REPORTS
Detection of Pulsed Gamma Rays
Above 100 GeV from the Crab Pulsar
The VERITAS Collaboration; E. Aliu,1 T. Arlen,2 T. Aune,3 M. Beilicke,4 W. Benbow,5 A. Bouvier,3
S. M. Bradbury,6 J. H. Buckley,4 V. Bugaev,4 K. Byrum,7 A. Cannon,8 A. Cesarini,9
J. L. Christiansen,10 L. Ciupik,11 E. Collins-Hughes,8 M. P. Connolly,9 W. Cui,12 R. Dickherber,4
C. Duke,13 M. Errando,1 A. Falcone,14 J. P. Finley,12 G. Finnegan,15 L. Fortson,16 A. Furniss,3
N. Galante,5 D. Gall,22 K. Gibbs,5 G. H. Gillanders,9 S. Godambe,15 S. Griffin,17 J. Grube,11
R. Guenette,17 G. Gyuk,11 D. Hanna,17 J. Holder,18 H. Huan,19 G. Hughes,20 C. M. Hui,15
T. B. Humensky,19 A. Imran,21 P. Kaaret,22 N. Karlsson,16 M. Kertzman,23 D. Kieda,15
H. Krawczynski,4 F. Krennrich,21 M. J. Lang,9 M. Lyutikov,12 A. S Madhavan,21 G. Maier,20
P. Majumdar,2 S. McArthur,4 A. McCann,17* M. McCutcheon,17 P. Moriarty,24 R. Mukherjee,1
P. Nuñez,15 R. A. Ong,2 M. Orr,21 A. N. Otte,3* N. Park,19 J. S. Perkins,5 F. Pizlo,12 M. Pohl,20,25
H. Prokoph,20 J. Quinn,8 K. Ragan,17 L. C. Reyes,19 P. T. Reynolds,26 E. Roache,5
H. J. Rose,6 J. Ruppel,25 D. B. Saxon,18 M. Schroedter,5* G. H. Sembroski,12 G. D. Şentürk,27
A. W. Smith,7 D. Staszak,17 G. Tešić,17 M. Theiling,12 S. Thibadeau,4 K. Tsurusaki,22
J. Tyler,17 A. Varlotta,12 V. V. Vassiliev,2 S. Vincent,15 M. Vivier,18 S. P. Wakely,19 J. E. Ward,8
T. C. Weekes,5 A. Weinstein,21 T. Weisgarber,19 D. A. Williams,3 B. Zitzer7
ulsars were first discovered more than 40
years ago (1) and are now believed to be
rapidly rotating, magnetized neutron stars.
Within the corotating magnetosphere, charged
particles are accelerated to relativistic energies
and emit nonthermal radiation from radio waves
through gamma rays. Although this picture reflects the broad scientific consensus, the details are
still very much a mystery. For example, a number
of models exist that can be distinguished from
each other on the basis of the location of the acceleration zone. Popular examples include the outergap model (2–5), the slot-gap model (6, 7), and
the pair-starved polar-cap model (8–10). One way
to better understand the dynamics within the magnetosphere is through observation of gamma rays
emitted by the accelerated particles.
All of the detected gamma-ray pulsars in (11)
exhibit a break in the spectrum between a few
hundred MeV and a few GeV, with a rapidly fading flux above the break. The break energy is related to the maximum energy of the particles and
to the efficiency of the pair production. Mapping
the cutoff can help to constrain the geometry of
the acceleration region, the gamma-ray radiation
mechanisms, and the attenuation of gamma rays.
Previous measurements of the spectral break are
statistically compatible with an exponential or subexponential cutoff, which is currently the most
favored shape for the spectral break.
