In the Classroom Semiconductor Nanocrystals: A Powerful Visual Aid for Introducing the Particle in a Box Tadd Kippeny, Laura A. Swafford, and Sandra J. Rosenthal* Department of Chemistry, Vanderbilt University, Nashville, TN 37235; *[email protected] Physical chemistry is an intimidating subject for many organometallic synthesis, ligand field theory, and group students, especially those who have an aversion to mathemat- theory are discussed. ics. For these students it is important to provide visual dem- The “particle in a box” is often the first problem students onstrations of difficult concepts, lest they become frustrated encounter in their studies of quantum mechanics. The series and lost in a sea of equations. Several curriculum reform of CdSe nanocrystals shown in Figure 1 is a powerful visual groups have also advocated the importance of placing course demonstration of the particle in a box. These nanocrystals were content in the context of real scientific, societal, or techno- chemically synthesized to have a desired size. They have the logical problems (1). Many of these efforts have focused on same wurtzite crystal structure as bulk CdSe, but consist of introductory chemistry courses. While physical chemistry only a few hundred to a few thousand atoms (Figures 2 and students are more advanced, students at all levels appreciate 3). As the nanocrystal size increases, the energy of the first the connection between course content and the world they excited state decreases, qualitatively following particle-in-a- encounter outside of the classroom. Quantum mechanics is box behavior (Figure 4). unlike thermodynamics (engines, refrigeration) and kinetics This size dependence and the emergence of discrete elec- (catalysis, enzymes); it is difficult for students to see the “real tronic states from the continuum of levels in the valence and world” significance of quantum mechanics or to visualize conduction bands of the bulk semiconductor result from quantum mechanical phenomena. With the advent of nano- quantum confinement; hence semiconducting nanocrystals are technology, the construction of devices on a nanometer scale, also referred to as “quantum dots”. In bulk CdSe, the electron– it is not only possible but important to connect quantum hole pair created upon absorption of a photon maintains a mechanical course content with technological problems. characteristic distance known as the bulk Bohr exciton radius. Semiconductor nanocrystals, nanoscale building blocks for CdSe has a Bohr exciton radius of ~56 Å (3), so for nano- new materials and technologies (2), provide a means for con- crystals smaller than 112 Å in diameter the electron and hole necting lecture content in quantum mechanics to real-world cannot achieve their desired distance and become particles technological problems. Nanocrystals can also be incorpo- trapped in a box. The discrete electronic transitions that rated into freshman chemistry lectures, where atomic spec- emerge from the continuum can be labeled as atomic transi- troscopy is introduced, and into inorganic chemistry, where tions (4). Because the energy of these transitions can be tuned by size, nanocrystals have also earned the nickname of “arti- ficial atoms”. This article provides an overview of methods by which nanocrystals can be prepared and describes in detail the preparation of CdSe nanocrystals by the pyrolysis of organo- metallic precursors. It then discusses the energy shift of CdSe nanocrystals with size and surveys some applications of nanocrystals. Downloaded via UNIV OF TEXAS AT DALLAS on March 3, 2020 at 17:36:36 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. Figure 1. CdSe nanocrystals dissolved in toluene. The size of the nanocrystals progressively increases from 19 Å on the left to 60 Å on the right. A color version of this figure is shown on p 1027. Figure 2. Transmission electron micrograph of single CdSe nano- Figure 3. Model of a wurtzite CdSe nanocrystal. This model was crystals viewed looking (left) down the C3v axis and (right) parallel constructed from transmission electron micrographs such as those to the C3v axis. in Figure 2. 1094 Journal of Chemical Education • Vol. 79 No. 9 September 2002 • JChemEd.chem.wisc.edu In the Classroom Nanocrystal Fabrication ing. The sol gel can be processed as a porous ceramic xerogel of high surface area, which can serve as a catalytic support. Semiconductor nanocrystals can be fabricated by a variety Here the CdS particles could catalyze CO2 fixation (8). of methods, including vapor deposition, ion implantation, Alternatively the sol–gel solution could be spun into an sol–gel methods, micelle methods, and organometallic optical fiber. For instance, a fiber containing PbS particles synthesis. Vapor deposition techniques for the growth of could be used for nonlinear optics. quantum dots, including atomic-vapor deposition and The micelle method of fabricating nanocrystals is per- chemical-vapor deposition, rely