Slow Charge Relaxation in Ionizable Alkanethiols and Its Role in

Feb 9, 2009 - Xian Ning Xie,*,† Sankaran Sivaramakrishnan,‡ Q. Song,‡ Xingyu Gao,‡ Peter K.-H Ho,‡. C. K. Ong,‡ and Andrew Thye Shen Wee*,...
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J. Phys. Chem. C 2009, 113, 3683–3690

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Slow Charge Relaxation in Ionizable Alkanethiols and Its Role in Modulating Electric Characteristics of Molecules and Passivated Gold Nanoparticles Xian Ning Xie,*,† Sankaran Sivaramakrishnan,‡ Q. Song,‡ Xingyu Gao,‡ Peter K.-H Ho,‡ C. K. Ong,‡ and Andrew Thye Shen Wee*,†,‡ NUS Nanoscience and Nanotechnology InitiatiVe (NUSNNI) and Department of Physics, National UniVersity of Singapore, 2 Science DriVe 3, Singapore, 117542 ReceiVed: September 1, 2008; ReVised Manuscript ReceiVed: January 6, 2009

This work addresses the electric characteristics associated with the slow ionization relaxation in ionizable alkylthiols which are widely used as passivation ligands for gold nanoparticles (NPs). It is observed that reversible ionic motion under a cycling voltage induces highly transient current with strong hysteresis loop. The ionization and relaxation of alkanethiols is responsible for the buildup and reversal of an internal bias, and thus leads to capacitive and charge memory effect in the electric conduction of the molecules. The role of ligand ionization in the electric behavior of passivated gold NPs is also demonstrated. The slow ionic process allows for both capacitive and resistive conduction and can be used to regulate the charge transport through the Au NP film. Our results represent a novel charge conduction mechanism governed by ligands in the NP/molecule binary system and would find new applications in molecular electronics and NP-based memory and sensor devices. Introduction Alkanethiols represent an important family of molecules which are of particular interest in self-assembly and molecular electronics. These molecules can form a robust self-assembled monolayer (SAM)1 on metal (e.g., Au, Hg, etc.) surfaces through the strong metal-sulfur (Au-S) covalent bond. The formation of stable SAM allows for reproducible characterization of the structural,2-4 electrochemical,5,6 electric,7-15 and dielectric16,17 properties of alkanethiol molecules. It is found that SAMs act as an electron tunneling medium in a metal-alkanethiol-metal junction. Key parameters including conductivity, electron tunneling coefficient, and rate of electron transfer of alkanethiols are available in the literature.5-15 Interesting phenomenon such as current rectification14,15 by fixed dipoles or asymmetrically placed redox centers in SAMs has been observed. In addition to their electronic properties, the ionic nature of alkanethiols is also reported. Impedance spectra obtained at low-frequency region indicated good ionic insulation behavior of alkanethiols in electrolyte solutions.18 Dielectric measurements16,17 yielded the capacitance, electron-transfer resistance, and average dielectric constant of alkanethiols. Dielectric loss peaks located in different frequency ranges were observed and attributed to molecular dipole relaxation and interfacial effect.16 In addition, the preparation of metal-alkanethiol contact by different methods and the electric behavior of the contact are also reported.19-21 The interest in alkanethiols grows further in recent years with the emergence of NP-based electronics. Due to the formation of SAM on Au surface, alkanethiols provide excellent passivation for Au NPs. Ligand passivation prevents the agglomeration of NPs such that the size of NPs and thus their properties can be controlled.22-24 To suit different processing and applica* To whom correspondence should be addressed. E-mail: phyweets@ nus.edu.sg (A.T.S.W); [email protected] (X.N.X). † NUSNNI. ‡ Department of Physics.

