Hauke Lehmann

Metal Domain Size Dependent Electrical Transport in Pt-CdSe Hybrid Nanoparticle Monolayers

Thin films prepared of semiconductor nanoparticles are promising for low-cost electronic applications such as transistors and solar cells. One hurdle for their breakthrough is their notoriously low conductivity. To address this, we precisely decorate CdSe nanoparticles with platinum domains of one to three nanometers in diameter by a facile and robust seeded growth method. We demonstrate the transition from semiconductor to metal dominated conduction in monolayered films. By adjusting the platinum content in such solution-processable hybrid, oligomeric nanoparticles the dark currents through deposited arrays become tunable while maintaining electronic confinement and photoconductivity. Comprehensive electrical measurements allow determining the reigning charge transport mechanisms.

Solution-Grown Nanowire Devices for Sensitive and Fast Photodetection

Highly sensitive and fast photodetector devices with CdSe quantum nanowires as active elements have been developed exploiting the advantages of electro- and wet-chemical routes. Bismuth nanoparticles electrochemically synthesized directly onto interdigitating platinum electrodes serve as catalysts in the following solution-liquid-solid synthesis of quantum nanowires directly on immersed substrates under mild conditions at low temperature. This fast and simple preparation process leads to a photodetector device with a film of nanowires of limited thickness bridging the electrode gaps, in which a high fraction of individual nanowires are electrically contacted and can be exposed to light at the same time. The high sensitivity of the photodetector device can be expressed by its on/off ratio or its photosensitivity of more than 10(7) over a broad wavelength range up to about 700 nm. The specific detectivity and responsivity are determined to D* = 4 × 10(13) Jones and R = 0.32 A/W, respectively. The speed of the device reflects itself in a 3 dB frequency above 1 MHz corresponding to rise and fall times below 350 ns. The remarkable combination of a high sensitivity and a fast response is attributed to depletion regions inside the nanowires, tunnel-junction barriers between nanowires, and Schottky contacts at the electrodes, where all of these features are strongly influenced by the number of photogenerated charge carriers.

Synthesis and Characterization of Monodisperse Metallodielectric SiO2@Pt@SiO2 Core-Shell-Shell Particles.

Metallodielectric nanostructured core-shell-shell particles are particularly desirable for enabling novel types of optical components, including narrow-band absorbers, narrow-band photodetectors, and thermal emitters, as well as new types of sensors and catalysts. Here, we present a facile approach for the preparation of submicron SiO2@Pt@SiO2 core-shell-shell particles. As shown by transmission and scanning electron microscopy, the first steps of this approach allow for the deposition of closed and almost perfectly smooth platinum shells onto silica cores via a seeded growth mechanism. By choosing appropriate conditions, the shell thickness could be adjusted precisely, ranging from ∼3 to ∼32 nm. As determined by X-ray diffraction, the crystalline domain sizes of the polycrystalline metal shells were ∼4 nm, regardless of the shell thickness. The platinum content of the particles was determined by atomic absorption spectroscopy and for thin shells consistent with a dense metal layer of the TEM-measured thickness. In addition, we show that the roughness of the platinum shell strongly depends on the storage time of the gold seeds used to initiate reductive platinum deposition. Further, using polyvinylpyrrolidone as adhesion layer, it was possible to coat the metallic shells with very homogeneous and smooth insulating silica shells of well-controlled thicknesses between ∼2 and ∼43 nm. After depositing the particles onto silicon substrates equipped with interdigitated electrode structures, the metallic character of the SiO2@Pt particles and the insulating character of the SiO2 shells of the SiO2@Pt@SiO2 particles were successfully demonstrated by charge transport measurements at variable temperatures.

Quantized conductance and evidence for zitterbewegung in InAs spin filters

We present measurements of the electron transport in top-gated InAs spin-filter cascades. The cascades consist of a first filter stage that acts as a polarizer, a center wire, and a second filter stage that acts as an analyzer. Conductance quantization indicates quasi-ballistic transport in these rather large devices. Oscillations of the conductances of the second filters outputs with the strength of an in-plane magnetic field perpendicular to the center wire provide evidence of the so-called zitterbewegung and substantiate the interpretation of the conductance imbalance at the second filter as the consequence of a spin polarization.

