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Abstract
The quantum impact of magnetic and optical effects on electrical properties exhibited by multiwall carbon nanotubes decorated with bimetallic nanoparticles integrated by platinum and nickel was analyzed. An external magnetic field causes sensitive magneto-quantum conductivity in the samples and it can be described by the Aharonov-Bohm effect. The nature of the nanoparticles decorating the carbon nanostructures shifted the characteristic quantum response for the magneto-conductive measurements. Magnetically-controlled changes in optical phonons were considered to be responsible for the bistable hysteretic system. A laser-induced-phononic process derives in a strong modification of the electrical properties exhibited by the sample.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Bistable systems based on advanced materials like carbon nanotubes (CNT) promise fascinating applications related to low-dimensional organic devices. Diversity of two-state CNT systems has been envisioned in impacting areas, such as biology [1], optics [2] and optomechanics [3]. Particularly, the multiwall carbon nanotubes (MWCNT) present a high modification in their electronic band structure sensitive to an external magnetic field due to the carbon-carbon interactions between the concentric walls. This behavior is related to the Aharonov-Bohm effect responsible for induced quantum states in carbon nanostructures related to a remarkable phononic excitation [4]. This principle has a specific control when the refractive index of a photonic material is modulated, and it generates a gauge potential for photons that is nonreciprocal [5]. Creating an accurate modulation phase in the Aharonov-Bohm propagation we can promote a photonic Aharonov-Bohm effect [6]. This result can be conceived as an important variation dependent on the electron-phonon interaction in CNT by optical phonons in a tight-binding model approximation [7]. In addition to the thermal properties in isolated MWCNT resulting from their tube-tube coupling, a significant phonon scattering process including all the acoustic bands can be revealed in a bulk sample characteristics [8–10]. These carbon allotropes present a high optical absorbance that makes them good candidates for storing photonic energy and transduce it in thermo-conductive phenomena [11]. The importance of phononic excitation in CNT allows the implementation of terahertz power amplification mechanisms [12], communication devices [13], and spectroscopy systems [14]. By these principles, the magneto-conductivity exhibited by MWCNT thin films is interesting for developing effective phonon-photon scattering in an Aharonov-Bohm flux [15]. Furthermore, MWCNT systems can be enhanced by supporting magnetic nanoparticles (NPs) such as Co, Fe, Ni and Pt, maintaining the potential applications in nanoscale functions [16]. The inclusion of metallic NPs to the surface of MWCNT automatically increases the density and electrical conductivity of their interphases [17, 18]. Although, from multimetallic NPs decoration can be regulated diverse properties dependent on collective chemical and engineering effects; that correspond to each element in the sample configuration [19]. The use of Pt NPs in MWCNT (Pt/MWCNT) has demonstrated the potential enhancement of photoconductive phenomena derived from unique plasmonic effects [20]. Besides, the nonlinear optical properties for Pt/MWCNT composites can be considered for designing all-optical and nanophotonic instruments [21]. Moreover, Ni incorporated to CNT present magnetization at room temperature featuring a strong hysteresis attractive for developing bistable systems [22]. It can be considered that the combination of the characteristics exhibited by the integration of Ni and Pt in MWCNT nanostructures pointed out the possibility to control magnetic and photoconductivity effects with advantages related to thermo-optical energy transfer phenomena induced by optical nonlinearities.
Bistable solutions for engineering problems like magnetic memory operations, multivibrators, or multiphotonic functions, can be employed for developing nano-antennas [23], optical polarization commutators [24] and all-optical switching devices [25]. These applications can be assisted by metallic NPs or MWCNT regarding their spin–orbit coupling able to induce optical bistability. With these motivations, this paper has been devoted to analyze phonon propagations in MWCNT decorated with particular bimetallic NPs based on Ni and Pt. We explored the bistable behavior of the samples for studying the participation of quantum processes.
2. Materials and methods
2.1 Sample synthesis and characterization
Sets of MWCNT were employed to be decorated with Ni and Pt at 5% wt each one. The MWCNT were synthetized by using a chemical vapor deposition (CVD) technique with a solution of toluene/ferrocene at 850 °C and a gas flow rate of 2 l/min of Argon. To reduce some remnant of Fe from the synthesis, the MWCNT were mixed in HNO3 at 100 °C for one day. Additionally, the nanotubes were filtered and dried to eliminate the nitric acid. The samples supporting Ni-Pt NPs (Ni-Pt/MWCNT) were prepared by an in-situ CVD route [26]: at first, the Ni precursor (C10H14NiO4, 98% of purity, EMD Millipore) was mixed with the Pt precursor ([CH3-COCHCO-CH3]2Pt, 97% of purity, Sigma Aldrich) and with MWCNT. The resulting materials were exposed to 180 °C for 10 minutes in a tube reactor. Finally, the condensation of the NPs was carried out at 400 °C for 10 minutes in the reactor under Ar gas flow rate of 100 cm3/min [27]. The final products were deposited in film form between two SiO2 plates with two carbon electrodes in direct contact with the nanotubes for recording conductivity. The distance between electrodes was 1 cm. The approximate thickness of the selected films was ellipsometrically estimated in about 1 μm.
