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The design and development of advanced devices based on metamaterials to control the transmission of acoustic waves is a hot topic. An important class of these metamaterials is based on phononic crystals with Locally Resonant Structure, included in those commonly known as Locally Resonant Sonic Materials. In these metamaterials, wave control is basically performed by two mechanisms: internal (or local) resonances in the scatterers that form the phononic crystal, and Bragg bandgaps due to structural periodicity. Their main control feature is the resonance peaks forming additional stop-bands away from the Bragg frequency, mainly in the low frequency regime. For some applications, coupling of the two phenomena is necessary to create a broad transmission gap. However, when both are located in close frequency ranges, some destructive interferences can occur. In this paper, the authors develop a comprehensive numerical model of periodic arrays of Hemholtz resonators, which explains in detail the physical mechanisms of this destructive interference and, simultaneously, allows the reproduction of the consequences of the interference. The numerical results are supported by experimental tests., Postprint (published version)

Density functional theory (DFT) calculations of polyethylene (PE) HVDC cable insulation have been performed for systems containing four different chemical impurities: acetophenone, cumene, $\alpha$-methyl styrene and $\alpha$-cumyl alcohol. Systems were generated by molecular dynamics (MD) equilibration at four different temperatures relevant for cable insulation applications: 277 K, 293 K, 343 K and 363 K. With the goal of gaining better measure of variations in hole and electron traps energies, four initial configurations were also stochastically generated at each temperature, which yielded four different final configurations after equilibration. The counterpoise correction scheme was implemented for DFT calculations, by distributing ghost atoms thought any empty pockets of space in between the PE chains. The PBE functional was selected for DFT simulations. The resulting band gaps were in agreement with those of earlier GGA-based studies, and thus lower by 3 eV than empirical band gaps. For all impurities, the first HOMO state and the first two LUMO states were generally located on the impurity molecule, forming one hole trap and two electron traps, but certain configurations generated increased electron trap numbers, or eliminated hole traps. No dependence could be derived between temperature and trap depth for either electron or hole traps. Mean electron trap energies were largely in agreement with results from earlier studies, they were deepest for acetophenone, and they varied by as much as 0.6 eV between different configurations. Hole traps are universally shallow and vary by up to 0.7 eV between configurations, and are similar in depth for all impurities. Results suggest that electron trap depths correlate with the presence of molecular features such as oxygen atoms and conjugated double bonds. The dependence of trap depth on the spatial configuration of the impurity molecule suggests that results could be improved by more precise quantum mechanical treatmen, Täthetsfunktionalteori (DFT) har använts för beräkningar av isolering till HVDC kablar som består av polyeten innehållande fyra olika kemiska orenheter: acetofenon, kumen, alfa-metylstyren och alfa-kumylalkohol. System att studera genererades genom molekylärdynamisk ekvilibrering vid fyra olika temperaturer relevanta för tillämning till kabelisolering: 277 K, 293 K, 343 K och 363 K. För att få ett mått på de variationer som existerar i energierna på hål- och elektronfällor genererades stokastiskt fyra initialkonfigurationer vid varje temperatur, vilket fyra olika konfigurationer efter relaxering. Motviktskorrigering implementerades för DFT-beräkningar, genom att fördela "spökatomer" i de tomrum som bildas mellan PE-kedjorna i den amorfa fasen. PBE-funktionalen användes för DFT-simuleringar. De resulterande bandgapen stämde överens med tidigare GGA-baserade studier, och var därmed runt 3 eV smalare än empiriskt uppmätta bandgap. För alla orenheter var det första HOMO-tillståndet och de två första LUMO-tillstånden i allmänhet placerade på orenheten, vilket resulterade i en hålfälla och två elektronfällor, men vissa konfigurationer gav upphov till fler elektronfällor, eller eliminerade hålfällorna. Inget samband kunde härledas mellan temperaturen och djupet på fällorna för vare sig elektron- eller hålfällor. Medelvärdet på elektronfällornas energier överensstämde till stor del med resultat från tidigare studier, energierna var högst för acetofenon, och de varierade med så mycket som 0,6 eV mellan olika konfigurationer. Hålfällorna var genomgående grunda, varierade med upp till 0,7 eV mellan olika konfigurationer, och hade likartat djup för alla orenheter. Resultaten indikerar att variationerna elektronfällornas medeldjup uppstår på grund av orenheternas olika molekylära uppbyggnad: förekomst av syreatomer och konjugerade dubbelbindningar i orenheterna leder till djupare elektronfällor. Det faktum att djupet på elektron- och hålfällor varierar mellan olika rumsliga konf

