Dynamics of CO2-Plasticized Electron Transport in Au Nanoparticle

3778

J. Phys. Chem. C 2007, 111, 3778-3785

Dynamics of CO2-Plasticized Electron Transport in Au Nanoparticle Films: Opposing Effects of Tunneling Distance and Local Site Mobility Jai-Pil Choi, Melissa M. Coble, Matthew R. Branham, Joseph M. DeSimone, and Royce W. Murray* Kenan Laboratories of Chemistry, UniVersity of North Carolina, Chapel Hill, North Carolina 27599-3290 ReceiVed: December 5, 2006; In Final Form: January 12, 2007

The electron-transport properties of unlinked and linked solid-state films of very small “Au38” nanoparticles (monolayer protected clusters, or MPCs) remarkably change in opposing directions upon contact with increasing pressures of CO2 gas (0∼6.6 MPa). Electronic conductivities (σEL) of dropcast, unlinked Au MPC films increase up to 31-fold with increasing CO2 pressure, while σEL of dithiol-linked Au MPC films decreases with increasing CO2 pressure. In conductivity by electron hopping between electron donor and acceptor Au MPC cores, the organic protecting ligands serve as a solvent shell whose properties influence the dynamics of electron transport. Exposure of Au MPC films to CO2 and consequent swelling by CO2 (or organic vapor) sorption into the monolayers changes two key factors: core edge-to-edge electron tunneling distances (dHOP) and a previously underappreciated effect of dimension and/or frequency of local core thermal motions. Swelling induces increases in both, but depending on which factor is dominant, the net σEL can increase or decrease. In unlinked films under increasing CO2 pressure, increased thermal motions of Au MPC cores and their monolayers enhance electron-hopping rates more than swelling-induced increases in dHOP decrease them. The net effect is increasing σEL (i.e., increased local mobility negates increased average tunneling distances). In contrast, in dithiol-linked films local thermal motions are constrained by the dithiol linker between Au MPC cores, leaving CO2 sorption-induced swelling and increase in dHOP as the more dominant factor; σEL now decreases with increasing CO2 pressure.

Introduction

can be written as

An important goal in nanoparticle science is understanding of the electronic, magnetic, optical, and chemical properties of nanometer-sized metal and semiconductor-based objects as their dimensions are reduced from bulk to atomic (or molecular) materials.1 Research over the past several years on gold nanoparticles that are protected by dense monolayers of organic thiolate ligands (termed monolayer protected clusters, or MPCs) has identified MPCs with Au cores of a few tens to several hundreds of atoms. Their size-dependent properties, such as electrochemical charging,2 band gap,3 and electron-transport dynamics,4-6 are gradually becoming mapped out. At Au75,7 discrete electronic states have emerged as a consequence of quantum confinement effects.8 Gold nanoparticles are promising components of future nanoscale electronic devices9 and sensors,10 owing to their stability, synthetic versatility, and controllable surface functionalities. Electron hopping between nanoparticles is a likely component of many nanoscale applications and it is important to understand this phenomenon. For a sufficiently small nanoparticle, electron hopping in a solid-state film can be experimentally defined as single electron exchanges between neighbor electron donor and acceptor nanoparticles. We have demonstrated the bimolecularity of electron hopping between the cores of Au MPC samples prepared in known mixed valent states (i.e., where the charge carrier population is defined). The hopping transport reaction * Corresponding author. E-mail: [email protected].

kEX

MPCx0 + MPCy1+ 98 MPCx1+ + MPCy0

(1)

where kEX, the bimolecular rate constant, remains (ideally) constant for various relative proportions of zero and +1 MPC core charge states, and x and y denote neighbor lattice sites. The hopping rate is maximized when equal amounts of MPC0 and MPC1+ are contained in the MPC film.4,11 This reaction and the consequent value of kEX is expected12 to involve electron tunneling13,14 between neighbor gold nanoparticle cores with the thiolate ligands bound to the core serving as an electron tunneling medium and a kind of solvent shell. The electronic conductivities of (nonlinked, nonplasticized) gold MPC films exhibit4 the expected exponential relation between conductivity and tunneling distance (with electronic coupling constant of ca. 0.8 Å-1 at 30 °C) as the latter is varied using different thiolate ligands -S(CH2)nCH3 (n ) 4 to 16). The potential of high-pressure carbon dioxide (CO2) as a solvent for sustainable technology15 is well known. Physical properties of CO2 (density, viscosity, dielectric constant, etc.) vary over a considerable range with changing pressure. Both liquid and supercritical CO2 phases have been employed to control and manipulate the physical properties of polymers during synthesis16 and processing.17 The phenomena involve CO2 sorption, induced polymer swelling, and plasticization, topics that have been extensively reported on.18,19 Our laboratory has used20,21 sorption from high-pressure CO2 (gas phase) to

10.1021/jp068349h CCC: $37.00 © 2007 American Chemical Society Published on Web 02/14/2007