One of the most powerful pulsars in gamma
rays is the Crab pulsar (12, 13), PSR J0534 + 2200,
which is the remnant of a historical supernova
P
that was observed in the year 1054. It is located
at a distance of 6500 T 1600 light-years (1 lightyear = 9.46 × 1015 m) and has a rotation period
of ~33 ms, a spin-down power of 4.6 × 1038
erg s−1, and a surface magnetic field of 3.8 ×
1012 G (14, 15). Attempts to detect pulsed gamma rays above 100 GeV from the Crab pulsar
began decades ago (16). Before the work reported here, the highest energy detection was at
25 GeV (17). At higher energies, near 60 GeV,
only hints of pulsed emission have been reported
in two independent observations (17, 18). Although Fermi-LAT measurements of the Crab
pulsar spectrum are consistent, within the errors
of the measurements, with a power law with an
exponential cutoff at about 6 GeV (13), the flux
measurements above 10 GeV are systematically
higher than the fit with an exponential cutoff,
which hints that the spectrum is indeed harder
than a power law with an exponential cutoff
(13, 17). However, the sensitivity of the previous
data was insufficient to allow a definite conclusion about the spectral shape.
We observed the Crab pulsar with the Very
Energetic Radiation Imaging Telescope Array
System (VERITAS) for 107 hours between September 2007 and March 2011. VERITAS is a
ground-based gamma-ray observatory composed
of an array of four atmospheric Cherenkov telescopes located in southern Arizona, USA (19).
VERITAS has a trigger threshold of 100 GeV.
Most of the data, 77.7 hours, were recorded after the relocation in summer 2009 of one of
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We report the detection of pulsed gamma rays from the Crab pulsar at energies above
100 giga–electron volts (GeV) with the Very Energetic Radiation Imaging Telescope Array
System (VERITAS) array of atmospheric Cherenkov telescopes. The detection cannot be explained
on the basis of current pulsar models. The photon spectrum of pulsed emission between
100 mega–electron volts and 400 GeV is described by a broken power law that is statistically
preferred over a power law with an exponential cutoff. It is unlikely that the observation can
be explained by invoking curvature radiation as the origin of the observed gamma rays above
100 GeV. Our findings require that these gamma rays be produced more than 10 stellar radii
from the neutron star.
the VERITAS telescopes, which resulted in a
lower energy threshold and better sensitivity of
the array. We processed the recorded atmospheric
shower images with a standard moment analysis (20) and calculated the energy and arrival
direction of the primary particles (21). We then
rejected events caused by charged cosmic-ray
events. For gamma rays, the distribution of the
remaining, or selected, events as a function of
energy peaks at 120 GeV. In the pulsar analysis,
for each selected event, we first transformed the
arrival time to the barycenter of the solar system
and then calculated the spin phase of the Crab
pulsar from the barycentered time using contemporaneously measured spin-down parameters (22, 23). All steps in the analysis have been
cross-checked by an independent software package and are explained in detail in the supporting
online material (SOM). We applied the H test
(24) to evaluate periodic emission at the frequency of the Crab pulsar (SOM). This yielded a test
value of 50, which corresponded to a significance
of 6.0 SD that pulsed emission is present in
the data.
The phase-folded event distribution, hereafter
pulse profile, of the selected VERITAS events is
shown in Fig. 1. The most significant structures
are two pulses with peak amplitudes at phase 0.0
and phase 0.4. These coincide with the locations
1
Department of Physics and Astronomy, Barnard College,
Columbia University, NY 10027, USA. 2Department of Physics
and Astronomy, University of California, Los Angeles, CA
90095, USA. 3Santa Cruz Institute for Particle Physics and
Department of Physics, University of California, Santa Cruz,
Santa Cruz, CA 95064, USA. 4Department of Physics, Washington University, St. Louis, MO 63130, USA. 5Fred Lawrence
Whipple Observatory, Harvard-Smithsonian Center for Astrophysics,
Amado, AZ 85645, USA. 6School of Physics and Astronomy,
University of Leeds, Leeds LS2 9JT, UK. 7Argonne National Laboratory, 9700 South Cass Avenue, Argonne, IL 60439, USA. 8School
of Physics, University College Dublin, Belfield, Dublin 4, Ireland.
