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Effects of Single Atom N-substitution in Molecular Skeleton on Fabricated Film Quality and Memory Device Performance Cheng Zhang, Yang Li, Qi-Jian Zhang, Hua Li, Qing-Feng Xu, Jing-Hui He, and Jian-Mei Lu Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01345 • Publication Date (Web): 14 Feb 2018 Downloaded from http://pubs.acs.org on February 25, 2018

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Crystal Growth & Design

Effects of Single Atom N-substitution in Molecular Skeleton on Fabricated Film Quality and Memory Device Performance Cheng Zhang, Yang Li, Qijian Zhang, Hua Li*, QingFeng Xu, Jinghui He, Jianmei Lu*

College of Chemistry, Chemical Engineering and Materials Science, Collaborative Innovation Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215123, P. R. China

KEYWORDS: conjugated small molecules, annealing, single atom substitution, ternary memory device performance.

ABSTRACT: In this paper, two conjugated organic small molecules, NITP and NITZ, were designed and synthesized. NITZ is just altered by single atom N-substitution from NITP thiophene group to thiazole group in heterocyclic aryl ring of the skeleton. The morphology and intermolecular stacking style of the fabricated nano-films were analyzed by atomic force microscopy (AFM), grazing-incidence small-angle x-ray scattering (GISAXS) and x-ray diffraction (XRD). It demonstrates that both molecules prefer highly ordered packing after annealing and the crystallite orientation is even more superior by N-substitution. As a result, both memory devices exhibit ternary memory performance. This study highlights that the thiazole group 1 ACS Paragon Plus Environment

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also constitutes a good candidate for π-bridge of organic semiconducting molecules and it really provides a simple and refined strategy for designing multilevel memory materials by single atom substitution.

INTROUCTION Nowadays, the ultra-high density data-storage (UHDDS) researches have attracted much attention for the advent of information explosion era. Several storage methods have already achieved great progress, such as ferroelectric storage storage

5,6

, electric storage

1-4

, phase change

7,8

, etc. In all of these fields, organic molecules have

achieved much more attention due to their excellent characteristics such as high density, low-cost, easy-processing, flexibility and reproducibility

9,10

. Up to now,

modifying molecular structure has been proved a useful strategy to optimize memory performances

11,12

. Many organic molecules have been designed and studied, for

instance, by tuning conjugation length or planarity, the number of electron donating (donors) groups and withdrawing (acceptors) groups

13-16

. However, these strategies

are normally accompanied by many other variables, such as molecular planarity and dimension, molecular polarity, band gaps, intermolecular stacking style or forces, and etc. These multiple variables are confused to set up the clear relationship between the molecular structure and the device performance, which is perhaps contrary to our original intention of controlling a single variable to improve one aspect of device performance. Therefore, decreasing variables in the whole device system is helpful to establish clear relationship between molecular structure and device characteristic. Several studies about single-heteroatom substitution in our group have been carried 2 ACS Paragon Plus Environment

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out to investigate memory mechanisms, and have found that the memory properties could be switched from binary to ternary, from dynamic DRAM to SRAM or from high threshold voltage to lower threshold voltage

17-19

. However, these changes were

restricted to the chalcogen elements, such as O, S and Se, which would especially lead to different atomic radius and molecular polarity.

Figure 1. Chemical structures (top) and GGA-predicted (unrestricted BLYP/DNP level) conformations of a) NITP and b) NITZ in front view (middle) and side view (bottom) illustrating the torsional backbone twist

Herein, two conjugated organic small molecules, named NITP and NITZ (Figure 1a), were designed with naphthalene imide and cyano groups as two different electron acceptors. Thiophene group as internal π-bridge has been investigated a lot for its several advantages, such as optimization of the molecular conjugate skeleton, tuning of the molecular energy levels, and improvement of optical and electrical properties20-22. NITZ is just altered by single atom N-substitution from thiophene 3 ACS Paragon Plus Environment