Electron Transport in Au Nanoparticle Films swell and plasticize molecular electron donor-acceptor media in studies of their electron transport dynamics. The electron donor-acceptor room-temperature melts (formally molecular molten salts or ionic liquids) are based on synthetically combining redox-active moieties with polyethylene glycol oligomers.22 The polyether oligomers function as built-in viscous solvent shells around (electrochemically generated) donoracceptor reaction partners, and their fluidity exerts a substantial influence on the rates of electron self-exchanges and consequently on the dynamics of charge transport in the melt. Further, electron self-exchange rates between redox moieties and rates of their physical diffusion and of their counterions all increase with increased CO2 pressure and sorption and thus-induced plasticization. This is a compelling background for assigning local thermal motions of redox sites in soft materials - whether made from metal complexes or nanoparticles - a lead role in electron-hopping transport. This paper describes the effects of CO2 and organic vapor sorption on solid-state electron-hopping conductivities of films of very small core size Au nanoparticles.23 The Au nanoparticles were previously thought to have Au38 cores but from recent mass spectrometry results23 are now known to be a mixture of Au25 and Au38 cores. We label them here Au38 for simplicity. The nanoparticles bear either monolayers of solely phenylethanethiolate ligands (SC2Ph) or mixed monolayers of SC2Ph ligands and short, thiolated polyethylene glycol chains. We report on the electron-hopping conductivity (σEL) of three kinds of Au38 MPC films: (a) dropcast films of MPCs, which will be termed unlinked films, (b) films of the same kinds of MPCs but which are linked together by reactions with R,ω-alkanedithiols, which are termed dithiol-linked films, and (c) dropcast mixed-valent films (i.e., mixtures of Au380 and [Au38+ClO4-] MPCs). Exposure of these various MPC films to pressurized CO2 (and to organic solvent vapors) changes their electronic conductivities (the rates of electron-hopping transport) in remarkably different ways: σEL of unlinked films increases with increasing CO2 pressure, σEL of dithiol-linked films decreases with increasing CO2 pressures, and mixed-valent film σEL values change in different directions depending on the degree of mixed valency. (These MPC films contain no added electrolytes and have negligible ionic conductivity, as demonstrated by strictly ohmic current-potential responses at low potentials without electrolytic hysteresis effects. It is important to note that they thus differ from PEG-based melts in which ions are mobile on the experimental time frame and electrolytic reactions occur at the microelectrode/melt interface.20,21) Our analysis of the observed MPC film conductivity changes considers two opposing effects. In unlinked MPC films, sorption of CO2 or organic vapors is observed to increase electronhopping conductivity. The sorption swells the film, increasing the average core edge-to-edge electron tunneling distances (dHOP), which can decrease electron-hopping rates. At the same time, however, we reason that local short-range thermal motions of the unlinked MPC donor-acceptor reaction centers are enhanced in the swelled, softened film. If these oscillating translational motions are of amplitude sufficient to shorten the separations between neighbor MPC cores at sufficient frequencies, electron transport can become accelerated even though the average dHOP has increased. (Local thermal mobility implies both physical displacement of the MPC core, altering instantaneous values of dHOP and associated fluctuation in the intervening thiolate ligand shells.) On the other hand, in dithiol-linked films

J. Phys. Chem. C, Vol. 111, No. 9, 2007 3779 and in highly mixed valent films, those local thermal motions are suppressed and sharply constrained, by physical or electrostatic cross-linking of the cores, so that dHOP increases caused by film swelling lead to net decreases in conductivity. A number of previous papers have described effects of sorbed vapors on the electronic conductivity of MPC films. Some studies based on unlinked films describe decreases in electronhopping conductivity as vapors are sorbed,24 while others display either increase and decrease depending on the particular vapor.10c,25 The sizes of MPCs that have been used10c,24,25 are larger (up to 8 nm ) than those in the present paper (∼1.1 nm). Unlinked films of large MPCs (3∼8 nm) generally do not show increases in conductivity upon vapor sorption.24 Films made from linked-togther MPCs rather uniformly display decreases in conductivity upon vapor sorption.26,27 There is general agreement in these works regarding the importance of tunneling distances that can increase upon vapor sorption and swelling, thus decreasing conductivity. There is, however, no generally accepted explanation for increases in conductivity; the change in dielectric permittivity of the intervening MPC-MPC medium upon vapor sorption is one speculation.25 The importance of local thermal MPC mobility has not been considered, as far as we can ascertain. None of the previous works have undertaken the comparison of similar MPCs in linked and unlinked form that is needed to delineate this factor. Also, previous studies all involve MPCs with larger core sizes than studied here. Experimental Section Chemicals. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4‚ 3H2O) was prepared by a literature method.28,29 Phenylethanethiol (PhCH2CH2SH, 98%), poly(ethylene glycol) monomethyl ether (average molecular weight ) ca. 350, PEG350), sodium borohydride (NaBH4, 99%), cerium(IV) sulfate (Ce(SO4)2), tetra-n-octylammonium bromide (Oct4NBr, 98%), sodium perchlorate (NaClO4, 99.9%), 1,8-octanedithiol (97+%), 1,9-nonanedithiol (95%), and tetra-n-butylammonium perchlorate (Bu4NClO4, g99.0%) were obtained from Aldrich or Fluka (Milwaukee, WI). Toluene, n-hexane, n-heptane, methanol (optima grade), acetonitrile (CH3CN, optima grade), and dichloromethane (CH2Cl2, optima grade) were purchased from Fisher Scientific (Suwanee, GA). 1,10-Decanedithiol was purchased from TCI America (Portland, OR) and ethanol (absolute, 200 proof) was purchased from Aaper Alcohol and Chemical Company (Shelbyville, KY). All chemicals were used without further purification. Deionized water (>18MΩ) was prepared with a Millipore Nanopure water purification system. SFC/SFE grade of CO2 from Air Products (Allentown, PA) was used as received. Au38 MPC Synthesis, Ligand-Exchange Reaction, and Chemical Charging. The small core size Au38 MPCs and 350 MW methyl-terminated poly(ethylene glycol) thiol (HSPEG350) were synthesized as described previously.6,30 We note above that this material is now recognized as being a mixture of Au25 and Au38 MPCs.23 MPCs with mixed monolayers of -SC2Ph and -SPEG350 thiolate ligands were prepared by ligand exchange, starting with MPCs with solely -SC2Ph ligands. A 20 mg sample of MPCs dissolved in 5 mL CH2Cl2 was mixed with a 3-fold molar excess of HSPEG350 (relative to the amount of -SC2Ph ligands on the MPCs) and stirred for 24 h at room temperature. The solvent was removed under reduced pressure and the solid product washed with n-heptane several times. The exchanged product’s UV-vis absorption and voltammetry was unchanged. The

