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Twisted bilayer heterostructures (TBHs) are materials whose physical properties depend on the twist angle between the two layers of two-dimensional (2D) materials. Those heterostructures are not fo...

In this work, we study the Raman spectra of twisted bilayer graphene samples as a function of their twist-angles (θ), ranging from 0.03° to 3.40°, where local θ are determined by analysis of their associated moiré superlattices, as imaged by scanning microwave impedance microscopy. Three standard excitation laser lines are used (457, 532, and 633 nm wavelengths), and the main Raman active graphene bands (G and 2D) are considered. Our results reveal that electron–phonon interaction influences the G band’s linewidth close to the magic angle regardless of laser excitation wavelength. Also, the 2D band lineshape in the θ < 1° regime is dictated by crystal lattice and depends on both the Bernal (AB and BA) stacking bilayer graphene and strain soliton regions (SP) (Gadelha et al 2021 Nature 590 405–9). We propose a geometrical model to explain the 2D lineshape variations, and from it, we estimate the SP width when moving towards the magic angle.

In this work, we analyzed numerically a multiscale nanosystem based on sMIM on TBG. Spontaneous formation of a water-meniscus by the approximation between the tip-sample concentrates the microwave fields, reaching resolutions of up to 1nm.

Synthesis of novel material often times requires novel analysis and characterization techniques. The possibility of combining sputtering (SPU) and Atomic Layer Deposition (ALD) in the same chamber, Sputtering Atomic Layer Augmented Deposition (SALAD), has produced interesting meta-dielectric nanocomposite systems that have unique optical and electronic properties, which may find novel applications [1]. Scanning Microwave Impedance Microscopy (sMIM) is a relatively novel characterization tool which permits assessment of local impedance. More recently, the utilization of microwaves in the near field regime has been an exciting topic in the field of high-resolution microscopy. We were able to demonstrate 1 nm resolution using scanning Microwave Impedance Microscopy (sMIM) where a spontaneously forming water meniscus concentrated the microwave fields in small regions [2]. Here we analyzed numerically sMIM with Finite Element Method (FEM) to investigate complex metal-dielectric structures created in a SALAD system. sMIM measurements provide information on both real and imaginary parts of the reflected microwave signal, which can be associated with the local conductivity and permittivity. Yet, these quantities can be influenced by the local topography, so extraction of the electronic contribution is a challenge. In this work, we perform tip-surface distance scans in order to gain a better understanding of the substrate response and compare with the FEM results.

Twisted bilayer graphene is created by slightly rotating the two crystal networks in bilayer graphene with respect to each other. For small twist angles, the material undergoes a self-organized lattice reconstruction, leading to the formation of a periodically repeated domain1-3. The resulting superlattice modulates the vibrational3,4 and electronic5,6 structures within the material, leading to changes in the behaviour of electron-phonon coupling7,8 and to the observation of strong correlations and superconductivity9. However, accessing these modulations and understanding the related effects are challenging, because the modulations are too small for experimental techniques to accurately resolve the relevant energy levels and too large for theoretical models to properly describe the localized effects. Here we report hyperspectral optical images, generated by a nano-Raman spectroscope10, of the crystal superlattice in reconstructed (low-angle) twisted bilayer graphene. Observations of the crystallographic structure with visible light are made possible by the nano-Raman technique, which reveals the localization of lattice dynamics, with the presence of strain solitons and topological points1 causing detectable spectral variations. The results are rationalized by an atomistic model that enables evaluation of the local density of the electronic and vibrational states of the superlattice. This evaluation highlights the relevance of solitons and topological points for the vibrational and electronic properties of the structures, particularly for small twist angles. Our results are an important step towards understanding phonon-related effects at atomic and nanometric scales, such as Jahn-Teller effects11 and electronic Cooper pairing12-14, and may help to improve device characterization15 in the context of the rapidly developing field of twistronics16.

