The optoelectronic properties of single crystal CdTe thin films were investigated by photolumines... more The optoelectronic properties of single crystal CdTe thin films were investigated by photoluminescence spectroscopy, photoreflectance spectroscopy and variable angle spectroscopic ellipsometry. The room temperature bandgap was measured to be 1.51 eV and was consistent between spectroscopic measurements and previously reported values. Breadth of bandgap emission was consistent with high quality material. Low temperature photoluminescence spectra indicated a dominant emission consistent with bound exci-tons. Emissions corresponding to self-compensation defects, doping and contaminants were not found. Variable angle spectroscopic ellipsometry measurements over the near-UV to infrared range demonstrated sharp resonance peaks. All spectroscopic measurements indicate high quality thin film material of comparable or better quality than bulk CdTe.
Photovoltaic devices made from M-phthalocyanine and fullerene have been fabricated and characteri... more Photovoltaic devices made from M-phthalocyanine and fullerene have been fabricated and characterized by current–voltage response, lateral time-of-flight photoconductivity, UV-visible absorption and scanning electron microscopy. The effect of varying the central moi-ety on the photovoltaic performance is examined, and demonstrates that the monovalent and divalent phthalocyanines tend to yield higher efficiencies in blended structures, whereas the trivalent and tetravalent phthalocyanines tend to yield higher efficiencies in a bilayer structure. The apparent reason for the disparity is the measured decrease in the hole transport efficiency in trivalent and tetravalent phthalocyanine upon blending with C 60. Furthermore, the open circuit voltages of M-phthalocyanine/fullerene solar cells are grouped together according to the valency of the central moiety.
A new class of gold nanostructures has been fabricated on the (100), (111), and (110) surfaces of... more A new class of gold nanostructures has been fabricated on the (100), (111), and (110) surfaces of lattice-matched MgAl 2 O 4 substrates. The nanostructures were fabricated through a synthesis route where a thin gold film dewets, liquefies, and then slowly self-assembles. The supported nanostructures are intricately shaped, crystalline, and epitaxially aligned. Simulations based on a continuum elastic theory indicate that the self-assembly is driven by strained epitaxy and minimization of the surface free energy.
In this paper we investigate the effect of Ar concentration and shadow mask material on the etch ... more In this paper we investigate the effect of Ar concentration and shadow mask material on the etch rate, surface roughness, and micromask pit density and depth for reactive ion etching (RIE) of x-cut alpha quartz. The ratio of SF 6 to Ar is varied at constant power and pressure to find an optimum Ar concentration for thinning x-cut quartz while maintaining good surface quality. The recipe is used with an STS320 RIE system to produce smooth, through-hole-free, insulating quartz membranes of 7 μm thickness in selective locations on 100 μm thick x-cut quartz wafers. We demonstrate that surface quality is much improved by replacing at least 75% of the SF 6 by volume with Ar, and that for non-inductively coupled plasma RIE the standard mask material nickel is detrimental to surface quality for long etch times.
Previously we have described the deposition of vertically aligned wurtzite CdTe nanowires derived... more Previously we have described the deposition of vertically aligned wurtzite CdTe nanowires derived from an unusual catalytically driven growth mode. This growth mode could only proceed when the surface of the substrate was corrupted with an alcohol layer, although the role of the corruption was not fully understood. Here, we present a study detailing the remarkable role that this substrate surface alteration plays in the development of CdTe nanowires; it dramatically improves the size uniformity and largely eliminates lateral growth. These effects are demonstrated to arise from the altered surface's ability to limit Ostwald ripening of the catalytic seed material and by providing a surface unable to promote the epitaxial relationship needed to sustain a lateral growth mode. The axial growth of the CdTe nanowires is found to be exclusively driven through the direct impingement of adatoms onto the catalytic seeds leading to a self-limiting wire height associated with the sublimation of material from the sidewall facets. The work presented furthers the development of the mechanisms needed to promote high quality substrate-based vertically aligned CdTe nanowires. With our present understanding of the growth mechanism being a combination of selective area epitaxy and a catalytically driven vapour–liquid–solid growth mode, these results also raise the intriguing possibility of employing this growth mode in other material systems in an effort to produce superior nanowires.
