Clemens Burda

The electronic origin of the visible-light absorption properties of C-, N- and S-doped TiO2 nanomaterials.

The origin of the visible-light absorption of main-group element (C, N, S) doped TiO2 nanostructures is investigated via diffuse reflectance and valence band X-ray photoelectron spectroscopy. The synthesized C-, N-, and S-doped titania nanomaterials show an increased electron density of states above the valence band of TiO2, which explains the red-shifted light absorption of these potential photocatalysts and simultaneously suggests a lowered potential as photooxidants compared to Degussa P25 TiO2.

The unique role of nanoparticles in nanomedicine: imaging, drug delivery and therapy.

This critical review will present the role of nanoparticles (NPs) in the directions that are vital to the new field of nanomedicine, including imaging and drug delivery. We reflect on the physical properties that make NPs advantageous for in vivo efficacy, and review recent advances in major NP based biomedical applications. Critical questions of transport, uptake, and clearance will be discussed and illustrated through the success and opportunities of NP imaging and therapy on a photodynamic therapy (PDT) based NP system that has been developed in our lab over the past decade (540 references).

Plasmonic Cu2−xS Nanocrystals: Optical and Structural Properties of Copper-Deficient Copper(I) Sulfides

Cu(2-x)S (x = 1, 0.2, 0.03) nanocrystals were synthesized with three different chemical methods: sonoelectrochemical, hydrothermal, and solventless thermolysis methods in order to compare their common optical and structural properties. The compositions of the Cu(2-x)S nanocrystals were varied from CuS (covellite) to Cu(1.97)S (djurleite) through adjusting the reduction potential in the sonoelectrochemical method, adjusting the pH value in the hydrothermal method and by choosing different precursor pretreatments in the solventless thermolysis approach, respectively. The crystallinity and morphology of the products were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM), which shows that most of them might be of pure stoichiometries but some of them are mixtures. The obtained XRDs were studied in comparison to the XRD patterns of previously reported Cu(2-x)S. We found consistently that under ambient conditions the copper deficient Cu(1.97)S (djurleite) is more stable than Cu(2)S (chalcocite). Corroborated by recent computational studies by Lambrecht et al. and experimental work by Alivisatos et al. This may be the reason behind the traditionally known instability of the bulk Cu(2)S/CdS interface. Both Cu(2)S and the copper-deficient Cu(1.97)S have very similar but distinguishable electronic and crystal structure. The optical properties of these Cu(2-x)S NCs were characterized by UV-vis spectroscopy and NIR. All presented Cu(2-x)S NCs show a blue shift in the band gap absorption compared to bulk Cu(2-x)S. Moreover the spectra of these Cu(2-x)S NCs indicate direct band gap character based on their oscillator strengths, different from previously reported experimental results. The NIR spectra of these Cu(2-x)S NCs show a carrier concentration dependent plasmonic absorption.

Quantum Dot‐based Energy Transfer: Perspectives and Potential for Applications in Photodynamic Therapy

Abstract Quantum dots have emerged as an important class of material that offers great promise to a diverse range of applications ranging from energy conversion to biomedicine. Here, we review the potential of using quantum dots and quantum dot conjugates as sensitizers for photodynamic therapy (PDT). The photophysics of singlet oxygen generation in relation to quantum dot-based energy transfer is discussed and the possibility of using quantum dots as photosensitizer in PDT is assessed, including their current limitations to applications in biological systems. The biggest advantage of quantum dots over molecular photosensitizers that comes into perspective is their tunable optical properties and surface chemistries. Recent developments in the preparation and photophysical characterization of quantum dot energy transfer processes are also presented in this review, to provide insights on the future direction of quantum dot-based photosensitization studies from the viewpoint of our ongoing research.

Femtosecond Time-Resolved Transient Absorption Spectroscopy of CH3NH3PbI3 Perovskite Films: Evidence for Passivation Effect of PbI2

CH3NH3PbI3 perovskite layered films deposited on substrates with and without a titania support structure have been prepared and studied using time-resolved femtosecond transient absorption (fs-TA) spectroscopy in the visible light range (450-800 nm). The electron injection dynamics from the photoexcited perovskite layers to the neighboring film structures could be directly monitored via the transient bleaching dynamics of the perovskite at ∼750 nm and thus systematically studied as a function of the layer-by-layer architecture. In addition, for the first time we could spectrally distinguish transient bleaching at ∼750 nm from laser-induced fluorescence that occurs red-shifted at ∼780 nm. We show that an additional bleach feature at ∼510 nm appears when PbI2 is present in the perovskite film. The amplitudes of the PbI2 and perovskite TA peaks were compared to estimate relative amounts of PbI2 in the samples. Kinetic analysis reveals that perovskite films with less PbI2 show faster relaxation rates than those containing more PbI2. These fast dynamics are attributed to charge carrier trapping at perovskite grain boundaries, and the slower dynamics in samples containing PbI2 are due to a passivation effect, in line with other recently reported work.

