The Effects of Rapid The Effects of Rapid Laser Heating on an Laser Heating on an

Au(111) Sample.Au(111) Sample.

By Sean B. NeesBy Sean B. Nees

QuestionsQuestions

How does a scanning tunneling microscope How does a scanning tunneling microscope measure the thermal expansion of gold?measure the thermal expansion of gold?

How much does the gold sample expand when it How much does the gold sample expand when it is shot with the <80mW laser?is shot with the <80mW laser? How much of the expansion is due to the mica that How much of the expansion is due to the mica that

the gold sample is evaporated on?the gold sample is evaporated on? Is the platinum-iridium tip expanding when the laser is Is the platinum-iridium tip expanding when the laser is

shot?shot?

Is there an observable change to the gold Is there an observable change to the gold surface when it is shot with the laser?surface when it is shot with the laser?

Single Line Scanning Single Line Scanning

We use single line scans in order to We use single line scans in order to measure the amount that a sample measure the amount that a sample expands when it is shot by the laser.expands when it is shot by the laser.

By using data analysis software we are By using data analysis software we are able to create a graph of the thermal able to create a graph of the thermal expansion by dividing the image into expansion by dividing the image into sections. sections.

Effect of Rapid Laser Heating on Effect of Rapid Laser Heating on the Au(111) Samplethe Au(111) Sample

Attempting to prove calculated data with experimental Attempting to prove calculated data with experimental data:data: Using COMSOL, a thermodynamics software, I have calculated a Using COMSOL, a thermodynamics software, I have calculated a

10.712 K temperature increase of the Au(111) sample when it is 10.712 K temperature increase of the Au(111) sample when it is shot with the laser.shot with the laser.

In COMSOL, I created models with the exact dimensions of the In COMSOL, I created models with the exact dimensions of the sample and sample holder. I then applied a heat flux to the gold sample and sample holder. I then applied a heat flux to the gold surface.surface.

I have been attempting to confirm these calculated results with I have been attempting to confirm these calculated results with experimental data. experimental data.

I have been taking before and after images of my sample I have been taking before and after images of my sample in order to see if the laser creates a change or in order to see if the laser creates a change or reassembling of the surface molecules of the gold.reassembling of the surface molecules of the gold.

COMSOL ModelCOMSOL Model

Setup for ExperimentSetup for Experiment Wooden box constructed with insulating acoustic foam and Wooden box constructed with insulating acoustic foam and

cork board.cork board. BNC connector BNC connector Stabilizing Base MagnetsStabilizing Base Magnets Vice Grip mount with <80mW 532nm laserVice Grip mount with <80mW 532nm laser Isolating hanging springsIsolating hanging springs Air table BaseAir table Base Piezoelectric TubePiezoelectric Tube Sample holder with conical hole for shooting the laser Sample holder with conical hole for shooting the laser

through.through. Microtherm thin-sheet insulator is used between the sample holder Microtherm thin-sheet insulator is used between the sample holder

and the sample to and the sample to reduce the amount of heat being conductedreduce the amount of heat being conducted away away from the sample when it is shot with the laser.from the sample when it is shot with the laser.

Electronics for the STMElectronics for the STM

Conclusions From Experimental Conclusions From Experimental DataData

When the Au(111) sample is shot with the When the Au(111) sample is shot with the <80mW 532nm laser the Au(111) sample <80mW 532nm laser the Au(111) sample expands >6000 angstroms.expands >6000 angstroms.

There is no observed change and/or rearranging There is no observed change and/or rearranging of the gold surface molecules when shot with the of the gold surface molecules when shot with the laser. laser. From the before and after images we acquired from From the before and after images we acquired from

scanning there appeared to be no change.scanning there appeared to be no change. The taking of before and after images was difficult The taking of before and after images was difficult

because of thermal drift. The area scanned before the because of thermal drift. The area scanned before the laser was shot and the area scanned after the laser laser was shot and the area scanned after the laser shot usually are not the same location. The shooting shot usually are not the same location. The shooting of the laser itself creates a substantial amount of of the laser itself creates a substantial amount of thermal drift. thermal drift.

