Atomically thin two-dimensional (2D) materials have shown great potential for applications in nanoscale electronic and optical devices. A fundamental property of these 2D flakes that needs to be well characterized is the thermal expansion coefficient (TEC), which is instrumental to the dry transfer process and thermal management of 2D material-based devices. Yet, most of current studies of 2D materials' TEC extensively rely on simulations due to the difficulty of performing experimental measurements on an atomically thin, micron-sized, and optically transparent 2D flake. In this work, we present a three-substrate approach to characterize the TEC of monolayer molybdenum disulfide (MoS 2 ) using micro-Raman spectroscopy. The temperature dependence of the Raman peak shift was characterized with three different substrate conditions, from which the in-plane TEC of monolayer MoS 2 was extracted based on lattice symmetries. Independently from two different phonon modes of MoS 2 , we measured the in-plane TECs as (7.6±0.9)×10 -6 1/K and (7.4±0.5)×10 -6 1/K, respectively, which are in good agreement with previously reported values based on first principle calculations. Our work is not only useful for thermal mismatch reduction during material transfer or device operation, but provides a general experimental method that does not rely on simulations to study key properties of 2D materials.
The simultaneous imaging of magnetic fields and temperature (MT) is important in a range of applications, including studies of carrier transport 1-3 , solid-state material dynamics 1-6 , and semiconductor device characterization 7,8 . Techniques exist for separately measuring temperature (e.g., infrared (IR) microscopy 9 , micro-Raman spectroscopy 9 , and thermo-reflectance microscopy 9 ) and magnetic fields (e.g., scanning probe magnetic force microscopy 10 and superconducting quantum interference devices 11 ). However, these techniques cannot measure magnetic fields and temperature simultaneously. Here, we use the exceptional temperature 12,13 and magnetic field 14,15 sensitivity of nitrogen vacancy (NV) spins in conformally-coated nanodiamonds to realize simultaneous wide-field MT imaging. Our "quantum conformally-attached thermo-magnetic" (Q-CAT) imaging enables (i) wide-field, high-frame-rate imaging (100 -1000 Hz); (ii) high sensitivity; and (iii) compatibility with standard microscopes. We apply this technique to study the industrially important problem 8,16-18 of characterizing multifinger gallium nitride high-electron-mobility transistors (GaN HEMTs). We spatially and temporally resolve the electric current distribution and resulting temperature rise, elucidating functional device behavior at the microscopic level. The general applicability of Q-CAT imaging serves as an important tool for understanding complex MT phenomena in material science, device physics, and related fields.The NV center in diamond has attracted great interest because of its exceptional spin properties at room temperature, which exhibit outstanding nanoscale sensitivity to magnetic fields 14,19-23 and temperature 12,13 . NV centers located within nanodiamonds (NVNDs) have gained particular interest for applications including drug delivery 24 , thermal measurements of biological systems [25][26][27][28][29] , and scanning magnetometer tips 30,31 . The NVND's small size allows direct measurement of their local MT environment. These applications have motivated studies of NVND properties such as strain, magnetic and thermal sensitivity, and coherence time 32,33 . However, NVND properties differ for a given fabrication process 32 or surface treatment 34 . This variability of nanodiamond material parameters and orientations has presented challenges for wide-field imaging studies using NVNDs. In this work, we (i) develop a model that describes the optically detected magnetic resonance (ODMR) 35,36 spectrum of NVND ensembles as a function of magnetic field and temperature; (ii) perform statistical characterization of NVND parameters, specifically the variation in NVND thermal response with implications for NVND temperature sensing; (iii) use this NVND model and our statistical characterization to extend the capabilities of NV sensing by enabling wide-field imaging with deposited coatings of NVNDs (Q-CAT imaging); (iv) demonstrate our technique's capabilities by imaging the dynamic phenomenon of electromigration; and (v) perform wide-field MT...
High power density electronics are severely limited by current thermal management solutions which are unable to dissipate the necessary heat flux while maintaining safe junction temperatures for reliable operation. We designed, fabricated, and experimentally characterized a microfluidic device for ultra-high heat flux dissipation using evaporation from a nanoporous silicon membrane. With~100 nm diameter pores, the membrane can generate high capillary pressure even with low surface tension fluids such as pentane and R245fa. The suspended ultra-thin membrane structure facilitates efficient liquid transport with minimal viscous pressure losses. We fabricated the membrane in silicon using interference lithography and reactive ion etching and then bonded it to a high permeability silicon microchannel array to create a biporous wick which achieves high capillary pressure with enhanced permeability. The back side consisted of a thin film platinum heater and resistive temperature sensors to emulate the heat dissipation in transistors and measure the temperature, respectively. We experimentally characterized the devices in pure vaporambient conditions in an environmental chamber. Accordingly, we demonstrated heat fluxes of 665 ± 74 W/cm 2 using pentane over an area of 0.172 mm × 10 mm with a temperature rise of 28.5 ± 1.8 K from the heated substrate to ambient vapor. This heat flux, which is normalized by the evaporation area, is the highest reported to date in the pure evaporation regime, that is, without nucleate boiling. The experimental results are in good agreement with a high fidelity model which captures heat conduction in the suspended membrane structure as well as non-equilibrium and sub-continuum effects at the liquid-vapor interface. This work suggests that evaporative membrane-based approaches can be promising towards realizing an efficient, high flux thermal management strategy over large areas for high-performance electronics.
We present a high-heat-flux cooling device for advanced thermal management of electronics. The device incorporates nanoporous membranes supported on microchannels to enable thin film evaporation. The underlying concept takes advantage of the capillary pressure generated by small pores in the membrane, and minimizes the viscous loss by reducing the membrane thickness. The heat transfer and fluid flow in the device were modeled to determine the effect of different geometric parameters. With the optimization of various parameters, the device can achieve a heat transfer coefficient in excess of 0.05 kW/cm 2 -K while dissipating a heat flux of 1 kW/cm 2 . When applied to power electronics, such as GaN high electron mobility transistors, this membrane-based evaporative cooling device can lower the near junction temperature by more than 40 K compared to contemporary single-phase microchannel coolers.
We report an experimental investigation of the adsorption properties of two important small-pore metal–organic framework (MOF) materials recently identified for gas separation applications, through the development and use of a high-pressure/high-temperature quartz crystal microbalance (QCM) device. In particular, we characterize in detail the CO2, CH4, and N2 adsorption characteristics of the MOFs Cu(4,4′-(hexafluoroisopropylidene)bisbenzoate)1.5 (referred to as Cu–hfipbb) and zeolitic imidazolate framework-90 (ZIF-90). We first describe the construction of a QCM-based adsorption measurement apparatus. Single-component adsorption isotherms of CO2, CH4, and N2 in the two MOFs were then measured at temperatures ranging from 30 to 70 °C and pressures ranging from 0.3 to 110 psi. In both materials, the order of adsorption strength is CO2 > CH4 > N2. We find that adsorption in the 1-D channels of Cu–hfipbb can be well described by a single-site Langmuir model. On the other hand, adsorption in ZIF-90 follows a more complex behavior, commensurate with its pore structure consisting of large porous cages connected in three dimensions by small windows. The nongravimetric QCM-based measurement techniques are shown to be a valuable microanalytical tool for the study of molecular adsorption in MOFs.
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