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group into thiazole group in heterocyclic aryl ring of the skeleton. Because carbon and nitrogen atoms have similar atomic radius, so this change won’t induce huge variable in molecular spatial configuration, they almost have the same planarity according to the molecular simulation (Figure 1b). Therefore, any intermolecular stacking variables and device performance changes should be ascribed to the single atom substitution excluding the spatial configuration effect. Thiazole unit has already been reported in the respect of lower the HOMO and LUMO energy levels effectively by the electronegative imine nitrogen (C=N−C) in the penta-heterocycles compound23-29, which has promise to replace thiophene unit as a superior π-bridge in semiconducting material. To create efficient percolating pathways for hole and electron transport and leading to low operating voltages of memory device macroscopically, annealing was utilized to improve intermolecular organized stacking and film morphologies 30-32.

RESULTS AND DISSCUSSION Thermogravimetric analysis (TGA) was used to explore the thermal stability of the two molecules (SI， Figure S7). The thermal decomposition temperatures of 5% weight loss of NITP and NITZ were 261 ℃ and 278 ℃, respectively, indicating the molecules are stable enough for future practical applications. Obviously, the additional N-atom has begun to play an effective role on the thermo-stability. We speculate that the better thermo-stability is arising from the promoted intermolecular stacking.

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Figure 2. UV-Visible absorption spectra of the two compounds in CH2Cl2 solution and in film: (a) NITP; (b) NITZ

Figure 2 shows the UV-Visible absorption spectra of the two compounds in CH2Cl2 solution and in film. The maximum absorption values of NITP and NITZ films were observed red-shift according to their own spectra in solution, while the absorption values of NITP and NITZ of the annealed films were both blue-shifted by 9 nm and 15 nm. These results reveal that the molecules happened self-assembling from inter-molecular J-aggregation (head-to-tail) to H-aggregation (face-to-face) after annealing ，

which is advantageous to acquisition of higher charge carrier

mobility33-36. The side-by-side molecular H-type aggregation benefited to obtain more uniform structure and produce large area π-stacking between adjacent molecules. Besides, the absorption region broadened in film states compared with that in solution, manifesting that the intermolecular interactions were enhanced in the film state37-40. The blue-shift of the NITZ annealed film is larger than that of the NITP annealed film, which is mainly attributed to the better delocalization of electrons by inducing the electronegative imine nitrogen (C=N−C). According to UV-visible absorption spectra and cyclic voltammetry (CV) curves (SI，Figure S8), the molecular 5 ACS Paragon Plus Environment


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bandgaps and the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were calculated (SI, Table 1). These results indicate that thiazole unit lowered both HOMO and LUMO energy levels after the substitution of an atom in an extended π -system with a more electronegative atom. X-ray diffraction was used to further detect the intermolecular stacking changes before and after film annealing. As shown in Figure 3a, the annealed film of NITP appeared two distinct diffraction peaks at 4.92o and 9.79 o, corresponding to the feature molecular layer distances by 17.9 Å and 9.0 Å, respectively. Similarly, NITZ annealed film had multiple diffraction peaks at 4.63 o, 9.31o and 14.0 o, which could be corresponding to the spacing distances of 18.7 Å，9.5 Å and 6.3 Å. These results further illustrate that both molecules happened reassembly and stacked layer-by-layer during the thermal annealing process. The d-spacing of NITZ was larger than that of the NITP film and the diffraction peak of NITZ was at 21.0 o corresponding to the d-spacing of π-π stacking by 4.2 Å, which attributed to highly efficient pathways for the charge mobility and good memory performance. Thus the reason that thiazole group improves the efficiency of intermolecular stacking and has better long-range ordered arrangement is attributed to the single atom N-substitution. Grazing-incidence small-angle X-ray scattering (GISAXS) analysis was applied to reflect the crystalline properties of films. As shown angle of the incident beam (αi) was 0.20° X-ray

beam

respectively.

