| 1. |
Dawei Li, Qin-Yi Li, Koji Takahashi, Thermal resistance mapping along a single cup-stacked carbon nanotube with focused electron beam heating, International Journal of Heat and Mass Transfer, 10.1016/j.ijheatmasstransfer.2022.123418, 198, 123418, 2022.09, The structural non-uniformity in low-dimensional materials, including interfaces and defects, makes it highly desirable to map the thermal property distribution with a high spatial resolution. Meanwhile, eliminating the error of thermal contact resistance at the sample-sensor junction has remained a critical challenge in nanoscale thermal conductivity measurement. Here, we combine the electron beam (EB) heating with two suspended line-shaped heat flux sensors and have achieved the in-situ thermal resistance mapping along a single cup-stacked carbon nanotube (CNT) in a scanning electron microscope (SEM). The CNT is anchored between the two suspended metal lines, and the focused electron beam heats the CNT locally with a nanometer-range spatial resolution, while the two metal lines simultaneously measure the heat fluxes induced by the EB heating. By sweeping the focused EB along the CNT, we can obtain the spatially resolved thermal resistance, from which the true thermal conductivity of the CNT was extracted to be around 40 W/m·K without the thermal contact resistance error. This SEM-based in-situ thermal measurement method can accelerate high-resolution nanomaterials characterization and the elucidation of nanoscale heat transfer.. |
| 2. |
Li, Dawei; Li, Qin-Yi; Ikuta, Tatsuya; Takahashi, Koji, Concurrent thermal conductivity measurement and internal structure observation of individual one-dimensional materials using scanning transmission electron microscopy, APPLIED PHYSICS LETTERS, 10.1063/5.0079153, 120, 4, 2022.01, The thermal conductivity of individual nanomaterials can vary from sample to sample due to the difference in geometries and internal structures, and thus, concurrent structure observation and thermal conductivity measurement at the nanoscale are highly desired but challenging. Here, we have developed an experimental method that allows concurrently the in situ thermal conductivity measurement and the real-time internal structure observation of a single one-dimensional (1D) material using scanning transmission electron microscopy (STEM) in a scanning electron microscope. In this method, the two ends of the 1D nanomaterial are bonded on a tungsten probe and a suspended platinum nanofilm, respectively. The platinum nanofilm serves simultaneously as a heater and a resistance thermometer, ensuring highly sensitive thermal measurements. The platinum nanofilm is fabricated on the edge of the silicon wafer so that the electron beam can transmit through the 1D material and be detected by the STEM detector, which caters for real-time observation of the inner nanostructure. Using this method, we in situ measured the thermal conductivities of two cup-stacked carbon nanotubes and concurrently observed the internal hollow structures. We found that the sample with more structural disorders had a lower thermal conductivity. Our measurement method can pave the way to the sample-by-sample elucidation of the structure–property relationship for 1D materials..  |
| 3. |
Chen, Kuan-Ting; Li, Qin-Yi; Omori, Takeshi; Yamaguchi, Yasutaka; Ikuta, Tatsuya; Takahashi, Koji, Slip length measurement in rectangular graphene nanochannels with a 3D flow analysis, CARBON, 10.1016/j.carbon.2021.12.048, 189, 162-172, 2021.12, Although many molecular dynamics simulations have been conducted on slip flow on graphene, experimental efforts remain very limited and our understanding of the flow friction on graphene remains far from sufficient. Here, to accurately measure the slip length in rectangular nanochannels, we develop a 3D capillary flow model that fully considers the nonuniform cross-section velocity profile, slip boundary conditions, and the dynamic contact angle. We show that the 3D analysis is necessary even for a channel with a width/height ratio of 100. We fabricated graphene nanochannels with 45-nm depth and 5-μm width, and measured slip lengths of about 30–40 nm using this 3D flow model. We also reevaluated the slip-length data for graphene obtained from capillary filling experiments in the literature: 30 nm instead of originally claimed 45 nm for a 25-nm-deep channel, and 47 nm instead of 60 nm for an 8.5-nm-deep channel. We discover a smaller slip length than existing experimental measurements due to our full 3D flow analysis considered in our method. This work presents a rigorous analysis approach while also providing a better understanding of slip flow in graphene nanochannels, which will benefit further innovation in nanofluidic applications, including electronics cooling and biomedical chips.. |
| 4. |