In order to characterize the morphology and dispersion of Ni-Pt NPs onto MWCNT, Scanning Electron Microscopy (SEM; JEOL JSM-6701F) coupled with a microanalysis sensor for Energy-Dispersive X-ray Spectroscopy (EDX) was employed. The Ni and Pt incorporation were studied by an X-Ray diffraction system (XRD; Bruker D8 Focus).
2.2 Electrochemical impedance spectroscopy studies
Electrochemical impedance spectroscopy measurements for analyzing the electrical behavior of the Ni-Pt/MWCNT samples were undertaken. An Autolab/PGSTAT302N high power potentiostat/galvanostat system was employed with a sinusoidal voltage signal modulated from 102 to 105 Hz with 10 mV amplitude and 1 s of integration time. The complex part of the impedance was measured to describe the phase between input and output signals.
2.3 Photothermal effects
The change in temperature in the films was examined to know the influence of the mechanism that gives rise to the phononic phenomena. The sample was directly irradiated for 3 seconds at 10 Hz rate repetition with the Nd:YAG laser source. Since the ablation threshold was around 2 mJ/cm2, two irradiance conditions were selected to study the time-resolved photothermal response of the samples, corresponding to 2 MW/cm2 and 4 MW/cm2. These measurements can confirm the characteristic phonon distribution that promoted the nonlinear photothermal behavior. The experimental data was measured by an infrared sensor with 0.02 °C of resolution. The signal was conditioned by a low noise amplifier and high-resolution analog-digital converter of 17-bits.
2.4 Influence of optical irradiance on magneto-conductivity
The magneto-conductivity process in MWCNT decorated with metallic NPs is highly sensitive to optical irradiation [4]. In order to study the magneto-conductivity dependent on optical irradiation, the second harmonic of a Nd:YAG laser source at 532 nm wavelength and 4 ns pulse duration was employed. The incident beam presented 6 mm of diameter, the propagation vector was perpendicular to the surface of the sample and the irradiance was modulated in amplitude from 0–7 MW/cm2. Moreover, a magnetic field was induced perpendicular, but in the other side of the sample, with a solenoid which emitted a constant field from 0–0.3 Gauss. The magneto-conductivity data were acquired in the electrodes by an ATmega328 8 bits microcontroller. In Fig. 1(a) is shown the scheme of the experiment. In order to modulate the conductivity in the nanohybrid samples, an Aharonov-Bohm effect was induced with an external magnetic field to change the direction of the electron flux. The Aharonov-Bohm effect can be described as a magneto-quantum process where the speed and trajectory of electrons in motion can be modulated through a magnetic potential. The error bar in the conductivity measurements has a maximum value of ± 5%.
Fig. 1. (a) Magneto-conductivity experiment as a function of visible irradiance. (b) Aharonov-Bohm effect and phonon excited scheme.
3. Results and discussion
3.1 Sample synthesis and characterization
Figure 2(a) depicts a representative SEM image of MWCNT decorated with bimetallic Ni-Pt NPs; it can be observed that MWCNT exhibited a uniform diameter around 80–100 nm. These diameters are relatively high, so it is ensured that the dynamic time constant in the thermal response can be measured [28]. Moreover, the dispersed NPs were verified by EDX evaluations shown in Fig. 2 (b). The EDX results point out the elements: C, O, Fe, Ni and Pt. The Fe can be associated with the ferrocene used to growth the CNT. On the other hand, the oxygen was related to the functional groups (mainly OH and carboxylic) generated by the acid treatment. EDX detected a 5% in weight of Ni and Pt. Figure 2 (c) presents the XRD patterns of the Ni-Pt/MWCNT studied, the reflection in 2θ = 26.3° matches to the plane (002) of the graphitic structure of the MWCNT reported in the card JPCDS 012-0212. While, the reflection in 2θ = 43.5° corresponds to the plane (012), of the rhombohedra structure of NiO, reported in the card JCPDS 044-1159. The plane (111) of Pt which is located in 2θ = 40.0° proved the bimetallic characteristics of the nanostructures.