tions, NPs with varying hydrophilicity are necessary for selective dispersion in aqueous or organic medium. For this purpose, carboxylic acid group (O)CH-OH or COOH) and carbinol group (-CH2-OH) are often used as terminal groups for alkanethiol ligands.25-27 The ionizable COOH makes NPs soluble in water through the ionization reaction of COOH f COO- + H+, while the carbinol group allows NPs to be dispersible in organic solvents such as methanol.27 It is known that charge transport in passivated NPs not only depends on the size of NPs but also the structure of their passivation molecules.28-32 For example, the electron transfer rate in NPs deceases exponentially with the increase of the chain length of alkanethiols.28 The different Au-S coupling and trans/gauge conformations of ligands could lead to multiple series of peaks in the conductance histograms of alkanethiols.29 In this work, we discuss the impact of terminal groups on the electric and dielectric properties of alkanethiols and NPs passivated by the alkylthiol ligands. While most work emphasizes the electronic behavior of alkanethiols,7-15 the focus of this work is on the crucial role played by slow ionic processes in the molecules. We report the charge trapping and current hysteresis induced by the ionization and relaxation of COOHcontaining alkanethiols under a slowly changing field. To the best of our knowledge, this is the first detailed description of the transient current generation by ionic processes in alkanethiols, although there is a huge amount of reports on the electric and dielectric behaviors of alkanethiols in general.5-17 We present current-voltage (IV) hysteresis loops and dielectric loss spectra to illustrate the effect of slow ionic motion on the electric characteristics of alkanethiols. Moreover, we also demonstrate the crucial role of ionizable alkanethiols in modulating the capacitive and resistive IV behaviors of Au NPs passivated by the alkanethiol ligands. Because ionizable alkanethiols are widely used in molecular electronics and NP-based applications, an understanding of the ionization-induced charge storage and memory effect is both important and timely.

10.1021/jp810850e CCC: $40.75  2009 American Chemical Society Published on Web 02/09/2009

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Xie et al. SCHEME 1: Experimental Setup for Current-Voltage (IV) Curve Measurement of MUA and MUO Filmsa

Figure 1. Molecular formula and chain-like structure of MUA and MUO, respectively.

Recently, fast switching between high- and low-conductivity states has been observed in passivated NPs.33,34 The memory effect is related to charge transfer between NPs and passivation molecules, and it works in the nanosecond regime due to the electronic nature of the charge transfer. Here, we present a different memory mechanism, which is based on slow ionic process and works on the millisecond-second time scale. Such ionization-induced transient charge conduction plays a critical role in nanodevice operations. For example, hysteresis loops were reported for nanotube-based field-effect transistors, and were correlated to ionic storage in oxides, absorbed water, or charged polymers.35-37 Either advanced or retarded hysteresis was observed by different groups,32,35-37 indicating the complex impact of the ionic process on IV characteristics. Therefore, it is the aim of this work to provide a general explanation for the varied observations reported in the literature. We use two representative alkanethiols, 11-mercaptoundecanoic acid [MUA, SH-(CH2)10-COOH] and 11-mercaptoundecanol [MUO, SH-(CH2)10-CH2-OH],27,38-40 for a comparative study to illustrate the electric-regulation effect of the slow ionization in MUA. Both of the alkanethiols have the same -CH2-CH2- backbone and the same number of eleven carbon atoms (see Figure 1). The only difference is that MUA is terminated by the ionizable COOH group, while MUO is terminated by the nonionizable CH2-OH group. However, as will be shown below, such a difference in the terminal group could lead to distinct IV characteristics: strong hysteresis loops dominate the conduction in MUA, while static resistive conduction is observed in MUO. In the following paragraphs, we first compare the IV and dielectric loss characteristics between pure MUA and MUO films, and discuss the mechanism and equivalent electric circuit of the COOH-ionization-induced current hysteresis. We then show the current modulation behavior in MUA-passivated Au NPs, which is associated with the charge-storage memory effect of COOH ionization in the NP ensemble. The different time-dependence of hysteresis loop between pure MUA film and NP/MUA composite is also discussed. Results and Discussion IV curves were collected on MUA and MUO films respectively using the setup shown in Scheme 1 (see Experimental Section). Figure 2a displays the IV curves collected on the MUA film by sweeping Vex (see Scheme 1 for the polarity of Vex) in the cycle of -1 T 1 V at different rate f ) 0.01, 0.5, 2.5, and 5.0 Hz, respectively. It can be seen that the IVs are strongly time-dependent, and hysteresis loops are formed as a function of f. Both the current I and the area A of the loop increase when the cycling frequency f is higher. Figure 2b shows the close-up view of the IV collected with f ) 0.01 Hz, which looks like a straight line in Figure 2a due to its low current level. It is clear in Figure 2b that this IV also exhibits a hysteresis loop of A f B f C f D f E f F f A when the Vex cycle is forwarded (-1 f 1 V) and then reversed (1 f -1 V). The maximum current of the loop is ∼3 nA, which is two orders lower than

a An external DC (direct current) voltage Vex was cycled with certain rates f to obtain IV curves. For impedance measurements, the Vex was replaced with a HP 4192A Impedance Analysis setup. In this case, the complex impedance of the device was measured using a sinusoidal AC perturbation in the frequency range of 5 Hz to 1 MHz. Both the real part r and imaginary part ′′ of the dielectric were obtained from the spectra. The dielectric loss tangent was then calculated using Tan δ ) (ε′′)/(εr).

