Studies of self assembled monolayers on highly oriented pyrolytic graphite using scanning tunneling microscopy and computational simulation 2

CHAPTER 2 EXPERIMENTAL 2.1 Scanning Tunneling Microscopy STM There are many modern instruments for surface structural and chemical analysis such as the Field Ion Microscope FIM, the Fi

CHAPTER 2

EXPERIMENTAL

2.1 Scanning Tunneling Microscopy (STM)

There are many modern instruments for surface structural and chemical analysis

such as the Field Ion Microscope (FIM), the Field Emission Microscope (FEM),

Low-Energy Electron Diffraction (LEED), Scanning Electron Microscope (SEM),

Electron Spectroscopy for Chemical Analysis (ESCA), Transmission Electron

Microscope (TEM), etc The development of these techniques has played an

important role in the field of surface science However, each of these techniques has

certain strengths and limitations LEED and X-ray diffraction techniques rely on

large-scale order of the surface, and can at best give averaged information about local

and defect structures; SEM requires samples with strong corrugation or mass contrast

and its resolution is not high enough to resolve surface atoms High-resolution TEM

can in some cases resolve features with atomic dimensions of specially thinned

samples However this can be accomplished only by aligning the electron beam with

the rows of atoms in a crystalline lattice FEM and FIM are only able to probe the

two-dimensional geometry of the atomic structure on the surfaces of sharp tips with

radii less than 100nm In addition, sample preparation is rather complicated For FIM

the samples must be stable in high fields, thus limiting its general usefulness Other

surface analytical techniques, such as X-ray Photoemission Spectroscopy (XPS),

Ultraviolet Photoemission Spectroscopy (UPS) and Electron Energy Loss

Spectroscopy (EELS), can only provide spatially averaged information of the

electronic structures of the surfaces Moreover, some of the techniques mentioned

above require high-vacuum environment and can only provide indirect results or

strongly rely upon model systems for data interpretation Until the Scanning

Tunneling Microscope (STM) was introduced, it still remained a dream to directly

observe geometric and electronic surface structures at the atomic level at ambient

pressure and room temperature [1-3]

Compared to other surface analytical techniques, there are several reasons for the

diversity of STM and STM-based technological applications: STM can achieve lateral

and vertical resolutions of 0.1nm and 0.01nm, respectively, i.e., individual atoms and

molecules can be resolved The higher vertical resolution of STM relative to other

microscopes also offers advantages with regard to qualitative analysis of surface

roughness on a nanometer scale STM can be performed in different environments,

such as vacuum, air, low or high temperature, etc Samples can even be immersed in

water or other solutions under potential control In most cases, special techniques for

sample preparation are not required, and samples remain mostly free of damage With

these advantages, STM is especially suitable for in-situ electrochemical studies,

biological studies, and the evaluation of sample surface under various experimental

conditions

The other unique feature of the STM is its truly local interaction with the surface

under study at the atomic scale rather than the averaged properties of the bulk phase

or of large surface area This allows the study of individual surface adsorbates, surface

defects, surface reconstructions, and adsorption-induced surface reconstructions at

unprecedented resolution Three-dimensional images of the surface and the solid-fluid

interface in real space can be obtained in real time, independent of the sample’s

periodicity This capability allows in-situ imaging of some dynamical processes taking

place on surfaces and at the solid-fluid interface Local surface electronic properties

such as charge-density waves, the changes of surface barrier and energy gap, as well

as spectroscopic images, can be provided by STM

STM can be employed for the modification of a surface and for the manipulation

of atoms and molecules through tip sample interactions, opening up the prospects of

constructing atomic or molecular scale devices

2.1.1 Principle of the Scanning Tunneling Microscopy (STM)

Scanning tunneling microscope (STM) is a powerful technique for viewing

surfaces at the atomic level It probes the density of states of a material using

tunneling current The basic design of the STM is shown in Fig 2.1

Based on the concept of quantum tunneling, when a sharp conducting tip is

brought very close to the metallic or semiconducting surface ( 10Å), the wave

functions of the sample will overlap with the wave function of the tip If a bias

voltage V between the tip and surface is applied, the electrons can travel through the

energy barrier via a quantum-mechanical mechanism called tunneling to give rise to a

current

Fig 2.1 Schematic view of scanning tunneling microscope (Copyright: Michael Schmid, TU Wien)

