arXiv subjects

Thomas Weimann

Publications and source records attributed to Thomas Weimann.

At least 19 recordsLinked to original sources

arXiv2026

Ultrasensitive Real-Time Detection of SARS-CoV-2 Proteins with Arrays of Biofunctionalized Graphene Field-Effect Transistors

With the growing interest in graphene field-effect transistors (GFETs) for biosensing applications, there is a strong demand for strategies enabling flexible and multiplexed biofunctionalization, as well as highly parallel, real-time electronic readout integrated with microfluidic control. Here we present a methodology that addresses these challenges by enabling real-time, parallel monitoring of multiple GFETs integrated on a single microfabricated chip within an automated electronic and microfluidic platform. We demonstrate the capabilities of this approach through ultrasensitive detection of the SARS-CoV-2 spike (S) and nucleocapsid (N) proteins. GFET chips are functionalized via van der Waals assembly using 1 nm-thick molecular two-dimensional (2D) materials - carbon nanomembranes - which enable multiplexed biofunctionalization. The chips are integrated into a custom-developed microelectronic and microfluidic system that allows parallel, real-time, and automated measurements of 15 GFETs. We present in situ biofunctionalization of the GFETs with antibodies, followed by highly specific detection of the S- and N-proteins with limits of detection down to 10 aM and a dynamic range spanning four orders of magnitude. Owing to its versatility, the presented methodology is readily adaptable for sensing a wide range of biological and chemical targets.

physics.bio-ph↗

arXiv2023

High-Q magnetic levitation and control of superconducting microspheres at millikelvin temperatures

We report the levitation of a superconducting lead-tin sphere with 100 micrometer diameter (corresponding to a mass of 5.6 micrograms) in a static magnetic trap formed by two coils in an anti-Helmholtz configuration, with adjustable resonance frequencies up to 240 hertz. The center-of-mass motion of the sphere is monitored magnetically using a dc superconducting quantum interference device as well as optically and exhibits quality factors of up to 2.6e7. We also demonstrate 3D magnetic feedback control of the sphere's motion. The setup is housed in a dilution refrigerator operating at 15 millikelvin. By implementing a cryogenic vibration isolation system we can attenuate environmental vibrations at 200 hertz by approximately seven orders of magnitude. The combination of low temperature, large mass and high quality factor as well as adjustable resonance frequencies provides a promising platform for testing quantum physics in previously unexplored regimes with high mass and long coherence times.

quant-ph↗

arXiv2023

Universal scaling of adiabatic tunneling out of a shallow confinement potential

The ability to tune quantum tunneling is key for achieving selectivity in manipulation of individual particles in quantum technology applications. In this work we count electron escape events out of a time-dependent confinement potential, realized as a dynamic quantum dot in a GaAs/AlGaAs heterostructure. A universal scaling relation of the escape probability as a function of potential barrier rise time and depth is established and developed as a method to probe tunneling rates over many orders of magnitude reaching the limit of shallow anharmonic confinement. Crossover to thermally activated transport is used to estimate the single time-energy scale of the universal model. In application to metrological single electron sources, in-situ calibrated control signals greatly extend the accessible dynamical range for probing the quantization mechanism. Validation of the cubic potential approximation sets a foundation for microscopic modeling of quantum tunneling devices in the shallow confinement regime.

cond-mat.mes-hall↗

arXiv2022

Two electrons interacting at a mesoscopic beam splitter

The non-linear response of a beam splitter to the coincident arrival of interacting particles enables numerous applications in quantum engineering and metrology yet poses considerable challenge to achieve focused interactions on the individual particle level. Here we probe the coincidence correlations at a mesoscopic constriction between individual ballistic electrons in a system with unscreened Coulomb interactions and introduce concepts to quantify the associated parametric non-linearity. The full counting statistics of joint detection allows us to explore the interaction-mediated energy exchange. We observe an increase from 50\% up to 70\% in coincidence counts between statistically indistinguishable on demand sources, and a correlation signature consistent with independent tomography of the electron emission. Analytical modeling and numerical simulations underpin consistency of the experimental results with Coulomb interactions between two electrons counterpropagating in a dispersive quadratic saddle, and demonstrate interactions sufficiently strong, $U/(\hbar \omega) > 10$, to enable single-shot in-flight detection and quantum logic gates.

