Annealing Temperature-Dependent Terahertz ThermalâElectrical

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Functional Nanostructured Materials (including low-D carbon)

Annealing temperature dependent terahertz-thermal-electrical conversion characteristics of three-dimensional microporous graphene Meng Chen, Yingxin Wang, Jianguo Wen, Honghui Chen, Wenle Ma, Fei Fan, Yi Huang, and Ziran Zhao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20095 • Publication Date (Web): 16 Jan 2019 Downloaded from http://pubs.acs.org on January 17, 2019

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ACS Applied Materials & Interfaces

Annealing temperature dependent terahertz-thermalelectrical conversion characteristics of threedimensional microporous graphene Meng Chen,†,‡ Yingxin Wang,†,‡ Jianguo Wen⊥, Honghui Chen,§ Wenle Ma,§ Fei Fan,∥ Yi Huang,§,* Ziran Zhao†,* †Key

Laboratory of Particle & Radiation Imaging (Tsinghua University), Ministry of Education,

Department of Engineering Physics, Tsinghua University, Beijing 100084, China ⊥Nuctech

Company Limited, Beijing 100084, China

§National

Institute for Advanced Materials, Tianjin Key Laboratory of Metal and Molecule

Based Material Chemistry, Key Laboratory of Functional Polymer Materials, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), School of Materials Science and Engineering, Nankai University, Tianjin 300071, China ∥Key

Laboratory of Optical Information Science and Technology, Ministry of Education,

Institute of Modern Optics, Nankai University, Tianjin, 300071, China *Corresponding ‡These

authors. E-mail: [email protected] and [email protected]

authors contributed equally to this work

ABSTRACT: Three-dimensional microporous graphene (3DMG), possesses ultrahigh photon absorptivity and excellent photothermal conversion ability, and shows great potential in energy storage and photodetection, especially for the not well-explored terahertz (THz) frequency range.

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Here, we report on the characterization of THz-thermal-electrical conversion properties of 3DMG with different annealing treatments. We observe distinct behavior of bolometric and photothermoelectric responses varying with annealing temperature. Resistance-temperature characteristics and thermoelectric power measurements reveal that marked charge carrier reversal occurs in 3DMG as the annealing temperature changes between 600 and 800 °C, which can be well explained by Fermi-level tuning associated with oxygen functional group evolution. Benefiting from the large specific surface area of 3DMG, it has an extraordinary capability of reaching thermal equilibrium quickly and exhibits a fast photothermal conversion with a time constant of 23 ms. In addition, 3DMG can serve as an ideal absorber to improve the sensitivity of THz detectors and we demonstrate that the responsivity of a carbon nanotube device could be enhanced by 12 times through 3DMG. Our work provides new insight into the physical characteristics of carrier transport and THz-thermal-electrical conversion in 3DMG controlled by annealing temperature and opens an avenue for the development of highly efficient graphenebased THz devices. KEYWORDS: three-dimensional microporous graphene, terahertz detection, bolometric effect, photothermoelectric effect, oxygen functional group 1. Introduction Room-temperature (RT) terahertz (THz) detection is currently one of the major bottlenecks for the development of THz technology.1,2 Because of the relatively high frequency and low photon energy of THz radiation with respect to the adjacent microwave/millimeter-wave and infrared/visible spectral regions, respectively, conventional well-established electronic and photonic devices are unusable. Alternatively, bandgap-independent detectors based on bolometric,3 photothermoelectric (PTE),4 and pyroelectric effects5 show great potential for THz


