We present a general approach to the computational design of nanostructured chemical sensors. The... more We present a general approach to the computational design of nanostructured chemical sensors. The scheme is based on identification and calculation of microscopic descriptors (design parameters) which are used as input to a thermodynamic model to obtain the relevant macroscopic properties. In particular, we consider the functionalization of a (6,6) metallic armchair single-walled carbon nanotube (SWNT) by nine different 3d transition metal (TM) atoms occupying three types of vacancies. For six gas molecules (N 2 , O 2 , H 2 O, CO, NH 3 , H 2 S) we calculate the binding energy and change in conductance due to adsorption on each of the 27 TM sites. For a given type of TM functionalization, this allows us to obtain the equilibrium coverage and change in conductance as a function of the partial pressure of the "target" molecule in a background of atmospheric air. Specifically, we show how Ni and Cu doped metallic (6,6) SWNTs may work as effective multifunctional sensors for both CO and NH 3. In this way, the scheme presented allows one to obtain macroscopic device characteristics and performance data for nanoscale (in this case SWNT) based devices.
A thorough understanding of the electronic structure is a necessary first step for the design of ... more A thorough understanding of the electronic structure is a necessary first step for the design of nanoelectronics, chemical/bio-sensors, electrocatalysts, and nanoplasmonics using graphene. As such, theoretical spectroscopic tech-niques to describe collective excitations of graphene are of fundamental importance. Starting from density functional theory (DFT), linear response time dependent DFT in frequency-reciprocal space within the random phase approxima-tion (TDDFT-RPA) is used to describe the loss function −={ε−1(q, ω)} for isolated graphene. To ensure any spurious interactions between layers are removed, both a radial cutoff of the Coulomb kernel, and extra vacuum directly at the TDDFT-RPA level are employed. A combination of both methods is found to provide a correct description of the electron energy loss spectra of isolated graphene, at a significant reduction in computational cost compared to standard methods. 1 Introduction In the last ten years, graphene [1,2, 3,4,5,6,7,8,...
∗Nano-Bio Spectroscopy Group and ETSF Scientific Development Centre, Depto. Fı́sica de Materiales... more ∗Nano-Bio Spectroscopy Group and ETSF Scientific Development Centre, Depto. Fı́sica de Materiales, Universidad del Paı́s Vasco and DIPC, Avenida de Tolosa 72, E-20018 San Sebastián, Spain; Phone:+34 943 01 8288 Fax:+34 943 01 8390 E-mail: [email protected] †Nano-Bio Spectroscopy Group and ETSF Scientific Development Centre, Depto. Fı́sica de Materiales, Universidad del Paı́s Vasco, Centro de Fı́sica de Materiales CSIC-UPV/EHUMPC and DIPC, Av. Tolosa 72, E-20018 San Sebastián, Spain; Center for Atomic-scale Materials Design, Department of Physics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark; Phone:+45 4525 3209 Fax:+45 4593 2399 E-mail: [email protected] ‡Nano-Bio Spectroscopy Group and ETSF Scientific Development Centre, Depto. Fı́sica de Materiales, Universidad del Paı́s Vasco and DIPC, Avenida de Tolosa 72, E-20018 San Sebastián, Spain; E-mail: [email protected] §Nano-Bio Spectroscopy Group and ETSF Scientific Development Centre, Depto. Fı́sica...
The recent development of efficient chirality sorting techniques has opened the way to the use of... more The recent development of efficient chirality sorting techniques has opened the way to the use of singlewalled carbon nanotubes (SWCNTs) in a plethora of nanoelectronic, photovoltaic, and optoelectronic applications. However, to understand the excitation processes undergone by SWCNTs, it is necessary to have highly efficient and accurate computational methods to describe their optical and electronic properties, methods which have until now been unavailable. Here we employ linear combinations of atomic orbitals (LCAOs) to represent the Kohn-Sham (KS) wavefunctions and perform highly efficient time dependent density functional theory (TDDFT) calculations in the frequency domain using our LCAO-TDDFT-k-ω code to model the optical absorbance and energy loss spectra and spatial distribution of the exciton charge densities in SWCNTs. By applying the GLLB-SC derivative discontinuity correction to the KS eigenenergies, we reproduce the measured E 11 and E 22 transitions within σ 70 meV and the optical absorbance and electron energy loss spectra semiquantitatively for a set of fifteen semiconducting and four metallic chirality sorted SWCNTs. Furthermore, our calculated electron hole density difference ∆ρ(r, ω) resolves the spatial distribution of the measured excitations in SWCNTs. These results open the path towards the computational design of optimized SWCNT nanoelectronic, photovoltaic, and optoelectronic devices in silico.
