NHC-Catalyzed Chemoselective Reactions of Enals and Aminobenzaldehydes for Access to Chiral Dihydroquinolines

Article abstract of DOI:10.1002/anie.201909479

An N-heterocyclic carbene (NHC)-catalyzed reaction between α-bromoenals and 2-aminoaldehydes has been developed. Key steps include chemoselective reaction of the NHC catalyst with one of the aldehyde substrates (the bromoenal) to eventually generate an α,β-unsaturated acylazolium intermediate. Addition of the nitrogen atom of aminoaldehyde to the unsaturated azolium ester intermediate followed by intramolecular aldol reaction, β-lactone formation, and decarboxylation leads to chiral dihydroquinolines with high optical purity. The dihydroquinoline products, which are quickly prepared by using this method, can be readily transformed into a diverse set of functional molecules such as pyridines and chiral piperidines.

Full text of DOI:10.1002/anie.201909479

Letter
pubs.acs.org/NanoLett
Cite This: Nano Lett. 2019, 19, 1527−1533
Isotope Effect in Bilayer WSe2
Wei Wu,†,‡ Mayra Daniela Morales-Acosta,‡ Yongqiang Wang,§,∥ and Michael Thompson Pettes*,†,‡,∥
†Department of Mechanical Engineering, University of Connecticut, Storrs, Connecticut 06269, United States
‡Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269, United States
§Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
∥Center for Integrated Nanotechnologies (CINT), Materials Physics and Applications Division, Los Alamos National Laboratory,
Los Alamos, New Mexico 87545, United States
S
* Supporting Information
ABSTRACT: Isotopes of an element have the same electron
number but differ in neutron number and atomic mass.
However, due to the thickness-dependent properties in MX2
(M = Mo, W; X = S, Se, Te) transition metal dichalcogenides
(TMDs), the isotopic effect in atomically thin TMDs still
remains unclear especially for phonon-assisted indirect excitonic
transitions. Here, we report the first observation of the isotope
effect on the electronic and vibrational properties of a TMD
material, using naturally abundant NAWNASe2 and isotopically
pure 186W80Se2 bilayer single crystals over a temperature range
of 4.4−300 K. We demonstrate a higher optical band gap energy in 186W80Se2 than in NAWNASe2 (3.9 0.7 meV from 4.41 to
300 K), which is surprising as isotopes are neutral impurities. Phonon energies decrease in the isotopically pure crystal due to
the atomic mass dependence of harmonic oscillations, with correspondingly longer E2g and A2 phonon lifetimes than in the
1g
naturally abundant sample. The change in electronic band gap renormalization energy is postulated as being the dominant
mechanism responsible for the change in optical emission spectra.
KEYWORDS: Isotope engineering, band gap engineering, tungsten diselenide, transition metal dichalcogenide, photoluminescence,
Raman
in WSe2 and MoSe2 monolayers.11,24 Yet, an intrinsic route to
ith the discovery of graphene,1 the atomically thin
Wfamily of materials receive interest continuously. A wide tune the electronic band structure and phonon dispersion
application in electronics,2−5 optoelectronics,6−11 and quan-
relationship in these materials still remains unexplored.
tum phononics12−16 has been achieved with the expansion of
Recently, the tunability of van der Waals interactions and
anharmonic phonon scattering in bulk hexagonal boron nitride
(h-BN) has been proven via experimental observation of
changes in the electronic band gap and Raman signature due to
the isotope effect.25,26 However, the isotopic effect on phonon
and optoelectronic properties still remains unknown in the
TMD class of atomically thin materials. Here, we report iso-
topic engineering of the optical band gap and phonon energy
in atomically thin bilayer naturally abundant NAWNASe2 and
isotopically pure 186W80Se2 by combing X-ray diffraction and
temperature-dependent Raman and photoluminescence spec-
troscopy from ∼4 K to room temperature.
this class of materials to the MX2 (M = Mo, W; X = S, Se, Te)
transition metal dichalcogenides (TMDs) possessing large
band gaps compared with zero-gap graphene. TMDs are a
group of materials consisting of three atom-thick layers with
transition metals covalently bonded with chalcogens in trigonal
prismatic coordination geometry and adjacent layers bonded
by relatively weak van der Waals interactions. Compared to
bulk materials, atomically thin TMDs offer size and tunability
advantages over traditional materials for miniaturization of
electronic and optical devices,6,7,17 especially as their
optoelectronic and vibrational properties are highly dependent
on layer thickness.18,19 Thus, the precise manipulation of
electron and phonon band structure in atomically thin TMDs
materials is the key to widespread adoption in applications
including energy conversion.20 Reversible modification of the
WSe2 occurs naturally with five W isotopes27 and six Se
isotopes28 with dominant concentrations of 184W (30.64% at.)
