Flexible polar nanowires of Cs5BiP4Se12 from weak interactions between coordination complexes: Strong nonlinear optical second harmonic generation

Article abstract of DOI:10.1021/ja808242g

The Cs₅BiP₄Se₁₂ salt grows naturally as nanowires that crystallize in the polar space group PmC2₁, with a = 7.5357(2) A, b = 13.7783(6) A, c = 28.0807(8) A, and Z = 4 at 293(2) K. The compound features octahedra

Full text of DOI:10.1021/ja808242g

Published on Web 01/30/2009
Flexible Polar Nanowires of Cs5BiP4Se12 from Weak
Interactions between Coordination Complexes: Strong
Nonlinear Optical Second Harmonic Generation
In Chung,†,‡ Jung-Hwan Song,§ Joon I. Jang,§ Arthur J. Freeman,§
John B. Ketterson,§ and Mercouri G. Kanatzidis*,†,‡
Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824, and
Department of Chemistry and Department of Physics and Astronomy, Northwestern UniVersity,
EVanston, Illinois 60208
Received October 20, 2008; E-mail: m-kanatzidis@northwestern.edu
Abstract: The Cs5BiP4Se12 salt grows naturally as nanowires that crystallize in the polar space group Pmc21,
with a ) 7.5357(2) Å, b ) 13.7783(6) Å, c ) 28.0807(8) Å, and Z ) 4 at 293(2) K. The compound features
octahedral [Bi(P2Se6)2]5- coordination complexes that stack via weak intermolecular Se · · ·Se interactions
to form long, flexible fibers and nanowires. The Cs5BiP4Se12 fibers are transparent in the near- and mid-IR
ranges and were found to exhibit a nonlinear optical second harmonic generation response at 1 µm that
is approximately twice that of the benchmark material AgGaSe2. The material has a nearly direct band gap
of 1.85 eV and melts congruently at 590 °C. Ab initio electronic structure calculations performed with the
full-potential linearized augmented plane wave (FLAPW) method show that the band gap increases from
its local density approximation (LDA) spin-orbit coupling value of 1.15 eV to the higher value of 2.0 eV
when the screened-exchange LDA method is invoked and explain how the long nanowire nature of
Cs5BiP4Se12 emerges.
SiO2,10 TiO2,11 V2O512), metal chalcogenide binaries [MQ2 (M
) group 4-6 metals,13 Re;14 Q ) S, Se, Te), Bi2S315], nitrides
(BN,16 GaN17), and carbides (BC,18 SiC19). The synthetic
procedures used to obtain 1D nanostructures, however, can be
challenging and generally require special nonequilibrium condi-
tions such as discharging, chemical vaporization, laser vaporiza-
tion, or hydrocarbon pyrolysis. Consequently, they are limited
in terms of yield, phase purity, crystallinity, and uniformity.2
For example, the vapor-liquid-solid process, the most suc-
cessful method for producing large quantities of nanowires with
single-crystalline structures, requires a metal catalyst that may
contaminate the nanowires and potentially change their proper-
ties.20 The emerging solution-based techniques can provide high
reproducibility but involve structure-directing agents mostly
chosen by empirical trial and error. An interesting class is the
1. Introduction
The synthesis of one-dimensional (1D) inorganic nanostruc-
tures, such as nanowires and nanotubes, has attracted great
interest because of their promising new optical and electronic
properties that can be useful in technological applications.1,2
Synthetic efforts to create inorganic nanowires and nanotubes
involve either the exploration of new compounds with nanofea-
tures or downsizing existing materials to the nanoscale. Ex-
amples of the former include SbPS4,3 and examples of the latter
include metals (Au,4 Ag,5 Ni,6 Co, Cu,7 Bi8), oxides (ZnO,9
† Michigan State University.
‡ Department of Chemistry, Northwestern University.
§ Department of Physics and Astronomy, Northwestern University.
(1) Pauzauskie, P. J.; Yang, P. Mater. Today 2006, 9, 36–45. Li, Y.; Qian,
F.; Xiang, J.; Lieber, C. M. Mater. Today 2006, 9, 18–27. Hu, J. T.;
Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435–445.
(2) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates,
B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. AdV. Mater. 2003, 15, 353–389.
(3) Malliakas, C. D.; Kanatzidis, M. G. J. Am. Chem. Soc. 2006, 128,
6538–6539.
(12) Spahr, M. E.; Bitterli, P.; Nesper, R.; Muller, M.; Krumeich, F.; Nissen,
H. U. Angew. Chem., Int. Ed. 1998, 37, 1263–1265.
(13) Remskar, M. AdV. Mater. 2004, 16, 1497–1504. Tenne, R. Angew.
Chem., Int. Ed. 2003, 42, 5124–5132.
(4) Hulteen, J. C.; Jirage, K. B.; Martin, C. R. J. Am. Chem. Soc. 1998,
120, 6603–6604.
(14) Brorson, M.; Hansen, T. W.; Jacobsen, C. J. H. J. Am. Chem. Soc.
2002, 124, 11582–11583.
(5) Hong, B. H.; Bae, S. C.; Lee, C. W.; Jeong, S.; Kim, K. S. Science
2001, 294, 348–351.
(15) Cademartiri, L.; Malakooti, R.; O’Brien, P. G.; Migliori, A.; Petrov,
S.; Kherani, N. P.; Ozin, G. A. Angew. Chem., Int. Ed. 2008, 47, 3814–
3817.
(6) Tian, F.; Zhu, J.; Wei, D. J. Phys. Chem. C 2007, 111, 6994–6997.
(7) Thurn-Albrecht, T.; Schotter, J.; Kastle, C. A.; Emley, N.; Shibauchi,
T.; Krusin-Elbaum, L.; Guarini, K.; Black, C. T.; Tuominen, M. T.;
Russell, T. P. Science 2000, 290, 2126–2129.
(16) Chopra, N. G.; Luyken, R. J.; Cherrey, K.; Crespi, V. H.; Cohen, M. L.;
Louie, S. G.; Zettl, A. Science 1995, 269, 966–967.
(17) Li, H. W.; Chin, A. H.; Sunkara, M. K. AdV. Mater. 2006, 18, 216–
220.
(8) Li, Y. D.; Wang, J. W.; Deng, Z. X.; Wu, Y. Y.; Sun, X. M.; Yu,
D. P.; Yang, P. D. J. Am. Chem. Soc. 2001, 123, 9904–9905.
(9) Wu, J. J.; Liu, S. C.; Wu, C. T.; Chen, K. H.; Chen, L. C. Appl. Phys.
Lett. 2002, 81, 1312–1314.
(18) Ma, R.; Bando, Y. Chem. Mater. 2002, 14, 4403–4407.
(19) Yang, W.; Araki, H.; Hu, Q. L.; Ishikawa, N.; Suzuki, H.; Noda, T.
J. Cryst. Growth 2004, 264, 278–283.
(10) Nakamura, H.; Matsui, Y. J. Am. Chem. Soc. 1995, 117, 2651–2652.
(11) Hoyer, P. Langmuir 1996, 12, 1411–1413.
(20) Law, M.; Goldberger, J.; Yang, P. D. Annu. ReV. Mater. Res. 2004,
34, 83–122.
