In-depth Surface Studies of p-GaN:Cs Photocathodes by Combining Ex-Situ Analytical Methods with In-Situ X-Ray Photoelectron Spectroscopy

The photocathode is one of the key components of particle accelerator facilities that provides electrons for experiments in many disciplines such as biomedicine, security imaging, and condensed matter physics. The requirements for the electron emitting material, the so-called photocathode, are rather high because these materials should provide a high quantum efficiency, a low thermal emittance, a fast response, and a long operational lifetime. At present, none of the state-of-the-art photocathodes can fully meet all the desired requirements. Therefore, new materials that can be used as potential photocathodes are urgently needed for future developments in accelerator research. Semiconductor photocathodes such as cesium telluride are the preferred materials in particle accelerators. These photocathodes provide high quantum efficiencies of above 10 %, making them highly attractive. The crystal growth of cesium telluride, as a compound semiconductor photocathode, requires the deposition of cesium and tellurium on a suitable substrate with an ideal chemical ratio, which seems elaborate and difficult to handle. In contrast, III-V semiconductors, such as gallium arsenide and gallium nitride (GaN), represent another type of semiconductor photocathode. These commercially available semiconductors are already grown on a substrate and only require a thin film of cesium and optional oxygen to obtain a photocathode. An atomically clean surface is necessary to achieve a negative electron affinity surface, which is the main prerequisite for high quantum efficiency. In this work, p-GaN grown on sapphire by metal-organic chemical vapor deposition, was wet chemically cleaned, and transferred into an ultra-high vacuum chamber, where it underwent a subsequent thermal cleaning. The cleaned p-GaN samples were activated with Cs to obtain p-GaN:Cs photocathodes and their performance was monitored with respect to their quality, especially concerning their quantum efficiency and storage lifetime. The surface topography and morphology were examined ex-situ by atomic force microscopy and scanning electron microscopy in combination with energy dispersive X-ray spectroscopy. Treatments at different temperatures resulted in various quantum efficiency values and storage lifetimes. Moderate temperatures of 400–500 °C were found to be more beneficial for the p-GaN surface quality, which was reflected by achieving higher quantum efficiency values. After the thermal cleaning, the samples were activated with a thin layer of cesium at an average pressure of 1 x E-9 mbar. The surface morphology was studied with scanning electron microscopy and energy dispersive X-ray spectroscopy after the samples were thermally cleaned and activated with cesium. The results showed that the surface appeared inhomogeneous when the samples were cleaned at a high temperature above 600 °C. A thermal cleaning from the back side through the substrate represented another possibility but did not yield higher quantum efficiency values. An in-situ analysis method facilitates following and understanding the changes in the surface electronic states before, during, and after any treatment of p-GaN:Cs photocathodes. For this purpose, an X-ray photoelectron spectrometer was applied that was built into an ultra-high vacuum system to prepare and characterize photocathodes. It allowed the in-situ monitoring of the photocathode surfaces beginning immediately after their cleaning and throughout the activation and degradation processes. The realization of the adaption of an X-ray photoelectron spectroscopy chamber to the preparation chamber presented a significant constructional challenge. Thus, this work paid special attention to the technical aspects of in-situ sample transportation between these chambers without leaving the ultra-high vacuum environment. The p-GaN surface was cleaned with different solutions and studied by X-ray photoelectron spectroscopy and atomic force microscopy, revealing that cleaning with a so-called 'piranha' solution in combination with rinsing in ethanol works best for the p-GaN surface. A cleaning step that solely uses ethanol is also possible and represents a simple cleaning procedure that is manageable in all laboratories. Afterward, the cleaned p-GaN samples underwent a subsequential thermal vacuum cleaning at various temperatures to achieve an atomically clean surface. Each treatment step was followed by X-ray photoelectron spectroscopy analysis without leaving the ultra-high vacuum environment, revealing residual oxygen and carbon on the p- GaN surface. A thermal treatment under vacuum did not entirely remove these organic contaminations, although the thermal cleaning reduced their peak intensities. The remaining oxygen and carbon contaminants were assumed to be residuals derived from the metal-organic chemical vapor deposition process. After the cesium activation, a shift toward a higher binding energy was observed in the X-ray photoelectron spectroscopy spectra of the related photoemission peaks. This shift indicated that the cesium was successfully adsorbed to the p-GaN surface. Before the cesium activation, adventitious carbon at a binding energy of approximately 284 eV was found, which was also present after the cesium activation but did not shift in its binding energy. It was also shown that the presence of remaining carbon significantly influenced the photocathode’s quality. After the cesium deposition, a new carbon species at a higher binding energy (approximately 286 eV) appeared in the carbon 1s spectrum. This new species showed a higher binding energy than adventitious carbon and was identified as a cesium carbide species. This cesium carbide species grew over time, resulting in islands on the surface. The X-ray photoelectron spectroscopy data facilitated the elucidation of the critical role of thiscesium carbide species in photocathode degradation. Typically, the quantum efficiency of photocathodes decays exponentially. Conversely, an immense quantum efficiency loss was observed after the p-GaN:Cs photocathodes were studied by X-ray photoelectron spectroscopy. The origin of the quantum efficiency loss derived from X-rays as an external influence and was not caused by the sample’s transportation. Therefore, potential X-ray damages to the p-GaN:Cs photocathodes were investigated. These experiments showed that the adsorbed cesium and its adhesion to the p-GaN surface were strongly influenced by X-ray irradiation. The cesium photoemission peaks shifted toward a lower binding energy, while the relative cesium concentration did not. This shift indicated that X-ray irradiation accelerated the external aging of the p-GaN photocathodes and thus it was proposed to use lower X-ray beam power or cool the samples to prevent X-ray damage to cesiated photocathodes. This work shows that an exclusive activation with cesium is feasible and that a re-activation of the same sample is possible. Quantum efficiency values of 1–12% were achieved when the p-GaN, grown on sapphire, was activated. The capability of an X-ray photoelectron spectroscopy analysis allowed the in-situ monitoring of the photocathode surface and shed light on the surface compositions that changed during the photocathodes’ degradation process