and 80Se (49.61% at.). The strength of isotopic disorder in a
compound is given by the second-order mass variance
parameter g = ∑i,j[ci,j(1−Mi,j/Mi,avg)2] in transport calcula-
tions,29,30 where ci,j and Mi,j are the concentration and atomic
21
electronic band gap in mono- and bilayer crystals of MoS2
10,22
and WSe2
has been demonstrated via strain,23 in which
bilayer WSe2 showed a two-orders of magnitude enhancement
of photoluminescence response with uniaxial tensile strain.10
Another method to extrinsically tune optoelectronic properties
has been achieved through application of external electric fields
Received: October 23, 2018
Revised: February 12, 2019
Published: February 12, 2019
© 2019 American Chemical Society
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Figure 1. Spatially dependent characterization of an isotopically pure bilayer 186W80Se2 crystal. (a) Optical image and (b) atomic force microscopy
(AFM) characterization of 186W80Se2 on a 285 nm SiO2-on-Si substrate. (c) AFM height profile corresponding to the line shown in part b. Spatially
resolved Raman intensity at (d) 248.5 cm−1 (E2g mode), (e) 256.7 cm−1 (A1g mode), and (f) 305.9 cm−1 (A21g mode), where the vibrational modes
are depicted schematically. (g) Spatially resolved photoluminescence (PL) intensity at the peak emission wavelength of 1.58 eV.
Figure 2. Isotopic mass dependent Raman spectra of bilayer WSe2. Normalized Raman spectra of (a) naturally abundant NAWNASe2 and (b)
isotopically pure 186W80Se2 over the temperature range from 4.41 to 300 K. (c) Phonon frequency difference (Δω) of E2g (red), A1g (blue), and
A21g (green) modes between NAWNASe2 and 186W80Se2, where error is defined by the standard deviation of six measurements each with a different
spectral window initial point in order to account for the instrument uncertainty. Temperature-dependent phonon lifetime and full width at half-
maximum of the (d) E2g mode and (e) A2 mode from 4.41 to 300 K.
1g
mass of ith atom and the jth impurity, respectively, and Mi,avg is
the average atomic mass. Naturally occurring NAWNASe2 has a
molecular mass of 341.98u and g of 9.94 × 10−4, where u is the
unified atomic mass constant. The g value can be reduced by
more than 377 times to 2.63 × 10−6 with a molecular mass of
345.99u through isotopic purification using 186W and 80Se at
commercially available purification levels greater than 99.9%
at., indicating that isotopic effects may play an important role
in TMDs such as WSe2 since the natural system is intrinsically
disordered.
In this first work on the isotope effect in a TMD material,
both naturally abundant NAWNASe2 and isotopically pure
186W80Se2 bilayers were grown by chemical vapor deposition
(CVD) on ∼285 nm SiO2-coated silicon substrates similar to
our previous report.10 Since the optical band gap is extremely
sensitive to strain and crystalline quality,10,31,32 the NAWNASe2
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and 186W80Se2 bilayers are synthesized under identical growth
conditions (for details, see methods and the Supporting
Information), eliminating differences in structure caused by
thermal expansion mismatch with the substrate.33 To isolate
the isotopic effect from the layer number19,34 and edge effect35
contributions to the optical band gap, Figure 1a−c
demonstrates our bilayer WSe2 single crystals are synthesized
so that the top and bottom layers have the same lateral
dimensions. Atomic force microscopy (AFM) analysis shows a
uniform thickness of ∼1.3 nm over the entire crystallite for
both NAWNASe2 and 186W80Se2 bilayers. Figure 1d−f illustrates
that the Raman spectra of an isotopically pure bilayer
186W80Se2 crystallite is spatially uniform in intensity throughout
mode represents selenium atom vibrations along the out-of-
plane direction with ΔωA1g = 1.53
0.07 cm−1. The more
interesting mode here is A21g with ΔωA21g = 1.94 0.06 cm−1.
The A2 mode represents an interlayer vibration involving
1g
both tungsten and selenium atoms from different van der
Waals layers and only appears for two or more layers of
WSe2.38 We observe that the isotopic effect on the interlayer
out-of-plane A21g mode is larger than on the in-plane E2g mode
or the intralayer out-of-plane A1g mode, which arises from the
fact that the isotopic effect on the weak interlayer van der
Waals interaction is slightly larger than it is on the strong
intralayer W−Se covalent interaction.
