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Modifying Spin Injection Characteristics in the Co/Alq System by Using a Molecular Self-assembled Monolayer Hyuk-Jae Jang, Jun-Sik Lee, Sujitra Pookpanratana, Christina A. Hacker, Ich C. Tran, and Curt A Richter J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b01222 • Publication Date (Web): 19 May 2015 Downloaded from http://pubs.acs.org on May 20, 2015

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The Journal of Physical Chemistry

Modifying Spin Injection Characteristics in the Co/Alq3 System by Using a Molecular SelfAssembled Monolayer Hyuk-Jae Jang1,2,*, Jun-Sik Lee3, Sujitra J. Pookpanratana1, Christina A. Hacker1, Ich C. Tran4, and Curt. A. Richter1 1.

Semiconductor and Dimensional Metrology Division, National Institute of Standards and

Technology, 100 Bureau Drive, M.S. 8120, Gaithersburg, MD 20899 2.

Department of Physics, Wake Forest University, Winston-Salem, NC 27109

3.

Stanford Synchrotron Radiation Lightsource, SLAC National Accelerator Laboratory, Menlo

Park, CA 94025 4.

Lawrence Livermore National Laboratory, Livermore, CA 94550

ACS Paragon Plus Environment

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ABSTRACT: We present the results of experiments that explore the influence of molecular selfassembled monolayers (SAMs) on characteristics of spin injection into an organic semiconductor, Alq3 [tris-(8-hydroxyquinoline) aluminum] from a ferromagnetic metal, Co. Two different SAMs, MHA (16-mercaptohexadeconic acid; HS(CH2)15CO2H) and ODT (1Octadecanethiol; CH3(CH2)17SH) are inserted between Alq3 and Co layers and their effects on electronic structure hybridization and related changes in energy levels and spin dependent properties at the interface are investigated. X-ray photoelectron measurements of the surface structure for the MHA, ODT, and Alq3 organic layers provide bonding and chemical conformational information. Ultraviolet photoelectron spectroscopy (UPS) measurements reveal that both MHA and ODT treatments lower the work function of Co. X-ray magnetic circular dichroism (XMCD) spectra imply that SAMs reduce the hybridization between Co and Alq3 and furthermore, they enhance the spin magnetic moment of Co.

KEYWORDS: Interface engineering, Ferromagnetic metal, Organic semiconductor, Electronic structure hybridization, Energy level alignment, Orbital and spin magnetic moments

1. INTRODUCTION One of the key factors to realize spin injection and spin transport through organic semiconductors is the understanding and engineering of the spin-dependent phenomena at the interface between a ferromagnetic metal and an organic semiconductor.1, 2 Recent studies have shown that the electronic structure hybridization between a ferromagnetic metal and an organic semiconductor can affect the spin-dependent properties at the interface via spin-dependent


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confinement and the induced spin polarization of an organic semiconductor.1, 2 In addition, the introduction of an interlayer between a ferromagnetic metal and an organic semiconductor such as a thin layer of an oxide tunneling barrier, a low work function metal, or an organic film has been explored to improve spin injection efficiency via better energy band alignment for selective spin carrier injection (electrons or holes) and enhancement of spin polarization.3-5 However, the aforementioned methods are limited in providing a versatile way of tuning the band alignment and spin polarization simultaneously at the interface and thus it is necessary to explore other ways to adjust them. Here, we explore the use of self-assembled monolayers (SAMs) as an interlayer to tune and engineer the characteristics of spin injection into an organic semiconductor from a ferromagnetic metal. Our experimental results show that the appropriate choice of a SAM can change the work function as well as the magnetic moment of the ferromagnetic metal surface. Moreover, Fermi level pinning can play a critical role in determining energy level alignment at the interface between an organic semiconductor and a ferromagnetic electrode. SAMs are monolayer assemblies, formed spontaneously on an inorganic solid surface by chemisorption of molecular constituents from solution or gas phase.6 SAMs are technologically attractive for surface and interface engineering because they are known to offer simple fabrication processes for modifying the physical and chemical properties of the interface and the surface such as wetting and work function.6, 7 Tailoring the monolayer is well-studied where changing the head group of the molecules impacts the molecular-substrate interaction affecting the monolayer quality and changing the tail group of the molecules impacts the surface properties, such as wetting, electrostatics, etc.6- 9 The shift of the effective metal work function by SAMs can be achieved by many mechanisms including the push-back effect, charge transfer at the molecule-substrate bond, or through the inherent dipole of the molecule.8


