2004
J. Phys. Chem. C 2008, 112, 2004-2007
Thermochemical Hole Burning on TEA(TCNQ)2 Single Crystal at Varied Temperatures in UHV System Xiaoming Huang, Feng Lin, Wei Zhou, Liang Ren, Hailin Peng, and Zhongfan Liu* Center for Nanoscale Science and Technology (CNST), Beijing National Laboratory for Molecular Sciences (BNLMS), State Key Laboratory for Structural Chemistry of Unstable and Stable Species, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, P. R. China ReceiVed: July 25, 2007; In Final Form: October 16, 2007
The present article investigates a thermochemical hole burning (THB) effect on a charge transfer (CT) complex, triethylammonium bis-7,7,8,8-tetracyanoquinodimethane [TEA(TCNQ)2] in an ultrahigh vacuum (UHV) system with a variable temperature scanning tunneling microscopy (VT-STM). The THB behaviors in the UHV system are similar to those under ambient conditions, except that the average hole diameter becomes larger, which is attributed to fewer conduction paths of heat energy in the UHV system. As the specimen temperatures are reduced, the threshold voltage of hole formation increases and the volumes of the holes become smaller. The existence of such temperature dependence suggests that the hole formation is closely related to a STM current-induced localized heating effect. We also estimate the temperature rise and analyze the thermal gravity and mass spectroscopy (TG-MS) for detailed understanding of the thermochemical decomposition reaction.
Introduction Scanning probe microscopy (SPM) has potential application in ultrahigh-density data storage because of its ability to make local surface fabrication through the tip from the nanometer scale down to the atomic scale.1-4 Considering that a local change of the electric, magnetic, optical, force, and thermal properties of substrate surfaces may be generated by SPM tip operations, quite a lot of materials are expected to be suitable for SPM memory, following, as already demonstrated, a variety of storage schemes such as electric-field-induced charge separation,5-8 electrical bistability,9-11 magnetic-phase change,12,13 heat-induced physical deformation or phase change,14-16 localized oxidation,17 and so forth. Alternatively, because the changes occur on a highly localized area, usually from a few nanometers to tens of nanometers, it is also a challenge to clarify these physical and/or chemical processes by experimental methods. In our recent works, we have developed a number of ammonium- and morpholinium-TCNQ charge-transfer (CT) complexes as STM memory materials.18-20 The information bit is recorded as a nanometer-sized hole, and we can achieve about 1 Tb/cm2 storage density using single-walled carbon nanotube STM tips.21 The mechanism of hole formation is that STM current heating elevates the temperature of a localized surface area, leading to the thermochemical decomposition reaction of CT complexes. The low boiling-point decomposition products go out of the surface, leaving a nanometer-sized hole. We name this mechanism thermochemical hole burning (THB). Although our previous THB works, such as the studies on the relationship of voltage and the hole volume18 and the anisotropic THB phenomenon,22 can manifest the THB effect on the features of the holes, there is still a lack of direct evidence that supports the thermochemical reaction mechanism induced by STM current joule heating. * To whom correspondence should be addressed. Tel. & Fax: 00-8610-6275-7157. E-mail:
[email protected].