Here we present advances in the use of near field microwaves, in both high-resolution microwave microscopy of nanostructured materials, as well as on the processing of materials with high microwave power density in selected areas. For the former, we utilize a commercial Scanning Microwave Impedance Microscope (PrimeNano Inc.) and demonstrate 3nm feature size. We discuss the mechanisms for image contrast, as well as draw equivalent circuits for a variety of metallic, semiconductor and dielectric systems. For the latter, we built a homemade system that enabled the concentration of microwaves in areas smaller than 100um diameter. We report on the system performance, and demonstrate the use of the tool in monitoring materials changes as heating takes place, by measuring and analyzing the reflected microwave signal. In summary, we demonstrate the usage, limits and opportunities of harnessing microwave power in the near field to map and modify materials properties in small scales.

Molecular and atomic imaging required the development of electron and scanning probe microscopies to surpass the physical limits dictated by diffraction. Nano-infrared experiments and pico-cavity tip-enhanced Raman spectroscopy imaging later demonstrated that radiation in the visible range can surpass this limit by using scanning probe tips to access the near-field regime. Here we show that ultimate resolution can be obtained by using scanning microwave imaging microscopy to reveal structures with feature sizes down to 1~nm using a radiation of 0.1~m in wavelength. As a test material we use twisted bilayer graphene, which is not only a very important recent topic due to the discovery of correlated electron effects such as superconductivity, but also because it provides a sample where we can systematically tune a superstructure Moir\'e patterns modulation from below one up to tens of nanometers. By analyzing the tip-sample distance dynamics, we demonstrate that this ultimate 10$^8$ probe-to-pattern resolution can be achieved by using liquid immersion microscopy concepts and exquisite force control exerted on nanoscale water menisci., Comment: suppl. mat included, movies not included and available upon request by email to gilberto@dcc.ufmg.br

In this work we investigated the electronic states of InAsP/GaAs QDs using photoluminescence (PL) and photomodulated transmission (PT) experiments. PL measurements were performed as a function of temperature and excitation power. As the recombination energy of the InAsP/GaAs QDs resides in between the extremes InAs/GaAs and InP/GaAs QDs, these two materials were also analyzed. From PL experiments we could identify the recombination from the QDs ground state and the contribution of the InAsP and InP wetting layer (WL) with a macro PL technique. PL measurements of annealed samples allowed us to to clearly identify the WL electronic states. Specifically for the InP/GaAs QDs, the WL and QD recombination overlap as a single PL band. Photomodulated transmission spectroscopy (PT) showed spectral structure consistent with the QD and WL assignments given after the PL analysis.

Tidal Range Structures (TRS) have been considered for large-scale electricity generation for their potential ability to produce reasonably predictable energy without the emission of greenhouse gases. Once the main forcing components for driving the tides have deterministic dynamics, the available energy in a given TRS has been estimated, through analytical and numerical optimisation routines, as a mostly predictable event. This constraint imposes state-of-art flexible operation methods to rely on tidal predictions to infer best operational strategies for TRS, with the additional cost of requiring to run optimisation routines for every new tide. In this paper, a Deep Reinforcement Learning approach (Proximal Policy Optimisation through Unity ML-Agents) is introduced to perform automatic operation of TRS. For validation, the performance of the proposed method is compared with six different operation optimisation approaches devised from the literature, utilising the Swansea Bay Tidal Lagoon as a case study. We show that our approach is successful in maximising energy generation through an optimised operational policy of turbines and sluices, yielding competitive results with state-of-art optimisation strategies, with the clear advantages of requiring training once and performing real-time automatic control of TRS with measured ocean data only., 35 pages, 13 figures and 11 tables

In this work, different regions of a graphene device were exposed to a 30 keV helium ion beam creating a series of alternating strips of vacancy-type defects and pristine graphene. From magnetoconductance measurements as function of temperature, density of carriers and density of strips we show that the electron-electron interaction is important to explain the logarithmic quantum corrections to the Drude conductivity in graphene with vacancies. It is known that vacancies in graphene behave as local magnetic moments that interact with the conduction electrons and leads to a logarithmic correction to the conductance through the Kondo effect. However, our work shows that it is necessary to account for the non-homogeneity of the sample to avoid misinterpretations about the Kondo physics due the difficulties in separating the electron-electron interaction from the Kondo effect.

