Ing. Jaroslav Kuliček, Ph.D.

Research investigating the interface between biological organisms and nanomaterials nowadays requires multi-faceted microscopic methods to elucidate the interaction mechanisms and effects. Here we describe a novel approach and methodology correlating data from an atomic force microscope inside a scanning electron microscope (AFM-in-SEM). This approach is demonstrated on bacteria-diamond-metal nanocomposite samples relevant in current life science research. We describe a procedure for preparing such multi-component test samples containing E. coli bacteria and chitosan-coated hydrogenated nanodiamonds decorated with silver nanoparticles on a carbon-coated gold grid. Microscopic topography information (AFM) is combined with chemical, material, and morphological information (SEM using SE and BSE at varied acceleration voltages) from the same region of interest and processed to create 3D correlative probe-electron microscopy (CPEM) images. We also establish a novel 3D RGB color image algorithm for merging multiple SE/BSE data from SEM with the AFM surface topography data which provides additional information about microscopic interaction of the diamond-metal nanocomposite with bacteria, not achievable by individual analyses. The methodology of CPEM data interpretation is independently corroborated by further in-situ (EDS) and ex-situ (micro-Raman) chemical characterization as well as by force volume AFM analysis. We also discuss the broader applicability and benefits of the methodology for life science research.

Heterostructures of graphene and hexagonal boron nitride (G/h-BN) have been widely studied for controlling and utilizing graphene electronic properties. Here we characterize specific optical and electronic properties of G/h-BN heterostructures made of a high-quality single layer chemical vapor deposition (CVD) graphene laid over h-BN flakes, with focus on plasmonic effects. We compare the G/h-BN properties on Si and SiO2 substrates by micro-Raman spectroscopy mapping, Kelvin probe force microscopy, optical and atomic force microscopy. We observe highly enhanced Raman intensity (up to 280 %) from Si as well as graphene along the G/h-BN edge. It is attributed to localized concentration of electrons in graphene and suitable perpendicular orientation of plasmonic vibrations at the edge. The plasmonic Raman enhancement occurs under a visible light excitation (532 nm) and the effect can be tuned by the h-BN flake thickness (10–150 nm). The enhancement is specific to G/h-BN/Si structures, on G/h-BN/SiO2 structures the Raman signal is suppressed while I2D/IG ratio is increased. Vice versa, change of surface potential under visible light illumination (photovoltage) is on G/h-BN/Si negligible (within 10 mV) compared to the G/h-BN/SiO2 structures. These results open new prospects for broad utilization of localized visible plasmonic effects in graphene.

Plasma provides specific adjustment of solid-state surface properties offering an alternative to high temperature treatment. Herein, hydrogen plasma treatment of monocrystalline (0001) ZnO surface is studied in an inductively coupled plasma reactor with reduced capacitively coupled plasma mode. The crucial role of electrical grounding of the sample holder for plasma etching and related changes in the morphology, optical, and electrical properties of surfaces exposed to electron and ion bombardment are explained. The effects on the chemical composition of the surface are analyzed by X-ray photoelectron spectroscopy (XPS), optical properties by photoluminescence spectroscopy, topography, roughness, and surface measurements by atomic force microscopy (AFM) and Kelvin probe force microscopy (KPFM). All methods show altered ZnO surface properties before and after plasma treatment strongly depending on the electrical potential of the holder.

Organic-based photovoltaic devices emerged as a complementary technology to silicon solar cells with specific advantages in terms of cost, ease of deployment, semi-transparency, and performance under low and diffuse light conditions. In this work, thin-film boron-doped diamond (B:NCD) electrodes are employed for their useful op-tical, electronic, and chemical properties, as well as stability and environmental safety. A set of oligothiophene perylene diimide (nT-PDI) donor-acceptor chromophores is designed and synthesized in order to investigate the influence of the oligothiophene spacer length when the nT-PDI molecule is attached to a B:NCD electrode. The chromophores are anchored to the diamond surface via diazonium grafting followed by Sonogashira cross -coupling. X-ray photoelectron spectroscopy shows that the surface coverage decreases with increasing oligo-thiophene length. Density functional theory (DFT/TDDFT) calculations reveal the upright nT-PDI orientation and the most efficient photogenerated charge separation and injection to diamond for elongated oligothiophene chains (8T-PDI). Yet, the maximum photovoltage is obtained for an intermediate oligothiophene length (3T-PDI), providing an optimum between decreasing transport efficiency and increasing efficiency of charge separation and reduced recombination with increasing oligothiophene length. Holes transferred from nT-PDI to diamond persist there even after the illumination is switched off. Such features may be beneficial for application in solar cells.