in

impinged

Figure 3d, by

the

the deflection monochromatic

Figure 3 (b, c) showed the clear diffractions signal in 6 ACS Paragon Plus Environment

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the out-of-plane orientation and the values of Qz were 3.48 Å-1 and 3.38 Å-1

11,41

,

corresponding to the spacing distances 18.0 Å and 18.6 Å respectively. As GISAXS pattern in plane of the compounds shown in Figure 3e, NITZ had scattering peaks at 14.5 Å-1 and 17.9 Å-1 which obviously meant that NITZ had stronger π-π stacking interaction. The GISAXS results were very close to the XRD observed lamellar spacing data which had been calculated above. The XRD and GISAXS dates both show NITZ obtain better ordered arrangement by single atom N-substitution.

Figure 3. (a) X-ray diffraction patterns of the compounds in ITO films; (b,c) GISAXS pattern of NITP and NITZ films cast on ITO film after thermal annealing (The deflection angle of the incident beam (αi) was 0.20°); (d) NITZ Schematic diagram of GISAXS; (e) GISAXS pattern in plane of the compounds in ITO films

Because the surface morphology of the film takes an important role in the efficiency of optoelectronic devices, the film morphology was investigated by atomic 7 ACS Paragon Plus Environment


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force microscopy (AFM). The RMS (root-mean-square roughness) surface roughness of the NITP were 2.59 and it became 10.20 after annealing. After the replacement of single atom, the surface morphology f NITZ became smoother with a roughness of 1.84 and it became 5.37 after annealing. Though the roughness of the films became larger after annealing, the crystallinity was improved a lot (SI, Figure S10) to help to enhance the mobility of the charge carriers. The annealed film of NITZ obtained more ordered particles aggregated morphologies than that of NITP. The researches of nanostructure and film morphology imply that the fabricated annealed film quality is strongly affected by the single atom. In order to have further study on the effects of single N-atom in molecular skeleton, Fourier Transform Infrared spectroscopy (FI-IR) was carried out. As shown in Figure 4, the absorption peaks of NITP film had blue-shift by 4.2 cm-1 compared with that in powder state, which were ascribed to hydrogen bonding interaction of cyano groups, while the absorption peaks of NITZ film had blue-shift by 5.7 cm-1. The larger blue-shift reflects that thiazole unit with aryl group triggerred the formation of C_H…N excluding cyano groups42-44. Hence, the intramolecular H-bonds are formed after inducing C=N−C which has become an important factor for producing different properties between NITP and NITZ.


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Figure 4. Infrared spectrums tested by films and powder state of the two molecules: (a) NITP; (b) NITZ

I-V characteristics of the sandwiched ITO/Organic/Al devices were measured. The devices of the molecules without annealing showed binary performance attributing to amorphous arrangement of molecules (SI Figure S9). Interestingly, the annealed devices showed ternary memory behaviors (Figure 6) which conformed to the molecular design intention of two traps leading to ternary performance. At the first sweep (sweep 1) of Figure 6a, the current was increased from zero under the voltage from 0 V to −5 V, the compound NITP based devices was at a low conductive state marked with (OFF, ‘‘0’’) state; The current had a sharp transition from 9.8×10−6 A to 9.9×10−5 A when the voltage comes to −1.98 V, this state was called as intermediate conductive (ON1, ‘‘1’’) state; The second transition from 3.39×10−4 A to 0.01 A comes up with the pressure of −4.20 V, finally switched to the high-conductivity (ON2, ‘‘2’’) state. A voltage sweep from 0 to -5 V was applied in the sweep 2 keeping the device on the high-conductivity. The sweep 3 and 4 were taken on



