Rainer F. Mahrt

Probing the wave function delocalization in CdSe/CdS dot-in-rod nanocrystals by time- and temperature-resolved spectroscopy.

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.

Single Cesium Lead Halide Perovskite Nanocrystals at Low Temperature: Fast Single-Photon Emission, Reduced Blinking, and Exciton Fine Structure

Metal-halide semiconductors with perovskite crystal structure are attractive due to their facile solution processability, and have recently been harnessed very successfully for high-efficiency photovoltaics and bright light sources. Here, we show that at low temperature single colloidal cesium lead halide (CsPbX3, where X = Cl/Br) nanocrystals exhibit stable, narrow-band emission with suppressed blinking and small spectral diffusion. Photon antibunching demonstrates unambiguously nonclassical single-photon emission with radiative decay on the order of 250 ps, representing a significant acceleration compared to other common quantum emitters. High-resolution spectroscopy provides insight into the complex nature of the emission process such as the fine structure and charged exciton dynamics.

SU-8 for real three-dimensional subdiffraction-limit two-photon microfabrication

We report the inherent utility of two-photon-absorption (TPA) in the fabrication of real three-dimensional (3D) structures with subdiffraction-limit resolution, based on SU-8 as the threshold polymer media. We exploit the nonlinear velocity dependence of TPA photopolymerization as the shutter mechanism for disruptive 3D lithography. We show that low numerical aperture optics can be used for the rapid microfabrication of ultrahigh-aspect ratio photoplastic pillars, planes, and cage structures.

Energy transfer in hybrid organic/inorganic nanocomposites.

Chemically synthesized colloidal quantum dots can easily be incorporated into conjugated polymer host systems allowing for novel organic/inorganic hybrid materials combining the natural advantages from both organic as well as inorganic components into one system. In order to obtain tailored optoelectronic properties, a profound knowledge of the fundamental electronic energy transfer processes between the inorganic and organic parts is necessary. Previous studies have attributed the observed efficient energy transfer to a dipole-dipole coupling with Förster radii of about 50-70 A. Here, we report on resonant energy transfer of nonequilibrium excitons in an amorphous polyfluorene donor CdSe/ZnS core-shell nanocrystal acceptor system. By time-resolved photoluminescence (PL) spectroscopy, we have investigated the PL decay behavior of the primarily excited polyfluorene as a function of temperature. We show that the transfer efficiency drops from about 30% at room temperature to around 5% at low temperature. These results shed light on the importance of temperature-activated exciton diffusion in the energy transfer process. As a consequence the exciton has to migrate very close to the surface of the quantum dot in order to couple to the quantum dot. Hence, the coupling strength is much weaker than that anticipated in previous work and requires treatment beyond Förster theory.

Nearly Temperature-Independent Threshold for Amplified Spontaneous Emission in Colloidal CdSe/CdS Quantum Dot-in-Rods

Among the different photonic applications envisioned using semiconductor quantum dots (Qdots), Qdot lasers are one of the most investigated. In contrast to bulk materials or quantum wells (Qwells), the delta-like density of electronic states predicts a low, temperature-independent lasing threshold, [ 1 ] providing enhanced device performance compared to other gain media, especially at elevated temperatures. Amplifi ed spontaneous emission (ASE) and lasing have already been observed both in epitaxial [ 2 , 3 ] and colloidal [ 4 ] Qdots. The molecular beam epitaxy (MBE)-grown Qdots, mostly based on III-V materials, have recently reached a level mature enough to move toward commercialization. Nevertheless, colloidal Qdots offer an interesting low cost alternative, being synthesized by wet chemistry approaches at low temperatures (25–350 ° C) and standard pressure. However, here progress toward effi cient laser devices has been hampered by material quality and enhanced Auger recombination due to the small Qdot size (2–5 nm). Only recently, the latter bottleneck was overcome by the use of quasi-type II CdSe/CdS giant-shell Qdots [ 5 ] or anisotropic seeded quantum rods (Qdot-in-rods). [ 6 ] Indeed, here the Auger recombination time could be extended from 10–50 ps to over 10 ns, resulting for instance in biexciton photoluminescence (PL) with a near-unity quantum effi ciency (QE). [ 7 ] Additionally, due to the reduced contribution of Auger recombination, a low ASE threshold [ 5 ] in combination with a prolonged gain lifetime and a broad gain spectrum has been observed. [ 6 ] However, it should be noted that these quasi-type II heterostructures may also intrinsically be of better quality compared to previously investigated colloidal Qdot-based gain media: as the lattice mismatch between the CdS shell and the CdSe core is reduced to 4%

Controlling the Exciton Fine Structure Splitting in CdSe/CdS Dot-in-Rod Nanojunctions

We demonstrate control and tunability of the exciton fine-structure splitting by properly engineering a nanojunction consisting of a CdSe nanocrystal core and an asymmetric rod-like CdS shell. Samples with small core and/or thick rod diameters exhibit a strongly reduced fine-structure splitting resulting from a reduced electron-hole exchange interaction. These results shed light onto the electronic configuration of such nanosystems and, apart from being of fundamental interest, could enable the use of colloidal nanocrystals as a source of entangled photons.