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detection and have become commercially available owing to their advantages of broadband operation, uniform sensitivity against wavelength, good stability, and low cost. The rapid development of materials science provides many novel materials for improving the performance of photodetectors, such as graphene,6-15 carbon nanotubes (CNTs),16-22 and black phosphorus.23 Graphene has been demonstrated as an appealing material for photodetectors from many aspects.24-27 Gapless graphene shows an ultra-broadband absorption spectrum from the THz to the ultraviolet range because of its unique Dirac-like electronic states. The much lower heat capacity of graphene compared with that of bulk materials makes it easier to generate a large thermal gradient, and its high carrier mobility and scattering-free heat flow can help to deliver energy rapidly and achieve a fast response speed for thermal detectors. Despite all these advantages of graphene, the sensitivity of existing graphene-based THz detectors still needs to be enhanced. They display low sensitivity because graphene only has a monolayer of atoms and shows very weak photon absorption.28,29 To overcome the low absorption of the single-layer graphene (SLG), three-dimensional microporous graphene (3DMG), which is assembled with randomly oriented and interconnected monolayer graphene sheets and exhibits remarkable photon absorption, has attracted widespread interest for photothermal applications.30-35 Ren et al. used 3DMG as an absorber to harvest sunlight and achieved a solar–thermal conversion efficiency of 93.4%30. Giorgianni and coworkers focused on the photothermal acoustic effect of 3DMG, finding that 3DMG could be used as a fully digital loudspeaker with high efficiency and low distortion.31 In the field of visible photodetection, Ito et al. demonstrated that the absorption of 3D nanoporous graphene is over 40 times higher than that of monolayer graphene materials.36 They achieved an ultrahigh photoresponse of 3.10×104 A W−1 and excellent external quantum efficiency of 1.04×107% by


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optimal reductive treatment of the 3D nanoporous graphene. Recently, Huang and colleagues demonstrated that 3DMG displayed excellent absorption behavior in the frequency range from 0.1 to 1.2 THz with a large reﬂection loss of 28.6 dB even at an incidence of 45°.37 In addition, the ultralight mass density of the 3DMG brings it a rather small heat capacity, which takes great advantage for THz thermal detection.38,39 Considering the unique advantages of remarkable photon absorption, excellent photothermal conversion, and ultralight mass (low heat capacity), 3DMG is of interest for THz detection. However, there are few experimental studies to explore the THz-thermal-electrical conversion characteristics of 3DMG and even on how to control these properties. Here, we employ electrical transport and THz photoresponse measurements to characterize the bolometric and PTE response of 3DMG annealed at different temperatures (from 180 to 1000 °C). We investigate the evolution of the THz response of 3DMG changing with the annealing temperature and analyze the role of the content of random defects and oxygen functional groups based on carrier transport and thermoelectric measurements. We also evaluate the temporal performance of THz-thermal conversion in 3DMG and describe the substantial enhancement of the THz detection responsivity achieved by using 3DMG as an absorbing element to increase the temperature gradient in a CNT-based detector 2. Preparation and Characterization of 3DMG samples We fabricated 3DMG by a solvothermal method,37,39 which was followed by annealing and cutting treatments (details are provided in Supporting Information S1). The 3DMG samples annealed at temperatures of 180, 400, 600, 800, and 1000 °C are denoted as T180, T400, T600, T800, and T1000, respectively. Figure 1a–d show the scanning electron microscopy (SEM) images of the samples, revealing that the 3DMG samples consist of disordered interlaced


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graphene sheets assembled into a distinctive framework with a highly porous structure.

Figure 1. (a)–(c) Cross-sectional SEM images of 3DMG. (d) Average Raman spectra, (e) statistical Raman analysis, and (f) THz absorptivity of the 3DMG samples annealed at different temperatures.

For each 3DMG sample, one hundred of Raman spectra were measured at different positions of the sample surface to perform a statistical analysis. The average spectra are shown in Figure 1d. The D peak observed at 1350 cm−1 is attributed to the breathing modes of sp2 atoms in rings and the broadening of the D peak is caused by sp3-bonded carbon and the presence of defects in the graphite layer. The G peak at 1584 cm−1 represents the in-plane vibration of sp2-bonded carbon atoms, and will be broadened as the defect density increases.40-42 Figure 1e provides the ID/IG ratio and the full width at half-maximum (FWHM) of the G peak (ГG) extracted from all of


