Review: using rolled-up tubes for strain-tuning the optical properties of quantum emitters.

Rolled-up tubes based on released III-V heterostructures have been extensively studied and established as optical resonators in the last two decades. In this review, we discuss how light emitters (quantum wells and quantum dots) are influenced by the inherently asymmetric strain state of these tubes. Therefore, we briefly review whispering gallery mode resonators built from rolled-up III-V heterostructures. The curvature and its influence over the diameter of the rolled-up micro- and nanotubes are discussed, with emphasis on the different possible strain states that can be produced. Experimental techniques that access structural parameters are essential to obtain a complete and correct image of the strain state for the emitters inside the tube wall. In order to unambiguously extract such strain state, we discuss x-ray diffraction results in these systems, providing a much clearer scenario compared to a sole tube diameter analysis, which provides only a first indication of the lattice relaxation in a given tube. Further, the influence of the overall strain lattice state on the band structure is examined via numerical calculations. Finally, experimental results for the wavelength shift of emissions due to the tube strain state are presented and compared with theoretical calculations available in literature, showing that the possibility to use rolled-up tubes to permanently strain engineer the optical properties of build-in emitters is a consistent method to induce the appearance of electronic states unachievable by direct growth methods.

Reconfiguration of Amorphous Complex Oxides: A Route to a Broad Range of Assembly Phenomena, Hybrid Materials, and Novel Functionalities.

Reconfiguration of amorphous complex oxides provides a readily controllable source of stress that can be leveraged in nanoscale assembly to access a broad range of 3D geometries and hybrid materials. An amorphous SrTiO3 layer on a Si:B/Si1- x Gex :B heterostructure is reconfigured at the atomic scale upon heating, exhibiting a change in volume of ≈2% and accompanying biaxial stress. The Si:B/Si1- x Gex :B bilayer is fabricated by molecular beam epitaxy, followed by sputter deposition of SrTiO3 at room temperature. The processes yield a hybrid oxide/semiconductor nanomembrane. Upon release from the substrate, the nanomembrane rolls up and has a curvature determined by the stress in the epitaxially grown Si:B/Si1- x Gex :B heterostructure. Heating to 600 °C leads to a decrease of the radius of curvature consistent with the development of a large compressive biaxial stress during the reconfiguration of SrTiO3 . The control of stresses via post-deposition processing provides a new route to the assembly of complex-oxide-based heterostructures in 3D geometry. The reconfiguration of metastable mechanical stressors enables i) synthesis of various types of strained superlattice structures that cannot be fabricated by direct growth and ii) technologies based on strain engineering of complex oxides via highly scalable lithographic processes and on large-area semiconductor substrates.

Emergence of Supramolecular Order from Combined Linear Amphiphilic and Diphosphonate Molecules.

Self-assembled molecules exhibit key functionalities for the development of novel technologies and applications. Usually, molecular systems that exhibit long-range positional order are employed in their pure form. In this work, we observe that a combination of an amphiphilic molecule, tetradecyl-phosphonic acid (TPA), and a diphosphonate molecule with a similar length, 1,10-decyldiphosphonic acid (DdPA), induces distinct long-range ordered structures depending on the relative volume of dilutions used for drop coating. Starting from 0.2 mM diluted ethanol solutions of each molecule and combining both in distinct proportions that range from 1:20 to 20:1, we were able to identify periodic molecular structures that consist of three and five molecules of TPA and DdPA arranged in symmetries and were retrieved by synchrotron X-ray diffraction. The possibility of deterministically building up such structures can be further developed to induce surface and bulk behaviors that better suit applications such as coatings for chemical and biological studies, as well as to engineer layers used in organic electronic applications.

Aggregation-Induced Emission and Temperature-Dependent Luminescence of Potassium Perylenetetracarboxylate.

