Applied Surface Science 182 (2001) 338±344
Preparation and electrical characterisation of dodecanethiol monolayer protected silver nanoclusters M. Aslam, N.K. Chaki, I.S. Mulla, K. Vijayamohanan* National Chemical Laboratory, Physical and Materials Chemistry Division, Dr. Homi Bhabha Road, Pune 411008, India
Abstract Preparation and characterisation of dodecanethiol monolayer protected silver clusters (10±15 nm) (Ag-DDT) is reported here to illustrate its interesting low temperature electrical properties. These monolayer protected silver nanoclusters were found to undergo an insulating to metallic transition around 180 K, in agreement with the theoretical prediction of the disappearance of the Kubo gap at low temperature in these systems, where the interparticle spacing (0.5±1 nm) is less compared to the nanocluster dimensions. The XRD data suggest the formation of multilayered structure, and the equidistant peaks at smaller angle indicates a long-range ordering of nanoclusters. The UV±Vis absorption in solution indicates the presence of a surface plasmon band at 425 nm and cyclic voltammetry studies con®rm the accessibility of these passivated clusters for electron transport. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Dodecanethiol; Monolayer protected; Nanoclusters; Cyclic voltammetry
1. Introduction Recent years have seen large efforts in utilising organic molecules capable of self-assembled monolayer formation for preparing quantum dots experimentally [1±4]. The individual and collective electronic properties of these monolayer protected clusters (MPCs) have been investigated with respect to size/carrier con®nement, and several important devices for molecular electronics such as single electron transistors (SETs) have been developed [1±3]. One of the important advantages of using organic molecules is that since the con®nement of electrons wave functions causes discreteness of energy levels, the average spacing of successive quantum levels (Kubo gap, d) can be controlled by adjusting the length of the molecules to make a system metallic * Corresponding author. Tel./fax: 91-20-5893044. E-mail address:
[email protected] (K. Vijayamohanan).
or nonmetallic. Since d is given by 4Ef/3n, where Ef is the Fermi energy of the bulk metal and n the number of valence electrons in the nanoclusters, a change in n by size tuning can lead to the control of d compared to the value of thermal energy. For example, d varies with r 3 for spherical particles, as their density of states is proportional to the volume while spacing can vary linearly with the hydrocarbon chain length. The low temperature behaviour of these systems will be more interesting as the spacing d may become larger than kBT, and the life time, t of the electronic states will be much larger than h/d, thus making the system insulating. At the same time, orientational disorder of organic molecules can create localisation which may be seen from the temperature dependence. The word Coulomb blockade indicates the suppression of electron transfer through tunnelling when nanoclusters are dispersed in an insulating matrix. This basically arises due to the charge quantisation in nanoclusters capable of small quantum charge
0169-4332/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 ( 0 1 ) 0 0 4 4 5 - 7
M. Aslam et al. / Applied Surface Science 182 (2001) 338±344
¯uctuations when electron transfer can occur as only in units of e. When there is a large interparticle separation
D > 2r, the clusters behave like a Mott insulator with a Coulomb gap described by the charging energies of individual nanoparticle sites. On the other hand, when the distance between the adjacent clusters is very small
D ! 2r compared to their size, strong quantum mechanical exchange coupling can cause the disappearance of Coulomb gap, causing metal-to-insulator transition (MIT) [5±9]; disorder induced localisation can make the system still insulating (Anderson localisation). The tuning of exchange coupling can be correlated with engineering the electronic properties of the quantum dot solid. Since the charging energy scales inversely with particle size, the MIT is experimentally easier to observe in superlattices composed of the larger clusters. This is to be contrasted with transition metal oxides, where MIT is normally accomplished by the variation of charge carrier density through altering the composition using substitution chemistry. Although doping of charge carriers can, in principle, cause disorder in this system, this aspect is not taken care of when homogenous models like Hubbard model are used which consider only interaction effects. In contrast, the change in Coulomb gap due to size distribution can cause disorder induced changes in the electronic structure which is manifested in the case of Coulomb blockade nanostructures. In this study, we demonstrate the possibility of such a transition in dodecanethiol protected silver nanoclusters using the temperature dependent electrical resistivity measurement. We use UV±vis spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV) as the characterising tools. XRD reveals the superlattice like solid formation, which demonstrates the accessibility of these clusters for the electron transfer through such periodic arrangement. 2. Experimental Dodecanethiol capped silver nanoclusters were synthesised using the Brust synthesis route [10]. In brief, AgNO3 was dissolved in millimolar concentration in deionised water and dodecanethiol was taken
