; 1222
Learning Center
Plans & pricing Sign in
Sign Out
Your Federal Quarterly Tax Payments are due April 15th Get Help Now >>



  • pg 1
									Conference Session #B2                                                                                                   Paper # 1222


                      Mark Bradel (mjb169@pitt.edu), Stephen Conover (slc114@pitt.edu)
Abstract – In the modern world, many products implement                very physically flexib le material, whereas ITO is very
transparent conductors. Usually these transparent                      brittle. Th is paper will examine ITO and why it has been so
conductors employ indium tin oxide (ITO) technology, using             successful, its major applicat ions, its flaws and why it needs
it in everything from cell phones to freezers to cars. ITO has         to be replaced, its most probable successor, graphene, the
many unique features which have led to its widespread use.             challenges facing graphene, other alternatives to ITO, the
Unfortunately indium, the main component of this                       sustainability of all of these, and the future of transparent
transparent and conductive material, is becoming harder to             conductor technology.
find, and may even be depleted in the next few years, making
it highly unsustainable [1]. Thus it is important for scientists              INDIUM TIN O XIDE, A TRANSPARENT
and engineers to develop a more sustainable successor to                                CONDUCTOR
ITO. Over the past few years, many technologies have come
into the spotlight to succeed ITO. The most likely of these            Indiu m tin o xide, co mmonly known as ITO, is a transparent
technologies is graphene. Carbon nanotubes and zinc oxide              conductor commonly found in touch screen devices, as well
are both likely as well, and all three are more sustainable            as other electronic products. ITO has gained so much
than ITO. Graphene is similar in many ways to ITO, but has             success that is has become the “market standard” for touch
many features that make it better, such as its physical                screen technology [1]. Its success can be attributed to its
flexibility and its low cost [2]. The only real limitation to          ability to function as a transparent conductor with high
graphene technology is that it is still unknown how to mass-           transparency and low resistance. Having both of these
produce it. This paper will discuss ITO technology and the             qualities is highly desirable in the field of consumer
future of transparent conductors.                                      electronics, as well as in many other industries. Being
                                                                       transparent is obviously important, as it allows the user to
Key Words – Graphene, transparent conductor, ITO, Indium               see the screen underneath the conductive material on touch
Tin Oxide, Zinc Oxide.                                                 screen devices. The ability to conduct electricity is just as
                                                                       important, however, as it is why the screen only responds to
  THE N EED TO R EPLACE INDIUM TIN O XIDE                              human touch [4]. The reason why it only responds to human
                                                                       touch is because when a part of a human body comes into
Indiu m tin o xide has dominated the field of transparent              contact with the screen, it makes a co mp lete circuit. This is
conductors for almost forty years [3]. It has been used in             how the ITO screen can detect where on the screen the user
countless products, from car windshields, to flat-screen               is pressing, as it can electronically detect where the magnetic
televisions, and even touch screen cell phones [4]. However,           impulse of the human’s finger is located [4]. The attributes
due to its scarcity and rising price, ITO is unsustainable and         of transparency and the ability to conduct electricity are both
will therefore need to be replaced in the co ming years.               necessary for an effective transparent conductor. “An
Graphene and other technologies are in the working process             effective transparent conductor should have high electrical
of becoming the next ITO, a d ifficult task to acco mplish             conductivity combined with low absorption of visible light”
considering how successful ITO technology has been.                    [4]. Ho wever, having both of these two properties at once is
However, before graphene or any of these other technologies            not very common. There are only a handful of materials that
can replace ITO as the “market standard” for transparent               have both of these features, and ITO happens to be one of
conductors [1], they must be proven to meet the quality                the best, as it performs highly in both areas [4].
standard that has been set forth by ITO. This means they                   In order to have good conductivity, an object must have
must be both very transparent as well as conduct electricity           low resistance [4]. Th is is because resistivity is the inverse
just as easily as ITO, hence the term transparent conductor.           of conductivity, meaning that resistance slows down the
Graphene is the most cost effective and otherwise most                 speed of electron travel. ITO happens to have a resistance of
viable alternative to ITO, but it is currently incapable of            only 6 Ω/Square [4]. This is very low fo r a transparent
being mass produced. There are things that can be done to              conductor. That low resistance coupled with the visible
make graphene able to be mass -produced, such as                       absorption coefficient 0.04 for ITO shows that it is a very
controlling its tendency to cause irregularities in itself,            good transparent conductor [4].
