Sterically induced shape and crystalline phase control of GaP nanocrystals

Article abstract of DOI:10.1021/ja027575b

We demonstrate a novel synthetic scheme that can be used to control the crystalline phase and shape of GaP semiconductor nanocrystals. Our study shows that steric effects of surfactant ligands can modulate the crystalline phases and control the shapes of nanocrystals. The shape of the nanocrystals obtained varies from zero-dimensional spheres to one-dimensional rods via controlling the ratio between primary and tertiary alkylamines. III-V semiconductors (in our case: GaP) under 10 nm in width are first reported, and unique optical properties due to shape anisotropy are also observed. Copyright

Full text of DOI:10.1021/ja027575b

Published on Web 10/30/2002
Sterically Induced Shape and Crystalline Phase Control of GaP Nanocrystals
Yong-Ho Kim,† Young-wook Jun,† Byung-Ho Jun,‡ Sang-Min Lee,‡ and Jinwoo Cheon*,‡
Department of Chemistry, Korea AdVanced Institute of Science and Technology, Taejon 305-701, Korea, and
Yonsei UniVersity, Seoul 120-749, Korea
Received July 8, 2002
Semiconductor nanorods and nanowires have drawn great interest
not only because they exhibit novel optical properties arising from
dimensional anisotropy, but also because they can be utilized as
key materials in addressable two-terminal circuits for nanodevice
applications.1 For example, CdSe nanorods2 exhibit linearly polar-
ized emission, and InP nanowires3 can be used for polarization-
sensitive nanoscale photodetectors. Research on liquid-phase
synthesis of anisotropic nanocrystals has largely focused on II-VI
semiconductor systems,2,4-6 but other systems have been poorly
explored because their synthetic methodology is not well developed.
Figure 1. Large area TEM image (a) and HRTEM images (b, c) of GaP
spheres.
In particular, studies on colloidal III-V semiconductor nanocrystals
are very limited, despite the technological importance of these
materials; III-V semiconductors that are luminescent from the near-
infrared (e.g., InAs) to the blue (e.g., GaN) region are widely used
for optical applications, and their large exciton Bohr radii allow
them to show quantum confinement effects with relatively large
sizes.7 Colloidal quantum dots (e.g., InP, GaAs) and nanofibers
(e.g., InAs, GaP) with large widths (>∼100 nm) are examples of
the important progresses.8-10 Typical studies, however, tend to show
colloidal III-V nanocrystals either polydispersed or much larger
than quantum confinement region in size. Many regard the size
and shape control of III-V nanocrystals to be difficult because they
are hard to crystallize at low temperature (<400 °C); this is most
likely due to a greater degree of covalent bonding and absence of
Figure 2. Large area TEM image (a) and a HRTEM image (b) of GaP
nanorods.
suitable precursors.
In this paper, we demonstrate a surfactant driven shape-controlled
synthesis of gallium phosphide semiconductor nanocrystals using
thermal decomposition of a single molecular precursor, tris(di-tert-
butylphosphino)gallane (Ga(PtBu2)3), in a hot mixture of amine
stabilizers. In addition to their well-defined single crystallinities,
the shape of the obtained GaP nanocrystals can be varied from
nanospheres to rods with highly monodispersed size distributions
by controlling the type and amount of stabilizing surfactants.
GaP nanocrystals were grown by thermal decomposition of 100
distances are 3.094, 2.687 Å for the 111 and 200 directions,
respectively, which is consistent with known values of zinc blende
GaP.12 In addition, selected area electron diffraction (SAED) and
X-ray diffraction (XRD) patterns of the nanosphere reveal its
crystalline phase as the zinc blende.13
Under the same conditions as above, addition of HDA into TOA
leads to changes in the shape and crystalline phase of the GaP
nanocrystals. When a mixture of 8.7 mL of TOA and 4.8 g of HDA
(ratio ) 1:1) was used instead of neat TOA solution, a mixture of
nanospheres (70%) and nanorods (30%) of 42.3 nm (σ ≈ 16.2%)
in length and 7.9 nm (σ ≈ 6.3%) in width was produced.13 Finally,
when the stabilizer ratio was increased to 1:2.5, the formation of
nanorods (8 nm (σ ≈ 5.7%) in width and 45 nm (σ ≈ 22.6%) in
length) dominated (Figure 2). The lattice fringes of HRTEM (Figure
2b) that orient in the 002 direction with an ABABAB... stacking
sequence with an interplanar distance of 3.26 Å reveal that the rod
has a highly ordered single crystalline wurtzite structure.12 Con-
sistently, SAED and XRD patterns for nanorods show additional
peaks from wurtzite phase GaP and an intense 002 peak indicates
that the nanorods anisotropically grow along the c-axis of the
wurtzite phase.13
11
mg of Ga(PtBu2)3 in 2 mL of trioctylamine (TOA), which was
injected into a mixture of TOA and hexadecylamine (HDA) at 330
°C. After 72 h, the resulting solution was quenched with cold
toluene and treated with methanol to precipitate amber-colored
flocculates that were separated by centrifugation. Nanocrystals were
obtained as amber-colored powders and easily redispersed in toluene
or dichloromethane. Without further selection processes, obtained
nanocrystals are moderately monodisperse in size and shape.
