Nanotechnology and Spectroscopy ECE 198 Lab Manual

Transcription

1 Nanotechnology and Spectroscopy ECE 198 Lab Manual Introduction/Overview: The purpose of this lab will be to study the light emitting, transmitting, and absorption properties of various materials. The lab will be built around the AvaSpec- EDU-VIS handheld spectrometer. Over the two weeks of this lab, you will build a spectroscopic system capable of measuring absorption and luminescence. Using your optical set-up, you will measure the optical properties of some of the nanoparticles we have discussed in class (Sigma-Aldrich Lumidot CdSe quantum dot solutions). There will be six different quantum dot solutions, each held in glass cuvettes. The first experiment will be to determine the absorption properties of each of these solutions. Using a simple white light source, you will illuminate the dot solutions and record the absorption spectra for each cuvette. Each solution should have a distinct absorption spectrum, which you should be able to determine from your measurements. Next, you will illuminate each cuvette with an ultra violet light emitting diode (UV LED). You will use the LED to optically excite the quantum dot solutions. See if you can find the emission spectra of each of the dot solutions. The majority of the two weeks will be spent building a set up to perform the above optical measurements on the quantum dot solutions. While the actual measurements won t take particularly long, the alignment process will most likely take some time. You should play around with all of the components you are given and familiarize yourself with how they work and how they are used. By the second week, you should be very familiar with the AvaSpec spectrometer. You should have time towards the end of lab to try to use the optical set-up you have built to perform measurements of the optical properties of other materials. 1

2 Basics of Spectroscopy: Spectroscopy, technically, is the action of determining a property of a material as a function of energy. We will be doing optical spectroscopy, which means we will be concerned solely with the optical properties (absorption, scattering, reflection, transmission, luminescence) of our systems as a function wavelength (energy). In this course you have already built your own spectrometer, so you should already understand fairly well how the spectrometer component of this lab works. Thus, we can spend a little more time discussing what it is, exactly, that our spectrometer is measuring. An absorption measurement studies the ability of a sample to absorb light over a range of wavelengths. All materials, whether they are single atoms, molecules, solutions, or crystals, contain electrons. In the case of a single atom, the electrons are only allowed to take on very specific energies, which correspond to the s, p, d, etc orbitals which you learned about in high-school chemistry. For a single atom, the electrons can only exist at these energies. Now imagine you now add a second atom to this situation. If the atoms are very far from one another, then each atom has the same electron energy levels as our initial, isolated atom. However, if the atoms are close together, for instance, if they are bonded to form a molecule, your energy picture changes. The electrons from each atom (especially those in the outer orbitals, because they are more loosely bound to the nucleus), now see the presence of the electrons from the other atom. In fact, if the atoms are bound together, the outer electrons may well be shared between the two atoms. When this happens, the orbitals of the electrons from atom 1 overlap with the electron orbitals from atom 2. Once the electrons are shared, you cannot tell which electron belongs to atom 1 and which belongs to atom 2. The Pauli Exclusion Principle tells us that we cannot have two objects existing at the same energy at the same point in space. The electrons in our scenario, because their orbitals coincide, exist, to a certain extent, at the same point in space. In order to obey the Pauli Principle, the electron energies must shift slightly, so that one electron is at an energy slightly lower, and the other at an energy slightly higher, than their original energies in the isolated atoms. For the electrons closer to the atomic nuclei, this shift will be small. This is because they are tightly bound to the nucleus, and don t see the electrons from the other atom. For those loosely bound, the energy shift is greater. If we now add a third atom to the picture, the electron energies will need to shift again, so that we now have 3 different energies for each electron orbital. As you add more and more electrons, you get more and more energy levels. A typical crystal might have atoms in a cubic centimeter. Thus, for a square centimeter of material, you would expect ~ different electron energies for each energy level you had in your original, single atom. This is essentially what happens, except because you have so many energy levels, they all blend together to give you what is known as an Energy Band. When we talk about energy bands in solids, we are just referring to the equivalent of the energy levels we had in our single atom. The width of each band (in Energy) and the distance between bands (in Energy) in a solid are determined by the material of which the solid is composed. Additionally, one often speaks of Energy Gaps in solids. These gaps are simply the ranges of energies at which you cannot have electrons. The single atom equivalent 2

