Studies of self assembled monolayers on highly oriented pyrolytic graphite using scanning tunneling microscopy and computational simulation 7

CHAPTER 7 FORMATION OF 2D CRYSTALS ON HOPG 7.1 Motivation In materials science, a crystal is built up from regularly repeating ‘structural motifs’, which may be atoms, molecules, or gr

CHAPTER 7 FORMATION OF 2D CRYSTALS ON HOPG

7.1 Motivation

In materials science, a crystal is built up from regularly repeating ‘structural motifs’, which may be atoms, molecules, or groups of atoms, molecules, or ions [1] Traditionally, it is widely accepted that the repeating pattern of the crystal reproduces itself into three spatial dimensions (X, Y, Z), which leads to a three dimensional crystal

If the regularly repeating ‘structural motifs’ are restricted within a flat plane, they can only grow in two dimensions (X, Y) The resulting structures can be considered as crystals in two dimensions, and we call them two dimensional crystals (2D crystals) Although not as popular as 3D crystals, 2D crystals are not very rare in nature with surfaces as their favorite media [2] The studies of physical and chemical properties of two dimensional objects have become more and more important in daily life and industries Thus, the two dimensional systems are gaining great interest from many sectors of industries, including microelectronics, catalysis, etc

The investigations of the two-dimensional crystals are traditionally part of the surface studies Two-dimensional phases and phase transitions between them were first observed by Langmuir in the 1920s in the experiments with layers of organic molecules (salts of fatty acids) on the surface of a liquid It results in the indirect discovery of a crystalline phase in a two-dimensional system [2] For existence of

most 2D crystals in three-dimensional world, an external field is needed Most 2D crystals are formed on the surface of a liquid or a crystal One representative example

is the organic monolayers on the HOPG, which will be discussed further in this chapter

As a unique tool in the study of the physisorbed organic monolayers, the use of scanning tunneling microscopy and the scanning results had been discussed in detail

in reviews by De Feyter [3, 4], Wan [5] and Tao [6] But Matzger and his coworkers’ attempt is the first systematic work to link the organic monolayers on the HOPG with the 2D crystals [7] Since very little systematic work has been done towards the two dimensional crystals till now, we hope our research can make some useful contribution to this area

As defined [2], there are two classes of 2D structures based on the dependence of the adatoms’ period to the period of the substrate One is the commensurate structure with periods that are multiples of the substrate period, and anther is the incommensurate structure with at least one period incommensurate with the substrate’s period

In our research the HOPG was chosen as the substrate to support 2D crystals Its advantages include natural flatness; minimum interaction between the adsorbates and substrate; excellent conductivity, which allows us to study the monolayers using STM Unlike some metal and semiconductor surfaces, e.g gold and silicon, the sp2 carbons

of the graphite crystal are chemically inert They are unlikely to form covalent bonds with adsorbates Therefore the organic molecules on the graphite surface only

experience the weak molecule-substrate interactions, e.g van der Waals forces

Some examples of two dimensional crystals are: freestanding smectic liquid crystal films; lattices of adatoms adsorbed on metal surfaces, for example: La-W(112) system; overlayers on exfoliated graphite, e.g noble gases on the basal plane of graphite; and intercalated graphite, e.g bromine atoms implanted into the graphite [2] The number of 2D crystals is significantly increased as the new family member SAMs

on HOPG is counted in, since there are thousands of SAMs The development in the STM technology enhances the possibility of further investigation for 2D crystals system

7.2 Classification of crystals

The most common method is to classify 3D crystals according to their crystalline structures There are seven crystal lattice systems [8]: cubic (isometric), tetragonal, orthorhombic, hexagonal, trigonal, triclinic and monoclinic Another way is to categorize the 3D crystals according to their chemical and physical properties There are four types of them: covalent crystals, metallic crystals, ionic crystals, and molecular crystals

For the case of crystals in 2D, among 17 space groups (p1, pg, p2gg, p4, p3, p6, p2, pm, p2mg, p4gm, p31m, p3m1, c2mm, cm, p2mm, p4mm, and p6mm) [7], the examined 359 SAMs belong to 9 groups ( p1, pg, p2gg, p3, p6, p2, p2mg, c2mm, cm) However, SAMs in eight highly symmetric groups (p4, p4gm, p4mm, p3ml, p3lm, p6mm, p2mm, and pm) have not been observed The most possible reason is that the

number of the sample is insufficient to make the complete statistical studies Furthermore, HOPG surface lattice can affect the configuration of the adsorbates, especially for the molecules with long alkyl chain groups, through van der Waal’s forces (Chapter 4) Therefore the low symmetric space groups are always more popular

With regards to the classification of 2D crystals according to their chemical and physical properties, only the molecular crystals have been observed so far Suzuki et

al have inserted the bipyridinium cations between the carboxylic acid groups [9] But the crystal is not held together by the ionic bonds (electrostatic forces), it is still considered as a molecular crystal, which is held together by non-covalent interactions, like van der Waals forces or hydrogen bonding

