Electronic Properties of a Functionalized Noble Metal Nanoparticles

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Electronic Properties of a Functionalized Noble Metal Nanoparticles Covalent Network Ilaria Fratoddi, Roberto Matassa, Laura Fontana, Iole Venditti, Giuseppe Familiari, Chiara Battocchio, Elena Magnano, Silvia Nappini, Grigore Leahu, Alessandro Belardini, Roberto Li Voti, and Concita Sibilia J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b07176 • Publication Date (Web): 04 Aug 2017 Downloaded from http://pubs.acs.org on August 9, 2017

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The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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The Journal of Physical Chemistry

Electronic Properties of a Functionalized Noble Metal Nanoparticles Covalent Network I. Fratoddia*, R. Matassab, L. Fontanaa, I. Vendittia, G. Familiarib, C. Battocchioc, E. Magnanod, S. Nappinid, G. Leahue, A. Belardinie, R. Li Votie, C. Sibiliae a

Department of Chemistry, Sapienza University of Rome, P.le A. Moro 5, 00185, Rome Italy.

b

Department of Anatomical, Histological, Forensic and Orthopaedic Sciences, Section of Human Anat-

omy, Sapienza University of Rome, Via A. Borelli 50, 00161 Rome, Italy. c

Department of Physics, Unità INSTM and CISDiC University Roma Tre, Via della Vasca Navale 85,

00146 Rome, Italy d

IOM CNR, Laboratorio TASC S.S. 14, km 163.5, 34149 Basovizza, Trieste, Italy.

e

Department of Basic and Applied Sciences for Engineering, Sapienza University of Rome, Via Antonio

Scarpa 16, 00161 Rome, Italy.

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ABSTRACT: Functionalized Gold and Silver nanoparticles have been prepared and characterized by means of spectroscopic and morphological techniques together focused on electrical measurements on cast deposited films. The Au and Ag nanoparticles have been functionalized with a on purpose prepared conjugated dithiol, the 9,9-didodecyl-2,7-bis-thiofluorene (FL), giving rise to organic solvents soluble AuNPs-FL and AgNPs-FL samples, respectively. In the case of AuNPs-FL, well separated nanoparticles with average size of about 4 nm, assembled into bi-dimensionally monolayer network and with regular spatial distributions have been observed by TEM. Synchrotron Radiation-XPS data support the observed network formation, showing that all bifunctional ligands end-groups are covalently bonded to metal noble atoms at the nanoparticles surfaces. In order to investigate their interesting conduction properties, electrical measurements evidenced a non ohmic behavior in the case of AuNPs-FL thin film, with a conduction mechanisms that strongly depends on polarons and bipolarons along the carbon active chain belonging to the Fluorene bridge. By increasing the stacked layers of AuNPs-FL thin film, the conductivity behavior changed following approximately the ohmic law. On the contrary, AgNPs-FL shows higher conductivity and upon an ageing process, a diode behavior was observed, that opens perspectives as flexible optoelectronics devices.


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Introduction One of the tools underlying the basis of nanotechnology is the bottom - up approach in which the system is developed starting from small to large complex structures. Metal nanoparticles (MNPs) can be produced through chemical reactions from a limited number of molecular entities, atomic or ionic in suitable conditions [1,2], allowing an atomic precision control [3]. A fundamental role in potential applications is played by the functionalizing ligand; like as, organic thiols, [4,5,6] organometallic systems [7,8] or polymers [9] that can make NPs based colloids easily handle in organic or aqueous media [10]. The ability to prepare metal nanoparticles of a desired composition, size, and shape enables their resulting morpho-structural properties to be tailored into different self-assembled architectures fundamental for several advanced applications, [11,12, 13]. In particular, functionalized metal nanoparticles are transforming many research fields, from biomedicine [14,15] to catalysis and energy conversion [16] with the intellectual excitement associated to the chance of finding new materials with synergic physical chemical properties. The large 3D surface area of MNPs leads to high local concentrations of ligands on the surface, providing enhanced opportunities for optoelectronics [17]. Additionally, MNPs have been functionalized with optically active organic or organometallic molecules for enhancing the optical/electronic properties of the metallic core tuned together with the active optical behavior of the ligands [18]. Among the optical applications of MNPs, a growing interest increased during the last years; colloidal AuNPs have opened new perspectives for the development of theranostic agents in biomedicine, considering that their optical properties provide both diagnostic and therapeutic advantages [19]. AgNPs are widely studied as antimicrobial agents due to the release of silver ions under physiological conditions [20] and an understanding on the interactions on the surface is necessary to their handling. Network based on MNPs have attracted a lot of interest due to their electronic and optoelectronic properties [21, 22, 23]. The manipulation of colloidal MNPs allows their self-assembly into complex structures, 2D or 3D networks showing interesting collective properties [24]. In particular, the fundamental control of the growth stabilization of AuNPs and their spatial distribution has been recently reported for the formation ACS Paragon Plus Environment