3 Figure 1: Progression of electron energy levels as a function of number of atoms. would be the spaces between energy levels. Figure 1 shows the energy level to band progression as more and more atoms are added to the picture. With this picture alone, we can almost understand the processes of absorption and luminescence. The final keys to understanding optical spectroscopy are 1) understanding how electrons behave in these solids when there is no light incident upon the material and then 2) understanding what happens when we introduce photons to the picture. While electrons are allowed to exist anywhere in the energy bands of our solids, they are limited in their energy by other factors. In introductory chemistry we learned that in a single atom, the orbital states are filled up with electrons in a specific order. This order is determined by the energy of an electron in one of these orbitals. Electrons always fill up the orbitals of lowest energy first, before they begin to fill the higher energy states. The same is true with solids. Each band in a solid can only hold N electrons, where N is the total number of atoms in the solid 1. The lower energy bands tend to be full of electrons. The highest energy bands have no electrons in them. In some cases, you may only have enough electrons in your solid to fill up the last band halfway. These materials are known as conductors. Sometimes the final band is entirely filled with electrons, these materials are known as semiconductors or insulators. The only difference between the two (semiconductors and insulators) is the distance from this top filled band (known as the Valence Band) to the next, empty band (the Conduction Band). For semiconductors, the energy difference between these bands is small. For insulators, it is quite large. When you shine light on a material, you are hitting that material with photons. The energy of these photons depends on the type of light you are using. If you are using a red HeNe laser, each photon in your laser beam has an energy of about 1.95eV. If you are using a UV light source, at, for instance, 350nm, your photons have energies of ~3.5eV each. Note: the energy of light can be described in numerous units. Wavelength (in units of nm, 1nm=1x10-9 m) is probably the most common, but many people also use wavenumber (in units of cm -1 ) and energy (units of electron volts, ev). The basics of energy units are 1 Actually, each energy level in the single atom can hold two electrons (one of spin up and one of spin down). If we extrapolate this to the many-atom case (N atoms), then each energy band will be able to hold 2N electrons. 3

4 described below. The two equations which will help you move between the various units used for measuring photon energies are: [ ] [ ] [ ] [ ] [ ] [ ] Here λ refers to the light s wavelength in meters, c is the speed of light in a vacuum (~3x108 m/s), ν is the light s frequency in Hz, E is Energy in Joules, and h is known as Planck s constant (6.62 x J s). Light energy is usually not written in Joules, but in electron Volts (ev). Sometimes, in spectroscopy, we use the units of wavenumber [cm -1 ] as well. The conversions from Joules to electron volts, electron volts to nanometers (wavelength) and electron volts to wavenumber (inverse centimeters) are, also [ ] [ ] and When these photons are incident on a solid, they can be absorbed by an electron. An electron can only absorb a photon s energy, however, if there is space available for the electron at an energy equal to its initial energy + the photon energy. Figure 2 depicts this process schematically. In Figure 2, the photons incident on the material are of energy h. In the situation depicted in Figure 2a), h > E g (the distance in energy between the conduction and valence bands, otherwise known as the bandgap of the material). In this case (2a), there is a lot of empty space in the Conduction band, so electrons from the valence band can absorb the photon s energy and get excited into the conduction band. This process absorbs the photon, which prevents it from making it through the solid. If, however, the photon is of an energy less than the energy of the bandgap (Figure 2b), the absorbed photon would excite a valence band electron to somewhere in the middle of the Figure 2: a) Incident photon is of greater energy than bandgap, b) incident photon is of lower energy than bandgap and c) incident photon is of greater energy than bandgap, but bands are full. 4

5 bandgap, where electrons are not allowed to exist. This transition is not allowed, which means the photon will not be absorbed. Thus a photon of energy less than the bandgap will travel straight through the sample. You might also wonder why we are only talking about the top two bands of a solid. If a photon tries to excite an electron from a band lower than the valence band (Figure 2c), it cannot excite that electron into the next band. This is because that band is full, so there is nowhere for the electron to be excited to. For an absorption measurement, one places the sample of interest directly in a beam of white light. White light, as you already know, has a spectrum that covers the entire visible light range. This means that your sample has photons of many different energies trying to pass through it. If one of these photons is of an energy greater than the bandgap, it may be absorbed, if not, it will simply pass through the sample. By measuring which photons make it through the sample, you can determine the absorbance of that sample. You will see peaks in absorbance at the energies which correspond to electron transitions between energy bands. Thus, an absorbance spectrum can give you information about the electronic structure of the material in question. We will be performing numerous absorption measurements in this lab. Additionally, this lab will also study light emission from materials. Figure 3 shows a schematic of the physical process of emission. In emission, instead of measuring the electrons we excite up from a lower energy band to a higher energy band, we are looking at the electrons which move down from a higher band to a lower band. In order to see emission, however, there must be electrons in the upper energy states, otherwise there will be no electrons to transition down, and thus, no light emitted. Figure 3: Schematic of the physical process of photoluminescence. There are various ways to put electrons into the higher energy bands so that they can transition down and emit light. We will be optically exciting our samples to move the electrons up in energy (photo-excitation). When we study the luminescence from a photo-excited sample, we are studying what is known as photoluminescence. The way this works is as follows: 1) A photon from an excitation source (usually a laser or a light emitting diode) is directed towards the sample. 2) This photon, if it is of an energy greater than the bandgap, can be absorbed and thus excites an electron into the conduction band. 5