Besides the harmonic oscillation of phonons, the contribu-
tion of van der Waals bond length on phonon frequency also
needs to be evaluated to support the statement above. The
phonon frequency changes due to strain can be defined as10
the entire crystallite for the E2g, A1g, and A2 Raman active
1g
modes, which is similar as our previous reported naturally
abundant bilayer NAWNASe2.10 Photoluminescence (PL) also
demonstrates spatially uniform peak intensity and in the entire
crystallite (Figure 1g), further indicating the uniformity of
atomic-level thickness. Room temperature X-ray diffraction
(XRD) analysis of NAWNASe2 and 186W80Se2 is shown in Figure
S1 of the Supporting Information. We have also conducted
Rutherford backscattering spectrometry (RBS) analysis to
obtain the elemental atomic ratios of the materials in this
report. Analyzing the scattering yield ratios between W and Se,
we obtain a W/Se ratio of 1:1.98 for both the naturally
abundant and isotopically enriched samples indicating
comparable stoichiometry and hence sample quality (Figure
S2, Supporting Information).
Atomic vibrations described as phonons have energies which
are dependent on the atomic mass, where frequency changes
stemming from isotopic substitution can be monitored by
Raman spectroscopy.36 Figure 2a,b shows the evolution of
optical phonon energies with temperature in NAWNASe2 and
186W80Se2 bilayers using Raman spectroscopy and 532 nm laser
excitation with a point-to-point resolution of ∼0.51 cm−1 using
an 1800 gr/mm grating. To minimize the instrumental
uncertainty, we conducted six sets of measurements with
spectral windows defined by initial points differing by 0.1 cm−1
to fully cover the point-to-point separation. The mean and
standard deviation of modeled peak positions and full width at
half-maximum (fwhm) obtained for each of these six spectra
was used as the uncertainty for each data point. To avoid any
influence from the slightly nonuniform temperature distribu-
tion on the sample mount in the optical cryostat, all Raman
spectra were aligned using the silicon substrate peak at each
nominal temperature.
Δωi,strain = −εγiωi where γ is the Gruneisen parameter, ε is the
̈
hydrostatic strain, and i represents the phonon mode. The
interlayer van der Waals bond length change can be treated as
Δc = εzzc, where Δc is the c-lattice parameter change due to
isotopic substitution, c is the natural abundance c-lattice
parameter, and εzz is the out-of-plane strain component. Thus,
the phonon frequency change due to isotopic substitution in
this work can be approximated as Δωi,strain = −γiωiΔc/c. By
adopting the experimental Gru
̈
neisen parameter of the A2
1g
mode in bilayer WSe2 as 0.35710 and the c-lattice parameters
obtained from XRD analysis, the phonon frequency changes
expected due to isotope induced changes in van der Waals
bond length can be estimated on the order of ΔωA21g,strain
=
0.04 cm−1, which is negligible compared to the measured
experimental frequency changes.
The phonon lifetime is an important parameter that
describes phonon scattering processes and can be estimated
from the fwhm of the Raman peak as26 τ = ℏ/Γ where τ is the
phonon lifetime, ℏ is the reduced Planck constant, and Γ is the
fwhm. Figure 2d,e illustrates that isotopically pure 186W80Se2
exhibits slightly longer phonon lifetimes for both the intralayer
in-plane E2g mode and the interlayer out-of-plane A2 mode,
1g
although this is still only marginally higher than experimental
uncertainty. The observed decrease of the phonon lifetime
with temperature arises from an increase of phonon occupancy
and phonon−phonon interaction with temperature.39 At 4.41
(300) K, the phonon lifetime of the E2g mode in bilayer
186W80Se2 is 2.90 0.02 (2.52 0.05) ps and ∼2.16 0.03
(1.32 0.09) ps for the A21g mode, which is 10.1 (4.42) % and
11.1 (2.77) % higher than the lifetimes of E2g and A1g modes in
bilayer NAWNASe2, respectively.
The frequency of optical lattice vibrations is expected to
decrease with heavier isotopic atomic mass according to a
simple one-dimensional harmonic oscillator model37 as ω =
Figure 3 shows the temperature dependent Raman shift of
the E2g, A1g, and A2 modes of bilayer NAWNASe2 and
1g
186W80Se2. As the temperature increases from 4.41 to 300 K,
[2C·ΣjMj−1 1/2 for zone center and ω = (2C/Mj)1/2, j = 1,2, for
]
the E2g mode frequency decreases by 1.31
0.03 cm−1 in
zone boundary phonons where C is the force constant and Mj
is the mass of the jth atom in the two-atom basis chain. The
experimental frequency difference between NAWNASe2 and
186W80Se2 bilayers for each Raman active mode is evaluated as
Δωi = ωi,NA − ωi,isotope, i = E2g, A1g, A21g, where ωi is the Raman
peak frequency of the ith Raman active mode. Figure 2c shows
that the phonon frequency globally red-shifts in the isotopic
samples over the entire temperature range from 4.41 to 300 K.
