Nano News: 08/03/15

Proving nanoparticles in sunscreen products

Cosmetics increasingly contain nanoparticles. One especially sensitive issue is the use of the miniscule particles in cosmetics, since the consumer comes into direct contact with the products. Sunscreen lotions for example have nanoparticles of titanium oxide. They provide UV protection: like a film made of infinite tiny mirrors, they are applied to the skin and reflect UV rays. But these tiny particles are controversial. They can penetrate the skin if there is an injury, and trigger an inflammatory reaction. Its use in spray-on sunscreens is also problematic. Scientists fear that the particles could have a detrimental effect on the lungs when inhaled. Even the effect on the environment has not yet been adequately researched. Studies indicate that the titanium oxide which has seeped into public beaches through sunscreens can endanger environmental balance. Therefore, a labeling requirement has been in force since July 2013, based on an EU Directive on cosmetics and body care products. If nano-sized ingredients are used in a product, the manufacturer must make this fact clear by adding “nano-” to the listed ingredient name. Due to requirements imposed by the legislature, the need for analysis methods is huge. Proving nanoparticles in sunscreen products

When a nanoparticle enters into plasma, a discontinuous signal occurs. Signal intensity correlates to particle size. (Image: Fraunhofer IGB) Determining particle sizes down to the tiniest scale Today’s electron microscope imaging processes, such as transmission-electron microscopy or scanning electron microscopy, are based on the properties of light dispersion. They are used to detect all particles present. They do not differentiate between a cell, a nanoparticle – or a piece of lint. These methods are ideally suited for the study of surface properties and shapes. “The light diffusion process and microscopy are not selective enough for a lot of studies, including toxicological examinations,” says Gabriele Beck-Schwadorf, scientist at the Fraunhofer Institute for Interfacial Engineering and Thin Films IGB in Stuttgart. The group manager and her team have advanced and refined an existing measurement method in a way that allows them to determine titanium nanoparticles within complex media consisting of several different components that are highly sensitive and delicate. Researchers measure individual particles by single particle, inductively coupled plasma mass spectroscopy (or SP-ICP-MS). “With this method, I determine mass. Titanium has an atomic mass of 48 AMUs (atomic mass units). If I set the spectrometer to that, then I can target the measurement of titanium,” explains Katrin Sommer, food chemist at IGB. With particle measurement, a suspension is sprayed into the plasma that contains both large and small particles in non-homogeneous distribution. The suspension has to be thinned out sharply so that one titanium dioxide particle after another can be detected and analyzed. Ions are formed out of these particles in hot plasma of about 7,000 Kelvins. They get to the spectrometer’s detector as an ion cloud, and are counted within the briefest measurement time of about three milliseconds. Signal intensity correlates to particle size. “We convert the intensity into nanometers. At the same time, we count particle signals, from which we calculate particle concentration with up to ten percent accuracy. We can establish exactly how many particles are of a specific size,” says Sommers, explaining the procedure. It was IGB scientists who originally developed the methods for measuring titanium oxide nanoparticles in wastewater. “But the process is generally suitable for complex media, and can also be applied to sunscreen lotions,” the researcher indicates. A unique feature of this approach: the IGB team performs the data analysis and data processing without specialized software. “We have statistically evaluated the raw data using a standard computer program, and thus can work irrespective of the producer. Compared to existing methods, SP-ICP-MS involves a rapid process that uses detection limits that extend down to the ultra-trace amount scale below ppm.” For example, one sample of just a few milliliters can be examined in about six minutes. Cosmetics makers, nanotechnology businesses, and consumers can benefit from the particle analysis for quality assurance of sun protection and body care products, but also use them for analyzing water, drinking water, and food. The researchers are planning to measure other nanoparticles in the future as well, such as silica dioxide. One can only determine whether a product contains silica dioxide through complex measurements. In order to establish the presence of nanoparticles, one must first determine their size or size distribution. Based on the EU’s definition, declaration requirements apply to a nanomaterial if at least 50 percent of the contained particles are of a size measuring between 1 and 100 nanometers (nm). Previous analysis methods are hitting their limits here. These make it possible to establish particle sizes only in pure solutions. They are not suited for analysis of complex media that one finds in modern cosmetics. In addition, nanoparticles with various chemical properties cannot be differentiated from each other this way.

Researchers demystify the ferroelectric properties observed in hafnium-oxide-based thin films