that of ∼300 nA of the loop collected at f ) 5.0 Hz. The width W of the loop obtained with the lowest frequency f ) 0.01 Hz is ∼1.8 V, and it decreases when f is increased (see Figure 2a). Figure 2c compares the IV loops obtained using Vex cycles of -6 T 6 V, -3 T 3 V, and -1 T 1 V at the same rate of f ) 0.5 Hz, respectively. It can be seen that when higher Vex is used, both the loop width W and the current I increase significantly. In contrast, the IVs collected on the MUO film under identical conditions does not exhibit the looping behavior discussed in Figure 2a-c. As shown in Figure 2d, the IVs collected at f ) 0.01 and 5 Hz, respectively, all pass through the origin without appreciable remanent voltage, indicating a static and resistive conduction in the MUO film. Such a resistive IV characteristic was always observed on MUO in our Vex cycles. The IVs in Figure 2d are nonohmic, suggesting that electron tunneling may be involved in charge transport throughout the MUO film. For an ideal alkanethiol SAM, its conductivity was reported to be is σ ) 6 × 10-15 S/cm.7 Using σ ) (I)/(Vex) · (d)/(A) (A ≈ 1.5 mm2 is the area of the Au electrode; d ) 500 nm is the thickness of MUO film), the conductivity σ of the MUO film was estimated to be σ ≈ 6.7 × 10-13 S/cm at Vex ) 1 V. This conductivity falls well within the range of conductivity obtained for similar systems by conducing probe atomic force microscopy (CP-AFM) and Hg-junction approach, respectively.7,8,11,12 These early works7,8,11,12 have shown that the conductivity of alkanethiols depends on the junction used in IV measurements, and a lower σ is associated with a larger junction contact area. It is clear in Figure 2 that the IVs of the MUA and MUO films are fundamentally different. As will be demonstrated below, the difference is due to the ionization of the COOH group of MUA, and the nA-order static leakage current does not affect the identification of µA-order transient current associated with COOH ionization. The results in Figure 2 show that the current through MUA is highly transient with a time-sensitive hysteresis, while that through MUO is static and free of hysteresis. As mentioned above, MUA and MUO are almost identical in structure except their different terminal groups. It is known that COOH is ionizable, and the zero-field ionization constant of MUA is up to 10-4,39 which is much higher than that of 10-7 commonly known for water. Therefore, the IV loops observed in Figure 2a-c can be attributed to the enhanced ionization of COOH in MUA under an external field Eex (or voltage Vex). As illustrated in Figure 3a and b, for example, when Vex ) -1 V is applied, the COOH is ionized into COO- and H+ ions, and the orientation of the ions is aligned in the direction of Vex. Due to the low electronic7,8 and ionic18 conductivity of alkanethiols, the COO- and H+ are trapped in double potential wells in the

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Figure 2. (a) Current-voltage (IV) hysteresis loops collected on the MUA film by ramping Vex in the cycle -1 T 1 V at a different rate of f ) 0.01, 0.5, 2.5, and 5.0 Hz, respectively. (b) Close-up view of the IV loop collected at f ) 0.01 Hz shown in (a). (c) Comparison of IV loops obtained using voltage cycles of -1 T 1 V, -3 T 3 V, and -6 T 6 V at the same rate of f ) 0.5 Hz, respectively. (d) IV curves collected on the MUO film by ramping Vex in the cycle of -2 T 2 V at f ) 0.01 and 5.0 Hz, respectively.

