A study on the synthesis and characterization of ternary semiconductor nanocrystals

Transcription

1 Honors Theses Chemistry Spring 2013 A study on the synthesis and characterization of ternary semiconductor nanocrystals Tyler J. Hurlburt Penrose Library, Whitman College Permanent URL: This thesis has been deposited to Whitman College by the author(s) as part of their degree program. All rights are retained by the author(s) and they are responsible for the content.

2 A STUDY ON THE SYNTHESIS AND CHARACTERIZATION OF TERNARY SEMICONDUCTOR NANOCRYSTALS by Tyler J. Hurlburt A thesis submitted in partial fulfillment of the requirements for graduation with Honors in Chemistry. Whitman College 2013

3 Certificate of Approval This is to certify that the accompanying thesis by Tyler J. Hurlburt has been accepted in partial fulfillment of the requirements for graduation with Honors in Chemistry. Steven Hughes Whitman College May 8, 2013 ii

4 Table of Contents Introduction. 1 Semiconductors... 2 Particle in a Box... 5 Advantages of Nanocrystal 8 Quantum Yield. 9 Previous Research. 12 Experimental Section 16 Results and Discussion 27 CIS Synthesis CGS Shell on CIS 29 CGSe Synthesis.. 31 Cation Exchange. 32 AIS Synthesis AGS Synthesis. 34 Changing of Stoichiometry. 38 Ligand Effects.. 40 Growth Time. 43 Conclusions Acknowledgements.. 49 References. 50 iii

5 List of Figures 1. Stokes shift Photoluminescence of a semiconductor Indirect band gap Example of the particle in a box Core/shell nanocrystal system Type I and type II heterostructures Chalcopyrite crystal structure of I-III-VI semiconductors Band gaps for several of the materials being studied Photoluminescence of CIS at various stages of growth UV/Vis absorption spectra of CIS at various stages of growth Photoluminescence of CIS before and after shelling with ZnS Photoluminescence of CIS/ZnS at various stages of growth Photoluminescence of CGS XRD of one-pot synthesis of CIS/CGS Photoluminescence of CGS shelling of small CIS seeds Photoluminescence of CGS shelling of large CIS seeds Photoluminescence of CGSe nanocrystals Photoluminescence of nanoparticles before and after cation exchange to cadmium Photoluminescence of AIS nanocrystals Photoluminescence of AGS nanocrystals grown with different amounts of time before sulfur injection Photoluminescence of AGS nanocrystals grown with and without vacuum Photoluminescence of AGS nanocrystals grown at different temperatures Photoluminescence of AGS nanocrystals with different Ag:Ga ratios XRD pattern for 0.5:1 silver to gallium AGS nanocrystals. 39 iv

6 25. Photoluminescence of AGS particles grown in the presence of three different ligands Photoluminescence of AGS particles grown in the presence of different ratios of stearic acid and hexanoic acid TEM images of nanocrystals grown in the presence of different ratios of stearic acid and hexanoic acid PL and absorption spectra for AGS particles taken at various times of growth XRD patterns for AGS particles grown for 5 min, 1 hour, and 2 hours TEM images of AGS particles grown for 5 min, 1 hour, and 2 hours 45 v

7 Introduction Currently, the lighting market is a billion dollar industry and there is a move away from incandescent and fluorescent light bulbs to light emitting diodes (LEDs) for general lighting purposes. 1 When developing lighting for rooms, a warm white light is desired. Unfortunately, most LEDs today have a blue hint to them, which most consumers find to be cold. 2 This is due to the fact that in the spectrum of light emitted there is a large peak in the blue region with no emission in the green and red regions. 2 One way to fix this problem is to coat the LED with a phosphor (a material that shows luminescence) that absorbs light in the ultraviolet and blue region very well and then reemits that light at longer wavelengths. 2 Typically a blue LED will be coupled with either one material that emits in the yellow region or two materials: one that emits in the green region and one that emits in the red. Usually, the added red phosphor causes the two phosphor system to produce a warmer white light than the single phosphor system. Unfortunately, adding an additional phosphor also increases the heat generated by the system, making these systems more inefficient. 3 The most common materials used to make these phosphors are rare-earth metals which can be costly and inefficient. 1 Additionally, many of the rare earth oxides used to make the phosphors are experiencing worldwide shortages, furthering the need to find an alternative material. An emerging trend in research is to develop semiconductor nanocrystals to downshift the light. 1,4,5

8 Figure 1. Stokes shift. This downshifting of the blue light comes from the Stokes shift of the material. Stokes shift is the difference between the peak wavelength of the absorption and emission spectra of a substance. 6 A typical Stokes shift has an emitted photon with less energy than the absorbed photon (see Figure 1); conversely a substance with an anti-stokes shift would emit a photon with greater energy than the absorbed photon, causing a blue-shift of the light. 7 The Stokes shift is the principle of all LED phosphors because of the ability to alter the wavelength of the light. Semiconductors: A semiconductor is a material with properties between those of conductors and insulators. All solids have a valence band, which the valence electrons normally occupy, and a conduction band, which excited electrons occupy. The energy between these two bands, where other energy states are forbidden, is called the band gap. In a conductor, these two bands are touching or even 2

9 overlapping such that valence electrons can lie in the conduction band without needing to be promoted, where they can then travel around the bulk material. In an insulator the bands are different in energy, so a great amount of energy is required to excite an electron form the valence band into the conduction band. 8 Semiconductors have an energy gap between the bands (band gap) that is commonly in the range of the energy held by photons in the visible light spectrum. Shining light of sufficient energy on to a semiconductor can promote an electron from the valence band to the conduction band leaving an electron hole in the valence band. 8 This electron-hole pair is commonly referred to as an exciton. After excitation, the exciton is free to travel in the material in the conduction band, allowing for conduction. The ability of an exciton to travel is measured by both the exciton mobility and lifetime. The lifetime of an exciton is the time between excitation and recombination, for CdSe nanocrystals the exciton lifetime is on the range of 6 ns. 9 A more mobile exciton with a longer lifetime can conduct much better than a less mobile exciton. 8 A long lifetime is not always desirable for uses in LEDs as more time allows for a greater chance of an exciton to travel to the surface of the nanocrystal. Figure 2. Photoluminescence of a semiconductor. In this case: silver gallium sulfide 3

