Search

Search Results

• 21. [Article] Existence of High-Order Superlattices in Orientationally Ordered C_{60}

  We correlate the presence or absence of high-order modulated structures in solid C60 with the differential scanning calorimetry signature of the orientational ordering transition and other diagnostic data. ...

  Citation × Citation Citation Abstract Copy Title: Existence of High-Order Superlattices in Orientationally Ordered C_{60} Author: Fischer, J. E., Luzzi, D. E., Kniaź, K., McGhie, A. R., Ricketts-Foot, D. A., Romanow, W. R., Vaughan, Gavin B. M., Li, Dayin, Smith, Allan L., Strongin, Robert M., Cichy, M. A., Bard, Laurent, Smith, Amos B., III Year: 1993 We correlate the presence or absence of high-order modulated structures in solid C60 with the differential scanning calorimetry signature of the orientational ordering transition and other diagnostic data. We have searched for, but not observed, the 2a face-centered cubic structure recently observed in low-temperature electron and single-crystal x-ray diffraction. We conclude that the intrinsic ground state is the previously reported simple cubic structure [space groupPa3¯, a(14 K)=14.06 Å], and that other structures may be stabilized by defects or impurities. Title: Existence of High-Order Superlattices in Orientationally Ordered C_{60} Author: Fischer, J. E., Luzzi, D. E., Kniaź, K., McGhie, A. R., Ricketts-Foot, D. A., Romanow, W. R., Vaughan, Gavin B. M., Li, Dayin, Smith, Allan L., Strongin, Robert M., Cichy, M. A., Bard, Laurent, Smith, Amos B., III Year: 1993 We correlate the presence or absence of high-order modulated structures in solid C60 with the differential scanning calorimetry signature of the orientational ordering transition and other diagnostic data. We have searched for, but not observed, the 2a face-centered cubic structure recently observed in low-temperature electron and single-crystal x-ray diffraction. We conclude that the intrinsic ground state is the previously reported simple cubic structure [space groupPa3¯, a(14 K)=14.06 Å], and that other structures may be stabilized by defects or impurities. Close

• 22. [Article] Unusual Thermal Stability of a Site-Ordered MC60 Rocksalt Structure (M=K, Rb, or Cs).

  X-ray diffraction and differential scanning calorimetry of MxC60, with x∼1 and M=K, Rb, or Cs, reveal an unusual T-dependent phase sequence. A low-symmetry ground state is found, while the high-T limit ...

  Citation × Citation Citation Abstract Copy Title: Unusual Thermal Stability of a Site-Ordered MC60 Rocksalt Structure (M=K, Rb, or Cs). Author: Zhu, Q., Zhou, O., Fischer, J. E., McGhie, Andrew R., Romanow, W. R., Strongin, Robert M., Cichy, M. A., Smith, Amos B., III Year: 1993 X-ray diffraction and differential scanning calorimetry of MxC60, with x∼1 and M=K, Rb, or Cs, reveal an unusual T-dependent phase sequence. A low-symmetry ground state is found, while the high-T limit is an ordered rocksalt structure in which only the octahedral sites are occupied. The unusual high-T stability of this ordered phase is attributed to the entropy of molecular orientational disorder and/or thermal disorder of the alkali-metal ions within the octahedral sites. Unique to KxC60 with x≥1.4, we find at intermediate temperatures an fcc site-disordered lattice gas phase with random occupancy of tetrahedral and octahedral sites, which is thus isostructural with superconducting K3C60. Title: Unusual Thermal Stability of a Site-Ordered MC60 Rocksalt Structure (M=K, Rb, or Cs). Author: Zhu, Q., Zhou, O., Fischer, J. E., McGhie, Andrew R., Romanow, W. R., Strongin, Robert M., Cichy, M. A., Smith, Amos B., III Year: 1993 X-ray diffraction and differential scanning calorimetry of MxC60, with x∼1 and M=K, Rb, or Cs, reveal an unusual T-dependent phase sequence. A low-symmetry ground state is found, while the high-T limit is an ordered rocksalt structure in which only the octahedral sites are occupied. The unusual high-T stability of this ordered phase is attributed to the entropy of molecular orientational disorder and/or thermal disorder of the alkali-metal ions within the octahedral sites. Unique to KxC60 with x≥1.4, we find at intermediate temperatures an fcc site-disordered lattice gas phase with random occupancy of tetrahedral and octahedral sites, which is thus isostructural with superconducting K3C60. Close

• 23. [Article] Structure and Phase Transition of the 6,5-Annulene Isomer of C_{61}H_{2}

  We have used differential scanning calorimetry and x-ray diffraction to study the crystalline phases of the 6,5-annulene isomer of C61H2. At 308 K the methylenated fullerenes are orientationally disordered ...