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reversed scanning, they remained in the ON2 state. The device of ITO/NITP/Al exhibited typical WORM (Write-Once-Read-Many times) ternary memory behavior. Similarly, the other device based on NITZ was also investigated and it showed the similar behavior compared of NITP. Though the devices based on the two molecules both showed the ternary memory performance, the Ion/Ioff ratio of NITP and NITZ were 1:10:103 and 1:102:105. The Ion/Ioff ratio of NITZ was significantly improved after the single-atom substitution and would be more favorable for practical application. After the modification from thiophene group to thiazole group in molecular structure, the average ON1 state and ON2 state switching threshold voltage of NITZ were lower and more stable than NITP by a set of data statistics (Figure 5b), respectively. Thus, the power consumption could be decreased after single atom N-substitution. The retention time of the memory devices in the three conductance states are shown in Figure S13, the NITZ-based device remained more stable than NITP after a series of data writing operations and could obviously read “OFF” “ON1” and “ON2” state from the devices. These results strongly proved that single atom N-substitution in molecular skeleton has great effects on memory device performance.


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Figure 5. (a) Current-voltage (I-V) characteristics of sandwich memory devices after annealing; (b) The statistical data of ON1 state switching threshold voltages (Vth1) and ON2 state switching threshold voltages (Vth2) based on NITP and NITZ devices EXPERIMENTAL SECTION

Figure 6.

The AFM patterns of the organic compound thickness in the memory

device (The scan size for the image is 10 ×10 um2) The specific scale ITO glass was pre-cleaned by ultra-sonicating for 20 min each in deionized water, acetone, and ethanol, in sequence. The molecule was fused and vaporized onto the glass in high vacuum evaporation equipment of JZ-ZF400 forming 130-thickness layer (Figure 6). The 100 nm thickness Al layer as the top electrode was thermally evaporated onto the organic surface through the shadow mask under the pressure of about 10-9 bar. The shadow masks are all customized and the area of each circular aperture is 0.0134 mm2. Then, the films were gone through the process of thermal annealing treatment at 80℃ for 12 h in vacuum oven. Eventually, all the films were fabricated into ITO/Molecule/Al sandwich memory devices. CONCLUSIONS



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In general, two organic small molecules have been designed and synthesized with a signal atom difference from C to N. The electronegative imine nitrogen (C=N−C) was introduced in respect of the lower HOMO and LUMO energy levels effectively. The backbone coplanarity was improved little showed by DFT calculation, and molecular internal molecules packing and crystalline orientation were enhanced by annealing which was beneficial for the establishment of efficient pathways for charge transport. Thus, thiazole-based device gained lower threshold voltages and more stable performance. Besides, our work also demonstrated that the thiazole ring compared with thiophene ring constituted a good candidate for conjugating π-bridge to build efficient organic semiconducting material. The simple and efficient strategy of designing new materials or improving the electrical properties of OMD by single atom substitution would be pushed forward in future work. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on Materials，the instrument of measurements, synthesis and characterization of 1H NMR spectrums and

13

C NMR spectrums, optical and electrochemical properties of

the two molecules, X-ray diffraction patterns of NITZ and NITP unannealed films, infrared spectrums of the two molecules, proposed mechanism of memory effect（PDF） AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. 12 ACS Paragon Plus Environment

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*E-mail: [email protected]. ORCID Jianmei Lu: 0000-0003-2451-7154 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. REFERENCES (1)

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Effects of Single Atom N-substitution in Molecular Skeleton on Fabricated Film Quality and Memory Device Performance Cheng Zhang, Yang Li, Qijian Zhang, Hua Li*, QingFeng Xu, Jinghui He, Jianmei Lu** Table of Contents

Novelty: Molecule NITP was changed to molecule NITZ by only one N-atom substitution in whole molecular skeleton, the sandwiched device based on molecule NITZ exhibited lower power consumption and more stable ternary data-storage performances. This result indicates that

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the proper alternative of atom in molecular structure would be a useful strategy for promotion of organic semiconductor material performances.

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Effects of Single Atom N-Substitution in the Molecular Skeleton on

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