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of 2D-network [25]. Literature demonstrates that nanoparticle networks can be utilized as a template structure to incorporate single molecules and studied for example as photoconducting materials [26, 27]. In this arrangement, the nanoparticles act as electronic contacts to the molecules. By varying parameters such as size or interparticle distance, the electronic behavior of the MNPs network can be substantially tuned and controlled. In general, when MNPs are physically in proximity each other into regularly packed aggregates, the interparticle coupling effect shows tunable optical properties and enhancement of photocurrents [28]. The versatility and possible applications of well-organized networks as switching device have been investigated on the charge transport related to the networks of metal nanocrystals [29]. Among optical properties, silver nanoparticles self assembled into multilayered films were utilized to enhance the optically stimulated luminescence (OSL) emission, and to increase the sensitivity of the radiation detectors. [30] Moreover, the integration of nanotechnology with photonic technologies allows the development of innovative methods of diagnostics and drug delivery. Recent developments of gold nanoparticles applications have been also exploited as contrast media for intravascular imaging by using photoacoustic measurements [31] and for the detection of inorganic, organic or biological analyses [32]. Plasmonic colloidal nanostructures are arguably among the most promising nanomaterials in the optoelectronic, biophotonic, sensing and biomedical fields due to their ability to support localized surface plasmons when excited by electromagnetic radiation. Nonlinear optical properties, using second and third harmonic generation processes, have been enhanced through local field resonances corresponding to the surface plasmon excitations of the MNPs [33]. Regarding electronic properties of hybrid organic-metallic systems, research efforts in organic molecules conduction have been dedicated to develop flexible electronic devices focusing on high level of miniaturization. Starting from MacDiarmid’s seminal papers, e.g. ref. [34, 35], conduction and electronic characteristics were studied in different configurations. A semiconductor behaviour with an energy gap between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) of 5.03 eV was found for Fluorene [36, 37]. Generally speaking, the conductance of organic materials is limited by charge transport mechanism typical of disorder materials, where conducACS Paragon Plus Environment

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tive islands are surrounded by regions that acts as a barrier [38,39]. An approach used for enhancing the conductivity in organic chains is to covalently incorporate metal centres between the organic monomers [39, 40]. Herein, we focus our research on advanced devices functionalized noble metal nanoparticles (MNPs) based networks suitable for advanced optoelectronics and biophotonics applications. Layered films of metal-organic material, self-assembled into 2D-network have been prepared. Different metal nanoparticles (Au, Ag) were covalently interconnected by same organic bifunctional thiol ligand, i.e. 9,9didodecyl-2,7-bis-thiofluorene (FL) that allows an easy solubility on the nanoparticle networks [5]. Particular attention has been given to obtain controlled optical properties with tuned absorption plasmon band. Recent studies show that the AuNPs network undergo important structural changes upon thermal annealing: the FL bridges became well ordered due to dominating cubic packing of AuNPs [41]. In the present work gold and silver nanoparticles functionalized with FL have been prepared (namely AuNPsFL and AgNPs-FL) and studied by means of different techniques, both optical and morphological one. such as Spectroscopic investigation (UV-Vis), Photoluminescence (PL), electron microscopy and Synchrotron Radiation-XPS techniques have been used to to ascertain the double behavior of the bifunctional ligand acting, as a physical bridge between metal nanoparticles and as photo-conductivity nanotool. In order to investigate the interesting electrical conduction properties of the MNPs network, electrical measurements have been carried out on cast deposited films.

Esperimental Section Materials and Methods Optical spectroscopy, i.e. UV-Vis absorption and emission spectra, were run in CH2Cl2 solution by using 1 cm quartz cells with a Varian Cary 100 Scan UV-Vis spectrophotometer. UV-Visible emission measurements were performed on a Cary Eclipse Spectrophotometer. A Xenon arc lamp was used as excitaACS Paragon Plus Environment

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tion source. FTIR and FIR spectra have been recorded as films deposited by casting from CH2Cl2 solutions using KRS-5 cells, with a Bruker Vertex 70 spectrophotometer. 1H-NMR, and