6 3) The electron will move very quickly down to the bottom of the energy band, usually by means of vibrations in the sample. This movement to the bottom of the band is non-radiative relaxation, meaning no photons are emitted. Non-radiative relaxation is a much faster process than most radiative (light emitting) transitions, which is why it happens first. However, the energy transitions associated with these vibrations are small. At the bottom of the band, the electron can no longer transition downward in energy non-radiatively, because it is prevented from doing so by the bandgap, which is much larger in energy than any of the vibrational transitions. In order to move back down to the lower energy band, where the electron wants to be (everything always wants to be at the lowest possible energy), it must execute a radiative transition. When this happens, the electron moves back down into the valence band, and emits a photon of energy equal to the distance between the two bands (the value of the energy gap) Non-radiative transitions, which play such an important role in the luminescence process, also help to explain a phenomenon known as the Stokes Shift. Basically, the Stokes shift is the energy difference between the peak of a material s absorption spectrum and its emission spectrum. If you think about it, when an electron absorbs light, it can be excited from anywhere in the valence band to anywhere in the conduction band. Typically the peak of the absorption spectrum corresponds to a transition from somewhere inside the valence band to somewhere inside the conduction band. Luminescence, however, because of the non-radiative transitions we discussed earlier, usually occurs from the edge of the conduction band to the edge of the valence band. Thus the peak of the absorption spectrum ends up being at a larger energy than the peak of the emission spectrum. For most of this lab, you will be playing around with quantum dots. Quantum dots are of great interest to scientists these days. They have special properties which bulk materials do not have. In order to understand quantum dots, let start with the bulk solid energy band diagram from Figure 1. We arrived at this picture by adding more and more atoms to our single atom picture until we saw the energy levels smear out into bands. We will now work backwards from this picture. We will remove atoms until we are left with only about a couple hundred atoms arranged in a sphere. This will be our quantum dot. Interestingly, the material still has a band structure, with both a conduction and valence band. However, this band structure is not the only thing which influences the energies which the electrons are allowed to take. Because the sample is so small, the electrons see (are affected by) the surface of the dot. Before, in the bulk solid, we had so many atoms that most of the electrons were contained well away from the edge of our sample and never saw the edges. Now, the electrons in the upper energy levels (loosely bound to the atoms) see the edges of the sample. In fact, the electrons are essentially squeezed by the dots boundaries. This leads to an effect known as quantization. The electrons in the valence band and conduction band are only allowed to take on certain, quantized energies. This is a picture very similar to the single atom picture we discussed earlier, and for this reason, quantum dots are often referred to as artificial atoms. The end result of this quantization is that 6

7 the emission and absorption spectra of quantum dots are very sharp (since instead of being allowed to move between wide bands of energy, electrons are forced to transition between discrete states). For this reason, quantum dots are excellent emitters and absorbers of light. You will get a chance to see this for yourself over the course of this lab. Before we start the lab itself, let s familiarize ourselves with the equipment we will be using. Equipment: This lab will center about the Avantes AvaSpec-EDU-VIS handheld spectrometer. This device allows you too look at light intensity as a function of wavelength/energy. The basic physics behind the device is actually fairly simple, and is really no different than the system you built in the first two weeks of this course. Light Collection Basics: The light you are interested in analyzing is collected by means of an optical fiber. The optical fiber is we are using is 1m long and has a 50μm diameter core which guides the light to the spectrometer opening. It is covered in a orange protective sheath. Please Note: The fiber is flexible, but there is a limit to the bend radius. Please try not to bend the fiber when you are using the spectrometer set up. A bend radius of ~6 is as small as you should go (1-2 loops). The fiber we are using has a numerical aperture (NA) of 0.22, which essentially means that it can only accept light entering the fiber at an angle of ~12.5 or less. This is actually an important parameter for building an efficient optical spectroscopy set-up. To a large extent, it determines the collection optics which can be placed in front of the fiber. However, this aspect of spectroscopy is a little out of the range of this course, and we should be able to get good results without worrying too much about the NA of the fiber. Spectrometer Basics: A schematic of a typical mini-spectrometer is shown in Figure 4. While this diagram is actually from one of Avantes competitors (Ocean Optics), the set-up of the two are likely nearly identical (and Avantes doesn t publish their lay-out, so we are showing the Ocean Optics version). The light incident on the open end of the fiber travels through the fiber, the other end of which is connected to the body of the spectrometer at position (1). The light passes into the spectrometer at (2). For many mini-spectrometers, position (2) is where the entrance slit of the spectrometer would be located, the width of which affects the ultimate resolution of the spectrometer. The Aventes system we are using does not have a slit, so our resolution is limited by the size of the fiber core we use. The light entering the spectrometer is then reflected off of a collimating mirror (4). This mirror collimates the light (this means the beam of light is neither expanding in size nor focusing as it travels. At this point, the beam of light traveling through the spectrometer consists of all the light collected by the fiber. The collimating mirror directs the light towards a grating (5), which spatially separates the polychromatic (many-colored) beam into its spectral components. In this regard, one can think of a diffraction grating as a 7

