Article
pubs.acs.org/JPCC
3-D Atomic-Scale Mapping of Manganese Dopants in Lead Sulfide
Nanowires
Dieter Isheim,†,‡ Jason Kaszpurenko,§ Dong Yu,§ Zugang Mao,† David N. Seidman,†,‡ and Ilke Arslan*,∥,⊥
†
Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, Illinois 60208,
United States
‡
Northwestern University Center for Atom-Probe Tomography (NUCAPT), 2220 Campus Drive, Evanston, Illinois 60208,
United States
§
Physics Department, University of California, Davis, California 95616, United States
∥
Department of Chemical Engineering and Materials Science, University of California, Davis, California 95616, United States
⊥
Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, United States
ABSTRACT: Dopants in nanowires, whether intentional or unintentional,
can ultimately control the material’s properties and, therefore, need to be
understood on the atomic scale. We study vapor−liquid−solid grown
manganese-doped lead sulfide nanowires by atom-probe tomography for the
first time for lead salt materials. The three-dimensional chemical concentration maps at the atomic scale demonstrate a radial distribution profile
of Mn ions, with a concentration of only 0.18 and 0.01 at. % for MnCl2 and
Mn-acetate precursors, respectively. The ability to characterize these small
concentrations of dopant atoms in Pb1−xMnxS nanowires (x = 0.0036 and
0.0002), important for spintronic and thermoelectric devices, sets a platform
for similar analyses for all nanostructures. First-principles calculations confirm that Mn atoms substitute for Pb in the PbS structure.
■
INTRODUCTION
Dilute magnetic semiconductors (DMSs)1 not only are an ideal
test bed for exploring spin-related transport and magnetization
but also have the potential to revolutionize the next electronics
generation at faster speeds and reduced energy consumption by
exploiting spins instead of charges as the main information
carriers. To date, the main focus of the DMS research is on wideband-gap semiconductors, where a high Curie temperature has
been predicted theoretically2 and observed experimentally.3
Alternatively, narrow-band-gap DMSs are less studied and may
provide distinct perspectives on magnetotransport mechanisms,
which are complex and not well-understood.4,5 Low-dimensional
DMSs, such as quantum dots (QDs) and nanowires (NWs),
facilitate device miniaturization and may also have enhanced
magnetoresistance (MR) and higher Curie temperatures, as the
quantum confinement effect and restricted transport direction
can strengthen the exchange interaction between impurity spins
and conduction charges. The QDs, even in the absence of
transition-metal ions, exhibit MR effects because of squeezing of
electron wave functions6 and spin blockade.7 Addition of local
spins into low-dimensional semiconductors can create an effective local magnetic field of 100−1000 T and a giant Zeeman
splitting on the order of 10−100 meV.8 NWs, at the smallest
dimension with electrical connections, provide an ideal platform
for studying magnetotransport.
In this article, we report on the successful vapor−liquid−
solid (VLS) growth of Pb1−xMnxS NWs and their atomic-scale
© 2012 American Chemical Society
characterization in three dimensions using atom-probe
tomography (APT).9−14 PbS has been employed in photodetectors and is recognized as a promising material for photovoltaics.15,16 PbS has several unique properties, such as a
narrow band gap (0.41 eV at room temperature), high dielectric
constant (190), and a large exciton Bohr radius (20 nm). Mn2+
ions are chosen as the dopants because (1) their half-filled d
orbital yields a maximum sp−d exchange interaction with the
conduction electrons and (2) Mn2+ ions act neither as donors
nor as acceptors in PbS if Mn2+ substitutes Pb2+ and thus will
not affect the carrier type and concentration in PbS.
