WO2022164307A1

WO2022164307A1 - Method for obtaining electrodes with nanospaces from suspended nanofibres of glassy carbon

Info

Publication number: WO2022164307A1
Application number: PCT/MX2022/050004
Authority: WO
Inventors: Sergio Omar Martínez Chapa; Arnoldo SALAZAR SOTO; Marc Josef MADOU; Alejandro MONTESINOS CASTELLANOS
Original assignee: Instituto Tecnológico y de Estudios Superiores de Monterrey
Priority date: 2021-01-27
Filing date: 2022-01-12
Publication date: 2022-08-04
Also published as: MX2021001079A

Abstract

The present invention relates to electrodes with nanospaces between 4 and 17 nm obtained from suspended nanofibres of glassy carbon with a length greater than 2 μm, designed for application in the area of nanotechnology in the manufacture of nanosensors.

Description

METHOD FOR OBTAINING ELECTRODES WITH NANOSPACE FROM SUSPENDED GLASS CARBON NANOFIBERS

FIELD OF THE INVENTION

The present invention is related to the application of nanotechnology to obtain electrodes with nanospaces, more specifically with a method for obtaining electrodes with nanospaces from suspended vitreous carbon nanofibers.

BACKGROUND OF THE INVENTION

Electrodes separated by a nanometric distance (nanogaps), known as nanogap electrodes, have been widely applied in the fundamental study of electrical properties, and the manipulation of submicron particles and molecules. To meet both objectives, a nanogap electrode must have a separation distance comparable to the size of the manipulated object. Nanogap electrodes with 20 nm nanogaps, matching the size of the nanogap to the length of the target DNA strands, have been proposed to study the electrical conductivity of individual bases. In view of the potential applications, various investigations have been carried out for the manufacture of nanogap electrodes.

have proposed methods such as mechanical break joint (MBJ), focused electron beam (FBI), electromigration, photolithography, among others. Unfortunately, these techniques still face technical challenges and suffer from poor performance, poor scalability, complex processing protocols, and/or high equipment cost.

One of the most used materials in the manufacture of nanogap electrodes has been gold. However, it has been described that gold has various drawbacks for the construction of nanospaces, among which the lack of stability, expansion of the nanospace, surface energy and high mobility of the material are mentioned. In order to avoid the problems presented with the use of gold, various materials with potential applications in the construction of electrodes with nanospaces have been sought. One example is nanostructured carbon, which has advantages in terms of resistance to electromigration, better stability at or above room temperature, and easier binding to a greater variety of molecules for biosensing applications. Fabrication of nanogapped electrodes from carbon for molecular-scale sensing applications is typically achieved by temperature-activated electrical decomposition of carbon nanotubes (CNTs), graphene sheets, or glassy carbon nanofibers (GCNFs). In various studies, it was found that the electrical decay mechanism of these carbon devices is related to both the electron transport in the sample and the experimental conditions.

Carbon nanotubes and graphene sheets exhibit higher electrical and thermal conductivities than those exhibited by glassy carbon nanofibers, consequently the first two are the subject of a greater number of studies. However, glassy carbon nanofibers exhibit good resistance to chemical attack, thermal stability, biocompatibility and greater simplicity in their production. Making them an attractive alternative for the manufacture of stable carbon-based nanodevices. A common method for the synthesis of glassy carbon nanofibers is to obtain them from a polymer through a two-part process consisting of: electrospinning, to produce the polymeric nanofibers, and pyrolysis, where the nanofibers are carbonized with an appropriate polymeric carbon precursor. . A limitation of the conventional methods for the manufacture of nanogap electrodes from nanostructured carbon nanofibers lies in the fact that they necessarily need nanofibers of small lengths, with values less than 2 pm, since if longer lengths are used, the space exceeds the nanometer order. However, obtaining nanofibers with lengths less than 2 pm is complicated.

Therefore, there is a need to develop a method for obtaining electrodes with nanospaces that can start from suspended glassy carbon nanofibers with lengths greater than 2 pm, for its application in the study of the electrical properties of particles and molecules, where electrodes with nanospaces have greater stability that prevents their dilation after storage.

