Home | People
| Projects | Techniques
Characteristics of Au−Alkanedithiol−Au Junctions formed via
Nanotransfer Printing (nTP), J. R. Niskala, W. C. Rice, R. C. Bruce, T.
J. Merkel, F. Tsui, W. You, J.
Am. Chem. Soc. 134,
metal (MMM) junctions, though technically challenging, is
desirable for both fundamental investigations and applications
of molecule-based electronics. In this study, we employed the
nanotransfer printing (nTP) technique using perfluoropolyether
(PFPE) stamps to print Au thin films onto self-assembled
monolayers (SAMs) of alkanedithiol formed on Au
thin films. We show that the resulting MMM junctions form
permanent and symmetrical tunnel junctions, without the need for an
additional protection layer between the top metal electrode
and the molecular layer. This type of junction makes it possible for
direct investigations into the electrical properties of the
molecules and the metal−molecule interfaces. Dependence of transport
properties on the length of the alkane molecules and the
area of the printed Au electrodes has been examined systematically.
From the analysis of the current−voltage (I−V) curves using
the Simmons model, the height of tunneling barrier associated with the
molecule (alkane) has been determined to be 3.5 +/- 0.2
eV, while the analysis yielded an upper bound of 2.4 eV for the
counterpart at the interface (thiol). The former is consistent with
the theoretical value of ∼3.5−5.0 eV. The measured I−V curves show
scaling with respect to the printed Au electrode area with
lateral dimensions ranging from 80 nm to 7 μm. These results
demonstrate that PFPE-assisted nTP is a promising technique for
producing potentially scalable and permanent MMM junctions. They also
demonstrate that MMM structures (produced by the
unique PFPE-assisted nTP) constitute a reliable test bed for exploring
Imaging and elastometry
of blood clots using magnetomotive optical coherence tomography and
labeled platelets (Invited), A. L. Oldenburg, G. Wu, D. Spivak, F.
Tsui, A. S. Wolberg, T. H.
J. Sel. Topics
Quantum Electron. 18(3),
methods for imaging and assessment of vascular
defects are needed for directing treatment of cardiovascular
pathologies. In this paper, we employ magnetomotive optical coherence
tomography (MMOCT) as a platform both to detect and to
measure the elasticity of blood clots. Detection is enabled through
the use of rehydrated, lyophilized platelets loaded with
iron oxides (SPIO-RL platelets) that are functional infusion
agents that adhere to sites of vascular endothelial damage.
Evidence suggests that the sensitivity for detection is improved
over threefold by magnetic interactions between SPIOs inside RL
platelets. Using the same MMOCT system, we show how elastometry
of simulated clots, using resonant acoustic spectroscopy, is
correlated with the fibrin content of the clot. Both methods are
based upon magnetic actuation and phase-sensitive optical monitoring
of nanoscale displacements using MMOCT, underscoring
its utility as a broad-based platform to detect and measure the
molecular structure and composition of blood clots.
morphology of dopant decomposition in Mn and Co codoped Ge epitaxial
films, J. R. Riley, D. E. Perea, L. He, F. Tsui, L. J. Lauhon, J. Phys. Chem. C 116,
probe tomography (APT) was used to quantify inhomogeneities in the
distribution of Mn and Co in doped epitaxial Ge thin films for which
X-ray diffraction (XRD)
studies indicate single phase material. The segregation of dopants into
Co and Mn-rich regions
with characteristic sizes was evident upon visual inspection of the APT
reconstruction and a
frequency distribution analysis of the concentration of Co, Mn, and Ge
atoms verified that the
composition fluctuations exceeded those of a random alloy.
Isoconcentration surfaces were
generated to establish the connectedness of regions enriched in Mn that
have been proposed to
enhance the Curie temperature in dilute magnetic semiconductors. The
important contributions that APT can make to the understanding of
magnetism in these
Epitaxial growth of
(111) thin films
the compositional range
around the Heusler alloy Co2MnSi, L. He, B. A. Collins, F.
Tsui, Y. S. Chu, J. Vac. Sci. Tech. B 29, 03C124
growth and structural properties of Cox
thin films on Ge (111)
substrates, including the Heusler alloy Co2
(111) have been studied
using combinatorial molecular beam epitaxy (MBE) techniques. In situ
reflection high energy electron diffraction and ex situ x-ray
diffraction experiments show that high quality coherent MBE growth with
fcc (111) stacking can be achieved over a relatively large composition
space that includes Co2
MnSi. The highest
structural and chemical
ordering is observed near the composition of Co0.63
rather than that at the Heusler stoichiometry of Co2
The in-plane crystallographic axis of the fcc film exhibits a 60 degree
respect to that of the Ge substrate. The rotation appears to be
originated at the film-substrate interface, as a result of the symmetry
and stacking of the Ge (111) surface reconstruction.