3780 J. Phys. Chem. C, Vol. 111, No. 9, 2007 relative numbers of -SC2Ph and -SPEG350 ligands in the sample studied were judged from 1H NMR to be about 5:19. Mixed-valent charging of Au38 MPCs was achieved with Ce(IV), vigorously stirring for various times, 10 mL of aqueous 5 mM Ce(SO4)2/0.1 M NaClO4 with 5 mL of CH2Cl2 containing ∼0.1 mM Au38 MPCs and 0.05 M Bu4NClO4. The aqueous solution was decanted, the CH2Cl2 removed at room temperature under reduced pressure, and the mixed-valent Au38 MPC samples were washed five times with 10 mL methanol to remove Bu4NClO4. The relative proportions of Au380 and Au381+ depend on the reaction time and are determined from rest potentials (EREST) of solutions of the mixed valent MPCs, using the Nernst equation and the formal potential of the Au380/1+ charge state couple, as before.4,11 Supporting Information, Table S-1 gives data on rest potentials and charge state compositions. Fabrication of High-Pressure Platform (HPP) Electrode. The HPP electrode (Supporting Information, Figure S-1) for use in a bath of CO2 gas was based on exposing the tips of two gold wires (0.5 mm diameter, Goodfellow) in an insulating surface. The gold wires were first connected to 22 gauge magnet wire (Belden) with silver epoxy (Epo-Tek H2OE, Epoxy Technology Inc.); the latter bears an insulating coating to avoid shorting. The two wires were then potted in a 1/4 inch stainless steel tube with an epoxy resin (glycidyl end capped poly[bisphenol A-co-epichlorohydrin], Mn ca. 377; cross-linked with 14 wt % 1,3-phenylenediamine; all Aldrich). The epoxy-sealed end of the tube was ground until the gold wire tips were exposed in the epoxy platform, which was then polished smooth with alumina (successively smaller grade from 1 µm to 0.05 µm, Buehler). High-Pressure σEL Measurement Cell. The high-pressure electron-hopping conductivity measurement cell (Supporting Information, Figure S-1) was constructed from a 316-stainless steel tube with a cavity volume of 25 mL. Sapphire windows (1 in. diameter and 3/8 in. thick, Crystal Systems) were mounted onto opposing ends of the cell, and held in place with hollow brass bolts and Teflon O-rings. Three 1/16 in. taper seal ports (High-Pressure Equipment) were used to connect CO2 inlet/ outlet tubes and a thermocouple to the cell. One 1/4 inch NPT port was tapped into the cell for the high-pressure two-electrode platform probe. The cell temperature was maintained at 25 ( 0.5 °C during measurements, and cell pressure was monitored using an output pressure transducer (model THE AP 121DV, Sensotec). High-pressure CO2 was introduced to the cell using a syringe pump (model 260D, Isco). As a safety precaution, a head equipped with a 48 MPa rupture disk (High-Pressure Equipment) is attached in the plumbing between the pump and the cell. High-Pressure Swelling Cell. CO2 sorption and consequent swelling volume of a phenylethylene thiol phase was measured using a previously described31 high-pressure cell. Briefly, this cell (6.35 mm width, 15.87 mm depth, and 15.87 mm height) is designed to constrain swelling of a sample to one dimension, so that a volume change can be easily quantified with optical images taken by a digital camera. An image capture board (Scion Corp LG3) and NIH imaging software were used to correlate the number of pixels occupied by the sample volume in the image with the calibration of the pixels. Because of the large sample volume (∼0.5 mL), a long equilibration time (∼24 h) was allowed prior to measurement. The large sample size required made use of the phenylethylene thiol necessary (rather than Au38 MPCs); the organic volume fraction of the MPCs is roughly 88% so this is a reasonable approximation.

