Gerardo Morell

Crystalline phases at the p‐ to n‐type transition in Cu‐ternary semiconducting films

We report here a study of the Raman spectra of ternary Cu–In–S and Cu–In–Se polycrystalline film compounds as a function of the x=[In]/{Cu]+[In]} ratio. Using these spectra we were able to identify, with high resolution in x, the phases present in the films. We found that the single phase of chalcopyrite CnInSe2 exists over the fairly wide composition range of 0.48⩽x⩽0.55, and that the lattice disorder increases with the increase of In content. No such single phase range was found for the Cu–In–S films. Considering the electrical properties of these materials around x=0.5, it is concluded that the native defect model accounts for the electrical properties of the Cu–In–Se films but does not account simply for the electrical properties of the Cu–In–S films.

Raman study of the network disorder in sputtered and glow discharge a‐Si:H films

We have carried out a comprehensive study of the Raman spectra of a‐Si:H films produced by the glow discharge (GD) and radio frequency sputtering (RFS) deposition techniques. The results show that the short‐range disorder (bond‐angle deviation), as measured by the width of the TO band (ΓTO), is larger in RFS than in GD a‐Si:H films. The intermediate‐range disorder (dihedral angle deviation), as measured by the ratio of the intensity of the TA band to that of the TO band (ITA/ITO), is generally larger in RFS than in GD a‐Si:H films. However, while the ITA/ITO values of RFS films remain relatively close to those of GD films when the interior is probed, the near surface of RFS films shows much larger values evidencing the existence of a significant disorder gradient along the growth axis. Together, these results indicate that the network order and homogeneity of RFS amorphous silicon is lower than those of GD for substrate temperatures that produce the hydrogenated material. These structural differences are ...

Advance in Novel Boron Nitride Nanosheets to Nanoelectronic Device Applications

We report low-temperature synthesis of large-scale boron nitride nanosheets (BNNSs) and their applications for high-performance Schottky diode and gas sensor. Ten minutes of synthesis with a short-pulse-laser-produced plasma deposition technique yields a large amount of highly flat, transparent BNNSs. A basic reason for using short-pulse plasma beams is to avoid nanosheet thermal ablation or have low heat generated. Consequently, it greatly reduces the stress and yield large, flat BNNSs. The average size of obtained BNNS is around 10 μm and thickness is around 1.7 nm. Carbon element has been used for doping BNNSs and achieving BNNSs-based Schottky diode and gas sensing device. Typical current versus voltage characteristics of diode are examined. The breakdown reverse voltage is around -70 V. This probably indicates that the breakdown electric field of BNNSs-based diode is up to 1 × 10(8) V/cm. Sensing behavior of BNNSs-based gas sensor toward methane diluted with dry air is also characterized. The response time and recovery time are around 3 and 5 s at the operating temperature of 150 °C. Relatively, the sensor has poor sensitivity to oxygen gas.

Study of the electron field emission and microstructure correlation in nanocrystalline carbon thin films

Nanocrystalline carbon thin films were deposited by hot-filament chemical vapor deposition using a 2% concentration of methane in hydrogen. The films were deposited on molybdenum substrates under various substrate biasing conditions. A positive bias produced a continuous flow of electrons from the filament onto the substrate, while a negative bias caused the substrate to be bombarded with positive ions. Films were also grown under no bias, for comparison. Differences in the electron field emission properties (turn-on fields and emitted currents) of these films were characterized. Correspondingly, microstructural differences were also studied, as characterized with atomic force microscopy and Raman spectroscopy. Films grown under electron bombardment showed lower turn-on fields, smoother surfaces, and smaller grains than those grown under ion bombardment or no bias. A correlation between the enhanced emission properties and the nanocrystalline carbon material produced by the low-energy particle bombardment...

L-cysteine capped ZnS:Mn quantum dots for room-temperature detection of dopamine with high sensitivity and selectivity

Dopamine (DA) is one of the most important catecholamine neurotransmitters of the human central nervous system, and is involved in many behavioral responses and brain functions. Below normal DA levels in biological fluids can lead to different neurodegenerative conditions. For excess DA levels, a failure in energy metabolism is indicated. In this study, a facile room-temperature phosphorescence sensor is developed to detect DA based on l-cysteine capped Mn doped ZnS quantum dots (l-cys ZnS:Mn QDs). The QDs display a prominent orange emission band peaking at ~598nm, which is strongly quenched upon addition of DA in alkaline medium. The sensor exhibits a linear working range of ~0.15-3.00μM, and a limit of detection of ~7.80nM. These results are explained in terms of a pH-dependent electron transfer process, in which the oxidized dopamine quinone functions as an efficient electron acceptor. The QDs-based sensor shows a high selectivity to DA over common interfering biomolecules (including some amino acids, ascorbic acid, chloride and glucose). The sensor has been successfully applied for the detection of DA in urine samples, yielding recoveries as high as 93%. Our findings indicate that our developed sensor exhibits high sensitivity and reproducibility to determine DA even in biological fluids where DA is at low levels, e.g., in the central nervous system, which is the usual clinical profile of a neurodegenerative disorder associated to the Parkinsons disease.

Single-crystal γ-MnS nanowires conformally coated with carbon.

