Laura Fumagalli

Calibrated complex impedance and permittivity measurements with scanning microwave microscopy

We present a procedure for calibrated complex impedance measurements and dielectric quantification with scanning microwave microscopy. The calibration procedure works in situ directly on the substrate with the specimen of interest and does not require any specific calibration sample. In the workflow tip-sample approach curves are used to extract calibrated complex impedance values and to convert measured S11 reflection signals into sample capacitance and resistance images. The dielectric constant of thin dielectric SiO2 films were determined from the capacitance images and approach curves using appropriate electrical tip-sample models and the εr value extracted at f = 19.81 GHz is in good agreement with the nominal value of εr ∼ 4. The capacitive and resistive material properties of a doped Si semiconductor sample were studied at different doping densities and tip-sample bias voltages. Following a simple serial model the capacitance-voltage spectroscopy curves are clearly related to the semiconductor depletion zone while the resistivity is rising with falling dopant density from 20 Ω to 20 kΩ. The proposed procedure of calibrated complex impedance measurements is simple and fast and the accuracy of the results is not affected by varying stray capacitances. It works for nanoscale samples on either fully dielectric or highly conductive substrates at frequencies between 1 and 20 GHz.

Nanoscale capacitance imaging with attofarad resolution using ac current sensing atomic force microscopy

Nanoscale capacitance imaging with attofarad resolution (∼1xa0aF) of a nano-structured oxide thin film, using ac current sensing atomic force microscopy, is reported. Capacitance images are shown to follow the topographic profile of the oxide closely, with nanometre vertical resolution. A comparison between experimental data and theoretical models shows that the capacitance variations observed in the measurements can be mainly associated with the capacitance probed by the tip apex and not with positional changes of stray capacitance contributions. Capacitance versus distance measurements further support this conclusion. The application of this technique to the characterization of samples with non-voltage-dependent capacitance, such as very thin dielectric films, self-assembled monolayers and biological membranes, can provide new insight into the dielectric properties at the nanoscale.

Nanoscale measurement of the dielectric constant of supported lipid bilayers in aqueous solutions with electrostatic force microscopy

We present what is, to our knowledge, the first experimental demonstration of dielectric constant measurement and quantification of supported lipid bilayers in electrolyte solutions with nanoscale spatial resolution. The dielectric constant was quantitatively reconstructed with finite element calculations by combining thickness information and local polarization forces which were measured using an electrostatic force microscope adapted to work in a liquid environment. Measurements of submicrometric dipalmitoylphosphatidylcholine lipid bilayer patches gave dielectric constants of ε(r) ~ 3, which are higher than the values typically reported for the hydrophobic part of lipid membranes (ε(r) ~ 2) and suggest a large contribution of the polar headgroup region to the dielectric response of the lipid bilayer. This work opens apparently new possibilities in the study of biomembrane electrostatics and other bioelectric phenomena.

Direct measurement of the dielectric polarization properties of DNA

Significance The strength of DNA–DNA and DNA–ligand electrostatic interactions crucially depends on the electric polarizability of DNA, represented by its dielectric constant. This has remained unknown owing to the lack of experimental techniques able to measure it. Here, we experimentally determined the dielectric constant of double-stranded DNA in a native condensed state inside a single bacteriophage as well as the dielectric constants of the protein shell and tail that compose the viral capsid using scanning force microscopy. We supported the experimental data by theoretically determining the DNA dielectric constant using atomistic simulations. Both approaches yield a dielectric constant of DNA around 8, sensibly higher than commonly assumed, thus revealing a DNA intrinsic property essential for realistic computational description of DNA. The electric polarizability of DNA, represented by the dielectric constant, is a key intrinsic property that modulates DNA interaction with effector proteins. Surprisingly, it has so far remained unknown owing to the lack of experimental tools able to access it. Here, we experimentally resolved it by detecting the ultraweak polarization forces of DNA inside single T7 bacteriophages particles using electrostatic force microscopy. In contrast to the common assumption of low-polarizable behavior like proteins (εr ∼ 2–4), we found that the DNA dielectric constant is ∼8, considerably higher than the value of ∼3 found for capsid proteins. State-of-the-art molecular dynamic simulations confirm the experimental findings, which result in sensibly decreased DNA interaction free energy than normally predicted by Poisson–Boltzmann methods. Our findings reveal a property at the basis of DNA structure and functions that is needed for realistic theoretical descriptions, and illustrate the synergetic power of scanning probe microscopy and theoretical computation techniques.