8 Figure 4. Schematic of USB2000-FL spectrometer reflecting prism. The light beam which originated from the fiber, is sent off of one final focusing mirror (6), which sends the spectrally separated light onto the charge-coupled device (CCD) chip (8). In your spectrometer lab, you rotated the grating while keeping the single element detector fixed. In the mini-spectrometer, the grating is fixed in position, and detector (the CCD) has many elements, which can detect the different colors of light simultaneously, as described below. This is the reason why the USB spectrometer can take many scans per second, while each scan for your spectrometers took ~minutes. Because the light has been expanded spectrally, each wavelength of light contained within your initial input beam is incident on the CCD at a different position (pixels) on the CCD chip. Each one of those positions on the CCD is understood, by the software, to correspond to a given wavelength. The AvaSpec CCD chip is a linear array of 2048 pixels, and can detect incident photons in the wavelength range from 350nm to 850nm. When a photon is incident on the CCD, it can be absorbed and if it is, it will most likely generate an electron at the pixel where it is absorbed. In the software, the CCD is told how long to absorb photons for. For instance, if you set the integration time for your CCD to 500ms, the CCD will absorb photons for 500 milliseconds. All of the photons captured during this time are stored (as charge) on the pixel of the CCD where they are captured. Each pixel is 14μm x 200μm, and can hold up to 62,500 electrons. After 500 milliseconds, the CCD will begin to clock out this stored charge. Figure 5 depicts the clocking out of charge on a CCD chip. Each pixel has three electrodes. These electrodes are biased, during light collection, to create an energy bucket which collects the charge generated by the photons. When you want to read this charge, the voltages are shifted between the three electrodes over each pixel in order to move the stored charge over to the next pixel. The voltages continue to this shift in this manner and the buckets of electrons are moved, pixel by pixel, across the CCD. At some point the charge reaches the end of the pixels, where it is clocked out and read. The amount of charge held in each pixel is measured sequentially by the accompanying CCD circuitry. As each pixel is clocked out, the charge is measured and recorded. The magnitude of charge read corresponds to the amount of light incident on the pixel clocked out. Because for any given spectrometer it is known which pixel corresponds to which wavelength, plotting charge vs. pixel# is equivalent to plotting light intensity vs. wavelength. 8

9 Figure 5: (a) Electrode/gate layout for a line of 3 pixels (b) Time lapse schematic of energy potential and charge transfer of one pixel (c) Clock voltages for 3 electrodes as a function of time The time it takes for the CCD to clock out and count every pixel is referred to as the read out time. For the AvaSpec, the time it takes to read out the entire CCD array (2048 pixels) and transfer the data to memory is on the order of milliseconds! The CCD chip in the spectrometer is not too different than what you might have in your cell-phone camera. In your cell-phone, each of the pixels has one of three filters on it, which allow red, green or blue light to be absorbed. This is how color images are generated from your CCD camera when it is placed in the image plane of the focusing optics of your cell-phone. For the CCD used in the spectrometer, there is no image, instead, all the light that is collected is spread so that each wavelength goes to a different part of the CCD. 9

10 Using the AvaSoft7USB2 Software for Data collection The spectrometer is controlled by Avantes AvaSoft7USB2 software. All the lab computers have the software installed. In the start menu go to Local Disk (C:) AvaSoft7USB2 avaspec77usb2. This should start the spectrometer software. Optical Equipment: For this lab you will be required to build both emission and absorption spectroscopy set-ups. For the former, you will need to utilize a white light source as your incident light. In order to maximize your absorption signal this light should be focused onto the sample you are studying, so that as much light as possible travels through the sample. You will need one lens to focus your white light onto your sample and then two lenses to collect the transmitted light and focus it into the collection fiber for the spectrometer. For the emission, or photoluminescence, experiments, you will need to direct your exciting light onto the sample with a lens (or two), and the collect the emitted light from the surface of the sample with a lens and focus it into the fiber. The excitation source for the photoluminescence experiment will be an ultraviolet light emitting diode. A white light lamp, which you have already used in the spectrometer lab, will be used for the absorption measurements. The quantum dots you will be studying are kept in 6, unlabelled, vials. They appear, for all intents and purposes, similar to water with some food coloring (though they are much more expensive). 10

11 The Lab The following experiments are meant to take between one and two weeks of lab time. You will be asked to build an optical set-up in the first week. Once you have built a functioning spectroscopy set-up, make sure you record the position of all of your optics carefully, so that in the subsequent week, if you must reconstruct the system, you can do so as fast as possible. You should be able to keep your set-up from week one to week two. Other groups will use the spectrometer and sources, but they shouldn t bother your optics. A good resource for understanding the principles involved in the construction of this optical set-up can be found at where you will find explanations of fundamental optical principles and more complicated problems, such as focusing light into a fiber. However, we really want you to spend time getting a physical feel for how optical components work. Try to spend as much time as possible actually playing around with the lenses, finding their focal points, maximizing your light intensity, etc. In this lab we will characterize our quantum dots using absorption and transmission experiments. We will build an optical set-up which can accommodate both of these experiments. In order to do so, we will use the cage/rail system available from ThorLabs. Using a cage/rail system has the significant advantage of making alignment less difficult than it otherwise would be with all table-mounted optics. Figure 6 shows a rough schematic of our optical set-up. The set-up basically consists of a T. At one branch of the T we will place our detection system. This consists of a Figure 6: Schematic of optical set up for absorption and luminescence experiments. 11