When only trioctylamine (TOA) was used as a stabilizer,
spherical particles were formed. Low magnification TEM images
of GaP nanospheres show that nanocrystals obtained were highly
monodisperse with the size of 8.1 nm (σ ≈ 5.4%) (Figure 1). High-
resolution TEM (Figure 1b,c) analyses show measured interplanar
From these observations, a low HDA to TOA ratio seems to
favor the formation of zinc blende nanospheres, but a high
concentration of HDA leads to the formation of the wurtzite phase
* To whom correspondence should be addressed. E-mail: jcheon@
alchemy.yonsei.ac.kr.
† Korea Advanced Institute of Science and Technology.
‡ Yonsei University.
9
13656
J. AM. CHEM. SOC. 2002, 124, 13656-13657
10.1021/ja027575b CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1. Proposed Mechanism for Surfactant Driven Steric
Effects on the Crystalline Phases (a, b) and Rod Growth (c) of
GaP Nanocrystals
Figure 3. Absorption and photoluminescence spectra of GaP rods (a) and
spheres (b).
and also induces anisotropic growth of the nanocrystals. These
results can be understood as arising from steric effects of the
stabilizers during crystal growth. GaP crystals have two different
crystalline phases14 which are in rotational isomerism - the
thermodynamically stable zinc blende structure is a staggered
conformation with 111 directions, and the kinetically stable
wurtzite structure is an eclipsed conformation with 002 directions
(Scheme 1).15 Kinetic stability of the wurtzite structure is induced
by strong dipole interaction of incoming GaP monomers with
surface GaP lattice atoms. Because stabilizers dynamically bind to
the crystal surfaces during the GaP crystal growth, the conformation
of crystal structures is highly affected by changing the stabilizer.
When the highly bulky tertiary amines (e.g., TOA) are used as
stabilizers, staggered conformation is favored, minimizing steric
hindrance between these ligands and GaP lattices (Scheme 1, path
A), and zinc blende GaP is preferred rather than wurtzite. In
contrast, when an excess amount of less sterically hindered HDA
is added to TOA, the rotational barrier between GaP-HDA
complexes and GaP lattices is reduced. Therefore, the formation
of the kinetically stable wurtzite GaP is now facilitated (Scheme
1, path B) under the kinetic growth regime induced by a high
monomer concentration.15,16
Moreover, the steric difference between these two stabilizers
seems to induce the anisotropic growth of the wurtzite GaP. It is
likely that, when wurtzite seeds are formed, sterically bulky TOA
selectively binds to the other faces (e.g., 100 and 110 faces) with
staggered conformation rather than to 002 faces and blocks growth
on these faces.17 On the other hand, GaP-HDA complexes continu-
ously supply monomers on the 002 faces with high surface energy
and therefore promote the growth in the c-direction of a rod structure
(Scheme 1c).
The nanocrystals obtained showed unique spectroscopic features
originating from quantum mechanical effects.18 The absorption
spectra exhibited strong shoulders (3.48 and 3.46 eV for spheres
and rods, respectively) and shallow tails that are attributed to direct
and indirect transition, respectively.19 The photoluminescence
maximum was 2.94 eV for spheres corresponding to the direct
transition of 8 nm GaP, while red shift (2.79 eV) was observed for
rods (8 × 45 nm), which may arise from shape anisotropy (Figure
3).6,20
In summary, a novel route for shape and crystalline phase control
of GaP nanocrystals has been successfully developed using a single-
molecular precursor approach. Even without selection processes,
the nanocrystals obtained are decently monodisperse in size and
shape. Our results are the first liquid-phase syntheses of anisotropic
III-V semiconductors within a quantum-confined region with a
width of less than 10 nm.