Several routes to synthesize Mn-doped PbS and PbSe nanostructures have been attempted, including solution synthesis,17,18
molecular-beam epitaxy (MBE),19 and the fusion method in a
glass matrix.20 One major difficulty of doping nanostructures is
that the impurities tend to migrate to the surface by a selfpurification process.21 Additionally, there has been a dearth of
effective characterization techniques to determine the spatial
distribution of the impurity ions, which has a great impact on the
electronic and magnetic properties of the nanostructures.22−24
Specifically, magnetic ions that segregate at the surface have
much weaker interactions with conduction electrons, and magnetic ions forming clusters result in secondary phases, which can
Received: January 5, 2012
Revised: February 13, 2012
Published: February 16, 2012
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convolute the study of the DMS properties. Standard analysis
tools, such as energy-dispersive X-ray spectroscopy (EDS), X-ray
diffraction (XRD), electron paramagnetic resonance (EPR), and
Fourier transform infrared spectroscopy (FTIR), have been used
to investigate DMS nanostructures.17,18,25 These techniques are,
however, not applicable to low dopant concentrations and/or the
small quantity of samples from a typical VLS growth. Even when
effective, these analysis instruments leave open questions concerning dopant location and uniformity. The atom-probe
tomograph (APT) provides three-dimensional (3-D) chemical
mapping of a single NW with atomic-scale spatial and chemical
resolution and, therefore, is the instrument of choice for
analyzing DMS NWs. In APT, a single NW is dissected atomby-atom and atomic plane-by-plane employing either highvoltage pulses or picosecond ultraviolet (UV) laser pulses. A twodimensional (2-D) position-sensitive detector records the spatial
positions of the evaporated atoms in a NW and in parallel their
chemical identities via time-of-flight mass spectrometry, that is,
mass-to-charge state ratios. The depth scale (z dimension) is
determined from the chronological sequence in which ions arrive
at the detector. APT has been applied in several cases to study
the nanoscale compositions of NWs,26 but no publication for this
family of NWs using APT exists to date. The APT studies are
complemented by first-principles calculations to confirm the
substitutional site preference of Mn atoms within the PbS
structure.
Figure 1. Synthesis and characterization of (Pb1−xMn x)S NWs. (a)
Schematic of the tube furnace. (b) Scanning electron microscope
image of hyperbranched NWsa dense network of NWs. (c)
Scanning electron microscope image of hyperbranched
NWs(Pb1−xMn x)S NWs using MnAc2 as precursor. (d) X-ray
diffraction pattern of pure PbS NWs and (Pb1−xMn x)S NWs grown
with different precursors. Note that the peak at 2θ = 33° and the broad
shoulder near this peak, indicated by an asterisk, are from the Si
substrate.
■
METHODS
Preparation of Mn-Doped PbS NWs. MnCl2·4H2O
(99%) and anhydrous 98% Mn(II) acetate are both purchased
from Alfa Aesar. MnCl2 does not contain carbon and thereby
reduces the possible contamination of the NWs. Its melting
point (654 °C) requires a high growth temperature, which
often causes the NWs to kink.27 MnAc2 with a significantly
lower melting temperature (210 °C) yields straight and dense
NWs even at a weight ratio of MnAc2 to PbCl2 as high as 1:1.
In a typical growth procedure, a quartz tube was evacuated to a
base pressure of 80 mTorr and then was filled with N2 to
ambient pressure. An alumina boat of PbCl2 (99.999%, Alfa
Aesar) was placed in the center of the quartz tube, and a boat of
sulfur (99.999%, Alfa Aesar) was placed upstream near the
mouth of the tube furnace (1:2 ratio of PbCl2 to S). A Si wafer
(⟨100⟩ test grade, University Wafers) was placed at 8−12 cm
downstream from the tube’s center. One of two precursors,
MnCl2 or Mn (II) acetate, was inserted between PbCl2 and S
(Figure 1a). N2 flowing at 150 sccm was maintained during the
entire growth procedure. Once a peak furnace temperature, Tp,
was achieved, a short burst of H2 (30−90 at 1 sccm) was flowed
into the system to initiate the formation of Pb nanoparticles on
the Si wafer: Tp was varied from 600 to 670 °C, depending on
the precursor. The sample was maintained at Tp for a total of
15 min, and then the sample was cooled to room temperature
over 3−4 h.
Atom-Probe Tomography Measurements. To prepare
individual NWs for analysis by APT, a micromanipulator in a
dual-beam FIB microscope was used to mount individual NWs
on silicon microposts. To accomplish this, the NWs were first
attached to the micromanipulator needle with a small (∼1 ×
1 μm2) patch of Pt that is deposited with the electron beam in
the FIB from a Pt metalorganic precursor gas. Next, the NWs
were removed from the silicon wafer by slowly translating the
micromanipulator needle until the NWs detached. Figure 2a
displays a pure PbS NW attached to the micromanipulator
needle after having been detached from a hyperbranched NW
bundle. The NW was then aligned with the long axis of a silicon micropost, and then attached to it, as shown in Figure 2b.
