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Mixed‐dimensional systems host interesting phenomena that involve electron and ion transport along or across the interface, with promising applications in optoelectronic and electrochemical devices. Herein, a heterosystem consisting of a graphene monolayer with a colloidal Ag2S nanocrystal film atop, in which both ions and electrons are involved in photoelectrical effects, is studied. An investigation of the transport at the interface in different configurations by using a phototransistor configuration with graphene as a charge‐transport layer and semiconductor nanocrystals as a light‐sensitive layer is performed. The key feature of charge transfer is investigated as a function of gate voltage, frequency, and incident light power. A simple analytical model of the photoresponse is developed, to gain information on the device operation, revealing that the nanocrystals transfer electrons to graphene in the dark, but the opposite process occurs upon illumination. A frequency‐dependence analysis suggests a fractal interface between the two materials. This interface can be modified using solid‐state electrochemical reactions, leading to the formation of metallic Ag particles, which affect the graphene properties by additional doping, while keeping the photoresponse. Overall, these results provide analytical tools and guidelines for the evaluation of coupled electron/ion transport in hybrid systems. [ABSTRACT FROM AUTHOR]

We propose here a new colloidal approach for the synthesis of both all-inorganic and hybrid organic-inorganic lead halide perovskite nanocrystals (NCs). The main limitation of the protocols that are currently in use, such as the hot injection and the ligand-assisted reprecipitation routes, is that they employ PbX2 (X = Cl, Br, or I) salts as both lead and halide precursors. This imposes restrictions on being able to precisely tune the amount of reaction species and, consequently, on being able to regulate the composition of the final NCs. In order to overcome this issue, we show here that benzoyl halides can be efficiently used as halide sources to be injected in a solution of metal cations (mainly in the form of metal carboxylates) for the synthesis of APbX[sub 3] NCs (in which A = Cs+, CH3NH3+, or CH(NH[sub 2])[sub 2]+). In this way, it is possible to independently tune the amount of both cations and halide precursors in the synthesis. The APbX3 NCs that were prepared with our protocol show excellent optical properties, such as high photoluminescence quantum yields, low amplified spontaneous emission thresholds, and enhanced stability in air. It is noteworthy that CsPbI3 NCs, which crystallize in the cubic α phase, are stable in air for weeks without any postsynthesis treatment. The improved properties of our CsPbX3 perovskite NCs can be ascribed to the formation of lead halide terminated surfaces, in which Cs cations are replaced by alkylammonium ions. [ABSTRACT FROM AUTHOR]

The integration of colloidal nanocrystals with polymers adds optoelectronic functionalities to flexible and mechanically robust organic films. In particular, self-assembled structures of nanocrystals in polymers can act as functional components enhancing, for instance, transport or optical properties of the hybrid material. This study presents Cu2Te hexagonal nanodisks that assemble into ribbons with a face-to-face configuration in poly(3-hexylthiophene-2,5-diyl) through a controlled solvent evaporation process. The ribbons form weaving patterns that create 3D networks fully embedded in the thin polymer film at high nanodisk concentration. The photoresponse of these composite films measured in a layered vertical geometry demonstrates increased photocurrent with increasing nanocrystal loading. This study attributes this behavior to the presence of networks of Cu2Te nanodisks that form a bulk heterojunction with the semiconducting polymer, which improves exciton dissociation and the overall photoelectric response. [ABSTRACT FROM AUTHOR]

A novel colloidal synthesis of copper selenide nanosheets (NSs) with lateral dimensions of up to 3 μm is developed. This material is used for the fabrication of flexible conductive films prepared via simple drop-casting of the NS dispersions without any additional treatment. The electrical performance of these coatings is benchmarked against copper selenide spherical nanocrystals (SNCs) in order to demonstrate the advantage of 2D morphology of the NSs for flexible electronics. In this contest, Cu2−xSe SNC films exhibit higher conductivity but lower reproducibility due to the formation of cracks leading to discontinuous films. Furthermore, the electrical properties of the films deposited on different flexible substrates following their bending, stretching and folding are studied. A comparison of Cu2−xSe SNC and CuSe NS films reveals an increased stability of the CuSe NS films under mechanical stress applied to the samples and their improved long-term stability in air. [ABSTRACT FROM AUTHOR]

