A. F. Lopeandia

Sensitive power compensated scanning calorimeter for analysis of phase transformations in small samples

We have designed and developed a sensitive scanning calorimeter for use with microgram or submicrogram, thin film, or powder samples. Semiconductor processing techniques are used to fabricate membrane based microreactors with a small heat capacity of the addenda, 120nJ∕K at room temperature. At heating rates below 10K∕s the heat released or absorbed by the sample during a given transformation is compensated through a resistive Pt heater by a digital controller so that the calorimeter works as a power compensated device. Its use and dynamic sensitivity is demonstrated by analyzing the melting behavior of thin films of indium and high density polyethylene. Melting enthalpies in the range of 40–250μJ for sample masses on the order of 1.5μg have been measured with accuracy better than 5% at heating rates ∼0.2K∕s. The signal-to-noise ratio, limited by the electronic setup, is 200nW.

In situ nanocalorimetry of thin glassy organic films

In this work, we describe the design and first experimental results of a new setup that combines evaporation of liquids in ultrahigh vacuum conditions with in situ high sensitivity thermal characterization of thin films. Organic compounds are deposited from the vapor directly onto a liquid nitrogen cooled substrate, permitting the preparation and characterization of glassy films. The substrate consists of a microfabricated, membrane-based nanocalorimeter that permits in situ measurements of heat capacity under ultrafast heating rates (up to 10(5) K/s) in the temperature range of 100-300 K. Three glass forming liquids-toluene, methanol, and acetic acid-are characterized. The spikes in heat capacity related to the glass-transition temperature, the fictive temperature and, in some cases, the onset temperature of crystallization are determined for several heating rates.

Review on measurement techniques of transport properties of nanowires.

Physical properties at the nanoscale are novel and different from those in bulk materials.

Storing magnetic information in IrMn/MgO/Ta tunnel junctions via field-cooling

In this paper, we demonstrate that in Ta/MgO/IrMn tunneling junctions, containing no ferromagnetic elements, distinct metastable resistance states can be set by field cooling the devices from above the Neel temperature (TN) along different orientations. Variations of the resistance up to 10% are found upon field cooling in applied fields, in-plane or out-of-plane. Well below TN, these metastable states are insensitive to magnetic fields up to 2 T, thus constituting robust memory states. Our work provides the demonstration of an electrically readable magnetic memory device, which contains no ferromagnetic elements and stores the information in an antiferromagnetic active layer.

Evaluation of Growth Front Velocity in Ultrastable Glasses of Indomethacin over a Wide Temperature Interval

Ultrastable thin film glasses transform into supercooled liquid via propagating fronts starting from the surface and/or interfaces. In this paper, we analyze the consequences of this mechanism in the interpretation of specific heat curves of ultrastable glasses of indomethacin for samples with varying thickness from 20 nm up to several microns. We demonstrate that ultrastable films above 20 nm have identical fictive temperatures and that the apparent change of onset temperature in the specific heat curves originates from the mechanism of transformation and the normalization procedure. An ad hoc surface normalization of the heat capacity yields curves which collapse into a single one irrespective of their thickness. Furthermore, we fit the surface-normalized specific heat curves with a heterogeneous transformation model to evaluate the velocity of the growth front over a much wider temperature interval than previously reported. Our data expands previous values up to Tg + 75 K, covering 12 orders of magnitude in relaxation times. The results are consistent with preceding experimental and theoretical studies. Interestingly, the mobility of the supercooled liquid in the region behind the transformation front remains constant throughout the thickness of the layers.