10 All semiconductors have a finite exciton lifetime, meaning that eventually the excited electron will fall back down to the valence band. When this occurs, the electron will drop in energy from the lowest energy state in the conduction band to the highest energy state in the valence band, emitting a photon with energy equivalent to the band gap. 8 Thus, a semiconductor emits light with energy equal to its band gap, as shown in Figure 2. Figure 3. An indirect band gap. Energy is plotted against crystal momentum. In order for an electron to be promoted to the conduction band it needs a photon of sufficient energy coupled with a phonon to overcome the difference in crystal momentum. Band gaps come in two categories: direct and indirect. In direct band gaps, the excited electron and its corresponding hole have the same crystal momentum in their respective bands. 8 This means that the lowest energy state in the conduction band is directly above the highest energy state in the valence band, allowing for a photon with energy equal to the band gap to be sufficient to promote an electron. In this case, crystal momentum is typically zero and radiative recombination is common. For indirect band gaps, shown in Figure 3, 4

11 the highest state in the valence band and the lowest state in the conduction band occur at different crystal momentums. This means that not only must an electron gain sufficient energy from a photon to be promoted, but it must also gain the right amount of momentum via a lattice vibration or phonon. 8 This same principle applies to the recombination of an electron and a hole, making a much slower two-step process thereby decreasing the likelihood of radiative recombination. For this reason, semiconductors with direct band gaps are much more widely used for optical devices. 8 Particle in a box: Nanocrystals are crystalline particles with a diameter less than 100 nanometers. 10 Due to their very small size, nanocrystals can exhibit a phenomenon known as quantum confinement. Quantum confinement is a phenomenon by which the size of the band gap for a semiconductor is determined by the size of the particle. 11 It effectively is a real-world example of the particle in a box. The particle in the box refers an electron confined in a potential well of length a. In the simplest model, inside the well there is a potential energy of zero, while anywhere outside the box has an infinite potential energy. This means that the probability of finding an electron outside the box is zero and inside is 1. It follows that the probability of finding the particle at the edges of the box is also zero. Consider the time-independent Schrödinger equation for a one dimensional potential well defined in the x-direction: -ħ 2 /2m d 2 ψ/dx 2 + V(x)ψ(x) = Eψ(x) 5

12 Where ħ is Planck s constant, m is the mass of the particle, ψ(x) is the wavefunction of the particle at location x, V(x) is the potential of the particle at x, and E is the total energy. Applying the boundary conditions imposed (ψ(0)=0,ψ(a)=0) to the general solution of the time-independent Schrödinger equation ψ(x) = Asin(k x)+bcos(k x) where A, B, and k are constants, gives the wavefunction of the particle as ψ(x) = Asin(k x). Differentiating the wavefunction gives us dψ/dx = kacos(k x) and d 2 ψ/dx 2 = -k 2 Asin(k x). We see that d 2 ψ/dx 2 = -k 2 ψ Where using the Schrodinger equation, k can be solved for k = (8π 2 me/h 2 ) 1/2 giving ψ(x) = Asin((8π 2 me/h 2 ) 1/2 x). Applying the boundary conditions again gives us ψ(x) = Asin(nπ/a x) where n is an integer. We can find A by normalizing the wavefunction. This gives A = (2/L) 6

13 and thus ψ(x) = (2/L) sin(nπ/a x). Solving for the allowed energies in the box, we find: E n = (n 2 h 2 )/(8ma 2 ) This shows that the energy levels in the box are defined by the size of the box, a. Decreasing the size of the box causes the energy of the respective levels to increase, also increasing the gaps between levels. In terms of nanocrystals, the smaller particle, the smaller a, causes the gap between the valence and conduction band to increase, resulting in a photon with greater energy being necessary to promote an electron and a photon with greater energy being emitted when the exciton recombines. 8 Blue > Red Figure 4. As an example of the particle in a box, the larger the nanocrystal, the redder the light emitted. Since the photon emitted has a greater energy for smaller particles, it follows that smaller particles emit bluer light. This quantum confinement leads to the tunability of the light emitted from nanocrystals. Previous research has shown 7

14 that it is quite easy to tune the emission of nanocrystals based on the size of the particles with larger particles emitting redder light, as in Figure 4. 4 Advantages of nanocrystals: Nanocrystals exhibit several very significant advantages over other materials used in LEDs. Perhaps the biggest advantage of nanocrystals is the fact that they can be synthesized in a colloidal system. 12 This bottom-up approach allows for scalable synthesis. 13 In a bottom-up synthesis, the particles are formed by the nucleation of just a few atoms followed by the controlled growth on to the outer surface of the crystal, until the final particle is formed. For the sake of comparison, in a top-down approach, you start with a piece of material and etch down until you have your desired particle. This top-down method is far more inefficient and difficult to scale. Nanocrystals are also highly absorbing, especially in the UV to blue region, 14 which is beneficial for downshifting applications, such as LEDs (and also extremely useful for solar cells since much of the sun s power lies in the UV). 8 For LEDs it is helpful for the nanocrystals to be highly absorbing so that enough photons are captured from the shorter wavelength regions to allow for adequate emission at the longer wavelengths. If they were not highly absorbing, it would be necessary to add a greater number of the particles in order to get the same effect, increasing the heat generated and making the LED more inefficient. Due to the incredibly small size of nanocrystals, they have a very high surface to volume ratio. 15 This is not particularly beneficial for use in LEDs, but it 8

15 provides a tremendous advantage for use in catalysis. Very small nanocrystals can almost be considered entirely surface, which is where catalysis occurs, theoretically making nanocrystals efficient catalysts with promising results towards water splitting in particular. 16,17 Quantum yield: One of the main measures of the efficiency of a semiconductor nanocrystal is the photoluminescence quantum yield. Photoluminescence quantum yield is defined as the ratio of the photons absorbed to the photons emitted. The greater the quantum yield, the brighter the emitted light. Typically quantum yields less than 1% are not visible to the naked eye, while yields in the 80% region are considered to be extremely good. 4,18 The main reason that an exciton would fail to reemit a photon would be due to traps. 4 These traps are energy levels that occur in the normally forbidden region of a band gap which then cause a multi-step recombination process whereby no photon with the energy of the band gap is emitted. The most significant source of these traps is from the energy levels associated with the surface of the materials, both on the exterior of the nanocrystal and at any crystal defects in the interior of the particle due to the presence of atoms that are not bonded to the proper number of other atoms. 8 An important factor for reducing these traps, and thereby increasing the quantum yield of the nanocrystals, is surface passivation. 1 The particles must have a ligand bonded to the surface of the crystal that passivates the surface, 9

16 effectively keeping the exciton from seeing the surface. In terms of the particle in a box, the passivating ligand increases the height of the walls of the well. The same ligand can also act as a surfactant, allowing for better solvation. 1 Without passivation the nanocrystals will aggregate, making larger particles, and then precipitate instead of staying suspended for long periods of time due to the size of the particles. Another implication of aggregation is the formation of more allowed energy levels in the forbidden region thereby creating traps and reducing quantum yield. Figure 5. A basic core/shell structure for nanocrystals. Another way to increase quantum yield of a nanocrystal is to have a core/shell system. 4 This works by having a core of one semiconductor material shelled by another as in Figure 5. For LED applications, the outer shell must have a band gap that is larger than that of the core with the valence band of the shell lower than that of the core and the conduction band of the shell higher than that of the core. 14 This allows for only photons with a higher energy to be absorbed promoting an electron up to the level of the shell s conduction band. 10

