Cathodoluminoscopic Spectroscopy at RPI

Transcription

1 Cathodoluminoscopic Spectroscopy at RPI JRSC 3C26 Basic Operations Manual J.D. Price and J.M. Hanchar February 18, 2002

2 ii Introducing the Luminoscope... 1 Cathodoluminoscope setup 12 steps to shooting electrons... 3 Using WinSpec The art of spectrography... 4 Acquisition setup... 4 Establishing background correction... 5 Calibration check... 5 Viewing the spectrum... 5 Acquisition over a large range of wavelengths... 6 Using WinView Crude digital imaging... 6 When you re through... 7 Appendix 1 Calibration data... 8 Appendix 2 Default Settings (as of Aug. 7, 2001)... 9 Acquisition Menu... 9 EXPERIMENTAL SETUP... 9 Calibration Menu USAGE Spectrograph Menu MOVE DEFINE CALIBRATE Setup Menu... 11

3 Introducing the Luminoscope Luminescence is a general term for the emission of non-thermal radiation from a material, generally in the visible range, but including the infrared and ultraviolet spectrum. The emission is the result of incident energy imparted to the material, occurring when the material has certain patterns of electron energy levels which may be related to the type of structure, the presence of trace amounts of elements which serve as activators, or certain crystal defects. Luminescence includes phosphorescence and fluorescence; the former refers to samples which radiate energy for 10-8 seconds or more after removal of the excitation source, the latter to samples which cease to radiate 10-8 seconds or less after removal of the source. Cathodoluminescence refers to excitation using an electron source. A cathodoluminocope composed of a cathode ray tube, a vacuum chamber with a lead-glass porthole, and a compound microscope. A mechanical vacuum pump provides a reasonable vacuum to the chamber, and a power control unit controls the current and voltage applied to the filament within the tube. When current and voltage are applied to the tube, the filament produces electrons which are accelerated into the vacuum chamber. Small magnets control the direction of the electron path, which may be focused onto a sample within the chamber. In addition to the above components, the RPI luminoscope also has a Princeton Instruments spectrograph and charge coupled device Figure 1. The RPI Cathodoluminoscope spectroscopy system: the sample chamber (detail in figure 2) is attached to stage of an Olympus BH-2 polarizing compound microscope with trinocular head. Spectrograph and CCD are connected through the trinocular optical tube. Power source switches for the spectrograph and Hg lamp (calibration standard) are also indicated. (CCD, also called the detector). The spectrograph consists of two surfaces on a pivot; one is a mirrored grating surface with inclined facets, which spread-out incident light into a spectral range, in order to identify discrete wavelengths in the electromagnetic energy spectra, the other is a flat mirrored surface that reflects the image in the microscope. The CCD is able to digitize the spectrum or an image as distributed by the spectrograph. To drive all of this, the RPI luminoscope has a Pentium-based computer running Roper Scientific s WinSpec and WinView programs with the Windows 98 platform and a Princeton Instruments driver computer. Please note that an electron beam will generate energies above the visible electromagnetic spectrum, including x-rays. The lead-glass and brass fittings of the sample chamber are sufficient to prevent x-rays from exiting the chamber. It is therefore important that any damage to the sample holder, particularly the window be repaired prior to further use of the machine.

4 2 Figure 2. Detail of the sample chamber. Note that vacuum pump valve is shown in open position (parallel to the vacuum line) Figure 3. Front panel of the power control unit for the cathode ray tube, which includes a vacuum gauge for the sample chamber.

