741© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com

1 . Introduction

Zinc oxide (ZnO) can form a variety of single-crystalline

nanostructures including nanowires, nanorods, nanobelts,

nanoplates, and nanorings. [ 1,2 ] One-dimensional (1D) ZnO

nanostructures such as nanowires and nanorods are prom-

ising, especially due to their mechanical fl exibility and

physical directionality along the longitudinal c-axis that

enable integration of ZnO into featured nanodevices. Many

functional characteristics of 1D ZnO nanomaterials have

been demonstrated and exploited for device applications.

For instance, piezoelectricity of ZnO nanowires has been

In Situ Monitoring of Laser-Assisted Hydrothermal Growth of ZnO Nanowires: Thermally Deactivating Growth Kinetics

Jung Bin In , Hyuk-Jun Kwon , Daeho Lee , Seung Hwan Ko , and Costas P. Grigoropoulos *

extensively explored for application in nanogenerators. [ 2 ]

Due to the wide direct band gap (3.37 eV) and large exciton

binding (60 meV), 1D ZnO nanomaterials can be also applied

to (opto-) electronic devices including ultra violet (UV) light

sensors, [ 3 ] lasers, [ 4 ] and light emitting devices. [ 5 ] Recently,

integration of ZnO nanowires as photoanodes in dye sensi-

tized solar cells attracted signifi cant attention, related to solar

energy technology. [ 6,7 ]

Various synthesis methods have been suggested to grow

this versatile nanomaterial in a controlled and effi cient way.

Among them, physical vapor deposition and hydrothermal

methods are most prevailing. In physical vapor deposition,

a powder form of Zn or ZnO source is vaporized at high

temperatures and subsequently deposited on the substrate

placed downstream. This method is capable of epitaxial

growth of large-scale ZnO nanowire arrays, and nanoscale

patterning of nanowires can be achieved when combined

with gold catalyst templates (vapor-liquid-sold (VLS) mech-

anism [ 4,8–10 ] ). However, growth temperatures of the physical

vapor deposition are too high to be applied to the polymeric

substrates that are ubiquitous to modern fl exible device tech-

nology. Moreover, the growth system is relatively expensive

and hard to maintain. In contrast, hydrothermal methods

enable ZnO nanowire growth in an aqueous precursor

solution and require only moderate process temperatures

(generally below 100 °C under atmospheric pressure). DOI: 10.1002/smll.201301599

The laser-assisted hydrothermal growth kinetics of a cluster of ZnO nanowires are studied based on optical in situ growth monitoring. The growth yields are orders of magnitude higher than those of conventional hydrothermal methods that use bulk heating. This remarkable improvement is attributed to suppression of precursor depletion occurring by homogeneous growth reactions, as well as to enhanced mass transport. The obtained in situ data show gradually decaying growth kinetics even with negligible precursor consumption. It is revealed that the growth deceleration is caused by thermal deactivation resulting from heat dissipation through the growing nanowires. Finally, it is demonstrated that the tailored temporal modulation of the input power enables sustained growth to extended dimensions. These results provide a key to highly effi cient use of growth precursors that has been pursued for industrial use of this functional metal oxide semiconductor.

ZnO Nanowires

Dr. J. B. In, H.-J. Kwon, Dr. D. Lee, Prof. C. P. GrigoropoulosDepartment of Mechanical Engineering University of California Berkeley Berkeley , CA , 94720-1740 , USAEmail: cgrigoro@berkeley.edu

Prof. S. H. Ko[+]

Department of Mechanical EngineeringKorea Advanced Institute of Science and Technology (KAIST) 291 Daehak-ro , Yuseong-gu , Daejeon , 305–701 , Korea [+]Present address: Department of Mechanical and Aerospace Engineering, Seoul National University, Gwanak-ro 1, Gwanak-gu, Seoul, 151-744, Korea

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Furthermore, this wet process is relatively cheap and easily

scalable. Despite these promising features, the conven-

tional hydrothermal method exhibits very low growth yields

as well as slow growth rates (i.e., a few micrometers long

from overnight growth). [ 11 ] (From now on, the conventional

hydrothermal growth is designated as the established solu-

tion growth method where the thermal activation is pro-

vided by bulk heating of the solution.) This low growth

effi ciency can hinder ZnO nanomaterials from being com-

mercially available and readily applicable to the aforemen-

tioned devices. Yet, the kinetic aspects of hydrothermal

growth of ZnO nanowires (or nanorods) and the origin

of such constrained growth yields have not been actively

investigated.

