University of Ljubljana
Faculty of Mathematics and Physics
Department of Physics
Seminar
GOLD
Author:
Nuša Pukšič
Mentor:
doc. dr. Denis Arčon
Abstract:
An overview is given of the physical properties of gold. The focus is mainly on bulk,
but nanoparticles and nanowires are also included to some degree. The main properties reviewed are structures of gold, band structure and optical properties. A sketch of
transport and chemical properties is also presented. In the end, an overview of uses of
gold in different sectors is added.
Ljubljana, November 2007
Contents
1 Introduction
2
2 Relativistic Effects
2
3 Structures of Gold
3.1 Crystal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3.2 Clusters and Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . .
3.3 Nanowires . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
3
3
4
6
4 Band Structure
4.1 Derivation of Band Structure . . . . . .
4.1.1 Tight-Binding Method . . . . . .
4.1.2 Augmented Plane Wave Method
4.2 Density of States . . . . . . . . . . . . .
4.3 Fermi Surface . . . . . . . . . . . . . . .
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7
7
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8
9
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5 Optical Properties
10
5.1 Salient Optical Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
5.2 Surface Plasmon Resonance . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6 Transport Properties
13
6.1 Electrical Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
6.1.1 Quantized Conductance of Gold Nanowires . . . . . . . . . . . . . 14
6.2 Thermal Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
7 Chemical Properties
15
8 Application
16
8.1 Electronics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.2 Medicine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
8.3 Chemical Industry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9 Conclusion
18
1
1
Introduction
Gold has been known and highly valued since prehistoric times. It may have been the
first metal used by humans. [1]
Although nowadays the price of some platinum group metals can be much higher,
gold has long been considered the most desirable of precious metals and its value has
been used as the standard for many currencies in history. [2]
The main goal of the alchemists was to produce gold from other substances — reputedly by the interaction with a mythical substance called the philosopher’s stone.
Although they never succeeded in this attempt, they promoted an interest in what can
be done with substances, and this laid a foundation for today’s chemistry. [2]
The symbol representing gold is Au (from Latin word aurum, meaning glowing dawn).
It has an atomic number of 79 and an atomic mass of 196.96655 g/mol. An atom of gold
is composed of 79 electrons, 79 protons and 118 neutrons.
Many isotopes have been synthethised (atomic masses ranging from 170 to 205), but
apart from the regular 197 Au none are occurring naturally, since most have a half-life
shorter than a day, with few exceptions (the longest-living being 195 Au with a half-life
of 186 days). [1]
The physics of gold is now well established and it is the purpose of this seminar to
give you an overview of its most important physical properties (structural details, band
structure and optical properties in particular) and applications.
2
Relativistic Effects
Since relativistic effects play a major role in the case of gold and will be continually
addressed throughout this seminar, they represent a good place to start. [3, 4]
Gold is in the last row of the periodic table containing stable elements and only four
stable elements (mercury, thallium, lead and bismuth) have larger atomic masses.
Let’s first consider the Bohr “solar system” model of the atom. Because of the size
of the nucleus, the electrons of the gold atom are subjected to an intense electrostatic
attraction. We can derive an approximation that the electrons in the 1s orbital would
have to orbit with a velocity of 1.6 108 m/s to have sufficient kinetic energy not to fall
into the nucleus, which is more than half the speed of light.
According to Einstein’s equation
mo
1 − v 2 /c2
mr = p
(1)
the electron’s mass increases by about 20% above the rest mass, mo .
Quantum mechanics replaces the Bohr orbits with a probability distribution of the
electron’s position, with the Bohr orbit radius interpreted as the distance from the nucleus where the peak probability occurs. The relativistic increase in mass of the electron
causes a relativistic contraction of its orbit, since the radius of an orbit is inversely
proportional to mass for constant angular momentum.
From the Bohr model, you might expect this effect to be significant only for the
innermost electrons, but all s shells must be orthogonal to each other in the radial
direction, which is why the contraction of the first shell influences all others. The higher
angular momentum p, d, f , and g orbitals have their probability peaks farther from
2
the nucleus, and hence are less affected by relativistic contraction (and since they are
orthogonal to the s orbitals by virtue of their angular components, they are not affected
by s shell contraction).
