Light trapping and guidance in plasmonic nanocrystals

We illustrate the possibility of light trapping and funneling in periodic arrays of metallic nanoparticles. A controllable minimum in the transmission spectra of such constructs arises from a collective plasmon resonance phenomenon, where an incident plane wave sharply localizes in the vertical direction, remaining delocalized in the direction parallel to the crystal plane. Using hybrid arrays of different structures or different materials, we apply the trapping effect to structure the eigenmode spectrum, introduce overlapping resonances, and hence direct the light in space in a wavelength-sensitive fashion.     

A rational approach to emission ratio enhancement of chemodosimeters via regulation of intramolecular charge transfer

The sensitivity as well as dynamic range of a ratiometric probe is determined by the ratio of emission intensities at two wavelengths. Thus, it is highly desirable to acquire a large ratiometric fluorescence response at two wavelengths. However, ratiometric fluorescent signals are intrinsic characteristics of the particular probe-analyte interactions. The design for fluorescent probes with a large ratiometric signal remains a challenging task. There is still a lack of a proper approach to enhance the ratiometric fluorescence response for fluorescent chemodosimeters. Herein, we introduced a novel strategy to increase the emission ratios of a chemodosimeter via modulation of intramolecular charge transfer.     

Electronic structure of 1 to 2 nm diameter silicon core/shell nanocrystals: surface chemistry, optical spectra, charge transfer, and doping

Static and time-dependent density functional calculations, geometrically optimized and including all electrons, are described for silicon nanocrystals as large as Si(87)H(76), which contains 163 atoms. We explore and predict the effect that different sp(3) passivation schemes-F or H termination, thin oxide shell, or alkane termination-have on the HOMO and LUMO, on the optical spectra, and on electron transfer properties. Electronegativity comparisons are a useful guide in understanding the observed deviation from the simple quantum size effect model. Nanocrystals containing Al or P impurity atoms, either on the surface or in the interior, are explored to understand electrical doping in strongly quantum-confined nanocrystals. Surface dangling bonds are found to participate in internal charge transfer with P atom dopant electrons.     

Interfacial charge transfer absorption: Application to metal–molecule assemblies

Optically induced charge transfer between adsorbed molecules and a metal electrode was predicted by Hush to lead to new electronic absorption features, but has been only rarely observed experimentally. Interfacial charge transfer absorption (IFCTA) provides information concerning the barriers to charge transfer between molecules and the metal/semiconductor and the magnitude of the electronic coupling and could thus provide a powerful tool for understanding interfacial charge-transfer kinetics. Here, we utilize a previously published model [C. Creutz, B.S. Brunschwig, N. Sutin, J. Phys. Chem. B 109 (2005) 10251] to predict IFCTA spectra of metal–molecule assemblies and compare the literature observations to these predictions. We conclude that, in general, the electronic coupling between molecular adsorbates and the metal levels is so small that IFCTA is not detectable. However, few experiments designed to detect IFCTA have been done. We suggest approaches to optimizing the conditions for observing the process.     

Resonant electron transfer and luminescent enhancement in a toluene suspension of Si nanocrystals

Efficient resonant electron transfer from the surface bonding structure to the conduction band of quantum confined Si nanocrystals is observed by Si nanocrystals in a toluene suspension. Based on the electron transfer mechanism, the enhancement of photoluminescence originates from the band-to-band recombination in the p-type Si nanocrystals suspended in a toluene solution. The energy levels of the electrons in the Si nanocrystals chemisorbed with toluene molecules are calculated using the method of linear combination of atomic orbitals, and the characteristics of the obtained density of states is in good agreement with the observed photoluminescence properties.     

Intramolecular charge transfer and electronic absorption and luminescence spectra of fluoroquinolinones

TD-DFT study on the effect of donor and acceptor substituents on molecular orbital localization and charge distribution in fluoroquinolinone molecules showed that their photoexcitation is accompanied by electron density redistribution over particular fragments. Depending on the protolytic form, frontier molecular orbitals are localized on different fragments, whereas variation of substituents weakly affects localization of these orbitals.     

