A modified critical point model dielectric function for graphene is derived here and used to analyze spectroscopic ellipsometry data obtained over a wide spectral range from 3 to 9 eV. Critical point and exciton resonance energies are extracted and discussed. Our findings indicate that epitaxial graphene on SiC to exhibits equivalent exciton behavior to that of suspended graphene. We further apply our model dielectric function to evaluate dielectric function data for highly oriented pyrolytic graphite reported in the literature. Excellent agreement is found between the critical point model developed here and the literature data even for the low energy spectral range up to 1 eV.

A methodology is described for atomistic simulations of shock-compressed materials that incorporates quantum nuclear effects on the fly. We introduce a modification of the multi-scale shock technique (MSST) that couples to a quantum thermal bath described by a colored noise Langevin thermostat. The new approach, which we call QB-MSST, is of comparable computational cost to MSST and self-consistently incorporates quantum heat capacities and Bose-Einstein harmonic vibrational distributions. As a first test, we study shock-compressed methane using the ReaxFF potential. The Hugoniot curves predicted from the new approach are found comparable with existing experimental data. We find that the self-consistent nature of the method results in the onset of chemistry at 40% lower pressure on the shock Hugoniot than observed with classical molecular dynamics. The temperature change associated with quantum heat capacity is determined to be the primary factor in this shift.

The effectiveness of lightweight aggregate (LWA) as an internal curing agent (ICA) to reduce concrete shrinkage is evaluated for repair concrete used in cultural heritage works (RCCHW) using curing periods of 30 days. Normal weight aggregate is replaced by LWA at volume replacement levels ranging from 10 to 14%. The mixtures contain Portland cement maintaining the paste content at approximately 24.1% of concrete volume. Comparisons are made with mixtures containing low-absorption granite and high-absorption limestone normal weight coarse aggregates. At the replacement levels used in this study, LWA results in a small reduction in concrete density, no appreciable effect on concrete compressive strength, and a decrease in concrete shrinkage for drying periods up to 30 days. With a curing period of 14 days, all mixtures with LWA exhibited less shrinkage than the mixtures with either low- or high-absorption normal weight aggregates.

In this paper, we demonstrate deposition methods and conditions that allow the control of the electrical properties of doped ZnTe grown by RF magnetron sputtering using both nitrogen and copper as dopants. The carrier density of the films was characterized using a van der Pauw Hall effect measurement method. We demonstrate how the concentration of nitrogen in the plasma during the growth of the film impacts the conductivity of the ZnTe films. Films with hole concentrations in excess of 1018 cm-3 and a high degree of crystallinity were successfully grown. Similarly, we demonstrate that the hole concentration in the Cu-doped ZnTe can be varied by varying the amount of copper introduced in the films. We also observe that annealing the copper doped ZnTe films increases the carrier density, whereas annealing the nitrogen doped ZnTe films causes a decrease in carrier concentration and conductivity.

Optical absorption efficiency, an important metric for sensing, radiometric and energy harvesting applications, has been studied theoretically and experimentally in porous, ordered nanostructures, including multi-walled- (MW) carbon nanotubes (CNTs) and single-walled- (SW) CNTs. We have characterized the absorption efficiencies in the 350 nm -7000 nm wavelength range of vertically aligned MWCNT arrays with high site densities synthesized directly on metallic substrates using a plasma-enhanced (PE)- chemical vapor deposition (CVD) process. Our ultra-thin absorbers exhibit a reflectance as low as ∼ 0.02 % (100 X lower than the benchmark). Such high efficiency absorbers are particularly attractive for radiometry, as well as energy harnessing applications. This work increases the portfolio of materials that can be integrated with such absorbers due to the potential for reduced synthesis temperatures arising from a plasma process. Optical modeling calculations were conducted that enabled a determination of the extinction coefficient in the films.

Papers in the Appendix were published in electronic format as Volume 1534

A major goal in the field of regenerative medicine is to improve our understanding of how biomaterial properties affect cells of the immune system. Systematic variation of defined chemical properties could help to understand which factors determine and modulate cellular responses. A series of copolymers poly[acrylonitrile-co-(N-vinylpyrrolidone)]s (P(AN-co-NVP)) served as model system, in which increasing hydrophilicity was adjusted by increasing the content related to the NVP based repeating units (nNVP) (0, 4.6, 11.8, 22.3, and 29.4 mol%). The influence of increasing nNVP contents on cellular response of human primary monocyte derived dendritic cells (DC), which play a key role in the initiation of immune responses, was investigated. It was shown using the LAL-Test as well as a macrophage-based assay, that the materials were free of endotoxins and other microbial contaminations, which could otherwise bias the readout of the DC experiments. The increasing nNVP content led to a slightly increased cell death of DC, whereas the activation status of DC was not systematically altered by the different P(AN-co-NVP)s as demonstrated by the expression of co-stimulatory molecule and cytokine secretion. Similarly, under inflammatory conditions mimicked by the addition of lipopolysaccharides (LPS), neither the expression of co-stimulatory molecules nor the release of cytokines was influenced by the different copolymers. Conclusively, our data showed that this class of copolymers does not substantially influence the viability and the activation status of DC.