Density functional theory (DFT) calculations of polyethylene (PE) HVDC cable insulation have been performed for systems containing four different chemical impurities: acetophenone, cumene, $\alpha$-methyl styrene and $\alpha$-cumyl alcohol. Systems were generated by molecular dynamics (MD) equilibration at four different temperatures relevant for cable insulation applications: 277 K, 293 K, 343 K and 363 K. With the goal of gaining better measure of variations in hole and electron traps energies, four initial configurations were also stochastically generated at each temperature, which yielded four different final configurations after equilibration. The counterpoise correction scheme was implemented for DFT calculations, by distributing ghost atoms thought any empty pockets of space in between the PE chains. The PBE functional was selected for DFT simulations. The resulting band gaps were in agreement with those of earlier GGA-based studies, and thus lower by 3 eV than empirical band gaps. For all impurities, the first HOMO state and the first two LUMO states were generally located on the impurity molecule, forming one hole trap and two electron traps, but certain configurations generated increased electron trap numbers, or eliminated hole traps. No dependence could be derived between temperature and trap depth for either electron or hole traps. Mean electron trap energies were largely in agreement with results from earlier studies, they were deepest for acetophenone, and they varied by as much as 0.6 eV between different configurations. Hole traps are universally shallow and vary by up to 0.7 eV between configurations, and are similar in depth for all impurities. Results suggest that electron trap depths correlate with the presence of molecular features such as oxygen atoms and conjugated double bonds. The dependence of trap depth on the spatial configuration of the impurity molecule suggests that results could be improved by more precise quantum mechanical treatmen, Täthetsfunktionalteori (DFT) har använts för beräkningar av isolering till HVDC kablar som består av polyeten innehållande fyra olika kemiska orenheter: acetofenon, kumen, alfa-metylstyren och alfa-kumylalkohol. System att studera genererades genom molekylärdynamisk ekvilibrering vid fyra olika temperaturer relevanta för tillämning till kabelisolering: 277 K, 293 K, 343 K och 363 K. För att få ett mått på de variationer som existerar i energierna på hål- och elektronfällor genererades stokastiskt fyra initialkonfigurationer vid varje temperatur, vilket fyra olika konfigurationer efter relaxering. Motviktskorrigering implementerades för DFT-beräkningar, genom att fördela "spökatomer" i de tomrum som bildas mellan PE-kedjorna i den amorfa fasen. PBE-funktionalen användes för DFT-simuleringar. De resulterande bandgapen stämde överens med tidigare GGA-baserade studier, och var därmed runt 3 eV smalare än empiriskt uppmätta bandgap. För alla orenheter var det första HOMO-tillståndet och de två första LUMO-tillstånden i allmänhet placerade på orenheten, vilket resulterade i en hålfälla och två elektronfällor, men vissa konfigurationer gav upphov till fler elektronfällor, eller eliminerade hålfällorna. Inget samband kunde härledas mellan temperaturen och djupet på fällorna för vare sig elektron- eller hålfällor. Medelvärdet på elektronfällornas energier överensstämde till stor del med resultat från tidigare studier, energierna var högst för acetofenon, och de varierade med så mycket som 0,6 eV mellan olika konfigurationer. Hålfällorna var genomgående grunda, varierade med upp till 0,7 eV mellan olika konfigurationer, och hade likartat djup för alla orenheter. Resultaten indikerar att variationerna elektronfällornas medeldjup uppstår på grund av orenheternas olika molekylära uppbyggnad: förekomst av syreatomer och konjugerade dubbelbindningar i orenheterna leder till djupare elektronfällor. Det faktum att djupet på elektron- och hålfällor varierar mellan olika rumsliga konf

Machine-learning models are capable of capturing the structure-property relationship from a dataset of computationally demanding ab initio calculations. Over the past two years, the Organic Materials Database (OMDB) has hosted a growing number of calculated electronic properties of previously synthesized organic crystal structures. The complexity of the organic crystals contained within the OMDB, which have on average 82 atoms per unit cell, makes this database a challenging platform for machine learning applications. In this paper, the focus is on predicting the band gap which represents one of the basic properties of a crystalline material. With this aim, a consistent dataset of 12 500 crystal structures and their corresponding DFT band gap are released, freely available for download at https://omdb.mathub.io/dataset. An ensemble of two state-of-the-art models reach a mean absolute error (MAE) of 0.388 eV, which corresponds to a percentage error of 13% for an average band gap of 3.05 eV. Finally, the trained models are employed to predict the band gap for 260 092 materials contained within the Crystallography Open Database (COD) and made available online so that the predictions can be obtained for any arbitrary crystal structure uploaded by a user., QC 20200918

Machine‐learning models are capable of capturing the structure–property relationship from a dataset of computationally demanding ab initio calculations. Over the past two years, the Organic Materials Database (OMDB) has hosted a growing number of calculated electronic properties of previously synthesized organic crystal structures. The complexity of the organic crystals contained within the OMDB, which have on average 82 atoms per unit cell, makes this database a challenging platform for machine learning applications. In this paper, the focus is on predicting the band gap which represents one of the basic properties of a crystalline material. With this aim, a consistent dataset of 12 500 crystal structures and their corresponding DFT band gap are released, freely available for download at https://omdb.mathub.io/dataset. An ensemble of two state‐of‐the‐art models reach a mean absolute error (MAE) of 0.388 eV, which corresponds to a percentage error of 13% for an average band gap of 3.05 eV. Finally, the trained models are employed to predict the band gap for 260 092 materials contained within the Crystallography Open Database (COD) and made available online so that the predictions can be obtained for any arbitrary crystal structure uploaded by a user.

Phononic metamaterials are capable ofmanipulatingmechanicalwave propagation in applications ranging from nanoscale heat transfer to noise and vibration mitigation. The design of phononic metamaterials to control lowfrequency vibrations, such as those induced by ground transportation and low-amplitude seismic waves, however, remains a challenge. Here we propose a new design methodology to generate seismic metamaterials that can attenuate surface waves below 10 Hz. Our design concept evolves around the engineering of the multilayered soil, the use of conventional construction materials, and operational construction constraints. The proposed seismic metamaterials are constructed by periodically varying concrete piles in the host multi-layered soil. We first validate the design concept and the numerical models by performing a lab-scale experiment on the low-amplitude surface wave propagation in a finite-size seismic metamaterial. To the best of the Authors' knowledge, this is one of the few attempts made to date to experimentally understand the vibration mitigation capability of seismic metamaterials.We then numerically demonstrate that the multi-layered seismic metamaterials can attenuate surface waves over a wide frequency range, with the incident wave energy being confined within the softest layer of the shallow layered seismic metamaterials. In addition to the localized wave energy distribution, deep layered seismic metamaterials exhibit broadband cut-off band gaps up to 7.2 Hz due to the strongly imposed constraint between piles and surrounding soil. Furthermore, these cut-off band gaps strongly depend on the constraint between the piles and the bottomlayer of the soil and hence can be tuned by tailoring the foundation stiffness.We also evidence the possibility to create constant wave band gaps by introducing hollow concrete piles with pile volume fraction b10% in the deep layered seismic metamaterials. The findings reported here open new avenues to protect enginee

The fundamental energy gap of a periodic solid distinguishes insulators from metals and characterizes low-energy single-electron excitations. However, the gap in the band structure of the exact multiplicative Kohn-Sham (KS) potential substantially underestimates the fundamental gap, a major limitation of KS density-functional theory. Here, we give a simple proof of a theorem: In generalized KS theory (GKS), the band gap of an extended system equals the fundamental gap for the approximate functional if the GKS potential operator is continuous and the density change is delocalized when an electron or hole is added. Our theorem explains how GKS band gaps from metageneralized gradient approximations (meta-GGAs) and hybrid functionals can be more realistic than those from GGAs or even from the exact KS potential. The theorem also follows from earlier work. The band edges in the GKS one-electron spectrum are also related to measurable energies. A linear chain of hydrogen molecules, solid aluminum arsenide, and solid argon provide numerical illustrations.

