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NF-RAD: Near-Field Radiation <br/>Absorption and Scattering by Nanoparticles on Surfaces

Final Report Summary - NF-RAD (NF-RAD: Near-Field Radiation Absorption and Scattering by Nanoparticles on Surfaces)

The process of diagnosis and fabricating structures 1 - 100 nanometres in size, which defines 'nanoscale,' is a challenging endeavor. Implementation of engineering processes, measurement and control of such processes requires further understanding of the nanoscale physics and chemistry, with significantly different approaches than those in the molecular or macroscale regimes. This mesoscale regime is governed by the unusual characteristics of nanosize structures and particles. This is due to their small size and high percentage of atoms in surface states, which yield unique properties that differ from those of the same bulk materials (Roco, 1998; Rather, 1988; Shipway et al., 2000). Particularly, nano-metallic particles or colloids can give rise to many unprecedented optical, electronic, and structural properties if the composition, structure, shape, and size distribution of nano-building-blocks can be carefully controlled (Shipway et al., 2000). To be able to engineer 'bottom-up' processes at nanoscales, new approaches need to be developed for measurement and visualisation of workpieces with all details. These measurements must be in real-time and non-intrusive and can be best achieved with the proper use of near-field wave interactions between different structures. The objective of this Marie Curie International Reintegration Grant (IRG) NF-RAD has been to establish a research programme based on near-field radiation fundamentals and applications at Ozyegin University in Istanbul, Turkey.

The principal investigator (PI: Menguc) of this project has been involved with several characterisation and manufacturing research and education activities over the last two decades, including the Radiative Transfer Laboratory (RTL), University of Kentucky, Lexington, United States of America (USA), and recently at Ozyegin University, Istanbul Turkey. His past research has been funded by several agencies in the USA, including the National Science Foundation, Kentucky Science and Technology Council, Department of Energy, and General Electric. At Ozyegin University, with the help of the current Marie Curie research programme he has initiated a comprehensive research effort as well. All these studies have included the applications of 'near-field radiation transfer,' and carried in collabourative efforts with a few other Turkish universities and the University of Kentucky, in the USA.

This research has direct impact on:

(a) diagnosis of nano-particles and structures (the development of surface-wave scattering systems for characterisation of particles on surfaces);
(b) nano-scale manufacturing (atomic force microscopy (AFM) tip-based directed self-assembly and nano-scale cooling); and
(c) energy harvesting (hybrid PV-TPV, i.e. photovoltaic and thermophotovoltaic) energy conversion devices).

In each of these three areas, Menguc has extensive research experience; also in (b) and (c) he has pending patent applications. All these problems are directly related to the solution of Maxwell equations with the correct boundary conditions and with the right optical properties. Yet, in all these applications the governing equations are not well established and the required solution techniques have not been perfected. In addition, diagnosis of nano-structures and the relevant processes are not yet possible with high accuracy.

With the help of the Marie Curie IRG from the European Seventh Framework Programme (FP7) 'People' Work Programme, a new research programme on 'near-field radiation transfer' has been initiated at Ozyegin University (OzU) in Istanbul Turkey. The current research programme has mostly focused on the theoretical foundations. Yet, no theoretical work should be carried on in isolation; on the contrary it should be closely integrated to parallel experimental efforts. Partial effort has been spent on the development of experimental infrastructure, with additional funding from the Turkish Scientific and Technological Council (Tubitak).

Project focus areas and accomplishments

This Marie Curie IRG research programme had clear focus on the near field radiation transfer, absorption and scattering concepts for the development of new engineering devices and processes, with four application areas:

i) nano-scale particle characterisation;
ii) nano-scale manufacturing;
iii) nano-scale cooling;
iv) near-field radiative transfer for energy harvesting.

