Scanning Probe Microscopy and Spectroscopy
The laboratory runs five scanning probe microscopes (SPM) offering variable temperature (40 K to room temperature) and low temperature (5 K) conditions in ultrahigh vacuum (UHV). While the variable temperature scanning tunneling microscope (STM) mainly allows the study of temperature-dependent structural transitions, our low-temperature STMs offer the stability required for scanning tunneling spectroscopy (STS) based electronic characterization and manipulation of nanostructures. Two of our setups combine low temperature STM and non-contact atomic force microscopy (nc-AFM) to allow for state-of-the-art structural and electronic characterization at low temperature with ultimate spatial stability. An additional dedicated scanning probe microscope offers STM and AFM characterization under ambient conditions and in liquids.
To access the ultrafast dynamics and probe optical properties with atomic resolution, our THz-STM/Laser lab combines a custom-built optical setup with a state-of-the-art low-temperature (5 K) scanning probe microscope. Phase-locked, single-cycle THz pulses drive state-selective electron tunneling in the STM junction in a time-controlled fashion. High intensity THz pulses are generated by optical rectification in lithium niobate using a 1030 nm Yb fiber laser, and provide picosecond voltage transients at repetition rates up to 40 MHz. We use electro-optical sampling in the far-field and photoemission sampling in the near-field to characterize the transient waveform. Precise control of the THz field strength and carrier-envelope phase achieves state selectivity of the tunneling process. THz pump-probe measurements can access material dynamics at picosecond to nanosecond time scales. The optical setup is complemented by a spectrometer for ultrasensitive luminescence detection.
Photoelectron Spectroscopy and Diffraction
Our scanning probe instruments are supplemented by a state-of-the-art photoelectron spectroscopy (PES) system, which provides complementary sample characterization capabilities regarding chemical, electronic and structural properties. Our laboratory-based PES system is equipped with computerized full-hemisphere goniometer manipulators for angle-scanned PES. This allows to perform x-ray photoelectron spectroscopy (XPS) and diffraction (XPD) experiments using MgKα or AlKα excitation, as well as valence band spectroscopy (UPS, ARPES) using ultraviolet He I or He II radiation. For dedicated band structure measurements the system is equipped with a Scienta R3000 electron analyzer providing 2D detection (angle and energy) and sample cooling to 24 K. Furthermore, it is connected to a dedicated low-temperature (5 K) STM/nc-AFM instrument. As a founding consortium member, we also have privileged access to the PEARL endstation of the Swiss Light Source to perform photon-energy and angle-scanned high-resolution XPS/XPD/ARPES measurements.
In addition to our analytical UHV SPM and PES systems, we dispose of a variety of dedicated sample growth and preparation tools. For the efficient on-surface synthesis of nanostructures we have developed a fully automated growth chamber, the so-called “GNR reactor”. This system automatizes substrate surface preparation, molecular precursor evaporation (up to 6 different molecules) and temperature activated chemical reactions. The GNR reactor can reach a throughput of 12 samples per day. For the CVD growth of carbon nanomaterials we use a commercial 2” Black Magic system from AIXTRON with Graphene and Carbon Nanotube growth kits. With the corresponding growth protocols this system can produce dense forests of single-walled CNTs or sparse vertically aligned CNTs.
For preparatory work and ex-vacuum investigations, we have a dedicated laboratory space equipped for experiments in liquids and electrolytes, but also for the preparation of STM-tips and the determination of molecule sublimation temperatures and deposition rates. Besides two chemical fume hoods our chemistry lab houses several other pieces of equipment. For the preparation of SPM tips, the lab comprises a setup for the electrochemical etching of PtIr-tips in a salt melt and of W-tips in aqueous electrolyte. To determine sublimation temperature and deposition rate of molecules prior to UHV experiments, the lab hosts a fully automatized high-vacuum system equipped with calibrated evaporation source and quartz crystal microbalance.