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CHARACTERIZING QUANTUM DOTS FOR USE IN DETECTING SUBSURFACE DAMAGE Wesley B. Williams1, Brigid A. Mullany1, Patrick J.Moyer2, Wesley C Parker2 and Mark H. Randles3 1 Department of Mechanical Engineering and Engineering Science University of North Carolina Charlotte Charlotte, NC, USA 2 Department of Physics and Optical Science University of North Carolina Charlotte Charlotte, NC, USA 3 Northrop Grumman Synoptics Charlotte, NC, USA SUMMARY Polishing, lapping and grinding are finishing processes used to achieve critical surface parameters in a variety of precision applications including optical and electronic components. As these processes remove aberrations on the surface through mechanical and chemical interactions they may induce a stressed layer of cracks below the surface. This subsurface damage can degrade the performance of a final product by creating optical aberrations due to diffraction, premature failure in oscillating components and a reduction in the laser damage threshold of high energy optics [1]. These defects are typically detected by etching or dimpling the surface to expose the underlying damage, both of which are destructive tests. The authors propose a non-destructive method for assessing subsurface damage in a polished or lapped glass sample. The method consists of tagging the abrasive slurry with quantum dots (nanometer scale semiconductors that fluoresce at a given wavelength when subject to laser excitation) and using a confocal microscope to map the depth of fluorescence signal in the sample. The quantum dots are believed to be sufficiently small (with diameters of 2-5nm) [2] that they will be able to travel into sample defects if they are open to the surface during the polishing process. Prior to polishing samples with quantum dot tagged slurries, it is important to characterize the performance of the confocal microscope and the expected fluorescent response in the samples. Toward this end, glass samples were exposed to high concentration solutions of quantum dots for varying times before being cleaned. It was

found that samples exposed for the longest duration (270 minutes) had high localized levels of fluorescent response, indicating that quantum dots had been adsorbed onto the surface. BACKGROUND Subsurface Damage Subsurface damage (SSD) is a layer of defects and stressed material that exists beneath an apparently smooth surface that provides no indication of the damage that lies beneath. These defects can negatively impact the performance of optical components by introducing optical aberrations, if aggravated they can propagate to the surface [1], or by reducing the laser induced damage threshold (LIDT), a measure of how much energy can be passed through an optical component without risking catastrophic failure [3]. Subsurface damage is seen to correlate with the rate of material removal [4]. As such, one theory is that localized compressive stress fields in the region of the traveling abrasive particle can cause brittle fractures which are further aggravated by the tensile stress fields that trail the abrasive particle. These fractures may be covered up by mechanisms such as material flow [5] or redeposition [6] or they may simply not be open to the surface in the absence of localized stresses. Without visual indications of the damage at surface, SSD is difficult to detect. Traditionally it has been assessed by etching to remove the topmost layer of the surface, exposing the defects that lie beneath [7], as well as a related technique of dimpling [1]. While relatively simple

to perform, both tests are destructive and subject to the lateral resolution limitations of optical microscopy in assessing the subsurface damage. Confocal Microscopy Confocal microscopy, pioneered by Minsky, consists of a setup where light from outside the focal plane is largely prevented from reaching the light detector [8]. This rejection of light outside the focal plane, coupled with exclusion of light from points adjacent to the focal point reduces haze and increases the sharpness of the image [9]. By scanning and assembling an array of these points, a 2-D image can be formed and a series of these 2-D images (or optical slices) can be stacked to form a 3-D representation of the subject. Confocal microscopy is utilized for its ability to image planes beneath the surface of a sample [10] which lends it to biological imaging applications where it is used to acquire optical sections of living specimens which have been treated with fluorescent dyes. The lateral resolution improvements over conventional microscopes as well as the ability to focus beneath a sample surface lead to its use in the inspection of surface topographies for semiconductors and subsurface integrity of transparent materials [11] [12]. Quantum Dots Quantum dots are nanoscale semiconductors that fluoresce when subject to excitation. They have been designed to restrict the motion of conduction band electrons by carefully controlling the particle size [13]. By restricting the size of the particle the conduction band electrons are forced into discrete energy states when excited, versus the bulk material which allows for a multitude of energy states. This restriction results in more narrow energy emission spectra when the semiconductors are excited. These dots offer several advantages over organic fluorescent dyes such as rhodamine, the most important of which are; broad excitation spectra, more intense fluorescence, narrower emission spectra, and increased resistance to photobleaching (diminishing of the fluorescent response in successive excitations) [14]. EXPERIMENTAL SETUP The confocal microscope used consists of three main components; the excitation source, the

photon detector and the sample stage which interact as shown in Figure 1 below. Focal Point Sample Interface Cover Slip

