Symposium CP04 : Interfacial Science and Engineering—Mechanics, Thermodynamics, Kinetics and Chemistry

Solute segregation in nanocrystalline metals has been widely explored as a route to improve the thermal stability of fine grain microstructures. Efforts have revealed that segregation may not always be uniform or subtle, thereby shifting the microstructural stabilization mechanism [1,2]. Delineating the subtle differences between these two mechanisms maybe be possible through improved understanding of mass transport along solute decorated grain boundaries. In this work, we present very recent observations of grain boundary diffusion with concomitant microstructural evolution by employing a thin film diffusion triple configuration. A 10nm layer of pure Al then a 500nm layer of a nanocrystalline, highly textured Ni-Cr alloy are deposited via DC magnetron sputtering on to a coarse-grained Ni-Cr alloy substrate of the same nominal composition. These diffusion triples are annealed under flowing Ar at 600C and the asymmetric diffusion profile is extracted via STEM EDX and APT characterization. Comparison of these diffusion data are presented against similar thin film diffusion triples containing <1at% Y in the nanocrystalline films. Despite nearly identical initial microstructures, there are significant differences between these diffusion profiles, with implications for microstructural stability as well as environmental resistance.

[1] Darling, K. A., M. Rajagopalan, M. Komarasamy, M. A. Bhatia, B. C. Hornbuckle, R. S. Mishra, and K. N. Solanki. "Extreme creep resistance in a microstructurally stable nanocrystalline alloy." Nature 537, no. 7620 (2016): 378.
[2] Abdeljawad, Fadi, Ping Lu, Nicolas Argibay, Blythe G. Clark, Brad L. Boyce, and Stephen M. Foiles. "Grain boundary segregation in immiscible nanocrystalline alloys." Acta Materialia 126 (2017): 528-539.

The coffee-ring effect refers to the phenomena in which a drop of a colloidal suspension evaporates on a substrate and deposits an ordered ring-structure due to the outward radial evaporative flow. This phenomena has been well-investigated due to numerous practical applications including ink-jet printing, microscale separations, and sensing/diagnostics. However, the phenomena has been less quantified for a crystallizing solution. When a drop of salt solution is evaporated, the salt becomes supersaturated and crystals begin to emerge at the substrate. The emerging crystals alter the local wettability of the substrate and fundamentally alter fluid dynamics of evaporation, which in turn alters the form of deposit left behind. Here, we investigate the role of energetic interactions between the substrate, crystals, and solution. We systematically vary the substrate interfacial chemistry by deposition of silane groups of various surface energies, and use four different crystal systems to probe this effect. We find that crystals with favorable interactions with the underlying substrate chemistries alter deposit morphology, while non-interacting crystals behave similarly to colloidal particles, and develop phase diagrams predicting this behavior. These results can be used for fine-tuned control and understanding of crystalline evaporative deposits.

Magnesium metal is potential candidate for high strength to weight ratio alloys with wide application in aerospace and automotive industries. However, poor corrosion resistance under ambient environmental conditions is the bottleneck for industrial deployment. Designing passivation layers and/or corrosion resistance alloys require fundamental understanding of the corrosion process. The traditional ex-situ spectroscopic measurements of polycrystalline metal surface with ubiquitous surface impurities provided indistinct view of the corrosion process. To clearly distinguish the mechanism and sequence of corrosion process, we employed in-situ cryo-based x-ray photoelectron spectroscopy (XPS) measurements on well-defined pure and aqueous solution exposed Mg-single crystal surfaces in combination with ab initio atomistic modelling studies. Clean Mg (0001) surfaces were carefully exposed to pure and sodium chloride mixed water and the subsequent interfacial reactions were studies through integrated experimental and theoretical approach. This study provides atomistic view of magnesium hydroxide nucleation as main product of the corrosion process. Under salt conditions, the competitive nucleation process between magnesium hydroxide and magnesium chloride were observed along with the formation of magnesium chloride hydrate and magnesium hydroxide hydrate phases. By combining the energy requirements from computational modelling and the electronic states of corrosion products, the mechanism and sequence of corrosion process on Mg metal will be discussed.

The increasing research of nanowires (NWs) as building blocks for electronic, optoelectronic, energy devices and quantum computing is driven by the unique physical, and chemical properties originate from their nanoscale dimensionality. Catalyzed surface-guided NWs offer the possibility to control their position, direction and crystallographic orientation, which eventually leads to high-performance devices. To adequately control these features and gain predictive abilities, a deeper understanding of the growth mechanism of surface-guided NWs is required. Here, we experimentally and theoretically study the kinetics of planar catalyzed NWs. We present a model that considers two main regimes of the growth rate of NWs depending on their thickness: the Gibbs–Thompson regime, which dominates the growth of thinner NWs, and the surface-diffusion-induced regime, which dominates the growth of thicker ones. By developing this kinetic model and fitting it to the experimental kinetic data, we determine the dimensionality of the surface diffusion. We observe a good correlation between the model and the results for surface-guided ZnSe and ZnS NWs grown on sapphire. The dimensionality of the surface diffusion value was determined to be ~1.5 and ~1.8 for ZnSe and ZnS NWs, respectively, in contrast to the value of 1 for vertical NWs growth models, supporting the difference between the two growth morphologies. The newly developed model distinguishes between the growth mechanisms of horizontal and vertical NWs, underscores the important role of the substrate in the horizontal growth, and provides new insights into the mechanism of surface-guided NWs growth. Understanding the growth mechanism introduces the possibility of controlling the growth directions and crystallographic orientations of the aligned NWs accurately and helps gain some prediction abilities of the NWs properties, leading to better nanowire-based electronic and optoelectronic devices.

Organic-inorganic hybrid perovskites, such as the prototypical methylammonium lead iodide (CH₃NH₃PbI₃ or MAPbI₃), have emerged as promising light absorbers in photovoltaic (PV) cells or as emitters in light-emitting diodes (LEDs). The strikingly high energy conversion efficiency up to ~ 20% ensures hybrid perovskites as the key component of highly efficient solar cells. However, they generally suffer from moisture instability, which limits the long-term use of perovskite-based devices in ambient environment. In this work, in order to improve the surface moisture stability of MAPbI₃ in common humidity atmosphere, more specifically, to discover better ligands under a specific coverage on the [MAI]⁰ surface to greatly improve the moisture stability and decrease the ion dissociation rate, we have applied molecular dynamics (MD) simulations and reaction kinetics theory to model the ion dissociation process and estimate the associated free energy barrier with and without ligand passivation. We have developed a new MD force field for MAPbI₃, which can match the density functional theory (DFT)-predicted elastic properties, experimental water contact angle on MAPbI₃, and DFT-predicted water infiltration and adhesion energies. We design MAPbI₃ with ligand-passivated surfaces by replacing MA⁺ with ligands composed of long-chain alkyl-ammoniums. Ligands with different chain lengths, such as CH₃(CH₂)_nNH₃⁺ (n = 3, 5, 7), and under different surface coverages (σ = 25%, 50%, 75%, 100%), are considered here. We discover that ligand passivation can greatly help protect MA⁺ on the surface due to the much higher dissociation free energy barriers of these ligands compared to that of MA⁺. For iodine ions, ligand passivation can also shield them from water contacts, except for long-chain ligands, such as CH₃(CH₂)_nNH₃⁺ (n = 5, 7) under full surface coverage (σ = 100%), due to the reduced dissociation free energy barriers of long-chain ligands. As an interesting finding, the reduced dissociation free energy barriers for long-chain ligands under high surface coverages could be explained by their larger tendencies to micellize, which serves as additional driving force for their dissociation. This work significantly motivates future experimental efforts in designing new surface ligands to improve the moisture stability of hybrid perovskites.