Band-edge exciton fine structure of small, nearly spherical colloidal CdSe/ZnS quantum dots

The exciton fine structure of small (2-3.5 nm) wurtzite (WZ) and zincblende (ZB) CdSe quantum dots (Qdots) has been investigated by means of nanosecond and picosecond time-resolved photoluminescence spectroscopy, at temperatures ranging from 5 K to room temperature. For both crystal structures, we observe a similar dark-bright energy level splitting of 2.4-5 meV, with a larger splitting corresponding to smaller Qdots. In addition, spectrally resolved streak camera images collected at 5 K reveal the presence of a third state, split from the lower dark-bright manifold by 30-70 meV, again independently of the crystal structure of the Qdots. The data thus reveal that small WZ and ZB CdSe Qdots are optically indistinguishable. This contrasts with theoretical calculations within the effective-mass approximation, which, in the limit of spherical Qdots, yield a different fine structure for both. However, experimental and theoretical results converge when taking the Qdot shape into account. With transmission electron microscopy, we determined that our Qdots are prolate, with an aspect ratio of 1.15:1. Incorporating this value into our calculations, we obtain a similar fine structure for both WZ and ZB Qdots. Moreover, the opposite sign of the crystal field and shape anisotropy in CdSe suggests that the lowest energy level in small CdSe Qdots has an angular momentum projection F = 0, in contrast with (perfectly) spherical Qdots, where the lowest level corresponds to the dark ±2 state. From the experimental and theoretical data we conclude that shape anisotropy and exchange interactions dominate over the crystal field anisotropy-induced splitting in this size range.

Lasing in organic circular grating structures

Optically pumped organic polymer lasers are fabricated by spin coating a thin polymer film onto a nanopatterned SiO2 circular-grating surface-emitting distributed Bragg reflector. For certain grating parameters, we observe a peak inside the stop band that leads to lasing with a reduced threshold. An analytical model, based on the transfer-matrix method, has been developed to investigate the origin of this peak. The theoretical results are in good agreement with the experimental findings.

Bright triplet excitons in caesium lead halide perovskites

Nanostructured semiconductors emit light from electronic states known as excitons. For organic materials, Hund’s rules state that the lowest-energy exciton is a poorly emitting triplet state. For inorganic semiconductors, similar rules predict an analogue of this triplet state known as the ‘dark exciton’. Because dark excitons release photons slowly, hindering emission from inorganic nanostructures, materials that disobey these rules have been sought. However, despite considerable experimental and theoretical efforts, no inorganic semiconductors have been identified in which the lowest exciton is bright. Here we show that the lowest exciton in caesium lead halide perovskites (CsPbX3, with X = Cl, Br or I) involves a highly emissive triplet state. We first use an effective-mass model and group theory to demonstrate the possibility of such a state existing, which can occur when the strong spin–orbit coupling in the conduction band of a perovskite is combined with the Rashba effect. We then apply our model to CsPbX3 nanocrystals, and measure size- and composition-dependent fluorescence at the single-nanocrystal level. The bright triplet character of the lowest exciton explains the anomalous photon-emission rates of these materials, which emit about 20 and 1,000 times faster than any other semiconductor nanocrystal at room and cryogenic temperatures, respectively. The existence of this bright triplet exciton is further confirmed by analysis of the fine structure in low-temperature fluorescence spectra. For semiconductor nanocrystals, which are already used in lighting, lasers and displays, these excitons could lead to materials with brighter emission. More generally, our results provide criteria for identifying other semiconductors that exhibit bright excitons, with potential implications for optoelectronic devices.

Evidence for bandedge lasing in a two-dimensional photonic bandgap polymer laser

Optically pumped organic semiconductor lasers are fabricated by spincoating a thin film of a methyl-substituted ladder-type poly-(para-phenylene) onto a nanopatterned circular grating distributed Bragg reflector. Varying the period of the grating allows for tunability of the laser wavelength. Comparing the emission spectra below and above the threshold shows that the lasing is spectrally located at the bandedge of the Bragg dip. This is in excellent agreement with the calculated dispersion relation of the laser system.