which in turn renders itself useless as a conductor. There are             A visible absorption coefficient is a measurement of how
also things that can be done which would make graphene                 strongly a chemical species absorbs light at a given
more like ITO, such as lowering its resistance. Despite these          wavelength [4]. Basically, the lower the absorption
flaws, graphene does have an advantage over ITO, as it is a            coefficient, the more transparent it is. This is a nice value fo r
Uni versity of Pittsburgh                                                                            S wanson School of Engi neering
Eleventh Annual Freshman Conference                                                                                      April 9, 2011
                                                                                                                        Mark Bradel
                                                                                                                    Stephen Conover

a transparent conductor, but there are other transparent               ZnO is a step in the right direction, considering the future of
conductors in this range, such as cadmiu m tin oxide                   indiu m supplies.
(Cd 2 SnO4 ). Cd 2 SnO4 has a resistance of 7.2 Ω/square and a
visible absorption coefficient of 0.02 [4]. This makes it seem             THE M AJOR FLAWS OF INDIUM AND ITO
as if Cd 2 SnO4 could compete with ITO for the top of the
market of touch-sensitive products. However there is a big             ITO is one of the best, if not the best, transparent conductor
issue with Cd 2 SnO4 , as Cad miu m (Cd) is to xic and a               currently in the market. Ho wever, it will not be around
carcinogen [4]. This means that while indiu m is also                  forever, as its rising costs and scarcity will eventually drive
poisonous as a metal [5], it is not a carcinogen and is                companies to find better and more cost effective and
therefore a much safer consumer-level product, since it is             sustainable alternatives. Part of the reason for its scarcity has
not poisonous when formed into ITO.                                    to do with the fact that there is no such thing as an indium
                                                                       mine [4]. This is because the “concentration in minerals is
         O THER M AJOR APPLICATIONS OF                                 too low to allow economic extraction only for the value of
           TRANSPARENT CONDUCTORS                                      the indium [4]” where indiu m is found. The way to find this
                                                                       “rare and expensive element,” is to obtain it “as a byproduct
Touch screens are not the only application of transparent              of the mining of ores for their content of other metals such
conductors. Transparent conductors are seen in energy-                 as zinc and lead” [4]. This essentially means that indium is
efficient windows, and these “low emissivity” windows are              only found in small deposits while min ing for other
the largest use for transparent conductors [4]. They are also          minerals. This shows that the reason why there are no
used in oven windows, to maintain surface temperature that             indiu m mines is simply because there is so little of it
is safe to touch [4]. They are used in front surface electrodes        available, making the price of indium not worth the cost of
for solar cells and flat panel displays [4]. Automat ically            mining it. And because the concentration of indiu m is so
dimming rear-view mirrors, window defrosters, and                      small, it is very hard to judge how much of it is in any
transparent radio antennas on windshields are all uses of              location [4]. This makes indiu m ore one of the most
transparent conductors for automobiles [4]. They are also              expensive raw materials to obtain, as the price of raw
used to keep frost off of freezer d isplay cases, and they can         materials for transparent conductors generally increase in
be made into transparent electromagnetic shields and even              this order: Cd <Zn < Ti < Sn < Ag < In [4]. Indiu m is at the
be made into invisible security circuits on windows [4].               end of this list because of its scarcity and difficulty in
These are only a few of ITO’s many applicat ions.                      mining. The price of indiu m is currently at the high price
Transparent conductors are in many different aspects of life,          and peaked at $1000/kg [1], and it is only expected to go up.
and in many cases, they are in seemingly unlikely places.                 Indiu m also has flaws other than scarcity and price. ITO
    One of the most interesting and highly applicable uses of          is one of the best transparent conductors in the market, but it
transparent conductors is in flat panel displays (FPDs).               is extremely fragile due to its brittleness. If ITO is bent,
“The many different styles of FPDs all use transparent                 torqued, or cracked in any way, its conductivity can go down
conductors as a front electrode” [4]. Essentially, this means          dramat ically, or it can even lose its conductivity altogether
that transparent conductors are used where the LCD or                  [6]. Th is is shown in Figure 1 [7].
plasma screen pixels meet their sources of electricity.