OBJECTS OF THE INVENTION

Taking into account the shortcomings of the prior art, it is an object of the present invention to provide electrodes with nanogaps obtained from suspended glassy carbon nanofibers with a length greater than 2 pm.

Another object of the present invention relates to a method for obtaining electrodes with nanospaces from suspended glassy carbon nanofibers with a length greater than 2 pm.

Finally, an additional object of the present invention refers to a nanosensor comprising electrodes with nanospaces obtained from suspended glassy carbon nanofibers with a length greater than 2 pm.

BRIEF DESCRIPTION OF THE INVENTION

For this, a first aspect of the present invention refers to an electrode with nanospace, where the nanospace has a length between 4 and 17 nm, which is obtained from suspended glassy carbon nanofibers with a length greater than 2 pm.

A second aspect of the present invention refers to a method for obtaining electrodes with nanospaces between 4 and 17 nm, which comprises a step of sublimating a suspended glassy carbon nanofiber with a length greater than 2 pm under reduced pressure conditions.

A third aspect of the present invention refers to a nanosensor comprising at least one electrode with nanospaces between 4 and 17 nm obtained from suspended glassy carbon nanofibers with a length greater than 2 pm.

DESCRIPTION OF THE FIGURES

The novel aspects that are considered characteristic of the present invention will be set forth with particularity in the appended claims. However, the features and benefits of the same will be better understood in the examples, when read in relation to the attached figures, where:

Figure 1 illustrates the process of thinning and decomposition of a suspended glassy carbon nanofiber.

Figure 2 illustrates the resistance evolution for the thinning and degradation process under different conditions, where A is dry air at atmospheric pressure, B is CO2 at atmospheric pressure, C is high vacuum after purging the chamber with dry air. , and D is high vacuum after purging the chamber with CO2.

Figure 3 illustrates SEM micrographs showing the smooth and rough surfaces of suspended glassy carbon nanofibers broken under different conditions, where A is dry air at atmospheric pressure, B is CO2 at atmospheric pressure, C is high vacuum in a chamber containing was previously purged with dry air, and D is high vacuum after purging the chamber with CO2.

DETAILED DESCRIPTION OF THE INVENTION

The present invention exhibits certain advantages over the state of the art, among which it can be mentioned that the electrodes with nanospaces between 4 and 17 nm are obtained from suspended vitreous carbon nanofibers with a length greater than 2 pm, which result stable at room temperature and compatible with various materials due to the characteristics of the material used in its manufacture. Likewise, the methodology used is fast, simple and scalable.

Therefore, the present invention refers firstly to an electrode with a nanogap, where the nanogap has a length between 4 and 17 nm, which is obtained from suspended glassy carbon nanofibers with a length greater than 2 pm. Preferably the nanogaps are less than 10 nm in length.

In a preferred embodiment of the present invention, the reduced pressure is between 0.001 and 0.003 Pa.

In another preferred embodiment of the present invention, the step of sublimating a suspended vitreous carbon nanofiber with a length greater than 2 pm is carried out with an increase in the resistance of the nanofiber greater than 0.05%. The increase in resistance greater than 0.05% can be achieved by applying a voltage in a range between 0.2 and 3.8 V, with increments of 0.2 V.

Suspended glassy carbon nanofibers with a length greater than 2 pm are preferably obtained according to the following steps: (i) coating a base with a layer of a polymeric carbon precursor with a thickness between 10 and 30 pm; (ii) heating the coated base until the solvent is removed from the polymeric precursor; (iii) selectively exposing the base to UV rays; (iv) depositing the obtained nanofibers on an electrospinning platform, which is adjusted to a distance from the needle to the collector of at least 1 mm and to a voltage to suspend them; and (v) heating the suspended nanofibers. More preferably, the step of (i) coating a base with a layer of a carbon polymeric precursor includes centrifugation between 3000 to 4000 rpm for between 25 to 30 s. The thickness of the polymeric carbon precursor is 20 pm. The polymeric carbon precursor is negative photoresist based on eight epoxy groups. The stage of (ii) heating the coated base until the solvent is eliminated from the polymeric precursor is achieved at a temperature between 90 and 100 °C for between 4.5 and 5.5 minutes. The stage of (i¡¡) selectively exposing the base to UV rays is achieved in a time between 3 to 5 s. The stage of (iv) deposing the nanofibers to suspend them is achieved at a voltage between 100 to 200 V. The stage of (v) heating the suspended nanofibers is achieved at a temperature between 800 and 1000 °C under an inert N2 atmosphere. for at least 1 hour.

