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2D materials

Single-layer materials are the focus of research for very versatile applications including nano-sized strain gauges, nanocrystalline TiO2 coatings for body implants, and the influence of atomic step terraces on growth phenomena, or e.g. the study of 2D material crystallinity of the anode or cathode component for faster and more efficient energy transfer in batteries.

In this section, we show how various measurement solutions and different technologies from Anton Paar play an important role in the characterization of 2D materials such as temperature-controlled grazing incidence, small-angle X-ray scattering (GISAXS) to study the thermal properties of self-assembled thin-film structures, atomic force microscopy (AFM) using a sharp tip for determination of dimensions of atomic steps, surface zeta potential to study protein adsorption on TiO2 anatase coatings, or use of gas pycnometry to measure skeletal density of a series of graphenes with varying crystallinity.

Characterization of atomic steps using atomic force microscopy

Introduction

No surface is perfectly flat. Even on highly polished surfaces, atomic step structures are present. Wafers are a typical example for such extremely flat surfaces. Obviously, it is impossible to cut, grind or polish an individual atom. Therefore, any deviation of the surface orientation from the atomic plane results in the formation of terraces on the surface, with atomic steps between them. The step edges are important for growth phenomena, like film formation, nucleation and epitaxy. The height of an atomic step depends on the atomic lattice of the material and also on the crystallographic plane that is parallel to the surface. Here we report on measurements of 12 atomic steps of silicon carbide with Tosca 400 AFM. 

Experimental

The described atomic force microscopy investigations were performed by using contact mode. The measurements were performed in ambient atmosphere and supported by an active anti-vibration isolation table and an acoustic enclosure. Both components minimize external sources of noise and are part of the recommended setup of the Tosca series AFM. The investigated atomic steps sample was a silicon carbide sample featuring 12 defined atomic steps.

Results and discussion

The investigated sample is a commercial reference sample for AFM, made of silicon carbide (SiC) and (0001) orientation. The results can be seen in Figure 1. In total, 12 different planes were detected. During data analysis in Tosca Analysis, the dataset was leveled parallel to a selected plane in order to display the step structure optimally and to enable the step height analysis. Successively, 11 atomic steps were detected and evaluated using the automatic step height analysis included in the Tosca Analysis software package. The absolute height of each plane and the step height are shown in the table. The average height of 11 steps was found to be 0.76 ±0.02 nm.

Figure 1: 3D rendering of the topography of SiC reference sample with eleven atomic steps of 0.76 nm step height.

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Effect of thermal annealing on the surface morphology of functional coatings for displays (studied by means of AFM)

Introduction

Metal layers play an important role in the visual performance of the display assembly. In order to optimize the visual performance, a low-reflectance metallization using an additional layer of highly absorbing, low-reflecting molybdenum-oxide is an option. The Mo-O system is known to contain two stable oxides, MoO2 and MoO3, as well as several Magnéli phases with intermediate Mo/O ratios. All these oxides have different colors and cover a wide range of electrical and optical properties depending on their oxidation state. Typically, the coatings are produced by magnetron sputtering processes and exhibit an amorphous microstructure. During further processing, the amorphous films were exposed to high temperatures up to 400 °C which can lead to crystallization of the films, accompanied by increased surface roughness and undesired changes of the optical properties. Here we report on the effect of thermal annealing on the surface roughness of optical Mo-oxide films.

Experimental

MoOx films were deposited by magnetron sputtering on a display glass substrate in pure Ar atmosphere using an electrically conductive ceramic MoOx target. Sputtered films in the as-deposited state show an amorphous microstructure and the desired optical properties. As the film surface was very smooth (Ra<1 nm), AFM measurements were conducted to investigate the surface morphology, topography and the surface roughness of MoOx films. Two different samples were investigated, the as-deposited state and a sample annealed in vacuum at 450 °C for 1 hour. For both samples, atomic force microscopy was performed on a Tosca 400 AFM. All AFM data were acquired using tapping mode with an Arrow-NCR cantilever with a typical resonance frequency of 285 kHz and a force constant of 42 N/m. The experiments were performed in ambient atmosphere. 

Results and discussion

The two samples analyzed were an as-deposited coating (top) and an annealed coating sample (bottom). The height images can be seen on the left side; the respective phase images are displayed on the right. Comparing Figure 1b and 1d, the phase image is homogeneous for the as-deposited state (1b). This indicates that mechanical properties were very homogeneously distributed. In contrast, the annealed coating (1d) shows areas with significant differences in the phase image. Furthermore, the phase contrast image reveals the formation of a network structure in the matrix of the coating. The histogram, which is given next to the images, was significantly wider for the annealed coating – which further supports the conclusion regarding higher inhomogeneity and the formation of different domains with different mechanical properties compared to the as-deposited state.

