@article{sakon_ribeill_garguilo_perkins_weninger_nemanich_2009, title={Fluorescence quenching effects of nanocrystalline diamond surfaces}, volume={18}, ISSN={["1879-0062"]}, DOI={10.1016/j.diamond.2008.10.028}, abstractNote={Undoped diamond has conductive properties when terminated by hydrogen and exposed to air or aqueous solution. Here, it is shown that nanocrystalline diamond, fabricated with hydrogen termination and deposited on quartz substrates using chemical vapor deposition, significantly quenched the fluorescence of adsorbed, dye-labeled fibrinogen protein in aqueous solutions at near neutral pH. Smaller levels of quenching were observed from oxygen terminated NCD surfaces. We suggest that these near-surface fluorescence quenching effects may arise from surface conductance effects in hydrogen terminated NCD. It is also shown that despite bulk quenching effects, single molecules of fibrinogen could be imaged on nanocrystalline diamond surfaces using epi-fluorescence techniques.}, number={1}, journal={DIAMOND AND RELATED MATERIALS}, author={Sakon, John J. and Ribeill, Guilhern J. and Garguilo, Jacob M. and Perkins, Jarnes and Weninger, Keith R. and Nemanich, Robert J.}, year={2009}, month={Jan}, pages={82–87} }
 @article{garguilo_hong_edwards_nemanich_simon_2007, title={The surface oxidation potential of melanosomes measured by free electron laser-photoelectron emission microscopy}, volume={83}, ISSN={["1751-1097"]}, DOI={10.1562/2006-09-11-RA-1037}, abstractNote={A technique for measuring the photoionization spectrum and the photoelectron emission threshold of a microscopic structured material is presented. The theoretical underpinning of the experiment and the accuracy of the measurements are discussed. The technique is applied to titanium silicide nanostructures and melanosomes isolated from human hair, human and bovine retinal pigment epithelium cells, and the ink sac of Sepia officinalis. A common photothreshold of 4.5 ± 0.2 eV is found for this set of melanosomes and is attributed to the photoionization of the eumelanin pigment. The relationship between the photoionization threshold and the electrochemical potential referenced to the normal hydrogen electrode is used to quantify the surface oxidation potential of the melanosome. The developed technique is used to examine the effect of iron chelation on the surface oxidation potential of Sepia melanosomes. The surface oxidation potential is insensitive to bound Fe(III) up to saturation, suggesting that the metal is bound to the interior of the granule. This result is discussed in relation to the age‐dependent accumulation of iron in human melanosomes in both the eye and brain.}, number={3}, journal={PHOTOCHEMISTRY AND PHOTOBIOLOGY}, author={Garguilo, Jacob and Hong, Lian and Edwards, Glenn S. and Nemanich, Robert J. and Simon, John D.}, year={2007}, pages={692–697} }
 @article{hong_garguilo_anzaldi_edwards_nemanich_simon_2006, title={Age-dependent photoionization thresholds of melanosomes and lipofuscin isolated from human retinal pigment epithelium cells}, volume={82}, ISSN={["1751-1097"]}, DOI={10.1562/2006-03-14-RA-846}, abstractNote={Abstract Melanosomes and lipofuscin were isolated from 14-, 59-, and 76-year-old, human retinal pigment epithelium specimens and examined. The morphological features of these samples were studied by scanning electron microscopy and atomic force microscopy, and the photoionization properties were examined by photoelectron emission microscopy. Ovoid- and rod-shaped melanosomes were observed. The size of the granules and the distribution between the two shapes show no significant age-dependent change. However, there is a higher occurrence of irregularly shaped aggregates of small round granules in older samples which suggests degradation or damage to melanosomes occurs with age. The melanosomes from the 14-year-old donor eye are well characterized by a single photoionization threshold, 4.1 eV, while the two older melanosomes exhibit two thresholds around 4.4 and 3.6 eV. Lipofuscin from both young and old cells show two thresholds, 4.4 and 3.4 eV. The similarity of the potentials observed for aged melanosomes and lipofuscin suggest that the lower threshold in the melanosome sample reflects lipofuscin deposited the surface of the melanosome. The amount, however, is not sufficient to alter the density of the melanosome, and therefore these granules do not separate in a sucrose gradient at densities characteristic of the typical melanolipofuscin granule. These data suggest that thin deposits of lipofuscin on the surface of retinal pigment epithelium melanosomes are common in the aged eye and that this renders the melanosomes more pro-oxidant.}, number={6}, journal={PHOTOCHEMISTRY AND PHOTOBIOLOGY}, author={Hong, Lian and Garguilo, Jacob and Anzaldi, Laura and Edwards, Glenn S. and Nemanich, Robert J. and Simon, John D.}, year={2006}, pages={1475–1481} }