The advantages of the present invention will be better understood from the following examples, which are presented solely for illustrative purposes to allow a full understanding of the non-illustrated modalities that can be put into practice based on the detailed description made above.

EXAMPLE 1

This example describes the process for the production of suspended vitreous carbon nanofibers, where the first step was to deposit a thin layer of a polymeric carbon precursor, photoresist negative SU-8 2025 with a thickness of 20 pm on a type of silicon. N wafer (100 mm) with a 1 pm thermal oxide layer by spin coating at a speed of 4000 rpm for 30 s. The coated surface was baked for 5 minutes at 95°C on a hot plate (HS61) in order to remove the solvent. The pattern was then defined by selective exposure to ultraviolet rays (UV light) for 4 s. Unexposed segments were removed by immersing the wafer in a developer solution, leaving behind a six-walled device.

The second step involved the deposition of the fiber obtained by SU-8 2025, where Electromechanical Spinning (EMS) was used on a Newport uFab electrospinning platform tailored to the platform. By setting the needle-to-collector distance to approximately 1 mm and the voltage to 400 V, high control of the position of a single polymer fiber on the supporting walls was achieved. Finally, the entire device, including the wall support and suspended fibers, was placed inside a PEO 601 oven for pyrolysis. During this process, the structures were heated up to 900 °C in an inert N2 environment, which resulted in the loss of non-carbon atoms from SU-8 and producing glassy carbon structures.

Once the glassy carbon nanofibers were obtained, they were subjected together with the support to various experimental conditions to obtain electrodes with nanospaces. Conditions were controlled via a 3 L chamber connected to a Pfeiffer HiPace 80 vacuum turbopump with electrical feed connections and gas inlet lines. In the following examples, the tests carried out under four different conditions are shown: (i) in a chamber filled with pressurized dry air atmosphere, (ii) in a chamber filled with CO2 at atmospheric pressure, (iii) under high vacuum in a chamber that was previously purged with dry air, and (iv) under high vacuum in a chamber previously purged with CO2.

EXAMPLE 2

In this essay, the formation of an electrode with nanogaps through the method of the present invention is described, using a program for the application of voltage that executes a loop controlled by feedback and monitors the changes of electrical resistance (AR) in the nanofiber. fiber. An indication of the effects caused by heating the nanofibers from this AR value when the nanofiber is electrically stimulated. Therefore, the AR parameter was selected as the main indicator of the changes that occur in the nanofibers. A negative AR indicates a decrease in electrical resistance as the fiber is heated, as expected from carbon's negative coefficient of resistance. A positive AR represents mass loss leading to thinning and eventual breakage of the nanofibers. The voltage application process was started by applying a low voltage of 0.2 V and the resistance was measured. Subsequently, the voltage was increased in 0.1 to 0.2 V increments until a limiting value AR was observed, above which a reduction in fiber cross-sectional area began to occur. To avoid an uncontrolled breakdown process, abrupt changes in AR were avoided. From the tests, it was found that a positive electrical resistance change greater than AR = 0.05% is adequate to allow the system to reach the activation temperature necessary to form a constriction, while preventing the rupture process from taking place. in an uncontrolled way.

