We describe the use of catalytically self-propelled microjets (dubbed micromotors) for degrading organic pollutants in water via the Fenton oxidation process. The tubular micromotors are composed of rolled-up functional nanomembranes consisting of Fe/Pt bilayers. The micromotors contain double functionality within their architecture, i.e., the inner Pt for the self-propulsion and the outer Fe for the in situ generation of ferrous ions boosting the remediation of contaminated water.The degradation of organic pollutants takes place in the presence of hydrogen peroxide, which acts as a reagent for the Fenton reaction and as main fuel to propel the micromotors. Factors influencing the efficiency of the Fenton oxidation process, including thickness of the Fe layer, pH, and concentration of hydrogen peroxide, are investigated. The ability of these catalytically self-propelled micromotors to improve intermixing in liquids results in the removal of organic pollutants ca. 12 times faster than when the Fenton oxidation process is carried out without catalytically active micromotors. The enhanced reaction–diffusion provided by micromotors has been theoretically modeled. The synergy between the internal and external functionalities of the micromotors, without the need of further functionalization, results into an enhanced degradation of nonbiodegradable and dangerous organic pollutants at small-scale environments and holds considerable promise for the remediation of contaminated water.

There is a great interest in reducing the toxicity of the fuel used to self-propel artificial nanomachines. Therefore, a method to increase the efficiency of the conversion of chemicals into mechanical energy is desired. Here, we employed temperature control to increase the efficiency of microjet engines while simultaneously reducing the amount of peroxide fuel needed. At physiological temperatures, i.e. 37 °C, only 0.25% H(2)O(2) is needed to propel the microjets at 140 μm s(-1), which corresponds to three body lengths per second. In addition, at 5% H(2)O(2), the microjets acquire superfast speeds, reaching 10 mm s(-1). The dynamics of motion is altered when the speed is increased; i.e., the motion deviates from linear to curvilinear trajectories. The observations are modeled empirically.

No abstract

We report the direct measurement of the persistent current carried by a single electron by means of magnetization experiments on self-assembled InAs=GaAs quantum rings. We measured the first Aharonov-Bohm oscillation at a field of 14 T, in perfect agreement with our model based on the structural properties determined by cross-sectional scanning tunneling microscopy measurements. The observed oscillation magnitude of the magnetic moment per electron is remarkably large for the topology of our nanostructures, which are singly connected and exhibit a pronounced shape asymmetry. DOI: 10.1103/PhysRevLett.99.146808 PACS numbers: 73.21.La, 73.23.Ra, 78.67.Hc In quantum mechanics, particular attention is paid to phenomena occurring due to the phase coherence of charge carriers in doubly connected (ring) topologies. Electrons confined to a submicron ring manifest a topologically determined quantum-interference phenomenon, known as the Aharonov-Bohm (AB) effect [1], as a result of the oscillatory behavior of their energy levels as a function of an applied magnetic field. This behavior is usually associated with the occurrence of oscillatory persistent currents in the ring [2 -4]. Experimental evidence for AB oscillations has been detected in the mesoscopic regime in metallic [5,6] and semiconducting [7,8] rings, containing many electrons. We address the occurrence of the AB effect in defect-free self-assembled semiconductor nanostructures [9][10][11][12][13]. The ability to fill nanostructures with only a few (1-2) electrons offers the unique possibility to detect magnetic field induced oscillations in the persistent current carried by single electron states. We report the first direct measurement by means of ultrasensitive magnetization experiments of the oscillatory persistent current carried by a single electron in self-assembled InAs/GaAs ''volcanolike'' nanostructures. Remarkably, this single electron current occurs even in the absence of an opening [14] in our nanostructures, which is required for the AB effect in the standard treatment [1]. The magnetic field at which the first oscillation in the magnetic moment arises is much higher than expected from the diameter of the quantum rings as determined by atomic force microscopy [13]. However, the experiments are in good agreement with a model based on the structural parameters as determined with cross-sectional scanning tunneling microscopy (XSTM) measurements.The persistent current was determined via the magnetic moment of electrons in a highly homogeneous ensemble of InAs self-assembled nanostructures. The sample was grown by molecular beam epitaxy and contains 29 mutually decoupled periods [ Fig. 1(a)] [15]. Each period consists of a nanostructured InAs layer, between two 24 nm GaAs layers, and a 2 nm doped (7 10 16 cm ÿ3 Si) GaAs layer that provides electrons to the InAs nanostructures. We used a one-dimensional Poisson solver [16] to estimate the average number of electrons per nanostructure to be about 1.5. Considering the two possible spin orientations we ...

