Generating electricity using graphene nanodrums

Abstract

A voltage is induced when grapheme nanodrums (a graphene membrane on a nanopore array in a silicon oxide substrate) upheave/sink. The magnitude of the induced voltage is closely related to the extent to which the graphene membrane is bent, while its sign depends on the upheaval/sinking movement of the graphene nanodrums.

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Introduction

In recent years, to realize the independent, sustainable and green power supply of nano/micro electromechanical systems (N/MEMS), harvesting energy directly from the ambient environment has attracted much attention from both scientific and industrial areas.^1,2 Graphene, which is a single-atom-layer carbon crystal with extraordinary electronic, mechanical, optical and optoelectronic properties,^3–5 has been demonstrated to be responsible for the conversion of some ambient energy into electrical energy.⁶ Theoretical and experimental studies have shown that flow of water or other polar liquids over graphene generates a net potential difference and associated electric current in the graphene along the flow direction.^7–9

Graphene nanodrums, a single- or n-layer graphene membrane suspending on the top of a nanopore array, have been researched more extensively.^10–12 Inui et al. examined the actuation of graphene nanodrums using a molecular dynamics simulation.¹³ Graphene resonator has been also explored using the structure of graphene nanodrums.^14,15 Recently, Singh et al. shot microwave photons at the graphene nanodrums to explore the way graphene in these nanodrums moves, and they inferred that these graphene nanodrums could act as memory chips in a quantum computer in the future.¹⁶ Whether graphene nanodrums can be used in the self-powered system?

In this work, we firstly show that a voltage on the order of a few millivolts can be produced by driving the n-layer graphene nanodrums to upheave and vibrate using thermal expansion. The value of induced voltage depends on the upheaval height, but has no matter with the external current bias and is not caused by the temperature difference between electrodes. When a gas flow is blown onto the graphene nanodrums and the graphene membrane sinks into the nanopores, the voltage is also obtained except that the direction is reversed. By the atomic force microscopy (AFM) technology, the vibration characteristics of graphene nanodrums are investigated. The mechanism of electricity generation is explored, some possible origins of our observations are discussed, including the electromagnetic induction and the bending-induced band gap in graphene nanodrums.

Experimental

Our graphene samples were grown on pretreated copper foil under ambient pressure by chemical vapour deposition (CVD)^2,17–19 (ESI, Fig. S1†), and identified to be 3–4 layer by Raman characterization (ESI, Fig. S2†).^20,21 A 2-by-2-cm array of nanopores (diameter: 1 μm, depth: 200 nm) was patterned in silicon oxide (SiO₂) epilayer (300 nm thickness) on silicon (Si) substrate by lithography and inductively coupled plasma (ICP) etching. Then the as-grown graphene sample was transferred onto the array of nanopores, forming many graphene nanodrums (ESI†), whose scanning electron microscope (SEM) images are shown in Fig. 1a. We attached two electrodes to the surface of the graphene nanodrums with metal cables using silver epoxy to form an ohmic contact.²

Fig. 1 (a) A typical SEM image of graphene nanodrums (a graphene membrane on a nanopore array in silicon oxide substrate). The diameter and depth of individual nanopore are 1.0 and 0.2 μm, respectively as schematically shown in the inset. (b) A voltage is induced when the graphene nanodrums upheave or sink. The magnitude of induced voltage is closely related to the extent to which the graphene nanodrum membrane is bent, while its sign is dependent on the upheaval/sinking movement of the graphene nanodrums.

The experimental set-ups are schematically depicted in Fig. 1b. In Fig. 1b, when the graphene membrane remains still on the array of nanopores, no voltage is induced. When the graphene nanodrums were put on a heating stage, and the heating stage is used to drive the graphene membrane to upheave and vibrate through thermal expansion of air, a voltage is induced. According to setting the set-up parameters, the value and change rate of temperature are controlled. The electrical conductivity and induced voltage between the two electrodes of graphene nanodrums are investigated. A semiconductor characterization system called Keithley 4200-SCS was used for measuring in real time.

Results and discussion

As shown in Fig. 2a, the I–V curve of the graphene nanodrums can be well fitted using a linear function whether it is in thermal environment or not. But, after the device is put in thermal environment, the slope of I–V curve becomes smaller, indicating that the electrical conductance decreases. Furthermore, the electrical conductance of graphene nanodrums at room temperature (26.5 °C) and 200.0 °C can be calculated correspondingly by the slopes of I–V curves, which are ∼2 × 10⁻³ and ∼1.25 × 10⁻³ s, respectively. It is found that a voltage can be induced when the air in the nanopores was driven to upheave and vibrate the graphene membrane using thermal expansion; the induced voltage is shown in Fig. 2b. The value of induced voltage depends on the heating temperature. In the temperature range of 26.5 to 200.0 °C, the induced voltage improves continuously and reaches to the maximum of 150 μV at 200.0 °C, and keeps unchanged when the temperature remains constant at 200.0 °C. With the decrease of the heating temperature, the induced voltage goes down.