Electron dynamics in gold and gold–silver alloy nanoparticles: The influence of a nonequilibrium electron distribution and the size dependence of the electron–phonon relaxation

Electron dynamics in gold nanoparticles with an average diameter between 9 and 48 nm have been studied by femtosecond transient absorption spectroscopy. Following the plasmon bleach recovery after low power excitation indicates that a non-Fermi electron distribution thermalizes by electron–electron relaxation on a time scale of 500 fs to a Fermi distribution. This effect is only observed at low excitation power and when the electron distribution is perturbed by mixing with the intraband transitions within the conduction band (i.e., when the excitation wavelength is 630 or 800 nm). However, exciting the interband transitions at 400 nm does not allow following the early electron thermalization process. Electron thermalization with the lattice of the nanoparticle by electron–phonon interactions occurs within 1.7 ps under these conditions, independent of the excitation wavelength. In agreement with the experiments, simulations of the optical response arising from thermalized and nonthermalized electron distri...

TiO2 nanoparticles as functional building blocks.

Lixia Sang,† Yixin Zhao,‡ and Clemens Burda* †Key Laboratory of Enhanced Heat Transfer and Energy Conservation, Ministry of Education and Key Laboratory of Heat Transfer and Energy Conversion, Beijing Municipality, College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100124, China ‡School of Environmental Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China Center for Chemical Dynamics and Nanomaterials Research, Department of Chemistry, Case Western Reserve University, 10900 Euclid Avenue, Cleveland, Ohio 44106, United States

Development of plasmonic semiconductor nanomaterials with copper chalcogenides for a future with sustainable energy materials

The development of copper chalcogenide based nanomaterials as promising candidates for sustainable energy materials, due to their environmental compatibility and low toxicity, is presented and discussed in this review. Different solution syntheses have recently been developed for the low-cost preparation of copper chalcogenide nanocrystals, and their unique properties derived from copper deficiencies were investigated with much progress. The notorious compositional instability and defect formation of copper chalcogenide nanocrystals have been healed by forming donor–acceptor pairs in ternary and quaternary chalcogenide based nanocrystals, especially in copper zinc tin sulfides (CZTS). In addition to their use as light absorbers in solar cells, copper chalcogenide nanocrystals have also been utilized in different applications, such as electrode materials in Li ion batteries and high efficiency counter electrodes in dye/quantum dot sensitized solar cells as well as for NIR photothermal therapy. All relate to their unique copper deficiency properties. The copper chalcogenide based nanomaterials are believed to be sustainable materials for future energy applications once the syntheses and property investigations have led to a more complete understanding of their physics.

Emergent properties resulting from type-II band alignment in semiconductor nanoheterostructures.

The development of elegant synthetic methodologies for the preparation of monocomponent nanocrystalline particles has opened many possibilities for the preparation of heterostructured semiconductor nanostructures. Each of the integrated nanodomains is characterized by its individual physical properties, surface chemistry, and morphology, yet, these multicomponent hybrid particles present ideal systems for the investigation of the synergetic properties that arise from the material combination in a non-additive fashion. Of particular interest are type-II heterostructures, where the relative band alignment of their constituent semiconductor materials promotes a spatial separation of the electron and hole following photoexcitation, a highly desirable property for photovoltaic applications. This article highlights recent progress in both synthetic strategies, which allow for material and architectural modulation of novel nanoheterostructures, as well as the experimental work that provides insight into the photophysical properties of type-II heterostructures. The effects of external factors, such as electric fields, temperature, and solvent are explored in conjunction with exciton and multiexciton dynamics and charge transfer processes typical for type-II semiconductor heterostructures.

Deep Penetration of a PDT Drug into Tumors by Noncovalent Drug-Gold Nanoparticle Conjugates

Efficient drug delivery to tumors is of ever-increasing importance. Single-visit diagnosis and treatment sessions are the goal of future theranostics. In this work, a noncovalent PDT cancer drug-gold nanoparticle (Au NP) conjugate system performed a rapid drug release and deep penetration of the drug into tumors within hours. The drug delivery mechanism of the PDT drug through Au NPs into tumors by passive accumulation was investigated via fluorescence imaging, elemental analysis, and histological staining. The pharmacokinetics of the conjugates over a 7-day test period showed rapid drug excretion, as monitored via the fluorescence of the drug in urine. Moreover, the biodistribution of Au NPs in this study period indicated clearance of the NPs from the mice. This study suggests that noncovalent delivery via Au NPs provides an attractive approach for cancer drugs to penetrate deep into the center of tumors.