Hybrid nanostructures are an emerging class of systems that aim to exploit the interactions between different components at the nanoscale. Their raison d’etre is to address problems by combining the best characteristics of their individual parts: such combinations greatly expand the range of functionalities and applications available. Ultimately, we can imagine how one can push the miniaturization level to the nanoscale by creating “all in one” devices out of single such hybrid nanostructures.
Carbon Nanotube-based hybrids
This class of nanomaterials serves as good examples of the principles outlined above.
Single-walled carbon nanotubes (SWCNTs) have well-researched, extraordinary physical and electronic properties. By integrating them as one part of a more complex, hybrid system one can take them further, into domains for which they were not previously thought to be suitable. See below how this can be achieved.
New functionalities for molecular electronics and spintronics
SWCNTs can serve as templates for the encapsulation inside their hollow core of inorganic nanowires or molecular systems. Because of the extremely narrowness of the resulting structures, of the order 1 nm, these are ultimate-scale nanomaterials, with properties that can reflect this ultimate size of matter. Extreme quantum behaviour and confinement effects of the encapsulated component can result.
The interaction between the nanotube and non-nanotube components of the hybrid system can further be used as an appealing route to modifying or producing specific nanotube properties in a controlled way.
As nanoprobes for nanomedicine and medical diagnostics
Using various derivatisation techniques, one can make nanotubes biocompatible, and complete them with functionalities required for their detection in cells, tissues and ultimately, in living bodies.
This research topic involves low-dimensional nanomaterials such as Carbon Nanotube-based Hybrids (as above), and makes use of novel Nanofabrication and Scanning probe microscopy (SPM) techniques. As explained below, Nanostenciling is the nanofabrication technique of choice for integrating such fragile nanostructures into devices.
Fig. 1 Device on perforated membrane incorporating an individual nanotube hybrid. The configuration allows HRTEM, SPM and electronic transport measurements.
Correlated electronic transport, SPM and structural investigations
These measurements are performed on the same individual carbon nanotube-based hybrid, formed by filling a SWCNT with either inorganic compounds or molecular systems. To measure different properties of the same nanostructure using different techniques is very challenging at the nanoscale.
We developed a powerful novel strategy that allows us to correlate the specific atomic structure of these hybrids (as seen in High Resolution Transmission Electron Microsocy, i.e. HRTEM) to their physical properties, and further, to their behaviour in devices. This is based on unconven-tional nano-fabrication techniques, such as Focussed Ion Beam (FIB) and nanostenciling (see below). The resulting devices (Figure 1) can be subsequently used in a number of configurations, for example for optical or magnetic investigations. A broad range of encapsulated materials are being investigated in Dr. Ilie's group. A variety of quantum phenomena occur as a result of nanocrystal or molecule encapsulation. Figures 1-3 show the example of a hybrid carbon nanotube system showing negative differential resistance.
Fig. 3 (above) Potential modulation at the nanotube walls. This effect highlights a more general way to locally confine electrons.
Fig. 2 Permanent dipoles that can form in an ionic encapsulate induce sizeable electrostatic potential modulation on the nanotube, producing a potential well with energy barriers. This results in negative differential resistance in the device.
Further experimental and theoretical aspects of recent work involve elements of spin transport and magnetism in low dimensional systems. Movies showing spin rotation effects are here and here. You may need to download the WindowsMedia Video 9 VCM in order to be able to play these movies.
To shed light on the phenomena related to these composite nano-materials, optical spectroscopic techniques as well as theoretical simulations are being employed.
"Effects of KI encapsulation in single-walled carbon nanotubes by Raman and optical absorption spectroscopy", Journal of Physical Chemistry B 110 (28), 13848 (2006)
"Thermal stability and reactivity of metal halide filled single-walled carbon nanotubes", Journal of Physical Chemistry B 110 (13), 6569 (2006)
A variety of scanning probe microscopy techniques based on both scanning tunnelling microscopy (STM/STS) and atomic force microscopy (AFM), as well as Dual AFM/STM are being used to access the local properties of these novel nanomaterials. At the same time Dr. Ilie is interested in developing and applying new nanofabrication techniques based on scanning probes, and specifically nanostenciling, a technique with unique capabilities.