It consists of scanning tip, piezoelectric controlled scanner, sample-to-tip control, and data

processing component

The direction of the electron flow depends on the sign of the applied bias on the

sample For a positive bias, electrons flow from occupied states of the tip to

unoccupied states of the sample and the direction is reversed for the opposite polarity

By keeping the tunneling current constant with an electronic feedback circuit, a fixed

tip-sample gap distance can be maintained as the tip is scanned laterally across the

sample Plots of the tip height z versus lateral positions x and y can then be generated

and such images yield information about the electronic structure and topography of

the surface being analyzed

The tunneling current is proportional to the local density of states (LDOS) near

the Fermi level The current is also proportional to the exponential of the separation d

between the tip and the sample:I e2d Such relationship gives STM the ability to

study surfaces with high sensitivity to height changes due to individual atoms

Even though the principle of a scanning tunneling microscope is not very

complicated, many factors must be taken into consideration in the design to ensure a

stable and reliable performance Optimum functioning of an STM device requires

tip-to-sample position control with picometer precision, a fine positioning capability

in three dimensions, high scanning speed, and simplicity of operation These

requirements have to be satisfied in the presence of building vibrations with up to

micrometer-size amplitudes, electric noise, thermal drift, creep and hysteresis of the

piezoelectric translation elements and other perturbations [1, 2]

In our experiment we use the commercial available product Nanoscope IIIa

provided by Veeco Asia This microscope system has both AFM and STM functions

which can be operated in air or organic solutions [4]

2.1.2 Vibration Isolation of the STM

The frame of the instrument is always subjected to vibrations transmitted from

the ground or the air Since the tunneling current depends exponentially on the gap

between tip and sample, even the smallest vibrations can affect the stability of the

instrument For many materials, especially metals, the atomic corrugations observed

in the constant current STM mode will typically be 0.01 nm Therefore, a good

vibration-isolation system is very important for a well-functioning STM In the

process of minimization of the sensitivity of an STM to vibrations from the building,

ventilation ducts and people’s motion, primary attention has to be given to the

frequency range between 1 and 100 Hz Increasing the inherent resonance frequency

of the STM body and employing a vibration damping system are two major ways to

isolate vibrations In our STM system, the vibration isolation is achieved by a massive

platform rest on inner tubes which are supported by the compressed air as shown in

Fig 2.2

Fig 2.2 Picture of a typical working platform of STM in our lab: the scanner is placed on a

vibration-isolated platform The metal cover was used to block the electromagnetic wave The

floating platform was supported by high pressure air gas

On the platform there is a small plate which is supported by the spring, where the

scanning tunneling microscope base is located The base and the STM head are

covered with a metal bell to isolate electromagnetic field from the environment

Furthermore the whole working platform is covered by a huge and heavy acoustic

hood By all these means the vibrations from building and air can be minimized as

much as possible [1, 2, 4]

2.1.3 Preparation of the STM Tips

The reliable fabrication of tunneling probe tips is critical for the proper operation

of STM The size, shape and chemical identity of the tip influence not only the

resolution and shape of an STM scan, but also the measured electronic structure The

microstructure of the tip is the key to atomic resolution because the tunneling current

depends exponentially on the gap distance It is necessary to have a single site of

closest atomic approach for stable operation, as anomalous imaging artifacts will

appear when simultaneous tunneling occurs through multiple atoms on the tip This is

commonly referred to as double-tip imaging

STM tips are typically fabricated from metal wires of tungsten (W),

platinum-iridium (Pt-Ir), or gold (Au) and sharpened by mechanical grinding, cutting

with a wire cutter or razor blade, “controlled” crashing, field emission/evaporation,

ion million, fracture, or electrochemical etching [1, 2]

Preparation of Tungsten Tips

The preferred method for preparing tungsten STM tips is the electrochemical

etching method There are two ways by which this can be done: Alternating-Current

(AC) or Direct-Current (DC) etching according to the applied potential Each

procedure gives a different tip shape; the AC etched tips have a conical shape and

much larger cone angles than the DC etched tips The hyperboloid-shaped DC etched

tips are much sharper than AC etched tips and are preferable for high-resolution STM

imaging

Fig 2.3 Schematic view of the etching machine: it consists of ring-shape Pt electrode, power

supply, and wire holder where the sample being etched is fastened The etchant surface just touchs

the Pt-ring

Fig 2.3 illustrates the details of the electrochemical cell used in DC etching,

which contains 100 mL of 2M NaOH or KOH The W wire to be etched is placed in

the center of the cell and serves as the anode It is mounted on a micrometer so that its

position relative to the surface of the electrolyte can be precisely adjusted The

counter electrode (or cathode) consists of a Pt ring which surrounds the anode

When a DC voltage of 13V is applied to the anode, bubbles can be observed

emerging at the cathode/solution interface The overall electrochemical reaction is