 @article{ye_hong_garguilo_pawlak_edwards_nemanich_sarna_simon_2006, title={Photoionization thresholds of melanins obtained from free electron laser-photoelectron emission microscopy, femtosecond transient absorption spectroscopy and electron paramagnetic resonance measurements of oxygen photoconsumption}, volume={82}, ISSN={["1751-1097"]}, DOI={10.1562/2006-01-02-RA-762}, abstractNote={ABSTRACT}, number={3}, journal={PHOTOCHEMISTRY AND PHOTOBIOLOGY}, author={Ye, Tong and Hong, Lian and Garguilo, Jacob and Pawlak, Anna and Edwards, Glenn S. and Nemanich, Robert J. and Sarna, Tadeusz and Simon, John D.}, year={2006}, pages={733–737} }
 @article{bush_garguilo_zucca_albertini_zecca_edwards_nemanich_simon_2006, title={The surface oxidation potential of human neuromelanin reveals a spherical architecture with a pheomelanin core and a eumelanin surface}, volume={103}, ISSN={["0027-8424"]}, DOI={10.1073/pnas.0604010103}, abstractNote={Neuromelanin (NM) isolated from the substantia nigra region of the human brain was studied by scanning probe and photoelectron emission microscopies. Atomic force microscopy reveals that NM granules are comprised of spherical structures with a diameter of ≈30 nm, similar to that observed forSepiacuttlefish, bovine eye, and human eye and hair melanosomes. Photoelectron microscopy images were collected at specific wavelengths of UV light between 248 and 413 nm, using the spontaneous-emission output from the Duke OK-4 free electron laser. Analysis of the data establishes a threshold photoionization potential for NM of 4.5 ± 0.2 eV, which corresponds to an oxidation potential of −0.1 ± 0.2 V vs. the normal hydrogen electrode (NHE). The oxidation potential of NM is within experimental error of the oxidation potential measured for human eumelanosomes (−0.2 ± 0.2 V vs. NHE), despite the presence of a significant fraction of the red pigment, pheomelanin, which is characterized by a higher oxidation potential (+0.5 ± 0.2 V vs. NHE). Published kinetic studies on the early chemical steps of melanogenesis show that in the case of pigments containing a mixture of pheomelanin and eumelanin, of which NM is an example, pheomelanin formation occurs first with eumelanin formation predominantly occurring only after cysteine levels are depleted. Such a kinetic model would predict a structural motif with pheomelanin at the core and eumelanin at the surface, which is consistent with the measured surface oxidation potential of the ≈30-nm constituents of NM granules.}, number={40}, journal={PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA}, author={Bush, William D. and Garguilo, Jacob and Zucca, Fabio A. and Albertini, Alberto and Zecca, Luigi and Edwards, Glenn S. and Nemanich, Robert J. and Simon, John D.}, year={2006}, month={Oct}, pages={14785–14789} }
 @article{kock_garguilo_nemanich_2005, title={Field enhanced thermionic electron emission from sulfur doped nanocrystalline diamond films}, volume={14}, ISSN={["1879-0062"]}, DOI={10.1016/j.diamond.2004.12.056}, abstractNote={Thermal activation of field enhancement based emitters can provide efficient means of lowering the emission barrier, thus enabling high current density electron sources. Microwave plasma assisted chemical vapour deposition was employed to synthesize sulfur doped nanocrystalline diamond films with various sulfur/carbon concentrations. Electron emission at elevated temperatures was characterized by direct imaging of the emission utilizing electron emission microscopy. Sulfur doped nanocrystalline diamond films exhibit electron emission from singular sites indicating a non-uniform distribution of the local field enhancement. The threshold field for electron emission changes significantly by varying the sulfur/carbon concentration in the gas phase. At elevated temperatures the emission is strongly enhanced but remains confined to the intense emission sites.}, number={3-7}, journal={DIAMOND AND RELATED MATERIALS}, author={Kock, FAM and Garguilo, JM and Nemanich, RJ}, year={2005}, pages={704–708} }