In this work, we performed the THB experiment on a typical CT complex TEA(TCNQ)2 in a UHV system with VT-STM. By varying the specimen temperatures from room temperature to 215 K, we found that the threshold voltage of hole formation on TEA(TCNQ)2 was highly temperature-dependent. The lower the specimen temperature we gave, the higher the threshold voltage required for hole formation. Furthermore, we gave an analysis of the thermal diffusion process induced by STM current joule heating, which clearly elucidated how the thermochemical reaction led to the hole formation. Experimental Section TEA(TCNQ)2 was prepared according to the method of Melby.23 The experiments of hole formation and imaging were performed in a UHV system with a base pressure of about 1.0 × 1010 mbar on an Omicron VT-STM, or in air under ambient conditions on a Nanoscope IIIa, Digital Instruments for a comparison. Mechanically cut Pt/Ir (80/20) tips were used. The TEA(TCNQ)2 single crystals were fixed on sample plates with highly conducting silver adhesive. Thermochemical hole burning was performed using a computer-controlled program. The tip was moved to a desired place, giving a voltage pulse to the tunneling gap while the feedback was turned off. After that, the feedback was switched on and the tip went to another setting place for data writing. Low temperatures were achieved with liquid N2 using a continuous-flow cryostat. The TG-MS experiments were carried out on a Setaram TGA92 thermogravimentric analyzer combined with a quadrupole mass spectrometer (Balzers OmniStar 200), which provided online monitoring of the mass loss of the sample and the formation of positive ions in the range of 1-300 amu. Results and Discussion The TEA(TCNQ)2 crystal consists of alternately stacked TCNQ anion layers and TEA cation layers along the crystal a
10.1021/jp0758566 CCC: $40.75 © 2008 American Chemical Society Published on Web 01/19/2008
Thermochemical Hole Burning
J. Phys. Chem. C, Vol. 112, No. 6, 2008 2005
Figure 2. STM images of a 4 × 4 hole array formed on TEA(TCNQ)2 by voltage pulses 6 V × 100 µs under (a) ambient conditions, (b) UHV conditions. Image conditions: -0.10 V, 0.1 nA for a, and -0.08 V, 0.6 nA for b, constant-current mode.
Figure 1. (a) Arrangement of TCNQ molecules of the bc surface of TEA(TCNQ)2 taken from the bulk crystallographic parameters. (b) A typical large-scale STM image of the bc surface; the inset is a highresolution image. Image conditions: -0.08 V, 0.6 nA, constant-current mode.
axis. One electron, on average, transfers from each TEA molecule to two TCNQ molecules, which makes the TCNQ and TEA layers conductive and insulating, respectively. Because of the strong π-π overlapping interaction between face-to-face stacking TCNQ molecules, the bc surface terminated with TCNQ molecules (Figure 1a) is the largest surface for the crystal. Figure 1b shows a STM image of the bc surface, which is molecularly flat and usually extends to the order of micrometer scale. The inset of Figure 1b shows a molecular-resolution STM image. The dimensions of the surface unit cell are b ) 0.8 nm, c ) 1.5 nm, and the angle γ between the b and c directions is 70.0°, which is in agreement with the crystallographic data obtained from X-ray diffraction b ) 0.79 nm, c ) 1.37 nm, γ ) 71.9°.24 The formation of hole arrays or data writing is performed by applying a higher voltage pulse between the STM tip and the single crystal. On TEA(TCNQ)2, we can achieve the hole size down to a few nanometers under a suitable voltage pulse, and the writing reliability reaches 100%. Figure 2a and b shows two STM images of regular 4 × 4 hole arrays formed on the bc surface of TEA(TCNQ)2 at room temperature with the pulse condition of 6 V × 100 µs, under ambient conditions and in a UHV system, respectively. Usually, under the same pulse conditions, the diameter of a hole formed under ambient conditions is smaller than that in the UHV system, for example, 41 nm versus 57 nm in Figure 2, while the depth of a hole is
almost the same for the two STM working surroundings. The differences of the hole can be explained well on the basis of the thermal effect induced by STM current. Under ambient conditions, besides the heat conduction in the crystal, there are some conductive paths on the surface, such as air conduction or heat energy dissipated by some water trace, which is related to the humidity in air.25 These paths make the heat energy loss quicker, but they do not exist in the UHV system definitely. Such reasons lead to the smaller hole diameter observed under ambient conditions (see the Supporting Information). The heat conduction within the crystal is almost identical for the two working surroundings; therefore, it is natural that the hole depth is almost the same. Under higher STM voltage and current, apart from the local heating, other effects may also exist that can lead to the hole formation, such as the localized oxidation, electric