Surface Science 605 (2011) 1861–1865

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Surface Science

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Thin films of molecule-based charge transfer complex cobalt tetracyanoethylene: Insitu X-ray photoemission study

Pramod Bhatt ⁎, S.M. YusufSolid State Physics Division, Bhabha Atomic Research Centre, Mumbai 400 085, India

⁎ Corresponding author.E-mail address: prabhatt@barc.gov.in (P. Bhatt).

0039-6028/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.susc.2011.06.024

a b s t r a c t

a r t i c l e i n f o

Article history:Received 9 February 2011Accepted 27 June 2011Available online 2 July 2011

Keywords:Molecule-based magnetsThin filmsX-ray photoelectron SpectroscopyPhysical vapor deposition

Thin films of molecule-based charge transfer magnet, cobalt tetracyanoethylene [Co(TCNE)x, x~2] consisting ofthe transition metal Co, and an organic molecule viz. tetracyanoethylene (TCNE) have been deposited by usingphysical vapor deposition method under ultra-high vacuum conditions at room temperature. X-rayphotoelectron spectroscopy (XPS) technique has been used extensively to investigate the electronic propertiesof the Co(TCNE)x thin films. The XPS measurements show that the prepared Co(TCNE)x films are clean, andoxygen free. The stoichiometries of the films, based on atomic sensitive factors, are obtained, and yields a ~1:2ratio between metal Co and TCNE for all films. Interestingly, the positive shift of binding energy position for Co(2p), and negative shifts for C(1s) and N(1s) peaks suggest a charge-transfer from Co to TCNE, and cobalt isassigned to its Co(II) valence state. In the valence band investigation, the highest occupied molecular orbital(HOMO) of Co(TCNE)x is found to be at ~2.4 eV with respect to the Fermi level, and it is derived either from theTCNE− singly occupiedmolecular orbital (SOMO) or Co(3d) states. The peaks located at ~6.8 eV and ~8.8 eV aredue to TCNE derived electronic states. The obtained core level and valence band results of Co(TCNE)x, films arecompared with those of V(TCNE)x thin film magnet: a well known system ofM(TCNE)x type of organic magnet,and important points regarding their electronic properties have been brought out.

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1. Introduction

Molecule-based charge transfer magnetic complex consisting of atransition metal, and an organic compound viz. TCNE form a uniqueclass ofmaterials denotedbyM(TCNE)x· yS (whereM=V,Mn, Fe, Co, orNi, and TCNE = tetracyanoethylene, S =solvent: dichloromethane,acetonitrile, or tetrahydrofuran) [1–7]. This class of magnet is verypopular because of the presence of room temperaturemagnetism in oneof its members, namely vanadium tetracyanoethylene, V(TCNE)x·y(CH2Cl2) [1]. This vanadium-based compound, prepared in a solution ofCH2Cl2 (dichloromethane) was the first room-temperature magnet(reported in 1991) to have a magnetic ordering temperature of ~400 K;a finding which accelerated the research in the field of TCNE-basedorganic magnets. Magnetism in this compound results from theexchange interaction between the unpaired electron in the π* orbitalof [TCNE]− (spin 1/2) and the metal ion, V2+ (3/2). On the basis of theexperimental results of the X-ray absorption spectroscopy study, it wasproposed that the vanadiumhas 2+oxidation state, and each V2+ ion iscoordinated by six N ions; most likely in a slightly distorted octahedralenvironment at room-temperaturewithanaveragedistanceof 2 Å [8]. V(TCNE)x·y(CH2Cl2), also possesses semiconducting property (conduc-tivity 10−3 S cm−1 at room temperature), andmagnetotransport study

