Kuang-Hui Li

Chemical insight into origin of forming-free resistive random-access memory devices

We demonstrate the realization of a forming-step free resistive random access memory (RRAM) device using a HfOx/TiOx/HfOx/TiOx multilayer structure, as a replacement for the conventional HfOx-based single layer structure. High-resolution transmission electron microscopy (HRTEM), along with electron energy loss spectroscopy (EELS) analysis has been carried out to identify the distribution and the role played by Ti in the RRAM stack. Our results show that Ti out-diffusion into the HfOx layer is the chemical cause of forming-free behavior. Moreover, the capability of Ti to change its ionic state in HfOx eases the reduction-oxidation (redox) reaction, thus lead to the RRAM devices performance improvements.

Influence of TMAl preflow on AlN epitaxy on sapphire

The trimethylaluminum (TMAl) preflow process has been widely applied on sapphire substrates prior to growing Al-polar AlN films by metalorganic chemical vapor deposition. However, it has been unclear how the TMAl preflow process really works. In this letter, we reported on carbons significance in the polarity and growth mode of AlN films due to the TMAl preflow. Without the preflow, no trace of carbon was found at the AlN/sapphire interface and the films possessed mixed Al- and N-polarity. With the 5 s preflow, carbon started to precipitate due to the decomposition of TMAl, forming scattered carbon-rich clusters which were graphitic carbon. It was discovered that the carbon attracted surrounding oxygen impurity atoms and consequently suppressed the formation of AlxOyNz and thus N-polarity. With the 40 s preflow, the significant presence of carbon clusters at the AlN/sapphire interface was observed. While still attracting oxygen and preventing the N-polarity, the carbon clusters served as randomly distrib...

Valence and conduction band offsets of β-Ga2O3/AlN heterojunction

Both β-Ga2O3 and wurtzite AlN have wide bandgaps of 4.5–4.9 and 6.1 eV, respectively. We calculated the in-plane lattice mismatch between the (−201) plane of β-Ga2O3 and the (0002) plane of AlN, which was found to be 2.4%. This is the smallest mismatch between β-Ga2O3 and binary III-nitrides which is beneficial for the formation of a high quality β-Ga2O3/AlN heterojunction. However, the valence and conduction band offsets (VBO and CBO) at the β-Ga2O3/AlN heterojunction have not yet been identified. In this study, a very thin (less than 2 nm) β-Ga2O3 layer was deposited on an AlN/sapphire template to form the heterojunction by pulsed laser deposition. High-resolution X-ray photoelectron spectroscopy revealed the core-level (CL) binding energies of Ga 3d and Al 2p with respect to the valence band maximum in individual β-Ga2O3 and AlN layers, respectively. The separation between Ga 3d and Al 2p CLs at the β-Ga2O3/AlN interface was also measured. Eventually, the VBO was found to be −0.55 ± 0.05 eV. Consequent...

Band alignment of B0.14Al0.86N/Al0.7Ga0.3N heterojunction

Owing to large bandgaps of BAlN and AlGaN alloys, their heterojunctions have the potential to be used in deep ultraviolet and power electronic device applications. However, the band alignment of such junctions has not been identified. In this work, we investigated the band-offset parameters of a B0.14Al0.86 N/Al0.7Ga0.3N heterojunction grown by metalorganic vapor phase epitaxy. These specific compositions were chosen to ensure a sufficiently large band offset for deep ultraviolet and power electronic applications. High resolution transmission electron microscopy confirmed the high structural quality of the heterojunction with an abrupt interface and uniform element distribution. We employed high resolution X-ray photoemission spectroscopy to measure the core level binding energies of B 1s and Ga 2p3/2 with respect to the valence band maximum of B0.14Al0.86N and Al0.7Ga0.3N layers, respectively. Then, we measured the energy separation between the B 1s and Ga 2p3/2 core levels at the interface of the hetero...

Thermodynamic photoinduced disorder in AlGaN nanowires

In this study, we examine thermodynamic photoinduced disorder in AlGaN nanowires through their steady-state and transient photoluminescence properties. We correlate the energy exchange during the photoexcitation and photoemission processes of the light–solid reaction and the generation of photoinduced entropy of the nanowires using temperature-dependent (6 K to 290 K) photoluminescence. We observed an oscillatory trend in the generated entropy of the system below 200 K, with an oscillation frequency that was significantly lower than what we have previously observed in InGaN/GaN nanowires. In contrast to the sharp increase in generated entropy at temperatures close to room temperature in InGaN/GaN nanowires, an insignificant increase was observed in AlGaN nanowires, indicating lower degrees of disorder-induced uncertainty in the wider bandgap semiconductor. We conjecture that the enhanced atomic ordering in AlGaN caused lower degrees of disorder-induced uncertainty related to the energy of states involved ...

