future mi projects

8/3/2019 Future MI Projects

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Future Projects onMI Instrument

May 1, 2006


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Ul

timate Goal

While experiments done on ourUHV/LTSTM provide great insight into chemical

systems, the operating conditions are notpractical for real world application. The advantages of the MI instrument is

that it works in an ambient environment(i.e. room temp. and at 1 atm.), whichallows for easy application to industrialprocessing conditions.


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In situ ST

M We are unable to achieve

atomic resolution (except forHOPG) on the MI instrumentdue to the ease with which the

metal surface can becomecontaminated in air(hydrocarbons and water).

Sonnenfield and Hansma in1986 were the first to use STMto study a surface immersed ina liquid.1

In 1990, Magnussen et al.achieved atomic resolution ona metal surface.1

Figure from Ref. 2


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D

evel

opment of In Situ ST

M Depended on three advances1:

The development of the STM by Binnig and Rohrer

The development of surface preparation methods inambient conditions.

The development of methods and materials to coatthe STM tip and to couple the STM with a

biopotentiostat.This technique provides information on surface

processes such as phase transitions in adlayers on amolecular and atomic level.


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Comparing UHV and In Situ

Images of Au (herringbones)

Image of Au(111) under 0.1 MHClO4 solution1

Image of Au(111) underUHV


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Comparing UHV and In Situ

Images of Au (atomic res.)

Flame-annealed Au(111) underclean mesitylene3

Image of Au(111) underUHV


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Comparing Ambient and In Situ

Images ofHOPG

Image ofHO

PG underphenyloctane2Image ofHOPG in air

File: 3-9-06HOPG009


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Comparing Ambient and In Situ

Images of Molecules on Au

L-cyseteine molecules on Au(111)under perchlorate solution4

C10, C12 SAM on Au(111) in air

File: 3-15-06AuMicaSAMVap028


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El

ectroch

emistry in ST

M Schematic of a sample molecule

coadsorbed with referencemolecules on a substrate asprobed by an STM tip.

RE and CE represent thereference and counter electrodes,respectively.

Vsub and Vbias are the substratepotential (with respect to thereference electrode) and the tip-substrate bias voltage,respectively, which are controlledindependently by a bipotentiostat.5


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El

ectroch

emistry in ST

M Because the charge transfer event central to

electrochemical reactivity occurs within a fewatomic diameters of the electrode surface, thedetailed arrangement of atoms and molecules atthis interface strongly controls the correspondingelectrochemical activity1.

Cycling the potential causes significant changes

in th

e surface topograph

y, from ch

angingh

owmolecules adsorb to the surface to causingreconstructions of the metal atoms themselves.


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Insul

atingT

ips Because the faradaic background from a bare

metal wire immersed in solution can approach

several

mill

iamps of current wh

il

e tunnel

ingcurrents are typically on the order of nanoamps,the STM tip must be insulated.

The tip is insulated by coating all but the very

end with

an insul

ator so th

at th

e tunnel

ingcurrent will not be overcome by theelectrochemical background.1

A variety of materials may be used to coat thetip, specifically wax and nail polish.


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T

ipE

tch

ing Extremely sharp tips withlow aspect ratios

are prepared by chemically etching the tip

in a 1 M basic solution (KOH). The etching current, which depends on the

area of immersed wire and applied voltage

is adjusted to an initial value. This process produces a neck shape near

the air-solution interface.6


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T

ipE

tch

ing As the etching proceeds,

the neck-like regionbecomes thinner and

thinner, and eventuallythe lower portion dropsoff.

This causes an abruptdecrease in the current.

A very sh

arp tip with

asmall protrusion at theend can be made byswitching off the circuit asthe current abruptlydrops.6


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W

ax Insul

ation ofT

ips Most common method uses Apiezon-brand wax The sharp etched tips are mounted vertically on

a manipul

ator. A copper plate is heated and used to melt thewax.

A rectangular slit in the plate provides atemperature gradient for the melted wax.

The tip is brought from underneath the slit bymeans of the manipulator.6


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W

ax Insul

ation ofT

ips The tip is first moved slowly into the hot

wax and allowed to attain a thermal

equilibrium and uniform wetting. The tip is then raised through the wax and

allowed to break the top surface region ofthe melt.

