Templates as reactants Synthesis of nanostructures of ZnTe, Te by alllona

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									                                                                                 Selected Results


Templates as reactants: Synthesis of nanostructures of ZnTe, Te, ZnAl2 O4
               and ZnAl2 O4 -Te from Zn(TePh)2 ·TMEDA
                    L. L. Zhao, E. Pippel, H. Hofmeister, and M. Steinhart
                                      in cooperation with
                                    M. Yosef and S. Schlecht
                                a                     u
                 Freie Universit¨ t Berlin, Institut f¨ r Chemie und Biochemie

One-dimensional nanostructures have received      bient conditions. After the solvent had evap-
much attention due to their distinct properties   orated, the templates with the precursor inside
and potential applications in nanoscale elec-     the pores were heated to 280◦ C and kept at this
tronics and optoelectronics. The use of porous    temperature for 2 h. Then, the samples were
templates containing aligned nanopores is a       heated to 500◦ C, annealed at this temperature
versatile method to fabricate arrays of aligned   for 24 h or 48 h, and slowly cooled to room
nanowires [1]. However, it remains still          temperature.
a challenge to extend the range of func-             If as-wetted samples were annealed in vac-
tional target materials that can be formed        uum for 48 h at 500◦ C, ZnTe nanowires were
into nanowires with low defect density and        obtained, which could be released by etching
high aspect ratios. To this end, we have ex-      the templates with aqueous KOH. Figure 1a
ploited ordered porous alumina [2] not only       shows a representative transmission electron
as a shape-defining mould but also as a re-        microscopy (TEM) image of a ZnTe nanowire
actant. Depending on the experimental con-        with a diameter of 25 nm, corresponding to
ditions applied, one-dimensional nanostruc-       that of the template pores. The selected area
tures consisting of an entire set of functional   electron diffraction (SAED) pattern (Fig. 1a,
target materials are accessible from one sin-     inset) of a ZnTe nanowire segment evidences
gle source precursor [3, 4]. Arylchalcogeno-      the single-crystalline nature as it contains only
lates of group 12 and 14 metals are a ver-        indexed Bragg reflections. Growth direction
satile class of such single-source precursors     of the ZnTe nanowire is (1-1-1). The HRTEM
with similar molecular architecture and ther-     image of a ZnTe nanowire (Fig. 1b) shows the
mal properties. As example in case, we show                                      ˚
                                                  (111) lattice planes (d = 3.48A) of cubic ZnTe
that Zn(TePh)2 ·TMEDA (TMEDA: tetram-             (space group F-43m) (JCPDS entry No. 15-
ethylethylenediamine) [5] and self-ordered        147).
porous alumina templates represent a con-
struction kit for the synthesis of single-           When the wetted samples were annealed in
crystalline nanowires of different types of       air for 24 h at 500◦ C, single-crystalline Te
                                                  nanowires 25 nm in diameter surrounded by
target materials with varying morphology.
Zn(TePh)2 ·TMEDA in the pores of ordered          an ∼8 nm thick shell consisting of polycrys-
porous alumina membranes may yield single-        talline ZnAl2 O4 were obtained (Fig. 2). The
crystalline nanowires of zinc telluride (ZnTe),   ZnAl2 O4 -Te core/shell nanowire has an over-
zinc spinel/tellurium (ZnAl2 O4 -Te) and ele-     all diameter of 35 - 40 nm, which is larger
mental tellurium (Te) or ZnAl2 O4 nanotubes.      than that of the template pores. This find-
                                                  ing demonstrates that the pore walls act as
  In a general synthetic procedure for the        chemical reactants in the formation of the
preparation of the nanostructures a solution      core/shell nanowires. We suggest the fol-
of 5 wt-% Zn(TePh)2 ·TMEDA in chloroform          lowing mechanism: After the thermolysis of
(CHCl3 ) was dropped onto ordered porous alu-     Zn(TePh)2 ·TMEDA, the initially formed ZnTe
mina with a pore diameter of 25 nm under am-      is transformed by ambient oxygen, resulting in



38
Selected Results


                                                      nanowires can be obtained (Fig. 3a). The
                                                      SAED patterns of the Te nanowire (Fig. 3a,
                                                      inset) show only Bragg reflections that can
                                                      be assigned to trigonal tellurium (space group
                                                      P31 21 , JCPDS entry No. 5-669). This gives
                                                      evidence of the single-crystalline nature of the
                                                      Te wires and the lack of relevant structural dis-
                                                      order in these products. Alternatively, the re-
                                                      moval of the Te core by etching with aqueous
                                                      KOH at 90◦ C for 4 hours yields nanotubes of
                                                      pure ZnAl2 O4 . As-obtained isolated ZnAl2 O4
Fig. 1: (a) TEM image and SAED pattern (in- nanotubes at low magnification are seen in Fig.
set, zone axis [1 -1 2]) of a single-crystalline ZnTe
                                                      3b.
nanowire. (b) HRTEM image of a ZnTe nanowire
                                       ˚
showing (111) lattice planes (d = 3.48 A).

the formation of ZnO and elemental Te within
the pores.
 2Zn2+ + 2T e2− + O2 −→ 2ZnO + 2T e (1)
Freshly formed active ZnO reacts with the
alumina template, as previously reported by
Wang and Wu [6], to form the polycrystalline
ZnAl2 O4 shell. The formation of the compact
and single-crystalline Te core is greatly facili-
                                                      Fig. 3: (a) TEM image and SAED pattern (in-
tated by the fact that elemental Te with a melt-
                                                      set, zone axis [-1 1 3]) of a single-crystalline Te
ing point of 452◦ C is formed in its liquid state     nanowire. (b) ZnAl2 O4 nanotubes at low magnifi-
at the annealing temperature of 500◦ C. A slow        cation.
cooling process following the annealing step
leads to the formation of single-crystalline Te         The example discussed above demonstrates
structures inside the spinel shell.                   that the concept of reactive templates signifi-
                                                      cantly extends the range of approaches to the
                                                      preparation of one-dimensional nanostructures
                                                      having high structural quality.

                                                      References

                                                       [1] C. R. Martin, Science 266, 1961 (1994)
                                                       [2] H. Masuda, K. Fukuda, Science 268,1466 (1995)
                                                                                                  o
                                                       [3] L. Zhao, M. Yosef, M. Steinhart, P. G¨ ring, H.
                                                                            o
                                                           Hofmeister, U. G¨ sele, and S. Schlecht, Angew.
                                                           Chem. Int. Ed. 45, 311 (2006)
                                                       [4] L. Zhao, M. Yosef, E. Pippel, H. Hofmeister,
Fig. 2: HRTEM image of a ZnAl2 O4 /Te core-shell
                                                                              o
                                                           M. Steinhart, U. G¨ sele and S. Schlecht, Angew.
nanowire. The lattice fringes of the Te core corre-        Chem. Int. Ed., in press.
                                   ˚
spond to the (101) planes (d = 3.1A).
                                                       [5] Y. Jun, C. S. Choi, and J. Cheon, Chem. Commun
                                                           1, 101 (2001)
   Sonification of the core/shell nanowires re-
                                                       [6] Y. Wang and K. Wu, J. Am. Chem. Soc. 127, 9687
sulted in the removal of the ZnAl2 O4 crys-
                                                           (2005)
tallites, and liberated single-crystalline Te

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