The fundamental energy gap of a periodic solid distinguishes insulators from metals and characterizes low-energy single-electron excitations. However, the gap in the band structure of the exact multiplicative Kohn-Sham (KS) potential substantially underestimates the fundamental gap, a major limitation of KS density-functional theory. Here, we give a simple proof of a theorem: In generalized KS theory (GKS), the band gap of an extended system equals the fundamental gap for the approximate functional if the GKS potential operator is continuous and the density change is delocalized when an electron or hole is added. Our theorem explains how GKS band gaps from metageneralized gradient approximations (meta-GGAs) and hybrid functionals can be more realistic than those from GGAs or even from the exact KS potential. The theorem also follows from earlier work. The band edges in the GKS one-electron spectrum are also related to measurable energies. A linear chain of hydrogen molecules, solid aluminum arsenide, and solid argon provide numerical illustrations.

The fundamental energy gap of a periodic solid distinguishes insulators from metals and characterizes low-energy single-electron excitations. However, the gap in the band structure of the exact multiplicative Kohn-Sham (KS) potential substantially underestimates the fundamental gap, a major limitation of KS density-functional theory. Here, we give a simple proof of a theorem: In generalized KS theory (GKS), the band gap of an extended system equals the fundamental gap for the approximate functional if the GKS potential operator is continuous and the density change is delocalized when an electron or hole is added. Our theorem explains how GKS band gaps from metageneralized gradient approximations (meta-GGAs) and hybrid functionals can be more realistic than those from GGAs or even from the exact KS potential. The theorem also follows from earlier work. The band edges in the GKS one-electron spectrum are also related to measurable energies. A linear chain of hydrogen molecules, solid aluminum arsenide, and solid argon provide numerical illustrations.

Tuning the electronic structure of metal–organic frameworks is the key to extending their functionality to the photocatalytic conversion of absorbed gases. Herein we discuss how the band edge positions in zeolitic imidazolate frameworks (ZIFs) can be tuned by mixing different imidazole‐based linkers within the same structure. We present the band alignment for a number of known and hypothetical Zn‐based ZIFs with respect to the vacuum level. Structures with a single type of linker exhibit relatively wide band gaps; however, by mixing linkers of a low‐lying conduction edge with linkers of a high‐lying valence edge, we can predict materials with ideal band positions for visible‐light water splitting and CO2 reduction photocatalysis. By introducing copper in the tetrahedral position of the mixed‐linker ZIFs, it would be possible to increase both photo‐absorption and the electron–hole recombination times.

Locally-resonant (LR) phononic meta-materials and structures have a potential for wide band-gaps of elastic wave attenuation in low frequency range. In this paper, we investigate a use of continuum beam resonators suspended periodically on an Euler–Bernoulli beam. In a mathematical analysis based on Floquet–Bloch’s theorem, we describe the dispersive characteristics of flexural wave attenuation. Our results show richer dispersion properties in the LR structures with periodically attached continuum resonators of distributed degrees of freedom than those with the conventional force-only resonators. In particular, we identify the appearance of a wide composite band gap of local resonance and Bragg-scattering types and its potentials for low-frequency applications.

Tuning the electronic structure of metal–organic frameworks is the key to extending their functionality to the photocatalytic conversion of absorbed gases. Herein we discuss how the band edge positions in zeolitic imidazolate frameworks (ZIFs) can be tuned by mixing different imidazole‐based linkers within the same structure. We present the band alignment for a number of known and hypothetical Zn‐based ZIFs with respect to the vacuum level. Structures with a single type of linker exhibit relatively wide band gaps; however, by mixing linkers of a low‐lying conduction edge with linkers of a high‐lying valence edge, we can predict materials with ideal band positions for visible‐light water splitting and CO2 reduction photocatalysis. By introducing copper in the tetrahedral position of the mixed‐linker ZIFs, it would be possible to increase both photo‐absorption and the electron–hole recombination times.

Tuning the electronic structure of metal–organic frameworks is the key to extending their functionality to the photocatalytic conversion of absorbed gases. Herein we discuss how the band edge positions in zeolitic imidazolate frameworks (ZIFs) can be tuned by mixing different imidazole‐based linkers within the same structure. We present the band alignment for a number of known and hypothetical Zn‐based ZIFs with respect to the vacuum level. Structures with a single type of linker exhibit relatively wide band gaps; however, by mixing linkers of a low‐lying conduction edge with linkers of a high‐lying valence edge, we can predict materials with ideal band positions for visible‐light water splitting and CO2 reduction photocatalysis. By introducing copper in the tetrahedral position of the mixed‐linker ZIFs, it would be possible to increase both photo‐absorption and the electron–hole recombination times.

Tuning the electronic structure of metal–organic frameworks is the key to extending their functionality to the photocatalytic conversion of absorbed gases. Herein we discuss how the band edge positions in zeolitic imidazolate frameworks (ZIFs) can be tuned by mixing different imidazole‐based linkers within the same structure. We present the band alignment for a number of known and hypothetical Zn‐based ZIFs with respect to the vacuum level. Structures with a single type of linker exhibit relatively wide band gaps; however, by mixing linkers of a low‐lying conduction edge with linkers of a high‐lying valence edge, we can predict materials with ideal band positions for visible‐light water splitting and CO2 reduction photocatalysis. By introducing copper in the tetrahedral position of the mixed‐linker ZIFs, it would be possible to increase both photo‐absorption and the electron–hole recombination times.

