Synthesis of Hierarchical ZnO/CdSSe Heterostructure Nanotrees

A two-step chemical vapor deposition procedure is here employed to prepare tree-like hierarchical ZnO/CdSSe hetero-nanostructures. The structures are ...

University of Delaware

The dynamics of heterogeneous electron transfer (ET) from the polycyclic aromatic chromophore perylene to nanostructured TiO2 anatase was investigated for two different anchor groups with transient absorption spectroscopy in an ultrahigh vacuum. Data from ultraviolet photoelectron spectroscopy and from linear absorption spectroscopy showed that the donor state of the chromophore was located around 900 meV above the lower edge of the conduction band. With the wide band limit fulfilled the rate of the heterogeneous ET reaction was only controlled by the strength of the electronic coupling and not reduced by Franck-Condon factors. Two different time constants for the electron transfer, i.e., 13 and 28 fs, were measured with carboxylic acid and phosphonic acid as the respective anchor groups. The difference in the ET time constants was explained with the different extension of the donor orbital onto the respective anchor group to reach the empty electronic states of the semiconductor. The time constants were extracted by means of a simple rate equation model. The validity of applying this model on this ultrafast time scale was verified by comparing the rate equation model with an optical Bloch equation model.

Role of molecular anchor groups in molecule-to-semiconductor electron transfer.

We present a Kerr-gated microscope capable of collecting diffraction-limited 2D fluorescence images with sub-100 fs time resolution. The concept is based on the insertion of a solid-state optical Kerr gate into a wide-field microscope. In addition to the considerably improved temporal resolution, the wide-field design allows for simultaneous tracking of several objects and ultrafast fluorescence lifetime imaging of doped and heterogeneous surfaces. The ultrafast fluorescence dynamics of gold nanoparticles is presented as an example of the capabilities of the instrument.

Femtosecond Kerr-gated wide-field fluorescence microscopy.

Optical Kerr gating is widely used in ultrafast measurements ranging from pulse characterization to spectroscopy and microscopy. We examined the efficiency and the temporal response of three cubic lattice Kerr media, YAG, GGG and BGO, and compared them with the well studied fused silica (fast response, low efficiency) and STO (high efficiency, slow response). YAG and GGG emerged as superior materials for ultrafast spectroscopy and microscopy applications thanks to their fast Kerr response and considerably higher gating efficiency than silica at low gating energies. Importantly, it was found that in collinear geometry all tested materials except STO are capable of reaching nearly 100% transmission.

Efficiency and temporal response of crystalline Kerr media in collinear optical Kerr gating.

A viologen derivative, 1,1'-di-p-tolyl-(4,4'-bipyridine)-1,1'-diium dichloride (DTV(2+)), was studied in solution and encapsulated in cucurbit[7]uril (CB7), a macrocyclic host. Upon encapsulation, DTV(2+) exhibited dramatically enhanced fluorescence. Aqueous solutions of DTV(2+) were weakly fluorescent (Φ = 0.01, τ < 20 ps), whereas the emission of the DTV(2+)@2CB7 complex was enhanced by 1 order of magnitude (Φ = 0.12, τ = 0.7 ns) and blue-shifted by 35 nm. Similar properties were observed in the presence of NaCl. DTV(2+) in a poly(methyl methacrylate) matrix was fluorescent with a spectrum similar to that observed for the complex in solution. (1)H NMR and UV-vis titrations indicated that the DTV(2+)@2CB7 complex is formed in aqueous solutions with complexation constants K(1) = (1.2 ± 0.3) × 10(4) M(-1) and K(2)= (1.0 ± 0.4) × 10(4) M(-1) in water. Density functional theory and configuration interaction singles calculations suggested that the hindrance of the rotational relaxation of the S(1) state of DTV(2+) caused by encapsulation within the host or a polymer matrix plays a key role in the observed emission enhancement. The absorption and emission spectra of DTV(2+)@2CB7 in water exhibited a large Stokes shift (ΔSt ~ 9000 cm(-1)) and no fine structure. DTV(2+) is a good electron acceptor [E°(DTV(2+)/DTV(•+)) = -0.30 V vs Ag/AgCl] and a strong photooxidant [E°(DTV*(2+)/DTV(•+)) = 0.09 V vs NHE]).