Figure 1: The relativistic contraction of 6s shell in the elements Cs (Z=55) to Rn (Z=86). The
pronounced local maximum of the contraction at gold, makes Au a unique element, even from
this point of view. [4]
A half-filled s orbital contracts more than a filled one, which is why Cs is smaller
than Ba. Minima in the lanthanoid series are at La (5d1 .6s2 ), Ce (4f 1 .5d1 .6s2 ) and Gd
(6s2 .4f 7 .5d1 ), all due to a new shell appearing. In the d block, from Lu to Ir, 6s is filled
before 5d, which is not true for Pt (5d9 .6s1 ) and Au (5d10 .6s1 ) and the minimum is again
due to a half-filled s shell.
The main relativistic effect in gold is the proximity (energy-wise) of 5d and 6s orbitals. This results in pronounced sd hybridization and in a significant shortening of the
interatomic bond distances, which increases the overlap of the 5d orbitals of neighbouring
atoms, making 5d electrons more delocalised.
3
3.1
Structures of Gold
Crystal
The monovalent metals are generally divided into two classes: the alkali metals (Li, Na,
K, Rb, Cs) and the noble metals (Cu, Ag, Au). The former crystallise in body-centered
cubic and the later in face-centered cubic crystal structures (Fig. 2).
Gold has a unit cell side of 0.407 nm and a distance to the nearest neighbour of 0.288
nm. From the mass of a single atom (A/NA ) and unit cell properties (4 atoms per cell
of size (0.407 nm)3 ), we can derive the density of gold:
196.97
4
g/cm3 = 19.3 g/cm3 .
23
6.02 10 (0.407 10−7 )3
(2)
This makes gold rather dense compared to other common materials (see Table 1 for
comparison), mainly due to its close-packed structure and large atomic mass.
3
Figure 2: Crystal structure of gold. [5]
Table 1: Densities of common metals. [6]
metal
aluminum
zinc
iron
copper
silver
lead
mercury
gold
density [g/cm3 ]
2.70
7.13
7.87
8.96
10.49
11.36
13.55
19.32
Gold’s very high ductility1 and malleability2 are also connected to its crystal structure. 1 g of gold can be drawn into 5 km of wire or beaten cold into a translucent sheet
of 1 m2 , which cannot be achieved with any other material. [2]
Face-centered cubic, body-centered cubic and hexagonal close-packed crystalline forms
allow deformation of the crystals to occur under severe stress by block-like movement
of adjacent sections of the crystals, which causes a disarrangement of the atoms along
the slip plane. This increases the resistance to further slip and eventually causes movement in that plane to stop. On further increase of stress, slip will occur along another
plane until all the potential planes have slipped to their limit. Generally slip occurs in
the most densely populated atomic planes of a crystal and in the direction of the most
closely packed line within that plane. In the face-centered cubic lattice slip occurs along
four such families of planes, in the body-centered cubic in three and in the hexagonal
close-packed lattice, slip occurs along one family of planes only. The metals which crystallise with a face-centered cubic lattice will be recognised as being the most ductile.
[7]
Contributing to gold’s extreme ductility and malleability is also its inertness (tarnish
would constrain the displacement of atoms at the surface).
3.2
Clusters and Nanoparticles
At the nanometric scale the surface to volume ratio dramatically increases and the fact
that the relative number of under-coordinated gold atoms on the surface also increases,
1
2
Degree of extension which takes place before failure of a material in tension.
The extent to which a material can undergo deformation in compression before failure.
4
can lead to deformed structures not found in bulk. In this section size-dependence of
structure will be presented.
The nanoparticles of average size of about 30 nm (Fig. 3) still possess a well defined
atomic arrangement resembling the face-centered cubic structure occurring in bulk gold,
but with a considerable local structural disorder.
The atomic ordering in particles with average size around 3 nm already departs from
the face-centered cubic one in a substantial way. The particles are spherical in shape,
with atoms at the surface less densely packed as those at the core of the particle.
Figure 3: TEM images of gold nanoparticles with average size of 30 nm (left) and 3 nm (right).
[8]
Figure 4: Geometry of Au−
N clusters. Several isomers are shown for each size, the ground state
is labelled by “A” in each case. [11]
For Au55 it was concluded, that it has a small number of definite structures, but
none of any particular symmetry. [9]
Proposed ground state structure of Au20 is tetrahedral, which is a fragment of the
face-centered cubic lattice of bulk gold with a small structural relaxation and is thus a
5
unique molecule with atomic packing similar to that of bulk gold.