A necessary first step in the development of technologies such as artificial photosynthesis is un... more A necessary first step in the development of technologies such as artificial photosynthesis is understanding the photoexcitation process within the basic building blocks of naturally occurring light harvesting complexes (LHCs). The most important of these building blocks in biological LHCs such as LHC II from green plants are the chlorophyll a (Chl a) and chlorophyll b (Chl b) chromophores dispersed throughout the protein matrix. However, efforts to describe such systems are still hampered by the lack of computationally efficient and accurate methods that are able to describe optical absorption in large biomolecules. In this work, we employ a highly efficient linear combination of atomic orbitals (LCAOs) to represent the Kohn-Sham (KS) wave functions at the density functional theory (DFT) level and perform time-dependent density functional theory (TDDFT) calculations in either the reciprocal space and frequency domain (LCAO-TDDFT-k-ω) or real space and time domain (LCAO-TDDFT-r-t) of the optical absorption spectra of Chl a and b monomers and dimers. We find that our LCAO-TDDFT-k-ω and LCAO-TDDFT-r-t calculations reproduce results obtained with a plane-wave (PW) representation of the KS wave functions (PW-TDDFT-k-ω) but with a significant reduction in computational effort. Moreover, by applying the Gritsenko, van Leeuwen, van Lenthe, and Baerends solid and correlation derivative discontinuity correction Δx to the KS eigenenergies, with both LCAO-TDDFT-k-ω and LCAO-TDDFT-r-t methods, we are able to semiquantitatively reproduce the experimentally measured photoinduced dissociation results. This work opens the path to first principles calculations of optical excitations in macromolecular systems.
Understanding the interaction of light with molecules physisorbed on substrates is a fundamental ... more Understanding the interaction of light with molecules physisorbed on substrates is a fundamental problem in photonics, with applications in biosensing, photovoltaics, photocatalysis, plasmonics, and nanotechnology. However, the design of novel functional materials in silico is severely hampered by the lack of robust and computationally efficient methods for describing both molecular absorbance and screening on substrates. Here we employ our hybrid G 0 [W 0 + ∆W]-BSE implementation, which incorporates the substrate via its screening ∆W at both the quasiparticle G 0 W 0 level and when solving the Bethe-Salpeter equation (BSE). We show this method can be used to both efficiently and accurately describe the absorption spectra of physisorbed molecules on metal substrates and thereby tailor the molecule's absorbance by altering the surface plasmon's energy. Specifically, we investigate how the optical absorption spectra of three prototypical π-conjugated molecules: benzene (C 6 H 6), terrylene (C 30 H 16) and fullerene (C 60), depends on the Wigner-Seitz radius r s of the metallic substrate. To gain further understanding of the light-molecule/substrate interaction, we also study the bright exciton's electron and hole densities and their interactions with infrared active vibrational modes. Our results show that (1) benzene's bright E 1 1u exciton at 7.0 eV, whose energy is insensitive to changes in r s , could be relevant for photocatalytic dehydrogenation and polymerization reactions, (2) terrylene's bright B 3u exciton at 2.3 eV hybridizes with the surface plasmon, allowing the tailoring of the excitonic energy and optical activation of a surface plasmon-like exciton, and (3) fullerene's π − π * bright and dark excitons at 6.4 and 6.8 eV hybridize with the surface plasmon, resulting in the tailoring of their excitonic energy and the activation of both a surface plasmon-like exciton and a dark quadrupolar mode via symmetry breaking by the substrate. This work demonstrates how a proper description of interfacial light-molecular/substrate interactions enables the prediction, design, and optimization of technologically relevant phenomena in silico.
The internal quantum efficiency (IQE) of an organic photovoltaic device (OPV) is proportional to ... more The internal quantum efficiency (IQE) of an organic photovoltaic device (OPV) is proportional to the number of free charge carriers generated and their conductivity, per absorbed photon. However, both the IQE and the quantities that determine it, for example, electron-hole binding, charge separation, electron-hole recombination, and conductivity, can only be inferred indirectly from experiments. Using density functional theory, we calculate the excited-state formation energy, charge transfer, and zero-bias conductance in the singlet ground state and triplet excited state across polymer/fullerene and polymer/single walled carbon nanotube (SWNT) OPV donor/acceptor bulk heterojunctions. Specifically, we compare polythiophene (PT) and poly(3-methylthiophene-2,5-diyl) (P3MT) as donors and C 60 chains with (6,4), (6,5), and (10,5) SWNTs as acceptors. We find the conductivity increases substantially for both the excited triplet relative to the singlet ground state and for PT compared with P3MT due to the increased charge transfer and the resulting improvement in donor/acceptor level alignment. Similarly, the (6,4) SWNT, with a larger SWNT band gap and greater conductivity than fullerenes, provides the highest conductivities of 5 and 9% of the theoretical maximum for electron and hole carriers, respectively. This work has important implications for both the optimization of polymer/SWNT bulk heterojunctions and the design of new OPV bulk heterojunctions in silico.
