Effects of electric current on individual graphene oxide sheets combining in situ transmission electron microscopy and Raman spectroscopy.

Graphene oxide (GO) is currently the object of extensive research because of its potential use in mass production of graphene-based materials, but also due to its tunability which holds great promise for new nanoscale electronic devices and sensors. To obtain a better understanding of the role of GO in electronic nano-devices, the elucidation of the effects of electrical current on a single GO sheet is of great interest. In this work, in situ transmission electron microscopy is used to study the effects of the electrical current flow through single GO sheets using an scanning tunneling microscope holder. In order to correlate the applied current with the structural properties of GO, Raman spectroscopy is carried out and data analysis is used to obtain information regarding the reduction grade and the disorder degree of the GO sheets before and after the application of current.

Tailoring Single-Mode Random Lasing of Tin Halide Perovskites Integrated in a Vertical Cavity.

The development of random lasing (RL) with predictable and controlled properties is an important step to make these cheap optical sources stable and reliable. However, the design of tailored RL characteristics (emission energy, threshold, number of modes) is only obtained with complex photonic structures, while the simplest optical configurations able to tune the RL are still a challenge. This work demonstrates the tuning of the RL characteristics in spin-coated and inkjet-printed tin-based perovskites integrated into a vertical cavity with low quality factor. When the cavity mode is resonant with the photoluminescence (PL) peak energy, standard vertical lasing is observed. More importantly, single mode RL operation with the lowest threshold and a quality factor as high as 1 000 (twenty times the quality factor of the resonator) is obtained if the cavity mode lies above the PL peak energy due to higher gain. These results can have important technological implications toward the development of low-cost RL sources without chaotic behavior.

The role of oxidative debris on graphene oxide films.

We study the effect of oxidative impurities on the properties of graphene oxide and on the graphene oxide Langmuir-Blodgett films (LB). The starting material was grupo Antolín nanofibers (GANF) and the oxidation process was a modified Hummers method to obtain highly oxidized graphene oxide. The purification procedure reported in this work eliminated oxidative impurities decreasing the thickness of the nanoplatelets. The purified material thus obtained presents an oxidation degree similar to that achieved by chemical reduction of the graphite oxide. The purified and non-purified graphene oxides were deposited onto silicon by means of a Langmuir-Blodgett (LB) methodology. The morphology of the LB films was analyzed by field emission scanning microscopy (FE-SEM) and micro-Raman spectroscopy. Our results show that the LB films built by transferring Langmuir monolayers at the liquid-expanded state of the purified material are constituted by close-packed and non-overlapped nanoplatelets. The isotherms of the Langmuir monolayer precursor of the LB films were interpreted according to the Volmer's model.

Inkjet-printed h-BN memristors for hardware security.

Inkjet printing electronics is a growing market that reached 7.8 billion USD in 2020 and that is expected to grow to ∼23 billion USD by 2026, driven by applications like displays, photovoltaics, lighting, and radiofrequency identification. Incorporating two-dimensional (2D) materials into this technology could further enhance the properties of the existing devices and/or circuits, as well as enable the development of new concept applications. Along these lines, here we report an easy and cheap process to synthesize inks made of multilayer hexagonal boron nitride (h-BN)-an insulating 2D layered material-by the liquid-phase exfoliation method and use them to fabricate memristors. The devices exhibit multiple stochastic phenomena that are very attractive for use as entropy sources in electronic circuits for data encryption (physical unclonable functions [PUFs], true random number generators [TRNGs]), such as: (i) a very disperse initial resistance and dielectric breakdown voltage, (ii) volatile unipolar and non-volatile bipolar resistive switching (RS) with a high cycle-to-cycle variability of the state resistances, and (iii) random telegraph noise (RTN) current fluctuations. The clue for the observation of these stochastic phenomena resides on the unpredictable nature of the device structure derived from the inkjet printing process (i.e., thickness fluctuations, random flake orientations), which allows fabricating electronic devices with different electronic properties. The easy-to-make and cheap memristors here developed are ideal to encrypt the information produced by multiple types of objects and/or products, and the versatility of the inkjet printing method, which allows effortless deposition on any substrate, makes our devices especially attractive for flexible and wearable devices within the internet-of-things.

Functionalization of reduced graphite oxide sheets with a zwitterionic surfactant.

