Laser Photoreduction of Graphene Aerogel Microfibers: Dynamic Electrical and Thermal Behaviors.

This work reports the dynamic behaviors of graphene aerogel (GA) microfibers during and after continuous wave (CW) laser photoreduction. The reduction results in one-order of magnitude increase in the electrical conductivity. The experimental results reveal the exact mechanisms of photoreduction as it occurs: immediate photochemical removal of oxygen functional groups causing a sharp decrease in electrical resistance and subsequent laser heating that facilitates thermal rearrangement of GO sheets towards more graphene-like domains. X-ray and Raman spectroscopy analysis confirm that photoreduction removes virtually all oxygen and nitrogen containing functional groups. Interestingly, a dynamic period immediately following the end of laser exposure shows a slow, gradual increase in electrical resistance, suggesting that a proportion of the electrical conductivity enhancement from photoreduction is not permanent. A two-part experiment monitoring the resistance changes in real-time before and after photoreduction is conducted to investigate this critical period. The thermal diffusivity evolution of the microfiber is tracked and shows an improvement of 277 % after all photoreduction experiments. A strong linear coherency between thermal diffusivity and electrical conductivity is also uncovered. This is the first known work to explore both the dynamic electrical and thermal evolution of a GO-based aerogel during and after photoreduction.

Significantly reduced c-axis thermal diffusivity of graphene-based papers.

Owing to their very high thermal conductivity as well as large surface-to-volume ratio, graphene-based films/papers have been proposed as promising candidates of lightweight thermal interface materials and lateral heat spreaders. In this work, we study the cross-plane (c-axis) thermal conductivity (k c ) and diffusivity (α c ) of two typical graphene-based papers, which are partially reduced graphene paper (PRGP) and graphene oxide paper (GOP), and compare their thermal properties with highly-reduced graphene paper and graphite. The determined α c of PRGP varies from (1.02 ± 0.09) × 10-7 m2 s-1 at 295 K to (2.31 ± 0.18) × 10-7 m2 s-1 at 12 K. This low α c is mainly attributed to the strong phonon scattering at the grain boundaries and defect centers due to the small grain sizes and high-level defects. For GOP, α c varies from (1.52 ± 0.05) × 10-7 m2 s-1 at 295 K to (2.28 ± 0.08) × 10-7 m2 s-1 at 12.5 K. The cross-plane thermal transport of GOP is attributed to the high density of functional groups between carbon layers which provide weak thermal transport tunnels across the layers in the absence of direct energy coupling among layers. This work sheds light on the understanding and optimizing of nanostructure of graphene-based paper-like materials for desired thermal performance.

Nonmonotonic thickness-dependence of in-plane thermal conductivity of few-layered MoS2: 2.4 to 37.8 nm.

Recent first-principles modeling reported a decrease of in-plane thermal conductivity (k) with increased thickness for few layered MoS2, which results from the change in phonon dispersion and missing symmetry in the anharmonic atomic force constant. For other 2D materials, it has been well documented that a higher thickness could cause a higher in-plane k due to a lower density of surface disorder. However, the effect of thickness on the k of MoS2 has not been systematically uncovered by experiments. In addition, from either experimental or theoretical approaches, the in-plane k value of tens-of-nm-thick MoS2 is still missing, which makes the physics on the thickness-dependent k remain ambiguous. In this work, we measure the k of few-layered (FL) MoS2 with thickness spanning a large range: 2.4 nm to 37.8 nm. A novel five energy transport state-resolved Raman (ET-Raman) method is developed for the measurement. For the first time, the critical effects of hot carrier diffusion, electron-hole recombination, and energy coupling with phonons are taken into consideration when determining the k of FL MoS2. By eliminating the use of laser energy absorption data and Raman temperature calibration, unprecedented data confidence is achieved. A nonmonotonic thickness-dependent k trend is discovered. k decreases from 60.3 W m-1 K-1 (2.4 nm thick) to 31.0 W m-1 K-1 (9.2 nm thick), and then increases to 76.2 W m-1 K-1 (37.8 nm thick), which is close to the reported k of bulk MoS2. This nonmonotonic behavior is analyzed in detail and attributed to the change of phonon dispersion for very thin MoS2 and a reduced surface scattering effect for thicker samples.

Identifying the Crystalline Orientation of Black Phosphorus by Using Optothermal Raman Spectroscopy.