We report for the first time the fabrication of single-crystal metastable manganese sulfide nanowires (γ-MnS NWs) conformally coated with graphitic carbon via chemical vapor deposition technique using a single-step route. Advanced spectroscopy and electron microscopy techniques were applied to elucidate the composition and structure of these NWs at the nanoscale, including Raman, XRD, SEM, HRTEM, EELS, EDS, and SAED. No evidence of α-MnS and β-MnS allotropes was found. The γ-MnS/C NWs have hexagonal cross-section and high aspect ratio (∼1000) on a large scale. The mechanical properties of individual γ-MnS/C NWs were examined via in situ uniaxial compression tests in a TEM-AFM. The results show that γ-MnS/C NWs are brittle with a Youngs modulus of 65 GPa. The growth mechanism proposed suggests that the bottom-up fabrication of γ-MnS/C NWs is governed by vapor-liquid-solid mechanism catalyzed by bimetallic Au-Ni nanoparticles. The electrochemical performance of γ-MnS/C NWs as an anode material in lithium-ion batteries indicates that they outperform the cycling stability of stable micro-sized α-MnS, with an initial capacity of 1036 mAh g(-1) and a reversible capacity exceeding 503 mAh g(-1) after 25 cycles. This research advances the integration of carbon materials and metal sulfide nanostructures, bringing forth new avenues for potential miniaturization strategies to fabricate 1D core/shell heterostructures with intriguing bifunctional properties that can be used as building blocks in nanodevices.

Free standing graphene-diamond hybrid films and their electron emission properties

Free standing graphene-diamond hybrid films have been fabricated using saturated hydrocarbon polymers as seeding material by hot filament chemical vapor deposition technique. The films are characterized with x-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron energy loss spectroscopy (EELS). The XRD shows the characteristic diffraction peaks of both diamond and graphene. The Raman spectrum shows the characteristic band of diamond at 1332 cm−1 and D, G, and 2D bands of graphene at 1349, 1592, and 2687 cm−1, respectively. Both SEM and TEM depict the presence of diamond and graphene in the films. The EELS recorded in the carbon K-edge region also shows the signature peaks of diamond and graphene. The free standing hybrid films exhibit a remarkably low turn-on field of about 2.4 V/μm and a high emission current density of 0.1 mA/cm2. Furthermore, emission currents are stable over the period of 7 days. The superior field emission...

Characterization of the silicon network disorder in hydrogenated amorphous silicon carbide alloys with low carbon concentrations

Abstract The network disorder of amorphous Si 1 − x C x :H films, containing carbon concentrations below 25 at.%, has been studied by means of Raman spectroscopy. Two different radiations were employed to excite the Raman scattering of these materials, one that is strongly (458 nm) absorbed and another that is weakly (581 nm) absorbed. The variation in probed depth attained with these excitation radiations together with the significant differences observed in the Raman spectra excited with them indicate the existence of silicon network disorder inhomogeneities at the short-range (bond angle) and intermediate-range (dihedral angle) levels along the axis perpendicular to the films which are considerably larger than those existing in device-quality a-Si:H. Such inhomogeneities are observed to develop abruptly at the smallest carbon concentration. The findings explain the various previous conflicting reports in the literature regarding the behavior of the Si-network disorder in these alloys. When excited with strongly absorbed radiations (e.g., 514 and 488 nm), the Raman spectra do not show a definite trend as a function of carbon concentration, although it can still be concluded that there is less order than in a-Si:H. On the other hand, the Raman spectra excited with weakly absorbed radiation show that the order actually improves in the bulk of Si-rich a-SiC:H films. The above findings indicate that silicon is forming compact clusters in the interior of the films on this compositional regime. Besides, our results show that hydrogen dilution of the gas mixture during film growth helps improve the short-range order in the bulk of the films with the smallest carbon concentrations, but has no definite effect over the intermediate-range order or over the short-range order at the near surface. It is also found that the spectral intensity enhancement observed in these materials with increasing carbon content is due to a scattering volume effect.

Photovoltaic effect in a wide-area semiconductor-ferroelectric device

Millimeter-diameter planar devices of glass/ZnO:Al/BiFeO3/La0.67Sr0.33CoO3 (LSCO) heterostructures were fabricated by pulsed laser deposition (PLD) techniques. Diode-like behavior with high short-circuit current (SSC ∼ 4 mA/cm2) and open-circuit voltage (OCV ∼ 0.22 V) was obtained under the illumination of about 1% of maximum solar energy. Impedance spectroscopy revealed that electrode/dielectric interface and grain-boundary conduction are mainly responsible for the photo-current. Electrode/dielectric interface, grain boundary impedance, and low-frequency ac conductivity change by almost three orders of magnitude under weak light. Relaxation time of the photo-carriers changes from 80 ms to 96 μs suggesting that with optimal collecting instruments, one should expect currents several orders higher.

Role of sp2 C cluster size on the field emission properties of sulfur-incorporated nanocomposite carbon thin films

The electron field emission properties of sulfur-incorporated nanocomposite carbon thin films grown by hot-filament chemical vapor deposition were investigated as a function of film microstructure. The in-plane correlation length (La) of the sp2 C clusters in these films was determined from the intensity ratio of the D and G bands [I(D)/I(G)] in the visible Raman spectra using a phenomenological model. The turn-on field was found to decrease with increasing sp2 C cluster size in the range of 0.8–1.4 nm. The lowest turn-on field found was 4.0 V/μm corresponding to films having sp2 C clusters of around 1.4 nm and conductivity of 30 Ω−1 cm−1. These findings are discussed in terms of a reduced field emission barrier brought about by the incorporation of sulfur and the need for relatively longer conductive paths capable of withstanding the relatively large emission currents.