In-depth Surface Studies of p-GaN:Cs Photocathodes by Combining Ex-Situ Analytical Methods with In-Situ X-Ray Photoelectron Spectroscopy

The photocathode is one of the key components of particle accelerator facilities that provides electrons for experiments in many disciplines such as biomedicine, security imaging, and condensed matter physics. The requirements for the electron emitting material, the so-called photocathode, are rather high because these materials should provide a high quantum efficiency, a low thermal emittance, a fast response, and a long operational lifetime. At present, none of the state-of-the-art photocathodes can fully meet all the desired requirements. Therefore, new materials that can be used as potential photocathodes are urgently needed for future developments in accelerator research. Semiconductor photocathodes such as cesium telluride are the preferred materials in particle accelerators. These photocathodes provide high quantum efficiencies of above 10 %, making them highly attractive. The crystal growth of cesium telluride, as a compound semiconductor photocathode, requires the deposition of cesium and tellurium on a suitable substrate with an ideal chemical ratio, which seems elaborate and difficult to handle. In contrast, III-V semiconductors, such as gallium arsenide and gallium nitride (GaN), represent another type of semiconductor photocathode. These commercially available semiconductors are already grown on a substrate and only require a thin film of cesium and optional oxygen to obtain a photocathode. An atomically clean surface is necessary to achieve a negative electron affinity surface, which is the main prerequisite for high quantum efficiency. In this work, p-GaN grown on sapphire by metal-organic chemical vapor deposition, was wet chemically cleaned, and transferred into an ultra-high vacuum chamber, where it underwent a subsequent thermal cleaning. The cleaned p-GaN samples were activated with Cs to obtain p-GaN:Cs photocathodes and their performance was monitored with respect to their quality, especially concerning their quantum efficiency and storage lifetime. The surface topography and morphology were examined ex-situ by atomic force microscopy and scanning electron microscopy in combination with energy dispersive X-ray spectroscopy. Treatments at different temperatures resulted in various quantum efficiency values and storage lifetimes. Moderate temperatures of 400–500 °C were found to be more beneficial for the p-GaN surface quality, which was reflected by achieving higher quantum efficiency values. After the thermal cleaning, the samples were activated with a thin layer of cesium at an average pressure of 1 x E-9 mbar. The surface morphology was studied with scanning electron microscopy and energy dispersive X-ray spectroscopy after the samples were thermally cleaned and activated with cesium. The results showed that the surface appeared inhomogeneous when the samples were cleaned at a high temperature above 600 °C. A thermal cleaning from the back side through the substrate represented another possibility but did not yield higher quantum efficiency values. An in-situ analysis method facilitates following and understanding the changes in the surface electronic states before, during, and after any treatment of p-GaN:Cs photocathodes. For this purpose, an X-ray photoelectron spectrometer was applied that was built into an ultra-high vacuum system to prepare and characterize photocathodes. It allowed the in-situ monitoring of the photocathode surfaces beginning immediately after their cleaning and throughout the activation and degradation processes. The realization of the adaption of an X-ray photoelectron spectroscopy chamber to the preparation chamber presented a significant constructional challenge. Thus, this work paid special attention to the technical aspects of in-situ sample transportation between these chambers without leaving the ultra-high vacuum environment. The p-GaN surface was cleaned with different solutions and studied by X-ray photoelectron spectroscopy and atomic force microscopy, revealing that cleaning with a so-called 'piranha' solution in combination with rinsing in ethanol works best for the p-GaN surface. A cleaning step that solely uses ethanol is also possible and represents a simple cleaning procedure that is manageable in all laboratories. Afterward, the cleaned p-GaN samples underwent a subsequential thermal vacuum cleaning at various temperatures to achieve an atomically clean surface. Each treatment step was followed by X-ray photoelectron spectroscopy analysis without leaving the ultra-high vacuum environment, revealing residual oxygen and carbon on the p- GaN surface. A thermal treatment under vacuum did not entirely remove these organic contaminations, although the thermal cleaning reduced their peak intensities. The remaining oxygen and carbon contaminants were assumed to be residuals derived from the metal-organic chemical vapor deposition process. After the cesium activation, a shift toward a higher binding energy was observed in the X-ray photoelectron spectroscopy spectra of the related photoemission peaks. This shift indicated that the cesium was successfully adsorbed to the p-GaN surface. Before the cesium activation, adventitious carbon at a binding energy of approximately 284 eV was found, which was also present after the cesium activation but did not shift in its binding energy. It was also shown that the presence of remaining carbon significantly influenced the photocathode’s quality. After the cesium deposition, a new carbon species at a higher binding energy (approximately 286 eV) appeared in the carbon 1s spectrum. This new species showed a higher binding energy than adventitious carbon and was identified as a cesium carbide species. This cesium carbide species grew over time, resulting in islands on the surface. The X-ray photoelectron spectroscopy data facilitated the elucidation of the critical role of thiscesium carbide species in photocathode degradation. Typically, the quantum efficiency of photocathodes decays exponentially. Conversely, an immense quantum efficiency loss was observed after the p-GaN:Cs photocathodes were studied by X-ray photoelectron spectroscopy. The origin of the quantum efficiency loss derived from X-rays as an external influence and was not caused by the sample’s transportation. Therefore, potential X-ray damages to the p-GaN:Cs photocathodes were investigated. These experiments showed that the adsorbed cesium and its adhesion to the p-GaN surface were strongly influenced by X-ray irradiation. The cesium photoemission peaks shifted toward a lower binding energy, while the relative cesium concentration did not. This shift indicated that X-ray irradiation accelerated the external aging of the p-GaN photocathodes and thus it was proposed to use lower X-ray beam power or cool the samples to prevent X-ray damage to cesiated photocathodes. This work shows that an exclusive activation with cesium is feasible and that a re-activation of the same sample is possible. Quantum efficiency values of 1–12% were achieved when the p-GaN, grown on sapphire, was activated. The capability of an X-ray photoelectron spectroscopy analysis allowed the in-situ monitoring of the photocathode surface and shed light on the surface compositions that changed during the photocathodes’ degradation process