Transparent conductors are used here for their ability to
carry that electricity but still be transparent to the consumers
who are watching television. However, “etchability is a very
important consideration in forming patterns in the
transparent conductor electrode,” meaning how easy it is to
be formed to fit your specific applicat ion is an important
factor of choosing the best transparent conductor [4]. ITO
happens to be relatively easy to etch, making it a great
application for FPDs [4]. “Lo w resistance is another factor
favoring ITO in very finely patterned displays, since the ITO
layer can be made very thin, thus the etched topog raphy
remains fairly smooth” [4]. This means that ITO is highly
favorable because it is so thin that the surface of the FPD
would still remain s mooth, as they could be rough if a
thicker transparent conductor was used. ITO is unfortunately
not perfect, as zinc oxide (ZnO), is both lower in cost and
has easier etchability than ITO [4]. This means that ZnO
might replace ITO in future displays. Replacing ITO with
Uni versity of Pittsburgh                                                                          S wanson School of Engi neering
Eleventh Annual Freshman Conference                                                                                  April 9, 2011
                                                                                                                      Mark Bradel
                                                                                                                  Stephen Conover

                                                                       transparent conductors because ITO films fail under bending
                                                                       [3].” If transparent conductors are going to continue being
                                                                       used in their plethora of applications, ITO needs to be
                                                                       replaced by a transparent conductor that has all of these

                                                                       GRAPHENE, THE M OST V IAB LE ALTERNATIVE
                                                                           FOR TRANSPARENT CONDUCTORS

                                                                       Graphene is one of the major contenders to succeed ITO
                                                                       because of its many similarities to ITO. It meets the
                                                                       requirements of source 3 being “lightweight, flexib le, cheap,
                                                                       and compatible with large-scale manufacturing methods ,”
                                                                       making it a major contender [3,8]. On top of these, graphene
                                                                       is an extremely sustainable substance, since carbon is one of
                                                                       the most abundant elements on earth. Graphene is made
                                                                       entirely of graphite, or carbon atoms, which are highly
                                                                       abundant, and therefore a cheap raw material to obtain [8].
                                                                       Graphite is the material in pencils that is used to write, and
                                                                       graphene is just a single layer of atoms of this substance,
                                                                       making a sheet [8]. It is easy to see then, how graphene is
                                                                       cheap and can be found so easily, especially compared to
                                                                           Graphene, like ITO, is a transparent conductor, and can
                                                                       be produced in large sheets of hexagonal carbon atom lattice
                                                                       [8]. There are issues with this, though. Graphene must retain
                                                                       this hexagonal lattice, or pattern, at all t imes in order to
                          FIGURE 1                                     conduct electricity [8], as shown in Figure 2, image (a)
  CONDUCTIVITY TESTS COMPARING ITO AND GRAPHENE BEFORE AND             below. If this lattice breaks for one hexagon, it would ruin
                       AFTER BENDING                                   the whole sheet, as a run of this irregular pattern would form
                                                                       all the way down the sheet [8]. A run of a bro ken pattern is
    The two graphs in Figure 1 show ITO and graphene and               shown in Figure 2, images (b) and (c).
