2, AgBiS2, AgInSe2) semiconductors, their alloys, core/shell, and Janus heterostructures (Table 1). Newly emerging infrared QDs also include metal−halide perovskite nanocrystals such as CsSnI3,13 CsSnxPb1−xI3,14 FAPbI3,15 and CsxFA1−xPbI316 QDs. The wide tunability of these infrared QDs to different spectral regions, which is extremely rare in most material systems, has been demonstrated for energies spanning the NIR, SWIR, and MWIR regions (Figure 1). The emission wavelengths of InAs QDs, for example, can readily cover most of the SWIR region.17 The SWIR spectral region is particularly attractive as modern optical telecommunication operates almost exclusively in the C-band (1530−1565 nm) of the SWIR due to low Rayleigh scattering of optical fibers and the need for high-power transmission. In addition to the considerable progress in the synthesis and the modification of QD surfaces, both the development of infrared characterization techniques as well as the demonstration of infrared QDs in exploratory devices have seen significant advances. For instance, high performing infrared QDLEDs based on PbS QDs recently achieved an external quantum efficiency of ∼7.9% at ∼1400 nm.18 The development of single nanocrystal spectroscopy in the infrared has led to the measurement of the fundamental emission spectrum line width from a single infrared QD, which was found to be as narrow as 52 meV.17 As an effective light absorber, infrared QDs are being explored as a promising strategy for overcoming the Shockley−Queisser (S−Q) limit in single-junction solar cells due to efficient multiple-exciton generation (MEG).1 Infrared QD absorber layers could also be incorporated as a low-cost,
he quantum confinement size effect has driven the development and exploration of colloidal semiconductor nanocrystals, or quantum dots (QDs), as emerging candidate materials for solution-processable optoelectronic applications, including light-emitting devices, photovoltaic cells, and photodetectors.1−7 Different from conventional organic molecules where the photophysical properties are altered by building and/or breaking chemical bonds, the optical and electrical properties of colloidal QDs can readily be tuned by controlling their physical dimensions (size, shape). Although the development of UV−visible QDs has passed its infancy, leading to a few commercial products, the advancement of infrared QDs remains relatively immature, due to challenges associated with both synthesis and the methods for infrared characterization. However, colloidal infrared QDs offer exceptional promise for infrared active optoelectronic devices, for example, in photovoltaics, infrared light-emitting diodes (LEDs), telecommunications, in vivo imaging, and many other applications.1,8−10 The past decade has brought tremendous progress in the development of infrared QDs due to the strong demand for processable, tunable infrared active material systems. Infrared QDs are colloidal semiconductor nanocrystals that absorb or emit beyond the UV−vis region of the electromagnetic spectrum, including near-infrared (NIR, 0.70−1.4 μm), short-wavelength infrared (SWIR, 1.4−3 μm), midwavelength infrared (MWIR, 3−8 μm), and long-wavelength infrared (LWIR, 8−15 μm) (CIE classification11). Although localized surface plasmon resonances in semiconductor nanocrystals also exhibit size-dependent and tunable resonant absorption in the infrared, they are excluded here as they have been reviewed recently elsewhere.12 Infrared QDs are generally found in groups IV (Si, Ge, GeSn), IV−VI (PbS, PbSe, PbTe), © XXXX American Chemical Society
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Table 1. Summary of Representative Infrared Quantum Dot (QD) Materials QDs
exciton peak (nm)
emission peak (nm)
fwhm (nm)
size (nm)
synthetic method
PLQY (%)
ref
Si Ge PbS PbSe PbTe PbTe CdTe/CdS InAs/CdSe/CdS InSb Cd3P2 Ag2S Ag2Se HgSe/CdS HgTe CuInS2/ZnS CuInSe2/ZnS AgInSe2 AgBiS2 CsSnI3 CsSnxPb1−xI3 FAPbI3 CsxFA1−xPbI3