Using materials with properties similar to those of cells and microorganisms together with innovative fabrication methods, soft and smart microrobots can be developed, with increased adaptability and flexibility toward in vivo applications. These tiny robots are designed to carry out difficult tasks such as noninvasive microsurgery, diagnosis and therapy in complex environments, including viscous media and intricate channels. Moreover, the novel property of the soft materials to respond to stimuli has paved the way for the creation of reconfigurable and smart microrobots with both actuation and function (e.g., sensing, drug delivery) capabilities. This feature article aims to give an overview of the different soft and smart swimming microrobots (less than 1 mm in all dimensions), highlighting some aspects of new materials, their development and the challenges in their processing to obtain highly functional microrobots.

Offermans, P.; Koenraad, P.M.; Wolter, J.H.; Granados, D.; Garcia, J.M.; Fomin, V.; Gladilin, V.N.; Devreese, J.T.

Together with the well-known ferro- and antiferromagnetic ordering, nature has created a variety of complex helical magnetic configurations. Here, 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.

We derive a nonsymmetrized 8-band effective-mass Hamiltonian for quantumdot heterostructures (QDHs) in Burt's envelope-function representation. The 8×8 radial Hamiltonian and the boundary conditions for the Schrödinger equation are obtained for spherical QDHs. Boundary conditions for symmetrized and nonsymmetrized radial Hamiltonians are compared with each other and with connection rules that are commonly used to match the wave functions found from the bulk k ·p Hamiltonians of two adjacent materials. Electron and hole energy spectra in three spherical QDHs: HgS/CdS, InAs/GaAs, and GaAs/AlAs are calculated as a function of the quantum dot radius within the approximate symmetrized and exact nonsymmetrized 8×8 models. The parameters of dissymmetry are shown to influence the energy levels and the wave functions of an electron and a hole and, consequently, the energies of both intraband and interband transitions.PACS numbers: 73.20. Dx, 73.40.Kp, 73.40.Lq This model explicitly includes eight bands around the Γ point of the Brillouin zone, namely, electron, heavy-, light-, and spin-orbit split-off hole bands (each of them is twice-degenerate due to the spin), and treats all other bands as remote. Along with more simple models, the 8×8 k ·p Hamiltonian has been used to investigate different QDs (see, e. g. .Recently, one has begun to apply multiband effective-mass Hamiltonians to investigate elastic, electronic, and optical properties of multilayer nanostructures such as quantumdot heterostructures (QDHs): CdS/HgS 11 , InAs/GaAs 12,13 , GaAs/Al x Ga 1−x As 14,15 , and CdS/HgS/CdS/H 2 O 16,17 . However, it should be emphasized, that multiband k ·p Hamiltonians are derived for homogeneous bulk materials, i.e. under the assumption that all effective-mass parameters are constant. This is important, because at a certain step of the derivation, wavenumbers k are declared as operatorsp/h that do not commute with the functions of coordinates. But, at the heterointerfaces of the multilayer nanostructures, there occurs an abrupt change of effective-mass parameters from their values in one material to those in the adjacent material. Inside a thin transitional layer that contains the heterointerface, the ordering of the differential operators and coordinate-dependent effective-mass parameters in the multiband Hamiltonian becomes crucial. In QDs with an infinitely high confining potential for electrons and holes, all components of the wave function vanish at the heterointerface, and there remains a possibility of applying the bulk multiband k ·p Hamiltonian straightforwardly. [3][4][5][6][8][9][10][11] There are two ways to proceed from QDs to QDHs.(i) The first way is to use an appropriate bulk multiband Hamiltonian for each constituent material separately, and then to match the obtained homogeneous solutions at the abrupt heterojunctions applying the connection rules (CRs) that are usually obtained by imposing the continuity of the wave function envelopes and of the normal to the heterointerface component of the velocity. 11,16...