Supplementary information

Nanoscale Size-Selective Deposition ofNanowires by Micrometer Scale Hydrophilic
Patterns
Yong He,1 Kazuki Nagashima,1 Masaki Kanai,1 Gang Meng,1 Fuwei Zhuge,1Sakon Rahong,1 Xiaomin Li,2 Tomoji Kawai 1 and Takeshi Yanagida 1

1The

Institute of Scientific and Industrial Research, Osaka University, 8-1

Mihogaoka Ibaraki, Osaka, 567-0047, Japan
2 Shanghai

Institute of Ceramics, Chinese Academy of Sciences, Shanghai,

200050, China

S1S1. Gibbs free energy calculation for the present systems
Here we describe the details of free energy calculations for the total energy change ofnanowire from oil to oil/water interface: [1, 2] The free energy gain can be calculated byusing the following formula:∆G=LA(γnw-γno)-LCγow

where L, A, and C are the nanowire length, arc length (nanowire/water interface) and
chord length (water/oil/nanowire interface) of nanowire cross section, respectively. The
values of the interfacial tensions between the nanowire and water γnw, the interfacial
tensions between the nanowire and oil γno, and the interfacial tensions between the oiland water γow are 11.4 mJ/m2, 6.7 mJ/m2, and 45.0 mJ/m2, respectively, based on theprevious reports [1].

S2. Measurements of water layers on hydrophilic patterns
The water layer formed onto the hydrophilic patterns plays an important role on thenanowire deposition phenomena in the present study. Although the direct measurementof the water layer is ideal to reveal the role, it is difficult to directly measure thenanoscale height of water layer due to the evaporation event during the measurements.
For example, as reported by A. J. Petsi and V. N. Burganos, [1] the change of waterlayer height for the water droplet with fixed contact line can be expressed as:

dh0/dt=-(J0/ρ)(θc-θccosθc)/(sinθc-θccosθc)

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where h0 is the height of water layer, t is time, J0 is the evaporation flux, ρ is the water
density and θc is the contact angle. Assuming the constant contact angle, the change ofwater layer height by evaporation event is constant and independent on the water
droplet size. If the change of water layer height is several tens nm scale, the contributionof water evaporation is negligible for sub-millimeter water thickness but involves
serious error for tens to hundreds nanometer water layer height. In fact, the error bars
for the height of water layer became larger as the hydrophilic pattern width decreased.
To minimize the contribution of water evaporation, we conducted the measurement onlyat millimeters scaled pattern. Fig. SI shows the measured water layer height distribution
data as a function of hydrophilic pattern width. The water layer height was defined as
the height data at the middle top of water layer on the hydrophilic patterns. The
relationship between the water layer height and the hydrophilic pattern width was foundto be liner on the log-log plots, as seen in Fig. SI. This proportional relationshipbetween the water droplet size and the height of water layer is theoretically supported
by Ref. 3 as following equations:h0=Rb(1-cosθc)/sinθcwhere Rb is the half-width of the wetted area. In addition, we found the goodconsistency between the estimated height of water layer in figure 1d and theexperimental results of nanowire alignment in figure 2d. Specifically, the deposition
probability for 10 m hydrophilic pattern with the estimated height of water layer of ca.
500 nm was kept almost 100 % in the nanowire diameter range of 100-625 nm, whilethat for 2 m hydrophilic pattern with the estimated height of water layer of ca. 100 nm

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decreased when the nanowire diameter was over 200 nm. These data strongly supportsthat our estimation of the height of water layer is qualitatively collect.

Fig. SI The microscopy image of water droplets formed on the hydrophilic pattern and measured
data of water layer heights on the hydrophilic patterns. The data was averaged from each 10measurements.

S3. Fabrication and characterization details of Si nanowire arrays
Si nanowires with the diameters ranged from 100 to 625 nm were fabricated bymetal-assisted chemical etching method [4, 5]. The schematic illustration can be seen inFig. SIIa. The diameter size of Si nanowires was defined by controlling Au meshpattern sizes on the silicon substrate. First, photoresist (AZ5206:AZ5200=2:1) was
coated on p-type Si substrate (resistivity = 0.01-0.02 Ω) by spin-coating with therotation speed of 5500 rpm for 60 sec, and subsequently the coated substrate was baked
at 90 °C for 3 min. The array patterns ranged from 100-625 nm were fabricated byperforming nano-imprint lithography on the resist-coated substrate. The pattern sizes
were defined by pattern sizes of polydimethylsiloxane (PDMS) molds, which weremade via Si master mold [4, 6]. After performing nano-imprint lithography, theS4

patterned photoresist layer was solidified at 120 °C for 5 min. Then, Au metal layer
with the thickness of 30 nm was deposited onto the patterned substrate by using RFsputtering.