We show how surfaces can be designed to redirect the momentum of the spreading lamella causing them to lift-off into 3-dimensional shapes thereby dramatically reducing the contact area. We design superhydrophobic surfaces with an in-plane discontinuity leading to the accumulation of vertical momentum resulting in the out-of-plane ejection of the lamella into water bowls. We demonstrate a two-fold reduction in the heat transfer between a cold rain and a warm surface. These insights can be broadly applied to other transport phenomena involving mass and energy exchange to limit heat loss under precipitation, icing of surfaces, reduce salt deposition on a surface exposed to ocean spray, or inhibit the formation of a water film on wings or wind turbine blades.

Reactivity and Ion transfer processes across heterogeneous interfaces is the backbone of electrochemical devices such as batteries and dye sensitized solar cells. The complexity of reactivity and charge transfer across interfaces often arise from convoluted relations between surface chemistry (such as defect density, crystallographic orientation and functional groups) and exogenous drivers such as temperature and electric field. For example, technologically significant nanocrystalline TiO₂ with diverse surface chemistry renders a complex interfacial processes. To correlate the surface chemistry of TiO₂ with interfacial processes (reactivity and ion transfer), we performed a multimodal spectroscopic analysis in combination with ab-initio molecular dynamics (AIMD) calculations. The interfacial processes of anatase and rutile phase with varying surface chemistry against lithium bis(trifluoromethylsulfone)imide (Li-TfSI) salt were analyzed. We observed that, ubiquitous oxygen defects at TiO₂ surface regime enhance the molecular decomposition of TFSI anions. The thermally driven splitting of –CF3 groups resulting in titanium oxyfluoride type bonding (O-Ti-F) at the near surface regime were observed in AIMD calculations and supported by multimodal spectroscopic measurements. The presence of free electrons due to fluoride ion transfer to TiO₂ surface were detected using low temperature electron paramagnetic resonance (EPR) analysis. In addition, separated Li⁺ from such a weakly coordinating anion, pass in through preferential transfer pathways across nanoparticle surfaces. The thermodynamics and kinetics of the molecular decomposition and associated Li⁺ ion transfer processes were derived from AIMD and high resolution nuclear magnetic resonance (NMR) analysis.

Complex oxide heterostructures possess a wide range of functional electronic and magnetic properties including metal-insulator transitions, superconductivity, ferroelectricity, and colossal magnetoresistance effects. At epitaxial interfaces formed between atomically thin complex oxide films, electronic, chemical and structural interactions can be used to effectively tune the physical properties of these materials. Using a combination of atomic-scale controlled thin film synthesis and high resolution synchrotron diffraction based imaging, we show that structural distortions the interfaces between polar La_1-xSr_xMnO₃ films and non-polar substrates can be effectively tuned by chemical modifications at these interfaces to control ferromagnetism in [001]-oriented
La_1-xSr_xMnO₃ films with thickness less than 1 nm. We show that atomic-scale chemical control at polar/non polar oxide interfaces provides a powerful route to engineer novel electronic and magnetic phenomena at complex oxide interfaces.

Adhesion and interface formation are essential processes in friction, particle aggregation, and grain boundary formation. It is generally accepted that adhesion is anisotropic and the adhesion force depends on the mutual orientation of attracting interfaces. However, direct experimental measurement of the adhesion force and its orientation dependence has never been realized. Here, by combining in situ environmental transmission electron microscopy and atomic force microscopy, we directly measured the adhesion between two rutile TiO₂ (001) surfaces with controlled mutual orientation. The results show that the adhesion force is primarily van de Waals attraction between the two surfaces and is highest when the lattices of the two surfaces are aligned, demonstrating strong orientation-dependence. Moreover, the orientation dependence is not influenced by surface hydration; though, the adhesion force drops considerably with increasing water vapor pressure. Our work demonstrates a powerful method for quantitative study of nanoscale interface formation and strength, providing new insights to the driving force of orientated attachment of particles and friction anisotropy.

Early grain boundary engineering work has demonstrated how significantly material properties can be improved with tailored microstructures. Unfortunately, continued improvement in this area will require complete structure-property relationships of grain boundaries, which are currently lacking. We describe tools and efforts to obtain both crystallographic structure-property relationships and atomic structure-property relationships. And since the two are must be combined to facilitate future design efforts, we detail efforts to find correlations between them. The properties of interest in this work are thermodynamic properties, like grain boundary energy, as well as kinetic properties, like grain boundary mobility.

The nonequilibrium nature of amorphous solids such as metallic glasses (MGs) shares intriguing commonalities with grain boundaries in nanocrystalline (NC) materials, owing largely to the multiplicity of energy states inherent to disorder. Here, we report on experimental studies of MG and NC materials and novel synthesis and processing routes for controlling the structural state – and as a consequence, the mechanical properties. Relaxation processes in disordered materials that facilitate atomic reconfigurations toward a lower energy state, such as low temperature annealing, have been shown to enhance mechanical strength while promoting shear localization. A particular focus in this talk will be on strategies for rejuvenation of disorder with the goal of suppressing shear localization and endowing damage tolerance. Parallels between our results and rejuvenation processes in glassy systems will be discussed in the context of controlling metastable structural configurations through novel processing routes.

Polymorphism in crystallography means the ability of a solid material to exist in more than one crystalline form, and the forms differ in physical and chemical properties even though their chemical components are identical. Recently, polymorphism in two-dimensional (2D) materials has been attracting interest in obtaining diverse properties that cannot be obtained in 3D forms [1, 2]. In this study, we have found 2D polymorphic nanostructures self-organized inside silicon ingots grown by Czchralski (CZ)-method; they are composed of different 2D periodic units just along Σ9{111}/{115} grain boundaries (GBs) with a 〈110〉 tilt axis. The atomistic structure of the 2D polymorphic GBs, as well as their segregation ability, are determined by a 3D chemical nanoanalysis technique using atom probe tomography (APT) combined with scanning transmission electron microscopy (STEM) and ab-initio calculations [3].

STEM revealed that, Σ9{111}/{115} GBs have a periodic interface structure with a period of 2 nm along the GBs, and the GB unit is composed of Σ9{221}-like and Σ9{114}-like nanofacets involving 2 〈110〉 reconstructed bonds (named type-I GBs in this paper) [4]. Ab initio calculations reveal the GB energy of 0.6 J/m², that is higher than the GB energy of Σ9{221} (0.18 J/m²) and Σ9{114} (0.36 J/m²) GBs. Also, the 〈110〉 bonds are rather longer in comparison with the 〈111〉 bonds in bulk Si. Thus, those long bonds would induce an energy in addition to the energy of Σ9{221}-like and Σ9{114}-like nanofacets.

The GB unit in type-I GBs is changed by annealing at 1100^oC. The GB unit in the annealed GBs, that was observed in a cast-grown Si ingot [5], is composed of Σ3{111}-like nanofacets involving 4 〈110〉 bonds, with the period of 2 nm (type-II GBs). Type-II GBs can be introduced by forming stacking faults of low GB energy (~0 J/m²) on the Σ9{221}-like and Σ9{114}-like nanofacets in type-I GBs. The GB energy of type-II GBs is estimated to be 0.45 J/m². Therefore, type-I GBs change into type-II GBs so as to remove Σ9{221}-like and Σ9{114}-like nanofacets of high GB energy, via the formation of Σ3{111}-like nanofacets of low GB energy, even though 2 〈110〉 bonds of high-energy are added in each GB unit.

APT reveals the oxygen segregation ability of type-I GBs (0.04-0.06 atoms/nm²) and that of type-II GBs (0.03 atoms/nm²). The ratio of the abilities is the same as the ratio of the GB energy of type-I and type-II GBs; i.e., the ability is proportional to the GB energy, as expected [3, 4]. We will discuss potential controls of the 2D polymorphic nanostructures, as well as of their segregation ability, by changing the growth condition.

