Epitaxial Growth of Hexagonal Boron Nitride on Ir(111)

Published: December 05, 2011

r 2011 American Chemical Society 157 dx.doi.org/10.1021/jp207571n | J. Phys. Chem. C 2012, 116, 157–164

ARTICLE

pubs.acs.org/JPCC

Epitaxial Growth of Hexagonal Boron Nitride on Ir(111)Fabrizio Orlando,†,‡ Rosanna Larciprete,§ Paolo Lacovig,|| Ilan Boscarato,^ Alessandro Baraldi,†,‡ andSilvano Lizzit*,||

†Physics Department and Center of Excellence for Nanostructured Materials, University of Trieste, Via A. Valerio 2, 34127 Trieste, Italy‡IOM-CNR, Laboratorio TASC, AREA Science Park, S.S. 14 Km 163.5, 34149 Trieste, Italy§CNR-ISC, Via Fosso del Cavaliere 100, 00133 Roma, Italy

)Sincrotrone Trieste S.C.p.A., AREA Science Park, S.S. 14 Km 163.5, 34149 Trieste, Italy^Department of Chemical Sciences, University of Trieste, Via A. Valerio 2, 34127 Trieste, Italy

’ INTRODUCTION

Ultrathin films grown on metal surfaces have attracted con-siderable attention because of their potential application in nano-technology. One of the main interests is represented by thepossibility of using these systems as templates for arrangingmolecules in a controlled ordered fashion,1,2 therefore openingthe door to the engineering of large-scale nanodevices.3 In thiscontext, the single layer of h-BN is considered a promisingcandidate, especially after the discovery by Corso et al.4 ofthe formation of a self-assembled nanostructure, the so-callednanomesh, on the Rh(111) surface. The interest in h-BN ismotivated by its appealing properties, such as the excellentthermal stability4 and, in particular, the large band gap5 whichmakes it an insulator isostructural to graphene,6 the new challen-ging material for future nanoelectronics. Recently, it has beenshown that using h-BN as a substrate for graphene electronicsleads to an improvement of the device performances in termsof charge carrier mobility.7

Among the several ways to synthesize a single layer of h-BN,the most common method used to obtain high-quality films is bychemical vapor deposition (CVD) of a molecular precursor, e.g.,benzene-like B3N3H6, at transition metal (TM) surfaces. Most ofthese studies concern the growth on substrates with the same C3v

symmetry as the h-BN, i.e., (111) and (0001) hexagonal sur-faces of face-centered cubic (fcc) and hexagonal closed-packed(hcp) crystals, respectively. Important examples are Ni(111),8�11

Pt(111),8,12 Pd(111),8,13 Ru(0001),14 andRh(111).4,15 In addition,

there are several investigations on h-BN monolayers grown ontopofmoreopen surfaces, such asNi(110),16Cr(110),17 Pd(110),18

and Mo(110).19

Most of the research focuses on the characterization of themorphology and electronic structure of the resulting h-BN films,but little is known on the processes that lead chemisorbedborazine molecules to form an extended and long-range orderedh-BN layer. The analysis of the island boundaries onNi(111)20,21

and Rh(111)15 suggests that the B�N bond breaks during theself-assembly process. However, up to date only a few investiga-tions report on the adsorption and interaction of borazine withmetallic surfaces.22�25 Depending on the substrate, two adsorp-tion geometries were identified: borazine bound with the mole-cule ring either perpendicular (Pt(111)22,23) or parallel to thesubstrate (Au(111)23 and Ru(0001)24). This behavior resemblesthat of benzene (C6H6) adsorption on TMs, although the C6H6

molecules adsorb prevalently in flat geometry.26�29 An exhaus-tive picture of the chemisorption properties of borazine onmetallic substrates is therefore still missing, motivating furtherstudies on this topic.

In the present article, we report on the formation of theh-BN layer on Ir(111) focusing on the low-temperatureadsorption and the dissociation of borazine molecules up to

Received: August 7, 2011Revised: November 17, 2011

ABSTRACT: The formation of a hexagonal boron nitride(h-BN) layer through dissociation of borazine (B3N3H6) mol-ecules on Ir(111) has been investigated by a combination ofX-ray photoelectron spectroscopy, near-edge X-ray absorptionfine structure, temperature-programmed desorption, and low-energy electron diffraction. At low temperature (T = 170 K),molecular borazine adsorption occurs with the plane of thebenzene-like ring parallel to the substrate. Dehydrogenation isobserved at temperatures higher than 250 K and extends up to900 K, with a maximum H2 desorption rate around 300 K.Besides dehydrogenation, room temperature adsorption of borazine leads to the formation of atomic and molecular fragments dueto the break-up of part of the BN bonds. The epitaxial growth of h-BN starts at temperature higher than 1000 K where an extendedand long-range ordered layer is obtained. The presence of a corrugation in the h-BN layer withmoir�e periodicity of (13� 13)/(12� 12)BN/Ir unit cell is reflected in the double component structure of the B 1s and N 1s core level spectra.

158 dx.doi.org/10.1021/jp207571n |J. Phys. Chem. C 2012, 116, 157–164

The Journal of Physical Chemistry C ARTICLE

the completion of the h-BN single layer. Using in situ high-energy resolution photoelectron spectroscopy, in combinationwith complementary surface science techniques, we find that atlow temperature borazine molecules adsorb intact on Ir(111)with the molecule ring parallel to the surface. Molecular dis-sociation occurs near room temperature, with dehydrogenationaccompained also by partial cracking of the benzene-like ring, aprocess which precedes the formation of the BN layer.

