Elsevier

Vacuum

Volume 162, April 2019, Pages 199-207
Vacuum

The promotion of Indeno [1, 2-b] flourene-6, 12 dione thin film to be changed into stable aromatic compound under the effect of annealing treatment

https://doi.org/10.1016/j.vacuum.2019.01.039Get rights and content

Highlights

  • A vacuum evaporation technique performed films of homogeneous and undissociated.

  • The IR of I[1,2-b]F dione films revealed the deficiency of C═O for annealed films.

  • The effect of annealing is expected to improve the charge-transport ability.

  • The negligible of oxidation layers verifies the performance of high vacuum technique.

  • This material has an excellent predilection to be used as optoelectronic materials.

Abstract

Thin films of Indeno [1, 2-b] flourene-6,12-dione (I[1, 2-b]F dione) were performed by the thermal vacuum evaporation technique. The identifications of a thin film such as structure, molecular structure and surface morphological were determined before and after annealing temperature using Fourier-transform infrared (FTIR), X-Ray diffreaction (XRD) and Scan electron microscopy (SEM). FTIR spectrum showed a diminishing of C═O bonds after annealing treatment. The analysis of XRD clarified that I[1, 2-b]F dione in its powder form is orthorhombic with space group P222. On the other hand, the as-deposited thin films were realized by amorphous nature, while the annealed I[1, 2-b]F dione thin films showed polycrystalline nature. The calculation of the average crystallite sizes proved that the annealed I[1, 2-b]F dione thin films were characterized by their nanostructure property. The SEM images illustrated that the aggregation of the particles has the formation of the nano-rod shape associated with average width in the range of 120–150 nm for the as-deposited thin film, while the average width of the branched nano-rod shape for the annealed thin films was observed to be in the range of 150–200 nm. The transmittance and reflectance spectrum at normal incidence of light in the range 190–2500 nm were employed to discuss the optical properties of as-deposited and the annealed I[1, 2-b]F dione thin films. Our conceptions elucidated that the optical transition type of I[1, 2-b]F dione films is the indirect allowed transition. The values of the fundamental energy gap for the as-deposited and the annealed films at 473 K were calculated out be 3.41 and 3.70 eV respectively. The dispersion of the refractive index was studied according to the single oscillator model. Moreover, the third-order nonlinear susceptibility and the nonlinear refractive index for I[1, 2-b]F dione thin films were calculated.

Introduction

The π-Conjugated small molecules encompass a significant investigated group of semiconductor materials that have attracted intensive attention during the last few decades due to the low-cost, mechanical flexibility and thermal stability [[1], [2], [3]]. Also, the presence of π-Conjugated system assists widely in the absorption of photons, and thus produces photo-induced free carriers of charge [4,5].

Most of the literature were interested in studying the aromatic π-Conjugated molecules since these types of organic materials are distinguished by easy synthesis [6]. Furthermore, these materials are known by their high resistance to the deformation of its molecular structure as it exposed to different surrounding conditions [6]. In particular, the aromatic π-conjugated skeleton preserved mostly its unique molecular structure when it attaches with a peripheral group to form a new type of organic material [7].