17 This sort of band gap pair is termed a type I heterostructure (also called a nested heterostructures) as shown in Figure 6(a). Because electrons move toward states at lower energies while holes tend to move towards higher energy states, the exciton will travel into the shell where the conduction band is lower and the valence band is higher. This increases the likelihood that a photon will be emitted with the proper energy. This allows for excellent confinement, since the entire surface of the particle will be composed of the material with a greater band gap. Figure 6. (a) A type I heterostructure. In this case both the excited electron and the hole would move toward the material on the right. (b) A type II heterostructure. In this case an excited electron would move to the lower energy conduction band of the material on the right, while the hole would move up in energy toward the material on the left. In type II (or staggered) heterostructures, the band gap of the shell material does not fully enclose that of the core material. Either the conduction band of the shell is lower than that of the core or the valence band of the shell is higher than that of the core, shown in Figure 6(b). Type II heterostructures lead to exciton separation, since the hole or electron will remain in the shell material, while the other particle will move toward the core material. This sort of charge separation may be beneficial for making solar cells, but very detrimental towards LEDs as the exciton pair would no longer recombine. 11

18 For this core/shell system to work properly, the two materials must have lattice structures that are similar enough to allow for proper bonding at the interface between the two materials. 18 If the two materials have a lattice mismatch greater than about 10%, there will be poor shelling. Poor shelling is the result of the formation of cracks and gaps in the crystal structure due to strain. This leads to an increase in the amount of traps and thus decreasing the quantum yield of the nanocrystals. 4 However, if the lattices match up sufficiently, they will be able to stretch slightly at the interface, decreasing the strain and to allow for good deposition of the shell material. Previous research: Previous research has shown that it can be relatively easy to tune the emission of nanocrystals by controlling the size of the particles. One of the main ways to control the size of the nanoparticles synthesized is to limit their growth time. 4 The longer the particles are grown, the bigger they will get and thus the longer the wavelength of emission will be. The vast majority of previous research on semiconductor nanocrystals has centered on studying binary nanocrystals, or crystals with two different elements. 2,14,15 These materials are generally either II-VI semiconductors such as CdSe and PbS or III-V semiconductors such as InAs and GaAs. 2,14,19 There has also been research done on the ternary nanocrystals, mostly of the I-III-VI form such as CuInS 2 (CIS) and AgInS 2 (AIS) All of these materials have band gaps in the 1-3 ev range making them suitable semiconductors for light 12

19 applications. However, most of the binary semiconductors contain toxic elements, particularly the II-VI s which contain cadmium and lead. 5 It is more difficult to make and market practical LEDs containing these materials due to their environmental effects, especially since they are banned in many regions. 5 Luckily, the ternary semiconductors tend to be made of far less toxic elements. 20 Recent research has shown that the band gap of a material, particularly for materials with at least three elements, can also be tuned by varying the stoichiometry of the constituent elements in the material. 5,24-26 These materials are all commonly used as the core material in a core/shell system. The most popular shelling material for all of the various nanocrystals is ZnS due to its wide band gap (3.8 ev) 27 which allows for nested heterostructures in almost all cases, its favorable lattice match to a wide range of materials (a=5.409 Å) 28, and its relatively inexpensive cost due to the natural abundance of zinc and sulfur. 4,5,14 Because ZnS has a very large band gap, only photons in the UV region have enough energy to promote an electron to the conduction band. This means that any nanocrystal with a ZnS shell would not be able to absorb blue light. Since we are looking to make a material that can absorb well in the blue region and then emit at longer wavelengths, our research looks to find an efficient way to shell a CIS core with another ternary compound instead of ZnS. These ternary I-III-VI materials typically have a chalcopyrite structure (Figure 7) with unit cell constants of a=5.52 Å and c= Å for CIS. 13

20 Cu Cu Cu S In Cu Cu In S Cu In S In S In Cu In S In Cu S In Cu S In Cu S Cu In Cu Cu Figure 7. Chalcopyrite structure of I-III-VI semiconductors. For CIS, copper is in pink, indium in blue, and sulfur in yellow. We will look into using both CuGaS 2 (CGS) and AgGaS 2 (AGS) as a shelling material. CGS shows promise because of a very good lattice match with CIS and the possibility of a graded shell where the interior of the nanocrystal is CIS and the shell is CGS, but there is also a region between the two with a mix of indium and gallium. For the AGS shell, the band gaps for these two materials line up very nicely, as seen in Figure 8, and they have a decent lattice match which is better than that of CIS and ZnS. By using a system that has two ternary materials, there is also that added tunability of the band gaps based on stoichiometry of the constituent elements. In order to achieve this, we will first find a good way to synthesize and control the CIS cores. We will then look into synthesizing CGS and AGS on their own, with a greater focus on the AGS particles due to a lack of previous research on the material, before moving on to attempting to shell the CIS particles in CGS and AGS. 14

21 Figure 8. Band gaps for several of the materials being studied. 15

22 Experimental Section Materials: Gallium(III) acetylacetonate (99.99%), copper(i) iodide (99.999%), indium(iii) acetate (99.99%), indium(iii) acetylacetonate (99.99%), indium(iii) chloride (99.999%), zinc stearate (90%), trioctylphosphine oxide (99%), trioctylphosphine (97%), 1-dodecanethiol (96%), oleylamine (70%), octylamine (99%), 1-octadecene (90%), tert-dodecanethiol (98.5%) were purchased from Sigma-Aldrich. Lead(II) nitrate (99.9%), lead(ii) chloride (100.0%), zinc acetate (99.9%), zinc chloride (99.1%), silver acetate (99%), sulfur powder (99.98%), and hexanoic acid (98%) were all purchased from J.T. Baker Chemical Co. Silver nitrate (99%) was purchased from EMD. Pyridine (99%) and toluene was purchased from Mallinckrodt. Toluene (99.5%) was also purchased from VWR, as was methanol (99.8%) and isopropanol (99%). Methanol (99.8%) was also purchased from Alfa Aesar. Selenium powder (99.99%) was purchased from the General Chemical Company. Stearic acid and cadmium chloride were provided by the Whitman College Chemistry Department Stockroom. All chemicals were used as received. Methods: One-pot synthesis of CIS/ZnS: A 25 ml three-neck round bottom flask was loaded with indium(iii) acetate (In(Ac) 3 ) (0.5 mmol), CuI ( mmol), and 1- dodecanethiol (1-DDT) (5 ml). A 20 ml scintillation vial was loaded with zinc stearate (2 mmol), 1-DDT (0.5 ml), and 1-octadecene (ODE) (2 ml). The zinc stearate solution was heated to 190 C to promote dissolution. The copper and 16