7.3 Proposed process of formation of the 2D crystals

The theory of growth of perfect crystals has been well developed For the growth

of a three dimensional crystal, it mainly occurs at steps of monomolecular height on its surfaces, and the probability for the formation of these two-dimensional nuclei is a very sensitive function of the supersaturation [10]

The principle of the crystal growth was employed to study the growth of SAMs

on HOPG Although the graphite surface has a high degree of perfection, they still have steps and defects, as shown in Fig 7.1

Fig 7.1 STM image of HOPG shows steps-like structures (Height profile, 172nm×172nm)

Two possible mechanisms for the growth of two dimensional crystals on the HOPG surface were proposed In the first mechanism, like most crystal growing environment, the steps with kinks are the best adsorption sites for the adsorbates The adsorbates start to occupy these adsorption sites when the supersaturated solution was deposited onto the HOPG surface It is followed by the growth of two dimensional islands with a dense structure around the first few adsorbed molecules with the increase in surface coverage, until the substrate is covered by a continuous monolayer

In the second mechanism, the molecules randomly physisorb on the graphite surface when the supersaturated solution was deposited onto the HOPG surface With

increasing number of adsorbates on the surface, the randomly distributed molecules start to pack closely to form a monolayer The first proposed mechanism is more likely to occur during the monolayer formation process, as discussed in Chapter 4 based on our experimental results In addition, the fact that only partial surface being covered with the SAMs, which are always beside the steps, suggests that mechanism one is more likely to occur On the other hand, if the second proposed mechanism was the correct, we should be able to observe HOPG surfaces that are fully covered by the densely packed monolayers, which were not observed in any experiments

The first mechanism can be divided into two steps: at the first step, the organic molecules are physisorbed on the HOPG surface; at the second step, the molecules start nucleation to form SAMs - 2D crystals (Fig 7.2)

Fig 7.2 Illustration of supersaturated solution deposited onto the HOPG surface

Consider the equilibrium between the HOPG surface empty sites S, the occupied sites SP, and the particles P in the solution, Langmuir equation is applied:

S + P  SP (7-1) Equilibrium constant K is given by

][

[

]

[

P

S

SP

K  (7-2)

Let’s consider the second step: formation of 2D crystals The top view of the surface

is shown in the Fig 7.3

Fig 7.3 Top view of HOPG surface with molecules forming SAMs (‘locked’ molecules) and

‘unlocked’ molecules

In this two dimensional model, the number of adsorption sites is N The number

of the ‘unlocked’ molecules is SP as discussed in equation (7-1) The number of the

‘locked molecules’ forming SAMs is M

Consider at the equilibrium

N + SP  M (7-3) The equilibrium constant c is given by the equation:

]

][

[

]

[

SP

N

M

c (7-4)

Rearrange (7-2) and (7-4):

] ][

][

[

]

[

N P

S

K

M

c (7-5)

Density of the adsorption sites on step [N] is a property of the graphite crystal, which is considered as not a variable but a constant [M] is the proportional to molecules of monolayers, therefore it is proportional to surface coverage  [S] is proportional to (1-) Thus:

] )[

1

(

]

[

1

P N

K

c









 (7-6) Rearrange (7-6):

] [

1 ] [

1 ]

[

1

1

P

k P

N





 (7-7)

where

] [

1

N Kc

k  (7-8)

Based on the proposed mechanism the relationship between the surface coverage

of SAMs and the solution concentration [P] was derived  is an equilibrium constant which is determined by many factors (as we will discuss in later part of this chapter)

7.4 Conditions for Formation of 2D crystals

Through numerous experiments carried out in our group, both failed and successful, together with literature studies, the attempt to figure out the conditions for formation of 2D crystals on HOPG was made Based on the equation

] [

1 ]

[

1

1

P N





 (7-7), the value of the coverage  can be used to qualitatively describe the possibility that a certain molecule can form SAMs on HOPG If formation can take place easily, its coverage  tends to be 1 On the contrary, if the

molecule is unlikely to form SAMs on HOPG surface, the coverage  tends to be 0

7.4.1 Concentration

It is very obvious that the possibility of forming SAMs on HOPG is highly concentration dependent

]

[

1

P

k





When the concentration of the molecule increases,

]

[P

k

will decrease, which leads to

the increase in the surface coverage  In our studies of stearic acid SAMs on HOPG surface at room temperature, the possibility of observing SAMs at a lower concentration solution (~10 mg/mL in phenyloctane) was lower than that of 30mg/mL solution Meanwhile the solutions at higher concentrations, e.g 50mg/mL and 80mg/mL, could be observed easily