1) M. Wu, et al., Phys. Rev. B 96 (2017) 205411.
2) M. Yoshida, et al., Nano Lett. 17 (2017) 5567.
3) Y. Ohno, et al., J. Microsc. 268 (2017) 230.
4) Y. Ohno, et al., Appl. Phys. Lett. 110 (2017) 062105.
5) A. Stoffers, et al., Phys. Rev. Lett. 115, 235502 (2015).
Acknowledgments: This work is supported by JST/CREST (Grant No. JPMJCR17J1, 2017-2023)

The scaling of behavior in ferroelectrics remains a topic of substantial interest given the need for device miniaturization and fundamental research. In recent years, a suite of reports has shown that the bulk piezoelectric functionality of thin ferroelectric films (approximately 30 nm or less) have a strong dependence on the surface chemical and structural state. Furthermore, these works suggest surface screening is a critical factor in understanding the bulk properties and switching behavior in thin film form, but precise mechanisms of this coupling are lacking. A key question that remains is the role of intrinsic as opposed to extrinsic screening on the properties of the thin ferroelectric films. In this work, we use BaTiO₃ (BTO) grown via pulsed laser deposition (PLD) as a model material system to explore the correlation between surface state and bulk properties. To carefully probe the surface and bulk properties of BTO thin films, we use a combination of atomically resolved scanning tunneling microscopy (STM), non-contact atomic force microscopy (AFM), piezoresponse force microscopy (PFM), X-ray diffraction (XRD), and in-situ X-ray photoelectron spectroscopy (XPS). The aforementioned characterization techniques provide an experimental pathway to evaluate the correlation between the surface state and bulk piezoelectric properties.

An assembly of hemispherical Ag nanoparticles is prepared by solid-state dewetting of thin Ag film deposited on the sapphire substrate. The in-situ nanomechanical compression testing of the particles with a flat diamond punch inside the scanning electron microscope demonstrates the deformation behavior typical for the nucleation-controlled plasticity: high elastic deformation followed by an abrupt particles collapse. The latter is associated with the dislocations nucleation in otherwise pristine particle. The average contact pressure in the contact zone at the onset of dislocation-controlled plasticity is about 8 GPa, and does not depend on particle size. This observation supports the hypothesis that the pseudoelasticity of much smaller Ag nanoparticles observed by Sun et al. [Nat. Mater. 2014, 13, 1007] is intrinsically related to their ultrahigh strength. A stress-induced diffusion along the particle-substrate and particle-punch interfaces is identified as a factor controlling the pseudoelastic deformation. The corresponding diffusion model allows estimating the room-temperature self-diffusion coefficient of Ag along the Ag-W and Ag-Zirconia interfaces, which was quite close to the estimated value of the grain boundary self-diffusion coefficient in Ag. Based on this finding, the map of pseudoelastic deformation of several metals is proposed.

Semiconductor heterostructures enable band-gap engineering by combining two or more dissimilar materials, and are essential for many optoelectronic applications such as multi-junction solar cells, lasers and light-emitting diodes. However, differences in lattice parameters between film and substrate give rise to interfacial misfit strain. Misfit dislocations are introduced beyond a certain critical thickness, which will likely severely limit device performance by acting as sites for electron-hole pair recombination. Identifying the sources of these defects and understanding their evolution as a function of film thickness would possibly lead to identifying a path for minimizing defects in such heterostructures by optimizing growth conditions, film thickness, etc. In this study, the creation and evolution of structural defects in low-mismatched (misfit strain ∼0.6%) GaAs (cap)/GaAs_0.92Sb_0.08/GaAs(001) heterostructures grown by molecular beam epitaxy have been investigated using transmission electron microscopy as well as high-resolution x-ray diffraction and atomic force microscopy (AFM). Three distinct stages of strain relaxation were identified in GaAs_0.92Sb_0.08 films with thicknesses in the range of 50-4000 nm, whereas 50-nm-thick GaAs capping layers exhibited only initial Stage-I relaxation. Aberration-corrected electron microscopy revealed that glide-set dissociated 60° dislocations were primarily formed during Stage-I relaxation. Lomer edge dislocations with compact core structure and curved dislocations prevalently with edge character formed in Stage-II and Stage-III of relaxation. Evolution of dislocation density at cap/film interfaces showed a strong correlation with the evolution of surface morphology at the growth front. Even though misfit strain was highest at the cap/film interface of the heterostructure with 4000-nm-thick film, dislocation density was lower in comparison with heterostructures with 1000-nm-thick and 2000-nm-thick films. AFM results indicated that smoothening of the growth front in the heterostructure with 4000-nm-thick film caused a decrease in the areal density of surface troughs which may act as preferred sites for heterogeneous nucleation of dislocation half-loops. Detailed consideration of different sources for threading dislocations and the experimental evidence of correlation between decreased dislocation density and decreased density of surface troughs led to the conclusion that heterogeneously nucleated surface half-loops are the major source of threading dislocations in these low-mismatched heterostructures [1].
Reference
[1] A. Gangopadhyay, A. Maros, N. Faleev, D.J. Smith, Strain relaxation in low-mismatched GaAs/GaAs_1-xSb_x/GaAs heterostructures, Acta Mater. 162 (2019) 103-115.

III-V-Bismides are attractive materials for near- and mid-IR device applications due to their dramatic decrease in band gap with small incorporations of bismuth. The growth space of GaAs_1-xBi_x has been explored by molecular beam epitaxy, with the highest obtained bismuth incorporation being x=0.22 [1]. However, resulting film quality has been a barrier to incorporating high bismuth content materials into optoelectronic devices. GaAsBi films are typically grown thin (<50 nm) on GaAs under-layers to prevent strain relaxation from the epitaxial mismatch. The resulting compressively strained films have exhibited a variety of phase separation characteristics, including lateral composition modulations [2], droplet induced composition inhomogeneity [3], and CuPt_B-type atomic ordering [4]. We have explored growing on InGaAs buffer layers as a method for reducing epilayer compressive strain, suppressing surface droplet formation, and eliminating vertical phase separation through the bismide film layers.

GaAsBi epilayers were grown by solid-source molecular beam epitaxy on a Veeco GENxplor MBE using a valved As₄ source and an effusion cell for bismuth. Samples were characterized by HRXRD to determine the average bismuth content and the strain state of the layers. AFM was used to analyze the droplet formation on the surface of the films. TEM and HAADF STEM were used to qualitatively characterize the bulk phase separation. By comparing samples grown on GaAs to samples grown on InGaAs buffers, we will draw connections between film strain, surface droplet formation, and phase separation. We are able to reduce the degree of phase separation that occurs in GaAsBi films, by growing on InGaAs buffer layers; thereby increasing film homogeneity and general suitability for optoelectronic applications.

[1] R.B. Lewis, M. Masnadi-Shirazi, and T. Tiedje Appl. Phys. Lett., 101 082112 (2012)
[2] E. Luna, M. Wu, J. Puustinen, M. Guina, and A. Trampert, J. Appl. Phys., 117, 185302 (2015)
[3] C. R. Tait, L. Yan, and J. M. Millunchick, Appl. Phys. Lett., 111, 042105, (2017)
[4] A. G. Norman, R. France, and A. J. Ptak, J. Vac. Sci. Technol. B, 29, 03C121, (2011)

C54 titanium disilicide (TiSi₂) has attracted great attention in Fin Field-effect transistor (FinFET) application, since it has accompanied with promising advantages, such as low resistivity, outstanding reliability and low mobility in silicon substrate. In order to further investigating the structural properties, we observed the formation process of C54-TiSi₂ by in-situ TEM in this study. During the heating process, Ti atoms diffused into Si_xGe_1-x substrate and nucleated afterward to form C54-TiSi₂ . It is remarkable that we also observed the occurance of a novel type of texture-axiotaxy between C54-TiSi₂ (110) plane and Si (001) plane at around 800 celcius degree. Axiotaxy is convinced to be produced by the strain implemented on the low index planes. Additionally, the elemental distribution information was provided by Energy Dispersive Spectrometer (EDS) analysis with scanning transmission electron microscopy (STEM) image. With this atomic scale analysis, we are able to establish the connection between microstructure and properties, further enhance the performance of Complementary Metal-Oxide-Semiconductor (CMOS) technology.

