Since 2009 IBM scientist Emanuel Loertscher has been designing and supervising the building of six state-of-the-art "noise-free" laboratories for ultra-sensitive nanotechnology research. Now, after having worked for some time more as an architect, he is turning back to his original research in nanotechnology, investigating the use of individual molecules as electronic building blocks.
Emanuel provided answers to some questions about a commentary he provided to a June 2013 special focus edition of Nature Nanotechnology on molecular electronics and about the field in general.
Emanuel Loertscher of IBM Zurich by the media feed-in port
in his noise-free laboratory.
Q: Your commentary for Nature Nanotechnology presents an overview of both the historic and recent research done in the field of molecular electronics. How close are we to achieving the early visions of the 1970s?
Emanuel Loertscher: In the 1970s, Ari Aviram and Mark Ratner proposed employing single molecules as functional building blocks in electronic circuits. This is still a revolutionary concept as molecules are nanometer-sized objects, unlike some other nanoscale building blocks where at least one dimension is much larger than 10 nm, sometimes even micrometers in size. However, this makes it very challenging to appropriately contact molecules and to wire them to a circuit.
Nevertheless, over the last 10 years, various tools have been developed that enable transport at the single-molecule level to be studied. Thereby, transport effects far beyond the seminal one, that tried to mimic semiconductor device mechanisms, have been discovered by exploring the quantum mechanics of molecules – such as quantum interference observable even at room temperature – and all of them are considered very attractive for future electronics. However, all these experimental realizations of molecular electronics are still at a very fundamental stage, and far away from practical implementation into electronic circuits.
Q: What is still required to make molecular electronics a viable candidate for future electronics?
EL: As molecules are typically one nanometer in size, they are comparable to the atoms composing the electrodes. Therefore all the details of the molecular junctions matter but are not yet under experimental control. This is particularly true when operating the devices at elevated temperatures or higher electric fields as the metal atoms of the electrode start to move around. In that sense, it's essential to develop a platform where the electrodes are stable under these conditions, leading to better reproducibility of the electrical characteristics and an enhanced long-term stability of the device.
Graphene is clearly the material of choice as it's stable at room temperature and electronically compatible with organic molecules. I'm convinced that the development of graphene structures will synergetically lead to improved molecular electronic devices in the near future.
Q: What do you think the first applications will be for electronic devices made of molecules?
EL: As mentioned, even though 40 years have elapsed, molecular electronics is still in its embryonic state mainly due to a lack of control at the atomic scale. Here, it has to be mentioned that all nanoelectronic concepts will face this challenge when they are ultimately scaled to the atomic level.
Regarding applications, ensemble junctions can be envisioned since single-molecule junctions are prone to large variabilities. Due to the attractive non-linear transport properties of molecules, such as very abrupt switching, I can imagine the first applications to be for computer memory.
Q: Are you currently exploring any research in this area?
EL: One of the largest areas of focus for IBM in this field is designing what we call "the next switch." Moore's Law is going to hit a wall shortly as we can no longer reduce the size of transistors every 18 months. Therefore we are currently investigating the role of molecular electronics to potentially offer novel device concepts, which are ultimately scaled and provide electronic functionalities beyond those of the transistor.
More specifically, we are studying functional molecules where electrical resistance can be switched when the molecule is contacted by two electrodes in a two-terminal device. In contrast to many other devices where a third electrode is needed, the two-terminal concept enables ultimate scaling and therefore the highest integration densities for memory applications.
We are also trying to work towards more realistic devices where the molecules are embedded and sealed in small pores and contacted in cross-bar structures. Just recently I've started a complementary activity using nano-optical means in plasmonic structures to study the assembly and vibrational properties of molecules inside the junction with the goal to correlate optical and electrical experiments.
Q: You've recently completed the design and development of the "noise-free" Labs at the new Binnig and Rohrer Nanotechnology Center. Can these high-tech rooms advance the nanotech field?
EL: The noise-free labs represent a unique environment as all disturbances relevant for our experiments are screened simultaneously to an unprecedented level. For molecular electronics, it enables more advanced characterizations of molecules than before, such as studying their thermoelectrical or optical properties. However, the main issue of the junction's control and stability cannot be solved by going to an ultra-silent environment. As mentioned above, there's no way around developing more stable platforms.
Q: Your outlook summary in Nature Nanotechnology leaves some mystery as to when we will begin to see such devices. What would be your best guess?
EL: The outlook is consciously written in an open way as many promises made in the past did not come true – leading to a legitimate disillusionment, especially in the semiconductor community about single-molecule electronics.
On the other hand, organic electronics – dealing with large ensembles of molecules in contrast to single-molecule electronics – has demonstrated that molecular building blocks can truly compete in terms of fabrication costs with today's predominant technologies. Also, when the roadmap hits dimensions below 5 nm, only molecules will offer accurate control over atomic distances and can still be made identical. This is the point in time when a true paradigm shift from top-down to bottom-up fabrication will take place.
— Chris Sciacca is the manager of communications at IBM Zurich Research Labs, Zurich, Switzerland.