The tip is moved sideways out of the slitso as to leave the very end of the tipunperturbed.6


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Procedure forWax Insulation of

Tips

From Ref. 6


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Images ofW

axC

oatedT

ips

SE

M image ofEC

ST

M tips, insul

ated with

doubl

e (a

) andsingle (b) pulling methods7


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N

ail

Pol

ish

Insul

ation ofT

ips Multiple articles cited using nail polish to

coat their tips, however the exact coating

procedure could not be found.


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Reconstructions Metal surfaces in UHV reconstruct in order

to minimize their surface energy. The extent of reconstruction is strongly

dependent on the work function of themetal.

Th

e el

ectroch

emical

environment offers anopportunity to systematically vary theelectronic state of a surface, through theapplication of potential and the influence of

adsorbed species in solution.1


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Adsorption

Adsorption induces changes in the workfunction modifications of the surface dipolarlayer

particularly if significant charge transfer occursbetween the adsorbate and surface

measurements of yield critical information

on the degree of charge reorganization uponadsorption

= adsorbate covered - clean


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Au Reconstructions Reconstructions can be removed electrochemically by

placing the electrode at sufficiently positive potential. The removal of reconstruction can be attributed to the

adsorption of electrolyte anions at higher potentials. Cycling the potential to a region where the herringbone

reconstruction is removed and then back revealschanges in the shape of the step edges on the surface,showing that the extra material required in thecompressed structure is taken from and returns to thestep edges.1


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Images of Au Reconstructions

TypicalAu(111) 23 X 3 reconstruction pattern.The image was obtained for Au under purewater at 0 mV.8

Typical image of Au(111) after thetransformation. The image was obtained for Auunder water after the surface potential was

raised to 400 mV.8


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Sul

fate on Au (111) Sulfate is known to form a (3 x 7)R19.1 structure on

Au(111) The coadsorption ofH3O+ ions is necessary to stabilize

the ordered oxoanion adlattices. Both species in H2SO4, sulfate (10%) and bisulfate

(90%) have 3 free oxygen atoms to interact with thesurface. The distance between them (2.47 ) is of thesame order of magnitude as the distance between Auatoms (2.88 ), so their geometrical arrangementmatches that of the Au (111) surface.9


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Sul

fate on Au (111) The reason for the presence of non-

uniform anion-anion distances is the

formation ofH-bridge bonds between theoxygen atoms of the oxoanions and thecoadsorbed H3O+ ions.9


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Images of Sul

fate on Au(111) In situ STM image (10x10 nm2) of a

Au(111) electrode in 0.1 M H2SO4showing both the (3 x 7)R19.1sulfate structure, (upper and lower

parts) and the (1x1) substrate (middlepart).

The potential was switched from 0.80to 0.65 V and then back to 0.80 V atthe points marked by the arrows.

The triangles and circles drawn on themiddle part of the image represent thepositions of the sulfate and hydroniumions, respectively.9


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Images of Sul

fate on Au(111) (B) Model of the(3 x

7)R19.1 sulfatestructure on Au(111) in

0.1 M H2SO4 The H3O+ ions are placed

on top of the Au atoms. Every H3O+ adsorbed can

form 3 H-bridge bondswith the oxygen atoms ofsurrounding sulfate ions.9


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Intro.T

oC

ycl

icV

ol

tammetry The voltage is swept

between two values

at a fixed rate, whenthe voltage reachesV2 the scan isreversed and the

voltage is swept backto V1.11


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Intro.T

oC

ycl

icV

ol

tammetry In the forward sweep, as the voltage is swept

further to the right (to more reductive values) acurrent begins to flow and eventually reaches apeak before dropping. To rationalize thisbehavior we need to consider the influence ofvoltage on the equilibrium established at theelectrode surface. If we consider electrochemicalreduction, the rate of electron transfer is fast incomparison to the voltage sweep rate.11 (i.e.Fe3+ Fe2+)


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Intro.T

oC

ycl

icV

ol

tammetry When the scan is

reversed we simply moveback through the

equilibrium positionsgradually convertingelectrolysis product backto reactant.(Fe2+ Fe3+)The current flow is now

from the solution speciesback to the electrode andso occurs in the oppositesense to the forwardsweep.11


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Cyclic voltammogram of Au(111) in

0.1 M H2SO4 The peak at 0.55 V is

attributed to the lifting ofthe (23 x 3)

reconstruction th

at takesplace in the lowerpotential region.