Figure 1: Comparison of height image (left) and phase contrast image (right) of the as-deposited state (top) and annealed coating (bottom) generated by tapping mode

Characterization of 2D nanostructures of self-assembled gold nanoparticles

Introduction

Colloidal nanoparticles are widely used due to their specific electronic, chemical and biological properties. E.g. colloidal gold (Au) may form two- and three-dimensional structures comprising monolayers of hexagonally-packed Au nanoparticles. Such materials are promising systems for, e.g. nano-sized strain gauges. The applicability of such thin-film materials may of course be limited by the mechanical or thermal stability. Hence, an in-situ method like temperature-controlled grazing-incidence small-angle X-ray scattering (GISAXS) is an ideal tool for studying the thermal properties of such self-assembled thin-film structures.

Experimental

A homogeneous layer of hexagonally arranged Au nanoparticles was deposited on Si wafer substrates by using a Langmuir-Schaefer method. The obtained thin films were measured with an Anton Paar SAXSpoint system equipped with a dedicated GISAXS stage and heating module. Structural changes of the sample were monitored from ambient temperature up to 220 °C. For precise evaluation of GISAXS scattering patterns, BornAgain [1] is currently the most advanced software – it allows for simulation of GISAXS data. With a dedicated data export routine, GISAXS data acquired with the SAXSpoint system can directly be transferred to the BornAgain platform.

Results and discussion

At room temperature, the acquired GISAXS pattern showed the self-assembled Au monolayer to have a typical hexagonal close-packed (hcp) arrangement. In in-situ heating GISAXS experiments, this structure was found to be stable to temperatures of up to 180 °C, proving the good stability of this system. Above 180 °C, a transformation of the small Au nanoparticles to larger sizes was observed, resulting in a stable nanostructure above 195 °C. This proves that in-situ GISAXS is a valuable tool to monitor the structure, stability and structural changes of such nanosized 3D structures.

For detailed evaluation, the GISAXS patterns of the sample exhibiting hcp arrangements were automatically exported and analyzed with the BornAgain software. The fit calculated with regard to the experimental data revealed a particle size of around 6 nm for the Au nanoparticles, and a lattice constant of around 8 nm [2].

Figure 1: In-situ temperature-dependent GISAXS results of Au nanoparticle monolayers at room temperature (left) and at 220 °C (right)

Figure 2: Experimental GISAXS results of the Au nanoparticle thin-film structure (left) and fit of the BornAgain software to the experimental data (right).

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References:

  1. www.bornagainproject.org
  2. K. Vesgo, M. Jergel, P. Siffalovic, M. Kotlar, Y. Halahovets, M. Hodas, M. Pelletta, E. Majkova, Sensors and Actuators A: Physical 241 (2016) 87-95.

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Protein adsorption on nanocrystalline TiO₂ coatings

Introduction

Ti-based materials are among the materials most commonly used for hard tissue replacement, as they provide suitable mechanical and chemical properties. This study focuses on the adsorption of proteins on TiO2 anatase coatings and the influence of UV irradiation prior to protein adsorption. The reported results give detailed insights into the surface zeta potential characterization of nanocrystalline TiO2 coatings for body implants, and thereby provide a basis for studies related to the hemocompatibility and biocompatibility of this material class.

Experimental

Commercially pure (cp) and hydrothermally treated (HT) TiO2 disks were used as a base material for conduction of zeta potential measurements and adsorption kinetic studies with a SurPASS. PBS served as an electrolyte solution. Ph titration curves were obtained using HCl as an acid and NaOH as a base. Bovine serum albumin (BSA) was used as a protein for investigation of time-resolved adsorption.

Results and discussion

All samples had an isoelectric point (IEP, pH value where the zeta potential is 0 mV and a charge reversal takes place) at acidic pH, and a negative zeta potential at physiological pH, with the highest negative zeta potential occuring for the cp sample. This observation can be related to the amorphous oxide structure of HT samples, which altered density and stability of surface hydroxyl groups.

The exposure to BSA led to a shift of the IEP to  a pH value of around 5, confirming full coverage of all investigated TiO2 surface with the globular protein. Even though the shape of the titration curves did not allow any further conclusions on the adsorption behavior, the recoded adsorption kinetics provided deeper insights and revealed the fastest rate of adsorption for the UV pre-irratiated HT sample, with gradually lower rates down to the cp sample.

