4364
J. Phys. Chem. C 2008, 112, 4364-4371
Investigation of the Interfaces of Tris-(8-hydroxyquinoline) Aluminum with Ag and Al Using Surface Raman Spectroscopy Robert J. Davis and Jeanne E. Pemberton* Department of Chemistry, UniVersity of Arizona, 1306 E. UniVersity BouleVard, Tucson, Arizona 85721 ReceiVed: October 26, 2007; In Final Form: December 21, 2007
Surface Raman spectroscopy in ultrahigh vacuum is used to interrogate interfaces formed between tris-(8hydroxyquinoline) aluminum (Alq3) and vapor-deposited Ag and Al. Vapor deposition of Ag onto Alq3 films results in preservation of the Alq3 structure with evolution of simple surface enhancement of Alq3 spectral intensities. Changes in the relative intensities of ring breathing and ring stretching modes of Alq3 upon deposition of Ag are consistent with weak interaction of Ag with the conjugated ring of the ligand. In contrast, vapor deposition of Al onto Alq3 films results in the appearance of new vibrational modes, but only for Al coverages >10 Å mass thickness, consistent with the formation of an Al-Alq3 adduct. At these coverages, slight surface enhancement of spectral intensities is also observed along with growth of several broad modes consistent with partial graphitization of the organic film.
Introduction Tris-(8-hydroxyquinoline) aluminum (Alq3) is a widely used electron-transport/light-emitting layer in organic light-emitting devices (OLEDs).1 To achieve low operating voltage and high efficiency, charge injection across each of the interfaces must be optimized. The interface of organic layers, such as Alq3, with metallic cathodes is of particular interest because of the dissimilarity of the materials involved. The use of low work function metals such as K, Li, Ca, Mg, and Al as cathodes is a common method for lowering the energy barrier for charge injection from the cathode to the lowest unoccupied molecular orbital (LUMO) of the organic layer.2 However, despite the importance of Alq3 as an electron transport layer in OLEDs, little is known about the molecular structure of its interfaces with metals. Alq3-metal interfaces have been studied extensively using X-ray photoelectron spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS),3-9 and near-edge X-ray adsorption.10 However, few investigations have directly probed the molecular structure of interfaces formed between Alq3 and vapor-deposited low work function metals used to form ohmic contacts. In UPS studies of interfaces of Al, Ca, and Mg with Alq3,3,5,6,8,9 the formation of a gap state shifted from the Alq3 HOMO has been observed. In contrast, in similar UPS studies of the interface of the more noble metals Cu, Ag, and Au with Alq3, a gap state is not observed. Comparison of the interaction of reactive metals and noble metals with Alq3 is thus a useful strategy to separate true chemical reactions from other interfacial phenomena that might be occurring. Of all low work function metals used as electrodes with Alq3, Al is particularly promising as it is relatively cheap and easy to handle. Bulk Al has a relatively low work function of 4.3 eV,11 yet when incorporated into devices with Alq3, poor device performance with high driving voltage and low efficiency is often observed. This poor performance has been attributed to strong interaction between Al and Alq3.4 To mitigate this * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: 520-621-8245. Fax: 520-621-8248.
interaction, insulating layers of LiF2 and MgF212 have been utilized between the metal and the Alq3 layers; this strategy has produced improvements in device performance, although little is known about how these insulating layers function to improve charge injection across the interface. In contrast to Al, a noble metal such as Ag has a slightly higher work function of 4.7 eV11 and has been confirmed by UPS to alter the electronic structure of Alq3 little.13 These observations illustrate that, despite similar work functions, Ag and Al contacts may have vastly different interactions with Alq3. Since these metal contacts are generally vapor deposited onto Alq3 films in device fabrication, any chemistry that occurs early in the deposition process prior to bulk metal electrode formation will occur between Alq3 and individual metal atoms. Thus, the observed interactions between Al or Ag and Alq3 are more appropriately considered on the basis of atomic ionization potentials of ∼6.0 eV for Al and ∼7.5 eV for Ag.11 These values indicate that an electron is more easily transferred from Al to Alq3 than from Ag to Alq3. As shown in this report, a molecular interpretation of the interaction of vapor-deposited Al and Ag on Alq3 on the basis of their atomic ionization potentials and the electron affinity of Alq3 better fits experimental observations than an interpretation based on bulk metal work functions. The interaction between Alq3 and low work function metals has been explained traditionally by two mechanisms, charge transfer or formation of an organometallic adduct. Charge transfer from a low work function metal to Alq3 is often used to explain formation of a new electronic state observed between the Fermi level and the Alq3 HOMO. On the basis of Mulliken electronegativity values, an electron should be donated from the metal to Alq3, forming the Alq3- anion.14 Theoretical studies have shown that Li donates an electron into the quinolate ligand π system forming Alq3-. This anion is then bound through strong ionic interaction of its oxygen atoms with Li+.14 In contrast, Curoni and Andreoni suggest that Al forms an organometallic adduct with Alq3 in which Al is covalently bonded to Alq3 oxygen atoms, resulting in substantially reduced ionic bonding character compared to Li.
