Optical and photoelectrical studies of gold nanoparticle-decorated C60 films

Thin Solid Films 518 (2010) 1737–1743

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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

Optical and photoelectrical studies of gold nanoparticle-decorated C60 films N.L. Dmitruk a,⁎, O.Yu. Borkovskaya a, S.V. Mamykin a, D.O. Naumenko a, V. Meza-Laguna b, E.V. Basiuk (Golovataya-Dzhymbeeva) c,1, I. Puente Lee d a

Institute for Physics of Semiconductors, National Academy of Sciences of Ukraine, 45 Nauki Prospect, Kyiv 03028, Ukraine Instituto de Ciencias Nucleares, Universidad Nacional Autónoma de México (UNAM), Circuito Exterior, Ciudad Universitaria, A. P. 70-186, C. P. 04510 México D.F., Mexico Centro de Ciencias Aplicadas y Desarrollo Tecnológico, Universidad Nacional Autónoma de México (UNAM), Circuito exterior S/N Ciudad Universitaria, A. P. 70-186, C. P. 04510 México D.F., Mexico d Facultad de Química, UNAM, Circuito de la Investigación Científica, Ciudad Universitaria, 04510 México D.F., Mexico b c

a r t i c l e

i n f o

Article history: Received 8 April 2009 Received in revised form 27 November 2009 Accepted 27 November 2009 Available online 4 December 2009 Keywords: Fullerene C60 Thin films Gold nanoparticles Optical properties Barrier structures Photocurrent

a b s t r a c t Optical and photoelectrical studies were performed on octane-1,8-dithiol cross-linked fullerene films, with supported gold nanoparticles (C60–DT–Au). According to high-resolution transmission electron microscopy observations, the average size of obtained gold nanoparticles was about 5 nm, and the shape was spherical. The comparative investigation of optical properties of pristine and cross-linked with octane-1,8-dithiol C60 films, decorated with gold nanoparticles, found the difference in the extinction coefficient spectra, which was observed also in the photocurrent spectra of barrier heterostructure Au/C60/Si. The analysis of dark current–voltage characteristics for Au/C60/Si heterostructures showed that the model for them includes the barrier at the C60/Si interface and internal barriers in the C60 layer, caused by the trapping centers. The hopping mechanism of the current transport in the C60 layer was supplemented with the Poole–Frenkel emission process on these centers, with the barrier height greater for the fullerene C60 film cross-linked with octane-1,8-dithiol. © 2009 Elsevier B.V. All rights reserved.

1. Introduction The optical properties of metal nanoparticles have been extensively studied by many research groups (see, for example, [1,2] and references therein), and found a lot of applications. For example, the selection of nanoparticles for achieving efficient contrast for biological and cell imaging applications, as well as for photothermal therapeutic applications, is based on them [3]. Since those physical properties are strongly dependent on the shape and size of nanoparticles, the technological applications of such structures are connected with the detail characterization of these parameters from growth to device processing [4]. Another significant strategy on the way toward better application of nanoparticles in nanotechnology is controlling the nanoparticle size, shape, and detailed modelling of their effects on functional properties [5]. In nanoscience and nanotechnology, for the development of plasmonic nanomaterials, chemical and biological sensors, the important task must be the combination of the above mentioned strategies: characterization and controlling the size and shape of metal nanoparticles. It is well known that gold is one of the most studied materials due to many advantages such as chemical stability, catalytic activity of small size particle, and biocompatibility [3,5]. It was found that gold ⁎ Corresponding author. Tel.: + 38 44 525 64 86; fax: +38 44 525 83 42. E-mail address: [email protected] (N.L. Dmitruk). 1 Tel.: + 52 55 5622 8602x1150; fax: +52 55 5622 8651. 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.11.083

