SUPPLEMENTARY INFORMATION

Transcription

1 SUPPLEMENTARY INFORMATION Conductance Measurements The conductance measurements were performed at the University of Aarhus. The Ag/Si surface was prepared using well-established procedures [1, 2]. After a pristine monolayer was formed, the room temperature conductance was measured using recently developed mono-cantilever multi-point probes [3, 4]. Details of the conductance measurements are given below. Following this, CoPc was dosed by means of a simple thermal evaporator. The doser was calibrated using two methods. For the clean surface, and throughout the deposition, the LEED pattern from the Ag/Si substrate was photographed and the intensity analysed. From the attenuation of the substrate spots, it was found that the growth mode was close to layer-by-layer growth and the dose rate could be extracted. Additionally, a quartz crystal micro-balance was used to confirm the calibration. The deposition rate was around 0.02 monolayers (ML) per minute. Using the doser calibration methods described above a (predominantly systematic) uncertainty of up to 30 % is present. Therefore an alternative approach was also used. While depositing > 1ML of CoPc, the Ag/Si substrate was held at 300 C, which ensures that only the first layer sticks and is, moreover, well-ordered [5]. By exactly one monolayer we mean that the Ag/Si surface is covered by one layer of flat-lying CoPc molecules as described in Ref. [5]. By preparing a film in this way, it is possible to make an objective comparison with the conductance measurements performed on films prepared with the substrate at room temperature. However, in order to do this, it is necessary to estimate the film thickness corresponding to one monolayer. According to our definition of one monolayer, its thickness is 0.3 nm. The blue, filled data point in Fig. 1 of the paper was obtained from the monolayer prepared by this method. The instrumentation for conductance measurements and our monolithic nano-scale probes are described elsewhere [3, 6]. The cantilever used here was a TiW-coated 12-contact monocantilever with the innermost four contacts placed 500 nm apart. The principal measurements presented here were performed using these innermost contacts. However, measurements with larger and unequal spacings were also performed. By varying the probe spacing, and by using unequal probe spacings, the dimensionality of the system can be probed [7]. nature nanotechnology 1

2 A single conductance measurement is performed by physically contacting the 12-contact probe to the surface, selecting four contacts, then linearly increasing the current through the outer two of these four contacts whilst measuring the potential difference across the inner two. By doing this, the surface structure at each contact is almost certainly destroyed - thus the current is not injected into a well defined layer. However, in order to travel between the contacts, the current passes through undamaged structure, thus the measurement represents that of the undamaged surface and contains no information on the injection into the layer. In fact, the detailed geometric and electronic structure of the contact area is irrelevant as long as the contacts are well-conducting themselves and not too large compared to the spacing between them [4, 7]. For the present measurements, the current is ramped linearly from -5 µa to 5 µa, and then back to -5 µa. Any charging effects are seen as a phase delay in the potential difference measurements. The ramp time is selected such that there is no such delay. Multiple cycles (typically > 5) are made in order to increase the signal to noise ratio. Typically each data point is based on around 100 such measurements. Also, as an added test before and after such a measurement, a current was passed from each probe, in turn, to the substrate to test the quality and consistency of the contacts. The data presented here is the result of several sets of measurements, using multiple Ag/Si substrate preparations. In each case the clean surface conductance is first measured, followed by several CoPc depositions. Thus all conductance measurements and each data point are the result of multiple independent measurement procedures. The uncertainties indicated represent the standard deviation in the measurement set. In addition to the data presented here, many of the data points were re-measured using various contact spacings up to 4 µm as well as using unequal spacings. For the larger contact spacings, the measured conductance agreed (within experimental uncertainties) with the data presented in Fig 1 of the paper. In order for the conductance measurements to be independent of the probe spacing, the sample must behave as a two-dimensional sheet [7, 8]. This provides additional verification that the measurements are dominated by the transport properties of the surface layer. 2 nature nanotechnology

