Francesc Alzina

Magnetotransport in disordered graphene exposed to ozone: From weak to strong localization

We present a magnetotransport study of graphene samples into which a mild disorder was introduced by exposure to ozone. Unlike the conductivity of pristine graphene, the conductivity of graphene samples exposed to ozone becomes very sensitive to temperature: it decreases by more than three orders of magnitude between 100 and 1 K. By varying either an external gate voltage or temperature, we continuously tune the transport properties from the weak to the strong localization regime. We show that the transition occurs as the phase coherence length becomes comparable to the localization length. We also highlight the important role of disorder-enhanced electron-electron interaction on the resistivity.

Reduction of the thermal conductivity in free-standing silicon nano-membranes investigated by non-invasive Raman thermometry

We report on the reduction of the thermal conductivity in ultra-thin suspended Si membranes with high crystalline quality. A series of membranes with thicknesses ranging from 9 nm to 1.5 μm was investigated using Raman thermometry, a novel contactless technique for thermal conductivity determination. A systematic decrease in the thermal conductivity was observed as reducing the thickness, which is explained using the Fuchs-Sondheimer model through the influence of phonon boundary scattering at the surfaces. The thermal conductivity of the thinnest membrane with d = 9 nm resulted in (9 ± 2) W/mK, thus approaching the amorphous limit but still maintaining a high crystalline quality.

A one-dimensional optomechanical crystal with a complete phononic band gap

Recent years have witnessed the boom of cavity optomechanics, which exploits the confinement and coupling of optical and mechanical waves at the nanoscale. Among their physical implementations, optomechanical (OM) crystals built on semiconductor slabs enable the integration and manipulation of multiple OM elements in a single chip and provide gigahertz phonons suitable for coherent phonon manipulation. Different demonstrations of coupling of infrared photons and gigahertz phonons in cavities created by inserting defects on OM crystals have been performed. However, the considered structures do not show a complete phononic bandgap, which should enable longer lifetimes, as acoustic leakage is minimized. Here we demonstrate the excitation of acoustic modes in a one-dimensional OM crystal properly designed to display a full phononic bandgap for acoustic modes at 4 GHz. The modes inside the complete bandgap are designed to have high-mechanical Q-factors, limit clamping losses and be invariant to fabrication imperfections.

Lifetimes of Confined Acoustic Phonons in Ultrathin Silicon Membranes

We study the relaxation of coherent acoustic phonon modes with frequencies up to 500 GHz in ultrathin free-standing silicon membranes. Using an ultrafast pump-probe technique of asynchronous optical sampling, we observe that the decay time of the first-order dilatational mode decreases significantly from ~4.7 ns to 5 ps with decreasing membrane thickness from ~194 to 8 nm. The experimental results are compared with theories considering both intrinsic phonon-phonon interactions and extrinsic surface roughness scattering including a wavelength-dependent specularity. Our results provide insight to understand some of the limits of nanomechanical resonators and thermal transport in nanostructures.

Phonons in slow motion: dispersion relations in ultrathin Si membranes.

We report the changes in dispersion relations of hypersonic acoustic phonons in free-standing silicon membranes as thin as ∼8 nm. We observe a reduction of the phase and group velocities of the fundamental flexural mode by more than 1 order of magnitude compared to bulk values. The modification of the dispersion relation in nanostructures has important consequences for noise control in nano- and microelectromechanical systems (MEMS/NEMS) as well as opto-mechanical devices.

A novel contactless technique for thermal field mapping and thermal conductivity determination: two-laser Raman thermometry.