During the epitaxial bottom up growth of nanowire (NW) arrays, occasional kinks in growth direction can lead to intersecting and consequently self-welded crystalline connections between NWs. In order to study these self-welded metallurgical NW junctions, a NW bridge device architecture which requires no post-growth processing was used to grow and stabilize Si NW junctions. Scanning Photocurrent Microscopy (SPCM) was used to study the optoelectronic properties of the NW junctions as well as the characteristics of the NW bridge devices. SPCM measurements show a bias dependent photocurrent (PC) response at the NW junction indicating local band bending at this location. A decay of the PC response away from the junction is also seen in the secondary NW channel ensuring an electrical connection. These junction properties may be important for ensemble NW optical devices.

We report a systematic study of polarization and magnetic field effects on the optical response of Fe3O4-silicone elastomer composite. The Fe3O4 particles were aligned in a silicone elastomer matrix with an external static magnetic field. Films of composites containing 5wt% of 20nm ≤ d ≤ 30nm Fe3O4 particles aligned in- and out-of-plane in the elastomer host were prepared. The optical spectra of the films were measured with the Perkin-Elmer Lambda 950 UV/vis/NIR spectrometer. We observed a systematic redshift in the optical response of the outof-plane composite films with increasing static magnetic field strength, which saturated near 600 Gauss. We obtained a maximum redshift of ∼46 nm at 600 Gauss. The observed redshift in the optical response of the out-of-plane composite film is attributed to the effect of the magnetic field. This facilitated the formation of the highly aligned particles that induced strong electric dipole in the aligned particles. Interestingly, there were no observable shifts with increasing magnetic field strength in the in-plane films, suggesting that the orientation (polarization) of the magnetic dipole and the induced electric dipole play a crucial role in the optical response.

Recently, Nb-Si based alloys have attracted considerable attention as potential candidate materials for ultra-high temperature applications, because of their low densities and high melting points. However, it is still very difficult to obtain materials with a good balance of high-temperature strength and room-temperature toughness. To address this issue, microstructure control is considered to be a promising method. In applying microstructure control to Nb-Si based alloys with a eutectic reaction (L → Nbss + Nb3Si) and a eutectoid reaction (Nb3Si → Nbss + Nb5Si3), the key is the control of Nb3Si phase stability. Nbss (Nb solid solution) is considered as a ductile phase. In previous reports, it was revealed that different elements had different effects on the stability of Nb3Si. In particular, Mo and W (>3 at %) destabilize the Nb3Si phase, while Ti and Ta stabilize it, and Zr acts as an accelerator for decomposition of Nb3Si. On the other hand, Cr is known to enhance the formation of the ductile Nbss phase. In the present study, we investigated the effects of adding combinations of stabilizing, destabilizing, and accelerating elements with Cr, such as Cr and W, Cr and Ta, Cr and Zr. According to SEM observation, different microstructures were obtained with different combination of additives, and the fracture toughness at room temperature of these samples were also evaluated to reveal the effects of the microstructure on the mechanical properties of Nb-Si based alloys.

Scanning Thermal Microscopy measurements with a resistive microprobe electrically heated were performed for different probe temperatures, for probe free in air and in contact with various specimens. The measured relative difference of Joule power dissipated in the probe when tip is in contact with a sample and when it is free in air is studied for different magnitude of the electrical current that heats the probe. A variation of this signal, never outlined before, is observed. A predictive modeling is used to explain these results and identify from the experimental data the global thermal conductance of the probe-sample thermal exchange for experiments performed in ambient conditions.

Relatively low efficiency is one of the main obstacles to overcome in the engineering of organic bulk heterojunction (BHJ) solar cells. Reduced graphene oxide (RGO), which has high conductivity, has been proposed to enhance the function of PCBM in the interfacial dissociation of excitons, but incorporating it into the hydrophobic photoactive polymers has proved challenging. Here we describe a novel technique for incorporating Au nanoparticles (AuNp) into the structure of the RGO. The AuNps then interact with the sulfur groups on the photoactive polymer component, while the RGO interacts via π – π stacking with the chemically similar PCBM, thereby anchoring the complex to the polymer interface. Graphene oxide was synthesized and then reduced in the presence of a gold salt. The resulting gold-functionalized RGO (AuRGO) sheets were characterized using TGA, FTIR, and TEM. The AuRGO was not soluble in chlorobenzene; however, in the presence of P3HT, the AuRGO dissolved, suggesting a reaction between the gold and the sulfur of the P3HT via a metal-thiolate bond. At 2 mg/ml, AuRGO increased the solar cell efficiency approximately 50% over the control, but higher concentrations produced large, columnar structures which blocked the electrode from having a uniform contact with the active layer.