ABSTRACT The optoelectronic properties of single crystal CdTe thin films were investigated by pho... more ABSTRACT The optoelectronic properties of single crystal CdTe thin films were investigated by photoluminescence spectroscopy, photoreflectance spectroscopy and variable angle spectroscopic ellipsometry. The room temperature bandgap was measured to be 1.51 eV and was consistent between spectroscopic measurements and previously reported values. Breadth of bandgap emission was consistent with high quality material. Low temperature photoluminescence spectra indicated a dominant emission consistent with bound excitons. Emissions corresponding to self-compensation defects, doping and contaminants were not found. Variable angle spectroscopic ellipsometry measurements over the near-UV to infrared range demonstrated sharp resonance peaks. All spectroscopic measurements indicate high quality thin film material of comparable or better quality than bulk CdTe.
The striatum and thalamus are subcortical structures intimately involved in addiction, and the mo... more The striatum and thalamus are subcortical structures intimately involved in addiction, and the morphology and microstructure of these has been studied in murine models of cocaine addiction. However, human studies using non- invasive MRI has shown inconsistencies in morphology using volumetric analysis. In our study, we used MRI-based volumetric and novel shape analysis, as well as a novel fast diffusion kurtosis imaging sequence to study the morphology and microstructure of striatum and thalamus in crack cocaine addiction (CA) compared to matched healthy controls (HC). We did not find significant differences in volume and mean kurtosis (MKT) between groups. However, we found significant contraction of nucleus accumbens in CA compared to HC. We also found significant age related changes in volume and MKT of CA in striatum and thalamus that are contrary to those seen in normal brain aging. Our findings suggest that the use of finer methods and sequences is needed to characterize morphological and microstructural changes in cocaine addiction, and that brain changes in cocaine addiction are related to age.
ABSTRACT The etching of native oxides on compound semiconductors is an important step in the prod... more ABSTRACT The etching of native oxides on compound semiconductors is an important step in the production of electronic and optoelectronic devices. Although it is known that the native oxide on CdTe can be etched through a rinsing in purified water, a deeper investigation into this process has not been done. Here we present results on both surface morphology changes and reaction rates for purified water etching of the native oxide on heteroepitaxial CdTe thin films, as studied by atomic force microscopy and x-ray photoelectron spectroscopy. Together with a characterization of both the structure and stoichiometry of the initial native oxide, we show how an altering of the pH-level of the etchant will affect the etching rates. If oxide regrowth was allowed, constant etching rates could be observed for all etchants, while a logarithmic decrease in oxide thickness was observed if regrowth was inhibited. Both acidic and basic etchants proved to be more efficient than neutral water.
ABSTRACT Photovoltaic devices made from M-phthalocyanine and fullerene have been fabricated and c... more ABSTRACT Photovoltaic devices made from M-phthalocyanine and fullerene have been fabricated and characterized by current–voltage response, lateral time-of-flight photoconductivity, UV-visible absorption and scanning electron microscopy. The effect of varying the central moiety on the photovoltaic performance is examined, and demonstrates that the monovalent and divalent phthalocyanines tend to yield higher efficiencies in blended structures, whereas the trivalent and tetravalent phthalocyanines tend to yield higher efficiencies in a bilayer structure. The apparent reason for the disparity is the measured decrease in the hole transport efficiency in trivalent and tetravalent phthalocyanine upon blending with C60. Furthermore, the open circuit voltages of M-phthalocyanine/fullerene solar cells are grouped together according to the valency of the central moiety.
A new class of gold nanostructures has been fabricated on the (100), (111), and (110) surfaces of... more A new class of gold nanostructures has been fabricated on the (100), (111), and (110) surfaces of lattice-matched MgAl(2)O(4) substrates. The nanostructures were fabricated through a synthesis route where a thin gold film dewets, liquefies, and then slowly self-assembles. The supported nanostructures are intricately shaped, crystalline, and epitaxially aligned. Simulations based on a continuum elastic theory indicate that the self-assembly is driven by strained epitaxy and minimization of the surface free energy.