12 fiber leading to the spectrometer. At the other two ends of the T we will place our light sources. The rails that make up each branch of the T consist of 4 rods, onto which one can slide mounts housing lenses or filters. After construction, you should have a set-up which can perform both photoluminescence and absorption measurements without readjustment of any optics. Chassis The first piece to put together is the sample housing, which sits at the middle of the T. There are two pieces, both located in the Cage Centerpiece drawer, which screw together to form the centerpiece. The quantum dot solutions will sit on the bottom surface inside the housing. The round piece has an 8-32 hole on the bottom into which a post can be screwed. Use a post and post holder to mount the centerpiece to the breadboard. To create the branches of the T, use the rods located in the Cage Rails drawer. Each branch will have 4 rods, which screw into the centerpiece and hold 1 cage mounts for lenses and adapters. The two source branches use 8 rails, while the detector branch uses 4 rails. Be careful building using the cage system, most of the cage/rail components have extremely small screws that can fall into the holes of the optical tables, from which they are basically irretrievable. White Light Source (Transmission) Our white light source will be the fiber lamp we used in the spectrometer lab. Use the large fiber adapter (located in Adapters ) that you used in the spectrometer lab to mount the fiber into a 1 cage mount. Before putting it in the cage setup, however, we must put in a lens that will focus the white light on the quantum dots. Try different lenses by holding them in front of the white light output (using gloves!). You need to be able to focus the white light down to a spot size smaller than the cuvette (since you want all of your light to interact with the dots). You also want to be able to focus in a relatively short distance (since we don t have any 3ft rails!). Pick a lens that focuses the light well, place it in one of the 30mm brackets, and secure in place with a retaining ring. Please use the red spanner wrench from ThorLabs when you tighten any of the retaining rings. Don t tighten the rings too tightly. You need only to keep the lens vertical and secure, nothing more. Slide the lens mount onto the rail system. NOTE: Never touch any optical components on their faces. Your fingers have natural oils on them and these oils will remain on the lenses and filters. Sometimes the oil is simply difficult to clean, but other times it can actually damage the surface of the optical component. Wear Nitrile gloves when handling optics. If this is not possible, always hold optics by the outer edge. Once the lens is in the cage rail system, you should be able to move the bracket with the lens back and forth along the rails. Next, add the mount containing the white light fiber adapter. Both brackets should be able to slide freely along the rails. Most likely you will want to place an aperture in front of the white light source, but before the lens, as 12

13 shown in Figure 6. This is going to keep stray light from the white light source from overwhelming your data, and will make focusing a lot easier. The quantum dots will sit in the middle of the centerpiece, so we need the light to focus to that point. With a business card or other piece of paper, find the focal point of the white light. Move the lens and light source until you have positioned the focal point of the light directly in the middle of the sample space. Lock the brackets in place with the set screws. Spectrometer (Detection) The input to the Spectrometer is the 1m orange fiber you see attached to the spectrometer. The fiber will be mounted using the fiber mount (in the Adapters drawer). The adapter can mount into the cage system, but we want to be able to move the fiber s position as necessary to capture the maximum signal. Therefore, place the adapter in a normal LMR1 optics mount and mount it using a post, post holder, and base. NOTE: Do not touch the tip of the fiber. The fiber tip is designed to be extremely smooth and clean. If you touch the tip, you could deposit oils on the tip or damage the actual surface. Both of these actions will be detrimental to your light collection, and thus your results, not to mention your classmates results. The light will be expanding from the center of the cage setup, so we need to refocus it to the fiber. Place a lens in a second bracket and secure in place with a retaining ring. Place this bracket on the collection arm of the optical set up and use it to collimate the white light. Then, place another lens in the cage setup to focus the light to a point. Again, using a business card, move the lens until you have centered the focus of the collected light into the fiber collector. Block the fiber collector with a piece of paper so as not to overload the CCD. You will have to choose the right lenses to use here, so don t lock things down until you know you are collecting and refocusing as much light as possible. Your absorption spectroscopy set up is now complete. The white light should be focused at the space where you plan on putting the sample, after which it should expand until it hits the collection lenses, which will collimate the light and then focus it onto the fiber collector. UV Diode (Luminescence) For the luminescence set-up, place the UV diode in the LED mount (Adapters drawer) and put it in a cage rail bracket. Before you put the UV LED onto the rails, add two lenses (of your choice) in front of where the UV LED will go, to collimate and focus the UV light onto the sample. Turn on the power supply (without hooking it up). Set the voltage to 4V, which is the correct voltage to power the LED. Turn off the power supply. Attach the red and black leads from the power supply to the LED. The red (power) lead should attach to the longer lead on the LED. Turn the power supply back on. The LED should light up blue. Adjust the power as necessary to change the intensity of the light, but be sure not to burn out the 13