Recently, Yeo et al. [ 12 ] demonstrated rapid site-selective

growth of ZnO nanowires and its potential application to UV

sensors. By irradiating ZnO seeded substrates with a focused

laser beam as a heat source, they could grow a cluster of ZnO

nanowires up to ≈ 10 μ m in length for 30 min, which is signifi -

cantly faster than growth speeds of the conventional hydro-

thermal growth. This study implies that the feasible growth

rates achievable in hydrothermal growth are actually much

higher than those previously reported. In this respect, it is

important to understand the growth kinetics of such effi cient

laser growth and thereby identify limiting factors of the con-

ventional hydrothermal growth. The obtained knowledge will

be valuable not only in improving growth kinetics of laser

growth but also in developing a new effi cient large-area syn-

thesis technique.

In this paper, we demonstrate in situ optical imaging

of the ZnO nanowire clusters growing by the laser-assisted

hydrothermal method. The localized laser heating minimizes

homogeneous growth reactions and provides a unique chance

for direct optical observation of the growing nanowires; oth-

erwise, optically dispersive reaction products would render

the liquid medium translucent. [ 11 ] We fi nd that the nanowire

growth can reach length up to 60 μ m within an hour. On the

basis of the in situ data, we discuss the underlying mechanism

of the ultra-high yield of the laser-assisted hydrothermal

growth. We also use the in situ data for study of the growth

kinetics and to explore growth deceleration and cessation

mechanisms of the laser-assisted hydrothermal growth of

ZnO nanowires. Finally, our in situ growth kinetics reveals

a pronounced effect of thermal transport during the crystal

growth, which has been rarely observed in other (near-) iso-

thermal growth of 1D nanomaterials.

2 . Results and Discussion

2.1 . Laser-Assisted Hydrothermal Growth

For this study, the growth substrates with a light absorp-

tion layer (Au/Cr; 90 nm/ 80 nm) on top were coated with

ZnO crystalline seeds (diameter: ∼ 6 nm in average) prior

to growth. This method hindered spontaneous nucleation

of additional growth sites and thereby we could selectively

capture growth kinetics of the nanowires emanating from

the pre-defi ned seeds. As shown in Figure 1 a, b, the growth

substrate was horizontally placed in our custom-built trans-

parent growth reactor that was equipped with optical win-

dows. Liquid inlet and outlet channels were introduced to the

reactor so that the precursor solution could be replenished,

replaced or removed without losing the original position of

the laser beam spot. However, we note that during a single

growth run we did not replenish the precursor, so the growth

was conducted basically in a closed reactor system. The fl ow

channels were used only for etching experiments that will

be discussed later. We used an almost ubiquitous combina-

tion of precursors: an aqueous mixture solution of equimolar

Figure 1. (a) Photo image of the in situ growth monitoring setup. (b) Schematic of the growth reactor. The nanowire cluster snapshot is the in situ image captured during ZnO nanowire growth that corresponds to Figure (c–f). The upper half shows the mirror image of the real nanowire cluster. (c) SEM image (side view) of a representative ZnO nanowire cluster grown with a laser power of 26.3 mW for 1 hr (scale bar: 10 μ m). (d) Magnifi ed SEM image of the marked area of (c) (scale bar: 2 μ m). (e) SEM image (top view) of the same sample (scale bar: 10 μ m). (f) Magnifi ed SEM image of the marked area of (e) (scale bar: 2 μ m).

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(25 mmol each) zinc nitrate (Zn(NO 3 ) 2 , 98%, Sigma-Aldrich)

and hexamethyltetramine (HMTA; (CH 2 ) 6 N 4 , ≥ 99.0%,

Sigma-Aldrich). Polyethylenimine (PEI; C 2 H 5 N) has been

also frequently used to enhance the aspect ratio of hydro-

thermally grown ZnO nanowires. In this study, however,

we assume that PEI induces different growth kinetics, for

instance by suppressing radial growth and homogeneous

nucleation processes. [ 6,13 ] Hence, we did not use PEI in our

experiments to capture more fundamental growth kinetics.

A laser beam (continuous, λ = 532 nm) was focused ver-

tically onto the substrate. All growth experiments were con-

ducted under ambient conditions. It should be noted that in

the hydrothermal growth of this study, light irradiation was

directed to the backside of the light-absorbing layer (see

Figure S1 in the Supporting Information); thus, the opposite

front side (or the seed side) was exposed to the precursor

solution. This is because the direct front irradiation can opti-

cally interfere with the growing nanowires, resulting into mul-

tiple scattering and backscattering into the ambient. [ 12 ] In this

case, understanding of the growth mechanism can be further

hampered by transient light absorption (or scattering) that

evolves with elongation of nanowires. Moreover, the direct

irradiation can also involve possible photolytic interaction

with the precursor molecules or the material deposits. There-

fore, we adopted the back irradiation scheme to mitigate the

listed complications. As a result, the role of the laser irradia-

tion essentially reduced to a localized heat source for thermal

activation of the growth process. (See the Experimental Sec-

tion for more details of the laser growth experiment.)