their conductivity versus voltage before and after bending
has occurred. In the test (a) in Figure 1, there is not much
difference between the two materials. ITO only carried
slightly less current than graphene during the test. But in test
(b), after being bent, ITO carried considerably less current
than graphene. This is due to ITO’s brittle nature. ITO starts
losing conductivity at approximately 1.25% of strain fro m
tension and at about 1.75% of strain fro m co mpression [2].                              a
This explains why in test b ITO performed so poorly. This is
a major flaw of ITO that “the mean critical strain for the
compression tests was 1.7%” [2]. Losing conductivity after
less than 2% of strain is placed on the thin film is not a
desirable quality, as it makes the material a lot less versatile
than a more physically flexible transparent conductor could
    According to one source, “the next generation of
optoelectronic devices requires transparent conductive
electrodes to be lightweight, flexible, cheap, and compatib le
with large-scale manufacturing methods, in addition to being
                                                                                                FIGURE 2
conductive and transparent” [3]. This puts more emphasis on                           LATTICE STRUCTURES OF GRAPHENE
why ITO may not be the best material for the job, as it does
not meet the requirements of “flexible” or “cheap.” In fact,              Notice in Figure 2 above that the lattice is six-sided
“these requirements severely limit the use of ITO as                   (hexagonal). The hexagonal shape is the state in which it can
Uni versity of Pittsburgh                                                                       S wanson School of Engi neering
Eleventh Annual Freshman Conference                                                                                  April 9, 2011
                                                                                                                         Mark Bradel
                                                                                                                     Stephen Conover

conduct electricity and remain transparent [8]. In images (b)           of creating it is. Th is is why there is such a broad spectrum
and (c) of Figure 2, a co mbination of seven-sided shapes               of measured resistances of graphene, since it is difficult to
(heptagons) and five-sided shapes (pentagons) are what                  make an ideal sheet [1].
cause the run. Image (b) shows just two enclosures in this                  This paper has already discussed the issues in lattice
shape. Image (c) shows how this pattern is continued in one             irregularities and the losses of conductivity which could
direction to infinity, or the end of the sheet. Notice how the          result from such irregularities. Unfo rtunately, there are other
sheet to the left and the right of the irregularity bend towards        issues to take into consideration while producing graphene.
each other. It is this bending that allows the pattern to               It turns out that it is difficult to keep a sheet of graphene just
continue, and it will do this until it has reached the end of           that – a sheet. Due to van der Waals, or intermo lecular,
the sheet [8]. Th is is one of the main inhib itors of mass -           forces between the carbon atoms that make up the graphene,
producing graphene, as it is hard to make large sheets                  the sheet will tend to either restack itself and make graphite,
without an error like the one demonstrated above, as it                 or to roll up and crate carbon nano tubes [9]. “The
would create a run that would break the hexagonal lattice for           prevention of aggregation,” or stacking, “is of particular
the whole sheet.                                                        importance for graphene sheets because most of their unique
    Graphene, like ITO, is ext remely t ransparent. In fact, they       properties are only associated with individual sheets [9].” So
both absorb only 2.3% of visible light, making them equally             if the sheets stack, they lose the properties which make
transparent [1]. This is good news for graphene, as it can              graphene worth producing in the first place. Obviously, this
compete in the transparency end of being a transparent                  is something that must be overcome before graphene can be
conductor spectrum. However, it still must be proven                    mass produced.
adequate as a conductor.                                                    Since it is not desirable for the sheets of graphene to
   As stated earlier, ITO has resistance of 6 Ω/sq, and is 10           stack and hence lose their individual qualit ies, the next topic
to 30 Ω/sq on average [4]. Graphene has a much higher                   that must be addressed is how to prevent this from
resistance than ITO, with its lowest ever recorded value at             happening all together. One way to do this would be to
280 Ω/sq [1]. On average, it is usually much closer to 350              oxidize the graphite before extracting the graphene. The
Ω/sq [1]. Developers need to overcome this high resistance              graphene extracted fro m that graphite would then be
and resistivity before anyone should anticipate seeing it on            oxidized graphene [9]. The purpose of this is that once the
any consumer leve l products.                                           graphene becomes graphene oxide, it will no longer be able
   One major advantage graphene does have over ITO is                   to stack and create graphite, which is optimal. The process
that it is much mo re flexible, wh ile still maintain ing its           starts with “che mical o xidation of graphite to [make it ]
conductivity and transparency [7]. In Figure 1, it is clearly           hydrophilic graphite oxide, which can be readily exfo liated
seen that the bending of graphene does little to its                    as individual graphene oxide” [9]. Th is kind of graphene
conductivity. “Upon flexing, the conductivity of indium tin             will not aggregate, or stack. Ho wever, there is a serious issue
oxide decreases 3 orders of magnitude, while the                        with this, as graphene oxide is electrically insulating,
conductivity of a graphene-carbon nano tube hybrid                      mean ing it cannot conduct electricity. This is obviously not a
electrode remains stable” [1]. This is an extremely                     good result, as it is exactly the opposite of what was
significant figure and a major advantage over ITO. Since                intended to be accomplished. Luckily, it “can be converted
ITO is so brittle and loses its conductivity when bent, it can          back to conducting graphene by chemical reduction, fo r
only ever be in products which are both inflexible and flat.            example, using hydrazine” [9]. This means that by
Graphene could be used in future products that have yet to              introducing another chemical to reduce the graphene, or to
be invented due to this added dimension of versatility. If              make it unoxidized, it can be made a conductor again.