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separately in toluene in a 1:2 ratio. After vigorous stirring for 30 min, this biphasic mixture was reduced slowly by dropwise addition of aqueous 0.1 M NaBH4 solution. The stirring was continued till the transfer of metallic ions to nonaqueous layer was complete. The layer was separated and dried under N2 atmosphere at 50±608C. The particle size was determined using UV±Vis absorption maximum to be ranging from 8 to 12 nm and was con®rmed by high resolution transmission electron microscopy (HRTEM). The average spacing between the clusters was found to be 0:65 0:02 nm. The optical absorption spectra were recorded using a Shimadzu, UV-2101 PC spectrophotometer with 2 nm spectral resolution. The dried samples were redissolved in dry toluene (25 mg of dried clusters in 50 ml of toluene) before taking the spectra. The XRD patterns were taken on RIKAGU mini¯ux JAPAN instrument using Cu Ka radiation
l 1:5404 A. Cyclic voltammograms were obtained using a Scanning Potentiostat Model 362 and a Recorder Model RE0151 using a standard three electrode cell comprising Pt as a working electrode, a platinum foil as counter electrode and saturated calomel electrode (SCE) as the reference electrode. Electrochemical studies were carried out on a 0.1 mM dodecanethiol monolayer protected silver clusters at different scan rates in a (1:1) toluene/acetonitrile mixed solvent containing 0.1 M tetrabutyl ammonium tetra¯uoroborate. The electrical resistivity measurements were performed between the temperatures 300 and 16 K under vacuum better than 10 5 Torr (APD Cryogenic System) on the pellets using the standard four-probe method. All the experiments were repeated several times with different nanocluster samples prepared and stored under identical conditions. 3. Results and discussions Fig. 1 shows the comparison of optical spectra of the capping agent alone and the capped metal clusters. For thiol capped silver nanoclusters, the characteristic surface plasmon band around 425 nm is observed similar to the reported results for silver nanoclusters using several other preparation conditions [11]. The broadening of the spectra arises due to the smaller size of the particles than the mean free path of the electron
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Fig. 1. UV±Vis spectra of: (a) free thiol; (b) Ag-DDT particles in toluene.
(>52 nm for silver). A rough estimation of the particle size on the basis of available optical absorption for silver cluster indicates the average size of 14 nm [11]. This is in excellent agreement with the results of TEM. Fig. 2 shows the X-ray diffractogram of dodecanethiol protected silver nanoclusters. In addition to the expected diffuse peaks arising from the planes of silver atom (arrow marked) within individual nanocluster cores, we found a sequence of equidistant intense sharp Bragg peaks in the small angle region which indicates a multilayered structure. These peaks arise from spacings between the planes of the nanoclusters within a giant, three-dimensional (3D) superlattice with a repeated order; a crystal of nanoclusters [12]. The bulk Ag re¯ections seem to be the part of the such periodic layered structure. Although the diameter of the clusters from TEM is not greatly different, the XRD peaks are narrower suggesting that the particles are composed of several such unit cells. We can attribute such a type of superstructure formation to interdigitation of alkyl chains. Therefore, we can index the pattern with unit a cell dimension larger than the core cluster size revealed by TEM (not shown here). However, further studies will explore the correlation of alkanethiol lengths and temperature on such arti®cial lattices, one can further tailor-make the multilayered structure by using
different combination of chain length and functional group and can reveal how the ordering of hydrocarbon chains is thermodynamically favourable. CV is an important tool to understand the electron transfer properties of these clusters, as totally passivated (i.e. insulating) clusters will not show any redox behaviour. Fig. 3 shows cyclic voltammograms of the blank and silver MPCs dispersed in the solvent at a scan rate of 500 mV/s with Pt as the working electrode. The voltammograms of dodecanethiol monolayer protected Ag clusters are unusually broad perhaps due to the large ohmic resistance associated with the insulating nature of the thiol layers on the metal clusters. Alternatively, broad voltammograms can arise due to a distribution of redox potentials corresponding to cluster distribution [13]. This characteristic feature of Ag clusters may be due to the size and number of thiolate ligands on the cluster. The blank CV is featureless while the silver octanethiol MPC shows a single cathodic peak at 0.03 V and two anodic peaks at 0.94 and 0.44 V, respectively. The anodic peaks may be due to the oxidation of Ag clusters while the cathodic peak is due to reductive desorption of thiol. Both the cathodic and anodic peaks decrease in intensity with the number of scans and disappear gradually (8th cycle) since more and more Ag clusters are assumed to be oxidised with increasing number of cycles. The quasi-reversible
M. Aslam et al. / Applied Surface Science 182 (2001) 338±344
Fig. 2. The Cu Ka XRD pattern of Ag-DDT nanoclusters. The arrow marks indicate the (1 1 1) and (2 2 0) planes of bulk silver.