cond-mat.mes-hall↗

arXiv2020

Traceably Calibrated Scanning Hall Probe Microscopy at Room Temperature

Fabrication, characterization and comparison of gold and graphene micro- and nano-size Hall sensors for room temperature scanning magnetic field microscopy applications is presented. The Hall sensors with active areas from 5 $μ$m down to 50 nm were fabricated by electron-beam lithography. The calibration of the Hall sensors in an external magnetic field revealed a sensitivity of 3.2 mV/(AT) $\pm$ 0.3 % for gold and 1615 V/(AT) $\pm$ 0.5 % for graphene at room temperature. The gold sensors were fabricated on silicon nitride cantilever chips suitable for integration into commercial scanning probe microscopes, allowing scanning Hall microscopy (SHM) under ambient conditions and controlled sensor-sample distance. The height dependent stray field distribution of a magnetic scale was characterized using a 5 $μ$m gold Hall sensor. The uncertainty of the entire Hall sensor based scanning and data acquisition process was analyzed allowing traceably calibrated SHM measurements. The measurement results show good agreement with numerical simulations within the uncertainty budget.

physics.ins-det↗

arXiv2020

Grazing incidence-X-ray fluorescence for a dimensional and elemental characterization of well-ordered nanostructures

The increasing importance of well-controlled ordered nanostructures on surfaces represents a challenge for existing metrology techniques. To develop such nanostructures and monitor complex processing constraints fabrication, both a dimensional reconstruction of nanostructures and a characterization (ideally a quantitative characterization) of their composition is required. In this work, we present a soft X-ray fluorescence-based methodology that allows both of these requirements to be addressed at the same time. By applying the grazing-incidence X-ray fluorescence technique and thus utilizing the X-ray standing wave field effect, nanostructures can be investigated with a high sensitivity with respect to their dimensional and compositional characteristics. By varying the incident angles of the exciting radiation, element-sensitive fluorescence radiation is emitted from different regions inside the nanoobjects. By applying an adequate modeling scheme, these datasets can be used to determine the nanostructure characteristics. We demonstrate these capabilities by performing an element-sensitive reconstruction of a lamellar grating made of Si$_3$N$_4$, where GIXRF data for the O-K$α$ and N-K$α$ fluorescence emission allows a thin oxide layer to be reconstructed on the surface of the grating structure. In addition, we employ the technique also to three dimensional nanostructures and derive both dimensional and compositional parameters in a quantitative manner.

physics.app-ph↗

arXiv2018

Lateral heterostructures of two-dimensional materials by electron-beam induced stitching

We present a novel methodology to synthesize two-dimensional (2D) lateral heterostructures of graphene and MoS2 sheets with molecular carbon nanomembranes (CNMs), which is based on electron beam induced stitching. Monolayers of graphene and MoS2 were grown by chemical vapor deposition (CVD) on copper and SiO2 substrates, respectively, transferred onto gold/mica substrates and patterned by electron beam lithography or photolithography. Self-assembled monolayers (SAMs) of aromatic thiols were grown on the gold film in the areas where the 2D materials were not present. An irradiation with a low energy electron beam was employed to convert the SAMs into CNMs and simultaneously stitching the CNM edges to the edges of graphene and MoS2, therewith forming a heterogeneous but continuous film composed of two different materials. The formed lateral heterostructures possess a high mechanical stability, enabling their transfer from the gold substrate onto target substrates and even the preparation as freestanding sheets. We characterized the individual steps of this synthesis and the structure of the final heterostructures by complementary analytical techniques including optical microscopy, Raman spectroscopy, atomic force microscopy (AFM), helium ion microscopy (HIM), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) and find that they possess nearly atomically sharp boundaries.

cond-mat.mes-hall↗

arXiv2014

Functional single-layer graphene sheets from aromatic monolayers

We demonstrate how self-assembled monolayers of aromatic molecules on copper substrates can be converted into high-quality single-layer graphene using low-energy electron irradiation and subsequent annealing. We characterize this two-dimensional solid state transformation on the atomic scale and study the physical and chemical properties of the formed graphene sheets by complementary microscopic and spectroscopic techniques and by electrical transport measurements. As substrates we successfully use Cu(111) single crystals and the technologically relevant polycrystalline copper foils.

cond-mat.mes-hall↗

arXiv2014

All-carbon vertical van der Waals heterostructures: Non-destructive functionalization of graphene for electronic applications