N/A N/A 850−1800 1100−4000 1009−2054 1009−2054 N/A 780−1300 1200−1750 N/A N/A N/A 1200−1500 1100−2500 N/A N/A N/A 1000 (onset) N/A N/A N/A N/A
690−995 980−1140 1000−1700 1200−4100 1100−2150 1100−2150 480−820 970−1500 1300−1850 650−1200 ∼1200 1080−1330 1300−1650 1200−3700 550−815 700−1000 800−1300 N/A 950 700−850 770−780 650−800
∼150 250−425 ∼200 50−100 ∼150 ∼150 200 100−200 100−200 ∼100 110−150 >200 N/A ∼80 ∼100 45 ∼50
3.0−7.4 2−13 2.5−7.2 3−17 2.6−8.3 2.6−8.3 6−11 2−7 3.3−6.5 3.5−4.5 5.4−10 3.1−3.9 3−5.7 3−12 2−4 2−5 9−23 4.6 ∼10 11−14 10−15 ∼10
nonthermal plasma hot injection hot injection hot injection hot injection hot injection aqueous synthesis continuous injection heating up hot injection aqueous synthesis hot injection hot injection aqueous synthesis heating up heating up heating up hot injection hot injection hot injection hot injection hot injection
2−60 70 60−70
21,22 23 24 25,26 27 27 28 29 30 31 32 33 34 35 36 37 38 39 13 14 15 16
exciting applications of infrared QDs are discussed with a focus on manipulating photon conversion to excitons, to other forms of photons, and to molecular bonds upon photoexcitation. We conclude with an outlook on future directions in infrared QDs.
SYNTHESIS OF COLLOIDAL INFRARED QDS Colloidal Synthesis. The most common synthetic approach to achieve colloidal infrared QDs is colloidal solution-phase synthesis, which attempts to control the divergent stages of nucleation from the subsequent growth of the nascent particles (LaMer mechanism40) in a solution containing both metal and anion precursors. Often mimicked from cadmium chalcogenide QDs, infrared QDs are typically prepared with similar precursor chemistries (metal-oleate complex, oleyamine, 1-octadecene, etc.), yet achieving the same degree of synthetic control remains a challenge.41 Therefore, advances in the colloidal synthesis of infrared QDs are strongly tied to the development of the requisite precursor chemistries. Here, we illustrate recent progress in infrared QD colloidal synthesis by focusing on the precursor chemistry for the relatively well-developed lead chalcogenide QDs and InAs QDs that provide extraordinary control over crystal size and shape.
Figure 1. Emission wavelength range of representative infrared QDs discussed here.
solution-processable, low-band-gap layer within a multijunction solar cell. Solar cells based on a single junction of infrared PbS QDs have achieved a record power conversion efficiency of 12%.19 In addition, efficient MEG in photoanodes fabricated from PbS QDs have demonstrated the production of solar fuels (H2) with over 100% external quantum yield in the blue region of the solar spectrum.20 Other emerging applications using infrared QDs are being explored. For instance, when coupled with a functional organic chromophore, infrared QDs can be employed for efficient photon up-conversion because of effective triplet−triplet energy transfer initiated at the organic−inorganic interface.17 We begin by describing recent progress on the synthetic strategies for finely controlling the size and composition of infrared QDs and for producing heterostructures. We then discuss the advances in infrared emitting applications, focusing on the development of spectroscopic characterization techniques including photoluminescence quantum yield, single nanocrystal spectroscopy, and photocarrier lifetimes. Other
Advances in the colloidal synthesis of infrared quantum dots are strongly tied to the development of the requisite precursor chemistries. Traditional colloidal infrared metal chalcogenide QDs can be prepared using a solution-based organometallic route. The typical metal precursor is a soluble metal oxide source, the sulfur source is bis(trimethylsilyl)sulfide (TMS),43 and the selenium/ tellurium source is elemental selenium/tellurium25 in 1octadecene. The size of the product QDs is controlled by varying reaction conditions including the precursor and ligand concentrations, the injection and growth temperatures, and B