Figure 2. Mounting procedure for attaching a NW to a prefabricated
silicon micropost: (a) An individual PbS NW from a hyperbranched
bundle is attached to the micromanipulator needle with an electronbeam deposited Pt patch and then detached from the bundle. (b) The
NW is translated and then aligned axially with a silicon micropost and
next affixed to it with an electron-deposited Pt patch.
A small (∼2 × 2 μm2) patch of Pt was then deposited with the
electron beam across the segment of the NW that overlapped the
silicon micropost to affix it: Figure 2b displays a NW mounted on
a silicon micropost. After the NW was mounted on the micropost, the micromanipulator was detached by translating the
manipulator needle to break the Pt patch that connected the NW
to the needle.
APT analyses were performed using a local-electrode atom-probe
(LEAP) tomograph LEAP 4000XSi (Cameca, Madison, WI).
Laser-assisted evaporation of the surface atoms from a microtip was achieved using an applied voltage of ∼2 kV dc and
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picosecond ultraviolet (UV) laser pulses with a wavelength of
355 nm. During APT analyses, the MnCl2 precursor grown
NWs were maintained at 80 K, while the pure PbS and
Mn-acetate precursor grown NWs were held at 30 K. The base
pressure of the ultra-high-vacuum (UHV) chamber was
2 × 10−11 Torr during the analyses, and a laser pulse energy
of 25 pJ was employed at a pulse repetition rate of 100 or
250 kHz to achieve a target evaporation rate of 0.002−0.01
atom pulse−1. Three-dimensional reconstructions and data evaluation were performed using Cameca’s IVAS3.4 code. The
reconstruction parameters were fitted to the diameters of the
NWs measured by scanning electron microscopy (SEM), 88,
80, or 110 nm for the pure PbS, MnCl2, and Mn-acetate precursor grown NWs, respectively. Since the atom probe analyzes
only the central part of the tip specimen, the actual reconstructions of the analyzed volumes have a smaller diameter.
First-Principles Calculations. Density-functional theory
calculations are used to determine the Mn site preference in the
PbS crystal structure, the substitutional energies for a Mn atom
replacing a Pb or a S atom, respectively, which employ the
plane-wave total-energy methodology with the generalized
gradient approximation28 using the Vienna Ab initio Simulation
Package (VASP).29−33 We employ projector augmented wave
(PAW) potentials with spin-polarized technique.34 Unless
otherwise specified, all structures are fully relaxed with respect
to volume as well as all internal atomic coordinates. We
carefully considered the convergence of results with respect to
energy cutoff and k-points. A plane-wave basis set was used
with an energy cutoff of 300 eV to represent the Kohn−Sham
wave functions. The summation over the Brillouin zone for the
bulk structures is performed on a 0.13 (1/Å) spacing
Monkhorst−Pack k-point mesh for all calculations. We use a
supercell with 2 × 2 × 2 unit cells with 32 Pb and 32 S atoms
for calculating the substitutional energies. The total energies of
the fully relaxed supercells are calculated before and after Pb
substituting in either Pb or S sublattice site.
Figure 3. (a) APT mass spectrum of a pure PbS NW. Portions of APT
mass spectra for NWs grown as (b) pure PbS, (c) PbS doped using a
MnCl2 precursor, and (d) PbS doped using a Mn-actetate precursor.
RESULTS AND DISCUSSION
The synthesis of Mn-doped PbS NWs was performed using a
modified approach of earlier research (Figure1a).35,36 We have
chosen MnCl2 and Mn(II) acetate as precursors to dope PbS
NWs, respectively. The growth yielded dense networks and/or
hyperbranched NWs that have diameters of 50−100 nm and are
tens of micrometers long (Figure 1b,c). XRD confirmed the
crystallinity and composition of the Pb1−xMnxS NWs (Figure 1d).
Individual NWs were mounted for APT analysis, on
prefabricated silicon microposts in a dual-beam focused ionbeam (FIB) microscope with a direct mounting technique (see
Figure 2) that does not involve any cutting with the Ga ion
beam, unlike conventional lift-out techniques.37,38 The advantage is that the NWs do not need to be exposed to the Ga-ion
beam, and therefore, no protective cover layers need to be
deposited. Figure 2a displays a pure PbS NW attached to a
micromanipulator needle after having been detached from a
hyperbranched NW bundle, and Figure 2b exhibits a NW
attached to a micropost ready for APT analysis.
