Franciszek Krok - Academia.edu (original) (raw)

Papers by Franciszek Krok

Progress in Surface Science, 2015

Nanostructure formation by single slow highly charged ion impacts can be associated with high den... more Nanostructure formation by single slow highly charged ion impacts can be associated with high density of electronic excitations at the impact points of the ions. Experimental results show that depending on the target material these electronic excitations may lead to very large desorption yields in the order of a few 1000 atoms per ion or the formation of nanohillocks at the impact site. Even in ultra-thin insulating membranes the formation of nanometer sized pores is observed after ion impact. In this paper, we show recent results on nanostructure formation by highly charged ions and compare them to structures and defects observed after intense electron and light ion irradiation of ionic crystals and Graphene. Additional data on energy loss, charge exchange and secondary electron emission of highly charged ions clearly show that the ion's charge state dominates the defect formation at the surface.

Annual Report of National Institute for Fusion Science, 2014

The AIII-BV semiconductors, due to their unique properties, are considered for decades as a promi... more The AIII-BV semiconductors, due to their unique properties, are considered for decades as a promising material to overcome the limitations of the silicone semiconductor devices. One of the important aspects in the AIII-BV semiconductor technology is gold-semiconductor interactions at the nanoscale, since gold is widely used to catalyze the growth of AIII-BV structures and it is also an important material for the electronic device mainly used as electrodes. Here, we report, by investigation of the nanostructures formed in the process of thermally induced Au self-assembly on various AIII-BV semiconductor surfaces (InSb(001), InAs(001), InP(001), GaSb(001), GaAs(001), GaP(001)), how Au interacts with each material at atomic scale as provided by the atomically resolved aberration corrected HAADF STEM measurements. The Au diffusion into bulk AIII-BV crystal lattice was seen only for InSb crystal, and investigated at atomic level experimentally by Machine Learning HAADF image quantificati...

Atomic collision research in Japan, 1996

Applied Surface Science, 2021

Repeated sputtering and annealing are standard preparation methods for obtaining a stoichiometric... more Repeated sputtering and annealing are standard preparation methods for obtaining a stoichiometric TiO 2 (1 1 0) surface for surface science experiments. However, both processes result in a reduction in TiO 2 crystal when used separately, leading to the modification of the physical and chemical properties of oxide materials. Our investigation aims to determine how these two processes affect the electronic properties of the surface and subsurface regions at the nanometer scale. To accomplish this goal, we utilized local microscopy (Kelvin probe force microscopy and local-conductivity atomic force microscopy) and spectroscopy methods (X-ray photoelectron spectroscopy and secondary ion mass spectrometry). We found that repeated sputtering and annealing does, in fact, affect both the conductivity and work function of the surface. The work function, as well as conductivity, increase with increasing number of cycles, but then reach a plateau. Furthermore, we show that the way the surface is prepared, using multiple cycles or one cycle of equivalent ion-beam fluence, matters. We attribute the differences in the crystal properties to the dynamics of stoichiometric changes during sputtering and subsequent annealing which we illustrate using secondary ion mass spectroscopy, which shows that after multiple cycles the subsurface layer is modified, even though XPS shows a stoichiometric surface.

Crystals, 2020

This study investigates the impact of extended defects such as dislocations on the electronic pro... more This study investigates the impact of extended defects such as dislocations on the electronic properties of SrTiO3 by using a 36.8° bicrystal as a model system. In order to evaluate the hypothesis that dislocations can serve as preferential reduction sites, which has been proposed in the literature on the basis of ab initio simulations, as well as on experiments employing local-conductivity atomic force microscopy (LC-AFM), detailed investigations of the bicrystal boundary are conducted. In addition to LC-AFM, fluorescence lifetime imaging microscopy (FLIM) is applied herein as a complementary method for mapping the local electronic properties on the microscale. Both techniques confirm that the electronic structure and electronic transport in dislocation-rich regions significantly differ from those of undistorted SrTiO3. Upon thermal reduction, a further confinement of conductivity to the bicrystal boundary region was found, indicating that extended defects can indeed be regarded as...

