The overall goal of this protocol is to control the creation of 1D and quasi-1D C60 structures on rippled graphene to form hybrid C60/graphene nanostructures by manipulating physical thermal evaporation and annealing conditions. This method can help answer key questions in the molecular self-assembly field about preparing self-assembled molecular nanostructures on 2D materials. The main advantage of this technique is that it is versatile and easy to perform, allowing us to prepare organic thin films with different structures by tuning temperature and time.
To begin assembling the Knudsen cell, clean the cell components and mount two seven-inch-long threaded steel support rods in holes drilled through a four-pin CF flanged stainless steel power feedthrough. Mount three two-inch-long hollow copper rods and a four-inch-long hollow copper rod on the feedthrough pins. Fix a one-centimeter-thick ceramic crosspiece with a hole through its center 2.5 inches from the top of the support rods using soft copper wire.
Then, slide a quartz glass tube into a tungsten wire spring with lengths of tungsten wire at the top and bottom. Fit the bottom of the glass tube into the matching hole in the ceramic piece. Fix the glass tube in place by securing the top between the support rods with soft copper wire.
Insert the wire from the bottom of the spring into the long, hollow copper rod. Use a side cutter to pinch the top of the rod shut to hold the wire in place. Insert the wire from the top of the spring into the short, hollow copper rod in a couple with the long rod.
Pinch the top of the rod shut around the wire. Then, obtain a three-inch length of insulated type K thermocouple wire with 0.5 inches of the insulation removed from each end. Twist together the exposed wires at one end of the thermocouple.
Firmly wedge the twisted wire between the quartz glass tube and the ceramic piece so that the wire is immobilized against the bottom of the tube. Insert the exposed nickel-chromium wire into one of the remaining hollow copper rods, and pinch the top of the rod shut around the wire. Insert the exposed nickel-aluminum wire into the last hollow copper rod, and pinch the rod shut around the wire.
Lastly, fit a hollow copper columniform shield over the assembly to sit firmly on the power feedthrough. Transfer a rippled graphene on copper foil substrate from the sample storage carousel to the annealing plate in the ultra-high vacuum preparation chamber of a scanning tunneling microscope. Ensure that the chamber has reached a base pressure below four times 10 to the minus 10 millibars.
Increase the substrate temperature to 400 degrees Celsius at about 100 degrees Celsius per hour, and anneal the substrate for 12 hours. Once the graphene annealing has started, load approximately 50 milligrams of 99.5%C60 powder into the quartz tube of the Knudsen cell. Mount the Knudsen cell in the CF flange of one branch of the load lock of the STM.
Pump down the load lock for 10 hours using a mechanical pump and a turbo pump in sequence. Meanwhile, once the graphene substrate has annealed for 12 hours, begin cooling the substrate to room temperature at 100 degrees Celsius per hour. Once the graphene substrate has begun cooling, confirm that the load lock pressure is in the range of 10 to the minus eight millibar or 10 to the minus seven torrs.
Then, connect a power supply to the power feedthrough pins connected to the top and bottom of the tungsten spring. Apply a voltage across the tungsten wire by tuning the current such that the C60 source is heated to 250 degrees Celsius at 1.5 degrees Celsius per minute. Anneal the C60 source at 250 degrees Celsius for two hours.
Then, heat the source to 300 degrees Celsius at about one to two degrees Celsius per minute, and anneal the source at that temperature for 30 minutes. Afterwards, decrease the source temperature to 270 degrees Celsius for the deposition. Once the C60 source has reached 270 degrees Celsius and the atomically clean graphene substrate has cooled to room temperature, move the annealing plate with graphene substrate to the load lock transfer position.
Open the valve between the preparation chamber and the load lock. Transfer the graphene substrate to the load lock, and place the substrate face down over the Knudsen cell. Allow C60 to deposit onto the graphene substrate for one minute.
Then, transfer the C60/graphene sample to the UHV preparation chamber. Ensure that the UHV preparation chamber is below four times 10 to the minus 10 millibars. Heat the C60/graphene sample to 150 degrees Celsius at 3.1 degrees Celsius per minute, and anneal the sample at that temperature for two hours.
Then, transfer the sample to the main chamber, and scan the sample with STM. Lastly, heat the sample to 210 degrees Celsius at 3.1 degrees Celsius per minute, anneal the sample at that temperature for two hours, and acquire another STM scan. A quasi-1D C60 chain structure was observed after annealing the newly deposited C60/graphene sample at 150 degrees Celsius.
The chain structure growth was induced by the well-defined linear periodic modulation of the underlying rippled graphene surface. The C60 molecules typically formed bimolecular or trimolecular chains in a hexagonal close packed manner, with a characteristic intrachain C60-C60 spacing of 0.87 nanometers and an average C60-C60 spacing of 1.00 nanometers overall. A second annealing at 210 degrees Celsius resulted in increased surface mobility of the C60 molecules, allowing self-assembly into a more compact, hexagonal close packed quasi-1D stripe structure oriented along the same direction as the chains.
Widths of three to eight molecules per stripe were observed, with six being the most common. The stripes formed as staggered, narrow terraces with no spaces between the stripes. The lateral inter-row spacing at the boundaries between terraces was 0.75 nanometers, while the molecules within the terrace planes retained the characteristic C60-C60 spacing of 0.87 nanometers.
3D modeling of the chain and stripe structures revealed that six-row stripes had nearly the same lateral periodicity as one unit of the chain structure, which was defined as a bimolecular cell and a trimolecular cell plus interchain spacing. We first had the idea for this method when we were trying to deposit C60 on a graphene-copper substrate. Once mastered, this technique can be done in 24 hours if it is performed properly.
While attempting this procedure, remember to accurately calibrate the thermocouple to ensure that you reach the precise molecular source deposition temperature. Following this procedure, other methods like scanning tunneling spectroscopy can be performed to answer additional questions about the electronic properties of the molecular nanostructures. After its development, this technique paved the way for researchers of electronic properties of semiconductor molecules to explore guiding adsorbates into different configurations without manual manipulation.
After watching this video, you should have a good understanding of how to prepare organic thin films by physical thermal deposition.