Fluorescence enhancement of di-p-tolyl viologen by complexation in cucurbit[7]uril.

The general case of a heterogeneous electron-transfer reaction is realized by ultrafast electron transfer from a light-absorbing molecule to a wide continuum of electronic acceptor states, realizing the so-called wide band limit. Experimental data obtained for perylene dye/TiO(2) systems confirm the predictions of fully quantum mechanical model calculations of the dynamics. The energy distribution of the injected electron shows an energy loss due to excitations of high-energy (quantum) vibrational modes in the ionized perylene moiety. The electron-transfer mechanism is non-adiabatic and the reaction is ultrafast, for example, with a time constant of 9 fs for the COOH anchor-bridge group. The underlying strong coupling of the electronic states to high-energy vibrational modes is a characteristic feature of sensitizer molecules.

Ultrafast photoinduced electron transfer at electrodes: the general case of a heterogeneous electron-transfer reaction.

The behavior of excitons and excess electrons in the confined space of a molecular polyoxotitanate cluster Ti17(μ4-O)4(μ3-O)16(μ2-O)4(OPr(i))20 (in short Ti17) was studied using femtosecond pump-probe transient absorption, pulse radiolysis, and fluorescence spectroscopy. Due to pronounced quantum size effects, the electronic spectra of the exciton, Ti17*, and the excess electron carrying radical anion, Ti17(•-), are blue-shifted in comparison with bulk TiO2 and have maxima at 1.91 and 1.24 eV, respectively. The 0.7 eV difference in the position of the absorption maxima of Ti17* and Ti17(•-) indicates the presence of strong Coulomb interaction between the conduction band electron and the valence band hole in the ∼1 nm diameter cluster. Ground state Raman spectra and the vibronic structure of the fluorescence spectrum point to the importance of the interfacial ligand modes in the stabilization and localization of the fully relaxed exciton. Four pentacoordinate Ti sites near the surface of the cluster appear to play a special role in this regard. Solvent polarity has only a minor influence on the spectral behavior of Ti17*. Exciton recombination in Ti17 is faster than in anatase nanoparticles or mesoporous films. The kinetics exhibits three components, ranging from less than 1 ps to 100 ps, which are tentatively assigned to the geminate recombination within the core of the cluster and to the decay of the surface stabilized charge transfer exciton. A persistent long-lived component with τ > 300 ps may indicate the involvement of intraband dark states, i.e., triplet excitons (3)Ti17*.

Excitons and excess electrons in nanometer size molecular polyoxotitanate clusters: electronic spectra, exciton dynamics, and surface states.

Ultrafast time-resolved measurements were performed on a novel pentafluorophenyl substituted 5,5-dimethyl phlorin derivative in solution and when attached to TiO2 colloidal films. The complex excited state dynamics of this porphyrinoid after S1 and S2 excitation was compared at different wavelengths and can be assigned to several subsequent relaxation mechanisms. The difference between excited state dynamics in the free molecule and when attached to an electron accepting electrode was measured. For both cases the dynamics was compared after excitation to the S1 and the S2 state. For the free molecule in solution an intermediate relaxation step was identified and assigned to a buckling motion of the tetrapyrrole ring. On the electrode, heterogeneous electron transfer (HET) times from both states were very similar and around 50 fs. Surprisingly, the large difference in the density of acceptor states that are resonant with the respective donor level of the molecule does not significantly influence HET dynamics. This result indicates that HET proceeds into intermediate transition states that are different from steady state surface states obtained from experiments or computations. The density of states (DOS) of these transient acceptor states appears not to be directly related to the corresponding surface or bulk DOS.