Even smaller clusters can be produced and observed. A number of calculated optimized low-energy structures for Au−
N (N = 4 − 14) are shown in Fig. 4. The occurrence
of energetically favourable planar structures is unique to gold clusters and is related to
strong relativistic effects in their bonding. The sd hybridization favours planar bonding,
which allows larger delocalization of 5d electrons. Resulting smaller kinetic energies of
5d electrons override the potential energy contributions from higher coordination (3D).
For N = 12 a coexistence of both planar and 3D structures was observed. At N = 13
and more, the 3D structures become preferable to planar ones. This is not obvious in
Fig. 4, because the method used in these calculations slightly overestimates the stability
of the planar structures. [11]
3.3
Nanowires
As the scale of microelectronics continues to shrink, the spotlight remains on quantum
wires.
Figure 5: STM images of a contact while withdrawing the tip (a-f) and of a linear strand of
gold atoms (g), coloured for better contrast. [20]
The dark lines in Fig. 5/a-f represent rows of gold atoms suspended between two
contacts. A mechanically sharpened gold tip was positioned close to a gold island, that
has been deposited on a thin copper wire. The tip was dipped into the island and
slowly withdrawn. The bridge between the tip and the island thins during withdrawal
and appears to be reorganizing itself into stable structures, with rows of gold atoms
disappearing one-by-one, so that the neck maintains a regular and uniform thickness.
[20]
Freestanding linear strands of gold atoms were observed (Fig. 5/g), with gold atoms
spaced at as much as 0.35 − 0.40 nm intervals, much larger than the nearest neighbour
spacing in crystalline gold. [20]
6
4
Band Structure
Ground state electron configuration of gold can be expressed as
[Xe].4f 14 .5d10 .6s1
and the shell structure as
2.8.18.32.18.1.
4.1
Derivation of Band Structure
Derivation of band structure is a combination of theoretical calculations and measurements. In the case of gold, achieving agreement with measurements3 requires a significant
effort. But since the band structure influences many other properties of a material, this
section is dedicated to shedding some light to the complex issue of the band structure of
gold.
4.1.1
Tight-Binding Method
In the simple tight-binding approximation [12], we regard a solid as a collection of weakly
interacting neutral atoms. The core of the method is in converting the overlapping atomic
orbitals of adjacent atoms into actual electronic levels of a metal.
We are solving the Schrödinger equation
Hψ(r) = (Hat + ∆U (r))ψ = ε(k)ψ(r)
(3)
where Hat is the atomic Hamiltonian, corresponding to atomic energy levels En and
atomic wave functions ψn (r).
We attempt to write the solutions as a linear combination of atomic wave functions
ψn (r) as:
ψ(r) =
X
R
eikR
X
bn ψn (r − R).
(4)
n
In the first approximation, we will only take into account the half-filled 6s orbitals
in gold. The eigenvalue equation is reduced to
ε(k) = Es − t
X
eikR ,
(5)
R
where Es is the energy of the 6s atomic level (and equals the Fermi energy εF ) and
Z
t=−
drφ∗ (r)∆U (r)φ(r − R), φ(r) =
X
bn ψn (r).
(6)
n
The nearest neighbours to the gold atom in the system origin are at
a
a
a
(±1, ±1, 0), (±1, 0, ±1), (0, ±1, ±1),
2
2
2
(7)
hence the 6s band energy:
ε(k) = Es − 4t(cos
ax
ay
ax
az
ay
az
cos
+ cos
cos
+ cos
cos ).
2
2
2
2
2
2
7
(8)
Figure 6: 6s band structure as derived from the tight-binding method, represented using standard symmetry points of the 1st Brillouin zone. Energy scale is in units of t, zero is set to
Es .
Using the s-band-width value from more advanced calculations (relativistic), we can
estimate the value of t for gold:
t ∼ 2.0 eV.
(9)
However, this is not very accurate. To get a better-fitting result, we would have to
take into account the five 5d bands, which are in the case of gold, close to the valence
6s band. This means we would first have to derive 6 equivalent bands by hybridization
and then solve a 6×6 secular problem.