Figure 1 c–f shows the scanning electron microscope

(SEM) images of a representative ZnO nanowire cluster

grown by the focused laser beam (power: 26.3 mW/ beam

diameter d : 32.6 μ m) for 1 h. The cluster has a pseudo-hem-

ispherical shape with decreasing lengths of the nanowires

from the spot center to the perimeter. This trend refl ects

the lateral thermal gradient induced by the Gaussian beam

shape. It is well known that the competitive growth mecha-

nism between randomly oriented nanowires results in growth

alignment to the normal direction of the substrate (Figure 1 f).

In our spot-wise growth, however, edge effects are so pro-

nounced that the nanowires near the perimeter of the cluster

grow inclined to the horizontal direction (Figure 1 d). These

morphological features confi rm random crystalline orienta-

tions of our initial seeds. [ 14,15 ]

2.2 . In Situ Monitoring of Growth Kinetics and Data Post-Processing

In order to explore the in situ growth kinetics, a simple

imaging system (consisting of a 20× objective lens, a zoom

lens, and a charge-coupled device (CCD) camera) was

directed perpendicularly to the reactor window (Figure 1 a).

Here we adjusted the magnifi cation of the zoom lens so that

1 CCD pixel corresponded to about 1 μ m in real length. This

optical confi guration enabled direct side-view observation

of a growing nanowire cluster. The oblique illumination of a

light-emitting device (LED) provided high-contrast images

that benefi ted from the light scattering incurred by the ZnO

nanowire cluster. The image data were saved into a movie fi le

for the next post-processing.

Figure 2 a pictorially describes the post-processing with

real image data and the corresponding kinetic curve. Image

processing was conducted to detect the boundary edges that

defi ne the shape of the nanowire cluster. In this study, we

focus on axial growth kinetics of ZnO nanowires at the center

of the cluster, so the transient edge position of the center

was traced and converted to height information. Finally, the

kinetic data were presented as a “nanowire height vs. growth

time” plot. However, it should be noted that we do not show

the kinetic data obtained during the fi rst 60 s after the laser

shutter was opened. This is because the initially small volume

of the nanowire cluster does not scatter suffi cient light,

resulting in a poor image contrast. The following image pro-

cessing on this blurry image mostly failed to provide reliable

Figure 2. (a) Illustration of postprocessing with real data. The set of snapshots shows the in situ optical images and post-processed outline images that were captured at the corresponding time. All the images have been rotated by 180°. (The real growth occurred downward with the base of the nanowires pinned to the fl ipped seed surface.) The kinetic data were collected from the highest pixel position of each image. (b) Comparison of the apparent growth rate to other hydrothermal growth data (fi nal length vs. growth time) reported from different groups. Morin et al . took advantage of a continuous fl ow reactor (CFR) to mitigate precursor depletion and their growth yield is relatively high.

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information on the edge positions. The accuracy of the meas-

urement was evaluated based on comparison to SEM of the

samples. The deviation was estimated less than 1.5 μ m in all

samples and the edge information showed a great match with

real morphologies (see Figure S2 in the Supporting Informa-

tion). Additional information on the image processing is pro-

vided in the Experimental Section.

2.3 . Rapid Ultra-High Yield Growth Kinetics

The laser-assisted hydrothermal growth is distinguished by

starkly rapid growth speed and the resulting high growth

yield. (In this paper, we consider growth yield based on the

laser irradiation area.) For instance, focused laser irradia-

tion of 45.7 mW ( d : 36.3 μ m) produced almost 60 μ m-long

nanowires for an hour (Figure 2 a). The growth rates can

increase further with higher laser power, but the laser power

is limited below the level of bubble generation. As shown

in Figure 2 b, the apparent growth rate (or growth yield) of

our laser-assisted hydrothermal growth is remarkably fast:

tens of micrometers for an hour, which is approximately two orders of magnitude higher than other conventional hydro-

thermal growth rates obtained under similar conditions. [ 14–20 ]

The growth parameters of the compared studies are listed in

Table S1 of Supporting Information.