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the Raman spectra. The values of ГG are in the range of 60~75 cm-1, indicating that the samples contain a high concentration of oxygen functional groups and disorder.43-45 As the annealing temperature increases, the ID/IG ratio increases from 0.94 for T180 to 1.70 for T1000. Combing the ГG and the ID/IG ratio, the size of defect-free domain LD which represents the mean distance between two defects can be quantitatively calculated. The detailed calculation process and analysis on the Raman spectra are given in Supporting Information S2. The values of LD for T180~T1000 are 1.29, 1.31, 1.32, 1.38 and 1.56 nm, respectively. The results indicate that as the annealing temperature increases, the defect density of the 3DMG decreases. This is because the annealing treatment partially restores the conjugated structure of 3DMG and removes lots of the oxygen functional groups. X-ray photoelectron spectroscopy (XPS) was also used to further clarify the chemical evolution that occurred during the annealing treatment. XPS survey scans (Figure 2a) show that the content of oxygen functional groups in 3DMG gradually decreases with increasing annealing temperature. The C1s/O1s atomic ratios are 6.3, 8.0, 14.9, 20.7, and 23.9 for T180, T400, T600, T800, and T1000, respectively. Multipeak fitting of the C1s spectra was performed using a Gaussian–Lorentzian peak shape after performing a Shirley background correction,46-48 as shown in Figure 2b–f. For 3DMG, the dominant peak at 284.6 eV arises from the non-oxygenated rings (C–C and C=C bonds), and the secondary peak at 285.7 eV originates from the C–O bonds of hydroxyl, ether, and epoxide functional groups. Two small peaks at 287. 6 and 289.0 eV are caused by the C=O bonds in carbonyl groups and O=C-O of carboxyl moieties. As the annealing temperature increases, the area under the primary peak (284.6 eV) increases, whereas the areas of other peaks related to the oxygen functional groups decrease markedly, confirming the removal of oxygen functional groups and the simultaneous restoration of the conjugated structure


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of 3DMG. It is noted that even when the annealing temperature was as high as 1000 °C, the C–O bonds were not removed completely, as shown in Figure 2f.

Figure 2. (a) XPS survey scans of 3DMG samples annealed at different temperatures. C1s spectra of (b) T180, (c) T400, (d) T600, (e) T800, and (f) T1000.

To evaluate the THz absorption properties, we measured the absorptivity of 3DMG samples with 1 mm thickness under different thermal treatments at 2.52 and 3.11 THz, as illustrated in


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Figure 1f. Details on the measurement method can be found in Supporting Information S3. The absorptivity increases remarkably with the annealing temperature in the range of 600~ 1000°C, which can be explained by the notable change of the conductivity. A more specific discussion on the relationship between the absorptivity and annealing temperature is provided in Supporting Information S4. The absorptivity of T1000 reaches up to 96.1%. This remarkable absorption behavior of 3DMG can be explained by two factors.37 First, the reflection at the interface between air and 3DMG is very weak and can be ignored. This is caused by both the wrinkled surface and ultrahigh porosity of 3DMG. The wrinkled, honeycomb-like surface effectively avoids specular reflection and the ultrahigh porosity (larger than 99.9%35,37) leads to a very low effective dielectric constant and ensures that the majority of incident THz radiation can enter into the interior of 3DMG. Second, the pore sizes of 3DMG are estimated to be about several tens of micrometers, as shown in the SEM images, which are comparable to the wavelength of THz radiation. Such a structure can create a strong scattering of THz radiation, enhance its interaction with 3DMG, and thus lead to a high absorption. 3. Results and discussion 3.1 THz bolometric response. Absorbed THz photons in graphene gives rise to efficient carrier heating.8,49 The hot carriers are significant for bolometric detection which utilizes the temperature dependence of the resistivity and is expected to be straightforwardly associated with the strong THz absorption for 3DMG. Both the temperature coefficient of resistance (TCR) and the THz-thermal conversion efficiency are critical to the bolometric performance of 3DMG. Here the TCR values of our 3DMG samples were measured in the temperature range of 25–80 °C by the four-electrode method. The measurement details are presented in Supporting Information S5. Figure 3a shows the resistivity as a function of annealing temperature for the


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samples heated at different temperatures. At 25 °C (other heating temperatures showed similar behavior), the resistivity of T180 is 1.0 kΩ·cm. With increasing annealing temperature, the resistivity first increases and then decreases. The resistivity of T600 reaches 1.6 kΩ·cm, whereas that of T1000 is only 34.8 Ω·cm. Figure 3b shows the TCR values of different samples at various heating temperatures, which are calculated by:

TCR 

1 dR R dT

(1)

where R and T are the resistance and heating temperature, respectively. The TCR values of all the samples are negative, indicating that 3DMG samples annealed at all temperatures behave as semiconductors. For samples annealed at higher temperature, the TCR values are lower, which is because the removal of the oxygen functional groups and restoration of the sp2 domains at higher annealing temperature limit the change of the carrier density so as to make the material more stable. The TCR of T180 at 25 °C is −0.48 % K−1, and that of T1000 is −0.1% K−1. These values are consistent with former reports of the TCR of SLG.50,51 As the heating temperature rises, the TCR values continuously increase. At 80 °C, the TCR of T180 reaches −0.65% K−1. This is because that at higher heating temperature, the carriers became more active and less likely to remain in the covalent bonds. Then the THz-thermal conversion performance of the 3DMG was tested with a configuration depicted in the inset of Figure 3c. The samples were suspended and connected to two metal electrodes with good ohmic contact. (Specific fabrication and measurement methods can be found in Supporting Information S6, and the current–voltage curves are provided in Supporting Information S7.) The temperature distributions over the 3DMG samples under THz illumination were recorded with an infrared camera to evaluate their THz-thermal conversion abilities. (Thermal images are given in Supporting Information S8). Figure 3c plots the extracted


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maximum temperature rise of different samples upon 2.52-THz irradiation. At the same incident power level, the temperature rises of T180, T400, and T600 were similar and those of T800 and T1000 were obviously larger. For instance, T1000 achieved a huge temperature rise of 109.3 °C at a power density of 19 mW·mm-2, highlighting the remarkable THz absorption and thermal conversion ability of 3DMG. Supporting Information S9 provides several thermal images of SLG on different substrates under THz illumination with the same power level. The absorption in these cases mainly depends on the substrate.28,52,53 A substrate with a high absorptivity and low thermal conductivity will result in a larger temperature rise for SLG, but it is still far smaller than that of 3DMG.

Figure 3. (a) Resistivities of the 3DMG samples at different heating temperatures. (b) TCR of the 3DMG samples annealed at different temperatures. (c) Temperature rise of the 3DMG samples under THz illumination with different power densities. Inset is a schematic diagram of the suspended 3DMG device. (d) Resistance variation of the 3DMG device channel under 19-mW·mm2, 2.52-THz illumination.


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Figure 3d shows the bolometric responses of the devices in the inset of Figure 3c, namely the channel resistance variation with THz on and off, including the measured results (specifiic IV curves were given in Supporting Information S7) and that calculated from the data in Figure 3b,c. The measured results have a similar trend but lower values comparing to the calculations. This may arise from that the THz spot only covered a part of the channel, and the average temperature rise over the whole channel was actually lower than that shown in Figure 3c. The relative change of the resistance of T180, T400, T600, T800 and T1000 are measured to be 6.97%, 5.22%, 4.44%, 5.09% and 4.88%, respectively. According to the data presented in Figure 3c, T180 has a smaller THz-thermal conversion efficiency than others, but it achieved the largest THz bolometric response, benefiting from its larger TCR (0.65%, at 80 °C). This TCR value is larger than that of conventional metallic bolometer materials (TCR of ~0.3% K−1).54 Although the TCR is not as large as that of the semiconducting bolometer materials (TCR of 2–4% K−1),54 the 3DMG shows advantages in terms of higher absorptivity, excellent photo-thermal conversion efficiency, and low heat capacity, which can lead to a high THz-thermal conversion efficiency. 3.2 THz PTE response. The hot carriers excited in graphene can also lead to a PTE response. PTE effect, caused by the diffusion of the hot carriers in the channel with a gradient temperature distribution formed by a local THz illumination or an asymmetrical device structure49,52. The photovoltage generated in each device due to the PTE effect is given as:55,56 T

V   SdT  S T

(2)

T0

where ΔT is the temperature difference between the two electrodes, and S is the thermoelectric power (Seebeck coefficient) of the channel material. For p-type materials, holes will diffuse from the hot side to the cold side, making the electrical potential at the cold side higher and thus