10.1021/jp710375c CCC: $40.75 © 2008 American Chemical Society Published on Web 02/27/2008
Tris-(8-hydroxyquinoline) Aluminum Vibrational spectroscopy studies reported on metal-Alq3 interfaces,15-17 an IR and Raman study of K-Alq3 complexes, suggest the presence of an ionic complex in which, following a charge-transfer reaction, Alq3- is present and associated with K+ counter-ions. Compared with Li and K, Al has a diminished tendency for electron-transfer based on electron affinities, and to date, no definitive proof of charge transfer has been shown for Alq3 interfaces with Al, Ca, or Mg. The formation of organometallic adducts due to chemical reaction at the interface has also been proposed to explain spectroscopic observations. Results from DFT calculations indicate two probable interactions between Al and Alq3. One interaction involves Al coupled to the benzene ring of the 8-hydroxyquinolate ligand, while the second has Al bound to two O atoms of the ligands.9 These DFT calculations suggest the latter structure to be slightly more stable. XPS measurements of Al-Alq3 interfaces confirm changes in chemical bonding of Alq3 upon contact with Al.6,8 Shen, Kahn, and Schwartz observed the formation of a new N 1s component shifted to lower BE by 1.8 eV and an O 1s component shifted to higher BE by 1.4 eV compared with bare Alq3. These observations are consistent with reactive conversion of Alq3 to a new species in which charge transfer has occurred from Al to the 8-hydroxyquinolate ligand, with the charge localized on the N and the deposited Al bound to the ligand.6 Unfortunately, because of the multitude of potential chemically distinct Al and C atoms in the product, changes in the Al 2p and C 1s levels are difficult to interpret, making further identification of new species at the interface challenging. The deposition of hot, reactive metals onto organic layers such as Alq3 was previously considered to be a potential additional source of chemical and morphological disruption of the organic film. Toward this end, studies have been undertaken on interfaces formed from Alq3 deposited onto metals used for ohmic contacts in devices. Core level shifts similar to those observed for the converse deposition sequence have been observed in XPS studies.6,18,19 However, the reactivity of organic-on-metal systems appears to be limited to a narrower interfacial region because of the lack of diffusion of the metal into the organic film.6 The high reactivity of these metal-organic interfaces further complicates efforts to characterize them. The potential for oxide formation by low work function metals requires the use of ultrahigh vacuum (UHV) to eliminate extrinsic interface reactions due to contamination.18 This study of the interaction of Alq3 with deposited metals is a continuation of previous work on the interaction of low work function metals with simple organic molecules.20 Raman spectroscopy in UHV allows structural changes to be monitored in situ while maintaining precise environmental control of these reactive systems. Raman spectroscopy has the high specificity and sensitivity required to observe changes in structure and bonding at metal-organic interfaces. In addition, the presence of certain metals at the interface can lead to significant surface enhancement of the optical signal, increasing the sensitivity of spectroscopic probes of the structure and reactivity of the first few layers of the interfacial region. Experimental Section Ultrahigh Vacuum Chamber. Alq3 films and Alq3-metal films were prepared and characterized in a custom-built ultrahigh vacuum (UHV) chamber20 that consists of a sample preparation chamber and an analysis chamber, separated by a 6” gate valve, providing a stable pressure and continuously clean environment
J. Phys. Chem. C, Vol. 112, No. 11, 2008 4365 in the analysis chamber, as shown in Figure S1 of Supporting Information. A 300 L s-1 turbomolecular pump, an ion pump, and two titanium sublimation pumps maintain a base pressure of 7 × 10-11 Torr; working pressures were seldom above 10-9 Torr. The ion and titanium sublimation pumps are liquid nitrogen-cooled to remove condensable gases (primarily H2O). Chemically polished Ag stubs were mounted on a 1.5 m Advanced Research Systems DE-204B closed-cycle He-cooled cryogenic translatable arm that could be cooled to a base operating temperature of 20 K. This arm can be translated between the sample preparation and the analysis chambers at temperature. Temperature was controlled in these studies using a Cryo-Con Model 34 cryogenic temperature controller using a type K, Chromel-AuFe thermocouple at the bottom of the cold finger within close proximity of the stub. Materials. 8-Hydroxyquioline, aluminum salt (99.995%) and aluminum foil (99.999%) were obtained from Aldrich. Silver wire (99.999%) was obtained from Alpha Aesar. Alq3 was degassed at 250 °C at 10-9 Torr for 6 h prior to deposition. A 1.25 cm diameter Ag (111) disk (99.999%, Alfa Aesar) was used as the underlying substrate for all experiments. Procedures. The procedure for obtaining chemically polished Ag surfaces has been outlined previously.21,22 Alq3 films were prepared on Ag substrates held at 300 K, similar to the conditions used in the fabrication of OLED devices. The resulting Alq3 thin films show no structural changes by Raman spectroscopy upon annealing to 400 K, consistent with the formation of amorphous or R-Alq3.23 Ag and Al were deposited after degassing onto the Alq3-Ag sample held at ∼60 K from Al2O3-coated W/Ta crucibles. Film deposition was monitored differentially using a multi-film deposition monitor (Maxtek TM-400) and a gold-coated quartz sensor crystal (Maxtek SC101). The sensor was held at room temperature for Alq3 deposition but cooled with liquid N2 during metal deposition; reported values of coverage represent mass thicknesses of the metal assuming bulk density. Raman spectra were acquired by moving the sample stub into the glass cell at the end of the analysis chamber. Radiation of 514.5 nm with 20 mW power at the sample was provided by a Coherent Innova 350C Ar+ laser. Light was incident on the sample at 60° and scattered photons were collected at 30° with respect to the surface normal. Collected radiation was collimated and focused onto the front entrance slit of a single monochromator (Spex 270M) with a 1200 gr/mm grating and set for a 5 cm-1 spectral bandpass. A holographic SuperNotch Plus filter (Kaiser Optical Systems) was used to reduce Rayleigh scattering. A 1340 × 400 pixel, thinned, back-illuminated CCD (Roper Scientific model 400-EB) held at -95 °C was used for detection. Typical integration times varied from 10 to 30 min per acquisition. Spectra were calibrated using Ar+ emission lines. Peak fitting of Raman spectra was accomplished with Grams/ 32 using 100% Gaussian peaks. Peaks at frequencies associated with intense Alq3 modes were fit using peak frequencies ((5 cm-1) and relative intensities ((10%) determined by Halls and Aroca23 with full-width at half-maximum (fwhm) values of 15 cm-1 ((10 cm-1). Modes associated with graphitic carbon were fit using five broad bands using peak frequencies ((10 cm-1), relative intensities ((50%) and fwhm ((50%) described by Ferrari and Robertson.24 All additional spectral features were fit with broad bands that were allowed to vary in frequency by (10 cm-1, in fwhm by (10 cm-1 and intensity by (50%. Results and Discussion Alq3 Films on Ag Substrates. The Raman spectrum of a 150 Å thick Alq3 film deposited onto a chemically polished
4366 J. Phys. Chem. C, Vol. 112, No. 11, 2008
Figure 1. Raman spectra of (a) 150 Å Alq3 film on Ag at 60 K and after Ag deposition of (b) 2, (c) 5, (d) 10, and (e) 20 Å mass thickness.