Choi et al.

Figure 1. Dependence of σEL of a solid-state Au38 film on CO2 and He pressure. An equilibration time of 20 min was allowed at each CO2 or He pressure. Inset is a typical i-E response of a Au38 film at a scan rate of 100 mV/s.

Film Preparation and σEL Measurements. For σEL measurements, various Au38 films were prepared by either dropcasting or dithiol-linking on the HPP electrode or on a gold interdigitated array (IDA) electrode (Abtech, Richmond, VA). Unlinked films were prepared by serially casting and drying two droplets (20 µL) of an MPC solution (10 mg/0.5 mL CH2Cl2). Dithiol-linked films form spontaneously on the electrodes when they are dipped into an acetonitrile solution (5 mL) containing 15 mg Au38 MPCs and a 2-fold molar excess of dithiol (relative to -SC2Ph ligands) for 24 h. The IDA electrode has 50 interdigitated fingers, 20 µm wide, 3 mm long, 180 nm high, and spaced apart by 20 µm gap. Films prepared are ∼1 µm thick (unless otherwise mentioned); film thicknesses were measured by stylus profilometry (Tencor Alpha-Step 100). σEL of films was measured from linear segments of currentpotential (i-E) plots using

σEL )

∆i C ∆E CELL

(2)

where CCELL is the cell constant. CCELL (1.36 cm-1) of the IDA electrode was estimated as previously reported.11 The effective cell constant of the wire tip HPP electrode, CCELL ) 642.1 cm-1, was calibrated from the conductance (∆i/∆E) of a Au38 MPC sample that had been measured using the IDA electrode. For measurements of σEL under CO2 pressure, an HPP electrode coated with unlinked or dithiol-linked MPCs was maintained under vacuum (-0.1 MPa) for 12 h in the highpressure σEL measurement cell to ensure film dryness. CO 2 was introduced into the cell in incremental pressures up to 6.6 MPa, giving 20 min of equilibration time at each pressure prior to σEL measurement. A flow cell and IDA electrodes were employed to measure σEL under hexane or ethanol vapors (Supporting Information, Figure S-2). Saturated vapor streams were generated by bubbling dry N2 through pure hexane or ethanol solvent. The saturated vapor stream of constant flow was diluted by metering with a dry N2 stream. At each fraction, 20 min was allowed as equilibration time for partitioning vapors. Results and Discussion σEL of Au38 Films23 at Varied CO2 Pressures. Figure 1, inset shows that a (1 V scan produces a linear current-potential response, without hysteresis. This is seen at all CO2 pressures and tested potential scan rates (5∼1000 mV/s), indicating that

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J. Phys. Chem. C, Vol. 111, No. 9, 2007 3781

TABLE 1: Electronic Conductivity (σEL) Results for Unlinked and Dithiol Linked Au38 MPC Films film type Au38 Au38-PEG350 C8-linked Au38 C9-linked Au38 C10-linked Au38

σEL (Ω-1 cm-1) 0 MPa 6.6 MPa 6.1 × 10-9 6.6 × 10-7 1.0 × 10-10 5.7 × 10-11 3.7 × 10-11

1.9 × 10-7 3.5 × 10-6 5.1 × 10-11 3.3 × 10-11 2.4 × 10-11

kHOP (s-1)

δ (nm)

CF (M)

0 MPa

6.6 MPa

2.4a 2.8d 2.5c 2.6c 2.7c

0.17a 0.10b 0.14d 0.13d 0.11d

1.0 × 103 1.3 × 105 18 10 7

3.1 × 104 6.4 × 109 9.3 6 4.5

a Ref 11. bEstimated by pycnometry using n-heptane as displacement liquid. cCalculated by summing MPC-core diameter (2rCORE) and dithiol ligand length. dEstimated from CF ) (0.7(103))/(4/3 π (δ/2)3 NA) where NA is Avogadro’s number and 0.7 is the fill factor for a hexagonally close-packed film.

