CHEM 4101 New directions in Ring, Chain and Clusters
Dr Jawwad A. Darr xt 34345
Size Matters! 1m 1mm S.A =12.6 m 2 V = 4.2 m 3 S.A/V ~3 1nm S.A =12.6 x10 -6 m 2 V = 4.2 x10 -9 m 3 S.A/V ~3000 S.A =12.6 x10 -18 m 2 V = 4.2 x10 -27 m 3 S.A/V ~3000000000 SA = 4( ) r 2 V = 4/3.( )r 3
N = 2 Total atoms = 4 Surface atoms = 4 N = 3 Tetrahedron Cube N = 2 Total atoms = 8 Surface atoms = 8 Total atoms = 10 Surface atoms = 10 N = 3 Total atoms = 27 Surface atoms = 26
Relationship between the number of shells N and the total number of atoms and surface atoms for different shapes
Size and Shape effects : SELECTIVITY
fcc metal gold having nanometer size should be a cuboctahedron.
Accordingly, for increasing particle sizes the relative amount of surface atoms, especially the fraction of dense (1 1 1) planes increases as well.
In a catalytic reaction such as reduction of the , -unsaturated aldehyde (shown above), the selectivity to a specific product may be seen to increase with particle size.
Size and Shape effects : SELECTIVITY General scheme for the hydrogenation of , -unsaturated aldehydes as acrolein (R = H) or crotonaldehyde (R = CH 3 ).
This could imply that on the closed packed structure of the (1 1 1) surface the C=O group is preferentially activated, whereas sites of low coordination, mostly present on small particles as corners and edges, strongly favor the activation of the C=C group
If hydrogenation of the C=O group of the , -unsaturated aldehyde is favored by face atoms, the increased fraction of dense (1 1 1) planes of the larger gold particles should give higher formation rates of the allylic alcohol.
THIS IS WHAT OCCURS.
Semiconductors are a cornerstone of the modern electronics industry and make possible applications such as the Light Emitting Diode and personal computer.
Semiconductors derive their great importance from the fact that their electrical conductivity can be greatly altered via an external stimulus (voltage, photon flux, etc), making semiconductors critical parts of many different kinds of electrical circuits and optical applications.
Quantum dots are unique class of semiconductor because they are so small, ranging from 2-10 nanometers (10-50 atoms) in diameter. At these small sizes quantum dots have unprecedented tunability which can be exploited for applications.
Quantum wires, wells, and dots are semiconductor crystals whose size, in one, two, and three dimensions respectively, are limited to atomic sizes; a few nanometers.
A Q-dot, is a sphere with a radius of few nm that can confine an electron in zero-dimension; hence the name "dot". The other nano crystals, Q-wires and Q-wells, are equally limited in size, and can confine electrons in one and two dimensions.
What makes Q-dots interesting, both for their science as well as their application, is that their physical properties are effectively controlled by their nano-dimensional size
Quantum dots (nanocrystals) are a special class semiconductors, which are composed of groups of II-VI, III-V, or IV-VI materials. Examples include CdS, CdSe, ZnSe
BULK SEMICONDUCTOR MATERIAL: the dimensions of the semiconductor crystal are much larger than the Exciton Bohr Radius.
Q-DOTS : 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 can be called a quantum dot. This affects the absorptive and emissive behaviour of the semiconductor material
The electrons in quantum dots have a range of energies. 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 is different for each bulk material composition)
Q-Dots BULK CRYSTAL Q-DOT
Size Dependent Control of Bandgap in Quantum Dots Q-dot
Because Q-dots' e- energy levels are discrete rather than continuous, changing a few atoms to the quantum dot has the effect of altering the boundaries of the bandgap.
Changing the geometry of the shape 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 Q-dot will be energetically larger than bulk ; therefore described as "blue shifted" because the e- must fall a greater distance in terms of energy releasing a "bluer" wavelength of light.
Hence, it is possible to control the colour with great precision as we can control size during synthesis.
Quantum Dots Higher E Lower wavelength Lower E Higher wavelength
Density of States
As the material in question is viewed from the bulk and atomic levels we can see that the average spacing that exists between consecutive energy levels (kubo gap) goes from a smaller to larger value.
As the kubo gap increases there is also a decrease in the density of states located at the Fermi level.
The kubo gap can also have an effect on the properties associated with the material. It is possible to control the kubo gap which will then dictate if the system is metallic or insulating at a particular temperature.
(The kubo gap measured in meV or millielectron volts)
Size, size distribution, aspect ratio.
