Materials Today in collaboration with Thermo Scientific are offering a short 45 minute webinar to introduce how Raman spectroscopy can help explore properties of graphene materials.

The presentation will include an overview of several significant applications of Raman spectroscopy:

• measuring thickness
• monitoring chemical properties
• monitoring physical properties

This webinar will be helpful for anyone who is either just starting working with graphene materials or wanting to learn more about Raman spectroscopy.

When: July 28 2011, 16h00 (BST) / 11h00 (EST)
Register here

(I) Mussels attach to hard surfaces in the marine intertidal zone with the byssus. (II) Byssal threads are extensible fibers with a hard and rough-textured protective cuticle (scanning electron microscopy). The knobby morphology of the cuticle originates from granular inclusions embedded in a continuous matrix. (III) The amount of dopa-iron complexes was found to be much higher in the granules than the matrix, which likely leads to their differences in mechanical performance during stretching. (Image: Matt Harrington, Max Planck Institute of Colloids and Interfaces)

According to their findings, local accumulation of iron-mediated cross-links creates hard knobs within an extensible matrix containing much fewer of these molecular bridges. Such a design could be an interesting concept for developing novel abrasion-resistant, highly extensible coatings. Their finding were published in Science.

Mussels thrive in rocky seashore habitats, in spite of the enormous physical demands present there. This is in no small part due to the evolution of the byssus, which mussels employ to tether themselves to accessible surfaces.

The individual byssal threads that compose the byssus are stiff, but stretchy and are fashioned by the mussel in a process resembling injection molding. Byssal threads are depended upon for dissipating the energy of crashing waves and also for resisting abrasive damage from water-borne debris. To this end, threads are sheathed with a thin and knobby outer cuticle; a biological polymer, which exhibits epoxy-like hardness, while straining up to 100% without cracking.

Matthew Harrington, a researcher who worked on the project and Humboldt fellow at the Max Planck Institute for Colloids and Interfaces explains the motivation for studying the byssus cuticle: “Protective coatings are important for prolonging the lifetime of materials and devices. However, considering that hardness and extensibility are seldom coupled in engineered polymers or composites, understanding how one protects a flexible substrate becomes quite important.” Byssal cuticles have a knobby appearance due to inclusions of submicron-sized granular structures in an apparently continuous matrix. Submicron-sized tears that form in the matrix during stretching of the cuticle are believed to hinder the formation of larger cracks that could lead to material failure.

Central to understanding the peculiar mechanical behaviour of the cuticle are the high concentration of iron ions in the cuticle and the presence of an uncommon modification of the amino acid tyrosine known commonly as dopa. Dopa is found at high concentrations in the main cuticle component, mussel foot protein-1 (mfp-1). Dopa is distinguished from typical amino acids due to its impressive affinity for complexing with transition metal ions, particularly iron. As Admir Masic, a scientist at the Max Planck Institute for Colloids and Interfaces who worked on the project, explains, “when 2-3 dopa residues complex with a single iron ion, they create an incredibly stable complex that can be utilized to cross-link structural proteins.” These metal-protein complexes have a high breaking force (nearly half that of covalent bonds), but unlike covalent bonds they are reversibly breakable, making them ideal for creating sacrificial cross-links.

Using a technique known as in situ Raman spectroscopy to probe the chemical composition of the cuticle, the researchers provided the first direct evidence that the cuticle is a protein-based polymeric scaffold stabilized by dopa-iron complexes. Moreover, it was discovered that the distribution of dopa-iron complexes is clustered, with areas of high density coinciding with the granular inclusions and low density with the inter-granular matrix. These observations, coupled with previous mechanical observations suggest that the densely cross-linked granules function as hard inclusions and the less cross-linked matrix functions in a sacrificial manner, allowing bonds to break prior to catastrophic failure.

“Nature has evolved an elegant solution to a problem that engineers are still struggling with; namely, how to combine the properties of abrasion resistance and high extensibility in the same material”, says Peter Fratzl, director of the biomaterials department at the Max Planck Institute for Colloids and Interfaces. Apparently, the cuticle achieves this through a careful tailoring of protein-metal chemistry and the submicron organization of cross-link density. “Conceivably, this same strategy could be applied in engineered polymers and composites.”

