Table Of ContentHernándezMainetetal.NanoscaleResearchLetters2012,7:80
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NANO EXPRESS Open Access
TiN nanoparticles: small size-selected fabrication
and their quantum size effect
Luis Carlos Hernández Mainet1,2*, Luis Ponce Cabrera1, Eugenio Rodriguez1, Abel Fundora Cruz2,
Guillermo Santana3, Jorge Luis Menchaca4 and Eduardo Pérez-Tijerina4
Abstract
Size-selected TiN nanoclusters in the range of 4 to 20 nm have been produced by an ionized cluster beam, which
combines a glow-discharge sputtering with an inert gas condensation technique. With this method, by controlling
the experimental conditions, it was possible to produce nanoparticles with a high control in size. The size
distribution of TiN nanoparticles was determined before deposition by mass spectroscopy and confirmed by
atomic force microscopy. The size distribution was also analyzed using a high-resolution transmission electron
micrograph. The photoluminescence [PL] spectra of TiN nanoparticles at different sizes were also experimentally
investigated. We reported, for the first time, the strong visible luminescence of TiN nanoparticles on Si (111) wafer
due to the reduced size. We also discussed the PL intensity as a function of the nanoparticle size distribution.
Introduction Metal nanoparticles are used in a broad spectrum of
Metal nanoparticles 1 to 100 nm in size and 102 to 108 applications such as in biomedicine [4], optoelectronics
atom aggregates (known as clusters) have demonstrated [5],solar cells [6], andanti-wear coatings[7]. Nowadays,
differentphysical-chemicalpropertiesfromtheirbulk.The theyareinvolvedinmanyproductsandappliedinseveral
reasonsforthesepropertiescanbeattributedtothelarge technologies. Most metal nanoparticles’ production pro-
portionofsurfaceatomsandquantumsizeeffect,whichis cesses require a precise control of narrow range size. In
causedbythereducedsizeinthreedimensions.Whenthe particular, especial conditions are necessary to produce
cluster is very small, the number of atoms at surfaces or verysmallsize-selectednanoparticlesatanindustrialscale
grainboundariesiscomparabletothenumberofatomsin [8].Fromatechnologicalviewpoint,metalclusterscanbe
thecrystallinelattice.Also,withthedecreaseofthecluster regardedastheprecursorstoanewgenerationofnanos-
size,theelectronicpropertiesstarttochange.Thiseffectis tructuredmaterialsanddevices.Thefabricationofnano-
called the ‘quantum confinement effect’, which can be particles by controlling theirsizeand shapeisone ofthe
observed as a shift in the optical bandgap or exciton challengingtasksfornanotechnology.
energy depending on the nanoparticle diameter. Several Among different techniques to obtain nanoparticles,
investigations have beencarried out tostudythe particle ionizedclusterbeamdeposition[ICBD]hasbeenreceiving
sizeeffectsontheirphysical-chemicalproperties.Atypical greatattentionduetoitscontrolofsize-selectednanopar-
exampleisthat themeltingtemperatureofnanoparticles ticles[9].Thenoveltechniquecombinesplasmasputtering
stronglydependsonthesizeandshapeandissubstantially and gas aggregation to produce nanoclusters from a few
lower than the bulk melting temperature [1-3]. For atomstoafewthousandatoms.Thegeneralsetupofthis
nanoscience,thestudyofsize-selectednanoclusterisvery system includes magnetronsputtering,aclusteraggrega-
importanttounderstandthefundamentalsofsolidphysics tionzone,amassfilter,andadepositionchamber.Usinga
in a nanometric scale because it is the bridge in the gap magnetron discharge, hot atoms are generated by Ar+
betweenindividualatomsandcondensedmatter. bombardmentonthetargetsurface.Theatomsarecooled
and condensed in a cold inert gas to create the clusters.
Theclustersizecanbecontrolledbyadjustingthesputter
*Correspondence:[email protected] yield, gas pressure, volume of the cluster growth region,
1LaboratoryofLaserTechnology,CICATA-IPN,Altamira,Tamaulipas,89600, and bias voltage. A mass filter located along the central
Mexico
axisof the system allows for selectionof the cluster size.