23 indium mixture was heated to 100 C with stirring and under a vacuum (<100 mtorr) and then held at 100 C for 45 min. The solution was then backfilled with argon and heated to above 200 C. Starting around 200 C, the solution began to change color from yellow to orange and then to red as the temperature increased. When the reaction mixture reached the desired color, the zinc stearate solution was added dropwise at a rate which kept the temperature nearly constant (~1 ml/min). After the addition of the zinc stearate solution, the reaction mixture was heated at 240 C for 1 hour. The solution was then cooled, cleaned, and collected as described in the cleaning section; all syntheses followed this same cleaning process. Cleaning process: The reaction mixture was cooled by squirting the flask with water. Once at room temperature, the solution was added to centrifuge tubes and an equal part of 50:50 methanol/isopropanol (IPA) was added. The mixture was shaken and then centrifuged at 12,500 rpm for 5 min. The supernatant was discarded and the solid product was collected in toluene. CIS synthesis (method A): CIS nanoparticles were synthesized by using the method described above, but instead of injecting a zinc stearate solution when the desired color was achieved, the solution was cooled. CIS synthesis (method B): A 25 ml three-neck round bottom flask was loaded with In(Ac) 3 (1 mmol), CuI (1 mmol), and ODE (10 ml). The mixture was stirred under a vacuum (<100 mtorr) for 30 min and then backfilled with argon and heated to 150 C. 1-DDT (2 ml) was then rapidly injected causing the solution to turn yellow. The solution was then heated to 280 C. During this heating the 17

24 solution went from yellow to orange to red to black. The solution was held at 280 C for 30 min. Two-pot synthesis of CIS/ZnS: A portion of the final product of CIS from method A (1.5 ml) was loaded in a 25 ml three-neck round bottom flask. The toluene was blown off with argon and light heating (50 C). Once the toluene was blown off, ODE (5 ml) was added to the flask (4 g of trioctylphosphine oxide (TOPO) were alternatively used) and the solution was degassed for 30 min (vacuum <100 mtorr). Meanwhile, a 20 ml scintillation vial was loaded with zinc stearate (2 mmol), 1-DDT (0.5 ml), and ODE (2 ml). The zinc stearate solution was heated to 190 C to aid in dissolution. The CIS solution was heated to 100 C while still under vacuum and then backfilled with argon and heated to 230 C. At 230 C, the zinc stearate solution was added dropwise at a rate which kept the temperature nearly constant (~1 ml/min). After the addition of the zinc stearate solution, the reaction mixture was heated at 240 C for 1 hour. Two-pot synthesis of CIS/CGS (method A): A portion of the final product of CIS from method A (1.5 ml) was loaded in a 25 ml three-neck round bottom flask. The toluene was blown off with argon and light heating (50 C). Once the toluene was blown off, gallium(iii) acetylacetonate (Ga(acac) 3 ) (0.5 mmol), copper(ii) acetylacetonate (Cu(acac) 2 ) (0.5 mmol), TOPO (1.75 mmol), and ODE (5 ml) were all added to the flask. The mixture was then heated to 100 C under argon, degassed for 30 min, and then heated to 230 C under argon. The solution was held at 230 C for 1 hour. 18

25 Two-pot synthesis of CIS/CGS (method B): A portion of the final product of CIS from method A (1.5 ml) was loaded in a 25 ml three-neck round bottom flask. The toluene was blown off with argon and light heating (50 C). Once the toluene was blown off, Ga(acac) 3 (0.5-1 mmol), Cu(acac) 2 (0.5-1 mmol), TOPO ( mmol), and ODE (5 ml) were all added to the flask. The mixture was degassed for min at 40 C, and then heated to 150 C under argon. A mixture of 1-DDT (0.25 ml) and t-ddt (1.75 ml) heated to 150 C was rapidly injected. The solution was heated to 245 C and held there for min. CGS synthesis: A 25 ml three-neck round bottom flask was loaded with Ga(acac) 3 (1 mmol), Cu(acac) 2 (1 mmol), TOPO (3.5 mmol), and ODE (10 ml). The mixture was stirred under vacuum at room temperature for 30 min and then backfilled with argon and heated to 150 C. A mixture of 1-DDT (0.25 ml) and t- DDT (1.75 ml) was rapidly injected. The solution was then heated to 280 C for 30 min. Two-pot synthesis of CIS/CGS (method C): A portion of the final product of CIS from method A (1.5 ml) was loaded in a 25 ml three-neck round bottom flask. The toluene was blown off with argon and light heating (50 C). Once the toluene was blown off, Ga(acac) 3 (0.5 mmol), Cu(acac) 2 (0.5 mmol), TOPO (1.75 mmol), and ODE (5 ml) were all added to the flask and the synthesis for CGS described above was followed. One-pot synthesis of CIS/CGS: A 25 ml three-neck round bottom flask was loaded with In(Ac) 3 (0.5 mmol), CuI ( mmol), and 1-DDT (5 ml) and degassed at room temperature. A 20 ml scintillation vial was loaded with 19

26 Ga(acac) 3 (1 mmol) and 1-DDT (20 ml). The copper and indium solution was heated to 100 C under vacuum and then backfilled with argon and heated to 280 C. During heating to 280 C, the solution changed color from orange to black at 210 C. At this point the gallium solution (10 ml) was added dropwise. The solution was heated at 280 C for 30 min. Two-pot synthesis of CIS/AGS: A portion of the final product of CIS from method A (1.5 ml) was loaded in a 25 ml three-neck round bottom flask. The toluene was blown off with argon and light heating (50 C). Once the toluene was blown off, Ga(acac) 3 (1 mmol), Ag(ac) (1 mmol), and 1-DDT (5 ml) were all added to the flask. The solution was degassed at room temperature for 30 min (<100 mtorr) and then backfilled with argon and heated to 150 C. A mixture of 1- DDT (0.25 ml) and tert-dodecanethiol (t-ddt) (1.75 ml) heated to 150 C was rapidly injected. The solution was heated at 150 C for 2 hours. CGSe synthesis: Oleylamine (20 ml) was pumped under vacuum (<100 mtorr) at 80 C for 16 hours turning slightly golden in color. A 25 ml three-neck round bottom flask was loaded with selenium powder (0.4 mmol) and the degassed oleylamine (8 ml). The solution was heated at 120 C under vacuum for 30 min turning golden brown and then backfilled with argon and heated at 250 C for 30 min which turned the solution orange. A separate three-neck flask was loaded with Cu(acac) 2 (0.2 mmol), Ga(acac) 3 (0.2 mmol), and degassed oleylamine (5 ml). This solution was heated at 80 C under vacuum for 1 hour giving a dark blue-green color. This copper and gallium solution (5 ml) was rapidly injected 20