’EXPERIMENTAL SECTION

All measurements were performed at the SuperESCA beam-line30,31 of Elettra, the synchrotron radiation facility in Trieste,Italy. The ultrahigh vacuum system, with a background pressurebetter than 2 � 10�10 mbar, is equipped with a sputter ion gunfor sample cleaning, a mass spectrometer, low-energy electrondiffraction (LEED) optics, and a Phoibos hemispherical electronenergy analyzer (150 mm mean radius) with an in-house devel-oped delay line detector.

The Ir(111) substrate was cleaned by repeated cycles of Ar+

sputtering (T = 300 K, p = 2� 10�6 mbar) and annealings in O2

(p = 5� 10�7 mbar) between 500 and 1070 K, followed by a finalflash annealing to 1400 K to remove residual oxygen. The samplecleanliness and ordering were verified by monitoring the pre-sence of eventual contaminants, such as C or O, and by checkingthe quality of the (1 � 1) LEED pattern, which exhibited sharpspots and a low background.

For the preparation of the h-BN layer, we used thermal decom-position of benzene-like borazine, which was synthesized follow-ing the procedure described by Wideman et al.32 Borazine wasstored below 250 K at all times to avoid degradation. Prior todeposition, borazine was regularly outgassed by freeze�thawcycles to pump off the residual vapor in the frozen state. Coveragesare reported in monolayers (ML), where 1 ML corresponds to thesurface density of the Ir atoms in the Ir(111) surface, i.e., to 1.57 �1015 atoms cm�2.

High-energy resolution fast X-ray photoelectron spectros-copy (XPS) Ir 4f7/2, B 1s, and N 1s core-level spectra werecollected using photon energies of 130, 284, and 500 eV, respec-tively, with an overall energy resolution (electron energy analyzer andX-ray monochromator) ranging from 40 to 100 meV. The surfacenormal, the incident beam direction, and the electron emissiondirection are all in the same horizontal plane, with the angle betweenthe photon beamand the electron energy analyzer fixed at 70�. All thebinding energies (BEs) presented in this work are referenced to theFermi level. The core level spectra were fitted using Doniach��Sunji�c(DS)33 functions convoluted with a Gaussian. In addition, a linearbackground was included in the fit. The lineshape parameters are theLorentzian width (Γ), the Anderson singularity index (α), and theGaussian width (G).

Near-edge X-ray absorption fine structure (NEXAFS) spectrawere acquired at the B and N K-edge in Auger yield mode atphoton incidence angles of θph = 0� and 70� with respect to thesample surface normal. In this case the spectra were normalizedto the incident beam intensity, monitored with a gold meshintercepting the beam.

Temperature-programmed desorption (TPD) spectra wereacquired in the range 170�1150 K, using a heating rate of∼3 K s�1. During desorption experiments, the sample was placedin front of the mass spectrometer equipped with a Feulner cup34

to enhance the surface signal. Quoted borazine exposures are givenin units of Langmuir (1 L = 1 � 10�6 Torr s�1).

’RESULTS

Low-Temperature Borazine Adsorption. Figure 1(a) showstime-lapsed B 1s spectra acquired during exposure of the Irsubstrate to B3N3H6 at 170 K. Up to ∼1 L the sequence ofspectra displays only one component (Bmol) at 189.7 eV. Athigher dose, Bmol diminishes in intensity and shifts (∼0.2 eV)progressively toward lower BEs. In parallel, a new component(Bm) emerges at ∼190.3 eV. This new feature grows continu-ously with increasing borazine exposure indicating the formationof a condensed multilayer structure, in agreement with theassigment made for analogous systems, e.g., benzene adsorbedon Pd(111).35 The integrated peak intensities of the twopopulations as obtained by the fitting procedure are shown inFigure 1(b). The lineshape of the two components has been keptfixed for each spectrum of the uptake sequence. This fittingprocedure gave low residual modulation, proving the existence ofno more than two nonequivalent molecular species. The con-tinuous shift of the Bm component (∼0.6 eV) toward high BEswith increasing borazine exposure most likely arises from finalstate effects.36,37 In particular, if compared to the first-layerchemisorbed molecules, the core-hole screening by the metalsurface becomes less efficient as the thickness of the multilayerfilm increases.N and B K-edge absorption spectra for low B3N3H6 exposure

(1.1 L) and different θph are reported in Figure 2(a). The NK-edge spectra show four π-related sharp resonances, A1, A2, A3,and A4 at 398.2, 399.3, 400.7, and 402.0 eV, respectively. In thephoton energy range 405�415 eV, instead, two σ-related broadpeaks B and C can be observed. By comparison with borazineelectronic structure,38 A1 and A3 resonances are assigned totransitions from theN 1s orbital toπ*(e00) andπ*(a002)molecularorbitals. These features show a pronounced angular dependence,being clearly visible at grazing incidence (black curve, θph = 70�)and almost completely absent at normal incidence (red curve,θph = 0�). Therefore, these peaks are related to molecules thatadsorb with the plane of the hexagonal ring parallel to the surface.A2 and A4 are assigned to π*(e00) and π*(a002) transitions too.However, these π*-resonances can be observed at both incidenceangles, indicating that some molecules are tilted away from thesurface. These assignments are supported by the 3 eV shift