Heteroepitaxial thin film growth on substrates with large lattice mismatch is an important area of crystal growth research to overcome applications-based restrictions on substrate choice. For some material systems, one crystalline phase may exhibit growth of domains with different epitaxial relationships to the substrate or two phases may grow simultaneously with different orientations. This process, known as double epitaxy, creates thin film materials with many grain interfaces at fixed angles with respect to the substrate. This work demonstrates double epitaxy in FeSe, an important superconducting compound with a complex phase diagram which is highly sensitive to stoichiometry. Conditions exist during pulsed laser deposition (PLD) that lead to the simultaneous epitaxial growth of the tetragonal (β-FeSe) and hexagonal Fe₇Se₈ (γ-FeSe) phases of FeSe in intermingled domains throughout the film, creating many phase boundary interfaces. We investigate the impact of PLD parameters on the double epitaxy of this interface-dominated material and identify the conditions under which double epitaxy is suppressed.. The lattice parameters of β-FeSe and γ-FeSe are unsuitable for lattice-matching epitaxial growth on (001) MgO. In the case of β-FeSe, the lattice parameter (a = 3.672-3.769 Å, depending on stoichiometry) differs from that of MgO (a = 4.211 Å) by more than the 9% normally tolerated for conventional epitaxy. The cubic structure of MgO, however, exhibits various domain matching epitaxial relationships with the various FeSe crystalline phases. This opens opportunities for engineering the internal interfacial boundaries of different FeSe phases by controlling the rotation of the epitaxial domains with processing parameters. A KrF excimer laser (1.4-3.4 J/cm²) was used to ablate a pressed, sintered target synthesized from FeSe powder. Targets were sintered in sealed quartz ampules at 700°C for 12 hours. Targets were ablated in high vacuum (10^-6 torr) with substrate temperatures in the 350-550°C range. FeSe films grown at temperatures between 350°C and 450°C with laser fluence fixed at 3.4 J/cm² showed the simultaneous epitaxial growth of both (001)-oriented β-FeSe and (101)-oriented γ-FeSe. The relative fraction of these two phases changes smoothly from majority β-FeSe at 350°C to majority γ-FeSe at 450°C. The β-FeSe phase grows aligned with the substrate and the γ-FeSe phase grows in two rotated domains, one dominant domain aligned with the substrate and a minor domain rotated by 45°. Atomic force microscopy (AFM) reveals the granular surface morphology with apparent grain sizes of 220-300 nm in films grown at 350°C and 450°C. The film grown at 400°C has a nearly equal fraction of each phase and the apparent grain size is found in the 120-200 nm range. The smaller grain size points to a larger number of phase interfaces, corresponding to an estimated number density of β-FeSe/γ-FeSe interfaces of 3.5x10⁹ cm^-2. Substrate temperature of 550°C results in a change in epitaxial orientation of β-FeSe from (001) to (101)-oriented. The (101) oriented β-FeSe grows in three domains, one aligned with the substrate and two others rotated by ±30°. AFM of the (101)-oriented FeSe shows large, micron scale grains leading to a much smaller grain interface number density than the biphasic films on the order of 10³ cm^-2. This growth technique shows the orders of magnitude scalability of in-plane interfaces of these epitaxially-oriented grains. At 550°C, reducing the laser fluence to 1.4 J/cm² increased the relative fraction of the γ-FeSe phase. The parameters of the PLD plasma plume are expected to influence the simultaneous epitaxial growth of the two phases by delivering a locally non-stoichiometric flux. Measurements of space and time resolved ion densities of the PLD plume will be presented and correlated with the emergence of γ-FeSe to determine the impact of the relative density of arriving ions on phase crystallization.

Grain boundary complexion engineering was recently introduced as a framework to be used to design grain boundaries and govern bulk material properties. The main conceptual tools of grain boundary complexion engineering are grain boundary complexion equilibrium diagrams and grain boundary complexion time-temperature-transformation (TTT) diagrams. Importantly, when the thermodynamic and kinetic diagrams are used in tandem, scientists and engineers have the ability to tailor classical processing conditions (e.g. chemistry, pressure, temperature, and time) to intelligently control grain boundary complexions, and specifically, complexion transitions from one complexion type to another. The ability to control complexion transitions is particularly important because a complexion transition could theoretically convert a tough material to a brittle material or an insulator to a conductor. Overall, the first segment of this talk will review the concepts of grain boundary complexion engineering and outline various approaches to develop the necessary complexion equilibrium and time-temperature-transformation diagrams. The second segment of this talk will summarize a grain boundary complexion engineering case study of silica and rare-earth oxide doped boron suboxide armor ceramics. Boron suboxide is being investigated as a promising armor ceramic because of its low density and high hardness, however, boron suboxide exhibits poor sinterability and fracture toughness. This case study outlines how grain boundary complexion engineering is used to control the thermodynamics and kinetics of grain boundary complexion transformations to improve sinterability and enhance fracture toughness. Boron suboxide powders were co-doped with silica and rare-earth oxide additives and the resultant microstructures were analyzed using advanced electron microscopy techniques to characterize grain boundary structures and quantify grain boundary segregation. The main observations include clear differences in microstructural evolution between the silica and rare-earth dopants, modified atomic structures with different excess grain boundary segregation concentrations, and improvements in bulk hardness.

Ni/Al reactive nanolaminates are systems of current scientific interest due to their highly customizable exothermic combustion characteristics. The high combustion temperatures and wave speeds attainable by these systems have found industrial application in soldering microelectronic components and have been proposed as microinitiators in military ordnance. Ni/Al nanolaminates are typically synthesized through a physical vapor deposition (PVD) technique, allowing precise control of nanostructure which has been shown to have significant impacts on combustion characteristics. The most fundamental structural attribute is the bilayer thickness, which is defined as the combined thickness of one Ni and one Al layer. This thickness has been shown to affect both the combustion temperature and wave speed of the reaction. Correlated with the bilayer thickness, the thickness of an individual material layer has been shown to affect grain size. The PVD process forms columnar grains perpendicular to nanolaminate layers, which have average grain diameters on the order of the individual layer thickness. The current work investigates the role of grain size on reaction rates and peak temperatures. Decreasing grain dimeter provides increased rates of solid-state diffusion and subsequent solid/liquid dissolution, increasing overall reaction rates. Two bilayer thicknesses are evaluated in this work, on twice the thickness of the other, revealing stronger grain size dependence in nanolaminates with larger bilayer thicknesses.

Nickel silicide (NiSi) thin films have various advantages for microelectronics processes, such us low resistivity and low formation temperature. Therefore, it has been widely used in semiconductor devices. The formation of NiSi thin film mainly depends on the mechanism of diffusion between Ni and Si. In order to understand its formation kinetics, in-situ transmission electron microscopy (TEM) is a powerful tool to study the diffusion behavior during the process. In this work, we successfully observed the process of diffusion by in-situ TEM. We chose silicon-germanium (Si_0.7Ge_0.3) as strain layer to improve the electronic performance. During the heating process, Ni atoms would diffuse into Si_0.7Ge_0.3 layer at 400°C in the duration of 10 minutes heating. The HRTEM results and the corresponding FFT-DP demonstrated that the product was NiSi. In addition, we found that there is a specific orientation relationship of [120]_NiSi//[01-1]_Si0.7Ge0.3 and (002)_NiSi//(1-1-1)_Si0.7Ge0.3. Furthermore, the Energy Dispersive Spectrometer (EDS) analysis established the elemental information for both as-grown and after-annealed samples, giving strong evidence for the formation of the NiSi. These results provide us useful information to grow low resistivity NiSi thin film.