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pnictogen precursors are P(SiMe3)3 and As(SiMe3)3 because of their high reactivities.53 However, these highly reactive precursors also result in poor size distributions due to the failure to separate the nucleation and growth stages. Bawendi and coworkers use a less reactive precursor, As(GeMe3)3, to achieve a superior size distribution of InAs QDs, yet the size tunability remains only moderately successful.54 Unexpectedly, the community has recently realized that changes in precursor reactivity play a rather minor role in the growth of III−V QD as compared to IV−VI QDs, and as a result, it is difficult to grow larger III−V QDs.41,55,56 Bawendi and co-workers proposed keeping the arsine precursor concentration low by controlling the precursor supply via a syringe pump during the synthesis.29 Using this approach, they successfully tuned the precursor conversion kinetics independently during the nucleation and growth stages (Figure 3e,f). Their modified synthesis strategy results in the continuous growth of InAs QDs with narrow emission (380 °C) gives ternary alloy InxGa1−xP and InxGa1−xAs QDs.58 The precursor chemistry and reaction kinetics of growing infrared III−V QDs are dramatically different from II−VI or IV−VI QDs and require further understanding. Rational control of the size, size distribution, and crystal phase of infrared QDs by designing molecular precursors is an important future direction that will require better understanding of precursor reaction kinetics across different reaction matrices. Gas-Phase Synthesis. Gas-phase synthesis of colloidal semiconductor nanocrystals is an equally viable approach to synthesizing infrared QDs and is one of the earliest methods for obtaining dispersible nanocrystals.59 Unlike wet-chemical methods, gas-phase synthetic methods are not limited by the boiling point of organic solvents and thus are capable of highenergy process environments that increase the scope of possible material formulations. Gas-phase synthetic techniques exist in many forms,60 and, in general, vapor-phase precursors are fragmented via high-energy decomposition (thermal, laser, plasma, etc.), which generates reactive intermediates. These small atomic and molecular intermediates nucleate into clusters and small nanoparticles, which then undergo agglomeration and subsequent growth (via additional agglomeration or surface growth depending on the method) to form QDs. The energetic distribution of intermediates depends on the generation mechanism. Techniques in which heat or light sources generate the intermediates, such as flame or laser pyrolysis, create welldefined, narrow energetic distributions.61 In this case, the synthesis is described to be near thermal equilibrium. Thermal nonequilibrium processes are just the opposite: the kinetic energy of reactants is widely distributed. Within the sphere of thermal nonequilibrium syntheses, Infrared QDs in both confined62 and nonconfined63 regimes have been synthesized through nonthermal plasma methods (Figure 2c). Here, we highlight a few recent advances in group IV and III−V NIR materials synthesized through nonthermal plasma methods. Silicon QDs are most frequently produced using the nonthermal plasma method. Typically, such plasmas are formed at low pressure (0.1 to 10 Torr) where an easily ionizable carrier gas (He, Ar, etc.) flows through a chamber inside of an electric field (capacitively or inductively coupled), which generates an
growth time. Although such a synthetic strategy readily produces colloidal infrared metal chalcogenide QDs with variable sizes (Figure 2a), obtaining well-controlled size distributions remains
Figure 2. (a) Absorption spectra of the typical infrared quantum dot (QD), PbSe, with diameters ranging from 3.3 to 8.1 nm, showing strong quantum-confined first exciton shifted absorption. Reprinted with permission from ref 42. Copyright 2012 Elsevier. Typical synthetic apparatus used in the preparation of infrared QDs by (b) colloidal synthesis and (c) nonthermal plasma synthesis. (c) Reprinted from ref 77. Copyright 2018 American Chemical Society.
