Papers by Pradip Basnet, Ph.D.
ChemInform, Aug 1, 2016
Visible Light-Induced Photoepoxidation of Propene by Molecular Oxygen over Chromia-Silica Catalys... more Visible Light-Induced Photoepoxidation of Propene by Molecular Oxygen over Chromia-Silica Catalysts.-Highly dispersed chromate species on silica are found to catalyze the oxidation of propene to propene oxide by molecular oxygen under UV irradiation and even under visible light irra
Journal of Nanoelectronics and Optoelectronics, Oct 1, 2019
ACS applied electronic materials, Mar 8, 2023
Understanding the resistance switching behavior of oxide-based memristive devices is critical for... more Understanding the resistance switching behavior of oxide-based memristive devices is critical for evaluating their usefulness in nonvolatile memory and/or in artificial neural networks. Oxide memristors often employ bi-or multilayered metal oxide thin films for improved performance compared to devices with a single-metal-oxide active layer. However, a clear understanding of the mechanisms that lead to improved performance for specific combinations of oxide thin films is still missing. Herein, we fabricated two types of bilayered heterostructure devices, with HfO x /AlO y and AlO y /HfO x bilayer films sandwiched between Au electrodes. Electrical responses of these bilayer devices reveal a digital set and an analog reset transition process. Single-layer HfO x and AlO y devices are also examined as control samples to validate the switching mechanism. The role of bilayered heterostructures is investigated using both the experimental and simulated results. Our results suggest that synergistic switching performance can be achieved with a proper combination of these materials, optimized structures, and proper test conditions. These results open the avenue for designing more efficient double-or multilayered memristive devices for an analog response.
Advances in Materials Science and Engineering, Nov 7, 2019
Field effect transistors (FETs) with their Boolean operation have been the fundamental building b... more Field effect transistors (FETs) with their Boolean operation have been the fundamental building block of digital computers. While this digital framework is excellent at performing complex arithmetic and logic calculations, it lags far behind the human brain in key areas such as adaptivity, generalization, and pattern recognition. Neuromorphic computation using nanoscale adaptive oxide devices or memristors is a very promising alternative to this framework. Oxides of transition metals such as hafnium (HfO2) are proven to be excellent candidate materials for these devices, which show non-volatile memory and analog switching characteristics, while employing only two voltage terminals. This work focuses on the computational investigation of local and temporal variation of voltage, current and temperature, so that the coupled nature of heat transfer and current flow through these devices is analyzed and the effects of substrate materials, set/reset voltages on relative on-state/off-state resistances, current magnitude and filament temperature are determined. These analyses are expected to provide insightful information on switching mechanisms of these adaptive oxides and warrant their scaled-up implementation for neuromorphic computing.
AIP Advances, Mar 1, 2020
Neuromorphic computation using nanoscale adaptive oxide devices or memristors is a very promising... more Neuromorphic computation using nanoscale adaptive oxide devices or memristors is a very promising alternative to the conventional digital computing framework. Oxides of transition metals, such as hafnium (HfOx), have been proven to be excellent candidate materials for these devices, because they show non-volatile memory and analog switching characteristics. This work presents a comprehensive study of the transport phenomena in HfOx based memristors and involves the development of a fully coupled electrothermal and mass transport model that is validated with electrical and thermal metrology experiments. The fundamental transport mechanisms in HfOx devices were analyzed together with the local and temporal variation of voltage, current, and temperature. The effect of thermal conductivity of substrate materials on the filament temperature, voltage ramp rate, and set/reset characteristics was investigated. These analyses provide insight into the switching mechanisms of these oxides and allow for the prediction of the effect of device architecture on switching behavior.
