Dane Stevenson, Brett Gyarfas, and K.W. HippsDepartment of Chemistry and Materials Science Program, Washington State University, Pullman, WA, 99164

INTRODUCTIONINTRODUCTIONThe overall objective was to examine the structure and electronic properties of an organic molecular materials on Au(111) or graphite in an epitaxial molecular layer. Ultimately these studies were to be conducted with scanning tunneling microscopy(STM) and scanning tunneling spectroscopy (STS). The original objective was to study trimesic acid (TMA). We did not succeed in analyzing TMA on Au(111) in air with STM primarily because of the lack of epitaxy. Therefore, the focus shifted to perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA). Using the STM to analyze PTCDA layers deposited on the Au(111) and graphite, we quickly determined that the PTCDA was a more appropriate molecule for this experiment. Though there exists published material similar to we have done to date,1-3 our aim is to produce the first documented scanning tunneling spectroscopy (STS) study of of PTCDA on graphite.

References:1)M. Mobus, N. Karl, and T. Kobayashi. J. Crys. Growth, 116, 495 (1992).2)C. Ludwig, B. Gompf, J. Peterson, R. Strohmaier, and W. Eisenmenger. Z. Phys. B, 93, 365, 373 (1994).3)M. Toerker, T. Fritz, H. Proehl, F. Sellam, and K. Leo. Surface Science,491, 255-264 (2001)

Trimesic Acid PTCDA

THE DEPOSITION PROCESSTHE DEPOSITION PROCESSThe process used to deposit thin films of PTCDA onto Au(111) and graphite was carried out in a bell jar deposition chamber. The chamber itself allows for three different surfaces to be deposited on, as well as for three different deposited materials. As part of this study, a mask was designed and created to securely hold the Au(111) and graphite substrates while the samples were deposited on them. Once the substrates are in place, the chamber is pumped down to 10-3 Torr. The diffusion pump is then used in conjunction with an ion gauge to lower the pressure to 3E-7. At this pressure, the samples are heated, along with the substrate, until a desired rate of deposit (0.008 nm/sec in this case) is reached, which is displayed using a thin film monitor. In our case, we found that heating the graphite to a temperature of 200°C prior to PTCDA deposition was effective in “baking off” all the water and any contaminants. The graphite was then cooled to 120°C and deemed ready for deposit. To reach our desired deposition rate, the PTCDA was heated to temperatures around 550°C.

Vacuum System Pumping for Vapor Deposition Homemade Mask for Deposition Chamber Inside the Deposition Chamber

Pico+ Molecular Imaging STM

CCD Camera

Stage Control

Sample Stage

STM Scanner

Sample

Stage Securing Plastic Mask

HOW STM WORKSHOW STM WORKSThe basic idea behind a scanning tunneling microscope is using afeedback loop to establish constant current contours as a tip runs across the surface of a sample. In this case, our sample was placed on a metal sample plate and held in place by a piece of plastic specially designed and created for this project. A wire is then placed on the sample to apply a voltage relative to the tip. With the sample in place, a motor controls the height of the stage holding the sample and brings the sample closer to the platinum-iridium tip (attached to the scanner) until it reaches a preset current. The setpoint current is decided and set by the user and in this case was set to 300 pA. The bias voltage applied to the sample was 800 mV. Once the tunneling current has been reached, the scanner moves the tip across the surface of the sample and records changes in the surface based on changes in the current. The computer connected to the scanner and controller translates the changes in the current and displays on-screen the topography of the sample. It is important to note that in order to work successfully, the sample must be able to pass a current (must be at least semi-conducting), or no current is sensed by the tip and it will will crash right into the sample.

Alpha Configuration

Beta Configuration

RESULTSRESULTSSeveral attempts were made to deposit a single layer of PTCDA on the graphite before finally discovering an optimal thickness. Slowly increasing our thickness of deposition, starting at 2 Å and moving incrementally to 6 Å, we found 5 Å to be the optimal thickness of PTCDA. For STM imaging, we originally used 300 pA and 800 mV as the setpoint current and bias voltage, respectively. Upon discovering our tip was interfering with the PTCDA layer, we found the most effective setpoint to be closer to 200 pA and the bias voltage to be in between 500 and 600mV. As can be seen from the captured image, using these optimal parameters, we were able to obtain images as good or better than those published documents. Recently, we have begun using a scanner that allows us to image with a 30pA setpoint, virtually eliminating the danger of tip disruption with the PTCDA layer.

CONCLUSIONCONCLUSIONThough we began the experiment with TMA in mind, it led us to PTCDA which has proven more successful. As we continue to revise our efforts and procedures, we continue to see higher quality images and surfaces of PTCDA. As we discovered shortly after our work with PTCDA, some published material exists1-3 that has served as an excellent guide as to what we should expect to see in regards to PTCDA formations. Because what we have done to this point has been published, we have been forced to take the research a step further into the scanning tunneling spectroscopy realm. As far as we could find, no complete spectroscopy study of PTCDA in thin films has been reported. As we have nearly perfected our procedure with temperatures, deposition rate, and STM imaging, our goal with STS will be to analyze the electronic structure and bias dependent conductivity of PTCDA in thin films not otherwise studied until now.

Dane (Using STM)

Brett (Cutting Tips)