indicates that the electrons in valence and conduction bands are spin-polarized, which can be exploits for future magnetic spintronicsapplications [9–11]. Apart from V(TCNE)x, the other compounds in theclass ofM(TCNE)x·yS which could be useful for spintronics applicationsare Co(II) based compounds, Co(TCNE)x and VxCo1−x(TCNE) because ofthe presence of large magnetocrystalline anisotropy in Co(II). Thepresence of large magnetocrystallince anisotropy should lead to anenhanced coercivity [12]. However, the main problemwith this class ofcompounds is the insolubility of the bulk powder in organic solvents aswell as contaminations present due to the solvent, precursor andorganometallic compounds used for synthesis. It was proposed that astrong binding between V and TCNE, and a 3-D network structure areresponsible for the insolubility of this magnet in organic solvents [8]. Inorder to reduce such difficulties of the bulk powders, and to improvetechnological utility of this compound, thin films of vanadiumtetracyanoethylene with stiochiomentry, V(TCNE)x, x~2 were synthe-sizedusing the chemical vapordeposition (CVD) technique [13–15]. Thefilms of V(TCNE)x prepared by CVD technique made them free fromcontaminations, and any defects caused by the use of solvents. TheCVD technique was later extended to deposit thin films using moremetal combinations (Cr, Nb andMo) at high vacuum (HV) condition.The HV-CVD technique partially solves the contamination and defectproblems [16]. On the other hand, thin films of M(TCNE)x areextremely air-sensitive, which makes them difficult to deposit asclean, and oxygen-free films even at high vacuum conditions. Thissituation makes it adverse for true commercialization, where the

Fig. 1. XPS wide scan spectra of Co(TCNE)x thin films along with pure metallic Co filmand sputtered clean Au substrate. Wide scan spectrum of pure TCNE is also shown forcomparison.

1862 P. Bhatt, S.M. Yusuf / Surface Science 605 (2011) 1861–1865

magnetic films need to be significantly more resistant to degradationby ambient atmosphere (air). Recently, a new deposition techniquebased on ultra high vacuum conditions (UHV) has been reported forthe preparation of V(TCNE)x film, known as physical vapordeposition (PVD) technique [17]. This technique provides air stableand contamination free films of organic magnets of M(TCNE)x withthe possibilities of tailoring physical properties just by tuningphysical parameters during film deposition. We have recently[18,19] synthesized oxygen and solvent or precursor-defect-freethin films of Fe(TCNE)x and Ni(TCNE)x in situ at UHV conditions atroom temperature using PVD technique and investigated theirelectronic and magnetic properties. In the present paper, we focuson the synthesis of oxygen free thin films of Co(TCNE)x, by PVDtechnique, and investigated their electronic properties using X-rayphotoelectron spectroscopy (XPS). In addition, the experimentallyobserved core level and valence band results of the present Co(TCNE)x films are compared with that of Fe(TCNE)x and V(TCNE)xfilms. The derived results of electronic properties are useful tounderstand the intertwined magnetic properties of other M(TCNE)xclass of materials as well.

2. Experimental

Thin films of Co(TCNE)x are deposited in situ on sputter-cleanedgold substrate by using PVD technique under UHV conditions at roomtemperature. The organic compound, TCNE, as received from Sigma-Aldrich, is introduced into the vacuum chamber with the help of aUHV leak valve and a gas handling system to deposit thin film of TCNE[27]. Polycrystalline gold substrates are cleaned in situ using argon ionsputtering prior to deposition of TCNE film. The same gas handlingsystem is used for depositing thin films of Co(TCNE)x on sputter-cleaned polycrystalline gold substrates by introducing the TCNEmolecules inside the vacuum chamber using UHV leak valve, and thenallowed the TCNEmolecules to react with physical vapor deposited Co(99.9% purity) using Omicron® PVD source. The detailed method ofpreparation of organic magnets of V(TCNE)x by using PVD techniquecan be found elsewhere [17]. The thin films of Co(TCNE)x with twodifferent thicknesses of ~3 and ~5 nm are deposited on polycrystallineAu substrates. The thicknesses of the films are determined by usingthe attenuation length of the XPS substrate Au (4f) signals, in thefollowing formula [20]:

Ifilm = Isubstrate exp −t= λ Sinθð Þ½ �

where t is the thickness of the film, Ifilm is the intensity of the Ausubstrate after film deposition and Isubstrate is the intensity of the pureAu substrate. λ is the attenuation length of electrons (~4.2±0.14 nm)[21] with θ being the photoelectron take off angle (45°) of electronemission between surface and analyzer axis. The thickness calculatedusing this method suggests that both the films are very thin havingthickness in the nanometer range (b10 nm), but note that the errormargin of ±1 nm in this type of estimate. During the film depositionthe deposition rate, time, and the substrate temperature areb0.5 Å min−1, 1.5–2 h and room temperature, respectively. In orderto avoid contamination during deposition, the system is thoroughlybaked to a temperature up to 200 °C for more than 12 h to achieve abackground pressure of 5×10−10 mbar. A base pressure of about8×10−8 mbar is kept constant during the film deposition. Absence ofO(1 s) in the XPS wide scan spectrum (Fig. 1) confirms clean andoxygen free Co(TCNE)x films.

The Co(TCNE)x films are characterized with X-ray, as well asultraviolet photoelectron spectroscopy (XPS and UPS), using a highresolution UHV spectrometer (SCIENTA) at a base pressure of10−10 mbar. The system is equipped with a hemispherical electronenergy analyzer, a monochromatized He resonance lamp for UPS-He I=21.2 eV or He II=40.8 eV, and a non-monochromatized Al Kα

X-ray source for XPS [(Al-source)=1486.6 eV] measurement. Theresolution for the UPSmeasurements is 0.1 eV, whichwas determinedfrom the Fermi-edge of a clean Au reference sample. The XPSresolution is 1.2 eV, obtained from the full width at half maximumof the Au (4f7/2) core-level line. No background subtraction method isadapted. The raw data of XPS core level spectra of Co(TCNE)x filmshave been presented.

3. Results and discussion

Fig. 1 presents the XPS wide scan spectra of Co(TCNE)x films alongwith those of pure Co film, TCNE film, and sputtered clean Au substrate.It is clearly seen that wide scan spectra of both the Co(TCNE)x films(~3 nm and ~5 nm) contain no signals from atomic species other thanthe expected Co, C and N, suggesting the formation of clean and oxygenfree Co(TCNE)x films. The Au (4f) core level peaks can be seen for ~3 nmCo(TCNE)x film due to the thinness of the films. Au (4f7/2) has been usedto for the reference of the sample for charging effect [22,23], and it wasfound that no charging occurred during the XPS measurements. In thisstudy, the binding energy reference of Au (4f7/2)was set on 84.0 eVwitha fixed doublet separation of 3.67 eV. Sample charging generally occursfor non-conducting samples due to the emission of photoelectronsattached to the sample surface during X-ray irradiation. Once thephotoelectrons are emitted out of the non-conducting sample surface, apositive charge zonewill establish quickly in the sample surface. A non-conducting sample does not have sufficient delocalized conductionband electrons available in order to neutralize the positive holes leftbehind during the photoeffect. As a result, a positive potential buildsnear the sample surface, which retards the outgoing electrons, makingthem lose some of their original kinetic energy resulting a positive shifttowards higher binding energies. This charging leads to a shift of theXPS spectrum on the energy scale, and inhomogeneous charging indifferent regions leads to a peak broadening. The XPS wide scanspectra are used here to calculate the film stoichiometry based onatomic sensitive factors [24], and it yielded an ~1:2 ratio betweenmetal Co and TCNE for all Co(TCNE)x thin films (i.e. x∼2.1±0.3).Itcan be noted here that the error margin is large (~10%) in this type ofestimate. The comparison of XPS wide scan spectra of TCNE and Co(TCNE)x suggests the formation of the Co(TCNE)x thin films.However, in order to further confirm the oxidation state, filmstoichiometry and chemical compound formation, X-ray core levelspectroscopy has been performed.