Current polarized Raman-based techniques for identifying the crystalline orientation of black phosphorus suffer significant uncertainty and unreliability because of the complex interference involving excitation laser wavelength, scattering light wavelength, and sample thickness. Herein, for the first time, we present a new method, optothermal Raman spectroscopy (OT-Raman), for identifying crystalline orientation. With a physical mechanism based on the anisotropic optical absorption of the polarized laser and the resulting heating, the OT-Raman can identify the crystalline orientation explicitly, regardless of excitation wavelength and sample thickness, by Raman frequency-power differential Φ (=∂ω/∂P). The parameter Φ has the largest (smallest) value when the laser polarization is along the armchair (zigzag) direction. The OT-Raman technique is robust and is able to identify the crystalline orientation of BP samples with thicknesses up to 300 nm at a minimum and potentially as high as 1200 nm.

Ultra-Low-Density Carbon Nanotube Aerogel Film for Fast and Sensitive Bolometric Sensing.

In various applications, infrared (IR) detectors with quick responses and high sensitivity at room temperature are essential. This work synthesizes carbon nanotube aerogel films (CAFs) with an ultra-low density of 1.33 mg cm-3. Transient electrothermal (TET) technology is used to characterize the thermal and electrical transport of CAFs in the temperature range of 320 to 10 K. CAF has record-low thermal conductivity (2.5 mW m-1 K-1 at 320 K) and thermal diffusivity (2.24 × 10-6 m2 s-1 at 320 K) in vacuum. The TCR of CAF is -0.11%/K at 295 K, which is 57% higher than that of the MWCNT films. In addition, the comprehensive bolometric performance of carbon nanotube aerogels is tested and analyzed, including the photothermal response, resistivity responsivity, and response time to lasers of a broad spectrum from ultraviolet to near-infrared. The relative responsivity of CAF to lasers of different wavelengths is found to be consistent. The response time of CAF with 200 µm suspended length is measured to be as short as 2.95-3.03 ms (framing rate of 330-339 per second). In addition, the resistive response of the CAF sample to a blackbody radiator and the radiation of the human hand also shows good sensitivity and repeatability. These results demonstrate the promising application of CAF as a sensitive and fast-response uncooled bolometer.

Flexible Large-Area Graphene Films of 50-600 nm Thickness with High Carrier Mobility.

Bulk graphene nanofilms feature fast electronic and phonon transport in combination with strong light-matter interaction and thus have great potential for versatile applications, spanning from photonic, electronic, and optoelectronic devices to charge-stripping and electromagnetic shielding, etc. However, large-area flexible close-stacked graphene nanofilms with a wide thickness range have yet to be reported. Here, we report a polyacrylonitrile-assisted 'substrate replacement' strategy to fabricate large-area free-standing graphene oxide/polyacrylonitrile nanofilms (lateral size ~ 20 cm). Linear polyacrylonitrile chains-derived nanochannels promote the escape of gases and enable macro-assembled graphene nanofilms (nMAGs) of 50-600 nm thickness following heat treatment at 3,000 °C. The uniform nMAGs exhibit 802-1,540 cm2 V-1 s-1 carrier mobility, 4.3-4.7 ps carrier lifetime, and > 1,581 W m-1 K-1 thermal conductivity (nMAG-assembled 10 µm-thick films, mMAGs). nMAGs are highly flexible and show no structure damage even after 1.0 × 105 cycles of folding-unfolding. Furthermore, nMAGs broaden the detection region of graphene/silicon heterojunction from near-infrared to mid-infrared and demonstrate higher absolute electromagnetic interference (EMI) shielding effectiveness than state-of-the-art EMI materials of the same thickness. These results are expected to lead to the broad applications of such bulk nanofilms, especially as micro/nanoelectronic and optoelectronic platforms.

Temperature Dependence of Thermal Conductivity of Giant-Scale Supported Monolayer Graphene.

Past work has focused on the thermal properties of microscale/nanoscale suspended/supported graphene. However, for the thermal design of graphene-based devices, the thermal properties of giant-scale (~mm) graphene, which reflects the effect of grains, must also be investigated and are critical. In this work, the thermal conductivity variation with temperature of giant-scale chemical vapor decomposition (CVD) graphene supported by poly(methyl methacrylate) (PMMA) is characterized using the differential transient electrothermal technique (diff-TET). Compared to the commonly used optothermal Raman technique, diff-TET employs joule heating as the heating source, a situation under which the temperature difference between optical phonons and acoustic phonons is eased. The thermal conductivity of single-layer graphene (SLG) supported by PMMA was measured as 743 ± 167 W/(m·K) and 287 ± 63 W/(m·K) at 296 K and 125 K, respectively. As temperature decreased from 296 K to 275 K, the thermal conductivity of graphene was decreased by 36.5%, which can be partly explained by compressive strain buildup in graphene due to the thermal expansion mismatch.