In-depth Surface Studies of p-GaN:Cs Photocathodes by Combining Ex-Situ Analytical Methods with In-Situ X-Ray Photoelectron Spectroscopy

The photocathode is one of the key components of particle accelerator facilities that provides electrons for experiments in many disciplines such as biomedicine, security imaging, and condensed matter physics. The requirements for the electron emitting material, the so-called photocathode, are rather high because these materials should provide a high quantum efficiency, a low thermal emittance, a fast response, and a long operational lifetime. At present, none of the state-of-the-art photocathodes can fully meet all the desired requirements. Therefore, new materials that can be used as potential photocathodes are urgently needed for future developments in accelerator research. Semiconductor photocathodes such as cesium telluride are the preferred materials in particle accelerators. These photocathodes provide high quantum efficiencies of above 10 %, making them highly attractive. The crystal growth of cesium telluride, as a compound semiconductor photocathode, requires the deposition of cesium and tellurium on a suitable substrate with an ideal chemical ratio, which seems elaborate and difficult to handle. In contrast, III-V semiconductors, such as gallium arsenide and gallium nitride (GaN), represent another type of semiconductor photocathode. These commercially available semiconductors are already grown on a substrate and only require a thin film of cesium and optional oxygen to obtain a photocathode. An atomically clean surface is necessary to achieve a negative electron affinity surface, which is the main prerequisite for high quantum efficiency. In this work, p-GaN grown on sapphire by metal-organic chemical vapor deposition, was wet chemically cleaned, and transferred into an ultra-high vacuum chamber, where it underwent a subsequent thermal cleaning. The cleaned p-GaN samples were activated with Cs to obtain p-GaN:Cs photocathodes and their performance was monitored with respect to their quality, especially concerning their quantum efficiency and storage lifetime. The surface topography and morphology were examined ex-situ by atomic force microscopy and scanning electron microscopy in combination with energy dispersive X-ray spectroscopy. Treatments at different temperatures resulted in various quantum efficiency values and storage lifetimes. Moderate temperatures of 400–500 °C were found to be more beneficial for the p-GaN surface quality, which was reflected by achieving higher quantum efficiency values. After the thermal cleaning, the samples were activated with a thin layer of cesium at an average pressure of 1 x E-9 mbar. The surface morphology was studied with scanning electron microscopy and energy dispersive X-ray spectroscopy after the samples were thermally cleaned and activated with cesium. The results showed that the surface appeared inhomogeneous when the samples were cleaned at a high temperature above 600 °C. A thermal cleaning from the back side through the substrate represented another possibility but did not yield higher quantum efficiency values. An in-situ analysis method facilitates following and understanding the changes in the surface electronic states before, during, and after any treatment of p-GaN:Cs photocathodes. For this purpose, an X-ray photoelectron spectrometer was applied that was built into an ultra-high vacuum system to prepare and characterize photocathodes. It allowed the in-situ monitoring of the photocathode surfaces beginning immediately after their cleaning and throughout the activation and degradation processes. The realization of the adaption of an X-ray photoelectron