nanoparticles with plasmon-resonant absorption in the near-infrared can be used to destroy cancerous tumors in mice. Such nanoparticles exhibit strong optical scattering and absorption at visible and nearinfrared wavelengths due to localized surface-plasmon resonance. This is a classical effect in which the light's electromagnetic field drives the collective oscillations of the nanoparticle's free electrons into resonance (see, for example, [6]). It is no doubt that the useful properties of noble-metal nanoparticles are determined by their geometry. Jain et al. [3] found that the magnitude of extinction and the relative contribution of scattering to the extinction rapidly increase by increasing the size of gold nanospheres from 20 to 80 nm. Gold nanospheres studied in this work [3] in the size range about 40 nm showed an absorption cross-section 5 orders higher than conventional absorbing dyes, while the magnitude of light scattering by 80nm gold nanospheres was 5 orders higher than the light emission from strongly fluorescing dyes. However, the variation in the plasmon wavelength maximum of nanospheres, from 520 to 550 nm, was too limited to be useful for in vivo applications [3]. The different spectroscopic techniques such as surface-enhanced Raman spectroscopy, a variety of nonlinear scattering measurements and timeresolved measurements, have been applied [7] to determine the optical properties of metal nanoparticles, but the extinction, absorption, and scattering were always and are still the primary optical properties of interest. In spite of the shapes and sizes, the metallic nanoparticles can be characterized using electron and scanning probe microscopy, and in

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some cases, it is possible to determine the optical properties of individual nanoparticles [8,9]. A lot of complicating factors exists in understanding the nanoparticle optical properties. The presence of a supporting substrate, the solvent molecules around the particles, a distance between particles can have significant influence on the extinction spectra [7]. The absorption of light in different materials, doped with very few Au atoms, is caused by excitation of the above mentioned plasmons and plasmon polaritons. The wavelength of the surface (localized) plasmons depends not only on size and shape, but also on topology and the dielectric environment of gold clusters in glasses [10,11] and other media [12]. Since gold nanoclusters already proved a considerable potential for the application in nanoelectronic devices, the attachment of gold nanoclusters to fullerene molecules already appeared as a useful way to prepare nanohybrids with interesting properties. A photoemission study of C60 adsorption on the Au(100) surface showed that the lowest unoccupied molecular orbital of fullerene undergoes a strong hybridization with the Au d-bands [13]. Such nanohybrid materials can be based on metallic nanocenters, gold clusters commonly, attached to fullerene C60 molecules with the aid of compounds that contain sulfur atoms [14–16]. In our previous works [17,18] we developed an approach where fullerene C60 films were exploited as molecular templates for metal cluster superstructure. The chemical bonding of metal nanoparticles to fullerene support reduced considerably the undesirable coalescence effects. We have used chemically cross-linked fullerene supports with an aliphatic bifunctional amine and thiol as linkers, to bind and immobilize silver and gold nanoparticles. The stable nanocomposite films have been obtained, uniformly decorated with Ag and Au nanoparticles. The measuring of optical properties in [19] showed that the diamine treatment changed recombination properties of fullerene films. It was found that the decoration of C60 pristine or cross-linked with silver nanoparticles leads to decreasing in photoluminescence intensity, and to band gap decreasing by about 0.1 eV, but had a weak influence on electrical and photoelectrical properties of Au/C60/Si barrier structures [19]. In the present work, we studied the optical and photoelectrical properties of fullerene C60 films cross-linked with octane-1,8-dithiol and decorated with gold nanometer-sized particles.

Along with the cross-linking of C60 films, and decorating them with gold nanoparticles, we performed for some comparison the same reactions with pristine fullerene powder. 2.2. Optical and photoelectric characterization A comparative investigation of Si-supported fullerene C60 films, namely pristine (C60/Si), cross-linked with octane-1,8-dithiol C60 films (C60–DT/Si), as well as the above mentioned films decorated with gold nanoparticles (C60–Au/Si and C60–DT–Au/Si, respectively), was carried out at room temperature in air. For their optical characterization the reflectance spectra were measured in the wavelength range of λ=400– 1000 nm at variable angles of incidence of p- and s-polarized light. The fullerene layer thickness value and its optical parameters (refractive index n, and extinction coefficient k at complex refractive index ň=n−ik) were determined by fitting of experimental dependencies with

