4 research outputs found
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 (TiS<sub>2</sub>) nanosheets
(NSs) using abundant low-cost molecular precursors. The obtained TiS<sub>2</sub> 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
Charge Carrier Trapping and Acoustic Phonon Modes in Single CdTe Nanowires
Semiconductor nanostructures produced by wet chemical synthesis are extremely heterogeneous, which makes single particle techniques a useful way to interrogate their properties. In this paper the ultrafast dynamics of single CdTe nanowires are studied by transient absorption microscopy. The wires have lengths of several micrometers and lateral dimensions on the order of 30 nm. The transient absorption traces show very fast decays, which are assigned to charge carrier trapping into surface defects. The time constants vary for different wires due to differences in the energetics and/or density of surface trap sites. Measurements performed at the band edge compared to the near-IR give slightly different time constants, implying that the dynamics for electron and hole trapping are different. The rate of charge carrier trapping was observed to slow down at high carrier densities, which was attributed to trap-state filling. Modulations due to the fundamental and first overtone of the acoustic breathing mode were also observed in the transient absorption traces. The quality factors for these modes were similar to those measured for metal nanostructures, and indicate a complex interaction with the environment
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 H<sub>2</sub> 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 H<sub>2</sub> generation rate of 58.06 ± 3.59 μmol h<sup>–1</sup> g<sup>–1</sup> 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 H<sub>2</sub> generation rates with
an important outcome being direct evidence for heterojunction-influenced
charge transfer enhancements of relevant chemical reduction processes
Molybdenum Carbamate Nanosheets as a New Class of Potential Phase Change Materials
We
report for the first time the synthesis of large, free-standing,
Mo<sub>2</sub>O<sub>2</sub>(μ-S)<sub>2</sub>(Et<sub>2</sub>dtc)<sub>2</sub> (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