Felix Vietmeyer

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.

Single nanowire extinction spectroscopy.

Here we show the first direct extinction spectra of single one-dimensional (1D) semiconductor nanostructures obtained at room temperature utilizing a spatial modulation approach. (1) For these materials, ensemble averaging in conventional extinction spectroscopy has limited our understanding of the interplay between carrier confinement and their electrostatic interactions. (2-4) By probing individual CdSe nanowires (NWs), we have identified and assigned size-dependent exciton transitions occurring across the visible. In turn, we have revealed the existence of room temperature 1D excitons in the narrowest NWs.

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.

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 shells 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.

Direct Measurement of Single CdSe Nanowire Extinction Polarization Anisotropies.

The origin of sizable absorption polarization anisotropies (ρabs) 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 (ρexc) studies, which only indirectly measure ρabs. In this manuscript, we directly measure the band edge extinction polarization anisotropies (ρext) 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 k·p theory used to model the electronic structure of CdSe NWs.

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 via 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.

Supercontinuum spatial modulation spectroscopy: Detection and noise limitations

Supercontinuum spatial modulation spectroscopy is a facile tool for conducting single molecule/particle extinction spectroscopy throughout the visible and near infrared (420-1100 nm). The techniques capabilities are benchmarked using individual Au nanoparticles (NPs) as a standard since they are well studied and display a prominent plasmon resonance in the visible. Extinction spectra of individual Au NPs with diameters (d) ranging from d ~ 8 to 40 nm are resolved with extinction cross sections (σ(ext)) of σ(ext) ~ 1 × 10(-13)-1 × 10(-11) cm(2). Corresponding signal-to-noise ratios range from ~30 to ~1400. The techniques limit of detection is determined to be 4.3 × 10(-14) cm(2) (4.3 nm(2)). To showcase supercontinuum spatial modulation spectroscopys broader applicability, extinction spectra are acquired for other model systems, such as individual single-walled carbon nanotubes (SWCNTs) and CdSe nanowires. We show for the first time extinction spectra of individual (8,3) and (6,5) SWCNTs. For both chiralities, their E11 [(8,3) 1.30 eV (952 nm); (6,5) 1.26 eV (986 nm)] and E22 [(8,3) 1.86 eV (667 nm); (6,5) 2.19 eV (567 nm)] excitonic resonances are seen with corresponding cross sections of σ(ext) ~ 10(-13) cm(2) μm(-1).

II-VI heterostructures obtained by encapsulation of colloidal CdSe nanowires by molecular beam epitaxy deposition of ZnSe

In this work, 10 and 20 nm diameter colloidal CdSe nanowires (NWs) have been successfully incorporated into ZnSe layers grown by molecular beam epitaxy on GaAs substrates. Atomic force microscopy and scanning electron microscopy were carried out to study the surface morphology of the samples. Transmission electron microscopy and energy-dispersive x-ray analysis were also carried out to study the chemical composition and lattice structure around the embedded CdSe NWs. Finally, single NW photoluminescence (PL) spectroscopy and PL imaging were used to study the optical properties of ZnSe films containing CdSe NWs. The experimental results confirm the presence of embedded CdSe NWs in the ZnSe films and show that the embedded NWs remain optically active.

Molybdenum Carbamate Nanosheets as a New Class of Potential Phase Change Materials

We report for the first time the synthesis of large, free-standing, Mo2O2(μ-S)2(Et2dtc)2 (MoDTC) nanosheets (NSs), which exhibit an electron-beam induced crystalline-to-amorphous phase transition. Both electron beam ionization and femtosecond (fs) optical excitation induce the phase transition, which is size-, morphology-, and composition-preserving. Resulting NSs are the largest, free-standing regularly shaped two-dimensional amorphous nanostructures made to date. More importantly, amorphization is accompanied by dramatic changes to the NS electrical and optical response wherein resulting amorphous species exhibit room-temperature conductivities 5 orders of magnitude larger than those of their crystalline counterparts. This enhancement likely stems from the amorphization-induced formation of sulfur vacancy-related defects and is supported by temperature-dependent transport measurements, which reveal efficient variable range hopping. MoDTC NSs represent one instance of a broader class of transition metal carbamates likely having applications because of their intriguing electrical properties as well as demonstrated ability to toggle metal oxidation states.