spectroscopy chamber to the preparation chamber presented a significant constructional challenge. Thus, this work paid special attention to the technical aspects of in-situ sample transportation between these chambers without leaving the ultra-high vacuum environment. The p-GaN surface was cleaned with different solutions and studied by X-ray photoelectron spectroscopy and atomic force microscopy, revealing that cleaning with a so-called 'piranha' solution in combination with rinsing in ethanol works best for the p-GaN surface. A cleaning step that solely uses ethanol is also possible and represents a simple cleaning procedure that is manageable in all laboratories. Afterward, the cleaned p-GaN samples underwent a subsequential thermal vacuum cleaning at various temperatures to achieve an atomically clean surface. Each treatment step was followed by X-ray photoelectron spectroscopy analysis without leaving the ultra-high vacuum environment, revealing residual oxygen and carbon on the p- GaN surface. A thermal treatment under vacuum did not entirely remove these organic contaminations, although the thermal cleaning reduced their peak intensities. The remaining oxygen and carbon contaminants were assumed to be residuals derived from the metal-organic chemical vapor deposition process. After the cesium activation, a shift toward a higher binding energy was observed in the X-ray photoelectron spectroscopy spectra of the related photoemission peaks. This shift indicated that the cesium was successfully adsorbed to the p-GaN surface. Before the cesium activation, adventitious carbon at a binding energy of approximately 284 eV was found, which was also present after the cesium activation but did not shift in its binding energy. It was also shown that the presence of remaining carbon significantly influenced the photocathode’s quality. After the cesium deposition, a new carbon species at a higher binding energy (approximately 286 eV) appeared in the carbon 1s spectrum. This new species showed a higher binding energy than adventitious carbon and was identified as a cesium carbide species. This cesium carbide species grew over time, resulting in islands on the surface. The X-ray photoelectron spectroscopy data facilitated the elucidation of the critical role of thiscesium carbide species in photocathode degradation. Typically, the quantum efficiency of photocathodes decays exponentially. Conversely, an immense quantum efficiency loss was observed after the p-GaN:Cs photocathodes were studied by X-ray photoelectron spectroscopy. The origin of the quantum efficiency loss derived from X-rays as an external influence and was not caused by the sample’s transportation. Therefore, potential X-ray damages to the p-GaN:Cs photocathodes were investigated. These experiments showed that the adsorbed cesium and its adhesion to the p-GaN surface were strongly influenced by X-ray irradiation. The cesium photoemission peaks shifted toward a lower binding energy, while the relative cesium concentration did not. This shift indicated that X-ray irradiation accelerated the external aging of the p-GaN photocathodes and thus it was proposed to use lower X-ray beam power or cool the samples to prevent X-ray damage to cesiated photocathodes. This work shows that an exclusive activation with cesium is feasible and that a re-activation of the same sample is possible. Quantum efficiency values of 1–12% were achieved when the p-GaN, grown on sapphire, was activated. The capability of an X-ray photoelectron spectroscopy analysis allowed the in-situ monitoring of the photocathode surface and shed light on the surface compositions that changed during the photocathodes’ degradation process