We present a route to non-destructive functionalization of graphene via assembly of vertical all-carbon van der Waals heterostructures. To this end, we employ singlelayer graphene (SLG) sheets grown by low-pressure methane CVD on Cu foils and large-area dielectric ~1 nm thick amino-terminated carbon nanomembranes (NH2-CNMs) generated by electron-beam-induced crosslinking of aromatic self-assembled monolayers. We encapsulate SLG sheets on oxidized silicon wafers with NH2-CNMs via mechanical stacking and characterize structural, chemical and electronic properties of the formed heterostructures by Raman spectroscopy and X-ray photoelectron spectroscopy as well as by electric and electromagnetic transport measurements. We show that functional amino groups are brought in close vicinity of the SLG sheets and that their transport characteristics are not impaired by this functionalization; moreover, we demonstrate a functional response of the heterostructure devices to the protonation of the amino groups in water. Due to its relative simplicity, the suggested approach opens broad avenues for implementations in graphene-based electronic devices where non-destructive chemical functionalization of graphene is required (e.g., for engineering electrical transducers for chemical and bio-sensing) or as complementary dielectric to graphene in hieratical heterostructures.

cond-mat.mes-hall↗

arXiv2014

Bilayer Graphene Quantum Dot Defined by Topgates

We investigate the application of nanoscale topgates on exfoliated bilayer graphene to define quantum dot devices. At temperatures below 500 mK the conductance underneath the grounded gates is suppressed, which we attribute to nearest neighbour hopping and strain-induced piezoelectric fields. The gate-layout can thus be used to define resistive regions by tuning into the corresponding temperature range. We use this method to define a quantum dot structure in bilayer graphene showing Coulomb blockade oscillations consistent with the gate layout.

cond-mat.mes-hall↗

arXiv2013

A self-referenced single-electron quantized-current source

With the anticipated redefinition of the international system of units (SI) the base units will be linked to fundamental constants of nature [1]. As for the electrical base unit "Ampere", it will be linked to the elementary charge e, requiring a corresponding quantum standard [2, 3]. Many concepts for such a standard have been investigated [4-14] relying on controlling the time-dependent tunnelling of electrons. However, the stochastic nature of quantum mechanical tunnelling intrinsically evokes uncontrolled deviations from the nominally quantized current. Alternatively, the counting of electrons [15, 16] has been explored but is severely limited in current amplitude and uncertainty by the low detector bandwidth. The late M. Wulf proposed [17] that this fundamental problem of electrical quantum metrology could be overcome by combining serial single-electron pumps with charge detectors allowing the generation of a quantized current and the in-situ detection of its stochastic deviations. Here, we experimentally demonstrate such quantized-current generation with in-situ detection of tunnelling errors at low frequencies and a reduction of the total current uncertainty by more than one order of magnitude. After frequency scaling this should enable a validated primary standard for the redefined SI base unit Ampere.

cond-mat.mes-hall↗

arXiv2013

Nb nano superconducting quantum interference devices with high spin sensitivity for operation in magnetic fields up to 0.5\,T

We investigate electric transport and noise properties of microstrip-type submicron direct current superconducting quantum interference devices (dc SQUIDs) based on Nb thin films and overdamped Josephson junctions with a HfTi barrier. The SQUIDs were designed for optimal spin sensitivity $S_μ^{1/2}$ upon operation in intermediate magnetic fields $B$ (tens of mT), applied perpendicular to the substrate plane. Our so far best SQUID can be continuously operated in fields up to $B\approx\pm50\,\rm{mT}$ with rms flux noise $S_{Φ,\rm w}^{1/2}\leq250\,\rm{nΦ_0/Hz^{1/2}}$ in the white noise regime and spin sensitivity $S_μ^{1/2}\leq29\,\rm{μ_B/Hz^{1/2}}$. Furthermore, we demonstrate operation in $B=0.5\,\rm{T}$ with high sensitivity in flux $S_{Φ,\rm w}^{1/2}\approx680\,\rm{nΦ_0/Hz^{1/2}}$ and in electron spin $S_μ^{1/2}\approx79\,\rm{μ_B/Hz^{1/2}}$. We discuss strategies to further improve the nanoSQUID performance.

