CH676 Physical Chemistry: Principles and Applications. CH676 Physical Chemistry: Principles and Applications

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1 CH676 Physical Chemistry: Principles and Applications

2 Crystal Structure and Chemistry Synthesis of Tetrahexahedral Platinum Nanocrystals with High-Index Facets and High Electro-Oxidation Activity Na Tian et al. Science , 732, 2007

3 Optically and Chemically Encoded Nanoparticle Materials for Bio-applications El-Sayed Professor of Chemistry at Georgia Tech Chad A. Mirkin Professor of Chemistry at Northwestern University

4 Zero-Dimensional Nanostructures: Nanoparticles Semiconductor nanocrystals Much of the development of nanostructures is related to semiconductors. Band structure is the heart of semiconductor materials Many intriguing properties (optical and electrical) are direct results of the band structure change at the nanometer scale. Displays LEDs Life Sciences Thermoelectrics Photonics & Telecommunications Security Inks Photovoltaics

5 Exciton Bohr Radius and Band Gap Quantum dots are also made out of semiconductor material. The concepts of energy levels, bandgap, conduction band and valence band still apply. However, there is a major difference. Excitons have an average physical separation between the electron and hole, referred to as the Exciton Bohr Radius. This physical distance is different for each material. In bulk, the dimensions of the semiconductor crystal are much larger than the Exciton Bohr Radius, allowing the exciton to extend to its natural limit. However, if the size of a semiconductor crystal becomes small enough that it approaches the size of the material's Exciton Bohr Radius, then the electron energy levels can no longer be treated as continuous - they must be treated as discrete, meaning that there is a small and finite separation between energy levels. This situation of discrete energy levels is called quantum confinement, and under these conditions, the semiconductor material ceases to resemble bulk, and instead can be called a quantum dot. This has large repercussions on the absorptive and emissive behavior of the semiconductor material. Si

6 Bohr radius and band gap change Because quantum dots' electron energy levels are discrete, the addition or subtraction of just a few atoms to the quantum dot has the effect of altering the boundaries of the bandgap. Changing the geometry of the surface of the quantum dot also changes the bandgap energy, owing again to the small size of the dot, and the effects of quantum confinement. The bandgap in a quantum dot will always be energetically larger; therefore, we refer to the radiation from quantum dots to be "blue shifted" reflecting the fact that electrons must fall a greater distance in terms of energy and thus produce radiation of a shorter, and therefore "bluer" wavelength.

7 Bohr radius and band gap change As with bulk semiconductor material, electrons tend to make transitions near the edges of the bandgap. However, with quantum dots, the size of the bandgap is controlled simply by adjusting the size of the dot. Because the emission frequency of a dot is dependent on the bandgap, it is therefore possible to control the output wavelength of a dot with extreme precision. In effect, it is possible to tune the bandgap of a dot, and therefore specify its "color" output depending on the needs of the applications. Prof. Brus Prof. Alivisatos Prof. Bawendi

8 Bohr radius and band gap change As with bulk semiconductor material, electrons tend to make transitions near the edges of the bandgap. However, with quantum dots, the size of the bandgap is controlled simply by adjusting the size of the dot. Because the emission frequency of a dot is dependent on the bandgap, it is therefore possible to control the output wavelength of a dot with extreme precision. In effect, it is possible to tune the bandgap of a dot, and therefore specify its "color" output depending on the needs of the applications.

9 Quantum Dots

10 Quantum Dots Quantum Dots Narrow FWHM and Tunable Emission Pattern The peak emission wavelength is bell-shaped (Gaussian) and occurs at a longer wavelength than the lowest absorption energy exciton peak. (Stoke's Shift). The bandwidth of the emission spectra (Full Width at Half Maximum; FWHM) stems from the temperature, natural spectral line width of the quantum dots, and the size distribution of the population of quantum dots within a solution or matrix material. Spectral emission broadening due to size distribution is known as inhomogeneous broadening and is the largest contributor to the FWHM. Narrower size distributions yield smaller FWHM. For CdSe, a 5% size distribution corresponds to ~ 30nm FWHM. Quantum Dots - Molecular Coupling Colloidally prepared quantum dots are free floating and can be attached to a variety of molecules via metal coordinating functional groups. These groups include thiol, amine, nitrile, phosphine, phosphine oxide, phosphonic acid, carboxylic acid or others ligands. By bonding appropriate molecules to the surface, the quantum dots can be dispersed or dissolved in nearly any solvent or incorporated into a variety of inorganic and organic films. Quantum Dots High Quantum Yield and Brightness The percentage of absorbed photons that result in an emitted photon is called Quantum Yield (QY). QY is controlled by the existence of nonradiative transition of electrons and holes between energy levels- transitions that produce no electromagnetic radiation. Nonradiative recombination largely occurs at the dot's surface and is therefore greatly influenced by the surface chemistry. Adding Shells to Quantum Dots It is established that capping a core quantum dot with a shell (several atomic layers of an inorganic wide band semiconductor) reduces nonradiative recombination and results in brighter emission, provided the shell is of a different semiconductor material with a wider bandgap than the Core semiconductor material. The higher QY of Core-Shell quantum dots comes about due to changes in the surface chemistry of the core quantum dot. The surface of quantum dots that lack a shell has both free (unbonded) electrons, in addition to crystal defects.