graphene is to be the future of transparent conductors, then            Unfortunately, these “Chemically Converted Graphene
there will be many new and notably more flexib le products              (CCG) sheets obtained through this method precipitate as
that appear.                                                            irreversible agglo merates owing to their hydrophobic
                                                                        nature” [9]. Essentially this means that once this method is
 THE C AREFUL D EVELOPMENT OF G RAPHEN E                                performed to reduce the graphene oxide, it beco mes really
                                                                        difficult to work with, because it is insoluble by water and
The previous section discussed graphene being a single layer            other organic compounds [9]. Also, other processes to
of carbon atoms in the shape of a lattice, and the possibilities        reduce graphene, such as vapor phase reduction, have been
of it creating a break in the lattice and ruining the sheet.            shown to make it back into sheets of graphene, while
There are more issues that go into creating sheets of                   retaining its ease of use [7]. However, these displayed
graphene than just these, however. Since developers are                 relatively poor conductivity [7], wh ich is a serious issue,
trying to get a single atomic layer fro m graphite and keep the         considering graphene already has poor conductivity. It
hexagonal lattice, it can be painstakingly tricky to deal with          seems that if graphene is to lose its ability to stack while
[1]. Because it is so tricky to deal with, it can often yield           retaining its qualities of being a transparent conductor,
many different results depending on how perfect the method
Uni versity of Pittsburgh                                                                            S wanson School of Engi neering
Eleventh Annual Freshman Conference                                                                                    April 9, 2011
                                                                                                                      Mark Bradel
                                                                                                                  Stephen Conover

scientists are going to have to look to other methods to                   Metal nanowires have also performed well in lab tests.
accomplish this.                                                       These metal nanowires are laid out in similar structures to
    Another topic worth exploring whether it is possible or            CNTs, but are made out of highly conductive metals like
not for graphene to have higher conductivity. Again, there             silver [3]. The nanowires are laid out in ordered arrays to
are a few processes on how to potentially do this. One that            make sheets [3]. In all of the lab experiments, the nanowires
has been researched and proven effective is to chemically              have shown extreme similarit ies to the statistics of ITO. The
dope the graphene. The method of chemically doping                     transparency of the metal nanowires has been measured at
involves coating the graphene with a vapor of an anion,                86% transparency, which is very similar to that of graphene,
which is a negatively charged ion [7]. In this process,                mean ing it is visually up to the standards of ITO [3]. The
“simp le treatment with thionyl chloride (SOCl2 ) vapor is             resistance was measured at 16 Ω/square, which is in the 10-
often employed as a means of anion doping and does not                 30 Ω/square range of ITO, so all around, these metal
significantly affect the optical transmittance of (carbon nano         nanowires seem to be a good competitor of ITO [3]. In fact,
tube) films” [7]. Th is means that by using this doping                nanowires made with surfactant-free silver can even be
process on the carbon nano tubes, or rolls of graphene, it can         made to be flexible, and when this is used in a nanowire
increase the ability for it to carry current, wh ile not               grating structure, it is an “ideal configuration for device
drastically affecting its transparency. The process starts by          applications” [3]. This means that when the metal nanowires
gauging the resistance of the sheet before the doping [7]. In          are made fro m surfactant-free silver, it can be made into a
this experiment, the sheet starts at 636 Ω/square [7]. Next ,          flexib le material that, when organized, is perfect for use in
the sheet is spun at 1500 revolutions per minute while                 electronic devices. This makes these metal nanowires a
introduced to the SOCl2 vapor at room temperature for                  serious competitor of graphene for the replacement of ITO
fifteen minutes [7]. After the test concludes, the graphene is         as the most popular transparent conductor.
