Massimo Macucci

Water-based and biocompatible 2D crystal inks for all-inkjet-printed heterostructures

Exploiting the properties of two-dimensional crystals requires a mass production method able to produce heterostructures of arbitrary complexity on any substrate. Solution processing of graphene allows simple and low-cost techniques such as inkjet printing to be used for device fabrication. However, the available printable formulations are still far from ideal as they are either based on toxic solvents, have low concentration, or require time-consuming and expensive processing. In addition, none is suitable for thin-film heterostructure fabrication due to the re-mixing of different two-dimensional crystals leading to uncontrolled interfaces and poor device performance. Here, we show a general approach to achieve inkjet-printable, water-based, two-dimensional crystal formulations, which also provide optimal film formation for multi-stack fabrication. We show examples of all-inkjet-printed heterostructures, such as large-area arrays of photosensors on plastic and paper and programmable logic memory devices. Finally, in vitro dose-escalation cytotoxicity assays confirm the biocompatibility of the inks, extending their possible use to biomedical applications.

MODELING AND MANUFACTURABILITY ASSESSMENT OF BISTABLE QUANTUM-DOT CELLS

We have investigated the behavior of bistable cells made up of four quantum dots and occupied by two electrons, in the presence of realistic confinement potentials produced by depletion gates on top of a GaAs/AlGaAs heterostructure. Such a cell represents the basic building block for logic architectures based on the concept of quantum cellular automata (QCA) and of ground state computation, which have been proposed as an alternative to traditional transistor-based logic circuits. We have focused on the robustness of the operation of such cells with respect to asymmetries derived from fabrication tolerances. We have developed a two-dimensional model for the calculation of the electron density in a driven cell in response to the polarization state of a driver cell. Our method is based on the one-shot configuration-interaction technique, adapted from molecular chemistry. From the results of our simulations, we conclude that an implementation of QCA logic based on simple “hole arrays” is not feasible, because...

SIMULATION OF ELECTRONIC PROPERTIES AND CAPACITANCE OF QUANTUM DOTS

The chemical potential and the capacitance of a 2D circular model quantum dot have been investigated for GaAs, InSb, and Si material parameters, covering a range from a few nanometers to micrometer dimensions. The Schrodinger equation has been solved self‐consistently, with the inclusion of many‐body effects, using a local density approximation as well as the optimized Krieger‐Li‐Iafrate exchange potential. Gate structures are included by use of the method of images. We have focused on quantum deviations from classical electrostatic capacitive behavior and found such deviations to be significant even for the material parameters of silicon for feature sizes smaller than 30 nm. The most striking features of quantum dot capacitance are signatures of the dot symmetry analogous to the orbital grouping in atoms: we find structure in the dot capacitance arising from quantum effects in correspondence with the filling of each group of energy‐degenerate orbitals. We also cover the influence of a magnetic field perp...

Shot noise in resonant tunneling structures

We propose a quantum mechanical approach to noise in resonant tunneling structures, that can be applied in the whole range of transport regimes, from completely coherent to completely incoherent. In both limiting cases, well known results which have appeared in the literature are recovered. Shot noise reduction due to both Pauli exclusion and Coulomb repulsion, and their combined effect, are studied as a function of the rate of incoherent processes in the well (which are taken into account by means of a phenomenological relaxation time), and of temperature. Our approach allows the study of noise in a variety of operating conditions (i.e., equilibrium, sub-peak voltages, second resonance voltages), and as a function of temperature, explaining experimental results and predicting interesting new results, such as the dependence of noise on filled emitter states and the prediction of both increasing and decreasing shot noise with increasing temperature, depending on the structure. It also allows the determination of the major contributions to shot noise suppression by performing noise measurements at the second resonance voltage.

Atomistic Boron-Doped Graphene Field-Effect Transistors: A Route toward Unipolar Characteristics

We report fully quantum simulations of realistic models of boron-doped graphene-based field-effect transistors, including atomistic details based on DFT calculations. We show that the self-consistent solution of the three-dimensional (3D) Poisson and Schrödinger equations with a representation in terms of a tight-binding Hamiltonian manages to accurately reproduce the DFT results for an isolated boron-doped graphene nanoribbon. Using a 3D Poisson/Schrödinger solver within the non-equilibrium Greens function (NEGF) formalism, self-consistent calculations of the gate-screened scattering potentials induced by the boron impurities have been performed, allowing the theoretical exploration of the tunability of transistor characteristics. The boron-doped graphene transistors are found to approach unipolar behavior as the boron concentration is increased and, by tuning the density of chemical dopants, the electron-hole transport asymmetry can be finely adjusted. Correspondingly, the onset of a mobility gap in the device is observed. Although the computed asymmetries are not sufficient to warrant proper device operation, our results represent an initial step in the direction of improved transfer characteristics and, in particular, the developed simulation strategy is a powerful new tool for modeling doped graphene nanostructures.