Ag substrate held at 60 K is shown in Figure 1. Several intense bands from Alq3, similar to those reported by Halls and Aroca,23 can be seen in this spectrum. The bands at 1595 and 1394 cm-1 correspond to the ring stretching and (ring stretching + CH bending) modes of the 8-hydroxyquinolate (8-HQ) ligand. In addition, lower frequency bands are present because of the ring breathing mode at 757 cm-1, the (Al-O stretching + Al-N stretching + ring deformation) combination modes at 647 and 543 cm-1, the (ring deformation + Al-O stretching) combination mode at 525 cm-1, and a ring deformation mode at 505 cm-1. The frequency of observed vibrational bands along with the film deposition conditions support the formation of an amorphous thin film of Alq3 composed primarily of the merisomer, the more stable of the two isomers of Alq3. Thin films of Alq3 vapor-deposited at 300 K have been shown to lack any X-ray diffraction up to a thickness of several micrometers, beyond which characteristic diffraction peaks of R-Alq3 are observed, suggesting that thin films of Alq3 are amorphous, because of the inherent polymorphism of Alq3, with a preference for formation of R-Alq3.25 Cooling of amorphous thin films deposited at 300 to 4.2 K has been shown by Brinkmann et al. to retain their amorphous structure while shifting the fluorescence maximum to lower wavelength, consistent with a slight ordering in the film resulting in greater overlap between the π systems of the 8-HQ ligand in neighboring molecules.25 Similarities to the calculated Raman spectrum of peak frequencies and relative intensities for the Alq3 film on chemically polished Ag confirms retention of Alq3 integrity upon vapor deposition as well as the lack of any charge transport or other reactive process between Alq3 and the Ag substrate. This behavior is consistent with the expected lack of reactivity based on the relative electron affinities and work functions of Alq3 and Ag. Postdeposition of Ag onto Thin Alq3 Films. In contrast to the deposition of Alq3 onto Ag, the deposition of Ag onto 150 Å thick Alq3 films supported on Ag substrates held at 60 K does induce significant changes in Raman spectral features. A complete listing of all bands and their assignments23 is provided in Table 1. Bands are attributed to either native Raman-active modes or IR-active modes that have been Raman-activated
Davis and Pemberton because of a loss in symmetry of Alq3 upon deposition of Ag. Although deposition of 2 Å mass thickness of Ag onto Alq3 (Figure 1b) does not induce observable spectral changes, the deposition of 5 Å (Figure 1c) produces an intensity increase in the four major bands at 1595, 1394, 757, and 525 cm-1 associated with the pristine Alq3 film. Several new modes not clearly identifiable in the pristine Alq3 film on Ag also appear after 5 Å Ag deposition, with the most intense observed at 1500, 1060, 862, and 470 cm-1. At Ag coverages >5 Å (Figure 1ce), all modes increase in intensity up to Ag coverages of 20 Å, consistent with surface enhancement by the deposited metal. This effect is shown quantitatively by the plot in Figure 2 as an increase in intensity of all modes for Ag coverages between 5 and 20 Å. For Ag coverages >20 Å, all modes decrease in intensity, consistent with formation of a thick metallic overlayer that reduces scattering efficiency and increases reflectivity of the composite film. The notable enhancement of Alq3 Raman modes for Ag coverages >5 Å is attributed to the formation of small Ag particles with metallic properties at the metal-organic interface with concomitant inducement of electromagnetic enhancement through coupling of the exciting radiation with surface plasmons of the Ag particles. In addition to a change in overall intensity, intensity ratios also change compared with those of the pristine Alq3 film, with peaks at 1595 and 757 cm-1 increasing relative to those at 1394 and 525 cm-1. These changes in relative intensity and the appearance of Raman-inactive modes at 1497 and 790 cm-1 (assigned to ring stretching and CH wagging modes, respectively)23 are consistent with a reduction in symmetry of Alq3 due to weak interaction of the deposited Ag with the aromatic ring system of the 8-HQ ligand. Although the observation of surface enhancement for Ag deposited on Alq3 has been previously reported,26 this is the first report of an accompanying substantial change in relative intensities of the Raman modes. This observation suggests that deposited Ag atoms interact with Alq3, leading to a lowering in symmetry, although this interaction preserves the basic attributes of Alq3 structure and bonding and is not consistent with charge transfer or reactive destruction of Alq3. It should be noted here that the Alq3 film ordering induced by cooling of the thin film is not expected to significantly impact the interaction of Alq3 with deposited Ag; however, the reduced temperature may reduce the diffusion of Ag atoms at the metal/organic interface, limiting the width of the interfacial region. Postdeposition of Al onto Alq3 Films. Al has a low atomic ionization potential that supports electron transfer to the conjugated 8-HQ ring and is widely considered to undergo strong interaction with the organic layer.4,9,18,19,27 Figure 3 shows Raman spectra at 60 K of a pristine 150 Å Alq3 film before Al deposition (Figure 3a) and after deposition