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Fig. 3. Cyclic voltammogram taken at 500 mV/s in 1:1 toluene:acetonitrile mixture with 0.1 M TBAF6P using Pt as working and SCE as reference electrode. The Ag-DDT clusters were dispersed in the medium.
peaks corresponding to the oxidation and reduction of Ag clusters observed at higher scan rates is tentatively assigned to redox processes on clusters
Agn ! Ag n e . The approximate surface coverage as calculated from the area under the anodic and cathodic peaks is 1:68 10 10 and 5:44 10 10 mol, respectively, implying that the cathodic currents are larger than the anodic currents. There is also a possibility of silver deposition on Pt working electrode on con®ning the cycling. The double layer capacitance for the Pt working electrode in this electrolyte at 0.4 V is found to increase from 76 mF/cm2 (blank) to 433 mF/cm2 (dodecanethiol MPC) presumably because of the organisation of clusters on the interface contributing to an increase in the dielectric constant. Since the above redox accessibility of silver nanoclusters depends on the time constant of the organised system, a variation in the sweep rate is
expected to alter the electron transfer properties of clusters. Fig. 4 shows the CV of Ag-dodecanethiol MPC at different scan rates. At a scan rate of 20 mV/s there are two anodic peaks at 1.2 and 1.43 V, respectively. However, as the scan rate increases the anodic peaks disappear and a cathodic peak starts to appear at 0.45 V increasing in intensity. For example, at a scan rate of 200 mV/s the 1st cycle shows a pronounced cathodic peak at 0.2 V and two minor anodic peaks at 0.3 and 0.8 V, respectively. The cathodic peak gradually decreases in intensity with the number of cycles while the anodic peak disappears completely in the 2nd cycle itself. At a scan rate of 500 mV/s both anodic and cathodic peaks are highly prominent and decrease in intensity with the increasing number of cycles. This behaviour, attributed to the monolayer fragmentation to form silver sulphide on the surface, needs further investigations.
M. Aslam et al. / Applied Surface Science 182 (2001) 338±344
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Fig. 4. Scan rate dependent voltammogram for the Ag-DDT cluster after dispersion on toluene using Pt as working Electrode. SCE is used as the reference electrode.
Fig. 5 shows the temperature dependence of resistivity (r) for dodecanethiol capped silver clusters, their room temperature (300 K) r values being 12 MO cm. Particularly, interesting is the initial temperature (200±300 K) independence
dr=dT 0:015 O cm=K, characteristic of weak electron± phonon coupling, shown for Ag nanoclusters. More interestingly, as the temperature decreases, metallic
Fig. 5. Temperature (K) vs. resistivity (r) plot for Ag-DDT clusters between 300 and 140 K. Ag-DDT indicates dodecanethiol capped silver clusters.