Revealing microstructure and dislocation behavior in BAlN/AlGaN heterostructures

The KAUST authors acknowledge the support of the GCC Research Program REP/1/3189-01-01, Baseline BAS/1/1664-01-01, and Equipment BAS/1/1664-01-07. The work at QU was supported by the GCC Research Program GCC-2017-007. The work at the Georgia Institute of Technology was supported in part by DARPA under Grant No. W911NF-15-1-0026 and NSF under Grant No. DMR-1410874. R.D.D. acknowledges the additional support of the Steve W. Chaddick Endowed Chair in Electro-Optics and Georgia Research Alliance.

Using Mosaicity to Tune Thermal Transport in Polycrystalline Aluminum Nitride Thin Films

The effect of controlling the c-axis alignment (mosaicity) to the cross-plane thermal transport in textured polycrystalline aluminum nitride (AlN) thin films is experimentally and theoretically investigated. We show that by controlling the sputtering conditions we are able to deposit AlN thin films with varying c-axis grain tilt (mosaicity) from 10° to 0°. Microstructural characterization shows that the films are nearly identical in thickness and grain size, and the difference in mosaicity alters the grain interface quality. This has a significant effect to thermal transport where a thermal conductivity of 4.22 vs 8.09 W/mK are measured for samples with tilt angles of 10° versus 0° respectively. The modified Callaway model was used to fit the theoretical curves to the experimental results using various phonon scattering mechanisms at the grain interface. It was found that using a non-gray model gives an overview of the phonon scattering at the grain boundaries, whereas treating the grain boundary as an array of dislocation lines with varying angle relative to the heat flow, best describes the mechanism of the thermal transport. Lastly, our results show that controlling the quality of the grain interface provides a tuning knob to control thermal transport in polycrystalline materials.

Effect of TMAl pretreatment of sapphire substrate on the properties of MOCVD grown AlN epilayers