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Aliphatic thiol SAMs are among the most extensively investigated types due to (i) their easy preparation, (ii) their relatively high stability mediated by the strong covalent bond between a sulfur head group and a metal as well as by van der Waals interaction between molecules, and (iii) their ability to tune physical and chemical properties of metallic surfaces simply by changing their tail groups of different functionalities.6, 8 For our study, we chose two different types of thiol SAMs, MHA (16-mercaptohexadeconic acid; HS(CH2)15CO2H) and ODT (1-Octadecanethiol; CH3(CH2)17SH). Both ODT and MHA have the same sulfur head group so that both SAMs bond well to the metallic surfaces such as Au.8, 10 However, these molecules are known to provide quite different physical and chemical properties on the Au surface due to the different tail groups; MHA increases the Au work function and makes the Au surface hydrophilic.8, 10-12 In contrast, ODT decreases the Au work function and makes the Au surface hydrophobic.8, 10, 11 We investigated how the insertion of these SAMs (ODT and MHA) between Co and Alq3 [tris-(8-hydroxyquinoline) aluminum] affects the electronic structure hybridization, the energetics and band offset, and the spindependent properties at the interface. We chose Co and Alq3 in our study because they are widely studied materials for a spin injector and an organic spin transport medium, respectively.14, 13

We found that both ODT and MHA decrease the work function at the Co film surface and

prevent hybridization between Co and Alq3. In addition, both SAMs modified the magnetic moment of Co. 2. EXPERIMENTAL SECTION 2.1. Sample preparation


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We prepared the 6 samples, Co, MHA/Co, ODT/Co, Alq3/Co, Alq3/MHA/Co, and Alq3/ODT/Co. For the deposition of Co and Alq3, thermal evaporation was performed in a vacuum deposition system with a base pressure of ≈ 7×10-6 Pa (≈ 5×10-8 Torr). The Co thickness was 15 nm and that of Alq3 was 2.5 nm in all samples. First, Co thin films were prepared on thermally oxidized (300 nm thick SiO2) silicon substrates as described in the top of Figure 1. The SAMs (both MHA and ODT) were prepared by solution immersion in a glove box under Ar gas that is directly connected to the vacuum deposition chamber so that there is no exposure to ambient air between deposition steps in order to avoid any oxidation and minimize contamination to the Co surface. After the deposition of the SAMs, X-ray photoelectron spectroscopy (XPS) confirmed successful attachment of the SAMs to the Co surface (see Supporting Information). Based on the S 2p spectrum of MHA/Co, the spectra was deconvoluted into two chemical contributions where the energetic positions are consistent with bound14 and free thiol.15, 16 This strongly suggests that the MHA is able to attach to the Co surface with the sulfur end or the carboxylate end. For the deposition of Alq3 thin films, a sublimed grade 99.995 % trace metal basis was used. The deposition of Alq3 was performed simultaneously on all samples. Thus, Alq3/Co, Alq3/MHA/Co, and Alq3/ODT/Co samples were all fabricated in the same run. The film thickness was monitored during deposition by a calibrated quartz crystal monitor. The deposition rates were ≈ 0.09 nm/s for Co and ≈ 0.03 nm/s for Alq3. 2.2. X-ray and ultraviolet photoelectron spectroscopy Ex-situ UV photoelectron spectroscopy (UPS) and XPS measurements were performed by using commercial instrument equipped with a hemispherical electron analyzer in a base pressure of ≈ 2.67 × 10-7 Pa (≈ 2 × 10-9 Torr) or less. The UPS measurements were performed by using the He I excitation line (21.2 eV), and monochromatized Al Kα served as the X-ray source. Samples