9
School of Physics, National University of Ireland Galway, University Road, Galway, Ireland. 10Physics Department, California
Polytechnic State University, San Luis Obispo, CA 94307, USA.
11
Astronomy Department, Adler Planetarium and Astronomy
Museum, Chicago, IL 60605, USA. 12Department of Physics,
Purdue University, West Lafayette, IN 47907, USA. 13Department
of Physics, Grinnell College, Grinnell, IA 50112–1690, USA.
14
Department of Astronomy and Astrophysics, 525 Davey Lab,
Pennsylvania State University, University Park, PA 16802, USA.
15
Department of Physics and Astronomy, University of Utah,
Salt Lake City, UT 84112, USA. 16School of Physics and Astronomy, University of Minnesota, Minneapolis, MN 55455, USA.
17
Physics Department, McGill University, Montreal, Quebec H3A
2T8, Canada. 18Department of Physics and Astronomy and the
Bartol Research Institute, University of Delaware, Newark, DE
19716, USA. 19Enrico Fermi Institute, University of Chicago,
Chicago, IL 60637, USA. 20Deutsches Elektronen Synchrotron,
Platanenallee 6, 15738 Zeuthen, Germany. 21Department of Physics and Astronomy, Iowa State University, Ames, IA 50011, USA.
22
Department of Physics and Astronomy, University of Iowa,
Van Allen Hall, Iowa City, IA 52242, USA. 23Department of Physics
and Astronomy, DePauw University, Greencastle, IN 46135–0037,
USA. 24Department of Life and Physical Sciences, Galway-Mayo
Institute of Technology, Dublin Road, Galway, Ireland. 25Institut für
Physik und Astronomie, Universität Potsdam, 14476 Potsdam-Golm,
Germany. 26Department of Applied Physics and Instrumentation,
Cork Institute of Technology, Bishopstown, Cork, Ireland. 27Department of Physics, Columbia University, New York, NY 10027.
*To whom correspondence should be addressed. E-mail:
nepomuk.otte@gmail.com (A.N.O.); mccann@hep.physics.
mcgill.ca (A.M.); schroedter@veritas.sao.arizona.edu (M.S.)
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of the main pulse and interpulse, hereafter P1 and
P2, which are the two main features in the pulse
profile of the Crab pulsar throughout the electromagnetic spectrum. We characterized the pulse
profile using an unbinned maximum-likelihood
fit (SOM). In the fit, the pulses were modeled
with Gaussian functions, and the background
was determined from the events that fell between
phases 0.43 and 0.94 in the pulse profile (referred to as the off-pulse region). The positions
of P1 and P2 in the VERITAS data thus lie at
the phase values – 0.0026 T 0.0028 and 0.3978 T
0.0020, respectively, and are shown by the vertical lines (Fig. 1). The full widths at half maximum (FWHM) of the fitted pulses are 0.0122 T
0.0035 and 0.0267 T 0.0052, respectively. The
pulses are narrower by a factor of two to three
than those measured by the Fermi Large Area
Telescope (Fermi LAT) at 100 MeV (13) (Fig. 1).
If gamma rays observed at the same phase are
emitted by particles that propagate along the same
magnetic field line (25) and if the electric field in
the acceleration region is homogeneous, then a
possible explanation of the observed narrowing
Counts per Bin
A
VERITAS > 120 GeV
3700
3600
3500
3400
3200
2000 -1
1500
Fermi > 100 MeV
-0.5
0
-0.5
0
0.5
500
0
-1
1950
Counts per Bin
B
VERITAS > 120 GeV
1900
P1
1850
0.5
1950
1750
1700
1700
1650
1650
1600
1600
1550
1550
Counts per Bin
1500
-0.05
Fermi > 100 MeV
0
0.05
0.1
Phase
1000
500
-0.1
P2
1850
1750
0
VERITAS > 120 GeV
1900
1800
1500
1
Phase
C
1800
1500
2000-0.1
-0.05
0
0.05
0.1
600
0.3
0.35
Fermi > 100 MeV
Fig. 1. Pulse profile of the Crab pulsar. Phase 0 is the position of P1 in radio.