At the limits of physical representation of bits, novel opportunities arise, in particular leveraging the granular nature of charges, photons and atoms. One interesting application is the generation of truly random numbers. The need of a true random sequence of numbers is strategic for a variety of applications, ranging from the game industry to cryptography. Physical sources that rely on natural phenomena spanning radioactive decay, chaotic oscillators, thermal and quantum noise have their own merit, but for the purposes of integration, attributes such as bandwidth and power consumption, need to be accounted for. Here we evaluate transition metal oxide two terminal devices, memristors operating near the quantum of conductance and negative differential resistance metal-insulator transition devices, as potential candidates for a solid state source. In particular, the caveats of each implementation will be covered, such as the necessity of postprocessing and scalability.

This paper presents a 18 GHz cm-Wave measurement-based campaign and path-loss analysis and a comparison with the traditional Alpha-Beta (AB) and the Close-in (CI) propagation models. A suburban scenario in the city of Belo Horizonte, Minas Gerais, Brazil, was considered and measurements were done considering LOS and NLOS conditions. The recorded samples data were processed and an outdoor propagation path loss was obtained, and then fitted in a curve to compare with the reference propagation models. It was observed a path loss exponent of 4.737 for the AB model, an exponent of 2.923 and 2.472 for the CI model with reference distances of 1 m and 2.5 m, respectively. A Beam Combining Gain (BCG) analysis was also performed and a mean value of 3.4 dB was obtained when using the 3 strongest components of two measurement points.

Metal–insulator–metal (MIM) structures based on titanium dioxide have demonstrated reversible and non-volatile resistance-switching behavior and have been identified with the concept of the memristor. Microphysical studies suggest that the development of sub-oxide phases in the material drives the resistance changes. The creation of these phases, however, has a number of negative effects such as requiring an elevated voltage, increasing the device-to-device variability, damaging the electrodes due to oxygen evolution, and ultimately limiting the device lifetime. In this work we show that the deliberate inclusion of a sub-oxide layer in the MIM structure maintains the favorable switching properties of the device, while eliminating many of the negative effects. Electrical and microphysical characterization of the resulting structures was performed, utilizing X-ray and electron spectroscopy and microscopy. In contrast to structures which are not engineered with a sub-oxide layer, we observed dramatically reduced microphysical changes after electrical operation.

A key requirement for using memristors in circuits is a predictive model for device behavior that can be used in simulations and to guide designs. We analyze one of the most promising materials, tantalum oxide, for high density, low power, and high-speed memory. We perform an ensemble of measurements, including time dynamics across nine decades, to deduce the underlying state equations describing the switching in Pt/TaOx/Ta memristors. A predictive, compact model is found in good agreement with the measured data. The resulting model, compatible with SPICE, is then used to understand trends in terms of switching times and energy consumption, which in turn are important for choosing device operating points and handling interactions with other circuit elements.

We developed an apparatus to measure the work function using a field emission retarding potential (FERP) technique. In this paper, we describe the basics of FERP and the measurement procedures to determine the work function of indium tin oxide (ITO) and aluminum thin films, which are used as electrodes in a large number of Organic Electronic devices. The system developed allows for rapid screening of candidate materials and surface processes for appropriate engineering of device electronic properties.

We present in detail a thermodynamic equilibrium model for the growth of nanostructures on semiconductor substrates in heteroepitaxy and its application to germanium deposition on silicon. Some results of this model have been published previously, but the details of the formulation of the model are given here for the first time. The model allows the computation of the shape and size distributions of the surface nanostructures, as well as other properties of the system. We discuss the results of the model, and their incorporation into a nanostructure diagram that summarizes the relative stability of domes and pyramids in the bimodal size distributions. Copyright © 2007 American Scientific Publishers. All rights reserved.

Much of traditional metallurgy depends on manipulating thermodynamic and kinetic constraints to achieve the desired microstructure. It is becoming apparent that it may be possible to exploit similar principles to achieve designer self-assembled nanostructures on surfaces. A key question in determining equilibrium island sizes is whether epitaxial growth occurs with commensurate lattice parameters (with corresponding elastic strain), or whether the islands are relaxed. In the case of TiSi 2 islands on Si(001), we can show both by STM and by TEM that the C49 phase is incommensurate, and that the islands have the relaxed silicide lattice parameters. The STM images show this both from the dimensions of surface reconstructions and from subtle current imaging contrast that we attribute to the periodic nature of the island/substrate interface. The TEM evidence comes primarily from analysis of moire fringes, and the electron diffraction data confirm the presence of the C49 phase.