measured to have a new resistivity of 240 Ω/square [7]. That               Another possible successor to ITO is zinc o xide, or ZnO.
is considerably lower, a whole 394 Ω/square less resistivity           ZnO is most likely going to replace ITO as a transparent
than when it started. The light transmittance only went down           electrode in flat panel displays like LCD telev isions [10].
fro m 88% to 86%, which is not enough to truly worry about             ZnO doped with galliu m or alu minum has recently attracted
[7]. While 240 Ω/square is still considerably higher than              a lot of attention, “as a potential to replace ITO due to low
what ITO is, experiments like th is show promise for                   cost, nontoxicity, and high stability…with good electrical
graphene and possible ways to make it a serious competitor             and optical properties” [10]. Th is doped ZnO has shown a
for ITO.                                                               lot of promise, as it can compete with ITO in transparency
    While graphene has advantages over ITO, it is by no                and conductivity, but has the added bonus of being cheaper
means ready to replace it. With more research, graphene                to produce and is extremely stable and not poisonous. As far
could prove to be the material to replace ITO as the “market           as transparent electrodes go, doped ZnO is one of the best
standard,” but as of right now, ITO is still the best available.       available [11]. One main reason why ZnO is considered
                                                                       superior to ITO in this field in particular is that, “doped ZnO
          O THER ALTERNATIVES TO ITO                                   films are more stable than ITO films” [11]. This is an
                                                                       important quality of an element that needs to remain stable
Although graphene is one of the most widely d iscussed                 in numerous consumer products throughout the product’s
alternative to ITO, there are other possible replacements.             lifetime. Other than in flat panel d isplays, “another
One of these possible replacements are carbon nanotubes                specialized application, doped ZnO films can be
(CNTs) [3]. Carbon nanotubes perform best in photovoltaic              advantageously used with thin-film solar cells” [10]. This
devices, or devices for collecting solar power, as well as in          adds another dimension to ZnO, making it more versatile as
display devices [3]. CNTs, like graphene, are made entirely            a transparent conductor. This is, “because ZnO films with a
of carbon atoms, making them inexpensive to produce [3].               textured surface structure are more easily produced on
However, since they have the same chemical makeup, they                relatively low temperature substrates than textured SnO 2
share a lot of the same issues. For example, CNTs have a               films” [11]. The tin o xide, o r SnO2 , films are what are
low amount of conductivity across bond sites between other             usually used now for solar cells, but the ZnO can be
CNTs, meaning larger sheets would have low conductivity                produced both cheaper and easier than the SnO2 . ZnO has
as a whole [3]. Ho wever if the CNT sheet is made to be 100            many qualities that will most likely p ropel it to lead the
nanometers thick, it can be brought all the way down to 10             replacement of ITO in the flat panel display market, as well
Ω/square, wh ich is in ITO’s average range [3]. There is a             as venture into other fields.
disadvantage to this, however. If a CNT sheet is 100
nanometers thick, the added thickness severely interferes                        THE ISSUE OF S USTAINAB ILITY
with its transparency [3]. CNTs could potentially be a
candidate for future replacement of ITO, but they must first           Sustainability is something that all engineers and scientists
overcome the issue of high resistivity at low thickness.               must take into deep consideration, especially when
Uni versity of Pittsburgh                                                                         S wanson School of Engi neering
Eleventh Annual Freshman Conference                                                                                  April 9, 2011
                                                                                                                            Mark Bradel
                                                                                                                        Stephen Conover

designing products that will be produced on a large-scale.            be able to make up for the loss of ITO whether individually
This is because if over-manufacturing depletes these                  or in some co mbination. On top of being able to perform at
materials over time to the point where the materials become           the same level, o r close to the same level, as ITO, these
too scarce and expensive to use. Hence, manufacturers,                many materials are both cheaper and safer to work with. As
especially for large-scale products, should only use those            stated earlier in this paper, indium is poisonous [5].