field evaporation,26 or electron bombardment.27 However, our THB experiment in the UHV system (Figure 2b) strictly excludes the possibility of the localized oxidation mechanism that often occurs under humid conditions because there is no oxygen and water herein. In principle, the change of the specimen temperature has no influence on the hole if the hole formation is induced by the electric field evaporation and the electron bombardment. On the contrary, the local heating effect, which leads to the thermal decomposition of TEA(TCNQ)2, is directly correlated to the change of the specimen temperatures. If we cool the TEA(TCNQ)2 substrate below room temperature, then it will need a larger amount of heat energy to reach the decomposition temperature point. That is to say, the hole should be formed under a larger pulse voltage and current. For this purpose, we carried out the THB experiments in a UHV-VTSTM system. Figure 3a-d shows a series of STM images that display 4 × 4 hole arrays on the surface of TEA(TCNQ)2 obtained at the same voltage pulse conditions but at different specimen temperatures of 290, 265, 240, and 215 K, respectively. The pulse voltage in each figure ramps up from 4 to 7 V as indicated. At a temperature of 290 K (Figure 3a), a complete 4 × 4 hole array is achieved, whereas the hole size becomes bigger from the bottom to the top hole row, which obviously contributes to the increasing heating energy. When the temperature is decreased to 265 K (Figure 3b), the smallest four holes at the bottom row produced by the 4 V pulse voltage are indiscernible. Figure 3c (240 K) and d (215 K) indicates that the threshold pulse voltage for the formation of holes becomes larger and larger with the decrease of the specimen temperatures, 6 V for 240 K, 7 V for 215 K, below which the holes cannot be produced on the TEA(TCNQ)2 surface. The specimen temperature also affects the depth and size of the hole. Figure 4a is the height profiles along the red arrow
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Figure 3. STM images of hole arrays obtained under the same voltage pulse conditions but at different temperatures: (a) 290 K; (b) 265 K; (c) 240 K; (d) 215 K. In each frame, from the bottom to top hole row, the pulse voltages are 4, 5, 6, and 7 V, respectively. The pulse width is 200 µs. Image conditions: -0.08 V, 0.6 nA, constant-current mode.
directions indicated in Figure 3a and c. The holes at 290 K are deeper than the holes at 240 K obviously. This phenomenon is the same for all voltage conditions from 4 to 7 V at temperatures from 290 to 215 K. Figure 4b shows the hole volume as a function of specimen temperature under different pulse voltages, 5, 6, and 7 V, respectively. With the increase of temperature an approximate linear relationship was obtained, which clearly indicates that under the same voltage pulse the higher the temperature, the larger the hole volume. The above THB behavior at reduced temperatures can be explained in the following thermal process
Qin ) Qout + Qrise + Qreact
(1)
where Qin is the total energy input supplied by the STM voltage pulse, Qout is the heat energy transmitted away, Qrise is the energy used to increase the temperature of heating area, and Qreact is the heat energy used in the thermal decomposition reaction. In theory, because the resistivity of the semiconducting TEA(TCNQ)2 is much higher than the metal STM tip, the energy input is dissipated predominantly in the sample via the jouleheating effect. Therefore, Qin can be written as Qin ) RIVt where R is a constant, I is the STM current, V is the pulse voltage, and t is the pulse width. Because the experiment is done in the UHV system, the heat conduction via air or liquid-film is avoided.24 Also considering the lower value of the thermal conductivity for TEA(TCNQ)2, Qout approximates to be neglected. Qrise is determined by the temperature difference between the decomposition temperature point, Tdecomposition, and the specimen temperature, Tspecimen, and can be estimated by the following equation Qrise ) cm(Tdecomposition - Tspecimen) where c and m are the average heat capacity and the mass of the heating area, respectively. If the specimen temperature is lowered, then Qrise will become larger. Obviously, the larger Qrise requires more joule heat supplied by a higher voltage pulse. As a result, the threshold voltage of hole formation increases with the decrease of the specimen temperature, which is consistent with the result obtained in Figure 3. Qreact is the heat energy needed to create a hole. Hence, Qreact is proportional to the hole volume.18 By substituting the equation
Figure 4. (a) Height profiles along red arrow directions shown in Figure 3a and c. (b) Dependence of the hole volume on the specimen temperature under the different pulse voltages, 5, 6, and 7 V. The hole volume was normalized by the maximum volume value. The solid line is the linear fit of the data.
