Article

Strain-Induced Exciton Transition Energy Shift in CdSe Nanoplatelets: The Impact of the Organic Ligand Shell

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Abstract

We study the influence of surface passivating ligands on optical and structural properties of zinc blende CdSe nanoplatelets. Ligand exchange of native oleic acid with aliphatic thiol or phosphonic acid on the surface of nanoplatelets results in the large shift of the exciton transition for up to 240 meV. Ligand exchange also leads to structural changes (strain) in the nanoplatelet’s core analysed by wide-angle X-ray diffraction. By correlating experimental data with theoretical calculations we demonstrate that the exciton energy shift is mainly caused by a ligand-induced anisotropic transformation of the crystalline structure altering the well width of the CdSe core. Further the exciton reduced mass in these CdSe quantum wells is determined by a new method and agrees well with expected values substantiating that ligand-strain induced changes in the colloidal quantum well thickness are responsible for the observed spectral shifts. Our findings are important for theoretical modeling of other anisotropically strained systems and demonstrate an approach to tune optical properties of 2D semiconductor nanocrystals over a broad region thus widening the range of possible applications of AIIBVI nanoplatelets in optics and optoelectronics.

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... 38 Several reports have also shown that surface ligands induce a tetragonal deformation of the NPL with respect to the ideal zinc blende structure, with a stress-induced deformation of the short axis whose magnitude depends on the ligands. 35,[39][40][41] We have previously shown that CdSe NPL can assemble into a variety of structures depending on the assembly conditions such as giant micro-needles 42 or (twisted) nano-ribbons. 43,44 In these assemblies, NPL are stacked one on top of each other in a face to face fashion. ...
... For example, a simple deformation of the NPL along the direction perpendicular to the basal [001] plane is not enough to make a polar axis unique. Thus, the tetragonal deformation of the NPL, with a compression of the short axis and a biaxial tension within the perpendicular plane, that has been evidenced recently by several groups, 35,[39][40][41] cannot account for the emergence of a permanent dipole. Indeed, the mid-plane of the NPL is still a mirror plane and the four 〈111〉 symmetry axes that cancel each other are still present in this deformed structure. ...
... Ligand exchange from the native oleic acid to phosphonic acid or thiols has been shown to distort the crystal lattice significantly with relative variations of lattice parameters which could reach 4%. 39 These important strains are consistent with previous studies which have shown that CdSe NPL could adopt various curved conformations depending on the surface ligand and their crystallographic structure. 44,79,80 Due to the very thin nature of the NPL, even the small stress exerted by the ligands at their surface can result in large deformations. ...
Article
Zinc-blende CdSe semiconducting nanoplatelets (NPL) show outstanding quantum confinement properties thanks to their small, atomically-controlled, thickness. For example, they display extremely sharp absorption peaks and ultra-fast recombination rates that make them very interesting objects for optoelectronic applications. However, the presence of a ground-state electric dipole for these nanoparticles has not yet been investigated. We therefore used transient electric birefringence (TEB) to probe the electric dipole of 5-monolayer thick zinc-blende CdSe NPL with a parallelepipedic shape. We studied a dilute dispersion of isolated NPL coated with branched ligands and we measured, as a function of time, the birefringence induced by DC and AC field pulses. The electro-optic behavior proves the presence of a large dipolar moment (> 245 D) oriented along the length of the platelets. We then induced the slow face-to-face stacking of the NPL by adding oleic acid. In these stacks, the in-plane dipole components of consecutive NPL cancel whereas their normal components add. Moreover, interestingly, the excess polarizability tensor of the NPL stacks gives rise to an electro-optic contribution opposite to that of the electric dipole. By monitoring the TEB signal of the slowly-growing stacks over up to a year, we extracted the evolution of their average length with time and we showed that their electro-optic response can be explained by the presence of a 80 D dipolar component parallel to their normal. In spite of the $\bar{4}$3m space group of bulk zinc-blende CdSe, these NPL thus bear an important ground-state dipole whose magnitude per unit volume is twice that found for wurtzite CdSe nanorods. We discuss the possible origin of this electric dipole, its consequences for the optical properties of these nanoparticles, and how it could explain their strong stacking propensity that severely hampers their colloidal stability.
... This may indicate that DPA as a ligand incorporated into the NPLs structures forming an additional half-monolayer. Absorbance peak at 400 nm corresponding to 3.1 eV is characteristic for 5.5 monolayers of CdS NPLs [13]. ...
... This may indicate that DPA as a ligand incorporated into the NPLs structures forming an additional half-monolayer. Absorbance peak at 400 nm corresponding to 3.1 eV is characteristic for 5.5 monolayers of CdS NPLs [13]. Additional half-monolayer caused the NPLs to be more rigid. ...
... at 400 nm corresponding to 3.1 eV is characteristic for 5.5 monolayers of CdS NPLs [13]. Additional half-monolayer caused the NPLs to be more rigid. ...
Article
Full-text available
In this paper, the study of surface modification of two-dimensional (2D), non-luminescent CdS nanoplates (NPLs) by thiol-containing ligands is presented. We show that a process of twophase transfers with appropriate ligand exchange transforms non-luminescent NPLs into spherical CdS nanoparticles (NPs) exhibiting a blue photoluminescence with exceptionally high quantum yield ~90%. In the process, transfer from inorganic solvent to water is performed, with appropriately selected ligand molecules and pH values (forward phase transfer), which produces NPs with modified size and shape. Then, in reverse phase transfer, NPs are transferred back to toluene due to surface modification by combined Cd (OL)2 and Cd (Ac)2. As a result, spherical NPs are formed (average diameter between 4 and 6 nm) with PL QY as high as 90%. This is unique for core only CdS NPs without inorganic shell.
... En utilisant la loi de Bragg sur le pic à (111) : où d est la distance entre deux plans cristallographiques, θ de demi-angle de déviation, n l'ordre de diffraction et λ la longueur d'onde des rayons X, il est constaté que le paramètre de maille des nanoplaquettes est de 0,613 nm alors qu'il est de 0,608 nm pour le matériau massif de CdSe. Ce paramètre expérimental, plus élevé, est dû à la pression qu'exercent les ligands à la surface des nanoplaquettes, subissant ainsi une distorsion tétragonal [41]. Le pic (220) fait apparaître deux contributions : un pic fin qui peut être attribué à de la diffraction suivant des plans perpendiculaires aux grandes dimensions latérales (plans rouges sur la Figure 6b) tandis que, celui plus large, est attribué à la diffraction de familles de plans contenant l'épaisseur (plans bleus sur la Figure 6b). ...
... Cela s'explique par la présence des ligands de surface qui induisent des contraintes dans le cristal. Les ligands oléates étendent le paramètre de maille dans les dimensions latérales [41]. Plus l'épaisseur des NPLs augmentent, moins les NPLs subissent les contraintes imposées par les ligands de surface. ...
... Dans un premier temps nous émettons l'hypothèse que les ligands acétates du précurseur de cadmium sont à l'origine de cette absence de croissance. En effet, les ligands de surface, plus particulièrement les carboxylates, contraignent la surface des NPLs [41]. En considérant également les contraintes imposées par la différence de paramètre de maille entre CdS et HgTe (10%), il n'est pas surprenant que la croissance du cadmium sur le plan de soufre ne se fasse pas. ...
Thesis
Dans un contexte de développement de photo-détecteurs et lasers dans le domaine de l’infrarouge, les nanocristaux colloïdaux de chalcogénures de mercure sont des candidats prometteurs pour l’imagerie infrarouge à bas coût. Des nanoplaquettes de chalcogénures de cadmium sont développées depuis 10 ans. Ces nanoparticules 2D présentent des propriétés optiques originales dans le visible, du fait de leur confinement quantique limité à une di-mension. L’objectif de cette thèse est de syn-thétiser des nanoplaquettes à base de chalco-génures de mercure afin d’obtenir des proprié-tés optiques fines dans l’infrarouge. Pour cela, nous avons utilisé l’échange cationique au mercure sur des nanoplaquettes de chalcogé-nures de cadmium, la synthèse directe n’étant pas encore démontrée.Dans une première partie, ce manuscrit expose la synthèse et la caractérisation de nanopla-quettes fines de 2 et 3 monocouches de HgTe et HgSe. Les nanoplaquettes de HgTe présen-tent des propriétés optiques exceptionnelles dans le proche infrarouge (largeur à mi-hauteur de 57 meV pour une émission aux alentours de 1,5 eV).Par la suite, l’échange cationique sur des na-noplaquettes plus épaisses de CdSe a été étu-dié. La limitation de la diffusion des atomes de mercure sur 2 plans cationiques dans l’épaisseur permet d’obtenir des hétérostruc-tures cœur/coque de CdSe/HgSe présentant des propriétés optiques et électroniques décor-rélées.Pour finir, cet échange cationique a été utilisé pour créer de nouvelles hétérostructures cœur/coque de HgTe/CdS et cœur/couronne HgSe/HgTe permettant de jouer sur la délocali-sation des fonctions d’onde des porteurs de charge.
