Morphology-dependent field emission properties and wetting behavior of ZnO nanowire arrays.

0.76 MB·English

by  Yao, Lujun

Yaoetal.NanoscaleResearchLetters2011,6:74
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NANO EXPRESS Open Access
Morphology-dependent field emission properties
and wetting behavior of ZnO nanowire arrays
Lujun Yao1, Maojun Zheng1,2*, Li Ma3, Wei Li3, Mei Li3, Wenzhong Shen1,2
Abstract
The fabrication of three kinds of ZnO nanowire arrays with different structural parameters over Au-coated silicon
(100) by facile thermal evaporation of ZnS precursor is reported, and the growth mechanism are proposed based
on structural analysis. Field emission (FE) properties and wetting behavior were revealed to be strongly
morphology dependent. The nanowire arrays in small diameter and high aspect ratio exhibited the best FE
performance showing a low turn-on field (4.1 V/μm) and a high field-enhancement factor (1745.8). The result also
confirmed that keeping large air within the films was an effective way to obtain super water-repellent properties.
This study indicates that the preparation of ZnO nanowire arrays in an optimum structural model is crucial to FE
efficiency and wetting behavior.
Introduction fabrication. Too closely and too densely spaced nanos-
ZnO has been considered as one of the most important tructures are both not favorable to construct FE nanode-
electronic and photonic material because of its wide vices. On the other hand, another significant application
direct bandgap (3.37 eV) and large exciton binding of ZnO related to the geometric effects is the wettability
energy (60 meV). Extensive researches have been devel- [16,17], which might bring great advantages in a wide
oped on the growth of quasi one-dimensional (1D) ZnO variety of applications in daily life, industry, and agricul-
nanostructures [1,2] including nanowires, nanotubes, ture. The vertically aligned nanostructures involving a
nanobelts, and nanoneedles. Meanwhile, these 1D ZnO large amount of trapped air within the films and their
nanostructures have been widely applied as room tem- high roughness have been proved to be potential for the
perature UV detector [3], transparent conductive elec- building of hydrophobic surfaces, various surfaces of
trodes [4], sensors [1,5-7], and solar cells [8]. Recently, ZnO nanostructured arrays showing lotus-like water-
various inorganic semiconductor nanostructures have repellent properties have been prepared in the past
been the focus of the researches on the studies of FE years [16,18,19].
properties [9] and wetting behavior [10], including the However, many previous efforts in the large-scale
well-aligned 1D ZnO nanostructured arrays which have fabrication of ZnO nanowire or nanorod arrays have
attracted great attention as promising field emission been achieved by physical evaporation of the mixture
(FE) sources [1,11-14] due to their negative electron affi- of ZnO and graphite powders, chemical vapor
nity [15], chemical stability, tip geometry, or apex struc- deposition using Zn powder as the source materials, or
ture. A crucial factor to influence FE performance low-temperature hydrothermal synthesis with the pre-
includes the interspacing between individual nanowires prepared colloidal ZnO nanocrystals as the grown
or nanorods, and aspect ratio. The manner in which seeds. In this article, a novel fabrication of ZnO nano-
these structural parameters could be controlled during wire arrays with different structural parameters over
self-organized growth processes has developed into a Au-coated silicon (100) by facile thermal evaporation
challenging and technological problem for nanostructure of ZnS precursors is reported. The nanowire diameter
and growth speed were controlled by changing the
thickness of coated Au film layer together with
*Correspondence:[email protected]
1LaboratoryofCondensedMatterSpectroscopyandOpto-ElectronicPhysics, substrate locations. The authors studied the morphol-
DepartmentofPhysics,ShanghaiJiaoTongUniversity,Shanghai200240, ogy-dependent FE performance, and first revealed that
People’sRepublicofChina.