of Al layers ranging in mass thickness from 5 to 20 Å. Figure 3b shows the spectrum after Al deposition of 5 Å. All Alq3 modes in this spectrum exhibit a 50-60% intensity decrease compared with those from the pristine Alq3 thin film (Figure 4). In addition, new modes appear at 1546, 1215, 1062, 951, and 580 cm-1 that are not consistent with modes for Alq3, although these modes appear in frequency regions consistent with structures closely related to 8-hydroxyquinoline and/or with Alq3 modes that are not normally Raman active. The broad bands at 543 and 647 cm-1 increase in intensity and are in a spectral region consistent with ν(Al-O) vibrations as well as δ(Al-O-C) modes.23 The assignment of these broad bands to new combination [ν(Al-O) + δ(Al-O-C)] modes is consistent with formation of an Al-Alq3 adduct in which the
Tris-(8-hydroxyquinoline) Aluminum
J. Phys. Chem. C, Vol. 112, No. 11, 2008 4367
TABLE 1: Raman Peak Frequencies and Assignments for a Pristine Alq3 Film and after Deposition of 20 Å of Ag and Al peak frequency (cm-1) a
literature Alq3
150 Å Alq3
20 Å Ag on Alq3
1595
1605 1595
1424
1500 1471 1424
20 Å Al on Alq3 1620
1605 1595 1500 1471 1424 1394 1386
1394
1394 1386
1595, 1584 1560 1526 1503 1465 1405 1394 1355
1335 1285 1230
1335 1285 1230
1173 1133 1117
1173 1133 1117
1060 1036
1060
1060 1036
1284 1216 1215 1207
1100 1062 998
950
950
918 862 825 804 789 757 647
918 862 825 804 789 757 647
951
757 647
596
596
577 541
577 543
878 825 790 757 647 580
525 504 470 428 404 364 a
543 525 505 470
525 505 470 428 404 364
543 525 505 468
mode assignmentb νring graphite D′-band νring νring graphite G-band νring νring νring νring + δ(C-H) graphite G-band νring + δ(C-H) δ(C-H) + νring graphite D-band νring + ν(C-O) + δ(C-H) δ(C-H) ν(C-N) + δ(C-H) ν(C-N) ν(C-N) + δ(C-H) δ(C-H) δ(C-H) ν(C-O) + δ(C-H) graphite D-band ν(C-O) + δ(C-H) δ(C-H) δ(C-H)wag δ(C-H)wag ν (Al-N) + δ(C-H)wag δring + ν (Al-N) δ(C-H)wag δ(C-H)wag δring δ(C-H)wag νring [ν(Al-O) + ν(Al-N) + δring]Alq3, [ν(Al-O) + δ(Al-O-C)]adduct CH torsion graphitic δring δring + δ(Al-O-C) [ν(Al-O) + ν(Al-N) + δring]Alq3, [ν(Al-O) + δ(Al-O-C)]adduct δring + ν(Al-O) δring CH torsion δring δring + ν(Al-O) δring + δ(Al-O-C)
From ref 23. b Assignments for Alq3 from ref 23 and those from graphite from refs 24 and 28.
deposited Al atom binds to the 8-HQ ligand O atom through its lone pair of electrons, as depicted in the drawing of the proposed Al-Alq3 products for the fac- and mer-isomers in Figure 5a,b, respectively. The product shown in Figure 5a is consistent with the lowest energy structure calculated by Andreoni and Curioni for the Al-(fac-Alq3) adduct in which the Al was proposed to bind only to 8-HQ O atoms.14 These researchers reported that Al-(fac-Alq3) is more stable than the corresponding Al-(mer-Alq3) shown in Figure 5b because of 3-coordinate binding compared with 2-coordinate binding. These authors also theorized that mer-Alq3, the more stable Alq3 isomer that is presumed to be dominant in thin films, may convert to fac-Alq3 upon interaction with deposited Al.14 However, the multitude of spectral changes in the spectral region of the Raman spectrum associated with Al-N and Al-O bonding upon Al deposition (Figure 3) prohibits definitive confirmation of the conversion from mer- to fac-Alq3. Thus, it is assumed that either Al-(fac-Alq3) or Al-(mer-Alq3) or both are possible products. Further work is needed to identify spectral characteristics of the product isomers. The formation of new Al-O bonding proposed on the basis of the increased Raman bands at 543 and 647 cm-1 is further supported by the XPS observations of Shen, Kahn, and
Schwartz.19 These researchers observed a new O 1s component in this system, shifted by 1.4 eV to higher binding energy relative to pristine Alq3, with increasing Al deposition, and argue that this new feature is consistent with a decrease in electron density of O due to binding with Al.19 Neither the structure of the calculated Al-Alq3 adduct14 nor the XPS observations19 support bonding between deposited Al and 8-HQ N atoms. Thus, based on the predicted structure of the Al-Alq3 adduct and the experimental XPS results for the O 1s and N 1s core levels, the Raman bands at 543 and 647 cm-1 are confidently assigned to [ν(Al-O) + δ(Al-O-C)] combination modes resulting from binding of deposited Al to 8-HQ O atoms. Additional evidence for Al-O bonding with an Al coverage of 5 Å is the observation of a new mode at 1062 cm-1. This mode is assigned as a ν(C-O) that is shifted from the ν(C-O) of Alq3 at 1117 cm-1.23 The 55 cm-1 shift to lower frequency is consistent with an increase in reduced mass due to the bonding of Al to 8-HQ O atoms. Although the ν(C-O) is too weak to observe in the Raman spectrum of pristine Alq3 (Figure 3a), this mode is also observed in the surface enhanced spectrum of Alq3 after 20 Å Ag deposition (Figure 1e). The increase in
4368 J. Phys. Chem. C, Vol. 112, No. 11, 2008
Figure 2. Plot of normalized Raman intensity as function of metal coverage for Ag deposited onto Alq3 for [ring deformation + ν(AlO)] at 525 cm-1 (9), [ν(Al-O) + ν(Al-N)] at 647 cm-1 (red B), νring at 757 cm-1 (green 2), δ(CH) at 1060 cm-1 (blue 1), νring at 1394 cm-1 (light blue +), and νring at 1595 cm-1 (pink solid left-pointing triangle). All modes normalized to intensities of respective bands in pristine Alq3.