dielectric medium (see Figure 3c). The depth Ea of the potential well is related to the static dielectric susceptibility or dielectric constant εr of MUA in the form of Ea ∝ εr.41 As will be shown in Figure 4, the ionization of COOH leads to a large contribution to the effective dielectric constant of MUA in the low frequency range of f < 30 Hz. The relaxation time τ of ionized species in amorphous films is an exponential function of Ea, for example, τ ∝ exp((Ea)/(kT)),16 where k is the Boltzmann constant and T is the temperature. Because εr is significantly enhanced by ionization (see Figure 4), Ea would be large and, thus, the time τ for ionic charge relaxation (typically 0.01-100 s)42 would be much longer than that for molecular dipole relaxation (from ms16 to µs42). Therefore, in our experimental frequency range of f ) 0.01-5.0 Hz (see Figure 2), when the external voltage Vex is reduced to zero, the induced internal bias Vin could still remains due to the slow ionic relaxation. Such ionization relaxation leads to a series RC circuit shown in Figure 3d in which the capacitor C is associated with Vin induced by MUA ionization, and R is the total resistance of the electrode/MUA/ electrode assembly (note R is not constant due to tunneling). On the basis of the equivalent RC circuit shown in Figure 3d, the transient IV loops observed in Figure 2a-c can be attributed to the charging and discharging current of the circuit. For example, at point A in Figure 2b, the application of Vex ) -1 V induces an opposite internal bias Vin (see Figure 3b), and the circuit current I is determined by Vex - Vin (e.g., I ∝ Vex -

Figure 3. (a) Schematic showing the MUA molecules under Vex ) 0 before the ionization of COOH. (b) The ionization of the COOH group under Vex ) -1 V. An internal bias Vin and field Ein is therefore induced by the ionization of COOH. The Vin and Ein are in opposite direction to the external bias Vex and field Eex. (c) The formation of potential barrier Ea in MUA as a result of the ionization of COOH. (d) Equivalent electric RC circuit in MUA at the moment when Vex ) -1 V. The capacitor C is in series with the resistor R, and C is induced by the ionization of COOH.

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Figure 4. (a) r ∼ f and ′′ ∼ f relations obtained for the MUA and MUO film, respectively; r and ′′ are the real and imaginary part of the dielectric constant, respectively; f is the frequency of the AC power source used for collecting the above curves. (b) Dielectric loss tangent collected for the MUA and MUO films, respectively.

Vin). In A f B region, Vex > Vin, I (I < 0) follows the negative sign of Vex. At point B, |Vex| ) |Vin| ) 0.87 V, so the current is zero (I ) 0). When |Vex| < 0.87 V, Vex - Vin < 0, consequently I changes its sign from negative to positive (see B f C range in Figure 2b). At point C, Vex ) 0 V, I is fully determined by Vin. The current in A f C range is dominated by the discharging of the RC series. In the next quarter-cycle of Vex ) 0 f 1 V (see CfD range in Figure 2b), the RC is being charged by Vex. In a RC circuit, the transient current I is frequency-dependent, for example, I ) C(dVex)/(dt); f ∝ (dVex)/(dt), which is often associated with ferroelectricity and dielectric relaxation.43-45 So, higher current is observed with increasing Vex cycling frequency f as shown in Figure 2a. Indeed, when the difference between the positive and negative current Idif (e.g., the distance between C and F points in Figure 2b) was plotted as a function of f using the data obtained for the -1 T 1 V voltage cycle (some IVs are shown in Figure 2a), a linear Idif ∼ f relation was obtained as shown in Figure S1 in Supporting Information i (see SI-i). The linear Idif ∼ f plot in Figure S1 is an outcome of the rate-dependent transient current I ) C[(dVex)/(dt)],43-45 and is an indication that ionic charging/discharging dominates the IVs of the MUA film. Despite the dominance of transient current, there is still appreciable leakage current observed in Figure 2b (see C f D portion of the IV). The maximum leakage current is 1.1 nA in the voltage range of 0-1 V and is in the same order as that observed for MUO film shown in Figure 2d. This similarity shows that both the MUA and MUO films prepared here exhibit roughly the same intrinsic leakage current. This is expected because both of the films are amorphous, and of similar thickness and device configuration. Since the ionization-induced transient current is f-dependent and is typically in the order of µA, it is well separated from the f-independent and nA-order background leakage current. Therefore, it is clear that carboxylic ionization and capacitive hysteresis dominates the IVs of MUA, while the IVs of the MUO film are static and resistive. As will be demonstrated in Figure 4, apart from molecular dipole polarization, there is no ionization-enhanced dielectric effect in MUO molecules. Therefore, the MUO film is equivalent to a pure resistive conduction medium with voltage-dependent resistance due to possible tunneling as shown in Figure 2d.