Micron, 2020

Analysis of microscope images is a tedious work which requires patience and time, usually done ma... more Analysis of microscope images is a tedious work which requires patience and time, usually done manually by the microscopist after data collection. The results obtained in such a way might be biased by the human who performed the analysis. Here we introduce an approach of automatic image analysis, which is based on locally applied Fourier Transform and Machine Learning methods. In this approach, a whole image is scanned by a local moving window with defined size and the 2D Fourier Transform is calculated for each window. Then, all the Local Fourier Transforms are fed into Machine Learning processing. Firstly, a number of components in the data is estimated from Principal Component Analysis (PCA) Scree Plot performed on the data. Secondly, the data are decomposed blindly by Non-Negative Matrix Factorization (NMF) into interpretable spatial maps (loadings) and corresponding Fourier Transforms (factors). As a result, the microscopic image is analyzed and the features on the image are automatically discovered, based on the local changes in Fourier Transform, without human bias. The user selects only a size and movement of the scanning local window which defines the final analysis resolution. This automatic approach was successfully applied to analysis of various microscopic images with and without local periodicity i.e. atomically resolved High Angle Annular Dark Field (HAADF) Scanning Transmission Electron Microscopy (STEM) image of Au nanoisland of fcc and Au hcp phases, Scanning Tunneling Microscopy (STM) image of Au-induced reconstruction on Ge(001) surface, Scanning Electron Microscopy (SEM) image of metallic nanoclusters grown on GaSb surface, and Fluorescence microscopy image of HeLa cell line of cervical cancer. The proposed approach could be used to automatically analyze the local structure of microscopic images within a time of about a minute for a single image on a modern desktop/notebook computer and it is freely available as a Python analysis notebook and Python program for batch processing.

Scientific Reports, 2019

We investigate the thermal reduction of TiO2 in ultra-high vacuum. Contrary to what is usually as... more We investigate the thermal reduction of TiO2 in ultra-high vacuum. Contrary to what is usually assumed, we observe that the maximal surface reduction occurs not during the heating, but during the cooling of the sample back to room temperature. We describe the self-reduction, which occurs as a result of differences in the energies of defect formation in the bulk and surface regions. The findings presented are based on X-ray photoelectron spectroscopy carried out in-operando during the heating and cooling steps. The presented conclusions, concerning the course of redox processes, are especially important when considering oxides for resistive switching and neuromorphic applications and also when describing the mechanisms related to the basics of operation of solid oxide fuel cells.

Materials Science and Engineering: C, 2019

Scientific Reports, 2019

Electroreduction experiments on metal oxides are well established for investigating the nature of... more Electroreduction experiments on metal oxides are well established for investigating the nature of the material change in memresistive devices, whose basic working principle is an electrically-induced reduction. While numerous research studies on this topic have been conducted, the influence of extended defects such as dislocations has not been addressed in detail hitherto. Here, we show by employing thermal microscopy to detect local Joule heating effects in the first stage of electroreduction of SrTiO3 that the current is channelled along extended defects such as dislocations which were introduced mechanically by scratching or sawing. After prolonged degradation, the matrix of the crystal is also electroreduced and the influence of the initially present dislocations diminished. At this stage, a hotspot at the anode develops due to stoichiometry polarisation leading not only to the gliding of existing dislocations, but also to the evolution of new dislocations. Such a formation is c...

Nano Letters, 2017

The quantitative composition of metal alloy nanowires on InSb(001) semiconductor surface and gold... more The quantitative composition of metal alloy nanowires on InSb(001) semiconductor surface and gold nanostructures on germanium surface is determined by blind source separation (BSS) machine learning (ML) method using non negative matrix factorization (NMF) from energy dispersive X-ray spectroscopy (EDX) spectrum image maps measured in a scanning electron microscope (SEM). The BSS method blindly decomposes the collected EDX spectrum image into three source components, which correspond directly to the X-ray signals coming from the supported metal nanostructures, bulk semiconductor signal and carbon background. The recovered quantitative composition is validated by detailed Monte Carlo simulations and is confirmed by separate cross-sectional TEM EDX measurements of the nanostructures. This shows that SEM EDX measurements together with machine learning blind source separation processing could be successfully used for the nanostructures quantitative chemical composition determination.