The E2g mode corresponds to intralayer tungsten and selenium
atoms vibrating against each other in the hexagonal basal plane,
in which the phonon frequency difference between NAWNASe2
and 186W80Se2 bilayers is ΔωE2g = 1.38 0.05 cm−1. The A1g
NAWNASe2 and by and 1.22 0.03 cm−1 in 186W80Se2. The A1g
mode frequency drops a similar amount over the same
temperature range, 1.58 0.11 cm−1 and 1.53 0.06 cm−1 in
NAWNASe2 and 186W80Se2, respectively. The A2 mode
1g
frequency drops by nearly twice as much, 2.58 0.03 cm−1
and 2.45
0.04 cm−1 in NAWNASe2 and 186W80Se2 bilayers,
respectively. This behavior can be explained by the positive
thermal expansion coefficient (TEC) of both in-plane and out-
of-plane unit cell parameters.40,41 We observe that the out-of-
plane vibrational modes, A1g and A21g, have ∼1.2 and ∼1.9
times higher frequency changes than the in-plane vibration
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(Figure 4c). This trend is similar to the PL spectra blue-shift
with increasing atomic mass phenomenon observed in other
isotopically purified indirect band gap semiconductors, where
the indirect band gap renormalization energy changes ∼4.4
meV between h-10BN and h-NABN,25 and ∼2.2 meV between
70Ge and 76Ge.42
The mechanisms responsible for this observable change in
emission energy are indirect band gap renormalization43 and
isotopic phonon energy shift.25 When evaluating the
normalized electronic states in an indirect band gap semi-
conductor, the band gap renormalization energy is inversely
proportional to the square root of effective mass and depends
on the zero-point vibrational energy, calculated with all
phonon modes at zero temperature. The isotope effect on
indirect band gap renormalization energy has been proven in
bulk h-BN.25 However, the phonon replicas of MX2 TMDs can
only be obtained by resonant Raman spectroscopy, rather than
the nonresonant spectra which is used here and in the previous
h-BN report.25 Another approach to estimate the renormaliza-
tion energy is to extrapolate the linear relationship between
temperature and band gap energy at high temperature, which is
up to 800 K for bulk h-BN25 and 1000 K for Ge.43 Comparing
to h-BN, the atomically thin MX2 TMDs are extremely
sensitive to oxygen,44 so there has been no available report of
the PL spectra above 400 K of atomically thin WSe2. The band
gap renormalization energy is proportional to μ−1/2, and thus
can be calculated as δEgα = δEgNA(μα/μNA 1/2, where α
)
represents the isotope and μ indicates the reduced mass. Since
both bilayer WSe2 and WS2 are indirect semiconductors,
sharing the same lamellar structure and having similar band
gaps, we adopt the experimental band gap renormalization
energy of bilayer WS2 bilayer for bilayer NAWNASe2, δEgNA
∼
500 meV.45 The change of band gap renormalization energy in
the isotopically enriched sample can then be estimated as δEgα
− δEgNA ≈ 2.99 meV. Therefore, the isotopic band gap
renormalization dominates the PL band gap shift which is
measured as 3.9 0.7 meV in this study.
Figure 3. Temperature dependence of Raman active mode peak
positions in isotopically engineered bilayer WSe2. Raman shift for the
(a) E2g, (b) A1g, and (c) A21g modes of naturally abundant NAWNASe2
(solid symbols) and isotopically pure 186W80Se2 (open symbols).
It is also important to evaluate the contribution of van der
Waals bond length change to optical band gap energy, as out-
of-plane compressive strain has been shown to decrease the
indirect optical band gap energy in bilayer WSe2.10 A similar
trend has also been predicted for bilayer MoS2, in which the
indirect band gap decreases with shorter c-lattice parameter.46
This is contrary to our observation of the higher optical band
gap in bilayer 186W80Se2 which possesses a smaller c-lattice
parameter than NAWNASe2. Therefore, we conclude that the
van der Waals bond length is not the dominant factor resulting
in the optical band gap energy change with isotopic
purification in this work. The temperature evolution of the
optical band gap is shown in Figure 4d, which decreases from
1.649 eV at 4.41 K to 1.599 eV at 300 K for NAWNASe2 and
from 1.652 eV at 4.41 K to 1.574 eV at 300 K for 186W80Se2.