Preparation and Bolometric Responses of MoS2 Nanoflowers and Multi-Walled Carbon Nanotube Composite Network.

Due to their broadband optical absorption ability and fast response times, carbon nanotube (CNT)-based materials are considered promising alternatives to the toxic compounds used in commercial infrared sensors. However, the direct use of pure CNT networks as infrared sensors for simple resistance read-outs results in low sensitivity values. In this work, MoS2 nanoflowers are composited with CNT networks via a facile hydrothermal process to increase the bolometric performance. The thermal diffusivity (α) against temperature (T) is measured using the transient electro-thermal (TET) technique in the range of 320 K to 296 K. The α-T curve demonstrates that the composite containing MoS2 nanoflowers provides significant phonon scattering and affects the intertube interfaces, decreasing the α value by 51%. As the temperature increases from 296 K to 320 K, the relative temperature coefficient of resistance (TCR) increases from 0.04%/K to 0.25%/K. Combined with the enhanced light absorption and strong anisotropic structure, this CNT-MoS2 composite network exhibits a more than 5-fold greater surface temperature increase under the same laser irradiation. It shows up to 18-fold enhancements in resistive responsivity ((Ron - Roff)/Roff) compared with the pure CNT network for a 1550 nm laser at room temperature (RT).

Nitrogen-Doped Unusually Superwetting, Thermally Insulating, and Elastic Graphene Aerogel for Efficient Solar Steam Generation.

By removing the oxygen-containing functional groups, thermal treatment in inert gas has been widely reported to improve the hydrophobicity of carbon materials. However, this work reports a contrary phenomenon for the nitrogen-doped graphene aerogel (NGA). As the temperature of thermal treatment increases from 200 to 1000 °C, NGA becomes more and more hydrophilic and the superwetting property remains for weeks in air. To uncover this unusual phenomenon, the effect of nitrogen doping is studied through both experiment and MD simulations. The effects of air exposure and air humidity are further investigated in detail to illustrate the whole physical picture clearly. The superwetting behavior is attributed to the preferential adsorption of water molecules to the nitrogen-doped sites, which significantly inhibits airborne hydrocarbon adsorption. In combination with the excellent properties including mechanical elasticity, high light absorption, and good thermal insulation, an efficient photothermal and solar steam generation performance is demonstrated by using NGA-600 as the photothermal material, presenting a high energy conversion efficiency of 86.2% and good recycling stability.

Effect of temperature on Raman intensity of nm-thick WS2: combined effects of resonance Raman, optical properties, and interface optical interference.

Temperature dependent Raman intensity of 2D materials features very rich information about the material's electronic structure, optical properties, and nm-level interface spacing. To date, there still lacks rigorous consideration of the combined effects. This renders the Raman intensity information less valuable in material studies. In this work, the Raman intensity of four supported multilayered WS2 samples are studied from 77 K to 757 K under 532 nm laser excitation. Resonance Raman scattering is observed, and we are able to evaluate the excitonic transition energy of B exciton and its broadening parameters. However, the resonance Raman effects cannot explain the Raman intensity variation in the high temperature range (room temperature to 757 K). The thermal expansion mismatch between WS2 and Si substrate at high temperatures (room temperature to 757 K) make the optical interference effects very strong and enhances the Raman intensity significantly. This interference effect is studied in detail by rigorously calculating and considering the thermal expansion of samples, the interface spacing change, and the optical indices change with temperature. Considering all of the above factors, it is concluded that the temperature dependent Raman intensity of the WS2 samples cannot be solely interpreted by its resonance behavior. The interface optical interference impacts the Raman intensity more significantly than the change of refractive indices with temperature.

Distinguishing Optical and Acoustic Phonon Temperatures and Their Energy Coupling Factor under Photon Excitation in nm 2D Materials.

Under photon excitation, 2D materials experience cascading energy transfer from electrons to optical phonons (OPs) and acoustic phonons (APs). Despite few modeling works, it remains a long-history open problem to distinguish the OP and AP temperatures, not to mention characterizing their energy coupling factor (G). Here, the temperatures of longitudinal/transverse optical (LO/TO) phonons, flexural optical (ZO) phonons, and APs are distinguished by constructing steady and nanosecond (ns) interphonon branch energy transport states and simultaneously probing them using nanosecond energy transport state-resolved Raman spectroscopy. ΔT OP -AP is measured to take more than 30% of the Raman-probed temperature rise. A breakthrough is made on measuring the intrinsic in-plane thermal conductivity of suspended nm MoS2 and MoSe2 by completely excluding the interphonon cascading energy transfer effect, rewriting the Raman-based thermal conductivity measurement of 2D materials. G OP↔AP for MoS2, MoSe2, and graphene paper (GP) are characterized. For MoS2 and MoSe2, G OP↔AP is in the order of 1015 and 1014 W m-3 K-1 and G ZO↔AP is much smaller than G LO/TO↔AP. Under ns laser excitation, G OP↔AP is significantly increased, probably due to the reduced phonon scattering time by the significantly increased hot carrier population. For GP, G LO/TO↔AP is 0.549 × 1016 W m-3 K-1, agreeing well with the value of 0.41 × 1016 W m-3 K-1 by first-principles modeling.

