Sub-picosecond injection of electrons from excited [Ru(2,2′-bipy-4,4′-dicarboxy)₂(SCN)₂] into TiO₂ using transient mid-infrared spectroscopy

We have used femtosecond pump-probe spectroscopy to time resolve the injection of electrons into nanocrystalline TiO2 film electodes under ambient conditions following photoexcitation of the adsorbed dye, [Ru(4,4’-dicarboxy-2,2’-bipyridine)2(NCS)2] (N3). Pumping at one of the metal-to-ligand charge transfer adsorption peaks and probing the absorption of electrons injected into the TiO2 conduction band at 1.52 µm and in the range of 4.1 to 7.0 µm, we have directly observed the arrival of the injected electrons. Our measurements indicate an instrument-limited ~50-fs upper limit on the electron injection time under ambient conditions in air. We have compared the infrared transient absorption for noninjecting (blank) systems consisting of N3 in ethanol and N3 adsorbed to films of nanocrystalline Al2O3 and ZrO2, and found no indication of electron injection at probe wavelengths in the mid-IR (4.1 to 7.0 µm). At 1.52 µm interferences exist in the observed transient adsorption signal for the blanks

Real Time Spectroscopic Ellipsometry Analysis of First Stage CuIn1-xGaxSe2 Growth: Indium-Gallium Selenide Co-Evaporation

Real time spectroscopic ellipsometry (RTSE) has been applied for in-situ monitoring of the first stage of copper indium-gallium diselenide (CIGS) thin film deposition by the three-stage co-evaporation process used for fabrication of high efficiency thin film photovoltaic (PV) devices. The first stage entails the growth of indium-gallium selenide (In1-xGax)₂Se₃ (IGS) on a substrate of Mo-coated soda lime glass maintained at a temperature of 400 °C. This is a critical stage of CIGS deposition because a large fraction of the final film thickness is deposited, and as a result precise compositional control is desired in order to achieve the optimum performance of the resulting CIGS solar cell. RTSE is sensitive to monolayer level film growth processes and can provide accurate measurements of bulk and surface roughness layer thicknesses. These in turn enable accurate measurements of the bulk layer optical response in the form of the complex dielectric function ε = ε₁ - iε₂, spectra. Here, RTSE has been used to obtain the (ε₁, ε₂) spectra at the measurement temperature of 400 °C for IGS thin films of different Ga contents (x) deduced from different ranges of accumulated bulk layer thickness during the deposition process. Applying an analytical expression in common for each of the (ε₁, ε₂) spectra of these IGS films, oscillator parameters have been obtained in the best fits and these parameters in turn have been fitted with polynomials in x. From the resulting database of polynomial coefficients, the (ε₁, ε₂) spectra can be generated for any composition of IGS from the single parameter, x. The results have served as an RTSE fingerprint for IGS composition and have provided further structural information beyond simply thicknesses, for example information related to film density and grain size. The deduced IGS structural evolution and the (ε₁, ε₂) spectra have been interpreted as well in relation to observations from scanning electron microscopy, X-ray diffractometry and energy-dispersive X-ray spectroscopy profiling analyses. Overall the structural, optical and compositional analysis possible by RTSE has assisted in understanding the growth and properties of three stage CIGS absorbers for solar cells and shows future promise for enhancing cell performance through monitoring and control

Experimental and theoretical investigation of electronic structure in colloidal indium phosphide quantum dots

We present detailed electronic structure calculations, based on an atomistic pseudopotential approach, for a 42 Å InP QD; theoretical results are correlated to results of experimental measurements. To better understand energy loss dynamics and mechanisms for hot carriers in QDs, we investigate relaxation between electronic levels in InP QDs with diameters significantly smaller than twice the Bohr exciton radius. Energy level spacings exceeding the optical phonon energy present the opportunity for slowed charge carrier cooling (phonon bottleneck effect). However, interaction between confined electrons and holes provides an alternate, efficient relaxation route. Therefore, we expect sufficiently fast charge separation to weaken the Coulomb interaction and decrease the electronic energy loss rate. We study electronic relaxation in InP QDs using variations of subpicosecond transient absorption spectroscopy. 1 Introduction Nanomaterials exhibit unique physical and chemical properties, and offer the potential to enhance the efficiency of solar energy conversion through utilization of hot carriers to either increase the photovoltage or to increase the photocurrent via impact ionizatio

Photophysics of (CdSe)ZnS colloidal quantum dots in an aqueous environment stabilized with amino acids and genetically-modified proteins †

Using a combination of two amino acids, histidine and N-acetyl-cysteine, to replace the original organic capping groups of (CdSe)ZnS quantum dots, water-soluble and highly luminescent (CdSe)ZnS quantum dots have been successfully prepared at pH 8. Characterization by steady-state and time-resolved photoluminescence spectroscopy, and transient absorption spectroscopy, demonstrate that the electronic properties of these quantum dots exceed those of the original as-synthesized samples dissolved in a more-conventional organic solvent. Furthermore, these amino acid-stabilized quantum dots have been assembled onto a cellulose substrate via cellulose binding proteins that specifically bind to cellulose and was genetically engineered to harbor dual hexahistidine tags at the N-and C-termini to confer binding with the zinc(II) on the quantum dot surface. The spectroscopic measurements show that the protein-bound quantum dots continue to retain their desirable electronic properties when bound on the substrate. Meanwhile, the specific and very selective binding properties of the proteins have remained effective