wetting behavior of ZnO nanowire arrays in different
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©2011Yaoetal;licenseeSpringer.ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommonsAttribution
License(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,andreproductioninanymedium,
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void ratios, which confirmed that a large amount of air Results and discussions
kept within the films would be an effective way to Structural and compositional characterization of ZnO
obtain super water-repellent properties. nanowire arrays
Figure 1 shows the X-ray diffraction patterns used to
Experimental assess the overall structure and phase purity. All posi-
The fabrication was performed using a two-end open tions of the peaks can be readily indexed to the hexago-
quartz tube connected to a rotary vacuum pump and a nal wurtzite ZnO with lattice constants a = 3.25 Å and
gas inlet through a vacuum coupling. The silicon (100) c = 5.21 Å (JCPDS card No. 80-0075). In particular, we
substrates prepared for samples A, B, and C were soni- can see that (002) peak located at about 34.4° is much
cated in acetone, washed with de-ionized (DI) water, stronger than the others for all of the three samples,
and dried with nitrogen. Then, Au film layers were which means these nanowire arrays have a preferential
deposited on these substrates by ion sputtering from the orientations in the c-axis direction. Moreover, it is
Au target (99.999%) using an ion sputter coater (Hitachi clearly seen that the peak intensity of sample B is the
E-1045, Hitachi Co., Tokyo, Japan.). The target-substrate strongest among the three products, whereas the sample
distance was about 30 mm, and the pressure of sputter- A has the weakest peak intensity. The reason can be
ing chamber was pumped down to 6 Pa before deposi- attributed to ZnO film thickness as well as their void
tion. The coating rate depending on discharge current ratios, which can be obtained from Table 1. The sample
was kept at 6 nm/min. The three kinds of above-men- B which has a thick film with small void ratio shows
tioned substrates were sputtered for 50, 50, and 15 s, higher peak intensity than the other two samples.
respectively. The corresponding thicknesses of Au film The surface chemical composition of all the three
layers are about 50, 50, and 15 Å. Growth procedures ZnO nanowire arrays have been characterized by means
were conducted by thermal evaporation of commercially of XPS to detect any trace of impurities in the samples
available high purity ZnS powder and graphite powder and to assess the vertical compositional homogeneity, as
with equal molar ratio, which was placed at the center
of the quartz tube furnace. Silicon substrates were
placed downstream about 5 cm (samples B and C) and
upstream about 5 cm (sample A) away from the source
materials to collect the products. Subsequently, we
introduced an Ar gas flow of 80 sccm, and a fixed pres-
sure at about 150 Torr was applied. The tube furnace
was then heated to 750°C quickly and maintained at this
peak for 30 min. After it cooled down naturally to room
temperature, all the substrates appeared dark gray indi- Sample(cid:3)A(cid:3)
cating the deposition.
Themorphologyandcrystalstructureswerecharacter- 2)
0
izedbyfieldemissionscanningelectronmicroscope(FE- 0
(
SEM, PhilipsSirion200) and X-ray diffractometer (Bru-
ker-AXS system) with Cu Ka radiation (l = 1.5406 Å).
)
0 )
TwhireesaurrrfaaycsewchaesmaincaallyczoemdpboysXitiPoSn(oKfrtahteosseAZXnOISnUalntora- (10 (101 02) Sample(cid:3)B
1
DLD) with a power of 150 W. A monochromatic Al Ka (
X-raysource(1486.6eV)wasoperatedinaconstantana-
lyzerenergymode.Watercontactangle(CA)andsliding
angle were measured using an optical contact-angle
meter system (Data Physics Instrument GmbH,
Germany) at ambient temperature. FE properties were
carriedoutemployinga two-parallel-plate configuration
in an ultrahigh vacuum chamber (5 × 10-7 Pa). In brief,
Sample(cid:3)C
samples were stuck onto a stainless-steel sample stage
using conducting glue to act as the cathode, while
20 30 40 50 60
another parallel stainless steel plate served as the anode
2Theta(degree)
with a fixed cathode-anode distance of 300 μm. The
emission current was monitored via a Keithley 485 Figure 1 XRD patterns of the three kinds of ZnO nanowire
arrays.
picoammeter.