We study the photoabsorption properties of photoactive bulk polymer/ fullerene/nanotube heterojun... more We study the photoabsorption properties of photoactive bulk polymer/ fullerene/nanotube heterojunctions in the near-infrared region. By combining pump-probe spectroscopy and linear response time-dependent density functional theory within the random phase approximation (TDDFT-RPA) we elucidate the excited state dynamics of the E 11 transition within (6,5) and (7,5) single-walled carbon nanotubes (SWNTs) and combined with poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) in P3HT/PCBM/SWNT blended samples. We find the presence of a photoinduced absorption (PA) peak is related mainly to the width of the photobleach (PB) peak and the charge carrier density of the SWNT system. For mixed SWNT samples, the PB peak is too broad to observe the PA peak, whereas within P3HT/PCBM/SWNT blended samples P3HT acts as a hole acceptor, narrowing the PB peak by exciton delocalization, which reveals a PA peak. Our results suggest that the PA peak originates from a widening of the band gap in the presence of excited electrons and holes. These results have important implications for the development of new organic photovoltaic heterojunctions including SWNTs.
We perform state-of-the-art calculations for a prototypical dye sensitized solar cell: catechol o... more We perform state-of-the-art calculations for a prototypical dye sensitized solar cell: catechol on rutile TiO 2 (110). Catechol is often used as an anchoring group for larger more complex organic and inorganic dyes on TiO 2 and forms a type II heterojunctions on TiO 2 (110). In particular, we compare quasiparticle (QP) G 0 W 0 with hybrid exchange correlation functional (HSE) density functional theory (DFT) calculations for the catecholrutile TiO 2 (110) interface. In so doing, we provide a theoretical interpretation of ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) experiments for this prototypical system. Specifically, we demonstrate that the position, presence, and intensity of peaks associated with catechol's HOMO, intermolecular OH-O bonds, and interfacial hydrogen bonds to the surface bridging O atoms (O br H-C and O br H-O) may be used to fingerprint deprotonation of catechol's OH anchoring groups. Furthermore, our results suggest deprotonation of these groups, while being nearly isoenergetic at high coverages, may significantly increase the photovoltaic efficiency of catechol-TiO 2 (110) interfaces.
X-ray photoelectron spectroscopy combined with first-principles modeling is a powerful tool for d... more X-ray photoelectron spectroscopy combined with first-principles modeling is a powerful tool for determining the chemical composition and electronic structure of novel materials. Of these, graphene is an especially important model system for understanding the properties of other carbon nanomaterials. Here, we calculate the carbon 1s core level binding energy of pristine graphene using two methods based on density functional theory total energy differences: a calculation with an explicit core-hole, and an all-electron extension of the delta self-consistent field (SCF) method. We study systematically their convergence and computational workload, and the dependence of the energies on the chosen exchange-correlation functional. The SCF method is computationally more expensive, but gives consistently higher C 1s energies. Although there is a significant functional dependence, the binding energy calculated using the PBE functional is found to be remarkably close to what has been measured for graphite.
We extend the two-fluid, two-dimensional hydrodynamic model, describing the collective response o... more We extend the two-fluid, two-dimensional hydrodynamic model, describing the collective response of a multiwalled carbon nanotube, to include solid dielectric media embedded inside, or surrounding the nanotube. In the case of aluminum, we study plasmon hybridization in the UV range and find that the nanotube's low-energy, quasiacoustic plasmons are remarkably stable, whereas the shift of the high-energy + plasmons strongly depends on the gap between the nanotube and the aluminum boundary. Moreover, we study the stopping force on fast ions moving inside nanotubes encapsulated in channels of insulating, amorphous carbon, and metallic materials, with special attention paid to nickel. Finally, our study of the dynamical image potential for fast ions shows that a nanotube will effectively shield the influence of an embedded nickel wire.
Narrow diameter tubes and especially ͑6,5͒ tubes with a diameter of 0.75 nm are currently one of ... more Narrow diameter tubes and especially ͑6,5͒ tubes with a diameter of 0.75 nm are currently one of the most studied carbon nanotubes because their unique optical and especially luminescence response makes them exceptionally suited for biomedical applications. Here we report on a detailed analysis of the electronic structure of nanotubes with ͑6,5͒ and ͑6,4͒ chiralities using a combined experimental and theoretical approach. From high-energy spectroscopy involving x-ray absorption and photoemission spectroscopy the detailed valence-and conduction-band response of these narrow diameter tubes is studied. The observed electronic structure is in sound agreement with state of the art ab initio calculations using density-functional theory.
