Directional Roll-up of Nanomembranes Mediated by Wrinkling

We investigate the relaxation of rectangular wrinkled thin films intrinsically containing an initial strain gradient. A preferential rolling direction, depending on wrinkle geometry and strain gradient, is theoretically predicted and experimentally verified. In contrast to typical rolled-up nanomembranes, which bend perpendicular to the longer edge of rectangular patterns, we find a regime where rolling parallel to the long edge of the wrinkled film is favorable. A non-uniform radius of the rolled-up film is well reproduced by elasticity theory and simulations of the film relaxation using a finite element method.Comment: 4 pages, 4 figure

Electronic and optical properties of quantum wells embedded in wrinkled nanomembranes

The authors theoretically investigate quantum confinement and transition energies in quantum wells (QWs) asymmetrically positioned in wrinkled nanomembranes. Calculations reveal that the wrinkle profile induces both blue- and redshifts depending on the lateral position of the QW probed. Relevant radiative transistions include the ground state of the electron (hole) and excited states of the hole (electron). Energy shifts as well as stretchability of the structure are studied as a function of wrinkle amplitude and period. Large tunable bandwidths of up to 70 nm are predicted for highly asymmetric wrinkled QWs.Comment: 3 pages, 4 figures. The following article has been submitted to Applied Physics Letters. After it is published, it will be found at http://apl.aip.or

Bending and wrinkling as competing relaxation pathways for strained free-hanging films

An equilibrium phase diagram for the shape of compressively strained free-hanging films is developed by total strain energy minimization. For small strain gradients {\Delta}{\epsilon}, the film wrinkles, while for sufficiently large {\Delta}{\epsilon}, a phase transition from wrinkling to bending occurs. We consider competing relaxation mechanisms for free-hanging films, which have rolled up into tube structures, and we provide an upper limit for the maximum achievable number of tube rotations.Comment: 4 pages, 4 figure

Rolled-up tubes and cantilevers by releasing SrRuO₃-Pr₀̣₇Ca₀̣₃MnO₃ nanomembranes

Three-dimensional micro-objects are fabricated by the controlled release of inherently strained SrRuO3/ Pr0.7Ca0.3MnO3/SrRuO3 nanometer-sized trilayers from SrTiO3(001) substrates. Freestanding cantilevers and rolled-up microtubes with a diameter of 6 to 8 μm are demonstrated. The etching behavior of the SrRuO3 film is investigated, and a selectivity of 1:9,100 with respect to the SrTiO3 substrate is found. The initial and final strain states of the rolled-up oxide layers are studied by X-ray diffraction on an ensemble of tubes. Relaxation of the sandwiched Pr0.7Ca0.3MnO3 layer towards its bulk lattice parameter is observed as the major driving force for the roll-up of the trilayers. Finally, μ-diffraction experiments reveal that a single object can represent the ensemble proving a good homogeneity of the rolled-up tubes. PACS: 81.07.-b; 68.60.-p; 68.37.Lp; 81.16.Dn

Rolled-Up Nanotech: Illumination-Controlled Hydrofluoric Acid Etching of AlAs Sacrificial Layers

<p>Abstract</p> <p>The effect of illumination on the hydrofluoric acid etching of AlAs sacrificial layers with systematically varied thicknesses in order to release and roll up InGaAs/GaAs bilayers was studied. For thicknesses of AlAs below 10 nm, there were two etching regimes for the area under illumination: one at low illumination intensities, in which the etching and releasing proceeds as expected and one at higher intensities in which the etching and any releasing are completely suppressed. The &#8220;etch suppression&#8221; area is well defined by the illumination spot, a feature that can be used to create heterogeneously etched regions with a high degree of control, shown here on patterned samples. Together with the studied self-limitation effect, the technique offers a way to determine the position of rolled-up micro- and nanotubes independently from the predefined lithographic pattern.</p

Elucidation of photovoltage origin and charge transport in Cu2O heterojunctions for solar energy conversion

Heterojunctions between p type cuprous oxide Cu2O and suitable n type layers stand out as some of the best performing and abundant oxide photoabsorbers currently available for the generation of hydrogen with photoelectrochemical cells or conversion of solar energy to electricity. In this contribution, we used drift diffusion semiconductor modeling to investigate the mechanism governing the charge transport in TiO2 Ga2O3 Cu2O and TiO2 Al ZnO Cu2O heterojunctions. The simulated photovoltage 0.9 V for TiO2 Ga2O3 Cu2O agrees well with the measured value of 1.0 V and the governing mechanism is identified to be thermionic emission of electrons across the Ga2O3 TiO2 interface. By modeling an optimized increase in Ga2O3 donor concentration, a photovoltage improvement of only 0.1 V is achievable, whereas further optimizing the electron affinity of Ga2O3 may lead to more significant improvement approaching 0.7 V. The optimized electron affinity of Ga2O3 enabled a simulated photovoltage of 1.6 V, close to the theoretical limit for Cu2O with a 2.17 eV bandgap energy. Additionally, we find that simulations can reproduce the measured photovoltage of TiO2 Al ZnO Cu2O only when an interface recombination layer at the Al ZnO Cu2O interface is included in the model. Our findings enable detailed understanding of the charge transport mechanism in Cu2O heterojunctions and offer various design directions for further photovoltage improvemen

Energy Diagram of Semiconductor/Electrolyte Junctions

In electrochemistry and photoelectrochemistry, it is most common to refer the voltage to a reference electrode (RE) or in a two-electrode cell with respect to a counterelectrode (CE).(1, 2) However, in a semiconductor film that is composed of several junctions, as in semiconductor nanoheterostructures including catalytic layers, it is convenient to use the energy scale with reference to the vacuum level (VL).(3) This type of reference in the energy diagram allows one to track the local variations of energy, electrostatic potential, and Fermi energy inside the solid, and it is the standard representation in solid-state electronics as well as in materials characterization involving ultraviolet photoelectron spectroscopy (UPS) measurements.(4, 5) In contrast, the measurement with respect to RE only takes the voltage at a point on the surface of the solid. Therefore, the relationship between the two pictures can lead to some confusion. In this Guest Commentary, we provide a consistent set of definitions and a diagram that allow one to combine both types of conventions. It is far from the scope of this paper to define the physical meaning of all quantities involved, but we do provide a set of mathematical relationships that allow us to relate the different expressions of voltage and energies such as the flatband potential and the energy (or potential) of the semiconductor conduction band at the surface

Folding Patterns in Partially Delaminated Thin Films

Michael Ortiz and Gustavo Gioia showed in the 90s that the complex patterns arising in compressed elastic films can be analyzed within the context of the calculus of variations. Their initial work focused on films partially debonded from the substrate, subject to isotropic compression arising from the difference in thermal expansion coefficients between film and substrate. In the following two decades different geometries have been studied, as for example anisotropic compression. We review recent mathematical progress in this area, focusing on the rich phase diagram of partially debonded films with a lateral boundary condition