We demonstrate a promising new material system for ionic resistive switches: nitride memristors. The switching material is an AlN film, deposited using atomic layer deposition (ALD), and the electrodes can be TiN, Pt or Al. A variety of materials characterizations were performed to determine the structure, composition and impurities of the AlN films.

We characterized the conduction mechanisms in thin sputtered films of three representative binary Me–O (Me=Ta, W, and Nb) systems as a function of oxygen content, by combining in situ chemical state and electronic band structure studies from X-ray photoemission with temperature-dependent transport measurements. Despite certain differences, these amorphous films all displayed Fermi glass behavior following an oxidation-induced transition from metallic to hopping conduction, down to a sub-percolation threshold. The electron localization estimated from the band structure was in good agreement with that from the transport measurements, and the two were used to construct phase diagrams of conduction in the degree of oxidation-conductivity coordinates, which should prove important in the design of resistive switching and other electronic devices.

Ostwald ripening is an interesting phenomenon that takes place in a variety of systems in nature, including nanostructured materials. A large number of nanoscale systems are formed by a mixture, which has to be considered for a realistic description of its evolution upon equilibrium. Here, a thermodynamic-based model is used to explain the intermixing contribution to Ostwald ripening phenomenon in a two-components system (AxB1–x). By considering the mixing entropy on the general growth rate equation of particles, ripening is maximized for an alloy with x = 0.5. As an example, Ge–Si incoherently strained epitaxial islands were analyzed. A selective chemical etching was used to reveal the complex effect of intermixing in islands’ final composition profile. Alloying was found to take place primarily through surface diffusion, as revealed by the etching experiments.

The logical relationship between two previously defined “memory resistors” is revealed by constructing and experimentally demonstrating a three-terminal memistor equivalent circuit using two two-terminal memristors. A technique is then presented, using nanoimprint lithography in combination with angle evaporation, to fabricate a single nanoscale device with a footprint of 4F2, where F is the minimum lithographic feature size, that can be operated as either a two-terminal lateral memristor or a three-terminal memistor inside a crossbar structure. These devices exhibit repeatable bipolar nonvolatile switching behavior with up to 103 ON/OFF conductance ratios, as well as the desired three-terminal behavior.

The quantification of the strain/stress state with high spatial resolution of nanostructured materials is essential for several applications and recent technologies. Several methods have been proposed and applied to address the strain state of such materials, for example, the traditional strain state analysis from high-resolution transmission electron microscopy (HRTEM) images. However, most of these methods fail when dealing with alloyed strained nanostructured materials. Such systems presents both intrinsic (due to chemical variations) and extrinsic (due to externally imposed stresses) strain which make proper analysis difficult. Here is presented a new methodology for high-resolution elastic behavior (strain, stress, elastic energy) assessment of alloyed strained nanostructures based on quantitative high-resolution transmission electron microscopy. The application of this technique to quantify the elastic behavior of the model system Si−Ge:Si(001) shows that this methodology arises as a remarkable tool...

Hybrid reconfigurable logic circuits were fabricated by integrating memristor-based crossbars onto a foundry-built CMOS (complementary metal-oxide-semiconductor) platform using nanoimprint lithography, as well as materials and processes that were compatible with the CMOS. Titanium dioxide thin-film memristors served as the configuration bits and switches in a data routing network and were connected to gate-level CMOS components that acted as logic elements, in a manner similar to a field programmable gate array. We analyzed the chips using a purpose-built testing system, and demonstrated the ability to configure individual devices, use them to wire up various logic gates and a flip-flop, and then reconfigure devices.

The knowledge of composition and strain with atomic resolution is of utmost importance for the understanding of the chemical and electronic properties of alloyed nanostructures, and they can only be extracted in a self-consistent fashion. As an example, several works have addressed the issue of strain and chemical composition on self-assembled epitaxial islands using different techniques such as X-ray diffraction, scanning probe microscopy and transmission electron microscopy (TEM). However, limited information is available on the 3D chemical composition of such nanostructures. Here, we demonstrate the use of a quantitative high-resolution transmission electron microscopy (QHRTEM) technique to obtain two-dimensional (2D) projected chemical maps of epitaxially grown Ge−Si/Si(001) islands, with high spatial resolution, at different crystallographic orientations. Combining these data with iterative simulation, the reconstruction of the three-dimensional (3D) chemical arrangement on the strained Ge−Si/Si(001) islands was realized. This methodology can be applied for a large variety of strained crystalline systems, such as nanowires, epitaxial islands, quantum dots and wells, and partially relaxed heterostructures.