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C-NMR, spectra

were recorded on a Varian 300 MHz instrument with solvent peaks as reference. FESEM images have been acquired with the Auriga Zeiss instrument (resolution 1 nm, applied voltage 6–12 kV) on freshly prepared films drop cast from CH2Cl2 solution on a metallic sample holder. Transmission electron microscopy observations were performed using a FEI-TITAN operating @300 kV and ZEISS EM10 @60 kV. XPS analysis was performed with a homemade instrument, consisting on a preparation and an analysis UHV chamber, (resolution of 1.0 eV as measured at the Ag 3d5/2 core level). The used X-ray radiation is a non-monochromatised Mg-Kα (1253.6 eV). The spectra were energy referenced to the C1s signal of aliphatic C atoms having a binding energy BE = 284.70 eV. Atomic ratios were calculated from peak intensities by using Scofield’s cross section values and calculated λ factor [42]. Curve-fitting analysis of the C1s, Au4f, S2p, spectra was performed using Voigt profiles as fitting functions, after subtraction of a Shirley-type background [43]. The spectra have been acquired on films cast or spin deposited from CHCl3 and CH2Cl2 solvents on TiO2/Si (111) wafers. SR-XPS experiments were carried out at the BACH (Beam line for Advanced DiCHroism) beam line at the ELETTRA synchrotron facility of Trieste (Italy). The BACH beam line exploits the intense radiation emitted from an undulator front-end. XPS data were collected in fixed analyzer transmission mode (pass energy = 30 eV), with the monochromator entrance and exit slits fixed at 30 µm. Photon energy of 380 eV and 600 eV were used for C1s, S2p, Au4f and Ag3d spectral regions with energy resolution 0.22 eV. Calibration of the energy scale was made referencing all the spectra to the gold Fermi edge of an Au reference sample, and the Au4f7/2, was always found at 83.96 eV. Curve-fitting analysis of the C1s, S2p, Au4f and Ag 3d spectra was performed using Gaussian curves as fitting functions. S2p3/2,1/2, Au4f7/2,5/2 and Ag3d5/2,3/2 doublets were fitted by using the same full width at half-maximum (FWHM) for each pair of components of the same core level, a spin–orbit splitting of, respectively, 1.2 eV, 3.7 eV and 6.0eV and branching ratios S2p3/2/S2p1/2 = 2/1, Au4f7/2/Au4f5/2 = 4/3 and Ag3d5/2/Ag3d3/2 = 3/2. When ACS Paragon Plus Environment

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several different species were individuated in a spectrum, the same FWHM value was used for all individual photoemission bands. Conductivity measurements on films were carried out with a Keithley 595 quasistatic CV meter on cast deposited solutions on interdigitated electrodes implemented on passivated silicon substrate (SiO2/Si) [44]. The interdigitated electrodes consisted of 40 pairs of 5 mm long gold electrodes, with gap and width of 20 µm, while the total thickness was 100 nm.

Chemicals Deionized water, obtained from Zeener Power I Scholar-UV (18.2 MΩ), was degassed for 15 minutes with Argon, before use. Anhydrous solvents were purchased from Sigma-Aldrich Co.: Dichloromethane, Chloroform, Ethanol, Toluene, Petroleum Ether, Diethyl Ether, 1,3-Dimethy-2-imidazolidinone(DMI), Chloroform-d (CDCl3). Reagent were purchased from Sigma-Aldrich Co and used without further purification: 9,9-didodecyl-2,7-dibromofluorene, Sodium methanethiolate, Acetyl chloride, Tetrachloroauric(III) acid trihydrate (HAuCl4·3H2O), Silver nitrate (AgNO3), Tetraoctylammonium bromide (TOAB), Sodium borohydride (NaBH4), Anhydrous Sodium sulfate (Na2SO4).Column chromatography was carried out on silica gel Merck (60 Ǻ, 70-230 mesh), and thin-layer chromatography (TLC) on aluminium sheets precoated with silica gel 60 F254 (Merck).

Synthesis of metal nanoparticles functionalized with 9,9-didodecyl-2,7-di(thioacetyl)fluorene (AuNPsFL and AgNPs-FL): Gold nanoparticles stabilized with thiolate were prepared in analogy to our previous studies on similar compounds, [5, 45] with Metal/FL molar ratios 2:1, 1:1, 1:2. The synthesis of new silver nanoparticles is herein reported for the first time: 0.0901 g (0.552 mmol) of AgNO3 was dissolved in deionized water (5 mL) and mixed with 0.3703 g, (0.674 mmol) of tetraoctylammonium bromide (TOAB) previously dissolved in 10 mL toluene. After mixing, 0.2086 g (0.276 mmol) of FL thiolate dissolved in 10 mL of toluene was added. An aqueous solution of sodium ACS Paragon Plus Environment

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borohydride (0.176 g, 5.52 mmol) in deionized water (5 mL) was rapidly added with vigorous stirring and maintained under stirring for 3 h. Then the organic phase was separated, and the suspension washed with water. The organic phase was reduced to 2 mL in a rotary evaporator, and 40 mL of ethanol were added. The mixture was kept overnight at -18 °C and then centrifuged at 5000 rpm for 15 min to remove excess thiol and TOAB. The surnatant was eliminated and the precipitate was washed by centrifugation at 13400 rpt for 10 min, with ethanol for 10 times. After the removal of the surnatant, a brown-violet solution of AgNPs-FL nanoparticles was obtained, soluble in the common organic solvents (yield 43%, 0.0387 g). UV-Vis (CH2Cl2), λmax (nm): 407. Fluorescence (CH2Cl2), λmax (nm): 470 (λexc= 400 nm).

