Suspended graphene and carbon nanotube device arrays by bottom-up assembly

Low-dimensional allotropes of carbon, namely the 1D carbon nanotubes (CNTs)and 2D graphene, posses a fantastic combination of superlative electronic, mechanical and optical properties. Despite the fact that high-performance nano-carbon devices have been demonstrated, they have yet to make a mark for themselves in real-world applications. The primary barrier to commercialization lies in the limited reproducibility and scalability of conventional (top-down) technologies when extended to nano-scale objects, to fabricate electronic devices, sensors, actuators, and other such architectures. While these conventional routes have yielded testable proof-of-concept devices, we would need a new, unconventional approach to fabricate these devices on a large scale at high integration densities. Simultaneously, such a method must be compatible with existing CMOS technology despite their unconventional nature, in order to achieve commercial viability and technological co-existence.

In the case of CNTs, the most promising approach is the bottom-up integration of active CNT elements into pre-defined locations using alternating-current (A/C) dielectrophoresis (DEP). DEP can be used in combination with CNT sorting methods such as density-gradient ultracentrifugation (DGU) to produce high-density arrays of only-semiconducting or even single-chirality CNT devices, making it the only commercially viable technique that can overcome the polydispersity problem in CNTs for device applications. Due to strong counteracting surface-tension forces during drop drying, DEP-based device fabrication has so far been limited to substrate-supported devices, i.e., where the active nano-carbon element has to lie on a substrate upon deposition and not freely suspended between the two electrodes.

In almost every case of CNT or graphene electronics-based device that has been studied to date, suspended devices have significantly outperformed substrate-supported devices. Devices such as resonators can only function in suspended configuration. Since every atom in a CNT or graphene is a surface atom, the properties and performance of nano-carbon devices are severely perturbed by interactions with substrates. Typical effects include heavy doping, lower mobility due to enhanced scattering, higher 1/f noise and consequently lower signal to noise ratios, and lower sensitivity in sensor applications since some surface area is obscured by the substrate.

We will carry out first attempt at large-scale bottom-up assembly and integration of nano-carbon spin-valves, comprised of ferromagnetic electrodes and graphene/CNT spin-channel, which will demonstrate a viable route for future nano-electronic circuits based on quantum-computing. A spin-valve is formed by tailoring the source and drain electrodes to switch at different fields. The substrate-gate will be used to modulate the spin current. The suspended configuration is expected to eliminate substrate-scattering and improve the spin-coherence length.

NEMS devices, such as resonators for mass-sensors, are only possible with suspended CNT/Graphene devices, which hold great promise owing to the excellent combination of mechanical and electronic properties of CNT/graphene; however, an array of such resonators is required to have high quality factor, and this project will demonstrate a scalable route to fabricating such resonator arrays.

The devices proposed in this project, particularly nano-carbon based sensors, are critical components in future energy and environment based industries. A number of leading UK and global companies are currently seeking next-generation sensors for such applications, for example, hydrogen sensors, sensors for high-radiation environments like nuclear reactors or sensors for detection of trace quantities of toxic or environmentally hazardous gasses in emissions.

A number of next-generation devices, such as single-electron transistors, spin-transistors, non-volatile memories and sensors with single-atom sensitivity, are waiting in the wings for the development of the fabrication and assembly technologies which will enable them to interface and integrate with existing micro-electronics and attain commercial viability. The current, so called 'top-down' fabrication methodology is not optimal and may not be suitable for the assembly and integration of nano-electronic devices. The approach that will be adopted and developed in this project represents the new 'bottom-up' paradigm which is critical to bridge this gap between promising performance of individual next-generation devices and their successful commercialization into practical, everyday applications.

The University of Manchester provides a unique opportunity to deliver maximum impact in the field of Graphene and CNT research. The recent EPSRC Science and Innovation Award (2009) for graphene research has allowed for a critical mass of scientists, including new academics such as myself, to come together and create a vibrant and collaborative research community. The profile of UoM has been further enhanced by the recent awarding of the 2010 Nobel Prize in Physics to Profs. A. Geim and K. Novoselov. They will play a major role in increasing the visibility and impact of the results of this project in the scientific and industrial community, in addition to contributing their scientific expertise and facilities to this project.

Technological impact: A number of leading UK and global companies are currently seeking next-generation sensors for applications such as hydrogen sensors, sensors for high-radiation environments like nuclear reactors or sensors for detection of trace quantities of toxic or environmentally hazardous gasses in emissions. These sensors can either operate on the principle of electronic sensing, or electro-mechanical sensing, both of which will be explored during the course of this project. While this has been demonstrated at an individual device level, the 'bottom-up' technology that will be developed in this project defines the way forward to scale-up the fabrication and integration of these devices into commercially viable sensors that can be deployed in the field.

Economic impact: There is significant potential for UK-based nanotechnology industry to benefit from the success of this project. I have already initiated discussions with UMIP, The University of Manchester's Intellectual Property Commercialisation Company, towards exploring various opportunities to commercially exploit the end products of this research programme. This includes filing European and international patents, as well as collaborations and licensing agreements with UK industrial partners. This project is expect to yield proof of concept devices such as sensors and resonators, whose tremendous general and niche applications offer the possibility of spin-out companies and to attract R&D investment in the UK from global industries and venture capitalists.

Societal impact: The people involved directly or indirectly with this project will be exposed to state-of-the art technologies and techniques, in particular in nanofabrication and advanced characterization techniques. This expertise and experience will be invaluable training for the next generation of UK engineers, scientists and academics, especially considering the tremendous potential for nanotechnology based industry in the UK. One of the primary motivations for the development of nanotechnology in sensor and NEMS applications is that such devices are expected to operate with significantly lower power consumption. This carries with it a tremendous impact on society, which relies increasingly every day on current power-hungry electronic devices for computing and entertainment needs.