Savas Delikanli

Remote control of ion channels and neurons through magnetic-field heating of nanoparticles

Recently, optical stimulation has begun to unravel the neuronal processing that controls certain animal behaviours. However, optical approaches are limited by the inability of visible light to penetrate deep into tissues. Here, we show an approach based on radio-frequency magnetic-field heating of nanoparticles to remotely activate temperature-sensitive cation channels in cells. Superparamagnetic ferrite nanoparticles were targeted to specific proteins on the plasma membrane of cells expressing TRPV1, and heated by a radio-frequency magnetic field. Using fluorophores as molecular thermometers, we show that the induced temperature increase is highly localized. Thermal activation of the channels triggers action potentials in cultured neurons without observable toxic effects. This approach can be adapted to stimulate other cell types and, moreover, may be used to remotely manipulate other cellular machinery for novel therapeutics.

Amplified Spontaneous Emission and Lasing in Colloidal Nanoplatelets

Colloidal nanoplatelets (NPLs) have recently emerged as favorable light-emitting materials, which also show great potential as optical gain media due to their remarkable optical properties. In this work, we systematically investigate the optical gain performance of CdSe core and CdSe/CdS core/crown NPLs having different CdS crown size with one- and two-photon absorption pumping. The core/crown NPLs exhibit enhanced gain performance as compared to the core-only NPLs due to increased absorption cross section and the efficient interexciton funneling, which is from the CdS crown to the CdSe core. One- and two-photon absorption pumped amplified spontaneous emission thresholds are found as low as 41 μJ/cm(2) and 4.48 mJ/cm(2), respectively. These thresholds surpass the best reported optical gain performance of the state-of-the-art colloidal nanocrystals (i.e., quantum dots, nanorods, etc.) emitting in the same spectral range as the NPLs. Moreover, gain coefficient of the NPLs is measured as high as 650 cm(-1), which is 4-fold larger than the best reported gain coefficient of the colloidal quantum dots. Finally, we demonstrate a two-photon absorption pumped vertical cavity surface emitting laser of the NPLs with a lasing threshold as low as 2.49 mJ/cm(2). These excellent results are attributed to the superior properties of the NPLs as optical gain media.

Room temperature ferromagnetism in Mn-doped CdS nanorods

Mn-doped CdS nanorods synthesized by solution phase chemistry demonstrate robust ferromagnetic properties at and above room temperature. The nanorods show large coercivity, possibly originating from the shape anisotropy. The hysteresis measurements reveal strong temperature dependence. Possible origin of the ferromagnetism is discussed.

Mn(2+)-Doped CdSe/CdS Core/Multishell Colloidal Quantum Wells Enabling Tunable Carrier-Dopant Exchange Interactions.

In this work, we report the manifestations of carrier-dopant exchange interactions in colloidal Mn(2+)-doped CdSe/CdS core/multishell quantum wells. The carrier-magnetic ion exchange interaction effects are tunable through wave function engineering. In our quantum well heterostructures, manganese was incorporated by growing a Cd0.985Mn0.015S monolayer shell on undoped CdSe nanoplatelets using the colloidal atomic layer deposition technique. Unlike previously synthesized Mn(2+)-doped colloidal nanostructures, the location of the Mn ions was controlled with atomic layer precision in our heterostructures. This is realized by controlling the spatial overlap between the carrier wave functions with the manganese ions by adjusting the location, composition, and number of the CdSe, Cd1-xMnxS, and CdS layers. The photoluminescence quantum yield of our magnetic heterostructures was found to be as high as 20% at room temperature with a narrow photoluminescence bandwidth of ∼22 nm. Our colloidal quantum wells, which exhibit magneto-optical properties analogous to those of epitaxially grown quantum wells, offer new opportunities for solution-processed spin-based semiconductor devices.