Fig. 2 (a) The current–voltage characteristics of graphene nanodrums at 26.5 °C and 200.0 °C, respectively. (b) The dynamic characteristics of the induced voltage of graphene nanodrums during heating, at constant temperature (200.0 °C) and during decreasing temperature. Three cycles are shown. (c) (d) The dynamic characteristics of the voltage of the device when positive and negative current bias 800 nA (c) and −800 nA (d) are applied, respectively. The two voltage curves have the same variation trend, indicating that there is an electromotive force in the circuit. (e) The dynamic characteristics of voltage when the temperature of heating stage is increased from 26.5 °C to 200.0 °C at the rates of 0.5 °C s⁻¹ and 0.2 °C s⁻¹, respectively. (f) Comparison of voltage versus time before and after the device is rotated 180 degrees horizontally at the heating stage. The two curves show opposite variation trends, indicating that the voltage is not induced by a temperature difference at the two electrodes.

To further determine the relationship between the induced voltage and the external current bias, we have measured the induced voltages at positive and negative current biases, respectively. When the current bias is set to 800 nA, the output voltage of graphene nanodrum device is displayed in Fig. 2c; when the current bias is set to −800 nA, the output voltage of graphene nanodrum device is displayed in Fig. 2d. Obviously, the voltage curve when the current bias is −800 nA is only a translation of that when the current bias is 800 nA. The jump values and directions of output voltage of graphene nanodrums are similar in both the cases. If the graphene nanodrum device is only a resistor in the closed loop, the induced voltage curve when the current bias is 800 nA should be different to that when the current bias is −800 nA. In our experiment, it is shown that the induced voltage hasn't obvious change when the temperature of heating stage was defined at a certain value despite that the current bias varies in the negative and positive range. Hence the induced voltage is independent of the external current bias, and the device is an electromotive force rather than a resistor.

It is demonstrated that the changing rate of induced voltage is dependent on that of the temperature of heating stage. In Fig. 2e, two temperature rates (0.2 and 0.5 °C s⁻¹) are defined. Obviously, during the same time interval, the change of induced voltage is larger when the temperature rate is 0.5 °C s⁻¹. So, it can be concluded that the induced voltage changes faster when the temperature rate is larger.

What are the mechanisms of the interesting observations? Due to the use of heating stage, the thermoelectric effect of graphene is firstly considered. But this possibility is soon ruled out. The thermal air of heating stage moves from bottom to up, and the two electrodes are on a plane, there is no cause of temperature difference. To provide better evidence, we perform a follow-on experiment, which is shown in Fig. 2f. The graphene nanodrum device is rotated 180 degrees horizontally, and the positions of the two electrodes on the heating stage are exchanged precisely. V₁ and V₂ are defined as the output voltages before and after the positions of the two electrodes being exchanged, respectively. Suppose that the voltage measured is induced by the temperature difference between the two electrodes, the voltages of V₁ (black line) and V₂ (red line) should have the same variation trend. However, the V₂ jumps to the opposite direction after the exchange of positions. According to the report²² that a thermopower of ∼100 μV K⁻¹ as measured at 500 K on few layered graphene films, a temperature difference of 1.5 K between the two electrodes is required to give rise to a voltage difference of 150 μV. In order to rule out the possibility of the voltage generation from thermoelectric effect, the temperatures of two electrodes are measured and no stable temperature difference is found (ESI, Fig. S5†). Hence, the possibility of the voltage generation from thermoelectric effect can be ruled out.

In Fig. 3a, the AFM image of one graphene nanodrum at 26.5 °C is shown. Along the dashed line in Fig. 3a, the height profile of one graphene nanodrum is investigated. The step height at the edge of the graphene nanodrum is about 25 nm. According to the theory of thermal expansion, the volume change (ΔM) of air enclosed in a graphene nanodrum can be calculated by the following equation:

ΔM = ΔT·M·β (1)

where ΔT is the change of thermal temperature, M is the air volume before temperature change, β is the thermal expansion coefficient. When the thermal temperature of heating stage is changed from room temperature into 50.0, 75.0, 100.0 and 125.0 °C in turn, the corresponding volume of air enclosed in a graphene nanodrum should improve and drive the graphene membrane to upheave and vibrate. This phenomenon is seen obviously in our experiment, the height profiles of graphene membrane raises gradually, as shown in Fig. 3b. The corresponding AFM images at 50.0, 75.0, 100.0 and 125.0 °C are shown in Fig. S3.† Hence, we conclude that the graphene membrane on our graphene nanodrums upheaves from the centres of nanopores when driven by thermal expansion, and the upheaval height increases continuously with the rise of heating temperature. However, during the process of upheaving and vibrating of the graphenes, the contact surface of graphene and SiO₂ has no change. So the triboelectric effect²³ also can't explain the generation of electricity in our experiment.

Fig. 3 (a) An AFM image of a typical graphene nanodrum. (b) Schematic diagram of height profiles along the dashed line in (a) when the temperature ranges from 26.5 to 125.0 °C, indicating that the graphene membrane upheaves gradually as the temperature rises.