  Citation × Citation Citation Abstract Copy Title: Structure and Phase Transition of the 6,5-Annulene Isomer of C_{61}H_{2} Author: Lommen, Andrea N., Heiney, Paul A., Vaughan, Gavin B. M., Stephens, Peter W., Liu, Dengfa, Li, Dayin, Smith, Allan L., McGhie, Andrew R., Strongin, Robert M., Brard, Laurent, Smith, Amos B., III Year: 1994 We have used differential scanning calorimetry and x-ray diffraction to study the crystalline phases of the 6,5-annulene isomer of C61H2. At 308 K the methylenated fullerenes are orientationally disordered in a face-centered cubic lattice with lattice parameter 14.19±0.02 Å. Below Tc=290 K the symmetry is lowered to Pa3¯, a simple-cubic structure. At 20 K the lattice parameter is 14.06±0.02 Å, with substantial statistical disorder remaining. The transition temperature in C61H2 is slightly higher than in C60O and higher still than in C60, but the three compounds exhibit orientationally ordering transitions between qualitatively similar phases. Title: Structure and Phase Transition of the 6,5-Annulene Isomer of C_{61}H_{2} Author: Lommen, Andrea N., Heiney, Paul A., Vaughan, Gavin B. M., Stephens, Peter W., Liu, Dengfa, Li, Dayin, Smith, Allan L., McGhie, Andrew R., Strongin, Robert M., Brard, Laurent, Smith, Amos B., III Year: 1994 We have used differential scanning calorimetry and x-ray diffraction to study the crystalline phases of the 6,5-annulene isomer of C61H2. At 308 K the methylenated fullerenes are orientationally disordered in a face-centered cubic lattice with lattice parameter 14.19±0.02 Å. Below Tc=290 K the symmetry is lowered to Pa3¯, a simple-cubic structure. At 20 K the lattice parameter is 14.06±0.02 Å, with substantial statistical disorder remaining. The transition temperature in C61H2 is slightly higher than in C60O and higher still than in C60, but the three compounds exhibit orientationally ordering transitions between qualitatively similar phases. Close

• 24. [Article] Neutron-Scattering Study of Librations and Intramolecular Phonons in Rb_{2.6}K_{0.4}C_{60}

  We report the results of inelastic neutron-scattering measurements on Rb2.6K0.4C60. Librational modes were observed as broad peaks with maxima between 4.1 and 4.7 meV, as the temperature is lowered from ...

  Citation × Citation Citation Abstract Copy Title: Neutron-Scattering Study of Librations and Intramolecular Phonons in Rb_{2.6}K_{0.4}C_{60} Author: Reznik, D., Kamitakahara, W. A., Neumann, D. A., Copley, J. R. D., Fischer, J. E., Strongin, Robert M., Cichy, M. A., Smith, Amos B., III Year: 1994 We report the results of inelastic neutron-scattering measurements on Rb2.6K0.4C60. Librational modes were observed as broad peaks with maxima between 4.1 and 4.7 meV, as the temperature is lowered from 300 to 12 K. As in K3C60, no change in the width or position of the librational peak was observed when the sample was cooled through the superconducting transition. Thus any coupling of the librations to electronic states is small. The magnitude of the orientational potential barrier was estimated from the librational peak frequency. A flat background observed in the low-energy inelastic-scattering spectra is ascribed to two-phonon scattering. The density of states of intramolecular modes is similar to that of previously studied M3C60 compounds; modes at 53 and 66 meV in pure C60 are not observed in Rb2.6K0.4C60. Title: Neutron-Scattering Study of Librations and Intramolecular Phonons in Rb_{2.6}K_{0.4}C_{60} Author: Reznik, D., Kamitakahara, W. A., Neumann, D. A., Copley, J. R. D., Fischer, J. E., Strongin, Robert M., Cichy, M. A., Smith, Amos B., III Year: 1994 We report the results of inelastic neutron-scattering measurements on Rb2.6K0.4C60. Librational modes were observed as broad peaks with maxima between 4.1 and 4.7 meV, as the temperature is lowered from 300 to 12 K. As in K3C60, no change in the width or position of the librational peak was observed when the sample was cooled through the superconducting transition. Thus any coupling of the librations to electronic states is small. The magnitude of the orientational potential barrier was estimated from the librational peak frequency. A flat background observed in the low-energy inelastic-scattering spectra is ascribed to two-phonon scattering. The density of states of intramolecular modes is similar to that of previously studied M3C60 compounds; modes at 53 and 66 meV in pure C60 are not observed in Rb2.6K0.4C60. Close