ACS applied nano materials, Mar 13, 2019
Hybrid nanocomposites of Cu2O–(R)TiO2, CuO–(R)TiO2, and Cu3TiO4–(R)TiO2 (where R represents the r... more Hybrid nanocomposites of Cu2O–(R)TiO2, CuO–(R)TiO2, and Cu3TiO4–(R)TiO2 (where R represents the rutile phase of TiO2) nanopowders (NPs) were produced via solid state reaction followed by 20 h of ball milling; their photocatalytic activities were evaluated for methylene blue (MB) degradation under visible light intensity (∼65 mW/cm2) and compared to Degussa P25 under both ultraviolet (UV) and visible light irradiations. The highest MB degradation rate under the visible light irradiation was observed to be 0.271 ± 0.010 h–1 for Cu2O–(R)TiO2 NPs, which was 2.5 times higher than that of P25, while under UV illumination both the Cu2O–(R)TiO2 and Cu3TiO4–(R)TiO2 NPs were slightly less active than that of the P25, and CuO–(R)TiO2 was the least active among all. The solar energy conversion performance of the CuxO–(R)TiO2 NPs was observed to be controlled by the applied potentials. The highest stable cathodic photocurrent density (6.3 μA/cm2) was observed for Cu2O–(R)TiO2 NPs at a low negative bias voltage (−0.3 V...
Nanoscale, 2017
The figure of merits (FOMs) of plasmonic sensors can be pushed to a new level (∼700 RIU−1) based ... more The figure of merits (FOMs) of plasmonic sensors can be pushed to a new level (∼700 RIU−1) based on the differential polarization transmission spectra of elliptical nanoholes.
Catalysis Science & Technology, 2016
Please note that technical editing may introduce minor changes to the text and/or graphics, which... more Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal's standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
RSC Advances, 2019
A comprehensive investigation on the equilibrium geometry, relative stability, vibrational spectr... more A comprehensive investigation on the equilibrium geometry, relative stability, vibrational spectra, and magnetic and electronic properties of neutral tantalum clusters (Ta n , n ¼ 2-17) was performed using density functional theory (DFT). We perform a study of the size dependence and correlations among those descriptors of parameters, and showed these could provide a novel way to confirm and predict experimental results. Some new isomer configurations that have never been reported before for tantalum clusters were found. The growth behaviors revealed that a compact geometrical growth route is preferred and develops a body-centered-cubic (BCC) structure with the cluster size increasing. The perfectly fitted functional curve, strong linear evolution, and obvious odd-even oscillation behavior proved their corresponding properties depended on the cluster size. Multiple demonstrations of the magic number were confirmed through the correlated relationships with the relative stability, including the second difference in energy, maximum hardness, and minimum polarizability. An inverse evolution trend between the energy gap and electric dipole moment and strong linear correlation between ionization potentials and polarizability indicated the strong correlation between the magnetic and electronic properties. Vibrational spectroscopy as a fingerprint was used to distinguish the ground state among the competitive geometrical isomers close in energy. The charge density difference isosurface, density of states, and molecular orbitals of selected representative clusters were analyzed to investigate the difference and evolutional trend of the relative stability and electronic structure. In addition, we first calculated the ionization potential and magnetic moment and compared these with the current available experimental data for tantalum clusters.
Journal of Materials Chemistry C, 2020
While two-terminal HfOX (x<2) memristor devices have been studied for ion transport and current e... more While two-terminal HfOX (x<2) memristor devices have been studied for ion transport and current evolution, there have been limited reports on the effect of the long-range thermal environment on their performance. In this work, amorphous-HfOX based memristor devices on two different substrates, thin SiO 2 (280 nm)/Si and glass, with different thermal conductivities in the range from 1.2 to 138 W/m-K were fabricated. Devices on glass substrates exhibit lower reset voltage, wider memory window and, in turn, a higher performance window. In addition, the devices on glass show better endurance than the devices on the SiO 2 /Si substrate. These devices also show nonvolatile multi-level resistances at relatively low operating voltages which is critical for neuromorphic computing applications. A Multiphysics COMSOL computational model is presented that describes the transport of heat, ions and electrons in these structures. The combined experimental and COMSOL simulation results indicate that the long-range thermal environment can have a significant impact on the operation of HfOx-based memristors and that substrates with low thermal conductivity can enhance switching performance.