The shaded histograms show the VERITAS data. The pulse profile in (A) is
shown twice for clarity. The dashed horizontal line shows the background level
estimated from data in the phase region between 0.43 and 0.94. (B and C)
Expanded views of the pulse profile with a finer binning than in (A) and are
centered at P1 and P2, which are the two dominant features in the pulse profile
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0.4
0.45
0.5
Phase
400
200
0
0.3
0.35
0.4
0.45
0.5
Phase
Phase
70
1
Phase
1000
Counts per Bin
Counts per Bin
Counts per Bin
3100
of the Crab pulsar. The data above 100 MeV from the Fermi LAT (13, 30) are
shown beneath the VERITAS profile. The vertical dashed lines in the panels (B)
and (C) mark the best-fit peak positions of P1 and P2 in the VERITAS data. The
solid black line shows the result of an unbinned maximum-likelihood fit of
Gaussian functions to the VERITAS pulse profile (described in text). The peak
positions for the Fermi-LAT and the VERITAS data agree within uncertainties.
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E2 dF/dE (MeV cm-2 s-1)
10-3
10-4
10-5
10-6
χ2
10-7
20
15
10
5
0
VERITAS, this work
Fermi (Abdo et al, 2010)
MAGIC (Aliu et al. 2008)
MAGIC (Albert et al. 2008)
CELESTE (De Naurois et al. 2002)
STACEE (Oser et al. 2001)
HEGRA (Aharonian et al. 2004)
Whipple (Lessard et al. 2000)
Broken power law fit
Exponential cutoff fit
3
10
Power law with exponential cutoff
5
104
10
4
10
6
10
Energy (MeV)
Broken power law
10
3
10
5
10
6
Energy (MeV)
Fig. 2. Spectral energy distribution (SED) of the Crab pulsar in gamma rays. VERITAS flux measurements
are shown by the bowtie. The dotted line enclosed by the bow tie gives the best-fit power-law spectrum and
the statistical uncertainties, respectively, for the VERITAS data using a forward-folding method. The solid
red circles show VERITAS flux measurements using a different spectral reconstruction method (SOM). FermiLAT data (13) are given by green squares, and the MAGIC flux point (17) by the solid reddish triangle. The
open symbols are upper limits from the CErenkov Low-Energy Sampling and Timing Experiment (CELESTE)
(31), the High-Energy-Gamma-Ray Astronomy (HEGRA) experiment (32), MAGIC (18), Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) (33), and Whipple (29). The result of a fit of the VERITAS
and Fermi-LAT data with a broken power law is given by the solid line, and the result of a fit with a
power-law spectrum multiplied with an exponential cutoff is given by the dashed line. Below the SED,
we plot c2 values to visualize the deviations of the best-fit parameterization from the Fermi-LAT and
VERITAS flux measurements.
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VOL 334
A(E/E0)aexp(–E/Ec), which is a good parameterization of the Fermi LAT (13) and MAGIC
(17) data. The Fermi-LAT and MAGIC data can
be equally well parameterized by a broken power
law, but those data are not sufficient to significantly distinguish between a broken power law
and an exponential cutoff. The VERITAS data,
on the other hand, clearly favor a broken power
law as a parameterization of the spectral shape.
The fit of the VERITAS and Fermi-LAT data
with a broken power law of the form A(E/E0)a/
[1 + (E/E0)a-b] results in a c2 value of 13.5 for
15 degrees of freedom with the fit parameters
A = (1.45 T 0.15stat ) × 10−5 TeV −1 cm−2 s−1, E0 =
4.0 T 0.5stat GeV, a = –1.96 T 0.02stat and b =
–3.52 T 0.04stat (Fig. 2). A corresponding fit with
a power law and an exponential cutoff yields a
c2 value of 66.8 for 16 degrees of freedom. The
fit probability of 3.6 × 10−8 derived from the c2
value excludes the exponential cutoff as a viable
parameterization of the Crab pulsar spectrum.