Fulllistofauthorinformationisavailableattheendofthearticle
©2012HernándezMainetetal;licenseeSpringer.ThisisanOpenAccessarticledistributedunderthetermsoftheCreativeCommons
AttributionLicense(http://creativecommons.org/licenses/by/2.0),whichpermitsunrestricteduse,distribution,andreproductionin
anymedium,providedtheoriginalworkisproperlycited.
HernándezMainetetal.NanoscaleResearchLetters2012,7:80 Page2of9
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Theclustersare acceleratedtowardthe substrate surface TiNclustersismeasuredbeforedepositionbymassspec-
byabiasvoltageapplication.Finally,clustersjointogether, trometry and after by transmission electron microscopy
whetherduringtheflighttothesubstrate oratthetarget [TEM]andAFMtechniques. Thecrystallinestructureof
surface,toformthenanoparticles. theTiNnanoparticlesisfurtherconfirmedbythemicro-
Titanium nitride [TiN] has been generally applied in graph fast Fourier transform [FFT] analysis. A statistical
industrial coatings with high demands on hardness and detailedanalysisoftheTiNnanoparticlesizedistribution
adhesion as well as high thermal stability and good con- asafunctionofbiasvoltageisperformedbyHRTEM.The
ductivity[10].Duetothisimportantfeature,TiNhasbeen PLspectraofTiNnanoparticlesatdifferentsizesarealso
widely used as a hard and protective coating for cutting investigatedexperimentally.Wereportedforthefirsttime,
tools or in electronic devices. Based on the properties of thestrongvisibleluminescenceofTiNnanoparticlesonSi
thebulkmaterials,TiNnanoparticlesarebeingusedasan (111)waferduetothereducedsize.
additive element in protective coatings to enhance the
adhesion properties [11] and a catalyst support material Experimental details
for noble metals for application in PEM fuel cells [12]. Cluster production and co-deposition
Several techniques, both chemical vapor deposition and A NanoSys 500 deposition system, built by Mantis
physicalvapordeposition,havebeenusedtodepositTiN Deposition Ltd (Thame, Oxon, UK), was used to pro-
coatings.However,theindustrialproductionofTiNnano- duce size-selected TiN nanoclusters [19]. Figure 1 shows
particles is still beginning to take its first steps. ICBD is a schematic diagram of the experimental setup. The fab-
onetechniquethatcanbeappliedtoproduce TiN nano- rication of noble metal nanoclusters using Nanosys 500
particles [13-16]. The investigation of TiN nanocluster has been described elsewhere by other authors [18]. The
depositionbyICBDcanhelptoimprovetheproductionof difference in producing TiN clusters is the use of a reac-
nanomaterials and to understand their physicochemical tive gas (N ) aggregation process.
2
propertiesatananoscale. Insidetheclustergenerationzone,theTiNclusterswere
The fabrication of nanoparticles is an elaborate proce- produced by sputtering a Ti target in an inert gas atmo-
dure. The following characterization is a complex but sphereofAr+andbyaddingdirectlyoverthesputtertar-
important task. Atomic force microscopy [AFM] and get a small flux of nitrogen gas (20 sccm). When the Ti
high-resolution transmission electron microscopy targetissputtered,theclustersareformedbymultiplecol-
[HRTEM] have demonstrated to be powerful techniques lisionsbetweenthehigh-densityparticles(sputteredparti-
to determinate the size of nanoparticles below 10 nm cles and also argon and nitrogen gasses). During cluster
[17,18].Inaddition,photoluminescence[PL]spectroscopy formation,thesputterchamberiskeptatalowtempera-
has emerged as an important tool for studying the lumi- turebyacoolantmixture(2°C),andthepressurewasset
nescence of nanoparticles. The origin of such lumines- at 1 × 10-4 Torr. The residence time within the aggrega-
cenceisoftenassociatedtoquantumconfinementeffects tion zone can be diversified by varying the length of the
inwhichthepositionofthePLenergypeakdependsfun- aggregationregionwiththelinearmotiondrive.Theclus-
damentallyonthenanoparticlesize. ter size can be controlled by varying the principal para-
In this paper, TiN nanoparticles with a narrow-sized meters: the sputter power, flow of gasses, and the
range are deposited on Si (111) substrates at room tem- aggregate zone length (variableusinga linear drive).The
perature by ICBD method. The size distribution of the clusters flow together with the argon gas through a
Figure1Schemeofthenanoclustersystemdeposition.