2. Experimental details 2.1. Materials and chemical procedures Fullerene C60 powder from MER Corp. (99.5% purity), octane-1,8dithiol (98% purity), hydrogen tetrachloroaureate (III) HAuCl4 trihydrate (99.9% purity), 2-propanol from Aldrich, and citric acid from Baker were used as received. The C60 fullerene films were deposited by sublimation method onto silicon Si(100) wafers (size of ca 10 × 10 mm), as well as onto transmission electron microscopy (TEM) grids, in a vacuum chamber at a pressure of 8.2 × 10− 4 Pa and the C60 source temperature of 250 °C, without heating the substrates. To perform the cross-linking of deposited fullerene films with dithiol, and the following decoration with gold nanoparticles, we employed the procedures described previously in [17]. The samples (silicon and TEM grid-supported C60 films, preliminarily degassed in vacuum) were placed into a Pyrex glass reactor along with a small drop of octane-1,8-dithiol, and the thiol addition reaction was carried out at a pressure of ca. 133 Pa and 140 °C for 3 h. For the deposition of gold nanoparticles onto dithiol-functionalized films, the fullerene samples were placed into 10 ml of 2-propanol. Then two solutions, of HAuCl4 in 2-propanol and of citric acid in 2-propanol, were simultaneously added drop-wise while the system was vigorously stirred at room temperature for 30 min. After finishing the deposition process, the samples were washed with 2-propanol, dried and stored under vacuum at room temperature.

Fig. 1. SEM images and EDS spectra for Si/C60Au (a, b) and Si/C60DT–Au (c) films.

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theoretical ones, calculated within the framework of a one-layer model for pristine or functionalized C60 films. Parameters obtained allowed to calculate the spectral dependencies of the absorption coefficient (α=4πk/λ) and the spectra of light transmittance in layered barrier structures metal(Au)/fullerene/Si. The photodiode structures were fabricated by vacuum evaporation of a semitransparent gold electrode layer (25.5 nm thickness) through an opaque mask (opening diameter of 1.3 mm) onto pristine and functionalized C60 films, with ohmic contact deposited all over the rear side of the n-Si substrate. 2.3. Analytical instruments Transmission electron microscopy observations of the grid-supported samples were carried out on a JEOL 2010 instrument, operating at 200 kV. Scanning electron microscopy (SEM) with energy dispersive X-ray spectroscopy (EDS) was carried out on a JEOL JSM-5900 instrument operating at 20 kV. The spectral dependencies of the light reflectance for C60/Si heterostructures and the photocurrent of Au/C60/Si photodiode structures were measured with the automated arrangement based on IKS-12 spectrometer equipped with a device for angular dependence of reflectance measurement, with a Glan prism as a polarizer and with the calibrated Si photodetector. 3. Results and discussion In our previous work [17] we demonstrated that the dithiolfunctionalized, cross-linked fullerene C60 film can be employed as a

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support for stable and homogeneous deposition of gold nanoparticles due to binding mechanism through a strong coordination attachment between Au nanoclusters and sulfur donor atoms of the functionalized fullerene, that was also supported by density functional theory calculations [17]. Here, before studying optical and photoelectrical properties of obtained gold nanoparticle-decorated fullerene films, the comparative morphology study of fullerene samples, by means of scanning electron microscopy with energy dispersive X-ray spectroscopy, and transmission electron microscopy, was employed. While SEM images (Fig. 1) present the formation of big gold clusters onto pristine fullerene film, where Au particles tend to undergo strong coalescence, the gold nanoparticles linked on dithiolfunctionalized fullerene film are spread uniformly, without largescale aggregation on the surface. EDS spectra of pristine and crosslinked fullerene films after deposition of gold particles, revealed the peaks of gold at ca. 2.12 keV, 9.7 keV and 11.5 keV for both kinds of films, and also a sharp peak at ca. 2 keV due to sulfur in the crosslinked fullerene films sample. We also performed the studies of obtained gold nanoparticles by means of TEM, applying for comparison of different kinds of C60 supports: powder and thin film, both functionalized by octane-1,8dithiol linker, where a similar deposition technique was employed. Interestingly, while a variety of shapes of gold nanoparticles, apparently cubic and spherical, was distinguished on powder sample, with an average particle size around 10–15 nm (Fig. 2), in the case of dithiol cross-linked C60 film we found only spherical-shape nanoparticles of less than 5 nm in diameter, where the nanoparticles were well separated from each other (Fig. 3).