The deposition of water solution containing colloidal nanorods on superhydrophobic substrates leads to the formation of a suspended drop. After evaporation, a robust 3D nanocrystal assembly remains and can be handled independently from the substrate, for example for the use as color converters. Optical fluorescence imaging reveals a hollow dome‐shape of the microstructure, and X‐ray diffraction confirms the material as CdSe/CdS nanorods with low contamination. [ABSTRACT FROM AUTHOR]

Abstract: Inorganic nanoparticles are among the most investigated nano-objects nowadays, both in fundamental science and in various technological applications. In this paper, we describe the physical properties of nanoparticles when their shape is elongated, i.e. rod-like or wire-like. In most cases, we will analyze the transition in the physical properties when the shape of the nanoparticles evolves from spherical to rod-like. [Copyright &y& Elsevier]

Colloidal core/shell nanocrystals are key materials for optoelectronics, enabling control over essential properties via precise engineering of the shape, thickness, and crystal structure of their shell. Here, the growth protocol for CdS branched nanocrystals is applied on CdSe nanoplatelet seeds and bone‐shaped heterostructures are obtained with a highly anisotropic shell. Surprisingly, the nanoplatelets withstand the high growth temperature of 350 °C and structures with a CdSe nanoplatelet core that is overcoated by a shell of cubic CdS are obtained, on top of which tetrahedral CdS structures with hexagonal lattice are formed. These complex core/shell nanocrystals show a band‐edge emission around 657 nm with a photoluminescence quantum yield of ≈42% in solution, which is also retained in thin films. Interestingly, the nanocrystals manifest simultaneous red and green emission and the relatively long wavelength of the green emission indicates charge recombination at the cubic/hexagonal interface of the CdS shell. The nanocrystal films show amplified spontaneous emission, random lasing, and distributed feedback lasing when the material is deposited on suitable gratings. This work stimulates the design and fabrication of more exotic core/shell heterostructures where charge carrier delocalization, dipole moment, and other optical and electrical properties can be engineered. [ABSTRACT FROM AUTHOR]

We studied the carrier confinement in tetrapod-shaped colloidal CdTe nanocrystals by means of absorption, photoluminescence, and photoluminescence excitation spectroscopy at room and cryogenic temperatures. The spectra show features characteristic of the tetrapod shape together with a clear dependence on the dominant confinement parameter, i.e., the diameter of the tetrapod arm. Theoretical calculations based on an envelope-function approximation and using the exact tetrapod shape have been performed to assign the observed spectral features. Oscillator strength and size dependence of the transitions energy have been calculated showing a direct correlation between the oscillator strength and the nanocrystal shape. [ABSTRACT FROM AUTHOR]

PbS nanocrystals are an important narrow-gap material for solar cells and photodetectors. Nevertheless, their application may be limited because device performance can be affected by atmospheric conditions. Indeed, the presence of oxygen and/or water can degrade the active layers, possibly leading to device failure. Strategies to address this issue are therefore actively explored. Here we report a solution-processed PbS quantum dot solar cell, consisting of a PbS-silane functionalized reduced graphene oxide (PbS-rGO) layer on top of the PbS absorber film, which enhances device stability, especially when the solar cells are exposed to moisture. Power conversion efficiency (PCE) measurements demonstrate a slower degradation under continuous illumination for solar cells with PbS-rGO. When storing the samples under saturated water vapor, differences are even more remarkable: with PbS-rGO the solar cells essentially maintain their initial PCE, while the PCE of the PbS reference devices is reduced by 50% after 5 days. Scanning electron microscopy, energy dispersive X-ray and X-ray photoelectron spectroscopy reveal the damage to the PbS films and the formation of PbSO x crystals in the PbS reference devices. Such crystals are not observed in the PbS-rGO devices, further supporting the importance of the PbS-rGO barrier layer. [ABSTRACT FROM AUTHOR]