Figure 2. Change of the swelling volume (V - V0) relative to the initial volume (V0) of HSC2Ph at 25 °C as a function of CO2 pressures. A time of 24 h was allowed for each equilibration of CO2-sorption.

there is no significant electrolysis or net physical mass transport on the experimental timescales. The electronic conductivities, σEL, measured from the slopes of curves like Figure 1, inset are nearly the same whether measured under He (6.1 × 10-9 Ω-1 cm-1), at any He pressure up to 6.6 MPa (Figure 1 -b-), in air (6.3 × 10-9 Ω-1 cm-1), or under modest (-0.1 MPa) vacuum (5.9 × 10-9 Ω-1cm-1). Introduction of pressurized CO2 (from 0 to 6.6 MPa) causes a substantial increase in the electron-hopping conductivity σEL of the Au38 film (Figure 1 -9-).32 σEL of the Au38 film is enhanced by 31-fold (6.1 × 10-9 Ω-1 cm-1 at 0 MPa vs 1.9 × 10-7 Ω-1 cm-1 at 6.6 MPa). CO2 sorption and swelling with associated increased electron tunneling distances should promote a decrease in the electron-hopping rate, so that effect clearly is not dominant in the experiment. The original 0 MPa conductivity is restored at return to 0 MPa, but with some hysteresis.32 Pressurized He causes no change in conductivity; we infer that it has more limited solubility in the organic monolayer, and/or is a less potent plasticizer of the MPCs compared to CO2. Swelling of the Au38 MPC sample under CO2 pressure was estimated by measuring the swelling of its organic monolayer constituent, namely phenylethylene thiol, as shown in Figure 2. Up to 7.2 MPa, the volume increase is linear, meaning that the linear partition isotherm of Henry’s Law applies. (Some thiol dissolution is probably responsible for the small decrease in apparent swollen HSC2Ph volume when the CO2 phase liquifies at 7.2 MPa in Figure 2.) The increase in HSC2Ph volume at 6.6 MPa was 36%. That CO2 gas would exhibit a substantial solubility in this medium is not surprising given previous33 work on the solubility of aromatic compounds in supercritical CO2 for example. Assuming that CO2 sorbs equally into the -SC2Ph ligand phase of Au38 MPCs, expansion of that phase would

Figure 3. Dependence of absorbance at 680 nm of Au38 film cast on a sapphire window on CO2 and He pressures. An equilibration time of 1.5 h was allowed at each CO2 or He pressure. Inset gives the electronic absorption spectra of an Au38 film on sapphire under various CO2 pressures. The cast film area exceeds that of the light beam.

increase the average core center-to-center separation (δ) from 2.4 nm in unswollen Au38 MPCs (Table 1) by a small increment, to 2.6 nm at 6.6 MPa of CO2. Another swelling measurement was based on the optical absorbance of a film of Au38 MPCs as a function of CO2 pressure. Figure 3, inset shows Au38 MPC absorbance spectra at various CO2 pressures. The spectra retain absorbance features typical of this nanoparticle, a band at 690 nm and shoulder at 795 nm, in its dilute solution spectra (Supporting Information, Figure S-3). The sapphire window cuts off observations below 500 nm. The film absorbance at 680 nm decreases with increasing CO2 pressure by ∼13% at 6.6 MPa of CO2. Both lateral and thickness swelling of the film surely occur; only the former would decrease the quantity of MPC cores in the beam. Assuming only lateral swelling leads to an estimate of δ (core center-to-center) ) 2.5 nm at 6.6 MPa CO2. (An alternative explanation that the observed change in absorbance reflects instead an influence of the sorbed low-dielectric CO2 medium on the MPC core electronic transition responsible for the 680 nm band, is inconsistent with other observations that the Au38 extinction coefficient at 680 nm decreases with increasing solvent polarity.34) There is, again, no effect of pressurized He on the MPC optical properties. The data in Figures 2 and 3 both indicate that swelling of the Au38 film is relatively minor in terms of effect on δ. Using the data in Figure 2, a change from δ ) 2.4-2.6 nm by swelling by sorption from 6.6 MPa CO2 corresponds in terms of average tunneling distance (dHOP, core edge-to-edge) to a change from 1.3 to 1.5 nm. Were this 0.2 nm (2 Å) change in electron tunneling distance the only operative effect, the rate of electron-

3782 J. Phys. Chem. C, Vol. 111, No. 9, 2007

Figure 4. Change of σEL of an Au38 MPC film contacted with partial pressures (relative to saturation) of ethanol (blue line) and hexane (red line) vapors. Carrier gas is dry nitrogen (N2).