Qin Yi Li, Tianli Feng, Wakana Okita, Yohei Komori, Hiroo Suzuki, Toshiaki Kato, Toshiro Kaneko, Tatsuya Ikuta, Xiulin Ruan, Koji Takahashi, Enhanced Thermoelectric Performance of As-Grown Suspended Graphene Nanoribbons, ACS nano, 10.1021/acsnano.9b03521, 13, 8, 9182-9189, 2019.08, Conventionally, graphene is a poor thermoelectric material with a low figure of merit (ZT) of 10-4-10-3. Although nanostructuring was proposed to improve the thermoelectric performance of graphene, little experimental progress has been accomplished. Here, we carefully fabricated as-grown suspended graphene nanoribbons with quarter-micron length and â40 nm width. The ratio of electrical to thermal conductivity was enhanced by 1-2 orders of magnitude, and the Seebeck coefficient was several times larger than bulk graphene, which yielded record-high ZT values up to â0.1. Moreover, we observed a record-high electronic contribution of â20% to the total thermal conductivity in the nanoribbon. Concurrent phonon Boltzmann transport simulations reveal that the reduction of lattice thermal conductivity is mainly attributed to quasi-ballistic phonon transport. The record-high ratio of electrical to thermal conductivity was enabled by the disparate electron and phonon mean free paths as well as the clean samples, and the enhanced Seebeck coefficient was attributed to the band gap opening. Our work not only demonstrates that electron and phonon transport can be fundamentally tuned and decoupled in graphene but also indicates that graphene with appropriate nanostructures can be very promising thermoelectric materials.. |
| 5. |
Qin-Yi Li, Ryo Matsushita, Yoko Tomo, Tatsuya Ikuta, Koji Takahashi, Water Confined in Hydrophobic Cup-Stacked Carbon Nanotubes beyond Surface-Tension Dominance, The Journal of Physical Chemistry Letters, 10.1021/acs.jpclett.9b00718, 2019.06. |
| 6. |
Qinyi Li, Koji Takahashi, Xing Zhang, Frequency-domain Raman method to measure thermal diffusivity of one-dimensional microfibers and nanowires, International Journal of Heat and Mass Transfer, 10.1016/j.ijheatmasstransfer.2019.01.057, 539-546, 2019.05, Thermal property measurement of individual micro- and nano-materials has been very challenging and the development of measurement methods is crucial for the experimental investigation of microscale and nanoscale heat transfer. Here we present a noncontact frequency-domain Raman method to measure thermal diffusivity of individual 1D microfibers and nanowires without the need of knowing laser absorptivity. Cosine-wave modulated laser is used to heat the sample, while the laser-intensity-weighted spatiotemporal average temperature is simultaneously detected from the sample's Raman band shift. Transient heat conduction models under periodic heating are established and analytically solved in the frequency domain with considerations of the Gaussian laser distribution and thermal contact resistance. By varying the laser modulation frequency as well as the laser spot size, we can eliminate the laser absorptivity by a normalization technique and extract the thermal diffusivity with high sensitivity. Typically, if the thermal diffusivity is on the order of 10 −4 m 2 /s, we need to use the modulation frequencies on the order of 10 Hz to measure millimeter long microfibers, and ∼MHz frequencies to measure micrometer long nanowires. We also demonstrate that any kind of periodic laser modulation can be decomposed to a series of cosine modes and readily analyzed by this frequency-domain approach, which can greatly broaden the applications of transient Raman techniques.. |
| 7. |
Qinyi Li, Koki Katakami, Tatsuya Ikuta, Masamichi Kohno, Xing Zhang, Koji Takahashi, Measurement of thermal contact resistance between individual carbon fibers using a laser-flash Raman mapping method, Carbon, 10.1016/j.carbon.2018.09.034, 141, 92-98, 2019.01, Thermal contact resistance (TCR) between individual carbon fibers (CFs) can dominate heat dissipation rates in CF-based composite materials. Here, we develop a totally non-contact “laser-flash Raman mapping” method to simultaneously measure the TCR at the CF-CF junction and their thermal conductivities. Laser power is used to heat the sample and the laser absorptivity is experimentally determined by a transient laser-flash Raman technique. The laser heating positions are changed along two connected CFs, and the change of temperature rise with varying positions is in-situ measured from the temperature dependent Raman band shifts. The high spatial resolution of the micro-Raman mapping allows direct observation of the abrupt jump of thermal resistance at the CF-CF junction, from which we extracted the TCR as well as the thermal conductivity. The laser absorptivity of the 11 μm-diameter CFs is measured to be 0.12 ± 0.03, the thermal conductivities of the individual CFs are around 200 W/mK, and the TCR of the CF-CF junction is (2.98 ± 0.92) × 105 K/W. This work provides indispensable knowledge for the design of CF-based composite for thermal management, and the novel non-contact measurement method can stimulate characterization and manipulation of contact/interface heat conduction between various micro- and nano-materials.. |
