5 research outputs found
Ultrafast Transient Terahertz Conductivity of Monolayer MoS<sub>2</sub> and WSe<sub>2</sub> Grown by Chemical Vapor Deposition
We have measured ultrafast charge carrier dynamics in monolayers and trilayers of the transition metal dichalcogenides MoS<sub>2</sub> and WSe<sub>2</sub> using a combination of time-resolved photoluminescence and terahertz spectroscopy. We recorded a photoconductivity and photoluminescence response time of just 350 fs from CVD-grown monolayer MoS<sub>2</sub>, and 1 ps from trilayer MoS<sub>2</sub> and monolayer WSe<sub>2</sub>. Our results indicate the potential of these materials as high-speed optoelectronic materials
Electron-Beam Patterning of Polymer Electrolyte Films To Make Multiple Nanoscale Gates for Nanowire Transistors
We
report an electron-beam based method for the nanoscale patterning
of the polyÂ(ethylene oxide)/LiClO<sub>4</sub> polymer electrolyte.
We use the patterned polymer electrolyte as a high capacitance gate
dielectric in single nanowire transistors and obtain subthreshold
swings comparable to conventional metal/oxide wrap-gated nanowire
transistors. Patterning eliminates gate/contact overlap, which reduces
parasitic effects and enables multiple, independently controllable
gates. The method’s simplicity broadens the scope for using
polymer electrolyte gating in studies of nanowires and other nanoscale
devices
Ultralow Surface Recombination Velocity in InP Nanowires Probed by Terahertz Spectroscopy
Using transient terahertz photoconductivity measurements,
we have
made noncontact, room temperature measurements of the ultrafast charge
carrier dynamics in InP nanowires. InP nanowires exhibited a very
long photoconductivity lifetime of over 1 ns, and carrier lifetimes
were remarkably insensitive to surface states despite the large nanowire
surface area-to-volume ratio. An exceptionally low surface recombination
velocity (170 cm/s) was recorded at room temperature. These results
suggest that InP nanowires are prime candidates for optoelectronic
devices, particularly photovoltaic devices, without the need for surface
passivation. We found that the carrier mobility is not limited by
nanowire diameter but is strongly limited by the presence of planar
crystallographic defects such as stacking faults in these predominantly
wurtzite nanowires. These findings show the great potential of very
narrow InP nanowires for electronic devices but indicate that improvements
in the crystallographic uniformity of InP nanowires will be critical
for future nanowire device engineering
Optimizing the Energy Offset between Dye and Hole-Transporting Material in Solid-State Dye-Sensitized Solar Cells
The
power-conversion efficiency of solid-state dye-sensitized solar
cells can be optimized by reducing the energy offset between the highest
occupied molecular orbital (HOMO) levels of dye and hole-transporting
material (HTM) to minimize the loss-in-potential. Here, we report
a study of three novel HTMs with HOMO levels slightly above and below
the one of the commonly used HTM 2,2′,7,7′- tetrakisÂ(<i>N</i>,<i>N</i>-di-<i>p</i>-methoxyphenylamino)-9,9′-spirobifluorene
(spiro-OMeTAD) to systematically explore this possibility. Using transient
absorption spectroscopy and employing the ruthenium based dye Z907
as sensitizer, it is shown that, despite one new HTM showing a 100%
hole-transfer yield, all devices based on the new HTMs performed worse
than those incorporating spiro-OMeTAD. We further demonstrate that
the design of the HTM has an additional impact on the electronic density
of states present at the TiO<sub>2</sub> electrode surface and hence
influences not only hole- but also electron-transfer from the sensitizer.
These results provide insight into the complex influence of the HTM
on charge transfer and provide guidance for the molecular design of
new materials
Metasurface spectrometers beyond resolution-sensitivity constraints
Optical spectroscopy plays an essential role across scientific research and industry for non-contact materials analysis1-3, increasingly through in-situ or portable platforms4-6. However, when considering low-light-level applications, conventional spectrometer designs necessitate a compromise between their resolution and sensitivity7,8, especially as device and detector dimensions are scaled down. Here, we report on a miniaturizable spectrometer platform where light throughput onto the detector is instead enhanced as the resolution is increased. This planar, CMOS-compatible platform is based around metasurface encoders designed to exhibit photonic bound states in the continuum9, where operational range can be altered or extended simply through adjusting geometric parameters. This system can enhance photon collection efficiency by up to two orders of magnitude versus conventional designs; we demonstrate this sensitivity advantage through ultra-low-intensity fluorescent and astrophotonic spectroscopy. This work represents a step forward for the practical utility of spectrometers, affording a route to integrated, chip-based devices that maintain high resolution and SNR without requiring prohibitively long integration times