16 research outputs found
Luminous Efficiency of Axial In<sub><i>x</i></sub>Ga<sub>1–<i>x</i></sub>N/GaN Nanowire Heterostructures: Interplay of Polarization and Surface Potentials
Using
continuum elasticity theory and an eight-band <b>k</b>·<b>p</b> formalism, we study the electronic properties
of GaN nanowires with axial In<sub><i>x</i></sub>Ga<sub>1–<i>x</i></sub>N insertions. The three-dimensional
strain distribution in these insertions and the resulting distribution
of the polarization fields are fully taken into account. In addition,
we consider the presence of a surface potential originating from Fermi
level pinning at the sidewall surfaces of the nanowires. Our simulations
reveal an in-plane spatial separation of electrons and holes in the
case of weak piezoelectric potentials, which correspond to an In content
and layer thickness required for emission in the blue and violet spectral
range. These results explain the quenching of the photoluminescence
intensity experimentally observed for short emission wavelengths.
We devise and discuss strategies to overcome this problem
Strain Engineering of Nanowire Multi-Quantum Well Demonstrated by Raman Spectroscopy
An analysis of the strain in an axial
nanowire superlattice shows
that the dominating strain state can be defined arbitrarily between
unstrained and maximum mismatch strain by choosing the segment height
ratios. We give experimental evidence for a successful strain design
in series of GaN nanowire ensembles with axial In<sub><i>x</i></sub>Ga<sub>1–<i>x</i></sub>N quantum wells. We
vary the barrier thickness and determine the strain state of the quantum
wells by Raman spectroscopy. A detailed calculation of the strain
distribution and LO phonon frequency shift shows that a uniform in-plane
lattice constant in the nanowire segments satisfactorily describes
the resonant Raman spectra, although in reality the three-dimensional
strain profile at the periphery of the quantum wells is complex. Our
strain analysis is applicable beyond the In<sub><i>x</i></sub>Ga<sub>1–<i>x</i></sub>N/GaN system under
study, and we derive universal rules for strain engineering in nanowire
heterostructures
Nanowires Bending over Backward from Strain Partitioning in Asymmetric Core–Shell Heterostructures
The flexibility and
quasi-one-dimensional nature of nanowires offer
wide-ranging possibilities for novel heterostructure design and strain
engineering. In this work, we realize arrays of extremely and controllably
bent nanowires comprising lattice-mismatched and highly asymmetric
core–shell heterostructures. Strain sharing across the nanowire
heterostructures is sufficient to bend vertical nanowires over backward
to contact either neighboring nanowires or the substrate itself, presenting
new possibilities for designing nanowire networks and interconnects.
Photoluminescence spectroscopy on bent-nanowire heterostructures reveals
that spatially varying strain fields induce charge carrier drift toward
the tensile-strained outside of the nanowires, and that the polarization
response of absorbed and emitted light is controlled by the bending
direction. This unconventional strain field is employed for light
emission by placing an active region of quantum dots at the outer
side of a bent nanowire to exploit the carrier drift and tensile strain.
These results demonstrate how bending in nanoheterostructures opens
up new degrees of freedom for strain and device engineering
Spontaneous Nucleation and Growth of GaN Nanowires: The Fundamental Role of Crystal Polarity
We experimentally investigate whether crystal polarity
affects
the growth of GaN nanowires in plasma-assisted molecular beam epitaxy
and whether their formation has to be induced by defects. For this
purpose, we prepare smooth and coherently strained AlN layers on 6H-SiC(0001)
and SiC(0001̅) substrates to ensure a well-defined polarity
and an absence of structural and morphological defects. On N-polar
AlN, a homogeneous and dense N-polar GaN nanowire array forms, evidencing
that GaN nanowires form spontaneously in the absence of defects. On
Al-polar AlN, we do not observe the formation of Ga-polar GaN NWs.