Fig. 2. TEM images of different magnifications (a,b) of gold nanoparticles deposited onto C60 powder functionalized by octane-1,8-dithiol. The obtained nanoparticles, supported on fullerene powder present two different geometries: spherical (c) and cubic (d).

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Fig. 4. Optical parameters, n (a) and k (b) for C60 (1), C60–DT (2), C60–Au (3), and C60–DT– Au (4) layers on the Si substrate.

Fig. 3. TEM images of spherical gold nanoparticles deposited on C60–DT films.

For obtained fullerene films with the immobilized spherical gold nanoparticles of the controlled size and shape, we performed optical and photoelectrical studies. Spectra of optical constants, n and k, for pristine and functionalized C60 films in the spectral range of 400–1100 nm are shown in Fig. 4 (a, b). The more essential difference is observed for the k value. Below 480 nm the presence of octane-1,8-dithiol molecules produces a slight increase in extinction spectrum, which is further enhanced when gold nanoparticles are deposited. Some increase of k in the range of λ ≈ 550–740 nm, and the appearance of absorption (k) in the range of λ ≈ 740–1000 nm for C60–DT–Au film, are found as well. The authors of [20] observed that for gold nanoparticles of spherical geometry the absorption spectrum shows a wider structure from 300 nm to 800 nm, showing a shoulder at about 520 nm. Spectra of the light absorption coefficients α = 4πk/λ in coordinates (α · hν)2 on hν (corresponding to so-called direct band-to-band transition) and their linear approximations for these films are shown in Fig. 5. The values of intercepts of these lines on the hν axis, treated as the band gap values, are presented in Table 1 together with the determined thicknesses of the C60 layers. These results are obtained in the framework of the one-layer model for functionalized C60 films. Since the observed decrease of the C60 band gap value due to corresponding treatments does not exceed the experimental errors, the influence of the hybridization effects [21], if they occur, is too small and/or takes place only in a thin upper layer of the C60 film. The determined parameters of the investigated films were used for the calculation of spectra of the light transmittance into the Si substrate through the C60 (Fig. 6a) and Au/C60 (Fig. 6b) layers. Au layer with thickness of 25.5 nm was evaporated onto the C60 surface

Fig. 5. Spectra of the light absorption coefficients α in coordinates (α · hν)2 vs hν for C60 (1), C60–DT (2), C60–Au (3), and C60–DT–Au (4) layers on the Si substrate.

to obtain Au/C60/Si diode structures for the studies of photoelectric and electric properties. The corresponding spectra of the short-circuit photocurrent, expressed as the external quantum efficiency Qext, (that is, the number of photocurrent carriers generated in Au/C60/Si structure by one photon of the incident light) normalized to the area of the diodes Au/C60/Si with pristine and functionalized C60 layers, are shown in Fig. 7. Corresponding spectra of internal quantum efficiency Qint (that

Table 1 Parameters of pristine and treated C60 layers. Sample

Thickness, nm

Band gap, eV

C60 (1) C60–DT (2) C60–Au (3) C60–DT–Au (4)

80.0 ± 0.4 82.3 ± 0.4 81.7 ± 0.5 90.6 ± 0.5

2.34 ± 0.02 2.33 ± 0.02 2.33 ± 0.02 2.32 ± 0.02

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Fig. 8. Spectra of internal quantum efficiency, normalized to the area of the diode, for Au/C60/Si (1), Au/C60–DT/Si (2), Au/C60–Au/Si (3), and Au/C60–DT–Au/Si (4) structures.