Magnetic and contrast
properties of labeled platelets using magnetomotive optical
coherence tomography, A. L. Oldenburg, C. M. Gallippi, F.
Tsui, T. C.
Nichols, K. N. Beicker, R. K. Chhetri, D. Spivak, A. Richardson,
T. H. Fischer, Biophys. J. 99,
article introduces a new functional imaging paradigm that uses optical
coherence tomography (OCT) to detect rehydrated, lyophilized platelets
(RL platelets) that are in the preclinical trial stage and contain
superparamagnetic iron oxides (SPIOs) approved by the U.S. Food and
Drug Administration. Platelets are highly functional blood cells that
detect and adhere to sites of vascular endothelial damage by forming
primary hemostatic plugs. By applying magnetic gradient forces, induced
nanoscale displacements (magnetomotion) of the SPIO-RL platelets are
detected as optical phase shifts in OCT. In this article, we
characterize the iron content and magnetic properties of SPIO-RL
platelets, construct a model to predict their magnetomotion in a tissue
medium, and demonstrate OCT imaging in tissue phantoms and ex vivo pig
arteries. Tissue phantoms containing SPIO-RL platelets exhibited
>3 dB contrast/noise ratio at ≥ 1.5 x 109
OCT imaging was performed on ex vivo porcine arteries after infusion of
SPIO-RL platelets, and specific contrast was obtained on an artery that
was surface-damaged (P < 10-6
). This may
enable new technologies
for in vivo monitoring of the adherence of SPIO-RL platelets to sites
of bleeding and vascular damage, which is broadly applicable for
assessing trauma and cardiovascular diseases.
Study of magnetic
anisotropy and magnetization reversal using the quadratic
effect in epitaxial CoxMnyGez(111)
films, P. K. Muduli, W. C. Rice, L. He, B. A. Collins, Y. S. Chu, F.
Tsui, J. Phys. Conden. Matter 21, 296005
anisotropy, magnetization reversal and the magnetooptic Kerr effect in
have been studied over a range of compositions between 0 and
50 at.% of Ge and between 1 and 3 in the Co to Mn atomic ratio,
including the Heusler alloy Co2
MnGe. A strong
Kerr effect has been observed within a narrow region of composition
centered around the Co to Mn atomic ratio of 2, which has been used to
probe and quantify the magnetic anisotropy and magnetization reversal
of the system. The anisotropy is sixfold with a weak uniaxial
component, and it exhibits sensitive dependence on composition,
especially on the atomic ratio between Co and Mn. The magnetization
reversal process is consistent with the single-domain Stoner–Wohlfarth
dependence of magnetic anisotropy and quadratic magnetooptical
effect in epitaxial films of the Heusler alloy Co2MnGe, P.
K. Muduli, W. C. Rice, L. He, F. Tsui, J. Magn. Magn. Mater. 320, L141
anisotropy and magnetooptic Kerr effect for epitaxial films of
grown on Ge (1 1 1) substrates have been studied
systematically in the compositional vicinity of the Heusler alloy
MnGe. A large quadratic magnetooptic Kerr
effect has been observed
within a narrow region of composition centered around the Co to Mn
atomic ratio of 2. The effect has been used to probe and quantify the
magnetic anisotropy of the system, which is shown to have a strong
sixfold in-plane component accompanied by a weak uniaxial component at
room temperature. These properties are shown to depend sensitively on
atomic ratio between Co and Mn, indicating the presence of an intrinsic
stability and strain states in Co and Mn-doped Ge (001) epitaxial
films, B. A. Collins, Y. S. Chu, L. He, Y.
S. Vogt, F. Tsui, Phys. Rev. B
investigation of structural, chemical, and magnetic properties of Co
and Mn-doped Ge (001) as
a function of doping concentration reveals that codoping with Co can
dramatically reduce phase separation and
diffusion of Mn within the Ge lattice while it magnetically complements
Mn. The measured strain states
indicate the critical role played by substitutional Co with its strong
tendency to dimerize with interstitial Mn.
