Helmut Baumgart

Thermoelectric materials, such as Sb₂Te₃, offer a sustainable means of converting waste heat into electricity, enabling advancements in power generation and refrigeration. Sb₂Te₃, a widely recognized p-type thermoelectric material, is particularly promising for energy-harvesting applications. Thin films of Sb₂Te₃ synthesized via Atomic Layer Deposition (ALD) provide scalability and environmental benefits, making them ideal for microelectronic devices. This study employs the Hall effect to investigate the electrical and transport properties of Sb₂Te₃ thin films. These films were deposited using a thermal ALD reactor, with SbCl₃ and Bis(trimethylsilyl)telluride serving as precursors. To ensure adequate vapor pressure for deposition, SbCl₃ was preheated to ~75°C. Using the van der Pauw method, the Hall coefficient, carrier concentration, and mobility were measured across varying temperatures and magnetic fields. The findings confirm Sb₂Te₃ as a p-type semiconductor, dominated by hole carriers. The observed carrier concentration and mobility align with thermoelectric requirements, offering valuable insights into optimizing electronic properties of Sb₂Te₃ for efficient energy conversion technologies.

Ion implantation emerges as a cutting-edge method for defect engineering in crystalline materials, offering unparalleled precision in modulating thermal conductivity. In this study, we employ argon (Ar) ion implantation to manipulate the thermal and phonon transport properties of the thermoelectric material antimony telluride (Sb2Te3). By systematically varying ion implantation doses and energies, we achieve controlled defect structures, characterized comprehensively using advanced techniques such as Transmission Electron Microscopy (TEM), X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), and atomic force microscopy (AFM). The results reveal that defect-induced structural changes, including point defects, dislocations, and amorphous-like regions, significantly suppress thermal conductivity, laying the foundation for enhanced thermoelectric efficiency. This work establishes Ar ion implantation as a scalable, tunable, and transformative approach to defect engineering, unlocking new horizons for thermoelectric materials in energy conversion and advanced thermal management applications.

Atomic Layer Deposition (ALD) inherently yields intrinsic, undoped semiconducting films, necessitating the incorporation of controlled impurity levels to convert them into extrinsic semiconductors. While several in-situ doping strategies—such as delta doping during the ALD process—have been explored, post-deposition doping via ion implantation offers superior control over both dose and doping profile. This study examines the effects of Chromium (Cr⁺) ion implantation at 180 keV on the properties of lead chalcogenide (PbTe) thin films, with the goal of enhancing their thermoelectric figure of merit. A fluence of 5 × 10¹⁵ ions/cm² was employed to achieve an estimated Cr doping concentration of 1%. SRIM (Stopping and Range of Ions in Matter) simulations were used to optimize the implantation energy to align the dopant distribution peak with the ALD film thickness. PbTe thin films were synthesized via ALD on silicon substrates with native oxide layers, using lead(II) bis(2,2,6,6-tetramethyl-3,5-heptanedionato) (Pb(C₁₁H₁₉O₂)₂) and (trimethylsilyl) telluride ((Me₃Si)₂Te) as the precursors for lead and tellurium, respectively. Nitrogen served as both the carrier and purge gas. Prior to deposition, substrate surfaces were functionalized to ensure consistent chemisorption of the ALD precursors. The ALD process was conducted over a temperature range of 130°C to 170°C, with precursor delivery temperatures of 170°C for lead and 45°C for tellurium. The deposition chamber was maintained at a base pressure of 500 mTorr. Comprehensive characterization of the PbTe films was carried out before and after ion implantation. Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS) were used to evaluate film morphology and composition X-ray Diffraction (XRD) analysis confirmed the formation of polycrystalline films with simple cubic structure. Atomic Force Microscopy (AFM) provided detailed information on surface topology, including RMS roughness, grain height, and average grain size. Electrical properties were assessed through temperature-dependent resistivity (ρ(T)), electrical conductivity, Hall mobility, and Seebeck coefficient measurements to evaluate the effects of Cr ion implantation on the thermoelectric performance of the PbTe films.