Tuning the electronic structure of metal–organic frameworks is the key to extending their functionality to the photocatalytic conversion of absorbed gases. Herein we discuss how the band edge positions in zeolitic imidazolate frameworks (ZIFs) can be tuned by mixing different imidazole‐based linkers within the same structure. We present the band alignment for a number of known and hypothetical Zn‐based ZIFs with respect to the vacuum level. Structures with a single type of linker exhibit relatively wide band gaps; however, by mixing linkers of a low‐lying conduction edge with linkers of a high‐lying valence edge, we can predict materials with ideal band positions for visible‐light water splitting and CO2 reduction photocatalysis. By introducing copper in the tetrahedral position of the mixed‐linker ZIFs, it would be possible to increase both photo‐absorption and the electron–hole recombination times.

Tuning the electronic structure of metal–organic frameworks is the key to extending their functionality to the photocatalytic conversion of absorbed gases. Herein we discuss how the band edge positions in zeolitic imidazolate frameworks (ZIFs) can be tuned by mixing different imidazole‐based linkers within the same structure. We present the band alignment for a number of known and hypothetical Zn‐based ZIFs with respect to the vacuum level. Structures with a single type of linker exhibit relatively wide band gaps; however, by mixing linkers of a low‐lying conduction edge with linkers of a high‐lying valence edge, we can predict materials with ideal band positions for visible‐light water splitting and CO2 reduction photocatalysis. By introducing copper in the tetrahedral position of the mixed‐linker ZIFs, it would be possible to increase both photo‐absorption and the electron–hole recombination times.

Tuning the electronic structure of metal–organic frameworks is the key to extending their functionality to the photocatalytic conversion of absorbed gases. Herein we discuss how the band edge positions in zeolitic imidazolate frameworks (ZIFs) can be tuned by mixing different imidazole‐based linkers within the same structure. We present the band alignment for a number of known and hypothetical Zn‐based ZIFs with respect to the vacuum level. Structures with a single type of linker exhibit relatively wide band gaps; however, by mixing linkers of a low‐lying conduction edge with linkers of a high‐lying valence edge, we can predict materials with ideal band positions for visible‐light water splitting and CO2 reduction photocatalysis. By introducing copper in the tetrahedral position of the mixed‐linker ZIFs, it would be possible to increase both photo‐absorption and the electron–hole recombination times.

Locally-resonant (LR) phononic meta-materials and structures have a potential for wide band-gaps of elastic wave attenuation in low frequency range. In this paper, we investigate a use of continuum beam resonators suspended periodically on an Euler–Bernoulli beam. In a mathematical analysis based on Floquet–Bloch’s theorem, we describe the dispersive characteristics of flexural wave attenuation. Our results show richer dispersion properties in the LR structures with periodically attached continuum resonators of distributed degrees of freedom than those with the conventional force-only resonators. In particular, we identify the appearance of a wide composite band gap of local resonance and Bragg-scattering types and its potentials for low-frequency applications.

The design and practical viability of an intermediate band solar cell incorporating low dimensional structures made with dilute nitrogen alloys of III-V semiconductors is investigated theoretically and experimentally. Modeling of optical and electronic properties of the proposed materials and device is undertaken. The study suggests potential for this new class of single junction devices to yield efficiencies and radiation tolerance comparable or exceeding those of existing multi-junction devices., The original document contains color images.

In the past three years, our research project sponsored by AFOSR has been focused on developing low bandgap semiconducting polymers as both the donor and acceptor materials for BHJ solar cells. Research effort were also devoted to establish structure-property relationship and develop approaches to optimize light harvest, charge separation and charge collection, thus the overall power conversion efficiency (PCE). We have made very important contributions to the field and developed a series of new semiconducting polymers for highly efficient ternary polymer solar cells. We also discovered cooperative plasmonic effect of dual precious metal nanoparticles in enhancing PCE of BHJ solar cells. Multiwall carbon nanowire were introduced to significantly enhance the charge collection. These results have had significant impact to the area. Several polymers developed in our lab are now widely investigated around world. Seven of the papers published are highly cited as indicated by SCI., The original document contains color images.

Graphene, the super carbon , is now accepted as wonder material with new physics and it has caused major breakthrough in nanotechnology[1-3]. Graphene has been the main candidate for applications in semiconductor electronics and in other related fields [4-7]. After a decade of the discovery of graphene [7] , one may have to answer the following questions. Are the key technological huddles overcome successfully to realise the full applications of graphene? What is the current status of the graphene based devices or Electronics ? How the graphene market business is going to evolve in the coming years? What kind of graphene based products can one expect in the near future?. The main challenge in graphene is the opening of the band gap in the semimetallic electronic structure. In the bandstructure of graphene which is dominated by Dirac description, valence and conduction bands cross the Fermi level at a single point (K points) in the Brillouin zone and hence it is a is a zero gap semiconductor or a semimetal with zero density of states . Due to the linear dispersion of energy bands and appearance of Dirac cones near the K point and Fermi level the electrons behave like massless Dirac fermions [1]. For applications in nanoelectronic devices a suitable band gap is essential. Opening a suitable bandgap in graphene will allow it to be used in the semiconductor industry for the fabrication of new 2-dimensional nanoelectronic devices such as solar cells, liquid crystal devices, and nanosized transistor prototypes [8].

[EN] The propagation of intense acoustic waves in a periodic medium (sonic crystal) is numerically studied. The medium consists in a structured fluid, formed by a periodic array of fluid layers with alternating linear acoustic properties and quadratic nonlinearity coefficient. The spacing between layers is of the order of the wavelength; therefore Bragg effects such as band gaps appear. We show that the interplay between strong dispersion and nonlinearity influences wave propagation. The classical waveform distortion process typical of intense acoustic waves in homogeneous media can be strongly altered when nonlinearly generated harmonics lie inside or close to band gaps.

We report and explain the photoluminescence spectra emitted from silicon solar cells with heavily-doped layers at the surface. A micro-photoluminescence spectroscopy system is employed to investigate the total spectrum emitted from both the heavily-doped layer and the silicon substrate with micron-scale spatial resolution. The two regions of the device give rise to separate photoluminescence peaks, due to band-gap narrowing effects in the highly-doped layer. Two key parameters, the absorption depth of the excitation wavelength, and the sample temperature, are shown to be critical to reveal the separate signatures from the two regions. Finally, this technique is applied to locally diffused and laser-doped regions on silicon solar cell pre-cursors, demonstrating the potential value of this micron-scale technique in studying and optimizing locally doped regions.