... 7,8 At the nanoscale, surface chemistry is a tool of choice to control the NPLs shape. [9][10][11][12][13] For instance, NPLs unfold as their surface is covered with halides ligands co-stabilized by amines. 11 In that respect, in addition to the tuning of NPLs core composition or the synthesis of heterostructures, 14 surface chemistry appears as a promising tool for the design of original optical properties for those nanomaterials. ...
... 470 nm) for thiolates capped 3 ML CdSe NPLs. 3,10,22 The redshift of the optical features arises from two effects: (i) the partial delocalization of the exciton wavefunction over the sulfide planes and (ii) a change from a contractile strain to an extensional strain in the thickness direction which induces an increase in the lattice parameter and a decrease of the confinement. The transmission electron microscopy (TEM) pictures (Figure 1 c-e) unveil helices with radius R increasing from 6.5 nm to 13 nm as the aliphatic chain length increases from 2 to 18 carbons. ...
... [30][31][32] Here, thiolate groups bridge two surface cadmiums to provide a tetrahedral environment. 10,33 Sulfides induce compressive stress both in the [110] and [11 ̅ 0] directions on the two opposite wide facets of CdSe NPLs. Thus NPLs curl with their top or bottom facets inside the helices with equal probability (Figure 1a, Supp Info Figure S10-S11). ...
... En ce qui concerne l'échange de ligands sur film, un film de nanocristaux est formé puis plongé dans une solution contenant des ligands organiques courts ou anioniques en fort excès. [66]. Ce travail met en évidence l'influence des ligands et de leur nature sur les contraintes à la surface des nanoplaquettes. ...
... Antanovich et al. [66], se sont également intéressés à la nature de la surface sur les propriétés optiques de nanoplaquettes de CdSe. Les carboxylates initiaux ont été échangés par des thiols (hexadecanethiol) et des acides phosphoniques (acide hexadecylphosphonic). Ils ont pu observer un décalage de l'absorption jusqu'à 240 meV attribué dans ce cas à une modification des contraintes à la surface des nanoplaquettes, mesurée par diffraction des rayons X, et a une délocalisation partielle de la fonction d'onde sur les ligands dans le cas du thiol. ...
... Les ligands de surface peuvent induire des contraintes dans le matériau. Antanovich et al. [66] ont montré par diffraction des rayons X que les carboxylates induisent une diminution de l'épaisseur ainsi qu'une augmentation du paramètre de maille dans les autres directions. Par conséquent, les carboxylates induisent une extension des dimensions latérales de la nanoplaquette comme représenté sur la Figure 85 c. par les flèches. ...
Thesis
Les nanoplaquettes de chalcogénures de cadmium présentent des propriétés optiques uniques résultant d’une épaisseur contrôlée à l’échelle de la monocouche atomique. Elles apparaissent comme une nouvelle classe de nanomatériaux à fort potentiel applicatif. L’objectif de cette thèse est, par le design de la particule inorganique et de la chimie de surface, de faire émerger de nouvelles propriétés optiques.Dans un premier temps des hétérostructures cœur/couronne de CdSe/CdSe1-xTex ont été synthétisées et ont montré une bi-émission à l’échelle de la nanoplaquette unique. Cette bi-émission émerge d’une compétition entre la localisation de l’électron dans le cœur de CdSe induite par le décalage entre les bandes de conduction et d’une recombinaison directe dans la couronne favorisée par la grande énergie de liaison de l’exciton dans les nanoplaquettes.Des nanoplaquettes de CdSe dopées à l’argent ont été synthétisées dans un second temps. Le dopage, fait par échange cationique partiel, créé des pièges émissifs dans la largeur de bande interdite. La quantité d’argent incorporée permet un contrôle progressif de la couleur d’émission du vert au rouge grâce à la coexistence de la recombinaison de bord de bande et de l’émission liée à l’argent. Ces niveaux d’argent sont fortement liés à CdSe et se situent 340 meV au-dessus de la bande de valence du CdSe massif.Enfin, la modification de chimie de surface par un mélange de bromures et d’oleylamine diminue les contraintes de surface et améliore la passivation de surface. Ceci induit un décalage, au-delà du confinement discret, de la longueur d’onde d’émission et une augmentation du rendement quantique jusqu’à 75%.
... 5 At present, CdSe NPLs are routinely synthesized with passivating ligands that have been shown to affect their photophysical properties. 6,7 Consistent with computational reports, 8,9 experiments also suggest 6,7 that NPLs exhibit liganddependent strain profiles. However, a detailed understanding of the relationship between ligands, atomic structure, and photophysical properties is still missing. ...
... QP,bulk strain bulk conf self (6) where ΔE strain bulk , E conf , and E self are defined as in eqs 3−5. A value of 2.5 eV for V 0 was assumed for all NPL thicknesses, as this was the average value found to best match our DFT data over the entire thickness range. ...
... In addition to rationalizing experimental results on core− shell heterostructures, eq 6 can be used to understand the changes in the quasiparticle energies of NPLs due to different ligands. 6,7 Passivating ligands have all similar dielectric constants (ε out ) of approximately two; therefore, differences in dielectric contrast due to ligands can be neglected. However, the biaxial strain due to surface stress and the barrier height (V 0 ) do vary as a function of the ligand type. ...
... Although, these type of NPLs allow for the control over emission wavelength, their growth is time consuming and complicated, which hampers large-scale production of NPLs. Another approach for controlling emission wavelength of cadmium chalcogenide NPLs is based on strain engineering [21][22][23]. Antanovich and co-workers demonstrated that surface ligand induced strain in CdSe NPLs can shift the exciton transition energy up to 240 meV, which makes strain engineering suitable for tuning opto-electronic properties of the NPLs [21]. The possibility of tuning exciton transition energy of CdTe NPLs in a wide range was realized by covering the NPLs with thiol-containing ligands [24]. ...
... Another approach for controlling emission wavelength of cadmium chalcogenide NPLs is based on strain engineering [21][22][23]. Antanovich and co-workers demonstrated that surface ligand induced strain in CdSe NPLs can shift the exciton transition energy up to 240 meV, which makes strain engineering suitable for tuning opto-electronic properties of the NPLs [21]. The possibility of tuning exciton transition energy of CdTe NPLs in a wide range was realized by covering the NPLs with thiol-containing ligands [24]. ...
... Another example of ligand effect on exciton band energies was shown for CdSe nanosheets [25]. Even though ligand engineering is much more facile method compared to synthesis of alloyed and heterostructured NPLs, the replacement of native ligands of NPLs usually results in a significant reduction or complete quenching of photoluminescence [21]. Therefore, the development of new methods for tuning optical properties of cadmium chalcogenide NPLs is of great interest for different practical applications. ...
Article
Colloidal cadmium chalcogenide nanoplatelets (NPLs) possess unique properties such as ultrapure emission and giant-oscillator strength originating from their atomically-flat surfaces and one-dimensional carrier confinement. However, unlike quantum dots, the NPLs do not provide continuously tunable absorption and emission, which restricts the range of their possible applications. In this paper, we report a new approach for tuning opto-electronic properties of cadmium chalcogenide NPLs. We demonstrate that incorporation of CdSe NPLs into a poly(methyl methacrylate) (PMMA) matrix leads to red-shifting of all the low energy excitonic transitions and band-edge emission of the NPLs. The polymer matrix induced red-shift of the band-edge emission depends on thickness of the NPLs and ranges from 109 to 253 meV for 4.5 and 2.5 monolayer thick CdSe NPLs, respectively. In the case of CdSe/CdS core-shell NPLs, the polymer matrix induced red-shift of emission peak strongly depends on thickness of the shell and becomes negligible for thick shell CdSe/CdS core-shell NPLs. Possible explanations for the observed effect are presented. The demonstrated here simple approach allows one to tune optical and electronic properties of cadmium chalcogenide NPLs without altering their sizes and composition, which makes it interesting for different practical applications.
... Moreover, in these 2D systems, the ligands nature can be easily modified through standard ligands exchange procedures. [20][21][22] We demonstrated previously 15 that in the framework of continuum elasticity, the frequency of the breathing vibration of a free CdSe NPL depends on its density and elastic constant ( in the case of cubic structure) according to (1) where is the thickness of the NPL expressed as a function of the lattice parameter and , i.e. the longitudinal speed of sound in the NPL. The presence of the native ligands can be taken into account through the inertial mass load they induce. ...
... These value were deduced from the bulk material considering a zincblende structure, but it is an estimation. In particular, several works have shown that the NPLs crystalline structure is slightly different from zinc-blende structure.21,Top. TEM images of the NPLs (3+1MLs) morphology as a function of the ligand. ...
Preprint
The influence of ligands on the low frequency vibration of different thicknesses cadmium selenide colloidal nanoplatelets is investigated using resonant low frequency Raman scattering. The strong vibration frequency shifts induced by ligand modifications as well as the sharp spectral linewidths make low frequency Raman scattering a tool of choice to follow ligand exchange as well as the nano-mechanical properties of the NPLs, as evidenced by a carboxylate to thiolate exchange study. Apart from their molecular weight, the nature of the ligands, such as the sulfur to metal bond of thiols, induces a modification of the NPLs as a whole, increasing the thickness by one monolayer. Moreover, as the weight of the ligands increases, the discrepancy between the massload model and the experimental measurements increase. These effects are all the more important when the number of layers is small and can only be explained by a modification of the longitudinal sound velocity. This modification takes its origin in a change of lattice structure of the NPLs, that reflects on its elastic properties. These nanobalances are finally used to characterize ligands affinity with the surface using binary thiols mixtures, illustrating the potential of low frequency Raman scattering to finely characterize nanocrystals surfaces.