on a lattice mismatch between haps the most approachable for the undergraduate labora- the deposited substance and a crystalline substrate. The lattice tory. A method for the synthesis of CdS nanocrystals has been mismatch induces strain, which is relieved by cluster formation. developed by Vossmeyer et al. (9). A cadmium salt is dissolved An example is Ge deposition on Si, which leads to the forma- in a thioglycerol–water solution whose pH is adjusted to 11.2 tion of Ge pyramids. Repeating the deposition of Si and Ge before the addition of dihydrogen sulfide with vigorous stirring. into a multilayer film enhances the regularity in position and The particle size is controlled by the amount of dihydrogen size of the quantum dots in a “self-organized” way (5, 6 ). The sulfide added as well as the temperature and duration of further resulting three-dimensional structure of dots could potentially heating. To improve the size distribution, the samples are size- communicate electronically but without wires. selectively precipitated. In the ion implantation technique for forming nano- The synthesis of nanocrystals by the pyrolysis of organo- crystals, the host matrix, for example crystalline SiO2, first metallic reagents, first developed by Murray et al. (10), is the undergoes a blanket implant of one of the elements, for ex- best chemical procedure for producing large quantities of ample As. A high-energy ion beam is then used to implant II-VI and III-V quantum dots in a processable form, with a the second element, for example Ga. Implantation creates a narrow size distribution and with no vacancies in the nano- supersaturated solution of an impurity in the near surface crystal. The method used to prepare the CdSe nanocrystals region, and thermal annealing leads to precipitation and a displayed in Figure 1 is outlined in Figure 5 (10, 11).1 First stock broad size distribution of nanocrystals. Recent developments solutions of the Cd and Se precursors are prepared and stored using a finely focused ion beam overcome this size distribution in a glove box. Ten milliliters of Cd(CH3)2 is vacuum limitation at the expense of a substantial increase in time for transferred to a 25-mL round-bottom flask on a Schlenk line fabrication. The spot size for the finely focused ion beam and then immediately transferred to the glove box while 2 can be as small as 70 Å, and thermally induced atomic maintaining vacuum, as Cd(CH3)2 is pyrophoric. A 0.96-g clustering can result in crystallites of considerably smaller size. portion of Se powder is complexed with 100 mL of Sol–gel methods for fabricating nanocrystals also result tributylphosphine (TBP) and this solution is stored in the µ in quantum dots that are embedded in a matrix. A variety of box until use. Just before the reaction, 166 L of Cd(CH3)2 nanocomposites have been synthesized by this technique (7). is mixed with 10 mL of the Se–TBP solution. This solution One example is a cadmium sulfide–silica gel nanocomposite. degrades with time and cannot be stored. The organocadmium Cadmium nitrate is dissolved in conventional sol–gel for- and selenium reagents are injected via a large-bore syringe mulations using tetraethylorthosilicate. This is followed by into 12 g of trioctylphosphine oxide (TOPO) at 360 °C, and reaction with dihydrogen sulfide. The resulting particles vary in nanocrystal nucleation occurs (Warning: 360 °C is above the size from 16 to 100 Å, depending upon the weight percentage flash point of TOPO, and this reaction must be done under of CdS in the silica gel and the conditions of thermal anneal- inert atmosphere). The temperature of the reaction mixture Reaction solution (1.9:1 Cd:Se): µ 0 166 L Cd(CH3)2, 0.096 g Se and 10 mL TBP are injected into 12 g TOPO Inject thermocouple organometallic and TBP = tributylphosphine Cd and Se controller column P[CH3(CH2)3]3 fractional reagents here TOPO = trioctylphosphine oxide O=P[CH3(CH2)7]3 360 °C crystal nucleation, 300 °C crystal growth TOPO Isolation by methanol precipitation Figure 5. Procedure for synthesizing high-quality, monodisperse Figure 4. Absorption and emission spectra of CdSe nanocrystals. CdSe nanocrystals by the pyrolysis of organometallic precursors. As the size of the nanocrystal increases both the absorption and This open-atmosphere setup is designed to be used in a glove box. emission shift to higher wavelengths. The reaction can be performed outside a glove box under argon. JChemEd.chem.wisc.edu • Vol. 79 No. 9 September 2002 • Journal of Chemical Education 1095 In the Classroom is immediately reduced to 300 °C, allowing the nanocrystals In the case of an electron–hole pair, the Hamiltonian is to grow to the desired size, at which point the reaction is 2 2 stopped by removing the heating mantle from the reaction Ᏼ ᎑ h ∇ 2 h ∇ 2 = e – h + V Se,Sh (4) vessel. The vials in Figure 1 show a range of growth times from 8π2m 8π2m 0 minutes (19 Å, furthest left) to 3 hours (60 Å, furthest e h right).