The detection of gamma-ray emission above
100 GeV provides strong constraints on the
gamma-ray radiation mechanisms. If one assumes
a balance between acceleration gains and radiative losses by curvature radiation, the break in
the gamma-ray spectrum is expected to be at Ebr =
150 GeV h3/4 sqrt(x), where h is the acceleration
efficiency (h < 1) and x is the radius of curvature
in units of the light-cylinder radius (28) (SOM).
Only in the extreme case of an acceleration field
that is close to the maximum allowed value and a
radius of curvature that is close to the light-cylinder
radius would it be possible to produce gamma-ray
emission above 100 GeV with curvature radiation.
It is, therefore, unlikely that curvature radiation is
the dominant production mechanism of the observed gamma-ray emission above 100 GeV. A
plausible different radiation mechanism is inverseCompton scattering that has motivated previous
searches for pulsed very high energy emission, e.g.,
(29). With regard to the overall gamma-ray production, two possible interpretations are that a single
emission mechanism alternative from curvature
radiation dominates at all gamma-ray energies or
that a second mechanism becomes dominant above
the spectral break energy. It might be possible to
distinguish between the two scenarios with higherresolution spectral measurements above 10 GeV.
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averaged spectrum because no “bridge” emission, which is observed at lower energies, is seen
between P1 and P2 in the VERITAS data. However, the existence of a constant flux component
that originates in the magnetosphere cannot be
excluded and would be indistinguishable from
the gamma-ray flux from the nebula. Figure 2
shows the VERITAS phase-averaged spectrum
together with measurements made with Fermi
LAT and the Major Atmospheric Gamma-ray Imaging Cherenkov telescope (MAGIC). In the energy range between 100 GeV and 400 GeV
measured by VERITAS, the energy spectrum
is well described by a power law F(E) = A(E/150
GeV)a, with A = (4.2 T 0.6stat + 2.4syst – 1.4syst) ×
10−11 TeV −1 cm−2 s−1 and a = –3.8 T 0.5stat T
0.2syst. At 150 GeV, the flux from the pulsar is
~1% of the flux from the nebula. The detection
of pulsed gamma-ray emission between 200 GeV
and 400 GeV, the highest energy flux point, is
only possible if the emission region is at least
10 stellar radii from the star’s surface (26). Using
calculations from (27), the emission region can even
be constrained to be at least 30 to 40 stellar radii.
Combining the VERITAS data with the FermiLAT data we can place a stringent constraint on
the shape of the spectral turnover. The previously favored spectral shape of the Crab pulsar
above 1 GeV was an exponential cutoff F(E) =
is that the region where acceleration occurs tapers.
However, detailed calculations are necessary to
explain fully the observed pulse profile.
Along with the observed differences in the
pulse width, the amplitude of P2 is larger than
P1 in the profile measured with VERITAS, in
contrast to what is observed at lower gamma-ray
energies where P1 dominates (Fig. 1). It is known
that the ratio of the pulse amplitudes changes as
a function of energy above 1 GeV (13) and becomes near unity for the pulse profile integrated
above 25 GeV (17). To quantify the relative intensity of the two peaks above 120 GeV, we integrated the pulsed excess between phase – 0.013
and 0.009 for P1 and between 0.375 and 0.421
for P2. This is the T2 SD interval of each pulse
as determined from the maximum-likelihood fit.
The ratio of the excess events and thus the intensity ratio of P2/P1 is 2.4 T 0.6. If one assumes
that the differential energy spectra of P1 and P2
above 25 GeV can each be described with a
power law, F(E) ~ E a and that the intensity
ratio is exactly unity at 25 GeV (17), then the
spectral index a of P1 must be smaller than the
spectral index of P2 by aP2 – aP1 = 0.56 T 0.16.