14 LED. Realistically, you shouldn t need to go to much higher voltage than 4V to get a good spectrum. Move the lenses so that the UV light is focused at the same point as your white light. The position of the lenses will determine the depth of the focal point. You will notice that the focal length of the lens seems to bear little to no relationship to the actual distance from the lens to the focal point. This is because the LED has a built in lens, which affects the actual focal distance of the set up. During your experiments, you will see a strong peak at 400nm from the UV LED. You might also notice a peak in emission intensity at 800 nm. This peak does not actually come from 800nm light, it is a figment of our spectrometer s imagination. What happens is that in the spectrometer, the incoming light is diffracted by the grating (as explained earlier, and in more detail in the spectrometer lab manual) and each wavelength is sent to areas on the CCD chip. However, the diffraction effect is not such that 100% of the light at wavelength x is sent to the x position of the CCD. Some of the x light will be sent to the 2x position on the spectrometer due to an effect known as second order diffraction. There is nothing one can really do about this, but we should be aware of it, especially when looking at strong signals. Figure 7: Emission spectrum from UV Diode, showing both the LED light emission at 400nm and the second order diffraction peak, which is actually 400nm light that has been sent to the 800nm pixels. Although the light emission between 450nm and 700nm is weak when compared to the diode signal, when compared to our luminescence signal, this light can be significant. Placing Quantum Dots Put an empty vial in the sample holder such that the light from both sources goes through the vial. You may need to adjust the focus of the white light source, the UV source, and the fiber collector, so that the collected light from each source goes to the fiber tip. You may have to first do this fine align with the white light source, and then turn off the white light source and align the UV source. This may take a few iterations. Keep in mind that the vast majority of the UV light will pass straight through the vial, 14

15 and only a small fraction will be scattered to the collection optics. In fact, the amount scattered may be so small that you cannot see it by eye, even with the lights off. If this is the case, align the UV light as best you can to the center of the sample vial, and move on (we can do the fine align before the luminescence experiment). When you are finished, you should be able to have both sources on, focusing at the same place, and being collected at the same place. Figure 8: Screen shot of AvaSoft software Once you have both the LED and the white light aligned and focused to the sample space of the spectroscopy set up and you have the sample mount fixed, you are ready to start taking data. Make sure you note carefully in your lab book the optical set-up you have built, including distances. The spectrometer should not be attached to the computer by its USB cable. Plug in the USB cable and wait for the sound which denotes the computers acknowledgement of the device. Open the software and press Start. You should see a spectrum on the screen. The spectrum (if your light sources are off) will be a spectrum of the room light. The peaks you see are the emission peaks corresponding to the fluorescent lighting in the room. Turn on the white light source. It will most likely overload the CCD, especially if your system is well-aligned. This presents something of a problem for our set-up. First, overloading the spectrometer is bad idea, in general, since the CCD can take some time to recover. More importantly, an overloaded CCD will give us no information about the absorbance of our material. Adjust the white light source power to prevent overloading the CCD. This means you will have to turn down the output power of the source (or decrease the CCD integration time) until the entire spectrum fits on the screen and you don t see it truncated at the top of the spectrum. Turn off the room lights and the white light beam. Look at your spectrum in the software. You will probably still have some light showing up on the spectrum, this is known as your dark spectrum. The one thing you want to make sure of is that your dark spectrum is reproducible. During the course of your experiment you will need to turn on the light and then turn them back off. You want to ensure that your lights off state is exactly the same every time. I would suggest turning off all the lights in the room and closing the curtain. 15

16 When you are happy with the darkness of the room, select the black box in the tool bar, labeled Save Dark. A box will pop up confirming the dark spectrum has been saved. The dark spectrum you have just saved is the background noise of your experiment. Because the white light source is blocked, this data contains no valuable information. You will only be interested in the light collected in addition to the dark spectrum. In the setup menu, select Subtract Saved Dark. The light emission should now be essentially flat and 0 across the spectrum. Unblock the white light source. You should now see a broad emission spectrum. If you change your integration time now you will have to go back and take a new dark spectrum. Any time you change the Integration Time you must do this!! Place an empty square vial of the quantum dot solution in the sample holder and fix the height of the vial so that the white light goes through the vial above the QD solution. Make sure the white light hits the vial directly on a face. If the light is off center, the vial can act as a lens, and direct your light away from the collection optics, which will cost you in terms of signal strength. Adjust the integration time and the lamp intensity so that you have a peak of about 3500 counts on your spectrum. The spectrum you now see is that of your white light source minus the light absorbed by the vial. This will be your reference. The reason we make this the reference, and not just open air, is because we are interested in the QD signal. Because the QDs are held in the vial, we want to get rid of whatever effects may be coming from the vial. If we take the spectrum of the vial and compare it to the spectrum of the vial with the dots in it, we can figure out what effect is from the dots alone. Once you have your dark spectrum saved and the white light spectrum through the empty portion of the vial peaked at 3500, you will want to store the reference spectrum. Select the white box in the tool bar, called Save Reference. If at some point you change any parameter of your set up (move a lens, move a light source, change the lamp intensity, or change the integration time), you will need to take, store, and save a new reference. Above the spectrum window, select Absorbance mode (see Figure 8). The spectrum display window will stop showing light intensity as a function wavelength, and instead will display Absorbance as a function of wavelength. Although the CCD is still taking the same data, the software is now displaying that data differently. Move the sample so that the light is now going through the quantum dot solution. You should see an absorbance spectrum on your screen with a defined peak. The software calculates the absorbance at a given wavelength λ as follows: S λ = Signal with sample in light path R λ = Reference signal D λ = Dark signal A λ = Absorbance A λ = -log 10 [(S λ D λ )/(R λ - D λ )] When you ask the software to display absorbance, it calculates A λ for every point in the spectrum, using the data you have stored in your reference and dark spectrums. If your absorbance spectrum goes negative at some point, this means that the expression 16

