Nanoscale Science and Technology is a fast developing field which deals with the characteristics and properties of features with dimensions of 100 nm or less. In the nanoscale, electrical, optical, magnetic, thermal and mechanical properties of materials might be radicaly different from those in the macroscale. Nanotechnologists strive after the control of such phenomena.
Furthermore, future technologies will have to deal with the consequences of confinement and the interactions between the ever decreasing nanoscopic parts composing cutting edge mechanical and electronic devices. Nowadays, efficiency can no longer be understood without the appropriate use of well characterised and functionalised surfaces that provide an intelligent interface between materials and their environment.
In our lab, we are developing an understanding of the mechanisms behind energy dissipation and electrical and thermal conductivity in the nanoscale with a combination of scanning probe and electric probe microscopy and femto second (fs) laser technology. Our vision is to provide the background for the successful development of science and technology in the nanoscale with a view to rapid implementation in industry.
A new investigation technique for the study of energy transport phenomena at the nano scale
Furthermore, detailed understanding of energy transfer at the nanoscale calls for improved experimental techniques. The use of ultrashort light pulses lasting tens of femtoseconds allows scientists to probe physical processes over ever shorter time and length scales. Short pulses allow for the interrogation of physical systems with fine temporal resolution, and this translates into correspondingly fine spatial resolution.
The most common way such measurements are carried out is known as the pump-probe technique, a two step measurement. In the first step, a pulse of light (the “pump” pulse) impinges on a sample, depositing energy over a short period of time. The deposited energy causes a change in the sample which can be correlated with a change in its optical properties (reflectivity, absorptivity or transmissivity). In the second step, the “probe” pulse (or a continuous beam) arrives and probes the state of the sample. Conventionally a difference in optical path lengths between the two pulses results in a variable time delay that can be used to study the evolution of a physical process. When reflectance is measured, the method is often called transient thermo-reflectance (TTR) or time-domain thermo-reflectance (TDTR). TDTR has been an effective tool to extensively study thermal transport in thin films and bulk materials, liquids, transport across material interfaces, and anisotropic heat conduction in films.
Researchers at the Laboratory for Energy and Nano Sciences at the Masdar Institute of Science and Technology have recently presented a substantial improvement to the commonly used TDTR, a frequency-domain thermo reflectance (FDTR) method that combines the advantages of TDTR with the relative experimental simplicity of modulated photo-thermal methods. By varying the modulation frequency of the “pump” pulse over a range up to 50 kHz - 20 MHz, one obtains a frequency-domain measurement that yields the same parameters as TDTR with similar or improved sensitivity, including the thermal conductivity of bulk materials and films in the sub-micron range and the thermal boundary conductance between layers. In addition, FDTR can reliably extract thermal conductivity, boundary conductance, and the volumetric heat capacity of a sample simultaneously, while TDTR can only be used to obtain two parameters. The FDTR method can be implemented with an ultrafast pulsed laser system, allowing easy switching between FDTR and TDTR, or with continuous-wave (CW) lasers, allowing for a simpler, less expensive system without a mechanical delay stage.
Currently, researchers at LENS are investigating the use of the FDTR method for the development of a non-destructive investigation tool for the characterization of thin films which is of interest to the photovoltaic industry. The FDTR method allows researchers to develop less expensive characterization equipment that can provide a reliable way to monitor thin films density and porosity. Work is under progress to develop quantitative non-destructive characterization equipment based on the FDTR method. This work has the potential to deliver better monitoring tools that can be advantageous in the photovoltaic industry. These tools allow for accessible characterization that may facilitate the development of new large scale production processes such as Masdar PV.
The FDTR method is also being employed to investigate the way to improve the efficiency of thermoelectric energy conversion . Solar-thermoelectric technology first converts solar energy into thermal energy and then into electricity through solid-state thermoelectric devices. Compared to solar photovoltaics, solar-thermoelectrics have potentially superior performance in efficiency, cost, temperature sensitivity, energy storage, system size and offer the possibility of roof-top applications. By means of the FDTR light is being shed to the importance of interfacial thermal transport in thermoelectric energy conversion materials. This has the potential to lead towards the development new materials and may further lead to the development of an entirely new industry in Abu Dhabi.