Selecting appropriate codopants that form energetically stable dimers
in a semiconductor host is shown to be
a viable approach, thus demonstrating the feasibility for engineering
stable doped magnetic semiconductors.
and characterization of a ternary epitaxial film of Co and Mn doped Ge
(001), F. Tsui, B. A. Collins, L. He, A. Mellnik, Y. Zhong,
S. Vogt, Y. S. Chu, Appl. Surf. Sci.
molecular beam epitaxy synthesis and properties
of a ternary epitaxial film of Co and Mn co-doped Ge grown on Ge (001)
substrate. Structural effects were examined in situ by reflection
high-energy electron diffraction and ex situ by microbeam X-ray
diffraction techniques, and magnetic properties were probed by using
magnetooptic Kerr effect. Ternary epitaxial phase diagrams have been
studied for total doping concentrations up to 30 at.%, where regions of
coherent epitaxy and rough disordered growth and those of near room
temperature ferromagnetic ordering have been identified.
diffraction studies of combinatorial epitaxial Ge (001) thin-films on
Ge (001) substrates, Y. Zhong, Y. S. Chu, B. A.
Collins, F. Tsui, Appl.
Surf. Sci. 254,
report high-resolution X-ray diffraction studies of combinatorial
epitaxial Ge (001) thin-films with varying doping concentrations of Co
and Mn grown on Ge (001) substrates. The crystalline structure of the
epitaxial thin-film has been determined using crystal-truncation rod
(CTR) measurements and fitting analysis. By analyzing the fine
interference fringes in the CTR intensity profile, strain sensitivity
of ~0.003% has been achieved. Using this method, the evolution of
interfacial structures has been quantified as a function of doping
Epitaxial growth of
L. He, B. A. Collins, F. Tsui, Y. Zhong, S. Vogt, Y.S. Chu, J. Vac. Sci.
Tech. B 25,
epitaxial growth of (FeCo)x
films on Ge and GaAs (001) substrates has been studied systematically
with x in the range between 0 and 17 at. %, using combinatorial
molecular beam epitaxy (MBE) techniques. Complementary doping using the
two transition metal dopants into Ge (001) during MBE growth is shown
to produce high quality coherent epitaxial films for transition metal
concentrations as high as 11 at. %. As the doping level
increases, rough growth occurs, which is accompanied by an increasing
amount of stacking faults along the <111> directions. The
crystal lattice that resulted from the rough growth exhibits a large
out-of-plane tetragonal distortion. There are no detectable secondary
phases up to a combined transition metal concentration of 17
at. %. The behaviors are shown to be invariant with respect to
the choice of substrates.
diffraction study of disorders in epitaxial films of the Heusler alloy
B. A. Collins,
Y. Zhong, Y. S. Chu, L. He, F. Tsui, J.
Tech. B 25,
authors report a study of structural and chemical disorders in a
ternary combinatorial epitaxial film of Cox
in the composition range that includes the Heusler alloy Co2
using microbeam anomalous x-ray diffraction techniques. The structural
and chemical ordering of the alloy has been found to be extremely
stable over a large composition range, while elemental site swapping
and sublattice vacancies have been identified. A model of anomalous
diffraction around the Co and Ge edges is presented and shown to make
possible the identification and quantification of these disorders in an
Structure and magnetism
epitaxial films, F. Tsui, L. He, D. Lorang, A. Fuller, Y. S. Chu, A.
S. Vogt, Appl.
Surf. Sci. 252, 2512
structural and magnetic phase diagrams of Coa(1-x)
epitaxial films near the composition of Heusler alloy Co2
have been studied using combinatorial molecular beam epitaxy
techniques. Epitaxial growth on Ge (111) has been stabilized over
nearly the entire composition range of the ternary system. Epitaxial
constraints play an important role for the small number of observed
structural phases. The variety of high-temperature magnetic properties,
and the structural and chemical compatibilities with group IV elements
make the ternary system promising for the science and applications of
combinatorial molecular beam epitaxy, F. Tsui,L. He, Rev. Sci.
considerations for implementing combinatorial approach to
molecular beam epitaxy (MBE) are discussed, focusing on the key issues
relevant to conventional MBE synthesis using solid sources and
characterization. The primary objective for implementing combinatorial
approach is to make MBE do more, more able to carry out controlled and
systematic work in large parameter space, without sacrificing any
existing capabilities of conventional MBE. Methods for accomplishing
this by integrating current instrumentation technology are described.
combinatorial materials by scanning X-ray fluorescence microscopy using
microfocused synchrotron X-ray beam, S. Vogt, Y. S. Chu, A. Tkachuk, P.
Ilinski, D. A. Walko, F. Tsui,
Surf. Sci. 223,
high-throughput scanning X-ray fluorescence (XRF) microscopy setup
using a microfocused synchrotron X-ray beam, which is optimized for
in-parallel X-ray characterization of composition and crystalline
structure of combinatorial samples. We present X-ray fluorescence
elemental maps of a full ternary CoxMnyGe1−x−y
composition-spread thin film and discuss the quantitative analysis
method used for obtaining the ternary composition.
carbide superlattice and nucleation of carbon nanotubes, F. Tsui, P. A.