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Table 1The structural parameters ofthe threekinds ofnanowire arrays
Sample Diameter(nm) Length(μm) Aspectratio Density(μm-2) Voidratio(%) E (V/μm) b CA
to
A 300 6 20 1.3 90.8 8.4 1209.5 142.1±1°
B 600 25 41.7 0.57 83.9 5.8 1566.7 94.8±1°
C 80 25 312.5 4.2 97.9 4.1 1745.8 154.3±1°
shown in Figure 2. The insets are the high resolution
spectra recorded for the Zn and O regions. Binding (a) 1.0x105
eaT12nd0ph4ve1e4re/2gn.r8ietaeiessrtpVieoweua(ceslstraliemvcreacepraclbbleoiiobrnAndrda)ei,wtnde1igdt0wh2ewi1nbt.ih6teinhratdghnriieendessgp1poe0eecf4nat4keZ.rts8nogaye2ttVhop1ef3(0s/2s2a2i8mg1a4n.np7.a6ldelaefZBnVon)dr,. y (C/s) 68..00xx110044 5255O 1s30535540n (LMM)O 1sZn (LMM) Zn 2pO (KLL)3/2 Zn 2p1/2 Zn 2s
1021.7 and 1044.9 eV (sample C). The photoelectron sit Z
slbopinceadctitenrdagaoetnfe5Or3g01i.es6s,ion5f3t0thh.e4e,atahsn-rpderee5sp3aa0mr.5epdeleVtsh,arrreeeespssieamcmtiilpvalerelsya.nwdTehrinee Inten 24..00xx110044 Zn 3dZn 3pZn 3s C 1s Zn 2p3/2 Zn 2p1/2
total agreement with the standard values of ZnO. The 1020 1035 1050
0.0
above XRD and XPS results revealed that our prepara- 0 200 400 600 800 1000 1200
tion method supplied pure surface compositions of all Binding Energy (eV)
the three ZnO products, not as the same as the wet che-
mical approaches induced doping type in ZnO nanos-
(b) 2.0x105
toscsrearfuFnomtcitshgtpsuuet-lhrresteeeeshsc3rtA[tei2ilpe,ot0ren,Bke2dsai,1nel(]andt.(nhtstsdehotehufCepbZ,eqnrpluOeoaisnwtpeneelapc)cnhataonainvrweaedlicl)rytteeFh.raEeTisri-rtrhSiacecEyosMmw,rroeweirlmslhp-pihaacoolhgnilgeodpsngirineoeedgs-f sity (C/s)11..05xx110055 525O 1s530535540Zn (LMM)O 1sZn (LMM) Zn 2pO (KLL)3/2 Zn 2p1/2 Zn 2s
nanowires of samples A and B are shown in large-scale, n
e p
tehveerysilsiicnognlesnuabnsotwrairteesw, aasndseltfh-aelrigenwedaspneropebnednidcuinlagrotor Int5.0x104 Zn 3dZn 3Zn 3s C 1s Zn 2p3/2 Zn 2p1/2
interconnects between themselves. For the sample C,
0.0 1020 1040 1060
some of ZnO nanowires with small diameters with high
0 200 400 600 800 1000 1200
aspect ratios are too weak to be standing up, leading to
Binding Energy (eV)
a little inclined morphology. The detailed structural
parameters of samples A, B, and C are listed in Table 1.
TaFdrfteheneihrgsirdenepuinvierrtaftcehitatlsieimv4oivzeanecser)losay.aarg.nAcsreTodenwudshepelieudavlonlmornbdelauedesestutireiantllorttyhgstaniernnialobtegsrfuunedmtb1giaeaDstebdmhtcorsnhtaeuoattaateennrtr3hoeisl0soesm06atcr,tn,auh6dtf2ciio0c5otlr0kue,n,nnrmsaeaegnsnso(tdsshdrhehpsas2o8hsi5w0onobflnμnodeAmmegiifnnuy-,, (ensity (C/s)c)4681....0000xxxx111100004445 525O 1s530 535 540 Zn (LMM) O 1sZn (LMM) Zn 2pO (KLL)3/2 Zn 2p1/2 Zn 2s
emluetchidoadtsedanadlosnevgewrailthmtohdeelasdtvhaantcheamveenbteeonf pprroeppaorsaetdioinn Int 2.0x104 Zn 3dZn 3pZn 3s C 1s Zn 2p3/2 Zn 2p1/2
the previous study [22]. Here, the growth mechanism of
ZnO nanowire arrays using ZnS precursor was involved 0.0 1020 1040 1060
based on the conventional vapor-liquid-solid (VLS) 0 200 400 600 800 1000 1200
using metal catalyst as a starting material [23,24], and Binding Energy (eV)
the schematic diagram is shown in Figure 4b. The Figure2X-rayphotoelectronspectraof theas-preparedZnO
growth stages might be briefly described as follows. Au nanowirearrays.(a)sampleA,(b)sampleB,and(c)sampleC.The
insetsarethecorrespondingZn2pandO1sspectra.
