Enhancing composition control of alloy nanoparticles from gas aggregation source by in operando optical emission spectroscopy

The use of multicomponent targets allows the gas‐phase synthesis of a large variety of alloy nanoparticles (NPs) via gas aggregation sources. However, the redeposition of sputtered material impacts the composition of alloy NPs, as demonstrated here for the case of AgAu alloy NPs. To enable NPs with tailored Au fractions, in operando control over the composition of the NPs is in high demand. We suggest the use of optical emission spectroscopy as a versatile diagnostic tool to determine and control the composition of the NPs. A strong correlation between operating pressure, intensity ratio of Ag and Au emission lines, and the obtained NP compositions is observed. This allows precise in operando control of alloy NP composition obtained from multicomponent targets

Enhancing Reliability of Studies on Single Filament Memristive Switching via an Unconventional cAFM Approach

Memristive devices are highly promising for implementing neuromorphic functionalities in future electronic hardware, and direct insights into memristive phenomena on the nanoscale are of fundamental importance to reaching this. Conductive atomic force microscopy (cAFM) has proven to be an essential tool for probing memristive action locally on the nanoscale, but the significance of the acquired data frequently suffers from the nonlocality associated with the thermal drift of the tip in ambient conditions. Furthermore, comparative studies of different configurations of filamentary devices have proven to be difficult, because of an immanent variability of the filament properties between different devices. Herein, these problems are addressed by constraining the memristive action directly at the apex of the probe through functionalization of a cAFM tip with an archetypical memristive stack, which is comprised of Ag/Si3N4. The design of such functionalized cantilevers (entitled here as "memtips") allowed the capture of the long-term intrinsic current response, identifying temporal correlations between switching events, and observing emerging spiking dynamics directly at the nanoscale. Utilization of an identical memtip for measurements on different counter electrodes made it possible to directly compare the impact of different device configurations on the switching behavior of the same filament. Such an analytical approach in ambient conditions will pave the way towards a deeper understanding of filamentary switching phenomena on the nanoscale

Memristive Dynamics of Ag-based Nanostructures for Neuromorphic Systems

In this thesis, an unconventional cAFM approach was developed to enhance basic research on nanoscale memristive systems. This approach intends to integrate the memristive system at the apex region of a cantilever, to achieve an efficient nanoscale localization of the memristive action. Two memristive systems were investigated via this approach: An individual Ag-filament evolved from a continuous active electrode and a AgPt-NP system embedded in SiOxNy. Moreover, AgPt and AgAu alloy-NPs embedded in SiO2 are investigated regarding their applicability as fundamental building units for diffusive memristive switching via regular cAFM. Furthermore, concepts are discussed to expand the functionalities of alloy-NP based memristive devices towards memsensing, meaning that the diffusive switching dynamics can be modulated by external stimuli. This is done by serial connection of alloy-NP based diffusive memristive systems and wide-bandgap semiconductors (TiO2 and ZnO) that incorporate UV-sensitivity to the circuit. Moreover, two approaches for NP-based memristive networks were investigated. It is shown, that AgAu-NPs are able to implement memristive switching locally in a higher level network created by random assembly of carbon nanotubes. Finally, networks of Ag-NPs poised at the percolation threshold were investigated with respect to their emergent collective behavior. An important finding of this thesis is, that critical dynamics and long-range temporal correlations in Ag-NP networks are not disturbed upon presence or absence of a matrix

Tailoring the Optical Properties of Sputter-Deposited Gold Nanostructures on Nanostructured Titanium Dioxide Templates Based on In Situ Grazing-Incidence Small-Angle X-ray Scattering Determined Growth Laws

Gold/titanium dioxide (Au/TiO$_2$) nanohybrid materials have been widely applied in various fields because of their outstanding optical and photocatalytic performance. By state-of-the-art polymer templating, it is possible to make uniform nanostructured TiO$_2$ layers with potentially large-scale processing methods. We use customized polymer templating to achieve TiO$_2$ nanostructures with different morphologies. Au/TiO$_2$ hybrid thin films are fabricated by sputter deposition. An in-depth understanding of the Au morphology on the TiO$_2$ templates is achieved with in situ grazing-incidence small-angle X-ray scattering (GISAXS) during the sputter deposition. The resulting Au nanostructure is largely influenced by the TiO$_2$ template morphology. Based on the detailed understanding of the Au growth process, characteristic distances can be selected to achieve tailored Au nanostructures at different Au loadings. For selected sputter-deposited Au/TiO$_2$ hybrid thin films, the optical response with a tailored localized surface plasmon resonance is demonstrated