An additional measure was implemented to avoid the uncontrolled rupture of the nanofibers by limiting the value to which the tension could be increased (Vmax). Before the start of the program, an initial Vmax was selected based on previous experiments with fibers of similar diameters and lengths. The voltage application program automatically adjusted the time in each voltage step between 1 and 4 s to further control the energy imparted on the fiber. Once AR exceeded the threshold value, electrical stimulation was interrupted and a new cycle of voltage application began. The process was repeated until an open circuit was detected, indicative of the formation of a nanogap. Based on the assumption that as the nanofibers were gradually reduced, the condition in which AR > 0.05% was reached occurred at a lower limit voltage (Vth), the maximum voltage was reduced in each cycle of voltage application, such as Vmax = Vth -0.2, to follow this condition. Therefore, at any given point during the experiment the thinned fiber that in a previous iteration caused the >R> 0.05% condition to be met, the voltage feed was cut off. Figure 1 shows the process of thinning and decomposition of a suspended glassy carbon nanofiber, where A represents a fiber of 44.5 pm length and 1.4 pm diameter that thinned to a nanoconstriction of 351 nm represented by B. Later , voltage was applied until an open circuit was detected, with the concomitant production of a 20-nm nanogap, as exhibited in C. In addition to the voltage application, another important factor for the fabrication of nanogaps smaller than 10 nm is the choice of experimental conditions for fiber rupture. The carbon can lose mass on heating by two main mechanisms: (i) an oxidation reaction with O2 or CO2 and (¡i) sublimation at high temperatures, estimated at approximately 4000 K at atmospheric pressure and 3000 K in high vacuum.

EXAMPLE 3

In the present test, conditions were established for the formation of an electrode with nanogaps in a chamber at atmospheric pressure in the presence of dry air and CO2, using the voltage application procedure described in the previous example.

The voltage was increased from 0.2 to Vmax = 3.8 V, with 0.2 V increments, and the resistance first decreased from 18 to 10 kQ and then increased at the end of this voltage range. Once the AR > 0.05% condition was detected, the voltage application was stopped, Vmax was reduced to 3.6 V, and the process was repeated. From 500 to 1750 s, a slow and gradual increase in electrical resistance to 3.6 V was recorded, followed by an abrupt increase in resistance at the end of this period. Figure 2, letter A shows the evolution of the resistance for the thinning process of the nanofibers under these conditions.

This behavior was always observed for experiments in air at atmospheric pressure and could be explained in terms of the two main factors that consume suspended glassy carbon nanofibers: oxidation and sublimation. Oxidation, which is due to the presence of O2 in the chamber, is an expected process because the autoignition temperature of carbon is approximately 973 K. This observation suggests that, even at temperatures below the sublimation temperature of carbon, the fiber could be thinned by burning. When the temperature reaches the sublimation temperature of carbon, which is estimated to be about 4000 K under atmospheric pressure, the thinning mechanism switches from an oxidation-driven reaction to a sublimation, producing a rapid and abrupt jump in electrical resistance, making it difficult to breaking control. Using the method described above for ten suspended glassy carbon nanofibers with a mean length of 43.3 pm under these conditions, nanogaps with a mean size of 154 nm with a standard deviation of 43.3 nm were obtained.

To avoid reactions due to the presence of O2 in the experimental setup, the tests were also performed at atmospheric pressure in a chamber filled with CO2. Figure 2, letter B shows a typical graph for suspended glassy carbon nanofibers treated under these conditions, with an initial Vmax = 4.8 V, from 0 to 500 s. Two cycles of voltage application were performed to avoid rapid rupture of the nanofiber. Then, at approximately 650 s, with the maximum voltage set to 4.4 V, a much faster increase in electrical resistance was observed than the test performed in the presence of dry air. In this case, the reactions were promoted not by the presence of O2 but by CO2. From the comparison of the resistance slopes of figure 2, letters A and B it can be immediately inferred that the fiber was consumed at a higher rate in CO2 than in the presence of dry air. This is most likely because the reaction in the chamber filled with CO2 (100% CO2 content) (i.e. C + CO2 => 2CO) was thermodynamically favored at temperatures above about 1600 K and the reaction rate was high compared to the reaction rate in air. (21% O2 content) (ie 2C + O2 => 2CO).