... Très couramment des ligands carboxylates de type L-X sont utilisés pour stabiliser les NPLs de CdSe en fin de synthèse. Ces ligands peuvent être ensuite remplacés par d'autres fonctions comme des thiols ou des phosphonates 26 . La Figure 8 schématise l'interaction d'un ligand carboxylate avec la surface d'une nanoplaquette de CdSe. ...
... Un décalage des propriétés optiques vers le rouge est observé, preuve de l'influence des ligands sur le confinement. En 2017, Antanovich et al. 26 ont mis en évidence l'influence structurale d'un changement de ligands. En remplaçant les carboxylates par de l'hexandecanethiol (HDT) ou de l'acide hexadecylphosphonique (HDPA), ils ont observé un décalage du premier pic excitonique jusqu'à 240 meV. ...
Thesis
Les nanoplaquettes de chalcogénures de cadmium sont des semiconducteurs de la famille II-VI, dont l’épaisseur est contrôlée à la monocouche atomique près, permettant ainsi un contrôle fin de leurs propriétés optiques. Ces matériaux peuvent s’étendre sur une centaine de nanomètres et présenter une épaisseur de quelques nanomètres.Lorsque les nanoplaquettes de CdSe 3 monocouches sont passivées par des ligands halogénures à température ambiante grâce à un précurseur de CdX2 (X = Cl, Br, I), l’énergie surfacique diminue. En chauffant (160 °C), des monomères de CdSe se dissolvent des bords des nanoplaquettes pour cristalliser sur les grandes faces. Ici, les nanoplaquettes servent elles-mêmes de réservoir de chalcogénure La modification de la chimie de surface permet donc l’obtention d’objets plus épais jusqu’à 9 monocouches et monodisperses. La versatilité de cette méthode a été prouvée sur d’autres chalcogénures de cadmium.De plus, la compréhension du mécanisme de dissolution/recristallisation a permis de développer un outil de croissance de coque, d’épaisseur contrôlée. La synthèse d’homo- et hétérostructures originales a ainsi été effectuée. Pour la première fois, une couche de CdTe a pu être synthétisée sur des nanoplaquettes de CdSe et CdTe. Enfin, des nanoplaquettes de CdSe à marches au comportemet uniqueont aussi été synthétisées. Ces dernières constituent le premier exemple de semiconducteurs avec un confinement induisant un alignement de bande de type I intraparticulaire et sans contrainte structurale.Un autre aspect de mon travail s’est porté sur la compréhension de la structure électronique des nanoplaquettes de HgTe. Nous avons systématiquement exploré leurs diagrammes de phase en fonction du confinement, de la pression et de la température. Nos résultats montrent qu’en fonction de la pression, les nanoplaquettes (confinement fort) et les nanocristaux (confinement plus faible) de HgTe ont un comportement similaire au massif: la largeur de bande interdite augmente avec la pression. En revanche, en fonction de la température, le régime de confinement est déterminant. En diminuant la température de 300 K à 10 K, la largeur de bande interdite diminue pour les nanocristauxs les plus gros.Cette diminution est de moins en moins importante pour les nanocristauxs les plus confinés, jusqu’aux nanoplaquettes pour lesquelles la largeur de bande interdite augmente. La modélisation de cet effet a permis de mettre en évidence le rôle de la seconde bande de conduction, qui pour les nanoplaquettes, modifie la courbure de la première bande de conduction lorsque la température diminue.
... The research focus relates to their promising nonlinear properties, like for instance their very high TPA coefficients σ (2) or their bandgap and spectral tunability via nanostructure size 17,[21][22][23] . Recently it has been demonstrated that anisotropic confined systems, like II-VI semiconductor nanoplatelets, [23][24][25][26][27][28][29] with strong z-confinement but weak lateral confinement exhibit due to their large coherence volumina and their small exciton Bohr radii (high exciton binding energies) extremely high two-photon absorption cross sections of up to 10 8 GM. 5 They scale quadratically with the area of the 2D structures. 21 These structures offer unprecedented nonlinearities (both per particle or per unit volume) with respect to the twophoton absorption coefficient β (reaching ∼100 times the bulk value). ...
Article
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We investigate broadband two-photon absorption autocorrelators based on II-VI semiconductor nanoplatelets as an alternative to common second harmonic generation based techniques. As compared to bulk materials the exceptionally high enhancement of two-photon absorption in these 2D structures results in very efficient two-photon absorption based autocorrelation detected via PL emission. We compare the results with TPA autocorrelation in CdS bulk as well as SHG based autocorrelation in β-barium borate. We show that CdSe nanoplatelet based autocorrelation can exceed the efficiency of conventional methods by two orders in magnitude, especially for short interaction length, and allows a precise pulse-width determination. We demonstrate that very high two-photon absorption cross sections of the nanoplatelets are the basis for this effective TPA autocorrelation. Based on our results with II-VI nanoplatelets efficient broadband autocorrelation with more than ∼100 nm bandwidth and very high sensitivity seems feasible.
... The (220) reflection of CdSe NPLs was found as a combination of broad and narrow peaks (Fig. 4). Splitting of the (220) reflection for cadmium chalcogenide NPLs was attributed to the contribution of the planes lying in basal planes of NPLs and planes oriented to thickness of NPLs [39,45]. In contrast to CdSe NPLs, the (220) reflection was found to be symmetrical and sharp for all series of CdSe/CdS CSNs, which means both a reduction of translational symmetry along the [220] axis due to growth of the shell and/or increase in thickness of samples, which makes different planes to be equivalent (Figs. ...
Article
In this study, we analyzed the effect of the composition and thickness of shell material on morphology, structure and optical properties of ultrathin CdSe/CdS, CdSe/CdS/ZnS and CdSe/ZnS core-shell nanoplatelets. It is shown that the growth of CdS and ZnS shells on ultrathin CdSe nanoplatelets with excitonic absorption at 463 nm leads to the formation of flat CdSe/CdS and CdSe/CdS/ZnS and folded CdSe/ZnS core-shell heterostructures, respectively. The tuning of the first excitonic transition in a range of 510–635 nm is shown by changing the composition and thickness of the shell material. Despite large lateral sizes, the CdSe/CdS and CdSe/CdS/ZnS core-shell nanoplatelets showed narrow emission bands with about 18–21 nm width. Emission width of the CdSe/ZnS heterostructures is shown to be thickness dependent and broader than that of CdSe/CdS and CdSe/CdS/ZnS heterostructures. Such control of the electronic structure of 2D core-shell nanoparticles, simply by changing the thickness and composition of the shell material may be attractive for the light-emitting device technology and lasers. 50-days Share Link: https://authors.elsevier.com/c/1YUcx53WXrDKE
... consisting of Stokes (first term) and anti-Stokes contributions including acoustical and optical phonon scattering. The coupling elements are g β α j,q q q , see Eqs. (14)(15)(16)(17)(18)(19). Note, that we include only first order, single phonon processes in our calculations. ...
Article
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CdSe nanoplatelets can be synthesized with different lateral sizes, very small nanoplatelets have almost quantum dot like features (almost discrete exciton states), while very large are expected to have properties of colloidal quantum wells (exciton continuum). However, nanoplatelets can be in an intermediate confinement regime with a rich substructure of excitons, which is neither quantum dot like nor an ideal 2D exciton. In this manuscript, we discuss the experimental transition energies and relaxation dynamics of exciton states in CdSe platelets with varying lateral dimensions and compare them with a microscopic theoretical model including exciton-phonon scattering. The model takes special care of the interplay of confinement and Coulomb coupling in the intermediate regime showing strong changes with respect to simple weak or strong confinement models \NLchange{by solving the full four dimensional lateral factorization free exciton wavefunction}. Depending on the platelet size broad resonances previously attributed to just ground and excited states are actually composed of a rich substructure of several exciton states in their temporal dynamics. We show that these factorization free exciton states can explain the spectral features observed in photoluminescence experiments. Furthermore we demonstrate that the interplay of exciton bright and dark states provides principle insights into the overall temporal relaxation dynamics, and allow tuning the exciton cooling via lateral platelet size. Our results and theoretical approach are directly relevant for understanding e.g. the size tuneability of lasing, excitonic cooling dynamics or light harvesting applications in these and similar 2D systems of finite lateral size.
... More surface-sensitive GIWAXS measurements of superlattices of lead sulfide and cadmium selenide quantum dots show a similar peak [13][14][15] . Explanations include unique alloys at the quantum dot surfaces 16,17 , amorphous glass slides 18,19 , oxide formation 9,10 , forbidden reflections 20,21 , and unreacted organic precursors such as indium myristate and lead oleate 22,23 . Most reports opt to ignore this peak entirely, either with a lack of explanation or failing to include it in the reported spectrum [13][14][15]24,25 . ...
