C. Parks Cheney

University of Tennessee, Knoxville, Tennessee, United States

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Publications (12)33.41 Total impact

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    ABSTRACT: Recent studies indicated that noncompensated cation-anion codoping of wide-band-gap oxide semiconductors such as anatase TiO_{2} significantly reduces the optical band gap and thus strongly enhances the absorption of visible light [W. Zhu et al., Phys. Rev. Lett. 103, 226401 (2009)PRLTAO0031-900710.1103/PhysRevLett.103.226401]. We used soft x-ray spectroscopy to fully determine the location and nature of the impurity levels responsible for the extraordinarily large (∼1 eV) band gap reduction of noncompensated codoped rutile TiO_{2}. It is shown that Cr/N codoping strongly enhances the substitutional N content, compared to single element doping. The band gap reduction is due to the formation of Cr 3d^{3} levels in the lower half of the gap while the conduction band minimum is comprised of localized Cr 3d and delocalized N 2p states. Band gap reduction and carrier delocalization are critical elements for efficient light-to-current conversion in oxide semiconductors. These findings thus raise the prospect of using codoped oxide semiconductors with specifically engineered electronic properties in a variety of photovoltaic and photocatalytic applications.
    Physical Review Letters 01/2014; 112(3):036404. · 7.73 Impact Factor
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    ABSTRACT: The electronic structure of the Rh-based CuRh0.9Mg0.1O2 oxide thermoelectric compound has been studied with a multitechnique approach consisting of photoemission, x-ray absorption, and x-ray emission spectroscopies. The data indicate that the region of the valence band in the proximity of the Fermi level is dominated by Rh-derived states. These findings outline the importance of the electronic structure of the Rh ions for the large thermoelectric power in CuRh0.9Mg0.1O2 at high temperature.
    Physical review. B, Condensed matter 03/2013; 87(12). · 3.77 Impact Factor
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    ABSTRACT: An aptamer-coated, piezoresistive, microcantilever-based biosensor has been fabricated and tested as presented here to measure the relative concentration of freebase cocaine in solution. Aptamers were used as the cocaine receptor due to their known high selectivity and affinity for the target molecules in buffered solutions. A Wheatstone bridge composed of four microcantilevers was utilized in converting the biological signal to an electrical signal. The approach here provides a prototype that will lead to a compact and low cost device that will be applicable for in-vivo applications. The performance of this sensor is demonstrated in an in-vitro experiment conducted by flowing a cocaine solution in distilled water over the microcantilever arrays. The response of the device had a limit of detection of 1 ng/ml, which is comparable to the conventional method of cocaine detection in the body performed using gas/liquid chromatography and mass spectrophotometry.
    Sensor Letters 03/2012; 10(3-4):850-855. · 0.52 Impact Factor
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    ABSTRACT: We use core-valence-valence Auger spectra to probe the Coulomb repulsion between holes in the valence band of Fe pnictide superconductors. By comparing the two-hole final-state spectra to density functional theory calculations of the single-particle density of states, we extract a measure of the electron correlations that exist in these systems. Our results show that the Coulomb repulsion is highly screened and can definitively be considered as weak. We also find that there are differences between the 1111 and 122 families and even a small variation as a function of the doping x in Ba(Fe{sub 1-x}Coâ)âAsâ. We discuss how the values of the hole-hole Coulomb repulsion obtained from our study relate to the onsite Coulomb parameter U used in model and first-principles calculations based on dynamical mean field theory and establish an upper bound for its effective value. Our results impose stringent constraints on model-based phase diagrams that vary with the quantity U or U/W by restricting the latter to a rather small range of values.
    Physical Review B 01/2012; 85(23). · 3.66 Impact Factor
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    ABSTRACT: A major obstacle toward employing TiO2 as an efficient photoactive material is related to its large optical band gap, strongly limiting visible light absorption. Substitutional doping with both donors and acceptors (co-doping) potentially leads to a significant band gap reduction, but the effectiveness of the co-doping approach remains limited by the low solubility of dopants inside TiO2. Here we show that nanostructured Cr and N co-doped TiO2 thin films can be obtained by Supersonic Cluster Beam Deposition (SCBD) with a high concentration of dopants and a strongly reduced band gap. Complementary spectroscopic investigations show that doping effectively occurs into substitutional lattice sites, inducing dopant levels in the gap that are remarkably delocalized. The high surface-to-volume ratio, typical of SCBD nanostructured films, likely facilitates the dopant incorporation. The present results indicate that SCBD films are highly promising photoactive nanophase materials.
    The Journal of Physical Chemistry C 01/2012; 116(116):311. · 4.84 Impact Factor