We measured the gamma-ray spectrum above
100 GeV by combining the pulsed excess in the
phase regions around P1 and P2. This can be
considered a good approximation of the phase-
References and Notes
1. A. Hewish, S. J. Bell, J. D. H. Pilkington, P. F. Scott,
R. A. Collins, Nature 217, 709 (1968).
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(1986).
3. R. W. Romani, Astrophys. J. 470, 469 (1996).
4. K. Hirotani, Astrophys. J. 652, 1475 (2006).
5. A. P. S. Tang, J. Takata, J. Jia, K. S. Cheng, Astrophys. J.
676, 562 (2008).
6. J. Arons, Astrophys. J. 266, 215 (1983).
7. A. G. Muslimov, A. K. Harding, Astrophys. J. 588, 430
(2003).
8. M. Frackowiak, B. Rudak, Adv. Space Res. 35, 1152 (2005).
9. A. K. Harding, V. V. Usov, A. G. Muslimov, Astrophys. J.
622, 531 (2005).
10. C. Venter, A. K. Harding, L. Guillemot, Astrophys. J. 707,
800 (2009).
11. A. Abdo et al., Astrophys. J. 187 (suppl.), 460 (2010).
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12. J. M. Fierro, P. F. Michelson, P. L. Nolan, D. J. Thompson,
Astrophys. J. 494, 734 (1998).
13. A. Abdo et al., Astrophys. J. 708, 1254 (2010).
14. R. N. Manchester, G. B. Hobbs, A. Teoh, M. Hobbs,
Astron. J. 129, 1993 (2005).
15. ATNF Pulsar Catalogue, www.atnf.csiro.au/people/pulsar/
psrcat/.
16. T. C. Weekes et al., Astrophys. J. 342, 379 (1989).
17. E. Aliu et al.; MAGIC Collaboration, Science 322, 1221
(2008).
18. J. Albert et al., Astrophys. J. 674, 1037 (2008).
19. J. Holder et al., Astropart. Phys. 25, 391 (2006).
20. A. M. Hillas, in Proceedings of the 19th International
Cosmic Ray Conference, La Jolla, CA, 11 to 23 August
1985, p. 445 (ICRC, La Jolla, 1985).
21. P. Cogan, in Proceedings of the 30th International
Cosmic Ray Conference, Mérida, Mexico, 3 to 7 July
2007, vol. 3, p. 1385 (ICRC, Mérida, 2008).
22. A. G. Lyne et al., Mon. Not. R. Astron. Soc. 265, 1003 (1993).
23. Jodrell Bank Crab Pulsar Monthly Ephemeris,
www.jb.man.ac.uk/~pulsar/crab.html.
24.
25.
26.
27.
O. C. de Jager, Astrophys. J. 436, 239 (1994).
X.-N. Bai, A. Spitkovsky, Astrophys. J. 715, 1282 (2010).
M. G. Baring, Adv. Space Res. 33, 552 (2004).
K. J. Lee et al., Mon. Notic. Roy. Astron. Soc. 405, 2103
(2010).
28. M. Lyutikov, A. N. Otte, A. McCann, arXiv:1108.3824
(2011).
29. R. W. Lessard et al., Astrophys. J. 531, 942 (2000).
30. The Fermi-LAT pulse profile of the Crab pulsar above
100 MeV that is shown in Fig. 1 is not the original one
from reference (13) but one that has been calculated
with an updated ephemerides that corrects for a small
phase offset that has been introduced in the original
analysis http://fermi.gsfc.nasa.gov/ssc/data/access/lat/
ephems/0534+2200/README.