We present an alternative methodology for calculating the quasi-particle energy, energy loss, and... more We present an alternative methodology for calculating the quasi-particle energy, energy loss, and optical spectra of a molecule deposited on graphene or a metallic substrate. To test the accuracy of the method it is first applied to the isolated benzene (C 6 H 6) molecule. The quasiparticle energy levels and especially the energies of the benzene excitons (triplet, singlet, optically active and inactive) are in very good agreement with available experimental results. It is shown that the vicinity of the various substrates (pristine/doped graphene or (jellium) metal surface) reduces the quasiparticle HOMO-LUMO gap by an amount that slightly depends on the substrate type. This is consistent with the simple image theory predictions. It is even shown that the substrate does not change the energy of the excitons in the isolated molecule. We prove (in terms of simple image theory) that energies of the excitons are indeed influenced by two mechanisms which cancel each other. We demonstrate that the benzene singlet optically active (E 1u) exciton couples to real electronic excitations in the substrate. This causes it substantial decay, such as Γ ≈ 174 meV for pristine graphene and Γ ≈ 362 meV for metal surfaces as the substrate. However, we find that doping graphene does not influence the E 1u exciton decay rate.
The Editors would like to thank all those people who have offered their help by reviewing manuscr... more The Editors would like to thank all those people who have offered their help by reviewing manuscripts for Journal of Modern Optics. The success of the Journal is largely dependent on the quality of reviews we receive, and so your assistance in assuring high standards in this way is greatly appreciated. The Editors would like to extend a special thanks to those reviewers marked (*) below who, on three or more occasions in 2011, have provided timely and insightful reviews that have greatly assisted the Editors in deciding what to publish, and in helping authors to improve their manuscripts.
Pristine single-walled carbon nanotubes (SWCNTs) are rather inert to O 2 and N 2 , which for low ... more Pristine single-walled carbon nanotubes (SWCNTs) are rather inert to O 2 and N 2 , which for low doses chemisorb only on defect sites or vacancies of the SWCNTs at the ppm level. However, very low doping has a major effect on the electronic properties and conductivity of the SWCNTs. Already at low O 2 doses (80 L), the X-ray photoelectron spectroscopy (XPS) O 1s signal becomes saturated, indicating nearly all the SWCNT's vacancies have been oxidized. As a result, probing vacancy oxidation on SWCNTs via XPS yields spectra with rather low signal-to-noise ratios, even for metallicity-sorted SWCNTs. We show that, even under these conditions, the first principles density functional theory calculated Kohn-Sham O 1s binding energies may be used to assign the XPS O 1s spectra for oxidized vacancies on SWCNTs into its individual components. This allows one to determine the specific functional groups or bonding environments measured. We find the XPS O 1s signal is mostly due to three O-containing functional groups on SWCNT vacancies: epoxy (C 2 >O), carbonyl (C 2 >C=O), and ketene (C=C=O), as ordered by abundance. Upon oxidation of nearly all the SWCNT's vacancies, the central peak's intensity for the metallic SWCNT sample is 60% greater than for the semiconducting SWCNT sample. This suggests a greater abundance of O-containing defect structures on the metallic SWCNT sample. For both metallic and semiconducting SWCNTs, we find O 2 does not contribute to the measured XPS O 1s spectra.
Journal of chemical theory and computation, Jan 31, 2016
Optimizing the photovoltaic efficiency of dye-sensitized solar cells (DSSC) based on staggered ga... more Optimizing the photovoltaic efficiency of dye-sensitized solar cells (DSSC) based on staggered gap heterojunctions requires a detailed understanding of sub-band gap transitions in the visible from the dye directly to the substrate's conduction band (CB) (type-II DSSCs). Here, we calculate the optical absorption spectra and spatial distribution of bright excitons in the visible region for a prototypical DSSC, catechol on rutile TiO2(110), as a function of coverage and deprotonation of the OH anchoring groups. This is accomplished by solving the Bethe-Salpeter equation (BSE) based on hybrid range-separated exchange and correlation functional (HSE06) density functional theory (DFT) calculations. Such a treatment is necessary to accurately describe the interfacial level alignment and the weakly bound charge transfer transitions that are the dominant absorption mechanism in type-II DSSCs. Our HSE06 BSE spectra agree semiquantitatively with spectra measured for catechol on anatase TiO...
The discovery of carbon nanotubes by Iijima in 1991 has created a torrent of new research activit... more The discovery of carbon nanotubes by Iijima in 1991 has created a torrent of new research activities. Research on carbon nanotubes ranges from studying their fundamental properties, such as their electron band structure and plasma frequencies, to developing new applications, such as self-assembled nano-circuits and field emission displays. Robust models are now needed to enable a better understanding of the electronic response of carbon nanotubes. We use timedependent density functional theory to derive a two-fluid two-dimensional (2D) hydrodynamic model describing the collective response of a multiwalled carbon nanotube with dielectric media embedded inside or surrounding the nanotube. We study plasmon hybridization of the nanotube system in the UV range, the stopping force for ion channelling, the dynamical image potential for fast ions, channelled diclusters and point dipoles, and the energy loss for ions with oblique trajectories. Comparisons are made of results obtained from th...