Figure 1. (a) Selection of B 1s fast-XPS core level spectra of the B3N3H6

uptake on Ir(111) at 170 K together with the fitting components (Bmol,molecular borazine; Bm, multilayer structure). (b) Evolution of the intensityof theB1s components as a functionof the borazine exposure. The intensitiescorresponding to each spectrum are normalized to the saturation value.



between π*(e00) and π*(a002) resonances, which is quite close tothe value reported for gas-phase B3N3H6 (3.3 eV).38 Theinterpretation of the B K-edge spectra is in line with this analysis.Also for the B resonances it is possible to single out a polariza-tion-dependent (A1 and A3) and a polarization-independent(A2) contribution of π* symmetry. However, the B K-edgeresonances are quite broad, and the exact peak positions arenot well-defined, except for the A2 and the A3 componentslocated at 190.5 and 191.5 eV, respectively. At exposures of 1.1 L,the low coverage adsorption state reaches saturation, while themultilayer state only starts to be populated (Figure 1). Hence, weattribute the more intense A1 and A3 resonances of the flat-lyingborazine to the Bmol component and associate the tilted geome-try to Bm.NEXAFS spectra of the borazine-saturated (3.3 L) Ir surface

are reported in Figure 2(b). A quite similar behavior is foundboth for N and B K-edges. As compared to the low B3N3H6 dose,in this case the A2 resonance develops into a strong componentdetected at both incidence angles. In addition, A1 appears only atgrazing incidence. These findings suggest a preferential growth of

the tilted population, in accordance with the development of themultilayer structure. The close similarity between the gas-phaseborazine EELS38 and our multilayer NEXAFS spectra can beexplained considering that the disorder of the multilayer is quitesimilar to the random molecular orientation in the gas phase.This further supports our assignment of π* transitions to differentadsorption geometries.From Molecular Adsorption to Hexagonal BN. Figure 3

shows the B 1s spectrum after 25 L of borazine exposure at T =330 K. The spectrum can be decomposed into three contribu-tions: B0, Bmol, and Bad, at BEs of 190.5, 189.6, and 188.6 eV,respectively. The inset in Figure 3 displays the intensity evolutionof these components as a function of borazine exposure. Thelineshapes of the three peaks were fixed during the fitting analysisof the whole sequence. At the beginning of the uptake, only Bmolis observed, while the other two components rise at slightlyhigher exposures (above 0.3 L). It is interesting to note that B0and Bad start their growth simultaneously, although with differentrates, thus suggesting a common origin. At saturation, above 2 L,these species are 52% (B0) and 32% (Bad) of the concentration ofthe Bmol peak. A possible attribution of B0 and Bad to nonequiva-lent B atomswithin the borazinemolecule appears unlikely, becausethe ratio B0/Bad is not constant during the uptake. On the otherhand, the high intensity of these components and the expectedlow density of surface defects rule out also the possibility of pre-ferential adsorption at defective sites. Moreover, induced photo-dissociation has to be excluded since the photoemission spectrado not undergo modification with time when the surface is ex-posed to the photon beam. Hence, the existence of multiplepeaks at 330 K in the B 1s spectra suggests that at this tem-perature a fraction of borazine molecules decomposes. The BE ofthe Bmol peak is close to the value reported in the previous sectionfor borazine adsorption at low temperature. This contribution istherefore assigned to borazine molecules which adsorb intact onIr(111). Conversely, molecular dissociation leads to the appear-ance of B0 and Bad. It is interesting to note that (i) afterdeposition at room temperature the intensity of the Bad compo-nent is approximately 17% of the total B 1s signal (Figure 3) andthat (ii) by heating the system to obtain a h-BN layer (as will beexplained later on) the photoemission intensity decreases by

Figure 2. N and B K-edge NEXAFS spectra for the Ir(111) exposed to(a) 1.1 L, thinner curves, and (b) 3.3 L, thicker curves, B3N3H6 at 170 K.

Figure 3. B 1s spectrum obtained after B3N3H6 saturation at T = 330 K(25 L) together with the fitting components (Bad, atomic boron; Bmol,molecular borazine; B0, molecular fragments). The inset shows theuptake curve for the three components, normalized to the saturationvalue, as obtained by fitting the B 1s spectra. Note that the saturation isreached already at about 1.8 L.



almost the same amount, the drop being of about 15%. There-fore, this reduction can be reasonably explained in terms of areduced amount of boron atoms on the surface. In addition, theBad peak location is close to the value of 188.4 eV reported fordiluted boron in Rh(111).39 From the above considerations, it istherefore realistic to explain the Bad component as due to singleboron atoms and B0 as due to molecular fragments.To confirm that B3N3H6 dissociation occurs on the sample

surface, we performed a series of TPD experiments on theborazine-saturated surface at low temperature. Figure 4 showsthe resulting H2 thermal desorption spectra (m/e = 2) from the Irsurface exposed to 11 L of B3N3H6 at 170 K. This spectrumshows a broad desorption feature resulting from dissociation ofthe adsorbed B3N3H6 layer. The dehydrogenation starts alreadyat 250 K and goes on over a broad temperature range up to 900 K,with a maximum desorption rate at about 330 K. These findingsare similar to those obtained on B3N3H6/Pt(111) by Simonsonet al.,23 reporting a broad H2 desorption peak with a maximum at285 K. In addition, the temperature of 330 K falls within thetemperature range of H2 desorption from the Ir(111) surfaceexposed to hydrogen, which lies between 290 and 370 Kdepending on the initial coverage.40 Therefore, our desorptiondata are compatible with a borazine dehydrogenation processfollowed by a second-order recombinative H2 desorption.Figure 5 shows the sequence of B 1s and Ir 4f7/2 spectra mea-

