Calculations by three researchers at the University of São Paulo (USP) have shown that by combining two of the most interesting materials recently discovered by physics, graphene and phosphorene, one can build a transistor that dissipates minimal energy. Measuring only a few nanometers (millionths of a millimeter) in size, the device works thanks to a special way of combining the two materials that preserves the characteristics of each. José Padilha, Adalberto Fazzio and Antônio José Roque da Silva showed that, contrary to what happens in current silicon transistors, the electrons in an electrical current lose almost no energy when passing from a sheet of graphene to a sheet of phosphorene, nor when moving in the opposite direction.
The prediction, published in the February 2015 issue of Physical Review Letters, was confirmed in the laboratory by a team at the National University of Singapore (NUS), with the participation of Brazilian Antônio Castro Neto, director of the 2D Advanced Materials Center and the Graphene Research Center at NUS.
Transistors, which are the basis of today’s computers, act like power switches that turn bulbs on and off. The “on” and “off” states represent the zeros and ones in binary code, the language of computers. More recent microprocessors contain from 1 to 2 billion transistors, each measuring 45 nanometers in length, made of silicon-based materials. These transistors are connected to each other—and to the other electronic components of the microprocessor—via metal wires (gold or copper).
When passing from the wires to the transistors and from these to the wires, the electrons in the electrical current lose part of their energy in the form of heat due to the contact resistance between the metal and the semiconductor. Currently this heat does not affect microprocessor operation. However, if the miniaturization trend of these components continues as it has in recent decades, the situation could become complicated. “There may come a point where the heat dissipated will burn the device or prevent its operation,” explains Padilha. Now a professor at the Federal University of Paraná, Jandaia do Sul Advanced Campus, the researcher performed the calculations demonstrating the possibility of building transistors of graphene and phosphorene during a period as a post-doctoral researcher at USP, supervised by Fazzio and Silva.
In recent years, researchers from various centers have believed that the solution to the contact problem would involve graphene. Discovered in 2004, this material is made up of carbon atoms placed in a hexagonal pattern, with a thickness of one atom. Electrons pass through graphene thousands of times faster than through silicon, and with minimal loss of energy.
“The only problem is that graphene is not a semiconducting material like silicon,” explains Padilha. Transistors are made of semiconducting materials because they allow control of the passage of electrons and creation of zeros and ones in a computer. Semiconductors only transport electrons with energy above a certain limit. In a transistor, this limit acts like a barrier that can be raised or lowered with the help of an electric field. This adjustable barrier—which either lets the electrons through or blocks them—allows this property to be used to encode binary information. “If graphene acted this way, it would be the perfect material,” says Padilha.
This limitation has led researchers the world over to search for other materials made of a single atomic layer. Several were discovered, but the most interesting material now is the one identified most recently: phosphorene. Consisting of a single atomic layer (monolayer) of phosphorus, phosphorene does not let electrons move as fast as graphene does, but they still move faster than in silicon. The advantage of phosphorene is that it is a semiconductor. In December 2013 Silva began discussing with Padilha and Fazzio the idea of investigating what the ideal contact would be between a phosphorene transistor and an electric circuit. “Phosphorene loses its semiconducting properties if soldered to copper or gold wires in a conventional circuit,” explains Padilha. “Additionally, the contact with the atoms in the metallic wires would lead to dissipation of the electrons’ energy in the form of heat.”
Padilha, Fazzio and Silva proposed solving the problem by replacing the metal wire contact with a layer of graphene superimposed on that of the phosphorene. While the contact between the wires and phosphorene are through chemical bonds between atoms, the phosphorene and graphene layers are connected by a low-intensity attractive force called the Van Der Waals interaction. Despite being weak, this electromagnetic force allows graphene and phosphorene atoms to share their electrons, without the electronic properties of one material interfering in those of the other.
Having found the solution, Padilha, Fazzio and Silva calculated the behavior of electrons in the transistor. This is a difficult task, since the electrons do not act like tiny balls that move within the device. Rather, they are a quantum mixture of wave and particle whose behavior is described by mathematical equations that take months to solve using computer superclusters. The results published in Physical Review Letters show that the phosphorene and graphene “sandwich” acts like a transistor that loses very little energy through its contacts and can be “turned on” or “turned off” by an electric field.
At almost the same time, a team of physicists led by Barbaros Özyilmaz at NUS built a transistor similar to that envisioned by the Brazilians in a laboratory. The difference is that the layers of phosphorene, which act like a semiconductor, and the two strips of graphene, used as the contact between the transistor and the rest of the circuit, made of silicon electronic devices, are covered by a layer of hexagonal boron nitride. This material protects the other layers from oxygen in the air. The transistor worked perfectly in tests. “We obtained the best results of all phosphorene devices built,” states Antônio Castro Neto. A theoretical physicist and researcher working on the project “Graphene: photonics and opto-electronics: UPM-NUS” collaboration as part of the FAPESP São Paulo Excellence Chair (SPEC) program, and based at the MackGraphe Center, at the Mackenzie Presbyterian University, Castro Neto collaborated on analysis of the experimental data, which confirmed the predictions of the USP group.
According to Padilha, the same calculations could lead to combinations of sheets of graphene and other monolayer semiconductors. “We made a transistor, but we could develop a solar cell whose electrons, excited by sunlight, would transfer from the semiconducting layer to the graphene layer with almost no loss of energy,” says Padilha. “Many people are exploring combinations of bidimensional materials, such as these, to produce structures with new properties,” concludes Silva.
Electronic, magnetic and transport properties of nanostructures (No. 2010/16202-3); Grant mechanism: Thematic Project; Principal investigator: Adalberto Fazzio (IF-USP); Investment: R$ 1,327,201.88 (FAPESP – for the entire project).
PADILHA, J. E. et al. Heterostructure of phosphorene and graphene: Tuning the schottky barrier and doping by electrostatic gating. Physical Review Letters. V. 114. Feb. 12, 2015.
AVSAR, A. et al. Air-stable transport in graphene-contacted, fully encapsulated ultrathin black phosphorus-based field-effect transistors. ACS Nano. V. 9, No. 4 Mar. 4, 2015.