• 25. [Article] Coherence Peak and Superconducting Energy Gap in Rb_{3}C_{60} Observed by Muon Spin Relaxation

  Muon spin relaxation resulting from spin exchange scattering of endohedral muonium (μ+e-) with thermal electronic excitations has been observed in the fullerene superconductor Rb3C60. The temperature dependence ...

  Citation × Citation Citation Abstract Copy Title: Coherence Peak and Superconducting Energy Gap in Rb_{3}C_{60} Observed by Muon Spin Relaxation Author: Kiefl, R. F., MacFarlane, W. A., Chow, K. H., Dunsiger, S., Duty, T. L., Johnston, T. M. S., Schneider, J. W., Sonier, J., Brard, Laurent, Strongin, Robert M., Fischer, J. E., Smith, Amos B., III Year: 1993 Muon spin relaxation resulting from spin exchange scattering of endohedral muonium (μ+e-) with thermal electronic excitations has been observed in the fullerene superconductor Rb3C60. The temperature dependence of T1-1 shows a coherence peak just below Tc and can be fit to the conventional Hebel-Slichter theory for spin relaxation in a superconductor with a broadened BCS density of states. The average energy gap for electronic excitations, Δ/kB=53(4) K or 2Δ/ kBTc=3.6(3), is consistent with the BCS weak coupling limit. Title: Coherence Peak and Superconducting Energy Gap in Rb_{3}C_{60} Observed by Muon Spin Relaxation Author: Kiefl, R. F., MacFarlane, W. A., Chow, K. H., Dunsiger, S., Duty, T. L., Johnston, T. M. S., Schneider, J. W., Sonier, J., Brard, Laurent, Strongin, Robert M., Fischer, J. E., Smith, Amos B., III Year: 1993 Muon spin relaxation resulting from spin exchange scattering of endohedral muonium (μ+e-) with thermal electronic excitations has been observed in the fullerene superconductor Rb3C60. The temperature dependence of T1-1 shows a coherence peak just below Tc and can be fit to the conventional Hebel-Slichter theory for spin relaxation in a superconductor with a broadened BCS density of states. The average energy gap for electronic excitations, Δ/kB=53(4) K or 2Δ/ kBTc=3.6(3), is consistent with the BCS weak coupling limit. Close

• 26. [Article] Thermal Effects of Microinverter Placement on the Performance of Silicon Photovoltaics

  Typical installation of a microinverter in a plane parallel position on the back side of a monocrystalline silicon photovoltaic (PV) panel can lead to differential heating of the PV cells immediately above ...

