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A team from IBM Research-Zurich and the Universities of Basel and Zurich developed a new technique to fabricate microelectronic devices by electrically contacting molecules at two well-defined termini. The method allows molecular integration on conventional silicon chips and by standard fabrication methods as used in semiconductor industry. The technique is based on depositing a nanoparticle layer, as reported today in the journal Nature. This advance can be used to employ ultimately scaled building blocks with novel functionalities for applications in electronics or sensing.
As the miniaturization of silicon technology slowly continues, devices which offer more than the on/off functionality of a transistor become more attractive. Molecular devices offer exactly this advantage, allowing a chemical tailoring of the device response. This is useful not only for emerging computing paradigms such as neuromorphic computing, but also for highly specific sensors based on molecular recognition or quantum effects. These devices are reality in the laboratory, but the step towards first commercial applications has been severly hampered by the technological difficulties to nondestructively integrate and mass-manufacture them with high yield.
Leveraging silicon fabrication
Top view of an array of molecular monolayer devices fabricated on a four inch silicon wafer with individual contact pads for electrical characterization of single pores
We address this limitation with a technique that allows molecules to be compartmentalized into dielectric pores and electrically contacted. A type of sandwich construction is used in which a layer of molecules is brought into contact with metallic electrodes from above and below. The lower electrode consists of a layer of platinum lithographically patterned onto a conventional silicon wafer, which is coated with a non-conducting material. Tiny pores of arbitrary diameters are then etched into this dielectric layer to expose a defined area of the bottom electrode.
We then take advantage of the ability of certain molecules to self-assemble onto surfaces. When placing the wafer in a solution of molecules, these molecules self-assemble on the open platinum areas. The resulting monolayer of upright, regularly spaced molecules is electrically connected to the bottom electrode by the covalent anchoring groups.
Gentle top-contact formation
Single pore (indicated by the arrow and the dotted circle) of 10 micrometer diameter, visible through the top nanoparticle, bulk metal contact that seals the molecular monolayer film to provide ambient stability
To perform the crucial step of creating a top-contact without destroying the molecular layer, a film of nanoparticles is deposited directly onto the molecules. This layer allows then a bulk electrode to be added using conventional methods, creating thousands of stable metal-molecule-metal junctions without compromising the properties of the molecules. The new technique resolves the problems that previously hampered the creation of electrical contacts to molecules, including high contact resistances or short circuits by filaments penetrating the film. Building blocks fabricated by this reliable, inexpensive, and mass-production-compatible method can be operated under ambient conditions and provide long-term stability.
Towards molecular-based devices
We demonstrate the approach using alkane-dithiol molecules made up of carbon, hydrogen, and sulfur (the state-of-the-art benchmarking system), but it can be applied to a variety of other molecular systems. This work may therefore open avenues for integrating molecular compounds into solid-state devices with various novel applications, including new electronic devices and instruments in the fields of sensor technology.
The project was funded by the Swiss National Science Foundation (SNF) by the National Center of Competence in Research (NCCR) Molecular Systems Engineering and National Research Program 62.
Metallic nanoparticle contacts for high-yield, ambient-stable molecular-monolayer devices
Gabriel Puebla-Hellmann, Koushik Venkatesan, Marcel Mayor, and Emanuel Lörtscher
Nature, volume 559, pages 232–235 (2018), doi: 10.1038/s41586-018-0275-z