‘V6’-POM
‘V18’-POM
‘P2V3W15’-POM
“More than Moore” the interface between synthetic inorganic chemistry and condensed matter physics Molecular electronics is attracting a lot of attention as a means of enabling continued reductions in feature size in data storage and processing devices. Dr Kirill Monakhov and his colleagues synthesise stimuli-responsive molecules, characterise them, and apply them on substrate surfaces, work which could open up new possibilities in highly sought after ‘More than Moore’ information technology. The microelectronics industry developed rapidly over the second half of the twentieth century and beyond, as continued down-scaling of CMOS devices opened up wider commercial opportunities. However, further miniaturisation down to the sub-10 nm regime (so-called ‘quantum limit’) and below is growing ever more challenging, prompting researchers to explore other avenues; Dr Kirill Monakhov and his colleagues (Prof Rainer Waser in Jülich and Prof Bernd Abel in Leipzig) are investigating the field of molecular electronics. “We produce coordination compounds suitable for molecular deposition experiments and nanoscale imaging on surfaces. We try to design and then synthesise molecules that are likely to exhibit many discrete and thermodynamically stable oxidation states,” he explains. Among these molecules are biocompatible vanadium-oxo clusters (polyoxovanadates) and their heteropoly derivatives from a class of polyoxometalates (hereinafter referred to as POMs), which have beneficial structure–property characteristics. “For example, usually negatively charged polyoxovanadates (charged balanced by e.g. quaternary ammonium cations or alkali
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metal cations) feature a striking interplay of molecular charge, redox states and spin states,” says Dr Monakhov. “These can be manipulated by micro-spectroscopic means to address specific goals, not only in the domain of IT devices, but also in molecular biochemistry and biophysics.”
Molecular synthesis and surface studies Researchers at Dr Monakhov’s laboratory carry out multiple tasks, from preparing and characterising molecules to immobilising and electrically accessing them on substrate surfaces, work which could hold important implications for the future development of molecule-based computer memory cells in the electronics industry. The first step here is to produce the markedly different molecular structural motifs, including the development of conceptually new metal complexes and their supramolecular assemblies. “The molecules are designed to have a level of stability against air and moisture, solubility and a specific functionality. The latter could be a low molecular charge or the charge neutrality of a POM building block for example, or you might have organic ligation growth, that
provides a source of stabilisation on surfaces,” outlines Dr Monakhov. The next step after synthesis of these coordination compounds is to investigate their suitability for adsorption as intact molecules on surfaces, which is one of the crucial prerequisites for their applicability. “We usually explore deposition of our nonvolatile molecules in solution on different substrate surfaces under ultra-high vacuum – the surfaces can be conductive. Gold substrates are basically the first choice, due to the ease of handling,” continues Dr Monakhov. The team is also investigating molecular deposition and charge transport characteristics on semi-conductive surfaces, as they aim to demonstrate compatibility with CMOS devices. Different surface-sensitive methods are used here to elucidate the structure and properties of molecular adsorbates in the electrode environment. “We employ various different techniques for sub-molecular resolution imaging and the analysis of molecule–surface interfaces, from scanning tunnelling microscopy (STM) to Grazing-incidence small-angle X-ray scattering (GISAXS),” explains Dr Monakhov. “First, we want to determine the adsorption type, the agglomeration tendency, the distribution and the oxidation state of deposited molecules.”
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Two terminal fundamental cell setup. Three terminal practical cell setup.
By applying scanning tunnelling spectroscopy on individual molecules, Dr Monakhov and colleagues are able to detect electron transport. “We can look at how molecules are conductive and whether their molecular conductivity can be tracked as a function of individual metal redox states. Their reversible switching at room temperature and a low bias voltage is the major goal,” he says. This work is central to assessing the suitability and processability of these single molecules as components of multiple-state resistive (memristive) switching devices; one possible application is in braininspired neuromorphic computing, for example. “Different computer memory cells are currently available on the market, based for example on charge-based, magnetic or optical storage. We go beyond these von Neumann architectureoriented concepts of constructing binary devices,” explains Dr Monakhov.
Resistive Random Access Memory (ReRAM) In collaboration with the group of Prof Waser at the Peter Grünberg-Institute (PGI-7) of the Forschungszentrum Jülich, Dr Monakhov’s lab is now investigating the concept of redoxbased resistive switching memories (see photograph opposite). These could bring some significant benefits over existing data storage and processing techniques in terms of low power consumption, higher scalability, non-volatility and lower switching time. The ReRAM cells are based on a relatively simple structure that helps to improve cost
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efficiency. “In our model two-terminal cell setup we just have two electrodes – a surface as bottom and an STM-tip as top – and the molecules inbetween,” says Dr Monakhov. The long-term goal is to develop a moleculebased memristive switching device – a type of electrical component – operated by many discrete, stable and electrically accessible metal redox states, which could open up some extremely interesting possibilities. “Indeed, we want to implement such devices in the area of artificial intelligence,” says Dr Monakhov. “This would be for specific high dense, non-
we would effectively replace a large number of the energy-intensive transistors by such memristive devices, ultimately improving efficiency and increasing storage capacity,” outlines Dr Monakhov. The microelectronics industry is currently dominated by metal-oxide-semiconductor structures, so shifting to molecular ‘More than Moore’ nanoelectronics could have farreaching effects in this area. “In a memristive device, writing of information would occur at an increased potential in the range of a few volts, whereas reading of information would
We produce coordination compounds suitable for molecular deposition experiments and nanoscale imaging on surfaces. We try to design and then synthesise molecules that are likely to exhibit many discrete and
thermodynamically stable oxidation states.