17 (S λ D λ )/(R λ - D λ )> 0 which means that you are getting more light (at some wavelengths) through your sample than you are through your reference. This is not possible in our experiment, and is usually an indication that the set-up changed between the reference and sample spectra. If this happens, it is probably a good idea to start over with a new dark spectrum and a new reference spectrum. However, there will always be slight changes as you move the dot solution so that light goes through the dots and not the vial, if you still can see a good spectrum, and the signal doesn t go too far negative, you can work with what you have. Note everything in your lab book. When you have a good absorbance spectrum on the screen save the spectrum by clicking the Save Experiment icon. The file will save to the data folder in the AvaSoft7USB2 folder where the software is stored. The file is given a file type and name that is unique to this software, and can only be viewed with the software. Since we care about the actual data points, we want to convert the file to a file type we can work with. Choose File -> Convert Graph -> To ASCII. This will save the file with the same name and with another unique file type,.t*t, with the * dependant on the type of graph. However, this file can be opened in notepad to view the data. Rename the file and maybe even save to a different file format. Perform similar measurements with the other five QD solution vials. It will be your decision as to whether you wish to take a new reference and dark spectrum for each QD solution. If your set-up moves during the exchange of samples, you will probably have to redo the reference measurements, if you are able to switch the dots without much disturbance to the set-up, you may use the same reference spectrum. Note: Additionally, if the signal/spectrum you are viewing on the screen is noisy, you can increase the number of averages you take, which will smooth out your spectrum. Simply increment the box labeled averages. When you do this, the software will automatically take multiple spectra and then average them. You have now taken the absorbance spectrum of all six of our quantum dot solutions. You should see absorption peaks at different energies for each of the samples. Note all of your results. Think about the results you see for wavelength values λ<450nm and λ>900nm. What is happening here? As a quick aside, it is worth noting that the absorption measurement you have just performed can also give you a Transmission spectrum. If you select Transmission Mode from the Spectrometer menu, the plot will show percentage transmission as a function of wavelength. The value for transmission is calculated using the equation: %T λ = [(S λ - D λ ) / (R λ D λ )] x 100% When you store a Transmission spectrum as sample data, the software stores the exact same spectrum as it would for an absorbance measurement. The only difference between the two is in the way the spectrum is displayed. You can now turn off the white light source. You will now use the luminescence setup you have built. It should already be aligned to the quantum dot solution held in the sample space. In the software, switch back to scope mode. You will probably want to take a new dark spectrum at this point. Store your dark spectrum and save it. It is important that all of your dark spectra are saved separately. When you come back later 17

18 on and wish to look at your data, you will not be able to view the absorbance data on the screen unless you also open the correct dark and reference files. When you save data, the software always just saves the intensity plot (light intensity vs. wavelength); it can only create absorbance plots with the reference and dark spectra associated with that scan. Make sure you note in your lab book which dark and reference scans go with which data files. For your luminescence experiments you will be using the UV diode as your excitation source. Turn off the room lights and take, store and save a dark spectrum. Turn on the diode to 4V. The light from the diode should be focused at the center of the vial of dot solution. The light emitted from the vial should be focused to the tip of the collection fiber. Look at the spectrum in the software (using Scope minus Dark Mode). You should see a very strong emission peak corresponding to the QD luminescence (you will most likely also see an even stronger peak from the UV diode). If the CCD is overloading you may have to lower the integration time. If this is the case, remember to take a new dark spectrum, and save and store this spectrum. If your signal is weak, you can increase your integration time, or you may need to fine align your system. Either way, a new dark spectrum will be necessary. Take photoluminescence spectra for each of your quantum dot solutions. Save each of the spectra with the solution number in the file name. At this point your spectroscopy set up should be able to switch between luminescence and absorption measurements without any adjustment of the optics. At this point in the lab, you may have some time left until the end of your lab session. Try to take absorption and emission spectra of materials other than quantum dots. For instance: 1) As I am sure you know, plants contain a chemical known as chlorophyll. This is the chemical that converts sunlight into energy for the plant. Obviously, in order to do this, chlorophyll must be able to absorb light. Now that you are masters of spectroscopy, light absorption measurements are something you can do with ease. It turns out that chlorophyll can be extracted from leaves rather easily. As an experiment, you could try to extract chlorophyll from a spinach leaf or a leaf from outside Everitt Lab and study its optical properties. This can be done by putting a bunch of leafs in a cuvette with Isopropyl Alcohol and mashing it up. Think about what your results tell you about chlorophyll and more generally, the process of photosynthesis. Does the chlorophyll emit light as well as absorb it? Why? 2) Every one has slightly different hair. Can you think of a way to quantify these differences using your spectrometer set-up? How would you set up this experiment? If you have some hair to spare, try to measure the spectra from your hair and see if the results are what you thought they would be. Note your work in your lab book. 3) Feel free to experiment with anything you can find: soda, soap, juice, skin flakes, etc. All of these have spectra associated with them. Some you may be able to detect on your set up, some of them you won t. Try taking the emission and absorption spectra of various things you can find around the lab or that you 18