As in the case of the reaction in air, in the presence of CO2 a sharp and unexpected increase in electrical resistance was observed at 1400 s, indicating that sublimation was activated due to the high temperature, which caused fiber breakage. Ten suspended glassy carbon nanofibers with an average length of 50 pm were thinned and broken, resulting in gaps with an average size of approximately 144 nm and a standard deviation of 13.5 nm.

EXAMPLE 4

In the present test, conditions were established for the formation of an electrode with nanogaps in a high vacuum chamber in the presence of dry air and CO2, using the voltage application procedure described in example 2.

In order to further reduce the size of the nanogap, a method of applying voltage to suspended glassy carbon nanofibers in high vacuum (2xio ^-5 mbar) was used. Figure 2, letter C shows a typical graph of the applied voltage and resistance evolution of a suspended glassy carbon nanofiber during voltage stimulation. As noted, the process ran for 10 cycles without experiencing the unpredictability seen at atmospheric pressure. When performing the experiments under vacuum conditions, ten fibers of 45 pm length with a mean nanospace size of 102 nm and a standard deviation of 52.7 nm were obtained. In this scenario, fiber thinning is primarily related to sublimation, which makes it easier to control the process. In contrast, the thinning/decomposition of suspended glassy carbon nanofibers under atmospheric pressure depends on the reactions of C with O2 or CO2, as well as the sublimation process. As fiber breakage approaches, these oxidation reactions accelerate the breakage process, whereas in the absence of reactive gases such as O2 or CO2, only sublimation is expected to influence the breakage process. Therefore, the voltage application method exhibits excellent performance under high vacuum conditions, resulting in small nanogaps. Furthermore, sublimation of the nanofibers could occur at lower temperatures (approximately 3000 K) under these conditions. When even smaller nanogap sizes are reached, it can be expected that as the constrictions in the nanofibers become thinner, the process becomes more difficult to control. As discussed in the previous examples, the presence of O2 or CO2 can lead to reactions that promote burning of the nanofibers. In a high vacuum of about 2xio ^-5 mbar, although the speed of the oxidation reactions is reduced, there may still be traces of CO2 and O2, which may have an effect on the reduction/breakdown mechanism of the nanofibers.

To remove any remaining oxygen, it was proposed to purge the chamber where the nanofibers were deposited with CO2 before performing the high vacuum experiments. As shown in figure 2, letters C and D, the resistance response was similar in both high vacuum processes (with or without CO2 purge). However, the nanogaps produced after purging the chamber with CO2 were significantly smaller, with an average size of 9.8 nm and a standard deviation of 7.4 nm for ten suspended glassy carbon nanofibers with an average length of 52 pm. Considering a stringent gap of 10 nm or less as the target separation, a 50% yield was achieved for the ten nanofibers broken under high vacuum after CO2 purging; however, all fibers were below 20 nm. The stability of carbon nanogaps was tested by performing SEM analysis of several samples after they were stored at room temperature for approximately 6 months. They do not show any shape change or nanospace separation, demonstrating the good stability of these structures.

From these results, it can be concluded that the use of the reported voltage application procedure under high vacuum in a chamber previously purged with CO2 provided a good approach for the fabrication of nanogaps of less than 10 nm in suspended glassy carbon nanofibers. without the need for fibers that are less than 2 pm long. In the case of high vacuum conditions, the sublimation temperature for an air-filled chamber is expected to be lower than that for a chamber at atmospheric pressure (approximately 3000 K), and the process would be driven primarily by sublimation of the nanofiber. A further reduction in the size of the nanogap can be achieved by considering the trace particles that remain in the chamber after vacuum conditions are reached. In the case of air, the presence of small amounts of O2 can cause exothermic reactions that release energy in the system, causing the nanofiber to continue burning even after the electrical stimulation stops. On the contrary, for CO2, the remaining particles undergo an endothermic reaction that consumes energy from the system, thus contributing to a better control over fiber breakage, with the concomitant production of nanogaps of less than 10 nm.