Investigation of temperature-dependent photoluminescent properties of potassium perylene-3,4,9,10-tetracarboxylate (K4PTC), a molecule with no internal rotational degrees of freedom, shows aggregation-induced enhanced emission at room temperature. The different excitonic emission processes are dependent of temperature, some of which quenches in an intermediate temperature range (from 50 to 150 K). The exciton excited states switching phenomenon from "dark" to "bright" states is observed and its explained using Herzberg-Teller selection rule. K4PTC is a molecule comparable to the size of its precursor, perylene-3,4,9,10-tetracarboxylic anhydride (PTCDA) and is highly soluble in water, contrary to PTCDA, which is poorly soluble in most solvents. Powder x-ray diffraction measurements corroborate a lesser degree of ordering of bulk K4PTC compared to bulk PTCDA. The green luminescent molecule could, in principle, be used as a biomarker, or in photodynamic therapy, if further studies show relatively low toxicity.

Compression of Vectors for Small Interfering RNAs Delivery: Toward Oral Administration of siRNA Lipoplexes in Tablet Forms.

Currently, most nonviral nucleic acid vectors are in the form of colloidal suspensions administered primarily parenterally. This type of formulation and the mode of administration impose strong constraints such as the size of the administered vectors or the production of sterile preparations. The tablet form provides access to easy oral administration, well accepted by patients; As regards nucleic acid vectors, a dry form represents an advance in terms of stability. Using an optimized lipid-based small interfering RNA-delivery system, we studied the tabletability of a liquid suspension of these vectors. We optimized the conditions of freeze-drying by choosing excipients and process, allowing for the conservation of both the gene-silencing efficacy of the formulated siRNAs and the supramolecular structure of the lipid particulate system. Gene-silencing efficacy was assayed on luciferase-expressing cells and the structure of the siRNA vector in freeze-dried and tablet forms was examined using small-angle X-ray scattering (SAXS) synchrotron radiation. The freeze-dried powders were then mixed with excipients necessary for the good progress of the compression by allowing for a regular supply of the matrix and the reduction of friction. The compression was carried out using a rotary press simulator that allows for complete monitoring of the compression conditions. After compression, formulated siRNAs retained more than 60% of their gene-silencing efficacy. Within the tablets, a specific SAXS signal was detectable and the lamellar and cubic phases of the initial liquid suspension were restored after resuspension of siRNA vectors by disintegration of the tablets. These results show that the bilayer lipid structures of the particles were preserved despite the mechanical constraints imposed by the compression. If such a result could be expected after the freeze-drying step, it was never shown, to our knowledge, that siRNA-delivery systems could retain their efficacy and structure after mechanical stress such as compression. This opens promising perspectives to oral administration of siRNA as an alternative to parenteral administration.

Paclitaxel-Loaded pH-Sensitive Liposome: New Insights on Structural and Physicochemical Characterization.

A long-circulating and pH-sensitive liposome containing paclitaxel (SpHL-PTX) was recently developed by our group. Once in an acidic environment, for example, tumors, these liposomes undergo destabilization, releasing the encapsulated drug. In this way, the aim of this study was to evaluate the molecular and supramolecular interactions between the lipid bilayer and PTX in similar biological environment conditions. High-sensitivity analyses of SpHL-PTX structures were obtained by the small-angle X-ray scattering technique combined with other techniques such as dynamic light scattering, asymmetric flow field-flow fractionation, transmission electron microscopy, and high-performance liquid chromatography. The results showed that PTX incorporation in the liposomal bilayer clearly leads to changes in supramolecular organization of dioleoylphosphatidylethanolamine (DOPE) molecules, inducing the formation of more ordered structures. Changes in supramolecular organization were observed at lower pH, indicating that pH sensitivity was preserved even in the presence of fetal bovine serum proteins. Furthermore, morphological and physicochemical characterization of SpHL-PTX evidenced the formation of nanosized dispersion suitable for intravenous administration. In conclusion, a stable nanosized dispersion of PTX was obtained at pH 7.4 with suitable parameters for intravenous administration. At lower pH conditions, the pH sensitivity of the system was clearly evidenced by changes in the supramolecular organization of DOPE molecules, which is crucial for the delivery of PTX into the cytoplasm of the targeted cells. In this way, the results obtained by different techniques confirm the feasibility of SpHL as a promising tool to PTX delivery in acidic environments, such as tumors.

