Ayelet Teitelboim

Exciton Quenching Due to Copper Diffusion Limits the Photocatalytic Activity of CdS/Cu2S Nanorod Heterostructures.

The formation of donor/acceptor junctions in hybrid nanomaterials is predicted to enhance photocatalytic activity as compared to single-component semiconductor systems. Specifically, nanomaterials containing a junction of n-type cadmium sulfide (CdS) and p-type copper sulfide (Cu2S) formed via cation exchange have been proposed as potential photocatalysts for reactions such as water splitting. Herein, we study the elemental distribution of Cu within these nanostructures using analytical transmission electron microscopy techniques. The resulting effects of this elemental distribution on photocatalytic activity and charge dynamics were further studied using a model photoreduction reaction and transient absorption spectroscopy. We find that copper diffusion in the hybrid nanostructure quenches the exciton lifetime and results in low photocatalytic activity; however, this effect can be partially mitigated via selective extraction. These results provide a deeper understanding of the physical processes within these hybrid nanostructures and will lead to more rational design of photocatalyst materials.

Probing the Interaction of Quantum Dots with Chiral Capping Molecules Using Circular Dichroism Spectroscopy

Circular dichroism (CD) induced at exciton transitions by chiral ligands attached to single component and core/shell colloidal quantum dots (QDs) was used to study the interactions between QDs and their capping ligands. Analysis of the CD line shapes of CdSe and CdS QDs capped with l-cysteine reveals that all of the features in the complex spectra can be assigned to the different excitonic transitions. It is shown that each transition is accompanied by a derivative line shape in the CD response, indicating that the chiral ligand can split the exciton level into two new sublevels, with opposite angular momentum, even in the absence of an external magnetic field. The role of electrons and holes in this effect could be separated by experiments on various types of core/shell QDs, and it was concluded that the induced CD is likely related to interactions of the highest occupied molecular orbitals of the ligands with the holes. Hence, CD was useful for the analysis of hole level–ligand interactions in quantum semiconductor heterostructures, with promising outlook toward better general understanding the properties of the surface of such systems.

Broadband Near-Infrared to Visible Upconversion in Quantum Dot–Quantum Well Heterostructures

Upconversion is a nonlinear process in which two, or more, long wavelength photons are converted to a shorter wavelength photon. It holds great promise for bioimaging, enabling spatially resolved imaging in a scattering specimen and for photovoltaic devices as a means to surpass the Shockley-Queisser efficiency limit. Here, we present dual near-infrared and visible emitting PbSe/CdSe/CdS nanocrystals able to upconvert a broad range of NIR wavelengths to visible emission at room temperature. The synthesis is a three-step process, which enables versatility and tunability of both the visible emission color and the NIR absorption edge. Using this method, one can achieve a range of desired upconverted emission peak positions with a suitable NIR band gap.

Colloidal Double Quantum Dots

Pairs of coupled quantum dots with controlled coupling between the two potential wells serve as an extremely rich system, exhibiting a plethora of optical phenomena that do not exist in each of the isolated constituent dots. Over the past decade, coupled quantum systems have been under extensive study in the context of epitaxially grown quantum dots (QDs), but only a handful of examples have been reported with colloidal QDs. This is mostly due to the difficulties in controllably growing nanoparticles that encapsulate within them two dots separated by an energetic barrier via colloidal synthesis methods. Recent advances in colloidal synthesis methods have enabled the first clear demonstrations of colloidal double quantum dots and allowed for the first exploratory studies into their optical properties. Nevertheless, colloidal double QDs can offer an extended level of structural manipulation that allows not only for a broader range of materials to be used as compared with epitaxially grown counterparts but also for more complex control over the coupling mechanisms and coupling strength between two spatially separated quantum dots. The photophysics of these nanostructures is governed by the balance between two coupling mechanisms. The first is via dipole-dipole interactions between the two constituent components, leading to energy transfer between them. The second is associated with overlap of excited carrier wave functions, leading to charge transfer and multicarrier interactions between the two components. The magnitude of the coupling between the two subcomponents is determined by the detailed potential landscape within the nanocrystals (NCs). One of the hallmarks of double QDs is the observation of dual-color emission from a single nanoparticle, which allows for detailed spectroscopy of their properties down to the single particle level. Furthermore, rational design of the two coupled subsystems enables one to tune the emission statistics from single photon emission to classical emission. Dual emission also provides these NCs with more advanced functionalities than the isolated components. The ability to better tailor the emission spectrum can be advantageous for color designed LEDs in lighting and display applications. The different response of the two emission colors to external stimuli enables ratiometric sensing. Control over hot carrier dynamics within such structures allows for photoluminescence upconversion. This Account first provides a description of the main hurdles toward the synthesis of colloidal double QDs and an overview of the growing library of synthetic pathways toward constructing them. The main discoveries regarding their photophysical properties are then described in detail, followed by an overview of potential applications taking advantage of the double-dot structure. Finally, a perspective and outlook for their future development is provided.