of Qin and Qrise into eq 1 and neglecting Qout, we can obtain the expression of Qreact in the following
Qreact ) RIVt - cmTdecompostion + cmTspecimen
(2)
Under the same voltage pulse, the former two terms are constant; thus, Qreact is proportional to Tspecimen. This is consistent with the result obtained in Figure 4b, where the linear relationship is achieved between the hole volume and the specimen temperature. Because the above experiments in the UHV system prove that the thermal effects induced by STM current are responsible for the hole formation, the estimation of the local temperature rise can be done. The organic crystal of TEA(TCNQ)2 exhibits a semiconducting behavior because of its special constituent and structure (Figure 1a). The electrical conductivities along the a, b, and c directions are σa ) 1 × 10-3 Ω-1 cm-1, σb ) 4 × 10-2 Ω-1 cm-1, and σc ) 4 Ω-1 cm-1, respectively.23 These values are several orders of magnitude lower than that of normal metals. Under a large STM pulse voltage, the induced tunneling current density can be as high as 108 Am-2.20 Therefore, it is sufficient to cause a significant current heating effect on the specimen surface because of the relatively smaller electrical conductivities (the larger resistivities). Alternatively, the thermal conductivity κc, which is basically isotropic, is 0.4 W m-1 Κ-1 at 300 Κ.28 This relatively lower value makes the heat energy
Thermochemical Hole Burning
J. Phys. Chem. C, Vol. 112, No. 6, 2008 2007 Acknowledgment. This work was supported by NSFC (90301006,50521201)andMOST(2007CB936203,2006CB932403, 2006CB932602). Supporting Information Available: Thermochemical hole burning on TMHDA-I2-(TCNQ)2 in liquid 1-octanol. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes
Figure 5. TG-MS curves of the TEA(TCNQ)2 single crystal, in which the TG curve corresponds to the left-hand ordinate and the numbers are the m/z values of ions of the gaseous products.
confined to a small area without transmitting off, resulting in a higher localized temperature rise. The maximum temperature rise in the crystal can be estimated from the formula ∆T ) FcJ2l2/2κ,29 where Fc is the electrical resistivity of the crystal, J is the current density, and l is the radius of the heating area. From this equation, ∆T is in the range of about 200-300 °C. The decomposition reaction of TEA(TCNQ)2 must be taken into account when the temperature increases 200-300 °C. Figure 5 shows the TG-MS analysis plots of TEA(TCNQ)2 taken at a heating rate of 5 °C/min. From the TG curve, the first step of mass loss, which corresponds to the formation of gaseous products, takes place at about 220 °C. The ion current signals m/z ) 26 and 27, indicated by the red and green curves, are identified as HCN by comparing with the standard mass spectrum. The second step of mass loss, which is not sharp, occurs at about 250-270 °C. The ion current signals m/z ) 58, 86, and 101 can be identified as the fragments of TEA. Hence, we can say that the joule heat induced by the tunneling current is localized to a small surface area, at which the temperature is increased such a degree that the thermochemical decomposition of TEA(TCNQ)2 occurs and subsequently the volatile decomposition products go out of the surface, leaving a hole on the crystal surface. In summary, we report the thermochemical hole burning performance of TEA(TCNQ)2 in the UHV-VT-STM system, which facilitates gaining insight into the THB mechanism. The hole size under ambient conditions is smaller than that in the UHV system under the same voltage pulse, which is attributed to the more heat conduction paths that transmit the heat energy away. By lowering the specimen temperatures in the UHV system, we did observe that the decomposition temperature cannot be reached at the lower pulse voltage, and thus the holes are not formed. This phenomenon gives a direct correlation between the decomposition reaction and the joule heat induced by STM current, therefore strongly supporting the THB mechanism. The TG-MS experiment indicates that the decomposition temperature of TEA(TCNQ)2 occurs at about 200-300 °C, which is consistent with the theoretical estimation of the temperature rise.
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