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were sealed in the Ar-filled glovebox, and loaded into the XPS instrument with 30 s to 60 s of exposure to air. The spectral area of the XPS core levels are determined after a Shirley background correction and correcting for their respective photoionization cross section.17 2.3. X-ray magnetic circular dichroism spectroscopy X-ray absorption spectroscopy (XAS) and its associated magnetic circular dichroism (XMCD) data have been used for studying both the electronic and magnetic characteristics of ferromagnetic metal surfaces.18 Notably, a recent development of the XMCD sum rules has allowed one to determine the spin and orbital magnetic moments of ferromagnetic metals, which are directly related to the spin polarization at the interface when spin carriers are injected from a ferromagnetic metal into an organic semiconductor.18 We utilized XMCD spectroscopy to investigate the change of spin-dependent properties at the surface of Co when SAMs are deposited on it. XAS and XMCD measurements on all samples were carried out at beamlines BL 8-2 and 13-1 at the Stanford Synchrotron Radiation Lightsource (SSRL), respectively. After the preparation, samples were sealed in argon and shipped to SSRL for XAS and XMCD measurements. We obtained the XMCD spectra by reversing the polarity (right- or left-circular) of the incident photon beam, and by changing the direction of the external magnetic (Hmax = ±0.2 T) field at a fixed polarity. The degree of circular polarization was ≈ 95 %. 3. RESULTS AND DISCUSSION 3.1. UPS and XPS analysis We examined the electronic structure of the Alq3-Co interface and the impact of inserting monolayers between the organic semiconductor and ferromagnetic material. Figure 1 displays


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the UPS spectra obtained from ODT, MHA, and 2.5 nm of Alq3 directly on Co as well as the bilayer systems of Alq3/ODT/Co and Alq3/MHA/Co where the organic semiconductor was evaporated onto the SAM-Co surface. Examination of the secondary electron cut-off (SECO) region yields the work function for these surfaces. The work function is obtained by subtracting the binding energy from the excitation energy (21.2 eV). For reference, the argon-sputter cleaned cobalt surface yielded a work function of 4.81 (± 0.05) eV, while cobalt with a native oxide produced a work function of 4.61 (± 0.05) eV. The values of the work function of each sample are listed in Table 1. Deposition of Alq3 onto Co decreased the surface work function to 3.46 (± 0.05) eV. Previous studies have reported the work function of the Alq3/Co surface between 3.6 eV19 to 4.4 eV.20 The work function of MHA and ODT on cobalt were found to be 3.5 (± 0.05) eV and 3.6 ± (0.05 eV), respectively. A previous report11 of these monolayers on gold found the MHA-Au work function to be 5.34 eV and a hexadecanethiol-Au work function of 4.84 eV. The work function of these monolayers are quite different on cobalt than on Au; this difference could be due to several reasons, including different interfacial bonding, different packing density, and in the case of MHA, both the –SH and –COOH functional groups reacting with the cobalt surface. The deposition of Alq3 onto MHA- and ODT-treated Co further decreases the work function by 0.1 eV. The minimal changes in the Alq3 work function for the differing SAM terminations on cobalt suggests that the Alq3-SAM-cobalt interface is pinned where the SAMs have little impact on the electronic properties of Alq3. Previous studies showed that when Alq3 is deposited directly onto Co, the new surface has a work function less than the starting substrate’s due to a large interface dipole caused by strong interaction between Co and Alq3 which is enabled by the Alq3 molecules interacting with a metal surface via their oxygen atoms.19 All the work functions of Alq3/Co, Alq3/MHA/Co, and


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Alq3/ODT/Co surfaces are similar (within 0.05 eV), and are slightly lower (≈ 0.1 eV) than those of MHA and ODT treated Co surfaces. It appears that the Alq3 is not influenced by any dipole induced from either ODT or MHA, since dΦsample/dΦsubstrate is close to zero. This is further evidenced by the measurement of the highest occupied molecular orbital as discussed below. The work function shift suggests that both MHA and ODT treatments may move the Fermi level of the Co surface closer to the lowest unoccupied molecular orbital (LUMO) level of Alq3, thus lowering the barrier for electron injection. The low energy region of the UPS spectra displayed in Figure 1 are indicative of the occupied molecular orbitals. The HOMO onset of the Alq3/Co sample was located around 2.0 eV below the Fermi level, which is consistent with a previous report.19 Alq3 deposited onto ODT- and MHA-treated Co have only a slightly larger HOMO onset, 2.05 (±0.05) eV and 2.1 (±0.05) eV, respectively. The values of the HOMO onset of each Alq3 deposited sample are listed in Table 2. The HOMO onset (or hole injection barrier) does not linearly vary with the substrate work function (MHA/Co or ODT/Co), further indicating Alq3 energy level pinning when deposited onto SAM-treated substrates. Fermi level pinning was previously observed in a weakly interacting interface between the electrode and Alq3 with work functions greater than 4.8 eV.21 Fermi level pinning is a common phenomenon in organic semiconductors, and has been an intense topic of research due to its direct impact on the utility of organic-based electronics.22 In another reported study,21 the work function of the Co surface lies within the vacuum level alignment regime, however, our findings here indicate otherwise. In addition to altering the electronic properties at the interface, the addition of a SAM also alters the hydrophobicity and chemical reactivity of the cobalt surface. The presence of SAMs strongly influences the early Alq3 growth at the interface.7 Uniform and smooth films are formed


