My watch list


Scientists observe nanowires as they grow

X-ray experiments reveal exact details of self-catalysed growth for the first time

DESY, Satishkumar Kulkarni/Thomas Keller

Nano forest: 'Aerial' view of the tiny wires growing on a silicon wafer, captured with a scanning electron microscope at the DESY NanoLab.

12-Feb-2018: At DESY's X-ray source PETRA III, scientists have followed the growth of tiny wires of gallium arsenide live. Their observations reveal exact details of the growth process responsible for the evolving shape and crystal structure of the crystalline nanowires. The findings also provide new approaches to tailoring nanowires with desired properties for specific applications. The scientists, headed by Philipp Schroth of the University of Siegen and the Karlsruhe Institute of Technology (KIT), present their findings in the journal Nano Letters. The semiconductor gallium arsenide (GaAs) is widely used, for instance in infrared remote controls, the high-frequency components of mobile phones and for converting electrical signals into light for fibre optical transmission, as well as in solar panels for deployment in spacecraft.

To fabricate the wires, the scientists employed a procedure known as the self-catalysed Vapour-Liquid-Solid (VLS) method, in which tiny droplets of liquid gallium are first deposited on a silicon crystal at a temperature of around 600 degrees Celsius. Beams of gallium atoms and arsenic molecules are then directed at the wafer, where they are adsorpted and dissolve in the gallium droplets. After some time, the crystalline nanowires begin to form below the droplets, whereby the droplets are gradually pushed upwards. In this process, the gallium droplets act as catalysts for the longitudinal growth of the wires. "Although this process is already quite well established, it has not been possible until now to specifically control the crystal structure of the nanowires produced by it. To achieve this, we first need to understand the details of how the wires grow," emphasises co-author Ludwig Feigl from KIT.

To observe the growth as it takes place, Schroth's group installed a mobile experimental chamber, specially developed by KIT for X-ray experiments and partially funded by the Federal Ministry of Education and Research (BMBF), in the brilliant X-ray beam of DESY's synchrotron radiation source PETRA III at experimental station P09. At one-minute intervals the scientists took X-ray pictures, which allowed both the internal structure and the diameter of the growing nanowires to be simultaneously determined. In addition, they measured the fully-grown nanowires using the scanning electron microscope at the DESY NanoLab. “To ensure the success of such complex measurements, an extensive period of growth characterisation and optimisation at the UHV Analysis Lab at KIT was a prerequisite,” explains co-author Seyed Mohammad Mostafavi Kashani from University of Siegen.

Over a period of about four hours, the wires grew to a length of some 4000 nanometres. One nanometre (nm) is one millionth of a millimetre. However, not only did the wires become longer during this time, but also thicker: their diameter increased from an initial 20 nm to up to 140 nm at the top of the wire, still making them around 500 times thinner than a human hair.

"One rather exciting feature is that the images taken under the electron microscope show the nanowires to have a slightly different shape," says co-author Thomas Keller from DESY NanoLab. Although the wires were thicker at the top than at the bottom, just as indicated by the X-ray data, the diameter measured under the electron microscope was larger in the lower region of the wire than what was observed using X-rays.

"We found out that the growth of the nanowires is not only due to the VLS mechanism but that a second component also contributes, which we were able to observe and quantify for the first time in this experiment. This additional sidewall growth lets the wires gain width," explains Schroth. Independently of VLS growth, the vapour deposited material also attaches itself directly to the side walls, particularly in the lower region of the nanowire. This additional contribution can be determined by comparing the X-ray measurements taken early on during the growth of the wire, with the electron microscope measurement after growth has ended.

Furthermore, the gallium droplets are constantly becoming larger as further gallium is added in the course of the growth process. Using growth models, the scientists were able to deduce the shape of the droplets, which had also been affected by the increasing droplet size. The effect of this is far-reaching: "As the droplet changes in size, the angle of contact between the droplet and the surface of the wires also changes. Under certain circumstances, the wire then suddenly continues growing with a different crystal structure," says Feigl. Whereas the fine nanowires initially crystallise in a hexagonal, so-called wurtzite structure, this behaviour changes after some time and the wires adopt a cubic zinc blende structure as they continue to grow. This change is important when it comes to applications, since the structure and shape of the nanowires have important consequences for the properties of the resulting material.

Such detailed findings not only lead to a better understanding of the growth process; they also provide approaches for customising future nanowires to have special properties for specific applications - for example to improve the efficiency of a solar cell or a laser.

Original publication:
"Radial Growth of Self-Catalyzed GaAs Nanowires and the Evolution of the Liquid Ga-Droplet Studied by Time-Resolved in situ X‐ray Diffraction"; Philipp Schroth, Julian Jakob, Ludwig Feigl, Seyed Mohammad Mostafavi Kashani, Jonas Vogel, Jörg Strempfer, Thomas F. Keller, Ullrich Pietsch, and Tilo Baumbach; Nano Letters; 2018

Facts, background information, dossiers

  • X-rays

More about Deutsches Elektronen-Synchroton DESY

More about KIT

  • News

    3-D-printing of glass now possible

    Glass is one of mankind's oldest materials. It was used as far back as in ancient Egypt and ancient Rome and has found a place now also in manufacturing technology of the 21st century. An interdisciplinary team at the KIT led by mechanical engineer Dr. Bastian E. Rapp developed a process us ... more

    Record for Perovskite/CIGS Tandem Solar Module

    Thin-film technologies can dramatically reduce the cost of next-generation solar modules. Whereas their production cost is low, it is in particular the combination of complementary absorber materials in a tandem solar module that increases the power conversion efficiency. At the PSCO intern ... more

    Nanofur for oil spill cleanup

    Some water ferns can absorb large volumes of oil within a short time, because their leaves are strongly water-repellent and, at the same time, highly oil-absorbing. Researchers of KIT, together with colleagues of Bonn University, have found that the oil-binding capacity of the water plant r ... more

q&more – the networking platform for quality excellence in lab and process

The q&more concept is to increase the visibility of recent research and innovative solutions, and support the exchange of knowledge. In the broad spectrum of subjects covered, the focus is on achieving maximum quality in highly innovative sectors. As a modern knowledge platform, q&more offers market participants one-of-a-kind networking opportunities. Cutting-edge research is presented by authors of international repute. Attractively presented in a high-quality context, and published in German and English, the original articles introduce new concepts and highlight unconventional solution strategies.

> more about q&more

q&more is supported by:

Your browser is not current. Microsoft Internet Explorer 6.0 does not support some functions on Chemie.DE