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The core level peaks corresponding to C(1s) and N(1s) for all Co(TCNE)x thinfilm samples are shown in Fig. 2. Alongwith these, the C(1s)and N(1s) core level peaks of pure TCNE and V(TCNE)x thin film are alsoshown. In the C(1s) core level spectrum of pure TCNE, a single peak atbinding energy position of ~287.7 eV is observed, which containscontributions from both cyano carbons and the vinyl carbons. However,the difference in binding energy of these two carbon species is only~0.4 eV [25], which makes them difficult from being resolved as twoseparate peaks. In the case of Co(TCNE)x films, C(1s) core level peaks areobserved at low binding energy position of ~286.0 eV, suggesting moreelectron rich environment compared to pure TCNE [26]. Moreover,the C(1s) core level peaks in Co(TCNE)x films appear with a shoulderfeature at a lower binding energy of ~285.2 eV. The observedshoulder could be mainly attributed to (i) Co-bonding defect siteswhich are mainly originated from residual by-products from thechemical reaction between precursors, and (ii) excess Co metalwhich is bonded to the vinyl carbon of TCNE and not coordinating tothe nitrogen of the cyano groups [10,16,27]. Since pure metal andTCNE are only used in the filmmade by PVD technique, the possibilityof defects in the films, therefore, can be ruled out. The trends in the N(1s) spectra closely follow those observed in the C(1s) spectra. Sincethere is only one type of nitrogen atom present in the TCNEmolecule,single N(1s) core level peak for pure TCNE is observed at bindingenergy position of ~400.2 eV. The N(1s) core level peaks for both Co(TCNE)x films are observed at the same binding energy positions of~398.6 eV, which mainly originated from cobalt coordinated nitro-gen. Some nitrogen atoms remain uncoordinated since each TCNEmolecule contains four N atoms [8]. The uncoordinated nitrogen ofTCNE is responsible for the asymmetric N(1s) core level peak. Theposition as well as asymmetry of the C(1s) and N(1s) core level peaksof Co(TCNE)x is very identical to that of the TCNE− derived peak ofRb-intercalated TCNE [27]. The N(1s) core levels in Co(TCNE)x arebroadened as compared to the N(1s) of pure TCNE. Similarbroadening was observed in case of thin films of V(TCNE)x as well.The C(1s) and N(1s) core level peaks for V(TCNE)x film are observedat 285.9 and 398.6 eV, and follow the same trends that are observedfor Co(TCNE)x films. Moreover, a clear shift of ~1.7 and ~1.6 eV in thebinding energy positions of C(1s) andN(1s), respectively, is observedfor V(TCNE)x/Co(TCNE)x thin films when compared to the values forpure TCNE. The shifts are, in fact, towards the lower binding energy side

Fig. 2. Core level spectra of C(1s) and N(1s) peaks of Co(TCNE)x thin films with respectto pure TCNE and V(TCNE)x thin films.

in C(1s) and N(1s) core level peaks of Co(TCNE)x thin films suggest thatTCNE has been chemically reacted with Co metal.