Because this method is better suited for studying insulators and transition metals
and does not reproduce the band structure of gold accurately, we will not go that far.
4.1.2
Augmented Plane Wave Method
Realistic calculations are actually carried out using the so-called (relativistic) augmented
plane wave method [12]: the crystal potential is approximated by a muffin-tin form,
which is spherically symmetric within spheres (the Slater spheres) surrounding each of
the atoms of the crystal lattice and by a constant (which can be taken to be zero) in the
regions between the Slater spheres (the interstitial regions).
The augmented plane waves φ are defined in the interstitial region as plane waves
and in the atomic region as solutions to the Schrödinger equation in the non-relativistic
case and as solutions to the Dirac equation in the relativistic case. φ are continuous
at the boundary between atomic and interstitial regions (the derivatives, in general, are
not).
In the relativistic case the degeneracy of the d bands is split due to spin and the
difference of contractions of the 5d and 6s bands brings them energetically closer together.
3
Agreement can be judged with comparison of Fermi surface parameters, specific heat constant, x-ray
spectra or optical properties.
8
Figure 7: Band structure derived with the non-relativistic [13] (left) and the relativistic [14]
(right) augmented plane wave method.
4.2
Density of States
The density of states gn (ε) and the band structure are connected with an explicit relation
Z
gn (ε) =
Sn (ε)
dS
1
,
3
4π |∇εn (k)|
(10)
where n denotes a given band. [12]
Figure 8: Density of states histogram derived from the band structure calculated with the
relativistic augmented plane wave method. [14]
The density of states function is then positive in an energy interval [ε1 , ε2 ] and zero
elsewhere, the width of the interval is called the bandwidth. When bands overlap, the
9
density of states intervals do as well. In gold all the five filled 5d bands and the half-filled
6s band overlap and the density of states for all 11 electrons is shown in Fig. 8.
For the half-filled bands εF lies in-between ε1 and ε2 . Materials with half-filled
valence bands or, in other words, a non-zero density of states at εF are conductors.
4.3
Fermi Surface
For each partially filled band there is a surface in k-space separating the occupied levels
from the unoccupied. The set of all such surfaces is called a Fermi surface and is
determined by εn (k) = εF , where εF is the Fermi energy. [12]
Figure 9: The Fermi surface of gold. [15]
The monovalent metals have the simplest of all Fermi surfaces, since all the bands
are completely filled or empty except for a single half-filled conduction band. Of the two
groups of monovalent metals, the noble metals have the more complex topology of Fermi
surfaces, because of the influence of the filled d bands. [12]
5
5.1
Optical Properties
Salient Optical Properties
Optical properties can be derived from the Maxwell equation for conductive medium
∇2 E = µσ
∂E
∂2E
+ µ² 2 .
∂t
∂t
(11)
The solutions are electromagnetic waves that will experience attenuation proportional
to µσ as they propagate. [12]
The salient optical properties of a material are specified by its complex index of
refraction N , which differs from the common index of refraction n, by incorporating an
imaginary part related to the extinction coefficient k:
N = n + ik,
(12)
or with the complex dielectric function ²:
N2 = ²
(n + ik)2 = ²0 + i²00
10
(13)
Figure 10: Reflectance (top), refractive index and extinction coefficient (middle) and real and
imaginary part of dielectric function (bottom) of gold as functions of photon energy. [16]
11
For any medium optically equivalent to vacuum ²0 = n2 − k 2 equals 1 and if there is
no absorption, ²00 = 2nk equals 0. The product nk is called the absorption coefficient.
Metallic or conductive materials have non-zero k and n which can vary wildly with the
wavelength. [12]
When a plane wave is normally incident from vacuum on a medium with dielectric
function ² is the fraction of power reflected given by
¯
¯
¯1 − N ¯
(1 − n)2 + k 2
¯
¯
.
r=¯
¯=
¯1 + N ¯
(1 + n)2 + k 2
(14)
Measured optical properties of gold are shown in Fig. 10.
The colour of materials is due to absorption of light. The sharp cut-off in the reflectance of gold at photon energy 2.4 eV is what gives gold its colour. The photon energy
of 2.4 eV is equivalent to wavelengths of about 520 nm and the cut-off means that gold
absorbs light in the blues and violets of the visible spectra and reflects the rest, which
makes it appear golden.