HernándezMainetetal.NanoscaleResearchLetters2012,7:80 Page3of9
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Figure2Massspectrumof(TiN)+ clusters.
n
Figure3SizedistributionofTiNnanoclustersrecordedbyaMesoQmassfilterat4.1,5.3,and7.1nm.
HernándezMainetetal.NanoscaleResearchLetters2012,7:80 Page4of9
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Figure4AFMimagesofTiNnanoparticles.(a)Aggregationofclustersduetocoagulationofneighboringclusters(timeofdepositionis360
s),(b)magnification,and(c)heightprofileof7.1nm.
variable orifice towards a mass filter. The mass distribu-
tion ismonitoredinsitu by a MesoQmassfilter(Mantis
Deposition LTD, Thame, Oxon, UK) before the deposi-
tion. This mass filter has been specifically added for the
purpose ofhigh-resolutionmeasurement and filteringof
nanoclustersbetween1×103and3×107amu.Atypical
massdistributionspectrumoftheobtainedTiN+clusters
is presented in Figure 2. The mean cluster diameter was
estimated from the cluster masses by using the specific
densityandthemolarvolumeoftheTiN.
Once thesize clustersare selected bythe MesoQmass
filter,theyareaccelerated byapplyinga biasvoltage (V )
b
toasubstrateinahighvacuumwithabasepressureof10-
8 Torr. The nanoparticles were deposited onto silicon
wafer.Thesubstrateswerecleanedinsuccessiveultrasonic
baths of acetone and isopropyl alcohol. The depositions
wereperformedatroomtemperaturewithoutanyheating
andwereapplieddifferentbiasvoltages(3and6kV).The
nanoparticlesizewascontrolledbyregulatingthemagne-
tron power, gas flow (Ar and N ), and aggregation zone
2
length.Theseparameterswerevariedtoproduceparticles
ofdifferentsizesontothesubstrate.
Characterization of TiN nanoparticles
Thesizedistributionandmorphologicalcharacterization
wereperformedbyAFManalysis,usinga(Veeco Instru-
ments Inc., Plainview, NY, USA) multimode scanning
probemicroscopeinhardtappingmode.Thesamplesur-
face was scanned at 1 Hz and within 1 μm2. The Image
Processing and Data Analysis software (Version 2.1.15)
by TM Microscope (Camarillo, CA, USA) was used for
imageanalysis.
For TEM characterizations, TiN nanoparticles were
grown on copper grids. Samples were characterized by
HRTEM using a 300-kV FEI Titan 80-300 STEM/TEM
microscope(FEICo.,Hillsboro,OR,USA).TheHRTEM
images were used to study the TiN crystalline structure
bymicrographFFTanalysis.Adetailedstatisticalanalysis Figure5AFMheighthistogramsforTiNnanoparticlesfiltered
of TiN nanoparticles after deposition was performed by byMesoQ.
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measuring several hundreds of nanoparticles using the
HRTEM micrographs. The size distribution, the nearest
neighbordistance(d ),andthecoveredareaonthesur-
NN
face were extracted from these calculations. The proce-
dure wascarried out by manually outliningthe particles
from several dozens of low- and high-resolution TEM
images. Once digitized and saved in the proper format,
the imagewasprocessedusingthe Gatan DigitalMicro-
graph and Mathematical software (Gatan, Inc., Pleasan-
ton,CA,USA).
PL studies were carried out at room temperature in a
conventionalPLsystem.AnHe-Cdlaser(l=325 nmat
16mW)wasemployedastheexcitationsource.Theout-
going radiation from the sample was focused on the
entrance slitofa 50-cmActonmonochromator(Prince-
ton Instruments, Trenton, NJ, USA). The detection was
carriedoutusingaPrincetonInstrumentphotomultiplier
tubetoaphotoncounter.Allthespectrawerecorrected
forthespectralresponseofthesystem.