In contrast, the anti-aromatic π-conjugated organic compounds are highly unstable and easily reactive. Indeed, the anti-aromatic compounds may achieve stability by changing their shape or either by breaking some of the π-conjugated bonds [8]. One of the examples of anti-aromatic π-conjugated organic compounds is Indeno fluorenes ([1,2-b]IF), which is comprised of three six-membered rings and two five-membered carbocycles [9]. This type of hydrocarbon-scaffold might loss the stability since s-indacene, a molecule is extremely active to react and therefore becomes isolated, is the unique structure of Indeno fluorenes scaffold [8]. On the other hand, it was found that [1,2-b]IF is characterized by its high electron affinity due to the presence of sp2 carbon atoms at 6, and 12 sites of the two five-membered carbocycles (See Fig. 1) [10]. Thus, [1,2-b]IF scaffold can serve as an electron-accepting or ambipolar material for organic electronics [11]. Indeed, the changing of the anti-aromatic five-membered carbocycles of [1,2-b]IF into aromatic carbocycles could be accessible by accepting two extra electrons, to achieve closed-shell structure, and thus both five-membered rings become aromatic, and a 22 π-electron species is generated [12]. Similarly, s-indacene could be reduced, and the resultant anion radical and dianion characterized. In addition, it was observed that the attachment of the carbon atoms (6, 12) of Indeno fluorenes with oxygen atoms had been very recently incorporated in n-type OFETs with very promising performances and stability allowing their incorporations in n-type organic field effect transistor applications [10]. Indeed, the new formation produced from the bridge link of carbon atoms (6, 12) with oxygen atoms, as shown in Fig. 2, is known as Indeno[1, 2-b]flourene-6, 12 dione (I[1, 2-b]F dione).

The literature of Ivan Martinez et al. [10], may help to understand the mechanism of electron transition between the HOMO–LUMO gap of I[1, 2-b]F dione. The key result of their work affirmed that the presence of the highest electron density was observed over the positions 6, 12 of carbon atoms. This result is evidence that the major transition of electrons derives from the contribution of electrons attributed with the specific octet carbons marked at 6 and 12 positions. Thereby, the orbits of the referred carbons (6, 12) are likely preferred (HOMO; π-orbital). Even that, there are two suggested of (LUMO; π*-orbital): the nearby carbon atoms or either the octet oxygen atom, one can easily conclude that the oxygen orbit is the most successful (LUMO; π*-orbital) according to the rules of polarity [13].

To shed more of light on the ability of I[1, 2-b]F dione to preserve its stability under the effect of heating; the annealing procedure could be utilized as a useful tool for this purpose. On the other hand, the annealing treatment provides us with the comprehensive picture about the specific site of the variant changes that could be happened in the skeleton of I[1, 2-b]F dione, and thus to examine the stability of this compound under the impact of heating. Therefore, the physical properties such as linear and non-linear optical properties of I[1, 2-b]F dione are expected to be varied under the effect of annealing treatment. Thus, we believe that the present study will greatly participate in determining the main atoms of I[1, 2-b]F dione which can be easily diminished under the effect of heating. In facts, these diminishing atoms have the willingness to be isolated or probably to react with other free radical molecules. This, in turn, should afford better insight and understanding when designing materials for electron-transporting and optoelectronic applications in devices. Consequently, the present research aims to study the influence of annealing temperature on the structure and optical properties of I[1, 2-b]F dione thin film.

Section snippets

Experiment technique

The powered of I[1, 2-b]F dione was obtained from a well-known company “Sigma-Aldrich company”. Thin films of I[1, 2-b]F dione were fabricated by thermal vacuum evaporating procedure. This procedure involves consequences steps. Firstly, the atmospheric pressure has to be reduced to an acceptable limit (in the order of 10−5 Pa), and HHV Auto 306 coating unit was applied to attain this value of pressure [14]. The system was kept under vacuum for 3 h to guarantee the evaporation of the material

Structural properties

FTIR technique is used as a fundamental and accessible method to identify a molecular structure for organic materials. The FTIR bands and the corresponding assignment spectra of the powder, as-deposited and annealed of I [1, 2-b]F dione thin films in the region of 400–4000 cm−1 have been given in Table 1. From the comparison of the spectra of powder and films of I [1, 2-b]F dione, it was observed similar types of bond mechanism attributed approximately with the same absorption wavelength for

Conclusions

Homogenous thin films of I [1, 2-b]F dione were successfully carried out by using vacuum thermal evaporation technique. The investigation of XRD pattern clarified that I[1, 2-b]F dione in powder form has a polycrystalline nature with orthorhombic structure. The monograph of topographies showed the impact of annealing procedure on the reconfiguration of I [1, 2-b]F dione molecules. Whilst, the increase of the nano-rod width have been result from the variation of the angular rotation between

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