The cut-off is attributed to the direct electron transmission from 5th filled 5d band
to the half-filled 6s band at the Fermi level near the high symmetry point L in the 1st
Brillouin zone. [14, 16]
5.2
Surface Plasmon Resonance
Plasmons are collective oscillations of the free electron gas density. They can also couple
with a photon to create plasma polaritons. Surface plasma polaritons (or surface plasmons) are surface electromagnetic waves that propagate parallel along a metal-dielectric
(or metal-vacuum) interface. [1, 17]
If we describe each material as a homogeneous continuum with a given dielectric
constant, for surface plasmons to exist, the real part of the dielectric constant of the
metal must be negative and its magnitude must be greater than that of the dielectric.
This condition is met in the IR-visible wavelength region for air-metal and water-metal
interfaces.
Since nanoparticles have a much larger portion of the substance at the surface, there
is more potential for surface plasmon resonance. The material’s surface shape controls
the types of surface plasmons that can couple to it and propagate across it, which is why
a change in shape or size causes a change in colour.
Figure 11: As gold nanoparticles increase in size, their colour changes from red to blue. From
left to right, size increases from 20 nm to 250 nm. [17]
Small gold nanoparticles absorb light in the blue-green portion of the spectrum. As
particle size increases, the wavelength of surface plasmon resonance related absorption
12
shifts to longer wavelengths and ultimately, when nearing the bulk limit, into the IR.
With mixing of particles of different sizes, their colour can be systematically varied from
pink through violet to blue.
It has been known for centuries that adding gold to glass and heating the mixture
makes it reddish and it is due to small golden clusters forming in the glass. Red stained
windows for old cathedrals were made that way. [2]
6
Transport Properties
The ability of gold to efficiently transfer heat and electricity is bettered only by copper
and silver.
6.1
Electrical Conductivity
The Drude theory of metals is a kinetic theory of a gas of conduction electrons, moving
against a background of heavy immobile ions. The density of the electron gas is given
by
n=
NA Zρm
,
A
(15)
where NA , Z, ρm and A are Avogadro’s number, number of conduction electrons per
atom, mass density and the atomic mass of the element. Also widely used measure of
electronic density is rs , defined as the radius of a sphere whose volume is equal to the
volume per conduction electron:
1
4πrs2
V
= =
, rs =
N
n
3
Ã
3
4πn
!1/3
.
(16)
Free electron densities for gold (at room temperature and atmospheric pressure) are then
as follows:
n = 5.90 1022 /cm3 , rs = 0.159 nm,
rs
= 3.01,
ao
where ao is the Bohr radius (0, 0529 nm). [12]
Table 2: Specific resistivity of common metals at 300 K. [18]
metal
mercury
iron
zinc
aluminum
gold
copper
silver
ρ [10−6 Ωm]
0.958
0.098
0.059
0.028
0.022
0.018
0.016
13
(17)
Figure 12: Temperature dependence of electrical conductivity of gold. [19]
The finite conductivity of metals is entirely due to deviations in the lattice from
perfect periodicity. The most important deviation is that associated with the thermal
vibrations of the ions around their equilibrium positions, present even in a perfect sample
free from such crystal imperfections as impurities, defects and boundaries. Assuming,
that the scattering is dominated by processes in which an electron absorbs or emits a
single phonon, and knowing that the number of phonons is proportional to temperature,
we can conclude, that since the number of scatterers increases linearly with temperature,
so will the resistivity. At low temperatures, the rate at which current degrades is no
longer simply proportional to the scattering rate, since the scattering becomes more
concentrated in the forward direction (low energy phonons can only change electron’s
velocity or direction for a small amount) and less effective in degrading the current. [12]
6.1.1
Quantized Conductance of Gold Nanowires
Quantum point contacts4 typically display a conductance quantized in steps of
Go =
2e2
≈ 13 kΩ−1
h
(18)
and were also observed for gold [20, 21].
