Direct Measurement of Single CdSe Nanowire Extinction Polarization Anisotropies

The origin of sizable absorption polarization anisotropies (ρ_abs) in one-dimensional (1D) semiconductor nanowires (NWs) has been debated. Invoked explanations employ either classical or quantum mechanical origins, where the classical approach suggests dielectric constant mismatches between the NW and its surrounding environment as the predominant source of observed polarization sensitivities. At the same time, the confinement-influenced mixing of states suggests a sizable contribution from polarization-sensitive transition selection rules. Sufficient evidence exists in the literature to support either claim. However, in all cases, these observations stem from excitation polarization anisotropy (ρ_exc) studies, which only indirectly measure ρ_abs. In this manuscript, we directly measure the band edge extinction polarization anisotropies (ρ_ext) of individual CdSe NWs using single NW extinction spectroscopy. Observed polarization anisotropies possess distinct spectral features and wavelength dependencies that correlate well with theoretical transition selection rules derived from a six-band <i>k</i>·<i>p</i> theory used to model the electronic structure of CdSe NWs

Directed evolution of a bright variant of mCherry: Suppression of non-radiative decay by fluorescence lifetime selections

The approximately linear scaling of fluorescence quantum yield (QY) with fluorescence lifetime (τ) in fluorescent proteins (FPs) has inspired engineering of brighter fluorophores based on screening for increased lifetimes. Several recently developed FPs such as mTurquoise2, mScarlet and FusionRed-MQV which have become useful for live cell imaging are products of lifetime selection strategies. However, the underlying photophysical basis of the improved brightness has not been scrutinized. In this study, we focused on understanding the outcome of lifetime-based directed evolution of mCherry, which is a popular red-FP (RFP). We identified four positions (W143, I161, Q163, and I197) near the FP chromophore that can be mutated to create mCherry-XL (eXtended Lifetime: QY = 0.70; τ =3.9 ns). The threefold higher quantum yield of mCherry-XL is on par with that of the brightest RFP to date, mScarlet. We examined selected variants within the evolution trajectory and found a near-linear scaling of lifetime with quantum yield and consistent blue-shifts of the absorption and emission spectra. We find that the improvement in brightness is primarily due to a decrease in the non-radiative decay of the excited state. In addition, our analysis revealed the decrease in non-radiative rate is not limited to the blue-shift of the energy gap and changes in the excited state reorganization energy. Our findings suggest that non-radiative mechanisms beyond the scope of energy-gap models such the Englman-Jortner are suppressed in this lifetime evolution trajectory

Direct Observation of Single Layer Graphene Oxide Reduction through Spatially Resolved, Single Sheet Absorption/Emission Microscopy

Laser reduction of graphene oxide (GO) offers unique opportunities for the rapid, nonchemical production of graphene. By tuning relevant reduction parameters, the band gap and conductivity of reduced GO can be precisely controlled. In situ monitoring of single layer GO reduction is therefore essential. In this report, we show the direct observation of laser-induced, single layer GO reduction through correlated changes to its absorption and emission. Absorption/emission movies illustrate the initial stages of single layer GO reduction, its transition to reduced-GO (rGO) as well as its subsequent decomposition upon prolonged laser illumination. These studies reveal GO’s photoreduction life cycle and through it native GO/rGO absorption coefficients, their intrasheet distributions as well as their spatial heterogeneities. Extracted absorption coefficients for unreduced GO are α405 nm ≈ 6.5 ± 1.1 × 104 cm–1, α520 nm ≈ 2.1 ± 0.4 × 104 cm–1, and α640 nm ≈ 1.1 ± 0.3 × 104 cm–1 while corresponding rGO α-values are α405 nm ≈ 21.6 ± 0.6 × 104 cm–1, α520 nm ≈ 16.9 ± 0.4 × 104 cm–1, and α640 nm ≈ 14.5 ± 0.4 × 104 cm–1. More importantly, the correlated absorption/emission imaging provides us with unprecedented insight into GO’s underlying photoreduction mechanism, given our ability to spatially resolve its kinetics and to connect local rate constants to activation energies. On a broader level, the developed absorption imaging is general and can be applied toward investigating the optical properties of other two-dimensional materials, especially those that are nonemissive and are invisible to current single molecule optical techniques.Fil: Sokolov, Denis A.. University Of Notre Dame-Indiana; Estados UnidosFil: Morozov, Yurii V.. University Of Notre Dame-Indiana; Estados Unidos. Taras Shevchenko National University of Kiev; RusiaFil: McDonald, Matthew P.. University Of Notre Dame-Indiana; Estados UnidosFil: Vietmeyer, Felix. University Of Notre Dame-Indiana; Estados UnidosFil: Hodak, Jose Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Kuno, Masaru. University Of Notre Dame-Indiana; Estados Unido