5 3 Cathodoluminoscope setup 12 steps to shooting electrons Set up of the cathodoluminoscope is uncomplicated provided that you are familiar with the basic pieces of the machine. If you have not done so already, please familiarize yourself figures 1-3 prior to starting the set up instructions. 1. Turn on vacuum pump, under the right end of the table. The switch is located on the power cord near the plug. Close the pump valve on right rear of sample chamber (the pump valve is closed when the handle is at a right angle to the flow direction) and open sample chamber valve. 2. Place the sample in the chamber. The sample holder will accept samples of 1 cm or less in height with dimensions of 7.5 x 4.5 cm or less. Whenever possible, make sure the sample is flat in the holder. Depending on the sample height, glass plates may need to be inserted between the sample and the holder to insure correct focusing distance. The holder may contain up to three petrographic thin sections, but two is recommended. 3. Close the chamber and open the close the sample chamber bleed valve and the pump valve. 4. Turn the control knob on the Luminoscope box to pumpdown. Note, the microscope lamp is plugged into the luminoscope box, and thus the control knob must be turned at least to pumpdown to turn on the microcope lamp. 5. Switch on the controller (black box with RS Princeton Instruments on face, located above the computer CPU case) and the spectrometer (small black box with red switch). 6. Allow the sample chamber to pump down to millitorrs (see vacuum gauge, figure 2). This usually takes ten minutes, which is adequate time for the electronic components to warm up. If the vacuum takes a long time to get to 100 millitorrs, then it is most likely that there is a leak in the o-ring on the Pb glass window. Try rotating the yellow ring around the glass to reseat the O- ring. If that fails to remedy the vacuum leak, replace the o-ring (spare o-rings may be ordered through Don Marshall ( ) for a modest cost. 7. As the vacuum pump evacuates the chamber, fully slide in the trinocular shutter, turn on the microscope lamp, and focus the view (see caution, right). 8. Switch the control knob to the regulated position. This regulates voltage and current by stabilizing the vacuum. 9. Switch on the electron tube by flipping the switch on the lower left of the Luminoscope face panel. 10. Slowly increase current to 0.7 ma. (Good current strong CL in many materials with least amount of damage). 11. Switch the meter knob to volts, and slowly turn the voltage knob until the meter reads 12 kv. Caution: the microscope was not specifically designed for use with the sample chamber; therefore exercise care when focusing or switching objective lenses. Do not let either objective lens touch the lead glass on the top of the sample holder. Such abuse will damage the objective lens and possibly crack the glass (an item worth more than $200). If you are unable to focus the sample, you need to open the sample chamber, and elevate the sample. 12. Adjust deflection magnets, on stage to center beam on the specimen under the objective. Loosen the magnet set screws to slide magnets in and out from lens area. While looking through the oculars of the microscope, move both magnets until you achieve the brightest intensity of luminescence in the sample.

6 4 Using WinSpec The art of spectrography Please make certain that you have carefully followed instructions concerning the setup of the luminoscope prior to making measurements. Before acquiring spectra or images, make certain that you have powered the controller (black box with RS Princeton Instruments on face) and the spectrometer (small black box with red switch). Note: the following provides the basic setup suitable for most users of the RPI luminoscope. More specific information on the instrumentation software is available in the WinSpec manual. Please cautiously undertake any deviations from the settings outlined below - some changes can greatly affect performance. The default settings are listed in Appendix 2 for reference. Acquisition setup Prior to software adjustments, you will want to close down the mechanical slit to 0.25 mm to increase the spectrographic resolution. 1. Start WinSpec32 (double click on desktop icon). Note: if you have recently (in the last minute) powered the spectrometer, you may encounter an error message indicating that the computer is not communicating with the spectrometer (ACTION SP-150 on COM1). This message pops up if the motor is initializing. Wait one minute and then click "Retry." 2. On the menu bar at the top of the window, find and click on Setup. On the resulting pull-down menu, select Detector Temperature and check that the specified temperature is -20 to -25 o C. Wait for the current temperature to reach the set temperature. Shortly thereafter the current temperature will lock. 3. Returning to the menu bar at the top of the window, click on Acquisition and select Experimental Setup from the resulting menu. By default, the Main tab will appear. Specify an Exposure Time (the time the shutter is to remain open). This will vary depending on the intensity of the light you are analyzing 0.1 seconds is good for really bright objects, such as the Hg lamp, 10 seconds is more appropriate for darker emitters, such as many geological materials. Set Number of Spectra to 1, and under the CCD Readout select Use Region of Interest and under accumulations, set Number to 1 4. Now click on the Data File Tab, and enter a name for the spectrum in text box at the top. Before entering the name, please click on the [...] box adjacent to the name box, and browse for or set up a folder to place your files (e.g. C:\Program Files\Princeton Instruments\WinSpec32\yourdata). In most cases you will want to not select Auto Increment File Name; enabling this feature will sequentially enumerate spectrum files. You may select whether you wish to Overwrite or Append files with the same name, and you may request confirmation to prevent overwriting files you wish to retain. In general, under Auto-save and prompts, select Don't auto-save or ask to save and leave the Use New Window for Each Run box unchecked. 5. Now click on the ROI Setup tab (note: these settings only apply if Use Region of Interest is selected on Main tab- step 3) and set Edit Pattern to 1. Select Spectroscopy Mode. Under Wavelength (the number of columns defined by x pixels, where 21 pixels = 1nm), set Start to 1, End to 576, and Group to 1 (This samples the full CCD - 1 to 576 pixels, a required setting for spectroscopy). Under Slit (software slit - the number of rows defined by y pixels), the Start may be any number from 1 to 384, and the Height may be any number between 1 and 384. Due to distortion of the spectra at the edges of the CCD, the best place to collect data is in the center. Consider setting Start to 192 and Height to 1; sampling the single centerline of pixels on the CCD. 6. Hit Store button to save your settings! 7. Now click on the Data Corrections Tab, and under Arithmetic make certain that Background is checked, and that a file name for the background appears in the following box (e.g. Back). To

