The Nobel Prize in Chemistry 2007

"for his studies of chemical processes on solid surfaces"

Gerhard Ertl Germany Fritz-Haber-Institut der Max-Planck-Gesellschaft Berlin, Germany b. 1936

Surface Science

Surface Science --- parallel with the emerging nanoscience/nanotechnology, which somehow is represented by the research of nanoparticles

• Young-Laplace equation, and Ostwald ripening for growth of particles; all dealing with chemical potentials, materials transformation kinetics, etc.

• Langmuir ’s work on surface adsorption and desorption, helping interpret ting lots of surface phenomena like catalysis in terms of kinetics.

• Atomic scale understanding and improvement of catalysis (represented by Ertl, the 2007 Novel laureate in chemistry), with the advancement in surface analysis instrumentation of the late 20th century.

• 19th century

• 20th century

• 21st century

• Concepts of particles or colloids, though nobody called them nanoparticles that time.

• Later 20th century, people started to name them nanoparticles because they could be made really small in the range of 1-100 nm, and there have been ways to protect these tiny particles with protecting groups capped onto the surface. Nanoparticles are not thermodynamically stable due to their small size and large surface atoms (unsaturated with coordination).

• With the advanced microscopy instrumentation, physical and chemical properties, as well as the chemical modification of nanoparticles have been extensively studied in the past decades.

Nanoparticles represent a unique class of materials that are not thermodynamically stable, but can still sustain long enough time for practical application, where the excess of surface energy was compensated by the surface capping of the protection groups.

What is Surface?• Surface --- the boundary layer between any two phases, which could be a solid, a liquid, or a

gas or vacuum. However, the modern surface science usually refers to the surface between of a solid in contact with a liquid or gas/vacuum, as such a surface or interface is critical for many phenomena and applications, including catalysis, friction, corrosion, membranes, etc.

• Surface differs substantially from the interior of the solid both in chemical composition and physical properties (e.g., chemical potential): unsaturated atoms for binding or adsorption, more exposure to air or liquid, etc.

• As the dimension of a solid becomes smaller (in nanometer range), the ratio of surface atoms increases dramatically. For a 3 nm CdSe nanoparticle, ~ 50% atoms are on surface, while for a large bulk material, the surface layer is generally only a tiny fraction of the total solid.

CdSe

Non-fluorescentas atoms Fluorescent when

assembled into particles

See the slide below for the tunable emission color of CdSe nano-particles.

Question: for such small particles, how to prevent them from growing into larger particles or even bulk chunk crystals through the Ostwald ripening? Small particles are not thermodynamically stable due to the small radius and thus the large surface energy.

Answer: capping the particles surface with long alkane groups so as to protect them from growing into larger ones.

Surface analysis

• Surface characterization is crucial for explaining many surface phenomena and improving the properties of many solid materials. Typical examples include1. catalysis (adsorption, surface reaction);2. electronic device (p-n junctions, interfacial CT);3. materials (crystalline structure);4. corrosion and adhesion mechanisms,5. metal/alloy surface,6. embrittlement properties,7. behavior and function of biological membranes.

Types of surface measurement

Two major instrumentation techniques for surface analysis:1. Surface spectroscopy:

a) identifying the surface chemical species and determining the concentrations;b) providing both qualitative and quantitative (less used) information about the

composition of surface layer (a few to a few tens of Å depth).Two major spectroscopy techniques:X-ray photoelectron spectroscopy (XPS); Auger electron spectroscopy (AES) --- depth profile

2. Surface microscopy: a) imaging the surface and determining the morphology, atomic crystalline

structure, and other physical properties and features at different size scale (nanometers to micrometers);

b) 3-dimensional surface structure with high resolution.

Graphite surface: imaged by scanning tunneling microscope (STM)

STM atomic resolution on Highly Oriented Pyrolytic Graphite

STM Imaging of single crystal lattice: NaCl, the table salt

The aluminum supporting is shown black.

Nanoparticles --- provide the most tunable surface, a thermodynamically unstable state because of the large surface-to-interior atom ratio and the unsaturated surface coordination sites.

Nanoparticles --- represent one major class of materials studied in Nanoscience and Nanotechnology.

Nanoparticles --- enable numerous options for application in nano-scale devices, including optics, electronics, solar cells, sensors, etc.

Nanoparticles --- usually referred to size (diameter) in the range of 1-100 nanometer.

Nanoparticles: a unique case for materials and surface science

Nanoparticles: a unique manifestation

• Semiconductor nanospheres represent one of the most attractive nanostructures, and have a wide variety of applications in optoelectronics, magnetics, and biological applications.

• The size of nanoparticles can be tuned between individual molecules and the bulk counterparts.

• The nanosphere remains the same crystalline structure as the bulk crystal, but it shows unique size dependent physical and chemical properties, so called quantum size effect. See next slide.

Quantum Size Effect of Semiconductor Materials

Quantum size effect: an extensively research topic

1.As particle decreases in size, the bandgap increases,

approaching the energy difference between LUMO and HOMO for

the individual molecules; LUMO: lowest unoccupied molecular

orbital; HOMO: highest occupied molecular orbital.

2.For fluorescent semiconductor materials, like CdS, the different

bandgap leads to different emission wavelength (color);

3.By making different sizes of the particles, people can tune the

emission color across the whole visible region (see next slide).

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