P diagram provide essential information for the conditions of materials synthesis and crystal growth. Although many binary phase diagrams were reported in the last century, that for Sodium (Na) and Silicon (Si) has not yet been established. In 2009, our group has presented a Na–Si binary phase diagram with the results of thermal analyses and morphology observation. In the present study, we demonstrated the crystal growth of Si from the Na–Si solution based on the Na–Si phase diagram. As shown in the Na-Si binary phase diagram (Fig. 1), Si is dissolved in a Na melt at 1173 K. Since the boiling point of Na is 1154 K at 1 atm and the vapor pressure of Na is relatively high above 973 K, Na can be removed from the products by evaporation. The Na-Si mixture (molar ratio Na/Si = 3:2) was heated at 1173 K. Na evaporation changed the composition of the sample toward the liquidus line at around 55 mol% Si at 1173 K, allowing crystallization of supersaturated Si to begin. After Na evaporation, single crystal of Si was obtained as shown in Fig. 1. Likewise, various Si crystals such as Si film, porous bulk Si and Si micro-tube were prepared by using a Na-Si solution. Furthermore, the efficient removal of impurities in Si for the solar cell was demonstrated by dissolution and recrystallization in a Na melt at low temperature. Recently, we succeeded in the crystal growth of Si clathrates by using a Na-Sn flux. These compounds have been widely studied due to their unique open-framework structures of Si polyhedrons.T Cu-Fe-O system has a great technological interest in the copper industry, as well as the development of catalytic compounds and transparent devices. The CuFeO2 phase (delafossite) and CuxFe3-xO4 phase (spinel) exhibit remarkable electrical, magnetic, optical and optoelectrical properties. Therefore, an in-depth understanding of the stability of the delafossite structure becomes of particular interest for fundamental research and for instance, its applications to the development of efficient p-type TCOs. The purpose of this study is reviewed the structural and thermodynamic information and phase equilibria of the Cu-Fe-O system in addition to checking the consistency of the available thermodynamics models with the experimental data. First, several of these models based on the CALPHAD method were reviewed and differences were highlighted. Moreover, several experimental procedures were employed to establish the relationships among temperature, lattice parameter, and stoichiometry of mixed oxides. In situ HTXRD (High-temperature X-Ray Diffraction) and TGA/DTA measurements, Rietveld refinement were used to provide thermostructural information in the range of 50° to 1100°C from stoichiometric mixture of CuO and Fe2O3 single oxides. Plasma Sintering (SPS) followed by adjusted post-annealing treatments were used to stabilize delafossite phase in different Copper/Iron gradient and analyzed by Electron Probe Micro-Analyzer (EPMA). The HT-XRD demonstrated that the spinel phase started to be formed from 750° and increases the amount of Cu after 900°C (CuxFe3-xO4). In addition, the variation of lattice parameters of spinel phase was determined by Rietveld refinement and compared with those of different molar ratios. Contrary to all the models, EPMA coupled with local structural analysis showed that delafossite phase could be stabilized with a substantial degree of cationic non-stoichiometry. These results were related to available thermodynamics models providing an improved understanding of this system, new information has generated to implement the existing data. The need to develop and improve a new model is considered.T growth of high quality AlN epitaxial films relies on precise control of the initial growth stages. In this work, we investigated the influence of trimethylaluminum (TMAl) pretreatment of sapphire substrates on the properties, impurity incorporation and growth mode change of AlN films grown by metalorganic chemical vapor deposition (MOCVD). Without the pretreatment, no trace of carbon was found at AlN/sapphire interface and the residual oxygen resulted in N-polarity. With 5s pretreatments, carbon started to be incorporated, forming scattered carbon-rich zones due to the decomposition of TMAl. It was discovered that carbon attracted surrounding oxygen impurity atoms and consequently, suppressed the formation of N-polarity. With 40 s pretreatment, a significant presence of carbon clusters at the AlN/sapphire interface occurred, which attracted considerable co-existed oxygen. While preventing the N-polarity, the carbon clusters served as random masks to further induce a 3D growth mode, creating Al-polar AlN nanocolumns with different facets. The properties of AlN and epitaxial growth mode change are discussedHerein, we present, for a first time, an electrochromic film of WO3 fabricated on a ITO by etching process, adopting a low-cost, facile and template-free fabrication process. By using hydrothermal method, we obtained WO3 films with a simplified architechture (ITO/HCl/WO3) in which HCl supports WO3 to form adhesive layer. Compared to ITO/WO3 configuration, the ITO/ HCl/WO3 configuration exhibited a strong enhancement in terms of roughness, porosity, open-tunnel structure, current density and coloration efficiency (about 179cm2C-1). Moreover, electro-optical characterization illustrates high transmittance modulation (about 49% at 630 nm) with excellent stability, making it attractive for a practical application. Biography Dr. Anamika Vitthal Kadam has completed her PhD at the age of 31 years from Bharti Vidyapeeth University, Pune, MH, and India. She is working as Assistant Prof in D.Y. Patil Engg and Tech, Kolhapur, MH, India and having guideship of D.Y. Patil University. Se has published more than 25 papers in national and international journals and achieved a project under young scientist scheme with one minor research project. anamikasonavane@rediffmail.com Anamika V Kadam, Res. Rev. J Mat. Sci. 2017, 5:7 DOI: 