a significant challenge. As such, emphasis has been placed on understanding the mechanistic aspects of nucleation and growth and using this knowledge to develop new methods based on a rational design of tunable molecular precursors. Strategically, controlled synthesis of chalcogen-based QDs can be achieved by designing both metal and chalcogen molecular precursors. However, there has been rather limited success in preparing tunable metal precursors,44−46 and most reports focus on the design of tunable chalcogen molecules. The general design principle is that by altering the electronic and/or steric structure of the molecular precursors, nanocrystal formation kinetics can be modified, leading to the controlled manipulation of QD size and/or crystal phase. For instance, Owen and co-workers elegantly developed a family of substituted thioureas that can tune the conversion reactivity to PbS QDs by over 5 orders of magnitude, resulting in a desired QD size while simultaneously obtaining full reaction yield (Figure 3a−d).24 The reaction kinetics are tightly controlled by the thiourea electronic structure, and thereby, the size distribution of the resulting QDs can be finely tuned based on the starting thiourea precursors. Owen and co-workers further extended the chemistry to selenourea precursors, which enables the preparation of PbSe QDs with exceptional control of size and size distributions.47 Their demonstrated strategy has greatly advanced the control over the preparation of lead chalcogenide QD with varying size. Krauss and co-workers developed a series of secondary phosphine sulfide precursors to control the reactivity, achieving size control of PbS QDs with 100% conversion yield.48 Other tunable chalcogen precursors are also being explored for infrared QD synthesis that lead to crystalphase manipulation. For instance, by using substituted diorganyl dichalcogenides, Brutchey and co-workers are able to control the resulting crystal phase (chalcopyrite vs wurtzite) of CuInS2 and CuInSe2 QDs.49−52 In contrast to the rich chalcogen precursor chemistry for metal chalcogenide QDs, the anion precursor chemistry for infrared III−V QDs is much less developed. The development of III−V QDs lags behind IV−VI QDs largely due to the lack of suitable group V precursors. The most widely explored C
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Figure 3. (a,b) Classic LaMer mechanism of precursor-limited homogeneous nucleation and growth of colloidal quantum dots (QDs). (a) Monomer concentration ([ME]i) as a function of reaction time consists of three stages: monomer supersaturation (I), crystal nucleation (II), and crystal growth (III). (b) Metal and chalcogen precursors (MX2, ER2) react and supply monomers ([ME]i); this step is a rate-limiting step that controls the following nucleation and growth. (c) Reaction mechanism of PbS QDs using lead oleate and substituted thiourea compounds. (d) Effect of thiourea substitution pattern on the relative thiourea conversion rate constants. Panels a−d are reproduced with permission from ref 24. Copyright 2015 American Association for the Advancement of Science. (e) Time evolution of the precursor concentration during a combination of hot and continuous injection for a modified InAs QDs synthesis. Here, a single injection is followed by a slow, continuous supply of precursors, thus maintaining a prolonged size-focusing regime for InAs QDs. (f) Absorption spectra of InAs QDs using a continuous injection strategy. Panels e,f are reproduced with permission from ref 29. Copyright 2016 Nature Publishing Group.