Electric Field-Induced Emission Enhancement and Modulation in Individual CdSe Nanowires

CdSe nanowires show reversible emission intensity enhancements when subjected to electric field strengths ranging from 5 to 22 MV/m. Under alternating positive and negative biases, emission intensity modulation depths of 14 ± 7% are observed. Individual wires are studied by placing them in parallel plate capacitor-like structures and monitoring their emission intensities <i>via</i> single nanostructure microscopy. Observed emission sensitivities are rationalized by the field-induced modulation of carrier detrapping rates from NW defect sites responsible for nonradiative relaxation processes. The exclusion of these states from subsequent photophysics leads to observed photoluminescence quantum yield enhancements. We quantitatively explain the phenomenon by developing a kinetic model to account for field-induced variations of carrier detrapping rates. The observed phenomenon allows direct visualization of trap state behavior in individual CdSe nanowires and represents a first step toward developing new optical techniques that can probe defects in low-dimensional materials

Electric Field-Induced Emission Enhancement and Modulation in Individual CdSe Nanowires

CdSe nanowires show reversible emission intensity enhancements when subjected to electric field strengths ranging from 5 to 22 MV/m. Under alternating positive and negative biases, emission intensity modulation depths of 14 ± 7% are observed. Individual wires are studied by placing them in parallel plate capacitor-like structures and monitoring their emission intensities <i>via</i> single nanostructure microscopy. Observed emission sensitivities are rationalized by the field-induced modulation of carrier detrapping rates from NW defect sites responsible for nonradiative relaxation processes. The exclusion of these states from subsequent photophysics leads to observed photoluminescence quantum yield enhancements. We quantitatively explain the phenomenon by developing a kinetic model to account for field-induced variations of carrier detrapping rates. The observed phenomenon allows direct visualization of trap state behavior in individual CdSe nanowires and represents a first step toward developing new optical techniques that can probe defects in low-dimensional materials

Low temperature solution-phase growth of ZnSe and ZnSe/CdSe core/shell nanowires

High quality ZnSe nanowires (NWs) and complementary ZnSe/CdSe core/shell species have been synthesized using a recently developed solution-liquid-solid (SLS) growth technique. In particular, bismuth salts as opposed to pre-synthesized Bi or Au/Bi nanoparticles have been used to grow NWs at low temperatures in solution. Resulting wires are characterized using transmission electron microscopy and possess mean ensemble diameters between 15 and 28 nm with accompanying lengths ranging from 4-10 μm. Subsequent solution-based overcoating chemistry results in ZnSe wires covered with CdSe nanocrystals. By varying the shell's growth time, different thicknesses can be obtained and range from 8 to 21 nm. More interestingly, the mean constituent CdSe nanocrystal diameter can be varied and results in size-dependent shell emission spectra.Fil: Petchsang, Nattasamon. University of Notre Dame; Estados Unidos. Thailand Ministry of Education; Tailandia. Mahidol University; TailandiaFil: Shapoval, Liubov. Herzen State Pedagogical University Of Russia; RusiaFil: Vietmeyer, Felix. University of Notre Dame; Estados UnidosFil: Yu, Yanghai. University Of Wisconsin Madison;Fil: Hodak, Jose Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Instituto de Química, Física de los Materiales, Medioambiente y Energía; ArgentinaFil: Tang, I-Ming. Mahidol University; Tailandia. Thailand Ministry Of Education; TailandiaFil: Kosel, Thomas H.. University of Notre Dame; Estados UnidosFil: Kuno, Masaru. University of Notre Dame; Estados Unido

Synthetic Strategy and Structural and Optical Characterization of Thin Highly Crystalline Titanium Disulfide Nanosheets

Two-dimensional (2D) nanomaterials have recently received significant attention because of their attractiveness for use in many nanostructured devices. Layered transition-metal dichalcogenides are of particular interest because reducing their dimensionality causes changes in their already anisotropic physical and chemical properties. The present study describes the first bottom-up solution-phase synthesis of thin highly crystalline titanium disulfide (TiS₂) nanosheets (NSs) using abundant low-cost molecular precursors. The obtained TiS₂ NSs have average dimensions of ∼500 nm × 500 nm in the basal plane and have thicknesses of ∼5 nm. They exhibit broad absorption in the visible that tails out into the near-infrared. The obtained results demonstrate new opportunities in synthesizing low-dimensional 2D nanomaterials with potential use in various photochemical energy applications

Direct Observation of Single Layer Graphene Oxide Reduction through Spatially Resolved, Single Sheet Absorption/Emission Microscopy