Data in Brief, 2016

Data included in this article are associated with the research article entitled 'Protocol of sing... more Data included in this article are associated with the research article entitled 'Protocol of single cells preparation for time-of-flight secondary ion mass spectrometry' (Bobrowska et al., 2016 in press) [1]. This data file contains topography images of single melanoma cells recorded using atomic force microscopy (AFM). Single cells cultured on glass surface were subjected to the proposed sample preparation protocol applied to prepare biological samples for time-of-flight secondary ion mass spectrometry (ToF SIMS) measurements. AFM images were collected step-by-step for the single cell, after each step of the proposed preparation protocol. It consists of four main parts: (i) paraformaldehyde fixation, (ii) salt removal, (iii) dehydrating, and (iv) sample drying. In total 13 steps are required, starting from imaging of a living cell in a culture medium and ending up at images of a dried cell in the air. The protocol was applied to melanoma cells from two cell lines, namely, WM115 melanoma cells originated from primary melanoma site and WM266-4 ones being the metastasis of WM115 cells to skin.

Materials Chemistry and Physics, 2017

 We fabricated the nanotubular TiO 2 arrays by electrochemical anodic oxidation of titanium  We... more  We fabricated the nanotubular TiO 2 arrays by electrochemical anodic oxidation of titanium  We investigated the growth of hydroxyapatite coatings on nanotubular TiO 2 substrates  The crystallization of hydroxyapatite was performed by chelate decomposition method  The efficiency of apatite formation depends on TiO 2 substrate morphology and crystallinity

Journal of Diabetes Research, 2016

The aim of this study was to check the relationship between the density of urinary EVs, their siz... more The aim of this study was to check the relationship between the density of urinary EVs, their size distribution, and the progress of early renal damage in type 2 diabetic patients (DMt2). Patients were enrolled to this study, and glycated hemoglobin (HbA1c) below 7% was a threshold for properly controlled diabetic patients (CD) and poorly controlled diabetic patients (UD). Patients were further divided into two groups: diabetic patients without renal failure (NRF) and with renal failure (RF) according to the Glomerular Filtration Rate. Density and diameter of EVs were determined by Tunable Resistive Pulse Sensing. Additionally, EVs were visualized by means of Transmission and Environmental Scanning Electron Microscopy. Nano-liquid chromatography coupled offline with mass spectrometry (MALDI-TOF-MS/MS) was applied for proteomic analysis. RF had reduced density of EVs compared to NRF. The size distribution study showed that CD had larger EVs (mode) than UD (115 versus 109 nm;p<0.05...

Physical Review Letters, 2016

Polycapillary x-ray focusing devices are built from hundreds of thousands of bent glass microcapi... more Polycapillary x-ray focusing devices are built from hundreds of thousands of bent glass microcapillaries that are stacked into hexagonal arrays. We show that intrinsic point defects of the optics (missing or larger capillaries) lead to the formation of multiple x-ray images of an object, which was positioned in the focal plane. These images can be recorded in parallel, and provide spatial resolution that is limited by the defect size and not by the focal spot size. In a proof-of-principle experiment, we demonstrate sub-micron resolution that has never been achieved with polycapillary focusing optics. Tailored optics with a controlled distribution of "defects" could be used for multimodal nanoscale x-ray imaging with laboratory setups.

Conference Record of the Twenty-Eighth IEEE Photovoltaic Specialists Conference - 2000 (Cat. No.00CH37036), 2000

The porous silicon (PS) layers on the front surface of n+ /p monocrystalline, textured silicon so... more The porous silicon (PS) layers on the front surface of n+ /p monocrystalline, textured silicon solar cells were investigated with the aim to improve the performance of standard screen-printed silicon solar cells. The application of the PS layer is very promising because simultaneous, antireflection coating and surface passivation can be obtained in one chemical process. In this work the improvement

Physical Review Letters, 2003

Noncontact atomic force microscopy (NC-AFM) has been used to study the c8 2 InSb(001) and the c8 ... more Noncontact atomic force microscopy (NC-AFM) has been used to study the c8 2 InSb(001) and the c8 2 GaAs(001) surfaces prepared by sputter cleaning and annealing. Atomically resolved tipsurface interaction maps display different characteristic patterns depending on the tip front atom type. It is shown that representative AFM maps can be interpreted consistently with the most recent structural model of A III B V 001 surface, as corresponding to the A III sublattice, to the B V sublattice, or to the combination of both sublattices.