Early identification of the specific proteins, which indicate delayed healing are critical to long-term success of the patient and the development of new treatments. It is the objective of this research to correlate changes in the proteome of wounds with clinical outcome and develop technology to sense these changes. Protein Imprinted Xerogels with Integrated Emission Sites (PIXIES) and porous polymer photonic bandgap (PBG) structures are being integrated to sense the wound bed in real time. Flexible structures have been developed to conform to the anatomy of the wound bed.

Hafnium oxide (HfO2) films were grown by sputter-deposition by varying deposition temperature (Ts) in a wide range of 25?700?C. The deposited HfO2 films were characterized by studying their growth behavior, microstructure, and optical properties. Characterization of the films employing a wide range of analytical techniques indicate a clear functional relationship between processing conditions, structure, morphology, and optical properties. HfO2 films were amorphous at Ts 6 200?C, at which point a structural transformation occurs. HfO2 films grown at Ts200?C were nanocrystalline, stabilized in a monoclinic structure. The nanocrystalline HfO2 films exhibit a strong (111) texturing. The average crystallite size of HfO2 films increased from10 nm to 20 nm with increasing Ts. Electron and atomic force microscopy measurements also correlated with the crystalline behavior, as well as an evenly distributed network of spherical shaped crystallites for the nanocrystalline HfO2 films. Density (ƿ) of HfO2 films probed using X-ray reflectivity and ellipsometry data analysis indicate that the q values are strongly dependent on Ts; ƿ varied in the range of 7.36?9.14 g/cm3. The higher end of ƿ values were noted only for HfO2 films grown at relatively higher Ts indicating the crystalline nature accounts for the q improvement. The band gap values of the films varied in the range of 5.78?6.17(?0.03) eV for Ts = 25?700?C. Index of refraction at 550 nm increased from 1.80 to 2.09, which also correlates with the characteristic feature of improved structural order, packing density of HfO2 films with increasing Ts. Based on the observed results, a correlation between growth conditions, microstructure and optical constants is established., Published in Optic Materials v37 p621-628, 2014.

We have demonstrated that a low band gap D-A conjugated polymer P3, that absorbs in the NIR (800 nm), can be fabricated into spherical nanoparticles (nano-P3) using Pluronic F127 as a soft template. Nano-P3 (67 nm) was found to be very stable in aqueous media. A heating efficiency curve showed that it took 7 g of nano-P3 to change the temperature of the solution by 20 deg C after one minute. This number is at a lower concentration than other donor acceptor polymer nanoparticles, such as nano-PCPDTBSe and nano-PCPDTBT, which may be due to P3 having a 20% greater molar absorptivity than PCPDTBSe. The nanoparticles, in the absence of NIR light, showed no significant toxicity towards CT26 colorectal cancer cells at concentrations of 5-200 g/mL. Cell viability assays showed that in the presence of NIR light, nano-P3 was shown to generate significant heating to destroy colorectal cancer cells at very low nanoparticle conentration ( 15 g/mL). Using the lowest possible concentration for photothermal ablation is advantageous for translating D-A ECPNs to clinical applications., The original document contains color images.

Poly(aryl-ether-ether-ketone) (PEEK) is a primary candidate for a high temperature capacitor dielectric due to its outstanding mechanical strength and thermal stability. However, its breakdown strength is unsatisfactory (400 MV/m) and its volumetric resistivity drops significantly at electric fields above 100 MV/m. Our main objective is to study the effect of chemical impurities, such as water molecules and chain ends, on impurity states in the band gap of PEEK using density functional theory (DFT) to gain insight on the factors impacting electrical resistivity under a high field, which may correlate to low breakdown strength. Our focus is to determine the correct geometry of the unit cell and develop a methodology for computing the band gap of PEEK accurately. We used dispersion-corrected DFT to optimize various starting geometries and determined the correct one based on a comparison with experimental crystallographic data. The band gap was determined from the projected density of states, which is calculated using the HSE06 hybrid exchange-correlation functional. The original HSE06 formulation overestimates the band gap of PEEK by 33% compared to the experimental value. So, to obtain a more accurate functional form, we conducted a systematic study on the parameter space of the functional by varying the exchange-correlation mixing and screening coefficients., The original document contains color images. Prepared in collaboration with the University of Texas at El Paso, TX.

We apply ultrafast x-ray diffraction with femtosecond temporal resolution to monitor the lattice dynamics in a thin film of multiferroic BiFeO3 after above-band-gap photoexcitation. The sound-velocity limited evolution of the observed lattice strains indicates a quasi-instantaneous photoinduced stress which decays on a nanosecond time scale. This stress exhibits an inhomogeneous spatial profile evidenced by the broadening of the Bragg peak. These new data require substantial modification of existing models of photogenerated stresses in BiFeO3: the relevant excited charge carriers must remain localized to be consistent with the data., The original document contains color images. Published in Physical Review Letters, v112 article ID 097602, 3 Mar 2014. Sponsored in part by DoE.

The focus of this research was to acquire definitive experimental data on predominant point defects in three important wide-band-gap semiconductors. Hyperfine interactions in electron paramagnetic resonance spectra were used to characterize the neutral nitrogen acceptor in zinc oxide, to identify a silicon interstitial impurity in titanium dioxide, and to determine the electronic structure of the singly ionized sulfur vacancy in stannous hexathiohypodiphosphate (SPS). Research on the basic properties of these technologically important materials plays a crucial role in the development of advanced optical and electronic systems. Zinc oxide is an electro-optic material with the potential to produce highperformance electronics and also ultraviolet detectors and emitters. The angular dependencies of axial and basal 67Zn hyperfine lines were used to extract 67Zn hyperfine interaction parameters for the neutral nitrogen acceptor in zinc oxide. These data establish the exact nature of the ground-state wave function associated with this important acceptor. The experimental information obtained in this study about the ground-state wave function will allow computational theorists to model the nitrogen acceptor and determine whether it can successfully lead to p-type conduction in zinc oxide. The search for p-type zinc oxide is one of the most important basic questions in present-day research on wide-band-gap semiconductors., The original document contains color images.