27 into the selenium solution causing the solution to immediately turn black. The solution was heated at 250 C for 1 hour. Cation exchange: Stock solutions of 0.15 mmol CuCl 2 2H 2 O, 0.15 mmol AgNO 3, 0.15 mmol CdCl 2, 0.15 mmol PbCl 2, 0.15 mmol Pb(NO 3 ) 2, 0.15 mmol CuI, 1.5 mmol AgNO 3 and 0.25 mmol TOP in methanol were each prepared. A stock solution of CIS nanoparticles as prepared by method A was diluted to have an absorbance at 350 nm of A series of 8 ml scintillation vials were loaded with the CIS stock solution ( ml) and toluene (1 ml). The cadmium stock solution or one of the lead stock solutions (1-4 ml) and an equal part of the TOP solution were added to the vial and the solution was then centrifuged with methanol. The small amount of product was collected in toluene (2 ml). A series of 8 ml scintillation vials were loaded with toluene (2 ml), methanol (0-0.3 ml), and one of the copper or silver stock solutions (2-6 ml). This solution was added to the cadmium exchanged product from before. The solution was centrifuged with methanol and the product was collected in toluene. AGS synthesis (method A): A 25 ml three-neck round bottom flask was loaded with Ga(acac) 3 (0.4 mmol), Ag(ac) (0.4 mmol), and 1-DDT (5 ml). The mixture was heated at 150 C under argon for 2 hours at which time the solution was black. At this point a solution of sulfur powder (3 mmol) in 1-DDT (5 ml) heated to 170 C was rapidly injected. The solution was then heated at 150 C for another 2 hours. AGS synthesis (method B): A 25 ml three-neck round bottom flask was loaded with Ga(acac) 3 (0.4 mmol), Ag(ac) (0.4 mmol), and 1-DDT (5 ml). The mixture 21

28 was stirred under vacuum (<200 mtorr) at room temperature for 15 min and then heated to 100 C under vacuum. The solution was then backfilled with argon and heated at 150 C for 30 min. At this point a solution of sulfur powder (3 mmol) in 1-DDT (5 ml) heated to 170 C was rapidly injected. The solution was then heated at 230 C for 15 min. AGS synthesis (method C): A 25 ml three-neck round bottom flask was loaded with Ga(acac) 3 ( mmol), AgNO 3 ( mmol), and 1-DDT (5 ml). The mixture was stirred under vacuum (<200 mtorr) at room temperature for 30 min and then heated to 100 C under vacuum. The solution was then backfilled with argon and heated to above 150 C. At the point where the solution reached a light yellow or orange color, a separate solution of sulfur powder (3 mmol) in 1-DDT (5 ml) heated to 170 C was rapidly injected. The solution was then heated at either 150 C or 250 C for hours. AGS synthesis (method D): AGS nanoparticles were synthesized using method B, but replacing Ag(ac) with AgNO 3. AGS synthesis (method E): A 25 ml three-neck round bottom flask was loaded with Ga(acac) 3 (0.4 mmol), AgNO 3 (0.4 mmol), and 1-DDT (5 ml). The mixture was heated to 150 C under argon at which time the solution was light yellow. At this point a solution of sulfur powder (3 mmol) in 1-DDT (5 ml) heated to 170 C was rapidly injected turning the solution black. The solution was then heated at 150 C for another 2 hours and the solution had turned to a dark red color. 22

29 AGS synthesis (method F): AGS nanoparticles were synthesized following method C but with the addition of ligands ( mmol). Ligands used were octylamine, or a mixture or hexanoic acid and stearic acid of varying ratios. One-pot synthesis of CIS/AGS: A 25 ml three-neck round bottom flask was loaded with In(Ac) 3 (0.5 mmol), CuI ( mmol), and 1-DDT (5 ml) and degassed at room temperature. A 20 ml scintillation vial was loaded with Ga(acac) 3 (0.4 mmol), Ag(NO) 3 (0.4 mmol), and 1-DDT (5 ml) and another was loaded with sulfur powder (3 mmol) and 1-DDT (5 ml); both of these solution were heated to 130 C. The copper and indium solution was heated to 100 C under vacuum and then backfilled with argon and held at 100 C for 30 min. The solution was then heated to 230 C. Around 210 C, the solution had turned red. At this point, 2.5 ml of the other two solutions were added dropwise over two minutes. The solution was then heated at 230 C for 30 min. Two-pot synthesis of CIS/AGS (Method A): A portion of the final product of CIS from method A (1.5 ml) was loaded in a 25 ml three-neck round bottom flask. The toluene was blown off with argon and light heating (50 C). The product was redissolved in 1-DDT (5 ml) and heated to 125 C giving a red solution. Another three-neck flask was loaded with Ga(acac) 3 (0.4 mmol), AgNO 3 (0.4 mmol), and 1-DDT (5 ml). The mixture was heated to 150 C under argon at which time the solution was light yellow. At this point, the CIS solution was rapidly injected, immediately turning the solution black. The solution was heated at 150 C for 2 hours. 23

30 Two-pot synthesis of CIS/AGS (Method B): A portion of the final product of CIS from method A (1.5 ml) was loaded in a 25 ml three-neck round bottom flask. The toluene was blown off with argon and light heating (50 C). The product was redissolved in 1-DDT (2 ml) and heated to 125 C giving a red solution. Another three-neck flask was loaded with Ga(acac) 3 (0.4 mmol), AgNO 3 (0.4 mmol), and 1-DDT (5 ml). The mixture was heated to 150 C under argon at which time the solution was light yellow. At this point, the CIS solution was rapidly injected, immediately turning the solution black. Once the solution reached 150 C again, a solution of sulfur powder (3 mmol) in 1-DDT (5 ml) heated to 170 C was rapidly injected. The solution was heated at 150 C for 2 hours. AIS synthesis: A 25 ml three-neck round bottom flask was loaded with In(ac) 3 (0.4 mmol), silver(i) acetate (0.4 mmol), and 1-DDT (5 ml). The mixture was heated at 150 C under argon for 2 hours at which time the solution was yellow. At this point a solution of sulfur powder (3 mmol) in 1-DDT (5 ml) heated to 170 C was rapidly injected turning the solution an opaque red-orange. The solution was then heated at 150 C for another 2 hours. One-pot AGS/ZnS synthesis (Method A): Method C for synthesizing AGS particles was followed using 0.2 mmol AgNO 3 and 0.4 mmol Ga(acac) 3, but once 250 C was reached, ml of a solution of zinc stearate (3 mmol) in 1-DDT (1 ml) and ODE (4 ml) heated to 170 C was added dropwise at a rate that kept the temperature constant. The solution was then heated at 250 C for 2 hours. 24