sured at room temperature after annealing the borazine-saturatedsurface to increasing temperatures, ranging from 470 to 1370 K.B0 grows with temperature and moves toward lower BEs, whileBmol and Bad intensity gets reduced. Upon heating to 620 K, anew component (B1, from spectrum (3)) at∼190.8 eV has to beincluded in the analysis to achieve a low residual modulation. Theintensity of this component increases progressively up to theannealing temperature of 1120 K (spectrum (5)). Between 1120and 1370 K, a complete suppression of Bmol and Bad is observed,while the remaining peaks (B0 and B1) shift toward lower BE by∼0.3 eV. At this point, a h-BN layer is formed, as confirmed by B0and B1 peak positions and lineshapes which are quite close tothose corresponding to the best h-BN layer obtained in thepresent work (see next section). The analysis of the integral areaof the B 1s core level region indicates a drop of about 15% goingfrom spectrum (1) to (6), as mentioned above. According to thisresult, for the same sequence of spectra the signal from the N 1sregion diminishes by about 20% (not shown here). This reduc-tion can be attributed to a decrease of the B and N species onthe surface, although photoelectron diffraction effects cannot be

excluded. In the former case, the diffusion of atomic boron intothe bulk and/or desorption of NHx or N2 species

41 seem to bethe most plausible processes.Ir core level spectra for the same heating sequence are shown

on the right side of Figure 5. As previously reported,42 the Ir 4f7/2spectrum of the clean surface exhibits two well-resolved compo-nents: a low BE peak centered at 60.28 eV, assigned to photo-emission from Ir atoms of the topmost layer (Irs), and a high BEfeature at 60.83 eV due to atoms in the deeper layers, i.e., bulkatoms (Irb). The two-component analysis gives best-fit values of160 (360) meV for the Lorentzian width, 0.16 (0.21) for theasymmetry parameter, and 230 (120) meV for the Gaussianwidth of the bulk (surface) component. The lineshape para-meters and the measured surface core level shift (SCLS), i.e., theBE difference between the surface and the bulk peak, of �540meV are in good agreement with the previous experimental42 andtheoretical findings.43 Upon borazine deposition, a new broadfeature (Iri) grows between the surface and the bulk componentsat 60.61 eV, paralleled by a drop of the Irs peak intensity and aslight reduction of its SCLS to �505 meV (spectrum (1) inFigure 5). For the adsorbate-induced component Iri, the samelineshape parameters of the clean surface component were used.Upon increasing the annealing temperature, Iri continuously de-creases, while Irs almost recovers the intensity of the clean surface

Figure 4. Background subtracted thermal desorption spectra obtainedduring heating (3 K s�1) of a B3N3H6 saturated (11 L at 170 K) surface.H2 desorbs between 250 and 900 K, with a maximum desorption rateat 330 K.

Figure 5. High-energy resolution B 1s and Ir 4f7/2 core level spectra forthe clean surface (bottom) and B3N3H6 saturated (25 L, T = 330 K) Irsurface (1) and after fast annealing to the listed temperatures (2�6).The different spectral contributions as extracted from the peak fitanalysis are represented by the colored peaks. All curves are plottedafter linear background removal.



and Irs moves toward lower BE eventually reaching a SCLS of about�535 meV, which is very close to the value of the clean Ir(111). Atthe same time, as expected, the BE position of the bulk peak re-mains fixed.Single Hexagonal BN Layer. The growth of a single h-BN

layer was achieved by borazine on the Ir(111) surface at a sampletemperature of 1170 K. Exposures above 20 L were needed togrow an extended and long-range ordered layer. Panel (a) inFigure 6 shows the LEED pattern from h-BN/Ir(111): first-ordersubstrate spots are surrounded by a six-fold arrangement ofdiffraction spots, indicating the formation of a coincidence latticebetween the h-BN layer and the Ir(111) substrate (for a sche-matic drawing of themoir�e superlattice see for instance Figure 3Ain ref 4). The line profile analysis (panel (b)) reveals a super-structure with a periodicity of (13� 13) BN units (a= 0.250 nm)on top of (12 � 12) Ir unit cells (a = 0.272 nm).The high-resolution N 1s, B 1s, and Ir 4f7/2 core level spectra

of h-BN/Ir(111) are shown in panel (c). The Ir spectrum relativeto the h-BN layer obtained by dosing borazine at high tempera-ture is similar to the onemeasured for the annealed surface previously

saturated with borazine at room temperature. In fact, the Ir spectrumin Figure 6 displays the same three components: Irb, Irs, and Iri. How-ever, the intensity of the adsorbate-induced component Iri is higher inthis case, as can be judged by comparing the analogous peak of spec-trum (6) in Figure 5.The N 1s and B 1s photoemission spectra display a quite similar

double peak structure: both signals are dominated by awell-resolvedmain component located at 397.52 eV (N0) and 189.84 eV (B0). Inaddition, a second weak contribution emerges at 398.62 eV (N1)and 190.61 eV (B1). To shed light on the origin of the spectralfeatures, we monitored in situ the evolution of N 1s and B 1s corelevel regions during borazine uptake at 1170 K.Figure 7 shows the B 1s XPS spectra in panel (a), while panel