| 8. |
Qinyi Li, Kailun Xia, Ji Zhang, Yingying Zhang, Qunyang Li, Koji Takahashi, Zhang Xing, Measurement of specific heat and thermal conductivity of supported and suspended graphene by a comprehensive Raman optothermal method, Nanoscale, 9, 10784-10793, 2017.07, The last decade has seen the rapid growth of research on two-dimensional (2D) materials, represented by graphene, but research on their thermophysical properties is still far from sufficient owing to the experimental challenges. Herein, we report the first measurement of the specific heat of multilayer and monolayer graphene in both supported and suspended geometries. Their thermal conductivities were also simultaneously measured using a comprehensive Raman optothermal method without needing to know the laser absorption. Both continuous-wave (CW) and pulsed lasers were used to heat the samples, based on consideration of the variable laser spot radius and pulse duration as well as the heat conduction within the substrate. The error from the laser absorption was eliminated by comparing the Raman-measured temperature rises for different spot radii and pulse durations. The thermal conductivity and specific heat were extracted by analytically fitting the temperature rise ratios as a function of spot size and pulse duration, respectively. The measured specific heat was about 700 J (kg K)−1 at room temperature, which is in accordance with theoretical predictions, and the measured thermal conductivities were in the range of 0.84–1.5 × 103 W (m K)−1. The measurement method demonstrated here can be used to investigate in situ and comprehensively the thermophysical properties of many other emerging 2D materials.. |
| 9. |
Qinyi Li, Wei Gang Ma, Xing Zhang, Laser flash Raman spectroscopy method for characterizing thermal diffusivity of supported 2D nanomaterials, International Journal of Heat and Mass Transfer, 10.1016/j.ijheatmasstransfer.2015.12.065, 95, 956-963, 2016.04, 2D nanomaterials have been attracting extensive research interests due to their superior properties and the accurate thermophysical characterization of 2D materials is very important for nanoscience and nanotechnology. This paper presents a transient "laser flash Raman spectroscopy" method for measuring the thermal diffusivity of 2D nanomaterials in the supported form without knowing the laser absorption coefficient. Square pulsed laser rather than continuous laser is used to heat the sample and the accumulated Raman signals are used to determine the time-averaged temperature rise of both the supported 2D material and the substrate. The laser absorption coefficient can be eliminated by comparing the temperature rises measured with different laser spot sizes and laser pulse durations. The method sensitivity is also analyzed by case studies for typical 2D nanomaterials. This method is useful for measuring the thermophysical properties of 2D materials in the most applicable forms and figuring out the difference between the supported and free-standing 2D material.. |
| 10. |
Qinyi Li, Koji Takahashi, Hiroki Ago, Xing Zhang, Tatsuya Ikuta, Takashi Nishiyama, Kenji Kawahara, Temperature dependent thermal conductivity of a suspended submicron graphene ribbon, Journal of Applied Physics, 10.1063/1.4907699, 117, 6, 2015.02, Thermophysical characterization of graphene is very important for both fundamental and technological research. While most of the existing thermal conductivity measurements are for graphene sheets with sizes larger than 1 μm, the thermal conductivities for suspended submicron graphene ribbons are still very few, although the thermal conductivity of graphene ribbons at the submicron scale is predicted to be much smaller than large graphene and strongly size dependent for both length and width due to the 2D nature of phonon transport. Here, we report the temperature dependent thermal conductivity of a 169-nm wide and 846-nm long graphene ribbon measured by the electrical self-heating method. The measured thermal conductivities range from (12.7 ± 2.95) W/m/K at 80 K to (932 ± 333) W/m/K at 380 K, being (349 ± 63) W/m/K at 300 K, following a ∼ T2.79 law for the full temperature range of 80 K to 380 K and a ∼ T1.23 law at low temperatures. The comparison of the measured thermal conductance with the ballistic transport limit indicates diffusive transport in this narrow and short ribbon due to phonon-edge as well as phonon-defect scattering. The data were also combined with an empirical model to predict possible width dependence of thermal conductivity for suspended graphene ribbons. These results help understand the 2D phonon transport in suspended submicron graphene ribbons and provide knowledge for controlling thermophysical properties of suspended graphene nanoribbons through size manipulation.. |