  Citation × Citation Citation Abstract Copy Title: Thermal Effects of Microinverter Placement on the Performance of Silicon Photovoltaics Author: Day, Nicholas U., Reinhart, Chase C., DeBow, Shaun, Smith, Matthew K., Sailor, David J., Johansson, Erik, Wamser, Carl C. Year: 2016 Typical installation of a microinverter in a plane parallel position on the back side of a monocrystalline silicon photovoltaic (PV) panel can lead to differential heating of the PV cells immediately above the microinverter by as much as 4 °C. Rotation of the microinverter to a perpendicular position allows the microinverter itself to run cooler by about 4 °C and completely removes the distinctive heat signature on the panels. Because the thermal effects of the microinverters are significant for only two of the 72 cells on the panel, changes in DC power output from the panels are not detectable. However, lower microinverter temperatures increase microinverter efficiency by about 0.65%, such that overall AC power production is increased by about 0.09%. In addition to these small improvements to be gained by the repositioning of microinverters, there are also potential long-term concerns that nonuniform heating may lead to accelerated degradation in the affected area of the panel. Title: Thermal Effects of Microinverter Placement on the Performance of Silicon Photovoltaics Author: Day, Nicholas U., Reinhart, Chase C., DeBow, Shaun, Smith, Matthew K., Sailor, David J., Johansson, Erik, Wamser, Carl C. Year: 2016 Typical installation of a microinverter in a plane parallel position on the back side of a monocrystalline silicon photovoltaic (PV) panel can lead to differential heating of the PV cells immediately above the microinverter by as much as 4 °C. Rotation of the microinverter to a perpendicular position allows the microinverter itself to run cooler by about 4 °C and completely removes the distinctive heat signature on the panels. Because the thermal effects of the microinverters are significant for only two of the 72 cells on the panel, changes in DC power output from the panels are not detectable. However, lower microinverter temperatures increase microinverter efficiency by about 0.65%, such that overall AC power production is increased by about 0.09%. In addition to these small improvements to be gained by the repositioning of microinverters, there are also potential long-term concerns that nonuniform heating may lead to accelerated degradation in the affected area of the panel. Close

• 27. [Article] Pressure-induced insulating state in an organic superconductor

  The electronic-transport properties of the quasi-two-dimensional organic superconductor β″–(BEDT-TTF)₂SF₅CH₂CF₂SO₃, where BEDT-TTF stands for bisethylenedithio-tetrathiafulvalene, have been investigated ...

  Citation × Citation Citation Abstract Copy Title: Pressure-induced insulating state in an organic superconductor Author: Gard, Gary L., Mohtasham, Javid, Schlueter, J. A., Pfleiderer, C., Wosnitza, J., Hagel, J. Year: 2003 The electronic-transport properties of the quasi-two-dimensional organic superconductor β″–(BEDT-TTF)₂SF₅CH₂CF₂SO₃, where BEDT-TTF stands for bisethylenedithio-tetrathiafulvalene, have been investigated in magnetic fields up to 15 T and under hydrostatic pressure up to about 14 kbars. Shubnikov–de Haas data reveal a nonmonotonic pressure dependence of the holelike Fermi surface, a roughly linear increase of the electron g factor, and an approximately linear decrease of the cyclotron effective mass. By assuming that the latter reflects the pressure-induced reduction of the superconducting coupling parameter λ the rapid reduction of the superconducting transition temperature Tc(p) can be reasonably well described by the modified McMillan equation. Above about 12 kbars the material becomes insulating with an activated resistive behavior. This first-order metal-insulator transition has a hysteresis of about 3 kbars. This unexpected behavior is assumed to be of structural origin, although clear changes of electronic band-structure properties precede the phase transition. Title: Pressure-induced insulating state in an organic superconductor Author: Gard, Gary L., Mohtasham, Javid, Schlueter, J. A., Pfleiderer, C., Wosnitza, J., Hagel, J. Year: 2003 The electronic-transport properties of the quasi-two-dimensional organic superconductor β″–(BEDT-TTF)₂SF₅CH₂CF₂SO₃, where BEDT-TTF stands for bisethylenedithio-tetrathiafulvalene, have been investigated in magnetic fields up to 15 T and under hydrostatic pressure up to about 14 kbars. Shubnikov–de Haas data reveal a nonmonotonic pressure dependence of the holelike Fermi surface, a roughly linear increase of the electron g factor, and an approximately linear decrease of the cyclotron effective mass. By assuming that the latter reflects the pressure-induced reduction of the superconducting coupling parameter λ the rapid reduction of the superconducting transition temperature Tc(p) can be reasonably well described by the modified McMillan equation. Above about 12 kbars the material becomes insulating with an activated resistive behavior. This first-order metal-insulator transition has a hysteresis of about 3 kbars. This unexpected behavior is assumed to be of structural origin, although clear changes of electronic band-structure properties precede the phase transition. Close

• 28. [Article] SIMPOL.1: a simple group contribution method for predicting vapor pressures and enthalpies of vaporization of multifunctional organic compounds

  The SIMPOL.1 group contribution method is developed for predicting the liquid vapor pressure poL (atm) and enthalpy of vaporization Δ Hvap (kJ mol-1) of organic compounds as functions of temperature (T). ...