The optoelectronic properties of single crystal CdTe thin films were investigated by photolumines... more The optoelectronic properties of single crystal CdTe thin films were investigated by photoluminescence spectroscopy, photoreflectance spectroscopy and variable angle spectroscopic ellipsometry. The room temperature bandgap was measured to be 1.51 eV and was consistent between spectroscopic measurements and previously reported values. Breadth of bandgap emission was consistent with high quality material. Low temperature photoluminescence spectra indicated a dominant emission consistent with bound exci-tons. Emissions corresponding to self-compensation defects, doping and contaminants were not found. Variable angle spectroscopic ellipsometry measurements over the near-UV to infrared range demonstrated sharp resonance peaks. All spectroscopic measurements indicate high quality thin film material of comparable or better quality than bulk CdTe.
Photovoltaic devices made from M-phthalocyanine and fullerene have been fabricated and characteri... more Photovoltaic devices made from M-phthalocyanine and fullerene have been fabricated and characterized by current–voltage response, lateral time-of-flight photoconductivity, UV-visible absorption and scanning electron microscopy. The effect of varying the central moi-ety on the photovoltaic performance is examined, and demonstrates that the monovalent and divalent phthalocyanines tend to yield higher efficiencies in blended structures, whereas the trivalent and tetravalent phthalocyanines tend to yield higher efficiencies in a bilayer structure. The apparent reason for the disparity is the measured decrease in the hole transport efficiency in trivalent and tetravalent phthalocyanine upon blending with C 60. Furthermore, the open circuit voltages of M-phthalocyanine/fullerene solar cells are grouped together according to the valency of the central moiety.
A new class of gold nanostructures has been fabricated on the (100), (111), and (110) surfaces of... more A new class of gold nanostructures has been fabricated on the (100), (111), and (110) surfaces of lattice-matched MgAl 2 O 4 substrates. The nanostructures were fabricated through a synthesis route where a thin gold film dewets, liquefies, and then slowly self-assembles. The supported nanostructures are intricately shaped, crystalline, and epitaxially aligned. Simulations based on a continuum elastic theory indicate that the self-assembly is driven by strained epitaxy and minimization of the surface free energy.
In this paper we investigate the effect of Ar concentration and shadow mask material on the etch ... more In this paper we investigate the effect of Ar concentration and shadow mask material on the etch rate, surface roughness, and micromask pit density and depth for reactive ion etching (RIE) of x-cut alpha quartz. The ratio of SF 6 to Ar is varied at constant power and pressure to find an optimum Ar concentration for thinning x-cut quartz while maintaining good surface quality. The recipe is used with an STS320 RIE system to produce smooth, through-hole-free, insulating quartz membranes of 7 μm thickness in selective locations on 100 μm thick x-cut quartz wafers. We demonstrate that surface quality is much improved by replacing at least 75% of the SF 6 by volume with Ar, and that for non-inductively coupled plasma RIE the standard mask material nickel is detrimental to surface quality for long etch times.
Previously we have described the deposition of vertically aligned wurtzite CdTe nanowires derived... more Previously we have described the deposition of vertically aligned wurtzite CdTe nanowires derived from an unusual catalytically driven growth mode. This growth mode could only proceed when the surface of the substrate was corrupted with an alcohol layer, although the role of the corruption was not fully understood. Here, we present a study detailing the remarkable role that this substrate surface alteration plays in the development of CdTe nanowires; it dramatically improves the size uniformity and largely eliminates lateral growth. These effects are demonstrated to arise from the altered surface's ability to limit Ostwald ripening of the catalytic seed material and by providing a surface unable to promote the epitaxial relationship needed to sustain a lateral growth mode. The axial growth of the CdTe nanowires is found to be exclusively driven through the direct impingement of adatoms onto the catalytic seeds leading to a self-limiting wire height associated with the sublimation of material from the sidewall facets. The work presented furthers the development of the mechanisms needed to promote high quality substrate-based vertically aligned CdTe nanowires. With our present understanding of the growth mechanism being a combination of selective area epitaxy and a catalytically driven vapour–liquid–solid growth mode, these results also raise the intriguing possibility of employing this growth mode in other material systems in an effort to produce superior nanowires.