Additionally, the surface of the resulting suspended glassy carbon nanofibers after the thinning and decomposition processes was analyzed. In Figure 3, the SEM images of four different fibers are presented. It can be clearly seen that experiments performed under atmospheric pressure of air and CO2 (letters A and B, respectively) resulted in a smooth, almost polished surface. This effect can be attributed to the reactions that drive fiber combustion before sublimation, since the oxidation of fibers in air and even in CO2 has been previously reported and used to treat the surface of carbon nanofibers. . On the other hand, in high vacuum cases, the sublimation-driven process can be considered responsible for breaking bonds in suspended disordered glassy carbon nanofibers, leading to the very rough surfaces seen in Figure 3, letters C. and d.

In accordance with the above, it can be seen that the method for obtaining electrodes with nanospaces from suspended vitreous carbon nanofibers with a length greater than 2 pm has been designed for application in the area of nanotechnology, specifically, in the manufacture of nanosensors, and it will be evident to any person skilled in the art that the modalities of the invention as described above and illustrated in the accompanying drawings, are only illustrative and not limiting of the present invention, since they are possible numerous major changes in its details without departing from the scope of the invention. For example, it is possible to use different polymeric carbon precursors to obtain suspended nanofibers of glassy carbon and follow the same method described here for the fabrication of electrodes with nanogaps.

Therefore, the present invention should not be considered as restricted except as required by the prior art and by the scope of the appended claims.

Claims

CLAIMS NOVELTY OF THE INVENTION

1. An electrode with nanospace, characterized in that the nanospace has a length between 4 and 17 nm, which is obtained from suspended glassy carbon nanofibers with a length greater than 2 pm.

2. The electrode with nanospaces according to claim 1, further characterized in that the nanospaces have a length of less than 10 nm.

3. A method for obtaining electrodes with nanospaces between 4 and 17 nm, characterized in that it comprises a step of sublimating a suspended glassy carbon nanofiber with a length greater than 2 pm under reduced pressure conditions.

4. The method according to claim 3, further characterized in that the reduced pressure is between 0.001 and 0.003 Pa.

5. The method according to claim 3, further characterized in that the step of sublimating a suspended vitreous carbon nanofiber with a length greater than 2 pm is carried out with an increase in the resistance of the nanofiber greater than 0.05%.

6. The method according to claim 5, further characterized in that the increase in resistance greater than 0.05% is achieved by applying a voltage in a range between 0.2 and 3.8 V with increments of 0.2 V.

7. The method according to claim 4, further characterized in that the suspended vitreous carbon nanofibers with a length greater than 2 pm are obtained according to the following steps: (i) coating a base with a layer of a polymeric precursor of carbon with a thickness between 10 and 30 pm; (ii) heating the coated base until the solvent is removed from the polymeric precursor; (iii) selectively exposing the base to UV rays; (iv) depositing the obtained nanofibers on an electrospinning platform, which is adjusted to a distance from the needle to the collector of at least 1 mm and to a voltage to suspend them; and (v) heating the suspended nanofibers.

8. The method according to claim 7, further characterized in that the step of (i) coating a base with a layer of a polymeric carbon precursor includes a centrifugation of between 3000 to 4000 rpm for between 25 to 30 s.

9. The method according to claim 7, further characterized in that the thickness of the polymeric carbon precursor is 20 pm.

10. The method according to claim 7, further characterized in that the carbon polymeric precursor is negative photoresist based on eight epoxy groups.

11. The method according to claim 7, further characterized in that the step of (¡i) heating the coated base until the solvent of the polymeric precursor is eliminated is achieved at a temperature of between 90 to 100 °C for between 4.5 to 5.5 minutes .

12. The method according to claim 7, further characterized in that the step of (i¡¡) selectively exposing the base to UV rays is achieved in a time of between 3 to 5 s.

13. The method according to claim 7, further characterized in that step (iv) deposing the nanofibers to suspend them is achieved at a voltage of between 100 to 200 V.

14. The method according to claim 7, further characterized in that the step of (v) heating the suspended nanofibers is achieved at a temperature between 800 and 1000 °C under an inert N2 atmosphere for at least 1 hour.

15. A nanosensor characterized in that it comprises at least one electrode with nanospaces between 4 and 17 nm obtained from suspended glassy carbon nanofibers with a length greater than 2 pm.