7 5 begin with, leave Flatfield unchecked. Under CCD Blemishes, leave Remove unchecked and Off selected for Cosmic Ray Removal. 8. No parameters should be active under the ADC tab. 9. Click on Timing Tab and set Timing Mode to Free Run and Shutter Control to Normal Select Safe Mode (async). Set the Delay Time to Seconds and make certain Wait For TTL is unselected. Set Edge Trigger to +edge. 10. None of the items should be selected under the Processes Tab. 11. Click OK box at base of the window. Establishing background correction Now you are almost ready for acquisition. But before acquiring a spectrum, you must acquire a background. The background is the energy read by the CCD sees when the shutter is closed, and will be subtracted out from your observed spectra. 12. On the main menu bar, click on Acquisition, and select Acquire Background from the drop-down menu. A window will open and a background spectrum will appear after the prescribed acquisition time. The resulting spectrum should be a lot of noise with a low distribution of counts around the mean observation. What is focus (Acquistion menu or green button)? Focus is a command that repeats acquisition until stopped. It is designed to allow an operator to tune the focal length of the trinocular tube or the spectrograph-ccd tube without having to hit acquire each time. Although most users should not attempt to adjust these focal lengths, one may use focus in liu of acquire. Calibration check Prior to evaluation of an unknown sample, you will want to acquire a spectrum on a standard source to verify the calibration. In general, unless the spectrograph or CCD are moved, the machine remains calibrated. However, a quick check is always warranted. Please proceed with steps 13-23, which will also acquaint you with standard operation of the machine. 13. Find the mercury vapor wand lamp and power supply. Place the lamp under the low power objective lens on the microscope, and pull the trinocular shutter open (fully extended). If you have not done so already, adjust the mechanical slit opening to 0.25 µm. Warning, the lamp is a source of UV radiation, so refrain from staring at the lamp. Additionally, be careful not to pinch the wand lamp between the objective lens and the stage. 14. With the lamp in place, go to the spectrograph menu and select move. Specify an angle of 540, a position such that it is within 13 nm of a known peak. Click on Acquisition and select Acquire from the drop-down menu, or click the button with ACQ in a green circle on the custom tool bar. The shutter will click open and then close after the preset amount of time, and the data will display in the active window. If properly calibrated, you will see a significant peak at nm (see appendix 1). Viewing the spectrum If the spectrum is too small to observe of too large to fit in the window, you may rescale the data to fit. 15. Click on Display on the menu bar, and select Autoscale or Auto 5%-95% (or use one of the two buttons on the bottom far left of the spectrum) to fit the data to the window height. Autoscale will place the background on the x-axis and set the scale to the tallest peak in view. Auto 5%-95% will leave a margin on the top and bottom.