9
10.1021/ja808242g CCC: $40.75
2009 American Chemical Society
J. AM. CHEM. SOC. 2009, 131, 2647–2656 2647

A R T I C L E S
Chung et al.
series of compounds LiMo3Se3,21 KMPS4 (M ) Ni, Pd),22 and
nanowaveguide-based applications.33 In this regard, Cs5BiP4Se12,
which is soluble in polar solvents and presumably processable,
is a potential candidate. The rather surprising finding is that
the repeating unit giving rise to the nanofibers is just the simple
octahedral coordination complex [Bi(P2Se6)2]5-. The driving
force for the 1D morphology is the directional association of
these complexes at a supramolecular level via weak Se · · ·Se
interactions and Coulomb interactions with Cs+ countercations.
Ab initio density functional theory calculations using the full-
potential linearized augmented plane wave (FLAPW) method34
confirm the experimental band gap and also explain the nanowire
nature of Cs5BiP4Se12. To the best of our knowledge,
Cs5BiP4Se12 presents a rare example of a polar nanowire.
NaNb2PS10,23 which possess quasi-1D anionic chains of
1
1
1/∞[Mo3Se31-], /∞[MPS41-], and /∞[Nb2PS101-], respectively,
isolated by alkali metal countercations. These materials are
synthesized as bulk crystals, however, and form nanowires and/
or nanotubes only when dissolved in polar organic solvents such
as N-methylformamide (NMF) and dimethyl sulfoxide, where
their crystal lattices disassemble into structurally and electroni-
cally identical molecular wires.
Most of the known 1D nanostructures are simple atomic or
binary phases. More complex multinary inorganic solids in this
class are still rare, yet such materials can exhibit important
properties, such as superconductivity,24 giant magnetoresis-
tance,25 ferroelectricity,26 liquid crystallinity,27 and optical
nonlinearity.28 We note that polar nanowires are rare. Recently,
we showed that low-dimensional chalcogenides with polar
2. Experimental Section
2.1. Reagents. The reagents employed in this work were used
as obtained: Cs metal, analytical reagent, Johnson Matthey/AESAR
Group, Seabrook, NH; red phosphorus powder, -100 mesh, Morton
Thiokol, Inc., Danvers, MA; Se, 99.9999%, Noranda Advanced
Materials, Quebec, Canada; N,N-dimethylformamide (DMF), ACS
reagent grade, Mallinckrodt Baker Inc., Paris, KY; diethyl ether,
ACS reagent grade, anhydrous, Columbus Chemical Industries,
Columbus, WI. The Cs2Se starting material was prepared by
reacting stoichiometric amounts of the elements in liquid ammonia
under N2.
2.2. Synthesis. The synthesis of pure Cs5BiP4Se12 micro/
nanofibers was achieved by reacting a 2.5:1:4:9.5 Cs2Se/Bi/P/Se
mixture under vacuum in a fused-silica tube at 850 °C for 5 days
followed by washing with degassed DMF under a N2 atmosphere
(∼80% yield based on Bi). Alternatively, the title compound could
be obtained by reacting a stoichiometric mixture of the same
reagents in a fused-silica tube at 850 °C for 3 h. Stoichiometric
mixtures with shorter reaction times typically produced tiny needles
instead of micro/nanofibers. Energy dispersive spectroscopy (EDS)
analysis showed average compositions “Cs4.9BiP3.8Se11.6” and
“Cs4.8BiP3.9Se11.8” for the respective synthetic conditions. The single
crystals are stable in DMF, alcohol, and air for up to two weeks.
They are soluble in NMF to give deep-orange solutions. As-prepared
Cs5BiP4Se12 micro/nanowires were dispersed in EtOH and im-
mediately formed an orange colloidal solution upon shaking a
couple of times. For transmission electron microscopy (TEM)
studies, the resulting suspension was diluted, sonicated, centrifuged
at 4000 rpm for 1 h, and filtered by a 0.2 µm syringe filter to obtain
evenly dispersed fibers rather than aggregated ones.
2.3. Physical Measurements. 2.3.1. Powder X-ray Diffrac-
tion. Phase-purity X-ray diffraction analyses were performed using
a calibrated CPS 120 INEL powder X-ray diffractometer (Cu KR
graphite monochromatized radiation) operating at 40 kV and 20
mA and equipped with a position-sensitive detector with a flat
sample geometry.
2.3.2. Scanning Electron Microscopy. Scanning electron mi-
croscopy (SEM) semiquantitative analyses and morphology images
of the compounds were obtained with a JEOL JSM-35C scanning
electron microscope (SEM) equipped with a Tracor Northern EDS
detector.
2.3.3. Transmission Electron Microscopy. TEM samples were
diluted with ethanol and irradiated by ultrasonication. TEM and
high-resolution TEM (HRTEM) images were obtained with JEOL
JEM 2200 FS field-emission transmission electron microscope.
2.3.4. Solid-State UV-Vis Spectroscopy. Optical diffuse re-
flectance measurements were performed at room temperature using
a Shimadzu UV-3101 PC double-beam, double-monochromator
spectrophotometer operating in the 200-2500 nm region. The
30
structures, such as K2P2Se6,29 LiAsS2 and CsZrPSe6,31 can
give rise to strong nonlinear optical responses and are promising
in meeting technological needs in the IR and near-IR regions
of the spectrum.
Here we describe the new compound Cs5BiP4Se12, which
intrinsically grows as nanowires and is obtained using a simple
synthetic method. The compound is a semiconductor and shows
a wide optical-transparency range through the mid- and near-
IR regions as well as a strong second harmonic generation
(SHG) response at 1 µm. Integrated photonic networks with
high wavelength-conversion efficiencies in the IR region
(especially at 1.3-1.5 µm) are used in the broadband Internet
communications industry.32 Materials with a large nonlinearity
and index of refraction are desirable for fiber-optic- and
(21) Song, J. H.; Messer, B.; Wu, Y. Y.; Kind, H.; Yang, P. D. J. Am.
Chem. Soc. 2001, 123, 9714–9715. Venkataraman, L.; Lieber, C. M.
Phys. ReV. Lett. 1999, 83, 5334–5337. Davidson, P.; Gabriel, J. C.;
Levelut, A. M.; Batail, P. Europhys. Lett. 1993, 21, 317–322. Tarascon,
J. M.; Disalvo, F. J.; Chen, C. H.; Carroll, P. J.; Walsh, M.; Rupp, L.
J. Solid State Chem. 1985, 58, 290–300.
(22) Sayettat, J.; Bull, L. M.; Gabriel, J. C. P.; Jobic, S.; Camerel, F.; Marie,
A. M.; Fourmigue, M.; Batail, P.; Brec, R.; Inglebert, R. L. Angew.
Chem., Int. Ed. 1998, 37, 1711–1714.
(23) Camerel, F.; Gabriel, J. C. P.; Batail, P.; Davidson, P.; Lemaire, B.;
Schmutz, M.; Guilk-Krzywicki, T.; Bourgaux, C. Nano Lett. 2002, 2,
403–407.
(24) Cava, R. J. Science 1990, 247, 656–662. Sleight, A. W. Science 1988,
242, 1519–1527.
(25) Jin, S.; Tiefel, T. H.; Mccormack, M.; Fastnacht, R. A.; Ramesh, R.;
Chen, L. H. Science 1994, 264, 413–415. Moritomo, Y.; Asamitsu,
A.; Kuwahara, H.; Tokura, Y. Nature 1996, 380, 141–144.
