Crystalline Molybdenum Oxide Thin-Films for Application as Interfacial Layers in Optoelectronic Devices

Transcription

1 Supporting Information Crystalline Molybdenum Oxide Thin-Films for Application as Interfacial Layers in Optoelectronic Devices André L. F. Cauduro 1,*, Roberto dos Reis 2, Gong Chen 2, Andreas K. Schmid 2, Christophe Méthivier 3, Horst-Günter Rubahn 1, Léo Bossard-Giannesini 4, Hervé Cruguel 4, Nadine Witkowski 4, and Morten Madsen 1,** 1 NanoSYD, University of Southern Denmark, Alsion 2, 6400-Sønderborg, Denmark 2 National Center for Electron Microscopy, The Molecular Foundry, LBNL, Berkeley, CA, 94720, US 3 Sorbonne Universités, UPMC Univ Paris 06, CNRS UMR 7197, Laboratoire de Réactivité de Surface (LRS), 4 place Jussieu, Paris, France 4 Sorbonne Universités, UPMC Univ Paris 06, UMR CNRS 7588, Institut des Nanosciences de Paris (INSP), 4 place Jussieu, Paris, France * Electronic mail: cauduro@mci.sdu.dk ** Electronic mail: madsen@mci.sdu.dk S-1

2 Supplementary note#1. XPS, Peak-Force AFM and LEED of the specimen displayed in the main text. Figure S1, S2, S3 and S4 show the complementary analysis of the sample within the main text of the article. Figure S1 shows the XPS C1s evolution with temperature, indicating that after post-annealing at 500 o C the surface is totally clean with no evidence of carbon. Figure S2 shows, on the other hand, the O1s peak evolution, indicating that the O intensity increases as a function of temperature. Here the C contamination was mostly like screening off the photoemitted electrons from the O1s, explaining why the O1s peaks increases with temperature- although here the defect band d1 increases together with the work function, indicating a trade-off between carbon concentration and defect density. Figure S3 displays the mechanical analysis of the surface after post-annealing in UHV at 500 o C. This is the first evidence that the surface is composed by a variety of oxides composed by different stoichiometry. Lastly, Figure S4 displays the LEED pattern where no evidence of a crystalline structure is seen. Supplementary information: XPS analysis. Figure S1. Temperature evolution of the carbon C1s peak via XPS. The dashed line is a guide to the eye and it represents the reduction of the carbon concentration (at %) as a function of annealing temperature. At 500 o C no carbon contamination could be quantified. S-2

3 Figure S2. Temperature evolution of the oxygen O1s peak via XPS. The O 1s peak increases with the temperature, being related to the decrease of the C1s peak. Carbon acts as screening the photoelectrons from O 1s sub shell. Supplementary information: Peak-Force AFM. Figure S3. Mechanical properties of the MoO x nanoaggregates formed at 500 o C. (a) Topography image, (b) color map of the Log of the elastic modulus of the sample based on the Derjaguin, Muller and Toropov (DMT) model. 1 (c) Color map of the adhesion forces of the surface. The adhesion map indicates regions composed by a stiffer MoO x phase, which is depicted by the dark-blue regions shown in (c). S-3

4 Supplementary material information of the surface microstructure: LEED Figure S4. Low-energy electron diffraction (LEED) pattern. LEED taken from the sample flash annealed at 500 º C showing no evidence of surface crystallization. Supplementary note#2. XPS and UPS measurements of the highly reduced sample: Towards the formation of MoO 2. Figure S5 and S6 and Table S1 display the valence band, Mo 3d 5/2 and XPS corelevel analysis of the MoO x (x~2.6) thin-film that was further reduced at 500 o C. Here, the intention is to confirm if the origin of the raise of the DOS at the E F formed at 500 o C (Figure 1c) is consistent, showing now the formation of a very reduced metallic-oxide with larger amount of, specially, Mo +4 and Mo +5 oxidation states. Work function measurements were done by evaluating the electron cut-off energy, as displayed in Figure S5a, which demonstrates a decrease from ~6.3 ev (Figure 1a and1b in the main text) down to ~6.1 ev for the highly reduced MoO x. Valence-band edge displayed in Figure S5b (and inset) indicates a higher density of filled states near the Fermi edge, indicating a clear metallic character of the sample, towards the formation of MoO 2 thin-films (metallic-oxide). 2 As explained by Greiner et al. 2 substantial S-4

5 changes in the density of states near the Fermi level are expected to change dramatically the carrier concentration and therefore the conductivity of the surface, but yet quite deep electronic levels are present in these metallic-oxides, being quite interesting materials for acting as hole injection/extraction of a variety of organic, inorganic and hybrid compounds. 3 We have shown recently that by just varying the [O] concentration in the direct-current reactive sputtering process, one can change e.g. the resistivity by 5 orders of magnitude on the as-deposited amorphous MoO x thin-films 4 via defect engineering during the growth process. Figure S5. UPS spectrum of the highly reduced MoOx (x~2.6). The left panel (a) depicts the photoemission onset and the right panel (b) displays the density of filled states close to the valence band edge and the inset shows in detail the region near the E F. Work function evaluation was done by linear extrapolation of the electron cut-off region to zero energy and then by subtracting this value to the photon energy (21.22 ev). The black line represents the linear fitting. A clear metallic character after further annealing the sample is observed: The valence band extends up to the Fermi level, and now possesses a higher DOS near the E F in comparison with the one shown in Figure 1c. S-5

6 Figure S6. XPS spectrum for the highly reduced MoO x (x~2.6). A reduced MoO x thin-film oxide with an increased concentration of Mo +4 is seen, standing for the higher DOS at the E F depicted in Figure S3. Table S1. XPS fitting parameters and corresponding work function evaluated from a different specimen further reduced at 500 o C. T [ o C] 500 o C ev (18.49%) a Mo 3d 5/2 Binding energy [ev] Peak area (%) AVG a oxidation state FWHM [ev] a (+4/+5/+6) Φ [ev] Mo +4 Mo +5 Mo ev eV /1.70/ (41.51%) (40.00%) a Asymmetric Gaussian-Lorentzian peaks were used to fit all XPS spectra using Shirley background. Movie S1. Sets of LEEM images in Mirror Electron Microscopy (MEM) mode showing the sample's morphology as the landing energy is swept from 0.1 V up to 8.0 V. Work function maps shown in the manuscript were evaluated from the same area seen in this movie. References (1) Derjaguin, B. V.; Muller, V. M.; Toropov, Y. P. Effect of Contact Deformations on the Adhesion of Particles. J. Colloid Interface Sci. 1975, 53, (2) Greiner, M. T.; Helander, M. G.; Wang, Z. B.; Tang, W. M.; Qiu, J.; Lu, Z. H. A Metallic S-7

7 Molybdenum Suboxide Buffer Layer for Organic Electronic Devices. Appl. Phys. Lett. 2010, 96, (3) Greiner, M. T.; Helander, M. G.; Tang, W.-M.; Wang, Z.-B.; Qiu, J.; Lu, Z.-H. Universal Energy-Level Alignment of Molecules on Metal Oxides. Nat. Mater. 2012, 11, (4) Fernandes Cauduro, A. L.; Fabrim, Z. E.; Ahmadpour, M.; Fichtner, P. F. P.; Hassing, S.; Rubahn, H.; Madsen, M. Tuning the Optoelectronic Properties of Amorphous MoOx Films by Reactive Sputtering. Appl. Phys. Lett. 2015, 106, S-7