The impedance Z of a RC series is defined to be Z ) R2 + XC2, where R and XC are the contribution of the resistor and capacitor, respectively.46 Our impedance measurements yielded similar Z (values obtained at frequency f ) 7) for the Au electrode/MUA/Si and Au electrode/MUO/Si devices (see Scheme 1), respectively. However, the phase angle φ of the two devices is quite different, being -54° for the former and -6° for the latter. This suggests a strong capacitive component in the MUA-based device, as in a RC series, φ is related to R and XC in the form of tan φ ) - (XC)/(R) (e.g., φ ) -90° for a pure capacitor46). The negligibly small phase angle of -6° measured for the MUO-based device is indicative of a resistordominated conduction, in agreement with the resistive IV characteristics observed in Figure 2d. The impedance results further confirm our interpretation of the IV hysteresis loops on the basis of the proposed ionization-induced RC circuit as discussed in Figures 2 and 3. Figure 4a compares the εr ∼ f relation collected on the MUA and MUO films, respectively. The εr value of MUO is measured to ∼2.2 at f ) 5.0 Hz, in agreement with the values of 2.1-3.9 reported previously for MUO molecules.39,40 The εr value of MUA should in principle be similar to that of MUO, as both of the molecules are made of the same hydrocarbon backbone, and thus should exhibit similar molecular dipolar behavior. Indeed, εr values of 2.1-4.339,40 have been reported for MUA in the literature. However, a large εr, up to 6 times that of MUO is observed for MUA in the low frequency region of f < 30 Hz, as shown in Figure 4a. The εr of MUA decreases rapidly from 12 at f ) 5.0 Hz to 2.5 at f ) 30 Hz. Such a loss behavior originates from the relaxation of ionic species42 and can be attributed to the ionization and relaxation of COOH in MUA molecules. Accordingly, the imaginary part ε′′ of the dielectric constant of MUA also exhibits a dielectric enhancement in the same frequency range which is characteristic of slow ionic motions42 (see AB region in the inset of Figure 4a). Figure 4b displays the dielectric loss tangent Tan δ (Tan δ ) (ε′′)/(εr)), where δ is the dielectric loss angle. In agreement with the general features of alkanethiols,16 the loss spectrum of MUO exhibits two broad peaks centered at 4 × 102 Hz and 4.5 × 104 Hz, respectively. The first peak is due to the relaxation of molecular dipoles, and

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Figure 5. (a) IV loops collected on MUA using Vex cycles of -3 T 3 V at rate of f ) 5, 10, and 56 Hz, respectively. (b) Constant polarity of Vin in the Vex cycle due to the fast cycling rate of 56 Hz.

the second one has multiple origins such as molecular dipole, intermolecular interactions, and interfacial effect.16 On the spectrum of MUA, there is a significant ionic contribution to the dielectric loss in the low frequency range (see AB region in Figure 4b). The ionization relaxation is so strong that it makes the peak of molecular dipole at 4 × 102 Hz invisible on the spectrum. In Figure 2a-c, the IV hysteresis loops were best observed using Vex cycling rate of 0.01-5.0 Hz, which just lies in the frequency region of f < 30 Hz for slow ionic motion. Therefore, the results in Figure 4 confirm that the ionization and relaxation of COOH under a changing filed is responsible for the hysteresis and transient current generation observed for MUA molecules in Figure 2. Figure 5 illustrates the IV hysteresis behavior when the voltage cycling frequency f is higher than the typical ionization relaxation rate of MUA. Here, we compare three IV loops collected by cycling Vex in the form of Vex ) -3 T 3 V at a rate of f ) 5, 10, and 56 Hz, respectively. In this frequency range, as f increases, the width W of the loop decreases significantly, and W is measured to be 1.8, 1.2, and 0.7 V for f ) 5, 10, and 56 Hz, respectively. The observation suggests that when the change of Vex is faster than the response time of ionization and relaxation, the buildup of the internal bias Vin is largely reduced due to insufficient time for ionization during the voltage cycle. It is also noted in Figure 5a that both the forward and reverse curves obtained at f ) 56 Hz are located within the same negative region of Vex. This is because under a high frequency of f ) 56 Hz, the Vex changes so fast that the polarity of the initial Vin induced in the Vex ) -3 f 0 V forward quarter-cycle remains unchanged in the next Vex ) 0 f 3 V quarter-cycle (see Figure 5b). That is, before Vin could reverse its polarity through the slow reorientation of ions, Vex has already completed its cycle. So the IV is systematically shifted toward the negative side of Vex as shown in Figure 5a. The above IVs were collected under ambient conditions. To verify if water adsorption and subsequent double layer formation on the electrodes are contributing to the hysteresis, additional experiments were carried out in vacuum. In a control experiment, the Au/MUA/Si device was loaded into a small vacuum