The extraordinary growth improvement might have

been attributed to possible unconventional morphologies of

nanowires that can affect apparent axial growth kinetics. [ 21 ]

However, the structures of the nanowires grown by the laser

method do not seem to be signifi cantly altered. As shown in

Figure 1 f, the produced nanowires maintain the typical prism

structure with the basal plane (0001) normal to the growth

direction. In Figure 3 , we also observe morphological evo-

lution with growth time that has been commonly shown in

the conventional hydrothermal growth. Since axial growth is

accompanied by relatively slow, albeit evident radial growth,

longer nanowires have larger diameters. [ 14,17 ] As nanowire

lengths exceed inter-nanowire distances, some of the inclined

nanowires stop growing because of spatial interference

with other neighboring nanowires. As a result, the number

density decreases with longer growth,

which can be accounted for by geometrical

selection. [ 15 ]

In addition, the growth might be

affected by the underlying gold surface.

For instance, Xu et al. reported seedless

growth of vertically aligned ZnO nanow-

ires on an annealed thin gold fi lm. [ 22 ] In

our laser growth, however, most of the

nanowires were randomly oriented as

aforementioned. Moreover, a control

experiment (isothermal growth with the

same substrate at 95 °C) produced very

short nanowires ( ≈ 520 nm/h) in accord-

ance with observations of isothermal

hydrothermal growth. Thus, here we

confi rm that the gold fi lm does not infl u-

ence hydrothermal growth kinetics under

the examined conditions. From these fi ndings, we postulate

that the ultra-high yield of the laser-assisted growth is due

to amplifi ed driving force for crystallization (that is, higher

supersaturation) at the growth interface.

The most fundamental difference of our laser-assisted

growth from other conventional hydrothermal processes

lies in the thermal environment of the growth reaction zone.

Thus, we focus on thermal aspects of the laser-assisted growth

to elucidate the mechanism of the ultra-high yield growth.

In the case of conventional hydrothermal growth, the entire

pool of the precursor solution is subject basically to the same

growth temperature. Mostly, the boundary layer thickness of

precursor concentration is much smaller than reactor dimen-

sions. Therefore, the thermally activated precursor molecules

can spontaneously nucleate and homogeneously grow into

ZnO crystals dispersed in liquid before they diffuse into the

heterogeneous growth sites of the substrate. Indeed, many

studies have reported signifi cant precursor consumption by

homogeneous growth reactions. [ 19,23 ]

In principle, the aforementioned challenge can be over-

come either by using a continuous fl ow reactor (CFR) system

that continuously provides fresh nutrients to keep a con-

stant growth condition, [ 19,24 ] or by minimizing the volume

of the heat-affected zone while maintaining a suffi ciently

high temperature at the growth interface. The laser-assisted

hydrothermal growth offers a unique opportunity that cor-

responds to the latter. As the laser beam is focused onto the

substrate, a temperature gradient develops from the thin light

absorption layer down to the liquid medium. Consequently,

the reaction area is confi ned to the proximity of the sub-

strate surface and the growing ZnO nanowires. This growth

localization suppresses precursor consumption from homoge-

neous reactions in liquid far from the substrate and thereby

improves effi ciency of precursor utilization.

In addition, mass transport of precursor molecules can be

signifi cantly facilitated in the spot-wise laser-assisted growth.

In a dilute precursor solution, mass transport can be an

important rate-limiting step of growth kinetics. For instance,

under a similar level of precursor concentration (16 mmol),

Boecker et al. [ 17 ] reported a higher growth yield near the edge

of seeded substrate region, which is a typical phenomenon of

Figure 3. A series of SEM images (top view at the center of the laser spot) of nanowires grown with varying growth time (from 5 to 3600 s). The calculated power density of laser irradiation on the image area is approximately 5.9 kW cm −2 . All the images are at the same magnifi cation. The scale bar of the bottom right image indicates 400 nm.

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diffusion-limited growth. In the case of our spot-wise growth

by focused laser, such diffusion enhancement can be ampli-

fi ed. [ 25,26 ] Finally, laser heating enables a rapid temperature

ramp to a growth point due to the small volume of the heat-

affected zone. In contrast, conventional reaction baths can

experience long transient heat-up, depending on the heat

capacity of the precursor pool. This advantageous effect of

rapid heating was explored in previous studies of microwave-

assisted hydrothermal growth. [ 27,28 ] Possible superheating

at the growth spot can result in rapid growth. [ 12 ] However,

our enhanced growth kinetics was obtained in a wide range

of laser power (as will be shown later in Figure 5 ). In this

regard, here we do not consider superheating as a differenti-

ating factor.