(26) Scott, J. F.; Araujo, C. A. Science 1989, 246, 1400–1405. Dearaujo,
C. A. P.; Cuchiaro, J. D.; Mcmillan, L. D.; Scott, M. C.; Scott, J. F.
Nature 1995, 374, 627–629. Hemberger, J.; Lunkenheimer, P.; Fichtl,
R.; von Nidda, H. A. K.; Tsurkan, V.; Loidl, A. Nature 2005, 434,
364–367.
(27) Gabriel, J. C. P.; Camerel, F.; Lemaire, B. J.; Desvaux, H.; Davidson,
P.; Batail, P. Nature 2001, 413, 504–508. Miyamoto, N.; Nakato, T.
AdV. Mater. 2002, 14, 1267–1270. Michot, L. J.; Bihannic, I.; Maddi,
S.; Funari, S. S.; Baravian, C.; Levitz, P.; Davidson, P. Proc. Natl.
Acad. Sci. U.S.A. 2006, 103, 16101–16104. Miyamoto, N.; Yamada,
Y.; Koizumi, S.; Nakato, T. Angew. Chem., Int. Ed. 2007, 46, 4123–
4127.
(28) Bordui, P. F.; Fejer, M. M. Annu. ReV. Mater. Sci. 1993, 23, 321–
379.
(29) Chung, I.; Malliakas, C. D.; Jang, J. I.; Canlas, C. G.; Weliky, D. P.;
Kanatzidis, M. G. J. Am. Chem. Soc. 2007, 129, 14996–15006.
(30) Bera, T. K.; Song, J.-H.; Freeman, A. J.; Jang, J. I.; Ketterson, J. B.;
Kanatzidis, M. G. Angew. Chem., Int. Ed. 2008, 47, 7828–7832.
(31) Banerjee, S.; Malliakas, C. D.; Jang, J. I.; Ketterson, J. B.; Kanatzidis,
M. G. J. Am. Chem. Soc. 2008, 130, 12270–12272.
(32) Dorn, R.; Baums, D.; Kersten, P.; Regener, R. AdV. Mater. 1992, 4,
464–473. Ma, H.; Jen, A. K. Y.; Dalton, L. R. AdV. Mater. 2002, 14,
1339–1365. Yamada, H.; Shirane, M.; Chu, T.; Yokoyama, H.; Ishida,
S.; Arakawa, Y. Jpn. J. Appl. Phys., Part 1 2005, 44, 6541–6545.
(33) Knight, J. C. Nature 2003, 424, 847–851.
(34) Wimmer, E.; Krakauer, H.; Weinert, M.; Freeman, A. J. Phys. ReV. B
1981, 24, 864–875.
9
2648 J. AM. CHEM. SOC. VOL. 131, NO. 7, 2009

Strong SHG in Flexible Polar Nanowires of Cs5BiP4Se12
A R T I C L E S
Table 1. Crystallographic Data and Refinement Details for
instrument was equipped with an integrating sphere and controlled
by a personal computer. BaSO4 was used as a 100% reflectance
standard. The sample was prepared by grinding the crystals to a
powder that was spread on a compacted bed of BaSO4 powder
standard material. The generated reflectance-versus-wavelength data
were used to estimate the band gap of the material by converting
reflectance to absorption data35 according to the Kubelka-Munk
equation: R/S ) (1 - R)2/(2R), where R is the reflectance and R
and S are the absorption and scattering coefficients, respectively.
2.3.5. Raman Spectroscopy. Raman spectra were recorded on
a Holoprobe Raman spectrograph equipped with a charge-coupled
device (CCD) camera detector using 633 nm radiation from a HeNe
laser for excitation and a resolution of 4 cm-1. The laser power at
the sample was estimated to be ∼5 mW, and the focused laser beam
diameter was ∼10 µm. A total of 128 scans was sufficient to obtain
spectra of good quality.
Cs5BiP4Se12
formula
crystal system
space group
Cs5BiP4Se12
orthorhombic
Pmc21 (No. 26)
a ) 7.5357(2)
b ) 13.7783(6)
c ) 28.0807(8)
4
unit cell dimensions (Å)
Z
V (Å3)
2915.59(17)
4.431
dcalcd (g cm-3
)
crystal dimensions (mm3)
0.1189 × 0.0006 × 0.0006
T (K)
λ (Å)
293(2)
0.71073
27.392
3304
µ (mm-1
F(000)
)
θmax (deg)
27.392
total/unique reflections
Rint
18838/6306
0.0443
246
2.3.6. Infrared Spectroscopy. FT-IR spectra were recorded on
solids in a CsI or KBr matrix. The samples were ground with dry
CsI or KBr into a fine powder and pressed into translucent pellets.
The spectra were recorded in the far-IR (600-100 cm-1) and mid-
IR (500-4000 cm-1) regions with 4 cm-1 resolution using a Nicolet
740 FT-IR spectrometer equipped with a TGS/PE detector and
silicon beam splitter.
no. of parameters
refinement method
full-matrix least-squares on F2
0.0626/0.1151
0.0790/0.1207
1.077
a
b
final R1 /wR2 [I > 2σ(I)]
R1/wR2 (all data)
goodness of fit on F2
largest diff. peak/hole (e Å-3
)
2.198/-3.339
2.3.7. Differential Thermal Analysis. Differential thermal
analysis (DTA) experiments were performed on a Shimadzu DTA-
50 thermal analyzer. A sample (∼30 mg) of ground crystalline
material was sealed in a silica ampule under vacuum. A similar
ampule of equal mass filled with Al2O3 was sealed and placed on
the reference side of the detector. The sample was heated to 800 at
10 °C min-1, and after 1 min, it was cooled at a rate of -10 °C
min-1 to 50 °C. The residues of the DTA experiments were
examined by powder X-ray diffraction. Reproducibility of the results
was confirmed by running multiple heating/cooling cycles. The
melting and crystallization points were measured at the minimum
of the endothermic peak and the maximum of the exothermic peak.
2.3.8. X-ray Crystallography. Since Cs5BiP4Se12 crystallized
in the form of a very thin microwire that is a bundle of individual
nanowires, it was extremely hard to find single crystals suitable
for X-ray diffraction studies. Most of the single crystals studied
showed substantially diffused peaks. Intensity data for Cs5BiP4Se12
were collected at 293(2) K on a STOE IPDS II diffractometer with
Mo KR radiation operating at 50 kV and 40 mA with a 34 cm
diameter imaging plate. Data collection at lower temperatures was
not useful because the N2 stream affected the very thin wire-shaped
single crystal. Individual frames were collected with a 15 min
exposure time and a 1.0 ω rotation. The X-AREA, X-RED, and
X-SHAPE software package was used for data extraction and
integration and to apply empirical and analytical absorption
corrections. The SHELXTL software package was used to solve
and refine the structure. The most satisfactory refinement was
obtained with the polar space group Pmc21. The Cs(8) atom was
modeled as split into Cs(8A) and Cs(8B) sites. Their occupancy
ratio was refined to 95:5. The Flack parameter x was refined to 0.
The parameters for data collection and the details of the structural
refinement are given in Table 1. Fractional atomic coordinates and
atomic displacement parameters for the structure are listed in Table
2, and selected bond distances and angles are provided in Tables 3
and 4, respectively. All of the atoms except Cs(8) were refined to
full occupancy.
b
a R1 ) ∑||Fo| - |Fc||/∑|Fo|. wR2 ) [∑w(Fo - Fc2)2/∑w(Fo )2]1/2
.