Because STM scan maximum size is 240nm240nm, it is difficult to determine the surface coverage  precisely with studying such small surface area Combining the experimental observation and equation (7-10), a rough diagram [Fig 7.4] of the surface coverage  vs [P] was proposed

Fig 7.4 Propsed diagram of surface coverage  vs solution concentration [P]: an derived  vs [P] equation based on our proposed model

Besides the concentration, the surface coverage  is also dependent on the equilibrium constants In our model the equilibrium constant  consists of many factors:

] [

1 ]

[

1

1

P N





 (7-7) where [N] is the density of the adsorption sites on the graphite crystal surface; K is the equilibrium constant of reaction (7-1); c is the equilibrium constant of reaction (7-3)

7.4.2 The surface structure of HOPG and [N]

[N] is the density of the kinks on the step of the graphite crystal surface which is the nature of the graphite crystal There are several grades of commercially available HOPG crystals: ZYA, ZYB, ZYD and ZYH [11] The different mosaic angle and lateral grain size of these crystals cause the variation of the density of steps Therefore

the density of the [N] is dependent on the grade of the HOPG In general, surface coverage  will increase when the density of step [N] on HOPG surfaces increases During our STM experiments the HOPG crystals were grade ZYB which was suitable for critical experimental requirements Other grade crystals have not been used in SAMs studies

7.4.3 Equilibrium constant K of adsorbate/surface interaction

7.4.3.1 Molecular size

Consider the equilibrium between the HOPG surface empty sites S, the occupied sites SP, and the particles P in the solution (Refer to Fig 7.2), Langmuir equation is applied:

S + P  SP (7-1) Equilibrium constant K is given by

]

][

[

]

[

P

S

SP

K  (7-2)

The equation (7-1) is the equilibrium between the particles P in solution and physisorbed particles SP on the HOPG surfaces It is obvious that more products SP will be formed when the interaction between the substrate and the adsorbate is stronger Therefore the value of k can be considered as an index of the strength of the adsorbate/substrate interaction The main contribution of the adsorbate/substrate interaction is from the van der Waals forces between the adsorbed molecules and HOPG surfaces

In equation (7-11), surface coverage  increases with increasing in k In other

words, molecules which experience stronger adsorbate/substrate interaction have higher chance to form SAMs on HOPG For example, the monolayers of n-C32H66 which could be easily localized on the HOPG surface, while monolayers of n-C17H36

were not as clear as n-C32H66 [12] This was because n-C17H36 exhibits higher surface mobility than n-C32H66 due to the weaker adsorbate/substrate interaction In previous studies [16, 17] the molecules were substituted with long alkyl chain to increase the molecular size, which in turn increases the interaction between the molecule and substrate and possibility for molecules to form SAMs on HOPG Therefore larger molecule has larger equilibrium constant k of adsorbate/surface interaction

7.4.3.2 Flatness

Not only the molecules size matters, but also does its geometrical structure As shown in Chapter 5, the derivatives of DDPER comprise large side groups have their flat center deformed More than that, the bulky side groups also hindered the major parts of adsorbates from contacting with the substrates In the review by Matzger A.J [7] most molecules are almost planar Therefore the flatness of the sample molecule will increase the equilibrium constant k of adsorbate/surface interaction because of the better surface contact

7.4.4 Equilibrium constant c of adsorbate/adsorbate interaction

Consider the equilibrium between the adsorption site N, ‘unlocked’ adsorbates SP and ‘locked’ adsorbates M based on the model in Fig 7.3:

N + SP  M (7-4) The equilibrium constant c is given by the equation:

]

][

[

]

[

SP

N

M

c (7-5)

Similar to K, the value of c can be considered as an index of the strength of the adsorbate/adsorbate interactions In equation (7-11), surface coverage  increases with increasing adsorbate/adsorbate interaction c Unlike the adsorbate/substrate interactions, which are mainly van der Waals forces, the interactions between the adsorbates arise from several interactions, namely: van der Waals forces, hydrogen bonding, dipole-dipole interaction, and etc

7.4.4.1 Molecular Structures and Symmetry

Linear organic molecules including saturated and unsaturated n-alkanes, n-alkyl acids, and n-alcohols exhibit good ability of forming SAMs on HOPG surfaces The linear molecules with longer alkyl skeleton are more stable on HOPG surfaces than analogs with shorter alkyl skeleton [12] While for molecules with same length, their structural difference induces the difference in the possibility of SAMs formation For example, either stearic acid (n-C17H35COOH) or elaidic acid (n-C17H33COOH) can form stable SAMs on the HOPG surface, but SAMs formed by oleic acid (n-C17H33COOH) can not be observed at room temperature [18, 19] Elaidic acid is a trans fatty acid while oleic acid is its cis isomer In 3D space, the elaidic acid crystal [17] consists of straight aligned acid molecules Its melting temperature at one atmosphere is 44°C For its cis isomer - oleic acid, the crystal consists of bended