Figure 3. Peak fitting of Raman spectra of (a) 150 Å Alq3 films in Figure 4 after Al deposition of (b) 5, (c) 10, (d) 15, and (e) 20 Å mass thickness. Raw Raman spectrum (solid line), overall fit envelope (dashed black line), Alq3 modes (dashed red line), Al-Alq3 reaction product modes (dashed green line), and graphitic carbon modes (dashed blue line).
intensity of the ν(C-O) after 5 Å Al deposition is rationalized as an increase in polarizability of the C-O due to binding of Al.
Davis and Pemberton
Figure 4. Plot of normalized Raman intensity as function of metal coverage for Al on Alq3 for the graphitic δring at 580 cm-1 (9), ν(AlN) at 951 cm-1 (red B), ν(C-O) at 1062 cm-1 (green 2), graphitic D-band at 1355 cm-1 (blue 1), νring at 1394 cm-1 (light blue +), graphitic G-band at 1560 cm-1 (pink solid left-pointing triangle), and νring at 1595 cm-1 (yellow solid right-pointing triangle). All modes normalized to intensity of νring at 1394 cm-1 of pristine Alq3.
The new modes observed at 1546 and 580 cm-1 with an Al coverage of 5 Å are in the frequency regions associated with Alq3 ring stretching and ring deformation modes, respectively. The decrease in frequency of this mode relative to that for the pristine Alq3 film at 1595 cm-1 is explained by binding of Al to the Alq3 complex, resulting either in an increase in mass and/ or in a lowering of ring electron density, leading to a decrease in the force constant for this mode. The absence of any observable ν(Al-C) modes in the spectrum of Figure 3b suggests that Al does not bond directly to the ring. Thus, the only remaining rationalization for an effect on ring breathing frequency must be an indirect electronic effect such as removal of ring electron density by interaction of Alq3 with deposited Al. Interestingly, no frequency shift is observed for the ring stretch at 1394 cm-1, although this mode does decrease in intensity relative to the bands at 1595 and 525 cm-1 after deposition of 5 Å Al. This 1394 cm-1 band appears in the spectrum because of resonance coupling of the two rings in 8-HQ;23 thus, this mode might be expected to decrease in intensity upon removal of electron density due to the resulting reduction in resonance coupling between the rings. The band observed at 1215 cm-1 after deposition of 5 Å Al is attributed to a ν(C-N) that is decreased in frequency relative to the 1230 cm-1 frequency of pristine Alq3.23 This shift can be explained either as an increase in reduced mass due to the bonding of Al to C or N or as a reduction in electron density of the 8-HQ pyridyl ring. As noted above, the presence of Al-N bonding is not supported by previous theoretical or experimental work, and no evidence for Al-C bond formation is observed in the Raman spectrum. Thus, the shift in the ν(C-N) mode to lower frequency must be attributed to a reduction in pyridyl ring electron density. The shifts in the ring stretching and ν(C-N) modes after 5 Å Al deposition is consistent with interaction of Alδ+ with the π system of the pyridyl ring, leading to a decrease in electron density of the pyridyl ring. This decrease in electron density results in weakening of the ring CdC and C-N bonds and loss of resonance coupling between the two 8-HQ rings. The π system of the pyridyl ring is more destabilized toward this electron density decrease than the π system of the benzene ring due to the presence of the N heteroatom. Given the geometry
Tris-(8-hydroxyquinoline) Aluminum
J. Phys. Chem. C, Vol. 112, No. 11, 2008 4369
Figure 6. Proposed structure for Al inserted into R-crystal phase of mer-Alq3. C, N, O, and Al atoms of Alq3 shown in gray, blue, red and green, respectively; vapor deposited Al atom shown in yellow. H atoms omitted for clarity.
