Influence of the cluster constituents' reactivity on the desorption/ionization process induced by neutral SO2 clusters.

The influence of the chemical nature of the cluster constituents on the desorption/ionization process was investigated for desorption/ionization induced by neutral SO2 clusters (DINeC). The polar clusters act as a transient matrix in which the desorbed analyte molecules are dissolved during the desorption process. For drop-cast samples, the desorption/ionization efficiency was found to be largely independent of the pH value of the initial solution the samples were prepared from; positive ions were almost always dominant and no multiply charged negative ions were observed. The results were traced back to the interaction of SO2 with water present in the samples. Both H/D exchange experiments and surface charge measurements showed that SO2 from the cluster beam interacts with water on and in the sample forming sulfurous acid. The latter then acts as an efficient proton supply leading to an enhanced ionization efficiency. The results demonstrate the possibility to control the ionization efficiency when using reactive cluster constituents in desorption-based ionization methods such as DINeC and cluster-based secondary ion mass spectrometry.

InGaN/GaN nanowires as a new platform for photoelectrochemical sensors - detection of NADH.

InGaN/GaN nanowire heterostructures are presented as nanophotonic probes for the light-triggered photoelectrochemical detection of NADH. We demonstrate that photogenerated electron-hole pairs give rise to a stable anodic photocurrent whose potential- and pH-dependences exhibit broad applicability. In addition, the simultaneous measurement of the photoluminescence provides an additional tool for the analysis and evaluation of light-triggered reaction processes at the nanostructured interface. InGaN/GaN nanowire ensembles can be excited over a wide wavelength range, which avoids interferences of the photoelectrochemical response by absorption properties of the compounds to be analyzed by adjusting the excitation wavelength. The photocurrent of the nanostructures shows an NADH-dependent magnitude. The anodic current increases with rising analyte concentration in a range from 5µM to 10mM, at a comparatively low potential of 0mV vs. Ag/AgCl. Here, the InGaN/GaN nanowires reach high sensitivities of up to 91µAmM-1cm-2 (in the linear range) and provide a good reusability for repetitive NADH detection. These results demonstrate the potential of InGaN/GaN nanowire heterostructures for the defined conversion of this analyte paving the way for the realization of light-switchable sensors for the analyte or biosensors by combination with NADH producing enzymes.

Nonpolar m-plane GaN/AlGaN heterostructures with intersubband transitions in the 5-10 THz band.

This paper assesses intersubband (ISB) transitions in the 1-10 THz frequency range in nonpolar m-plane GaN/AlGaN multi-quantum-wells deposited on free-standing semi-insulating GaN substrates. The quantum wells (QWs) were designed to contain two confined electronic levels, decoupled from the neighboring wells. Structural analysis reveals flat and regular QWs in the two perpendicular in-plane directions, with high-angle annular dark-field scanning transmission electron microscopy images showing inhomogeneities of the Al composition in the barriers along the growth axis. We do not observe extended structural defects (stacking faults or dislocations) introduced by the epitaxial process. Low-temperature ISB absorption from 1.5 to 9 THz (6.3-37.4 meV) is demonstrated, covering most of the 7-10 THz band forbidden to GaAs-based technologies.

Integration of an opto-chemical detector based on group III-nitride nanowire heterostructures.

The photoluminescence intensity of group III nitrides, nanowires, and heterostructures (NWHs) strongly depends on the environmental H(2) and O(2) concentration. We used this opto-chemical transducer principle for the realization of a gas detector. To make this technology prospectively available to commercial gas-monitoring applications, a large-scale laboratory setup was miniaturized. To this end the gas-sensitive NWHs were integrated with electro-optical components for optical addressing and read out within a compact and robust sensor system. This paper covers the entire realization process of the device from its conceptual draft and optical design to its fabrication and assembly. The applied approaches are verified with intermediate results of profilometric characterizations and optical performance measurements of subsystems. Finally the gas-sensing capabilities of the integrated detector are experimentally proven and optimized.

Electrical transport properties of Ge-doped GaN nanowires.

The conductivity and charge carrier concentration of single GaN nanowires (NWs) doped with different concentrations of Ge were determined by four-point resistivity and temperature-dependent Seebeck coefficient measurements. We observed high carrier concentrations ranging from 9.1 × 10(18) to 5.5 × 10(19) cm(-3), well above the Mott density of 1.6 × 10(18) cm(-3), and conductivities up to 625 S cm(-1) almost independent of the NW diameter. The weak temperature dependence of the conductivity between 2 and 10 K could be assigned to the formation of an impurity band. For the sample with the highest conductivity metallic behaviour was found, indicated by a positive temperature coefficient of the resistivity. The near band edge emission analyzed by micro-photoluminescence spectroscopy showed only a small increase of the peak width up to 70 meV and no spectral shift for carrier concentrations up to 5.5 × 10(19) cm(-3). The latter was attributed to the simultaneous influence of band filling, band gap renormalization, and strain.