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when Alq3 is deposited on SAMs (such as MHA) that create a hydrophilic surface. Due to spinodal dewetting, Alq3 congregates in smaller areas when it is deposited onto hydrophobic surfaces (such as ODT/Co surfaces) leading to thin films that show islands and corrugation as well as regions of exposed substrate. 7,10 Direct confirmation of Alq3 on the surfaces is confirmed by the N- and Al-derived core levels shown in the XPS spectra in Figure 2. The core level spectra are normalized to the low binding energy background to correct for slight variations of sample height to enable intensity comparison of the different samples. The intensity of the N 1s spectra differs for Alq3 on Co, ODT, and MHA, which implies that the initial growth of Alq3 depends on the surface treatment of the Co substrate.23 This observation of the variation of the Alq3 growth on bare and SAM-treated Co surfaces is also confirmed in the Co-derived XPS spectra. The Co 3s intensity decreases, as expected, when it is functionalized with SAMs and after Alq3 deposition, as shown in Figure 2b. Both the Al 2s and Co 3s photoelectron lines are shown within the same measurement window which demonstrates that the Co substrate can still be detected after SAM and Alq3 deposition. As in the case for the N 1s spectra, the Al 2s intensity fluctuates between the bare and SAM-treated Co substrates. To probe the Alq3 surface coverage and geometry, the ratio of Al atoms to Co atoms ([Al]:[Co]) was determined by using the Al 2s and Co 3s combination, and the Al 2p and Co 3p combination (not shown). The determination of [Al]:[Co] is accurate since the transmission function of the spectrometer is nearly constant in this narrow energy window (< 20 eV). We find that the [Al]:[Co] of Alq3/Co, Alq3/ODT/Co, and Alq3/MHA/Co are 0.15 ± 0.01, 0.09 ± 0.03, and 0.28 ± 0.05, respectively. The Alq3 exposure is the same for all three samples since they were fabricated within the same deposition run, but the varying amounts of [Al]:[Co] detected at the surface strongly suggests that the wetting properties of the SAMs induce different initial growth of the Alq3 layer. The


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largest [Al]:[Co] is found with the Alq3/MHA/Co sample, which is consistent with the hydrophilic (wetting) properties of MHA7,24, 25 and/or moderate wetting properties of thiolterminated SAMs26 (in the case for an “upside down” MHA attachment) leading to uniform Alq3 coverage. The smallest [Al]:[Co] is found with the Alq3 deposited onto an ODT/Co substrate, where the ODT-modified surfaces are hydrophobic27 and consistent with the expectation of dewetting and corrugated Alq3 films that leave areas of exposed substrate.7 AFM data also support our argument as described in Figure S1. Similar variations are observed in the HOMO features of the UPS spectra, where the magnitude of the HOMO peak of the Alq3/ODT/Co sample is much smaller than those of other two samples. Thus, in addition to serving as a physical barrier between Co and Alq3, the SAMs induce different structural interfacial properties. These two effects impact the degree of hybridization between the cobalt ferromagnet and Alq3. 3.2. XMCD analysis In order to gain insight on the influence of SAMs on the effective spin polarization of our Co/Alq3 systems, we performed XMCD measurements. Figure 3 shows the Co XMCD spectra at L2,3-edge on 6 different surfaces, Co, MHA/Co, ODT/Co, Alq3/Co, Alq3/MHA/Co, and Alq3/ODT/Co. All XMCD spectra have a shape very similar to metallic Co18 indicating that the Co layer was not contaminated by the top layer – e.g., there is no evidence of cobalt oxidation. Also, we estimated the ratio of the orbital magnetic moment (µL) to the spin magnetic moment (µS) of Co from each sample by using the XMCD sum rule,18 showing the calculated µL/µS ratios of those surfaces in the inset of Figure 3. Very interestingly, despite the fact that the XMCD line shapes of all surfaces look nearly the same, we found that the µL/µS ratio is quite different from one surface to the other. When the Co surface was treated with ODT or MHA, there was only a slight enhancement in the µL/µS ratio compared to that of bare Co surface (≈ 0.104). In contrast,