film layers coated on Si substrates would crack to Au
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3(cid:3)(cid:645)m(cid:3) 6(cid:645)m 3(cid:3)(cid:645)m(cid:3)
3(cid:3)(cid:645)m(cid:3) 6(cid:3)(cid:645)m(cid:3) 3(cid:3)(cid:645)m(cid:3)
Figure 3 The tiltedandcross-sectional(in the corresponding belowpanel) FE-SEM imagesof (a) sample A,(b) sampleB,and(c)
sampleC.
nanoparticles with an elevated temperature and serve as Figure 4c shows the top-view SEM images of
catalyst, and it reacted with ZnS vapor to form Au-Zn-S Au-coated silicon substrates after annealing at 750°C for
alloy liquid droplets. The alloy liquid droplets could 30 min in the absence of source materials, but with the
absorb oxygen atoms, or react with oxygen atoms from other experimental conditions unchanged. The Au film
air to yield ZnO molecules. Consequently, the formation layer melted into separated nanoparticles with different
of ZnO may be expressed by the corresponding chemi- sizes evenly distributed on the surface of Si substrates,
cal reaction [24] which are about 200-300 nm in diameter for the sam-
ples A and B, but only about 40-50 nm for the sample
ZnS(g)+O2(g)↔ZnO(s)+SO2(g) (1) C. It illustrates that thicker Au film layer leads to larger
(c)
sample A sample B sample C
Figure4ThegrowthofZnOnanowirearrays.(a)Theschematicdiagramofexperimentalsetup,(b)growthmechanismofZnOnanowire
arrays,and(c)top-viewSEMimagesofAucatalystonSisubstratesafterannealingat750°Cfor30minintheabsenceofsourcematerials.The
AufilmthicknessesofsamplesA,BandCareabout50,50,and15Å,respectively.Thescalebarsareall1μm.
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Au nanoparticles during the initiated growth process, in be 5.3 eV, the estimated b of the samples A, B, and C
agreement with the previous study [14]. According to are 1209.5, 1566.7, and 1745.8, respectively. Based on
the VLS growth mechanism, the nanowire’s diameter is the above discussions, it can be seen that the sample C
defined by the Au nanoparticle’s diameter, which was has the best FE efficiency including the lowest E and
to
observed by the fact that the sample B with Au film the highest b.
layer about 50 Å has the nanowire with larger diameter Many former studies have demonstrated that FE per-
than that of the sample C coated with Au film of 15 Å. formance of ZnO nanostructured arrays can be signifi-
However, diameters of all these nanowires were cantly enhanced through either changing geometry
observed to be larger than the corresponding Au nano- configuration, achieving rational spatial distribution of
particle sizes because of the coarsening effect resulting the emitting centers, or increasing the aspect ratio
from the formation of a supersaturated Au-Zn-S alloy [13,14,30]. The relationship of b and aspect ratio l/r is
liquid droplets. However, the sample A was located proposed by an empirical model [31]
upstream, although it has the same Au nanoparticle size
formed during the initiated growth as sample B, the =b(l/r +h)0.9[1−exp(−as/l)] (4)
captured ZnS vapor would be less than that located in
the downstream, leading to insufficiency of zinc vapor where l, r, and s are the length, radius, and the inter-
so that the growth speed was decreased and the coar- spacing of ZnO nanowires, respectively; h is an alter-
sening effect would not be remarkable. able parameter which can be adjusted to fit the
experimental data. It is obvious that the field-enhance-
FE properties ment factor b can be decided by the aspect ratio and
The FE properties were measured on the three kinds of the interspacing of nanowires. The sample C has the
ZnO nanowire arrays with different structural para- nanowires up to 25 μm in length but only tens of
meters. They were measured one after the other under nanometres in diameter; the aspect ratio as high as
exactly the same conditions. Figure 5a, c, e depicts the 312.5 could explain for its excellent FE properties.
