Manipulating photon−plasmon coupling in microtube cavities
In situ generation of silver nanoparticles for selective coupling between localized plasmonic resonances and whispering-gallery modes is demonstrated by spatially resolved laser dewetting on microtube surfaces. Our work provides a convenient way to manipulate photon-plasmon coupling in three-dimensional micro-cavities, which is of interest for optical tuning abilities and control of enhanced light-matter interaction.
Chemotherapeutic micromotor for targeted drug delivery
A sperm-driven micromotor acts as a targeted drug delivery system to potentially treat diseases in the female reproductive tract. More precisely, the sperm cell of the micromotor system is first loaded with an anticancer drug (DOX-HC). Then the system is guided by a synthetic magnetic harness to an in-vitro cultured tumor spheroid. Finally, the drug is delivered locally into the tumour once the sperm cell is freed by an integrated mechanical release mechanism.
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Cancer treatment by dual-action biogenic microdaggers
We present dual-action biogenic microbots with a dagger-like morphology for cellular microsurgery and drug delivery. These biocompatible micromotors allow magnetically controlled drilling into a target cell along with the sustained release of a loaded drug. This study highlights “selective targeting and destruction” of harmful cells in living systems and advances the understanding of microscale interactions at the cellular level.
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High performance tubular GMI sensors
Arrays of rolled-up on-chip-integrated giant magneto-impedance (GMI) sensors equipped with pick-up coils are demonstrated. The geometrical transformation of an initially planar layout into a tubular 3D architecture stabilizes favorable azimuthal magnetic domain patterns. This work creates a solid foundation for further development of CMOS compatible GMI sensorics for magnetoencephalography.
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Ultra-compact helical antennas for smart system implants
Ultra-compact helical antennas with a total length five times smaller compared to their conventional dipole counterparts are demonstrated to operate in the Industry-Scientific-Medical radio band. The antennas can be implanted by standard medical syringes and inter-antenna as well as antenna-smartphone communication is reported. Our work highlights the potential of helical antennas for medical applications as components of smart system implants.
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- Surgical Tribune (November 2015) URL
Magnetism in Möbius rings
In physics of nanomagnetism curvature-induced magnetic effects are subject of exciting current topical studies. In this framework we demonstrate that the magnetic Möbius ring is a unique object because it unites two classes of geometrical effects, namely, topologically induced magnetization patterning and curvature induced chirality symmetry breaking. With this work we complete the broad theoretical studies of various physical phenomena related to the Mobius geometry by including magnetic phenomena. This work was carried out in close collaboration with the Taras Shevchenko National University and the Bogolyubov Institute for Theoretical Physics in Kiev, Ukraine.
Domain patterns of magnetic rolled-up microtubes
Rolled-up magnetic microtubes display spiral-like, longitudinally or azimuthally magnetized domain patterns. The rolled-up geometry offers an elegant possibility for tailoring the fundamental magnetic interactions at the nanoscale in three-dimensions. The novel magnetic-domain patterns have a strong impact on their magnetic response and transport properties and could be attractive for future magnetoimpedance-based field sensors.
Stretching quantum dots till they become light
We demonstrate the creation of a light-hole exciton ground state by applying elastic stress to an initially unstrained quantum dot. The signature is clearly distinct from that of the well-known heavy-hole exciton and consists of three orthogonally polarized bright optical transitions and a fine-structure splitting of hundreds of microelectron volts between in-plane and out-of-plane components. Our work paves the way for the exploration of the fundamental properties of three-dimensionally confined light-hole states in quantum technologies and was carried out in collaboration with the Johannes Kepler Universität in Linz, the Kavli Institute at the TU Delft and the Max-Planck Institute for Solid State Research in Stuttgart.
Y.H. Huo et al., Nat. Phys. 10, 46 (2014) URL PDF
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Making quantum dots electronically symmetric
The lack of structural symmetry which usually characterizes semiconductor quantum dots lifts the energetic degeneracy of the bright excitonic states and hampers severely their use as high-fidelity sources of entangled photons. We demonstrate experimentally and theoretically that it is always possible to restore the excitonic degeneracy by the simultaneous application of large strain and electric fields. This is achieved by using one external perturbation to align the polarization of the exciton emission along the axis of the second perturbation, which then erases completely the energy splitting of the states. This result, which holds for any quantum dot structure, highlights the potential of combining complementary external fields to create artificial atoms meeting the stringent requirements posed by scalable semiconductor-based quantum technology.
