Supporting Information
True Vapor–Liquid–Solid Process Suppresses
Unintentional Carrier Doping of Single Crystalline Metal
Oxide Nanowires

Hiroshi Anzai1, Masaru Suzuki1, Kazuki Nagashima1, Masaki Kanai1, ZetaoZhu1, Yong He1, Mickaël Boudot1, Guozhu Zhang1, Tsunaki Takahashi1,Katsuichi Kanemoto2, Takehito Seki3, Naoya Shibata3 and Takeshi Yanagida1*

1 Institute for Materials Chemistry and Engineering, Kyushu University, 6-1

Kasuga-Koen, Kasuga, Fukuoka 816-8580, Japan
2 Department of Physics, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku, Osaka 558-8585, Japan
3 Institute of Engineering Innovation, University of Tokyo, 2-11-16

Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan*yanagida@cm.kyushu-u.ac.jp

S1

S1. Models for electrical conductivity of tapered SnO2 nanowires.
As shown in Figure S1, we consider the three models to understand the electricalproperties of tapered SnO2 nanowire, including 1) “homogeneous conductionmodel”, 2) “surface depletion model”, and 3) “VS-LS model”. In the former twomodels (homogeneous model and surface depletion model), the electricalresistance is inversely proportional to the cross-sectional area. Note that in thesurface depletion model, the observed nanowire resistance might be higher thanthat for the homogeneous model due to the presence of depletion layer at thecontact between electrode and nanowire. The experimental data in Figure 2c ofmain manuscript highlighted that the resistance change cannot be explained byeither the homogeneous model or the surface depletion model, since theresistance value changed almost 3 orders when the nanowire diameter ranged
from 58 nm to 135 nm. This indicates that the electrical conduction in SnO2nanowires is spatially inhomogeneous. Thus, it seems that the VS-LS modelseems to be more appropriate to explain the spatial conductivity of tapered
nanowires in terms of the conductive outer VS crystal and the insulative inner LS
crystal.
Model 1. homogeneous model
uniform conduction

Model 2. surface depletion modelsurface depletion area
+uniform conduction

Model 3. VS-LS modelouter conduction area
+inner insulative area

Figure S1 Models of electrical conductivity of tapered SnO2 nanowire.

S2

S2. Spatial distribution analysis of electrical conductivity of tapered SnO 2
nanowire when varying the oxygen partial pressure during crystal growth.
Here, we show the spatial distribution analysis of electrical conductivity within atapered SnO2 nanowire when varying the oxygen partial pressure. Since metaloxide nanowires are composed of metal and oxygen elements, the sidewall
crystal growth can be introduced by oxygen flux variation. Figure S2 (a) shows
oxygen flux (oxygen partial pressure: PO2) dependence on the diameter of SnO2
nanowires. These SnO2 nanowires were grown under the conditions, including
the Sn flux (8.0×1017 cm2s-1), the growth temperature (750oC) and Au catalysts (1nm). It should be noted that the SnO2 nanowires growth was not observable atthis Sn flux range when PO2 was lower than 10-4 Pa. While the nanowire growthoccurred at PO2 >10-4 Pa, the sidewall growth was not seen at PO2 <10-3 Pa. Thisis consistent with the results in Figure 1f of main manuscript, in which the sidewallarea starts to grow with Sn flux over 15×1017 cm-2s-1 and PO2 of 10-3 Pa. The clear
sidewall growth was seen when PO2 >10-2 Pa. Figure S2 (b) shows the FESEM
image of single SnO2 nanowire device bridged by multi-nanoelectrodes. TheSnO2 nanowire was grown at PO2 10-2 Pa. The gap size was 430 nm. Figure S2(c) shows the measured spatial distribution data of 4-probe resistances in tapered
SnO2 nanowire. The resistances changed by 3 orders when the nanowire
diameter changed from 37 nm to 51 nm. As a comparison, the calculated
resistance values by assuming the homogeneous electrical conduction are also
shown in the figure. (The data was calibrated with resistance value at 37 nm) Thecomparison clearly highlights that the electrical conduction in SnO2 nanowire
cannot be explained by the homogeneous conduction model, and the VS crystal

S3

is more conductive than LS crystal even when the VS crystal is created by oxygenflux. Thus these results highlight that the electrical conduction observed intapered SnO2 nanowire was not due to the excess Sn flux supply, and the VS
and LS interfaces play crucial role on the unintentional doping event during crystal
growth.