Highly luminescent CuGaxIn1−xSySe2−y nanocrystals from organometallic single-source precursors

The solution-based thermolysis of the organometallic single-source precursors [(iPr3PCu)4(MeGa)4S6] (1) and [(iPr3PCu)4(MeIn)4Se6] (2) is investigated. Multinary semiconductor nanocrystals with sizes in the range of 3–10 nm are obtained by a simple heating-up process in a solvent mixture of long-chain alkyl thiols and amines. The thiols also serve as a sulfur source and capping ligand for the nanoparticles. By mixing 1 and 2, nanocrystal compositions in the range from CuGaS2 to CuInSSe are accessible. Surface passivation of the nanocrystals with ZnS results in high stability and bright photoluminescence (PL). PL maxima are observed in the spectral range between 600 and 800 nm, depending on the size and composition of the nanocrystals. The highest PL quantum yields exceed 50% and are observed for a gallium to indium ratio of 1 : 5.

All-inorganic colloidal upconversion quantum dots (Conference Presentation)

Upconversion (UC) is a nonlinear process in which two, or more, long wavelength photons are converted to a shorter wavelength photon. This process is based on sequential absorption of two or more photons, involving metastable, long lived intermediate energy states, thus is not restricted to ultrashort pulsed excitation. Hence, requirements for UC processes are long lived excited states, a ladder like arrangement of energy levels and a mechanism inhibiting cooling of the hot charge carrier. UC holds great promise for bioimaging, enabling to perform multiphoton imaging in scattering specimen at very low powers. Rare-earth-doped nanocrystals, the most commonly used ones for UC, typically require a minimal particle diameter of several tens of nanometers and have a limited action spectrum. Here, we present a novel luminescence upconversion nano-system based on colloidal semiconductor double quantum dots, consisting of a NIR-absorbing component and a visible emitting component separated by a tunneling barrier in a spherical onion-like geometry. These dual near-infrared and visible core/shell/shell PbSe/CdSe/CdS nanocrystals are shown to efficiently upconvert a broad range of NIR wavelengths up to 1.2 microns to visible emission at room temperature, covering a spectral range where there are practically no alternative upconversion systems. The particle diameter is less than ten nanometers, and the synthesis enables versatility and tunability of both the visible emission color and the NIR absorption edge. The physical mechanism for upconversion in this type of structures, as well as potential advances and extensions on this system will be discussed.

Broadband near-infrared to visible upconversion in quantum dot-quantum well heterostructures(Conference Presentation)

Upconversion (UC) is a nonlinear process in which two, or more, long wavelength photons are converted to a shorter wavelength photon. This process is based on sequential absorption of two or more photons, involving metastable, long lived intermediate energy states, thus is not restricted to upconversion of coherent laser radiation as a non-coherent process. Hence, requirements for UC processes are long lived excited states, a ladder like arrangement of energy levels and a mechanism inhibiting cooling of the hot charge carrier. UC holds great promise for bioimaging, enabling spatially resolved imaging in a scattering specimen and for photovoltaic devices as a mean to surpass the Shockley-Queisser efficiency limit. Here, we present a novel luminescence upconversion nano-system based on colloidal semiconductor double quantum dots, consisting of a NIR-emitting component and a visible emitting component separated by a tunneling barrier in a spherical onion-like geometry. These dual near-infrared and visible emitting core/shell/shell PbSe/CdSe/CdS nanocrystals are shown to upconvert a broad range of NIR wavelengths to visible emission at room temperature, covering a spectral range where there are practically no alternative upconversion systems. The synthesis is a three-step process, which enables versatility and tunability of both the visible emission color and the NIR absorption edge. Using this method one can achieve a range of desired upconverted emission peak positions with a suitable NIR band gap. The physical mechanism for upconversion in this structure, as well as possible extensions and improvements will be discussed. 1 (1) Teitelboim, A.; Oron, D. ACS Nano 2015, acsnano.5b05329.

Quantum dots: using the known as well as exploring the unknown

Super-resolution microscopy, the imaging of features below the Abbe diffraction limit, has been achieved by a number of methods in recent years. Each of these methods relies on breaking one of the assumptions made in the derivation of the diffraction limit. While uniform spatial illumination, linearity and time independence have been the most common cornerstones of the Abbe limit broken in super-resolution modalities, breaking the ‘classicality of light’ assumption as a pathway to achieve super-resolution has not been shown. Here we demonstrate a method that utilizes the antibunching characteristic of light emitted by Quantum Dots (QDs), a purely quantum feature of light, to obtain imaging beyond the diffraction limit. Measuring such high order correlations in the emission of a single QD necessitates stability at saturation conditions while avoiding damage and enhanced blinking. This ability was facilitated through new understandings that arisen from exploring the QD ‘blinking’ phenomena. We summarize here two studies that contributed to our current understanding of QD stability.