To gain a deeper insight into the interesting phenomenon mentioned above, a control experiment is performed, in which we used the same graphene nanodrums as that in Fig. 1b but changed the external force into gas flow, the experimental set-up is shown in Fig. 4a. When the nitrogen flow is impinging on the devices, all the cables are fixed properly before the measurement such that the cables are not swinging. When a nitrogen flow is blown vertically onto the center of the device from above, a voltage is also induced but the direction is opposite to that in Fig. 2b, as shown in Fig. 4b. Moreover, it is found that the induced voltage in Fig. 4b is one order smaller than that in Fig. 2b. Why? We consider that the upheaval amplitude of the graphene membrane on the nanopores when driven by thermal expansion is larger than the sinking depth when driven by nitrogen flow from above.

Fig. 4 (a) Schematic illustration of the control experiment. In the control experiment, a nitrogen flow is blown vertically onto the device and the induced voltage is monitored. (b) The dynamic characteristics of the induced voltage when the nitrogen flow is blown vertically onto the device. Three cycles are shown. (c) The dynamic characteristics of induced voltage versus the nitrogen flow velocity, which is controlled by the pressure of nitrogen. The induced voltage increases as the nitrogen flow velocity increases. (d) The dynamic characteristics of voltage when a nitrogen flow is blown vertically onto a device with graphene on SiO₂/Si substrate without nanopores. The induced voltage is hardly observed.

The nitrogen flow is from a cylinder and the velocity is controlled by rotating the low-pressure regulator. When we rotate the low-pressure regulator of nitrogen cylinder continually, it is found that the induced voltage improves with the increase of flow velocity; the results are shown in Fig. 4c. However, in Fig. 4d, no voltage can be induced when nonporous SiO₂/Si substrate is used, which shows that the structure of nanopore array is critical to the electricity generation.

Our experimental results can be described as the bending-induced voltage in graphene nanodrum devices. The magnitude of the measured voltage difference is closely related to the extent to which the graphene nanodrum membrane is bent. The sign of the voltage depends on the upheaval/sinking movement of the graphene nanodrums. One possible reason is that a band gap of graphene can be opened up to ∼0.5 eV when it's bent.²⁴ The opening of the band gap in the area where the graphene is bent may lead to charge redistribution in the graphene, and hence give rise to a measurable voltage difference at a macroscopic scale, provided that anisotropic coverage of graphene on nanopores is satisfied. We have checked the morphologies of the graphene nanodrums with AFM and found that there are some anisotropic coverages for individual nanodrum (ESI, Fig. S4†), but there are hundreds of nanodrums in each device and the anisotropic coverage cancels out each other. On the other hand, the mechanism of bending-induced band gap cannot be used to explain the observation of sign change of induced voltage for upheaval and sinking movement of the devices.

Another possible reason may be the electromagnetic induction. The graphene membrane on the nanopore array of SiO₂ consists of hundreds of 3–4-layer graphene flakes and each flake has a size of about several microns. At the edges of these graphenes flakes, the local magnetic moments have been demonstrated experimentally.^25–30 When the graphene flakes are driven to upheave or sink by an external force, an induced electromotive force is obtained, which is proportional to the rate of the change of the magnetic flux enclosed by the circuit according to Faraday's law. The observation of sign change of induced voltage for upheaval and sinking movement of the devices can be well explained. However, this proposal for generating electricity suffers from the fact that nonzero voltages are found at steady states as shown in Fig. 2b and 4b. One possible explanation may be that at the “steady” state, the amplitude of the vibration of graphene nanodrums is different for that at the central and the circumferential parts (which is in contact with the substrate) of the graphene nanodrums.

It should be noted that at present the underlying mechanism is not known and deserves more theoretical and experimental works.

Conclusions

In conclusion, we reported that electricity can be generated from the n-layer graphene nanodrums when the graphene membrane is driven to upheave or sink by an external force. To the best of our knowledge, this is the first time a graphene nanodrum device is shown to convert mechanical energy into electricity. The possible mechanisms, including the bending-induced voltage and the electromagnetic induction, are discussed. The experimental results indicate that the graphene nanodrums have various potential applications, including the thermo detector, the sensor of gas flow velocity and energy-harvesting device.

Acknowledgements

The authors sincerely thank National Centre for Nanoscience and Technology for providing the experimental facility and technical support. This work was supported by National Science Foundation of China (Grant nos 10774032, 90921001), Key Knowledge Innovation Project of the Chinese Academy of Sciences on Water Science Research, Instrument Developing Project of the Chinese Academy of Sciences (Grant no. Y2010031).

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Footnotes

† Electronic supplementary information (ESI) available: Synthesis of graphene; fabrication of nanopore array and device; Raman spectra of graphene; AFM images of individual graphene nanodrum; anisotropic coverage of graphene on individual nanopore; temperatures of two electrodes; voltage measurements. See DOI: 10.1039/c5ra00174a

‡ These authors contributed equally to this work.

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