We determined the enthalpic and entropic contributions to the thermodynamics of coherently strained nanocrystals grown via deposition of pure Ge on Si(001) surfaces at 600 and 700 degrees C by analyzing their composition profile and local strain. We found that the free energy associated with the entropy of mixing, which drives GexSi1-x alloy formation, was significantly larger than the relaxation enthalpy that produces the islands. Thus, entropy plays a significant role in the evolution of the size and shape of the islands during growth through the strong thermodynamic drive to form an alloy.

Tantalum oxide memristors can switch continuously from a low-conductance semiconducting to a high-conductance metallic state. At the boundary between these two regimes are quantized conductance states, which indicate the formation of a point contact within the oxide characterized by multistable conductance fluctuations and enlarged electronic noise. Here, we observe diverse conductance-dependent noise spectra, including a transition from 1/f2 (activated transport) to 1/f (flicker noise) as a function of the frequency f, and a large peak in the noise amplitude at the conductance quantum GQ=2e2/h, in contrast to suppressed noise at the conductance quantum observed in other systems. We model the stochastic behaviour near the point contact regime using Molecular Dynamics–Langevin simulations and understand the observed frequency-dependent noise behaviour in terms of thermally activated atomic-scale fluctuations that make and break a quantum conductance channel. These results provide insights into switching mechanisms and guidance to device operating ranges for different applications., Oxide memristors exhibit noise in excess of 2–4 orders of magnitude above the baseline at quantized conductance states. Here, the authors measure anomalous electrical noise at these states in tantalum oxide memristors and relate it to thermally-activated atomic fluctuations by numerical simulations.

Self-assembled nanocrystals grown by epitaxy represent a viable way to mass produce quantum structures exhibiting novel electronic properties. In order to infer the electronic properties of such systems, knowledge of both composition and strain is necessary. Here, detailed maps of these quantities were obtained by anomalous X-ray diffraction for samples grown at 600 °C and 700 °C by chemical vapor deposition and molecular beam epitaxy, respectively – two different temperatures and growth techniques. From these maps, the elastic energies stored in the islands were evaluated. It was found that the elastic energy is concentrated at the island borders, regardless of the growth temperature or method. In contrast, the regions with the lowest elastic energy were the island core and top. These results provide insight into the mechanisms that govern the growth and evolution of strained nanocrystals.

To study the effect of phosphorus on the evolution of Ge islands on Si(001), phosphorus-doped islands were annealed in a H2 ambient and undoped islands were annealed in a PH3/H2 ambient. In both cases phosphorus stabilizes the island structure and reduces coarsening during annealing. The changes of island shape during annealing are consistent with phosphorus influencing the thermodynamic properties of the islands, while the slower decrease in the density of islands when phosphorus is present is consistent with phosphorus kinetically retarding surface diffusion of Ge atoms.

When Ge is deposited epitaxially on Si, the strain energy from the lattice mismatch causes the Ge to form distinctive, three-dimensional islands. The shape of the islands is determined by the energies of the surface facets, facet edges, and interfaces. When phosphorus is added during chemical vapor deposition of Ge, the surface energies change, modifying the island shapes and sizes. Three different island shapes are found for doped layers, as for undoped layers; however, each doped island type is smaller than the corresponding undoped island type. The intermediate-size doped islands are of the same family as the undoped multifaceted “dome” structures, but are considerably smaller; they also have a narrow size distribution. The largest doped islands are related to the defective “superdomes” found for undoped islands, but are bounded by a smaller number of facets, creating pyramidal-shaped structures with their edges aligned along 〈110〉 directions. The distribution of Ge among the different island types dep...