resources that are sustainable. ITO is already produced on a          However, of the many different transparent conductors we
large-scale, even though supplies of indiu m are dimin ishing         researched, only one of them is dangerous, being Cd 2 SnO4 ,
rapidly [4]. This will in turn render continued mass -                and this is why it was ruled out as a possible successor to
production of ITO imposs ible. Therefore finding a                    ITO [5]. These materials are also cheaper, which is easy to
sustainable alternative to ITO is a very important matter.            say when the price of indium is $1000 per kilogram [1]. The
    Of these alternatives, graphene is one of the most                price is a factor of more than five hundred times less
sustainable. Carbon is one of the most abundant elements on           expensive. For examp le the price of zinc for ZnO is only
earth, and since graphene is made up of only carbon ato ms,           $1.30 per kilogram [10]. The price of graphite, which is
it is therefore one of the most sustainable products on earth         what makes up graphene, is only $2.00 per kilogram [11].
[8]. As far as the other alternatives to ITO are considered,          Using these cheap materials and cheap development
carbon nanotubes are just as sustainable as graphene, since           practices, industries will be able to manufacture the same
they are both made of only carbon atoms. This is probably             products for a fraction of the price of ITO, which will make
one of the highest levels of sustainability attainable,               these types of products available to much more people. Plus,
considering the abundance and availability of carbon. On the          both of these alternatives are sustainable.
other hand, metal nanowires are not very sustainable. They               The fact that graphene is a flexib le material will lead to
are made of silver, which is a precious metal [3]. Th is means        many different products that are not available today. These
that in order to make metal nanowires, manufacturers must             products could include curved flat panel displays, curved
pay the high price of silver, as well as compete with jewelers        energy-efficient windows, curved solar panels, and many
for supplies of silver. Silver is also not exactly in high            other products that have yet to be invented [1,10]. ZnO also
supply, especially in co mparison to carbon. However, if              seems to be a very potential successor to ITO in the flat
developers can find a way to make these metal nanowires               panel display field. It actually is very similar in both
with a more sustainable metal than silver, then metal                 transparency and conductivity now, but has the added bonus
nanowires could be a safe alternative to ITO. Zinc o xide, o r        of being cheap [1,11]. It is for these reasons that ZnO will
ZnO, is another alternative to ITO which is considered                most likely rep lace ITO in the flat panel display market.
sustainable. The price for raw materials generally increases             Indiu m t in o xide has been a very successful material
in this order: Cd < Zn < Ti < Sn < Ag < In [4]. Indiu m is the        which has been used in many products. However, its high
most expensive, while zinc is the second least expensive.             cost and low availability will eventually lead to its extinction
This is because zinc is both more abundant and more easily            in the global market of transparent conductors. But the end
attainable than indium. Because of this, ZnO can be a very            of ITO will not mean the end of transparent conductors as a
sustainable alternative to ITO.                                       whole, as there are many transparent conductors that are in
    To keep up with consumers’ ever-increasing demand,                the process of being researched, which in time will be able
manufacturers, engineers, and scientists alike must                   to replace ITO. The most prominent of these many different
concentrate their efforts on making products that both                transparent conductors is graphene, due to its similarity to
perform well and are co mposed of sustainable resources. In           ITO as well as its cheapness and added features of flexib ility
the case of transparent conductors, it is clear that ITO is not       and sustainability. Other transparent conductors may surpass
sustainable. However, graphene, carbon nanotubes, and zinc            ITO, such as zinc o xide, metal nanowires, and carbon
oxide are all safe alternatives that can be used to replace           nanotubes. The future is not so dark for transparent
ITO.                                                                  conductors, and enough research will eventually bring about
                                                                      the successor of the field’s arguably most successful
THE FUTUR E OF TRANSPARENT CONDUCTORS                                 transparent conductor to date: ITO.