Films of a few layers in thickness of reduced graphite oxide (RGO) sheets functionalized by the zwitterionic surfactant N-dodecyl-N,N-dimethyl-3-ammonio-1-propanesulfonate (DDPS) are obtained by using the Langmuir-Blodgett method. The quality of the RGO sheets is checked by analyzing the degrees of reduction and defect repair by means of X-ray photoelectron spectroscopy, atomic force microscopy (AFM), field-emission scanning electron microscopy (SEM), micro-Raman spectroscopy, and electrical conductivity measurements. A modified Hummers method is used to obtain highly oxidized graphite oxide (GO) together with a centrifugation-based method to improve the quality of GO. The GO samples are reduced by hydrazine or vitamin C. Functionalization of RGO with the zwitterionic surfactant improves the degrees of reduction and defect repair of the two reducing agents and significantly increases the electrical conductivity of paperlike films compared with those prepared from unfunctionalized RGO.

2D PEA2SnI4 Inkjet-Printed Halide Perovskite LEDs on Rigid and Flexible Substrates.

Lead-free PEA2SnI4-based perovskite LEDs are successfully inkjet-printed on rigid and flexible substrates. Red-emitting devices (λmax = 633 nm) exhibit, under ambient conditions, a maximum external quantum efficiency (EQEmax) of 1% with a related brightness of 30 cd/m2 at 10 mA/cm2.

An investigation on solid state reactions in heat treated Au/Pd thin films for electrodes applications.

The interface between metals and oxides is particularly interesting. It has been the main topic of many research projects to illustrate that their properties are highly dependent on the structure of the interface. Poor adhesion between gold films and oxides is well known. On the basis of solid solution between the materials, the main effect of a Pd thin film as the adhesion layer between substrate and Au was investigated. Pure palladium and gold films were grown by electron beam evaporation sequentially on the SrTiO3 (001) single crystal substrates. After different annealing time (from 0.06 to 1.8 ks) at 500 degrees C, the growth sequence of the compositions between Au and Pd evolved as a results of the solid solution were investigated by means of X-ray photoelectron spectroscopy (XPS), X-ray diffraction. Meanwhile the surface morphology and the surface topography were examined respectively by scanning electron microscopy (SEM) and atomic force microscopy (AFM) observation. The effect of an intermediate Pd adhesion layer and its influence on the interfacial properties and morphology of the Au layer is investigated.

Triple-twin domains in Mg doped GaN wurtzite nanowires: structural and electronic properties of this zinc-blende-like stacking.

We report on the effect of Mg doping on the properties of GaN nanowires grown by plasma assisted molecular beam epitaxy. The most significant feature is the presence of triple-twin domains, the density of which increases with increasing Mg concentration. The resulting high concentration of misplaced atoms gives rise to local changes in the crystal structure equivalent to the insertion of three non-relaxed zinc-blende (ZB) atomic cells, which result in quantum wells along the wurtzite (WZ) nanowire growth axis. High resolution electron energy loss spectra were obtained exactly on the twinned (zinc-blende) and wurtzite planes. These atomically resolved measurements, which allow us to identify modifications in the local density of states, revealed changes in the band to band electronic transition energy from 3.4 eV for wurtzite to 3.2 eV in the twinned lattice regions. These results are in good agreement with specific ab initio atomistic simulations and demonstrate that the redshift observed in previous photoluminescence analyses is directly related to the presence of these zinc-blende domains, opening up new possibilities for band-structure engineering.

A model for the response towards oxidizing gases of photoactivated sensors based on individual SnO2 nanowires.

The paper presents a quantitative model to elucidate the role of impinging photons on the final response towards oxidizing gases of light-activated metal oxide gas sensors. The model is based on the competition between oxygen molecules in air and oxidizing target gases (such as NO(2)) for the same adsorption sites: the surface oxygen vacancies (OV). The model fairly reproduces the experimental measurements of both the steady-state and the dynamic response of individual SnO(2) nanowires towards oxidizing gases. Quantitative results indicate that: (1) at room temperature NO(2) adsorbs onto OV more avidly than oxygen; (2) the flux of photons and the NO(2) concentration determine the partition of the two gas populations at the surface; and (3) the band-to-band generation of electron-hole pairs plays a significant role in the photodesorption process of gas molecules. The model also offers a methodology to estimate some fundamental parameters, such as the adsorption rates and the photodesorption cross sections of oxidizing molecules interacting with the nanowires' surface. All these results, enabled by the use of individual nanowires, provide deep insight about how to control the response of metal oxide nanowires towards oxidizing gases, paving the way to the development and consolidation of this family of low consumption conductometric sensors operable at room temperature.

Low-Power, High-Performance, Non-volatile Inkjet-Printed HfO2-Based Resistive Random Access Memory: From Device to Nanoscale Characterization.

