University of Bremen, Germany
Nanophononics is defined as the control of heat transport in the nanoscale. This kind of control would improve existing electronic devices, as well as lead to completely new applications in the fields of nano machines, electronics, refrigeration, energy transport and energy generation . It has been suggested that similar devices as those used for electronics can be developed for the control of phonon flows . The promise of phononics can bring changes as dramatic as the ones brought by electronics.
There are many important challenges that have to be overcome: experimental, computational and theoretical. Phonons do not have charge or mass, and are not conserved particles. This makes their experimental characterization and control very difficult. There are also many theoretical challenges, as we lack a fundamental understanding of thermodynamics at the nanoscale. Thus, many fundamental questions arise in this context: What is the appropriate definition of temperature here? What is the role of quantum effects in nanoscale heat transport? How do these questions affect experiments? How should nanophononic devices be designed? What materials are better suited for these devices? These are some of the fundamental unanswered questions. There are major computational challenges as well. First, the computational modeling of the experiments is very difficult due to the scale of these materials and devices. Also, many of the existing techniques rely on theoretical assumptions with an unreliable foundation and an unclear regime of validity.
This field is reaching a point where, to proceed forwards, more exchange between theory, computational methods and experiments is needed. It is also important to have researchers working on different computational methods to exchange ideas in order to bridge methodological gaps. A workshop that brings all these together will focus the research field, helping to identify challenges and goals. It will also help to define and structure the community.
Although the fundamentals of non-equilibrium thermodynamics are well established in the macroscopic scale, there are many open questions in the nanoscale . These questions have profound implications on how to transport and control heat in this length scale. Thus, much research has been done that shows that heat transport in nanowires is fundamentally different from the macroscopic scale. These results have been summarized in a comprehensive review by Y. Dubi and M. Di Ventra . These differences can lead to new devices. These devices require a new theoretical understanding and new experiments to characterize them.
There have been many theoretical developments to study heat transport in these devices, which include master equation approaches by D. Segal [5,6], molecular dynamic simulations , quantum Langevin equations , and non-equilibrium Green function techniques [9-12]. These have been applied to a variety of systems.
From these, important results have shown that the thermal conductance is affected by different factors. There are geometrical factors that have an impact on the conductivity. For example, it has been shown that there is a smooth thermal conductivity crossover from 1D to 3D as a function of the nanowires diameter. This was shown using the Keldysh Green function formalism . Also, the Landuaer approach was used to study the role of structural defects on phonon transport in quasi 1D solids . The spatial confinement of phonons is further influenced by the rigidity of the boundaries, which in turn affects phonon group velocity, phonon relaxation and ultimately, reduces the thermal conductivity .
Surface effects also play an important role. In a series of publications [15-18], N. Mingo and others studied phonon transport in different kinds of nanowires, semiconducting and dielectric, focusing on understanding the influence of the coating of the surface of the wire. The transition between ballistic and diffusive transport has been observed. These results combined the techniques of Green functions, semi-classical Boltzmann equation and calculations of the phononic band structure. There are many other publications that have studied how boundary effects and surface roughness of nanowires can influence the scattering of phonons as shown by A. Balandin and others [19-21]. There is also some evidence that surface roughness can be strongly detrimental to phonon transport, as shown in Ref. . Other studies support this view as well, but geometric factors make the picture more complex .
Other important systems being studied are molecular junctions. There, a molecule is placed between two thermal baths, and phononic heat transfer is studied. In general, this has been addressed mostly as special cases in connection to electron transport, as reported in an important paper by D. Segal, A. Nitzan and P. Hänggi , as well by others [24-29]. However, recently, some direct studies have looked at toy models with quantum mechanical effects, and noticed that the steady state heat transport was equivalent to the classical heat transport with temperature relaxation , leaving open many questions about the properties of heat flow far way from equilibrium. In addition, some other studies on the non-equilibrium dissipation of heat in molecular devices have also been performed [31-35]. New experiments have found out that ultrafast effects are crucial for heat transport in this scale .
Another frontier in nanophononics is the understanding of thermal conductance quantization. This is thermal energy transport quantization can be understood in analogy to charge transport quantization in dielectric quantum wires. (dielectric quantum wires at the end of the sentence sounds like referring to charge transport quantization, not to heat,maybe one can sandwich it: This is the analogy, in dielectric quantum wires, to charge transport quantization…or something like this)[26,49]. Although there have been some studies on quasi-1D systems as studied by K. Schwab et al.  and others [51,52], conclusive verification of thermal quantization is still an experimental challenge. There are predictions that thermal conductance quantization can occur in carbon nanotubes at low temperatures [29, 53].
The development of devices that control heat at this scale constitutes the ultimate goal of nanophononics . Heat diodes and heat transistors have been proposed theoretically by N. Li, B. Li and collaborators , by the group of Cassati  and others . D. Segal, and collaborators, studied molecular-chain versions of these in more details [23,24,40]. Proof-of-principle solid state versions of heat diodes have been built and characterized . Thermal rectification has been verified in other studies . Another important achievement in controlling heat in this scale is heat pumping , which was demonstrated experimentally .
The characterization of these devices has promoted the development of new experimental techniques pioneered by D. Cahill, K. Goodson, G. Mahan (Mahan is a theoretician), A. Majumdar and collaborators . This has led e.g. to the modification of atomic force microscopes for thermometry . Improvement to these methods lead to a technique dubbed Scanning Thermal Microscopy [46,47]. It has been shown that it can be used to characterize thermal conductivity . Although these devices are very promising, there are still many fundamental questions that need to be addressed [1,4].
Mads Brandbyge (Technical University of Denmark, Lyngby) - Organiser
Gianaurelio Cuniberti (Technical University Dresden) - Organiser
Thomas Frauenheim (University of Bremen) - Organiser
César A. Rodríguez-Rosario (University of Bremen, BCCMS) - Organiser