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Powder X-ray diffraction is one of the key techniques used to characterize the inorganic structure of colloidal nanocrystals. The comparatively low scattering factor of nuclei of the organic capping ligands and their propensity to be disordered has led investigators to typically consider them effectively invisible to this technique. In this report, we demonstrate that a commonly observed powder X-ray diffraction peak around $$q=1.4{\AA}^{-1}$$ q = 1.4 Å − 1 observed in many small, colloidal quantum dots can be assigned to well-ordered aliphatic ligands bound to and capping the nanocrystals. This conclusion differs from a variety of explanations ascribed by previous sources, the majority of which propose an excess of organic material. Additionally, we demonstrate that the observed ligand peak is a sensitive probe of ligand shell ordering. Changes as a function of ligand length, geometry, and temperature can all be readily observed by X-ray diffraction and manipulated to achieve desired outcomes for the final colloidal system.
... After this procedure, both absorption bands were signi cantly red-shifted, from λ = 510 nm and λ = 479 nm to respectively λ = 515 nm and λ = 485 nm. Such a red shift is typical for capping ligands such as aliphatic thiols coating CdSe nanoplatelets, 34 and thus indicates a successful ligand exchange procedure and that the nanoplatelets have not been damaged by oxidation during their functionalization. Indeed, oxygen can contribute to the formation of a CdO layer on the platelets surface, detectable by a blue shift of the UV-Vis absorption. ...
Preprint
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CdSe nanoplatelets are a recently discovered class of colloidal semiconducting nanocrystals. Atomic control over their thickness allows achieving control over quantum size effects, and in particular, these platelets exhibit monochromatic light emission because of the confinement of photo-generated excitons only in their thickness. These nanoplatelets can self-organize into supra-particular polymers, depending on their environment, which means that their shape anisotropy can be expressed at the microscale. Here, the self-assembly of semiconducting nanoplatelets is controlled remotely by light, in a dynamic nanoparticulate system that integrates light-responsive molecular switches covalently. Azobenzene ligands were thus designed to ( i ) be grafted on the nanoplatelets ( ii ) ensure their colloidal stability in chloroform when confined on their surface in the E -configuration. Upon irradiation, the ligands isomerize into their Z -configuration, leading to a modification of the dipolar moment of the particles and to the formation of one-dimensional stacks. The self-assembly is reversible, as thermal relaxation of the ligands yields the initial dispersion back. This reversible hybrid system can be used in the design of responsive optical systems, as illustrated by photo-patterning experiments leading to controlled spatial resolution of the luminescence intensity in thin films.
... However, this ligand exchange causes larger PL quenching. 82 In 2019, Dufour et al found that using of halide ligands can tune the emission properties and significantly improve PLQY. 83 The red shift leaded by ligand exchange is mainly considered to have two reasons. ...
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Compared with zero‐dimensional quantum dots and one‐dimensional nanorods, two‐dimensional nanoplatelets exhibit many unique optical properties. Due to strong quantum confinement in only one direction and the large lateral size, the CdSe nanoplatelets show large exciton binding energy, giant oscillator strength, narrow emission, large absorption cross‐section, and large optical gain. In this review, the development of synthesis technology of nanoplatelets is first briefly introduced. Subsequently, the optical properties of CdSe nanoplatelets and their heterostructures are reviewed, including absorption and photoluminescence, carrier dynamics, optical gain, and nonlinear properties. These optical properties are affected by size, material and structure. At last, the main applications of CdSe nanoplatelets in recent years are introduced, including light‐emitting diodes, lasers, light harvesting, photocatalysis, and sensing. CdSe nanoplatelets (NPLs) is a quasi‐two‐dimensional nanomaterial with many unique properties, such as narrow emission peak, large absorption cross‐section, large exciton binding energy and high optical gain. This review summarizes the optical properties and various applications of CdSe NPLs in recent years and point out some challenges, which will promote the further development of NPLs.
... The narrow spectral bandwidth of the intersubband absorptions in CQWs compared to epitaxial systems is particularly noteworthy. Recent work showing the critical role of surface termination on the lattice strain of CQWs, CQW band gap, and CQW transition line widths underlines the potential of colloidal systems to relax to a state with reduced bandwidth compared to epitaxial systems [66][67][68] . In the future, this can be explicitly explored with atomically precise epitaxial interfaces. ...
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Colloidal quantum wells are two-dimensional materials grown with atomically-precise thickness that dictates their electronic structure. Although intersubband absorption in epitaxial quantum wells is well-known, analogous observations in non-epitaxial two-dimensional materials are sparse. Here we show that CdSe nanoplatelet quantum wells have narrow (30–200 meV), polarized intersubband absorption features when photoexcited or under applied bias, which can be tuned by thickness across the near-infrared (NIR) spectral window (900–1600 nm) inclusive of important telecommunications wavelengths. By examination of the optical absorption and polarization-resolved measurements, the NIR absorptions are assigned to electron intersubband transitions. Under photoexcitation, the intersubband features display hot carrier and Auger recombination effects similar to excitonic absorptions. Sequenced two-color photoexcitation permits the sub-picosecond modulation of the carrier temperature in such colloidal quantum wells. This work suggests that colloidal quantum wells may be promising building blocks for NIR technologies.
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Colloidal CdSe nanoplatelets, with the electronic structure of quantum wells, self-assemble into lamellar stacks due to large co-facial van der Waals attractions. These lamellar stacks are shown to display coherent acoustic phonons that are detected from oscillatory changes in the absorption spectrum observed in infrared pump, electronic probe measurements. Rather than direct electronic excitation of the nanocrystals using a femtosecond laser, impulsive transfer of heat from the organic ligand shell, excited at C–H stretching vibrational resonances, to the inorganic core of individual nanoplatelets occurs on a time-scale of <100 ps. This heat transfer drives in-phase longitudinal acoustic phonons of the nanoplatelet lamellae, which are accompanied by subtle deformations along the nanoplatelet short axes. The frequencies of the oscillations vary from 0.7 to 2 GHz (3–8 μeV and 0.5–1 ns oscillation period) depending on the thickness of the nanoplatelets—but not their lateral areas—and the temperature of the sample. Temperature-dependence of the acoustic phonon frequency conveys a substantial stiffening of the organic ligand bonds between nanoplatelets with reduced temperature. These results demonstrate a potential for acoustic modulation of the excitonic structure of nanocrystal assemblies in self-assembled anisotropic semiconductor systems at temperatures at or above 300 K.
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Colloidal quantum wells, also called nanoplatelets, are nanoscopic materials displaying quantum confinement in two dimensions. Unlike colloidal quantum dots, colloidal quantum well ensembles have no inhomogeneous broadening due to an atomically-precise definition of the short axis, a fact which results in much narrower ensemble absorption and emission. Thus, colloidal quantum wells can translate many advantages of colloidal nanocrsytals or other solution-processable materials, such as scalable synthesis and substrate-agnostic deposition (particularly compared to epitaxial quantum wells), without sacrificing material uniformity. Due to very narrow photoluminescnece peaks, these materials have found a home in applications involving light emission, such as downconversion enhancement films, light-emitting diodes, and lasers, in which they represent some of the best performers among solution-cast materials. As argued in this review, the full spectrum of epitaxial quantum well devices offers a roadmap to other potential applications, such as detection, electronics, electro-optics, non-linear optics, or intersubband devices, in which only nascent efforts have been made.
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Colloidal two-dimensional cadmium chalcogenides nanoplatelets have recently emerged as a class of semiconductor nanoparticles with the narrowest emission and absorption excitonic bands that are of interest for optical applications. Here, we have developed a synthesis protocol for 2.5 monolayers (ML) thick CdSe nanosheets as a single population. We found that a two-step synthesis in the presence of water promoted the growth of atomically-thin nanosheets with high structural and morphological perfection. Using seeded-growth technique, we extended the lateral size of nanosheets up to 400 nm, which led to the formation of multiwall rolled-up nanostructures. Ligand exchange of native oleic acid, attached to Cd-rich (001) planes, with achiral thioglycolic acid and chiral N-acetylcysteine retains a scroll-like morphology of nanosheets, in contrast to thicker 3.5 ML population. A reorientation from the [110] to [100] folding direction was found during the change from an achiral to a chiral ligand. In the case of ligand exchange with chiral N-acetyl-L- or D-cysteine, we demonstrated that 2.5 ML CdSe nanosheets with 400 nm lateral size have circular dichroism with dissymmetry g-factor up to 3‧10-3. Strong circular dichroism found for colloidal CdSe nanosheets makes them a promising candidate for polarization-enabled applications, while the growth protocol of the thinnest CdSe nanosheets enriches the known synthesis methods of a set of CdSe nanoplatelet populations.
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We investigate the role of the stress arising between core and shell materials in colloidal CdSe/X hetero-nanoplatelets (X=ZnS,CdS,CdTe). The resulting strain distribution is calculated within the linear elastic regime, and its influence on the electronic structure with k·p theory. We show that strain shifts the energy of electrons and that of holes by several tens of meV. In structures with type-I band alignment the two shifts have opposite signs and the net effect on the exciton emission energy is small, but in type-II systems they add up. The strain response in colloidal NPLs is found to exhibit some differences as compared to that of epitaxial quantum wells, including sizable influence of lateral dimensions below 10 nm and potentially relevant effect of coupled strain-momentum terms of the Hamiltonian. We further show that asymmetric shell covering leads to bending of the nanoplatelet and tilted potential profiles along the strong confinement direction, analogous to a built-in electric field. We propose overcoating CdSe/CdS NPLs with an outer ZnS shell as a method to mitigate tunneling-induced redshift of emission via strain engineering.