Intraband absorption in self-assembled Ge-doped GaN/AlN nanowire heterostructures.

We report the observation of transverse-magnetic-polarized infrared absorption assigned to the s-p(z) intraband transition in Ge-doped GaN/AlN nanodisks (NDs) in self-assembled GaN nanowires (NWs). The s-p(z) absorption line experiences a blue shift with increasing ND Ge concentration and a red shift with increasing ND thickness. The experimental results in terms of interband and intraband spectroscopy are compared to theoretical calculations of the band diagram and electronic structure of GaN/AlN heterostructured NWs, accounting for their three-dimensional strain distribution and the presence of surface states. From the theoretical analysis, we conclude that the formation of an AlN shell during the heterostructure growth applies a uniaxial compressive strain which blue shifts the interband optical transitions but has little influence on the intraband transitions. The presence of surface states with density levels expected for m-GaN plane charge-deplete the base of the NWs but is insufficient to screen the polarization-induced internal electric field in the heterostructures. Simulations show that the free-carrier screening of the polarization-induced internal electric field in the NDs is critical to predicting the photoluminescence behavior. The intraband transitions, on the other hand, are blue-shifted due to many-body effects, namely, the exchange interaction and depolarization shift, which exceed the red shift induced by carrier screening.

Nanostructure and strain in InGaN/GaN superlattices grown in GaN nanowires.

The structural properties and the strain state of InGaN/GaN superlattices embedded in GaN nanowires were analyzed as a function of superlattice growth temperature, using complementary transmission electron microscopy techniques supplemented by optical analysis using photoluminescence and spatially resolved microphotoluminescence spectroscopy. A truncated pyramidal shape was observed for the 4 nm thick InGaN inclusions, where their (0001¯) central facet was delimited by six-fold {101¯l} facets towards the m-plane sidewalls of the nanowires. The defect content of the nanowires comprised multiple basal stacking faults localized at the GaN base/superlattice interface, causing the formation of zinc-blende cubic regions, and often single stacking faults at the GaN/InGaN bilayer interfaces. No misfit dislocations or cracks were detected in the heterostructure, implying a fully strained configuration. Geometrical phase analysis showed a rather uniform radial distribution of elastic strain in the (0001¯) facet of the InGaN inclusions. Depending on the superlattice growth temperature, the elastic strain energy is partitioned among the successive InGaN/GaN layers in the case of low-temperature growth, while at higher superlattice growth temperature the in-plane tensile misfit strain of the GaN barriers is accommodated through restrained diffusion of indium from the preceding InGaN layers. The corresponding In contents of the central facet were estimated at 0.42 and 0.25, respectively. However, in the latter case, successful reproduction of the experimental electron microscopy images by image simulations was only feasible, allowing for a much higher occupancy of indium adatoms at lattice sites of the semipolar facets, compared to the invariable 25% assigned to the polar facet. Thus, a high complexity in indium incorporation and strain allocation between the different crystallographic facets of the InGaN inclusions is anticipated and supported by the results of photoluminescence and spatially resolved microphotoluminescence spectroscopy.

Radical formation at the gallium nitride nanowire-electrolyte interface by photoactivated charge transfer.

We investigated the transfer of photogenerated charge carriers from GaN nanowires into a surrounding electrolyte by electron paramagnetic resonance (EPR) and fluorescence spectroscopy. Using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) as a spin trap we find that the formation of hydroxyl radicals dominates in acidic, neutral and moderately basic environments, while in an electrolyte with a pH of 13.5 the superoxide formation becomes detectable. We explain the two processes considering the redox potentials for radical formation in the electrolyte as well as the positions of the conduction and valence bands. The role of surface band bending and surface states in the semiconductor is discussed.

Oxidation of Fe(II) leads to increased C-2 methylation of pentacyclic triterpenoids in the anoxygenic phototrophic bacterium Rhodopseudomonas palustris strain TIE-1.

