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Frequency-domain probe beam deflection method for measurement of thermal conductivity of materials on micron length scale

Time-domain thermoreflectance and frequency-domain thermoreflectance (FDTR) have been widely used for non-contact measurement of anisotropic thermal conductivity of materials with high spatial resolution. However, the requirement of a high thermoreflectance coefficient restricts the choice of metal coating and laser wavelength. The accuracy of the measurement is often limited by the high sensitivity to the radii of the laser beams. We describe an alternative frequency-domain pump-probe technique based on probe beam deflection. The beam deflection is primarily caused by thermoelastic deformation of the sample surface, with a magnitude determined by the thermal expansion coefficient of the bulk material to measure. We derive an analytical solution to the coupled elasticity and heat diffusion equations for periodic heating of a multilayer sample with anisotropic elastic constants, thermal conductivity, and thermal expansion coefficients. In most cases, a simplified model can reliably describe the frequency dependence of the beam deflection signal without knowledge of the elastic constants and thermal expansion coefficients of the material. The magnitude of the probe beam deflection signal is larger than the maximum magnitude achievable by thermoreflectance detection of surface temperatures if the thermal expansion coefficient is greater than 5 × 10−6 K−1. The uncertainty propagated from laser beam radii is smaller than that in FDTR when using a large beam offset. We find a nearly perfect matching of the measured signal and model prediction, and measure thermal conductivities within 6% of accepted values for materials spanning the range of polymers to gold, 0.1–300 W/(m K).
Publications

Immunoglobulin adsorption and film formation on mechanically wrinkled and crumpled surfaces at submonolayer coverage

Understanding protein adsorption behavior on rough and wrinkled surfaces is vital to applications including biosensors and flexible biomedical devices.
Publications

Designing the Bending Stiffness of 2D Material Heterostructures

Abstract2D monolayers represent some of the most deformable inorganic materials, with bending stiffnesses approaching those of lipid bilayers. Achieving 2D heterostructures with similar properties would enable a new class of deformable devices orders of magnitude softer than conventional thin‐film electronics. Here, by systematically introducing low‐friction twisted or heterointerfaces, interfacial engineering is leveraged to tailor the bending stiffness of 2D heterostructures over several hundred percent. A bending model is developed and experimentally validated to predict and design the deformability of 2D heterostructures and how it evolves with the composition of the stack, the atomic arrangements at the interfaces, and the geometry of the structure. Notably, when each atomic layer is separated by heterointerfaces, the total bending stiffness reaches a theoretical minimum, equal to the sum of the constituent layers regardless of scale of deformation—lending the extreme deformability of 2D monolayers to device‐compatible multilayers.
Publications

Epitaxial growth and magnetic properties of kagome metal FeSn/elemental ferromagnet heterostructures

Binary kagome compounds TmXn (T = Mn, Fe, Co; X = Sn, Ge; m:n = 3:1, 3:2, 1:1) have garnered recent interest owing to the presence of both topological band crossings and flatbands arising from the geometry of the metal-site kagome lattice. To exploit these electronic features for potential applications in spintronics, the growth of high-quality heterostructures is required. Here, we report the synthesis of Fe/FeSn and Co/FeSn bilayers on Al2O3 substrates using molecular beam epitaxy to realize heterointerfaces between elemental ferromagnetic metals and antiferromagnetic kagome metals. Structural characterization using high-resolution x-ray diffraction, reflection high-energy electron diffraction, and electron microscopy reveals that the FeSn films are flat and epitaxial. Rutherford backscattering spectroscopy was used to confirm the stoichiometric window where the FeSn phase is stabilized, while transport and magnetometry measurements were conducted to verify metallicity and magnetic ordering in the films. Exchange bias was observed, confirming the presence of antiferromagnetic order in the FeSn layers, paving the way for future studies of magnetism in kagome heterostructures and potential integration of these materials into devices.
Publications

Anomalous Dimensionality‐Driven Phase Transition of MoTe2 in Van der Waals Heterostructure

AbstractPhase transition in nanomaterials is distinct from that in 3D bulk materials owing to the dominant contribution of surface energy. Among nanomaterials, 2D materials have shown unique phase transition behaviors due to their larger surface‐to‐volume ratio, high crystallinity, and lack of dangling bonds in atomically thin layers. Here, the anomalous dimensionality‐driven phase transition of molybdenum ditelluride (MoTe2) encapsulated by hexagonal boron nitride (hBN) is reported. After encapsulation annealing, single‐crystal 2H‐MoTe2 transformed into polycrystalline Td‐MoTe2 with tilt‐angle grain boundaries of 60°‐glide‐reflection and 120°‐twofold rotation. In contrast to conventional nanomaterials, the hBN‐encapsulated MoTe2 exhibit a deterministic dependence of the phase transition on the number of layers, in which the thinner MoTe2 has a higher 2H‐to‐Td phase transition temperature. In addition, the vertical and lateral phase transitions of the stacked MoTe2 with different crystalline orientations can be controlled by inserted graphene layers and the thickness of the heterostructure. Finally, it is shown that seamless Td contacts for 2H‐MoTe2 transistors can be fabricated by using the dimensionality‐driven phase transition. The work provides insight into the phase transition of 2D materials and van der Waals heterostructures and illustrates a novel method for the fabrication of multi‐phase 2D electronics.
Publications

Fluorinated Graphene Contacts and Passivation Layer for MoS2 Field Effect Transistors

AbstractRealizing a future of 2D semiconductor‐based devices requires new approaches to channel passivation and nondestructive contact engineering. Here, a facile one‐step technique is shown that simultaneously utilizes monolayer fluorinated graphene (FG) as the passivation layer and contact buffer layer to 2D semiconductor transistors. Monolayer graphene is transferred onto the MoS2, followed by fluorination by XeF2 gas exposure. Metal electrodes for source and drain are fabricated on top of FG‐covered MoS2 regions. The MoS2 transistor is perfectly passivated by insulating FG layer and, in the contacts, FG layer also acts as an efficient charge injection layer, leading to the formation of Ohmic contacts and high carrier mobility of up to 64 cm2 V−1 s−1 at room temperature. This work shows a novel strategy for simultaneous fabrication of passivation layer and low‐resistance contacts by using ultrathin functionalized graphene, which has applications for high performance 2D semiconductor integrated electronics.

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