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We study the impact of organic surface ligands on the electronic structure and electronic band edge energies of quasi-two-dimensional (2D) colloidal cadmium selenide nanoplatelets (NPLs) using density functional theory. We show how control of the ligand and ligand-NPL interface dipoles results in large band edge energy shifts, over a range of 5 eV for common organic ligands with minor effect on the NPLs band gaps. Using a model self-energy to account for the dielectric contrast and an effective mass model of the excitons, we show that the band edge tunability of NPLs together with the strong dependence of the optical band gap on NPL thickness can lead to favorable photochemical and optoelectronic properties.
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Three-monolayer CdSe nanoplatelets having the composition (CdSe)3[Cd(OAc)0.77(oleate)1.23]0.78, large lateral dimensions, minimal strain distortions, and the zinc-blende crystal structure undergo Z-type to L-type ligand exchange with ethylenediamine, affording NPLs of composition (CdSe)3[en]0.67 (en = ethylenediamine). The L-type ligation of the (CdSe)3[en]0.67 (en = ethylenediamine) NPLs is exchanged for Z-type ligation with Cd(oleate)2, Cd(OAc)2, CdCl2, Zn(oleate)2, and ZnCl2, giving NPLs having near to the ideal compositions of (CdSe)3[MX2]. All of the Z-type to L-type and L-type to Z-type ligand exchanges are kinetically slow, requiring several hours to reach completion, suggesting that a considerable surface reconstruction is required. In contrast, three-monolayer NPLs having a significant rolling distortion, and four- and five-monolayer NPLs having small lateral dimensions are unstable to ethylenediamine, and Z-type to L-type ligand exchange is not achieved.
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Treatment of CdSe nanoplatelets with halide salts induces a bathochromic shift in the absorption resonances that does not occur in quasi-spherical quantum dots of the same composition. The optical shifts are not strongly sensitive to the cation moiety, which may be organic or inorganic. The magnitude of the energy shift is largest for thinner nanoplatelets, with bathochromic shifts as large as 240 meV observed for 3 monolayer nanoplatelets. This effect is driven by a tetragonal distortion of the zinc blende lattice in response to ligand exchange. The expansion of the lattice in the shortest nanoplatelet axis results in the observed red-shifts due primarily to relaxation of quantum confinement, with secondary contributions from strain.
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Among colloidal nanocrystals, 2D nanoplatelets (NPL) made of II-VI compounds, appear as a special class of emitters with especially narrow photoluminescence signal. However the PL signal in the case of NPL is only tunable by discrete step. Here we demonstrate that doping is a viable path to finely tune the color of this NPL from green to red, making them extremely interesting as phosphor for wide gamut display. In addition using a combination of luminescence spectroscopy, tight binding simulation, transport and photoemission, we provide a consistent picture for the Ag+ doped CdSe NPL. The Ag activated state is strongly bound and located 340 meV above the valence band of the bulk material. The Ag dopant induces a relative shift of the Fermi level toward the valence band by up to 400 meV but preserve the n-type nature of the material.
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The dynamics of intersubband relaxation are critical to quantum well technologies such as quantum cascade lasers and quantum well infrared photodetectors. Here, intersubband relaxation in CdSe colloidal quantum wells, or nanoplatelets, is studied via pump-push-probe transient spectroscopy. An initial interband pump pulse is followed by a secondary infrared push excitation, resonant with intersubband absorption, which promotes electrons from the first conduction band of the quantum well to the second conduction band. A probe pulse monitors subsequent electron cooling to the band edge of the quantum well. Using this technique, intersubband relaxation is studied as a function of critical variables such as colloidal quantum well size and thickness, surface ligand chemistry, temperature, and excitation pulse intensity. Larger quantum well sizes, judicious selection of surface ligand chemistry (e.g., thiolates), low temperatures, and elevated push pulse fluences slow intersubband relaxation. However, compared to resonant intraband relaxation in colloidal quantum dots (up to hundreds of picoseconds), intersubband relaxation in colloidal quantum wells is rapid (<1 ps) under all examined conditions. These experiments indicate that rapid relaxation is driven by both LO phonon and surface scattering. The short time scale of relaxation observed in these materials may hinder intersubband technologies such as mid-infrared detectors, although such rapid relaxation may prove valuable in optical switching.
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Organic ligands are essential in the growth of monodisperse colloidal inorganic nanocrystals and can be leveraged to create a wide variety of shapes and sizes. Inorganic nanocrystals coated with surfactant-like organic molecules have a vast range of properties that arise from the combination of the individual components. In this Review, we discuss the role that the tails of the organic ligands play in the synthesis and properties of colloidal nanocrystals, particularly the collective effects of the organic ligands on the surface. Ligand–ligand interactions influence the thermodynamic and kinetic properties of the nanocrystals, as well as alter their colloidal stability. These interactions should inform the conceptualization of new nanocrystal syntheses as they influence the surface energy of the colloid, and these interactions should play a role in subsequent assembly strategies to prepare nanocrystal superlattices, which are driven by interparticle interactions. Inorganic nanocrystals coated with surfactant-like organic molecules have a vast range of properties arising from the combination of their components. In this Review, the role of the organic ligands on the synthesis of colloidal nanocrystals is discussed with a focus on the tails of the ligands and their collective effects on the surface.
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Semi-conducting nanoplatelets are two-dimensional nanoparticles whose thickness is in the nanometer range and controlled at the atomic level. They have come up as a new category of nanomaterial with promising optical properties due to the efficient confinement of the exciton in the thickness direction. In this perspective, we first describe the various conformations of these 2D nanoparticles which display a variety of bent and curved geometries and present experimental evidences linking their curvature to the ligand-induced surface stress. We then focus on the assembly of nanoplatelets into superlattices to harness the particularly efficient energy transfer between them, and discuss different approaches that allow for directional control and positioning in large scale assemblies. We emphasize on the fundamental aspects of the assembly at the colloidal scale in which ligand-induced forces and kinetic effects play a dominant role. Finally, we highlight the collective properties that can be studied when a fine control over the assembly of nanoplatelets is achieved.
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A lot of substantial innovation in advancement of microfluidic field in recent years to produce nanoparticle reveals a number of distinctive characteristics for instance compactness, controllability, fineness in process, and stability along with minimal reaction amount. Recently, a prompt development, as well as realization in production of nanoparticles in microfluidic environs having dimension of micro to nanometers and constituents extending from metals, semiconductors to polymers, has been made. Microfluidics technology integrates fluid mechanics for production of nanoparticles having exclusive with homogenous sizes, shapes, and morphology, which are utilized in several bioapplications such as biosciences, drug delivery, healthcare, including food engineering. Nanoparticles are usually well‐known for having fine and rough morphology because of their small dimensions including exceptional physical, biological, chemical, and optical properties. Though the orthodox procedures need huge instruments, costly autoclaves, use extra power, extraordinary heat loss, as well as take surplus time for synthesis. Additionally, this is fascinating in order to systematize, assimilate, in addition, to reduce traditional tools onto one platform to produce micro and nanoparticles. The synthesis of nanoparticles by microfluidics permits fast handling besides better efficacy of method utilizing the smallest components for process. Herein, we will focus on synthesis of nanoparticles by means of microfluidic devices intended for different bioapplications. This article is protected by copyright. All rights reserved
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The effects of halide‐ligand exchange and Cu and Ag doping are studied on structural, optical, and electrical properties of four monolayer CdSe nanoplatelet (NPL) and NPL thin films. Combinational study shows that NH4Cl‐treatment on CdSe NPL and NPL thin films show tetragonal lattice distortion of NPL, side‐to‐side attachment between NPLs, bathochromic shift in absorption spectra, and complete quenching of band‐edge and dopant‐induced emissions. First‐principle calculations reveal that Cl creates states below valence band maximum while Ag and Cu dopants create acceptor‐like states, explaining the change of their optical property. Field‐effect transistors are fabricated to investigate the effect of doping and reduced interplatelet distance on electrical properties of CdSe NPL thin films, demonstrating Cu and Ag dopants mitigate n‐type character of CdSe NPL thin films. Temperature‐dependent electrical characterization is conducted to further understand charge transport behavior depending on the existence of dopants. This work provides scientific information on the influence of surface chemistry and impurity doping on quantum confined semiconductors and new directions for the design of high‐performance nanomaterial‐based electronic and optoelectronic devices. The effect of ligand exchange and impurity doping on structural, optical, and electrical properties of four monolayer CdSe nanoplatelet (NPL) and NPL thin films is studied using both experimental and theoretical methods. Field‐effect transistors are fabricated to investigate the charge transport behavior of CdSe NPL thin films depending on the existence of dopants.