  Citation × Citation Citation Abstract Copy Title: SIMPOL.1: a simple group contribution method for predicting vapor pressures and enthalpies of vaporization of multifunctional organic compounds Author: Pankow, James F., Asher, William E. Year: 2008 The SIMPOL.1 group contribution method is developed for predicting the liquid vapor pressure poL (atm) and enthalpy of vaporization Δ Hvap (kJ mol-1) of organic compounds as functions of temperature (T). For each compound i, the method assumes log10poL,i (T)=∑kνk,ibk(T) where νk,i is the number of groups of type k, and bk (T) is the contribution to log10poL,i (T) by each group of type k. A zeroeth group is included that uses b0 (T) with ν0,i=1 for all i. A total of 30 structural groups are considered: molecular carbon, alkyl hydroxyl, aromatic hydroxyl, alkyl ether, alkyl ring ether, aromatic ether, aldehyde, ketone, carboxylic acid, ester, nitrate, nitro, alkyl amine (primary, secondary, and tertiary), aromatic amine, amide (primary, secondary, and tertiary), peroxide, hydroperoxide, peroxy acid, C=C, carbonylperoxynitrate, nitro-phenol, nitro-ester, aromatic rings, non-aromatic rings, C=C–C=O in a non-aromatic ring, and carbon on the acid-side of an amide. The T dependence in each of the bk (T) is assumed to follow b(T)=B1/T+B2+B3T+B4ln T. Values of the B coefficients are fit using an initial basis set of 272 compounds for which experimentally based functions po L,i=fi (T) are available. The range of vapor pressure considered spans fourteen orders of magnitude. The ability of the initially fitted B coefficients to predict poL values is examined using a test set of 184 compounds and a T range that is as wide as 273.15 to 393.15 K for some compounds. σFIT is defined as the average over all points of the absolute value of the difference between experimental and predicted values of log10poL,i (T). After consideration of σFIT for the test set, the initial basis set and test set compounds are combined, and the B coefficients re-optimized. For all compounds and temperatures, σFIT=0.34: on average, poL,i (T) values are predicted to within a factor of 2. Because d(log10 poL,i (T))d(1/T) is related to the enthalpy of vaporization ΔHvap,i, the fitted B provide predictions of ΔHvap,i based on structure. Title: SIMPOL.1: a simple group contribution method for predicting vapor pressures and enthalpies of vaporization of multifunctional organic compounds Author: Pankow, James F., Asher, William E. Year: 2008 The SIMPOL.1 group contribution method is developed for predicting the liquid vapor pressure poL (atm) and enthalpy of vaporization Δ Hvap (kJ mol-1) of organic compounds as functions of temperature (T). For each compound i, the method assumes log10poL,i (T)=∑kνk,ibk(T) where νk,i is the number of groups of type k, and bk (T) is the contribution to log10poL,i (T) by each group of type k. A zeroeth group is included that uses b0 (T) with ν0,i=1 for all i. A total of 30 structural groups are considered: molecular carbon, alkyl hydroxyl, aromatic hydroxyl, alkyl ether, alkyl ring ether, aromatic ether, aldehyde, ketone, carboxylic acid, ester, nitrate, nitro, alkyl amine (primary, secondary, and tertiary), aromatic amine, amide (primary, secondary, and tertiary), peroxide, hydroperoxide, peroxy acid, C=C, carbonylperoxynitrate, nitro-phenol, nitro-ester, aromatic rings, non-aromatic rings, C=C–C=O in a non-aromatic ring, and carbon on the acid-side of an amide. The T dependence in each of the bk (T) is assumed to follow b(T)=B1/T+B2+B3T+B4ln T. Values of the B coefficients are fit using an initial basis set of 272 compounds for which experimentally based functions po L,i=fi (T) are available. The range of vapor pressure considered spans fourteen orders of magnitude. The ability of the initially fitted B coefficients to predict poL values is examined using a test set of 184 compounds and a T range that is as wide as 273.15 to 393.15 K for some compounds. σFIT is defined as the average over all points of the absolute value of the difference between experimental and predicted values of log10poL,i (T). After consideration of σFIT for the test set, the initial basis set and test set compounds are combined, and the B coefficients re-optimized. For all compounds and temperatures, σFIT=0.34: on average, poL,i (T) values are predicted to within a factor of 2. Because d(log10 poL,i (T))d(1/T) is related to the enthalpy of vaporization ΔHvap,i, the fitted B provide predictions of ΔHvap,i based on structure. Close

• 29. [Article] Copper Nanowires Synthesis and Self-Assembly for Interconnect Applications

  One-dimensional (1D) nanomaterial self-assembly offers an excellent approach to the fabrication of highly complex nanodevices. Despite considerable effort and research, precisely controlling the orientation ...