Source: original article

An educational webinar from Materials Today is coming soon:

• Topic: Introduction to Raman Spectroscopy as a Characterization Tool for Graphene and Carbon Nanostructures
• When: Tuesday March 9th, 2010
• Time: 16h00 GMT, 11h00 EST
• Duration: 1 hour

Registration is free. Simply, follow this link to reserve your place.

Raman spectroscopy is a laser light scattering technique, if you like a form of vibrational spectroscopy that records vibrations of covalent bonds and provides detailed molecular information, ideal in the elucidation of carbon nanomaterials.

There are references to the use of carbon nanomaterials in just about every area of material science today because of the amazing range of properties offered by these materials. Raman spectroscopy has emerged from this early work as one of the key characterization tools for understanding this novel class of material. This webinar will introduce you to what Raman spectroscopy can tell you about carbon nanotubes, graphene, and other carbon nanomaterials.

If you are working with Carbon nanomaterials or looking to begin working with Carbon nanomaterials and you are not familiar with Raman spectroscopy then this webinar would be a great way to learn about what Raman can offer to Carbon nanomaterial characterization.

Nanotechnology and nanoscience have certainly been one of the most popular fields of research in the last decade. Manufacturing processes are now able to carry out the deterministic synthesis of nanostructures with properties radically different from their macroscopic forms, enabling the realization of previously unthinkable devices. Despite these advances, few nanofabrication techniques feature the required characteristics for the commercial manufacturing of these new products in an effective fashion since they are either too slow, or too expensive and complex.

This page is a summary of my doctoral thesis project, accomplished at the Laboratory for multiscale mechanics at the École Polytechnique de Montréal (LM2, mechanical engineering). The main objective of this project was to develop a new manufacturing technique allowing the local synthesis of nanostructures on a surface for their eventual integration into nanodevices. The desired process has to be selective, reproducible, versatile, simple, fast and inexpensive for potential industrial utilization. Moreover, the manufacturing process must have a minimal environmental impact for sustainable development.

To implement the required specifications, a laser process combining the characteristics of laser-induced chemical liquid deposition (LCLD) and of sol-gel synthesis was proposed. The technique is very simple and consists of three steps. A precursor solution is first prepared. Next, a droplet of a controlled volume is transferred on a substrate by means of a micropipette. The droplet is then irradiated using a laser emitting in the infrared to induce the fast synthesis of nanostructures.

ZnO nanostructure synthesis by LCLD

In order to prove the feasibility of the manufacturing concept for the synthesis of nanostructures, the new process was used for the synthesis of zinc oxide (ZnO) nanostructures. ZnO has become one of most studied nanomaterials in the five last years as it presents very interesting properties for optoelectronics and sensing applications, while being synthesizable in a plethora of nanoscale morphologies.

Using laser processing, the deposition of coatings of several square millimetres of various nanostructures (nanorods, nanowires, porous films of nanoparticles) was carried out. In particular, nanorods with an average width of 300 nm and a length of two micrometers with hexagonal cross-sections and almost atomically flat surfaces were synthesized (see figure 1). Nanowires with diameters of approximately 50 nm and lengths exceeding four micrometers were also grown. This constituted an innovation among the laser processing techniques, as only laser-induced chemical vapour deposition (LCVD) and pulsed laser deposition (PLD) had been used to produce ZnO, and just in the form of thin films and nanoparticles.

