Optical XAFS of ZnO Nanowires at the Zn K-Edge and
Related Phenomena
Franziskus Heigl1, X.H. Jeff Sun1, Simone Lam1, Tsun-Kong Sham1, Robert
Gordon2, Dale Brewe3, Richard Rosenberg3, Gopal Shenoy3, Mikhail
Yablonskikh4, Janay MacNaughton4, and Alex Moewes4
1. Department of Chemistry, University of Western Ontario
2. Department of Physics, Simon Fraser University
3. Advanced Photon Source, Argonne National Laboratory
4. Department of Physics, University of Saskatchewan
Abstract. We report x-ray excited optical luminescence (XEOL) from one-dimensional nanostructures of ZnO excited
with photon energies across the Zn K-edge. The optical luminescence shows an UV and a green emission band
characteristic of near band edge and defect emission, respectively. The optical channels were used in turn to monitor the
Zn K-edge XAFS to high k values. The densities of states of oxygen character in the valence band were also studied
with x-ray emission spectroscopy (XES). The Zn K-edge decay dynamics was examined with time-resolved x-ray
excited optical luminescence.
Keywords: Zn K-edge, ZnO (0001), ZnO nanostructures, time-resolved XEOL
PACS: 61.10.Ht, 61.66.Fn, 42.70.Qs, 01.30.Cc
INTRODUCTION
The rich morphology of ZnO nanostructures, from
simple nanowire [1] to complex tetrapod [2], has
drawn considerable attention. In addition to its
potential application in optoelectronics, ZnO
nanostructures also show promise for the development
of nanolasers [2]. Recently, we contributed several
new members to this incredible family [3].
Remarkably, these structures are distinct in their x-ray
excited optical luminescence (XEOL) and exhibit
anisotropic emission with O K-edge excitation [4].
Here, we report XEOL results from two
nanostructures, a nearly-perfect nanoneedle (NN)
single crystal and a defect-filled nanowire (NW), with
excitations at the Zn K-edge in both energy and time
domain. Time-resolved x-ray excited optical
luminescence (TRXEOL) [5,6] uses the time structure
of the storage ring (APS top-up: 100 ps pulses and 153
ns repetition rate).
A ZnO (0001) single crystal was also studied for
comparison. In addition, we also show that XEOL and
XES (x-ray emission) can be used for nanostructure
studies. Fig. 1 shows the schematic for XEOL and
XES in semiconductor or insulator. Upon the
absorption of x-rays, electrons in all core and valence
levels accessible will be excited to a previously
unoccupied bound (resonance), quasi-bound (multiplescattering) or continuum (ionization) state. The decay
FIGURE 1. Schematic for XEOL and XES.
of the corehole and secondary processes result in
electrons in the conduction band and holes in the
valence band or in the traps in the band gap. These e-h
pairs can recombine radiatively emitting optical
photons. For shallow core levels, the fluorescence
decay involves electrons from the valence band,
providing densities of states information if the
fluorescence x-ray is measured with a high resolution
monochromator (XES).
Intensity (arb. units)
20000
EXPERIMENTAL
ZnO nano needle
fast (0 -10 ns)
15000
10000
slow (10 ns - 130 ns) x 10
ungated
5000
0
The preparation of the ZnO NN and NW was
described recently [3]. Fig. 2 shows the near-perfect
surface of the NN compared with the rough surface of
NW in high resolution TEM images. ZnO (0001), a ~
5 x 5 x1 mm crystal was obtained commercially.
Intensity (arb. units)
100
200
8
7
6
5
4
3
2
1
0
-1
400
500
600
700
800
900
ZnO nanowire
h ν = 9800 eV
0-90 ns
20-90 ns
100
a)
300
200
300
400
500
600
700
800
900
Intensity (arb. units)
5
4.5x10
5
4.0x10
5
3.5x10
5
3.0x10
5
2.5x10
5
2.0x10
5
1.5x10
5
1.0x10
4
5.0x10
0.0
Fig. 3 shows the time-gated XEOL from NN and
NW with fast and slow time windows together with
the un-gated XEOL from the ZnO (0001) single
crystal. It is apparent from Fig. 3 that there is a narrow
near band-gap emission at ~ 383 nm followed by a
broadband defect emission peaked at ~ 489 nm and
~541 nm for all specimens. However, the branching
ratio varies dramatically with the ZnO NN having the
most intense near band-gap emission and the ZnO
(0001) single crystal the least. Time-gated results
indicate that the near band-gap emission is very fast
while the broadband emission is relatively slow as
confirmed by the decay curves of the ZnO NN shown
in Fig. 4 where the ~ 383 nm decay is faster than our
time resolution (~ 2 ns, mainly determined by the
PMT). Note that there is a blue shift relative to the
bulk attributable to quantum size effect. The slower
decay at 486 nm shows a single exponential behavior
except at the first 10-20 ns. This behavior is consistent
with a recent observation [4]. More information awaits
better statistics and detailed analysis. Qualitatively, the
slow decay almost certainly results from effective
energy transfer from the super excited state to the
chromophore, the defect luminescence center.
300
400
500
600
700
800
900
FIGURE 3. XEOL and TRXEOL from ZnO NN (top), ZnO
NW (middle) and ZnO(0001) (bottom) excited at 9800 eV
(just above the Zn K-edge).
70000
Intensity (arb. units)
RESULTS AND DISCUSSION
200
Wavelength (nm)
60000
ZnO nano-needle
50000
40000
30000
486 nm
20000
10000
383 nm
0
20
40
60
80
100
120
140
Time channel (ns)
FIGURE 4. Decay curves of emission from ZnO NN; the
reversed display results from the use of the signal as the start
and the light pulse as the stop. [5-7]
We now examine the optical XAFS, the XAFS
monitored with a selected optical emission (Fig. 5).