Figure 5. Proposed Al-Alq3 adduct structures in which Al is bonded to Alq3 through ligand O atoms for (a) fac-isomer of Alq3 and (b) merisomer of Alq3. C, H, N, O, and Al atoms of Alq3 shown in gray, white, blue, red and green, respectively; vapor deposited Al atom shown in yellow.
of the proposed Al-(mer-Alq3) product in Figure 5b, the π density that interacts with Alδ+ must be donated by an adjacent Alq3 molecule, thereby satisfying the presumed Alδ+ coordination number of three. The proximity of the deposited Al atom in the Al-(mer-Alq3) adduct to the π system of 8-HQ in a neighboring Alq3 for R-(mer-Alq3) can be seen more clearly in the R-(mer-Alq3) crystal structure in Figure 6. The R-(mer-Alq3) structure is the lowest temperature crystal structure for Alq3 that most likely resembles the amorphous films prepared here.25 Further evidence for interaction of at least one pyridyl ring with the inserted Al atom is the increase in frequency of the ν(Al-N) mode from 91823 to 951 cm-1. This shift is rationalized as a strengthening of the Al-N bond due to electron donation from the inserted Al to the pyridyl ring, thereby shortening and strengthening the Al-N bond involving the adducted Al.14 This explanation is supported by the XPS observations of Shen, Kahn, and Schwartz that the N 1s core level shifts to lower binding energy upon Al deposition, consistent with an increase in electron density on the N.19 Although on the basis of the Al atomic ionization potential and the Alq3 electron affinity electron transfer from Al to Alq3 is predicted, no spectral evidence is observed for formation of
the Alq3 anion after 5 Å Al deposition. The presence of additional electron density in the 8-HQ π system might be expected to result in a shift to higher frequency for the ring stretch at 1595 cm-1 and the ν(C-N) at 1230 cm-1. In fact, shifts in the opposite direction are actually observed, consistent with a decrease in ring electron density. This behavior suggests that a simple charge-transfer model is insufficient to explain the interaction between Al and Alq3. The deposition of 10 to 20 Å of Al onto an Alq3 film at 60 K (Figures 3c-e) produces significant spectral changes compared to the 5 Å Al coverage (Figure 3b). Most significant are several broad modes between 1100 and 1650 cm-1 that appear after deposition of 10 Å Al and become increasingly prominent with increasing Al deposition. Peak fitting of this region to 100% Gaussian bands reveals two intense bands at 1560 and 1355 cm-1 whose breadth and frequency are consistent with the presence of graphitic carbon.24 In addition, the broad band observed at 580 cm-1 corresponds to a weak out-of-plane mode of amorphous graphite previously observed in sp3-rich forms.28 The peak at 1560 cm-1 is attributed to the G-band of graphitic carbon, a mode associated with in-plane stretching in continuous graphitic sheets.24 The known low-frequency asymmetry of the G-band is fit using an additional Gaussian band at 1400 cm-1. The peak at 1355 cm-1 is attributed to the D-band of graphitic carbon, a ring breathing mode that is forbidden in perfect graphite and whose presence indicates disorder within the graphite.24 As with the G-band, the known low-frequency asymmetry of the D-band is fit using an additional broad band at 1100 cm-1. An additional graphitic mode is present at 1620 cm-1, assigned to the D′-band, an overtone mode associated with the presence of polycrystalline graphite in small grains.24 The ratio of peak intensities of the fits for the D-band to the G-band is ∼0.5 for all Al coverages between 10 and 20 Å, indicating the presence of amorphous or nanocrystalline graphite.24 The D′-band at 1620 cm-1 increases in intensity with Al coverage indicating the clustering of graphite into larger sheets.24
4370 J. Phys. Chem. C, Vol. 112, No. 11, 2008 A similar progressive conversion to graphitic carbon has been reported for the deposition of Al onto C60 thin films.29,30 In addition to the appearance of graphitic carbon modes at Al coverages of 10 to 20 Å, modes associated with the AlAlq3 product at 1215, 1062, 951, 647, and 543 cm-1 also intensify. The ring stretching mode at 1546 cm-1, observed for the Al-Alq3 product with an Al coverage of 5 Å, is, however, difficult to observe for 10-20 Å Al coverage because of the appearance of the graphitic carbon G-band in this region. As shown in Figure 4, the Al-Alq3 product modes at 1062 and 951 cm-1 increase somewhat linearly with Al deposition of 5, 10, and 15 Å before leveling off at Al coverages of 15-20 Å with a concomitant decrease in intensity of the ring stretching modes at 1394 and 1595 cm-1 of pristine Alq3. This indicates that formation of the Al-Alq3 adduct is limited to the interfacial region and that the reaction is limited by the availability of Alq3 within this interface. The graphitic G-band at 1560 cm-1 and D-band at 1355 cm-1 also increase linearly with Al coverage between 5 and 20 Å, although the slope of the intensity increase with coverage of these modes is greater than what is observed for modes of the Al-Alq3 adduct. The intensity-Al coverage behavior of the graphite modes allows speculation about the sequence of interfacial events that eventually lead to formation of graphitic carbon. The initial Al deposited on the Alq3 film is apparently accommodated by diffusion into the Alq3 film, presumably forming Al-Alq3 adducts. This adduct formation occurs to a currently unknown, albeit relatively shallow, depth within the Alq3 film. After deposition of ∼5 Å of Al, Alq3 within this interfacial region becomes saturated with Al in terms of its capacity to form AlAlq3 adducts, and further adduct formation is limited by the diffusion of Al into the solid. This process is hypothesized to become kinetically limiting such that additional Al atoms deposited undergo electron-transfer reactions with interfacial Al-Alq3 adducts before having time to diffuse into the solid to find free Alq3 with which to form adducts. The additional electrons transferred to the Al-Alq3 adducts at the interface are thought to create radical species which promote graphitization. This sequence of events, although plausible based on the data presented here, remains to be verified by additional detailed studies and, hence, must be viewed as speculative. Such studies are underway in this laboratory and will be reported at a later date. Several bands not previously assigned to the Al-Alq3 adduct or to graphitic carbon also appear and increase in intensity upon deposition of 10-20 Å Al. These bands appear at frequencies consistent with vibrational modes of Alq3 and