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when a thin layer of Alq3 was deposited on the Co, the µL/µS ratio (≈ 0.335) was greatly enhanced relative to the bare Co surface. However, the enhancement of µL/µS ratio upon the deposition of Alq3 thin layer becomes weaker when the SAMs are inserted between Alq3 and Co, indicating that SAMs may prevent the hybridization between Co and Alq3, which will be further discussed in detail in the next paragraphs on individual values of µL and µS. Individual values of µL and µS of the 6 different surfaces show further details on how the attachment of SAMs to Co influences the spin-dependent properties of the surface as illustrated in Figure 4. For the determination of µL and µS, nh = 2.49 was used as suggested in Ref. 9 for Co, where nh is the total number of 3d holes. It is known that the µS and µL at the surface of ferromagnetic transition metals can be quite different from their bulk values due to symmetry breaking, d band narrowing, and a larger value of the density of states at the Fermi level.28 In addition, when a layer of a ferromagnetic transition metal is in contact with a layer of another material, the values of µS and µL in the magnetic layer can be altered because electron hybridization at the interface can give rise to charge transfer across the interface, resulting in a change of the density of states near the Fermi level.28, 29 Recently, it was reported that the electronic structure hybridization between a ferromagnetic metal and Alq3 can induce charge transfer and magnetic coupling across the interface creating a huge interface dipole and the modification of their magnetic states.2 The values of both µS and µL of our bare Co film displayed in Figure 4 are about 20% smaller than originally reported values determined by the XMCD sum rules18, but they are considerably higher (about 57 % higher in µS and about 28 % higher in µL) than recently reported values.5 The difference in measured values from previous reports may come from the difference in exact details of prepared Co surfaces such as bonding structure between Co atoms because these thin films were deposited to be either amorphous or


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polycrystalline in nature and also they were prepared on different substrates. In Figure 4a, one can see that the trend of the change in µL of our systems coincides with that of µL/µS ratio. The value of µL of ferromagnetic transition metal thin films is known to be more sensitive to the orbital hybridization effect at the interface than that of µS because these systems have almost saturated µS.5, 28 Therefore, the change in µL/µS ratio is governed mainly by the µL. We observed a large increase in µL of Alq3/Co sample compared to the bare Co which is consistent with the hybridization between Alq3 and cobalt. However, both SAM coated samples, Alq3/MHA/Co and Alq3/ODT/Co showed smaller increases in µL than Alq3/Co, which implies that SAMs impede the electronic structure hybridization between Co and Alq3 as stated earlier. Based on the data of our Alq3/Co, MHA/Co, and ODT/Co samples, one may reach the conclusion that the change in µL of these three systems was simply due to the alteration of the Co 3d shell occupation number induced by their Co–O and Co–S bonds. As mentioned earlier, Alq3 molecules interact with a metal surface via their oxygen atoms.19 Also, the hybridization between Co and ODT occurs through the Co–S bond, but at the interface of Co and MHA, both Co–S and Co–O bonds exist. Thus, given our data, it appears that the Co–S bond has a very little impact on µL, while the Co– O bond boosts it. However, as previous studies suggest, in order to elucidate the origins of the change in µS and µL of our systems, many different factors such as angle, shape, and energy of the bonding between two materials, and the structures of molecules should be clarified and considered because all of these factors will modify the crystal field around Co atoms at the interface and thus cause the change in their 3d orbitals resulting in the modification of their electronic and magnetic structures.30-34 As displayed in Figure 4b, our MHA/Co and ODT/Co films showed a change in µS similar to µL. A relatively large increase in µS of Co was detected when MHA was deposited, but only a slight