Laser reduction of graphene oxide (GO) offers unique opportunities for the rapid, nonchemical production of graphene. By tuning relevant reduction parameters, the band gap and conductivity of reduced GO can be precisely controlled. In situ monitoring of single layer GO reduction is therefore essential. In this report, we show the direct observation of laser-induced, single layer GO reduction through correlated changes to its absorption and emission. Absorption/emission movies illustrate the initial stages of single layer GO reduction, its transition to reduced-GO (rGO) as well as its subsequent decomposition upon prolonged laser illumination. These studies reveal GO’s photoreduction life cycle and through it native GO/rGO absorption coefficients, their intrasheet distributions as well as their spatial heterogeneities. Extracted absorption coefficients for unreduced GO are α_405 nm ≈ 6.5 ± 1.1 × 10⁴ cm^–1, α_520 nm ≈ 2.1 ± 0.4 × 10⁴ cm^–1, and α_640 nm ≈ 1.1 ± 0.3 × 10⁴ cm^–1 while corresponding rGO α-values are α_405 nm ≈ 21.6 ± 0.6 × 10⁴ cm^–1, α_520 nm ≈ 16.9 ± 0.4 × 10⁴ cm^–1, and α_640 nm ≈ 14.5 ± 0.4 × 10⁴ cm^–1. More importantly, the correlated absorption/emission imaging provides us with unprecedented insight into GO’s underlying photoreduction mechanism, given our ability to spatially resolve its kinetics and to connect local rate constants to activation energies. On a broader level, the developed absorption imaging is general and can be applied toward investigating the optical properties of other two-dimensional materials, especially those that are nonemissive and are invisible to current single molecule optical techniques

Direct Observation of Spatially Heterogeneous Single-Layer Graphene Oxide Reduction Kinetics

Graphene oxide (GO) is an important precursor in the production of chemically derived graphene. During reduction, GO’s electrical conductivity and band gap change gradually. Doping and chemical functionalization are also possible, illustrating GO’s immense potential in creating functional devices through control of its local hybridization. Here we show that laser-induced photolysis controllably reduces individual single-layer GO sheets. The reaction can be followed in real time through sizable decreases in GO’s photoluminescence efficiency along with spectral blueshifts. As-produced reduced graphene oxide (rGO) sheets undergo additional photolysis, characterized by dramatic emission enhancements and spectral redshifts. Both GO’s reduction and subsequent conversion to photobrightened rGO are captured through movies of their photoluminescence kinetics. Rate maps illustrate sizable spatial and temporal heterogeneities in sp2 domain growth and reveal how reduction “flows” across GO and rGO sheets. The observed heterogeneous reduction kinetics provides mechanistic insight into GO’s conversion to chemically derived graphene and highlights opportunities for overcoming its dynamic, chemical disorder.Fil: McDonald, Matthew P. . University Of Notre Dame-indiana; Estados UnidosFil: Eltom, Ahmed . University Of Waterloo; CanadáFil: Vietmeyer, Felix . University Of Notre Dame-indiana; Estados UnidosFil: Thapa, Janak . Illinois Wesleyan University; Estados UnidosFil: Morozov, Yurii V. . Taras Shevchenko National University of Kiev; UcraniaFil: Sokolov, Denis A. . University Of Notre Dame-indiana; Estados UnidosFil: Hodak, Jose Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires; ArgentinaFil: Vinodgopal, Kizhanipuram . North Carolina Central University; Estados UnidosFil: Kamat, Prashant V. . University Of Notre Dame-indiana; Estados UnidosFil: Kuno, Masaru . University Of Notre Dame-indiana; Estados Unido

Photocatalytic Hydrogen Generation Efficiencies in One-Dimensional CdSe Heterostructures

To better understand the role nanoscale heterojunctions play in the photocatalytic generation of hydrogen, we have designed several model one-dimensional (1D) heterostructures based on CdSe nanowires (NWs). Specifically, CdSe/CdS core/shell NWs and Au nanoparticle (NP)-decorated core and core/shell NWs have been produced using facile solution chemistries. These systems enable us to explore sources for efficient charge separation and enhanced carrier lifetimes important to photocatalytic processes. We find that visible light H₂ generation efficiencies in the produced hybrid 1D structures increase in the order CdSe < CdSe/Au NP < CdSe/CdS/Au NP < CdSe/CdS with a maximum H₂ generation rate of 58.06 ± 3.59 μmol h^–1 g^–1 for CdSe/CdS core/shell NWs. This is 30 times larger than the activity of bare CdSe NWs. Using femtosecond transient differential absorption spectroscopy, we subsequently provide mechanistic insight into the role nanoscale heterojunctions play by directly monitoring charge flow and accumulation in these hybrid systems. In turn, we explain the observed trend in H₂ generation rates with an important outcome being direct evidence for heterojunction-influenced charge transfer enhancements of relevant chemical reduction processes