2.4 . Growth Deceleration

Notwithstanding the benefi cial outcomes of the laser growth,

the laser-assisted growth is still limited by gradually deac-

tivating kinetics as already shown in Figure 2 a. Here we

explore the underlying deactivation mechanism. First, we

revisit the aforementioned precursor consumption and quan-

titatively evaluate its effect on growth kinetics based on our

in situ kinetic data. In Figure 4 a, run #1 and #2 show repre-

sentative kinetic curves obtained with the same laser power

(53.8 mW) but different laser beam diameters (36.3 μ m and

39.6 μ m, respectively). The nanowires of run #1 grew faster

and longer than run #2, obviously due to the higher laser

energy density at the spot center. Meanwhile, the growth rate

of run #1 decelerated faster than that of run #2. This expo-

nentially decaying growth kinetics resembles the growth

model that Zhou et al. [ 23 ] proposed to account for signifi cant

precursor consumption of their homogeneous ZnO nanowire

growth. However, calculation of the precursor utilization

promptly proves that this is not the case in our laser growth.

Even by a conservative calculation, consumed precursor for

a single laser growth experiment is estimated less than 0.1%

of the entire precursor (solution volume: 3 cc). More obvi-

ously, when we repeated the same laser growth (run #3)

immediately following run #2, we reproduced almost iden-

tical growth kinetics even without replenishing the precursor

solution. From these fi ndings, we safely rule out growth decay

by precursor depletion.

Another possibility could be growth inhibition by accu-

mulating poisonous materials such as secondary zinc-based

material deposits [ 15 ] and ionic metal impurities. [ 29 ] These

materials can cover active growth sites hindering access of

precursor molecules even with existing supersaturation. In

order to examine the validity of the poisoning theory, we

conducted a sequence of in situ growth−etching−regrowth

experiments. The etching was to remove possible poisons

so that the following regrowth could reactivate the previous

decaying growth kinetics. First, we grew a nanowire cluster

long enough to show clear growth deactivation. Then, the

laser was turned off and the precursor solution was removed.

A continuous fl ow of deionized water was applied to rinse

the growth substrate and the reactor. For etching, a dilute

HCl (0.5 mmol) was introduced to the reactor in order to

slightly etch the growth front. According to the recorded in

situ images, the nanowires were etched away from the tips by

approximately 1 μ m for 1 h. After the second rinse, the iden-

tical but fresh precursor solution was introduced again and

the regrowth was fi nally conducted under the same condition.

The growth kinetics obtained from the etching experiment is

shown in Figure 4 b. As shown by the SEM images, the etching

process roughened the nanowire surface. By regrowth, how-

ever, the original hexagonal prism structure was completely

recovered. Importantly, the regrowth kinetics does not show

distinct reactivation by this etching-regrowth scheme. Rather,

it appears that the original trend is extended by the regrowth.

Therefore, we suggest that the poisoning mechanism cannot

account for the growth deactivation of the laser-assisted

growth at least for the examined growth time (1 h).

Up to this point, it has been presumed that the tempera-

ture at the growth front changes negligibly during growth.

Considering thermal confi nement of focused laser irradia-

tion, however, we expect a signifi cant temperature gradient

in the reaction zone that can impact on the resulting growth

kinetics. [ 30 ] Indeed, our fi nite element method (FEM) simula-

tion study (COMSOL Multiphysics) suggests that the length

Figure 4. (a) Representative in situ kinetic curves that were obtained with different beam diameters (run #1: 36.3 μ m, run #2, #3: 39.6 μ m) under the same laser irradiation power (53.8 mW). The inset shows the SEM image of Run #1 (scale bar: 20 μ m). The maximum height is about 58.8 μ m (SEM measurement). (b) Kinetic data of the etching experiment. The insets show SEM of ZnO nanowires before and after the regrowth. Both scale bars indicate 1 μ m.

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full papersscale of the heat-affected zone induced by the focused laser

beam is comparable to the size of a nanowire cluster (see

Figure S3 in Supporting Information). Then we can raise a

question about how the temperature of the growth interface

(especially the tip of the growing nanowire) will respond to

the evolving thermal environment.

One of the relatively few discussions about the infl u-

ence of a temperature gradient on growth kinetics of 1D

nanomaterials is the study of Louchev et al . [ 31 ] They pro-

posed that the kinetic deceleration in growth of carbon

nanotubes (CNTs) by chemical vapor deposition (CVD)