When a linear strand connects two reservoirs biased by dE = eU to produce a current
dI, then dI/dE equals 2e/h. This is because the electrons that move in a linear chain
of gold atoms, with a momentum in the range (p, p + dp) can only carry a current of
dI = 2e
∂E dp
,
∂p h
(19)
where ∂E/∂p is the velocity of the electron at the Fermi level and dp/h is the number
of electrons and because the number of allowable conduction channels, determined by
the electron motion, normal to the strand axis, is one for a single strand. The double
4
Generally metallic structures in which a neck of atoms, few atomic diameters wide, bridges two
electrical contacts.
14
Figure 13: Conductance as a function of the displacement of the two gold electrodes with
respect to each other. A long plateau with conductance near 1Go is observed and (after a jump
to tunnelling) the electrode needs to return by a little more than the length of the plateau to
come back into contact. [21]
strand, then, has twice the unit conductance, if the interaction between the strands is
not strong.
A conductance curve obtained while stretching a gold nanocontact is presented in
Fig. 13. The steps are results of the structural rearrangements and the remarkable
length of the last plateau at the value of about one conductance quantum coincides with
already mentioned stretched linear strands of gold atoms.
6.2
Thermal Conductivity
Thermal and electrical conductivity are related, since the same electrons conduct both,
heat and electricity. The relation is approximated by the Lorentz number [12]:
2
k
π 2 kB
=
= 2.44 10−8 WΩ/K2 .
Tσ
3 e2
7
(20)
Chemical Properties
Gold’s resistance to tarnishing and other chemical reactions is also due to relativistic
effects. With a single 6s electron, one might expect gold to be highly reactive. Gold’s
6s orbital, however, is relativisticaly contracted toward the nucleus, and its electron
has a high probability to be among the electrons of the filled inner shells. Only the
most reactive substances can tug gold’s 6s1 electron out from where it’s hiding among
the others, hence not only the colour of gold, but its immunity from tarnishing and
corrosion. [3, 6, 23]
15
Figure 14: Temperature dependence of thermal conductivity of gold. [22]
Table 3: Thermal conductivity [18] and Lorentz numbers [12] of common metals at 300 K.
metal
lead
iron
zinc
aluminum
gold
copper
silver
k
[Wcm−1 K−1 ]
0.35
0.80
1.16
2.37
3.17
4.01
4.29
k/T σ
[10−8 WΩ/K2 ]
2.53
2.88
2.30
2.19
2.36
2.29
2.38
Gold is stable in air under normal conditions and does not react with water or aqueous
bases. However, it does dissolve in aqueous cyanide solutions in the presence of air.
Gold reacts with chlorine, Cl2 , or bromine, Br2 , to form the trihalides: gold(III)
chloride, AuCl3 , or gold(III) bromide, AuBr3 , respectively, or with iodine, I2 , to form
the monohalide gold(I) iodine, AuI.
Solutions of chlorine, Cl2 , and trimethylammonium chloride, [NHMe3 ]Cl, in acetonitrile, MeCN, dissolve gold. Aqua regia, a mixture of hydrochloric acid, HCl, and
concentrated nitric acid, HNO3 , in a 3:1 ratio, also dissolves gold. The name aqua regia
was coined by alchemists because of its ability to dissolve gold - the “king of metals”.
8
Application
With its beauty, permanence and rarity gold is still the material of choice for the fabrication of religious artefacts, decorative articles and jewellery. However, the unique
chemical and physical properties offered by this precious metal are increasingly being
sought for use in a growing number of industrial and medical applications. [1, 2, 24, 25]
16
8.1
Electronics
One area that has seen significant growth in the use of gold is electronics. Examine
the battery connections of any mobile phone and these are almost certainly gold-plated
contacts. Similarly, within computers there are usually gold-plated edge connectors.
Gold bonding wires are used extensively within semiconductor packages, gold thick film
inks are applied in the fabrication of hybrid circuits. Also, some DVDs and recordable
CD-Rs have thin gold metallisations.
Many new uses of gold in electronic devices are visible in people’s everyday lives,
such as the use of gold plating and bonding wires in the rapidly increasing smart-card
market. Other uses are not so visible, such as the increasing use of gold in automotive
electronics. Here, gold is used in a growing range of applications, including ignition
control electronics, anti-lock brakes and electronic fuel injection. Where safety and
reliability are important, gold can be relied upon to perform as required, even after
many years of inactivity. So, for example, crash sensors for airbags are a classic example
of the type of important application for which gold contacts are quite indispensable.