Instead, sparse N-polar GaN nanowires grow embedded in a Ga-polar
GaN layer. These N-polar GaN nanowires are shown to be accidental
in that the necessary polarity inversion is induced by the formation
of Si<sub><i>x</i></sub>N. The present findings thus demonstrate
that spontaneously formed GaN nanowires are irrevocably N-polar. Due
to the strong impact of the polarity on the properties of GaN-based
devices, these results are not only essential to understand the spontaneous
formation of GaN nanowires but also of high technological relevance
Radial Stark Effect in (In,Ga)N Nanowires
We study the luminescence of unintentionally
doped and Si-doped
In<sub><i>x</i></sub>Ga<sub>1–<i>x</i></sub>N nanowires with a low In content (<i>x</i> < 0.2) grown
by molecular beam epitaxy on Si substrates. The emission band observed
at 300 K from the unintentionally doped samples is centered at much
lower energies (800 meV) than expected from the In content measured
by X-ray diffractometry and energy dispersive X-ray spectroscopy.
This discrepancy arises from the pinning of the Fermi level at the
sidewalls of the nanowires, which gives rise to strong radial built-in
electric fields. The combination of the built-in electric fields with
the compositional fluctuations inherent to (In,Ga)N alloys induces
a competition between spatially direct and indirect recombination
channels. At elevated temperatures, electrons at the core of the nanowire
recombine with holes close to the surface, and the emission from unintentionally
doped nanowires exhibits a Stark shift of several hundreds of meV.
The competition between spatially direct and indirect transitions
is analyzed as a function of temperature for samples with various
Si concentrations. We propose that the radial Stark effect is responsible
for the broadband absorption of (In,Ga)N nanowires across the entire
visible range, which makes these nanostructures a promising platform
for solar energy applications
Polarity-Induced Selective Area Epitaxy of GaN Nanowires
We present a conceptually
novel approach to achieve selective area
epitaxy of GaN nanowires. The approach is based on the fact that these
nanostructures do not form in plasma-assisted molecular beam epitaxy
on structurally and chemically uniform cation-polar substrates. By <i>in situ</i> depositing and nitridating Si on a Ga-polar GaN
film, we locally reverse the polarity to induce the selective area
epitaxy of N-polar GaN nanowires. We show that the nanowire number
density can be controlled over several orders of magnitude by varying
the amount of predeposited Si. Using this growth approach, we demonstrate
the synthesis of single-crystalline and uncoalesced nanowires with
diameters as small as 20 nm. The achievement of nanowire number densities
low enough to prevent the shadowing of the nanowire sidewalls from
the impinging fluxes paves the way for the realization of homogeneous
core-shell heterostructures without the need of using <i>ex situ</i> prepatterned substrates
A Comparative Proteomic Study of Human Skin Suction Blister Fluid from Healthy Individuals Using Immunodepletion and iTRAQ Labeling
Aberrations in skin morphology and functionality can
cause acute
and chronic skin-related diseases that are the focus of dermatological
research. Mechanically induced skin suction blister fluid may serve
as a potential, alternative human body fluid for quantitative mass
spectrometry
(MS)-based proteomics in order to assist in the understanding of the
mechanisms and causes underlying skin-related diseases. The combination
of abundant-protein removal with iTRAQ technology and multidimensional
fractionation techniques improved the number of identified protein
groups. A relative comparison of a cohort of 8 healthy volunteers
was thus recruited in order to assess the net variability encountered
in a healthy scenario. The technology enabled the identification,
to date, of the highest number of reported protein groups (739) with
concomitant relative quantitative data for over 90% of all proteins
with high reproducibility and accuracy. The use of iTRAQ 8-plex resulted
in a 66% decrease in protein identifications but, despite this, provided
valuable insight into interindividual differences of the healthy control
samples. The geometric mean ratio was close to 1 with 95% of all ratios
ranging between 0.45 and 2.05 and a calculated mean coefficient of
variation of 15.8%, indicating a lower biological variance than that
reported for plasma or urine. By applying a multistep sample processing,
the obtained sensitivity and accuracy of quantitative MS analysis
demonstrates the prospective value of the approach in future research
into skin diseases
A Comparative Proteomic Study of Human Skin Suction Blister Fluid from Healthy Individuals Using Immunodepletion and iTRAQ Labeling
Aberrations in skin morphology and functionality can
