IU Indianapolis ScholarWorks :: Browsing by Author "Cheng, Ruihua"

Browsing by Author "Cheng, Ruihua"

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A High Sensitivity Custom-Built Vibrating Sample Magnetometer
(MDPI, 2022-08) Phillips, Jared Paul; Yazdani, Saeed; Highland, Wyatt; Cheng, Ruihua; Physics, School of Science
This work details the construction and optimization of a fully automated, custom-built, remote controlled vibrating sample magnetometer for use in spintronics related research and teaching. Following calibration by a standard 6 mm diameter Ni disc sample with known magnetic moment, hysteresis measurements of Nd-Fe-B thin films acquired by this built vibrating sample magnetometer were compared to the data taken using a commercial superconducting quantum interference device and showed very similar results. In plane and out of plane magnetic hysteresis data acquired for 25 nm Fe thin films are also presented. The developed vibrating sample magnetometer is able to achieve a sensitivity approaching 1 × 10−5 emu. Further alterations to the design that may improve beyond this limit are also discussed.
Au nanoparticle assembly on cnts using flash induced solid-state dewetting
(2015-04-28) Kulkarni, Ameya; Ryu, Jong Eun; Agarwal, Mangilal; Xie, Jian; Cheng, Ruihua
Carbon Nanotubes (CNTs) are used extensively in various applications where substrate are required to be possessing higher surface area, porosity and electrical and thermal conductivity. Such properties can be enhanced to target a particular gas and biochemical for efficient detection when CNT matrix is functionalized with Nanoparticles (NPs). Conventional functionalization involves harsh oxidation repeated washing, filtration and sonication, which induce defects. The defects lead to hindered mobility of carriers, unwanted doping and also fragmentation of the CNTs in some cases. In this document we demonstrate functionalization of CNT with Au nanoparticles on a macro scale under dry and ambient condition using Xenon ash induced solid-state dewetting. A sputtered thin film was transformed into nanoparticles which were confirmed to be in a state of thermodynamic equilibrium. We worked on 3 nm, 6 nm, 9 nm, 15 nm, 30 nm initial thickness of thin films. Xenon ash parameters of energy, number of pulse, duration of pulse, duration of gap between consecutive pulses were optimized to achieve complete dewetting of Au thin films. 3 nm deposition was in the form of irregular nano-islands which were transformed into stable nanoparticles with a single shot of 10 J/cm2 of 2 ms duration. 6 nm and 9 nm deposition was in form of continues film which was also dewetted into stable nanoparticles with a single pulse but with an increased energy density of 20 J/cm2 and 35 J/cm2 respectively. In case of 15 nm and 30 nm deposition the thin film couldn't be dewetted with a maximum energy density of 50 J/cm2, it was observed that 3 and 4 pulses of 2 ms pulse duration and 2 ms gap duration with an energy density of 50 J/cm2 were required to completely dewet the thicker films. However irregularity was induced in the sizes of the NPs due to Ostwald ripening phenomenon which causes smaller particle within a critical difiusion length to combine and form a larger particle during or after dewetting process. For comparison, the Au thin films were also dewetted by a conventional process involving annealing of samples until the thin film was fully transformed into NPs and the size of NPs seized to grow. Scanning electron microscope (SEM) was used to characterize the samples. Thermodynamic stability of the particles was confirmed with statistical analyses of size distribution after every additional pulse.
Controlled short time large scale synthesis of magnetic cobalt nanoparticles on carbon nanotubes by flash annealing
(AIP, 2020) Mosey, Aaron; Yue, Lanping; Gaire, Babu; Ryu, Jong Eun; Cheng, Ruihua; Physics, School of Science
Nanopatterned arrays of discrete cobalt nanostructures showing characteristic parameter-dependent sizes are formed from continuous thin films on a carbon nanotube substrate using millisecond pulsed intense UV light. The nanoparticles exhibit ferromagnetic behavior with magnetic remanence and coercivity depending on the particle size. The end-state particle size is shown to be a function of initial thin film thickness and excitation energy and is therefore tunable. The evolutionary process from continuous thin films to a discrete morphology is thermodynamically driven by the large surface energy difference between metastable thin films and the underlying carbon nanotube substrate. Evidence of the Danielson model of the dewetting process is observed. These arrays can find applications as platforms for the self-assembly of magnetically susceptible materials, such as iron or nickel nanostructures, into a conduction matrix for applications in energy extraction from a latent heat storage device.