Results and Discussions AuNPs-FL and AgNPs-FL: optical and electrical characterization. The modern fashion of wet-chemistry synthesis via a reduction route (HAuCl4·3H2O and NaBH4) represents one of the most used sample preparation for producing metallic core-shell systems using small organic molecules. Actually, wet-chemistry approach is still unclear on how the nucleation phase evolve into nanocrystal object and scientists effort is still devoting to understand the evolution growth. Additional complexity might occurs when functionalizing ligands increase in dimension from smallest (thiols) to large molecules, because the increasing of carbon atoms provide a steric barrier for a homogeneous growth for obtaining a regular metal-organic network in the solution phase. Similarly, controlling MNPs growth in the presence of the dithiol ligand 9,9-didodecyl-2,7-bis-thiofluorene (FL) in liquid medium may represents a challenging task. Furthermore, it should be noticed that a modest variation in the size, shape, and neighbor distance of MNPs-FL network changes dramatically the fundamental optical and electronic properties. In order to involve appropriate methods for interconnecting and pattering assemblies of the individual MNP-FL, Au and Ag nanoparticles have been prepared in the presence of the ACS Paragon Plus Environment

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π-conjugated dithiol FL ligand (Figure 1a), in order to achieve stable and soluble nanoparticles. The use of the chemical reduction procedure of the HAuCl4 precursor in the presence of NaBH4 as the reducing agent, allowed to isolate the AuNPs-FL and AgNPs-FL obtaining a very reproducible yield of highly stable and purifiable MNPs-FL compounds (up to 40% wt). For obtaining a well-organized interconnection network, the Metal/ligand molar ratio was varied in the range 2:1-1:2. The functionalized noble metal nanoparticles have been characterized in the visible range of the UV-Vis spectrum for AgNPs-FL and AuNPs-FL networks in liquid medium. In the case of AuNPs-FL nanoparticles, an absorption broad peak has been observed at around 525 nm typical of small AuNPs and any significant shift of the wavelength by varying the gold/ligand molar ratio was not observed. AuNPs solution exhibited the typical red-violet color and remarkably high stability in organic solvents. Interestingly, the synthesized silver nanoparticles have shown a clear increase value of the Plasmon resonance by increasing the FL amount, shifting slightly toward the longer wavelength region from the value of 445 nm (for the 2/1 molar ratio) toward 485 nm (for the 1/2 molar ratio, Figure 1b) this effects may be due to the interaction among NPs and their inter-coupling effects [24]. The synthesized nanoparticles have also shown fluorescence properties: the used excitation wavelength were 520 nm for AuNPs and 400 nm for the AgNPs, the emission peak are reported in Supporting Information. The wavelength of the AuNPs-FL sample exhibits a slightly wavelength shift of about ∆λ= 37 nm, whereas the AgNPs-FL sample shows pronounced wavelength shift of ∆λ= 70 nm.


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Figure 1: (a) chemical sketch of MNPs functionalized with FL bifunctional ligand and (b) UV-Vis spectra in dichloromethane of AgNPs-FL samples.

Morphological Identification of AuNPs-FL and AgNPs-FL nanoparticles. This section aims at pooling information from an appropriate morphological characterization useful for tailoring the wet-chemistry synthesis to achieve desirable electron-optical properties. To obtain insight into the self-assembly behavior of this hybrid material at the nanoscopic scale, a combination of electron microscopy techniques (TEM and SEM) and image processing techniques have been exploited in order to obtain all possible experimental information for exploring the morphometric features of the nanoparticles assembled in an organized network [46]. Morphological and morphometric characterizations of well-separated noble nanoparticles linked at the surface by 9,9-Didodecyl-2,7-bisthiofluorene are able to arrange into an extensive network, as reported in Figure 2. AuNPs-FL network gathered by bright-field ACS Paragon Plus Environment