Figure 1: pH dependence of zeta potential for cp Ti and HT- and UV-treated Ti before and after BSA adsorption

Figure 2: Time-resolved adsorption kinetics of BSA on the cp and HT non-irradiated and pre-irradiated titanium samples

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Source:

S. Novak et al., Biomed. Mater. 10 (2015) 045012

Surface characterization of self-assembled monolayers deposited on gold substrates

Introduction

Self-assembled monolayers (SAMs) are spontaneously formed assemblies of organic molecules adsorbed on solid surfaces, and enable the control of their physiochemical properties. This type of nanomaterial is an attractive candidate for tailoring the surface properties of biological and biotechnological surfaces, like biosensors, as it doesn’t influence the sample thickness or the materials’ bulk electrical properties. This study focuses on the investigation of organic SAMs with different functionalities and the characterization of the surface zeta potential. This relationship can provide valuable information on the SAMs electrostatic interaction with nano-sized materials, and can trigger the adsorption and desorption of the latter.

Experimental

Investigated samples:
Thiol-based SAMs with pure COOH, pure NH2, and mixed functionality deposited on gold substrates.

Characterization technique:
Measurements of the surface zeta potential were conducted with the SurPASS electro-kinetic analyzer equipped with Ag/AgCl electrodes and the adjustable gap cell for planar samples. An electrolyte 0.1 mM NaCl solution was used. Ph titration curves were obtained by adding an acid (HCl) and a base (NaOH) to the electrolyte solution.

Results and discussion

Thiol-based SAMs with different ratios of amine and carboxyl terminal groups were characterized according to their surface zeta potential. The study of zeta potential versus pH value revealed a clear shift in the isoelectric point (IEP, pH value where the zeta potential is 0 mV and a charge reversal takes place) with changing ratio of functional groups. The higher the amine content, the further the IEP was shifted from acidic towards neutral pH. However, the increase in IEP was not linear with the ratio of terminal groups; still, the contribution of the COOH functionality dominated. This can be related to the fact that the carbon chain of COOH is longer and therefore more easily accessible compared to the chain of NH2 as depicted in Figure 2.

Figure 1: Zeta potential versus pH of pure COOH SAMs, pure NH2 SAMs, and mixed SAMs.

Figure 2: Schematic depiction of SAMs deposited on a gold substrate with carboxyl and amine terminal groups

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Source:

*Shyue et al., Phys. Chem. Chem. Phys. 11 (2009) 6199-6204 

Crystallinity of graphene and relation to battery electron transfer properties

Introduction

Improving electrode battery components to achieve faster and more efficient energy transfer is an important part of battery material research and design. In particular, crystallinity is a critical property of solid battery electrode components because it enables ions to conduct efficiently through the anode and cathode instead of slowing down in the amorphous domains. The higher the crystallinity of the anode or cathode component, the more efficient the electron transfer through and between the components. Here we use gas pycnometry to measure the skeletal density of a series of graphenes with varying crystallinity, where the higher skeletal density correlates to a more crystalline material.

Experimental

Characterization techniques: Skeletal density measurements of a series of graphenes were performed using an Anton Paar Ultrapyc 5000 gas pycnometer. Because of the porous nature of these carbons, the samples were vacuum degassed for 3 hours at room temperature on an external degasser before being transferred to the instrument sample cell. The samples were then conditioned inside the pycnometer using a flow of helium purge gas for 3 min. The gas expansions were performed using helium from a starting pressure of 1.17 bar.

Results and discussion

The skeletal densities for 3 different electrode carbons were found to be 2.117 g/cm3, 2.139 g/cm3, and 2.152 g/cm3. The data in the table below show that density can be measured with excellent repeatability, <1% for all samples. Comparing the 3 samples, the density increases from Sample A to Sample B to Sample C. It is known that increased crystallinity correlates with an increase in skeletal density, so Sample C was the most crystalline of these 3 samples of electrode carbon. The higher the crystallinity, the more efficient that material is for electron transfer, making Sample C also the best choice for a battery anode.

Sample

Density (g/cm3)

Repeatability (%)

 

Run 1

Run 2

Run 3

Average

 

A

2.120

2.114

2.118

2.117

0.11

B

2.138

2.139

2.139

2.139

0.02

C

2.144

2.158

2.154

2.152

0.27

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