could be attributed to thermal cool-down at the cata-

lyst tips of the growing CNT array. Analogously, the

growing ZnO nanowires can act as effective thermal fi ns

that enhance the heat transfer rate from the beam spot to

the solution medium. Even though emphasized phonon-

boundary scattering reduces the thermal conductivity of

ZnO nanowires by more than one order, ZnO nanowires

still have a higher thermal conductivity ( ∼ 10 W m −1 K −1 ,

when the diameter is ∼ 200 nm) [ 32 ] than water. Furthermore,

an amplifi ed cooling effect is reasonably expected in solution

growth, due to signifi cantly higher heat dissipation of water

(quite generally, 200−1500 W m −2 K −1 in free convection and

0.6−0.7 W m −1 K −1 in conduction) than that of general gases

(2−25 W m −2 K −1 in free convection and 0.02−0.2 W m −1 K −1

in conduction). [ 33 ] (See the Supporting Information for fur-

ther discussion on thermal transport processes.) In contrast

to CVD processes, the growth temperature of the laser

method is sustained from the thin light-absorbing fi lm that

has a small heat capacity. Thus, the temperature at the base as

well as at the tip of the nanowire can immediately respond to

heat loss through the nanowire structure, resulting in growth

deceleration.

In order to fi nd experimental evidence on the thermally

deactivated growth, we conducted systematic growth experi-

ments (1 hour of growth time each) by changing the laser

power. The beam diameter was set at 39.6 μ m. Figure 5 shows

each kinetic curve and the resulting trend of the growth

heights. As shown in Figure 5 a, the growth rate is obviously

proportional to laser power due to the increased growth tem-

perature. At a higher temperature, the growth reactions shift

forward with the increased concentration of OH − ion most

likely caused by faster decomposition of HMTA. [ 11,23 ] Figure 5 b

also indicates that the nanowire height (obtained by 1 h

growth) increases linearly with higher laser power. Nanow-

ires could not be grown below a specifi c level of laser power,

which should be related to the equilibrium temperature of

the growth reactions. [ 20 ]

Interestingly, the growth with the lowest laser power

(19.8 mW) generated secondary deposits near the rim area

of the growth spot. The SEM images in Figure 5 b show that

the secondary material covers the nanowires that already

grew, implying that this material nucleated and grew at a

later stage of the laser growth. However, growth with higher

laser power did not generate such secondary deposits for the

same growth time. Furthermore, when we reduced the laser

power less than 18.1 mW we found that the secondary mate-

rial completely covered the beam spot area without nanow-

ires observable by SEM. The material identity is under our

investigation, but we could fi nd morphological resemblance

from several studies on hydrothermal generation of zinc

hydroxide precipitates. [ 34,35 ] Especially in accordance with

our observation, Olson et al. reported possible zinc hydroxide

nucleation on the ZnO nanowires at a reduced level of super-

saturation. [ 15 ] Therefore, we postulate that this secondary

material is indicative of low growth temperatures.

If this is the case, the secondary material should be also

generated under any higher laser power by extended growth;

eventually, the enhanced heat transfer of longer nanowires

will reduce the tip temperature to the level responsible for

generation of the secondary material. Indeed, when we con-

ducted a laser growth experiment at suffi ciently high laser

power (57.7 mW, d : 39.6 μ m) for 7 h, we found a similar mate-

rial deposit growing from the center vertex of the nanowire

cluster. Figure 6 a–c shows the in situ snapshots, ex situ optical

microscope (bright fi eld), and SEM images of the nanowire

cluster, respectively. Figure 6 a especially reveals that the

secondary material started to grow from about 3 h after the

initiation of the laser irradiation. Here, we note that this sec-

ondary material can be regarded as a growth poison from the

Figure 5. (a) In situ kinetic curves obtained with different laser powers ( d : 39.6 μ m). The SEM images show the side views of each sample. The scale bars commonly indicate 20 μ m. (The size of every image box corresponds to 60 μ m × 60 μ m.) The maximum height of each cluster measured by SEM is: (i) 49. 41 μ m, (ii) 40.35 μ m, (iii) 32.03 μ m, (iv) 20.24 μ m. (b) Plot of fi nal height (1 h growth) vs laser power. This plot shows the height data obtained by SEM measurement.

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perspective of poisoning theory. [ 15 ] As previously mentioned

in our etching and regrowth experiment, we believe that this

poisoning mechanism manifests itself rather at the fi nal stage

of the ZnO growth and thereby it is more related to a perma-

nent growth termination.

To further support the proposed thermal deactivation,

we conducted a laser growth experiment by increasing the

laser power stepwise from 26.3 mW to 53.8 mW every 10 min

during a single growth run. Figure 7 shows the corresponding

in situ kinetic data. The lower curve shows the regular growth

with constant laser power (26.3 mW), and the upper curve

indicates the power-manipulated growth run. Indeed, the

increasing laser power delayed deceleration of growth speed.