100100(nm)Nanowire
diameter (nm)Nanowire diameter

a80
80
6060

40
40
20
2000
10-5 1.E-04
10-4 1.E-03
10-3 1.E-02
10-2 1.E-01
10-1 1.E+00
100
1.E-05Oxygen flux
Oxygenflux(Pa)
(Pa)

c1

1.E+121012
4probe resistance (Ohm)
4-probe Resistance (W)b1011
1.E+112337nm

1010
1.E+10109
1.E+09108
1.E+0851nm

107
1.E+071µm

PO2: 10-2Pa
106
1.E+060112233

4Position
Position

Figure S2 a) Morphology variation of tapered SnO2 nanowire when varying
the oxygen partial pressure during growth, b) SEM image of fabricatedmulti-probe device to measure the spatial distribution of electricalresistivity of tapered SnO2 nanowire, and c) Spatial distribution data of 4probe resistance with comparison to homogeneous electrical conduction
model.

S4S3. XRD analysis of tapered and untapered SnO2 nanowires.
We performed x-ray diffraction (XRD) analysis (Cu K =1.5418 Å) to evaluate thecrystallinity of tapered and untapered SnO2 nanowires, Figure S3 shows XRD patternsof tapered SnO2 nanowires (purple line, Sn flux: 27.2×1017 cm-2s-1) and untapered SnO2nanowires (red line, Sn flux: 10.4×1017 cm-2s-1). Only SnO2 {101} peaks were observed
for both samples in the scan range of 20-90°, indicating the single crystalline nature ofboth fabricated SnO2 nanowires. However, there are no significant difference between
the two nanowires on the XRD data since the XRD data is spatially averaged.

Log Intensity (a.u.)

SnO2 (101)

Al2O3 (110)

Sn flux: 10.4x1017 cm-2s-1
Sn flux: 27.2x1017 cm-2s-1

3032

34363840

2Theta (degree)

Figure S3. XRD data of tapered and untapered SnO2 nanowires.

S5S4. Ultraviolet-visible (UV-vis) absorption analysis of tapered anduntapered SnO2 nanowires.
We performed an ultraviolet-visible (UV-vis) absorption spectroscopy measurement for
tapered and untapered SnO2 nanowires. The wavelength range of this measurement
was 190-900 nm. The absorption spectra of the SnO2 nanowires were obtained bysubtracting the background signals from a both side polished Al2O3 (110) substrate.
Figure S4 shows field-emission scanning electron microscopy (FESEM) images of
untapered (Sn flux: 8.73×1017 cm-2S-1) and tapered (Sn flux: 52.3×1017 cm-2s-1) SnO2nanowires grown onto both side polished Al2O3 (110) substrates. Figure S4 also shows
the absorption spectra as a function of incident light energy. These spectra werenormalized by adjusting at the maximum intensity. The results showed that tapered SnO2nanowires have the stronger absorption at the energy less than 3 eV compared withuntapered SnO2 nanowires. This indicates that more energy states exist within the bandgap (3.6 eV) for the tapered SnO2 nanowires.

Sn flux: 8.73x1017 cm-2s-1

Sn flux: 52.3x1017 cm-2s-1(a.u.)
AbsorptionAbsorption (a.u.)

Sn flux: 8.73x1017 cm-2s-1
Sn flux: 52.3x1017 cm-2s-1

500nm

500nm

1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5

Energy(eV)Energy (eV)Figure S4. UV-vis absorption spectroscopy data of tapered and untapered
SnO2 nanowires.

