Mechanooptic Regulation of Photoconduction in Functionalized Carbon Nanotubes Decorated with Platinum

1 Sección de Estudios de Posgrado e Investigación, ESIME ZAC, Instituto Politécnico Nacional, 07738 México, DF, Mexico 2Departamento de Ingenieŕıa Metalurgia y Materiales, ESIQIE, Instituto Politécnico Nacional, 07300 México, DF, Mexico 3 Instituto Politécnico Nacional, ESIQIE, 07738 México, DF, Mexico 4Centro de Investigación en Ciencia Aplicada y Tecnologı́a Avanzada Unidad Querétaro, Instituto Politécnico Nacional, 76090 Santiago de Querétaro, QRO, Mexico 5 Departamento de Fı́sica y Matemáticas, Universidad Iberoamericana, Prolongación Paseo de la Reforma 880, Lomas de Santa Fe, 01219 México, DF, Mexico


Introduction
One of the most significant scientific interest related to study carbon nanostructures, comes fro m their particular advantages for designing nanosystems that can be improved by a number of materials [1]. A mong some examp les devoted to investigate high sensitive instrumentation of signals, diverse reports have been dedicated to describe mechanical [2][3][4][5] or electrical [6][7][8] tasks that highlight the extraord inary properties exhib ited by carbon nanotubes (CNTs).
Several preparation methods for evaluating the morphological and structural characteristics of CNTs have been carried out [9,10]; and also, the possibility to influence the resulting physical functions of advanced materials based on CNTs seems to be attractive [11].
Moreover, it is notable that the adjustment of singular physical features during the processing route of different samples has been accomplished by implementing hybrid organic -inorganic materials mainly constituted by CNTs [12][13][14][15].
In many fields of nanotechnology, distinct kinds of CNTs have drawn considerable attention regarding their outstanding optical and electrical parameters [16,17]. In addition, the great importance of the third order optical nonlinearit ies of CNTs has also originated to consider them as exceptional materials for proposing all-optical systems displaying powerful absorptive and refract ive nonlinear effects [18][19][20][21][22]. Apparently, the development of nonlinear lo w-d imensional compound materials is a crucial step towards the improvement of all-optical nanotechnology [23].
So, in regards to the fascinating mechanical, optical and electrical response associated to CNTs, an opportunity to use them for developing mu lti-functional s mart materials can be contemplated.
The research progress of the photoconductive and mechano -optical response with CNTs and other metal elements have originated that different configurations can be considered as ideal candidates for next-generation of flexib le transparent conducting films [ 24]. Decoration of CNTs by metallic nanoparticles (NPs) is attractive because the resulting physical characteristics exh ibited by the individual co mponents can be enhanced [25][26][27]. Using electrochemical deposition methods, a good control on size and densities of Pt NPs or bimeta llic Pt-Ru NPs decorating CNTs has been accomplished [28,29]. On the other hand, microwave-assisted hydrothermal synthesis has allo wed the preparation of Pd NPs, Ni NPs, and Sn NPs decorated on CNTs [30]. Following a polyol process, homogeneous distributions of Au NPs, Ag NPs, Pt NPs, Pd NPs, as well as bimetallic PtPd NPs and PtRu NPs, have been deposited on CNTs [31][32][33][34][35]. And it has been pointed out that the optical transmittance and electrical conductivity of the resulting samples can be controlled by the deposition time of the processing route [35]. Moreover, in order to form heterojunctions of CNTs to metal NPs, eletron-beam systems have been employed [36][37][38], and Fe NPs, Co NPs, Ni NPs, and FeCo Nps, decorating CNT with improved electrical and mechanica l properties have been obtained [39].
With this motivation, in this research an attempt has been made to further investigate the potential applications of the photoconductive and mechano-optic response of multiwall CNTs (MWCNTs). Experimental results associated to functionalization, electrical, photoconductive and nonlinear optical absorption phenomena that were successfully enhanced by platinum decoration of the studied samples are presented.

MWCNTs: synthesis and functionalization
MWCNTs were produced by thermal decomposition of Toluene (Fermont, 99.9% purity) and Ferrocene (Sigma-Aldrich, 98% purity) in a tubular reactor at T = 850 °C, P tot = 80.1 kPa for 40 min. Ferrocene was dissolved in Toluene to form the source solution in a 1/39 mol ratio. The solution was nebulized as microdroplets and carried into the reactor by Ar gas with a flow rate of 2.5 L/min. [40]. MWCNTs were functionalized in 3:1 v/v mixture of sulfuric (30 mL, 95-97%) and nitric acid (10 mL, 65%) under sonication (42 kHtz) for 15 min at room temperature. After functionalization, MWCNTs were repeatedly washed in distilled water, centrifuged and dried in vacuum. [41]. The quality of functionalized MWCNTs (f-MWCNTs) was investigated by Raman Spectroscopy (Jobin Yvon Horiba).