Developing an approach to improve beta-phase properties in ferroelectric pvd-hfp thin films
(2020-05) Dale, Ashley S.; Cheng, Ruihua; Petrache, Horia; Wassall, Stephen
Improved fabrication of poly(vinylindenefluoride)-hexafluoropropylene (PVDF-HFP) thin films is of particular interest due to the high electric coercivity found in the beta-phase structure of the thin film. We show that it is possible to obtain high-quality, beta-phase dominant PVDF-HFP thin films using a direct approach to Langmuir-Blodgett deposition without the use of annealing or additives. To improve sample quality, an automated Langmuir-Blodgett thin film deposition system was developed; a custom dipping trough was fabricated, a sample dipping mechanism was designed and constructed, and the system was automated using custom LabVIEW software. Samples were fabricated in the form of ferroelectric capacitors on substrates of glass and silicon, and implement a unique step design with a bottom electrode of copper with an aluminum wetting layer and a top electrode of gold with an aluminum wetting layer. Samples were then characterized using a custom ferroelectric measurement program implemented in LabVIEW with a Keithley picoammeter/voltage supply to confirm electric coercivity properties. Further characterization using scanning electron microscopy and atomic force microscopy confirmed the improvement in thin film fabrication over previous methods.
Direct observation of the magnetic anisotropy of an Fe(II) spin crossover molecular thin film
(IOPP, 2023-07) Dale, Ashley S.; Yazdani, Saeed; Ekanayaka, Thinlini K.; Mishra, Esha; Hu, Yuchen; Dowben, Peter A.; Freeland, John W.; Zhang, Jian; Cheng, Ruihua; Physics, School of Science
In this work, we provide clear evidence of magnetic anisotropy in the local orbital moment of a molecular thin film based on the SCO complex [Fe(H2B(pz)2)2(bipy)] (pz = pyrazol−1−yl, bipy = 2,2'−bipyridine). Field dependent x-ray magnetic circular dichroism measurements indicate that the magnetic easy axis for the orbital moment is along the surface normal direction. Along with the presence of a critical field, our observation points to the existence of an anisotropic energy barrier in the high-spin state. The estimated nonzero coupling constant of ∼2.47 × 10−5 eV molecule−1 indicates that the observed magnetocrystalline anisotropy is mostly due to spin–orbit coupling. The spin- and orbital-component anisotropies are determined to be 30.9 and 5.04 meV molecule−1, respectively. Furthermore, the estimated g factor in the range of 2.2–2.45 is consistent with the expected values. This work has paved the way for an understanding of the spin-state-switching mechanism in the presence of magnetic perturbations.
ECSTM Studies of the Electrocatalyst Stability for the AAEM Fuel Cell
(Office of the Vice Chancellor for Research, 2010-04-09) Xu, Qingmin; Cheng, Ruihua; Thornberry, Courtney; Chen, Rongrong
Alkaline fuel cells (AFC) have come to the forefront of fuel cell research due to the friendlier environment they provide to the cell’s components in comparison to acid-based Proton Exchange Membrane (PEM) fuel cells. The AFC shows real world application of 60% efficiency, but suffers from long term degradation due to the formation of carbonate precipitates formed from carbon dioxide. A solid-state form of the AFC, the alkaline anion exchange membrane (AAEM) fuel cell, is under development to overcome the degradation, due to the usage of liquid potassium hydroxide (KOH) or sodium hydroxide (NaOH) electrolytes in the AFC. Also, the AFC are known to have a higher rate of contamination and therefore need higher purity fuel than their acidic counterparts. This problem is eliminated by the AAEM fuel cell. The cathode, which consists of the catalyst, ionomer and current supports in the AAEM fuel cell or the AFC, is the key component that determines the cell’s performance and stability. The material found to work best for the AAEM fuel cell is platinum (Pt). The issue with Pt as a catalyst material for these fuel cells is that is it very cost prohibitive for mass production. Therefore, other metals are being investigated to find a material with less cost, but perform as well as the Pt in AAEM fuel cells. Several theories have been proposed as to the cause of cathode degradation. It was found that an increase in current density, temperature and ligand (OH-) concentration accelerated corrosion of catalysts and carbon supports. Studies have been done on the catalyst material of Pt, as well as the highly oriented pytolytic graphite (HOPG). HOPG is a carbon-based material that Pt is deposited upon. So far, most of these studies were done in acid media. The objective of this work is to develop an in situ electrochemical scanning tunneling microcopy (ECSTM) method for characterizing stability of nano-Pt and HOPG substrate under operation conditions of an AFC. Future research will characterize the stability of other metal nanostructure in an attempt to find cheaper and effective alternatives to Platinum.