Figure 1: Example of nanorods grown by LCLD

Caracterization and optimization of the process

One of the secondary objectives of this thesis was to improve the properties of the deposits for one of the target applications of ZnO, photoluminescent devices. For this reason, a parametric study was carried out during which the influence of the laser-related parameters (irradiation time, intensity) and of the solution-related parameters (precursor, additives, concentration) on morphology and crystallinity was studied. The use of scanning electron microscopy (SEM) and X-ray diffraction (XRD) showed that an increase in laser intensity and irradiation time increased nanostructure length and crystallite size (see figure 2A). Transmission electron microscopy (TEM) demonstrated that the nanorods grew along the c axis of the crystal lattice, at the apex of randomly oriented ZnO crystals forming a seed layer on the substrate. Additionally, an increase in precursor concentration was found to increase the thickness of this seed layer and the introduction of additives in the solution had the effect of promoting the vertically aligned growth of nanorods. Raman and photoluminescence (PL) spectroscopy also showed that the deposits were of high quality with few crystalline defects. In particular, the PL spectroscopy results gave evidence that the ZnO nanostructured deposits produced by the laser process were good for ultraviolet emission applications with the presence of an intense peak at 390 nm (see figure 2B). The extensive characterization of the samples also allowed the development of a qualitative growth model for the laser-grown ZnO nanostructures inspired by the growth and nucleation models used for conventional chemical synthesis in a liquid medium.

Figure 2: Microstructure of samples A)XRD, B)PL

This project led to the development of a new technique allowing the synthesis of high quality ZnO nanostructures in a few seconds whereas the traditional techniques of chemical synthesis need several hours. Once the reproducibility and selectivity problems are resolved, this very promising technique could easily be upgraded for the production of nanodevices such as UV nanolasers or light emitting diodes. Three scientific articles were published during this project:

[1] Fauteux, C., Smirani, R., Pegna, J., El Khakani, M. A., Therriault, D., Fast synthesis of ZnO nanostructures by laser-induced chemical liquid deposition, Applied Surface Science, vol. 225, 2009, pp 5359-5362;

[2] Fauteux, C., El Khakani, M. A. , Pegna, J., Therriault, D., Influence of solution parameters for the fast growth of ZnO nanostructures by laser-induced chemical liquid deposition, Applied physics A: Materials Science and Processing, vol.94, 2009, pp. 819-829;

[3] Fauteux, C., Longtin, R, Pegna, J., Therriault, D., Fast Synthesis of ZnO Nanostructures by Laser-Induced Decomposition of Zinc Acetylacetonate, Inorganic Chemistry, v. 46, 2007, pp. 11036-11047;

Author: Christian Fauteux

Freeform fabrication is a type of manufacturing process that allows the fabrication of 3D objects with a large variety of shapes, structure and composition. Laser-assisted chemical vapor deposition (LCVD) is a freeform fabrication technique that allows the deposition of micro-scale structures at a high growth rate with the flexibility of conventional CVD.

The LCVD process

During chemical vapour deposition (CVD), energy is given to a gas (the precursor gas) which contains the atom that we want to deposit. This energy dissociated the gaseous molecules by a series of chemical reactions in order to obtain a solid product (the deposit) on a surface where we want the deposition to occur (the substrate). This can be conceptualized by the following equation which describes the deposition of the gaseous species AB_u :
AB_u →  A(↓) + u B(↑)

The energy can be given to the gas by many different means: heating of the substrate, plasma, lamp, laser, etc. Chemical reactions can occur in the gas itself (homogeneous reactions) and at the surface of the substrate (heterogeneous reactions). The two most important parameters in a CVD process are temperature and pressure since they strongly influence the growth rate. Two possible cases can occur: the growth can be controlled by surface reactions (kinetically controlled regime, the growth rate increases with temperature) or by diffusion (mass-transport controlled regime, the growth rate increases with pressure).

LCVD has been used to produce a large variety of products (fibers, thin films, microcoils, band-gap structures, etc.) made also from a large variety of materials (C, B, BN, W, WC, Si, SiO₂, Ti, TiN, Mo, etc.).^[1,2,3,4] As for conventional CVD, the advantage of this method over the other microfabrication processes is its ability to use many precursor gases without the need to modify heavily the deposition system. It is also possible to obtain very high growth rates compared to other processes. Indeed, while the deposition surface is small, linear growth rates up to 0.3 mm/s have been reported for the growth of carbon fibers by LCVD.^[5] LCVD is also a freeform fabrication process which allows the direct generation of 2D or 3D structures without any additional patterning steps. The two most used LCVD techniques are direct writing, where the laser beam is scanned in 2D on the surface of the substrate, and 3D-LCVD, where the laser focus is moved in 3D above the substrate after the start of growth on its surface. It is also possible to focus the laser slightly above the surface of the substrate in order to induce homogeneous deposition in the gas.