8 6 16. To determine the exact position and intensity of any part of the spectrum, click on the graph. Then click on View on the menu bar, and select Info or click on the info icon (a yellow "i" in a window) on the custom toolbar for the Info window. Acquisition over a large range of wavelengths You will note that the simple acquisition described above covers a 37 nm of the complete mercury spectrum. To conduct a thorough test of the calibration, you must do so using the Step and Glue procedure. This procedure takes a series of sequential 30 nm spectra over a specified range and produces a compiled spectrum. 17. Click on Acquisition and select Step and Glue... from the menu. 18. Under Glued Output Data File Name, specify a name for the final spectrum (e.g. HgLine). By clicking on the [...] box you may browse or create a folder for this file. 19. Under Gluing Parameters, state a starting value and an ending value. The RPI spectrometer uses a 1800 nm grooved plate with a 500 nm blaze; optimal settings for looking at the nm part of the spectrum. Keep that in mind when constructing a spectrum. Specify a minimum overlap of each spectrum; higher values are more time consuming. 20. Under Raw Data Files Setup, specify a File Names Template (this will be the prefix to all of the individual 30 nm spectra files). 21. Specify a File Increment Value (a value of 1 creates a nice sequence) that will be automatically placed on each 30 nm spectrum file. 22. Check the box prior to Reset After Each Run 23. When finished, click Run. The computer starts by moving to the center of the first 30nm spectrum, opens the shutter, counts for the prescribed time, closes the shutter and displays the spectrum. Then it moves to the center of the second 30 nm spectrum and repeats until the range in the gluing parameters is covered. When finished, a new window will open and the entire spectra will be displayed. If you are acquiring a spectrum on the Hg lamp for calibration verification, and the spectrum is not as presented in figure A1 (appendix 1), please consult Daniele Cherniak or Jon Price. Do not attempt to rectify the calibration without further instruction. Using WinView Crude digital imaging The CCD may also be used as a digital imaging device. However, there are several limitations to imaging on the RPI scope. 1. While the CCD is optimized to examine spectra with detailed resolution, it is a little too crude for producing high-resolution images. 2. The CCD is strictly a grayscale digitizer, so the lovely hues produced by CL activators are lost. 3. The pressure-sealed sample chamber compromises microscope objective height, and thus high-powered lenses are not an option. 4. While the turret is two sided, the mirrored side (1200 BLZ=MIRnm) is not an optimal mirror for imaging. While the best images are acquired using a separate camera, WinView may be used to capture crude pictures of Cl distribution, or (if the 1800 grating is employed) an optical resolution of diffraction peaks. Use of WinView is fairly straightforward, particularly as it looks A glitch after shifting back an forth from WinSpec to WinView, you may find that one or both of the programs will no longer restart. In that case, depress Ctrl-Alt-Delete simultaneously one time, find one of these programs on the list of running programs, and select end task. Repeat until all are removed from the list. Double click on the icon of the program you wish to start. and operates almost exactly like WinSpec. However, it is important to be mindful of the fact that it employs the settings file from WinSpec, and changes made here will affect WinSpec operation.

9 7 If you have already configured WinSpec as specified previously (such that you are able to obtain a calibration Hg spectra, then there are only a few things to change. If you have not configured Winspec, return to the previous section and do so before proceeding with this section. 1. If you wish to image what you see in the ocular lenses, you need to rotate the mirror side of the turret into place. In Winpec (reopen it if you have closed it) go to the Spectrograph Menu, and select MOVE..., and specify the 1200 BLZ=MIRnm. Set move to at 0.0. If you wish instead to image a particular peak in the spectra, got to the Spectrograph Menu, and select MOVE..., and keep the 1800 BLZ=500nm selected, and specify the peak angle in the box. 2. Open the mechanical slit to 3mm by rotation the vernier knob on the spectrograph. 3. Launch Winview (double click on desktop icon). 4. On the menu bar at the top of the window, click on Acquisition and select Experimental Setup from the resulting menu. As in Winspec, the Main tab page appears in front. Specify an Exposure Time. Again, this will vary depending on the intensity of the CL from the sample; 0.1 seconds is good for really bright objects, 10 seconds is more appropriate for many geological materials. Under the CCD Readout select Use Full chip. 5. Because you have changed the acquisition dimension of the chip from the single pixel side setting in the previous section (ROI) to the full chip, you must acquire a background. On the main menu bar, click on Acquisition, and select Acquire Background from the drop-down menu. A window will open and a background spectrum will appear after the prescribed acquisition time. The result will be a rather dark image. 6. Select Acquire on the Acquisition menu, and the image will appear in a separate window. Adjust shutter time (step 4) if necessary. When you re through 1. Switch off the electron tube with the toggle switch on the lower left side of the control panel. 2. Turn down the current and voltage knobs fully counter-clockwise 3. Turn the control knob to pump down. 4. Close the pump valve (rotate until it is 90 o to the line) 5. Open the chamber bleed valve to vent the sample chamber. 6. Switch off the pump. 7. Turn off the spectrometer and the Princeton Instruments control. 8. Shutdown the computer.