2
2
Table 2. Atomic Coordinates (×104) and Equivalent Isotropic
Parameters (×103 Å2) for Cs5BiP4Se12 at 293(2) Ka b
,
atom
x
y
z
Ueq
Cs(1)
Cs(2)
Cs(3)
Cs(4)
Cs(5)
Cs(6)
Cs(7)
Cs(8A)
Cs(8B)
Cs(9)
Cs(10)
Bi(1)
Bi(2)
P(1)
P(2)
P(3)
P(4)
P(5)
P(6)
P(7)
P(8)
Se(1)
Se(2)
Se(3)
Se(4)
Se(5)
Se(6)
Se(7)
Se(8)
Se(9)
Se(10)
Se(11)
Se(12)
Se(13)
Se(14)
Se(15)
Se(16)
5000
5000
5000
5000
5000
0
1495(2)
1511(2)
1680(2)
5132(2)
5212(2)
1356(1)
1675(2)
1680(2)
1710(20)
4953(2)
7879(2)
1681(1)
3406(1)
50(7)
1862(1)
85(1)
10(1)
17(1)
22(1)
20(1)
21(1)
9(1)
20(1)
21(1)
21(1)
17(1)
22(1)
6(1)
10(1)
15(2)
12(2)
7(1)
9(2)
8(2)
6808(1)
3542(1)
1653(1)
5139(1)
3247(1)
6780(1)
6823(12)
209(1)
3559(1)
4219(1)
1777(1)
3042(3)
8775(3)
5492(3)
4835(3)
1187(3)
737(3)
2829(3)
2417(3)
7718(1)
3155(1)
3753(1)
4122(1)
5873(1)
5210(1)
36(2)
4423(1)
1005(1)
1933(1)
3(1)
0
0
-1080(60)
0
0
5000
0
5000
5000
5000
5000
0
594(6)
2792(6)
3750(6)
871(6)
2200(6)
4521(7)
5922(7)
382(2)
1658(3)
7873(3)
24(2)
3137(2)
1308(2)
4745(2)
3322(2)
83(2)
1339(2)
1846(3)
3025(2)
4821(4)
3734(2)
6631(2)
5509(2)
0
0
0
7(2)
18(2)
16(2)
19(1)
12(1)
21(1)
15(1)
12(1)
9(1)
21(1)
12(1)
11(1)
10(1)
22(1)
13(1)
29(1)
20(1)
17(1)
11(1)
2540(3)
5000
5000
2597(3)
2594(3)
5000
5000
2633(3)
2406(3)
0
0
2378(3)
0
2390(3)
2467(3)
0
962(1)
2.3.9. Electronic Band Structure Calculations. The electronic
structure calculations were performed using the highly precise
3564(1)
2576(1)
2605(1)
1653(1)
(35) (a) Wendlandt, W. W.; Hecht, H. G. Reflectance Spectroscopy;
Interscience Publishers: New York, 1966. (b) Kortu¨m, G. Reflectance
Spectroscopy: Principles, Methods, Applications; Springer: Berlin,
1969. (c) Tandon, S. P.; Gupta, J. P. Phys. Status Solidi 1970, 38,
363–367. (d) Chung, D. Y.; Choi, K. S.; Iordanidis, L.; Schindler,
J. L.; Brazis, P. W.; Kannewurf, C. R.; Chen, B. X.; Hu, S. Q.; Uher,
C.; Kanatzidis, M. G. Chem. Mater. 1997, 9, 3060–3071. (e) Stephan,
H. O.; Kanatzidis, M. G. Inorg. Chem. 1997, 36, 6050–6057.
a The occupancies of Cs(8A) and Cs(8B) are 95 and 5%, respectively.
All other sites are fully occupied. b Ueq is defined as one-third of the trace
of the orthogonalized Uij tensor.
FLAPW method34 within the density functional theory scheme. The
experimentally obtained lattice parameters and atomic coordinates
9
J. AM. CHEM. SOC. VOL. 131, NO. 7, 2009 2649

A R T I C L E S
Chung et al.
Table 3. Selected Bond Distances (Å) for Cs5BiP4Se12 at 293(2) K
Bi(1)-Se(2)
Bi(1)-Se(4)
Bi(1)-Se(6)
Bi(1)-Se(8)
P(1)-Se(1)
P(1)-Se(2)
P(2)-Se(3)
P(2)-Se(4)
P(3)-Se(5)
P(3)-Se(6)
P(4)-Se(7)
P(4)-Se(8)
P(1)-P(2)
2.986(3)
Bi(2)-Se(10)
Bi(2)-Se(12)
Bi(2)-Se(14)
Bi(2)-Se(16)
P(5)-Se(9)
P(5)-Se(10)
P(6)-Se(11)
P(6)-Se(12)
P(7)-Se(13)
P(7)-Se(14)
P(8)-Se(15)
P(8)-Se(16)
P(5)-P(6)
2.882(4)
2.927(3) × 2
2.953(3) × 2
2.914(3) × 2
2.917(3)
2.175(5) × 2
2.192(8) × 2
2.118(8)
2.832(3)
2.937(3) × 2
2.149(5) × 3
2.238(10)
2.114(9) × 2
2.226(5) × 3
2.159(5) × 2
2.193(8)
2.148(8) × 2
2.206(5) × 2
2.241(12) × 2
2.268(12)
2.214(5) × 2
2.105(10)
2.219(7) × 2
2.165(5) × 2
2.220(9)
2.224(11)
2.252(13)
P(3)-P(4)
P(7)-P(8)
Table 4. Selected Bond Angles (deg) for Cs5BiP4Se12 at 293(2) Ka
Figure 1. Structure of two crystallographically independent [Bi(P2Se6)2]5-
anions. Bi (yellow), P (gray), and Se (pink) atoms are labeled and selected
bond distances (Å) shown.
Se(2)-Bi(1)-Se(4)
Se(2)-Bi(1)-Se(6)
Se(2)-Bi(1)-Se(8)
Se(4)-Bi(1)-Se(4)#1
Se(4)-Bi(1)-Se(6)
Se(4)-Bi(1)-Se(8)
Se(4)#1-Bi(1)-Se(8)
Se(6)-Bi(1)-Se(8)
Se(8)-Bi(1)-Se(8)
84.21(7) P(1)#7-P(2)-Se(3)#5 111.6(4)
168.91(10) Se(10)-Bi(2)-Se(12)
101.78(7) Se(10)-Bi(2)-Se(14)
86.67(7)
92.05(8)
76.45(10) Se(10)-Bi(2)-Se(16) 178.10(11)
87.09(7) Se(12)-Bi(2)-Se(12)#2 74.74(10)
104.06(6) Se(12)-Bi(2)-Se(14) 104.44(6)
174.01(7) Se(12)-Bi(2)-Se(14)#2 178.51(9)
incident laser pulse of 300 µJ was focused onto a spot 500 µm in
diameter using a 3 cm focal length lens. The corresponding incident
photon flux was ∼10 GW cm-2. The SHG signal was collected
from the excitation surface in the reflection geometry and focused
onto a fiber-optic bundle. The output of the fiber-optic bundle was
coupled to the entrance slit of a Spex Spec-One 500 M spectrometer
and detected using a nitrogen-cooled CCD camera. The data
collection time was 20 s. Single-crystal Cs5BiP4Se12 and AgGaSe2
samples were ground and separated into various size ranges using
sieves. Samples were placed in capillary tubes and measured. The
SHG intensity of Cs5BiP4Se12 was compared with that of the
86.98(7) Se(12)-Bi(2)-Se(16)
94.84(8)
74.79(9) Se(14)-Bi(2)-Se(14)#2 76.36(11)
Se(1)#3-P(1)-Se(1)#4 119.2(4) Se(14)-Bi(2)-Se(16)
Se(1)#3-P(1)-Se(2) 109.5(3) P(1)#7-P(2)-Se(4)#7
86.45(8)
104.5(3)
105.2(4)
110.4(4)
105.7(3)
106.0(3)
104.5(4)
111.3(4)
105.1(3)
106.1(3)
107.5(4)
105.2(3)
106.1(4)
109.6(5)
104.7(3)
Se(3)#5-P(2)-Se(4)#6 113.3(3) P(2)#3-P(1)-Se(2)
Se(4)#6-P(2)-Se(4)#7 108.9(4) P(3)-P(4)-Se(7)#5
Se(5)-P(3)-Se(5)#1 114.3(4) P(3)-P(4)-Se(8)
Se(5)-P(3)-P(6)
112.6(2) P(4)-P(3)-Se(5)
Se(7)#5-P(4)-Se(8)#1 113.3(3) P(4)-P(3)-Se(6)
Se(8)-P(4)-Se(8)#1 107.9(3) P(5)-P(6)-Se(11)
Se(9)-P(5)-Se(9)#2 113.0(4) P(5)-P(6)-Se(12)
37
benchmark material AgGaSe2 in the range 500-1000 nm.
