The high-cost of fabrication of nanohole arrays for extraordinary optical transmission, surface-plasmon-resonance-based
sensors, inhibits their widespread commercial adoption. Production typically involves the application of
small-area patterning techniques, such as focused-ion-beam milling, and electron-beam lithography onto high-cost
gold-coated substrates. Moving to lower-cost manufacturing is a critical step for applications such as the detection of
environmental oil-leaks, or water quality assurance. In these applications, the sensitivity requirements are relatively
low, and a bio-compatible inert surface, such as gold, is unnecessary. We report on the optical response of
aluminum-coated nano-bucket arrays fabricated on flexible polyethylene terephthalate substrates. The arrays are
fabricated using an economical roll-to-roll UV-casting process from large sheets of nickel templates generated from
master quartz stamps. The nano-featured surface is subsequently coated with 50 nm of thermally-evaporated
aluminum. The roll-to-roll production process has a 97% yield over a 600 m roll producing nano-buckets with 240
nm diameters, 300 nm deep, with a 70° taper. When exposed to a series of refractive index standards (glucose
solutions), changes in the locations of the resonance transmission peaks result in optical sensitivities as high as 390
± 20 nm/RIU. The peak transmission is approximately 5% of illumination, well within the sensitivity requirements
of most common low-cost detectors.
An application friendly technique to increase the surface area of the ionomer membrane such as Aquivion™ has been
developed. By utilizing existing micro-fabrication technologies, square pillars were fabricated onto glass and silicon
substrates. In combination with a low cost heat press, the glass and silicon stamps were used to successfully hot emboss
micro-features onto the ionomer membrane. Consequently, the surface area of the Aquivion™ membrane was drastically
increased enabling potential improvement of sensing and energy storage technologies. Preliminary results show
successful fabrication of devices with systematic higher surface area and an improved capacitance.
We have investigated the molecular beam epitaxial growth and characterization of InN nanowires. Detailed optical and electrical transport studies confirm that nondoped InN nanowires can exhibit extremely low (< 1015 cm-3) residual electron density. Furthermore, the near-surface Femi-level was measured to be 0.4 to 0.5 eV above the valence band maximum (VBM), suggesting the absence of Fermi-level pinning and surface electron accumulation. These features are fundamentally different from those of n-type degenerate InN nanowires or InN epilayers. The absence of surface electron accumulation was also observed in Mg-doped InN nanowires, where p-type conduction was directly measured via Mg-doped InN nanowire field-effect transistors. Furthermore, the near-surface Fermi-level can be tuned from 0.1 eV to 1 eV above the VBM, i.e., from p-type degenerate to n-type degenerate through controlled Mg and Si dopant incorporations, a first demonstration for any semiconducting nanowire structures.
Surface plasmon resonance sensors are a popular technology for the optical detection of surface adsorption, for
applications ranging from drug-development to pathogen detection. Here, we will discuss the integration of nano-hole
arrays to provide high-sensitivity detection, with a lower detection limit, speed and cost. Calculations will be presented
that suggest that in-hole detection is more sensitive than detecting binding from the surface around the nano-holes. In-hole
detection also has the benefit of increased speed due to rapid diffusive transport, yet it provides the challenge of
microfluidic/nanofluidic integration. We will outline our recent efforts to produce nano-hole arrays with through-hole
detection.
Amorphous silicon photodiodes are increasingly being used as fundamental components in digital diagnostic medical imaging system including large area chest radiography, mammography and real time fluoroscopy. The intrinsic a-Si:H material (i-a-Si:H), commonly deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD), is well known to suffer from both light and bias stress induced instabilities over time that can result in an increase in dark current and a decrease in photoconductivity. In contrast, research in Hot-Wire Chemical Vapor Deposition (HWCVD) indicates that a-Si:H films grown by HWCVD can have superior physical and electronic properties to those grown by PECVD.
In this research, we report on the material properties and stability of i-a-Si:H material by comparing the photoconductivity degradation of the HWCVD and PECVD films over time. Then, we discuss the p-i-n diode fabrication process and examine the leakage and photo-current degradation in the HWCVD and PECVD photodiode structures over time via bias and time stress measurements. Also, we investigate the quantum efficiency degradation over time in a-Si:H p-i-n detectors grown by PECVD.