Graphene Aerogel Based Bolometer for Ultrasensitive Sensing from Ultraviolet to Far-Infrared.

This work uncovers that free-standing partly reduced graphene aerogel (PRGA) films in vacuum exhibit extraordinarily bolometric responses. This high performance is mainly attributed to four structure characteristics: extremely low thermal conductivity (6.0-0.6 mW·m-1·K-1 from 295 to 10 K), high porosity, ultralow density (4 mg·cm-3), and abundant functional groups (resulting in tunable band gap). Under infrared radiation (peaked at 5.8-9.7 µm), the PRGA film can detect a temperature change of 0.2, 1.0, and 3.0 K of a target at 3, 25, and 54 cm distance. Even through a quartz window (transmissivity of ∼0.98 in the range of 2-4 µm), it can still successfully detect a temperature change of 0.6 and 5.8 K of a target at 3 and 28 cm distance. At room temperature, a laser power as low as 7.5 µW from a 405 nm laser and 5.9 µW from a 1550 nm laser can be detected. The detecting sensitivity to the 1550 nm laser is further increased by 3-fold when the sensor temperature was reduced from 295 K to 12 K. PRGA films are demonstrated to be a promising ultrasensitive bolometric detector, especially at low temperatures.

Green Production of Regenerated Cellulose/Boron Nitride Nanosheet Textiles for Static and Dynamic Personal Cooling.

Personal cooling technology using functional clothing that could provide localized thermal regulation instead of cooling the entire space is regarded as a highly anticipated strategy to not only facilitate thermal comfort and human health but also be energy-saving and low-cost. The challenge is how to endow textiles with prominent cooling effect whenever the wearer is motionless or sportive. In this study, high content of edge-selective hydroxylated boron nitride nanosheets (BNNSs) up to 60 wt % was added into a biodegradable cellulose/alkaline/urea aqueous solution, and then regenerated cellulose (RCF)/BNNS multifilaments were successfully spun in a simple, low-cost, and environmentally friendly process, which was demonstrated to serve as both static and dynamic personal cooling textile. Typically, excellent axial thermal conductivity of RCF/BNNS filament rendered that body-generated heat could directly escape from skin to the outside surface of the textile by means of thermal conduction, achieving a much better static personal cooling result through continuous thermal radiation. Besides, synergistic effect between excellent heat dissipation capability and good hygroscopicity also resulted in much better dynamic cooling effect once the wearer is doing some sports, whose efficiency was even better than commercial hygroscopic textiles such as cotton and RCF.

High-Performance N-Type Carbon Nanotube Composites: Improved Power Factor by Optimizing the Acridine Scaffold and Tailoring the Side Chains.

Single-walled carbon nanotubes (SWCNTs)/organic small molecules (OSMs) are promising candidates for application in thermoelectric (TE) modules; however, the development of n-type SWCNT/OSMs with high performance is lagging behind. Only a few structure-activity relationships of OSMs on SWCNT composites have been reported. Recently, we find that the n-type acridone/SWCNT composites display high power factor (PF) values at high temperature but suffer from low PFs at room temperature. Here, the performance of SWCNT composites containing an acridine derivative (AD) as well as its analogues with different counterions (Cl-, SO42- and F-) and lengths of alkyl chains (ADLA1-2 and ADLA4-5) is reported. Among the composites, SWCNT/ADLA4 with no counterions exhibits the highest PF value of 195.2 µW m-1 K-2 at room temperature, which is 4.9 times higher than that of SWCNT/ADTAd (39.8 µW m-1 K-2), indicating that the acridine scaffold and the lengths of alkyl chains contribute to the dramatic changes in the TE performance. In addition, SWCNT/ADLA4 exhibits high PF values at all the temperatures we investigate, which range from 154.7 to 230.7 µW m-1 K-2. Furthermore, a TE device consisting of five pairs of p (the pristine SWCNTs)-n (SWCNT/ADLA4) junctions is assembled, generating a relatively high open-circuit voltage (41.7 mV) and an output power of 1.88 µW at a temperature difference of 74.8 K. Our results suggest that structural modifications might be an effective way to advance the development of TE materials.