Carrier-dopant exchange interactions in Mn-doped PbS colloidal quantum dots

Carrier-dopant exchange interactions in Mn-doped PbS colloidal quantum dots were studied by circularly polarized magneto-photoluminescence. Mn substitutional doping leads to paramagnetic behavior down to 5 K. While undoped quantum dots show negative circular polarization, Mn doping changes its sign to positive. A circular polarization value of 40% was achieved at T = 7 K and B = 7 tesla. The results are interpreted in terms of Zeeman splitting of the band edge states in the presence of carrier-dopant exchange interactions that are qualitatively different from the s,p-d exchange interactions in II-VI systems.

Near‐Unity Emitting Copper‐Doped Colloidal Semiconductor Quantum Wells for Luminescent Solar Concentrators

Doping of bulk semiconductors has revealed widespread success in optoelectronic applications. In the past few decades, substantial effort has been engaged for doping at the nanoscale. Recently, doped colloidal quantum dots (CQDs) have been demonstrated to be promising materials for luminescent solar concentrators (LSCs) as they can be engineered for providing highly tunable and Stokes-shifted emission in the solar spectrum. However, existing doped CQDs that are aimed for full solar spectrum LSCs suffer from moderately low quantum efficiency, intrinsically small absorption cross-section, and gradually increasing absorption profiles coinciding with the emission spectrum, which together fundamentally limit their effective usage. Here, the authors show the first account of copper doping into atomically flat colloidal quantum wells (CQWs). In addition to Stokes-shifted and tunable dopant-induced photoluminescence emission, the copper doping into CQWs enables near-unity quantum efficiencies (up to ≈97%), accompanied by substantially high absorption cross-section and inherently step-like absorption profile, compared to those of the doped CQDs. Based on these exceptional properties, the authors have demonstrated by both experimental analysis and numerical modeling that these newly synthesized doped CQWs are excellent candidates for LSCs. These findings may open new directions for deployment of doped CQWs in LSCs for advanced solar light harvesting technologies.

Time-resolved photoluminescence study of CdSe/CdMnS/CdS core/multi-shell nanoplatelets

We used photoluminescence spectroscopy to resolve two emission features in CdSe/CdMnS/CdS and CdSe/CdS core/multi-shell nanoplatelet heterostructures. The photoluminescence from the magnetic sample has a positive circular polarization with a maximum centered at the position of the lower energy feature. The higher energy feature has a corresponding signature in the absorption spectrum; this is not the case for the low-energy feature. We have also studied the temporal evolution of these features using a pulsed-excitation/time-resolved photoluminescence technique to investigate their corresponding recombination channels. A model was used to analyze the temporal dynamics of the photoluminescence which yielded two distinct timescales associated with these recombination channels. The above results indicate that the low-energy feature is associated with recombination of electrons with holes localized at the core/shell interfaces; the high-energy feature, on the other hand, is excitonic in nature with the holes confined within the CdSe cores.