                                                                                               R EFERENCES
Although industry has made a market for a wide variety of
                                                                      [1] R. Kaner, J. Wassei. (2010, March 20). “Graphene, a promising
products using ITO technology, the fact that ITO is running           transparent conductor.” Materials Today. Vol. 13, Issue 13. pp.52-59.
out will not hurt the industry very much. Having enough               [Online].                                                     Available:
foresight to see that the world is running out of indiu m,            http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6X1J-
scientists and engineers have been conducting research to             4YN57GY
                                                                      [2] Z. Chen, S. Chua, B. Cotterell, E. Guenther, W. Wang. (2001, August
find a sustainable replacement for this technology. Luckily,          15). “A mechanical assessment of flexible optoelectronic devices.” Thin
possible replacements have been found. Although they are              Solid Films. Vol. 394, Issues 1-2. pp.201-205. [Online]. Available:
not ready for mass-production quite yet, these products will
Uni versity of Pittsburgh                                                                            S wanson School of Engi neering
Eleventh Annual Freshman Conference                                                                                    April 9, 2011
                                                                                                         Mark Bradel
                                                                                                     Stephen Conover

[3] A. Kumar and C. Zhou. (2010, November 14). “The Race to Replace
Tin-Doped Indium Tin Oxide: Which Material Will Win?.” ACS Nano.
[Online]. Available: http://pubs.acs.org/doi/pdf/10.1021/nn901903b
[4] R. Gordon. (2000, August). “Criteria for Choosing T ransparent
Conductors.”         Materials Research Society. [Online]. Available:
[5] P. Patel-Predd. (2009, January). “The Trouble With Touch Screens.”
IEEE.        [Online].      Available:      http://spectrum.ieee.org/consumer-
[6] A. Hartley. (2010, October 27). “Are we running out of touchscreens?.”
Tech Radar. [Online]. Available: http://www.techradar.com/news/phone-
[7] M. Allen, L. Chen, R. Kaner, K. Nelson, V. T ung, J. Wassei, and Y.
Yang. (2009, April 10). “Low-Temperature Solution Processing of
Graphene−Carbon Nanotube Hybrid Materials for High-Performance
Transparent Conductors.” American Chemical Society. [Online]. Available:
[8] P. Kim. 2010, September 23). “Graphene: Across the border.” Nature
Materials.                          [Online].                         Available:
http://www.nature.com/nmat /journal/v9/n10/full/nmat2862.html
[9] S. Gilje, D. Li, R. Kaner, M. Müller, and G. Wallace. (2008, January
27). “Processable aqueous dispersions of graphene nanosheets.” Nature
Nanotechnology.                         [Online].                     Available:
[10] O. Bamiduro, H. Mustafa, R. Mundle, R. B. Konda, and A. K. Pradhan.
(2007, June 20). “ Metal-like conductivity in transparent Al:ZnO films.”
American         Institute     of       Physics.       [Online].      Available:
[11] T. Minami. (2005, March 15). “Transparent conducting oxide
semiconductors for transparent electrodes.” Institute of Physics Publishing.
[Online].               Available:               http://iopscience.iop.org/0268-

                  ADDITIONAL R ESOURCES
T. Hudson. (2008, July 1). “ RIP LCD Displays. Invented: 1971 - Extinct:
2017.” [Online]. Available: http://slashdot.org/~tomhudson/journal/206171
There have been many influential forces that guided us
through this writing process . We would like to thank Dave
Grohl for h is continuous assistance in persevering
throughout this daunting task. Thom Yorke provided the
drive to be individualistic in the paper. Matthew Bellamy
provided the inspiration to write about our subject
throughout the duration of the assignment. We would also
like to thank Jaime Escalante for making us realize that we
need to have desires in order to achieve our goals.

Uni versity of Pittsburgh                                                              S wanson School of Engi neering
Eleventh Annual Freshman Conference                                                                      April 9, 2011

To top