Physical Review B, 2007

We have investigated the formation of gold nanoclusters during submonolayer deposition on atomica... more We have investigated the formation of gold nanoclusters during submonolayer deposition on atomically flat KBr and RbI ͑001͒ ionic-crystal substrates, as well as on substrates patterned with two-dimensional ͑2D͒ nanoscale pits produced by electron stimulated desorption. In this way, it is possible to produce atomic steps free from stress and local charging, which are normally considered the reason for enhanced cluster nucleation. Easy nucleation of the Au clusters inside the pits at the lower side of the atomic step edge is not observed, while nucleation of the Au nanoclusters is found to occur preferentially at the upper step edges. Moreover, we observe that gold atoms landing inside the bottom of the pits are able to escape from them by means of thermally activated upward diffusion at the step edges. We propose that the preferential Au nucleation sites at the edges of 2D pits are activated by F centers produced in electronic processes used for nanopatterning of the ionic crystals.

Physical Review B, 2008

We report on high-resolution potential imaging of heterogeneous surfaces by means of Kelvin probe... more We report on high-resolution potential imaging of heterogeneous surfaces by means of Kelvin probe force microscopy, working in frequency modulation mode ͑FM-KPFM͒, performed in ultrahigh vacuum. To study the limits of potential and lateral resolutions in FM-KPFM, we have investigated clean surface of compound semiconductor InSb͑001͒ and the same surface with some submonolayer coverages of KBr and Au. It was found that long-and short-range bias-dependent interactions, acting between the tip and the surface, could be detected and that both interactions contribute to the measured contact potential difference ͑CPD͒ signal. On the one hand, when only the long-range electrostatic interactions between the tip and the surface are active, the CPD map provides the distribution of the local surface potential on the imaged sample with the lateral resolution and the correctness of the measured values depending on the measurement conditions. For this case, the experimental findings were compared with the predictions of theoretical calculations based on a realistic model for the cantilever-sample geometry. On the other hand, when the short-range and bias-dependent interactions are detected, FM-KPFM provides even the sub-nanometer contrast in the CPD signal. In this situation, however, the measured CPD signal is not related to the sample surface potential but reflects the properties of the front tip atom-surface atom interactions.

Progress in Surface Science, 2015

Nanostructure formation by single slow highly charged ion impacts can be associated with high den... more Nanostructure formation by single slow highly charged ion impacts can be associated with high density of electronic excitations at the impact points of the ions. Experimental results show that depending on the target material these electronic excitations may lead to very large desorption yields in the order of a few 1000 atoms per ion or the formation of nanohillocks at the impact site. Even in ultra-thin insulating membranes the formation of nanometer sized pores is observed after ion impact. In this paper, we show recent results on nanostructure formation by highly charged ions and compare them to structures and defects observed after intense electron and light ion irradiation of ionic crystals and Graphene. Additional data on energy loss, charge exchange and secondary electron emission of highly charged ions clearly show that the ion's charge state dominates the defect formation at the surface.

Annual Report of National Institute for Fusion Science, 2014

The AIII-BV semiconductors, due to their unique properties, are considered for decades as a promi... more The AIII-BV semiconductors, due to their unique properties, are considered for decades as a promising material to overcome the limitations of the silicone semiconductor devices. One of the important aspects in the AIII-BV semiconductor technology is gold-semiconductor interactions at the nanoscale, since gold is widely used to catalyze the growth of AIII-BV structures and it is also an important material for the electronic device mainly used as electrodes. Here, we report, by investigation of the nanostructures formed in the process of thermally induced Au self-assembly on various AIII-BV semiconductor surfaces (InSb(001), InAs(001), InP(001), GaSb(001), GaAs(001), GaP(001)), how Au interacts with each material at atomic scale as provided by the atomically resolved aberration corrected HAADF STEM measurements. The Au diffusion into bulk AIII-BV crystal lattice was seen only for InSb crystal, and investigated at atomic level experimentally by Machine Learning HAADF image quantificati...

