Synthetic Metals 144 (2004) 285–289
The preparation of nanograin Ag–TCNQ thin films by vacuum evaporation with post-heat treatment Qun Zhang∗ , Weijun Wang, Gangfeng Ye, Xuejian Yan, Zhuangjian Zhang, Zhongyi Hua Department of Materials Science, Fudan University, Shanghai 200433, China Received 14 July 2003; received in revised form 5 March 2004; accepted 4 April 2004 Available online 25 May 2004
Abstract Ag–TCNQ organometallic complex with single phase and uniform grains in nanometer scale were prepared by vacuum evaporation and post-heat treatment. The grain size of the films was decreased by introducing the post-heat treatment process to about 50 nm. By applying a ramp voltage onto the film through STM probe tip in air at room temperature, the film will transfer from high impedence to low impedence at about 2.0 V. A writing dot of about 70 nm in diameter, corresponding to the low impedence state, was obtained after applying a pulse voltage of 5.0 V in amplitude and 5.0 ms in duration. © 2004 Elsevier B.V. All rights reserved. Keywords: Electrical bistable; Organometallic complex; Nanometer-scale grains; Heat treatment
1. Introduction There is a rapidly growing interest in the molecular electronic materials with electrical bistable functions for the potential applications in switching devices and ultrahigh density memory devices. Many researches focused on the organometallic charge transfer complexes of metal (M = Ag, Cu, etc.)-7,7,8,8-tetracyanoquinodimethane (TCNQ), because of their reversible and rapid bistable electrical switching from a high impedance to a low impedance state in the films, where TCNQ acts as an electron acceptor molecule with metal as an electron donor. Potember et al. ascribed the low impedance in the “on state” to the formation of neutral non-stoichiometric TCNQ◦ (neutral TCNQ molecules), and Raman band of TCNQ◦ was detected [1–4]. Meanwhile, ever since the invention of the scanning tunneling microscopy (STM), with its ability to image surfaces at the atomic scale, many attempts on applying the technique to perform ultrahigh density data storage were made. A wide variety of schemes have been demonstrated for using the STM/AFM to modify as well as to image surfaces with spatial resolution ranging from 1 to 100 nm [5], though their readout speeds are still lower from the practical speed (faster than 1 Mb/s). Therefore, it is necessary to investigate
the bistable state transition behavior of M–TCNQ films modified by SPM. In order to realize the electrical switching in extremely small area or the bistable state transition in ultrahigh-density data storage uniformly, the basic requirement is to prepare thin films with smooth and uniform surfaces. It was reported that M–TCNQ films prepared by wet-chemical method are polycrystalline, with grain size in micrometers [6,7]. Grain size in Ag–TCNQ films prepared by conventional vacuum thermal deposition is larger than 150 nm [8]. Therefore, it is important to prepare M–TCNQ films with fine grains in nanometer scale. By using vacuum evaporation and post-heat treatment, Ag–TCNQ films with single phase and uniform nanograins were successfully prepared, and bistable transition phenomenon between high and low impedence states were observed. In this paper, The vacuum evaporation and post-heat treatment conditions for preparing Ag–TCNQ films are reported. The post-heat treatment effect on the grain size of the film is demonstrated and discussed, and the electrical bistable transition property is investigated by STM.
2. Experimental 2.1. Film preparation
∗
Corresponding author. Tel.: +86-21-6564-2642; fax: +86-21-6564-2682. E-mail address:
[email protected] (Q. Zhang). 0379-6779/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2004.04.012
Ag and TCNQ films are prepared by conventional vacuum evaporation on HOPG and ITO-coated glass substrates kept
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at room temperature. Ag was evaporated by molybdenum boat and its evaporation rate was controlled at 0.1–0.6 nm/s, TCNQ was evaporated by quartz crucible and its evaporation rate was controlled at 0.35–0.5 nm/s. Ag–TCNQ films were obtained by evaporating Ag first, and then evaporating TCNQ, finally through complexation reaction by heat treating the films in air at 80–150 ◦ C for 5–60 min. 2.2. Measurement methods Transmittance curves at visible light region were measured by transmittance spectrometer set up by our laboratory. The crystalline structure of the films were examined by a Cu K␣ X-ray diffractometer (Rigaku D/MAX-rB). The chemical states of oxygen and nitrogen between the as-deposited and heat treated Ag–TCNQ films were checked by X-ray electron spectroscopy (LH SKL-12). The morphology of the films were evaluated by atomic force microscopy (PSI AutoProbe CP). Measurement of the current–voltage characteristics of the films were performed in air at room temperature by STM (AJ Nanoview II), with signal generator (HP Agilent 33120A) and oscilloscope (HP Agilent 54622A) externally connected.