Figure 3a is an APT mass spectrum of a Mn-acetate grown
NW. Sulfur is observed exclusively as molecular ions 64,65,66,68S2+,
64,65,66,68 2+
S 2 , and 96,97,98,100 S 3 + , and also as part of
442,444,445,446,447,448
Pb2S2+, 238,239,240PbS+, 268,270,271,272,273,274PbS2+,
302,303,304,305,306
PbS3+, and 334,335,336PbS4+ molecular ions. Manganese is detected as 55Mn2+ and 55Mn+ ions, and Pb as singly or
doubly charged ions, 204,206,207,208Pb+ and 204,206,207,208Pb2+. The
evaporation of molecular ions is frequently observed when studying compound semiconductor materials with APT.39−41 Figure 3
is a comparison of the 25−40 amu ranges of three mass spectra
for (b) pure PbS, (c) MnCl2, and (d) Mn-acetate precursor
grown NWs, respectively. The different base temperature did not
affect the mass spectrum significantly, as is evident, for instance,
from the similarity of the 64S2+ peaks for all three specimen types
in Figure 3b. The mass spectrum of pure PbS has a small peak
at 28 amu, corresponding to a 17 ± 2 at. ppm impurity concentration of either 28Si+ or 28N2+ molecular ions; the first is
possibly from the Si substrate used for the NW growth and the
latter possibly from residual gas in the UHV chamber. The mass
spectra for MnCl2 and Mn-acetate precursor grown NWs show
distinctive peaks at 27.5 amu for 55Mn2+. A quantitative analysis
of the peaks at 32 and 33 amu demonstrates that they are 64S22+
and 66S22+ dimeric ions: The two most abundant S isotopes are
32
S and 34S with 94.99% and 4.25% isotopic abundances.42
Therefore, the most abundant molecular weights of S2 are 64
(32S + 32S) and 66 (32S + 34S) and have 90.23% and 8.07% relative isotopic abundances, calculated from the isotopic
abundances using the binomial permutations of the individual
isotopes. In the doubly charged state, the dominant peaks are at
32 and 33 amu. The absence of a peak at 34 amu demonstrates
that the peaks at 32 and 33 are exclusively S2 dimeric ions.
■
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Table 1. Concentrations Measured by Atom-Probe Tomography for NWs Grown as (a) Pure PbS, (b) PbS Doped with Mn
Using MnCl2 Precursor Material, and (c) PbS Doped with Mn Using Mn-actetate as Precursor Material
concentrationa (at. %)
number of atoms
b
NW type
Mn
pure PbSc
MnCl2 grownd
Mn-acetate grown
ND
5587
5461
b
Pb
S
Mn
Pb
S
3 467 888
1 536 589
23 619 551
3 606 598
1 568 638
24 379 463
ND
0.18 ± 0.02
0.0114 ± 0.0002
48.61 ± 0.02
49.40 ± 0.03
49.20 ± 0.01
51.39 ± 0.02
50.43 ± 0.03
50.79 ± 0.01
a
The concentration error given is the statistical uncertainty calculated from the total number of atoms collected using σ = [c(1 − c)/N]1/2, where c is
the measured concentration and N is the number of atoms. bND = not detected. cTrace impurities of 17 ± 2 at. ppm of either Si or N2 were detected
in this sample. dNumber of atoms corrected for time-of-flight mass spectrum cutoff effect: only 34% of the Pb and 48% of the S atoms were detected
in the mass range below 200 amu.
The similarity between the 64S22+ and 66S22+ peaks in the three
spectra is indicative of similar evaporation conditions for the
different APT experiments. This fact is used to correct the
overall compositions for samples for which the time-of-flights of
the heavier (>200 amu) molecular ions were longer than the
time-of-flight window. For instance, the mass spectrum for the
MnCl2 precursor grown NW was recorded up to 200 amu,
therefore, capturing only 34% of the Pb and 48% of the
S atoms. The concentration values for the MnCl2 grown NW
listed in Table 1 were calculated using the corrected number
of atoms.
Table 1 presents the measured overall compositions for pure
PbS, and MnCl2 and Mn-acetate precursor grown NWs. The
measured PbS stoichiometry of the base material is Pb48.6S51.4
for the pure PbS NW, Pb49.4S50.4 for the MnCl2 precursor
grown NW, and Pb49.2S50.8 for the Mn-acetate precursor grown
NW. The Mn concentration is 0.18 ± 0.02 at. % in the MnCl2
grown NW, and 0.0114 ± 0.0002 at. % in the NW grown with
the Mn-acetate precursor. Mn-acetate has a low melting point
(210 °C) and may have completely evaporated prior to the
formation of the NWs, leading to a lower Mn concentration.