This grant was the first to the PI to use first principles quantum mechanics to characterize key atomic-scale elementary processes involved in photocatalysis by novel materials comprised of abundant elements. In particular, we performed computer simulations to evaluate the efficacy of doped and alloyed first-row transition metal oxides for use as catalysts for renewable fuels production. Our work began with extensive testing of methods to calculate properties of interest in photocatalytic production of fuels. These preliminary studies were necessary to establish a quantitatively accurate set of theories to employ when performing photocatalyst modeling, which entails a complicated series of events starting from sunlight absorption to produce electronic excited states (electron-hole pairs), subsequent electron-hole pair separation and transport, followed by redox chemistry at the surface of the catalyst to produce fuels. Properties to be optimized include the material s band gap (which determines the amount of sunlight absorbed), valence and conduction band edge positions (which control whether, e.g., photocatalytic water splitting is thermodynamically allowed), electron-hole pair lifetimes and charge carrier mobilities (which determine whether the charge carriers survive to reach an interface where they can react), and overpotentials for redox reactions at the surfaces of catalysts (which largely control the kinetics of the photoelectrocatalysis). We have examined all of these properties for a number of candidate catalyst materials, and have elucidated important design principles from these studies for future optimization of photocatalysts.

A circular monopole antenna fed with a coplanar waveguide is designed as an ultra-wideband (UWB) antenna for ground-penetrating radar. The performance of the antenna is enhanced by adding an electromagnetic band gap (EBG) surface on one side and a monopole director on the other side. The EBG surface can be a single-resonance structure or dual or multi-resonance structure for wider bandwidth. The enhanced design shows higher and flatter gain across the wideband, with different levels of improvements, depending on the choice of the additional layers, which also impacts the overall profile of the antenna. Simulation results of a number of designs are presented, along with measurement results of one design. Comparison of measured and simulated results is presented and shows good agreement. The target frequency band for the design is 0.7 3.0 GHz., The original document contains color images.

Following the discovery of graphene, there has been increased interest in materials that allow for the construction of two-dimensional (2D) devices. In this project we will focus on 1H-molybdenum disulfide (MoS2), which, like graphene, is a monolayer; unlike graphene, however, it has a band gap. 1H-MoS2 differs from silicon in that its band gap is direct and, like graphene, can be mechanically exfoliated (ME) to isolate it as a single molecular layer. We will lay the groundwork required for fabricating field-effect transistors and address the effects that the material microstructure has on transistor properties by comparing the quality of ME-MoS2 to MoS2 grown by chemical vapor deposition (CVD-MoS2). The layer count and material quality will be analyzed using Raman spectroscopy, photoluminescence spectroscopy, and atomic force microscopy (AFM) mapping, after which the transistors will be built using e-beam lithography., The original document contains color images.

The design and optimization of higher efficiency Copper Indium Gallium Selenide (CIGS) solar cells are investigated in this thesis. Optimizing the thickness layers of a cell for various band gaps was conducted in order to design a cell that exceeds the current industry efficiency record of 20.8%. Silvaco provides a modeling program called ATLAS that is specifically designed to model semiconductor devices. ATLAS was used to model a CIGS cell that is currently being produced to verify the validity of the model. Various thicknesses were then swept to determine the optimum thickness for a given band gap. Solar spectrum intensity varies by location around the Earth. Optimizing CIGS cells for various band gaps yields higher overall power output when dealing with drastic climate and location variations. Cells for five band gaps ranging from 1.14 eV to 1.69 eV were optimized in this thesis. The highest achieved efficiency was for a band gap of 1.69 eV with an overall theoretical efficiency of 22.4%.

We report on discovery of and strategies for designing new complex oxide materials in which two normally contradictory properties coexist, and its publication. In one example, switchable polar character (ferroelectricity) arising from non-centrosymmetric structure is achieved in a narrow-gap material through design and synthesis of cation and oxygen vacancy-driven tuning of band gap without loss of polar character. Shown for a single phase solid solution ferroelectric oxide perovskite (K,Ba),(Ni,Nb)O_(3-delta), this material exhibits a compositionally tunable and direct band gap in the range of 1.1 3.8 eV, with potential for novel nonlinear light-matter applications in addition to high-efficiency photovoltaic solar energy conversion. In a second example we report, with collaborators, on the discovery, synthesis and properties of a completely new family of non-perovskite complex oxide that exhibits ferroelectric and antiferromagnetic order. We report on publication of a new thin film synthesis strategy that enables, through epitaxial stabilization, low-energy and highly scalable growth of single-crystal heteroepitaxial complex oxide thin films featuring low-temperature deposition and a brief annealing under wellcontrolled conditions. Finally, we describe progress in using inelastic light scattering, in conjunction with other techniques, to characterize and quantify oxygen vacancy concentrations, including their effects on structure and electronic phase transitions.

In this study, first, we prepared ternary component semiconductor powders of ZnmIn2S3+m (ZIS) by using microwave-assisted hydrothermal method. Microwaveassisted hydrothermal process is a facile way to produce nanomaterials at a shorter reaction time with less secondary impurities while changing the amount of zinc in a series of solid solutions. With little adjustment of the ratios of [Zn]/[In], the hydrogen production rate of the photocatalysts, particle sizes, and band gap are significantly different. In the following experiments, the core-shell of nanoshell@SiO2, as well as the nanostructure of photocatalyst, were further investigated. Solar energy in the visible-light range is expected to be absorbed by the photocatalyst first without any interference from the metal nanoshells. The presence of metal nanoshells as the core can absorb the solar energy in the IR and visible-light region ranging from 500 nm to 900 nm. Our data showed that the plasmonic-enhanced photocatalytic activity was a function of the absorption of Au nanoshells. At the absorption wavelength of 500 nm of the Au nanoshells, the enhancement of hydrogen production was probably due to the plasmonic effect. As opposed to the absorption wavelength at 900 nm, the enhancement was due to thermal effect. Furthermore, the nanoshell absorbing at 700 nm has the highest hydrogen production rate which may be due to both the plasmonic and thermal effect. Thickness of the silica layer also influenced hydrogen evolution. Steady-state photoluminescence measurement was used to investigate the electron-hole recombination mechanism. Results demonstrated that efficient electron-hole charge separation can be achieved when Au nanoshells were incorporated. Finally, the core-shell with various [Zn]/[In] compositions was used to identify the interaction between the Au nanoshell and the photocatalyst.