We report on the growth of quantum dot (QD) layers of InAsP alloys buried in GaAs by low-pressure metalorganic chemical vapor deposition. Ternary QDs were obtained by the addition of a PH3 flux during the InAs QD growth, exhibiting recombination energies lying between those of InAs and InP QDs. The electronic properties of these QDs, as evaluated by photoluminescence spectroscopy, could be tailored by varying both the growth rate and the PH3 flux for a constant AsH3 flux. The morphology of these QDs was investigated by transmission electron microscopy from which the formation of an InAsP ternary alloy QDs was inferred. Based on electron microscopy results, the fundamental role of As incorporation on the morphology of and on the defect nucleation associated to InAsP QDs could be then evaluated. From this optical–structural combined analysis, we were able to identify the growth conditions that produce good quality InAsP QDs embedded in GaAs.

Ordered arrays of self-assembled ErSi 2 xSmSi 2 and Dysi 2 nanowires aligned along Si [1 1 0] have been grown at 600°C on Si(00 1) vicinal substrates. The nanowires grow perpendicular to the Si dimer rows and are 1.5-5 nm wide, approximately 2 monolayers high and up to 1 μm long. These nanowires were characterized in situ with scanning tunneling microscopy.

We report electronic transport measurements on two-dimensional electron gases in a Ga[Al]As heterostructure with an embedded layer of InAs self-assembled quantum dots. At high InAs dot densities, pronounced Altshuler-Aronov-Spivak magnetoresistance oscillations are observed, which indicate short-range ordering of the potential landscape formed by the charged dots and the strain fields. The presence of these oscillations coincides with the observation of a metal-insulator transition, and a maximum in the electron mobility as a function of the electron density. Within a model based on correlated disorder, we establish a relation between these effects.

Sub-monolayer amounts of Er deposited onto Si (001)react with the substrate to form epitaxial nanowires of crystalline ErSi2. The growth of uniaxial structures occurs because the different crystal structures of ErSi2 and Si have a good lattice match along one Si crystallographic axis but a significant mismatch along the perpendicular Si axis. The nucleation, growth, and subsequent evolution of ErSi2 nanowires were investigated as functions of erbium coverage on the Si (001) surface, annealing time, and annealing temperature. Low annealing temperatures (620 °C) and times (5 min) produced ErSi2 nanowires with widths of a few nanometers, heights less than one nanometer, and lengths of several hundred nanometers. For longer annealing times at low temperature, the surface roughened without significant ripening of the wires. Annealing at intermediate temperatures (∼700 °C) caused stacking faults to form along the long axis of the nanowires and their lengths to ripen. At high temperature (800 °C), the wires broke apart into short segments with stacking faults.

Adding a phosphorus-containing species during chemical vapor deposition of Ge islands on Si (0 0 1) modifies the island sizes and shapes, primarily by changing the surface energies and the relative surface energies of different surface facets. Three distinct island shapes occur, but the island types and their sequence of formation differ from those found with undoped Ge islands. The addition of phosphorus decreases the size of the multifaceted “domes”—the island shape that has a favored island size, providing an additional method for controlling the islands. The largest islands have a steep pyramidal structure not seen for undoped islands.

InGaAs/GaAs self-assembled quantum dots in the presence of a two-dimensional electron gas were studied by micro-photoluminescence. Several sharp optical emission lines attributed to individual quantum dots were observed. These lines exhibit a blue shift and a broadening as the electron gas density is increased. We discuss the origin of the blue shift considering the variation of the built-in electric field in the asymmetric geometry of the dots. The observed broadening of the quantum dot emission lines is an indication of coupling between the two-dimensional electron gas and the electrons in the quantum dot.

We report electronic transport measurements on two-dimensional electron gases in a Ga[Al]As heterostructure with an embedded layer of InAs self-assembled quantum dots. At high InAs dot densities, Altshuler–Aronov–Spivak oscillations are observed. The presence of these oscillations correlates with the observation of a metal–insulator transition, and with the existence of a maximum in the electron mobility as a function of the electron density. These results indicate hexagonal short-range ordering of the charged InAs dots.

The capability to tune the dopant concentrations of oxygen vacancies and other impurities in atomic layer deposited oxide films is crucial for engineering contemporary electronic device properties. For memristor applications, the composition of the oxides needs to be tuned within a certain range with process windows different from the traditional studies. Accordingly, the unusal growth modes, such as the non-self-limited growth, become valuable to explore as demonstrated here.