ACS Applied Materials & Interfaces, Nov 6, 2013
Using a unique oblique angle co-deposition technique, well aligned arrays of Ag nanoparticle embe... more Using a unique oblique angle co-deposition technique, well aligned arrays of Ag nanoparticle embedded TiO 2 composite nanorods have been fabricated with different concentrations of Ag. The structural, optical and photocatalytic properties of the composite nanostructures are investigated using a variety of experimental techniques and compared with those of pure TiO 2 nanorods fabricated similarly. Ag nanoparticles are formed in the composite nanorods, which increase the visible light absorbance due to localized surface plasmon resonance. The Ag concentrations and the annealing conditions are found to affect the size and the density of Ag nanoparticles and their optical properties. The Ag nanoparticle embedded TiO 2 nanostructures exhibit enhanced photocatalytic activity compared to pure TiO 2 under visible-or UV-light illumination. Ag plays different roles in assisting the photocatalysis with different light sources. Ag can be excited and inject electrons to TiO 2 , working as an electron donor under visible light. While under UV illumination, Ag acts as electron acceptor to trap the photogenerated electrons in TiO 2. Due to the opposite electron transfer direction under UV and visible light, the presence of Ag may not result in a greater enhancement in the photocatalytic performance.
ACS Applied Materials & Interfaces, Mar 13, 2013
Both Fe2O3 thin films and nanorod arrays are deposited using electron beam evaporation through no... more Both Fe2O3 thin films and nanorod arrays are deposited using electron beam evaporation through normal thin film deposition and oblique angle deposition (OAD) and are characterized structurally, optically, and photocatalytically. The morphologies of the thin films are found to be arrays of very thin and closely packed columnar structures, while the OAD films are well-aligned nanorod arrays. All films were determined to be in the hematite phase (α-Fe2O3), as confirmed by both structural and optical characterization. Texture measurements indicate that films have similar growth modes where the [110] direction aligns with the direction of material growth. Under visible light illumination, the thin film samples were more efficient at photocatalytically degrading methylene blue, while the nanorod arrays were more efficient at inactivating E. coli O157:H7. The size of the targeted agent and the different film morphologies result in different reactant/surface interactions, which is the main factor that determines photoactivity. Furthermore, an analytic mathematical model of bacterial inactivation based on chemotactic bacterial diffusion and surface deactivation is developed to quantify and compare the inactivation rate of the samples. These results indicate that α-Fe2O3 nanorods are promising candidates for antimicrobial applications and are expected to provide insight into the development of better visible-light antimicrobial materials for food products and processing environments, as well as other related applications.
ACS Applied Materials & Interfaces, Oct 23, 2018
Transition metal dichalcogenide (TMD)-based vertical Schottky heterostructures have recently show... more Transition metal dichalcogenide (TMD)-based vertical Schottky heterostructures have recently shown promise as a next generation device for a variety of applications. In order for these devices to operate effectively, the interface between the TMD and metal contacts must be well-understood and optimized. In this work, the interface between synthesized MoS2 and gold or platinum metal contacts is explored as a function of MoS2 film quality to understand Fermi level pinning effects. Raman, X-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy are used to physically characterize both MoS2 and MoS2/metal interface. Metal/MoS2/metal purely vertical heterostructure cross-point devices were fabricated to explore the injection behavior across the Schottky barrier formed between MoS2 and the metal. The temperature dependence of the device behavior is used to understand injection mechanisms, and modeling is performed to verify the injection mechanisms across the interface barrier. By combining both physical characterization with electrical results and modeling, Fermi level pinning is investigated as a function of macroscopic MoS2 quality. Low-quality MoS2 was found to exhibit much stronger pinning than high-quality films, which is consistent with an observed increase in covalency of the metal/MoS2 interface. Additionally, MoS2 was found to pin gold much more strongly than platinum, which is consistent with an increased covalent interaction between MoS2 and gold. These results show that the synthesis temperature and, therefore, the quality of MoS2 dramatically impacts Fermi level pinning and the resultant current-voltage characteristics of Schottky barrier-mediated devices.
