Frank Mendoza

Switchable photovoltaic effect in bilayer graphene/BiFeO3/Pt heterostructures

We report the switchable photovoltaic effects in graphene/BiFeO3/Pt heterostructures. Pure phase polycrystalline BiFeO3 films were deposited on Pt/TiO2/SiO2/Si substrates by pulse laser deposition. A bilayer graphene was transferred onto the BiFeO3 film which serves as transparent conducting electrodes. The heterostructures showed switchable photovoltaic effect depending on ferroelectric polarization directions indicating depolarization field induced separation of photo-generated carriers. The open circuit voltage (VOC) and short circuit current density (JSC) were measured to be ∼110 mV, ∼92 μA/cm2 in positive polarity and similar values were obtained when the polarity was reversed. The JSC and VOC also showed rapid response (<100 ms) as a function of light exposure time.

Conformal coating of ferroelectric oxides on carbon nanotubes

Nano-size barium strontium titanate Ba0.7Sr0.3TiO3 (BST) maize-bead?like structures were conformally coated on bamboo-like carbon nanotubes (BCNTs) grown on copper substrates. Initially BCNTs were fabricated by hot-filament chemical vapor deposition (HF-CVD) techniques on Cu substrates. Later, BST was deposited on BCNT/Cu by pulsed laser deposition (PLD) techniques. Surface morphology, cross-sectional image and topography of the BCNTs and BCNT-BST maize-like nanostructures were investigated by X-ray, field-emission scanning electron microscopy (FE-SEM), Raman spectroscopy and tunneling electron microscopy (TEM) techniques. BST-BCNT hybrid structures provide an additional degree of freedom to interconnect the oxides, which in turn provides [3D] geometry for functionality. There had been a popular misconception that oxides cannot grow on CNT at high temperature; this long-standing problem has now been solved. The present unregistered nanostructures can be used as sensors; however, if these structures are made as registered arrays, they can be used as nonvolatile memory elements and high-energy density capacitors.

Ultraviolet photosensitivity of sulfur-doped micro- and nano-crystalline diamond

The room-temperature photosensitivity of sulfur-doped micro-, submicro-, and nano-crystalline diamond films synthesized by hot-filament chemical vapor deposition was studied. The structure and composition of these diamond materials were characterized by Raman spectroscopy, scanning electron microscopy, and x-ray diffraction. The ultraviolet (UV) sensitivity and response time were studied for the three types of diamond materials using a steady-state broad UV excitation source and two pulsed UV laser radiations. It was found that they have high sensitivity in the UV region (as high as 109 s−1 mV−1 range), a linear response in a broad spectral range below 320 nm, photocurrents around ∼10−5 A, and a short response time better than 100 ns, which is independent of fluency intensity. A phenomenological model was applied to help understand the role of defects and dopant concentration on the materials’ photosensitivity.The room-temperature photosensitivity of sulfur-doped micro-, submicro-, and nano-crystalline diamond films synthesized by hot-filament chemical vapor deposition was studied. The structure and composition of these diamond materials were characterized by Raman spectroscopy, scanning electron microscopy, and x-ray diffraction. The ultraviolet (UV) sensitivity and response time were studied for the three types of diamond materials using a steady-state broad UV excitation source and two pulsed UV laser radiations. It was found that they have high sensitivity in the UV region (as high as 109 s−1 mV−1 range), a linear response in a broad spectral range below 320 nm, photocurrents around ∼10−5 A, and a short response time better than 100 ns, which is independent of fluency intensity. A phenomenological model was applied to help understand the role of defects and dopant concentration on the materials’ photosensitivity.