Lattice vibrational modes in cadmium chalcogenide nanocrystals (NCs) have a strong impact on the carrier dynamics of excitons in such confined systems and on the optical properties of these nanomaterials. A prominent material for light emitting applications are CdSe/CdS core-shell dot-in-rods. Here we present a detailed investigation of the acoustic phonon modes in such dot-in-rods by nonresonant Raman spectroscopy with laser excitation energy lower than their bandgap. With high signal-to-noise ratio in the frequency range from 5-50 cm-1, we reveal distinct Raman bands that can be related to confined extensional and radial-breathing modes (RBM). Comparison of the experimental results with finite elements simulation and analytical analysis gives detailed insight into the localized nature of the acoustic vibration modes and their resonant frequencies. In particular, the RBM of dot-in-rods cannot be understood by an oscillation of a CdSe sphere embedded in a CdS rod matrix. Instead, the dot-in-rod architecture leads to a reduction of the sound velocity in the core region of the rod, which results in a redshift of the rod RBM frequency and localization of the phonon induced strain in vicinity of the core where optical transitions occur. Such localized effects potentially can be exploited as a tool to tune exciton-phonon coupling in nanocrystal heterostructures. [ABSTRACT FROM AUTHOR]

In this Letter we report patterningof colloidal nanocrystal filmsthat combines direct e-beam (electron beam) writing with cation exchange.The e-beam irradiation causes cross-linking of the ligand moleculespresent at the nanocrystal surface, and the cross-linked moleculesact as a mask for further processing. Consequently, in the followingstep of cation exchange, which is performed by directly dipping thesubstrate in a solution containing the new cations, the regions thathave not been exposed to the electron beam are chemically transformed,while the exposed ones remain unchanged. This selective protectionallows the design of patterns that are formed by chemically differentnanocrystals, yet in a homogeneous nanocrystal film. Spatially resolvedcompositional analysis by energy-dispersive X-ray spectroscopy (EDS)corroborates that the selective exchange occurs only in the nonirradiatedregions. We demonstrate the utility of this lithography approach byfabricating conductive wires and luminescent patterns in CdSe/CdSnanocrystal films by converting nonirradiated regions to Cu2–xSe/Cu2–xS. Furthermore,we show that X-ray irradiation too can lead to protection from cationexchange. [ABSTRACT FROM AUTHOR]

We report a phosphine-free colloidalsynthesis of CuInxGa1–xS2(CIGS) nanocrystals (NCs) by heatinga mixture of metal salts, elementalsulfur, octadecene, and oleylamine. In contrast with the more commonlyused hot injection, this procedure is highly suitable for large-scaleNC production, which we tested by performing a gram-scale synthesis.The composition of the CIGS NCs could be tuned by varying the In andGa precursor ratios, and the samples showed a composition-dependentband gap energy. The average particle size was scaled from 13 to 19nm by increasing the reaction temperature from 230 to 270 °C.Two concomitant growth mechanisms took place: in one, covellite (CuS)NCs nucleated already at room temperature and then incorporated increasingamounts of In and Ga until they evolved into chalcopyrite CIGS NCs.In the second mechanism, CIGS NCs directly nucleated at intermediatetemperatures. They were smaller than the NCs formed by the first mechanism,but richer in In and Ga. In the final sample, obtained by prolongedheating at 230–270 °C, all NCs were homogeneous in sizeand composition. Attempts to replace the native ligands on the surfaceof the NCs with sulfur ions (following literature procedures) resultedin only around 50% exchange. Films prepared using the partially ligandexchanged NCs exhibited good homogeneity and an ohmic dark conductivityand photoconductivity with a resistivity of about 50 Ω·cm. [ABSTRACT FROM AUTHOR]

The article presents that study that investigates the efficacy of phosphine-free synthesis of p-type copper(I) Selenide nanocrystals as reductant of Selenium and ligand for nanocrystals in the U.S. Researchers found the method to be effective because they have avoided the use of phospines in synthesizing Copper(I)-Selenide. Moreover, they found that nanocrystals manifest distinct optical absorption should in ultraviolet.

The photoconductive properties of nanocrystal solids have so far been described by an empirical model based on phenomenological parameters such as sheet resistance and voltage barrier or by a microscopic approach that considered the local exciton ionization but disregarded the transport pathway needed to harvest the charges. Here, we have studied the photocurrent in thin films of densely packed nanocrystals using scanning photocurrent microscopy with a spatial resolution of 200 nm, and we present a microscopic model to fit our data that includes both the exciton ionization process and the influence of the charge transport pathway of the photoexcited charges. With this theoretical framework, physical parameters such as the product of mobility and lifetime of the charge carriers in the nanocrystal film can be assessed. [ABSTRACT FROM AUTHOR]