(b) displays the intensity behavior of the two components. Thespectral contributions grow together with a constant B1/B0 ratioof about 0.10. Similar results have been obtained for the Ncomponents (not shown here). The fact that the ratio betweenthe two components is constant during the growth processsuggests that B1 and N1 are not related to layer defects. On thebasis of these findings and according to ref 44, the spectralcontributions are attributed to h-BN regions differently interact-ing with the Ir substrate: N0 and B0 are associated to the fractionof the BN film weakly bounded to the substrate atoms, repre-sented by Irs, while N1 and B1 are attributed to h-BN regionsstrongly interacting with the Ir surface, represented by Iri.A quantitative interpretation of the overlayer morphology

cannot be directly extracted from XPS spectra presented inFigure 6 and Figure 7 since the intensity of the photoemissionpeaks can be strongly affected by diffraction effects. To properlyevaluate the intensity ratios N1/N0 and B1/B0, we thus followedthe procedure proposed in ref 44, measuring the photoemissionspectra at different photon energies, corresponding to a kineticenergy range of 40�200 eV. The average value ofN1/N0 andB1/B0obtained in this way attests at 0.17, which well compares to therelative weight of the iridium adsorbate-induced feature withrespect to the total surface signal, Iri/(Iri + Irs), i.e., 0.16. Similartrends were found also for the corrugated h-BN layer grown ontop of Rh(111) and Ru(0001) surfaces.44 This result is in accor-dance with the overall assignment of the different components: N0,

Figure 6. (a) LEED pattern of the h-BN/Ir(111) taken at E = 77 eV. Irspots are surrounded by the h-BN spots; the satellite spots reflect theperiodicity of the moir�e. (b) Line profile of the diffraction pattern alongthe (01) direction. Tomeasure this profile, the sample has beenmoved afew degrees off normal. (c) Ir 4f7/2, B 1s, and N 1s core level spectratogether with the spectral contributions resulting from the peak-fitanalysis of h-BN/Ir(111) resulting from dosing B3N3H6 at T = 1170 K.N 1s fitting parameters (Γ, α,G): N0 and N1 (100 meV, 0.08, 800 meV).B 1s fitting parameters (Γ, α, G): B0 and B1 (100 meV, 0.11, 760 meV).

Figure 7. B3N3H6 deposition on Ir(111) at 1270 K. (a) Selection ofhigh-energy resolution B 1s spectra measured during the uptake. (b)Evolution of the integrated intensities for the two spectral contributionsresulting from fitting of the B 1s spectra. The upper part shows the B1/B0ratio, the initial fluctuations are due to the poor signal-to-noise ratio atlow coverage.



B0, and Irs stem from the atoms in the low-interacting regions whileN1, B1, and Iri to the atoms residing in the regions where the h-BNlayer strongly interacts with the Ir substrate.The geometry of the BN lattice on the Ir surface was examined

by recording the N K-edge spectra at two photon incidenceangles. As can be seen in Figure 8, the marked angular depen-dence allows us to distinguish between π* resonances (A0, A, andA00) and σ* features (B and C). The complete absence of π*components at normal incidence indicates that the h-BN lattice islying flat on the substrate. According to the work by Preobra-jenski et al. for h-BN growth on selected metal surfaces,44 A0 andA00 resonances are attributed to h-BN regions significantlyinteracting with the Ir atoms. Both these resonances occur dueto the orbital mixing between the Ir d orbitals and the h-BN πstates. In particular,A0 can be assigned to excitations to adsorption-induced gap states of h-BN,45 while A00 relates to excitations tointerlayer conduction-band states.9 On the other hand, the πresonance A is detected also in the bulk h-BN NEXAFSspectra9,44 and is thus related to the h-BN site weakly bound tothe metal substrate.

’DISCUSSION

Two different geometries were identified for low temperatureborazine adsorption on clean Ir(111): a flat geometry, assignedto the Bmol component, and a tilted configuration, attributed tothe Bm peak (Figure 1). Because of the fundamental role playedby the availability of space in the molecular adsorption mechan-ism, an estimate of the maximum saturation coverage is in order.From the LEED analysis the B andN coverages for the best h-BNlayer resulted to be 1.17 ML. The intensity of the B 1s core levelmeasured on this surface can be used to evaluate the boroncoverage at low temperature. This method results in a B coverageof 0.6 ML at the saturation limit of the Bmol component, reachedat 1.1 L (Figure 1), i.e., to a molecular coverage of about 0.2 ML.Considering the borazine molecule as a regular hexagon, wecalculated a van der Waals area of 32.74 Å2 46 corresponding to asurface density of 3.05� 1014 molecules cm�2. Accordingly, thetheoretical borazine saturation coverage is 0.19 ML. This valuemust be considered only as a crude approximation of themaximum packing since it does not take into account photo-electron diffraction effects, that could modulate the photoemis-sion intensity, or the adsorbate�substrate registry. Nevertheless,the good agreement between theoretical and experimental values

indicates that: (i) the 1.1 L exposed Ir(111) is completely satura-ted with an extended close-packed parallel molecular configura-tion and (ii) the saturation coverage is determined by borazinelateral interactions. We propose therefore that the most favorableconfiguration for the low-temperature adsorption is with themolecular plane, defined by the hexagonal ring, parallel orientedto the Ir substrate (Bmol component in XPS spectra) and boundto it via π bonds. The subsequent surface crowding leads to theoccupancy of all these sites and, above 0.20 ML, inhibits addi-tional planar adsorption, forcing the molecules to anchor in tiltedconfigurations and, eventually, to form a condensed structure ontop of the first chemisorbed layer (Bm). Under similar growthconditions, the borazine molecule was proposed to lie flat onRu(0001),24 Re(0001),25 and Au(111)23 substrates. On thecontrary, in previous studies based on vibrational spectroscopya vertical bonding geometry on Pt(111) was also reported.22�24