Atomic collision research in Japan, 1996

Applied Surface Science, 2021

Repeated sputtering and annealing are standard preparation methods for obtaining a stoichiometric... more Repeated sputtering and annealing are standard preparation methods for obtaining a stoichiometric TiO 2 (1 1 0) surface for surface science experiments. However, both processes result in a reduction in TiO 2 crystal when used separately, leading to the modification of the physical and chemical properties of oxide materials. Our investigation aims to determine how these two processes affect the electronic properties of the surface and subsurface regions at the nanometer scale. To accomplish this goal, we utilized local microscopy (Kelvin probe force microscopy and local-conductivity atomic force microscopy) and spectroscopy methods (X-ray photoelectron spectroscopy and secondary ion mass spectrometry). We found that repeated sputtering and annealing does, in fact, affect both the conductivity and work function of the surface. The work function, as well as conductivity, increase with increasing number of cycles, but then reach a plateau. Furthermore, we show that the way the surface is prepared, using multiple cycles or one cycle of equivalent ion-beam fluence, matters. We attribute the differences in the crystal properties to the dynamics of stoichiometric changes during sputtering and subsequent annealing which we illustrate using secondary ion mass spectroscopy, which shows that after multiple cycles the subsurface layer is modified, even though XPS shows a stoichiometric surface.

Crystals, 2020

This study investigates the impact of extended defects such as dislocations on the electronic pro... more This study investigates the impact of extended defects such as dislocations on the electronic properties of SrTiO3 by using a 36.8° bicrystal as a model system. In order to evaluate the hypothesis that dislocations can serve as preferential reduction sites, which has been proposed in the literature on the basis of ab initio simulations, as well as on experiments employing local-conductivity atomic force microscopy (LC-AFM), detailed investigations of the bicrystal boundary are conducted. In addition to LC-AFM, fluorescence lifetime imaging microscopy (FLIM) is applied herein as a complementary method for mapping the local electronic properties on the microscale. Both techniques confirm that the electronic structure and electronic transport in dislocation-rich regions significantly differ from those of undistorted SrTiO3. Upon thermal reduction, a further confinement of conductivity to the bicrystal boundary region was found, indicating that extended defects can indeed be regarded as...

Micron, 2020

Analysis of microscope images is a tedious work which requires patience and time, usually done ma... more Analysis of microscope images is a tedious work which requires patience and time, usually done manually by the microscopist after data collection. The results obtained in such a way might be biased by the human who performed the analysis. Here we introduce an approach of automatic image analysis, which is based on locally applied Fourier Transform and Machine Learning methods. In this approach, a whole image is scanned by a local moving window with defined size and the 2D Fourier Transform is calculated for each window. Then, all the Local Fourier Transforms are fed into Machine Learning processing. Firstly, a number of components in the data is estimated from Principal Component Analysis (PCA) Scree Plot performed on the data. Secondly, the data are decomposed blindly by Non-Negative Matrix Factorization (NMF) into interpretable spatial maps (loadings) and corresponding Fourier Transforms (factors). As a result, the microscopic image is analyzed and the features on the image are automatically discovered, based on the local changes in Fourier Transform, without human bias. The user selects only a size and movement of the scanning local window which defines the final analysis resolution. This automatic approach was successfully applied to analysis of various microscopic images with and without local periodicity i.e. atomically resolved High Angle Annular Dark Field (HAADF) Scanning Transmission Electron Microscopy (STEM) image of Au nanoisland of fcc and Au hcp phases, Scanning Tunneling Microscopy (STM) image of Au-induced reconstruction on Ge(001) surface, Scanning Electron Microscopy (SEM) image of metallic nanoclusters grown on GaSb surface, and Fluorescence microscopy image of HeLa cell line of cervical cancer. The proposed approach could be used to automatically analyze the local structure of microscopic images within a time of about a minute for a single image on a modern desktop/notebook computer and it is freely available as a Python analysis notebook and Python program for batch processing.

Scientific Reports, 2019

We investigate the thermal reduction of TiO2 in ultra-high vacuum. Contrary to what is usually as... more We investigate the thermal reduction of TiO2 in ultra-high vacuum. Contrary to what is usually assumed, we observe that the maximal surface reduction occurs not during the heating, but during the cooling of the sample back to room temperature. We describe the self-reduction, which occurs as a result of differences in the energies of defect formation in the bulk and surface regions. The findings presented are based on X-ray photoelectron spectroscopy carried out in-operando during the heating and cooling steps. The presented conclusions, concerning the course of redox processes, are especially important when considering oxides for resistive switching and neuromorphic applications and also when describing the mechanisms related to the basics of operation of solid oxide fuel cells.