chamber (∼10-3 torr, see SI-ii), and the chamber was baked at 100 °C for 1 h to remove the water absorbed in the film. After natural cooling of the chamber to room temperature, IV curves were collected in situ and were compared with those (see Figure 2a-c) obtained under ambient conditions. It can be seen in Figure S2b that the hysteresis obtained in ambient and vacuum, respectively, is quantitatively similar. The result shows that the hysteresis is not of an electrochemical origin on the basis of water adsorption and double layer formation. Instead, it is an intrinsic characteristic of the MUA molecule, and is a manifestation of the COOH ionization and relaxation under a changing field. We further demonstrate the effect of the slow ionization and relaxation in MUA on the IV characteristics of MUA-passivated Au NPs. Details about the synthesis of Au NPs (3.3 nm in diameter) can be found in our previous work.27 The good passivation of Au NPs is evidenced by our PES (photoelectron spectroscopy) results showing the shift of the Au 4f core level spectra to higher binding energy due to the formation of Au-S bonds (see SI-iii). Figure 6a displays the IV curves collected on MUA-passivated Au NPs using Vex cycles of -1 T 1 V at a rate of f ) 0.05, 0.5, 1.0, and 5.0 Hz, respectively. It can be seen that hysteresis loops similar to those of pure MUA shown in Figure 2a are also formed, and the transient current increases when higher f is used. Figure 6b compares the IV loops collected using Vex cycles of -3 T 3 V and -1 T 1 V at the same f ) 0.5 Hz, respectively. It is noted that there is a significant difference between the loops observed for pure MUA molecules and MUA-passivated NPs. For the pure MUA film, the loop width W decreases as the cycling rate f increases (see Figure 2a). Figure 6c illustrates this W-f dependence of pure MUA, which is characteristic of the RC series (see the inset) already discussed in Figures 3 and 5. In this case, with the increase of f, the time available for MUA ionization and internal bias buildup is shorter, so Vin is smaller and W would be narrower. In contrast, opposite W-f relation is observed for MUApassivated NPs. As shown in Figure 6a and c, W increases with higher f, for example, W ) 0.13 V at f ) 0.25 Hz and W ) 0.72 V at f ) 2.5 Hz. We propose that such a W-f dependence

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Figure 6. (a) IV loops collected on MUA-passivated Au NPs using Vex cycles of -1 T 1 V at a rate of f ) 0.05, 0.5, 1.0, and 5.0 Hz, respectively. (b) Comparison of IV loops collected on MUA-passivated Au NPs using -1 T 1 V and -3 T 3 V cycles at the same rate of f ) 0.5 Hz, respectively. (c) Different W ∼ f dependence observed for pure MUA film and MUA-passivated Au NPs, respectively, when cycling Vex in the form of -1 T 1 V. Inset shows the electric circuit formed by pure MUA film. (d) Structure of the MUA-passivated Au NP assembly and its equivalent electric circuit. Compared to the circuit formed by pure MUA film, an additional resistance RNP/MUA is introduced due to contact resistance of the NP and MUA interface.

is due to the introduction of an additional contact resistance caused by the NP/MUA interface. As shown in Figure 6d, the passivation of single NPs leads to the formation of many NP/ MUA interfaces, and electrons will have to overcome the interface barrier for charge conduction, thus resulting in an interface resistance RNP/MUA. Therefore, on the basis of the circuit of pure MUA, a new electric circuit is formed (see Figure 6d) in which the capacitor C associated with MUA ionization is connected to RNP/MUA in parallel. In this circuit, when f is lower, C is almost open, and current flows mainly through the resistive path of R +RNP/MUA. Consequently, the capacitive hysteresis effect is minimal as shown in Figure 6a for f ) 0.05 Hz. When f increases, the R + C path starts to contribute current and introduce hysteresis as a result of its charging and discharging. The higher the f, the greater the capacitive contribution would be. Therefore, the width W of hysteresis loops broadens with increasing f as observed in Figure 6a and c for MUA-passivated

Au NPs. The results show that the slow ionic motion plays a crucial role in regulating the IV characteristic of MUA and MUA-passivated Au NPs. Moreover, the same ionizationinduced memory and hysteresis effect could cause different IV behaviors when the composition of the film is changed. In the case of Au NPs, the ionization induces a capacitive effect which, in combination with RNP/MUA, modulates the charge transport across the entire Au NP film. Depending on the frequency f, both transient capacitive conduction and static resistive conduction can be obtained. The results shown in Figure 6 were obtained on a MUA-passivated Au NP film with a NP volume fraction of 0.7.27 It can be envisaged that by adjusting the volume fraction of Au NPs, the NP/MUA interface density can be tuned to obtain RNP/MUA with different resistance. This would enable continuously tunable IV characteristic ranging from that of a simple RC series to that of more complicated circuits as shown in Figure 6d.

