© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1 www.advmat.de www.MaterialsViews.com wileyonlinelibrary.com COMMUNICATION Francesco Rossella,* Caterina Soldano, Vittorio Bellani, and Matteo Tommasini Metal-Filled Carbon Nanotubes as a Novel Class of Photothermal Nanomaterials Dr. F. Rossella, Prof. V. Bellani Dipartimento di Fisica “A. Volta” and CNISM Università degli Studi di Pavia Via Bassi 6, 27100 Pavia, Italy E-mail: francesco.rossella@unipv.it Dr. C. Soldano Dipartimento di Chimica e Fisica Università degli Studi di Brescia Via Valotti 9, 25121 Brescia, Italy Prof. M. Tommasini Dipartimento di Chimica Materiali e Ingegneria Chimica “G. Natta” Politecnico di Milano Piazza Leonardo da Vinci 32, 20133 Milano, Italy DOI: 10.1002/adma.201104393 Metal-filled and decorated carbon nanotubes (CNTs) represent a class of hybrid carbon-based nanostructures systems with enor- mous interest for applications in several fields, ranging from nanoelectronics and spintronics to nanomedicine and magnetic storage. [1–5] First, CNTs are ideal supporting materials for metal nano- particle catalysts in electrochemical and fuel cells, [6] and repre- sent also a template for the attachment of metal nanoparticles or nanowires for hydrogen (and for bio- and chemical) sensing applications. [7] On the one hand, metal nanoparticles have been widely used as contrast agents for simultaneous cell imaging and photothermal cancer therapy. [8] On the other hand, it has been recently reported on the fabrication of metal nanoparticle/ CNT hybrids with unique optical properties as well as biocom- patibility with application as efficient dark field light scattering agents for cancer cell imaging. [9] Secondly, nanostructured metal-carbon systems are extremely appealing for optical applications since metal-dielectric inter- faces of arbitrary geometries can support charge density oscil- lations similar to the surface plasmons typical of planar inter- faces (i.e., localized Mie plasmons). [10,11] Hence, the coupling between the plasmonic modes of metal nanoparticles and the tube is of fundamental interest and may be efficiently used for light harvesting applications. In fact, arrays of hybrid metal- carbon nanostructures can enhance the plasmonic coupling between the metal nano-objects, and represent excellent exam- ples of engineered plasmonic devices. Very recently, hybrid cobalt cluster-filled anodized alumina template multi-walled carbon nanotubes arrays have been proposed as platforms for the development of photonic band gap materials due to their enhanced optical response. [12] Third, the thermal properties of CNTs have been recently used for unique and special applications, mostly exploiting the thermal gradients established within the tube itself. For example, the direct use of thermal gradients to induce mass transport ( thermophoresis) allows microscale manipulation and control of flow in nanofluidic devices, driving liquids and solids in nanochannels, such as water nanodroplets confined inside a single- and double-walled nanotubes. [13] Moreover, thermal gradient-driven nanoelectromechanical systems (NEMS) rep- resent a recent example of cutting-edge applications of CNTs. In fact, Barreiro et al. [14] have fabricated an artificial nanomotor applying 0.1 mA to a multi-walled CNT with a cargo attached to an ablated outer wall, demonstrating that the thermal gradient electrically established along the nanotube was the driving force of the cargo motion. Here, using a contactless method based on the local laser excitation combined with micro-Raman scattering spectros- copy, [15,16] we show that individual cobalt cluster-filled multi- walled nanotubes (Co-MWNTs) present a strong increase of the temperature at the cobalt cluster sites, when illuminated by visible light, as a result of the enhanced plasmonic light absorp- tion at the cobalt cluster surface, which behaves as a radiator for the CNT. Around this location, a temperature gradient ∇ T is generated along the tube, with a measured lower limit of the order of 100 K/ μm. Our work provides the experimental proof- of-principle that metal-filled carbon nanotubes can represent a novel class of photothermal nanomaterials, with potential appli- cations in nanomedicine for cell imaging and thermal therapy, as well as platforms for light-activated, thermal gradient-driven devices or actuators. Moreover, applications as recording media for heat-assisted magnetic recording can be envisioned. Visible laser light ( λ = 632 nm) is used to locally generate heat and simultaneously measure the Stokes and anti-Stokes Raman scattering spectra in the wavenumber range from –1800 cm -1 to +1800 cm -1 . This allows estimating the local temperature of individual tubes within the laser spot region from the ratio of the intensities of the Stokes and anti-Stokes Raman signals. In fact, this ratio depends on the population of the vibrational levels involved in the scattering processes, which is a function of the local temperature of the sample according to Boltzmann statistics (the lower the Stokes/anti-Stokes ratio, the higher the temperature). In all our samples, we observed a large decrease of the Stokes/anti-Stokes ratio in correspondence of the cluster site, for which we estimate a temperature increase with respect to the unfilled areas of approximately 15%. Figure 1 (inset) shows the scanning electron microscopy (SEM) image of one of the Co-MWNT investigated samples dis- persed on a SiO 2 /Si substrate, with a length of 10 μm, a dia- meter of ∼370 nm and a wall thickness d ≅ 10 nm. This structure is due to the CVD growth process in alumina template, where the entire inner surface of the template pores acts as catalyst for the growth of small carbon islands, thus resulting in multiple and simultaneous growth on the whole available surface. The Adv. Mater. 2012, DOI: 10.1002/adma.201104393