S6

S5. Principal component analysis (PCA) of EELS spectra for crosssectional SnO2 nanowires
In order to reveal a spatial distribution of the spectrum deviation, the EELS
spectra were obtained from two dimensional regions (9×11 pixels for the VS-richnanowire and 7×9 pixels for the VS-less nanowire). The VS-rich and the VS-lessSnO2 nanowire were fabricated with Sn flux 47.3×1017 cm2s-1 and 12.3 ×1017
cm2s-1. Data acquisition time for each pixel is 5 sec. During the acquisitions, theprobe scanned inside each pixel. The pixel sizes for VS-rich and VS-lessnanowire are 4.3 nm and 2.0 nm, respectively. After the energy-drift correction,
principal component analysis was performed for the 99 and the 63 spectra
dataset, respectively. Figure S5 shows the principal component analysis (PCA)
for the cross-sectional (a) VS-rich and (b) VS-less SnO2 nanowire. The figurecontains the data including the extracted EELS components (Component 1 andComponent 2), the HAADF image and the amplitude maps created byComponent 1 and Component 2. For VS-rich nanowire (figure S5 (a)), theComponent 2 contains two significant peaks corresponding to O-K edge, in which
intensity ratio of the 2nd peak to the 1st one is higher than Component 1. Spectra
withhighand

lowintensity

ratio

indicate

stoichiometric

SnO2andnonstoichiometric SnO2-δ (Figure 3c), respectively. Therefore, the darker pixels
located at surface area in the amplitude map for Component 2 of VS-richnanowire indicate the inhomogeneous stoichiometry of SnO2 and the presenceof unintentional doping at the surface area. On the other hand, for VS-lessnanowire (figure S5 (b)), intensities of two small peaks corresponding to O-K
edge in Component 2 are comparable to random noise around the peaks and theS7

homogeneous stoichiometric distribution was seen in the amplitude map. It
indicates the spectra data of VS-less nanowire exhibit no spatial dependence.
Combined with the result of spatially resolved EELS spectra shown in figure 3,
we conclude that the unintentional doping is drastically suppressed in the VS-lessSnO2 nanowire.

aVS-rich
SnO2 nanowire

bVS-less
SnO2 nanowire

Figure S5 Principal component analysis (PCA) of EELS spectra for crosssectional SnO2 nanowires. Figure S5a shows the data of VS-rich SnO2nanowire, ad Figure S5b shows the data of VS-less SnO2 nanowire.

S8S6. Calculation details of MD simulations
The molecular dynamics (MD) simulation can express time-evolution of motion(position and translation speed) for each of particles which interact with eachother through short-range Lenard-Jones (L-J) interaction. In this simulation
system, four types of particles (A, B, A’ and C) were utilized to model vapor-liquidsolid (VLS) growth process. Here, A and C can make AC compound in solid
phase which expresses an oxide formation, A’ is an impurity atom, and B is
assigned as solvent liquid atom. Following L-J type potential is used as 2-bodyinteraction potential except the interaction between A-A and C-C.
12𝜎𝜎∅(𝑟𝑖𝑗 ) = 4𝜀 [( 𝑟 𝑖𝑗 )6− ( 𝑟 𝑖𝑗 ) ] + ∅𝑐𝑜𝑟𝑟 (𝑟𝑖𝑗 )𝑖𝑗𝑖𝑗(1)

Here, 𝑟𝑖𝑗 means distance between the interacted two particles (center-to-centerdistance), 𝜎𝑖𝑗 is sum of the respective particles radii and 𝜀 expresses the bonding
energy constant defined for respective particle combination, BB, AB, CB andACindependently. A’ particle is identical with A particle, except for the bonding
energy with C particle, A’C(≡ 'AC). To accelerate the calculation speed, longrange interaction is neglected in this program code with inducing cut-off distance,𝑅𝑐 (set to be 𝑅𝑐=3𝜎𝑖𝑗), and  (𝑟𝑖𝑗) is set to be zero with 𝑟𝑖𝑗>𝑅𝑐. The correction termcorr is set to satisfy the continuity of  and ddr at 𝑟𝑖𝑗=𝑅𝑐.
For the potential between A-A and C-C, other modified L-J potential is used.
𝜎12∅(𝑟𝑖𝑗 ) = 4𝜀 [(𝑟 𝑖𝑗 )𝑖𝑗−𝑙0𝜎6− (𝑟 𝑖𝑗 ) ] + ∅𝑐𝑜𝑟𝑟 (𝑟𝑖𝑗−𝑙0 )𝑖𝑗−𝑙0S9(2)Here, 𝑙0 is offset distance and 𝜀𝐴𝐴 (=𝜀𝐶𝐶) is energy constant. Potential (2) and 𝑙0were introduced to stabilize the rock-salt structure of AC solid phase. 𝑙0 is set tobe 𝑙0 ≡ 𝑙𝑎 (√2 − 1), where 𝑙a is the lattice constant of AC rock-salt crystal.
The size of simulation box is set to be LLL, where L is the system height(set =2 in this article). Here the typical system size is set to be L = 32 la. Initially,
the substrate with four layer-thick solid consisting of A and C particles with rocksalt order is fixed on the bottom wall. Above the substrate, a flat layer of B-particle
solvent is placed in the simulation of the VLS process, while no solvent is added
to the VS process. The side walls have a periodic boundary condition. The
temperature of the system is controlled on the top and bottom walls. The particle
bath densities of A particles Asup, of A’ particles A’sup, and of C particlesCsup are
set independently, and each species is supplied from the top wall. All physicalparameters treated in this program are normalized by 𝜀𝐴𝐶, the diameter of A
particle 𝜎𝐴, and the particle mass m. This normalization leads to dimensionlesstemperature and time parameters of 𝑘𝐵𝑇/𝜀𝐴𝐶 and 𝑡√𝜀𝐴𝐶 ⁄𝑚⁄𝜎𝐴 , respectively. In thisarticle, the value of each L- parameter is set as follows: A’=c=A, B=1.44,