The 0.18% Mn concentration corresponds to an average distance of 4 nm between neighboring Mn ions.
Figure 4 displays the 3-D APT reconstructions obtained for
the three types of NWs. The reconstruction volumes are 37 ×
38 × 389, 35 × 35 × 166, and 50 × 50 × 1444 nm3 and contain
7 074 659, 1 267 365, and 48 004 475 atoms, respectively, for
(a) pure PbS, (b) MnCl2, and (c) Mn-acetate precursor grown
NWs. The 3-D distributions of Mn atoms are displayed next to
the reconstructions containing all elements for the Mn-doped
NWs. Projections of the Mn atoms along the z axis of the reconstruction volumes are displayed in Figure 4d,e. It is seen
that the Mn atoms are fairly uniformly distributed, with a
possible higher concentration around the central axis of the
NWs. This indicates that Mn is most likely incorporated
through the catalytic droplet and not from the surface. The
distribution of Mn atoms in the NWs is seen in greater detail in
the concentration profiles (Figure 5). Figure 5a,b shows axial
concentration profiles taken along the z axis of the NWs. The
dashed line in the graphs is the average Mn concentration for
each NW listed in Table 1, 0.18 ± 0.02 and 0.0114 ± 0.0002
at. % for the MnCl2 and Mn-acetate precursor grown NWs,
respectively. Figure 5a,b demonstrates that the Mn concentration along the z axis of the NW is uniform. Figure 5c,d
displays radial Mn concentration profiles averaged over the
entire analyzed length for (c) the MnCl2 and (d) Mn-acetate
precursor grown NWs. The Mn concentration is higher along
the central portion of the NWs and continuously decreases
toward the surface of the NWs. The dashed lines represent
the overall average Mn concentrations. The radial distribution
Figure 4. Three-dimensional atom-probe tomographic reconstructions of sections of NWs grown as (a) pure PbS, (b) PbS doped
with Mn using a MnCl2 precursor, and (c) PbS doped with Mn
using Mn-actetate as precursor. The reconstructed volumes are 37
× 38 × 389, 35 × 35 × 166, or 50 × 50 × 1444 nm3 in size and
contain 7 074 659, 1 267 365, or 48 004 475 atoms, respectively. In
(a) and (c), only 20% of Pb and S atoms are displayed for clarity. In
(d) and (e), the distributions of Mn atoms are displayed in a
projection along the axis of the NWs (z axis) over the total length
of the reconstructions in (b) and (c), with 5587 Mn atoms in (d)
and 5461 Mn atoms in (e).
of Mn dopants with a higher concentration at the center is an
unexpected result and one that will be explored more deeply
in a future publication.
Understanding the distribution of Mn atoms on an atomic
scale also requires knowledge of the site preference of Mn
atoms within the PbS lattice. If Mn atoms dissolve interstitially,
then charge balance should result in n-type doping, as each
interstitial Mn atom donates two electrons. This is inconsistent
with the experimental observation that Mn-doped PbS NWs
remain p-type (Figure 6). Thus, we only consider the possibility
that Mn atoms dissolve substitutionally. As a measure of the
PbS
PbS
and EMn→S
site preference, the substitutional energies EMn→Pb
for a Mn atom replacing a Pb or a S atom, respectively, are
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atom, so x = 1/32 and y = 1/32). The quantity μ is the
chemical potential, and the values of Pb, S, and Mn are −3.697,
−0.512, and −1.053 eV atom−1. The calculated results demonstrate that Mn atoms prefer strongly to substitute for Pb on the
PbS
Pb sublattice sites of PbS, as EMn→Pb
is significantly smaller than
PbS
EMn→S (Table 2).
PbS
PbS
Table 2. Substitutional Energies, EMn→Pb
and EMn→S
,
Determined by First-Principles Calculationsa
(Pb1−xMnx)S
Pb(S1−yMny)
PbS
EMn→Pb,S
(eV atom−1)
average atomic
force (eV Ǻ −1)
average atomic
displacement (Ǻ )
−0.982
1.598
0.0128
0.0484
0.1091
0.1886
a
Since the simulation cell contains 32 Pb and 32 S atoms, x = 1/32
and y = 1/32 in this calculation for substituting one Pb or S atom by a
Mn atom.
