4 research outputs found
Phonon-Assisted Field Emission in Silicon Nanomembranes for Time-of-Flight Mass Spectrometry of Proteins
Time-of-flight (TOF) mass spectrometry
has been considered as the
method of choice for mass analysis of large intact biomolecules, which
are ionized in low charge states by matrix-assisted-laser-desorption/ionization
(MALDI). However, it remains predominantly restricted to the mass
analysis of biomolecules with a mass below about 50 000 Da.
This limitation mainly stems from the fact that the sensitivity of
the standard detectors decreases with increasing ion mass. We describe
here a new principle for ion detection in TOF mass spectrometry, which
is based upon suspended silicon nanomembranes. Impinging ion packets
on one side of the suspended silicon nanomembrane generate nonequilibrium
phonons, which propagate quasi-diffusively and deliver thermal energy
to electrons within the silicon nanomembrane. This enhances electron
emission from the nanomembrane surface with an electric field applied
to it. The nonequilibrium phonon-assisted field emission in the suspended
nanomembrane connected to an effective cooling of the nanomembrane
via field emission allows mass analysis of megadalton ions with high
mass resolution at room temperature. The high resolution of the detector
will give better insight into high mass proteins and their functions
Thermally Driven Field Emission from Zinc Oxide Wires on a Nanomembrane Used as a Detector for Time-of-Flight Mass Spectrometry
Mass spectrometry
is a crucial technology in numerous applications,
but it places stringent requirements on the detector to achieve high
resolution across a broad spectrum of ion masses. Low-dimensional
nanostructures offer opportunities to tailor properties and achieve
performance not reachable in bulk materials. Here, an array of sharp
zinc oxide wires was directly grown on a 30 nm thin, free-standing
silicon nitride nanomembrane to enhance its field emission (FE). The
nanomembrane was subsequently used as a matrix-assisted laser desorption/ionization
time-of-flight mass spectrometry detector. When ionized biomolecules
impinge on the backside of the surface-modified nanomembrane, the
currentemitted from the wires on the membrane’s front
sideis amplified by the supplied thermal energy, which allows
for the detection of the ions. An extensive simulation framework was
developed based on a combination of lateral heat diffusion in the
nanomembrane, heat diffusion along the wires, and FE, including Schottky
barrier lowering, to investigate the impact of wire length and diameter
on the FE. Our theoretical model suggests a significant improvement
in the overall FE response of the nanomembrane by growing wires on
top. Specifically, long thin wires are ideal to enhance the magnitude
of the FE signal and to shorten its duration for the fastest response
simultaneously, which could facilitate the future application of detectors
in mass spectrometry with properties improved by low-dimensional nanostructures
Power Dissipation of WSe<sub>2</sub> Field-Effect Transistors Probed by Low-Frequency Raman Thermometry
The
ongoing shrinkage in the size of two-dimensional (2D) electronic circuitry
results in high power densities during device operation, which could
cause a significant temperature rise within 2D channels. One challenge
in Raman thermometry of 2D materials is that the commonly used high-frequency
modes do not precisely represent the temperature rise in some 2D materials
because of peak broadening and intensity weakening at elevated temperatures.
In this work, we show that a low-frequency E<sub>2g</sub><sup>2</sup> shear mode can be used to accurately
extract temperature and measure thermal boundary conductance (TBC)
in back-gated tungsten diselenide (WSe<sub>2</sub>) field-effect transistors,
whereas the high-frequency peaks (E<sub>2g</sub><sup>1</sup> and A<sub>1g</sub>) fail to provide reliable
thermal information. Our calculations indicate that the broadening
of high-frequency Raman-active modes is primarily driven by anharmonic
decay into pairs of longitudinal acoustic phonons, resulting in a
weak coupling with out-of-plane flexural acoustic phonons that are
responsible for the heat transfer to the substrate. We found that
the TBC at the interface of WSe<sub>2</sub> and Si/SiO<sub>2</sub> substrate is ∼16 MW/m<sup>2</sup> K, depends on the number
of WSe<sub>2</sub> layers, and peaks for 3–4 layer stacks.
Furthermore, the TBC to the substrate is the highest from the layers
closest to it, with each additional layer adding thermal resistance.
We conclude that the location where heat dissipated in a multilayer
stack is as important to device reliability as the total TBC
Bimodal Phonon Scattering in Graphene Grain Boundaries
Graphene has served as the model
2D system for over a decade, and the effects of grain boundaries (GBs)
on its electrical and mechanical properties are very well investigated.
However, no direct measurement of the correlation between thermal
transport and graphene GBs has been reported. Here, we report a simultaneous
comparison of thermal transport in supported single crystalline graphene
to thermal transport across an individual graphene GB. Our experiments
show that thermal conductance (per unit area) through an isolated
GB can be up to an order of magnitude lower than the theoretically
anticipated values. Our measurements are supported by Boltzmann transport
modeling which uncovers a new bimodal phonon scattering phenomenon
initiated by the GB structure. In this novel scattering mechanism,
boundary roughness scattering dominates the phonon transport in low-mismatch
GBs, while for higher mismatch angles there is an additional resistance
caused by the formation of a disordered region at the GB. Nonequilibrium
molecular dynamics simulations verify that the amount of disorder
in the GB region is the determining factor in impeding thermal transport
across GBs