High-carbon steels (pearlitic steel) are widely used in the fields, including bridge cable, automobile tire cord and high-rise building, owing to the outstanding high strength and low cost. Micro-alloying is the common method to increase the strength of pearlite steels. In the simplest Fe-C-M (M: Mn, Si, Cr, Ni) ternary system, the effects of alloying elements have been extensively studied. For example, Mn, as the austenite stabilized element, delays the pearlitic transformation, improves the stability of cementite and retards the spheroidization of cementite lamellar; Si, as the non-carbide forming element, refines the lamellar structure of pearlite, increases tensile strength, improve softening resistance during tempering, and etc. However, in the complicated multicomponent system the partition and interactive effects among alloying elements needs to be attention, especially the interaction of alloying elements during the eutectoid transformation need to be further studied.
Therefore, in the present work the atomic interaction among Si, C and Mn in Fe-C-Mn-Si quaternary system during the eutectoid transformation is investigated using atom-probe tomography combining first-principle calculations. Two types of pearlitic carbon steels with different Si contents are prepared. At the initial stage of eutectoid transformation, atom probe tomography observations indicate that Si is enriched at the ferrite side of ferrite-cementite interface, while Mn is enriched at the cementite side. This interfacial segregation phenomenon gradually diminishes or disappears as the transformation proceed. Based on the results of first-principle calculations, Si moves from the core of cementite layer to the ferrite side of ferrite-cementite interface can obviously enhance the chemical bonding and stability of pearlite system. In the same manner, Mn atom moves from the core of ferrite layer to the ferrite-cementite interface, which improves the thermal stability of pearlite system. Therefore, Si/Mn segregates at each side of ferrite-cementite interface at the initial stage of pearlitic transformation. Furthermore, the partitioning ratio of Mn in high Si steel is higher than that in low Si steel, leading to more Mn partition into cementite. Based on the results of first-principles calculations, due to the strong repulsive force between Si and Mn at the interface of pearlite, the enrichment of Si in ferrite promotes the partitioning of Mn into cementite.

We have performed an ab initio study of tensorial elastic properties of the interface states in Ni₃Si associated with the Σ5(210) grain boundary (GB). Ni₃Si is well known structural material with a large potential for high temperature applications in corrosive atmospheres, which however suffers by low ductility with tendency to intergranular fracture. Rather complex tensorial elasto-chemical aspects of the studied periodic GB approximants were found in the case of states with different atoms at the GB plane (either only Ni atoms (Σ5(210)^Ni,Ni) or both Si and Ni atoms (Σ5(210)^Si,Ni)). The Σ5(210)^Ni,Ni GB state is predicted to have only a slightly lower interface energy of the two studied variants. The elastic constants are found to depend very sensitively on the GB plane chemical composition. In particular, the GB variant containing both Ni and Si atoms at the interface is shown to be unstable with respect to a shear deformation (one of the elastic constants, C₅₅, is negative). This instability is occurrs for a rectangular-parallelepiped supercell obtained when applying a standard coincidence-lattice construction. Our elastic-constant analysis allowed us to identify a shear-deformation mode reducing the energy and eventually to obtain a mechanically stable ground-state characterized by a shear-deformed parallelepiped supercell. Nevertheless, a three-/eight-fold reduction of the elastic constant C₅₅ (when compared with the bulk value) is identified as the crucial weakest link of the mechanical stability for the studied GB states. We have also partly stabilized this GB interface state by Al substituent replacing Si atoms at the GB.
Next, an origin of this elastic softening and instability of the rectangular-parallelepiped Σ5(210)^Si,Ni GB variant is discussed in terms of chemical inter-atomic interactions described by the crystal orbital Hamilton population (COHP). Lattice-dynamics properties represented by projected force-constant matrices on the unit vector along each bonding direction were considered as well. Such complex analysis reveals a weak interaction far from the GB interface between the Ni atoms in the 3^rd plane and the Si atoms in the 5^th plane. However, this bond weakening is a consequence of a very strong interaction between the Si atoms in the GB plane and Ni atoms in the 3rd plane off the GB interface. The same strong interaction was not observed when Si atom at the GB is replaced by Al. Thus the strong interaction near the GB plane makes this GB variant mechanically unstable.
Our study clearly shows the importance of anisotropic elastic-constant treatment as well as interatomic-interaction analysis for next studies of interface states close to GBs when determining origin of their mechanical (in-)stability. The sensitivity of tensorial elasto-chemical properties, which was additionally illustrated by studying the impact caused by Al atoms substituting Si atoms at the GB interface plane, paves a new way towards a solute-controlled design of GB-associated interface states with controlled tensorial elastic properties and stability.

For decades, diamond has shown superior properties that make it desirable for novel electronic materials. However, fabricating large size high quality single crystal diamond wafers faces multiple challenges. Lateral outgrowth and mosaic growth of CVD diamond make it highly possible for large size diamond wafers to be realized. Mesoscale characterization on the interface and surface of the grown CVD diamond is key to understanding how to eventually fabricate large size high quality single crystal diamond wafers.

In this work, mosaic and lateral single crystal diamonds were grown by Microwave Plasma Assisted Chemical Vapor Deposition (MPACVD) on High Pressure High Temperature (HPHT) seeds and characterized by a micro x-ray diffraction (µ-XRD) mapping technique with a 300 µm slit to show the evolution of diamond crystal structure information during growth and provide feedback on growth strategies. By understanding how crystal structure evolves during growth we can understand the mechanisms linking processing conditions the structure and properties of grown material.

Our results demonstrate after growth and regrowth by MPACVD, variation in the structure and distributions of these variations evolves towards homogeneity. Measurements of the (400) phase for mosaic and lateral growth samples by high-resolution x-ray rocking curve (HRXRC) and 2θ-ω µ-mapping technique. The overall misorientation of the merging CVD layers is shown to decrease with the regrowth from 0.225° (bottom layer plate) to 0.092° (uppermost layer plate). µ-HRXRC mapping results show a low mosaicity and μ-HR 2θ-ω scans show a low average Full Width at Half Maximum (FWHM), 0.032° and high crystallinity. Typically lateral outgrowth shows a uniform misorientation at the growth front, ranging from 0.092° to 0.035°. In contrast, our mosaically joined samples show a non-uniform distribution in misorientation at the merging interface, displaying a biaxial deviation parallel and perpendicular to the interface. The preliminary data shows that after regrowth, this biaxial distribution tends toward uniformity, with regions of independent preferred orientation overtaken by a single, new orientation. We will discuss this phenomenon in terms of step-flow overgrowth and energetically favorable states of the diamond crystalline phase during epitaxial lateral overgrowth in the extreme plasma environment.

Continuum models will be critical towards the computational characterization of the diffuse layer at electrochemical interfaces. Recently, a number of different approaches have been proposed. Here we will present a new class of methods based on a planar averaged approximation and the Gouy-Chapman theory for electrochemical interfaces [1]. These models potentially constitute a powerful set of tools for investigating one-dimensional surfaces, with several advantages over their more generalized counterparts. They exploit the analytic solution for the one-dimensional complex electrostatic problem, are computationally inexpensive, and are not affected by any imposed periodicity. Charged metal solvent interfaces and their experimentally recorded differential capacitance values present a benchmark to compare candidate models. We will present the theory, implementation, testing, and applications.

[1] F. Nattino, M. Truscott, N. Marzari, and O. Andreussi "Continuum models of the electrochemical diffuse layer in electronic-structure calculations" under review by J. Chem. Phys. (2018), available at https://arxiv.org/abs/1810.09797

Ethylene is a high-value chemical that is an essential building block for polymers and other designer molecules. Producing ethylene from ethane (a more plentiful feedstock) is, therefore, a high-value activity, but one that requires a lot of energy. The use of catalysts to more efficiently perform chemical reactions, for example by lowering energy barriers for reaction pathways, therefore has the potential to save billions of dollars annually at production scales. The oxide Mo-V-Nb-Te-O (M1) is one such catalyst, but one whose efficacy degrades at elevated temperatures. In this work, we aim to understand the dynamics of ethylene on the surface of M1 to inform strategies for improving M1’s efficacy or lower its optimal operating temperature. Specifically, we develop tools that facilitate molecular modeling of organic molecules including ethane and ethylene on M1 surfaces and use them to study surface dynamics. We implement a combination of the OPLS-AA and UFF force fields and measure surface residency times of ethylene and ethane as a function of temperature. These measurements show ethane preferentially adsorb near Vanadium and Tellurium sites, informing strategies for increasing adsorption selectivity towards Vanadium. We investigate candidates for surface patterning and discuss anticipated changes in catalyst effectiveness.