ABSTRACT The optoelectronic properties of single crystal CdTe thin films were investigated by pho... more ABSTRACT The optoelectronic properties of single crystal CdTe thin films were investigated by photoluminescence spectroscopy, photoreflectance spectroscopy and variable angle spectroscopic ellipsometry. The room temperature bandgap was measured to be 1.51 eV and was consistent between spectroscopic measurements and previously reported values. Breadth of bandgap emission was consistent with high quality material. Low temperature photoluminescence spectra indicated a dominant emission consistent with bound excitons. Emissions corresponding to self-compensation defects, doping and contaminants were not found. Variable angle spectroscopic ellipsometry measurements over the near-UV to infrared range demonstrated sharp resonance peaks. All spectroscopic measurements indicate high quality thin film material of comparable or better quality than bulk CdTe.
The striatum and thalamus are subcortical structures intimately involved in addiction, and the mo... more The striatum and thalamus are subcortical structures intimately involved in addiction, and the morphology and microstructure of these has been studied in murine models of cocaine addiction. However, human studies using non- invasive MRI has shown inconsistencies in morphology using volumetric analysis. In our study, we used MRI-based volumetric and novel shape analysis, as well as a novel fast diffusion kurtosis imaging sequence to study the morphology and microstructure of striatum and thalamus in crack cocaine addiction (CA) compared to matched healthy controls (HC). We did not find significant differences in volume and mean kurtosis (MKT) between groups. However, we found significant contraction of nucleus accumbens in CA compared to HC. We also found significant age related changes in volume and MKT of CA in striatum and thalamus that are contrary to those seen in normal brain aging. Our findings suggest that the use of finer methods and sequences is needed to characterize morphological and microstructural changes in cocaine addiction, and that brain changes in cocaine addiction are related to age.
ABSTRACT The etching of native oxides on compound semiconductors is an important step in the prod... more ABSTRACT The etching of native oxides on compound semiconductors is an important step in the production of electronic and optoelectronic devices. Although it is known that the native oxide on CdTe can be etched through a rinsing in purified water, a deeper investigation into this process has not been done. Here we present results on both surface morphology changes and reaction rates for purified water etching of the native oxide on heteroepitaxial CdTe thin films, as studied by atomic force microscopy and x-ray photoelectron spectroscopy. Together with a characterization of both the structure and stoichiometry of the initial native oxide, we show how an altering of the pH-level of the etchant will affect the etching rates. If oxide regrowth was allowed, constant etching rates could be observed for all etchants, while a logarithmic decrease in oxide thickness was observed if regrowth was inhibited. Both acidic and basic etchants proved to be more efficient than neutral water.
ABSTRACT Photovoltaic devices made from M-phthalocyanine and fullerene have been fabricated and c... more ABSTRACT Photovoltaic devices made from M-phthalocyanine and fullerene have been fabricated and characterized by current–voltage response, lateral time-of-flight photoconductivity, UV-visible absorption and scanning electron microscopy. The effect of varying the central moiety on the photovoltaic performance is examined, and demonstrates that the monovalent and divalent phthalocyanines tend to yield higher efficiencies in blended structures, whereas the trivalent and tetravalent phthalocyanines tend to yield higher efficiencies in a bilayer structure. The apparent reason for the disparity is the measured decrease in the hole transport efficiency in trivalent and tetravalent phthalocyanine upon blending with C60. Furthermore, the open circuit voltages of M-phthalocyanine/fullerene solar cells are grouped together according to the valency of the central moiety.
A new class of gold nanostructures has been fabricated on the (100), (111), and (110) surfaces of... more A new class of gold nanostructures has been fabricated on the (100), (111), and (110) surfaces of lattice-matched MgAl(2)O(4) substrates. The nanostructures were fabricated through a synthesis route where a thin gold film dewets, liquefies, and then slowly self-assembles. The supported nanostructures are intricately shaped, crystalline, and epitaxially aligned. Simulations based on a continuum elastic theory indicate that the self-assembly is driven by strained epitaxy and minimization of the surface free energy.
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