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Driven by the unique optoelectronic features arising from the strong quantum confinement along the thickness direction, rapid development of CdSe‐based nanoplatelets (NPL) has accomplished facile bandgap tunability over a broad spectrum via different preparation protocols or various postsynthesis treatments. The anisotropic geometry of NPLs also stimulates the exploitation of self‐assembled CdSe NPL superstructures to enhance light extraction efficiency in display technologies and achieves polarized lasing performance or even electrically pumped laser by adjusting the transition dipole orientation. Although several review articles about the general optical properties of CdSe NPLs have been published, none of them focus on the ability of spectral tunability and precise control of orientations in the solid‐state phase of CdSe NPLs. Herein, the band structure engineering approaches in the CdSe NPL family are systematically summarized and the optical properties and device performance metrics of these deliberately engineered NPLs are compared. Then, the recent advanced studies of the motivation and assembly methods of geometrically anisotropic CdSe NPL superlattices are discussed. Finally, the current challenges and future outlook on the controlled modification of optical features and the influences of the approaches in the aspect of CdSe NPL–based devices’ performance are highlighted. Several strategies of manipulation of imperative optical properties, including emission tunability and distribution of the transition dipole orientation, in the case of well‐known CdSe nanoplatelets are discussed herein. These summarized synthesis protocols and postsynthesis treatments shed new light on the design of colloidal 2D materials with desired optical properties.
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The influence of ligands on the low frequency vibration of cadmium selenide colloidal nanoplatelets of different thicknesses is investigated using resonant low frequency Raman scattering. The strong vibration frequency shifts induced by ligand modifications as well as sharp spectral linewidths make low frequency Raman scattering a tool of choice to follow ligand exchange as well as the nano-mechanical properties of the NPLs, as evidenced by a carboxylate to thiolate exchange study. Apart from their molecular weight, the nature of the ligands, such as the sulfur to metal bond of thiols, induces a modification of the NPLs as a whole, increasing the thickness by one monolayer. Moreover, as the weight of the ligands increases, the discrepancy between the mass-load model and the experimental measurements increase. These effects are all the more important when the number of layers is small and can only be explained by a modification of the longitudinal sound velocity. This modification takes its origin in a change of the lattice structure of the NPLs, that reflects on their elastic properties. These nanobalances are finally used to characterize ligand affinity with the surface using binary thiol mixtures, illustrating the potential of low frequency Raman scattering to finely characterize nanocrystal surfaces.
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Semiconductor CdSe/CdS core-shell nanoplatelets exhibit narrow and intense absorption and photoluminescence spectra in the visible range, which makes them suitable for numerous applications in optoelectronics. Of particular interest is the preparation and optical characterization of thin films with an accurately controlled amount of nanoplatelets. Here we report on the use of spectroscopic ellipsometry for investigating the optical properties of ultrathin films composed of a single layer of negatively charged CdSe/CdS core-shell nanoplatelets prepared by the electrostatic layer-by-layer deposition on SiO 2/Si substrates. Combining the ellipsometric spectra with atomic force microscopy measurements, we were able to infer the nanoplatelet film extinction spectra which was found to exhibit the two characteristic exciton peaks albeit blueshifted relative to the colloidal nanoplatelets.
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Nanoplatelets (NPLs)-colloidally synthesized, spatially anisotropic, two-dimensional semiconductor quantum wells-are of intense interest owing to exceptionally narrow transition line widths, coupled with solution processability and bandgap tunability. However, given large surface areas and undercoordinated bonding at facet corners and edges, excitation under sufficient intensities may induce anisotropic structural instabilities that impact desired properties. We employ time-resolved X-ray diffraction to study the crystal structure of CdSe NPLs in response to optical excitation. Photoexcitation induces greater out-of-plane than in-plane disordering in 4 and 5 monolayer (ML) NPLs, while 3 ML NPLs display the opposite behavior. Recovery dynamics suggest that out-of-plane cooling slightly outpaces in-plane cooling in 5 ML NPLs with recrystallization occurring on indistinguishable time scales. In comparison, for zero-dimensional CdSe nanocrystals, disordering is isotropic and recovery is faster. These results favor the use of NPLs in optoelectronic applications, where they are likely to exhibit superior performance over traditional, zero-dimensional nanocrystals.
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The chirality of inorganic semiconductor nanocrystals (NCs) was demonstrated to be sensitive to the particle structures and chiral molecules capped on the NC surfaces. In this work, we found the native achiral ligands used for the synthesis of NCs could also influence their circular dichroism (CD) response. With different native ‘growth’ ligands but similar semiconductor cores, the CD spectra of NCs exhibited mirror images by using the same enantiopure cysteine (Cys) ligands, indicating the inversion of NC chirality. In addition, distinct anisotropic dissymmetry factors (g-factor) were observed. With appropriate native ligands, we prepared chiral cysteine stabilized spherical CdSe NCs (quantum dots, QDs) with the highest g-factor ever reported.
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We investigate the lateral size tunability of the exciton diffusion coefficient and mobility in colloidal quantum wells by means of line width analysis and theoretical modeling. We show that the exciton diffusion coefficient and mobility in laterally finite 2D systems like CdSe nanoplatelets can be tuned via the lateral size and aspect ratio. The coupling to acoustic and optical phonons can be altered via the lateral size and aspect ratio of the platelets. Subsequently the exciton diffusion and mobility become tunable since these phonon scattering processes determine and limit the mobility. At 4 K the exciton mobility increases from ∼ 4 × 10³ cm² V⁻¹ s⁻¹ to more than 1.4 × 10⁴ cm² V⁻¹ s⁻¹ for large platelets, while there are weaker changes with size and the mobility is around 8 × 10¹ cm² V⁻¹ s⁻¹ for large platelets at room temperature. In turn at 4 K the exciton diffusion coefficient increases with the lateral size from ∼ 1.3 cm² s⁻¹ to ∼ 5 cm² s⁻¹, while it is around half the value for large platelets at room temperature. Our experimental results are in good agreement with theoretical modeling, showing a lateral size and aspect ratio dependence. The findings open up the possibility for materials with tunable exciton mobility, diffusion or emission line width, but quasi constant transition energy. High exciton mobility is desirable e.g. for solar cells and allows efficient excitation harvesting and extraction.
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In recent years, microfluidic technologies have emerged as a powerful approach for the advanced synthesis and rapid optimization of various solution‐processed nanomaterials, including semiconductor quantum dots and nanoplatelets, and metal plasmonic and reticular framework nanoparticles. These fluidic systems offer access to previously unattainable measurements and synthesis conditions at unparalleled efficiencies and sampling rates. Despite these advantages, microfluidic systems have yet to be extensively adopted by the colloidal nanomaterial community. To help bridge the gap, this progress report details the basic principles of microfluidic reactor design and performance, as well as the current state of online diagnostics and autonomous robotic experimentation strategies, toward the size, shape, and composition‐controlled synthesis of various colloidal nanomaterials. By discussing the application of fluidic platforms in recent high‐priority colloidal nanomaterial studies and their potential for integration with rapidly emerging artificial intelligence‐based decision‐making strategies, this report seeks to encourage interdisciplinary collaborations between microfluidic reactor engineers and colloidal nanomaterial chemists. Full convergence of these two research efforts offers significantly expedited and enhanced nanomaterial discovery, optimization, and manufacturing.
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Recent experiments suggest that the photoluminescence line width of CdSe nanoplatelets (NPLs) and core/shell CdSe/CdS NPLs may be broadened by the presence of shakeup (SU) lines from negatively charged trions. We carry out a theoretical analysis, based on effective mass and configuration interaction (CI) simulations, to identify the physical conditions that enable such processes. We confirm that trions in colloidal NPLs are susceptible of presenting SU lines up to 1 order of magnitude stronger than in epitaxial quantum wells, stimulated by dielectric confinement. For these processes to take place, trions must be weakly bound to off-centered charge traps, which relax symmetry selection rules. Charges on the lateral sidewalls are particularly efficient to this end. Our simulations display a single strong SU replica in most instances, which agrees well with experiments on CdSe NPLs, but suggests that the multipeaked emission reported for core/shell CdSe/CdS NPLs must involve other factors beyond SU processes. We propose emission from a metastable spin triplet trion state may be responsible. Understanding the origin of SU processes may open paths to rational design of NPLs with narrower line width.
Article
We study the nonlinear optical response of CdSe/CdS nanoplatelets (NPLs) in the vicinity of heavy hole (hh) and light hole (lh) exciton resonances by means of the two color pump-probe technique. The population of hh and lh excitonic states leads to the decrease of the light absorption at both resonances under resonant excitation of one of them. Increase of the pump power leads to the saturation of the absorption. This effect allows us to estimate the exciton total lifetime.