hopping conductivity should decrease by ca. 7-fold, assuming35 an electronic coupling term βd ≈ 1 Å-1. Instead, at 6.6 MPa CO2 the conductivity increased by 31-fold (Figure 1). Values of δ of course represent only equilibrium averages, and by themselves do not inform on changes in the amplitude and frequency of local thermal motions of the core and its monolayer ligand bath that are enhanced by CO2 plasticization. For example, a 31-fold increase in electron-hopping rate at 6.6 MPa CO2 could occur by shortening of the Au38 core edge-to-edge distance by only ca. 0.3 nm through oscillating local core translations of frequency higher than the hopping rate of ca. 104 s-1 (Table 1). This small distance of thermal motion of the MPC cores is not very different from the CO2-induced change (ca. 0.2 nm) in the average dHOP. Other Plasticization Effects on Electron Transport. The effect of CO2 sorption on the electron-hopping conductivity of films of unlinked Au38 MPCs is roughly replicated by sorption of other small molecules. Figure 4 shows effects on conductivity by exposing unlinked films to vapors of hexane and ethanol. (We have previously observed, by quartz crystal microbalance measurements, the sorption of ethanol by network polymer films containing larger-core MPCs.36,37) The Au38 MPCs are poorly soluble in liquid hexane and ethanol. Figure 4 shows that the changes in Au38 electron-hopping rates at different partial pressures of these molecules are even larger than seen for CO2 sorption; σEL increases by 135-fold in nearly saturated ethanol vapor and by 47-fold on exposure to hexane vapor. (Currentpotential responses were again linear with no hysteresis.) The optical absorbances of films of unlinked Au38 MPCs (on glass slides) immersed in liquid ethanol and hexane for 20 min (Supporting Information, Figure S-4) decreased by 12 and 5%, respectively. The larger apparent (lateral) swelling in ethanol is consistent with the larger increase in MPC film conductivity in ethanol vapor (Figure 4) and with the notion that plasticization and local thermal motions are enhanced by a larger volume fraction of sorbed small molecules. The fluidity of MPC ligand monolayers and the response of film conductivity to sorbed CO2 can also be manipulated by choice of the ligands employed in the MPC monolayer. The phenylethanethiolate ligands of Au38 MPCs were partly replaced (exchanged) by thiolated poly(ethylene glycol) to produce mixed monolayer MPCs (Au38-PEG350).6 PEG materials are wellknown for preparing molecular melts22and are CO2-philic.20,21 CO2-induced swelling of PEG is well documented.19,38

Choi et al.

Figure 5. Dependence of σEL of Au38 and Au38-PEG350 films on CO2 pressures. An equilibration time of 20 min was allowed at each CO2 pressure.

An unlinked MPC film prepared from Au38-PEG350 is expected (in the absence of CO2) to have greater local thermal mobility than do MPCs in other unlinked Au38 films, owing to the softening effect of the PEG ligands themselves.20-22 A greater conductivity is indeed observed as seen in Figure 5 and Table 1. In the absence of pressurized CO2, σEL of the Au38PEG350 film (6.6 × 10-7 Ω-1cm-1) is ca. 102-fold larger than that of an unlinked Au38 MPC film (6.1 × 10-9 Ω-1cm-1). Figure 5 shows that σEL of the Au38-PEG350 film again increases with increased CO2 pressures, but by a smaller factor (5-fold at 6.6 MPa CO2) than was the case (31-fold) for Au38 MPCs. It is plausible that the relative enhancement in local thermal mobility and in consequent electron-hopping rate is less when there is a substantial pre-existing degree of local translational mobility. Although not measured, the Au38-PEG350 film is also probably more swelled than that of Au38 MPCs, so a larger contravening increase in electron-hopping distances may play an added role. Another consideration conceivably relevant to the experiments in Figures 1 and 4 is the possible effect of changing the dielectric environment of the MPC on its electron-transfer properties. Perfusion of CO2 into the MPC monolayer should lower the effective dielectric constant of the MPC core medium bath, which could increase the hopping rate as in Figure 1. However, addition of the more polar thiolated PEG to the monolayer should then have the opposite effect, which is not observed (Figure 5, Table 1). For this reason we believe previous explanations of MPC film conductivity changes based on dielectric properties of sorbed organic vapors are less attractive than local thermal mobility changes. Electron-hopping conductivity is enhanced11 by the addition of Au381+ states (doping) to the film. That the difference in conductivity between films of Au38 MPCs and of Au38-PEG350 might reflect an unintentional mixed valency in the latter was ruled out by evaluating rest (EREST) and formal39 (E°′) potentials of their solutions, as done previously.4,11 EREST and E°′ of Au38 MPCs (in CH2Cl2) are -0.47 and -0.36 V, respectively, (vs 10 mM AgNO3/Ag in CH3CN); corresponding values for Au38PEG350 MPCs (in CH3CN) are -0.39 and -0.26 V, respectively. Applying the Nernst equation40 to these results shows that both MPC materials contain only about 1% of 1+ charged nanoparticles. First-order electron-hopping rate constants, kET (s-1), can be estimated from MPC film conductivities by assuming a cubiclattice model, as used previously for redox polymers and

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alkanethiolate-protected nanoparticles4,5,11,41

kHOP )

