Oliver G. Schmidt

Nanotechnology. Thin solid films roll up into nanotubes.

The rigorous size miniaturization of nanotechnology is continually generating new applications and new physical effects. We show here that nanotubes can be formed from thin solid films of almost any material at almost any position, once these films are released from their substrate. This exceptional design flexibility has useful implications, including for fluid transportation and capillarity on the nanometre scale, as well as offering the opportunity to extend fundamental investigations to a new diversity of materials, material systems and geometries.

Catalytic Microtubular Jet Engines Self‐Propelled by Accumulated Gas Bubbles

Strain-engineered microtubes with an inner catalytic surface serve as self-propelled microjet engines with speeds of up to approximately 2 mm s(-1) (approximately 50 body lengths per second). The motion of the microjets is caused by gas bubbles ejecting from one opening of the tube, and the velocity can be well approximated by the product of the bubble radius and the bubble ejection frequency. Trajectories of various different geometries are well visualized by long microbubble tails. If a magnetic layer is integrated into the wall of the microjet engine, we can control and localize the trajectories by applying external rotating magnetic fields. Fluid (i.e., fuel) pumping through the microtubes is revealed and directly clarifies the working principle of the catalytic microjet engines.

Cu-Si nanocable arrays as high-rate anode materials for lithium-ion batteries.

There is a surge in developing rechargeable lithium-ion batteries (LIBs) with higher energy densities and higher rate performance for application in powering future advanced communications equipment and electric vehicles (EVs). [ 1–6 ] The development of the electrode materials is essential for the improvement of the electrochemical properties of LIBs. [ 7–10 ] Among various anode materials tested for LIBs, Si has triggered signifi cant research effort because of its low Li-uptake potential and the high theoretical capacity (4200 mA h g − 1 ). [ 6 , 11–19 ] However, the main disadvantage that restricts the application of Si is the large volume changes of Si during Li + insertion and extraction, which results in a pulverization of the Si particles, a peeling off the current connection network, and, consequently, a rapid capacity decline upon cycling. [ 11–17 ] To overcome this issue, Si nanostructures, such as Si nanowires and nanotubes, have been fabricated. [ 6 , 11 , 18–23 ] The procedures for the fabrication of the Si nanostructures have also been well developed. [ 24–26 ] These nanostructures can provide spaces to accommodate the large volume variation during charge and discharge processes and thus allow for facile strain relaxation, which prevents pulverization upon lithium insertion. [ 11–19 , 27 ] The cycle stability of the Si anode has been signifi cantly improved by using these nanostructures. [ 11–17 , 27 ] Nevertheless, the rate capability of these materials highly needed for EVs is still not satisfying. This is possibly due to the lack of favorable electronic conductivity and the continuous growth of the unstable solid electrolyte interphase (SEI) at the Si/electrolyte interface upon cycling. Therefore, a new design for the structure of the Si anode is in high demand to achieve both longer cycling life and higher rate capability. Our previous work suggested that the application of nanocable structures in LIBs electrodes can signifi cantly improve the batteries’ electrochemical performance, especially the high

Self-propelled micromotors for cleaning polluted water.

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.

Catalytic Janus motors on microfluidic chip: deterministic motion for targeted cargo delivery.

We fabricated self-powered colloidal Janus motors combining catalytic and magnetic cap structures, and demonstrated their performance for manipulation (uploading, transportation, delivery) and sorting of microobjects on microfluidic chips. The specific magnetic properties of the Janus motors are provided by ultrathin multilayer films that are designed to align the magnetic moment along the main symmetry axis of the cap. This unique property allows a deterministic motion of the Janus particles at a large scale when guided in an external magnetic field. The observed directional control of the motion combined with extensive functionality of the colloidal Janus motors conceptually opens a straightforward route for targeted delivery of species, which are relevant in the field of chemistry, biology, and medicine.

