Q&A: Field-induced conductance switching

IBM 7nm node test chip
Industry experts consider 7 nanometer (nm) technology crucial to meeting the anticipated demands of future cloud computing and big data systems, cognitive computing, mobile products and other emerging technologies.
Microprocessors utilizing 22 nm and 14 nm technology power today’s servers, cloud data centers and mobile devices, and 10 nm technology is well on the way to becoming a mature technology, but the challenges dramatically increase when going below 7 nm or even 5 nm.
At dimensions below 5 nm it becomes increasingly difficult to achieve reasonably high on/off current ratios, mainly because of increasing leakage paths and less control over the doping atoms. To explain the challenge, consider a leaky water faucet even after closing the valve as far as possible, water continues to drip. This is similar to today’s transistor, in that energy is constantly "leaking" or being lost or wasted in the off-state. For these reasons, scientists around the world are exploring novel materials including carbon nanotubes, graphene, 2D layered materials, phase-change materials or single molecules to discover novel type of switching mechanisms with enhanced control over transport at nanometer dimensions.
Appearing today in the peer-review journal NatureNanotechnology, scientists at IBM Research, the University of Zurich and the University of Vienna are reporting on a novel concept to modulate the current through a less-than-3 nm long molecule by more than three orders of magnitude a critical step to control transport for future electric circuits, with applications in signal processing, logic data manipulation, data storage or neuromorphic networks.
On the eve of its publication, Field-induced ConductanceSwitching by Charge-state Alternation in Organometallic Single-MoleculeJunctions, lead scientist Emanuel Lörtscher addresses a few questions.

Let’s get straight to the most pressing question, have you discovered the so-called “next switch”? 

Emanuel Loertscher
Emanuel Lörtscher (EL): I wouldn’t say so because of the following reason: In our work, we employ redox-active metal centers embodied in a single molecular compound. The novelty is that the charge-state of these 3 nm long molecules can be addressed solely by the external field present in a simple two terminal geometry, changing the conductance by more than three orders of magnitude simply by adding one extra charge onto the molecule. This abrupt switching behavior and its large influence on transport are quite astonishing for a molecular-intrinsic mechanism that does furthermore not require a gate electrode, like in case of a standard CMOS transistor. In that sense, such functional molecular compounds are very promising switches.

For the ideal “next switch” however, substantially higher “on” currents are needed, a requirement that a single molecule cannot fulfill and will never as its intrinsic degrees of freedom – the intrinsic functionality get lost upon coupling it too strongly to electrodes. Ensembles of molecules however, could potentially do that task and may serve as the active “next switching component,” causing switching and hysteresis for memory applications. Even a few hundred molecules will occupy a footprint which is still sufficiently small to be technologically attractive. The well-defined, microscopic integration of multiple molecules into circuits is however, still a vision. 

In the paper you state that have achieved an abrupt switching ratio with high-to-low current of more than 1,000, which outperformes all previously explored molecular-intrinsic conductance-switching mechanisms. What was the previous level and why is this ratio so important?
EL: For electronic devices, the high-to-low current ratio defines in principle the possible range of applications the higher the “better” for circuit functionality, fault tolerance, storage contrast, etc. Usually in single molecules, the conductance could be varied only by a factor of 3 – 50, employing the existing mechanisms such as rotation or conformational changes. 

To go beyond that level and to reach technologically relevant high-to-low ratios of 1,000, more advanced mechanisms had to be developed. We employ intrinsic charge states of redox-active metal centers that we place directly into the transport pathway of electrons to achieve the largest possible influence on the conductance – a concept that has been optimized over the years and finally realized by our chemistry partners form the University of Zurich. On the sub-5 nm range, the performance achieved in these organometallic compounds can compete even with other technologies that usually suffer from severe scaling limitations. 
Abrupt switching ratio with high-to-low current of more than 1,000
was achieved

It appears that the “secret sauce" to your design was the introduction of molybdenum. How did you decide to include it in your device?
EL: This is correct. The big performance improvements arose by using molybdenum as central metal atom. This was pure instinct of the chemists combined with physicist’s hope to cause more pronounced effects due to the high magnetic anisotropy of Mo (molybdenum). The abrupt behavior found, however, made it hard to understand exactly why molybdenum causes such unique effects – distinctly different from Fe (iron) and Ru (ruthenium). 

Calculations performed by our colleagues from the University of Vienna finally provided a microscopic picture that agreed excellently with our experimental findings: The Mo has a spin-polarized ground state, which creates a localized molecular orbital that is weakly coupled and can be charged by a slow, decoherent hopping process, resulting in the huge switching and hysteresis we discovered experimentally. So in simple terms, we use quantum mechanical effects to create a highly functional molecule acting as an efficient current switch and storage element with unprecedented performance.
In the summary you cite future memory applications for your device. What are the advantages of using molecules for that task?
EL: This work represents fundamental research that assesses the potential of novel concepts for future electronics. Talking about applications is therefore quite speculative, keeping all the technical and economical boundary conditions in mind. The large conductance alternation and the hysteresis caused by adding a single extra charge to the molecule, however, is a finding with large potential for various applications, namely in data storage or neuromorphic networks, simply by its basic two-terminal addressing and non-linear electrical response function.

In fact, charge and spin states in molecules are quantum mechanical mechanisms that can ideally be employed to provide novel complex functionality not used in today’s transistor technology. These capabilities can be tailored and embodied in scaled compounds that can all be produced identically, due to the chemical synthesis used as a cheap, bottom-up fabrication process with sub-atomic control.
What’s next for your research?
EL: As we discussed above, molecular compounds have truly large potential to be used as highly functional electronic building blocks. But their reliable implementation represents the main show stopper for practical usage since the dawn of molecular electronic concepts 40 years ago. In order to overcome this technological hurdle, we are currently working (in the framework of NCCR MSE) on implementing ensembles of molecules reliably in nanopores, on a solid-state and wafer-scale platform for robust operation under ambient conditions.

This will keep us busy for the next few years – but all this incremental success, together with the new horizons IBM is currently looking at, keep us motivated every day.

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