10 8 Appendix 1 Calibration data Wavelength (nm) Mercury * ** * * * ( ) * ( ) ( ) ( ) ( ) *indicates strong line ** indicates strongest line for the element Figure A1. CL spectrum for mercury.

11 9 Appendix 2 Default Settings (as of Aug. 7, 2001) The text in appendix 2 is formatted as follows: Bold - user entered parameters Underlined - software determined parameters Italized - comments by document author Acquisition Menu EXPERIMENTAL SETUP Here follows a typical setup for spectra acquisition (worked for Hg lamp) MAIN TAB Exposure time sec Number of Images: 1 CCD Readout Use region of Interest selected (or use full chip) Accumulations Number: 1 DATA FILE TAB Name: Spectrum01 C:\Program Files\Princeton Instruments\WinSpec32\Data Auto Increment File Name Enable unchecked Overwrite/Append Overwrite existing files Confirm before overwriting checked Auto-save and prompts Don't auto-save or ask to save Use new window for each run unchecked ROI SETUP TAB These settings only apply if "Use region of Interest" is selected on MAIN TAB Edit Pattern: 1 Number stored: 1 Imaging Mode unselected Spectroscopy Mode selected Wavelength Spectroscopy requires full width of CCD - 1 to 576 pixels Start: 1 End: 576 Group: 1 Slit Start: 192 Height: 1 Use Software Binning unselected DATA CORRECTIONS TAB Arithmetic Background checked Back Flatfield unchecked CCD Blemishes Remove unchecked

12 10 Cosmic Ray Removal Off selected Calibration Menu USAGE Calibration Mode Auto Spectro selected Calibration Units nm Save as Default Spectrograph Menu MOVE... Only initiate movement when the spectrometer motor is not already moving - listen for it Grating: 1800 BLZ=500nm (for spectrometry) or 1200 BLZ=MIRnm (for imaging) Move to: user's choice DEFINE... MAIN TAB Action SP150 on COM1 Use for Auto-spectro Calibration checked Excitation (doesn't apply unless using a laser source) Laser Wavelength: 0 nm Warn when crossing Laser Line: unselected GRATINGS TAB Grating Grooves Name BLZ=500nm Spectrometer side of turret 1800 Grating with 500 nm Blaze BLZ=MIRnm Mirror side of turret (imaging) Turret:1 Display warning on grating change unchecked (more of a hassel than its worth - just observe warnings listed under "Spectrograph, Move" Slew selected (Scan doesn't seem to work) CONNECT TAB COM1 Comm settings... button Baud: 9600 Data bits: 8 Par: None Stop bits: 1 Use Flow control: Unchecked CALIBRATE... warning: this is the spectrometer calibration - settings change spec angle and signal reception! Do not change these values unless you know what you are doing! Calibration based on Hg lamp, and standardized to a Dy-doped Zircon, J.D. Price, March 21, Spectrograph to calibrate ACTION SP150 on COM1

13 11 Grating to Calibrate 1800 BLZ=500nm Detector Pixel Width: 22 Magnification: 1 Grating movement slew selected (scan doesn't seem to work) Offset... button Selected grating: 1800 BLZ=500nm Reference Wavelength: (strong peak on the left side of the spectrometer range) Offset Value: Adjust... button Selected grating: 1800 BLZ=500nm Reference Wavelength: (intense peak on right of the calibration spectrum) Adjust Value: Dispersion... button Selected grating: 1800 BLZ=500nm Geometrics Focal Length (mm): Inclusion Angle: Detector Angle: Dispersion Calibration Lower Reference Wavelength: Higher Reference Wavelength: Next Target Wavelength: Setup Menu HARDWARE (refers to CCD controller - black box labled RS Princeton Instruments) CONTROLLER/CAMERA TAB ST133 version 3 EEV 576x384 [3ph] (setting on [6ph] will only utillize right half of screen) small shutter NTSC (shouldn't matter - this parameter refers to video display andwe don't have a video unit) Do not select user defined chip and timing! DISPLAY TAB Rotate Checked Reversed Checked Flip Checked image on CCD is rotated, reversed, flipped from optical image INTERFACE TAB Type: High-speed PCI the following are fixed parameters Interupt Level: IRQ 3 I/O address: 0x0d400 I/O address 2: d000 H I/O address 3: b800 H CLEANS/SKIPS TAB

14 Cleans Number of Cleans: 1 Number of Strips per Clean: 576 Vertical Skips: 2 Number of Blocks: 5 12