Self-Propelled Nanotools

We describe nanoscale tools in the form of autonomous and remotely guided catalytically self-propelled InGaAs/GaAs/(Cr)Pt tubes. These rolled-up tubes with diameters in the range of 280-600 nm move in hydrogen peroxide solutions with speeds as high as 180 μm s(-1). The effective transfer of chemical energy to translational motion has allowed these tubes to perform useful tasks such as transport of cargo. Furthermore, we observed that, while cylindrically rolled-up tubes move in a straight line, asymmetrically rolled-up tubes move in a corkscrew-like trajectory, allowing these tubes to drill and embed themselves into biomaterials. Our observations suggest that shape and asymmetry can be utilized to direct the motion of catalytic nanotubes and enable mechanized functions at the nanoscale.

Precise control of thermal conductivity at the nanoscale through individual phonon-scattering barriers

The ability to precisely control the thermal conductivity (kappa) of a material is fundamental in the development of on-chip heat management or energy conversion applications. Nanostructuring permits a marked reduction of kappa of single-crystalline materials, as recently demonstrated for silicon nanowires. However, silicon-based nanostructured materials with extremely low kappa are not limited to nanowires. By engineering a set of individual phonon-scattering nanodot barriers we have accurately tailored the thermal conductivity of a single-crystalline SiGe material in spatially defined regions as short as approximately 15 nm. Single-barrier thermal resistances between 2 and 4 x 10(-9) m(2) K W(-1) were attained, resulting in a room-temperature kappa down to about 0.9 W m(-1) K(-1), in multilayered structures with as little as five barriers. Such low thermal conductivity is compatible with a totally diffuse mismatch model for the barriers, and it is well below the amorphous limit. The results are in agreement with atomistic Greens function simulations.

Dynamics of Biocatalytic Microengines Mediated by Variable Friction Control

We describe the motion of self-propelled hybrid microengines containing catalase enzyme covalently bound to the cavity of rolled-up microtubes. The high efficiency of these hybrid microengines allows them to move at a very low concentration of peroxide fuel. The dynamics of the catalytic engines is mediated by the generation of front-side bubbles, which increase the drag force and make them turn. The specific modification of the inner layer of microtubes with biomolecules can lead to other configurations to generate motion from different chemical fuels.

Formation of carbon-induced germanium dots

A very small amount of pre-deposited C on a Si substrate causes island formation after epitaxial growth of less than 2 monolayers Ge. These C-induced Ge dots can be as small as 10 nm in lateral size and 1 nm in height. Their areal density is 1011 cm−2. Intense photoluminescence signal from these small Ge quantum dots is observed reaching a maximum for 2.1±0.3 monolayers of Ge. In the initial stages of island formation, the optical transition of the wetting layer is blue-shifted by strain compensation effects. We propose spatially indirect mechanisms of radiative recombination between electrons confined in the underlying wetting layer and holes confined in the Ge islands.A very small amount of pre-deposited C on a Si substrate causes island formation after epitaxial growth of less than 2 monolayers Ge. These C-induced Ge dots can be as small as 10 nm in lateral size and 1 nm in height. Their areal density is 1011 cm−2. Intense photoluminescence signal from these small Ge quantum dots is observed reaching a maximum for 2.1±0.3 monolayers of Ge. In the initial stages of island formation, the optical transition of the wetting layer is blue-shifted by strain compensation effects. We propose spatially indirect mechanisms of radiative recombination between electrons confined in the underlying wetting layer and holes confined in the Ge islands.

Microbots Swimming in the Flowing Streams of Microfluidic Channels

We describe the motion of self-propelled catalytic Ti/Fe/Pt rolled-up microtubes (microbots) in the microchannels of a microfluidics system. Their motion is precisely controlled by a small magnetic field, and the transport of multiple spherical microparticles into desired locations is achieved. The microbots are powerful enough to propel themselves against flowing streams. The integration of “smart and powerful” microbots into microchip systems can lead to multiple lab-on-a-chip functions such as separation of cells and biosensing.