Observation of partial relaxation mechanisms via anisotropic strain relief on epitaxial islands using semiconductor nanomembranes.

In this work we attempt to directly observe anisotropic partial relaxation of epitaxial InAs islands using transmission electron microscopy (TEM) and synchrotron x-ray diffraction on a 15 nm thick InAs:GaAs nanomembrane. We show that under such conditions TEM provides improved real-space statistics, allowing the observation of partial relaxation processes that were not previously detected by other techniques or by usual TEM cross section images. Besides the fully coherent and fully relaxed islands that are known to exist above previously established critical thickness, we prove the existence of partially relaxed islands, where incomplete 60° half-loop misfit dislocations lead to a lattice relaxation along one of the 〈110〉 directions, keeping a strained lattice in the perpendicular direction. Although individual defects cannot be directly observed, their implications to the resulting island registry are identified and discussed within the frame of half-loops propagations.

Observation of strain-free rolled-up CVD graphene single layers: toward unstrained heterostructures.

Single layer graphene foils produced by chemical vapor deposition (CVD) are rolled with self-positioned layers of InGaAs/Cr forming compact multi-turn tubular structures that consist on successive graphene/metal/semiconductor heterojunctions on a radial superlattice. Using elasticity theory and Raman spectroscopy, we show that it is possible to produce homogeneously curved graphene with a curvature radius on the 600-1200 nm range. Additionally, the study of tubular structures also allows the extraction of values for the elastic constants of graphene that are in excellent agreement with elastic constants found in the literature. However, our process has the advantage of leading to a well-defined and nonlocal curvature. Since our curvature radius lies in a range between the large radius studied using mechanical bending and the reduced radius induced by atomic force microscopy experiments, we can figure out whether bending effects can be a majoritary driving force for modifications in graphene electronic status. From the results described in this work, one can assume that curvature effects solely do not modify the Raman signature of graphene and that strain phenomena observed previously may be ascribed to possible stretching due to the formation of local atomic bonds. This implies that the interactions of graphene with additional materials on heterostructures must be investigated in detail prior to the development of applications and devices.

Ursolic acid incorporation does not prevent the formation of a non-lamellar phase in pH-sensitive and long-circulating liposomes.

Ursolic acid (UA) is a triterpene found in different plant species that has been shown to possess significant antitumor activity. However, UA presents a low water solubility, which limits its biological applications. In this context, our research group has proposed the incorporation of UA in long-circulating and pH-sensitive liposomes (SpHL-UA).These liposomes, composed of dioleylphosphatidylethanolamine (DOPE), cholesteryl hemisuccinate (CHEMS), and distearoylphosphatidylethanolamine-polyethylene glycol2000 (DSPE-PEG2000), were shown to be very promising carriers for UA. Considering that the release of UA from SpHL-UA and its antitumor activity depend upon the occurrence of the lamellar to non-lamellar phase transition of DOPE, in the present work, the interactions of UA with the components of the liposomes were evaluated, aiming to clarify their role in the structural organization of DOPE. The study was carried out by differential scanning calorimetry (DSC) and small-angle X-ray scattering (SAXS) under low hydration conditions. DSC studies revealed that DOPE phase transition temperatures did not shift significantly upon UA addition. On the other hand, in SAXS studies, a different pattern of DOPE phase organization was observed in the presence of UA, with the occurrence of the cubic phase Im3m at 20 °C and the cubic phase Pn3m at 60 °C. These findings suggest that UA interacts with the lipids and changes their self-assembly. However, these interactions between the lipids and UA were unable to eliminate the lamellar to non-lamellar phase transition, which is essential for the cytoplasmic delivery of UA molecules from SpHL-UA.

Temperature-induced coexistence of a conducting bilayer and the bulk-terminated surface of the topological insulator Bi2Te3.