3. Results and discussions
TCNQ layers and thereby the Ag–TCNQ structure film be formed. The X-ray diffraction investigation of the films formed on HOPG before and after heat treatment was performed. Fig. 2(a) shows the diffraction pattern of the as-deposited Ag and TCNQ double layer film. Three peaks are observed, in which 2θ = 25.78◦ and 27.51◦ (triangle marked) are the diffraction peaks of TCNQ, while peak at 2θ = 38.12◦ (square marked) corresponding to the diffraction of Ag (1 1 1). It indicates that the as-deposited film is TCNQ and Ag double-layer structure. After heat treated at 120 ◦ C for 30 min, the two peaks of TCNQ disappeared, instead of circle marked peaks corresponding to the Ag–TCNQ [9], as shown in Fig. 2(b), Indicating that heat treatment promote the complexation reaction between the TCNQ and Ag and thereby forming the Ag–TCNQ crystalline structure film. The result of Ag (1 1 1) peak still existed in Fig. 2(b) makes it clear that the mol ratio of TCNQ:Ag is not 1:1 in the sample, there existed excessive Ag. Fig. 3 shows the XPS spectra of O-1s of as-deposited and heat treated films. It is known that the peak at 530.5 eV should be attributed to the dissolve or disordered oxygen in the lattice of TCNQ or Ag–TCNQ, and the peak at 533.0 eV is mainly due to the contribution of OH groups. From the
Intensity (arb. unit)
Ag film of about 2 nm in thickness and TCNQ film of about 36 nm in thickness were prepared on glass substrate, respectively. Corresponding transmittance curves at visible light range are shown in Fig. 1(a) and (b). The transmittance curve of first Ag second TCNQ double layer as-deposited film is shown in Fig. 1(c). Obviously, the transmittance curve is the product of curve (a) and curve (b). After heat treated at about 120 ◦ C in air for 30 min, the transmittance curve (Fig. 1(d)) becomes quite different from that of Fig. 1(c), there is a transparent window from 450 to 550 nm, showing green–yellow colour, which is similar to that prepared by chemical method [6]. It implies that the complexation reaction be occurred between Ag and
Fig. 2. XRD diagram of Ag–TCNQ films. (a) As-deposited Ag and TCNQ double layer film (b) Ag–TCNQ film after heat treated at 120 ◦ C for 30 min (䉱) TCNQ (䊏) Ag and (䊉) Ag–TCNQ.
as-deposited heat-treated
O 1s
200
100
540
Fig. 1. The transmittance curves at visible light range. (1) 2 nm thick Ag film (2) 36 nm thick TCNQ film (3) 2 nm Ag film + 36 nm TCNQ film and (4) Ag–TCNQ film after heat treated at 120 ◦ C in air for 30 min.
535
530
525
Binding Energy (eV) Fig. 3. XPS spectra of O-1s for as-deposited and heat treated film.
Intensity (arb. unit)
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(a)
287
N 1s as-deposited post-heat-treated
405
400
395
Binding Energy (eV) 2
Intensity (arb. unit)
200
(b) 2.6eV 3 4 2.0eV 1
1
2 0
405
400
395
Binding Energy (eV) Fig. 4. XPS spectra of N-1s for Ag–TCNQ film (a) as-deposited and post-heat treated curves, and (b) the fitted components of N-1s spectrum for post-heat treated Ag–TCNQ film.
curves we know that there is no obvious changes in oxygen chemical state or content. The oxygen detected in the films might exist on the surface of the sample. Meanwhile, there is no clear peak at 528.5 eV, which is typically observed in transition metal oxides. In case of oxidation brought by heat treatment, a peak at there should be observed. Combining the XRD result of no mixture diffraction peaks, we know that heat treatment process in the air only play the role of promoting the reaction to form the Ag–TCNQ complex and re-crystallization. For comparison, Fig. 4(a) shows the N-1s spectra of as-deposited and post-heat treated Ag–TCNQ films. The spectrum of heat treated sample broadens at low binding energy side, which implies that new component caused by electrons transfer from Ag to TCNQ existed. The new broadened peak can be fitted by two main peaks and their corresponding shake-up peaks, two of them are originally existed at 399.1 and 401.7 eV, with a interval of 2.6 eV, while the other two of them are existed at low binding energy side of 398.1 and 400.1 eV, with a interval of 2.0 eV, as shown in Fig. 4(b). This fact indicated that the electron transfer from Ag to TCNQ has happened, which causes the change of chemical state of N atoms in the film surface.
Fig. 5. Images of Ag–TCNQ (1 × 1 ) films formed on HOPG, taken at atomic force of 1 nN. (a) As-deposited film (b) are the films heat treated at 120 ◦ C for 15, 30 and 60 min, respectively.