  Citation × Citation Citation Abstract Copy Title: Copper Nanowires Synthesis and Self-Assembly for Interconnect Applications Author: Darmakkolla, Srikar Rao Year: 2017 One-dimensional (1D) nanomaterial self-assembly offers an excellent approach to the fabrication of highly complex nanodevices. Despite considerable effort and research, precisely controlling the orientation and positioning of nanowires (NWs) on a large-scale area and assembling into a functional device is still a state of the art problem. This thesis focuses on the dimensionally controlled copper nanowires (Cu NWs) synthesis, and magnetic field assisted self-assembly of cupronickel nanowires (Cu/Ni NWs) into interconnect structures on a carbon doped silicon dioxide (CDO) wafer. CDO is a low dielectric constant (k) material used for copper interconnects in multilayered complex integrated circuits (ICs). Here, a strong affinity of copper (Cu) and nickel (Ni) to thiol (-SH) functional groups were exploited to strongly adhere the nanowires (Cu/Ni NWs) onto the CDO substrate. Thiol (-SH) functionalization of the CDO surface was achieved via a series of reactions involving (1) esterification of the surface exposed ≡Si-OH functional group to its triflate (≡Si-O-Tf), (2) reduction of triflate to ≡Si-H using DIBAL-H, and (3) hydrosilylation of ≡Si-H using 2-propene thiol (≡Si-(CH2)3-SH) in a photochemical reaction. The thiol functionalization of CDO surface enhances the interaction of Cu/Ni NWs with strong chemical bonds. The same reaction scheme was also used in the functionalization of the hydrophilic (Si-OH) surface to the hydrophobic long alkyl chain derivatized (≡Si-CH2-(CH2)16-CH3) surface. This long alkyl chain modified surface acts as an excellent moisture resistant film, which helps to maintain the low-k value of CDO. The dimensionally controlled Cu NWs were synthesized by a wet chemical approach. Optimization of the reducing agent, hydrazine (N2H4), controlled the surface morphology of nanowires (NWs). Interestingly, the high concentration of reducing agent produced particle decorated and/or with a rough NW surface, and conversely decreasing its concentration resulted in a comparatively thin, particle-free and smooth surface. The reaction temperature affected the aspect ratio (Length/Diameter) of the NWs. As the reaction temperature increased from 60 to 90 °C, the aspect ratio decreased from 140 to 21. Controlling the orientation of Cu NWs in a magnetic field was accomplished by coating them with a thin layer (~20 nm) of ferromagnetic nickel (Ni). This Ni-coated NWs showed an excellent degree of alignment (half-width ≈10 degrees) in the direction of an applied magnetic field over a large surface area at field strength as low as 2500 Gauss. Also, the Ni coating helped in protecting the copper core from oxidation resulting in better electrical wire-to-wire contacts. A nanowire-based interconnect channel was fabricated by combining magnetic field assisted alignment and deposition of aligned NWs on a thiol-modified and photolithography patterned CDO substrate. The NWs, deposited in the trenches, strongly bonded to the thiol-derivatized CDO substrate while an acetone wash removed loosely bound NWs on the photoresist surface. In electrical characterization, the directionally well-aligned Cu/Ni NWs channel displayed surprisingly two-fold higher conductivity than randomly