Final Report Summary - TO COME (Towards CMOS-compatible molecular electronics)
Summary of the objective
The microelectronic industry has gone through significant changes from 'just' shrinking devices and increasing the density to introducing new materials such as high-k dielectrics and metal gates. Significant resources are invested in research and development (R&D) for continuous improvement of silicon-based complementary metal-oxide-semiconductor (CMOS) devices by developing new materials and new device architectures. However, further miniaturisation poses ever more complex challenges and appears to be neither technically nor economically feasible. The 2009 International Technology Roadmap of Semiconductors (ITRS) predicts that the industry will reach these fundamental limits within about 10-year time. In order to increase computing performance beyond CMOS, we need to develop new technologies and strategies. This includes the use of novel materials and device concepts, innovative device architectures, and smart integration schemes. In the first step, however, a new technology to become successful must still be compatible with the existing silicon / CMOS technology to be integrated in a hybrid platform for specialised tasks at a first stage. Electronic devices fabricated from junctions with bridging self-assembled molecular monolayers (SAMs) appear to be ideal candidates for future nanoelectronics as they offer unparalleled design flexibility and scalability not found in inorganic material systems. The major focus of research in this area has been on metal-molecule-metal junctions. A variety of studies have shown that through novel design and selection of the molecular structure, end groups, and metal electrode materials, important operational functions can result, including negative differential resistance, bistable switching, and chemical transduction. The primary goal of this project is to establish a suitable platform of CMOS-compatible electrodes using alternative noble metals. The most suitable candidates amongst them are palladium (Pd) which are known to possess reduced surface mobility with respect to gold (Au). Furthermore, it is CMOS-compatible and are available in relative abundance. Electrodes from Pd will be developed for their use in future devices based on single or ensembles of molecules.
This will be achieved by addressing the following tasks:
(1) Fundamental studies using in situ scanning tunnelling microscopic techniques will evaluate the morphological and electronic coupling of molecules to these materials resulting from various chemical 'anchor' groups.
(2) Study of chemical assemblies of functional molecules on Pd electrodes and characterise their functional behaviour (assembly, transport properties, etc.). These studies will be carried out in situ on molecular systems formed from liquid phase deposition.
(3) Develop first realisation of small-scale devices incorporating Pd electrodes and functional molecules and investigate charge transport across the molecular junctions using conductive atomic force microscopy (AFM).
Summary of the work
In order to understand the molecular scale process at the liquid-solid interface, it is necessary to develop the appropriate tools that are capable of recording dynamic molecular scale phenomena. Therefore, it was the primary task to set up the instrumentation capability such as scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS) that allows the visualisation of single molecular morphology and recording of electronic signature within a liquid medium. Suitable fabrication techniques for preparing ultra-flat metal substrates of Pd were necessary in order to study SAM films and later on apply the protocol to develop functional devices. A thorough investigation of the surface roughness of the as-prepared metal thin films is central to this study as one of the requirements on the bottom electrode previously unresolved is the roughness of the free metal surface that will later be covered with molecules. This surface must have a roughness far below the molecular length scales. Deposition and heat treatment methods will be screened using scanning probe techniques to measure the roughness quantitatively. The knowledge generated from the above-discussed experimental packages will be applied in integrating organic molecules with CMOS-compatible molecular electronic platforms.
Significant results and achievements
(1) Developed STM and STS instrumentation capability with exceptional lateral resolution (2 nm) at room temperature under ambient conditions.
(2) The designed set-up allows high-speed recording of molecular process such as Oswald ripening of molecular layers (C60 self-assembled monolayers) and the pre-amplifier allows detection of ultra-low tunnelling current (2 pA). This is very advantageous as the STM tip does not induced molecular motion.
(3) The custom built liquid cell setup is designed in a manner as to permit measurements in harsh solvents such as tetra-hydro-furan (THF) as its made of Teflon. The in situ STM / STS measurements are housed in the state-of-the-art noise free labs at IBM Research - Zurich.
(4) The scanning system has minimal drift and has a scan range of 7 µm2, this facilitates large area analysis of molecular films and evaluate substrate quality over large-fields.
(5) Ultra-flat metal thin films (50 - 100 nm) of gold and Pd were fabricated on halide support platforms with atomic scale smoothness over the metal grains. This novel method is cost-effective and circumvents complex processing steps associated with other techniques such as template-stripping and flame annealing.
(6) The metal thin films had fewer surface defects and yielded exceptionally large grain areas (approximately 1.5 µm2 ) with a root-mean-square (rms) surface roughness of approximately 0.5 nm measured over a 25 µm2 sample area.
(7) Upon fabricating ultra-flat metal films, molecular adsorption was studied using the in situ STM setup. The motion of single molecules was recorded over time and the evolution of self-assembled fullerene molecular layers was augmented. The fullerene molecular layers are formed in closely packed structures in order to minimise the overall interfacial energy. Molecular units adsorbed on ultra-flat metals were observed to form defect-free (pinhole defects) molecular over layers. This is a crucial step as it restricts electrical shorts between the top and bottom electrode through the molecular layers when integrated in metal-molecule-metal type architectures.