From kinetic to collective behavior in thermal transport on semiconductors and semiconductor nanostructures

We present a model which deepens into the role that normal scattering has on the thermal conductivity in semiconductor bulk, micro, and nanoscale samples. Thermal conductivity as a function of the temperature undergoes a smooth transition from a kinetic to a collective regime that depends on the importance of normal scattering events. We demonstrate that in this transition, the key point to fit experimental data is changing the way to perform the average on the scattering rates. We apply the model to bulk Si with different isotopic compositions obtaining an accurate fit. Then we calculate the thermal conductivity of Si thin films and nanowires by only introducing the effective size as additional parameter. The model provides a better prediction of the thermal conductivity behavior valid for all temperatures and sizes above 30 nm with a single expression. Avoiding the introduction of confinement or quantum effects, the model permits to establish the limit of classical theories in the study of the thermal conductivity in nanoscopic systems.

Heat transfer in symmetric U-shaped microreactors for thin film calorimetry

We describe the results of two-dimensional finite difference analysis of the thermal profile, in both transient and steady state, of a symmetric U-shape designed high-sensitive nanocalorimeter. The thin film calorimeter, with a heat capacity of 100 nJ K−1 at room temperature, consists of a 180 nm thick freestanding silicon-rich nitride membrane on which thin film heaters and sensors are deposited. Simulated temperature profiles are in good agreement with in situ experimental data obtained at the heater and sensor locations. The first-order solid-to-liquid transition of indium films, from a few A to hundreds of nm thick, was used as an experimental reference of the thermal profiles obtained from the 2D modeling. Temperature differences inside the sample region induced by the symmetric U-shape design of the Pt heaters limit the use of the nanocalorimeter to two different heating rate regimes. At low heating rates, β < 10 K s−1, especially with a thermal layer, the temperature profile is reasonably flat so that small samples can be characterized in power compensation mode. At heating rates faster than 4 × 104 K s−1 the nanocalorimeter works in adiabatic mode and measures transitions occurring in the sample directly loaded underneath the heater.

Thermal conductivity of group-IV semiconductors from a kinetic-collective model.

The thermal conductivity of group-IV semiconductors (silicon, germanium, diamond and grey tin) with several isotopic compositions has been calculated from a kinetic-collective model. From this approach, significantly different to Callaway-like models in its physical interpretation, the thermal conductivity expression accounts for a transition from a kinetic (individual phonon transport) to a collective (hydrodynamic phonon transport) behaviour of the phonon field. Within the model, we confirm the theoretical proportionality between the phonon–phonon relaxation times of the group-IV semiconductors. This proportionality depends on some materials properties and it allows us to predict the thermal conductivity of the whole group of materials without the need to fit each material individually. The predictions on thermal conductivities are in good agreement with experimental data over a wide temperature range.

Tailoring thermal conductivity by engineering compositional gradients in Si1-xGex superlattices

The transport properties of artificially engineered superlattices (SLs) can be tailored by incorporating a high density of interfaces in them. Specifically, SiGe SLs with low thermal conductivity values have great potential for thermoelectric generation and nano-cooling of Si-based devices. Here, we present a novel approach for customizing thermal transport across nanostructures by fabricating Si/Si1−xGex SLs with well-defined compositional gradients across the SiGe layer from x = 0 to 0.60. We demonstrate that the spatial inhomogeneity of the structure has a remarkable effect on the heat-flow propagation, reducing the thermal conductivity to ∼2.2 W·m−1·K−1, which is significantly less than the values achieved previously with non-optimized long-period SLs. This approach offers further possibilities for future applications in thermoelectricity.

Nanocalorimetric analysis of the ferromagnetic transition in ultrathin films of nickel

We report on in situ heat capacity measurements (370–800K) using quasiadiabatic ultrafast differential scanning nanocalorimetry in thin films (1–200nm) of Ni grown by electron beam evaporation. The heat capacity shows a broad peak with a rounded maximum that is attributed to the decrease of long-range interactions in the ferromagnetic to paramagnetic phase transition of Ni. The calorimetric data exhibit a reduction of the Curie temperature as the thickness of the films (or the average grain size) decreases. The magnitude of the jump in specific heat at TC scales with the number of surface or interface atoms.