the original report of gas-phase InP synthesis,78 achieved visibleemitting QDs. The phosphine (PH3) and trimethylindium(gallium) (In(CH3)3 and Ga(CH3)3) precursors used in these approaches begs the question of whether other readily available semiconductor precursor gases such as arsine (AsH3) could be used to generate lower band gap InAs QDs. In addition, the production of binary compound semiconductor Cu2S QDs via nonthermal plasma synthesis79 suggests that further development could yield ternary I−III−VI materials such as CuIn(S/ Se)2 and provide an important new avenue for making infrared tunable QDs. Although the number of reports on nonthermal plasma-synthesized QDs is small compared to those prepared via wet-chemical methods, the ability to access high-energy, kinetic regimes rather than thermodynamic reaction conditions provides unique possibilities in preparing covalent, alloyed, and doped infrared QDs. Stability and Core/Shell Structure. A major hindrance for infrared QDs compared to UV−vis QDs is their instability under ambient environments. Lead chalcogenide QDs, in particular, degrade quickly in air due to the oxidation of chalcogen atoms (Te is less stable than Se which is less stable than S). Extensive research efforts have been devoted to improving the air stability of lead chalcogenide QDs, including surface chlorine treatment,80 in situ passivation using halide precursors,81 postsynthetic metal halide treatments,82 and phosphonic acid passivation.83 Interestingly, we find that a direct cation exchange from CdSe84 or ZnSe85 to PbSe QDs significantly improves their air stability, enabling air-stable PbSe-based QD solar cells, for example. Such a strategy has also been demonstrated for Ag2Se QDs.86 Another important strategy to improve the air stability is by growing an inorganic epitaxial shell, forming core/shell heterostructures. The formation of type-I band alignment further reduces surface dangling bonds that serve as surface trap states, thus leading to improved light emission. The core/ shell strategy has been widely explored for the synthesis of bright, stable infrared QDs such as PbS/CdS,4 PbSe/CdSe,87
ion-rich environment. Silane (SiH4) precursor gas is passed though the plasma where it is decomposed into radical fragments that enable QD nucleation and growth. Quantum dot charging effects from the ionizing plasma atmosphere inhibit QD agglomeration, which typically yields single-crystalline QDs. Depending on the specific parameters (pressure, gas ratios, plasma power, gas residence time in plasma) the QD size, size distribution, morphology, and surface termination can be tuned.61−64 For a more detailed description of nonthermal plasma synthesis of QDs, the interested reader is pointed to ref 61. The product QD powder is uniformly terminated by reactive molecular fragments (typically hydrogen) that can readily be transformed with known chemistries21 to produce well-defined, dispersible, luminescent colloidal Si QDs with a wide array of possible ligand binding groups.65 Germanium QDs,61,63,66−69 as well as silicon/germanium alloys are also readily accessible.70,71 Recently, efforts by Kortshagen and co-workers to modify the QD surface with a secondary precursor gas injection after initial QD nucleation and growth have successfully produced Ge/Si core/shell structures and demonstrated how lattice mismatch strain between silicon and germanium can be used to tune the optical properties of the Ge QD core in the NIR region.72 Doping Si with phosphorus (n-type) or boron (p-type) also has been demonstrated, exhibiting plasmonic features in the mid-IR range that scale in intensity with the dopant incorporation.73,74 Doping is an exciting development to access the mid-IR range as band edge optical activity limits Si and Ge QDs to the NIR. Interestingly, when simultaneously doped with both n-type and p-type carriers, these so-called codoped n,p-Si QDs display sub-band-gap photoluminescence energy, indicative of donor−acceptor pair recombination.75,76 This phenomenon has only been demonstrated in ion-beam implantation samples and may be a desirable path for nonthermal plasma synthesis to access lower energy IR wavelengths in Si-based QDs. Binary and alloy III−V InP, GaP, and InxGa1−xP were recently prepared by us through nonthermal methods77 and, like D
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Figure 4. (a) X-type ligand exchange in PbS quantum dots (QDs) in which surface-bound oleate is displaced by cinnamic acid molecules. Reprinted with permission from ref 95. Copyright 2016 Royal Society of Chemistry Journals. (b) Absorbance spectra of PbS QDs after ligand exchange with cinnamic acid molecules, showing enhanced optical absorbance. (c) Enhanced absorbance parameter plotted against ligand optical gap. Panels b,c reprinted from ref 96. Copyright 2018 American Chemical Society. (d) Chemical structures and computed vacuum electronic dipoles of different functionalized cinnamate ligands: 4-CN-CA− = 4-cyanocinnamate; 4-CF3−CA− = 4-trifluoromethylcinnamate; 3,5-F-CA− = 3,5-difluorocinnamate; 4-H-CA− = cinnamate; 2,6-F-CA− = 2,6-difluorocinnamate; 4-OCH3−CA− = 4-methoxycinnamate; 4N(CH3)2-CA− = 4-dimethylaminocinnamate; OA− = oleate. (e) Band edge energies of PbS QD films after treatments with different fluorinated cinnamic acid ligands. Panels d,e reprinted with permission from ref 97. Copyright 2017 Nature Publishing Group.