10.4172/2321-6212-C1-011T half-metallic magnets find broad applications in spintronics owing to the optimized magnetization and magnetic anisotropy. Herein, a low-temperature thermal decomposition method is utilized to grow new stabilized tetragonal ferrite films. Tetragonal Fe3O4-based film possesses high saturation magnetizations of ~1 Tesla and tetragonal Co-doped Fe3O4-based film exhibits high energy product of ~10.9 MGOe with perpendicular magnetocrystalline anisotropy. A combined experimental and first-principles study reveals that carbon interstitials (Ci B) and oxygen vacancies (VO) form Ci -VO pairs which stabilize the tetragonal phase and meanwhile enhancing the magnetization. The high magnetization is attributed to the spin flipping on FeA as a result of the Ci -VOinduced atomic migration and lattice distortion. The novel stabilized tetragonal ferrite films with high and tunable magnetization and magnetic anisotropy largely extends the applications of half-metallic spinel ferrites and novel energy harvest devices.T environmental pollution caused by massive carbon dioxide emissions has become one of the main obstacles to the national health and economic development. It is now an urgent problem to develop novel CO2 conversion catalysts. According to current research, Cu electrode is reported to be the best CO2 reduction catalyst among the commonly used metal electrodes. However, bulk Cu electrode is also faced with a few problems like high overpotential, poor selectivity on products and low reaction efficiency due to scaling relationships. In order to develop new Cu-based CO2 reduction catalysts, we will focus on geometric optimization of low dimensional nanomaterials and study their catalytic performances. The introduction of Cu atom, Cu2 dimer, Cu nanowires and nano-flakes to one or two dimensional organic or inorganic systems may bring unique catalytic characteristic and break the limits of bulk Cu electrode. These well-distributed Cu nanostructures are easier to controll and may show novel physical and chemical effects including size effect, geometric effect, substrate effect, magnetic effect, curvature effect and spatial confinement effect, which may improve CO2 catalytic reduction.E and environmental drivers are leading to exchanger weight reduction for automotive applications. The consequence is a material downgauging and some critical conditions reached. One of the exchanger’s main failure modes is induced by transient differential dilatations between the exchanger components. In this work, a detailed characterization of the cyclic damage mechanisms in car heat exchangers has been undertaken in order to improve their reliability. The studied material is a very thin (<0.3 mm) aluminum sheet composed of 3 layers (4XXX /3XXX/ 4XXX) (figure 1) compared to the same aluminium sheet made of 1 layer (3XXX), brazed in similar conditions to those of real components. Fatigue tests at constant stress amplitude have been performed at room temperature to show the influence of the clad in the fatigue resistance. Tensile properties between the clad alloy and the unclad alloy are strictly identical. However, the Wölher curve shows a high fatigue resistance for the unclad material compared to the cladded one. (Figure 2). Fractography analysis on the 3 layer alloy reveals that the crack initiation is intergranular on the clad side and occurs on the specimen face because the clad is harder than the core which weakened the grain boundaries leading to multifissuration initiation and propagation. Cracks stopped for about 50% of the fatigue life then there is a transgranular propagation of the crack until failure. Concerning the unclad material, the elements Cu, Fe, Si, Mn and Ti are better distributed in the alloy thickness leading to a less modified surface state and decreasing the site initiation number and therefore the multifissuration. Thanks to microstructure, tomography and surface rupture analysis a fatigue damage mechanisms can be proposed, showing the residual clad layer key influence on the crack initiation.B top-contact organic field effect transistors (OFET’s) have been fabricated on flexible substrates. Organic/inorganic materials are used as gate dielectric to enhance the output and transfer characteristics of the fabricated devices. Rutile titania nanoparticles (NP’s) were prepared using solvothermal technique and incorporated into poly vinyl alcohol (PVA) to improve the capacitance and therefore dielectric constant of the host matrix. The composite films were exposed to ozone treatment and the gold contacts were thermally made on top of the films through a shadow mask. The gate dielectric was treated with a self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS) and then an active layer of copper phthalocyanine (CuPc ) was deposited on top of the films. The output and transfer characteristics of the fabricated FET’s were measured using semiconductor parameter analyzer. OFET’s treated with a SAM of OTS exhibited higher mobility, on-off current ratio, and lower threshold voltage than the devices without a SAM of OTS treatment.C (Cu) is an essential metal in biological systems; however, at concentrations beyond threshold limits, not only can it kill aquatic organisms but also it can become highly toxic to humans. Copper can bind onto certain organic ligands via coordination mechanisms. Electrochemical grafting of aryl diazonium derivatives have successfully been used to modify substrates by introducing layers of various organic functional groups onto metallic and semiconducting substrate surfaces.1 Strategies involving functionalization of substrates with large-molecular-weight oligomers and peptides via diazonium grafting routes for extraction of heavy metal ion (HMI) pollutants have been reported. Some of these methods involve introduction of chelating groups in more than a single step. However, a simpler one-step quick grafting of low-molecular-weight HMI chelating agents may not only present some cost reduction advantages