3 NEXAFS and photoemission measurements NEXAFS measurements In order to ascertain the orientation of the molecules we performed near-edge x-ray absorption fine structure (NEXAFS) measurements on monolayer and thicker films of CoPc on Ag/Si. The Ag/Si surface was prepared as above, and CoPc molecules (Sigma-Aldrich) were further purified by sublimation before use. The molecules were evaporated from an evaporator held at C. One monolayer was prepared in the same way as for the conductance measurements. For a multilayer the substrate was held at room temperature and molecules were evaporated with the evaporator held at the higher end of the temperature range quoted, with a deposition time of one hour. This resulted in a film thickness of 100 ± 5 Å, determined using the method outlined in Ref. [9] by monitoring the intensity of the Si 2p photoemission peak before and after deposition of the multilayer, at a photon energy of 920 ev. The NEXAFS measurements were performed via Auger electron yield with linear horizontal polarized light at several beamlines: the BACH beamline at ELETTRA [10]; beamline D1011 at MAX-lab [11]; and beamline SX700 at ASTRID. Photon energy calibration was established by measuring the well-known Si 2p core level in photoemission at various photon energies in the N K-edge range, or when that failed to generate a clear signal for the thicker films, the Ta 4f level of the sample holder was used. All spectra were normalized to the beam current and monolayer spectra were normalized to a NEXAFS spectrum obtained on the clean Ag/Si substrate prior to molecule deposition. All measurements were performed at room temperature. The determination of molecular orientations on surfaces using NEXAFS with linearly polarized light is well established [12]. The theory is applicable when localized electronic dipole resonances are excited by linearly polarized light near threshold. In organic molecules these resonances correspond to transitions from atomic 1s core levels into unoccupied π or σ levels which are highly localized on the individual molecules. For molecules with planar aromatic rings, like the phthalocyanines, the lowest unoccupied molecular orbital (LUMO) is a π orbital formed by the antibonding combination of atomic p z orbitals and it is oriented perpendicular to the plane of the molecule. The dipole excitation from a spherically symmetric 1s level into the LUMO can therefore be represented by a vector O, nature nanotechnology 3

4 characterized by a polar angle α with respect to the surface normal, and an azimuthal angle φ defined by the projection of O onto the surface and the plane of x-ray incidence, spanned by the surface normal and the dominant electric field vector component E. The incident light is characterized by an angle θ between E and the surface normal, or equivalently, by the angle between the substrate and the incoming x-ray beam. The equations can be simplified as done by Stöhr and Outka for threefold or higher symmetries, which removes the effect of the azimuthal angle φ. The resonance intensity is then given by: [12] I = A(cos 2 θ cos 2 α sin2 θ sin 2 α) I = 1A 2 sin2 α (1) I PI + (1 P )I, (2) where A is the angle integrated cross section. The intensity I arises from the minor electric field component perpendicular to the plane of x-ray incidence (along the y axis), in the case that the polarization factor P of the light is less than 1. At each angle, measurements of the maximum intensity of the LUMO peak were taken, which were then plotted with the theoretical curves, as in Fig. 2. The value for the angle α between the vector perpendicular to the CoPc molecular plane and the surface normal is then read as α 16 for the monolayer, which means that the molecules are lying virtually horizontally on the Ag/Si surface. For the multilayer we find an angle α = 64, which translates as a stacking angle of 26 between adjacent molecules and between the molecular plane and the surface normal (as outlined in Fig. 1 of the paper), indicative of α-phase CoPc. Photoemission measurements The photoemission measurements were obtained at the beamlines D1011 at MAX-lab and SGM-1 at ASTRID (similar measurements on a pristine monolayer were obtained at beamline BACH at Elettra). Co 2p photoemission spectra for the Li-doped films were taken at hν = 920 ev and in normal emission. Co 2p spectra for the CoPc films were taken using a MgKα x-ray source (hν = 1254 ev). Li doping was performed using a welldegassed Li getter source operating at 6.5 A (SAES getters). The Li concentration was determined by comparing the intensity of the Li 1s and Co 3p photoemission peaks, measured at hν = 140 ev and in normal emission. The valence band spectra were measured at 4 nature nanotechnology