We present a novel contactless technique for thermal conductivity determination and thermal field mapping based on creating a thermal distribution of phonons using a heating laser, while a second laser probes the local temperature through the spectral position of a Raman active mode. The spatial resolution can be as small as 300 nm, whereas its temperature accuracy is ±2 K. We validate this technique investigating the thermal properties of three free-standing single crystalline Si membranes with thickness of 250, 1000, and 2000 nm. We show that for two-dimensional materials such as free-standing membranes or thin films, and for small temperature gradients, the thermal field decays as T(r) ∝ ln(r) in the diffusive limit. The case of large temperature gradients within the membranes leads to an exponential decay of the thermal field, T ∝ exp[ - A·ln(r)]. The results demonstrate the full potential of this new contactless method for quantitative determination of thermal properties. The range of materials to which this method is applicable reaches far beyond the here demonstrated case of Si, as the only requirement is the presence of a Raman active mode.

Ultra-thin free-standing single crystalline silicon membranes with strain control

We report on fabrication and characterization of ultra-thin suspended single crystalline flat silicon membranes with thickness down to 6 nm. We have developed a method to control the strain in the membranes by adding a strain compensating frame on the silicon membrane perimeter to avoid buckling after the release. We show that by changing the properties of the frame the strain of the membrane can be tuned in controlled manner. Consequently, both the mechanical properties and the band structure can be engineered, and the resulting membranes provide a unique laboratory to study low-dimensional electronic, photonic, and phononic phenomena.

Electrocatalytic tuning of biosensing response through electrostatic or hydrophobic enzyme–graphene oxide interactions

The effect of graphene oxidative grades upon the conductivity and hydrophobicity and consequently the influence on an enzymatic biosensing response is presented. The electrochemical responses of reduced graphene oxide (rGO) have been compared with the responses obtained from the oxide form (oGO) and their performances have been accordingly discussed with various evidences obtained by optical techniques. We used tyrosinase enzyme as a proof of concept receptor with interest for phenolic compounds detection through its direct adsorption onto a screen-printed carbon electrode previously modified with oGO or rGO with a carbon-oxygen ratio of 1.07 or 1.53 respectively. Different levels of oGO directly affect the (bio)conjugation properties of the biosensor due to changes at enzyme/graphene oxide interface coming from the various electrostatic or hydrophobic interactions with biomolecules. The developed biosensor was capable of reaching a limit of detection of 0.01 nM catechol. This tuning capability of the biosensor response can be of interest for building several other biosensors, including immunosensors and DNA sensors for various applications.

Two-Dimensional Phononic Crystals: Disorder Matters

The design and fabrication of phononic crystals (PnCs) hold the key to control the propagation of heat and sound at the nanoscale. However, there is a lack of experimental studies addressing the impact of order/disorder on the phononic properties of PnCs. Here, we present a comparative investigation of the influence of disorder on the hypersonic and thermal properties of two-dimensional PnCs. PnCs of ordered and disordered lattices are fabricated of circular holes with equal filling fractions in free-standing Si membranes. Ultrafast pump and probe spectroscopy (asynchronous optical sampling) and Raman thermometry based on a novel two-laser approach are used to study the phononic properties in the gigahertz (GHz) and terahertz (THz) regime, respectively. Finite element method simulations of the phonon dispersion relation and three-dimensional displacement fields furthermore enable the unique identification of the different hypersonic vibrations. The increase of surface roughness and the introduction of short-range disorder are shown to modify the phonon dispersion and phonon coherence in the hypersonic (GHz) range without affecting the room-temperature thermal conductivity. On the basis of these findings, we suggest a criteria for predicting phonon coherence as a function of roughness and disorder.

Hypersonic phonon propagation in one-dimensional surface phononic crystal

Hypersonic, thermally activated surface acoustic waves propagating in the surface of crystalline silicon patterned with periodic stripes were studied by Brillouin light scattering. Two characteristic directions (normal and parallel to the stripes) of surface acoustic waves propagation were examined exhibiting a distinctive propagation behavior. The measured phononic band structure exhibits diverse features, such as zone folding, band gap opening, and hybridization to local resonance for waves propagating normal to the stripes, and a variety of dispersive modes propagating along the stripes. Experimental results were supported by theoretical calculations performed using finite element method.