6RTσEL 10-3F2δ2CF

(4)

where F is the Faraday constant, δ is the core center-to-center distance, and CF is the concentration of MPCs in the film. This relation normalizes for the roles of nanoparticle concentration and electron-hopping distance in transport and improves comparisons between different MPC materials. The concentration of Au cores (CF) is 0.10 M for Au38-PEG350 MPCs (Table 1) with an electron-hopping distance of 2.8 nm. Although its concentration is lower and δ larger than those in Au38 MPC films (0.17 M and 2.4 nm respectively), the Au38-PEG350 film exhibits a 102-fold faster electron-hopping rate. Again, this reflects the strong role of local thermal mobility in the electrontransfer mechanism that controls electron transport in the film. Electron-Hopping Transport Through Dithiol-Linked Films. All of the preceding data refer to unlinked Au38 MPC films and their variants. We turn next to films composed of Au38 MPCs but in which the MPC cores are linked together by R,ω-alkanedithiols (HS-R-SH, RdC8H16, C9H18, and C10H20). The linking occurs by ligand exchange in which opposite ends of a given dithiol displace -SC2Ph ligands from two neighbor MPCs. The populations of dithiolate ligands in the film that are only monolinked, or are doubly linked to a single MPC core, are unknown as we do not have a developed assay for this. That some dithiols do link (and cross-link) MPCs together is, however, manifest by the formation of a dense, insoluble film on most surfaces in contact with the exchanging solution. The measured average edge-to-edge spacing (dHOP) in unlinked Au38 MPC films,11 1.3 nm, is smaller than that for dithiol linked ones if dHOP is assumed to equal the extended chain lengths42 of the dithiolate ligands, which are 1.4, 1.5, and 1.6 nm for -SC8S-, -SC9S-, and -SC10S- dithiolates, respectively. The σEL electron-hopping conductivities of dithiol-linked films are remarkably lower than those for unlinked MPC films (Table 1). For example, compared with σEL (6.1 × 10-9 Ω-1 cm-1) of an unlinked Au38 MPC film, σEL (3.7 × 10-11 Ω-1 cm-1) of a -SC10S- dithiolate-linked film is lowered by ca. 160-fold. Under 6.6 MPa CO2, the difference climbs to ca. 8 × 102-fold. These changes are much larger than could be anticipated from the relatively small changes in average MPC separation (Table 1). As anticipated, cross-linking of MPCs with dithiol is highly effective at reducing their local thermal mobilities, relative to those of unlinked ones. MPCs linked with different chainlength dithiols exhibit small increments of conductivity with decreasing dithiolate linker chainlength. The changes (Table 1) per dithiolate linker methylene unit are between 2- and 1.5-fold, which is somewhat less but not much, than the 2.6-fold change expected for 1.2 nm increments (methylene unit length) of tunneling distance based on βd ) 1 Å-1. The smaller than estimated change may be due to partial folding of the dithiolate linker (assumed in the estimate to be extended), and/or by our choice of βd value. Further, in sharp contrast and opposite to Figures 1 and 4, dithiol-linked MPC films under CO2 exhibit conductivities (Figure 6) that decrease with increasing CO2 pressure. The decreases in σEL seen in Figure 6 are presumed to occur through the agency of increased electron tunneling distances. The reduced fluidity of the monolayer structures around the core sites, as compared to unlinked films, must restrict the amplitude and/or frequency of local thermal fluctuations that shorten the MPC core edge-to-edge distance. The result is dominance of

Figure 6. Dependence of σEL of dithiol-linked Au38 films on CO2 pressures. Linker dithiols are indicated on the figure: C8- ) -(CH2)8S-, C9- ) -S(CH2)9S-, and C10- ) -S(CH2)10S-. An equilibration time of 20 min was allowed at each CO2 pressure.

electron-hopping rates by tunneling distances. While the dithiollinkers constrain local thermal motions, a modest amount of swelling, accompanied by extension of any pre-existing folding of the dithiolate linker chains, appears to still be possible. We attribute the CO2-induced decreases in σEL (2-fold for the -SC8Sdithiolate linker and 1.5-fold for the -SC10S- dithiolate linker, Table 1) to such swelling. Equation 4 presumes uniform packing of the MPC nanoparticles, which is undoubtedly an approximation as perfect packing is unlikely. Variations in packing disorder among the different ways films are formed may contribute to differences in their conductivities. Packing disorder will invariably elongate average tunneling distances and increase the incidence of nonconductive voids. In unlinked films, with or without PEG350 content, these effects must be modest because the measured film densities (i.e., δ values) are not far from ideal. For the dithiollinked films, direct measurement of film density and thus dHOP is not available. It seems reasonable, however, that as linked films are assembled in a chaotic process that includes both oligomer formation in the bulk solution and subsequent grafting to a surface film, as well as monomer-by-monomer film growth, that defects and voids are common in linked MPC packing. We cannot separate out the defect issue at this time; in the dithiollinked films it could well play as important a role in the electronhopping conductivity as the suppression of local thermal motions discussed above. Another packing issue that would affect solely the unlinked Au38-PEG350 films could be nonsphericity of these MPCs owing to microphase segregation of the two kinds of ligands. We have no data to assess whether such segregation occurs in these materials or not. The effect of irregular packing would however decrease σEL, not increase it relative to -SC2Ph-only MPCs. The issue then is whether the increase in conductivity caused by the PEG350 thiolate ligands might have been even larger were no packing defects to exist. Electron-Hopping Transport Through Mixed Valent Films. Mixed valency in a solid film has the effect of linking MPCs by electrostatic interactions to the counterions of charged MPCs. We used previously developed methods to prepare mixed valent films of Au38 MPCs, containing demonstrably variable proportions of Au38 cores with zero and +1 charge states, and estimated the degree of mixed valency as reported before. The detailed data are summarized in Supporting Information, Table S-1. As expected, we see that electron transport rates are