Electrochemical behavior of tin foil anode in half cell and full cell with sulfur cathode
(Elsevier, 2019-01) Cui, Yi; Li, Tianyi; Zhou, Xinwei; Mosey, Aaron; Guo, Wei; Cheng, Ruihua; Fu, Yongzhu; Zhu, Likun; Mechanical Engineering, School of Engineering and Technology
Tin-based (Sn) metal anode has been considered an attractive candidate for rechargeable lithium batteries due to its high specific capacity, safety and low cost. However, the large volume change of Sn during cycling leads to rapid capacity decay. To address this issue, Sn foil was used as a high capacity anode by controlling the degree of lithium uptake. We studied the electrochemical behavior of Sn foil anode in half cell and full cell with sulfur cathode, including phase transform, morphological change, discharge/charge profiles and cycling performance. Enhanced cycling performance has been achieved by limiting the lithiation capacity of the Sn foil electrode. A full cell consisting of a pre-lithiated Sn foil anode and a sulfur cathode was constructed and tested. The full cell exhibits an initial capacity of 1142 mAh g−1 (based on the sulfur mass in the cathode), followed by stable cycling performance with a capacity retention of 550 mAh g−1 after 100 cycles at C/2 rate. This study reports a potential prospect to utilize Sn and S as a combination in rechargeable lithium batteries.
The Emergence of the Local Moment Molecular Spin Transistor
(IOP, 2020-05) Hao, Guanhua; Cheng, Ruihua; Dowben, P. A.; Physics, School of Science
Local moment molecular systems have now been used as the conduction channel in gated spintronics devices, and some of these three terminal devices might even be considered molecular spin transistors. In these systems, the gate voltage can be used to tune the molecular level alignment, while applied magnetic fields have an influence on the spin state, altering the magnetic properties, and providing insights to the magnetic anisotropy. More recently, the use of molecular spin crossover complexes, as the conduction channel, has led to devices that are both nonvolatile and have functionality at higher temperatures. Indeed, some devices have now been demonstrated to work at room temperature. Here, several molecular transistors, including those claiming to use single molecule magnets (SMM), are reviewed.
Evidence of dynamical effects and critical field in a cobalt spin crossover complex
(Royal Society of Chemistry, 2022-01) Ekanayaka, Thilini K.; Wang, Ping; Yazdani, Saeed; Phillips, Jared Paul; Mishra, Esha; Dale, Ashley S.; N'Diaye, Alpha T.; Klewe, Christoph; Shafer, Padraic; Freeland, John; Streubel, Robert; Wampler, James Paris; Zapf, Vivien; Cheng, Ruihua; Shatruk, Michael; Dowben, Peter A.; Physics, School of Science
The [Co(SQ)2(4-CN-py)2] complex exhibits dynamical effects over a wide range of temperature. The orbital moment, determined by X-ray magnetic circular dichroism (XMCD) with decreasing applied magnetic field, indicates a nonzero critical field for net alignment of magnetic moments, an effect not seen with the spin moment of [Co(SQ)2(4-CN-py)2].
Improvement and use of radiative transfer models to assess lunar space weathering and mechanisms for swirl formation
(2015-06-15) Liu, Dawei; Li, Lin; Jacinthe, Pierre-André; Wang, Lixin; Cheng, Ruihua; Johnson, Daniel
This dissertation focuses on quantification of submicroscopic iron of different sizes, mineral abundance and grain size of lunar soils using Hapke's radiative transfer model. The main objective is to explore implications of these results for assessing the relative importance of solar wind implantation versus micrometeorite impacts for lunar space weathering as well as three hypotheses (solar wind deflection, comet impact and dust transport) for swirl formation on the Moon. Results from this study can help to make connections between ordinary chondritic meteorites and asteroids, and put physical and chemical constraints on heating processes in the early solar system.

Browsing by Author "Cheng, Ruihua"

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