We present a general approach to the computational design of nanostructured chemical sensors. The... more We present a general approach to the computational design of nanostructured chemical sensors. The scheme is based on identification and calculation of microscopic descriptors (design parameters) which are used as input to a thermodynamic model to obtain the relevant macroscopic properties. In particular, we consider the functionalization of a (6,6) metallic armchair single-walled carbon nanotube (SWNT) by nine different 3d transition metal (TM) atoms occupying three types of vacancies. For six gas molecules (N 2 , O 2 , H 2 O, CO, NH 3 , H 2 S) we calculate the binding energy and change in conductance due to adsorption on each of the 27 TM sites. For a given type of TM functionalization, this allows us to obtain the equilibrium coverage and change in conductance as a function of the partial pressure of the "target" molecule in a background of atmospheric air. Specifically, we show how Ni and Cu doped metallic (6,6) SWNTs may work as effective multifunctional sensors for both CO and NH 3. In this way, the scheme presented allows one to obtain macroscopic device characteristics and performance data for nanoscale (in this case SWNT) based devices.
A thorough understanding of the electronic structure is a necessary first step for the design of ... more A thorough understanding of the electronic structure is a necessary first step for the design of nanoelectronics, chemical/bio-sensors, electrocatalysts, and nanoplasmonics using graphene. As such, theoretical spectroscopic tech-niques to describe collective excitations of graphene are of fundamental importance. Starting from density functional theory (DFT), linear response time dependent DFT in frequency-reciprocal space within the random phase approxima-tion (TDDFT-RPA) is used to describe the loss function −={ε−1(q, ω)} for isolated graphene. To ensure any spurious interactions between layers are removed, both a radial cutoff of the Coulomb kernel, and extra vacuum directly at the TDDFT-RPA level are employed. A combination of both methods is found to provide a correct description of the electron energy loss spectra of isolated graphene, at a significant reduction in computational cost compared to standard methods. 1 Introduction In the last ten years, graphene [1,2, 3,4,5,6,7,8,...
∗Nano-Bio Spectroscopy Group and ETSF Scientific Development Centre, Depto. Fı́sica de Materiales... more ∗Nano-Bio Spectroscopy Group and ETSF Scientific Development Centre, Depto. Fı́sica de Materiales, Universidad del Paı́s Vasco and DIPC, Avenida de Tolosa 72, E-20018 San Sebastián, Spain; Phone:+34 943 01 8288 Fax:+34 943 01 8390 E-mail: [email protected] †Nano-Bio Spectroscopy Group and ETSF Scientific Development Centre, Depto. Fı́sica de Materiales, Universidad del Paı́s Vasco, Centro de Fı́sica de Materiales CSIC-UPV/EHUMPC and DIPC, Av. Tolosa 72, E-20018 San Sebastián, Spain; Center for Atomic-scale Materials Design, Department of Physics, Technical University of Denmark, DK-2800 Kgs. Lyngby, Denmark; Phone:+45 4525 3209 Fax:+45 4593 2399 E-mail: [email protected] ‡Nano-Bio Spectroscopy Group and ETSF Scientific Development Centre, Depto. Fı́sica de Materiales, Universidad del Paı́s Vasco and DIPC, Avenida de Tolosa 72, E-20018 San Sebastián, Spain; E-mail: [email protected] §Nano-Bio Spectroscopy Group and ETSF Scientific Development Centre, Depto. Fı́sica...
The recent development of efficient chirality sorting techniques has opened the way to the use of... more The recent development of efficient chirality sorting techniques has opened the way to the use of singlewalled carbon nanotubes (SWCNTs) in a plethora of nanoelectronic, photovoltaic, and optoelectronic applications. However, to understand the excitation processes undergone by SWCNTs, it is necessary to have highly efficient and accurate computational methods to describe their optical and electronic properties, methods which have until now been unavailable. Here we employ linear combinations of atomic orbitals (LCAOs) to represent the Kohn-Sham (KS) wavefunctions and perform highly efficient time dependent density functional theory (TDDFT) calculations in the frequency domain using our LCAO-TDDFT-k-ω code to model the optical absorbance and energy loss spectra and spatial distribution of the exciton charge densities in SWCNTs. By applying the GLLB-SC derivative discontinuity correction to the KS eigenenergies, we reproduce the measured E 11 and E 22 transitions within σ 70 meV and the optical absorbance and electron energy loss spectra semiquantitatively for a set of fifteen semiconducting and four metallic chirality sorted SWCNTs. Furthermore, our calculated electron hole density difference ∆ρ(r, ω) resolves the spatial distribution of the measured excitations in SWCNTs. These results open the path towards the computational design of optimized SWCNT nanoelectronic, photovoltaic, and optoelectronic devices in silico.