arranged NWs channel. Title: Copper Nanowires Synthesis and Self-Assembly for Interconnect Applications Author: Darmakkolla, Srikar Rao Year: 2017 One-dimensional (1D) nanomaterial self-assembly offers an excellent approach to the fabrication of highly complex nanodevices. Despite considerable effort and research, precisely controlling the orientation and positioning of nanowires (NWs) on a large-scale area and assembling into a functional device is still a state of the art problem. This thesis focuses on the dimensionally controlled copper nanowires (Cu NWs) synthesis, and magnetic field assisted self-assembly of cupronickel nanowires (Cu/Ni NWs) into interconnect structures on a carbon doped silicon dioxide (CDO) wafer. CDO is a low dielectric constant (k) material used for copper interconnects in multilayered complex integrated circuits (ICs). Here, a strong affinity of copper (Cu) and nickel (Ni) to thiol (-SH) functional groups were exploited to strongly adhere the nanowires (Cu/Ni NWs) onto the CDO substrate. Thiol (-SH) functionalization of the CDO surface was achieved via a series of reactions involving (1) esterification of the surface exposed ≡Si-OH functional group to its triflate (≡Si-O-Tf), (2) reduction of triflate to ≡Si-H using DIBAL-H, and (3) hydrosilylation of ≡Si-H using 2-propene thiol (≡Si-(CH2)3-SH) in a photochemical reaction. The thiol functionalization of CDO surface enhances the interaction of Cu/Ni NWs with strong chemical bonds. The same reaction scheme was also used in the functionalization of the hydrophilic (Si-OH) surface to the hydrophobic long alkyl chain derivatized (≡Si-CH2-(CH2)16-CH3) surface. This long alkyl chain modified surface acts as an excellent moisture resistant film, which helps to maintain the low-k value of CDO. The dimensionally controlled Cu NWs were synthesized by a wet chemical approach. Optimization of the reducing agent, hydrazine (N2H4), controlled the surface morphology of nanowires (NWs). Interestingly, the high concentration of reducing agent produced particle decorated and/or with a rough NW surface, and conversely decreasing its concentration resulted in a comparatively thin, particle-free and smooth surface. The reaction temperature affected the aspect ratio (Length/Diameter) of the NWs. As the reaction temperature increased from 60 to 90 °C, the aspect ratio decreased from 140 to 21. Controlling the orientation of Cu NWs in a magnetic field was accomplished by coating them with a thin layer (~20 nm) of ferromagnetic nickel (Ni). This Ni-coated NWs showed an excellent degree of alignment (half-width ≈10 degrees) in the direction of an applied magnetic field over a large surface area at field strength as low as 2500 Gauss. Also, the Ni coating helped in protecting the copper core from oxidation resulting in better electrical wire-to-wire contacts. A nanowire-based interconnect channel was fabricated by combining magnetic field assisted alignment and deposition of aligned NWs on a thiol-modified and photolithography patterned CDO substrate. The NWs, deposited in the trenches, strongly bonded to the thiol-derivatized CDO substrate while an acetone wash removed loosely bound NWs on the photoresist surface. In electrical characterization, the directionally well-aligned Cu/Ni NWs channel displayed surprisingly two-fold higher conductivity than randomly arranged NWs channel. Close