The transmission of normally incident light through arrays of subwavelength holes (nanoholes) in gold thin films is enhanced at the wavelengths that satisfy the surface plasmon (SP) resonance condition. The enhanced transmission is accompanied by strong field localization and has potential for applications in several fields, ranging from quantum information processing to nanolithography. In this work, arrays of nanoholes were used as chemical sensors to monitor the binding of organic and biological molecules to metallic surfaces. In a first approach, the interaction between the adsorbate with the metallic nanostructure modified the SP resonance conditions, leading to a shift in the wavelength of maximum transmission. The sensitivity of this substrate was found to be 400 nm RIU-1 (refractive index units), which is comparable to other grating-based surface plasmon resonance devices. The array of nanoholes was also integrated into a microfluidic system and the characteristics of the solution flow and detection systems were evaluated. The second approach to sensor development using this class of substrate involved the observation of enhanced spectroscopic signal from species located within the SP field. Surface-enhanced Raman scattering and surface- enhanced fluorescence spectroscopy were observed from adsorbed dyes. The enhanced spectroscopic signal was dependent on the fabrication parameters of the array. The largest enhancement was observed when the periodicity of the nanoholes matched the energy of the laser excitation. Among the main advantages of this substrate for chemical sensing is the collinear optical geometry. This simplifies the alignment with respect to the traditional reflection arrangement used in SPR sensing.
Bimetallic films have been found to be promising direct write binary and grayscale photomask materials, as they turn transparent after laser exposure. Current structural analysis shows that the laser exposure is an oxidation process. The amount of the oxidized metal created during the laser writing process is related to the laser power, which in turn, determines the gray level (OD) of the exposed film. New exposure conditions have greatly increased the transparency of exposed films (down to 0.18 OD at 365 nm). Furthermore, this extended to deeper UV (300 nm). As the transparency of exposed area changes with the laser exposure power, grayscale photomasks can be created with the bimetallic films, and 3D structures can be produced in the substrate. Interference lithography has been used to investigate the bimetallic films resolution limit, which can generate much finer structures. Lines of 100-180 nm wide were successfully created on silicon and silicon dioxide. Aluminum thin films were found to turn transparent (0.28 OD) after laser exposure with high power, indicating that Al can also be a potential direct-write photomask material.
KEYWORDS: Absorbance, Spectral resolution, Spectrometers, Statistical analysis, In vivo imaging, Fiber optics, Control systems, Absorption, Signal attenuation, Biological research
We report the development of a microspectrophotometer system for use on micro samples of mitochondrial respiratory pigments. A novel optical fiber set-up uses visible spectrophotometry to monitor the reduction of mitochondrial electron carriers. Data is presented for the reduction of cytochrome-c and for the effect of temperature on the levels of complex II/III activity from the mitochondria of rat liver. This in-vivo simulation of the reduction of cytochrome-c can be observed using a fiber optic probe which requires less than twenty (mu) l of sample for analysis. The key features of the system are: front end adaptability, high sensitivity and fast multispectral acquisition which are essential for the biological reactions which are observed.
The in situ measurement of surface roughening is important to the characterization or control of a variety of dynamic processes, including polishing, etching, film growth, strain relaxation, phase transitions and interdiffusion. Solid and liquid surface roughness statistics are being studied using optical, x-ray, electron and atom diffraction. While average roughness can be measured by specular scattering of x-rays, for example, only non-specular, diffuse diffraction can detect lateral correlations in atomic scale roughness and quantify the surface height-height correlation function or power spectral density function in real time. In this paper, the capabilities and application of diffuse diffraction of photons for in situ measurement of roughening statistics are reviewed.
The continuing demand for non-invasive tools for use in clinical diagnosis has created the need for flexible and innovative optical systems which satisfy current requirements. We report the development of a microspectrophotometer system for use on mitochondrial respiratory pigments. This novel optical fiber set-up uses visible spectrophotometry to monitor the reduction of mitochondrial electron carriers. Preliminary data is presented for the reduction of cytochrome-c by two methods. In the first, cytochrome-c was reduced in isolation using sodium dithionite. The second was an in-vivo simulation of the reduction of cytochrome-c using the mitochondrial extract from rat liver. The key features of the system are; front end adaptability, high sensitivity and fast scanning capabilities which are essential for the rapid biological reactions which are observed.
The surface morphology and degree of strain relaxation in compositionally graded InxGa1-xAs epilayers grown on GaAs substrates can be controlled by the growth conditions. InxGa1-xAs epilayers grown as compositionally step-graded buffers on (001) GaAs by solid-source molecular beam epitaxy at a constant substrate temperature and GaAs growth rate (>= 500 degree(s)C, 0.9 micrometers /hr) undergo a transition from two to three dimensional morphology as the indium concentration is increased from x equals 0.3 to x equals 0.4. The strain relaxation is asymmetric in the <110> in- plane directions and severe roughening with the formation of cusps occurs preferentially in the (110) direction. When the substrate growth temperature is lowered successively for each InGaAs composition and the Ga beam flux lowered for the growth of the x equals 0.4 layer, the surface morphology becomes two-dimensional and the strain relaxation symmetric and greater than 90% in the two <110> directions.
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