Subsequently to the samples characterization, the phonon band structure of an isolated MWCNT was analyzed by using the Virtual NanoLab software [29]. The simulation basis was a MWCNT with 3 concentric walls and armchair quiral angle mode and no impurities, to maintain the conductive behavior of the sample. Moreover, we substituted the 10% of the carbon atoms in the external wall for Ni atoms in order to approximate the exclusive influence of the Ni incorporation to the MWCNT (Ni/MWCNT). Analogously, 10% of carbon atoms were replaced for 5% Ni and 5% Pt to compute the Ni-Pt/MWCNT sample. In Fig. 3 (a) is shown the phonon density of states (DOS) of the isolated carbon/metal nanohybrids. We observed a maximum density around ∼60 THz in which free phonons could contribute to an enhancement in the thermal quantum activity. From the numerical results can be deduced a collective phonon excitation at higher energies. These quantum effects can be used in the design of nonlinear thermo-optical systems controlled by NPs concentration.
Considering remarkable changes for the different samples in the DOS results, we experimentally compared the electrical response of the samples evaluated by electrochemical impedance spectroscopy. Figure 3(b) illustrates the complex impedance of the samples as a function on the electrical frequency that corresponds to a purely conductive behavior for the Ni-Pt/MWCNT sample. An experimental error bar in the data are expected to be within ± 15% For the case of a representative Ni/MWCNT sample response, an inductive effect can be observed as an increase in the electrical reactance to a maximum value close to 4.5 kHz, and then the sample becomes capacitive with a decreasing reactance behavior for higher frequencies. Figure 3 (a) pointed out important differences in phonon DOS in this electrical spectrum region derived by the incorporation of Ni to the sample. According to these results, it can be considered that the contribution of the Ni NPs is the main responsible for the electrical reactance of the nanohybrids transporting electrons inside the nanotubes.
The photothermal dynamical response of the studied samples is shown in Figs. 4(a) and 4(b). We observed a nonlinear characteristic when the samples were irradiated with 2 MW/cm2 and then with 4 MW/cm2. At low irradiance the Ni-Pt/MWCNT sample presents a small change in temperature, opposite to what happens at high irradiance in which the Ni-Pt/MWCNT sample promoted a bigger increment in temperature. The differences in the thermal transport were attributed to the phononic scatter in each sample and also to the nonlinear optical absorption. The experimental data was fitted by using the two-dimensional heat-transport equation, since the thickness of the film was considered to be negligible in comparison to its surface [11].
Fig. 4. Transient photothermal response of change in temperature of two selected irradiances in the studied samples (a) 2 MW/cm2 and (b) 4 MW/cm2. Marks are associated with experimental data and lines with numerical approximation.
A remarkable change in temperature due to optical irradiance in nanostructure systems gives as a consequence a measurable change in electrical impedance. In Fig. 5 is shown the variation in conductivity for each sample at the same irradiated conditions that the previous experiment. Each mark in the experimental plot represents an average of 10 measurements. Although there is a strong dispersion in the experimental data, the minimum and maximum values in the plots were reproducible and they were clearly identified far from the error bar in our measurements. The behavior exhibited by the trends shown in Fig. 5 is mainly associated with the thermal carriers inside the MWCNT and the inference of metallic NPs, which classically can be all related to the temperature coefficient of resistance (TCR) [30]. The continuum lines in Fig. 5 were fitted by using the method of least squares and considering the equation ${R_f} = {R_0}(TCR \times \Delta T + 1)$ where Rf and R0 are the final and initial electrical resistance of the sample. The coefficients which better approximate the experimental data were −0.008 K−1 and 0.025 K−1; respectively.
Fig. 5. Change in conductivity of the studied samples as a function on thermo-optical activity (a) Ni/MWCNT (b) Ni-Pt/MWCNT.
Since the electrical characteristics can be controlled by phonons, we can expect a modulation in the magneto-conductivity process exhibited by MWCNT by the assistance of metallic NPs incorporation [4]. In experimental results we achieved a considerable change in magnetic properties by the inclusion of low concentration of metallic particles in the walls of the MWCNT. The quantum process that depicts the effective-mass approximation of an electron in motion through a carbon nanotube by the presence of a weak magnetic field is the Aharonov-Bohm effect. The repercussion of this effect is a change in the conductivity and by a magnetic response by a quantum-intenference phenomenon. The conductivity as a function on a magnetic field can be expressed [31]:
where e is the elementary charge, $\hbar$ is the Plank constant, and ΛK ± are defined by:Fig. 6. Phonon scatter in magneto-conductivity of Ni/MWCNT sample. The marks indicate the experimental data, and the continuous lines correspond to the better curve fitting.