Figure 1: illustration of the LCVD process with the laser beam focused on the substrate surface

The use of LCVD to deposit cylindrical carbon rods from methane and ethane was reported in 1972 by Nelson and Richardson.^[6] Leyendecker and Bäuerle then improved the process by using an Ar+ laser and additional gases, such as ethylene and acetylene.^[7] It became clear that the strong temperature gradient induced at the tip of the rod influenced the shape and structure of the deposits. Indeed, the normal growth rate is maximized at the center of the tip and decreases as we go further from this point.^[8] This creates radially different growth conditions. On the basis of observations of the surface of the rods and of polished longitudinal sections with an electronic microscope, it was then suggested that the rods were made of pyrolitic graphite with a columnar layered structure.^[8] Investigation of LCVD-drawn carbon lines by Raman spectroscopy showed that the surface of the carbon lines (covered by nodules from homogeneous nucleation) consisted of glassy carbon and the interior of ordered pyrolitic graphite.^[3]

In the case of LCVD, the laser beam is focused (beam diameter of w) on the substrate and oriented orthogonally to the surface of the substrate. It induces a temperature rise ΔT(r):
ΔT(r) = ΔT₀ exp(r² / w²)

where ΔT₀ is a constant function of the parameters of the laser beam. Since the pressures used for this project where atmospheric or sub-atmospheric, we were theoretically in the kinetic regime and the linear growth rate R can be written like this:
R(r) ~ exp(-E_a / k T(r))

where E_a is the global activation energy of the deposition reaction and k the Boltzmann constant. When the pressure or temperature increases, homogeneous reactions are starting to happen in the gas and this approximation is no longer valid.

According to previously published experimental and theoretical results on LCVD grown carbon fibers,^[2,7,8] the growth rate increases with laser power and pressure while the diameter of the fibers increases with laser power but is independent of pressure. These relations are valid for atmospheric and sub-atmospheric pressures. When the pressure is higher (HP-LCVD), it was reported that the diameter of the fibers decreases with increasing pressure.^[5] This is happening because of the cooling effect of the fast movement of the gas near the fiber. Indeed, since the growth rate is very high at high pressure, a convection phenomenon is going on near the deposition area: the gas must quickly fill the space that has been emptied by the deposition. A good part of the heat produced by the laser is then evacuated in the gas before the completion of the deposition, which reduces the size of the reaction zone.

Few theoretical models for the growth of LCVD fibers are available. Most models are very complex, are limited to specific deposition conditions and don’t allow clear results that resemble the experimental observations.^[2,10] However, from Arnold’s model,^[10] we can gather that the deposition of the fibers is occurring by the accumulation of layers at the tip of the fiber and that once the reaction is started, it no longer depends on the substrate’s nature. Figure 1 illustrates a deposition model for LCVD fibers. The layers have a elliptic or parabolic shape that can be observed with an electronic microscope (SEM) when a fiber is fractured (see Figure 2 and Figure 3).

Figure 2: tip of a LCVD fiber

Characterization of the microstructure of LCVD deposits

Research on LCVD and its possibilities has been pursued for more than 20 years and it had become important to characterize the internal structure of the deposits in order to achieve a better understanding of the process. Investigation of the cross-sectional microstructure of carbon fibers grown by LCVD using Raman spectroscopy has been done. During the first part of the project,^[11] a radial Raman analysis was done on a single fiber. Spectra were taken at regular intervals to cover the whole diameter of a fiber. Key parameters of the Raman spectra were studied (G, D, D’ peaks and D/G intensity ratios) and provided insight on the microstructure of the fibers. Indeed, it was discovered that the microstructure changed radially with a well-graphitized center, a nanocrystalline graphite edge and an amorphous carbon surface.

Figure 3: longitudinal view of a LCVD fiber

The second part of the project,^[12] involved the same kind a radial Raman analysis but several fibers, grown at different laser powers and precursor gas pressures, were studied. This deepened our understanding of the fiber’s microstructure and allowed the discovery of experimental parameters dependencies:

• An increase in precursor gas pressure, increasing the linear growth rate, was accompanied by a decrease in graphitization of the fiber;
• An increase in laser power, increasing the growth temperature and linear growth rate, first induced an increase in graphitization followed by a decrease in graphitization as the growth rate becomes too high.