Normalized Intensity (arb. units)
XEOL and TRXEOL experiments were conducted
at the PNC-XOR of the Advanced Photon Source at
Argonne National Laboratory as described previously
[7]. XES was obtained at BL8 of the Advanced Light
Source at Lawrence Berkeley National Laboratory.
398 nm
100
FIGURE 2. HRTEM of (a) nanoneedle and (b) nanowire.
ZnO(0001)
489 nm
541 nm
ZnO(0001) Time-gated optical XAFS
5.0
4.5
20 -150 ns
4.0
3.5
0 -20 ns
3.0
9600
9800
10000
10200
10400
10600
Photon Energy (eV)
FIGURE 5. The time-gated optical XAFS recorded from the
ZnO (0001) single crystal.
We see from Fig. 5 that the XAFS in PLY for
ZnO(0001) is inverted (same for the un-gated spectra).
This inversion is due to saturation effects for a thick
crystal [5]. At the Zn K-edge, new de-excitation
channels (fluorescence and Auger) turn on, allowing
some energy to escape the surface without contributing
to the radiative de-excitation optical channel [5].
We now compare the ZnO(0001) optical XAFS
with those of ZnO NN and ZnO NW where the most
intense emission, band-gap and defect luminescence,
respectively was used to monitor the XAFS.
10000
5000
Normalized Intensity (Arbitrary Units)
Normalized Intensity (Arbitrary Units)
1.0
550.0 eV (ZnO NP)
550.0 eV (ZnO powder)
548.1 eV (ZnO NW)
543.4 eV (ZnO NW)
ZnO powder (TEY)
ZnO NP (TEY)
ZnO NW (TEY)
0.5
0
-3.0
0.0
PLY (arb. units)
490 495 500 505 510 515 520 525 530 535 540 545
-5.0
9600
9700
1.5
PLY(arb. units)
540
545
550
555
560
565
Excitation Energy (eV)
ZnO(0001) Time-gated
9800
9900
Photon Energy (eV)
The narrow densities of states of the valence band
are consistent with a sharp near band-gap
luminescence. It will be of great interest to monitor the
RIXS and the XEOL simultaneously, and as a function
of temperature.
ZnO nanoneedle
(383 nm )
ACKNOWLEDGMENTS
1.0
ZnO nanowire
(486 nm )
0.5
0.0
9600
535
FIGURE 7. O K-edge XES (left) and XANES (right) of
ZnO nanostructures. The excitation energy and detection
modes are noted.
0 -20 ns
-4.0
-4.5
Excitation Energy (eV)
20 -150 ns
-3.5
9700
9800
9900
Photon Energy (eV)
FIGURE 6. Optical XAFS of ZnO (0001) (inverted) and asrecorded ZnO NN (383 nm) and ZnO NW (486 nm).
The ZnO NN and ZnO NW optical XAFS exhibit
normal, nearly identical XANES, indicating that the
specimens are thin and the energy absorbed is
effectively transferred to the chromophore. Thus PLY
is proportional to the absorption coefficient.
Finally, we propose that XEOL and XES (Fig.1)
are connected and can be investigated simultaneously.
The questions of interest are (i) where the electrons
and the holes are when they recombine radiatively and
(ii) what the effect of a RIXS (resonant inelastic x-ray
scattering) final state is on the optical de-excitation
channel. We will attempt to answer these questions
experimentally. For now we show in Fig.7 the XES
and XANES at the O K-edge for some of the ZnO
nanostructures. Note that powder and NP samples used
in the measurement are close in behavior to those of
ZnO bulk and ZnO NN respectively.
Fig. 7 shows that the densities of states above and
below the band gap are essentially the same for the
bulk ZnO (0001) crystal and the nanostructures with a
slight narrowing in the latter.
Research at UWO and U Sask. are supported by
NSERC, CFI and CRC of Canada; the PNC-CAT/APS
was partially supported by an NSERC MFA grant. The
Advanced Photon Source was supported by the U.S.
Department of Energy under Contract No. W-31-109ENG-38. The Advanced Light Source was supported
by the U.S. Department of Energy under contract No.
DE-AC03-76SF00098.
REFERENCES
1.
2.
3.
4.
5.
6.
7.
M. H. Huang, S. Mao, H. Feick, H.Q. Yan, Y.Y. Wu,
H. Kind, E. Weber, R. Russo, P.D. Yang., Science, 292,
1897 (2001).
H.Q. Yan, R.R. He, J. Pham, P.D. Yang, Adv. Mater.,
15, 402 (2003).
X.H. Sun, S. Lam, T.K. Sham, F. Heigl, A. Jurgensen,
and N.B. Wong, J. Phys, Chem. B 109, 3120 (2005).
R.A. Rosenberg and G.K. Shenoy, L.-C. Tien, D,
Norton, and S. Pearton, X.T. Zhou and T.K. Sham,
Appl. Phys. Lett., in press.
A. Rogalev and J. Goulon, in CHEMICAL
APPLICATIONS OF SYNCHROTRON RADIATION,
Part II: X-ray Applications, edited by T.K. Sham
(World Scientific Publishing Co., Singapore, 2002),
Vol. 12B, p. 707.
11. R.A. Rosenberg, G.K. Shenoy, F. Heigl, S.-T. Lee,
P.-S. G. Kim, X.-T. Zhou, and T.K. Sham, Appl. Phys.
Lett. 87, 253105 (2005).
F. Heigl, A. Jürgensen, X.-T. Zhou, S. Lam, M.
Murphy, J.Y. P. Ko, T.K. Sham, R. A. Rosenberg, R.
Gordon, D. Brewe, T. Regier, L. Armelao. SRI 2006
Daegu, Korea, AIP Proc. in press.