Slow Charge Relaxation in Ionizable Alkanethiols In the Introduction, we introduced the different hysteresis32,35-37 reported earlier in the literature. The results presented in this work may provide a general explanation for these varied observations. The hysteresis is governed by the interplay of internal bias buildup, ionic relaxation, bias reversal, and the rate of the cycling external field. The transient current also depends on the relative values of resistance R, RNP/MUA, and the capacitor C of the circuit shown in Figure 6d. For instance, when R f 0, an advanced hysteresis would be obtained because the components in Figure 6d work as a parallel RNP/MUAC circuit, and the discharge of C causes the current increment of the return sweep. On the other hand, one would observe a retarded hysteresis at the same frequency if RNP/MUA T ∞. In this case, the circuit consists of R and C in series, and the discharge of capacitor C in the return cycle would generate an opposite current, thus leading to reduced total current. In addition, such advanced or retarded hysteresis could also be caused by the interplay between the voltage cycling rate f and the timescales of ion motion and internal bias reversal associated with the ionic species in the device assembly. In conclusion, we have investigated the electric characteristic of the slow ionization and relaxation in ionizable alkanethiol molecules. The formation of hysteresis loops in IV curves was observed, and the mechanism for the transient current generation was proposed. The voltage- and frequency-dependence of hysteresis was discussed in terms of the capacitive charge storage effect associated with molecular ionization and relaxation. Moreover, the significant role of ionic motion in determining the electric behavior of MUA-passivated Au NPs was demonstrated. In addition to the electron-transfer-induced memory effect in the nanosecond region,33,34 the slow ionizationinduced charge storage and regulation would create new possibilities in molecular electronics and NP-based devices operated in the millisecond-second regime. Experimental Section MUA and MUO chemicals were purchased from Aldrich. MUA was dissolved in toluene (60 mg/mL) at 60 °C, and then the solution was spin-coated onto a p-type silicon (Si) substrate to obtain the MUA thin film. Similarly, the MUO film was prepared by first dissolving MUO in methanol (40 mg/mL in concentration) at 60 °C, and then spin-coating the solution onto the same type of Si substrate. The average thickness d of the MUA and MUO films is roughly the same of 500 nm as measured using a Tencor Alpha-Step 500 surface profiler. Gold (Au) thin film (∼40 nm in thickness) was evaporated onto the MUA and MUO films respectively using an electron beam evaporator (Leybold Univex 300). The area of the Au electrode on the MUA or MUO film is A ≈ 1.5 mm2, as shown in Scheme 1. For IV curve collection, the device (see Scheme 1) was connected to an external DC (direct current) voltage Vex, which was supplied by a conductive atomic force microscope (cAFM) module.47-49 The cAFM module allows for Vex cycling rate of 0.01-112 Hz, and high current detection sensitivity of pA. The maximum current limit of the module is ∼0.3 µA. Both forward and reverse traces of all the IV curves were recorded for comparison of their hysteresis effect. For impedance and dielectric characterization, the same device was connected to a HP 4192A Impedance Analysis setup instead of the DC voltage Vex. The frequency dependence of dielectric properties of the device was obtained in the frequency range of 5-106 Hz using an AC (alternating current) voltage source in the form of sine wave (see Scheme 1). MUA-passivated Au nanoparticles (NPs; diameter ) 3.3 nm) were prepared according to the procedures