Low-power, high-performance metal-insulator-metal (MIM) non-volatile resistive memories based on HfO2 high- k dielectric are fabricated using a drop-on-demand inkjet printing technique as a low-cost and eco-friendly method. The characteristics of resistive switching of Pt (bottom)/HfO2/Ag (top) stacks on Si/SiO2 substrates are investigated in order to study the bottom electrode's interaction with the HfO2 dielectric layer and the resulting effects on resistive switching. The devices show low Set and Reset voltages, high ON/OFF current ratio, and relatively low switching current (∼1 µA), which are comparable to the characteristics of current commercial CMOS memories. In order to understand the resistive switching mechanism, direct structural observation is carried out by field-emission scanning electron microscopy (FE-SEM) and high-resolution transmission electron microscopy (HRTEM) on cross-sectioned samples prepared by focused ion beam (FIB). In addition, electron energy loss spectroscopy (EELS) inspections discard a silver electro-migration effect.

Low-Cost Fabrication of Printed Electronics Devices through Continuous Wave Laser-Induced Forward Transfer.

Laser-induced forward transfer (LIFT) is a direct-writing technique that allows printing inks from a liquid film in a similar way to inkjet printing but with fewer limitations concerning ink viscosity and loading particle size. In this work, we prove that liquid inks can be printed through LIFT by using continuous wave (CW) instead of pulsed lasers, which allows a substantial reduction in the cost of the printing system. Through the fabrication of a functional circuit on both rigid and flexible substrates (plastic and paper), we provide a proof-of-concept that demonstrates the versatility of the technique for printed electronics applications.

A Study of Carbon Nanofibers and Active Carbon as Symmetric Supercapacitor in Aqueous Electrolyte: A Comparative Study.

Symmetric supercapacitors are fabricated by carbon nanofibers (CNF) and activated carbon (AC) using similar proportions of 7 wt% polyvinylidene fluoride (PVDF) polymer binder in an aqueous electrolyte. In this study, a comparison of porous texture and electrochemical performances between CNFs and AC based supercapacitors was carried out. Electrodes were assembled in the cell without a current collector. The prepared electrodes of CNFs and AC present Brunauer-Emmett-Teller (BET) surface area of 83 and 1042 m2/g, respectively. The dominant pore structure for CNFs is mesoporous while for AC is micropore. The results showed that AC provided higher specific capacitance retention up to very fast scan rate of 500 mV/s. AC carbon had a specific capacitance of 334 F/g, and CNFs had 52 F/g at scan rate 5 mV/s in aqueous solution. Also, the results indicate the superior conductivity of CNFs in contrast to AC counterparts. The measured equivalent series resistance (ESR) showed a very small value for CNFs (0.28 Ω) in comparison to AC that has an ESR resistance of (3.72 Ω). Moreover, CNF delivered higher specific power (1860 W/kg) than that for AC (450 W/kg). On the other hand, AC gave higher specific energy (18.1 Wh/kg) than that for CNFs (2 Wh/kg).This indicates that the AC is good for energy applications. Whereas, CNF is good for power application. Indeed, the higher surface area will lead to higher specific capacitance and hence higher energy density for AC. For CNF, lower ESR is responsible for having higher power density.Both CNF and AC supercapacitor exhibit an excellent charge-discharge stability up to 2500 cycles.

Energetics and carrier transport in doped Si/SiO2 quantum dots.

In the present theoretical work we have considered impurities, either boron or phosphorous, located at different substitutional sites in silicon quantum dots (Si-QDs) with diameters around 1.5 nm, embedded in a SiO2 matrix. Formation energy calculations reveal that the most energetically-favored doping sites are inside the QD and at the Si/SiO2 interface for P and B impurities, respectively. Furthermore, electron and hole transport calculations show in all the cases a strong reduction of the minimum voltage threshold, and a corresponding increase of the total current in the low-voltage regime. At higher voltages, our findings indicate a significant increase of transport only for P-doped Si-QDs, while the electrical response of B-doped ones does not stray from the undoped case. These findings are of support for the employment of doped Si-QDs in a wide range of applications, such as Si-based photonics or photovoltaic solar cells.

Chemoresistive sensing of light alkanes with SnO2 nanocrystals: a DFT-based insight.

Density functional theory (DFT) modelling of the alkane-SnO2 surface interaction correctly predicts the results of the chemoresistive alkane sensing tests, provided that the highly reduced nature of the SnO2 nanocrystal surface is properly inserted in the model.