The XPS results clearly indicate that B3N3H6 moleculesundergo dissociation on Ir(111) at T = 330 K. However, thepeaks corresponding to atomic boron and molecular fragmentscan be detected only for exposures higher than 0.3 L, when theslope of the Bmol intensity curve decreases (inset in Figure 3).These findings indicate that borazine decomposition does notoccur in the initial stage of the adsorption; i.e., the first adsorbedmolecules keep their benzene-like ring intact, giving rise to theBmol component. Borazine dissociation takes place only above0.3 L, leaving dehydrogenated molecules and molecular frag-ments (B0) and boron adatoms (Bad) on the surface. A tempera-ture as high as 930 K was suggested for the B�N bond breakingprocess taking place on Rh(111).15 Here, instead, the presence ofatomic boron indicates that the borazine ring cracks already at330 K. This value compares to the detection of atomic B and Nspecies by Auger electron spectroscopy of the Re(0001) surfacedosed with borazine and annealed to 570 K.25 Increasing thesubstrate temperature to 470 K results in further moleculardissociation, as depicted by the intensity drop of Bmol and theincrease of B0 components (Figure 5). This process results also ina decrease of Bad, indicating a depletion of boron adatoms. Giventhis information, and the fact that the overall B 1s signal isreduced by 15% at the end of the annealing cycle, we suggest thatthe boron atoms undergo bulk diffusion. A similar behavior hasbeen previously observed for boron adsorbed on Pd(111)47 andNi(100).48 Above T = 620 K, this trend in the intensity evolutionof B0, Bmol, and Bad continues, and in addition, a new feature (B1)grows. Our findings can be explained by comparing the h-BNgrowth process on Rh(111).15 By using scanning tunneling micro-scopy, Dong et al. evidenced that a first reorganization intonanometer-scale clusters takes place at T = 690 K, while theh-BN structure emerges already belowT = 900K. According to thispicture, we intepret the observed changes in our XPS spectra asthe earliest rearrangement of dehydrogenatedmolecules and frag-ments to form islands with h-BN morphology. These islands arealready characterized by regions differently interacting with the Irsurface, i.e., boron and nitrogen atoms strongly (B1 and N1) andweakly bonded (B0 and N0) to the substrate. As the temperatureis further increased, B0 and B1 gradually shift toward lower BEs,finally reaching a position which resembles the value obtained forour best h-BN layer, while the Bmol andBad components disappearwhen an extended layer is formed. Hence, the evolution, in termsof BE position and intensity of B0 component together with theappearance of B1, reflects the coalescence of single dehydroge-nated BN rings and molecular fragments which ends up with thecompletion of the h-BN layer. Accordingly, this process is reflected

Figure 8. N K-edge NEXAFS spectra of the h-BN single layer onIr(111).



in the hydrogen desorption curve in Figure 5. After initial surfacedecomposition of borazine, as indicated by the main desorptionfeature around room temperature, dehydrogenation extends overa wide temperature range. This continuous desorption is dueto the dehydrogenation of the intact-ring molecules and of themolecular fragments adsorbed on the surface.

The variations of the adsorbate chemical composition andstructure lead to measurable changes also in the Ir 4f7/2 spectra.The SCLS of the clean Ir(111) surface is primarily due to thereduced coordination of first-layer Ir atoms with respect to thebulk, which determines a narrowing of the surface d-band thatresults in a shift of the surface component Irs toward lower BEs.Upon borazine adsorption at T = 330 K the Ir 4f7/2 spectrumshows a decrease of Irs together with the appearance of Iri, whichstems from the Ir atoms interacting with the chemical speciesadsorbed on the substrate. It is well-known that atomic andmolecular adsorption on TM surfaces typically induces a broad-ening of the substrate d-band, together with a lowering of thedensity of states in the Fermi level region. For TMs with a morethan half-filled d-band this results in a shift toward higher BEs ofthe d-band of the surface atoms interacting with the adsorbatewith respect to that of the clean atoms, which is reflected in themeasured adsorbate-induced SCLSs.49 This effect applies also tothe case of Iri. Upon increasing the annealing temperature, Iridecreases, while the clean surface peak Irs gains intensity. Thisbehavior clearly reflects the reorganization of the adsorbate layerfrom a disordered structure, composed of several chemical spe-cies strongly interacting with the Ir surface, into an ordered h-BNlayer, which on the average is weakly interacting with the substrate.Similar findings have been obtained for the related hexagonallattice of graphene grown on Ir(111).43,50 In that case, upon ethy-lene adsorption at room temperature and subsequent annealings,43

a third feature between surface and bulk components due to Irfirst-layer atoms differently interacting with the adsorbed specieswas found. Moreover, as in our case, increasing the substratetemperature resulted in a continuous reduction of this peakparalleled by the growth of the clean surface component. Thisbehavior has been explained as a result of the evolution of thedifferent species on the surface from ethylene, strongly interact-ing with the substrate, to the formation of a graphene layer, whichis very weakly interacting with Ir(111).43 These considerationscan be applied also to the h-BN growth process on Ir(111).

