Colloidal semiconductor quantum structures allow controlling the strong confinement of charge carriers through material composition and geometry. Besides being a unique platform to study fundamental effects, these materials attracted considerable interest due to their potential in opto-electronic and quantum communication applications. Heteronanostructures like CdSe/CdS offer new prospects to tailor their optical properties as they take advantage of a small conduction band offset allowing tunability of the electron delocalization from type-I toward quasi-type-II. Here, we report on a detailed study of the exciton recombination dynamics in CdSe/CdS heterorods. We observed a clear size-dependent radiative lifetime, which can be linked to the different degree of electron wave function (de)localization. Moreover, by increasing the temperature from 70 to 300 K, we observed a considerable increase of the radiative lifetime, clearly demonstrating a reduction of the conduction band offset at higher temperatures. Understanding and controlling electron delocalization in such heterostructures will be pivotal for realizing efficient and low-cost photonic devices.

We propose a direct method to determine absorption anisotropy of colloidal quantum rods. In this method, the rods are aligned in solution by using an alternating electric field and we measure simultaneously the resulting average change in absorption. We show that a frequency window for the electric field exists in which the change in absorbance as a function of field strength can be analyzed in terms of the quantum-rod dipole moment and the absorption coefficient for light that is polarized parallel or perpendicular to the long axis of the rod. The approach is verified by measuring the absorbance change of CdSe rods at 400 nm as a function of field strength, where we demonstrate excellent agreement between experiment and theory. This enables us to propose improved values for the CdSe quantum-rod extinction coefficient. Next, we analyze CdSe/CdS dot-in-rods and find that the absorption of the first exciton transition is fully anisotropic, with a vanishing absorption coefficient for light that is polarized perpendicularly to the long axis of the rods.

We analyze the optical properties of CdTe quantum dots, including the sizing curve, the absorption coefficient, and the oscillator strength of the band gap transition, by combining absorption spectroscopy, elemental analysis, and electron microscopy imaging. At short wavelengths, the absorption coefficient spectrum is still affected by quantum confinement, yet a largely constant value, close to that of bulk CdTe, is found at around 410 nm. At shorter wavelengths, remaining quantum confinement effects on the CdTe E 1 transition are present even for the largest quantum dots studied (11 nm). For the band gap transition, we find an integrated absorption coefficient μ gap that scales almost proportionally to the inverse of the quantum dot volume. Especially for the smaller diameters, deviations up to a factor of 3 are found as compared to widely used literature values. The corresponding oscillator strength f gap is almost size-independent in the diameter range 3−7 nm. The correspondence between radiative lifetimes predicted based on f gap and literature values is discussed.

Pb cations in PbS quantum rods made from CdS quantum rods by successive complete cationic exchange reactions are partially re-exchanged for Cd cations. Using STEM-HAADF, we show that this leads to the formation of unique multiple dot-in-rod PbS/CdS heteronanostructures, with a photoluminescence quantum yield of 45−55%. We argue that the formation of multiple dot-in-rods is related to the initial polycrystallinity of the PbS quantum rods, where each PbS crystallite transforms in a separate PbS/CdS dot-in-dot. Effective mass modeling indicates that electronic coupling between the different PbS conduction band states is feasible for the multiple dotin-rod geometries obtained, while the hole states remain largely uncoupled. C olloidal quantum rods (QRs) are an attractive class of nanocrystals. Either grown out of a single material or as a heterostructure, the combination of quantum confinement and shape anisotropy results in materials with tunable, anisotropic opto-electronic properties. A typical example is heterogeneous CdSe/CdS dot-in-rods.