InAs/CdSe,88 InAs/CdSe/ZnSe,89 etc. We believe that the stability issue of these infrared QDs can be successfully addressed.
employed to determine the equilibrium constants and free energy of the exchange reaction at different extents of exchange (Figure 4a).95 In situ characterization of solution-phase ligand exchange reactions provide important insights that enable precise and rationally control of the ligand shell in infrared QDs. Thus far, such in situ characterization tools of surface ligand exchange reactions remain mostly for X-type ligand exchanges.
SURFACE CHEMISTRY OF COLLOIDAL INFRARED QUANTUM DOTS The surface chemistry of QDs plays a paramount role in their electronic and photophysical properties and, hence, their practical utility. Understanding QD surface chemistry has advanced considerably where models used to describe metal− ligand complexes (covalent bond classification method) are now applied to QDs. Surface ligands are generally categorized as Xtype, L-type, and Z-type ligands depending on the number of donating electrons.90 In addition to designing novel inorganic and organic surface ligands, recent advances in the surface chemistry of infrared QDs has focused in two main directions: (1) quantitative determination and (2) practical demonstration. (1) There have been significant efforts to understand the ligand exchange mechanisms by quantitatively probing ligand exchange isotherms and extracting equilibrium constants and binding energies via spectroscopic techniques. (2) Researchers continue to explore how ligand−QD interactions impact the electronic and photophysical properties of infrared QDs. Conventionally, QD surface ligands are modified to meet certain functionality via a ligand exchange reaction, leaving the general thermodynamic parameters unknown. This knowledge deficit stems from the lack of general in situ characterization techniques that are able to probe the exchange reaction. Recently, broad implementation of NMR techniques in colloidal nanocrystals has largely enabled the determination of fundamental thermodynamic constants of ligand exchange reactions.91−94 For instance, quantitative NMR spectroscopy, together with spectrophotometric absorption titrations, can be
In situ characterization of solutionphase ligand exchange reactions provide important insights that enable precise and rationally control of the ligand shell in infrared quantum dots. In addition, the research community is exploring the impact of surface ligands on the QDs electronic and photophysical properties. Early research focused primarily on improving charge transport properties within infrared QD arrays for optoelectronic applications. Extensive reports have demonstrated that the electronic coupling within QD arrays can be improved by exchanging the native ligands for short organic/ inorganic ligands, giving improved photovoltaic performance.98−100 Recently, unexpected insights are finding a larger role of surface chemistry on the core QD/ligand properties. For instance, Gigli and co-wokers showed that the electronic states of the ligand and core QDs can be mixed when using arenethiolate ligands, resulting in an enhanced optical absorption of the QD/ligand complex.101 Similarly, we demonstrated that the nature of the silicon-ligand bond at Si QD surfaces tunes the resulting optical properties.65 Bawendi, Bulovic, and co-workers demonstrated that the frontier energy E
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Figure 5. (a) Illustrative description of photoluminescence line width in quantum dot (QD) emitters. An ensemble emission spectrum contains the intrinsic single-emitter spectrum coupled with the interparticle inhomogeneities. The single-emitter spectrum is a combination of spectral dynamics, exciton−phonon coupling, and emission from exciton fine-structure states. Single nanocrystal spectroscopy is the key to unveil the intrinsic photoluminescence property of QDs. Reprinted from ref 109. Copyright 2016 American Chemical Society. (b) Short-wavelength infrared (SWIR) s-PCFS setup for InAs/CdSe QDs. (c) Gaussian fits of the single InAs/CdSe QD (green) and ensemble (black) spectral correlations. The average single InAs/CdSe nanocrystal fluorescence line width is shown as narrow as 52 meV. (d) Full width at half-maximum of InAs/CdSe QD series as a function of CdSe shell thickness. Panels b−d reprinted from ref 17. Copyright 2018 American Chemical Society.