31 Two-pot AGS/ZnS synthesis (Method B): A portion of the final product of AGS from method C (1.5 ml) was centrifuged with methanol and the product was collected in ODE (5 ml). The solution was stirred under vacuum (<100 m Torr) at room temperature for 30 min and then backfilled with argon and heated to 80 C or 220 C. A solution of zinc stearate (0.4 mmol) in 1-DDT (1 ml) and ODE (4 ml) heated to 90 C was added dropwise (~1 ml/min). The solution was then heated at 220 C for 1 hour. Ligand Exchange: A series of 24 scintillation vials (8 ml) were each half filled with toluene. Each vial had a portion of AGS nanoparticles as prepared by method C added to them: six vials had a low concentration of AGS with a 1:1 Ag/Ga ratio added; six had a low concentration of AGS with a 1:2 Ag/Ga ratio added; six had a high concentration of AGS with a 1:1 Ag/Ga ratio added; six had a high concentration of AGS with a 1:2 Ag/Ga ratio added. Each of these vials had three drops of one of the following ligands added: hexanoic acid, pyridine, 1- DDT, trioctylphosphine (TOP), octylamine, or oleylamine. The samples were analyzed by PL after being left to sit for a month. Photoluminescence Measurements: Approximately 1 ml of the final nanocrystal solution was diluted in approximately 5 ml toluene. This diluted nanocrystal solution was analyzed by a Jasco FP-6200 Fluorescence Spectrophotometer. The excitation wavelength was set to 340 nm and a scan rate of 125 nm/min was used. 25

32 UV/Vis Measurements: The same diluted solution used for the photoluminescence measurements was used. UV/Vis spectra were obtained using a StellerNet EPP2000. Elemental Analysis: A series of silver standards were made using silver nitrate in 2% HNO 3 with silver concentrations ranging from 0.12 ppm to 1.00 ppm. A series of gallium standards were made using gallium acac in 2% HNO 3 with gallium concentrations ranging from 50 ppm to 200 ppm. These standards were analyzed using a Perkin-Elmer AAnalyst 400. Nanocrystals were dried by gentle heating while having argon blown over the solvent. The solid nanocrystals were then dissolved in 1 ml water with 5 ml concentrated HNO 3. These nanocrystal solutions were diluted to fall within the concentration range for each metal being studied and then analyzed on the same FAAS. Powder X-Ray Diffraction: Nanocrystal solutions were centrifuged with methanol to crash out the particles and then blown over with argon to remove any remaining solvent. The XRD pattern was obtained using an Oxford Diffraction Nova X-ray diffractometer with Cu Kα source and an Onyx CCD detector using the powder diffraction setting. A 100 second integration time was used. TEM Imaging: TEM imaging of the nanocrystals was provided by the Portland State CEMN. 26

33 Results and Discussion CIS synthesis: Previous research has shown that CIS/ZnS nanoparticles could be produced with a high photoluminescence quantum yield. 3 We were able to replicate these results synthesizing particles with a photoluminescence quantum yield (71%) similar to that seen in the literature. It was also found that we could readily synthesize nanocrystals with varying band gaps by altering their growth time. CIS nanocrystals could be grown with photoluminescence either in a range around 420 nm or around 620 nm. As seen in Figure 9, as the growing time increases the 420 nm peak diminishes and the 620 nm peak gets bigger. By cooling the reaction immediately once the desired highest temperature was reached ( C), it was possible to tune the ratio of these peaks. During heating of the reaction mixture in this range, the color of the solution would change from yellow to orange to red and then to a very dark blood red showing an increase in the size of the particles. Figure 9. Photoluminescence of CIS at various stages of growth. Figure 10. UV/Vis absorption spectra of CIS at various stages of growth. 27

34 CIS particles on their own could be tuned by stopping the growth when desired or a ZnS shell could be added by adding a zinc stearate solution dropwise when the desired color (and thus desired size) was reached. The majority of the red shifting of the CIS particles was found to occur while the temperature was increasing, although the crystals did continue to grow while the temperature was held constant. The longer growth also resulted in a greater intensity of photoluminescence, as shown in Figure 9. During this growth, the emergence of a slight shoulder in the absorption was found to appear around 470 nm as in Figure 10 as the band gap of the particles decreased with the increasing particle size. Figure 11. Photoluminescence of CIS before and after shelling with ZnS. Figure 12. Photoluminescence of CIS/ZnS at various stages of growth. Time is after ZnS injection. It was seen that adding the ZnS shell to the CIS nanocrystals always resulted in particles that emitted near 570 nm, with no change based on the size of the CIS cores. Since the resulting shelled materials always had photoluminescence near 570 nm, we sometimes found a blue shift of the emitted light even though the shelled particle would be bigger than the unshelled CIS (Figure 11). This blue shifting would become more pronounced as the reaction 28

35 time increases as shown in Figure 12. Again, the photoluminescence became more intense the longer the growth was allowed to run (Figure 12). The absorption did not change significantly upon the addition of the ZnS shell or during the growth of this shell as shown in Figure 10. CGS shell on CIS: Copper gallium sulfide (CGS) was picked as a potential shelling material for CIS due to the very good lattice match (a=5.523 Å for CIS and a=5.35 Å for CGS) between the two materials. 27 CGS particles were first synthesized on their own. The CGS nanoparticles were found to emit in the nm range and have a peak near 420 nm and a very distinct shoulder around 500 nm as seen in Figure 13. Figure 13. Photoluminescence of CGS nanocrystals All attempts at making a CIS/CGS nanoparticle system via one pot syntheses resulted in nanocrystals with no photoluminescence in the visible range. This is likely due to the poor alignment of the band gaps of the two materials, as CGS has a higher valence band and a higher conduction band than 29

36 CIS, 28 leading to a separation of the electron-hole pair. While the photoluminescence was very poor, the x-ray diffraction pattern for CGS particles grown using a one pot synthesis fit an average of the CIS and CGS patterns as shown in Figure 14. This would indicate that an alloy of CIS and CGS could have been produced instead of a core/shell system; more research on this is needed. Figure 14. XRD of one-pot synthesis of CIS/CGS. Experimentally found pattern for CIS/CGS is in red, pattern for bulk CIS is in blue and to the left of the CIS/CGS peaks, and bulk CGS is in green and to the right of the CIS/CGS peaks. Shelling CIS seeds with a CGS shell via a two pot synthesis gave mixed results. When the shelling was done on small CIS seeds (CIS particles that were grown for a short amount of time, which gave a photoluminescence peak at 420 nm and a shoulder at 470 nm), it was found that adding a CGS shell resulted in blue shifting of the main peak and red shifting the shoulder. This effectively 30