cause acute
and chronic skin-related diseases that are the focus of dermatological
research. Mechanically induced skin suction blister fluid may serve
as a potential, alternative human body fluid for quantitative mass
spectrometry
(MS)-based proteomics in order to assist in the understanding of the
mechanisms and causes underlying skin-related diseases. The combination
of abundant-protein removal with iTRAQ technology and multidimensional
fractionation techniques improved the number of identified protein
groups. A relative comparison of a cohort of 8 healthy volunteers
was thus recruited in order to assess the net variability encountered
in a healthy scenario. The technology enabled the identification,
to date, of the highest number of reported protein groups (739) with
concomitant relative quantitative data for over 90% of all proteins
with high reproducibility and accuracy. The use of iTRAQ 8-plex resulted
in a 66% decrease in protein identifications but, despite this, provided
valuable insight into interindividual differences of the healthy control
samples. The geometric mean ratio was close to 1 with 95% of all ratios
ranging between 0.45 and 2.05 and a calculated mean coefficient of
variation of 15.8%, indicating a lower biological variance than that
reported for plasma or urine. By applying a multistep sample processing,
the obtained sensitivity and accuracy of quantitative MS analysis
demonstrates the prospective value of the approach in future research
into skin diseases
A Comparative Proteomic Study of Human Skin Suction Blister Fluid from Healthy Individuals Using Immunodepletion and iTRAQ Labeling
Aberrations in skin morphology and functionality can
cause acute
and chronic skin-related diseases that are the focus of dermatological
research. Mechanically induced skin suction blister fluid may serve
as a potential, alternative human body fluid for quantitative mass
spectrometry
(MS)-based proteomics in order to assist in the understanding of the
mechanisms and causes underlying skin-related diseases. The combination
of abundant-protein removal with iTRAQ technology and multidimensional
fractionation techniques improved the number of identified protein
groups. A relative comparison of a cohort of 8 healthy volunteers
was thus recruited in order to assess the net variability encountered
in a healthy scenario. The technology enabled the identification,
to date, of the highest number of reported protein groups (739) with
concomitant relative quantitative data for over 90% of all proteins
with high reproducibility and accuracy. The use of iTRAQ 8-plex resulted
in a 66% decrease in protein identifications but, despite this, provided
valuable insight into interindividual differences of the healthy control
samples. The geometric mean ratio was close to 1 with 95% of all ratios
ranging between 0.45 and 2.05 and a calculated mean coefficient of
variation of 15.8%, indicating a lower biological variance than that
reported for plasma or urine. By applying a multistep sample processing,
the obtained sensitivity and accuracy of quantitative MS analysis
demonstrates the prospective value of the approach in future research
into skin diseases
A Comparative Proteomic Study of Human Skin Suction Blister Fluid from Healthy Individuals Using Immunodepletion and iTRAQ Labeling
Aberrations in skin morphology and functionality can
cause acute
and chronic skin-related diseases that are the focus of dermatological
research. Mechanically induced skin suction blister fluid may serve
as a potential, alternative human body fluid for quantitative mass
spectrometry
(MS)-based proteomics in order to assist in the understanding of the
mechanisms and causes underlying skin-related diseases. The combination
of abundant-protein removal with iTRAQ technology and multidimensional
fractionation techniques improved the number of identified protein
groups. A relative comparison of a cohort of 8 healthy volunteers
was thus recruited in order to assess the net variability encountered
in a healthy scenario. The technology enabled the identification,
to date, of the highest number of reported protein groups (739) with
concomitant relative quantitative data for over 90% of all proteins
with high reproducibility and accuracy. The use of iTRAQ 8-plex resulted
in a 66% decrease in protein identifications but, despite this, provided
valuable insight into interindividual differences of the healthy control
samples. The geometric mean ratio was close to 1 with 95% of all ratios
ranging between 0.45 and 2.05 and a calculated mean coefficient of
variation of 15.8%, indicating a lower biological variance than that
reported for plasma or urine. By applying a multistep sample processing,
the obtained sensitivity and accuracy of quantitative MS analysis
demonstrates the prospective value of the approach in future research
into skin diseases