Near field radiative transfer involves structures in 'close proximity' to each other. These structures can be particles, agglomerates, pillars, carbon nanotubes or nanowires, or any AFM tip tapping on them. In these cases, the light-matter interaction cannot be simply described using the classical ray-tracing, scattering or radiative transfer approaches. In this context, 'close proximity' refer to distances fraction of the dominant wavelength of radiation exchange, which would be under 100 nm. These problems may be tackled with the solution of the Maxwell equations, provided that all interference, polarisation and tunneling effects are properly accounted for in the formulations. The most important aspect to be considered is the effect of the surface waves, which enhance the radiative exchange significantly. Depending on a material, these waves prevail significantly within a few-hundred nanometers.

These areas are discussed separately below and published papers are listed. These papers are the ones that acknowledge the Marie Curie NF-RAD grant. In addition, several conference papers were presented and talks were delivered; they are listed below. First, we provide a brief overview of these papers. The first paper, PAP-1, is the extended abstract of a key-note lecture by Menguc, delivered recently at the International Workshop on Nano- /Micro-Thermal Radiation in Japan.

Surface-wave based nanoparticle characterisation:

Understanding of interaction of surface waves with the objects within close proximity would pave ways to better diagnostics of small nanosize particles in situ, from their far-field polarised scattering signals (Aslan et al., 2005; Videen et al., 2005; Venkata et al., 2007; Francoeur et al., 2007). The same effects can also be used to enhance the energy transfer between different surfaces and structures built on them. This enhanced energy transfer is strongly spectral in nature, and can be used to build advanced energy harvesting concepts, such as thermophotovoltaic cells (Francoeur et al., 2008b). In addition, the same concept can be adapted for directed self assembly approaches (Hawes et al., 2008).

This research programme developed under the Marie Curie IRG NF-RAD at Ozyegin University is the primary research focus of this project. This has computational emphasis on the development of a comprehensive computer algorithm to investigate the particle-on-surface absorption and scattering problems with and without the presence of a tip (representing an atomic force microscopy tip). Initial work in this direction was performed by Ms Nazli Donmezer. Her Master thesis focused on the improvement of the discrete dipole approximation for particles on surfaces. She received her Master thesis at the Middle East Technical University, even though she has spent her entire second year at Ozyegin University. (The reason for that was the Master programme at Ozyegin University was not established right away, but waited 2011). We released these findings in a conference paper (Donmezer et al., 2009).

This computational work was initially extended by Vincent Loke who joined Menguc at Ozyegin University as a post-doctoral fellow. He and Menguc have also worked on a different and original formulation of the problem, as articulated in three separate papers. This formulation, named Discrete Dipole Approximation with Surface Interaction (DDA-SI) is considered as a Matlab toolbox application, and has been outlined in the Journal of Optical Society of America A (Loke and Menguc, 2010; PAP-1) and chosen to Spotlight in Optics for free access. The other paper appeared in the Journal of Quantitative Spectroscopy and Radiative Transfer (Loke et al, 2011, PAP-2). These studies were combined with the efforts of Kentucky and Germany groups; it is currently being considered for publication in JQSRT (PAP-3).

Loke and Menguc's extension of the standard discrete dipole approximation (DDA) method to account for interaction between light and scatterers in the proximity of a planar substrate surface was novel and paved the way for more controlled diagnosis and manufacturing at the nano-scale. The DDI-SI formulation involved the inclusion of reflection terms calculated using Sommerfeld integration which decomposed the expanding spherical waves from the dipoles into planar and cylindrical components. The cylindrical waves expand in the direction parallel to the substrate and are therefore unaltered. The planar waves, on the other hand, expand in the normal direction and are subject to the Fresnel reflection coefficients. Because the permittivities of the materials in question are complex numbers, the complex integration paths needed to avoid the branch cuts.

The discrete dipole approximation with surface interaction (DDA-SI) was successfully benchmarked against known results, namely by Taubenblatt and Tran (1993). Subsequently, we conducted simulations for the AFM probe in the proximity of a nanoparticle illuminated by an evanescent wave. The simulations demonstrated the phenomenon of near-field coupling whereby the latent energy of the evanescent field that would otherwise remain on the substrate surface can be tunnel or be drawn up to the particle and AFM probe. Under optimal AFM probe and particle separation, a highly intense and localised field was observed in the nanoparticle and AFM probe.