Ferroelectric materials have applications in next-generation electronics devices from optoelectronic modulators and random access memory to piezoelectric transducers and tunnel junctions. Now researchers at Tokyo Institute of Technology report insights into the properties of epitaxial hafnium-oxide-based (HfO2-based) thin films, confirming a stable ferroelectric phase up to 450°C ("Growth of epitaxial orthorhombic YO1.5-substituted HfO2 thin film"). As they point out, “This temperature is sufficiently high for HfO2-based ferroelectric materials to be used in stable device operation and processing as this temperature is comparable to those of other conventional ferroelectric materials.” Reports of ferroelectric properties in thin films of substituted hafnium-oxide - where some ions were replaced with other metals - have attracted particular interest because these films are already used in electronics and are compatible with the silicon fabrication techniques that dominate the industry. However attempts to study the crystal structure of HfO2-based thin films in detail to understand these ferroelectric properties have met with challenges due to the random orientation of the polycrystalline films. In order to obtain thin films with a well-defined crystal orientation, Takao Shimizu, Hiroshi Funakubo and colleagues at Tokyo Institute of Technology turned to a growth approach that had not been tried with HfO2-based materials before - epitaxial film growth. They then used a range of characterisation techniques — including x-ray diffraction analysis and wide-area reciprocal space mapping — to identify changes in the crystal structure as the yttrium content increased. They found a change from a low- to a high-symmetry phase via an interim orthorhombic phase with increasing yttrium from -15 % substituted yttrium oxide. Further studies confirmed that this orthorhombic phase is ferroelectric and stable for temperatures up to 450°C. They conclude, “The present results help to clarify the nature of ferroelectricity in HfO2-based ferroelectric materials and also its potential application in various devices.” $x-ray diffraction patterns$ The x-ray diffraction patterns with inclination angle of 45° observed for 0.07YO1.5-0.93HfO2 film measured from room temperature to 600°C. (b) The integrated intensity of the 111 super-spot of 0.07YO1.5-0.93HfO2 film as a function of temperature. Background Hafnium oxide thin films The dielectric constant (high-κ) of HfO2 has previously attracted interest for use in electronics components such as dynamic random-access memory (DRAM) capacitors and is already used for high-κ gates in devices. As a result its compatibility with the CMOS processing that dominates current electronics fabrication is already known. Ferroelectric properties have been reported in HfO2 thin films with some hafnium ions substituted by different types of ions including yttrium, aluminium and lanthanum, as well as silicon and zirconium. The researchers studied HfO2 films substituted with the yttrium oxide YO1.5 as ferroelectric properties have already been reported in films of this material. Epitaxial growth Well-defined crystal orientation with respect to the substrate can be obtained in epitaxially grown films but the process usually requires high temperatures. Due to the tendency to decompose into non-ferroelectric phases HfO2 are usually prepared by crystallization of amorphous films. The researchers used pulsed laser deposition to prepare epitaxially grown HfO2-based films without destroying the ferroelectric phase. The films were grown on yttria-stabilised zirconia and were around 20 nm thick. Crystal phases and characterization HfO2 exists in a stable low-symmetry monoclinic phase, where the structure resembles rectangular prism with a parallelogram base. This structure changes to a high-symmetry cubic or tetragonal structured phase through a metastable orthorhombic phase. Monoclinic, cubic and tetragonal crystalline structures have inversion canter, which rules out ferroelectric properties. Therefore the researchers focused on the orthorhombic. The coexistence of several phases in HfO2 further complicates studies of crystal structure, making it yet more desirable to obtain films with well-defined crystal orientations. Prior to the current work it was still unclear whether epitaxial growth of HfO2-based films was possible. Previous work had used transmission electron microscopy and simultaneous convergent beam electron diffraction to confirm the existence of the orthorhombic phase, but more detailed analysis of the crystalline structure proved difficult due to the random polycrystalline orientation. With the epitaxially grown thin films the researchers were able to use x-ray diffraction analysis and wide-area reciprocal space mapping measurements to identify the orthorhombic phase. They then used aberration-corrected annular bright-field and high angle annular dark field scanning transmission electron microscopy to confirm that the orthorhombic phase was ferroelectric.

Parity effect observed in graphene

Kensuke Kobayashi (Professor, Graduate School of Science, Osaka University) and Sadashige Matsuo (Assistant Professor, School of Engineering, The University of Tokyo), in a collaborative research effort with a research group led by Teruo Ono (Professor, Institute for Chemical Research, Kyoto University) and Kazuhito Tsukagoshi (Lead Researcher, International Center for Materials Nanoarchitectonics, National Institute for Materials Science), have theoretically projected and successfully proven through experimentation the parity effect of the quantum Hall edge transport in graphene antidot devices with pn junctions (PNJs). Graphine, or single-layered graphite, has properties of both metals and semiconductors. This group confirmed that the parity effect in graphene antidot devices has a good analogy to optical systems. This means various quantum interference devices can be produced by using the quantum hall edge transport with pn junctions (, "Parity effect of bipolar quantum Hall edge transport around graphene antidots").

(a) and (b): Schematic picture of the chirality of the quantum Hall edge states around a single antidot when the number of PNJs (N) is (a) even and (b) odd. The present study has established that the conductance is essentially different between the two cases, namely the parity effect. (c) Optical image of the device. The inset shows that this device has a single open window (an antidot) shown by the white curves. We tuned the top gate voltages of these two top gate electrodes, marked as a and b, in order to experimentally realize the cases with N = 0, 1, 2, and 3. Abstract We discover the parity effect of the quantum Hall edge transport in graphene, which is a new ubiquitous phenomenon in quantum Hall edge transport in massless Dirac electron systems. First, we theoretically study a graphene device with an antidot and multiple pn junctions (PNJs) and have obtained a new compact formulae to show a significant parity effect regarding the number of PNJs. Then we have experimentally realized such graphene devices to confirm the new formulae. Our achievement is the first to establish the parity effect on bipolar quantum Hall edge transport in massless Dirac electron systems and is an important step forward to design new electron interferometer devices using graphene.