Atomically precise Au25(GSH)18 nanoclusters versus plasmonic Au nanocrystals: Evaluating charge impetus in solar water oxidation

Atomically precise metal nanoclusters (NCs) have been deemed as an emerging class of metal nanomaterials owing to fascinating size-dependent physicochemical properties, discrete energy band structure, and quantum confinement effect, which are distinct from conventional metal nanoparticles (NPs). Nevertheless, metal NCs suffer from photoinduced self-oxidative aggregation accompanied by in-situ transformation to metal NPs, markedly reducing the photosensitization of metal NCs. Herein, maneuvering the generic instability of metal NCs, we perform the charge transport impetus comparison between atomically precise metal NCs and plasmonic metal NPs counterpart obtained from in-situ self-transformation of metal NCs in photoelectrochemical (PEC) water splitting reaction. For conceptual demonstration, we proposed two quintessential heterostructures, which include TNTAs-Au₂₅ heterostructure fabricated by electrostatically depositing glutathione (GSH)-protected Au₂₅(GSH)₁₈ NCs on the TiO₂ nanotube arrays (TNTAs) substrate, and TNTAs-Au heterostructure constructed by triggering self-transformation of Au₂₅(GSH)₁₈ NCs to plasmonic Au NPs in TNTAs-Au₂₅ via calcination. The results indicate that photoelectrons produced over Au₂₅ NCs are superior to hot electrons of plasmonic Au NPs in stimulating the interracial charge transport toward solar water oxidation. This is mainly ascribed to the significantly accelerated carrier transport kinetics, prolonged carrier lifespan, and substantial photosensitization effect of Au₂₅ NCs compared with plasmonic Au NPs, resulting in the considerably enhanced PEC water splitting performance of TNTAs-Au₂₅ relative to plasmonic TNTAs-Au counterpart under visible light irradiation. Our work would provide important implications for rationally designing atomically precise metal NCs-based photosystems toward solar energy conversion.     

High-efficiency carrier multiplication and ultrafast charge separation in semiconductor nanocrystals studied via time-resolved photoluminescence

We demonstrate novel methods for the study of multiple exciton generation from a single photon absorption event (carrier multiplication) in semiconductor nanocrystals (or nanocrystal quantum dots) that are complementary to our previously reported transient absorption method. By monitoring the time dependence of photoluminescence (PL) from CdSe nanocrystals via time-correlated single photon counting, we find that carrier multiplication is observable due to the Auger decay of biexcitons. We compare these data with similar studies using transient absorption and find that the two methods give comparable results. In addition to the observation of dynamical signatures of carrier multiplication due to the Auger decay, we observe spectral signatures of multiple excitons produced from the absorption of a single photon. PL spectra at short times following excitation with high-energy photons are red-shifted compared to the single-exciton emission band, which is consistent with previous observations of significant exciton-exciton interactions in nanocrystals. We then show using a combination of transient absorption and time-resolved PL studies that charge transfer between a nanocrystal and a Ru-based catalyst model compound takes place on a time scale that is faster than Auger recombination time constants, which points toward a possible design of donor-acceptor assemblies that can be utilized to take advantage of the carrier multiplication process.     

Synthesis of lead chalcogenide nanocrystals and study of charge transfer in blends of PbSe nanocrystals and poly(3-hexylthiophene)

Nearly monodisperse lead chalcogenide (PbE, E = S, Se, or Te) semiconductor quantum dots of controllable shape have been produced via a novel synthesis which includes the occurrence of in situ formed Pb(0) particles. Tunable size and shape are achieved through appropriate choice of the precursor type and the stabilizer. As precursor, we use, on the one hand, lead oxide or lead acetate, on the other hand, tellurium, selenium, or sulfur powder dissolved in trioctylphosphine (TOP), tributylphosphine (TBP), or 1-octadecene (ODE). Oleic acid (OA) and various amines, as well as TOP and TBP are used for stabilization. With respect to possible application in hybrid solar cells, the surface of as-synthesized spherical PbSe nanocrystals was investigated by nuclear magnetic resonance (NMR), mass spectrometry (MS) and thermogravimetric analysis (TGA). As an important result, it was found that the surface is not mostly covered by oleic acid after synthesis, but by a phosphorus compound. We also applied a ligand exchange procedure with hexylamine and found evidence for the successful attachment of hexylamine to the nanocrystal surface. Additionally, charge separation between these nanoparticles and the conjugated polymer poly(3-hexylthiophene) (P3HT) is studied by electron spin resonance and photoinduced absorption spectroscopy. The spectra obtained suggest that charges can be produced successfully by photoinduced charge transfer.     

Efficient charge transfer and separation of TiO2@NiCo-LDH core-shell nanowire arrays for enhanced photoelectrochemical water-splitting

Journal of Solid State Electrochemistry - Efficient charge transfer and separation play a significant role in determining the photoelectrochemical (PEC) water-splitting performance of...     