(8) In addition to mapping the morphology of single molecules we established protocols to study the electronic properties of novel molecular scale systems such as fullerene dumbbells. These molecules (shown in schematic) were obtained through a collaborator work with the chemical synthesis groups at the University of Madrid. We have mapped the morphology and electronic signature in liquid medium for these molecules and were able to resolve the molecular resonance peaks.
(9) We have developed a novel methodology to study the intrinsic molecular properties by partially decoupling the molecules from the metal platforms. In order to decouple the molecules from their respective platforms we synthesised insulating organic molecules (n - tetradecane) using a room-temperature based spray deposition technique.
(10) This handheld spray deposition technique is currently installed at the chemical laboratories at IBM Research - Zurich where nanoscale materials (nanowires, graphene, MoS2) dispersed in a liquid phase can be homogeneously deposited and studied using AFM, scanning electron microscopy (SEM) techniques. A fine spray nozzle (0.1 mm diameter) allows precise deposition of nanowires at desired locations. This was achieved by successfully calibrating the rate of material flow and spraying angle. This technique has been applicable to forming top-electrode in molecule tunnel junction devices. The spray deposition technique permits controlled fabrication of the insulating molecular layers with nanometre-level thickness control.
(11) Through the presence of the insulating organic layers with extremely low dielectric constant fullerene dumbbells were decoupled both structurally and electronically from the underlying ultra-flat metal (Au and Pd) platforms. This decoupling effect was substantiated by the evolution of sharp molecular states with well-defined conductance gaps (shown in STS curve, inset shows STS spectra on fullerene dumbbells directly in contact with the metal surface). This technique has also been extended to vapour phase deposited molecular systems such as pentacene molecular layers which has been demonstrated to be decoupled from the metal substrate. This methodology has important implications extending from basic molecular electronics research to fabricating organic gates in thin film transistors.
(12) We have initiated the conductance imaging AFM (C-AFM) technique here at IBM Research - Zurich and have applied this technique to study the electronic landscape of metal thin and to probe the charge transport through molecules trapped in lithographically designed nanopores.
(13) The open architecture of the nanopore devices have been examined using the C-AFM technique and the on-chip characterisation of the molecules integrated in the devices are being evaluated for their molecular functionality.
(14) The molecular adsorption on the ultra-flat metal platforms is not affected after the fabrication and selective etching of the oxide layer to trap the molecules. Such platforms can now be readily made in which a wide range of molecules can be analysed. Currently experiments are underway to complete a manuscript of the experiments conducted on this on-chip characterisation of molecular systems.
The microelectronic industry has gone through significant changes from 'just' shrinking devices and increasing the density to introducing new materials such as high-k dielectrics and metal gates. Significant resources are invested in research and development (R&D) for continuous improvement of silicon-based complementary metal-oxide-semiconductor (CMOS) devices by developing new materials and new device architectures. However, further miniaturisation poses ever more complex challenges and appears to be neither technically nor economically feasible. The 2009 International Technology Roadmap of Semiconductors (ITRS) predicts that the industry will reach these fundamental limits within about 10-year time. In order to increase computing performance beyond CMOS, we need to develop new technologies and strategies. This includes the use of novel materials and device concepts, innovative device architectures, and smart integration schemes. In the first step, however, a new technology to become successful must still be compatible with the existing silicon / CMOS technology to be integrated in a hybrid platform for specialised tasks at a first stage. Electronic devices fabricated from junctions with bridging self-assembled molecular monolayers (SAMs) appear to be ideal candidates for future nanoelectronics as they offer unparalleled design flexibility and scalability not found in inorganic material systems. The major focus of research in this area has been on metal-molecule-metal junctions. A variety of studies have shown that through novel design and selection of the molecular structure, end groups, and metal electrode materials, important operational functions can result, including negative differential resistance, bistable switching, and chemical transduction. The primary goal of this project is to establish a suitable platform of CMOS-compatible electrodes using alternative noble metals. The most suitable candidates amongst them are palladium (Pd) which are known to possess reduced surface mobility with respect to gold (Au). Furthermore, it is CMOS-compatible and are available in relative abundance. Electrodes from Pd will be developed for their use in future devices based on single or ensembles of molecules.
This will be achieved by addressing the following tasks:
(1) Fundamental studies using in situ scanning tunnelling microscopic techniques will evaluate the morphological and electronic coupling of molecules to these materials resulting from various chemical 'anchor' groups.
(2) Study of chemical assemblies of functional molecules on Pd electrodes and characterise their functional behaviour (assembly, transport properties, etc.). These studies will be carried out in situ on molecular systems formed from liquid phase deposition.
(3) Develop first realisation of small-scale devices incorporating Pd electrodes and functional molecules and investigate charge transport across the molecular junctions using conductive atomic force microscopy (AFM).