Materials Science and Engineering: C, 2019

Scientific Reports, 2019

Electroreduction experiments on metal oxides are well established for investigating the nature of... more Electroreduction experiments on metal oxides are well established for investigating the nature of the material change in memresistive devices, whose basic working principle is an electrically-induced reduction. While numerous research studies on this topic have been conducted, the influence of extended defects such as dislocations has not been addressed in detail hitherto. Here, we show by employing thermal microscopy to detect local Joule heating effects in the first stage of electroreduction of SrTiO3 that the current is channelled along extended defects such as dislocations which were introduced mechanically by scratching or sawing. After prolonged degradation, the matrix of the crystal is also electroreduced and the influence of the initially present dislocations diminished. At this stage, a hotspot at the anode develops due to stoichiometry polarisation leading not only to the gliding of existing dislocations, but also to the evolution of new dislocations. Such a formation is c...

Nano Letters, 2017

The quantitative composition of metal alloy nanowires on InSb(001) semiconductor surface and gold... more The quantitative composition of metal alloy nanowires on InSb(001) semiconductor surface and gold nanostructures on germanium surface is determined by blind source separation (BSS) machine learning (ML) method using non negative matrix factorization (NMF) from energy dispersive X-ray spectroscopy (EDX) spectrum image maps measured in a scanning electron microscope (SEM). The BSS method blindly decomposes the collected EDX spectrum image into three source components, which correspond directly to the X-ray signals coming from the supported metal nanostructures, bulk semiconductor signal and carbon background. The recovered quantitative composition is validated by detailed Monte Carlo simulations and is confirmed by separate cross-sectional TEM EDX measurements of the nanostructures. This shows that SEM EDX measurements together with machine learning blind source separation processing could be successfully used for the nanostructures quantitative chemical composition determination.

Data in Brief, 2016

Data included in this article are associated with the research article entitled 'Protocol of sing... more Data included in this article are associated with the research article entitled 'Protocol of single cells preparation for time-of-flight secondary ion mass spectrometry' (Bobrowska et al., 2016 in press) [1]. This data file contains topography images of single melanoma cells recorded using atomic force microscopy (AFM). Single cells cultured on glass surface were subjected to the proposed sample preparation protocol applied to prepare biological samples for time-of-flight secondary ion mass spectrometry (ToF SIMS) measurements. AFM images were collected step-by-step for the single cell, after each step of the proposed preparation protocol. It consists of four main parts: (i) paraformaldehyde fixation, (ii) salt removal, (iii) dehydrating, and (iv) sample drying. In total 13 steps are required, starting from imaging of a living cell in a culture medium and ending up at images of a dried cell in the air. The protocol was applied to melanoma cells from two cell lines, namely, WM115 melanoma cells originated from primary melanoma site and WM266-4 ones being the metastasis of WM115 cells to skin.

Materials Chemistry and Physics, 2017

 We fabricated the nanotubular TiO 2 arrays by electrochemical anodic oxidation of titanium  We... more  We fabricated the nanotubular TiO 2 arrays by electrochemical anodic oxidation of titanium  We investigated the growth of hydroxyapatite coatings on nanotubular TiO 2 substrates  The crystallization of hydroxyapatite was performed by chelate decomposition method  The efficiency of apatite formation depends on TiO 2 substrate morphology and crystallinity

Journal of Diabetes Research, 2016

The aim of this study was to check the relationship between the density of urinary EVs, their siz... more The aim of this study was to check the relationship between the density of urinary EVs, their size distribution, and the progress of early renal damage in type 2 diabetic patients (DMt2). Patients were enrolled to this study, and glycated hemoglobin (HbA1c) below 7% was a threshold for properly controlled diabetic patients (CD) and poorly controlled diabetic patients (UD). Patients were further divided into two groups: diabetic patients without renal failure (NRF) and with renal failure (RF) according to the Glomerular Filtration Rate. Density and diameter of EVs were determined by Tunable Resistive Pulse Sensing. Additionally, EVs were visualized by means of Transmission and Environmental Scanning Electron Microscopy. Nano-liquid chromatography coupled offline with mass spectrometry (MALDI-TOF-MS/MS) was applied for proteomic analysis. RF had reduced density of EVs compared to NRF. The size distribution study showed that CD had larger EVs (mode) than UD (115 versus 109 nm;p<0.05...