Electronic state dependence of heterogeneous electron transfer: injection from the S1 and S2 state of phlorin into TiO2.

This Perspective discusses recent developments in ultrafast electron transfer dynamics at interfaces between organic and inorganic materials. Heterogeneous electron transfer (HET) is a key process in important fields like catalysis and solar energy conversion. Furthermore, the solid state nature of the systems gives control over relevant parameters and allows for investigating excited state dynamics and electron transfer processes in unprecedented detail. Progress in synthesis, sample preparation, and instrumentation makes it possible to provide experimental proof of recent prediction from theory concerning the adiabaticity of the reaction and the influence of coherence. A short recapitulation of the field is followed by a discussion of recent experimental efforts that allowed for studying HET, particularly focusing on the influence of energetics and vibrational dynamics.

Photoinduced Ultrafast Heterogeneous Electron Transfer at Molecule-Semiconductor Interfaces.

A new tree-like ZnO/CdSSe nanocomposite with CdSSe branches grown on ZnO nanowires prepared via a two-step chemical vapor deposition is presented. The nanotrees (NTs) are vertically aligned on a substrate. The CdSSe branches result in strong visible light absorption and form a type-II heterojunction with the ZnO stem that facilitates efficient electron transfer. A combination of photoluminescence spectroscopy and lifetime measurements indicates that the NTs are promising materials for applications that benefit from a Z-scheme charge transfer mechanism. Vertically aligned branched ZnO nanowires can provide direct electron transport pathways to substrates and allow for efficient charge separation. These advantages of nanoscale hierarchical heterostructures make ZnO/CdSSe NTs a promising semiconductor material for solar cells, and other opto-electronic devices.

Efficient Z-scheme charge separation in novel vertically aligned ZnO/CdSSe nanotrees.

A Kerr-gated microscope capable of imaging ultraviolet luminescence with femtosecond time resolution has been developed. The system allows the spatial, spectral, and temporal measurement of UV-emitting samples. The instrumentation was optimized for emission collection in the UV, resulting in sub 90 fs time resolution of gated signals. ZnO nanowires were used to demonstrate the performance of the instrument. The evolution of the emission from a single nanowire was tracked via ultrafast transient spectroscopy and through sequential imaging. Transient dynamics were extracted from a region of intense emission on a single ZnO nanowire. This technique is a powerful tool capable of contactless ultrafast measurements of charge carrier dynamics in single nanoparticles.

Ultraviolet femtosecond Kerr-gated wide-field fluorescence microscopy.

Cyclic tetrapyrroles are the active core of compounds with crucial roles in living systems, such as hemoglobin and chlorophyll, and in technology as photocatalysts and light absorbers for solar energy conversion. Zinc-tetraphenylporphyrin (Zn-TPP) is a prototypical cyclic tetrapyrrole that has been intensely studied in past decades. Because of its importance for photochemical processes the optical properties are of particular interest, and, accordingly, numerous studies have focused on light absorption and excited-state dynamics of Zn-TPP. Relaxation after photoexcitation in the Soret band involves internal conversion that is preceded by an ultrafast process. This relaxation process has been observed by several groups. Hitherto, it has not been established if it involves a higher lying "dark" state or vibrational relaxation in the excited S2 state. Here we combine high time resolution electronic and vibrational spectroscopy to show that this process constitutes vibrational relaxation in the anharmonic S2 potential.

Lars Gundlach

Department of Chemistry & Biochemistry,
Department of Physics & Astronomy

University of Delaware

Synthesis of Hierarchical ZnO/CdSSe Heterostructure Nanotrees

Lars Gundlach

.css-o250i3{font-size:18px;font-family:Open Sans,sans-serif;line-height:21.6px;}Department of Chemistry & Biochemistry,Department of Physics & Astronomy

University of Delaware

Synthesis of Hierarchical ZnO/CdSSe Heterostructure Nanotrees

Department of Chemistry & Biochemistry,
Department of Physics & Astronomy