J.V. Anguita

Deposition of polycrystalline thin films with controlled grain size

Difficulties in controlling the grain size and size distribution in polycrystalline thin films are a major obstacle in achieving efficient performance of thin film devices. In this paper we describe a sputtering technology that allows the control of the grain size and size distribution in sputtered films without the use of seed layers, substrate heating or additives. This is demonstrated for three different materials (Cr, NiFe and FeMn) via transmission electron microscopy imaging and grain size analysis performed using the cumulative percentage method. The mean grain size was controlled only via the sputtering rate. We show that higher sputtering rates promote the growth of larger grains. Similar trends were obtained in the standard deviation, which showed a clear reduction with the sputtering rate.

Ultra-broadband light trapping using nanotextured decoupled graphene multilayers

A study of graphene on metal nanotextured surfaces for ultra-broadband light trapping, blackbody absorbers, and thermal imaging. The ability to engineer a thin two-dimensional surface for light trapping across an ultra-broad spectral range is central for an increasing number of applications including energy, optoelectronics, and spectroscopy. Although broadband light trapping has been obtained in tall structures of carbon nanotubes with millimeter-tall dimensions, obtaining such broadband light–trapping behavior from nanometer-scale absorbers remains elusive. We report a method for trapping the optical field coincident with few-layer decoupled graphene using field localization within a disordered distribution of subwavelength-sized nanotexturing metal particles. We show that the combination of the broadband light–coupling effect from the disordered nanotexture combined with the natural thinness and remarkably high and wavelength-independent absorption of graphene results in an ultrathin (15 nm thin) yet ultra-broadband blackbody absorber, featuring 99% absorption spanning from the mid-infrared to the ultraviolet. We demonstrate the utility of our approach to produce the blackbody absorber on delicate opto-microelectromechanical infrared emitters, using a low-temperature, noncontact fabrication method, which is also large-area compatible. This development may pave a way to new fabrication methodologies for optical devices requiring light management at the nanoscale.

Thermal stability of plasma deposited thin films of hydrogenated carbon–nitrogen alloys

The need to grow high quality semiconducting hydrogenated amorphous carbon (a-C:H) thin films to allow n-type electronic doping by nitrogenation has lead us to deposit films with low paramagnetic defect density (1017 cm−3). The films were grown on the earthed electrode of a radio frequency driven plasma enhanced chemical vapor deposition system using methane, helium and a range of nitrogen concentrations as the precursor gases. The deposited films are shown to be polymer like. Changes in the chemical structure and relative bond fractions as a function of the nitrogen flow rate into the plasma chamber and ex situ annealing are reported. Particular attention is paid to changes in the film structure after annealing at 100 °C, since an increase in the E04 optical band gap is observed as a function of nitrogen flow after the anneal. This suggests a decrease in the defect density of the film.The need to grow high quality semiconducting hydrogenated amorphous carbon (a-C:H) thin films to allow n-type electronic doping by nitrogenation has lead us to deposit films with low paramagnetic defect density (1017 cm−3). The films were grown on the earthed electrode of a radio frequency driven plasma enhanced chemical vapor deposition system using methane, helium and a range of nitrogen concentrations as the precursor gases. The deposited films are shown to be polymer like. Changes in the chemical structure and relative bond fractions as a function of the nitrogen flow rate into the plasma chamber and ex situ annealing are reported. Particular attention is paid to changes in the film structure after annealing at 100 °C, since an increase in the E04 optical band gap is observed as a function of nitrogen flow after the anneal. This suggests a decrease in the defect density of the film.

The microstructural dependence of the opto-electronic properties of nitrogenated hydrogenated amorphous carbon thin films

Abstract The microstructural properties of nitrogenated hydrogenated amorphous carbon (a-C:H:N) thin films deposited using a chemical vapour deposition system are analysed in order to evaluate their impact on the opto-electronic properties. Electron energy loss spectroscopy is used to reconstruct a joint density of states (JDOS) for the a:C:H:N films and thus used to examine the microstructure. Information obtained from optical absorption is then used to confirm the JDOS. Using this JDOS we attempt to predict the variation expected in the electronic conduction as a function of nitrogen content. The electrical data obtained for the a:C:H:N thin films appear to be bulk controlled as opposed to metal contact dominated for films that are deposited on the driven electrode and are more diamond-like in character. The bulk electronic properties at high fields are fitted to different types of conduction behaviour in order to obtain the best fits to the data. From this study it is observed that a Poole–Frenkel type fit is best for films that have a diamond-like structure. For the films that have a polymeric structure which are deposited on the earthed electrode the conductivities are very much lower, and consistent with the lower defect densities observed in the microstructural study. It is possible that the conduction in these films are Schottky barrier controlled.