Se(9)-P(5)-Se(10)
111.8(3) P(6)-P(5)-Se(9)
3. Results and Discussion
Se(11)-P(6)-Se(12) 113.3(2) P(6)-P(5)-Se(10)
Se(12)-P(6)-Se(12)#2 108.1(4) P(7)-P(8)-Se(15)
Se(13)-P(7)-Se(14) 114.2(3) P(7)-P(8)-Se(16)
Se(14)-P(7)-Se(14)#2 108.5(5) P(8)-P(7)-Se(13)
Se(15)-P(8)-Se(15)#2 118.3(4) P(8)-P(7)-Se(14)
Se(15)-P(8)-Se(16) 110.6(3)
3.1. Synthesis and Crystal Structure. In spite of the nanowire
microstructure and fibrous nature of the material, which made
the selection of single crystals very challenging, we succeeded
in determining the structure using single-crystal X-ray crystal-
lography on very thin fibers. The new compound Cs5BiP4Se12
adopts the polar space group Pmc21. It features discrete
molecular [Bi(P2Se6)2]5- anions (Figure 1), which are isolated
by Cs+ cations (Figure 2a). The anions are isostructural to the
a Symmetry transformations used to generate equivalent atoms: (#1)
1
1
-x + 1, y, z; (#2) -x, y, z; (#3) -x + 1, -y, z - /2; (#4) x, -y, z - /2;
1
1
(#5) -x + 1, -y + 1, z + /2; (#6) x, -y, z + /2; (#7) -x + 1, -y, z +
1/2.
38
39
[In(P2Se6)2]5-
and [P(P2Se6)2]5-
anions. There are two
were employed for the calculations. The core and valence states
were treated fully relativistically and scalar relativistically, respec-
tively. The spin-orbit interaction was also included self-consistently
by a second variational method.36 A 5 × 3 × 1 mesh of special
k-points was used in the irreducible Brillouin zone, and the energy
cutoffs for the interstitial plane-wave basis and the star functions
were 13.0 and 144.0 Ry, respectively. The muffin-tin radii of P,
Se, Cs, and Bi were 1.9, 2.0, 2.7, and 2.8 bohr, respectively.
2.3.10. Nonlinear Optical Property Measurements. We used
the frequency-tripled output of a passive-active mode-locked Nd:
YAG laser with a pulse width of ∼15 ps and a repetition rate of
10 Hz to pump an optical parametric amplifier (OPA). The OPA
generated vertically polarized pulses in the ranges 400-685 and
737-3156 nm. In order to determine the SHG intensity as a function
of the excitation energy, we tuned the wavelength of the incident
light from 1000 to 2000 nm. In this range, the spectral bandwidth
of the linearly polarized light from the OPA was rather broad, ∼2
meV full width at half-maximum. However, the phase-space
compression phenomena ensured effective SHG where lower-energy
portions were exactly compensated by the higher-energy parts,
thereby satisfying both energy and momentum conservation. The
crystallographically unique Bi atoms, and each octahedrally
coordinated Bi3+ center is capped by two chelating tridentate
[P2Se6]4- units from opposite directions (Figure 1). Although
the respective [Bi(P2Se6)2]5- molecules are acentric because
of the distorted [BiSe6] octahedra, the polar structure arises from
the noncentrosymmetric packing of Cs+ ions. As expected from
the polar crystal class mm2 of the space group Pmc21, the
coordination environment of the Cs atoms results in a macro-
scopic alignment of the dipole moments along the c axis (Figure
2b), resulting in the polar nature.40 The absence of a mirror
plane perpendicular to the 21 screw axis creates the polar
structural arrangement of the compound. The Bi-Se distances
are normal, ranging from 2.832(3) to 2.986(3) Å. The P-Se
distances range from 2.114(9) to 2.241(12) Å. The P-P
(37) Nikogosyan, D. N. Nonlinear Optical Crystals: A Complete SurVey;
Springer-Science: New York, 2005.
(38) Chondroudis, K.; Chakrabarty, D.; Axtell, E. A.; Kanatzidis, M. G.
Z. Anorg. Allg. Chem. 1998, 624, 975–979.
(39) Chung, I.; Jang, J. I.; Gave, M. A.; Weliky, D. P.; Kanatzidis, M. G.
Chem. Commun. 2007, 4998–5000.
(36) MacDonald, A. H.; Pickett, W. E.; Koelling, D. D. J. Phys. C 1980,
13, 2675–2683.
(40) Halasyamani, P. S.; Poeppelmeier, K. R. Chem. Mater. 1998, 10, 2753–
2769.
9
2650 J. AM. CHEM. SOC. VOL. 131, NO. 7, 2009

Strong SHG in Flexible Polar Nanowires of Cs5BiP4Se12
A R T I C L E S
Rb4P6Se1243 display similarly short Se · · ·Se interactions. The
dipole associated with the noncentrosymmetric structure is
parallel to the c axis, which is perpendicular to the nanowire
direction. Other polar nanowire systems include II-IV semi-
conductors such as ZnO, CdS, and CdSe and the III-V
semiconductors GaAs, InP, etc. In these systems, the dipole is
generally aligned along the nanowire axis. Cs5BiP4Se12 is a rare
example where the dipole is perpendicular to the long axis.
By comparing polychalcophosphate flux reaction conditions
that favor Cs5Bi(P2Se6)2 vis-a`-vis Cs8Bi4(P2Se6)5,44 we can gain
deeper insight into the evolution of the structure. The latter was
synthesized in a more basic 1:1.5:2 Cs2Se/P2Se5/Se alkali
polyselenophosphate flux at 460 °C, giving a remarkably
complicated layered structure of 2/∞[Bi4(P2Se6)58-]. In contrast,
the simpler molecular complex [Bi(P2Se6)2]5-in the title com-
pound was obtained in a less basic 1:1.6:3.8 Cs2Se/P2Se5/Se
flux at 850 °C. In general, less basic conditions or lower flux
temperatures tend to generate longer or extended fragments.45
Hence, we assume that the temperature plays a more dominant
role than the higher flux basicity in stabilizing Cs5Bi(P2Se6)2.