Experimentelle Untersuchungen zur Auslegung von Entspannungsverdampfern

In vielen Bereichen der Verfahrenstechnik werden Entspannungsverdampfer (Flash- Verdampfer) eingesetzt, beispielsweise zur Entsalzung von Meerwasser (Multi-stageflash, MSF). Dabei werden zur Phasentrennung nach dem Entspannungsorgan (Drosselventil) oft Abscheider mit großen Durchmessern eingesetzt, um die Dampfaufstiegsgeschwindigkeiten gering zu halten und somit den Tropfenmitriss zu minimieren. Dies kann hohe Anlagenkosten und einen hohen Platzbedarf verursachen. In der Literatur fehlen allgemein anerkannte Auslegungsrichtlinien für Entspannungsverdampfer. In dem Beitrag werden experimentelle Ergebnisse einer neuartigen Bauform von Abscheidern präsentiert, die zeigen, dass Entspannungsverdampfer wesentlich kompakter als bisher angenommen realisiert werden können

Experimental study on low temperature desalination by flash evaporation in a novel compact chamber design

The production of potable water in dry areas nowadays is mainly done by the desalination of seawater. State ofthe art desalination plants usually are built with high production capacities and consume a lot of electrical energy or energy from primary resources such as oil. This causes difficulties in rural areas, where no infrastructure is available neither for the plants' energy supply nor the distribution of the produced potable water. To address this need, small, self-sustaining and locally operated desalination plants came into the focus of research. In this work, a novel flash evaporator design is proposed which can be driven either by solar power or by low temperature waste heat. It offers low operation costs as well as easy maintenance. The results of an experimental setup operated with water at a feed flow rate of up to 1600 l/h are presented. It is shown that the proof of concept regarding efficient evaporation as well as efficient gas-liquid separation is provided successfully. The experimental evaporation yield counts for 98% of the vapor content that is expected from the vapor pressure curve of water. Neither measurements of the electrical conductivity of the gained condensate, nor the analysis of the vapor flow by optical methods show significant droplet entrainment, so there are no concerns regarding the purity of the produced condensate for the use as drinking water