A necessary first step in the development of technologies such as artificial photosynthesis is un... more A necessary first step in the development of technologies such as artificial photosynthesis is understanding the photoexcitation process within the basic building blocks of naturally occurring light harvesting complexes (LHCs). The most important of these building blocks in biological LHCs such as LHC II from green plants are the chlorophyll a (Chl a) and chlorophyll b (Chl b) chromophores dispersed throughout the protein matrix. However, efforts to describe such systems are still hampered by the lack of computationally efficient and accurate methods that are able to describe optical absorption in large biomolecules. In this work, we employ a highly efficient linear combination of atomic orbitals (LCAOs) to represent the Kohn-Sham (KS) wave functions at the density functional theory (DFT) level and perform time-dependent density functional theory (TDDFT) calculations in either the reciprocal space and frequency domain (LCAO-TDDFT-k-ω) or real space and time domain (LCAO-TDDFT-r-t) of the optical absorption spectra of Chl a and b monomers and dimers. We find that our LCAO-TDDFT-k-ω and LCAO-TDDFT-r-t calculations reproduce results obtained with a plane-wave (PW) representation of the KS wave functions (PW-TDDFT-k-ω) but with a significant reduction in computational effort. Moreover, by applying the Gritsenko, van Leeuwen, van Lenthe, and Baerends solid and correlation derivative discontinuity correction Δx to the KS eigenenergies, with both LCAO-TDDFT-k-ω and LCAO-TDDFT-r-t methods, we are able to semiquantitatively reproduce the experimentally measured photoinduced dissociation results. This work opens the path to first principles calculations of optical excitations in macromolecular systems.
Understanding the interaction of light with molecules physisorbed on substrates is a fundamental ... more Understanding the interaction of light with molecules physisorbed on substrates is a fundamental problem in photonics, with applications in biosensing, photovoltaics, photocatalysis, plasmonics, and nanotechnology. However, the design of novel functional materials in silico is severely hampered by the lack of robust and computationally efficient methods for describing both molecular absorbance and screening on substrates. Here we employ our hybrid G 0 [W 0 + ∆W]-BSE implementation, which incorporates the substrate via its screening ∆W at both the quasiparticle G 0 W 0 level and when solving the Bethe-Salpeter equation (BSE). We show this method can be used to both efficiently and accurately describe the absorption spectra of physisorbed molecules on metal substrates and thereby tailor the molecule's absorbance by altering the surface plasmon's energy. Specifically, we investigate how the optical absorption spectra of three prototypical π-conjugated molecules: benzene (C 6 H 6), terrylene (C 30 H 16) and fullerene (C 60), depends on the Wigner-Seitz radius r s of the metallic substrate. To gain further understanding of the light-molecule/substrate interaction, we also study the bright exciton's electron and hole densities and their interactions with infrared active vibrational modes. Our results show that (1) benzene's bright E 1 1u exciton at 7.0 eV, whose energy is insensitive to changes in r s , could be relevant for photocatalytic dehydrogenation and polymerization reactions, (2) terrylene's bright B 3u exciton at 2.3 eV hybridizes with the surface plasmon, allowing the tailoring of the excitonic energy and optical activation of a surface plasmon-like exciton, and (3) fullerene's π − π * bright and dark excitons at 6.4 and 6.8 eV hybridize with the surface plasmon, resulting in the tailoring of their excitonic energy and the activation of both a surface plasmon-like exciton and a dark quadrupolar mode via symmetry breaking by the substrate. This work demonstrates how a proper description of interfacial light-molecular/substrate interactions enables the prediction, design, and optimization of technologically relevant phenomena in silico.
The internal quantum efficiency (IQE) of an organic photovoltaic device (OPV) is proportional to ... more The internal quantum efficiency (IQE) of an organic photovoltaic device (OPV) is proportional to the number of free charge carriers generated and their conductivity, per absorbed photon. However, both the IQE and the quantities that determine it, for example, electron-hole binding, charge separation, electron-hole recombination, and conductivity, can only be inferred indirectly from experiments. Using density functional theory, we calculate the excited-state formation energy, charge transfer, and zero-bias conductance in the singlet ground state and triplet excited state across polymer/fullerene and polymer/single walled carbon nanotube (SWNT) OPV donor/acceptor bulk heterojunctions. Specifically, we compare polythiophene (PT) and poly(3-methylthiophene-2,5-diyl) (P3MT) as donors and C 60 chains with (6,4), (6,5), and (10,5) SWNTs as acceptors. We find the conductivity increases substantially for both the excited triplet relative to the singlet ground state and for PT compared with P3MT due to the increased charge transfer and the resulting improvement in donor/acceptor level alignment. Similarly, the (6,4) SWNT, with a larger SWNT band gap and greater conductivity than fullerenes, provides the highest conductivities of 5 and 9% of the theoretical maximum for electron and hole carriers, respectively. This work has important implications for both the optimization of polymer/SWNT bulk heterojunctions and the design of new OPV bulk heterojunctions in silico.