The phase transformation of spinodal decomposition proceeds without nucleation and is affected by the alloy composition, temperature, interfaces and gradient energy, as well as the presence of lattice strain. As a consequence, a coherent spinodal can be depressed well below the chemical spinodal within the miscibility gap. Phase separation from a solid solution within the spinodal leads to the formation of characteristic composition wavelengths. In the nickel-based alloy system, a nanolaminate structure is used to initially create an artificial composition fluctuation with unique nanoscale wavelengths. The direct measurement of diffusivity at low temperatures in Cu-Ni and Cu-Ni(Fe), from the spinodal towards room temperature, requires sensitivity to the nanoscale fluctuations in composition. For this purpose, x-ray diffraction scans are used to assess changes in the short-range order of the composition fluctuation and the corresponding changes in the gradient energy, from which an evaluation of lattice distortion effects reveals a peak in strain energy for 2-3 nm composition wavelengths.

Silicon nanoparticles-based inks were investigated in respect of their suitability for photovoltaic and thermoelectric applications. Nanoparticles with a diameter ranging between 20 to 150 nm were functionalized in order to avoid oxidation as well as having a good stability in suspension. After inkjet-printing and drying, they were annealed up to 1000 °C under nitrogen atmosphere by both rapid thermal and microwave annealing. The influence of the annealing treatment on the structural, electrical, optical and thermal properties was investigated by Raman, SEM, electrical and optical measurements. SEM and Raman demonstrate evolution of the microstructure at temperature as low as 600 °C. Optical, electrical and thermal properties depend strongly on the annealing temperature and tend to exhibit a modification of physical properties above 800 °C when the smallest nanoparticles begin to melt. The annealing method has been identified to be of primary importance on the layer microstructure and its thermal behavior.

In this work we report on the characteristics of GaAs/AlGaAs heterostructures with a symmetric double two-dimensional electron gas (D-2DEG). Optical characterization was made by room temperature photoreflectance (PR) spectroscopy as well as electrical properties were determinated using the quantum Hall effect measurements at 2K. In order to study the surface effects on the conduction band profile, three samples with different GaAs cap layer thickness (25, 60 and 80 nm) were grown by the molecular beam epitaxy. Photoreflectance spectra at room temperature show the wide-period Franz-Keldysh oscillations between 1.42 and 1.70 eV originated by the surface electric field. The analysis of these oscillations shows that the surface electric field varies from 503 to 120 kV/cm whereas the thickness of the cap layer increases that was produced by the reduction of the depletion zone near the surface. Using QHE measurements we found that electron density increases if the surface electric field decreases.

It is well established that controlled high-temperature annealing of hydrogen silsesquioxane leads to the formation of small spherical silicon nanocrystals (∼3 nm). The present study outlines an investigation into the influence of annealing time and temperature. After prolonged annealing, crystal surfaces thermodynamically self-optimize to form a variety of faceted structures (e.g., cubic, truncated trigonal and hexagonal structures).

Organic solar cells consisting of Phthalocyaninatocopper (PcCu) as donor and the Buckminsterfullerene C60 as acceptor molecule were prepared by physical vapor deposition as planar or bulk heterojunctions. The devices were studied by IV-characterization as well as intensity-modulated photovoltage spectroscopy to determine the average lifetime of charge carriers formed subsequent to light absorption. An increasing charge carrier lifetime was determined for an increasing PcCu-content in the films. Back transfer of electrons at the undesired contact of C60 with PEDOT:PSS as well as recombination following hole trapping in interface states in the contact of PcCu with C60 or in isolated domains of PcCu are discussed as possible origins.

A sophomore level Materials Engineering course entitled, “Materials, Ethics, & Society” at Cal Poly analyzes the interactions between technology and society, and emphasizes the communication of societal and ethical impacts of technology to diverse audiences. Students study materials in a historical context, not only to highlight specific materials science concepts, but also to explore societal-technology connections. Starting from the Stone Age, advances in civilizations have come about through discoveries and new uses of materials. World geography, history, and culture become intertwined with the casting of metals, alloy development, ceramics, and phase diagrams. Students highlight the social relevancy of materials throughout history, and seek parallel themes in today’s world. While learning about the development of the atomic bomb, students also examine the ethical dilemmas of the scientists and the current NSPE Code of Ethics for Engineers. In addition, the role of the engineering profession is examined with the NAE Engineering Grand Challenges and current news items. Students investigate how materials engineering can help society through examples of appropriate technology solutions, such as designing porosity in ceramics for water filters and food storage pots. Students develop their analytical and communication skills by discussing C.P. Snow’s “two cultures” and debating a rationale for scientific literacy. Students are also trained in informal science learning in preparation for NanoDays with young and diverse audiences. At the same time, they learn about nano-scale science and technology principles, and the associated societal and ethical implications. The course culminates with student-created videos for the general public that highlight a material or technology of importance to society and integrates the material science with ethical, environmental, and societal dimensions.