In the last part of the thesis, we studied the growth of 4-DMAPENI molecules on SiO2 surfaces. Basically, we found that the growth of such structures depends on the diffusion of molecules.
Introduction
- Organic Semiconductors
- Fundamental of Thin Film Growth
- Inorganic Thin Film Growth
- Organic Thin Film Growth
- Organic Molecules
- References
Due to the instability of the organic molecular stack, these types of structural formations are observed. As the molecules diffuse upward, the local surface slope of the surface structure changes.
Experimental techniques and data analysis
Growth Techniques of self-assembled
- Substrate Preparation
- Thin Film Growth Techniques
- Evaporation of organic molecules
2.3 (a) Schematic diagram of Knudsen cell together with DN40CF flange, thermocouple and electrical connection, (b) Different components of home developed Knudsen cell and its total structure. The substrate was positioned so that the surface was normal to the direction of the molecular beam.
Characterization Techniques
- Structural Characterizations
Scanning Tunneling Microscopy (STM)
2.5:(a) Schematic diagram of the STM operation (b) Energy diagram of the tunnel contact of the STM tip and the metal sample. According to the lateral resolution, 90% of the tunneling current flows through the gap between the last atom of the tip and the atom of the surface.
Atomic Force Microscopy (AFM)
In constant height mode, the special variation of the cantilever is directly used to generate the topographic image, because the height of the scanner is fixed. In the non-contact mode, the detected signal comes from the amplitude variation of the cantilever vibrating near the sample surface.
X-ray Scattering
Homogeneous or uniform elastic strain in the (hkl) direction can also be calculated from the shift in the diffraction peak positions and the dhkl spacing of the unstrained crystal. X-ray diffraction patterns of the nanostructures were obtained using a commercial XRD setup (TTRAX III, RIGAKU 2500) using a Cu Ka1 (λ = 1.5406 Å) radiation with nickel filter. All data were taken at room temperature in thin-film mode of the XRD at a sweep angle of 4°.
Here r0 is the classical electron radius Å), ρe is the electron density of the medium, μl is the linear absorption coefficient of the incident photons in the medium.21, 22. 2.11(a), the reflected intensity can be calculated by applying the boundary condition for the electric and magnetic fields at each interface. The oscillations are the result of constructive interferences between the reflected waves at the interfaces, and their period gives the thickness 𝐷𝐷𝑗𝑗, of the respective layer.
Therefore, GIDX is a surface-sensitive diffraction method, and by varying the angle of incidence, the probed depth of the material can be adjusted. schematic representation of grazing incidence geometry is shown in Fig. c) and (d) show a side and top view, respectively, of the GIDX geometry.
Field-Emission Scanning Electron Microscopy…
Because the instrument operates in high vacuum (less than 10-7 mbar), there is no scattering along the column, which helps prevent discharges within the instrument. These types of electron emitters can produce up to 1000 times the emission of a tungsten filament. A secondary electron detector is placed close to the sample and provides a morphological image of the sample.
A secondary electronic detector is attached to the objective lens (in-lens detector), which distinguishes the image from the conventional image.
Transmission Electron Microscopy
- Spectroscopic Measurements
After the electrons leave the electron gun, they are collimated by electromagnetic condenser lenses, focused by objective lenses. FESEM is also equipped with a special objective lens that projects the magnetic field under the lens. As in X-ray diffraction, the electron diffraction patterns are spot patterns from monocrystalline films, ring patterns from randomly oriented crystallites with fine grains, and superimposed ring and spot patterns from polycrystalline films with larger grains containing some preferred orientation (textured films).
A camera consisting of a yttrium aluminum garnet (YAG) scintillator connected by optical fiber to an image intensifier, connected to a high-resolution television tube camera, provides a total magnification of 30 million (microscope magnification~ 1.5×106 and the camera magnification: 20). Due to a limited penetration depth of electrons in solids, the sample should be very thin: the acceptable thickness is about 100-1000Å. We used top view TEM and high resolution TEM (HRTEM) and selective area electron diffraction (SAED) pattern for the work presented in this thesis.
In this section, we briefly discuss the instruments used for the spectroscopic measurement of the sample.
UV-Vis Spectroscopy
- Introduction
- Experimental
- Clean Surfaces of Silicon
- Si(111) surface reconstruction
- Growth of Ag on Si
- Results and Discussions
- Morphological Analysis
- Statistical analysis of surface morphology
- Conclusions
- References
The electron density profile due to fitting is shown in the inset of the graph. In the analysis process, we have predicted different scaling exponents for the electronic growth. Growth exponent (β) and dynamic exponent (1/z) are calculated from the slope of the a and b curve respectively.
