Spanish scientists have shown that a molecule of a simple organic substance, nonadiine-1,8, can be used as a molecular diode. This world’s smallest diode also proved to be very efficient, and unlike previously created molecular diodes, it can operate at room temperature.
One of the main directions in the development of electronics is miniaturization. Electronic circuits and their components are getting smaller and smaller. However, it is possible to reduce the usual electronic circuits for us based on silicon, germanium and other semiconductor materials (see Semiconductors) only up to a certain limit. Therefore, for a couple of decades now, intensive developments have been carried out in the field of molecular electronics, in which individual molecules serve as electronic components. Already known are molecular wires, molecular gates, molecular diodes and molecular transistors.
Researchers at the University of Barcelona working with Ismael Díez-Pérez’s group were able to demonstrate that a single nonadiine-1,8 molecule on a silicon substrate acts like a diode (Figure 1). This molecule consists of only nine carbon atoms and twelve hydrogen atoms (CnineH12). Thus, the created diode is very small even by the standards of molecular electronics.
Perhaps some readers, after reading the word “diode”, imagined small glowing lights – light-emitting diodes. However, in this case, we do not mean a light source, but a device that passes an electric current in one direction and blocks its flow in the opposite direction. Such devices are one of the basic components of electronic circuits.
Nonadiine-1,8 belongs to the class of terminal diines – molecules containing two carbon-carbon triple bonds, which are located at opposite ends of the carbon chain. Such terminal diins were synthesized in the second half of the 20th century and found application, for example, in the production of synthetic polymers. At the macroscopic level, neither nonadiine-1.8 nor structurally related compounds conduct electric current. The researchers used a hydrocarbon molecule with two triple bonds not as an ordinary electrical contact, but as a contact for tunneling current, which occurs when a charge carrier “breaks through” between the electrodes – a tunnel junction (electron tunneling is best known).
A diyne molecular diode is attached to a partially hydrogenated silicon surface containing Si – H bonds. This happens as a result of a hydrosilylation reaction initiated by ultraviolet radiation – the addition of a Si – H bond to a multiple bond at one of the ends of the nonadiine molecule. (This reaction proceeds similarly to the reactions studied at school for the addition of hydrogen H – H or hydrogen chloride H – Cl to double or triple bonds.) The second triple bond – at the other end of the diyne molecule – remains free for the possibility of electrical contact with external electrons. This allows the initially symmetric molecule to behave differently with electrons moving in opposite directions – towards or away from the substrate.
To test the operation of the diode, the researchers developed a special experimental methodology, which they called the “blinking test”. The silicon surface with nonadiine-1,8 molecules attached to it was studied using a scanning tunneling microscope. When a microscope probe made of gold came into contact with a molecule of nonadiine (Fig. 2, a), the electric circuit was closed, and an abrupt increase in the current strength – “flickering” (Fig. 2, b) was recorded. By periodically changing the polarity of the voltage, the researchers confirmed the one-sided conductivity of the diode (Fig. 2, c).
Starting with the study of a silicon surface covered with hundreds or dozens of nonadiine-1,8 molecules, by increasing the accuracy of scanning and measurements, the researchers were able to quickly adapt the method to study electrical contact with the participation of one single molecule. Interestingly, molecules of similar structure, for example, nonin-1, which has only one triple bond, cannot act as electrical contacts. Probably, the second triple bond of nonadiine or the hydrogen atom bound to it (forming a straight C≡C – H line with the last two carbon atoms) plays the role of a kind of antenna facilitating the tunneling electron transfer.
The created diode is interesting not only in size – its efficiency is unique for electronic components of this type. The efficiency of diodes is usually determined by the value of the rectification factor – the ratio of forward current to reverse current. For the new diode, the rectification coefficient reaches about 4000. This is two orders of magnitude higher than the rectification coefficient of the first molecular diode, obtained in 2009 with the participation of Ismael Diez-Perez himself, as well as Ivan Oleinik from the Institute of Chemical Physics named after A.I. N. N. Semenov RAS (I. Díez-Pérez et al., 2009. Rectification and stability of a single molecular diode with controlled orientation). In addition, a diode made of nonadiine-1.8 is highly stable – it can work at room temperature, while the diodes of 2009 (several molecules with one-sided conductivity obtained at that time were various combinations of aromatic rings) could work only at temperatures close to absolute zero (about 10 K).
The high efficiency and stability of the new diode suggests that the combination of the capabilities of organic chemistry and the proven approaches to working with silicon microcircuits can serve as the basis for a real breakthrough in molecular electronics. Of course, the mass application of such molecular diodes is still a long way off – despite the fact that the new diode is much better than its predecessors, before using such devices in electronic circuits, it is necessary to increase both thermal stability, current stability, and lifetime. Nevertheless, the researchers are confident that extending the stable operation time of a molecular diode from a few seconds to several months is a very realistic prospect. In any case, the results of studying the molecular diode will be useful in the near future – the developed and successfully used “flickering method” can be useful for studying the laws of electric charge transfer and for other systems in which the surface of a metal or semiconductor will be modified by organic molecules of various structures.
A source: Albert C. Aragonès, Nadim Darwish, Simone Ciampi, Fausto Sanz, J. Justin Gooding & Ismael Díez-Pérez. Single-molecule electrical contacts on silicon electrodes under ambient conditions // Nature Communications… 2017.8 DOI: 10.1038 / ncomms15056.