LEED images

When the LEED images were obtained the images were recorded on a digital camera which was mounted on a stand which was in turn mounted on the external flange of the reverse view LEED screen, to maintain the same distance for all images. To extract the spot spacing for the images, line profiles were taken using ImageJ [88]; the diffraction spot spacing were measured as shown in figure 3.1. The measured and textbook bases are equivalent as they can be transformed into each other. The A bases will be along the direction of the spot splitting from the vicinal surfaces, as Pt has a face centred cubic (fcc) lattice structure the A and B bases should be equal. Both A and B bases were collected from all images, with all possible bases were collected from each image and were then averaged to account for any spread caused by errors in the angle of incidence. The spot splitting distance was measured along the A basis direction.

All other photo editing performed on the LEED images was also performed in ImageJ which was limited to cropping and altering colour scales. LEED was also used to align sample to AES experiments as the electron beam is approximately normal to the surface when the undifferentiated spot is on the centre of the screen.

It is necessary to say here that the set up for obtaining the LEED images did undergo some changes during the work. Two different chambers were used during the course of this study with two separate, though same model LEED screens was used. The main difference in the set up was on the manipulators which made exact positioning for the LEED experiments difficult. Even though reproducibility of position in the individual chamber can be achieved, a direct comparison of the LEED patterns is not possible. This is exasperated by one of the drives on one of the manipulators routinely becoming unseated for the manipulator and having to be reseated on several occasions during experiments. However the camera and its mount were not altered during the course of the experiments and it is for this reason that as will be seen later that the spot separations are measured in pixels. As the pixels in all diagrams will be same as measured by ImageJ, all the beam energies were kept at 100 V for all the images in the work.

3.2. Data analysis methods Growth and formation of wires and capping layer

Figure 3.1: Schematic diagram of the LEED images observed with the A (black) and B (red) basis vectors. The solid lines are the LEED basis vectors shown in textbook form and the dashed lines are the bases that were measured due to the electron gun blocking the view of the undifferentiated spots. These are the same as the text book bases as the bases can be translated onto each other through addition of the text book bases. For vicinal surface the spot splitting was measured along the A basis direction.

3.2.2 STM images

All STM images that were obtained and were processed in WSxM [89], though other pro- grams such as Gwyddion [90] could also be used. For all images shown in this study a local plane was taken on the image, the local place was placed on the terraces observed in the STM images to account for the slope observed in the image that did not have any physical meaning apart from the sample angle plane to the x-y plane of the piezo scanner plane. In some of the images line profiles are taken, with the lines chosen to be perpendicular to the step edge, which for the (997) and (13 13 11) surfaces will be along the [11¯2] direction.

The step direction will appear to vary in images; this is due to the rotation of the imaging frame, to ensure that the features that are seen in the images are real and to reduce the noise seen in images.

3.3. Pt(997) Growth and formation of wires and capping layer

3.2.3 AES

AES has the simplest data analysis of all the techniques in this study, the peak positions were observed in the experimental spectra and compared to the peak position from the reference spectra [91] and for the specific elements studied here in appendix C.

3.3

Pt(997)

3.3.1 Introduction

When growth is performed by self-assembly as is the case here; the crystalline order of the substrate surface is very important as it provides the template for all subsequent growth layers. The order of the Pt(997) surface was probed with two methods LEED and STM.

LEED probes the long range order of the surface (of the order of 50 µm due to the electrons coherence), due its nature as a diffraction technique requires many regularly spaced structures and the size of the beam which is of the order ofmm. The expected LEED pattern of the (997) surface is the hexagonal 1×1 pattern seen from the (111) surface but with a spot splitting. As diffraction techniques image the surface in reciprocal space the LEED spots the are separated by large distances are do the lattice spacing of the surface and the spot splitting is due to the order from the regularly spaced steps on the vicinal surface the spot splitting distance is inversely proportional to the interval between the atomic steps [92].

STM investigates the surface electronic structure, as metals are being investigated in this study where the electrons are more free than in semiconductors, so the images obtained from metals will be approximately due to the topography of the surface. The STM images expected for the (997) surface would be a flat terrace regions with a width of 2 nm separated by a step of 2.25˚Aas outlined in figure 1.3.