Shape, preferred orientation of crystals.
Surface change (zeta potential).
The nature of the core, nature of the shell, nature of final coat.
What colour is Gold? www.primidi.com www.contrarianedge.com Lycurgus Cup (British Museum)
What colour is Gold?
Faraday in the 1850s performed groundbreaking studies of nanoscale gold particles in aqueous solution.
He established the first quantitative basis for the area, noting that these colloidal metal sols “pseudo-solutions” are thermodynamically unstable, and that the individual gold nanoparticles must be stabilized kinetically against aggregation. Once the nanoparticles coagulate, the process cannot be reversed. Remarkably, Faraday also identified the very essence of the nature of colloidal, nanoscale particles of metals; specifically, for the case of gold, he concluded in 1857!)
“ the gold is reduced in exceedingly fine particles which becoming diffused, produce a beautiful fluid ... The various preparations of gold whether ruby, green, violet or blue... consist of that substance in a metallic divided state”.
During that century, colloidal phenomena played a pivotal role in the genesis of physical chemistry by establishing a connection between descriptive chemistry and theoretical physics.
Metals are good conductors because electrons form a “cloud” and experience tells us that metals are shiny.
Thus, photons of light cannot be absorbed by the atomic cores because they are blocked by the electron cloud. Consequently, photons are reflected back to the eye producing the sheen associated with metals.
If we imagine electrons in the electron cloud as a wave with a certain energy value, it is possible for light of the same wavelength to be absorbed by the electron cloud, producing “resonance”
When a metal absorbs light of a resonant wavelength it causes the electron cloud to vibrate, dissipating the energy.
This process usually occurs at the surface of a material (as metals are not usually transparent to light) and is therefore called surface plasmon resonance (plasmons are the name for the oscillations of the electron cloud).
there are certain wavelengths where photons are not reflected, but instead are absorbed and converted into surface plasmon resonance (electron cloud vibrations).
For gold these wavelengths occur in the IR portion of the spectrum (outside the visible range) it therefore appears to reflect most light and is shiny.
Nanoparticles have extremely number of atoms at their surfaces. More surface area means more potential for surface plasmon resonance.
Nanoparticles can experience SPR in the visible portion of the spectrum.
This means that a certain portion of visible wavelengths will be absorbed, while another portion will reflected. The portion reflected will lend the material a certain color. E.g. nanoparticles can absorb light in the blue-green portion of the spectrum (~400-500 nm) while red light (~700 nm) is reflected, yielding a deep red color (Figure 5, left).
As particle size increases, the wavelength of SPR related absorption shifts to longer, redder wavelengths. This means that red light is now adsorbed, and bluer light is reflected, yielding particles with a pale blue or purple colour.
As particle size continues to increase toward the bulk limit, SPR wavelengths move into the IR portion of the spectrum and most visible wavelengths are reflected. This gives the nanoparticles clear or translucent colour
Nanoparticles for testing
An example of nanoparticles in sensing certain home pregnancy tests.
Nanoparticles (< 50 nm) are bound to antibodies complementary to a hormone produced by pregnant women Latex microspheres are also bound to antibodies for the hormone.
When the stick is submerged in urine flow, if the hormone is present it will bind to the microspheres (~ 500 μm) and nanoparticles causing aggregates to form. The solution then passes through a paper filter.
If the pregnancy hormone is present, the aggregates will be trapped by the filter producing a colored product. If the pregnancy hormone is not detected, the nanoparticles will pass through the filter because of their small size.
this test does not rely explicitly on plasmon resonance to create the signal, the deep red color of the nanoparticles employed results directly from SPR.
Other tests (using SPR): nanoparticles start out as large aggregates that are blue. If the complementary DNA base is present, the nanoparticles will bind to that base instead of each other and the aggregates will dissolve producing a deep red color. Can use a spectrometer to follow this.
The method is most often used in a quantitative way to determine concentrations of an absorbing species in solution A versus c is plotted), using the Beer-Lambert law:
where A is the measured absorbance, I 0 is the intensity of the incident light at a given wavelength, I is the transmitted intensity, L the pathlength through the sample, and c the concentration of the absorbing species. For each species and wavelength, ε is a constant known as the molar absorptivity or extinction coefficient.
When white light passes through or is reflected by a colored substance, a characteristic portion of the mixed wavelengths is absorbed. The remaining light will then assume the complementary color to the wavelengths absorbed.
Thus, absorption of 420-430 nm light renders a substance yellow, and absorption of 500-520 nm light makes it red. Green is unique in that it can be created by absoption close to 400 nm as well as absorption near 800 nm.