High-temperature deposition of borazine leads to the forma-tion of a more extended h-BN single layer compared to thatobtained by heating the substrate saturated at room temperature.Indeed, the XPS integral area shows that the h-BN film obtainedby the latter procedure covers an area which is about 58% that ofour best layer, achieved with the former growth technique. This isfurther supported by comparison of the corresponding Ir 4f7/2spectra: for the room temperature deposition, the lower intensityof Iri with respect to Irs is due to a higher fraction of uncoveredsurface. At room temperature, in fact, part of the hydrogen atomsare attached to borazine molecules, which therefore occupy agreater area than the hexagon in the h-BN configuration.15 Thepresence of more than one surface component in the Ir 4f7/2 corelevel, as well as that of two components in the B 1s and N 1sspectra, reflects different strength of interaction of the h-BN layerwith the substrate. In fact, while Iri is related to substrate regionsstrongly interacting with the h-BN lattice, the main surface com-ponent Irs has a SCLS of�540meV, which is practically identicalto that of the clean surface, denoting the small interaction betweenthese surface atoms and the h-BN thin film. Interestingly, the

interaction between Ir(111) and the graphene layer is quite dif-ferent, and no adsorbate-induced components are detected in theIr spectra.43 The strength of the chemical bonding gives rise tosubstantial changes also in the morphology of the overlayer.44

Indeed, according to ref 44, N1 and B1 spectral features are rela-ted to the layer atoms that are close to the substrate, while themain N0 and B0 contributions correspond to the elevated part ofthe h-BN overlayer. Since the chemical bonding between h-BNand Ir(111) is known to be halfway between that of Pt(111),weakly interacting, and Rh(111),44 strongly interacting, we there-fore expect a rather poor degree of corrugation of the h-BN film.

’CONCLUSIONS

We have presented a study of the interaction of borazine withthe Ir(111) surface, from low-temperature adsorption to dissociationand formation of a h-BN single layer. Experimental data for borazineadsorbed on Ir(111) at T = 170 K indicate that borazine adsorbsmolecularly with the plane of the ring parallel to the substrate; themolecular saturation coverage is estimated to be 0.20ML. Subsequentannealing of the borazine-covered surface results in a continuousdehydrogenation from ∼250 up to ∼1000 K, with a maximum H2

desorption rate at 330K, andeventually ends upwith the formationofthe h-BN layer. AboveT = 330 K themolecule ring cracks, leading tothe formation of molecular fragments and atomic species, as revealedby the detection of atomic boron. Deposition of borazine on the Irsurface kept at high temperature leads to the formationof an extendedh-BN layer. LEED images of the single h-BN film show a moir�epattern consistentwith the formation of a (13� 13)/(12� 12) BN/Ir coincidence structure. XPS and NEXAFS measurements indicatethe presence of h-BN regions differently interacting with the Ir surface.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

’ACKNOWLEDGMENT

We thank J. Ka�spar, S. Fornarini, and R. Zanoni for precioussupport in the production of borazine and for stimulating dis-cussions. A.B. acknowledges financial support from the Univer-sity of Trieste under the program FRA2009.

’REFERENCES

(1) Dil, H.; Lobo-Checa, J.; Laskowski, R.; Blaha, P.; Berner, S.;Osterwalder, J.; Greber, T. Science 2008, 319, 1824.

(2) Berner, S.; Corso, M.; Widmer, R.; Groening, O.; Laskowski, R.;Blaha, P.; Schwarz, K.; Goriachko, A.; Over, H.; Gsell, S.; Schreck, M.;Sachdev, H.; Greber, T.; Osterwalder, J. Angew. Chem. 2007, 119, 5207.

(3) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671.(4) Corso, M.; Auw€arter, W.; Muntwiler, M.; Tamai, A.; Greber, T.;

Osterwalder, J. Science 2004, 303, 217.(5) Watanabe, K.; Taniguchi, T.; Kanda, H.Nat. Mater. 2004, 3, 404.(6) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183.(7) Dean, C. R.; Young, A. F.; Meric, I.; Lee, C.; Wang, L.;

Sorgenfrei, S.; Watanabe, K.; Taniguchi, T.; Kim, P.; Shepard, K. L.;Hone, J. Nat. Nanotechnol. 2010, 5, 722.

(8) Nagashima, A.; Tejima, N.; Gamou, Y.; Kawai, T.; Oshima, C.Phys. Rev. Lett. 1995, 75, 3918.

(9) Preobrajenski, A. B.; Vinogradov, A. S.;M�artensson, N. Phys. Rev.B 2004, 70, 165404.

(10) Auw€arter, W.; Kreutz, T.; Greber, T.; Osterwalder, J. Surf. Sci.1999, 429, 229.



(11) Grad, G. B.; Blaha, P.; Schwarz, K.; Auw€arter, W.; Greber, T.Phys. Rev. B 2003, 68, 085404.(12) Cavar, E.; Westerstr€om, R.; Mikkelsen, A.; Lundgren, E.;

Vinogradov, A.; Ng, M. L.; Preobrajenski, A.; Zakharov, A.; M�artensson,N. Surf. Sci. 2008, 602, 1722.(13) Morscher, M.; Corso, M.; Greber, T.; Osterwalder, J. Surf. Sci.

2006, 600, 3280.(14) Goriachko, A.; He; Knapp, M.; Over, H.; Corso, M.; Brugger,

T.; Berner, S.; Osterwalder, J.; Greber, T. Langmuir 2007, 23, 2928.(15) Dong, G.; Fourr�e, E. B.; Tabak, F. C.; Frenken, J. W. M. Phys.