Finite-size effects and analytical modeling of electrostatic force microscopy applied to dielectric films.

A numerical analysis of the polarization force between a sharp conducting probe and a dielectric film of finite lateral dimensions on a metallic substrate is presented with the double objective of (i) determining the conditions under which the film can be approximated by a laterally infinite film and (ii) proposing an analytical model valid in this limit. We show that, for a given dielectric film, the critical diameter above which the film can be modeled as laterally infinite depends not only on the probe geometry, as expected, but mainly on the film thickness. In particular, for films with intermediate to large thicknesses (>100 nm), the critical diameter is nearly independent from the probe geometry and essentially depends on the film thickness and dielectric constant following a relatively simple phenomenological expression. For films that can be considered as laterally infinite, we propose a generalized analytical model valid in the thin-ultrathin limit (<20-50 nm) that reproduces the numerical calculations and the experimental data. Present results provide a general framework under which accurate quantification of electrostatic force microscopy measurements on dielectric films on metallic substrates can be achieved.

CMOS fully compatible microwave detector based on MOSFET operating in resistive regime

A microwave detector featuring full compatibility with standard CMOS process is presented. It is based on the channel resistance nonlinearity of a MOSFET operating in ohmic regime. The detecting sensitivity is shown to be tuned to below mW power by properly setting the bias voltage of the gate and of the drain of the transistor. Experiments with 180-nm gate length transistor have confirmed detecting operation up to 34GHz. The absence of additional technological steps required for the detector fabrication with respect to a standard CMOS process opens the realm of RF monitoring in products at virtually no cost.

Electron transport through supported biomembranes at the nanoscale by conductive atomic force microscopy.

We present a reliable methodology to perform electron transport measurements at the nanoscale on supported biomembranes by conductive atomic force microscopy (C-AFM). It allows measurement of both (a) non-destructive conductive maps and (b) force controlled current-voltage characteristics in wide voltage bias range in a reproducible way. Tests experiments were performed on purple membrane monolayers, a two-dimensional (2D) crystal lattice of the transmembrane protein bacteriorhodopsin. Non-destructive conductive images show uniform conductivity of the membrane with isolated nanometric conduction defects. Current-voltage characteristics under different compression conditions show non-resonant tunneling electron transport properties, with two different conduction regimes as a function of the applied bias, in excellent agreement with theoretical predictions. This methodology opens the possibility for a detailed study of electron transport properties of supported biological membranes, and of soft materials in general.

Nondestructive thickness measurement of biological layers at the nanoscale by simultaneous topography and capacitance imaging

Nanoscale capacitance images of purple membrane layers are obtained simultaneously to topography in a nondestructive manner by operating alternating current sensing atomic force microscopy in jumping mode. Capacitance images show excellent agreement with theoretical modeling and prove to be a noninvasive method for measuring the thickness of purple membrane layers beyond the single monolayer limit with nanoscale lateral spatial resolution. With the ability of spatially resolving the capacitance while preserving the sample from damaging, this technique can be applied for nanoscale thickness measurement of other biological layers and soft materials in general.

dc modulation in field-effect transistors operating under microwave irradiation for quantum readout

With a view to using microwaves to excite the single-spin resonance of an electron trapped in a defect at the Si∕SiO2 interface of a metal-oxide-semiconductor field-effect transistor (MOSFET), we report on the experimental evidence for a stationary current in such devices operated under microwave radiation. The stationary current is examined as a function of the microwave power and of the operating voltage of the MOSFET. The transistor behavior is reproduced by a model exploiting the nonlinearity of the MOSFET channel resistance as a component of the circuit coupled with the electromagnetic field. We conclude that, in operating a MOSFET under microwaves, one has to pay attention to the generation of spurious stationary currents that may alter the likelihood to observe spin-dependent phenomena in the random telegraph signal observed in a MOSFET.

Theory of amplitude modulated electrostatic force microscopy for dielectric measurements in liquids at MHz frequencies

A theoretical analysis of amplitude modulated electrostatic force microscopy (AM-EFM) in liquid media at MHz frequencies, based on a simple tip-sample parallel plate model, is presented. The model qualitatively explains the main features of AM-EFM in liquid media and provides a simple explanation of how the measured electric forces are affected by: the frequency of the applied voltage, the tip-sample distance, the ionic concentration, the relative dielectric constant of the solution, and the relative dielectric constant and thickness of the sample. These results provide a simple framework for the design of AM-EFM measurements for localized dielectric characterization in liquid media.