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(BF) transmission electron microscopy (TEM) shows the bi-dimensional behavior of the nanoparticles, forming a 2D self-assembled network (Figure 2a). Consistent with TEM observations, scanning electron microscopy of multilayer film shows bright nanometric regions overlapping a flat surface constituted of brightest nanoparticles (Figure 2b). The brightest regions or nebulosus could be attributed to the photo-luminescent property of an extra amount of FL molecules interacting with the electron beam accelerated at lowest extra high tension (EHT). The nanoparticles of brightest intensity placed at the top of the thin film seem to form mono-layers organized in a 2D stacking network, which favorite the escape of the secondary electrons from the surface by extra electron reflections with respect to the grounded dark regions. The entire observed surface shows well-separated nanoparticles of slightly different dimensions. Appropriate imaging analyses have been exploited to perform accurate quantitative measurements of size-shape and of spatial distribution of the individual nanoparticles observed into TEM image, paying attention to the statistical analysis [7,47]. Since the nanoparticles seem assembled into bidimensionally monolayer network with regular spatial distributions, a quantitative morphometric measurements and a possible topological relationship among the NPs have been investigated. AuNPs-FL distribution has been quantified on a probed area of 75 nm by 75 nm. The morphometric analysis, conducted on Figure 2a, has identified NPs of quasi-spherical shape with evident rough surfaces due to the FL molecules as covering shell. The measured diameter of the nanoparticles provided an average value of 4.15 ± 0.33 nm. An image analysis technique has been applied to quantify the distribution among surface-to-surface nearest-neighbor distances (ds). [5, 48] The neighbor-distance analysis among AuNPsFL have provided a value of ds = 1.45 ± 0.21 nm. This measured value is about the length of the 9,9Didodecyl-2,7-bisthiofluorene bridge that links covalently the surfaces of the metal nanoparticle, as sketched in Figure 1a. BF-TEM image clearly shows a single-layer of well separated AgNPs-FL network (Figure 2c). The morphometric analysis has identified an average diameter of 3.75 ± 0.16 nm of the AgNPs-FL sample. The estimated a neighbor-distance distribution is centered around ds = 1.56 ± 0.41 nm that could correspond to the length of FL linker. SEM observation of multilayer film still shows quasi-smooth surface in ACS Paragon Plus Environment

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Figure 2d. The morphological evolution of the two different noble metal nanoparticle functionalized with same FL organic spacer shows a slightly difference in shape and in spatial distribution. AuNPs-FL sample clearly shows nanoparticles well aligned into network and dimensionally larger than AgNPs-FL sample. The small silver nanoparticles seems to self-aggregate into a bi-dimensional network, but preferentially organization into mono-dimensional chains can be also observed.

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Figure 2. Morphological Identification of AuNPs-FL and AgNPs-FL nanoparticles self-assembled into a network. (a) BF-TEM image of single-layer of AuNPs-FL network. (b) SEM image illustrating the surface morphology of a multilayer of AuNPs-FL network. (c) BF-TEM image of single-layer of AgNPs-FL 12 ACS Paragon Plus Environment

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network. Inset: high magnification of AgNPs-FL nanoparticles. (d) SEM image illustrating the surface morphology of a multilayer of AgNPs-FL network.

SR-XPS of AuNPs-FL and AgNPs-FL. SR-XPS measurements were performed on AuNPs-FL and AgNPs-FL samples with the aim to ascertain whether the bifunctional ligands always act as bridges between metal nanoparticles, or some are attached on one side leaving the thiolate end group free, similarly to the investigation already reported for AuNPs-FL in reference [5]. Measurements were performed at C1s, S2p and Au4f, Ag3d core levels (all BE values, FWHM and atomic ratios estimated for both samples are reported in Table 1 in the Supporting Information). The measured C1s core level spectra confirm (Figure 3a and c) the stability of the fluorenyl dithiols molecular structure. C1s signals are composites for all samples, and by applying a peak-fitting procedure at least three components can be individuated, corresponding respectively to aromatic C atoms (C1: 284.7 eV), aliphatic carbons (C2: 285.0 eV), and C atoms of the terminal thiol group (R-C–S) (C3: about 286.2 eV); the atomic ratio between the three C1s spectral components is C1:C2:C3 = 1: 2.4:0.15 for AuNPs-FL andC1:C2:C3 = 1: 2.5:0.13 for AgNPs-FL, both in excellent agreement with the theoretical C1:C2:C3 = 1:2.5:0.2 atomic ratio expected from the molecular structure. S2p and Au4f, Ag3d signals provide useful information about the interaction between the fluorenyl dithiol and the metallic cluster. Au4f spectrum of AuNPs-FL and Ag3d spectrum of AgNPs-FL are respectively shown in Figure 4 a,b; both spectra appear structured, and by following a peak-fitting procedure two pairs of spin–orbit components can be individuated for both signals. The first feature in Au4f spectrum (Au4f7/2BE= 83.91 eV) is associated with metallic gold, and is due to gold atoms in the bulk of nanoparticles. Similarly, the first component in Ag3d spectrum (Ag3d5/2 BE = 368.10 eV) is due to metallic silver atoms at the NP core. The spin–orbit pair of lower intensity at higher BE values that can be observed in both spectra (Au4f7/2 BE = 84.46 eV BE, Ag3d5/2 BE = 368.76 eV) are indicative for positiveACS Paragon Plus Environment

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ly charged metal atoms, as expected for surface atoms bonded to sulfur [49, 50]. S2p spectra provide complementary information about the chemical bond between fluorenyl dithiols and surface metal atoms of AuNPs and AgNPs in agreement with the well-organized MNPs network observed using electron microscopy techniques. As shown in Figure 3 b,d, both S2p spectra show a single pair of spin-orbit components at BE values typical of sulfur atoms covalently bonded to metals (S2p3/2 BE = 162.80 eV). If some thiol end-groups were free, a spin-orbit pair at higher BE values (S2p3/2 BE =163.5– 164.0 eV) would have been found in S2p spectra [45]. Since this signal is not observed, SR-XPS data suggest that all bifunctional ligands end-groups are chemically bonded to metal atoms at the nanoparticles surfaces, giving rise to networks.