Results
Size distribution of TiN nanoparticles
Figure3displaysthetypicalsizedistributionrecordedby
themassfilterforthreesamplescontainingnanoparticles
withdifferentdiameters.Themeannanoclusterdiameter
was obtained after fitting a Gaussian function to the
experimental data (Figure 3, solid lines). The figure dis-
playsthenanoclustershavinganarrowsizedistributionof
4.1,5.3,and7.1nmwithafullwidthathalfmaximumof
1.2, 1.4, and 2.1 nm, respectively. Moreover, a good
separationofpeaksisclearlyobserveddemonstratingthat
themassfiltercanresolvethenanoparticlediameterwith
a difference at approximately 1 nm. Hence, the filtering
processallowstheselectionofparticleswithahighresolu-
tion in size higher than 1 nm, which could be used in
either scientific research or technological development.
Varying the critical parameters on the system (gas flow,
partial gaspressure, magnetron power, aggregation zone
length), we were able to produce small nanoparticles at
different sizes: 4.1 ± 0.2, 5.3 ± 0.4, 6.2 ± 0.2, 7.1 ± 0.3,
14.9±0.7,and20.3±0.8nm.
After the deposition of TiN nanoparticles on Si (111)
wafer, the morphology and size distribution were ana-
lyzed using the AFM images. Figure 4 shows a typical
AFM image of TiN nanoparticles. From the figure, it is
possible to observe the aggregation of nanoparticles,
whichcouldbe explaineddue to clusterslandingon top
of each other before the layer is completed. For longer
deposition times, it is expected that even more clusters
get aggregated. The magnification in Figure 4a, b was
usedtomeasuretheheightprofilesoffournanoparticles Figure 6 HRTEM micrographs and the corresponding FFT of
TiNnanoparticlesgrownbyICBDtechnique.(a)7.1nm,(b)6.2
(Figure4c).TheheightshowsaroughlyGaussianprofile,
nm,and(c)4.1nm.Inset:thecorrespondingFFTshowingthe[111,
which allows to consider the nanoparticle as quasi-
200],and[220]directionsoftheTiNFCCstructure.
spherical.
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Figure7Biasvoltage,sizedistribution,andnearestneighbordistance.(a)HRTEMmicrographofTiNnanoparticlesgrownat3kVbias
voltage,(b)thedistributionofsize,and(c)thenearestneighbordistance.
AFMheighthistogramsforTiNnanoparticlesatdiffer- ThenanoparticlesizewasdirectlymeasuredbyHRTEM
entfilteredsizesareshowninFigure5.Thesehistograms micrograph.Figure6showsTiNnanoparticleswithsizes
shownarrowheightdistributionswithaveragesof15.5nm of7.1(Figure6a),6.2(Figure6b),and4.1nm(Figure6c),
(Figure 5a) and 23.0 nm (Figure 5b). Usually in an AFM wherethetwo-dimensionalatomarrangementandcircu-
measurement,thedistanceinthez-axisisdirectlyrelated lar shape can be observed. The nanoparticle size was
tothenanoparticlesize.However,inthexyplane,thedis- directly measured from the HRTEM micrographs using
tance is enlarged regarding the real spatial dimensions suitabletoolsoftheimageprocessingsoftware.Thecrys-
becausethewidthoftheAFMtipisdistortedbythecom- tallinestructureoftheTiNnanoparticleswasfurthercon-
bination of the nanoparticle shape and tip geometry. In firmed by FFT analysis (insert in the figures). The FFT
thiscase, the widths of height histogramsdonotrefer to showsadiffractionpatterncorrespondingtoplanesofthe
the real sizes of nanoparticles. Therefore, the results of TiN face-centered cubic [FCC] structure with a lattice
sizedistributionforbothmassfilterandAFMheightpro- parameter of4.2417 Ǻ [20]: (111) [d =2.529 Å],(200)
111
files support the effective mass filter to control the TiN [d =2.134Ǻ],and(220)[d =1.561Å].Accordingto
200 220
nanoparticlesize. the AFM height profiles and plan-view TEM image, we
Figure8Biasvoltage,sizedistribution,andnearestneighbordistance.(a)HRTEMmicrographofTiNnanoparticlesgrownat6kVbias
voltage,(b)thedistributionofsize,and(c)thenearestneighbordistance.