The two sharp peaksaround 1.0 V are due tothe formation of an

ordered sulfate structureat more positivepotentials.10


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UnderpotentialDeposition

The electrodeposition of a metal on a foreignmetal at potentials less negative than theequilibrium potential of the deposition reaction.

Such a process is energetically unfavorable andit can occur only because of a strong interactionbetween the two metals, with their interactionenergy changing the overall energetics tofavorable. Consequently, only one (very seldomtwo) monolayer can be deposited this way, andthis is a very convenient way to produce well-controlled monolayer deposits.12


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UnderpotentialDeposition

Upd monolayers are formed by the deposition oflow work function metals onto high work functionmetals.

The monolayer originates from a relativelystrong adatom-substrate bond formed using lessenergy than required for adatom-adatom bondsformed during bulk deposition.

One of the most intriguing aspects of upd is theanion dependence, which derives fromcoadsoprtion of the anion and the adatom.1


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UnderpotentialDeposition ofCu on

Au (111) One of the first examples of atomic

resolution in the electrochemical

environment was Cu monolayers on Au(111) in H2SO4.

Three different structures are seen before

bulk Cu deposition.1


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Images ofUnderpotential

Deposition ofCu on Au (111) At positive potentials

(+300 mV), the bare

Au(111) surface isseen.1


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Deposition ofCu on Au (111) Ordered adlayer with

(3 x 3)R30

structure, ascribed tocoadsorbed sulfate.

Formed between 200and 100 mV.1


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Deposition ofCu on Au (111) FullCu monolayer in

registry (1x1) with

Au(111).1 At 5 mV


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UnderpotentialDeposition ofCu on

Au (111) Different solutions of anions give rise to different

structures on the electrode surface.

Cl- anions form both (2 x 2) and (5 x 5)incommensurate structures depending on theconc. of the anion.

On otherlow Miller index faces of Au, Cu doesnot exhibit the pronounced dependence on thetype and conc. of anion.1


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Conclusions

In situ STM allows for atomic resolution under ambientconditions.

Electrochemical STM can be used to understand theelectrochemical double layer and to correlate detailedstructure of the electrode surface with the double-layerstructure and ultimately with electrochemical response.

Studies of the upd processes reveal a rich structural andreactive chemistry, the detailed nature of which isdependent on potential, available anions, substrateorientation, and substrate identity.1


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References

1) Gewirth, A. A.; Niece, B. K. Chem. Rev. 1997, 97, 1129-1162.2) De Feyter, S.; Gesquiere, A.; Abdel-Mottaleb, M. M.; Grim, P. C.; De Schryver, F. C.; Meiners,

C.; Sieffert, M.; Valiyaveettil, S.; Mullen, K. Acc. Chem. Res. 2000, 33, 520-531.3) Han, W.; Li, S.; Lindsay, S. M.; Gust, D.; Moore, T. A.; Moore, A. L. Langmuir. 1996, 12, 5742-

5744.4) Dakkouri, A. S.; Kolb, D. M.; Edelstein-Shima, R.; Mandler, D. Langmuir. 1996, 12, 2849-2852.5) Tao, N. J. Phys. Rev. Lett. 1996, 76, 4066-4069.6) Nagahara, L. A.; Thundat, T.; Lindsay, S. M. Rev. Sci. Instrum. 1989, 60, 3128-3130.7) Kazinczi, R.; Szocs, E.; Kalman, E.; Nagy, P. Appl. Phys. A. 1998, 66, S535-S538.8) Tao, N.J.; Lindsay, S. M. J. Appl. Phys. 1991, 70, 5141-5143.9) Cuesta, A.; Kleinert, M.; Kolb, D. M. Phys. Chem. Chem. Phys. 2000, 2, 5684-5690.10) Climent, V.; Coles, B. A.; Compton, R. G. J. Phys. Chem. B 2001, 105, 10669-10673.

11) http://www.cartage.org.lb/en/themes/sciences/Chemistry/Electrochemis/Electrochemical/CyclicVoltammetry/CyclicVoltammetry.htm

12) http://www.corrosion-doctors.org/Dictionary/Dictionary-U.htm

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