3.3.2 Pt(997) LEED

The substrate used for the majority of samples in this study is vicinal Pt(997). This surface is made up of Pt(111) terraces each of monoatomic step height, the terrace size is uniform as the step edge surfaces repel each other and mean that terrace widths are prone to being very uniform.

As can be seen can be seen from figure 3.2, the Pt(997) surface is well ordered. The spot splitting seen in the images is caused by the stepped structure; the sharpness of the spots with the presence of spot splitting indicates that the surface is well ordered. The

3.3. Pt(997) Growth and formation of wires and capping layer

Figure 3.2: A representative LEED image of a clean Pt(997) surface at E=100 eV. The spot splitting observed here indicates that there is long range step structure.

spot splitting also indicated that the surface is clean as it has been reported that any impurities cause the step edges to become pinned [93] and will not reach its local terrace order. As LEED gives an image of the surface in reciprocal space the large spacing between the terraces causing spot splitting as shown in table 3.1. The spot splitting is approximately 1/8 of the spacing between the 1×1 spots which indicates that the terrace size being in good agreements as that expected as the spacing of the step structure is 8 time the nearest neighbour distance in Pt.

The LEED images show on the large scale that the Pt(997) surface to be well ordered as demonstrated by the sharp diffraction spots with the presence of the spot splitting.

Sample A(pixels) B(pixels) Splitting (pixels) Clean Pt(997) 285±11 288±1 29±3

Table 3.1: A, and B bases with the observed spot splitting obtained from LEED images from a clean Pt(997) surface.

3.3. Pt(997) Growth and formation of wires and capping layer

3.3.3 STM study of clean Pt(997)

STM was performed on a clean Pt(997) with a representative image presented here, in figure 3.3 the Pt(997) is shown to be well ordered with a regular stepped structure. The regular step structure again indicates the surface is free from impurities.

When an image is obtained on a smaller area as shown in figure 3.4 the terraced size agrees with those reported previously. This is clearer in the profile taken along the line shown in figure 3.4 which was chosen to be perpendicular to the step edge and to be along the [11¯2] direction in figure 3.5. It also shown the step height is monoatomic also in agreement with what is expected from a Pt(997) surface. The terrace width of 2.59 nm with a step hight of 1.8 ˚Athe difference from the ideal values of 2.02nm and 2.45 ˚Arespectively can be due to inaccuracies in the piezo calibration.

3.3. Pt(997) Growth and formation of wires and capping layer

Figure 3.3: 100×100nm image of the

(997) surface showing the regularity of the step structure. Scan parameters V=- 1 V I=.1 nA.

Figure 3.4: 6.5×6.5nmimage of the (997)

surface, the line shown indicates from where the line profile was taken from, and

this should be along the [11¯2] direction.

Scan parameters V=-1 V I=.1 nA.

Figure 3.5: Profile obtained along line shown in figure 3.4, the step structure with monoatomic steps can be clearly shown.

3.4. Pt(111) LEED study Growth and formation of wires and capping layer

3.3.4 Conclusion

As can be shown the Pt(997) surface used in this study are well ordered with a regular spaced stepped surface over both large and short ranges as shown by the LEED and STM respectively. The values compare well to the values that have been reported previously and show that the crystal surfaces obtained are suitable for the self-assembly growth of atomic wires.

3.4

Pt(111) LEED study

3.4.1 Introduction

To study how the Co and Au would form on the Pt(997) surface, the lattice constants of adlayers that were grown on top of a Pt(111) surface was studied. It serves as a reminder that the terraces of the Pt(997) surface are a (111) surface and the adlayers will grow in a similar fashions to the Pt(997) with the Co forming as islands in a layer by layer fashion at defects [94] or as nanostripes at the step edges [47] and the Au growing in a layer by layer fashion [95, 96] with 1 ML islands forming the first layer.

The clean Pt(111) surface was studied along with: Pt(111) with a 2 ML Co layer; Pt(111) with a 5 ML Au coverage; and 5 ML Au/2 ML Co/Pt(111). The Co coverage of 2 ML was chosen so as to allow the Co to completely cover the Pt(111) surface giving a LEED image representative of the Co structure before dislocations form at 3 ML thickness [94]. The 5 ML Au coverage was chosen as it was the capping layer that was used most throughout this work as will be seen later in chapter 5.