Technological advances in the electronics industry have resulted in our digital cameras, computers, and cell phones drastically reducing in size. In order to maintain this momentum, there is a strong drive towards research into nanoelectronics, to produce smaller and smaller electronic components for these devices.
This is inevitably leading towards a state where the most fundamental components meet the ultimate size limit - the size of individual atoms and molecules.
Diodes are critical for a broad range of applications from radios, logic gates, light-emitting devices, and photodetectors. Diodes are components allowing current flow in a single direction around an electrical circuit but not in the other direction.
In order that a molecule functions that way, it must be physically asymmetric, with significantly different chemical structures or compositions at each end of the molecule.
Researchers are attempting to extend Moore's Law by taking electronic components down to the molecular level. Image Credit: National Science Foundation
Diblock Molecular Diode
In 2003, a team of researchers led by Professor Luping Yu at the Materials Research Center contributed significantly towards the development of nanoelectronics by the synthesis and physical characterization of the first nanoscale polymeric molecular system that functions similar to a semiconductor p-n junction diode. These molecular diodes are unique as they are synthesized chemically, enabling structural variation which allows them to be tuned easily.
Yu’s main strategy was to make a molecular material that had two separate sections by combining an electron-rich oligo-thiophene segment with an electron-poor oligo-thiazole segment. These segments are efficient hole and electron transfer materials, respectively. The molecular structure features an in-built electronic symmetry equivalent to a semiconductor p-n junction.
First, different alkyl side chains with contrasting water affinity were added to the segments to cause the molecules to integrate at the solvent/water interface, from where they can be transferred to a single molecule layer on a solid surface.
Scanning tunneling microscopy was used to determine the electrical properties of this diblock molecular material. An asymmetric current vs voltage curve was observed for the diblock thiazole-thiophene compound with a turn-on voltage observed at around +1.0 V.
This asymmetry implies that current moves more easily in the forward direction than in the backward direction - just like a diode. Identical measurements on an analogous thiophene-thiophene compound that was synthesized to enable comparison resulted in a much smaller current and an almost symmetric I-V curve.
In the next stage of the study, the basic diblock oligomer units were inserted into a self-assembled monolayer (SAM) of alkanethiols on a gold surface, to allow for measurements of the compound's rectifying effect. STM imaging showed the areas where molecules were inserted as bright spots.
An asymmetric I-V curve was obtained. These rectifying conjugated molecules will enable the fabrication of molecular-scale electronic circuits that will help design nanoscale logic circuits. Based on this diblock system, it will be possible to have several electronic and structural property variations resulting in made-to-order diodes.
In 2009, N.J. Tao and his team at Arizona State University were able to produce a single molecular which functioned as a diode. They reviewed previous studies into creating molecular diodes, reporting that previous efforts had focused on macromolecular structures and thin films, rather than single-molecule designs.
Tao's team also performed a detailed comparison of electron transport in symmetric and asymmetric molecules. In the case of asymmetric molecules, the current travels both ways like in an ordinary resistor. This is a useful observation; however, it is tougher to duplicate a diode, which conducts well in one direction only.
Tao’s team achieved this feat using a property known as AC modulation. A periodically varying mechanical perturbation was applied to the molecule. In case a molecule is bridged across two electrodes it responds in a distinct way when compared to its response if there was no molecule.
The team made use of conjugated molecules wherein atoms are connected with alternating multiple and single bonds. These molecules demonstrate high electrical conductivity and have asymmetrical ends that can spontaneously form covalent bonds with metal electrodes to form a closed circuit.
Molecular Rod Diode
In 2005, researchers from Northwestern University, Evanston designed and developed a molecular rod that contains two weakly coupled electronic π–systems with shifted energy levels. It was expected that this asymmetry in energy levels would result in diode-like asymmetries in electrical properties.
Sulfur-gold bonds immobilize the individual molecules between the electrodes of a mechanically controlled break junction. This setup allowed the electronic properties of the molecular rod to be studied.
The results showed current-voltage characteristics similar to a diode. In contrast, control experiments done with symmetrical molecular rods with two identical π-systems did not show significant asymmetries in the transport properties.
The underlying transport mechanism was studied by combining phenomenological arguments with density functional theory-based calculations.
According to the theoretical analysis, the bias dependence of the molecule polarizability feeds back to the current resulting in an asymmetric shape of the current-voltage characteristics, similar to a semiconductor diode.
Diodes consisting of a single molecule have the potential to transform the face of electronics. along with other nanoscale electronic components, they will enable the rapid miniaturization of electronic circuits and devices.
The research discussed above has led to major breakthroughs in the field of molecular electronics. Whichever methods are used to create these nanoelectronic components commercially, they will represent a complete paradigm shift in electronics, shifting from a technology based almost entirely on silicon to a much more diverse ecosystem of complex molecular systems and advanced materials.
Sources and Further Reading