19 brought to lab this week. Note your experiments and your results in your lab book. Lab Write Up The lab report for this week s lab will be in the form of a journal manuscript. In particular, look at the journal Applied Physics Letters (APL) for inspiration. APL publishes short (3-4 page) letters describing experimental and theoretical work in Physics, Electrical engineering, Nanoscience, Materials science, Optics, etc. It is a wellrespected journal and has historical importance, as the home of numerous groundbreaking papers, including many from this institution! Each group member will turn in their own lab-report. For this lab report, assume you are reporting on a new type of quantum nanostructure, which you are investigating using optical absorption and emission spectroscopy. Since you don t know the size or material make-up of your quantum dots, you will have to use your data to make informed and reasonable conclusions. You may not be able to describe your materials perfectly, and there may be uncertainty or noise in your experimental results. This MUST be reported. Everything you state in the paper must be verifiable from your data. If you are guessing, you must note this, and explain the uncertainty inherent in your guess. Please note that a scientific guess is very different than what you might be used to calling a guess. If a scientist cannot be certain about something, but has strong experimental evidence to support a conjecture, she may note that the data suggests a certain conclusion, but that there could be other possibilities or uncertainty in the experiment. She will then elaborate on these. The general format of an APL paper is as follows: Title: Come up with a descriptive, succinct title for your work. Authors and Affiliations: List all people who contributed in a meaningful way to the work presented, as well as their affiliations. Abstract: In about 100 words, summarize the work you are going to present, why it is interesting, and what the results are. Manuscript Body: The manuscript body is, for APL, usually one large block of text. However, you may find it helpful to divide the manuscript body into the following sections: Introduction: Give a larger picture view of the field of research your work falls into. In this case, the lab is a nanotechnology and spectroscopy lab, so spend a couple paragraphs talking about nanotechnology, spectroscopy, and in particular, quantum dots. Why they are interesting, what they might be used for, etc. Experimental Set-up: Describe the experiments you ran to get the data. A figure here might help people understand what you did. 19

20 Results and Discussion: Here is where you show your data, explain how you generated the data (dark subtraction, etc). For instance, if you decide to show absorbance, or transmittance, explain how you calculate the absorbance. Try to show as much data as possible. You will probably show at least 2 figures here. However, showing 6 figures for PL from each sample, and 6 figures with absorbance from each sample, would be overkill. You don t have room for this. You might want to show all six absorbance data sets, and all six PL data sets on the same plot. Then maybe you would show a figure with the absorbance and PL of a single sample (your best one) on a single plot. Describe the data, i.e: we see distinct peaks from each of the six samples at wavelengths of.. Now you are ready to discuss the data. Try to explain to the reader what each figure is telling you about the samples. What is happening physically, in the PL or absorbance process? What do the differences in the data tell you? Conclusion: Take a paragraph or two to summarize what you did, what your results are, what they tell you, and why it is interesting. Figures: Your paper should have anywhere from 3-5 figures. Make these as clear as possible for the reader to understand. This means: color coding and legends, using LEGIBLE axis titles and numbering, and any labeling that might help the reader understand your work better. All figures should include figure captions that tell the reader what you are showing in the figure, and pointing out the most important features of the figure, i.e. Note the 6 different PL peaks covering the wavelength range or A clear shift in emission is seen for each of the 6 samples, etc. In general, Excel is a crappy plotting program. I believe students at UIUC have free access to Origin, which is the plotting program of choice for most experimental scientists. Even MATLAB is preferable to Excel. Do not use someone else s figures!! This is a no-no. You cannot take figures from the internet (or the lab manual) for your manuscript. References: You must reference all sources of information you use in your manuscript! In most APL articles, we try to reference the work that went before us, and give credit to others working on similar effects or devices. However, I do not want this lab-report to turn into a research paper, so you are not required to cite everyone working in the field of nanotechnology. However, if in the course of preparing the manuscript, you come across sources which helped you to understand the topic, or you find papers of people doing similar work, these should be referenced. Formatting: Please try to replicate the APL 2-column format and font size, if possible. I have linked to a Word template on the course website. You may need to adjust fonts, etc., but this should serve as a starting point and save you a lot of time. 20