• 30. [Article] Investigation into Effects of Instability and Reactivity of Hydride-Passivated Silicon Nanoparticles on Interband Photoluminescence

  While silicon has long been utilized for its electronic properties, its use as an optical material has largely been limited due to the poor efficiency of interband transitions. However, discovery of visible ...

  Citation × Citation Citation Abstract Copy Title: Investigation into Effects of Instability and Reactivity of Hydride-Passivated Silicon Nanoparticles on Interband Photoluminescence Author: Radlinger, Christine Marie Year: 2017 While silicon has long been utilized for its electronic properties, its use as an optical material has largely been limited due to the poor efficiency of interband transitions. However, discovery of visible photoluminescence (PL) from nanocrystalline silicon in 1990 triggered many ensuing research efforts to optimize PL from nanocrystalline silicon for optical applications. Currently, use of photoluminescent silicon nanoparticles (Si NPs) is commercially limited by: 1) the instability of the energy and intensity of the PL, and 2) the low quantum yield of interband PL from Si NPs. Herein, red-emitting, hydrogen-passivated silicon nanoparticles (H-Si NPs) were synthesized by thermally-induced disproportionation of a HSiCl3-derived (HSiO1.5)n polymer. The H-Si NPs produced by this method were then subjected to various chemical and physical environments to assess the long-term stability of the optical properties as a function of changing surface composition. This dissertation is intended to elucidate correlations between the reported PL instability and the observed changes in the Si NP surface chemistry over time and as a function of environment. First, the stability of the H-Si NP surface at slightly elevated temperatures towards reactivity with a simple alkane was probed. The H-Si NPs were observed by FT-IR spectroscopy to undergo partial hydrosilylation upon heating in refluxing hexane, in addition to varying degrees of surface oxidation. The unexpected reactivity of the Si surface in n-hexane supports the unstable nature of the H-Si NP surface, and furthermore implicates the presence of highly-reactive Si radicals on the surfaces of the Si NPs. We propose that reaction of alkene impurities with the Si surface radicals is largely responsible for the observed surface alkylation. However, we also present an alternate mechanism by which Si surface radicals could react with alkanes to result in alkylation of the surface. Next, the energy and intensity stability of the interband PL from H-Si NPs in the presence of a radical trap was probed. Upon addition of (2,2,6,6,-tetramethyl-piperidin-1-yl)oxyl (TEMPO), the energy and intensity of the interband transition was observed to change over time, dependent on the reaction conditions. First, when the reaction occurred at 4ºC with minimal light exposure, the interband transition exhibited a gradual hypsochromic shift to between 595 nm and 655 nm, versus the λmax of the original low energy emission peak at 700 nm, depending on the amount of TEMPO in the sample. Second, when the reaction proceeded at room temperature with frequent exposure to 360 nm irradiation, the original interband transition at 660 nm was quenched while a new peak at 575 nm developed. Based on all the data collected and analyzed, we assign the 595 -- 655 nm transition as due to interband exciton recombination from Si NPs with reduced diameters relative to the original Si NPs. We furthermore assign the 575 nm transition as due to an oxide-related defect state resulting from rapid oxidation of photo-excited Si NPs. Title: Investigation into Effects of Instability and Reactivity of Hydride-Passivated Silicon Nanoparticles on Interband Photoluminescence Author: Radlinger, Christine Marie Year: 2017 While silicon has long been utilized for its electronic properties, its use as an optical material has largely been limited due to the poor efficiency of interband transitions. However, discovery of visible photoluminescence (PL) from nanocrystalline silicon in 1990 triggered many ensuing research efforts to optimize PL from nanocrystalline silicon for optical applications. Currently, use of photoluminescent silicon nanoparticles (Si NPs) is commercially limited by: 1) the instability of the energy and intensity of the PL, and 2) the low quantum yield of interband PL from Si NPs. Herein, red-emitting, hydrogen-passivated silicon nanoparticles (H-Si NPs) were synthesized by thermally-induced disproportionation of a HSiCl3-derived (HSiO1.5)n polymer. The H-Si NPs produced by this method were then subjected to various chemical and physical environments to assess the long-term stability of the optical properties as a function of changing surface composition. This dissertation is intended to elucidate correlations between the reported PL instability and the observed changes in the Si NP surface chemistry over time and as a function of environment. First, the stability of the H-Si NP surface at slightly elevated temperatures towards reactivity with a simple alkane was probed. The H-Si NPs were observed by FT-IR spectroscopy to undergo partial hydrosilylation upon heating in refluxing hexane, in addition to varying degrees of surface oxidation. The unexpected reactivity of the Si surface in n-hexane supports the unstable nature of the H-Si NP surface, and furthermore implicates the presence of highly-reactive Si radicals on the surfaces of the Si NPs. We propose that reaction of alkene impurities with the Si surface radicals is largely responsible for the observed surface alkylation. However, we also present an alternate mechanism by which Si surface radicals could react with alkanes to result in alkylation of the surface. Next, the energy and intensity stability of the interband PL from H-Si NPs in the presence of a radical trap was probed. Upon addition of (2,2,6,6,-tetramethyl-piperidin-1-yl)oxyl (TEMPO), the energy and intensity of the interband transition was observed to change over time, dependent on the reaction conditions. First, when the reaction occurred at 4ºC with minimal light exposure, the interband transition exhibited a gradual hypsochromic shift to between 595 nm and 655 nm, versus the λmax of the original low energy emission peak at 700 nm, depending on the amount of TEMPO in the sample. Second, when the reaction proceeded at room temperature with frequent exposure to 360 nm irradiation, the original interband transition at 660 nm was quenched while a new peak at 575 nm developed. Based on all the data collected and analyzed, we assign the 595 -- 655 nm transition as due to interband exciton recombination from Si NPs with reduced diameters relative to the original Si NPs. We furthermore assign the 575 nm transition as due to an oxide-related defect state resulting from rapid oxidation of photo-excited Si NPs. Close