Astrid Besmehn

Resistive switching of rose bengal devices: A molecular effect?

The resistive switching behavior of devices consisting of aluminum top electrode, molecular layer (rose bengal), and bottom electrode (zinc oxide and indium tin oxide) is examined. By measuring the current versus voltage dependence of these devices for various frequencies and by systematically varying the composition of the device, we show that the switching is an extrinsic effect that is not primarily dependent on the molecular layer. It is shown that the molecular layer is short circuited by filaments of either zinc oxide or aluminum and that the switching effect is due to a thin layer of aluminum oxide at the zinc oxide/aluminum interface.

Scaling Potential of Local Redox Processes in Memristive SrTiO

In this work, we address the following question: Where do the resistive switching processes take place in memristive thin-film devices of the single crystalline model material Fe-doped SrTiO3? We compare resistive switching induced by the tip of the atomic force microscope on the bare thin-film surface with the switching properties observed in memristive devices with Pt top electrode. In order to close the gap between these two approaches, we combine conductive-tip atomic force microscopy with a delamination technique to remove the top electrode of Fe-doped SrTiO3 metal-insulator-metal (MIM) structures to gain insight into the active switching interface with nanoscale lateral resolution. This enables us to prove the coexistence of a filamentary and area-dependent switching process with opposite switching polarities in the same sample. The spatially resolved analysis by transmission electron microscopy and photoelectron spectromicroscopy gives us some hints that the two switching types take place in device regions with different defect density and significant stoichiometry difference.

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Recent studies indicate that small amyloid-β peptide (Aβ) oligomers are the major toxic species responsible for development and progression of Alzheimers disease (AD). Therefore, we suggest that the number of Aβ oligomers in body fluids is the most direct and relevant biomarker for AD. Determination of the Aβ oligomer content of cerebrospinal fluid (CSF) samples from 14 AD patients and 12 age-matched controls revealed a clear distinction between both groups. All samples of the control group showed homogenously low numbers of Aβ oligomers, while the samples of the AD group exhibited significantly higher levels of Aβ oligomers. The Aβ oligomer numbers correlated with the patients Mini-Mental State Examination scores. This indicates that the quantity of Aβ oligomers in CSF reflects the severity of the disease and that Aβ oligomers play a crucial role in AD pathology and in turn can be used as a diagnostic biomarker.

Thin-Film Devices

Environmentally friendly nanocrystals (NCs) such as InP are in demand for various applications, such as biomedical labeling, solar cells, sensors, and light-emitting diodes (LEDs). To fulfill their potential applications, the synthesis of such high-quality green InP NCs required further improvement so as to achieve better stability, higher brightness NCs, and also to have a more robust synthesis route. The present study addresses our efforts on the synthesis of high-quality In(Zn)P/ZnS core-shell NCs using an air- and moisture-stable ZnS single molecular precursor (SMP) and In(Zn)P cores. The SMP method has recently emerged as a promising route for the surface overcoating of NCs due to its simplicity, high reproducibility, low reaction temperature, and flexibility in controlling the reaction. The synthesis involved heating the In(Zn)P core solution and Zn(S2CNR2) (where R = methyl, ethyl, butyl, or benzyl and referred to as ZDMT, ZDET, ZDBT, or ZDBzT, respectively) in oleylamine (OLA) to 90-250 °C for 0.5-2.5 h. In this work, we systematically studied the influence of different SMP end groups, the complex formation and stability between the SMP and oleylamine (OLA), the reaction temperature, and the amount of SMP on the synthesis of high-quality In(Zn)P/ZnS NCs. We found that thiocarbamate end groups are an important factor contributing to the low-temperature growth of high-quality In(Zn)P/ZnS NCs, as the end groups affect the polarity of the molecules and result in a different steric arrangement. We found that use of SMP with bulky end groups (ZDBzT) results in nanocrystals with higher photoluminescence quantum yield (PL QY) and better dispersibility than those synthesized with SMPs with the shorter alkyl chain groups (ZDMT, ZDET, or ZDBT). At the optimal conditions, the PL QY of red emission In(Zn)P/ZnS NCs is 55 ± 4%, which is one of the highest values reported. On the basis of structural (XAS, XPS, XRD, TEM) and optical characterization, we propose a mechanism for the growth of a ZnS shell on an In(Zn)P core.

The amyloid-β oligomer count in cerebrospinal fluid is a biomarker for Alzheimer's disease.

Integration of molecule-capped gold nanoparticles (AuNP) into nanoelectronic devices requires detailed knowledge about the AuNP-electrode interface. Here, we report the pH-dependent adsorption of amine or carboxylic acid-terminated gold nanoparticles on platinum or gold/palladium (30% Pd) alloy, respectively. We synthesized amine-terminated AuNP, applying a new solid phase supported approach, as well as AuNP exhibiting carboxylic acid as terminal groups. The pH-induced agglomeration of the synthesized AuNP was investigated by UV-vis, DLS, and ζ-potential measurements. Depending on the pH and the ionic strength of the AuNP solution a preferential adsorption on the different metals occurred. Thereby, we demonstrate that by choosing the appropriate functional group and adjusting the pH as well as the ionic strength a directed binding can be achieved, which is an essential prerequisite for applications of these particles in nanoelectronics. These findings will pave the way for a controlled designing of the interface between molecule-capped AuNP and metallic electrodes for applications in nanoelectronics.