Acknowledgment. This work was financially supported by
Korea Ministry of Commerce, Industry, and Energy (nanostan-
dardization project 10001948) and the Advanced Backbone IT
Technology Development Project (IMT-2000-B3-2) of Korea. We
thank KBSI for TEM data.
Supporting Information Available: XRD, SAED, and EDAX of
GaP nanocrystals. A TEM image of mixed shapes of nanocrystals
(PDF). This material is available free of charge via the Internet at http://
pubs.acs.org.
References
(1) (a) Alivisatos, A. P. J. Phys. Chem. 1996, 100, 13226. (b) Hu, J.; Odom,
T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (c) Cui, Y.; Wei,
Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289.
(2) Hu, J.; Li, L.; Yang, W.; Manna, L.; Wang, L.; Alivisatos, A. P. Science
2001, 292, 2060.
(3) Wang, J.; Gudiksen, M. S.; Duan, X.; Cui, Y.; Lieber, C. M. Science
2001, 293, 1455.
(4) (a) Peng, X.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich,
A.; Alivisatos, A. P. Nature 2000, 404, 59. (b) Manna, L.; Scher, E. C.;
Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (c) Peng, A.; Peng,
X. G. J. Am. Chem. Soc. 2001, 123, 183. (d) Peng, A.; Peng, X. G. J.
Am. Chem. Soc. 2001, 123, 1389. (e) Jun, Y.; Lee, S.-M.; Kang, N.-J.;
Cheon, J. J. Am. Chem. Soc. 2001, 123, 5150. (f) Jun, Y.; Jung, Y.; Cheon,
J. J. Am. Chem. Soc. 2002, 124, 615.
(5) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425.
(6) (a) Li, L. S.; Hu, J. T.; Yang, W. D.; Alivisatos, A. P. Nano Lett. 2001,
1, 349. (b) Mohamed, M. B.; Burda, C.; El-Sayed, M. A. Nano Lett. 2001,
1, 589.
(7) Gaponenko, S. V. Optical Properties of Semiconductor Nanocrystals;
Cambridge University Press: Cambridge, 1998.
(8) (a) Micic O. I.; Curtis, C. J.; Jones, K. M.; Sprague, J. R.; Nozik, A. J. J.
Phys. Chem. 1994, 98, 4966. (b) Micic, O. I.; Ahrenkiel, S. P.; Bertram,
D.; Nozik, A. J. Appl. Phys. Lett. 1999, 75, 478.
(9) Olshavsky, M. A.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc.
1990, 112, 9238.
(10) Trentler, T. J.; Goel, S. C.; Hickman, K. M.; Viano, A. M.; Chiang, M.
Y.; Beatty, A. M.; Gibbons, P. C.; Buhro, W. E. J. Am. Chem. Soc. 1997,
119, 2172.
(11) Ga(P-tBu2)3 was synthesized with modification of a literature method.
See: Arif, A. M.; Benac, B. L.; Cowley, A. H. J. Chem. Soc., Chem.
Commun. 1986, 1543.
(12) JCPDS cards, No. 32-0397 (zinc blende GaP) and No. 80-0002 (wurtizte
GaP).
(13) See Supporting Information.
(14) Yeh, C.-Y.; Lu, Z. W.; Froyen, S.; Zunger, A. Phys. ReV. B 1992, 46,
10086.
(15) Narayanan, V.; Mahajan, S. Philos. Mag. A 2000, 80, 555.
(16) In comparison, surfactant effects on crystalline phases in diffusion
controlled growth of CdSe have been documented. See: Qu, L.; Peng,
X. J. Am. Chem. Soc. 2002, 124, 2049.
(17) This is because wurtzite structure has eclipsed conformation in 002
directions, while it has staggered conformation in other directions.
(18) The exciton Bohr radii of GaP are 1.45 nm for indirect transition and 5.5
nm for direct transition. See ref 19.
(19) Nozik, A. J.; et al. J. Phys. Chem. 1995, 99, 7754.
(20) Rama Krishna, M. V.; Friesner, R. A. J. Chem. Phys. 1991, 95, 8309.
JA027575B
9
J. AM. CHEM. SOC. VOL. 124, NO. 46, 2002 13657