Presumably, for a given flux composition, the products formed
at high reaction temperatures of 800-1000 °C differ from those
formed at intermediate temperatures of 300-600 °C.
3.2. Nanowires. The fibrous nature of Cs5BiP4Se12 is apparent
by visual inspection and in SEM images. When we examined
the as-prepared product, we observed aggregates of fiber
bundles. As-prepared microfibers are typically millimeters in
length and submicrons to tens of nanometers in thickness (Figure
3a). The thicker fibers are bundles of thinner nanowires. Thus,
in this article, we refer to the as-prepared fibers as microfibers,
in that mostly they are not single-nanowire strands. The
microfibers can readily be split into ever thinner fibers by
physical contact. TEM images of a relatively thick single
microfiber confirmed that the microfibers consist of individual
nanowires with uniform thickness and alignment with an interval
of 2.9 nm, which is close to the dimension of the crystal-
lographic c axis (Figure 3b). This creates the extreme difficulty
in finding single crystals suitable for X-ray diffraction studies.
Most of the crystals we examined showed extensive diffuse
scattering normal to the a* axis, consistent with the nanosize
dimensions of the coherence length along the b and c axes (i.e.,
perpendicular to the fiber). The diffuse scattering is caused by
the sliding of individual nanowires and chains past each other.
This observation indicates that most of the microfibers are
aggregated bundles of nanowires rather than genuine microfibers
with a singe domain.
Figure 2. (a) Structure of Cs5BiP4Se12 viewed down the a axis (down the
fiber direction). Bi (yellow), P (gray), and Se (pink) atoms are labeled.
Large blue circles are Cs atoms. (b) Coordination environment of Cs atoms
in a polyhedral representation showing the alignment of the macroscopic
dipole moment along the c axis. (c) Polyhedral and (d) ball-and-stick
The Cs5BiP4Se12 microfibers can be dispersed in EtOH under
a N2 atmosphere to form a colloidal suspension of nanowires.
The resulting colloid was diluted, ultrasonicated, and centrifuged
to obtain evenly dispersed nanowires. A representative TEM
image of a nanowire is shown in Figure 3c, and the selected-
area electron diffraction (SAED) pattern (Figure 3c inset) shows
the crystalline nature of the nanowires. EDS analysis of the
sample examined by TEM showed the Cs, Bi, P, and Se to be
present in the expected ratio (Figure 3d). The distribution of
representations of a pseudo-2D [Bi(P2Se6)2]5- chain viewed along the [021]
j
direction. The green dashed lines depict the short nonbonding interactions
[3.478(3) Å] between Se(5) and Se(16). In (c), light polyhedra depict [BiSe6]
octahedra and dark ones [P2Se6]4- units; Cs atoms have been omitted.
distances of 2.224(11) to 2.252(11) Å are typical. Noteworthy
is the unusually short intermolecular Se(5) · · ·Se(16) nonbonding
interaction at 3.478(3) Å, which is much shorter than the van
der Waals radii sum of 3.8 Å.41 This weak interaction is
important in enabling the compound to organize in an infinite
pseudo-1D structure along the a axis (Figure 2c,d). However,
the ionic interaction of Cs+ with [Bi(P2Se6)2]5- is also prominent,
as we will argue later. Low-dimensional compounds such as
NbSe3, APSe6 (A ) K, Rb, Cs),42 K2P2Se6,29 and molecular
(42) Chung, I.; Do, J.; Canlas, C. G.; Weliky, D. P.; Kanatzidis, M. G.
Inorg. Chem. 2004, 43, 2762–2764.
(43) (a) Chung, I.; Karst, A. L.; Weliky, D. P.; Kanatzidis, M. G. Inorg.
Chem. 2006, 45, 2785–2787. (b) Kanatzidis, M. G.; Huang, S. P. Inorg.
Chem. 1989, 28, 4667–4669.
(44) McCarthy, T. J.; Hogan, T.; Kannewurf, C. R.; Kanatzidis, M. G.
Chem. Mater. 1994, 6, 1072–1079.
(45) Kanatzidis, M. G.; Sutorik, A. C. Prog. Inorg. Chem. 1995, 43, 151–
265.
(41) Bondi, A. J. Phys. Chem. 1964, 68, 441–451.
9
J. AM. CHEM. SOC. VOL. 131, NO. 7, 2009 2651

A R T I C L E S
Chung et al.
Figure 4. (a) DTA curve and (b) X-ray powder diffraction patterns of the
pristine material and the sample obtained after DTA.
Figure 5. Solid-state optical absorption spectrum of Cs5BiP4Se12.
must have a bonding character and play a key role in determin-
ing the emergent fiber behavior in this material, which arises
from the packing of simple coordinating complexes of
[Bi(P2Se6)2]5-. To confirm the phase identity of the nanowires
imaged by TEM, the colloidal suspension was drop-cast, dried,
and examined by powder X-ray diffraction. The patterns taken
matched those of the theoretical simulation based on the single-
crystal structure refinement and were fully indexed (Figure 3g),
confirming that no phase degradation occurred during the
dispersion process. The chemical composition obtained by EDS
analysis was close to Cs5BiP4Se12.
Figure 3. (a) Representative SEM image of Cs5BiP4Se12 microfibers,
demonstrating their flexible texture. (b) TEM image of a bundle of individual
nanowires, showing their uniform alignment. The lateral spacing between
nanowires was measured to be 2.9 nm, which is close to the crystallographic
c axis of 2.808 nm. (c) TEM image and (inset) SAED pattern of a nanowire.
(d) EDS analysis of nanowires (Cs, blue; Bi, orange; P, gray; Se, red). (e)
HRTEM image of individual Cs5BiP4Se12 nanowires dispersed in EtOH.
(f) Tangled texture of a Cs5BiP4Se12 nanowire, depicting its striking
flexibility. (g) (top) Powder X-ray diffraction pattern of a dried Cs5BiP4Se12
colloidal suspension and (bottom) the theoretical calculation. Major peaks
are indexed.
DTA of Cs5BiP4Se12 at a rate of 10 °C min-1 showed melting
at 590 °C upon heating and crystallization at 559 °C upon
cooling (Figure 4a). The powder X-ray diffraction patterns
before and after melting/recrystallization were identical (Figure
4b), indicating congruent melting. The solid-state optical
absorption spectrum (Figure 5) revealed a sharp absorption edge
and a band gap of 1.85 eV, which is in good agreement with
its dark-red color. The nearly vertical rise of the absorption
coefficient suggests a direct band gap material. The nature of
this gap is discussed below in conjunction with the electronic
nanowire diameters is ∼3-9 nm (for a single nanowire, d ≈
2.8 nm). Typical long Cs5BiP4Se12 nanowires showed a striking
flexibility and tangled texture (Figure 3e,f), which is extraor-
dinary considering the single-crystalline nature of the fibers.
The flexible character likely arises from the weak intra- and
interchain interactions created by the short nonbonding Se · · ·Se
contacts between [Bi(P2Se6)2]5- anions, which allow the nanow-
ires to slide past one another and bend. These weak interactions
9
2652 J. AM. CHEM. SOC. VOL. 131, NO. 7, 2009

Strong SHG in Flexible Polar Nanowires of Cs5BiP4Se12
A R T I C L E S
structure calculations. In comparison, the extended polymeric
Cs8Bi4(P2Se6)5 compound shows a sharp optical gap at 1.44
eV.44
grow more rapidly along the a axis (i.e., the apparent nanofiber
growth direction).