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In the large field of research on nanoplatelets (NPLs), their strong tendency to self‐assemble into ordered stacks and the resulting changes in their properties are of great interest. The assembly reveals new characteristics such as the charge carrier transport through the NPL assembly or altered optical properties. In particular, a reduced distance should enhance the charge carrier transport due to higher electronic coupling of neighboring NPLs, and therefore, is the focus of this work. To modify the inter‐particle distances, the straightforward method of ligand exchange is applied. Various CdSe and CdSe/CdX (hetero‐) NPLs serve as building blocks, which not only display different material combinations but also different types of heterostructures. The surface‐to‐surface distance between the stacked NPLs can be reduced to below 1 nm, thus, to less than the half compared to assemblies of pristine NPLs. Moreover, for certain NPLs stacking is only enabled by the ligand exchange. To characterize the ligand exchanges and to investigate the influences of the reduced distances, photo‐electrochemical measurements, fluorescence spectroscopy, energy dispersive X‐ray spectroscopy, nuclear magnetic resonance, and X‐ray photoelectron spectroscopy are performed. It is possible to show higher photocurrents for smaller distances, indicating enhanced charge transport ability within those stacks. Nanoplatelets possess a large tendency to self‐assemble into ordered stacks. The influence of the assembly on the properties is thereby of great interest. This work focuses on the minimization of the inter‐particle distance via different ligand exchanges. Further, the influence of the smaller distances on the charge carrier transport within these assemblies are investigated through photo‐electrochemical measurements.
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We introduce new oxygen- and moisture-proof polymer matrices based on polyisobutylene (PIB) and its block copolymer with styrene (poly(styrene-block-isobutylene-block-styrene), PSt-b-PIB-b-PSt) for encapsulation of colloidal semiconductor nanocrystals. In order to prepare transparent and processable composites, we developed a special procedure of the nanocrystal surface engineering including ligand exchange of parental organic ligands to inorganic species followed by attachment of specially designed short-chain PIB functionalized with amino-group (PIB-NH2). The latter provides excellent compatibility of the particles with the polymer matrices. As colloidal nanocrystals, we chose CdSe nanoplatelets (NPLs), since they possess a large surface and thus are very sensitive to the environment, in particular in terms of their limited photostability. The encapsulation strategy is quite general and can be applied to a wide variety of semiconductor nanocrystals, as demonstrated on the example of PbS quantum dots. All obtained composites exhibited excellent photostability being tested in a focus of a powerful white-light source, as well as exceptional chemical stability in a strongly acidic media. We compared these properties of the new composites with those of widely used polyacrylate based materials, demonstrating the superiority of the former. The developed composites are of particular interest for application in optoelectronic devices, such as color-conversion light emitting diodes (LEDs), laser diodes, luminescent solar concentrators, etc.
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The emergence of chirality is a central issue in chemistry, materials science, and biology. In nanoparticle assemblies, chirality has been shown to arise through a few different processes, but chiral organizations composed of plate-like nanoparticles, a class of material under scrutiny due to their wide applicative potential, have not yet been reported. We show that ribbons of stacked board-shaped cadmium selenide (CdSe) nanoplatelets (NPLs) twist upon the addition of oleic acid ligand, leading to chiral ribbons that reach several micrometers in length and display a well-defined pitch of ~400 nm. We demonstrate that the chirality originates from surface strain caused by the ligand because isolated NPLs in dilute solution undergo a transition from a flat to a twisted shape as the ligand coverage increases. When the platelets are closely stacked within ribbons, the individual twist propagates over the whole ribbon length. These results show that a ligand-induced mechanical stress can strongly distort thin NPLs and that this stress can be expressed at a larger scale, paving the way to stress engineering in assemblies of nanocrystals. Such a structural change resulting from a simple external stimulus could have broad implications for the design of sensors and other responsive materials.
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Colloidal nanocrystals of fully inorganic cesium lead halide (CsPbX3, X = Cl, Br, I, or combinations thereof) perovskites have attracted much attention for photonic and optoelectronic applications. Herein, we demonstrate a facile room-temperature (e.g., 25 oC), ligand-mediated reprecipitation strategy for systematically manipulating the shape of CsPbX3 colloidal nanocrystals, such as spherical quantum dots, nanocubes, nanorods, and nanoplatelets. The colloidal spherical quantum dots of CsPbX3 were synthesized with photoluminescence (PL) quantum yield values up to >80%, and the corresponding PL emission peaks covering the visible range from 380 to 693 nanometer. Besides spherical quantum dots, the shape of CsPbX3 nanocrystals could be engineered into nanocubes, one-dimensional nanorods, and two-dimensional few-unit-cell-thick nanoplatelets with well-defined morphology by choosing different organic acid and amine ligands via the reprecipitation process. The shape-dependent PL decay lifetimes have been determined to be several to tens to hundreds of nanoseconds. Our method provides a facile and versatile route to rational control the shape of the CsPbX3 perovskites nanocrystals, which will create opportunities for applications such as displays, lasing, light-emitting diodes, solar concentrators, and photon detection.
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We present a method for the determination of the average number of polymer molecules on the surface of AIIBVI luminescent core-shell nanocrystals (CdSe/ZnS, ZnSe/ZnS quantum dots and CdS/ZnS nanorods) encapsulated with amphiphilic polymer. Poly(maleic anhydride-alt-1-tetradecene) (PMAT) was quantitatively labeled with amino-derivative of fluorescein and the average amount of PMAT molecules per single nanocrystal determined using optical absorption of the dye in the visible spectral range. The average amount of PMAT molecules is growing linearly with the surface area of all studied nanocrystals. However, surface density of monomer units increases nonlinearly with the surface area due to the increased competition between PMAT molecules for Zn-hexanethiol surface binding sites. The average value of zeta potential ζ = -35 mV was found to be independent of the size, shape and chemical composition of nanocrystals at fixed buffer parameters (carbonate-bicarbonate buffer, pH 9.5 and 5 mM ionic strength). This finding is expected to be used for the determination of surface density of remaining carboxyl groups in PMAT-encapsulated nanocrystals.
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Increasing the fraction of 1H,1H,2H,2H-perfluorodecanethiol (PFDT) in the mixed-PFDT/oleate ligand shell of a PbS quantum dot (QD) dramatically reduces the permeability of the ligand shell to alkyl-substituted benzoquinones (s-BQs), as measured by a decrease in the efficiency of collisional photoinduced electron transfer. Replacing only 21% of the oleates on the QD surface with PFDT reduces the yield of photo-oxidation by tetramethyl BQ by 68%. Experiments with s-BQ quenchers of two different sizes reveal that the degree of protection provided by the PFDT-doped monolayer, relative to a DT-doped monolayer at similar coverage, is due to both size exclusion (PFDT is larger and more rigid than DT), and the oleophobicity of PFDT. This work demonstrates the usefulness of fluorinated ligands in designing molecule-selective and potentially corrosion-inhibiting surface coatings for QDs for applications as robust emitters or high fidelity sensing platforms.
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All nanomaterials share a common feature of large surface-to-volume ratio, making their surfaces the dominant player in many physical and chemical processes. Surface ligands - molecules that bind to the surface - are an essential component of nanomaterial synthesis, processing and application. Understanding the structure and properties of nanoscale interfaces requires an intricate mix of concepts and techniques borrowed from surface science and coordination chemistry. Our Review elaborates these connections and discusses the bonding, electronic structure and chemical transformations at nanomaterial surfaces. We specifically focus on the role of surface ligands in tuning and rationally designing properties of functional nanomaterials. Given their importance for biomedical (imaging, diagnostics and therapeutics) and optoelectronic (light-emitting devices, transistors, solar cells) applications, we end with an assessment of application-targeted surface engineering.
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Exciting characteristic features of nanomaterials in contrast to their bulk phase require shape control of morphologies at the nanometer scale. Surfactants, a unique class of surface active molecules, possess remarkable ability to control crystal growth and direct it in shape and size controlled manner. The fine-tuning of the desired morphologies can be achieved by controlling the surfactant architecture as well as its self-assembly behavior. This review highlights the correlation between the surfactant properties and their ability in designing nanomorphologies. In terms of future perspectives, the methodologies can also be implemented to design biologically sustainable nanomaterials for their possible use in nanomedicine.
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We developed a new chemical strategy to enhance the stability of lead selenide nanocrystals (PbSe NCs) against oxidation, through the surface passivation by P-O- moieties. In the synthesis of PbSe NCs, tris(diethylamino)phosphine (TDP)-selenide (Se) was used as a Se precursor, and the resulting PbSe NCs withstood long-term air exposure while showing little nearly no sign of oxidation. Nuclear magnetic resonance (NMR) spectroscopy reveals that TDP derivatives passivate the surface of PbSe NC. Through a series of ligand cleavage reactions, we found that the TDP derivatives are bound on NC surface through the P-O- moiety. Based on such understanding, it turned out that direct addition of various PAs during the synthesis of PbSe NCs also results in the NCs whose absorption spectrum remains nearly intact after air exposure for weeks. The P-O- moieties render the NCs stable in the operation of field effect transistors, suggesting that our findings can enable the use of air stable PbSe NCs in wider array of optoelectronic applications.
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CdSe/CdTe heterojunction nanorods with type II staggered band offset can allow directional and efficient separation of photogenerated charge carriers. However, CdTe nanocrystals can often be easily oxidized even with postsynthesis processing in air, which can then lead to charge traps that negate the benefits of the type II band offset. Here, we introduce a simple ligand exchange method to replace the native ligands on CdSe/CdTe heterojunction nanorods with 1-octanethiol resulting in improved photoluminescence and good stability in air. Transient absorption measurements reveal that electron transfer from CdTe to CdSe remains efficient/fast (∼400 fs) despite the hole trapping nature of thiol ligands for CdSe. Absorption bleach arising from CdTe-to-CdSe electron transfer can be observed out to 1 μs even after days of storage in air, an order of magnitude longer than heterojunction nanorods with native ligands that are processed with anhydrous solvents under air-free conditions and kept air-free. This improved stability/robustness that preserves efficient charge separation translates to enhanced photocurrent generation especially with respect to contribution from photoexcitation of CdTe transitions. (Graph Presented).