Topological insulators such as Bi2Se3 and Bi2Te3 have extremely promising transport properties, due to their unique electronic behavior: they are insulators in the bulk and conducting at the surface. Recently, the coexistence of two types of surface conducting channels has been observed for Bi2Se3, one being Dirac electrons from the topological state and the other electrons from a conventional two-dimensional gas. As an explanation for this effect, a possible structural modification of the surface of these materials has been hypothesized. Using scanning tunneling microscopy we have directly observed the coexistence of a conducting bilayer and the bare surface of bulk-terminated Bi2Te3. X-ray crystal truncation rod scattering was used to directly show the stabilization of this epitaxial bilayer which is primarily composed of bismuth. Using this information, we have performed density functional theory calculations to determine the electronic properties of the possible surface terminations. They can be used to understand recent angular resolved photoemission data which have revealed this dual surface electronic behavior.

Scanning Tunneling Spectroscopy Method for the Prediction of Semiconductor Heterojunction Performance as a Prequel for Device Development.

The prediction of semiconductor device performance is a persistent challenge in materials science, and the ability to anticipate useful specifications prior to construction is crucial for enhancing the overall efficiency. In this study, we investigate the constituents of a solar cell by employing scanning tunneling microscopy (STM) and spectroscopy (STS). Through our observations, we identify a spatial distribution of the dopant type in thin films of materials that were designed to present major p-doping for germanium sulfide (GeS) and dominant n-doping for tin disulfide (SnS2). By generating separate STS maps for each semiconductor film and conducting a statistical analysis of the gap and doping distribution, we determine intrinsic limitations for the solar cell efficiency that must be understood prior to processing. Subsequently, we fabricate a solar cell utilizing these materials (GeS and SnS2) via vapor phase deposition and carry out a characterization using standard J-V curves under both dark/illuminated irradiance conditions. Our devices corroborate the expected reduced efficiency due to doping fluctuation but exhibit stable photocurrent responses. As originally planned, quantum efficiency measurements reveal that the peak efficiency of our solar cell coincides with the range where the standard silicon solar cells sharply decline. Our STS method is suggested as a prequel to device development in novel material junctions or deposition processes where fluctuations of doping levels are retrieved due to intrinsic material characteristics such as the occurrence of defects, roughness, local chemical segregation, and faceting or step bunching.

Structural and Electronic Response of Multigap N-Doped In2Se3: A Prototypical Material for Broad Spectral Optical Devices.

The production of controlled doping in two-dimensional semiconductor materials is a challenging issue when introducing these systems into current and future technology. In some compounds, the coexistence of distinct crystallographic phases for a fixed composition introduces an additional degree of complexity for synthesis, chemical stability, and potential applications. In this work, we demonstrate that a multiphase In2Se3 layered semiconductor system, synthesized with three distinct structures─rhombohedral α and ß-In2Se3 and trigonal δ-In2Se3─exhibits chemical stability and well-behaved n-type doping. Scanning tunneling spectroscopy measurements reveal variations in the local electronic density of states among the In2Se3 structures, resulting in a compound system with electronic bandgaps that range from infrared to visible light. These characteristics make the layered In2Se3 system a promising candidate for multigap or broad spectral optical devices, such as detectors and solar cells. The ability to tune the electronic properties of In2Se3 through structural phase manipulation makes it ideal for integration into flexible electronics and the development of heterostructures with other materials.

Evidence of thickness-dependent surface-induced ferroelectricity in few-layer germanium sulfide obtained via scanning tunneling spectroscopy.

The discovery of ferroelectricity in two-dimensional van der Waals materials has sparked enormous interest from the scientific community, due to its possible applications in next-generation nanoelectronic devices, such as random-access memory devices, digital signal processors, and solar cells, among others. In the present study, we used vapor phase deposition to synthesize ultrathin germanium sulfide nano-flakes on a highly oriented pyrolytic graphite substrate. Nanostructures of variable thicknesses were characterized using scanning tunneling microscopy and spectroscopy. Tunneling currents under forward and backward biases were measured as a function of nano-flake thickness. Remarkably, we clearly observed a hysteresis pattern, which we attributed to surface ferroelectric behavior, consistent with the screening conditions of polarization charges. The effect increases as the number of layers is reduced. This experimental result may be directly applicable to miniaturized memory devices, given the two-dimensional nature of this effect.

