Project description: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.
Project description:Transfer printing of thin-film nanoelectronics from their fabrication wafer commonly requires chemical etching on the sacrifice of wafer but is also limited by defects with a low yield. Here, we introduce a wafer-recyclable, environment-friendly transfer printing process that enables the wafer-scale separation of high-performance thin-film nanoelectronics from their fabrication wafer in a defect-free manner that enables multiple reuses of the wafer. The interfacial delamination is enabled through a controllable cracking phenomenon in a water environment at room temperature. The physically liberated thin-film nanoelectronics can be then pasted onto arbitrary places of interest, thereby endowing the particular surface with desirable add-on electronic features. Systematic experimental, theoretical, and computational studies reveal the underlying mechanics mechanism and guide manufacturability for the transfer printing process in terms of scalability, controllability, and reproducibility.
Project description:Vanadium dioxide (VO2) thin films of different thicknesses were prepared by regulating the deposition time (2, 2.5, 3, and 3.5 h). The impact of deposition time on the microstructure, surface morphology, and cross-section morphology was investigated. The results showed that the grain size increased with the film thickness. Meanwhile, the influence of film thickness on the residual stress was evaluated by X-ray diffraction. The phenomenon of "compressive-to-tensile stress transition" was illustrated as the thickness increased. The change of dominant mechanism for residual stress was used for explaining this situation. First, the composition of residual stress indicates that growth stress play a key role. Then, the effect of "atomic shot peening" can be used to explain the compressive stress. Lastly, the increased grain size, lower grain boundary density, and "tight effect" in the progress of film growth cause tensile stress.
Project description:A wafer scale bulge testing system has been constructed to study the mechanical properties of thin films and microstructures. The custom built test stage was coupled with a pressure regulation system and optical profilometer which gives high accuracy three-dimensional topographic images collected on the time scale of seconds. Membrane deflection measurements can be made on the wafer scale (50-150 mm) with up to nanometer-scale vertical resolution. Gauge pressures up to 689 kPa (100 psi) are controlled using an electronic regulator with and accuracy of approximately 0.344 kPa (0.05 psi). Initial testing was performed on square diaphragms 350, 550, and 1200 microm in width comprised of 720+/-10 nm thick low pressure chemical vapor deposited silicon nitride with approximately 20 nm of e-beam evaporated aluminum. These initial experiments were focused on measuring the system limitations and used to determine what range of deflections and pressures can be accurately measured and controlled. Gauge pressures from 0 to approximately 8.3 kPa (1.2 psi) were initially applied to the bottom side of the diaphragms and their deflection was subsequently measured. The overall pressure resolution of the system is good (approximately 350 Pa) but small fluctuations existed at pressures below 5 kPa leading to a larger standard deviation between deflection measurements. Analytical calculations and computed finite element analysis deflections closely matched those empirically measured. Using an analytical solution that relates pressure deflection data for the square diaphragms the Young's modulus was estimated for the films assuming a Poisson's ratio of v=0.25. Calculations to determine Young's modulus for the smaller diaphragms proved difficult because the pressure deflection relationship remained in the linear regime over the tested pressure range. Hence, the calculations result in large error when used to estimate the Young's modulus for the smaller membranes. The deflection measurements of three 1200 x 1200 microm(2) Si(3)N(4-x) membranes were taken at increased pressures (>25 kPa) to increase nonlinearity and better determine Young's modulus. This pressure-deflection data were fit to an analytical solution and Young's modulus estimated to be 257+/-3 GPa, close to those previously reported in literature.
Project description:Au-Au surface activated bonding (SAB) using ultrathin Au films is effective for room-temperature pressureless wafer bonding. This paper reports the effect of the film thickness (15-500 nm) and surface roughness (0.3-1.6 nm) on room-temperature pressureless wafer bonding and sealing. The root-mean-square surface roughness and grain size of sputtered Au thin films on Si and glass wafers increased with the film thickness. The bonded area was more than 85% of the total wafer area when the film thickness was 100 nm or less and decreased as the thickness increased. Room-temperature wafer-scale vacuum sealing was achieved when Au thin films with a thickness of 50 nm or less were used. These results suggest that Au-Au SAB using ultrathin Au films is useful in achieving room-temperature wafer-level hermetic and vacuum packaging of microelectromechanical systems and optoelectronic devices.