Cathode: 6H2O + 6e-  3H2 (g) + 6OH- SRP = -2.45 V

Anode: W(s) + 8OH-  WO42- + 4H2O + 6e- SRP = +1.05 V

W(s) + 2OH- + 2H2O  WO42- + 3H2(g) E0 = -1.43 V

where SRP stands for standard reduction potential The above reaction involves the

oxidative dissolution of W to soluble tungstate (WO42-) anions at the anode, and the

reduction of water to form bubbles of hydrogen gas and OH- ions at the cathode

Actually, the reaction mechanism is much more complex than indicated by the above

equations and the potential required to drive an electrochemical reaction is usually

higher than that calculated from standard electrode potentials [1, 5, 6, 7]

Several factors affect the etching process Due to the surface tension of the

aqueous solution, a meniscus is formed around the wire once it is placed into the

electrolyte It is primarily the shape of the meniscus which determines the aspect ratio

and overall shape of the tip The shorter the meniscus is, the smaller the aspect ratio

becomes A low aspect ratio is important in reducing vibration in the tip during

scanning As the reaction proceeds, the change in the surface area of the wire and in

the fluid disturbances may result in the variation of the meniscus height To avoid

oddly shaped tips, the meniscus height should be kept at the same position by

adjusting the micrometer during etching

Usually, a quick automatic cutoff circuit is used to cut off the potential to avoid

over-etching The cutoff time of the etching has a significant effect on the radius of

curvature of the tip: the faster the cutoff time, the sharper the tip Because OH- is

consumed in the reaction, it is necessary to replace NaOH solution periodically The

tip-drop-off time increases with the decrease of the OH- concentration

The chemically etched tungsten tips are very sharp but can be oxidized easily in

air to form tungsten-oxide layer that has much higher resistance than the desired

tunneling gap resistance It leads to tip crashing sometimes Thus the tungsten tips are

not the most suitable tips for our experiment conducted in air, although they work

perfectly in our UHV STM system The etching system we used here is W-TEK

purchased from Omicron Technology

Preparation of Pt-Ir Tips

Platinum, although a soft metal, is a material preferred over tungsten because it is

inert to oxidation The addition of Ir to form a Pt/Ir alloy adds stiffness while

maintaining a chemically inert material Pt-Ir tips are widely employed, too,

particularly in atmospheric and electrochemical environments

Mechanical shearing is the most common approach for fabricating Pt-Ir tips In

spite of the variation in shape, many experiments had proven that atomic resolution

can be achieved using the mechanically fabricated Pt-Ir tips Although resolution

requirements are usually not as stringent for highly topographic samples, wide-area

scans place unique restrictions on the tip morphology For such samples, symmetric,

controlled-geometry tips with small radii of curvature and high aspect ratios are

necessary to minimize the convolution of the tip shape into the acquired image

However, asymmetric or double tips are often formed during mechanical shearing of

the Pt-Ir wire, resulting in misleading sample topography To make the tip as sharper

as possible, we usually cut the Pt-Ir wire with very large seizers, with the angle 

equals to 45° At the end of cutting, we pull the seizers away from the Pt-Ir tip along

the wire This step helps to form a sharper tip at the moment when the tip is broken

into two pieces Usually the tips cut by this method are always quite sharp, and can be

readily used in STM experiments

Fig 2.4 Illustration of the tip cutting procedure: the vertical line represents Pt-Ir wire; the crossing

line represents the seizers Angle  equals to 45 When the tip is nearly dropping off from the

wire, the seizers should move along the arrow so that a sharp tip could be produced

Meanwhile several in-situ tip treatments were already used at the birth of STM by

the inventors: by gently touching the tip with the sample surface, the resolution can be

improved; or by exposing the tip to high electric fields, of the order of 109V/cm, the

tips become sharpened There are additional methods to improve the tip sharpness

when the experiment is carried out in air The freshly prepared STM tips are always

washed with absolute ethanol to remove the possible organic contents either from the

seizers or environment The tip is engaged onto surface and then withdrawn from

surface for several times Such movements help to remove the possible contaminants