TheCo(2p) core level spectra for both (~3 nmand~5 nm)Co(TCNE)xfilms are presented in Fig. 3, and have been used to determine theoxidation state of Co. The Co(2p) core level spectra of pure Cofilm is alsoshown in Fig. 3 for reference. Two strong peaks, observed at bindingenergy positions of ~782.7 and ~798.6 eV for both Co(TCNE)x films aredue to the spin-orbit coupling originated Co(2p3/2) and Co(2p1/2) peaks,almost similar to the Co(2p) peaks observed at ~777.9 and ~793.1 eV forpuremetallic Co[28–30]. In addition, thepeak shoulders appearingat thebinding energy positions of 788.7 and ~804.3 eV corresponds to Co(2p)satellite peaks, originated due to a shake-up process. The shake-upprocess is produced by a monopole excitation of an electron brought onby the creation of a core hole, and is associatedwith ligand-metal type ofcharge-transfer excitation [31,32]. The low binding energy shoulderpeak observed at ~779.0 eV is shifted to slightly higher binding energyside when compared with pure Co metal. This could be due to excess ofmetallic cobalt, which is chemically bonded to the vinyl carbon of TCNE,results shifts in the binding energy position. The excess amount ofmetallic Co has also been reflected in the core level spectra of C(1s) of Co(TCNE)xfilms, asdescribedpreviously. It has beenobserved that themainCo(2p3/2) core level peaks for bothCo(TCNE)xfilms are shiftedby ~4.9 eVtowards higher binding energy side with respect to pure elementalbinding energy positions of Co(2p). Similarly, satellite peaks are alsoshifted towards higher binding energy side with respect to pureelemental Co(2p). The observed binding energy positions of Co(2p)core level and corresponding satellite peaks of Co(TCNE)x films matchedwell with previously-reported binding energy values of Co(II) in cobaltoxides. Moreover, a energy difference of 15.9 eV between Co(2p1/2) andCo(2p3/2) is observed for ~3 nm Co(TCNE)x film, which comparesfavorably with the value of 16 eV reported by Brinen et al. for Co(II) inCoA12O4[33]. The same energy difference of ~15.9 eV is also evident for~5 nm Co(TCNE)x thin film, suggesting that cobalt is present in Co(II)states. The intense satellite structure on the high binding-energy side ofthe main core level peaks for Co(TCNE)x films also demonstrates thatcobalt is present as Co(II) states [34]. The core level shift (chemical shift)is a significant parameter which provides the most direct informationabout the change of the character of the bonding due to compoundformation, anddirection of charge transfer between themetal and ligand.Therefore, the observed positive shift of ~4.9 eV for Co(2p3/2) peak,and negative shifts of ~1.7 eV and ~1.6 eV for C(1s) and N(1s) peaks,respectively, confirm the charge transfer from Co to TCNE in Co(TCNE)x. Moreover, based on the estimated stoichiometry of x~2(calculated from XPS data), the binding energy positions of Co(2p)core levels and their corresponding satellite peaks, it is confirmed

Fig. 3. Co(2p) core level spectra of Co and Co(TCNE)x films. Co(2p) core level spectra ofpure Co film is shown for reference.

Fig. 5. Comparison of valence band spectra of Co(TCNE)x films with Fe(TCNE)x film.Valence band of pure Co film is shown for reference. The arrows indicate the peakpositions of the molecular orbitals.

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that the Co is in 2+ valence state, and there is no featurecorresponding to metallic Co in either Co(TCNE)x spectra.