37 extended the distance between these two peaks, see Figure 15. However, when the shelling was done on CIS seeds which were allowed to grow to where they were emitting at 640 nm, the resulting photoluminescence was quenched as shown in Figure 16. This is likely due to the fact that the larger particles had a more rigid lattice, so there would be a greater amount of strain at the interface. XRD analysis of the two-pot shelling did not give a pattern indicating an alloy, as it did for the one-pot shelling, but a pattern indicating a third structure. Figure 15. Photoluminescence of CGS shelling of small CIS seeds, before and after shelling. Figure 16. Photoluminescence of CGS shelling of large CIS seeds, before and after shelling. CGSe Synthesis: Because copper gallium selenide (CGSe) has a band gap that would form a nested heterostructure as the core material when shelled with either AGS or CGS, we looked at synthesizing CGSe nanocrystals as an alternative to CIS. The resulting particles gave no emission in the visible region as shown in Figure 17. Coupling this with the fact that the synthesis called for pumping the solvent under 31

38 a high vacuum for 16 hours, we decided to not look into this material any farther at this time. Figure 17. Photoluminescence of CGSe nanocrystals. Cation exchange: Cation exchange is a process where a metal cation in a crystal structure is replaced by a different metal cation. This often involves metals of different charges such as Ag + replacing Cd 2+, for this to happen two silver cations take the place of one cadmium ion. 29 This is possible for nanocrystals because of the small size. 26 Attempts were made at using cation exchange on CIS nanoparticles to potentially incorporate silver into the particles. First, a solution of nanocrystals was added to a solution containing either cadmium or lead ions which would give structures of Cd 0.5 InS 2 and Pb 0.5 InS 2. It was found that this process quenched the emission of the nanocrystals (Figure 18), indicating that cadmium and lead may have moved into the crystal structure. We expect the cadmium and lead ions to exchange with the softer copper ions instead of the harder indium ions, due to the fact that two In 3+ ions would need to be replaced with three Cd 2+ ions. 32

39 Figure 18. Photoluminescence of nanoparticles before and after cation exchange to cadmium. Prior to cation, particles are CIS. Unfortunately, subsequent cation exchange from either of the cadmium or lead containing particles to the corresponding silver or original copper nanocrystals did not show any increase in photoluminescence. There has been very little research done on cation exchange with ternary nanocrystals, so the effectiveness of these processes is questionable and difficult to measure. Since another cation is present (indium) it is possible that the process which works well for binary nanocrystals for both reaction directions will not work for ternary nanocrystals, especially at the usually easy conditions seen in cation exchange of binary nanocrystals

40 AIS synthesis: Figure 19. Photoluminescence of AIS nanocrystals. Due to the fact that silver indium sulfide (AIS) is a more well-studied material than AGS (which we are very interested in for shelling of CIS) and because it should behave similarly, we looked at synthesizing AIS nanocrystals as a jumping off point. The resulting particles were found to have photoluminescence with a sharp narrow peak at 390 nm and a wide peak at 490 nm (Figure 19). The XRD pattern matched very well to that of bulk AIS. Due to the odd emission spectra of AIS and a reported band gap around 2 ev, 27,28 which is smaller than what we are looking for, this material was not explored further at this time. AGS synthesis: Based on the fact that silver gallium sulfide has a valence band lower than that of CIS and a conduction band higher than that of CIS (band gap of 1.53 ev for CIS and 2.7 ev for AGS) 27,28, AGS was chosen as a potential shelling 34

41 material for CIS nanoparticles. Additionally, the lattice mismatch between the two materials is fairly low at 4% (lattice constant a=5.52 Å for CIS and 5.75 Å for AGS). 27,28 Attempts at first shelling the CIS particles in AGS following a one-pot synthesis very similar to the shelling of CIS in ZnS resulted in particles with no photoluminescence. Since there has been little previous research done on AGS nanoparticles, before using it as a shelling material, we wanted to better understand the synthetic conditions for growing AGS nanoparticles on their own in order to better understand how to grow it as a shell. Over the course of this study many different syntheses were run to varying degrees of success. Figure 20. Photoluminescence of AGS nanocrystals grown for no time and for 2 hours before sulfur injection In the first syntheses, the metal precursors in 1-DDT were heated to 150 C during which the solution turned clear red-orange and then light yellow. The solution was then held at 150 C for 2 hours, during which time the solution turned dark brown, before the sulfur in 1-DDT was injected into the solution. This 35

42 resulted in photoluminescence with many different peaks. By letting the metal precursors in 1-DDT heat at 150 C for an extended period of time it is likely that some AGS particles were being formed by using the sulfur from the solvent. This likely lead to the formation of many differently sized particles upon the injection of the sulfur as well as possibly created other compositions such as AgS and GaS. When the sulfur was instead injected immediately when the reaction mixture turned light yellow around 150 C only one photoluminescence peak emerges as shown in Figure 20. For this reason it is important to not let the metal precursors heat for too long prior to the nucleation injection. Figure 21. Photoluminescence of AGS nanocrystals grown with and without vacuum at the beginning of the synthesis Due to suspicion that residual moisture and oxygen may play a role in the reaction kinetics, degassing of the reaction mixture prior to heating and during the first part of the heating process was then added to the synthesis. The reaction mixture was degassed at room temperature for min. and then held 36

43 under vacuum during heating until 100 C. At this point the flask was backfilled under a flow of argon. It was found that doing so resulted in a more narrow emission peak that was slightly blue-shifted for similar reaction conditions as can be seen in Figure 21. As hypothesized, this is likely due to a smaller presence of water and other oxidizers. Figure 22. Photoluminescence of AGS nanocrystals grown at different temperatures. The next syntheses included heating the reaction mixture to higher temperatures. In the earlier syntheses, after injecting the sulfur, the solution was heated at 150 C. It was found that by increasing the temperature after the injection to 250 C a fairly large blue-shift in the emission of the particles was seen as shown in Figure 22. The reason for this blue-shift is not understood and requires further study. 37

44 Changing of stoichiometry: Previous research has shown that the band gap of ternary nanocrystals can be slightly tuned based on the stoichiometry of the metals in the material. 4 In typical AGS the ratio of silver to gallium is 1:1. AGS nanoparticles with a higher relative concentration of gallium were first synthesized by reducing the amount of silver precursor in the synthesis. It was seen that having more gallium present than silver resulted in a final product that was more yellow, even approaching green, than the typical orange of normal AGS. Additionally, these particles resulted in the broad, low photoluminescence peaks shown in Figure 23. In a similar fashion, AGS particles with a greater relative amount of silver were also synthesized by increasing the amount of silver precursor present in the synthesis. These particles resulted in a final solution that was more dark red or black, also with very broad and low photoluminescence. Figure 23. Photoluminescence of AGS nanocrystals with different Ag:Ga ratios. 38

