SUPPLEMENTARY INFORMATION

Transcription

1 SUPPLEMENTARY INFORMATION Electroluminescence from a single nanotube-molecule-nanotube junction Christoph W. Marquardt, Sergio Grunder, Alfred Błaszczyk, Simone Dehm, Frank Hennrich, Hilbert v. Löhneysen, Marcel Mayor, Ralph Krupke Supplementary Information nature nanotechnology 1

2 supplementary information Process for the fabrication of the Si trench Equipment: Oxford Instruments Plasmalab 80Plus. Etching parameters: Process 1: 25 sccm CHF 3 and 25 sccm Ar, 30 mtorr, 200 W, 1:30 min Process 2: 30 sccm O 2, 50 mtorr, 200 W, 3 min Electroluminescence measurement setup The electroluminescence experiments were performed in a Zeiss AxioTech Vario microscope setup with an Oxford Instruments HiResII optical cryostat as central element. The emitted light is collected by a Zeiss LD "Plan-Neofluar" 40x Objective (cover glas corrected, Numerical Aperture 0.6) through a Spectrosil-B window in the cryostat and detected by an Acton Research SpectraPro 2150i spectrograph with a Princeton Instruments PIXIS 256E Silicon CCD camera. The spectrograph can operate in the reflection mode, with a mirror to take real-space images, or in the spectroscopy mode, with a diffraction grating (750nm blaze wavelength, 300 grooves/mm). The dark current of the CCD detector yields about 2 counts per pixel per hour, and allows integrating the signal over extended periods. We estimate the sensitivity of the setup on the basis of the quantum efficiency and amplifier gain of the CCD, the efficiency of the grating, the geometrical constraints of the microscope optics (optical path), and by assuming an isotropic emitter. The sensitivity s is about 103 and 105 emitted photons per count, in reflection and diffraction mode, respectively, and the bandwidth per pixel Δλ is 0.5 nm. The relative spectral response of the system has been measured by a calibrated halogen lamp and all recorded spectra were corrected accordingly. The spatial and spectral resolution are 0.7 μm and 1.5 nm, respectively. The measurements were done under vacuum at a base pressure of <10-6 mbar and biased with an Agilent 4155C Precission Semiconductor Analyzer. 2 nature nanotechnology

3 supplementary information Fig. S1. Discrimination between electrophoretic and dielectrophoretic motion of dissolved molecules in electric fields: A drop of molecule solution was placed onto interdigitated Auelectrodes. During the drying process a dc voltage bias of 1V was applied. An additional sample was prepared under identical conditions but without voltage bias. The as prepared samples were characterized by scanning the sample with laser light at 633 nm and measuring the integrated fluorescence signal (636 nm 740 nm). The fluorescence spatial maps (identical intensity scales) show an enhanced deposition of molecules at the edges of the electrodes for the sample dried under bias (B) compared to the sample dried without bias (A). Interestingly we do not observe a difference in intensity for electrodes at positive and negative polarity, indicated by the red and blue bars. This is a clear indication that the molecules migrate due to an induced dipole moment (dielectrophoresis) and not due to charging (electrophoresis). nature nanotechnology 3

4 supplementary information Fig. S2. Sensitivity of molecules to air under excitation with light: Temporal evolution of the integrated fluorescence (FL)-intensity measured on a molecule film on SiO2 in air and under Ar flow (excitation at 633 nm, FL integration nm, see inset). In air the FL intensity drops rapidly and irreversibly with a characteristic time constant of ~ 2 s. Under Ar flow the FL is more stable and decreases with a characteristic time constant of ~ 50 s. The excited state of the molecule is apparently quenched by oxygen in air. Thus we perform the electroluminescence experiments in vacuum. 4 nature nanotechnology

5 supplementary information Fig. S3. Failure of NT-M-NT junctions by spontaneous gap closure: (A) Current I versus time t measured on a NT-M-NT junction. At t = 600 s, I increases abruptly from ~ 50 na to ~ 20 μa, a current level typical for an intact metallic nanotube, and hence indicates a spontaneous gap closure. (B) Electroluminescence spectrum acquired during a similar gap closure. The spectrum is significantly different from the spectrum of the NT-M-NT junction shown in Fig. 3E in terms of peak position and width, and near-infrared intensity (solid lines: fits to data). nature nanotechnology 5

6 supplementary information Fig. S4. (A) Fluorescence (FL) spectra of the molecule dissolved in various solvents with different dielectric constants ε (see Inset of B). (B) FL-peak position as a function ε. Included is the data point from the electroluminescence (EL) measurement of the NT-M-NT junction (Fig. 3E) under high-vacuum (assuming ε 1). While the FL-peak positions vary marginally around 630 nm, the wavelength of the EL-peak differs significantly and can not be explained by an interpolation of the data towards ε 1. 6 nature nanotechnology

7 supplementary information Fig. S5. Temperature dependent fluorescence (FL) spectra of molecules on an HOPG surface under HV conditions, excited at 532 nm (coloured lines). The FL intensity increases with decreasing temperature, and the peak at 677 nm becomes narrower. In addition a shoulder at ~ 750 nm appears. The fluorescence spectra at 4.2 K shows the expected mirror symmetry to the absorption spectra measured in CH 2 Cl 2 (dashed line). nature nanotechnology 7