J. Nanosci. Nanotech.
metal-carbon systems with known stable compounds, carbide
nanocrystals self-organize epitaxially on metal surfaces to form
two-dimensional arrays during carbon deposition. The process is
energetically driven by the competition between the strain and surface
energies, and it appears to play an important role in the nucleation of
single-walled carbon nanotubes. Interplay between energetics and
kinetics controls carbon precipitation from the superlattice, such that
the length scale of the carbide and superlattice appears to control the
size and morphology of the precipitates. Furthermore, carbon
precipitates appear to be "seedlings" of carbon nanotubes grown on top
of the carbide nanocrystals. These findings reveal that the nucleation
of carbon nanotubes is a nonequilibrium process and that a stable
carbide superlattice can be used as an ordered template of carbon
saturated "roots" for nucleating nanotube bundles with controlled
diameter, spacing, and perhaps chirality.
Evidence for strain
compensation in stabilizing epitaxial growth of highly doped germanium,
F. Tsui, L He, A. Tkachuk, S. Vogt, Y. S. Chu, Phys.
Rev. B 69, 081304(R)
report on a study of the epitaxial phase diagram of Co- and Mn-doped
Ge(001) magnetic semiconductors. Complementary doping using dopants
from different groups of elements can compensate for the effects of
lattice strain caused by the doping species. Reducing lattice mismatch
with the Ge host has been shown to be the key to stabilizing epitaxial
growth and suppressing phase separation at higher doping levels.
Applying this approach to other multidopant systems opens new prospects
for tailoring highly doped electronic materials.
investigation of CoMnGe combinatorial epitaxial thin films using
microfocused synchrotron X-ray, Y. S. Chu, A. Tkachuk, S. Vogt,
P. Ilinski, D. A. Walko, D. C. Mancini, E. M. Dufrense, L. He, F. Tsui,
Surf. Sci. 223,
discuss important experimental considerations and high-throughput
synchrotron-based techniques for structural characterization of binary
and ternary composition-spread thin films. We apply these techniques to
obtain detailed structural phase diagrams of CoMnGe ternary alloy
approach: A useful tool for studying epitaxial processes in doped
magnetic semiconductors, F. Tsui, Y. S. Chu,
(Feature Article) 189 (2004).
describe the recent discovery of promising new Ge-based magnetic
semiconductors and heterostructures using combinatorial molecular-beam
epitaxy for the science and applications of spintronics. We discuss key
experimental considerations for implementing combinatorial synthesis
and characterization, and highlight important findings in epitaxial
films of (100) Ge doped by Co and Mn, specifically the ternary
epitaxial phase-diagram, and the novel magnetic and
electrical-transport phenomena. We illustrate the natural "marriage"
between the controlled synthesis and combinatorial approach, and
demonstrate the usefulness of the approach for studying complex
synthesis of single-crystalline iron phosphide
nanorods/nanowires, C. Qian, F. Kim, L. Ma, F. Tsui, P. Yang, and J.
Am. Chem. Soc. 126 (4),
Novel Ge-based magnetic
semiconductors, F. Tsui, L. He, L. Ma, A. Tkachuk, Y. S. Chu, K.
Nakajima, T. Chikyow, Phys.
Rev. Lett. 91
rectification effect in a Ge-based magnetic
heterojunction, F. Tsui, L. Ma, and L. He, Appl.
Phys. Lett. 83
carbide superlattice during carbon deposition on Mo, F. Tsui and P. A.
Rev. Lett. 89
diagramming of epitaxial films, Y. K. Yoo and F. Tsui, MRS
magnetic phase diagram of epitaxial La0.67Sr0.33MnO3
thin films, F. Tsui, M. C. Smoak, T. K. Nath, and C. B. Eom, Appl.
Phys. Lett. 76,
susceptibility of multi-walled carbon nanotubes, F. Tsui, J. Lei, and
O. Zhou, Appl.
Phys. Lett. 76,
strain states and crystallographic domain structures of
epitaxial colossal magnetoresistive La0.8Ca0.2MnO3
thin films, R. A. Rao, D. Lavric. T. K. Nath, C. B. Eom, L. Wu, and F.
Phys. Lett. 73,
and layer-by-layer growth of epitaxial Rh, F. Tsui, J. Wellman, C.
Uher, and R. Clarke, Phys.
Rev. Lett. 76,