Nanomaterial Design
Magnetic microhelix coil structuresWe 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.E. J. Smith et al., Physical Review Letters 107, 097204 (2011) URL PDF | ||
Cardboard rolls on the nanoscaleEverybody 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.P. Cendula et al., Nano Letters 11, 236 (2011) URL PDF | ||
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.F. Ding et al., Nano Letters 10, 3453 (2010) URL PDF | ||
Direct laser writing of nanoscale light-emitting diodesWe 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.O. Makarovsky et al., Advanced Materials 22, 3176 (2010) URL PDF | ||
 | Electromechanical tuning of quantum dot emission energiesElastic 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.F. Ding et al., Physical Review Letters 104, 067405 (2010) URL PDF | |
 | Epitaxial quantum dots in stretchable optical microcavitiesArrays 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.T. Zander et al., Optics Express 17, 22452 (2009) URL PDF | |
 | Tuneable electronic shell structure of GaAs quantum dotsSelf-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.L. Wang et al., Physical Review B 80, 085309 (2009) URL PDF
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 | Ultrathin AlN/GaN nanomembranesScanning 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 moleculesSelf-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.L. Wang et al., Advanced Materials 21, 2601 (2009) URL PDF | |
From wrinkling to rollingWe 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.P. Cendula et al., Physical Review B 79, 085429 (2009) URL PDF | ||
New planar hybrid heterostructures and superlatticesWe 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.T. Zander et al., Applied Physics Letters 94, 053102 (2009) URL PDF | ||
Self-organisation into nanochannel networksHighly 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.A. Malachias et al., ACS Nano 2: 1715 (2008) URL PDF This work was also highlighted in: Lab on a Chip 8, 1979 (2008) URL PDF | ||
Nanochannel arrays by deterministic layer wrinklingWe 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.Y. F. Mei et al., Advanced Materials 19, 2124 (2007) URL PDF | ||
Semiconductor/metal radial superlatticesStrained 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.Ch. Deneke et al., Applied Physics Letters 90, 263107 (2007) URL PDF | ||
Quantum wells illuminate the strain state of nanomembranesThe 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.Y. F. Mei et al., Nano Letters 7, 1676 (2007) URL PDF | ||
Perfectly resonant quantum dotsLaser 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.A. Rastelli et al., Applied Physics Letters 90, 073120 (2007) URL PDF | ||
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