Inverted Type-I CdS/CdSe Core/Crown colloidal quantum ring

Inverted Type-I quantum rings (QRs) are a recently developed class of nanostructure in which a lower bandgap material is laterally grown as a crown of same thickness around a higher bandgap nanoplatelet (NPL) core [l·2]· e.g. core/crown CdS/CdSe as shown in Fig. 1(a). For this work, we have colloidally synthesized 3 monolayer (ML) thick CdS/CdSe QR samples using seed-mediated method, by using 3ML CdS NPLs as the core seed [3]. Subsequently, continuous injection of Se precursor, elemental Se dissolved in octadecene (ODE), Cd precursor and Cd(CH3CO2)2 led to the growth of CdSe crown. The ring width was controlled by tuning the injection amount to regulate the extent of the lateral size of the crown coating. A TEM image of the 3ML CdS/CdSe QRs is shown in Fig. 1(b)· which suggests atomically flat lateral growth of CdSe on CdS NPLs, as it has larger average lateral size (30±10 nm) compared to the CdS core only NPLs. We have measured the photoluminescence (PL) and absorption spectra of CdS/CdSe QRs at different phases of crown growth. Fig. 1(c) shows the PL and absorption spectra for two contrasting cases having a (i) thin, and a (ii) thick CdSe crown. As the crown thickness increases, there is lesser extent of lateral confinement, and thus a red-shift in the PL peak position. Also the linewidth of the emission spectra narrows down as more CdSe is deposited in the crown. This is owing to the randomly dispersed CdSe islands of varying sizes formed around the CdS NPLs at the initial phase of the CdSe crown growth resulting in a normal distribution of energy levels. For a sufficiently thick CdSe crown, the electronic structure and optical properties of CdS/CdSe QR is identical to CdSe NPLs alone, characterized by narrower linewidths. Moreover, we have verified that the emission peak of the 3ML CdS/CdSe QRs is spectrally tunable between the peak emissions of 3ML CdS NPL core only (382 nm) and 3 ML CdSe NPL core only (462 nm). In terms of the absorption spectra, as we begin to grow the CdSe crown, there is a gradual emergence of the primary and secondary CdSe absorption peaks at 460 nm (electron-heavy hole) and 431 nm (electron-light hole) respectively, as can be seen in Fig. 1(c-ii). On the other hand, for thin CdSe crowns, these peaks are undeterminable, while the overlapping primary and secondary absorption peaks of CdS at 382 nm (electron-heavy hole) and 373 nm (electron-light hole) are more prominent, as can be seen in Fig. 1(c-i).

Time resolved photoluminescence study of magnetic CdSe/CdMnS/CdS core/multi-shell nanoplatelets

Colloidal semiconductor nanoplatelets (NPLs) are quasi 2D-nanostructures that are grown and processed inexpensively using a solution based method and thus have recently attracted considerable attention. We observe two features in the photoluminescence spectrum, suggesting two possible recombination channels. Their intensity ratio varies with temperature and two distinct temperature regions are identified; a low temperature region (10K < T < 90K) and a high temperature region (90K < T < 200K). This ratio increases with increasing temperature, suggesting that one recombination channel involves holes that are weakly localized with a localization energy of 0.043meV. A possible origin of these localized states are energy-variations in the xy-plane of the nanoplatelet. The presence of positive photoluminescence circular polarization in the magnetically-doped core/multi-shell NPLs indicates a hole-dopant exchange interaction and therefore the incorporated magnetic Manganese ions act as a marker that determines the location of the localized hole states.1 Time-resolved measurements show two distinct timescales (τfast and τslow) that can be modeled using a rate equation model. We identify these timescales as closely related to the corresponding recombination times for the channels. The stronger hole localization of one of these channels leads to a decreased electron-hole wave function overlap and thus a decreased oscillator strength and an increased lifetime. We show that we can model and understand the magnetic interaction of doped 2D-colloidal nanoplatelets which opens a pathway to solution processable spin controllable light sources.

sp-d Exchange Interactions in Wave Function Engineered Colloidal CdSe/Mn:CdS Hetero-Nanoplatelets

In two-dimensional (2D) colloidal semiconductor nanoplatelets, which are atomically flat nanocrystals, the precise control of thickness and composition on the atomic scale allows for the synthesis of heterostructures with well-defined electron and hole wave function distributions. Introducing transition metal dopants with a monolayer precision enables tailored magnetic exchange interactions between dopants and band states. Here, we use the absorption based technique of magnetic circular dichroism (MCD) to directly prove the exchange coupling of magnetic dopants with the band charge carriers in hetero-nanoplatelets with CdSe core and manganese-doped CdS shell (CdSe/Mn:CdS). We show that the strength of both the electron as well as the hole exchange interactions with the dopants can be tuned by varying the nanoplatelets architecture with monolayer accuracy. As MCD is highly sensitive for excitonic resonances, excited level spectroscopy allows us to resolve and identify, in combination with wave function calculations, several excited state transitions including spin-orbit split-off excitonic contributions. Thus, our study not only demonstrates the possibility to expand the extraordinary physical properties of colloidal nanoplatelets toward magneto-optical functionality by transition metal doping but also provides an insight into the excited state electronic structure in this novel two-dimensional material.