The theory and simulation results were published in a journal article (Loke and Menguc, 2010) which was selected for Spotlight on Optics (by Optical Society of America). Although, we used different materials in the simulations, this supports the phenomenon observed in experiments by Hawes et al. (2008) where the AFM probe was used to selectively melt nanoparticles sitting on BK7 glass, under evanescent wave illumination. We provide ongoing support for experiments by by determining the parameters (probe-particle distance, materials, shape etc.) for optimal field intensity (Loke and Menguc, 2010).

The applicability of DDA-SI goes beyond what was intended in this project. We also demonstrated plasmonic resonance of arrays of Ag and Au nanoparticles on BK7 glass. We are in the process of packaging writing the user guide for the public release of DDA-SI coinciding with our recent publication (Loke, Nieminen and Menguc, 2011). Ongoing work is being conducted to extend the capability for stratified planar surfaces as substrates often have one or more coatings of dissimilar substances. Here, reflections from multiple surfaces need to be accounted for. As the materials that make up the layers may have complex permittivity, the Sommerfeld complex integral will potentially have additional branch cuts. The problem, solution and testing process is non-trivial.

As discussed above, these studies have allowed us to collabourate with European and American groups. Vincent Loke has spent about six months in Bremen, Germany with Dr Thomas Wriedt and later spent two months at the University of Kentucky, Lexington, USA with Dr Todd Hastings (who still has collabourative research projects with Dr Menguc).

Near-field radiation transfer between parallel plates:

Although this work was not specifically mentioned in the original proposal, Menguc has worked on the near-field radiation transfer problem along with Mathieu Francoeur (his former PhD student) and Rodolphe Vaillon (his colleague at INSA, Lyon, France). They have published five papers and made a number of presentations. Menguc has also been working on the experimental aspects of the problem along with two Master students: David Kurt Webb (with Prof. Hakan Erturk at Bogazici University, Istanbul) and Zafer Artvin (with Prof. Tuba Okutucu at ODTU/METU, Ankara). These students are not paid by this grant; however, the Marie Curie IRG is acknowledged in publications as the principle of the work is similar and Menguc's involvement had to be accounted for in these studies.

In the papers published in the Journal of Applied Physics (JAP) and the Journal of Physics D (JoPD), we explored the possibilities of tuning near-field radiative heat transfer via coupling of surface phonon-polaritons in thin silicon carbide (SiC) films. We first performed this task by computing the local density of electromagnetic states within the nanometric gap formed between two thin SiC layers (JAP). Results showed that the near-field thermal spectrum emitted is not only a function of the structure of the emitter, but also a function of the geometry and property of the receiver. We also studied near-field radiative heat transfer by calculating the radiative heat flux between two thin SiC films (JoPD). The simulations clearly demonstrated that it is possible to tune the radiative flux by varying the thicknesses of the layers and their separation distance. The analyses performed in the JAP and JoPD open the path toward the design of nanostructures selectively emitting and absorbing near-field thermal radiation, which is extremely important for nanoscale-gap thermophotovoltaic (nano-TPV) power generation. The main results from the JAP and JoPD have been presented and published at META 10.

This work was later extended to the design of nano-thermophotovoltaic cells. In addition, a design scheme and regime map was discussed in a prestigious Physical Review-B paper. Finally, a more detailed discussion of nanoscale heat transfer with heat generation was the topic of a paper written along with two collaborators of Menguc.

We presented a poster at the ELS-XII and RAD-VI regarding nano-TPV devices. Simulation results clearly shown that nano-TPV systems proposed so far in the literature are unrealistic due to excessive heating of the cell converting thermal radiation into electricity. This work suggested that it is crucial to design nanostructures that will allow near-field radiative heat exchanges in a selective manner.

Near-field radiation transfer including magnetic effects:

We are also working with Dr Kursat Sendur of Sabanci University, Istanbul, on modelling nano-antennas using DDA with magnetic capability. In addition, we are extending standard DDA, which only caters for electric dipoles, for the inclusion of magnetic dipoles applied to nano-sized electromagnetic antennas. The results of the work will be published pending the outcome. In addition, Menguc is in the PhD committee of one Dr Sendur's student's (Erdem Ogut) and Dr Sendur will serve in the PhD committee of Azadeh Didari, a PhD student of Prof. Menguc.