High Pass Filter

Single Photon Counting Module

Laser Source

FIGURE 1. Arrangment of key components in the confocal microscope. Excitation is provided at 470 nm wavelength by a PicoQuant PDL 800 diode laser driver pulsed at 10 MHz. The light illuminates the sample causing fluorescent material that is present to fluoresce. The light from the fluorescence is reflected through a corner cube to a long pass filter. The 538 nm long-pass filter is used to reject any reflected light from the laser. Finally, the small aperture of the EG&G SPCM singlephoton avalanche diode acts as the pinhole in a traditional confocal microscope to reject fluorescence outside of the focal plane. The sample stage is a flexure based design that keeps the optics stationary while moving sample during the scan [15]. The sample stage accommodates samples which can fit on a 25 mm × 25 mm glass cover slip (used to prevent contaminating the optics with fluorescent material) and provides motion in x, y and z directions with ranges of 64.5 µm, 49.7 µm and 31.5 µm respectively [15]. PROCEDURE A quantum dots solution was prepared consisting of “Aloe Green” EviDots from Evident Technologies diluted with toluene and acetone to a concentration of 0.4 nmol/mL. These dots are cadmium-selenium cores with a zinc-sulfide shell and have a diameter of approximately 2.5 nm [2] with absorption and emission peaks at 540 nm and 553 nm respectively. Samples of Corning 0215 glass were cleaned with isopropyl alcohol (IPA) moistened tissues

The samples were then scanned on the confocal microscope at 3 locations and multiple focal planes into each sample. The first scan was taken at the interface between the sample and the coverslip at each location, then additional images were taken at 5 µm intervals into the cover slip (up to 10 µm) and 2 µm intervals into the sample (up to 10 µm) by finely adjusting the sample stage with piezoelectric actuators. At each focal plane, the fluorescent response was measured over a 40 µm×40 µm area with a total of 256×256 data points.

40 µm

Relative Fluorescence

RESULTS The fluorescent response data from each focal plane was analyzed in a MATLAB program that normalized the data with respect to the maximum fluorescent response measured on a glass sample without exposure to quantum dots. A representative scan of fluorescent response is shown below in Figure 2.

40 µm FIGURE 2. Image from the confocal microscope showing fluorescence in a sample exposed to quantum dots for 270 minutes prior to cleaning. The relative fluorescence scale is established with the maximum signal in a control sample (untreated by quantum dots) being equal to a value of one.

The fluorescent values below the normalized value of 1 were discarded as fluctuations were within the measurement noise of the sample. The remaining data was then examined to look for the maximum value and mean value of data points above the threshold as well as the percentage of the data points in the total scan that exceeded the threshold. The average values (across the 3 locations and multiple focal planes on the samples) and associated deviations are listed in Table 1. TABLE 1. Normalized fluorescent response in glass samples exposed to quantum dot solution. Sample

Maximum above threshold

Mean above threshold

% data above threshold

1.4 (σ=0.2)

1.2 (σ=0.1)

0.01% (σ=.008%)

1.9 (σ=0.9)

1.4 (σ=0.2)

0.03% (σ=.03%)

12.0 (σ=17.7)

2.4 (σ=1.4)

0.9% (σ=1.2%)

10.1 (σ=6.4)

2.3 (σ=0.6)

5.7% (σ=5.4%)

Cleaned after 15 minutes Cleaned after 75 minutes Cleaned after 270 minutes Sample Not Cleaned

Samples with a fluorescent response above the threshold were then analyzed to see how the fluorescence varied with the position of the focal plane. Examples of the fluorescent response for samples cleaned after 75 and 270 minutes are shown in Figure 3 below. Maximum Relative Fluorescence wrt Focal Plane Z-position

Maximum Relative with respect to (270Fluorescence minute exposure) Focal Plane Z-position (75 and 270 minute exposure) 70

Cover Slip ← Relative R e la t ivFluorescence e F lu o r e s c e

before 2 drops of quantum dot solution were placed on the surface. The samples were then divided into 4 groups. The first was cleaned after 15 minutes with IPA moistened tissues. The second group was cleaned in the same manner after 75 minutes while the third group as cleaned after 270 minutes. The final group was left uncleaned. Each sample surface was covered with a glass coverslip to prevent contamination.