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bringing the material a positive S. While for n-type materials, electrons are the dominant carriers, and the S is negative. Figure 4a depicts the temporal photovoltage responses of the 3DMG devices to 2.52-THz radiation focused on the positive end. We find that the responses of T180, T400, and T600 are negative and the amplitudes decreases with increasing annealing temperature. In contrast, the opposite behavior was observed for T800 and T1000. They show positive responses and the amplitude of T1000 is evidently larger than the others. Using Equation (2), we calculate the S values of the 3DMG samples, as shown in Figure 4b (the detailed calculation process can be found in Supporting Information S10). T180 has a positive S value of 21.5 μV K−1, and thus behaves as a p-type semiconductor. As the annealing temperature goes up, S gradually diminishes and becomes negative, indicating that the 3DMG transforms into an n-type material. The photovoltage responsivities are calculated as −2.9, −2.2, −1.4, 2.1, and 5.1 mV W−1 for T180, T400, T600, T800, and T1000, respectively. In general, the S value of T180 is larger than that of T1000, but the absorptivity and temperature rise of the latter is stronger. Therefore, the response of T1000 is more sensitive than that of T180. The noise equivalent power (NEP) and specific detectivity D*of the 3DMG devices were also measured to evaluate their key figure of merits. Detailed measurement and calculation results are provided in Supporting Information S11. Undoubtedly, the response of T1000 is optimal, with a NEP of 700 nW/Hz0.5 and a D* of 2.5×105 cmHz0.5/W.” 3.3 Annealing temperature dependent charged carrier characteristics. As mentioned above, the 3DMG samples annealed at low and high temperatures possess oppositive S values, indicating that efficient Fermi-level (EF) tuning and distinct carrier type reversal occurred in the annealing process. This can be explained by the evolution of the oxygen functional groups and the restoring of the conjugated structure in 3DMG. Oxygen functional groups can withdraw or


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Figure 4. (a) Temporal PTE responses of 3DMG samples to 2.52-THz radiation with the same incident power. (b) Responsivity and Seebeck coefficient of 3DMG samples. Inset is a schematic of the changes in the energy band structure of the samples with increasing annealing temperature.

donate electron density through inductive and resonance effects.57 Both C=O and O=C–O groups, in conjugation with an aromatic ring, can withdraw electron density by the resonance effect as well as by the inductive effect. The C-O bonds of hydroxyl, ether, and epoxide groups withdraw electron density through the induction effect and donate electron density through the resonance effect. When these groups are bonded to an sp2-hybridized carbon ring, the resonance effect dominates and the groups act as strong electron donors. When they are bonded to the structure through an sp3-hybridized carbon atom, the resonance effect will be suppressed and therefore the groups will withdraw electron density.57-60 Before the annealing treatment, the oxygen functional groups withdraw electron density overall and lead to p-type 3DMG. As the annealing temperature rises, the oxygen functional groups are gradually removed. As the XPS results of Figure 2 suggest, when the annealing temperature was higher than 800 °C, the C=O and O=C–O groups were almost completely removed, whereas C–O groups are still present. Meanwhile, the sp2 carbon domain is enlarged, as illustrated by the Raman spectra, which implies that a higher proportional of the remaining C–O groups are bonded to carbon atoms through sp2 hybridization than is the case for samples annealed at lower temperatures. As the annealing temperature


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increased, the resonance effect became dominant and the carriers changed to electrons, resulting in n-type 3DMG. This evolution process is consistent with the resistance variation shown in Figure 3a. T180 contains abundant C=O, O=C–O, and sp3-hybridized C–O groups. These groups withdraw electrons, produce free holes, and make the 3DMG a p-type material. As the annealing temperature increased, the C=O and O=C–O groups were gradually removed and the bonding state of C–O also changed from sp2 to sp3 hybridized. As a consequence, the ability of the groups to withdraw electron density becomes weaker, the content of free holes decreases, and the whole sample tends to become electrically neutral. For T600, the density of p-type carriers is still higher than that of n-type ones, but the resistivity is already very large. When the annealing temperature continues to rise, C=O and O=C–O groups are almost entirely removed, and the C– O groups are changed to electron donors because of their change of hybridization. As a result, the electron density increases and the whole 3DMG sample converts to n-type. From the viewpoint of energy band structure shown in the inset of Figure 4b, the free carriers of T180 are holes, and its EF is below the bandgap. At higher annealing temperature, the content of free holes in 3DMG drops, EF moves up and the electrical resistivity gradually increases. For samples annealed in the range between 600 and 800 °C, EF crosses the bandgap, the free carriers change to electrons, and 3DMG transforms from p-type to n-type. At even higher annealing temperature, EF keeps on moving upward, the density of free carriers continues to increase and the electrical resistivity decreases. Within the ambient temperature range, the mobility of electrons is larger than that of holes. In addition, as the annealing temperature rises and the oxygen functional groups are removed, the conjugated structure of 3DMG is restored, which would promote carrier transport. Therefore, the resistivity of T1000 is far lower than that of T180.