Project description:Graphene films, fabricated by chemical vapor deposition (CVD) method, have exhibited superiorities in high crystallinity, thickness controllability, and large-scale uniformity. However, most synthesized graphene films are substrate-dependent, and usually fragile for practical application. Herein, a freestanding graphene film is prepared based on the CVD route. By using the etchable fabric substrate, a large-scale papyraceous freestanding graphene fabric film (FS-GFF) is obtained. The electrical conductivity of FS-GFF can be modulated from 50 to 2800 Ω sq-1 by tailoring the graphene layer thickness. Moreover, the FS-GFF can be further attached to various shaped objects by a simple rewetting manipulation with negligible changes of electric conductivity. Based on the advanced fabric structure, excellent electrical property, and high infrared emissivity, the FS-GFF is thus assembled into a flexible device with tunable infrared emissivity, which can achieve the adaptive camouflage ability in complicated backgrounds. This work provides an infusive insight into the fabrication of large-scale freestanding graphene fabric films, while promoting the exploration on the flexible infrared camouflage textiles.
Project description:Remarkable improvements in both structural and optical properties of wafer-scale hexagonal boron nitride (h-BN) films grown by metal-organic chemical vapor deposition (MOCVD) enabled by high-temperature post-growth annealing is presented. The enhanced crystallinity and homogeneity of the MOCVD-grown h-BN films grown at 1050 °C is attributed to the solid-state atomic rearrangement during the thermal annealing at 1600 °C. In addition, the appearance of the photoluminescence by excitonic transitions as well as enlarged optical band gap were observed for the post-annealed h-BN films as direct consequences of the microstructural improvement. The post-growth annealing is a very promising strategy to overcome limited crystallinity of h-BN films grown by typical MOCVD systems while maintaining their advantage of multiple wafer scalability for practical applications towards two-dimensional electronics and optoelectronics.
Project description:Engineered metamaterials offer unique functionalities for manipulating the spectral and spatial properties of electromagnetic waves in unconventional ways. Here, we report a novel approach for making reconfigurable metasurfaces capable of deflecting electromagnetic waves in an electronically controllable fashion. This is accomplished by tilting the phase front of waves through a two-dimensional array of resonant metasurface unit-cells with electronically-controlled phase-change materials embedded inside. Such metasurfaces can be placed at the output facet of any electromagnetic radiation source to deflect electromagnetic waves at a desired frequency, ranging from millimeter-wave to far-infrared frequencies. Our design does not use any mechanical elements, external light sources, or reflectarrays, creating, for the first time, a highly robust and fully-integrated beam-steering device solution. We demonstrate a proof-of-concept beam-steering metasurface optimized for operation at 100?GHz, offering up to 44° beam deflection in both horizontal and vertical directions. Dynamic control of electromagnetic wave propagation direction through this unique platform could be transformative for various imaging, sensing, and communication applications, among others.
Project description:A systematic study of various metal-insulator transition (MIT) associated phases of VO2, including metallic R phase and insulating phases (T, M1, M2), is required to uncover the physics of MIT and trigger their promising applications. Here, through an oxide inhibitor-assisted stoichiometry engineering, we show that all the insulating phases can be selectively stabilized in single-crystalline VO2 beams at room temperature. The stoichiometry engineering strategy also provides precise spatial control of the phase configurations in as-grown VO2 beams at the submicron-scale, introducing a fresh concept of phase transition route devices. For instance, the combination of different phase transition routes at the two sides of VO2 beams gives birth to a family of single-crystalline VO2 actuators with highly improved performance and functional diversity. This work provides a substantial understanding of the stoichiometry-temperature phase diagram and a stoichiometry engineering strategy for the effective phase management of VO2.
Project description:It has been reported that mitochondrial metabolic and biophysical parameters are associated with degenerative diseases and the aging process. To evaluate these biochemical parameters, current technology requires several hundred milligrams of isolated mitochondria for functional assays. Here, we demonstrate manufacturable wafer-scale mitochondrial functional assay lab-on-a-chip devices, which require mitochondrial protein quantities three orders of magnitude less than current assays, integrated onto 4'' standard silicon wafer with new fabrication processes and materials. Membrane potential changes of isolated mitochondria from various well-established cell lines such as human HeLa cell line (Heb7A), human osteosarcoma cell line (143b) and mouse skeletal muscle tissue were investigated and compared. This second generation integrated lab-on-a-chip system developed here shows enhanced structural durability and reproducibility while increasing the sensitivity to changes in mitochondrial membrane potential by an order of magnitude as compared to first generation technologies. We envision this system to be a great candidate to substitute current mitochondrial assay systems.