Taking into account the magneto-conductive behavior of MWCNT under nanosecond irradiation, it can be contemplated to induce controlled transitions dependent on variable magnetic fields. Afterwards, it is noticeable that a bistable electrical function could be effectively accomplished by a magneto-optical field. In this regard, Fig. 7 exemplifies the conductivity of the carbon/metal nanohybrids, resulting from increasing and decreasing the magnetic fields by a solenoid under the influence of nanosecond pulses with 2 MW/cm2 at a repetition rate of 10 Hz. The magnetic field activates the bistable response exhibited by the MWCNT compounds and it can be observed that the induced states present a reversible behavior. The employed peak optical irradiance is close the ablation threshold but the absence of optical damage was guarantee in our measurement by optical spectroscopy. Well-dispersed Pt NPs incorporated in MWCNT increased the electrical conductivity as it could be expected [33]. Furthermore, the increased electrical conductivity could efficiently enhance the catalytic activity due to the improvement of electron transfer. In Table 1 are summarized the experimental results in order to visualize and compare the physical properties of the samples.
Fig. 7. Magneto-conductive bistability in MWCNT thin film decorated with (a) Ni at 10% wt, (b) Ni-Pt at 5% wt each element. The direction of the arrows exhibit the evolution of the magnetic field.
Table 1. Comparison of the parameters obtained from numerical and experimental results.
There have been previously described optical bistable mechanisms by carbon allotropes such as bidirectional plasmonic switching [34,35] and the improvement in the electrical conductivity by the influence of metallic NPs [36–39]. Considerable advances in nanotechnology by adding monometallic or bimetallic NPs in MWCNT have been reported towards the optical detection of biological conditions [40]. Ni/MWCNT nanostructures can enhance the hardness properties of rotors by electrodeposited coating [41]; furthermore, MWCNT used for gas sensing devices have been selected to discriminate aromatic from non-aromatic organic compounds in ambient with the participation of Pt in low-dimensional particles [42]. Moreover, Ni-Co and Ni-Au bimetallic incorporations have been proven to be able to solve medical and chemical actions by the synergy of the particles [43, 44]. With regard to the magnetic behavior, selectable nonlinear Kerr transmittance and quantum propagation in Pt/MWCNT films have been denoted to encrypt signals [4]. Polarization-selectable optical devices in MWCNT have been also proposed to optimize the attenuation coefficient of the transmitted light by modulating an external magnetic field [45], and magneto-phonon coupling in resonant MWCNT for quantum entanglement [46]. On the other hand, the development of systems based on Ni-Pt/MWCNT seems to be feasible to improve the oxygen reduction activity in cathodes [47], memory storage in semiconductors [48], transistor logic technologies [49], or in multi-magnetic spin transport applications [50]. In particular, in this work, it is highlighted the remarkable magneto-conductivity bistable properties of Ni-Pt /MWCNT samples by analyzing the phonon-photon interactions.
Bistable systems coordinated by the increment and decrement of a weak magnetic field are suitable to be designed by considering MWCNT/metal nanohybrids. The conductive characteristics of organic compounds are highly flexible by adding metallic NPs. The quantum Aharonov-Bohm effect in both studied samples was revealed in similar experimental conditions. The bistable nature of the studied systems was numerical demonstrated by a nonlinear first order differential equation [51]. The main results shown in this work open the possibility for developing controlled nonlinear resonators by phonon-photon interactions [52]. Moreover, the method can contribute to the detection of phonons by exciting CNT close to the ablation boundary for quantum applications.
4. Conclusions
Within this work is described the influence of heat transfer in bistable magneto-conductivity effects exhibited by MWCNT decorated with monometallic Ni NPs and bimetallic Ni with Pt NPs. Measurements in photothermal and thermo-conductive effects induced by nanosecond pulse irradiation revealed an effective phonon-photon interaction in the samples. Noninvasive changes in conductivity were induced by the Aharonov-Bohm effect and they were modulated by nanosecond pulses at 532 nm wavelength. This process is disturbed by characteristic phonon effects in the nanohybrid samples. The bistable system assisted by the Aharonov-Bohm effect has immediate applications for developing nanoscale magnetic moment sensors.
Funding
Consejo Nacional de Ciencia y Tecnología (CONACYT) (CB-2015-251201); Instituto Politécnico Nacional (IPN) (SIP-2018).
Acknowledgments
The authors kindly acknowledge the financial support from the Instituto Politécnico Nacional, the Consejo Nacional de Ciencia y Tecnología and the Centro de Nanociencias y Micro y Nanotecnología from Instituto Politécnico Nacional.
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