Molecular wiring of nanocrystals: NCS-enhanced cross-surface charge transfer in self-assembled Ru-complex monolayer on mesoscopic oxide films

We report on rapid ambipolar cross-surface charge transfer within self-assembled monolayers (SAM) of the heteroleptic Ru-complexes cis-RuLL'(NCS)(2) (L = 2,2'-bipyridyl-4,4'-dicarboxylic acid, L' = 4,4'-dinonyl-2,2'-bipyridyl) (1) and cis-RuLL' '(NCS)(2) (L = 2,2'-bipyridyl-4,4'-dicarboxylic acid, L' = 4,4'-dimethyl-2,2'-bipyridyl) (2) on the surface of mesoscopic insulating oxide films. The bipyridyl ligands of the Ru-complex transport electrons, while the NCS groups plays a pivotal role in mediating surface confined hole percolation. Molecular dynamics calculations show the NCS ligands of 1 and 2 to orient in a fashion that enhances the overlap of the HOMOs of neighboring ruthenium complexes. Using ab initio Hartree-Fock calculations the electronic coupling matrix element for intermolecular hole exchange at the surface is estimated to be 0.13 eV. Cyclic voltammetry as well as spectroelectrochemical and impedance measurements performed with a series of other Ru-complexes confirmed the control of the cross surface charge transfer by the molecular structure. Complex 2 shows the highest percolation rate, the surface hole diffusion coefficient being 1.1 x 10(-8) cm(2)/s. The effects of the ligand properties, such as denticity, geometry, and size, on the intermolecular charge transport are discussed in detail.     

Assignment of charge transfer absorption band in optical absorption spectra of the adsorbate-silver colloid system

In this paper we reported the UV-visible-NIR optical absorption properties of silver colloid, employed as a high efficient substrate in surface-enhanced Raman spectra (SERS), under various conditions. Experimental results revealed that the new absorption band, usually appearing in the longer wavelength region due to the addition of molecules, was related to the direct adsorption of molecules on colloidal silver surface. When the adsorption occurred, this new band would appear. Once the molecules were desorbed from silver surface, the new band could not be observed. Some evidences inferred that the new absorption band was associated with the effect of charge-transfer transition between adsorbates and colloidal silver particles, while not with the effect of the surface plasma resonance due to the silver particles aggregation which was usually attributed to in previous research work.     

Surface charge transfer doping of graphene using a strong molecular dopant CN6-CP

Surface charge transfer doping of graphene plays an important role in graphene-based electronics due to its simplicity, high doping efficiency, and easy-controllability. Here, we demonstrate the effective surface charge transfer hole doping of graphene by using a strong p-type molecular dopant hexacyano-trimethylene-cyclopropane (CN6-CP). The CN6-CP exhibits a very high intrinsic work function of 6.37 eV, which facilitates remarkable electron transfer from graphene to CN6-CP as revealed by in situ photoelectron spectroscopy investigations. Consequently, hole accumulation appears in the graphene layer at the direct contact with CN6-CP. As evidenced by Hall effect measurements, the areal hole density of graphene significantly increased from 8.3 × 10¹² cm^?2 to 2.21 × 10¹³ cm^?2 upon 6 nm CN6-CP evaporation. The CN6-CP acceptor with strong p-doping effect has great implications for both graphene-based and organic electronics.     

Signature OH absorption spectrum from cluster models of solvation: a solvent-to-solute charge transfer state

Ab initio electronic structure theory calculations on cluster models support the characterization of the signature absorption spectrum of a solvated hydroxyl OH radical as a solvent-to-solute charge transfer state modulated by the hydrogen-bonding environment. Vertical excited states in OH(H2O)n clusters (n = 0-7, 16) calculated at the TDDFT level of theory (with companion calculations at the EOM-CCSD level of theory for n 相似文献   

Synthesis,characterization, and hydrolysis of PVAc-PS-PVAc via charge transfer polymerization

An ABA triblock copolymer of polyvinyl acetate-b-polystyrene-b-polyvinyl acetate (PVAc-PS-PVAc) was successfully synthesized with a binary system composed of polystyrene with N,N-dimethylaniline end groups (PS_da) and benzophenone to initiate the polymerization of vinyl acetate under UV irradiation. The PS_da was obtained by capping the living polystyrene macrodianion with p-(dimethylamino) benzaldehyde in excess. The PVA-PS-PVA could then be obtained by hydrolysis of PVAc-PS-PVAc in the sodium ethoxide benzene solution. The intermediates and desirable copolymers were characterized by GPC, IR, and ¹H-NMR in detail. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2595–2600, 1999     