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canstronglyconsidertheshapeofnanoparticlesasquasi- from 20 to 15.5 nm can be observed. However, when
spherical. the diameter decreases down to 5.4 nm, a strong visible
The size distribution, cover surface, and nearest neigh- PL is clearly seen. The origin of this strong PL is only
bor distance were also statistically analyzed as a function due to the nanocluster size confinement because in
of the bias voltage. TiN nanoparticles produced at 3 and transition metals, the light emission is normally very
6 kV bias voltage on TEM grids are shown in Figures 7a weak due to ultrafast nonradiative decay and the
and 8a, respectively. The covered surface by the nano- absence of a bandgap [21]. The PL has been previously
particles at 3 kV is around 8.2%, while at 6 kV, they reported in another transition metal nanoparticle with a
cover only 2.1% of the surface. size below 10 nm [22]. However, this is the first report
Also, the figures display the corresponding size distri- in TiN nanoparticles.
butions (Figures 7b and 8b) and the nearest neighbor It can also be observed in Figure 10 that the strong PL
distances (Figures 7c and 8c). In Figure 7a, b, it can be is deconvoluted by three Gaussians, which correspond
seen that the nanoparticles have an average diameter of to the emission at different wavelengths. According to
5.7 nm and a distance between particles of 6.7 nm. the bulk band structure studies, these emissions could
When the bias voltage is increased at 6 kV, the average be related to the transition between L1 and Gamma1.
diameter and the d are enlarged to 6.7 nm and 22.9 This is because the emission energies are close to the
NN
nm, respectively. The increase of the nanoparticle size difference of energy band (2.3 ± 0.3 eV) [23]. In this
and the nearest neighbor distance when the bias voltage case, we considered that the observation of three emis-
is raised can be explained by the coalescence of two or sions is due to the size distribution on the surface
more nanoparticles, as shown in Figure 9. The bias (observed in AFM and HRTEM). A first attempt to
polarization increases the cluster kinetic energy towards explain this phenomenon is shown in Figure 11.
the substrate and enhances the nanoparticles’ mobility A fitting equation (Figure 11, inset) and the HRTEM
on the surface, allowing that one nanoparticle could diameter distribution of TiN nanoparticles at 3 kV were
reach other nanoparticles. used to describe the PL intensity. It can be noticed that
energy distribution, as a function of nanoparticle size, is
Quantum confinement effect of the reduced size of TiN in an agreement with PL peak position. The slight dif-
nanoparticles ference (2.5 eV) of both the theoretical energy band and
Figure 10 displays the spectral PL of TiN nanoparticles the PL peaks are due to the quantum confinement
at different sizes: 20, 15.5, and 5.4 nm. In the figure, the effect, where the energy band is enlarged when the
increase of the PL intensity when the diameter decreases nanoparticle size becomes smaller (for spherical parti-
cles) [24,25]. Thus, in TiN, the PL peaks at a higher
energy displacement with the nanoparticle size. For lar-
ger sizes, the energy displacement is less, (peaks or
shoulders at approximately 2.6 eV) while for smaller
sizes, the energy displacement is greater (peaks at
approximately 3.0 eV).
Conclusions
Size-selected TiN nanoparticles were produced on Si
(111) substrates at room temperature by ICBD method.
The critical parameters of the system were tuned to
obtain small nanoparticles at different size: 4.1 ± 0.2, 5.3
± 0.4, 6.2 ± 0.2, 7.1 ± 0.3, 14.9 ± 0.7, and 20.3 ± 0.8 nm.
The nanoparticle size was controlled during the produc-
tion process by a mass filter with a high resolution.
After deposition, the size distribution was statistically
analyzed using AFM images and was in excellent agree-
ment with the filter mass.
The TiN nanoparticle size was directly measured by
HRTEM micrograph, and the crystalline structure was
confirmed by the FFT patterns. The nanoparticle shape
Figure 9 HRTEM micrograph displays a nanoparticle (red was considered as quasi-spherical according to the AFM
contour)conformedbynanoparticleswithdifferentsizes height profiles and plan-view TEM image. The size dis-
(whitecontour).
tribution, cover surface, and nearest neighbor distance
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Figure10PhotoluminescencespectraofTiNnanoparticleswithdiametersof20,15.5,and5.4nm.
Figure11EnergydistributionbyfittingequationofthePLspectrum.
HernándezMainetetal.NanoscaleResearchLetters2012,7:80 Page9of9
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