Grigorios Itskos

Efficient electrical spin injection from a magnetic metal/tunnel barrier contact into a semiconductor

We report electrical spin injection from a ferromagnetic metal contact into a semiconductor light emitting diode structure with an injection efficiency of 30% which persists to room temperature. The Schottky barrier formed at the Fe/AlGaAs interface provides a natural tunnel barrier for injection of spin polarized electrons under reverse bias. These carriers radiatively recombine, emitting circularly polarized light, and the quantum selection rules relating the optical and carrier spin polarizations provide a quantitative, model-independent measure of injection efficiency. This demonstrates that spin injecting contacts can be formed using a widely employed contact methodology, providing a ready pathway for the integration of spin transport into semiconductor processing technology.

Analysis of the transport process providing spin injection through an Fe/AlGaAs Schottky barrier

Electron-spin polarizations of 32% are obtained in a GaAs quantum well via electrical injection through a reverse-biased Fe/AlGaAs Schottky contact. An analysis of the transport data using the Rowell criteria demonstrates that single-step tunneling is the dominant transport mechanism. The current–voltage data show a clear zero-bias anomaly and phonon signatures corresponding to the GaAs-like and AlAs-like LO phonon modes of the AlGaAs barrier, providing further evidence for tunneling. These results provide experimental confirmation of several theoretical analyses, indicating that tunneling enables significant spin injection from a metal into a semiconductor.

Lead Halide Perovskites and Other Metal Halide Complexes As Inorganic Capping Ligands for Colloidal Nanocrystals

Lead halide perovskites (CH3NH3PbX3, where X = I, Br) and other metal halide complexes (MXn, where M = Pb, Cd, In, Zn, Fe, Bi, Sb) have been studied as inorganic capping ligands for colloidal nanocrystals. We present the methodology for the surface functionalization via ligand-exchange reactions and the effect on the optical properties of IV–VI, II–VI, and III–V semiconductor nanocrystals. In particular, we show that the Lewis acid–base properties of the solvents, in addition to the solvent dielectric constant, must be properly adjusted for successful ligand exchange and colloidal stability. High luminescence quantum efficiencies of 20–30% for near-infrared emitting CH3NH3PbI3-functionalized PbS nanocrystals and 50–65% for red-emitting CH3NH3CdBr3- and (NH4)2ZnCl4-capped CdSe/CdS nanocrystals point to highly efficient electronic passivation of the nanocrystal surface.

Reduction of spin injection efficiency by interface defect spin scattering in ZnMnSe/AlGaAs-GaAs spin-polarized light-emitting diodes.

We report the first experimental demonstration that interface microstructure limits diffusive electrical spin injection efficiency across heteroepitaxial interfaces. A theoretical treatment shows that the suppression of spin injection due to interface defects follows directly from the contribution of the defect potential to the spin-orbit interaction, resulting in enhanced spin-flip scattering. An inverse correlation between spin-polarized electron injection efficiency and interface defect density is demonstrated for ZnMnSe/AlGaAs-GaAs spin-LEDs with spin injection efficiencies of 0 to 85%.

Quantifying electrical spin injection: Component-resolved electroluminescence from spin-polarized light-emitting diodes

The spin-polarized light-emitting diode (spin-LED) is a very effective tool for accurately quantifying electrical spin injection in a model independent manner. We resolve and identify various components which occur in the electroluminescence (EL) spectra of GaAs quantum-well-based spin-LEDs, and examine the circular polarization of each. While a number of components exhibit significant circular polarization, the values do not necessarily reflect the electrical spin injection efficiency. We show that a reliable measure of spin injection efficiency can be obtained only if one takes care to spectroscopically resolve and accurately identify the free exciton or free carrier components of the EL spectrum, and exclude other components.

Electrical spin injection across air-exposed epitaxially regrown semiconductor interfaces

We have fabricated spin-polarized light-emitting diode structures via epitaxial regrowth of Zn1−xMnxSe on air-exposed surfaces of AlyGa1−yAs/GaAs quantum wells. No passivation procedures were used to protect or prepare the III–V surface. The electroluminescence is strongly circularly polarized due to the electrical injection of spin-polarized electrons from the ZnMnSe contact into the GaAs quantum well. An analysis of the optical polarization yields a lower bound of 65% for the spin injection efficiency. These results demonstrate the robustness of the spin injection process in the diffusive transport regime, and attest to the practicality of manufacturing semiconductor-based spin injection devices.