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    ABSTRACT: Magnesium-doped rhodium oxides with formula unit CuRh1-xMgxO2 and delafossite-type structure exhibit a high thermoelectric figure of merit at elevated temperatures. The electronic structure of CuRh1-xMgxO2 has been studied with x-ray emission spectroscopy (XES), x-ray absorption spectroscopy (XAS), and photoemission spectroscopy (PES). The data reveal that the states at the Fermi level are Rh-derived. Measurements carried out by changing the orientation of the linear photon polarization further indicate that the Rh states have a more localized character along the c-axis, consistent with the layered crystal structure. Given the similarity of the electronic configurations of Co and Rh, these data provide solid experimental evidence that the orbital degrees of freedom of the d^6 ionic configuration of the states rooted in transport are key for explaining the thermoelectric properties of oxide materials.
    03/2011;
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    ABSTRACT: Here we present a study based on ARPES and X-ray absorption spectroscopies in order to unveil the electronic structure evolution upon Co-doped in BaFe2As2 high Tc superconductors, for the doping levels x=(0,6,8,12,22)%. This study focuses on two points: i) the effective role of Co at different doping levels; ii) the shift upon doping of the band structure. X-ray absorption experiments carried out at the Co L23 edge highlight the chemical state of cobalt at the various doping levels, thus unveiling its role as charge donor. The orbital selectivity of polarization dependent ARPES is used to show the filling evolution of each band. The spectra have been collected in different geometries along the gammaX and gammaM high symmetry crystallographic directions and at two different photon energies. The experimental results show a general rearrangement of the charge within the various orbitals upon doping, with a non rigid band shift.
    03/2011;
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    ABSTRACT: The orbital symmetries of electron-doped iron-arsenide superconductors Ba(Fe1−xCox)2As2 have been measured with x-ray absorption spectroscopy. The data reveal signatures of Fe d electron itinerancy, weak electronic correlations, and a high degree of Fe-As hybridization related to the bonding topology of the Fe dxz+yz states, which are found to contribute substantially at the Fermi level. The energies and detailed orbital character of Fe and As derived unoccupied s and d states are found to be in remarkably good agreement with the predictions of standard density-functional theory.
    Physical Review B 02/2010; 81(10). · 3.66 Impact Factor
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    ABSTRACT: Traditional alcohol studies measure blood alcohol concentration to elucidate the biomedical factors that contribute to alcohol abuse and alcoholism. These measurements require large and expensive equipment, are labor intensive, and are disruptive to the subject. To alleviate these problems, we have developed an implantable, wireless biosensor that is capable of measuring alcohol levels for up to six weeks. Ethanol levels were measured in vivo in the interstitial fluid of a Wistar rat after administering 1 g/kg and 2 g/kg ethanol by intraperitoneal (IP) injection. The data were transmitted wirelessly using a biosensor selective for alcohol detection. A low-power piezoresistive microcantilever sensor array was used with a polymer coating suitable for measuring ethanol concentrations at 100% humidity over several hours. A hydrophobic, vapor permeable nanopore membrane was used to screen liquid and ions while allowing vapor to pass to the sensor from the subcutaneous interstitial fluid.
    Sensors and Actuators B Chemical 04/2009; 138(1):264-269. · 3.84 Impact Factor
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    ABSTRACT: Sensitive and inexpensive sensors play a significant role in the analysis of drugs and drug metabolites. Specifically, reliable in vivo detection of cocaine and cocaine metabolites serves as a useful tool in research of the body's reaction to the drug and in the treatment of the drug addiction. We present here a promising cocaine biosensor to be used in the human body. The sensor's active element consists of piezoresistive microcantilevers coated with an oligonucleotide-based aptamer as the cocaine binder. In vitro cocaine detection was carried out by flowing a cocaine solution over the microcantilevers. Advantages of this device are its low power consumption, its high sensitivity, and its potential for miniaturization into an implantable capsule. The limit of detection for cocaine in distilled water was found to be 1 ng/ml.
    03/2008;
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    ABSTRACT: Real-time, in vivo measurements were taken in the interstitial fluid of a Wistar rat after administering 2.5 g/kg ethanol by intraperitoneal injection. A low-power piezoresistive microcantilever sensor array was used with polymer coatings suitable for measuring ethanol concentrations at 100% humidity over several hours. A hydrophobic, vapor permeable nanopore membrane was used to screen liquid and ions while allowing vapor to pass to the sensor that was implanted into the saline environment presented by the interstitial fluid. The real-time measurements followed the time scale of previous blood ethanol concentration data.
    Applied Physics Letters 01/2007; 90(1):013901-013901-3. · 3.79 Impact Factor
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    ABSTRACT: We report detection of ethanol in a phosphate buffered saline (PBS) solution using a low-power piezoresistive microcantilever-based system that has the potential to be used in the human body. PBS was used to simulate interstitial fluid and a permeable hydrophobic membrane was employed to transport ethanol vapor to the sensor while blocking the liquid and ions of the PBS. Commercial gold-coated cantilevers were functionalized with polymers for optimal ethanol response. Advantages of this device are its low-power consumption, its high sensitivity, and its capabilities for miniaturization into an implantable capsule. The limit of detection for ethanol in PBS was found to be less than 100 ppm or 8 mg/dl.
    Review of Scientific Instruments 09/2006; 77(9):095101-095101-5. · 1.60 Impact Factor