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We investigate the optical absorption properties of colloidal CdSe nanoplatelets and compare them to CdSe quantum dots. Starting from inductively coupled plasma–atomic emission spectroscopy (ICP-AES) measurements on their intrinsic absorption coefficients μi, we compare these results with a theoretical approach by a continuum absorption Lorentz local field model. We show that the platelets’ intrinsic absorption coefficients μi are strongly thickness and aspect ratio dependent, which results in the possibility to tune the absorption properties of this material class by the lateral size and thickness. The continuum intrinsic absorbance of the platelets is considerably larger if compared with quantum dots making them more efficient absorbers with higher light–matter interaction that is essential for their use in, for example, solar cells. The obtained μi values can be used for concentration determination of CdSe nanoplatelets in solution and solid films which is essential for all optical experiments with controlled generated population density upon optical excitation.
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There have been multiple demonstrations of amplified spontaneous emission (ASE) and lasing using colloidal semiconductor nanocrystals. However, it has been proven difficult to achieve low thresholds suitable for practical use of nanocrystals as gain media. Low-threshold blue ASE and lasing from nanocrystals is an even more challenging task. Here, we show that colloidal nanoplatelets (NPLs) with electronic structure of quantum wells can produce ASE in the red, yellow, green, and blue regions of the visible spectrum with low thresholds and high gains. In particular, for blue emitting NPLs, the ASE threshold is 50 µJ/cm2, lower than any reported value for nanocrystals. We then demonstrate red, yellow, green, and blue lasing using NPLs with different thicknesses. We find that the lateral size of NPLs does not show any strong effect on the Auger recombination rates and, correspondingly, on the ASE threshold or gain saturation. This observation highlights qualitative difference of multiexciton dynamics in CdSe NPLs and other quantum-confined CdSe materials, such as quantum dots and rods. Our measurements of the gain bandwidth and gain lifetime further support the prospects of colloidal NPLs as solution-processed optical gain materials.
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We report a comprehensive study on the two-photon absorption cross sections of colloidal CdSe nano-platelets, nano-rods and quantum-dots of different sizes by the means of z-scan and two-photon excitation spectroscopy. Platelets show ten times more efficient two-photon absorption in the quasi-continuum than nano-rods or dots. This unexpectedly strong shape dependence goes beyond the effect of local fields. Further, we discuss the yet unconsidered influence of laser pulse's spectral width on measured TPA cross sections. We also demonstrate that the larger the particles' aspect ratios are (long 1D rods or large 2D platelets), the greater is the confinement related electronic contribution to the increased two-photon absorption. Both, electronic confinement and local field effects make platelets unique two-photon absorbers with outstanding cross sections of up to 10(7) GM, ideally suited for two-photon imaging. The obtained results are confirmed by two independent techniques as well as a new self-referencing method. A comparison with CdS nanoparticles suggests a universal, material independent mechanism of two-photon absorption enhancement.
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Discrete nature of thickness and flat basal planes of two-dimensional (2D) nanostructures display unique diffraction features. Their origin was uncovered by a new analysis method of powder x-ray diffraction, which reveals thickness and lattice orientation of the 2D nanostructures. Results indicate necessity of adoption of a different unit cell from the corresponding bulk crystal with the same internal atomic packing. For CdSe 2D nanostructures with zinc-blende atomic packing, pseudo tetragonal lattices are adequate, instead of face-centered cubic.
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Colloidal nanoplatelets, quasi-two-dimensional quantum wells, have recently been introduced as colloidal semiconductor materials with the narrowest known photoluminescence linewidth (~10 nm). Unfortunately, these materials have not been shown to have continuously-tunable emission, but rather emit at discrete wavelengths that depend strictly on atomic-layer thickness. Herein, we report a new synthesis approach that overcomes this issue: by alloying CdSe colloidal nanoplatelets with CdS, we finely-tune the emission spectrum while still leveraging atomic-scale thickness control. We proceed to demonstrate light emitting diodes with sub-bandgap turn-on voltages (2.1 V for a device emitting at 2.4 eV) and the narrowest electroluminescence spectrum (FWHM ~12.5 nm) reported for colloidal semiconductor LEDs.
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Here, we systematically investigated the spontaneous and stimulated emission performances of solution-processed atomically flat quasi-2D nanoplatelets (NPLs) as a function of their lateral size using colloidal CdSe core NPLs. We found that the photoluminescence quantum efficiency of these NPLs decreases with increasing lateral size while their photoluminescence decay rate accelerates. This strongly suggests that nonradiative channels prevail in the NPL ensembles having extended lateral size, which is well-explained by growing number of defected NPL sub-population. In the case of stimulated emission the role of lateral size in NPLs influentially emerges both in the single- and two-photon absorption (1PA and 2PA) pumping. In the amplified spontaneous emission measurements, we uncovered that the stimulated emission thresholds of 1PA and 2PA exhibit completely opposite behavior with increasing lateral size. The NPLs with larger lateral sizes exhibited higher stimulated emission thresholds under 1PA-pumping due to the dominating defected sub-population in larger NPLs. On the other hand, surprisingly, larger NPLs remarkably revealed lower 2PA-pumped amplified spontaneous emission thresholds. This is attributed to the observation of "gaint" 2PA cross-section overwhelmingly growing with increasing lateral size and reaching record high levels higher than 106 GM, at least an order of magnitude stronger than colloidal quantum dots and rods. These findings suggest that the lateral size control in the NPLs, which is commonly neglected, is essential to high-performance colloidal NPL optoelectronic devices in addition to the vertical monolayer control.
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Colloidal nanocrystals (NCs, i.e., crystalline nanoparticles) have become an important class of materials with great potential for applications ranging from medicine to electronic and optoelectronic devices. Today's strong research focus on NCs has been prompted by the tremendous progress in their synthesis. Impressively narrow size distributions of just a few percent, rational shape-engineering, compositional modulation, electronic doping, and tailored surface chemistries are now feasible for a broad range of inorganic compounds. The performance of inorganic NC-based photovoltaic and light-emitting devices has become competitive to other state-of-the-art materials. Semiconductor NCs hold unique promise for near- and mid-infrared technologies, where very few semiconductor materials are available. On a purely fundamental side, new insights into NC growth, chemical transformations, and self-organization can be gained from rapidly progressing in situ characterization and direct imaging techniques. New phenomena are constantly being discovered in the photophysics of NCs and in the electronic properties of NC solids. In this Nano Focus, we review the state of the art in research on colloidal NCs focusing on the most recent works published in the last 2 years.
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Semiconductors are at the basis of electronics. Up to now, most devices that contain semiconductors use materials obtained from a top down approach with semiconductors grown by molecular beam epitaxy or chemical vapor deposition. Colloidal semiconductor nanoparticles have been synthesized for more than 30 years now, and their synthesis is becoming mature enough that these nanoparticles have started to be incorporated into devices. An important development that recently took place in the field of colloidal quantum dots is the synthesis of two-dimensional (2D) semiconductor nanoplatelets that appear as free-standing nanosheets. These 2D colloidal systems are the newborn in the family of shaped-controlled nanoparticles that started with spheres, was extended with rods and wires, continued with tetrapods, and now ends with platelets.
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Colloidal semiconductor quantum wells, also commonly known as nanoplatelets (NPLs), have arisen among the most promising materials for light generation and harvesting applications. Recently, NPLs have been found to assemble in stacks. However, their emerging characteristics essential to these applications have not been previously controlled or understood. In this report, we systematically investigate and present excitonic properties of controlled column-like NPL assemblies. Here, by a controlled gradual process, we show that stacking in colloidal quantum wells substantially increases exciton transfer and trapping. As NPLs form into stacks, surprisingly we find an order of magnitude decrease in their photoluminescence quantum yield while the transient fluorescence decay is considerably accelerated. These observations are corroborated by ultra-efficient Förster resonance energy transfer (FRET) in the stacked NPLs, in which exciton migration is estimated to be in the ultra-long range (>100 nm). Homo-FRET (i.e., FRET among the same emitters) is found to be ultra-efficient reaching levels as high as 99.9% at room temperature owing to the close-packed collinear orientation of the NPLs along with their large extinction coefficient and small Stokes shift, resulting in a large Förster radius of ~13.5 nm. Consequently, the strong and long-range homo-FRET boosts exciton trapping in non-emissive NPLs, acting as exciton sink centers, quenching photoluminescence from the stacked NPLs due to rapid nonradiative recombination of the trapped excitons. Rate equation based model, which considers the exciton transfer, the radiative and nonradiative recombination within the stacks, shows an excellent match with the experimental data. These results show the critical significance of stacking control in NPL solids, which exhibit completely different signatures of homo-FRET as compared to that in the colloidal nanocrystals due to the absence of inhomogeneous broadening.