16 research outputs found
Poole-Frenkel Effect and Phonon-Assisted Tunneling in GaAs Nanowires
We present electronic transport measurements of GaAs nanowires grown by
catalyst-free metal-organic chemical vapor deposition. Despite the nanowires
being doped with a relatively high concentration of substitutional impurities,
we find them inordinately resistive. By measuring sufficiently high
aspect-ratio nanowires individually in situ, we decouple the role of the
contacts and show that this semi-insulating electrical behavior is the result
of trap-mediated carrier transport. We observe Poole-Frenkel transport that
crosses over to phonon-assisted tunneling at higher fields, with a tunneling
time found to depend predominantly on fundamental physical constants as
predicted by theory. By using in situ electron beam irradiation of individual
nanowires we probe the nanowire electronic transport when free carriers are
made available, thus revealing the nature of the contacts
Suppression of mid-infrared plasma resonance due to quantum confinement in delta-doped silicon
The classical Drude model provides an accurate description of the plasma
resonance of three-dimensional materials, but only partially explains
two-dimensional systems where quantum mechanical effects dominate such as
P:-layers - atomically thin sheets of phosphorus dopants in silicon
that induce novel electronic properties beyond traditional doping. Previously
it was shown that P:-layers produce a distinct Drude tail feature in
ellipsometry measurements. However, the ellipsometric spectra could not be
properly fit by modeling the -layer as discrete layer of classical
Drude metal. In particular, even for large broadening corresponding to
extremely short relaxation times, a plasma resonance feature was anticipated
but not evident in the experimental data. In this work, we develop a physically
accurate description of this system, which reveals a general approach to
designing thin films with intentionally suppressed plasma resonances. Our model
takes into account the strong charge density confinement and resulting quantum
mechanical description of a P:-layer. We show that the absence of a
plasma resonance feature results from a combination of two factors: i), the
sharply varying charge density profile due to strong confinement in the
direction of growth; and ii), the effective mass and relaxation time anisotropy
due to valley degeneracy. The plasma resonance reappears when the atoms
composing the -layer are allowed to diffuse out from the plane of the
layer, destroying its well-confined two-dimensional character that is critical
to its novel electronic properties
Observation of Space-Charge-Limited Transport in InAs Nanowires
Recent theory and experiment have suggested that space-charge-limited
transport should be prevalent in high aspect-ratio semiconducting nanowires. We
report on InAs nanowires exhibiting this mode of transport and utilize the
underlying theory to determine the mobility and effective carrier concentration
of individual nanowires, both of which are found to be diameter-dependent.
Intentionally induced failure by Joule heating supports the notion of
space-charge-limited transport and proposes reduced thermal conductivity due to
the nanowires polymorphism
Impact of Casimir force in molecular electronic switching junctions
Despite significant progress In synthesizing several new molecules and many promising single device demonstrations, wide range acceptance of molecular electronics as an alternative to CMOS technology has been stalled not only by controversial theories of a molecular device's operation, for example the switching mechanism, but also by our inability to reproducibly fabricate large arrays of devices. In this paper, we investigate the role of Casimir force as one of the potential source of a wide range of discrepancies in the reported electrical characteristics and high rate of device shorting in molecular electronic switching junctions fabricated by sandwiching a molecular monolayer between a pair of planar metal electrodes
Nanoscale Infrared Spectroscopy: Improving the Spectral Range of the Photothermal Induced Resonance Technique
Photothermal induced resonance (PTIR) is a new technique
which
combines the chemical specificity of infrared (IR) spectroscopy with
the lateral resolution of atomic force microscopy (AFM). PTIR requires
a pulsed tunable laser for sample excitation and an AFM tip to measure
the sample expansion induced by light absorption. The limited tunability
of commonly available laser sources constrains the application of
the PTIR technique to a portion of the IR spectrum. In this work,
a broadly tunable pulsed laser relying on a difference frequency generation
scheme in a GaSe crystal to emit light tunable from 1.55 μm
to 16 μm (from 6450 cm<sup>–1</sup> to 625 cm<sup>–1</sup>) was interfaced with a commercial PTIR instrument. The result is
a materials characterization platform capable of chemical imaging,
in registry with atomic force images, with a spatial resolution that
notably surpasses the light diffraction limit throughout the entire
mid-IR spectral range. PTIR nanoscale spectra and images allow the
identification of compositionally and optically similar yet distinct
materials; organic, inorganic, and composite samples can be studied
with this nanoscale analog of infrared spectroscopy, suggesting broad
applicability. Additionally, we compare the results obtained with
the two tunable lasers, which have different pulse lengths, to experimentally
assess the recently developed theory of PTIR signal generation
Sonochemical approach for rapid growth of zinc oxide nanowalls
The sonochemical process drives chemical reactions with sound fields by creating extraordinarily high density of energy, pressure and temperatures. The process resulted in a number of unexpected chemical species and thought-provoking results in the recent past. In this paper, we present a new sonochemical approach to synthesize ZnO (zinc oxide) nanowalls (NWalls) on aluminum and alumina coated substrates at room ambient conditions. We achieved highly dense and uniform ZnO NWalls in areas that are coated with Al or Al2O3 (alumina). The synthesis process was shown not to occur on Si, SiO2, Cr, or Ag surfaces. A series of experiments on understanding the growth kinetics offers detailed insight into the growth dynamics over time. Photoluminescence (PL) measurements, UV Vis spectroscopy, and SEM-EDS results confirm NWalls composed of crystalline ZnO that are formed via Al assisted growth induced by phase transformations under extraordinary pressure, temperature, and chemical growth kinetics. The chemical growth method as reported here, is applicable to arbitrary substrates coated with an Al thin film. We demonstrate the applications of the as-formed NWalls in UV photoconductors and gas sensors
Nanoscale imaging and spectroscopy of plasmonic modes with the PTIR technique
The photothermal induced resonance (PTIR) method measures the near-field absorption in gold plasmonic structures. PTIR absorption images show the interference between the bright and dark modes of asymmetric split ring resonators (ASRRs). The plasmonic absorption spectra of the ASRRs individual arcs are obtained locally, confirming the collective nature of the plasmonic excitation. Scale bars are 500 nm