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increase was measured when ODT was deposited. However, contrary to the change in µL, no change or rather a small reduction in µS of the Co surface was observed when Alq3 was deposited. A slight reduction of the magnetic moment of Fe (or Co) was previously reported and this was attributed to the N atoms of Alq3 gaining a magnetic moment from Fe (or Co) atoms at the interface despite the comparatively large distance between N and Fe (or Co) atoms.2 A decrease in µS despite a large enhancement in µL is possible and it was also observed in other ferromagnetic/nonmagnetic thin film system due to orbital hybridization and a change in the 3d occupation number of the ferromagnetic material.28 For example, if the 3d shell of a Co atom at the surface is initially half-full and then it loses one electron (or two electrons) when the Co comes in contact with an Alq3 molecule, the µL of Co would increase, but the µS would decrease.35 When both Co–O and Co–S bonds exist at the interface, the µL and µS of Co atoms can be altered in different ways depending on which atom they are bonded to and thus the overall change in the µL and µS obtained in our data would be the sum of outcomes resulting from both bonds. A recent theoretical study of methanethiol adsorbed on a Co (0001) surface found that the magnetic moment of the Co surface does not change after the adsorption of the molecule.31 Methanethiol is anchored onto the Co surface via a Co–S bond, similar to our ODT on Co film. The study showed that both bound and unbound Co atoms to S atoms exist at the interface in the methanethiol/Co system and the Co atoms bound to S atoms generate a slightly smaller magnetic moment than a bare Co surface, but the Co atoms unbound to S atoms produce a larger magnetic moment31 and thus those two effects compensate each other, and as a result, no quenching of the magnetic moment of the Co surface occurs. This result is found to be due to the difference in the change of the minority spin population by a different charge transfer between the molecule and Co in bound and unbound atoms.31 An increase in µS of MHA coated Co surface in our system


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which has both Co–O and Co–S bonds seems to suggest that Co atoms bound to O atoms next to Co–S bonds may increase the magnetic moment more than unbound Co atoms. Since SAMs prevent the electronic structure hybridization between Co and Alq3, we found that both SAM coated samples maintained higher values of µS even after Alq3 deposition as displayed in the data of Alq3/MHA/Co, Alq3/ODT/Co, and Alq3/Co in Figure 4b. Higher spin polarization in Co can lead to higher spin injection into Alq3 and it is known that µS corresponds to the net spinpolarization integrated over the unoccupied Co 3d band.36,37 Our data imply that the SAMmodified Co surface, especially the MHA-modified surface can help spin injection into Alq3 by increasing the spin polarization at the interface. 4. CONCLUSIONS We have carried out a study of how the insertion of a SAM (ODT and MHA) between Co and Alq3 influences the electronic structure hybridization, the energetics and band offset, and the spin-dependent characteristics at the interface by using XPS, UPS, and XMCD. Our experimental results show that both ODT and MHA decrease the work function of Co, thus producing a small injection barrier for electron spins. However, our data revealed that the Fermi level pinning between Co and Alq3 can prevent further decrease in the surface work function even after the SAM deposition. In addition, we found that the orbital magnetic moment (µL) of Co became smaller when either ODT or MHA was inserted between Co and Alq3, implying that SAMs prevent hybridization between Co and Alq3. The spin magnetic moment (µS) of Co, a main contributor to the spin polarization at the interface can be enhanced by inserting a SAM between Co and Alq3. SAMs can be used to engineer critical interface properties between metallic surfaces and organic materials. Altering the details of the self-assembled molecule such as the head group and tail group, can change band energetics, morphology, and spin-dependent


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properties at the interface of a ferromagnetic metal and an organic semiconductor. Thus, the use of SAMs for interlayer engineering can be a versatile way of tuning spin injection characteristics into an organic semiconductor.


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Figure 1. UPS spectra of MHA/Co, ODT/Co, Alq3/Co, Alq3/MHA/Co, and Alq3/ODT/Co samples. The secondary electron cutoff region in panel (a) indicates the work function. Panel (b) shows the low binding energy region near the Fermi energy, where the HOMO center is indicated with a dashed line. Molecular structures of MHA, ODT, and Alq3 and schematic view of Alq3/SAM/Co sample are illustrated in the top.