morphology-dependent emission current density J on However, the aspect ratios of the samples A and B are
the electric field E at cathode-anode distance of 300 μm 20 and 41.7, respectively, indicating that b is not line-
for all the measurements. For the sample C, the turn-on arly increasing with the aspect ratio, which could be
field (E ) defined as the electric field required for reach- attributed to the screening effect. From the experimen-
to
ing emission current density to 0.1 μA/cm2 (0.001 μA/ tal results, it can be observed that the Eto and b values
mm2) is 4.1 V/μm. It is lower than those of ZnO nanor- were all not proportional to their nanowire densities
ods (5.3 V/μm) [25] and ZnO nanotubes (7.0 V/μm) (revealed in Table 1), we could conclude that nanowire
[26], whereas for the samples A and B they are about density was not the essence in deciding the FE effi-
8.4 and 5.8 V/μm, respectively. The E values can be ciency of nanostructured arrays, and that it was indis-
to
clearly read from their corresponding insets. For further pensable to consider the aspect ratio including the tip
understanding of FE current-voltage characteristics, it is morphology and the relative void ratio.
demonstrated by the Fowler-Nordheim (F-N) equation
[27-29] Wetting behavior
Wettability was studied by examining water CA on the
J =(A2E2 /Φ)exp[−BΦ32(E)−1] (2) surfaces of three kinds of ZnO nanowire arrays. Photo-
graphs of water droplet on the three representative ZnO
films with different surface morphologies are shown in
ln(J /E2)=ln(A2 /Φ)−BΦ32 /E (3) Figure 6. The DI water droplets of about 5 μL were
placed on the surfaces, and the CAs of the samples A
where J and E are the current density, and the applied and B were measured to be about 142.1° and 94.8°,
electric field, respectively. F is the work function of respectively. However, nearly spherical droplet at the
emitting materials. A and B are constants with the microscopic level with a measured CA value as high as
values of 1.56 × 10-10 AeV/V2 and 6.83 × 103 eV-3/2/μm. 154.3° in average was obtained for the sample C, which
Figure 5b, d, f presents that the F-N lines are all have reveals the superhydrophobic properties. The surface
nearly linear relationship, indicating that the electron presents a stable character in air, with the CA showing
emission is indeed caused by a vacuum tunneling. b is no apparent change for up to 15 min, and the water
the field-enhancement factor defined as the ratio of the droplet eventually evaporates on the surface of the ZnO
local electric field at the tip of a nanowire to the macro- nanowire quasi-arrays without any obvious sinking into
scopic electric field, can be estimated from the slope of the film. To investigate their different wetting behaviors,
F-N plots. Assuming the work function of bulk ZnO to surface structure-induced transition may be crucial. The
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(a)(cid:3) (b)
0.05
-8
0.004
0.04 2m)
m0.002
2m)0.03 uA/ -12
m J(0.000 )
2
(uA/0.02 6E (V/u8m) 10 n(J/E
J0.01 l-16
0.00
-20
6 8 10 12 0.08 0.12 0.16 0.20
(c)(cid:3) E (V/um) (d) 1/E (um/V)
0.12
0.002
0.10
2m) -8
0.08 m0.001
2) A/
m u
uA/m 00..0046 J(0.000 4 5 6 2(J/E)-12
( n
J E (V/um) l-16
0.02
0.00 -20
4 6 8 0.12 0.18 0.24
E (V/um) 1/E(um/V)
(e)(cid:3) (f)
1.0
0.002
-4
0.8
2m)
0.001
m
2)0.6 A/ -8
m u
m J(
0.000 )
A/0.4 2E
u 2 3 4 J/ -12
J(0.2 E (V/um) ln(
-16
0.0
2 4 6 8 0.1 0.2 0.3 0.4
E (V/um)
1/E (um/V)
Figure5FEpropertiesof(a,b)sampleA,(c,d)sampleB,and(e,f)sampleC.Thecorrespondinginsetsarethemagnifiedpartsshowing
theE valuesclearly.
to
authors present the corresponding structural models greatly by the fractional areas of solid (f ) versus air
1
according to the three samples (shown in Figure 5, the pockets (f )
2
below panel), which clearly shows the different void
ratios induced by their different diameters and interspa- cos= f1cos1− f2, (f1+ f2 =1) (5)
cing of the aligned nanowires. Theoretically, a thorough
understanding of the superhydrophobic phenomenon Here, θ and θ are the corresponding water CAs on
1
can be obtained from the Cassie and Baxter equation rough and smooth surfaces. Evidently, the CA varies
[32], and the CA for a composite surface is influenced with the amount of air trapped within the voids among
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sample(cid:3)A(cid:3) sample(cid:3)B sample(cid:3)C
Figure6PhotographsofthemeasuredCAvaluesof(a)sampleA,(b)sampleB,and(c)sampleC.Structuralschematicsareshownin
belowpanel,respectively.