31. M. de Naurois et al., Astrophys. J. 566, 343 (2002).
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Acknowledgments: This research is supported by grants from
the U.S. Department of Energy, NSF, and the Smithsonian
Institution; by Natural Sciences and Engineering Research
Kumar Varoon,* Xueyi Zhang,* Bahman Elyassi, Damien D. Brewer, Melissa Gettel,†
Sandeep Kumar,‡ J. Alex Lee,§ Sudeep Maheshwari,|| Anudha Mittal, Chun-Yi Sung,
Matteo Cococcioni, Lorraine F. Francis, Alon V. McCormick, K. Andre Mkhoyan, Michael Tsapatsis¶
Thin zeolite films are attractive for a wide range of applications, including molecular sieve
membranes, catalytic membrane reactors, permeation barriers, and low-dielectric-constant
materials. Synthesis of thin zeolite films using high-aspect-ratio zeolite nanosheets is desirable
because of the packing and processing advantages of the nanosheets over isotropic zeolite
nanoparticles. Attempts to obtain a dispersed suspension of zeolite nanosheets via exfoliation
of their lamellar precursors have been hampered because of their structure deterioration and
morphological damage (fragmentation, curling, and aggregation). We demonstrated the synthesis
and structure determination of highly crystalline nanosheets of zeolite frameworks MWW and MFI.
The purity and morphological integrity of these nanosheets allow them to pack well on porous
supports, facilitating the fabrication of molecular sieve membranes.
igh-aspect-ratio zeolite single crystals
with thickness in the nanometer range
(zeolite nanosheets) are desirable for applications including building blocks for heterogeneous catalysts (1–3) and the fabrication of
thin molecular sieve films and nanocomposites
for energy-efficient separations (4). They could
H
Department of Chemical Engineering and Materials Science,
University of Minnesota, 151 Amundson Hall, 421 Washington
Avenue Southeast, Minneapolis, MN 55455, USA.
*These authors contributed equally to this work.
†Present address: Department of Chemical and Environmental
Engineering, University of California, Riverside, 1175 West
Blaine Street, Riverside, CA 92507, USA.
‡Present address: Material Analysis Laboratory, Intel Corporation, Hillsboro, OR 97124, USA.
§Present address: Department of Chemical and Biomolecular
Engineering, Rice University, MS-362, 6100 Main Street,
Houston, TX 77005, USA.
||Present address: Schlumberger Doll-Research, Schlumberger
Limited, 1 Hampshire Street, Cambridge, MA 02139, USA.
¶To whom correspondence should be addressed. E-mail:
tsapatsis@umn.edu
72
also be of fundamental importance in probing
the mechanical, electronic, transport, and catalytic
properties of microporous networks at the nanoscale (5, 6). Despite steady advances in the preparation and characterization of layered materials
containing microporous layers and of their pillared and swollen analogs (1–3, 7–17), the synthesis of suspensions containing discrete, intact,
nonaggregated zeolite nanosheets has proven
elusive because of structural deterioration and/or
aggregation (18) of the lamellae upon exfoliation.
Here, we report the isolation and structure determination of highly crystalline zeolite nanosheets
of the MWW and MFI structure types, and we
demonstrated the use of their suspensions in the
fabrication of zeolite membranes.
MWW and MFI nanosheets were prepared
starting from their corresponding layered precursors
ITQ-1 (1) and multilamellar silicalite-1 (3), respectively. Before exfoliation by melt blending with
polystyrene (weight-average molecular weight =
45000 g/mol), ITQ-1 was swollen according to a
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Supporting Online Material
www.sciencemag.org/cgi/content/full/334/6052/69/DC1
Materials and Methods
SOM Text
Figs. S1 to S4
References (34–36)
10 May 2011; accepted 19 August 2011
10.1126/science.1208192
previously reported procedure (18); multilamellar
silicalite-1 was used as made. Melt blending was
performed under a nitrogen environment in a corotating twin screw extruder with a recirculation
channel (19). The polystyrene nanocomposites
obtained by melt blending were characterized by
x-ray diffraction (XRD), and microtomed sections
were imaged by transmission electron microscopy (TEM) to reveal the presence of exfoliated
MWW and MFI nanosheets embedded in the
polymer matrix (figs. S1 and S2).