To better understand how the discrete molecules give rise to
the nanowire nature of Cs5BiP4Se12, the binding energies of the
molecular [Bi(P2Se6)2]5- units in each crystallographic direction
were investigated with the LDA and Perdew-Burke-Ernzerhof
(PBE)48 generalized gradient approximation (GGA) forms of
the exchange-correlation potential. We considered three different
models in which the coordinates of two molecular units were
taken from the original crystal structure (Figure 2a) in such a
way that they have intermolecular interactions for different
orientations in the b-c plane (Figure 7a,b) and along the a axis
(Figure 7c). For these models, the binding energies were
estimated by examining the total energy as a function of the
distance between the two molecular units.
The LDA calculations yielded binding energies of 1.90, 2.07,
and 4.13 eV for the first, second, and third models in Figure 7,
respectively, while the GGA calculations showed significantly
smaller binding energies of 0.84, 0.87, and 3.35 eV, respectively.
The difference between the binding energies from the LDA and
the GGA comes mainly from the van der Waals interactions.
While the strongest interactions are the ionic interactions
between the molecular [Bi(P2Se6)2]5- units, there are also van
der Waals closed-shell interactions, for which the LDA in
general shows largely overestimated binding energies of the
bonds. The GGA, in contrast, may give over- or underestimated
or even no binding energies, depending on the particular GGA
form [i.e., Becke-Lee-Yang-Parr (BLYP)49 or PBE] em-
ployed.50 Even with the deficiency of these exchange-correlation
approximations for the van der Waals interactions, both the LDA
and GGA showed significantly stronger bonds in the third
model, which is the interaction along the fiber direction. This
is, therefore, the fastest-growing direction, which is consistent
with the nanofiber nature found experimentally. The electronic
structure calculations on Cs5BiP4Se12 clearly demonstrate the
existence of dominating interactions along specific directions.
3.4. Spectroscopy and Nonlinear Optical Response. The
Raman spectrum of Cs5BiP4Se12 is very active, with shifts at
118, 143, 172, 221, 300, 433, and 467 cm-1 (Figure 8a). The
peak at 221 cm-1 is unambiguously assigned to the locally A1g
symmetric stretching mode of the PSe3 unit (half of the [P2Se6]4-
ligand).21 The peaks below 200 cm-1 are possibly related to
Bi-Se stretching excitations. The other shifts can be attributed
to PSe3 stretching modes.
3.3. Electronic Structure Calculations. To better understand
the experimental findings, and in particular to explore the origin
and nature of the band gap results, the electronic structure was
calculated using the FLAPW method with the screened-
exchange local density approximation (sX-LDA) and Hedin-
Lundqvist46 (LDA) forms of the exchange-correlation potential.
The sX-LDA method has been successfully applied to a wide
range of semiconductors and shows a great improvement in the
excited electronic states in terms of correctly determining the
band gap and band dispersion.47 While the LDA calculations
yield band gaps (Eg) of 1.48 eV without spin-orbit coupling
(SOC) and 1.15 eV with SOC included, the electronic structure
calculated using the sX-LDA method with SOC predicts the
band gap to be 2.0 eV, which is close to the experimental
value of 1.85 eV. The valence band maximum (VBM) and the
conduction band minimum (CBM) occur along the T-Y
direction and give rise to an indirect band gap. However, the
almost plateaulike narrow band dispersion (e5 meV) along the
T-Y direction causes a large joint density of states at 2.0 eV,
which is responsible for the very sharp absorption edge observed
experimentally (Figure 5). The narrow band dispersions (e.g.,
e0.05 eV for the valence top band along Γ-Z-T-Y) indicate
the molecular nature of the compound. The relatively strong
dispersion (∼0.24 eV for the valence top band) along the Γ-X
direction compared with the other directions arises from the
Se· · ·Se intermolecular interactions along the nanowire direction
(the a axis in Figure 2a).
The angular-momentum-resolved densities of states (DOS)
for the individual atoms reveal that the p-p mixing has a strong
effect on the electronic structure and the energy-level ordering.
The strong covalent interactions between mainly P and Se p
orbitals form the lower energy levels (at -6 to -4 eV) in the
valence band. The energy states derived from the relatively weak
hybridization between the Bi and Se p orbitals are located
between -4 and -2 eV. The dominating Se p character from
-2 eV to the VBM can be attributed to the lone-pair states of
Se and the ionic bonding between Cs and Se. In this energy
range, the p-orbital contributions of other elements are negligibly
small, as shown in Figure 6d. The CBM shows strong Bi p
character, which is highly affected by the SOC (Figure 6a,b).
Therefore, the band gap excitation observed in the electronic
spectrum of the compound, shown in Figure 5, is due to direct
transitions from Se p to Bi p states.
The far-IR spectrum shows peaks at 150, 181, 221, 300,
415, 440, and 502 cm-1 (Figure 8b), similar to those of
KBiP2Se6, which also includes [P2Se6]4- anions.51 Cs5-
BiP4Se12 is optically transparent from the far-IR through the
mid-IR to below the band edge in the visible region (18.7 to
0.67 µm; Figure 8c), corresponding to the region above the
[P2Se6]4- stretching absorptions. Wide optical transparency
is a critical property for application of nonlinear optical
(NLO) materials and optical fibers in the IR region. Infrared
NLO materials with high SHG response are highly desirable,
for example, in telecommunications (near-IR at 1.3-1.6
It is unusual for discrete molecules to form nanofibers. Careful
inspection of the crystal structure (Figures 2a and 7) shows that
Cs atoms along the a axis (the fiber direction) have more anionic
Se atoms available with which to interact; this gives stronger
ionic interactions along the a axis than along the other directions.
For instance, Figure 7d shows three different [Bi(P2Se6)2]5-
anions (I, II, and III), with blue cirles around atoms neighboring
within 4.7 Å of a Cs atom. Because they contain larger numbers
of such neighboring atoms, molecular units I and II along the
a axis have much stronger ionic interactions with the Cs than
does molecular unit III in the b-c plane (Figure 7d). As a result,
those ionic interactions can predominantly propagate along the
a axis, which in turn gives a better chance for crystal seeds to
(48) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77,
3865–3868.
(49) Becke, A. D. J. Chem. Phys. 1988, 88, 2547–2553. Lee, C. T.; Yang,
W. T.; Parr, R. G. Phys. ReV. B 1988, 37, 785–789.
(50) Montanari, B.; Ballone, P.; Jones, R. O. J. Chem. Phys. 1998, 108,
6947–6951. Wu, Q.; Yang, W. T. J. Chem. Phys. 2002, 116, 515–
524.
(46) Hedin, L.; Lundqvist, B. I. J. Phys. C 1971, 4, 2064–2083.
(47) Asahi, R.; Mannstadt, W.; Freeman, A. J. Phys. ReV. B 1999, 59, 7486–
7492. Bylander, D. M.; Kleinman, L. Phys. ReV. B 1990, 41, 7868–
7871.
(51) Breshears, J. D.; Kanatzidis, M. G. J. Am. Chem. Soc. 2000, 122,
7839–7840.
9
J. AM. CHEM. SOC. VOL. 131, NO. 7, 2009 2653

A R T I C L E S
Chung et al.