to 16% at 1.5 μm.29,104 Recently, Bawendi and co-workers found a PLQY of 82% in InAs/CdSe/CdS QDs at 970 nm.29 Although researchers often compare the QD PLQY with other infrared emitters, historically, there is a general lack of robust methodology to determine the absolute PLQY for infrared QDs accurately. Another key parameter for the infrared technologies is the fluorescence line width, which is governed by size inhomogeneity as well as the intrinsic “nature” of the QD. Although the size inhomogeneity of infrared QDs has been significantly improved due to recent synthetic advances for some materials (IV−VI primarily), the development of infrared singleparticle spectroscopy provides a powerful tool to investigate the intrinsic fluorescence line width and blinking mechanisms of infrared QDs.105 Infrared Photoluminescence Quantum Yield Determination. Direct comparison of different infrared QD materials relies on the precise determination of the absolute PLQY. The easiest way to determine PLQY involves comparing the emission intensity of a known standard with the emission intensity of the sample of interest, where ideally both samples possess similar absorbances at the excitation wavelength and exhibit similar emission spectra. However, unlike the rich library of visible standards, only a few efficient and reliable emitters are available in the NIR region, resulting in large errors in determining infrared PLQYs. Therefore, a direct and reliable measurement of infrared PLQY needs to be developed. By using an integrating sphere, the PLQY of a large variety of sizes of PbS and PbSe QDs could be measured.106 A direct measurement is more robust and
levels of PbS QDs can be shifted by up to 0.9 eV using different chemical ligand exchange treatments.102 They employed this capability to engineer the band alignment within QD solids and used that strategy to develop a PbS QD-based solar cell with a record power conversion efficiency in 2014.103 However, the tunability of the QD band edges could only be achieved in solidstate ligand exchanges, limiting its utility toward a larger application space. Recently, we developed a solution-phase ligand exchange procedure using functionalized cinnamate ligands (small dipolar ligands) that can tune the band edge positions of PbS QDs over 2.0 eV (Figure 4e) while retaining their solution colloidal stability.97 Furthermore, we find that these cinnamate ligands also lead to optical absorption enhancement, which scales linearly with the optical gap of the ligand (Figure 4b,c).96 Our results indicate that the ligand/QD coupling occurs equally efficiently between the QD and ligand HOMO and their respective LUMO levels.
RECENT ADVANCES IN INFRARED EMITTING APPLICATIONS General Infrared Emitting Properties. The tunable infrared emission wavelength makes QDs attractive for telecommunication and biological imaging. The most important characteristic of infrared emitting QDs is their photoluminescence quantum yield (PLQY). Among common infrared QD emitters, lead chalcogenides (PbX; X = S, Se, Te) and InAs have the highest reported PLQYs with PbS ranging from 60% at 1 μm to 30% at 1.5 μm and InAs ranging from as high as 37% at 1 μm F
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become increasingly sophisticated. As first predicted by Nozik and co-workers,110 the relaxation or cooling rate of hot carriers can be dramatically slowed in QDs as compared to bulk semiconductors due to the so-called phonon bottleneck effect. They anticipated that when the quantized levels in QDs are separated in energy by more than the fundamental phonon energy, the cooling of hot carriers would require multiphonon processes and, thus, should be significantly slowed relative to relaxation in the bulk where the energy levels in the conduction and valence bands are continuous. However, the cooling dynamics are rather controversial with some experiments supporting a phonon bottleneck while other measurements contradict it. Indeed, there are a number of scattering processes, including Auger recombination, electron−hole scattering, multiphonon emission, interaction with the vibrational levels of the ligands, and deep-level trapping, that can circumvent the phonon bottleneck in QDs, leading to the different relaxation times that vary from one to several tens of picoseconds.1 Along these lines, the rates of Auger processes and the inverse Auger process, MEG, are significantly enhanced due to carrier confinement and the concomitantly increased e−−h+ Coulomb interaction. The ultrafast MEG rate (