Enhanced Accident Tolerant Fuels (ATFs) are defined as nuclear fuels that can tolerate a severe loss of active cooling in the reactor core for a considerably longer period of time than the current UO₂ – zirconium alloy fuel system, while maintaining, or preferably improving, the fuel performance during normal operations and operational transients. The Critical Heat Flux (CHF) is one of the key parameters used to evaluate the thermal performance of a fuel product and to establish safety and operational margins. Preliminary studies for the proposed ATF concepts indicate that their cladding surface characteristics may result in substantial impacts in their CHF behavior. This work aims to investigate the surface characteristics of ATF cladding tubes of C26M, and APMT (both FeCrAl alloys) manufactured using industrial procedures. A comparison of their surface characteristics with current fuel cladding of Zircaloy-2 and Zircaloy-4 will be presented. Various roughness parameter, contact angle, and oxide layer thickness will be assessed using advanced materials examination techniques such as Profilometry, Atomic Force Microscopy, Contact Angle Goniometry and Scanning Electron Microscopy. Recent research has evidenced that surface roughness and contact angle may play an important role in the CHF enhancement. Further work will be focused on conducting a series of CHF experiments, at low and high pressure, with advanced temperature and fluid measurement instrumentation to obtain high fidelity qualitative and quantitative measurements of the thermal-hydraulic behavior of the ATF cladding under evaluation.

Hybrid organic-inorganic perovskites (HOIPs) such as methylammonium lead iodide (MAPI) and methylammonium lead bromide (MAPBr) have attracted much attention as solution-processed semiconductors that could be used in high performance optoelectronic devices such as photovoltaics, light emitting diodes, lasers, and X-Ray detectors. These materials exhibit long carrier diffusion lengths, large absorption coefficients, and high photoluminescence quantum yields. For commercial applications, however, HOIPs suffer from significant humidity-induced degradation. Here, we report the stabilizing and destabilizing effect of B-site substitutional doping in HOIP materials, and discuss how surface doping at the B-site can induce preferential crystal orientation.
Substitutional doping of MAPI with Bi was found to lead to a stabilizing effect at high humidity, but a destabilizing effect at lower humidity. With 5 mol% Bi in MAPI, UV-Vis-NIR absorptance spectroscopy and X-ray diffraction (XRD) showed that at 90% relative humidity, the films took 72 h to fully degrade compared to 24 h for the undoped MAPI films. At the lower humidity of 60% RH, the undoped MAPI films were more stable than the Bi-doped MAPI films. The MAPI films were stable for 14 days; whereas, the Bi-doped MAPI films degraded after only 10 days.
While MAPI is a promising photovoltaic material, certain tandem cell and LED applications require wider band gap HOIPs, such as MAPBr. However, the random orientation of grains in MAPBr thin films made from commonly employed synthetic methods may hinder these devices’ effectiveness. Here we report the incorporation of Ag into MAPBr films induces a preferential (100) crystallographic orientation on any substrate and increases crystal grain size of thin film cubic MAPBr. X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray spectroscopy (EDS), and time-of-flight secondary ion mass spectrometry (TOF-SIMS) revealed that Ag accumulates at the interfaces of the MAPBr film, which appears to be the reason for the preferred orientation of the film on the substrate. Elemental maps and grazing incidence wide angle X-ray scattering (GIWAXS) of the Ag-doped films also showed the emergence of secondary methylammonium silver bromide phases. These regions in the film could create recombination centers in photovoltaic devices (PVs). PVs were fabricated with MAPBr and Ag-doped MAPBr films. Despite the differences in crystallographic orientation and crystal grain size, there were no observed differences in the performance of these perovskite PVs up to 2 mol% Ag incorporation, at which point PV performance became worse with increasing silver content.

Cholesterol is an amphiphilic molecule present in cellular membranes at different concentrations depending on the cell’s role and functionality. It is uniquely structured as its hydrophilic head is much smaller than its bulky hydrophobic tail. When present in a phospholipid leaflet cholesterol behaves according to the umbrella model, filling the gaps in between phospholipid molecules and reducing the area per lipid. This provides an ordering and condensing effect for the membrane. The effect of cholesterol on cell membranes has been investigated and it has been seen to minimize its permeability and move the membrane from a liquid disordered to a liquid ordered state. These mechanics may be measured through the measurement of the interfacial tensions of lipid monolayers and bilayers with and without cholesterol.
In this work, the droplet interface bilayer (DIB) technique is used to investigate cholesterol’s effect on surface tension and the membrane structure. Based on lipid-coated aqueous droplets adhering in an oil medium, the DIB method enables linking droplets mechanics and membrane mechanics for interpreting changes in the membrane structure through droplet-droplet adhesion and the angles of contact at the droplet intersections. It is a purely fluidic method where the droplets are minimally constrained and the membrane is free to expand dependent on the interfacial tensions. By observing the changes in the droplets’ adherence while measuring the membrane properties through electrophysiology, one can calculate membrane’s tension, specific capacitance, applied stress and resulting strain.
Pendant drop tensiometry was used to investigate the changes in the monolayer surface tension with increasing amount of cholesterol. Higher monolayer tensions were observed with increasing cholesterol concentrations, along with increasing bilayer tension and adhesion energy. Next, membrane properties with varying cholesterol were measured. The membranes exhibited a non-linear reduction in thickness with the increasing cholesterol mole fractions. To understand cholesterol’s impact on membrane’s structure and rigidity, an electric field was gradually increased across the membrane until failure providing membrane compression beyond the equilibrium dimensions. The membrane thickness along with the corresponding electrical stress were measured for each voltage. Plotting the pressure versus membrane thickness for different cholesterol concentrations shows that cholesterol does not affect the critical stress at failure. However, it significantly reduces membrane thinning. Adding 30% cholesterol reduces membrane thinning by more than half. This indicated that cholesterol enhances the stiffness of the membrane, making it more rigid and resistant to transverse compression.

Controllability of the shape, size, and internal structure of block copolymer (BCP) particles as well as their uniformity is crucial to determine their utility and functionality in the practical applications. Here, we demonstrate the particle restructuring by solvent engineering (PRSE) strategy that combines membrane emulsification and solvent annealing processes, to produce monodisperse particles using functional BCPs with controlled size, shape, and inner structure. Importantly, the advantage of our PRSE approach is the general applicability to various types of functional BCPs including polystyrene-block-poly(1,4-butadiene) (PS-b-PB), polystyrene-block-polydimethylsiloxane (PS-b-PDMS), and polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP). PRSE starts with producing monodisperse BCP spheres in a wide range of particle size (from hundreds of nanometers to tens of several microns) using membrane emulsification, followed by successful transformation to shape-anisotropic BCP particles by solvent annealing under neutral wetting condition. Monodispersity of particle size was maintained during the PRSE, and the shape transformations to both prolate and oblate shaped ellipsoids were successfully achieved. Our approach was effective in controlling the aspect ratio (AR) of particles over wide ranges, from 1.0 - 2.5 and 1.0 - 5.0 for prolate and oblate ellipsoids, respectively. Also, observed AR of the particles was well-supported by theoretical calculation based on the model describing the particle elongation. Further investigation on the shape transformation kinetics during the PRSE revealed that the morphology transformation was driven by reorientation of BCP microdomains, whose kinetics was strongly associated with the overall molecular weight of BCP and the annealing time.

Surface energy and hydro-affinity play a significant role in device manufacturing and semiconductor processing. Surface reactivity, bonding and passivation is as critical as structure and composition for gate oxides, ohmic contact formation, heterostructures, opto-electronic and piezo-electronic device integration.