include peaks in the region of ring deformations at 468 and 505 cm-1, CH wags at 790, 825, and 998 cm-1, a (CH wag + ring deformation) at 878 cm-1, a [δ(C-H) + ν(C-N)] at 1215 cm-1, a δ(C-H) at 1284 cm-1, and ring stretching modes at 1465, 1503, 1526, and 1584 cm-1.23 All new peaks are associated with CH wags, CH bends, ring deformation, or ring stretching modes of the ring system of 8-HQ. Other studies in this laboratory on the Raman spectroscopy of Al deposited onto benzene thin films suggest an η6 π interaction in which Al binds to the face of the electronrich ring.31 This interaction produces several new vibrational modes at 655, 780, 868, 950, 1027, 1475, and 1572 cm-1 that are similar in frequency to those observed here for Al deposited onto Alq3, suggesting a similar interaction. DFT simulations performed by Morikawa and co-workers on the Al-Alq3 adduct also indicate bonding of the initial layer of deposited Al atoms to mer-Alq3 through O atoms, with subsequent Al layers leading to the formation of weak Al-C and Al-N bonding between
Davis and Pemberton adatoms and the 8-HQ ring system, stabilized by the formation of Al clusters.32 Finally, a comment on the temperature conditions under which these studies were performed and how these relate to device fabrication conditions is warranted. The studies reported here were done for film temperatures of 60 K to ensure a high sticking coefficient of Al on the Alq3 film. Although this temperature is not believed to significantly alter the structures of the Alq3 film or products formed between Al and Alq3, cooling of the samples may result in reduced diffusion of Al, limiting the depth of the reaction with Alq3 and creating a narrower metal/organic interfacial region than may be formed in real devices produced at room temperature. In order to completely understand the nature of the interaction of Al with Alq3, further experimental work is needed in which the interactions of Al with simple organic constituents of Alq3, including benzene, phenol, pyridine, quinoline, and 8-hydroxyquinoline, are characterized. These studies are currently underway in this laboratory and will be reported at a later date. Conclusions Raman spectroscopy of vapor-deposited thin Alq3 films on Ag substrates confirms the retention of Alq3 molecular structure and the absence of strong reaction between Alq3 and Ag substrates. In contrast, the deposition of Ag onto Alq3 produces considerable enhancement of all Raman modes as well as the appearance of previously Raman-inactive modes due to local symmetry reduction. A significant change in relative intensity of the major Alq3 ring breathing modes suggests for the first time a significant (but nonreactive) association between deposited Ag and Alq3. The relative inertness of the Ag-Alq3 interface is consistent with the stability predicted by the atomic ionization potential of Ag and the electron affinity of Alq3. Raman spectra of Alq3 films after deposition of 5 to 20 Å Al exhibit significant spectral changes. With 5 Å of Al, the decrease in intensity of all major Alq3 modes coupled with the appearance of new modes support the partial reactive conversion of Alq3 to a new product in which Al is bound to Alq3 through the O atoms of 8-HQ. As proposed by Andreoni and Curioni, merAlq3, the more stable Alq3 isomer, may undergo isomerization upon contact with deposited Al to form the Al-(fac-Alq3) adduct, the more stable isomer for the Al-Alq3 adduct.14 However, a reduction in symmetry of the Al-N and Al-O modes of Alq3 expected for a conversion from mer-Alq3 to facAlq upon Al doping is not readily observable because of the appearance of new ν(Al-O) and shifts in the ν(Al-N) upon bonding with the deposited Al. The formation of this Al-Alq3 adduct is further supported by previous theoretical calculations of possible Al-Alq3 structures and XPS results detailing the electronic consequences of the interaction of Al with Alq3.14,19 The appearance and growth of graphitic carbon modes at 1560 and 1355 cm-1 for Al coverages of 10-20 Å indicate a degradation pathway for the conversion of Alq3 to graphitic carbon. At low Al coverage, this graphitic carbon is in the form of small nanocrystalline or amorphous clusters that convert to larger continuous graphitic sheets as Al coverage increases. This graphite is thought to be the result of degradation and not sputtering by hot metal atoms, since the Raman spectrum for the Ag-Alq3 system shows no evidence of similar graphitization despite the higher Ag deposition temperature of 857 °C compared with 677 °C for Al.11 The presence of graphite at the Al-Alq3 interface has profound implications for interfacial electronic structure and conductivity in functioning electronic devices. The graphitization reaction observed in this study at
Tris-(8-hydroxyquinoline) Aluminum 60 K is expected to be retained at room-temperature, commensurate with the conditions used for device fabrication, although diffusion may limit the extent of the reaction and broadness of the interfacial region at 100K. The multitude of spectral changes observed due to the reactive conversion of Alq3 to an adduct species at low Al coverage, with additional graphitization at higher Al coverage, indicates a more chemically complex and varied interfacial region than previously reported. The observed spectral changes for these metal-organic systems speak to the power of surface Raman spectroscopy for molecular characterization of metal-organic interfaces. The specificity of vibrational spectroscopy for monitoring changes in molecular structure coupled with the sensitivity provided by resonance enhancement and possible electromagnetic enhancement at the metal-organic interface make Raman spectroscopy an ideal tool for probing such interfaces. Acknowledgment. The authors gratefully acknowledge support of this research by the National Science Foundation (CHE-0317114). Supporting Information Available: Custom-built UHV Raman system. This material is available free of charge via the internet at http://pubs.acs.org. References and Notes (1) Tang, C. W.; Vanslyke, S. A. Appl. Phys. Lett. 1987, 51, 913. (2) Mason, M. G.; et al. J. Appl. Phys. 2001, 89, 2756. (3) Choong, V. E.; Mason, M. G.; Tang, C. W.; Gao, Y. Appl. Phys. Lett. 1998, 72, 2689. (4) Le, Q. T.; Yan, L.; Gao, Y.; Mason, M. G.; Giesen, D. J.; Tang, C. W. J. Appl. Phys. 2000, 87, 375. (5) Rajagopal, A.; Kahn, A. J. Appl. Phys. 1998, 84, 355. (6) Shen, C.; Kahn, A.; Schwartz, J. J. App. Phy. 2001, 89, 449. (7) Turak, A.; Grozea, D.; Feng, X. D.; Lu, Z. H.; Aziz, H.; Hor, A. M. Appl. Phys. Lett. 2002, 81, 766.