Switch on the high thermal conductivity of graphene paper.

This work reports on the discovery of a high thermal conductivity (κ) switch-on phenomenon in high purity graphene paper (GP) when its temperature is reduced from room temperature down to 10 K. The κ after switch-on (1732 to 3013 W m-1 K-1) is 4-8 times that before switch-on. The triggering temperature is 245-260 K. The switch-on behavior is attributed to the thermal expansion mismatch between pure graphene flakes and impurity-embedded flakes. This is confirmed by the switch behavior of the temperature coefficient of resistance. Before switch-on, the interactions between pure graphene flakes and surrounding impurity-embedded flakes efficiently suppress phonon transport in GP. After switch-on, the structure separation frees the pure graphene flakes from the impurity-embedded neighbors, leading to a several-fold κ increase. The measured κ before and after switch-on is consistent with the literature reported κ values of supported and suspended graphene. By conducting comparison studies with pyrolytic graphite, graphene oxide paper and partly reduced graphene paper, the whole physical picture is illustrated clearly. The thermal expansion induced switch-on is feasible only for high purity GP materials. This finding points out a novel way to switch on/off the thermal conductivity of graphene paper based on substrate-phonon scattering.

Thermal Diffusivity of a Single Carbon Nanocoil: Uncovering the Correlation with Temperature and Domain Size.

The helical geometries and polycrystalline-amorphous structure of carbon nanocoils (CNCs), an exotic class of low-dimensional carbon nanostructures, distinguish them from carbon nanotubes and graphene. These distinct structures result in very different energy transport from that in carbon nanotubes and graphene, leading to important roles in applications as wave absorbers, near-infrared sensors, and nanoelectromechanical sensors. Here we report a systematic study of the thermal diffusivity (α) and conductivity (κ) of CNCs from 290 to 10 K and uncover their property-structure aspects. Our room-temperature α study reveals a correlation between α and the line diameter (d): α = (5.43 × 104 × e-d/37.7 + 9.5) × 10-7 m2/s. Combined with the Raman-based grain size (La) characterization, α and La are correlated as α = [81.2 × (La - 3.32)1.5 + 9.5] × 10-7 m2/s. With temperature decreasing from 290 K to 10 K, α has a 1-1.6-fold increase, and κ shows a peak around 75 K. To best understand the defect level and polycrystalline-amorphous structure of CNCs, the thermal reffusivity (Θ = α-1) of CNCs is studied and compared with that of graphite and graphene foam from 290 K down to 10 K. Very interestingly, CNC's Θ linearly decreases with decreased temperature, while Θ of graphite and graphene foam have an exponential decrease. The extrapolated 0 K-limit Θ is determined by low-momentum phonon scattering and gives a structure domain size of CNC samples (d = 455, 353, and 334 nm) of 1.28, 2.03 and 3.24 nm. These sizes are coherent with the X-ray diffraction results (3.5 nm) and the Raman spectroscopy study and confirm the correlation among d, La, and α.

The defect level and ideal thermal conductivity of graphene uncovered by residual thermal reffusivity at the 0 K limit.

Due to its intriguing thermal and electrical properties, graphene has been widely studied for potential applications in sensor and energy devices. However, the reported value for its thermal conductivity spans from dozens to thousands of W m(-1) K(-1) due to different levels of alternations and defects in graphene samples. In this work, the thermal diffusivity of suspended four-layered graphene foam (GF) is characterized from room temperature (RT) down to 17 K. For the first time, we identify the defect level in graphene by evaluating the inverse of thermal diffusivity (termed "thermal reffusivity": Θ) at the 0 K limit. By using the Debye model of Θ = Θ0 + C× e(-θ/2T) and fitting the Θ-T curve to the point of T = 0 K, we identify the defect level (Θ0) and determine the Debye temperature of graphene. Θ0 is found to be 1878 s m(-2) for the studied GF and 43-112 s m(-2) for three highly crystalline graphite materials. This uncovers a 16-43-fold higher defect level in GF than that in pyrolytic graphite. In GF, the phonon mean free path solely induced by defects and boundary scattering is determined as 166 nm. The Debye temperature of graphene is determined to be 1813 K, which is very close to the average theoretical Debye temperature (1911 K) of the three acoustic phonon modes in graphene. By subtracting the defect effect, we report the ideal thermal diffusivity and conductivity (κideal) of graphene presented in the 3D foam structure in the range of 33-299 K. Detailed physics based on chemical composition and structure analysis are given to explain the κideal-T profile by comparing with those reported for suspended graphene.