High-energy physics research, industrial sterilizing, and material processing depend extensively on electron beam accelerators. Exit windows are crucial components of such electron accelerator systems, maintaining vacuum integrity inside the machine while providing mechanical strength, thermal stability, and radiation resistance at the beam-target interface. In this study, thin metallic films of Ni, Ti, Cr, and V were explored for use in electron beam exit windows, and their properties were compared with the properties of their bulk counterparts. Simulation results of metal foils predicted Ti to exhibit less beam power dissipation compared to Ni. However, Ni possesses superior mechanical and structural properties compared to Ti. The performance of these films under electron beam irradiation was examined by depositing thin layers of these films on silicon and metallic substrates using magnetron sputtering and exposing them to e-beam irradiation in a controlled setup. The deposited films were subjected to a dose of approximately 66 kGy at a beam energy of 10 MeV and characterized prior to and postexposure to the beam using field emission scanning electron microscopy, atomic force microscopy, x-ray diffraction (XRD), and nanoindentation. Particular emphasis was given to characteristics like the grain structure, surface morphology, dislocation density, and hardness. XRD patterns revealed irradiation-induced changes in peak intensities, while the crystallinity remained largely unchanged. Nanoindentation results showed that the pristine and irradiated Ti and Ni films were twice as hard when compared to bulk Ti and Ni samples, regardless of the substrate type used (Si or bulk Ti, or Ni). These results emphasize the superior mechanical properties of thin metal films compared to their bulk counterparts. This study advances the optimization of thin film materials for robust and efficient e-beam applications, ensuring improved durability and operational reliability.

We report a new synthesis method for multiple-walled nested thin-film nanostructures by combining hydrothermal growth methods with atomic layer deposition (ALD) thin-film technology and sacrificial films, thereby increasing the surface-to-volume ratio to improve the sensing performance of novel ZnO gas sensors. Single-crystal ZnO nanorods serve as the core of the nanostructure assembly and were synthesized hydrothermally on fine-grained ALD ZnO seed films. Subsequently, the ZnO core nanotubes were coated with alternating sacrificial coaxial 3D wrap-around ALD Al₂O₃ films and ALD ZnO films. Basically, the center nanorod was coated with an ALD 3D wrap-around Al₂O₃ sacrificial layer to realize a nested coaxial ZnO thin-film nanotube. To increase the surface-to-volume ratio of the nested multiple-film nanostructure, both the front and backside of the nested coaxial ZnO films must be exposed by selectively removing the intermittent Al₂O₃ sacrificial films. The selective removal of the sacrificial films exposes the front and backside of the free-standing ZnO films for interaction with target gases during sensing operation while steadily increasing the surface-to-volume ratio. The sensing response of the novel ZnO gas sensor architecture with nested nanotubes achieved a maximum 150% enhancement at low temperature compared to a conventional ZnO nanorod sensor.

Volatile Organic Compounds (VOCs) encompass a very wide range of organic compounds characterized by high vapor pressure at lower temperatures, which gives them significant volatility already at room temperatures. They are present in the living and work environments. Long-term indoor exposure to VOCs can cause fatigue, headache, nausea, eye, nose, and throat irritation in sensitive persons, but also damage the central nervous system and other organs. Because of their widespread occurrence in our environment, there is a need for good detectors and sensors for VOCs. We have investigated the perturbation of the physico-chemical the properties of Zinc Oxide (ZnO) and Metal-Organic Frameworks (MOFs) in the presence of VOCs, and developed a novel approach for the detection of such VOCs utilizing Broadband Dielectric Spectroscopy (BDS) techniques. In this talk, we will review the current state-of-the-art in MOF sensing research to provide a context for the novel BDS metrology we have developed, discuss our effort to elucidate the sensing mechanisms of HKUST-1 MOF films with BDS, and showcase the capabilities of BDS for metal-oxide sensing research advancement. Specifically, we will focus on the use of surface anchored MOF (SURMOF)– as opposed to free running powdery MOFs, as sensing agents. Our results show that the SURMOF sensing agent is stable, more robust, and more sensitive in detecting VOCs, than traditional metal oxide (MOS) agents even at temperatures well below 100 °C.

We report a new synthesis method for multiple walled nested thin film nanostructures by combining hydrothermal growth methods with Atomic Layer Deposition (ALD) thin film technology and sacrificial films thereby increasing the surface-to-volume ratio to improve the sensing performance of novel ZnO gas sensors. Single crystal ZnO nanorods serve as the core of the nanostructure assembly and were synthesized hydrothermally on fine-grained ALD ZnO seed films. Subsequently the ZnO core nanotubes were coated with alternating sacrificial co-axial 3-D wraparound ALD Al2O3 films and ALD ZnO films. Basically, an ALD 3-D wrap-around Al2O3 sacrificial layer was coating the center nanorod to realize a nested coaxial ZnO thin film nanotube. To increase the surface-to-volume ratio of the nested multiple film nanostructure both front and backside of the nested coaxial ZnO films must be exposed by selectively removing the intermittent Al2O3 sacrificial films. The selective removal of the sacrificial films exposes the front and backside of the free-standing ZnO films for interacting with target gases during sensing operation while steadily increasing the surface-to-volume ratio. The sensing response of the novel ZnO gas sensor architecture with nested nanotubes achieved a maximum 150% enhancement at low temperature compared to a conventional ZnO nanorod sensor.