 @article{wang_gupta_garguilo_liu_qin_nemanich_2005, title={Growth and field emission properties of small diameter carbon nanotube films}, volume={14}, ISSN={["1879-0062"]}, DOI={10.1016/j.diamond.2004.10.003}, abstractNote={Vertically aligned carbon nanotube films with diameters smaller than 5 nm, high densities up to 1012/cm2, and lengths of ∼ 5–8 μm were deposited by microwave plasma-assisted chemical vapor deposition. Experiments show that, by continuous reduction in the thickness of the iron film (i.e., ∼0.3–0.5 nm), small diameter carbon nanotubes can be achieved with diameters that ranged from 1–5 nm, and the films are comprised of both single- and double-wall nanotubes. The electron field emission properties of the films were investigated by variable distance field emission and temperature-dependent field electron emission microscopy (T-FEEM). The films showed an emission site density of ∼104/cm2 and a threshold field of 2.8 V/μm similar to multiwalled nanotubes (1.9 V/μm). In addition, they also exhibited a temperature dependence of the emission site intensity.}, number={3-7}, journal={DIAMOND AND RELATED MATERIALS}, author={Wang, YY and Gupta, S and Garguilo, JM and Liu, ZJ and Qin, LC and Nemanich, RJ}, year={2005}, pages={714–718} }
 @article{gupta_wang_garguilo_nemanich_2005, title={Imaging temperature-dependent field emission from carbon nanotube films: Single versus multiwalled}, volume={86}, ISSN={["1077-3118"]}, DOI={10.1063/1.1850616}, abstractNote={Field emission properties of vertically aligned single- and multiwalled carbon nanotube films at temperatures up to 1000°C are investigated by electron emission microscopy, enabling real-time imaging of electron emission to provide information on emission site density, the temporal variation of the emission intensity, and insight into the role of adsorbates. The nanotube films showed an emission site density of 104∼105∕cm2, which is compared to the areal density (from 1012–1013∕cm2to108–109∕cm2). At ambient temperature, the emission indicated temporal fluctuation (∼6%–8%) in emission current with minimal changes in the emission pattern. At elevated temperatures, the emission site exhibited an increase in emission site intensity. From the experimental observations, it is proposed that the chemisorbed molecules tend to desorb presumably at high applied electric fields (field-induced) in combination with thermal effects (thermal-induced) and provide a contrasting comparison between semiconducting (single-walled) and metallic (multiwalled) nanotubes.}, number={6}, journal={APPLIED PHYSICS LETTERS}, author={Gupta, S and Wang, YY and Garguilo, JM and Nemanich, RJ}, year={2005}, month={Feb} }
 @article{samokhvalov_hong_liu_garguilo_nemanich_edwards_simon_2005, title={Oxidation potentials of human eumelanosomes and plheomelanosomes}, volume={81}, ISSN={["1751-1097"]}, DOI={10.1562/2004-07-23-RC-245.1}, abstractNote={Abstract Eumelanosomes and pheomelanosomes isolated from black and red human hair, respectively, were studied by photoelectron emission microscopy (PEEM). PEEM images were collected at various wavelengths between 207 and 344 nm, using the spontaneous emission output of the Duke OK-4 free electron laser (FEL). Analysis of the FEL-PEEM data revealed ionization thresholds of 4.6 and 3.9 eV corresponding to oxidation potentials of −0.2 and +0.5 V vs normal hydrogen electrode for eumelanosomes and pheomelanosomes, respectively. The difference in oxidation potential is attributed to the pigment content of the melanosome, namely whether it contains primarily eumelanin and pheomelanin. The effect of added melanosomes on the reduction of Fe(III)-cytochrome showed pheomelanosomes are stronger reducing agents than eumelanosomes, consistent with the measured oxidation potentials. The FEL-PEEM experiment offers to be an important new approach for quantifying the effects of age, oxidation and metal accumulation on the oxidation potentials of intact melanosomes.}, number={1}, journal={PHOTOCHEMISTRY AND PHOTOBIOLOGY}, author={Samokhvalov, A and Hong, L and Liu, Y and Garguilo, J and Nemanich, RJ and Edwards, GS and Simon, JD}, year={2005}, pages={145–148} }
 @article{garguilo_koeck_nemanich_xiao_carlisle_auciello_2005, title={Thermionic field emission from nanocrystalline diamond-coated silicon tip arrays}, volume={72}, ISSN={["2469-9969"]}, DOI={10.1103/physrevb.72.165404}, abstractNote={Thermionic field emission properties of nitrogen doped ultrananocrystalline diamond (UNCD) coated silicon tip arrays are examined using thermionic field emission electron microscopy (TFEEM). Nitrogen doping has been shown to enhance the emission properties of diamond by the introduction of a donor level $1.7\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ below the conduction band minimum. The field enhancing geometry of the films initiates accelerated electron emission at the tipped structures which may be beneficial to thermionic energy converter design where space charge effects can significantly limit attainable current densities. Two temperature regimes of electron emission are observed; $600--800\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, where the emission is enabled because of the H passivation and $900--1100\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, where the emission is attributed to tunneling from nitrogen related states through the barrier of a clean diamond surface.}, number={16}, journal={PHYSICAL REVIEW B}, author={Garguilo, JM and Koeck, FAM and Nemanich, RJ and Xiao, XC and Carlisle, JA and Auciello, O}, year={2005}, month={Oct} }