We study the photoabsorption properties of photoactive bulk polymer/ fullerene/nanotube heterojun... more We study the photoabsorption properties of photoactive bulk polymer/ fullerene/nanotube heterojunctions in the near-infrared region. By combining pump-probe spectroscopy and linear response time-dependent density functional theory within the random phase approximation (TDDFT-RPA) we elucidate the excited state dynamics of the E 11 transition within (6,5) and (7,5) single-walled carbon nanotubes (SWNTs) and combined with poly(3-hexylthiophene-2,5-diyl) (P3HT) and [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) in P3HT/PCBM/SWNT blended samples. We find the presence of a photoinduced absorption (PA) peak is related mainly to the width of the photobleach (PB) peak and the charge carrier density of the SWNT system. For mixed SWNT samples, the PB peak is too broad to observe the PA peak, whereas within P3HT/PCBM/SWNT blended samples P3HT acts as a hole acceptor, narrowing the PB peak by exciton delocalization, which reveals a PA peak. Our results suggest that the PA peak originates from a widening of the band gap in the presence of excited electrons and holes. These results have important implications for the development of new organic photovoltaic heterojunctions including SWNTs.
We perform state-of-the-art calculations for a prototypical dye sensitized solar cell: catechol o... more We perform state-of-the-art calculations for a prototypical dye sensitized solar cell: catechol on rutile TiO 2 (110). Catechol is often used as an anchoring group for larger more complex organic and inorganic dyes on TiO 2 and forms a type II heterojunctions on TiO 2 (110). In particular, we compare quasiparticle (QP) G 0 W 0 with hybrid exchange correlation functional (HSE) density functional theory (DFT) calculations for the catecholrutile TiO 2 (110) interface. In so doing, we provide a theoretical interpretation of ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES) experiments for this prototypical system. Specifically, we demonstrate that the position, presence, and intensity of peaks associated with catechol's HOMO, intermolecular OH-O bonds, and interfacial hydrogen bonds to the surface bridging O atoms (O br H-C and O br H-O) may be used to fingerprint deprotonation of catechol's OH anchoring groups. Furthermore, our results suggest deprotonation of these groups, while being nearly isoenergetic at high coverages, may significantly increase the photovoltaic efficiency of catechol-TiO 2 (110) interfaces.
X-ray photoelectron spectroscopy combined with first-principles modeling is a powerful tool for d... more X-ray photoelectron spectroscopy combined with first-principles modeling is a powerful tool for determining the chemical composition and electronic structure of novel materials. Of these, graphene is an especially important model system for understanding the properties of other carbon nanomaterials. Here, we calculate the carbon 1s core level binding energy of pristine graphene using two methods based on density functional theory total energy differences: a calculation with an explicit core-hole, and an all-electron extension of the delta self-consistent field (SCF) method. We study systematically their convergence and computational workload, and the dependence of the energies on the chosen exchange-correlation functional. The SCF method is computationally more expensive, but gives consistently higher C 1s energies. Although there is a significant functional dependence, the binding energy calculated using the PBE functional is found to be remarkably close to what has been measured for graphite.
We extend the two-fluid, two-dimensional hydrodynamic model, describing the collective response o... more We extend the two-fluid, two-dimensional hydrodynamic model, describing the collective response of a multiwalled carbon nanotube, to include solid dielectric media embedded inside, or surrounding the nanotube. In the case of aluminum, we study plasmon hybridization in the UV range and find that the nanotube's low-energy, quasiacoustic plasmons are remarkably stable, whereas the shift of the high-energy + plasmons strongly depends on the gap between the nanotube and the aluminum boundary. Moreover, we study the stopping force on fast ions moving inside nanotubes encapsulated in channels of insulating, amorphous carbon, and metallic materials, with special attention paid to nickel. Finally, our study of the dynamical image potential for fast ions shows that a nanotube will effectively shield the influence of an embedded nickel wire.
Narrow diameter tubes and especially ͑6,5͒ tubes with a diameter of 0.75 nm are currently one of ... more Narrow diameter tubes and especially ͑6,5͒ tubes with a diameter of 0.75 nm are currently one of the most studied carbon nanotubes because their unique optical and especially luminescence response makes them exceptionally suited for biomedical applications. Here we report on a detailed analysis of the electronic structure of nanotubes with ͑6,5͒ and ͑6,4͒ chiralities using a combined experimental and theoretical approach. From high-energy spectroscopy involving x-ray absorption and photoemission spectroscopy the detailed valence-and conduction-band response of these narrow diameter tubes is studied. The observed electronic structure is in sound agreement with state of the art ab initio calculations using density-functional theory.