We performed experiments to test the hypothesis that partial counter-doping of sulfur-hyperdoped silicon would create a partially filled intermediate band, which could be used for sub-bandgap photodetection. We fabricated counter-doped Si:S:B of well-controlled crystal quality and dopant concentration-depth profile, and we made three independent measurements of photoconductivity: one with contacts, and two without contacts. These measurements were per-formed in collaboration with the Persans group at RPI, the Buonassisi group at MIT, and the Warrender group at Ben t Labs. In all cases, the sub-bandgap photoresponse is negligibly small. We conclude from these results that the lifetime of photo-excited carriers is very small-- less than about 10 ns. Sub-bandgap photodetection using counter-doped silicon does not appear to be promising. The optoelectronic response appears to be less desirable than those of certain deep-level transition metals in Si, which appear much more promising for future investigation.

The Department of Defense has expressed interest in thorium dioxide (ThO2) and uranium dioxide (UO2) as possible candidates for use as special nuclear material in designing neutron detectors. Both materials have large neutron interaction cross sections. Uranium dioxide is particularly attractive due to its semiconducting properties and a relatively small band gap of 2 eV. Both materials fluoresce under ionizing radiation making them candidates for scintillating detectors. Three Ux:Th1-xO2 (x= 0.00, 0.01, 0.22) hydrothermally grown single crystals were examined using cathodoluminescence to interrogate the changing electronic properties of ThO2 as it became an alloy. Both depth-resolved and temperature- dependent cathodoluminescence studies were performed to examine the crystal structure and the defects present. An ultra-high vacuum system operating at 10 to the minus 8th power Torr was used with electron beam energies ranging from 2 to 14 keV. Spectra were taken on all three samples before and after a proprietary chemical cleaning process involving a crown ether/picric acid solution was applied to the crystals to remove surface contaminants. Spectral deconvolution of the spectra showed evidence of both direct and indirect gap photon transitions from the O 2p to Th 6d at 4.2 eV and 4.8 eV respectively. Uranium-doped spectra showed evidence of the midgap O 2p to U 5f quadrupole transition and O 2p to U 6d transition at 5 eV., The original document contains color images.

The objective of the project is to develop silicon-based group IV heterostructure lasers by the incorporation of another group IV element of Sn. We have made significant progress toward the milestones described in the proposal. Notable achievements are: (a) direct bandgap group IV materials (cooperated with Sn) is achieved and the first state-of-the-art group IV light-emitting diode (GeSn p-i-n) operated at near infrared with direct emission is demonstrated (Appl. Phys. Lett. 102, 182106 (2013). Editor s Picks on Semiconductor Research from APL (2014).), and (b) photodetector with best clarity around the near to mid-infrared region is illustrated (Appl. Phys. Lett. 103, 231907 (2013), and references within). Our researches are a major milestone towards the realization of the group IV based photonic devices.

In the past year, we successfully demonstrated several phenomena related to light localization, light beam manipulation, and light signal routing in free-space as well as in optically-induced photonic bandgap structures, including elimination of transverse instability in stripe solitons by one-dimensional lattices, linear and nonlinear control of optical beams in optical periodic structures. In particular, we predicted and demonstrated a new class of nonconventional edge states in optically induced honey-comb lattices - an optical analog of graphene, which has been published in Nature Material and Physics Review Letters. Our work was reported in a number of news media as well as featured in PRL cover. We are happy to report that we have made significant progress in this funded project with already many scientific research papers published in top-rated refereed journals (See attached list of publications that acknowledged the support from AFOSR). In addition, several students have actively involved in this project, and close collaboration has been maintained with other principal investigators supported by AFOSR., The original document contains color images.

Topological insulators exhibit metallic surface states populated by massless Dirac fermions with spin-momentum locking, where the carrier spin lies in-plane, locked at right angles to the carrier momentum. Here, we show that a charge current produces a net spin polarization via spin-momentum locking in Bi2Se3 films, and this polarization is directly manifested as a voltage on a ferromagnetic contact. This voltage is proportional to the projection of the spin polarization onto the contact magnetization, is determined by the direction and magnitude of the charge current, scales inversely with Bi2Se3 film thickness, and its sign is that expected from spin-momentum locking rather than Rashba effects. Similar data are obtained for two different ferromagnetic contacts, demonstrating that these behaviours are independent of the details of the ferromagnetic contact. These results demonstrate direct electrical access to the topological insulators surface-state spin system and enable utilization of its remarkable properties for future technological applications., Published in NATURE NANOTECHNOLOGY, p1-7, 2014. The original document contains color images.

A method for growing single crystal arrays of semimetallic vertical and horizontal erbium antimonide nanorods, nanotrees, and nanosheets embedded within a semiconducting gallium antimonide matrix was investigated. The nanostructures form simultaneously with the matrix and have epitaxial, coherent interfaces with no evidence of stacking faults or dislocations as observed by high-resolution transmission electron microscopy. By combining molecular beam epitaxy growth and in-situ scanning tunneling microscopy, images of the growth surface one atomic layer at a time showed that the nanostructured composites form via a surface mediated self-assembly mechanism that is controlled entirely at the growth front and is not a product of bulk diffusion or bulk segregation. These highly tuneable nanocomposites show promise for direct integration into epitaxial semiconductor device structures, and also provide a unique system in which to study the atomic scale mechanisms for nucleation and growth., The original document contains color images.