This behavior can be modeled using the empirical Varshni
relation47 as Eg(T) = Eg(T = 0) − (αT2)/(β + T) where Eg(T
= 0) is the band gap energy at 0 K, T is the absolute
temperature, and α and β are adjustable constants. We
obtained Eg(T = 0) = 1.650 eV, α = 0.365 meV/K, and β =
93.93 K for NAWNASe2, and Eg(T = 0) = 1.654 eV, α = 0.379
meV/K, and β = 112.96 K for 186W80Se2. Coefficients of
determination were 0.991 and 0.992 for NAWNASe2 and
186W80Se2, respectively. The slight difference between our
experimental data and the empirical modeling at low
temperature arises from the quadratic temperature dependence
mode (E2g), respectively, which arises from an out-of-plane
TEC approximately 1.6 times higher than the in-plane TEC.41
Electronic transitions from filled to empty states must
conserve electron momentum. Bilayer WSe2 is known as a
semiconductor with an indirect electronic band gap,19 in which
the conduction band minimum at the Σ-point (CBMΣ) and
valence band maximum at the K-point (VBMK) are not at the
same electron momentum in reciprocal space. Since the
momentum of photons is negligible, especially in the
wavelength range used in this work, the transition between
CBMΣ and VBMK must involve phonon assistance to obey the
rule of momentum conservation, which means the band gap
can be affected by the isotope effect. Furthermore, phonon
energy tuning by the isotope effect may also result from a
change in wave vector arising from a shift of the PL band gap
energy. Figure 4 reports the temperature dependent PL spectra
for bilayer NAWNASe2 and 186W80Se2. To avoid uncertainty
arising from nonidentical local heating by the excitation laser,
all the samples were characterized under the same conditions
including laser power and acquisition time at each temperature.
Surprisingly, we observe a higher optical band gap energy in
bilayer 186W80Se2 compared with bilayer NAWNASe2 where the
PL spectra of the isotopically pure sample blue-shifts by 3.9
0.7 meV over the entire temperature range from 4.41 to 300 K
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Figure 4. Isotopic mass dependent photoluminescence and optical band gap. Normalized photoluminescence spectra over a temperature range
from 4.41 to 300 K for (a) naturally abundant bilayer NAWNASe2 and (b) isotopically pure bilayer 186W80Se2. (c) Temperature dependence of the
band gap of NAWNASe2 and 186W80Se2, where modeling results using the empirical Varshni relation are given by solid and dashed lines, respectively.
(d) Difference in optical band gap between NAWNASe2 and 186W80Se2 as a function of temperature.
of the empirical Varshni relation, whereas theoretical and
experimental observations are reported to exhibit T4 depend-
ence at low temperature.43,48,49
izations, and analytical modeling; W.W. and M.D.M.-A.
performed X-ray diffraction spectroscopy and analysis. The
manuscript was written by M.T.P. and W.W. with contribu-
tions from all authors.
In conclusion, we have experimentally demonstrated the
isotope effect on the phonon frequency, phonon lifetime, and
optical band gap energy in an atomically thin TMD through
temperature dependent spectroscopy of naturally abundant
and isotopically pure bilayer WSe2. We have postulated a new
mechanism by which the electronic band gap energy and
phonon dispersion can be tuned in this material by isotopic
enrichment. The outcomes of this study should help stimulate
future investigations and theoretical predictions of phonon and
electronic property modification in other isotopically enriched
van der Waals material systems, such as the isotope effect on
thermal conductivity.36,50−53
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the U.S. National Science
Foundation under Award No. CAREER-1553987 (M.T.P.,
W.W.). This work was performed, in part, at the Center for
Integrated Nanotechnologies, an Office of Science User
Facility operated for the U.S. Department of Energy (DOE)
Office of Science. Los Alamos National Laboratory, an
affirmative action equal opportunity employer, is managed by
Triad National Security, LLC for the U.S. Department of
Energy’s NNSA, under Contract 89233218CNA000001. This
work was performed, in part, at the Harvard University Center
for Nanoscale Systems (CNS), a member of the National
Nanotechnology Coordinated Infrastructure Network
(NNCI), which is supported by the U.S. National Science
Foundation under Award No. 1541959.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.nano-
lett.8b04269.
■
S
Additional synthesis, structural characterization, and
experimental details (PDF)
AUTHOR INFORMATION
Corresponding Author
*E-mail: pettesmt@lanl.gov.
ORCID
Michael Thompson Pettes: 0000-0001-6862-6841
Author Contributions
This project was conceived and led by M.T.P.; W.W. and
M.T.P. conducted WSe2 synthesis, experimental character-
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