A turning point in the graphitization level of the carbon fibers was thus observed when the laser power is increased. The knowledge gathered during this research project allows the growth of carbon fibers with desired microstructural properties and further comprehension of the growth process.

Mechanical properties of LCVD deposits

A second project was begun to investigate the local and bulk mechanical properties of LCVD fibers. The goal was to directly relate the mechanical properties to the fibre microstructure.

The investigation of the local mechanical properties was achieved by radial nanoindentation^[13] of the fiber cross-sections. The bulk mechanical properties were determined through three-point microbending^[14] experiments carried out with the use of a micromanipulator. A clear mechanical properties dependence on experimental parameters was found. The mechanical properties investigated (Young’s modulus, the hardness, the tensile and fracture strengths as well as the bending resistance) will help us determine the type and the extent of applications.

Nanoindentation

Nanoindentation is a near surface characterisation technique used mainly to determine the mechanical properties of thin films. A sharp indenting tip moves deeper in a material as the applied force is increases and the apparatus measures the degree of response of the material. Load-displacement curves are used to determine the materials stiffness and hardness.

In the case of LCVD produced fibers, a radial change in mechanical properties was observed. The fibers produced at atmospheric and sub-atmospheric pressures have low hardness and elastic modulus. The elastic modulus decreases, from edge to center, as the microstructure changes radially from nanocrystalline to more graphitic structure. The local mechanical properties are directly related to the microstructure induced by the Gaussian laser intensity distribution. Varying the deposition parameters showed that the local elastic modulus and hardness increases with amorphitization. This is achieved by increasing the precursor pressure and by increasing the laser power above a specific value.

Three-point microbending

Bulk mechanical properties investigation was done by micromechanical testing using a high-precision micromanipulator. During three-point bend testing (Figure 4), the fibers showed an elastic response with no residual strain upon unloading. The low pressure LCVD fibers have poor mechanical properties when compared to other forms of carbon and when compared to HP-LCVD or other commercially produced carbon fibers. It is believed that the fiber’s structural weakness is due to their layered graphitic nature. Again, the mechanical properties improve with amorphization. Internal structural defects due to difficult process control occur at higher pressures and laser powers. Furthermore, a rough surface morphology, due to the presence of surface nodules could lead stress concentrations. This in turn would reduce the overall strength of the fibers.

Figure 4: typical fiber bending during three-point bend testing

Laser assisted surface-bound growth of carbon nanofibers

This poster reviews the recent advances in laser assisted surface-bound growth of carbon nanofibers:
Recent advances in laser assisted surface-bound growth of carbon nanofibers: from vertical nanofibers arrays to thin films and nanocomposite coatings (pdf)

Carbon nanofibers and nanotubes are grown by selectively heating catalytic metal nanoparticles in the presence of a gaseous hydrocarbon precursor with a low power (1-5 W) argon ion laser beam (488 nm). The nanofibers are the result of atomic carbon self-assembly initially into graphene sheets, which are essentially large planar 2D molecules, and subsequently into various arrangements of graphene sheets.

The prime innovation reported herein deals with the improved control over nanofiber characteristics during their synthesis and their assembly into various surface coatings. Assemblies in the form of thin nanofiber films, vertically aligned nanofiber arrays, horizontally aligned nanofiber mats and carbon-carbon nanocomposite coatings were synthesized locally on the surface of a porous alumina substrate using a nickel catalyst. Scanning electron microscopy and Raman spectroscopy were used to characterize the different assemblies whereas individual nanofiber texture and nanotexture were assessed by transmission electron microscopy.

Irradiation time ranges leading to the formation of specific assemblies have been identified and are presented. The different pre- or post-synthesis strategies used to control diameter, location and alignment are described. Also, the growth mode and the nanofiber shaping mechanism are discussed. Recommendations on how to control nanofiber characteristics such as shape and internal structure by choosing the appropriate process parameters are provided.

