Ion etching of ultranarrow structures

We describe the use of Polymethylmethacrylate as both electron beam sensitive resist and ion etch mask for high-resolution pattern transfer. By using high-resolution electron beam lithography, chemically assisted ion beam etching, and in-situ metallization, we have fabricated ultra-narrow gates with lateral dimensions below 20 nm, spaced with < 50 nm pitch on high mobility 2D electron gas material. This technique, which is thought to provide extremely small lateral electron depletion lengths and well defined confienement potentials, allows us to produce new and more complicated structures for the study of quantum transport

Microfabrication below 10 nm

We describe a new electron beam lithography method for producing structures with lateral sizes smaller than the incident beam diameter. These patterns are transferred into GaAs/AlGaAs, InGaAs/GaAs and InGaAs/InP quantum well heterostructures using chemically assisted ion beam etching, thereby forming uniform arrays of pillars with lateral dimensions at or below 10 nm. To correlate the sizes of such structures with our exposure and development conditions, reflection electron microscopy observations are used

Direct measurement of the transmission matrix of a mesoscopic conductor

We have developed an experimental approach which permits evaluation of the entire transmission matrix of a mesoscopic conductor. Results are presented from two new investigations enabled by this technique: (a) We study ballistic multiprobe conductors in the limit of weak probe coupling, and (b) we image modal features in the distribution function of electrons emerging from a quantum point contact

Ion etching of ultranarrow structures

We describe the use of Polymethylmethacrylate as both electron beam sensitive resist and ion etch mask for high-resolution pattern transfer. By using high-resolution electron beam lithography, chemically assisted ion beam etching, and in-situ metallization, we have fabricated ultra-narrow gates with lateral dimensions below 20 nm, spaced with < 50 nm pitch on high mobility 2D electron gas material. This technique, which is thought to provide extremely small lateral electron depletion lengths and well defined confienement potentials, allows us to produce new and more complicated structures for the study of quantum transport

Imaging dielectric relaxation in nanostructured polymers by frequency modulation electrostatic force microscopy

We have developed a method for imaging the temperature-frequency dependence of the dynamics of nanostructured polymer films with spatial resolution. This method provides images with dielectric compositional contrast well decoupled from topography. Using frequency-modulation electrostatic-force-microscopy, we probe the local frequency-dependent (0.1–100 Hz) dielectric response through measurement of the amplitude and phase of the force gradient in response to an oscillating applied electric field. When the phase is imaged at fixed frequency, it reveals the spatial variation in dielectric losses, i.e., the spatial variation in molecular/dipolar dynamics, with 40 nm lateral resolution. This is demonstrated by using as a model system; a phase separated polystyrene/polyvinyl-acetate (PVAc) blend. We show that nanoscale dynamic domains of PVAc are clearly identifiable in phase images as those which light-up in a band of temperature, reflecting the variations in the molecular/dipolar dynamics approaching the glass transition temperature of PVAc

Are transport anomalies in "electron waveguides" classical?

We test the implicit prediction of a recent classical model that transport anomalies in ballistic multiprobe conductors—the last plateau, quenching, and the negative bend resistance—should scale with Fermi momentum, pF. Data from junctions defined by adiabatic lateral potentials (having corners with radii of curvature rJ>ħ/pF) qualitatively scale classically; departures observed are attributed to momentum-memory loss from random scattering. With more abrupt (square-cornered) junctions, however, nonclassical pF dependence emerges

Experimental measurement of scattering coefficients in mesoscopic conductors

We describe a general experimental approach yielding the entire transmission matrix of a multiprobe mesoscopic conductor. Results are presented for several new investigations with ballistic semiconductor nanostructures enabled by this technique. We measure the transmission coefficients for an open cross junction employing a sample design which is an almost literal realization of the Landauer-Büttiker model and verify that these transmission coefficients are consistent with Hall and bend resistance anomalies obtained by resistance measurements at low magnetic fields. Other investigations utilize a pinched cross junction in which two of the probes are separated from the channel by quantum point contacts. This series arrangement of point contacts allows a sensitive momentum spectroscopy of the emitted distribution. Even in the case of a single propagating mode, through the constrictions we observe modal features in the outgoing distribution. This indicates that the potential is spatially nonadiabatic even though conductance quantization is observed. This pinched geometry also allows the first fully characterized realization of weakly coupled probes

Fv antibodies to aflatoxin B1 derived from a pre-immunized antibody phage display library system

The production and characterization of recombinant antibodies to aflatoxin B[SUB1] (AFB[SUB1]), a potent mycotoxin and carcinogen is described. The antibody fragments produced were then applied for use in a surface plasmon resonance-based biosensor (BIAcore), which measures biomolecular interactions in 'real-time'. Single chain Fv (scFv) antibodies were generated to aflatoxin B1 from an established phage display system, which incorporated a range of different plasmids for efficient scFv expression. The scFv's were used in the development of a competitive ELISA, and also for the development of surface plasmon resonance (SPR)-based inhibition immunoassays. They were found to be suitable for the detection of AFB[SUB1], in this format, with the assays being sensitive and reproducible

Contemporary challenges in school recruitment for criminological survey research: lessons from the international self-report delinquency study in England, Germany, the Netherlands, and the United States

Several multiwave cross-national surveys have experienced drops in school participation for youth health and risk behavior (HRB) surveys in Western European countries. This article considers explanations for the challenge in recruiting schools for surveys in England, Germany, the Netherlands, and the United States and the most important lessons learned during school recruitment for the third wave of the International Self-Report Delinquency Study in these four countries. Comparing school response rates for international academic surveys with those focused on HRB, schools have been increasingly less likely to participate in HRB surveys over the past two decades. However, considerable variation within and across surveys and countries suggests there are numerous influences on school recruitment, and there may be facilitators on which researchers could capitalize. We conclude that when planning future school-based HRB surveys, researchers should consider multiple strategies to engage schools from the outset, tailored to regional and national settings