Tuning resistive switching on single-pulse doped multilayer memristors.

Short-period multilayers containing ultrathin atomic layers of Al embedded in titanium dioxide (TiO(2)) film-here called single-pulse doped multilayers-are fabricated by atomic layer deposition (ALD) growth methods. The approach explored here is to use Al atoms through single-pulsed deposition to locally modify the chemical environment of TiO(2) films, establishing a chemical control over the resistive switching properties of metal/oxide/metal devices. We show that this simple methodology can be employed to produce well-defined and controlled electrical characteristics on oxide thin films without compound segregation. The increase in volume of the embedded Al(2)O(3) plays a crucial role in tuning the conductance of devices, as well as the switching bias. The stacking of these oxide compounds and their use in electrical devices is investigated with respect to possible crystalline phases and local compound formation via chemical recombination. It is shown that our method can be used to produce compounds that cannot be synthesized a priori by direct ALD growth procedures but are of interest due to specific properties such as thermal or chemical stability, electrical resistivity or electric field polarization possibilities. The monolayer doping discussed here impacts considerably on the broadening of the spectrum of performance and technological applications of ALD-based memristors, allowing for additional degrees of freedom in the engineering of oxide devices.

Unravelling the molecular structure and packing of a planar molecule by combining nuclear magnetic resonance and scanning tunneling microscopy.

The determination of the molecular structure of a porphyrin is achieved by using nuclear magnetic resonance (NMR) and scanning tunneling microscopy (STM) techniques. Since macroscopic crystals cannot be obtained in this system, this combination of techniques is crucial to solve the molecular structure without the need for X-ray crystallography. For this purpose, previous knowledge of the flatness of the reagent molecules (a porphyrin and its functionalizing group, a naphthalimide) and the resulting molecular structure obtained by a force-field simulation are used. The exponents of the I-V curves obtained by scanning tunneling spectroscopy (STS) allow us to check whether the thickness of the film of molecules is greater than a monolayer, even when there is no direct access to the exposed surface of the metal substrate. Photoluminescence (PL), optical absorption, infrared (IR) reflectance and solubility tests are used to confirm the results obtained here with this NMR/STM/STS combination.

Alpha-tocopheryl succinate and doxorubicin-loaded liposomes improve drug uptake and tumor accumulation in a murine breast tumor model.

Liposomes composed of a rigid bilayer have high plasma stability; however, they can be challenged in efficacy due to complications in releasing the encapsulated drug as well as being internalized by the tumor cell. On the other hand, fusogenic liposomes may fuse with the plasmatic membrane and release encapsulated material directly into the cytoplasm. In a previous study, fusogenic liposomes composed of alpha-tocopheryl succinate (TS) and doxorubicin (DOX) were developed (pHSL-TS-DOX). These stabilized tumor growth and reduced toxicity compared to a commercial formulation. In the present study, we investigated whether cellular uptake or DOX accumulation in the tumor could justify the better performance of the pHSL-TS-DOX formulation. Release, deformability, and DOX plasmatic concentration studies were also carried out. pHSL-TS-DOX showed an adequate release profile and demonstrated characteristics of a deformable formulation. Data from apoptosis, cell cycle, and nuclear morphology studies have shown that the induction of cell death caused by pHSL-TS-DOX occurred more quickly. Higher DOX cellular uptake and tumor accumulation were observed when pHSL-TS-DOX was administered, demonstrating better drug delivery capacity. Therefore, better DOX uptake as well as tumor accumulation explain the great antitumor activity previously demonstrated for this formulation.

Thermal stability and ordering study of long- and short-alkyl chain phosphonic acid multilayers.