MnO octahedral nanocrystals and MnO@C core-shell composites: synthesis, characterization, and electrocatalytic properties

We present a simple and facile synthesis of MnO octahedral nanocrystals and MnO@C core-shell composite nanoparticles. The synthesis is accomplished by a single-step direct pyrolysis of cetyltrimethylammonium permanganate in specially made Let-lock union cells. The products are characterized by HRSEM, HRTEM, Raman spectroscopy, and cyclic voltammetry (CV). The product consists mainly of octahedral MnO nanocrystals and MnO coated with carbon (MnO@C). The core-shell particles are observed only when the core size is smaller than 150 nm. The shape of the nanocrystals can be controlled by varying parameters such as reaction temperature and duration. As the temperature increases from 600 to 800 degrees C, the octahedral MnO crystals observed are without any carbon shell. The effect of time and temperature on the octahedral MnO nanocrystal formation is described. The electrocatalytic activities of the products are studied for oxygen reduction reaction in aqueous basic medium and are compared with bulk MnO. The MnO nanocrystals and core-shell composites exhibit higher activity than that of bulk MnO.     

Supramolecular fullerene/porphyrin charge transfer interaction studied by absorption spectrophotometric method

A detailed UV–Vis spectrometric and thermodynamic studies were done to look insight into the nature of molecular interactions of the electron donor–acceptor complexes of C₆₀ and C₇₀ with 5,10,15,20-tetrakis(octadecyloxyphenyl)-21H,23H-porphyrin (1) in chloroform and toluene. Charge transfer (CT) absorption bands were located in the visible region and vertical ionization potential of 1 was determined utilizing CT transition energy. Low values of oscillator and transition dipole strengths suggested that the complexes were almost of neutral character in ground states. The high binding constant value for the C₇₀–1 complex indicated high selectivity of 1 molecule towards C₇₀. Experimental as well as theoretically determined of enthalpies of formation value substantiated the trend in K values for fullerene–1 complexes.     

HEK-TCNQ的光诱导电荷转移与HEK-DDQ的基态电荷转移

合成了电荷转移复合物HEK-TCNQ和HEK-DDQ(HEK=9-hydroxyethylcarbazole)。拉曼光谱和吸收光谱测定表明: 光电导为10^-^1^1s.cm^-^1的HEK-TCNQ在514.5nm激光照射下可发生电荷转移, 生成HEK^+TCNQ^-, 其光电导显著增大。X射线结构分析和红外光谱表明: HEK与DDQ之间基态电荷转移量为0.1~0.2。     

Relating absorption, emission, and resonance Raman spectra with electron transfer rates in photoinduced charge transfer systems: promises and pitfalls

In molecules or complexes in which a photoinduced electronic transition corresponds to a formal electron transfer process, there exists a close relationship between the theoretical expressions for the absorption, fluorescence, and resonance Raman spectra and the nonphotochemical return electron transfer rate in the coupling limit. In this paper, the theoretical expressions cast in both the sum-over-vibrational-eigenstates picture and the equivalent time-domain picture are reviewed. Several recent efforts by ourselves and other groups to make use of such relationships in the analysis of experimental spectroscopic and electron transfer rate data are then described and critiqued. Finally, prospects for the further development of this approach are discussed, as are some of its limitations and potential pitfalls.     

Insights on charge transfer doping and intrinsic phonon line shape of carbon nanotubes by simple polymer adsorption

Doping of individual single-walled carbon nanotubes via noncovalent adsorption of polyethylenimine which converts p-type semiconducting nanotubes into n-type is examined by micro-Raman studies. Distinctively different responses are observed in metallic and in semiconducting nanotubes. Very little or no changes in the radial breathing and the disorder modes are observed upon polymer adsorption on semiconducting carbon nanotubes indicating noncovalent nature of this process. Tangential G-band spectral downshift of up to approximately 10 cm(-)(1) without line broadening is observed for semiconducting tubes suggesting similar magnitude of electron transfer as commonly observed in electrochemical doping with alkali metals. Strong diameter dependence is also observed and can be explained by thermal ionization of charge carriers with activation barrier that scales as the energy gap of the semiconducting nanotubes. In contrast, metallic nanotubes exhibit very different behavior with significant line broadening of the G-band and concurrent enhancement of the disorder mode. In certain cases, initially symmetric Lorentzian line shapes of the G-band features with narrow line widths similar to semiconducting tubes are converted to a broad, asymmetric Breit-Wigner-Fano line shape. Implications on the effects of electron injection and the local chemical environment on the intrinsic line shape of isolated carbon nanotubes are discussed.