Triboelectric nanogenerators: providing a fundamental framework

A new model which comprehensively explains the working principles of contact-mode triboelectric nanogenerators (TENGs) based on Maxwells equations is presented. Unlike previous models which are restricted to known simple geometries and derived using the parallel plate capacitor model, this model is generic and can be modified to a wide range of geometries and surface topographies. We introduce the concept of a distance-dependent electric field, a factor not taken into account in previous models, to calculate the current, voltage, charge, and power output under different experimental conditions. The versatility of the model is demonstrated for non-planar geometry consisting of a convex–concave surface. The theoretical results show excellent agreement with experimental TENGs. Our model provides a complete understanding of the working principles of TENGs, and accurately predicts the output trends, which enables the design of more efficient TENG structures.

Multi-Functional Carbon Fibre Composites using Carbon Nanotubes as an Alternative to Polymer Sizing

Carbon fibre reinforced polymers (CFRP) were introduced to the aerospace, automobile and civil engineering industries for their high strength and low weight. A key feature of CFRP is the polymer sizing - a coating applied to the surface of the carbon fibres to assist handling, improve the interfacial adhesion between fibre and polymer matrix and allow this matrix to wet-out the carbon fibres. In this paper, we introduce an alternative material to the polymer sizing, namely carbon nanotubes (CNTs) on the carbon fibres, which in addition imparts electrical and thermal functionality. High quality CNTs are grown at a high density as a result of a 35 nm aluminium interlayer which has previously been shown to minimise diffusion of the catalyst in the carbon fibre substrate. A CNT modified-CFRP show 300%, 450% and 230% improvements in the electrical conductivity on the ‘surface’, ‘through-thickness’ and ‘volume’ directions, respectively. Furthermore, through-thickness thermal conductivity calculations reveal a 107% increase. These improvements suggest the potential of a direct replacement for lightning strike solutions and to enhance the efficiency of current de-icing solutions employed in the aerospace industry.

Amorphous carbon thin films

Publisher Summary This chapter focuses on the evolution of a carbon (C) thin films and maps out the significant contributions to the field by numerous research laboratories. The impact of the microstructure and growth on the optical and electrical properties is also examined in the chapter. New results show how ion implantation allows a methodology to delocalize gap states within these films. Carbon is unique in its structure by being able to form one of the strongest materials known to man—diamond—or one that is soft—graphite—by virtue of the way in which each atom bonds to another. All of these variations are made possible by the three different bond hybridizations that are available to carbon. Diamond-like carbon (DLC) should generally be reserved for polycrystalline or nanocrystalline carbon films, whereas amorphous carbon films should generally fall into the categories of polymer such as amorphous carbon (PAC), graphite-like amorphous carbon (GAC), diamond-like amorphous carbon (DAC), tetrahedral amorphous carbon (TAC), and nano-composite amorphous carbon (NAC).

Photoluminescence from polymer-like hydrogenated and nitrogenated amorphous carbon films

The mechanism of photoluminescence (PL) in hydrogenated amorphous carbon (a-C:H), and nitrogenated and hydrogenated amorphous carbon (a-C:H:N) thin films grown by radio frequency driven plasma enhanced chemical vapor deposition is still a subject of much debate. In this work, we investigate the PL signal obtained from a-C:H and a-C:H:N films, paying particular attention to the effect of nitrogen flow rate during growth, and postgrowth, ex situ annealing on the PL properties of the films. We also correlate the PL spectra to the electronic structure of the films. The films had a low paramagnetic defect density (1017 cm−3). The PL spectra were obtained using the 488 nm (2.54 eV) line of an argon ion laser, as the excitation source. It was observed that the nitrogenation of the films leads to the creation of new bands in the PL signal, which were correlated to the bond fraction of CN triple bonds, as measured by infrared spectroscopy.

Low temperature growth of gallium nitride

Abstract Radio frequency reactive sputtering was used to produce gallium nitride films on glass and silicon substrates at close to room temperature. The films were analysed by transmission electron microscopy and Rutherford backscattering spectroscopy. The films were found to consist of nanocrystalline wurtzite GaN with c -axis-oriented columnar grains growing perpendicular to the substrate. Varying the N 2 :Ar sputtering gas ratio was found to have little effect on the grain size. Annealing the films at 400°C was found to increase the E 1 (TO) signal observed by Fourier transform IR spectroscopy and to reduce the porosity of the columnar structure.

Efficient coupling of optical energy for rapid catalyzed nanomaterial growth: high-quality carbon nanotube synthesis at low substrate temperatures.

The synthesis of high-quality nanomaterials depends on the efficiency of the catalyst and the growth temperature. To produce high-quality material, high-growth temperatures (often up to 1000 °C) are regularly required and this can limit possible applications, especially where temperature sensitive substrates or tight thermal budgets are present. In this study, we show that high-quality catalyzed nanomaterial growth at low substrate temperatures is possible by efficient coupling of energy directly into the catalyst particles by an optical method. We demonstrate that using this photothermal-based chemical vapor deposition method that rapid growth (under 4 min, which includes catalyst pretreatment time) of high-density carbon nanotubes can be grown at substrate temperatures as low as 415 °C with proper catalyst heat treatment. The growth process results in nanotubes that are high quality, as judged by a range of structural, Raman, and electrical characterization techniques, and are compatible with the requirements for interconnect technology.