Nanomechanics.
Mechanical deformation of nanostructured materials leads to the emergence of phenomena that can be utilized to study unexplored physical concepts and engineer novel devices. We exploit these principles to enable the realization of technologies including ultra-thin speakers, energy efficient electromechanical switches, and tunable lasers. To achieve these new device architectures and facilitate exploration of the nanoscale, we continuously develop nanofabrication and characterization techniques.
Team Members
An Ultra-Thin Flexible Loudspeaker Based on a Piezoelectric Micro-Dome Array.
Ultra-thin, lightweight, high-performance, low-cost and energy-efficient loudspeakers that can be deployed over a wide area have become increasingly attractive to both traditional audio systems and emerging applications such as active noise control and immersive entertainment. In this paper, a thin-film loudspeaker is proposed based on an active piezoelectric layer embossed with an array of microscale domes. Actuation of these freestanding domes contributes to excellent sound generation by the loudspeaker, for example, 86 dB sound pressure level (SPL) at 30-cm distance with 25-V (RMS) excitation at 10 kHz, regardless of the rigid surface on which it is bonded. The acoustic performance is further tunable by designing the dome dimensions. The proposed loudspeaker also exhibits high bandwidth, which extends its prospects into the ultrasonic range. The loudspeaker weighs only 2 g, is 120 μm thick and can be manufactured at low cost. These advantages make the proposed loudspeaker a promising candidate for ubiquitous applications in existing and emerging industrial and commercial scenarios.
Han J., Lang J.H., Bulović V., IEEE Transactions on Industrial Electronics (2022)
Molecular Platform for Fast Low-Voltage Nanoelectromechanical Switching.
The use of molecules as active components to build nanometer-scale devices inspires emerging device concepts that employ the intrinsic functionality of molecules to address longstanding challenges facing nanoelectronics. Using molecules as controllable-length nanosprings, here we report the design and operation of a nanoelectromechanical (NEM) switch which overcomes the typical challenges of high actuation voltages and slow switching speeds for previous NEM technologies. Our NEM switches are hierarchically assembled using a molecular spacer layer sandwiched between atomically smooth electrodes, which defines a nanometer-scale electrode gap and can be electrostatically compressed to repeatedly modulate the tunneling current. The molecular layer and the top electrode structure serve as two degrees of design freedom with which to independently tailor static and dynamic device characteristics, enabling simultaneous low turn-on voltages (sub-3 V) and short switching delays (2 ns). This molecular platform with inherent nanoscale modularity provides a versatile strategy for engineering diverse high-performance and energy-efficient electromechanical devices.
Research Projects.
Low-Voltage Switches.
Low-Voltage and Stiction-Free Nanoelectromechanical Squitches
Farnaz Niroui, Mayuran Saravanapavanantham, Annie Wang, Vladimir Bulović
Collaborators: Ellen Sletten, Wen Jie Ong, Timothy Swager, Jeffrey Lang
Nanoelectromechanical (NEM) switches have emerged as a promising competing technology to the conventional complementary metal-oxide semiconductor (CMOS) transistors. NEM switches can exhibit abrupt switching behavior with large on-off current ratios and near-zero off-state leakage currents. However, they typically require large operating voltages exceeding 1 V and suffer from failure due to stiction. To address these challenges, we propose an electromechanical switch, referred to as a “squitch”, based on a switching gap composed of a molecular film sandwiched between conductive contacts. In this design, an applied voltage between the electrodes provides sufficient electrostatic force to compress the molecular film. As the molecules are compressed, the distance between the electrodes is reduced, causing an exponential increase in the tunneling current to turn on the device. The molecular layer helps formation of nanoscale switching gaps that promote lowering of the actuation voltage. Concurrently, the elastic restoring force in the compressed molecular film helps overcome surface adhesive forces during operation to prevent stiction-induced failure.
Related publications and links
F. Niroui, A.I. Wang, E. M. Sletten, Y. Song, J. Kong, E. Yablonovitch, T. M. Swager, J. H. Lang, and V. Bulović, “Tunneling Nanoelectromechanical switches based on compressible molecular thin films,” ACS Nano, vol. 9, 7886-7894 (2015).
F. Niroui, E.M. Sletten, P.B. Deotare, A.I. Wang, T.M. Swager, J.H. Lang, and V. Bulović, “Controlled fabrication of nanoscale gaps using stiction,” in Proc. 28th IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 85-88 (2015).
F. Niroui, P.B. Deotare, E.M. Sletten, A.I. Wang, E. Yablonovitch, T.M. Swager, J.H. Lang, and V. Bulović, “Nanoelectromechanical tunneling switches based on self-assembled molecular layers,” in Proc. 27th IEEE International Conference on Micro Electro Mechanical Systems (MEMS), 1103-1106 (2014).
Electrically Tunable Lasers.
Electrically Tunable Organic Vertical-Cavity Surface-Emitting Lasers
Wendi Chang, Apoorva Murarka, Annie Wang, Vladimir Bulović
Since their invention in the 1950s, laser systems are an example of a ubiquitous technology that enabled a wide range of applications. From Blu-ray to surgeries, lasers also created the foundation of the modern field of photonics and spectroscopy. However, creating a compact, dynamically tunable laser in the visible wavelengths is still an undeveloped problem. A wavelength tunable lasing device would enable many fields of research and technology, including spectroscopy and remote sensing. Laser wavelength tuning using standard nonlinear optics techniques requires high power and large equipments. While small compact lasers with a range of visible emission wavelengths has been fabricated on a single device, the emission is predetermined during fabrication and cannot be dynamically tuned.
Drawing inspiration from developments in micro-electro-mechanical systems (MEMS) of tunable inorganic, infrared lasers, we demonstrate a organic vertical-cavity surface-emitting laser (VCSEL) with dynamic wavelength-tunability in the visible wavelengths. Since standard inorganic semiconductor fabrication methods such as lithography and etching cannot be applied to soft, organic materials, we develop a composite membrane contact-transfer printing technique to fabricate an array of microcavities. Each vertical cavity is formed by a bottom DBR substrate and a suspended top silver mirror. By applying a voltage across top gold contact and bottom indium tin oxide (ITO) electrode, electrostatic pressure deflects the top membrane, changing the cavity length and laser emission wavelength. Beyond tunable lasing, the same device in could enable many applications as an all-optical pressure sensing array, where the cavity length change is correlated with pressure difference across each membrane.
Figure: (a) Schematic of device structure; voltage applied between the top gold and bottom ITO allows membrane deflection due to electrostatic force. (b) Optical interferometry difference image between 20V and 0V applied bias on an array of devices as imaged from the top membrane. (c) Comparison of cavity mode emission peak shift with membrane deflection. (d) Difference profile between applied bias and 0V bias show controllable membrane deflection.
Related publications and links
W. Chang*, A. Murarka*, A. Wang*, G. M. Akselrod, C. Packard, J. Lang, and V. Bulovic, “Electrically tunable organic vertical-cavity surface-emitting laser,” Appl. Phys. Lett., 105, 073303 (2014).
W. Chang*, A. Wang*, A. Murarka*, J. Lang, and V. Bulovic, "Transfer-Printed Composite Membranes for Electrically-Tunable Organic Optical Microcavities,” Micro Electro Mechanical Systems (MEMS), 2014 IEEE 27th International Conference on.