cond-mat.supr-con↗

arXiv2012

Counting statistics for electron capture in a dynamic quantum dot

We report non-invasive single-charge detection of the full probability distribution $P_n$ of the initialization of a quantum dot with $n$ electrons for rapid decoupling from an electron reservoir. We analyze the data in the context of a model for sequential tunneling pinch-off, which has generic solutions corresponding to two opposing mechanisms. One limit considers sequential "freeze out" of an adiabatically evolving grand canonical distribution, the other one is an athermal limit equivalent to the solution of a generalized decay cascade model. We identify the athermal capturing mechanism in our sample, testifying to the high precision of our combined theoretical and experimental methods. The distinction between the capturing mechanisms allows to derive efficient experimental strategies for improving the initialization.

cond-mat.mes-hall↗

arXiv2012

Precision quantization of Hall resistance in transferred graphene

We show that quantum resistance standards made of transferred graphene reach the uncertainty of semiconductor devices, the current reference system in metrology. A large graphene device (150 \times 30 \mum2), exfoliated and transferred onto GaAs, revealed a quantization with a precision of (-5.1 \pm 6.3) \times 10-9 accompanied by a vanishing longitudinal resistance at current levels exceeding 10 \muA. While such performance had previously only been achieved with epitaxially grown graphene, our experiments demonstrate that transfer steps, inevitable for exfoliated graphene or graphene grown by chemical vapor deposition (CVD), are compatible with the requirements of high quality quantum resistance standards.

cond-mat.mes-hall↗

arXiv2011

Graphene p-n junction Arrays as Quantum-Hall Resistance Standards

We demonstrate a device concept to fabricate resistance standards made of quantum Hall series arrays by using p-type and n-type graphene. The ambipolar nature of graphene allows fabricating series quantum Hall resistors without complex multi-layer metal interconnect technology, which is required when using conventional GaAs two-dimensional electron systems. As a prerequisite for a precise resistance standard we confirm the vanishing of longitudinal resistance across a p-n junction for metrological relevant current levels in the range of a few \muA.

cond-mat.mes-hall↗

arXiv2011

Magneto-Transport Properties of Exfoliated Graphene on GaAs

We studied the magneto-transport properties of graphene prepared by exfoliation on a III V semiconductor substrate. Tuneability of the carrier density of graphene was achieved by using a doped GaAs substrate as a back-gate. A GaAs/AlAs multilayer, designed to render the exfoliated graphene flakes visible, also provides the required back-gate insulation. Good tuneability of the graphene carrier density is obtained, and the typical Dirac resistance characteristic is observed despite the limited height of the multilayer barrier compared to the usual SiO2 oxide barrier on doped silicon. In a magnetic field weak localization effects as well as the quantum Hall effect of a graphene monolayer are studied.

cond-mat.mes-hall↗

arXiv2011

One nanometer thin carbon nanosheets with tunable conductivity and stiffness

We present a new route for the fabrication of ultrathin (~1 nm) carbon films and membranes, whose electrical behavior can be tuned from insulating to conducting. Self-assembled monolayers of biphenyls are cross-linked by electrons, detached from the surfaces and subsequently pyrolized. Above 1000K, the cross-linked aromatic monolayer forms a mechanically stable graphitic phase. The transition is accompanied by a drop of the sheet resistivity from ~10^8 to ~10^2 kOhm/sq and a mechanical stiffening of the nanomembranes from ~10 to ~50 GPa. The technical applicability of the nanosheets is demonstrated by incorporating them into a microscopic pressure sensor

cond-mat.mes-hall↗

arXiv2011

Conversion of self-assembled monolayers into nanocrystalline graphene: Structure and electric transport

Graphene-based materials have been suggested for applications ranging from nanoelectronics to nanobiotechnology. However, the realization of graphene-based technologies will require large quantities of free-standing two-dimensional (2D) carbon materials with tuneable physical and chemical properties. Bottom-up approaches via molecular self-assembly have great potential to fulfil this demand. Here, we report on the fabrication and characterization of graphene made by electron-radiation induced cross-linking of aromatic self-assembled monolayers (SAMs) and their subsequent annealing. In this process, the SAM is converted into a nanocrystalline graphene sheet with well defined thickness and arbitrary dimensions. Electric transport data demonstrate that this transformation is accompanied by an insulator to metal transition that can be utilized to control electrical properties such as conductivity, electron mobility and ambipolar electric field effect of the fabricated graphene sheets. The suggested route opens broad prospects towards the engineering of free-standing 2D carbon materials with tuneable properties on various solid substrates and on holey substrates as suspended membranes.

cond-mat.mes-hall↗