Real-time insight into nanostructure evolution during the rapid formation of ultra-thin gold layers on polymers

Ultra-thin metal layers on polymer thin films attract tremendous research interest for advanced flexible optoelectronic applications, including organic photovoltaics, light emitting diodes and sensors. To realize the large-scale production of such metal–polymer hybrid materials, high rate sputter deposition is of particular interest. Here, we witness the birth of a metal–polymer hybrid material by quantifying in situ with unprecedented time-resolution of 0.5 ms the temporal evolution of interfacial morphology during the rapid formation of ultra-thin gold layers on thin polystyrene films. We monitor average non-equilibrium cluster geometries, transient interface morphologies and the effective near-surface gold diffusion. At 1 s sputter deposition, the polymer matrix has already been enriched with 1% gold and an intermixing layer has formed with a depth of over 3.5 nm. Furthermore, we experimentally observe unexpected changes in aspect ratios of ultra-small gold clusters growing in the vicinity of polymer chains. For the first time, this approach enables four-dimensional insights at atomic scales during the gold growth under non-equilibrium conditions

In Situ Monitoring of Scale Effects on Phase Selection and Plasmonic Shifts during the Growth of AgCu Alloy Nanostructures for Anticounterfeiting Applications

Tailoring of plasmon resonances is essential for applications in anti-counterfeiting. This is readily achieved by tuning the composition of alloyed metal clusters; in the most simple case binary alloys. Yet, one challenge is the correlation of cluster morphology and composition with the changing optoelectronic properties. Hitherto, the early stages of metal alloy nanocluster formation in immiscible binary systems like silver and copper has been accessible by molecular dynamics simulations and transmission electron microscopy. Here, we investigate in real-time the formation of supported silver, copper and silver-copper-alloy nanoclusters during sputter deposition on poly(methyl methacrylate) by combining in situ surface sensitive X-ray scattering with optical spectroscopy. While following the transient growth morphologies, we quantify the early stages of phase separation at the nanoscale, follow the shifts of surface plasmon resonances and quantify the growth kinetics of the nanogranular layers at different thresholds. We are able to extract the influence of scaling effects on the nucleation and phase selection. The internal structure of the alloy cluster shows a copper-rich core/silver-rich shell structure, since the copper core yields a lower mobility and higher crystallization tendency than the silver fraction. We compare our results to molecular dynamics simulation and transmission electron microscopy data. This demonstrates a route to tailor accurately the plasmon resonances of nanosized, polymer supported clusters which is a crucial prerequisite for anti-counterfeiting

Revealing the growth of copper on polystyrene- block -poly(ethylene oxide) diblock copolymer thin films with in situ GISAXS

Copper (Cu) as an excellent electrical conductor and the amphiphilic diblock copolymer polystyrene-block-poly(ethylene oxide) (PS-b-PEO) as a polymer electrolyte and ionic conductor can be combined with an active material in composite electrodes for polymer lithium-ion batteries (LIBs). As interfaces are a key issue in LIBs, sputter deposition of Cu contacts on PS-b-PEO thin films with high PEO fraction is investigated with in situ grazing-incidence small-angle X-ray scattering (GISAXS) to follow the formation of the Cu layer in real-time. We observe a hierarchical morphology of Cu clusters building larger Cu agglomerates. Two characteristic distances corresponding to the PS-b-PEO microphase separation and the Cu clusters are determined. A selective agglomeration of Cu clusters on the PS domains explains the origin of the persisting hierarchical morphology of the Cu layer even after a complete surface coverage is reached. The spheroidal shape of the Cu clusters growing within the first few nanometers of sputter deposition causes a highly porous Cu–polymer interface. Four growth stages are distinguished corresponding to different kinetics of the cluster growth of Cu on PS-b-PEO thin films: (I) nucleation, (II) diffusion-driven growth, (III) adsorption-driven growth, and (IV) grain growth of Cu clusters. Percolation is reached at an effective Cu layer thickness of 5.75 nm