Nanoscaled materials demonstrate the characteristic blue shift of absorption maxima compared to bulk materials.
High resolution transmission electron microscopy enables individual nanoscale cluster grains to be examined. the resolution is sufficient to enable measurement of the interlayer separation of the layers.
A beam of electrons is transmitted through an ultra thin specimen, interacting with it.
An image is formed from the electrons transmitted through the specimen, magnified and focused by an objective lens and appears on an imaging screen, a fluorescent screen or tv / film / CCD camera
You can see objects to the order of a few angstrom (10-10 m). For example, you can study small details in the cell or different materials down to near atomic levels.
via TEM is a powerful structural method. The advantages of electron diffraction over other methods, e.g., x-ray or neutron, arise from the extremely short wavelength (2 pm), strong atomic scattering, and the ability to examine tiny volumes of matter (10 nm 3 ).
ZnO CoFe 2 O 4 Co 3 O 4 Fe 2 O 3 CuO Ca 10 (PO 4 ) 6 (OH) 2 Ce x Zr 1-x O 2 CO 3 -HA
X-ray powder diffraction patterns of nanoscaled materials show very broadened peaks. These can be put into the Scherer equation which makes use of the width of the peak at half height to get an estimation of the average crystallite size. Nanoscaled materials show interesting effects from 1-20nm in size, this equates to particles with 50- 10,000 atoms.
Scherrer (1918) first observed that small crystallite size could give rise to peak broadening. He derived a well-known equation for relating the crystallite size to the peak width, which is called the Scherrer formula:
 t = 0.9 /(B.cos B )
where t is the averaged dimension of crystallites; λ is the wavelength of X-ray; and B is the integral breadth of a reflection (in radians 2θ) located at 2θ.
B = 0.5 º, λ = 0.154 nm, 2θ = 27º, θ = 13.5 º; cos (13.5) = 0.972.
360 º = 2 x 3.142 radians 0.5 degrees = 0.5 x 2 x 3.142/(360) = .00873 radians. therefore t = 16.3 nm
PXRD – Peak broadening in nano
BET Surface area
The BET stands for Brunauer, Emmett and Teller
Apply vacuum to “clean” the sample surface
A special apparatus allows gases to be added (Nitrogen) and the sample to be cooled in a fixed volume
We know PV = nRT
V constant (apparatus fixed ), thus, the changes in the system as due to change in n or due to change in gas pressure.
Fix the T at liquid nitrogen temperature 77k
When gas is let into apparatus and some of it will be absorbed onto the sample, and some will stay in gas phase. After a time, equilibrium will be established (absorb/desorb)
BET Surface area
Repeat this previous step at different pressures until a plateau is reached. i.e. no more will absorb on surface
Plot volume of gas absorbed (stp) versus “relative gas pressure” (P/Po, where Po = saturation pressure)
the area covered by a N 2 molecule at 77k is 162 square Angstroms. We can figure out how many are required to completely cover the surface of the powder - hence get surface area !!
BET Surface area Plot of the adsorption and desorption curve for a powdered material Monolayer used to calculate BET surface area (stage 2) Pore filling Nanoparticles = >300 m 2 g -1 Surface area of a sphere = 4 r 2 volume = 4 r 3 /3 relative pressure equals one, the gas molecules adsorbing onto the solid surface are equaled by the number of gas molecules escaping or desorbing
BET Surface area The adsorbed gas molecules at various stages in the adsorption cycle. Stage 1 shows the isolated sites on the sample surface as they begin to adsorb the gas molecules at low P. Stage 2 shows the formation of the monolayer of adsorbed gas molecules. Stage 3 shows some of the sample pores being filled by the adsorbed gas. Stage4 the complete coverage of the sample surfaceand the complete tilling of its pores
DYNAMIC LIGHT SCATTERING
we know that particles move under Brownian motion when we look at them under a microscope.
When a coherent source of light (such as a laser) having a known frequency is directed at the moving particles, the light is scattered, but at a different frequency. T
The shift is termed a Doppler shift or broadening, and the concept is the same for light when it interacts with small moving particles.
For the purposes of particle measurement, the shift in light frequency is related to the size of the particles causing the shift. Due to their higher average velocity, smaller particles cause a greater shift in the light frequency than larger particles.
It is this difference in the frequency of the scattered light among particles of different sizes that is used to determine the sizes of the particles present
Can detect particles of around 1nm and above and get “hydrodymanic raius” which can differ considerably from XRD and other methods