Hwan Sung Choe

Anisotropic in-plane thermal conductivity of black phosphorus nanoribbons at temperatures higher than 100 K

Black phosphorus attracts enormous attention as a promising layered material for electronic, optoelectronic and thermoelectric applications. Here we report large anisotropy in in-plane thermal conductivity of single-crystal black phosphorus nanoribbons along the zigzag and armchair lattice directions at variable temperatures. Thermal conductivity measurements were carried out under the condition of steady-state longitudinal heat flow using suspended-pad micro-devices. We discovered increasing thermal conductivity anisotropy, up to a factor of two, with temperatures above 100 K. A size effect in thermal conductivity was also observed in which thinner nanoribbons show lower thermal conductivity. Analysed with the relaxation time approximation model using phonon dispersions obtained based on density function perturbation theory, the high anisotropy is attributed mainly to direction-dependent phonon dispersion and partially to phonon–phonon scattering. Our results revealing the intrinsic, orientation-dependent thermal conductivity of black phosphorus are useful for designing devices, as well as understanding fundamental physical properties of layered materials.

Ferroelectrically Gated Atomically Thin Transition-Metal Dichalcogenides as Nonvolatile Memory

Ferroelectrically driven nonvolatile memory is demonstrated by interfacing 2D semiconductors and ferroelectric thin films, exhibiting superior memory performance comparable to existing thin-film ferroelectric field-effect transistors. An optical memory effect is also observed with large modulation of photoluminescence tuned by the ferroelectric gating, potentially finding applications in optoelectronics and valleytronics.

Variable range hopping electric and thermoelectric transport in anisotropic black phosphorus

Black phosphorus (BP) is a layered semiconductor with a high mobility of up to ∼1000 cm2 V−1 s−1 and a narrow bandgap of ∼0.3 eV, and shows potential applications in thermoelectrics. In stark contrast to most other layered materials, electrical and thermoelectric properties in the basal plane of BP are highly anisotropic. To elucidate the mechanism for such anisotropy, we fabricated BP nanoribbons (∼100 nm thick) along the armchair and zigzag directions, and measured the transport properties. It is found that both the electrical conductivity and Seebeck coefficient increase with temperature, a behavior contradictory to that of traditional semiconductors. The three-dimensional variable range hopping model is adopted to analyze this abnormal temperature dependency of electrical conductivity and Seebeck coefficient. The hopping transport of the BP nanoribbons, attributed to high density of trap states in the samples, provides a fundamental understanding of the anisotropic BP for potential thermoelectric appl...

Stress compensation for arbitrary curvature control in vanadium dioxide phase transition actuators

Due to its thermally driven structural phase transition, vanadium dioxide (VO2) has emerged as a promising material for micro/nano-actuators with superior volumetric work density, actuation amplitude, and repetition frequency. However, the high initial curvature of VO2 actuators severely obstructs the actuation performance and application. Here, we introduce a “seesaw” method of fabricating tri-layer cantilevers to compensate for the residual stress and realize nearly arbitrary curvature control of VO2 actuators. By simply adjusting the thicknesses of the individual layers, cantilevers with positive, zero, or negative curvatures can be engineered. The actuation amplitude can be decoupled from the curvature and controlled independently as well. Based on the experimentally measured residual stresses, we demonstrate sub-micron thick VO2 actuators with nearly zero final curvature and a high actuation amplitude simultaneously. This “seesaw” method can be further extended to the curvature engineering of other microelectromechanical system multi-layer structures where large stress-mismatch between layers are inevitable.

Black Arsenic: A Layered Semiconductor with Extreme In‐Plane Anisotropy

2D layered materials have emerged in recent years as a new platform to host novel electronic, optical, or excitonic physics and develop unprecedented nanoelectronic and energy applications. By definition, these materials are strongly anisotropic between the basal plane and cross the plane. The structural and property anisotropies inside their basal plane, however, are much less investigated. Black phosphorus, for example, is a 2D material that has such in-plane anisotropy. Here, a rare chemical form of arsenic, called black-arsenic (b-As), is reported as a cousin of black phosphorus, as an extremely anisotropic layered semiconductor. Systematic characterization of the structural, electronic, thermal, and electrical properties of b-As single crystals is performed, with particular focus on its anisotropies along two in-plane principle axes, armchair (AC) and zigzag (ZZ). The analysis shows that b-As exhibits higher or comparable electronic, thermal, and electric transport anisotropies between the AC and ZZ directions than any other known 2D crystals. Such extreme in-plane anisotropies can potentially implement novel ideas for scientific research and device applications.

Quantifying van der waals interactions in layered transition metal dichalcogenides from pressure-enhanced valence band splitting

van der Waals (vdW) forces, despite being relatively weak, hold the layers together in transition metal dichalcogenides (TMDs) and play a key role in their band structure evolution, hence profoundly affecting their physical properties. In this work, we experimentally probe the vdW interactions in MoS2 and other TMDs by measuring the valence band maximum (VBM) splitting (Δ) at K point as a function of pressure in a diamond anvil cell. As high pressure increases interlayer wave function coupling, the VBM splitting is enhanced in 2H-stacked MoS2 multilayers but, due to its specific geometry, not in 3R-stacked multilayers, hence allowing the interlayer contribution to be separated out of the total VBM splitting, as well as predicting a negative pressure (2.4 GPa) where the interlayer contribution vanishes. This negative pressure represents the threshold vdW interaction beyond which neighboring layers are electronically decoupled. This approach is compared to first-principles calculations and found to be widely applicable to other group-VI TMDs.