these nanowire arrays. The nanostructured films with to elucidate the physical essence of the FE performance
high void ratio would keep larger fraction of air trapped and wetting behavior related to the corresponding
within the voids and greatly increase the air/water inter- nanostructured arrays.
face, the effectively cause the increase of water CA. For
the samples A, B, and C, the void ratios are roughly cal-
Abbreviations
culated to be about 90.8, 83.9, and 97.9%, respectively,
CA:contactangle;DI:de-ionized;FE-SEM:fieldemissionscanningelectron
using the formula: h = (1 - Nπr2) × 100%, assuming microscope;F-N:Fowler-Nordheim;VLS:vapor-liquid-solid.
that those nanowires for each sample have the same
length and cylindrical shape. Here, N and r, respectively, Acknowledgements
ThisstudywassupportedbytheNaturalScienceFoundationofChina(Grant
represent the density (nanowires/μm2) and average Nos.10874115and10734020),theNationalMajorBasicResearchProjectof
radius of nanowires listed in Table 1. The results 2010CB933702,ShanghaiNanotechnologyResearchProjectof0952nm01900,
ShanghaiKeyBasicResearchProjectof08JC1411000,andtheResearchfund
demonstrated a qualitative analysis that larger void ratio
fortheDoctoralProgramofHigherEducationofChina.Theauthors
could play an effective approach to increase CA values sincerelythankProfessorD.PYuandProfessorQ.Zhao(theStateKey
for the three sample surfaces which are all ZnO nano- LaboratoryforMesoscopicPhysics,andElectronMicroscopyLaboratory,
SchoolofPhysics,PekingUniversity)fortheirhelpinFEmeasurements.
wire arrays with same preferential orientations in the
c-axis direction. However, decreasing the surface free Authordetails
energy by coating with low surface energy molecules is 1LaboratoryofCondensedMatterSpectroscopyandOpto-ElectronicPhysics,
DepartmentofPhysics,ShanghaiJiaoTongUniversity,Shanghai200240,
also greatly regarded as the other point to obtain super- People’sRepublicofChina.2KeylaboratoryofArtificialStructuresand
hydrophobic surfaces [33,34], even if the void ratio is QuantumControl(MinistryofEducation),DepartmentofPhysics,Shanghai
not large enough. The sliding behavior of the sample C JiaoTongUniversity,Shanghai200240,People’sRepublicofChina.3School
ofChemistry&ChemicalTechnology,ShanghaiJiaoTongUniversity,
was also performed by fixing the sample on the platform Shanghai200240,People’sRepublicofChina.
of OCA CA system, a 5-μL water droplet was dropped
on its surface and the system tilted until the water dro- Authors’contributions
LYparticipatedinthedesignofthestudy,carriedoutthetotalexperiment,
plet rolled off. Then a SA of 7.3° in average was
performedthestatisticalanalysisaswellasdraftedthemanuscript.MZ
obtained, showing super water-repellent properties. participatedinthedesignofthestudy,givedthetheoreticaland
These properties could be used for self-cleaning func- experimentalguidance,performedthestatisticalanalysis,andgavethe
correctionsofmanuscript.LMparticipatedinthedesignofexperimental
tions, antifog, or other fields.
sectionandsuppliedthehelpinexperiment.WLandMLmainlyhelpedto
carryoutthemeasurementofCAandslidingangles.WShelpedtoamend
Conclusions themanuscriptandtheanalysisofFEproperties.
Three kinds of large scale ZnO nanowire arrays with
Competinginterests
different aspect ratios and void ratios were fabricated Theauthorsdeclarethattheyhavenocompetinginterests.
using facile thermal evaporation route using ZnS source
Received:2August2010 Accepted:12January2011
materials. Experimental results demonstrated that ZnO
Published:12January2011
nanowire arrays with larger aspect ratio and proper den-
sity have better FE properties including lower turn-on References
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