The use of Langmuir films of hydrophobic colloidal nanoparticles (NPs) on water surfaces followed by their deposition on a solid substrate is an increasingly popular way of forming 2D nanoparticle superstructures. [1][2][3] This is mainly inspired by practical advantages. Langmuir-Blodgett (LB) or LangmuirSchaefer (LS) deposition, in which the Langmuir film is transferred either vertically (LB) or horizontally (LS) to a substrate, leads to well-defined, large-area NP monolayers with a tunable particle density and is not limited to flat substrates. Especially for colloidal quantum dots (QDs), this is a highly attractive processing method because applications such as light-emitting diodes (LEDs), [4] photodetectors, [5] biosensors, [6] and light-harvesting devices [7] require highquality QD mono-or multilayers. On the other hand, Langmuir films of amphiphilic molecules exhibit a rich and intriguing phase behavior. Langmuir films of saturated carboxylic acids, for example, show a first-order phase transition between a gaseous and a condensed phase that appears as a plateau in the pressure-area p-A isotherm. [8] Similar observations on colloidal NP Langmuir films are rare. Experiments and simulations involving further compression of a NP monolayer usually resulted in layer buckling, [9] or gave only indirect indications of phase transitions. [10,11] Here, we show that the continued compression of monolayers of a variety of colloidal QDs leads to a sequence of plateaus in the p-A isotherm. Using transmission electron microscopy (TEM) and atomic force microscopy (AFM), we show that these plateaus correspond to the consecutive formation of a QD double and triple layer. Moreover, we argue that these transformations correspond to phase transitions in the Langmuir film that can be rationalized based on thermodynamic considerations.A typical p-A isotherm of 4.0 nm CdSe QDs (Q-CdSe, standard deviation (s) of 6.9 %) on the air/water interface contains different regions (Figure 1). Regions I and II show the usual transition from monolayer islands into a full monolayer.[3] Around 17 mN m À1 the slope of the isotherm decreases, indicating layer collapse (i.e. transfer of particles out of the monolayer). However, further compression only leads to a minor pressure increase, creating a plateau in the isotherm (III). Similar plateaus have been reported in studies of Langmuir layers of amphiphilic molecules [12] and hydrophilic Au nanocrystals (NCs), [10] and indicate a phase transition in the layer. When the surface area of the trough is reduced to half the area of the initial full monolayer (A 1 ), a second increase of the surface pressure is observed (IV), followed by another plateau (V) and again a pressure increase (VI) starting at about A 1 /3. With bigger CdSe/CdS core/shell particles (6.4 nm, s = 5.4 %), the first plateau occurs at a slightly higher pressure (21 mN m À1 ) and is not entirely flat. No second plateau was observed with these particles. In both systems, further compression is irreversible once the first plateau i...

The use of Langmuir films of hydrophobic colloidal nanoparticles (NPs) on water surfaces followed by their deposition on a solid substrate is an increasingly popular way of forming 2D nanoparticle superstructures. [1][2][3] This is mainly inspired by practical advantages. Langmuir-Blodgett (LB) or Langmuir-Schaefer (LS) deposition, in which the Langmuir film is transferred either vertically (LB) or horizontally (LS) to a substrate, leads to well-defined, large-area NP monolayers with a tunable particle density and is not limited to flat substrates. Especially for colloidal quantum dots (QDs), this is a highly attractive processing method because applications such as light-emitting diodes (LEDs), [4] photodetectors, [5] biosensors, [6] and light-harvesting devices [7] require highquality QD mono-or multilayers. On the other hand, Langmuir films of amphiphilic molecules exhibit a rich and intriguing phase behavior. Langmuir films of saturated carboxylic acids, for example, show a first-order phase transition between a gaseous and a condensed phase that appears as a plateau in the pressure-area p-A isotherm. [8] Similar observations on colloidal NP Langmuir films are rare. Experiments and simulations involving further compression of a NP monolayer usually resulted in layer buckling, [9] or gave only indirect indications of phase transitions. [10,11] Here, we show that the continued compression of monolayers of a variety of colloidal QDs leads to a sequence of plateaus in the p-A isotherm. Using transmission electron microscopy (TEM) and atomic force microscopy (AFM), we show that these plateaus correspond to the consecutive formation of a QD double and triple layer. Moreover, we argue that these transformations correspond to phase transitions in the Langmuir film that can be rationalized based on thermodynamic considerations.A typical p-A isotherm of 4.0 nm CdSe QDs (Q-CdSe, standard deviation (s) of 6.9 %) on the air/water interface contains different regions (Figure 1). Regions I and II show the usual transition from monolayer islands into a full monolayer. [3] Around 17 mN m À1 the slope of the isotherm decreases, indicating layer collapse (i.e. transfer of particles out of the monolayer). However, further compression only leads to a minor pressure increase, creating a plateau in the isotherm (III). Similar plateaus have been reported in studies of Langmuir layers of amphiphilic molecules [12] and hydrophilic Au nanocrystals (NCs), [10] and indicate a phase transition in the layer. When the surface area of the trough is reduced to half the area of the initial full monolayer (A 1 ), a second increase of the surface pressure is observed (IV), followed by another plateau (V) and again a pressure increase (VI) starting at about A 1 /3. With bigger CdSe/CdS core/shell particles (6.4 nm, s = 5.4 %), the first plateau occurs at a slightly higher pressure (21 mN m À1 ) and is not entirely flat. No second plateau was observed with these particles. In both systems, further compression is irreversible once the first plateau...

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