Understanding and controlling the optical and electrical properties of the solar cells, from the absorber layer to the complete devices, is one of the key elements for engineering high-efficiency devices. Such an understanding, especially the correlation between device performance and optical-structural-morphological properties of film stack, is still lacking in the field of cadmium selenide/telluride alloy-based solar cells. Here, we report confocal Raman and photoluminescence microscopy study results obtained through exciting both film and glass sides of cadmium selenide and cadmium telluride bilayer device stacks treated with cadmium chloride. We show that the device stack, especially the glass side, losses significant charge carries to the recombination arising from uniformity issues related to material composition, energetics, and defects. Furthermore, there is a high energy tail emission peak originating from CdTe, and CdSe can suppress it significantly under CdCl-2 treatment.

Properties and functions of various ZnO materials are intensively investigated in biological systems for diagnostics, therapy, health risks assessment as well as bactericidal and decontamination purposes. Herein, the interface between ZnO and biological environment is studied by characterizing adsorption of bovine serum albumin (BSA) and fetal bovine serum (FBS) using atomic force microscopy with CF4-treated tips. Similar molecular morphologies (thickness around 2 nm) yet different binding forces to ZnO (10–25 nN) are observed. These observations are corroborated by atomic scale simulations of BSA on (0001) ZnO surface using force-field method and showing rearrangements of Zn surface atoms. Such binding may have an impact also on other properties of ZnO–BSA complex.

MXenes have drawn considerable attention in the past decade, thanks to their attractive properties such as metallic conductivity and surface hydrophilicity. While MXenes form highly stable dispersions in water, it can act as a limitation for certain applications such as photoactive layer in photovoltaic devices. In this work, delaminated MXenes of aqueous solution type Ti3C2 were prepared first, and then we have used a solvent-exchange technique to prepare suspensions of MXenes in three polar aprotic solvents namely, DMSO, DMF and NMP. Upon spin-coating under the same conditions, each solvent variation yields a different thin film morphology – in terms of particle size and surface coverage, as evidenced from AFM investigations. While the MXenes in DMSO yielded large aggregated particles with µm-sized islands in the film, MXenes in DMF and NMP were found to form films with well-dispersed MXene sheets in the size range 250 nm-50 nm and 80 nm-10 nm, respectively. This study also provides additional insights into the microstructure and opto-electronic properties of the MXene thin films using correlative Raman microscopy and photoluminescence spectroscopy. The information provided by this study on the variation in the properties depending on the solvent used to process and spin-cast the films are important for evaluating MXenes in thin film device applications.

Zinc oxide nanoparticles have been synthesized using non-thermal atmospheric pressure plasma (ZnO-NTP). We investigated the behavior of these ligand-free as a colloid suspension using different solvents, from deionized water to physiological saline and microbial culture broth. We found that the zeta potential of ZnO-NTP became more negative after exposure to microbial culture broth relative to water, which suggests increased colloid stability. Photoluminescence spectra of ZnO-NTP were similar regardless of liquid type, yet optical and fluorescent images of samples showed different agglomeration behavior depending on liquid type. Scanning electron microscopy images revealed large agglomerates of ZnO-NTP interacting with the surface of bacteria cells, ranging in size from 200 nm up to 2 µm. We also studied effect of sub-lethal concentrations of ZnO-NTP on bacteria under illumination. There was no significant difference in viable bacteria concentration after 24h exposure to 10 µg/mL ZnO-NTP relative to untreated control irrespective of sample illumination.

Perovskites are one of the most intensively studied photovoltaic materials nowadays. Microscopic studies can provide useful information about material roughness, conductivity, structure, mechanical and opto-electronic properties as well as about kinetic effects from short to long time scale for understanding and improving the photovoltaic performance. Here, time-resolved photovoltage was measured by Kelvin probe in the dark and under white light illumination. Morphology was characterized by optical and atomic force microscopy. We identify the impact of charge transporting layers (CTLs) on structural and opto-electronic properties of MAPbI3 perovskite with different ratio of MAI and PbI2.