Conducted and radiated interference measurements in the line-pantograph system

We present the results of a measurement campaign aimed at investigating electromagnetic interference (EMI) phenomena in the interaction between overhead railway power supply lines and pantograph. In order to obtain such data, an experimental setup was assembled in a shielded room, consisting in a short section of overhead line and a full scale pantograph. One of the most interesting results consists in the observation of a very significant (both from the qualitative and the quantitative point of view) difference between the EMI behavior in the switch-on (pantograph going up) and switch-off (pantograph going down) transients.

Very sensitive measurement method of electron devices current noise

The problem of measuring very low levels of current noise in bipoles (linear or not) is dealt with, and a measurement technique is proposed. This technique allows the measurement of noise power spectra 6-10 dB lower than the equivalent input power spectrum of the amplified necessary to perform the measurement. An improvement of 16-20 dB in the sensitivity is obtained with respect to the one of conventional methods, which, for an acceptable accuracy, require the noise of the bipole under test to be 10 dB larger than the equivalent input one of the amplifier. The present method is based on the accurate measurement of the amplifier transimpedance with respect to the input current noise sources and on the precise evaluation and subtraction of the contribution from all the spurious sources to the total noise. The whole procedure is implemented by means of a dual-channel signal analyzer and almost completely automated. The technique has been tested by using it to measure the power spectra of the noise given by known generators, of the Nyquist noise produced by bipoles made up of resistors and capacitors, and of shot noise in p-n junctions. The experimental results agree very well with theoretical predictions. >

The k center dot p method and its application to graphene, carbon nanotubes and graphene nanoribbons: the Dirac equation

The k.p method is a semi-empirical approach which allows to extrapolate the band structure of materials from the knowledge of a restricted set of parameters evaluated in correspondence of a single point of the reciprocal space. In the first part of this review article we give a general description of this method, both in the case of homogeneous crystals (where we consider a formulation based on the standard perturbation theory, and Kanes approach) and in the case of non-periodic systems (where, following Luttinger and Kohn, we describe the single-band and multi-band envelope function method and its application to heterostructures). The following part of our review is completely devoted to the application of the k.p method to graphene and graphene-related materials. Following Andos approach, we show how the application of this method to graphene results in a description of its properties in terms of the Dirac equation. Then we find general expressions for the probability density and the probability current density in graphene and we compare this formulation with alternative existing representations. Finally, applying proper boundary conditions, we extend the treatment to carbon nanotubes and graphene nanoribbons, recovering their fundamental electronic properties.

Thermal behavior of quantum cellular automaton wires

We investigate the effect of finite temperature on the behavior of logic circuits based on the principle of quantum cellular automata (QCA) and of ground state computation. In particular, we focus on the error probability for a wire of QCA cells that propagates a logic state. A numerical model and an analytical, more approximate model are presented for evaluation of the partition function of such a system and, consequently, of the desired probabilities. We compare the results of the two models, assess the limits of validity of the analytical approach, and show that error probabilities depend on the ratio of the energy splitting between the ground state and first excited state to the thermal energy kT. We then provide estimates of the maximum operating temperature for a few relevant cases, and discuss possible approaches for increasing it.

Huge Conductance Peak Caused by Symmetry in Double Quantum Dots

We predict a huge interference effect contributing to the conductance through large ultra-clean quantum dots of chaotic shape. When a double-dot structure is made such that the dots are the mirror-image of each other, constructive interference can make a tunnel barrier located on the symmetry axis effectively transparent. We show (via theoretical analysis and numerical simulation) that this effect can be orders of magnitude larger than the well-known universal conductance fluctuations and weak-localization (both less than a conductance quantum). A small magnetic field destroys the effect, massively reducing the double-dot conductance; thus a magnetic field detector is obtained, with a similar sensitivity to a SQUID, but requiring no superconductors.