→ Sample

60 50 40

270 min

30 20 10

75 min

0 -10

-8

-6

-4

-2

0

2

4

6

8

Position Position(microns) (microns)

FIGURE 3. Relative Fluorescence with Respect to Position in the Sample. Negative values denote focal planes in the coverslip, while positive values are in the glass sample.

10

In the case of the sample exposed for 270 minutes, there is a peak in the fluorescent response near the interface. This value decreases as the focal plane is moved into the cover slip (negative values, moving left) and as the focal plane is moved into the sample (positive values, moving right). Such a decrease indicates that the fluorescent material is no longer in the plane of focus and its fluorescence is increasingly prevented from reaching the photon counting module. DISCUSSION The samples cleaned after 15 and 75 minutes showed minor increases in fluorescent response over the control samples and only in a small percentage of the data points. The samples cleaned after 270 minutes showed an extreme increase in the fluorescent response, but once again only in a small percentage of points. The results of these tests illustrate the expected behavior of glass samples exposed to quantum dots in solution, independent of a polishing process. Exposure to quantum dots for less than 75 minutes prior to being wiped clean should not result in an appreciable adsorption of quantum dots onto the surface. Thus if significant number of quantum dots are retained in sample that was polished with a quantum dot tagged slurry for 75 minutes or less, the retention of quantum dots can reasonably be attributed to the influence of polishing mechanics. Polishing with quantum dot tagged slurries is underway. ACKNOWLEDGEMENTS This material is based upon the work supported by the Nation Science Foundation under Grant No. 0620783. Any opinions, findings and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation. Northrop Grumman, Synoptics has been an invaluable industrial partner with Bryan Stanley, Kevin Stevens and Adam Dittli making notable contributions to this work. REFERENCES [1] Lindquist, A., S.D. Jacobs, and A. Feltz. Surface Preparations for Rapid Measurement of Sub-surface Damage Depth. in Science of Optical Finishing Topical Meeting. 1990. Monterey, Calif.

[2] Quantum Dots Features. Quantum Dots Explained 2008 [cited 2008 April 14]; Available from: http://www.evidenttech.com/ quantum-dots-explained/quantum-dotfeatures.html. [3] Genin, F.Y., et al., Role of light intensitification by cracks in optical breakdown on surfaces. J. Opt. Soc. Am. A, 2001. 18(10): p. 2607-2616. [4] Lambropoulos, J.C., et al., Non-contact estimate of grinding-induced subsurface damage, in Optical manufacturing and testing III. 1999: Denver, Colorado. p. 41-50. [5] Bowden, F.P. and T.P. Hugher, Physical Properties of Surfaces IV-Polishing, Surface Flow and the Formation of the Beilby Layer. Proceedings of the Royal Society of London. Series A, Mathematical and Physical Sciences, 1937. 160(903): p. 575-587. [6] Adam, N.K., The Polishing of Surfaces. Nature, 1927. 119(2987): p. 162-163. [7] Beilby, G.T., Aggregation and Flow of Solids. 1921, London: Macmillan and Company Limited. [8] Minsky, M., Microscopy Apparatus, U.S.P. Office, Editor. 1961: USA. [9] Semwogerere, D. and E.R. Weeks, Confocal Microscopy, in Encyclopedia of Biomaterials and Biomedical Engineering. 2005, Taylor & Francis. [10] Hocken, R., N. Chakraborty, and C. Brown, Optical Metrology of Surfaces. CIRP annals, 2005. 54(2): p. 705-719. [11] Winn, A.J., et al., Examination of microhardness indentation-induced subsurface damage in alumina platelet reinforced borosilicate glass using confocal scanning laser microscopy. Journal of Microscopy, 1997. 186(1): p. 35-40. [12] Fine, K.R., et al., Non-destructive, real time direct measurement of subsurface damage. Proceedings of SPIE, 2005. 5799: p. 105110. [13] Reed, M.A., Quantum Dots, in Scientific American. 1993. p. 118-123. [14] Nanomaterials catalog Vol 7. 2005, Evident Technologies. [15] Elliott, K., Development of a Versatile Scanning System for Multi-Probe Biomedical Measurements, in Department of Mechanical Engineering and Engineering Science. 2008, University of North Carolina at Charlotte: Charlotte, NC.