Solar-blind field-emission diamond ultraviolet detector

We report our studies on the responsivity of sulfur-doped diamond films to ultraviolet radiation using two types of device configurations: the planar configuration with electrodes directly on the diamond surface, and the electron field emission configuration with a bias electrode suspended above the diamond surface. Diamond films of different grain sizes were employed: microcrystalline diamond, sub-microcrystalline diamond, and nanocrystalline diamond. The responsivity values of diamond films in the field emission configuration reached ∼10 mA/W at around 220 nm, which is ∼40% higher than that of the planar configuration. These responsivity values of diamond films are comparable to those of commercially available photodiodes in the wavelength range of 210–300 nm, but with the advantage of being solar blind. The responsivity data were correlated with the bandgap structure of sulfur-doped diamond.

Study on the optical and electrical properties of tetracyanoethylene doped bilayer graphene stack for transparent conducting electrodes

We report the optical and electrical properties of chemically-doped bilayer graphene stack by tetracyanoethylene, a strong electron acceptor. The Tetracyanoethylene doping on the bilayer graphene via charge transfer was confirmed by Raman spectroscopy and Infrared Fourier transform spectroscopy. Doped graphene shows a significant increase in the sheet carrier concentration of up to 1.520 × 1013 cm−2 with a concomitant reduction of the sheet resistance down to 414.1 Ω/sq. The high optical transmittance (ca. 84%) in the visible region in combination with the low sheet resistance of the Tetracyanoethylene-doped bilayer graphene stack opens up the possibility of making transparent conducting electrodes for practical applications.

Binder Free SnO2-CNT Composite as Anode Material for Li-Ion Battery

Tin dioxide-carbon nanotube (SnO2-CNT) composite films were synthesized on copper substrates by a one-step process using hot filament chemical vapor deposition (HFCVD) with methane gas (CH4) as the carbon source. The composite structural properties enhance the surface-to-volume ratio of SnO2 demonstrating a desirable electrochemical performance for a lithium-ion battery anode. The SnO2 and CNT interactions were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared-attenuated total reflectance (ATR-FTIR) spectroscopy. Comprehensive analysis of the structural, chemical, and electrochemical properties reveals that the material consists of self-assembled and highly dispersed SnO2 nanoparticles in CNT matrix. The process employed to develop this SnO2-CNT composite film presents a cost effective and facile way to develop anode materials for Li-ion battery technology.

Cold cathode emission studies on topographically modified few layer and single layer MoS2 films

Nanostructured materials, such as carbon nanotubes, are excellent cold cathode emitters. Here, we report comparative field emission (FE) studies on topographically tailored few layer MoS2films consisting of ⟨0001⟩ plane perpendicular (⊥) to c-axis (i.e., edge terminated vertically aligned) along with planar few layer and monolayer (1L) MoS2films. FE measurements exhibited lower turn-on field Eto (defined as required applied electric field to emit current density of 10 μA/cm2) ∼4.5 V/μm and higher current density ∼1 mA/cm2, for edge terminated vertically aligned (ETVA) MoS2films. However, Eto magnitude for planar few layer and 1L MoS2films increased further to 5.7 and 11 V/μm, respectively, with one order decrease in emission current density. The observed differences in emission behavior, particularly for ETVA MoS2 is attributed to the high value of geometrical field enhancement factor (β), found to be ∼1064, resulting from the large confinement of localized electric field at edge exposed nanograins. Emission behavior of planar few layers and 1L MoS2films are explained under a two step emission mechanism. Our studies suggest that with further tailoring the microstructure of ultra thin ETVA MoS2films would result in elegant FE properties.

Hot Filament Chemical Vapor Deposition: Enabling the Scalable Synthesis of Bilayer Graphene and Other Carbon Materials

The hot filament chemical vapor deposition (HFCVD) technique is limited only by the size of the reactor and lends itself to be incorporated into continuous roll-to-roll industrial fabrication processes. We discuss the HFCVD reactor design and the interplay between the reactor parameters, such as filament and substrate temperatures, filamentto-substrate distance, and total pressure. Special attention is given to the large-area synthesis of bilayer graphene on copper, which is successfully grown by HFCVD with transmittance greater than 90% in the visible region and no gaps. We also discuss the HFCVD synthesis of carbon nanotubes, microcrystalline diamond, and nanocrystal‐ line diamond.