PTCDI-Ph on SiO2 Surfaces: Growth
Introduction
Furthermore, these molecules are the building blocks of low-cost, high-performance organic devices1-4 such as field-effect transistors5, solar cells6-9, light-harvesting arrays10, 11 and light-emitting diodes12-14. Out of all π-conjugated organic semiconductors, perylenetetracarboxyldiimides (PTCDI) and their derivatives have attracted more attention in recent years for their extraordinary thermal and photochemical stability15, 16 It has also been used as optical switch17, photoreceptors, chemical sensors,18 etc. Due to weak π−π interaction between molecules, the packing of molecules within the active channel of the devices has been very important to increase the charge carrier mobility.
Therefore precise control over morphology and other properties are the main challenging issues in film growth and. One can adjust their HOMO-LUMO gap by attaching different functional groups to the core.21 On the other hand, the diffusion of molecules plays a very important role in self-assembly. In this chapter, we have discussed the self-assembled growth of the perylene-diimide derivative, N,N´-di-phenyl perylene diimide tetracarboxylic (PTCDI-Ph) of long terraces on SiO2 substrate.
Structural, electrical and optical properties of the films grown at different substrate temperatures are studied.
Experimental
Results and Discussions
- Studies of surface morphology
- Structural Characterizations
- Optical Characterizations
5.2: (a) AFM image of the terrace (b) Line scan profile of different layers of the flat top strip. The calculated activation energy for increasing the length (El) and width (Ed) of the terraces is found to be 0.21 eV and 0.34 eV, respectively. To study the vertical growth of terraces with substrate temperature, we calculated the roughness width (w) from the sample grown at different substrate temperatures.
We observed Bragg peaks up to the third order, indicating a multilayer structure with a well-ordered vertical arrangement of molecules in the films. The molecules are connected laterally by the width of the terraces, which is significantly weaker. As a result, the molecular interaction along the length of the terraces is stronger than along the width.
Higher substrate temperature increases the diffusion of the molecules and possibly the diffusion of small terraces, which merge with the larger terraces.
Absorption property
In this section, we discuss the optical properties of the PTCDI-Ph molecular structure grown on the SiO2 substrate. Two other less intense peaks centered at 553 nm and 588 nm appear in the thin film.
Photoluminescence properties
However, the second peak was not observed for the samples grown at elevated substrate temperature. The ratio of the intensities of these two peaks is calculated and its variation with respect to substrate temperature is shown in figure. It confirms that the state contribution corresponds to 1.93 eV observed at 643 nm and decreases with substrate temperature.
Time-Resolved Photoluminescence properties
- Photoresponse of Molecular Terraces
- Conclusions
- References
Interestingly, it is found that as the substrate temperature increases, the highest contributing component of the decay constant (τ1) is increasing. This means that the density of state corresponding to the 640 nm emission is decreasing with increasing substrate temperature. From the nature of the curve it is clear that up to ~2 volts in both cases (Dark and Photo), the current is very small.
A typical photocurrent spectra of the sample measured at 4V bias condition is shown in the figure. This property of the material can be used to design photodetector in the visible region. The photoresponse of the PTCDI-Ph film measured under excitation by 475 nm light is shown in Fig.
This observation confirms that molecular interaction along the length of the terraces is stronger than along the width.
Self-assembled growth of organic ribbons
- Introduction
- Experimental Details
- Results and Discussions
- Study of morphology
- Optical characterizations
- Conclusions
- References
Due to the increase in temperature of the substrate, the diffusion of molecules in the films increases. These molecules can come from cast cells during growth or mass diffusion in ribbons from films. Therefore, the molecules located in the already formed ribbons can diffuse along the length of the ribbons and the length increases.
On the other hand, molecules from the wetting layer can also diffuse to increase the length of the ribbons. The activation energy of the molecule is 0.21 eV, calculated from the slope of the curve. The current is expected to increase further as the length of the ribbons increases.
This may be due to poor intermolecular interaction between molecules along the width of the bands.
Summery and Conclusions
We observed a transition from stationary to non-stationary growth, followed by the formation of a 2D layer into a 3D mound. However, the formation of the TC-PTCDIC8 pile is due to the limited diffusion of molecules by the barrier at the step edges. Ag film growth kinetics were studied at room temperature with different coverages.
It is interesting to note that the vertical growth rate is slower than the lateral growth and in this case too, the rate is found to be 0.02 that of the TC-PTCDI-C8 mound growth. We observed three different roughness scaling exponents, indicating three different growth mechanisms that maintain the same growth and dynamic exponents. From the study of growth kinetics, we calculated the activation energy for this type of growth, which showed a higher activation energy along the width of the terraces than along the length.
In this thesis, we have studied the growth kinetics of various self-assembled structures in different dimensions.