3784 J. Phys. Chem. C, Vol. 111, No. 9, 2007

Figure 7. Dependence of σEL of the mixed-valent Au38 films on CO2 pressures. Percent of Au381+ in the films, as determined from EREST and the Nernst equation, is indicated on the figure.

enhanced by the addition of +1 states because the electronhopping reaction is bimolecular (eq 1). The interesting effect is how the different mixed valencies respond differently to CO2 sorption. Figure 7 shows how σEL of Au38+1/0 MPC films varies with their mixed valency and as a function of CO2 pressure. With no CO2 pressurization, σEL increases with the population of +1 states (1-42%) as known from previous work.11 This effect is quantitatively attributable to the second-order kinetics of the electron-hopping reaction. When CO2 pressure is increased, σEL changes in three different ways. For very low proportions of the +1 state (1%, lower curve), conductivity increases with increasing CO2 pressure much like Figure 1. At 22% of +1 states, conductivity mildly decreases with increasing CO2 pressure and then reaches a minimum and mildly increases. At higher +1 proportions, 32 and 42%, σEL decreases at higher CO2 pressures, by factors respectively of 0.7 and 0.6, and flattens out at the highest CO2 pressure. We propose that the results in Figure 7 represent a transition from effects seen in Figure 1, where changes in local thermal mobility provided the major consequence of CO2 sorption, to those seen in Figure 6, where constraints in local mobility are imposed by binding interactions between MPCs. In Figure 7, the electrostatic binding interactions between +1 charge state MPCs and their counterions presumably would increase from 1-42%. The sorption of CO2 is most at the highest CO2 pressures and local thermal mobilities are most enhanced there, which causes the minimum in the 22% results and the flattening out at high CO2 pressures. Conclusions The present results highlight electron-transport properties of linked vs unlinked Au38 nanoparticle films. The rate of electron hopping through the film as CO2 (or organic vapor) is sorbed is proposed to be mainly governed by two opposing factors: conductivity decrease caused by increased tunneling distance between MPC cores and conductivity increase caused by enhancement of local thermal motions of MPC cores and/or their surrounding monolayers, as illustrated in Figure 8. These two factors play against one another differently, depending on whether the MPCs are linked together or not. Unlinked films maximally allow local thermal motions, which appear to be sufficient in amplitude and/or frequency to allow favorable electron tunneling pathways that overwhelm the average increase

Choi et al.

Figure 8. Schematics illustrating (a) local short-range thermal motions of MPC cores and (b) quenching (or suppressing) of such thermal motions by dithiol-linking. R ) PhC2S- or -SPEG350; L ) C8-, C9-, or C10-dithiol.

in tunneling distance caused by film swelling. This result clearly shows that average distances between nanoparticles, especially unlinked nanoparticles, are a poor basis for interpretation of tunneling-based electron transport rates through nanoparticle films. The increased local thermal mobility consequence of CO2 sorption is strongly suppressed by dithiol linking between MPC cores. Then, tunneling distances become a significantly important factor in film conductivity. It is useful at this point to look back at some previous reports on how organic vapors (analogous to CO2 sorption) affect electron-hopping conductivity of MPC films. For the most part, the previous observations are entirely consistent with the results presented here. MPC films that are linked together by dithiols,26a,c,27a,c,43 metal carboxylate coordination,36,37 and hydrogen bonding25b exhibit decreases in conductivity upon vapor sorption (and presumed swelling). (Air brush-deposited, unlinked films studied as gas chromatography detectors,10c,24c however, exhibit decreased conductivity upon organic vapor exposure.) The present report is the first to show that a general increase in conductivity upon small molecule sorption of both polar and nonpolar type occurs for unlinked films and is the first to show for a given nanoparticle chemistry that the direction of response to sorption can be ordained by linking the nanoparticles together or not. Finally, connection needs to be drawn between the present results and a theoretical paper44 that dealt with the interplay between the restraining effects of static percolation on charge transport in redox polymers and the enhancing effects of redox site mobility on reducing the impact of static percolation rules. Substantial local site mobility produced transport more akin to free electron-hopping diffusion. Knowing of this insightful previous paper was of value in our analysis of the present data. Acknowledgment. This work was supported in part by grants from the National Science Foundation and the Office of Naval Research. Supporting Information Available: Table of charge-state composition of mixed-valent Au38 MPCs; diagrams of a highpressure σEL measurement cell, a high-pressure platform (HPP) electrode, and a flow cell; UV-vis absorption spectrum of Au38 in CH2Cl2; UV-vis absorption spectra of Au38 films in ethanol and hexane; and cyclic voltammograms of Au38 and Au38-

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