Rev. Lett. 2010, 104, 096102.(16) Greber, T.; Brandenberger, L.; Corso,M.; Tamai, A.; Osterwalder,

J. e-J. Surf. Sci. Nanotechnol. 2006, 4, 410.(17) M€uller, F.; H€ufner, S.; Sachdev, H. Surf. Sci. 2008, 602, 3467.(18) Corso, M.; Greber, T.; Osterwalder, J. Surf. Sci. 2005, 577, L78.(19) Allan, M.; Berner, S.; Corso, M.; Greber, T.; Osterwalder, J.

Nanoscale Res. Lett. 2007, 2, 94.(20) Auw€arter, W.; Suter, H. U.; Sachdev, H.; Greber, T. Chem.

Mater. 2004, 16, 343.(21) Auw€arter, W.; Muntwiler, M.; Osterwalder, J.; Greber, T. Surf.

Sci. 2003, 545, L735.(22) Simonson, R.; Trenary, M. J. Electron Spectrosc. Relat. Phenom.

1990, 54�55, 717.(23) Simonson, R.; Paffett, M.; Jones, M.; Koel, B. Surf. Sci. 1991,

254, 29.(24) Paffett, M.; Simonson, R.; Papin, P.; Paine, R. Surf. Sci. 1990,

232, 286.(25) He, J.-W.; Goodman, D. Surf. Sci. 1990, 232, 138.(26) Koel, B. E.; Crowell, J. E.; Mate, C. M.; Somorjai, G. A. J. Phys.

Chem. 1984, 88, 1988.(27) Lehwald, S.; Ibach, H.; Demuth, J. E. Surf. Sci. 1978, 78, 577.(28) Jakob, P.; Menzel, D. Surf. Sci. 1989, 220, 70.(29) Grassian, V. H.; Muetterties, E. L. J. Phys. Chem. 1987, 91, 389.(30) Baraldi, A.; Comelli, G.; Lizzit, S.; Kiskinova, M.; Paolucci, G.

Surf. Sci. Rep. 2003, 49, 169.(31) Baraldi, A.; Barnaba, M.; Brena, B.; Cocco, D.; Comelli, G.;

Lizzit, S.; Paolucci, G.; Rosei, R. J. Electron Spectrosc. Relat. Phenom. 1995,76, 145.(32) Wideman, T.; Sneddon, L. G. Inorg. Chem. 1995, 34, 1002.(33) Doniach, S.; Sunjic, M. J. Phys. C 1970, 3, 285.(34) Feulner, P.; Menzel, D. J. Vac. Sci. Technol. 1980, 17, 662.(35) Lee, A. F.; Lambert, R. M.; Goldoni, A.; Baraldi, A.; Paolucci, G.

J. Phys. Chem. B 2000, 104, 11729.(36) Chiang, T.-C.; Kaindl, G.; Mandel, T. Phys. Rev. B 1986,

33, 695.(37) Tillborg, H.; Nilsson, A.; Hernn€as, B.; M�artensson, N.; Palmer,

R. E. Surf. Sci. 1993, 295, 1.(38) Doering, J. P.; Gedanken, A.; Hitchock, A. P.; Fischer, P.;

Moore, J.; Olthoff, J. K.; Tossell, J.; Raghavachari, K.; Robin, M. B. J. Am.Chem. Soc. 1986, 108, 3602.(39) M€uller, F.; H€ufner, S.; Sachdev, H.; Gsell, S.; Schreck, M. Phys.

Rev. B 2010, 82, 075405.(40) Engstrom, J. R.; Tsai, W.; Weinberg, W. H. J. Chem. Phys. 1987,

87, 3104.(41) Weststrate, C.; Bakker, J.; Gluhoi, A.; Ludwig, W.; Nieuwenhuys,

B. Catal. Today 2010, 154, 46.(42) Bianchi, M.; Cassese, D.; Cavallin, A.; Comin, R.; Orlando, F.;

Postregna, L.;Golfetto, E.; Lizzit, S.; Baraldi, A.New J. Phys.2009, 11, 063002.(43) Lacovig, P.; Pozzo, M.; Alf�e, D.; Vilmercati, P.; Baraldi, A.;

Lizzit, S. Phys. Rev. Lett. 2009, 103, 166101.(44) Preobrajenski, A.; Nesterov, M.; Ng, M. L.; Vinogradov, A.;

M�artensson, N. Chem. Phys. Lett. 2007, 446, 119.(45) Preobrajenski, A. B.; Krasnikov, S. A.; Vinogradov, A. S.; Ng,

M. L.; K€a€ambre, T.; Cafolla, A. A.; M�artensson, N. Phys. Rev. B 2008, 77,085421.(46) The following internuclear bond distances were used: B�N,

1.44 Å; B�H, 1.20 Å; N�H, 1.02 Å. The H (aromatic) van der Waalsradii is 1.00 Å.51

(47) Krawczyk, M. Appl. Surf. Sci. 1998, 135, 209.(48) Desrosiers, R.M.; Greve, D.W.;Gellman, A. J. J. Vac. Sci. Technol.

A 1997, 15, 2181.(49) Baraldi, A. J. Phys.: Condens. Matter 2008, 20, 093001.(50) Lizzit, S.; Baraldi, A. Catal. Today 2010, 154, 68.(51) Bondi, A. J. Phys. Chem. 1964, 68, 441.

Date post:	12-Dec-2016
Category:	Documents
Upload:	silvano
View:	212 times
Download:	0 times

Epitaxial Growth of Hexagonal Boron Nitride on Ir(111)

Documents