This work was chosen as PRL Editors' suggestion and selected for a Viewpoint in Physics (October 1, 2012) URL
Self - Elongating Nanowires
We report the self-assembled growth of Ge nanowires with a height of only 3 unit cells and a length of up to 2 micrometers by means of molecular beam epitaxy. Compared to nanowires grown by catalytic methods, the catalyst-free Ge nanowires we obtained exhibit an outstanding uniformity in their lateral size, they lie horizontally along well-defined crystallographic directions, and they are monolithically integrated into the silicon substrate. In view of their exceptionally small and self-defined cross section, these Ge wires hold promise for the realization of hole systems with exotic properties and provide a new development route for silicon-based nanoelectronics.
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Magnetic microhelix coil structures
We design and investigate three-dimensional microhelix coil structures that are radial-, corkscrew-, and hollow-bar-magnetized. The magnetization configurations of the differently magnetized coils are experimentally revealed by probing their specific dynamic response to an external magnetic field. Helix coils offer an opportunity to realize microscale geometries of the magnetic toroidal moment, observed so far only in bulk multiferroic materials.
Cardboard rolls on the nanoscale
Everybody knows that cardboard paper can be a highly anisotropic material. You can easily bend or roll it in one direction and it is stiff in the other. If you take a close look you will find that the paper is periodically buckled along one direction. We have now exploited this phenomenon on the nanoscale to define the roll-up direction of ultra-thin membranes on a substrate surface. Given the abundance of fabrication methods to create thin corrugated films (including graphene), our work will help to realize novel 3D tubular nanostructures with well-controlled position, orientation, material composition, and exciting functionalities.
Stretch graphene, get more details...
We are continuously expanding the knowledge of how controllable external stresses, as a basic physical technique, modify the properties and unveil interesting physics of nanomaterials. Graphene, a one atom thick carbon sheet, can be studied in more details with our recently developed piezoelectric-actuator based technique. Controllable biaxial stress, which does not change the relative positions of the Dirac cones, is applied to graphene. The key mechanical characteristics of graphene, the Grueneisen parameters, are extracted from Raman spectroscopy. We also observe that the frequency of the 2D peak is not exactly twice that of the D peak, as predicted previously by theory. The appealing feature of our technique is that it allows exerting strain on demand, which promises new opportunities to study the strain-related behaviors of graphene with unprecedented details.
Direct laser writing of nanoscale light-emitting diodes
We have fabricated sub-micrometer light emitting diodes (LEDs) in a mesoscopic semiconductor structure by means of a focused laser beam. The local heating produced by the beam allows spatially controlled diffusion of mobile interstitial manganese ions out of a GaMnAs layer towards an underlying quantum well heterostructure. This activates a nanoscale region of the LED to emit light at a bias well below the threshold voltage for emission from the non-annealed regions. The technique,which provides real-time in-situ control of the nanostructures during their formation, may represent an alternative to deep etching for defining narrow current channels in mesoscopic devices.
Electromechanical tuning of quantum dot emission energies
Elastic mechanical strain is a powerful control tool for engineering the electronic states in quantum dots. With a simple electro-mechanical device we apply in-plane biaxial stress to a 200-nm-thick GaAs membrane containing InAs quantum dots. The relative energy levels of the exciton and biexciton states can be tuned to emit photons with exactly the same color. This observation may lead to the implementation of a recently proposed concept for the generation of entangled photon pairs. The strain tuning technique adds a new degree of freedom to the field of semiconductor nanostructures, and may inspire exciting future experiments in other fields.
Epitaxial quantum dots in stretchable optical microcavities
Arrays of GaAs microring optical resonators with embedded quantum dots are placed on top of piezoelectric actuators, which allow the microcavities to be reversibly “stretched” or “squeezed” by applying relatively large biaxial stresses at low temperatures. The emission energy of both QDs and optical modes red- or blue- shift depending on the strain sign, with the QD emission shifting more rapidly than the optical mode with applied strain. Remarkably, excitonic emissions from different QDs are observed to shift at different rates, implying that this technique can be used to bring spatially separated excitons into resonance.