Beam exit windows are crucial components of any particle accelerator as they provide an interface between the beamline vacuum and target material at atmospheric media. For high beam power machines, special materials and designs are required to withstand high radiation and mechanical loads, while minimizing energy loss during transition and maximizing window lifetime. This research investigates the impact of electron beam exposure to bulk CrMnV and CrMnTiV high entropy alloys (HEAs) with the primary goal of identifying suitable candidate materials for the design of robust and durable exit window settings. The selection criteria include intrinsic characteristics, power dissipation, and mechanical responses. According to the thermodynamic calculations, both equiatomic CrMnV and the addition of 7% of Ti with equiatomic CrMnV yield solid-solutions phases. The structural and mechanical properties of CrMnV and CrMnTiV samples were tested using field emission scanning electron microscopy, atomic force microscopy, scanning electron microcopy with energy dispersive x-ray spectroscopy, x-ray diffraction, and nanoindentation before and after exposure to a dose of ∼66 kGy from a 10 MeV e-beam accelerator. Despite exhibiting beam transmission characteristics comparable to Cr and V, the indentation hardness of HEAs exceeded that of the Cr and V samples by five to six times. The examination of the CrMnTiV irradiated samples revealed organized deformation patterns depicting new features, which we suspect twinning and twin boundaries due to the addition of Ti to CrMnV. Ti, a hexagonal-close-packed crystal structure, is commonly known for deformation twinning behavior.

In this research, magnetron sputtering is employed to fabricate thin films of high entropy alloys. The target materials are derived from VCrMn and VCrMnTi systems. The focus is to pinpoint suitable candidates for robust and long-lasting beam exit windows in e-beam accelerators. The selection process considers their inherent characteristics, their capacity to dissipate power, and how they respond to irradiation in terms of mechanical traits like hardness, ductility, defect density, and strength. For the bulk alloy samples of VCrMn and VCrMnTi, a sequence of nanoindentation tests has been executed to evaluate hardness and elastic modulus. The outcomes reveal that both the hardness and modulus are notably greater than those of any individual metal samples. It is anticipated that fabricated alloy films with same compositions will exhibit robust qualities in terms of mechanical properties, including ductility, defect density, and strength. The incorporation of additional titanium (Ti) alloying elements into the high entropy alloys (HEAs) of VCrMn is projected to enhance electron transmission. The fabricated films have undergone thorough analysis and testing to comprehend their microstructural attributes and chemical composition which involves the utilization of SEM with EDS, FESEM, AFM to investigate microstructural and surface properties of the films. Subsequently, X-ray diffraction has been employed to acquire insights into the morphology and lattice orientation of the films.

This study aims to explore the impact of e-beam irradiation on the mechanical and structural characteristics of four different polycrystalline metals (Ni, Cr, V, and Ti), in both bulk and thin film forms. Thin metal films are fabricated using magnetron sputtering. The primary goal is to identify suitable metal candidates for growing thin films, which could serve as exit windows for e-beam accelerators. This selection is based on their properties, power dissipation capabilities, and the effects of irradiation on mechanical attributes such as hardness, ductility, defect density, and strength. For the bulk polycrystalline metal samples, a series of nanoindentation tests has been conducted before and after e-beam irradiation to investigate changes in hardness and elastic modulus. Additionally, to compare performance with the bulk samples, thin metal films are subjected to the same procedure before and after irradiation, allowing for the evaluation of film hardness, moduli, and creep properties. Results indicate that irradiated bulk samples of Ni, Cr, and V become harder, whereas Ti experiences softening. There is no significant irradiation effect on modulus for all for polycrystalline metal samples. To understand the underlying reasons behind these effects, both the fabricated metal films and bulk metal samples undergo comprehensive characterization tests for their microstructural properties, elastic behavior, and chemical composition. This involves the utilization of SEM with EDS, FESEM and AFM to analyze microstructure and surface attributes of the films, followed by X-ray diffraction to gain insight into film morphology and lattice orientation.