 @article{kock_garguilo_nemanich_2004, title={Direct correlation of surface morphology with electron emission sites for intrinsic nanocrystalline diamond films}, volume={13}, ISSN={["1879-0062"]}, DOI={10.1016/j.diamond.2004.01.008}, abstractNote={Abstract Field emission properties of nanocrystalline diamond films show low turn-on fields resulting in significant electron emission at low applied fields originating from individual sites with an emission site density of ∼104/cm2. We have employed a high resolution electron emission microscope operating in field emission mode to image the spatial distribution of emission sites for intrinsic nanocrystalline diamond thin films. The location of individual emission sites has been directly correlated to the surface morphology probed by scanning electron microscopy. Surface topography measurements show fine structured features consisting of micron and submicron domains separated by grain boundaries. No preferred topographic features that would account for field emission can be detected suggesting that the electronic structure of the grains and their boundaries under a high electric field has to be considered in order to account for the observed emission characteristics.}, number={4-8}, journal={DIAMOND AND RELATED MATERIALS}, author={Kock, EM and Garguilo, JM and Nemanich, RJ}, year={2004}, pages={1022–1025} }
 @article{wang_tang_koeck_brown_garguilo_nemanich_2004, title={Experimental studies of the formation process and morphologies of carbon nanotubes with bamboo mode structures}, volume={13}, ISSN={["0925-9635"]}, DOI={10.1016/j.diamond.2004.01.009}, abstractNote={Carbon nanotubes (CNT) were synthesized by microwave plasma chemical vapor deposition, and the formation process and morphologies of bamboo mode structures were systematically analyzed. Thin films of Fe on Si substrates were used as the catalyst, and the CNT films were characterized with electron microscopy, Raman spectroscopy, and Auger electron spectroscopy. For growth up to 15 min, the films grow with vertically aligned CNT with evidence of amorphous carbon at the top surface. For longer growth times the films exhibit a layer of amorphous carbon and a CNT mat on top of the aligned carbon nanotube ‘forest.’ Transmission electron microscopy measurements displayed multiwalled CNT with bamboo structure and encapsulated tips some of which contained catalyst particles. Two kinds of bamboo mode structures were observed: cone shaped, and cylindrical. The results indicate that the CNT growth is predominantly of the base growth mode, and the formation of the compartments was attributed to the difference in the bulk and surface diffusion of carbon species at the catalyst.}, number={4-8}, journal={DIAMOND AND RELATED MATERIALS}, author={Wang, YY and Tang, GY and Koeck, FM and Brown, B and Garguilo, JM and Nemanich, RJ}, year={2004}, pages={1287–1291} }
 @article{garguilo_davis_buddie_kock_nemanich_2004, title={Fibrinogen adsorption onto microwave plasma chemical vapor deposited diamond films}, volume={13}, ISSN={["1879-0062"]}, DOI={10.1016/j.diamond.2003.10.008}, abstractNote={This study investigates the haemocompatibility of diamond films and attempts to correlate the results to structural characteristics and surface termination. The samples evaluated consisted of polycrystalline and nanocrystalline diamond films, single crystal diamond, titanium and silicon. Raman spectroscopy detailed the sp3 and sp2 bonding configurations while surface morphology was imaged using atomic force microscopy. Initial contact angles of deionized water were obtained using the sessile drop method. Samples exposed to a hydrogen plasma, and thus hydrogen terminated, became more hydrophobic while samples oxidized by a nitric acid etch became more hydrophilic. The adsorption process of the human protein fibrinogen was then studied on each of the samples. The water contact angle for the MPCVD and single crystal diamond samples did not change, indicating little protein adsorption. Titanium and silicon samples became more hydrophilic as a result of adhered fibrinogen protein.}, number={4-8}, journal={DIAMOND AND RELATED MATERIALS}, author={Garguilo, JM and Davis, BA and Buddie, M and Kock, FAM and Nemanich, RJ}, year={2004}, pages={595–599} }