is, the number of current carriers generated by one photon of the light transmitted into Si) (Fig. 8) were calculated dividing Qext (λ) spectra by spectra of the light transmittance T2 into Si through the Au/C60 layers (Fig. 6b). The observed Qint values greater than unity are due to the planar contribution of the carriers generated by light outside the diode area (planar component of photocurrent). The efficiency of this photocurrent component is determined by the transmission of light into Si through the C60 layer T3 (Fig. 6a). Besides, the contribution of the photocurrent, generated in the C60 layer can be seen in the spectral range of λ ≤ 530 nm as the shoulder of Qext (λ) dependencies and maximum of Qint (λ) ones. In the last dependencies this contribution is intensified by the ratio of T1/T2, where T1 — is the transmittance of

light into the C60 layer through the Au film, shown in Fig. 6b, similarly as it was mentioned in [19]. Although the values of photocurrent (Qext) for diodes of the same structure may differ, the dependencies shown in Fig. 8 are typical for the investigated structures. For interpretation of the photocurrent spectra, the barrier characteristics of investigated structures were determined from the dark current–voltage (I–V) dependencies, measured in the direct current regime. The dark current–voltage characteristics for these diode structures are presented in Fig. 9. They show the evidence for the barrier existence on the C60/n-Si interface (with forward direction corresponding to V N 0 at Au electrode) causing the collection of photocarriers generated in Si as the main source of photocurrent. The analysis of dark I–V characteristics showed that forward I–V characteristics agree well with theoretical ones calculated in the framework of model taking into account emission, recombination, tunnel and ohmic (shunt) components of the current flow. In that case, similarly as in [22], the current component, interpreted as tunnel, is the dominant one in a voltage range up to ∼2 V. The series resistance Rs value is caused mainly by the C60 layer resistance, since the resistance of the Si substrate layer together with ohmic contact to it is lesser by two orders of value. So, the specific resistance, determined for all the investigated fullerene films, placed between the Au contact layer and Si substrate, ranges from 106 to 107 Ω cm, that agrees well with known data [23]. The ohmic character of the C60 layer resistance suggests the hopping mechanism of the current flow for this voltage range at room temperature. At the same time, a more accurate fitting of the experimental and theoretical characteristics was obtained under the assumption of the Poole–Frenkel mechanism of the current transport in the C60 layer, which suggested the field-

Fig. 7. Spectra of external quantum efficiency, normalized to the area of the diode, for Au/C60/Si (1), Au/C60–DT/Si (2), Au/C60–Au/Si (3), and Au/C60–DT–Au/Si (4) structures.

Fig. 9. Dark I–V characteristics for Au/C60/Si (1), Au/C60–DT/Si (2), Au/C60–Au/Si (3), and Au/C60–DT–Au/Si (4) structures.

Fig. 6. Spectra of light transmittance: (a) into Si substrate through the C60 (1), C60–DT (2), C60–Au (3), and C60–DT–Au (4) layers; (b) through the Au/C60 layers (1–4) and through the Au layers (1′–4′) for Au/C60/Si (1, 1′), Au/C60–DT/Si (2, 2′), Au/C60–Au/Si (3, 3′), and Au/C60–DT–Au/Si (4, 4′) structures on Si.

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enhanced thermal excitation of electrons trapped with local centers into the conduction band [24]. Corresponding I–V dependence was expressed as: rffiffiffiffiffiffiffiffiffiffiffiffiffi!  
  U 2a U qU V I = C U− exp ⋅ exp −1 + U− n T n nkT Rsh