To obtain a dispersion of these nanosheets,
the nanosheet-polystyrene nanocomposites were
placed in toluene and sonicated. After polymer
dissolution and removal of the larger particles
by centrifugation, the dispersions, containing approximately 1.25% w/w polymer and 0.01% w/w
nanosheets, were used to prepare samples for
TEM and atomic force microscopy (AFM) examination, by drying a droplet on TEM grids and
freshly cleaved mica surfaces, respectively (the
AFM sample was calcined in air at 540°C to
remove polymer). Low-magnification TEM images of high-aspect-ratio MWW and MFI nanosheets reveal their flakelike morphology (Fig. 1,
A and B). The uniform contrast from isolated
nanosheets suggests uniform thickness, whereas
the darker areas can be attributed to overlapping
of neighboring nanosheets. Although lattice fringes
are not easily visible in the high-resolution TEM
(HRTEM) images of the nanosheets (figs. S3A
and B), they do exist, as confirmed by their fast
Fourier transform (FFT) (figs. S3C and D). In
addition, electron diffraction (ED) from single
MWWand MFI nanosheets (Fig. 1, C to E, and G)
and XRD data obtained from calcined powders
of MWW and MFI nanosheets (Fig. 2, A and B)
confirm that the nanosheets are highly crystalline materials of the MWW and MFI type, respectively. The thin dimensions of MWW and
MFI nanosheets, as expected, are along the c
and b axes, respectively, as indicated from the
FFT of the HRTEM images and the ED data.
AFM measurements, calibrated using steps
formed on freshly cleaved mica (20), revealed
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Dispersible Exfoliated Zeolite
Nanosheets and Their Application
as a Selective Membrane
Council of Canada; by Science Foundation Ireland (SFI
10/RFP/AST2748); and by the Science and Technology
Facilities Council in the United Kingdom. We acknowledge
the excellent work of the technical support staff at the
Fred Lawrence Whipple Observatory and at the collaborating
institutions in the construction and operation of the
instrument. A.N.O. was supported in part by a Feodor-Lynen
fellowship of the Alexander von Humboldt Foundation.
We are grateful to M. Roberts and A. Lyne for providing us
with Crab-pulsar ephemerides before the public ones
became available.
Detection of Pulsed Gamma Rays Above 100 GeV from the Crab Pulsar
The VERITAS Collaboration, E. Aliu, T. Arlen, T. Aune, M. Beilicke, W. Benbow, A. Bouvier, S. M. Bradbury, J. H. Buckley, V.
Bugaev, K. Byrum, A. Cannon, A. Cesarini, J. L. Christiansen, L. Ciupik, E. Collins-Hughes, M. P. Connolly, W. Cui, R.
Dickherber, C. Duke, M. Errando, A. Falcone, J. P. Finley, G. Finnegan, L. Fortson, A. Furniss, N. Galante, D. Gall, K. Gibbs,
G. H. Gillanders, S. Godambe, S. Griffin, J. Grube, R. Guenette, G. Gyuk, D. Hanna, J. Holder, H. Huan, G. Hughes, C. M. Hui,
T. B. Humensky, A. Imran, P. Kaaret, N. Karlsson, M. Kertzman, D. Kieda, H. Krawczynski, F. Krennrich, M. J. Lang, M.
Lyutikov, A. S Madhavan, G. Maier, P. Majumdar, S. McArthur, A. McCann, M. McCutcheon, P. Moriarty, R. Mukherjee, P.
Nuñez, R. A. Ong, M. Orr, A. N. Otte, N. Park, J. S. Perkins, F. Pizlo, M. Pohl, H. Prokoph, J. Quinn, K. Ragan, L. C. Reyes, P.
T. Reynolds, E. Roache, H. J. Rose, J. Ruppel, D. B. Saxon, M. Schroedter, G. H. Sembroski, G. D. Sentürk, A. W. Smith, D.
Staszak, G. Tesic, M. Theiling, S. Thibadeau, K. Tsurusaki, J. Tyler, A. Varlotta, V. V. Vassiliev, S. Vincent, M. Vivier, S. P.
Wakely, J. E. Ward, T. C. Weekes, A. Weinstein, T. Weisgarber, D. A. Williams and B. Zitzer
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DOI: 10.1126/science.1208192