Ulrich Quaade

Metal ammine complexes for hydrogen storage

The hopes of using hydrogen as an energy carrier are severely dampened by the fact that there is still no safe, high-density method available for storing hydrogen. We investigate the possibility of using metal ammine complexes as a solid form of hydrogen storage. Using Mg(NH3)6Cl2 as the example, we show that it can store 9.1% hydrogen by weight in the form of ammonia. The storage is completely reversible, and by combining it with an ammonia decomposition catalyst, hydrogen can be delivered at temperatures below 620 K.

Mechanism of single atom switch on silicon

Abstract We demonstrate single atom switch on silicon which operates by displacement of a hydrogen atom on the silicon (100) surface at room temperature. We find two principal effects by which the switch is controlled: a pronounced maximum of the switching probability as function of sample bias and a preferred direction of switching as function of STM tip position. Based on first principles calculations, we show that this behaviour is due to a novel mechanism involving an electronic excitation of a localized surface resonance.

Adsorption behavior and current–voltage characteristics of CdSe nanocrystals on hydrogen-passivated silicon

Using scanning tunneling microscopy and spectroscopy we have studied both the geometric distribution and the conduction properties of organic shell capped CdSe nanocrystals adsorbed on hydrogen-passivated Si(100). At submonolayer concentrations, the nanocrystal distribution on the surface was found to be highly nonhomogeneous, with an aggregation of most of the nanocrystals into islands of monolayer thickness. I–V spectra collected on nanocrystals adsorbed on n- and p-type substrates showed a strong difference in the conduction behavior, caused by the substrate: CdSe nanocrystals on n-Si:H caused a widening of the surface band gap by 1 eV with respect to the gap of the substrate, while a significant narrowing of the gap was observed for nanocrystals on p-Si:H. This experimental result could be explained by modeling the system as a metal–insulator–semiconductor (MIS) diode. Using this model we have found that the current through the MIS junction is limited by the nanocrystals only in one bias direction, wh...

Low temperature methane oxidation on differently supported 2 nm Au nanoparticles

Low temperature CH4 oxidation was studied on 2 nm gold nanoparticles supported on various metaloxides. The differences in reaction rates for the different systems suggest that the support material has an effect on the activity. From TEM analysis, we found that the gold particles were stable in size during the reaction. In addition to full oxidation to CO2, traces of C2H6 were detected when Au/TiO2 was used, indicating limited partial CH4 oxidation. TiO2 was found to be the best support for gold nanoparticles both in terms of activity and gold particle stability.

Microsystem with integrated capillary leak to mass spectrometer for high sensitivity temperature programmed desorption

Temperature programmed desorption (TPD) is a method for obtaining information about quantities and binding properties of adsorbed species on a surface. A microfabricated flow system for TPD with an integrated capillary leak to a mass spectrometer is presented. The use of an integrated capillary leak minimizes dead volumes in the system, resulting in increased sensitivity and reduced response time. These properties make the system ideal for TPD experiments in a carrier gas. With CO desorbing from platinum as model system, it is shown that CO desorbing in 105 Pa of argon from as little as 0.5 cm2 of platinum foil gives a clear desorption peak. By using the microfabricated flow system, TPD experiments can be performed in a carrier gas with a sensitivity approaching that of TPD experiments in vacuum.

Single-atom reversible recording at room temperature

A single hydrogen atom can be reversibly switched between two symmetric sites on a silicon dimer at the surface of Si(100) using a scanning tunnelling microscope (STM). This is a model binary switch for silicon-based atom-scale reversible data storage at room temperature. In this paper we investigate two important aspects of using this single-atom switch as a memory device. First, the switching is electron stimulated, and through detailed modelling the switching probability per electron is accurately deduced. Second, we have investigated the possibilities for desorbing single hydrogen atoms to construct ordered arrays of switches to manufacture a memory device. Two desorption mechanisms have been considered: the well known electron-induced desorption at negative sample bias and a novel mechanism probably involving elastic deformation of the tip. For both mechanisms mechanical stability of the STM is of crucial importance. With our equipment it was possible to create a row of four switches in a controlled way.

Fabrication and modeling of narrow capillaries for vacuum system gas inlets

Micrometer-sized cylindrical capillaries with well-controlled dimensions are fabricated using deep reactive ion etching. The flow through the capillaries is experimentally characterized for varying pressures, temperatures, and diameters. For the parameters used, it is shown that the Knudsen number is in the intermediate flow regime, and Knudsen’s expression for the flow fit the data well. The flow properties of the capillaries make them ideal for introducing gas into vacuum systems and in particular mass spectrometers.

Imaging and manipulation of a polar molecule on Ag(1 1 1)

A scanning tunneling microscope (STM) was applied to image and laterally manipulate isolated phosphangulene molecules on Ag(1 1 1) at 6 K. Atomic-resolution images clearly revealed three characteristic types of appearances (three-lobed, fish and bump shape) for the adsorbed molecules, which could correspond to three distinct binding configurations. From a detailed analysis of the relative distance between neighboring three-lobed molecules we determine the adsorption site. Applying the lateral manipulation technique we demonstrate that the molecule can be pulled, slid or pushed by the tip on the surface. Accompanying with the reposition, molecular rotation and/or changing of binding configurations can also be induced. It is found that the dipole moment of the molecule has minor effects on its lateral movement. The results demonstrate that due to many degrees of freedom for large molecules, their manipulating processes can be much more complex in comparison with atoms and small molecules. Such information can shed light on the involved mechanisms on a molecular scale.