Photoconductive measurements
Electrical measurements were evaluated using an Autolab/PGSTAT302N high power potentiostat/galvanostat. The impedance spectrum was measured with a 10 mV signal and an integration time of 1 s. The photoconduction on the samples was separately investigated at 445 nm and 532 n m wavelengths with continuous wave (CW) lasers providing 1 W of average power. The incident polarization of the optical beam was aligned to coincide with the path in measurement.
The electrodes used for these experiments were in direct co ntact with the sample; they were located in the neighborhood of the diameter of the incident beam. The conductivity was measured using two metallic electrodes separated by a distance of 5 mm for each studied sample.

Linear optical response
The linear absorption spectrum of the samp les was acquired with a Perkin Elmer XLS UV-visib le spectrophotometer.

Photoconduction and nonlinear optical response
The optical transmittance and photoconduction in the thin films were measured by means of a high irradiance single beam trans mittance experiment. A 532 n m wavelength with 1 ns pulse duration was mon itored using the second harmonic of a Nd-YA G laser source Continuum Model SL II-10.     It has been previously reported that the incorporation of Pt will increase the conductivity exhibited by decorated carbon nanotubes [43]. The electrical results for our studied samples are shown in Fig. 4. As it could be expected, a considerable enhancement in the resulting alternating current (AC) was derived fro m the incorporation of Pt in the nanotubes. But it is worth noting that the functionalization of the tubes originates a remarkable inhibit ion of the conductive response;

Results and Discussion
probably because this process may promote the quenching of free holes that generates a change in the conductive phenomena.
j   , ω is the angular frequency of the AC of electrons; R and R 1 represent the electric resistances, X C is the capacitive reactance and C is the capacitance. Best fitting parameters are p resented in Table 1. Table 1. Electrical parameters in the studied samples. Taking into account the data described in Table 1, for the higher electrical frequencies plotted in Figs. (5) and (6), the photoconductive response in the Pt/f-MWCNTs shows a stronger capacitance behavior associated to the change in their monotonically increase in conductivity. This behavior is consistent with the fact that some electrical charges could be stored by the excitat ion of Pt ions that are also incorporated in the tubes. Regarding these photoconductive results, it can be considered that the functionalizat ion ought to originate a modification of meta-stable electronic states that results in a decrease of the conductivity.
On the other hand, the experimental setup illustrated in Fig. 1    Pt/f-MWCNTs is evidently described by the transmittance results plotted in Fig. 9. This TPA phenomenon, together with the enhancement in the photoconductivity at higher repetition rates of nanosecond pulses, could be associated with an important contribution of multi -photonic interactions that results after the incorporation Pt in the tubes.
In order to explain the observed nonlinear optical response, we considered the dipole approximation in a system of N two-level ato ms per unit volu me under a high optical irrad iance.
where m represents the atomic dipole mo ment, Δ=ω-ω 21 is the detuning of the frequency ω of the incident radiation, 1/T 1 represents the population loss through radiative and non -radiative processes of the upper level, and 1/T 2 represents the rate of polarization loss for the off-d iagonal matrix elements. Fro m Eqs. (3) and (4)

Time [sec]
We calculated the heat-transference generated by optical irrad iation in propagation through the sample by using [47], here T is the temperature, which is a function of the estimated depth in our sample z=1 μm. As a first approximation, we considered the thermal conductivity = 200 W/mºK, the density = 1×10 -3 Kg/cm 3 , and the heat capacity C=1×10 3 J/KgºK for MWCNTs previously reported [48]. In our case the time of irradiat ion t= 5 s, the linear absorption coefficient =2×10 6 m -1 and I the optical intensity. The estimated results indicate that an instantaneous temperature change of approximately 180º K can be expected after the propagation of each pulse. However experimental measurements allow us to state that long duration temperature changes (after at least one second) of about 2º K were detected in agreement with our calculations .
The possibility to enhance the nonlinear optical response and the electronic features of CNTs samples by the modification of meta-stable electronic levels appears to be attractive. What is more, we consider that potential applications for tailoring the optical and the electrical response associated to diverse materials can be also improved by the incorporation of NPs capable to change electrical and optical interactions . Regarding the contribution of the absorptive nonlinearities to the electrical p roperties of the studied samples, potential applications for developing optoelectronic nanosystems based on decorated CNTs can be contemplated.

Conclusion
A noticeable enhancement in the electrical and photoconductive response exh ibited by f-MWCNTs nanotubes was achieved by platinum decoration. Apparently, the incorporation of platinum NPs on MWCNTs originates a modification in the non-resonant electronic levels that can play an important role in the resulting photoconductive and two-photon absorption effects. A simp le mechano-optic effect based on the photoconductive response exhib ited by the studied samples was observed.