volatile and intelligent memories, which could realize data storage and processing in the same material through the additive character of available molecular redox states. This will allow us to substantially reduce internal data transfer, from which classical von Neumann architectures suffer, and furthermore improve performance of IT systems.” One other possible application is in digital medicine. “We can also think about implementing the molecular memristive functionalities into neurostimulators. Thus,
occur at significantly lower voltages in the millivolt range. This is amazing, because the envisaged molecule-orchestrated read and write operations would enable us to sustainably increase the performance and energy efficiency of nanoelectronic devices, while keeping on down-scaling,” continues Dr Monakhov.
Moving fundamentals towards the application The long-term prospects are very exciting, yet there are still some technical issues to deal
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with first, which are high on the agenda for Dr Monakhov and his colleagues. The current focus in research is on understanding how the coordination compounds behave on various conductive and semi-conductive surfaces, and how their structural and electronic properties respond to adsorption and further manipulation by the electric field of a scanning tunnelling microscope. “The next step beyond that will be to integrate single molecules into specific nano-gaps, in order to somehow electrically contact the molecules (see figure to left),” says Dr Monakhov. This remains one of the major challenges in the wider molecular electronics field, and Dr Monakhov says that it is central to the development of practical devices relevant to the needs of industry. “We need to find procedures by which we can address the stable and reproducible electrical contacts to single molecules that are wired to nano-gap electrodes,” he continues. “The usage of a three-terminal setup implying two electrodes and a contact substrate surface is moreover the key to determining the retention and switching time of the molecular metal redox states and the endurance of the potential single molecule-based memristive device.” However, one significant question that needs to be explored by chemists and physicists is how to conveniently embed the single molecules in nano-gaps, without ‘losing’ their inherent molecular orbital structure, as compared to their bulk state. “For this reason, not only the structure– property relationships of coordination
compounds should be fine-tuned, but also the nano-gap electrodes require optimisation by both chemical and physical means,” says Dr Monakhov. Molecular electronics brings together elements of many different fields, and while Dr Monakhov specialises in chemical synthesis and surface engineering, he says other disciplines also play an indispensable role in driving progress. “There are people from different backgrounds, including for example micro-spectroscopists, theoretical chemists, and materials scientists. Close collaboration with them is very important with respect to fundamental understanding and active control of molecular electron transport processes and for future industrial applications,” he outlines. This research is quite high-risk and exploratory in nature, yet this also means that the potential gains are correspondingly high. While the current focus in research is on the development of processable coordination compounds, the characterisation of their properties, the bottom-up surface modification and the electrical addressability of fabricated hybrid materials, Dr Monakhov is also fully aware of the wider picture. “We want to move all these underlying research questions and goals towards practical implementation, and we’re looking towards the next steps in this yet fundamental area. We’re looking for opportunities to do technology-focused research, outside the framework of our surface-oriented work,” he says.
SwitchSpinPOV Development of Heteropolyvanadate Spin Clusters as Candidates for Future Redox-Based Memory Devices
Project Objectives
Project studies lie at the interface between the synthetic inorganic chemistry and condensed matter physics. They target the fundamental understanding of multiple-state resistive (memristive) switching at the level of individual polyoxovanadates and their organically modified derivatives immobilised on conductive and semi-conductive surfaces. The farreaching goal is to integrate the developed redoxactive molecules into the industrially relevant “More than Moore” technology setups.
Project Funding
The project is supported by the Emmy Noether programme of the Deutsche Forschungsgemeinschaft (DFG).
Project Partners
• Professor Rainer Waser (JARA-FIT, Forschungszentrum Jülich und RWTH Aachen University)
Contact Details
Project spokesperson: Dr Kirill Monakhov Laboratory for Switchable Surfaces and Spintronics Leibniz Institute of Surface Engineering (IOM) Permoserstraße 15 04318 Leipzig Germany T: +49 (0)341 235 3364 E: kirill.monakhov@iom-leipzig.de W: https://www.iom-leipzig.de W: https://monakhovlab.jimdofree.com/ W: https://twitter.com/monakhovlab?lang=en
Kirill Monakhov
Maria Glöß (PhD in the group of Kirill Monakhov) and Dr Marco Moors (PostDoc in the group of Rainer Waser) perform a model STM experiment with polyoxovanadate molecules immobilised on a gold substrate surface at the UHV oxide cluster tool located at the Peter Grünberg Institute (PGI-7) of the FZ Jülich.
Kirill Monakhov currently holds a Group Leader position at the Leibniz Institute of Surface Engineering (IOM) in Leipzig. He received his Dr. rer. nat. degree at the Heidelberg University in 2010 and spent then several years as a postdoctoral fellow at the University of Strasbourg and the RWTH Aachen University. He was the recipient of the prestigious DFG Emmy Noether Fellowship in 2015 and of the Academia Europaea Burgen Scholarship in 2011.
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