11 Quantum Dots LEDs Quantum dots are valued for displays, because they emit light in very specific gaussian distributions. This can result in a display that more accurately renders the colors that the human eye can perceive. Quantum dots also require very little power since they are not color filtered. Life Sciences In modern biological analysis, various kinds of organic dyes are used. However, with each passing year, more flexibility is being required of these dyes, and the traditional dyes are often unable to meet the expectations. One of the most immediately obvious being brightness (owing to the high extinction co-efficient combined with a comparable quantum yield tofluorescentdyesaswellastheirstability. It has been estimated that quantum dots are 20 times brighter and 100 times more stable than traditional fluorescent reporters. Thermoelectrics Photonics & Telecommunications Security Inks Photovoltaics Quantum dots may be able to increase the efficiency and reduce the cost of today's typical silicon photovoltaic cells. For example, quantum dots of lead selenide can produce as many as seven excitons from one high energy photon of sunlight (7.8 times the bandgap energy). Quantum dot photovoltaics would theoretically be cheaper to manufacture. The generation of more than one exciton by a single photon is called multiple exciton generation (MEG) or carrier multiplication.

12 Introduction: Bottom-up Thermodynamic Approach (v.s. Kinetic Approach): 1.Generation of supersaturation 2.Nucleation 3.Subsequent growth Solvent; Precursor; Capping Agent; Reducing Agent

13 Nuecleation - Free Energy

14 Nuecleation - Free Energy This equation indicates that high initial concentration or supersaturation, low viscosity and low critical energy barrier favor the formation of a large number of nuclei. For a given concentration of solute, a larger number of nuclei mean smaller sized nuclei.

15 Growth Generation of growth species Diffusion of the growth species from bulk to the growth surface Adsorption of the growth species onto the growth surface Surface growth through irreversible incorporation of growth species onto the solid surface Diffusion / Growth

16 Important factors in metallic Nanoparticle Synthesis Reduction conditions Stronger reduction faster reactions smaller nanoparticles and narrower distributions Weaker reduction slower reactions Stabilizer Wider distribution (if further or secondary nucleation forms) Narrow distribution (if no further nucleation forms diffusion limited growth) Stabilization Diffusion barrier

17 Choice of reduction agent Choice of reduction agents and reaction conditions determine nucleation/growth thermodynamics and kinetics; product size, shape/morphologies are sensitive to T, reduction agent, ph, etc. Turkevich, J., et al, Discuss. Faraday Soc. 11, 55, 1951

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21 Synthesis of semiconductor nanoparticles Thermo decomposition Non-oxide semiconductor (some oxides as well) Temporally discrete nucleation (by rapid increase in the reagent concentrations) Oswald ripening (to form uniform size distributions) Size selective precipitation applied Hot Injection & Heat-Up Sol-gel processing Oxide semiconductor (Versatile) Forced hydrolysis Controlled release of constituent ions Alkoxide & Chloride Si(OR) 4 + H 2 O HO-Si(OR) 3 + R-OH Condensation (OR) 3 Si-OH + HO Si-(OR) 3 [(OR) 3 Si O Si(OR) 3 ] + H-O-H Polymerization

22 Synthesis of semiconductor nanoparticles Hot Injection Precursors For Cd, Me 2 Cd For S: (TMS) 2 S For Se: TOP Se Procedures 1. Hot TOPO solution (320 ºC) 2. Cd and chalogenide precursors in TOPO Capping Agents Polar head bind to the surface (non-polar solvents) Control the growth process Stabilize colloidal suspension Passivate semiconductor surface Capping agent can be exchanged

23 Hot Injection - Synthesis of semiconductor nanoparticles Precursors For Cd, Me 2 Cd For S: (TMS) 2 S For Se: TOP Se Procedures 1. Hot TOPO solution (320 ºC) 2. Cd and chalogenide precursors in TOPO

24 Heat Up Synthesis of semiconductor nanoparticles Precursors Metal-oleate Complex Procedures 1. Matel-oleate, oleic acid, and 1-octadecene 2. Heat mix solution (320 ºC) Capping Agents Polar head bind to the surface (non-polar solvents) Control the growth process Stabilize colloidal suspension Passivate semiconductor surface 70 C ~4 hours 36 g (40 mmol) of the iron-oleate complex 5.7 g of oleic acid (20 mmol, Aldrich, 90%) 200 g of 1-octadecene (Aldrich, 90%) 10.8 g of FeCl 3 6H 2 O (40 mmol, Aldrich, 98%) 36.5 g of sodium oleate (120 mmol, TCI, 95%) 80 ml ethanol + 60 ml D.I. water ml hexane Waxy solid form M OOCR M + RCOO M OOCR MO + RC O M + MO MOM 320 C (3.3 C /min) ~30 min

25 Heat Up Synthesis of semiconductor nanoparticles Precursors Metal-oleate Complex Procedures 1. Matel-oleate, oleic acid, and 1-octadecene 2. Heat mix solution (320 ºC)

26 Heat Up Synthesis of semiconductor nanoparticles Precursors Metal-oleate Complex Procedures 1. Matel-oleate, oleic acid, and 1-octadecene 2. Heat mix solution (320 ºC)

27 Hot Injection & Heat Up Synthesis of semiconductor nanoparticles