ð1Þ

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi q , d is the thickness of the investigated 4πεε0 d fullerene films, Rsh is the shunt resistance, n is the ideality factor, which V−IRs applied to the space charge region determines the part of voltage n in Si at the C60/Si interface, and C is the proportionality constant, value of which depends on the barrier height at the C60/Si interface, on the internal barriers heights, and the density of the trapping centers in C60. The dependencies of C vs n for different diodes of the investigated structures are shown in Fig. 10. Two different dependencies can be seen: one is for diodes with C60 or C60–Au layers and the other one for the structures with C60–DT and C60–DT–Au layers. So, a greater internal barrier height value (or lesser density of the trapping centers) can be suggested for the Au/C60–DT structure with respect to the Au/C60 one, caused by dithiol functionalization of C60 films. It will be noted that more corrected discrimination of the mechanisms of the current flow in such heterostructures requires further investigation. To evaluate the difference in Qext (λ) spectra for the structures with pristine and functionalized C60 films the spectral dependencies of their ratios are shown in Fig. 11 (a, b). The results for two different diodes of every structure are presented here. To distinguish the peculiarities caused by functionalization of C60 films from those associated with the unhomogeneities over the area of one plate with set of diodes, the ratios of Qext for diodes of the same structure are presented in Fig. 11c. It is seen that the deposition of gold nanoparticles onto the C60 films, cross-linked with octane-1,8-dithiol, results in increase of photocurrent in the range of λ ≈ 600–900 nm (Fig. 11b), while the deposition of Au nanoparticles onto pristine C60, or the cross-linking of C60 with octane-1,8-dithiol itself (Fig. 11a), decreases Qext in this spectral range. The spectral range of this photocurrent enhancement coincides with the one of the extinction coefficient increase (Fig. 4), calculated within the range of one-layer model. So, the light transmittance into Si, calculated both for the diode area (T2), and under the C60 layer (T3), is lesser for the C60–DT–Au film than for C60–DT and C60 in this spectral range. Therefore, such increase of the photocurrent may be caused either by photogeneration of current carriers in the C60–DT–Au layer, or by the nontaken into account increasing of light transmittance into Si due to a surface plasmon resonance excitation in the Au nanoparticles. Distinguishing between these effects requires further investigations. where U =V −IRs, a =

q k

Fig. 11. Spectra of ratios of the external quantum efficiencies for diodes of structures with different C60 treatment (a, b) and for diodes of the same structure (c): a) Au/C60–Au/Si to Au/C60/Si (1,2), Au/C60–DT/Si to Au/C60/Si (3,4); b) Au/C60–DT–Au/Si to Au/C60–DT/Si (1-3), Au/C60–DT–Au/Si to Au/C60/Si (4); c) Au/C60/Si (1), Au/C60–DT/Si (2), Au/C60–Au/Si (3), and Au/C60–DT–Au/Si (4).

4. Conclusions

Fig. 10. The dependencies of C vs n, obtained from Eq. (1), for different diodes of the investigated structures.

The comparative investigation of optical properties of pristine and cross-linked with octane-1,8-dithiol C60 films decorated with gold nanoparticles of spherical shape, found out the difference in the extinction coefficient spectra, which is more essential in the case of C60–DT–Au films. These peculiarities of k (λ) spectra influence differently the photocurrent spectra of the barrier heterostructure Au/C60/Si. Thus, an increase of absorptance for the C60–DT and C60– DT–Au films with respect to the C60 and C60–Au ones in the spectral range of λ b 600 nm, manifests itself in a decrease of the photocurrent of corresponding barrier structures. It seems that this smaller value of photocurrent in the case of C60–DT–Au films results from a lesser transmittance of light into Si, both under the diode area (Fig. 6b), and through the C60–DT–Au layer outside the diode (Fig. 6a). At the same time, an additional region of the light absorptance with maximum at λ ∼ 800 nm for C60–DT–Au films coincides with the region of the

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photocurrent increase. The last effect is probably caused by the gold nanoparticles deposited on the C60 film cross-linked with octane-1,8dithiol, and respectively, by an additional photogeneration of the current carriers in Si due to a surface plasmon excitation in Au nanoparticles. The analysis of dark I–V characteristics for the Au/C60/Si heterostructures has shown that the more likely model for them includes besides the barrier at the C60/Si interface, also internal barriers in the C60 layer, caused by the trapping centers. So, the hopping mechanism of the current transport in the C60 layer is supplemented with the Poole–Frenkel emission process on these centers, the barrier height for which is greater in the case of dithiol cross-linking of the C60 film.

[6] [7] [8] [9] [10] [11] [12]

[13] [14] [15] [16]

Acknowledgements

[17]

Financial support from the National Autonomous University of Mexico (grant DGAPA IN103009) and from the National Council of Science and Technology of Mexico (grant CONACYT-56420) is greatly appreciated.

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[20]

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