skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Graphene Charge Transfer, Spectroscopy, and Photochemical Reactions

Abstract

This project focused on the special electronic and optical properties of graphene and adsorbed molecular species. Graphene makes an excellent substrate for current collection in nanostructured photovoltaic designs. Graphene is almost transparent, and can be used as a solar cell window. It also has no surface states, and thus current is efficiently transported over long distances. Progress in graphene synthesis indicates that there will soon be practical methods for making large pieces of graphene for devices. We now need to understand exactly what happens to both ground state and electronically excited molecules and Qdots near graphene, if we are going to use them to absorb light in a nano-structured photovoltaic device using graphene to collect photocurrent. We also need to understand how to shift the graphene Fermi level, to optimize the kinetics of electron transfer to graphene. And we need to learn how to convert local graphene areas to semiconductor structure, to make useful spatially patterned graphenes. In this final report, we describe how we addressed these goals. We explored the question of possible Surface Enhanced Raman spectroscopy from molecular Charge Transfer onto Graphene substrates. We observed strong hole doping of graphene by adsorbed halogens as indicated by the shiftmore » of the graphene G Raman band. In the case of iodine adsorption, we also observed the anionic species made by hole doping. At low frequency in the Raman spectrum, we saw quite intense lines from I 3 - and I 5 - , suggesting possible SERS. We reported on Fresnel calculations on this thin film system, which did not show any net electromagnetic field enhancement.« less

Authors:
 [1]
  1. Columbia Univ., New York, NY (United States)
Publication Date:
Research Org.:
Columbia Univ., New York, NY (United States)
Sponsoring Org.:
USDOE Office of Science (SC), Basic Energy Sciences (BES) (SC-22)
OSTI Identifier:
1341618
Report Number(s):
DOE-Columbia-SC0006521-1
DOE Contract Number:
SC0006521
Resource Type:
Technical Report
Country of Publication:
United States
Language:
English
Subject:
77 NANOSCIENCE AND NANOTECHNOLOGY; graphene

Citation Formats

Brus, Louis. Graphene Charge Transfer, Spectroscopy, and Photochemical Reactions. United States: N. p., 2017. Web. doi:10.2172/1341618.
Brus, Louis. Graphene Charge Transfer, Spectroscopy, and Photochemical Reactions. United States. doi:10.2172/1341618.
Brus, Louis. Tue . "Graphene Charge Transfer, Spectroscopy, and Photochemical Reactions". United States. doi:10.2172/1341618. https://www.osti.gov/servlets/purl/1341618.
@article{osti_1341618,
title = {Graphene Charge Transfer, Spectroscopy, and Photochemical Reactions},
author = {Brus, Louis},
abstractNote = {This project focused on the special electronic and optical properties of graphene and adsorbed molecular species. Graphene makes an excellent substrate for current collection in nanostructured photovoltaic designs. Graphene is almost transparent, and can be used as a solar cell window. It also has no surface states, and thus current is efficiently transported over long distances. Progress in graphene synthesis indicates that there will soon be practical methods for making large pieces of graphene for devices. We now need to understand exactly what happens to both ground state and electronically excited molecules and Qdots near graphene, if we are going to use them to absorb light in a nano-structured photovoltaic device using graphene to collect photocurrent. We also need to understand how to shift the graphene Fermi level, to optimize the kinetics of electron transfer to graphene. And we need to learn how to convert local graphene areas to semiconductor structure, to make useful spatially patterned graphenes. In this final report, we describe how we addressed these goals. We explored the question of possible Surface Enhanced Raman spectroscopy from molecular Charge Transfer onto Graphene substrates. We observed strong hole doping of graphene by adsorbed halogens as indicated by the shift of the graphene G Raman band. In the case of iodine adsorption, we also observed the anionic species made by hole doping. At low frequency in the Raman spectrum, we saw quite intense lines from I3- and I5- , suggesting possible SERS. We reported on Fresnel calculations on this thin film system, which did not show any net electromagnetic field enhancement.},
doi = {10.2172/1341618},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Tue Jan 31 00:00:00 EST 2017},
month = {Tue Jan 31 00:00:00 EST 2017}
}

Technical Report:

Save / Share:
  • Quantitative studies have been made of effects of sulfur compounds on photoreduction of benzophenone by amines. Aromatic mercaptans and disulfides are converted to photostationary equilibrium concentrations of the two forms, and retard photoreduction very efficiently, to a small extent by light absorption and quenching of triplet (approximately 10%), to a large extent (approximately 60%) by the repeated hydrogen transfer reactions, and the remainder by quenching of the charge transfer complex. Aliphatic disulfides are reduced, mercaptans are not oxidized, the two states are not equilibrated, and photoreduction by amines is accelerated by aliphatic mercaptans. The acceleration is attributed to catalysis ofmore » proton transfer in the charge-transfer complex. Ratios of rate constants for reduction of amine-derived radicals by mercaptans to oxidation by ketone are obtained. Effects of light absorption, triplet quenching and hydrogen transfer are calculated in retardation by mercaptans of photoreduction by alcohols. In reduction by 2-propanol and acetophenone, more hydrogen transfer is observed than would be calculated, indicating that thiyl radicals, in addition to ketone triplet, abstract hydrogen. In reduction by benzhydrol, retardation is due entirely to light absorption and quenching. Benzophenone ketyl radical is too highly stabilized for hydrogen abstraction to compete with radical demerization. Rate constants for abstraction of hydrogen from aromatic mercaptans by the ketyl radicals are estimated. In photoreduction of fluorenone by substituted dimethylanilines, low quantum yield due to an electron donating substituent is increased by decrease in polarity of solvent, and low quantum yield due to electron withdrawing substituents is increased by increase in polarity of solvent. The results are attributed to effects of substituents and solvent polarity on extent of charge transfer in the charge transfer complex.« less
  • Absorption of ultraviolet or visible light, or high energy radiation, may lead to highly reactive free radicals. Thiols affect the reactions of these radicals in the following ways: (1) transfer of hydrogen from sulfur of the thiol to a substrate radical, converting the radical to a stable molecule, and the thiol to a reactive thiyl radical; and (2) transfer of hydrogen from a substrate radical or molecule to thiyl, regenerating thiol. The thiol is thus used repeatedly and a single molecule may affect the consequences of many quanta. Three effects may ensue, depending upon the system irradiated: (1) the substratemore » radicals may be converted by thiol-thiyl to the original molecules, and protection against radiation damage is afforded. (2) The radicals may be converted to molecules not identical with the starting materials, and in both cases damage caused by radical combination processes is prevented. (3) Product yields may be increased where the initial radicals might otherwise regenerate starting materials. It was shown that rates of reaction of excited species can be correlated with triplet energies and reduction potentials, and with ionization potentials, that amines are very reactive toward excited carbonyl compounds of all types, and that yields of products from these reactions can be increased by thiols, leading to increased efficiency in utilization of light.« less
  • The relative importance of light absorption, quenching of triplet, and hydrogen transfer repair has been examined in retardation by mercaptans of photoreduction of aromatic ketones by alcohols. In the reduction of benzophenone by 2-propanol, retardation is efficient and, after correction for the first two effects, is due entirely to hydrogen-transfer repair, as indicated by deuterium labeling. In reduction of acetophenone by ..cap alpha..-methylbenzyl alcohol, repair by hydrogen transfer is also operative. In reduction of benzophenone by benzhydrol, retardation is less efficient and is due to quenching, as the ketyl radical does not abstract hydrogen from mercaptan rapidly in competition withmore » coupling. Deuterium isotope effects are discussed in terms of competitive reactions. Photoreduction of benzophenone by 2-butylamine and by triethylamine is retarded by aromatic mercaptans and disulfides. Of the retardation not due to light absorption and triplet quenching by the sulfur compounds, half is due to hydrogen-transfer repair, as indicated by racemization and deuterium labeling. The remainder is attributed to quenching by the sulfur compound of the charge-transfer-complex intermediate. Photoreduction by primary and secondary amines, but not by tertiary amines, is accelerated by aliphatic mercaptans. The acceleration is attributed to catalysis of hydrogen transfer by the mercaptan in the charge-transfer complex. The effect is large in hydrocarbon solvent, less in polar organic solvents and absent in water.« less
  • A systematic study of intramolecular photoassociation and photoinduced charge transfer (CT) was initiated in bichromophoric systems of M-X-M, where two identical aromatic hydrocarbons M are joined by X=CH[sub 2], O, NH, etc. Dinaphthylamines, dinaphthylethers, and dinaphthylmethanes in nonpolar solvents form triplet excimers, following inter system crossing of singlets to the triplet manifold; in polar solvents, the molecule forms an intramolecular CT state. The interchromophore interaction study was extended to N-phenyl-2-naphthylamine. The lowest excited singlet states of the dinaphthylamines were studied by semiempirical quantum chemical methods. Exciplex formation was studied in excited states of jet-cooled van der Waals complexes, such asmore » fluorene/substituted benzenes and 1-cyanonaphthalene-aliphatic amines.« less
  • Aluminosilicate zeolites provide an excellent host for photochemical charge separation. Because of the constraints provided by the zeolite, the back electron transfer from the reduced acceptor to the oxidized sensitizer is slowed down. This provides the opportunity to separate the charge and use it in a subsequent reaction for water oxidation and reduction. Zeolite-based ruthenium oxide catalysts have been found to be efficient for the water splitting process. This project has demonstrated the usefulness of zeolite hosts for photolytic splitting of water.