behaviour is observed suggesting a fast ordering in the monolayers, perhaps arising from the fragility of the defects in the organic layer on the Ag surface. In addition, a shoulder before the transition is also seen, which can be considered a ®rst-order transition owing to the disordered state of the Ag MPCs. Further studies are necessary to unravel the exact origin of these phenomena and we have observed such a transition for similar Cu and Au clusters. The transition temperature for these ordered nanosized clusters may be correlated to the Kubo gap through the Fermi energy of the respective metals as follows. The magnitude of d can be approximately estimated based on the experimentally determined size of the cluster. For example, d for our clusters is of the order of about 2 meVand kB T > d even at room temperature
kB T 25 meV. For bigger clusters having about 103 atoms, the charging energy can be calculated using the expression, E
r e2 =2C
r, where C is the capacitance of the individual nanocluster. Considering the clusters as `spheres with a thin dielectric shell', C(r) can be calculated with the help of equation, C 4pe0 er rR=R r
1 er , using known value of er, r and R (where R r length of the organic molecule). Taking er as 3 (justi®ed on the basis of experimentally measured values of 2.8±3.1 for hydrocarbon chain lengths having 10±14 carbon atoms [14]), the capacitance values are found to be of the order of 2 aF. The charging energy varies
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between 50 and 150 meV which is ®ve times less than that of the particles of the 3 nm size showing charging energy of the order of 0.3 eV [9]; and hence an easier path (very low Kubo gap) for the transfer of electron exists making these metallic. This con®rms the prediction of Henrichs et al. [15] that bigger particles will be easily accessible for the metallic transition at low temperature. The experimentally observed value of D/2r for these clusters is in excellent agreement with the earlier predictions of the Anderson transition to a delocalised electronic phase when D=2r < 1:4 [16]. The nature of the electron transport was further investigated using various thermally activated hopping models [5] in the initial region prior to Tc. The application of r / exp
E=kT and its ®tting to Motts variable range-hopping (VRH) model, r / exp
T0 =Tf , where T0 b=kB N
Exd [5], allowed us to calculate the hopping distance, which was found to be comparable to the lattice spacing. We have calculated localisation radius using T0 b=kB N
Exd , where N(E) is the density of states at Fermi level, x the localisation radius of states near the Fermi level, d the dimensionality considered as 3 and b a numerical constant (21.2 for 3D) characteristic for the system. The value of f, related to the dimensionality of the system, is close to 0.5 for these clusters. This is in excellent agreement with the ordered nature of the superlattice [17]. 4. Conclusions In conclusion, our experimental data provides strong evidence that dodecanethiol capped silver clusters, show reversible insulator-to-metal transition when the interparticle separation is much smaller than the size of clusters. Structural characterisation using XRD and TEM supports the interdigitation of organic molecules leading to the formation of a superlattice. Electron transport through the insulating organic molecule and the Ag MPCs array is substantiated
by the reversible peak in cyclic voltammogram. This type of hierarchical design of organoinorganic nanostructures with multiple length scale has several promising application in molecular electronics. Acknowledgements MA and NKC would like to thank the Council of Scienti®c and Industrial Research (CSIR), Delhi, for the award of Senior and Junior Research Fellowships, respectively. References [1] Y. Xia, B. Gates, Y. Yin, Y. Lu, Adv. Mater. 12 (10) (2000) 693. [2] W.P. Halpen, Rev. Mod. Phys. 58 (1981) 533. [3] C.P. Collier, R.J. Saykally, J.J. Shiang, S.E. Henrichs, J.R. Heath, Science 277 (1997) 1978. [4] K. Vijayamohanan, M. Aslam, Appl. Biochem. Biotech., in press. [5] N.F. Mott, E.A. Davis, Electronic Processes in Noncrystalline Materials, Clarendon Press, Oxford, 1971. [6] F. Remacle, R. Levine, J. Am. Chem. Soc. 122 (2000) 4084. [7] J. Lambe, R.C. Jaklevic, Phys. Rev. Lett. 22 (1969) 1371. [8] R.E. Cavicchi, R.H. Silsbee, Phys. Rev. B 37 (1988) 706. [9] J.J. Shiang, J.R. Heath, C.P. Collier, R.J. Saykally, J. Phys. Chem. B 102 (1998) 3425. [10] M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, J. Chem. Soc., Chem. Commun. 7 (1994) 801. [11] A. Henglein, Acc. Chem. Res. 97 (1993) 5457. [12] R.L. Whetten, M.N. Sha®gullin, J.J. Khoury, T.G. Schaff, I. Vezmer, M.M. Alvarez, A. Wilkinson, Acc. Chem. Res. 14 (1999) 297. [13] E. Sabatini, I. Rubinsten, J. Phys. Chem. 91 (1987) 2974. [14] E.E. Polymeropoulos, J. Sagiv, J. Chem. Phys. 69 (5) (1978) 1836. [15] S. Henrichs, C.P. Collier, R.J. Saykally, Y.R. Shen, J.R. Heath, J. Am. Chem. Soc. 122 (17) (2000) 4077. [16] F. Remacle, C.P. Collier, J.R. Heath, R.D. Levine, Chem. Phys. Lett. 291 (1998) 453. [17] M. Aslam, I.S. Mulla, K. Vijayamohanan, Appl. Phys. Lett. 79 (5) (2001) 689.