The valence band spectroscopy provides a more precise tool forinvestigating minute changes in the valence electronic structure. Thevalence band spectrum of Co(TCNE)x film, recorded using He-I light, isshown in Fig. 4. The inset of Fig. 4 shows the valence band spectrum inthe lower binding energy region for better illustration of peaks in thefrontiers of electronic bands. Three peaks are observed in the valenceband region of 0 to 10 eV for both Co(TCNE)x films. The two peaksobserved at ~6.8 and ~8.8 eV are due to TCNE-derived electronicstates, and the third peak located at ~2.4 eV (below the Fermi level) isthe highest occupied molecular orbital (HOMO) of Co(TCNE)x. Itshould be noted here that the peaks observed in the frontier electronicstructures of M(TCNE)x type of compounds are mainly dominatedeither by the 3d band of transition metals or by the hybridizationbetween transition metals and TCNE molecule. Thus, the single bandobserved at ~2.4 eV for Co(TCNE)x film could be either derived fromthe Co(3d) states or from TCNE− singly occupied molecular orbital.However, further investigations are necessary to confirm the presenceof Co(3d) states at ~2.4 eV. Whereas, in the case of V(TCNE)x film,three peaks are observed between ~1 eV to 4 eV range (fig. notshown). These peaks are assigned to the destabilized HOMO at~3.5 eV, singly occupiedmolecular orbital (SOMO) of TCNE at ~1.5 eV,and a hybridized state of V(3d)-TCNE ligand orbital at ~1 eV[16,17,35]. Similarly, in the case of Fe(TCNE)x film, three peaks areobserved in the frontier electronic structures (shown in Fig. 5). Thepeak, HOMO, observed at ~1.7 eV is derived from the SOMO of TCNEmolecule, whereas the peak appearing at higher binding energy of~4.5 eV is Fe(3d)-derived unlike to V(TCNE)x, where the HOMO ismainly derived from V(3d) states [18]. The absence of hybridized stateor strong metal (Co)-legend (TCNE) interaction in Co(TCNE)x film couldalsobe responsible for the lowermagnetic ordering temperature (~44 K)for Co(TCNE)x. A comparison between valence band of Co(TCNE)x, withFe(TCNE)x, and Co films is also shown in Fig. 5. It is clearly observed thatthe peaks located at ~6.8 and ~8.8 eV for Co(TCNE)x and Fe(TCNE)xrespectively, are only TCNE-derived [35]. Moreover, no metallicsignature of pure Co is evident in Co(TCNE)x films as Co(3d) band doesnot lie on the Fermi level, and is shifted by more than ~1 eV towardshigher binding energy side. However, more investigations, such asresonant photoelectron spectroscopy, X-ray absorption spectroscopy,and X-ray magnetic circular dichroism are needed further to investigateelectronic properties of Co(TCNE)x type molecule-based magnets.

4. Conclusion

Molecule-based thin film magnets of Co(TCNE)x are synthesizedat UHV conditions using physical vapor deposition method, and

Fig. 4. Valence band spectra of ~3 nm, Co(TCNE)x films. Arrow indicates peak position ofthe molecular orbitals.

their electronic properties are investigated by using XPS technique.XPS wide scan spectra show that the Co(TCNE)x thin films do nothave any contaminations either from oxygen, or solvents or evenprecursor. The XPS derived stoichiometry points to a 1:2 ratiobetween Co and TCNE resulting in a Co(TCNE)x, x~2 film. The XPScore level study shows the oxidation state of Co is 2+. Moreover, thepositive shifts for Co(2p) and negative shifts for C(1s) and N(1s)suggest a charge transfer from Co to TCNE leading to a formation ofCo(TCNE)x compound. XPS valence band investigation reveals thatthe HOMO of Co(TCNE)x is located at ~2.4 eV with respect to theFermi level and could be either derived from the TCNE− singlyoccupied molecular orbital (SOMO) or Co(3 d) states. The peaks at~6.8 eV and ~8.8 eV are due to TCNE derived electronic states ascompared to valence band of TCNE molecule. The XPS valence bandanalysis also suggests the absence of metallic Co in the sample. Inaddition, comparing the XPS data of Co(TCNE)x with V(TCNE)x andFe(TCNE)x, it has been observed that the absence of hybridized stateor strong metal (Co)-legend (TCNE) interaction in Co(TCNE)x couldbe responsible for the lower magnetic ordering temperature of~44 K for the Co(TCNE)x compound.

In summary, the present paper contains following importantaspects related to electronic property investigations of clean andoxygen free thin films of Co(TCNE)x, prepared using physical vapordeposition at UHV conditions, which would be beneficial for synthesisand characterization of M(TCNE)x type of organic magnets.

(i) The experimental observations of core level and valence bandspectroscopy of clean Co(TCNE)x thin films along with pure Coand TCNE molecule.

(ii) Comparison of XPS core level and valence band of Co(TCNE)xfilms with those of V(TCNE)x; a well know system ofM(TCNE)xtype of compound.

Acknowledgment

Authors would like to thank Prof. Mats Fahlman for his support inthe XPS measurements, which are carried out at Linkoping University,Sweden.

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