New light from hybrid inorganic-organic emitters

We present the highlights of a research programme on hybrid inorganic?organic light emitters. These devices combine recent developments in III?V nitride technology (including UV emitting micro-arrays and specifically tailored quantum wells) with conjugated polymers to access the entire visible spectrum. Two types of devices are studied, those based on down conversion of the quantum well emission by radiative transfer and those based on non-radiative resonant energy transfer. The spectral and operating characteristics of the devices are described in detail. Selectable colour micro-arrays and bar emitters are demonstrated. The nature of the non-radiative energy transfer process has also been studied and we find transfer efficiencies of up to 43% at 15?K, with a 1/R2 dependence on the distance between quantum well and polymer layer, suggesting a plane?plane interaction. The relative importance of the non-radiative resonant energy transfer process increases with temperature to be up to 20 times more efficient, at 300?K, than the radiative transfer process.

Oblique Hanle measurements of InAs/GaAs quantum dot spin-light emitting diodes

We report on studies of electrical spin injection from ferromagnetic Fe contacts into semiconductor light emitting diodes containing single layers of InAs∕GaAs self-assembled quantum dots (QDs). An oblique magnetic field is used to manipulate the spin of the injected electrons in the semiconductor. This approach allows us to measure the injected steady-state spin polarization in the QDs, Pspin as well as estimate the spin losses in the QD spin detector. After subtraction of magneto-optical effects not related to spin injection, we measured a Pspin of 7.5% at 15 K and estimated an injected spin polarization before QD recombination of around 20%.

Single crystal coordinating solvent exchange as a general method for the enhancement of the photoluminescence properties of lanthanide MOFs

The discovery of new methods for the post-synthesis modification of materials is essential in order to establish suitable strategies for the tuning of their properties in a rational manner. Here we present a series of single-crystal-to-single-crystal (SCSC) transformations for the flexible [Eu2(CIP)2(DMF)2(H2O)2] (UCY-8) [H3CIP = 5-(4-carboxybenzylideneamino)isophthalic acid] and rigid [Eu2(N-BDC)3(DMF)4] (EuN-BDC) (H2N-BDC = 2-amino-1,4-benzene dicarboxylic acid) Metal–Organic Frameworks (MOFs) that involve the replacement of their coordinating solvent molecules by terminally ligating organic molecules with multiple functional groups including –OH, –SH, –NH– and –NH2 or their combinations, chelating ligands, and two different organic compounds. The capability of the flexible MOF, which contains small pores and channels (<4 A), to exchange its coordinating solvent molecules by relatively bulky molecules (such as pyridine, 2-hydroxymethyl-phenol, etc.) is shown to be the result of its breathing capacity. Remarkably, the rigid MOF is also highly capable of replacing its coordinating solvent molecules by bulky ligands, despite its small pores (2–5 A) and lack of structural flexibility. Interestingly, the insertion of some organic ligands into the rigid MOF results in a significant modification of its framework structure and substantial expansion of its potential void space. Not only a plethora of exchanged analogues of these MOFs have been isolated and crystallographically characterized, but also, in some cases, a tremendous enhancement of their Eu3+-based photoluminescence (PL) signals, lifetimes and quantum yields (up to ∼16 times) compared to those of the pristine materials has been observed due to the replacement of terminal solvents by organic ligands being efficient sensitizers for the Eu3+ ion. Overall this work indicates that the Single Crystal Coordinating Solvent Exchange (SCCSE) can be applied as a general post-synthetic modification method for LnMOFs and also constitutes a highly efficient strategy for the enhancement of the Ln3+-based PL.

Novel BODIPY-based conjugated polymers donors for organic photovoltaic applications

Five new polymers based on the 4,4′-difluoro-4-bora-3a,4a-diaza-s-indacene core (BODIPY) chromophore moiety have been synthesized as low bandgap polymers for optoelectronic applications. The polymers exhibited high solubility in common organic solvents and optical bandgaps ranging from 1.7–2 eV. The materials were characterized using NMR, UV-Vis, steady state and time-resolved photoluminescence and the energy levels were examined using electrochemistry and validated using quantum chemical calculations. Finally, a representative BODIPY derivativeu2006:u2006PCBM blend was examined in terms of photovoltaic properties. Preliminary device performance parameters as a function of photo-active layer thickness and composition are reported and discussed, relating to power conversion efficiency values.