The Princeton Nanoscale Microscopy Laboratory (PNML)
Located in the basement of Jadwin Hall, PNML is a state-of-the-art laboratory designed for high precision measurements in condensed matter physics. This space houses a host of different instruments including scanning tunneling microscopes (STM) that have been moved from the University of Illinois at Urbana-Champaign, where our group had been from 1998 to the summer of 2005. The Princeton space is designed with three ultra-quiet rooms, which consist of double wall enclosures built around a massive plinth that is floating to isolate the experiments. Such a design leads to substantial reductions of both acoustic noise (because of the double enclosure) and seismic vibration (by floating the massive 35 Ton floors). One of these ultra-quiet rooms also has radio frequency shielding to make it possible to carry out experiments which are sensitive to radio frequency interference. Overall, the combination of acoustic, vibrational, and RF isolation make this an ideal space for a host of experiments in physics, including high resolution microscopy and spectroscopy measurements with STMs. The lab space has been designed in collaboration with Wilson Architects
Ultra-low Temperature Scanning Tunneling Microscope (ULT-STM)
PNML’s newest experimental facility is the ultra-low temperature scanning tunneling microscope (ULTSTM), which is capable of investigating both single crystal samples and gated device samples at UHV pressures and at a base temperature of 10 millikelvin. This well-controlled sample environment enables us to perform extremely high-resolution (~50μeV), spectroscopic studies of the local electronic mechanisms responsible for a myriad of exotic superconducting and topological systems. Within this single, integrated design, a custom-built dilution refrigerator surrounds a cooled UHV tube chamber, which houses our STM, with specially designed electrical connections at low temperatures to connect to an interchangeable STM head module. The UHV tube chamber and surrounding cryostat hang from a two-stage passive vibration isolation system, consisting of a 7-ton floated granite instrumentation table upon a 30-ton concrete plinth platform.
This vibration isolation system, in conjunction with the surrounding soundproof and RF-shielded infrastructure, facilitates the exceptionally quiet and stable conditions necessary for our surface-sensitive measurements.
Additionally, the microscope is outfitted with a superconducting vector magnetic, submerged within a 240L liquid He cryostat, capable of supporting 10 days of continuous operation. Our vector magnet is capable of inducing a 1T magnetic field in any direction, or a 9T magnetic field along the microscope’s z-axis, allowing us to perform high-field and directional-dependence studies of magnetism and superconductivity. This microscope is singular in these capabilities, which are necessary and sufficient for directly addressing the goals of this proposal.
The ULTSTM’s versatility stems from its modular design, which serves two primary purposes (see top right figure). First, it allows the microscope head module to move between low-temperature and room-temperature experimental stages at any time without compromising cryogenics or vacuum, maximizing our ability to efficiently identify the proper surface preparation and device fabrication methods before proceeding with high-resolution experiments at low temperatures. Second, and more importantly, it allows us to select from a gamut of sample holder designs and microscope capabilities, and to assemble this ideal experimental apparatus in situ. Microscope modularity enables us to take full advantage of a number of highly developed scanning probe technique, each with its own specific instrumentation tailored for measuring a particular physical quantity. This ability allows us to cater the functionality of our facility on-demand to perform the experiments best suited to each material of interest without risk of sample contamination or degradation.
UHV Ultra-Low Temperature, High Magnetic Field Dilution Fridge STM
PNML’s dilution fridge-based UHV STM represents one of the most technically advanced STMs in existence, capable of high-performance operation at a base temperature of 20 mK and in magnetic fields up to 14 T. This state-of-the-art instrument is among the very few ultra-low temperature STMs that maintains an UHV environment during sample exchange and thermal cycling, thus accelerating turn-around time and enabling the entire suite of ultra-clean surface and tip preparation procedures.
The instrument’s entire assembly of the cryostat, UHV chambers, and vibration isolation structure is shown in Fig. 2 (left) and the custom cryogenic insert (Oxford Instruments) with home-built microscope head is shown in Fig. 2 (right). The meticulous, multi-year realization of this complex undertaking has rewarded our lab with access to unprecedented areas of temperature-field phase space and to high B/T ratios that are critical to revealing the quantum effects at ultra-low temperatures.
This instrument is designed to be compatible with a vacuum suitcase for transferring samples from MBE growth into the STM in a short period of time. This instrument will be modified with low temperature electronics for the purpose of local capacitance measurements, to probe local quantum capacitance of the devices in this program.
Variable Temperature Scanning Tunneling Microscope (VT-STM)
We have constructed two variable temperature STMs capable of performing measurements in ultra-high vacuum (UHV) from room temperature down to 10 K. These system are equipped with both sputtering and evaporation sources for in-situ preparation of ultra-thin films. A great deal of care has gone into the design of this system to ensure thermal compensation, so as to avoid the typical problems associated with thermal drift. This instrument has been specifically designed to be able to track the same area of a sample surface at atomic scales as we vary the temperature. In this system we also have the added versatility by allowing the exchange of STM tips without venting the UHV system. This apparatus, with its ability to perform STM over a wide range of temperatures, is being used extensively in our investigations of correlated electronic states. We plan to continue to use these instruments to study a variety of phase transition phenomena.
Molecular Beam Epitaxy (MBE) instrument
Molecular beam epitaxy (MBE) is a process for growing thin, epitaxial films of a wide variety of materials, ranging from oxides to semiconductors to metals. It allows for controlled growth of 2D materials monolayer by monolayer, producing very clean, ordered samples for scanning tunneling microscopy (STM) experiments. The advantage of our new MBE system (completed December 2016) is that the PNML researchers will be able to precisely control and engineer sample growth while monitoring crystal structure through the diffraction pattern produced when an electron beam is reflected off of the sample. This allows custom engineering of new 2D material systems. The power of STM lies in imaging how electronic wavefunctions vary spatially on a surface with atomic resolution. Combined with the MBE, STM becomes a powerful tool to study topological superconductors and other phenomena like the quantum Hall effect which require very clean 2D films. The MBE’s vacuum chamber attains one of the purest laboratory vacuums achievable.
2D Material and Device Fabrication
Fabrication of two-dimensional (2D) materials and devices is key to many of our ongoing projects. We have constructed instrumentation for isolating flakes of 2D monolayer samples and for their transfer and stacking to making the devices for a variety of experiments. The devices often consist of patterned chips that have been created in various nano-fabrication facilities on the Princeton campus onto which we stamp the 2D structures. The challenge for STM experiments is that such devices will have to be atomically clean on their surfaces, making these studies more challenging than typical transport measurements of 2D devices.
For fabrication of 2D devices, motorized micro-manipulators underneath an optical microscope are used to control the translational and rotational alignment of the 2D sheets. The transfer station consists of an optical microscope with ultra-long working distance objectives as well as two sets of translation stages to assist during sample pick-up and drop-off processes. The translation stages, the microscope objectives, and focus is fully motorized so as not to disturb the pick-up process requiring micrometer accuracy.
So far, we have used these techniques in air for creating graphene samples but for the longer term we will be creating such capabilities inside a glove box for air sensitive samples beyond graphene. Many-layered crystalline materials are chemically sensitive to oxygen and atmospheric moisture. This sensitivity to environmental factors critically increases when those crystals are thinned down to the two-dimensional limit of one atomic layer, and so the crystals must be contained in the inert atmosphere of a nitrogen or argon glove box. Our glove box is designed to interface with a UHV suitcase which docks with the different STM instrumentation in our lab.