Libor Novák

In-situ observation of 〈110〉 oriented Ge nanowire growth and associated collector droplet behavior

Using in-situ microscopy, we show that germanium nanowires can be grown by a vapor-liquid-solid process in 〈110〉 directions both on Ge(100) and Ge(111) substrates if very low supersaturation in the collector droplet is ensured. This can be provided if thermal evaporation is utilized. Such a behavior is also in agreement with earlier chemical vapor deposition experiments, where 〈110〉 oriented wires were obtained for very small wire diameters only. Our conclusions are supported by in-situ observations of nanowire kinking towards 〈111〉 direction occurring more frequently at higher evaporation rates.

Controlled faceting in 〈110〉 germanium nanowire growth by switching between vapor-liquid-solid and vapor-solid-solid growth

We show that the hexagonal cross-section of germanium nanowires grown in the 〈110〉 direction by physical vapor deposition is a consequence of minimization of surface energy of the collector droplet. If the droplet is lost or solidified, two {001} sidewall facets are quickly overgrown and the nanowire exhibits a rhomboidal cross-section. This process can be controlled by switching between the liquid and solid state of the droplet, enabling the growth of nanowires with segments having different cross-sections. These experiments are supported by in-situ microscopic observations and theoretical model.

New approaches to in-situ heating in FIB/SEM systems

Over last decades significant effort has been made on in-situ heating experiments inside SEM and FIB/SEM chambers. Traditional way is to use low vacuum environment in the entire chamber. Although this valuable approach brings various undeniable advantages, new state of the art experiments coincide with new requirements, such as rapid changes in temperature, high-vacuum operation to maximize experiment cleanliness, ultra-high resolution SEM imaging and on top of it adaptable geometry in order to investigate sample’s crystallography and composition changes using EBSD and EDS detectors. In this contribution we introduce an integration of two new modules fulfilling these requirements by allowing in-situ heating in FIB/SEM systems under high vacuum conditions. Moreover, heating in high vacuum combined with injection of selected gases was also proven capable of providing sample surface oxidation [1] or reduction (Figure 1), [2].

MEMS-based Heating Element for in-situ Dynamical Experiments on FIB/SEM Systems

In-situ observation of microstructural evolution of solids such as recrystallization, grain growth and phase changes in SEM is important for various fields of material science and industry research. This technique requires reliable discrimination of differently oriented crystal phases combined with useful spatial and temporal resolution and with fast and precise control of specimen temperature. While the requirements on spatial and temporal resolution are satisfied by current SEMs with resolution below 1 nm and 100 Hz frame rate, existing heating holders for bulk samples only allow for heating rates up to 300oC per minute (5oC/s). Long ramping time, which is required during heating experiments done using these devices, may cause unwanted sample changes (e.g. oxidation or recrystallization) before the temperature range of interest is reached. Thermal radiation of massive heating holders decreases quality of material contrast imaging as the commonly used detectors of backscattered electrons become saturated by thermally emitted photons. MEMS-based heating holder [1], [2] in combination with in-situ site specific sample preparation using a FIB/SEM system brings significant improvement in instrumentation for in-situ heating experiments inside the SEM chamber.

Study of Stability and Structural Changes Occurring during High Thermal Load of the High Voltage Cathode Material by In Situ Scanning Electron Microscopy

One of the most progressive battery systems which are used in portable devices, electric vehicles and energy storage systems are Li-Ion batteries. However, currently used cathode materials are close to their limits and in the coming years they are no longer able to meet growing energy demands. Many research groups focus their interest on modifications of existing cathode materials in order to improve their parameters. Some of them search for new types of cathode materials which could replace currently used cathode materials. The result of one of those efforts was the development of the cathode material LiNi0.5Mn1.5O4 [1]. This material is based on the LiMn2O4 where manganese is partially replaced by nickel, this allows to charge the cathode material up to 5 V. Potential of LiNi0.5Mn1.5O4 against lithium is 4.7 V i.e. 1 – 1.5 V increase in respect to standard cathode materials. With this combination of high potential and theoretical capacity 148 mAh/g, LiNi0.5Mn1.5O4 exhibits high gravimetric energy density approaching 700 Wh/kg which is approximately 20 % more than gravimetric energy density of LiCoO2 and about 30 % more than in the case of the cathode material LiFePO4. Moreover, LiNi0.5Mn1.5O4 is also stable during long term cycling and exhibits good stability at higher current loads because of the spinel structure; however, it still suffers by dissolution of manganese into the electrolyte during cycling at higher temperatures which leads to defects in the structure and capacity decrease [2].

The synergic effect of atomic hydrogen and catalyst spreading on Ge nanowire growth orientation and kinking

In nanowire-based three-dimensional device architecture (e.g. gate-all-around nanowire field effect transistors) it is required to intentionally grow nanowires at precisely determined location and, additionally, perpendicular to the substrate. The former requirement was successfully met utilizing the so-called Vapor-Liquid-Solid (VLS) mechanism, where a eutectic catalyst droplet is used to localize the semiconductor material nucleation and growth to the droplet-substrate (liquid-solid) interface. The growth direction of VLS-grown semiconductor nanowires and its manipulation is, however, a complex task attracting a lot of attention [1]. It was demonstrated that nanowires exhibit a preferential growth direction (usually <111>) irrespective of the substrate crystallographic orientation. Schmidt et al. analyzed the nanowire growth considering surface free energies of the growth interface and nanowire sidewalls [2]. This concept based solely on thermodynamics introduced two possibilities to control the nanowire growth direction; either by catalyst engineering (thus potentially changing the solid-liquid interface energy) or by passivating nanowire sidewalls with different adsorbates (solid-vapor interface energy). Another success of Schmidt’s hypothesis is in explaining an intriguing experimental observation that small diameter nanowires (usually <20 nm) prefer to grow in <110> directions, while the larger diameter ones grow in <111> directions. This has been observed in many material systems grown from vapor phase molecular precursors, including Si, Ge, III-Vs and II-VIs nanowires.