Experimental study on low temperature desalination by flash evaporation in a novel compact chamber design

The production of potable water in dry areas nowadays is mainly done by the desalination of seawater. State of the art desalination plants usually are built with high production capacities and consume a lot of electrical energy or energy from primary resources such as oil. This causes difficulties in rural areas, where no infrastructure is available neither for the plants’ energy supply nor the distribution of the produced potable water. To address this need, small, self-sustaining and locally operated desalination plants came into the focus of research. In this work, a novel flash evaporator design is proposed which can be driven either by solar power or by low temperature waste heat. It offers low operation costs as well as easy maintenance. The results of an experimental setup operated with water at a feed flow rate of up to 1,600 l/h are presented. It is shown that the proof of concept regarding efficient evaporation as well as efficient gas-liquid separation is provided successfully. The experimental evaporation yield counts for 98 % of the vapor content that is expected from the vapor pressure curve of water. Neither measurements of the electrical conductivity of the gained condensate, nor the analysis of the vapor flow by optical methods show significant droplet entrainment, so there are no concerns regarding the purity of the produced condensate for the use as drinking water

Data to Impact on various cleaning procedures on p-GaN surfaces

This folder "XPS data" contains original and evaluated XPS data (.vms) on a p-GaN sample which was treated at various temperatures and underwent Ar+ irradiation. Furthermore, the folder "REM Images" contains REM images (.tif) and EDX data (.xlsx) on the used excessively treated sample. All images that are published in the main manuscript are collected as .tif files in the folder "images"

Viscoelastic and self-healing behavior of silica filled ionically modified poly(isobutylene-co-isoprene) rubber

Rubber composites were prepared by mixing bromobutyl rubber (BIIR) with silica particles in the presence of 1-butylimidazole. In addition to pristine (precipitated) silica, silanized particles with aliphatic or imidazolium functional groups, respectively, were used as filler. The silanization was carried out either separately or in situ during compounding. The silanized particles were characterized by TGA, 1H-29Si cross polarization (CP)/MAS NMR, and Zeta potential measurements. During compounding, the bromine groups of BIIR were converted with 1-butylimidazole to ionic imidazolium groups which formed a dynamic network by ionic association. Based on DMA temperature and strain sweep measurements as well as cyclic tensile tests and stress-strain measurements it could be concluded that interactions between the ionic groups and interactions with the functional groups of the silica particles strongly influence the mechanical and viscoelastic behavior of the composites. A particularly pronounced reinforcing effect was observed for the composite with pristine silica, which was attributed to acid-base interactions between the silanol and imidazolium groups. In composites with alkyl or imidazolium functionalized silica particles, the interactions between the filler and the rubber matrix form dynamic networks with pronounced self-healing behavior and excellent tensile strength values of up to 19 MPa. This new approach in utilizing filler-matrix interactions in the formation of dynamic networks opens up new avenues in designing new kinds of particle-reinforced self-healing elastomeric materials with high technological relevance

Dataset to publication: Influence of surface carbon on the performance of cesiated p-GaN photocathodes with high quantum efficiency

This data repository contains analyzed data files of the shown figures. Please see the info.txt files in the individual directories and the original manuscript for guidance. Manuscript title: Influence of surface carbon on the performance of cesiated p-GaN photocathodes with high quantum efficiency Original datasets of the surface study of p-GaN: after wet chemical cleaning, after Cs deposition, during the degradation, and the influence of X-rays on p-GaN:Cs photocathodes. The folder contains datasets from origin, excel and XPS files (.vms) The XPS data were evaluated by using CasaXPS