Figure 3. SR-XPS spectra : a), c) C1s and b), d) S2p spectra of AuNPs-FL and AgNPs-FL samples ACS Paragon Plus Environment

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Figure 4. a)Au4f and b) Ag3d SR-XPS spectra of AuNPs-FL and AgNPs-FL samples, respectively.

Electrical characterization Established experimentally the best spatial distribution of MNPs-FL covalently connected into 2Dnetwork by controlling chemical functionalities of a wet-chemistry approach supported by electron microscopy and SR-XPS techniques, MNPs-FL thin film with Au/FL of 1/1 and Ag/FL of 1/1 molar ratio have been characterized for their electronic applications.


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Electrical measurements performed on AuNPs-FL films deposited on interdigitated electrodes at room temperature are shown in Figure 5. Current-Voltage (I/V) response curve of an AuNPs-FL single layer thin film of a 2D regular network is plotted in figure 5a (see also TEM images, Figure 2a). Measurements were carried out under dark and light visible illumination conditions with halogen lamp (QTH lamp, 50 W, illumination distance of 5 cm) and the photoresponse did not show relevant electrical differences. The I/V characteristic is completely symmetrical with respect to the polarity of the applied voltage. The curve follows non ohmic relation, typical of the hopping theory of conduction in doped polymers [34, 51, 52]. In general, it should be noted that the conduction mechanisms not only depend by free carriers through doping effect, but also by the formation of polarons and bipolarons along π bridges of the active carbon chain of Fluorene bridge [37]. At low macroscopic mobility of free carriers, hopping mechanism among adjacent polymeric chains requires a certain activation energy E0 of about 2-3 eV from the occupied levels to the lowest unoccupied energy levels [53, 54]; whereas the movement of the charge along a conjugation length is responsible for the highest conductivity [55]. In our case, the hopping effect is given by the presence of electrons from the AuNPs acting as electrons reservoirs of diffusing free carriers into a bi-dimensional network. Similar expectation has been observed by Hermosa et al. in ref. 39 demonstrating that transport electrical carriers in high order one-dimensional metal organic is more efficient than in 1D disorder chain polymer. Therefore, at low dimensional of well-organized metalloorganic network the hopping mechanism of conduction is dominant with respect to the metallic ohmic conductance. By using the following equation derived from previous studies [39, 56] we fitted the I/V experimental curve of Figure 5a:

I = G V + I ℎ Γ 1 + , (1)

where Γ is the incomplete upper Gamma function, is the bias voltage, is the room temperature, is the Boltzmann constant, is the modulus of the charge of the electron. We were able to estimate the following set of physical parameters:

= 1/(140 ± 20 'Ω) represents a parallel conductance at low


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voltage, * = 0.20 ± 0.02 , is the current scale factor, - = 5.9 is the critical exponent for the density of states [39,56], 0 = 0.02 is the fraction of the voltage drop across the high impedance junction [56]. The corresponding voltage drop =

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= 2.6 is related to the activation energy 4 =

that enable the hopping mechanism in the conduction along the FL molecules in thin film AuNPs-FL sample, allowing the overpassing the Homo-Lumo gap [36, 37]. To investigate the electronic properties changing the dimensionality of the AuNPs-FL network from bidimensional into a three-dimensional architectures, the I/V characteristic curve of an AuNPs-FL network deposited as thick film (approx. 1 micron) have been displayed in Figure 5b (see SEM image Figure 2b). In this case, by fitting the data with eq.1, we obtain larger current scale factor * = 2.4 ± 0.2 , and a even larger conductance

= 1/(1.4 ± 0.2 'Ω), with similar materials parameters -

and 0. By comparing the different dimensional contributions at the current, the ohmic conductance is dominant with respect the hopping contribution into 3D-network, thus the activation energy is less critical compared to the previous low dimensional systems since the electrons can easily pass from one layer directly to another one. The overall behavior can be well approximated by an ohmic law with a resistance of 1.24 ± 0.20 'Ω and conductivity of 567189 = 0.12 :;:12.