Also it confirms that the bonds between Ag and TCNQ be near cyan end. Fig. 5 shows the 1 × 1 morphologies of Ag–TCNQ films prepared on HOPG, about 50 nm in thickness, taken by AFM at atomic force of 1 nN. Fig. 5(a) and (b) are the images of two films, one is as-deposited and the other is post-heat treated at 120 ◦ C for 30 min, respectively. It was found that as-deposited film exhibits large grain size of several hundreds of nanometre magnitude and thus rough surface, ascribed to the TCNQ molecule groups. After heat treated, the film showing the uniform and nano-flat surface was obtained, with grain size in order of film thickness of about 50 nm. It implies that the complexation reaction between bottom Ag layer and top TCNQ layer was occurred under the post-heat treatment condition, which causing the Ag atoms move into TCNQ molecule groups and forming Ag–TCNQ molecules, changing the chemical state of nitrogen atoms as detected by XPS and reducing the grain size of the molecule groups simultaneously. The line and region analysis results in Fig. 5 was concluded in Table 1. In-line measurement, the Rp–v gives the maximum peak-to-valley distance within the selected A–A height profile. The Rp–v value is 45.4 nm for as-deposited film, whereas 3.66 nm for post-heat treated film. In surface
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Table 1 The analysis results of height profiles and region measurements for as-deposited and post-heat treated Ag–TCNQ films Sample
Line
Height (nm)
Rp–v (nm)
RMS (nm)
As-deposited Post-heat treated
A–A
in Fig. 4(a) A–A in Fig. 4(b)
21.2 0.209
45.4 3.66
13.5 0.513
Sample
Region
Ave rough
Rp–v (nm)
RMS (nm)
As-deposited Post-heat treated
1 × 1 in Fig. 4(a) 1 × 1 in Fig. 4(b)
68.0 4.50
10.6 0.506
region measurements, the root-mean-squared (RMS) roughness is given by the standard deviation of the data, determined using the standard definition. For the surface roughness of 1 × 1 area shown by Fig. 5, it was found that RMS of as-deposited film is about 10.6 nm, while that for post-heat treated film is about 0.5 nm. For a certain film thickness, there exist an appropriate post-heat treatment condition to complete the complexation reaction. Shorter heat treatment time makes complexation reaction in the films insufficient, while longer heat treatment time makes Ag–TCNQ molecules agglomerate and crack. In this work, we also found that the higher post-heat treatment temperature will cause the TCNQ films evaporation, and lower post-heat treatment temperature causes the complexation reaction insufficient. The appropriate post-heat treatment parameters are 120 ◦ C and about 30 min for about 50 nm thick films. In order to measure the electrical properties of Ag–TCNQ films, a ramp voltage of 0–5.0 V was applied to the STM probe tip and the I–V characteristic curve was recorded in Fig. 6. From the curve we know that when the voltage is less than 2.0 V, the tunneling current was small and increased slightly with the ramp voltage increasing, showing the film is in the high impedence state. After the voltage becomes larger than 2.0 V, the tunneling current increased rapidly with the ramp voltage increasing, which indicates that the film at the very small area transferred from high impedence state to the low impedence state, and there exist a high and low impedence state transition threshold voltage of about 2.0 V.
Fig. 6. I–V characteristic curve of the heat treated Ag–TCNQ film, obtained when applying a ramp voltage of 0–5.0 V onto the STM probe tip.
8.24 0.386
Because the ramping voltage was applied from external circuits, and at the same time the feedback circuit of the STM was stopped, the distance between the probe tip and the sample surface was kept unchanged. Therefore, the bistable state transition phenomenon from high impedence to low impedence is the intrinsic property of the Ag–TCNQ films. Fig. 7(a) shows the STM image of the heat treated Ag–TCNQ film, obtained in the constant current mode with the bias voltage of 50 mV and tunneling current of 0.5 nA. By means of applying a pulse voltage of 5.0 V in
Fig. 7. The STM image of the heat treated Ag–TCNQ film (a) obtained in the constant current mode with the bias voltage of 50 mV and tunneling current of 0.5 nA. (b) A bright writing dot of about 70 nm in diameter was obtained by means of applying a pulse voltage of 5.0 V in amplitude and 5.0 ms in duration, corresponding to the low impedence state was obtained, as shown on the left of the image. The bright writing dot in size of about 100 nm × 200 nm on the right of the image was obtained after applying the same pulse voltage and moving around the probe tip.
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amplitude and 5.0 ms in duration, a writing dot of about 70 nm in diameter, corresponding to the low impedence state was obtained, as shown on the left of the image in the Fig. 7(b). The large bright writing dot in size of about 100 nm × 200 nm on the right of the image of Fig. 7(b) was obtained after applying the same pulse voltage and moving around the probe tip. Thus, the electrical bistable transition phenomenon of Ag–TCNQ film was confirmed. 4. Conclusions Ag–TCNQ organometallic complex film with nano grain size of about 50 nm and single phase was prepared by conventional vacuum evaporation and post-heat treatment. There exits an appropriate heat treatment condition corresponding to the thickness of the film. When applying a ramp voltage of 0–5.0 V, the bistable state transitions from high impedence to low impedence phenomenon was observed, and the writing dot of about 70 nm in diameter was recorded. Acknowledgements The authors are grateful to Prof. Q.J. Zhang, Mr. L.Z. Kong for XPS measurements and valuable discussions.
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