We present an alternative methodology for calculating the quasi-particle energy, energy loss, and... more We present an alternative methodology for calculating the quasi-particle energy, energy loss, and optical spectra of a molecule deposited on graphene or a metallic substrate. To test the accuracy of the method it is first applied to the isolated benzene (C 6 H 6) molecule. The quasiparticle energy levels and especially the energies of the benzene excitons (triplet, singlet, optically active and inactive) are in very good agreement with available experimental results. It is shown that the vicinity of the various substrates (pristine/doped graphene or (jellium) metal surface) reduces the quasiparticle HOMO-LUMO gap by an amount that slightly depends on the substrate type. This is consistent with the simple image theory predictions. It is even shown that the substrate does not change the energy of the excitons in the isolated molecule. We prove (in terms of simple image theory) that energies of the excitons are indeed influenced by two mechanisms which cancel each other. We demonstrate that the benzene singlet optically active (E 1u) exciton couples to real electronic excitations in the substrate. This causes it substantial decay, such as Γ ≈ 174 meV for pristine graphene and Γ ≈ 362 meV for metal surfaces as the substrate. However, we find that doping graphene does not influence the E 1u exciton decay rate.
The Editors would like to thank all those people who have offered their help by reviewing manuscr... more The Editors would like to thank all those people who have offered their help by reviewing manuscripts for Journal of Modern Optics. The success of the Journal is largely dependent on the quality of reviews we receive, and so your assistance in assuring high standards in this way is greatly appreciated. The Editors would like to extend a special thanks to those reviewers marked (*) below who, on three or more occasions in 2011, have provided timely and insightful reviews that have greatly assisted the Editors in deciding what to publish, and in helping authors to improve their manuscripts.
Pristine single-walled carbon nanotubes (SWCNTs) are rather inert to O 2 and N 2 , which for low ... more Pristine single-walled carbon nanotubes (SWCNTs) are rather inert to O 2 and N 2 , which for low doses chemisorb only on defect sites or vacancies of the SWCNTs at the ppm level. However, very low doping has a major effect on the electronic properties and conductivity of the SWCNTs. Already at low O 2 doses (80 L), the X-ray photoelectron spectroscopy (XPS) O 1s signal becomes saturated, indicating nearly all the SWCNT's vacancies have been oxidized. As a result, probing vacancy oxidation on SWCNTs via XPS yields spectra with rather low signal-to-noise ratios, even for metallicity-sorted SWCNTs. We show that, even under these conditions, the first principles density functional theory calculated Kohn-Sham O 1s binding energies may be used to assign the XPS O 1s spectra for oxidized vacancies on SWCNTs into its individual components. This allows one to determine the specific functional groups or bonding environments measured. We find the XPS O 1s signal is mostly due to three O-containing functional groups on SWCNT vacancies: epoxy (C 2 >O), carbonyl (C 2 >C=O), and ketene (C=C=O), as ordered by abundance. Upon oxidation of nearly all the SWCNT's vacancies, the central peak's intensity for the metallic SWCNT sample is 60% greater than for the semiconducting SWCNT sample. This suggests a greater abundance of O-containing defect structures on the metallic SWCNT sample. For both metallic and semiconducting SWCNTs, we find O 2 does not contribute to the measured XPS O 1s spectra.
Journal of chemical theory and computation, Jan 31, 2016
Optimizing the photovoltaic efficiency of dye-sensitized solar cells (DSSC) based on staggered ga... more Optimizing the photovoltaic efficiency of dye-sensitized solar cells (DSSC) based on staggered gap heterojunctions requires a detailed understanding of sub-band gap transitions in the visible from the dye directly to the substrate's conduction band (CB) (type-II DSSCs). Here, we calculate the optical absorption spectra and spatial distribution of bright excitons in the visible region for a prototypical DSSC, catechol on rutile TiO2(110), as a function of coverage and deprotonation of the OH anchoring groups. This is accomplished by solving the Bethe-Salpeter equation (BSE) based on hybrid range-separated exchange and correlation functional (HSE06) density functional theory (DFT) calculations. Such a treatment is necessary to accurately describe the interfacial level alignment and the weakly bound charge transfer transitions that are the dominant absorption mechanism in type-II DSSCs. Our HSE06 BSE spectra agree semiquantitatively with spectra measured for catechol on anatase TiO...
The discovery of carbon nanotubes by Iijima in 1991 has created a torrent of new research activit... more The discovery of carbon nanotubes by Iijima in 1991 has created a torrent of new research activities. Research on carbon nanotubes ranges from studying their fundamental properties, such as their electron band structure and plasma frequencies, to developing new applications, such as self-assembled nano-circuits and field emission displays. Robust models are now needed to enable a better understanding of the electronic response of carbon nanotubes. We use timedependent density functional theory to derive a two-fluid two-dimensional (2D) hydrodynamic model describing the collective response of a multiwalled carbon nanotube with dielectric media embedded inside or surrounding the nanotube. We study plasmon hybridization of the nanotube system in the UV range, the stopping force for ion channelling, the dynamical image potential for fast ions, channelled diclusters and point dipoles, and the energy loss for ions with oblique trajectories. Comparisons are made of results obtained from th...
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Papers by Duncan Mowbray