J. Phys. Chem. C, Vol. 113, No. 9, 2009 3689 detailed in our previous work.27 The NPs were spin-coated on the same Si substrate used for pure MUA or MUO film deposition as shown above. The thickness of the NP film is ∼100 nm, and its volume fraction in the film is 0.7.27 Au electrode with 1.2 mm2 dimension was then deposited on the NP film for device fabrication. The same IV curve measurement procedures used above for pure MUA or MUO film were applied to the Au NP film. Acknowledgment. This work is supported by the NUS Nanoscience and Nanotechnology Initiative (NUSNNI), the National University of Singapore. Professor X.-S. Wang (Dept. Phys., NUS) is gratefully acknowledged for insights on the equivalent electric circuits for various hysteresis loops. Supporting Information Available: Linear Idif ∼ f relation, comparison of IVs collected in ambient and vacuum, respectively, and PES (photoelectron spectroscopy) characterization of MUA-passivated Au NPs. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ulman, A. Thin Films: Self-Assembled Monolayers of Thiols, Academic Press: San Diego, CA, 1998. (2) Nishi, N.; Hobara, D.; Yamamoto, M.; Kakiuchi, T. J. Chem. Phys. 2003, 118, 1904. (3) Loglio, F.; Schweizer, M.; Kolb, D. M. Langmuir 2003, 19, 830. (4) Poirier, G. E. Chem. ReV. 1997, 97, 1117. (5) Boldt, F. M.; Baltes, N.; Borgwarth, K.; Heinze, J. Surf. Sci. 2005, 597, 51. (6) York, R. L.; Slowinski, K. J. Electroanal. Chem. 2003, 327, 550– 551. (7) Rampi, M. A.; Schueller, O. J. A.; Whitesides, G. M. Appl. Phys. Lett. 1998, 72, 1871. (8) Holmlin, R. E.; Haag, R.; Chabinyc, M. L.; Ismagilov, R. F.; Cohen, A. E.; Terfort, A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2001, 123, 5075. (9) Slowinski, K.; Majda, M. J. Electroanal. Chem. 2000, 491, 139. (10) Fan, F. R. F.; Yang, J. P.; Cai, L. T.; Price, D. W.; Dirk, S. M.; Kosynkin, D. V.; Yao, Y. X.; Rawlett, A. M.; Tour, J. M.; Bard, A. J. J. Am. Chem. Soc. 2002, 124, 5550. (11) Wold, D. J.; Haag, R.; Rampi, M. A.; Frisbie, C. D. J. Phys. Chem. B 2002, 106, 2813. (12) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103. (13) Chu, C. W.; Na, J. S.; Parsons, G. N. J. Am. Chem. Soc. 2005, 129, 2287. (14) Chabinyc, M. L.; Chen, X.; Holmlin, R. E.; Jacobs, H.; Skulason, H.; Frisbie, C. D.; Mujica, V.; Ratner, M. A.; Rampi, M. A.; Whitesides, G. M. J. Am. Chem. Soc. 2002, 124, 11730. (15) Aviram, A.; Ratner, M. A. Chem. Phys. Lett. 1974, 29, 277. (16) Wang, B.; Luo, J.; Wang, X.; Wang, H.; Hou, J. G. Langmuir 2004, 20, 5007. (17) Slowinski, K.; Hong, H. K. Y.; Majda, M. J. Am. Chem. Soc. 1999, 121, 7257. (18) Boubour, E.; Lennox, R. B. Langmuir 2000, 16, 4222. (19) Haick, H.; Ambrico, M.; Ghabboun, J.; Ligonzo, T.; Cahen, D. Phys. Chem. Chem. Phys. 2004, 6, 4538. (20) Haick, H.; Niitsoo, O.; Ghabboun, J.; Cahen, D. J. Phys. Chem. C 2007, 111, 2318. (21) Haick, H.; Ghabboun, J.; Cahen, D. Appl. Phys. Lett. 2005, 86, 042113. (22) Baletto, F.; Ferrando, R. ReV. Mod. Phys. 2005, 77, 371. (23) Link, S.; El-Sayed, M. Annu. ReV. Phys. Chem. 2003, 54, 331. (24) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (25) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (26) Chen, S.; Kimura, K. Langmuir 1999, 15, 1075. (27) Sivaramakrishnan, S.; Chia, P.-J.; Yeo, Y.-C.; Chua, L.-L.; Ho, P. K.-H. Nat. Mater. 2007, 6, 149. (28) Terrill, R. H.; Postlethwaite, T. A.; Chen, C.-H.; Poon, C.-D.; Terzis, A.; Chen, A.; Hutchison, J. E.; Clark, M. R.; Wignall, G.; Londono, J. D.; Superfine, R.; Falvo, M., Jr.; Samulski, E. T.; Murray, R. W. J. Am.Chem. Soc. 1995, 117, 12537. (29) Li, C.; Pobelov, I.; Wandlowski, T.; Bagrets, A.; Arnold, A.; Evers, F. J. Am. Chem. Soc. 2008, 130, 318.

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