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.Comment: 19 pages, 4 figures, final published version, Nano Letters, 201

Highly Aligned Bacterial Nanocellulose Films Obtained During Static Biosynthesis in a Reproducible and Straightforward Approach

Bacterial nanocellulose (BNC) is usually produced as randomly-organized highly pure cellulose nanofibers films. Its high water-holding capacity, porosity, mechanical strength, and biocompatibility make it unique. Ordered structures are found in nature and the properties appearing upon aligning polymers fibers inspire everyone to achieve highly aligned BNC (A-BNC) films. This work takes advantage of natural bacteria biosynthesis in a reproducible and straightforward approach. Bacteria confined and statically incubated biosynthesized BNC nanofibers in a single direction without entanglement. The obtained film is highly oriented within the total volume confirmed by polarization-resolved second-harmonic generation signal and Small Angle X-ray Scattering. The biosynthesis approach is improved by reusing the bacterial substrates to obtain A-BNC reproducibly and repeatedly. The suitability of A-BNC as cell carriers is confirmed by adhering to and growing fibroblasts in the substrate. Finally, the thermal conductivity is evaluated by two independent approaches, i.e., using the well-known 3 ω -method and a recently developed contactless thermoreflectance approach, confirming a thermal conductivity of 1.63 W mK −1 in the direction of the aligned fibers versus 0.3 W mK −1 perpendicularly. The fivefold increase in thermal conductivity of BNC in the alignment direction forecasts the potential of BNC-based devices outperforming some other natural polymer and synthetic materials. Bacteria confined and statically incubated for a few days biosynthesized bacterial nanocellulose (BNC) nanofibers in a single direction without entanglement. The obtained film is highly oriented within the total volume of the film, and it shows a five-fold increase in thermal conductivity in the parallel direction forecasting the potential of BNC-based devices outperforming some other natural polymer and synthetic materials

Nanoparticle shape anisotropy and photoluminescence properties : Europium containing ZnO as a model case

The precise control over electronic and optical properties of semiconductor (SC) materials is pivotal for a number of important applications like in optoelectronics, photocatalysis or in medicine. It is well known that the incorporation of heteroelements (doping as a classical case) is a powerful method for adjusting and enhancing the functionality of semiconductors. Independent from that, there already has been a tremendous progress regarding the synthesis of differently sized and shaped SC nanoparticles, and quantum-size effects are well documented experimentally and theoretically. Whereas size and shape control of nanoparticles work fairly well for the pure compounds, the presence of a heteroelement is problematic because the impurities interfere strongly with bottom up approaches applied for the synthesis of such particles, and effects are even stronger, when the heteroelement is aimed to be incorporated into the target lattice for chemical doping. Therefore, realizing coincident shape control of nanoparticle colloids and their doping still pose major difficulties. Due to a special mechanism of the emulsion based synthesis method presented here, involving a gelation of emulsion droplets prior to crystallization of shape-anisotropic ZnO nanoparticles, heteroelements can be effectively entrapped inside the lattice. Different nanocrystal shapes such as nanorods, -prisms, -plates, and -spheres can be obtained, determined by the use of certain emulsification agents. The degree of morphologic alterations depends on the type of incorporated heteroelement M, concentration, and it seems that some shapes are more tolerant against doping than others. Focus was then set on the incorporation of Eu³⁺ inside the ZnO particles, and it was shown that nanocrystal shape and aspect ratios could be adjusted while maintaining a fixed dopant level. Special PL properties could be observed implying energy transfer from ZnO excited near its band-gap (3.3 eV) to the Eu³⁺ states mediated by defect luminescence of the nanoparticles. Indications for an influence of shape on photoluminescence (PL) properties were found. Finally, rod-like Eu@ZnO colloids were used as tracers to investigate their uptake into biological samples like HeLa cells. The PL was sufficient for identifying green and red emission under visible light excitation

Optical properties of low-dimensional semiconductor nanostructures under high pressure