Physical Review Letters, 2016

Polycapillary x-ray focusing devices are built from hundreds of thousands of bent glass microcapi... more Polycapillary x-ray focusing devices are built from hundreds of thousands of bent glass microcapillaries that are stacked into hexagonal arrays. We show that intrinsic point defects of the optics (missing or larger capillaries) lead to the formation of multiple x-ray images of an object, which was positioned in the focal plane. These images can be recorded in parallel, and provide spatial resolution that is limited by the defect size and not by the focal spot size. In a proof-of-principle experiment, we demonstrate sub-micron resolution that has never been achieved with polycapillary focusing optics. Tailored optics with a controlled distribution of "defects" could be used for multimodal nanoscale x-ray imaging with laboratory setups.

Conference Record of the Twenty-Eighth IEEE Photovoltaic Specialists Conference - 2000 (Cat. No.00CH37036), 2000

The porous silicon (PS) layers on the front surface of n+ /p monocrystalline, textured silicon so... more The porous silicon (PS) layers on the front surface of n+ /p monocrystalline, textured silicon solar cells were investigated with the aim to improve the performance of standard screen-printed silicon solar cells. The application of the PS layer is very promising because simultaneous, antireflection coating and surface passivation can be obtained in one chemical process. In this work the improvement

Physical Review Letters, 2003

Noncontact atomic force microscopy (NC-AFM) has been used to study the c8 2 InSb(001) and the c8 ... more Noncontact atomic force microscopy (NC-AFM) has been used to study the c8 2 InSb(001) and the c8 2 GaAs(001) surfaces prepared by sputter cleaning and annealing. Atomically resolved tipsurface interaction maps display different characteristic patterns depending on the tip front atom type. It is shown that representative AFM maps can be interpreted consistently with the most recent structural model of A III B V 001 surface, as corresponding to the A III sublattice, to the B V sublattice, or to the combination of both sublattices.

Physical Review B, 2007

We have investigated the formation of gold nanoclusters during submonolayer deposition on atomica... more We have investigated the formation of gold nanoclusters during submonolayer deposition on atomically flat KBr and RbI ͑001͒ ionic-crystal substrates, as well as on substrates patterned with two-dimensional ͑2D͒ nanoscale pits produced by electron stimulated desorption. In this way, it is possible to produce atomic steps free from stress and local charging, which are normally considered the reason for enhanced cluster nucleation. Easy nucleation of the Au clusters inside the pits at the lower side of the atomic step edge is not observed, while nucleation of the Au nanoclusters is found to occur preferentially at the upper step edges. Moreover, we observe that gold atoms landing inside the bottom of the pits are able to escape from them by means of thermally activated upward diffusion at the step edges. We propose that the preferential Au nucleation sites at the edges of 2D pits are activated by F centers produced in electronic processes used for nanopatterning of the ionic crystals.

Physical Review B, 2008

We report on high-resolution potential imaging of heterogeneous surfaces by means of Kelvin probe... more We report on high-resolution potential imaging of heterogeneous surfaces by means of Kelvin probe force microscopy, working in frequency modulation mode ͑FM-KPFM͒, performed in ultrahigh vacuum. To study the limits of potential and lateral resolutions in FM-KPFM, we have investigated clean surface of compound semiconductor InSb͑001͒ and the same surface with some submonolayer coverages of KBr and Au. It was found that long-and short-range bias-dependent interactions, acting between the tip and the surface, could be detected and that both interactions contribute to the measured contact potential difference ͑CPD͒ signal. On the one hand, when only the long-range electrostatic interactions between the tip and the surface are active, the CPD map provides the distribution of the local surface potential on the imaged sample with the lateral resolution and the correctness of the measured values depending on the measurement conditions. For this case, the experimental findings were compared with the predictions of theoretical calculations based on a realistic model for the cantilever-sample geometry. On the other hand, when the short-range and bias-dependent interactions are detected, FM-KPFM provides even the sub-nanometer contrast in the CPD signal. In this situation, however, the measured CPD signal is not related to the sample surface potential but reflects the properties of the front tip atom-surface atom interactions.