towards devising kits for HMI extraction but also permit the fabrication of relatively thinner layers with optimal surface grafting with excellent chelation efficiency. Silicon is one of the most abundant materials on the earth’s crust and its suitable surface chemistry has motivated organic functionalization efforts towards developing a wide range of applications. The purpose of this study is to explore a one-step functionalization strategy for introducing carboxymethylthio (CMT) chelating groups via direct electrografting of the diazonium cation 4-[(carboxymethyl)thio]benzenediazonium cation, (4-CMTBD), onto Si surface, leading to fabrication of the Si-(4-CMTB) surface. The investigation of Cu chelation is also studied.I the present study, the microstructure of newly developed magnesium matrix composites reinforced with SiC nanoparticles was investigated. To produce Mg-SiC nanocomposites, magnesium powder and various volume fractions of SiC particles with an average diameter of 50 nm were co-milled by a high energy planetary ball mill. The milled powder was compacted by a hydraulic hand-operated press followed by cold isostatic pressing and sintering. Finally, the nanocomposites were hot extruded to eliminate porosity and achieve full density. Scanning electron microscopy, energy dispersive x-ray analysis and x-ray diffraction were used to characterize the microstructure and texture of the magnesium matrix and the distribution of the SiC-reinforcements after extrusion. Further, transmission electron microscopy analyses were performed to study the grain size of the magnesium matrix and the interface between the SiC nanoparticles and the magnesium matrix. All developed nanocomposites revealed a uniform distribution of the SiC nanoparticles in the magnesium matrix. No evidence of porosity or interfacial products between the SiC nanoparticles and the magnesium matrix were found, indicating a well-bonded interface. The used powder metallurgy techniques allow to produce dense nanostructured Mg-SiC nanocomposites.S of the Problem: Epoxy coatings topic is experiencing a continuous renewal and still presents a great potential to produce new advanced functional materials exhibiting self-healing, shape memory or other functional properties such as transparent-toopaque transitions. Epoxy coatings can be widely formulated by tailoring the epoxy resin/hardener partners upon the performance requirements for the end product. However, these materials exhibit naturally low impact resistance because of their high crosslinking density. The usual approach to toughen epoxy thermosets is to add either elastomers or thermoplastic modifiers, but this is also lowering their overall mechanical performances. Methodology & Theoretical Orientation: New epoxy-silicone coating formulations are proposed based on diglycidyl ether of bisphenol-A epoxy resin (DGEBA) and 5-amino-1,3,3-trimethylcyclohexanemethanamine (IPDA) as hardener. Several block and grafted copolymers with a silicone part were added to the epoxy matrix or to epoxy-silicone blends, at different silicone contents. Their effect on the morphology and dispersion effectiveness was studied by scanning electron microscopy (SEM). The influence of liquid silicone inclusions on epoxy curing kinetics and on final thermomechanical properties of epoxy-modified networks was investigated using differential scanning calorimetry (DSC) and dynamic mechanical thermal analysis (DMTA). Thermal stabilities of the new formulations were analyzed by thermogravimetric analysis (TGA). Findings: The morphological evolution of epoxy-toughened networks was used to understand and explain the differences in curing kinetics and impact properties of the epoxy-modified networks with different contents of liquid silicone. Conclusion & Significance: A new strategy for preparing epoxy coatings was presented. Varying the chemical structure of silicone copolymers allows tailoring the morphology and morphological evolution of the silicone inclusions during curing and so, the impact resistance of the epoxy-silicone modified coatings.B is a complex, highly organized living organ forming the structural framework of the body. It is a naturally existing composite that is composed of an inorganic mineral phase of hydroxyapatite (60% by weight) and an organic phase of mainly type I collagen. Bone defects are a serious illness that may result after a pathological process has destroyed vital components of the bone. Recently, Injectable hydrogels have been used in bone regenerative applications, because of their high tissue-like water content and moldable features. Such injectable hydrogels are of particular significance because drugs, cells, proteins, and bioactive agents can be essentially incorporated into polymer solutions before administration. In this work antimicrobial injectable hydrogel scaffolds based on a biopolymer matrix composed of collagen, reinforced with the nanohydroxyapatite (nHA), were prepared. The chemical structure, morphology, and swelling ratio as well as mechanical and viscoelastic properties of the prepared hydrogel scaffolds were investigated. For drug-release tests, gentamicin, an antibiotic drug, was entrapped within the scaffold, and the drug-release profile was examined The results revealed that the incorporation of the nHA particles into the collagen hydrogel enhanced the mechanical and biodegradable properties and also cause a reduction in both the hydrogel porosity and swelling ratio. Furthermore, the rheological studies showed that the collagen/nHA hydrogel scaffolds is non-Newtonian viscoelastic material with more elastic dominance and exhibited higher stiffness. These properties make the injectable hydrogel of potential interest as biomimetic scaffold for bone regeneration operations in diverse applications. Consequently, this collagen/nHA hydrogel scaffold will provide an opportunity to translate lab research to the market and to apply the principles of tissue engineering in the clinical settings.