Here the laser beam was focused tightly, so the beam spot size

was 32.6 μ m. Interestingly, the fi rst increase of laser power

at 600 sec (from 26.3 mW to 33.4 mW) did not generate a

bubble due to effi cient heat dissipation through the already

grown nanowires. Otherwise, the 33.4 mW laser power would

instantly generate a bubble by excessive heat. This result vali-

dates the role of the growing nanowires as thermal fi ns. Con-

sequently, we confi rm the presence of thermal cool-down at

the growth front that is responsible for the observed growth

deceleration and the subsequent growth termination.

2.5 . Effi cient Hydrothermal Growth

Our results provide profound implications on further devel-

opment for effi cient hydrothermal synthesis of ZnO nanow-

ires. (1) The localized heat-affected zone develops over time

to a suffi cient spatial extent for sustaining the related growth

reactions up to tens of micrometers of the nanowire height.

In this regard, the relatively large volume of conventional

chemical bath reactor systems and thereby the amount of

wasted precursor can be reasonably reduced. Rather, in-

line precursor replenishment will be a more effi cient way

to support further extended growth time or continuous

batch runs. (2) While the growth rate increases with higher

light power, boiling phenomena limit the possible enhance-

ment by further increase of light power. However, pressur-

izing the growth reactor will suppress boiling and allow a

wider window of laser power for faster growth. (3) We also

expect that the laser heating will enable reproducible growth

on substrates not only because it decouples substrate growth

kinetics from complicated homogeneous reaction kinetics [ 11 ]

but also because it is free of effects of reactor walls. (4) As

this is a purely thermal approach, one can still apply previ-

ously invented chemical growth modifi cation (i.e. improving

aspect ratio by using chemical capping agents [ 1,6 ] ). (5) The

proposed method does not necessarily require laser irra-

diation; in practice, the technique is readily compatible with

other rapid thermal processes such as Joule-heating of thin

conductive elements, [ 36 ] which can be a promising solution

for large-area growth. In this case, we also expect thermally

deactivating growth kinetics (see 1D thermal fi n model in

Supporting Information). (6) A relatively large beam spot

and the accompanying heat diffusion limit the resolution of

selective growth that is crucial for single-nanowire device fab-

rication. A high magnifi cation process can improve the reso-

lution to a degree, but the required tight control hinders its

practical use in industry. Moreover, the demonstrated facile

coating of crystalline seeds results in uncontrolled orientation

of nanowires. To resolve this dimensional restriction, density-

controlled accurate positioning of seed material should be

pursued. [ 37 ] (7) When it comes to the total energy consump-

tion necessary for mass production of nanowires (J/kg),

the energy effi ciency of laser growth can hardly match that of

a well-insulated conventional growth system. This is because

Figure 7. The lower curve indicates growth kinetics with constant laser power (26.3 mW). The upper curve indicates growth with stepwise increasing laser power (every 10 min). Each inset shows the SEM of the corresponding nanowire cluster. We note that the lower right inset is identical to Figure 1 c. Both of the images have been resized at the same scale (scale bar: 20 μ m).

Figure 6. (a) In situ optical images of the ZnO nanowire cluster grown for the extended growth time (7 hours). A secondary material starts to grow at the cluster top center (*) from about 3 hr. (b) Ex situ optical microscope image (bright fi eld) of the fi nal structure. (c) SEM image of the corresponding cluster (scale bar: 40 μ m). All the images have been rotated by 180° from the real growth confi guration.

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J. B. In et al.

748 www.small-journal.com © 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

full papersof the limited energy effi ciency of the laser system itself,

wasted light by refl ection, and heat dissipation to the solu-

tion medium. In this respect, we envision another possibility

of using even sunlight as a light source for this photo-thermal

process. [ 38 ] For instance, sunlight irradiation on 1 cm 2 can

provide about 135 mW light power, which can be reasonably

utilized as a local heat source of our technique when focused.

3 . Conclusion

In summary, we have demonstrated ultra-high yields of

laser-assisted hydrothermal growth of ZnO nanowires and

the related growth kinetics. In situ monitoring of the growth

kinetics reveals that the laser growth enables effi cient use of

the growth precursor by suppressing homogeneous growth

reactions. In contrast to the conventional hydrothermal

growth where the precursor solution and the growing

nanowires are subject to an isothermal environment, local-

ized heating of laser induces growth deceleration stemming

from the enhanced heat transfer (that is, cooling) through the

growing nanowire thermal fi ns. As the growth temperature

decreases to the level of the secondary material deposition,

possible growth poisoning can affect the growth kinetics and

result in growth termination. This growth deactivation can be

mitigated by providing the growth sites with sustained heat

(i.e. increasing light power). Our results therefore provide

a fundamental basis for development of a high-effi ciency

hydrothermal growth system.