10

150

AuNPs-FL thick film fit

AuNPs-FL thin film fit Intensity Current [µA]

100

Intensity Current [nA]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50 0 S

-50

S

S

S

S

S

S S

S

S S

S S

S

S

S S

S

S

S

S S S

S S

S S

-100

S SS

S

S

S S S S S SS

S S S S

S S S S

SS SS S

S

SS S

S SS

S

SS

S

S S

S S S

S

S S

S S

S

S S

S

S S

0

S S S

S

S

S S

S S S S SS S

S

S

S

S

S S S

-5

S

S

(a)

-5

0

Voltage [V]

5

SS

S S S SS

S S S

S S S

SSS

SS

S S S S

10

-10 -10

S

S

-5

S

S

S S

S

S

0

S S

S S S

S

S S S

S S

S S S S S S S

S S

S S S

SS S S S S S

S S S S

S S S

S S

S SS

SSS

S

S

S

S

S

S S S S S SS

SS S

S

S S

SSS

S

S S S S S

S

S

S

SS S

S

S

S

-150 -10

SS

S S

S SS S

S S

SSS

S S SS

SS S

S S

S S

S S S S S SS

SSS

S

SSS

S S S S S

S

S S SS

S S S

SS S S S S

S S

S

SS

S

S

S S

S

SS

S S

S SS S S

S

SS S

S

S

SSS

S S SS

SSS

S

S S S S S S SS

S S S S S

SS

S S S

S S S

S

S S S SS

S

S SS

S

S

S

S

S

S S S

S S

S

S

S S

S

S S

S S S S S

S S S S

S S S S

S S S

S

S S S

SS S

S

S

S SS

S S S SS S

S

S SS

S

S

S

S

S S

S

S

S S

S

5

S S S S S

S S

5

S S

S S

(b)

S S

10

Voltage [V]


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Page 18 of 35

Figure 5: Electrical measurements of AuNPs-FL (a) thin and (b) thick films. Both data sets are fitted by eq.1.

In order to further increase the conductivity property of the MNPs-FL material, conductivity measurements of AgNPs-FL were performed on a film of about 1 micron in thickness (Figure 6) (see also TEM and SEM images in Figure 2 c,d). The electrical behavior gives a resistance of 6.4 ± 0.3 Ω and a conductivity of 56=189 = 23.4 :;:12. Illumination with halogen lamp didn’t increase the overall conductivity as shown in Figure 6a. This fact shows that the free carriers gives by MNPs are dominant with respect the carriers induced by light on the FL molecules [36, 37]. AgNPs-FL sample shows a reduction of the conductivity during I/V measurements in dark condition as shown in Fig. 6b (black squares), probably due to an oxidation process. Consequently, the I/V characteristic curve lost symmetry driven by rectification effect typical of Schottky metal-semiconductor barrier [51, 57, 58] of organic diodes [57, 59, 60]. The resistance of AgNPs-FL sample for larger positive voltage is around 4.9 ± 0.2 'Ω and a conductivity of 56=189 = 0.031 :;:12 was calculated. The rectification effect is due to the selective oxidation of AgNPs close to the positive electrode during the forward bias condition, forming a structure composed by an organic layer inserted between two electrodes with different work functions [58, 61], the metallic one and the oxidized one. Under light illumination the resistance was reduced down to 3.3 ± 0.2 'Ω as shown in Fig. 6b (red circles) with a conductivity of 56=189 = 0.045 :;:12 . Hence, the free carriers generated by AgNPs are partially inhibited by oxidation effect, while charge carriers induced by illumination in the FL molecules consistently contribute to the overall conductivity. As discussed in [58, 62] Schottky behavior can be analyzed by using thermionic emission theory [63], described by:

* = *> ?@A B − 1D ACS Paragon Plus Environment

(2) 18

Page 19 of 35

Where Is is the reverse saturation current and is expressed as: *> = ,,∗ @A −

FG

(3)

e is the electron charge, V is the applied voltage, A* is the effective Richardson constant, A is the effective junction area, T is the absolute temperature, k is the Boltzmann constant, n is called the ideality factor (n=1 for perfect inorganic semiconductor diode and differs from unity for organic semiconductors) and HI is the barrier height. The value of n is determined from the slope of the linear region of the forward bias semi log I/V characteristics through the relation:

J

= J(KL M)

(4)

In our case, the best fit for Figure 6b gives *> = 0.46 ± 0.06 N, and

B

= 0.18 ± 0.01 12 at room

temperature. The best fit of the I/V experimental curve reveals that the Schottky model considering two electrodes with different work functions, is consistent with our system. In our case one work function is due to the contact between the AgNPs and one of the two interdigitated gold electrodes, while the second work function is given by the contact between the oxidized AgNPs and the other interdigitated gold electrode.