In this work, the effect of doping species upon Si(100) total surface energy, γ^T, hydro-affinity, and reactivity, is investigated for three n-type dopants, phosphorus, arsenic or antimony, and two p-type dopants, boron or gallium, [2] by new quantitative measurements via Three Liquid Contact Angle Analysis (3LCAA) [1] correlated with Ion Beam Analysis (IBA), as a function of concentration.

3LCAA and the van Oss-Chaudhury-Good (vOCG) theory yield γ^T from three interactions, the Lifshitz-van der Waals interaction energy γ^LW, the interaction energy with electron donors γ^-, and acceptors γ⁺, measured via the contact angles of three liquids.

A new automated image analysis algorithm, DROP™, [3], yields contact angles within <1° and γ^T within 3% in minutes. DROP™’s accuracy makes possible quantitative analysis of γ^T, γ^LW, γ^-, and γ⁺ as a function of doping species and concentrations.

Ion Beam Analysis (IBA) can detect oxygen coverage and elemental composition. High resolution IBA combes ��(O16, O16)�� 3.039±0.01 MeV nuclear resonance with <111> channeling detects oxygen within 5%, or about 6x10¹⁴ at/cm². Rutherford Backscattering Spectrometry (RBS) combined with <111> axial channeling can detect dopants with a sensitivity that increases quadratically with their atomic number, thus ~4x10²⁰ boron/cm³, or 1 at.%; ~4x10¹⁹ phosphorus/cm³, or 0.1 at.%; ~1x10¹⁹ gallium/cm³, or 0.02 at.%; ~1x10¹⁹ arsenic/cm³, or 0.02 at.%; and ~4x10¹⁸ antimony/cm³, or 0.01 at.% [2]. Oxygen coverage measured by IBA, along with doping levels and dopant species, can be correlated with γ^T, γ^LW, γ^-, and γ⁺ measured by 3LCAA on both as-implanted and Rapid Thermal Annealed (RTA) Si(100). Si(100) amorphizes with increasing ion dose and defect concentrations, which catalyzes native oxide formation. During RTA, Si(100) recrystallizes and incorporates dopants into substitutional lattice sites. However, native oxide formation and dopant segregation modify final native oxide composition and thickness, as well as total surface energy and the surface interactions with donors and acceptors.

Higher dopant concentrations in both p-type and n-type doped Si(100) result in larger oxygen coverage and thus thicker native oxides, as measured by IBA, and lower surface energies as measured by 3LCAA. As expected, resulting surfaces are more hydrophobic and less reactive than as-implanted Si(100).

3LCAA also finds that as Si resistivity increases, γ^T also increases, showing that lower doping concentrations result in a more hydrophilic and more reactive surface [3]. Implanted Si is then etched, which removes surface oxides, and recrystallized by RTA to electrically activate dopants, regrowing a new native oxide. Longer RTA processes cause thicker oxides to form, along with higher segregation of n-type dopants at the Si-SiO₂ interface. Hence, electrically activated n-type dopants enhance oxidation. The opposite effect is observed for recrystallized p-type doped Si, which shows thinner native oxides than as-implanted Si. 3LCAA, combined with IBA, can accurately measure changes to native oxide surface energies, hydro-affinity and reactivity as a function of dopant species and dopant concentration. These insights from 3LCAA expand understanding of dopants’ influence upon native oxide formation and of surface engineering for heterostructure formation by heteroepitaxy, conventional wafer bonding and NanoBonding™ [3].

[1] US Patents #9,018,077; #9,589,801, Herbots N. et al (2015); (2017)
[2] P. A. Cullen, Ph.D, Massachusetts Institute of Technology PhD Thesis (1991)
[3] US Patents Pending, Nicole Herbots, Saaketh Narayan, Jack Day, et al. (2018)

Ultrahigh-pressure (HP) polymorphs such as diamond exhibit unique physical properties. There are many HP polymorphs that can be recovered to the ambient pressure to be utilized in industry. However, some of the HP polymorphs that are thermodynamically stable under HP (> 10 GPa) transforms into amorphous structure or different crystalline phases during decompression. If this back-transformation can be suppressed, we can obtain some useful HP polymorphs to be utilized in future. As a possible solution, we combined the epitaxial thin film growth technique with HP synthesis method using a multi-anvil apparatus.

We first investigated a HP-phase, α-PbO₂-type titanium dioxide (α-TiO₂, orthorhombic, a = 0.454 nm, b = 0.549 nm, c = 0.491 nm). Unfortunately, most of the reported data was about the product in form of powder, and only a few reports about the fabrication of single crystals are currently available. In particular, single-phase α-TiO₂ epitaxial thin films has not been reported. In this study, we report the fabrication of the high-quality single-phase α-TiO₂(100) epitaxial thin films.

Thin films of epitaxial rutile TiO₂(100) (thickness: ~100 nm) were deposited as precursors on Al₂O₃(001) (5 mm in diameter and 0.5 mm in height) using pulsed laser deposition. HP treatment for thin films was performed using a Kawai-type multi-anvil high-pressure apparatus. The precursor thin film was heated up to 1000°C under HP of 8 GPa, and then kept for 0.5 h. After the heating step, cooling was started down to room temperature (RT), followed by decompression.

As a result, a successful single-phase α-TiO₂(100) epitaxial thin film has been obtained. It should be stressed here that rocking-curve full width at half maximum of the 200 peak showed a quite small value of 0.113°, indicating very high crystallinity. Our present study indicates that HP treatment to thin film samples allows us to fabricate high-quality HP-phase epitaxial thin films.

It is well-known that mono-dispersed colloidal particles self-assemble into hexagonal close-packed (HCP) structures, under appropriate conditions, which are also known as two-dimensional (2D) colloidal crystals. While various techniques, such as sedimentation, confined convective self-assembly, dip coating, drop casting, evaporative drying, electrophoretic deposition, self-assembly at the gas-liquid interface and Langmuir−Blodgett (LB) technique, inkjet printing etc. has been used for making such arrays, spin coating has its own advantage due to direct compatibility with microelectronic processing, pattern uniformity and requirement of very small amount of colloidal dispersion. We first show that monolayer colloidal crystal formation by spin coating is possible only in a narrow parametric window involving spin speed, substrate wettability, particle size, and substrate pattern geometry, in case a topographically patterned substrate is used for template guided assembly. Subsequently, I will highlight a facile colloidal transfer printing method that relies on UVO mediated degradation of a sacrificial PMMA thin film. The method is capable of transferring both hexagonal closed pack (HCP) and template guided non-HCP arrays of inorganic or polymeric colloids onto any hydrophobic or hydrophilic target surface, which can be smooth, rough, or even curved. The method does not require any surface modification of the target substrate or any medium to facilitate the transfer. Finally, I will show that in certain cases the transferred particle array can significantly enhance the anti-reflection property of the target surface and can also be used as a template for growing patterned Zinc Oxide Nano Rod array which exhibit Self Cleaning property.
Key References:
M. Banik and R. Mukherjee, ACS Omega, 3, 13422, 2018.
M. Banik, N. Bhandaru and R. Mukherjee, Chem Comm. 54, 3484, 2018

Nanostructured carbons are common components of electrodes used for energy-storage and
-conversion applications to impart enhanced electronic conductivity that sustains high-rate operation, to support dispersed active materials (e.g., electrocatalytic metals and oxides), and to contribute to the physical structure and integrity of the electrode. Often overlooked in the design and characterization of such electrode structures is the critical nature of electronic/chemical/physical interactions at the junction of carbon with the nanoscale phases that impart desired storage or electrocatalytic functionality. Our own experience with such multifunctional compositions shows that we still “leave on the table” too much energy and power performance as a consequence of sub-optimal interfacial design. In order to explore these fundamental questions, we step back from the complexity of 3D electrode architectures to planar electrode configurations that otherwise mimic the material characteristics of practical electrode structures. Carbon films deposited onto planar substrates by vapor-phase pyrolysis of benzene and related monomers serve as model interfaces that we then modify with charge-storing metal oxides, electroactive polymers, and metal colloids [1]. The resulting 2D interfaces are characterized by classical electroanalytical methods to determine fundamental properties such as electron-transfer rate constants and impedance-derived response times. These substrates are also amenable to interrogation by scanning probe microscopy, including in-situ monitoring of conductivity and surface morphology under potential/current control. Lessons learned from these model 2D interfaces are readily applied to the redesign of practical 3D electrode structures in next-generation electrochemical capacitors and batteries.