45 These particles were analyzed via flame atomic absorption spectroscopy in order to determine if the ratio of starting material actually resulted in nanoparticles with a corresponding ratio. It was found that the ratio of the metal precursors used in the synthesis fairly closely leads to the desired ratio of metals in the resulting nanoparticles as shown in Table 1. XRD analysis of nanocrystals with 0.5:1 silver to gallium shows the same pattern as bulk AGS as shown in Figure 24. This means that in the chalcopyrite structure the more prevalent metal must be taking up the lattice locations of the less common metal. This is possible on the nanoscale because the small size of the particles allows for crystals with less rigid lattices giving way to intermediate crystal structures. Precursor Ratio Ratio in particles Error 1:1 1.02: :1 1.91: :1 0.52: Table 1. Experimentally determined Ag:Ga ratios (and error) of various AGS nanocrystals with different ratios of metal precursors. Figure 24. XRD pattern for 0.5:1 silver to gallium AGS particles with the pattern for bulk AGS overlaid. 39

46 Ligand effects: In the early syntheses of AGS, the dodecanethiol used as the solvent for the synthesis was also the passivating ligand and surfactant for the nanocrystals. Because the AGS particles had trouble staying suspended, additional surfactants/ligands were added to the system. Octylamine was the first ligand used for synthesis. It was found that adding octylamine resulted in extreme bubbling of the reaction mixture during the heating process. While the photoluminescence of these particles was found to be decent (Figure 25), the extreme bubbling resulted in a loss of precursor as they were deposited on the sides of the condenser, making octylamine a poor choice for these syntheses. Figure 25. Photoluminescence of AGS particles grown in the presence of three different ligands: octylamine in blue, hexanoic acid in red, and stearic acid in green. We then looked at using carboxylic acids as the surfactants because they are cheap, commonly used for binary nanocrystals, and come in a variety of chain lengths. Both hexanoic acid and stearic acid were used individually and 40

47 together in various ratios to get a mix of short and long chains. A series of reactions were carried out with: only stearic acid; 80% stearic acid, 20% hexanoic acid; 60% stearic acid, 40 % hexanoic acid; 40% stearic acid, 60% hexanoic acid, 20% stearic acid, 80% hexanoic acid, and only hexanoic acid. In these reactions the total amount of ligand was equal to the total amount of metal precursor. The 100% hexanoic acid resulted in particles with broad and weak photoluminescence with a peak at 540 nm most likely due to poor size control. The 100% stearic acid resulted in a more narrow emission centered on 535 nm. The mixtures of hexanoic acid and stearic acid all gave somewhat similar broad emissions with peaks between 460 and 480 nm as seen in Figure 26. Out of all of these, only the 80/20 and 20/80 ratios gave particles that were able to stay suspended in solution. All of the reactions run with stearic acid resulted in what appears to be noisy emission curves, but previous research has shown that these regular and repetitive smaller peaks may be due to phonon assisted transitions. 32 Figure 26. Photoluminescence of AGS particles grown in the presence of different ligands systems with stearic acid and hexanoic acid. 41

48 a b c d e f Scale bar is at 20 nm Figure 27. TEM images with inset picture of nanocrystals in solution. Nanocrystals were prepared with: (a) 100% stearic acid; (b) 80% stearic acid, 20% hexanoic acid; (c) 60% stearic acid, 40% hexanoic acid; (d) 40% stearic acid, 60% hexanoic acid; (e) 20% stearic acid, 80% heaxanoic acid; (f) 100% hexanoic acid. Increased scattering is observed in samples (a), (c), (d), and (f), due to increased aggregation. Each of the products from these reactions was analyzed by TEM, shown in Figure 27. It was seen that the 100% stearic acid, 80% stearic acid, and 100% hexanoic acid all gave very poor size and shape control. The 60/40, 40/60, and 20/80 ratios all gave significantly improved shape and size control. Based on the size (about 5 nm in diameter) and shape control seen in the TEM images, as well as the ability to stay suspended (which indicates less aggregation), it was determined that the 20% stearic acid and 80% hexanoic acid was the optimal ligand system to be used for further modifications. The total amount of ligand used was then doubled to see if the amount of ligand has a significant effect as having a greater amount of the ligands present could result in a decrease in aggregation. This resulted in particles that gave a 42

49 decent emission centered at 525 nm with the same phonon assisted transitions seen. Oleic acid, a commonly used surfactant, replaced the stearic acid giving similar results. While no TEM images of these particles have been taken yet, there is no appreciable improvement or detriment in the photoluminescence of these nanocrystals. Growth time: Figure 28. PL and absorption spectra for AGS particles with 20% stearic acid, 80% hexanoic acid taken at various times of growth. Absorption did not change with time of growth. The time of growth during the synthesis of AGS nanoparticles was studied to find if there was an optimal growth time by taking aliquots of the reaction mixture at various times during the synthesis. A ligand system of 20% stearic acid and 80% hexanoic acid was used. The reaction was allowed to run for 2 hours at 250 C. Aliquots of the reaction were taken at 5 min., 15 min., 30 min., 1hour, and a last aliquot at 2 hours right before cooling the solution. 43

50 A general red shifting of the emission for the particles from 490 nm to 520 nm was seen during the first hour of growth (Figure 28). This is consistent with what is typically seen, as the particles would be getting larger as time goes on, leading to a smaller band gap and thus redder emission. This would allow for a slight tunability of the emission wavelength of AGS nanoparticles based on growth time. Figure 29. XRD patterns for AGS particles with 20% stearic acid, 80% hexanoic acid for growth of 5 min, 1 hour, and 2 hours. From the first hour to the second hour, there was a blue shift from 520 nm to 500 nm, shown in Figure 28. The reason for this shift is not understood at this time. The XRD patterns for the particles grown for 5 min. and 1 hour show significant Scherrer broadening which is to be expected for nanoparticles, while 44

51 the XRD pattern for 2 hours of growth shows very sharp and distinct peaks that match bulk AGS (Figure 29). This is also consistent with TEM images for these particles shown in Figure 30. At 5 min. of growth, the particles are about 4 nm in diameter or smaller and very spherical. At 2 hours, many of the particles are at least 10 nm in size with most of the particles having non-spherical shapes and a large amount of aggregation between particles. At these sizes and shapes the particles begin to exhibit some bulk properties as seen in the XRD pattern. Scale bar at 20 nm. Figure 30. TEM images of AGS particles with 20% stearic acid, 80% hexanoic acid grown for: (a) 5 min; (b) 2 hours. 45