Consultable des del TDXTítol obtingut de la portada digitalitzadaEn el transcurso de este trabajo se han estudiado las propiedades ópticas y vibracionales de nanoestructuras semiconductoras de baja dimensionalidad por medio de espectroscopia Raman y fotoluminiscencia combinadas con la técnica de altas presiones hidrostáticas en una celda de diamantes. Los sistemas materiales investigados pueden ser clasificados en tres grupos: aleaciones de SiGe, puntos cuánticos de Ge/Si, y puntos cuánticos de CdSe/ZnCdMgSe. Las propiedades vibracionales de las aleaciones de SiGe fueron estudiadas investigando la dependencia en composición de los potenciales de deformación ópticos (K11 y K12). Con esta finalidad se crecieron capas estresadas de SiGe/Si(100) utilizando la técnica de epitaxia por haces moleculares, las cuales fueron posteriormente estudiadas combinando espectroscopia Raman con la técnica de altas presiones. Una vez obtenidos estos potenciales de deformación calculamos la dependencia en composición de parámetros muy utilizados en la literatura como sean el strain-shift coefficient o el parámetro de Grüneisen. La ventaja principal de este método respecto de otros es que es independiente del sistema material estudiado presentándose como una solución a la determinación de la dependencia en composición de los potenciales de deformación. El estado de strain de puntos cuánticos autoensamblados Ge/Si como función del espesor de cap layer sobre los mismos fue investigado utilizando la dependencia con presión de sus vibraciones ópticas (fonones). A medida que se aumenta el espesor del cap layer sobre los puntos cuánticos observamos una transición en el estado de strain de los mismos de una situación biaxial a hidrostática. Este resultado provee un método conveniente para determinar la forma del tensor de strain en sistemas de puntos cuánticos. El estado de strain de puntos cuánticos de CdSe/ZnCdMgSe también fue investigado utilizando espectroscopía Raman. Estimamos la importancia relativa del estado de strain de los mismos frente a los efectos de confinamiento en lo que refiere la dependencia del primer nivel excitado. Finalmente, observamos interdifusión de Mg de la barrera hacia los puntos cuánticos de CdSe en condiciones de resonancia, que fueron logradas mediante la aplicación de presión hidrostática al sistema

Phonon Transport in the Gigahertz to Terahertz Range: Confinement, Topology and Second Sound

Perspectives on studying phonon transport in the gigahertz to terahertz range with Brillouin light scattering and frequency-domain thermoreflectance.Transport of heat and hypersound with gigahertz (GHz) to terahertz (THz) phonons is crucial for heat management in electronics, mediating signal processing with microwave radiation, thermoelectrics, and various types of sensors based on nanomechanical resonators. Efficient control of heat and sound transport requires new materials, novel experimental techniques, and a detailed knowledge of the interaction of phonons with other elementary excitations. Wave-like heat transport, also known as second sound, has recently attracted renewed attention since it provides several opportunities for overcoming some of the limitations imposed by diffusive transport (Fourier’s regime). The frequency-domain detection of GHz-to-THz phonons can be carried out in a remote, non-destructive, and all-optical manner. The ongoing development of nanodevices and metamaterials made of low-dimensional nanostructures will require spatially resolved, time-resolved, and anisotropic measurements of phonon-related properties. These tasks can be accomplished with Brillouin light scattering (BLS) and various newly developed variants of this method, such as pumped-BLS. In the near future, pumped-BLS is expected to become useful for characterizing GHz topological nanophononics. Finally, second-sound phenomena can be observed with all-optical methods like frequency-domain thermoreflectance.This project has received funding from the European Union’s Horizon 2020 Research and Innovation Programme (No. 101003436) and the Polish National Science Centre (No. UMO-2018/31/D/ST3/03882). J.S.R. acknowledges financial support from the Spanish Ministerio de Economıa, Industria y Competitividad for its support through Grant No. CEX2019-000917-S (FUNFUTURE) in the framework of the Spanish Severo Ochoa Centre of Excellence program and Grant No. PID2020-119777GB-I0016 (THERM2MAIN)