A multifunctional micro-electro-opto-mechanical (MEOM) platform based on phase-transition materials

Along with the rapid development of hybrid electronic-photonic systems, multifunctional devices with dynamic responses have been widely investigated for improving many optoelectronic applications. For years, microelectro-opto-mechanical systems (MEOMS), one of the major approaches to realizing multifunctionality, have demonstrated profound reconfigurability and great reliability. However, modern MEOMS still suffer from limitations in modulation depth, actuation voltage, or miniaturization. Here, we demonstrate a new MEOMS multifunctional platform with greater than 50% optical modulation depth over a broad wavelength range. This platform is realized by a specially designed cantilever array, with each cantilever consisting of vanadium dioxide, chromium, and gold nanolayers. The abrupt structural phase transition of the embedded vanadium dioxide enables the reconfigurability of the platform. Diverse stimuli, such as temperature variation or electric current, can be utilized to control the platform, promising CMOS-compatible operating voltage. Multiple functionalities, including an active enhanced absorber and a reprogrammable electro-optic logic gate, are experimentally demonstrated to address the versatile applications of the MEOMS platform in fields such as communication, energy harvesting, and optical computing.

A 0.2 V Micro-Electromechanical Switch Enabled by a Phase Transition

Micro-electromechanical (MEM) switches, with advantages such as quasi-zero leakage current, emerge as attractive candidates for overcoming the physical limits of complementary metal-oxide semiconductor (CMOS) devices. To practically integrate MEM switches into CMOS circuits, two major challenges must be addressed: sub 1 V operating voltage to match the voltage levels in current circuit systems and being able to deliver at least millions of operating cycles. However, existing sub 1 V mechanical switches are mostly subject to significant body bias and/or limited lifetimes, thus failing to meet both limitations simultaneously. Here 0.2 V MEM switching devices with ≳106 safe operating cycles in ambient air are reported, which achieve the lowest operating voltage in mechanical switches without body bias reported to date. The ultralow operating voltage is mainly enabled by the abrupt phase transition of nanolayered vanadium dioxide (VO2 ) slightly above room temperature. The phase-transition MEM switches open possibilities for sub 1 V hybrid integrated devices/circuits/systems, as well as ultralow power consumption sensors for Internet of Things applications.

Compensated thermal conductivity of metallically conductive Ta-doped TiO2

Electrical and thermal conductivities of epitaxial, high-quality Ta-doped TiO2 (Ta:TiO2) thin films were experimentally investigated in the temperature range of 35–375 K. Structurally identified as the anatase phase, degenerate Ta doping leads to high electrical conductivity in TiO2, reaching >105 (Ω-m)−1 at 5 at. % of Ta, making it a potential candidate for indium-free transparent conducting oxides. In stark contrast, Ta doping suppresses the thermal conductivity of TiO2 via strong phonon-impurity scattering imposed by the Ta dopant which has a high mass contrast with Ti that it substitutes. For instance, the near-peak value shows a >50% reduction, from 9.0 down to 4.4 W/m-K, at just 2 at. % doping at 100 K. Interestingly, further Ta doping beyond 2 at. % no longer reduces the measured total thermal conductivity, which is attributed to a high electronic contribution to thermal conduction that compensates the alloy-scattering loss, as well as possibly the renormalization of phonon dispersion relation in the heavy doping regime originating from doping-induced lattice stiffening. As a result, at high Ta doping, TiO2 exhibits high electrical conductivity without much degradation of thermal conductivity. For example, near room temperature, 5 at. % Ta doped TiO2 shows over 3 orders of magnitude enhancement in electrical conductivity from undoped TiO2, but with only less than 10% reduction in thermal conductivity. The metallic Ta:TiO2 maintaining reasonable good thermal conductivity might find application in energy devices where good conduction to both charge and heat is needed.

Enhancing Modulation of Thermal Conduction in Vanadium Dioxide Thin Film by Nanostructured Nanogaps

Efficient thermal management at the nanoscale is important for reducing energy consumption and dissipation in electronic devices, lab-on-a-chip platforms and energy harvest/conversion systems. For many of these applications, it is much desired to have a solid-state structure that reversibly switches thermal conduction with high ON/OFF ratios and at high speed. Here we describe design and implementation of a novel, all-solid-state thermal switching device by nanostructured phase transformation, i.e., modulation of contact pressure and area between two poly-silicon surfaces activated by microstructural change of a vanadium dioxide (VO2) thin film. Our solid-state devices demonstrate large and reversible alteration of cross-plane thermal conductance as a function of temperature, achieving a conductance ratio of at least 2.5. Our new approach using nanostructured phase transformation provides new opportunities for applications that require advanced temperature and heat regulations.