This work was mentioned here as during the course of this study we had the chance to cooperate with Dr Sendur, who is also a MCI grantee.

Students and collaborators in the project

During this project, we partially funded four researchers at Ozyegin University. In addition, a few other students have participated to this research without being funded by the Marie Curie IRG NF-RAD.

The first student at Ozyegin University was Ms Nazli Donmezer, who joined our group in middle 2009. Nazli, a Turkish student, has completed her Master degree on Mechanical Engineering at Middle East Technical University (METU) in the Summer of 2010 (as the graduate programs at Ozyegin University was established in fall 2010). After that she went to the USA to start her PhD at Georgia Institute of Technology in Atlanta, Georgia. Her Master thesis was to develop a computer programme to help characterisation of nano-size particles on surfaces. Her work was guided by MP Menguc and Assistant Professor Tuba Okutucu of METU, Ankara, Turkey.

In early 2010, Dr Vincent Loke, an Australian, has joined our group at Ozyegin University as a post-doctoral fellow (research associate). He stayed on longer than one year; after that he went to the University of Kentucky in Lexington, USA for two months and then to the University of Bremen, in Bremen, Germany. During his stay in Istanbul, we published two papers. In addition, one paper was published from his collabourative work in Lexington and two others came out of the collabourative work in Bremen.

The third student was Mr David Kurt Webb, an American. He was mostly paid from an accompanying Tubitak grant on experimental research. However, his work on literate search was directly related to the fundamentals of near-field radiative transfer between parallel plates. He has received his Master thesis from Bogazici University in Istanbul. His work was guided by MP Menguc and Assistant Professor Hakan Erturk of Bogazici University.

The fourth student is Ms Azadeh Didari, an Iranian. She is currently working on her PhD at Ozyegin University on near field radiation fundamentals. Unfortunately, the funding from this project is over, and I need to find extra funding for her.

In addition to these students, I have continued collabourating with my PhD student at the University of Kentucky, Dr Mathieu Francoeur, who is currently an Assistant Professor of Mechanical Engineering at the University of Utah, Salt Lake City, Utah, USA. We have written several papers with him after I joined Ozyegin University in 2009. In almost all of these papers, Dr Rodolphe Vaillon of CNRS and INSA Lyon, France was also a co-author.

Note that after arriving to Istanbul in 2009, we have started an ad-hoc research collabouration with three other researchers from two other Istanbul Universities. Menguc, Drs Kursat Sendur and Ali Kosar from Sabanci University, and Dr Hakan Erturk from Bogazici University have established the so-called 'Istanbul collabourative' which has been instrumental in guiding several students from all three universities. They have also applied for a patent (Sendur, Kosar, Menguc) which started from the discussions during Istanbul Collabourative meetings. Finally, we have written, published and in process of publishing several papers.

Explanation on the use of the resources and collabourations

Most of the funding from this Marie Curie IRG is spent on a post-doctoral fellow, Dr Vincent Loke. He has spent all of his efforts on the project and wrote two papers. The computer program DDI-SI will be readily available to public during 2011. Partial funding was provided to Professor Menguc during this research. Initially, some funding was spent on Nazli Donmezer. In 2011 - 2012, we spent some funding on Azadeh Didari, who is a PhD student.

During the course of this study, Menguc has collabourated extensively with Dr Kursat Sendur of Sabanci University. He is also a recipient of Marie Curie IRG, and synergetic activities between the PI and him resulted significant discussions. Menguc and Sendur have established a grass-roots research collabouration named Istanbul collabourative in Istanbul to address scientific issues related to nano-scale thermal and magnetic effects. This work will continue in the years to come with the inclusion of other researchers from different Istanbul universities.

In addition, Menguc has been instrumental in the preparation of an FP7 proposal (NETFAST) submitted in March 2010 with several French, German, Spanish researchers. Unfortunately, this proposal was not funded.