Figure 15: Gold-plated circuit board contacts (left) and gold-plated smart card contacts arranged on a strip for use in smart card manufacture (right). [2]
Longer term innovations for future information technology hardware will, however,
be in the form of highly integrated electronic devices on the nanometre scale. Gold is an
indispensable element for nanoscale electronic components because of its resistance to
oxidation and its mechanical robustness. Among other metals, only silver and platinum
offer similar properties, but silver can be too reactive and platinum is significantly more
expensive than gold. In the area of nanoelectronics, work on using gold nanowires as
potential connectors in nanodevices is underway in research centres around the world.
8.2
Medicine
The excellent biocompatibility, malleability and resistance to corrosion of gold provides
benefits for use inside the body and was used in dental applications by The Etruscans
as far back as the seventh century BC.
The ideal dental alloy is one that is easy to manipulate but is strong, stiff, durable
and resistant to tarnish and corrosion. A typical crown and bridge alloy may contain
60-75% gold. Additional elements are added to improve castability or to control thermal
expansion.
Currently, there are a number of direct applications of gold in medical devices such as
wires for pacemakers. Gold possesses a high degree of resistance to bacterial colonisation
and because of this it is the material of choice for implants that are at risk of infection.
17
Figure 16: X-ray picture of a person with a pacemaker. [24]
In the past few decades the properties of gold compounds have been of interest as
potential cancer treatments. Currently, the most broadly used treatments for many types
of cancers are platinum-based drugs, but these can have serious side effects.
Because of the high opacity of gold to x-rays, gold nanoparticles are being considered
for use as intravenous contrast enhancers in medical imaging.
Figure 17: X-ray of kidneys in a live mouse 60 minutes after intravenous injection of (a) gold
nanoparticles or (b) iodine contrast medium. Arrow: 100 nm ureter. [25]
8.3
Chemical Industry
Gold has been overlooked as a possible catalyst until very recently, as its use requires
careful preparation centered on achieving a very small particle size (less than 5nm). Gold
catalysts operate effectively at temperatures much lower than that of other precious metal
catalysts and in some cases offer alternative cost effective solutions to the platinum group
metals. Fuel cell and odour reduction applications are possible, as well as the potential
to remove nitrogen oxides from diesel engine exhaust gases. [2]
9
Conclusion
Nowadays the physics of bulk gold is well known and uses well established.
A lot of research power is centered on physics of gold nanoparticles and nanowires.
Nanoparticles, in particular, seem to offer many possibilities for use in different sectors.
The common consensus on gold being a valuable metal and human fascination with
it will obviously continue.
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References
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[5] www.cartage.org.lb/en/themes/Sciences/Physics/mainpage.htm (11/2007)
[6] www.chemicalelements.com (11/2007)
[7] G. S. Lambert, Transactions and Proceedings of the Royal Society of New Zealand
77, 122, (1948-49)
[8] V. Petkov et al, Phys. Rev. B 72, 195402, (2005)
[9] H. Häkkinen et al, Phys. Rev. Lett. 93, 093401, (2004)
[10] J. Li et al, Science 299, 864, (2003)
[11] H. Häkkinen et al, J. Phys. Chem. A 107, 6168, (2003)
[12] N. W. Ashcroft, D. N. Mermin: Solid State Physics, (1976)
[13] S. Kupratakuln et al, J. Phys. C: Solid State Phys. 2, 1886, (1969)
[14] M. G. Ramchandani, J. Phys. F: Met. Phys. 1, 169-176, (1971)
[15] www.phys.ufl.edu/fermisurface (11/2007)
[16] G. B. Irani et al, Phys. Rev B 6, 2904, (1972)
[17] www.nbi.dk (11/2007)
[18] environmentalchemistry.com (11/2007)
[19] hypertextbook.com/facts/2004/JennelleBaptiste.shtml (11/2007)
[20] H. Ohnishi et al, Nature 395, 780, (1998)
[21] A. I. Yanson et al, Nature 395, 783, (1998)
[22] www.efunda.com/materials/elements (11/2007)
[23] www.webelements.com (11/2007)
[24] missinglink.ucsf.edu (11/2007)
[25] www.nsec.ohio-state.edu (11/2007)
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