J. Rodriguez-Viejo

Cathodoluminescence and photoluminescence of highly luminescent CdSe/ZnS quantum dot composites

We report room-temperature cathodoluminescence and photoluminescence spectra originating from ZnS overcoated CdSe nanocrystals, 33 and 42 A in diameter, embedded in a ZnS matrix. The thin-film quantum dot composites were synthesized by electrospray organometallic chemical vapor deposition. Cathodoluminescence and photoluminescence are dominated by the sharp band-edge emission characteristic of the initial nanocrystals. The emission wavelength can be tuned in a broad window (470–650 nm) by varying the size of the dots. The cathodoluminescence intensity depends on the crystallinity of the ZnS matrix and the voltage and current density applied.

Evidence of photo- and electrodarkening of (CdSe)ZnS quantum dot composites

We present a study of the kinetics of photoluminescence (PL) and cathodoluminescence (CL) degradation of semiconductor quantum dot composites, formed by highly luminescent (CdSe)ZnS core-shell nanocrystals embedded in a ZnS matrix. The photoluminescence and cathodoluminescence spectra indicate that both emissions originate from the same near band-edge state of the nanocrystals. We observe a strong decrease in the PL and CL intensities with time. Photoluminescence experiments carried out at high laser fluences (0.5–10 mJ/cm2 per pulse) show that the PL intensity decay with time depends on the size of the nanocrystals and the nature of the surrounding matrix. For instance, close-packed films showed a much slower decay than composite films. The cathodoluminescence intensity degradation is enhanced at lower temperatures. Partial recoveries of the CL signal have been achieved after thermal annealing at temperatures around 120 °C, which indicates that activation of trapped carriers can be induced by thermal sti...

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.

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.

Anomalous Transformation of Vapor-Deposited Highly Stable Glasses of Toluene into Mixed Glassy States by Annealing Above Tg.

Vapor-deposited glasses have recently emerged as a remarkable new class of materials that can form much denser and stable glasses than those obtained by cooling the liquid. These new amorphous materials reach lower regions of the energy landscape and may impact important technologies that use vapor-deposition. Here, we report on the formation of a glass with two distinct glassy states obtained through the partial annealing of highly stable vapor-deposited glassy films of toluene. The resulting glass exhibits two clear heat capacity overshoots with different onset and fictive temperatures. The transformation times of the ultrastable glass are around 10(5) times slower than the structural relaxation time (τα) of supercooled liquid toluene. We show that the nature of the transformed glass depends on the annealing temperature above Tg. This finding suggests the formation of distinct supercooled liquids at temperatures slightly above Tg during the transformation of the highly stable glass. Our results are compatible with the existence of polyamorphism in toluene.

Cross-plane thermal conductivity reduction of vertically uncorrelated Ge∕Si quantum dot superlattices

A drastic reduction in temperature dependent cross-plane thermal conductivity κ⊥ occurs in Ge quantum dot superlattices (QDSLs), depending on the vertical correlation between dots. Measurements show at least a twofold decrease of κ⊥ in uncorrelated dot structures as compared to structures with the same Si spacer of 20nm but good vertical dot alignment. The observed impact of disorder on the conductivity provides an alternative route to reduce the thermal conductivity of QDSLs. The results of this work have implications for the development of highly efficient thermoelectric materials and on-chip nanocooling devices.

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.

Formation of Pd2Si on single-crystalline Si (100) at ultrafast heating rates: An in-situ analysis by nanocalorimetry

The kinetics of intermediate phase formation between ultrathin films of Pd (12 nm) and single-crystalline Si (100) is monitored by in-situ nanocalorimetry at ultrafast heating rates. The heat capacity curves show an exothermic peak related to the formation of Pd2Si. A kinetic model which goes beyond the conventional linear-parabolic growth to consider independent nucleation and lateral growth of Pd2Si along the interface and vertical growth mechanisms is developed to fit the calorimetric curves. The model is used to extract the effective interfacial nucleation/growth and diffusion coefficients at the unusually high temperatures of silicide formation achieved at very fast heating rates.

Kinetics of silicide formation over a wide range of heating rates spanning six orders of magnitude

Kinetic processes involving intermediate phase formation are often assumed to follow an Arrhenius temperature dependence. This behavior is usually inferred from limited data over narrow temperature intervals, where the exponential dependence is generally fully satisfied. However, direct evidence over wide temperature intervals is experimentally challenging and data are scarce. Here, we report a study of silicide formation between a 12 nm film of palladium and 15 nm of amorphous silicon in a wide range of heating rates, spanning six orders of magnitude, from 0.1 to 105 K/s, or equivalently more than 300 K of variation in reaction temperature. The calorimetric traces exhibit several distinct exothermic events related to interdiffusion, nucleation of Pd2Si, crystallization of amorphous silicon, and vertical growth of Pd2Si. Interestingly, the thickness of the initial nucleation layer depends on the heating rate revealing enhanced mass diffusion at the fastest heating rates during the initial stages of the re...

Acoustic-like dynamics of amorphous drugs in the THz regime

The high frequency dynamics of Indomethacin and Celecoxib glasses has been investigated by inelastic x-ray scattering, accessing a momentum-energy region still unexplored in amorphous pharmaceuticals. We find evidence of phonon-like acoustic dynamics, and determine the THz behavior of sound velocity and acoustic attenuation. Connections with ordinary sound propagation are discussed, along with the relation between fast and slow degrees of freedom as represented by non-ergodicity factor and kinetic fragility, respectively.