ATTOCALMAT · Atto-calorimetric tools to explore material properties in the nanoscale

Atto-calorimetric tools to explore material properties in the nanoscale

Nanoscale phenomena where the surface/interfaces or the small dimensions play a predominant role in the physical properties have become increasingly important in the last decade. Many characterization techniques have been adapted to face new challenges and understand new phenomena and calorimetry is no exception. Calorimetry is a widespread technique used in the characterization of phase transitions, reaction kinetics and thermodynamics in a variety of domains including materials science, physics, chemistry and biology. Its ability to provide the heat involved during phase changes is a unique feature that drives most of the cutting-edge applications of calorimetry. Many problems in nanoscience and material engineering involving surface or near-surface processes could significantly benefit from the information provided by this technique for future technological applications. The project ATTOCALMAT aims to develop and improve calorimetric tools, offering new research possibilities. The technique should offer the same performance of thin film scanning calorimetry (TDSC), which benefits from ultrafast heating rates to increase energy resolution. In the approach proposed here, while increasing the heating rate in a very narrow temperature interval, the sample is maintained in quasi-isothermal conditions. This way, heat capacity can be monitored as a function of external variables, such as magnetic field (magnetocalorimetric experiments) or time (absorption-desorption experiment), while the average sample temperature is quasi-constant. The very high sensitivity of the technique allows the analysis of samples with dimensions of few nm, enabling systematic studies of size effects. During the project, the basic experimental setup was mounted. A liquid-helium immersion cryostat (granted by Institute Neel- CNRS) was adapted, fabricating an ultra-high vacuum chamber with a thermostatic holder to load the nanocalorimeters. The temperature of the mount holder can be controlled at a given temperature, with temperature fluctuations 1mK in the full range of temperature, from 4 to 500K. Recently and with the aim of performing magnetocalorimetric measurements, the cryostat has been equipped with both a superconducting coil able to generate fields up to 4T (built in collaboration with Institute Neel- CNRS) and a secondary coil with small impedance to generate sinusoidal magnetic field excitations. The newly developed calorimetric technique, s-pulse nanocalorimetry, considers promoting local heating in a sample, around a reference temperature, while measuring heating rates. Like in TDSC, ultra fast heating rates are the key for the high resolution, and to keep the temperature scans small faster electronics for sourcing and signal conditioning have been designed and fabricated. They are based on commercial INA103, with setting times of few microseconds, enabling current pulses of 10 µs. The simplicity of the technique permits the use of devices with different sensing areas. Performing differential measurements sensitivities (Cp/Cp) up to 10-5 are currently reached. The instrumentation has been synchronized using Labview software, using a background program with the pulse heating routine that works while other external variables are controlled. Nowadays, measurements of heat capacity can be done while fixing external variables (magnetic field, gas pressure, temperature,…). This is one of the main achievements of the present project and represents a step forward to analyze size effects on the heat capacity. During the project new calorimetric chips, based on free-standing membranes ensuring very light heat capacities like TDSC devices, have been designed and fabricated. The designs have considered the new microsecond timescale to accommodate the diffusion characteristics. The final design of the devices was previously optimized by modeling the thermal behavior using COMSOL multiphysics software. Designs with reduced sensing areas to increase the selectivity have been successfully fabricated (collaboration with IMB-CNM-CSIC), defining the heater/sensing elements by e-beam lithography. Between the designs fabricated one can finds calorimeters with sensing areas ranging from 0.01 mm2 to 0.1m2. For these devices heat capacity addenda have been measured reporting values of ~8nJ/K and ~90fJ/K, respectively. Accounting the standard sensitivity, in the smaller devices sensitivities in the aJ/K range can be reached. In parallel to the heat capacity measurement, suspended structures to measure heat flux and thermal conductivities in nanoscale Si samples have been also developed. Besides the development of new instrumentation, this first period has been used to master the growth of different materials for nanocalorimetric characterization: EuO, NiO and CoO. Especial effort has been devoted on the study of growth of EuO by different methods: sputtering at UAB-MATGAS, by molecular beam epitaxy in collaboration with Francis Bitter Lab (MIT) and by pulsed laser deposition in collaboration with University of Tweente. The technical difficulties on building up the magnetocalorimetric setup and especially with the fabrication of the superconducting coil have delayed the studies as function of magnetic field. Currently, experiment on antiferromagnetic systems (NiO, CoO…), ferromagnetic (Co, NiPd), exchange bias systems (Co/CoO) are in progress. This fact enabled the application of this technique to different systems not considered in the initial application, such as thin films of IrMn/MgO/Ta (collaboration with UC Berkeley) or ultra-stable glasses of organic substances, i.e., indomethacine. In the second period, an initial exploration of the 2D-3D transition of Ge grown by MBE was pursued in collaboration with ICMAB-CSIC. For that purpose, a new insertion arm, incorporating the sample holder for the nanocalorimeters, was embedded in the MBE setup. Nanocalorimeters with monocrystalline Si layers as substrate were used in those experiments. The first experiments using thin-film scanning calorimetry were non-conclusive and a clear transition was not observed. However, the stringent conditions required for the epitaxial growth have delayed the final results. The complete realization of pulse-heating and the new small area devices with high-sensitivity will enhance sensitivity and new measuremsnt will be carried out outside the duration of the project. However, taking profit of the monocrystalline Si surfaces the interface reaction between Pd, Ni and Si to form silicides was successfully investigated. In addition, simultaneous nanocalorimetric and synchrotron radiation experiments conducted recently provided new insights into the formation mechanism of the silicide phase.

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