Understanding the role of single molecular ZnS precursors in the synthesis of In(Zn)P/ZnS nanocrystals.

In this work the effect of multivalency on the stability of NIR-absorbing HAuNSs and AuNRs functionalized by mono-, bi- and tridentate polyethyleneglycol (PEG) thiol ligands is reported. Comparison of commercially-available monodentate and self-synthesized bi- and tridentate methoxy terminated thiol-polyethyleneglycol ligands having molecular weights of around 5000 Da shows the stability increase of HAuNSs and AuNRs for bi- and tridentate ligands, attributed to the multivalency of the ligands. The stability was explored according to three different aspects: (1) stability towards competition reactions with the strong binding ligand dithiothreitol, (2) resistance towards oxidative Au dissolution with potassium cyanide, and (3) colloidal stability, tested by the addition of NaCl. Our PEGylation approach leads to AuNRs where the CTAB concentration is below the detection limit of the performed analytical methods, which is vital for any clinical applications. Furthermore, we found strikingly high biocompatibility after PEGylation for both particle types whereby we observed no significant difference in cytotoxicity comparing the mono-, bi- and tridentate PEGylated species.

Differential Adsorption of Gold Nanoparticles to Gold/Palladium and Platinum Surfaces

This report presents a systematic study on the effect of zinc (Zn) carboxylate precursor on the structural and optical properties of red light emitting InP nanocrystals (NCs). NC cores were assessed using X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS), energy-dispersive X-ray spectroscopy (EDX), and high-resolution transmission electron microscopy (HRTEM). When moderate Zn:In ratios in the reaction pot were used, the incorporation of Zn in InP was insufficient to change the crystal structure or band gap of the NCs, but photoluminescence quantum yield (PLQY) increased dramatically compared with pure InP NCs. Zn was found to incorporate mostly in the phosphate layer on the NCs. PL, PLQY, and time-resolved PL (TRPL) show that Zn carboxylates added to the precursors during NC cores facilitate the synthesis of high-quality InP NCs by suppressing nonradiative and sub-band-gap recombination, and the effect is visible also after a ZnS shell is grown on the cores.

Multivalency of PEG-thiol ligands affects the stability of NIR-absorbing hollow gold nanospheres and gold nanorods

Resistively switching devices are considered promising for next-generation nonvolatile random-access memories. Today, such memories are fabricated by means of top-down approaches applying thin films sandwiched between nanoscaled electrodes. In contrast, this work presents a bottom-up approach disclosing for the first time the resistive switching (RS) of individual TiO2 nanoparticles (NPs). The NPs, which have sizes of 80 and 350 nm, respectively, are obtained by wet chemical synthesis and thermally treated under oxidizing or vacuum conditions for crystallization, respectively. These NPs are deposited on a Pt/Ir bottom electrode and individual NPs are electrically characterized by means of a nanomanipulator system in situ, in a scanning electron microscope. While amorphous NPs and calcined NPs reveal no switching hysteresis, a very interesting behavior is found for the vacuum-annealed, crystalline TiO(2-x) NPs. These NPs reveal forming-free RS behavior, dominantly complementary switching (CS) and, to a small degree, bipolar switching (BS) characteristics. In contrast, similarly vacuum-annealed TiO2 thin films grown by atomic layer deposition show standard BS behavior under the same conditions. The interesting CS behavior of the TiO(2-x) NPs is attributed to the formation of a core-shell-like structure by re-oxidation of the reduced NPs as a unique feature.

Effect of Zinc Incorporation on the Performance of Red Light Emitting InP Core Nanocrystals

We report the formation of thiol nanopatterns on SAM covered silicon wafers by converting sulfonic acid head groups via e-beam lithography. These thiol groups act as binding sites for gold nanoparticles, which can be enhanced to form electrically conducting nanostructures. This approach serves as a proof-of-concept for the combination of top-down and bottom-up processes for the generation of electrical devices on silicon.

Resistive Switching of Individual, Chemically Synthesized TiO2 Nanoparticles

Sr4Ru2O9 has a P6¯2C space group and differs from the Ruddelsden–Popper series of the strontium ruthenate family where Ru is distinctly present in Ru+5 state with no Jahn–Teller activity. A significant increase in magnetic moment and coercivity is observed along with lowering of resistivity, especially around a critical concentration of x=0.2 in Sr4−xLaxRu2−xMnxO9. Such an unusual effect is correlated to the mixed magnetic pair effect arising out from the variable oxidation states of Ru as suggested by x-ray photoelectron spectroscopic studies. Our results substantiate the long-range ferromagnetism observed for Ruthenium-substituted three-dimensional manganites.