Laser-induced control on the size and the phase transition in
the chemically etched Ge nanoparticles
Ali Karatutlu *,1,2, William Little1, Osman Ersoy1, Yuanpeng Zhang1 and Andrei Sapelkin1
1
Centre for Condensed Matter and Materials Physics, School of Physics and Astronomy, Queen Mary, University of
London, London, E1 4NS, UK
2 Electrical and Electronics Engineering, Bursa Orhangazi University, 16310 YILDIRIM/Bursa, Turkey
Keywords Free-standing Ge nanoparticles, p-Ge, explosive crystallization, phase transformation, HeNe laser, photothermal therapy
1. Supplementary file
A TEM measurement was taken to confirm the size of the as-prepared free-standing Ge nanoparticles and is shown
in Figure 1S. The size of the as-prepared sample was found to be 10 ± 3 nm.
Figure S1. (a) A bright field TEM micrograph and (b) the size distribution of fresh free-standing Ge nanoparticles out of 60
nanoparticles. The size distribution represents the mean size of free-standing Ge nanoparticles as 10 ± 3 nm.
Table S1 shows time of the laser exposure, size of free-standing Ge nanoparticles and corresponding FWHM for
the each size.
* Corresponding author: a.karatutlu@qmul.ac.uk, ali.karatutlu@bou.edu.tr, Phone:+904448268-1166, Fax: +902242114406
1
Table S1. Size and FWHM of Ge nanoparticles versus time of the He-Ne laser exposition at a wavelength of 633 nm. Laser
power was measured to be 2 mW just before the hit of the sample.
Time of
Size of Ge
FWHM of
exposure (min)
nanoparticles
Raman shift
(nm)
(cm-1)
1 (fresh)
7.21
10.6
5
8.94
9.14
10
9.00
9.10
15
8.69
9.29
20
9.18
9.00
30
12.70
7.89
35
23.81
7.31
40
Bulk
5.91
Figure S2. Raman shift of bulk Ge (purchased from Sigma-Aldrich with % 99.9 purity) and that of the Ge nanoparticles
exposed to 40 minutes of the laser beam.
2
Raman shift of the each exposure step from 40 min to 100 min is shown in Figure S3. Soon after completion of
the transformation from the nanoparticle to the bulk form, oxidation comes forth. Raman measurement of
germanium oxides were reported to be different previously depending on the structural phase of the oxides
which were further confirmed by neutron and X-ray studies 1. For instance, Raman spectrum of alpha-quartz
type GeO2 (-GeO2) mainly give rises at 442 cm-1 (symmetric stretching mode within tetrahedral GeO 4 group)
whereas that of Rutile type of GeO2 has the strongest band at 700 cm-1 (Ge-O stretching mode within
octahedral GeO6 group) 2. On the other hand, Raman bands of amorphous glassy form of GeO 2 show rather
broader peak width relative to its crystalline counterparts at 420 cm-1 (O-Ge-O, symmetric stretching of
bridging oxygen) and shoulders at 327 cm-1 (Ge “deformation” motion within the tetrahedral network) , and at
500-620 cm-1 range (bending modes) 1.
Figure S3. Raman spectra of Ge nanoparticles exposed to a He-Ne laser light with an excitation wavelength of 633 nm (laser
power = 2mW) from 40 minutes to 100 minutes. At the end of 40 minutes, Ge nanoparticles were transformed to bulk Ge with
some degree of oxidation. By the time of the laser exposure from 40 minutes to 100 minutes, bulk Ge was transformed to –
type GeO2. This transformation was shown via vanishing of Ge-Ge TO phonon mode at 302 cm-1 and domination of Ge-O
stretching mode at 433 cm-1. The measurements was collected at ambient environment.
Therefore, Raman spectroscopy was used as a fingerprint to infer the structural form of the oxide in case of further
laser exposition. A single peak at 433 cm-1 in Figure 4 was assigned to Ge-O stretching mode of alpha-quartz (GeO2) phase of GeO2. The complete transformation from bulk Ge to -GeO2 was gradually occurred by 100 min
of the exposure with respective to the growth of the as-prepared sample.
3
The growth of Ge qdots and the transformation to -type GeO2 are depicted in Figure S4. The scheme illustrates
that the size of Ge qdots after synthesis can be controlled by the exposure of He-Ne laser light at 2 mW laser
power.
Figure S4. Schematic of the growth of free-standing Ge nanoparticles by a 2 mW He-Ne laser (depicted with a pink circle).
At first, Ge nanoparticles with a size of 7.2 nm were increased in size to 23.8 nm at the end of 35 minutes. Then, at 40 minutes,
Ge nanoparticles were transformed to bulk Ge and finally converted to -type GeO2 at the end of 100 minutes. The size of
nanoparticles were estimated using phonon confinement model
3,4
based on Ge TO modes as shown in Figure 3. Ge
nanoparticles at the end of the 35 minutes laser exposure were oxidized which is neglected in the schematic for clarity.
The etching mechanism is illustrated in Figure S5 for Si and Ge and differentiated in Ge by a re-nucleation on npGe which is considered to form a-pGe.
In case of transition from the disordered state of Ge nanoparticles to its crystalline state triggered by the
laser exposure, the energy is considered to be released as represented in Figure S5.
4
Figure S5. Schematic representation of energy states where A and B are referred to crystalline and amorphous state (strainfree) of germanium respectively. C and D are referred to amorphous state (strained) and amorphous state (triggered)
respectively. ∆εf is the free-energy difference between B and A. ∆𝜀𝑓′ is the free-energy difference between C and A. ∆εo is the
activation energy for the transition from B to A. ∆𝜀𝑜′ is the activation energy for the transition from C to A. ∆εi and ∆εe are the
internal strain energy and the external triggering energy respectively where the latter is referred in our case to the energy
supplied by the laser exposure. The picture was reproduced from reference 5.
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