[1] J. F. Parker, G. E. Kamm, A. D. McGovern, P. A. DeSario, D. R. Rolison, and J. W. Long, Langmuir, 33 (2017) 9415–9425.

Myriads of electrode materials with interesting nanostructures have been fabricated for a range of electrochemical applications. Beyond the increases in their surface area, unconventional properties have been observed along with the development of new approach and methodologies to analyze and utilize such properties. For instance, once a redox-active molecule diffuses into the nanoscale cavity of the electrode, it frequently collides with the inner electrode surface before it escapes the cavity, which is termed nano-confinement effect. This effect can specifically amplify the signals of slow electron transfer reactions (e.g., glucose oxidation) and has been established as a new mechanism for sensing the target materials that have slow electron transfer kinetics.

In this study, we fabricated a Au nanopillar electrode and devised a new electrochemical platform using redox cycling reaction to utilize the nano-confinement effect of the Au nanopillar electrode, which is applicable even to the fast electron transfer reactions as demonstrated by facilitating the electrochemical reaction of a specific biomarker (i.e., pyocyanin which has fast electron transfer kinetics) of bacterial infection (i.e., Pseudomonas aeruginosa). The Au nanopillar electrode was fabricated by first treating a polymer substrate with plasma to render an array of polymer nanopillars on the substrate and then depositing Au onto the plasma-treated substrate to the appropriate thickness to form nanoscale inter-pillar space. The Au nanopillar surface was modified with the redox cycling counterpart of pyocyanin so that the pyocyanin confined in the inter-pillar space underwent repetitive cycles of reduction (by the electrode) and oxidation (by the redox cycling counterpart tethered to electrode surface) reactions resulting in the amplification of the pyocyanin signal. This redox cycling-assisted signal amplification was clearly observed with the Au nanopillar electrode with higher amplification by taller Au nanopillars while the amplification was insignificant for the flat Au electrode, demonstrating the nano-confinement effect of Au nanopillar electrodes. As a result, the sensitivity in measuring pyocyanin was improved by the Au nanopillar electrode compared with the flat Au electrode. We believe that this new platform based on the surface-bound redox cycling reaction could overcome the limit in applicability of the nano-confinement effect.

Liquid thin films generate intriguing flow regimes and under specific conditions, lead to disintegration and de-wetting with the formation structures of submicron feature size. This concept has been well utilized as a non-lithographic route for creating meso-scale structures, where the structures can be ordered by guiding the instability pathway using a template. We show that patterns present on the surface of a polymer thin film (polystyrene) on a flat substrate can result in an ordered dewetted morphology under certain conditions. The pre-patterned polymer thin film undergoes pattern directed rupture along the thinnest parts of the film when the initial local thickness over these zones (h_rm) is reduced to a limiting thickness (h_lim = 10 nm). In addition, depending on the periodicity of the imprinted patterns, the wavelength of instability corresponding to h_rm must be lower than the width of the patterned grooves (l_s). A morphology phase diagram is constructed which indicates a transition from the surface tension induced flattening to the ordered pattern directed rupture. The versatility of this technique is in its ability to form myriad of aligned meso-patterns starting from a simple grating structure on the film surface. We further extend this concept to polystyrene-poly(methylmethacrylate) bilayers with a patterned polymer-polymer interface, where the instability is more complex due to coupled deformation of multiple, confined interfaces. This leads to creation of exotic patterns such as submerged, embedded and core shell structures, which are beyond the capability of standard lithography methods. The morphology of the evolving patterns is controlled by several parameters including the initial film thickness (h_F), pre-pattern amplitude (h_st), duration of solvent vapor exposure and wettability of the stamp used for pre-patterning the film. The evolution can be interrupted at any intermediate stage thereby achieving patterns on demand, which are well ordered as well as significantly miniaturized as compared to features obtained from dewetting of a flat film of same initial thickness.
References
1. N. Bhandaru, P. S. Goohpattader, D. Faruqui, R. Mukherjee, and A. Sharma, Langmuir 31, 3203−3214 (2015).
2. N. Bhandaru, A. Das, R. Mukherjee, Nanoscale 8, 1073-1087 (2016).

In this paper we report high voltage MOS and Schottky Diode CV techniques for silicon and SiC power devices. 4H Silicon carbide is a wide bandgap semiconductor suitable for high voltage power electronics and RF applications due to high avalanche breakdown critical electric field, and thermal conductivity. The performance of various power devices, which may include MOSFET and Static Induction Transistor (SIT), can be affected by the deep level traps in the substrate and the oxide interfacial defects. We have characterized deep level trap (High Voltage Schottky Diode HF CV) and oxide interface trap densities (High Voltage HF MOS CV), measured the device channel doping profile for both 4H SiC and silicon, gate metal workfunction, and simulated the effects on DC/AC performance.

TCAD simulation is done for a high voltage RF SIT. Pinch voltage (V_Pinch, as defined by the gate voltage required to turn the device off for a certain applied drain voltage) may be affected by the interface charges. Generally speaking, the V_Pinch is very sensitive to the channel doping concentration.

If the oxide / SiC interface charges are positive charges, the V_Pinch is higher. Please notice the gain is also reduced – this may affect the AC performance.

The depletion width is derived from converting the measured Schottky Diode high frequency CV to Depletion Width vs. Voltage. The 1/C² vs. V plot may also be used to extract the gate metal work function.

SIT gate metal workfunction is extracted to be 5.42V.

SIT channel doping profile is generated by converting the high voltage HF Schottky Diode CV to 1/C² vs. V plot. When the frequency varies from 100 KHz to 1MHz, there is a small shift of the extracted doping profile. The difference between the measured doping concentrations at these voltages may be caused by the deep level traps in the 4H SiC epitaxial layer. When the test frequency is lower than 100 KHz, there is no significant shift. The SIT channel doping profile is extracted from the HF Schottky Diode CV. The deep level trap density is the difference between the two doping profiles.

The Surface Potential at the thermal oxide / 4H SiC interface vs. gate voltage is derived by the integration technique for the measured HF MOS CV.

Oxide / Silicon or 4H SiC interface trap density may be extracted by comparing the measured high-frequency MOS CV and a theoretical, defect-free high frequency MOS CV:

D_it (eV^-1×cm^-2) = 1/q ×C_ox × d(Delta_V_G)/ dØ_S (1)

Where q is the electron charge, C_ox is the oxide capacitance, D_it is the oxide interface defect density, Delta_V_G is the gate voltage difference between measured and theoretical HF MOS CVs, corresponding to the same capacitance in the CV plot, and Ø_S is the surface potential at the SiC / oxide interface. Surface Potential vs. Delta_V_G and Interface Trap Density vs. Surface Potential plots are presented in this paper.

Summary:

Oxide / Si or 4H SiC interface trap density, deep level trap density, device channel doping profile, gate metal workfunction are characterized with high voltage (100V), high frequency (up to 1MHz) MOS and Schottky Diode CV techniques. TCAD simulation shows the effects of these defects on DC and AC performance. The deep level trap density of 4H SiC (average) = 1×10¹⁵ cm^-3 about 1 order of magnitude higher than lightly doped silicon (= 1×10¹⁴ cm^-3). The interface state density of 4H SiC is (average) = 4×10¹¹ cm^-2 eV^-1, also about 10 times higher than silicon.