Tuneable electronic shell structure of GaAs quantum dots
Self-assembled quantum dots (QDs) usually form on top of a thin planar wetting layer (WL), whose properties can be hardly tuned independently from those of the QDs. We now studied QDs for which the WL thickness can be arbitrarily controlled. For fixed QD shape, a systematic decrease in the energy separation between ground and excited states of QDs is observed when the WL thickness is increased. This rather surprising phenomenon can be seen as a cross talk between QD vertical and lateral confinement potential.
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Ultrathin AlN/GaN nanomembranes
Scanning electron micrographs of ultrathin AlN/GaN nanomembranes self-assembled into various geometries such as tubes, spirals, and curved sheets on Si(111). These freestanding structures contain nanopores with sizes from several to tens of nanometers within nanomembranes of 20−35 nm nominal thickness and can find application in molecular separation or artificial blood capillaries.
Y. F. Mei et al., ACS Nano 3, 1663 (2009) URL PDF
Self-assembled quantum dot molecules
Self-assembled semiconductor quantum dot molecules (QDMs) obtained by epitaxial growth are reviewed. A comprehensive overview of the development and current stage of the research on QDMs composed of vertically (in the growth direction) or laterally (in the growth plane) aligned QDs is provided. The cover shows a 2D photoluminescence intensity map from a self-assembled lateral QDM in an electric field applied along the molecular axis. The coupling of the two QDs is evidenced by intricate spectral line anticrossings, indicated by dotted lines.
From wrinkling to rolling
We have explored the change-over from wrinkling to rolling for compressively strained thin solid films. For small strain gradients across the film thickness the layer wrinkles whereas for large strain gradients it rolls up into a nanotube. Our theory provides an upper limit for the maximum achievable rotations of the film and is therefore of uttermost importance for many applications such as on-chip self-wound capacitors and coils.
New planar hybrid heterostructures and superlattices
We have invented an entirely new approach to create hybrid material layer stacks, which cannot be produced by any other technology. Hybrid layers are rolled up into a multi-winding tube on a substrate surface, and subsequently pressed down into a planar geometry. This leads to a hybrid superlattice out of single crystalline semiconductors and polycrystalline metals.
Self-organisation into nanochannel networks
Highly ordered semiconductor nanochannel networks are fabricated using a combination of standard optical lithography and a self-organization of a pre-stressed nanomembrane. The "patchwork" shown here demonstrates the ability to tune the channel density and periodicity by varying both intrinsic and extrinsic parameters of the material and the lithographic process. Such self-organized nanochannel networks could be useful as nanofluidic devices in laboratory-on-a-chip applications.
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Nanochannel arrays by deterministic layer wrinkling
We have developed a method that exploits the deterministic wrinkling and a subsequent bond-back of a semiconductor layer to create well-defined and versatile nanochannel networks. In these networks, the periodicity and the positions of the branch channels can be tuned and controlled by changing the width of the partially released layers and by applying appropriate lithography. We demonstrate nanofluidic transport as well as femto-litre filling and emptying of individual wrinkles on a standard semiconductor substrate. The technique is compatible to advanced Si and CMOS technology, and the top wrinkling layer can easily be made optically or electronically active.
Semiconductor/metal radial superlattices
Strained rolled-up heterostructures allow for the creation of radial superlattices incorporating crystalline and non-crystalline material. These superlattices can be used in new functionalities including flexible optical ring resonators and x-ray waveguides. In this letter, we explore mainly semiconductor/oxide/metal and semiconductor/metal radial superlattices. An investigation of the radial structure using transmission electron microscopy clearly shows a periodic layering of semiconductor material and metal, while a chemical analysis reveals the detailed layer structure.
Quantum wells illuminate the strain state of nanomembranes
The strain state of a deterministically wrinkled nanomembrane has been accurately analyzed by incorporating an embedded quantum well into the layer and subsequently using micro-photoluminescence (μ-PL) spectroscopy to investigate the shift of the transition energy. The investigations reveal that while the bent nanomembrane exhibits a shifted transition energy, the bonded back layer displays a fully relaxed state. An enhancement of the light emission is found in the wrinkled areas and is well explained by interference contrast theory.
Perfectly resonant quantum dots
Laser processing is used as a post-growth method to tune, within a broad spectral range and with resolution-limited accuracy, the confined energy states of single quantum dots (QDs). The same laser is used as a heat source (at high power) and as a probe (at low power) to controllably blue-shift the emission of selected QDs and immediately control the result of the processing. The method, which can be virtually applied to any material system, opens the way to the fabrication of QDs with identical emission energies.