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3.4 Temporal characteristics of THz-thermal conversion. The PTE response of a graphene device originates from the diffusion of the hot carriers, which are generated by the photon-carrier interaction and the carrier-carrier scattering. This process is ultrafast, generally hundreds of femtoseconds or less.9,11,61 After removing the irradiation, the hot carriers will be cooled down by carrier-phonon scattering with a decay time of several picoseconds.9,11.61 Such short rising and decay times bring the graphene PTE detectors a large intrinsic bandwidth of several hundreds of GHz. However, for the detector with a channel length much larger than the cooling length of the hot carriers (several micrometers or less10,25,62,63), the response speed is dominated by the thermal equilibrium process of the device which mainly depends on the thermal conductivity and heat capacity of the channel and substrate materials, the shape and size of the channel, and the heat transfer efficiency between the device and the environment. The rising time and the decay time are nearly equal and can be extracted by exponential fitting to the photovoltage curves (The comparison results are given in Supporting Information S12). Figure 5 shows the fitting results of the rising time. From T180 to T1000, the response speed first slows down and then gets fast. The time constants of T180, T600, and T1000 were 71, 115, and 23 ms, respectively. For the 3DMG devices proposed in our work, the heat conduction and equilibrium should be considered as the main factors when analyzing the time response characteristics, rather than the hot carrier diffusion effect. The thermal conductivity of carbon nanomaterials mainly depends on phonon–phonon scattering because the strong covalent sp2 bonding results in efficient heat transfer by lattice vibrations.64-66 The 3DMG samples with higher annealing temperature contained fewer impurities and defects, and the conjugated structure was partly restored, which led to larger phonon thermal conductivity and faster thermal equilibrium process. From this perspective, the response time should shorten with increasing annealing temperature.


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For T800 and T1000, the response times are relatively short, in good agreement with this analysis. However, for T180, T400, and T600, the response time lengthened with increasing annealing temperature. This is because the large doping concentration, i.e., high content of oxygen functional groups, in these samples makes the electron thermal conductivity a substantial contributor to the thermal conductivity. According to the Wiedemann–Franz law,64 the electron thermal conductivity is proportional to the electrical conductivity. The electron thermal conductivity of T180 is larger than that of T600, which outweighs the disadvantage of its relatively lower phonon thermal conductivity. Thus, its response speed is faster than that of T600. Generally, the response time has a negative correlation with the length of the channel; i.e., the longer the channel, the slower the response. Considering that although the channel length in our devices is on the millimetric scale, the response speed of 3DMG is very fast, which can satisfy the requirements for real-time imaging. Many reported devices with a channel length of the same scale have time constants longer than 1 s.52,67-69 In contrast, the time constants of our 3DMG samples are of the order of 23–115 ms, which are obviously shorter than those of SLGand CNT-based devices with similar channel sizes. This can be explained by the unique structure of 3DMG. The wrinkled and porous structure of 3DMG endows it with a huge specific surface area, which allows effective heat dissipation, compared to its small mass, and results in a very short response time (further discussion is provided in Supporting Information S13). On contrary, the heat states of SLG and CNTs mainly depend on the underlying solid substrate, which takes a longer time to achieve heat balance than the porous 3DMG and slows the response speed of the whole device. 3.5 Enhancement of the THz detector response using 3DMG. The outstanding absorption and photothermal conversion ability of 3DMG make it an ideal absorber to enhance the response


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Figure 5. THz-thermal response times of 3DMG samples. Dots are measured photovoltage curves, and solid lines are the corresponding exponential fits. Inset shows the relationship between the time constant and the annealing temperature.