 @article{koeck_garguilo_nemanich_2004, title={On the thermionic emission from nitrogen-doped diamond films with respect to energy conversion}, volume={13}, ISSN={["0925-9635"]}, DOI={10.1016/j.diamond.2004.06.027}, abstractNote={Thermionic energy converters utilize thermal energy and efficiently transform it into more useful electrical energy. A key aspect in thermionic energy conversion is the emission of electrons at elevated temperatures, where the electron emitter is separated from the collector by a vacuum gap and a voltage is generated due to the temperature difference between the emitter and collector. In this study, nitrogen-doped diamond films with a negative electron affinity surface have been synthesized with plasma-assisted chemical vapor deposition, and the electron emission has been imaged using high-resolution electron emission microscopy. This study reports the measurement of a thermovoltage and current, i.e. energy conversion, at temperatures considerably less than 1000 °C.}, number={11-12}, journal={DIAMOND AND RELATED MATERIALS}, author={Koeck, FAM and Garguilo, JM and Nemanich, RJ}, year={2004}, pages={2052–2055} }
 @misc{samokhvalov_garguilo_yang_edwards_nemanich_simon_2004, title={Photoionization threshold of eumelanosomes determined using UV free electron laser - Photoelectron emission microscopy}, volume={108}, ISSN={["1520-5207"]}, DOI={10.1021/jp046701q}, abstractNote={The application of UV-free electron laser photoelectron emission microscopy (UV-FEL PEEM) to measure the threshold photoelectron spectrum and photoionization potential for human eumelanosomes is described. The origin of potential artifacts and the limitations of the technique are discussed and their potential effects on the measured photoionization potential are quantified. The UV-FEL-PEEM images collected on human eumelanosomes isolated from black hair show that the organelle is photoionized by UV-B radiation. The photoionization threshold is determined to be 4.6 ± 0.2 eV. This result provides new insight into the origin of the differences between the photoionization and oxygen photoconsumption action spectra for eumelanins.}, number={42}, journal={JOURNAL OF PHYSICAL CHEMISTRY B}, author={Samokhvalov, A and Garguilo, J and Yang, WC and Edwards, GS and Nemanich, RJ and Simon, JD}, year={2004}, month={Oct}, pages={16334–16338} }
 @article{kock_garguilo_nemanich_gupta_weiner_morell_2003, title={Spatial distribution of electron emission sites for sulfur doped and intrinsic nanocrystalline diamond films}, volume={12}, ISSN={["1879-0062"]}, DOI={10.1016/S0925-9635(02)00365-5}, abstractNote={We have investigated high sp2 content intrinsic and sulfur doped nanocrystalline diamond films to study field emission properties by electron emission microscopy operated in different modes. Electron emission microscopy enables real time imaging of the electron emission from a surface with a lateral resolution of ∼15 nm. The nanocrystalline intrinsic diamond films exhibit electron emission at room temperature from localized emission sites with weak temperature dependence, and a density of ∼103–104/cm2. In contrast, sulfur doped diamond films show similar emission characteristics at room temperature, but at elevated temperatures the emission significantly increases from the localized regions and a thermionic component is identified in the I/V dependence. We discuss the role of S-donor states to explain the enhanced emission of the S-doped nanocrystalline diamond.}, number={3-7}, journal={DIAMOND AND RELATED MATERIALS}, author={Kock, FAM and Garguilo, JM and Nemanich, RJ and Gupta, S and Weiner, BR and Morell, G}, year={2003}, pages={474–480} }
 @article{kock_garguilo_brown_nemanich_2002, title={Enhanced low-temperature thermionic field emission from surface-treated N-doped diamond films}, volume={11}, ISSN={["1879-0062"]}, DOI={10.1016/S0925-9635(02)00006-7}, abstractNote={Nitrogen-doped diamond films have been synthesized for application as a low-temperature thermionic field-emission cathode. The critical result of this study is the observation of uniform electron emission from UV photo-excitation and from thermionic field emission for films terminated with hydrogen or a 0.3-nm Ti layer. The samples were imaged with photoelectron emission microscopy (PEEM) and thermionic field-emission electron microscopy (T-FEEM) at temperatures up to 900 °C, and the electron emission current was recorded vs. the applied voltage. Hydrogen-passivated films show enhanced electron emission, but become unstable at elevated temperatures, while Ti-terminated films showed similar enhanced emission at temperatures up to 950 °C. Temperature-dependent I/V measurements show strongly increased electron emission at higher temperatures, suggesting that electron emission originates from the conduction band. These results indicate a promising new material for the production of low-temperature, high-brightness electron sources.}, number={3-6}, journal={DIAMOND AND RELATED MATERIALS}, author={Kock, FAM and Garguilo, JM and Brown, B and Nemanich, RJ}, year={2002}, pages={774–779} }