Hopanoids are among the most widespread biomarkers of bacteria that are used as indicators for past and present bacterial activity. Our understanding of the production, function, and distribution of hopanoids in bacteria has improved greatly, partly due to genetic, culture-independent studies. Culture-based studies are important to determine hopanoid function and the environmental conditions under which these compounds are produced. This study compares the lipid inventory of Rhodopseudomonas palustris strain TIE-1 under anoxic photoautotrophic conditions using either H2 or Fe(II) as electron donor. The high amount to which adenosylhopane is produced irrespective of the used electron donor suggests a specific function of this compound rather than its exclusive role as an intermediate in bacteriohopanepolyol biosynthesis. C-2 methylated hopanoids and tetrahymanol account for as much as 59% of the respective C-2 methylated/non-methylated homologs during growth with Fe(II) as electron donor, as compared with 24% C-2 methylation for growth with H2 . This observation reveals that C-2 methylated hopanoids have a specific function and are preferentially synthesized in response to elevated Fe(II) concentrations. The presence of C-2 methylated pentacyclic triterpenoids has commonly been used as a biosignature for the interpretation of paleoenvironments. These new findings suggest that increased C-2 methylation may indicate anoxic ferrous conditions, in addition to other environmental stressors that have been previously reported.

Optical properties of GaN-based nanowires containing a single Al(0.14)Ga(0.86)N/GaN quantum disc.

The optical properties of wurtzite GaN nanowires containing single Al0.14Ga0.86N/GaN quantum discs of different thickness have been investigated. The dependence of the photoluminescence (PL) transition energy on the quantum disc thickness and the thickness of a lateral AlGaN shell has been simulated in the framework of a three-dimensional effective mass model, accounting for the presence of a lateral AlGaN shell, strain state and the piezoelectric and spontaneous polarization. The predicted transition energies are in good agreement with the statistics realized on more than 40 single nanowire emission spectra and PL spectra of ensembles of nanowires. The emission spectra of the single quantum discs exhibit a Lorentzian shape with a homogeneous line width as low as 3 meV. Finally, we discuss the dependence of the interband transition energy on diameter.

Electrochemical properties of GaN nanowire electrodes--influence of doping and control by external bias.

We report on the electrochemical characteristics of GaN nanowire (NW) ensembles grown by plasma-assisted molecular beam epitaxy on Si111 substrates and on the influence of Si and Mg doping. The NW electrochemical properties in terms of surface capacitance (C(S)), surface resistance (R(S)) are extracted from electrochemical impedance spectra. While Mg doping of GaN NWs does not cause a significant variation of these quantities, an increase of the Si concentration leads to an increase of C(S) and a simultaneous decrease of R(S), indicating the presence of charge carriers in the NWs. According to the extracted values for R(S) and C(S) the NWs are classified into resistive and conductive. For conductive NWs charge transfer to a ferricyanide redox couple in the electrolyte is demonstrated and the ensemble average of the flatband voltage was determined. Variation of the lateral surface potential due to application of an external bias via the electrolyte is demonstrated.

GaN nanodiscs embedded in nanowires as optochemical transducers.

The photoluminescence (PL) response of GaN/AlGaN nanowire heterostructures (NWHs) to hydrogen and oxygen between room temperature and 300 °C is reported. Exposure of Pt-coated NWHs to H2 leads to an increase of the PL intensity attributed to the suppression of surface recombination by local dipole fields of adsorbed atomic hydrogen. When exposed to O2, uncoated NWHs show a decrease in PL intensity that is assigned to enhanced non-radiative recombination. The detection limits are below 5 ppm at 150 °C.

Gallium nitride electrodes for membrane-based electrochemical biosensors.

We report on the deposition of planar lipid bilayers (supported membranes) on gallium nitride (GaN) electrodes for potential applications as membrane-based biosensors. The kinetics of the lipid membrane formation upon vesicle fusion were monitored by simultaneous measurements of resistance and capacitance of the membrane using AC impedance spectroscopy in the frequency range between 50 mHz and 50 kHz. We could identify a two-step process of membrane spreading and self-healing. Despite its relatively low resistance, the membrane can be modeled by a parallel combination of an ideal resistor and capacitor, indicating that the membrane efficiently blocks the diffusion of ions.

Real-time precision refractometry: new approaches.

We introduce two new approaches for near-real-time, high-precision tracking of the refractive index of the ambient atmosphere. The methods can be realized at low cost and are expected to have important practical application in those accurate dimensional metrology applications employing interferometry in air. A valuable potential application is the control of step-and-repeat mask positioning for integrated circuit production in which temporal stability time scales over days can be crucial. Extension of the methods to absolute index measurement is discussed.

Acid phosphatase for monitoring prostatic carcinoma. Comparison of radioimmunoassay and enzymatic techniques.

We compared a commercial radioimmunoassay kit with an enzymatic assay for prostatic acid phosphatase in monitoring the progression or remission of disease in 27 patients with prostatic cancer. In 5 of the 18 patients whose disease progressed, and in 4 of the 9 whose disease responded to treatment, the change was reflected better by the radioimmunoassay. In no case was the enzymatic assay better. Radioimmunoassay for prostatic acid phosphatase may be an effective and sensitive way to monitor the course of carcinoma of the prostate.