2,5

200

AgNPs-FL dark AgNPs-FL under light

100

0

S

S

S

S

S

S S

S

S

S S S

S S

S

S

S S

S S

SSS

S S

S

S

S S S

S S S

SS

S

S S

S S

S

S S S

S S

S S

S

S S S

S SS

S

S S S S

-200

S S

S

S S S S S S

S S

S

S S

S S

S

S S S

S S

S S

SS

S S

SS

S S S

S S

S

S S S

S S

S S

S S

S S S

S S

S S

S S

S

S

S S S

SS

S S

S HS

S

S

S S

S S

S S S S

SS S S S

S S

S S

S S S S

S S S

S S

SSS S S

S S

S

S S

-100

S

AgNPs-FL dark AgNPs-Fl light

2,0

Intensity Current [µA]

Intensity Current [µA]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


S S

1,0

0,5

0,0

S S

(b)

S

S

S

fit

1,5

S

(a) -0,5

-1,5

-1,0

-0,5

0,0

0,5

1,0

1,5

-10

-5

0

5

10

Voltage [V]

Voltage [V]


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Page 20 of 35

Figure 6: Electrical measurements of AgNPs-FL thick film in dark (black) condition and under light (red) before (a) and after (b) oxidation process.

Therefore, the current is not only carried by free carriers (electrons and holes) as in semiconductors, but also by the formation of polarons and bipolarons. As the applied voltage is increased, the formation of polarons and bipolarons increases rapidly, contributing to higher values of current through the sample.

Conclusions In conclusion, the electrical properties of AuNPs-FL thin film follows a non ohmic behavior, with a conduction mechanisms that strongly depends on polarons and bipolarons along π bridges of the carbon chain of Fluorene bridge. In this sample, morphometric TEM studies evidenced well-separated nanoparticles assembled into bi-dimensionally monolayer network with regular spatial distributions with average size of about 4 nm. SR-XPS data suggest that all bifunctional ligands end-groups are chemically bonded to metal atoms at the nanoparticles surfaces, giving rise to networks. The presence of the FL linker allows an easy solubility and a spacing of the AuNPs of about 1.5 nm that in turn has an effect on the electrical non ohmic behavior. By increasing the number of layers as shown by SEM study on thick film, and measuring its conductivity, the AuNPs-FL sample can be well approximated by an ohmic law with a resistance of 1.24 ± 0.20 'Ω and conductivity of 567189 = 0.12 :;:12 that can be compared with literature data [64] on AuNPs layers without stabilizing linkers. TEM and SEM studies on AgNPs-FL samples evidenced an irregular distance distribution that in turn affected the electrical behavior that is not dependent on the film thickness. In the case of AgNPs-FL, the electrical behavior gives a resistance of 6.4 ± 0.3 Ω with two orders of magnitude less with respect to AuNPs-FL. Upon the oxidation process the resistance of AgNPs-FL increases of about three orders of magnitude and the thick film shows a diode behavior. The overall resistance decreases upon illumina-


20

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tion, thanks to the presence of the organic bridges in the network, and optoelectronic properties became evident.

ACKNOWLEDGMENTS The authors gratefully acknowledge the Sapienza University of Rome, Ateneo Sapienza 2016 project for financial support. The authors are grateful for the support of the European Research Infrastructure EUMINA fab (funded under the FP7 specific program Capacities, Grant Agreement Number 226460) and the Advanced Microscopy Laboratory (AML) in CRANN for the provision of their facilities and expertise, Trinity College Dublin, Ireland. The authors are also pleased to thank Ezio Battaglione of the Electron Microscopy Laboratory “Pietro M. Motta”, located at Sapienza University of Rome, for him fruitful discussions and experimental help in TEM measurements.

Supporting Information Supporting Information is available. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author Ilaria Fratoddi email: [email protected].


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Figure 1: (a) chemical sketch of MNPs functionalized with FL bifunctional ligand and (b) UV-Vis spectra in dichloromethane of AgNPs-FL samples. 80x99mm (300 x 300 DPI)



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Figure 2. Morphological Identification of AuNPs-FL and AgNPs-FL nanoparticles self-assembled into a network. (a) BF-TEM image of single-layer of AuNPs-FL network. (b) SEM image illustrating the surface morphology of a multilayer of AuNPs-FL network. (c) BF-TEM image of single-layer of AgNPs-FL network. Inset: high magnification of AuNPs-FL nanoparticles. (d) SEM image illustrating the surface morphology of a multilayer of AgNPs-FL network. 52x48mm (300 x 300 DPI)


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Figure 3. SR-XPS spectra : a), c) C1s and b), d) S2p spectra of AuNPs-FL and AgNPs-FL samples 64x52mm (300 x 300 DPI)



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Figure 4. a)Au4f and b) Ag3d SR-XPS spectra of AuNPs-FL and AgNPs-FL samples, respectively. 73x68mm (300 x 300 DPI)


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Figure 5: Electrical measurements of AuNPs-FL (a) thin and (b) thick films. Both data sets are fitted by eq.1. 70x99mm (300 x 300 DPI)



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Figure 6: Electrical measurements of AgNPs-FL thick film in dark (black) condition and under light (red) before (a) and after (b) oxidation process. 70x99mm (300 x 300 DPI)


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Graphic for TOC 22x10mm (300 x 300 DPI)


Electronic Properties of a Functionalized Noble Metal Nanoparticles

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