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

Nanoarchitecture Effects on Persistent Room Temperature Photoconductivity and Thermal Conductivity in Ceramic Semiconductors: Mesoporous, Yolk–Shell, and Hollow ZnO Spheres

A gas-phase approach is applied to synthesize a set of spherical particles with mesoporous, yolk−shell, or hollow character. A special arrangement of the ZnO lattice results in a polar character of the particle shell, and this facilitates effective separation of electrons and holes on different sides of the interface

A single-source precursor route to anisotropic halogen-doped zinc oxide particles as a promising candidate for new transparent conducting oxide materials

Correccions a aquest document es poden consultar a a https://ddd.uab.cat/record/185756The license is subject to the Beilstein Journal of Nanotechnology terms and conditions: http://www.beilstein-journals.org/bjnanoNumerous applications in optoelectronics require electrically conducting materials with high optical transparency over the entire visible light range. A solid solution of indium oxide and substantial amounts of tin oxide for electronic doping (ITO) is currently the most prominent example for the class of so-called TCOs (transparent conducting oxides). Due to the limited, natural occurrence of indium and its steadily increasing price, it is highly desired to identify materials alternatives containing highly abundant chemical elements. The doping of other metal oxides (e.g., zinc oxide, ZnO) is a promising approach, but two problems can be identified. Phase separation might occur at the required high concentration of the doping element, and for successful electronic modification it is mandatory that the introduced heteroelement occupies a defined position in the lattice of the host material. In the case of ZnO, most attention has been attributed so far to n-doping via substitution of Zn²+ by other metals (e.g., Al³+). Here, we present first steps towards n-doped ZnO-based TCO materials via substitution in the anion lattice (O²− versus halogenides). A special approach is presented, using novel single-source precursors containing a potential excerpt of the target lattice 'HalZn·Zn₃O₃' preorganized on the molecular scale (Hal = I, Br, Cl). We report about the synthesis of the precursors, their transformation into halogene-containing ZnO materials, and finally structural, optical and electronic properties are investigated using a combination of techniques including FT-Raman, low- T photoluminescence, impedance and THz spectroscopies

Nanoparticle shape anisotropy and photoluminescence properties : Europium containing ZnO as a model case

The precise control over electronic and optical properties of semiconductor (SC) materials is pivotal for a number of important applications like in optoelectronics, photocatalysis or in medicine. It is well known that the incorporation of heteroelements (doping as a classical case) is a powerful method for adjusting and enhancing the functionality of semiconductors. Independent from that, there already has been a tremendous progress regarding the synthesis of differently sized and shaped SC nanoparticles, and quantum-size effects are well documented experimentally and theoretically. Whereas size and shape control of nanoparticles work fairly well for the pure compounds, the presence of a heteroelement is problematic because the impurities interfere strongly with bottom up approaches applied for the synthesis of such particles, and effects are even stronger, when the heteroelement is aimed to be incorporated into the target lattice for chemical doping. Therefore, realizing coincident shape control of nanoparticle colloids and their doping still pose major difficulties. Due to a special mechanism of the emulsion based synthesis method presented here, involving a gelation of emulsion droplets prior to crystallization of shape-anisotropic ZnO nanoparticles, heteroelements can be effectively entrapped inside the lattice. Different nanocrystal shapes such as nanorods, -prisms, -plates, and -spheres can be obtained, determined by the use of certain emulsification agents. The degree of morphologic alterations depends on the type of incorporated heteroelement M, concentration, and it seems that some shapes are more tolerant against doping than others. Focus was then set on the incorporation of Eu³⁺ inside the ZnO particles, and it was shown that nanocrystal shape and aspect ratios could be adjusted while maintaining a fixed dopant level. Special PL properties could be observed implying energy transfer from ZnO excited near its band-gap (3.3 eV) to the Eu³⁺ states mediated by defect luminescence of the nanoparticles. Indications for an influence of shape on photoluminescence (PL) properties were found. Finally, rod-like Eu@ZnO colloids were used as tracers to investigate their uptake into biological samples like HeLa cells. The PL was sufficient for identifying green and red emission under visible light excitation

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