J. Phys. Chem. C, Vol. 112, No. 11, 2008 4371 (8) Yan, L.; Mason, M. G.; Tang, C. W.; Gao, Y. L. Appl. Surf. Sci. 2001, 175, 412. (9) Zhang, R. Q.; Lu, W. C.; Lee, C. S.; Hung, L. S.; Lee, S. T. J. Chem. Phys. 2002, 116, 8827. (10) Yokoyama, T.; Ishii, H.; Matsuie, N.; Kanai, K.; Ito, E.; Fujimori, A.; Araki, T.; Ouchi, Y.; Seki, K. Synth. Met. 2005, 152, 277. (11) CRC Handbook of Chemistry and Physics, 64th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1984. (12) Park, Y.; Lee, J.; Lee, S. K.; Kim, D. Y. App. Phys. Lett. 2001, 79, 105. (13) Hill, I. G.; Makinen, A. J.; Kafafi, Z. H. Appl. Phys. Lett. 2000, 77, 1825. (14) Curioni, A.; Andreoni, W. J. Amer. Chem. Soc. 1999, 121, 8216. (15) Sakurai, Y.; Hosoi, Y.; Ishii, H.; Ouchi, Y.; Salvan, G.; Kobitski, A.; Kampen, T. U.; Zahn, D. R. T.; Seki, K. J. Appl. Phys. 2004, 96, 5534. (16) Sakurai, Y.; Salvan, G.; Hosoi, Y.; Ishii, H.; Ouchi, Y.; Seki, K.; Kampen, T. U.; Zahn, D. R. T. Appl. Surf. Sci. 2002, 190, 382. (17) Sakurai, Y.; Yokoyama, T.; Hosoi, Y.; Ishii, H.; Ouchi, Y.; Salvan, G.; Kobitski, A.; Kampen, T. U.; Zahn, D. R. T.; Seki, K. Synth. Met. 2005, 154, 161. (18) Shen, C. F.; Hill, I. G.; Kahn, A. AdV. Mater. 1999, 11, 1523. (19) Shen, C.; Kahn, A.; Schwartz, J. J. Appl. Phys. 2001, 89, 449. (20) Hawkridge, A. M.; Pemberton, J. E. J. Am. Chem. Soc. 2003, 125, 624. (21) Smolinski, S.; Zelenay, P.; Sobkowski, J. J. Electroanal. Chem. 1998, 442, 41. (22) Tiani, D. J.; Pemberton, J. E. Langmuir 2003, 19, 6422. (23) Halls, M. D.; Aroca, R. Can. J. Chem. 1998, 76, 1730. (24) Ferrari, A. C.; Robertson, J. Phys. ReV. B 2000, 61, 14095. (25) Brinkmann, M.; Gadret, G.; Muccini, M.; Taliani, C.; Masciocchi, N.; Sironi, A. J. Am. Chem. Soc. 2000, 122, 5147. (26) Salvan, G.; Sakurai, Y.; Kobitski, A. Y.; Scholz, R.; Astilean, S.; Kampen, T. U.; Zahn, D. R. T.; Ishii, H.; Seki, K. Appl. Surf. Sci. 2002, 190, 371. (27) Yanagisawa, S.; Morikawa, Y. Jpn. J. Appl. Phys. 2006, 45, 413. (28) Tamor, M. A.; Vassell, W. C. J. Appl. Phys. 1994, 76, 3823. (29) Hebard, A. F.; Eom, C. B.; Iwasa, Y.; Lyons, K. B.; Thomas, G. A.; Rapkine, D. H.; Fleming, R. M.; Haddon, R. C.; Phillips, J. M.; Marshall, J. H.; Eick, R. H. Phys. ReV. B 1994, 50, 17740. (30) Khalid, F. A.; Beffort, O.; Klotz, U. E.; Keller, B. A.; Gasser, P.; Vaucher, S. Acta Mater. 2003, 51, 4575. (31) McKee, M. L. J. Phys. Chem. 1991, 95, 7247. (32) Takeuchi, K.; Yanagisawa, S.; Morikawa, Y. Sci. Tech. AdV. Mater. 2007, 8, 191.