5 hν = 40 ev in normal emission. [1] Wells, J. W., Kallehauge, J. F., & Hofmann, P. Thermal switching of the electrical conductivity of Si(111)( 3 3 )Ag due to a surface phase transition. J. Phys. Cond. Matt. 19, (2007). [2] Crain, J. N., Gallagher, M. C., McChesney, J. L., Bissen, M., & Himpsel, F. J. Doping of a surface band on Si(111) 3 3-Ag. Phys. Rev. B 72, (2005). [3] Gammelgaard, L. et al. A complementary metal-oxide-semiconductor compatible monocantilever 12-point probe for conductivity measurements on the nanoscale. Appl. Phys. Lett. 93, (2008). [4] Hofmann, P. & Wells, J. W. Surface-sensitive conductance measurements. J. Phys. Cond. Matt. 21, (2009). [5] Upward, M. D., Beton, P. H., & Moriarty, P. Adsorption of cobalt phthalocyanine on Ag terminated Si(111). Surf. Sci. 441, (1999). [6] Wells, J. W., Kallehauge, J. F., Hansen, T. M., & Hofmann, P. Disentangling surface, bulk and space-charge layer conductivity: Si(111)(7x7). Phys. Rev. Lett. 97, (2006). [7] Wells, J. W., Kallehauge, J. F., & Hofmann, P. Surface-sensitive conductance measurements on clean and stepped semiconductor surfaces: Numerical simulations of four point probe measurements. Surf. Sci. 602, (2008). [8] Smits, F. Measurement of sheet resistivities with the four-point-probe. Bell System Technical Journal 37, (1958). [9] Petraki, F., Papaefthimiou, V., & Kennou, S. The electronic structure of Niphthalocyanine/metal interfaces studied by x-ray and ultraviolet photoelectron spectroscopy. Organic Electronics 8, Oct (2007). [10] Zangrando, M. et al. Polarized high-brilliance and high-resolution soft x-ray source at ELET- TRA: The performance of beamline BACH. Rev. Sci. Inst. 75, (2004). [11] Nyholm, R., Svensson, S., Nordgren, J., & Flodström, A. A soft x-ray monochromator for the MAX synchrotron radiation facility. Nucl. Inst. Meth. Phys. Res, A 246, (1986). [12] Stöhr, J. & Outka, D. A. Determination of molecular orientations on surfaces from the angular dependence of near-edge X-ray absorption fine-structure spectra. Phys. Rev. B 36, (1987). nature nanotechnology 5

6 FIG. 1: Nitrogen K-edge NEXAFS spectra showing the dependence of the π and σ intensity on the polar incidence angle θ of the light. The presented spectra have been normalized to coincide on the high photon energy side and the cross-over point between the π and σ regions (405.2 ev). Left: monolayer spectra taken at Elettra. Right: multilayer spectra obtained at MAX-lab. FIG. 2: Normalized LUMO intensity at measured angles of θ from Fig. 1 plotted alongside theoretical curves giving a value of α 16 for the measured monolayers and a value of α = 64 for the multilayer. Intensity (arb. units) CoPc monolayer: N K-edge NEXAFS π* σ* θ = 90 o 78 o 68 o 58 o 48 o 38 o 28 o 18 o 8 o Intensity (arb. units) CoPc multilayer: N K-edge NEXAFS π* σ* θ = 90 o 70 o 50 o 30 o 20 o Photon Energy (ev) Fig 1 of the supporting online material for manuscript NNANO B 6 nature nanotechnology

7 Intensity P*I +(1-P)I α = 0 o 30 o 40 o 50 o 60 o Monolayer: α = 16 o Multilayer: α = 64 o 1.0 Data sets: BACH, Elettra - monolayer D1011, MAX-lab - monolayer D1011, MAX-lab - multilayer SX700, Astrid - multilayer o Angle Θ between incoming light and surface Fig 2 of the supporting online material for manuscript NNANO B nature nanotechnology 7