(a)

(b) Alq3/ODT/Co

Intensity Norm. to Background

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Alq3/MHA/Co Alq3/ODT/Co

Alq3/Co Alq3/MHA/Co Al 2s Co Alq3/Co

406

404

Co 3s

402

400

398

Binding Energy [eV]

396 128

120

112

104

96

Binding Energy [eV]


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Figure 2. High-resolution XPS spectra of the (a) N 1s and (b) Al 2s and Co 3s spectra of Alq3 deposited onto bare Co (green), MHA/Co (magenta), and ODT/Co (crimson). The bare Co substrate (bottom) is shown in (b) as a spectral reference.

L2 Alq3 / ODT / Co

Alq3 / MHA / Co Alq3 / Co

Intensity [a. u.]

ODT / Co MHA / Co Co

760

780

800

Alq3/ODT/Co

Alq3/Co

ODT/Co

0.0

Co

0.1

MHA/Co

L3

0.2

Alq3/MHA/Co

0.3

µ L/µ S

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


820

Photon Energy [eV]

Figure 3. XMCD spectra obtained from Co L2,3 XAS measurements reversing the saturation magnetization of Co films for Co, MHA/Co, ODT/Co, Alq3/Co, Alq3/MHA/Co, and Alq3/ODT/Co samples. Calculated ratios of the orbital magnetic moment (µL) to the spin magnetic moment (µS), µL/µS based on XMCD sum rules for all samples are plotted in the inset.


17


Uncertainty of estimated µL, µS, and their ratio is less than 5 %. Red error bars represent the maximum uncertainty of data, 5 %.

0.4 1.4

Alq3/ODT/Co

Alq3/Co

ODT/Co

0.8

MHA/Co

1.0

Alq3/MHA/Co

1.2

Co

Alq3/MHA/Co

Alq3/Co

ODT/Co

Co

0.1

MHA/Co

0.2

Alq3/ODT/Co

µ S [ µ B]

0.3

µ L [ µ B]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 26

0.0

(a)

(b)

Figure 4. Estimated (a) orbital magnetic moment (µL) and (b) the spin magnetic moment (µS) for Co, MHA/Co, ODT/Co, Alq3/Co, Alq3/MHA/Co, and Alq3/ODT/Co samples. µB is Bohr magneton. Red error bars represent the maximum uncertainty of data, 5 %.

Work function [eV]

Co

MHA/Co

ODT/Co

Alq3/Co

4.81

3.50

3.60

3.46

Alq3/MHA/Co Alq3/ODT/Co 3.40

3.5


18

Page 19 of 26

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Table 1. Measured surface work functions of Co (Ar+ sputtered), MHA/Co, ODT/Co, Alq3/Co, Alq3/MHA/Co, and Alq3/ODT/Co samples. The uncertainty of each data was ± 0.05 eV.

HOMO onset [eV]

Alq3/Co

Alq3/MHA/Co

Alq3/ODT/Co

2.00

2.10

2.05

Table 2. Measured HOMO onset values of Alq3 in Alq3/Co, Alq3/MHA/Co, and Alq3/ODT/Co samples. The uncertainty of each data was ± 0.05 eV.

AUTHOR INFORMATION Corresponding Author *[email protected]

ACKNOWLEDGMENT The authors would like to acknowledge Dr. D. J. Gundlach and Prof. O. D. Jurchescu for supporting device preparation. Synchrotron studies were carried out at the SSRL, a Directorate of SLAC and an Office of Science User Facility operated for the US DOE Office of Science by Stanford University. J.-S.L. acknowledges support by the Department of Energy, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, under contract DE-AC0276SF00515.


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Page 20 of 26

ASSOCIATED CONTENT Supporting Information Atomic Force Microscopy (AFM) data on morphology difference of Alq3 films deposited on different SAM coated surfaces (Figure S1). Infrared (IR) absorption spectra of MHA and ODT SAMs on a cobalt/gold substrate (Figure S2). XPS S 2p spectra of ODT on Co and MHA on Co before and after Alq3 deposition (Figure S3). This information is available free of charge via the Internet at http://pubs.acs.org.

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Table of Contents Graphic and Synopsis


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