Figure 6. Band structures calculated using (a) LDA (Eg ) 1.45 eV), (b) LDA with SOC (Eg ) 1.15 eV), and (c) sX-LDA with SOC (Eg ) 2.0 eV) [in (c),
results are shown for only a small part of the Brillouin zone, namely, Z-T-Y-Γ]. (d) Projected densities of states for p orbitals of the individual elements
(Bi, P, Se) calculated using sX-LDA with SOC.
µm);52 sensing for organic and inorganic molecules,53 includ-
ing chemical-warfare agents,54 biohazards,55 explosives,56 and
pollutant detection57 in the molecular fingerprint region
(2-12 µm); and medical surgery applications (mid-IR at 10.6
µm).58
Because of the polar crystal structure of Cs5BiP4Se12, we
investigated its NLO properties. Using a modified Kurtz
powder method,59 we measured the SHG response for
polycrystalline samples with sizes of 45-63 µm using 1.2-2
µm fundamental idler radiation from a tunable OPA. The
SHG response could be observed only for wavelengths longer
than 0.79 µm because of optical absorption above the band
edge. Because the SHG intensity increased with increasing
wavelength up to 1 µm, the upper limit of the tunable laser
used in the SHG experiments, the SHG response of
Cs5BiP4Se12 appears to be affected by optical absorption over
the whole detection range (Figure 9a). Accordingly, the SHG
intensities of Cs5BiP4Se12 are expected to increase continu-
ously with increasing wavelength, reaching a plateau when
the second harmonic enters the optically transparent region.
The SHG intensities obtained were compared with those of
AgGaSe2, which is a representative NLO material for IR
applications (Figure 9a).37 All of the samples were similarly
prepared with the same range of particle sizes, and identical
laser settings were used. The SHG intensity of Cs5BiP4Se12
is ∼2 times larger than that of AgGaSe2 at 1 µm, showing
good performance in converting short-wave IR (Figure 9b).
(52) Islam, M. N. Phys. Today 1994, 47, 34–40.
(53) Tittel, F. K.; Richter, D.; Fried, A. Top. Appl. Phys. 2003, 89, 445–
510.
(54) Pushkarsky, M. B.; Webber, M. E.; Macdonald, T.; Patel, C. K. N.
Appl. Phys. Lett. 2006, 88, 044103.
(55) Pestov, D.; Wang, X.; Ariunbold, G. O.; Murawski, R. K.; Sautenkov,
V. A.; Dogariu, A.; Sokolov, A. V.; Scully, M. O. Proc. Natl. Acad.
Sci. U.S.A. 2008, 105, 422–427.
(56) Pushkarsky, M. B.; Dunayevskiy, I. G.; Prasanna, M.; Tsekoun, A. G.;
Go, R.; Patel, C. K. N. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 19630–
19634.
(57) Pushkarsky, M.; Tsekoun, A.; Dunayevskiy, I. G.; Go, R.; Patel,
C. K. N. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 10846–10849.
(58) Jean, B.; Bende, T. Top. Appl. Phys. 2003, 89, 511–544.
(59) Dougherty, J. P.; Kurtz, S. K. J. Appl. Crystallogr. 1976, 9, 145–158.
Kurtz, S. K.; Perry, T. T. J. Appl. Phys. 1968, 39, 3798–3813.
9
2654 J. AM. CHEM. SOC. VOL. 131, NO. 7, 2009

Strong SHG in Flexible Polar Nanowires of Cs5BiP4Se12
A R T I C L E S
Figure 9. (a) Relative SHG intensities of Cs5BiP4Se12 (b) to AgGaSe2
([) over a wide range of wavelengths. (b) SHG responses of Cs5BiP4Se12
(blue) and AgGaSe2 (red) at 1 µm.
examined, and the corresponding particle size of 38.5 ( 6.5
µm could be regarded as the coherence length of Cs5BiP4Se12.
Despite this, such materials can be useful through “random”
quasi-phase matching.60 It should be noted that a modified
Kurtz powder method may not be suitable for determining
SHG intensity for samples with a microwire morphology.
The Kurtz powder method assumes a relationship between
the SHG intensity and particle size. However, the typical
diameter of as-prepared Cs5BiP4Se12 is less than 1 µm, so
the sieves used to differentiate the size distribution did not
work well in this case. For this reason, the measured SHG
response reported here could be underestimated. As a result,
to better understand SHG in Cs5BiP4Se12, the use of larger
single-crystal samples is required.
Figure 7. (a-c) Three different models taken from the original crystal
structure (Figure 2a) for use in the binding energy calculations (see the
text). In (a) and (b), the two molecular units have interactions in the b-c
plane, while those in (c) interact along the a-axis. (d) Three molecular
[Bi(P2Se6)2]5- units (I, II, and III), in which atoms neighboring within 4.7
Å of the Cs atom are circled in blue. Pink, purple, gray, and green balls
represent Cs, Bi, P, and Se atoms, respectively.
4. Concluding Remarks
The new compound Cs5BiP4Se12 naturally forms long,
flexible fibers. The compound reflects the rich structural
chemistry of Bi with [PyQz]n- chalcophosphate ligands. The
packing mode of the [Bi(P2Se6)2]5- molecules and the weak
Se · · · Se interactions between molecules are responsible for
the self-formed long, flexible nanowires, which organize into
fibers. Therefore, because of this natural tendency, the
Cs5BiP4Se12 nanowires can be obtained pure and in high yield
without any complex chemical or physical processes. The
nanowire morphology emerges from specific intermolecular
secondary interactions, which could not be predicted a priori.
This discovery implies that innate 1D or 2D nanostructures
may be rationally approached from crystal structure consid-
erations. This is indicated by the significant differences in
binding energies along different crystallographic directions
and explains how the ostensible molecular compound grows
naturally into nanofibers. The result suggests the possibility
of theoretical modeling and prediction of which structural
species can intrinsically produce nanofibers without complex
preparation processes. Cs5BiP4Se12 is widely transparent in
the near- and mid-IR, over the range from 18.8 to 0.67 µm,
and it exhibits a relatively strong SHG response, which is
∼2 times larger than that of AgGaSe2 at 1 µm. The compound
is a nearly direct band gap semiconductor with a very sharp
absorption edge, and it melts congruently. This material is
promising for further in-depth investigations of its NLO
properties.
Figure 8. (a) Raman, (b) far-IR, and (c) far-IR/mid-IR/vis absorption
spectra of Cs5BiP4Se12. A wide transparency range of Cs5BiP4Se12
between 18.7 µm in the far-IR region and 0.67 µm in the visible region
is shown.
The SHG intensity reached a maximum for 32-45 µm-sized
particles and decreased for larger sizes. Thus, Cs5BiP4Se12
is type-I non-phase-matchable in the spectral range we
(60) Baudrier-Raybaut, M.; Haidar, R.; Kupecek, P.; Lemasson, P.;
Rosencher, E. Nature 2004, 432, 374–376.
9
J. AM. CHEM. SOC. VOL. 131, NO. 7, 2009 2655

A R T I C L E S
Chung et al.
Acknowledgment. Financial support from the National Sci-
ence Foundation (Grants DMR-0801855, FRG-0703382, and
0306731 U.S./Ireland cooperation) and the Northwestern Uni-
versity Materials Research Center (under NSF Grant DMR-
0520513) for J.-H.S., A.J.F., J.I.J., and J.B.K. is gratefully
acknowledged.
Supporting Information Available: X-ray crystallographic file
in CIF format. This material is available free of charge via the
Internet at http://pubs.acs.org.
JA808242G
9
2656 J. AM. CHEM. SOC. VOL. 131, NO. 7, 2009