of THz detectors. To prove this idea, we performed a demonstration experiment as illustrated in Figure 6a. A CNT fiber with a length of 5 mm and a diameter of 100 µm was suspended between two metal electrodes with a small block of 3DMG (T1000, 1.2 mm1 mm0.5 mm) adhered to one side. Figure 6b compares the photovoltage when THz radiation illuminated on either side of the CNT fiber. The photovoltage of the 3DMG side (0.73 mV) was 12 times higher than that of the other side (0.06 mV). The responsivity, NEP and D* of the 3DMG-enhanced side were evaluated to be 46.8 mV/W, 14 nW/Hz0.5, and 7.7×106 cmHz0.5/W, respectively (Detailed measurements and calculations can be found in Supporting Information S11, and the comparison between the 3DMG devices and some recent reported carbon-nanomaterials-based THz detectors is given in Supporting Information S14). To further confirm the experimental results from the viewpoint of thermal conversion, the temperature distribution under THz illumination was measured by an infrared camera, as shown in the inset of Figure 6a. The thermal images were taken from the back side of the 3DMG (the side contact with the CNT fiber), and the temperature data were extracted along the CNT fiber. The maximum temperature rise at the CNT side was


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only 0.8 °C, whereas that at the 3DMG side was 14 °C (17.5 times higher), which is responsible for the more pronounced photovoltage generation for the 3DMG side of this device. According to the photovoltage and temperature rise values, the CNT Seebeck coefficient was calculated to be 75 μV·K−1, which was in good agreement with that reported in the literatures.18,70,71 This experiment successfully demonstrated the utilization of the strong absorption and photothermal conversion properties of 3DMG for THz detection.

Figure 6. (a) Temperature distribution profile with background subtraction extracted along the CNT fiber under 13-mW·mm-2, 2.52-THz illumination on the 3DMG and CNT sides of the device. Schematic diagram of the measurement setup and the thermal images are shown in the insets. The arrows beside the thermal images indicate the position of the CNT fiber. (b) Temporal photovoltage responses of the two sides.

4. Conclusion The bolometric and PTE responses of 3DMG samples annealed at different temperatures were systematically investigated at THz frequencies. As the annealing temperature increased, the oxygen functional groups were removed and the conjugated structure was restored gradually, leading to extraordinary evolution in carrier characteristics of 3DMG. The Fermi-level shifted from below to above the bandgap, and the p-type charge carriers were transformed to n-type when the annealing temperature changed from 600 and 800 °C. The physical origins of this


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process were well understood based on XPS and Raman spectroscopy, and confirmed by the carrier transport and thermoelectric power measurements. These results imply that controlling the annealing temperature is an effective and convenient way to tune the THz-thermal-electrical conversion properties of 3DMG. Meanwhile, 3DMG showed an outstanding ability in terms of thermal equilibrium rate due to its large effective heat dissipation area and ultralow density. A fast PTE response with the time constant of 23 ms was achieved for 3DMG annealed at 1000 °C. Combining the superior THz absorption and photothermal conversion ability of 3DMG, the response of a CNT-based THz detector was enhanced by 12 times by using 3DMG as an absorber. Our work opens a new direction for applications of 3DMG in developing promising THz photothermal and optoelectronic devices. ASSOCIATED CONTENT Supporting Information. Material preparation; Defect density analysis; Method and system for measuring the THz absorptivity of 3DMG; Analysis of the relationship between the conductivity and annealing temperature; Device for measuring the electrical resistivity and TCR of 3DMG; Device measuring the bolometric and PTE responses of 3DMG; I-V characteristics and bolometric responses of the 3DMG samples; Thermal images of the 3DMG samples; Thermal images of the SLG on different substrates; Calculation method of the Seebeck coefficient; Noise characteristics of the PTE responses of the 3DMG devices; Analysis on the response time of the PTE effect of the 3DMG devices; Comparison of the 3DMG devices and recent reported carbonnanomaterials-based THz detectors; Analysis of the thermal equilibrium of the 3DMG in time domain. (PDF)


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] and [email protected] Author Contributions Ziran Zhao and Yi Huang conceived the idea. Meng Chen and Yingxin Wang led the design, fabrication, and measurements of the 3DMG devices and co-wrote the manuscript. Jianguo Wen assisted the experiments. Honghui Chen and Wenle Ma fabricated the materials. Fei Fan provided many constructive suggestions to improve this work. Notes The authors declare no competing financial interest. ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from National Natural Science Foundation of China (No. U1633202, 21875114), the Ministry of Science and Technology of China (2016YFA0200200, 2017YFA0701002, 2017YFC0803601), and 111 Project (B18030). REFERENCES (1)

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