Summary of the work
In order to understand the molecular scale process at the liquid-solid interface, it is necessary to develop the appropriate tools that are capable of recording dynamic molecular scale phenomena. Therefore, it was the primary task to set up the instrumentation capability such as scanning tunnelling microscopy (STM) and scanning tunnelling spectroscopy (STS) that allows the visualisation of single molecular morphology and recording of electronic signature within a liquid medium. Suitable fabrication techniques for preparing ultra-flat metal substrates of Pd were necessary in order to study SAM films and later on apply the protocol to develop functional devices. A thorough investigation of the surface roughness of the as-prepared metal thin films is central to this study as one of the requirements on the bottom electrode previously unresolved is the roughness of the free metal surface that will later be covered with molecules. This surface must have a roughness far below the molecular length scales. Deposition and heat treatment methods will be screened using scanning probe techniques to measure the roughness quantitatively. The knowledge generated from the above-discussed experimental packages will be applied in integrating organic molecules with CMOS-compatible molecular electronic platforms.
Significant results and achievements
(1) Developed STM and STS instrumentation capability with exceptional lateral resolution (2 nm) at room temperature under ambient conditions.
(2) The designed set-up allows high-speed recording of molecular process such as Oswald ripening of molecular layers (C60 self-assembled monolayers) and the pre-amplifier allows detection of ultra-low tunnelling current (2 pA). This is very advantageous as the STM tip does not induced molecular motion.
(3) The custom built liquid cell setup is designed in a manner as to permit measurements in harsh solvents such as tetra-hydro-furan (THF) as its made of Teflon. The in situ STM / STS measurements are housed in the state-of-the-art noise free labs at IBM Research - Zurich.
(4) The scanning system has minimal drift and has a scan range of 7 µm2, this facilitates large area analysis of molecular films and evaluate substrate quality over large-fields.
(5) Ultra-flat metal thin films (50 - 100 nm) of gold and Pd were fabricated on halide support platforms with atomic scale smoothness over the metal grains. This novel method is cost-effective and circumvents complex processing steps associated with other techniques such as template-stripping and flame annealing.
(6) The metal thin films had fewer surface defects and yielded exceptionally large grain areas (approximately 1.5 µm2 ) with a root-mean-square (rms) surface roughness of approximately 0.5 nm measured over a 25 µm2 sample area.
(7) Upon fabricating ultra-flat metal films, molecular adsorption was studied using the in situ STM setup. The motion of single molecules was recorded over time and the evolution of self-assembled fullerene molecular layers was augmented. The fullerene molecular layers are formed in closely packed structures in order to minimise the overall interfacial energy. Molecular units adsorbed on ultra-flat metals were observed to form defect-free (pinhole defects) molecular over layers. This is a crucial step as it restricts electrical shorts between the top and bottom electrode through the molecular layers when integrated in metal-molecule-metal type architectures.
(8) In addition to mapping the morphology of single molecules we established protocols to study the electronic properties of novel molecular scale systems such as fullerene dumbbells. These molecules (shown in schematic) were obtained through a collaborator work with the chemical synthesis groups at the University of Madrid. We have mapped the morphology and electronic signature in liquid medium for these molecules and were able to resolve the molecular resonance peaks.
(9) We have developed a novel methodology to study the intrinsic molecular properties by partially decoupling the molecules from the metal platforms. In order to decouple the molecules from their respective platforms we synthesised insulating organic molecules (n - tetradecane) using a room-temperature based spray deposition technique.
(10) This handheld spray deposition technique is currently installed at the chemical laboratories at IBM Research - Zurich where nanoscale materials (nanowires, graphene, MoS2) dispersed in a liquid phase can be homogeneously deposited and studied using AFM, scanning electron microscopy (SEM) techniques. A fine spray nozzle (0.1 mm diameter) allows precise deposition of nanowires at desired locations. This was achieved by successfully calibrating the rate of material flow and spraying angle. This technique has been applicable to forming top-electrode in molecule tunnel junction devices. The spray deposition technique permits controlled fabrication of the insulating molecular layers with nanometre-level thickness control.
(11) Through the presence of the insulating organic layers with extremely low dielectric constant fullerene dumbbells were decoupled both structurally and electronically from the underlying ultra-flat metal (Au and Pd) platforms. This decoupling effect was substantiated by the evolution of sharp molecular states with well-defined conductance gaps (shown in STS curve, inset shows STS spectra on fullerene dumbbells directly in contact with the metal surface). This technique has also been extended to vapour phase deposited molecular systems such as pentacene molecular layers which has been demonstrated to be decoupled from the metal substrate. This methodology has important implications extending from basic molecular electronics research to fabricating organic gates in thin film transistors.
(12) We have initiated the conductance imaging AFM (C-AFM) technique here at IBM Research - Zurich and have applied this technique to study the electronic landscape of metal thin and to probe the charge transport through molecules trapped in lithographically designed nanopores.
(13) The open architecture of the nanopore devices have been examined using the C-AFM technique and the on-chip characterisation of the molecules integrated in the devices are being evaluated for their molecular functionality.
(14) The molecular adsorption on the ultra-flat metal platforms is not affected after the fabrication and selective etching of the oxide layer to trap the molecules. Such platforms can now be readily made in which a wide range of molecules can be analysed. Currently experiments are underway to complete a manuscript of the experiments conducted on this on-chip characterisation of molecular systems.