Journal of materials chemistry. A, Materials for energy and sustainability, 2014
Applied Physics Letters, Feb 17, 2014
Metal oxides (MOs) nanostructures, as visible light active photocatalysts, are widely used in app... more Metal oxides (MOs) nanostructures, as visible light active photocatalysts, are widely used in applications such as solar energy conversion, biocidal coating, and environmental remediation. This dissertation investigates the morphological, structural, optical, and photocatalytic properties of some visible light active MO photocatalysts, WO3, CuxO (x = 1, 2), and α-Fe2O3 nanostructures fabricated by the dynamic shadowing growth for above mentioned applications. Dye adsorption and photodegradation property of WO3 nanorods (NRs) has first been studied. The porous and amorphous WO3 NRs are observed to exhibit superior methylene blue (MB) adsorption capability in aqueous solution due to its large specific surface area and active surface functionality. The adsorption of MB on the surface of WO3 NRs are well described by Langmuir isotherm behavior. Photocatalytic MB degradation with WO3 NRs under UV irradiation is observed to be relatively higher than that under visible light. The solar water splitting reaction in a photoelectrochemical (PEC) cell and photocatalytic dye degradation behavior of CuxO (x=1,2) NR arrays have been studied under visible light irradiation. Both single phase Cu2O and CuO, and mixed phase Cu2O/CuO polycrystalline NRs are observed to exhibit excellent visible light induced photocatalytic activity for both cationic (MB) and anionic (methyl orange) dye degradation. When used as a photocathode, they also show good PEC performance, especially for the mixed phase Cu2O/CuO NRs. The maximum stable photocurrent density is observed to be -0.24 mA/cm2 under a simulated solar light (AM 1.5G) at an applied bias potential of -0.5 V (versus Ag/AgCl reference electrode). About 20% and 44% incident photon-to-current conversion efficiency are obtained at incident light wavelengths λ = 500 nm and 400 nm respectively. Finally, both MB degradation and bactericidal activities are studied for the α-Fe2O3 NR arrays and films under the ambient light conditions. The α-Fe2O3 NR arrays annealed at 350 °C exhibit an enhanced bactericidal performance in inactivating E. coli O157:H7, when compared to α-Fe2O3 film. Mathematical models are used to correlate the observed photocatalytic activities.
Applied Physics Letters, Feb 10, 2020
Filamentary adaptive oxide devices based on HfOx are a promising technology for neuromorphic comp... more Filamentary adaptive oxide devices based on HfOx are a promising technology for neuromorphic computing applications. The resistance of these devices depends on the concentration of oxygen vacancies in the filament region. Local temperature rise from joule heating plays a significant role in the movement of oxygen ions, making thermal management crucial to reliable performance. In this work, the role of the substrate thermal conductivity on the analog performance was investigated at biologically realistic pulse widths. Au/Ti/HfOx/Au adaptive oxide devices were fabricated on substrates with two orders of magnitude difference in thermal conductivity. A lower thermal conductivity substrate dissipates heat more slowly, resulting in a large initial change in resistance from a single operation pulse; which is detrimental to the desired analog behavior. The results were validated by a COMSOL Multiphysics ® model that models the flow of heat in both samples. Main Text As the scaling limit for silicon-based transistors approaches, developing new materials that can be used for non-von Neumann architectures becomes increasingly important. Since the first demonstration of an adaptive oxide device based on TiO2 in 2008 by Strukov et al. 1 , many studies have demonstrated resistive switching of various binary transition metal oxides. 1-10 These materials exhibit a non-volatile change in resistance by modulating the concentration of oxygen
Catalysis Today, Jul 1, 2016
By using the glancing angle deposition technique and post-deposition annealing, Fe 2 O 3-TiO 2 co... more By using the glancing angle deposition technique and post-deposition annealing, Fe 2 O 3-TiO 2 core-shell nanorod arrays with specific crystalline states can be designed and fabricated. The Fe 2 O 3-TiO 2 core-shell samples annealed at temperatures greater than 450°C formed α-Fe 2 O 3 and anatase TiO 2 , and showed higher catalytic efficiency for the degradation of methylene blue (MB) under visible light illumination when compared with pure anatase TiO 2 or α-Fe 2 O 3 nanorod arrays. Solar conversion of carbon dioxide and water vapor in the presence of Fe 2 O 3-TiO 2 coreshell nanorod arrays was also investigated. Carbon monoxide, hydrogen, methane, and methanol along with other hydrocarbons were produced after only several hours' exposure under ambient
ChemistrySelect, Jun 1, 2016
Narrow bandgap semiconductor CuBi 2 O 4 microstructures such as three dimensional hierarchical mi... more Narrow bandgap semiconductor CuBi 2 O 4 microstructures such as three dimensional hierarchical microspheres, micro-flowers, one dimensional microrods and nanorods were fabricated through a facile hydrothermal synthesis approach by systematically changing the synthesis conditions. Their optical properties and photocatalytical performances under visible light illumination were characterized. The hierarchical micro-flower shaped CuBi 2 O 4 sample showed a high photodecay rate of 0.114 AE 0.002 h À1 with 1 mg/ml CuBi 2 O 4 concentration for methylene blue (30 mM). Under the illumination of an AM 1.5G solar simulator, a 0.02 mA/cm 2 stable cathodic photocurrent density was observed at a low negative bias voltage (-0.25 V vs (Ag/AgCl)). The micro-flower shaped CuBi 2 O 4 sample also showed a high antibacterial effect against Escherichia coli, with~93 % bacteria reduction after 6 h illumination by a commercial white LED light (10 mW/cm 2). These results indicate that CuBi 2 O 4 can be a promising candidate for visible light driven energy conversion and antimicrobial applications.