New DataNew Data

From Earlier Scans, there appeared to be no From Earlier Scans, there appeared to be no change in the surface of the gold or change in the surface of the gold or molecules/thiols on the surface.molecules/thiols on the surface.

I have taken scans that we do see a change in I have taken scans that we do see a change in image after the laser is shot. The change does image after the laser is shot. The change does not occur in the surface of the gold, but rather in not occur in the surface of the gold, but rather in the thiols on the gold.the thiols on the gold.

These scans are inconclusive. Many new These scans are inconclusive. Many new questions and need for changes in scanning questions and need for changes in scanning procedure have arisen because of these scans.procedure have arisen because of these scans.

Earlier Before and After ImagesEarlier Before and After Images

When we shot the laser and then took a second scan, When we shot the laser and then took a second scan, the the subsequentsubsequent scan had drifted away from its original scan had drifted away from its original position. The only possible change was that junk position. The only possible change was that junk molecules had been removed from the surface.molecules had been removed from the surface.

Recent Before and After ImagesRecent Before and After Images

INSERT IMAGES WHEN YOU GET THEM FROM BECKYINSERT IMAGES WHEN YOU GET THEM FROM BECKY There are features on the gold (thiols) which appear to have There are features on the gold (thiols) which appear to have

changed when the sample was shot with the laser. The molecular changed when the sample was shot with the laser. The molecular integrity of the gold surface remained unchanged.integrity of the gold surface remained unchanged.

Additional Questions/Things To Be Additional Questions/Things To Be AddressedAddressed

After shooting the laser how long do the effects After shooting the laser how long do the effects on the sample occur? Does the energy added to on the sample occur? Does the energy added to the sample continue to change after the laser the sample continue to change after the laser has been turned off and for how long?has been turned off and for how long?

Does the laser negatively impact the tip as to Does the laser negatively impact the tip as to make atoms from the tip fall off or create a tip make atoms from the tip fall off or create a tip that is less sharp?that is less sharp?

Changes must be made in order to allow for the Changes must be made in order to allow for the laser to reach its highest amount of energy laser to reach its highest amount of energy output. We have to turn off the laser before the output. We have to turn off the laser before the laser has reached its maximum temperature. laser has reached its maximum temperature.

Additional Questions/Things To Be Additional Questions/Things To Be Addressed Cont’dAddressed Cont’d

Another change that has to be done instead of Another change that has to be done instead of taking just a single before and after image. I now taking just a single before and after image. I now have to take a series of scans. This way we can have to take a series of scans. This way we can see change in the sample over time to see if the see change in the sample over time to see if the effects on the sample are not only immediate.effects on the sample are not only immediate.

I have to calculate, using calculus, the amount of I have to calculate, using calculus, the amount of time that it takes for the energy applied by the time that it takes for the energy applied by the laser heat flux to leave the sample.laser heat flux to leave the sample.

I also have to do COMSOL calculations to see I also have to do COMSOL calculations to see how long it takes for the sample to reach its how long it takes for the sample to reach its highest temperature gain. highest temperature gain.

Further ResearchFurther Research Set up the laser so that the laser is not run on batteries. Set up the laser so that the laser is not run on batteries.

Set up the laser so that it can be plugged into the wall Set up the laser so that it can be plugged into the wall thus ensuring that the laser is always receiving the same thus ensuring that the laser is always receiving the same amount of power (batteries lose power over time).amount of power (batteries lose power over time).

Use optics to focus the laser.Use optics to focus the laser. Adjust the clamp so that it holds the laser more sturdily Adjust the clamp so that it holds the laser more sturdily

and ensures that the laser does not move when the and ensures that the laser does not move when the scope box is closed (sometimes when the box is opened scope box is closed (sometimes when the box is opened we can see that the laser is not pointing where it was we can see that the laser is not pointing where it was initially aimed).initially aimed).

Scan other samples (C60 and C70 buckey balls on gold Scan other samples (C60 and C70 buckey balls on gold and octanethiols on gold) and rapidly heat them with a and octanethiols on gold) and rapidly heat them with a laser.laser.