Type-II strained layer superlattices (SLSs) offer a broad range of design degrees of freedom to help optimize their properties as absorber layers of infrared photon detectors. We theoretically examine a new class of mid-wavelength infrared (2-5 micron bandpass) Type-II structures with two-layer InGaSb/InPSb and four-layer InAs/GaSb/InAs/InPSb SLS periods. Phosphorous-containing SLSs are a promising approach to improving infrared photon detector performance due to providing a new set of material properties, including favorable valence band offsets. P-based SLSs of four-layer type InAs/GaSb/InAs/InPSb were found to be among the best 5-micron gap SLSs that we have modeled. Among the studied designs, the lowest dark current in an ideal structure is predicted for a four-layer 23.6 Angstrom InAs/20 Angstrom GaSb/23.6 Angstrom InAs/60 Angstrom InP0.62Sb0.38 SLS. Its predicted ideal dark current is about 35 times lower than an n-type HgCdTe-based photodiode absorber and six times lower than a p-type HgCdTe one for the same bandgap, temperature, and dopant concentration. We also discuss a defect mitigation strategy that involves positioning the SLS gap in an energy range that avoids defect levels and show how this applies to the aforementioned P-containing SLS., Published in IEEE Journal of Selected Topics in Quantum Electronics, v19 n5, Sep/Oct 2013.

The goal of this research effort is to implement dynamically-tuneable Electromagnetic BandGap (EBG) circuits for filtering in an integrated circuit (IC) fabrication processes leveraging a technique previously developed for Printed Circuit Board (PCB).

InAs/InGaSb superlattice (SL) materials are an excellent candidate for infrared photodiodes with cut-off wavelengths beyond 15 micrometer, i.e. in the very long infrared wavelength (VLWIR) range. There are relatively few options for high performance infrared detectors to cover wavelengths longer than 15 micrometer, especially for operating temperatures above 15K. There are a variety of possible superlattice designs that will cover the VLWIR wavelength range, including designs with and without indium alloying of the GaSb layers. Transport modeling has shown that alloy scattering should not be a dominant factor in these superlattices so our focus is on designs with 25% indium in the gallium antimonide to achieve energy band gaps less than 50 meV with a superlattice period on the order of 68 A. Similar to the work reported on InAs/GaSb LWIR and VLWIR superlattices, our designs employ InGaSb layers less than 7 monolayers in width. While the superlattice designs are strain balanced to the GaSb substrate, care was also taken to minimize strain spikes in the interfacial regions. High resolution transmission electron microscope images were analyzed to create strain mapping profiles of the SL layers and interfaces. By focusing on a narrow set of VLWIR SL designs, the deposition parameters for the molecular beam epitaxial SL growth could be carefully optimized., Prepared in collaboration with University of Dayton, Dept. of Physics, Dayton, OH.

The original goal of this three-year project was to investigate bandgap-tunable bilayer-graphene microribbons allowing continuously tunable photodetection in the IR bands. Due to the government sequestration, the 2nd and 3rd years increments toward this project have been defunded. Here we are using our 1st year result as our final performance report. To this end we designed, fabricated, and characterized in Year One a device with suspended graphene microribbons, and found that fully suspended CVD-grown graphene devices are dominated by the photoelectric effect, which is promising towards CVD-grown graphene photodetectors approaching THz cut-off frequencies., The original document contains color images.

The collaborative efforts were aimed at designing and fabricating tunable phoXonic band gap materials by self assembly of block copolymers \201BCP\202. We also characterized both the structures and the resultant properties and made critical comparisons with simulations. The tunable properties of stimulus-responsive materials currently attract great interest in a variety of technological applications. Photonic gels are a new class of these materials that can be tuned to reflect different wavelengths of light. Controlling this reflected color via temperature-induced and solvent-quality induced changes of self-assembled photonic materials is important for their application in sensors and as displays. We found that the thermochromic behavior of a lamellar block copolymer poly(styrene-b-2-vinylpyridine) (PS-P2VP) photonic gel originated from a temperature-induced change in the pKa of the P2VP blocks. Control was obtained through the manipulation of the solution pH. The findings of this work provide the basis for understanding and controlling the properties of thermochromic block copolymers fostering their use in technologically relevant applications.

Molybdenum disulfide (MoS2) field effect transistors (FET) were fabricated on atomically smooth large-area single layers grown by chemical vapor deposition. The layer qualities and physical properties were characterized using high-resolution Raman and photoluminescence spectroscopy, scanning electron microscopy, and atomic force microscopy. Electronic performance of the FET devices was measured using field effect mobility measurements as a function of temperature. The back-gated devices had mobilities of 6.0 cm(expn 2)/V s at 300 K without a high-k dielectric overcoat and increased to 16.1 cm(expn 2)/V s with a high-k dielectric overcoat. In addition the devices show on/off ratios ranging from 10(expn 5) to 10(expn 9)., Prepared in cooperation with the Department of Electrical Engineering, Oregon State University, Corvallis and the Department of Mechanical Engineering and Materials Science, Rice University, Houston, TX. Published in Applied Physics Letters, v102 article ID 193107, 16 May 2013.

This research effort involved studies on chemically synthesized quantum dots, epitaxially created quantum dots and graphene thin films. For this report we have gathered together seven research papers on chemically synthesized quantum dots that were published in the last two years. Titles are: Analysis of Thermal Band Gap Variations of PbS Quantum Dots by Fourier Transform Transmission and Emission Spectroscopy; Supercritical Fluid Deposition of Uniform PbS Nanoparticle Films for Energy-Transfer Studies; Lead Sulfide Quantum Dot Synthesis, Deposition, and Temperature Dependence Studies of the Stokes Shift; Fourier Spectroscopy on PbS Quantum Dots; PbS Nanoparticles: Synthesis, Supercritical Fluid Deposition, and Optical Studies; Emission of Precipitation Deposited PbS Quantum Dots on Polyethylen Terephthalate; Stability Studies of Lead Sulfide Colloidal Quantum Dot Films on Glass and GaAs Substrates.