Direct current-biased InAs spin-filter cascades

We study dc-biased spin-transport in InAs two-stage spin-filter cascades. The cascades allow all-electrical generation and detection of spin-polarized currents in an all-semiconductor device. The application of a dc bias simplifies the interpretation of the experimental results, enhances the signal-to-noise ratio, and paves the way for more definite measurements in magnetic fields.

Attachment of Colloidal Nanoparticles to Boron Nitride Nanotubes

There is a strong interest to attach nanoparticles noncovalently to one-dimensional systems like boron nitride nanotubes to form composites. The combination of those materials might be used for catalysis, in solar cells, or for water splitting. Additionally, the fundamental aspect of charge transfer between the components can be studied in such systems. We report on the synthesis and characterization of nanocomposites based on semiconductor nanoparticles attached directly and noncovalently to boron nitride nanotubes. Boron nitride nanotubes were simply integrated into the colloidal synthesis of the corresponding nanoparticles. With PbSe, CdSe, and ZnO nanoparticles, a wide range of semiconductor band gaps from the near-infrared to the ultraviolet range was covered. A high surface coverage of the boron nitride nanotubes with these semiconducting nanoparticles was achieved, while it was found that a similar in situ approach with metallic nanoparticles does not lead to proper attachment. In addition, possibl...

Point contact Andreev spectroscopy of epitaxial Co2FeSi Heusler alloys on GaAs (001)

The predicted half-metallicity of Co2FeSi in combination with its high Curie temperature of above 980 K makes this Heusler alloy interesting for spinelectronics. Thin Co2FeSi films are grown by molecular-beam epitaxy on GaAs (001) with a close lattice match. We present a study of point-contact measurements on different films, varying in thickness between 18 nm and 48 nm and in substrate temperature during deposition between 100 °C and 300 °C. Transport spin polarizations at the Fermi level are determined from differential conductance curves obtained by point-contact Andreev-reflection spectroscopy. A maximum transport spin polarization of about 60% is measured for a 18 nm thin Co2FeSi film grown at 200 °C.

Metal nanoparticle film–based room temperature Coulomb transistor

A new transistor concept exploits the colloidal synthesis of metal nanoparticles and their Coulomb charging energy. Single-electron transistors would represent an approach to developing less power–consuming microelectronic devices if room temperature operation and industry-compatible fabrication were possible. We present a concept based on stripes of small, self-assembled, colloidal, metal nanoparticles on a back-gate device architecture, which leads to well-defined and well-controllable transistor characteristics. This Coulomb transistor has three main advantages. By using the scalable Langmuir-Blodgett method, we combine high-quality chemically synthesized metal nanoparticles with standard lithography techniques. The resulting transistors show on/off ratios above 90%, reliable and sinusoidal Coulomb oscillations, and room temperature operation. Furthermore, this concept allows for versatile tuning of the device properties such as Coulomb energy gap and threshold voltage, as well as period, position, and strength of the oscillations.

Growth and Characterization of Ferromagnetic Alloys for Spin Injection

Spin electronics with semiconductor/ferromagnet hybrids is a topic of ongoing interest. We review developments in hybrid spintronics and give an overview of achievements in efficient spin injection from ferromagnetic metals into semiconductors. The focus of this work is on thin Heusler films grown on semiconductor substrates. Ni2MnIn films are deposited on a variety of substrates by coevaporation of nickel and the alloy MnIn. The almost perfect lattice match between Ni2MnIn and InAs qualifies this alloy for basic research in spintronics. Point-contact Andreev spectroscopy serves to quantify the spin polarization relevant to transport. Nanopatterning of Ni2MnIn electrodes with electron-beam lithography and lift-off processing is examined. In this context, the influence of post-growth annealing on the film’s morphology and crystal structure is studied in situ using transmission-electron microscopy. The electrodes are completed by a copper strip to form a lateral spin-valve. In first measurements in local geometry we have detected the spin-valve effect.