The combination of different properties of materials, especially when it is done on the nanometer scale, is the key to advanced functional materials. This, however, requires processes that enable one to modify surfaces and to deposit materials in a controlled manner. A new chemical approach to atomic layer deposition and its potential for the fabrication of multifunctional nanomaterials will be briefly outlined for the case of coated carbon nanotubes.

Introduction

The controlled deposition and growth of high purity metal oxide thin films on various supports has tremendous implications not only in fundamental fields, such as surface science, but also in applied fields. In microelectronics, for example, new processes and approaches towards the production of ultra-thin metal oxides are sought after. Future device scaling and strict leakage current requirements in low-power applications require a reduction of the equivalent gate oxide thickness (EOT) and hence, methods for the deposition of thin films of high dielectric constant materials. The controlled deposition of thin films onto various supports is also of high interest in fields such as catalysis and gas sensing, where the modification of the surface, which acts as an interface between a material and its surrounding, is the key to advanced properties and the design of multi-functional materials.

A noble and economic procedure for the formation of thin films is Atomic Layer Deposition (ALD). ALD is a form of chemical vapor deposition (CVD) in which the reaction between precursor materials is separated into successive surface reactions. In this manner, the precursor materials are kept separate until the adsorbed species react at the surface in a self-limiting process, i.e. without the presence of a gas phase reaction [1]. As a consequence, ALD offers excellent surface conformity. Inherent to the process is the possibility to accurately control the thickness of the deposited film at almost atomic level simply by counting the number of deposition cycles. ALD is already the privileged technique used in semiconductor industry for the growth of high-κ dielectric materials (for example in DRAM trenches which are characterized by a high aspect ratio).

Recently, we introduced a new non-aqueous sol-gel approach [2-4] for the ALD of metal oxides[5]. By using carboxylic acids as oxygen source instead of the more traditionally used ones (e.g. water, ozone, oxygen, etc.) it was shown that vanadium, titanium and hafnium oxide can be grown from the respective metal alkoxide precursors down to temperatures as low as 50ºC. The surface reaction leading to the M-O-M bond formation is self-limited and takes place via an ester elimination condensation step. As such, this approach allows the deposition of metal oxide thin films that contain no impurities such as halides and only a very low amount of residual carbon [6]. Hence, no post treatment is required.

ALD on CNTs

Carbon nanotubes (CNTs) are certainly the most appealing objects in nanotechnology due to their peculiar electrical and mechanical properties and their high surface area and chemical stability. As such they are ideally suited as support for other materials or molecular species for applications ranging from biotechnology to catalysis. For example, the electrical properties of CNTs are very sensitive to the surrounding environment. The presence of some gaseous molecules, either donating or accepting electrons, causes an alteration of their conductivity. This property makes them suitable for integration in nanoscale conductivity-based devices for gas-sensing. The functionalization of CNTs permits to control or enhance the sensitivity and the selectivity towards specific molecules. However, the coating of CNTs with metal oxides of a well defined and controllable thickness is still very challenging. Recently, we demonstrated that our ALD approach permits to homogeneously coat CNTs on the outer and inner surfaces with a nanometric thick film of metal oxide at a so far unprecedented quality [7, 8].

The morphology of the obtained hybrid materials is nicely revealed by scanning electron microscopy (SEM) images recorded using the secondary (SE) or back scattering electron (BSE) detector, respectively (fig. 1). Whilst the secondary electron image recorded at low primary electron acceleration voltage (2 keV) reveals structural details of the coating, the backscattering images show the location and distribution of the high Z material (i.e. V, Ti, Hf) on the tubes. A brighter contour in the inside and outside of the tube thus proves that the carbon nanotubes are indeed coated with a material of larger electronic density. In order to highlight the deposited film, SEM images were also recorded from regions that show defects in the coating (fig. 1c). Observation of a large number of tubes leads to the conclusion that mechanical deformation of the tubes during SEM sample preparation, or mechanical contact between tubes during the deposition are mainly responsible for such defects. In the case of the TEM images (fig. 2), the contrast rich, darker regions on the outer and inner walls of the CNTs correspond to the metal oxide layers deposited by the ALD process. The coating, only a few nanometers thick, is uniform along the whole surface of the tubes and presents approximately the same thickness at the inner and outer surface. High resolution TEM and electron diffraction experiments reveal that the as-deposited oxide films are amorphous (fig. 2f). In order to analyze the elemental composition and to prove the quality of the as deposited films, electron energy loss spectrometric techniques (EELS and EELS mapping) were performed in the TEM. Bright field images and the corresponding EELS elemental maps, using the L_II and L_III ionization edges of vanadium (fig. 2 a,b,d,e) have been recorded for different tubes. The elemental maps clearly prove that the tubes are coated not only on the outer surface but also on the inside. Only the inner cavities in bamboo-like tubes remain inaccessible for the precursor vapors and hence, uncoated (this is clearly seen in fig. 2c). A more detailed chemical analysis is provided by EELS spectra recorded from the titania and vanadia coated tubes. It could be shown that the as deposited vanadium oxide film corresponds, in terms of oxidation state and elemental ratio, to V₂O₄. In the case of the titania, the shape of the oxygen K edge resembles the one of rutile TiO₂. The high quality of the coating was further outlined by the low carbon content in the film [8].