The

residualphotoresist

wereremovedbysonication

withinN,N-dimethylformamide solution for 30 min. The fabricated patterned substrate wasimmersed into an etchant solution of 4.60 M hydrofluoric (HF) and 0.44 M hydrogenperoxide (H2O2). This etching process time was controlled to achieve the desired lengthof Si nanowires. Then the etched substrates were rinsed by Milli-Q water.

The scanning electron microscope (SEM) images of Au patterns and the fabricated Sinanowires with the diameter of 200 nm were shown in Fig. SIIb and S1c. Fig. SIId
shows the TEM image of fabricated Si nanowires. The selected area diffraction patternwas shown in the inset. The TEM and diffraction data demonstrated that the fabricatedSi nanowires were single crystals. The length of Si nanowires was controlled to beabout 4.5 μm by controlling the etching time. The surface of fabricated Si nanowires
was then chemically modified by 1% chloromethyl(trichloro)silane in a chloroform andhexadecane (1:4) solution for 2 hours at room temperature. The chemically modified Si
nanowires could be well dispersed within 1, 4-dichlorobutane by ultra-sonication, asshown in the inset of Fig. SIIe. Fig. SIIe shows the length distribution of Si nanowiresin solvent after ultra-sonication process to remove the nanowires from the substrate.
As can be seen, the length data for 80.4 % of all Si nanowires were within the range
between 4 and 5.5 μm.

S5aPDMS
mold

Reactive ion etching

Sputtering

Au layer

Photoresist

Si NW array

Si substrate

Soft lithography
patterning

Residual resist
removeAu layer
deposition

Sonication lift-off

Metal assisted
chemical etching

Fig. SII (a) Schematic illustration of fabrication of Si nanowire arrays. (b) SEM images of fabricatedpatterned Au layer on Si substrate. (c) SEM images of Si nanowires. (d) TEM images and selective
area electron diffraction (SAED) pattern (inset) of Si nanowires. (e) Length distribution of Sinanowires dispersed in 1, 4-dichlorobutane.

S4. Nanowire deposition probability when varying blade-coating cycles

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The effect of blade-coating cycles on the nanowire deposition probability was shownin Fig. SIIIa. When blade-coating cycles increased, the nanowire deposition probability
increased from 12 % to 98 %, as shown in Fig. SIIIb. In these experiments, thenanowire diameter, the pattern size, the coating speed and temperature were set to be
625 nm, 3 μm x 10 μm, 10 mm/s and 20 °C, respectively.

a5 cycles

40 cycles

20 cycles

80 cycles

50µm

LessMore

100
100808060Y Axis Title
4wmLbDeposition probability (%)

Coating cycles

6040402020000204060

80

100

0 20 40 60 80 100
X AxisT TitleCoating cycles

Fig. SIII (a) Dark field optical microscopy images of aligned Si nanowires when varying
blade-coating cycles. (b) Nanowire deposition probability data as a function of blade-coatingcycles.

S5. Deposition probability and the number of nanowires within a single patternwhen varying pattern widthS7

Fig. SIVa and SIVb show the optical microscopy images of aligned Si nanowires ofthe diameter of 200 nm on the hydrophilic patterns with the widths of 3 μm and 1 μm.
Although the nanowire deposition probability decreased from 90.5 % to 71.8 % when
the hydrophilic pattern width decreased from 3 μm and 1 μm, the percentage of single
nanowire deposition per a single pattern increased from 46.3 % to 68.5 %, as shown inFig. IV4c. The number of nanowires per pattern was measured for 700 patterns and theblade-coating in these experiments was performed for five cycles.

Although our theory does not predict that only a single nanowire is deposited onto
each pattern, a single nanowire deposition can be achieved by precisely controlling theheight of water layer. As shown in Figure SVb and SVd, when the 5 μm hydrophilicpattern was used for nanowire alignment, the height of water layer was large enough,therefore many nanowires could be deposited onto each hydrophilic pattern. When theheight of water layer decreased, the number of nanowire per pattern reduced, as shownin Figure SVa and SVc. In principle, more perfect single nanowire alignment can beachieved by precisely controlling the height of water layer. It can be modulated not onlyby the pattern size, but also by the environmental temperature and humidity.

Fig. SIV Dark field optical microscopy images of aligned Si nanowire on (a) 3 μm and (b) 1 μm
width patterns. (c) The distribution data of numbers of deposited nanowires per a pattern.

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Fig. SV (a,b) Dark field optical microscopy images and (c,d) SEM images of aligned Si
nanowire on (a,c) 2 μm and (b,d) 5 μm width patterns. (e) Schematic of nanowire alignment ondifferent pattern width.

References
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Phys. Rev. E 72, 047301 (2005).
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4. Huang, Z., Geyer, N., Werner, P., de Boor, J. & Goesele, U. Metal-Assisted
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