4 . Experimental Section

Seed Substrate Preparation : We deposited a thin Au/Cr light-absorption layer (90 nm/ 80 nm) on a quartz glass wafer (560 μ m thick) by electron beam evaporation. A seed solution consisting of crystalline ZnO nanoparticle seeds (size: ∼ 6 nm, 0.019% (wt.), prepared by the modifi ed procedure [ 39 ] of Pacholski method [ 40 ] ) was spin-coated on the substrate at 3000 rpm for 1 min. Then the coated surface was dried by blowing a nitrogen gas, and heated on a hot plate (120 °C) for 3 min to enhance adsorption of the particles. Each coating step was repeated 3 times to increase the seed density. After completion of the coating procedure, the coated wafer was split into pieces. The pieces possibly under an edge effect of spin coating were selectively removed. The opposite (or irradiation) side of the quartz wafer was cleaned with an ethanol-soaked cotton swab to make an optically clean surface.

Laser-Assisted Hydrothermal Growth : A growth substrate was mounted on a polydimethylsiloxane (PDMS) block, and this assembly was placed in the growth reactor built with trans-parent glass slides to allow optical imaging. We used two sepa-rate syringe pumps to inject and remove solutions. The growth precursor ( ∼ 3 mL) was introduced through a 0.22 μ m fi lter unit (Millipore). As illustrated in Figure 1 b, this precursor volume lev-eled the solution free surface at the substrate position without submerging the substrate. This leveling was necessary to get a consistent focus without interference due to the different

refractive index of the solution. A glass slide capped the reactor to minimize evaporation of the solution. The measured evapora-tion during laser irradiation was negligible ( ∼ 15 μ m/h decrease in level). A continuous diode-pumped solid-state laser ( λ = 532 nm, LambdaPro) was used for laser irradiation. We applied different combinations of absorptive neutral density (ND) fi lters to control the laser power. Each laser power was measured with a power meter (Coherent) and it showed a stable output (Figure S1b) for the general growth time (1 hour). A plano-convex lens (LA1951-A, Thorlabs) was placed above the reactor to focus the laser beam on the Au/Cr absorption layer. The size (1/e 2 ) of the focused beam spot was approximately 32.6 μ m in diameter and could be enlarged by adjusting the working distance (lens-to-object dis-tance). Whenever a new sample was loaded, the working distance was carefully set with multiple alignment stages and an optical micrometer (LS7030M, Keyence). When we applied laser on the substrate, the growth spot was positioned closer to the camera side for better image quality, but kept about 150 μ m away from the substrate edge to avoid possible enhanced heat transfer very near the edge. As very limited area of the substrate was used for growth, multiple growth experiments could be conducted in a single batch. We especially note that the series of growth experi-ments in this study was conducted on a single substrate sample for consistency.

In Situ Growth Monitoring System and Image Processing : In conjunction with the laser-assisted growth of ZnO nanowire clusters, we adopted a rather straightforward optical imaging method to explore real-time growth kinetics of ZnO nanowires. The system consists of basic components: a 20× objective lens, a zoom lens, a CCD camera (resolution: 640 × 480, frame rate: 29.98 fps) for image capture, and a LED ( λ = 375 nm, LED370E, Thorlabs) for oblique illumination. A higher magnifi cation might help get a better resolution, but the accompanying reduced depth of focus undermined the image quality due to the 3-dimen-sional structure of the nanowire cluster. A fl uorescence imaging fi lter ( λ = 460 nm/ fwhm = 60 nm, MF460–60, Thorlabs) was installed in the monitoring optics to selectively cut off the laser light ( λ = 532 nm) from the light-scattered image. A recorded video that corresponds to run #3 of Figure 4 a is provided in Sup-porting Information.

Post-processing of the Image Data : The original movie data (recorded for 1 hour) has a large amount of data, so the movie was reduced to 1/64 size by using Microsoft Windows Movie Maker. For image processing we used a built-in function “edge()” of MATLAB to detect edges from the raw image data. We did not con-duct additional data processing (such as data smoothing) on the fi nal kinetic curves. Thus, the seemingly stepwise increase in the kinetic curves presented in this paper does not refl ect real abrupt growth behavior but originates from the limited spatial resolution of the growth monitoring system.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

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In Situ Monitoring of Laser-Assisted Hydrothermal Growth of ZnO Nanowires

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Acknowledgements

This work was supported by the NSF SINAM NSEC. J.B.I. thanks Dr. Sang-Gil Ryu for advice on laser experimentation.

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Received: May 24, 2013Revised: July 14, 2013Published online: September 20, 2013

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