Long-range order evolution of self-assembled phosphonic acid multilayers as a function of temperature is studied here for two molecules with different alkyl chain length. By using synchrotron conventional diffraction, distinct order configurations are retrieved on phosphonic acid multilayers and their thermodynamic behavior monitored by energy-dispersive diffraction. This later technique allows us to observe the system behavior near order-disorder temperatures, as well as to determine the most stable configurations in the range from room temperature up to 120 °C. Planar order is also addressed by wide-angle X-ray scattering (WAXS) transmission experiments. Order parameter phase diagrams are built based on the experimental results, showing the dominant configuration at each temperature. The multilayer molecular long-range order retrieved from the experiments is corroborated by first principles calculations based on the Density Functional Theory. The bulk configurations depicted in this work are produced by molecule-molecule interactions and allow for future comparisons with the behavior of ordered molecules in few-monolayers configurations, commonly used in organic devices, where the presence of surfaces and interfaces strongly affects the molecule packing.

Spheroplexes: Hybrid PLGA-cationic lipid nanoparticles, for in vitro and oral delivery of siRNA.

Vectorized small interfering RNAs (siRNAs) are widely used to induce gene silencing. Among the delivery systems used, lipid-based particles are the most effective. Our objective was the development of novel lipid-polymer hybrid nanoparticles, from lipoplexes (complexes of cationic lipid and siRNAs), and poly (lactic-co-glycolic acid) (PLGA), using a simple modified nanoprecipitation method. Due to their morphology, we called these hybrid nanoparticles Spheroplexes. We elucidated their structure using several physico-chemical techniques and showed that they are composed of a hydrophobic PLGA matrix, surrounded by a lipid envelope adopting a lamellar structure, in which the siRNA is complexed, and they retain surface characteristics identical to the starting nanoparticles, i.e. lipoplexes siRNA. We analyzed the composition of the particle population and determined the final percentage of spheroplexes within this population, 80 to 85% depending on the preparation conditions, using fluorescent markers and the ability of flow cytometry to detect nanometric particles (approximately 200 nm). Finally, we showed that spheroplexes are very stable particles and more efficient than siRNA lipoplexes for the delivery of siRNA to cultured cells. We administered spheroplexes contain siRNAs targeting TNF-α to mice with ulcerative colitis induced by dextran sulfate and our results indicate a disease regression effect with a response probably mediated by their uptake by macrophages / monocytes at the level of lamina propria of the colon. The efficacy of decreased level of TNF-α in vivo seemed to be an association of spheroplexes polymer-lipid composition and the specific siRNA. These results demonstrate that spheroplexes are a promising hybrid nanoparticle for the oral delivery of siRNA to the colon.

Mapping the local dielectric constant of a biological nanostructured system.

The aim of this work is to determine the varying dielectric constant of a biological nanostructured system via electrostatic force microscopy (EFM) and to show how this method is useful to study natural photonic crystals. We mapped the dielectric constant of the cross section of the posterior wing of the damselfly Chalcopteryx rutilans with nanometric resolution. We obtained structural information on its constitutive nanolayers and the absolute values of their dielectric constant. By relating the measured profile of the static dielectric constant to the profile of the refractive index in the visible range, combined with optical reflectance measurements and simulation, we were able to describe the origin of the strongly iridescent wing colors of this Amazonian rainforest damselfly. The method we demonstrate here should be useful for the study of other biological nanostructured systems.

Sub-diffractional cavity modes of terahertz hyperbolic phonon polaritons in tin oxide.

Hyperbolic phonon polaritons have recently attracted considerable attention in nanophotonics mostly due to their intrinsic strong electromagnetic field confinement, ultraslow polariton group velocities, and long lifetimes. Here we introduce tin oxide (SnO2) nanobelts as a photonic platform for the transport of surface and volume phonon polaritons in the mid- to far-infrared frequency range. This report brings a comprehensive description of the polaritonic properties of SnO2 as a nanometer-sized dielectric and also as an engineered material in the form of a waveguide. By combining accelerator-based IR-THz sources (synchrotron and free-electron laser) with s-SNOM, we employed nanoscale far-infrared hyper-spectral-imaging to uncover a Fabry-Perot cavity mechanism in SnO2 nanobelts via direct detection of phonon-polariton standing waves. Our experimental findings are accurately supported by notable convergence between theory and numerical simulations. Thus, the SnO2 is confirmed as a natural hyperbolic material with unique photonic properties essential for future applications involving subdiffractional light traffic and detection in the far-infrared range.