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Papers by Pradip Basnet, Ph.D.
Dye adsorption and photodegradation property of WO3 nanorods (NRs) has first been studied. The porous and amorphous WO3 NRs are observed to exhibit superior methylene blue (MB) adsorption capability in aqueous solution due to its large specific surface area and active surface functionality. The adsorption of MB on the surface of WO3 NRs are well described by Langmuir isotherm behavior. Photocatalytic MB degradation with WO3 NRs under UV irradiation is observed to be relatively higher than that under visible light.
The solar water splitting reaction in a photoelectrochemical (PEC) cell and photocatalytic dye degradation behavior of CuxO (x=1,2) NR arrays have been studied under visible light irradiation. Both single phase Cu2O and CuO, and mixed phase Cu2O/CuO polycrystalline NRs are observed to exhibit excellent visible light induced photocatalytic activity for both cationic (MB) and anionic (methyl orange) dye degradation. When used as a photocathode, they also show good PEC performance, especially for the mixed phase Cu2O/CuO NRs. The maximum stable photocurrent density is observed to be -0.24 mA/cm2 under a simulated solar light (AM 1.5G) at an applied bias potential of -0.5 V (versus Ag/AgCl reference electrode). About 20% and 44% incident photon-to-current conversion efficiency are obtained at incident light wavelengths λ = 500 nm and 400 nm respectively.
Finally, both MB degradation and bactericidal activities are studied for the α-Fe2O3 NR arrays and films under the ambient light conditions. The α-Fe2O3 NR arrays annealed at 350 °C exhibit an enhanced bactericidal performance in inactivating E. coli O157:H7, when compared to α-Fe2O3 film. Mathematical models are used to correlate the observed photocatalytic activities.
as methanol (CH3OH) and amino acetonitrile (NH2CH2CN). Interstellar dust, composed of graphite, carbon and/or silicate compounds, is also an important constituent in the interstellar medium. Approximately 1 % of the mass of the interstellar medium is in the form of dust and the remainder is in gas. These dust grains, with dimensions of a few tenths of microns, play a vital role in influencing the physical and chemical state of the interstellar medium -- from the thermodynamics and chemistry of the gas, to the dynamics of star formation. For
instance, the heating and cooling mechanisms in GMCs are based on the collisional heating and cooling and radiative cooling and heating which takes place in the coupled gas-grain system. Moreover, the surfaces of dust grain catalyze chemical reaction pathways leading to the formation of both simple (eg. H2) and complex (eg. CH3OH) molecules. Models which include only thermal desorption predict that molecules in the gaseous phase of the GMCs should rapidly freeze out, or desorb, onto the surfaces of dust grains on time scales much shorter than the lifetime of the GMC. At the mean temperatures of the
GMCs, however, we need to include non-thermal desorption mechanisms because the thermal energy is significantly less than the surface molecular binding energy and hence the thermal desorption rate is negligible. We calculate the number of far ultraviolet (FUV) photons produced from cosmic-ray protons in ionization equilibrium by the Prasad-Tarafdar effect, in which neutral H2 is collisionally excited by high-energy cosmic-ray-ionized electrons, and by recombination.
Using this background FUV field as a boundary condition, we, for the first time, consider the effect of radiative precursors from interstellar shocks. Specifically, we demonstrate that the post-shock compression enhances the production of FUV photons in a post-shock region where the dust grains slip relative to the gas. Inclusion of this process in our models results in the enhancement of photodesorption of molecules from grain surfaces. Finally, we estimate the total gas-phase abundances in GMCs including the combined effect
of adsorption and the newly-discovered effect of shock-enhanced FUV photodesorption.
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