AA=CC =0.2, BB =0.45, AB=0.493 and CB=0.258. Temperature is set to be 0.27.

S10

S7. Comparison with previous works on the resistivity of na(2)nowires

106
1.E+05This work

Electrical resistivity (Wcm)

1.E+04
104

resistivity

1.E+03

1.E+02102
1.E+01
1.E+00100
1.E-01
1.E-0210-2

Previous works

1.E-03
1.E-0410-4005050100150200250300

diameter(nm)Nanowire
diameter (nm)Figure S7 Comparison between the present SnO2 nanowires and previous
works on the electrical resistivity.

Figure S7 shows the comparison between this work and the previous works as
to the electrical resistivity of SnO2 nanowires for various nanowire diameters.
Fabricated SnO2 nanowire in this study showed the almost 3-4 orders ofmagnitude higher electrical resistivity compared with previous works includingAppl. Phys. Lett. 107, 233503 (2015), Adv. Funct. Mater. 21, 474 (2011), Mater. Today 14, 42 (2011), Solid
State Commun. 130, 313 (2004), Appl. Phys. Lett. 89, 093114 (2006), Nano Lett. 4, 403 (2004), ACS
Sensor. 1, 997 (2016), Nanotechnology 17, 5577 (2006), J. Mater. Chem. 22, 24012 (2012), Adv. Funct.
Mater. 25, 3157 (2015), Appl. Phys. Lett. 95, 053105 (2009), ACS Appl. Mater. Interfaces 3, 3097 (2011),
Adv. Mater. 15, 1754 (2003), Biosensors and Bioelectronics 25, 2436 (2010), Sens. Actuators B 121, 3(2007). This is because the unintentional doping effect induced by VS crystal

growth could not be avoided. In this study, the unintentional doping wasdrastically suppressed by strictly controlling the occurrence of VS crystal growth.

S11

S8. Oxygen partial pressure effect on the electrical resistivity of SnO2-
thin films deposited onTemperature: 300ºC
Oxygen pressure 10-3-10Pa (in Ar 10Pa)
On Al2O3 (11-20) substrate
Deposition time 30min

SnO2 thin film

(Wcm)
film resistivity
ThinResistivity(Wcm)102101
1.E+01100
1.E+00

1.E-0110-1
1.E-02
10-2

Al2O3 (11-20)
substrate

1.E-0310-3
10-4 1.E-03
10-3 1.E-02
10-2 1.E-01
10-1 1.E+00
100 1.E+01101 102
1.E-04Oxygenpartialpressure(Pa)Oxygenpressure(Pa)Figure S8 Effect of oxygen partial pressure on the electrical resistivity ofSnO2- thin films.

Figure S8 shows the effect of oxygen partial pressure on the electrical resistivityof SnO2- films grown in Al2O3 (11-20) substrate. The deposition temperature andtime are 300oC and 30 min, respectively. The oxygen partial pressure is ranged
from 10-3 to 10 Pa with the constant total pressure 10 Pa of Ar gas. Four-probe
measurements were performed to measure the electrical resistivity of fabricatedfilms. As can be seen in the figure, the film resistivity decreased with decreasing
the oxygen partial pressure.

S12