Literature

[1] F. T. Wallenberger, P. C. Nordine, M. Boman, Inorganic fibers and microstructures directly from the vapor phase, Comp. Sci. Technol. 51 (1994), 193-212

[2] D. Bäuerle, Laser Processing and Chemistry, 2nd ed. Berlin (1996), Springer, 649 pages

[3] H. Westberg, M. Boman, A.-S. Norekrans, J.-O. Carlsson, Carbon growth by thermal laser-assisted chemical vapour deposition, Thin Solid Films 215 (1992), 126-133

[4] M. C. Wanke, O. Lehmann, K. Müller, Q. Wen, M. Stuke, Laser Rapid Prototyping of Photonic Band-Gap Microstructures, Science 275 (1997), 1284-1286

[5] F. T. Wallenberger, P. C. Nordine, Strong, pure and uniform carbon fibers obtained directly from the vapor phase, Science 260 (1993), 66-68

[6] L. S. Nelson, N. L. Richardson, Formation of thin rods of pyrolytic carbon by heating with a focused carbon dioxide laser, Mater. Res. Bull. 7 (1972), 971-976

[7] G. Leyendecker, D. Bäuerle, P. Geittner, H. Lydtin, Laser induced chemical vapor deposition of carbon, Appl. Phys. Lett. 39 : 11 (1981), 921-923

[8] G. Leyendecker, H. Noll, D. Bäuerle, P. Geittner, H. Lydtin, Rapid determination of apparent activation energies in chemical vapor deposition, J. Electrochem. Soc. 130:1 (1983), 157-160

[9] M. Lax, Temperature rise induced by a laser beam, J. Appl. Phys. 48 (1977), 3919-3924

[10] N. Arnold, E. Thor, N. Kirichenko, D. Bäuerle, Pyrolitic LCVD of fibers: A theoretical description, Appl. Phys. A. 62 (1996), 503-508

[11] C. Fauteux, J. Pegna, Radial characterization of 3D-LCVD carbon fibers by Raman spectroscopy, Applied physics A: Materials science and processing 78 (2004), 883-888 (abstract) (pdf)

[12] C. Fauteux, R. Longtin, J. Pegna, M. Boman, Microstructure and growth mechanism of laser grown carbon microrods as a function of experimental parameters, Journal of applied physics 95 (2004), 2737-2743 (abstract) (pdf)

[13] R. Longtin, C. Fauteux, E. Coronel, U. Wiklund, J. Pegna, M. Boman, Nanoindentation of carbon microfibers deposited by laser-assisted chemical vapor deposition, Applied physics A: Materials science and processing 79 (2004), 573-577 (abstract) (pdf)

[14] R. Longtin, C. Fauteux, J. Pegna, M. Boman, Micromechanical testing of carbon fibers deposited by low pressure laser-assisted chemical vapor deposition, Carbon 42 (2004), 2905-2913 (abstract) (pdf)

[15] R. Longtin, Synthèse et caractérisation de recouvrements organisés de nanofibres de carbone par dépôt chimique en phase vapeur assiste par laser [Ph.D. dissertation]. Canada: Ecole Polytechnique, Montréal (Canada); 2007. Available from: Dissertations & Theses: A&I. Accessed November 14, 2008, Publication Number: AAT NR29203.

[16] R. Longtin, L.-P. Carignan, C. Fauteux, D. Therriault, J. Pegna, Laser-assisted synthesis of carbon nanofibers: from arrays to thin films and coatings, Surface and Coatings Technology, vol. 202, pp. 2661–2669, 2008.

[17] R. Longtin, L.-P. Carignan, C. Fauteux, D. Therriault, J. Pegna, Selective area synthesis of aligned carbon nanofibers by laser-assisted catalytic chemical vapor deposition, Diamond & Related Materials, vol. 16, pp. 1541–1549, 2007.

[18] R. Longtin, C. Fauteux, R. Goduguchinta, J. Pegna, Synthesis of carbon nanofiber films and nanofiber composite coatings by laser-assisted catalytic chemical vapor deposition, Thin Solid Films, vol. 515, pp. 2958-2964, 2007.

Authors: Christian Fauteux and Rémi Longtin