The average atomic forces and displacements at the first
nearest-neighbor distance associated with the local stresses and
strains resulting from the substitution of Mn on the Pb and S
sublattice sites are listed in Table 2. We note that no atomic
force was measured for the relaxed PbS phase prior to Mn site
substitution. The substitution of Mn on the Pb sublattice sites
results in smaller values of the atomic force and displacement
than substitution of Mn at the S sites, and therefore, Mn prefers
the Pb sublattice sites. This result is also consistent with the
electrical measurements of single (Pb1−xMnx)S NW devices,
which exhibit similar p-type carrier concentrations as pure PbS
NWs (Figure 6).
Figure 5. Axial (z axis) concentration profiles of Mn for NWs grown
as (a) PbS doped with Mn using a MnCl2 precursor and (b) PbS
doped with Mn using a Mn-acetate precursor. Radial concentration
profiles of Mn for NWs grown as (c) PbS doped with Mn using a
MnCl2 precursor and (d) PbS doped with Mn using a Mn-actetate
precursor. All profiles are averaged over the entire cross section along
the length of the reconstruction volume in Figure 4.
calculated utilizing density functional theory. The substitutional
PbS
PbS
energies, EMn→Pb
and EMn→S
in PbS are defined as by
PbS
tot
EMn
→ Pb = [(E(Pbx Mn1 − x)S + nPbμPb)
(1)
PbS
tot
EMn
→ S = [(EPb(S1 − yMn y) + nSμS)
tot + n μ )]/n
− (EPbS
Mn Mn
Mn
CONCLUSIONS
■
AUTHOR INFORMATION
NWs of pure and Mn-doped PbS have been grown by the VLS
technique and characterized with subnanoscale spatial resolution and atomic-scale chemical sensitivity in 3-D utilizing
atom-probe tomography. The results indicate an absence of Mn
for the pure PbS NW, and Mn dopant concentrations of 0.18 or
0.01 at. % for the NWs grown with MnCl2 or Mn-acetate precursors, respectively. The Mn dopants are found to be distributed uniformly along the axial length (z axis) of the NW but
have a varying radial concentration, with the highest concentration in the center of the NWs and a decreasing concentration
toward the surfaces of the NWs. This level of detailed characterization and sensitivity to the extremely small dopant concentrations in these nanostructures cannot be achieved by any
other method. Furthermore, these are the first APT results
reported on NWs of the lead salt family, and this article defines
the conditions for their successful analyses. First-principles
calculations confirm that Mn has a strong preference for
occupying lattice sites on the Pb sublattice of the PbS structure.
These measurements and calculations open the door to a new
level of understanding of doped NWs and will contribute to the
optimized synthesis and function of future nanodevices.
Figure 6. Current−voltage characteristics at various gate voltages for a
field effect transistor (FET) incorporating a single PbS NW grown
with the MnAc2 precursor, showing the p-type nature of the NW. We
extracted a hole concentration of ∼1018 cm−3 and mobility of ∼40
cm2/(V s). The gate voltages (Vg) from top to bottom are 4, 2, 1, 0,
−1, −2, and −5 V.
tot + n μ )]/n
− (EPbS
Mn Mn
Mn
■
Corresponding Author
(2)
*E-mail: [email protected].
where the Etot's are the calculated total energies before and after
substitution, n is the number of substitutional atoms, and x and
y are the fractions substituted. We utilize only one substitutional
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
The NW growth and characterization were supported by U.C.
Davis Startup Funds and a Hellman Fellowship. D.Y. and J.K.
thank C. Miller and K. Yoha for their assistance in NW growth.
Atom-probe tomographic measurements were performed at the
Northwestern University Center for Atom-Probe Tomography
(NUCAPT). The LEAP tomograph was purchased and
upgraded with funding from NSF-MRI (DMR-0420532) and
ONR-DURIP (N00014-0400798, N00014-0610539, N000140910781) grants. We also gratefully acknowledge the Initiative
for Sustainability and Energy at Northwestern (ISEN) for
grants to upgrade the capabilities of NUCAPT. This research
made use of the EPIC facility (NUANCE Center) at
Northwestern University. The Pacific Northwest National
Laboratory is operated by Battelle for the U.S. Department of
Energy under contract DE-AC05-76RL01830.
■
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