The researchers from the General Physics Institute named after A.M. Prokhorov of RAS, Skoltech and Tokio University presented a unique method for creating graphene nano-ribbons aligned at the macro-level, i.e., oriented strictly in one direction across the entire surface of the sample. Such structures are viewed as a promising foundation for future electronic and optoelectronic appliances: from flexible displays and high-sensitivity sensors to mini processors, where precise monitoring of light and electric current moving directions is vital.
Graphene nano-ribbons are narrow graphene stripes with the width of just several atoms. Thanks to such geometry, they behave like one-dimensional systems (SISO systems), their properties change drastically depending on the orientation relative to impinging light or external field. For several years nano-ribbons are called candidates for the new generation of transistors, high-sensitivity optic elements and sensors. But to make their unique features manifest in real appliances, many nano-ribbons need to be laid strictly in parallel. Even a slight deviation or “spreading” destroy anisotropy – the oriented nature of optic response. Indeed, nano-ribbons may grow oriented on metal substrates, but when they are moved, the order is violated. That is why the researchers proposed a more refined approach: they took the already aligned matrix made of single-layer carbon nanotubes and synthetized nano-ribbons directly inside these tubes, which perform as ideal right-lined nanochannels.
The synthesis process consisted of two stages. At first, an organic precursor was introduced into nanotubes in vacuum – 4.4’’-dibrom-p-terphenyl in the form of a chain consisting of three benzene rings with two atoms of bromine on the sides. When heated up to 320 °С, the molecules were losing bromine and connected into narrow three-atoms nano-ribbons. Then the temperature was raised up to 750 °С, and the adjacent narrow ribbons joined into broader ones consisting of six rows. Their growth was limited by the walls of the nanotubes not allowing for sidewise diversion, hence, all the formed nano-ribbons were automatically aligned along the axes – in the same way as the carbon tubes were initially oriented.
To make sure that nano-ribbons were formed and maintained their orientation, the researchers applied several methods of analysis. Transmission electron microscopy (TEM) showed thin graphene stripes inside the tubes, which could be in curved form in certain places due to space limitations. But polarized Raman spectroscopy was the key instrument. This method allows for measuring scattering light intensity at different angles between the laser polarization direction and nano-ribbons orientation. It turned out that the key signals of nano-ribbons become significantly stronger, when the laser is polarized along their direction, and they practically disappear is the sample is rotated 90 degrees. Such conspicuous anisotropy decisively demonstrates that nano-ribbons are aligned in one direction and fully follow the orientation of the nanotubes’ matrix.
Theoretical calculations performed within the Density Functional Theory (DFT) provided another confirmation. Simulation forecasted the frequencies of major atomic oscillations in the nano-ribbon and accurately reproduced the observed angular dependence of Raman signals coinciding with the experimental data.
The developed method opens the opportunity for creating macroscopic films with controlled direction of electronic and optical responses. Such materials are extremely needed for detectors, optical elements and nano-electronic appliances, where orientation of the structure is of critical importance. Further it may be used for synthetizing other types of graphene nano-ribbons and for exploring the impact of directional organization on the properties of future appliances.