Scanning Tunneling Microscopy and Spectroscopy (Low- and Room Temperature) (To be updated)
Modifications in the density of states due to chemical derivatization, as well as other quantum effects are being investigated for a variety of Single-Walled Carbon Nanotube-based Hybrids, and other low-dimensional systems. This uses Low-Temperature STM to achieve atomic resolution and local, atomic site-dependent spectroscopy of the density of states (STS).
Fig. 4 TEM and Low-Temperature Scanning Tunnelling Microscopy on inorganic core-SWCNT hybrids. (To be updated)
UHV Dynamic non-contact Atomic Force Microscopy
nc-AFM spectroscopy (force and energy dissipation) can be used to reveal material contrast. Forces due to nano-scale van der Waals interactions can be made to decrease by combining a Ag core and a carbon nanotube shell in the Ag@SWNT system. This specific behaviour was attributed to a significantly different effective dielectric function compared to the individual constituents, which was evaluated using a core-shell optical model (Figure 3(b)). Energy dissipation measurements showed that by filling the nanotube the dissipation increased, therefore filled and unfilled nanotubes can be discriminated based on force and dissipation measurements. Figure 3(c) shows spectroscopic maps (created from spectroscopic curves taken every 1nm) of a bundle from the Ag filled material. These were obtained for different positions of the tip relative to the set-point at which the bundle was imaged. Region (f), with the lowest force and highest dissipation, is attributed to the Ag filled part of a nanotube.
A. Ilie et al., Physical Review B 74 (23), 235418 (2006) (cond-mat/0505010).
Fig 5(a) HRTEM of silver nanowires encapsulated in SWCNTs (Ag@SWCNT). (Image courtesy of Dr. J. Sloan).
Fig 5(b) Top panel: complex dielectric function for Ag, and corresponding to the
transverse direction for a (23,0) SWNT. Bottom panel: effective dieletric
function for the Ag core/SWNT core-shell system.
Fig 5(c) Spectroscopic maps (1x1nm2resolution). The two top images are frequency maps, while the bottom images are dissipation maps.
Nanostenciling (Dynamic Shadow-Mask) Technique
Dynamic shadow-mask is an unconventional nano-fabrication technique based on the principles of Atomic Force Microscopy, and which allows the transfer of materials (e.g. by evaporation) through a stencil-cantilever or stencil-mask. The stencil moves over a surface and "draws" patterns. Coupled with UHV characterisation microscopies such a facility has the potential to become a universal tool for nanoscience, and a variety of its applications are shown in the panel below.
"Dynamic shadow mask technique: a universal tool for nanoscience", S. Egger, A. Ilie, Y. Fu, J. Chongsatien, D-J. Kang, M. E. Welland, Nano Letters 5, 15 (2005)
We recently succeeded in using the technique for the first time to connect pre-existing nanostructures. Nanostenciling has unprecendented applicability for thin/narrow, and fragile (soft matter and biological) systems, for which no other nanofabrication technique was found entirely satisfactory.
"Integration of individual nanoscale structures into devices using dynamic nanostenciling", S. Egger, A. Ilie, S. Machida and T. Nakayama, Nano Letters 7, 3399 (2007)
Fig. 6 Principle of dynamic nanostenciling (top row, first panel) and examples of nanostructures that can be integrated in devices using this technique: nanotubes and soft, supramolecular organic assemblies (such as J-aggregates of porphyrins). Bottom row, first two panels show various nano-structures that can be uniquely produced by moving the stencil-cantilever according to complex motion or with variable speed.
Dr. Ilie's current collaborators involve an expanding number of groups, with leading research institutions from the UK , Europe and Japan.
Updated: Sunday 20 April 2008
A. Ilie - firstname.lastname@example.org